Signal Transduction: GPCRs Flashcards
Define the following:
Endocrine Signaling
Paracrine Signaling
Autocrine Signaling
Endocrine Signaling - “long distance signaling” - hormone secreted into blood that travels to target tissue, long distance
Paracrine Signaling - “local signaling” - target cell is very close to signaling cell
Autocrine Signaling - signaling cell is the target cell; seen in many proliferating cells
What are the general principles of signaling?
signaling molecule binds extracellular domain of specific receptor on target cell
the receptor transduces the signal internally…
ultimately a Second Messenger is
multiple intracellular molecules are activated/deactivated, leading to a Signaling Cascade” which permits Signal Amplification
Ultimately the signal is terminated by various mechanisms
In general, what are GPCRs? What are their structures and basic mechanisms of action?
GPCRs = Guanine Protein (G-Protein)-Coupled Receptors
- 7-pass transmembrane proteins, with N- terminus in extracellular space and C-terminus in intracellular space
- when activated, GPCRs associate with and activate Trimeric G-Proteins (Gα, Gβ, Gγ subunits)
- stimulatory G-proteins (Gαs) activate Effector to produce 2nd Messenger; inhibitory G-Proteins (Gαi) inhibit Effector from producing 2nd Messenger; Stimulatry
Describe the process of GPCR activation/inactivation
1) ligated GPCR binds Trimeric G-Protein:
GPCR + Gβγα-GDP –> GPCR + Gβγα-GDP
2) this binding causes Gα to undergo conformational change which leads to dissociation of GDP from it:
GPCR-Gβγα-GDP –> GPCR-Gβγα + GDP
3) GTP binds Gα:
GPCR-Gβγα + GTP –> GPCR-Gβγα-GTP
4) Binding of GTP to Gα causes dissociation of Gα from both GPCR and from Gβγ
GPCR-Gβγα-GTP –> GPCR + Gβγ + Gα-GTP
5) Gα-GTP binds and activates a membrane-bound Effector, causing it to generate a 2nd Messenger:
Gα-GTP + Effector –> Gα-GTP-Effector –> Gα-GTP-Effector + 2nd Messenger
6) Gα hydrolizes its bound GTP to GDP, inactivating it:
Gα-GTP –> Gα-GDP (inactive)
7) Gα-GDP dissociates from Effector and reassociates with Gβγ:
Gα-GDP + Gβγ –> Gβγα-GDP
cAMP/PKA Pathway - REVISIT!!!!
GPCR –> AC –> cAMP
- cAMP –> activates PKA –> phosphorylates and activates CREB (a TF) –> phosphorylated CREB enters nucleus and binds to basal transcription machinery associated with genes containing a CRE consensus sequence –> increasing transcription of these genes
- cAMP –> MAPK
What are three examples of GPCR-activated Effectors and 2nd Messengers?
1) Adenylyl Cyclase (AC)
- -> cAMP (from AMP) –> PKA
2) Guanalyl Cyclase (GC)
- -> cGMP (from GMP) –> PKG –> opoens cation channels in Rod Cells (eye)
3) Phospholipase C (PLC)
- -> DAG –> PKC
- -> IP3 –> activates Ca2+ release channels in ER
What are the effects of Epinephrine, ACTH, and Glucagon on Adipose tissue?
Hormones: Epinephrine, ACTH, Glucagon
Target Tissue: Adipose
Effects:
- ↑ hydrolysis of triglycerides
- ↓ AA uptake
Mechanism:
GPCR –> Gαs –> AC –> ↑ cAMP
What are the effects of Epinephrine, Norepinephrine (NE), and Glucagon on Liver
Hormones: Epi, NE, Glucagon
Target Tissue: Liver
Effects: increased [Glc]
- ↑ Glycogenolysis (Glycogen –> ↑ Glc)
- ↓ glycogen synthesis
- ↑ AA uptake
- ↑ Gluconegenesis (AAs –> Glc)
Mechanism:
GPCR –> Gαs –> AC –> ↑ cAMP
What are the effects of Epinephrine on Cardiac Muscle?
Hormones: Epi
Target Tissue: Cardiac Muscle
Effect:
-↑ contraction rate
Mechanism:
GPCR –> Gαs –> AC –> ↑ cAMP
What is the effect of Epi on Skeletal Muscle?
