Orai1 interacts with STIM1 and mediates capacitative Ca2+ entry in mouse pulmonary arterial smooth muscle cells

The transmembrane protein LbRSG from the recretohalophyte Limonium bicolor enhances salt gland development and salt tolerance

Salt glands are the unique epidermal structures present in recretohalophytes, plants that actively excrete excess Na+ by salt secretory structures to avoid salt damage. Here, we describe a transmembrane protein that localizes to the plasma membrane of the recretohalophyte Limonium bicolor. As virus-induced gene silencing of the corresponding gene LbRSG in L. bicolor decreased the number of salt glands, we named the gene Reduced Salt Gland. We detected LbRSG transcripts in salt glands by in situ hybridization and transient transformation. Overexpression and silencing of LbRSG in L. bicolor pointed to a positive role in salt gland development and salt secretion by interacting with Lb3G16832. Heterologous LbRSG expression in Arabidopsis enhanced salt tolerance during germination and the seedling stage by alleviating NaCl-induced ion stress and osmotic stress after replacing or deleting the (highly) negatively charged region of extramembranous loop. After screened by immunoprecipitation-mass spectrometry and verified using yeast two-hybrid, PGK1 and BGLU18 were proposed to interact with LbRSG to strengthen salt tolerance. Therefore, we identified (highly) negatively charged regions in the extramembrane loop that may play an essential role in salt tolerance, offering hints about LbRSG function and its potential to confer salt resistance.

Basal Vascular Smooth Muscle Cell Tone in eNOS Knockout Mice Can Be Reversed by Cyclic Stretch and Is Independent of Age

Introduction and Aims: Endothelial nitric oxide synthase (eNOS) knockout mice develop pronounced cardiovascular disease. In the present study, we describe the alterations in aortic physiology and biomechanics of eNOS knockout and C57Bl/6 control mice at 2–12 months of age, including a thorough physiological investigation of age and cyclic stretch-dependent VSMC contractility and aortic stiffness.

Methods and Results: Peripheral blood pressure and aortic pulse wave velocity were measured in vivo, and aortic biomechanical studies and isometric contractions were investigated ex vivo. Age-dependent progression of aortic stiffness, peripheral hypertension, and aortic contractility in eNOS knockout mice was absent, attenuated, or similar to C57Bl/6 control mice. Voltage-gated calcium channel (VGCC)-dependent calcium influx inversely affected isometric contraction and aortic stiffening by α₁-adrenergic stimulation in eNOS knockout mice. Baseline aortic stiffness was selectively reduced in eNOS knockout mice after ex vivo cyclic stretch exposure in an amplitude-dependent manner, which prompted us to investigate cyclic stretch dependent regulation of aortic contractility and stiffness. Aortic stiffness, both in baseline conditions and after activation of vascular smooth muscle cell (VSMC) contraction, was reduced with increasing cyclic stretch amplitude. This cyclic stretch dependency was attenuated with age, although aged eNOS knockout mice displayed better preservation of cyclic stretch-dependency compared to C57Bl/6 control mice. Store operated calcium entry-medicated aortic stiffening as induced by inhibiting sarcoplasmic reticulum calcium ATPase pumps with 10 µM CPA was most pronounced in the aorta of aged mice and at low cyclic stretch amplitude, but independent of eNOS. Basal aortic tonus and VSMC depolarization were highly dependent on eNOS, and were most pronounced at low cyclic stretch, with attenuation at increasing cyclic stretch amplitude.

Conclusion: eNOS knockout mice display attenuated progression of arterial disease as compared to C57Bl/6 control mice. Basal VSMC tone in eNOS knockout mice could be reduced by ex vivo exposure to cyclic stretch through stretch-dependent regulation of cytosolic calcium. Both baseline and active aortic stiffness were highly dependent on cyclic stretch regulation, which was more pronounced in young versus aged mice. Other mediators of VSMC contraction and calcium handling were dependent on cyclic stretch mechanotransduction, but independent of eNOS.

Role of Store-Operated Ca2+ Entry in the Pulmonary Vascular Remodeling Occurring in Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a severe and multifactorial disease. PAH pathogenesis mostly involves pulmonary arterial endothelial and pulmonary arterial smooth muscle cell (PASMC) dysfunction, leading to alterations in pulmonary arterial tone and distal pulmonary vessel obstruction and remodeling. Unfortunately, current PAH therapies are not curative, and therapeutic approaches mostly target endothelial dysfunction, while PASMC dysfunction is under investigation. In PAH, modifications in intracellular Ca2+ homoeostasis could partly explain PASMC dysfunction. One of the most crucial actors regulating Ca2+ homeostasis is store-operated Ca2+ channels, which mediate store-operated Ca2+ entry (SOCE). This review focuses on the main actors of SOCE in human and experimental PASMC, their contribution to PAH pathogenesis, and their therapeutic potential in PAH.

A novel gene LbHLH from the halophyte Limonium bicolor enhances salt tolerance via reducing root hair development and enhancing osmotic resistance

BACKGROUND

Identifying genes involved in salt tolerance in the recretohalophyte Limonium bicolor could facilitate the breeding of crops with enhanced salt tolerance. Here we cloned the previously uncharacterized gene LbHLH and explored its role in salt tolerance.

RESULTS

The 2,067-bp open reading frame of LbHLH encodes a 688-amino-acid protein with a typical helix-loop-helix (HLH) domain. In situ hybridization showed that LbHLH is expressed in salt glands of L. bicolor. LbHLH localizes to the nucleus, and LbHLH is highly expressed during salt gland development and in response to NaCl treatment. To further explore its function, we heterologously expressed LbHLH in Arabidopsis thaliana under the 35S promoter. The overexpression lines showed significantly increased trichome number and reduced root hair number. LbHLH might interact with GLABRA1 to influence trichome and root hair development, as revealed by yeast two-hybrid analysis. The transgenic lines showed higher germination percentages and longer roots than the wild type under NaCl treatment. Analysis of seedlings grown on medium containing sorbitol with the same osmotic pressure as 100 mM NaCl demonstrated that overexpressing LbHLH enhanced osmotic resistance.

CONCLUSION

These results indicate that LbHLH enhances salt tolerance by reducing root hair development and enhancing osmotic resistance under NaCl stress.

