Calcium signalling in the endothelium

The glycocalyx and calcium dynamics in endothelial cells

The endothelial glycocalyx is a dynamic surface layer composed of proteoglycans, glycoproteins, and glycosaminoglycans with a key role in maintaining endothelial cell homeostasis. Its functions include the regulation of endothelial barrier permeability and stability, the transduction of mechanical forces from the vascular lumen to the vessel walls, serving as a binding site to multiple growth factors and vasoactive agents, and mediating the binding of platelets and the migration of leukocytes during an inflammatory response. Many of these processes are associated with changes in intracellular calcium levels that may occur through mechanisms that alter calcium entry in the endothelium or the release of calcium from the endoplasmic reticulum. Whether the endothelial glycocalyx can regulate calcium dynamics in endothelial cells is unresolved. Interestingly, during cardiovascular disease progression, changes in calcium dynamics are observed in association with the degradation of the glycocalyx and with changes in barrier permeability and vascular reactivity. Herein, we aim to provide a summarized overview of what is known regarding the role of the glycocalyx as a regulator of endothelial barrier and vascular reactivity during homeostatic and pathological conditions and to provide a perspective on how such processes may relate to calcium dynamics in endothelial cells, exploring a possible connection between components of the glycocalyx and calcium-sensitive pathways in the endothelium.

Cerebrovascular and neurological perspectives on adrenoceptor and calcium channel modulating pharmacotherapies

Adrenoceptor and calcium channel modulating medications are widely used in clinical practice for acute neurological and systemic conditions. It is generally assumed that the cerebrovascular effects of these drugs mirror that of their systemic effects - and this is reflected in how these medications are currently used in clinical practice. However, recent research suggests that there are distinct cerebrovascular-specific effects of these medications that are related to the unique characteristics of the cerebrovascular anatomy including the regional heterogeneity in density and distribution of adrenoceptor subtypes and calcium channels along the cerebrovasculature. In this review, we critically evaluate existing basic science and clinical research to discuss known and putative interactions between adrenoceptor and calcium channel modulating pharmacotherapies, the neurovascular unit, and cerebrovascular anatomy. In doing so, we provide a rationale for selecting vasoactive medications based on lesion location and lay a foundation for future investigations that will define neuroprotective paradigms of adrenoceptor and calcium channel modulating therapies to improve neurological outcomes in acute neurological and systemic disorders.

Carbenoxolone and 18β-glycyrrhetinic acid inhibit inositol 1,4,5-trisphosphate-mediated endothelial cell calcium signalling and depolarise mitochondria

BACKGROUND AND PURPOSE

Coordinated endothelial control of cardiovascular function is proposed to occur by endothelial cell communication via gap junctions and connexins. To study intercellular communication, the pharmacological agents carbenoxolone (CBX) and 18β-glycyrrhetinic acid (18βGA) are used widely as connexin inhibitors and gap junction blockers.

EXPERIMENTAL APPROACH

We investigated the effects of CBX and 18βGA on intercellular Ca²⁺ waves, evoked by inositol 1,4,5-trisphosphate (IP₃ ) in the endothelium of intact mesenteric resistance arteries.

KEY RESULTS

Acetycholine-evoked IP₃ -mediated Ca²⁺ release and propagated waves were inhibited by CBX (100 μM) and 18βGA (40 μM). Unexpectedly, the Ca²⁺ signals were inhibited uniformly in all cells, suggesting that CBX and 18βGA reduced Ca²⁺ release. Localised photolysis of caged IP₃ (cIP₃ ) was used to provide precise spatiotemporal control of site of cell activation. Local cIP₃ photolysis generated reproducible Ca²⁺ increases and Ca²⁺ waves that propagated across cells distant to the photolysis site. CBX and 18βGA each blocked Ca²⁺ waves in a time-dependent manner by inhibiting the initiating IP₃ -evoked Ca²⁺ release event rather than block of gap junctions. This effect was reversed on drug washout and was unaffected by small or intermediate K⁺ -channel blockers. Furthermore, CBX and 18βGA each rapidly and reversibly collapsed the mitochondrial membrane potential.

CONCLUSION AND IMPLICATIONS

CBX and 18βGA inhibit IP₃ -mediated Ca²⁺ release and depolarise the mitochondrial membrane potential. These results suggest that CBX and 18βGA may block cell-cell communication by acting at sites that are unrelated to gap junctions.

FK506 regulates Ca2+ release evoked by inositol 1,4,5-trisphosphate independently of FK-binding protein in endothelial cells

Background and Purpose

FK506 and rapamycin are modulators of FK‐binding proteins (FKBP) that are used to suppress immune function after organ and hematopoietic stem cell transplantations. The drugs share the unwanted side‐effect of evoking hypertension that is associated with reduced endothelial function and nitric oxide production. The underlying mechanisms are not understood. FKBP may regulate IP₃ receptors (IP₃R) and ryanodine receptors (RyR) to alter Ca²⁺ signalling in endothelial cells.

Experimental Approach

We investigated the effects of FK506 and rapamycin on Ca²⁺ release via IP₃R and RyR in hundreds of endothelial cells, using the indicator Cal‐520, in intact mesenteric arteries from male Sprague‐Dawley rats. IP₃Rs were activated by acetylcholine or localised photo‐uncaging of IP₃, and RyR by caffeine.

Key Results

While FKBPs were present, FKBP modulation with rapamycin did not alter IP₃‐evoked Ca²⁺ release. Conversely, FK506, which modulates FKBP and blocks calcineurin, increased IP₃‐evoked Ca²⁺ release. Inhibition of calcineurin (okadiac acid or cypermethrin) also increased IP₃‐evoked Ca²⁺ release and blocked FK506 effects. When calcineurin was inhibited, FK506 reduced IP₃‐evoked Ca²⁺ release. These findings suggest that IP₃‐evoked Ca²⁺ release is not modulated by FKBP, but by FK506‐mediated calcineurin inhibition. The RyR modulators caffeine and ryanodine failed to alter Ca²⁺ signalling suggesting that RyR is not functional in native endothelium.

