Store-operated calcium entry channels in pulmonary endothelium: the emerging story of TRPCS and Orai1

The Molecular Heterogeneity of Store-Operated Ca2+ Entry in Vascular Endothelial Cells: The Different roles of Orai1 and TRPC1/TRPC4 Channels in the Transition from Ca2+-Selective to Non-Selective Cation Currents

Store-operated Ca²⁺ entry (SOCE) is activated in response to the inositol-1,4,5-trisphosphate (InsP₃)-dependent depletion of the endoplasmic reticulum (ER) Ca²⁺ store and represents a ubiquitous mode of Ca²⁺ influx. In vascular endothelial cells, SOCE regulates a plethora of functions that maintain cardiovascular homeostasis, such as angiogenesis, vascular tone, vascular permeability, platelet aggregation, and monocyte adhesion. The molecular mechanisms responsible for SOCE activation in vascular endothelial cells have engendered a long-lasting controversy. Traditionally, it has been assumed that the endothelial SOCE is mediated by two distinct ion channel signalplexes, i.e., STIM1/Orai1 and STIM1/Transient Receptor Potential Canonical 1(TRPC1)/TRPC4. However, recent evidence has shown that Orai1 can assemble with TRPC1 and TRPC4 to form a non-selective cation channel with intermediate electrophysiological features. Herein, we aim at bringing order to the distinct mechanisms that mediate endothelial SOCE in the vascular tree from multiple species (e.g., human, mouse, rat, and bovine). We propose that three distinct currents can mediate SOCE in vascular endothelial cells: (1) the Ca²⁺-selective Ca²⁺-release activated Ca²⁺ current (I_CRAC), which is mediated by STIM1 and Orai1; (2) the store-operated non-selective current (I_SOC), which is mediated by STIM1, TRPC1, and TRPC4; and (3) the moderately Ca²⁺-selective, I_CRAC-like current, which is mediated by STIM1, TRPC1, TRPC4, and Orai1.

Relevance of stromal interaction molecule 1 (STIM1) in experimental and human stroke

Stroke represents a main cause of death and permanent disability worldwide. In the attempt to develop targeted preventive and therapeutic strategies, several efforts were performed over the last decades to identify the specific molecular abnormalities preceding cerebral ischemia and neuronal death. In this regard, mitochondrial dysfunction, autophagy, and intracellular calcium homeostasis appear important contributors to stroke development, as underscored by recent pre-clinical evidence. Intracellular calcium (Ca²⁺) homeostasis is regulated, among other mechanisms, by the calcium sensor stromal interaction molecule 1 (STIM1) and calcium release-activated calcium modulator (ORAI) members, which mediate the store-operated Ca²⁺ entry (SOCE). The activity of SOCE is deregulated in animal models of ischemic stroke, leading to ischemic injury exacerbation. We found a different pattern of expression of few SOCE components, dependent from a STIM1 mutation, in cerebral endothelial cells isolated from the stroke-prone spontaneously hypertensive rat (SHRSP), compared to the stroke-resistant (SHRSR) strain, suggesting a potential involvement of this mechanism into the stroke predisposition of SHRSP. In this article, we discuss the relevant role of STIM1 in experimental stroke, as highlighted by the current literature and by our recent experimental findings, and the available evidence in the human disease. We also provide a glance on future perspectives and clinical implications of STIM1.

Impact of Na+ permeation on collective migration of pulmonary arterial endothelial cells

