Myosin VI is required for structural integrity of the apical surface of sensory hair cells in zebrafish

Abstract

Unconventional myosins have been associated with hearing loss in humans, mice, and zebrafish. Mutations in myosin VI cause both recessive and dominant forms of nonsyndromic deafness in humans and deafness in Snell's waltzer mice associated with abnormal fusion of hair cell stereocilia. Although myosin VI has been implicated in diverse cellular processes such as vesicle trafficking and epithelial morphogenesis, the role of this protein in the sensory hair cells remains unclear. To investigate the function of myosin VI in zebrafish, we cloned and examined the expression pattern of myosin VI, which is duplicated in the zebrafish genome. One duplicate, myo6a, is expressed in a ubiquitous pattern during early development and at later stages, and is highly expressed in the brain, gut, and kidney. myo6b, on the other hand, is predominantly expressed in the sensory epithelium of the ear and lateral line at all developmental stages examined. Both molecules have different splice variants expressed in these tissues. Using a candidate gene approach, we show that myo6b is satellite, a gene responsible for auditory/vestibular defects in zebrafish larvae. Examination of hair cells in satellite mutants revealed that stereociliary bundles are irregular and disorganized. At the ultrastructural level, we observed that the apical surface of satellite mutant hair cells abnormally protrudes above the epithelium and the membrane near the base of the stereocilia is raised. At later stages, stereocilia fused together. We conclude that zebrafish myo6b is required for maintaining the integrity of the apical surface of hair cells, suggesting a conserved role for myosin VI in regulation of actin-based interactions with the plasma membrane.

A nonsense mutation in the gene encoding a zebrafish myosin VI isoform causes defects in hair-cell mechanotransduction

“…While this paper was being prepared for publication, another report of the zebrafish myosin VI isoforms appeared (37), with results largely consistent with ours.…”

Auditory Input to CNS Is Acquired Coincidentally with Development of Inner Ear after Formation of Functional Afferent Pathway in Zebrafish

“…2, 4) including the appearance of the macular HCs and the hair bundles (Fig. 2) (Haddon and Lewis, 1996;Riley et al, 1997;Haddon et al, 1998;Omori and Malicki, 2006), (2) the early expression of mechanotransduction-related molecules (Ernest et al, 2000;Seiler et al, 2004Seiler et al, , 2005Söllner et al, 2004), and (3) the early formation of presynaptic molecules responsible for synaptic transmission to the VIIIth nerve afferents (Fig. 2) Obholzer et al, 2008).…”

Myosin VI and VIIa distribution among inner ear epithelia in diverse fishes

“…Additionally, recent work shows that zebrafish possess two myosin VI genes, myo6a and myo6b (Kappler et al, 2004;Seiler et al, 2004). It was not known whether the myo6 antibody used here binds to zebrafish myo6a, myo6b, or to both proteins.…”

“…Since the myo6 antibody recognizes both myo6a and myo6b, it cannot be determined which protein is being labeled in the experiments described here. However, in situ hybridization in zebrafish with myo6a and myo6b-specific riboprobes indicates that only myo6b is expressed in the inner ear (Seiler et al, 2004;Coffin, 2005). Therefore, myo6 immunolocalization data in teleost fishes (zebrafish, American shad, and oscar) presented here is probably specific for myo6b.…”

“…In normal mammalian hair cells, Myo6 localizes to the cuticular plate region (apical cell surface at the base of the stereocilia) and is thought to function as a membrane anchor around individual stereocilia (Hasson et al, 1997). In the Myo6 mouse mutant Snell's waltzer, stereocilia develop normally but fuse shortly after birth (Avraham et al, 1995;Self et al, 1999), and a similar phenotype is seen in the zebrafish myo6b mutant, satellite (Kappler et al, 2004, Seiler et al, 2004. It is hypothesized that Myo6 functions to anchor the plasma membrane in between individual stereocilia and that in the absence of normal Myo6, the membrane "zips up," forming giant stereocilia which then degenerate (Hasson et al, 1997;Self et al, 1999).…”