Localization and function of TMC1, LHFPL5 and FLRT3 in mammalian auditory hair cells

Abstract

The channel that governs mechanotransduction (MT) by hair cells in the inner ear has been investigated intensively for four decades, but its precise molecular composition remains enigmatic. Transmembrane channel-like protein 1 (TMC1) was recently identified as a component of the MT channel, and lipoma HMGIC fusion partner-like 5 (LHFPL5) is considered to be part of the MT complex and may functionally couple the tip link to the MT channel. As components of the MT complex,TMC1 and LHFPL5 are expected to localize at the lower end of the tip link in hair cells, a notion generally supported by previous studies on neonatal mice. However, the localization of these two proteins, particularly in the hair cells of adult mice, remains incompletely elucidated. Because the localization of TMC1 and LHFPL5 at distinct developmental stages is essential for the understanding of their function and regulation, we used several approaches to examine the localization of them in neonatal and adult hair cells in the mouse. We report several notable findings: 1) TMC1 and LHFPL5 predominantly localize at the tips of the shorter rows of stereocilia in neonatal hair cells, which largely verifies the previously published findings in neonatal hair cells; 2) LHFPL5 persists in the hair bundle of hair cells after postnatal day 7 (P7),which clarifies the previously reported unexpected absence of LHFPL5 after P7 and supports the view that LHFPL5 is a permanent component in the MT complex; and 3) TMC1 and LHFPL5 remain at the tips of the shorter rows of stereocilia in adult outer hair cells, but in adult inner hair cells, TMC1 is uniformly distributed in both the tallest row and the shorter rows of stereocilia, whereas LHFPL5 is uniformly distributed in the shorter rows of stereocilia. These findings raise intriguing questions regarding the turnover rate, regulation, additional functions, and functional interaction of TMC1 and LHFPL5. Our study confirms the previous findings in neonatal hair cells and reveals several previously unidentified aspects of TMC1 and LHFPL5 localization in more mature hair cells. Additionally, we propose that fibronectin leucine rich transmembrane protein 3 (FLRT3) may act as one of the components of the ankle link complex, a structure to keep hair bundle coherent in neonatal hair cells. FLRT3 is expressed transiently at around the base of developing hair bundles in mice. Auditory brainstem recordings and distortion product measurements shows that FLRT3 deficient mice may have hearing impairment. Thus, our results indicate that FLRT3 may be required for the formation of the ankle link and the normal development of cochlear hair bundles.

Date of Award	2019
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology