Sp8 regulates inner ear development

Significance Deficits in hearing or balance are common and result from both developmental and environmental causes. Model organisms have contributed many fundamental insights into embryonic development and we have added Xenopus tropicalis as a new genetically tractable organism in the field of inner ear development. As a result of a forward genetic screen in X. tropicalis, we have identified specificity protein 8, a new initiator of ear development, and analyzed mutant phenotypes and molecular interactions with genes that are involved in inner ear development. Given morphological and genetic similarities between inner ears of frog and mammals, the establishment of a new in vivo model system amenable to genetic manipulation will provide an important new tool to study vertebrate ear development. A forward genetic screen of N-ethyl-N-nitrosourea mutagenized Xenopus tropicalis has identified an inner ear mutant named eclipse (ecl). Mutants developed enlarged otic vesicles and various defects of otoconia development; they also showed abnormal circular and inverted swimming patterns. Positional cloning identified specificity protein 8 (sp8), which was previously found to regulate limb and brain development. Two different loss-of-function approaches using transcription activator-like effector nucleases and morpholino oligonucleotides confirmed that the ecl mutant phenotype is caused by down-regulation of sp8. Depletion of sp8 resulted in otic dysmorphogenesis, such as uncompartmentalized and enlarged otic vesicles, epithelial dilation with abnormal sensory end organs. When overexpressed, sp8 was sufficient to induce ectopic otic vesicles possessing sensory hair cells, neurofilament innervation in a thickened sensory epithelium, and otoconia, all of which are found in the endogenous otic vesicle. We propose that sp8 is an important factor for initiation and elaboration of inner ear development.

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