Cell Communication and Signaling Lecture Notes PDF

Human Biology and Physiology (BMD 211) Lecture 4: Cell communication and signaling Assistant Prof. Raghda Ramadan 1  Midterm evaluation Lecture 5: Cell signaling II (Mechanisms of cell signaling)  4 www.simplypsyc...

Human Biology and Physiology (BMD 211) Lecture 4: Cell communication and signaling Assistant Prof. Raghda Ramadan 1  Midterm evaluation Lecture 5: Cell signaling II (Mechanisms of cell signaling)  4 www.simplypsychology.org/fight-flight-freeze-fawn.html Cell-cell communication A) Local communication Signals to target cells in the neighboring area 2. Synaptic 3. Autocrine and 1. Direct contact signaling paracrine signals Cell 1 Cell 2  Cell-cell communication B) Long distance communication Signals to target cells in distant area 1. Endocrine system 2. Nervous system Hormones Neurohormones  B) Long distance communication A) Local communication 7  Signal transduction pathway ❑ The process that converts a signal on a cell’s surface into a cellular response involved a series of steps that are collectively called signal transduction pathway 1. Reception ▪ A signaling molecule binds to a receptor protein, causing it to change shape 2. Transduction ▪ Cascades of molecular interactions relay signals from receptors to target molecules in the cell 3. Response ▪ Cell signaling leads to regulation of transcription or cytoplasmic activities 8 Signal transduction pathway In this signaling system, the hormone epinephrine (adrenaline) acts through a G protein-coupled receptor to activate a succession of relay molecules, including cAMP and two protein kinases (see Figure). The final protein activated is the enzyme glycogen phosphorylase, which uses inorganic phosphate to release glucose monomers from glycogen in the form of glucose 1-phosphate molecules. This pathway amplifies the hormonal signal: One receptor protein can activate approximately 100 molecules of G protein, and each enzyme in the pathway, once activated, can act on many molecules of its substrate, the next molecule in the cascade. Lecture 5: Cell signaling II 9 (Mechanisms of cell signaling ) Lecture outline 1. Receptor major types ▪ Ligand-gated Ion channels ▪ G-protein coupled receptors ▪ Kinase-linked receptors 2. Transduction mechanisms 3. Common signaling pathways ▪ JAK-STAT signaling pathway ▪ RAS/RAF/MAPK signaling pathway 10  Signal transduction pathway ❑ The process that converts a signal on a cell’s surface into a cellular response involved a series of steps that are collectively called signal transduction pathway 1. Reception ▪ A signaling molecule binds to a receptor protein, causing it to change shape 2. Transduction ▪ Cascades of molecular interactions relay signals from receptors to target molecules in the cell 3. Response ▪ Cell signaling leads to regulation of transcription or cytoplasmic activities 11  Signal transduction pathway 1. Reception ❑ Ligand-receptor interaction: ▪ Specific Ligands (key) bind to a specific receptor (lock) with a specific pose in the active site to allow for a specific action (cell response). Ex: In the case of the epinephrine circulating throughout the bloodstream in the fight light response, the hormone encounters many types of cells, but only certain target cells detect and react to the epinephrine molecule. A receptor protein on or in the target cell allows the cell to “hear” the signal and respond to it. The signaling molecule is complementary in shape to a specific site on the receptor and attaches there, like a key in lock. ▪ An agonist binds to the receptor and produces an effect within the cell. An antagonist may bind to the same receptor, but does not produce a response, instead it blocks that receptor to a natural agonist 12  Signal transduction pathway 1. Reception ❑ Target cell receptor location: ▪ Cell membrane: - Outer surface receptors (most common) - For mainly hydrophilic (water soluble) ligands that can't enter the cells (eg: proteins, amino acids, peptides..etc) ▪ Cytosolic or Nuclear: - In the cytosol or in the nucleus - For hydrophobic or very small ligands that can enter the cells (eg: steroids hormones, nitric oxide…etc) 13 Signal transduction pathway ❑ Major types of receptors: Membrane receptors Nuclear receptor 14 Major types of receptors Ligand-gated ion channels ▪ A ligand-gated ion channel is a type of membrane channel receptor containing a region that can act as a “gate”. The channel remains closed until a ligand binds to the receptor and the receptor changes shape. ▪ When a signaling molecule binds as a ligand to the channel receptor, the channel opens, allowing the flow of specific ions, such as Na+ or Ca2+. This change may directly affect the activity of the cell in some way (cellular response). ▪ When the ligand dissociate from the receptor, the channel closes and ions no longer enter the cell. 15 Major types of receptors Ligand-gated ion channels ▪ Ex: Neural release of acetyl choline neurotransmitters (Ligand) acting on nicotinic receptors (ligand-gated ion channels) allowing opening of Na+ channel and flow of Na+ into cells, which stimulates action potential for neuron communication (response). ▪ This type of neuron communication is essential in the nervous system function (and involved in learning and memory) Ligand gated ion channels 16 Major types of receptors Ligand-gated ion channels ▪ Ex2: Neural release of acetyl choline neurotransmitters (Ligand) acting on nicotinic receptors (ligand-gated ion channels) allowing opening of Na+ channel and flow of Na+ into cells, which stimulates Ca2+ release leading to skeletal muscles contraction (response). 