Vasoconstriction resulting from dynamic membrane trafficking of TRPM4 in vascular smooth muscle cells

Transient Receptor Potential Channels in Vascular Mechanotransduction

Transmural pressure and shear stress are mechanical forces that profoundly affect the smooth muscle cells (SMCs) comprising the vascular wall and the endothelial cells (ECs) lining the lumen. Pressure and flow are detected by mechanosensors in these cells and translated into appropriate responses to regulate blood pressure and flow. This review focuses on the role of the transient receptor potential (TRP) superfamily of cation channels in this process. We discuss how specific members of the TRP superfamily (TRPC6, TRPM4, TRPV1, TRPV4, and TRPP1) regulate the resting membrane and intracellular Ca2+ levels in SMCs and ECs to promote changes in vascular tone in response to intraluminal pressure and shear stress. Although TRP channels participate in vascular mechanotransduction, little evidence supports their intrinsic mechanosensitivity. Therefore, we also examine the evidence exploring the force-sensitive signal transduction pathways acting upstream of vascular TRP channels. Understanding the interplay between mechanosensors, force-induced signaling cascades, and TRP channels holds promise for the development of targeted therapies for diseases caused by vascular dysfunction.

Physiological functions and pathological involvement of ion channel trafficking in the vasculature

A variety of ion channels regulate membrane potential and calcium influx in arterial smooth muscle and endothelial cells to modify vascular functions, including contractility. The current (I) generated by a population of ion channels is equally dependent upon their number (N), open probability (Po) and single channel current (i), such that I = N.PO.i. A conventional view had been that ion channels traffic to the plasma membrane in a passive manner, resulting in a static surface population. It was also considered that channels assemble with auxiliary subunits prior to anterograde trafficking of the multimeric complex to the plasma membrane. Recent studies have demonstrated that physiological stimuli can regulate the surface abundance (N) of several different ion channels in arterial smooth muscle and endothelial cells to control arterial contractility. Physiological stimuli can also regulate the number of auxiliary subunits present in the plasma membrane to modify the biophysical properties, regulatory mechanisms and physiological functions of some ion channels. Furthermore, ion channel trafficking becomes dysfunctional in the vasculature during hypertension, which negatively impacts the regulation of contractility. The temporal kinetics of ion channel and auxiliary subunit trafficking can also vary depending on the signalling mechanisms and proteins involved. This review will summarize recent work that has uncovered the mechanisms, functions and pathological modifications of ion channel trafficking in arterial smooth muscle and endothelial cells.

Sodium-Permeable Ion Channels TRPM4 and TRPM5 are Functional in Human Gastric Parietal Cells in Culture and Modulate the Cellular Response to Bitter-Tasting Food Constituents

Gastric parietal cells secrete chloride ions and protons to form hydrochloric acid. Besides endogenous stimulants, e.g., acetylcholine, bitter-tasting food constituents, e.g., caffeine, induce proton secretion via interaction with bitter taste receptors (TAS2Rs), leading to increased cytosolic Ca2+ and cAMP concentrations. We hypothesized TAS2R activation by bitter tastants to result in proton secretion via cellular Na+ influx mediated by transient receptor potential channels (TRP) M4 and M5 in immortalized human parietal HGT-1 cells. Using the food-derived TAS2R agonists caffeine and l-arginine, we demonstrate both bitter compounds to induce a TRPM4/M5-mediated Na+ influx, with EC50 values of 0.65 and 10.38 mM, respectively, that stimulates cellular proton secretion. Functional involvement of TAS2Rs in the caffeine-evoked effect was demonstrated by means of the TAS2R antagonist homoeriodictyol, and stably CRISPR-Cas9-edited TAS2R43ko cells. Building on previous results, these data further support the suitability of HGT-1 cells as a surrogate cell model for taste cells. In addition, TRPM4/M5 mediated a Na+ influx after stimulating HGT-1 cells with the acetylcholine analogue carbachol, indicating an interaction of the digestion-associated cholinergic pathway with a taste-signaling pathway in parietal cells.

Co-activation of NMDAR and mGluRs controls protein nanoparticle-induced osmotic pressure in neurotoxic edema

BACKGROUND

Glutamate stimuli and hyperactivation of its receptor are predominant determinants of ischemia-induced cytotoxic cerebral edema, which is closely associated with protein nanoparticle (PN)-induced increases in osmotic pressure. Herein, we investigated the electrochemical and mechanical mechanisms underlying the neuron swelling induced by PNs via the co-activation of N-methyl-D-aspartate receptor subunit (NMDAR) and excitatory metabotropic glutamate receptors (mGluRs).

RESULTS

We observed that co-activation of ionic glutamate receptor NMDAR and Group I metabotropic mGluRs promoted alteration of PN-induced membrane potential and increased intracellular osmosis, which was closely associated with calcium and voltage-dependent ion channels. In addition, activation of NMDAR-induced calmodulin (CaM) and mGluR downstream diacylglycerol (DAG)/protein kinase C α (PKCα) were observed to play crucial roles in cytotoxic hyperosmosis. The crosstalk between CaM and PKCα could upregulate the sensitivity and sustained opening of sulfonylurea receptor 1 (SUR1)-transient receptor potential cation channel subfamily M member 4 (TRPM4) and transmembrane protein 16 A (TMEM16A) channels, respectively, maintaining the massive Na+/Cl- influx, and the resultant neuron hyperosmosis and swelling. Intracellular PNs and Na+/Cl- influx were found to be as potential targets for cerebral edema treatment, using the neurocyte osmosis system and a cerebral ischemic rat model.

CONCLUSIONS

This study highlights PNs as a key factor in "electrochemistry-tension" signal transduction controlling Na+/Cl- ion channels and increased osmotic pressure in ischemia-induced cytotoxic edema. Moreover, enhanced sensitivity in both Na+ and Cl- ion channels also has a crucial role in cerebral edema.

Vasodilators mobilize SK3 channels in endothelial cells to produce arterial relaxation

Endothelial cells (ECs) line the lumen of all blood vessels and regulate functions, including contractility. Physiological stimuli, such as acetylcholine (ACh) and intravascular flow, activate transient receptor potential vanilloid 4 (TRPV4) channels, which stimulate small (SK3)- and intermediate (IK)-conductance Ca2+-activated potassium channels in ECs to produce vasodilation. Whether physiological vasodilators also modulate the surface abundance of these ion channels in ECs to elicit functional responses is unclear. Here, we show that ACh and intravascular flow stimulate rapid anterograde trafficking of an intracellular pool of SK3 channels in ECs of resistance-size arteries, which increases surface SK3 protein more than two-fold. In contrast, ACh and flow do not alter the surface abundance of IK or TRPV4 channels. ACh triggers SK3 channel trafficking by activating TRPV4-mediated Ca2+ influx, which stimulates Rab11A, a Rab GTPase associated with recycling endosomes. Superresolution microscopy data demonstrate that SK3 trafficking specifically increases the size of surface SK3 clusters which overlap with TRPV4 clusters. We also show that Rab11A-dependent trafficking of SK3 channels is an essential contributor to vasodilator-induced SK current activation in ECs and vasorelaxation. In summary, our data demonstrate that vasodilators activate Rab11A, which rapidly delivers an intracellular pool of SK3 channels to the vicinity of surface TRPV4 channels in ECs. This trafficking mechanism increases surface SK3 cluster size, elevates SK3 current density, and produces vasodilation. These data also demonstrate that SK3 and IK channels are differentially regulated by trafficking-dependent and -independent signaling mechanisms in endothelial cells.

