Canal cristae growth and fiber extension to the outer hair cells of the mouse ear require Prox1 activity

The homeobox gene Prox1 is required for lens, retina, pancreas, liver, and lymphatic vasculature development and is expressed in inner ear supporting cells and neurons. We have investigated the role of Prox1 in the developing mouse ear taking advantage of available standard and conditional Prox1 mutant mouse strains using Tg(Pax2-Cre) and Tg(Nes-Cre). A severe reduction in the size of the canal cristae but not of other vestibular organs or the cochlea was identified in the E18.5 Prox1(Flox/Flox); Tg(Pax2-Cre) mutant ear. In these mutant embryos, hair cell differentiated; however, their distribution pattern was slightly disorganized in the cochlea where the growth of type II nerve fibers to outer hair cells along Prox1 expressing supporting cells was severely disrupted. In the case of Nestin-Cre, we found that newborn Prox1(Flox/Flox); Tg(Nestin-Cre) exhibit only a disorganized innervation of outer hair cells despite apparently normal cellular differentiation of the organ of Corti, suggesting a cell-autonomous function of Prox1 in neurons. These results identify a dual role of Prox1 during inner ear development; growth of the canal cristae and fiber guidance of Type II fibers along supporting cells in the cochlea.