Which of the following represent the CORRECT order of molecular steps involved in the movement of a growth cone towards a chemoattractive cue?

Question

BlackTom AI · Accepted Answer

Binding of a chemoattractant to its receptor triggers intracellular Ca2+ transients that regulate actin- and microtubule-binding proteins, leading to localized polymerization and directional growth-cone advance.

1. Relevant knowledge points and concepts
- Extracellular guidance cues (e.g., netrins, BDNF) bind specific receptors on the growth cone membrane. 
- Receptor activation induces intracellular second-messenger changes, notably local increases in Ca2+ and activation of kinases/phosphatases and small GTPases (Rac, Cdc42, Rho). 
- Ca2+ and these signaling pathways modulate the activity of actin-binding proteins (cofilin, profilin, Arp2/3, Ena/VASP) and microtubule-associated proteins (MAPs, +TIPs), altering polymerization dynamics. 
- Local polymerization of actin (filopodial/lamellipodial protrusion) and microtubule advance toward the cue produces turning toward the attractant.

2. Step-by-step reasoning
- Step 1: Extracellular cue binds its receptor—this is the initiating event and must occur first. 
- Step 2: Receptor activation triggers intracellular signaling cascades that change local Ca2+ concentration and activate downstream effectors. 
- Step 3: Elevated Ca2+ and effectors change the activity (phosphorylation, binding state) of actin- and tubulin-binding proteins. 
- Step 4: These altered regulators promote localized polymerization of actin filaments and microtubules toward the source, steering the growth cone.

3. Calculations
- No numeric calculations are required; the sequence is mechanistic.

4. Why the correct answer is correct
- Option e correctly orders initiation (binding) → signaling (Ca2+ changes) → regulation of cytoskeletal regulators → execution (polymerization). This matches the known causal chain in chemotropic growth-cone guidance.

5. Why other options are incorrect
- a and c place polymerization before receptor binding or signaling, violating causality. 
- b begins with Ca2+ changes before receptor binding, which is backward. 
- d places regulation before receptor binding, likewise incorrect.

6. Conclusion
- The correct sequence is option e: Binding of cue to its receptor → Changes in intracellular Ca2+ → Regulation of actin/tubulin-binding proteins → Polymerization toward the cue, matching the provided answer.

Which of the following represent the CORRECT order of molecular steps involved in the movement of a growth cone towards a chemoattractive cue?Single choice

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