Critical role for transient receptor potential channel TRPM4 in myogenic constriction of cerebral arteries

Roles of TRP Channels in Neurological Diseases

Transient receptor potential (TRP) proteins consist of a superfamily of cation channels that have been involved in diverse physiological processes in the brain as well as in the pathogenesis of neurological disease. TRP channels are widely expressed in the brain, including neurons and glial cells, as well as in the cerebral vascular endothelium and smooth muscle. Members of this channel superfamily show a wide variety of mechanisms ranging from ligand binding to voltage, physical, and chemical stimuli, implying the promising therapeutic potential of TRP in neurological diseases. In this review, we focus on the physiological functions of TRP channels in the brain and the pathological roles in neurological disorders to explore future potential neuroprotective strategies.

Implication of Potassium Channels in the Pathophysiology of Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a rare and severe cardiopulmonary disease without curative treatments. PAH is a multifactorial disease that involves genetic predisposition, epigenetic factors, and environmental factors (drugs, toxins, viruses, hypoxia, and inflammation), which contribute to the initiation or development of irreversible remodeling of the pulmonary vessels. The recent identification of loss-of-function mutations in KCNK3 (KCNK3 or TASK-1) and ABCC8 (SUR1), or gain-of-function mutations in ABCC9 (SUR2), as well as polymorphisms in KCNA5 (Kv1.5), which encode two potassium (K⁺) channels and two K⁺ channel regulatory subunits, has revived the interest of ion channels in PAH. This review focuses on KCNK3, SUR1, SUR2, and Kv1.5 channels in pulmonary vasculature and discusses their pathophysiological contribution to and therapeutic potential in PAH.

TRPM4 is overexpressed in breast cancer associated with estrogen response and epithelial-mesenchymal transition gene sets

Ion channels form an important class of drug targets in malignancies. Transient receptor potential cation channel subfamily M member 4 (TRPM4) plays oncological roles in various solid tumors. Herein, we examined TRPM4 protein expression profile by immunohistochemistry (IHC) in breast cancer cases compared with normal breast ducts, its association with clinico-demographical parameters, and its potential function in breast cancers by Gene Set Enrichment Analysis (GSEA). Data-mining demonstrated that TRPM4 transcript levels were significantly higher in The Cancer Genome Atlas series of breast cancer cases (n = 1,085) compared with normal breast tissues (n = 112) (p = 1.03 x 10⁻¹¹). Our IHC findings in tissue microarrays showed that TRPM4 protein was overexpressed in breast cancers (n = 83/99 TRPM4⁺; 83.8%) compared with normal breast ducts (n = 5/10 TRPM4⁺; 50%) (p = 0.022). Higher TRPM4 expression (median frequency cut-off) was significantly associated with higher lymph node status (N1-N2 vs N0; p = 0.024) and higher stage (IIb-IIIb vs I-IIa; p = 0.005). GSEA evaluation in three independent gene expression profiling (GEP) datasets of breast cancer cases (GSE54002, n = 417; GSE20685, n = 327; GSE23720, n = 197) demonstrated significant association of TRPM4 transcript expression with estrogen response and epithelial-mesenchymal transition (EMT) gene sets (p<0.01 and false discovery rate<0.05). These gene sets were not enriched in GEP datasets of normal breast epithelium cases (GSE10797, n = 5; GSE9574, n = 15; GSE20437, n = 18). In conclusion, TRPM4 protein expression is upregulated in breast cancers associated with worse clinico-demographical parameters, and TRPM4 potentially regulates estrogen receptor signaling and EMT progression in breast cancer.

KIR channels in the microvasculature: Regulatory properties and the lipid-hemodynamic environment

Basal tone and perfusion control is set in cerebral arteries by the sensing of pressure and flow, key hemodynamic stimuli. These forces establish a contractile foundation within arterial networks upon which local neurovascular stimuli operate. This fundamental process is intimately tied to arterial V_M and the rise in cytosolic [Ca²⁺] by the graded opening of voltage-operated Ca²⁺ channels. Arterial V_M is in turn controlled by a dynamic interaction among several resident ion channels, K_IR being one of particular significance. As the name suggests, K_IR displays strong inward rectification, retains a small outward component, potentiated by extracellular K⁺ and blocked by micromolar Ba²⁺. Cerebrovascular K_IR is unique from other K⁺ currents as it is present in both smooth muscle and endothelium yet lacking in classical regulatory modulation. Such observations have fostered the view that K_IR is nothing more than a background conductance, activated by extracellular K⁺ and which passively facilitates dilation. Recent work in cell model systems has; however, identified two membrane lipids, phosphatidylinositol 4,5-bisphosphate (PIP₂) and cholesterol, that interact with K_IR2.x, to stabilize the channel in the preferred open or silent state, respectively. Translating this unique form of regulation, recent studies have demonstrated that specific lipid-protein interactions enable unique K_IR populations to sense distinct hemodynamic stimuli and set basal tone. This review summarizes the current knowledge of vascular K_IR channels and how the lipid and hemodynamic impact their activity.

Intravascular flow stimulates PKD2 (polycystin-2) channels in endothelial cells to reduce blood pressure

PKD2 (polycystin-2, TRPP1), a TRP polycystin channel, is expressed in endothelial cells (ECs), but its physiological functions in this cell type are unclear. Here, we generated inducible, EC-specific Pkd2 knockout mice to examine vascular functions of PKD2. Data show that a broad range of intravascular flow rates stimulate EC PKD2 channels, producing vasodilation. Flow-mediated PKD2 channel activation leads to calcium influx that activates SK/IK channels and eNOS serine 1176 phosphorylation in ECs. These signaling mechanisms produce arterial hyperpolarization and vasodilation. In contrast, EC PKD2 channels do not contribute to acetylcholine-induced vasodilation, suggesting stimulus-specific function. EC-specific PKD2 knockout elevated blood pressure in mice without altering cardiac function or kidney anatomy. These data demonstrate that flow stimulates PKD2 channels in ECs, leading to SK/IK channel and eNOS activation, hyperpolarization, vasodilation and a reduction in systemic blood pressure. Thus, PKD2 channels are a major component of functional flow sensing in the vasculature.

KCTD5, a novel TRPM4-regulatory protein required for cell migration as a new predictor for breast cancer prognosis

Transient receptor potential melastatin 4 (TRPM4) is a Ca²⁺ -activated nonselective cationic channel that regulates cell migration and contractility. Increased TRPM4 expression has been related to pathologies, in which cytoskeletal rearrangement and cell migration are altered, such as metastatic cancer. Here, we identify the K⁺ channel tetramerization domain 5 (KCTD5) protein, a putative adaptor of cullin3 E3 ubiquitin ligase, as a novel TRPM4-interacting protein. We demonstrate that KCTD5 is a positive regulator of TRPM4 activity by enhancing its Ca²⁺ sensitivity. We show that through its effects on TRPM4 that KCTD5 promotes cell migration and contractility. Finally, we observed that both TRPM4 and KCTD5 expression are increased in distinct patterns in different classes of breast cancer tumor samples. Together, these data support that TRPM4 activity can be regulated through expression levels of either TRPM4 or KCTD5, not only contributing to increased understanding of the molecular mechanisms involved on the regulation of these important ion channels, but also providing information that could inform treatments based on targeting these distinct molecules that define TRPM4 activity.

Ion channels and myogenic activity in retinal arterioles

Retinal pressure autoregulation is an important mechanism that protects the retina by stabilizing retinal blood flow during changes in arterial or intraocular pressure. Similar to other vascular beds, retinal pressure autoregulation is thought to be mediated largely through the myogenic response of small arteries and arterioles which constrict when transmural pressure increases or dilate when it decreases. Over recent years, we and others have investigated the signaling pathways underlying the myogenic response in retinal arterioles, with particular emphasis on the involvement of different ion channels expressed in the smooth muscle layer of these vessels. Here, we review and extend previous work on the expression and spatial distribution of the plasma membrane and sarcoplasmic reticulum ion channels present in retinal vascular smooth muscle cells (VSMCs) and discuss their contribution to pressure-induced myogenic tone in retinal arterioles. This includes new data demonstrating that several key players and modulators of the myogenic response show distinctively heterogeneous expression along the length of the retinal arteriolar network, suggesting differences in myogenic signaling between larger and smaller pre-capillary arterioles. Our immunohistochemical investigations have also highlighted the presence of actin-containing microstructures called myobridges that connect the retinal VSMCs to one another. Although further work is still needed, studies to date investigating myogenic mechanisms in the retina have contributed to a better understanding of how blood flow is regulated in this tissue. They also provide a basis to direct future research into retinal diseases where blood flow changes contribute to the pathology.

