Tug of War at the Cell-Matrix Interface

Abstract

Mechanical interactions between a cell and its environment, or between cells, influence key developmental and physiologic processes as well as many aspect of disease (1Daley W.P. Yamada K.M. ECM-modulated cellular dynamics as a driving force for tissue morphogenesis.Curr. Opin. Genet. Dev. 2013; 23: 408-414Crossref PubMed Scopus (140) Google Scholar, 2Provenzano P.P. Keely P.J. Mechanical signaling through the cytoskeleton regulates cell proliferation by coordinated focal adhesion and Rho GTPase signaling.J. Cell Sci. 2011; 124: 1195-1205Crossref PubMed Scopus (365) Google Scholar). Indeed the ability of a cell to sense, produce, and respond to mechanical cues has emerged as a fundamental regulator of cell behaviors such as differentiation, proliferation, survival, and migration. In mechanical terms the interactions governing these behaviors are regulated by intracellular and extracellular physical events that are orchestrated by complex biochemical and mechanical signals. Focal adhesions are sites where transmembrane integrins cluster and link to a host of scaffolding and signaling proteins to form a connection between the actin cytoskeleton and the extracellular matrix. In this context these adhesions act as a primary structure to transduce force between the cell and its microenvironment. In fact, application of external force to adhesions or exposure to stiffer two-dimensional or three-dimensional environments has been reported to promote focal adhesion size and strength while Rho signaling to generate myosin-based contractile (i.e., traction) forces through the actin cytoskeleton promotes focal adhesion assembly (2Provenzano P.P. Keely P.J. Mechanical signaling through the cytoskeleton regulates cell proliferation by coordinated focal adhesion and Rho GTPase signaling.J. Cell Sci. 2011; 124: 1195-1205Crossref PubMed Scopus (365) Google Scholar, 3Geiger B. Spatz J.P. Bershadsky A.D. Environmental sensing through focal adhesions.Nat. Rev. Mol. Cell Biol. 2009; 10: 21-33Crossref PubMed Scopus (1878) Google Scholar, 4Oakes P.W. Gardel M.L. Stressing the limits of focal adhesion mechanosensitivity.Curr. Opin. Cell Biol. 2014; 30: 68-73Crossref PubMed Scopus (96) Google Scholar). As a consequence of the fundamental influence of these mechanical interactions between a cell and its environment, considerable efforts have been presented to dissect the physical and molecular mechanisms governing these processes from both an experimental and mathematical perspective. A seminal mathematical model of cell behavior was the migration model of DiMilla et al. (5DiMilla P.A. Barbee K. Lauffenburger D.A. Mathematical model for the effects of adhesion and mechanics on cell migration speed.Biophys. J. 1991; 60: 15-37Abstract Full Text PDF PubMed Scopus (473) Google Scholar) that incorporated cytoskeletal force generation via myosin-regulated contractility, defined cell polarization, and dynamic adhesions modeled as springs. However, this model assumed the cell was interacting with a homogeneous rigid substrate and as such did not account for cellular responses to environments of different stiffness or modulus. To account for differing mechanical properties in the cell environment, Chan and Odde (6Chan C.E. Odde D.J. Traction dynamics of filopodia on compliant substrates.Science. 2008; 322: 1687-1691Crossref PubMed Scopus (591) Google Scholar) presented the motor-clutch model for cell traction forces as a major advance to understand mechanical interactions between a cell and the extracellular matrix. Their approach notably employs a force-velocity relationship for actomyosin forces and accounts for elastic deformation in the substrate, while allowing spontaneous cell polarization to emerge (6Chan C.E. Odde D.J. Traction dynamics of filopodia on compliant substrates.Science. 2008; 322: 1687-1691Crossref PubMed Scopus (591) Google Scholar, 7Klank R.L. Decker Grunke S.A. Odde D.J. et al.Biphasic dependence of glioma survival and cell migration on CD44 expression level.Cell Rep. 2017; 18: 23-31Abstract Full Text Full Text PDF PubMed Scopus (55) Google Scholar). In this issue of Biophysical Journal, Weinberg et al. (8Weinberg S.H. Mair D.B. Lemmon C.A. Mechanotransduction dynamics at the cell-matrix interface.Biophys. J. 2017; 112: 1962-1974Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar) present another major advance in our understanding of cell-matrix mechanical interactions by interfacing the motor-clutch model framework to a mechanically evolving extracellular matrix. The motor-clutch model predicts that there is an optimal substrate stiffness where traction forces are maximal with a contaminant minimum in retrograde flow of cytoskeletal F-actin. That is, when the substrate is rigid the bonds between integrins and the substrate rupture frequently, resulting in a low transmission of forces (termed “frictional slippage”). In contrast, on softer substrates high traction forces arise from oscillatory “load-and-fail” dynamics (6Chan C.E. Odde D.J. Traction dynamics of filopodia on compliant substrates.Science. 