MyosinVIIa Interacts with Twinfilin-2 at the Tips of Mechanosensory Stereocilia in the Inner Ear

Abstract

In vertebrates hearing is dependent upon the microvilli-like mechanosensory stereocilia and their length gradation. The staircase-like organization of the stereocilia bundle is dynamically maintained by variable actin turnover rates. Two unconventional myosins were previously implicated in stereocilia length regulation but the mechanisms of their action remain unknown. MyosinXVa is expressed in stereocilia tips at levels proportional to stereocilia length and its absence produces staircase-like bundles of very short stereocilia. MyosinVIIa localizes to the tips of the shorter stereocilia within bundles, and when absent, the stereocilia are abnormally long. We show here that myosinVIIa interacts with twinfilin-2, an actin binding protein, which inhibits actin polymerization at the barbed end of the filament, and that twinfilin localization in stereocilia overlaps with myosinVIIa. Exogenous expression of myosinVIIa in fibroblasts results in a reduced number of filopodia and promotes accumulation of twinfilin-2 at the filopodia tips. We hypothesize that the newly described interaction between myosinVIIa and twinfilin-2 is responsible for the establishment and maintenance of slower rates of actin turnover in shorter stereocilia, and that interplay between complexes of myosinVIIa/twinfilin-2 and myosinXVa/whirlin is responsible for stereocilia length gradation within the bundle staircase.

Myosin VIIa and sans localization at stereocilia upper tip-link density implicates these Usher syndrome proteins in mechanotransduction

“…However, in the shaker-1 mice where MYO7A has a mutation in the motor domain that is predicted to limit its function (38), harmonin, sans, and MYO7A are still able to assemble at the UTLD. Previous reports suggested that MYO7A transports cargo to shorter stereocilia tips (39) and to the UTLD site (2). Although our results do not exclude the possibility that MYO7A serves as a stereocilia cargo transporter, the pattern of localization is more consistent with a role at the UTLD site.…”

β-Actin and Fascin-2 Cooperate to Maintain Stereocilia Length

“…For example, unconventional myosins, such as myosin IIIa, myosin 15a and myosin 7a that regulate stereocilia length (Belyantseva et al, 2005; Prosser et al, 2008; Salles et al, 2009) may move with different kinetics on each actin isoform. The capping protein twinfillin is another intriguing candidate because it localizes only to the tips of row 2 and 3 stereocilia, the same stereocilia that shorten in β-actin knockouts (Peng et al, 2009; Rzadzinska et al, 2009). …”

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.…”