| Many different cell types respond to substrate elasticity as sensitively as more well studied soluble or immobilized ligands, yet mechanisms by which mechanical cues are transduced to cells have been far less explored. Sufficient substrate stiffness, i.e.---Young's Modulus E, appears important to anchorage-dependent, contractile cells, relying on finite resistance to cell-generated forces to induce outside-in mechanical signals and maintain cell function. Muscle cells, in particular, transmit large actomyosin contractions to surrounding extracellular matrix (ECM), appearing mechanochemically sensitive very early as cell adhesion depends on substrate compliance and adhesive ligand density; limited spreading on soft gels (E ∼ 1kPa) is surprisingly insensitive to adhesive ligand density. On rigid matrices or substrates, cells produce isometric contractile efforts not conducive to cell function; in contrast, a more compliant matrix, i.e.---a collagen or polyacrylamide gel, permits cell contractions. Thus, longer-term cultures indicate that muscle cell striation and cardiomyocyte function is stiffness sensitive, with an intermediate range similar to passive muscle ( Emuscle ∼ 8--12kPa) being most favorable. Remarkably, mesenchymal stem cell differentiation also appears mechanosensitive, with cells adopting a myoblast-like elongation and expressing myogenic markers on substrates only near ESk Muscle, while matrices 10-fold softer or several fold stiffer generate cells which commit to neuronal or osteogenic lineages, respectively. Incomplete expression by mechanosensing is augmented by growth factors thus inducing full differentiation; chemical or physical stimulus alone cannot, and when altered, as in fibrotic scarring post-myocardial infarct, cell therapies are ineffective at regenerating functional tissue as the physical microenvironment inhibits contraction.; Underlying these mechanosensitive observations are key signaling 'sets-points.' Cytoskeletal proteins expression and phosphorylation, myosin II contractility and organization, and Rho-GTPases and their affectors when perturbed, alter cell function and can be fit to novel biphasic mechanochemical functions to explain the coupling observed in muscle and stem cells. Gene expression arrays and site specific amino acid labels from these cells implicate several mechanically active proteins and signaling pathways. Overall, our data implies that, in addition to soluble or immobilized ligands, tissue and/or matrix elasticity and structure is critical for musculoskeletal cell signaling, development, and subsequent therapies. |
