Mechanobiological insight into brain diseases based on mechanosensitive channels: Common mechanisms and clinical potential

BACKGROUND

As physical signals, mechanical cues regulate the neural cells in the brain. The mechanosensitive channels (MSCs) perceive the mechanical cues and transduce them by permeating specific ions or molecules across the plasma membrane, and finally trigger a series of intracellular bioelectrical and biochemical signals. Emerging evidence supports that wide-distributed, high-expressed MSCs like Piezo1 play important roles in several neurophysiological processes and neurological disorders.

AIMS

To systematically conclude the functions of MSCs in the brain and provide a novel mechanobiological perspective for brain diseases.

METHOD

We summarized the mechanical cues and MSCs detected in the brain and the research progress on the functional roles of MSCs in physiological conditions. We then concluded the pathological activation and downstream pathways triggered by MSCs in two categories of brain diseases, neurodegenerative diseases and place-occupying damages. Finally, we outlined the methods for manipulating MSCs and discussed their medical potential with some crucial outstanding issues.

RESULTS

The MSCs present underlying common mechanisms in different brain diseases by acting as the "transportation hubs" to transduce the distinct signal patterns: the upstream mechanical cues and the downstream intracellular pathways. Manipulating the MSCs is feasible to alter the complicated downstream processes, providing them promising targets for clinical treatment.

CONCLUSIONS

Recent research on MSCs provides a novel insight into brain diseases. The common mechanisms mediated by MSCs inspire a wide range of therapeutic potentials targeted on MSCs in different brain diseases.

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.

Is there a role of proinflammatory cytokines on degenerin-mediated cerebrovascular function in preeclampsia?

Preeclampsia (PE) is associated with adverse cerebrovascular effects during and following parturition including stroke, small vessel disease, and vascular dementia. A potential contributing factor to the cerebrovascular dysfunction is the loss of cerebral blood flow (CBF) autoregulation. Autoregulation is the maintenance of CBF to meet local demands with changes in perfusion pressure. When perfusion pressure rises, vasoconstriction of cerebral arteries and arterioles maintains flow and prevents the transfer of higher systemic pressure to downstream microvasculature. In the face of concurrent hypertension, loss of autoregulatory control exposes small delicate microvessels to injury from elevated systemic blood pressure. While placental ischemia is considered the initiating event in the preeclamptic cascade, the factor(s) mediating cerebrovascular dysfunction are poorly understood. Elevated plasma proinflammatory cytokines, such as tumor necrosis factor α (TNF-α) and interleukin-17 (IL-17), are potential mediators of autoregulatory loss. Impaired CBF responses to increases in systemic pressure are attributed to the impaired pressure-induced (myogenic) constriction of small cerebral arteries and arterioles in PE. Myogenic vasoconstriction is initiated by pressure-induced vascular smooth muscle cell (VSMC) stretch. Recent studies from our laboratory group indicate that proinflammatory cytokines impair the myogenic mechanism of CBF autoregulation via inhibition of vascular degenerin proteins, putative mediators of myogenic constriction in VSMCs. This brief review links studies showing the effect of proinflammatory cytokines on degenerin expression and CBF autoregulation to the pathological cerebral consequences of preeclampsia.

Epithelial Sodium Channel δ Subunit Is Expressed in Human Arteries and Has Potential Association With Hypertension

BACKGROUND

Elevated expression and increased activity of vascular epithelial sodium channel (ENaC) can result in vascular dysfunction in small animal models. However, there is limited or no knowledge on expression and function of ENaC channels in human vasculature. Hence, this study explored the expression and function of ENaC in human arteries and their association with hypertension.

METHODS

Human internal mammary artery (IMA) and aorta were obtained from cardiovascular patients undergoing coronary artery bypass graft surgery. Expression of the ENaC subunit was analyzed by polymerase chain reaction, Western blot, and immunohistochemistry. ENaC function was observed by patch-clamp electrophysiology in endothelial cells isolated from IMA. Levels of ENaC subunit expression levels were compared between arteries from normotensive, uncontrolled hypertensive, and controlled hypertensive patients.

RESULTS

For the first time, expression of α, β, γ, and δ was detected at mRNA and protein levels in human IMA and aorta. Single-channel patch-clamp recordings identified both αβγ- and δβγ-like channel conductance in primary endothelial cells isolated and cultured from IMA. Reduced expression of the δ subunit was observed in controlled hypertensive IMA, whereas reduced expression of γ-ENaC was observed in controlled hypertensive aorta.

CONCLUSIONS

These data suggest that functional ENaC channels are expressed in human arteries and their expression levels are associated with hypertension.

