SGNAL TRANSDUCTION PATHWAYS ,IP3-DAG PATHWAY; CAMP PATHWAY ; CHANNEL REGULATION; LIGAND GATED ION CHANNEL , JAK-STAT PATHWAY,Ras-Raf Pathway.

1. RECEPTOR –TRANSDUCTION
PATHWAYS ,STRUCTURE &
FUNCTION
PRESENTED BY :
HEENA PARVEEN
M.PHARMACOLOGY
ASST.PROFESSOR.

2. TYPES OF RECEPTOR
Receptors elicit many different types of cellular effect.
Based on molecular structure and the nature of this linkage (the
transduction mechanism),
we can distinguish four receptor types, or superfamilies .
Type 1: ligand-gated ion channels (also known as ionotropic
receptors).
These are membrane proteins with a similar structure to other ion
channels, and incorporate a ligand-binding (receptor) site, usually
in the extracellular domain.
Typically, these are the receptors on which fast neurotransmitters
act.
Examples include the nicotinic acetylcholine receptor (nAChR;
GABAA receptor; and glutamate receptors of the NMDA, AMPA and
kainate types.

3. Molecular structure of TYPE 1 RECEPTOR:
The nicotinic acetylcholine receptor, the first to be
cloned.
It is assembled from 4 different types of subunit, termed
α, β, γ and δ.
The four subunits show marked sequence homology, and
analysis of the hydrophobicity profile, which
determines which sections of the chain are likely to
form membrane-spanning α helices, suggests that they
are inserted into the membrane .
The pentameric structure (α2, β, γ, δ) possesses two
acetylcholine binding sites, each lying at the interface
between one of the two α subunits and its neighbour.

4. The two acetylcholine-binding sites lie on the
extracellular parts of the two α subunits.
One of the transmembrane helices (M2) from each of
the five subunits forms the lining of the ion channel.
The five M2 helices that form the pore are sharply
Linked inwards halfway through the membrane,
forming a constriction.
When acetylcholine molecules bind, the α subunits
twist, causing the kinked M2 segments to swivel out
of the way, thus opening the channel .

5. Structure of the nicotinic acetylcholine receptor (a typical ligand-gated ion channel) in
side view (left) and plan view (right).
The five receptor subunits (α2, β, γ, δ) form a cluster surrounding a central
transmembrane pore, the lining of which is formed by the M2 helical segments of each
subunit.

6. Receptors of this type control the fastest synaptic
events in the nervous system, in which a
neurotransmitter acts on the postsynaptic
membrane of a nerve or muscle cell and transiently
increases its permeability to particular ions.
Most excitatory neurotransmitters, such as
acetylcholine at the neuromuscular junction or
glutamate in the central nervous system, cause an
increase in Na+ and K+s permeability.
This results in a net inward current carried mainly by
Na+, which depolarises the cell and increases the
probability that it will generate an action potential.

7. The action of the transmitter reaches a peak in a
fraction of a millisecond, and usually decays within
a few milliseconds.
In contrast to other receptor families, no intermediate
biochemical steps are involved in the transduction
process.

9. ACTION POTENTIAL:
Action potentials are nerve signals. Neurons generate
and conduct these signals along their processes in
order to transmit them to the target tissues.
Upon stimulation, they will either be stimulated,
inhibited, or modulated in some way. structure and
all the types of the neurons with the following study
unit.
An action potential has several phases;
hypopolarization, depolarization, overshoot,
repolarization and hyperpolarization.

10. DEPOLARISATION:
The threshold potential opens voltage-gated sodium channels
and causes a large influx of sodium ions. This phase is called
the depolarization.
During depolarization, the inside of the cell becomes more and
more electropositive.
REPOLARISATION:
After the overshoot, the sodium permeability suddenly
decreases due to the closing of its channels.
The overshoot value of the cell potential opens voltage-gated
potassium channels, which causes a large potassium efflux,
decreasing the cell’s electropositivity.
This phase is the repolarization phase, whose purpose is to
restore the resting membrane potential.

11. Repolarization always leads first to hyperpolarization,
a state in which the membrane potential is more
negative than the default membrane potential.
But soon after that, the membrane establishes again
the values of membrane potential.

