Date of Award

2020

Document Type

Dissertation - MUSC Only

Degree Name

Doctor of Philosophy (PhD)

Department

Pathology and Laboratory Medicine

College

College of Graduate Studies

First Advisor

Hainan Lang

Second Advisor

Bradley A. Schulte

Third Advisor

Jeremy L Barth

Fourth Advisor

Amanda C. LaRue

Fifth Advisor

Amy Bradshaw

Sixth Advisor

Judy R. Dubno

Abstract

Age-related hearing loss (ARHL), or presbyacusis, is a prominent chronic degenerative disorder affecting older individuals. Characterization of ARHL pathology reveals degeneration of both sensory and non-sensory cells, as well as alteration of the cochlear microenvironment. It is unclear when and to what extent the tissue homeostasis of the cochlea is perturbed during aging. The progression of age-related neurodegenerative diseases is associated with an altered microenvironment reflective of chronic inflammatory signaling. Under these conditions, resident and recruited immune cells, such as microglia and macrophages, demonstrate aberrant cellular activity that may contribute to neurodegeneration. Two key elements of the innate immune response are effector immune cells, which include macrophages, and pathways of regulation, such as the complement system. Complement molecules are key mediators of the innate immune system, and dysregulation of their expression contributes to the progression of several neurodegenerative diseases. In the inner ear, macrophages have been identified to populate in various compartments of the cochlea, with a demonstrated role in auditory nerve (AN) refinement during development. Furthermore, previous studies have documented the cochlear macrophage response to noise-induced injury and sensory cell degeneration in animal models. However, our understanding of how an altered cochlear microenvironment may impact or result from altered resident immune cell activity is limited. This dissertation investigates if the aging inner ear demonstrates enhanced inflammatory signaling and aberrant macrophage cellular activity, thereby contributing to ARHL pathology. Examination of age-related alterations in the cochlear lateral wall (CLW) and AN was performed in both human and mouse specimens. The finding of morphological distinction of macrophages in the CLW and AN may be reflective of region-specific cellular functions. Additionally, macrophage number and morphological alterations consistent with activation were shown to increase with age in the AN. Evidence of enhanced inflammatory signaling in the aged cochlea was identified via the transcriptional profiles of the AN and CLW. Significant enrichment of the complement signaling pathway was identified in the aging AN and CLW. To further investigate the origin of complement signaling molecules in the cochlea, we performed transcriptomic analysis on the samples enriched for cochlear macrophages compared to the samples obtained from non-macrophage cochlear cells. Using this novel approach, we identified expression of central complement molecule (C3) in both macrophages and non-macrophage cochlear cells. Further, cell surface receptors that are documented to mediate macrophage/microglia recruitment and activation, such as C3ar1, C5ar1, Itgam, and Itgb2, were found to be expressed by cochlear macrophages. We next tested the hypothesis that reduction of complement activation has a protective effect on age-related hearing loss. Our data suggests that deficiency of C3 reduced the incidence of cochlear macrophage activation and recruitment in the aged AN and CLW. Although we did not observe improved hearing thresholds, quantitative analysis indicates protection for spiral ganglion sub- types when comparing C3-deficient mice relative to controls. Together, our investigations indicate over-expression of central complement molecule C3 in the aging cochlea contributes to changes in macrophage activity that are associated with pathophysiological alterations of the aged cochlea.

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