Evidence for functional and dynamic microcompartmentation of Cav-1/TRPV4/KCa in caveolae of endothelial cells

Endothelial mechanobiology in atherosclerosis

Cardiovascular disease (CVD) is a serious health challenge, causing more deaths worldwide than cancer. The vascular endothelium, which forms the inner lining of blood vessels, plays a central role in maintaining vascular integrity and homeostasis and is in direct contact with the blood flow. Research over the past century has shown that mechanical perturbations of the vascular wall contribute to the formation and progression of atherosclerosis. While the straight part of the artery is exposed to sustained laminar flow and physiological high shear stress, flow near branch points or in curved vessels can exhibit 'disturbed' flow. Clinical studies as well as carefully controlled in vitro analyses have confirmed that these regions of disturbed flow, which can include low shear stress, recirculation, oscillation, or lateral flow, are preferential sites of atherosclerotic lesion formation. Because of their critical role in blood flow homeostasis, vascular endothelial cells (ECs) have mechanosensory mechanisms that allow them to react rapidly to changes in mechanical forces, and to execute context-specific adaptive responses to modulate EC functions. This review summarizes the current understanding of endothelial mechanobiology, which can guide the identification of new therapeutic targets to slow or reverse the progression of atherosclerosis.

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Transient Receptor Potential Vanilloid 4: a Double-Edged Sword in the Central Nervous System

Transient receptor potential vanilloid 4 (TRPV4) is a nonselective cation channel that can be activated by diverse stimuli, such as heat, mechanical force, hypo-osmolarity, and arachidonic acid metabolites. TRPV4 is widely expressed in the central nervous system (CNS) and participates in many significant physiological processes. However, accumulative evidence has suggested that deficiency, abnormal expression or distribution, and overactivation of TRPV4 are involved in pathological processes of multiple neurological diseases. Here, we review the latest studies concerning the known features of this channel, including its expression, structure, and its physiological and pathological roles in the CNS, proposing an emerging therapeutic strategy for CNS diseases.

Treadmill exercise decreases cerebral edema in rats with local cerebral infarction by modulating AQP4 polar expression through the caveolin-1/TRPV4 signaling pathway

Rehabilitation therapy is beneficial for patients with ischemic stroke. Our previous study showed that treadmill training is conducive to neurological function in rats that underwent middle cerebral artery occlusion (MCAO). However, whether exercise benefits cerebral edema and the underlying mechanism remain unclear. This study investigated the influence of treadmill exercise on brain edema and the mechanism of its formation and elimination. The MCAO model was established with Sprague-Dawley (SD) rats, and lentivirus-mediated caveolin-1 shRNA was used to investigate the role of caveolin-1 in brain edema. As expected, we found that treadmill exercise has a beneficial effect on brain edema after stroke. Training led to a significant increase in the expression of caveolin-1 and TRPV4; and reduced brain water content and blood-brain barrier (BBB) damage. This treatment also changed the localization of aquaporin-4 (AQP4). Moreover, the effect of treadmill training on the polar expression of AQP4 differed over time. The results showed that early treadmill training inhibited the polar expression of AQP4, and later promoted its expression. However, the rats that were injected with the caveolin-1 shRNA lentivirus exhibited enhanced edema. Caveolin-1 shRNA eliminated the protective effect induced by exercise, which is consistent with the downregulation of TRPV4 expression. The findings indicate that treadmill training improves brain edema through the caveolin-1/TRPV4/AQP4 pathway.

Endothelial Cells and the Cerebral Circulation

Endothelial cells form the innermost layer of all blood vessels and are the only vascular component that remains throughout all vascular segments. The cerebral vasculature has several unique properties not found in the peripheral circulation; this requires that the cerebral endothelium be considered as a unique entity. Cerebral endothelial cells perform several functions vital for brain health. The cerebral vasculature is responsible for protecting the brain from external threats carried in the blood. The endothelial cells are central to this requirement as they form the basis of the blood-brain barrier. The endothelium also regulates fibrinolysis, thrombosis, platelet activation, vascular permeability, metabolism, catabolism, inflammation, and white cell trafficking. Endothelial cells regulate the changes in vascular structure caused by angiogenesis and artery remodeling. Further, the endothelium contributes to vascular tone, allowing proper perfusion of the brain which has high energy demands and no energy stores. In this article, we discuss the basic anatomy and physiology of the cerebral endothelium. Where appropriate, we discuss the detrimental effects of high blood pressure on the cerebral endothelium and the contribution of cerebrovascular disease endothelial dysfunction and dementia. © 2022 American Physiological Society. Compr Physiol 12:3449-3508, 2022.

Extremely low frequency electromagnetic stimulation reduces ischemic stroke volume by improving cerebral collateral blood flow

Extremely low frequency electromagnetic stimulation (ELF-EMS) has been considered as a neuroprotective therapy for ischemic stroke based on its capacity to induce nitric oxide (NO) signaling. Here, we examined whether ELF-EMS reduces ischemic stroke volume by stimulating cerebral collateral perfusion. Moreover, the pathway responsible for ELF-EMS-induced NO production was investigated. ELF-EMS diminished infarct growth following experimental stroke in collateral-rich C57BL/6 mice, but not in collateral-scarce BALB/c mice, suggesting that decreased lesion sizes after ELF-EMS results from improved collateral blood flow. In vitro analysis demonstrated that ELF-EMS increased endothelial NO levels by stimulating the Akt-/eNOS pathway. Furthermore, ELF-EMS augmented perfusion in the hind limb of healthy mice, which was mediated by enhanced Akt-/eNOS signaling. In healthy C57BL/6 mouse brains, ELF-EMS treatment increased cerebral blood flow in a NOS-dependent manner, whereas no improvement in cerebrovascular perfusion was observed in collateral-sparse BALB/c mice. In addition, ELF-EMS enhanced cerebral blood flow in both the contra- and ipsilateral hemispheres of C57BL/6 mice subjected to experimental ischemic stroke. In conclusion, we showed that ELF-EMS enhances (cerebro)vascular perfusion by stimulating NO production, indicating that ELF-EMS could be an attractive therapeutic strategy for acute ischemic stroke by improving cerebral collateral blood flow.

Histone deacetylase inhibitors (HDACi) increase expression of KCa2.3 (SK3) in primary microvascular endothelial cells

The small conductance calcium-activated potassium channel (KCa2.3) has long been recognized for its role in mediating vasorelaxation through the endothelium-derived hyperpolarization (EDH) response. Histone deacetylases (HDACs) have been implicated as potential modulators of blood pressure and histone deacetylase inhibitors (HDACi) are being explored as therapeutics for hypertension. Herein, we show that HDACi increase KCa2.3 expression when heterologously expressed in HEK cells and endogenously expressed in primary cultures of human umbilical vein endothelial cells (HUVECs) and human intestinal microvascular endothelial cells (HIMECs). When primary endothelial cells were exposed to HDACi, KCa2.3 transcripts, subunits, and functional current are increased. Quantitative RT-PCR (qPCR) demonstrated increased KCa2.3 mRNA following HDACi, confirming transcriptional regulation of KCa2.3 by HDACs. By using pharmacological agents selective for different classes of HDACs, we discriminated between cytoplasmic and epigenetic modulation of KCa2.3. Biochemical analysis revealed an association between the cytoplasmic HDAC6 and KCa2.3 in immunoprecipitation studies. Specifically inhibiting HDAC6 increases expression of KCa2.3. In addition to increasing the expression of KCa2.3, we show that nonspecific inhibition of HDACs causes an increase in the expression of the molecular chaperone Hsp70 in endothelial cells. When Hsp70 is inhibited in the presence of HDACi, the magnitude of the increase in KCa2.3 expression is diminished. Finally, we show a slower rate of endocytosis of KCa2.3 as a result of exposure of primary endothelial cells to HDACi. These data provide the first demonstrated approach to increase KCa2.3 channel number in endothelial cells and may partially account for the mechanism by which HDACi induce vasorelaxation.

