Activation of endoplasmic reticulum calcium leak by 2-APB depends on the luminal calcium concentration

Effects of intracellular Ca2+ on developmental potential and ultrastructure of cryopreserved-warmed oocyte in mouse

Maintaining appropriate intracellular calcium of oocytes is necessary to prevent ultrastructure and organelle damage caused by freezing and cryoprotectants. The present study aimed to investigate whether cryoprotectant-induced changes in the calcium concentrations of oocytes can be regulated to reduce damage to developmental potential and ultrastructure. A total of 33 mice and 1381 oocytes were used to explore the effects of intracellular calcium on the development and ultrastructures of oocytes subjected to 2-aminoethoxydiphenyl borate (2-APB) inhibition or thapsigargin (TG) stimulation. Results suggested that high levels intracellular calcium interfered with TG compromised oocyte survival (84.4 % vs. 93.4 %, p < 0.01) and blastocyst formation in fresh and cryopreservation oocytes (78.1 % vs. 86.4 %, and 60.5 % vs. 72.5 %, p < 0.05) compared with that of 2-APB pretreated oocytes in which Ca2+ was stabilized even though no differences in fertilization and cleavage was detected (p > 0.05). Examination by transmission electron microscopy indicated that the microvilli decreased and shortened, cortical granules considerably decreased in the cortex area, mitochondrial vesicles and vacuoles increased, and the proportion of vacuole mitochondria increased after oocytes were exposed to cryoprotectants. The cryopreservation-warming process deteriorated the negative effects on organelles of survival oocytes. By contrast, a low level of intracellular calcium mediated with 2-APB was supposed to contribute to the protection of organelles. These findings suggested oocyte injuries induced by cryoprotectants and low temperatures can be alleviated. More studies are necessary to confirm the relationship among Ca2+ concentration of the cytoplasm, ultrastructural injuries, and disrupted developmental potential in oocytes subjected to cryopreservation and warming.

FK506 impairs neutrophil migration that results in increased polymicrobial sepsis susceptibility

OBJECTIVE

This study aimed to investigate the effects of FK506 on experimental sepsis immunopathology. It investigated the effect of FK506 on leukocyte recruitment to the site of infection, systemic cytokine production, and organ injury in mice with sepsis.

METHODS

Using a murine cecal ligation and puncture (CLP) peritonitis model, the experiments were performed with wild-type (WT) mice and mice deficient in the gene Nfat1 (Nfat1^-/-) in the C57BL/6 background. Animals were treated with 2.0 mg/kg of FK506, subcutaneously, 1 h before the sepsis model, twice a day (12 h/12 h). The number of bacteria colony forming units (CFU) was manually counted. The number of neutrophils in the lungs was estimated by the myeloperoxidase (MPO) assay. The expression of CXCR2 in neutrophils was determined using flow cytometry analysis. The expression of inflammatory cytokines in macrophage was determined using ELISA. The direct effect of FK506 on CXCR2 internalization was evaluated using HEK-293T cells after CXCL2 stimulation by the BRET method.

RESULTS

FK506 treatment potentiated the failure of neutrophil migration into the peritoneal cavity, resulting in bacteremia and an exacerbated systemic inflammatory response, which led to higher organ damage and mortality rates. Failed neutrophil migration was associated with elevated CXCL2 chemokine plasma levels and lower expression of the CXCR2 receptor on circulating neutrophils compared with non-treated CLP-induced septic mice. FK506 did not directly affect CXCL2-induced CXCR2 internalization by transfected HEK-293 cells or mice neutrophils, despite increasing CXCL2 release by LPS-treated macrophages. Finally, the CLP-induced response of Nfat1^-/- mice was similar to those observed in the Nfat1^+/+ genotype, suggesting that the FK506 effect is not dependent on the NFAT1 pathway.

CONCLUSION

Our data indicate that the increased susceptibility to infection of FK506-treated mice is associated with failed neutrophil migration due to the reduced membrane availability of CXCR2 receptors in response to exacerbated levels of circulating CXCL2.