βENaC and ASIC2 associate in VSMCs to mediate pressure-induced constriction in the renal afferent arteriole

In independent studies, our laboratory has shown the importance of the degenerin proteins β-epithelial Na+ channel (βENaC) and acid-sensing ion channel 2 (ASIC2) in pressure-induced constriction (PIC) in renal interlobar arteries. Most, but not all, of the PIC response is abolished in mice lacking normal levels of βENaC or in ASIC2-null mice, indicating that the functions of βENaC and ASIC2 cannot fully compensate for the loss of the other. Degenerin proteins are known to associate and form heteromeric channels in expression systems, but whether they interact biochemically and functionally in vascular smooth muscle cells is unknown. We hypothesized that βENaC and ASIC2 interact to mediate PIC responses in renal vessels. To address this possibility, we 1) used biochemical approaches to show that βENaC associates into high-molecular-weight complexes and immunoprecipitants with ASIC2 in vascular smooth muscle cells and then 2) examined PIC in renal afferent arterioles in mice lacking normal levels of βENaC (βENaCm/m) or/and ASIC2 (ASIC2-/-) using the isolated afferent arteriole-attached glomerulus preparation. We found that the sensitivity of the PIC response (slope of the relationship between intraluminal pressure and percent myogenic tone) decreased to 26%, 27%, and -8% of wild-type controls in ASIC2-/-, βENaCm/m, and ASIC2-/-/βENaCm/m groups, respectively, suggesting that the PIC response was totally abolished in mice deficient in both ASIC2 and βENaC. Surprisingly, we found that resting internal diameters were 20-30% lower (60 mmHg, Ca2+ free) in ASIC2-/-/βENaCm/m (11.3 ± 0.5 µm) mice compared with control (14.4 ± 0.6 µm, P = 0.0007, independent two-tailed t test) or singly modified (15.7 ± 1.0 to 16.3 ± 1.1 µm) mice, suggesting compensatory vasoconstriction or remodeling. We then examined mean arterial blood pressure (MAP) using radiotelemetry and glomerular injury using histological examination of renal sections. We found that 24-h MAP was mildly elevated (+8 mmHg) in ASIC2-/-/βENaCm/m mice versus wild-type controls and the glomerular injury score was modestly increased by 38%. These findings demonstrate that myogenic constriction in afferent arterioles is dependent on normal expression of βENaC and ASIC2 and that mice lacking normal levels of ASIC2 and βENaC have mild renal injury and increased MAP.NEW & NOTEWORTHY Transmission of systemic blood pressure to delicate renal microvessels is a primary determinant of vascular injury in chronic kidney disease progression to end-stage renal disease. Here, we identified two degenerin family members, with an evolutionary link to mechanosensing, that interact biochemically and functionally to regulate systemic blood pressure and renal injury. Thus, degenerin proteins may serve as a target for the development of therapies to prevent or delay renal disease progression.

What Evolutionary Evidence Implies About the Identity of the Mechanoelectrical Couplers in Vascular Smooth Muscle Cells

Loss of pressure-induced vasoconstriction increases susceptibility to renal and cerebral vascular injury. Favored paradigms underlying initiation of the response include transient receptor potential channels coupled to G protein-coupled receptors or integrins as transducers. Degenerin channels may also mediate the response. This review addresses the 1) evolutionary role of these molecules in mechanosensing, 2) limitations to identifying mechanosensitive molecules, and 3) paradigm shifting molecular model for a VSMC mechanosensor.

Epithelial Sodium Channel and Salt-Sensitive Hypertension

The development of high blood pressure is influenced by genetic and environmental factors, with high salt intake being a known environmental contributor. Humans display a spectrum of sodium-sensitivity, with some individuals displaying a significant blood pressure rise in response to increased sodium intake while others experience almost no change. These differences are, in part, attributable to genetic variation in pathways involved in sodium handling and excretion. ENaC (epithelial sodium channel) is one of the key transporters responsible for the reabsorption of sodium in the distal nephron. This channel has an important role in the regulation of extracellular fluid volume and consequently blood pressure. Herein, we review the role of ENaC in the development of salt-sensitive hypertension, and present mechanistic insights into the regulation of ENaC activity and how it may accelerate sodium-induced damage and dysfunction. We discuss the traditional role of ENaC in renal sodium reabsorption and review work addressing ENaC expression and function in the brain, vasculature, and immune cells, and how this has expanded the implications for its role in the initiation and progression of salt-sensitive hypertension.

The mechanism of ions in pulmonary hypertension

Pulmonary hypertension（PH）is a kind of hemodynamic and pathophysiological state, in which the pulmonary artery pressure (PAP) rises above a certain threshold. The main pathological manifestation is pulmonary vasoconstriction and remodelling progressively. More and more studies have found that ions play a major role in the pathogenesis of PH. Many vasoactive substances, inflammatory mediators, transcription-inducing factors, apoptosis mediators, redox substances and translation modifiers can control the concentration of ions inside and outside the cell by regulating the activity of ion channels, which can regulate vascular contraction, cell proliferation, migration, apoptosis, inflammation and other functions. We all know that there are no effective drugs to treat PH. Ions are involved in the occurrence and development of PH, so it is necessary to clarify the mechanism of ions in PH as a therapeutic target for PH. The main ions involved in PH are calcium ion (Ca2+), potassium ion (K+), sodium ion (Na+) and chloride ion (Cl-). Here, we mainly discuss the distribution of these ions and their channels in pulmonary arteries and their role in the pathogenesis of PH.

