Endothelial signalling operates on a small‐world network to control vascular function
Authors: Lee Matthew D. , Buckley Charlotte , Zhang Xun , Wilson Calum , McCarron John G. 
Published: May 2022

The endothelium is the innermost layer of all blood vessels and it controls a host of cardiovascular functions including vascular contractility, hemostasis, inflammation and the exchange of nutrients and waste products between circulating blood and tissue. The importance of the endothelium is clear since changes in the behaviour of this single layer of cells underlies almost all cardiovascular disease. To control each cardiovascular function, the endothelium processes and responds to endless streams of information that originate from multiple sources (i.e. blood cells, hormones, neighboring endothelial cells or underlying smooth muscle cells). To process so much information, the endothelium utilises distributed sensing on spatially‐distinct populations of cells that are primed to detect specific activators. These spatially‐distinct populations communicate across the endothelial network to coordinate vascular responses. The nature of the endothelial network, and how communication is achieved, is currently not understood. Here, by examining the Ca2+ responses in thousands of endothelial cells in intact arteries, we show how the endothelial network operates.

Network organization controls overall system behaviour by determining the signal propagation speed, the robustness of the system to failures and attack, and the degree of synchronizability in the system. To determine the endothelial network structure, we used network (graph) theory. Graph theory has gained much attention due to its ability to quantify social, technological and biological systems, especially the connectivity and synchronisation of the nervous system. However, no studies have examined the network structure employed by the endothelium in the control of cardiovascular function. We analysed muscarinic‐ and histaminergic‐evoked Ca2+ activity from ~1000 endothelial cells from intact resistance arteries. Ca2+ signals were separately analysed in each of the 1000 individual endothelial cells using a custom‐written Python algorithm.

Ca2+ activity in active cells was cross‐correlated and compared to a stochastic model to statistically quantify network connections. We show highly‐correlated Ca2+ activities occurred in multiple, separate, cell clusters for each agonist. The network communication links between the clusters exhibited unexpectedly short path‐lengths, i.e. the number of links between active cells was significantly shorter than expected when active cells were randomly positioned. The number of connections between active cells (degree distribution) followed a power‐law relationship, revealing a scale‐free network topology. The path‐length and degree distribution reveal an endothelial network with a ‘small‐world’ configuration. The small‐world configuration confers particularly dynamic endothelial properties including high signal‐propagation speed, stability and a high degree of synchronizability.

These findings show that endothelial network design is effective for local and global efficiency in the interaction of cells, and for rapid and robust communication to efficiently control cardiovascular activity. The network organization explains how coordinated cell activity occurs across large regions of endothelium, despite sensing being distributed on spatially‐distinct populations of cells.