Project 2: Mechanistic studies of age-related hearing loss using animal models and human tissue

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PROJECT SUMMARY/ABSTRACT – PROJECT 2 Age-related hearing loss (presbyacusis) is a common neurodegenerative disorder that can be associated with loss and/or dysfunction of several specialized cell types in the cochlear lateral wall (metabolic presbyacusis) and the auditory nerve (neural presbyacusis). Project 2 of the Clinical Research Center aims to identify critical cellular and molecular mechanisms underlying human metabolic and neural presbyacusis using mouse models and post-mortem human temporal bones from younger and older donors. Dysregulation of microglia/macrophages and the complement cascade, two fundamental elements of the innate immune system, have been shown to play vital roles in several age-related neurodegenerative disorders. Our preliminary studies have revealed that cochlear macrophages may undergo structural and molecular alterations with increasing age, indicative of functional changes, and that these alterations are associated with pathological changes in the cochlear lateral wall microvasculature of aged mice. Sphingosine-1-phosphate (S1P), a lipid signaling molecule, regulates macrophage activity. Preliminary studies of gene expression patterns and age-related macrophage dysfunction in mouse cochlear tissue suggest a link between a reduction in S1P bioavailability with increasing age and macrophage dysfunction in the cochlear lateral wall. In addition, differential gene expression analysis in the mouse auditory nerve has identified major changes with increasing age in the innate immune response and complement cascade pathways. Recent studies of other neurodegenerative disorders have shown that complement dysregulation can lead to demyelination and neural degeneration. Based on these observations, our overarching hypothesis is that age-dependent dysregulation of the cochlear innate immune system contributes to the degeneration of specialized cells in the aging lateral wall and auditory nerve, leading to declines in auditory function consistent with metabolic and neural presbyacusis. Project 2 will 1) determine the relationship between S1P-mediated macrophage dysfunction and strial microvasculature degeneration in the lateral wall of aged mice (Aim 2.1); and 2) elucidate the links between age-related dysregulation of the complement system with degeneration and functional declines in auditory nerve fibers, in particular fibers with low spontaneous rates (Aim 2.2). Animal models of lateral wall and auditory nerve degeneration characterized in Project 2 will also be used to validate genetic and pathophysiology results obtained from human subjects studied in Projects 1, 3, and 4. Comparative studies of the expression patterns of these key immune response regulatory molecules will also be examined in human temporal bones, which are accessed through the Human Subjects Core (Core B). The ability to compare results from animal models, human temporal bones, and human subjects provides an unparalleled opportunity to address questions regarding the specific role of the cochlear macrophage dysfunction and complement regulation in vascular and myelinating glial pathophysiology associated with human presbyacusis.