Ca2+ signaling in vascular smooth muscle and endothelial cells in blood vessel remodeling: a review

Abstract

Vascular smooth muscle cells (VSMCs) and endothelial cells (ECs) act together to regulate blood pressure and systemic blood flow by appropriately adjusting blood vessel diameter in response to biochemical or biomechanical stimuli. Ion channels that are expressed in these cells regulate membrane potential and cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt) in response to such stimuli. The subsets of these ion channels involved in Ca²⁺ signaling often form molecular complexes with intracellular molecules via scaffolding proteins. This allows Ca²⁺ signaling to be tightly controlled in localized areas within the cell, resulting in a balanced vascular tone. When hypertensive stimuli are applied to blood vessels for extended periods, gene expression in these vascular cells can change dramatically. For example, alteration in ion channel expression often induces electrical remodeling that produces a depolarization of the membrane potential and elevated [Ca²⁺]_cyt. Coupled with endothelial dysfunction blood vessels undergo functional remodeling characterized by enhanced vasoconstriction. In addition, pathological challenges to vascular cells can induce inflammatory gene products that may promote leukocyte infiltration, in part through Ca²⁺-dependent pathways. Macrophages accumulating in the vascular adventitia promote fibrosis through extracellular matrix turnover, and cause structural remodeling of blood vessels. This functional and structural remodeling often leads to chronic hypertension affecting not only blood vessels, but also multiple organs including the brain, kidneys, and heart, thus increasing the risk of severe cardiovascular events. In this review, we outline recent advances in multidisciplinary research concerning Ca²⁺ signaling in VSMCs and ECs, with an emphasis on the mechanisms underlying functional and structural vascular remodeling.

Keywords: Ca2+ signaling; Endothelial cells; Hypertension; Macrophages; Monocytes; Vascular remodeling; Vascular smooth muscle cells.

Fig. 1

Progression and Classification of Vascular Remodeling. A When a stimulus is applied to an artery, stress factors (shear stress and circumferential wall stress) deviate from their normal steady state. The artery compensates for this perturbation by changing the vessel diameter through relaxation or contraction. If the stimulus is short-term, the change in vessel diameter is also temporary. B Conversely, if pro-hypertensive stimuli persists for a prolonged period, vascular component cells exhibit dramatic changes in transcription profiles, including ion channels that regulate intracellular Ca²⁺ signaling. This can result in electrical remodeling causing enhanced vasocontractility. At the same time, vasodilator capacity and endothelial barrier integrity are impaired. These changes cause functional remodeling. In addition, macrophages traffic to, and accumulate in the adventitia of blood vessels where they promote structural remodeling that enhances arterial stiffness and changes vessel diameter. In combination, these functional and structural remodeling lead to chronic hypertension. C Structural remodeling is classified on the basis of changes in vessel diameter and cross-sectional area. In hypertension, outward hypertrophic remodeling is observed in elastic arteries, whereas inward eutrophic remodeling or inward hypertrophic remodeling is observed in resistance arteries. In atherosclerosis, outward hypertrophic remodeling is observed in large vessels. EDHF: endothelium-derived hyperpolarizing factor, EGF: epidermal growth factor, ET-1: endothelin-1, NAd: noradrenaline