Involvement of TRPC1 and Cyclin D1 in Human Pulmonary Artery Smooth Muscle Cells Proliferation Induced by Cigarette Smoke Extract

Cigarette smoking contributes to the development of pulmonary artery hypertension (PAH). As the basic pathological change of PAH, pulmonary vascular remodeling is considered to be related to the abnormal proliferation of pulmonary artery smooth muscle cells (PASMCs). However, the molecular mechanism underlying this process remains not exactly clear. The aim of this research was to study the molecular mechanism of PASMCs proliferation induced by smoking. Human PASMCs (HPASMCs) were divided into 6 groups: 0% (control group), cigarette smoking extract (CSE)-treated groups at concentrations of 0.5%, 1%, 2%, 5%, 10% CSE respectively. HPASMCs proliferation was observed after 24 h. HPASMCs were divided into two groups: 0 (control group), 0.5% CSE group. The mRNA and protein expression levels of transient receptor potential channel 1 (TRPC1) and cyclin D1 in HPASMCs after CSE treatment were respectively detected by RT-PCR and Western blotting. The intracellular calcium ion concentration was measured by the calcium probe in each group. In the negative control group and TRPC1-siRNA transfection group, the proliferation of HPASMCs and the expression of cyclin D1 mRNA and protein were detected. Data were compared with one-way ANOVA (for multiple-group comparison) and independent t-test (for two-group comparison) followed by the least significant difference (LSD) test with the computer software SPSS 17.0. It was found that 0.5% and 1% CSE could promote the proliferation of HPASMCs (P<0.05), and the former was more effective than the latter (P<0.05), while 3% and above CSE had inhibitory effect on HPASMCs (P<0.05). The mRNA and protein expression levels of TRPC1 and cyclin D1 in 0.5% and 1% CSE groups were significantly higher than those in the control group (P<0.05), while those in 3% CSE group were significantly decreased (P<0.05). Moreover, the proliferation of HPASMCs and the expression of cyclin D1 mRNA and protein in TRPC1-siRNA transfection group were significantly reduced as compared with those in the negative control group (P<0.05). It was concluded that low concentration of CSE can promote the proliferation of HPASMCs, while high concentrations of CSE inhibit HPASMCs proliferation. These findings suggested that CSE induced proliferation of HPASMCs at least in part via TRPC1-mediated cyclin D1 expression.

Insights into Activation Mechanisms of Store-Operated TRPC1 Channels in Vascular Smooth Muscle

In vascular smooth muscle cells (VMSCs), the stimulation of store-operated channels (SOCs) mediate Ca2+ influx pathways which regulate important cellular functions including contraction, proliferation, migration, and growth that are associated with the development of vascular diseases. It is therefore important that we understand the biophysical, molecular composition, activation pathways, and physiological significance of SOCs in VSMCs as these maybe future therapeutic targets for conditions such as hypertension and atherosclerosis. Archetypal SOCs called calcium release-activated channels (CRACs) are composed of Orai1 proteins and are stimulated by the endo/sarcoplasmic reticulum Ca2+ sensor stromal interaction molecule 1 (STIM1) following store depletion. In contrast, this review focuses on proposals that canonical transient receptor potential (TRPC) channels composed of a heteromeric TRPC1/C5 molecular template, with TRPC1 conferring activation by store depletion, mediate SOCs in native contractile VSMCs. In particular, it summarizes our recent findings which describe a novel activation pathway of these TRPC1-based SOCs, in which protein kinase C (PKC)-dependent TRPC1 phosphorylation and phosphatidylinositol 4,5-bisphosphate (PIP2) are obligatory for channel opening. This PKC- and PIP2-mediated gating mechanism is regulated by the PIP2-binding protein myristoylated alanine-rich C kinase (MARCKS) and is coupled to store depletion by TRPC1-STIM1 interactions which induce Gq/PLCβ1 activity. Interestingly, the biophysical properties and activation mechanisms of TRPC1-based SOCs in native contractile VSMCs are unlikely to involve Orai1.

ORAI channels in cellular remodeling of cardiorespiratory disease

Cardiorespiratory disease, which includes systemic arterial hypertension, restenosis, atherosclerosis, pulmonary arterial hypertension, asthma, and chronic obstructive pulmonary disease (COPD) are highly prevalent and devastating diseases with limited therapeutic modalities. A common pathophysiological theme to these diseases is cellular remodeling, which is contributed by changes in expression and activation of ion channels critical for either excitability or growth. Calcium (Ca²⁺) signaling and specifically ORAI Ca²⁺ channels have emerged as significant regulators of smooth muscle, endothelial, epithelial, platelet, and immune cell remodeling. This review details the dysregulation of ORAI in cardiorespiratory diseases, and how this dysregulation of ORAI contributes to cellular remodeling.

ORAI Channels as Potential Therapeutic Targets in Pulmonary Hypertension

Pulmonary hypertension is a complex and fatal disease that lacks treatments. Its pathophysiology involves pulmonary artery hyperreactivity, endothelial dysfunction, wall remodelling, inflammation, and thrombosis, which could all depend on ORAI Ca²⁺ channels. We review the knowledge about ORAI channels in pulmonary artery and discuss the interest to target them in the treatment of pulmonary hypertension.

Transient Receptor Potential Melastatin-8 Activation Induces Relaxation of Pulmonary Artery by Inhibition of Store-Operated Calcium Entry in Normoxic and Chronic Hypoxic Pulmonary Hypertensive Rats

Pulmonary hypertension (PH) is characterized by enhanced vasoconstriction and vascular remodeling, which are attributable to the alteration of Ca²⁺ homeostasis in pulmonary arterial smooth muscle cells (PASMCs). It is well established that store-operated Ca²⁺ entry (SOCE) is augmented in PASMCs during PH and that it plays a crucial role in PH development. Our previous studies showed that the melastatin-related transient receptor potential 8 (TRPM8) is down-regulated in PASMCs of PH animal models, and activation of TRPM8 causes relaxation of pulmonary arteries (PAs). However, the mechanism of TRPM8-induced PA relaxation is unclear. Here we examined the interaction of TRPM8 and SOCE in PAs and PASMCs of normoxic and chronic hypoxic pulmonary hypertensive (CHPH) rats, a model of human group 3 PH. We found that TRPM8 was down-regulated and TRPM8-mediated cation entry was reduced in CHPH-PASMCs. Activation of TRPM8 with icilin caused concentration-dependent relaxation of cyclopiazonic acid (CPA) and endothelin-1 contracted endothelium-denuded PAs, and the effect was abolished by the SOCE antagonist Gd³⁺ Application of icilin to PASMCs suppressed CPA-induced Mn²⁺ quenching and Ca²⁺ entry, which was reversed by the TRPM8 antagonist N-(3-aminopropyl)-2-([(3-methylphenyl)methyl])-oxy-N-(2-thienylmethyl)benzamide hydrochloride salt (AMTB). Moreover, the inhibitory effects of icilin on SOCE in PA and PASMCs of CHPH rats were significantly augmented due to enhanced SOCE activity in PH. Our results, therefore, demonstrated a novel mechanism of TRPM8-mediated inhibition of SOCE in pulmonary vasculature. Because SOCE is important for vascular remodeling and enhanced vasoconstriction, down-regulation of TRPM8 in PASMCs of CHPH rats may minimize its inhibitory influence to allow unimpeded SOCE activity for PH development.

