Notch1 is required to maintain supporting cell identity and vestibular function during maturation of the mammalian balance organs

Abstract

The inner ear houses two sensory modalities: the hearing organ, located in the cochlea, and the balance organs, located throughout the vestibular regions of the ear. Both hearing and vestibular sensory regions are composed of similar cell types, including hair cells and associated supporting cells. Recently, we showed that Notch1 is required for maintaining supporting cell survival postnatally during cochlear maturation. However, it is not known whether Notch1 plays a similar role in the balance organs of the inner ear. To characterize the role of Notch during vestibular maturation, we conditionally deleted Notch1 from Sox2-expressing cells of the vestibular organs in the mouse at P0/P1. Histological analyses showed a dramatic loss of supporting cells accompanied by an increase in type II hair cells without cell death, indicating the supporting cells are converting to hair cells in the maturing vestibular regions. Analysis of 6-week old animals indicate that the converted hair cells survive, despite the reduction of supporting cells. Interestingly, measurements of vestibular sensory evoked potentials (VsEPs), known to be generated in the striolar regions of the vestibular afferents in the maculae, failed to show a response, indicating that NOTCH1 expression is critical for striolar function postnatally. Consistent with this, we find that the specialized type I hair cells in the striola fail to develop the complex calyces typical of these cells. These defects are likely due to the reduction in supporting cells, which have previously been shown to express factors critical for the striolar region. Similar to other mutants that lack proper striolar development, Notch1 mutants do not exhibit typical vestibular behaviors such as circling and head shaking, but do show difficulties in some vestibular tests, including the balance beam and forced swim test. These results indicate that, unlike the hearing organ in which the supporting cells undergo cell death, supporting cells in the balance regions retain the ability to convert to hair cells during maturation, which survive into adulthood despite the reduction in supporting cells. Significance StatementNotch signaling regulates the cell fate choices between hair cells and supporting cells during inner ear development. However, little is known about how Notch functions in the mammalian vestibular sensory organs once cell fate has been determined. Here, we examine the role of Notch1 in the maturing balancing organs. We show that deletion of Notch1 results in vestibular physiological and behavioral dysfunction by 3 months of age. Histological analyses reveal supporting cells are converting to type II hair cells in the utricle, and despite a loss of supporting cells, the hair cells survive to adulthood. Additionally, the striolar type I hair cells important for generating a VsEP response are decreased in number and not innervated properly. These results show that Notch continues to function in maintaining supporting cell identity in the vestibular organs postnatally, which may be important in strategies for hair cell regeneration.