Cellular Crowding Influences Extrusion and Proliferation to Facilitate Epithelial Tissue Repair

Jovany Jeomar Franco, Youmna Maryline Atieh, Chase Dallas Bryan, Kristen Marie Kwan, George Thomas Eisenhoffer Jr.

Posted on: 4 June 2018

Preprint posted on 17 May 2018

Article now published in Molecular Biology of the Cell at http://dx.doi.org/10.1091/mbc.e18-05-0295

How do biophysical cues control cell behaviour during tissue repair? This recent preprint employs high resolution live imaging of regenerating zebrafish tissue to provide new insight into the tension-induced changes underlying successful wound repair

Selected by Helen Weavers

Categories: biophysics, cell biology, developmental biology, genetics

Context

Following injury, tissues rapidly initiate a multi-step repair process to restore tissue architecture. Effective repair requires precise control of the cell number within the tissue – and this means that tissue-wide levels of cell proliferation and elimination must be tightly coordinated. Given that adjacent cells in epithelial tissues are physically linked, any changes to individual cells can have profound long-range mechanical and behavioural effects on surrounding cells. Although mechanical forces are well known to regulate cell behaviours during tissue morphogenesis¹, whether these mechanical forces play important roles in regulating cell behaviours across a repairing epithelial sheet is currently less well understood.

Key findings

In this interesting preprint, Franco et al use high resolution time-lapse imaging of zebrafish larvae to analyse the changes in cell behaviour that occur at an individual and tissue-wide level during epithelial repair. These live-imaging studies following surgical amputation of the zebrafish tail (Figure 1, A before and B after amputation) reveal there is a strict spatial regulation of cell elimination and proliferation across the regenerating tissue. Whilst injury causes a dramatic increase in the extrusion of non-apoptotic cells in crowded areas near the wound edge (see Figure 1; GFP labelled actin), cell proliferation occurs preferentially in separate non-crowded areas much further back. But are these distinct cell behaviours linked to changes in tissue mechanics? By both applying specialised CellFIT software²to infer cellular tension and measuring cell density, the authors could correlate changes in tension (Figure 2) and crowding with these specific cell behaviours.

Figure 1: Taken from Figure 1A of the preprint showing live imaging of (A) non-amputated and (B) amputated 4 day post-fertilization (dpf) zebrafish larvae expressing LifeAct-GFP.

Figure 2: Taken from Figure 1G-H of the preprint showing tension maps generated using CellFIT software from live-imaging of non-amputated (G) and amputated (H) zebrafish larvae.

How then, do epithelial cells detect such changes in crowding and tissue tension? Strikingly, experimental disruption of mechanically-regulated stretch-activated ion channels (SACs) caused dramatic effects on cell behaviour; by disrupting the cells ability to sense mechanical forces, cells failed to extrude from crowded areas of the tissue and these areas even experienced aberrant proliferation. Thus, successful tissue regeneration relies on the ability of epithelial cells to sense mechanical forces though the activity of SACs, in order to spatially and temporally coordinate cell extrusion and proliferation across a repairing epithelial sheet.

Future directions

This work raises many fascinating questions for future research. What signalling pathways lie downstream of the SAC activity in response to crowding or increased tissue tension to elicit these different cellular responses? Could such mechano-signalling pathways be exploited in the clinic to improve the repair of chronic wounds that fail to heal? Given my own interests in field of injury-induced inflammation, I’m also intrigued as to the fate of the extruded epithelial cells – are these eliminated cells cleared by phagocytic cells of the zebrafish innate immune system or do they persist and contribute to tissue repair in other ways? Moreover, if immune cells are involved, does phagocytic clearance of these cells have a critical role in the extrusion and regeneration process itself?

References

Heisenberg, CP. And Bellaiche, Y. 2013. Forces in tissue morphogenesis and patterning. Cell. 153:948-62.
Brodland, GW., Veldhuis, JH., Kim, S., Perrone, M., Mashburn, D. and Hutson, MS. 2014. CellFIT: a cellular force-inference toolkit using curvilinear cell boundaries. PloS One. 9:e99116.

Tags: live-imaging, zebrafish; regeneration; wound repair; tissue mechanics; morphogenesis; cell migration; cell death