Term
| What is the basic features of a GPCR? |
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Definition
1) Trimeric complex
2) G-alpha binds GTP and hydrolyzes it (GTPase)
3) GTP hydrolysis increases GTP-alpha binding affinity for GTP-beta and gamma (in other words, when GTP is bound,the affinity is less, and G-alpha can go bind another target protein) |
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Term
| Why are regulators of G protein signaling (RGS) crucial in G-protein signaling? Clinical relevance of this? |
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Definition
The GTPase activity of the G-alpha subunit is stimulated by its binding to an effector, but NOT EOUGH for rapid termination.
RGS molecules can thereby regulate sensitivity to GPCRs
RGS4 has been implicated in the prefrontal cortex-related symptoms schizophrenia, influencing GPCR and GABA transmission. |
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Term
| How does cholera toxin act in intestinal enterocytes? |
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Definition
Upon endocytosis, the toxin catalyzes the trasfer of ADP ribose from NAD+ to G-alpha proteins.
G-alpha can no longer hydrolyze bound GTP and remains chronically activity (active AC follows). Increased cAMP activates CFTR, causing chloride efflux and watery diarrhea. |
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Term
| How are MAPK pathways related to G-alpha signaling? |
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Definition
Remember, G-beta and G-gamma are associated to the plasma membrane via isoprenolization. When G-alpha-GTP leaves and binds other molecules, these subunits can activate MAPK through adaptor proteins such as Shc
G-alpha activated pathways can inhibit this G-beta/G-gamma induced MAPK pathways!. |
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Term
| How might GPCR deficits relate to tumor progression (hint. think PKC activity). |
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Definition
This is tricky, take 1 step at a time.
1) GTP-bound Gaq has less affinity for the tetrameric GPCR complex and increased affinity for its effector, PLC-beta.
2) PLC-b catalyzes the hydrolysis of PIP2 to increase cytosolic IP3 and membrane-associated DAG.
3) DAG, along with increased calcium concentration from IP3-bound calcium channels can activate PKC.
PKC is involved in cell proliferation and aberrant signaling can cause tumor development. |
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Term
| How is it that cholecystokinin (CCK) and Acetylcholine (Ach) both activate the secretion of digestive enzymes by pancreatic acinar cells through changes in intracellular Ca, and yet, CCK's effects last longer? |
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Definition
RGS molecules!
RGS4 binds muscarinic Ach receptors more tightly than CCK receptors. This tighter binding changes calcium kinetics in Ach receptors (shortens), and leads to a shorter response!
NOTE- RSGs are molecules that regulate deactivation of GTP-bound Ga proteins through ATPase activity. |
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Term
| Why do GTP-bound Gas proteins increase cAMP production, but GTP-bound Gai proteins decrease it? |
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Definition
| Both GTP-bound proteins interact with adenylyl cyclase (AC) in the plasma membrane, but Gas activates it and Gai inhibits it. |
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Term
1) How do trimeric G-proteins including the Gai-subunit regulate heart rate and beat-strength?
2) How does Pertussus toxin relate to this? |
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Definition
1) Short Answer- Open K+ channels to let out K+ and decrease cardiac cell excitability
Explanation- Interestingly, the Gai subunit is not what matter here. Ach binds to muscurinic GPCRs and when Gai dissociates and acts on AC to decrease cAMP, the Gb/Gy complex triggers the opening of K+ channels that let K+ flow out of heart cells, making them more difficult to depolarize and activate.
2) Pertussus Toxin (whooping cough) catalyzes ADP-ribosylation of Gai, stabilizing the GDP-bound form and limiting the subunits ability to deactivate AC. The overall effect is INCREASED cAMP |
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Term
| How do the actions of Pertussus toxin (PT) and Cholera toxin (CT) relate? |
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Definition
Both are involved in ADP-ribosylation of GPCR subunits, and both lead to aberrant cAMP signaling
CT acts on Gas, inhibiting the hydrolysis of GTP from Gas (active form)
PT acts on Gai, stabilizing its GDP-bound form, preventing it from interacting with AC and limiting cAMP (leads to increased cAMP). |
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Term
| Which G-protein subunit activates cyclic nucleotide-gated ion channels? What is the functional consequence of this action? |
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Definition
Golf
Golf is coupled to olfactory receptors in olfactory epithelia cells and is involved in odor detection.
