| Spiral ganglion neuron (SGN) loss, either dependent or independent of sensory hair cell (HC) loss, is a major cause of deafness, particularly in the ageing population. Cochlear implants (CI) are presently the only known "cure" for many forms of deafness. Nevertheless, successful function of a CI depends on the preservation of SGNs. In the early developing inner ear, the otocyst secretes a factor called Otocyst Derived Factor (ODF). ODF promotes directional neurite outgrowth and neuronal survival of the statoacoustic ganglion (SAG), the precursor of the auditory portion of the SAG that eventually forms the SG. Cytokine arrays and proteomic studies demonstrated that the bioactive components of ODF include Macrophage Migration Inhibitory Factor (MIF), which has been described as "pleiotropic" cytokine because of its multiple roles, including roles in the immune system and in neuronal development and regeneration. Based on its known roles and our preliminary data, we hypothesized that MIF plays a key instructional role, acting as a neurotrophin, in inner ear development. The goal of this dissertation project is to elucidate the role of MIF in inner ear neuronal development as well as the possibility of using this developmental information to study and to enhance inner ear neuronal regeneration. We found that, at low concentrations (5 pg/ml and 5 ng/ml), recombinant MIF alone supports both mouse and chick SAG directional neurite outgrowth and neuronal survival, and evokes a neuronal phenotype from mouse embryonic stem cells (ESC); at higher concentrations (500 ng/ml), MIF inhibits these functions. We also found that MIF is expressed in supporting cells (SC) of the inner ear and its receptor, CD74 is expressed on both SAG and SGN. In the MIF knock-out (KO) mice, abnormal development of both SC and hair cells (HC) as well as a significant hearing impairment in the high frequency region of the cochlea are seen with concomitant loss of SGN in this region of the cochlea. In addition, we observed that the neurites from SG explants extend directionally in culture toward the wild-type (WT) Organ of Corti (OC), but not in the isolated MIF KO OC. Finally, we found that blockade of either MIF or its receptor with antibody or RNAi respectively suppresses SAG neurite outgrowth and survival. Our study indicates that MIF functions as an essential component of normal inner ear neuronal development and innervation and could potentially be used for SGN retention or re-growth as well as to potentiate the function of a cochlear implant in the injured or diseased mammalian inner ear. |
