Cell Signaling through GCPRs and intracellular receptors.pptx
1. Cell Signaling through GPCRs and
intracellular receptors
Moderator: Dr. Sisay
Presenter: Zeleke Endalew
2. Objectives
Upon completion of this session, You should be able to
– Know the different types of Cell- Cell communications
– Know cell signaling mechanisms
– Comprehend how GPCRs are involved in cell signaling
– Know the different types of Second messengers involved in
intracellular cell signaling
3. Outline
• Cell- Cell communication
• Cell-surface receptors
• GPCRs
• Intracellular signaling mechanisms
• Second messenger system
4. CELL-TO-CELL COMMUNICATION
• Cells communicate with each other by sending signaling
molecules to each other.
• The cell that synthesizes and releases the chemical is the
signaling cell and the intended receiver cell is the target
cell.
5. CELL-TO-CELL …
In order to receive the signal, the target cell must have a
receptor molecule to which the signaling molecule can
bind.
Signaling molecules can be neurotransmitters (released
into synaptic clefts) and hormones.
7. Signaling types
• Hydrophobic signaling
molecules diffuse through the
plasma membrane and bind to
the receptors in the cytosol
• The receptor signal complex
then move into the nucleus
– Bind to transcription control
regions
– Activate or repress gene
expression
8.
9. Membrane receptors
are primarily integral membrane glycoproteins.
They are embedded in the phospholipid bilayer and have three domains,
an extracellular domain that protrudes into the extracellular space and has
binding sites for the signaling molecule,
a transmembrane domain that passes through the phospholipid bilayer, and
an intracellular domain that is located on the cytoplasmic aspect of the
phospholipid bilayer and contacts either peripheral proteins or cellular
organelles
transducing the extracellular contact into an intracellular event.
10. Membrane…
Channel-linked receptors bind a signaling molecule that
temporarily opens or closes the gate, permitting or
inhibiting the movement of ions across the cell membrane.
Examples include nicotinic acetylcholine receptors on
the muscle cell sarcolemma at the myoneural junction.
11. Membrane p…
Catalytic receptors are single-pass transmembrane
proteins.
• Their extracellular moiety is a receptor, and their
• cytoplasmic component is a protein kinase.
12. G-proteins
• A common way to translate a signal to a biologic effect
• Inactivated when binding GDP
• Activated when signal reaches a G-protein,
– the protein exchanges GDP for GTP.
13. G- proteins… cont’d
• Two principal groups involved in cell signaling
I. Small G-proteins
•involved in many cellular functions.
Members of the Rab family
– regulate the rate of vesicle traffic between the ER, the GA,
lysosomes, endosomes, and the cell membrane
14. • The Rho/Rac family
– Another family of small GTP-binding proteins
• mediates interactions between the cytoskeleton and cell
membrane.
• The Ras family
– regulates growth by transmitting signals from the cell membrane to the
nucleus
15. II. Heterotrimeric G-proteins
– the first to be identified.
– Made up of three subunits designated α, β, and γ
– α subunit is bound to GDP
16. Trimeric G-protein
• A trimeric G protein is composed of three
different polypeptide chains, called a, b, and g.
• The Gs-a chain binds and hydrolyzes GTP and
activates adenylyl cyclase.
17. Trimeric
• Stimulatory G-protein (Gs)
• Inhibitory G-Proteins (Gi)
• Phospholipase C activator G
Protein(Gq)
• Olfactory specific G-
protein(Golf)
• Transducin (Gt)
18. • G 12/13 :Control the formation of the actin component of
the cytoskeleton
– Facilitates migration of a cell
19.
20. Signaling via G-Protein-linked Cell-
Surface Receptors
G-protein-linked receptors
• transmembrane proteins
• associated with an ion channel or with an enzyme that is
bound to the cytoplasmic surface of the cell membrane.
– interact with heterotrimeric G protein [GTP]-binding
regulatory protein after binding of a signaling molecule.
• AKA Seven-helix receptors /serpentine receptors
21. G-PROTEIN…
• G protein-coupled receptors vary
– in the binding sites for their ligands
– for different types of G proteins inside the cell.
• GPCR proteins are all remarkably similar in structure.
• There are more than 800 GPCRs.
23. • Ligand binding
• Receptor undergoes conformational change
• Activation of resting heterotrimeric G-protein on the
cytoplasmic side.
– Enables it to bind to the G- 𝑎 protein subunit
– Dissociation of GDP and binding of GTP
– Dissociation of the G-ßŶ protein subunit
24. G-protein-linked receptors…
• In most cases, GTP-G𝑎 Subunit diffuse in the plasma
membrane bind to and activate effector protein.
• Free G-ßŶ protein subunit bind to and activate ion
channel or other target proteins.