Mechanical Strain-Mediated Tenogenic Differentiation of Mesenchymal Stromal Cells Is Regulated through Epithelial Sodium Channels

It has been suggested that mechanical strain may elicit cell differentiation in adult somatic cells through activation of epithelial sodium channels (ENaC). However, such phenomenon has not been previously demonstrated in mesenchymal stromal cells (MSCs). The present study was thus conducted to investigate the role of ENaC in human bone marrow-derived MSCs (hMSCs) tenogenic differentiation during uniaxial tensile loading. Passaged-2 hMSCs were seeded onto silicone chambers coated with collagen I and subjected to stretching at 1 Hz frequency and 8% strain for 6, 24, 48, and 72 hours. Analyses at these time points included cell morphology and alignment observation, immunocytochemistry and immunofluorescence staining (collagen I, collagen III, fibronectin, and N-cadherin), and gene expression (ENaC subunits, and tenogenic markers). Unstrained cells at similar time points served as the control group. To demonstrate the involvement of ENaC in the differentiation process, an ENaC blocker (benzamil) was used and the results were compared to the noninhibited hMSCs. ENaC subunits' (α, β, γ, and δ) expression was observed in hMSCs, although only α subunit was significantly increased during stretching. An increase in tenogenic genes' (collagen1, collagen3, decorin, tenascin-c, scleraxis, and tenomodulin) and proteins' (collagen I, collagen III, fibronectin, and N-cadherin) expression suggests that hMSCs underwent tenogenic differentiation when subjected to uniaxial loading. Inhibition of ENaC function resulted in decreased expression of these markers, thereby suggesting that ENaC plays a vital role in tenogenic differentiation of hMSCs during mechanical loading.

Pressure-induced constriction of the middle cerebral artery is abolished in TrpC6 knockout mice

Pressure-induced constriction (PIC) is an inherent response of small arteries and arterioles in which increases in intraluminal pressure evoke vasoconstriction. It is a critical mechanism of blood flow autoregulation in the kidney and brain. Degenerin (Deg) and transient receptor potential (Trp) protein families have been implicated in transduction of PIC because of evolutionary links to mechanosensing in the nematode and fly. While TrpC6 has been suggested to contribute to PIC signaling, direct supporting evidence is contradictory. Therefore, the aim of this study was to determine the importance of TrpC6 in PIC signaling using a mouse model lacking TrpC6. To address this aim, we evaluated graded pressure (20-90 mmHg), depolarization (4-80 mM KCl)-, and adrenergic receptor (phenylephrine; PE 10^-7-10^-4 M)-mediated constriction of isolated middle cerebral artery (MCA) segments from 9-wk-old male wild-type (TrpC6^+/+, n = 7) and homozygous null (TrpC6^-/-, n = 9) TrpC6 mice (Jackson Laboratories). Isolated MCA segments were cannulated and pressurized with physiological salt solution using pressure myography (Living Systems). Vasoconstrictor responses to KCl and PE were identical in TrpC6^-/- and TrpC6^+/+ mice. In contrast, PIC responses were totally abolished in TrpC6^-/- mice. At 90 mmHg, the calculated myogenic tone was -0.8 ± 0.5 vs. 10.7 ± 1.7%, P = 0.0002 in TrpC6^-/- and TrpC6^+/+ mice, respectively. Additionally, there were no changes in mechanical properties of circumferential wall strain and stress or morphological properties of wall thickness and wall-to-lumen ratio at 50 mmHg between TrpPC6^-/- and TrpC6^+/+ mice. Although these results demonstrate that TrpC6 is critical for the integrated PIC response, they do not identify whether TrpC6 acts as a mechanosensor or a downstream signaling component.NEW & NOTEWORTHY Pressure-induced, but not agonist-induced, vasoconstriction is abolished in the middle cerebral artery (MCA) of TrpC6 null mice. TrpC6 localization in dissociated cerebral vascular smooth muscle cells is primarily cytoplasmic and not associated with the surface membrane where a mechanoelectrical coupler might be expected. These findings suggest that TrpC6 is required for transduction of pressure-induced constriction in the MCA; however, its role as a mechanoelectrical coupler or downstream signal amplifier remains unresolved.

Interleukin-17 Reduces βENaC via MAPK Signaling in Vascular Smooth Muscle Cells

Degenerin proteins, such as the beta epithelial Na⁺ channel (βENaC), are essential in the intracellular signaling of pressure-induced constriction, an important vascular smooth muscle cell (VSMC) function. While certain cytokines reduce ENaC protein in epithelial tissue, it is unknown if interleukin-17 (IL-17), a potent pro-inflammatory cytokine, directly mediates changes in membrane-associated βENaC in VSMCs. Therefore, we tested the hypothesis that exposure to IL-17 reduces βENaC in VSMCs through canonical mitogen-activated protein kinase (MAPK) signaling pathways. We treated cultured rat VSMCs (A10 cell line) with IL-17 (1–100 ng/mL) for 15 min to 16 h and measured expression of βENaC, p38MAPK, c-jun kinase (JNK), and nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB). IL-17 reduced βENaC protein expression in a concentration-dependent fashion and increased phosphorylation of p38MAPK by 15 min and JNK by 8 h. NFκB was unaffected by IL-17 in VSMCs. IL-17 treatment reduced VSMC viability but had no effect on cell death. To determine the underlying signaling pathway involved in this response, VSMCs were treated before and during IL-17 exposure with p38MAPK or JNK inhibitors. We found that JNK blockade prevented IL-17-mediated βENaC protein suppression. These data demonstrate that the pro-inflammatory cytokine IL-17 regulates VSMC βENaC via canonical MAPK signaling pathways, raising the possibility that βENaC-mediated loss of VSMC function may occur in inflammatory disorders.

