The lhfpl5 Ohnologs lhfpl5a and lhfpl5b Are Required for Mechanotransduction in Distinct Populations of Sensory Hair Cells in Zebrafish.

Timothy Erickson,Itallia V Pacentine,Alexandra Venuto,Teresa Nicolson,Rachel Clemens

doi:10.3389/fnmol.2019.00320

Abstract

Hair cells sense and transmit auditory, vestibular, and hydrodynamic information by converting mechanical stimuli into electrical signals. This process of mechano-electrical transduction (MET) requires a mechanically gated channel localized in the apical stereocilia of hair cells. In mice, lipoma HMGIC fusion partner-like 5 (LHFPL5) acts as an auxiliary subunit of the MET channel whose primary role is to correctly localize PCDH15 and TMC1 to the mechanotransduction complex. Zebrafish have two lhfpl5 genes (lhfpl5a and lhfpl5b), but their individual contributions to MET channel assembly and function have not been analyzed. Here we show that the zebrafish lhfpl5 genes are expressed in discrete populations of hair cells: lhfpl5a expression is restricted to auditory and vestibular hair cells in the inner ear, while lhfpl5b expression is specific to hair cells of the lateral line organ. Consequently, lhfpl5a mutants exhibit defects in auditory and vestibular function, while disruption of lhfpl5b affects hair cells only in the lateral line neuromasts. In contrast to previous reports in mice, localization of Tmc1 does not depend upon Lhfpl5 function in either the inner ear or lateral line organ. In both lhfpl5a and lhfpl5b mutants, GFP-tagged Tmc1 and Tmc2b proteins still localize to the stereocilia of hair cells. Using a stably integrated GFP-Lhfpl5a transgene, we show that the tip link cadherins Pcdh15a and Cdh23, along with the Myo7aa motor protein, are required for correct Lhfpl5a localization at the tips of stereocilia. Our work corroborates the evolutionarily conserved co-dependence between Lhfpl5 and Pcdh15, but also reveals novel requirements for Cdh23 and Myo7aa to correctly localize Lhfpl5a. In addition, our data suggest that targeting of Tmc1 and Tmc2b proteins to stereocilia in zebrafish hair cells occurs independently of Lhfpl5 proteins.

Highlights

The mechano-electrical transduction (MET) complex of sensory hair cells is an assembly of proteins and lipids that facilitate the conversion of auditory, vestibular and hydrodynamic stimuli into electrical signals
Nor is there evidence that the Salmonid-specific whole genome duplication (WGD) (Allendorf and Thorgaard, 1984) lead to further expansion of the lhfpl5 family. These analyses suggest that the ancestral lhfpl5 gene was duplicated in the teleost WGD and that the lhfpl5 ohnologs were retained in all teleost species examined here
We provide support for the following points: (1) The ancestral lhfpl5 gene was likely duplicated as a result of the teleost WGD, leading to the lhfpl5a and lhfpl5b ohnologs described in this paper

Summary

Introduction

The mechano-electrical transduction (MET) complex of sensory hair cells is an assembly of proteins and lipids that facilitate the conversion of auditory, vestibular and hydrodynamic stimuli into electrical signals. Zebrafish lhfpl5a/b Required for Mechanotransduction and lower ends of the tip link respectively (Kazmierczak et al, 2007), the pore-forming subunits transmembrane channel-like proteins TMC1 and TMC2 (Kawashima et al, 2011; Pan et al, 2013, 2018), and the accessory subunits transmembrane inner ear (TMIE) and lipoma HMGIC fusion partner-like 5 (LHFPL5) (Xiong et al, 2012; Zhao et al, 2014; Cunningham and Müller, 2019) How these proteins are correctly localized to the sensory hair bundle and assemble into a functional complex is a fundamental question for understanding the molecular basis of how mechanotransduction occurs. A structure of the PCDH15 – LHFPL5 complex has been reported (Ge et al, 2018)

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Conclusion