GTP-bound Golf activates AC to increase cAMP, which binds cAMP-gated ion channels leading to the opening of Na channels that depolarize neurons. |
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Term
| How does Gt relate to rod photoreceptor cells in the retina? |
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Definition
Light-activated Rhodopsin leads to a confirmational change that activates Gt (Transducin).
GTP-bound transducin activates cGMP phosphodiesterase, closing of cGMP-dependent sodium channels, hyperpolarizion, and in this RARE case, neural firing (usually depolarization causes this)! |
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Term
True:False
GPCRs share a high degree of sequence homology in their 7 transmembrane domains, but not in their extracellular or cytoplasmic domains. |
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Definition
True!
This homology is used to break GPCRs into categories.
1) Many As: Rhodopsin-like
2) B: Secretin-like
3) Few Cs: Glutamate-like (very long extracellular domain) |
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Term
| What does specificity of cellular responses to GPCR activation depend on? |
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Definition
Short- integration of multiple signaling pathways, receptor-ligand affinity and expression and localization of downstream molecules.
Explanation
A cell may have many different GPCR types that are coupled to the same trimeric G-protein (i.e. different GPCR-ligand combinations may all increases cAMP through AC).
Therefore, specificity depends upon
1) Integration of multiple signaling pathways
2) Receptor affinity for ligands and signaling intermediaries
3) Expression levels and localization of signaling molecules |
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Term
1) Which G-proteins are coupled to each of the different catecholamine (dopamine, epinephrine, norepinephrine)-regulated GPCRs?
2) What are the differences is ligand-sensitity? |
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Definition
1) B1, B2, B3- Gas (increase cAMP)
alpha1- Gq (phospholipase to IP3 signaling)
alpha2- Gi (reduce cAMP)
2) alpha 1 and alpha 2- NE>EPI and no synthetic isoprotereon (ISO)
B1 and B2- high ISO
B1-NE=EPI
B2- EPI>NE
B3- NE>EPI |
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Term
| What does the differential ligand specificity of PACAP-27 and PACAP-38 tell us about GPCR signaling? |
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Definition
These are both ligands for the same receptor, but they exhibit different trimeric G-protein specificity.
Remember, PACAP-27 and PACAP-38 bind Gas with equal affinity, but only PACAP-38 couples well with Gq.
Therefore, only PACAP-38 can elevate cAMP and IP3-dependent calcium increases, even though it acts on the same pituitary receptor. |
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Term
| What is the significance of B-arrestin mutations in GPCR sensitization? |
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Definition
B-arrestin promotes clathrin-mediated endocytosis of activated GPCRs leading to recycling or lysosomal degredation
An absence would therefore cause over-activity of GPCRs and could lead to numerous issues such as over contractility of the heart. |
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Term
| Which GPCRs have been implicated in cancer (tumor-induced angiogensis and cell growth)? |
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Definition
Over-expression of Protease-activated Receptors (PARs)
Note- Thrombin cleaves the N-terminus of PARs, and the terminus acts as a "tethered" ligand to activate the receptor. Pars are associated with trimeric G-proteins and over expression of PARs makes these receptors hypersensitive to protease activation in the tumor environment. |
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Term
| How can tumor cells self-propagate by acting on GPCRS? |
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Definition
Through autocrine and paracrine secretion of specific neuropeptides produced by the cancerous cells themselves! Example is LPA in ovarian cancer.
This is often referred to as "persistence stimulation" |
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Term
| Provide an example of how genetic variation of GPCRs within a population contributes to determining risk for complex human disease? |
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Definition
B-adrenergic receptor polymorphisms in heart disease.
Racial differences in the intracellular domain AA composition exist, which seem to determine sensitivity to drug treatment (i.e. one group is more likely to have mutations and is therefore more likely to respond to certain drugs). |
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Term
| What are the 3 mechanisms underlying aberrant GPCR activation and hypersensitivity in cancer? |
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Definition
| 1) Constitutively active mutants, persistent activation by autocrin and paracrine signals, over-expression of PARs. |
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Term
| Why might GPCRs become hypersensitive to protease action and sometimes cause cancer? |
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Definition
Over-expression of Protease Activated Receptors (PARs).
These GPCRs are activated by thrombin-mediated N-terminal cleavage, exposing a "tethered" receptor which binds and activates to the receptors to alter downstream signaling through Gaq and Gai. |
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