Molecular and Cellular Bases of Physiology L03 - Summer 24 PDF

Molecular and Cellular Bases 3. Cell signaling Andre Azevedo, DVM, MSc Visiting Professor of Veterinary Physiology [email protected] Learning objectives for this lecture Describe the location of the receptors Describe the 3 most common cell surface receptors List the most important...

Molecular and Cellular Bases 3. Cell signaling Andre Azevedo, DVM, MSc Visiting Professor of Veterinary Physiology [email protected] Learning objectives for this lecture Describe the location of the receptors Describe the 3 most common cell surface receptors List the most important second messengers and their location Summarize how a GPCR works Describe how cAMP is formed and acts as a second messenger Describe how nuclear receptors (intracellular receptors) work Cell signals video https://www.youtube.com/watch?v=89W6uACEb7M Intracellular signaling pathway Cell-cell communication RECEPTOR A protein molecule that receives chemical signals from outside the cell The binding of a LIGAND activates the receptor, which in turn activates intracellular signaling pathways or systems Intracellular signal proteins process and distribute the signal to specific intracellular targets (effector proteins). Example of effector proteins: transcription regulator, ion channels, components of metabolic pathways, or parts of the cytoskeleton Receptor locations CELL SURFACE Hydrophilic signal molecules INTRACELLULAR In the cytoplasm or in the nucleus Hydrophobic signal molecules Types of cell surface receptors 1. LIGAND-GATED ION CHANNEL RECEPTORS (ionotropic) 2. G-PROTEIN COUPLED RECEPTORS (metabotropic) 3. ENZYME COUPLED RECEPTORS A. RECEPTOR TYROSINE KINASE (RTK) B. CYTOKINE RECEPTORS 4. INTEGRINS 5. TOLL-LIKE RECEPTORS Ligand-gated ion channel receptors Also called ionotropic receptors Ion channels that open in response to the binding of a ligand – last lecture! Integral proteins with a large-diameter pore through which ions can pass The ionic flow will change the membrane potential / trigger a cellular response Which ion could it be? G-protein coupled receptors GPCRs Also called metabotropic receptors Is an integral membrane protein composed of a single polypeptide chain that passes in and out of the plasma multiple times Bind to a particular type of G protein Mediate most responses to signals from the external world, as well as signals from other cells Hormones, neurotransmitters, and local mediators About 900 types in humans G-protein Membrane-associated trimeric protein that relays the signal into the cell interior 3 different subunits: α, β, and γ They bind directly to the cytoplasmic domain of GPCRs G-protein Molecular “on-off switches” The α subunit is a GTPase and has a GDP bound in its inactive state Gα subunit exchange GDP (guanosine diphosphate) for GTP (guanosine triphosphate) The activation dissociates β/γ complex from α subunit Both GTP-bound Gα and β/γ complex interact with targets I.e.: Enzymes or ion channels which relay the signal onward GTPase hydrolyzes GTP to GDP and becomes inactive again G-protein Their signaling is like a molecular relay race Later effectors Target Action 2nd messengers Intracellular signaling molecules generated in large amounts in response to receptor activation Diffuse away from their source and spread the signal to other parts of the cell By binding to and altering the behavior of selected signaling or effector proteins There are only a few second-messenger systems within animal cells Important second messengers: DIACYLGLYCEROL (DAG) membrane-associated water-insoluble molecule that diffuses from the plasma membrane into the intermembrane space where they can reach and regulate membrane-associated effector proteins CYCLIC ADENOSINE MONOPHOSPHATE (cAMP) INOSITOL 1,4,5-TRISPHOSPHATE (IP3) CALCIUM (Ca2+) water-soluble molecules located within the cytosol G-protein types Gs (stimulatory) cAMP signaling pathway AC = ADENYLYL CYCLASE PDE = PHOSPHODIESTERASE cAMP signaling pathway PROTEIN KINASE A (PKA) has catalytic and regulatory subunits GPCRs G proteins do not act exclusively by regulating the activity of enzymes They can directly activate or inactivate ion channels in the plasma membrane Alter the ion permeability and electrical excitability I.e.: Acetylcholine released by the vagus nerve reduces heart rate Gi inhibits Adenylyl cyclase while βγ subunits bind to K channels, that will open making depolarization of the cells more difficult GPCRs Very important to medicine Almost half of all known drugs work through GPCRs or the signaling pathways GPCRs activate The same signal can activate many different GPCRs I.e.: Epinephrine activates at least 9 types; Acetylcholine 5; and Serotonin 14. Hormonal responses mediated by cAMP TARGET TISSUE HORMONE MAJOR RESPONSE THYROID GLAND Thyroid-stimulating-hormone (TSH) Thyroid hormone synthesis and secretion ADRENAL CORTEX Adrenocorticotrophic hormone (ACTH) Cortisol secretion OVARY Luteinizing hormone (LH) Progesterone secretion MUSCLE Adrenaline Glycogen breakdown BONE Parathormone Bone resorption HEART Adrenaline Increase in heart rate and force of contraction LIVER Glucagon Glycogen breakdown KIDNEY Vasopressin Water resorption FAT Adrenalin, ACTH, glucagon, TSH Triglyceride breakdown Receptor tyrosine kinase Is the largest class of Enzyme-Coupled Receptors Receptor also act as enzymes They are transmembrane proteins with ligand-binding domain outside the plasma membrane and their cytosolic domain either has intrinsic enzyme activity or associates with an enzyme Each subunit of the receptor has only one transmembrane segment Growth factors act through RTK I.e.: Insulin, Insulin-like growth factor, Epidermal growth factor, Fibroblast growth factor, Vascular endothelial growth factor etc. Receptor tyrosine kinase DIMERIZE!!! PHOSPHORYLATE!!! Receptor tyrosine kinase The binding of a ligand causes the receptors to dimerize It brings the kinase domains close promoting their activation Leads to phosphorylation of tyrosine side chains* Create phosphotyrosine docking sites for various intracellular signaling proteins that relay the signal *some RTKs are not activated by phosphorylation – only by conformational changes from the interaction between the 2 kinases domains. EX: EGF Receptor tyrosine kinase Intracellular receptors Lipophilic signals are carried by proteins in the blood from which they dissociate before entering a target cell This small hydrophobic signal molecules diffuse directly across the plasma membrane of target cells bind to intracellular receptors that are transcription regulators receptors can be in the cytosol or in the nucleus (more often) I.e.: Steroid hormones, thyroid hormones, retinoid and vitamin D Intracellular receptors are both receptors and intracellular effectors of the signal They bind to their respective intracellular receptor proteins and alter the ability of these proteins to control the transcription of specific genes Some drugs use intracellular receptors as a target Fibrate drugs used to decrease blood lipids (ex: gemfibrozil, bezafibrate, fenofibrate) Glucocorticoids, steroidal contraceptives and hormone replacement therapies Glitazones (ex: Rosiglitazone, Pioglitazone) - used to treat type 2 diabetes mellitus, decrease lipids and inflammation Intracellular receptors Hormones x Receptors Don’t worry, this slide was intentionally left blank! Additional reading / videos Neurotransmitter identification and action: ionotropic receptors versus metabotropic receptors – link https://www.youtube.com/watch?v=Glu_T6DQuLU&t=114s https://www.youtube.com/watch?v=xT0mAQ4726s&t=5s https://www.youtube.com/watch?v=uuDm8nxPtQU https://www.youtube.com/watch?v=GskbODSxAU8 Questions?