Abstract
Actin filaments and microtubules create diverse cellular protrusions, but intermediate filaments, the strongest and most stable cytoskeletal elements, are not known to directly participate in the formation of protrusions. Here we show that keratin intermediate filaments directly regulate the morphogenesis of microridges, elongated protrusions arranged in elaborate maze-like patterns on the surface of mucosal epithelial cells. We found that microridges on zebrafish skin cells contained both actin and keratin filaments. Keratin filaments stabilized microridges, and overexpressing keratins lengthened them. Envoplakin and periplakin, plakin family cytolinkers that bind F-actin and keratins, localized to microridges, and were required for their morphogenesis. Strikingly, plakin protein levels directly dictate microridge length. An actin-binding domain of periplakin was required to initiate microridge morphogenesis, whereas periplakin-keratin binding was required to elongate microridges. These findings separate microridge morphogenesis into distinct steps, expand our understanding of intermediate filament functions, and identify microridges as protrusions that integrate actin and intermediate filaments.
Highlights
The three major classes of cytoskeletal elements—microtubules, actin filaments, and intermediate filaments—each have distinct mechanical and biochemical properties, suiting them to different functions
They form on a variety of mucosal epithelia in many animals, including the periderm layer of the zebrafish skin, where they are required for maintaining glycans on the skin surface (Pinto et al, 2019)
Microridges are primarily known as actin-based protrusions, ultrastructural studies have reported the occasional presence of Keratin intermediate filaments (IFs) within microridges (Uehara et al, 1991; Pinto et al, 2019)
Summary
The three major classes of cytoskeletal elements—microtubules, actin filaments, and intermediate filaments—each have distinct mechanical and biochemical properties, suiting them to different functions. Imaging periderm cells in live zebrafish expressing these reporters revealed that all Keratins localized in two distinct patterns within a cell: As expected, they formed a filamentous network filling cells; remarkably, they formed what appeared to be thick bundles in the pattern of microridges at the apical surface (Fig. 1A, S1). To observe Keratin localization at higher resolution, we used Super-Resolution Structured Illumination Microscopy (SIM) to image cells expressing the Krt17-GFP BAC reporter.
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