Reactive Oxygen Species-Induced TRPM2-Mediated Ca2+ Signalling in Endothelial Cells

Abstract

Endothelial cells form the innermost layer of blood vessels with a fundamental role as the physical barrier. While regulation of endothelial cell function by reactive oxygen species (ROS) is critical in physiological processes such as angiogenesis, endothelial function is a major target for interruption by oxidative stress resulting from generation of high levels of ROS in endothelial cells by various pathological factors and also release of ROS by neutrophils. TRPM2 is a ROS-sensitive Ca²⁺-permeable channel expressed in endothelial cells of various vascular beds. In this review, we provide an overview of the TRPM2 channel and its role in mediating ROS-induced Ca²⁺ signaling in endothelial cells. We discuss the TRPM2-mediated Ca²⁺ signaling in vascular endothelial growth factor-induced angiogenesis and in post-ischemic neovascularization. In particular, we examine the accumulative evidence that supports the role of TRPM2-mediated Ca²⁺ signaling in endothelial cell dysfunction caused by various oxidative stress-inducing factors that are associated with tissue inflammation, obesity and diabetes, as well as air pollution. These findings provide new, mechanistic insights into ROS-mediated regulation of endothelial cells in physiology and diseases.

Keywords: Ca2+ signaling; ROS; TRPM2 channel; angiogenesis; barrier dysfunction; endothelial cells; vascular diseases.

Figure 1

The structural properties of the TRPM2 channel and major mechanisms of channel activation by ROS. (A) A cartoon showing the tetrameric complex of the TRPM2 channel. Each subunit has a membrane topology of six transmembrane segments (S1–S6), with the S5 and S6 and re-entrant pore-forming loop between them (P-loop) lining the ion-conducting pore. The distal C-terminal NUDT9-H domain is engaged in ADPR binding. (B) The atomic structure of the human TRPM2 channel in closed state (regenerated from RXSB PDB: 6PU), viewed from parallel to the plasma membrane (left) or the extracellular side (right). Four subunits are shown in different colors. (C) Summary of ROS-induced activation of the TRPM2 channel. When applied extracellularly or generated extracellularly or intracellularly, ROS at high levels can induce generation of ADPR from NAD via the DNA repair mechanism in the nucleus mediated by PARP and PARG. ROS can also activate NADase in the mitochondria to generate ADPR. ADPR in turns gates the TRPM2 channel in the plasma membrane or in the lysosomes, resulting in Ca²⁺ influx or Ca²⁺ release, respectively, to elevate intracellular Ca²⁺ concentration. Abbreviations: NUDT9-H, NUDT9 homology; PARP, poly(ADPR) polymerase; PARG, poly(ADPR) glycohydrolase.

Figure 2

TRPM2-mediated Ca²⁺ signaling in VEGF-induced endothelial cell migration. VEGF induces endothelial cell migration (A) through TRPM2-mediated Ca²⁺ signaling mechanism in (B). VEGF induces NOX2-mediated generation of ROS that (or exposure to H₂O₂) activates PARP to generate ADPR and the TRPM2 channel. TRPM2-mediated Ca²⁺ influx activates c-Src kinase, which phosphorylates VE-cadherin to promote its internalization and disassembly of AJ. Abbreviations: VEGF, vascular endothelial growth factor; NOX2, NADPH oxidase 2; PARP, poly(ADPR) polymerase; VE, vascular endothelial; AJ, adherens junctions.

Figure 3

TRPM2-mediated Ca²⁺ signaling in endothelial cell death induced by ROS-inducing factors. (A) Generation of ROS induced by TNF-α (or exposure to H₂O₂) activates PARP to generate ADPR and the TRPM2 channel to induce Ca²⁺ influx. In addition, ROS activates PKCα and promotes its interaction with and thereby phosphorylation of TRPM2-S, leading to disassociation of TRPM2-S with, and disinhibition of, the TRPM2 channel. TRPM2-mediated Ca²⁺ influx activates caspase-3 and caspase-8, triggering apoptosis. (B) Generation of ROS by LPS, or generation of H₂O₂ by GO in the presence of glucose activates PARP1 to generate ADPR and the TRPM2 channel to mediate Ca²⁺ influx, resulting in activation of caspase-3 and apoptosis. Abbreviations: TNF-α, tumor necrosis factor-α; PARP, poly(ADPR) polymerase; PKC, protein kinase C; GO, glucose oxidase; LPS, lipopolysaccharide.

Figure 4

TRPM2-mediated Ca²⁺ signaling in endothelial barrier dysfunction and neutrophil trans-migration associated with inflammation and exposure to ultrafine particulate matters. (A) The cartoon illustrates attachment of neutrophils to endothelial cells and trans-endothelial migration through signaling mechanisms in response to tissue infection (B) or exposure to uPMs (C). (B) ROS generated by LPS-exposed endothelial cells (or exposure to H₂O₂) or generated via NOX in fMLP/LPS-treated neutrophils and released to endothelial cells, activates PARP1 and the TRPM2 channel. TRPM2-mediated Ca²⁺ influx induces c-Src activation and VE-cadherin phosphorylation to promote its internalization and disassembly of AJ, facilitating trans-endothelial migration of neutrophils. TRPM2-mediated Ca²⁺ influx also increase cell surface expression of P-selectin to recruit neutrophils. (C) ROS generated by uPM-exposed endothelial cells (or exposure to H₂O₂) activates PARP and the TRPM2 channel. TRPM2-mediated Ca²⁺ influx induces calpain activation, ZO1 protein degradation and loss of TJ, facilitating trans-endothelial migration of neutrophils. Abbreviations: fMLP, N-formylmethionyl-leucyl-phenylalanine; LPS, lipopolysaccharide; NOX, NADPH oxidase; PARP, poly(ADPR) polymerase; VE, vascular endothelial; AJ, adherens junctions; uPM, ultrafine particulate matters; ZO1, zona occludens-1; TJ, tight junctions.

Figure 5

TRPM2-mediated Ca²⁺ signaling in endothelial insulin resistance induced by palmitate associated with obesity. Exposure to palmitate induces ROS generation and PARP-dependent activation of the TRPM2 channel. TRPM2-mediated Ca²⁺ influx induces activation of CaMKII and PERK and expression of ATF4 and TPB3, leading to the inhibition of insulin-induced vasorelaxation that depends on eNOS-mediated generation of NO. Abbreviations: PARP, poly(ADPR) polymerase; CaMKII, Ca/calmodulin-dependent kinase II; TRB3, pseudo-kinase tribble 3; eNOS, endothelial NO synthase; NO, nitric oxide.

Figure 6

TRPM2-mediated Ca²⁺ signaling in mitochondrial dysfunction induced by high glucose associated with diabetes. Exposure to a high concentration of glucose induces ROS generation and PARP-dependent activation of the TRPM2 channel. TRPM2-mediated Ca²⁺ influx induces lysosomal release of Zn²⁺, and subsequent Zn²⁺ accumulation in the mitochondria recruits Drp-1 to drive mitochondrial fission, leading to mitochondrial fragmentation and dysfunction. Abbreviations: PARP, poly(ADPR) polymerase; Drp-1, dynamin-related protein-1.