Transient receptor potential (TRP) channels: Biosensors for redox environmental stimuli and cellular status

Transient receptor potential (TRP) channels are a family of cation channels that depolarizes the membrane potential and regulates intracellular concentrations of cations such as Ca²⁺. TRP channels are also known to function as "biosensors" to detect changes of the surrounding environment and cellular status. Lines of evidence have unveiled that numerous proteins are subject to redox modification and subsequent signaling. For example, TRPM2, TRPC5, TRPV1, and TRPA1 are known as redox sensors activated by hydrogen peroxide (H₂O₂), nitric oxide (NO), and electrophiles. Thus, these channels facilitate the influx of cations which in turn triggers the appropriate cellular responses against environmental redox stimuli and cellular redox status. In this review, we focus on the recent findings regarding the functions of TRP channels in relation to other ion channels, and other proteins which also go through redox modification of cysteine (Cys) residues. We aim to understand the structural and molecular basis of the redox-sensing mechanisms of TRP channels in exerting various functions under physiological conditions as well as pathological conditions such as cancer malignancy. Their future potential as drug targets will also be discussed.

A structural overview of the ion channels of the TRPM family

The TRPM (transient receptor potential melastatin) family belongs to the superfamily of TRP cation channels. The TRPM subfamily is composed of eight members that are involved in diverse biological functions such as temperature sensing, inflammation, insulin secretion, and redox sensing. Since the first cloning of TRPM1 in 1998, tremendous progress has been made uncovering the function, structure, and pharmacology of this family. Complete structures of TRPM2, TRPM4, and TRPM8, as well as a partial structure of TRPM7, have been determined by cryo-EM, providing insights into their channel assembly, ion permeation, gating mechanisms, and structural pharmacology. Here we summarize the current knowledge about channel structure, emphasizing general features and principles of the structure of TRPM channels discovered since 2017. We also discuss some of the key unresolved issues in the field, including the molecular mechanisms underlying voltage and temperature dependence, as well as the functions of the TRPM channels' C-terminal domains.

Mechanism of Membrane Depolarization Involved in α1A-Adrenoceptor-Mediated Contraction in Rat Tail and Iliac Arteries

Our previous studies have shown that phenylephrine-induced contraction of cutaneous arteries is primarily mediated via α_1A-adrenoceptors, but not α_1D-adrenoceptors that generally mediate vascular contraction, and that the larger part of the contraction is induced in a voltage-dependent Ca²⁺ channel (VDCC)-independent manner. Here, we investigated the mechanism underlying the smaller part of the α_1A-adrenoceptor-mediated contraction, i.e., VDCC-dependent one, in cutaneous arteries. Isometric contraction was measured with wire myograph in endothelium-denuded tail and iliac arterial rings isolated from male Wistar rats. LOE908 (10 µM), a cation channel blocker, partially inhibited the contraction induced by phenylephrine in tail and iliac arteries. Nifedipine (10 µM) further suppressed the phenylephrine-induced contraction that remained in the presence of LOE908 (10 µM) in iliac arteries but barely in tail arteries, suggesting that phenylephrine-induced depolarization in tail arteries is due to the activation of LOE908-sensitive cation channels. In iliac arteries, the contraction induced by A-61603, a specific α_1A-adrenoceptor agonist, was also partially inhibited by LOE908 (10 µM); however, nifedipine had little effect on the A-61603-induced contraction that remained in the presence of LOE908 (10 µM), suggesting that depolarization mediated via α_1A-adrenoceptors is due to the activation of LOE908-sensitive cation channels even in iliac arteries. These results suggest that membrane depolarization mediated via α_1Α-adrenoceptors is caused by cation influx through LOE908-sensitive cation channels. Less contribution of VDCC to phenylephrine-induced contraction in tail arteries compared to in iliac arteries is likely due to that α_1Α-adrenoceptor-mediated activation of VDCC is caused only by depolarization via cation influx through LOE908-sensitive cation channels.

The oncogenic roles of TRPM ion channels in cancer

Transient receptor potential (TRP) proteins are a diverse family of ion channels present in multiple types of tissues. They function as gatekeepers for responses to sensory stimuli including temperature, vision, taste, and pain through their activities in conducting ion fluxes. The TRPM (melastatin) subfamily consists of eight members (i.e., TRPM1-8), which collectively regulate fluxes of various types of cations such as K⁺ , Na⁺ , Ca²⁺ , and Mg²⁺ . Growing evidence in the past two decades indicates that TRPM ion channels, their isoforms, or long noncoding RNAs encoded within the locus may be oncogenes involved in the regulation of cancer cell growth, proliferation, autophagy, invasion, and epithelial-mesenchymal transition, and their significant association with poor clinical outcomes of cancer patients. In this review, we describe and discuss recent findings implicating TRPM channels in different malignancies, their functions, mechanisms, and signaling pathways involved in cancers, as well as summarizing their normal physiological functions and the availability of ion channel pharmacological inhibitors.

Calcium signals that determine vascular resistance

Small arteries in the body control vascular resistance, and therefore, blood pressure and blood flow. Endothelial and smooth muscle cells in the arterial walls respond to various stimuli by altering the vascular resistance on a moment to moment basis. Smooth muscle cells can directly influence arterial diameter by contracting or relaxing, whereas endothelial cells that line the inner walls of the arteries modulate the contractile state of surrounding smooth muscle cells. Cytosolic calcium is a key driver of endothelial and smooth muscle cell functions. Cytosolic calcium can be increased either by calcium release from intracellular stores through IP3 or ryanodine receptors, or the influx of extracellular calcium through ion channels at the cell membrane. Depending on the cell type, spatial localization, source of a calcium signal, and the calcium-sensitive target activated, a particular calcium signal can dilate or constrict the arteries. Calcium signals in the vasculature can be classified into several types based on their source, kinetics, and spatial and temporal properties. The calcium signaling mechanisms in smooth muscle and endothelial cells have been extensively studied in the native or freshly isolated cells, therefore, this review is limited to the discussions of studies in native or freshly isolated cells. This article is categorized under: Biological Mechanisms > Cell Signaling Laboratory Methods and Technologies > Imaging Models of Systems Properties and Processes > Mechanistic Models.

Increased TRPM4 Activity in Cerebral Artery Myocytes Contributes to Cerebral Blood Flow Reduction After Subarachnoid Hemorrhage in Rats

Cerebral blood flow (CBF) reduction underlies unfavorable outcomes after subarachnoid hemorrhage (SAH). Transient receptor potential melastatin-4 (TRPM4) has a pivotal role in cerebral artery myogenic tone maintenance and CBF regulation under physiological conditions. However, the role of TRPM4 in CBF reduction after SAH is unclear. In this study, we aimed at testing whether TRPM4 would contribute to CBF reduction after SAH in vivo and determining underlying mechanisms. Rat SAH model was established by stereotaxic injection of autologous nonheparinized arterial blood at the suprasellar cistern. A TRPM4 blocker, 9-phenanthrol (9-Phe), was infused through an intraventricular catheter connected to a programmed subcutaneous pump to evaluate the contribution of TRPM4 to SAH outcomes. TRPM4 expression and translocation in cerebral artery myocytes were detected by immunoblotting. Macroscopic currents in cerebral artery myocytes were determined by whole-cell patch clamp. Myogenic tone of cerebral arteries was studied by pressurized myography. Cortical and global CBFs were measured via laser Doppler flowmetry and fluorescent microspheres, respectively. After SAH, TRPM4 translocation and macroscopic current density increased significantly. Furthermore, TRPM4 accounted for a greater proportion of myogenic tone after SAH, suggesting an upregulation of TRPM4 activity in response to SAH. Cortical and global CBFs were reduced after SAH, but were restored significantly by 9-Phe, implying that TRPM4 contributed to CBF reduction after SAH. Collectively, these discoveries show that increased TRPM4 activity has a pivotal role in CBF reduction after SAH, and provide a novel target for the management of cerebral perfusion dysfunction following SAH.