TRPV4: Cell type-specific activation, regulation and function in the vertebrate eye

The architecture of the vertebrate eye is optimized for efficient delivery and transduction of photons and processing of signaling cascades downstream from phototransduction. The cornea, lens, retina, vasculature, ciliary body, ciliary muscle, iris and sclera have specialized functions in ocular protection, transparency, accommodation, fluid regulation, metabolism and inflammatory signaling, which are required to enable function of the retina-light sensitive tissue in the posterior eye that transmits visual signals to relay centers in the midbrain. This process can be profoundly impacted by non-visual stimuli such as mechanical (tension, compression, shear), thermal, nociceptive, immune and chemical stimuli, which target these eye regions to induce pain and precipitate vision loss in glaucoma, diabetic retinopathy, retinal dystrophies, retinal detachment, cataract, corneal dysfunction, ocular trauma and dry eye disease. TRPV4, a polymodal nonselective cation channel, integrate non-visual inputs with homeostatic and signaling functions of the eye. The TRPV4 gene is expressed in most if not all ocular tissues, which vary widely with respect to the mechanisms of TRPV4 channel activation, modulation, oligomerization, and participation in protein- and lipid interactions. Under- and overactivation of TRPV4 may affect intraocular pressure, maintenance of blood-retina barriers, lens accommodation, neuronal function and neuroinflammation. Because TRPV4 dysregulation precipitates many pathologies across the anterior and posterior eye, the channel could be targeted to mitigate vision loss.

Coronary Large Conductance Ca2+-Activated K+ Channel Dysfunction in Diabetes Mellitus

Diabetes mellitus (DM) is an independent risk of macrovascular and microvascular complications, while cardiovascular diseases remain a leading cause of death in both men and women with diabetes. Large conductance Ca2+-activated K+ (BK) channels are abundantly expressed in arteries and are the key ionic determinant of vascular tone and organ perfusion. It is well established that the downregulation of vascular BK channel function with reduced BK channel protein expression and altered intrinsic BK channel biophysical properties is associated with diabetic vasculopathy. Recent efforts also showed that diabetes-associated changes in signaling pathways and transcriptional factors contribute to the downregulation of BK channel expression. This manuscript will review our current understandings on the molecular, physiological, and biophysical mechanisms that underlie coronary BK channelopathy in diabetes mellitus.

The Transient Receptor Potential Vanilloid 4 Channel and Cardiovascular Disease Risk Factors

Vascular endothelial cells regulate arterial tone through the release of nitric oxide and other diffusible factors such as prostacyclin and endothelium derived hyperpolarizing factors. Alongside these diffusible factors, contact-mediated electrical propagation from endothelial cells to smooth muscle cells via myoendothelial gap junctions, termed endothelium-dependent hyperpolarization (EDH), plays a critical role in endothelium-dependent vasodilation in certain vascular beds. A rise in intracellular Ca²⁺ concentration in endothelial cells is a prerequisite for both the production of diffusible factors and the generation of EDH, and Ca²⁺ influx through the endothelial transient receptor potential vanilloid 4 (TRPV4) ion channel, a nonselective cation channel of the TRP family, plays a critical role in this process in various vascular beds. Emerging evidence suggests that the dysregulation of endothelial TRPV4 channels underpins endothelial dysfunction associated with cardiovascular disease (CVD) risk factors, including hypertension, obesity, diabetes, and aging. Because endothelial dysfunction is a precursor to CVD, a better understanding of the mechanisms underlying impaired TRPV4 channels could lead to novel therapeutic strategies for CVD prevention. In this mini review, we present the current knowledge of the pathophysiological changes in endothelial TRPV4 channels associated with CVD risk factors, and then explore the underlying mechanisms involved.

Endothelial pannexin 1-TRPV4 channel signaling lowers pulmonary arterial pressure in mice

Pannexin 1 (Panx1), an ATP-efflux pathway, has been linked with inflammation in pulmonary capillaries. However, the physiological roles of endothelial Panx1 in the pulmonary vasculature are unknown. Endothelial transient receptor potential vanilloid 4 (TRPV4) channels lower pulmonary artery (PA) contractility and exogenous ATP activates endothelial TRPV4 channels. We hypothesized that endothelial Panx1–ATP–TRPV4 channel signaling promotes vasodilation and lowers pulmonary arterial pressure (PAP). Endothelial, but not smooth muscle, knockout of Panx1 increased PA contractility and raised PAP in mice. Flow/shear stress increased ATP efflux through endothelial Panx1 in PAs. Panx1-effluxed extracellular ATP signaled through purinergic P2Y2 receptor (P2Y2R) to activate protein kinase Cα (PKCα), which in turn activated endothelial TRPV4 channels. Finally, caveolin-1 provided a signaling scaffold for endothelial Panx1, P2Y2R, PKCα, and TRPV4 channels in PAs, promoting their spatial proximity and enabling signaling interactions. These results indicate that endothelial Panx1–P2Y2R–TRPV4 channel signaling, facilitated by caveolin-1, reduces PA contractility and lowers PAP in mice.

Puerarin induces mouse mesenteric vasodilation and ameliorates hypertension involving endothelial TRPV4 channels

Puerarin (Pue) is an isoflavone derived from the root of Pueraria lobata, which has been widely used as food and a herb for treating cardiovascular and cerebrovascular diseases. Transient receptor potential vanilloid 4 (TRPV4), a Ca2+-permeable channel with multiple modes of activation, plays an important role in vascular endothelial function and vasodilation. However, no reports have shown the effects of Pue on TRPV4 channels and mouse small mesenteric arteries. In the present study, we performed a molecular docking assay by using Discovery Studio 3.5 software to predict the binding of Pue to TRPV4 protein. The activation of TRPV4 by Pue was determined by intracellular Ca2+ concentration ([Ca2+]i), live-cell fluorescent Ca2+ imaging and patch clamp assays. Molecular docking results indicated a high possibility of Pue-TPRV4 binding. [Ca2+]i and Ca2+ imaging assays showed that Pue activated TRPV4 channels and increased [Ca2+]i in TRPV4-overexpressing HEK293 (TRPV4-HEK293) cells and primary mouse mesenteric artery endothelial cells (MAECs). Patch clamp assay demonstrated that Pue stimulated the TRPV4-mediated cation currents. Additionally, Pue relaxed mouse mesenteric arteries involving the TRPV4-small-conductance Ca2+-activated K+ channel (SKCa)/intermediate-conductance Ca2+-activated K+ channel (IKCa) pathway, and reduced systolic blood pressure (SBP) in high-salt-induced hypertensive mice. Our study found for the first time that Pue acts as a TRPV4 agonist, induces endothelium-dependent vasodilation in mouse mesenteric arteries, and attenuates blood pressure in high-salt-induced hypertensive mice, highlighting the beneficial effect of Pue in treating endothelial dysfunction-related cardiovascular diseases.

Role of Transient Receptor Potential Vanilloid 4 in Vascular Function

Transient receptor potential vanilloid 4 (TRPV4) channels are widely expressed in systemic tissues and can be activated by many stimuli. TRPV4, a Ca²⁺-permeable cation channel, plays an important role in the vasculature and is implicated in the regulation of cardiovascular homeostasis processes such as blood pressure, vascular remodeling, and pulmonary hypertension and edema. Within the vasculature, TRPV4 channels are expressed in smooth muscle cells, endothelial cells, and perivascular nerves. The activation of endothelial TRPV4 contributes to vasodilation involving nitric oxide, prostacyclin, and endothelial-derived hyperpolarizing factor pathways. TRPV4 activation also can directly cause vascular smooth muscle cell hyperpolarization and vasodilation. In addition, TRPV4 activation can evoke constriction in some specific vascular beds or under some pathological conditions. TRPV4 participates in the control of vascular permeability and vascular damage, particularly in the lung capillary endothelial barrier and lung injury. It also participates in vascular remodeling regulation mainly by controlling vasculogenesis and arteriogenesis. This review examines the role of TRPV4 in vascular function, particularly in vascular dilation and constriction, vascular permeability, vascular remodeling, and vascular damage, along with possible mechanisms, and discusses the possibility of targeting TRPV4 for therapy.

