Sponge bHLH Gene Expression in Xenopus laevis Disrupts Inner Ear and Lateral Line Neurosensory Development and Otic Afferent Pathfinding

Basic helix–loop–helix (bHLH) transcription factors, such as those in the atonal family, are important in cellular fate determination. The expression of the sponge ortholog of the atonal bHLH gene family, AmqbHLH1, in Xenopus laevis previously resulted in the formation of ectodermal ectopic neurons. However, the extent to which these neurons persist through development and the effects on the inner ear and lateral line, which require a critical level and timing of bHLH genes, remains unexplored. To test these long-term effects, we injected various concentrations of AmqbHLH1 mRNA into X. laevis embryos and assessed neurosensory development at developmental stages coinciding with fully developed neurosensory structures. The expression of AmqbHLH1 mRNA in X. laevis resulted in a dose-dependent reduction in or loss of ears and the lateral line system without eliminating ectopic neurons. At the lowest concentrations examined, we found that inner ear neurosensory development consisted sometimes of only a few scattered hair cells in a single-layer epithelium. Furthermore, low concentrations of AmqbHLH1 mRNA affected inner ear afferent guidance. Our data suggest that the AmqbHLH1 gene has some anti-neurosensory abilities in frogs and that the overexpression of a single gene may not be sufficient for stable long-term transdifferentiation in cells.