Lecture 11: Growth Factors and Body Weight PDF

🥒 Lecture 11: Growth factors and Body Weight LO: Understand what growth factors are and how they function. Lecture 11: Growth factors and Body Weight 1  “Growth Factor”, “Cytokine”, and “Hormone” ar...

🥒 Lecture 11: Growth factors and Body Weight LO: Understand what growth factors are and how they function. Lecture 11: Growth factors and Body Weight 1  “Growth Factor”, “Cytokine”, and “Hormone” are often used interchangeably to describe proteins secreted by cells that affect specific processes in the body. Growth Factors: They are proteins secreted by most cells in the body Typically act in an autocrine or paracrine manner—either on the cells that produce them or on nearby cells Primary roles include stimulating cell growth, differentiation, survival, homeostasis, and tissue repair. In development → growth factors control proliferation, differentiation, apoptosis and morphogenesis In adults → embryogenesis or organogenesis Eg) IGF family, FGF family, TGF-B family Lecture 11: Growth factors and Body Weight 2  Cytokines Mainly produced by immune cells such as macrophages, B and T lymphocytes, mast cells, and endothelial cells, though some fibroblasts and stromal cells also produce them Regulate immune responses and cell migration Eg) Interleukin family, interferon family, tumour necrosis factor (TNF) family Hormones Hormones, especially protein or peptide hormones, are produced by glands and are transported either through the circulatory system or via a duct to distant target cells Regulate physiology and behaviour Eg) Growth hormone, gonadotropins (FSH, LH), Insulin, Leptin LO: Give examples of the major growth factor families involved in the regulation of body weight. Fibroblast Growth Factor (FGFs) - consisted of 22 members, typically organised into 3 groups: (1) Canonical FGFs: These family members act as true growth factors. They are produced locally in response to a signal and act in an autocrine or paracrine manner. (2) Non-secreted FGFs: Four FGFs lack signal peptides and are not secreted from cells. They bind to intracellular receptors and signal without secretion. (3) Endocrine FGFs: Three FGFs—FGF15, FGF21, and FGF23—act in an endocrine manner, traveling to distant target cells. Lecture 11: Growth factors and Body Weight 3  This lecture focus on FGF21. Signaling Pathways: FGFs bind to FGF receptors (purple on the slide) located on the surface of target cells. This binding activates several intracellular signaling pathways, including: STAT, AKT, MAP kinase These pathways lead to the transcription of specific target genes. Transforming growth factors (TGF-βs) - largest family of growth and differentiation factors, with 34 members. Lecture focusing on → Activin A, Activin B, Myostatin, BMP8B ⇒ plays a crucial muscle & fat mass Signalling Pathways: TGF-beta proteins bind to type 1 and type 2 TGF-βs receptors on the cell surface This forms a ligand-receptor complex that activates SMAD transcription factors (pink and orange on the slide) SMAD transcription factors are the primary mediators of TGF-beta cellular effects Lecture 11: Growth factors and Body Weight 4  LO: Describe how growth factors control the development, growth and function of: White Adipose Tissue, Brown Adipose Tissue, Skeletal Muscle Growth factors in adipogenesis and myogenesis Lecture 11: Growth factors and Body Weight 5  Specific growth factors determine whether a mesenchymal stem cell progresses down the muscle, white or brown adipose pathways ⇒ we’ll look at BMP7 and myostatin BMP7 → determines whether a brown pre-adipocyte can differentiate into a brown adipocyte BMP7 KO in mice → significantly reduced brown adipogenesis (arrowhead points to the brown adipose tissue in histological image) PCR sequencing showed little to absent UCP1 expression (required for brown adipose tissue thermogenesis) → indicate that the brown adipose tissue present in BMP7 KO mice may not be functional Lecture 11: Growth factors and Body Weight 6  Myostatin → inhibits the differentiation of myoblasts into skeletal muscle cells Genetically deleting myostatin in mice → mice shows double muscle phenotype or a double in muscle mass This is because myostatin is a body's major negative regulator of muscle growth, ie. myostatin puts a break on the development of skeletal muscle APPLICATION 1: Targeting myostatin-related growth factors to increase muscle mass Myostatin (TGF-B protein) is the bodies major negative regulator of muscle mass, Muscle mass is really controlled by the balance between protein synthesis and protein degradation or breakdown → myostatin influences both protein synthesis and protein degradation: Reduced protein synthesis combined with increased protein degradation leads to a decrease in muscle mass. Myostatin signalling pathway: This image illustrates myostatin binding to its type 1 and type 2 receptors on the surface of muscle cells… Myostatin binding activates the SMAD transcriptional pathway, particularly SMAD 2 and SMAD 3. Lecture 11: Growth factors and Body Weight 7  One effect of SMAD activation is a reduction in phosphorylation