Focal adhesion | EPFL Graph Search

In cell biology, focal adhesions (also cell–matrix adhesions or FAs) are large macromolecular assemblies through which mechanical force and regulatory signals are transmitted between the extracellular matrix (ECM) and an interacting cell. More precisely, focal adhesions are the sub-cellular structures that mediate the regulatory effects (i.e., signaling events) of a cell in response to ECM adhesion. Focal adhesions serve as the mechanical linkages to the ECM, and as a biochemical signaling hub to concentrate and direct numerous signaling proteins at sites of integrin binding and clustering. Structure and function Focal adhesions are integrin-containing, multi-protein structures that form mechanical links between intracellular actin bundles and the extracellular substrate in many cell types. Focal adhesions are large, dynamic protein complexes through which the cytoskeleton of a cell connects to the ECM. They are limited to clearly defined ranges of the cell, at which th

Group Decisions Influence Emergence and Regulation of Leaders during Collective Migration of Epithelial Cells

Medhavi Vishwakarma

Collective migration is a central event involving coordinated movement of several individuals. One of the critical events during collective migration is the emergence of leading individuals, who provide directional guidance for the group movement. While on the move, animals select their leaders by a collective decision making process, which require active participation of the followers. On the contrary, the prevalent view on collective cell migration, especially in the context of epithelial cells during wound healing, assumes a hierarchical leader-follower organization and belittles the contribution of follower cells in choosing or regulating the leaders. Furthermore, how the dynamics of cells located at the wound margin evolve as the wound heals, remains illusive. Here, we report and analyse distinct phases of collective migration during wound closure and demonstrate how cellular-level shared decision-making process and collective mechanical dynamics influence selection, regulation and kinematics of leader cells in these phases. We found that in the preparatory phase, before the initiation of migration (Phase 0), the selection of leader cells at the epithelial wound margin depends on the pre-migratory dynamics of the follower cells situated immediately behind the future leaders. Long before the prospective leaders actually start displaying their phenotypic peculiarities, cells behind them manifest stochastic augmentations in the traction forces and monolayer stresses, and display large perimeter-to-area ratio indicating a local unjamming in the followers much before the leaders are selected. Further, introducing an unjammed or fluidic follower at the back stimulates leader cell formation at the margin thereby indicating the role of collective bulk dynamics in leader cell selection. Interestingly, the length upto which cells cooperatively join forces, corresponds very well with the distance between the two emerging leaders and this mechano-biological control remains preserved even in the presence of geometric bias or physiological levels of chemical cue at the interface. Immediately after the initiation of migration (Phase 1), leaders show their distinct phenotypes and drive the cellular outgrowths. In this phase, pluricellular actin belt at the margin regulates the fraction of marginal leaders, which therefore remains unchanged, while the number of followers per leader increases with time. As the migration progresses, fraction of leader cells increases while the latter settles to a steady level set again by the length scale of cell-cell force transmission (Phase 2). Any perturbations in mechanical forces that modifies the force correlation lengths, invariably enforces a change in the number of followers per leader thereby modifying the time required to transit from one phase to the other. Furthermore, orientation of focal adhesions and persistence of cellular motions also display this phase specific behaviour. Together, these findings provide a novel system insight into collective cell migration and indicate integrative leader-follower interactions during wound closure. Given the physiological and pathological importance of leader cell formation in epithelial wound healing, in organogenesis and in metastatic migration of cancer cells, the system-view that the results offer here is anticipated to have to a long-standing impact on the design and discovery of avant-garde therapeutic strategies in future.

