Transient receptor potential channels in the vasculature

Abstract

The mammalian genome encodes 28 distinct members of the transient receptor potential (TRP) superfamily of cation channels, which exhibit varying degrees of selectivity for different ionic species. Multiple TRP channels are present in all cells and are involved in diverse aspects of cellular function, including sensory perception and signal transduction. Notably, TRP channels are involved in regulating vascular function and pathophysiology, the focus of this review. TRP channels in vascular smooth muscle cells participate in regulating contractility and proliferation, whereas endothelial TRP channel activity is an important contributor to endothelium-dependent vasodilation, vascular wall permeability, and angiogenesis. TRP channels are also present in perivascular sensory neurons and astrocytic endfeet proximal to cerebral arterioles, where they participate in the regulation of vascular tone. Almost all of these functions are mediated by changes in global intracellular Ca(2+) levels or subcellular Ca(2+) signaling events. In addition to directly mediating Ca(2+) entry, TRP channels influence intracellular Ca(2+) dynamics through membrane depolarization associated with the influx of cations or through receptor- or store-operated mechanisms. Dysregulation of TRP channels is associated with vascular-related pathologies, including hypertension, neointimal injury, ischemia-reperfusion injury, pulmonary edema, and neurogenic inflammation. In this review, we briefly consider general aspects of TRP channel biology and provide an in-depth discussion of the functions of TRP channels in vascular smooth muscle cells, endothelial cells, and perivascular cells under normal and pathophysiological conditions.

Figure 1.

Phylogenetic tree of the mouse TRP superfamily.

Figure 2.

Membrane topology and functional domains of TRP subunits.

Figure 3.

TRPV4 sparklets in endothelial cells and vascular smooth muscle cells. A: time-lapse image of a typical TRPV4 sparklet recorded from a primary cerebral artery endothelial cell using TIRF microscopy. The cell was stimulated with the selective TRPV4 agonist GSK1016790A (GSK; 100 nM). B: TRPV4 sparklets recorded from a tsA-201 cell transfected with TRPV4 (top) and from a native cerebral artery myocyte (bottom). The middle and right panels demonstrate the single channel-like behavior of these events. TRPV4 sparklets were stimulated with GSK. C: TRPV4 sparklets recorded from the intact endothelium of cerebral arteries from mice expressing the Ca²⁺-indicator protein GCaMP2 exclusively in the endothelium using high-speed, high-resolution confocal microscopy before and after stimulation with GSK. Experiments were performed in the presence of cyclopiazonic acid (CPA) to eliminate interference from ER Ca²⁺-release events. Representative traces indicating single channel-like events are shown below. [A from Sullivan and Earley (336). B from Mercado et al. (221). C from Sonkusare et al. (323), with permission from American Association for the Advancement of Science.]

Figure 4.

Ca²⁺-influx mechanisms mediated by TRP channels. A: receptor-operated Ca²⁺ entry (ROCE). Agonist binding to GPCRs stimulates the activity of PLC, which cleaves the membrane phospholipid PIP₂ into IP₃ and DAG. IP₃ binds to IP₃Rs on the ER/SR to cause release of Ca²⁺ into the cytosol. DAG directly activates Ca²⁺ influx through TRPC3, TRPC6, or TRPC7 channels on the plasma membrane. B: example trace of changes in intracellular Ca²⁺ following stimulation of a GPCR, showing the phasic increase resulting from ER/SR Ca²⁺ release and sustained elevation resulting from ROCE. C: store-operated Ca²⁺ entry (SOCE). Depletion of Ca²⁺ in the ER/SR causes clustering of STIM1 in the ER/SR membrane proximal to the plasma membrane. STIM1 interacts with Oria1 at the plasma membrane to promote Ca²⁺ influx. TRPC1, TRPC4, and/or TRPC5 channels, either independently or in association with STIM1/Oria1, may participate in SOCE. D: example of a typical SOCE experiment. Under Ca²⁺-free conditions, cells are treated with the SERCA inhibitor thapsigargin, causing Ca²⁺ to leak from the ER/SR into the cytosol. Reintroduction of Ca²⁺ to the bathing solution after ER/SR stores are depleted causes SOCE.

Figure 5.

TRP channels contribute to vascular smooth muscle cell contractility. Cation influx through TRP channels depolarizes the plasma membrane (ΔE_m), initiating Ca²⁺ influx through VDCCs. Elevated Ca²⁺ levels increase binding of calmodulin-Ca²⁺ (Cd-Ca²⁺) complexes to MLCK, stimulating phosphorylation of the regulatory myosin light chain (MLC₂₀). MLC₂₀ is dephosphorylated by myosin light-chain phosphatase (MLCP). Some studies suggest that Ca²⁺ influx through TRP channels can directly initiate myocyte contraction.

Figure 6.