Membrane Lipid-K IR 2.x Channel Interactions Enable Hemodynamic Sensing in Cerebral Arteries

Objective—

Inward rectifying K + (K IR ) channels are present in cerebral arterial smooth muscle and endothelial cells, a tandem arrangement suggestive of a dynamic yet undiscovered role for this channel. This study defined whether distinct pools of cerebral arterial K IR channels were uniquely modulated by membrane lipids and hemodynamic stimuli.

Approach and Results—

A Ba 2+ -sensitive K IR current was isolated in smooth muscle and endothelial cells of rat cerebral arteries; molecular analyses subsequently confirmed K IR 2.1/K IR 2.2 mRNA and protein expression in both cells. Patch-clamp electrophysiology next demonstrated that each population of K IR channels was sensitive to key membrane lipids and hemodynamic stimuli. In this regard, endothelial K IR was sensitive to phosphatidylinositol 4,5-bisphosphate content, with depletion impairing the ability of laminar shear stress to activate this channel pool. In contrast, smooth muscle K IR was sensitive to membrane cholesterol content, with sequestration blocking the ability of pressure to inhibit channel activity. The idea that membrane lipids help confer shear stress and pressure sensitivity of K IR channels was confirmed in intact arteries using myography. Virtual models integrating structural/electrical observations reconceptualized K IR as a dynamic regulator of membrane potential working in concert with other currents to set basal tone across a range of shear stresses and intravascular pressures.

Conclusions—

The data show for the first time that specific membrane lipid-K IR interactions enable unique channel populations to sense hemodynamic stimuli and drive vasomotor responses to set basal perfusion in the cerebral circulation.

Properties of single-channel and whole cell Cl- currents in guinea pig detrusor smooth muscle cells

Multiple types of Cl^- channels regulate smooth muscle excitability and contractility in vascular, gastrointestinal, and airway smooth muscle cells. However, little is known about Cl^- channels in detrusor smooth muscle (DSM) cells. Here, we used inside-out single channel and whole cell patch-clamp recordings for detailed biophysical and pharmacological characterizations of Cl^- channels in freshly isolated guinea pig DSM cells. The recorded single Cl^- channels displayed unique gating with multiple subconductive states, a fully opened single-channel conductance of 164 pS, and a reversal potential of -41.5 mV, which is close to the E_Cl of -65 mV, confirming preferential permeability to Cl^-. The Cl^- channel demonstrated strong voltage dependence of activation (half-maximum of mean open probability, V_0.5, ~-20 mV) and robust prolonged openings at depolarizing voltages. The channel displayed similar gating when exposed intracellularly to solutions containing Ca²⁺-free or 1 mM Ca²⁺. In whole cell patch-clamp recordings, macroscopic current demonstrated outward rectification, inhibitions by 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS) and niflumic acid, and insensitivity to chlorotoxin. The outward current was reversibly reduced by 94% replacement of extracellular Cl^- with I^-, Br^-, or methanesulfonate (MsO^-), resulting in anionic permeability sequence: Cl^->Br^->I^->MsO^-. While intracellular Ca²⁺ levels (0, 300 nM, and 1 mM) did not affect the amplitude of Cl^- current and outward rectification, high Ca²⁺ slowed voltage-step current activation at depolarizing voltages. In conclusion, our data reveal for the first time the presence of a Ca²⁺-independent DIDS and niflumic acid-sensitive, voltage-dependent Cl^- channel in the plasma membrane of DSM cells. This channel may be a key regulator of DSM excitability.

Functional Interaction among KCa and TRP Channels for Cardiovascular Physiology: Modern Perspectives on Aging and Chronic Disease

Effective delivery of oxygen and essential nutrients to vital organs and tissues throughout the body requires adequate blood flow supplied through resistance vessels. The intimate relationship between intracellular calcium ([Ca²⁺]_i) and regulation of membrane potential (V_m) is indispensable for maintaining blood flow regulation. In particular, Ca²⁺-activated K⁺ (K_Ca) channels were ascertained as transducers of elevated [Ca²⁺]_i signals into hyperpolarization of V_m as a pathway for decreasing vascular resistance, thereby enhancing blood flow. Recent evidence also supports the reverse role for K_Ca channels, in which they facilitate Ca²⁺ influx into the cell interior through open non-selective cation (e.g., transient receptor potential; TRP) channels in accord with robust electrical (hyperpolarization) and concentration (~20,000-fold) transmembrane gradients for Ca²⁺. Such an arrangement supports a feed-forward activation of V_m hyperpolarization while potentially boosting production of nitric oxide. Furthermore, in vascular types expressing TRP channels but deficient in functional K_Ca channels (e.g., collecting lymphatic endothelium), there are profound alterations such as downstream depolarizing ionic fluxes and the absence of dynamic hyperpolarizing events. Altogether, this review is a refined set of evidence-based perspectives focused on the role of the endothelial K_Ca and TRP channels throughout multiple experimental animal models and vascular types. We discuss the diverse interactions among K_Ca and TRP channels to integrate Ca²⁺, oxidative, and electrical signaling in the context of cardiovascular physiology and pathology. Building from a foundation of cellular biophysical data throughout a wide and diverse compilation of significant discoveries, a translational narrative is provided for readers toward the treatment and prevention of chronic, age-related cardiovascular disease.

Involvement of transient receptor potential melastatin 4 channels in the resting membrane potential setting and cholinergic contractile responses in mouse detrusor and ileal smooth muscles

Here, we investigated the effects of 9-hydroxyphenanthrene (9-phenanthrol), a potent and selective transient receptor potential melastatin 4 (TRPM4) channel blocker, on the resting membrane potential and cholinergic contractile responses to elucidate the functional role of TRPM4 channels in the contractile activities of mouse detrusor and ileal longitudinal smooth muscles. We observed that, 9-phenanthrol (3-30 µM) did not significantly inhibit high K⁺-induced contractions in both preparations; however, 9-phenanthrol (10 µM) strongly inhibited cholinergic contractions evoked by electrical field stimulation in detrusor preparations compared to inhibitions in ileal preparations. 9-Phenanthrol (10 µM) significantly inhibited the muscarinic agonist, carbachol-induced contractile responses and slowed the maximum upstroke velocities of the contraction in detrusor preparations. However, the agent (10 µM) did not inhibit the contractions due to intracellular Ca²⁺ release evoked by carbachol, suggesting that the inhibitory effect of 9-phenanthrol may primarily be due to the inhibition of the membrane depolarization process incurred by TRPM4 channels. On the other hand, 9-phenanthrol (10 µM) did not affect carbachol-induced contractile responses in ileal preparations. Further, 9-phenanthrol (10 µM) significantly hyperpolarized the resting membrane potential and decreased the basal tone in both detrusor and ileal muscle preparations. Taken together, our results suggest that TRPM4 channels are constitutively active and are involved in setting of the resting membrane potential, thereby regulating the basal tone in detrusor and ileal smooth muscles. Thus, TRPM4 channels play a significant role in cholinergic signaling in detrusor, but not ileal, smooth muscles.

TRP Channels as Sensors of Chemically-Induced Changes in Cell Membrane Mechanical Properties

Transient Receptor Potential ion channels (TRPs) have been described as polymodal sensors, being responsible for transducing a wide variety of stimuli, and being involved in sensory functions such as chemosensation, thermosensation, mechanosensation, and photosensation. Mechanical and chemical stresses exerted on the membrane can be transduced by specialized proteins into meaningful intracellular biochemical signaling, resulting in physiological changes. Of particular interest are compounds that can change the local physical properties of the membrane, thereby affecting nearby proteins, such as TRP channels, which are highly sensitive to the membrane environment. In this review, we provide an overview of the current knowledge of TRP channel activation as a result of changes in the membrane properties induced by amphipathic structural lipidic components such as cholesterol and diacylglycerol, and by exogenous amphipathic bacterial endotoxins.