Collective migration of endothelial cells is important for wound healing and angiogenesis. During such migration, each constituent endothelial cell coordinates its magnitude and direction of migration with its neighbors while retaining intercellular adhesion. Ensuring coordination and cohesion involves a variety of intra- and inter-cellular signaling processes. However, the role of permeation of extracellular Na⁺ in collective cell migration remains unclear. Here, we examined the effect of Na⁺ permeation in collective migration of pulmonary artery endothelial cell (PAEC) monolayers triggered by either a scratch injury or a barrier removal over 24 hours. In the scratch assay, PAEC monolayers migrated in two approximately linear phases. In the first phase, wound closure started with fast speed which then rapidly reduced within 5 hours after scratching. In the second phase, wound closure maintained at slow and stable speed from 6 to 24 hours. In the absence of extracellular Na⁺, the wound closure distance was reduced by >50%. Fewer cells at the leading edge protruded prominent lamellipodia. Beside transient gaps, some sustained interendothelial gaps also formed and progressively increased in size over time, and some fused with adjacent gaps. In the absence of both Na⁺ and scratch injury, PAEC monolayer migrated even more slowly, and interendothelial gaps obviously increased in size towards the end. Pharmacological inhibition of the epithelial Na⁺ channel (ENaC) using amiloride reduced wound closure distance by 30%. Inhibition of both the ENaC and the Na⁺/Ca²⁺ exchanger (NCX) using benzamil further reduced wound closure distance in the second phase and caused accumulation of floating particles in the media. Surprisingly, pharmacological inhibition of the Ca²⁺ release-activated Ca²⁺ (CRAC) channel protein 1 (Orai1) using GSK-7975A, the transient receptor potential channel protein 1 and 4 (TRPC1/4) using Pico145, or both Orai1 and TRPC1/4 using combined GSK-7975A and Pico145 treatment did not affect wound closure distance dramatically. Nevertheless, the combined treatment appeared to cause accumulation of floating particles. Note that GSK-7975A also inhibits small inward Ca²⁺ currents via Orai2 and Orai3 channels, whereas Pico145 also blocks TRPC4, TRPC5, and TRPC1/5 channels. By contrast, gene silence of Orai1 by shRNAs led to a 25% reduction of wound closure in the first 6 hours but had no effect afterwards. However, in the absence of extracellular Na⁺ or cellular injury, Orai1 did not affect PAEC collective migration. Overall, the data reveal that Na⁺ permeation into cells contributes to PAEC monolayer collective migration by increasing lamellipodial formation, reducing accumulation of floating particles, and improving intercellular adhesion.

Endothelial Ca2+ Signaling, Angiogenesis and Vasculogenesis: just What It Takes to Make a Blood Vessel

It has long been known that endothelial Ca²⁺ signals drive angiogenesis by recruiting multiple Ca²⁺-sensitive decoders in response to pro-angiogenic cues, such as vascular endothelial growth factor, basic fibroblast growth factor, stromal derived factor-1α and angiopoietins. Recently, it was shown that intracellular Ca²⁺ signaling also drives vasculogenesis by stimulation proliferation, tube formation and neovessel formation in endothelial progenitor cells. Herein, we survey how growth factors, chemokines and angiogenic modulators use endothelial Ca²⁺ signaling to regulate angiogenesis and vasculogenesis. The endothelial Ca²⁺ response to pro-angiogenic cues may adopt different waveforms, ranging from Ca²⁺ transients or biphasic Ca²⁺ signals to repetitive Ca²⁺ oscillations, and is mainly driven by endogenous Ca²⁺ release through inositol-1,4,5-trisphosphate receptors and by store-operated Ca²⁺ entry through Orai1 channels. Lysosomal Ca²⁺ release through nicotinic acid adenine dinucleotide phosphate-gated two-pore channels is, however, emerging as a crucial pro-angiogenic pathway, which sustains intracellular Ca²⁺ mobilization. Understanding how endothelial Ca²⁺ signaling regulates angiogenesis and vasculogenesis could shed light on alternative strategies to induce therapeutic angiogenesis or interfere with the aberrant vascularization featuring cancer and intraocular disorders.

Cortical Actin Dynamics in Endothelial Permeability

The pulmonary endothelial cell forms a critical semi-permeable barrier between the vascular and interstitial space. As part of the blood-gas barrier in the lung, the endothelium plays a key role in normal physiologic function and pathologic disease. Changes in endothelial cell shape, defined by its plasma membrane, determine barrier integrity. A number of key cytoskeletal regulatory and effector proteins including non-muscle myosin light chain kinase, cortactin, and Arp 2/3 mediate actin rearrangements to form cortical and membrane associated structures in response to barrier enhancing stimuli. These actin formations support and interact with junctional complexes and exert forces to protrude the lipid membrane to and close gaps between individual cells. The current knowledge of these cytoskeletal processes and regulatory proteins are the subject of this review. In addition, we explore novel advancements in cellular imaging that are poised to shed light on the complex nature of pulmonary endothelial permeability.

