Endothelial TRPV4 channels mediate dilation of cerebral arteries: impairment and recovery in cerebrovascular pathologies related to Alzheimer's disease

Mitochondrial dysfunction and neurological disorders: A narrative review and treatment overview

Mitochondria play a vital role in the nervous system, as they are responsible for generating energy in the form of ATP and regulating cellular processes such as calcium (Ca2+) signaling and apoptosis. However, mitochondrial dysfunction can lead to oxidative stress (OS), inflammation, and cell death, which have been implicated in the pathogenesis of various neurological disorders. In this article, we review the main functions of mitochondria in the nervous system and explore the mechanisms related to mitochondrial dysfunction. We discuss the role of mitochondrial dysfunction in the development and progression of some neurological disorders including Parkinson's disease (PD), multiple sclerosis (MS), Alzheimer's disease (AD), depression, and epilepsy. Finally, we provide an overview of various current treatment strategies that target mitochondrial dysfunction, including pharmacological treatments, phototherapy, gene therapy, and mitotherapy. This review emphasizes the importance of understanding the role of mitochondria in the nervous system and highlights the potential for mitochondrial-targeted therapies in the treatment of neurological disorders. Furthermore, it highlights some limitations and challenges encountered by the current therapeutic strategies and puts them in future perspective.

Impact of aging on vascular ion channels: perspectives and knowledge gaps across major organ systems

Individuals aged ≥65 yr will comprise ∼20% of the global population by 2030. Cardiovascular disease remains the leading cause of death in the world with age-related endothelial "dysfunction" as a key risk factor. As an organ in and of itself, vascular endothelium courses throughout the mammalian body to coordinate blood flow to all other organs and tissues (e.g., brain, heart, lung, skeletal muscle, gut, kidney, skin) in accord with metabolic demand. In turn, emerging evidence demonstrates that vascular aging and its comorbidities (e.g., neurodegeneration, diabetes, hypertension, kidney disease, heart failure, and cancer) are "channelopathies" in large part. With an emphasis on distinct functional traits and common arrangements across major organs systems, the present literature review encompasses regulation of vascular ion channels that underlie blood flow control throughout the body. The regulation of myoendothelial coupling and local versus conducted signaling are discussed with new perspectives for aging and the development of chronic diseases. Although equipped with an awareness of knowledge gaps in the vascular aging field, a section has been included to encompass general feasibility, role of biological sex, and additional conceptual and experimental considerations (e.g., cell regression and proliferation, gene profile analyses). The ultimate goal is for the reader to see and understand major points of deterioration in vascular function while gaining the ability to think of potential mechanistic and therapeutic strategies to sustain organ perfusion and whole body health with aging.

Transient Receptor Potential Vanilloid (TRPV4) channel inhibition: A novel promising approach for the treatment of lung diseases

Research on transient receptor potential vanilloid-4 (TRPV4) can provide a promising potential therapeutic target in the development of novel medicines for lung disorders. TRPV4 expresses in lung tissue and plays an important role in the maintenance of respiratory homeostatic function. TRPV4 is upregulated in life-threatening respiratory diseases like pulmonary hypertension, asthma, cystic fibrosis, and chronic obstructive pulmonary diseases. TRPV4 is linked to several proteins that have physiological functions and are sensitive to a wide variety of stimuli, such as mechanical stimulation, changes in temperature, and hypotonicity, and responds to a variety of proteins and lipid mediators, including anandamide (AA), the arachidonic acid metabolite, 5,6-epoxyeicosatrienoic acid (5,6-EET), a plant dimeric diterpenoid called bisandrographolide A (BAA), and the phorbol ester 4-alpha-phorbol-12,13-didecanoate (4α-PDD). This study focused on relevant research evidence of TRPV4 in lung disorders and its agonist and antagonist effects. TRPV4 can be a possible target of discovered molecules that exerts high therapeutic potential in the treatment of respiratory diseases by inhibiting TRPV4.

Blockage of TRPV4 Downregulates the Nuclear Factor-Kappa B Signaling Pathway to Inhibit Inflammatory Responses and Neuronal Death in Mice with Pilocarpine-Induced Status Epilepticus

The blockage of transient receptor potential vanilloid 4 (TRPV4) inhibits inflammation and reduces hippocampal neuronal injury in a pilocarpine-induced mouse model of temporal lobe epilepsy. However, the underlying mechanisms remain largely unclear. NF-κB signaling pathway is responsible for the inflammation and neuronal injury during epilepsy. Here, we explored whether TRPV4 blockage could affect the NF-κB pathway in mice with pilocarpine-induced status epilepticus (PISE). Application of a TRPV4 antagonist markedly attenuated the PISE-induced increase in hippocampal HMGB1, TLR4, phospho (p)-IκK (p-IκK), and p-IκBα protein levels, as well as those of cytoplasmic p-NF-κB p65 (p-p65) and nuclear NF-κB p65 and p50; in contrast, the application of GSK1016790A, a TRPV4 agonist, showed similar changes to PISE mice. Administration of the TLR4 antagonist TAK-242 or the NF-κB pathway inhibitor BAY 11-7082 led to a noticeable reduction in the hippocampal protein levels of cleaved IL-1β, IL-6 and TNF, as well as those of cytoplasmic p-p65 and nuclear p65 and p50 in GSK1016790A-injected mice. Finally, administration of either TAK-242 or BAY 11-7082 greatly increased neuronal survival in hippocampal CA1 and CA2/3 regions in GSK1016790A-injected mice. Therefore, TRPV4 activation increases HMGB1 and TLR4 expression, leading to IκK and IκBα phosphorylation and, consequently, NF-κB activation and nuclear translocation. The resulting increase in pro-inflammatory cytokine production is responsible for TRPV4 activation-induced neuronal injury. We conclude that blocking TRPV4 can downregulate HMGB1/TLR4/IκK/κBα/NF-κB signaling following PISE onset, an effect that may underlie the anti-inflammatory response and neuroprotective ability of TRPV4 blockage in mice with PISE.

Transient Receptor Potential (TRP) Channels in Pain, Neuropsychiatric Disorders, and Epilepsy

Pharmacomodulation of membrane channels is an essential topic in the study of physiological conditions and disease status. Transient receptor potential (TRP) channels are one such family of nonselective cation channels that have an important influence. In mammals, TRP channels consist of seven subfamilies with a total of twenty-eight members. Evidence shows that TRP channels mediate cation transduction in neuronal signaling, but the full implication and potential therapeutic applications of this are not entirely clear. In this review, we aim to highlight several TRP channels which have been shown to mediate pain sensation, neuropsychiatric disorders, and epilepsy. Recent findings suggest that TRPM (melastatin), TRPV (vanilloid), and TRPC (canonical) are of particular relevance to these phenomena. The research reviewed in this paper validates these TRP channels as potential targets of future clinical treatment and offers patients hope for more effective care.

The role of endothelial TRP channels in age-related vascular cognitive impairment and dementia

Transient receptor potential (TRP) proteins are part of a superfamily of polymodal cation channels that can be activated by mechanical, physical, and chemical stimuli. In the vascular endothelium, TRP channels regulate two fundamental parameters: the membrane potential and the intracellular Ca²⁺ concentration [(Ca²⁺)_i]. TRP channels are widely expressed in the cerebrovascular endothelium, and are emerging as important mediators of several brain microvascular functions (e.g., neurovascular coupling, endothelial function, and blood-brain barrier permeability), which become impaired with aging. Aging is the most significant risk factor for vascular cognitive impairment (VCI), and the number of individuals affected by VCI is expected to exponentially increase in the coming decades. Yet, there are currently no preventative or therapeutic treatments available against the development and progression of VCI. In this review, we discuss the involvement of endothelial TRP channels in diverse physiological processes in the brain as well as in the pathogenesis of age-related VCI to explore future potential neuroprotective strategies.

Role of the endothelial reverse mode sodium-calcium exchanger in the dilation of the rat middle cerebral artery during hypoosmotic hyponatremia

A decrease in serum sodium ion concentration below 135 mmol L^-1 is usually accompanied by a decrease in plasma osmolality (hypoosmotic hyponatremia) and leads to the disorder of intracranial homeostasis mainly due to cellular swelling. Recently, using an in vitro model of hypoosmotic hyponatremia, we have found that a decrease in sodium ion concentration in the perfusate to 121 mmol L^-1 relaxes the isolated rat middle cerebral artery (MCA). The aim of the present study was to explore the mechanism responsible for this relaxation. Isolated, pressurized, and perfused MCAs placed in a vessel chamber were subjected to a decrease in sodium ion concentration to 121 mmol L^-1. Changes in the diameter of the vessels were monitored with a video camera. The removal of the endothelium and inhibition of nitric oxide-dependent signaling or the reverse mode sodium-calcium exchanger (NCX) were used to study the mechanism of the dilation of the vessel during hyponatremia. The dilation of the MCA (19 ± 5%, p < 0.005) in a low-sodium buffer was absent after removal of the endothelium or administration of the inhibitor of the reverse mode of sodium-calcium exchange and was reversed to constriction after the inhibition of nitric oxide (NO)/cGMP signaling. The dilation of the middle cerebral artery of the rat in a 121 mmol L^-1 Na⁺ buffer depends on NO signaling and reverse mode of sodium-calcium exchange. These results suggest that constriction of large cerebral arteries with impaired NO-dependent signaling may be observed in response to hypoosmotic hyponatremia.