Chronic glucocorticoid exposure accelerates Aβ generation and neurotoxicity by activating calcium-mediated CN-NFAT1 signaling in hippocampal neurons in APP/PS1 mice

Glucocorticoid (GC) exposure can lead to deterioration of the structure and function of hippocampal neurons and is closely involved in Alzheimer's disease (AD). Amyloid-β (Aβ) overproduction is an important aspect of AD pathogenesis. Our study mainly investigated the mechanism of chronic GC exposure in accelerating Aβ production in primary cultured hippocampal neurons from APP/PS1 mice. The results indicated that chronic dexamethasone (DEX, 1 μM) significantly accelerated neuronal damage and Aβ accumulation in hippocampal neurons from APP/PS1 mice. Meanwhile, DEX exposure markedly upregulated APP, NCSTN, BACE1 and p-Tau/Tau expression in hippocampal neurons from APP/PS1 mice. Our study also indicated that chronic DEX exposure significantly increased intracellular Ca²⁺ ([Ca²⁺]_i) levels and the expressions of p-PLC, CN and NFAT1 in hippocampal neurons from APP/PS1 mice. We further found that stabilizing intracellular calcium homeostasis with 2-APB (50 μM) and SKF-96365 (10 μM) significantly alleviated neuronal damage and Aβ accumulation in chronic DEX-induced hippocampal neurons from APP/PS1 mice. Additionally, dual luciferase assays showed that NFAT1 upregulated NCSTN transactivation, which was further increased upon DEX treatment. This study suggests that chronic DEX exposure accelerates Aβ accumulation by activating calcium-mediated CN-NFAT1 signaling in hippocampal neurons from APP/PS1 mice, which may be closely related to the acceleration of AD.

PKC-Mediated Orai1 Channel Phosphorylation Modulates Ca2+ Signaling in HeLa Cells

The overexpression of the Orai1 channel inhibits SOCE when using the Ca²⁺ readdition protocol. However, we found that HeLa cells overexpressing the Orai1 channel displayed enhanced Ca²⁺ entry and a limited ER depletion in response to the combination of ATP and thapsigargin (TG) in the presence of external Ca²⁺. As these effects require the combination of an agonist and TG, we decided to study whether the phosphorylation of Orai1 S27/S30 residues had any role using two different mutants: Orai1-S27/30A (O1-AA, phosphorylation-resistant) and Orai1-S27/30D (O1-DD, phosphomimetic). Both O1-wt and O1-AA supported enhanced Ca²⁺ entry, but this was not the case with O1-E106A (dead-pore mutant), O1-DD, and O1-AA-E106A, while O1-wt, O1-E106A, and O1-DD inhibited the ATP and TG-induced reduction of ER [Ca²⁺], suggesting that the phosphorylation of O1 S27/30 interferes with the IP₃R activity. O1-wt and O1-DD displayed an increased interaction with IP₃R in response to ATP and TG; however, the O1-AA channel decreased this interaction. The expression of mCherry-O1-AA increased the frequency of ATP-induced sinusoidal [Ca²⁺]_i oscillations, while mCherry-O1-wt and mCherry-O1-DD decreased this frequency. These data suggest that the combination of ATP and TG stimulates Ca²⁺ entry, and the phosphorylation of Orai1 S27/30 residues by PKC reduces IP₃R-mediated Ca²⁺ release.

A comprehensive overview of the complex world of the endo- and sarcoplasmic reticulum Ca2+-leak channels

Inside cells, the endoplasmic reticulum (ER) forms the largest Ca²⁺ store. Ca²⁺ is actively pumped by the SERCA pumps in the ER, where intraluminal Ca²⁺-binding proteins enable the accumulation of large amount of Ca²⁺. IP₃ receptors and the ryanodine receptors mediate the release of Ca²⁺ in a controlled way, thereby evoking complex spatio-temporal signals in the cell. The steady state Ca²⁺ concentration in the ER of about 500 μM results from the balance between SERCA-mediated Ca²⁺ uptake and the passive leakage of Ca²⁺. The passive Ca²⁺ leak from the ER is often ignored, but can play an important physiological role, depending on the cellular context. Moreover, excessive Ca²⁺ leakage significantly lowers the amount of Ca²⁺ stored in the ER compared to normal conditions, thereby limiting the possibility to evoke Ca²⁺ signals and/or causing ER stress, leading to pathological consequences. The so-called Ca²⁺-leak channels responsible for Ca²⁺ leakage from the ER are however still not well understood, despite over 20 different proteins have been proposed to contribute to it. This review has the aim to critically evaluate the available evidence about the various channels potentially involved and to draw conclusions about their relative importance.

