Endothelial/Mesenchymal Stem Cell Crosstalk within Bioprinted Cocultures
The development of viable tissue surrogates requires a vascular network that sustains cell metabolism and tissue development. The coculture of endothelial cells (ECs) and mesenchymal stem cells (MSCs), the two key players involved in blood vessel formation, has been heralded in tissue engineering (TE) as one of the most promising approaches for scaffold vascularization. However, MSCs may exert both proangiogenic as well antiangiogenic role. Furthermore, it is unclear which cell type is responsible for the upregulation of angiogenic pathways observed in EC:MSC cocultures. There is disagreement on the proangiogenic action of MSCs, as they have also been shown to negatively affect the formation of capillary networks. To address these issues, we investigated the regulation of key angiogenic pathways in scaffolds hosting different EC:MSC ratios fabricated via extrusion-based bioprinting. Human ECs were cocultured with either rat or human MSCs and the regulation of fundamental angiogenic and arteriogenic pathways was analyzed through DNA, gene, and protein expression. The use of a hybrid human/rat coculture system facilitated pinpointing each cell type role in the regulation of specific genes, and showed that MSCs exert a dose-dependent inhibitory effect on the EC expression of angiogenic factors within the first 24h. Within a week of coculture, MSCs exert a proangiogenic effect, as corroborated in human/human bioprinted cocultures. Interestingly, juxtacrine signaling promoted secretion of the angiogenic factor VEGF in direct cocultures (EC and MSC co-encapsulated), while paracrine signaling encouraged secretion of the arteriogenic factor PGDF in indirect cocultures (adjacent bioprinting of EC-laden and MSC-laden scaffolds). Overall, the use of a bioprinted system to elucidate EC:MSC interplay allows rapid leveraging of the data for novel vascular TE applications. Despite the transitory negative effect early in the culture, MSC presence is necessary for the regulation of pathways involved in arteriogenesis. With further validation in vivo, this study provides a possible explanation to the controversial findings present in literature and shows how MSCs effect on angiogenic pathway regulation mimics the dynamics of blood vessel formation reported in literature and normally occurring in vivo.