Endocrinology Notes PDF

**Endocrinology Notes** **[Week 1 - Josef Köhrle]** **Lecture 1** **Modern endocrinology** has expanded to include other tissues and organs, like adipose tissue (leptin). Gut, liver, and muscles also have endocrine functions. **Endocrine feedback** systems operate through negative feedback loops...

**Endocrinology Notes** **[Week 1 - Josef Köhrle]** **Lecture 1** **Modern endocrinology** has expanded to include other tissues and organs, like adipose tissue (leptin). Gut, liver, and muscles also have endocrine functions. **Endocrine feedback** systems operate through negative feedback loops, where a change (e.g., blood glucose levels) triggers a response that counteracts it. For example, when blood glucose levels rise after a meal, the pancreas releases insulin to lower glucose levels. If glucose levels drop too low, the pancreas releases glucagon to raise them. Positive feedback loops in endocrinology are less common but still essential. They amplify and accelerate a response. For example, during childbirth, oxytocin triggers uterine contractions, which, in turn, stimulate the release of more oxytocin, for the baby to be born. **The hypothalamus and pituitary gland** in the brain are central components in many endocrine feedback loops. The hypothalamus produces **releasing hormones** that signal the pituitary gland to release or inhibit the secretion of various hormones, which, in turn, control the activity of other endocrine glands. **An endocrine axis** is a series of glands and tissues that work together to regulate hormonal secretion. Key components include **hypothalamus, pituitary, target gland and feedback.** **Common endocrine axes include:** - **Hypothalamic-Pituitary-Adrenal** (HPA) Axis: release of cortisol from the adrenal glands, particularly in response to stress. - **Hypothalamic-Pituitary-Thyroid** (HPT) Axis: secretion of thyroid hormones, which are essential for metabolic processes and energy regulation. - **Hypothalamic-Pituitary-Gonadal** (HPG) Axis: Regulates the production of sex hormones, including testosterone and estrogen, influencing reproductive and sexual development. - **Hypothalamic-Pituitary-Growth** (HPG) Axis: Controls the secretion of growth hormone, which plays a vital role in childhood growth and various metabolic processes in adulthood. **Many endocrine systems depend on circadian rhythms**, which influence the timing of hormone secretion. Many hormones, such as cortisol, melatonin, and certain growth hormones, follow a circadian pattern of secretion. For example, cortisol levels tend to peak in the morning to help with wakefulness and energy, while melatonin levels rise in the evening to promote sleep. Endocrinology is all about the precise regulation of hormone concentration, timing, and location within the body. Excess or deficiency of any hormonal regulation event at some point can lead to pathophysiology. The body receives different types of signals like neural, immune, endocrine, nutrition, environmental, microbiome and others. **!!!Aschheim-Zondek** reaction is the name for today's standard pregnancy test, that detects hCG (human chorion gonadotropin) in urine. Initially, in one of their publications they made a mistake, as hCG does not originate from the anterior pituitary but **it is produced by the chorion, an early placental structure**. Early pregnancy tests also included methods like the \"frog test,\" which involved injecting a woman\'s urine into a frog to detect hCG-induced changes in the frog\'s ovaries. In female mice upon subcutaneous injection of pregnant human\'s urine, enlargement of ovaries and maturation of ovarian follicles are observed. Today hCG is detected with standard immunoassays. **!!!hCG a similar structure to TSH, LH and FSH** (3 hormones produced by the ant. pit.). All have an alpha and beta subunit. **Alpha is the same** and beta differs. They are called **glycoproteohormones**, **are heavily glycosylated and have big corresponding GPCRS.** Non-mammal animals do not have hCG as they do not have a placenta. **Evolution of Glycoprotein Hormones (GPHs)** and receptors reveals a shared history with α and β subunits. GPH-A2B5 (thyrostiumulin) is a conserved precursor-GPH. GPHRs, present in diverse organisms, have distinct roles. In mosquitos, A2B5 receptor aids survival during oxidative stress from blood meals, showcasing diverse effects of GPHs in evolution. **The Hypothalamic-Pituitary-Thyroid (HPT) axis** starts in the hypothalamus in the brain after receiving signals from the CNS like cold exposure. As a response, it releases thyrotropin-releasing hormone (TRH), which signals thyrotroph cells in the anterior pituitary to produce thyroid-stimulating hormone (TSH - thyrotropin). TSH, in turn, stimulates the thyroid gland to release thyroxine (T4) and triiodothyronine (T3), two hormones that regulate metabolism and other bodily functions, like temperature homeostasis. **!!!Around the 12th week of pregnancy, hCG levels peak.** **High concentrations of hCG can compete with TSH** for binding to the TSH receptor. As a result, the normal negative feedback loop involving thyroid hormones (T3 and T4) is disrupted, leading to altered thyroid function. This can result in some pregnant women feeling less sensitive to cold. These are part of the body\'s evolutionary mechanisms to ensure optimal conditions for the developing fetus. Thyroid hormones are crucial for the baby\'s brain development, and during pregnancy, the body prioritizes delivering these hormones to the fetus, sometimes at the expense of the mother. **!!!Glycoproteohormones** (TSH, LH, FSH & hCG and thyrostimulin) **have a common a and a different b subunit**. b subunit alone cannot interact with the receptor, as the latter needs to \"see\" the heterodimer, but the b subunit defines the specificity and is recognized. We have 3 instead of 4 receptors as **hCG has the same receptor as LH**, needs to be at higher concentrations than LH to compete/activate the receptor effectively. **FSH receptor is the most specific - does not tolerate** any other ligand. As said LH receptor tolerates 2 ligands BUT **TSH receptor 4** **(tolerates the most ligands)**, TSH, TRAK, thyrostimulin (function unknown) and high levels of hCG. **TRAK** are thyrotropin receptor antibodies; they are autoantibodies responsible for the **Grave\'s disease**. To treat, take the thyroid out or destroy with iodine. **!!!Where is hCG coming from and why is it detectable in the urine?** In early development, the blastocyst attaches to the uterine lining, and its cells invade and differentiate into the **syncytiotrophoblast**. This tissue then secretes hCG into the maternal bloodstream, **signaling the body to maintain the corpus luteum (which produces progesterone to support early pregnancy), stimulating the ovaries to produce estrogen and progesterone, and supporting the overall pregnancy.** hCG levels in early pregnancy are very high, and some of it can spill into the mother\'s urine, making it detectable by pregnancy tests. **Arnold Berthold: first endocrine experiment**, he removes the testis and transplanted them to a previously castrated rooster, managing to restore the masculine phenotype. **He concluded that there must be a compound produced by the testis and that is transported via blood, that is powerful in low concentrations and impacts other tissues.** **Hormones -- Definition I**: - biochemical signaling compounds by specialized cells. - released into the bloodstream or interstitial space and can travel over long distances to reach their target. - exert their biological effects at very low concentrations (nano, pico, even femto). - their effects can vary based on their concentration. - specific - bind to receptors and activating pathways. - long-term and immediate effects. - not only activate but also inhibit (in one word modulate). **Endocrine glands lack ducts**, unlike exocrine glands that secrete their products through ducts into body cavities. Endocrine glands **rely on a rich blood supply** for both efficient hormone secretion and the reception of messenger molecules, nutrient and oxygen supply, as well as the removal of waste products. !!!Endocrine glands contain precursor molecules, or the biologically active **hormone stored** (TG stored in thyroid follicles). **Steroid hormones** cannot be stored as they would integrate into the plasma membrane and they **must be synthesized de novo all the time**. **In neuronal communication** direct electrical messages are sent into the endosynaptic cleft. In endocrine communication hormonal messages are sent via the bloodstream and target cells respond by expressing the necessary receptor. While the nervous and endocrine systems often collaborate to regulate physiology, some neurons can also function as endocrine cells. **Lecture 2 - Kohrle** A diagram of a human body Description automatically generated The main difference in males and females in the context of endocrinology is the reproductive system and sex hormones. However, sex has often an impact on other tissues; according to sex some axes are more present. For example, thyroid diseases, both benign and malignant, are much more prevalent in females. **Melatonin** is the hormone of darkness. It synchronizes the rhythm to darkness and drives darkness-related behavior. Humans are day-active animals and thus it is often mistakenly called the sleep hormone. For nocturnal animals, melatonin is higher when they are at their most active state. **New paradigms in endocrinology:** ANPs from the heart (blood pressure and fluid balance), vitamin-D from the skin, liver produces hepcidin (iron homeostasis), stomach ghrelin (stimulates appetite), subcutaneous adipose tissue produces leptin (body weight and energy balance), myokines are released by myocytes during contraction, phosphatonin is a hormone produced by osteocytes in bone and plays a crucial role in the regulation of phosphate and vitamin D metabolism, heme (has nuclear receptors that regulate circadian rhythms and metabolism). **→ The conclusion is that there are molecules that come from \"non-traditional\" endocrine organs and turn out to have endocrine functions.** **Irisin,** once thought to increase with exercise and counter adipose tissue, may not undergo the same process in humans as in mice. FNDC5, the human irisin precursor, may not activate during physical activity, with limited data in humans compared to mice. **White and brown adipose tissue:** The liver-secreted fibroblast growth factors (FGFs) have been implicated in the conversion of white to brown adipose tissue. The dynamics of the transition from white to brown adipocytes are not well-defined. Adipocytes, composed mainly of a large lipid droplet, produce hormones like leptin, with brown adipocytes characterized by multiple lipid droplets and high mitochondria density. **Adipocytes release adipokines, including leptin.** Among myokines there are also antagonists that signal back to the muscle to decrease function and size like **myostatin**. Cows with blocked/mutated myostatin are very muscular. **Hormone analytics** demand specialized tools due to their low concentrations. Monoclonal antibodies enable immunoassays like ELISA and RIA (using isotopes). Mass spectrometry has emerged as a gold standard, despite being costly and requiring pre-analytic preparation. Techniques apply to different samples. **Hormone multifaceted functions:** body functions, including metabolism and growth. At the cellular level, they control processes like division and gene transcription. Molecularly, hormones impact protein synthesis, enzyme activity, and gene regulation. Maintaining balance is crucial for homeostasis; disruptions can lead to hyperfunction or hypofunction, arising from changes in production, secretion, or receptor modulation. **Signal and information systems:** these 3 systems need to be in coordination. A compound can be a neurotransmitter, like catecholamines (adrenaline), when secreted into the neurosynaptic cleft and a hormone, when secreted by the adrenal gland. The body integrates signals also from nutrition and microbiota. **Chemically, hormones** have been identified in the form of peptides/proteins, fatty acids and cholesterol derived. Yet, no nucleic acid or sugar molecules have been found to function like hormones. **2 important messages:** - **Message 1: Neurons have hormone receptors!