17 Types of channels Gating of channels Ligand Voltage Mechanical Requires binding of specific Requires a specific gradient of Requires a specific tension to open chemical (ligand) to open electrical charge across the Found in smooth muscle cells membrane to open around arteries Ex: Acetyl choline binding to nicotinic receptors Ex: voltage gated calcium channels & voltage gated sodium channels Ca+ local anesthetic drug blocking voltage-gated sodium channels Signal transduction pathway ❑ Major types of receptors: Membrane receptors Nuclear receptor 19 Major types of receptors G-protein coupled receptors (GPCRs) Several components of chocolate closely related to neurotransmitters serotonin and dopamine, they exert their effects by binding to specific GPCRs in the brain, influencing mood. 20 Major types of receptors G-protein coupled receptors (GPCRs) GPCRs are the largest and most diverse family of membrane receptors in eukaryotes, found throughout the human body and involved in a wide range of physiological processes. Immune system Cardiovascular system Sensory organs Immune cells such as T & B cells, and cardiac muscle and blood vessel cells GPCRs play a key role in sensory macrophages express various GPCRs express several GPCRs perception 21 Major types of receptors G-protein coupled receptors (GPCRs) 22 Major types of receptors G-protein coupled receptors (GPCRs) ▪ Malfunctions of GPCRs associated with many human diseases, including cancer, cardiovascular and neurodegenerative disease (GPCRs are the target of around to 50% of all modern medicinal drugs). Beta-Blockers (e.g., Propranolol) Histamine H1 and H2 Receptor Antagonists (e.g., Ranitidine, Telfast) Target GPCR: Beta-adrenergic receptors (β-ARs) Target GPCR: Histamine H1 and H2 GPCRs 23  Major types of receptors G-protein coupled receptors G-protein coupled receptors (GPCRs) ❑Structure of GPCR ▪ The most important characteristic of GPCRs is that they have seven transmembrane alpha helices. ▪ The extracellular domains are responsible for ligand binding, while the intracellular domains interact with G proteins. ▪ G proteins are heterotrimeric proteins consisting of alpha, beta, and gamma subunits. 24 3D structure of GPCR  Major types of receptors G-protein coupled receptors (GPCRs) ❑How Do GPCRs work? ▪ GPCRs work with the help of a G protein ▪ In the inactive state, GDP is bound to the alpha subunit of G Li, J. et al. Nature 420, 716-717 (2002). protein. ▪ When an agonist (ligand) binds to a GPCR, it undergoes a conformational change that activates the G protein (GTP is The basic GPCR signaling pathway attached). involves: Ligand ▪ The activated G protein dissociates into alpha and beta- The GPCR gamma subunits, which then go on to activate downstream A heterotrimeric G protein effector proteins to stimulate a cellular response. A downstream effector proteins ▪ Binding of signaling molecules is reversible: Like other ligands, they bind and dissociate many times. 25 Signal transduction pathway ❑ Major types of receptors: Membrane receptors Nuclear receptor 26  Major types of receptors Kinase-linked receptors ❖ Receptor tyrosine kinases (RTK) Epidermal Growth Factor Receptor (EGFR) Vascular Endothelial Growth Factor Insulin Receptor Found on the surface of many cell types, Receptor (VEGFR) Primarily found on the surface of including skin cells Mainly present on the surface of vascular muscle, liver, and fat cells. Function: EGFR is involved in cell growth, endothelial cells (cells lining blood vessels). Function: They regulate glucose proliferation, and differentiation, making it Function: critical for angiogenesis, the homeostasis important for skin development and tissue formation of new blood vessels, and is repair. essential for tissue growth and repair. 27 Major types of receptors Kinase-linked receptors Examples of ligands that stimulate Receptor tyrosine kinases (RTK): 28  Major types of receptors Kinase-linked receptors ❖ Receptor tyrosine kinases (RTK) ▪ RTKs are one of the biggest classes of Kinase-linked receptors. ▪ There are around 60 RTKs in human and they can be found throughout the human body. ▪ RTK signaling plays a pivotal role in cell proliferation, differentiation, and survival. ▪ Mutations or overexpression of RTKs can lead to uncontrolled cell growth and are often associated with cancer. ▪ Many cancer drugs target RTKs to inhibit their signaling and slow tumor growth. ▪ EX: Breast cancer patients have a poor prognosis if their tumor cells harbor excessive levels of a receptor tyrosine kinase called HER2. Using molecular biological techniques, researchers have developed a protein drug called Herceptin that binds to HER2 on cells and inhibits cell division (antagonist), thus prevents further tumor development. 