Induction of neurogenin-1 expression by sonic hedgehog: Its role in development of trigeminal sensory neurons.

We have examined the roles of signaling molecules in the mechanisms underlying the induction of neurogenin (ngn)-1 expression. ngn-1 is a basic helix-loop-helix (bHLH) transcription factor, which is essential for the specification of trigeminal sensory neurons. Semiquantitative reverse transcriptase-polymerase chain reaction using cranial explants in organ cultures showed that sonic hedgehog (Shh) promotes ngn-1 expression. This promoting activity was not observed in other signaling molecules examined. The promotion of ngn-1 expression by Shh, furthermore, was inhibited by cyclopamine, a specific inhibitor of Shh signaling. Shh did not affect the expression of ngn-2, a bHLH transcription factor that plays an important role in the specification of epibranchial placode-derived sensory neurons. The expression levels of ngn-1 and ngn-2 decreased after fibroblast growth factor-2 treatment. These results suggest that Shh induces ngn-1 expression specifically and that expression of ngn-1 and ngn-2 is regulated by different mechanisms. The induction of ngn-1 expression by Shh suggests that this signaling molecule participates in the specification of trigeminal sensory neurons. We therefore examined the effect of Shh on the development of these neurons. Immunostaining using anti-ngn-1 demonstrated that Shh promotes ngn-1 expression in trigeminal neural crest cells. Trigeminal neural crest cells are derived from the posterior mesencephalon and the most-anterior rhombencephalon, and they contain a subset of precursors of trigeminal sensory neurons. Moreover, a subpopulation of trigeminal neural crest cells expressed the Shh receptor Patched. The number of cells that express Brn3a, a POU-domain transcription factor that plays an important role in differentiation of sensory neurons, also increased with Shh treatment. Our data suggest that Shh signaling is involved in the specification of trigeminal sensory neurons through the induction of ngn-1 expression. Furthermore, Shh promotes the differentiation of neural crest cells into trigeminal sensory neurons.