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.

Endothelial cell TRPA1 activity exacerbates cerebral hemorrhage during severe hypertension

Introduction: Hypoxia-induced dilation of cerebral arteries orchestrated by Ca2+-permeable transient receptor potential ankyrin 1 (TRPA1) cation channels on endothelial cells is neuroprotective during ischemic stroke, but it is unknown if the channel has a similar impact during hemorrhagic stroke. TRPA1 channels are endogenously activated by lipid peroxide metabolites generated by reactive oxygen species (ROS). Uncontrolled hypertension, a primary risk factor for the development of hemorrhagic stroke, is associated with increased ROS production and oxidative stress. Therefore, we hypothesized that TRPA1 channel activity is increased during hemorrhagic stroke. Methods: Severe, chronic hypertension was induced in control (Trpa1 fl/fl) and endothelial cell-specific TRPA1 knockout (Trpa1-ecKO) mice using a combination of chronic angiotensin II administration, a high-salt diet, and the addition of a nitric oxide synthase inhibitor to drinking water. Blood pressure was measured in awake, freely-moving mice using surgically placed radiotelemetry transmitters. TRPA1-dependent cerebral artery dilation was evaluated with pressure myography, and expression of TRPA1 and NADPH oxidase (NOX) isoforms in arteries from both groups was determined using PCR and Western blotting techniques. In addition, ROS generation capacity was evaluated using a lucigenin assay. Histology was performed to examine intracerebral hemorrhage lesion size and location. Results: All animals became hypertensive, and a majority developed intracerebral hemorrhages or died of unknown causes. Baseline blood pressure and responses to the hypertensive stimulus did not differ between groups. Expression of TRPA1 in cerebral arteries from control mice was not altered after 28 days of treatment, but expression of three NOX isoforms and the capacity for ROS generation was increased in hypertensive animals. NOX-dependent activation of TRPA1 channels dilated cerebral arteries from hypertensive animals to a greater extent compared with controls. The number of intracerebral hemorrhage lesions in hypertensive animals did not differ between control and Trpa1-ecKO animals but were significantly smaller in Trpa1-ecKO mice. Morbidity and mortality did not differ between groups. Discussion: We conclude that endothelial cell TRPA1 channel activity increases cerebral blood flow during hypertension resulting in increased extravasation of blood during intracerebral hemorrhage events; however, this effect does not impact overall survival. Our data suggest that blocking TRPA1 channels may not be helpful for treating hypertension-associated hemorrhagic stroke in a clinical setting.

A Novel Role of the TRPM4 Ion Channel in Exocytosis

Under physiological conditions, the widely expressed calcium-activated TRPM4 channel conducts sodium into cells. This sodium influx depolarizes the plasma membrane and reduces the driving force for calcium entry. The aberrant expression or function of TRPM4 has been reported in various diseases, including different types of cancer. TRPM4 is mainly localized in the plasma membrane, but it is also found in intracellular vesicles, which can undergo exocytosis. In this study, we show that calcium-induced exocytosis in the colorectal cancer cell line HCT116 is dependent on TRPM4. In addition, the findings from some studies of prostate cancer cell lines suggest a more general role of TRPM4 in calcium-induced exocytosis in cancer cells. Furthermore, calcium-induced exocytosis depends on TRPM4 ion conductivity. Additionally, an increase in intracellular calcium results in the delivery of TRPM4 to the plasma membrane. This process also depends on TRPM4 ion conductivity. TRPM4-dependent exocytosis and the delivery of TRPM4 to the plasma membrane are mediated by SNARE proteins. Finally, we provide evidence that calcium-induced exocytosis depends on TRPM4 ion conductivity, not within the plasma membrane, but rather in TRPM4-containing vesicles.

Pharmacological Modulation and (Patho)Physiological Roles of TRPM4 Channel-Part 2: TRPM4 in Health and Disease

Transient receptor potential melastatin 4 (TRPM4) is a unique member of the TRPM protein family and, similarly to TRPM5, is Ca²⁺ sensitive and permeable for monovalent but not divalent cations. It is widely expressed in many organs and is involved in several functions; it regulates membrane potential and Ca²⁺ homeostasis in both excitable and non-excitable cells. This part of the review discusses the currently available knowledge about the physiological and pathophysiological roles of TRPM4 in various tissues. These include the physiological functions of TRPM4 in the cells of the Langerhans islets of the pancreas, in various immune functions, in the regulation of vascular tone, in respiratory and other neuronal activities, in chemosensation, and in renal and cardiac physiology. TRPM4 contributes to pathological conditions such as overactive bladder, endothelial dysfunction, various types of malignant diseases and central nervous system conditions including stroke and injuries as well as in cardiac conditions such as arrhythmias, hypertrophy, and ischemia-reperfusion injuries. TRPM4 claims more and more attention and is likely to be the topic of research in the future.

Nitric Oxide Signals Through IRAG to Inhibit TRPM4 Channels and Dilate Cerebral Arteries

Nitric oxide (NO) relaxes vascular smooth muscle cells (SMCs) and dilates blood vessels by increasing intracellular levels of cyclic guanosine monophosphate (cGMP), which stimulates the activity of cGMP-dependent protein kinase (PKG). However, the vasodilator mechanisms downstream of PKG remain incompletely understood. Here, we found that transient receptor potential melastatin 4 (TRPM4) cation channels, which are activated by Ca²⁺ released from the sarcoplasmic reticulum (SR) through inositol triphosphate receptors (IP₃Rs) under native conditions, are essential for SMC membrane depolarization and vasoconstriction. We hypothesized that signaling via the NO/cGMP/PKG pathway causes vasodilation by inhibiting TRPM4. We found that TRPM4 currents activated by stretching the plasma membrane or directly activating IP₃Rs were suppressed by exogenous NO or a membrane-permeable cGMP analog, the latter of which also impaired IP₃R-mediated release of Ca²⁺ from the SR. The effects of NO on TRPM4 activity were blocked by inhibition of soluble guanylyl cyclase or PKG. Notably, upon phosphorylation by PKG, IRAG (IP₃R-associated PKG substrate) inhibited IP₃R-mediated Ca²⁺ release, and knockdown of IRAG expression diminished NO-mediated inhibition of TRPM4 activity and vasodilation. Using superresolution microscopy, we found that IRAG, PKG, and IP₃Rs form a nanoscale signaling complex on the SR of SMCs. We conclude that NO/cGMP/PKG signaling through IRAG inhibits IP₃R-dependent activation of TRPM4 channels in SMCs to dilate arteries.

SIGNIFICANCE STATEMENT

Nitric oxide is a gaseous vasodilator produced by endothelial cells that is essential for cardiovascular function. Although NO-mediated signaling pathways have been intensively studied, the mechanisms by which they relax SMCs to dilate blood vessels remain incompletely understood. In this study, we show that NO causes vasodilation by inhibiting the activity of Ca²⁺-dependent TRPM4 cation channels. Probing further, we found that NO does not act directly on TRPM4 but instead initiates a signaling cascade that inhibits its activation by blocking the release of Ca²⁺ from the SR. Thus, our findings reveal the essential molecular pathways of NO-induced vasodilation-a fundamental unresolved concept in cardiovascular physiology.

