Thapsigargin activates Ca²+ entry both by store-dependent, STIM1/Orai1-mediated, and store-independent, TRPC3/PLC/PKC-mediated pathways in human endothelial cells

Advanced glycation end products acutely impair ca(2+) signaling in bovine aortic endothelial cells

Post-translational modification of proteins in diabetes, including formation of advanced glycation end products (AGEs) are believed to contribute to vascular dysfunction and disease. Impaired function of the endothelium is an early indicator of vascular dysfunction in diabetes and as many endothelial cell processes are dependent upon intracellular [Ca²⁺] and Ca²⁺ signaling, the aim of this study was to examine the acute effects of AGEs on Ca²⁺ signaling in bovine aortic endothelial cells (BAEC). Ca²⁺ signaling was studied using the fluorescent indicator dye Fura-2-AM. AGEs were generated by incubating bovine serum albumin with 0–250 mM glucose or glucose-6-phosphate for 0–120 days at 37°C. Under all conditions, the main AGE species generated was carboxymethyl lysine (CML) as assayed using both gas-liquid chromatograph-mass spectroscopy and high-performance liquid chromatography. In Ca²⁺-replete solution, exposure of BAEC to AGEs for 5 min caused an elevation in basal [Ca²⁺] and attenuated the increase in intracellular [Ca²⁺] caused by ATP (100 μM). In the absence of extracellular Ca²⁺, exposure of BAEC to AGEs for 5 min caused an elevation in basal [Ca²⁺] and attenuated subsequent intracellular Ca²⁺ release caused by ATP, thapsigargin (0.1 μM), and ionomycin (3 μM), but AGEs did not affect extracellular Ca²⁺ entry induced by the re-addition of Ca²⁺ to the bathing solution in the presence of any of these agents. The anti-oxidant α-lipoic acid (2 μM) and NAD(P)H oxidase inhibitors apocynin (500 μM) and diphenyleneiodonium (1 μM) abolished these effects of AGEs on BAECs, as did the IP₃ receptor antagonist xestospongin C (1 μM). In summary, AGEs caused an acute depletion of Ca²⁺ from the intracellular store in BAECs, such that the Ca²⁺ signal stimulated by the subsequent application other agents acting upon this store is reduced. The mechanism may involve generation of reactive oxygen species from NAD(P)H oxidase and possible activation of the IP₃ receptor.

Requirement for both receptor-operated and store-operated calcium entry in N-formyl-methionine-leucine-phenylalanine-induced neutrophil polarization

Tissue penetration of neutrophils is a key process in many inflammatory diseases. In response to inflammatory stimuli such as N-formyl-methionine-leucine-phenylalanine (fMLP), neutrophils polarize and migrate towards the chemotactic gradient of the stimulus. Elevated intracellular Ca(2+) concentration is known to play a critical role in neutrophil polarization and migration; however, the exact mechanism remains elusive. Here, we demonstrated that fMLP stimulation caused not only store-operated calcium entry (SOCE), but also receptor-operated calcium entry (ROCE) in neutrophils by using both pharmacological and neutralizing monoclonal antibody approaches. We also investigated neither Rac2 nor Cdc42 activation could take place if either SOCE or ROCE was inhibited. This study thus provides the first evidence for coordination of Ca(2+) influx by SOCE and ROCE to regulate neutrophil polarization.

Store-operated Ca2+ entry in hippocampal neurons: Regulation by protein tyrosine phosphatase PTP1B