Tumor necrosis factor-α impairs cerebral blood flow in pregnant rats: role of vascular β-epithelial Na+ channel

Preeclampsia is a pregnancy-related disorder characterized by hypertension, vascular dysfunction and an increase in circulating inflammatory factors including the cytokine, tumor necrosis factor-α (TNF-α). Studies have shown that placental ischemia is associated with 1) increased circulating TNF-α, 2) attenuated pressure-induced cerebral vascular tone, and 3) suppression of β-epithelial Na⁺ channel (βENaC) protein in cerebral vessels. In addition to its role in epithelial Na⁺ and water transport, βENaC is an essential signaling element in transduction of pressure-induced (aka "myogenic") constriction, a critical mechanism of blood flow autoregulation. While cytokines inhibit expression of certain ENaC proteins in epithelial tissue, it is unknown if the increased circulating TNF-α associated with placental ischemia mediates the loss of cerebrovascular βENaC and cerebral blood flow regulation. Therefore, the purpose of this study was to test the hypothesis that increasing plasma TNF-α in normal pregnant rats reduces cerebrovascular βENaC expression and impairs cerebral blood flow (CBF) regulation. In vivo TNF-α infusion (200 ng/day, 5 days) inhibited cerebrovascular expression of βENaC and impaired CBF regulation in pregnant rats. To determine the direct effects of TNF-α and underlying pathways mediating vascular smooth muscle cell βENaC reduction, we exposed cultured VSMCs (A10 cell line) to TNF-α (1-100 ng/mL) for 16-24 h. TNF-α reduced βENaC protein expression in a concentration-dependent fashion from 0.1 to 100 ng/mL, without affecting cell death. To assess the role of canonical MAPK signaling in this response, VSMCs were treated with p38MAPK or c-Jun kinase (JNK) inhibitors in the presence of TNF-α. We found that both p38MAPK and JNK blockade prevented TNF-α-mediated βENaC protein suppression. These data provide evidence that disorders associated with increased circulating TNF-α could lead to impaired cerebrovascular regulation, possibly due to reduced βENaC-mediated vascular function.NEW & NOTEWORTHY This manuscript identifies TNF-α as a possible placental-derived cytokine that could be involved in declining cerebrovascular health observed in preeclampsia. We found that infusion of TNF-α during pregnancy impaired cerebral blood flow control in rats at high arterial pressures. We further discovered that cerebrovascular β-epithelial sodium channel (βENaC) protein, a degenerin protein involved in mechanotransduction, was reduced by TNF-α in pregnant rats, indicating a potential link between impaired blood flow and this myogenic player. We next examined this effect in vitro using a rat vascular smooth muscle cell line. TNF-α reduced βENaC through canonical MAPK-signaling pathways and was not dependent on cell death. This study demonstrates the pejorative effects of TNF-α on cerebrovascular function during pregnancy and warrants future investigations to study the role of cytokines on vascular function during pregnancy.

Regulation of ion channels in the microcirculation by mineralocorticoid receptor activation

The mineralocorticoid receptor (MR) has classically been studied in the renal epithelium for its role in regulating sodium and water balance and, subsequently, blood pressure. However, the MR also plays a critical role in the microvasculature by regulating ion channel expression and function. Activation of the MR by its endogenous agonist aldosterone results in translocation of the MR into the nucleus, where it can act as a transcription factor. Although most of the actions of the aldosterone can be attributed to its genomic activity though MR activation, it can also act by nongenomic mechanisms. Activation of this ubiquitous receptor increases the expression of epithelial sodium channels (ENaC) in both the endothelium and smooth muscle cells of peripheral and cerebral vessels. MR activation also regulates activity of calcium channels, calcium-activated potassium channels, and various transient receptor potential (TRP) channels. Modification of these ion channels results in a myriad of negative consequences, including impaired endothelium-dependent vasodilation, alterations in generation of myogenic tone, and increased inflammation and oxidative stress. Taken together, these studies demonstrate the importance of studying the impact of the MR on ion channel function in the vasculature. While research in this area has made advances in recent years, there are still many large gaps in knowledge that need to be filled. Crucial future directions of study include defining the molecular mechanisms involved in this interaction, as well as elucidating the potential sex differences that may exist, as these areas of understanding are currently lacking.

Acid sensing ion channels in rat cerebral arteries: Probing the expression pattern and vasomotor activity

AIMS

The recent identification of acid sensing ion channels (ASICs) in vascular beds suggests their possible involvement in modulating vasomotor tone. Therefore, we investigated the gene expression profiles of ASIC subtypes in the middle cerebral artery (MCA) of Wistar rats and the functional implication of ASICs in acidosis-induced relaxation as well as maintenance of resting tension.

MAIN METHODS

Real time PCR was employed to study the pattern of ASIC mRNA expression in the MCA wall in comparison with (i) matching brain tissue samples and (ii) arteries cultured for 24 h and 48 h. The functional implication regarding vasomotor response to acidosis and maintenance of resting tension was assessed using in vitro myography.

KEY FINDINGS

A robust mRNA expression of ASIC-1, -2 and -4 was found in brain tissue samples and to a lower extent in freshly isolated MCA. In the MCA wall, short term culture induced a down-regulation of ASIC-1 and -2 expression without any remarkable change in ASIC-4 expression. Acidosis induced a pH-related relaxation of freshly isolated MCA ring segments, being more pronounced after short term culture. Incubation with the ASIC blocker amiloride moderately enhanced acidosis-induced relaxation, in cultured MCAs somewhat stronger than in freshly isolated vessels. In addition, amiloride resulted in a decrease of resting tension, albeit only in freshly isolated MCA.