Conclusion and Implications

The hypertensive effects of the immunosuppressant drugs FK506 and rapamycin, while mediated by endothelial cells, do not appear to be exerted at the documented cellular targets of Ca²⁺ release and altered FKBP binding to IP₃ and RyR.

Reciprocality Between Estrogen Biology and Calcium Signaling in the Cardiovascular System

17β-Estradiol (E₂) is the main estrogenic hormone in the body and exerts many cardiovascular protective effects. Via three receptors known to date, including estrogen receptors α (ERα) and β (ERβ) and the G protein-coupled estrogen receptor 1 (GPER, aka GPR30), E₂ regulates numerous calcium-dependent activities in cardiovascular tissues. Nevertheless, effects of E₂ and its receptors on components of the calcium signaling machinery (CSM), the underlying mechanisms, and the linked functional impact are only beginning to be elucidated. A picture is emerging of the reciprocality between estrogen biology and Ca²⁺ signaling. Therein, E₂ and GPER, via both E₂-dependent and E₂-independent actions, moderate Ca²⁺-dependent activities; in turn, ERα and GPER are regulated by Ca²⁺ at the receptor level and downstream signaling via a feedforward loop. This article reviews current understanding of the effects of E₂ and its receptors on the cardiovascular CSM and vice versa with a focus on mechanisms and combined functional impact. An overview of the main CSM components in cardiovascular tissues will be first provided, followed by a brief review of estrogen receptors and their Ca²⁺-dependent regulation. The effects of estrogenic agonists to stimulate acute Ca²⁺ signals will then be reviewed. Subsequently, E₂-dependent and E₂-independent effects of GPER on components of the Ca²⁺ signals triggered by other stimuli will be discussed. Finally, a case study will illustrate how the many mechanisms are coordinated to moderate Ca²⁺-dependent activities in the cardiovascular system.

Calcium Mobilization in Endothelial Cell Functions

Endothelial cells (ECs) constitute the innermost layer that lines all blood vessels from the larger arteries and veins to the smallest capillaries, including the lymphatic vessels. Despite the histological classification of endothelium of a simple epithelium and its homogeneous morphological appearance throughout the vascular system, ECs, instead, are extremely heterogeneous both structurally and functionally. The different arrangement of cell junctions between ECs and the local organization of the basal membrane generate different type of endothelium with different permeability features and functions. Continuous, fenestrated and discontinuous endothelia are distributed based on the specific function carried out by the organs. It is thought that a large number ECs functions and their responses to extracellular cues depend on changes in intracellular concentrations of calcium ion ([Ca²⁺]_i). The extremely complex calcium machinery includes plasma membrane bound channels as well as intracellular receptors distributed in distinct cytosolic compartments that act jointly to maintain a physiological [Ca²⁺]_i, which is crucial for triggering many cellular mechanisms. Here, we first survey the overall notions related to intracellular Ca²⁺ mobilization and later highlight the involvement of this second messenger in crucial ECs functions with the aim at stimulating further investigation that link Ca²⁺ mobilization to ECs in health and disease.

Transient Receptor Potential Channels and Endothelial Cell Calcium Signaling

The vascular endothelium is a broadly distributed and highly specialized organ. The endothelium has a number of functions including the control of blood vessels diameter through the production and release of potent vasoactive substances or direct electrical communication with underlying smooth muscle cells, regulates the permeability of the vascular barrier, stimulates the formation of new blood vessels, and influences inflammatory and thrombotic processes. Endothelial cells that make up the endothelium express a variety of cell-surface receptors and ion channels on the plasma membrane that are capable of detecting circulating hormones, neurotransmitters, oxygen tension, and shear stress across the vascular wall. Changes in these stimuli activate signaling cascades that initiate an appropriate physiological response. Increases in the global intracellular Ca²⁺ concentration and localized Ca²⁺ signals that occur within specialized subcellular microdomains are fundamentally important components of many signaling pathways in the endothelium. The transient receptor potential (TRP) channels are a superfamily of cation-permeable ion channels that act as a primary means of increasing cytosolic Ca²⁺ in endothelial cells. Consequently, TRP channels are vitally important for the major functions of the endothelium. In this review, we provide an in-depth discussion of Ca²⁺ -permeable TRP channels in the endothelium and their role in vascular regulation. © 2019 American Physiological Society. Compr Physiol 9:1249-1277, 2019.

tmem33 is essential for VEGF-mediated endothelial calcium oscillations and angiogenesis

Angiogenesis requires co-ordination of multiple signalling inputs to regulate the behaviour of endothelial cells (ECs) as they form vascular networks. Vascular endothelial growth factor (VEGF) is essential for angiogenesis and induces downstream signalling pathways including increased cytosolic calcium levels. Here we show that transmembrane protein 33 (tmem33), which has no known function in multicellular organisms, is essential to mediate effects of VEGF in both zebrafish and human ECs. We find that tmem33 localises to the endoplasmic reticulum in zebrafish ECs and is required for cytosolic calcium oscillations in response to Vegfa. tmem33-mediated endothelial calcium oscillations are critical for formation of endothelial tip cell filopodia and EC migration. Global or endothelial-cell-specific knockdown of tmem33 impairs multiple downstream effects of VEGF including ERK phosphorylation, Notch signalling and embryonic vascular development. These studies reveal a hitherto unsuspected role for tmem33 and calcium oscillations in the regulation of vascular development.

Calcium signalling downstream of VEGF is essential for VEGF-induced angiogenesis. Here Savage et al. show that Transmembrane Protein 33 (TMEM33) is required for angiogenesis and the endothelial calcium response to VEGF, revealing a function for TMEM33 in multicellular organisms.