of AKT protein kinase, a key mediator in the IGF1 protein synthesis pathway → leads to decreased protein synthesis. SMAD activation also causes the nuclear translocation of FOXO3A, which leads to the induction of MERF1 and ATRG1. MERF1 and ATRG1 promote the ubiquitination and degradation of muscle proteins, contributing to muscle breakdown. ^^ As a result, the net effect of myostatin activity is decreased protein synthesis and increased protein degradation, leading to a reduction in muscle mass. Myostatin in Different Conditions Exercise: Myostatin levels fall, resulting in increased muscle mass. Inactivity (e.g., ICU patients): Myostatin levels rise, leading to muscle mass reduction. Specifically blocking myostatin signalling in adult mice results in an ~30% increase in muscle mass So understanding the biology of myostatin revealed its potential as a target for treating muscle wasting diseases Lecture 11: Growth factors and Body Weight 8  Important to note! - in adult life myostatin works in concert with other TGF- beta ligands - so it is necessary to block multiple TGF-beta proteins for more weight loss (not just 30%) Follistatin Follistatin is a molecule that binds to myostatin and the related proteins activin A and activin B with very high affinity - prevents these growth factors from interacting with their receptors & blocking their activity Experiment: Follistatin was delivered to the right tibialis anterior (TA) muscle of mice, while the contralateral TA muscle was injected with a control Results: muscle mass in the follistatin-treated muscle doubled compared to the control muscle over a period of about four weeks. The increase in muscle mass was driven by hypertrophy (growth of muscle fibers), as shown in the cross-sectional image. Indication: blocking multiple TGF-beta proteins (including myostatin, activin A, and activin B) is necessary to achieve a significant increase in muscle mass. Safety: no obvious detrimental effects from follistatin delivery at the histological level - the muscle architecture appeared normal. Lecture 11: Growth factors and Body Weight 9  Targeting additional ligands to myostatin The most likely additional ligands involved in the negative regulation of muscle mass are activin A and activin B. These ligands use the same receptors and downstream signaling pathways as myostatin. Experiment: Activin A and activin B antagonists were incorporated into an adeno-associated viral vector (AAV) and delivered to the tibialis anterior (TA) muscle of mice. Findings: Inhibiting Activin A or Activin B → muscle mass increased by approximately 20%, confirming that these growth factors negatively regulate muscle mass. Inhibiting Myostatin → greater increase in muscle mass (~45%) because myostatin is highly expressed in skeletal muscle and is the major negative regulator of muscle mass. Inhibiting All Three Growth Factors → Muscle mass increased by as much as 150% & the increase in muscle mass was driven by Lecture 11: Growth factors and Body Weight 10  hypertrophy of the muscle fibers treated muscles showed significant functional improvement → ankle torque increase the most when all three growth factors were inhibited APPLICATION 2: BMP8B and the control of brown adipose tissue thermogenesis Brown adipose tissue (BAT) overview: Found in significant amounts in rodents, particularly on the back of the neck. Although initially thought to disappaer after 1-2 years of age, BAT is present in adult in the supraclavicular, cervical, axillary, and paravertebral area Lecture 11: Growth factors and Body Weight 11  Thermogenesis: sympathetic activation leads to PKA signaling within these brown adipocytes. This signaling results in the release of free fatty acids from lipid droplets Free fatty acids pass through an uncoupling process within the mitochondria, under the control of UCP1. This process produces heat, a critical function in small animals and infant humans due to their large surface-to-volume ratios. Potential Impact of BAT Activation on Obesity Cold activation of brown adipose tissue was theorised to increase resting energy expenditure (REE) in an average adult by 127 kilocalories. If maintained for 30 days, this could result in the loss of 0.5 kilograms of adipose tissue. Lecture 11: Growth factors and Body Weight 12  BMP8B (TGF-B) BMP8B is TGF-beta protein produced by brown adipocytes. Highly expressed in mature brown adipocytes and testes BMP8B's Response to Thermogenic Stimuli: Cold exposure led to a more than 100-fold increase in BMP8B expression → indicating BMP8B's potential role as a thermogenic gene. Lecture 11: Growth factors and Body Weight 13  BMP8B’s Response to High-Fat-Diet Stimuli: Researchers knocked out BMP8B in a separate cohort of mice and placed these mice on a high-fat diet. BMP8B KO mice gained significantly more weight on the high-fat diet compared to the control mice BMP8B KO mice consumed less food than the controls, but their fat mass increased dramatically compared to other mice. BMP8B KO mice exhibited a major reduction in body temperature → suggesting a decrease in thermogenesis. Lecture 11: Growth factors