Moderate mechanical stimulation rescues degenerative annulus fibrosus by suppressing caveolin-1 mediated pro-inflammatory signaling pathway

Mechanical loading can induce or antagonize the extracellular matrix (ECM) synthesis, proliferation, migration, and inflammatory responses of annulus fibrosus cells (AFCs), depending on the loading mode and level. Caveolin-1 (Cav1), the core protein of caveolae, plays an important role in cellular mechanotransduction and inflammatory responses. In the present study, we presented that AFCs demonstrated different behaviors when subjected to cyclic tensile strain (CTS) for 24 h at a magnitude of 0%, 2%, 5% and 12%, respectively. It was found that 5% CTS had positive effects on cell proliferation, migration and anabolism, while 12% CTS had the opposite effects. Besides, cells exposed to interleukin-1β stimulus exhibited an increase expression in inflammatory genes, and the expression of these genes decreased after exposure to moderate mechanical loading with 5% CTS. In addition, 5% CTS decreased the level of Cav1 and integrin β1 and exhibited anti-inflammatory effects. Moreover, the expression of integrin β1 and p-p65 increased in AFCs transfected with Cav1 plasmids. In vivo results revealed that moderate mechanical stimulation could recover the water content and morphology of the discs. In conclusion, moderate mechanical stimulation restrained Cav1-mediated signaling pathway and exhibited anti-inflammatory effects on AFCs. Together with in vivo results, this study expounds the underlying molecular mechanisms on the effect of moderate mechanical stimulation on intervertebral discs (IVDs) and may provide a new therapeutic strategy for the treatment of IVD degeneration.

Polarized Proteins in Endothelium and Their Contribution to Function

Protein localization in endothelial cells is tightly regulated to create distinct signaling domains within their tight spatial restrictions including luminal membranes, abluminal membranes, and interendothelial junctions, as well as caveolae and calcium signaling domains. Protein localization in endothelial cells is also determined in part by the vascular bed, with differences between arteries and veins and between large and small arteries. Specific protein polarity and localization is essential for endothelial cells in responding to various extracellular stimuli. In this review, we examine protein localization in the endothelium of resistance arteries, with occasional references to other vessels for contrast, and how that polarization contributes to endothelial function and ultimately whole organism physiology. We highlight the protein localization on the luminal surface, discussing important physiological receptors and the glycocalyx. The protein polarization to the abluminal membrane is especially unique in small resistance arteries with the presence of the myoendothelial junction, a signaling microdomain that regulates vasodilation, feedback to smooth muscle cells, and ultimately total peripheral resistance. We also discuss the interendothelial junction, where tight junctions, adherens junctions, and gap junctions all convene and regulate endothelial function. Finally, we address planar cell polarity, or axial polarity, and how this is regulated by mechanosensory signals like blood flow.

Chloride intracellular channel 1 cooperates with potassium channel EAG2 to promote medulloblastoma growth

Ion channels represent a large class of drug targets, but their role in brain cancer is underexplored. Here, we identify that chloride intracellular channel 1 (CLIC1) is overexpressed in human central nervous system malignancies, including medulloblastoma, a common pediatric brain cancer. While global knockout does not overtly affect mouse development, genetic deletion of CLIC1 suppresses medulloblastoma growth in xenograft and genetically engineered mouse models. Mechanistically, CLIC1 enriches to the plasma membrane during mitosis and cooperates with potassium channel EAG2 at lipid rafts to regulate cell volume homeostasis. CLIC1 deficiency is associated with elevation of cell/nuclear volume ratio, uncoupling between RNA biosynthesis and cell size increase, and activation of the p38 MAPK pathway that suppresses proliferation. Concurrent knockdown of CLIC1/EAG2 and their evolutionarily conserved channels synergistically suppressed the growth of human medulloblastoma cells and Drosophila melanogaster brain tumors, respectively. These findings establish CLIC1 as a molecular dependency in rapidly dividing medulloblastoma cells, provide insights into the mechanism by which CLIC1 regulates tumorigenesis, and reveal that targeting CLIC1 and its functionally cooperative potassium channel is a disease-intervention strategy.

Dysregulation of TRPV4, eNOS and caveolin-1 contribute to endothelial dysfunction in the streptozotocin rat model of diabetes

Endothelial dysfunction is a common complication in diabetes in which endothelium-dependent vasorelaxation is impaired. The aim of this study was to examine the involvement of the TRPV4 ion channel in type 1 diabetic endothelial dysfunction and the possible association of endothelial dysfunction with reduced expression of TRPV4, endothelial nitric oxide synthase (eNOS) and caveolin-1. Male Wistar rats (350-450 g) were injected with 65 mg/kg i.p. streptozotocin (STZ) or vehicle. Endothelial function was investigated in aortic rings and mesenteric arteries using organ bath and myograph, respectively. TRPV4 function was studied with fura-2 calcium imaging in endothelial cells cultured from aortas from control and STZ treated rats. TRPV4, caveolin-1 and eNOS expression was investigated in these cells using immunohistochemistry. STZ-treated diabetic rats showed significant endothelial dysfunction characterised by impaired muscarinic-induced vasorelaxation (aortic rings: STZ-diabetics: Emax = 29.6 ± 9.3%; control: Emax = 77.2 ± 2.5% P˂0.001), as well as significant impairment in TRPV4-induced vasorelaxation (aortic rings, 4αPDD STZ-diabetics: Emax = 56.0 ± 5.5%; control: Emax = 81.1 ± 2.1% P˂0.001). Furthermore, STZ-diabetic primary aortic endothelial cells showed a significant reduction in TRPV4-induced intracellular calcium elevation (P˂0.05) compared with the control group. This was associated with significantly lower expression of TRPV4, caveolin-1 and eNOS and this was reversed by insulin treatment of the endothelial cultures from STZ -diabetic rats. Taken together, these data are consistent with the hypothesis that signalling through TRPV4, caveolin-1, and eNOS is downregulated in STZ-diabetic aortic endothelial cells and restored by insulin treatment.

Fluid flow as a driver of embryonic morphogenesis

Fluid flow is a powerful morphogenic force during embryonic development. The physical forces created by flowing fluids can either create morphogen gradients or be translated by mechanosensitive cells into biological changes in gene expression. In this Primer, we describe how fluid flow is created in different systems and highlight the important mechanosensitive signalling pathways involved for sensing and transducing flow during embryogenesis. Specifically, we describe how fluid flow helps establish left-right asymmetry in the early embryo and discuss the role of flow of blood, lymph and cerebrospinal fluid in sculpting the embryonic cardiovascular and nervous system.

The Importance of Mechanical Forces for in vitro Endothelial Cell Biology

Blood and lymphatic vessels are lined by endothelial cells which constantly interact with their luminal and abluminal extracellular environments. These interactions confer physical forces on the endothelium, such as shear stress, stretch and stiffness, to mediate biological responses. These physical forces are often altered during disease, driving abnormal endothelial cell behavior and pathology. Therefore, it is critical that we understand the mechanisms by which endothelial cells respond to physical forces. Traditionally, endothelial cells in culture are grown in the absence of flow on stiff substrates such as plastic or glass. These cells are not subjected to the physical forces that endothelial cells endure in vivo, thus the results of these experiments often do not mimic those observed in the body. The field of vascular biology now realize that an intricate analysis of endothelial signaling mechanisms requires complex in vitro systems to mimic in vivo conditions. Here, we will review what is known about the mechanical forces that guide endothelial cell behavior and then discuss the advancements in endothelial cell culture models designed to better mimic the in vivo vascular microenvironment. A wider application of these technologies will provide more biologically relevant information from cultured cells which will be reproducible to conditions found in the body.

Mechanisms underlying selective coupling of endothelial Ca2+ signals with eNOS vs. IK/SK channels in systemic and pulmonary arteries

KEY POINTS

Endothelial cell TRPV4 (TRPV4_EC ) channels exert a dilatory effect on the resting diameter of resistance mesenteric and pulmonary arteries. Functional intermediate- and small-conductance K⁺ (IK and SK) channels and endothelial nitric oxide synthase (eNOS) are present in the endothelium of mesenteric and pulmonary arteries. TRPV4_EC sparklets preferentially couple with IK/SK channels in mesenteric arteries and with eNOS in pulmonary arteries. TRPV4_EC channels co-localize with IK/SK channels in mesenteric arteries but not in pulmonary arteries, which may explain TRPV4_EC -IK/SK channel coupling in mesenteric arteries and its absence in pulmonary arteries. The presence of the nitric oxide-scavenging protein, haemoglobin α, limits TRPV4_EC -eNOS signalling in mesenteric arteries. Spatial proximity of TRPV4_EC channels with eNOS and the absence of haemoglobin α favour TRPV4_EC -eNOS signalling in pulmonary arteries.