Microvascular mechanisms limiting skeletal muscle blood flow with advancing age

Effective oxygen delivery to active muscle fibers requires that vasodilation initiated in distal arterioles, which control flow distribution and capillary perfusion, ascends the resistance network into proximal arterioles and feed arteries, which govern total blood flow into the muscle. With exercise onset, ascending vasodilation reflects initiation and conduction of hyperpolarization along endothelium from arterioles into feed arteries. Electrical coupling of endothelial cells to smooth muscle cells evokes the rapid component of ascending vasodilation, which is sustained by ensuing release of nitric oxide during elevated luminal shear stress. Concomitant sympathetic neural activation inhibits ascending vasodilation by stimulating α-adrenoreceptors on smooth muscle cells to constrict the resistance vasculature. We hypothesized that compromised muscle blood flow in advanced age reflects impaired ascending vasodilation through actions on both cell layers of the resistance network. In the gluteus maximus muscle of old (24 mo) vs. young (4 mo) male mice (corresponding to mid-60s vs. early 20s in humans) inhibition of α-adrenoreceptors in old mice restored ascending vasodilation, whereas even minimal activation of α-adrenoreceptors in young mice attenuated ascending vasodilation in the manner seen with aging. Conduction of hyperpolarization along the endothelium is impaired in old vs. young mice because of "leaky" membranes resulting from the activation of potassium channels by hydrogen peroxide released from endothelial cells. Exposing the endothelium of young mice to hydrogen peroxide recapitulates this effect of aging. Thus enhanced α-adrenoreceptor activation of smooth muscle in concert with electrically leaky endothelium restricts muscle blood flow by impairing ascending vasodilation in advanced age.

Advancing Age Decreases Pressure-Sensitive Modulation of Calcium Signaling in the Endothelium of Intact and Pressurized Arteries

Aging is the summation of many subtle changes which result in altered cardiovascular function. Impaired endothelial function underlies several of these changes and precipitates plaque development in larger arteries. The endothelium transduces chemical and mechanical signals into changes in the cytoplasmic calcium concentration to control vascular function. However, studying endothelial calcium signaling in larger arteries in a physiological configuration is challenging because of the requirement to focus through the artery wall. Here, pressure- and agonist-sensitive endothelial calcium signaling was studied in pressurized carotid arteries from young (3-month-old) and aged (18-month-old) rats by imaging from within the artery using gradient index fluorescence microendoscopy. Endothelial sensitivity to acetylcholine increased with age. The number of cells exhibiting oscillatory calcium signals and the frequency of oscillations were unchanged with age. However, the latency of calcium responses was significantly increased with age. Acetylcholine-evoked endothelial calcium signals were suppressed by increased intraluminal pressure. However, pressure-dependent inhibition of calcium signaling was substantially reduced with age. While each of these changes will increase endothelial calcium signaling with increasing age, decreases in endothelial pressure sensitivity may manifest as a loss of functionality and responsiveness in aging.

N-Ethylmaleimide Sensitive Factor (NSF) Inhibition Prevents Vascular Instability following Gram-Positive Pulmonary Challenge

Background

The Acute Respiratory Distress Syndrome (ARDS), remains a significant source of morbidity and mortality in critically ill patients. Pneumonia and sepsis are leading causes of ARDS, the pathophysiology of which includes increased pulmonary microvascular permeability and hemodynamic instability resulting in organ dysfunction. We hypothesized that N-ethylmaleimide sensitive factor (NSF) regulates exocytosis of inflammatory mediators, such as Angiopoietin-2 (Ang-2), and cytoskeletal stability by modulating myosin light chain (MLC) phosphorylation. Therefore, we challenged pulmonary cells, in vivo and in vitro, with Gram Positive bacterial cell wall components, lipoteichoic acid (LTA), and peptidoglycan (PGN) and examined the effects of NSF inhibition.

Methods

Mice were pre-treated with an inhibitor of NSF, TAT-NSF700 (to prevent Ang-2 release). After 30min, LTA and PGN (or saline alone) were instilled intratracheally. Pulse oximetry was assessed in awake mice prior to, and 6 hour post instillation. Post mortem, tissues were collected for studies of inflammation and Ang-2. In vitro, pulmonary endothelial cells were assessed for their responses to LTA and PGN.

Results

Pulmonary challenge induced signs of airspace and systemic inflammation such as changes in neutrophil counts and protein concentration in bronchoalveolar lavage fluid and tissue Ang-2 concentration, and decreased physiological parameters including oxygen saturation and pulse distention. TAT-NSF700 pre-treatment reduced LTA-PGN induced changes in lung tissue Ang-2, oxygen saturation and pulse distention. In vitro, LTA-PGN induced a rapid (<2 min) release of Ang-2, which was significantly attenuated by TAT-NSF700 or anti TLR2 antibody. Furthermore, TAT-NSF700 reduced LTA-PGN-induced MLC phosphorylation at low concentrations of 1–10 nM.

Conclusions

TAT-NSF700 decreased Ang-2 release, improved oxygen saturation and pulse distention following pulmonary challenge by inhibiting MLC phosphorylation, an important component of endothelial cell retraction. The data suggest that inhibition of NSF in pneumonia and sepsis may be beneficial to prevent the pulmonary microvascular and hemodynamic instability associated with ARDS.

