GPCR Signaling

Question 1

Endocrine signaling - what is it? Examples?

Answer 1

Hormone from distant tissue enters blood to reach distant target cell; seen in metabolic pathways

Ex) Insulin/glucagon; pituitary gland

Question 2

Paracrine signaling - what is it and what’s an example?

Answer 2

Signaling and responding cells are very close or adjacent.

Ex) Growth factors, smooth muscle relaxation

Question 3

Autocrine signaling

Answer 3

Signaling cell and the responding cell are the same cell

Seen in many proliferating cell types

Question 4

When is the G_<em>a</em> subunit active?

Answer 4

Active when bound to GTP

Question 5

Summarize the GPCR Signaling pathway

Answer 5

1. Ligand binds GPCR, changing its conformation
2. GPCR activates the trimeric G protein by triggering dissociation of GDP
3. G_a binds GTP and leaves the receptor to activate an effector
4. Effector hydrolyzes G_a’s GTP and generates a second messenger.
5. Second messenger activates some intracellular and downstream signaling pathway while the ianctivated G_a returns to the By subunits at the GPCR

Question 6

GPCR > Adenylyl cyclase (effector) > ____ > ____

Answer 6

GPCR > Adenylyl cyclase (effector) > cAMP (2nd msger) > PKA pathway

Question 7

GPCR > Guanylyl cyclase (effector) > ____ > ____

Answer 7

GPCR > Guanylyl cyclase (effector) > cGMP (2nd msger) > PKG pathway

Question 8

GPCR > Phospholipase cyclase (effector) > ____ > ____

Answer 8

GPCR > Phospholipase C (effector)

> DAG (2nd msger) > PKC pathway

&

> IP3 (2nd msger) > Ca²⁺ release from ER (also considered a 2nd msger)

Question 9

B adrenergic receptor vs a adrenergic receptor impact on cAMP

Answer 9

B-adrenergic (epinephrine) receptor activates G_<em>as</em> to activate adenylyl cyclase and increase cAMP

a-adrenergic receptoractivates G_ai to activate adenylyl cyclase anddecrease cAMP

Question 10

GPCR siganling activates or shuts down a pathway by

Answer 10

Stimulating or inhibiting an effector –> increases or decreases the 2nd msger

Question 11

Example of how different tissues respond differently to the same signal and GPCR pathway - Epinephrine

Answer 11

Epinephrine causes metabolism of fats in adipose tissue; breakdown of glycogen in liver and muscle; increases contractile rate in cardiac muscle; and fluid secretion in the intestine

Question 12

PKA pathway

Answer 12

1. cAMP binds PKA’s regulatory subunits
2. PKA’s catalytic subunit enter the nucleus to phosphorylate CREB
   
   1. Can also phoshporylate MAPK to affect changes in gene expression
3. CREB is a tf that binds to CRE response elements
4. CREB-binding proteins (CBP)/p300 tf’s bind CREB and associate with transcriptional machinery to mediate transcription of CRE-containing genes

Question 13

Cholera & diarrhea

Answer 13

Maintains G_<em>as</em> -> adenylyl cyclase -> cAMP -> PKA -> CFTR channel

Opening the CFTR channel in intestinal epithelial cells causes Cl- to flow into the intestinal lumen and water follows –> Watery diarrhea

Question 14

Bordatella pertussis & Whooping cough

Answer 14

Turns off G_<em>ai</em> -> Adenylyl cyclase -> cAMP -> PKA -> CFTR channel

Opening the CFTR channel in the lung causes fluid loss (as mucous secretion) in the lungs, causing whooping cough

Question 15

What GPCR signaling pathway is involved in normal heart contraction?

Answer 15

B-Adrenergic receptor / cAMP / PKA signaling

Question 16

How does the B-adrenergic/GPCR/cAMP/PKA signaling pathway cause heart contraciton?

Answer 16

1. The PKA pathway causes an intracellular Ca²⁺ from the sarcoplasmic reticulum into the cytosol.
2. Increased cytosolic Ca²⁺ generates systolic muscle contraction
3. Myofibril efflux of Ca²⁺ back into the SR causes relaxation

Question 17

Any __crease in the in the relative amplitude or ratio of cytosolic:SR Ca2+ pools weakens subsequent contractions

Answer 17

Decrease

Question 18

Decreased muscle contraction (systolic dysfunction) results from __creased B-adrenergic GPCR signaling

Answer 18

Increased B-Adrenergic GPCR signaling

Causes impaired Ca2+ release; defective termination of release; and/or defective calcium sequestration (diastolic dysfunction)

Question 19

How does propranolol work?

Answer 19

Propranolol is a beta-blocker- it inhibits B-Adrenergic signaling to re-establish the proper cytosolic:SR ratios of Ca²⁺ stores –> improves cardiac contractile function

Question 20

Smooth msucle relaxation illustrates what type of signaling?

Answer 20

Paracrine signaling among endothelial cells lining blood vessels

Question 21

What’s the ligand for smooth muscle relaxation?

Answer 21

1. Acetylcholine activates PLC to generate IP₃ and DAG
2. IP3 causes release of Ca²⁺
3. Ca2+ complexes with calmodulin
4. Calmodulin-Ca2+ activates nitric oxide synthase
5. Nitric oxide synthase produces NO
6. NO diffuses to the neighboring smooth msucle cell and activates guanylyl cyclase
7. Guanylyl cyclase –> cGMP
8. cGMP activates PKG
9. PKG causes smooth muscle relaxation and blood vessel dilation

Question 22

How does phospholipase C activate PKC to phosphorylate target proteins?

Answer 22

1. Phospholipase C cleaves PIP₂ into DAG and IP3
2. IP3 releases Ca²⁺ from the ER
3. Ca²⁺ brings PKC to the plasma membrane, where it is activated by DAG