EPFL2017

Adhesion of a tape loop

Harmeet Singh

In this work, we revisit experimentally and theoretically the mechanics of a tape loop. Using primarily elastic materials (polydimethylsiloxane, PDMS, or polycarbonate, PC) and confocal microscopy, we monitor the shape as well as the applied forces during an entire cycle of compression and retraction of a half-loop compressed between parallel glass plates. We observe distinct differences in film shape during the cycle; points of equal applied force or equal plate separation differ in shape upon compression or retraction. To model the adhesion cycle in its entirety, we adapt the 'Sticky Elastica' of [T. J. W. Wagner et al., Soft Matter, 2013, 9, 1025-1030] to the tape loop geometry, which allows a complete analytical description of both the force balance and the film shape. We show that under compression the system is generally not sensitive to interfacial interactions, whereas in the limit of large separation of the confining parallel plates during retraction the system is well described by the peel model. Ultimately, we apply this understanding to the measurement of the energy release rate of a wide range of different cross-linker ratio PDMS elastomer half-loops in contact with glass. Finally, we show how the model illuminates an incredibly simple adhesion measurement technique, which only requires a ruler to perform.

ROYAL SOC CHEMISTRY2020

CO-REGULATION OF ENDOTHELIAL AND MYOGENIC CELL FATES THROUGH INTERCELLULAR CROSSTALK IN SKELETAL MUSCLE

Umji Lee

Skeletal muscle is one of the most dynamic organs in the human body, which has a vigorous potential to regenerate and adapt to environmental changes. Depending on environments, skeletal muscle enables essential life activities to survive by controlling movement and metabolism. The functional and structural adaptations of skeletal muscle crosstalk with other physiological systems enabling nutrition and oxygen delivery, and the elimination of metabolic waste products. The work in this dissertation focuses on the intercellular signals produced by myogenic cells to dynamically remodel skeletal muscle during exercise or muscle repair, and the reciprocal signals from the local niche and the systemic environment that regulate myogenic cell fate and metabolism according to physiological needs. This complex crosstalk is dissected in three specific chapters where we study the crosstalk of myogenic cells with endothelial cells and the vasculature to coordinate local niche interactions and systemic crosstalks. First, we demonstrate that an exercise-induced myokine called apelin is produced by muscle fiber and mediates intercellular signaling to endothelial cells through the apelin receptor. In addition, through a yeast one-hybrid screen of transcription factor binding to the apelin promoter, we identified the myogenic transcription factor TEA domain family member 1 (Tead1) as a regulator of Apln transcription. We observed via single-cell transcriptomic analysis of regenerating skeletal muscle that Aplnr (Apelin receptor) is enriched in muscle endothelial cells, whereas Tead1 is enriched in myogenic cells. Myofiber-specific over-expression of Tead1 suppresses apelin secretion at the whole-body level, and apelin secretion via Tead1 knock-down in muscle cells stimulates endothelial cell proliferation in co-cultures. By showing that apelin peptide supplementation in vivo enhances endothelial cell expansion following muscle injury, we conclude that paracrine crosstalk in which apelin secretion controlled by Tead1 in myogenic cells influences endothelial remodeling during skeletal muscle repair. Secondly, we show how tissue-resident support cells affect muscle progenitor activation in different metabolic environments. Skeletal muscle progenitors (SKMP) reside in close proximity to supportive cell types in the stem cell niche and dynamically interact with each other to adapt to environmental changes. Here, we studied how different niche cell types affect SKMPs in response to glycemic levels. Using co-cultures of SKMPs and niche cells, we discovered that endothelial cells synergistically enhance SKMP proliferation in a low glycemic environment, while fibroblasts and macrophages had no effect. We observed that the crosstalk between SKMPs and endothelial cells was mediated by direct cell-cell contacts independent of soluble paracrine signals. Transcriptomic analysis revealed that the endothelial alpha/beta hydrolase N-Myc Downstream Regulated1 (NDRG1) is induced by a low glycemic environment and is associated with biological adhesion. SKMPs co-cultured with Ndrg1 knock-down endothelial cells lose their synergistic functional relationship in response to glucose levels. Therefore, our findings suggest that Ndrg1 is a key mediator of endothelial cell-mediated glycemic control of SKMPs and provides a link between systemic energy levels and the skeletal muscle stem cell niche. Lastly, I developed intravital imaging to monitor muscle stem cells and vasculature.

EPFL2021