Kevin K. Ohlemiller - US grants

Acquired hearing loss represents a complex interplay of genes and environment. Although there is much support for the existence of genes that influence the vulnerability of the cochlea to noise and ototoxins, few candidate genes or processes have been identified. One candidate process involves the generation and regulation of reactive oxygen species (ROS). Both chronic neurodegenerative disease and acute CNS injury involve elevated ROS, and deficiency of antioxidant enzymes promotes vulnerability to injury. We hypothesize that some genetic defects that predispose people to acquired hearing loss involve impairment of ROS regulatory mechanisms, rendering the cochlea more vulnerable to injury. We will apply hearing loss-prone and -resistant mouse models (C57BL/6, BALB/c, CBA/Ca), and 'knockout' mice deficient in antioxidant enzymes (superoxide dismutase and glutathione peroxidase), of carefully considered ages to the following Specific Aims: (1) Correlating the dynamics of cochlear ROS production following noise exposure with specific cochlear injury. We will establish the relation between the magnitude and time course of cochlear ROS production following acute noise exposure and cochlear injury, as measured by auditory brainstem responses, light and electron microscopy, and hair cell counts. (2) Identifying genetic influences on the relation between ROS production and noise-induced cochlear injury. We will determine the impact of genetic defects of hearing and ROS regulation on the relation between cochlear ROS production and noise-induced cochlear injury. (3) Uncovering the basis of genetic and age influences on the efficacy of antioxidants. We will determine the impact of age and genetic defects of hearing on the ability of exogenous antioxidants to attenuate both ROS production and noise-induced cochlear injury. Our experiments will establish how well the dynamics of ROS production predict cochlear injury, and whether progressive deafness genes may impair cochlear ROS regulation.

DESCRIPTION (provided by applicant): We and others have identified several genes that promote cochlear noise injury in mice, and whose homologues may promote similar injury in humans. We now have evidence for a major effect quantitative trait locus (QTL) in mice that influences not the extent of noise injury, but rather the cellular distribution of noise injury. Hours after a moderate noise exposure (4-45 kHz, 110 dB SPL, 2 hrs), CBA/J mice show a reduction in the endocochlear potential (EP), as well as characteristic pathology within stria vascularis, spiral ligament, and spiral limbus. Although the EP recovers over time, the injury to stria and limbus is permanent. C57BL/6J (B6) mice, by contrast, show no significant acute EP reduction, and minimal acute or permanent cellular pathology. B6/CBA F1 hybrid mice respond to noise in a manner similar to the CBA parent strain, suggesting that one or a few dominant loci govern all facets of the injury phenotype. N2 backcross mice show the same constellation of noise pathology, and suggest linkage of the noise phenotype to the region containing the agouti locus on mouse chromosome 2. We HYPOTHESIZE that the linkage interval includes a gene involved in ion transport through the lateral stria (basal and intermediate cells) and limbus. The gene may code for an ion channel whose conductance is down-regulated by hypoxia or oxidative stress. Our findings point to novel genetic modulation of cochlear ion homeostasis during noise stress. The gene(s) and processes involved may impact the long term stability of cochlear noise injury and the accumulation of injury that presents as presbycusis. Our SPECIFIC AIMS are 1) To determine the cellular basis of EP reduction after noise exposure in CBA mice, 2) To examine the correlation of noise-related cellular pathologies of stria, spiral ligament, and limbus that comprise the CBA phenotype, and 3) To identify candidate gene(s) underlying CBA versus B6 strain differences in the effects of noise. RELEVANCE TO PUBLIC HEALTH: The cochlea contains many cell types whose functions are not known, but probably help maintain an appropriate ionic environment so that sensory cells can survive and respond sensitively to sound. Our work points to one or more genes that profoundy impact the distribution of noise injury in non-sensory cells. The gene, and the processes in which it is involved, may affect the long-term stability of cochlear injury, and the accumulation of injury that may be diagnosed as presbycusis.

The advent of transgenic technology in rodents has yielded a goldmine of models offering insights into molecular and cellular aspects of sensory function and pathology. The vibrant research community at WUSM includes many investigators with a primary interest in sensory function and dysfunction. Moreover, because insights sometimes come from models intended for other purposes, all investigators must be encouraged to approach their models from a broad perspective, and should face as few roadblocks as possible in asking larger questions. The goal of the RCAVS Functional Testing Core (FTC) is to impart to investigators the ability to assess the auditory, vestibular, and visual systems of small animals in a uniform way, applying state-of-the-art methods and equipment. Accordingly, the FTC maintains and operates equipment serving the separate requirements of vestibular, auditory, and visual testing to make well conceived, comprehensive functional testing available to our colleagues within the Research Core Center and the larger research community. The Specific Aims of the FTC are: 1) To facilitate comprehensive sensory (inner ear and visual) testing in mouse and other small animal models. Auditory tests offered include auditory brainstem response (ABR) thresholds, ABR input/output analyses, ABR waveform and latency analysis, and ABR temporal and spectral masking profiles. In addition, distortion product otoacoustic emissions'(DPOAEs) provide information about outer hair cell motor function. Vestibular tests principally encompass tracking of eye movements through analysis of reflected light as animals are subjected to controlled rotations. Finally, our experience has shown that inner ear and visual pathology often coincide. We have published expertise in recording of flash electroretinograms (ERGs) and have incorporated this capability into our equipment and testing regime. All tests can either be done by Core staff, or we can train an investigator's own staff for perform tests. 2) To provide consultative services that support proper application of sensory tests and reporting of results. Upon request. Core staff assist investigators in interpretation of findings, statistical analyses, and presentation conventions. 3) To promote interactions and stimulate new research endeavors by RCAVS investigators and throughout WUSM. Research Core services are advertised on the WUSM intranet, and promoted in occasional forums. The RCAVS has sparked collaborations between members of several departments and continues to seed interactions between scientists in diverse disciplines.