The role of endothelial leak in pulmonary hypertension (2017 Grover Conference Series)

The canonical transient receptor potential 4 (TRPC4) protein contributes to the molecular make-up of endothelial store-operated calcium entry channels. Store-operated calcium entry is a prominent mode of calcium influx in endothelium. Store-operated calcium entry channels are activated by inflammatory mediators and growth factors, and in endothelium, this process induces inter-endothelial cell gaps that increase permeability. Pulmonary endothelium within extra-alveolar segments, including pulmonary arteries, is especially sensitive to the activation of store-operated calcium entry. Pulmonary arterial hypertension (PAH) is characterized by endothelial cell dysfunction in arteries. As one of the topics for the 2017 Grover Conference Series, we examined whether an endothelial cell permeability defect accompanies PAH and, if so, whether TRPC4 contributes to this defect. Through a series of studies conducted over the past five years, we find endothelial cell barrier dysfunction occurs early in the progression of experimental PAH. Endothelium within the arterial segment, and perhaps in other vascular segments, is highly susceptible to disruption secondary to both activation of store-operated calcium entry channels and high flow. This phenomenon partly depends upon TRPC4 channels. We discuss whether endothelial cell hyperpermeability is relevant to human disease, and more specifically, whether it is relevant to all groups of pulmonary hypertension.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

TRPC3-mediated Ca2+ signals as a promising strategy to boost therapeutic angiogenesis in failing hearts: The role of autologous endothelial colony forming cells

Endothelial progenitor cells (EPCs) are a sub-population of bone marrow-derived mononuclear cells that are released in circulation to restore damaged endothelium during its physiological turnover or rescue blood perfusion after an ischemic insult. Additionally, they may be mobilized from perivascular niches located within larger arteries' wall in response to hypoxic conditions. For this reason, EPCs have been regarded as an effective tool to promote revascularization and functional recovery of ischemic hearts, but clinical application failed to exploit the full potential of patients-derived cells. Indeed, the frequency and biological activity of EPCs are compromised in aging individuals or in subjects suffering from severe cardiovascular risk factors. Rejuvenating the reparative phenotype of autologous EPCs through a gene transfer approach has, therefore, been put forward as an alternative approach to enhance their therapeutic potential in cardiovascular patients. An increase in intracellular Ca²⁺ concentration constitutes a pivotal signal for the activation of the so-called endothelial colony forming cells (ECFCs), the only known truly endothelial EPC subset. Studies from our group showed that the Ca²⁺ toolkit differs between peripheral blood- and umbilical cord blood (UCB)-derived ECFCs. In the present article, we first discuss how VEGF uses repetitive Ca²⁺ spikes to regulate angiogenesis in ECFCs and outline how VEGF-induced intracellular Ca²⁺ oscillations differ between the two ECFC subtypes. We then hypothesize about the possibility to rejuvenate the biological activity of autologous ECFCs by transfecting the cell with the Ca²⁺ -permeable channel Transient Receptor Potential Canonical 3, which selectively drives the Ca²⁺ response to VEGF in UCB-derived ECFCs.

STIM1 Phosphorylation at Y361 Recruits Orai1 to STIM1 Puncta and Induces Ca2+ Entry

Store-operated Ca²⁺ entry (SOCE) mediates the increase in intracellular calcium (Ca²⁺) in endothelial cells (ECs) that regulates several EC functions including tissue-fluid homeostasis. Stromal-interaction molecule 1 (STIM1), upon sensing the depletion of (Ca²⁺) from the endoplasmic reticulum (ER) store, organizes as puncta that trigger store-operated Ca²⁺ entry (SOCE) via plasmalemmal Ca²⁺-selective Orai1 channels. While the STIM1 and Orai1 binding interfaces have been mapped, signaling mechanisms activating STIM1 recruitment of Orai1 and STIM1-Orai1 interaction remains enigmatic. Here, we show that ER Ca²⁺-store depletion rapidly induces STIM1 phosphorylation at Y361 via proline-rich kinase 2 (Pyk2) in ECs. Surprisingly, the phospho-defective STIM1-Y361F mutant formed puncta but failed to recruit Orai1, thereby preventing. SOCE Furthermore, studies in mouse lungs, expression of phosphodefective STIM1-Y361F mutant in ECs prevented the increase in vascular permeability induced by the thrombin receptor, protease activated receptor 1 (PAR1). Hence, Pyk2-dependent phosphorylation of STIM1 at Y361 is a critical phospho-switch enabling recruitment of Orai1 into STIM1 puncta leading to SOCE. Therefore, Y361 in STIM1 represents a novel target for limiting SOCE-associated vascular leak.

