Roles of microRNAs in zebrafish inner ear development

microRNAs (miRNAs) are a class of short, non-coding RNAs, which can bind to 3’ untranslated region of target messenger RNAs, and suppress their translation. Many miRNAs are expressed in hair cells of the inner ear and/or neuromasts on the lateral line of zebrafish, including the miR-183 family (miR-183, miR-96 and miR-182), and miR-200a. Recently, dominant progressive hearing loss in humans and mice is linked to mutations in the seed region of miR-96. However, functions of these miRNAs in the development of mechanosensory hair cells and neurons of the inner ear are unknown. I investigated the expression and functions of these miRNAs through in situ hybridization, and manipulations of the levels of them in zebrafish embryos, using synthesized miRNAs and antisense morpholino oligonucleotides. The miR-183 family is expressed abundantly in specific sensory cell types in the eye, peripheral olfactory organ and inner ear. In the inner ear, expression is robust in the mechanosensory hair cells and weak in the associated statoacoustic ganglion (SAG) neurons; both cell types can share a common lineage during development. miR-200a is expressed in olfactory bulb, hair cells in the inner ear, and the neuromasts. Knockdown of miR-183, miR-96, miR-182, and miR-200a causes reduced numbers of hair cells in the inner ear, smaller SAGs, defects in semicircular canals, and abnormal neuromasts on the posterior lateral line. However, the prosensory region of the posterior macula (PM) in the miR-183 family morphants, where the number of hair cells is reduced by ~50%, is not significantly impaired; while the prosensory region of the PM in the miR-200a morphants is decreased. In contrast, overexpression of miR-96 or miR-182 induces duplicated otocysts, ectopic or expanded sensory patches and extra hair cells; while morphogenesis of the SAG is adversely affected to different degrees. Furthermore, I utilized bioinformatics methods and in vitro assays to search and validate candidate target genes of the miRNAs. These findings suggest both overlapping and distinct roles for these miRNAs in cell fate determination in the inner ear, and these principles might apply to development of other sensory organs