Amyloid-β disrupts unitary calcium entry through endothelial NMDA receptors in mouse cerebral arteries

Transient increases in intracellular Ca2+ activate endothelium-dependent vasodilatory pathways. This process is impaired in cerebral amyloid angiopathy, where amyloid-β(1-40) accumulates around blood vessels. In neurons, amyloid-β impairs the Ca2+-permeable N-methyl-D-aspartate receptor (NMDAR), a mediator of endothelium-dependent dilation in arteries. We hypothesized that amyloid-β(1-40) reduces NMDAR-elicited Ca2+ signals in mouse cerebral artery endothelial cells, blunting dilation. Cerebral arteries isolated from 4-5 months-old, male and female cdh5:Gcamp8 mice were used for imaging of unitary Ca2+ influx through NMDAR (NMDAR sparklets) and intracellular Ca2+ transients. The NMDAR agonist NMDA (10 µmol/L) increased frequency of NMDAR sparklets and intracellular Ca2+ transients in endothelial cells; these effects were prevented by NMDAR antagonists D-AP5 and MK-801. Next, we tested if amyloid-β(1-40) impairs NMDAR-elicited Ca2+ transients. Cerebral arteries incubated with amyloid-β(1-40) (5 µmol/L) exhibited reduced NMDAR sparklets and intracellular Ca2+ transients. Lastly, we observed that NMDA-induced dilation of pial arteries is reduced by acute intraluminal amyloid-β(1-40), as well as in a mouse model of Alzheimer's disease, the 5x-FAD, linked to downregulation of Grin1 mRNA compared to wild-type littermates. These data suggest that endothelial NMDAR mediate dilation via Ca2+-dependent pathways, a process disrupted by amyloid-β(1-40) and impaired in 5x-FAD mice.

Calcium-Dependent Ion Channels and the Regulation of Arteriolar Myogenic Tone

Arterioles in the peripheral microcirculation regulate blood flow to and within tissues and organs, control capillary blood pressure and microvascular fluid exchange, govern peripheral vascular resistance, and contribute to the regulation of blood pressure. These important microvessels display pressure-dependent myogenic tone, the steady state level of contractile activity of vascular smooth muscle cells (VSMCs) that sets resting arteriolar internal diameter such that arterioles can both dilate and constrict to meet the blood flow and pressure needs of the tissues and organs that they perfuse. This perspective will focus on the Ca²⁺-dependent ion channels in the plasma and endoplasmic reticulum membranes of arteriolar VSMCs and endothelial cells (ECs) that regulate arteriolar tone. In VSMCs, Ca²⁺-dependent negative feedback regulation of myogenic tone is mediated by Ca²⁺-activated K⁺ (BK_Ca) channels and also Ca²⁺-dependent inactivation of voltage-gated Ca²⁺ channels (VGCC). Transient receptor potential subfamily M, member 4 channels (TRPM4); Ca²⁺-activated Cl⁻ channels (CaCCs; TMEM16A/ANO1), Ca²⁺-dependent inhibition of voltage-gated K⁺ (K_V) and ATP-sensitive K⁺ (K_ATP) channels; and Ca²⁺-induced-Ca²⁺ release through inositol 1,4,5-trisphosphate receptors (IP₃Rs) participate in Ca²⁺-dependent positive-feedback regulation of myogenic tone. Calcium release from VSMC ryanodine receptors (RyRs) provide negative-feedback through Ca²⁺-spark-mediated control of BK_Ca channel activity, or positive-feedback regulation in cooperation with IP₃Rs or CaCCs. In some arterioles, VSMC RyRs are silent. In ECs, transient receptor potential vanilloid subfamily, member 4 (TRPV4) channels produce Ca²⁺ sparklets that activate IP₃Rs and intermediate and small conductance Ca²⁺ activated K⁺ (IK_Ca and sK_Ca) channels causing membrane hyperpolarization that is conducted to overlying VSMCs producing endothelium-dependent hyperpolarization and vasodilation. Endothelial IP₃Rs produce Ca²⁺ pulsars, Ca²⁺ wavelets, Ca²⁺ waves and increased global Ca²⁺ levels activating EC sK_Ca and IK_Ca channels and causing Ca²⁺-dependent production of endothelial vasodilator autacoids such as NO, prostaglandin I₂ and epoxides of arachidonic acid that mediate negative-feedback regulation of myogenic tone. Thus, Ca²⁺-dependent ion channels importantly contribute to many aspects of the regulation of myogenic tone in arterioles in the microcirculation.

Alzheimer's disease and cerebrovascular pathology alter brain endothelial inward rectifier potassium (KIR 2.1) channels

Background and Purpose

Inward rectifier potassium (K_IR) channels are key effectors of vasodilatation in neurovascular coupling (NVC). K_IR channels expressed in cerebral endothelial cells (ECs) have been confirmed as essential modulators of NVC. Alzheimer's disease (AD) and cerebrovascular disease (CVD) impact on EC‐K_IR channel function, but whether oxidative stress or inflammation explains this impairment remains elusive.

Experimental Approach

We evaluated K_IR channel function in intact and EC‐denuded pial arteries of wild‐type (WT) and transgenic mice overexpressing a mutated form of the human amyloid precursor protein (APP mice, recapitulating amyloid β‐induced oxidative stress seen in AD) or a constitutively active form of TGF‐β1 (TGF mice, recapitulating inflammation seen in cerebrovascular pathology). The benefits of antioxidant (catalase) or anti‐inflammatory (indomethacin) drugs also were investigated. Vascular and neuronal components of NVC were assessed in vivo.

Key Results

Our findings show that (i) K_IR channel‐mediated maximal vasodilatation in APP and TGF mice reaches only 37% and 10%, respectively, of the response seen in WT mice; (ii) K_IR channel dysfunction results from K_IR2.1 subunit impairment; (iii) about 50% of K⁺‐induced artery dilatation is mediated by EC‐K_IR channels; (iv) oxidative stress and inflammation impair K_IR channel function, which can be restored by antioxidant and anti‐inflammatory drugs; and (v) inflammation induces K_IR2.1 overexpression and impairs NVC in TGF mice.

Conclusion and Implications

Therapies targeting both oxidative stress and inflammation are necessary for full recovery of K_IR2.1 channel function in cerebrovascular pathology caused by AD and CVD.

Role of TRP ion channels in cerebral circulation and neurovascular communication

The dynamic regulation of blood flow is essential for meeting the high metabolic demands of the brain and maintaining brain function. Cerebral blood flow is regulated primarily by 1) the intrinsic mechanisms that determine vascular contractility and 2) signals from neurons and astrocytes that alter vascular contractility. Stimuli from neurons and astrocytes can also initiate a signaling cascade in the brain capillary endothelium to increase regional blood flow. Recent studies provide evidence that TRP channels in endothelial cells, smooth muscle cells, neurons, astrocytes, and perivascular nerves control cerebrovascular contractility and cerebral blood flow. TRP channels exert their functional effects either through cell membrane depolarization or by serving as a Ca2+ influx pathway. Endothelial cells and astrocytes also maintain the integrity of the blood-brain barrier. Both endothelial cells and astrocytes express TRP channels, and an increase in endothelial TRP channel activity has been linked with a disrupted endothelial barrier function. Therefore, TRP channels can play a potentially important role in regulating blood-brain barrier integrity. Here, we review the regulation of cerebrovascular contractility by TRP channels under healthy and disease conditions and their potential roles in maintaining blood-brain barrier function.

Methyl-Beta-Cyclodextrin Restores KIR Channel Function in Brain Endothelium of Female Alzheimer's Disease Mice

Background:

As the sixth-leading cause of death in the United States, Alzheimer’s disease (AD) entails deteriorating endothelial control of blood flow throughout the brain. In particular, reduced inward-rectifying K⁺ (K_IR) channel function in animal models of aging and AD compromises endothelial function and optimal perfusion of brain parenchyma. Deficient endothelial K_IR channels may result from aberrant interaction with plasma membrane cholesterol as a primary regulator of membrane fluidity and ion channels.

Objective:

We tested the hypothesis that mild methyl-β-cyclodextrin (MβCD) treatment to reduce membrane cholesterol may restore endothelial K_IR channel function in brain endothelium of old AD mice.

Methods:

Membrane potential was continuously measured in isolated endothelial tubes from posterior cerebral arteries of young (1 to 3 months) and old (16 to 19 months) female 3xTg-AD mice before and after mild treatment with the cholesterol-removing agent MβCD (1 mmol/L). Elevated extracellular potassium ([K⁺]_E; 15 mmol/L) and NS309 (1μmol/L) activated K_IR and Ca²⁺-activated K⁺ (SK_Ca/IK_Ca) channels respectively before and after MβCD treatment.