29  Major types of receptors Kinase-linked receptors ❖ Receptor tyrosine kinases (RTK) ▪ RTK is a protein kinase—an enzyme that catalyzes the transfer of phosphate groups from ATP to another protein. The binding of a signaling molecule causes two receptor monomers to form a complex known as a dimer ▪ Upon ligand binding, such as a growth factor, one RTK may activate ten or more different transduction pathways and cellular responses, helping the cell regulate and coordinate many aspects of cell growth and cell reproduction. ▪ The ability of a single ligand-binding event to trigger so many pathways is a key difference between RTKs and GPCRs. Dimerization activates the tyrosine though adding a phosphate from an ATP molecule → activates the bound relay protein → triggering a transduction pathway, leading to a cellular response 30 Signal transduction pathway ❑ Major types of receptors: Membrane receptors Nuclear receptor 31  Signal transduction pathway ❑ The process that converts a signal on a cell’s surface into a cellular response involved a series of steps that are collectively called signal transduction pathway 1. Reception ▪ A signaling molecule binds to a receptor protein, causing it to change shape 2. Transduction ▪ Cascades of molecular interactions relay signals from receptors to target molecules in the cell 3. Response ▪ Cell signaling leads to regulation of transcription or cytoplasmic activities 32 Lecture outline 1. Receptor major types ▪ Ligand-gated Ion channels ▪ G-protein coupled receptors ▪ Kinase-linked receptors 2. Transduction mechanisms 33  Signal transduction pathway Phosphorylation cascade 1. Protein Phosphorylation and Dephosphorylation ▪ Many of the relay molecules in signal transduction pathways are protein kinases, and they often act on other protein kinases in the pathway through adding a phosphate to a target protein in a serios of events known as phosphorylation cascade. ▪ Phosphate group is removed by protein phosphatases from a target protein (mostly to turn it off). The process is known as dephosphorylation. Thus, turning off the signal transduction pathway In a phosphorylation cascade, a series of different proteins in a pathway are phosphorylated in turn, each protein adding a phosphate group to the next one in line. Here, phosphorylation activates each protein, and dephosphorylation returns it to its inactive form.  Signal transduction pathway 2. Small Molecules and Ions as The GPCR adenylyl cyclase cascade Second Messengers Second Messenger (cAMP): ▪ cAMP (cyclic Adenosine Mono-Phosphate) is produced by the enzyme adenylyl cyclase in response to the binding of extracellular signaling molecules to cell surface GPCRs. cAMP then activates a variety of downstream effector proteins, as in the adenylyl cyclase cascade (amplify signals) Calcium Ions and Inositol Trisphosphate (IP3 ) ▪ Calcium ions (Ca2+) and inositol trisphosphate (IP3) also function as second messengers in many signal transduction pathways. One signal molecule can activate amplified signal molecules 35 Signal transduction pathway 2. Small Molecules and Ions as Second Messengers Exanple: Regulation of glycogen metabolism by epinephrine in fight/flight response GPCR stimulation by epinephrine (ligand) leads to G protein activation → adenylyl cyclase activation (amplifier enzyme) → cAMP, (second messenger), activates protein kinase A → activation of phosphorylase kinase → activation of glycogen phosphorylase, which catalyzes the breakdown of glycogen to glucose1phosphate (response). 36  Signal transduction pathway ❑ The process that converts a signal on a cell’s surface into a cellular response involved a series of steps that are collectively called signal transduction pathway 1. Reception ▪ A signaling molecule binds to a receptor protein, causing it to change shape 2. Transduction ▪ Cascades of molecular interactions relay signals from receptors to target molecules in the cell 3. Response ▪ Cell signaling leads to regulation of transcription or cytoplasmic activities 37 Signal transduction pathway 3. Response ❑ Ultimately, a signal transduction pathway leads to the regulation of one or more cellular activities. The response at the end of the pathway may occur in the nucleus of the cell or in the cytoplasm. ❑ Why different cells when they are exposed to the same signaling molecule, they have different responses? Ex: Epinephrine stimulates the liver cell to break down glycogen, but the main response of the heart cell to epinephrine