Upregulation of transient receptor potential melastatin 4 (TRPM4) in ventricular fibroblasts from heart failure patients

The transient receptor potential melastatin 4 (TRPM4) is a Ca2+-activated nonselective monovalent cation channel belonging to the TRP channel superfamily. TRPM4 is widely expressed in various tissues and most abundantly expressed in the heart. TRPM4 plays a critical role in cardiac conduction. Patients carrying a gain-of-function or loss-of-function mutation of TRPM4 display impaired cardiac conduction. Knockout or over-expression of TRPM4 in mice recapitulates conduction defects in patients. Moreover, recent studies have indicated that TRPM4 plays a role in hypertrophy and heart failure. Whereas the role of TRPM4 mediated by cardiac myocytes has been well investigated, little is known about TRPM4 and its role in cardiac fibroblasts. Here we show that in human left ventricular fibroblasts, TRPM4 exhibits typical Ca2+-activation characteristics, linear current-voltage (I-V) relation, and monovalent permeability. TRPM4 currents recorded in fibroblasts from heart failure patients (HF) are more than 2-fold bigger than those from control individuals (CTL). The enhanced functional TRPM4 in HF is not resulted from changed channel properties, as TRPM4 currents from both HF and CTL fibroblasts demonstrate similar sensitivity to intracellular calcium activation and extracellular 9-phenanthrol (9-phen) blockade. Consistent with enhanced TRPM4 activity, the protein level of TRPM4 is about 2-fold higher in HF than that of CTL hearts. Moreover, TRPM4 current in CTL fibroblasts is increased after 24 hours of TGFβ1 treatment, implying that TRPM4 in vivo may be upregulated by fibrogenesis promotor TGFβ1. The upregulated TRPM4 in HF fibroblasts suggests that TRPM4 may play a role in cardiac fibrogenesis under various pathological conditions.

Age-dependent decrease in TRPM4 channel expression but not trafficking alters urinary bladder smooth muscle contractility

During development, maturation, or aging, the expression and function of urinary bladder smooth muscle (UBSM) ion channels can change, thus affecting micturition. Increasing evidence supports a novel role of transient receptor potential melastatin‐4 (TRPM4) channels in UBSM physiology. However, it remains unknown whether the functional expression of these key regulatory channels fluctuates in UBSM over different life stages. Here, we examined TRPM4 channel protein expression (Western blot) and the effects of TRPM4 channel inhibitors, 9‐phenanthrol and glibenclamide, on phasic contractions of UBSM isolated strips obtained from juvenile (UBSM‐J, 5–9 weeks old) and adult (UBSM‐A, 6–18 months old) male guinea pigs. Compared to UBSM‐J, UBSM‐A displayed a 50–70% reduction in total TRPM4 protein expression, while the surface‐to‐intracellular expression ratio (channel trafficking) remained the same in both age groups. Consistent with the reduced total TRPM4 protein expression in UBSM‐A, 9‐phenanthrol showed lower potencies and/or maximum efficacies in UBSM‐A than UBSM‐J for inhibiting amplitude and muscle force of spontaneous and 20 mM KCl‐induced phasic contractions. Compared to 9‐phenanthrol, glibenclamide also attenuated both spontaneous and KCl‐induced contractions, but with less pronounced differential effects in UBSM‐A and UBSM‐J. In both age groups, regardless of the overall reduced total TRPM4 protein expression in UBSM‐A, cell surface TRPM4 protein expression (~80%) predominated over its intracellular fraction (~20%), revealing preserved channel trafficking mechanisms toward the cell membrane. Collectively, this study reports novel findings illuminating a fundamental physiological role for TRPM4 channels in UBSM function that fluctuates with age.

Transient receptor potential channel-dependent myogenic responsiveness in small-sized resistance arteries

It is well documented that the inherent ability of small arteries and arterioles to regulate intraluminal diameter in response to alterations in intravascular pressure determines peripheral vascular resistance and blood flow (termed myogenic response or pressure-induced vasoconstriction/dilation). This autoregulatory property of resistance arteries is primarily originated from mechanosensitive vascular smooth muscle cells (VSMCs). There are diverse biological apparatuses in the plasma membrane of VSMCs that sense mechanical stimuli and generate intracellular signals for the contractility of VSMCs. Although the roles of transient receptor potential (TRP) channels in pressure-induced vasoconstriction are not fully understood to date, TRP channels that are directly activated by mechanical stimuli (e.g., stretch of VSMCs) or indirectly evoked by intracellular molecules (e.g., inositol trisphosphate) provide the major sources of Ca²⁺ (e.g., Ca²⁺ influx or release from the sarcoplasmic reticulum) and in turn, evoke vascular reactivity. This review sought to summarize mounting evidence over several decades that the activation of TRP canonical, TRP melastatin, TRP vanilloid, and TRP polycystin channels contributes to myogenic vasoconstriction.

Obligatory role for PKCδ in PIP2 -mediated activation of store-operated TRPC1 channels in vascular smooth muscle cells

KEY POINTS

In vascular smooth muscle cells (VSMCs), activation of Ca²⁺ -permeable store-operated channels (SOCs) composed of canonical transient receptor potential channel 1 (TRPC1) subunits mediates Ca²⁺ entry pathways that regulate contraction, proliferation and migration, which are processes associated with vascular disease. Activation of TRPC1-based SOCs requires protein kinase C (PKC) activity, which is proposed to phosphorylate TRPC1 proteins to promote channel opening by phosphatidylinositol 4,5-bisphosphate (PIP₂ ). We investigated the identity of the PKC isoform involved in activating TRPC1-based SOCs in rat mesenteric artery VSMCs. TRPC1-based SOCs were reduced by PKCδ inhibitors and knockdown of PKCδ expression. Store depletion induced interactions between TRPC1 and PKCδ and PKCδ-dependent phosphorylation of TRPC1. Furthermore, generation of store-operated interactions between PIP₂ and TRPC1 and activation of TRPC1-based SOCs by PIP₂ required PKCδ. These findings reveal that PKCδ activity has an obligatory role in activating TRPC1-based SOCs, through regulating PIP₂ -mediated channel opening.

ABSTRACT

In vascular smooth muscle cells (VMSCs), stimulation of Ca²⁺ -permeable canonical transient receptor potential channel 1 (TRPC1)-based store-operated channels (SOCs) mediates Ca²⁺ entry pathways that regulate cell contraction, proliferation and migration, which are processes associated with vascular disease. It is therefore important to understand how TRPC1-based SOCs are activated. Stimulation of TRPC1-based SOCs requires protein kinase C (PKC) activity, with store-operated PKC-dependent phosphorylation of TRPC1 essential for channel opening by phosphatidylinositol 4,5-bisphosphate (PIP₂ ). Experimental protocols used to activate TRPC1-based SOCs suggest that the PKC isoform involved requires diacylglycerol (DAG) but is Ca²⁺ -insensitive, which are characteristics of the novel group of PKC isoforms (δ, ε, η, θ). Hence, the present study examined whether a novel PKC isoform(s) is involved in activating TRPC1-based SOCs in contractile rat mesenteric artery VSMCs. Store-operated whole-cell cation currents were blocked by Pico145, a highly selective and potent TRPC1/4/5 channel blocker and T1E3, a TRPC1 blocking antibody. PKCδ was expressed in VSMCs, and selective PKCδ inhibitory peptides and knockdown of PKCδ expression with morpholinos oligomers inhibited TRPC1-based SOCs. TRPC1 and PKCδ interactions and phosphorylation of TRPC1 induced by store depletion were both reduced by pharmacological inhibition and PKCδ knockdown. In addition, store-operated PIP₂ and TRPC1 interactions were blocked by PKCδ inhibition, and PKCδ was required for PIP₂ -mediated activation of TRPC1 currents. These results identify the involvement of PKCδ in stimulation of TRPC1-based SOCs and highlight that store-operated PKCδ activity is obligatory for channel opening by PIP₂ , the probable activating ligand.