Results:

SK_Ca/IK_Ca channel function for producing hyperpolarization remained stable regardless of age group and MβCD treatment (ΔV_m: ∼–33 mV). However, as deficient during AD, K_IR channel function was restored (ΔV_m: –9±1 mV) versus pre-MβCD conditions (–5±1 mV); a progressive effect that reached –14±1 mV hyperpolarization at 60 min following MβCD washout.

Conclusion:

In female animals, MβCD treatment of brain endothelium selectively restores K_IR versus SK_Ca/IK_Ca channel function during AD. Thus, the endothelial cholesterol-K_IR channel interface is a novel target for ameliorating perfusion of the AD brain.

Reactive Oxygen Species and Endothelial Ca2+ Signaling: Brothers in Arms or Partners in Crime?

An increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) controls virtually all endothelial cell functions and is, therefore, crucial to maintain cardiovascular homeostasis. An aberrant elevation in endothelial can indeed lead to severe cardiovascular disorders. Likewise, moderate amounts of reactive oxygen species (ROS) induce intracellular Ca²⁺ signals to regulate vascular functions, while excessive ROS production may exploit dysregulated Ca²⁺ dynamics to induce endothelial injury. Herein, we survey how ROS induce endothelial Ca²⁺ signals to regulate vascular functions and, vice versa, how aberrant ROS generation may exploit the Ca²⁺ handling machinery to promote endothelial dysfunction. ROS elicit endothelial Ca²⁺ signals by regulating inositol-1,4,5-trisphosphate receptors, sarco-endoplasmic reticulum Ca²⁺-ATPase 2B, two-pore channels, store-operated Ca²⁺ entry (SOCE), and multiple isoforms of transient receptor potential (TRP) channels. ROS-induced endothelial Ca²⁺ signals regulate endothelial permeability, angiogenesis, and generation of vasorelaxing mediators and can be exploited to induce therapeutic angiogenesis, rescue neurovascular coupling, and induce cancer regression. However, an increase in endothelial [Ca²⁺]_i induced by aberrant ROS formation may result in endothelial dysfunction, inflammatory diseases, metabolic disorders, and pulmonary artery hypertension. This information could pave the way to design alternative treatments to interfere with the life-threatening interconnection between endothelial ROS and Ca²⁺ signaling under multiple pathological conditions.

Impaired Hippocampal Neurovascular Coupling in a Mouse Model of Alzheimer's Disease

Alzheimer’s disease (AD), the most common form of dementia, is characterized by neuronal degeneration and cerebrovascular dysfunction. Increasing evidence indicates that cerebrovascular dysfunction may be a key or an aggravating pathogenic factor in AD. This emphasizes the importance to investigate the tight coupling between neuronal activity and cerebral blood flow (CBF) termed neurovascular coupling (NVC). NVC depends on all cell types of the neurovascular unit within which astrocytes are important players in the progression of AD. Hence, the objective of this study was to characterize the hippocampal NVC in a mouse model of AD. Hippocampal NVC was studied in 6-month-old amyloid-beta precursor protein (APP) transgenic mice and their corresponding wild-type littermates using in vivo laser Doppler flowmetry to measure CBF in area CA1 of the hippocampus in response to Schaffer collaterals stimulation. Ex vivo two-photon microscopy experiments were performed to determine astrocytic Ca²⁺ and vascular responses to electrical field stimulation (EFS) or caged Ca²⁺ photolysis in hippocampal slices. Neuronal synaptic transmission, astrocytic endfeet Ca²⁺ in correlation with reactive oxygen species (ROS), and vascular reactivity in the presence or absence of Tempol, a mimetic of superoxide dismutase, were further investigated using electrophysiological, caged Ca²⁺ photolysis or pharmacological approaches. Whisker stimulation evoked-CBF increases and ex vivo vascular responses to EFS were impaired in APP mice compared with their age-matched controls. APP mice were also characterized by decreased basal synaptic transmission, a shorter astrocytic Ca²⁺ increase, and altered vascular response to elevated perivascular K⁺. However, long-term potentiation, astrocytic Ca²⁺ amplitude in response to EFS, together with vascular responses to nitric oxide remained unchanged. Importantly, we found a significantly increased Ca²⁺ uncaging-induced ROS production in APP mice. Tempol prevented the vascular response impairment while normalizing astrocytic Ca²⁺ in APP mice. These findings suggest that NVC is altered at many levels in APP mice, at least in part through oxidative stress. This points out that therapies against AD should include an antioxidative component to protect the neurovascular unit.

Silencing TRPV4 partially reverses the neurotoxic effects caused by excess Ketamine

Excessive use of Ketamine (KET) has a neurotoxic effect on the brain. This study explored the effect of Transient Receptor Potential Vanilloid 4 (TRPV4) on KET-induced neurotoxicity in the hippocampus. We extracted and identified rat hippocampal neuronal cells. The hippocampal neurons were treated with different concentrations (0, 0.1, 1, 10, 100, 300 and 1000 μmol/L) of KET (6, 12 and 24 hr). Cell viability was detected by cell counting Kit-8 (CCK-8), and TRPV4 expression was detected by quantitative Real Time-Polymerase Chain Reaction (qRT-PCR) and western blot. After silencing TRPV4, we tested cell viability and apoptosis. The contents of superoxide dismutase (SOD), glutathione (GSH), malondialdehyde (MDA), and catalase (CAT) were detected by colorimetry, and the contents of TNF-α, IL-1β, IL-6 and reactive oxygen species (ROS) were detected by Enzyme-Linked ImmunoSorbent Assay (ELISA). Finally, the expression levels of apoptosis-related proteins Bcl-2, Bax and Cleaved caspase-3, and phosphorylated-p65 (p-65), p65, phosphorylated-IκBα (p-IκBα) and IκBα were detected by qRT-PCR and western blot. KET inhibited the viability of hippocampal neurons in a dose-dependent manner, and up-regulated TRPV4 expression. SiTRPV4 inhibits KET-induced decrease in cell viability and promotes apoptosis. SiTRPV4 reduced MDA and ROS content, increased SOD, GSH and CAT levels. The release of proinflammatory factors TNF-α, IL-1β and IL-6 was also inhibited by siTRPV4. In addition, siTRPV4 up-regulated KET-induced Bcl-2 expression in hippocampal neurons, down-regulated Bax and Cleaved caspase-3, and inhibited the activation of the inflammatory pathway. Silencing TRPV4 partially reverses the neurotoxic effects induced by KET through regulating apoptosis-related proteins and p65/IκBα pathway.

Role of TRPV4 channel in vasodilation and neovascularization

The transient receptor potential vanilloid type 4 (TRPV4) channel, a Ca²⁺ -permeable nonselective cation channel, is widely distributed in the circulatory system, particularly in vascular endothelial cells (ECs) and smooth muscle cells (SMCs). The TRPV4 channel is activated by various endogenous and exogenous stimuli, including shear stress, low intravascular pressure, and arachidonic acid. TRPV4 has a role in mediating vascular tone and arterial blood pressure. The activation of the TRPV4 channel induces Ca²⁺ influx, thereby resulting in endothelium-dependent hyperpolarization and SMC relaxation through SK_Ca and IK_Ca activation on ECs or through BK_Ca activation on SMCs. Ca²⁺ binds to calmodulin, which leads to the production of nitric oxide, causing vasodilation. Furthermore, the TRPV4 channel plays an important role in angiogenesis and arteriogenesis and is critical for tumor angiogenesis and growth, since it promotes or inhibits the development of various types of cancer. The TRPV4 channel is involved in the active growth of collateral arteries induced by flow shear stress, which makes it a promising therapeutic target in the occlusion or stenosis of the main arteries. In this review, we explore the role and the potential mechanism of action of the TRPV4 channel in the regulation of vascular tone and in the induction of neovascularization to provide a reference for future research.

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.

The Calcium Signaling Mechanisms in Arterial Smooth Muscle and Endothelial Cells

The contractile state of resistance arteries and arterioles is a crucial determinant of blood pressure and blood flow. Physiological regulation of arterial contractility requires constant communication between endothelial and smooth muscle cells. Various Ca²⁺ signals and Ca²⁺ -sensitive targets ensure dynamic control of intercellular communications in the vascular wall. The functional effect of a Ca²⁺ signal on arterial contractility depends on the type of Ca²⁺ -sensitive target engaged by that signal. Recent studies using advanced imaging methods have identified the spatiotemporal signatures of individual Ca²⁺ signals that control arterial and arteriolar contractility. Broadly speaking, intracellular Ca²⁺ is increased by ion channels and transporters on the plasma membrane and endoplasmic reticular membrane. Physiological roles for many vascular Ca²⁺ signals have already been confirmed, while further investigation is needed for other Ca²⁺ signals. This article focuses on endothelial and smooth muscle Ca²⁺ signaling mechanisms in resistance arteries and arterioles. We discuss the Ca²⁺ entry pathways at the plasma membrane, Ca²⁺ release signals from the intracellular stores, the functional and physiological relevance of Ca²⁺ signals, and their regulatory mechanisms. Finally, we describe the contribution of abnormal endothelial and smooth muscle Ca²⁺ signals to the pathogenesis of vascular disorders. © 2021 American Physiological Society. Compr Physiol 11:1831-1869, 2021.

