A quantitative framework for endothelial cell state transition in response to shear stress

Endothelial cells (ECs) are integral to vascular function, experiencing shear stress as blood flows through the vessels they line. This mechanical stimulus influences their structure, functionality, and morphology. In this study, we use the re-alignment of human induced pluripotent stem cell-derived ECs (hiPSC-ECs) in response to fluid shear stress as a model for exploring collective cell behavior and transitions between distinct cell states. We performed 3D, live cell imaging on endogenously tagged hiPSC-ECs as the cells were subjected to a change in fluid shear stress. We found that under low shear stress, the hiPSC-ECs are elongated and align parallel relative to the direction of fluid flow as they migrate upstream. The cells also display distinct VE-cadherin puncta localized to the contacts of lateral cells. When subjected to high shear stress, the cells look more disorganized, migrated in all directions, and rarely developed VE-cadherin puncta. Furthermore, when the magnitude of shear stress is switched from high to low or vice versa, hiPSC-ECs changed their collective migration behavior, organization, and alignment accordingly. To quantify the transition between these different cell states, we developed a segmentation- and single cell tracking-free machine learning framework to extract unsupervised features from 2D time-lapse image data. We applied the recently developed "Langevin regression" method to learn a model of the stochastic dynamics governing the single-cell behaviors encoded by these features. By solving the stationary Fokker-Planck equation for the learned model, we can visualize the generalized potential landscape of the single-cell dynamics for the distinct experimental shear stress conditions. We expect that analyses of this landscape and the underlying dynamics will give insight on how the environmental cue of shear stress could influence collective migration and the cell state of hiPSC-ECs, improving our understanding of endothelial and cell biology.