TRPML1 channels initiate Ca2+ sparks in vascular smooth muscle cells

TRPML1 (transient receptor potential mucolipin 1) is a Ca²⁺-permeable, nonselective cation channel localized to the membranes of endosomes and lysosomes and is not present or functional on the plasma membrane. Ca²⁺ released from endosomes and lysosomes into the cytosol through TRPML1 channels is vital for trafficking, acidification, and other basic functions of these organelles. Here, we investigated the function of TRPML1 channels in fully differentiated contractile vascular smooth muscle cells (SMCs). In live-cell confocal imaging studies, we found that most endosomes and lysosomes in freshly isolated SMCs from cerebral arteries were essentially immobile. Using nanoscale super-resolution microscopy, we found that TRPML1 channels present in late endosomes and lysosomes formed stable complexes with type 2 ryanodine receptors (RyR2) on the sarcoplasmic reticulum (SR). Spontaneous Ca²⁺ signals resulting from the release of SR Ca²⁺ through RyR2s ("Ca²⁺ sparks") and corresponding Ca²⁺-activated K⁺ channel activity are critically important for balancing vasoconstriction. We found that these signals were essentially absent in SMCs from TRPML1-knockout (Mcoln1^-/- ) mice. Using ex vivo pressure myography, we found that loss of this critical signaling cascade exaggerated the vasoconstrictor responses of cerebral and mesenteric resistance arteries. In vivo radiotelemetry studies showed that Mcoln1^-/- mice were spontaneously hypertensive. We conclude that TRPML1 is crucial for the initiation of Ca²⁺ sparks in SMCs and the regulation of vascular contractility and blood pressure.

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.

The ion channel TRPM4 in murine experimental autoimmune encephalomyelitis and in a model of glutamate-induced neuronal degeneration

Transient receptor potential melastatin member 4 (TRPM4), a Ca²⁺-activated nonselective cation channel, has been found to mediate cell membrane depolarization in immune response, insulin secretion, cardiovascular diseases, and cancer. In murine experimental autoimmune encephalomyelitis (EAE), TRPM4 deletion and administration of glibenclamide were found to ameliorate clinical symptoms and attenuate disease progression. However, the exact role of TRPM4 in EAE, as well as the molecular mechanisms underlining TRPM4 contribution in EAE, remain largely unclear. In the present study, EAE was induced in WT C57BL/6 N mice using myelin oligodendrocyte glycoprotein 35–55 (MOG_35–55) and TRPM4 protein and mRNA expression were examined in spinal cord membrane extracts. Our results showed that TRPM4 protein and mRNA are upregulated in EAE, and that their upregulation correlated with disease progression. Moreover, newly-developed TRPM4 inhibitors, named compound 5 and compound 6, were shown to exert a better neuroprotection compared to currently used TRPM4 inhibitors in an in vitro model of glutamate-induced neurodegeneration. These results support the hypothesis that TRPM4 is crucial from early stages of EAE, and suggest that these more potent TRPM4 inhibitors could be used as novel protective therapeutic tools in glutamate-induced neurodegeneration.

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.

Angiotensin II reduces the surface abundance of KV 1.5 channels in arterial myocytes to stimulate vasoconstriction

KEY POINTS

Several different voltage-dependent K⁺ (K_V ) channel isoforms are expressed in arterial smooth muscle cells (myocytes). Vasoconstrictors inhibit K_V currents, but the isoform selectivity and mechanisms involved are unclear. We show that angiotensin II (Ang II), a vasoconstrictor, stimulates degradation of K_V 1.5, but not K_V 2.1, channels through a protein kinase C- and lysosome-dependent mechanism, reducing abundance at the surface of mesenteric artery myocytes. The Ang II-induced decrease in cell surface K_V 1.5 channels reduces whole-cell K_V 1.5 currents and attenuates K_V 1.5 function in pressurized arteries. We describe a mechanism by which Ang II stimulates protein kinase C-dependent K_V 1.5 channel degradation, reducing the abundance of functional channels at the myocyte surface.

ABSTRACT

Smooth muscle cells (myocytes) of resistance-size arteries express several different voltage-dependent K⁺ (K_V ) channels, including K_V 1.5 and K_V 2.1, which regulate contractility. Myocyte K_V currents are inhibited by vasoconstrictors, including angiotensin II (Ang II), but the mechanisms involved are unclear. Here, we tested the hypothesis that Ang II inhibits K_V currents by reducing the plasma membrane abundance of K_V channels in myocytes. Angiotensin II (applied for 2 h) reduced surface and total K_V 1.5 protein in rat mesenteric arteries. In contrast, Ang II did not alter total or surface K_V 2.1, or K_V 1.5 or K_V 2.1 cellular distribution, measured as the percentage of total protein at the surface. Bisindolylmaleimide (BIM; a protein kinase C blocker), a protein kinase C inhibitory peptide or bafilomycin A (a lysosomal degradation inhibitor) each blocked the Ang II-induced decrease in total and surface K_V 1.5. Immunofluorescence also suggested that Ang II reduced surface K_V 1.5 protein in isolated myocytes; an effect inhibited by BIM. Arteries were exposed to Ang II or Ang II plus BIM (for 2 h), after which these agents were removed and contractility measurements performed or myocytes isolated for patch-clamp electrophysiology. Angiotensin II reduced both whole-cell K_V currents and currents inhibited by Psora-4, a K_V 1.5 channel blocker. Angiotensin II also reduced vasoconstriction stimulated by Psora-4 or 4-aminopyridine, another K_V channel inhibitor. These data indicate that Ang II activates protein kinase C, which stimulates K_V 1.5 channel degradation, leading to a decrease in surface K_V 1.5, a reduction in whole-cell K_V 1.5 currents and a loss of functional K_V 1.5 channels in myocytes of pressurized arteries.

Hydrogen peroxide suppresses TRPM4 trafficking to the apical membrane in mouse cortical collecting duct principal cells

A Ca²⁺-activated nonselective cation channel (NSC_Ca) is found in principal cells of the mouse cortical collecting duct (CCD). However, the molecular identity of this channel remains unclear. We used mpkCCD_c14 cells, a mouse CCD principal cell line, to determine whether NSC_Ca represents the transient receptor potential (TRP) channel, the melastatin subfamily 4 (TRPM4). A Ca²⁺-sensitive single-channel current was observed in inside-out patches excised from the apical membrane of mpkCCD_c14 cells. Like TRPM4 channels found in other cell types, this channel has an equal permeability for Na⁺ and K⁺ and has a linear current-voltage relationship with a slope conductance of ~23 pS. The channel was inhibited by a specific TRPM4 inhibitor, 9-phenanthrol. Moreover, the frequency of observing this channel was dramatically decreased in TRPM4 knockdown mpkCCD_c14 cells. Unlike those previously reported in other cell types, the TRPM4 in mpkCCD_c14 cells was unable to be activated by hydrogen peroxide (H₂O₂). Conversely, after treatment with H₂O₂, TRPM4 density in the apical membrane of mpkCCD_c14 cells was significantly decreased. The channel in intact cell-attached patches was activated by ionomycin (a Ca²⁺ ionophore), but not by ATP (a purinergic P₂ receptor agonist). These data suggest that the NSC_Ca current previously described in CCD principal cells is actually carried through TRPM4 channels. However, the physiological role of this channel in the CCD remains to be further determined.

