The Role of atoh1a and sox2/sox3 During Zebrafish Posterior Lateral Line Development and Regeneration

Both the inner ear of mammalian and avian vertebrates and the posterior lateral line sense organ of zebrafish begin as ectodermal thickenings known as placodes and subsequently differentiate to give rise to all the complex sensory and neuronal architecture of each respective structure. Similar genetic and molecular signaling cascades govern the development of both of these structures. Mechanosensory hair cells of the avian inner ear and zebrafish lateral line, however, can regenerate from underlying support cells after loss whereas loss of mammalian mechanosensory hair cells is irreversible, resulting in deafness. Thus, differences in the regulation of sensory cell differentiation genes between mammal and avian/fish models may elucidate key evolutionary genetic/molecular discrepancies that enable fish/avian hair cell regeneration and/or preclude mammalian hair cell regeneration. Here, I will review previous studies comparing and contrasting the development of the inner ear and lateral line from placode to hair cell. I will also review and contrast avian and fish hair cell regeneration with the status of quiescent support cells in deafened mammals. Ultimately, my results highlighted a key difference in regulation between fish and mammals amongst the bHLH mechanosensory hair cell differentiation gene Atoh1 and the SRY-Box stem cell pluripotency gene Sox2. Whereas in the mammalian system Sox2 acts upstream of Atoh1 and is not known to be down-regulated in deafened mice, I demonstrated that in zebrafish, sox2 expression in the nascent neuromast required atoh1a gene function. In addition I demonstrated that a wave of sox gene downregulation followed hair cell ablation by acute neomycin exposure. Thus, I propose that downregulation of sox2 may be a necessary event for hair cell regeneration. As Sox2 maintains strong expression in deaf mice, its differential regulation might underlie differences in regenerative ability. I also report the discovery of a novel mutation in zebrafish atoh1a, fh282, which was predicted to structurally disrupt one of the DNA-binding helix domains. Whereas removal of atoh1a gene function by morpholino oligonucleotide injection ablated sox2 expression, sox2 expression and other sensory cell gene expression was normal in these mutant homozygotes despite the fact the fish never developed neuromast hair cells.