BILIRUBIN TOXICITY IN THE AUDITORY SYSTEM

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Human newborns are still at risk for brain damaged and hearing loss from bilirubin toxicity despite advances in the care and treatment of hyperbilirubinemia. The spectrum of bilirubin encephalopathy today ranges from classic kernicterus in premature, low-birth-weight infants, to more subtle conditions or the isolated sequelae of hearing loss and cognitive dysfunction. The incidence of impairment due to bilirubin toxicity, especially in the subtle or isolated conditions, is largely unknown because it is difficult to relate abnormalities that appear later in life to transient biochemical abnormalities that occur in the newborn period. Furthermore, the pathogenesis, localization of sites of auditory nervous system dysfunction, and the determinants of vulnerability and reversibility are still only partially understood despite decades of study. In a continuation of our successful use of brainstem auditory evoked potentials (BAEPs) in the Gunn rat model of bilirubin encephalopathy, we will combine noninvasive neurophysiological recordings with quantitative neuroanatomical studies, biochemical measurements, and immunohistochemistry to provide a cohesive synthesis of the localization, reversibility and pathogenesis of dysfunction due to bilirubin toxicity and its interaction with developmental processes. Electrophysiologic findings that occur soon after acute exposure to bilirubin toxicity will be compared to anatomic and biochemical measures. Interventions aimed at reversing acute bilirubin toxicity will be used to explore the time constraints of reversibility of the pathological process. Studies at different ages early in development will examine the vulnerability of different areas of the immature auditory and central nervous systems to bilirubin toxicity. We will continue our efforts to localize the specific site(s) of bilirubin-induced auditory nervous system dysfunction utilizing BAEPs, otoacoustic emissions, binaural interaction evoked potentials, and later-latency evoked potentials to assess damage to the cochlea and the central auditory nervous system, and verify our electrophysiologic results with anatomic and biochemical experiments. The resulting multidisciplinary approach is expected to provide new insights into the localization, pathogenesis, and reversibility of this disorder, and its effects on the auditory system. Understanding the complex relationships between electrophysiological, anatomical and biochemical processes in animal models of bilirubin encephalopathy should lead to improved noninvasive procedures for predicting, preventing, and treating the neurological and audiological sequelae of bilirubin toxicity in human newborns.GRANT=R03DC02094 The goal of the proposed research is to understand the molecular mechanisms controlling neurogenesis. Many pathologies involve the degeneration of the nervous system, including Alzheimer's, Huntington's and Parkinson's diseases. Similarly, spinal cord injuries can lead to paralysis due to lesions of the neuronal pathways. The majority of neurons in vertebrates are terminally differentiated and do not regenerate after damage. Understanding the processes involved in neuronal development and differentiation may ultimately yield therapies for these neuronal pathologies involving the repair or replacement of damaged neurons. The mammalian olfactory system is virtually unequaled for the study of neurogenesis in vertebrates. The olfactory sensory neurons are regenerated from stem cells throughout life. This process involves the extension of a dendrite to the mucosal surface and an axon to the olfactory bulb. The neurons from a single area of the olfactory epithelium express different populations of odorant receptors and synapse at different points on the central target. Thus, during the process of regeneration, neuronal connectivity must be tightly controlled. Because these neurons undergo a constant recapitulation of the neurogenesis observed during embryogenesis, they represent an excellent model system for the study of this process. Little is known about the molecular mechanisms controlling neurogenesis, and the experiments proposed focus on this process from a novel direction. Many neuronal growth factors are tyrosine kinases, suggesting the importance of tyrosine phosphorylation in neuronal growth and differentiation. Tyrosine phosphorylation has been proposed as a mechanism for controlling neuritogenesis. A newly identified class of factors, the protein tyrosine phosphatases (PTPs), present an unique avenue for probing neurogenesis. Since their identification in 1988, over forty different PTPs have been cloned; there are both receptor and intracellular types. Little is known about PTPs in neurons, and few substrates have been identified for any of the PTPs. With the clear importance of tyrosine phosphorylation in neuronal signal transduction pathways, it is likely that PTPs represent an important class of factors that regulate these pathways. The experiments proposed will identify and characterize PTPs expressed in the olfactory sensory neurons. Regeneration of the olfactory neurons will be induced to establish when during neurogenesis the PTPs are expressed. Enzymatic activities and subcellular localization will be determined to aid in the identification of possible substrates. Further, experiments are proposed to alter the activity of the PTPs to determine their function during neurogenesis. Using the olfactory neuroepithelium as a model system, these experiments will elucidate the role of PTPs in neurogenesis and provide information which should be applicable in other neuronal systems.