TRPM4-mediated control of FcεRI-evoked Ca(2+) elevation comprises enhanced plasmalemmal trafficking of TRPM4 channels in connective tissue type mast cells

TRPM4 proteins form Ca²⁺-activated non selective cation (CAN) channels that affect transmembrane Ca²⁺-influx by determining the membrane potential. Tight control of the intracellular Ca²⁺ concentration is essential for mast cell responses. In this study, we analyzed the expression of TRPM4 in peritoneal mast cells (PCMC) as a model for connective tissue type mast cells with respect to FcεRI-evoked calcium changes and the subcellular localization of fluorescently labeled TRPM4 using two viral transduction systems before and following antigen stimulation. Our results show that TRPM4 is expressed in PCMCs, is an essential constituent of the endogenous CAN channels in PCMCs and regulates antigen-evoked increases in intracellular calcium that are significantly enhanced in TRPM4-deficient PCMCs. Compared to PCMCs analyzed before antigen stimulation, the cells depict a substantially increased localization of TRPM4 proteins towards the plasma membrane after FcεRI stimulation. Thus, TRPM4 functions as a limiting factor for antigen evoked calcium rise in connective tissue type mast cells and concurrent translocation of TRPM4 into the plasma membrane is part of this mechanism.

Calcium Entry Through Thermosensory Channels

ThermoTRPs are unique channels that mediate Na(+) and Ca(2+) currents in response to changes in ambient temperature. In combination with their activation by other physical and chemical stimuli, they are considered key integrators of environmental cues into neuronal excitability. Furthermore, roles of thermoTRPs in non-neuronal tissues are currently emerging such as insulin secretion in pancreatic β-cells, and links to cancer. Calcium permeability through thermoTRPs appears a central hallmark for their physiological and pathological activities. Moreover, it is currently being proposed that beyond working as a second messenger, Ca(2+) can function locally by acting on protein complexes near the membrane. Interestingly, thermoTRPs can enhance and expand the inherent plasticity of signalplexes by conferring them temperature, pH and lipid regulation through Ca(2+) signalling. Thus, unveiling the local role of Ca(2+) fluxes induced by thermoTRPs on the dynamics of membrane-attached signalling complexes as well as their significance in cellular processes, are central issues that will expand the opportunities for therapeutic intervention in disorders involving dysfunction of thermoTRP channels.

Regulation of calcium channels in smooth muscle: new insights into the role of myosin light chain kinase

Smooth muscle myosin light chain kinase (MLCK) plays a crucial role in artery contraction, which regulates blood pressure and blood flow distribution. In addition to this role, MLCK contributes to Ca(2+) flux regulation in vascular smooth muscle (VSM) and in non-muscle cells, where cytoskeleton has been suggested to help Ca(2+) channels trafficking. This conclusion is based on the use of pharmacological inhibitors of MLCK and molecular and cellular techniques developed to down-regulate the enzyme. Dissimilarities have been observed between cells and whole tissues, as well as between large conductance and small resistance arteries. A differential expression in MLCK and ion channels (either voltage-dependent Ca(2+) channels or non-selective cationic channels) could account for these observations, and is in line with the functional properties of the arteries. A potential involvement of MLCK in the pathways modulating Ca(2+) entry in VSM is described in the present review.

Physiological functions of the TRPM4 channels via protein interactions

Transient Receptor Potential, Melastatin-related, member 4 (TRPM4) channels are Ca²⁺-activated Ca²⁺-impermeable cation channels. These channels are expressed in various types of mammalian tissues including the brain and are implicated in many diverse physiological and pathophysiological conditions. In the past several years, the trafficking processes and regulatory mechanism of these channels and their interacting proteins have been uncovered. Here in this minireview, we summarize the current understanding of the trafficking mechanism of TRPM4 channels on the plasma membrane as well as heteromeric complex formation via protein interactions. We also describe physiological implications of protein-TRPM4 interactions and suggest TRPM4 channels as therapeutic targets in many related diseases. [BMB Reports 2015; 48(1): 1-5]

Intravascular pressure enhances the abundance of functional Kv1.5 channels at the surface of arterial smooth muscle cells

Voltage-dependent potassium (K(v)) channels are present in various cell types, including smooth muscle cells (myocytes) of resistance-sized arteries that control systemic blood pressure and regional organ blood flow. Intravascular pressure depolarizes arterial myocytes, stimulating calcium (Ca(2+)) influx through voltage-dependent Ca(2+) (Ca(v)) channels that results in vasoconstriction and also K(+) efflux through K(v) channels that oppose vasoconstriction. We hypothesized that pressure-induced depolarization may not only increase the open probability of plasma membrane-resident K(v) channels but also increase the abundance of these channels at the surface of arterial myocytes to limit vasoconstriction. We found that K(v)1.5 and K(v)2.1 proteins were abundant in the myocytes of resistance-sized mesenteric arteries. K(v)1.5, but not K(v)2.1, continuously recycled between the intracellular compartment and the plasma membrane in contractile arterial myocytes. Using ex vivo preparations of intact arteries, we showed that physiological intravascular pressure through membrane depolarization or membrane depolarization in the absence of pressure inhibited the degradation of internalized K(v)1.5 and increased recycling of K(v)1.5 to the plasma membrane. Accordingly, by stimulating the activity of Ca(v)1.2, membrane depolarization increased whole-cell K(v)1.5 current density in myocytes and K(v)1.5 channel activity in pressurized arteries. In contrast, the total amount and cell surface abundance of K(v)2.1 were independent of intravascular pressure or membrane potential. Thus, our data indicate that intravascular pressure-induced membrane depolarization selectively increased K(v)1.5 surface abundance to increase K(v) currents in arterial myocytes, which would limit vasoconstriction.

Transient receptor potential channels in the vasculature

The mammalian genome encodes 28 distinct members of the transient receptor potential (TRP) superfamily of cation channels, which exhibit varying degrees of selectivity for different ionic species. Multiple TRP channels are present in all cells and are involved in diverse aspects of cellular function, including sensory perception and signal transduction. Notably, TRP channels are involved in regulating vascular function and pathophysiology, the focus of this review. TRP channels in vascular smooth muscle cells participate in regulating contractility and proliferation, whereas endothelial TRP channel activity is an important contributor to endothelium-dependent vasodilation, vascular wall permeability, and angiogenesis. TRP channels are also present in perivascular sensory neurons and astrocytic endfeet proximal to cerebral arterioles, where they participate in the regulation of vascular tone. Almost all of these functions are mediated by changes in global intracellular Ca(2+) levels or subcellular Ca(2+) signaling events. In addition to directly mediating Ca(2+) entry, TRP channels influence intracellular Ca(2+) dynamics through membrane depolarization associated with the influx of cations or through receptor- or store-operated mechanisms. Dysregulation of TRP channels is associated with vascular-related pathologies, including hypertension, neointimal injury, ischemia-reperfusion injury, pulmonary edema, and neurogenic inflammation. In this review, we briefly consider general aspects of TRP channel biology and provide an in-depth discussion of the functions of TRP channels in vascular smooth muscle cells, endothelial cells, and perivascular cells under normal and pathophysiological conditions.