** While neurons primarily communicate through neurotransmitters at synapses, some express receptors for hormones. - **Message 2: Endocrine cells have receptors for neurotransmitters & hormones!** Beyond responding to circulating hormones, endocrine cells also possess receptors for neurotransmitters. For example, adrenal medulla endocrine cells, receive signals from the sympathetic nervous system in the form of neurotransmitters, influencing the release of hormones like adrenaline. The endocrine system, through the release of hormones into the bloodstream, exhibits a high degree of versatility as hormones can travel throughout the body, affecting multiple target organs and tissues (**more systemic**). The nervous system typically employs neurotransmitters for cell-to-cell communication, and these molecules act in a **more localized manner**. Neurotransmitters are released at synapses, influencing the activity of the next neuron in a specific neural pathway. While the hypothalamus and pituitary gland are classic examples of neuroendocrine structures, **the gastrointestinal (GI) tract is also rich in neuroendocrine circuits**. In the GI tract, there is an extensive network of neuroendocrine cells, which release signaling molecules, including hormones, in response to factors such as nutrient availability. **!!!Types of hormonal regulation.** \* H = Hormone N = Neuropeptide R = Receptor  **Adenohypophysis** is not considered a neuronal component -- parenchymal tissue not part of the brain. !!!**From PSCs with different cocktails of TFs we reach 5 different cell types in the anterior pituitary, the -trophes**, getting stimulated (or inhibited) by releasing (or release-inhibiting) hormones and producing ACTH, PRL, GH, TSH, FSH and LH. Somatotrophs are the predominant cell type in the anterior pituitary, and growth hormone is the major hormone they produce.  **!!!The posterior pituitary receives its blood supply mainly from the inferior artery, whereas the** **anterior pituitary is supplied by both the superior and inferior** arteries, with a unique **portal system** connecting it to the hypothalamus for the efficient transport of regulatory hormones. **The posterior pituitary** houses magnocellular neurosecretory cells (MNCs), specialized neurons that have dendrites and project single axons to the posterior pituitary gland, where they release **oxytocin and vasopressin** (also known as antidiuretic hormone, ADH). Oxytocin is recognized for facilitating uterine contractions during childbirth, milk ejection during breastfeeding and for its angiogenic role in development. In contrast, vasopressin regulates water balance by influencing water reabsorption in the kidneys and vasoconstriction of blood vessels. **Folliculostellate (FS) cells are non-endocrine cells that make up a small percentage of the anterior pituitary** cell population. They are characterized by their star-shaped appearance and are involved in paracrine signaling, mediating communication between different cell types within the anterior pituitary, but also between the anterior pituitary and the immune system. A table with text and numbers Description automatically generated **!!!The Hypothalamus-Pituitary-Adrenal (HPA) axis** regulates the body\'s response to stress. **The process begins in the hypothalamus, where neurons release corticotropin-releasing hormone (CRH)** **in response to stress or a circadian rhythm**. **CRH travels to the pituitary gland, signaling the release of adrenocorticotropic hormone (ACTH). ACTH is released by the pituitary gland into the bloodstream and travels to the adrenal glands. There, it stimulates the adrenal glands to produce and release glucocorticoids, primarily cortisol.** Cortisol plays a crucial role in various physiological processes, including metabolism, immune response, and stress adaptation. **The secretion of ACTH and cortisol follows a circadian rhythm, with levels typically peaking in the early morning and reaching their lowest point in the late evening**. This rhythmic pattern is essential for maintaining the body\'s homeostasis and **adapting to the challenges of the day**. Additionally, both ACTH and cortisol are released in a **pulsatile** fashion, meaning they are not continuously secreted but rather in bursts or pulses. **Amplification** **refers to the process by which a small initial signal or stimulus is significantly increased or enhanced at each step of the endocrine pathway**, from the hypothalamus to the anterior pituitary gland, to the target organ, ultimately leading to a robust physiological response.  **Lecture 3** **!!!What are the different ways to stimulate hormone production and/or secretion?** 1\) **With signals coming from the bloodstream**, like calcium levels. An example is the production of PTH (parathyroid hormone, stored in secretory vesicles in chief cells). A drop in serum calcium signals PTH secretion, which regulates calcium homeostasis. Different types of messenger events can stimulate hormone production like ions (calcium) and metabolites\' concentration (glucose). 2\) **Classical neuronal way**: neuron reaches the adrenal medulla cells, synaptic cleft, ACH intermediate molecule, secretion of catecholamines into the nearby capillaries. **!!!And lastly, 3) in a neuroendocrine -- hormonal way, changes in the body are picked up by the CNS, which stimulates the hypothalamus to produce a releasing (or release-inhibiting) hormone of fM to pM concentration that is transported through the hypophyseal portal system to the anterior pituitary, which produces a hormone (FSH, LH, GH, PRL, ACTH, TSH) of nM order. This enters the bloodstream and reaches the target tissue (adrenal cortex, gonads, thyroid, liver etc.). Along this axis we see that the concentration of each response is increasing but the frequency can also be increased as the neurons can fire more frequently (amplification concepts).**  **Adrenaline (epinephrine) produced in the adrenal medulla** activates adrenergic receptors, classic GPCRs, initiating a signal transduction cascade. The signal is amplified resulting in increased heart rate, preparing the body for the \"fight or flight\" response to stress. **The 4 most important signaling receptor classes:** 1. **Ligand-Gated Ion Channels**: directly controlled by the binding of a specific ligand, leading to changes in ion channel conformation or opening/closing. Example: Neurotransmitter receptors, such as acetylcholine receptors. 2. **GPCRs**: signal transduction through G proteins upon ligand binding, leading to intracellular responses. Can be orthosteric (binding at the active site) or allosteric (binding at a different site, modulating receptor activity). **Receptors for glycoproteohormones**, catecholamines, amino acid-derived molecules, and small molecules like serotonin. 3. **RTKs:** Ligand binding activates intrinsic kinase activity, leading to phosphorylation of tyrosine residues and downstream signaling cascades. Again, orthosteric or allosteric regulation. Examples: **Insulin receptor**, epidermal growth factor receptor (EGFR). 4. **Nuclear Hormone Receptors** act as transcription factors upon ligand binding, modulating gene expression in the nucleus. Their ligands are non-hydrophilic hormones, including cholesterol-derived hormones (e.g., **vitamin D, sex hormones (estrogen, testosterone), thyroid hormones, glucocorticoids(cortisol)).** Transporters may be involved in the movement of hydrophobic hormones through the plasma membrane.  **Some neurotransmitters and hormones like estradiol can act via ion channels, leading to rapid (millisecond) responses.** The idea that hydrophobic hormones simply diffuse through the plasma membrane is not universally applicable. Transporters may play a role in facilitating the movement of certain hormones into cells. - **Type I nuclear hormone receptors**, **which mainly regulate steroid hormones**, form homodimers that serve as TFs. Before activation, these sit in the cytosol, in complexes with chaperone proteins, like heat shock protein 90 (Hsp90). Upon hormone binding, a conformational change occurs in the monomeric receptor, leading to the dissociation of the chaperone complex. The hormone-bound receptor then homodimerizes and enters the nucleus, where it exhibits TF activity, regulating gene transcription. - **Type II nuclear hormone receptors** **form heterodimers with retinoic acid receptors**. These receptors sit on genes, waiting for ligands to enter the nucleus. The activation process involves the ligand (hormone) entering the nucleus and occupying the ligand-binding site of the nuclear hormone receptor. This results in a conformational change, leading to either the inhibition or activation of transcription. - **Type III and IV nuclear hormone receptors** are less well-understood- unknown ligands.  !!!In the **canonical** pathway, estrogens, such as estradiol, enters the cell and binds to intracellular estrogen receptors. The estrogen-receptor complex translocates to the nucleus, binds to estrogen response elements (EREs), and, with the involvement of coactivators and corepressors, modulates the transcription of target genes. This process is considered relatively slower, taking around 30 minutes for significant activation. **In the non-canonical pathway**, estrogen interacts with membrane receptors, triggering rapid, non-genomic signaling events. **Firstly, some hormones exhibit minimal changes** - a more stable situation. Examples are **hormones with long half-lives, such as T4**, which has a half-life of 7-8 days in humans. T4 serves as a precursor to the active hormone, T3. The majority of T4 is protected from degradation, and only a small fraction is in its free form. T3, with a shorter half-life of 10-20 hours, may exhibit more pronounced day-night cycle variations. **Secondly, hormones can respond to specific stimuli, resulting in fluctuations**. For instance, **insulin secretion** is stimulated by food intake, leading to increased blood sugar levels and subsequently reduced glucagon. This exemplifies a regulatory feedback loop where hormone levels change in response to physiological needs, such as maintaining glucose homeostasis. **Thirdly, hormones like growth hormone demonstrate variations associated with** sleep, circadian rhythms, or **regular behavioral patterns**. **New GnIH regulators**: a \"kiss neuron\" releases a neuropeptide (kisspeptin), which signals GnRH neurons, influencing the production of FSH and LH. **GnIH neurons** have been found. The **estradiol receptor in the kiss neuron plays a role in adjusting the frequency of kisspeptin secretion.** **!!!The same gonadotroph cells produce both FSH and LH in the anterior pituitary. The decision on whether to release FSH or LH is influenced by feedback systems, with inhibin (only affects FSH) playing a key role in inhibiting FSH release. Also, the gonadotropins are affected differently by GnRH pulsatility.**  **TRH, having these chemical modifications, is protected from both sides**, from the specific TRH degradation enzyme. However, the enzyme can still regulate/degrade it. Neurotransmitters are also protected from ectopeptidases, usually by being stored in vesicles. Glycoprotein hormones undergo post-translational modifications, to enhance the hormones\' stability, bioactivity, and half-life in the bloodstream.  **Comments-Discussion** for the last couple of slides that he went by relatively quick: - Fatty-acid derived hormones, being short-lived, exhibit potent biological activity in regulating various physiological processes. - Plants employ phytohormones like ethylene to signal processes such as leaf abscission, influencing the shedding of leaves. - While pheromones are volatile compounds crucial for animal communication, their presence in humans is not as overt or easily discernible. - While carbon monoxide is not considered a volatile signaling molecule like pheromones, it does have physiological effects and can act as a signaling gas in the body **[Week 2]** **Lecture 1 -- Sarah Paisdzior (Part 1 - Transformation of hormonal signals)** **Hormone classification:** **Steroid**: cholesterol-derived, sex hormones and others (estradiol, testosterone, progesterone, cortisol, aldosterone) **Amine**: tyrosine-derived (L-DOPA intermediate, dopamine, epinephrine, norepinephrine, thyroid hormones, melatonin) **Peptide**: larger protein structures (TRH, IGF-1, GH, ACTH, vasopressin, oxytocin) **Protein**: even larger protein structures (prolactin, insulin) **Glycoproteohormones**: glycosylated protein (releasing hormones, FSH, LH, TSH, hCG) **Fatty acid-derived** -- Eicosanoids hormones: arachidonic acid-derived (prostaglandins, leukotriene). **An example of positive feedback regulation:** During childbirth, the positive feedback loop of oxytocin release is initiated by uterine contractions, stimulated by the stretching of the cervix, leading to **intensified contractions until the baby is delivered**.   1. **GPCRs** (see Annibale lecture) 2. **Enzyme-linked receptors**, **like RTKs** (or cytokine receptors), activate through ligand binding, causing receptor conformational changes and dimerization. This allows **tyrosine residue phosphorylation in receptor intracellular domain, creating docking sites for downstream signaling proteins** like MAPK, PI3/Akt, Ca signaling, and apoptosis. Refined activation involves positive cooperativity with a second ligand binding. Asymmetrical dimerization of the tyrosine kinase domain generates docking sites for SH2 domain-containing proteins, initiating downstream signaling. **Cytokine signaling through the JAK-STAT pathway:** Interleukins, like IL-6, bind to their respective receptors, inducing conformational changes in the receptor. Subsequently, JAKs associated with the cytoplasmic domain of the receptor, become activated. This leads to the phosphorylation of tyrosine residues on the receptor itself and on specific members of the STAT protein family. The phosphorylated STATs form dimers, and these activated dimers translocate to the nucleus. Inside the nucleus, they function as TFs, binding to STAT-binding sequences in gene promoters and regulating the gene expression (see image below). 3. **Channel-linked receptors** (like ion channel receptors): **Acetylcholine** binds to the receptor, which gets activated, Sodium flows into the cell and neutralizes the negative charge inside. Depolarization/activation occurs. 4. **Intracellular receptors** (nuclear -- not on plasma membrane). Four families (remember Kohler's lecture): The DNA-binding domain of nuclear receptors, often containing zinc finger motifs, interacts with specific genomic sequences known as Response Elements (REs). **Response Elements (REs) are specific sequences of DNA that serve as recognition sites for the nuclear receptor, typically located in the regulatory regions of target genes. The binding facilitates the regulation of gene expression by influencing the transcriptional activity.** **!!!T3 uptake involves thyroid hormone transporters, with a specific focus on the** **Monocarboxylate Transporter 8 (MCT8)**. In a clinical excursion, a patient exhibiting **developmental delay**, **mental retardation**, **muscular hypotonia** **and a lack of speech development was found to have an MCT8 mutation.** **Due to the mutation, T3 cannot efficiently enter the CNS**, **impairing the negative feedback mechanism. This results in elevated TSH and subsequently serum T3 levels**. Moreover, the inability of T3 to enter the CNS has significant implications for cerebral development, as **thyroid hormones play a crucial role in normal brain development and function.** **Thyroid hormone action extends beyond the classical pathways, involving non-canonical mechanisms** like integrin-mediated signaling, PLC-ERK pathway activation, and cytosolic thyroid hormone receptor interaction with PI3K pathway, both leading to expression of target genes, which may or may not be the same with the ones activated by the nuclear receptor. **Lecture 1 -- Sarah Paisdzior (Part 2 - Regulation of growth hormone synthesis and action)** For this lecture she told us to **know the somatotrophic axis very well**! This is a summary: The somatotrophic axis regulates the **growth and development of the body**, primarily through the secretion and action of **growth hormone** (GH) and insulin-like growth factor 1 (**IGF-1**). The process begins in the hypothalamus, where growth hormone-releasing hormone (GHRH) is synthesized and released. GHRH stimulates the anterior pituitary gland to secrete growth hormone (GH). GH, also known as somatotropin, enters the bloodstream and travels to target tissues throughout the body. In the liver and other peripheral tissues, GH stimulates the production of insulin-like growth factor 1 (IGF-1). **IGF-1 mediates the growth-promoting effects of GH**. **Both GH and IGF-1 exert negative feedback on the somatotrophic axis**. **Elevated levels of IGF-1 inhibit the release of GH, and GH itself negatively feedbacks on GHRH**. IGF-1 acts on target tissues, promoting cellular growth, proliferation, and differentiation. It plays a central role in bone growth, muscle development, and **overall somatic growth**. **Additional players in this axis are ghrelin and somatostatin.** **Ghrelin stimulates GH release**, especially in response to fasting (when you are hungry and the body waits to eat or when you see food because then you will have the energy to grow), while **somatostatin exerts inhibitory effects**, preventing excessive GH secretion and increases as a response to higher IGF-1. A diagram of a protein Description automatically generated **Pit-1**, also known as POU1F1, is a TF that plays a crucial role in the regulation of GH synthesis and **somatotroph cell differentiation.** **GHRH and ghrelin act as positive regulators**, stimulating GH release, with GHRH responding to various stimuli and ghrelin being released during fasting. **Somatostatin serves as a negative regulator**, inhibiting GH release. **GH is characterized by pulsatile secretion and regulates protein, fat, and carbohydrate metabolism. Its effects are not only direct but also indirect, mediated by Insulin-like Growth Factor 1 (IGF-1), also known as somatomedin, and its receptor.** **The growth hormone receptor (GHR)** **is an enzyme-associated receptor with a dimer structure, containing two JAK boxes. It activates the JAK-STAT pathway but can also engage PLCγ-MAPK.** GH binding to the extracellular domain of the GHR reorients the pre-existing homodimer so that one GHR subunit rotates relative to the other. This structural reorientation results in a repositioning of tyrosine kinases bound to the cytoplasmic domain of the receptors. **GH metabolic effects -- not restricted to the liver:** **GH exerts pleiotropic effects on carbohydrate, lipid, and protein metabolism.** In adipocytes, GH influences lipolysis, while in skeletal muscle and liver, it signals through the GHR and activates the JAK--STAT pathways, interacting with insulin signaling. **GH antagonizes insulin action** **by promoting adipocyte lipolysis.**  **GH interacts with GHR in hepatocytes increasing IGF-I secretion in different organs,** **although an autocrine/paracrine IGF-I production by those organs is also present**.  **Resemblances between Insulin and IGFs** allow cross-interactions **where IGFs can bind to their own receptors** (preferently) but also to an Insulin receptor (IR) with a lower specificity. The hybrid receptor shares components from both IR and IGF-IR. **!!!IGF-1 is not produced/secreted ONLY as a response to GH, but an autocrine/paracrine IGF-1 production by these organs is also present (IGFs can bind to their own receptors).** **Signaling of IGF1 receptor:** When IGF-1 binds to IGF-1R, it triggers the receptor to phosphorylate cellular proteins, which then interact with various signaling molecules like Akt, Ras/Raf, MEKs, and ERKs. The activation of the PI3K and Akt pathway by IGF-1 helps prevent apoptosis. Moreover, the IGF-1R/IGF axis positively influences cell cycle progression, through the PI-3K/Akt and/or ERK pathways.  **Wild-type Mouse (+/+):** the control group. **GH Mouse (GH):** overexpress GH but endogenous IGF-I. It is a giant mouse, with insulin resistance, shorter lifespan and larger muscle fibers. **GH Antagonist Mouse (GHA):** reduced GH signaling and impaired negative feedback. This is why GH and GHR levels are higher but IGF-1 levels lower. The GH indirect effects (mediated by IGF-1) are mostly affected. As a result, the muscle tissue is smaller with a dwarf-like appearance. **GH Receptor Knockout Mouse (GHR --/--):** here the GHR genes have been disrupted. Reduced GH signaling that impairs negative feedback and justifies very high GH (no feedback by IGF-1). These mice are very insulin-sensitive and tend to have an extended lifespan. **Isolated growth hormone deficiency (IGHD)** is a rare condition linked to genetic factors. It manifests in two main forms: **Complete IGHD** with autosomal recessive inheritance involves gene deletions and mutations, while **Partial IGHD** results from mutations in the donor-splice site of GH exon 4. These genetic variations disrupt the growth hormone pathway, affecting growth and development, with familial clustering suggesting a genetic basis. **4 patient cases:** - **1^st^ case: GH gene homozygous deletion -- complete loss of GH**   - **2^nd^ case: GHR mutation, GH is secreted but lacks a biological response -\> no IGF-1 production and GH high due to impaired feedback. Possible treatment IGF-1. Alternative name: GH insensitivity syndrome.**  - **3^rd^ case: abnormal ghrelin receptor (GHSR mutation), signal for GH is weaker (only positive signal now is coming from GHRH), IGF-1 can be close to normal, obesity later in life.**  **Remember: Ghrelin is produced in the stomach and acts on the growth hormone secretagogue receptor (GHSR) to stimulate the release of growth hormone** - **4^th^ case: Pit1 mutation: both GH and TSH are affected, common TF for the differentiation of anterior pituitary precursor cells thyro- and somato-trophs -\> Pit1 mutations.** **What can go wrong? Everything**  **Lecture 2 -- Sarah Paisdzior (Hypothalamic regulation of energy metabolism)** **Obesity**: worldwide problem, WHO defines it at **BMI \> 25**. Since the 70s it is increasing. Childhood obesity is also increasing. **Clinical picture - Marie**: obese and tall for her age. Body weight: 120 kg. Normal glucose, but **high insulin secretion** (resistance), **always hungry**. Only stops eating when belly hurts. Slim mother and dead father. No matter how much she exercises, she does not lose weight. **But what is her genetic defect???** (see at the end) **Jojo\'s weight fluctuation pattern**, characterized by cycles of gaining and losing weight, reflects a common challenge in weight management. **Both human and rodent evidence suggest the presence of a predetermined weight set point, indicating the body\'s tendency to return to a specific weight range despite intervention efforts.** **Monozygotic twins tend to exhibit similar weights, highlighting the strong genetic component in weight determination. The FTO gene has been identified as having a high correlation with obesity.