SIGNIFICANCE

Our results comprehensively describe ASIC subtype composition in the rat MCA in physiological and pathological conditions and strongly suggest the involvement of ASICs in the modulation of vasomotor responses under conditions of normal or decreased pH values.

Epithelial Na+ channel differentially contributes to shear stress-mediated vascular responsiveness in carotid and mesenteric arteries from mice

A potential "new player" in arteries for mediating shear stress responses is the epithelial Na⁺ channel (ENaC). The contribution of ENaC as shear sensor in intact arteries, and particularly different types of arteries (conduit and resistance), is unknown. We investigated the role of ENaC in both conduit (carotid) and resistance (third-order mesenteric) arteries isolated from C57Bl/6J mice. Vessel characteristics were determined at baseline (60 mmHg, no flow) and in response to increased intraluminal pressure and shear stress using a pressure myograph. These protocols were performed in the absence and presence of the ENaC inhibitor amiloride (10 µM) and after inhibition of endothelial nitric oxide synthase (eNOS) by N^ω-nitro-l-arginine methyl ester (l-NAME; 100 µM). Under no-flow conditions, amiloride increased internal and external diameters of carotid (13 ± 2%, P < 0.05) but not mesenteric (0.5 ± 0.9%, P > 0.05) arteries. In response to increased intraluminal pressure, amiloride had no effect on the internal diameter of either type of artery. However, amiloride affected the stress-strain curves of mesenteric arteries. With increased shear stress, ENaC-dependent effects were observed in both arteries. In carotid arteries, amiloride augmented flow-mediated dilation (9.2 ± 5.3%) compared with control (no amiloride, 6.2 ± 3.3%, P < 0.05). In mesenteric arteries, amiloride induced a flow-mediated constriction (-11.5 ± 6.6%) compared with control (-2.2 ± 4.5%, P < 0.05). l-NAME mimicked the effect of ENaC inhibition and prevented further amiloride effects in both types of arteries. These observations indicate that ENaC contributes to shear sensing in conduit and resistance arteries. ENaC-mediated effects were associated with NO production but may involve different (artery-dependent) downstream signaling pathways. NEW & NOTEWORTHY The epithelial Na⁺ channel (ENaC) contributes to shear sensing in conduit and resistance arteries. In conduit arteries ENaC has a role as a vasoconstrictor, whereas in resistance arteries ENaC contributes to vasodilation. Interaction of ENaC with endothelial nitric oxide synthase/nitric oxide signaling to mediate the effects is supported; however, cross talk with other shear stress-dependent signaling pathways cannot be excluded.

Juvenile growth reduces the influence of epithelial sodium channels on myogenic tone in skeletal muscle arterioles

Previous studies have documented that rapid juvenile growth is accompanied by functional changes in the arteriolar endothelium, but much less is known about functional changes in arteriolar smooth muscle over this period. In this study, we investigate the possible contribution of epithelial sodium channels (ENaC) to the myogenic behaviour of arterioles at two stages of juvenile growth. The effects of the ENaC inhibitor benzamil on different levels of myogenic tone were studied in isolated gracilis muscle arterioles from rats aged 21-28 days ("weanlings") and 42-49 days ("juveniles"). ENaC subunit expression in the arteriolar wall was also determined, and the interaction between ENaC and nitric oxide (NO) in regulating vascular tone was explored by combined use of benzamil and N^G -monomethyl-l-arginine (l-NMMA). At physiological pressures, both steady-state myogenic tone and the dynamic adjustments in this tone triggered by acute pressure changes were less in juvenile arterioles than in weanling arterioles. α, β and γ ENaC protein was present in arterioles at both ages, but benzamil only had an effect on myogenic tone in weanling arterioles. In these vessels, benzamil increased, rather than decreased, myogenic tone, and this effect was prevented by l-NMMA or endothelial removal. These findings suggest that although ENaC is present in gracilis muscle arterioles of both weanling and juvenile rats, it is not obligatory for the genesis of myogenic activity in these vessels at either age. However, ENaC activity can significantly modulate the level of myogenic tone through stimulation of endothelial NO release at an early stage of growth.

Involvement of Epithelial Na+ Channel in the Elevated Myogenic Response in Posterior Cerebral Arteries from Spontaneously Hypertensive Rats

Hypertension is characterized by increased peripheral vascular resistance which is related with elevated myogenic response. Recent findings have indicated that epithelial sodium channel (ENaC) is involved in mechanotransduction of the myogenic response. The purpose of this study was to investigate the involvement of ENaC in the elevated myogenic response of posterior cerebral arteries (PCAs) from spontaneously hypertensive rats (SHRs). Sixteen to eighteen weeks old male wistar kyoto rats (WKYs) and SHRs were used in this study. We found that wall to lumen (W/L) ratio was increased in the PCAs from SHRs compared with WKYs at the resting state. Interestingly, amiloride significantly inhibited myogenic response in the PCAs from SHRs and WKYs, however, the magnitude of the blockade was greater in SHRs. The transfection of γENaC-siRNA significantly reduced the expression of γENaC protein and inhibited myogenic response in the PCAs from SHRs. Furthermore, these data were supported by the findings that serum/glucocorticoid-induced kinase (Sgk1) and neural precursor cell-expressed developmentally downregulated gene 4-2 (Nedd4-2) were increased in SHRs compared with WKYs. Our results suggest that γENaC may play an important role in the elevated myogenic response in PCAs from SHRs.