Vascular Dysfunction in Diabetes and Obesity: Focus on TRP Channels

Transient receptor potential (TRP) superfamily consists of a diverse group of non-selective cation channels that has a wide tissue distribution and is involved in many physiological processes including sensory perception, secretion of hormones, vasoconstriction/vasorelaxation, and cell cycle modulation. In the blood vessels, TRP channels are present in endothelial cells, vascular smooth muscle cells, perivascular adipose tissue (PVAT) and perivascular sensory nerves, and these channels have been implicated in the regulation of vascular tone, vascular cell proliferation, vascular wall permeability and angiogenesis. Additionally, dysfunction of TRP channels is associated with cardiometabolic diseases, such as diabetes and obesity. Unfortunately, the prevalence of diabetes and obesity is rising worldwide, becoming an important public health problems. These conditions have been associated, highlighting that obesity is a risk factor for type 2 diabetes. As well, both cardiometabolic diseases have been linked to a common disorder, vascular dysfunction. In this review, we briefly consider general aspects of TRP channels, and we focus the attention on TRPC (canonical or classical), TRPV (vanilloid), TRPM (melastatin), and TRPML (mucolipin), which were shown to be involved in vascular alterations of diabetes and obesity or are potentially linked to vascular dysfunction. Therefore, elucidation of the functional and molecular mechanisms underlying the role of TRP channels in vascular dysfunction in diabetes and obesity is important for the prevention of vascular complications and end-organ damage, providing a further therapeutic target in the treatment of these metabolic diseases.

Endothelial Control of Cerebral Blood Flow

Since constant perfusion of blood throughout the brain is critical for neuronal health, the regulation of cerebral blood flow is complex and highly controlled. This regulation is controlled, in part, by the cerebral endothelium. In this review, multiple modes of endothelium-derived blood flow regulation is discussed, including chemical control of vascular tone, heterotypic and homotypic cell-cell interactions, second messenger signaling, and cellular response to physical forces and inflammatory mediators. Because cerebral small vessel disease is often associated with endothelial dysfunction and a compromised blood-brain barrier, understanding the endothelial factors that regulate vessel function to maintain cerebral blood flow and prevent vascular permeability may provide insights into disease prevention and treatment.

Cannabis sativa: Interdisciplinary Strategies and Avenues for Medical and Commercial Progression Outside of CBD and THC

Cannabis sativa (Cannabis) is one of the world’s most well-known, yet maligned plant species. However, significant recent research is starting to unveil the potential of Cannabis to produce secondary compounds that may offer a suite of medical benefits, elevating this unique plant species from its illicit narcotic status into a genuine biopharmaceutical. This review summarises the lengthy history of Cannabis and details the molecular pathways that underpin the production of key secondary metabolites that may confer medical efficacy. We also provide an up-to-date summary of the molecular targets and potential of the relatively unknown minor compounds offered by the Cannabis plant. Furthermore, we detail the recent advances in plant science, as well as synthetic biology, and the pharmacology surrounding Cannabis. Given the relative infancy of Cannabis research, we go on to highlight the parallels to previous research conducted in another medically relevant and versatile plant, Papaver somniferum (opium poppy), as an indicator of the possible future direction of Cannabis plant biology. Overall, this review highlights the future directions of cannabis research outside of the medical biology aspects of its well-characterised constituents and explores additional avenues for the potential improvement of the medical potential of the Cannabis plant.

Aging Alters Cerebrovascular Endothelial GPCR and K+ Channel Function: Divergent Role of Biological Sex

Age-related dementia entails impaired blood flow to and throughout the brain due, in part, to reduced endothelial nitric oxide signaling. However, it is unknown whether sex affects cerebrovascular Gq-protein-coupled receptors (GPCRs) and K+ channels underlying endothelium-derived hyperpolarization (EDH) during progressive aging. Thus, we simultaneously evaluated intracellular Ca2+ ([Ca2+]i) and membrane potential (Vm) of intact endothelial tubes freshly isolated from posterior cerebral arteries of young (4-6 mo), middle-aged (12-16 mo), and old (24-28 mo) male and female C57BL/6 mice. Purinergic receptor function (vs. muscarinic) was dominant and enhanced for [Ca2+]i increases in old females versus old males. However, Ca2+-sensitive K+ channel function as defined by NS309-evoked Vm hyperpolarization was mildly impaired in females versus males during old age. This sex-based contrast in declined function of GPCRs and K+ channels to produce EDH may support a greater ability for physiological endothelial GPCR function to maintain optimal cerebral blood flow in females versus males during old age. As reflective of the pattern of cerebral blood flow decline in human subjects, inward-rectifying K+ (KIR) channel function decreased with progressive age regardless of sex. Combined age-related analyses masked male versus female aging and, contrary to expectation, hydrogen peroxide played a minimal role. Altogether, we conclude a sex-based divergence in cerebrovascular endothelial GPCR and K+ channel function while highlighting a previously unidentified form of age-related endothelial dysfunction as reduced KIR channel function.

TRP Channels Regulation of Rho GTPases in Brain Context and Diseases

Neurological and neuropsychiatric disorders are mediated by several pathophysiological mechanisms, including developmental and degenerative abnormalities caused primarily by disturbances in cell migration, structural plasticity of the synapse, and blood-vessel barrier function. In this context, critical pathways involved in the pathogenesis of these diseases are related to structural, scaffolding, and enzymatic activity-bearing proteins, which participate in Ca²⁺- and Ras Homologs (Rho) GTPases-mediated signaling. Rho GTPases are GDP/GTP binding proteins that regulate the cytoskeletal structure, cellular protrusion, and migration. These proteins cycle between GTP-bound (active) and GDP-bound (inactive) states due to their intrinsic GTPase activity and their dynamic regulation by GEFs, GAPs, and GDIs. One of the most important upstream inputs that modulate Rho GTPases activity is Ca²⁺ signaling, positioning ion channels as pivotal molecular entities for Rho GTPases regulation. Multiple non-selective cationic channels belonging to the Transient Receptor Potential (TRP) family participate in cytoskeletal-dependent processes through Ca²⁺-mediated modulation of Rho GTPases. Moreover, these ion channels have a role in several neuropathological events such as neuronal cell death, brain tumor progression and strokes. Although Rho GTPases-dependent pathways have been extensively studied, how they converge with TRP channels in the development or progression of neuropathologies is poorly understood. Herein, we review recent evidence and insights that link TRP channels activity to downstream Rho GTPase signaling or modulation. Moreover, using the TRIP database, we establish associations between possible mediators of Rho GTPase signaling with TRP ion channels. As such, we propose mechanisms that might explain the TRP-dependent modulation of Rho GTPases as possible pathways participating in the emergence or maintenance of neuropathological conditions.

Modulation of TRPV4 and BKCa for treatment of brain diseases

As a member of transient receptor potential family, the transient receptor potential vanilloid 4 (TRPV4) is a kind of nonselective calcium-permeable cation channel, which belongs to non-voltage gated Ca²⁺ channel. Large-conductance Ca²⁺-activated K⁺ channel (BKCa) represents a unique superfamily of Ca²⁺-activated K⁺ channel (KCa) that is both voltage and intracellular Ca²⁺ dependent. Not surprisingly, aberrant function of either TRPV4 or BKCa in neurons has been associated with brain disorders, such as Alzheimer's disease, cerebral ischemia, brain tumor, epilepsy, as well as headache. In these diseases, vascular dysfunction is a common characteristic. Notably, endothelial and smooth muscle TRPV4 can mediate BKCa to regulate cerebral blood flow and pressure. Therefore, in this review, we not only discuss the diverse functions of TRPV4 and BKCa in neurons to integrate relative signaling pathways in the context of cerebral physiological and pathological situations respectively, but also reveal the relationship between TRPV4 and BKCa in regulation of cerebral vascular tone as an etiologic factor. Based on these analyses, this review demonstrates the effective mechanisms of compounds targeting these two channels, which may be potential therapeutic strategies for diseases in the brain.

