Skin cell mechanical ability for sealing epithelial gaps

Epithelial cells have a natural tendency to close gaps and this feature plays a crucial role in many biological processes such as embryological development and wound healing. Depending on the distribution of extracellular matrix, gap closure occurs through assembly of multicellular actin-based contractile cables and/or cell migration of border cells into the gap. In the absence of cell supporting environment, the closure of non-adhesive gaps is driven exclusively by contraction of multicellular actin based cables. Such epithelial gap closure through “purse-string contraction” that strongly depends on gap geometry results from a cellular “tug-of-war” at the gap edge.

HFSP Program Grant holders Benoit Ladoux and Chwee Teck Lim
authored on Thu, 19 March 2015

Skin not only provides an essential protective barrier against foreign materials and pathogens, but it also helps the body retain various fluids and electrolytes. When this barrier is damaged, the consequences can be devastating. Ulcers, internal bleeding and bacterial infections may result and the chances of these occurring increases the longer wounds remain open.

Fortunately, epithelial cell sheets are self-repairing. The moment the integrity of the barrier is compromised, cellular mechanisms are initiated to close the gap. Depending on the distribution of extracellular matrix, gap closure occurs through assembly of multicellular actin-based contractile cables and/or protrusive activity of border cells into the gap. Because the relative contribution of these two mechanisms relies on cell-substrate interactions, it is difficult to understand how gaps close in situations where the extra-cellular matrix is heterogeneous and/or poorly adherent. In such cases, the purse-string contraction of actin cables appears to be the crucial mechanism for gap closure but it remains poorly characterized.

Figure: Forces exerted by the cells surrounding the gap (dotted blue line) extend away at first, then direct towards the gap during contraction of the purse-string cable (red). (Copyright S. Wolf, Mechanobiology Institute, Singapore).

By using micropatterned substrates, we study the closure of circular gaps devoid of extracellular matrix protein and functionalized with a cell non-adhesive polymer within sheets of keratinocytes. We find that closure of such non-adherent gaps is driven exclusively by contraction of multicellular actin based cables. The ability to close these gaps is determined by geometrical cues such as size and curvature of the gap as well as intact intercellular junctions.

The cells at the edge of the non-adherent gap are still attached to the ECM. These epithelial skin cells then spread themselves out as far as possible towards the centre of the gap. Measuring the direction of force revealed that these cells are actually pushing away from the gap. While this may sound counter-intuitive, it actually stabilises the cells, in a similar manner to a cantilever bridge, where support at either end anchors the extension of the bridge into space until two sides eventually meet in the middle. Once the cells have spread as far as possible into the gap, the contractile ‘purse-string’ cable forms across the cells, encircling the gap. The force exerted by these cells is reversed and the cells begin to pull each other towards the centre of the gap, continually speeding up the contraction of the protein cable. As the cells move inwards to close the empty space, more contractile cables can reach out over the gap and connect to the other side. These cables can contract rapidly, leading to the formation of a suspended cell sheet over the gap, and complete closure of the wound. The ‘tug-of-war’ mechanism identified in this study provides a vivid demonstration of how cells exert directional forces to enhance biological processes.

One of the most important findings of this study is that some epithelial cells, e.g. renal epithelial cells, are unable to close such gaps as effectively and efficiently as keratinocytes. Consequently, such assays may provide experimental tools to probe and compare the mechanical properties of various tissues as well as determine the relative contribution of intracellular components to tension. Future work will be directed towards understanding precisely the origin of these differences. This will help us to evolve better strategies for wound healing for different types of tissues with different cell types and may lead to advances in wound repair, especially in cases where the ECM is compromised.

Reference

Mechanics of epithelial closure over non-adherent environments. Sri Ram Krishna Vedula, Grégoire Peyret, Ibrahim Cheddadi, Tianchi Chen, Agustí Brugués, Hiroaki Hirata, Horacio Lopez-Menendez, Yusuke Toyama, Luis Almeida, Xavier Trepat, Chwee Teck Lim, Benoit Ladoux. Nature Communications 2015 Jan 22;6:6111. doi: 10.1038/ncomms7111.

Pubmed link

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