STRUCTURE, LOCALIZATION AND FUNCTION OF MOUSE MYOSIN XVA IN THE INNER EAR
Friedman, Thomas B
Popper, Arthur N
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Recessive mutations of MYO15A encoding unconventional myosin XVA are associated with sensorineural deafness in both humans (DFNB3) and the shaker 2 phenotype in mice, which are deaf and exhibit circling behavior due to a vestibular abnormality. Myosin XVa is extensively alternatively spliced and encodes approximately thirty protein isoforms. In mouse hair cells that are homozygous for the shaker 2 (Myo15asupsh2) missense mutation, stereocilia are abnormally short due to a failure of their actin filamentous core to elongate. The stereocilia bundle of homozygous (Myo15asupsh2) hair cells lack the characteristic staircase architecture found in wild type hair cell stereocilia bundles. I show that the inability of mutant myosin XVa to deliver whirlin, a scaffold protein, to stereocilia tips underlies the stereocilia dysmorphology in hair cells from homozygous (Myo15asupsh2) mice. The introduction of exogenous myosin XVa homozygous (Myo15asupsh2) hair cells results in restoration of stereocilia elongation and staircase bundle formation. These results imply that the delivery of whirlin by myosin XVa is essential for stereocilia elongation and staircase formation. Using a series of GFP tagged myosin XVa expression constructs containing deletions of one or more domains, the regions necessary for localization to filopodia tips in COS-7 cells, as a model system, and to stereocilia tips of inner ear hair cells was determined. In COS-7 cells, the myosin XVa motor plus several combinations of domains of the myosin XVa tail are sufficient for filopodia tip targeting. The myosin XVa motor plus either the MyTh4sub1/FERMsub1 or MyTh4sub2/FERMsub2 domains are sufficient for stereocilia tip localization in inner ear hair cells. A preliminary assessment of the regions of myosin XVa necessary for the re-initiation of stereocilia elongation and staircase formation in homozygous (Myo15asupsh2) mutant inner ear hair cells is provided. These results provide a better understanding of the molecular mechanisms underpinning normal stereocilia morphogenesis.