Zebrafish pou3f3b controls saccular/auditory development and marks non-neuronal cells that delaminate from the otic vesicle to promote neuroblast maturation.

The zebrafish otic vesicle initially develops with only two sensory maculae, each with distinct functions. The anterior utricular macula is indispensable for vestibular function, while the posterior saccular macula is the primary auditory endorgan in zebrafish. The unique identities of these maculae are specified in the early otic vesicle by differing levels of Fgf vs. Shh signaling, but few downstream effectors have been identified. pou3f3b is the only saccule-specific marker known, but its function has not been established. We generated a knockout allele of pou3f3b and found that it causes a persistent delay in accumulation of saccular hair cells due to a failure to activate saccular expression of fgf3. In addition, saccular hair cells exhibit reduced expression of Otoferlin caused by ectopic expression of neurog1. Defects in saccular hair cell development are fully rescued by misexpressing fgf3 or knocking down neurog1. Misexpression of pou3f3b causes loss of utricular pax5 expression and further truncates neurog1 in the posterior otic vesicle but does not otherwise alter macular development. In addition to regulating saccular development, pou3f3b is also expressed in a previously undescribed population of non-neuronal cells that delaminate from the otic vesicle and migrate together with developing neuroblasts to promote their maturation. Mutant neuroblasts show a marked delay in activation of expression of neurod1, causing a transient delay in accumulation of mature SAG neurons. Thus pou3f3b is required for timely development of SAG neurons and saccular/auditory hair cells.