– Muscarinic Ach receptors
25. • Hydrolysis of GTP terminates Signaling
– Reassembly of Heterotrimeric G-proteins
• Binding of another ligand molecule causes repetition of
the cycle
26. CYCLIC AMP(cAMP)
• Second messenger
• formed from ATP
• converted to physiologically inactive 5’ AMP
27. cAMP…
• All of the direct effect of cAMP are mediated through its
activation of protein kinase A ,alias cAMP dependent
protein kinase
• cAMP activates protein kinase A by releasing its inhibitory
subunits
28. cAMP…
• Binding of a single
epinephrine molecule to one
GPCRs induce many
molecules of AC
• Two molecules of cAMP
activate one PKA
• Each activated PKA
phosphorylates and activates
multiple target proteins
29. GPCRs that activate or inhibit Adenylate
cyclase
• Prostaglandin E1 and
adenosine, when binding to
their respective G-protein
• activate the inhibitory Gi ,
whose active GTP G𝑎 𝑖
subunit binds to and inhibit
adenyl cyclase
– lowering the cellular response
30. • Cyclic-AMP-mediated protein phosphorylation was first
demonstrated in studies of glycogen metabolism in
skeletal muscle cells.
• Its synthesis and degradation in skeletal muscle cells are
regulated by adrenaline.
• the adrenal gland secretes adrenaline into the blood.
31. • Adrenaline acts by binding to b-adrenergic receptors on
the muscle cell surface,
– causing an increase in the level of cyclic AMP in the cytosol.
The cyclic AMP activates A-kinase, which phosphorylates two
other enzymes.
32. phosphorylase kinase, which was the first protein
kinase to be discovered (in 1956) phosphorylates the
enzyme glycogen phosphorylase;
activating the phosphorylase to release glucose residues
from the glycogen molecule.
33. • glycogen synthase
– inhibits the enzyme's activity, thereby shutting off glycogen
synthesis.
• an increase in cyclic AMP levels both stimulates
• glycogen breakdown and inhibits glycogen synthesis
– maximizing the amount of glucose available to the cell.
34. • CREB links AMP and PKA to activation of gene
transcription
– PKA activation stimulates the expression of many genes
• Leading to long term effect on cells
• e.g. PKA induces the expression of several enzymes involved in
gluconeogenesis
• The promotor segment of all genes regulated by PKA contains cAMP
Response element
35. • PKA moves to the nucleus
– phosphorylates the cAMP responsive element-binding protein
(CREB).
• This transcription factor then binds to DNA and alters
transcription of a number of genes.
36. • elevation of cAMP
• Release of active PKA catalytic
subunits
• some of the catalytic subunits
move into the nucleus.
• Phosphorylates serine-133 on
transcription factor called CREB.
• Phosphorylated CREB binds to
CRE containing target gene
• Also binds to CBP/P300(
coactivator )
– links CREB to RNA polymerase II
– stimulating gene transcription
37.
38. INTRACELLULAR Ca2+ AS A SECOND
MESSENGER
• Ca2+ regulates a very large number of physiologic
processes
• Many second messengers act by increasing the
cytoplasmic Ca2+ concentration
• IP3 is the major second messenger that causes Ca2+
release from the endoplasmic reticulum through the direct
activation of a ligand-gated channel, the IP3 receptor.
39. • The increase is produced by releasing Ca2+ from
– intracellular stores—primarily the endoplasmic reticulum
– by increasing the entry of Ca2+ into cells,
40. Ca2+ Functions as a Ubiquitous
Intracellular Messenger
• The first direct evidence that Ca2+ functions as an
intracellular mediator came from an experiment done in
1947
– the intracellular injection of a small amount of Ca2+ caused a
skeletal muscle cell to contract.
41. Two pathways of Ca2+ signaling
I. Depolarization of the plasma membrane causes an influx
of Ca2+ into the nerve terminal, initiating the secretion of
neurotransmitter;
– the Ca2+ enters through voltage-gated Ca2+ channels that
open when the plasma membrane of the nerve terminal is
depolarized by an invading action potential.
42. II. the binding of extracellular signaling molecules to cell-
surface receptors causes the release of Ca2+ from the ER.
• The events at the cell surface are coupled to the opening
of Ca2+ channels in the ER
– inositol trisphosphate.
44. • An activated receptor stimulates a trimeric G protein
called Gq, which in turn activates phospholipase C- b
– An enzyme that cleaves PIP2 to generate two products: inositol
trisphosphate and diacylglycerol
45. IP3 Couples Receptor Activation
to Ca2+ Release from the ER
.
• IP3 is a small water-soluble molecule that leaves the
plasma membrane and diffuses rapidly through the
cytosol.
– it releases Ca2+ from the ER by binding to IP3-gated Ca2+-release
channels in the ER membrane.
– The increase in cytosolic Ca2+ propagates the signal by
influencing the activity of Ca2+- sensitive intracellular proteins.
46. • Diacylglycerol has two potential signaling roles.
• cleaved to release arachidonic acid,
– can act as a messenger in its own.
– be used in the synthesis of eicosanoids .
• It activates a crucial serine/threonine protein kinase
– phosphorylates selected proteins in the target cell.
47. • The enzyme activated by diacylglycerol is called protein
kinase C (C-kinase, or PKC) because it is Ca2+-
dependent.
