Defining endothelial cell functional heterogeneity and plasticity using single-cell RNA-sequencing

Abstract

The state of the endothelium defines vascular health – ensuring homeostasis when intact and driving pathology when dysfunctional. The endothelial cells (ECs) which comprise the endothelium are necessarily highly responsive and reflect their local mechanical, biological, and cellular context, making a concrete definition of endothelial state elusive. Heterogeneity is the hallmark of ECs and has long been studied in the context of structural and functional differences between organs and vascular beds. More recently, local heterogeneity within vessels has also been demonstrated. Understanding the impact of local heterogeneity on EC function in health and disease requires complete description of the heterogeneous states occupied by ECs under physiologic conditions and methods for defining the components of and variability in plasticity in EC state in response to disease-relevant stimuli. Recent technological advances in transcriptional characterization with single-cell resolution including single-cell RNA-sequencing (scRNA-seq) enable high-resolution study of the EC response to a range of stimuli. The premise of this work is that by applying scRNA-seq we can more precisely define individual EC state in terms of functional correlates and differentiate heterogeneous EC responses to pathologically relevant stimuli singly and in combination. Thus, using droplet-based scRNA-seq profiling of the murine aorta we defined three distinct EC states present under physiological conditions in vivo: two major populations specialized for adhesion and lipid handling, and a third primarily lymphatic. These subpopulations persist and demonstrate a unified transcriptional response to Western diet, an environmental stressor. To further define the components of and heterogeneity in stimulus-induced EC plasticity, isolated ECs were examined using a controllable bioreactor system for flow culture and picowell-based scRNA-seq for transcriptional characterization. Shear stress was applied as a paradigmatic mechanical stimulus singly and in combination with rapamycin as a paradigmatic chemical stimulus. The modular components of shear-induced EC state, defined based on previously identified shear-responsive transcription factors, showed heterogeneous expression among shear-cultured ECs. Computational definition of the components of rapamycin-induced plasticity demonstrates that physiological shear-cultured ECs have a more complex plastic response relative to altered shear-cultured ECs. EC subpopulations with transcriptomes dominated by characteristic functions including mitosis and stress states were identified in the shear-cultured populations. These subpopulations recapitulate the altered-shear response to rapamycin but not the physiologic-shear response to rapamycin, suggesting that ECs in a physiologic state are maximally plastic while shear alteration or other stress stimuli induce transcriptional states that limit plasticity to a second stimulus. These results suggest that locally observed EC heterogeneity may be the result of variable and interdependent responses to multiple stimuli, and that EC dysfunction may limit pharmacologic manipulation. Further studies to investigate EC heterogeneity and its impact on plasticity are proposed using both scRNA-seq and other molecular phenotyping methods with the aim of understanding the translational relevance of these results.