Signaling at the cell surface and secondary messenger cascades

🔹 Modes of Cell Signaling

Q: What is endocrine signaling?

A: Signal (e.g., hormone) is released into the bloodstream and acts on distant target cells (e.g., insulin).

🔹 Modes of Cell Signaling

Q: What is paracrine signaling?

A: Signal acts on nearby cells within the same tissue (e.g., neurotransmitters).

🔹 Modes of Cell Signaling

Q: What is autocrine signaling?

A: Cell releases a signal that acts on itself (common in immune response and cancer).

🔹 Modes of Cell Signaling

Q: What is contact-mediated signaling?

A: Signal requires direct physical contact between cells (e.g., Notch-Delta pathway).

🔹 Hormone Receptor Occupancy

Q: Why does the hormone's effect exceed the receptor occupancy?

Why does the functional effect of hormone binding consistently exceed the bound fraction (occupancy) of the receptor by its cognate hormone?

A: Signal transduction includes amplification steps: binding of a few receptors activates many intracellular molecules (e.g., via second messengers like cAMP or IP₃).

🔹 GPCRs (G Protein-Coupled Receptors)

Q: What was the first structurally characterized GPCR?

A: Rhodopsin, from rod photoreceptor cells.

🔹 GPCRs (G Protein-Coupled Receptors)

Q: Name two important GPCRs in mammals.

A:

• β2-adrenergic receptor (β2AR) – regulates heart rate and airway relaxation

• Muscarinic acetylcholine receptor – modulates neural and cardiac function

🔹 GPCRs (G Protein-Coupled Receptors)

Q: What is the functional cycle of a GPCR?

A:

1. Ligand binds to GPCR

2. GPCR undergoes conformational change

3. Gα exchanges GDP for GTP

4. Gα and Gβγ dissociate and activate effectors (e.g., adenylyl cyclase)

5. GTP is hydrolyzed → Gα reassociates with Gβγ → cycle resets

🔹 Gα Subunit Functions

Q: What is the function of GTP and the GTPase activity in Gα subunit?

A:

• GTP activates Gα

• GTPase activity hydrolyzes GTP to GDP → inactivates Gα

• Acts as a built-in timer to limit signaling duration

🔹 Gα Subunit Functions

Q: What is the role of a GEF (Guanine Exchange Factor) in G-protein signaling?

A: GEF (often the GPCR itself) promotes GDP-GTP exchange, activating Gα.

🔹 Gα Subunit Functions

Q: What is the scale of β2AR's conformational change during activation?

A: ~14 Å movement of transmembrane helices rearranges the intracellular face to allow G protein binding.

🔹 Gα Subunit Functions

Q: What role does β2AR play in Gs activation?

A: β2AR activates Gs, a stimulatory G protein that activates adenylyl cyclase, increasing cAMP production.

🔹 Phototransduction

Q: What is the origin of the disc-filled outer segment in rod cells?

A: Derived from specialized plasma membrane infoldings, packed with rhodopsin for light detection.

🔹 Phototransduction

Q: What pigment does rhodopsin contain and how does it respond to light?

A: 11-cis-retinal, which undergoes photo-isomerization to all-trans-retinal, triggering rhodopsin activation.

🔹 Phototransduction

Q: What is the key ion channel in the visual cascade and how is it regulated?

A: Cyclic nucleotide-gated (CNG) channel, regulated by cGMP levels. Light → ↓cGMP → channel closes → hyperpolarization.

🔹 Receptor Tyrosine Kinases (RTKs)

Q: What activates RTKs?

A: Ligand binding induces dimerization, activating kinase domains for autophosphorylation.

🔹 Receptor Tyrosine Kinases (RTKs)

Q: Name two RTK examples.

A:

• EGFR (Epidermal Growth Factor Receptor)

• Insulin receptor

🔹 Notch Signaling

Q: How does the Notch receptor work?

A: Contact-mediated signaling: ligand (Delta) binding triggers proteolytic cleavage of Notch → intracellular domain enters nucleus → regulates gene expression.

🔹 JAK-STAT Pathway

Q: What does the STAT complex do in EPO signaling?

A: Erythropoietin (EPO) binds its receptor → activates JAK kinase → phosphorylates STAT, which dimerizes and enters the nucleus to activate genes for red blood cell production.

🔹 Transcription & Chromatin

Q: What does it mean that a transcription factor “opens” chromatin?

A: It recruits chromatin remodeling complexes or histone acetyltransferases (HATs) to make DNA more accessible for transcription by loosening nucleosomes.