Cis interaction between Semaphorin6A and Plexin-A4 modulates the repulsive response to Sema6A.

The correct navigation of axons to their targets depends on guidance molecules in the extra-cellular environment. Differential responsiveness to a particular guidance cue is largely an outcome of disparity in the expression of its receptors on the reacting axons. Here, we show that the differential responsiveness of sympathetic and sensory neurons to the transmembrane Semaphorin Sema6A is mainly determined by its co-expression in the responding neurons. Both sympathetic and sensory neurons express the Sema6A receptor Plexin-A4, but only sympathetic neurons respond to it. The expression of Sema6A counteracts this responsiveness and is detected only in sensory neurons. Remarkably, sensory neurons that lack Sema6A gain sensitivity to it in a Plexin-A4-dependent manner. Using heterologus systems, we show that the co-expression of Sema6A and Plexin-A4 hinders the binding of exogenous ligand, suggesting that a Sema6A-Plexin-A4 cis interaction serves as an inhibitory mechanism. Finally, we provide evidence for differential modes of interaction in cis versus in trans. Thus, co-expression of a transmembrane cue together with its receptor can serve as a guidance response modulator.