STRC Research

Stereocilia Actin Cytoskeleton

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Stereocilia Actin Cytoskeleton

Stereocilia are not microvilli. They’re highly organized F-actin bundles — each one a tightly cross-linked, paracrystalline array of actin filaments with barbed ends at the tip and pointed ends at the base. They’re some of the most mechanically specialized structures in the body.

The bundle is held together by three types of links: tip links (cadherin-23 + protocadherin-15 heterodimer), shaft connectors, and ankle links. The actin core itself is cross-linked by espin, fascin, and plastin-1 (fimbrin). At the base, the cuticular plate anchors the rootlets.

Why they’re vulnerable: OHC stereocilia don’t regenerate in mammals. You get the ones you were born with. Damage from noise, ototoxins, or genetic mutation is permanent.

STRC’s role: Stereocilin (STRC) is a non-motor protein secreted by OHCs. It locates at the tips of stereocilia and at the contact region between the tallest stereocilia row and the tectorial membrane. STRC links the stereocilia bundle to the overlying tectorial membrane and maintains the horizontal top connectors between adjacent stereocilia. Without STRC, the bundle deforms under mechanical load and the OHC loses its ability to amplify incoming sound waves.

The actin nucleation angle: WH2-containing proteins (WASP, WAVE2, spire) are part of the machinery that builds and maintains the F-actin core. The Chéreau et al. 2005 paper establishes the structural basis for how WH2 domains template filament nucleation without capping the filament. This machinery runs continuously in stereocilia to maintain the bundle despite ongoing turnover.

The practical implication: STRC loss doesn’t immediately destroy the actin bundle. The OHC survives for years without STRC in DFNB16 patients. That’s the therapeutic window. STRC gene replacement can still work in mature OHCs — the cells are intact, just functionally uncoupled from the tectorial membrane.

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