Arterial smooth muscle cell PKD2 (TRPP1) channels regulate systemic blood pressure

Systemic blood pressure is determined, in part, by arterial smooth muscle cells (myocytes). Several Transient Receptor Potential (TRP) channels are proposed to be expressed in arterial myocytes, but it is unclear if these proteins control physiological blood pressure and contribute to hypertension in vivo. We generated the first inducible, smooth muscle-specific knockout mice for a TRP channel, namely for PKD2 (TRPP1), to investigate arterial myocyte and blood pressure regulation by this protein. Using this model, we show that intravascular pressure and α₁-adrenoceptors activate PKD2 channels in arterial myocytes of different systemic organs. PKD2 channel activation in arterial myocytes leads to an inward Na⁺ current, membrane depolarization and vasoconstriction. Inducible, smooth muscle cell-specific PKD2 knockout lowers both physiological blood pressure and hypertension and prevents pathological arterial remodeling during hypertension. Thus, arterial myocyte PKD2 controls systemic blood pressure and targeting this TRP channel reduces high blood pressure.

Dehydroepiandrosterone inhibits ICa,L and its window current in voltage-dependent and -independent mechanisms in arterial smooth muscle cells

Dehydroepiandrosterone (DHEA) is an adrenal steroid hormone, which has the highest serum concentration among steroid hormones with DHEA sulfate (DHEAS). DHEA possesses an inhibitory action on glucose-6-phosphate dehydrogenase (G6PD), the first pentose-phosphate pathway enzyme that reduces NADP⁺ to NADPH. DHEA induced relaxation of high K⁺-induced contraction in rat arterial strips, whereas DHEAS barely induced it. We studied the effects of DHEA on L-type Ca²⁺ current ( I_Ca,L) of A7r5 arterial smooth muscle cells and compared the mechanism of inhibition with that produced by the 6-aminonicotinamide (6-AN) competitive inhibitor of G6PD. DHEA moderately inhibited I_Ca,L that was elicited from a holding potential (HP) of -80 mV [voltage-independent inhibition (VIDI)] and accelerated decay of I_Ca,L during the depolarization pulse [voltage-dependent inhibition (VDI)]. DHEA-induced VDI decreased peak I_Ca,L at depolarized HPs. By applying repetitive depolarization pulses from multiple HPs, novel HP-dependent steady-state inactivation curves ( f_∞-HP) were constructed. DHEA shifted f_∞-HP to the left and inhibited the window current, which was recorded at depolarized HPs and obtained as a product of current-voltage relationship and f_∞-HP. The IC₅₀ value of I_Ca,L inhibition was much higher than serum concentration. DHEA-induced VDI was downregulated by the dialysis of guanosine 5'- O-(2-thiodiphosphate), which shifted f_∞-voltage to the right before the application of DHEA. 6-AN gradually and irreversibly inhibited I_Ca,L by VIDI, suggesting that the inhibition of G6PD is involved in DHEA-induced VIDI. In 6-AN-pretreated cells, DHEA induced additional inhibition by increasing VIDI and generating VDI. The inhibition of G6PD underlies DHEA-induced VIDI, and DHEA additionally induces VDI as described for Ca²⁺ channel blockers. NEW & NOTEWORTHY Dehydroepiandrosterone, the most abundantly released adrenal steroid hormone with dehydroepiandrosterone sulfate, inhibited L-type Ca²⁺ current and its window current in aortic smooth muscle cells. The IC₅₀ value of inhibition decreased with the depolarization of holding potential to 15 µM at -20 mV. The inhibition occurred in a voltage-dependent manner as described for Ca²⁺ channel blockers and in a voltage-independent manner because of the inhibition of glucose-6-phosphate dehydrogenase.

Ion Channels in Pulmonary Hypertension: A Therapeutic Interest?

Pulmonary arterial hypertension (PAH) is a multifactorial and severe disease without curative therapies. PAH pathobiology involves altered pulmonary arterial tone, endothelial dysfunction, distal pulmonary vessel remodeling, and inflammation, which could all depend on ion channel activities (K⁺, Ca²⁺, Na⁺ and Cl^-). This review focuses on ion channels in the pulmonary vasculature and discusses their pathophysiological contribution to PAH as well as their therapeutic potential in PAH.

Role of age, Rho-kinase 2 expression, and G protein-mediated signaling in the myogenic response in mouse small mesenteric arteries

The myogenic response (MR) and myogenic tone (MT) in resistance vessels is crucial for maintaining peripheral vascular resistance and blood flow autoregulation. Development of MT involves G protein-coupled receptors, and may be affected by aging.

AIMS

(1) to estimate the mesenteric blood flow in myogenically active small mesenteric arteries; (2) to investigate the signaling from Gαq/11 and/or Gα12 activation to MT development; (3) to investigate the role of Rho-kinase 2 and aging on MT in mesenteric resistance arteries.

METHODS

we used pressure myography, quantitative real-time PCR, and immunolocalization to study small (<200 μm) mesenteric arteries (SMA) from young, mature adult, and middle aged mice.

RESULTS

Poiseuille flow calculations indicated autoregulation of blood flow at 60-120 mm Hg arterial pressure. Gαq/11 and Gα12 were abundantly expressed at the mRNA and protein levels in SMA. The Gαq/11 inhibitor YM-254890 suppressed MT development, and the Phosholipase C inhibitors U73122 and ET-18-OCH3 robustly inhibited it. We found an age-dependent increase in ROCK2 mRNA expression, and in basal MT. The specific ROCK2 inhibitor KD025 robustly inhibited MT in SMAs in all mice with an age-dependent variation in KD025 sensitivity. The inhibitory effect of KD025 was not prevented by the L-type Ca2+ channel activator BayK 8644. KD025 reversibly inhibited MT and endothelin-1 vasoconstriction in small pial arteries from Göttingen minipigs.

CONCLUSIONS

MT development in SMAs occurs through a Gαq/11 /PLC/Ca2+ -dependent pathway, and is maintained via ROCK2-mediated Ca2+ sensitization. Increased MT at mature adulthood can be explained by increased ROCK2 expression/activity.

Intraluminal pressure triggers myogenic response via activation of calcium spark and calcium-activated chloride channel in rat renal afferent arteriole

Myogenic contraction of renal arterioles is an important regulatory mechanism for renal blood flow autoregulation. We have previously demonstrated that integrin-mediated mechanical force increases the occurrence of Ca²⁺ sparks in freshly isolated renal vascular smooth muscle cells (VSMCs). To further test whether the generation of Ca²⁺ sparks is a downstream signal of mechanotransduction in pressure-induced myogenic constriction, the relationship between Ca²⁺ sparks and transmural perfusion pressure was investigated in intact VSMCs of pressurized rat afferent arterioles. Spontaneous Ca²⁺ sparks were found in VSMCs when afferent arterioles were perfused at 80 mmHg. The spark frequency was significantly increased when perfusion pressure was increased to 120 mmHg. A similar increase of spark frequency was also observed in arterioles stimulated with β₁-integrin-activating antibody. Moreover, spark frequency was significantly higher in arterioles of spontaneous hypertensive rats at 80 and 120 mmHg. Spontaneous membrane current recorded using whole cell perforated patch in renal VSMCs showed predominant activity of spontaneous transient inward currents instead of spontaneous transient outward currents when holding potential was set close to physiological resting membrane potential. Real-time PCR and immunohistochemistry confirmed the expression of Ca²⁺-activated Cl^- channel (Cl_Ca) TMEM16A in renal VSMCs. Inhibition of TMEM16A with T16Ainh-A01 impaired the pressure-induced myogenic contraction in perfused afferent arterioles. Our study, for the first time to our knowledge, detected Ca²⁺ sparks in VSMCs of intact afferent arterioles, and their frequencies were positively modulated by the perfusion pressure. Our results suggest that Ca²⁺ sparks may couple to Cl_Ca channels and trigger pressure-induced myogenic constriction via membrane depolarization.