and Body Weight 14  Central and peripheral effects of BMP8B on Brown Adipose Tissue (BAT) thermogenesis Peripherally, BMP8B ”primes” brown adipocytes to respond to sympathetic stimulation. Lecture 11: Growth factors and Body Weight 15  Centrally, BMP8B promotes sympathetic outflow to BAT. In a study - researcher delivered higher doses BMP8B centrally - key findings: BMP8B reduced the activation of AMP kinase (AMPK) in the ventromedial nucleus (VMN) This reduction in AMPK activation led to increased signaling in the lateral hypothalamic area (LHA). As a result, sympathetic activation of BAT was increased, leading to: increased thermogenesis, higher energy expenditure, decreased body weight Increased browning of white adipose tissue (WAT) Conclusion: by delivering BMP8B centrally, the researchers were able to decrease body weight in mice. APPLICATION 3: FGF-21 - metabolic wonder drug? FGF21 is primarily produced by the liver and is involved in fuel utilisation FGF21 effects on Fatty Acid Oxidation and Ketogenesis Overexpression of FGF21 in mice increases fatty acid oxidation and ketone production in the liver This pathway is activated during prolonged fasting, where the liver cannot produce sufficient glucose for the central nervous system. As a result, there is a reduction in fat depot size due to lipolysis in adipose tissue providing fatty acids for ketogenesi. Lecture 11: Growth factors and Body Weight 16  FGF21’s impact on Body Size and Growth Hormone Resistance Mice overexpressing FGF21 display a smaller body size, primarily due to growth hormone resistance. Consistent with an “energy-preserving phenotype” FGF21 overexpressing mice display a smaller body size, which is mainly due to growth hormone resistance: FGF21 inhibits the GH-IGF1 signaling axis in the liver, and protecting immune system Increases longevity to a similar extent as caloric restriction (mice live longer by as much as 40%) Lecture 11: Growth factors and Body Weight 17  FGF21 regulates glucose metabolism During re-feeding or overfeeding, FGF21 enhances insulin action and reduces glucose production The glucose-lowering effect is linked to reduced hepatic glucose output (gluconeogenesis) and increased glucose uptake in adipose tissue. FGF21 preserve of pancreatic beta cells fuction and survival In the fed state, blood glucose levels are lower, and insulin levels are higher in FGF21 overexpression mice. FGF21 increases the number of beta cells and promotes their survival. FGF21 potential for treating type 2 diabetes The regulation of beta cell function and the prevention of beta cell death are particularly relevant for type 2 diabetes therapy, where beta cell mass is often reduced, insulin secretion is impaired, and beta cell apoptosis is elevated. Lecture 11: Growth factors and Body Weight 18  FGF21’s ability to enhance beta cell survival positions it as a potential target for novel treatments for type 2 diabetes. FGF21 and thermogenesis FGF21 produced in white adipose tissue (WAT) act in an autocrine/paracrine manner to regulate browning and thermogenesis FGF21 also activates a thermogenic transcriptional program in brown adipose tissues It can also act centrally to increase sympathetic activation of brown adipose tissue Consistent with increased thermogenesis in WAT and BAT, FGF21 causes weight loss in obese rodents and monkeys Lecture 11: Growth factors and Body Weight 19  Limitations of growth factors as therapeutics to regulate body weight A key limitation of growth factor as a therapeutic is that growth factors like myostatin, activin, and BMPs are expressed widely in the body. Expecting systemic delivery of these antagonists to produce positive effects only in muscle is naive, as they can affect multiple tissues. Clinical concern… Early clinical trials with myostatin and activin antagonists in boys with muscular dystrophy were halted due to bleeding from mucosal surfaces in some participants. Off-target effects… BMP delivery, while beneficial for thermogenesis, could also affect bone growth. FGF21, despite its positive effects, has been shown to induce bone loss upon delivery. Lecture 11: Growth factors and Body Weight 20  Growth factor properties that can be improved using protein engineering techniques Pharmaceutical companies are working on methods to minimize the side effects of growth factor therapies. This is often achieved through protein engineering, where growth factors are modified to: Increase stability and in vivo half-life. Enhance affinity for signalling receptors. Improve bio-distribution and tissue penetration. Lecture 11: Growth factors and Body Weight 21  Another key challenge is reducing the production and purification costs of growth factors, which are typically higher than other drug classes. Eg) Growth Factor Modification for Wound Healing In the image provided, the natural growth factor has been modified to improve its intrinsic properties, such as receptor affinity. A peptide tag was added to increase the growth factor's retention in the wound microenvironment, allowing it to accelerate the healing process. Lecture 11: Growth factors and Body Weight 22