A force-sensitive signaling network involving TRPC6 and TRPM4 channels in cerebral artery smooth muscle cells. A stretch-sensing pathway in cerebral artery myocytes that includes 1) activation of AT₁R, Src tyrosine kinase, and PLC-γ1, leading to the generation of IP₃; and 2) Ca²⁺ influx through TRPC6 channels onto IP₃Rs, promoting CICR that results in activation of TRPM4 channels, which are responsible for pressure-induced depolarization of the plasma membrane. PM, plasma membrane; ΔV_m, change in membrane potential. [From Gonzales et al. (118).]

Figure 7.

Ca²⁺ influx through TRPV4 channels relaxes cerebral artery smooth muscle cells. Stimulation of Ca²⁺ influx through TRPV4 channels directly with 11,12-EET or through activation of G_q-coupled AT₁Rs with ANG II, leading to generation of DAG by PLC and AKAP150-dependent PKC activity, causes an increase in the frequency of Ca²⁺ sparks mediated by ryanodine receptors (RyRs) on the SR through a CICR mechanism. Increased generation of Ca²⁺ sparks causes smooth muscle cell membrane hyperpolarization and relaxation by increasing K⁺ efflux through BK channels. [Modified from Earley et al. (79), with permission from Wolters Kluwer Health, and Mercado et al. (221).]

Figure 8.

Mechanisms of endothelium-dependent vasodilation. In endothelial cells (EC), Ca²⁺ influx stimulates the activity of eNOS to generate NO, which diffuses through the IEL to cause relaxation of underlying vascular smooth muscle cells (SMC). COX activity generates PGI₂, which also relaxes SMC. A variety of other substances, collectively known as EDHF, can cause smooth muscle cell hyperpolarization and relaxation. K⁺ efflux hyperpolarizes the endothelial cell plasma membrane (ΔE_m). Electrotonic spread of this influence through myoendothelial gap junctions (MEGJs) promotes EDH, causing SMC relaxation.

Figure 9.

Ca²⁺ pulsars in the endothelium. A: time course of a three-dimensional Ca²⁺ pulsar originating from within a hole in the IEL (white circle) shown in the leftmost image. Scale bar, 5 μm. B: Ca²⁺ pulsars recorded from a pressurized artery (80 mmHg) expressing the Ca²⁺-indicator protein GCaMP2. An endothelial cell and its nucleus are outlined (dotted lines), with the initiation sites indicated by red arrows. Scale bar, 10 μm. C: model of the functional effects of endothelial Ca²⁺ pulsars. IP₃R-dense ER stores follow portions of the endothelial cell membrane that evaginate through holes in the IEL and interface with underlying SM cell membranes. Repetitive localized Ca²⁺ events (pulsars) originate from these deep Ca²⁺ stores, which are regionally delimited to the myoendothelial junction and the base of the endothelial cell. These ongoing dynamic Ca²⁺ signals are driven by constitutive IP₃ production and are inherently dependent on the level of endothelial stimulation. The left detail depicts a single endothelial projection through the IEL. IK (K_Ca3.1) channels in the plasma membranes of these endothelial projections are in very close proximity to Ca²⁺ pulsars, eliciting persistent Ca²⁺-dependent hyperpolarization of the membrane potential at the myoendothelial junctions. The right detail illustrates the endothelial influence on the smooth muscle membrane potential at the myoendothelial interaction site where Ca²⁺ pulsars activate K_Ca3.1 channels and hyperpolarize the endothelial membrane. This hyperpolarization can be transmitted to the SM through gap junction channels or by activation of SM K_ir channels by K⁺ ions released by endothelial K_Ca3.1 channels. Membrane hyperpolarization promotes relaxation of SM through a decrease in VDCC open probability. [From Ledoux et al. (180).]

Figure 10.

TRPV4 Ca²⁺ sparklets in endothelial cells initiate vasodilation. TRPV4 Ca²⁺ influx from the extracellular space through as few as 3–4 TRPV4 channels activates Ca²⁺-activated K⁺ (K_Ca) channels, resulting in K⁺ efflux from the cell. This hyperpolarizes the endothelial cell (EC) membrane, which directly hyperpolarizes smooth muscle cells (SMCs) via gap junctions located within myoendothelial junctions. IEL, internal elastic lamina; V_m, membrane potential. [From Sullivan and Earley (336).]

Figure 11.

TRP channels in perivascular cells. A: Ca²⁺ influx through TRPV1 or TRPA1 channels in perivascular nerves can stimulate the release of CGRP to cause arterial dilation. B: TRPV4 channels contribute to neurovascular coupling. Stimulation of metabotropic glutamate receptors (mGluR) on parenchymal astrocytes leads to the generation of IP₃, which causes the release of Ca²⁺ from the endoplasmic reticulum (ER), stimulating K⁺ efflux through BK channels. The resulting increase in [K⁺] in the space between the astrocytic endfoot and cerebral arteriole activates K⁺ efflux through K_IR channels present on vascular smooth muscle cells to cause dilation. Production of EETs or prostaglandins (e.g., PGE₂) may also contribute. Ca²⁺ influx through TRPV4 channels contributes to the propagation of Ca²⁺ waves throughout the endfoot.