Cocaine inhibits store-operated Ca2+ entry in brain microvascular endothelial cells: critical role for sigma-1 receptors

Sigma-1 receptor (Sig-1R) is an intracellular chaperone protein with many ligands, located at the endoplasmic reticulum (ER). Binding of cocaine to Sig-1R has previously been found to modulate endothelial functions. In the present study, we show that cocaine dramatically inhibits store-operated Ca(2+) entry (SOCE), a Ca(2+) influx mechanism promoted by depletion of intracellular Ca(2+) stores, in rat brain microvascular endothelial cells (RBMVEC). Using either Sig-1R shRNA or pharmacological inhibition with the unrelated Sig-1R antagonists BD-1063 and NE-100, we show that cocaine-induced SOCE inhibition is dependent on Sig-1R. In addition to revealing new insight into fundamental mechanisms of cocaine-induced changes in endothelial function, these studies indicate an unprecedented role for Sig-1R as a SOCE inhibitor.

Advanced age protects microvascular endothelium from aberrant Ca(2+) influx and cell death induced by hydrogen peroxide

KEY POINTS

Calcium signalling in endothelial cells of resistance arteries is integral to blood flow regulation. Oxidative stress and endothelial dysfunction can prevail during advanced age and we questioned how calcium signalling may be affected. Intact endothelium was freshly isolated from superior epigastric arteries of Young (∼4 months) and Old (∼24 months) male C57BL/6 mice. Under resting conditions, with no difference in intracellular calcium levels, hydrogen peroxide (H2 O2 ) availability was ∼1/3 greater in endothelium of Old mice while vascular catalase activity was reduced by nearly half. Compared to Old, imposing oxidative stress (200 μm H2 O2 ) for 20 min increased intracellular calcium to 4-fold greater levels in endothelium of Young in conjunction with twice the calcium influx. Prolonged (60 min) exposure to H2 O2 induced 7-fold greater cell death in endothelium of Young. Microvascular adaptation to advanced age may protect endothelial cells during elevated oxidative stress to preserve functional viability of the intima.

ABSTRACT

Endothelial cell Ca(2+) signalling is integral to blood flow control in the resistance vasculature yet little is known of how its regulation may be affected by advancing age. We tested the hypothesis that advanced age protects microvascular endothelium by attenuating aberrant Ca(2+) signalling during oxidative stress. Intact endothelial tubes (width, ∼60 μm; length, ∼1000 μm) were isolated from superior epigastric arteries of Young (3-4 months) and Old (24-26 months) male C57BL/6 mice and loaded with Fura-2 dye to monitor [Ca(2+) ]i . At rest there was no difference in [Ca(2+) ]i between age groups. Compared to Young, the [Ca(2+) ]i response to maximal stimulation with acetylcholine (3 μm, 2 min) was ∼25% greater in Old, confirming signalling integrity with advanced age. Basal H2 O2 availability was ∼33% greater in Old while vascular catalase activity was reduced by half. Transient exposure to elevated H2 O2 (200 μm, 20 min) progressively increased [Ca(2+) ]i to ∼4-fold greater levels in endothelium of Young versus Old. With no difference between age groups at rest, Mn(2+) quench of Fura-2 fluorescence revealed 2-fold greater Ca(2+) influx in Young during elevated H2 O2 ; this effect was attenuated by ∼75% using ruthenium red (5 μm) as a broad-spectrum inhibitor of transient receptor potential channels. Prolonged exposure to H2 O2 (200 μm, 60 min) induced ∼7-fold greater cell death in endothelium of Young versus Old. Thus, microvascular endothelium can adapt to advanced age by reducing Ca(2+) influx during elevated oxidative stress. Protection from cell death during oxidative stress will sustain endothelial integrity during ageing.

STIM1 positively regulates the Ca2+ release activity of the inositol 1,4,5-trisphosphate receptor in bovine aortic endothelial cells

The endothelium is actively involved in many functions of the cardiovascular system, such as the modulation of arterial pressure and the maintenance of blood flow. These functions require a great versatility of the intracellular Ca2+ signaling that resides in the fact that different signals can be encoded by varying the frequency and the amplitude of the Ca2+ response. Cells use both extracellular and intracellular Ca2+ pools to modulate the intracellular Ca2+ concentration. In non-excitable cells, the inositol 1,4,5-trisphosphate receptor (IP3R), located on the endoplasmic reticulum (ER), is responsible for the release of Ca2+ from the intracellular store. The proteins STIM1 and STIM2 are also located on the ER and they are involved in the activation of a store-operated Ca2+ entry (SOCE). Due to their Ca2+ sensor property and their close proximity with IP3Rs on the ER, STIMs could modulate the activity of IP3R. In this study, we showed that STIM1 and STIM2 are expressed in bovine aortic endothelial cells and they both interact with IP3R. While STIM2 appears to play a minor role, STIM1 plays an important role in the regulation of agonist-induced Ca2+ mobilization in BAECs by a positive effect on both the SOCE and the IP3R-dependent Ca2+ release.

High-throughput optical screening of cellular mechanotransduction

We introduce an optical platform for rapid, high-throughput screening of exogenous molecules that affect cellular mechanotransduction. Our method initiates mechanotransduction in adherent cells using single laser-microbeam generated micro-cavitation bubbles (μCBs) without requiring flow chambers or microfluidics. These μCBs expose adherent cells to a microTsunami, a transient microscale burst of hydrodynamic shear stress, which stimulates cells over areas approaching 1mm². We demonstrate microTsunami-initiated mechanosignalling in primary human endothelial cells. This observed signalling is consistent with G-protein-coupled receptor stimulation resulting in Ca²⁺ release by the endoplasmic reticulum. Moreover, we demonstrate the dose-dependent modulation of microTsunami-induced Ca²⁺ signalling by introducing a known inhibitor to this pathway. The imaging of Ca²⁺ signalling, and its modulation by exogenous molecules, demonstrates the capacity to initiate and assess cellular mechanosignalling in real-time. We utilize this capability to screen the effects of a set of small molecules on cellular mechanotransduction in 96-well plates using standard imaging cytometry.

