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. 2011 Jan 25;21(2):167-72.
doi: 10.1016/j.cub.2010.12.046. Epub 2011 Jan 13.

Regulation of stereocilia length by myosin XVa and whirlin depends on the actin-regulatory protein Eps8

Uri Manor  1 , Andrea Disanza, M'Hamed Grati, Leonardo Andrade, Harrison Lin, Pier Paolo Di Fiore, Giorgio Scita, Bechara Kachar
Affiliations

Regulation of stereocilia length by myosin XVa and whirlin depends on the actin-regulatory protein Eps8

Uri Manor et al. Curr Biol. .

Abstract

Myosin XVa (MyoXVa) and its cargo whirlin are implicated in deafness and vestibular dysfunction and have been shown to localize at stereocilia tips and to be essential for the elongation of these actin protrusions [1-4]. Given that whirlin has no known actin-regulatory activity, it remains unclear how these proteins work together to influence stereocilia length. Here we show that the actin-regulatory protein Eps8 [5] interacts with MyoXVa and that mice lacking Eps8 show short stereocilia compared to MyoXVa- and whirlin-deficient mice. We show that Eps8 fails to accumulate at the tips of stereocilia in the absence of MyoXVa, that overexpression of MyoXVa results in both elongation of stereocilia and increased accumulation of Eps8 at stereocilia tips, and that the exogenous expression of MyoXVa in MyoXVa-deficient hair cells rescues Eps8 tip localization. We find that Eps8 also interacts with whirlin and that the expression of both Eps8 and MyoXVa at stereocilia tips is reduced in whirlin-deficient mice. We conclude that MyoXVa, whirlin, and Eps8 are integral components of the stereocilia tip complex, where Eps8 is a central actin-regulatory element for elongation of the stereocilia actin core.

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Figures

Figure 1
Figure 1
Eps8 is essential for stereocilia elongation, and localizes to stereocilia tips in a MyoXVa-dependent manner. (A) Scanning electron microscope images show that Eps8 KO mouse stereocilia are shorter than wild-type (WT) stereocilia, similar to shaker-2 and whirler mouse stereocilia. (B) Eps8 (green) localizes to the tips of stereocilia in amounts scaled to length (see Figure S1), and also appears at the tips of microvilli in neighboring supporting cells. In shaker-2 stereocilia, Eps8 is undetectable above background, while in whirler mice Eps8 localizes to stereocilia tips, but in reduced amounts. Scale bars, 5 μm. See also Figure S1.
Figure 2
Figure 2
Overexpression of MyoXVa elongates stereocilia and enhances Eps8 tip-localization. (A) Overexpression of GFP-MyoXVa in a rat cochlear hair cell elongates stereocilia. The transfected cells showed elongation of both the top and middle row of stereocilia when compared to neighboring non-transfected control cells. (B) The relative length (RL) of the tallest (dark gray) and middle (light gray) row of stereocilia in control versus GFP-MyoXVa transfected hair cells. (C) A wild-type (WT) cochlear hair cell overexpressing GFP-MyoXVa (green) shows enhanced Eps8 labeling that colocalizes with MyoXVa. A shaker-2 vestibular hair cell expressing GFP-MyoXVa (green) shows Eps8 labeling at stereocilia tips, in contrast to neighboring non-transfected cells, which have undetectable levels of Eps8 labeling. Scale bars, 5 μm. See also Figure S2.
Figure 3
Figure 3
MyoXVa enhances Eps8 accumulation and actin protrusion elongation activity in heterologous systems. (A) A vestibular supporting cell expressing GFP-MyoXVa (green) shows enhanced labeling of Eps8 (red) in microvilli. Counterstained actin is shown in blue. Scale bar: 5 μm. (B) In Cos-7 cells, the relative intensity (RI) of endogenous Eps8 labeling (red) at filopodia tips versus the cell cortex is significantly increased at filopodia tips in cells expressing GFP-MyoXVa (green) when compared to non-transfected or GFP-MyoX (green) transfected cells. Scale bar: 2 μm. (C) Cos-7 cells overexpressing cherry-Eps8 (red) or MyoXVa (green) both show some elongation of filopodia, but when both are co-overexpressed, filopodia elongation is significantly enhanced. Scale bar: 2 μm. See also Figure S3.
Figure 4
Figure 4
MyoXVa, Eps8, and whirlin interact with one another in vitro. (A) Cartoon diagrams showing the MyoXVa and whirlin constructs used in the in vitro binding assays (IVBs). (B) Cartoon diagrams of the GST-tagged Eps8 fragments used in our binding assays. (C) Ponceau staining of GST-tagged Eps8 fragments containing the proline-rich domain (PR), the first 535 amino acids (1–535), and amino acids 535–821. (D) The proline-rich domain did not interact with GFP-MyoXVa, GFP-whirlin or cherry-MyoXVaΔ. The 1–535 fragment of Eps8 interacted with GFP-whirlin, while the 535–821 fragment interacted with both GFP-MyoXVa and cherry-MyoXVaΔ. When GFP-MyoXVa and GFP-whirlin were co-expressed, both halves (1–535 and 535–821) of Eps8 pulled down GFP-MyoXVa and GFP-whirlin. In contrast, when the cherry-MyoXVaΔ construct was co-expressed with GFP-whirlin, the 1–535 and 535–821 Eps8 fragments pulled down GFP-whirlin and cherry-MyoXVaΔ exclusively (respectively). (E) Cartoon diagram showing the interacting regions of the MyoXVa:Eps8:whirlin tripartite complex. See also Figure S4.
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