Mechanics of epithelial tissues during gap closure
Introduction
Epithelia have important roles in shaping tissues and organs during embryogenesis, as well as in protecting tissues from homeostasis loss during wound healing [1]. Many physiological and pathological processes involve the (re-)sealing of epithelial gaps. From single cell apoptosis to macroscopic wound, discontinuities of the epithelial barrier occur continuously throughout the lifetime of organisms and in various scales and geometries.
Our review hence focuses on how epithelium maintains its own integrity by examining diverse gap closure scenarios. Such discontinuities can arise either intrinsically (e.g. ventral closure and dorsal closure during development, cell extrusion during homeostasis maintenance) or extrinsically (e.g. physical and chemical injury, infection). Due to its physiological importance, a wide range of studies has strived to elucidate the mechanism of epithelial gap closure with both in vivo and in vitro techniques.
Various morphogenetic events require the collective migration of neighboring epithelium into an opening to form a continuous monolayer, including D. melanogaster dorsal closure, C. elegans ventral enclosure, eyelid closure, neural tube closure and trachea invagination [2, 4••, 5•, 6]. In all these processes, an actin cable assembles apically to form a contractile ‘purse-string’, and actin-based structures drive basal protrusion [7, 8, 9, 10]. Lessons learnt from other gap closure processes studied in vitro, thanks to their striking similarities, helped understand the analysis of tissue morphogenesis in vivo [3].
Wound healing takes place during embryogenesis but also during adult life after a stress, for instance a skin cut, asthma or acute lung injury in the airway system. Independent of the tissue, healing processes share similarities [11]. However, due to its prevalence and tissue accessibility, epidermal wound healing has been the most studied: a multi-step process including tissue growth and remodeling leading to the reconstruction of the wounded area [12]. In adult skin injuries, re-epithelization can last days, during which activated keratinocytes migrate collectively over the wound area, dragging their own basal lamina as they move forward [13]. Keratinocytes in the front remodel the underlying ECM by secreting proteolitic enzymes such as metalloproteinases and depositing new ECM proteins [14]. Cell crawling seems to be more prominent here, with leader cells extending broad lamellipodia [15, 16, 17]. Interestingly, wound healing mechanisms vary with the age of the tissue. Much attention has been devoted to the study of embryonic wound healing due to its lack of scarring, reminiscent of gap closure events during morphogenesis, typically by a purse-string mechanism including rapid recruitment and assembly of actin and myosin into a thick cable in neighboring cells around the wound [18, 19, 20].
Finally, a particular case of epithelial gap closure is apoptotic cell extrusion, in which a dying cell is excluded from an epithelial monolayer. Cell extrusion also occurs recurrently in adulthood during tissue turnover and homeostatic processes [21, 22, 23]. When one or more cells undergo apoptosis, a purse-string mechanism triggers contraction that squeezes the apoptotic cell out of the epithelium.
From the examples discussed above, it appears that two main mechanisms contribute to the restoration of the epithelial integrity: (1) acto-myosin cable contraction in a purse-string manner and (2) cell crawling driven by lamellipodial and/or filopodial protrusions. Sometimes one mechanism dominates but often the two are both present and not mutually exclusive, making it challenging to distinguish their individual contributions [24, 25•] (Table 1, Table 2). Fortunately, recent development of in vitro approaches allowed great progress in the understanding of the relative and synergistic effects of the two mechanisms as well as their regulation, by means of applying mechanical and geometrical constraints [25•, 26•, 27••, 28•, 29, 30•, 31].
Section snippets
The acto-myosin purse-string in epithelial gap closure
The purse-string mechanism is defined as the accumulation of actin and myosin II forming a contractile cable surrounding the rim of the gap [19]. It is involved in a large variety of situations related to epithelial gap closure.
Single cell wounding is a critical event that must be quickly addressed to avoid leakage of intracellular components and subsequent cell death [32]. Cell repair by purse-string mechanism is conserved from embryonic to adult tissue cells of mammalian and non-mammalian
The role of cell crawling in epithelial gap closure
The crawling mechanism requires the extension of a lamellipodium by leading edge cells, often switching from apico-basal polarity to front-rear polarity [51] (Figure 1IIa and IIb). This process was initially described in monolayer wounding experiments using mechanical removal of a strip of cells, that is, manual scraping with pipette tip or razor blade [16, 52, 53]. Other studies performed with damage-free stencil removal and surface patterning techniques have shown that gap closure can in fact
Coexistence and interplay between cell crawling and purse-string
Cell crawling and purse-string are both important for closing epithelial gaps, and one can be favored over the other depending on the experimental conditions, including the presence of dead factors, gap size and geometry. Importantly, the two mechanisms are not mutually exclusive (Table 2). For instance, even though wound healing has been shown to mainly depend on purse-string in embryos, the presence of cellular protrusions has also been reported, and both mechanisms are required for efficient
Conclusions and perspectives
Purse-string and cell crawling mechanisms have been proposed to drive epithelial gap closure, but a clear picture of their respective functions is masked by the complexity of the closure process and the variety of conditions. However, recent in vitro and in vivo experiments have shown that physical constraints, such as local tissue curvature are crucial to the regulation of gap closure mechanisms [25•, 27••, 70•]. Such coupling could be mediated by a differential organization of the actin
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
The authors thank Luis Almeida, Ester Anon, Chwee Teck Lim, Andrea Ravasio, Xavier Trepat and SRK. Vedula for helpful discussions. The authors would also like to thank Chun Xi Wong from MBI Science Communication Core for his help in the illustrations. Financial supports from the Human Frontier Science Programme (grant RGP0040/2012), the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no 617233, the Mechanobiology Institute and
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