You will be presented with the abstract, introduction, and discussion section from a biomedical research article. Your task is to create a summary that a layperson can understand, capturing the essence of the research article. Keep essential scientific terms, but ensure that the language remains clear and the concepts are explained in an uncomplicated manner.

Title: Mapping the dynamics of force transduction at cell–cell junctions of epithelial clusters}
Article:
Abstract:
Force transduction at cell-cell adhesions regulates tissue development, maintenance and adaptation. We developed computational and experimental approaches to quantify, with both sub-cellular and multi-cellular resolution, the dynamics of force transmission in cell clusters. Applying this technology to spontaneously-forming adherent epithelial cell clusters, we found that basal force fluctuations were coupled to E-cadherin localization at the level of individual cell-cell junctions. At the multi-cellular scale, cell-cell force exchange depended on the cell position within a cluster, and was adaptive to reconfigurations due to cell divisions or positional rearrangements. Importantly, force transmission through a cell required coordinated modulation of cell-matrix adhesion and actomyosin contractility in the cell and its neighbors. These data provide insights into mechanisms that could control mechanical stress homeostasis in dynamic epithelial tissues, and highlight our methods as a resource for the study of mechanotransduction in cell-cell adhesions.
Introduction:
Tissues undergo continuous rearrangements that require tightly balanced exchanges of mechanical forces between individual cells through cadherin-mediated cell–cell junctions. The importance of mechanical coupling between cells has been documented for diverse multi-cellular processes, including tissue morphogenesis during development( Mammoto and Ingber, 2010; Martin et al., 2010; Rauzi et al., 2010; Lecuit et al., 2011; Weber et al., 2012), collective migration during wound healing( Trepat et al., 2009; Tambe et al., 2011; Ng et al., 2012), epithelial and endothelial barrier functions( Tzima et al., 2005; Twiss et al., 2012), and cancer progression( Friedl et al., 2004; Bajpai et al., 2009). In all these processes, the spatiotemporal regulation of mechanical forces is essential for the maintenance of tissue integrity as well as for communication and coordination between cells. Nevertheless, it is yet unclear how the magnitude and length scale of cell–cell force transmission are modulated dynamically as a function of cell position and topological changes within a cellular ensemble. Moreover, as recent studies have begun to characterize the molecular basis of the mechano-responsiveness of the cadherin–catenin complex( Gomez et al., 2011; Leckband et al., 2011), it is important to understand how the recruitment and regulation of cadherin molecules at the adherens junctions are coupled to cell–cell force transmission. To address these questions, an assay is required that can measure cell–cell force transmission in situ and in time, which can be combined with high-resolution time-lapse imaging of protein recruitment and activity. We developed a method that utilizes high-resolution traction force microscopy( TFM) and time-lapse live cell microscopy to quantify cell–cell mechanical interactions in dynamic multi-cellular clusters, while maintaining the sub-cellular resolution needed to measure force exchange at the level of individual junctions. Our method expanded upon several pioneering studies, which inferred cell–cell force transmission from the regional imbalance of traction forces in cell pairs and three-cell clusters in linear configurations( Liu et al., 2010; Maruthamuthu et al., 2011; McCain et al., 2012; Tseng et al., 2012), by now providing the time and sub-cellular resolution needed to derive the spatiotemporal coordination of force exchanges in larger cell groups as well as the coupling of force and protein dynamics at individual adherens junctions. To achieve reconstruction of forces at individual cell–cell junctions and in cell clusters of generic configurations, we incorporated the thin-plate modeling approach of cells and finite element analysis of the plate deformation that was utilized by other studies to infer intercellular interactions in entire cell sheets( Trepat et al., 2009; Tambe et al., 2011, 2013; Hur et al., 2012). Using this assay, we quantitatively mapped the forces transmitted through individual cell–cell junctions of dynamic MCF10A epithelial cell clusters of various sizes and configurations over time. Our results revealed that the spatial distributions and dynamics of basal cell–cell force transmission correlated with morphogenetic events such as cell divisions. Along the cell junction, we showed with sub-interface resolution that the local mechanical stresses also correlated with localization of E-cadherin molecules. Expanding our approach to temporal force fluctuation analysis at the length scale of a cell, we found that force propagation through cells can be intercepted by cell anchorage to the extracellular matrix and by actomyosin contraction. Together these results provide a first glimpse of the dynamics of sub-cellular force exchanges that yield stress homeostasis in quiescent and proliferating tissues.