** The genetic landscape of obesity is complex, involving numerous genes, making it a **multifactorial condition with strong a genetic component**. A black and white text Description automatically generated **EXAM: !!!The leptin-melanocortin pathway**: Leptin, produced by adipose tissue in response to adequate energy stores, signals the hypothalamus by binding to leptin receptors (LEPR). This leads to the production of melanocortin peptides, like αMSH (alpha-melanocyte-stimulating hormone). αMSH then activates the melanocortin-4 receptors (MC4R) in the hypothalamus. This signal leads to satiety, reduced food intake, and increased energy expenditure -- basically the body's **"We have enough energy"**. Conversely, when leptin levels are low, the hypothalamus releases neuropeptide Y (NPY) and agouti-related peptide (AgRP), both appetite stimulants. NPY and AgRP inhibit the activity of MC4R receptors, diminishing their appetite-suppressing effects and promoting increased food intake and reduced feeling of satiety -- "**We need food**". **EXAM:** **!!!The anorexic (appetite-suppressing) pathway**: insulin, produced by the beta cells of the islets of Langerhans in the pancreas, promotes transport and uptake of glucose from blood to tissue, decreases release of glucose by the liver and decreases appetite. Leptin, produced by white adipose tissue, conveys information to the hypothalamus on the amount of energy stored in fat, by binding to LepR. This activates POMC neurons, inhibits AgrP/NPY neurons, and decreases appetite. Melanocyte-stimulating hormone (like α-MSH), a neuropeptide, produced by POMC/CART neurons in the arcuate nucleus after stimulation with leptin, further contributes to appetite suppression by activating melanocortin receptors (MC3/4R). **→ Decrease in appetite, energy balance.** Conversely, **the orexigenic pathway (appetite-stimulating)** involves hormones that stimulate appetite and counteract the anorexic effects. **Ghrelin**, produced by the stomach, is known as the \"hunger hormone\" and increases appetite by activating GHSR receptors in the hypothalamus. Ghrelin\'s release is stimulated by the sight of food. AgRP, produced by AgRP/NPY neurons in the arcuate nucleus after stimulation with Ghrelin, acts as an antagonist for MC4R receptors, increasing appetite. Neuropeptide Y, also produced by AgRP/NPY neurons, increases appetite by activating Y1R receptors. **→ Insulin, leptin and ghrelin are all circulating hormones.** **The ob/ob mouse model** **has a mutation in the ob gene (leptin gene) resulting in leptin deficiency**. These mice have obesity, hyperphagia and lack of satiety. With more than 50% of their body weight as fat, these mice do not enter puberty (hypogonadism). **In the absence of sufficient leptin, the hypothalamus interprets the situation as a low-energy state, leading to a reduction in GnRH secretion and subsequently LH and FSH.** The hyperphagia observed in ob/ob mice leads to elevated blood sugar levels, countered by high insulin levels, potentially contributing to insulin resistance later in life. **Energy, leptin and sexual maturation**: Adequate energy stores signal to the body that it is in a state conducive to supporting the energy-intensive process of sexual maturation. Leptin communicates information about the body\'s energy status to the hypothalamus, influencing the release of GnRH and subsequently regulating the reproductive axis. **Leptin receptors** **are enzyme-associated receptors** that signal through the activation of signal transducer and STAT3. However, the action of leptin can be inhibited by suppressor of SOCS3. **Leptin resistance**: biological activity of leptin is diminished, because: - low levels of leptin - inhibitory effects of SOCS3 - impaired transport of leptin to the specific neurons in the brain **Downstream signaling** includes MAPK and PI3K/Akt activation, resulting in cell proliferation and survival. **Treatment**: In cases where there is a defect in the LEPR gene, **setmelanotide** is an alternative MC4R ligand, that effectively overcomes the leptin receptor defect. **!!!Proopiomelanocortin (POMC)** is a precursor peptide that undergoes processing to produce various melanocortin peptides, including ACTH and αMSH. The melanocortin receptors (MC1R, MC2R, etc.) are GPCRs play diverse roles in different tissues. For instance, **MC1R is involved in determining skin pigmentation and hair color**, while **MC2R is primarily found in the adrenal gland, where it plays a crucial role in the synthesis and release of cortisol**. **MC3R and MC4R are, as discussed, expressed in the hypothalamus arcuate nucleus region to regulate appetite and food intake.** The **patient\'s presentation with obesity, pale complexion, and red hair**, along with the need for **cortisol supplementation**, suggests a defect in the melanocortin system. A mutation in POMC precursor peptide could be disrupting ACTH. ACTH normally stimulates the adrenal glands to produce cortisol. Without functional ACTH (MC2R not activated), cortisol synthesis is compromised, leading to adrenal insufficiency. The patient\'s paleness and red hair indicate that faulty a-MSH does not activate MC1R (skin pigmentation and hair color) → **general POMC defect**. Treatment: cortisol replacement. **MC4R gene mutations** can result in **early-onset obesity** and accelerated growth. The MC4R receptor is primarily expressed in the hypothalamus and the paraventricular nucleus of the brain. α-MSH and β-MSH are endogenous ligands. Patients with these mutations are characterized by reduced receptor activity and a higher ligand concentration needed for receptor activation. **Treatment: use of a melanocortin-4 receptor agonist, that mimics natural ligands. It binds and activates the faulty MC4R**, bypassing the need for the endogenous ligands or compensating for the reduced receptor activity. **Marie\'s condition involves a mutation in the MC4R gene**, specifically a heterozygous mutation that affects the receptor\'s signaling capabilities. MC4R activates adenylate cyclase (AC), that generates cAMP. In Marie\'s case, the mutation results in a receptor that has an **increased affinity for its ligand but fails to elicit the expected cellular response**, as indicated by the lack of cAMP accumulation. The mutated receptor interferes with the function of the normal one and **higher concentration of the ligand is required to achieve a response**. **MC4R mutations are identified as the most frequent monogenic cause of obesity.** **Semaglutide**, GLP-1 analog activates proopiomelanocortin POMC neurons, contributing to the satiety signal. This drug presents a promising avenue for managing obesity and related metabolic conditions in pediatric patients. **!!!Know well how the pathway works, all the players, and the cases/patients -- what can go wrong.** **Lecture 3 -- Sarah Paisdzior (Cases)** **Case 1**: - 5 yo girl - Obesity & hyperphagia - Her brother is also obese. - Parents are first degree cousins. - Only one in the family with pale skin and red hair - High leptin - normal function of thyroid, adrenal, kidney, and liver **Case 1 answer**: Her brother being also obese, and the incest suggests a genetic basis. Her red hair and pale skin point toward genetic mutations affecting pigmentation. The presence of high leptin levels in the absence of an effective signal raises suspicion of a disruption in the melanocortin pathway. Further emphasis on the normal adrenal function rules out certain adrenal disorders. Identified mutation in POMC exon 3, specifically in b-MSH, which binds to both MC1R (pigmentation) and MC4R (signals appetite suppression). **Case 2**: - 4 yo boy - Fast growth, aggressive behavior, deepening voice, penile enlargement and frequent erection - Very tall - Serum testosterone highly elevated - LH and FSH were prepubertal and normal for the age. - Normal adrenal gland - Many mature Leydig cells and seminiferous tubular development with spermatogenesis **Case 2 answer**: Features point toward **early puberty**, supported by heightened testosterone levels and advanced secondary sexual characteristics. The normal adrenal function rules out adrenal-related causes. **An LH receptor activating mutation** could explain the overactivity of the receptor, possibly requiring less ligand or functioning in the absence of a ligand (LH normally prepubertal). **Case 3**: - 3 yo boy. - Poor growth since birth. - Parents were first-degree cousins. - Miscarriages in the family history. - Poor response to sound. - GH, TH and sex hormones normal. - GH peaked after the administration of GHRH-arginine. **Case 3 answer**: The GH peak after GHRH-arginine administration and the hearing impairment point towards an **IGF-1 defect/mutation. The boy\'s deafness is attributed to the importance of IGF in cochlear hair cell production and the protection it provides against apoptosis in the inner ear.** Also, GHRH-arginine is a diagnostic test used to evaluate the function of the GH axis. In this case, the fact that GH peaks after the administration of GHRH-arginine suggests that the boy\'s GH axis is responsive (no hypothalamus or pituitary defect). **Case 4:** - Young pregnant woman presents in the 10^th^ week with nausea and vomiting. - Recent weight loss. - Previous pregnancy resulted in early miscarriage. - Tachycardia, excessive sweating, and hand tremor. - TSH undetectable - Very high T3 and T4 - No antibodies to thyroid peroxidase or thyrotropin receptor - After anti-thyroid medication for her hyperthyroidism, her condition improved rapidly. - No other problems until the end of pregnancy - The patient reported comparable problems in her mother when she was pregnant. **Case** **4 answer**: The diagnosis aligns with **pregnancy-related Graves' disease, a form of hyperthyroidism specific to pregnancy**. This condition is attributed to antibodies that act like TSH, leading to excessive TH production by TSHR continuous activation. Also, hCG can bind to multiple receptors, including TSHR. This interaction contributes to the hyperthyroid state in pregnancy-related Graves' disease. **Case 5**: - 6yo girl with severe growth failure - Her parents were first-degree cousins. - Born with low birth weight. - Mother normal height, father short - normal thyroid, adrenal, kidney, and liver function - GH and IGF-1 very low **Case 5 answer**: **Mutation in the GHRH receptor**. This mutation would likely impair the responsiveness of the pituitary gland to GHRH, leading to a deficiency in GH. The incest nature of the parents further supports the possibility of a genetic basis for this growth disorder in the child. **Case 6**: - Boy presents with severe muscular hypotonia and decreased reflexes. - Lack of mental development - Growth normal, wight below the normal range - No metabolic disease - No inflammation - TSH and T4 normal - T3 very high **Case 6 answer**: **These point toward an MCT8 mutation. MCT8 transports T3 into cells**. The normal TSH levels are maintained due to the feedback loop, allowing for lower TSH secretion despite elevated TRH. The observed clinical features like lack of mental development align with an MCT8 defect phenotype. **Case 7**: - 15 yo girl. - Obesity and hyperphagia. - Never entered normal puberty. - Her parents were first-degree cousins. - Mother overweight, father normal BMI. - Normal leptin. - normal function of thyroid, adrenal, kidney, and liver. **Case 7 answer**: **Leptin or LEPR mutation**. While leptin levels are normal, the biological activity of the hormone seems to be impaired. The clinical features, along with the familial and biochemical context, suggest a genetic basis for the observed obesity and pubertal delay in this adolescent girl, most likely mediated by mutations in either of these proteins. **Case 8**: - 6-month baby with delayed development. - His sister suffered congenital hypothyroidism, she is now 6yo and mentally retarded. - T3, T4 low, TSH low normal range. - No pituitary hormone deficiency. **Case 8 answer**: The diagnostic consideration is a **potential mutation in TSH**, as TSH mutations can be detected but may not be biologically active. Genetic base -- his sister suffered from congenital hypothyroidism. Low TH are responsible for this clinical phenotype. **Lecture 3 -- Robert Opitz - Thyroid Hormones** **EXAM: thyroid hormones (production, Dios, and what they're for)** Thyroid Hormones (TH) are critical for the regulation of growth, development and metabolism. The 2 major TH are T3 (triiodothyronine) and T4 (thyroxine) and have 3 and 4 atoms of iodine in their molecule respectively. **T4 is produced by the thyroid and is considered a prohormone because it is converted into the more active form, T3**, **in peripheral tissues**. **This conversion** occurs mainly in organs such as the liver and kidneys, **catalyzed by enzymes, including deiodinases, which remove one iodine atom from T4 to produce T3.