ABSTRACT

Spatially localized Ca²⁺ signals activate Ca²⁺ -sensitive intermediate- and small-conductance K⁺ (IK and SK) channels in some vascular beds and endothelial nitric oxide synthase (eNOS) in others. The present study aimed to uncover the signalling organization that determines selective Ca²⁺ signal to vasodilatory target coupling in the endothelium. Resistance-sized mesenteric arteries (MAs) and pulmonary arteries (PAs) were used as prototypes for arteries with predominantly IK/SK channel- and eNOS-dependent vasodilatation, respectively. Ca²⁺ influx signals through endothelial transient receptor potential vanilloid 4 (TRPV4_EC ) channels played an important role in controlling the baseline diameter of both MAs and PAs. TRPV4_EC channel activity was similar in MAs and PAs. However, the TRPV4 channel agonist GSK1016790A (10 nm) selectively activated IK/SK channels in MAs and eNOS in PAs, revealing preferential TRPV4_EC -IK/SK channel coupling in MAs and TRPV4_EC -eNOS coupling in PAs. IK/SK channels co-localized with TRPV4_EC channels at myoendothelial projections (MEPs) in MAs, although they lacked the spatial proximity necessary for their activation by TRPV4_EC channels in PAs. Additionally, the presence of the NO scavenging protein haemoglobin α (Hbα) within nanometer proximity to eNOS limits TRPV4_EC -eNOS signalling in MAs. By contrast, co-localization of TRPV4_EC channels and eNOS at MEPs, and the absence of Hbα, favour TRPV4_EC -eNOS coupling in PAs. Thus, our results reveal that differential spatial organization of signalling elements determines TRPV4_EC -IK/SK vs. TRPV4_EC -eNOS coupling in resistance arteries.

In vitro study of carbon black nanoparticles on human pulmonary artery endothelial cells: effects on calcium signaling and mitochondrial alterations

Human exposure to manufactured nanoparticles (NPs) is a public health concern. Endothelial cells lining the inner surface of arteries could be one of the primary targets for inhaled nanoparticles. Moreover, it is well known that alteration in calcium signaling is a critical event involved in the physiopathology of cardiovascular diseases. The objective of this study was to assess the role of oxidative stress in carbon black FW2 NPs-induced alteration in calcium signaling and mitochondria in human pulmonary artery endothelial cells. To this end, cells were exposed for 4 or 24 h to FW2 NPs (1-10 μg/cm²) and the following endpoints were studied: (i) production of ROS by fluorimetry and electron paramagnetic resonance, (ii) variation in intracellular calcium concentration by confocal microscopy, and (iii) mitochondrial alteration and apoptosis by confocal microscopy and transmission electronic microscopy. Exposure to FW2 NPs concentration-dependently increases oxidative stress, evidenced by the production of superoxide anion leading to an alteration in calcium content of intracellular organelles, such as endoplasmic reticulum and mitochondria activating, in turn, intrinsic apoptosis. This study provides evidence that FW2 NPs exposure impairs calcium signaling and mitochondria triggered by oxidative stress, and, thus, could act as a cardiovascular disease risk owing to the key role of calcium homeostasis in the control of vascular tone.

Caveolae in CNS arterioles mediate neurovascular coupling

Proper brain function depends on neurovascular coupling: neural activity rapidly increases local blood flow to meet moment-to-moment changes in regional brain energy demand¹. Neurovascular coupling is the basis for functional brain imaging², and its impairment is implicated in neurodegeneration¹. The underlying molecular and cellular mechanisms of neurovascular coupling remain poorly understood. The conventional view is that neurons or astrocytes release vasodilatory factors that act directly on smooth muscle cells (SMC) to induce arterial dilation and increase local blood flow¹. Here, using two-photon microscopy to image neural activity and vascular dynamics simultaneously in the barrel cortex of awake mice under whisker stimulation, we found that arteriolar endothelial cells (aECs) play an active role in mediating neurovascular coupling. We found that aECs, unlike other vascular segments of ECs in the CNS, have abundant caveolae. Acute genetic perturbations that eliminated caveolae in aECs, but not in neighboring SMCs, impaired neurovascular coupling. Strikingly, caveolae function in aECs is independent of the eNOS-mediated nitric oxide (NO) pathway. Ablation of both caveolae and eNOS completely abolished neurovascular coupling, whereas each single mutant exhibited partial impairment, revealing that caveolae-mediated pathway in aECs is a major contributor to neurovascular coupling. Our findings indicate that vasodilation is largely due to ECs that actively relay signals from the CNS to SMCs via a caveolae-dependent pathway.

Endothelial TRPV4 channels and vasodilator reactivity

Transient receptor potential vanilloid 4 (TRPV4) ion channels on the endothelial cell membrane are widely regarded as a crucial Ca²⁺ influx pathway that promotes endothelium-dependent vasodilation. The downstream vasodilatory targets of endothelial TRPV4 channels vary among different vascular beds, potentially contributing to endothelial cell heterogeneity. Although numerous studies have examined the role of endothelial TRPV4 channels using specific pharmacological tools over the past decade, their physiological significance remains unclear, mainly due to a lack of endothelium-specific knockouts. Moreover, the loss of endothelium-dependent vasodilation is a significant contributor to vascular dysfunction in cardiovascular disease. The activity of endothelial TRPV4 channels is impaired in cardiovascular disease; therefore, strategies targeting the mechanisms that reduce endothelial TRPV4 channel activity may restore vascular function and provide therapeutic benefit. In this chapter, we discuss endothelial TRPV4 channel-dependent signaling mechanisms, the heterogeneity in endogenous activators and targets of endothelial TRPV4 channels, and the role of endothelial TRPV4 channels in the pathogenesis of cardiovascular diseases. We also discuss potentially interesting future research directions that may provide novel insights into the physiological and pathological roles of endothelial TRPV4 channels.

Internalization and Transportation of Endothelial Cell Surface KCa2.3 and KCa3.1 in Normal Pregnancy and Preeclampsia

Altered redox state modulates the expression levels of endothelial K_Ca2.3 and K_Ca3.1 (K_Cas) in normal pregnancy (NP) and preeclampsia (PE), thereby regulating vascular contractility. The mechanisms underlying K_Cas endocytosis and transportation remain unknown. We investigated the regulation of K_Cas expression in plasma membrane (PM) during NP and PE. Cultured human uterine artery endothelial cells were incubated in serum from normal nonpregnant women and women with NP or PE, or in oxidized LDL-, or lysophosphatidylcholine- (LPC-) containing a medium for 24 hours. NP serum elevated PM levels of K_Cas and reduced caveolin-1 and clathrin levels. PE serum, oxidized LDL, or LPC reduced PM levels of K_Cas and elevated caveolin-1, clathrin, Rab5c, and early endosome antigen-1 (EEA1) levels. Reduced K_Cas levels by PE serum or LPC were reversed by inhibition of caveolin-1, clathrin, or EEA1. Catalase and glutathione peroxidase 1 (GPX1) knockdown elevated PM-localized K_Cas levels and reduced caveolin-1 and clathrin levels. Elevated K_Ca2.3 levels upon catalase and GPX1 knockdown were reversed by PEG-catalase treatment. An H₂O₂ donor reduced clathrin and Rab5c. In contrast, elevated clathrin, caveolin-1, or colocalization of caveolin-1 with K_Ca3.1 by PE serum or LPC was reversed by NADPH oxidase inhibitors or antioxidants. A superoxide donor xanthine+xanthine oxidase elevated caveolin-1 or Rab5c levels. We concluded that K_Cas are endocytosed in a caveola- or a clathrin-dependent manner and transported in a Rab5c- and EEA1-dependent manner during pregnancy. The endocytosis and transportation processes may slow down via H₂O₂-mediated pathways in NP and may be accelerated via superoxide-mediated pathways in PE.