Discussion:
Using our generalized cell–cell force measurement and analysis method, we quantified, spatially and temporally, the transmission of mechanical forces through cell–cell junctions of dynamic adherent epithelial clusters. Combined with image analyses of cell–cell junction configurations and rearrangements in the living cell clusters, our data provided unprecedented statistical power and time resolution to address questions regarding force distributions and fluctuations within cell clusters, as well as their coupling to the recruitment of molecular components of adherens junctions( Maruthamuthu et al., 2011). To our knowledge, the proposed method is the first to resolve cell–cell force transmission dynamically with sub-cellular resolution. Yet, there are limitations that need to be considered with re-implementations of the approach: first, our method relies on a simple thin-plate model of the cell cluster that assumes homogeneity in material property within the cluster. In reality, the mechanical properties of the cell cluster may not be linear and isotropic elastic, as there may be spatial heterogeneity arising from differences in sub-cellular structures and cellular contractility. The assumption of mechanical homogeneity has been made also in stress distribution analyses on an entire cell monolayer( Tambe et al., 2013). In that case, the low spatial resolution tends to average inhomogeneity. The higher the resolution in imaging and the finer the spatial scale of the FEM analysis, the less likely is the cell cluster well represented by a thin-plate. How severe the resulting prediction error is depends on the cluster configuration and the noise level in the traction forces. In the present study, we validated the resolution of the inferred cell–cell forces by cross-correlation analysis of the spatial force fluctuations with E-cadherin density fluctuations along the junctions. These tests confirmed that despite the simple thin-plate model, cell–cell forces are resolved at the length scale E-cadherin-GFP intensities that fluctuate along the cell junction( Figure 6). We therefore conclude that the model simplifications are in this case not severe enough to obscure spatial relationships between force and underlying molecular processes in cell junctions. Second, our method relies on E-cadherin-GFP signals for determining the location of cell–cell junctions. Imaging resolutions( lateral and axial) as well as diffuse signals in dynamic junctions may affect the precision of determining the cell–cell boundary locations and therefore the accuracy of cell–cell force calculations. Third, the method relies on enclosure of the entire cluster in the field of view. Without tiling the image acquisition, the cluster size is therefore limited to a maximum of 8–10 cells. It is important to note that increasing cluster size would also increase the error of force measurement, as the error of traction force microscopy sums up during the calculation of cell–cell forces. Thus for detailed analyses of small relative force variations in space and time, it is advised to restrict the cluster size. We have quantified cell–cell force dynamics in naturally forming clusters of up to nine MCF10A cells. Lastly, because the method derives cell–cell forces indirectly through measurement of cell–matrix traction forces, a fundamental assumption is that the measured traction forces fully capture all forces produced by the cells. Thus, the method implicitly neglects the dissipation of forces inside cells, which—in principle—may affect cell–cell junctional forces. However, experiments that attempted to determine the magnitude of dissipative forces suggest that these contributions may be small compared to the elastic forces analyzed by the present approach( Keren et al., 2009). One key observation we made is that, at the multi-cellular level, junctional force distributions fluctuate temporally and spatially in response to local variations in cell–matrix adhesions and cellular contractility. Although tissue reorganization has long been known as mechanically regulated, our method provides now the capability to monitor the dynamics of cell–cell junctional force exchanges during various morphogenetic events and thus the ability to correlate the induction and outcome of processes such as cell polarization( Blankenship et al., 2006), mitotic spindle orientation( Fink et al., 2011), cell migration( Toyama et al., 2008), and cell sorting( Landsberg et al., 2009), not only with putative surrogates of force generation( e. g., myosin-II localization) but also with actual force values. We show that mechanical forces may be balanced during major topology-changing events such as cell division by dynamic redistribution of cell–cell junctional forces within small cell clusters( Figure 4). The result suggests that force fluctuations at individual cell–cell junctions can be dynamically compensated by forces at other cell junctions. We speculate that this type of mechanical compensation occurs during cell division—and apoptosis—within larger tissues as well and may be the basis of tissue stress homeostasis to maintain mechanical integrity in a proliferating and deforming epithelium. Our method also provides insights into tissue mechanics by revealing that force transmission across a cell cluster is short-ranged, typically on the length scale of one cell diameter, due to the anchoring and scrambling of forces by cell–matrix adhesions and basal actomyosin contractility( Figure 9). The longer-range force transmission previously observed and required for tissue morphogenesis processes, such as collective cell migration( Tambe et al., 2011), is possible by the active regulation of force propagation. This can be accomplished by spatially coordinating the activation of actomyosin contractility above basal force levels, which yields a significant force exchange between neighboring cells, or by