is contraction, leading to a more rapid heartbeat. How do we account for this difference? The reason for signal specificity is that each cell type has different receptors, relay molecules, and effector proteins 38 Signal transduction pathway Signal Termination ❑ The cell has to stop responding to a particular signal in order to be able to respond to other signals. ❑ For this reason, the changes across the signal transduction pathway must all be reversible. ▪ Reception: the binding of a ligand to a receptor is reversible. To terminate the signal, the ligand dissociates from the receptor. ▪ Transduction: To terminate the signal, phosphorylated (active) kinases are dephosphorylated. ▪ Response: the effector proteins are dephosphorylated or degraded 39 Lecture outline 1. Receptor major types ▪ Ligand-gated Ion channels ▪ G-protein coupled receptors ▪ Kinase-linked receptors 2. Transduction mechanisms 3. Common signaling pathways ▪ JAK-STAT signaling pathway ▪ RAS/RAF/MAPK signaling pathway 40  Common signaling pathways ❖The JAK-STAT pathway ▪ The Janus kinase (JAK) - Signal Transducer and Activator of Transcription (STAT) pathway is a crucial signaling cascade in cell communication, mainly involved in transmitting signals from the cell surface to the nucleus in response to various extracellular signals, including cytokines and growth factors that activate Tyrosine kinase receptors (RTK). ▪ This pathway plays a fundamental role in regulating various cellular processes, such as cell growth, differentiation, and immune responses (dysregulated in cancer → abnormal cell growth and differentiation). 41  Common signaling pathways ❖The JAK-STAT pathway ▪ The STAT proteins are transcription factors. ▪ In unstimulated cells, STAT proteins are inactive in the cytosol. ▪ Stimulation of cytokine ligands binding to RTK receptors leads to the binding of STAT proteins to phospho-tyrosine binding sites on the receptor, where they are phosphorylated by the receptor associated JAK tyrosine kinases. ▪ The phosphorylated STAT proteins then dimerize and translocate to the nucleus, where they activate the transcription of target genes involved in cell growth, differentiation…etc. How can you detect abnormal activation of JAK-STAT pathway? 42  Common signaling pathways How can you detect abnormal ❖The JAK-STAT pathway activation of JAK-STAT pathway? Measuring the levels of phosphorylated JAK and STAT proteins (at the gene or the protein levels) assess the functional consequences of JAK-STAT signaling (e.g., cell proliferation assay). 43  Common signaling pathways ❖The JAK-STAT pathway Several drugs target the JAK-STAT pathway for various medical conditions, primarily autoimmune diseases and certain cancers: EX1: Tofacitinib (Xeljanz): a JAK1 and JAK3 inhibitor , primarily used for autoimmune conditions, and has been studied in clinical trials for various cancers, including solid tumors. EX2: Ruxolitinib (Jakafi): Ruxolitinib is a JAK1 and JAK2 inhibitor primarily used for the treatment of myelofibrosis, a myeloproliferative neoplasm. 44  Common signaling pathways ❖The JAK-STAT pathway 45  Common signaling pathways ❖ The RAS/RAF/MAPK signalling pathway ▪ MAP kinase signaling is one of the major pathways of signal transduction activated downstream of both receptor and nonreceptor tyrosine kinases. ▪ The central elements in the pathway are a family of serine/threonine kinases called the MAP kinases (mitogen-activated protein kinases) that are activated in response to a variety of growth factors and other signaling molecules. ▪ This crucial pathway plays a central role in regulating various cellular processes, including cell growth, proliferation, differentiation, survival, and metabolism. 46  Common signaling pathways ❖ The RAS/RAF/MAPK signalling pathway ▪ The initial step in MAP kinase signaling from growth factor receptors is activation of the GTP binding protein Ras protein (from rat sarcoma virus). ▪ Ras activates the Raf protein kinase, which in turn activates MEK and ERK. ▪ Activated ERK can translocate to the nucleus and phosphorylate transcription factors, altering gene expression involved in growth, proliferation and survival. Growth, proliferation and survival How can you detect abnormal activation of MAPK-ERK pathway? 47  Common signaling pathways ❖The RAS/RAF/MAPK signalling pathway ▪ Several drugs are targeting MAPK/ERK pathway for the management of cancer 48  Common signaling pathways The PI3K/Akt/mTOR pathway ❖The RAS/RAF/MAPK signalling pathway Lecture 6: Cell death and signaling Downward et a., Nat Med. 2008 Dec;14(12):1315-6. 49 Cell communication and signaling References 1. Campbell Biology, 11th edition, Chapter 11; Cell Communication, P. 212-233 2. Molecular Biology of the Cell, 6th edition, Chapter 15: Cell Signaling. P. 813 -888 https://www.khanacademy.org/science/ap-biology/cell-communication-and-cell- cycle/changes-in-signal-transduction-pathways/v/example-of-signal-transduction-pathway 50