** While T4 is released by the thyroid into the bloodstream, it is the T3 hormone that has a more direct and potent effect. Iodine is a trace element, whole geographic areas on earth are deprived and therefore, it is added to table salt. **Outer ring deiodinases (type I and II) remove the Iodine atom from the first ring, to convert T4 to T3**. **Type III deiodinases. Type III deiodinases remove iodine atoms from both the inner and outer rings of T4 and T3, converting them into inactive metabolites like reverse T3 (rT3) and T2.** THs undergo various modifications like deamination and decarboxylation, leading to the formation of derivatives such as TRIAC, TETRAC and thyronamines. These derivatives have reduced thyroid hormone activity. The thyroid gland is located in the neck and consists of two lobes and each lobe is made up of many follicles, which are the structural and functional units. The follicles contain thyroid cells called thyrocytes, which surround a central colloid-filled space. The colloid contains a gel-like substance where thyroid hormones are produced and stored. Located between or on the follicles, C-cells are responsible for producing calcitonin (calcium homeostasis). Blood vessels, including capillaries, supply the thyroid gland with nutrients and oxygen. → **Thyroid follicles are the functional subunits of thyroid tissue. A follicular organization of thyroid cells is strictly required for TH synthesis**. If you dissolve the organization and have thyrocytes in a 2D structure, put all the necessary molecules etc. you will never get THs.  **!!!EXAM:** Thyroglobulin (TG) is synthesized in the rough ER and follows the secretory pathway to enter the colloid in the lumen of the thyroid follicle by exocytosis. Iodide ions are circulating in low amounts in the blood and Na/I symporters pump them into the cell. In this way, thyrocytes accumulate Iodine ions in concentrations 40X compared to the blood. This accumulated iodide then enters the follicular lumen from the cytoplasm through the pendrin transporter. In the colloid, iodide (I^-^) is oxidized to iodine (I^0^) by an enzyme called thyroid peroxidase. Iodine (I^0^) is very reactive and iodinates TG at tyrosyl residues in its protein chain. The tyrosyl residues in the TG molecules provide the aromatic rings needed for TH production. In conjugation, adjacent tyrosyl residues are paired together. The entire complex re-enters thyrocytes by endocytosis. Proteolysis T4 and T3, which enter the blood by largely unknown mechanisms. Essentially the colloid is a protein matrix full of TG in different stages of iodination. In this follicular organ you have a rich T4 storage. When THs are needed, TSH signals the thyrocytes for T4 production, endocytosis and blood release. **The key function of the thyrocyte is to provide all the necessary substrates for TH synthesis, but the synthesis itself happens outside of the cells, in the colloid**. Normally, when you want to control a process, you would make it happen intracellularly -- this is anarchy! Why did evolution choose this? **Oxidation of iodide would be too harmful to occur inside the cell.** The oxidation reaction is catalyzed by thyroid peroxidase (TPO), using hydrogen peroxide (H2O2) as a substrate. H2O2 is very dangerous ROS and provided that large amounts of it are needed for TH production, it would be risky for that process to take place in the thyrocyte. **Iodide oxidization involves two reactions:** 1. **DUOX Reaction**: DUOX1 and DUOXA2, members of the NADPH oxidase family, collaborate to produce hydrogen peroxide by transferring electrons across the cell membrane, where molecular oxygen is then combined with these electrons to produce H2O2. This reaction occurs in the immediate vicinity of TPO. 2. **TPO Reaction**: Subsequently, the generated H2O2 is utilized by TPO in the iodination process. TPO catalyzes the oxidation of iodide (I^-^) to iodine (I^0^) using H2O2 as a substrate. These two reactions contribute to the localized and specific generation of H2O2 in the immediate vicinity of TPO, ensuring its availability for the iodination process and preventing its diffusion away. The main trophic hormone for the thyroid is TSH, produced by the thyrotrophes of the anterior pituitary. Subsequently, T4 (and some T3) is released into the bloodstream. **The T4 is the main mediator of the negative feedback loop in this axis**. **T3 is mainly produced in extrathyroid tissues, by deiodinases.** The term "goiter" describes the phenotype of an enlarged thyroid gland, which tries to compensate for impaired TH production. **For example, in Iodine deficiency conditions**, no T4 is produced. The negative feedback is deregulated, as T4 does not act on the hypothalamus and pituitary to reduce its release, leading to elevated levels of TRH and TSH. Elevated TSH levels stimulate the thyroid gland to grow and produce more thyroglobulin to compensate for the insufficient thyroid hormone synthesis. The thyroid gland enlarges as it tries to capture any available iodine and produce thyroid hormones. **TH levels are indicative of proper or improper thyroid gland function and offer insights into various health conditions:** 1\. **Total** T4 and total T3 refer to their overall concentrations in the blood, including both the free and protein-bound forms. Total T4 levels are higher than total T3, as thyrocytes produce more T4 than T3. 2\. **Free** T4 and T3 represent the biologically active forms - not bound to carrier proteins and available for cellular uptake. Free T4 and T3 provide a more accurate reflection of the thyroid hormone status at the tissue level. 3\. Thyroid hormones circulate in the blood bound to **carrier proteins**, primarily thyroxine-binding globulin (TBG), albumin, and transthyretin (prealbumin). These proteins help transport thyroid hormones through the bloodstream and contribute to their stability. The binding of thyroid hormones to these proteins affects their availability for cellular uptake. 4\. **Blood Sample Conclusions**: - Elevated TSH with low free T4 and free T3 may suggest hypothyroidism. - Low TSH with high free T4 and free T3 may indicate hyperthyroidism. - Elevated total T4 or T3 with normal free hormone levels may suggest an issue with thyroid-binding proteins.  **Thyroid hormones nuclear receptors always "sit" on the DNA** compared to other hormone receptors like estrogens. The estrogen receptor "waits" in the cytosol and when estradiol is present it moves to the nucleus and contributes to target gene expression. For TH receptors, this means that **in the absence of a ligand TH target genes' expression is blocked**.  **Thyroid hormones need transporters to be taken up by the cell and one example is MCT8**. Each cell type has a unique composition of TH transporters and deiodinases. When downregulation is needed, that often does not happen on the level of transporters but D3 deiodinase upregulation. Essentially, **D1/D2/D3 are the gatekeepers to decide how much T3 goes into the nucleus**. TH nuclear receptors are more broadly expressed, and they do not contribute as much in T3 levels activity regulation. **Transporters: MCT8**, MCT10, OATP1C1, LAT1, LAT2 and others. **Activation of T4 to T3 by DIO1 or DIO2 and Inactivation from T4 to rT3 and from T3 to T2 by DIO3**. Vertebrates have two TR genes: TRa (THRA gene) and TRb (THRB gene). **Resistance to thyroid hormone (RTH)** is a rare condition characterized by reduced responsiveness of target tissues to THs, despite elevated levels of circulating thyroid hormones. T4 levels may be lower than expected, contributing to the elevation of T3 levels. Elevated TSH shows the body attempting to stimulate the thyroid to produce more THs. **However, the target organs are resistant to TH action, leading to persistent elevation of TSH**. The neurological phenotype observed could be attributed to the reduced responsiveness of the brain and nervous system to thyroid hormones. **Symptoms may mimic hypothyroidism due to impaired thyroid hormone action**. Tachycardia is a symptom often associated with hyperthyroidism. In RTH, despite elevated circulating thyroid hormones, the heart and other tissues may not respond appropriately, leading to a paradoxical situation where features of both hypo- and hyperthyroidism can be present. **!!!MCT8 mutations can lead to THR**. The resulting condition is known as **Allan-Herndon-Dudley syndrome** (AHDS), with severe intellectual and motor disabilities. In AHDS, the impaired function of MCT8 hinders the transport of T3 into the cells, leading to reduced intracellular levels of active thyroid hormone. This reduction in cellular T3 levels contributes to the clinical features of hypothyroidism, even when there are elevated levels of T3 in the bloodstream. **[Week 4]** **Lecture 1 -- Stefan Mergler (Diabetes)** **Exam**: basics, type I and type II differences, role of corresponding hormones, insulin resistance **Definition**: Diabetes is a metabolic disorder characterized by abnormally high blood sugar levels, known as hyperglycemia. Blood sugar levels are typically controlled by insulin, a hormone secreted by the pancreas. The pancreas contains **islets of Langerhans**, which consist of different types of cells, including alpha cells that produce glucagon and beta cells that produce insulin. Insulin plays a crucial role in regulating blood sugar levels. **Type 1 Diabetes:** In Type 1 diabetes, the pancreas is almost completely destroyed, particularly the beta cells that produce insulin. This results in minimal to no insulin production. Individuals with Type 1 diabetes often require external insulin to manage blood sugar levels. **Type 2 Diabetes:** **Type 2 diabetes is more common and is characterized by insulin resistance.** Although the pancreas produces insulin, the insulin receptors are downregulated, leading to reduced effectiveness of insulin. Both lifestyle factors and genetics contribute to its development, but most cases are strongly associated with lifestyle choices.  **Symptoms:** Common symptoms of diabetes include excessive thirst and urination. The reabsorption of water by the kidneys, along with other metabolic changes, contributes to these symptoms. Uncontrolled diabetes can lead to various complications, including damage to blood vessels, nerves, and organs. Excess blood glucose is not tolerated by the body. Among others, it can damage the capillaries behind the eye (smallest vessels of the body) and lead to vision impairment. **Type 1 Diabetes**: - Also known as juvenile diabetes. - **Immune system-mediated** destruction of the insulin-producing beta cells in the islets of Langerhans. - Results in insulin deficiency. - Typically diagnosed in childhood or adolescence but can occur at any age. - Mainly immune-mediated, but sometimes the cause is idiopathic. **VS** **Type 2 Diabetes:** - Characterized by reduced insulin sensitivity in the early stages. - Leads to hyperglycemia (elevated blood sugar levels). - **Develops due to insulin resistance**, where cells do not respond effectively to insulin, and relatively reduced insulin secretion. - Often associated with lifestyle factors such as poor diet and lack of physical activity. - Commonly diagnosed in adulthood, but increasingly seen in younger populations due to lifestyle factors. Some studies suggested that individuals with Type 2 diabetes might face a slightly higher risk compared to those with Type 1 diabetes. This could be since people with **Type 2 diabetes often have other comorbidities such as obesity, cardiovascular disease, and hypertension.