Cell Signaling

Question 1

1. Describe principle types of detectors of extracellular signaling molecules.

Answer 1

-Ligand (or voltage)-gated ion channels,
-GPCR,
-Enzyme linked receptors (like receptor tyrosine-kinases), and
-Nuclear receptors (TFs activated by cell-penetrating signaling molecules).
I(LoV); G,E,N

Communication can be:

• Paracrine (target nearby),
• Autocrine (target is the signaling molecule),
• Endocrine (covers long distance), or synaptic.
• Lipophilic signaling molecules (like steroid hormones) can penetrate membrane, receptors can be intracellular, but can’t be stored in cell, release only controlled by synthesis, and slow signaling.
• Hydrophilic signaling molecules (like proteins) can’t penetrate membrane, receptor has to be on cell surface, can be stored in vesicles in cell, release controlled by vesicle release, and fast signaling.
• Pathways affect metabolism, gene expression, cell shape/motility, secretion/surface proteins.

Question 2

2. List other “tools” of signaling pathways, including at least three 2nd messengers.

Answer 2

Tool 2:

• Second messengers, which are small molecules released in response to the first messenger.
• Ca (stored in ER, SR)
• NO generated by NOS (cell permeable)
• cAMP generated by AC; cGMP produced by GC
• phosolipase C generates IP3 (released in cytosol) and DAG (membrane)

Tool 3:

• Other signaling steps: signaling proteins activated by receptors or 2nd messengers can activate other signaling/effector proteins.
• Protein modification =
• phosphorylation,
• acetylation (histones),
• glycosylation (trafficking),
• ubiquitinylation (marking for degradation),
• proteolytic cleavage (of inactive precursor)
• protein-protein binding and protein targeting
• GTP/GDP exchange = G-proteins coupled to receptors and small GTPases (ras).
• GTP is not a 2nd messenger because signaling through GTP does not depend on concentration.

Question 3

3. Describe at least three mechanisms for signal termination (including phosphodiesterases).

Answer 3

1) Extracellular signaling molecule can be:
- Diffused
- Inactivated/broken down extracellularly
- Uptaken into cells by Transporters (which can then be important drug targets)

2) Receptor
- Receptor can become desensitized to its ligand; there can be a reduction of binding or downstream signaling, or the receptor can be internalized. This allows for plasticity that affects later signals as body accustoms to how sensitive you are

3) 2nd Messengers
Second messengers go away:
-Ca –> ATP dependent pumps that pump it out of cell or into stores (like SR)
-metabolic modification (like cAMP or cGMP to AMP or GMP by phosphodiesterases).
-Drugs can target PDE5 for Viagra
-Normally GC produces cGMP using GTP –> turns on PKG kinase (which causes Ca release, smooth muscle relaxation and vasodilation and erection).
-PDE5 turns cGMP into GMP, which turns off this effect. Viagra inhibits PDE5 –> prolonged erection because cGMP not degraded.

4) -Phosphorylation by kinases and 5)dephosphorylation by phosphatases.
6) Protein binding/targeting –> lack of stimulus –> protein degradation.
7) Some signaling proteins have built-in terminators, like G-proteins and GTP binding proteins, which have slow GTPases to turn themselves off, reversing the active GTP-bound state. GAP speeds up this process (GTPase Activating Proteins).

Question 4

4. Evaluate a “pathway” for amplification and termination.

Answer 4

• Amplification generally causes a signal cascade.
• Happens when signaling molecule activates more than one downstream molecule; if this happens at multiple consecutive steps, you get a cascade.
• Signaling cascades depend on how many downstream target molecules are affected and how long does the original signaling molecule remain active.

Positive feedback loop:

• dangerous form of amplification –> usually coupled to negative feedback loop.
• Hormones can have large effects (measured by photodetection).
• Cooperative binding can cause sharpened response in that you need less [substrate] to make the reaction go.

Question 5

5. Identify “nodes” (such as calcium) and “modules” in a signaling pathway, and evaluate the potential for crosstalk in signal transduction.

Answer 5

• Signaling networks are generally very complex with lots of potential for cross-talk in signaling.
• They are more than just the sum of the pathways –> they help understand context of pathways.
• Sometimes simultaneous activity of several receptor types may be necessary to cause a certain outcome –> coincidence detection.
• Lots of possibilities –> some connections may require additional input stimulation, others the signaling protein may not be expressed, or expressed in the wrong location, or downstream signaling pathway doesn’t result in a function.

Nodes:

• Network that receive multiple inputs/outputs.
• Pathways can go through a node –> multiple pathways use the same players.
• Ca = very extensive node in signaling.

Modules:

• Groups of components/building blocks that function together; can sometimes be physically associated to form complexes.
• Multiple signals get processed together, generally result in one outcome.