Disentangling the role of TRPM4 in hippocampus-dependent plasticity and learning: an electrophysiological, behavioral and FMRI approach

Hippocampal long-term potentiation (LTP) has been extensively studied as a cellular model of learning and memory. Recently, we described a central function of the Transient Receptor Potential M4 (TRPM4) channel in hippocampal LTP in mice in vitro. Here, we used Trpm4 knock-out (Trpm4^-/-) rats to scrutinize TRPM4's role in the intact brain in vivo. After having confirmed the previous in vitro findings in mice, we studied hippocampal synaptic plasticity by chronic recordings in freely moving rats, hippocampus-dependent learning by a behavioral battery and hippocampal-cortical connectivity by fMRI. The electrophysiological investigation supports an involvement of TRPM4 in LTP depending on the induction protocol. Moreover, an exhaustive analysis of the LTP kinetics point to mechanistic changes in LTP by trpm4 deletion. General behavior as measured by open field test, light-dark box and elevated plus maze was inconspicuous in Trpm4^-/- rats. However, they showed a distinct deficit in spatial working and reference memory associated to the Barnes maze and T-maze test, respectively. In contrast, performance of the Trpm4^-/- in the Morris water maze was unaltered. Finally, fMRI investigation of the effects of a strong LTP induction manifested BOLD responses in the ipsilateral and contralateral hippocampus and the prefrontal cortex of both groups. Yet, the initial BOLD response in the stimulated hippocampal area of Trpm4^-/- was significantly enhanced compared to WT rats. Our findings at the cellular, behavioral and system level point to a relevant role for TRPM4 in specific types of hippocampal synaptic plasticity and learning but not in hippocampal-prefrontal interaction.

Structures of the calcium-activated, non-selective cation channel TRPM4

TRPM4 is a calcium-activated, phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P₂) -modulated, non-selective cation channel that belongs to the family of melastatin-related transient receptor potential (TRPM) channels. Here we present the electron cryo-microscopy structures of the mouse TRPM4 channel with and without ATP. TRPM4 consists of multiple transmembrane and cytosolic domains, which assemble into a three-tiered architecture. The N-terminal nucleotide-binding domain and the C-terminal coiled-coil participate in the tetrameric assembly of the channel; ATP binds at the nucleotide-binding domain and inhibits channel activity. TRPM4 has an exceptionally wide filter but is only permeable to monovalent cations; filter residue Gln973 is essential in defining monovalent selectivity. The S1-S4 domain and the post-S6 TRP domain form the central gating apparatus that probably houses the Ca²⁺- and PtdIns(4,5)P₂-binding sites. These structures provide an essential starting point for elucidating the complex gating mechanisms of TRPM4 and reveal the molecular architecture of the TRPM family.

Role of TRP Channels in Dinoflagellate Mechanotransduction

Transient receptor potential (TRP) ion channels are common components of mechanosensing pathways, mainly described in mammals and other multicellular organisms. To gain insight into the evolutionary origins of eukaryotic mechanosensory proteins, we investigated the involvement of TRP channels in mechanosensing in a unicellular eukaryotic protist, the dinoflagellate Lingulodinium polyedra. BLASTP analysis of the protein sequences predicted from the L. polyedra transcriptome revealed six sequences with high similarity to human TRPM2, TRPM8, TRPML2, TRPP1, and TRPP2; and characteristic TRP domains were identified in all sequences. In a phylogenetic tree including all mammalian TRP subfamilies and TRP channel sequences from unicellular and multicellular organisms, the L. polyedra sequences grouped with the TRPM, TPPML, and TRPP clades. In pharmacological experiments, we used the intrinsic bioluminescence of L. polyedra as a reporter of mechanoresponsivity. Capsaicin and RN1734, agonists of mammalian TRPV, and arachidonic acid, an agonist of mammalian TRPV, TRPA, TRPM, and Drosophila TRP, all stimulated bioluminescence in L. polyedra. Mechanical stimulation of bioluminescence, but not capsaicin-stimulated bioluminescence, was inhibited by gadolinium (Gd³⁺), a general inhibitor of mechanosensitive ion channels, and the phospholipase C (PLC) inhibitor U73122. These pharmacological results are consistent with the involvement of TRP-like channels in mechanosensing by L. polyedra. The TRP channels do not appear to be mechanoreceptors but rather are components of the mechanotransduction signaling pathway and may be activated via a PLC-dependent mechanism. The presence and function of TRP channels in a dinoflagellate emphasize the evolutionary conservation of both the channel structures and their functions.

Microtubule structures underlying the sarcoplasmic reticulum support peripheral coupling sites to regulate smooth muscle contractility

Junctional membrane complexes facilitate excitation-contraction coupling in skeletal and cardiac muscle cells by forming subcellular invaginations that maintain close (≤20 nm) proximity of ryanodine receptors (RyRs) on the sarcoplasmic reticulum (SR) with voltage-dependent Ca2+ channels in the plasma membrane. In fully differentiated smooth muscle cells, junctional membrane complexes occur as distributed sites of peripheral coupling. We investigated the role of the cytoskeleton in maintaining peripheral coupling and associated Ca2+ signaling networks within native smooth muscle cells of mouse and rat cerebral arteries. Using live-cell confocal and superresolution microscopy, we found that the tight interactions between the SR and the plasma membrane in these cells relied on arching microtubule structures present at the periphery of smooth muscle cells and were independent of the actin cytoskeleton. Loss of peripheral coupling associated with microtubule depolymerization altered the spatiotemporal properties of localized Ca2+ sparks generated by the release of Ca2+ through type 2 RyRs (RyR2s) on the SR and decreased the number of sites of colocalization between RyR2s and large-conductance Ca2+-activated K+ (BK) channels. The reduced BK channel activity associated with the loss of SR-plasma membrane interactions was accompanied by increased pressure-induced constriction of cerebral resistance arteries. We conclude that microtubule structures maintain peripheral coupling in contractile smooth muscle cells, which is crucial for the regulation of contractility and cerebral vascular tone.

TRPV4 channels contribute to renal myogenic autoregulation in neonatal pigs

Myogenic response, a phenomenon in which resistance size arteries and arterioles swiftly constrict or dilate in response to an acute elevation or reduction, respectively, in intravascular pressure is a key component of renal autoregulation mechanisms. Although it is well established that the renal system is functionally immature in neonates, mechanisms that regulate neonatal renal blood flow (RBF) remain poorly understood. In this study, we investigated the hypothesis that members of the transient receptor potential vanilloid (TRPV) channels are molecular components of renal myogenic constriction in newborns. We show that unlike TRPV1-3, TRPV4 channels are predominantly expressed in neonatal pig preglomerular vascular smooth muscle cells (SMCs). Intracellular Ca²⁺ concentration ([Ca²⁺]_i) elevation induced by osmotic cell swelling was attenuated by TRPV4, L-type Ca²⁺, and stretch-activated Ca²⁺ channel blockers but not phospholipase A₂ inhibitor. Blockade of TRPV4 channels reversed steady-state myogenic tone and inhibited pressure-induced membrane depolarization, [Ca²⁺]_i elevation, and constriction in distal interlobular arteries. A step increase in arterial pressure induced efficient autoregulation of renal cortical perfusion and total RBF in anesthetized and mechanically ventilated neonatal pigs. Moreover, intrarenal arterial infusion of the TRPV4 channel blockers HC 067047 and RN 1734 attenuated renal autoregulation in the pigs. These data suggest that renal myogenic autoregulation is functional in neonates. Our findings also indicate that TRPV4 channels are mechanosensors in neonatal pig preglomerular vascular SMCs and contribute to renal myogenic autoregulation.

Microvascular NADPH oxidase in health and disease

The systemic and cerebral microcirculation contribute critically to regulation of local and global blood flow and perfusion pressure. Microvascular dysfunction, commonly seen in numerous cardiovascular pathologies, is associated with alterations in the oxidative environment including potentiated production of reactive oxygen species (ROS) and subsequent activation of redox signaling pathways. NADPH oxidases (Noxs) are a primary source of ROS in the vascular system and play a central role in cardiovascular health and disease. In this review, we focus on the roles of Noxs in ROS generation in resistance arterioles and capillaries, and summarize their contributions to microvascular physiology and pathophysiology in both systemic and cerebral microcirculation. In light of the accumulating evidence that Noxs are pivotal players in vascular dysfunction of resistance arterioles, selectively targeting Nox isozymes could emerge as a novel and effective therapeutic strategy for preventing and treating microvascular diseases.