2008; 322: 1687-1691Crossref PubMed Scopus (591) Google Scholar). This biphasic behavior is in agreement with traction forces measured in embryonic chick forebrain neurons (9Elosegui-Artola A. Bazellières E. Roca-Cusachs P. et al.Rigidity sensing and adaptation through regulation of integrin types.Nat. Mater. 2014; 13: 631-637Crossref PubMed Scopus (219) Google Scholar), but contrasts with other observations showing a more monotonic behavior with traction forces increasing with substrate stiffness. One explanation for this disparity was elegantly demonstrated by Elosegui-Artola et al. (10Elosegui-Artola A. Oria R. Roca-Cusachs P. et al.Mechanical regulation of a molecular clutch defines force transmission and transduction in response to matrix rigidity.Nat. Cell Biol. 2016; 18: 540-548Crossref PubMed Scopus (403) Google Scholar), who show that the molecular composition of the clutch, and in particular, the levels of the protein talin in the focal adhesion unit, can define the response to substrate stiffness and account for biphasic and monotonic behaviors. An alternate explanation for the observed variability in the cellular response to substrate stiffness is presented by Weinberg et al. (8Weinberg S.H. Mair D.B. Lemmon C.A. Mechanotransduction dynamics at the cell-matrix interface.Biophys. J. 2017; 112: 1962-1974Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar), who suggest that modulation of the extracellular matrix and the resulting reciprocal mechanical events between the cell and evolving matrix can explain diverse behaviors in response to the mechanical properties of the cell environment. Focusing on fibronectin (a dominant extracellular matrix ligand that is critical to many aspects of development, normal physiology, and disease), the authors develop a model that accounts for dynamics across the actomyosin-focal adhesion-extracellular matrix (i.e., fibronectin) unit that predicts a behavior that is between the defined frictional slippage and load-and-fail behavior. To model evolving fibronectin behavior, Weinberg et al. (8Weinberg S.H. Mair D.B. Lemmon C.A. Mechanotransduction dynamics at the cell-matrix interface.Biophys. J. 2017; 112: 1962-1974Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar) utilize units of the three domains of fibronectin to incorporate length and model deformation as time-dependent Hookean spring behavior. Further, their model incorporates connectivity between cell adhesions and fibronectin, with actomyosin force transmission governed by the force-velocity relationship of the motor-clutch model. This elegantly allows for contractile forces to deform fibronectin to allow fibronectin assembly, perhaps by exposing cryptic binding sites. This approach ultimately leads to a cell-bound fibronectin matrix with three-dimensional structure, facilitating analysis of the time-evolving extracellular matrix as well as the operant cell biophysics and mechanics over time. In and of itself, this is a great advance in modeling cell behavior in mechanically complex microenvironments. However, the impact of the approach by Weinberg et al. (8Weinberg S.H. Mair D.B. Lemmon C.A. Mechanotransduction dynamics at the cell-matrix interface.Biophys. J. 2017; 112: 1962-1974Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar) goes beyond the findings presented in this article. It further opens the door for connecting physics-based models of cell behavior to environments that evolve over time or display nonlinear or anisotropic behaviors. It is hoped that such approaches will ultimately evolve to the level where they can account for multiple evolving extracellular matrix constituents that are spatially heterogeneous and also nonlinear in space and time. Indeed, achieving the ability to incorporate this level of complexity has the potential to allow for truly predictive cell models within in vivo settings. Another notable development is that the model by Weinberg et al. (8Weinberg S.H. Mair D.B. Lemmon C.A. Mechanotransduction dynamics at the cell-matrix interface.Biophys. J. 2017; 112: 1962-1974Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar) makes predictions on focal adhesion length by utilizing the distance between integrin connections to the modeled fibronectin molecule. It is well established that differences in focal adhesion area (typically via changes in length) is a meaningful measure of focal adhesion maturation and is related to the mechanics of the cell-matrix interaction (2Provenzano P.P. Keely P.J. Mechanical signaling through the cytoskeleton regulates cell proliferation by coordinated focal adhesion and Rho GTPase signaling.J. Cell Sci. 2011; 124: 1195-1205Crossref PubMed Scopus (365) Google Scholar, 3Geiger B. Spatz J.P. Bershadsky A.D. Environmental sensing through focal adhesions.Nat. Rev. Mol. Cell