Forceful closure: cytoskeletal networks in embryonic wound repair

Abstract

Embryonic tissues heal wounds rapidly and without scarring, in a process conserved across species and driven by collective cell movements. The mechanisms of coordinated cell movement during embryonic wound closure also drive tissue development and cancer metastasis; therefore, embryonic wound repair has received considerable attention as a model of collective cell migration. During wound closure, a supracellular actomyosin cable at the wound edge coordinates cells, while actin-based protrusions contribute to cell crawling and seamless wound healing. Other cytoskeletal networks are reorganized during wound repair: microtubules extend into protrusions and along cell-cell boundaries as cells stretch into damaged regions, septins accumulate at the wound margin, and intermediate filaments become polarized in the cells adjacent to the wound. Thus, diverse cytoskeletal networks work in concert to maintain tissue structure, while also driving and organizing cell movements to promote rapid repair. Understanding the signals that coordinate the dynamics of different cytoskeletal networks, and how adhesions between cells or with the extracellular matrix integrate forces across cells, will be important to elucidate the mechanisms of efficient embryonic wound healing and may have far-reaching implications for developmental and cancer cell biology.

FIGURE 1:

Cytoskeletal polarization during embryonic wound closure. (A) Filamentous actin (visualized using the actin-binding domain of Moesin tagged with green fluorescent protein [GFP]) contributes to the actomyosin cable around the wound, as well as to protrusions (arrowheads) during Drosophila embryonic wound closure. (B) Myosin accumulates at the wound edge and contributes to the formation of a contractile cable in a zebrafish embryo expressing nonmuscle myosin light-chain 12 (the myosin II regulatory subunit) tagged with GFP. (C) Immuno­fluorescence staining in a Xenopus embryo showing microtubule (green) alignment perpendicular to the wound edge (arrowheads). β-Catenin (magenta), an AJ component, is present in all cell–cell boundaries with the exception of the wound edge (dotted line). (D) The septin subunit Sept7 accumulates both at the wound edge and along cell–cell boundaries perpendicular to the wound edge. (A–D) Scale bars, 20 µm. (C, D) Reprinted with permission (Shindo et al., 2018).

FIGURE 2:

Actomyosin cable assembly requires adherens junction redistribution. (Left) Immediately after wounding an embryonic epithelium, AJs (blue) are almost continuous along all edges of the cells, and actin (green) is not polarized. (Center) Shortly after wounding, AJ components including E-cadherin, β-catenin, and α-catenin are removed from the wound edge, in a process mediated by polarized endocytosis. AJ components relocalize to former tricellular junctions around the wound, where actin polymerization (green) and myosin assembly (orange) begin. (Right) AJ removal from the wound edge continues as the actomyosin cable further assembles into a heterogeneous network around the wound and contracts, coordinating cell movements.

FIGURE 3:

Multiple cytoskeletal networks contribute to embryonic wound closure. Different cytoskeletal components contribute to wound healing in a variety of model organisms. Upon wounding, actin (green) and myosin (orange) become polarized in the cells adjacent to the wound, accumulating at the wound edge and forming a supracellular actomyosin cable around the wound. Cable contraction generates forces transmitted across cells by adherens junctions (not shown) and coordinates cell movements. Actin also forms protrusions that promote cell crawling and seamless wound zipping. Microtubules (cyan) are also present in protrusions, where they may facilitate cargo transport. In parallel, microtubules at cell–cell boundaries may aid in the elongation of the cells as they extend into the wound. Septins (magenta) localize to the wound edge and cell–cell boundaries, where they may coordinate actin and microtubule networks, as well as membrane reorganization. Finally, intermediate filaments (red) accumulate close to the wound edge, where they may exchange mechanical signals with the extracellular matrix.