Therapeutic potential of pharmacological agents targeting TRP channels in CNS disorders

Central nervous system (CNS) disorders like Alzheimer's disease (AD), Parkinson disease (PD), stroke, epilepsy, depression, and bipolar disorder have a high impact on both medical and social problems due to the surge in their prevalence. All of these neuronal disorders share some common etiologies including disruption of Ca²⁺ homeostasis and accumulation of misfolded proteins. These misfolded proteins further disrupt the intracellular Ca²⁺ homeostasis by disrupting the activity of several ion channels including transient receptor potential (TRP) channels. TRP channel families include non-selective Ca²⁺ permeable channels, which act as cellular sensors activated by various physio-chemical stimuli, exogenous, and endogenous ligands responsible for maintaining the intracellular Ca²⁺ homeostasis. TRP channels are abundantly expressed in the neuronal cells and disturbance in their activity leads to various neuronal diseases. Under the pathological conditions when the activity of TRP channels is perturbed, there is a disruption of the neuronal homeostasis through increased inflammatory response, generation of reactive oxygen species, and mitochondrial dysfunction. Therefore, there is a potential of pharmacological interventions targeting TRP channels in CNS disorders. This review focuses on the role of TRP channels in neurological diseases; also, we have highlighted the current insights into the pharmacological modulators targeting TRP channels.

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.

Channels that Cooperate with TRPV4 in the Brain

Transient receptor potential vanilloid 4 (TRPV4) is a nonselective Ca²⁺-permeable cation channel that is a member of the TRP channel family. It is clear that TRPV4 channels are broadly expressed in the brain. As they are expressed on the plasma membrane, they interact with other channels and play a crucial role in nervous system activity. Under some pathological conditions, TRPV4 channels are upregulated and sensitized via cellular signaling pathways, and this can cause nervous system diseases. In this review, we focus on receptors that cooperate with TRPV4, including large-conductance Ca²⁺-activated K⁺(BKca) channels, N-methyl-D-aspartate receptors (NMDARs), α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptors (AMPARs), inositol 1,4,5-trisphosphate receptors (IP3Rs), ryanodine receptors (RyRs), aquaporin 4 (AQP4), and other potential cooperative receptors in the brain. The data demonstrate how these channels work together to cause nervous system diseases under pathological conditions. The aim of this review was to discuss the receptors and signaling pathways related to TRPV4 based on recent data on the important physiological functions of TRPV4 channels to provide new clues for future studies and prospective therapeutic targets for related brain diseases.

Exercise training ameliorates cerebrovascular dysfunction in a murine model of Alzheimer's disease: role of the P2Y2 receptor and endoplasmic reticulum stress

Cerebrovascular dysfunction is a critical risk factor for the pathogenesis of Alzheimer's disease (AD). The purinergic P2Y2 receptor and endoplasmic reticulum (ER) stress are tightly associated with vascular dysfunction and the pathogenesis of AD. However, the protective effects of exercise training on P2Y2 receptor- and ER stress-associated cerebrovascular dysfunction in AD are mostly unknown. Control (C57BL/6, CON) and AD (APP/PS1dE9, AD) mice underwent treadmill exercise training (EX). 2-MeS-ATP-induced dose-dependent vasoreactivity was determined by using a pressurized posterior cerebral artery (PCA) from 10-12-mo-old mice. Human brain microvascular endothelial cells (HBMECs) were exposed to laminar shear stress (LSS) at 20 dyn/cm² for 30 min, 2 h, and 24 h. The expression of P2Y2 receptors, endothelial nitric oxide synthase (eNOS), and ER stress signaling were quantified by Western blot analysis. Notably, exercise converted ATP-induced vasoconstriction in the PCA from AD mice to vasodilation in AD+EX mice to a degree commensurate to the vascular reactivity observed in CON mice. Exercise reduced the expression of amyloid peptide precursor (APP) and increased the P2Y2 receptor and Akt/eNOS expression in AD mice brain. Mechanistically, LSS increased the expression of both P2Y2 receptor and eNOS protein in HBMECs, but these increases were blunted by a P2Y2 receptor antagonist in HBMECs. Exercise also reduced the expression of aberrant ER stress markers p-IRE1, p/t-eIF2α, and CHOP, as well as Bax/Bcl-2, in AD mice brain. Collectively, our results demonstrate for the first time that exercise mitigates cerebrovascular dysfunction in AD through modulating P2Y2 receptor- and ER stress-dependent endothelial dysfunction.NEW & NOTEWORTHY A limited study has investigated whether exercise training can improve cerebrovascular function in Alzheimer's disease. The novel findings of the study are that exercise training improves cerebrovascular dysfunction through enhancing P2Y2 receptor-mediated eNOS signaling and reducing ER stress-associated pathways in AD. These data suggest that exercise training, which regulates P2Y2 receptor and ER stress in AD brain, is a potential therapeutic strategy for Alzheimer's disease.

AT2R's (Angiotensin II Type 2 Receptor's) Role in Cognitive and Cerebrovascular Deficits in a Mouse Model of Alzheimer Disease

Antihypertensive medications targeting the renin-angiotensin system have lowered the incidence and progression of Alzheimer disease. Understanding how these medications function could lead to novel therapeutic strategies. AT4Rs (angiotensin IV receptors) have been associated with angiotensin receptor blockers' cognitive, cerebrovascular, and neuroinflammatory rescue in Alzheimer disease models. Yet, whether AT4Rs act alone or with AT2Rs remains unknown. Here, we investigated whether AT2Rs contribute to losartan's benefits and whether chronic AT2R activation could mimic angiotensin receptor blocker benefits in transgenic mice overexpressing familial Alzheimer disease mutations of the human APP (amyloid precursor protein). Losartan-treated mice (10 mg/kg per day, drinking water, 7 months) received intracerebroventricular (1 month) administration of vehicle or AT2R antagonist PD123319 (1.6 nmol/day). PD123319 countered losartan's benefits on spatial learning and memory, neurovascular coupling, and hampered those on oxidative stress and nitric oxide bioavailability. PD123319 did not oppose losartan's benefits on short-term memory and vasodilatory function and had no benefit on neuroinflammation or Aβ (amyloid β) pathology. Mice receiving either vehicle or selective AT2R agonist compound 21 (intracerebroventricular: 1 nmol/day, 1 month or drinking water: 10 mg/kg per day, 7 months), showed no improvement in memory, vasodilatory function, or nitric oxide bioavailability. Compound 21 treatment normalized neurovascular coupling, reduced astrogliosis independent of persisting microgliosis, and exacerbated oxidative stress in APP mice. Compound 21 reduced dense core Aβ plaques, but not diffuse plaques or Aβ species. Our findings suggest that targeting AT2Rs is not an ideal strategy for restoring Aβ-related cognitive and cerebrovascular deficits.

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.

Memory and cerebrovascular deficits recovered following angiotensin IV intervention in a mouse model of Alzheimer's disease

Angiotensin II type 1 receptor antagonists like losartan have been found to lower the incidence and progression to Alzheimer's disease (AD), as well as rescue cognitive and cerebrovascular deficits in AD mouse models. We previously found that co-administration of an angiotensin IV (AngIV) receptor (AT4R) antagonist prevented losartan's benefits, identifying AT4Rs as a possible target to counter AD pathogenesis. Therein, we investigated whether directly targeting AT4Rs could counter AD pathogenesis in a well-characterized mouse model of AD. Wild-type and human amyloid precursor protein (APP) transgenic (J20 line) mice (4.5 months old) received vehicle or AngIV (~1.3 nmol/day, 1 month) intracerebroventricularly via osmotic minipumps. AngIV restored short-term memory, spatial learning and memory in APP mice. AngIV normalized hippocampal AT4R levels, increased hippocampal subgranular zone cellular proliferation and dendritic arborization, and reduced oxidative stress. AngIV rescued whisker-evoked neurovascular coupling, endothelial- and smooth muscle cell-mediated cerebral vasodilatory responses, and cerebrovascular nitric oxide bioavailability. AngIV did not alter blood pressure, neuroinflammation or amyloid-β (Aβ) pathology. These preclinical findings identify AT4R as a promising target to counter Aβ-related cognitive and cerebrovascular deficits in AD.

Endothelial TRPV4 channels modulate vascular tone by Ca2+ -induced Ca2+ release at inositol 1,4,5-trisphosphate receptors

BACKGROUND AND PURPOSE

The TRPV4 ion channels are Ca²⁺ permeable, non-selective cation channels that mediate large, but highly localized, Ca²⁺ signals in the endothelium. The mechanisms that permit highly localized Ca²⁺ changes to evoke cell-wide activity are incompletely understood. Here, we tested the hypothesis that TRPV4-mediated Ca²⁺ influx activates Ca²⁺ release from internal Ca²⁺ stores to generate widespread effects.

EXPERIMENTAL APPROACH

Ca²⁺ signals in large numbers (~100) of endothelial cells in intact arteries were imaged and analysed separately.