Hypoxia selectively upregulates cation channels and increases cytosolic [Ca2+] in pulmonary, but not coronary, arterial smooth muscle cells

Ca²⁺ signaling, particularly the mechanism via store-operated Ca²⁺ entry (SOCE) and receptor-operated Ca²⁺ entry (ROCE), plays a critical role in the development of acute hypoxia-induced pulmonary vasoconstriction and chronic hypoxia-induced pulmonary hypertension. This study aimed to test the hypothesis that chronic hypoxia differentially regulates the expression of proteins that mediate SOCE and ROCE [stromal interacting molecule (STIM), Orai, and canonical transient receptor potential channel TRPC6] in pulmonary (PASMC) and coronary (CASMC) artery smooth muscle cells. The resting cytosolic [Ca²⁺] ([Ca²⁺]_cyt) and the stored [Ca²⁺] in the sarcoplasmic reticulum were not different in CASMC and PASMC. Seahorse measurement showed a similar level of mitochondrial bioenergetics (basal respiration and ATP production) between CASMC and PASMC. Glycolysis was significantly higher in PASMC than in CASMC. The amplitudes of cyclopiazonic acid-induced SOCE and OAG-induced ROCE in CASMC are slightly, but significantly, greater than in PASMC. The frequency and the area under the curve of Ca²⁺ oscillations induced by ATP and histamine were also larger in CASMC than in PASMC. Na⁺/Ca²⁺ exchanger-mediated increases in [Ca²⁺]_cyt did not differ significantly between CASMC and PASMC. The basal protein expression levels of STIM1/2, Orai1/2, and TRPC6 were higher in CASMC than in PASMC, but hypoxia (3% O₂ for 72 h) significantly upregulated protein expression levels of STIM1/STIM2, Orai1/Orai2, and TRPC6 and increased the resting [Ca²⁺]_cyt only in PASMC, but not in CASMC. The different response of essential components of store-operated and receptor-operated Ca²⁺ channels to hypoxia is a unique intrinsic property of PASMC, which is likely one of the important explanations why hypoxia causes pulmonary vasoconstriction and induces pulmonary vascular remodeling, but causes coronary vasodilation.

Orai1, 2, 3 and STIM1 promote store-operated calcium entry in pulmonary arterial smooth muscle cells

Previous studies have demonstrated that besides the classic canonical transient receptor potential channel family, Orai family and stromal interaction molecule 1 (STIM1) might also be involved in the regulation of store-operated calcium channels (SOCCs). An increase in cytosolic free Ca²⁺ concentration promoted by store-operated Ca²⁺ entry (SOCE) in pulmonary arterial smooth muscle cells (PASMCs) is a major trigger for pulmonary vasoconstriction and proliferation and migration of PASMCs. In this study, our data revealed the following: (1) in both rat distal pulmonary arteries and PASMCs, chronic hypoxia exposure upregulated the expression of Orai1 and Orai2, without affecting Orai3 and STIM1; (2) either heterozygous knockout of HIF-1α in mice or knockdown of HIF-1α in PASMCs abolished the hypoxic upregulation of Orai2, but not Orai1, suggesting the hypoxic upregulation of Orai2 depends on HIF-1α; and (3) using small interference RNA knockdown strategies, Orai1, 2, 3 and STIM1 were all shown to mediate SOCE in hypoxic PASMCs. Together, these results suggested that the components of SOCCs, including Orai1, 2, 3 and STIM1, may lead to novel therapeutic targets for the treatment of chronic hypoxia-induced pulmonary hypertension.

Hydrogen peroxide is a critical regulator of the hypoxia-induced alterations of store-operated Ca2+ entry into rat pulmonary arterial smooth muscle cells

To investigate the association between store-operated Ca2+ entry (SOCE) and reactive oxygen species (ROS) during hypoxia, this study determined the changes of transient receptor potential canonical 1 (TRPC1) and Orai1, two candidate proteins for store-operated Ca2+ (SOC) channels and their gate regulator, stromal interaction molecule 1 (STIM1), in a hypoxic environment and their relationship with ROS in pulmonary arterial smooth muscle cells (PASMCs). Exposure to hypoxia caused a transient Ca2+ spike and subsequent Ca2+ plateau of SOCE to be intensified in PASMCs when TRPC1, STIM1, and Orai1 were upregulated. SOCE in cells transfected with specific short hairpin RNA (shRNA) constructs was almost completely eliminated by the knockdown of TRPC1, STIM1, or Orai1 alone and was no longer affected by hypoxia exposure. Hypoxia-induced SOCE enhancement was further strengthened by PEG-SOD but was attenuated by PEG-catalase, with correlated changes to intracellular hydrogen peroxide (H2O2) levels and protein levels of TRPC1, STIM1, and Orai1. Exogenous H2O2 could mimic alterations of the interactions of STIM1 with TRPC1 and Orai1 in hypoxic cells. These findings suggest that TRPC1, STIM1, and Orai1 are essential for the initiation of SOCE in PASMCs. Hypoxia-induced ROS promoted the expression and interaction of the SOC channel molecules and their gate regulator via their converted product, H2O2.

Evidence that Orai1 does not contribute to store-operated TRPC1 channels in vascular smooth muscle cells

Ca²⁺-permeable store-operated channels (SOCs) mediate Ca²⁺ entry pathways which are involved in many cellular functions such as contraction, growth, and proliferation. Prototypical SOCs are formed of Orai1 proteins and are activated by the endo/sarcoplasmic reticulum Ca²⁺ sensor stromal interaction molecule 1 (STIM1). There is considerable debate about whether canonical transient receptor potential 1 (TRPC1) proteins also form store-operated channels (SOCs), and if they do, is Orai1 involved. We recently showed that stimulation of TRPC1-based SOCs involves store depletion inducing STIM1-evoked Gαq/PLCβ1 activity in contractile vascular smooth muscle cells (VSMCs). Therefore the present work investigates the role of Orai1 in activation of TRPC1-based SOCs in freshly isolated mesenteric artery VSMCs from wild-type (WT) and Orai1^−/− mice. Store-operated whole-cell and single channel currents recorded from WT and Orai1^−/− VSMCs had similar properties, with relatively linear current-voltage relationships, reversal potentials of about +20mV, unitary conductances of about 2pS, and inhibition by anti-TRPC1 and anti-STIM1 antibodies. In Orai1^−/− VSMCs, store depletion induced PLCβ1 activity measured with the fluorescent phosphatidylinositol 4,5-bisphosphate/inositol 1,4,5-trisphosphate biosensor GFP-PLCδ1-PH, which was prevented by knockdown of STIM1. In addition, in Orai1^−/− VSMCs, store depletion induced translocation of STIM1 from within the cell to the plasma membrane where it formed STIM1-TRPC1 interactions at discrete puncta-like sites. These findings indicate that activation of TRPC1-based SOCs through a STIM1-activated PLCβ1 pathway are likely to occur independently of Orai1 proteins, providing evidence that TRPC1 channels form genuine SOCs in VSMCs with a contractile phenotype.