Coordination of intercellular Ca(2+) signaling in endothelial cell tubes of mouse resistance arteries

OBJECTIVE

To test the hypothesis that Ca(2+) responses to GPCR activation are coordinated between neighboring ECs of resistance arteries.

METHODS

EC tubes were freshly isolated from superior epigastric arteries of C57BL/6 mice. Intercellular coupling was tested using microinjection of propidium iodide. Following loading with fluo-4 dye, intracellular Ca(2+) responses to ACh were imaged with confocal microscopy.

RESULTS

Cell-to-cell transfer of propidium iodide confirmed functional GJCs. A 1 μm ACh stimulus evoked Ca(2+) responses (9.8 ± 0.8/min, F/F(0) = 3.11 ± 0.2) which pseudo-line-scan analysis revealed as composed of Ca(2+) waves and spatially restricted Ca(2+) release events. A 100 nm ACh stimulus induced Ca(2+) responses of lower frequency (4.5 ± 0.7/min) and amplitude (F/F(0) = 1.95 ± 0.11) composed primarily of spatially restricted events. The time interval between Ca(2+) waves in adjacent cells (0.79 ± 0.12 s) was shorter (p < 0.05) than that between nonadjacent cells (1.56 ± 0.25 s). Spatially restricted Ca(2+) release events had similar frequencies and latencies between adjacent and nonadjacent cells. Inhibiting intracellular Ca(2+) release with 2-APB, Xestospongin C or thapsigargin eliminated Ca(2+) responses.

CONCLUSIONS

With moderate GPCR stimulation, localized Ca(2+) release events predominate among cells. Greater GPCR stimulation evokes coordinated intercellular Ca(2+) waves via the ER. Calcium signaling during GPCR activation is complex among cells, varying with stimulus intensity and proximity to actively signaling cells.

Blockade of p-selectin is sufficient to reduce MHC I antibody-elicited monocyte recruitment in vitro and in vivo

Donor-specific HLA antibodies significantly lower allograft survival, but as yet there are no satisfactory therapies for prevention of antibody-mediated rejection. Intracapillary macrophage infiltration is a hallmark of antibody-mediated rejection, and macrophages are important in both acute and chronic rejection. The purpose of this study was to investigate the Fc-independent effect of HLA I antibodies on endothelial cell activation, leading to monocyte recruitment. We used an in vitro model to assess monocyte binding to endothelial cells in response to HLA I antibodies. We confirmed our results in a mouse model of antibody-mediated rejection, in which B6.RAG1(-/-) recipients of BALB/c cardiac allografts were passively transferred with donor-specific MHC I antibodies. Our findings demonstrate that HLA I antibodies rapidly increase intracellular calcium and endothelial presentation of P-selectin, which supports monocyte binding. In the experimental model, donor-specific MHC I antibodies significantly increased macrophage accumulation in the allograft. Concurrent administration of rPSGL-1-Ig abolished antibody-induced monocyte infiltration in the allograft, but had little effect on antibody-induced endothelial injury. Our data suggest that antagonism of P-selectin may ameliorate accumulation of macrophages in the allograft during antibody-mediated rejection.

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.

The vascular barrier-protecting hawthorn extract WS® 1442 raises endothelial calcium levels by inhibition of SERCA and activation of the IP3 pathway

WS® 1442 has been proven as an effective and safe therapeutical to treat mild forms of congestive heart failure. Beyond this action, we have recently shown that WS® 1442 protects against thrombin-induced vascular barrier dysfunction and the subsequent edema formation by affecting endothelial calcium signaling. The aim of the study was to analyze the influence of WS® 1442 on intracellular calcium concentrations [Ca(2+)](i) in the human endothelium and to investigate the underlying mechanisms. Using ratiometric calcium measurements and a FRET sensor, we found that WS® 1442 concentration-dependently increased basal [Ca(2+)](i) by depletion of the endoplasmic reticulum (ER) and inhibited a subsequent histamine-triggered rise of [Ca(2+)](i). Interestingly, the augmented [Ca(2+)](i) did neither trigger an activation of the contractile machinery nor led to a barrier breakdown (macromolecular permeability). It also did not impair endothelial cell viability. As assessed by patch clamp recordings, WS® 1442 did only slightly affect endothelial Na(+)/K(+)-ATPase, but increased [Ca(2+)](i) by inhibiting the sarcoplasmic/endoplasmic reticulum Ca(2+) ATPase (SERCA) and by activating the inositol 1,4,5-trisphosphate (IP(3)) pathway. Most importantly, WS® 1442 did not induce store-operated calcium entry (SOCE), but even irreversibly prevented histamine-induced SOCE. Taken together, WS® 1442 prevented the deleterious hyperpermeability-associated rise of [Ca(2+)](i) by a preceding, non-toxic release of Ca(2+) from the ER. WS® 1442 interfered with SERCA and the IP(3) pathway without inducing SOCE. The elucidation of this intriguing mechanism helps to understand the complex pharmacology of the cardiovascular drug WS® 1442.

Protein kinase Cα phosphorylates a novel argininosuccinate synthase site at serine 328 during calcium-dependent stimulation of endothelial nitric-oxide synthase in vascular endothelial cells

Endothelial nitric-oxide synthase (eNOS) utilizes l-arginine as its principal substrate, converting it to l-citrulline and nitric oxide (NO). l-Citrulline is recycled to l-arginine by two enzymes, argininosuccinate synthase (AS) and argininosuccinate lyase, providing the substrate arginine for eNOS and NO production in endothelial cells. Together, these three enzymes, eNOS, AS, and argininosuccinate lyase, make up the citrulline-NO cycle. Although AS catalyzes the rate-limiting step in NO production, little is known about the regulation of AS in endothelial cells beyond the level of transcription. In this study, we showed that AS Ser-328 phosphorylation was coordinately regulated with eNOS Ser-1179 phosphorylation when bovine aortic endothelial cells were stimulated by either a calcium ionophore or thapsigargin to produce NO. Furthermore, using in vitro kinase assay, kinase inhibition studies, as well as protein kinase Cα (PKCα) knockdown experiments, we demonstrate that the calcium-dependent phosphorylation of AS Ser-328 is mediated by PKCα. Collectively, these findings suggest that phosphorylation of AS at Ser-328 is regulated in accordance with the calcium-dependent regulation of eNOS under conditions that promote NO production and are in keeping with the rate-limiting role of AS in the citrulline-NO cycle of vascular endothelial cells.

