Molecular Mechanisms of Clarin-1 and Clarin-2 Function in Mechanosensory Hair Cells

ABSTRACT Deafness is the most common form of sensory impairment in humans and frequently of genetic origin. Mutations in well over 100 genes have been linked to the disease. Many forms of deafness are caused by disruptions of the structure and function of the stereocilia bundle of cochlear hair cells, and in some instances directly affect components of the mechanotranduction (MET) machinery of hair cells. Notably, MET affects hair bundle morphogenesis, suggesting an interesting interplay between MET and morphogenesis. The study of genes linked to hearing loss has provided tremendous insight into the mechanisms of hair bundle development and MET, but we still lack an understanding of the function of many genes linked to deafness as well as of the molecular mechanisms that link MET and morphogenesis. This knowledge is important not only to unravel the molecular pathogenesis of hearing loss, but also to develop rational approaches for its treatment. The long-term goal of my laboratory is to elucidate the mechanisms that regulate the development and function of hair bundles of cochlear hair cells, and how hair bundle defects cause disease. Here we propose to study the function of the tetraspan proteins clarin1 (CLRN1) and clarin2 (CLRN2) in hair cells. Mutations that affect CLRN1&2 cause defects in hair bundle development and MET and lead to deafness. The central hypothesis of this proposal is that CLRN1&2 acts in common molecular pathways with several additional proteins that are affected in genetic forms of hearing loss to regulate hair bundle development and MET. We anticipate that CLRN1/2 regulate hair cell function by direct and indirect effects on the MET machinery of cochlear hair cells, but they may also have additional roles at hair cell synapses. Our hypothesis is supported by published and new data: (1) using newly generated mouse lines expressing epitope tagged Clrn1 and Clrn2, we show that the two proteins are localized to hair bundles; (2) published and new preliminary data show that hair bundles from Clrn1 mutant mice are severely disorganized shortly after birth. Hair bundles from Clrn2 mutant mice show degenerative changes partially resembling those in mice with mutations that affect the MET machinery; (3) MET is affected in Clrn1-/- and Clrn2-/- mutant mice, and abolished in Clrn1-/;Clrn2-/- double mutants; (4) new data show that CLRN1&2 bind to proteins linked to hair bundle development and MET. The subcellular distribution of some of these proteins depends on CLRN2.