ROK and Arteriolar Myogenic Tone Generation: Molecular Evidence in Health and Disease

The myogenic response is an inherent property of resistance arteries that warrants a relatively constant blood flow in response to changes in perfusion pressure and protect delicate organs from vascular insufficiencies and excessive blood flow. This fundamental phenomenon has been extensively studied aiming to elucidate the underlying mechanisms triggering smooth muscle contraction in response to intraluminal pressure elevation, particularly, Rho-associated kinase (ROK)-mediated Ca²⁺-independent mechanisms. The size of the resistance arteries limits the capacity to examine changes in protein phosphorylation/expression levels associated with ROK signaling. A highly sensitive biochemical detection approach was beneficial in examining the role of ROK in different force generation mechanisms along the course of myogenic constriction. In this mini review, we summarize recent results showing direct evidence for the contribution of ROK in development of myogenic response at the level of mechanotransduction, myosin light chain phosphatase inhibition and dynamic actin cytoskeleton reorganization. We will also present evidence that alterations in ROK signaling could underlie the progressive loss in myogenic response in a rat model of type 2 diabetes.

PICK1/calcineurin suppress ASIC1-mediated Ca2+ entry in rat pulmonary arterial smooth muscle cells

Acid-sensing ion channel 1 (ASIC1) contributes to Ca(2+) influx and contraction in pulmonary arterial smooth muscle cells (PASMC). ASIC1 binds the PDZ (PSD-95/Dlg/ZO-1) domain of the protein interacting with C kinase 1 (PICK1), and this interaction is important for the subcellular localization and/or activity of ASIC1. Therefore, we first hypothesized that PICK1 facilitates ASIC1-dependent Ca(2+) influx in PASMC by promoting plasma membrane localization. Using Duolink to determine protein-protein interactions and a biotinylation assay to assess membrane localization, we demonstrated that the PICK1 PDZ domain inhibitor FSC231 diminished the colocalization of PICK1 and ASIC1 but did not limit ASIC1 plasma membrane localization. Although stimulation of store-operated Ca(2+) entry (SOCE) greatly enhanced colocalization between ASIC1 and PICK1, both FSC231 and shRNA knockdown of PICK1 largely augmented SOCE. These data suggest PICK1 imparts a basal inhibitory effect on ASIC1 Ca(2+) entry in PASMC and led to an alternative hypothesis that PICK1 facilitates the interaction between ASIC1 and negative intracellular modulators, namely PKC and/or the calcium-calmodulin-activated phosphatase calcineurin. FSC231 limited PKC-mediated inhibition of SOCE, supporting a potential role for PICK1 in this response. Additionally, we found PICK1 inhibits ASIC1-mediated SOCE through an effect of calcineurin to dephosphorylate the channel. Furthermore, it appears PICK1/calcineurin-mediated regulation of SOCE opposes PKA phosphorylation and activation of ASIC1. Together our data suggest PKA and PICK1/calcineurin differentially regulate ASIC1-mediated SOCE and these modulatory complexes are important in determining downstream Ca(2+) signaling.

Regulator of G Protein Signaling 2: A Versatile Regulator of Vascular Function

Regulators of G protein signaling (RGS) proteins of the B/R4 family are widely expressed in the cardiovascular system where their role in fine-tuning G protein signaling is critical to maintaining homeostasis. Among members of this family, RGS2 and RGS5 have been shown to play key roles in cardiac and smooth muscle function by tightly regulating signaling pathways that are activated through Gq/11 and Gi/o classes of heterotrimeric G proteins. This chapter reviews accumulating evidence supporting a key role for RGS2 in vascular function and the implication of changes in RGS2 function and/or expression in the pathogenesis of blood pressure disorders, particularly hypertension. With such understanding, RGS2 and the signaling pathways it controls may emerge as novel targets for developing next-generation antihypertensive drugs/agents.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Blockade of ENaCs by amiloride induces c-Fos activation of the area postrema

Epithelial sodium channels (ENaCs) are strongly expressed in the circumventricular organs (CVOs), and these structures may play an important role in sensing plasma sodium levels. Here, the potent ENaC blocker amiloride was injected intraperitoneally in rats and 2h later, the c-Fos activation pattern in the CVOs was studied. Amiloride elicited dose-related activation in the area postrema (AP) but only ~10% of the rats showed c-Fos activity in the organum vasculosum of the lamina terminalis (OVLT) and subfornical organ (SFO). Tyrosine hydroxylase-immunoreactive (catecholamine) AP neurons were activated, but tryptophan hydroxylase-immunoreactive (serotonin) neurons were unaffected. The AP projects to FoxP2-expressing neurons in the dorsolateral pons which include the pre-locus coeruleus nucleus and external lateral part of the parabrachial nucleus; both cell groups were c-Fos activated following systemic injections of amiloride. In contrast, another AP projection target--the aldosterone-sensitive neurons of the nucleus tractus solitarius which express the enzyme 11-β-hydroxysteriod dehydrogenase type 2 (HSD2) were not activated. As shown here, plasma concentrations of amiloride used in these experiments were near or below the IC50 level for ENaCs. Amiloride did not induce changes in blood pressure, heart rate, or regional vascular resistance, so sensory feedback from the cardiovascular system was probably not a causal factor for the c-Fos activity seen in the CVOs. In summary, amiloride may have a dual effect on sodium homeostasis causing a loss of sodium via the kidney and inhibiting sodium appetite by activating the central satiety pathway arising from the AP.

