G Protein Coupled Receptors
Faraza Javed
Mphil Pharmacology
G Protein-Coupled Receptors
G protein-coupled receptors (GPCRs), also known
as seven-transmembrane domain receptors, 7TM
receptors, serpentine receptor, and G protein-linked
receptors (GPLR), constitute a large protein family
of receptors that sense molecules outside the cell and
activate inside signal transduction pathways and
ultimately, cellular responses.
They are called seven-transmembrane receptors because
they pass through the cell membrane seven times.
The ligands that bind and activate these receptors
include:
Light sensitive compounds
Hormones and
Neurotransmitters
That vary in size from small molecules
to peptides to large proteins.
Families of GPCR
3 Families:
A – Rhodopsin family
B - Secretin/Glucagon receptor family
eg. Peptide hormones.
C - Metabotropic Glutamate family
eg. GABAB , Glutamate.
Rhodopsin Receptor Family
RLR are a family of proteins comprise of G protein-
coupled receptors and are extremely sensitive to light.
It activates the G protein transducin (Gt) to activate
the visual phototransduction pathway.
Mutation of the rhodopsin gene is a major contributor to
various retinopathies.
Remaining receptors are liganded by
known Endogenous compounds.
Examples include receptor (FXR) farnesoid X receptor,
which is activated by bile acid, liver X
receptor (LXR), and peroxisome proliferator-
activated receptor (PPAR).
Secretin Receptor Family
The secretin-receptor family of GPCRs
include Vasoactive intestinal peptide receptors and
receptors for secretin, calcitonin and parathyroid
hormone/parathyroid hormone-related peptides.
These receptors activate adenylyl cyclase and
the phosphatidyl-inositol-calcium pathway.
Metabotropic Glutamate Family
The metabotropic glutamate receptors (mGluRs) are
family C GPCR that participate in the modulation of
synaptic transmission and neuronal excitability
throughout the central nervous system.
They have been subdivided into three groups, based on
intracellular signalling mechanisms.
Group I mGlu receptors (coupled to PLC and
intracellular calcium signalling).
Group II
Group III receptors
are negatively coupled to adenylyl cyclase.
These receptors are generally widely distributed
throughout the mammalian brain with high levels in
the cerebellum and thalamus.
Structure of G Protein
G proteins, also known as guanine nucleotide-binding
proteins, involved in transmitting signals and
function as molecular switches.
Their activity is regulated by factors that control their
ability to bind to and hydrolyze guanosine
triphosphate (GTP) to guanosine diphosphate (GDP).
When they bind GTP, they are 'on', and, when they
bind GDP, they are 'off '.
G protein complexes are
Made up of alpha (α), beta (β)
and gamma (γ) subunits.
Beta and gamma subunits
can form a stable dimeric
complex referred to as the
beta-gamma complex.
G proteins located within the cell are activated
by GPCRs that span the cell membrane. Inside the
cell, on the plasma membrane, G Protein binds GDP
when inactive and GTP when active. When the
GPCRs binds to a signal molecule, the receptor is
activated and changes shape, thereby allowing it to
bind to an inactive G Protein. When this occurs, GTP
displaces GDP which activates the G Protein.
The newly activated G Protein then migrates along the
cell membrane until it binds to adenylyl cyclase
which convert ATP to cAMP that leads to the next
step in the pathway and generates a cellular response.
After transduction, G Protein functions as a GTPase
and hydrolyzes the bound GTP which causes a
phosphate group to fall off. This regenerates GDP
and inactivates the G Protein and the cycle repeats.
G Protein Mediated Pathways
Secondary messenger Systems Involved In Signal
Transduction:
Adenylate cyclase cAMP mediated pathway
Phospholipase mediated pathway
cAMP Mediated Pathway
The cAMP-dependent pathway, also known as
the adenylyl cyclase pathway, is a G protein-coupled
receptor triggered signaling cascade used in cell
communication.
When a GPCR is activated by its extracellular ligand, a
conformational change is induced in the receptor that
is transmitted to an attached
intracellular heterotrimeric G protein complex.
The Gs alpha subunit of the stimulated G protein
complex exchanges GDP for GTP and is released
from the complex.
In a cAMP-dependent pathway, the activated
Gs alpha subunit binds to and activates an enzyme
called adenylyl cyclase, which, in turn, catalyzes the
conversion of ATP into (cAMP).
Gs cAMP Dependent Pathway
Increases in concentration of the second
messenger cAMP may lead to the activation of an
enzyme called protein kinase A (PKA).