ROS Signaling in the Pathogenesis of Acute Lung Injury (ALI) and Acute Respiratory Distress Syndrome (ARDS)

The generation of reactive oxygen species (ROS) plays an important role for the maintenance of cellular processes and functions in the body. However, the excessive generation of oxygen radicals under pathological conditions such as acute lung injury (ALI) and its most severe form acute respiratory distress syndrome (ARDS) leads to increased endothelial permeability. Within this hallmark of ALI and ARDS, vascular microvessels lose their junctional integrity and show increased myosin contractions that promote the migration of polymorphonuclear leukocytes (PMNs) and the transition of solutes and fluids in the alveolar lumen. These processes all have a redox component, and this chapter focuses on the role played by ROS during the development of ALI/ARDS. We discuss the origins of ROS within the cell, cellular defense mechanisms against oxidative damage, the role of ROS in the development of endothelial permeability, and potential therapies targeted at oxidative stress.

Endothelial hyperpermeability in severe pulmonary arterial hypertension: role of store-operated calcium entry

Here, we tested the hypothesis that animals with severe pulmonary arterial hypertension (PAH) display increased sensitivity to vascular permeability induced by activation of store-operated calcium entry. To test this hypothesis, wild-type and transient receptor potential channel 4 (TRPC4) knockout Fischer 344 rats were given a single injection of Semaxanib (SU5416; 20 mg/kg) followed by 3 wk of exposure to hypoxia (10% oxygen) and a return to normoxia (21% oxygen) for an additional 2-3 wk. This Semaxanib/hypoxia/normoxia (i.e., SU5416/hypoxia/normoxia) treatment caused PAH, as evidenced by development of right ventricular hypertrophy, pulmonary artery medial hypertrophy, and occlusive lesions within precapillary arterioles. Pulmonary artery pressure was increased fivefold in Semaxanib/hypoxia/normoxia-treated animals compared with untreated, Semaxanib-treated, and hypoxia-treated controls, determined by isolated perfused lung studies. Thapsigargin induced a dose-dependent increase in permeability that was dependent on TRPC4 in the normotensive perfused lung. This increase in permeability was accentuated in PAH lungs but not in Semaxanib- or hypoxia-treated lungs. Fluid accumulated in large perivascular cuffs, and although alveolar fluid accumulation was not seen in histological sections, Evans blue dye conjugated to albumin was present in bronchoalveolar lavage fluid of hypertensive but not normotensive lungs. Thus PAH is accompanied by a TRPC4-dependent increase in the sensitivity to edemagenic agents that activate store-operated calcium entry.

Molecular mechanisms in lipopolysaccharide-induced pulmonary endothelial barrier dysfunction

The confluent pulmonary endothelium plays an important role as a semi-permeable barrier between the vascular space of blood vessels and the underlying tissues, and it contributes to the maintenance of circulatory fluid homeostasis. Pulmonary endothelial barrier dysfunction is a pivotal early step in the development of a variety of high mortality diseases, such as acute lung injury (ALI). Endothelium barrier dysfunction in response to inflammatory or infectious mediators, including lipopolysaccharide (LPS), is accompanied by invertible cell deformation and interendothelial gap formation. However, specific pharmacological therapies aiming at ameliorating pulmonary endothelial barrier function in patients are still lacking. A full understanding of the fundamental mechanisms that are involved in the regulation of pulmonary endothelial permeability is essential for the development of barrier protective therapeutic strategies. Therefore, this review summarizes several important molecular mechanisms involved in LPS-induced changes in pulmonary endothelial barrier function. As for barrier-disruption, the activation of myosin light chain kinase (MLCK), RhoA and tyrosine kinases; increase of calcium influx; and apoptosis of the endothelium lead to an elevation of lung endothelial permeability. Additionally, the activation of Rac1, Cdc42, protease activated receptor 1 (PAR1) and adenosine receptors (ARs), as well as the increase of cyclic AMP and sphingosine-1-phosphate (S1P) content, protect against LPS-induced lung endothelial barrier dysfunction. Furthermore, current regulatory factors and strategies against the development of LPS-induced lung endothelial hyper-permeability are discussed.

