EphB2 regulates axonal growth at the midline in the developing auditory brainstem

Eph receptors play important roles in axon guidance at the midline. In the auditory system, growth of axons across the midline is an important determinant of auditory function. The avian cochlear nucleus, n. magnocellularis (NM), makes bilateral projections to its target, n. laminaris (NL). We examined the time course of NM axon growth toward the midline, the expression of Eph proteins at the midline during this growth, and the effects of Eph receptor misexpression on axonal growth across the midline. We found that NM axons reach the midline at E4. At this age, EphB receptors are expressed at the ventral floor plate. Expression extends dorsally to the ventricular zone beginning at E5. NM axons thus grow across the midline at a time when EphB receptor expression levels are low. Overexpression of EphB2 at E2 resulted in misrouted axons that deflected away from transfected midline cells. This effect was observed when midline cells were transfected but not when NM cells alone were transfected, suggesting that EphB2 acts non-cell autonomously and through reverse signaling. These data suggest an inhibitory role for midline Eph receptors, in which low levels permit axon growth and subsequently high levels prohibit growth after axons have crossed the midline.