The angiotensin II receptor type 1b is the primary sensor of intraluminal pressure in cerebral artery smooth muscle cells

KEY POINTS

The angiotensin II receptor type 1b (AT1 Rb ) is the primary sensor of intraluminal pressure in cerebral arteries. Pressure or membrane-stretch induced stimulation of AT1 Rb activates the TRPM4 channel and results in inward transient cation currents that depolarize smooth muscle cells, leading to vasoconstriction. Activation of either AT1 Ra or AT1 Rb with angiotensin II stimulates TRPM4 currents in cerebral artery myocytes and vasoconstriction of cerebral arteries. The expression of AT1 Rb mRNA is ∼30-fold higher than AT1 Ra in whole cerebral arteries and ∼45-fold higher in isolated cerebral artery smooth muscle cells. Higher levels of expression are likely to account for the obligatory role of AT1 Rb for pressure-induced vasoconstriction. ABSTRACT: Myogenic vasoconstriction, which reflects the intrinsic ability of smooth muscle cells to contract in response to increases in intraluminal pressure, is critically important for the autoregulation of blood flow. In smooth muscle cells from cerebral arteries, increasing intraluminal pressure engages a signalling cascade that stimulates cation influx through transient receptor potential (TRP) melastatin 4 (TRPM4) channels to cause membrane depolarization and vasoconstriction. Substantial evidence indicates that the angiotensin II receptor type 1 (AT1 R) is inherently mechanosensitive and initiates this signalling pathway. Rodents express two types of AT1 R - AT1 Ra and AT1 Rb - and conflicting studies provide support for either isoform as the primary sensor of intraluminal pressure in peripheral arteries. We hypothesized that mechanical activation of AT1 Ra increases TRPM4 currents to induce myogenic constriction of cerebral arteries. However, we found that development of myogenic tone was greater in arteries from AT1 Ra knockout animals compared with controls. In patch-clamp experiments using native cerebral arterial myocytes, membrane stretch-induced cation currents were blocked by the TRPM4 inhibitor 9-phenanthrol in both groups. Further, the AT1 R blocker losartan (1 μm) diminished myogenic tone and blocked stretch-induced cation currents in cerebral arteries from both groups. Activation of AT1 R with angiotensin II (30 nm) also increased TRPM4 currents in smooth muscle cells and constricted cerebral arteries from both groups. Expression of AT1 Rb mRNA was ∼30-fold greater than AT1 Ra in cerebral arteries, and knockdown of AT1 Rb selectively diminished myogenic constriction. We conclude that AT1 Rb , acting upstream of TRPM4 channels, is the primary sensor of intraluminal pressure in cerebral artery smooth muscle cells.

Regional heterogeneity in the mechanisms of myogenic tone in hamster arterioles

Myogenic tone is an important feature of arterioles and resistance arteries, but the mechanisms responsible for this hallmark characteristic remain unclear. We used pharmacological inhibitors to compare the roles played by phospholipase C (PLC; 10 μM U73122), inositol 1,4,5-trisphosphate receptors (IP₃Rs; 100 μM 2-aminoethoxydiphenylborane), protein kinase C (10 μM bisindolylmaleimide I), angiotensin II type 1 receptors (1 μM losartan), Rho kinase (10 nM-30 μM Y27632 or 300 nM H1152), stretch-activated ion channels (10 nM-1 μM Gd³⁺ or 5 μM spider venom toxin GsMTx-4) and L-type voltage-gated Ca²⁺ channels (0.3-100 μM diltiazem) in myogenic tone of cannulated, pressurized (80 cmH₂O), second-order hamster cremaster or cheek pouch arterioles. Effective inhibition of either PLC or IP₃Rs dilated cremaster arterioles, inhibited Ca²⁺ waves, and reduced global Ca²⁺ levels. In contrast, cheek pouch arterioles did not display Ca²⁺ waves and inhibition of PLC or IP₃Rs had no effect on myogenic tone or intracellular Ca²⁺ levels. Inhibition of Rho kinase dilated both cheek pouch and cremaster arterioles with equal efficacy and potency but also reduced intracellular Ca²⁺ signals in both arterioles. Similarly, inhibition of mechanosensitive ion channels with Gd²⁺ or GsMTx-4 produced comparable dilation in both arterioles. Inhibition of L-type Ca²⁺ channels with diltiazem was more effective in dilating cremaster (86 ± 5% dilation, n = 4) than cheek pouch arterioles (54 ± 4% dilation, n = 6, P < 0.05). Thus, there are substantial differences in the mechanisms underlying myogenic tone in hamster cremaster and cheek pouch arterioles. Regional heterogeneity in myogenic mechanisms could provide new targets for drug development to improve regional blood flow in a tissue-specific manner.NEW & NOTEWORTHY Regional heterogeneity in the mechanisms of pressure-induced myogenic tone implies that resistance vessels may be able to alter myogenic signaling pathways to adapt to their environment. A better understanding of the spectrum of myogenic mechanisms could provide new targets to treat diseases that affect resistance artery and arteriolar function.

Regulation of transient receptor potential melastatin 4 channel by sarcoplasmic reticulum inositol trisphosphate receptors: Role in human detrusor smooth muscle function

We recently reported key physiologic roles for Ca²⁺-activated transient receptor potential melastatin 4 (TRPM4) channels in detrusor smooth muscle (DSM). However, the Ca²⁺-signaling mechanisms governing TRPM4 channel activity in human DSM cells are unexplored. As the TRPM4 channels are activated by Ca²⁺, inositol 1,4,5-trisphosphate receptor (IP₃R)-mediated Ca²⁺ release from the sarcoplasmic reticulum represents a potential Ca²⁺ source for TRPM4 channel activation. We used clinically-characterized human DSM tissues to investigate the molecular and functional interactions of the IP₃Rs and TRPM4 channels. With in situ proximity ligation assay (PLA) and perforated patch-clamp electrophysiology, we tested the hypothesis that TRPM4 channels are tightly associated with the IP₃Rs and are activated by IP₃R-mediated Ca²⁺ release in human DSM. With in situ PLA, we demonstrated co-localization of the TRPM4 channels and IP₃Rs in human DSM cells. As the TRPM4 channels and IP₃Rs must be located within close apposition to functionally interact, these findings support the concept of a potential Ca²⁺-mediated TRPM4-IP₃R regulatory mechanism. To investigate IP₃R regulation of TRPM4 channel activity, we sought to determine the consequences of IP₃R pharmacological inhibition on TRPM4 channel-mediated transient inward cation currents (TICCs). In freshly-isolated human DSM cells, blocking the IP₃Rs with the selective IP₃R inhibitor xestospongin-C significantly decreased TICCs. The data suggest that IP₃Rs have a key role in mediating the Ca²⁺-dependent activation of TRPM4 channels in human DSM. The study provides novel insight into the molecular and cellular mechanisms regulating TRPM4 channels by revealing that TRPM4 channels and IP₃Rs are spatially and functionally coupled in human DSM.

Muscling in on TRP channels in vascular smooth muscle cells and cardiomyocytes

The human TRP protein family comprises a family of 27 cation channels with diverse permeation and gating properties. The common theme is that they are very important regulators of intracellular Ca²⁺ signaling in diverse cell types, either by providing a Ca²⁺ influx pathway, or by depolarising the membrane potential, which on one hand triggers the activation of voltage-gated Ca²⁺ channels, and on the other limits the driving force for Ca²⁺ entry. Here we focus on the role of these TRP channels in vascular smooth muscle and cardiac striated muscle. We give an overview of highlights from the recent literature, and highlight the important and diverse roles of TRP channels in the pathophysiology of the cardiovascular system. The discovery of the superfamily of Transient Receptor Potential (TRP) channels has significantly enhanced our knowledge of multiple signal transduction mechanisms in cardiac muscle and vascular smooth muscle cells (VSMC). In recent years, multiple studies have provided evidence for the involvement of these channels, not only in the regulation of contraction, but also in cell proliferation and remodeling in pathological conditions. The mammalian family of TRP cation channels is composed by 28 genes which can be divided into 6 subfamilies groups based on sequence similarity: TRPC (Canonical), TRPM (Melastatin), TRPML (Mucolipins), TRPV (Vanilloid), TRPP (Policystin) and TRPA (Ankyrin-rich protein). Functional TRP channels are believed to form four-unit complexes in the plasma, each of them expressed with six transmembrane domain and intracellular N and C termini. Here we review the current knowledge on the expression of TRP channels in both muscle types, and discuss their functional properties and role in physiological and pathophysiological processes.

