Summary– Neurons of the statoacoustic ganglion (SAG) have a complex patter of development. This paper focuses on how the transcription factor Tfap2a coordinates multiple signaling pathways to promote neurogenesis of these SAG neurons in the Zebrafish inner ear. Previous research has shown that Fgf initiates formation of SAG progenitors, but high levels of Fgf can halt this process. Notch signaling is also observed to decreases SAG development. Overexpression of Tfap2a caused an increase in neuroblast formation and an increase in the rate of differentiation. A rise in mature neuron formation was also seen, however, neuron death resulted soon after. Knockout of Tfap2a saw a decrease neuroblast formation and a reduced rate of differentiation with fewer than normal mature neurons. Blocking and increasing expression of Fgf and Notch by other means showed similar results to the addition and knockout of Tfap2a. This shows that induction of Tfap2a inhibits Fgf and Notch signaling. It was found that this was due to activating expression of BMP7a, an antagonist to Fgf and Notch. This demonstrates that a balance of BMP7a, Fgf, and Notch signaling is required for sustained accumulation of mature SAG neurons.
Basic Methods– Zebrafish embryos were heat shocked to allow for mis-expression of different genes, mainly Tfap2a . For the loss of function experiments, tfap2a was knocked out through injecting embryos at the 1-cell stage. Mutants were identified by characteristic phenotypes showing expected Mendelian frequencies. Treating embryos with LY411575 blocked Notch signaling and BMP was blocked by treatment with Dorsomorphin. Both in situ hybridization and antibody staining were used for visualization and statistical analysis was based on studet’s t-tests and one-way ANOVA.
Significance– We are currently trying to push our hESCs into an otic placode prior to differentiation into neurons. We are doing this through modulating BMP. This research shows us how different factors such as BMP, FGF, and Notch affect neurogenesis after we induce the placode. It is something to keep in mind while trying to induce the placode and when taking the next step of differentiation. Based upon how the overexpression of neurogenin produces neurons, the effects of BMP, Fgf, and Notch will be important in forming neurons.