Our findings reveal that the interaction of Sema6D, Plexin-A1, and Nr-CAM on chiasm cells inverts the sign of Sema6D signaling and represents a switch mechanism crucial for chiasm crossing. The deployment of distinct receptor subunits has been shown to switch axonal responses to an individual guidance molecule in many different neural systems. As one example, Sema3E is a repulsive cue for distinct populations of forebrain axons that express Plexin-D1 alone, but when Neuropilin1 and Plexin-D1 are coexpressed by these axons, Sema3E signaling promotes rather than retards growth (Chauvet SB431542 in vivo et al., 2007). In an analogous manner the expression of the netrin receptor, unc5, and
its vertebrate counterparts (UNC5H1-3) can change the response of its coreceptor unc40/DCC to the guidance factor unc-6/netrin from attraction to repulsion (Chisholm and Tessier-Lavigne, 1999, Culotti and Merz,
1998 and Hong et al., 1999). In these and other instances, a change in the receptor complex expressed by axons underlies the switch in response to guidance cues. The scenario we describe in this study is conceptually different in that the expression of Nr-CAM and Plexin-A1 by ligand-presenting midline cells triggers the switch in axonal response. Although we recognize the existence of other conceivable strategies for switching the response of RGCs to Sema6D (Figure S8), our data can best be explained by a model in which Nr-CAM Selleckchem BI6727 and Plexin-A1 on chiasm cells modulate the interaction of the Sema6D ligand with Nr-CAM and Plexin-A1 receptors many on RGCs. The fact that Plexin-A1 and Nr-CAM are expressed by the ligand-presenting cells (midline glia and chiasm neurons) as well as by RGCs poses the question
of how they change RGC axon interactions with Sema6D. One possibility is that Nr-CAM and Plexin-A1 on chiasm cells alter the conformation of the Nr-CAM and Plexin-A1 receptor system on RGCs. This altered receptor state would then transduce Sema6D signals in a manner different from that of Sema6D alone. An alternative idea is that the conformation of Sema6D on midline glia is changed by its interaction with Nr-CAM on midline glia and with Plexin-A1 on chiasm neurons, such that Sema6D association with the Nr-CAM/Plexin-A1 receptor system on RGC axons triggers growth rather than inhibition (Figure 8D). Evaluation in vivo of the phenotype of Sema6Dflox/flox, Plexin-A1flox/flox, and Nr-CAMflox/flox mice will require the construction of new and more selective means of gene inactivation in chiasm cells. Furthermore, although the detailed organization of the Sema6D/Nr-CAM/Plexin-A1 receptor complex is not yet known, the structural characterization of semaphorin ligands bound to plexin receptors ( Janssen et al., 2010 and Nogi et al., 2010) could provide insights into such potential molecular interactions at the chiasm.