Proteoglycans, ion channels and cell-matrix adhesion

Cell surface proteoglycans comprise a transmembrane or membrane-associated core protein to which one or more glycosaminoglycan chains are covalently attached. They are ubiquitous receptors on nearly all animal cell surfaces. In mammals, the cell surface proteoglycans include the six glypicans, CD44, NG2 (CSPG4), neuropilin-1 and four syndecans. A single syndecan is present in invertebrates such as nematodes and insects. Uniquely, syndecans are receptors for many classes of proteins that can bind to the heparan sulphate chains present on syndecan core proteins. These range from cytokines, chemokines, growth factors and morphogens to enzymes and extracellular matrix (ECM) glycoproteins and collagens. Extracellular interactions with other receptors, such as some integrins, are mediated by the core protein. This places syndecans at the nexus of many cellular responses to extracellular cues in development, maintenance, repair and disease. The cytoplasmic domains of syndecans, while having no intrinsic kinase activity, can nevertheless signal through binding proteins. All syndecans appear to be connected to the actin cytoskeleton and can therefore contribute to cell adhesion, notably to the ECM and migration. Recent data now suggest that syndecans can regulate stretch-activated ion channels. The structure and function of the syndecans and the ion channels are reviewed here, along with an analysis of ion channel functions in cell-matrix adhesion. This area sheds new light on the syndecans, not least since evidence suggests that this is an evolutionarily conserved relationship that is also potentially important in the progression of some common diseases where syndecans are implicated.

Distance constraints on activation of TRPV4 channels by AKAP150-bound PKCα in arterial myocytes

Vascular smooth muscle tone can be regulated by angiotensin II, which enhances TRPV4 channel activity via AKAP150-bound protein kinase C. Tajada et al. show that the effect of AKAP150 on TRPV4 channels is inversely proportional to the distance between them, which varies with sex and arterial bed.

TRPV4 (transient receptor potential vanilloid 4) channels are Ca²⁺-permeable channels that play a key role in regulating vascular tone. In arterial myocytes, opening of TRPV4 channels creates local increases in Ca²⁺ influx, detectable optically as “TRPV4 sparklets.” TRPV4 sparklet activity can be enhanced by the action of the vasoconstrictor angiotensin II (AngII). This modulation depends on the activation of subcellular signaling domains that comprise protein kinase C α (PKCα) bound to the anchoring protein AKAP150. Here, we used super-resolution nanoscopy, patch-clamp electrophysiology, Ca²⁺ imaging, and mathematical modeling approaches to test the hypothesis that AKAP150-dependent modulation of TRPV4 channels is critically dependent on the distance between these two proteins in the sarcolemma of arterial myocytes. Our data show that the distance between AKAP150 and TRPV4 channel clusters varies with sex and arterial bed. Consistent with our hypothesis, we further find that basal and AngII-induced TRPV4 channel activity decays exponentially as the distance between TRPV4 and AKAP150 increases. Our data suggest a maximum radius of action of ∼200 nm for local modulation of TRPV4 channels by AKAP150-associated PKCα.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

T-type Ca2+ channels and autoregulation of local blood flow

L-type voltage gated Ca²⁺ channels are considered to be the primary source of calcium influx during the myogenic response. However, many vascular beds also express T-type voltage gated Ca²⁺ channels. Recent studies suggest that these channels may also play a role in autoregulation. At low pressures (40-80 mmHg) T-type channels affect myogenic responses in cerebral and mesenteric vascular beds. T-type channels also seem to be involved in skeletal muscle autoregulation. This review discusses the expression and role of T-type voltage gated Ca²⁺ channels in the autoregulation of several different vascular beds. Lack of specific pharmacological inhibitors has been a huge challenge in the field. Now the research has been strengthened by genetically modified models such as mice lacking expression of T-type voltage gated Ca²⁺ channels (Ca_V3.1 and Ca_V3.2). Hopefully, these new tools will help further elucidate the role of voltage gated T-type Ca²⁺ channels in autoregulation and vascular function.

Mechanical activation of angiotensin II type 1 receptors causes actin remodelling and myogenic responsiveness in skeletal muscle arterioles

KEY POINTS

Candesartan, an inverse agonist of the type 1 angiotensin II receptor (AT1 R), causes a concentration-dependent inhibition of pressure-dependent myogenic tone consistent with previous reports of mechanosensitivity of this G protein-coupled receptor. Mechanoactivation of the AT1 R occurs independently of local angiotensin II production and the type 2 angiotensin receptor. Mechanoactivation of the AT1 R stimulates actin polymerization by a protein kinase C-dependent mechanism, but independently of a change in intracellular Ca2+ . Using atomic force microscopy, changes in single vascular smooth muscle cell cortical actin are observed to remodel following mechanoactivation of the AT1 R.

ABSTRACT

The Gq/11 protein-coupled angiotensin II type 1 receptor (AT1 R) has been shown to be activated by mechanical stimuli. In the vascular system, evidence supports the AT1 R being a mechanosensor that contributes to arteriolar myogenic constriction. The aim of this study was to determine if AT1 R mechanoactivation affects myogenic constriction in skeletal muscle arterioles and to determine underlying cellular mechanisms. Using pressure myography to study rat isolated first-order cremaster muscle arterioles the AT1 R inhibitor candesartan (10-7 -10-5  m) showed partial but concentration-dependent inhibition of myogenic reactivity. Inhibition was demonstrated by a rightward shift in the pressure-diameter relationship over the intraluminal pressure range, 30-110 mmHg. Pressure-induced changes in global vascular smooth muscle intracellular Ca2+ (using Fura-2) were similar in the absence or presence of candesartan, indicating that AT1 R-mediated myogenic constriction relies on Ca2+ -independent downstream signalling. The diacylglycerol analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG) reversed the inhibitory effect of candesartan, while this rescue effect was prevented by the protein kinase C (PKC) inhibitor GF 109203X. Both candesartan and PKC inhibition caused increased G-actin levels, as determined by Western blotting of vessel lysates, supporting involvement of cytoskeletal remodelling. At the single vascular smooth muscle cell level, atomic force microscopy showed that cell swelling (stretch) with hypotonic buffer also caused thickening of cortical actin fibres and this was blocked by candesartan. Collectively, the present studies support growing evidence for novel modes of activation of the AT1 R in arterioles and suggest that mechanically activated AT1 R generates diacylglycerol, which in turn activates PKC which induces the actin cytoskeleton reorganization that is required for pressure-induced vasoconstriction.

Calcium Channels in Vascular Smooth Muscle

Calcium (Ca²⁺) plays a central role in excitation, contraction, transcription, and proliferation of vascular smooth muscle cells (VSMs). Precise regulation of intracellular Ca²⁺ concentration ([Ca²⁺]_i) is crucial for proper physiological VSM function. Studies over the last several decades have revealed that VSMs express a variety of Ca²⁺-permeable channels that orchestrate a dynamic, yet finely tuned regulation of [Ca²⁺]_i. In this review, we discuss the major Ca²⁺-permeable channels expressed in VSM and their contribution to vascular physiology and pathology.

The role of transient receptor potential channels in hypertension and metabolic vascular damage

NEW FINDINGS

What is the topic of this review? Transient receptor potential (TRP) channels are highly implicated in the pathogenesis of hypertension and the regulation of metabolism. What advances does it highlight? Dysfunction of TRP channels leads to hypertension and metabolic disorders. Elucidating the role of TRP channels in hypertension and metabolic vascular damage would facilitate the design of novel target therapeutics for these intractable diseases. Intracellular Ca²⁺ homeostasis is critical for vascular function and the regulation of metabolism. Metabolic disorders are major risk factors for hypertension. A family of transient receptor potential (TRP) channels plays an important role in the regulation of cellular calcium signalling and cardiometabolic function. Emerging evidence indicates that TRP channels are highly implicated in the pathogenesis of hypertension and metabolic disorders. Dysfunction of TRP channels leads to hypertension and metabolic dysfunction. Activation of certain subtypes of TRP channels could attenuate metabolic vascular damage and alleviate hypertension. Therefore, elucidating the role of TRP channels in the physiological state and in cardiometabolic diseases will facilitate the design of novel targeted therapeutics for these intractable diseases.