The PKA enzyme is also known as cAMP-dependent
enzyme because it gets activated only if cAMP is
present. Many different cell responses are mediated
by cAMP. These include increase in heart rate,
cortisol secretion, and breakdown of glycogen and
fat.
GTP
GDP
GDP
GTP
ATP
cAMP
Cell response
AT
Protein
kinase
ADP
P
Inactive
protein
Active
protein
hormone
Adenylate cyclase
Signaling System
AC
RS
Inhibitor
Ri
This pathway can:
Activate enzymes and
Regulate gene expression
If cAMP-dependent pathway is not controlled, it can
ultimately lead to hyper-proliferation, which may
contribute to the development and/or progression
of cancer.
Alterations in number, structure or function of receptors
will lead to disorder in cellular signal transduction.
Up-regulation/hypersensitivity
Down-regulation/desensitization
Receptor Gene Mutation
Hyperthyroidism
Hyperthyroidism, often called overactive thyroid, is a
condition in which the thyroid gland produces and
secretes excessive amounts of the thyroid hormones
T3 and/or T4. Grave disease is the most common
cause of hyperthyroidism.
Mechanism: The thyrotropin receptor (TSH
receptor) responds to thyroid-stimulating hormone
and stimulates the production of thyroxine (T4)
and triiodothyronine (T3). The TSH receptor is a
member of the G protein-coupled receptor and is
coupled to the Gs protein. Mutation in TSHR gene
(chromosome 14q31) lead to the hyperactivation of
cAMP pathway results in hyperactivation of gland
and make progress towards the development of
tumor.
Treatment:
Antithyroid Medicine including Propylthiouracil,
Methimazole and Carbimazole.
Radioactive Iodine
Cholera Toxin
Cholera is an infection of the small intestine caused by
the bacterium Vibrio cholerae.
Mechanism:
When cholera toxin is released from the bacteria in the
infected intestine, it binds to the intestinal cells
known as enterocytes. Toxin enters, where it activates
the G protein Gs through an ADP-ribosylation
reaction that acts to lock the G protein in its GTP-
bound form, thereby continually stimulating
adenylate cyclase to produce cAMP.
Increased Gs activation leads to increased adenylate
cyclase activity, which increases the intracellular
concentration of cAMP to more than 100-fold over
normal and over-activates cytosolic PKA. These
active PKA then phosphorylate the cystic fibrosis
transmembrane conductance regulator (CFTR)
chloride channel proteins, which leads to ATP-
mediated efflux of chloride ions and leads to
secretion of H2O, Na+,K+, and HCO3- into
the intestinal lumen.
In addition, the entry of Na+ and consequently the entry
of water into enterocytes are diminished. The
combined effects result in rapid fluid loss from the
intestine, leading to severe dehydration.
G-protein modification—
cholera
lumen of intestine
GsCT
AC
cAMP ↑ ↑ ↑
Cl-H2O Na+
CT--Cholera toxin Gs ribosylation
Treatment:
Rehydration. The goal is to replace lost fluids and
electrolytes using a simple rehydration solution, oral
rehydration salts (ORS).
Intravenous fluids.
Antibiotics.
Zinc supplements.
Gi cAMP Dependent Pathway
Gi mainly inhibits the cAMP dependent pathway by
inhibiting adenylate cyclase activity, decreasing the
production of cAMP from ATP, which, in turn,
results in decreased activity of cAMP-dependent
protein kinase. Therefore, the ultimate effect of Gi is
the opposite of cAMP-dependent protein kinase.
When Gi receptors get activated, they release
activated G-protein βγ- subunits from
inactive heterotrimeric G protein complexes.
Gβγ dimeric protein interacts with GIRK channels to
open them so that they become permeable to
potassium ions, resulting in hyperpolarization of the
cell.
These receptors are primarily found on heart as well as
in brain.
Atrial fibrillation (abnormal heart rhythm) is
associated with shorter action potential duration and
believed to be affected by the G protein-gated
K+ channel, IK,Ach.
The IK,AChchannel, when activated by G proteins,
allows the flow of K+ across the plasma membrane
and out of the cell. This current hyperpolarizes the
cell, thus terminating the action potential.
In chronic atrial fibrillation there is an increase in this
inwardly rectifying current because of constantly
activated IK,ACh channels. Increase in the current
results in shorter action potential duration
experienced in chronic atrial fibrillation and leads to
the subsequent fibrillating of the cardiac muscle.