Store operated Ca(2+) entry (SOCE) replenishes intracellular Ca(2+) stores and activates a number of intracellular signalling pathways. Whilst several molecular components forming store operated Ca(2+) channels (SOCC) have been identified, their modulation in neurons remains poorly understood. Here, we extend on our previous findings and show that neuronal SOCE is modulated by tyrosine phosphorylation. Cyclopiazonic acid induced SOCE was characterised in hippocampal cultures derived from forebrain specific protein tyrosine phosphatase 1B knockout (PTP1B KO) mice and wild type (WT) litter mates using Fura-2 Ca(2+) imaging. PTP1B KO cultures expressed elevated SOCE relative to WT cultures without changes in cytoplasmic Ca(2+) homeostasis or depolarisation-induced Ca(2+) influx. WT and PTP1B KO cultures displayed similar pharmacological sensitivities towards the SOCE inhibitors gadolinium and 2-aminoethoxydiphenyl borate, as well as the tyrosine kinase inhibitor Ag126 indicating an augmentation of native SOCCs by PTP1B. Following store depletion WT culture homogenates showed heightened phospho-tyrosine levels, an increase in Src tyrosine kinase activation and two minor PTP1B species. These data suggest tyrosine phosphorylation gating SOCE, and implicate PTP1B as a key regulatory enzyme. The involvement of PTP1B in SOCE and its relation to SOCC components and mechanism of regulation are discussed.

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.

Activation of transient receptor potential canonical 3 (TRPC3)-mediated Ca2+ entry by A1 adenosine receptor in cardiomyocytes disturbs atrioventricular conduction

Although the activation of the A(1)-subtype of the adenosine receptors (A(1)AR) is arrhythmogenic in the developing heart, little is known about the underlying downstream mechanisms. The aim of this study was to determine to what extent the transient receptor potential canonical (TRPC) channel 3, functioning as receptor-operated channel (ROC), contributes to the A(1)AR-induced conduction disturbances. Using embryonic atrial and ventricular myocytes obtained from 4-day-old chick embryos, we found that the specific activation of A(1)AR by CCPA induced sarcolemmal Ca(2+) entry. However, A(1)AR stimulation did not induce Ca(2+) release from the sarcoplasmic reticulum. Specific blockade of TRPC3 activity by Pyr3, by a dominant negative of TRPC3 construct, or inhibition of phospholipase Cs and PKCs strongly inhibited the A(1)AR-enhanced Ca(2+) entry. Ca(2+) entry through TRPC3 was activated by the 1,2-diacylglycerol (DAG) analog OAG via PKC-independent and -dependent mechanisms in atrial and ventricular myocytes, respectively. In parallel, inhibition of the atypical PKCζ by myristoylated PKCζ pseudosubstrate inhibitor significantly decreased the A(1)AR-enhanced Ca(2+) entry in both types of myocytes. Additionally, electrocardiography showed that inhibition of TRPC3 channel suppressed transient A(1)AR-induced conduction disturbances in the embryonic heart. Our data showing that A(1)AR activation subtly mediates a proarrhythmic Ca(2+) entry through TRPC3-encoded ROC by stimulating the phospholipase C/DAG/PKC cascade provide evidence for a novel pathway whereby Ca(2+) entry and cardiac function are altered. Thus, the A(1)AR-TRPC3 axis may represent a potential therapeutic target.

Orai1 calcium channels in the vasculature

Orai1 was discovered in T cells as a calcium-selective channel that is activated by store depletion. Recent studies suggest that it is expressed and functionally important also in blood vessels, not only because haematopoietic cells can incorporate in the vascular wall but also because Orai1 is expressed and functional in vascular smooth muscle cells and endothelial cells. This article summarises the arising observations in this new area of vascular research and debates underlying issues and challenges for future investigations. The primary focus is on vascular smooth muscle cells and endothelial cells. Specific topics include Orai1 expression; Orai1 roles in store-operated calcium entry and ionic currents of store-depleted cells; blockade of Orai1-related signals by Synta 66 and other pharmacology; activation or regulation of Orai1-related signals by physiological substances and compartments; stromal interaction molecules and the relationship of Orai1 to other ion channels, transporters and pumps; transient receptor potential canonical channels and their contribution to store-operated calcium entry; roles of Orai1 in vascular tone, remodelling, thrombus formation and inflammation; and Orai2 and Orai3. Overall, the observations suggest the existence of an additional, previously unrecognised, calcium channel of the vascular wall that is functionally important particularly in remodelling but probably also in certain vasoconstrictor contexts.