Gating and regulation of the calcium release-activated calcium channel: Recent progress from experiments and molecular modeling

Calcium release-activated calcium (CRAC) channels are highly calcium ion (Ca²⁺)-selective channels in the plasma membrane. The transient drop of endoplasmic reticulum Ca²⁺ level activates its calcium sensor stromal interaction molecule (STIM) and then triggers the gating of the CRAC channel pore unit Orai. This process involves a variety of activities of the immune system. Therefore, understanding how the activation and regulation of the CRAC channel can be accomplished is essential. Here we briefly summarize the recent progress on Orai gating and its regulation by 2-aminoethoxydiphenylborate (2-APB) obtained from structural biology studies, biochemical and electrophysiological measurements, as well as molecular modeling. Indeed, integration between experiments and computations has further deepened our understanding of the channel gating and regulation.

Store-operated Ca2+ entry-dependent Ca2+ refilling in the endoplasmic reticulum in astrocytes

Astrocytes regulate various brain functions, for which Ca²⁺ release from the endoplasmic reticulum (ER) often play crucial roles. Because astrocytic ER Ca²⁺ release is robust and frequent, the ER Ca²⁺ refilling mechanism should be critical for ongoing Ca²⁺ signaling in astrocytes. In this study, we focused on the putative functional significance of store-operated Ca²⁺ entry (SOCE) in ER Ca²⁺ refilling. We expressed the ER luminal Ca²⁺ indicator G-CEPIA1er in astrocytes in acute cortical slices to directly monitor the decrease and recovery of ER Ca²⁺ concentration upon spontaneous or norepinephrine-induced Ca²⁺ release. Inhibition of SOCE significantly slowed the recovery of ER Ca²⁺ concentration after Ca²⁺ release in astrocytes. This delayed recovery resulted in a prolonged decrease in the ER Ca²⁺ content in astrocytes with periodic spontaneous Ca²⁺ release, followed by the attenuation of cytosolic Ca²⁺ responses upon Ca²⁺ release. Therefore, our results provide direct evidence for the physiological significance of SOCE in ER Ca²⁺ refilling after ER Ca²⁺ release.

l-ornithine activates Ca2+ signaling to exert its protective function on human proximal tubular cells

Oxidative stress and reactive oxygen species (ROS) generation can be influenced by G-protein coupled receptor (GPCR)-mediated regulation of intracellular Ca²⁺ ([Ca²⁺]_i) signaling. ROS production are much higher in proximal tubular (PT) cells; in addition, the lack of antioxidants enhances the vulnerability to oxidative damage. Despite such predispositions, PT cells show resiliency, and therefore must possess some inherent mechanism to protect from oxidative damage. While the mechanism in unknown, we tested the effect of l-ornithine, since it is abundantly present in PT luminal fluid and can activate Ca²⁺-sensing receptor (CaSR), a GPCR, expressed in the PT luminal membrane. We used human kidney 2 (HK2) cells, a PT cell line, and performed Ca²⁺ imaging and electrophysiological experiments to show that l-ornithine has a concentration-dependent effect on CaSR activation. We further demonstrate that the operation of CaSR activated Ca²⁺ signaling in HK-2 cells mediated by the transient receptor potential canonical (TRPC) dependent receptor-operated Ca²⁺ entry (ROCE) using pharmacological and siRNA inhibitors. Since PT cells are vulnerable to ROS, we simulated such deleterious effects using genetically encoded peroxide-induced ROS production (HyperRed indicator) to show that the l-ornithine-induced ROCE mediated [Ca²⁺]_i signaling protects from ROS production. Furthermore, we performed cell viability, necrosis and apoptosis assays, and mitochondrial oxidative gene expression to establish that presence of l-ornithine rescued the ROS-induced damage in HK-2 cells. Moreover, l-ornithine-activation of CaSR can reverse ROS production and apoptosis via mitogen-activated protein kinase p38 activation. Such nephroprotective role of l-ornithine can be useful as the translational option for reversing kidney diseases involving PT cell damage due to oxidative stress or crystal nephropathies.