Biol. 2009; 10: 21-33Crossref PubMed Scopus (1878) Google Scholar). As such, model predictions of focal adhesion length related to cell contractile forces and extracellular matrix mechanics have the ability to offer insight into the biophysics of clustered integrin complexes and the mechanics of focal adhesions over time. In the work by Weinberg et al. (8Weinberg S.H. Mair D.B. Lemmon C.A. Mechanotransduction dynamics at the cell-matrix interface.Biophys. J. 2017; 112: 1962-1974Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar), the model predicts a U-shaped dependence between the focal adhesion stress (calculated as a force-focal adhesion length ratio) and matrix stiffness (U-shaped moving from softer to more stiff substrates). This results from differences in the fraction of attached clutches with the number of fibronectin molecules and stiffness, ultimately resulting in comparable forces being transmitted along adhesion complexes of different length, thus resulting in different stress values. Interestingly, while a number of independent studies demonstrate that traction forces scale with increasing focal adhesions size, these observations are not ubiquitous, and as such, there is some disagreement in the field regarding whether or not the stress transmitted to a substrate across an adhesion scales with adhesion size (4Oakes P.W. Gardel M.L. Stressing the limits of focal adhesion mechanosensitivity.Curr. Opin. Cell Biol. 2014; 30: 68-73Crossref PubMed Scopus (96) Google Scholar). As much of the observed differences may be attributed to distinct cell types and their associated differences in the composition, structure, organization, and dynamics of key elements of the force linkage (i.e., myosin-actin-focal adhesion-matrix), the approach by Weinberg et al. (8Weinberg S.H. Mair D.B. Lemmon C.A. Mechanotransduction dynamics at the cell-matrix interface.Biophys. J. 2017; 112: 1962-1974Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar) with focal adhesion length incorporated into their model may be applied to a number of conditions to ultimately add insight into this debate. Further, it is likely to suggest experiments, and probably the much more powerful approach of iterative experiment and modeling, to dissect out these complex cell mechanics in response to different mechanical properties of its environment. Overall, the model of Weinberg et al. (8Weinberg S.H. Mair D.B. Lemmon C.A. Mechanotransduction dynamics at the cell-matrix interface.Biophys. J. 2017; 112: 1962-1974Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar) represents an important step in the development of approaches to capture increasingly complex cell and extracellular matrix behavior in both space and time. Of course, additional model development, as well as focused and iterative experiments with quantitative analysis and parameterization of the data, is needed. This is true for both the cell and the matrix. Many questions remain for incorporating additional physical elements and control systems into the physics-based model approach. Further, the addition of multiple adhesions units (e.g., multiple integrin complexes, CD44, etc.) in a given modeled cell, with full spatial organization to account for focal adhesion area and its changes in space and time, will further propel the field. This appears particularly relevant as recent studies in this area suggest distinct cell-matrix mechanics as a function of the type of adhesion complex (7Klank R.L. Decker Grunke S.A. Odde D.J. et al.Biphasic dependence of glioma survival and cell migration on CD44 expression level.Cell Rep. 2017; 18: 23-31Abstract Full Text Full Text PDF PubMed Scopus (55) Google Scholar, 9Elosegui-Artola A. Bazellières E. Roca-Cusachs P. et al.Rigidity sensing and adaptation through regulation of integrin types.Nat. Mater. 2014; 13: 631-637Crossref PubMed Scopus (219) Google Scholar). Likewise, this will be particularly germane as models expand to incorporate multiple extracellular matrices, each with distinct mechanical properties and organization. P.P.P. is supported by a Research Scholar Grant (No. RSG-14-171-01-CSM) from the American Cancer Society and the NIH (Nos. R01CA181385 and U54CA210190). Mechanotransduction Dynamics at the Cell-Matrix InterfaceWeinberg et al.Biophysical JournalMay 09, 2017In BriefThe ability of cells to sense and respond to mechanical cues from the surrounding environment has been implicated as a key regulator of cell differentiation, migration, and proliferation. The extracellular matrix (ECM) is an oft-overlooked component of the interface between cells and their surroundings. Cells assemble soluble ECM proteins into insoluble fibrils with unique mechanical properties that can alter the mechanical cues a cell receives. In this study, we construct a model that predicts the dynamics of cellular traction force generation and subsequent assembly of fibrils of the ECM protein fibronectin (FN). Full-Text PDF Open Archive