Agonist-dependent modulation of cell surface expression of the cold receptor TRPM8

The spatial and temporal distribution of receptors constitutes an important mechanism for controlling the magnitude of cellular responses. Several members of the transient receptor potential (TRP) ion channel family can regulate their function by modulating their expression at the plasma membrane (PM) through rapid vesicular translocation and fusion. The mechanisms underlying this regulation are not completely understood, and the contribution of vesicular trafficking to physiological function is unknown. TRPM8 receptors are expressed in mammalian peripheral sensory neurons and are essential for the detection of cold temperatures. Previously, we showed that TRPM8-containing vesicles are segregated into three main pools, immobile at the PM, simple diffusive and corralled-hopping. Here, we show that channel expression at the PM is modulated by TRPM8 agonists in F11 and HEK293T cells. Our results support a model in which the activation of TRPM8 channels, located at the PM, induces a short-lived recruitment of a TRPM8-containing vesicular pool to the cell surface causing a transitory increase in the number of functional channels, affecting intrinsic properties of cold receptor responses. We further demonstrate the requirement of intact vesicular trafficking to support sustained cold responses in the skin of mice.

Brain transient receptor potential channels and stroke

Transient receptor potential (TRP) channels have been increasingly implicated in the pathological mechanisms of CNS disorders. TRP expression has been detected in neurons, astrocytes, oligodendrocytes, microglia, and ependymal cells as well as in the cerebral vascular endothelium and smooth muscle. In stroke, TRPC3/4/6, TRPM2/4/7, and TRPV1/3/4 channels have been found to participate in ischemia-induced cell death, whereas other TRP channels, in particular those expressed in nonneuronal cells, have been less well studied. This review summarizes the current knowledge on the expression and functions of the TRP channels in various cell types in the brain and our current understanding of TRP channels in stroke pathophysiology. In an aging society, the occurrence of stroke is expected to increase steadily, and there is an urgent requirement to improve the current stroke management strategy. Therefore, elucidating the roles of TRP channels in stroke could shed light on the development of novel therapeutic strategies and ultimately improve stroke outcome.

Distinct modes of perimembrane TRP channel turnover revealed by TIR-FRAP

Transient Receptor Potential (TRP) channels form a broadly expressed and functionally diverse family of cation channels involved in various (patho)physiological processes. Whereas the mechanisms that control opening of TRP channels have been extensively studied, little is known about the transport processes of TRP channels to and within the plasma membrane. Here we used Total Internal Reflection--Fluorescence Recovery after Photobleaching (TIR-FRAP) to selectively visualize and bleach the fluorescently labeled TRP channels TRPV2 and TRPM4 in close proximity of the glass-plasma membrane interface, allowing detailed analysis of their perimembrane dynamics. We show that recovery of TRPM4 occurs via 200-nm diameter transport vesicles, and demonstrate the full fusion of such vesicles with the plasma membrane. In contrast, TRPV2 recovery proceeded mainly via lateral diffusion from non-bleached areas of the plasma membrane. Analysis of the two-dimensional channel diffusion kinetics yielded 2D diffusion coefficients ranging between 0.1 and 0.3 μm(2)/s, suggesting that these TRP channels move relatively unrestricted within the plasma membrane. These data demonstrate distinct modes of TRP channel turnover at the plasma membrane and illustrate the usefulness of TIR-FRAP to monitor these processes with high resolution.

Casein kinase-mediated phosphorylation of serine 839 is necessary for basolateral localization of the Ca²⁺-activated non-selective cation channel TRPM4

Transient receptor potential melastatin-like 4 (TRPM4) is a Ca(2+)-activated non-selective cation channel expressed in a wide range of human tissues. TRPM4 participates in a variety of physiological processes such as T cell activation, myogenic vasoconstriction, and allergic reactions. TRPM4 Ca(2+) sensitivity is enhanced by calmodulin (CaM) and phosphathydilinositol 4, 5-bisphosphate (PI(4,5)P2) binding, as well as, under certain conditions, PKC activation. However, information as to the mechanisms of modulation of this channel remains unknown, including direct identification of phosphorylation sites on TRPM4 and their role in channel features. Here, we use mass-spectrometric-based proteomic approaches (immunoprecipitation and tandem mass spectrometry) to unambiguously identify S839 as a phosphorylation site present on human TRPM4 expressed in a human cell line. Site-directed mutagenesis employing a serine to alanine mutation to eliminate phosphorylation, and a phospho-mimetic aspartate mutation, as well as biochemical and immunocytochemical experiments, revealed a role for S839 phosphorylation in the basolateral expression of TRPM4 channels in epithelial cells. Moreover, we demonstrated that casein kinase 1 (CK1) phosphorylates S839 and is responsible for the basolateral localization of TRPM4.

Depletion of 14-3-3γ reduces the surface expression of Transient Receptor Potential Melastatin 4b (TRPM4b) channels and attenuates TRPM4b-mediated glutamate-induced neuronal cell death

BACKGROUND

TRPM4 channels are Ca2+-activated nonselective cation channels which are deeply involved in physiological and pathological conditions. However, their trafficking mechanism and binding partners are still elusive.

RESULTS

We have found the 14-3-3γ as a binding partner for TRPM4b using its N-terminal fragment from the yeast-two hybrid screening. Ser88 at the N-terminus of TRPM4b is critical for 14-3-3γ binding by showing GST pull-down and co-immunoprecipitation. Heterologous overexpression of 14-3-3γ in HEK293T cells increased TRPM4b expression on the plasma membrane which was measured by whole-cell recordings and cell surface biotinylation experiment. Surface expression of TRPM4b was greatly reduced by short hairpin RNA (shRNA) against 14-3-3γ. Next, endogenous TRPM4b-mediated currents were electrophysiologically characterized by application of glutamate and 9-phenanthrol, a TRPM4b specific antagonist in HT-22 cells which originated from mouse hippocampal neurons. Glutamate-induced TRPM4b currents were significantly attenuated by shRNAs against 14-3-3γ or TRPM4b in these cells. Finally, glutamate-induced cell death was greatly prevented by treatment of 9-phenanthrol or 14-3-3γ shRNA.

CONCLUSION

These results showed that the cell surface expression of TRPM4 channels is mediated by 14-3-3γ binding, and the specific inhibition of this trafficking process can be a potential therapeutic target for glutamate-induced neuronal cell death.

Large conductance Ca2+-activated K+ channel (BKCa) α-subunit splice variants in resistance arteries from rat cerebral and skeletal muscle vasculature

Previous studies report functional differences in large conductance Ca²⁺ activated-K⁺ channels (BK_Ca) of smooth muscle cells (VSMC) from rat cerebral and cremaster muscle resistance arteries. The present studies aimed to determine if this complexity in BK_Ca activity may, in part, be due to splice variants in the pore-forming α-subunit. BK_Ca variants in the intracellular C terminus of the α-subunit, and their relative expression to total α-subunit, were examined by qPCR. Sequencing of RT-PCR products showed two α-subunit variants, ZERO and STREX, to be identical in cremaster and cerebral arteries. Levels of STREX mRNA expression were, however, significantly higher in cremaster VSMCs (28.9±4.2% of total α-BK_Ca) compared with cerebral vessels (16.5±0.9%). Further, a low level of BK_Ca SS4 α-subunit variant was seen in cerebral arteries, while undetectable in cremaster arteries. Protein biotinylation assays, in expression systems and arterial preparations, were used to determine whether differences in splice variant mRNA expression affect surface membrane/cytosolic location of the channel. In AD-293 and CHO-K1 cells, rat STREX was more likely to be located at the plasma membrane compared to ZERO, although the great majority of channel protein was in the membrane in both cases. Co-expression of β1-BK_Ca subunit with STREX or ZERO did not influence the dominant membrane expression of α-BK_Ca subunits, whereas in the absence of α-BK_Ca, a significant proportion of β1-subunit remained cytosolic. Biotinylation assays of cremaster and cerebral arteries showed that differences in STREX/ZERO expression do not alter membrane/cytosolic distribution of the channel under basal conditions. These data, however, revealed that the amount of α-BK_Ca in cerebral arteries is approximately 20X higher than in cremaster vessels. Thus, the data support the major functional differences in BK_Ca activity in cremaster, as compared to cerebral VSMCs, being related to total α-BK_Ca expression, regardless of differences in splice variant expression.

