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      Hydrogels with tunable stress relaxation regulate stem cell fate and activity

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          Abstract

          Natural extracellular matrices (ECMs) are viscoelastic and exhibit stress relaxation. However, hydrogels used as synthetic ECMs for three-dimensional (3D) culture are typically elastic. Here, we report a materials approach to tune the rate of stress relaxation of hydrogels for 3D culture, independently of the hydrogel’s initial elastic modulus, cell-adhesion-ligand density and degradation. We find that cell spreading, proliferation, and osteogenic differentiation of mesenchymal stem cells (MSCs) are all enhanced in cells cultured in gels with faster relaxation. Strikingly, MSCs form a mineralized, collagen-1-rich matrix similar to bone in rapidly relaxing hydrogels with an initial elastic modulus of 17 kPa. We also show that the effects of stress relaxation are mediated by adhesion-ligand binding, actomyosin contractility and mechanical clustering of adhesion ligands. Our findings highlight stress relaxation as a key characteristic of cell-ECM interactions and as an important design parameter of biomaterials for cell culture.

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          Most cited references59

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          Matrix elasticity directs stem cell lineage specification.

          Microenvironments appear important in stem cell lineage specification but can be difficult to adequately characterize or control with soft tissues. Naive mesenchymal stem cells (MSCs) are shown here to specify lineage and commit to phenotypes with extreme sensitivity to tissue-level elasticity. Soft matrices that mimic brain are neurogenic, stiffer matrices that mimic muscle are myogenic, and comparatively rigid matrices that mimic collagenous bone prove osteogenic. During the initial week in culture, reprogramming of these lineages is possible with addition of soluble induction factors, but after several weeks in culture, the cells commit to the lineage specified by matrix elasticity, consistent with the elasticity-insensitive commitment of differentiated cell types. Inhibition of nonmuscle myosin II blocks all elasticity-directed lineage specification-without strongly perturbing many other aspects of cell function and shape. The results have significant implications for understanding physical effects of the in vivo microenvironment and also for therapeutic uses of stem cells.
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            Tensional homeostasis and the malignant phenotype.

            Tumors are stiffer than normal tissue, and tumors have altered integrins. Because integrins are mechanotransducers that regulate cell fate, we asked whether tissue stiffness could promote malignant behavior by modulating integrins. We found that tumors are rigid because they have a stiff stroma and elevated Rho-dependent cytoskeletal tension that drives focal adhesions, disrupts adherens junctions, perturbs tissue polarity, enhances growth, and hinders lumen formation. Matrix stiffness perturbs epithelial morphogenesis by clustering integrins to enhance ERK activation and increase ROCK-generated contractility and focal adhesions. Contractile, EGF-transformed epithelia with elevated ERK and Rho activity could be phenotypically reverted to tissues lacking focal adhesions if Rho-generated contractility or ERK activity was decreased. Thus, ERK and Rho constitute part of an integrated mechanoregulatory circuit linking matrix stiffness to cytoskeletal tension through integrins to regulate tissue phenotype.
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              Role of YAP/TAZ in mechanotransduction.

              Cells perceive their microenvironment not only through soluble signals but also through physical and mechanical cues, such as extracellular matrix (ECM) stiffness or confined adhesiveness. By mechanotransduction systems, cells translate these stimuli into biochemical signals controlling multiple aspects of cell behaviour, including growth, differentiation and cancer malignant progression, but how rigidity mechanosensing is ultimately linked to activity of nuclear transcription factors remains poorly understood. Here we report the identification of the Yorkie-homologues YAP (Yes-associated protein) and TAZ (transcriptional coactivator with PDZ-binding motif, also known as WWTR1) as nuclear relays of mechanical signals exerted by ECM rigidity and cell shape. This regulation requires Rho GTPase activity and tension of the actomyosin cytoskeleton, but is independent of the Hippo/LATS cascade. Crucially, YAP/TAZ are functionally required for differentiation of mesenchymal stem cells induced by ECM stiffness and for survival of endothelial cells regulated by cell geometry; conversely, expression of activated YAP overrules physical constraints in dictating cell behaviour. These findings identify YAP/TAZ as sensors and mediators of mechanical cues instructed by the cellular microenvironment.
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                Author and article information

                Journal
                101155473
                30248
                Nat Mater
                Nat Mater
                Nature materials
                1476-1122
                30 October 2015
                30 November 2015
                March 2016
                30 May 2016
                : 15
                : 3
                : 326-334
                Affiliations
                [1 ]School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA
                [2 ]Wyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge MA 02138, USA
                [3 ]Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA
                [4 ]Dept. Orthopedic Surgery, Research Institute MOVE, VU University Medical Center, Amsterdam, The Netherlands
                [5 ]Gladstone Institute of Cardiovascular Disease, San Francisco
                [6 ]Julius Wolff Institute, Charité – Universitätsmedizin Berlin and Berlin-Brandenburg Center for Regenerative Therapies, Berlin
                Author notes
                [* ]Correspondence to: mooneyd@ 123456seas.harvard.edu
                [†]

                These authors contributed equally to this work.

                Article
                NIHMS733608
                10.1038/nmat4489
                4767627
                26618884
                424928ca-a693-42e4-9f2c-0a61fa65fe19

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                Materials science
                Materials science

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