Store-operated interactions between plasmalemmal STIM1 and TRPC1 proteins stimulate PLCβ1 to induce TRPC1 channel activation in vascular smooth muscle cells

KEY POINTS

Depletion of Ca2+ stores activates store-operated channels (SOCs), which mediate Ca2+ entry pathways that regulate cellular processes such as contraction, proliferation and gene expression. In vascular smooth muscle cells (VSMCs), stimulation of SOCs composed of canonical transient receptor potential channel 1 (TRPC1) proteins requires G protein α q subunit (Gαq)/phospholipase C (PLC)β1/protein kinase C (PKC) activity. We studied the role of stromal interaction molecule 1 (STIM1) in coupling store depletion to this activation pathway using patch clamp recording, GFP-PLCδ1-PH imaging and co-localization techniques. Store-operated TRPC1 channel and PLCβ1 activities were inhibited by STIM1 short hairpin RNA (shRNA) and absent in TRPC1-/- cells, and store-operated PKC phosphorylation of TRPC1 was inhibited by STIM1 shRNA. Store depletion induced interactions between STIM1 and TRPC1, Gαq and PLCβ1, which required STIM1 and TRPC1. Similar effects were produced with noradrenaline. These findings identify a new activation mechanism of TRPC1-based SOCs in VSMCs, and a novel role for STIM1, where store-operated STIM1-TRPC1 interactions stimulate Gαq/PLCβ1/PKC activity to induce channel gating.

ABSTRACT

In vascular smooth muscle cells (VSMCs), stimulation of canonical transient receptor potential channel 1 (TRPC1) protein-based store-operated channels (SOCs) mediates Ca2+ entry pathways that regulate contractility, proliferation and migration. It is therefore important to understand how these channels are activated. Studies have shown that stimulation of TRPC1-based SOCs requires G protein α q subunit (Gαq)/phospholipase C (PLC)β1 activities and protein kinase C (PKC) phosphorylation, although it is unclear how store depletion stimulates this gating pathway. The present study examines this issue by focusing on the role of stromal interaction molecule 1 (STIM1), an endo/sarcoplasmic reticulum Ca2+ sensor. Store-operated TRPC1 channel activity was inhibited by TRPC1 and STIM1 antibodies and STIM1 short hairpin RNA (shRNA) in wild-type VSMCs, and was absent in TRPC1-/- VSMCs. Store-operated PKC phosphorylation of TRPC1 was reduced by knockdown of STIM1. Moreover, store-operated PLCβ1 activity measured with the fluorescent phosphatidylinositol 4,5-bisphosphate/inositol 1,4,5-trisphosphate biosensor GFP-PLCδ1-PH was reduced by STIM1 shRNA and absent in TRPC1-/- cells. Immunocytochemistry, co-immunoprecipitation and proximity ligation assays revealed that store depletion activated STIM1 translocation from within the cell to the plasma membrane (PM) where it formed STIM1-TRPC1 complexes, which then associated with Gαq and PLCβ1. Noradrenaline also evoked TRPC1 channel activity and associations between TRPC1, STIM1, Gαq and PLCβ1, which were inhibited by STIM1 knockdown. Effects of N-terminal and C-terminal STIM1 antibodies on TRPC1-based SOCs and STIM1 staining suggest that channel activation may involve insertion of STIM1 into the PM. The findings of the present study identify a new activation mechanism of TRPC1-based SOCs in VSMCs, and a novel role for STIM1, in which store-operated STIM1-TRPC1 interactions stimulate PLCβ1 activity to induce PKC phosphorylation of TRPC1 and channel gating.

ASIC1-mediated calcium entry stimulates NFATc3 nuclear translocation via PICK1 coupling in pulmonary arterial smooth muscle cells

The development of chronic hypoxia (CH)-induced pulmonary hypertension is associated with increased pulmonary arterial smooth muscle cell (PASMC) Ca(2+) influx through acid-sensing ion channel-1 (ASIC1) and activation of the Ca(2+)/calcineurin-dependent transcription factor known as nuclear factor of activated T-cells isoform c3 (NFATc3). Whether Ca(2+) influx through ASIC1 contributes to NFATc3 activation in the pulmonary vasculature is unknown. Furthermore, both ASIC1 and calcineurin have been shown to interact with the scaffolding protein known as protein interacting with C kinase-1 (PICK1). In the present study, we tested the hypothesis that ASIC1 contributes to NFATc3 nuclear translocation in PASMC in a PICK1-dependent manner. Using both ASIC1 knockout (ASIC1(-/-)) mice and pharmacological inhibition of ASIC1, we demonstrate that ASIC1 contributes to CH-induced (1 wk at 380 mmHg) and endothelin-1 (ET-1)-induced (10(-7) M) Ca(2+) responses and NFATc3 nuclear import in PASMC. The interaction between ASIC1/PICK1/calcineurin was shown using a Duolink in situ Proximity Ligation Assay. Inhibition of PICK1 by using FSC231 abolished ET-1-induced and ionomycin-induced NFATc3 nuclear import, but it did not alter ET-1-mediated Ca(2+) responses, suggesting that PICK1 acts downstream of Ca(2+) influx. The key findings of the present work are that 1) Ca(2+) influx through ASIC1 mediates CH- and ET-1-induced NFATc3 nuclear import and 2) the scaffolding protein PICK1 is necessary for NFATc3 nuclear import. Together, these data provide an essential link between CH-induced ASIC1-mediated Ca(2+) influx and activation of the NFATc3 transcription factor. Identification of this ASIC1/PICK1/NFATc3 signaling complex increases our understanding of the mechanisms contributing to the vascular remodeling and increased vascular contractility that are associated with CH-induced pulmonary hypertension.

