Hair Cell Development in the Mammalian Cochlea

National Institute On Deafness And Other Communication Disorders

Investigators

Linked publications, trials & patents

Abstract

Auditory and vestibular function are dependent of the formation of a functional inner ear. While there are multiple components for both of these systems, this laboratory focuses on the development of the sensory epithelia, which contain mechanosensory hair cells and associated cells called supporting cells and on the innervation of those hair cells by neurons from the VIIIth (acousticovestibular) cranial nerve. All three of these cell types are derived from the otocyst, a placodal structure that forms adjacent to the hindbrain early in development. Identifying the factors that specify each of these cell types and then direct their assembly into functional units is a key goal of the Section on Developmental Neuroscience. During the previous year, different members of the laboratory have examined several different aspects of these developmental processes. A key step in understanding how different cell types within the inner ear are specified is to determine the developmental lineage relationships of different cell types. Developmental lineage plays a key role in dictating the extent and ability of cells to switch between phenotypes, a potential important step in potential therapies that require cellular regeneration. The small size of the inner ear and its relatively limited accessibility within the temporal bone of the skull had prevented comprehensive lineage studies for the inner ear. During the last year, in collaboration with the laboratory of Dr. Emma Andersson at the Karolinska Institute, we combined ultrasound guided surgery with lentiviral vectors to introduce lineage tracing constructs into the developing inner ear. Results revealed previously unknown lineage relationships between different cell types within different regions of the cochlea. These data will be useful in guiding the development of regenerative strategies. A unique functional aspect of the mammalian cochlea is the presence of a fluid filled tunnel, the tunnel of Corti, that extends along the length of the cochlear spiral. The tunnel of Corti provides stability and vibrational isolation that are crucial for normal auditory function. The tunnel of Corti is formed by two rows of unique cell types, called inner and outer pillar cells. To determine the factors that regulate pillar cell development, we collected single cells from the developing cochlea and then profiled gene expression using RNA sequencing. Subsequent bioinformatic analysis identified a group of transcription factors, Etv4, Etv5 and Etv1, as likely regulators of pillar cell formation. To test this hypothesis, we generated animals in which all three Etv genes had been deleted. Results indicated a specific loss of pillar cells in these mice. These results provide valuable insights regarding signaling molecules that might be used to induce regeneration of pillar cells in damaged human ears. In a separate study, we identified a transcription factor, Casz1 as expressed in developing hair cells. In collaboration with the laboratory of Botond Bonfi at the University of Iowa, we demonstrated the mice with a targeted deletion of Casz1 are deaf as a result of the progressive loss of hair cells as the animals mature. Using gene profiling approaches, we were able to demonstrate the Casz1 plays a role in the development of stereocilia, a unique structure found only in mechanosensory hair cells. In a separate project, we compared changes in gene expression in the presence and absence of the deafness gene, Atoh1, in mice. Results identified approximately 200 hair cell specific genes that are down-regulated in the absence of Atoh1. Among these is the transcription factor, Myt1. We generated targeted deletions of Myt1 which demonstrated an over-production of hair cells, suggested that Myt1 acts as a hair cell suppressor. The transcription factor Prox1 is expressed only in the region of the cochlea that will give rise to outer hair cells. To determine the role of Prox1 we generated a conditional deletion in this gene within the cochlea. Assessment of auditory function indicates significant hearing loss in these mice while phenotypic analysis indicates a loss of hair cells in adult mice. Using gene profiling techniques, we have determined that Prox1 plays a role in specifying unique cell types within the inner ear and that in the absence of Prox1 those cells undergo a phenotypic conversion. This conversion likely underlies the hearing loss in these mice. During development of the spiral ganglion neurons, all developing neurons express a transcription factor, Pou4f1. To determine the role of Pou4f1 in spiral ganglion development, we generated mice with a specific deletion in Pou4f1 and then used gene profiling approaches to determine how spiral ganglion development changes without Pou4f1. Results indicate significant changes in expression of genes related to the sonic hedgehog signaling pathway. The absence of sonic hedgehog has significant effects on overall formation of the inner ear, demonstrating a key role for spiral ganglion neurons and Pou4f1 in inner ear formation.