Secretion of fibrinolytic enzymes facilitates human mesenchymal stem cell invasion into fibrin clots.

Adult human mesenchymal stem cells (hMSC) are involved in wound healing and regeneration of mesodermal tissue, but the underlying homing mechanisms are not well understood. Fibrin clot formation is associated with most wound healing processes and potentially guides the recruitment of hMSC. The objective of this study is the investigation of a fibrinolytic capacity, which is required for hMSC to migrate into a wounded tissue and thus to contribute to tissue regeneration. Using RT-PCR, semiquantitative real-time PCR and ELISA, we detected key components of the fibrinolytic cascade, including the urokinase plasminogen activator (uPA) and its receptor (uPAR), the tissue plasminogen activator (tPA) and the plasminogen activator inhibitor (PAI), suggesting a strong fibrinolytic activity of hMSC. To test this activity in a functional assay, we cultured fibrin-embedded hMSC in vitro for 7 days. The cells efficiently dissolved the surrounding fibrin mesh into the fibrin degradation products, the fibrinopeptides. The fibrinolytic activity of hMSC and human dermal fibroblasts, known to be critically involved in skin wound extracellular matrix remodeling, was similar. Our results suggest that a high intrinsic fibrinolytic capacity of hMSC mediates the invasion into a fibrin clot of a wounded tissue.

Three-dimensional epidermis-like growth of human mesenchymal stem cells on dermal equivalents: contribution to tissue organization by adaptation of myofibroblastic phenotype and function.

Human mesenchymal stem cells (hMSC) are able to differentiate into mature cells of various mesenchymal tissues. Recent studies have reported that hMSC may even give rise to cells of ectodermal origin. This indication of plasticity makes hMSC a promising donor source for cell-based therapies. This study explores the differentiation potential of hMSC in a tissue-specific microenvironment simulated in vitro. HMSC were cultured air-exposed on dermal equivalents (DEs) consisting of collagen types I and III with dermal fibroblasts and subjected to conditions similar to those used for tissue engineering of skin with keratinocytes. Culture conditions were additionally modified by pre-treating the cells with 5-azacytidine or supplementing the medium with all trans retinoic acid (RA). HMSC were capable of adaptation to epidermis-specific conditions without losing their mesenchymal multipotency. However, despite the viability and evident three-dimensional epidermis-like growth pattern, hMSC showed a persistent expression of mesenchymal but not of epithelial markers, thus indicating a lack of epidermal (trans) differentiation. Further, electron microscopy and immunohistochemical analyses demonstrated that hMSC cultured under epidermis-specific conditions adopted a myofibroblastic phenotype and function, promoted in particular by air exposure. In conclusion, multipotent hMSC failed to differentiate into E-cadherin- or cytokeratin-expressing cells under optimized organotypic culture conditions for keratinocytes but differentiated into myofibroblast-like cells contracting the extracellular matrix, a phenomenon that was enhanced by RA and 5-azacytidine. These results indicate that hMSC might contribute to wound-healing processes by extracellular matrix reorganization and wound contraction but not by differentiation into keratinocytes.