Transient receptor potential canonical type 3 channels control the vascular contractility of mouse mesenteric arteries

Transient receptor potential canonical type 3 (TRPC3) channels are non-selective cation channels and regulate intracellular Ca²⁺ concentration. We examined the role of TRPC3 channels in agonist-, membrane depolarization (high K⁺)-, and mechanical (pressure)-induced vasoconstriction and vasorelaxation in mouse mesenteric arteries. Vasoconstriction and vasorelaxation of endothelial cells intact mesenteric arteries were measured in TRPC3 wild-type (WT) and knockout (KO) mice. Calcium concentration ([Ca²⁺]) was measured in isolated arteries from TRPC3 WT and KO mice as well as in the mouse endothelial cell line bEnd.3. Nitric oxide (NO) production and nitrate/nitrite concentrations were also measured in TRPC3 WT and KO mice. Phenylephrine-induced vasoconstriction was reduced in TRPC3 KO mice when compared to that of WT mice, but neither high K⁺- nor pressure-induced vasoconstriction was altered in TRPC3 KO mice. Acetylcholine-induced vasorelaxation was inhibited in TRPC3 KO mice and by the selective TRPC3 blocker pyrazole-3. Acetylcholine blocked the phenylephrine-induced increase in Ca²⁺ ratio and then relaxation in TRPC3 WT mice but had little effect on those outcomes in KO mice. Acetylcholine evoked a Ca²⁺ increase in endothelial cells, which was inhibited by pyrazole-3. Acetylcholine induced increased NO release in TRPC3 WT mice, but not in KO mice. Acetylcholine also increased the nitrate/nitrite concentration in TRPC3 WT mice, but not in KO mice. The present study directly demonstrated that the TRPC3 channel is involved in agonist-induced vasoconstriction and plays important role in NO-mediated vasorelaxation of intact mesenteric arteries.

TRPM4 channels couple purinergic receptor mechanoactivation and myogenic tone development in cerebral parenchymal arterioles

Cerebral parenchymal arterioles (PAs) have a critical role in assuring appropriate blood flow and perfusion pressure within the brain. They are unique in contrast to upstream pial arteries, as defined by their critical roles in neurovascular coupling, distinct sensitivities to chemical stimulants, and enhanced myogenic tone development. The objective of the present study was to reveal some of the unique mechanisms of myogenic tone regulation in the cerebral microcirculation. Here, we report that in vivo suppression of TRPM4 (transient receptor potential) channel expression, or inhibition of TRPM4 channels with 9-phenanthrol substantially reduced myogenic tone of isolated PAs, supporting a key role of TRPM4 channels in PA myogenic tone development. Further, downregulation of TRPM4 channels inhibited vasoconstriction induced by the specific P2Y4 and P2Y6 receptor ligands (UTPγS and UDP) by 37% and 42%, respectively. In addition, 9-phenanthrol substantially attenuated purinergic ligand-induced membrane depolarization and constriction of PAs, and inhibited ligand-evoked TRPM4 channel activation in isolated PA myocytes. In concert with our previous work showing the essential contributions of P2Y4 and P2Y6 receptors to myogenic regulation of PAs, the current results point to TRPM4 channels as an important link between mechanosensitive P2Y receptor activation and myogenic constriction of cerebral PAs.

Blood pressure regulation via the epithelial sodium channel: from gene to kidney and beyond

The epithelial sodium channel (ENaC) has long been recognized as playing a vital role in blood pressure (BP) regulation due to its involvement in fluid balance. The genes encoding the three ENaC subunits are likewise important contributors to hypertension, both in rare monogenic diseases and in the general population. The unusually high numbers of genetic variants associated with complex traits, including BP, that are located in non-coding areas suggest an involvement of these variants in regulatory functions. This may involve differential regulation of expression in different tissues. Emerging evidence indicates that the ENaC plays an important role in BP determination not only via its actions in the kidney, but also in other tissues commonly involved in BP regulation. The ENaC in the central nervous system is proposed to regulate BP via sympathetic nervous system activity. Recent evidence suggests that the ENaC contributes to vascular function and the myogenic response. Additional roles potentially include initiation of the baroreceptor reflex via ENaC in the baroreceptors and driving high salt intake with a 'taste for salt' via ENaC in the tongue. The present review describes the involvement of the ENaC in the determination of BP at a genetic and physiological level, detailing recent evidence for its role in the kidney and in other pertinent tissues.