Blocking IK,ACh channel activity could be a
therapeutic target in atrial fibrillation and is an area
under study.
Opioids are prescribed to treat chronic pain in
different diseases, GIRK channels are activated by
certain GPC opioid receptors, which leads to the
inhibition of nociceptive transmission, thus
functioning in pain relief.
Studies have shown that G proteins directly activate
GIRKs which were found to participate in
propagation of morphine-induced analgesia in
inflamed spines of mice. Research pertaining to
chronic pain management continues to be performed
in this field.
GPC Receptors
G Protein Receptors Signaling Pathway
GSBeta adrenergic
receptors, glucagon,
histamine, serotonin
Increase Adenylyl
cyclase CAMP
Excitatory effects
GiAlpha2 adrenergic
receptors, mAchR,
opioid, serotonin
Decrease Adenylyl
cyclase CAMP
Cardiac K+ channel
open- decrease heart
rate
GqmAchR, serotonin
5HT1C
PLC- IP3 , DAG
Increase Cytoplasmic Ca
GtRhodopsin and colour
opsins in retinal rod
and cone cells
Increase cGMP
phosphodiesterase.
Decrease cGMP
Gq Protein Coupled Receptor
Gq protein is a heterotrimeric protein subunit that
activates phospholipase C (PLC). PLC in turn
hydrolyzes Phosphatidylinositol 4,5-bisphosphate
(PIP2) to diacyl glycerol (DAG) and inositol
trisphosphate (IP3) signal transduction pathway. DAG
acts as a second messenger that activates Protein
Kinase C (PKC) and IP3 acts on calcium channels to
release calcium from stores and phosphorylation of
some proteins.
Cell Signaling Pathway: Activation of PKC through G
protein coupled receptor
Receptors that are Gq protein coupled include:
5-HT2 serotonergic receptors
Alpha-1 adrenergic receptor
Vasopressin type 1 receptors: 1A and 1B
Angiotensin II receptor type 1
Histamine H1 receptor
Metabotropic glutamate receptor, Group I
M1, M3, and M5 muscarinic receptors
Clinical Significance
Ligands targeting the mAChR that are currently approved
for clinical use include non-selective antagonists for the
treatment of Parkinson's disease, atropine (to dilate the
pupil), Scopolamine (used to prevent motion sickness),
and ipratropium (used in the treatment of COPD).
Pilocarpine can be given in glaucoma because it reduces
intraocular pressure by contraction of the ciliary
muscle, opening the trabecular meshwork and
allowing increased outflow of the aqueous humour
Gt Protein Coupled Receptors
Gt protein coupled receptors are found in photoreceptos
(rods and cons) of the eye.
Photoreceptors are light sensitive and responsible for
visual phototransduction process.
These encode a light stimulus as a chemical output.
Photoreceptor Cells
Two types of photoreceptors: rods and cones
Rods are very sensitive cells specialized for night
vision.
In bright light conditions the response of the rods is
saturated and cones, faster but less sensitive
photoreceptors, mediate day vision.
Phototransduction
Light activates the opsin molecules in the
photoreceptors (rhodopsin). Upon activation becomes
metarhodopsin II.
Metarhodopsin II activates transducin, a Gt protein.
GDP-bound inactive transducin will exchange GDP
for GTP. GTP-bound active transducin will increase
the activity of cGMP phosphodiesterase. The result is
decreased levels of cGMP in the cytoplasm.
Decreased levels of cGMP cause the closing of
cGMP-gated ion channels which will lead to
membrane hyperpolarization.
Disorders of Phototransduction
Bradyopsia (or ‘slow vision’) is a condition that results
from mutations in genes encoding the transducin-
inactivating protein RGS9 or the RGS9 anchor protein
(R9AP). This protein inactivates transducin during light
termination process.
Patients with bradyopsia have trouble adjusting to
changing light conditions, experiencing a temporary
blindness when first exposed to bright light.
Congenital Stationary Night Blindness is an inherited
disorder that affects rod photoreceptors and impairs
vision under low-light conditions.
This disorder may result from missense mutations in
the rhodopsin gene that cause the mutated rhodopsin
protein to constitutively activate transducin.
Persistent activation of the phototransduction cascade
limits the fidelity of the light response by rod
photoreceptors.
Retinitis Pigmentosa is an inherited disorder
characterized by degeneration of photoreceptor cells
and accumulation of retinal pigments.
This disorder, which often leads to blindness, can
result from mutations in a variety of genes expressed
in photoreceptors.
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