Detection of the proteins in a unique extracellular fluid of the mouse inner ear

In Japan, 10% of the population suffer from hearing loss, which impairs daily life; it is an urgent issue to identify the causative factors and develop the innovative therapies. Irreversible hearing loss stems primarily from disorders of the cochlea of the inner ear. The cochlea consists of three tubular structures. Two tubules contain perilymph, which exhibits an ionic composition similar to plasma. Another tubule is filled with endolymph, which shows a highly positive potential of +100 mV. This unique environment sensitizes hearing. Because the endolymph is packed in the narrow space and its quantity is small of ~1 µL in a mouse cochlea, the protein content remains unclear. To address this challenge, we developed a sophisticated approach to collect the endolymph from the cochlea of live mice. We fabricated a micropipette filled with a conductive organic solvent and inserted it into the cochlea, while monitoring the potential. As detecting the high potential of the endolymph, we aspirated the fluid into the pipette. The sample was then analyzed by LC-MS/MS. When comparing the result to the profile of the perilymph, we found that a few proteins including molecules involving lipid metabolism were enriched in the endolymph. The protein data described here may be useful for understanding of the mechanisms underlying hearing loss.