PHYSL 371: G-Protein Couple Receptors (GPCR)

what is the primary function of the CNS

to acquire, coordinate, and disseminate information by receiving a chemical signal, transmitting electrical information along an axon, and release a chemical signal

what is the main mode of neuronal communication

chemical transmission

what is used for chemical trasnmission

neurotransmitters and neuromodulators

neurontransmitter criteria

must be synthesized from neurons, must interact with receptors to produce effects, action must be terminated by an active mechanism

major neurotransmitters

ACh, DA, Glu, GABA, Noradrenaline, seretonin, Gly, adenosines

neuromodulators

messenger which modulate transmission by dampening and amplifying the ongoing synaptic response

what are the main categories of neuromodulators

neuropeptides, hormones, growth factors, cytokines, NO and CO2

where are NM synthesized

cell soma

where are NT synthsized

axon terminals

what is the response time of NM

slow response

what is the response time of NT

fast response

what terminates the actions of NM

enzymes

what terminates the action of NT

uptake mechanisms

what are receptors

protein structures that bind to ligands and produce a cellular response

what are different receptors classified based on

their location, function, and type of signal they detect/mediate

what 2 categories can receptors based on location be categorized into

1. intracellular receptors

2. cell surface receptors

where are intracellular receptors found

within the cytoplasm or nucleus

how do intracellular receptors function

bind to lipid soluble ligands that can cross the membrane so that the complex can directly influence gene expression by binding to specific DNA sequences

where are cell-surface receptors found

embedded in the cell membrane

how do cell-surface receptors function

they bind to ligands that are unable to cross the membrane like NT and NM to trigger a signal transduction that produces a functional response

what are the 3 categories of cell surface receptors

1. GPCRs

2. ligand-gated ion channels

3. enzyme-linked receptors

what is the largest family of cell surface receptors that can respond to a number of ligands

GPCR

how do GPCRs work

they trigger a cascade of intracellular signals via interactions with intermediate G-proteins when a ligand binds to the receptor on the extracellular side

what is the response time of GPCR

much slower responses

what are the major GPCR

muscarinic ACh, D1-D4, noradrenergic, mahority of seretonin, GABAb receptors, and some glutamate receptors (mGluR)

what cell surface receptor do almost all neuropeptides mediate their effects by activating

GPCR

how do ligand-gated ion channels work

the receptor is bound to a channel that can open or close in response to a specific ligand binding on it and allowing for specific ions to flow in and out of the cell leading to a functional response

what is the response time of ligand-gated ion channels

very fast

what are the major igand-gated ion channels

nicotinic ACh, GABAa, GABAc, one class or serotonin, 3 classes of glutamate and glycine receptors

what is the general structure of inotropic receptors

relatively large, multi-subunit complex composed of 4-5 individual proteins that come together to form an ion channel in the membrane

what do different subtypes of receptors do

confer different ligand affinity or ion conductance

where are enzyme-linked receptors found

embedded in the cell membrane

what are enzyme-linked receptors represented by

growth factors such as tyrosine kinase linked insulin-like growth factor-1 and insulin receptors

how do enzyme-linked receptors work

either through intrinsic enzymatic activity on their intracellular side leading to phosphorylation of other proteins to mediate signals or by directly associating with an enzyme

what is the response time for enzyme-linked receptors

much slower response time than any of the others (minutes to hours)

what do majority of GPCR mediate

olfactory responses to diverse odorants

what are GPCR targets for

drugs that can treat a variety of diseases

what do majority of the approved GPCR drugs target

orthosteric binding sites on the receptors

what role do majority of the GPCR in the brain play

cognition, mood, appetite, pain

what are common GPCR ligands

photons, ions, amines, peptides, growth factors, and lipids

what are the 5 main families of GPCRs

1. Glutamate

2. Rhodopsin

3. Adhesion

4. Frizzled/taste2

5. Secretin

GRAFS

glutamate GPCR category

include the metabatropic Glu and GABA receptors and are involved in the perception of sweet and bitter tastes (15 members)

Rhodopsin GPCR category

include the receptors for biogenic amines, peptides, signalling lipids, metal ions, and sensory stimuli such as light and odorants (701 members)

Adhesion GPCR category

poorly understood receptors with large extracellular amino-terminal domains and possess GPCR auto proteolysis-including domains (24 members)

Frizzled/Taste2 GPCR category

play roles in developmental biology, incompletely understood, different from conventional GPCRs

Secretin GPCR category

receptors include hormones glucagon, PTH and others

GPCR structure

7 transmembrane hydrophobic alpha helices with extracellular N terminal domains

what are the alpha helices connected by

3 intracellular hydrophilic loops (ICL1, ICL2, ICL3) and 3 extracellular hydrophilic loops (ECL1, ECL2, ECL3)

what is disparity in a.a chain length of different GPCR due to

variations in length of the extracellular portion involved in ligand recognition (intracellular portions that interacts with the G-protein has less variation

when do GPCR have high affinity for their ligands

when the receptor is coupled to the G-protein

when do GPCR have low affinity for their ligands

when the g-protein dissociates from the receptor

where is the agonist binding site of GPCR located

within a central pocket formed by the seven TM spanning segments and strategically positioned charged and polar a.a residues which help with ligand binding

where is the main site for coupling of the G-protein to the receptor

the 3rd intracellular loop

what happens when an agonist/ligand binds to the receptor

the receptor undergoes a conformational change leading to alterations at the cytoplasmic surface that facilitates its interaction with the heterotrimeric G-protein complex

