Unconventional Myosins in Fish Ears

Abstract

Unconventional myosins are critical motor proteins in the vertebrate inner ear. Mutations in both myosins VI and VIIa cause multiple forms of human hereditary deafness but the precise function of these proteins is unknown. This dissertation uses a comparative approach to better understand the role of myosins VI and VIIa in vertebrate ears. Gene expression and protein distribution for these two myosins is examined in the ears of evolutionarily diverse fishes. RT-PCR data shows that myo7a is expressed in the ears of all taxonomically diverse fish species examined here, and immunofluorescence reveals that myo7a protein is distributed throughout the sensory hair bundles of all inner ear regions. Myosin VI expression and distribution is more complex. Studies in other laboratories show that zebrafish (Danio rerio) have two myo6 paralogs with differing gene expression patterns. This dissertation extends previous findings by showing that all teleost fishes have two myo6 genes while non-teleost fishes and tetrapods have one, suggesting that myo6 duplication occurred in an ancestral teleost, probably during a genome-wide duplication. RT-PCR experiments suggest that both myo6 paralogs are expressed in teleost ears. mRNA localization with in situ hybridization shows, however, that myo6a is not expressed in sensory epithelia. Immunocytochemical data shows that myo6 protein is distributed throughout hair bundles in all inner ear end organs of the sea lamprey (Petromyzon marinus) and the zebrafish but is not found in utricular hair bundles in other fishes. While protein expression studies find that the myo6 antibody used in this dissertation binds to both myo6 proteins in the zebrafish, the gene expression studies suggest that only myo6b is expressed in hair cells, and therefore that this is differential distribution of a single protein. This dissertation adds depth to current studies of myo6-associated hereditary deafness and suggests that comparative studies between zebrafish and other fishes such as shad (Alosa sapidissima) that differ in myo6 protein distribution will help elucidate the function of this critical hair cell protein. Comparisons between the two myo6 paralogs will further aid in functional studies and shed light on evolutionary processes during the teleost radiation.