KEY RESULTS

Responses to the TRPV4 channel agonist GSK1016790A were heterogeneous across the endothelium. In activated cells, Ca²⁺ responses comprised localized Ca²⁺ changes leading to slow, persistent, global increases in Ca²⁺ followed by large propagating Ca²⁺ waves that moved within and between cells. To examine the mechanisms underlying each component, we developed methods to separate slow persistent Ca²⁺ rise from the propagating Ca²⁺ waves in each cell. TRPV4-mediated Ca²⁺ entry was required for the slow persistent global rise and propagating Ca²⁺ signals. The propagating waves were inhibited by depleting internal Ca²⁺ stores, inhibiting PLC or blocking IP₃ receptors. Ca²⁺ release from stores was tightly controlled by TRPV4-mediated Ca²⁺ influx and ceased when influx was terminated. Furthermore, Ca²⁺ release from internal stores was essential for TRPV4-mediated control of vascular tone.

CONCLUSIONS AND IMPLICATIONS

Ca²⁺ influx via TRPV4 channels is amplified by Ca²⁺ -induced Ca²⁺ release acting at IP₃ receptors to generate propagating Ca²⁺ waves and provide a large-scale endothelial communication system. TRPV4-mediated control of vascular tone requires Ca²⁺ release from the internal store.

Pharmacological inhibition of TRPV4 channels protects against ischemia-reperfusion-induced renal insufficiency in neonatal pigs

Renal vasoconstriction, an early manifestation of ischemic acute kidney injury (AKI), results in renal hypoperfusion and a rapid decline in kidney function. The pathophysiological mechanisms that underlie ischemia-reperfusion (IR)-induced renal insufficiency are poorly understood, but possibilities include alterations in ion channel-dependent renal vasoregulation. In the present study, we show that pharmacological activation of TRPV4 channels constricted preglomerular microvessels and elicited renal hypoperfusion in neonatal pigs. Bilateral renal ischemia followed by short-term reperfusion increased TRPV4 protein expression in resistance size renal vessels and TRPV4-dependent cation currents in renal vascular smooth muscle cells (SMCs). Selective TRPV4 channel blockers attenuated IR-induced reduction in total renal blood flow (RBF), cortical perfusion, and glomerular filtration rate (GFR). TRPV4 inhibition also diminished renal IR-induced increase in AKI biomarkers. Furthermore, the level of angiotensin II (Ang II) was higher in the urine of IR- compared with sham-operated neonatal pigs. IR did not alter renal vascular expression of Ang II type 1 (AT₁) receptors. However, losartan, a selective AT₁ receptor antagonist, ameliorated IR-induced renal insufficiency in the pigs. Blockade of TRPV4 channels attenuated Ang II-evoked receptor-operated Ca²⁺ entry and constriction in preglomerular microvessels. TRPV4 inhibition also blunted Ang II-induced increase in renal vascular resistance (RVR) and hypoperfusion in the pigs. Together, our data suggest that SMC TRPV4-mediated renal vasoconstriction and the ensuing increase in RVR contribute to early hypoperfusion and renal insufficiency elicited by renal IR in neonatal pigs. We propose that multimodal signaling by renal vascular SMC TRPV4 channels controls neonatal renal microcirculation in health and disease.

Transient receptor potential vanilloid 4 channels are important regulators of parenchymal arteriole dilation and cognitive function

OBJECTIVE

Hypertension-associated PA dysfunction reduces cerebral perfusion and impairs cognition. This is associated with impaired TRPV4-mediated PA dilation; therefore, we tested the hypothesis that TRPV4 channels are important regulators of cerebral perfusion, PA structure and dilation, and cognition.

METHODS

Ten- to twelve-month-old male TRPV4 knockout (WKY-Trpv4^em4Mcwi ) and age-matched control WKY rats were studied. Cerebral perfusion was measured by MRI with arterial spin labeling. PA structure and function were assessed using pressure myography and cognitive function using the novel object recognition test.

RESULTS

Cerebral perfusion was reduced in the WKY-Trpv4^em4Mcwi rats. This was not a result of PA remodeling because TRPV4 deletion did not change PA structure. TRPV4 deletion did not change PA myogenic tone development, but PAs from the WKY-Trpv4^em4Mcwi rats had severely blunted endothelium-dependent dilation. The WKY-Trpv4^em4Mcwi rats had impaired cognitive function and exhibited depressive-like behavior. The WKY-Trpv4^em4Mcwi rats also had increased microglia activation, and increased mRNA expression of GFAP and tumor necrosis factor alpha suggesting increased inflammation.

CONCLUSION

Our data indicate that TRPV4 channels play a critical role in cerebral perfusion, PA dilation, cognition, and inflammation. Impaired TRPV4 function in diseases such as hypertension may increase the risk of the development of vascular dementia.

High cholesterol triggers white matter alterations and cognitive deficits in a mouse model of cerebrovascular disease: benefits of simvastatin

Transgenic mice overexpressing transforming growth factor-β1 (TGF mice) display impaired cerebrovascular reactivity, cerebral hypoperfusion and neurovascular uncoupling, but no overt cognitive deficits until old age. Cardiovascular diseases are a major risk factor for vascular cognitive impairment and dementia (VCID). We investigated the impact of a high cholesterol diet (HCD) on cerebrovascular and cognitive function in adult (6 months) and aged (12 months) TGF mice, together with the potential benefit of simvastatin (SV), an anti-cholesterol drug with pleiotropic effects, in adult mice. HCD increased blood, but not brain, cholesterol levels in treated mice, which SV did not reduce. In WT mice, HCD induced small, albeit significant, impairment in endothelium-dependent dilatory function. In TGF mice, HCD worsened the established brain vessel dilatory dysfunction in an age-dependent manner and increased the number of string vessels in the white matter (WM), alterations respectively normalized and significantly countered by SV. HCD triggered cognitive decline only in TGF mice at both ages, a deficit prevented by SV. Concurrently, HCD upregulated galectin−3 immunoreactivity in WM microglial cells, a response significantly reduced in SV-treated TGF mice. Grey matter astrogliosis and microgliosis were not affected by HCD or SV. In the subventricular zone of adult HCD-treated TGF mice, SV promoted oligogenesis and migration of oligodendrocyte progenitor cells. The results demonstrate that an underlying cerebrovascular pathology increases vulnerability to cognitive failure when combined to another risk factor for dementia, and that WM alterations are associated with this loss of function. The results further indicate that myelin repair mechanisms, as triggered by SV, may bear promise in preventing or delaying cognitive decline related to VCID.

Simultaneous Measurements of Intracellular Calcium and Membrane Potential in Freshly Isolated and Intact Mouse Cerebral Endothelium

Cerebral arteries and their respective microcirculation deliver oxygen and nutrients to the brain via blood flow regulation. Endothelial cells line the lumen of blood vessels and command changes in vascular diameter as needed to meet the metabolic demand of neurons. Primary endothelial-dependent signaling pathways of hyperpolarization of membrane potential (Vm) and nitric oxide typically operate in parallel to mediate vasodilation and thereby increase blood flow. Although integral to coordinating vasodilation over several millimeters of vascular length, components of endothelium-derived hyperpolarization (EDH) have been historically difficult to measure. These components of EDH entail intracellular Ca²⁺ [Ca²⁺]i increases and subsequent activation of small- and intermediate conductance Ca²⁺-activated K⁺ (SKCa/IKCa) channels. Here, we present a simplified illustration of the isolation of fresh endothelium from mouse cerebral arteries; simultaneous measurements of endothelial [Ca²⁺]i and Vm using Fura-2 photometry and intracellular sharp electrodes, respectively; and a continuous superfusion of salt solutions and pharmacological agents under physiological conditions (pH 7.4, 37 °C). Posterior cerebral arteries from the Circle of Willis are removed free of the posterior communicating and the basilar arteries. Enzymatic digestion of cleaned posterior cerebral arterial segments and subsequent trituration facilitates removal of adventitia, perivascular nerves, and smooth muscle cells. Resulting posterior cerebral arterial endothelial "tubes" are then secured under a microscope and examined using a camera, photomultiplier tube, and one to two electrometers while under continuous superfusion. Collectively, this method can simultaneously measure changes in endothelial [Ca²⁺]i and Vm in discrete cellular locations, in addition to the spreading of EDH through gap junctions up to millimeter distances along the intact endothelium. This method is expected to yield a high-throughput analysis of the cerebral endothelial functions underlying mechanisms of blood flow regulation in the normal and diseased brain.