Regulation of mitochondrial fragmentation in microvascular endothelial cells isolated from the SU5416/hypoxia model of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a morbid disease characterized by progressive right ventricle (RV) failure due to elevated pulmonary artery pressures (PAP). In PAH, histologically complex vaso-occlusive lesions in the pulmonary vasculature contribute to elevated PAP. However, the mechanisms underlying dysfunction of the microvascular endothelial cells (MVECs) that comprise a significant portion of these lesions are not well understood. We recently showed that MVECs isolated from the Sugen/hypoxia (SuHx) rat experimental model of PAH (SuHx-MVECs) exhibit increases in migration/proliferation, mitochondrial reactive oxygen species (ROS; mtROS) production, intracellular calcium levels ([Ca2+]i), and mitochondrial fragmentation. Furthermore, quenching mtROS with the targeted antioxidant MitoQ attenuated basal [Ca2+]i, migration and proliferation; however, whether increased mtROS-induced [Ca2+]i entry affected mitochondrial morphology was not clear. In this study, we sought to better understand the relationship between increased ROS, [Ca2+]i, and mitochondrial morphology in SuHx-MVECs. We measured changes in mitochondrial morphology at baseline and following inhibition of mtROS, with the targeted antioxidant MitoQ, or transient receptor potential vanilloid-4 (TRPV4) channels, which we previously showed were responsible for mtROS-induced increases in [Ca2+]i in SuHx-MVECs. Quenching mtROS or inhibiting TRPV4 attenuated fragmentation in SuHx-MVECs. Conversely, inducing mtROS production in MVECs from normoxic rats (N-MVECs) increased fragmentation. Ca2+ entry induced by the TRPV4 agonist GSK1017920A was significantly increased in SuHx-MVECs and was attenuated with MitoQ treatment, indicating that mtROS contributes to increased TRPV4 activity in SuHx-MVECs. Basal and maximal respiration were depressed in SuHx-MVECs, and inhibiting mtROS, but not TRPV4, improved respiration in these cells. Collectively, our data show that, in SuHx-MVECs, mtROS production promotes TRPV4-mediated increases in [Ca2+]i, mitochondrial fission, and decreased mitochondrial respiration. These results suggest an important role for mtROS in driving MVEC dysfunction in PAH.

Biomechanical Forces and Oxidative Stress: Implications for Pulmonary Vascular Disease

Significance: Oxidative stress in the cell is characterized by excessive generation of reactive oxygen species (ROS). Superoxide (O₂^-) and hydrogen peroxide (H₂O₂) are the main ROS involved in the regulation of cellular metabolism. As our fundamental understanding of the underlying causes of lung disease has increased it has become evident that oxidative stress plays a critical role. Recent Advances: A number of cells in the lung both produce, and respond to, ROS. These include vascular endothelial and smooth muscle cells, fibroblasts, and epithelial cells as well as the cells involved in the inflammatory response, including macrophages, neutrophils, eosinophils. The redox system is involved in multiple aspects of cell metabolism and cell homeostasis. Critical Issues: Dysregulation of the cellular redox system has consequential effects on cell signaling pathways that are intimately involved in disease progression. The lung is exposed to biomechanical forces (fluid shear stress, cyclic stretch, and pressure) due to the passage of blood through the pulmonary vessels and the distension of the lungs during the breathing cycle. Cells within the lung respond to these forces by activating signal transduction pathways that alter their redox state with both physiologic and pathologic consequences. Future Directions: Here, we will discuss the intimate relationship between biomechanical forces and redox signaling and its role in the development of pulmonary disease. An understanding of the molecular mechanisms induced by biomechanical forces in the pulmonary vasculature is necessary for the development of new therapeutic strategies.

Endothelium-Dependent Hyperpolarization (EDH) in Diabetes: Mechanistic Insights and Therapeutic Implications

Diabetes mellitus is one of the major risk factors for cardiovascular disease and is an important health issue worldwide. Long-term diabetes causes endothelial dysfunction, which in turn leads to diabetic vascular complications. Endothelium-derived nitric oxide is a major vasodilator in large-size vessels, and the hyperpolarization of vascular smooth muscle cells mediated by the endothelium plays a central role in agonist-mediated and flow-mediated vasodilation in resistance-size vessels. Although the mechanisms underlying diabetic vascular complications are multifactorial and complex, impairment of endothelium-dependent hyperpolarization (EDH) of vascular smooth muscle cells would contribute at least partly to the initiation and progression of microvascular complications of diabetes. In this review, we present the current knowledge about the pathophysiology and underlying mechanisms of impaired EDH in diabetes in animals and humans. We also discuss potential therapeutic approaches aimed at the prevention and restoration of EDH in diabetes.

Transient Receptor Potential Channels and Endothelial Cell Calcium Signaling

The vascular endothelium is a broadly distributed and highly specialized organ. The endothelium has a number of functions including the control of blood vessels diameter through the production and release of potent vasoactive substances or direct electrical communication with underlying smooth muscle cells, regulates the permeability of the vascular barrier, stimulates the formation of new blood vessels, and influences inflammatory and thrombotic processes. Endothelial cells that make up the endothelium express a variety of cell-surface receptors and ion channels on the plasma membrane that are capable of detecting circulating hormones, neurotransmitters, oxygen tension, and shear stress across the vascular wall. Changes in these stimuli activate signaling cascades that initiate an appropriate physiological response. Increases in the global intracellular Ca²⁺ concentration and localized Ca²⁺ signals that occur within specialized subcellular microdomains are fundamentally important components of many signaling pathways in the endothelium. The transient receptor potential (TRP) channels are a superfamily of cation-permeable ion channels that act as a primary means of increasing cytosolic Ca²⁺ in endothelial cells. Consequently, TRP channels are vitally important for the major functions of the endothelium. In this review, we provide an in-depth discussion of Ca²⁺ -permeable TRP channels in the endothelium and their role in vascular regulation. © 2019 American Physiological Society. Compr Physiol 9:1249-1277, 2019.

Caveolae facilitate TRPV4-mediated Ca2+ signaling and the hierarchical activation of Ca2+-activated K+ channels in K+-secreting renal collecting duct cells

Transient receptor potential cation channel subfamily V member 4 (TRPV4)-mediated Ca²⁺ signaling induces early activation of small/intermediate Ca²⁺-activated K⁺ channels, SK3 (KCNN3) and IK1 (KCNN4), which leads to membrane hyperpolarization and enhanced Ca²⁺ influx, which is critical for subsequent activation of the large conductance Ca²⁺-activated K⁺ channel BK (KCNMA1) and K⁺ secretion in kidney cortical collecting duct (CCD) cells. The focus of the present study was to determine if such coordinated hierarchical/sequential activation of these channels in CCD was orchestrated within caveolae, a known microcompartment underlying selective Ca²⁺-signaling events in other cells. In K⁺-secreting mouse principal cell (PC) line, mCCDcl1 cells, knockdown of caveolae caveolin-1 (CAV-1) depressed TRPV4-mediated Ca²⁺ signaling and activation of SK3, intermediate conductance channel (IK1), and BK. Immunofluorescence colocalization analysis and coimmunoprecipitation assays demonstrated direct coupling of TRPV4 with each of the KCa channels in both mCCDcl1 and whole mouse kidney homogenates. Likewise, extending this analysis to CAV-1 demonstrates colocalization and direct coupling of CAV-1 with TRPV4, SK3, IK1, and BK, providing strong support for coupling of the channels in caveolae microdomains. Furthermore, differential expression of CAV-1 along the CCD was apparent where CAV-1 was strongly expressed within and along the cell borders of kidney PCs and intercalated cells (ICs), although significantly less in ICs. It is concluded that caveolae provide a key microdomain in PCs and ICs for coupling of TRPV4 with SK3, IK1, and BK that directly contributes to TRPV4-mediated Ca²⁺ signaling in these domains leading to rapid and sequential coupling of TRPV4-SK3/IK1-BK that may play a central role in mediating Ca²⁺-dependent regulation of BK and K⁺ secretion.

Treatment of hypertension by increasing impaired endothelial TRPV4-KCa2.3 interaction

The currently available antihypertensive agents have undesirable adverse effects due to systemically altering target activity including receptors, channels, and enzymes. These effects, such as loss of potassium ions induced by diuretics, bronchospasm by beta‐blockers, constipation by Ca²⁺ channel blockers, and dry cough by ACEI, lead to non‐compliance with therapies (Moser, 1990). Here, based on new hypertension mechanisms, we explored a new antihypertensive approach. We report that transient receptor potential vanilloid 4 (TRPV4) interacts with Ca²⁺‐activated potassium channel 3 (KCa2.3) in endothelial cells (ECs) from small resistance arteries of normotensive humans, while ECs from hypertensive patients show a reduced interaction between TRPV4 and KCa2.3. Murine hypertension models, induced by high‐salt diet, N(G)‐nitro‐l‐arginine intake, or angiotensin II delivery, showed decreased TRPV4‐KCa2.3 interaction in ECs. Perturbation of the TRPV4‐KCa2.3 interaction in mouse ECs by overexpressing full‐length KCa2.3 or defective KCa2.3 had hypotensive or hypertensive effects, respectively. Next, we developed a small‐molecule drug, JNc‐440, which showed affinity for both TRPV4 and KCa2.3. JNc‐440 significantly strengthened the TRPV4‐KCa2.3 interaction in ECs, enhanced vasodilation, and exerted antihypertensive effects in mice. Importantly, JNc‐440 specifically targeted the impaired TRPV4‐KCa2.3 interaction in ECs but did not systemically activate TRPV4 and KCa2.3. Together, our data highlight the importance of impaired endothelial TRPV4‐KCa2.3 coupling in the progression of hypertension and suggest a novel approach for antihypertensive drug development.