** **Gestational diabetes** stems from the body\'s inadequate insulin production to manage increased glucose levels required for the growing baby. Hormonal changes exacerbate blood sugar levels and pose risks of pregnancy complications. Even though it can be harmful, this type of diabetes is easily treated if you detect it on time. **Pathophysiology**: **persistent elevated blood glucose levels, leading to poor protein synthesis and metabolic derangements, including acidosis**. When the concentration of glucose in the blood surpasses the renal threshold (approximately 10 mmol/L), glucose is excreted in the urine, resulting in glycosuria. This glycosuria contributes to increased urine production (polyuria) and heightened fluid loss, leading to dehydration and an accompanying increase in thirst. **In the absence of insulin, the liver continues to release glucose into the bloodstream (gluconeogenesis), exacerbating hyperglycemia**. Type 1 diabetes typically requires exogenous insulin administration to manage blood glucose levels. In type 2 diabetes, produces even more glucose and releasing it into the bloodstream despite the body\'s insulin resistance. Additionally, impaired suppression of gluconeogenesis by insulin adds to the hyperglycemic state. **Somatostatin** **is produced by the delta cells of the pancreatic islets of Langerhans** and has an inhibitory effect on both insulin and glucagon: 1) it suppresses the release of insulin in response to elevated blood glucose levels. This helps prevent excessive insulin secretion and 2) it also inhibits the secretion of glucagon in response to low blood glucose levels to prevent the release of glucagon, which would otherwise stimulate the liver to produce and release glucose into the bloodstream.  **!!!Incretins**, **including GLP-1** and GIP, **are hormones released from the small intestine in response to ingested nutrients. They stimulate insulin release, promoting glucose uptake, and inhibiting the release of glucagon.** **The incretin effect**, characterized by an enhanced insulin response to oral glucose compared to intravenous administration, underscores the importance of incretins in controlling postprandial (after a meal) blood glucose levels. **Pharmaceutical agents targeting incretins, such as GLP-1 receptor agonists** and DPP4 inhibitors, are used in the management of Type 2 diabetes. **EXAM SLIDE!!! -- Involvement of ion channels in insulin release:** The sequence of events in the involvement of ion channels in insulin release begins with the uptake of glucose by beta cells through the GLUT2 transporter. Once inside the beta cells, glucose undergoes respiration, leading to the production of ATP. The increased ATP production alters the ATP to ADP ratio, resulting in the closure of potassium channels on the cell membrane. This closure causes depolarization of the cell membrane, leading to the opening of voltage-gated calcium channels. Calcium ions influx into the beta cells, raising the intracellular concentration of calcium. The elevated calcium concentration activates various processes, including the stimulation of insulin gene expression via CREB (cAMP response element-binding protein). Ultimately, this cascade of events culminates in the exocytosis of stored insulin, releasing it into the bloodstream to facilitate the uptake and utilization of glucose by target tissues. The fasting plasma glucose (**FPG**) test and the oral glucose tolerance test (**OGTT**) are diagnostic tests used to assess blood glucose levels and diagnose conditions such as diabetes. The **management** of Type 1 diabetes primarily involves insulin replacement therapy, either through injections or insulin pumps. Alongside insulin, strict dietary management is crucial, requiring individuals to track carbohydrate intake and monitor glucose levels regularly. Type 2 diabetes can occur later in life or, surprisingly, in younger individuals. Management strategies for Type 2 diabetes include a focus on proper diet and exercise, often aiming for weight loss. Additionally, glucose levels are managed, and in some cases, insulin replacement therapy may be prescribed. The onset of Type 2 diabetes, while often occurring later in life, is increasingly observed in younger populations. **Lecture 2 -- Stefan Mergler (Ca^2+^ signaling in endocrine cells)** (I was absent that day) The planar patch clamp technique, using a microchip, is vital for studying ion channels and cellular responses. Calcium (Ca) acts as a universal messenger, participating in various cellular processes, including proliferation, migration, and apoptosis. Ca homeostasis involves channels and pumps in plasma membrane, endoplasmic reticulum (ER), and mitochondria. Voltage-gated calcium channels (VOCC) include ligand-binding and voltage-gated types, crucial for cellular responses. TRP channels, responding to stimuli like temperature, play sensory roles and influence physiological responses. TRPV1, activated by capsaicin, is studied by measuring calcium influx. Fluorescence calcium measuring techniques demonstrate transient increases in calcium levels. TRP channel blockers modulate signaling, and nuclear fluorescence measurement characterizes channel responses. Planar patch clamp aids in precise current measurements. TRP channels, like TRPV1, respond to diverse stimuli, important for applications like treating lung edema. Calcium is crucial for hormone secretion, synchronizing cells and contributing to processes like insulin release in response to glucose. **Exam:** know the meaning of calcium, and of its concentration inside and outside the cell; strong electrochemical gradient; calcium is universal messenger, involved in a lot of bio processes (proliferation, cell migration)... it also plays a role in apoptosis (2 much ca cell can die). Ca equilibrium means that we have channels for calcium in cell and pumps for calcium out cell, same for compartments like ER. Don't know the details of subtype pumps etc. know how the hormone secretion works. Planar patch; ion channels, measure by the technique.  **Introduction to Calcium Signaling** Calcium ions (Ca²⁺) are fundamental to cellular processes across all living organisms. They act as messengers that facilitate numerous intracellular and extracellular events. Calcium signaling plays a dual role: - **First messenger:** Interacts directly with cell surface receptors. - **Second messenger:** Propagates signals inside the cell, amplifying the initial stimulus. **Why Calcium Is Important:** - Its versatility allows it to participate in critical biological functions, such as muscle contraction, neurotransmitter release, hormone secretion, and apoptosis. - Cells maintain a highly controlled concentration gradient of calcium, with extracellular levels being about 10,000 times higher than intracellular levels. This gradient is vital for calcium\'s signaling capabilities. **Historical Perspective on Calcium Signaling** The significance of calcium in biology has been known for over a century. Key historical milestones include: 1. **Ringer\'s Experiment (1883):** - Demonstrated that calcium was essential for heart muscle contraction. Ringer replaced blood in a frog heart preparation with saline, showing that adding calcium chloride restored contractility. 2. **Luigi Galvani (1794):** - Conducted experiments on frog legs and proved that electricity could induce muscle contraction, indirectly highlighting the role of ions like calcium. 3. **Hodgkin and Huxley (1939):** - Recorded intracellular action potentials in squid axons, providing insights into ionic currents, including those involving calcium. These foundational experiments paved the way for modern calcium research, revealing its essential roles in both signaling and homeostasis. **Calcium Homeostasis** Maintaining calcium levels within cells is crucial. Intracellular calcium levels are tightly regulated at nanomolar concentrations, while extracellular calcium levels are in the millimolar range. This stark difference underpins calcium\'s ability to act as a potent intracellular signal. **Mechanisms of Calcium Regulation:** 1. **Calcium Channels:** - **Voltage-operated calcium channels (VOCCs):** Open in response to changes in membrane potential, allowing calcium influx. - **Store-operated calcium channels (SOCs):** Trigger calcium entry when intracellular stores are depleted. 2. **Calcium Pumps:** - **Plasma Membrane Ca²⁺-ATPase (PMCA):** Pumps calcium out of the cell, critical for restoring low intracellular calcium after signaling events. - **Sarcoplasmic/Endoplasmic Reticulum Ca²⁺-ATPase (SERCA):** Sequesters calcium into intracellular stores like the endoplasmic reticulum (ER) or sarcoplasmic reticulum (SR). 3. **Sodium-Calcium Exchangers (NCX):** - Facilitates calcium removal by exchanging intracellular calcium for extracellular sodium, particularly in excitable cells like neurons. **Consequences of Dysregulation:** - Chronic elevations in intracellular calcium can lead to cellular stress, mitochondrial dysfunction, and apoptosis, contributing to diseases like diabetes and neurodegeneration. **Calcium-Binding Proteins** Proteins that bind calcium serve as interpreters of calcium signals, mediating downstream cellular responses. These proteins change their conformation upon calcium binding, enabling them to interact with other cellular components. **Key Calcium-Binding Proteins:** 1. **Calmodulin (CaM):** - Binds calcium and regulates multiple enzymes, ion channels, and signaling pathways. - Integral to processes like muscle contraction and memory formation. 2. **Annexins:** - Involved in membrane repair and vesicle trafficking. They mediate interactions between calcium and phospholipids. 3. **Gelsolin:** - Regulates actin filament assembly and disassembly, essential for cell motility and shape. 4. **Neuronal Calcium Sensor Proteins (NCS):** - Specialized for calcium signaling in neurons, playing roles in neurotransmitter release and synaptic plasticity. Diseases such as familial amyloidosis and cancer-associated retinopathy can arise from dysfunctional calcium-binding proteins. **Calcium and Hormone Secretion** Calcium is indispensable in endocrine processes, particularly in the secretion of hormones. The release of hormones from endocrine cells is often calcium-dependent and involves intricate signaling mechanisms. **How Calcium Regulates Hormone Secretion:** 1. **Voltage-Gated Calcium Channels:** - Depolarization of the cell membrane opens these channels, allowing calcium influx. - This triggers vesicle fusion with the plasma membrane and hormone release (e.g., insulin secretion by pancreatic beta cells). 2. **Calcium-Induced Calcium Release (CICR):** - Calcium entering the cell can stimulate further calcium release from intracellular stores, amplifying the signal. 3. **Store-Operated Calcium Entry (SOCE):** - When intracellular calcium stores are depleted, channels like STIM and Orai are activated to replenish calcium levels. **Example -- Insulin Secretion:** - Pancreatic beta cells rely on calcium influx to initiate pulsatile insulin release, which is crucial for blood glucose regulation. **Transient Receptor Potential (TRP) Channels** TRP channels are specialized ion channels that mediate calcium signaling in response to environmental stimuli. They are critical for sensory functions. **Functions of TRP Channels:** - **TRPV1 (Capsaicin Receptor):** Detects heat and spicy compounds like capsaicin. - **TRPM8:** Activated by cold temperatures, contributing to thermosensation. - **TRPA1:** Responds to mechanical stress and certain chemical irritants. These channels are widely expressed and contribute to taste, pressure sensing, and pain perception. **Calcium and Apoptosis** Calcium is a regulator of apoptosis, or programmed cell death. While moderate calcium signaling is essential for survival, sustained elevations in calcium can trigger apoptotic pathways. **Mechanisms Involved:** 1. **Mitochondrial Pathway:** - Calcium overload in mitochondria increases membrane permeability, releasing pro-apoptotic factors like cytochrome c. 2. **Calcineurin Pathway:** - Calcium activates calcineurin, a phosphatase that dephosphorylates proteins involved in cell death. Disruptions in calcium-regulated apoptosis are linked to conditions such as cancer and neurodegenerative diseases. **Calcium in Disease** The dysregulation of calcium signaling contributes to a range of diseases: 1. **Diabetes:** - Impaired calcium handling in pancreatic beta cells disrupts insulin secretion. 