Endothelial progenitor cells from peripheral blood support bone regeneration by provoking an angiogenic response.

Neovascularization is crucial for fracture healing and plays an important role in long-time graft survival in tissue engineering applications. Endothelial progenitor cells (EPCs) can be isolated from peripheral blood avoiding donor site morbidity, which makes them attractive for autologous cell-based engineering of neovessels. However, contradictory results are published concerning the vasculogenic potential of this cell type. We could previously show that implanted human endothelial vein cells (HUVECs) gave rise to the formation of a complex functional human neovasculature in a heterotopic (subcutaneous) as well as in an orthotopic (calvarial defect) model of severe combined immunodeficiency (SCID) mice, where vessel formation could even be increased by coimplanting mesenchymal stem cells (MSCs) functioning as perivascular cells. In this study, we investigated whether coimplantation of MSCs which have been predifferentiated in vitro into SMCs (SMC-MSCs) may enable pbEPCs to form blood vessels upon implantation and, if this would be the case, whether the resulting enhanced vascularization may support bone regeneration. For this purpose, pbEPCs and SMC-MSCs were mono- or cocultured in collagen matrices and seeded into scaffolds consisting of decalcified processed bovine cancellous bone (PBCB, Tutobone). Neovascularization and osteogenesis were evaluated using a calvarial bone defect-model in SCID mice. Our experiments could show that the missing vasculogenic potential of pbEPCs is not rescued by coimplantation of SMCs derived from MSCs predifferentiated along the vascular smooth muscle lineage. However, implantation of both cell types alone, or in combination induced an angiogenic response, which correlated in a positive manner with bone formation within the implants.

Human endothelial progenitor cells induce extracellular signal-regulated kinase-dependent differentiation of mesenchymal stem cells into smooth muscle cells upon cocultivation.

Neovascularization represents an important issue in tissue-engineering applications, since survival of implanted cells strongly relies on sufficient oxygen and nutrient supply. We have recently observed that human bone marrow-derived mesenchymal stem cells (MSCs) support neovessel formation originating from coimplanted endothelial cells (ECs) in vivo, suggesting that MSCs may function as perivascular cells by investing and stabilizing nascent EC-derived neovessels. In this study, we investigated EC-induced mural cell differentiation of MSCs in vitro. For this purpose, endothelial progenitor cells (EPCs) from two different origins, namely adult peripheral blood (pbEPCs) and neonatal cord blood (cbEPCs), or human umbilical vein endothelial cells (HUVECs), were cocultured with human MSCs to analyze the effect on MSC differentiation toward a smooth muscle cell (SMC)/pericyte phenotype. EPCs as well as HUVECs increased alpha-smooth muscle actin expression in MSCs upon cocultivation in a time-dependent manner. This effect was strongly dependent on direct cell-to-cell contact and extracellular signal-regulated kinase (ERK) signaling, but was not mediated by heterotypic gap junction communication. Beyond enhanced SMC marker gene expression in MSCs, EPCs also enhanced the functional characteristics of cocultured MSCs by increasing their ability to attach to EC tubes in vitro. In conclusion, our study has shown that EPCs from adult peripheral blood as well as from cord blood commit cocultivated MSCs toward an SMC/pericyte phenotype in a cell-contact- and ERK-dependent manner.