Augmented astrocyte microdomain Ca2+ dynamics and parenchymal arteriole tone in angiotensin II-infused hypertensive mice

Hypertension is an important contributor to cognitive decline but the underlying mechanisms are unknown. Although much focus has been placed on the effect of hypertension on vascular function, less is understood of its effects on nonvascular cells. Because astrocytes and parenchymal arterioles (PA) form a functional unit (neurovascular unit), we tested the hypothesis that hypertension-induced changes in PA tone concomitantly increases astrocyte Ca²⁺ . We used cortical brain slices from 8-week-old mice to measure myogenic responses from pressurized and perfused PA. Chronic hypertension was induced in mice by 28-day angiotensin II (Ang II) infusion; PA resting tone and myogenic responses increased significantly. In addition, chronic hypertension significantly increased spontaneous Ca²⁺ events within astrocyte microdomains (MD). Similarly, a significant increase in astrocyte Ca²⁺ was observed during PA myogenic responses supporting enhanced vessel-to-astrocyte signaling. The transient potential receptor vanilloid 4 (TRPV4) channel, expressed in astrocyte processes in contact with blood vessels, namely endfeet, respond to hemodynamic stimuli such as increased pressure/flow. Supporting a role for TRPV4 channels in aberrant astrocyte Ca²⁺ dynamics in hypertension, cortical astrocytes from hypertensive mice showed augmented TRPV4 channel expression, currents and Ca²⁺ responses to the selective channel agonist GSK1016790A. In addition, pharmacological TRPV4 channel blockade or genetic deletion abrogated enhanced hypertension-induced increases in PA tone. Together, these data suggest chronic hypertension increases PA tone and Ca²⁺ events within astrocytes MD. We conclude that aberrant Ca²⁺ events in astrocyte constitute an early event toward the progression of cognitive decline.

Mineralocorticoid receptor antagonism improves parenchymal arteriole dilation via a TRPV4-dependent mechanism and prevents cognitive dysfunction in hypertension

Hypertension and mineralocorticoid receptor activation cause cerebral parenchymal arteriole remodeling; this can limit cerebral perfusion and contribute to cognitive dysfunction. We used a mouse model of angiotensin II-induced hypertension to test the hypothesis that mineralocorticoid receptor activation impairs both transient receptor potential vanilloid (TRPV)4-mediated dilation of cerebral parenchymal arterioles and cognitive function. Mice (16-18 wk old, male, C57Bl/6) were treated with angiotensin II (800 ng·kg-1·min-1) with or without the mineralocorticoid receptor antagonist eplerenone (100 mg·kg-1·day-1) for 4 wk; sham mice served as controls. Data are presented as means ± SE; n = 5-14 mice/group. Eplerenone prevented the increased parenchymal arteriole myogenic tone and impaired carbachol-induced (10-9-10-5 mol/l) dilation observed during hypertension. The carbachol-induced dilation was endothelium-derived hyperpolarization mediated because it could not be blocked by N-nitro-l-arginine methyl ester (10-5 mol/l) and indomethacin (10-4 mol/l). We used GSK2193874 (10-7 mol/l) to confirm that in all groups this dilation was dependent on TRPV4 activation. Dilation in response to the TRPV4 agonist GSK1016790A (10-9-10-5 mol/l) was also reduced in hypertensive mice, and this defect was corrected by eplerenone. In hypertensive and eplerenone-treated animals, TRPV4 inhibition reduced myogenic tone, an effect that was not observed in arterioles from control animals. Eplerenone treatment also improved cognitive function and reduced microglia density in hypertensive mice. These data suggest that the mineralocorticoid receptor is a potential therapeutic target to improve cerebrovascular function and cognition during hypertension. NEW & NOTEWORTHY Vascular dementia is a growing public health issue that lacks effective treatments. Transient receptor potential vanilloid (TRPV)4 channels are important regulators of parenchymal arteriole dilation, and they modulate myogenic tone. The data presented here suggest that TRPV4 channel expression is regulated by the mineralocorticoid receptor (MR). MR blockade also improves cognitive function during hypertension. MR blockade might be a potential therapeutic approach to improve cerebrovascular function and cognition in patients with hypertension.

Cannabis Therapeutics and the Future of Neurology

Neurological therapeutics have been hampered by its inability to advance beyond symptomatic treatment of neurodegenerative disorders into the realm of actual palliation, arrest or reversal of the attendant pathological processes. While cannabis-based medicines have demonstrated safety, efficacy and consistency sufficient for regulatory approval in spasticity in multiple sclerosis (MS), and in Dravet and Lennox-Gastaut Syndromes (LGS), many therapeutic challenges remain. This review will examine the intriguing promise that recent discoveries regarding cannabis-based medicines offer to neurological therapeutics by incorporating the neutral phytocannabinoids tetrahydrocannabinol (THC), cannabidiol (CBD), their acidic precursors, tetrahydrocannabinolic acid (THCA) and cannabidiolic acid (CBDA), and cannabis terpenoids in the putative treatment of five syndromes, currently labeled recalcitrant to therapeutic success, and wherein improved pharmacological intervention is required: intractable epilepsy, brain tumors, Parkinson disease (PD), Alzheimer disease (AD) and traumatic brain injury (TBI)/chronic traumatic encephalopathy (CTE). Current basic science and clinical investigations support the safety and efficacy of such interventions in treatment of these currently intractable conditions, that in some cases share pathological processes, and the plausibility of interventions that harness endocannabinoid mechanisms, whether mediated via direct activity on CB₁ and CB₂ (tetrahydrocannabinol, THC, caryophyllene), peroxisome proliferator-activated receptor-gamma (PPARγ; THCA), 5-HT_1A (CBD, CBDA) or even nutritional approaches utilizing prebiotics and probiotics. The inherent polypharmaceutical properties of cannabis botanicals offer distinct advantages over the current single-target pharmaceutical model and portend to revolutionize neurological treatment into a new reality of effective interventional and even preventative treatment.

Shear stress sensitizes TRPV4 in endothelium-dependent vasodilatation

The aim of this study was to better understand the role of TRPV4 in the regulation of blood vessel dilatation by blood flow and activation of GPCRs. Using pressure myography, the dilator responses to the TRPV4 agonist GSK1016790A and to acetylcholine, were examined in rat cremaster arterioles exposed to either no shear stress or to 200 μl/min flow for 6 min. In control vessels GSK1016709A caused vasodilatation (pEC₅₀ 7.73 ± 0.12 M, ΔD_max 97 ± 3%) which was significantly attenuated by the TRPV4 antagonists GSK2193874 (100 nM) (pEC₅₀ 6.19 ± 0.11 M, p < 0.05) and HC067047 (300 nM) (pEC₅₀ 6.44 ± 0.12 M) and abolished by removal of the endothelium. Shear conditioned arterioles were significantly more sensitive to GSK1016790A (pEC₅₀ 8.34 ± 0.11, p < 0.05). Acetylcholine-induced vasodilatation (pEC₅₀ 7.02 ± 0.07 M, ΔD_max 93 ± 2%) was not affected by shear forces (pEC₅₀ 7.08 ± 0.07 M, ΔD_max 95 ± 1%). The dilator response to acetylcholine was unaffected by the TRPV4 antagonist GSK2193874 in control arterioles (pEC₅₀ 7.24 ± 0.07 M, ΔD_max 97 ± 2%). However, in shear treated arterioles, the acetylcholine-response was significantly attenuated by GSK2193874 (pEC₅₀ 6.25 ± 0.12 M, p < 0.05) indicating an induced interaction between TRPV4 and muscarinic receptors. TRPV4 antibodies localized TRPV4 to the endothelium and shear stress had no effect on its localisation. Finally, agonist activation of the M3 muscarinic receptor opened TRPV4 in HEK293 cells. We concluded that shear stress increases endothelial TRPV4 agonist sensitivity and links TRPV4 activation to muscarinic receptor mediated endothelium-dependent vasodilatation, providing strong evidence that blood flow modulates downstream signalling from at least one but not all GPCRs expressed in the endothelium.

Small vessels, dementia and chronic diseases - molecular mechanisms and pathophysiology

Cerebral small vessel disease (SVD) is a major contributor to stroke, cognitive impairment and dementia with limited therapeutic interventions. There is a critical need to provide mechanistic insight and improve translation between pre-clinical research and the clinic. A 2-day workshop was held which brought together experts from several disciplines in cerebrovascular disease, dementia and cardiovascular biology, to highlight current advances in these fields, explore synergies and scope for development. These proceedings provide a summary of key talks at the workshop with a particular focus on animal models of cerebral vascular disease and dementia, mechanisms and approaches to improve translation. The outcomes of discussion groups on related themes to identify the gaps in knowledge and requirements to advance knowledge are summarized.

Electrical dynamics of isolated cerebral and skeletal muscle endothelial tubes: Differential roles of G-protein-coupled receptors and K+ channels

Electrical dynamics of freshly isolated cerebral endothelium have not been determined independently of perivascular nerves and smooth muscle. We tested the hypothesis that endothelium of cerebral and skeletal muscle arteries differentially utilizes purinergic and muscarinic signaling pathways to activate endothelium‐derived hyperpolarization. Changes in membrane potential (V_m) were recorded in intact endothelial tubes freshly isolated from posterior cerebral and superior epigastric arteries of male and female C57BL/6 mice (age: 3‐8 months). V_m was measured in response to activation of purinergic (P2Y) and muscarinic (M₃) receptors in addition to small‐ and intermediate‐conductance Ca²⁺‐activated K⁺ (SK_Ca/IK_Ca) and inward rectifying K⁺ (K_IR) channels using ATP (100 μmol·L⁻¹), acetylcholine (ACh; 10 μmol·L⁻¹), NS309 (0.01‐10 μmol·L⁻¹), and 15 mmol·L⁻¹ KCl, respectively. Intercellular coupling was demonstrated via transfer of propidium iodide dye and electrical current (±0.5‐3 nA) through gap junctions. With similarities observed across gender, peak hyperpolarization to ATP and ACh in skeletal muscle endothelial tubes was ~twofold and ~sevenfold higher, respectively, vs cerebral endothelial tubes, whereas responses to NS309 were similar (from resting V_m ~−30 mV to maximum ~−80 mV). Hyperpolarization (~8 mV) occurred during 15 mmol·L⁻¹ KCl treatment in cerebral but not skeletal muscle endothelial tubes. Despite weaker hyperpolarization during endothelial GPCR stimulation in cerebral vs skeletal muscle endothelium, the capability for robust SK_Ca/IK_Ca activity is preserved across brain and skeletal muscle. As vascular reactivity decreases with aging and cardiovascular disease, endothelial K⁺ channel activity may be calibrated to restore blood flow to respective organs regardless of gender.

