Trichohyalin mechanically strengthens the hair follicle: multiple cross-bridging roles in the inner root shealth.

Trichohyalin is expressed in specialized epithelia that are unusually mechanically strong, such as the inner root sheath cells of the hair follicle. We have previously shown that trichohyalin is sequentially subjected to post-synthetic modifications by peptidylarginine deaminases, which convert many of its arginines to citrullines, and by transglutaminases, which introduce intra- and interprotein chain cross-links. Here we have characterized in detail the proteins to which it becomes cross-linked in vivo in the inner root sheath of the mouse hair follicle. We suggest that it has three principal roles. First, it serves as an interfilamentous matrix protein by becoming cross-linked both to itself and to the head and tail end domains of the inner root sheath keratin intermediate filament chains. A new antibody reveals that arginines of the tail domains of the keratins are modified to citrullines before cross-linking, which clarifies previous studies. Second, trichohyalin serves as a cross-bridging reinforcement protein of the cornified cell envelope of the inner root sheath cells by becoming cross-linked to several known or novel barrier proteins, including involucrin, small proline-rich proteins, repetin, and epiplakin. Third, it coordinates linkage between the keratin filaments and cell envelope to form a seamless continuum. Together, our new data document that trichohyalin is a multi-functional cross-bridging protein that functions in the inner root sheath and perhaps in other specialized epithelial tissues by conferring to and coordinating mechanical strength between their peripheral cell envelope barrier structures and their cytoplasmic keratin filament networks.