G-protein-coupled receptor participates in 20-hydroxyecdysone signaling on the plasma membrane

BACKGROUND

Animal steroid hormones are conventionally known to initiate signaling via a genomic pathway by binding to the nuclear receptors. The mechanism by which 20E initiates signaling via a nongenomic pathway is unclear.

RESULTS

We illustrate that 20E triggered the nongenomic pathway through a plasma membrane G-protein-coupled receptor (named ErGPCR) in the lepidopteran insect Helicoverpa armigera. The transcript of ErGPCR was increased at the larval molting stage and metamorphic molting stage by 20E regulation. Knockdown of ErGPCR via RNA interference in vivo blocked larval-pupal transition and suppressed 20E-induced gene expression. ErGPCR overexpression in the H. armigera epidermal cell line increased the 20E-induced gene expression. Through ErGPCR, 20E modulated Calponin nuclear translocation and phosphorylation, and induced a rapid increase in cytosolic Ca2+ levels. The inhibitors of T-type voltage-gated calcium channels and canonical transient receptor potential calcium channels repressed the 20E-induced Ca2+ increase. Truncation of the N-terminal extracellular region of ErGPCR inhibited its localization on the plasma membrane and 20E-induced gene expression. ErGPCR was not detected to bind with the steroid hormone analog [3H]Pon A.

CONCLUSION

These results suggest that ErGPCR participates in 20E signaling on the plasma membrane.

Endothelial and smooth muscle cell ion channels in pulmonary vasoconstriction and vascular remodeling

The pulmonary circulation is a low resistance and low pressure system. Sustained pulmonary vasoconstriction and excessive vascular remodeling often occur under pathophysiological conditions such as in patients with pulmonary hypertension. Pulmonary vasoconstriction is a consequence of smooth muscle contraction. Many factors released from the endothelium contribute to regulating pulmonary vascular tone, while the extracellular matrix in the adventitia is the major determinant of vascular wall compliance. Pulmonary vascular remodeling is characterized by adventitial and medial hypertrophy due to fibroblast and smooth muscle cell proliferation, neointimal proliferation, intimal, and plexiform lesions that obliterate the lumen, muscularization of precapillary arterioles, and in situ thrombosis. A rise in cytosolic free Ca(2+) concentration ([Ca(2+)]cyt) in pulmonary artery smooth muscle cells (PASMC) is a major trigger for pulmonary vasoconstriction, while increased release of mitogenic factors, upregulation (or downregulation) of ion channels and transporters, and abnormalities in intracellular signaling cascades are key to the remodeling of the pulmonary vasculature. Changes in the expression, function, and regulation of ion channels in PASMC and pulmonary arterial endothelial cells play an important role in the regulation of vascular tone and development of vascular remodeling. This article will focus on describing the ion channels and transporters that are involved in the regulation of pulmonary vascular function and structure and illustrating the potential pathogenic role of ion channels and transporters in the development of pulmonary vascular disease.

TRPC4 inactivation confers a survival benefit in severe pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is characterized by elevated pulmonary arterial pressure with lumen-occluding neointimal and plexiform lesions. Activation of store-operated calcium entry channels promotes contraction and proliferation of lung vascular cells. TRPC4 is a ubiquitously expressed store-operated calcium entry channel, but its role in PAH is unknown. We tested the hypothesis that TRPC4 promotes pulmonary arterial constriction and occlusive remodeling, leading to right ventricular failure in severe PAH. Severe PAH was induced in Sprague-Dawley rats and in wild-type and TRPC4-knockout Fischer 344 rats by a single subcutaneous injection of SU5416 [SU (semaxanib)], followed by hypoxia exposure (Hx; 10% O2) for 3 weeks and then a return to normoxia (Nx; 21% O2) for 3 to 10 additional weeks (SU/Hx/Nx). Although rats of both backgrounds exhibited indistinguishable pulmonary hypertensive responses to SU/Hx/Nx, Fischer 344 rats died within 6 to 8 weeks. Normoxic and hypertensive TRPC4-knockout rats recorded hemodynamic parameters similar to those of their wild-type littermates. However, TRPC4 inactivation conferred a striking survival benefit, due in part to preservation of cardiac output. Histological grading of vascular lesions revealed a reduction in the density of severely occluded small pulmonary arteries and in the number of plexiform lesions in TRPC4-knockout rats. TRPC4 inactivation therefore provides a survival benefit in severe PAH, associated with a decrease in the magnitude of occlusive remodeling.