spatially coordinating the de-activation of actomyosin contractility and/or cell–matrix adhesion, which may increase the transduction range of basal force levels beyond one cell diameter. The notion of active propagation of mechanical forces across tissues via spatial gradients in myosin-II activation is supported by studies of epithelial sheet migration( Ng et al., 2012) and collective invasion of cancer cell clusters( Hidalgo-Carcedo et al., 2011), in which an asymmetric distribution of actomyosin contractility promote cell cohesion. Based on our statistical analysis of over 90 cell clusters and over 250 cell–cell junctions in these clusters, it appeared that intercellular force transmission exhibits distinct spatial patterns in clusters cultured on soft vs stiff substrates, in which cell–cell forces are transmitted along cluster peripheral cells on stiff substrates, but through the cluster center on soft substrates( Figure 7). We speculate that this may also be a result of the active interplay between cell–cell adhesions, cell–matrix adhesions, and basal actomyosin contractility to transmit, anchor, and scramble forces. As previously reported by the Dufresne group( Mertz et al., 2012, 2013) and shown here in Figure 7 and Figure 8, focal adhesion numbers and traction force magnitudes are higher along the outer periphery of the cluster, suggesting that cell–cell forces are derived from cell–matrix forces that occur mainly at the cluster periphery. On stiffer substrates, where actomyosin contractility and traction force exertion are known to be high( Discher et al., 2005), the force anchoring and scrambling mechanisms are likely more active, preventing force transmission to the center of the cluster, whereas on softer substrates such mechanisms are less active, allowing force propagation through the cluster center. Although it is yet unclear how substrate stiffness precisely regulates cell–matrix adhesions, the cytoskeletal network, and traction force exertion in cell clusters, it has been shown that the differences induced by substrate stiffness can extend multiple cell layers into a cell sheet( Ng et al., 2012). Understanding how microenvironmental mechanical properties influence force transduction in epithelial cell groups may have implications not only for developmental processes but also for pathologies such as cancer where matrix stiffness is often altered( Paszek et al., 2005). Lastly, our cell–cell force microscopy method allowed us to correlate, in quantitative terms, cell–cell force transmission to intracellular molecular dynamics. We showed that, at the sub-cellular level, the variations in junctional forces can be coupled to turnover of E-cadherin-GFP at the junctions. Future studies can apply the same approach for analyses of other cell–cell junction molecules, including known force-responsive proteins, such as alpha-catenin( Yonemura et al., 2010), vinculin( le Duc et al., 2010; Huveneers et al., 2012; Thomas et al., 2013), and Eplin( Abe and Takeichi, 2008; Taguchi et al., 2011). With temporal and spatial resolutions at both the sub-cellular and multi-cellular levels, our method represents a tested toolkit for systematic investigation of the complex force homeostatic mechanisms required for the maintenance of a stress-resistant tissue.
The intestines , liver , and skin are all examples of organs that perform specific functions . Organs are comprised of tissues , which are themselves made up of cells . Epithelial tissue is one of the four basic types of tissue found in animals , and it occurs in almost every organ in the body . For example , epithelial tissue makes up the outermost layer of the skin , and the lining of the lungs and the intestines; the cells in epithelial tissues are attached to one another via ‘adhesion molecules’ . Organs and tissues need to be maintained throughout life in order for them to work properly . Epithelial cells in particular are very short-lived and must be constantly replaced . If epithelial tissue is cut or damaged in any way , the surrounding healthy epithelial cells must work together to repair the wound and restore the tissue's integrity . These processes require individual epithelial cells to communicate with one another . While chemical signals provide one means of cell-to-cell communication , cells also sense and respond to the physical presence of surrounding cells . In adults , organs and tissues generally do not change shape or size; as such there is a tightly balanced exchange of mechanical forces between the individual cells . Damage to the tissue causes a detectable change in these mechanical forces , which is sensed by nearby healthy epithelial cells and causes them to work towards healing the wound . While the importance of mechanical forces in maintaining tissue integrity is widely recognized , there were few tools to study these forces; this meant that mechanical communication through cell–cell adhesion sites was not well understood . Now Ng , Besser et al . describe the development and use of a new method for measuring and mapping the exchange of mechanical forces at cell–cell adhesion sites . Changes in the strength of the forces exchanged between cells could be measured across clusters of multiple cells or for specific parts of individual cells . Ng , Besser et al . found that when an epithelial cell in a cluster started to divide to form two new cells , the cell exerted less mechanical force on its neighboring cells . Ng , Besser et al . found that the forces exerted between cells were strongest when there was more of an adhesion molecule called E-cadherin in the cell surface membrane at the cell–cell adhesion sites . The opposite was also true , as these forces were weakest at cell–cell adhesion sites with fewer E-cadherin molecules . The new method and findings will now help to guide future studies into how mechanical forces are transmitted between living cells .