Inhibition of transient receptor potential vanilloid 4 decreases the expressions of caveolin-1 and caveolin-2 after focal cerebral ischemia and reperfusion in rats

This study aimed to investigate the effects of transient receptor potential vanilloid 4 (TRPV4) inhibition on blood-brain barrier (BBB) integrity and the expressions of caveolae structural proteins caveolin-1 and caveolin-2 in rats with focal cerebral ischemia and reperfusion. BBB permeability was assessed by Evans blue extravasation. The mRNA and protein expressions of caveolin-1 and caveolin-2 were determined by RT-PCR, Western blot and immunohistochemistry assays. We found that BBB permeability significantly increased and reaches its peak at 72 h of reperfusion in cerebral ischemia-reperfusion rats and is able to be ameliorated by administration of HC-067047, an antagonist of TRPV4. Additionally, it shows a significant upregulation of caveolin-1 and caveolin-2 expression in cerebral microvessels of ischemic tissue. However, treatment with HC-067047 was shown to downregulate caveolin-1 and caveolin-2 expression during cerebral ischemia-reperfusion. This study demonstrates that inhibition of TRPV4 ameliorates BBB leakage induced by ischemia-reperfusion injury through the downregulation of caveolin-1 and caveolin-2.

Vasoconstrictor stimulus determines the functional contribution of myoendothelial feedback to mesenteric arterial tone

KEY POINTS

In isolated resistance arteries, endothelial modulation of vasoconstrictor responses to α₁ -adrenoceptor agonists occurs via a process termed myoendothelial feedback: localized inositol trisphosphate (InsP₃ )-dependent Ca²⁺ transients activate intermediate conductance Ca²⁺ -activated K⁺ (IK_Ca ) channels, hyperpolarizing the endothelial membrane potential to limit further reductions in vessel diameter. We demonstrate that IK_Ca channel-mediated myoendothelial feedback limits responses of isolated mesenteric arteries to noradrenaline and nerve stimulation, but not to the thromboxane A₂ mimetic U46619 or to increases in intravascular pressure. In contrast, in the intact mesenteric bed, although responses to exogenous noradrenaline were limited by IK_Ca channel-mediated myoendothelial feedback, release of NO and activation of endothelial small conductance Ca²⁺ -activated K⁺ (SK_Ca ) channels in response to increases in shear stress appeared to be the primary mediators of endothelial modulation of vasoconstriction. We propose that (1) the functional contribution of myoendothelial feedback to arterial tone is determined by the nature of the vasoconstrictor stimulus, and (2) although IK_Ca channel-mediated myoendothelial feedback may contribute to local control of arterial diameter, in the intact vascular bed, increases in shear stress may be the major stimulus for engagement of the endothelium during vasoconstriction.

ABSTRACT

Constriction of isolated resistance arteries in response to α₁ -adrenoceptor agonists is limited by reciprocal engagement of inhibitory endothelial mechanisms via myoendothelial feedback. In the current model of feedback, agonist stimulation of smooth muscle cells results in localized InsP₃ -dependent Ca²⁺ transients that activate endothelial IK_Ca channels. The subsequent hyperpolarization of the endothelial membrane potential then feeds back to the smooth muscle to limit further reductions in vessel diameter. We hypothesized that the functional contribution of InsP₃ -IK_Ca channel-mediated myoendothelial feedback to limiting arterial diameter may be influenced by the nature of the vasoconstrictor stimulus. To test this hypothesis, we investigated the functional role of myoendothelial feedback in modulating responses of rat mesenteric resistance arteries to the adrenoceptor agonist noradrenaline, the thromboxane A₂ mimetic U46619, increases in intravascular pressure and stimulation of perivascular sympathetic nerves. In isolated arteries, responses to noradrenaline and stimulation of sympathetic nerves, but not to U46619 and increases in intravascular pressure, were modulated by IK_Ca channel-dependent myoendothelial feedback. In the intact mesenteric bed perfused under conditions of constant flow, responses to exogenous noradrenaline were modulated by myoendothelial feedback, but shear stress-induced release of NO and activation of endothelial SK_Ca channels appeared to be the primary mediators of endothelial modulation of vasoconstriction to agonists and nerve stimulation. Thus, we propose that myoendothelial feedback may contribute to local control of diameter within arterial segments, but at the level of the intact vascular bed, increases in shear stress may be the major stimulus for engagement of the endothelium during vasoconstriction.

Endothelial-dependent dilation following chronic hypoxia involves TRPV4-mediated activation of endothelial BK channels

Following chronic hypoxia (CH), the systemic vasculature exhibits blunted vasoconstriction due to endothelial-dependent hyperpolarization (EDH). Previous data demonstrate that subsequent to CH, EDH-mediated vasodilation switches from a reliance on SK_ca and IK_ca channels to activation of the endothelial BK_ca channels (eBK). The mechanism by which endothelial cell stimulation activates eBK channels following CH is not known. We hypothesized that following CH, EDH-dependent vasodilation involves a TRPV4-dependent activation of eBK channels. ACh induced concentration-dependent dilation in pressurized gracilis arteries from both normoxic and CH rats. Inhibition of TRPV4 (RN-1734) attenuated the ACh response in arteries from CH rats but had no effect in normoxic animals. In the presence of L-NNA and indomethacin, TRPV4 blockade attenuated ACh-induced vasodilation in arteries from CH rats. ACh elicited endothelial TRPV4-mediated Ca²⁺ events in arteries from both groups. GSK1016790A (GSK101, TRPV4 agonist) elicited vasodilation in arteries from normoxic and CH rats. In arteries from normoxic animals, TRAM-34/apamin abolished the dilation to TRPV4 activation, whereas luminal iberiotoxin had no effect. In CH rats, only administration of all three K_ca channel inhibitors abolished the dilation to TRPV4 activation. Using Duolink®, we observed co-localization between Cav-1, TRPV4, and BK channels in gracilis arteries and in RAECs. Disruption of endothelial caveolae with methyl-β-cyclodextrin significantly decreased ACh-induced vasodilation in arteries from both groups. In gracilis arteries, endothelial membrane cholesterol was significantly decreased following 48 h of CH. In conclusion, CH results in a functional coupling between muscarinic receptors, TRPV4 and K_ca channels in gracilis arteries.

Critical contribution of Na+-Ca2+ exchanger to the Ca2+-mediated vasodilation activated in endothelial cells of resistance arteries

Na⁺-Ca²⁺ exchanger (NCX) contributes to control the intracellular free Ca²⁺ concentration ([Ca²⁺]_i), but the functional activation of NCX reverse mode (NCXrm) in endothelial cells is controversial. We evaluated the participation of NCXrm-mediated Ca²⁺ uptake in the endothelium-dependent vasodilation of rat isolated mesenteric arterial beds. In phenylephrine-contracted mesenteries, the acetylcholine (ACh)-induced vasodilation was abolished by treatment with the NCXrm blockers SEA0400, KB-R7943, or SN-6. Consistent with that, the ACh-induced hyperpolarization observed in primary cultures of mesenteric endothelial cells and in smooth muscle of isolated mesenteric resistance arteries was attenuated by KB-R7943 and SEA0400, respectively. In addition, both blockers abolished the NO production activated by ACh in intact mesenteric arteries. In contrast, the inhibition of NCXrm did not affect the vasodilator responses induced by the Ca²⁺ ionophore, ionomycin, and the NO donor, S-nitroso- N-acetylpenicillamine. Furthermore, SEA0400, KB-R7943, and a small interference RNA directed against NCX1 blunted the increase in [Ca²⁺]_i induced by ACh or ATP in cultured endothelial cells. The analysis by proximity ligation assay showed that the NO-synthesizing enzyme, eNOS, and NCX1 were associated in endothelial cell caveolae of intact mesenteric resistance arteries. These results indicate that the activation of NCXrm has a central role in Ca²⁺-mediated vasodilation initiated by ACh in endothelial cells of resistance arteries.-Lillo, M. A., Gaete, P. S., Puebla, M., Ardiles, N. M., Poblete, I., Becerra, A., Simon, F., Figueroa, X. F. Critical contribution of Na⁺-Ca²⁺ exchanger to the Ca²⁺-mediated vasodilation activated in endothelial cells of resistance arteries.