Conformation of ryanodine receptor-2 gates store-operated calcium entry in rat pulmonary arterial myocytes

AIMS

Store-operated Ca(2+) entry (SOCE) contributes to a multitude of physiological and pathophysiological functions in pulmonary vasculatures. SOCE attributable to inositol 1,4,5-trisphosphate receptor (InsP3R)-gated Ca(2+) store has been studied extensively, but the role of ryanodine receptor (RyR)-gated store in SOCE remains unclear. The present study aims to delineate the relationship between RyR-gated Ca(2+) stores and SOCE, and characterize the properties of RyR-gated Ca(2+) entry in pulmonary artery smooth muscle cells (PASMCs).

METHODS AND RESULTS

PASMCs were isolated from intralobar pulmonary arteries of male Wister rats. Application of the RyR1/2 agonist 4-chloro-m-cresol (4-CmC) activated robust Ca(2+) entry in PASMCs. It was blocked by Gd(3+) and the RyR2 modulator K201 but was unaffected by the RyR1/3 antagonist dantrolene and the InsP3R inhibitor xestospongin C, suggesting RyR2 is mainly involved in the process. siRNA knockdown of STIM1, TRPC1, and Orai1, or interruption of STIM1 translocation with ML-9 significantly attenuated the 4-CmC-induced SOCE, similar to SOCE induced by thapsigargin. However, depletion of RyR-gated store with caffeine failed to activate Ca(2+) entry. Inclusion of ryanodine, which itself did not cause Ca(2+) entry, uncovered caffeine-induced SOCE in a concentration-dependent manner, suggesting binding of ryanodine to RyR is permissive for the process. This Ca(2+) entry had the same molecular and pharmacological properties of 4-CmC-induced SOCE, and it persisted once activated even after caffeine washout. Measurement of Ca(2+) in sarcoplasmic reticulum (SR) showed that 4-CmC and caffeine application with or without ryanodine reduced SR Ca(2+) to similar extent, suggesting store-depletion was not the cause of the discrepancy. Moreover, caffeine/ryanodine and 4-CmC failed to initiate SOCE in cells transfected with the ryanodine-binding deficient mutant RyR2-I4827T.

CONCLUSIONS

RyR2-gated Ca(2+) store contributes to SOCE in PASMCs; however, store-depletion alone is insufficient but requires a specific RyR conformation modifiable by ryanodine binding to activate Ca(2+) entry.

Hypoxic pulmonary vasoconstriction, carotid body function and erythropoietin production in adult rats perinatally exposed to hyperoxia

KEY POINTS

Adult animals that have been perinatally exposed to oxygen-rich atmospheres (hyperoxia), recalling those used for oxygen therapy in infants, exhibit a loss of hypoxic pulmonary vasoconstriction, whereas vasoconstriction elicited by depolarizing agents is maintained. Loss of pulmonary hypoxic vasoconstriction is not linked to alterations in oxygen-sensitive K(+) currents in pulmonary artery smooth muscle cells. Loss of hypoxic vasoconstriction is associated with early postnatal oxidative damage and corrected by an antioxidant diet. Perinatal hyperoxia damages carotid body chemoreceptor cell function and the antioxidant diet does not reverse it. The hypoxia-elicited increase in erythropoietin plasma levels is not affected by perinatal hyperoxia. The potential clinical significance of the findings in clinical situations such as pneumonia, chronic obstructive pulmonary disease or general anaesthesia is considered.

ABSTRACT

Adult mammalians possess three cell systems that are activated by acute bodily hypoxia: pulmonary artery smooth muscle cells (PASMC), carotid body chemoreceptor cells (CBCC) and erythropoietin (EPO)-producing cells. In rats, chronic perinatal hyperoxia causes permanent carotid body (CB) atrophy and functional alterations of surviving CBCC. There are no studies on PASMC or EPO-producing cells. Our aim is to define possible long-lasting functional changes in PASMC or EPO-producing cells (measured as EPO plasma levels) and, further, to analyse CBCC functional alterations. We used 3- to 4-month-old rats born and reared in a normal atmosphere or exposed to perinatal hyperoxia (55-60% O2 for the last 5-6 days of pregnancy and 4 weeks after birth). Perinatal hyperoxia causes an almost complete loss of hypoxic pulmonary vasoconstriction (HPV), which was correlated with lung oxidative status in early postnatal life and prevented by antioxidant supplementation in the diet. O2 -sensitivity of K(+) currents in the PASMC of hyperoxic animals is normal, indicating that their inhibition is not sufficient to trigger HPV. Perinatal hyperoxia also abrogated responses elicited by hypoxia on catecholamine and cAMP metabolism in the CB. An increase in EPO plasma levels elicited by hypoxia was identical in hyperoxic and control animals, implying a normal functioning of EPO-producing cells. The loss of HPV observed in adult rats and caused by perinatal hyperoxia, comparable to oxygen therapy in premature infants, might represent a previously unrecognized complication of such a medical intervention capable of aggravating medical conditions such as regional pneumonias, atelectases or general anaesthesia in adult life.

Ion channels and transporters as therapeutic targets in the pulmonary circulation

Pulmonary circulation is a low pressure, low resistance, high flow system. The low resting vascular tone is maintained by the concerted action of ion channels, exchangers and pumps. Under physiological as well as pathophysiological conditions, they are targets of locally secreted or circulating vasodilators and/or vasoconstrictors, leading to changes in expression or to posttranslational modifications. Both structural changes in the pulmonary arteries and a sustained increase in pulmonary vascular tone result in pulmonary vascular remodeling contributing to morbidity and mortality in pediatric and adult patients. There is increasing evidence demonstrating the pivotal role of ion channels such as K(+) and Cl(-) or transient receptor potential channels in different cell types which are thought to play a key role in vasoconstrictive remodeling. This review focuses on ion channels, exchangers and pumps in the pulmonary circulation and summarizes their putative pathophysiological as well as therapeutic role in pulmonary vascular remodeling. A better understanding of the mechanisms of their actions may allow for the development of new options for attenuating acute and chronic pulmonary vasoconstriction and remodeling treating the devastating disease pulmonary hypertension.

