Co-Emergence of Specialized Endothelial Cells from Embryonic Stem Cells

Cysteine cathepsins are altered by flow within an engineered in vitro microvascular niche

Throughout the process of vascular growth and remodeling, the extracellular matrix (ECM) concurrently undergoes significant changes due to proteolytic activity—regulated by both endothelial and surrounding stromal cells. The role of matrix metalloproteinases has been well-studied in the context of vascular remodeling, but other proteases, such as cysteine cathepsins, could also facilitate ECM remodeling. To investigate cathepsin-mediated proteolysis in vascular ECM remodeling, and to understand the role of shear flow in this process, in vitro microvessels were cultured in previously designed microfluidic chips and assessed by immunostaining, zymography, and western blotting. Primary human vessels (HUVECs and fibroblasts) were conditioned by continuous fluid flow and/or small molecule inhibitors to probe cathepsin expression and activity. Luminal flow (in contrast to static culture) decreases the activity of cathepsins in microvessel systems, despite a total protein increase, due to a concurrent increase in the endogenous inhibitor cystatin C. Observations also demonstrate that cathepsins mostly co-localize with fibroblasts, and that fibrin (the hydrogel substrate) may stabilize cathepsin activity in the system. Inhibitor studies suggest that control over cathepsin-mediated ECM remodeling could contribute to improved maintenance of in vitro microvascular networks; however, further investigation is required. Understanding the role of cathepsin activity in in vitro microvessels and other engineered tissues will be important for future regenerative medicine applications.

Temporal Dynamics of Gene Expression During Endothelial Cell Differentiation From Human iPS Cells: A Comparison Study of Signalling Factors and Small Molecules

Endothelial cell (EC) therapy may promote vascular growth or reendothelization in a variety of disease conditions. However, the production of a cell therapy preparation containing differentiated, dividing cells presenting typical EC phenotype, functional properties and chemokine profile is challenging. We focused on comparative analysis of seven small molecule-mediated differentiation protocols of ECs from human induced pluripotent stem cells. Differentiated cells showed a typical surface antigen pattern of ECs as characterized with flow cytometry analysis, functional properties, such as tube formation and ability to uptake acetylated LDL. Gene expression analysis by RNA sequencing revealed an efficient silencing of pluripotency genes and upregulation of genes related to cellular adhesion during differentiation. In addition, distinct patterns of transcription factor expression were identified during cellular reprogramming providing targets for more effective differentiation protocols in the future. Altogether, our results suggest that the most optimal EC differentiation protocol includes early inhibition of Rho-associated coiled-coil kinase and activation of cyclic AMP signaling, and inhibition of transforming growth factor beta signaling after mesodermal stage. These findings provide the first systematic characterization of the most potent signalling factors and small molecules used to generate ECs from human induced pluripotent stem cells and, consequently, this work improves the existing EC differentiation protocols and opens up new avenues for controlling cell fate for regenerative EC therapy.