The 133-kDa N-terminal domain enables myosin 15 to maintain mechanotransducing stereocilia and is essential for hearing

Abstract

The precise assembly of inner ear hair cell stereocilia into rows of increasing height is critical for mechanotransduction and the sense of hearing. Yet, how the lengths of actin-based stereocilia are regulated remains poorly understood. Mutations of the molecular motor myosin 15 stunt stereocilia growth and cause deafness. We found that hair cells express two isoforms of myosin 15 that differ by inclusion of an 133-kDa N-terminal domain, and that these isoforms can selectively traffic to different stereocilia rows. Using an isoform-specific knockout mouse, we show that hair cells expressing only the small isoform remarkably develop normal stereocilia bundles. However, a critical subset of stereocilia with active mechanotransducer channels subsequently retracts. The larger isoform with the 133-kDa N-terminal domain traffics to these specialized stereocilia and prevents disassembly of their actin core. Our results show that myosin 15 isoforms can navigate between functionally distinct classes of stereocilia, and are independently required to assemble and then maintain the intricate hair bundle architecture.DOI: http://dx.doi.org/10.7554/eLife.08627.001

Defective Gpsm2/Gαi3 signalling disrupts stereocilia development and growth cone actin dynamics in Chudley-McCullough syndrome

“…The failure of Whrn , Gpsm2 and Gnai3 cKOs stereocilia to elongate despite the localization of myosin 15, suggests that these proteins assemble in a macromolecular complex (including Eps8) to regulate actin polymerization at the stereocilia tips. Interestingly, it was recently shown that the distribution of whirlin on the tallest row of stereocilia was dependent upon the isoform 2 of myosin 15 (short form)38. It is therefore probable that Gpsm2 and Gα i3 are part of a preferential complex with myosin 15 isoform 2 and whirlin.…”

Stereocilia morphogenesis and maintenance through regulation of actin stability

“…Interestingly, the protein composition of stereocilia tips differs in the tallest row of the bundle compared to those in shorter rows. In particular, EPS8 is enriched in row 1 while EPS8L2 and TWF2 are largely restricted to rows 2 and 3 along with the long splice isoform of myosin XVa [2,24,26,27]. This localization pattern is likely important for generating or maintaining the staircase morphology of the bundle.…”

“…4C). The morphology of the mechanotransducing stereocilia is also particularly sensitive to perturbation of the actin cytoskeleton as loss or mutation of several cytoskeletal proteins, including the long isoform of myosin XVa, fascin-2, β-actin, Esp8L2, actin interacting protein 1 (AIP1) and ADF result in shortening of these stereocilia while not impacting the length of the non-mechanotransducing row [2,4,5,69,27]. …”

MyTH4-FERM myosins in the assembly and maintenance of actin-based protrusions

“…More recently, two Myo15a isoforms have been identified to serve differential roles in hair bundle morphology [93 •• ]. Isoform 1 contains a long N-terminal extension and targets to the tips of the shorter stereocilia rows [93 •• ].…”

“…More recently, two Myo15a isoforms have been identified to serve differential roles in hair bundle morphology [93 •• ]. Isoform 1 contains a long N-terminal extension and targets to the tips of the shorter stereocilia rows [93 •• ]. Conversely, the isoform lacking this extension, isoform 2, localizes to the longest stereocilia row and is sufficient to target the cargo proteins Eps8 and whirlin [93 •• ].…”