A PLCγ1-dependent, force-sensitive signaling network in the myogenic constriction of cerebral arteries

Maintaining constant blood flow in the face of fluctuations in blood pressure is a critical autoregulatory feature of cerebral arteries. An increase in pressure within the artery lumen causes the vessel to constrict through depolarization and contraction of the encircling smooth muscle cells. This pressure-sensing mechanism involves activation of two types of transient receptor potential (TRP) channels: TRPC6 and TRPM4. We provide evidence that the activation of the γ1 isoform of phospholipase C (PLCγ1) is critical for pressure sensing in cerebral arteries. Inositol 1,4,5-trisphosphate (IP3), generated by PLCγ1 in response to pressure, sensitized IP3 receptors (IP3Rs) to Ca(2+) influx mediated by the mechanosensitive TRPC6 channel, synergistically increasing IP3R-mediated Ca(2+) release to activate TRPM4 currents, leading to smooth muscle depolarization and constriction of isolated cerebral arteries. Proximity ligation assays demonstrated colocalization of PLCγ1 and TRPC6 with TRPM4, suggesting the presence of a force-sensitive, local signaling network comprising PLCγ1, TRPC6, TRPM4, and IP3Rs. Src tyrosine kinase activity was necessary for stretch-induced TRPM4 activation and myogenic constriction, consistent with the ability of Src to activate PLCγ isoforms. We conclude that contraction of cerebral artery smooth muscle cells requires the integration of pressure-sensing signaling pathways and their convergence on IP3Rs, which mediate localized Ca(2+)-dependent depolarization through the activation of TRPM4.

The TRPM4 channel inhibitor 9-phenanthrol

The phenanthrene-derivative 9-phenanthrol is a recently identified inhibitor of the transient receptor potential melastatin (TRPM) 4 channel, a Ca(2+) -activated non-selective cation channel whose mechanism of action remains to be determined. Subsequent studies performed on other ion channels confirm the specificity of the drug for TRPM4. In addition, 9-phenanthrol modulates a variety of physiological processes through TRPM4 current inhibition and thus exerts beneficial effects in several pathological conditions. 9-Phenanthrol modulates smooth muscle contraction in bladder and cerebral arteries, affects spontaneous activity in neurons and in the heart, and reduces lipopolysaccharide-induced cell death. Among promising potential applications, 9-phenanthrol exerts cardioprotective effects against ischaemia-reperfusion injuries and reduces ischaemic stroke injuries. In addition to reviewing the biophysical effects of 9-phenanthrol, here we present information about its appropriate use in physiological studies and possible clinical applications.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

Glycosylation of TRPM4 and TRPM5 channels: molecular determinants and functional aspects

The transient receptor potential channel, TRPM4, and its closest homolog, TRPM5, are non-selective cation channels that are activated by an increase in intracellular calcium. They are expressed in many cell types, including neurons and myocytes. Although the electrophysiological and pharmacological properties of these two channels have been previously studied, less is known about their regulation, in particular their post-translational modifications. We, and others, have reported that wild-type (WT) TRPM4 channels expressed in HEK293 cells, migrated on SDS-PAGE gel as doublets, similar to other ion channels and membrane proteins. In the present study, we provide evidence that TRPM4 and TRPM5 are each N-linked glycosylated at a unique residue, Asn⁹⁹² and Asn⁹³², respectively. N-linked glycosylated TRPM4 is also found in native cardiac cells. Biochemical experiments using HEK293 cells over-expressing WT TRPM4/5 or N992Q/N932Q mutants demonstrated that the abolishment of N-linked glycosylation did not alter the number of channels at the plasma membrane. In parallel, electrophysiological experiments demonstrated a decrease in the current density of both mutant channels, as compared to their respective controls, either due to the Asn to Gln mutations themselves or abolition of glycosylation. To discriminate between these possibilities, HEK293 cells expressing TRPM4 WT were treated with tunicamycin, an inhibitor of glycosylation. In contrast to N-glycosylation signal abolishment by mutagenesis, tunicamycin treatment led to an increase in the TRPM4-mediated current. Altogether, these results demonstrate that TRPM4 and TRPM5 are both N-linked glycosylated at a unique site and also suggest that TRPM4/5 glycosylation seems not to be involved in channel trafficking, but mainly in their functional regulation.

Intracellular acidification alters myogenic responsiveness and vasomotion of mouse middle cerebral arteries

Intracellular pH (pHi) in the vascular wall modulates agonist-induced vasocontractile and vasorelaxant responses in mesenteric arteries, whereas effects on myogenic tone have been unsettled. We studied the role of Na(+),HCO3(-) cotransporter NBCn1 in mouse isolated middle cerebral arteries and the influence of pHi disturbances on myogenic tone. Na(+),HCO3(-) cotransport was abolished in arteries from NBCn1 knockout mice and steady-state pHi ∼0.3 units reduced compared with wild-type mice. Myogenic tone development was low under control conditions but increased on treatment with the NO-synthase inhibitor N-nitro-L-arginine methyl ester (L-NAME). This effect of L-NAME was smaller in arteries from NBCn1 knockout than wild-type mice. Myogenic tone with L-NAME present was significantly lower in arteries from NBCn1 knockout than wild-type mice and was abolished by rho-kinase inhibitor Y-27632. The arteries displayed vasomotion, and this rhythmic contractile pattern was also attenuated in arteries from NBCn1 knockout mice. No differences in membrane potential or intracellular [Ca(2+)] were seen between arteries from NBCn1 knockout and wild-type mice. We propose that NO production and rho-kinase-dependent Ca(2+) sensitivity are reduced at low pHi in pressurized mouse middle cerebral arteries. This likely impedes the ability to adjust to changes in perfusion pressure and regulate cerebral blood flow.

TRPM4

TRPM4 is a Ca(2+)-activated nonselective cation channel. The channel is activated by an increase of intracellular Ca(2+) and is regulated by several factors including temperature and Pi(4,5)P2. TRPM4 allows Na(+) entry into the cell upon activation, but is completely impermeable to Ca(2+). Unlike TRPM5, its closest relative in the transient receptor potential family, TRPM4 proteins are widely expressed in the body. Currents with properties that are reminiscent of TRPM4 have been described in a variety of tissues since the advent of the patch clamp technology, but their physiological role is only beginning to be clarified with the increasing characterization of knockout mouse models for TRPM4. Furthermore, mutations in the TRPM4 gene have been associated with cardiac conduction disorders in human patients. This review aims to overview the currently available data on the functional properties of TRPM4 and the current understanding of its physiological role in healthy and diseased tissue.

