Membrane Function Signal Transduction I Introduction to Receptors
Membrane Function Signal Transduction
I. Introduction to Receptors & Signal Transduction
The Players l l l Signaling molecules Receptors G-proteins Second messenger systems Effector proteins
Signaling Molecules l l l Neurotransmitters Hormones Growth factors Drugs Other nomenclature w Ligand w Agonist / Antagonist
Receptors l l l Receptors are proteins associated with cell membranes Receptors “recognize” signaling molecules by binding to them. Binding of receptors by signaling molecules ---> Cell behavior change
Figure 1: Overview of Signaling Transmitters Hormones Growth Factors Hormones: Steroids Thyroid Transmitters Second Tyrosine Messangers Kinase Protein Kinases Ion Channels m. RNA Synthesis Protein Synthesis
Neurotansmitters: Biogenic Amines. l Catecholamines w Epinephrine w Norepinephrine w Dopamine l l Esters: Acetylcholine Indolamines w Histamine w 5 -HT
Neurotransmitters: Peptides l l l Substance P Neuropeptide Y (NPY) Enkephalins Somatostatin VIP
Neurotransmitters: Amino Acids l Excitatory w Glutamate w Aspartate l Inhibitory w g-aminobutyric acid (GABA) w Glycine
Neurotranmitters: Other l l l Nitric Oxide Arachadonic acid Carbon Monoxide PAF Zinc
The G-Proteins l l l Involved in most signaling processes Link receptor proteins to effector proteins. Trimeric proteins composed of a, b, and gsubunits.
Figure 2: G-Protein Cycling A R b a A g GDP A R R b a a g GTP (GTPase) -Pi A GTP R a b g GTP Adenylate Cyclase Phospholipase C Ion Channels Phospholipase A 2 Phosphodiesterase b g GDP
Functional G-Protein Units l GTP-activated a-subunit w produce second messenger w and/or opens ion channels. l bg-complexes w Initially thought to be inert. w Probably not inert w Exact role currently ill-defined.
Second messengers produced by G-protein activation. l Adenylate Cyclase w c. AMP l Phospholipase C (PLC) w Inositol triphosphate (IP 3) w Diacylglycerol (DAG) l Ion Channel Activity
Families of G-proteins l l l Unique structure of their a-subunits. bg subunits appear to be similar across families. Main families: w Gas w Gai w Gaq
II. c. AMP Second Messenger System
Figure 3: Adenylate Cyclase Ai As Adenylate Cyclase R 1 R 2 Gs GDP AMP Gi GTP ATP-Mg++ PDE GDP c. AMP C C Protein Reg C C Protein Kinase A (PKA) PKA Protein-P
Summary of Adenylate Cyclase Activation l Receptors which associate with Gs -type Gprotein w Stimulates adenylate cyclase. w Increases c. AMP l Receptors which associate with Gi -type Gprotein w Inhibit adenylate cyclase. w Decreases c. AMP
Summary of c. AMP action l l c. AMP exerts its effect by activating protein kinase A (PKA) PKA phosphorylates proteins w Enzymes, pumps, and channels w Phosphorylation can either increase or decrease activity depending on the protein.
Adenylate Cyclase l l Family of membrane spanning enzymes. Types I through IV have been well characterized. w Additional types probably exist. l Types differ with respect to activity modulation by other second messenger systems
Adenylate Cyclase Activity and Other Messenger Systems l l Kinases (PKA, PKC, other) can phosphorylate adenylate cyclase in some cells. Binding of adenylate cyclase by: w bg-subunits of other G-proteins w Ca++/calmodulin complexes l Allows other second messenger systems to interact with c. AMP system
III. The Phospholipase C Second Messenger System: IP 3 and DAG
Figure 4: Phospholipase C System Ca++ A R PLC Gq DAG PKC PIP 2 Protein IP 3 Ca++ Protein-P Endoplasmic Reticulum
Summary of the Phospholipase C Messengers l l Agonist binds receptor Occupied Receptor ---> activation of PLC (Gq -mediated) PLC Produces second messengers: IP 3 and DAG PLC activation associated with Ca++-channel activation
Action of IP 3 l l IP 3 binds to IP 3 -receptors on the endoplasmic reticulum Releases intracellular Ca++ stores.
