Endothelial Mechanotransduction, Redox Signaling and the Regulation of Vascular Inflammatory Pathways

Abstract

The endothelium that lines the interior of blood vessels is directly exposed to blood flow. The shear stress arising from blood flow is "sensed" by the endothelium and is "transduced" into biochemical signals that eventually control vascular tone and homeostasis. Sensing and transduction of physical forces occur via signaling processes whereby the forces associated with blood flow are "sensed" by a mechanotransduction machinery comprising of several endothelial cell elements. Endothelial "sensing" involves converting the physical cues into cellular signaling events such as altered membrane potential and activation of kinases, which are "transmission" signals that cause oxidant production. Oxidants produced are the "transducers" of the mechanical signals? What is the function of these oxidants/redox signals? Extensive data from various studies indicate that redox signals initiate inflammation signaling pathways which in turn can compromise vascular health. Thus, inflammation, a major response to infection or endotoxins, can also be initiated by the endothelium in response to various flow patterns ranging from aberrant flow to alteration of flow such as cessation or sudden increase in blood flow. Indeed, our work has shown that endothelial mechanotransduction signaling pathways participate in generation of redox signals that affect the oxidant and inflammation status of cells. Our goal in this review article is to summarize the endothelial mechanotransduction pathways that are activated with stop of blood flow and with aberrant flow patterns; in doing so we focus on the complex link between mechanical forces and inflammation on the endothelium. Since this "inflammation susceptible" phenotype is emerging as a trigger for pathologies ranging from atherosclerosis to rejection post-organ transplant, an understanding of the endothelial machinery that triggers these processes is very crucial and timely.

Keywords: endothelial mechanotransduction; inflammation; redox signals; revascularization; vascular disease.

Figure 1

(A) Schematic showing patterns of blood flow. The laminar or regular streamlined blood flow is high in the center of the vessel and faces drag or friction along the walls. Obstructive or stasis causes turbulent flow that is characterized by eddies and whirlpools. (B) Shear stress induced signaling involves sensing (mechanotransduction), transduction and transmission (ROS, NO), reception (cellular receptors), and physiological response (atherosclerosis, proliferation, angiogenesis, inflammation).

Figure 2

Virchow's Triad comprising of blood flow, vascular endothelial injury and the viscosity (constitution) of the blood are the three factors that are thought to contribute to thrombosis.

Figure 3

Schematic diagram to show laminar flow in a major vessel and disturbed flow at a bifurcation. The diagram illustrates how blood flow forms recirculatory patterns and eddies at the curvatures.

Figure 4

Experimental Models (in vitro and in situ) to study signaling with various patterns of flow. (A) In vitro flow system (parallel plate chamber) that allows for real time fluorescence microscopy with flow. Endothelial cells grown on coverslips are inserted into the chamber and kept under flow. After 24 h of flow adaptation, various signaling molecules can be monitored by the use of fluorescent dyes (that monitor membrane polarity or ROS production) and real time confocal microscopy. (B) Artificial capillary chamber consisting of narrow capillaries (polypropylene) to mimic blood vessels. Endothelial cells seeded into these capillaries and allowed to attach. During this attachment period, cells are fed via perfusion through abluminal ports. For flow adaptation, medium is perfused through luminal ports. (C) In situ (ex vivo) model of altered flow in the lung. Rat or mouse lungs cleared of blood are ventilated and perfused. The lungs are then preperfused with fluorescent dyes, placed on the stage of a microscope, and imaged for ROS, Ca²⁺, or NO, etc. (D) Cone-and-plate viscometer recapitulates the blood flow of the arterial system on endothelial cells. (E) In vitro model of disturbed flow whereby a step at the inlet in a parallel plate chamber can create eddies downstream of the step. (F) In vivo model of disturbed flow (partial ligation) or stop of flow (ligation) of arteries. Left: ligation or stents would lead to recirculation and eddies. Right: Complete ligation of femoral artery leads to stop of flow as would occur with a clot or thrombus. (G) Schema of a bypass graft. First (Step 1) the vessel is sutured and then dissected (Step 2) and cuffs are placed at the end, and segment of the vessel turned outwards to cover the cuff (Step 2). The cuff was kept in place with a suture (Step 3). Finally a vessel segment (vena cava vein) was grafted over the ends (Step 4). The larger diameter of the engrafted vein as compared to that of the smaller vessel, results in lowering of the shear stress in the engrafted vein. Lowered shear can facilitate an atherogenic phenotype [parts of this figure were originally published in Chatterjee et al. (2014), no permission required].

Figure 5

Mechanotransduction cascade in pulmonary vascular endothelium with stop of flow following the framework outlined in Figure 1. Mechanotransduction by endothelial cells occurs via the mechanosome complex comprising of caveolae-PECAM, VEGFR2 and VE-cadherin resulting in deactivation of KATP channel. This alteration in membrane potential results in activation of NOX2 and eNOS with consequent production of ROS and NO. These mediators result in vasodilation and revascularization probably as a signal to restore the stopped flow (originally published in Chatterjee and Fisher, , no permission required).

Figure 6

Schema showing mechanotransduction induced signaling as would occur with organ storage. ROS activates an inflammation cascade during storage. These predispose the graft to bind to immune cells that influx into it after transplant. ROS can also transform the lung endothelial cells as an antigen presenting cell (APC). The endothelium as APC can “present” donor antigens to the T-lymphocytes that come from the recipient. The APC-T cell interaction leads to cytokine release and cytolysis. (Originally published in Jungraithmayr et al., , permission to reproduce obtained from publisher).