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

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

Effects of chronic exposure to cigarette smoke on canonical transient receptor potential expression in rat pulmonary arterial smooth muscle

To clarify the possible mechanism of cigarette smoke (CS)-induced pulmonary hypertension and furthermore provide effective targets for prevention and treatment, the effects of chronic CS on rat pulmonary arterial smooth muscle in vivo and nicotine treatment on rat pulmonary arterial smooth muscle cells (PASMCs) in vitro were investigated. In this study, we demonstrated that chronic CS exposure led to rat weight loss, right ventricular hypertrophy, and pulmonary arterial remodeling. A fluorescence microscope was used to measure intracellular calcium concentration ([Ca(2+)]i) in rat distal PASMCs. Results showed that basal [Ca(2+)]i and store-operated calcium entry (SOCE) levels in PASMCs from 3- and 6-mo CS-exposed rats were markedly higher than those in cells from the unexposed control animals (the increases in 6-mo CS group were more significant than that in 3-mo group), accompanied with increased canonical transient receptor potential 1 (TRPC1) and TRPC6 expression at both mRNA and protein levels in isolated distal PA. Simultaneously, in vitro study showed that nicotine treatment (10 nM) significantly increased basal [Ca(2+)]i and SOCE and upregulated TRPC1 and TRPC6 expression in cultured rat distal PASMCs. TRPC siRNA knockdown strategies revealed that the elevations of basal [Ca(2+)]i and SOCE induced by nicotine in PASMCs were TRPC1 and TRPC6 dependent. These results suggested that chronic CS-induced changes in vascular tone and structure in PA and the development of pulmonary hypertension might be largely due to upregulation of TRPC1 and TRPC6 expression in PASMCs, in which nicotine played an important role.

Regulation of ca(2+) signaling in pulmonary hypertension

Understanding the cellular and molecular mechanisms involved in the development and progression of pulmonary hypertension (PH) remains imperative if we are to successfully improve the quality of life and life span of patients with the disease. A whole plethora of mechanisms are associated with the development and progression of PH. Such complexity makes it difficult to isolate one particular pathway to target clinically. Changes in intracellular free calcium concentration, the most common intracellular second messenger, can have significant impact in defining the pathogenic mechanisms leading to its development and persistence. Signaling pathways leading to the elevation of [Ca(2+)](cyt) contribute to pulmonary vasoconstriction, excessive proliferation of smooth muscle cells and ultimately pulmonary vascular remodeling. This current review serves to summarize the some of the most recent advances in the regulation of calcium during pulmonary hypertension.

STIM1-regulated Ca2+ influx across the apical and the basolateral membrane in colonic epithelium

In nonexcitable cells, store-operated Ca(2+) entry is the most important pathway for influx of extracellular Ca(2+) serving as a second messenger in the cytoplasm. The present study investigated the expression, localization and polar distribution of two key components of store-operated Ca(2+) entry identified, e.g., in lymphocytes or epithelial cell lines-STIM1 (stromal interacting molecule 1), working as a Ca(2+) sensor in the endoplasmic reticulum, and Orai1, working as the (or part of the) store-operated Ca(2+) channel in the plasma membrane-in a native intestinal epithelium, i.e., rat colon. Immunohistochemical investigations revealed expression of STIM1 and Orai1 in the rat colonic epithelium. Ca(2+) store depletion led to a translocation of STIM1 both to the basolateral as well as to the apical cell pole as observed by confocal microscopy. A Ca(2+) depletion/repletion protocol was used in Ussing chamber experiments to investigate the contribution of basolateral and apical store-operated Ca(2+) entry to the induction of anion secretion. These experiments revealed that Ca(2+)-dependent anion secretion was induced not only by basolateral Ca(2+) repletion but also, to a lesser extent, by apical Ca(2+) repletion. Both responses were suppressed by La(3+). The effect of basolateral Ca(2+) repletion was significantly inhibited by brefeldin A, a blocker of vesicular transport from the endoplasmic reticulum to the Golgi apparatus. In a final series of experiments, fura-2-loaded HT29/B6 cells were used. A carbachol-induced increase in the cytosolic Ca(2+) concentration was significantly reduced when cells were pretreated with siRNA against STIM1. In conclusion, these results demonstrate that STIM1 as a key component of intracellular Ca(2+) signaling is expressed by rat colonic epithelium and is involved in the regulation not only of basolateral but also of apical Ca(2+) influx.

Emerging roles for native Orai Ca2+ channels in cardiovascular disease

Orai proteins form highly calcium (Ca(2+))-selective channels located in the plasma membrane of both nonexcitable and excitable cells, where they make important contributions to many cellular processes. The well-characterized Ca(2+) release-activated Ca(2+) current is mediated by Orai1 multimers and is activated, upon depletion of inositol 1,4,5-trisphosphate-sensitive stores, by direct interaction of Orai1 with the endoplasmic reticulum Ca(2+) sensor, stromal interaction molecule 1 (STIM1). This pathway is known as capacitative Ca(2+) entry or store-operated Ca(2+) entry. While most investigations have focused on STIM1 and Orai1 in their store-dependent mode, emerging evidence suggests that Orai1 and Orai3 heteromultimeric channels can form store-independent Ca(2+)-selective channels. The role of store-dependent and store-independent channels in excitation-transcription coupling and the pathological remodeling of the cardiovascular system are beginning to come forth. Recent evidence suggests that STIM/Orai-generated Ca(2+) signaling couples to gene transcription and subsequent phenotypic changes associated with the processes of cardiac and vascular remodeling. This short review will explore the contributions of native Orai channels to heart and vessel physiology and their role in cardiovascular diseases.

Cross talk between plasma membrane Na(+)/Ca (2+) exchanger-1 and TRPC/Orai-containing channels: key players in arterial hypertension

Arterial smooth muscle (ASM) Na(+)/Ca(2+) exchanger type 1 (NCX1) and TRPC/Orai-containing receptor/store-operated cation channels (ROC/SOC) are clustered with α2 Na(+) pumps in plasma membrane microdomains adjacent to the underlying junctional sarcoplasmic reticulum. This arrangement enables these transport proteins to function as integrated units to help regulate local Na(+) metabolism, Ca(2+) signaling, and arterial tone. They thus influence vascular resistance and blood pressure (BP). For instance, upregulation of NCX1 and TRPC6 has been implicated in the pathogenesis of high BP in several models of essential hypertension. The models include ouabain-induced hypertensive rats, Milan hypertensive rats, and Dahl salt-sensitive hypertensive rats, all of which exhibit elevated plasma ouabain levels. We suggest that these molecular mechanisms are key contributors to the increased vascular resistance ("whole body autoregulation") that elevates BP in essential hypertension. Enhanced expression and function of ASM NCX1 and TRPC/Orai1-containing channels in hypertension implies that these proteins are potential targets for pharmacological intervention.

