Semaphorin 5B is a repellent cue for sensory afferents projecting into the developing spinal cord

Abstract

During vertebrate development, centrally projecting sensory axons of the dorsal root ganglia neurons first reach the embryonic spinal cord at the dorsolateral margin. Instead of immediately projecting into the grey matter, they bifurcate and extend rostrally and caudally to establish the longitudinal dorsal funiculus during a stereotyped waiting period of approximately 48 h. Collateral fibres then extend concurrently across multiple spinal segments and project to their appropriate targets within the grey matter. This rostrocaudal extension of sensory afferents is crucial for the intersegmental processing of information throughout the spinal cord. However, the precise cues that prevent premature entry during the waiting period remain to be identified. Here, we show that semaphorin 5B (Sema5B), a member of the semaphorin family of guidance molecules, is expressed in the chick spinal cord during this waiting period and dorsal funiculus formation. Sema5B expression is dynamic, with a reduction of expression apparent in the spinal cord concomitant with collateral extension. We show that Sema5B inhibits the growth of NGF-dependent sensory axons and that this effect is mediated in part through the cell adhesion molecule TAG-1. Knockdown of Sema5B in the spinal cord using RNA interference leads to the premature extension of cutaneous nociceptive axons into the dorsal horn grey matter. These premature projections predominantly occur at the site of dorsal root entry. Our results suggest that Sema5B contributes to a repulsive barrier for centrally projecting primary sensory axons, forcing them to turn and establish the dorsal funiculus.

Attractive and permissive activities of semaphorin 5A toward dorsal root ganglion axons in higher vertebrate embryos

“…Consistent with that previous report, 8 Sema5B was observed to be broadly distributed in the spinal cord at E10.5 ( Fig. 1G).…”

“…Based on the fact that Sema5B, another member of the Sema5 family in vertebrates, can repel DRG axons at early stages, 8 we further examined the expression pattern of Sema5B mRNA in order to compare it with that of Sema5A. Consistent with that previous report, 8 Sema5B was observed to be broadly distributed in the spinal cord at E10.5 ( Fig.…”

“…7 More recently, another group revealed that Sema5B is a repellent of DRG axons innervating the spinal cord. 8 On the other hand, the effect of Sema5A on DRG axonal guidance is not clear.…”

The role of Gpi-anchored axonal glycoproteins in neural development and neurological disorders

“…There remains some uncertainty about which semaphorins are critical to the guidance of spinal sensory axons in vivo: whereas similar defects are seen in Sema3A mutants in the central branches of nociceptive axons in NRP1 knockouts animals (Gu et al, 2003), loss of NRP1 or of Sema3A also affects the peripheral branches, whereas loss of CNTN2 does not (Law et al, 2008;Liu & Halloran, 2005), suggesting that the in vivo effects of CNTN2 loss may reflect sensitivities to other Semas, for example Sema5B (Liu et al, 2014).…”

Towards an understanding of semaphorin signalling in the spinal cord

“…Sema5B is expressed in the dorsal grey matter early in chick development, where it inhibits the initial entry of dorsal root ganglia axons into the grey matter (Liu et al, 2014). This inhibition is most likely mediated, at least in part, by the transient axonal glycoprotein 1 (TAG1) located on the growing axons (Liu et al, 2014).…”