51. • binding of Ca2+ enables calmodulin to bind to various
target proteins in the cell to alter their activity
• Ca2+/calmodulin has no enzymatic activity itself
• Instead acts by binding to and activating other proteins
52. • When an activated molecule of Ca2+/calmodulin binds to
its target protein, the calmodulin further changes its
conformation
• Targets that calmodulin regulate
– enzymes
– membrane transport proteins.
53. • Ca2+/ calmodulin binds to the plasma membrane
– activates Ca2+ pump that uses ATP hydrolysis to pump Ca2+
out of cells.
54. • Many effects of Ca2+, are more indirect
– catalyzed by a family of protein kinases called
Ca2+/calmodulin-dependent kinases (CaM-kinases).
• Some CaM-kinases phosphorylate transcription
regulators, such as the CREB protein, and in this way
activate or inhibit the transcription of specific genes
55. Some G Proteins Directly Regulate Ion
Channels
• G proteins do not act exclusively by regulating the activity
of membrane-bound enzymes
• they activate or inactivate ion channels in the plasma
membrane of the target cell
– altering the ion permeability and hence the electrical
excitability of the membrane
56. Vagal stimulation reduces HR
• acetylcholine receptors activate the Gi protein.
• the α subunit of Gi inhibits adenylyl cyclase.
• the βγ subunits bind to K+ channels in the heart muscle
cell plasma membrane and open them.
• Inhibition of Depolarization of cardiac myocytes.
57. Smell and Vision Depend on GPCRs
Smell and Vision Depend on GPCRs That Regulate Ion
Channels
Smell
– detected using specialized olfactory receptor neurons in the
lining of the nose.
– These cells use specific GPCRs called olfactory receptors
– displayed on the surface of the modified cilia that extend from
each cell
58. • The receptors act through cAMP.
• When stimulated by odorant binding, they activate an
olfactory-specific G protein (known as Golf)
– which in turn activates adenylyl cyclase.
– increase in cAMP opens cyclic-AMP-gated cation channels
– influx of Na+, which depolarizes the olfactory receptor neuron
– initiates a nerve impulse that travels along its axon to the brain
59. visual transduction
• visual transduction responses
– are the fastest G-protein-mediated responses
– receptor activation causes a fall in the level of the cyclic
nucleotide
– well studied in rod photoreceptors (rods)
60. visual transduction … cont’d
• rod photoreceptor
– outer and inner segments,
– a cell body,
– a synaptic region
• the rod passes a chemical signal to a retinal nerve cell
61. Rhodopsin
• Photosensitive molecules in the outer segment of the rod
• cyclic-GMP-gated cation channels located in the plasma
membrane surrounding the outer segment.
• Cyclic GMP bound to these channels keeps them open in
the dark.
62. visual trans…
• light-induced activation of rhodopsin molecules in the disc
membrane decreases the cyclic GMP concentration
– closes the cation channels in the surrounding plasma
membrane
• Light causes a hyperpolarization of the plasma membrane
63. • Rhodopsin is a member of the GPCR family
• the activating extracellular signal is not a molecule but a
photon of light
• The activated rhodopsin molecule then alters the
conformation of the G protein transducin (Gt)
– causing the transducin α subunit to activate cyclic GMP
phosphodiesterase.
– hydrolyzes cyclic GMP, so that cyclic GMP levels in the
cytosol fall.
64. • a light signal is converted into an electrical one, through a
hyperpolarization of the rod cell plasma membrane.
65. • A rhodopsin-specific protein kinase called rhodopsin
kinase (RK) phosphorylates the cytosolic tail of activated
rhodopsin
– inhibiting the ability of the rhodopsin to activate transducin.
– arrestin binds to the phosphorylated rhodopsin,
• further inhibiting rhodopsin’s activity.
66. • RGS protein binds to activated transducin
– stimulate transducin to hydrolyze its bound GTP to GDP
– which returns transducin to its inactive state.
67. Nitric Oxide as a signaling molecules
• NO Is a Gaseous Signaling Mediator That Passes
Between Cells
• acetylcholine stimulates NO synthesis by activating a
GPCR on the membranes of the endothelial cells
– IP3 synthesis and Ca2+ release ensues
– This leads to stimulation of an enzyme that synthesizes NO
68. Nitric oxide…
• NO diffuses out of the cell where it is produced and into
neighboring smooth muscle cells,
– causes muscle relaxation and thereby vessel dilation
69. • NO bind reversibly to iron in the active site of guanylyl
cyclase
– stimulating synthesis of cyclic GMP.
– increase cyclic GMP in the cytosol within seconds,
70. • The drug Viagra inhibits the cyclic GMP
phosphodiesterase
– increasing the amount of time that cyclic GMP levels remain
elevated in the smooth muscle cells of penile blood vessels
after NO production is induced.
– The cyclic GMP, in turn, keeps the blood vessels relaxed
71.
72. R E F R E N C E S
1. MOLECULAR BIOLOGY OF THE CELL, SIXTH
EDITION
2. CAMPBELL BIOLOGY, TWELFTH EDITION
3. GANONG’S REVIEW OF MEDICAL PHYSIOLOGY,
TWENTY-FIVTH EDITION
