Overexpression of Sponge BHLH MRNA In Xenopus Laevis Disrupts Inner Ear Neurosensory Development

To uncover the steps necessary to restore hearing loss in a more superior way than is currently available, it is necessary to obtain a more complete understanding of the genetic and molecular mechanisms of vertebrate mechanosensory development. Attempts to fully restore hearing are currently focused on manipulating adult ear cells in model organisms using specific basic helix-loop-helix (bHLH) genes. The main purpose of our study was to better understand the development of inner ear neurosensory pathfinding ability and determine if neuronal projections are controlled by diffusible cues from the hindbrain, or by cues from the neurons themselves. To address this we overexpressed an ancestral proneuronal bHLH gene from the sponge Amphimedon queenslandica in the two-cell stage embryo of Xenopus laevis. Following this we performed ear transplants from the injected animals to control animals as well as from control animals to the injected. It was found that when transplanting from an injected animal to a control the pathfinding ability of the neurons in the inner ear was derailed in the same way that it is derailed in an injected animal prior to transplantation. When transplanting from the control animal to an injected the inner ear neurons were not derailed. These findings suggest that cues from the hindbrain are not responsible for the pathfinding ability of inner ear afferents, but rather the neurons themselves affect the pathfinding ability. From this we determined that the overexpression of AmqbHLH affects the neurons ability to pathfind, likely due to a malfunction of the wnt/PCP pathway component, Frizzled3.