Action of DAG l l l Remains membrane associated. Activates Protein kinase C (PKC) which translocates from the cytosol to the membrane Activated PKC phosphorylates other proteins and alters their function state.
PLC System and Calcium l l PLC causes the IP 3 -mediated Calcium PLC also causes the influx of Ca++ binds one of a family of Ca++ binding proteins (calmodulin). Ca++/calmodulin complex w binds to yet other proteins and changes their functional activity.
IV. Guanylate Cyclase: c. GMP and Nitric Oxide As Second Messengers
Figure 5: Nitric Oxide and c. GMP Intracellular Ca++ Stores NO Synthetase Arginine Ca++ Membrane Bound Guanylate Cyclase C. M. Ca++ NO GTP Soluble Guanylate Cyclase NO + Citrulline GTP c. GMP PDE Ion Channels c. GMP-Dependent PK PDEase Activity GMP
NO is Membrane Soluble. l l l Diffusion to nearby cells Increase c. GMP levels in nearby cells Vascular endothelial cells and nearby smooth muscle cells.
V. SIGNALING BY ACETYLCHOLINE
Acetylcholine As a Neurotransmitter l l Both the central and peripheral nervous systems. Binds two broad classes of receptors: w Nicotinic receptors w Muscarinic receptors.
Nicotinic Receptor Features l Composed of 5 subunits: w 2 a, b, g and d. l l l Subunits are arranged to form a central cavity that extends across the membrane. Nicotinic receptors are also channels ACh-binding opens gates and allows ion fluxes across the channel
Figure 6: Nicotinic Receptor Channel Agonist Binding Site Gate
Subclasses of Nicotinic Receptors l Skeletal muscle (N 1 or Nm) w Unique a and b subunits l l Autonomic ganglia (N 2 or Ng). Both N 1 and N 2 are gene-product families not single receptor types.
Other Ligand-Gated Channels l l Structural and sequence similarity to nicotinic receptors. Example agonists for these channels include: w Serotonin (5 -HT) w Glutamate w GABA w Glycine
Muscarinic receptors l l l Muscarinic receptors are not channels. Operate through G-proteins to alter second messenger systems. 5 muscarinic subtypes have been cloned and sequenced (M 1, M 2, M 3, M 4, M 5).
Grouping Muscarinic Receptors l M 1, M 3, and M 5 receptors: Activate Phospholipase C through Gq. w PLC activation ---> increased IP 3 --> increased intracellular Ca++ w Increased intracellular Ca++ ---> Activation of Ca++-sensitive K+ & Cl- channels.
Grouping Muscarinic Receptors l M 2 and M 4 receptors w Gi -coupled inhibition of adenylate cyclase w Go or Gi -coupled regulation of certain Ca++ & K+ channels.
VI. Signaling by Epinephrine and Norepinephrine and Coupling Through Adrenergic Receptors
Three Families of Adrenergic Receptors: l l l b -receptors: Three subtypes b 1, b 2, and b 3. a 1 -receptors: Three subtypes a 1 A , a 1 B , and a 1 C a 2 -receptors: Three subtypes a 2 A , a 2 B , and a 2 C
. All adrenergic receptors appear to be coupled to cellular processes through G-proteins
Occupation of b - Adrenergic Receptors l l l Gs-mediated stimulation of adenylate cyclase Increased c. AMP Increased PKA activity.
Occupation of a 1 -Adrenergic Receptors l l Mechanistic details sketchy Possibly Gq-mediated PLC activation w Increases IP 3 and DAG for some subtypes (1 B)? w Activates Ca++-channels for other subtypes (1 A)?
Occupation of a 2 -Adrenergic Receptors l l l Gi -mediated inhibition of adenylate cyclase. Decreased c. AMP Decreased PKA activity.
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