Pathogenic role of store-operated and receptor-operated ca(2+) channels in pulmonary arterial hypertension

Pulmonary circulation is an important circulatory system in which the body brings in oxygen. Pulmonary arterial hypertension (PAH) is a progressive and fatal disease that predominantly affects women. Sustained pulmonary vasoconstriction, excessive pulmonary vascular remodeling, in situ thrombosis, and increased pulmonary vascular stiffness are the major causes for the elevated pulmonary vascular resistance (PVR) in patients with PAH. The elevated PVR causes an increase in afterload in the right ventricle, leading to right ventricular hypertrophy, right heart failure, and eventually death. Understanding the pathogenic mechanisms of PAH is important for developing more effective therapeutic approach for the disease. An increase in cytosolic free Ca²⁺ concentration ([Ca²⁺]_cyt) in pulmonary arterial smooth muscle cells (PASMC) is a major trigger for pulmonary vasoconstriction and an important stimulus for PASMC migration and proliferation which lead to pulmonary vascular wall thickening and remodeling. It is thus pertinent to define the pathogenic role of Ca²⁺ signaling in pulmonary vasoconstriction and PASMC proliferation to develop new therapies for PAH. [Ca²⁺]_cyt in PASMC is increased by Ca²⁺ influx through Ca²⁺ channels in the plasma membrane and by Ca²⁺ release or mobilization from the intracellular stores, such as sarcoplasmic reticulum (SR) or endoplasmic reticulum (ER). There are two Ca²⁺ entry pathways, voltage-dependent Ca²⁺ influx through voltage-dependent Ca²⁺ channels (VDCC) and voltage-independent Ca²⁺ influx through store-operated Ca²⁺ channels (SOC) and receptor-operated Ca²⁺ channels (ROC). This paper will focus on the potential role of VDCC, SOC, and ROC in the development and progression of sustained pulmonary vasoconstriction and excessive pulmonary vascular remodeling in PAH.

The roles of G proteins in the activation of TRPC4 and TRPC5 transient receptor potential channels

TRPC4 and TRPC5 channels are important regulators of electrical excitability in both gastrointestinal myocytes and neurons. Much is known regarding the assembly and function of these channels including TRPC1 as a homotetramer or a heteromultimer and the roles that their interacting proteins play in controlling these events. Further, they are one of the best-studied targets of G protein-coupled receptors and growth factors in general and Gαq protein coupled receptor or epidermal growth factor in particular. However, our understanding of the roles of Gαi/o proteins on TRPC4/5 channels is still rudimentary. We discuss potential roles for Gαi/o proteins in channel activation in addition to their known role in cellular signaling.

Myosin light chain kinase signaling in endothelial barrier dysfunction

Microvascular barrier dysfunction is a serious problem that occurs in many inflammatory conditions, including sepsis, trauma, ischemia-reperfusion injury, cardiovascular disease, and diabetes. Barrier dysfunction permits extravasation of serum components into the surrounding tissue, leading to edema formation and organ failure. The basis for microvascular barrier dysfunction is hyperpermeability at endothelial cell-cell junctions. Endothelial hyperpermeability is increased by actomyosin contractile activity in response to phosphorylation of myosin light chain by myosin light chain kinase (MLCK). MLCK-dependent endothelial hyperpermeability occurs in response to inflammatory mediators (e.g., activated neutrophils, thrombin, histamine, tumor necrosis factor alpha, etc.), through multiple cell signaling pathways and signaling molecules (e.g., Ca(++) , protein kinase C, Src kinase, nitric oxide synthase, etc.). Other signaling molecules protect against MLCK-dependent hyperpermeability (e.g., sphingosine-1-phosphate or cAMP). In addition, individual MLCK isoforms play specific roles in endothelial barrier dysfunction, suggesting that isoform-specific inhibitors could be useful for treating inflammatory disorders and preventing multiple organ failure. Because endothelial barrier dysfunction depends upon signaling through MLCK in many instances, MLCK-dependent signaling comprises multiple potential therapeutic targets for preventing edema formation and multiple organ failure. The following review is a discussion of MLCK-dependent mechanisms and cell signaling events that mediate endothelial hyperpermeability.

