Differentiaton of mesenchymal stem cells in response to mechanical and chemical factors

Mesenchymal stem cells (MSCs) have the ability to differentiate into the cells of many tissue types, such as bone, cartilage, fat, tendon and muscle. These cells are particularly of interest in cardiovascular applications, such as for a cell source for tissue-engineered vascular grafts. In order to use MSCs for stem cell therapies, we must better understand their differentiation pathways and the microenvironmental factors involved. Both chemical and mechanical factors are important in regulating MSC differentiation. Since transforming growth factor-beta1 (TGF-beta1) and mechanical strain are important in vascular development and the modulation of smooth muscle cell (SMC) phenotype, we studied the effect of these two factors as well as matrix rigidity on MSCs into SMCs. We showed that TGF-beta1 induced cell morphology changes in MSCs along with an increase in actin fibers. TGF-beta1 increased the expression of smooth muscle (SM) alpha-actin and decreased the expression of gelsolin. Over-expression of gelsolin inhibited the TGF-beta1-induced assembly of SM alpha-actin, while knocking down gelsolin expression enhanced the assembly of alpha-actin and actin filaments without significantly affecting alpha-actin expression. This study helped elucidate some of the underlying molecular mechanisms involved in MSC differentiation to SMCs via TGF-beta1.; Not only did TGF-beta1 regulate the expression of SMC markers in MSCs, but mechanical strain did as well. Different types of mechanical strain, equiaxial vs. uniaxial, induced differential responses. Cyclic equiaxial strain downregulated alpha-actin and SM-22alpha in MSCs and decreased alpha-actin in stress fibers. In contrast, cyclic uniaxial strain transiently increased the expression of alpha-actin and SM-22alpha. In addition, uniaxial but not equiaxial strain induced a transient increase of collagen I expression. These results suggest that uniaxial strain, which better mimics the type of mechanical strain experienced by SMCs, may promote MSC differentiation into SMCs.; Finally, we explored the effects of another mechanical factor, matrix rigidity, on MSCs in SMC differentiation, while also observing other cell phenotypes induced by matrix rigidity. MSCs expressed less SM markers with decreasing matrix rigidity, while TGF-beta1 did not induce SM markers in MSCs on softer substrates but did on rigid substrates. On the other hand, a soft matrix upregulated chondrogenic markers in MSCs, while augmenting the TGF-beta1-induced increase of these markers compared to on a rigid matrix. This finding shows that the same growth factor can have two different effects on the same cell depending on the rigidity of the substrate. Not only did chondrogenic markers increase on soft matrices but adipogenic and dopamine neural markers increased in MSCs as well, indicating that a certain rigidity is not specific to stimulating the differentiation of a particular cell type. In addition to regulating differentiation, matrix stiffness decreased MSC proliferation. These studies combined show that both chemical and mechanical factors are important for MSC proliferation and differentiation and that a combination of these factors can enhance differentiation into a particular lineage.