Role of the endothelial caveolae microdomain in shear stress-mediated coronary vasorelaxation

In this study, we determined the role of caveolae and the ionic mechanisms that mediate shear stress-mediated vasodilation (SSD). We found that both TRPV4 and SK channels are targeted to caveolae in freshly isolated bovine coronary endothelial cells (BCECs) and that TRPV4 and KCa2.3 (SK3) channels are co-immunoprecipitated by anti-caveolin-1 antibodies. Acute exposure of BCECs seeded in a capillary tube to 10 dynes/cm² of shear stress (SS) resulted in activation of TRPV4 and SK currents. However, after incubation with HC067047 (TRPV4 inhibitor), SK currents could no longer be activated by SS, suggesting SK channel activation by SS was mediated through TRPV4. SK currents in BCECs were also activated by isoproterenol or by GSK1016790A (TRPV4 activator). In addition, preincubation of isolated coronary arterioles with apamin (SK inhibitor) resulted in a significant diminution of SSD whereas preincubation with HC067047 produced vasoconstriction by SS. Exposure of BCECs to SS (15 dynes/cm² 16 h) enhanced the production of nitric oxide and prostacyclin (PGI₂) and facilitated the translocation of TRPV4 to the caveolae. Inhibition of TRPV4 abolished the SS-mediated intracellular Ca²⁺ ([Ca²⁺] _i ) increase in BCECs. These results indicate a dynamic interaction in the vascular endothelium among caveolae TRPV4 and SK3 channels. This caveolae-TRPV4-SK3 channel complex underlies the molecular and ionic mechanisms that modulate SSD in the coronary circulation.

The caveolae dress code: structure and signaling

Over the past decade, interest in caveolae biology has peaked. These small bulb-shaped plasma membrane invaginations of 50-80nm diameter present in most cell types have been upgraded from simple membrane structures to a more complex bona fide organelle. However, although caveolae are involved in several essential cellular functions and pathologies, the underlying molecular mechanisms remain poorly defined. Following the identification of caveolins and cavins as the main caveolae constituents, recent studies have brought new insight into their structural organization as a coat. In this review, we discuss how these new data on caveolae can be integrated in the context of their role in signaling and pathophysiology.

TRPV4: Molecular Conductor of a Diverse Orchestra

Transient receptor potential vanilloid type 4 (TRPV4) is a calcium-permeable nonselective cation channel, originally described in 2000 by research teams led by Schultz (Nat Cell Biol 2: 695-702, 2000) and Liedtke (Cell 103: 525-535, 2000). TRPV4 is now recognized as being a polymodal ionotropic receptor that is activated by a disparate array of stimuli, ranging from hypotonicity to heat and acidic pH. Importantly, this ion channel is constitutively expressed and capable of spontaneous activity in the absence of agonist stimulation, which suggests that it serves important physiological functions, as does its widespread dissemination throughout the body and its capacity to interact with other proteins. Not surprisingly, therefore, it has emerged more recently that TRPV4 fulfills a great number of important physiological roles and that various disease states are attributable to the absence, or abnormal functioning, of this ion channel. Here, we review the known characteristics of this ion channel's structure, localization and function, including its activators, and examine its functional importance in health and disease.

Dynamic coupling between TRPV4 and Ca2+-activated SK1/3 and IK1 K+ channels plays a critical role in regulating the K+-secretory BK channel in kidney collecting duct cells

The large-conductance Ca²⁺-activated K⁺ channel, BK (KCNMA1), is expressed along the connecting tubule (CNT) and cortical collecting duct (CCD) where it underlies flow- and Ca²⁺-dependent K⁺ secretion. Its activity is partially under the control of the mechanosensitive transient receptor potential vanilloid type 4 (TRPV4) Ca²⁺-permeable channel. Recently, we identified three small-/intermediate-conductance Ca²⁺-activated K⁺ channels, SK1 (KCNN1), SK3 (KCNN3), and IK1 (KCNN4), with notably high Ca²⁺-binding affinities, that are expressed in CNT/CCD and may be regulated by TRPV4-mediated Ca²⁺ influx. The K⁺-secreting CCD mCCDcl1 cells, which express these channels, were used to determine whether SK1/3 and IK1 are activated on TRPV4 stimulation and whether they contribute to Ca²⁺ influx and activation of BK. Activation of TRPV4 (GSK1016790A) modestly depolarized the membrane potential and robustly increased intracellular Ca²⁺, [Ca²⁺]_i Inhibition of both SK1/3 and IK1 by application of apamin and 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34), respectively, further depolarized the membrane potential and markedly suppressed the TRPV4-mediated rise in [Ca²⁺]_i Application of BK inhibitor iberiotoxin after activation of TRPV4 without apamin/TRAM-34 also reduced [Ca²⁺]_i and further intensified membrane depolarization, demonstrating BK involvement. However, the BK-dependent effects on [Ca²⁺]_i and membrane potential were largely abolished by pretreatment with apamin and TRAM-34, identical to that observed by separately suppressing TRPV4-mediated Ca²⁺ influx, demonstrating that SK1/3-IK1 channels potently contribute to TRPV4-mediated BK activation. Our data indicate a direct correlation between TRPV4-mediated Ca²⁺ signal and BK activation but where early activation of SK1/3 and IK1 channels are critical to sufficiently enhanced Ca²⁺ entry and [Ca²⁺]_i levels required for activation of BK.

Effects of shear stress on endothelial cells: go with the flow

Haemodynamic forces influence the functional properties of vascular endothelium. Endothelial cells (ECs) have a variety of receptors, which sense flow and transmit mechanical signals through mechanosensitive signalling pathways to recipient molecules that lead to phenotypic and functional changes. Arterial architecture varies greatly exhibiting bifurcations, branch points and curved regions, which are exposed to various flow patterns. Clinical studies showed that atherosclerotic plaques develop preferentially at arterial branches and curvatures, that is in the regions exposed to disturbed flow and shear stress. In the atheroprone regions, the endothelium has a proinflammatory phenotype associated with low nitric oxide production, reduced barrier function and increased proadhesive, procoagulant and proproliferative properties. Atheroresistant regions are exposed to laminar flow and high shear stress that induce prosurvival antioxidant signals and maintain the quiescent phenotype in ECs. Indeed, various flow patterns contribute to phenotypic and functional heterogeneity of arterial endothelium whose response to proatherogenic stimuli is differentiated. This may explain the preferential development of endothelial dysfunction in arterial sites with disturbed flow.

Inwardly rectifying K+ channels are major contributors to flow-induced vasodilatation in resistance arteries

KEY POINTS

Endothelial inwardly rectifying K⁺ (Kir2.1) channels regulate flow-induced vasodilatation via nitric oxide (NO) in mouse mesenteric resistance arteries. Deficiency of Kir2.1 channels results in elevated blood pressure and increased vascular resistance. Flow-induced vasodilatation in human resistance arteries is also regulated by inwardly rectifying K⁺ channels. This study presents the first direct evidence that Kir channels play a critical role in physiological endothelial responses to flow.

ABSTRACT

Inwardly rectifying K⁺ (Kir) channels are known to be sensitive to flow, but their role in flow-induced endothelial responses is not known. The goal of this study is to establish the role of Kir channels in flow-induced vasodilatation and to provide first insights into the mechanisms responsible for Kir signalling in this process. First, we establish that primary endothelial cells isolated from murine mesenteric arteries express functional Kir2.1 channels sensitive to shear stress. Then, using the Kir2.1^+/- heterozygous mouse model, we establish that downregulation of Kir2.1 results in significant decrease in shear-activated Kir currents and inhibition of endothelium-dependent flow-induced vasodilatation (FIV) assayed in pressurized mesenteric arteries pre-constricted with endothelin-1. Deficiency in Kir2.1 also results in the loss of flow-induced phosphorylation of eNOS and Akt, as well as inhibition of NO generation. All the effects are fully rescued by endothelial cell (EC)-specific overexpression of Kir2.1. A component of FIV that is Kir independent is abrogated by blocking Ca²⁺ -sensitive K⁺ channels. Kir2.1 has no effect on endothelium-independent and K⁺ -induced vasodilatation in denuded arteries. Kir2.1^+/- mice also show increased mean blood pressure measured by carotid artery cannulation and increased microvascular resistance measured using a tail-cuff. Importantly, blocking Kir channels also inhibits flow-induced vasodilatation in human subcutaneous adipose microvessels. Endothelial Kir channels contribute to FIV of mouse mesenteric arteries via an NO-dependent mechanism, whereas Ca²⁺ -sensitive K⁺ channels mediate FIV via an NO-independent pathway. Kir2 channels also regulate vascular resistance and blood pressure. Finally, Kir channels also contribute to FIV in human subcutaneous microvessels.