Update on vascular endothelial Ca2+ signalling: A tale of ion channels, pumps and transporters

A monolayer of endothelial cells (ECs) lines the lumen of blood vessels and forms a multifunctional transducing organ that mediates a plethora of cardiovascular processes. The activation of ECs from as state of quiescence is, therefore, regarded among the early events leading to the onset and progression of potentially lethal diseases, such as hypertension, myocardial infarction, brain stroke, and tumor. Intracellular Ca²⁺ signals have long been know to play a central role in the complex network of signaling pathways regulating the endothelial functions. Notably, recent work has outlined how any change in the pattern of expression of endothelial channels, transporters and pumps involved in the modulation of intracellular Ca²⁺ levels may dramatically affect whole body homeostasis. Vascular ECs may react to both mechanical and chemical stimuli by generating a variety of intracellular Ca²⁺ signals, ranging from brief, localized Ca²⁺ pulses to prolonged Ca²⁺ oscillations engulfing the whole cytoplasm. The well-defined spatiotemporal profile of the subcellular Ca²⁺ signals elicited in ECs by specific extracellular inputs depends on the interaction between Ca²⁺ releasing channels, which are located both on the plasma membrane and in a number of intracellular organelles, and Ca²⁺ removing systems. The present article aims to summarize both the past and recent literature in the field to provide a clear-cut picture of our current knowledge on the molecular nature and the role played by the components of the Ca²⁺ machinery in vascular ECs under both physiological and pathological conditions.

Orai1 determines calcium selectivity of an endogenous TRPC heterotetramer channel

RATIONALE

Canonical transient receptor potential 4 (TRPC4) contributes to the molecular composition of a channel encoding for a calcium selective store-operated current, I(SOC), whereas Orai1 critically comprises a channel encoding for the highly selective calcium release activated calcium current, I(CRAC). However, Orai1 may interact with TRPC proteins and influence their activation and permeation characteristics. Endothelium expresses both TRPC4 and Orai1, and it remains unclear as to whether Orai1 interacts with TRPC4 and contributes to calcium permeation through the TPRC4 channel.

OBJECTIVE

We tested the hypothesis that Orai1 interacts with TRPC4 and contributes to the channel's selective calcium permeation important for endothelial barrier function.

METHODS AND RESULTS

A novel method to purify the endogenous TRPC4 channel and probe for functional interactions was developed, using TRPC4 binding to protein 4.1 as bait. Isolated channel complexes were conjugated to anti-TRPC protein antibodies labeled with cy3-cy5 pairs. Förster Resonance Energy Transfer among labeled subunits revealed the endogenous protein alignment. One TRPC1 and at least 2 TRPC4 subunits constituted the endogenous channel (TRPC1/4). Orai1 interacted with TRPC4. Conditional Orai1 knockdown reduced the probability for TRPC1/4 channel activation and converted it from a calcium-selective to a nonselective channel, an effect that was rescued on Orai1 reexpression. Loss of Orai1 improved endothelial cell barrier function.

CONCLUSION

Orai1 interacts with TRPC4 in the endogenous channel complex, where it controls TRPC1/4 activation and channel permeation characteristics, including calcium selectivity, important for control of endothelial cell barrier function.

Why thioridazine in combination with antibiotics cures extensively drug-resistant Mycobacterium tuberculosis infections

Thioridazine (TDZ) in combination with antibiotics to which extensively drug-resistant Mycobacterium tuberculosis (XDR-TB) is initially resistant yields a cure. This is due to the fact that TDZ enhances the killing of intracellular M. tuberculosis by non-killing macrophages, inhibits the genetic expression of efflux pumps of M. tuberculosis that extrude antibiotics prior to reaching their intended targets, and inhibits the activity of existing efflux pumps that contribute to the multidrug-resistant phenotype of M. tuberculosis. The combination of these effects of TDZ probably contributes to the successful recent cures of XDR-TB cases when the phenothiazine TDZ is used in combination with antibiotics to which the patient with XDR-TB was initially unresponsive.