TRPV4: a Sensor for Homeostasis and Pathological Events in the CNS

Transient receptor potential vanilloid type 4 (TRPV4) was originally described as a calcium-permeable nonselective cation channel. TRPV4 is now recognized as a polymodal ionotropic receptor: it is a broadly expressed, nonselective cation channel (permeable to calcium, potassium, magnesium, and sodium) that plays an important role in a multitude of physiological processes. TRPV4 is involved in maintaining homeostasis, serves as an osmosensor and thermosensor, can be activated directly by endogenous or exogenous chemical stimuli, and can be activated or sensitized indirectly via intracellular signaling pathways. Additionally, TRPV4 is upregulated in a variety of pathological conditions. In this review, we focus on the role of TRPV4 in mediating homeostasis and pathological events in the central nervous system (CNS). This review is composed of three parts. Section 1 describes the role of TRPV4 in maintaining homeostatic processes, including the volume of body water, ionic concentrations, volume, and the temperature. Section 2 describes the effects of activation and inhibition of TRPV4 in the CNS. Section 3 focuses on the role of TRPV4 during pathological events in CNS.

The Role of Endothelial Ca2+ Signaling in Neurovascular Coupling: A View from the Lumen

BACKGROUND

Neurovascular coupling (NVC) is the mechanism whereby an increase in neuronal activity (NA) leads to local elevation in cerebral blood flow (CBF) to match the metabolic requirements of firing neurons. Following synaptic activity, an increase in neuronal and/or astrocyte Ca2+ concentration leads to the synthesis of multiple vasoactive messengers. Curiously, the role of endothelial Ca2+ signaling in NVC has been rather neglected, although endothelial cells are known to control the vascular tone in a Ca2+-dependent manner throughout peripheral vasculature.

METHODS

We analyzed the literature in search of the most recent updates on the potential role of endothelial Ca2+ signaling in NVC.

RESULTS

We found that several neurotransmitters (i.e., glutamate and acetylcholine) and neuromodulators (e.g., ATP) can induce dilation of cerebral vessels by inducing an increase in endothelial Ca2+ concentration. This, in turn, results in nitric oxide or prostaglandin E2 release or activate intermediate and small-conductance Ca2+-activated K⁺ channels, which are responsible for endothelial-dependent hyperpolarization (EDH). In addition, brain endothelial cells express multiple transient receptor potential (TRP) channels (i.e., TRPC3, TRPV3, TRPV4, TRPA1), which induce vasodilation by activating EDH.

CONCLUSIONS

It is possible to conclude that endothelial Ca2+ signaling is an emerging pathway in the control of NVC.

Transforming growth factor-β1 induces cerebrovascular dysfunction and astrogliosis through angiotensin II type 1 receptor-mediated signaling pathways

Transgenic mice constitutively overexpressing the cytokine transforming growth factor-β1 (TGF-β1) (TGF mice) display cerebrovascular alterations as seen in Alzheimer's disease (AD) and vascular cognitive impairment and dementia (VCID), but no or only subtle cognitive deficits. TGF-β1 may exert part of its deleterious effects through interactions with angiotensin II (AngII) type 1 receptor (AT1R) signaling pathways. We test such interactions in the brain and cerebral vessels of TGF mice by measuring cerebrovascular reactivity, levels of protein markers of vascular fibrosis, nitric oxide synthase activity, astrogliosis, and mnemonic performance in mice treated (6 months) with the AT1R blocker losartan (10 mg/kg per day) or the angiotensin converting enzyme inhibitor enalapril (3 mg/kg per day). Both treatments restored the severely impaired cerebrovascular reactivity to acetylcholine, calcitonin gene-related peptide, endothelin-1, and the baseline availability of nitric oxide in aged TGF mice. Losartan, but not enalapril, significantly reduced astrogliosis and cerebrovascular levels of profibrotic protein connective tissue growth factor while raising levels of antifibrotic enzyme matrix metallopeptidase-9. Memory was unaffected by aging and treatments. The results suggest a pivotal role for AngII in TGF-β1-induced cerebrovascular dysfunction and neuroinflammation through AT1R-mediated mechanisms. Further, they suggest that AngII blockers could be appropriate against vasculopathies and astrogliosis associated with AD and VCID.

Trpv4 involvement in the sex differences in blood pressure regulation in spontaneously hypertensive rats

Arterial pressure (AP) is lower in premenopausal women than in men of a similar age. Premenopausal women exhibit a lower sympathetic activity and a greater baroreceptor reflex; however, mechanisms controlling sex differences in blood pressure regulation are not well understood. We hypothesized that different neuronal functions in the cardiovascular centers of the brains of men and women may contribute to the sex difference in cardiovascular homeostasis. Our previous studies on male spontaneously hypertensive rats (SHRs) and their normotensive counterparts, Wistar Kyoto (WKY) rats, revealed that the gene-expression profile of the nucleus tractus solitarius (NTS), a region of the medulla oblongata that is pivotal for regulating the set point of AP, is strongly associated with AP. Thus, we hypothesized that gene-expression profiles in the rat NTS are related to sex differences in AP regulation. Because female SHRs clearly exhibit lower AP than their male counterparts of a similar age, we investigated whether SHR NTS exhibits sex differences in gene expression by using microarray and RT-qPCR experiments. The transcript for transient receptor potential cation channel subfamily V member 4 ( Trpv4) was found to be upregulated in SHR NTS in females compared with that in males. The channel was expressed in neurons and glial cells within NTS. The TRPV4 agonist 4-alpha-phorbol-12,13-didecanoate (4α-PDD) decreased blood pressure when injected into NTS of rats. These findings suggest that altered TRPV4 expression might be involved in the sex differences in blood pressure regulation.

TRPV4 (Transient Receptor Potential Vanilloid 4) Channel-Dependent Negative Feedback Mechanism Regulates Gq Protein-Coupled Receptor-Induced Vasoconstriction

OBJECTIVE

Several physiological stimuli activate smooth muscle cell (SMC) G_qPCRs (G_q protein-coupled receptors) to cause vasoconstriction. As a protective mechanism against excessive vasoconstriction, SMC G_qPCR stimulation invokes endothelial cell vasodilatory signaling. Whether Ca²⁺ influx in endothelial cells contributes to the regulation of G_qPCR-induced vasoconstriction remains unknown. Ca²⁺ influx through TRPV4 (transient receptor potential vanilloid 4) channels is a key regulator of endothelium-dependent vasodilation. We hypothesized that SMC G_qPCR stimulation engages endothelial TRPV4 channels to limit vasoconstriction.

APPROACH AND RESULTS

Using high-speed confocal microscopy to record unitary Ca²⁺ influx events through TRPV4 channels (TRPV4 sparklets), we report that activation of SMC α₁ARs (alpha₁-adrenergic receptors) with phenylephrine or thromboxane A₂ receptors with U46619 stimulated TRPV4 sparklets in the native endothelium from mesenteric arteries. Activation of endothelial TRPV4 channels did not require an increase in Ca²⁺ as indicated by the lack of effect of L-type Ca²⁺ channel activator or chelator of intracellular Ca²⁺ EGTA-AM. However, gap junction communication between SMCs and endothelial cells was required for phenylephrine activation or U46619 activation of endothelial TRPV4 channels. Lowering inositol 1,4,5-trisphosphate levels with phospholipase C inhibitor or lithium chloride suppressed phenylephrine activation of endothelial TRPV4 sparklets. Moreover, uncaging inositol 1,4,5-trisphosphate profoundly increased TRPV4 sparklet activity. In pressurized arteries, phenylephrine-induced vasoconstriction was followed by a slow, TRPV4-dependent vasodilation, reflecting activation of negative regulatory mechanism. Consistent with these data, phenylephrine induced a significantly higher increase in blood pressure in TRPV4^-/- mice.

CONCLUSIONS

These results demonstrate that SMC G_qPCR stimulation triggers inositol 1,4,5-trisphosphate-dependent activation of endothelial TRPV4 channels to limit vasoconstriction.