Regulation of cerebral artery smooth muscle membrane potential by Ca²⁺-activated cation channels

Arterial tone is dependent on the depolarizing and hyperpolarizing currents regulating membrane potential and governing the influx of Ca²⁺ needed for smooth muscle contraction. Several ion channels have been proposed to contribute to membrane depolarization, but the underlying molecular mechanisms are not fully understood. In this review, we will discuss the historical and physiological significance of the Ca²⁺-activated cation channel, TRPM4, in regulating membrane potential of cerebral artery smooth muscle cells. As a member of the recently described transient receptor potential super family of ion channels, TRPM4 possesses the biophysical properties and upstream cellular signaling and regulatory pathways that establish it as a major physiological player in smooth muscle membrane depolarization.

Lipid signaling cascades of orexin/hypocretin receptors

Orexins - orexin-A and orexin-B - are neuropeptides with significant role in regulation of fundamental physiological processes such as sleep-wakefulness cycle. Orexins act via G-protein-coupled OX1 and OX2 receptors, which are found, in addition to the central nervous system, also in a number of peripheral organs. Orexin receptors show high degree of signaling promiscuity. One particularly prominent way of signaling for these receptors is via phospholipase cascades, including the phospholipase C, phospholipase D and phospholipase A2 cascades, and also diacylglycerol lipase and phosphoinositide-3-kinase pathways. Most analyses have been performed in recombinant cells; there are indications of some of these cascades in native cells while the significance of other cascades remains to be shown. In this review, I present these pathways, their activation mechanisms and their physiological significance.

Oxidative stress-modulated TRPM ion channels in cell dysfunction and pathological conditions in humans

The transient receptor potential melastatin (TRPM) protein family is an extensive group of ion channels expressed in several types of mammalian cells. Many studies have shown that these channels are crucial for performing several physiological functions. Additionally, a large body of evidence indicates that these channels are also involved in numerous human diseases, known as channelopathies. A characteristic event frequently observed during pathological states is the raising in intracellular oxidative agents over reducing molecules, shifting the redox balance and inducing oxidative stress. In particular, three members of the TRPM subfamily, TRPM2, TRPM4 and TRPM7, share the remarkable feature that their activities are modulated by oxidative stress. Because of the increase in oxidative stress, these TRPM channels function aberrantly, promoting the onset and development of diseases. Increases, absences, or modifications in the function of these redox-modulated TRPM channels are associated with cell dysfunction and human pathologies. Therefore, the effect of oxidative stress on ion channels becomes an essential part of the pathogenic mechanism. Thus, oxidative stress-modulated ion channels are more susceptible to generating pathological states than oxidant-independent channels. This review examines the most relevant findings regarding the participation of the oxidative stress-modulated TRPM ion channels, TRPM2, TRPM4, and TRPM7, in human diseases. In addition, the potential roles of these channels as therapeutic tools and targets for drug design are discussed.

TRPM4 channels in smooth muscle function

The melastatin (M) transient receptor potential (TRP) channel TRPM4 is selective for monovalent cations and is activated by high levels of intracellular Ca(2+). TRPM4 is broadly distributed and may be involved in numerous functions, including electrical conduction in the heart, respiratory rhythm, immune response, and secretion of insulin by pancreatic β-cells. The significance of TRPM4 in smooth muscle cell function is reviewed here. Several studies indicate that TRPM4 channels are critically important for pressure-induced cerebral arterial myocyte depolarization and myogenic vasoconstriction as well as autoregulation of cerebral blood flow. Regulation of TRPM4 activity in arterial smooth muscle cells is complex and involves release of Ca(2+) from the sarcoplasmic reticulum through inositol 1,4,5-trisphosphate receptors and translocation of TRPM4 channels to the plasma membrane in response to protein kinase Cδ. TRPM4 is also present in colonic, urinary bladder, aortic, interlobar pulmonary and renal artery, airway, and corpus cavernosum smooth muscle cells, but its significance and regulation in these tissues is less well characterized.

Purinergic receptors regulate myogenic tone in cerebral parenchymal arterioles

Myogenic tone is a fundamental aspect of vascular behavior in resistance arteries. This contractile response to changes in intravascular pressure is critically involved in blood flow autoregulation in tissues such as the brain, kidneys, and heart. Myogenic tone also helps regulate precapillary pressure and provides a level of background tone upon which vasodilator stimuli act to increase tissue perfusion when appropriate. Despite the importance of these processes in the brain, little is known about the mechanisms involved in control of myogenic tone in the cerebral microcirculation. Here, we report that pharmacological inhibition of P2Y4 and P2Y6 pyrimidine receptors nearly abolished myogenic tone in cerebral parenchymal arterioles (PAs). Molecular suppression of either P2Y4 or P2Y6 receptors using antisense oligodeoxynucleotides reduced myogenic tone by 44%±8% and 45%±7%, respectively. These results indicate that both receptor isoforms are activated by increased intravascular pressure, which enhances the activity of voltage-dependent calcium channels and increases myogenic tone in PAs. Enhancement or inhibition of ectonucleotidase activity had no effect on parenchymal arteriolar myogenic tone, indicating that this response is not mediated by local release of nucleotides, but rather may involve direct mechanical activation of P2Y receptors in the smooth muscle cells.

TRPM4 cation channel mediates axonal and neuronal degeneration in experimental autoimmune encephalomyelitis and multiple sclerosis

In multiple sclerosis, an inflammatory disease of the central nervous system (CNS), axonal and neuronal loss are major causes for irreversible neurological disability. However, which molecules contribute to axonal and neuronal injury under inflammatory conditions remains largely unknown. Here we show that the transient receptor potential melastatin 4 (TRPM4) cation channel is crucial in this process. TRPM4 is expressed in mouse and human neuronal somata, but it is also expressed in axons in inflammatory CNS lesions in experimental autoimmune encephalomyelitis (EAE) in mice and in human multiple sclerosis tissue. Deficiency or pharmacological inhibition of TRPM4 using the antidiabetic drug glibenclamide resulted in reduced axonal and neuronal degeneration and attenuated clinical disease scores in EAE, but this occurred without altering EAE-relevant immune function. Furthermore, Trpm4(-/-) mouse neurons were protected against inflammatory effector mechanisms such as excitotoxic stress and energy deficiency in vitro. Electrophysiological recordings revealed TRPM4-dependent neuronal ion influx and oncotic cell swelling upon excitotoxic stimulation. Therefore, interference with TRPM4 could translate into a new neuroprotective treatment strategy.

Different effect of Rho kinase inhibition on calcium signaling in rat isolated large and small arteries

In addition to its role in the regulation of artery contraction, Rho kinase (ROCK) was reported to be involved in the cytosolic calcium response to vasoconstrictor agonists in rat aorta and superior mesenteric artery (SMA). However, it remains to be determined whether ROCK also contributes to calcium signaling in resistance arteries, which play a major role in blood pressure regulation. The investigation of the effect of ROCK inhibition on the calcium and contractile responses of rat resistance mesenteric artery (RMA), in comparison with aorta and SMA, indicated that the calcium response to noradrenaline was inhibited by the ROCK inhibitor Y-27632 in aorta and SMA but not in RMA. The effect of Y-27632 on the calcium signal was unaffected by cytochalasin-D. ROCK activation in noradrenaline-stimulated arteries was confirmed by the inhibition of myosin light chain phosphorylation by Y-27632. Moreover, noradrenaline-induced calcium signaling was similarly inhibited by nimodipine in aorta, SMA and RMA, but nimodipine sensitivity of the contraction increased from the aorta to the RMA, suggesting that the contraction was controlled by different sources of calcium. In pressurized RMA, Y-27632 and H-1152 depressed pressure-induced calcium responses and abolished myogenic contraction. These results stress the important differences in calcium signaling between conductance and resistance arteries.