Heterotrimeric G proteins, focal adhesion kinase, and endothelial barrier function

Ligands by binding to G protein coupled receptors (GPCRs) stimulate dissociation of heterotrimeric G proteins into Gα and Gβγ subunits. Released Gα and Gβγ subunits induce discrete signaling cues that differentially regulate focal adhesion kinase (FAK) activity and endothelial barrier function. Activation of G proteins downstream of receptors such as protease activated receptor 1 (PAR1) and histamine receptors rapidly increases endothelial permeability which reverses naturally within the following 1-2 h. However, activation of G proteins coupled to the sphingosine-1-phosphate receptor 1 (S1P1) signal cues that enhance basal barrier endothelial function and restore endothelial barrier function following the increase in endothelial permeability by edemagenic agents. Intriguingly, both PAR1 and S1P1 activation stimulates FAK activity, which associates with alteration in endothelial barrier function by these agonists. In this review, we focus on the role of the G protein subunits downstream of PAR1 and S1P1 in regulating FAK activity and endothelial barrier function.

T-cell-specific deletion of STIM1 and STIM2 protects mice from EAE by impairing the effector functions of Th1 and Th17 cells

T-cell function is dependent on store-operated Ca(2+) influx that is activated by the stromal interaction molecules (STIM) 1 and 2. We show that mice with T-cell-specific deletion of STIM1 or STIM2 are protected from EAE, a mouse model of multiple sclerosis (MS). While STIM1- and STIM2-deficient T cells could be successfully primed by autoantigen, they failed to produce the proinflammatory cytokines IL-17 and IFN-γ. STIM1-deficient T cells showed reduced expression of IL-23R, required for Th17 cell homeostasis, and had impaired chemokine-dependent T-cell migration caused by a lack of chemokine-induced Ca(2+) influx. Autoantigen-specific STIM1- or STIM2-deficient T cells failed to expand and accumulate in the CNS and lymph nodes following adoptive transfer to passively induce EAE, suggesting that autoantigen-specific restimulation or homeostasis of STIM1- and STIM2-deficient T cells are impaired. Combined deletion of both STIM1 and STIM2, previously shown to impair Treg development and function, completely protected mice from EAE. This indicates that, in the absence of Ca(2+) influx, autoreactive T cells are severely dysfunctional rendering Treg dispensable for the prevention of CNS inflammation. Our findings demonstrate that both STIM1 and STIM2 are critical for T-cell function and autoimmunity in vivo.

Myosin light chain kinase in microvascular endothelial barrier function

Microvascular barrier dysfunction is implicated in the initiation and progression of inflammation, posttraumatic complications, sepsis, ischaemia-reperfusion injury, atherosclerosis, and diabetes. Under physiological conditions, a precise equilibrium between endothelial cell-cell adhesion and actin-myosin-based centripetal tension tightly controls the semi-permeability of microvascular barriers. Myosin light chain kinase (MLCK) plays an important role in maintaining the equilibrium by phosphorylating myosin light chain (MLC), thereby inducing actomyosin contractility and weakening endothelial cell-cell adhesion. MLCK is activated by numerous physiological factors and inflammatory or angiogenic mediators, causing vascular hyperpermeability. In this review, we discuss experimental evidence supporting the crucial role of MLCK in the hyperpermeability response to key cell signalling events during inflammation. At the cellular level, in vitro studies of cultured endothelial monolayers treated with MLCK inhibitors or transfected with specific inhibiting peptides have demonstrated that induction of endothelial MLCK activity is necessary for hyperpermeability. Ex vivo studies of live microvessels, enabled by development of the isolated, perfused venule method, support the importance of MLCK in endothelial permeability regulation in an environment that more closely resembles in vivo tissues. Finally, the role of MLCK in vascular hyperpermeability has been confirmed with in vivo studies of animal disease models and the use of transgenic MLCK210 knockout mice. These approaches provide a more complete view of the role of MLCK in vascular barrier dysfunction.