STIM1 tyrosine-phosphorylation is required for STIM1-Orai1 association in human platelets

Stromal interaction molecule 1 (STIM1) is a key element of the store-operated Ca(2+) entry mechanism (SOCE). Recently, regulation of STIM1 by glycosylation and phosphorylation on serine/threonine or proline residues has been described; however other modes of phosphorylation that are important for activating SOCE in platelets, such as tyrosine phosphorylation, have been poorly investigated. Here we investigate the latency of STIM1 phosphorylation on tyrosine residues during the first steps of SOCE activation. Human platelets were stimulated and fixed at desired times using rapid kinetic assays instruments, and immunoprecipitation and western blotting techniques were then used to investigate the pattern of STIM1 tyrosine phosphorylation during the first steps of SOCE activation. We have found that maximal STIM1 tyrosine phosphorylation occurred 2.5s after stimulation of human platelets with thapsigargin (Tg). STIM1 localized in the plasma membrane were also phosphorylated in platelets stimulated with Tg. By using chemical inhibitors that target different members of the Src family of tyrosine kinases (SKFs), two independent signaling pathways involved in STIM1 tyrosine phosphorylation during the first steps of SOCE activation were identified. We finally conclude that STIM1 tyrosine phosphorylation is a key event for the association of STIM1 with plasma membrane Ca(2+) channels such as Orai1, hence it is required for conducting SOCE activation.

Calcium and endothelium-mediated vasodilator signaling

Vascular tone refers to the balance between arterial constrictor and dilator activity. The mechanisms that underlie tone are critical for the control of haemodynamics and matching circulatory needs with metabolism, and thus alterations in tone are a primary factor for vascular disease etiology. The dynamic spatiotemporal control of intracellular Ca(2+) levels in arterial endothelial and smooth muscle cells facilitates the modulation of multiple vascular signaling pathways. Thus, control of Ca(2+) levels in these cells is integral for the maintenance of tone and blood flow, and intimately associated with both physiological and pathophysiological states. Hence, understanding the mechanisms that underlie the modulation of vascular Ca(2+) activity is critical for both fundamental knowledge of artery function, and for the development of targeted therapies. This brief review highlights the role of Ca(2+) signaling in vascular endothelial function, with a focus on contact-mediated vasodilator mechanisms associated with endothelium-derived hyperpolarization and the longitudinal conduction of responses over distance.

Transient receptor potential canonical channels are required for in vitro endothelial tube formation

In endothelial cells Ca(2+) entry is an essential component of the Ca(2+) signal that takes place during processes such as cell proliferation or angiogenesis. Ca(2+) influx occurs via the store-operated Ca(2+) entry pathway, involving stromal interaction molecule-1 (STIM1) and Orai1, but also through channels gated by second messengers like the transient receptor potential canonical (TRPC) channels. The human umbilical vein-derived endothelial cell line EA.hy926 expressed STIM1 and Orai1 as well as several TRPC channels. By invalidating each of these molecules, we showed that TRPC3, TRPC4, and TRPC5 are essential for the formation of tubular structures observed after EA.hy926 cells were plated on Matrigel. On the contrary, the silencing of STIM1 or Orai1 did not prevent tubulogenesis. Soon after being plated on Matrigel, the cells displayed spontaneous Ca(2+) oscillations that were strongly reduced by treatment with siRNA against TRPC3, TRPC4, or TRPC5, but not siRNA against STIM1 or Orai1. Furthermore, we showed that cell proliferation was reduced upon siRNA treatment against TRPC3, TRPC5, and Orai1 channels, whereas the knockdown of STIM1 had no effect. On primary human umbilical vein endothelial cells, TRPC1, TRPC4, and STIM1 are involved in tube formation, whereas Orai1 has no effect. These data showed that TRPC channels are essential for in vitro tubulogenesis, both on endothelial cell line and on primary endothelial cells.