The activation state of the inositol 1,4,5-trisphosphate receptor regulates the velocity of intracellular Ca2+ waves in bovine aortic endothelial cells

Ca(2+) is a highly versatile second messenger that plays a key role in the regulation of many cell processes. This versatility resides in the fact that different signals can be encoded spatio-temporally by varying the frequency and amplitude of the Ca(2+) response. A typical example of an organized Ca(2+) signal is a Ca(2+) wave initiated in a given area of a cell that propagates throughout the entire cell or within a specific subcellular region. In non-excitable cells, the inositol 1,4,5-trisphosphate receptor (IP(3) R) is responsible for the release of Ca(2+) from the endoplasmic reticulum. IP(3) R activity can be directly modulated in many ways, including by interacting molecules, proteins, and kinases such as PKA, PKC, and mTOR. In the present study, we used a videomicroscopic approach to measure the velocity of Ca(2+) waves in bovine aortic endothelial cells under various conditions that affect IP(3) R function. The velocity of the Ca(2+) waves increased with the intensity of the stimulus while extracellular Ca(2+) had no significant impact on wave velocity. Forskolin increased the velocity of IP(3) R-dependent Ca(2+) waves whereas PMA and rapamycin decreased the velocity. We used scatter plots and Pearson's correlation test to visualize and quantify the relationship between the Ca(2+) peak amplitude and the velocity of Ca(2+) waves. The velocity of IP(3) R-dependent Ca(2+) waves poorly correlated with the amplitude of the Ca(2+) response elicited by agonists in all the conditions evaluated, indicating that the velocity depended on the activation state of IP(3) R, which can be modulated in many ways.

Calcium and electrical signalling along endothelium of the resistance vasculature

This MiniReview is focused on the nature of intercellular signalling along the endothelium that helps to co-ordinate blood flow control in vascular resistance networks. Vasodilation initiated by contracting skeletal muscle ascends from arterioles within the tissue to encompass resistance arteries upstream and thereby increase blood flow during exercise. In resistance vessels, acetylcholine microiontophoresis or intracellular current injection initiates hyperpolarization that conducts through gap junction channels (GJCs) along the vessel wall resulting in conducted vasodilation (CVD). Both ascending vasodilation and CVD are eliminated with endothelial cell (EC) disruption, pointing to common signalling events and mutual dependence upon EC integrity. As demonstrated by electrical coupling and dye transfer during intracellular recording, their longitudinal orientation and robust expression of GJCs enable ECs to play a predominant role in CVD. Once conduction is initiated, a major interest centres on whether CVD is purely passive or involves additional 'active' signalling events. Here, we discuss components for Ca²⁺ and electrical signalling with an emphasis on intercellular coupling through endothelial GJCs. We stress the importance of understanding relationships between intracellular Ca²⁺ dynamics, EC hyperpolarization and CVD while integrating findings from isolated ECs into more complex interactions in vivo. Whereas endothelial dysfunction accompanies cardiovascular disease and the components of intra- and inter-cellular signalling are increasingly well defined, little is known of how Ca²⁺ signalling and electrical conduction along microvascular endothelium are altered in diseased states. Thus, greater insight into how these relationships are governed and interact is a key goal for continued research efforts.

Air bubble contact with endothelial cells in vitro induces calcium influx and IP3-dependent release of calcium stores

Gas embolism is a serious complication of decompression events and clinical procedures, but the mechanism of resulting injury remains unclear. Previous work has demonstrated that contact between air microbubbles and endothelial cells causes a rapid intracellular calcium transient and can lead to cell death. Here we examined the mechanism responsible for the calcium rise. Single air microbubbles (50-150 μm), trapped at the tip of a micropipette, were micromanipulated into contact with individual human umbilical vein endothelial cells (HUVECs) loaded with Fluo-4 (a fluorescent calcium indicator). Changes in intracellular calcium were then recorded via epifluorescence microscopy. First, we confirmed that HUVECs rapidly respond to air bubble contact with a calcium transient. Next, we examined the involvement of extracellular calcium influx by conducting experiments in low calcium buffer, which markedly attenuated the response, or by pretreating cells with stretch-activated channel blockers (gadolinium chloride or ruthenium red), which abolished the response. Finally, we tested the role of intracellular calcium release by pretreating cells with an inositol 1,4,5-trisphosphate (IP3) receptor blocker (xestospongin C) or phospholipase C inhibitor (neomycin sulfate), which eliminated the response in 64% and 67% of cases, respectively. Collectively, our results lead us to conclude that air bubble contact with endothelial cells causes an influx of calcium through a stretch-activated channel, such as a transient receptor potential vanilloid family member, triggering the release of calcium from intracellular stores via the IP3 pathway.

An integrated mathematical model of thrombin-, histamine-and VEGF-mediated signalling in endothelial permeability

BACKGROUND

Endothelial permeability is involved in injury, inflammation, diabetes and cancer. It is partly regulated by the thrombin-, histamine-, and VEGF-mediated myosin-light-chain (MLC) activation pathways. While these pathways have been investigated, questions such as temporal effects and the dynamics of multi-mediator regulation remain to be fully studied. Mathematical modeling of these pathways facilitates such studies. Based on the published ordinary differential equation models of the pathway components, we developed an integrated model of thrombin-, histamine-, and VEGF-mediated MLC activation pathways.

