The Role of ADF and Cofilin in Auditory Sensory Cell Development

Jamis McGrath, Purdue University

Abstract

Over half the population above 70 years of age experiences hearing loss. Around one-third of the population with hearing loss can be attributed to noise-induced damage to the auditory end organ. Hearing, balance, and head movement are all sensed by sensory cells called hair cells found in the inner ear. Auditory hair cells transform sound vibrations into biological signals that can be processed by the nervous system and, importantly, are not replaced after cell death in mammals. Instead, these cells must be maintained throughout one’s life while functioning almost continuously. Protrusions called stereocilia extend from the apical surface to form a highly patterned and interlinked bundle. The shape of the bundle endows directional sensitivity necessary to reliably convert the peaks and troughs of wave stimuli. The stereocilia themselves are believed to regrow to some extent but if they degenerate completely, they are irreplaceable, removing a portion of the ion current generated during stimulation and, eventually, causing hearing loss. Thus, understanding how the pattern of the bundle is obtained and how stereocilia are maintained is important for preserving auditory function. Underlying the stereocilia bundle is a plate like structure which forms a structural base for the bundle and anchors the stereocilia to the cell. This mysterious structure must also be meticulously maintained as its physical properties are believed to facilitate bundle movements and transduction. The junctions of hair cells are specialized and limit the flux of ions across the sensory epithelium, providing an electrochemical gradient necessary for hair cell function. Structural protein networks formed by polymerized actin proteins from the backbones of the stereocilia, cuticular plate, and junctions. To better understand how these actin networks are regulated, actin depolymerizing factor (Adf, or Dstn) and paralog cofilin1 (Cfl1) genes were knocked out. The protein products of these genes bind actin filaments and disassemble them through severing and depolymerization. ADF/CFL1 proteins enrich at stereocilia tips and between the cuticular plate and perijunctional actin networks. Knockout of either gene alone causes progressive hearing loss but mostly compensated for each other until late into adulthood. Compound mutants were generated which revealed a high degree of compensation between ADF and CFL1. Stereocilia bundles were grossly mispatterned in many hair cells and this seemed to be guided by rearrangements in the cuticular plate caused by overgrowth of the actin network. Kinocilia placement was slightly but significantly altered as well, though it remains unclear if this is due to defective polarity signaling or was also affected by cuticular plate growth. ADF/CFL1 continued to regulate the size of the cuticular plate into adulthood. Loss of these proteins caused the medial edge of the cuticular plate to swell outward. Perijunctional actin accumulated in the lower portion of the apical junction complex. In the bundle, stereocilia were often wider in mutants and row 1 failed to elongate postnatally. Mechanotransducing stereocilia enriched ADF/CFL1 proteins at their tips where they increased the proportion of bound and unbound filament ends. ADF/CFL1 were found to be essential for auditory hair cell development and maintenance by limiting the growth of their actin networks.

Degree

Ph.D.

Advisors

Perrin, Purdue University.

Subject Area

Morphology|Acoustics|Audiology|Neurosciences

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