Ectopic expression of the Sema6A ectodomain or full-length Sema6A in motoneurons competes with BCC-derived Sema6A binding to motoneurons. The AP-tagged ectodomain of Sema6A binds to axons expressing PlexinAs. (a) Both commissural axons (open arrowhead) and motor axons (arrowhead) express PlexinAs  and bind the Sema6A ectodomain. (b) No binding of the AP-tag alone was detectable. (c) Ectopic expression of both the ectodomain of Sema6A (not shown) and the full-length myc-tagged form resulted in motoneurons streaming out of the spinal cord along the ventral roots (arrows). (d) Staining of the myc tag demonstrates expression of Sema6A in motor axons (arrowhead), consistent with a competitive role of motor axon-derived Sema6A with BCC-derived Sema6A in the periphery. (e) As seen after downregulation of either Sema6A in BCCs (compare to Figure 7f) or PlexinA1 in motoneurons (compare to Figure 7g), ectopic expression of Sema6A resulted in the aberrant formation of BCC clusters. Bar: 100 μm.