RESULTS

Our model was validated against experimental data for calcium release and thrombin-, histamine-, and VEGF-mediated MLC activation. The simulated effects of PAR-1, Rho GTPase, ROCK, VEGF and VEGFR2 over-expression on MLC activation, and the collective modulation by thrombin and histamine are consistent with experimental findings. Our model was used to predict enhanced MLC activation by CPI-17 over-expression and by synergistic action of thrombin and VEGF at low mediator levels. These may have impact in endothelial permeability and metastasis in cancer patients with blood coagulation.

CONCLUSION

Our model was validated against a number of experimental findings and the observed synergistic effects of low concentrations of thrombin and histamine in mediating the activation of MLC. It can be used to predict the effects of altered pathway components, collective actions of multiple mediators and the potential impact to various diseases. Similar to the published models of other pathways, our model can potentially be used to identify important disease genes through sensitivity analysis of signalling components.

Temperature effects on morphological integrity and Ca²⁺ signaling in freshly isolated murine feed artery endothelial cell tubes

To study Ca(2+) signaling in the endothelium of murine feed arteries, we determined the in vitro stability of endothelial cell (EC) tubes freshly isolated from abdominal muscle feed arteries of male and female C57BL/6 mice (5-9 mo, 25-35 g). We tested the hypothesis that intracellular Ca(2+) concentration ([Ca(2+)](i)) responses to muscarinic receptor activation would increase with temperature. Intact EC tubes (length: 1-2 mm, width: 65-80 μm) were isolated using gentle enzymatic digestion with trituration to remove smooth muscle cells. A freshly isolated EC tube was secured in a chamber and superfused at 24 (room temperature), 32, or 37°C. Using fura-2 dye, [Ca(2+)](i) was monitored (ratio of fluorescence at 340- to 380-nm wavelength) at rest and in response to bolus doses of ACh (20 nmol to 200 μmol). The morphological integrity of EC tubes was preserved at 24 and 32°C. Based on the Ca(2+) K(d) values we determined for fura-2 (174 nM at 24°C and 146 nM at 32°C), resting [Ca(2+)](i) remained stable for 180 min at both 24 and 32°C (27 ± 4 and 34 ± 2 nM, respectively), with peak responses to ACh (20 μmol) increasing from ∼220 nM at 24°C to ∼500 nM at 32°C (P < 0.05). There was no difference in responses to ACh between EC tubes from male versus female mice. When EC tubes were maintained at 37°C (typical in vivo temperature), resting [Ca(2+)](i) increased by ∼30% within 15 min, and gaps formed between individual ECs as they retracted and extruded dye, precluding further study. We conclude that EC tubes enable Ca(2+) signaling to be evaluated in the freshly isolated endothelium of murine feed arteries. While Ca(2+) responses are enhanced by approximately twofold at 32 versus 24°C, the instability of EC tubes at 37°C precludes their study at typical body temperature.

The mechanism of agonist induced Ca2+ signalling in intact endothelial cells studied confocally in in situ arteries

In endothelial cells there remain uncertainties in the details of how Ca²⁺ signals are generated and maintained, especially in intact preparations. In particular the role of the sarco-endoplasmic reticulum Ca²⁺-ATPase (SERCA), in contributing to the components of agonist-induced signals is unclear.

The aim of this work was to increase understanding of the detailed mechanism of Ca²⁺ signalling in endothelial cells using real time confocal imaging of Fluo-4 loaded intact rat tail arteries in response to muscarinic stimulation. In particular we have focused on the role of SERCA, and its interplay with capacitative Ca²⁺ entry (CCE) and ER Ca²⁺ release and uptake. We have determined its contribution to the Ca²⁺ signal and how it varies with different physiological stimuli, including single and repeated carbachol applications and brief and prolonged exposures.

In agreement with previous work, carbachol stimulated a rise in intracellular Ca²⁺ in the endothelial cells, consisting of a rapid initial phase, then a plateau upon which oscillations of Ca²⁺ were superimposed, followed by a decline to basal Ca²⁺ levels upon carbachol removal. Our data support the following conclusions: (i) the size (amplitude and duration) of the Ca²⁺ spike and early oscillations are limited by SERCA activity, thus both are increased if SERCA is inhibited. (ii) SERCA activity is such that brief applications of carbachol do not trigger CCE, presumably because the fall in luminal Ca²⁺ is not sufficient to trigger it. However, longer applications sufficient to deplete the ER or even partial SERCA inhibition stimulate CCE. (iii) Ca²⁺ entry occurs via STIM-mediated CCE and SERCA contributes to the cessation of CCE. In conclusion our data show how SERCA function is crucial to shaping endothelial cell Ca signals and its dynamic interplay with both CCE and ER Ca releases.

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.

Na+-Ca2+ exchanger contributes to Ca2+ extrusion in ATP-stimulated endothelium of intact rat aorta

The role of Na(+)-Ca(2+) exchanger (NCX) in vascular endothelium is still matter of debate. Depending on both the endothelial cell (EC) type and the extracellular ligand, NCX has been shown to operate in either the forward (Ca(2+) out)- or the reverse (Ca(2+) in)-mode. In particular, acetylcholine (Ach) has been shown to promote Ca(2+) inflow in the intact endothelium of excised rat aorta. Herein, we assessed the involvement of NCX into the Ca(2+) signals elicited by ATP in such preparation. Removal of extracellular Na(+) (0Na(+)) causes the NCX to switch into the reverse-mode and induced an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)), which disappeared in the absence of extracellular Ca(2+), and in the presence of benzamil, which blocks both modes of NCX, and KB-R 7943, a selective inhibitor of the reverse-mode. ATP induced a transient Ca(2+) signal, whose decay was significantly prolonged by 0Na(+), benzamil, DCB, and monensin while it was unaffected by KB-R 7943. Notably, lowering extracellular Na(+) concentration increased the sensibility to lower doses of ATP. These date suggest that, unlike Ach-stimulated ECs, NCX promotes Ca(2+) extrusion when the stimulus is provided by ATP in intact endothelium of rat aorta. These data show that, within the same preparation, NCX operates in both modes, depending on the chemical nature of the extracellular stimulus.