12. Type 2: G-protein-coupled receptors (GPCRs).
These are also known as metabotropic receptors or 7-
transmembrane-spanning (heptahelical) receptors.
They are membrane receptors that are coupled to
intracellular effector systems via a G-protein .
They constitute the largest family,and include
receptors for many hormones and slow
transmitters, for example the muscarinic
acetylcholine receptor (mAChR; , adrenergic
receptorsand chemokine receptors

13. MOLECULAR STRUCTURE OF G-PCR
G-protein-coupled receptors consist of a single
polypeptide chain of up to 1100 residues.
Their characteristic structure comprises seven
transmembrane α helices, similar to those of the ion
channels , with an extracellular N-terminal domain of
varying length, and an intracellular C-terminal domain.
GPCRs are divided into three distinct families.
They share the same seven-helix (heptahelical) structure,
but differ in other respects, principally in the length of
the extracellular N terminus and the location of the
agonist binding domain.

14. Family A is by far the largest, comprising most
monoamine, neuropeptide and chemokine
receptors.
Family B includes receptors for some other peptides,
such as calcitonin and glucagon.
Family C is the smallest, its main members being the
metabotropic glutamate and GABA receptors and
the Ca2+-sensing receptors.

16. The function of the G-protein. The G-protein consists of three subunits (α, β, γ), which are anchored to the membrane
through attached lipid residues. Coupling of the α subunit to an agonist-occupied receptor causes the bound GDP to
exchange with intracellular GTP; the α-GTP complex then dissociates from the receptor and from the βγ complex, and
interacts with a target protein (target 1, which may be an enzyme, such as adenylate cyclase, or an ion channel).
The βγ complex may also activate a target protein (target 2).
The GTPase activity of the α subunit is increased when the target protein is bound, leading to hydrolysis of the bound GTP
to GDP, whereupon the α subunit reunites with βγ

17. G-Protein subunits Main effectors
Gαs Many amine and other receptors
(e.g. catecholamines, histamine,
serotonin)
Stimulates adenylyl cyclase, causing increased
cAMP formation.
Gαi As for Gαs, also opioid,
cannabinoid receptors
Inhibits adenylyl cyclase, decreasing cAMP
formation.
Gαo As for Gαs, also opioid,cannabinoid
receptors
Limited effects of αsubunit (effects mainly due to
βγsubunits)
Gαq Amine, peptide and prostanoid
receptors
Activates phospholipase C, increasing production of
second messengers inositol trisphosphate and
diacylglycerol
Gβγ subunits All GPCRs As for Gα subunits (see above). Also:
• activate potassium channels
• inhibit voltage-gated calcium channels
• activate GPCR kinases (p. 40)
• activate mitogen-activated protein kinase cascade

18. Transduction pathways in GPCR’s
There are 3major effector pathways through which G-
protein coupled receptors function. They are:
a)Adenylcyclase:cAMP pathway
b)Phospholipase C: IP3-DAG Pathway
c)Channel Regulation

19. a)Adenylcyclase:cAMP pathway
cAMP is a nucleotide synthesised within the cell from ATP by the action of a
membrane-bound enzyme, adenylyl cyclase.
It is produced continuously and inactivated by hydrolysis to 5´-AMP, by the action of a
family of enzymes known as phosphodiesterases (PDEs).
Many different drugs, hormones and neurotransmitters act on GPCRs and produce their
effects by increasing or decreasing the catalytic activity of adenylyl cyclase, thus
raising or lowering the concentration of cAMP within the cell.
There are several different molecular isoforms of the enzyme, some of which respond
selectively to Gαs or Gαi

21. b) Phospholipase C: IP3-DAG Pathway
The phosphoinositide system, an important intracellular second messenger system, was
first discovered in the 1950s by Hokin and Hokin.
Activation of PLc hydrolyses the membrane phospholipid phosphatidylionositol 4.5-
bihosphate (PIP2),to generate the second messengers …….IP3 & DAG.
IP3 mobilises Ca+2 from intracellular depots & DAG enhances proten knase C
activation by Ca+2.
Ca+2 …Third messenger is a highly versatile regulator acting through CAM
(Calmodulin),PKc & other effectors.

23. c)Channel Regulation
G-protein-coupled receptors can control ion channel function directly by mechanisms
that do not involve second messengers such as cAMP or inositol phosphates. This
was first shown for cardiac muscle, but it now appears that direct G-protein-channel
interaction may be quite general .
The activated G-proteins can also open or close ionic channels specific for
Ca+2,K+/Na+2 & bring about Hyperpolarisation/ Depolarisation/ changes in
intracellular Ca+2 .
Eg: Gs opens Ca+2 channels in myocardium & skeletal muscles.
Gi & Go opens K+ channels in heart & smooth muscles as well as close Neuronal
Ca+2 channels.

