PROJECT SUMMARY
The development and function of hair cell stereocilia in the cochlea are essential for our sense of hearing, as
they play a pivotal role in detecting sound through their mechanosensory capabilities. Stereocilia, organized into
rows of graded height on the hair cell surface, are susceptible to damage caused by loud sounds and the aging
process, leading to irreversible hearing loss. The architecture of stereocilia is critical for detecting sound and is
regulated by a structural protein called actin that forms a rigid scaffold of filaments within these structures. My
overarching goal is to understand the regulatory mechanisms that govern actin filament growth and ensure the
correct development of stereocilia. In this proposal, I explore the function of the molecular motor protein myosin
15 (MYO15A) that controls actin filament assembly and stereocilia size. Mutations in MYO15A cause human
hereditary hearing loss, DFNB3, underlying the essential activity of this protein in the cochlea. The central focus
of my proposal is to discover how MYO15A and its associated proteins, known as the 'elongation complex' (EC),
can control actin polymerization and stimulate stereocilia growth. My preliminary data along with the published
work of others, have revealed a unique ability of MYO15A and EC proteins to form biomolecular condensate that
are hypothesized to form the stereocilia tip density that controls actin filament elongation. The properties of these
MYO15A-EC condensates are poorly understood, but highly relevant to stereocilia biology. In this proposal, I will
conduct a comprehensive characterization of the biophysical properties of MYO15A-EC tip density condensates
and explore their potential as reaction compartments optimized for actin filament growth. In Aim 1, I will measure
the material properties and microrheology of purified MYO15A-EC tip density condensates, revealing their
structural makeup. In Aim 2, I will test the hypothesis that MYO15A-EC tip-density condensates can potentiate
actin filament growth. This work will utilize cutting-edge experimental approaches including optical trapping force
spectroscopy and single-molecule microscopy techniques. The results from these experiments will reveal the
fundamental properties of MYO15A-EC tip density condensates and advance our understanding of how
stereocilia are built and maintained. Completion of this project will advance my long-term goal of manipulating
stereocilia biology therapeutically to treat hearing loss in patients.