Project overview

Skin wound healing re-establishes the epithelial barrier after disease or injury. During wound healing, keratinocytes surrounding the wound bed proliferate and migrate into the wound bed. Prior studies have shown that wound healing is controlled not only by biochemical signals but also by mechanical forces. In this project, we develop a cellular dynamic finite element model (CeldyFEM) to study the effects of different guidance mechanisms of cell migration and the influence of both biochemical and mechanical cues on re-epithelialization process during wound healing. Our study suggests that biochemical cues can guide keratinocyte migration with improved directionality and persistence, while mechanical cues can guide keratinocyte migration with improved coordination so neighboring keratinocyte migrate collectively and efficiently towards the wound bed. View more details of CeldyFEM in model background and our publication Zhao, et al., J. Royal Soc. Interface, 2017.

Model and Measurements

Model of the skin tissue, cell migration mechanisms under biochemical signal (chemotaxis, chemokinesis) or mechanical signal (cohesotaxis), and metrics to measure the efficiency of re-epithelialization.

Results

Re-epithelialization efficiency under different mechanisms

Re-epithelialization under cohesotaxis mechanism. Blue: keratinocyte; Light blue: migrating keratinocyte; Green: wound element.

Re-epithelialization under cohesotaxis mechanism. Blue: keratinocyte; Light blue: migrating keratinocyte; Green: wound element.

Results of re-epithelialization efficiency under different cell migration mechanisms showed that although biochemical cues act on the skin tissue locally, they can direct the migration of keratinocytes with higher persistence towards the wound bed. In contrast, mechanical cues affect the skin tissue globally and can better coordinate the collective migration of keratinocytes through local cell-cell contacts. Furthermore, the details of cell shape plays important role in guiding collective cell migration for in-time re-epithelialization.

Spatio-temporal patterns of cell proliferation under chemotaxis and cohesotaxis

Proliferation provides fresh new cells for wound repair. We study how different patterns of cell proliferation of a population of cells may arise from mechanical and biochemical guidance cues of cell migration and how these patterns of cell proliferation under chemotaxis and cohesotaxis affect the process of re-epithelialization. We divided the whole tissue of size 1600 micrometer times 700 micrometers into 56 blocks. Blocks directly covering the wound bed belong to the central region; blocks immediately neighboring the wound bed belong to the surrounding region. The time-course of re-epithelialization is divided into four intervals, 0 ~ 12 hours, 13 ~ 24 hours, 25 ~ 36 hours, 37 ~ 48 hours. The proliferation index of keratinocyte in one region at one time interval is then calculated as ratio of the number of keratinocytes that are newly generated inside that region during that time interval over the average number of keratinocytes inside that region during that time interval.

The spatial proliferating pattern under chemotaxis is more consistent with that of a previous experimental study compared to that under cohesotaxis, which showed that there exists a burst of cell proliferation at the wound margin during the early stage of re-epithelialization (Garlick, et al. Tissue En., 2007). The temporal pattern of keratinocyte proliferation under chemotaxis is consistent with the observation in recent experimental study, in which it was found that the epidermal proliferation index in the wound region remained at a high level, while proliferation in the surrounding region decreased significantly (Safferling, et al. J. Cell Biol., 2013). These similarities suggest that biochemical cues may play dominant roles in guiding migration of cells located in the wound surrounding region, while it plays negligible roles for cells located in regions distant from the wound bed.

Disturbed cell-cell adhesion under cohesotaxis

Cell-cell adhesions are critical in coordinating collective cell migration in both in vivo and in vitro studies. Studies provided evidence that mechanical tension are transmitted through E-cadherin from neighboring cells when responding to external mechanical stimulus. Many factors are known to be regulators of cell-cell adhesions. The inhibition of myosin-II rapidly decreases cadherin-based cell adhesiveness (Shewan, et al., Am. Soc. Cell Biol., 2005) and the depletion of αE-catenin significantly affects the transmission of mechanical force through cell-cell contacts (Borghi, et al., PNAS., 2012). We explored how disruption of cell-cell adhesion affect the re-epithelialization efficiency under the cohesotaxis where cells migrate following the contraction signal transmitted from neighboring cells. We first studied the effect of inhibition of myosin-II activity by decreasing the adhesion due to cadherin between keratinocytes. We then reduced the levels of αE-catenin depletion by gradually inhibiting the contraction signal transmission between keratinocytes.

We showed that the decrease of cell-cell adhesion intensity significantly perturbed the cell-cell coordination and reduced the migration directionality. This is in agreement with recent study showing that E-cadherin adhesions and the cortical F-actin cytoskeleton transduce mechanical force to side and rear cells which promotes direction sensing during collective cell migration (Cai, et al., Cell, 2014). We also showed that the inhibition of mechanical force transmission through cell-cell junctions significantly reduced the migration directionality and persistence and prolonged the re-epithelialization time. This characterized more similar with random migration. This suggested that the mechanism to switch from cohesotaxis to chemokinesis (random migration) may be the knockdown or down-regulation of cell-cell adhesion, such as actomyosin or catenin.