NS2: G-protein Coupled Receptors Notes PDF

NS2. G-protein coupled receptors Physiology IIB Andrea Yool [email protected] Objectives General principles of G-protein-coupled receptor (GPCR) signaling mechanisms Examples of G-protein coupled receptor signaling: -dir...

NS2. G-protein coupled receptors Physiology IIB Andrea Yool [email protected] Objectives General principles of G-protein-coupled receptor (GPCR) signaling mechanisms Examples of G-protein coupled receptor signaling: -direct (G protein binding) -second messenger binding -kinase-mediated phosphorylation Responses are defined by the ion channels that are activated or inhibited Ligand gated channels produce responses that are fast and localized. G protein coupled receptors produce responses that are relatively slower, smaller, longer in duration, and widespread within the cell. Cell-cell communication Cell surface receptors include: -Directly-gated ion channel (“ligand-gated”) -G-protein coupled receptor Other mechanisms include: – Tyrosine-kinase-linked receptors – Other enzymatic activity-linked receptors – Intracellular receptors (steroid hormones)  Mechanism of G-protein coupled receptor signaling Receptor-stimulated G proteins bind GTP, dissociate into active subunits that interact with downstream targets (effectors), and return to the resting state when GTP is hydrolysed into GDP (and Hawes et al., 2000. Peptides 21:961 phosphate, Pi). Mechanism of G-protein coupled receptor signaling G proteins have 3 subunits a and bg The a subunit binds GTP Separated subunits regulate signaling cascades Image from Sigma Aldrich Good details to know G-proteins, classified by functions Gs stimulates adenylyl cyclase Gi inhibits adenylyl cyclase Gt transducin (retina) activates phosphodiesterase Go regulates ion channels Gq couples to phospholipase Diversity of G-proteins There are multiple isoforms for each subunit, creating a vast array of combinatorial possi- bilities, and thus selectivity of signaling mechanisms. For your interest  GPCR diversity GPCRs (“metabotropic receptors”) are a superfamily of related genes. There are >1000 different genes for GPCRs. Needed to respond to diverse small molecules and neuropeptide hormones and transmitters. A subset use the same native transmitters (GABA, ACh); however, these GPCR and ligand- gated receptors differ in gene sequence, protein structure, function, pharmacology. Responses are defined by the ion channels that are activated (or inhibited) Different responses to the same transmitter depend on the type of receptor activated. selective pharmacological agents DIRECT GPCR cation or second messenger regulation of anion current an ion channel (K or Ca, typically)  Responses are defined by the ion channels that are activated (or inhibited) Example: Acetylcholine nicotine nAChR mAChR muscarine (nicotinic) (muscarinic) cation second messenger regulation of current an ion channel (K or Ca, typically) Responses are defined by the ion channels that are activated (or inhibited) Example: Acetylcholine GPCR typical effects increase Kir: inhibit mAChR decrease Kir: excite increase CaV: excite decrease CaV: inhibit Multiple levels of regulation by G proteins 1. Direct modulation of channel function by binding of a G protein 2. Modulation by synthesis of a second messenger that binds to the channel 3. Modulation by second-messenger- mediated activation of a kinase that phosphorylates the channel Direct modulation of ion channel function by G proteins Example: The M channel of cardiac atrium is regulated by the mAChR (muscarinic ACh receptor). Functional significance: Parasympathetic stimulation of the heart slows the rate (supporting the “rest and digest” goal of the autonomic nervous system).  Functional roles of K channels Kir channels are open at negative potentials and set the resting membrane potential. KV channels open at positive potentials, and repolarise cells after a stimulus. K channels restore and maintain the resting state. Parasympathetic fibers (vagus nerve) release ACh onto the cardiac atrial muscle cells. Go --> increased activity of Kir channels slower depolarisation yields a slower heart rate Modulation of ion channel function by binding of an intracellular second messenger ligand Signal transduction in the retina Functional significance: The mechanism for the detection of light in the vertebrate visual system. How does light change membrane potential? Photoreceptors at the back of the retina detect light  Seeing light “Dark current”: A continuous depolarising current flows through cGMP-gated cation channels in membranes of photoreceptors in the dark. In response to light, decreased cGMP and closure of the channels causes hyper- polarisation of the photoreceptor cell. The decrease in transmitter release is interpreted by the CNS as the detection of light. Photoreceptor signaling A photon (hv) is absorbed by a rhodopsin GPCR, thus activating the G-protein transducin. The GT subunit activates phosphodi- esterase (PDE), to breakdown cGMP into inactive GMP. cGMP- gated cation channels close. modified from Leskov et al., 2000  Modulation of channel function by kinase- mediated phosphorylation Increase in the rate and force of contraction in the heart Functional significance: The sympathetic nervous system prepares the body for a “fight or flight” response, which includes an increase in cardiac output. Sympathetic signaling in cardiac muscle Adrenaline binds to an adrenergic GPCR, – activates a G protein, which – regulates adenylate cyclase (an enzyme) which – increases the GS synthesis of cAMP. Sympathetic signaling in cardiac muscle cAMP activates PKA, a protein kinase that phosphorylates voltage-gated Ca channels, increasing their open probability, and thus increasing the intracellular Ca, promoting cardiac muscle contraction. Sympathetic signaling in cardiac muscle Enhanced Ca channel activity is seen as an increase in the duration of the plateau, corresponding to an increase in Ca influx and increased tension generation (strength of contraction).  Revision questions True or false? Ion channels are not influenced by intracellular second messenger signals. List three levels of control of channels by G- protein signaling cascades. What functions do GPCRs enable for determining neuronal patterns of activity?