Ferlins are large multi-C2 domain membrane proteins involved in membrane fusion and fission events. In this study we investigate the effects binding of the C2 domains of otoferlin, dysferlin and myoferlin have upon the structure of lipid bilayers. Fluorescence measurements indicate that multi-C2 domain constructs of myoferlin, dysferlin and otoferlin change the lipid packing of both small unilamellar vesicles and giant plasma membrane vesicles. The activities of these proteins were enhanced in the presence of calcium, and required negatively charged lipids like phosphatidylserine or phosphatidylglycerol for activity. Experiments on individual domains uncovered functional differences between the C2A domain of otoferlin as compared to dysferlin and myoferlin, and truncation studies suggest that the effects of each subsequent C2 domain on lipid ordering appear additive. Finally, we demonstrate that the activities of these proteins on membranes are insensitive to high salt concentrations, suggesting a non-electrostatic component to the interaction between ferlin C2 domains and lipid bilayers. Together, the data indicate that dysferlin, otoferlin, and myoferlin do not merely passively adsorb to membranes, but actively sculpt lipid bilayers, which would result in highly curved or distorted membrane regions that could facilitate membrane fusion, fission, or recruitment of other membrane trafficking proteins.

Sensory hair cells convert mechanical motion into chemical signals.Otoferlin, a six-C2 domain transmembrane protein linked to deafness in humans, is hypothesized to play a role in exocytosis at hair cell ribbon synapses. To date, however, otoferlin has been studied almost exclusively in mouse models, and no rescue experiments have been reported. Here we describe the phenotype associated with morpholino-induced otoferlin knockdown in zebrafish and report the results of rescue experiments conducted with full-length and truncated forms of otoferlin. We found that expression of otoferlin occurs early in development and is restricted to hair cells and the midbrain. Immunofluorescence microscopy revealed localization to both apical and basolateral regions of hair cells. Knockdown of otoferlin resulted in hearing and balance defects, as well as locomotion deficiencies. Further, otoferlin morphants had uninflated swim bladders. Rescue experiments conducted with mouse otoferlin restored hearing, balance, and inflation of the swim bladder. Remarkably, truncated forms of otoferlin retaining the C-terminal C2F domain also rescued the otoferlin knockdown phenotype, while the individual N-terminal C2A domain did not. We conclude that otoferlin plays an evolutionarily conserved role in vertebrate hearing and that truncated forms of otoferlin can rescue hearing and balance.H air cells couple mechanical motion to neurotransmitter release at synapses (1). In contrast to conventional neural synapses, hair cell synapses release neurotransmitter continuously and, in a graded manner (2), possess synaptic ribbons (2-4), and lack synaptophysin (5), complexin (6-9), Munc13 (10), and the calcium sensors synaptotagmin I and II (11). In place of synaptotagmin, it is believed that otoferlin may confer calcium sensitivity to evoke neurotransmitter release (12, 13). Otoferlin is a six-C2 domain transmembrane protein expressed in inner, outer, and vestibular hair cells, as well as restricted regions of the brain (13-16). In humans, missense mutations in otoferlin have been linked to hearing loss (17,18), and biochemical studies have determined that otoferlin binds calcium and lipids (12,19), as well as membrane trafficking proteins (12,(20)(21)(22)(23). Further, in vitro assays have demonstrated that otoferlin accelerates SNARE-mediated membrane fusion (12). Based upon this evidence, it is hypothesized that otoferlin functions as a calcium-sensitive regulator of neurotransmitter release in sensory hair cells.However, the results of several studies have raised questions related to otoferlin's function. For instance, despite otoferlin expression in vestibular hair cells, knockout mice show no balance defects (24, 25) despite reduced exocytosis in vestibular type I hair cells (24,26). This raises questions as to the importance of otoferlin in this system. Further, otoferlin did not rescue synchronous neurotransmitter release in synaptotagmin I knockout cultured neurons, indicating that otoferlin and synaptotagmin are not functionally redundant (2...

The precise spatial and temporal expression of genes is essential for proper organismal development. Despite their importance, however, many developmental genes have yet to be identified. We have determined that Fer1l6, a member of the ferlin family of genes, is a novel factor in zebrafish development. We find that Fer1l6 is expressed broadly in the trunk and head of zebrafish larvae and is more restricted to gills and female gonads in adult zebrafish. Using both genetic mutant and morpholino knockdown models, we found that loss of Fer1l6 led to deformation of striated muscle tissues, delayed development of the heart, and high morbidity. Further, expression of genes associated with muscle cell proliferation and differentiation were affected. Fer1l6 was also detected in the C2C12 cell line, and unlike other ferlin homologues, we found Fer1l6 expression was independent of the myoblast-to-myotube transition. Finally, analysis of cell and recombinant protein–based assays indicate that Fer1l6 colocalizes with syntaxin 4 and vinculin, and that the putative C2 domains interact with lipid membranes. We conclude that Fer1l6 has diverged from other vertebrate ferlins to play an essential role in zebrafish skeletal and cardiac muscle development.

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