Upregulation of an EphA receptor in a nasal retinal ganglion axon that normally expresses low levels of this receptor would ______________.

Question

BlackTom AI · Accepted Answer

EphA receptors on retinal ganglion cell (RGC) axons mediate repulsive interactions with ephrin-A ligands in the superior colliculus (SC); higher EphA expression increases sensitivity to ephrin-A and therefore increases repulsion from regions with high ephrin-A concentration. In the SC the ephrin-A ligand is expressed in a posterior-to-anterior gradient (highest posteriorly, lowest anteriorly), while RGC EphA receptors are normally graded across the temporal–nasal axis of the retina (highest in temporal RGCs, lowest in nasal RGCs). This complementary gradient mapping positions temporal axons (high EphA) more anteriorly and nasal axons (low EphA) more posteriorly.

1. Relevant knowledge points and concepts
- EphA receptors bind ephrin-A ligands and produce repulsive guidance.
- Ephrin-A in SC: high posteriorly → creates repulsive zone.
- Retinal EphA gradient: temporal high, nasal low → establishes anterior–posterior topography.

2. Step-by-step reasoning
- A nasal RGC normally has low EphA, so it tolerates higher ephrin-A and projects more posteriorly.
- Upregulating EphA on that nasal axon increases its sensitivity to ephrin-A.
- Because posterior SC expresses more ephrin-A, the now EphA-high axon experiences stronger repulsion from posterior regions.
- To avoid high-ephrin-A posterior zones, the axon will shift its termination zone anteriorly, where ephrin-A levels are lower.

3. Calculations
- No numeric calculation is required; the conclusion follows from qualitative gradient interactions (increased receptor → increased repulsion → shift toward lower ligand).

4. Why the correct answer is correct
- Answer (b) states the axon will project more anteriorly in the lateral SC. This matches the predicted effect of increasing EphA: greater avoidance of posterior (high-ephrin-A) SC and relocation to anterior regions with lower ephrin-A.

5. Why other options are incorrect
- (a) Medial projection is controlled by dorsal–ventral cues (e.g., ephrin-B gradients), not EphA–ephrin-A anterior–posterior mapping. 
- (c) Posterior projection is the opposite of what increased EphA causes (it would be repelled from posterior). 
- (d) Upregulation changes guidance sensitivity, so “no change” is wrong. 
- (e) Laterality (ipsilateral vs contralateral) is determined by midline crossing mechanisms, not by EphA level in this context.

6. Conclusion
These gradient-based repulsive interactions predict an anterior shift of the nasal axon when EphA is upregulated, confirming the provided correct answer: b) Make the axon project more anteriorly in the lateral superior colliculus.

Upregulation of an EphA receptor in a nasal retinal ganglion axon that normally expresses low levels of this receptor would ______________.单项选择题

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