Navigation of trochlear motor axons along the midbrain-hindbrain boundary by neuropilin 2

Abstract

Trochlear motor axons project dorsally along the midbrain-hindbrain boundary (MHB) to decussate at the dorsal midline. We report on the roles of neuropilin 2 and its ligands in the molecular mechanisms controlling this trajectory. In chick embryos, neuropilin 2 was expressed in the neuroepithelium of the dorsal isthmus in addition to the trochlear neurons,and Sema3F transcripts were localized along the caudal margin of the midbrain. Misexpression of Sema3F demonstrated that Sema3F displays repulsive activity in vivo that guides the trochlear motor axons along the MHB. An unexpected result was that misexpression of neuropilin 2 canceled the midbrain-evoked repulsion, allowing trochlear motor axons to cross the MHB and invade the tectum. A binding assay with neuropilin 2 ectodomain revealed the existence of neuropilin 2 ligands in the midbrain, which were masked by ectopic neuropilin 2. We therefore propose that neuropilin 2 neutralizes the repulsive activity in order to steer trochlear motor axons towards the dorsal decussation point. Taken together, our results suggest that the interaction of neuropilin 2 with its ligands has crucial roles for establishing trochlear trajectory along the MHB.

Neuropilin 2/semaphorin 3F signaling is essential for cranial neural crest migration and trigeminal ganglion condensation

“…2(D)]. This reciprocal pattern is also observed in chick (Watanabe et al, 2003;Osborne et al, 2005) and zebrafish (Yu and Moens, 2005).…”

“…1 npn2 expression is observed in the dorsal neural tube and neural folds before migration as well as on migrating cranial neural crest cells. In addition to staining in the neural crest, a prominent stripe of npn2 expression at the midbrain-hindbrain boundary (Watanabe et al, 2003) is also apparent, made …”

“…The prominent stripe of npn2 expression at the midbrain-hindbrain boundary [ Fig. 2(A)] (Watanabe et al, 2003) alternates with sema3F expression in stripes at the level of the caudal midbrain (Watanabe et al, 2003), r3, and r5 [ Fig. 2(D)].…”

Sema3D and Sema7A have distinct expression patterns in chick embryonic development

“…We sequenced approximately 60 clones and analyzed the sequence by nucleotide and translated blast analyses using the programs provided by National Center for Biotechnology Information (NCBI). Several clones were confirmed as identical to the previously reported chick Sema3D gene (Luo et al, 1995;Chilton and Guthrie, 2003;Watanabe et al, 2004;Jin et al, 2006). Another clone was found to be identical to the newly annotated Sema7A gene from the chick genomic sequence (accession number: XM_413687), except a stretch of 44 nucleotides (Fig.…”

Motor Axon Guidance of the Mammalian Trochlear and Phrenic Nerves: Dependence on the Netrin ReceptorUnc5cand Modifier Loci

“…Therefore, we anticipate that the modifier locus we have identified will be a component of a partially redundant or parallel signaling pathway, which differs in function or expression between B6 and SJL. Although netrin3, or any other modifier gene, could be influencing the signaling of other netrin receptors, it is also possible that components of another signaling pathway, such as the neuropilin-semaphorin III/D pathway, which has been shown to act as a chemorepulsive signal for this nerve (Chen et al, 2000;Giger et al, 2000;Sahay et al, 2003;Watanabe et al, 2004), are disrupted in B6 mice.…”