Signaling at purinergic P2X receptors

P2X receptors are membrane cation channels gated by extracellular ATP. Seven P2X receptor subunits (P2X(1-7)) are widely distributed in excitable and nonexcitable cells of vertebrates. They play key roles in inter alia afferent signaling (including pain), regulation of renal blood flow, vascular endothelium, and inflammatory responses. We summarize the evidence for these and other roles, emphasizing experimental work with selective receptor antagonists or with knockout mice. The receptors are trimeric membrane proteins: Studies of the biophysical properties of mutated subunits expressed in heterologous cells have indicated parts of the subunits involved in ATP binding, ion permeation (including calcium permeability), and membrane trafficking. We review our current understanding of the molecular properties of P2X receptors, including how this understanding is informed by the identification of distantly related P2X receptors in simple eukaryotes.

Ca2+ entry via alpha1G and TRPV4 channels differentially regulates surface expression of P-selectin and barrier integrity in pulmonary capillary endothelium

Pulmonary vascular endothelial cells express a variety of ion channels that mediate Ca(2+) influx in response to diverse environmental stimuli. However, it is not clear whether Ca(2+) influx from discrete ion channels is functionally coupled to specific outcomes. Thus we conducted a systematic study in mouse lung to address whether the alpha(1G) T-type Ca(2+) channel and the transient receptor potential channel TRPV4 have discrete functional roles in pulmonary capillary endothelium. We used real-time fluorescence imaging for endothelial cytosolic Ca(2+), immunohistochemistry to probe for surface expression of P-selectin, and the filtration coefficient to specifically measure lung endothelial permeability. We demonstrate that membrane depolarization via exposure of pulmonary vascular endothelium to a high-K(+) perfusate induces Ca(2+) entry into alveolar septal endothelial cells and exclusively leads to the surface expression of P-selectin. In contrast, Ca(2+) entry in septal endothelium evoked by the selective TRPV4 activator 4alpha-phorbol-12,13-didecanoate (4alpha-PDD) specifically increases lung endothelial permeability without effect on P-selectin expression. Pharmacological blockade or knockout of alpha(1G) abolishes depolarization-induced Ca(2+) entry and surface expression of P-selectin but does not prevent 4alpha-PDD-activated Ca(2+) entry and the resultant increase in permeability. Conversely, blockade or knockout of TRPV4 specifically abolishes 4alpha-PDD-activated Ca(2+) entry and the increase in permeability, while not impacting depolarization-induced Ca(2+) entry and surface expression of P-selectin. We conclude that in alveolar septal capillaries Ca(2+) entry through alpha(1G) and TRPV4 channels differentially and specifically regulates the transition of endothelial procoagulant phenotype and barrier integrity, respectively.

Microarray analysis of gene expression in mouse aorta reveals role of the calcium signaling pathway in control of atherosclerosis susceptibility

Inbred mouse strains C57BL/6J (B6) and C3H/HeJ (C3H) exhibit a marked difference in atherosclerotic lesion formation when deficient in apolipoprotein E (apoE(-/-)), and the arterial wall has been identified as a source of the difference in atherosclerosis susceptibility. In the present study, differences in gene expression in aortic walls of the two strains were analyzed by microarrays. Total RNA was extracted from the aorta of 6-wk-old female B6 and C3H apoE(-/-) mice fed a chow or Western diet. There were 1,514 genes in chow fed mice and 590 genes in Western fed mice that were found to be differentially expressed between the two strains. Pathway analysis of differentially expressed genes suggested a role for the calcium signaling pathway in regulating atherosclerosis susceptibility. Oxidized LDL (oxLDL) induced a dose-dependent rise in cytosolic calcium levels in B6 endothelial cells. oxLDL-induced monocyte chemoattractant protein-1 production was inhibited by pretreatment with calcium chelator EGTA or intracellular calcium trapping compound BAPTA, indicating that calcium ions mediate the effect of oxLDL on monocyte chemoattractant protein-1 induction. The present findings demonstrate involvement of the calcium signaling pathway in the inflammatory process of atherogenesis.

Microfilament and microtubule assembly is required for the propagation of inositol trisphosphate receptor-induced Ca2+ waves in bovine aortic endothelial cells

Ca2+ is a highly versatile second messenger that plays a key role in the regulation of numerous cell processes. One-way cells ensure the specificity and reliability of Ca2+ signals is by organizing them spatially in the form of waves that propagate throughout the cell or within a specific subcellular region. In non-excitable cells, the inositol 1,4,5-trisphosphate receptor (IP3R) is responsible for the release of Ca2+ from the endoplasmic reticulum. The spatial aspect of the Ca2+ signal depends on the organization of various elements of the Ca2+ signaling toolkit and varies from tissue to tissue. Ca2+ is implicated in many of endothelium functions that thus depend on the versatility of Ca2+ signaling. In the present study, we showed that the disruption of caveolae microdomains in bovine aortic endothelial cells (BAEC) with methyl-beta-cyclodextrin was not sufficient to disorganize the propagation of Ca2+ waves when the cells were stimulated with ATP or bradykinin. However, disorganizing microfilaments with latrunculin B and microtubules with colchicine both prevented the formation of Ca2+ waves. These results suggest that the organization of the Ca2+ waves mediated by IP3R channels does not depend on the integrity of caveolae in BAEC, but that microtubule and microfilament cytoskeleton assembly is crucial.