Molecular mechanisms of the cochlear response to noise and deafness.

Abstract

Studies described in this dissertation were designed to explain the molecular mechanisms of cochlear damage occurring in noise overstimulation and to regenerate the peripheral processes of the auditory nerve in deafness. To study the molecular mechanisms of the cochlear damage, the gene array technique was used. Gene array is a high throughput technique that allows us to examine very many genes in parallel. The first study was performed to validate the sensitivity of the gene array. Normal gene expression profiles were generated for the whole cochlea, cochlear subfractions and two central auditory brainstem regions (cochlear nucleus and inferior colliculus) of adult rats. We identified genes that were highly expressed in each region and were able to detect several genes that are known to be expressed in a specific cell type of the cochlea. For example, p27Kip1 was detected in the cochlear regions but not in the central nervous system (CNS) regions and known to be expressed only in the supporting cells of the mature cochlea. A greater number of differentially expressed genes were found when comparing cochlear and CNS regions than comparing the auditory regions and the hippocampus. Several families of insulin-like growth factor binding proteins, matrix metalloproteinases (MMPs) and tissue inhibitors of MMPs were among the genes highly expressed in the cochlea compared to the central nervous system regions. These results indicate that gene arrays can detect tissue-specific genes as well as genes differentially expressed among regions. The next set of studies determined differential gene expression in the rat cochlea following two levels of noise overstimulation. The higher intensity of noise induced major changes in expression of immediate early genes (IEGs), including genes for three transcription factors, <italic>c-fos, Egr1</italic>, and <italic>Nur77</italic>, and three cytokines, <italic>BTG2, IP10</italic> and <italic>LIF</italic>. These transcription factors are known to be upregulated by a variety of physiological stimuli such as growth factors, as well as in neuronal injury following ischemia-reperfusion. The cytokines are known to be induced in cell damage. They might be therefore induced as a result of the cell injury that occurs only at the higher level of noise overstimulation, and might be involved either in pathways leading to cell death or in pathways for repair and recovery. These results suggest that early responses of damage to the cochlea are similar to those of other tissues that respond to a variety of stimuli. Lastly, I examined the influences of different neurotrophic factors on cochlear neuron survival and process regeneration in deafness. Neurotrophic factors are survival and differentiation factors for neuronal cells. Treatment with BDNF alone triggered cell survival of the spiral ganglion cell as well as regeneration of the auditory nerve fibers, whereas FGF-1 alone affected neither cell survival nor peripheral process regeneration. So far, the most robust reappearance of peripheral processes was observed in a combined treatment of FGF-1 and BDNF compared to FGF-1 or BDNF alone. Further studies will be necessary to determine the most effective concentration and combination of these two factors as well as other neurotrophic factors.