Fig. 2

Ca²⁺ Signaling in Healthy Vascular Smooth Muscle Cells (VSMCs) and Endothelial Cells (ECs). A Diagrammatic illustration of the structure of blood vessels. ECs form myoendothelial junctions with VSMCs through holes/slits in the internal elastic lamina (IEL). The blue squares marked B-E in this figure define areas which are emphasized in the following panel (B to E). B Ca²⁺ channels involved in VSMC contraction. Ca_v1.2 channel functions as the main Ca²⁺-permeable channel. Stretch-sensitive ion channels, TRPM4/TRPC6, can activate Ca_v1.2 channels. TRPV4 channels are activated downstream of α₁AR. Gq protein-coupled receptors produce IP₃, which activates IP₃R, and causes an increase in [Ca²⁺]_cyt. C Ca²⁺ sparks activate nearby BK_Ca channels to induce STOCs and hyperpolarize the membrane potential. Caveolin1 and junctophilin2 are expressed in close proximity to RyR2 and BK_Ca channels. TRPV4 channels increase STOCs either by supplying Ca²⁺ directly to BK_Ca channels, or indirectly via Ca²⁺ sparks. K_v channels can hyperpolarize the membrane potential and suppress the activity of Ca_v1.2 channels. D ACh activates IP₃R or TRPV4 channels and increases [Ca²⁺]_cyt, resulting in NO and EDHF production. Physiological shear stress also activates TRPV4 channels to promote NO production. Stretch or shear sensitive Piezo1 channels activate TRPV4 channels via pannexin1/P2Y2 to promote NO production. Caveolae are important for NO and EDHF production by TRPV4 channels. E At the myoendothelial junction, Ca²⁺ influx through TRPV4 channels activates IK_Ca channels and eNOS to produce EDHF and NO, respectively. K_ir2.1 channels amplify the hyperpolarization response by IK_Ca/SK_Ca channels. Gap junctions formed by connexin 37/40/43 transmit hyperpolarization to VSMCs. VSMCs are also hyperpolarized due to active K⁺ efflux mediated by Na⁺/K⁺-ATPase and K_ir2.1 channels. AT₁R: AngII receptor type 1, DAG: diacylglycerol, EDHF: endothelium-derived hyperpolarizing factor, ER: endoplasmic reticulum, PM: plasma membrane, SR: sarcoplasmic reticulum, STOC: spontaneous transient outward current, α₁AR: α₁ adrenergic receptor

Fig. 3

Electrical remodeling in hypertensive VSMCs and ECs. A In hypertensive VSMCs, the expression and activity (cluster formation) of Ca_v1.2 channels are enhanced. In addition, increased expression of AT₁R and IP₃R and increased formation of the α₁AR-TRPV4 complexes can increase VSMC contractility. B Decreased activity of BK_Ca channels and dissociation of TRPV4 channel reduce STOCs. K_v channel expression also decreases, resulting in depolarization of the membrane potential. C In ECs, dissociation of TRPV4 channel and eNOS reduces NO production. Activation of Piezo1 channels can cause sustained Ca²⁺ influx from TRPV4 channels due to sustained high shear stress, enhancing endothelial permeability. In addition, Piezo1 channels activate a NF-κB pathway mediated by panexin1/P2Y2 in response to turbulent laminar flow. D In myoendothelial junctions, downregulation or oxidation of AKAP150 reduce TRPV4 channel activity and reduce EDHF and NO production. K_ir2.1 channel and gap junction component proteins are also downregulated in hypertensive ECs and VSMCs localized at the myoendothelial junction. AA: arachidonic acid, AT₁R: AngII receptor type 1, EDHF: endothelium-derived hyperpolarizing factor, EET: epoxytrienoic acid, PLA₂: phospholipase A₂, PM: plasma membrane, SR: sarcoplasmic reticulum, STOC: spontaneous transient outward current, α₁AR: α₁ adrenergic receptor

Fig. 4

Structural Remodeling mediated by Interactions between Vascular Component and Immune Cells. A Pressure load changes on blood vessels and/or Ang II initiate or promote the production of chemokines from ECs, VSMCs, and adventitial fibroblasts, leading to monocyte and macrophage infiltration into the vessel wall. ROS produced by macrophages and vascular component cells cause vascular damage. B CD8⁺ T cells and probably Th1 cells accumulate in the vessel wall through CCL5-CCR5 interactions, where they produce IFNγ and TNFα. In addition, Th17 cells produce IL-17 and superoxide. These cytokines and ROS both can promote hypertension. C Elevated local blood flow in the mesenteric artery increases circumferential stress in the vascular wall by stretching VSMCs and depolarizing the membrane potential. The scaffolding protein caveolin1 mediates the formation of a molecular complex consisting of Ca_v1.2 channels, CaMKK2, and CaMK1. Ca²⁺ influx through Ca_v1.2 channels can activate CaMKK2 and CaMK1, and this causes CaMK1 to translocate to the nucleus where it induces the transcription of genes that encodes chemokines and adhesion molecules such as CXCL1, CCL2, P-selectin, and VCAM1. As a result, monocytes and macrophages accumulate in the adventitia, leading to outward remodeling. BP: blood pressure, CaMKK2: Ca²⁺/CaM dependent kinase kinase2, PM: plasma membrane