Receptors & Cell Signaling

Question 1

Types of Signaling

Answer 1

1) Endocrine
2) Paracrine
3) Autocrine
4) Juxtacrine

Question 2

Cell signaling steps

Answer 2

1) Lipid/water soluble molecule (ligand) secreted in response to stimulus
2) Ligand binds to target cell protein receptor
3) Ligand-receptor complex activates or inhibits something
4) Dowstream effects occur, amplified by secondary messengers
5) Signal stops via ligand dissociation or inhibition

Question 3

Endocrine signaling

Answer 3

• Transported via blood
• Long distance signaling
• Example: Epinephrine, released by adrenal medulla and acts on heart
• Long lasting

Question 4

Paracrine

Answer 4

• Signal diffuses to neighboring cell
• Example: Testosterone
  1) Leydig cells secrete/synthesize
• Short-lived signal

Question 5

Autocrine

Answer 5

• Signal acts on cell that released it
• Example: Interleukins
  1) T cells secrete interleukin which promotes T cells to replicate during an immune response

Question 6

Direct/Juxtacrine

Answer 6

• Requires cell to cell contact
• Signal binds to signalling cell, which then binds to target cell receptor
• Example: HB-EGF
  1) In immune cells

Question 7

Hydrophilic signaling (and receptors)

Answer 7

-Can’t cross membrane, bind to receptors (epinephrine, insulin)
Receptors –> signal cascade via SECOND MESSENGERS (small, short-lived)
-Downstream response triggered
Receptors: G-protein-coupled receptors (GPCR), Receptor tyrosine kinases (RTKs)

Question 8

Lipophilic signaling

Answer 8

• Diffuse through membranes (steroids, thyroid hormones)
• Signal binds to receptors in the cell (cytosol, nucleus) -> formed complex acts as transcription factor
• Long lives (hours to days)

Question 9

Cytoplasmic receptors

Answer 9

• Heat-shock protein (HSP) dissociates when hormone-receptor complex (HRC) is formed
• HRC moves to nucleus —> initiates gene transcription via hormone response element (HRE)

Question 10

Nuclear receptors

Answer 10

• Already in nucleus, bound to DNA

- Signal hormone activates complex for interactions with proteins for gene transcription

Question 11

G-Protein Coupled Receptor structure (Important ion what clinical context)

Answer 11

Extracellular domain (ECD), Trans Membrane Domain (TM), Intracellular Domain (CD)

-TARGET FOR DRUG THERAPY!!!

Question 12

Steps of a G-Protein Signal Relay

Answer 12

1) Ligand binds to GCPR, incurring conformational change
2) GCPR binds to G-protein, acts as GEF (Guanidine Exchange Factor), GDP exchanged for GTP, activating G-protein
3) G-protein activates effector protein, which triggers cascade of secondary molecules

Question 13

Signal Desensitization Mechanisms

Answer 13

1) Hormone level drop
- Decreased cAMP and PKA activity
2) Removal of the signal molecule
3) Receptor isolation by endosome
4) Receptor destruction (endosome + lysosome + proteases)

Question 14

Receptor recycling

Answer 14

-Depends on quality of the receptor — good quality = fast or slow re-insertion to membrane. Poor quality results in enzyme degradation

Question 15

Types of GCPR Signaling

Answer 15

1) Gs — Adenylate cyclase (stimulates)
2) Gi — Adenylate cyclase (inhibits)
3) Gt — cGMP Phosphodiesterase (stimulates)
4) Gq — Phospholipase C (activates)

Question 16

Gs signaling steps

Answer 16

1) GPCR activated by ligand, which activates Gs-alpha via GTP -> GDP.
2) Gt-alpha activate Adenylate Cyclase via ATP
3) cAMP is activate by AC, which then activate PKA to target proteins
4) Phosphodiesterase (PDE) inactivates cAMP

Question 17

Gi signaling steps

Answer 17

1) GCPR is activated by ligand, which activates Gi-alpha.

2) Gi-alpha does not activate AC

Question 18

Gt signaling steps

Answer 18

1) GPCR activated by ligand, which activates Gt -alpha via GTP -> GDP.
2) Gt-alpha activate cGMP PDE via cGMP

KEY: how vision occurs…..Rhodopsin has cGMP PDE

Question 19

Gq signaling steps

Answer 19

1) GPCR activated by ligand, which activates Gq-alpha via GTP -> GDP.
2) Gq-alpha activates Phospholipase C
3) PLC release PIP, which combines with DAG to activate Protein Kinase C (PKC) -> phosphorylation of target proteins
4) IP3 breaks from PIP to release Ca2+ from Endoplasmic Reticulum, which activates Ca2+ calmodulin=dependent proteins

Question 20

Name the GCPR proteins for:

1) Epinephrine
2) Histamine
3) Epinephrine/norepinephrine
4) Dopamine
5) Acetylcholine
6) Light

Answer 20

1) Gs
2) Gs
3) Gi
4) Gi
5) Gq (q sounds like -choline)
6) Gt (Gt, more like G-see…light!!)

Question 21

Effect of cGMP PDE inhibition on cells

Answer 21

• Since CGMP PDE turns cGMP into 5’-GMP, cellular concentration of cGMP are increased, leading to smooth muscle relaxation and vasodilation (i.e. Viagra)
• Caffeine does the same and increases heart rate
• NITRIC OXIDE IS BAAAADDDDDD = Low BP

Question 22

How does Cholera toxin affect the body?

Answer 22

• Prevents activation of Gs-alpha
• AC continuously stimulated -> overproduction of cGMP
• Cl- channel opens in intestine via CTFR (cystic fibrosis) = loss of electrolytes and water in diarrhea

Question 23

How does Pertussin toxin affect the body?

Answer 23

• Prevents Gi activation by preventing dissociation of Gi-alpha from G-protein complex
• Prevents Gi from inhibiting AC
• cAMP build up —> loss of fluids and excessive mucus = Whooping cough

Question 24

What do antihistamines do?

Answer 24

Inhibit GCPR signaling by blocking effects of histamine to the H1 GCPR

Question 25

What is the structure of Receptor Tyrosine Kinase (RTK)?

Answer 25

• Integral protein

- ICD possesses TYROSINE KINASE ACTIVITY

Question 26

RTK Signaling steps

Answer 26

1) Ligand bind to RTK on the ECD
2) Kinase activated, tyrosine residues are phosphorylated
3) Docking proteins recognized phosphotyrosines and attach (dock)
4) Cascade activated to trigger protein, gene transcription activity via RAS-dependent or RAS-independent
5) Signal terminated

Question 27

What mediates RAS-dependent signaling

Answer 27

-MAPKinase family of enzymes

Question 28

RAS relevance to cancer

Answer 28

• Mutations with RAS gene cause cancer, as well as GTPase activity, causing it to REMAIN ACTIVE
• Involved in a wide variety of human cancers
• Target of drug Therapy (breast cancer drug Herceptin targets HER2, a binding protein for RTK

Question 29

Explain the importance of Small G proteins

Answer 29

• More than 150 in RAS family
• Importance in signal transduction from membrane receptors to effector proteins - Cell proliferation, survival, membrane rand transport
• Because they have intrinsic GTPase activity, MUTATIONS OFTEN LEAD TO CANCER