Auditory Hair Cell Mechanotransduction Channels Dynamically Shape the Mechanical Properties of Their Membrane Environment.

The plasma membrane is actively regulated by lipid transporters that create electrochemical gradients between leaflets, and passively by scramblases that dissipate these gradients. Membrane properties such as lipid packing are critical for the proper function of transmembrane proteins, particularly mechanosensitive ion channels. Mechanosensation is a key component of many sensory processes including balance, and hearing. Inner ear hair cells convert mechanical deflection of their hair bundles into electrical signals by gating mechanoelectrical transduction (MET) channels. Transmembrane channel-like proteins (TMCs) are an essential component of the hair cell MET complex, and part of a superfamily of molecules whose members are ion channels and/or lipid scramblases. TMCs are implicated as scramblases in hair cells, however no direct evidence separates scramblase activity from channel properties, nor is there clarity around how MET activity impacts the stereocilia environment. Here, using a novel viscosity sensor boron-dipyrromethene (BODIPY) 1c, this work probes stereocilia membrane viscosity and its relationship with TMC expression and MET current. Using developmental, genetic, electrophysiological and pharmacological tools, this work demonstrates that the MET complex directly regulates the stereocilia membrane viscosity. This work shows that phosphatidylserine externalization does not completely describe, nor solely represent TMC scramblase activity. Lipid flippase/floppase activity along with an MET independent scramblase are implicated in lipid remodeling. Together these data identify a dynamic regulation of stereocilia membrane hypothesized to modulate mechanotransduction channel properties.