Abstract
The molecular mechanisms underlying hair cell synaptic maturation are not well understood. Cadherin-23 (CDH23), protocadherin-15 (PCDH15) and the very large G-protein coupled receptor 1 (VLGR1) have been implicated in the development of cochlear hair cell stereocilia, while clarin-1 has been suggested to also play a role in synaptogenesis. Mutations in CDH23, PCDH15, VLGR1 and clarin-1 cause Usher syndrome, characterized by congenital deafness, vestibular dysfunction and retinitis pigmentosa. Here we show developmental expression of these Usher proteins in afferent spiral ganglion neurons and hair cell synapses. We identify a novel synaptic Usher complex comprised of clarin-1 and specific isoforms of CDH23, PCDH15 and VLGR1. To establish the in vivo relevance of this complex, we performed morphological and quantitative analysis of the neuronal fibers and their synapses in the Clrn1−/− mouse, which was generated by incomplete deletion of the gene. These mice showed a delay in neuronal/synaptic maturation by both immunostaining and electron microscopy. Analysis of the ribbon synapses in Ames waltzerav3J mice also suggests a delay in hair cell synaptogenesis. Collectively, these results show that, in addition to the well documented role for Usher proteins in stereocilia development, Usher protein complexes comprised of specific protein isoforms likely function in synaptic maturation as well.
Highlights
Inner hair cells (IHCs) are the cochlear mechano-electrical sensors that transduce sound waves into electrical currents [1,2]
Among the 9 genes associated with Usher syndrome (USH), mutations in the genes encoding adhesion molecules, cadherin-23 (CDH23) and protocadherin-15 (PCDH15), the very large G protein-coupled receptor 1 (VLGR1) and clarin-1 are associated to USH1D, USH1F, USH2C and USH3A, respectively [12,13,14,15]
Antibody qualification To establish the specificity of the isoforms recognized by each of these antibodies given the lack of true knockout mouse models [29,46,47,48,49], we employed two different approaches: specific knock down in UB/OC-1 cells (Fig. 1 and Fig. 2) and pre-adsorption of the antibody with the peptide immunogen followed by western blot analysis (Fig. 2)
Summary
Inner hair cells (IHCs) are the cochlear mechano-electrical sensors that transduce sound waves into electrical currents [1,2]. They are innervated by several synapses formed by type I afferent spiral ganglion neurons (SGNs), facing a synaptic ribbon surrounded by microvesicles, and lateral efferent fibers. Among the 9 genes associated with USH, mutations in the genes encoding adhesion molecules, cadherin-23 (CDH23) and protocadherin-15 (PCDH15), the very large G protein-coupled receptor 1 (VLGR1) and clarin-1 are associated to USH1D, USH1F, USH2C and USH3A, respectively [12,13,14,15].
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