Role of ASIC1 in the development of chronic hypoxia-induced pulmonary hypertension

Chronic hypoxia (CH) associated with respiratory disease results in elevated pulmonary vascular intracellular Ca(2+) concentration, which elicits enhanced vasoconstriction and promotes vascular arterial remodeling and thus has important implications in the development of pulmonary hypertension (PH). Store-operated Ca(2+) entry (SOCE) contributes to this elevated intracellular Ca(2+) concentration and has also been linked to acute hypoxic pulmonary vasoconstriction (HPV). Since our laboratory has recently demonstrated an important role for acid-sensing ion channel 1 (ASIC1) in mediating SOCE, we hypothesized that ASIC1 contributes to both HPV and the development of CH-induced PH. To test this hypothesis, we examined responses to acute hypoxia in isolated lungs and assessed the effects of CH on indexes of PH, arterial remodeling, and vasoconstrictor reactivity in wild-type (ASIC1(+/+)) and ASIC1 knockout (ASIC1(-/-)) mice. Restoration of ASIC1 expression in pulmonary arterial smooth muscle cells from ASIC1(-/-) mice rescued SOCE, confirming the requirement for ASIC1 in this response. HPV responses were blunted in lungs from ASIC1(-/-) mice. Both SOCE and receptor-mediated Ca(2+) entry, along with agonist-dependent vasoconstrictor responses, were diminished in small pulmonary arteries from control ASIC(-/-) mice compared with ASIC(+/+) mice. The effects of CH to augment receptor-mediated vasoconstrictor and SOCE responses in vessels from ASIC1(+/+) mice were not observed after CH in ASIC1(-/-) mice. In addition, ASIC1(-/-) mice exhibited diminished right ventricular systolic pressure, right ventricular hypertrophy, and arterial remodeling in response to CH compared with ASIC1(+/+) mice. Taken together, these data demonstrate an important role for ASIC1 in both HPV and the development of CH-induced PH.

Role of endogenous ENaC and TRP channels in the myogenic response of rat posterior cerebral arteries

AIMS

Mechanogated ion channels are predicted to mediate pressure-induced myogenic vasoconstriction in small resistance arteries. Recent findings have indicated that transient receptor potential (TRP) channels and epithelial sodium channels (ENaC) are involved in mechanotransduction. The purpose of this study was to investigate the role of TRP channels and ENaC in the myogenic response. Our previous study suggested that ENaC could be a component of the mechanosensitive ion channels in rat posterior cerebral arteries (PCA). However, the specific ion channel proteins mediating myogenic constriction are unknown. Here we found, for the first time, that ENaC interacted with TRPM4 but not with TRPC6 using immunoprecipitation and confocal microscopy.

METHODS AND RESULTS

Treatment with a specific βENaC inhibitor, amiloride, a specific TRPM4 inhibitor, 9-phenanthrol, and a TRPC6 inhibitor, SKF96365, resulted in inhibition of the pressure-induced myogenic response. Moreover, the myogenic response was inhibited in rat PCA transfected with small interfering RNA of βENaC, TRPM4, and TRPC6. Co-treatment with amiloride and 9-phenanthrol showed a similar inhibitory effect on myogenic contraction compared to single treatment with amiloride or 9-phenanthrol. The myogenic response was not affected by 9-phenanthrol or amiloride treatment in PCA transfected with βENaC or TRPM4 siRNA, respectively. However, pressure-induced myogenic response was fully inhibited by co-treatment with amiloride, 9-phenanthrol, and SKF96365, and by treatment with SKF96365 in PCA transfected with βENaC siRNA.

CONCLUSION

Our results suggest that ENaC, TRPM4, and TRPC6 play important roles in the pressure-induced myogenic response, and that ENaC and TRPM4 interact in rat PCA.

Afferent Arteriolar Responses to β,γ-methylene ATP and 20-HETE are not Blocked by ENaC Inhibition

Afferent arteriolar myogenic and tubuloglomerular feedback responses are critical for the proper maintenance of renal hemodynamics and water and electrolyte homeostasis. Adenosine triphosphate (ATP) P2X receptor activation and 20-hydroxyeicosatetraenoic acids (20-HETE) have been implicated in afferent arteriolar autoregulatory responses. Besides these two participants, members of the degenerin/epithelial Na⁺ channel (DEG/ENaC) family have been demonstrated to play a pivotal role in the afferent arteriolar myogenic response. The aim of this study was to determine if ENaC contributes to P2X receptor- or 20-HETE-mediated afferent arteriolar vasoconstriction. As previously demonstrated, afferent arteriolar diameter responses to increasing perfusion pressure from 100 to 160 mmHg were abolished by ENaC inhibitors amiloride or benzamil. Afferent arteriolar diameter decreased by 29% under control conditions and by 1% and 5% in the presence of amiloride or benzamil, respectively. The P2X receptor agonist β,γ-methylene ATP decreased afferent arteriolar diameter by 3 ± 1%, 7 ± 1%, 12 ± 2%, and 17 ± 3% in response to 0.1, 1, 10, and 100 μmol/L, respectively. ENaC inhibition did not alter the afferent arteriolar vasoconstrictor response to the P2X receptor agonist β,γ-methylene ATP. Like P2X receptor activation, 20-HETE dose-dependently decreased afferent arteriolar diameter and this vasoconstrictor response was not altered by the presence of ENaC inhibitors amiloride or benzamil. These results suggest that DEG/ENaC channels are required for afferent arteriolar autoregulatory responses; however, DEG/ENaC channels do not contribute to P2X receptor- or 20-HETE-mediated afferent arteriolar vasoconstriction.