Age-dependent impact of CaV 3.2 T-type calcium channel deletion on myogenic tone and flow-mediated vasodilatation in small arteries

KEY POINTS

Blood pressure and flow exert mechanical forces on the walls of small arteries, which are detected by the endothelial and smooth muscle cells, and lead to regulation of the diameter (basal tone) of an artery. CaV 3.2 T-type calcium channels are expressed in the wall of small arteries, although their function remains poorly understood because of the low specificity of T-type blockers. We used mice deficient in CaV 3.2 channels to study their role in pressure- and flow-dependent tone regulation and the possible impact of ageing on this role. In young mice, CaV 3.2 channels oppose pressure-induced vasoconstriction and participate in endothelium-dependent, flow-mediated dilatation. These effects were not seen in mature adult mice. The results of the present study demonstrate an age-dependent impact of CaV 3.2 T-type calcium channel deletion in rodents and suggest that the loss of CaV 3.2 channel function leads to more constricted arteries, which is a risk factor for cardiovascular disease.

ABSTRACT

The myogenic response and flow-mediated vasodilatation are important regulators of local blood perfusion and total peripheral resistance, and are known to entail a calcium influx into vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), respectively. CaV 3.2 T-type calcium channels are expressed in both VSMCs and ECs of small arteries. The T-type channels are important drug targets but, as a result of the lack of specific antagonists, our understanding of the role of CaV 3.2 channels in vasomotor tone at various ages is scarce. We evaluated the myogenic response, flow-mediated vasodilatation, structural remodelling and mRNA + protein expression in small mesenteric arteries from CaV 3.2 knockout (CaV 3.2KO) vs. wild-type mice at a young vs. mature adult age. In young mice only, deletion of CaV 3.2 led to an enhanced myogenic response and a ∼50% reduction of flow-mediated vasodilatation. Ni2+ had both CaV 3.2-dependent and independent effects. No changes in mRNA expression of several important K+ and Ca2+ channel genes were induced by CaV 3.2KO However, the expression of the other T-type channel isoform (CaV 3.1) was reduced at the mRNA and protein level in mature adult compared to young wild-type arteries. The results of the present study demonstrate the important roles of the CaV 3.2 T-type calcium channels in myogenic tone and flow-mediated vasodilatation that disappear with ageing. Because increased arterial tone is a risk factor for cardiovascular disease, we conclude that CaV 3.2 channels, by modulating pressure- and flow-mediated vasomotor responses to prevent excess arterial tone, protect against cardiovascular disease.

Hydrogen sulfide-induced vasodilation mediated by endothelial TRPV4 channels

Hydrogen sulfide (H₂S) is a recently described gaseous vasodilator produced within the vasculature by the enzymes cystathionine γ-lyase and 3-mercaptopyruvate sulfurtransferase. Previous data demonstrate that endothelial cells (EC) are the source of endogenous H₂S production and are required for H₂S-induced dilation. However, the signal transduction pathway activated by H₂S within EC has not been elucidated. TRPV4 and large-conductance Ca²⁺-activated K channels (BK channels) are expressed in EC. H₂S-induced dilation is inhibited by luminal administration of iberiotoxin and disruption of the endothelium. Calcium influx through TRPV4 may activate these endothelial BK channels (eBK). We hypothesized that H₂S-mediated vasodilation involves activation of TRPV4 within the endothelium. In pressurized, phenylephrine-constricted mesenteric arteries, H₂S elicited a dose-dependent vasodilation blocked by inhibition of TRPV4 channels (GSK2193874A, 300 nM). H₂S (1 μM) increased TRPV4-dependent (1.8-fold) localized calcium events in EC of pressurized arteries loaded with fluo-4 and Oregon Green. In pressurized EC tubes, H₂S (1 μM) and the TRPV4 activator, GSK101679A (30 nM), increased calcium events 1.8- and 1.5-fold, respectively. H₂S-induced an iberiotoxin-sensitive outward current measured using whole cell patch-clamp techniques in freshly dispersed EC. H₂S increased K⁺ currents from 10 to 30 pA/pF at +150 mV. Treatment with Na₂S increased the level of sulfhydration of TRPV4 channels in aortic ECs. These results demonstrate that H₂S-mediated vasodilation involves activation of TRPV4-dependent Ca²⁺ influx and BK channel activation within EC. Activation of TRPV4 channels appears to cause calcium events that result in the opening of eBK channels, endothelial hyperpolarization, and subsequent vasodilation.

Modulation of TRPV4 by diverse mechanisms

Transient receptor potential ion channels (TRP) are a superfamily of non-selective ion channels which are opened in response to a diverse range of stimuli. The TRP vanilloid 4 (TRPV4) ion channel is opened in response to heat, mechanical stimuli, hypo-osmolarity and arachidonic acid metabolites. However, recently TRPV4 has been identified as an ion channel that is modulated by, and opened by intracellular signalling cascades from other receptors and signalling pathways. Although TRPV4 knockout mice show relatively mild phenotypes, some mutations in TRPV4 cause severe developmental abnormalities, such as the skeletal dyplasia and arthropathy. Regulated TRPV4 function is also essential for healthy cardiovascular system function as a potent agonist compromises endothelial cell function, leading to vascular collapse. A better understanding of the signalling mechanisms that modulate TRPV4 function is necessary to understand its physiological roles. Post translational modification of TRPV4 by kinases and other signalling molecules can modulate TRPV4 opening in response to stimuli such as mechanical and hyposmolarity and there is an emerging area of research implicating TRPV4 as a transducer of these signals as opposed to a direct sensor of the stimuli. Due to its wide expression profile, TRPV4 is implicated in multiple pathophysiological states. TRPV4 contributes to the sensation of pain due to hypo-osmotic stimuli and inflammatory mechanical hyperalsgesia, where TRPV4 sensitizaton by intracellular signalling leads to pain behaviors in mice. In the vasculature, TRPV4 is a regulator of vessel tone and is implicated in hypertension and diabetes due to endothelial dysfunction. TRPV4 is a key regulator of epithelial and endothelial barrier function and signalling to and opening of TRPV4 can disrupt these critical protective barriers. In respiratory function, TRPV4 is involved in cystic fibrosis, cilary beat frequency, bronchoconstriction, chronic obstructive pulmonary disease, pulmonary hypertension, acute lung injury, acute respiratory distress syndrome and cough.In this review we highlight how modulation of TRPV4 opening is a vital signalling component in a range of tissues and why understanding of TRPV4 regulation in the body may lead to novel therapeutic approaches to treating a range of disease states.

Increasing TRPV4 expression restores flow-induced dilation impaired in mesenteric arteries with aging

The flow-stimulated intracellular Ca²⁺ concentration ([Ca²⁺]_i) rise in endothelial cells is an important early event leading to flow-induced blood vessel dilation. Transient receptor potential vanilloid subtype 4 (TRPV4), a Ca²⁺-permeable cation channel, facilitates the flow-stimulated [Ca²⁺]_i rise. To determine whether TRPV4 is involved in age-related flow-induced blood vessel dilation impairment, we measured blood vessel diameter and nitric oxide (NO) levels and performed Ca²⁺ imaging, immunoblotting, and immunostaining assays in rats. We found that the flow-induced and TRPV4 activator 4α-PDD-induced dilation of mesenteric arteries from aged rats were significantly decreased compared with those from young rats. The flow- or 4α-PDD-induced [Ca²⁺]_i rise was also markedly reduced in primary cultured mesenteric artery endothelial cells (MAECs) from aged rats. Immunoblotting and immunostaining results showed an age-related decrease of TRPV4 expression levels in MAECs. Additionally, the 4α-PDD-induced NO production was significantly reduced in aged MAECs. Compared with lentiviral GFP-treated aged rats, lentiviral vector delivery of TRPV4 increased TRPV4 expression level in aged MAECs and restored the flow- and 4α-PDD-induced vessel dilation in aged mesenteric arteries. We concluded that impaired TRPV4-mediated Ca²⁺ signaling causes endothelial dysfunction and that TRPV4 is a potential target for clinical treatment of age-related vascular system diseases.