24. These membrane receptors are quite different in structure and function
from either the ligand-gated channels or the GPCRs.
They play a major role in controlling cell division, growth,
differentiation, inflammation, tissue repair, apoptosis and immune
responses.
The main types are as follows:
Receptor tyrosine kinases (RTKs). These receptors have the basic
structure incorporating a tyrosine kinase moiety in the intracellular
region.
They include receptors for many growth factors, such as epidermal
growth factor and nerve growth factor, and also the group of Toll-like
receptors that recognise bacterial lipopolysaccarides and play an
important role in the body's reaction to infection …
The insulin receptor also belongs to the RTK class, although it has a
more complex dimeric structure.
Type 3: Enzymatic Receptors

25. Serine/threonine kinases:
This smaller class is similar in structure to RTKs but phosphorylate
serine and/or threonine residues rather than tyrosine.
The main example is the receptor for transforming growth factor (TGF).
Cytokine receptors:
These receptors lack intrinsic enzyme activity.
When occupied, they associate with, and activate, a cytosolic tyrosine
kinase, such as Jak (the Janus kinase) or other kinases.
Ligands for these receptors include cytokines such as interferons and
colony-stimulating factors(CSF) involved in immunological
responses.
Guanylyl cyclase-linked receptors:
These are similar in structure to RTKs, but the enzymic moiety is
guanylyl cyclase and they exert their effects by stimulating cGMP
formation.
The main example is the receptor for ANF .

26. Two well-defined signal transduction pathways are
summarised in The Ras/Raf pathway mediates the
effect of many growth factors and mitogens.
Ras, which is a proto-oncogene product, functions like a
G-protein, and conveys the signal (by GDP/GTP
exchange) from the SH2 domain protein, Grb, which is
phosphorylated by the RTK.
Activation of Ras in turn activates Raf, which is the first of
a sequence of three serine/threonine kinases, each of
which phosphorylates, and activates, the next in line.
The last of these, mitogen-activated protein (MAP)
kinase, phosphorylates one or more transcription
factors that initiate gene expression, resulting in a
variety of cellular responses, including cell division.

27. This three-tiered MAP kinase cascade forms part of
many intracellular signalling pathways involved in a
wide variety of disease processes, including
malignancy, inflammation, neurodegeneration,
atherosclerosis and much else.
The kinases form a large family, with different
subtypes serving specific roles. They are thought to
represent an important target for future
therapeutic drugs.
Many cancers are associated with mutations in the
genes coding for proteins involved in this cascade,
leading to activation of the cascade in the absence
of the growth factor signal .

28. A second pathway, the Jak/Stat pathway is involved in
responses to many cytokines.
Dimerisation of these receptors occurs when the cytokine
binds, and this attracts a cytosolic tyrosine kinase unit
(Jak) to associate with, and phosphorylate, the receptor
dimer.
Jaks belong to a family of proteins, different members
having specificity for different cytokine receptors.
Among the targets for phosphorylation by Jak are a
family of transcription factors (Stats).
These are SH2 domain proteins that bind to the
phosphotyrosine groups on the receptor-Jak complex,
and are themselves phosphorylated.
Thus activated, Stat migrates to the nucleus and activates
gene expression

29. Ras Raf Pathway

30. (Cytokine )JAK-STAT PATHWAY

31. TYPE 4: NUCLEAR RECEPTORS
The fourth type of receptors we will consider belong
to the nuclear receptor family.
By the 1980s, it was clear that receptors for steroid
hormones such as oestrogen and the
glucocorticoids were present in the cytoplasm of
cells and translocated into the nucleus after binding
with their steroid partner.
Other hormones, such as the thyroid hormone T3 and
the fat-soluble vitamins D and A (retinoic acid) and
their derivatives that regulate growth and
development, were found to act in a similar fashion.

32. Today, it is convenient to regard the entire nuclear
receptor family as ligand-activated transcription
factors that transduce signals by modifying gene
transcription.
The nuclear receptor superfamily consist of two main
classes-together with a third that shares some of
the characteristics of both.
Class I consists largely of receptors for the steroid
hormones, including the glucocorticoid and
mineralocorticoid receptors (GR and MR,
respectively), as well as the oestrogen,
progesterone and androgen receptors (ER, PR, and
AR, respectively).

33. Class I receptors generally recognise hormones that act in
a negative feedback fashion to control biological
events.
Class II nuclear receptors function in a slightly different
way. Their ligands are generally lipids already present to
some extent within the cell.
This group includes the peroxisome proliferator-activated
receptor (PPAR) that recognises fatty acids;
The liver oxysterol (LXR) receptor that recognises and acts
as a cholesterol sensor,
The farnesoid (bile acid) receptor (FXR), a xenobiotic
receptor (SXR; in rodents the PXR) that recognises a
great many foreign substances, including therapeutic
drugs, and
The constitutive androstane receptor (CAR), which not
only recognises the steroid androstane but also some
drugs such as phenobarbital.