TRPC1 and Orai1 interact with STIM1 and mediate capacitative Ca2+ entry caused by acute hypoxia in mouse pulmonary arterial smooth muscle cells

Previous studies in pulmonary artery smooth muscle cells (PASMCs) showed that acute hypoxia activates capacitative Ca2+ entry (CCE) but the molecular candidate(s) mediating CCE caused by acute hypoxia remain unclear. The present study aimed to determine if transient receptor potential canonical 1 (TRPC1) and Orai1 interact with stromal interacting molecule 1 (STIM1) and mediate CCE caused by acute hypoxia in mouse PASMCs. In primary cultured PASMCs loaded with fura-2, acute hypoxia caused a transient followed by a sustained rise in intracellular Ca2+ concentration ([Ca2+]i). The transient but not sustained rise in [Ca2+]i was partially inhibited by nifedipine. Acute hypoxia also increased the rate of Mn2+ quench of fura-2 fluorescence that was inhibited by SKF 96365, Ni2+, La3+, and Gd3+, exhibiting pharmacological properties characteristic of CCE. The nifedipine-insensitive rise in [Ca2+]i and the increase in Mn2+ quench rate were both inhibited in cells treated with TRPC1 antibody or TRPC1 small interfering (si)RNA, in STIM1 siRNA-transfected cells and in Orai1 siRNA-transfected cells. Moreover, overexpression of STIM1 resulted in a marked increase in [Ca2+]i and Mn2+ quench rate caused by acute hypoxia, and they were reduced in cells treated with TRPC1 antibody and in cells transfected with Orai1 siRNA. Furthermore, TRPC1 and Orai1 coimmunoprecipitated with STIM1 and the precipitation levels of TRPC1 and Orai1 were increased in cells exposed to acute hypoxia. Immunostaining showed colocalizations of TRPC1-STIM1 and Orai1-STIM1, and the colocalizations of these proteins were more apparent in acute hypoxia. These data provide direct evidence that TRPC1 and Orai1 channels mediate CCE through activation of STIM1 in acute hypoxic mouse PASMCs.

55th Bowditch Lecture: Effects of chronic hypoxia on the pulmonary circulation: role of HIF-1

When exposed to chronic hypoxia (CH), the pulmonary circulation responds with enhanced contraction and vascular remodeling, resulting in elevated pulmonary arterial pressures. Our work has identified CH-induced alterations in the expression and activity of several ion channels and transporters in pulmonary vascular smooth muscle that contribute to the development of hypoxic pulmonary hypertension and uncovered a critical role for the transcription factor hypoxia-inducible factor-1 (HIF-1) in mediating these responses. Current work is focused on the regulation of HIF in the chronically hypoxic lung and evaluation of the potential for pharmacological inhibitors of HIF to prevent, reverse, or slow the progression of pulmonary hypertension.

Chronic hypoxia upregulates pulmonary arterial ASIC1: a novel mechanism of enhanced store-operated Ca2+ entry and receptor-dependent vasoconstriction

Acid-sensing ion channel 1 (ASIC1) is a newly characterized contributor to store-operated Ca(2+) entry (SOCE) in pulmonary vascular smooth muscle (VSM). Since SOCE is implicated in elevated basal VSM intracellular Ca(2+) concentration ([Ca(2+)](i)) and augmented vasoconstriction in chronic hypoxia (CH)-induced pulmonary hypertension, we hypothesized that ASIC1 contributes to these responses. To test this hypothesis, we examined effects of the specific pharmacologic ASIC1a inhibitor, psalmotoxin 1 (PcTX1), on vasoconstrictor and vessel wall [Ca(2+)](i) responses to UTP and KCl (depolarizing stimulus) in fura-2-loaded, pressurized small pulmonary arteries from control and CH (4 wk at 0.5 atm) Wistar rats. PcTX1 had no effect on basal vessel wall [Ca(2+)](i), but attenuated vasoconstriction and increases in vessel wall [Ca(2+)](i) to UTP in arteries from control and CH rats; normalizing responses between groups. In contrast, responses to the depolarizing stimulus, KCl, were unaffected by CH exposure or PcTX1. Upon examining potential Ca(2+) influx mechanisms, we found that PcTX1 prevented augmented SOCE following CH. Exposure to CH resulted in a significant increase in pulmonary arterial ASIC1 protein. This study supports a novel role of ASIC1 in elevated receptor-stimulated vasoconstriction following CH which is likely mediated through increased ASIC1 expression and SOCE.

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.

New mechanisms of pulmonary arterial hypertension: role of Ca²⁺ signaling

Pulmonary arterial hypertension (PAH) is a severe and progressive disease that usually culminates in right heart failure and death if left untreated. Although there have been substantial improvements in our understanding and significant advances in the management of this disease, there is a grim prognosis for patients in the advanced stages of PAH. A major cause of PAH is increased pulmonary vascular resistance, which results from sustained vasoconstriction, excessive pulmonary vascular remodeling, in situ thrombosis, and increased pulmonary vascular stiffness. In addition to other signal transduction pathways, Ca(2+) signaling in pulmonary artery smooth muscle cells (PASMCs) plays a central role in the development and progression of PAH because of its involvement in both vasoconstriction, through its pivotal effect of PASMC contraction, and vascular remodeling, through its stimulatory effect on PASMC proliferation. Altered expression, function, and regulation of ion channels and transporters in PASMCs contribute to an increased cytosolic Ca(2+) concentration and enhanced Ca(2+) signaling in patients with PAH. This review will focus on the potential pathogenic role of Ca(2+) mobilization, regulation, and signaling in the development and progression of PAH.

Hypoxic pulmonary vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

STIM1/Orai1-mediated store-operated Ca2+ entry: the tip of the iceberg

Highly efficient mechanisms regulate intracellular calcium (Ca2+) levels. The recent discovery of new components linking intracellular Ca2+ stores to plasma membrane Ca2+ entry channels has brought new insight into the understanding of Ca2+ homeostasis. Stromal interaction molecule 1 (STIM1) was identified as a Ca2+ sensor essential for Ca2+ store depletion-triggered Ca2+ influx. Orai1 was recognized as being an essential component for the Ca2+ release-activated Ca2+ (CRAC) channel. Together, these proteins participate in store-operated Ca2+ channel function. Defective regulation of intracellular Ca2+ is a hallmark of several diseases. In this review, we focus on Ca2+ regulation by the STIM1/Orai1 pathway and review evidence that implicates STIM1/Orai1 in several pathological conditions including cardiovascular and pulmonary diseases, among others.

