Formation of adherens junctions leads to the emergence of a tissue-level tension in epithelial monolayers.
Bottom Line: Adherens junctions and desmosomes integrate the cytoskeletons of adjacent cells into a mechanical syncitium.Though much is known about the biological mechanisms underlying junction formation, little is known about how tissue-scale mechanical properties are established.As a consequence, inhibition of any of the molecular mechanisms participating in adherens junction initiation, remodelling and maturation significantly impeded the emergence of tissue-level tension in monolayers.
Affiliation: London Centre for Nanotechnology, University College London, London WC1H 0AH, UK Department of Physics, University College London, London WC1E 6BT, UK Engineering Doctorate Program, Department of Chemistry, University College London, London WC1H 0AJ, UK.Show MeSH
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Mentions: Using time-lapse imaging and time-resolved mechanical measurements, together with chemical and genetic perturbations, we have shown that the formation of intercellular junctions is accompanied by an increase in the apparent stiffness of monolayer–collagen composites that reflects the emergence of a tissue-level tension in reforming monolayers; a finding that is relevant to biological processes involving METs. Interestingly, recent traction force microscopy experiments have shown that total traction force increases linearly with the number of cells within colonies (Mertz et al., 2012), suggesting that the steady increase in tissue tension that we observed over the first 150 min after replating reflected a progressive increase in the number of cells interfaced with one another around the location of indentation. The establishment of tissue tension coincided with the assembly of adherens junctions and was sensitive to perturbation of the biological mechanisms involved in their formation (Fig. 8). Furthermore, experiments inhibiting cadherin-mediated adhesion suggested that the formation of intercellular adhesions was the main factor underlying increases in apparent stiffness following replating. Consistent with this, depolymerisation of the actin cytoskeleton inhibited the formation of intercellular junctions, thus abolishing the concomitant increases in monolayer apparent stiffness. Perturbation of the biological mechanisms leading to the assembly of adherens junctions prevented the formation of intercellular junctions and, consequently, the establishment of tissue tension. Furthermore, perturbation of junction maturation after the initial formation of contacts also disrupted the establishment of tissue-scale mechanics. By contrast, during the 150 min timecourse over which the most dramatic increases in apparent stiffness were observed, an intercellular network of intermediate filaments linked by desmosomes did not reform; its assembly after ∼300 min did not correlate with an increase in monolayer tension, and disruption of the interfacing of intermediate filaments with desmosomes did not affect monolayer tension in mature monolayers. As mutations to keratins and desmosomal proteins are known to increase the fragility of tissues (Getsios et al., 2004; Huen et al., 2002), our present results suggest that adherens junctions and desmosomes have distinct mechanical roles, with adherens junctions setting tissue tension and desmosomes governing the maximal deformation a tissue can withstand before failure [i.e. the ultimate strain (Huen et al., 2002)]. Although our study focussed on the role of intercellular junctions, traction stresses applied by individual cells through integrins also contribute to tissue tension, can influence cell–cell tension (Liu et al., 2010; Maruthamuthu et al., 2011) and are likely to be affected by the inhibitors used in this study. Thus, further work will be necessary to fully determine the respective contributions of integrin-mediated traction stresses and tension across intercellular junctions to the evolution of tissue tension during monolayer formation.
Affiliation: London Centre for Nanotechnology, University College London, London WC1H 0AH, UK Department of Physics, University College London, London WC1E 6BT, UK Engineering Doctorate Program, Department of Chemistry, University College London, London WC1H 0AJ, UK.