2. **Neurodegeneration:** - Chronic calcium elevation damages neurons, a hallmark of diseases like Alzheimer's. 3. **Calcium Sensor Protein Defects:** - Mutations in proteins like gelsolin can lead to conditions such as familial amyloidosis. **Calcium Research and Analytical Techniques** Advanced tools are essential for studying calcium signaling: 1. **Fluorescence Calcium Imaging:** - Uses calcium-sensitive dyes to visualize intracellular calcium levels in real time. 2. **Patch-Clamp Electrophysiology:** - Measures calcium currents through ion channels, revealing their functional properties. 3. **Mass Spectrometry (LC-MS):** - Identifies and quantifies calcium-related proteins and metabolites with high precision. **Conclusion** 1. Calcium is central to life, serving as a versatile messenger in diverse biological processes. 2. Its regulation is vital to maintaining cellular function and preventing diseases. 3. Research continues to uncover the complexity of calcium signaling and its implications for health and disease. **Lecture 3 -- Lutz Schomburg (Selenium)** Iodine and selenium are trace elements and important for thyroid hormones. Trace elements are not for energy provision but essential for pathways. Thyroid hormones dictate your basic metabolic rate, deciding how much energy you produce. Up to a point it was thought of as poison but then we realized its importance as a micronutrient (change in appreciation). Finland started this big selenium endeavor because of a 1982 paper which **associated Se deficient people with a greater risk for MI and CDV-deaths.** Selenium works through selenoproteins, a small group of our genome -- 25 genes. Selenium is not added but gets directly into the protein during ribosomal proteinosynthesis, in the form of selenocysteine (image). So that makes aa 21 and not 20. 3 important families: - SELENOP: Se transporters - Glutathione-Peroxidase (GPx): glutathiones in general help protect against ROS and oxidative stress, as they degrade peroxides. - DIO GPx3 and SELENOP present in serum sample, used to measure Se deficiency. **SELENOP knockout mice: infertile** (**peroxidase needed for sperm maturation**), poor bone quality, reduced growth, poor muscles and poor brain function, the brain was dependent on SELENOP. The knockout mouse also performed poorly on the rotator test. But if you add selenium in their water, they improve so indeed selenium supplementation works to "rescue" such a transport deficiency. Knockout mice also developed epileptic seizures, **selenium important for keeping the brain intact and preventing neurodegeneration.** Strong neurodegeneration, maybe not be able to speak and hear, low IQ, glass bones, muscular problem, very similar phenotype to the mouse model. **After selenium supplements drop in the number of epileptic seizures.** The knockout is for SELENOP, how is selenium supplementation helping? Evidence suggests that **the transporter is essential when you have low selenium supply, bringing the trace element to the important structures of the brain**. **2 diseases:** **Keshan Disease cardiomyopathy** is selenium dependent. You can develop it if you are Se deficient and get infected with Coxsackie virus. Mice with good Se status, even when infected they do not develop this disease. **Kashin disease**: reduced growth, problems with bones and joints, disproportionate growth of limbs. Again, this disease is preventable by sufficient Se levels. **!!!In areas with Se deficient soil autoimmune thyroid disease is observed.** **!!!Thyroid gland** is the only organ that actively produces peroxides, to remove iodide electrons and make it reactive, so it can bind to tyrosyl residues on TG and produce T4. But for thyrocytes to be protected, SELENOP and GPx3 are produced. So, the thyroid needs Se to protect itself from the "poison" that it is producing. **And this is why we see this high incidence of autoimmune thyroid disease**. Because if the peroxides are not contained and oxidize other cellular proteins, they create new epitopes that trigger immune responses (autoantibodies). **Pregnancy**: baby's needs are prioritized over mother's (evolution) so more and more selenium is going towards the baby. By the 3^rd^ trimester 1 in 3 pregnant women are Se deficient. **Se supplementation in pregnancy prevents postpartum thyroiditis.** Patient with **Sepsis** reduce their SELENOP production in the liver meaning that SELENOP is a negative acute-phase reactant, strongly suppressed in sepsis. This happens because evolutionarily sepsis would mean you have bacteria in the blood and the body wants to downregulate selenium, iron etc. for the body to fight bacteria and slow down their proliferation. Today, this contributes to patients staying more in ICUs and highlights the importance of NOT being Se deficient. Se deficiency is also associated with mortality risk from COVID-19. **During the Acute Phase Response (APR), the suppression of cytosolic Se binding proteins results in a continuous decline in serum selenium status and impaired selenium transport.** Se uses UGA codon, it is a stop codon normally, but the tRNA at its one end has structured bound by Sbp2 proteins. **Conclusion**: Se deficiency and low SELENOP cause impaired Se transport and multi-organ Se deficits, negatively affecting the three major information systems; immune, endocrine and CNS. Neurodegeneration occurs due to high peroxidases (in the absence of Selenium) that trigger ferroptosis in neurons. But it can also trigger ferroptosis in many immune cells, contributing to autoimmunity. Current consensus: despite some individual encouraging studies, others do not suggest such an association, so it has not (yet) been established as part of the standard guidelines. At least we know it's not toxic! **Se deficiency and cancer**: Se deficiency reduces the CRC risk by a factor of 2 and for HCC up to a factor of 10. Breast cancer study in Europe showed Se status to constitute a more precise prediction of mortality risk (biomarker of survival) as compared to lymph node involvement, tumor size or morphology. The cardiovascular disorders are linked with SELENOP deficiency. **Chronic fatigue syndrome**: symptoms similar to severe hypothyroidism, lack of Se, you cannot develop the deiodinases and thyroid hormone deficiency, no strength, lose important neurons, get brain fog. **Selenocysteine is replaced with other aa and thus you make a modified protein that can trigger autoantibodies, eventually leading to hypothyroidism and fatigue.** **[Week 5]** **Lecture 1 -- Lutz Schomburg (Ca homeostasis, osteoporosis and Vitamin D)** Parathyroid hormone (PTH) is a major regulator of calcium in the body. The parathyroid glands, located near the thyroid gland in the neck, produce PTH in response to low blood calcium levels.  **!!!Bone is also viewed as an endocrine organ**. It has 3 main cell types: 1) **osteoclasts**, multinucleated, responsible for bone resorption, breaking down bone tissue and releasing minerals, including calcium, into the bloodstream 2) **osteoblasts**, involved in bone formation and 3) **osteocytes**, mechanosensors embedded in the bone, provide signals for bone quality (endocrine function). If needed, the body strengthens or weakens the bone. Osteocytes produce FGF23, which is signaling towards the kidneys to regulate phosphate excretion. Osteocytes also produce SOST (sclerostin, inhibiting bone formation by suppressing BMPs and Wnt), BMPs and Wnt (bone formation and remodeling). Excessive signal intensity in these pathways can be pathogenic and lead to excessive bone formation. In **osteoporosis** you want to promote bone formation and therefore antibodies are used that block sclerostin (SOST). **Van Buchem syndrome**: rare genetic disorder, bone overgrowth, caused by SOST mutations. Without the inhibitory action of sclerostin, Wnt signaling, promotes bone formation. **Fibrodysplasia Ossificans Progressiva (FOP)**: rare and disabling genetic disorder, abnormal formation of bone in soft tissues. **Mutation in the ACVR1 gene, which encodes a BMP type I receptor. This makes the receptor abnormally sensitive to BMPs**, leading to ossification of soft tissues. Patients lose their joints as they are replaced with bone, can't eat - jaw is locked in place. **2 major bone components**: 1) hydroxyapatite is the basic mineral unit and 2) cells and ECM like collagens. If i place a bone in EDTA overnight the mineral structure will dissolve and the bone will become bendy and if i put it in a protease solution the \"wet\" component like cells and ECM will be gone, the bone will still be intact but very fragile, would turn to ashes with a single hit. **Glass bone people have mutations in genes that encode proteins of bone ECM.**  **!!!RANK is a surface receptor on osteoclasts**. In response to various signals like PTH, RANK-L is produced by osteoblasts, binds to RANK and activates osteoclasts. Osteoclasts degrade bone 1) by secreting acidic fluids to \"dig\" the bone mineral matrix and 2) by producing proteases, leading to the release of P and Ca ions as bone is degraded. To deactivate osteoclasts, osteoblasts secrete soluble receptor OPG, acting as a decoy receptor for RANK-L. **Calcitonin** by the thyroid gland also inhibits osteoclast activity. With ageing we have more osteoclast than osteoblast activity. Denosumab antibodies used for osteoporosis treatment inhibit RANK-L, impairing osteoclast activation. **Feedback axis: Low Ca -\> PTH secretion -\> PTH makes osteoblasts produce RANK-L -\> osteoclasts activated -\> bone resorption, Ca and P release -\> Ca normal levels -\> PTH inhibition- \> OPG by osteoblasts and calcitonin** **Osteocytes are mechanosensors** and respond to changes in stress or strain. Placing a weighted backpack on mice will lead to reduced feeding and weight loss, because the body will perceive this signal as extra resistance to the bones -- being overweight. Very low body fat leads to reduced leptin signaling. Leptin plays a role in reproductive function, and disruptions in leptin signaling can contribute to infertility.  **!!!PTH is derived from a pre-prohormone within the chief cells of the parathyroid glands:** transcription and translation of the pre-prohormone, which includes a signal peptide (pre) directs protein into the ER lumen. There, the signal peptide is cleaved off, resulting in the prohormone. The prohormone is then transported to the Golgi, where certain alterations lead to PTH. The active PTH is stored in secretory vesicles. The release of PTH is triggered when the serum calcium levels in the blood are low. **How is calcium concentration sensed?** The calcium-sensing receptor (CaSR) is a GPCR, sensing extracellular calcium concentrations. CaSR is expressed in various tissues, including the parathyroid glands, kidneys, intestines, and bone cells. When calcium is high, its ions bind to the CaSR inhibit signaling. The dissociation of these ions from the receptor removes the inhibitory signal and PTH secretion is stimulated. **Primary hyperparathyroidism**: excessive production of PTH. The **most common cause is a benign tumor or adenoma in one or more of the parathyroid glands**. This results in elevated levels of calcium in the blood (hypercalcemia). **In the symptoms bone pain is included, as osteoclasts are overactivated.