Enhanced store-operated Ca²+ entry and TRPC channel expression in pulmonary arteries of monocrotaline-induced pulmonary hypertensive rats

Pulmonary hypertension (PH) is associated with profound vascular remodeling and alterations in Ca(2+) homeostasis in pulmonary arterial smooth muscle cells (PASMCs). Previous studies show that canonical transient receptor potential (TRPC) genes are upregulated and store-operated Ca(2+) entry (SOCE) is augmented in PASMCs of chronic hypoxic rats and patients of pulmonary arterial hypertension (PAH). Here we further examine the involvement of TRPC and SOCE in PH with a widely used rat model of monocrotaline (MCT)-induced PAH. Rats developed severe PAH, right ventricular hypertrophy, and significant increase in store-operated TRPC1 and TRPC4 mRNA and protein in endothelium-denuded pulmonary arteries (PAs) 3 wk after MCT injection. Contraction of PA and Ca(2+) influx in PASMC evoked by store depletion using cyclopiazonic acid (CPA) were enhanced dramatically, consistent with augmented SOCE in the MCT-treated group. The time course of increase in CPA-induced contraction corresponded to that of TRPC1 expression. Endothelin-1 (ET-1)-induced vasoconstriction was also potentiated in PAs of MCT-treated rats. The response was partially inhibited by SOCE blockers, including Gd(3+), La(3+), and SKF-96365, as well as the general TRPC inhibitor BTP-2, suggesting that TRPC-dependent SOCE was involved. Moreover, the ET-1-induced contraction and Ca(2+) response in the MCT group were more susceptible to the inhibition caused by the various SOCE blockers. Hence, our study shows that MCT-induced PAH is associated with increased TRPC expression and SOCE, which are involved in the enhanced vascular reactivity to ET-1, and support the hypothesis that TRPC-dependent SOCE is an important pathway for the development of PH.

SOC and now also SIC: store-operated and store-inhibited channels

There is a specialized form of calcium influx that involves a close communication between endoplasmic reticulum and the channels at the plasma membrane. In one side store depletion activates channels known as store-operated channels (SOC), which are responsible of the well-studied store-operated calcium entry (SOCE). SOC comprises two different types of channels. Orai, which is exclusively activated by store depletion being the channel responsible of the calcium release-activated calcium current, and transient receptor potential canonical channel, which in contrast, is activated by store depletion only under specific conditions and carries nonselective cationic currents. On the other hand, it has been recently shown that store depletion also inhibits calcium channels. The first member identified, of what we named as store-inhibited channels (SIC), is the L-type voltage-gated calcium channel. Stores control both SOC and SIC by means of the multifunctional protein STIM1. The identification of SOC and SIC opens a new scenario for the role of store depletion in the modulation of different calcium entry pathways, which may satisfy different cellular processes.

Nanomolar potency and selectivity of a Ca²⁺ release-activated Ca²⁺ channel inhibitor against store-operated Ca²⁺ entry and migration of vascular smooth muscle cells

BACKGROUND AND PURPOSE

The aim was to advance the understanding of Orai proteins and identify a specific inhibitor of the associated calcium entry mechanism in vascular smooth muscle cells (VSMCs).

EXPERIMENTAL APPROACH

Proliferating VSMCs were cultured from human saphenous veins. Intracellular calcium was measured using fura-2, whole-cell current was recorded using patch-clamp and cell migration quantified in modified Boyden chambers. Subcellular protein localization was determined by microscopy. Isometric tension was recorded from mouse aortic rings.

KEY RESULTS

Molecular disruption and rescue experiments indicated the importance of Orai1 in calcium entry caused by store depletion evoked passively or by platelet-derived growth factor (PDGF), suggesting the presence of Ca(2+) release-activated Ca(2+) (CRAC) channels like those of the immune system. The CRAC channel blocker, S66, was a potent inhibitor of the VSMC signals, IC(50) 26 nM, which was almost two orders of magnitude greater than with leucocytes. S66 had no effect on PDGF- and ATP-evoked calcium release, overexpressed transient receptor potential canonical (TRPC)5 channels, native TRPC1/5-containing channels, stromal interaction molecule 1 clustering, non-selective cationic current evoked by store depletion and phenylephrine-evoked aortic contraction. S66 reduced PDGF-evoked VSMC migration while having only modest effects on cell proliferation and no effect on cell viability.

CONCLUSIONS AND IMPLICATIONS

The data suggest that Orai1 has a role in human VSMC migration, and that a CRAC channel inhibitor has high potency and selectivity for the associated calcium entry, suggesting a distinct characteristic of vascular CRAC channels and the potential for selective chemical suppression of vascular remodelling.

Calcium signaling in smooth muscle

Changes in intracellular Ca(2+) are central to the function of smooth muscle, which lines the walls of all hollow organs. These changes take a variety of forms, from sustained, cell-wide increases to temporally varying, localized changes. The nature of the Ca(2+) signal is a reflection of the source of Ca(2+) (extracellular or intracellular) and the molecular entity responsible for generating it. Depending on the specific channel involved and the detection technology employed, extracellular Ca(2+) entry may be detected optically as graded elevations in intracellular Ca(2+), junctional Ca(2+) transients, Ca(2+) flashes, or Ca(2+) sparklets, whereas release of Ca(2+) from intracellular stores may manifest as Ca(2+) sparks, Ca(2+) puffs, or Ca(2+) waves. These diverse Ca(2+) signals collectively regulate a variety of functions. Some functions, such as contractility, are unique to smooth muscle; others are common to other excitable cells (e.g., modulation of membrane potential) and nonexcitable cells (e.g., regulation of gene expression).

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

The ER Ca²+ sensor STIM1 and the Ca²+ channel Orai1 are key players in store-operated Ca²+ entry (SOCE). In addition, channels from the TRPC family were also shown to be engaged during SOCE, while their precise implication remains controversial. In this study, we investigated the molecular players involved in SOCE triggered by the SERCA pump inhibitor thapsigargin in an endothelial cell line, the EA.hy926. siRNA directed against STIM1 or Orai1 reduced Ca²+ entry by about 50-60%, showing that a large part of the entry is independent from these proteins. Blocking the PLC or the PKC pathway completely abolished thapsigargin-induced Ca²+ entry in cells depleted from STIM1 and/or Orai1. The phorbol ester PMA or the DAG analog OAG restored the Ca²+ entry inhibited by PLC blockers, showing an involvement of PLC/PKC pathway in SOCE. Using pharmacological inhibitors or siRNA revealed that the PKCeta is required for Ca²+ entry, and pharmacological inhibition of the tyrosine kinase Src also reduced Ca²+ entry. TRPC3 silencing diminished the entry by 45%, while the double STIM1/TRPC3 invalidation reduced Ca²+ entry by more than 85%. Hence, in EA.hy926 cells, TG-induced Ca²+ entry results from the activation of the STIM1/Orai1 machinery, and from the activation of TRPC3.