Cross-Linked Matrix Rigidity and Soluble Factors Induce Differentiation via Distinct but Overlapping Pathways

Stem cell differentiation is regulated by both soluble factors and the physical properties of extracellular matrix, but the extent to which differentiation pathways are distinct or overlap is often unclear. Here, the micromechanical stiffness of the collagenous bone surface together with broad compositional correlations with collagen-I across many soft tissues suggests enzymatic cross-linking of matrix correlates with nucleoskeletal protein lamin-A, with a retinoid receptor RARG, and with induction toward osteogenesis. Collagen films just 2 nm thick on mica were stiffened or not by transglutaminase cross-linking and used as minimal culture substrates for Mesenchymal stem cells (MSCs). Cells pulling on pristine nano-films visibly deformed and aligned with the collagen fibrils, but on cross-linked films, cells spread isotropically as if adhering to a substrate of greater effective stiffness. Cell nuclei also spread and stiffened, with an increase of lamin-A, nuclear localization of RARG, and upregulation of key early and late osteogenic factors. RARG antagonists also increased lamin-A, and enhanced osteogenesis on rigid substrates in vitro as well as in xenografts of MSCs in mice. A model of the underlying Mechanochemical Gene Circuit couples the sensitivity of stem cells to both insoluble and soluble factors, while a proteomic comparison underscores both differences and overlaps in differentiation pathways.