Hormones: Epi
Target Tissue: Skeletal Muscle
Effect:
-↑ Glycogenolysis( Glycogen –> ↑ Glc)
Mechanism:
GPCR –> Gαs –> AC –> ↑ cAMP
How does the bacterium Vibrio cholerae produce the symptoms of cholera?
Vibrio cholarea produces toxin which modifies Gαs subunit, making it constituitively active
–> PKA activates CFTR channel in intestinal epithelial cells –> CFTR channel allows Cl- to flow out of cells and into intestinal lumen, H20 follows it –> watery diarrhea, dehydration
How does the bacterium Bordatella pertussis produce the symptoms of Whooping Cough?
Bordatella pertussis produces toxin which modifies Gαi subunit, preventing GDP release from it and thereby inactivating it (no inhibiory G-protein activity)
-therefore AC –> ↑ cAMP –> PKA –> PKA activates CFTR in lungs –> mucous secretions on lungs
What are the major classes of mammalian Trimeric G-Proteins?
Gαs Gαi Gαolf Gαq Gαo Gαt
Gαs
Effector / 2nd Messenger
AC / ↑ cAMP
What are the associated Effector and 2nd Messenger for Gαs-containing trimeric G-Proteins? What are some examples of GPCRs associated with Gαs-containing G-Proteins?
Gαs (stimulatory)
Effector / 2nd Messenger:
AC / ↑ cAMP
Examples: β-Adrenergic Receptors = EPI RECEPTORS Glucagon receptors Vasopressin receptors Seratonin receptor
What are the associated Effector and 2nd Messenger for Gαi-containing trimeric G-Proteins? What are some examples of GPCRs associated with Gαi-containing G-Proteins?
Gαi (inhibitory)
Effector / 2nd Messenger:
AC (inhibits) / ↓ cAMP
Examples:
α2-Adrenergic Receptors
AND
Effector / 2nd Messenger:
K+ channels (inhibited by Gβγ) –> hyper polarizes Membrane Potential
Examples
mACHR’s (muscarinic ACh Receptors)
What are the associated Effector and 2nd Messenger for Gαolf-containing trimeric G-Proteins? What are some examples of GPCRs associated with Gαolf-containing G-Proteins?
Gαolf
Effector / 2nd Messenger:
AC (stimulates) / ↑ cAMP
Examples:
oderant receptors in nose
What are the associated Effector and 2nd Messenger for Gαq-containing trimeric G-Proteins? What are some examples of GPCRs associated with Gαq-containing G-Proteins?
Gαq
Effector / 2nd Messenger:
PLC / ↑ IP3, ↑ DAG
Examples:
α1-Adrenergic Receptors
What is the mechanism by which catecholamines such as Epinephrine affect Cardiac Muscle contraction?
(Lecture p. 9)
Epi binds β-Adrenergic Receptor (Gαs-affiliated GPCR) –> AC –> ↑ cAMP –> ↑ PKA
PKA phosphorylates RyR’s (Ca2+ channels in SR) and DHPRs (L-type channels in T-tubules) –> RELEASE of Ca2+ from SR –> ↑ [Ca2+] –> SYSTOLIC CONTRACTION!!!!
What is a biochemical mechanism that causes/contributes to Heart Failure?
CHRONIC ELEVATION of catecholamines
(how does this occur?)
—> –> ↑ AC/cAMP/PKA pathway
–>causes continuous release of Ca2+ from SR,
-Ca2+ cannot be restored to SR. Therefore, subsequent contractions are weaker
(but what about the Ca2+ that’s already in the cytosol? can’t it just re-bind to calmodulin and re-activate contraction?)
What are β-blockers? What are they used to treat? What is their mechanism of action in the treatment of cardiac problems?
β-blockers in the treatment of Heart Failure?
(Lecture p. 10)
β-blockers = β-Adrenergic receptor antagonists
-example: Propanolol
Treat:
HT, cardiac arrhythmias, MIs, Heart Failure
Mechanism:
–> inhibit AC/cAMP/PKA pathway –> allows Ca2+ to be restored to SR
(???)
Potential side effects:
-depressed cardiac function, reduced exercise tolerance
PLC/DAG/IP3/Ca2+ Pathway - REVISIT!!!!
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