Synthetic Peptides as cGMP-Independent Activators of cGMP-Dependent Protein Kinase Iα

PKG is a multifaceted signaling molecule and potential pharmaceutical target due to its role in smooth muscle function. A helix identified in the structure of the regulatory domain of PKG Iα suggests a novel architecture of the holoenzyme. In this study, a set of synthetic peptides (S-tides), derived from this helix, was found to bind to and activate PKG Iα in a cyclic guanosine monophosphate (cGMP)-independent manner. The most potent S-tide derivative (S1.5) increased the open probability of the potassium channel KCa1.1 to levels equivalent to saturating cGMP. Introduction of S1.5 to smooth muscle cells in isolated, endothelium-denuded cerebral arteries through a modified reversible permeabilization procedure inhibited myogenic constriction. In contrast, in endothelium-intact vessels S1.5 had no effect on myogenic tone. This suggests that PKG Iα activation by S1.5 in vascular smooth muscle would be sufficient to inhibit augmented arterial contractility that frequently occurs following endothelial damage associated with cardiovascular disease.

Hypertension: the missing WNKs

The With no Lysine [K] (WNK) family of enzymes are central in the regulation of blood pressure. WNKs have been implicated in hereditary hypertension disorders, mainly through control of the activity and levels of ion cotransporters and channels. Actions of WNKs in the kidney have been heavily investigated, and recent studies have provided insight into not only the regulation of these enzymes but also how mutations in WNKs and their interacting partners contribute to hypertensive disorders. Defining the roles of WNKs in the cardiovascular system will provide clues about additional mechanisms by which WNKs can regulate blood pressure. This review summarizes recent developments in the regulation of the WNK signaling cascade and its role in regulation of blood pressure.

The effects of bupivacaine combined with different adjuvants on block onset and duration and on ion channel expressions (SCN9A, TRPM) in sciatic nerve block in rats

BACKGROUND/AIM

The objective of this experimental study was to examine the effects of epinephrine, dexmedetomidine, and clonidine added as adjuvants to bupivacaine on block onset and effect times, as well as the effects on the Na+ and Ca+2 channel gene expressions, which may indicate cell damage in the sciatic nerve cell membrane.

MATERIALS AND METHODS

Rats were divided into five groups: Group S (sham), saline solution; Group B, bupivacaine; Group BD, bupivacaine + dexmedetomidine; Group BC, bupivacaine + clonidine; and Group BE, bupivacaine + epinephrine. For each group, 0.2 mL of local anesthetic was injected into the sciatic nerve bifurcation point of the right leg. Sensory (proprioceptive and nociceptive block) and motor block onset and ending times were recorded.

RESULTS

The shortest onset time for the examined sciatic block was observed in the BC group, whereas the longest sensory and motor block times were observed in the BD group. The present data suggest suppressed TRPM7 and increased TRPM2 in the groups other than the BE group.

CONCLUSION

Clonidine is more suitable for fast onset of peripheral nerve blocks, whereas the addition of dexmedetomidine is better in terms of duration. Because the SCN9A and TRPM2,4,7 expression ratios of the BE group showed the least amount of change, this group had the best cellular integrity.

Myogenic and metabolic feedback in cerebral autoregulation: Putative involvement of arachidonic acid-dependent pathways

The present paper presents a mechanistic model of cerebral autoregulation, in which the dual effects of the arachidonic acid metabolites 20-hydroxyeicosatetraenoic acid (20-HETE) and epoxyeicosatrienoic acids (EETs) on vascular smooth muscle mediate the cerebrovascular adjustments to a change in cerebral perfusion pressure (CPP). 20-HETE signalling in vascular smooth muscle mediates myogenic feedback to changes in vessel wall stretch, which may be modulated by metabolic feedback through EETs released from astrocytes and endothelial cells in response to changes in brain tissue oxygen tension. The metabolic feedback pathway is much faster than 20-HETE-dependent myogenic feedback, and the former thus initiates the cerebral autoregulatory response, while myogenic feedback comprises a relatively slower mechanism that functions to set the basal cerebrovascular tone. Therefore, assessments of dynamic cerebral autoregulation, which may provide information on the response time of the cerebrovasculature, may specifically be used to yield information on metabolic feedback mechanisms, while data based on assessments of static cerebral autoregulation represent the integrated functionality of myogenic and metabolic feedback.

Rab25 influences functional Cav1.2 channel surface expression in arterial smooth muscle cells

Plasma membrane-localized CaV1.2 channels are the primary calcium (Ca(2+)) influx pathway in arterial smooth muscle cells (myocytes). CaV1.2 channels regulate several cellular functions, including contractility and gene expression, but the trafficking pathways that control the surface expression of these proteins are unclear. Similarly, expression and physiological functions of small Rab GTPases, proteins that control vesicular trafficking in arterial myocytes, are poorly understood. Here, we investigated Rab proteins that control functional surface abundance of CaV1.2 channels in cerebral artery myocytes. Western blotting indicated that Rab25, a GTPase previously associated with apical recycling endosomes, is expressed in cerebral artery myocytes. Immunofluorescence Förster resonance energy transfer (immunoFRET) microscopy demonstrated that Rab25 locates in close spatial proximity to CaV1.2 channels in myocytes. Rab25 knockdown using siRNA reduced CaV1.2 surface and intracellular abundance in arteries, as determined using arterial biotinylation. In contrast, CaV1.2 was not located nearby Rab11A or Rab4 and CaV1.2 protein was unaltered by Rab11A or Rab4A knockdown. Rab25 knockdown resulted in CaV1.2 degradation by a mechanism involving both lysosomal and proteasomal pathways and reduced whole cell CaV1.2 current density but did not alter voltage dependence of current activation or inactivation in isolated myocytes. Rab25 knockdown also inhibited depolarization (20-60 mM K(+)) and pressure-induced vasoconstriction (myogenic tone) in cerebral arteries. These data indicate that Rab25 is expressed in arterial myocytes where it promotes surface expression of CaV1.2 channels to control pressure- and depolarization-induced vasoconstriction.

Membrane potential and Ca2+ concentration dependence on pressure and vasoactive agents in arterial smooth muscle: A model

A mathematical model incorporating junctional and stretch-activated microdomains and 37 protein components describes the myogenic response in arterial smooth muscle cells.

Arterial smooth muscle (SM) cells respond autonomously to changes in intravascular pressure, adjusting tension to maintain vessel diameter. The values of membrane potential (V_m) and sarcoplasmic Ca²⁺ concentration (Ca_in) within minutes of a change in pressure are the results of two opposing pathways, both of which use Ca²⁺ as a signal. This works because the two Ca²⁺-signaling pathways are confined to distinct microdomains in which the Ca²⁺ concentrations needed to activate key channels are transiently higher than Ca_in. A mathematical model of an isolated arterial SM cell is presented that incorporates the two types of microdomains. The first type consists of junctions between cisternae of the peripheral sarcoplasmic reticulum (SR), containing ryanodine receptors (RyRs), and the sarcolemma, containing voltage- and Ca²⁺-activated K⁺ (BK) channels. These junctional microdomains promote hyperpolarization, reduced Ca_in, and relaxation. The second type is postulated to form around stretch-activated nonspecific cation channels and neighboring Ca²⁺-activated Cl⁻ channels, and promotes the opposite (depolarization, increased Ca_in, and contraction). The model includes three additional compartments: the sarcoplasm, the central SR lumen, and the peripheral SR lumen. It incorporates 37 protein components. In addition to pressure, the model accommodates inputs of α- and β-adrenergic agonists, ATP, 11,12-epoxyeicosatrienoic acid, and nitric oxide (NO). The parameters of the equations were adjusted to obtain a close fit to reported V_m and Ca_in as functions of pressure, which have been determined in cerebral arteries. The simulations were insensitive to ±10% changes in most of the parameters. The model also simulated the effects of inhibiting RyR, BK, or voltage-activated Ca²⁺ channels on V_m and Ca_in. Deletion of BK β1 subunits is known to increase arterial–SM tension. In the model, deletion of β1 raised Ca_in at all pressures, and these increases were reversed by NO.