heterotrimeric G protein

largest family of GPCR tranducers and are composed of a nucleotide-binding alpha subunit and a dimer consisting of a beta and gamma subunit

when is a G-protein considered inactive

when the alpha subunit is bound to GDP

what happens to the G-protein when a ligand binds to the receptor

GDP is displaced by GTP causing an unstable complex which forces the G alpha subunit to break from the G beta gamma subunit

what do the dissociated G protein subunits go on to do

they interact with a wide range of downstream effectors to regulate ion channels, second messenger levels, protein kinases, etc. to impact different cellular functions

what happens after the G alpha subunit does its signalling

the GTPase activity leads to hydrolysis of GTP to GDP returning it back to its inactive state and re-associating with the G beta gamma subunit

what are the 4 main categories of the G alpha subunit

1. G alpha s

2. G alpha i

3. G alpha q/11

4. G alpha 12/13

why are there different G alpha subunits

each of them are targeted by specific ligand to interact with their own signalling pathway

what does the G alpha s subunit do

activates AC to increase cAMP and activates Ca2+ channels

what does the G alpha i subunit do

inhibits AC to decrease cAMP

what does the G alpha q/11 subunit do

activates PLC to hydrolyze membrane-bound PIP2 into DAG (remains membrane bound) and IP3 (diffuses into cytoplasm) that act as second messengers to induce Ca2+ release through the IP3 Ca2+ channel receptor

what does the G alpha 12/13 subunit do

activates Rho family monomeric GTPases to regulate the actin cytoskeleton

how many G protein subtypes can a GPCR activate

multiple

how many beta and gamma subunits are there

5 beta and 12 gamma

what are the primary effectors the G beta gamma subunit

various ion channels and sometimes on phosphorylations of proteins (isoforms of adenylate cyclase, PLC, and phosphoinositide 3 kinase)

what are some of the ion channels the G beta gamma subunit targets

inwardly rectifying K+ channel, P/Q, N types Ca2+ channels

4 types of ligand that can influence the activation of given receptor

1. full agonists

2. partial agaonists

3. inverse agonists

4. neutral antagonists

full agonists

ligand that shift the equilibrium in favour of the active state and achieve maximal activity

partial agonists

ligands that shift equilibrium in favour of the active state but only achieve partial activity

inverse antagonists

shift equilibrium in favour of the inactive state and decrease basal/constitutive activity as an agonist

antagonists

ligand that do not affect the equilibrium and suppress agonist effect

what can influence membrane fluidity and GPCR functioning

membrane phospholipids, sphingolipids, glycolipids, sterols, cholesterol

what happens after a ligand binds and G-proteins are activated

the receptor is rapidly sensitized to prevent their response through a process that involves phosphorylation of the receptor

how does the receptor become desensitized

the receptor become phosphorylated by kinases that phosphorylate ser/thr motifs in the receptors intracellular loops and C-terminal tail

what are the 2 types of receptor desensitization

1. homologous desensitization

2. heterologous desensitization

homologous desensitization

process in which an agonist-mediated activation of the receptor is phosphorylated by a unique class of ser/thr protein kinases

GRKs

GPCR kinases that are made up of ser/thr and mediate homologous desensitization

what are the 3 subfamilies of GRKs

1. GRK1 and GRK7

2. GRK4, GRK5, and GRK6

3. GRK2 and GRK3

GRK1 and GRK7

visual GRKs restricted to the retinal cells

GRK2 and GRK3

beta adrenergic receptor kinases because of their close association with beta adrenergic GPCRs

GRK4, GRK5, and GRK6

found throughout the body and GRK4 is found in the testes

where are GRK2 and GRK3 found

throughout the body

heterologous desensitization

activation of one type of receptor that activate a signal transduction of second messenger dependent kinases like PKA and PKC which lead to the desensitization of another type of receptor

what happens when GPCRs become phosphorylated by GRK

beta arrestins get recruited to these GPCR which interferes with the receptors ability to interact with the G alpha subunit again therefore preventing further signalling by G proteins

what happens when beta arrestins get recruited to GPCR

leads to the internalization of the receptor via clathrin which then translocates it to the intracellular compartment for recycling to the plasma membrane or lysosomes for degradation

what happens if a GPCR has high affinity for beta arrestin

it will become dreaded in lysosomes

what are some receptors that exhibit high beta arrestin affinity

vasopressin 2 receptor and angiotensin II receptor

what happens if a GPCR has low affinity for beta arrestin

it will be recycled back to the plasma membrane following internalization

what are some receptors that exhibit low beta arrestin affinity

beta 2 adrenergic receptors

what are the 4 subtypes of beta arrestin

1. beta arrestin 1

2. beta arrestin 2

3. beta arrestin 3

4. beta arrestin 4

where is beta arrestin 1 found

in the visual sensory system

where is beta arrestin 2 found

widely expressed in tissues and can be recruited by phosphorylated receptors to regulate subsequent physiological processes

where is beta arrestin 3 found

widely expressed in tissues and can be recruited by phosphorylated receptors to regulate subsequent physiological processes

where is beta arrestin 4

in the visual sensory system

what is another function of beta arrestins other than GPCR desensitization

they can initiate alternative signalling pathways by activating 20 different kinases (including MAP kinases, PI-3 kinase, AKT signalling pathways) involved cell cycle regulation/proliferation and cell death/survival

Orthosteric ligand binding site

the endogenous ligand binding site located on the extracellular site of the receptor

allosteric modulator

when the binding of a ligand differs from the orthosteric site