The mechano-electrical transduction (MET) process allows the transduction of mechanical information
from sound into electrical signals, and it is a fundamental step in cochlear system function. Failures in this
process lead to hearing loss and deafness. Understanding the basic properties of MET will lead to a better
understanding of deafness, leading to targeted treatments and therapies. MET takes place at the level of the
hair bundle and is mediated by tip links, extracellular proteins connecting shorter stereocilia to adjacent taller
stereocilia. Deflections of the hair bundle towards the tallest stereocilia row increase tip-link tension and open
MET channels that reside at the top of the shorter stereocilia. Although there is a large body of work regarding
lipid membrane modulation of mechanosensitive ion channels, there is a limited but growing body of data on
lipid modulation of cochlear hair cell MET.
The lipid environment can affect channels indirectly through changes in membrane mechanics, or directly
through individual lipid/protein interactions. PIP2, an endogenous phospholipid, modulates MET channel
properties, potentially through a direct interaction or indirectly by altering membrane mechanics. A stretch
activated channel modifier, GsMTx4 reduces the resting open probability (Po) of MET channel while also blocking
the increase in Po induced by lowering external calcium or depolarizing the hair cell, suggesting the lipid
membrane may be involved in modulating MET channel Po. The effect of voltage and calcium could be mediated
through changes in lipid packing due to multivalent ions interacting between adjacent lipids. Our recent direct
assessment of membrane diffusivity of individual stereocilium at a time using two-photon Fluorescent Recovery
after Photobleaching (FRAP) demonstrated that stereocilia membrane is sensitive to calcium and voltage but
not the soma, and MET channel Po co-varies with membrane diffusivity, supporting the hypothesis that the MET
channel can be modulated by membrane mechanics. However, due to spatial and temporal limitations of FRAP,
we were unable to monitor stereocilia membrane locally and dynamically.
To further test this hypothesis and overcome current technological limitations, I will combine
electrophysiology with live-cell fluorescence lifetime imaging (FLIM) of a novel viscosity sensor to examine the
membrane viscosity with improved spatio-temporal resolution for the first time in mammalian cochlea. I will
assess local and temporal changes in the stereociliary membrane viscosity with voltage, calcium, and membrane
components like cholesterol and PIP2 and correlate these effects to changes in MET channel Po. These studies
will enhance our basic understanding of the importance of lipid membrane in hair cell mechanotransduction.
Understanding the crucial components in the mechanical underpinnings of the stereocilia are both biophysically
and biologically relevant. The development and use of these new technologies will greatly advance my career
as an independent investigator and likely have broader applications in the auditory field and beyond.