** **When Ca is bound to CaSR both Gq and Gi G proteins contribute to suppression of PTH secretion**. When Ca dissociates from the receptor these inhibitory signals are lifted. Paradoxically, because of Gq activity high intracellular calcium inhibits PTH secretion. Usually high \[Ca\]int is associated with peptide secretion. **!!!Calcitonin** **is a peptide hormone synthesized in the C-cells of the thyroid. It opposes the actions of PTH, and its main function is to lower calcium levels in the blood either by inhibiting osteoclast activity or by decreasing calcium reabsorption in the kidneys**. However, it does not seem to be essential. We know that because: Thyroid-ectomized patients maintain their regular Ca^2+^-concentrations, no disease has so far been linked to the CT gene locus by GWAS. So, it might be of importance for fine tuning or during development.  A screenshot of a computer Description automatically generated  **!!!Vitamin D biosynthesis**: cholesterol in the skin, is exposed to UVB sunlight (1^st^ limiting reaction), which triggers its conversion to previtamin D3 (some can also originate from food sources). Previtamin D3 is transported to the liver, forming calcidiol. The conversion of calcidiol to the biologically active form calcitriol occurs in the kidneys. This is the 2^nd^ rate-limiting step. **PTH induces the expression of the enzyme for calcitriol formation promoting the absorption of calcium in the intestines and reabsorption of calcium in the kidneys.** It is a 3-organ system (skin, liver and kidneys). Active vitamin D (calcitriol) regulates its production with a feedback mechanism, inducing its own degradation when its levels are high. Calcitriol molecule seem to have 2 -OH groups but there is one more from the cholesterol basis, so in total we have 3 (-triol). **!!!VDR** is a nuclear hormone receptor, "sitting" on the DNA like RAR and THR. Vitamin-D enters the nucleus, binds to VDR, activating the VDR-RXR heterodimer, that now interacts with specific DNA sequences known as vitamin D response elements (VDREs) in the promoter regions of target genes, enabling their expression. On top of that, coactivators with histone acetyltransferase (HAT) activity, are recruited and promote a more open chromatin structure and facilitating the transcriptional machinery. When calcitriol is insufficient/absent, the VDR complex does not form properly. Histone deacetylases (HDACs) deacetylate local histones and lead to a more condensed chromatin structure, inhibiting gene transcription. **!!!Vitamin D supplements**: the hype does not seem to be justified. Correlation does not equal causation! Vitamin D is only one of the confounders and the tricky point is that people with efficient calcitriol levels also engage in other lifestyle choices as well, that all together have an impact on health and disease. The only direct association is supported by a study highlighting Vitamin D importance in autoimmune diseases in the elderly population. **Hypercalcemia in malignancy** is a condition where elevated levels of calcium are observed in the blood of cancer patients. One of the pathomechanisms involves the production of parathyroid hormone-related protein (PTHrP) by cancer cells. PTHrP has actions like PTH, including the ability to increase the release of calcium from bones and enhance calcium reabsorption in the kidneys. Cancer cells that produce PTHrP can contribute to hypercalcemia by promoting the release of calcium from bones and reducing renal excretion of calcium. The presence of hypercalcemia in cancer patients is often associated with a poorer prognosis and lower survival rates.  **Exam: look at the 3 main mechanisms (?)** The calcium, PTH, Vitamin D 3 homeostatic feedback loop. \| Download Scientific Diagram **Lecture 2 -- Eddy Rijntjes (Female reproduction)** **This is very complicated from Eddy's slides; I will study from the endo book mostly.**  A diagram of a person\'s reproductive system Description automatically generated **Phenotypic sex**: internal (urogenital system), external (genitalia and secondary sex characteristics), and at the level of the brain (brain sex, hormonal release pattern and behavior). - Genetic sex: defined in fertilization -- XX or XY - Gonadal sex: SF1 and SRY upregulate SOX9 leading to testis formation. In the absence of SRY gene and SF1 signaling, ovaries will develop. - Phenotypic sex: newly formed testes produce testosterone and AMH that guide Wolffian and Mullerian ducts towards male reproductive structures. DHT plays a role in male external genitalia. In the absence of Testo and AMH, Müllerian ducts undergo development into the oviduct and uterus. !!!Under the influence of Kiss 1 peptide, hypothalamic neurons release GnRH in a pulsatile fashion, which stimulates LH (favored by increased GnRH pulse) and FSH (favored by decreased GnRH pulse). Target cells for LH are theca and for FSH granulosa, two cells synergize to produce estradiol**.** In the follicular phase, estradiol has negative feedback on the axis but a bit prior to ovulation positive feedback, leading to a surge in LH (and FSH to a lesser extent), facilitating ovulation. In the luteal phase (after ovulation) the corpus luteum secretes progesterone, which along with inhibin, exerts negative feedback on the axis suppressing further GnRH, LH, and FSH release. **The estradiol produced in the follicular phase contributes to growth for endometrial lining, positive feedback for LH surge triggering ovulation and formation/function of the corpus luteum.** **!!!During the follicular phase, FSH stimulates the growth and maturation of ovarian follicles,** which produce estrogens, promoting the development of the endometrial lining in preparation for a potential pregnancy. As the follicular phase progresses and close to ovulation GnRH frequency increases. The surge in LH (preceding the FSH surge) is triggered by rising levels of estrogen (at that point estradiol exerts a positive feedback) and facilitates the release of a mature egg from the ovary. This marks the transition from the follicular phase to the luteal phase. The follicle that released the egg transforms into the corpus luteum, which produces progesterone. Progesterone, produced by the corpus luteum, exerts negative feedback on the hypothalamus and pituitary, suppressing further release of GnRH, LH, and FSH. This is a preparatory phase for pregnancy, as **progesterone supports the maintenance of the endometrial lining**. In males, LH peaks proceed the testosterone peak. But in females, estradiol peak due to positive feedback triggers more intense LH peak (LH was also present before and led to estradiol increase in the first place). **Hypogonadism** often involves inadequate secretion of gonadotropins. Primary in a disease means the "final" tissue in the axis is involved like gonads, secondary refers to the one above being pathological (pituitary) etc. It is obvious that **no estradiol means no ovulation and not pregnancy**. If in a case of hypogonadism, we can substitute LH with hCG, the problem is not in the gonadal level (ovaries). hCG mimics the action of LH and can stimulate the ovaries directly. When a woman is experiencing amenorrhea (absence of menstrual cycles) the first thing we'd look is FSH and LH. Low levels of LH and FSH might suggest a dysfunction at the level of the hypothalamus or pituitary, indicating a potential issue with the release of gonadotropins. If there are specific concerns about the hypothalamus or pituitary gland, an MRI may be recommended. **!!!Olfactory bulb hypoplasia**: we only have 4 to 6 thousand GnRH producing neurons and their precursor neurons start developing in the olfactory placode. A foul development (hypoplasia) of the olfactory tract due to gene mutation results in the GnRH precursor neurons not being able to reach the hypothalamus. So, no GnRH is produced, and that condition is called **Kallmann\'s syndrome** featured by low gonadotropins and their corresponding steroid hormones. Hyposmia is a distinctive feature of these patients. Putting it all together, the clinical presentation of delayed puberty, low estradiol, low gonadotropins, and olfactory abnormalities strongly suggests Kallmann syndrome. The MRI is crucial for ruling out structural lesions that might affect the function of the hypothalamus or pituitary gland.  **The neuropeptide kisspeptin** receives signals from various sources. Kisspeptin receptors, specifically GPR54 (KISS1R), are located on **GnRH** neurons. External signals, including developmental cues, hormonal feedback from the ovaries, and nutritional status indicators like leptin, converge on the hypothalamus and pituitary, influencing the pulsatile release of GnRH. **GnRH pulsatility and FSH/LH balance**: Short-term exposure to E2 prompts a negative feedback loop, slowing pulse frequency and reducing LH secretion. But, long-term E2 exposure initiates a positive feedback loop, amplifying LH responses. This modulation reveals that biological signals can be encoded not just by signal size but also by frequency.  The life cycle of an ovarian follicle begins with the primordial follicle, which consists of an immature egg (oocyte) surrounded by a single layer of granulosa cells. As the follicle progresses through various stages of development, it transforms into a primary follicle, then a secondary (preantral) follicle, and eventually into an antral follicle. **During each menstrual cycle, several antral follicles begin to develop, but typically only one will become the dominant follicle, which will ovulate, releasing the egg for potential fertilization. The others undergo atresia (degeneration).** **Histology question!!! Know the histology of the ovary.** A close-up of a microscope slide Description automatically generated  **An antral follicle** (or Graafian follicle) is a structure within the ovary that represents a maturing ovarian follicle. The ovarian follicle is the basic functional unit of the ovary, and it undergoes several stages of development during the menstrual cycle. It is characterized by the presence of a fluid-filled cavity called an antrum. This cavity forms within the developing follicle and contains follicular fluid. Antral follicles are at an intermediate stage of maturation. **Theca cell** layer is vascularized as you need cholesterol to start producing androgens. They express receptors for LH, the response to which is an increase in steroid acute regulatory protein (StAR) and the cleavage of the side chain of cholesterol. In the ER of the thecal cells, pregnenolone is converted to androstenedione, which diffuses across the basement membrane of the follicle into the follicular fluid from which it is taken up by **granulosa cells**. These cells express the FSH receptor which, when activated by FSH, brings about the aromatase that converts androstenedione to estrone. Action of another enzyme converts estrone to estradiol. Thus, for enough estrogen to be synthesized for maturation and ovulation of the follicle, both granulosa cells and thecal cells must be functional, and both gonadotrophins must be secreted in appropriate amounts. **Up to the secondary follicle stage, granulosa cells do not express FSH receptors. The early stages of follicular development, including primordial, primary, and early secondary follicles, occur in the absence of FSH stimulation**. These initial stages are primarily **gonadotropin independent**. But as the follicle progresses, both FSH and LH play crucial roles in supporting further growth, maturation, and eventual ovulation. **Follicular development** - In early gestation, female fetus\'s ovaries contain 500 to 1300 primordial germ cells -\> mitosis -\> 6 to 7 million. These initiate meiosis and arrest in meiotic prophase I. As birth approaches, they regress and become primordial follicles. Then, a substantial loss of germ cells results in 1-2 million primordia