Biological membranes organize and compartmentalize cell signaling into discrete microdomains, a process that often involves stable,
cholesterol-rich platforms that facilitate
protein-
protein interactions. Polarized cells with distinct apical and basolateral cell processes rely on such compartmentalization to maintain proper function. In the cochlea, a variety of highly polarized sensory and non-sensory cells are responsible for the early stages of sound processing in the ear, yet little is known about the mechanisms that traffic and organize signaling complexes within these cells. We sought to determine the prevalence, localization, and
protein composition of
cholesterol-rich
lipid microdomains in the cochlea.
Lipid raft components, including the scaffolding
protein caveolin and the
ganglioside GM1, were found in sensory, neural, and glial cells. Mass spectrometry of
detergent-resistant membrane (DRM) fractions revealed over 600 putative raft
proteins associated with subcellular localization, trafficking, and metabolism. Among the DRM constituents were several
proteins involved in human forms of
deafness including those involved in ion homeostasis, such as the
potassium channel KCNQ1, the
co-transporter SLC12A2, and
gap junction proteins GJA1 and GJB6. The presence of
caveolin in the cochlea and the abundance of
proteins in
cholesterol-rich DRM suggest that
lipid microdomains play a significant role in cochlear physiology.
BIOLOGICAL SIGNIFICANCE: Although mechanisms underlying
cholesterol synthesis, homeostasis, and compartmentalization in the ear are poorly understood, there are several lines of evidence indicating that
cholesterol is a key modulator of cochlear function. Depletion of
cholesterol in mature sensory cells alters calcium signaling, changes excitability during development, and affects the biomechanical processes in outer hair cells that are responsible for hearing acuity. More recently, we have established that the
cholesterol-modulator
beta-cyclodextrin is capable of inducing significant and
permanent hearing loss when delivered subcutaneously at high doses. We hypothesize that
proteins involved in cochlear homeostasis and otopathology are partitioned into
cholesterol-rich domains. The results of a large-scale proteomic analysis point to metabolic processes, scaffolding/trafficking, and ion homeostasis as particularly associated with
cholesterol microdomains. These data offer insight into the
proteins and
protein families that may underlie
cholesterol-mediated effects in sensory cell excitability and
cyclodextrin ototoxicity.