Oxytocin induces intracellular Ca2+ release in cardiac fibroblasts from neonatal rats

Pituitary neuropeptide oxytocin is increasingly recognised as a cardiovascular hormone, in addition to its many regulatory roles in other organ systems. Studies in atrial and ventricular myocytes from the neonatal and adult rats have identified synthesis of oxytocin and the expression of oxytocin receptors in these cells. In cardiac fibroblasts, the most populous non-myocyte cell type in mammalian heart, the oxytocin receptors have not been described before. In the present study, we have investigated the direct effects of oxytocin on intracellular Ca²⁺ dynamics in ventricular myocytes and fibroblasts from new born rats. In myocytes, oxytocin increased the frequency of spontaneous Ca²⁺ transients and decreased their amplitude. Our data suggest that oxytocin receptors are also present and functional in the majority of cardiac fibroblasts. We used selective oxytocin receptor inhibitor L-371,257 and a number of intracellular Ca ²⁺ release blockers to investigate the mechanism of oxytocin induced Ca²⁺ signalling in cardiac fibroblasts. Our findings suggest that oxytocin induces Ca²⁺ signals in cardiac fibroblasts by triggering endoplasmic reticulum Ca²⁺ release via inositol trisphosphate activated receptors. The functional significance of the oxytocin induced Ca²⁺ signalling in cardiac fibroblasts, especially for their activation into secretory active myofibroblasts, remains to be investigated.

Effects of 2-Aminoethyl Diphenylborinate, a Modulator of Transient Receptor Potential and Orai Channels in Subarachnoid Hemorrhage: An Experimental Study

BACKGROUND

Cerebral vasospasm remains a serious problem affecting morbidity and mortality in patients with subarachnoid hemorrhage (SAH) during neurosurgery. We aimed to demonstrate the role of the transient receptor potential channel and other channels for Ca²⁺ in the etiology of cerebral vasospasm using 2-aminoethyl diphenylborinate (2-APB) and the effective dose range of an unstudied pharmacological agent, which can limit vasospasm.

METHODS

We performed an experimental study using 32 Sprague-Dawley rats divided into 4 groups: sham group (n = 8), SAH group (n = 8), 2-APB group (SAH rats intraperitoneally administered with 0.5 mg/kg 2-APB; n = 8), and 2-APB-2 group (SAH rats intraperitoneally administered with 2 mg/kg 2-APB; n = 8). The rats were sacrificed after 24 hours, and superoxide dismutase, glutathione peroxidase, malondialdehyde, tumor necrosis factor-α, and interleukin-1β in the brain tissue and serum were measured. The histopathological investigation of brain tissue included measurement of the luminal diameter and wall thickness of the basilar artery (BA), and apoptotic cells in the hippocampus were counted after caspase staining.

RESULTS

Autologous arterial blood injection into the cisterna magna caused vasospasm in rats. 2-APB treatment increased the BA wall thickness and reduced the BA lumen diameter, inducing significant vascular changes. 2-APB also alleviated cell apoptosis at 24 hours after SAH.

CONCLUSION

In experimental SAH in rats, 2-APB treatment increased the BA wall thickness and reduced the BA lumen diameter, inducing significant vascular changes. 2-APB also alleviated cell apoptosis at 24 hours after SAH.

The Trans Golgi Region is a Labile Intracellular Ca2+ Store Sensitive to Emetine

The Golgi apparatus (GA) is a bona fide Ca²⁺ store; however, there is a lack of GA-specific Ca²⁺ mobilizing agents. Here, we report that emetine specifically releases Ca²⁺ from GA in HeLa and HL-1 atrial myocytes. Additionally, it has become evident that the trans-Golgi is a labile Ca²⁺ store that requires a continuous source of Ca²⁺ from either the external milieu or from the ER, to enable it to produce a detectable transient increase in cytosolic Ca²⁺. Our data indicates that the emetine-sensitive Ca²⁺ mobilizing mechanism is different from the two classical Ca²⁺ release mechanisms, i.e. IP₃ and ryanodine receptors. This newly discovered ability of emetine to release Ca²⁺ from the GA may explain why chronic consumption of ipecac syrup has muscle side effects.

Matrine attenuates endoplasmic reticulum stress and mitochondrion dysfunction in nonalcoholic fatty liver disease by regulating SERCA pathway

Background

Endoplasmic reticulum (ER) stress, which can promote lipid metabolism disorders and steatohepatitis, contributes significantly to the pathogenesis of nonalcoholic fatty liver disease (NAFLD). Calcium (Ca²⁺) homeostasis is considered to play a key role in ER stress. Matrine (Mat) has been applied for the treatment of hepatitis B, but its effect on NAFLD is still unknown, and there is no unified view of Mat on the regulation of ER stress in the previous literature.

Methods

The pharmacological effects were studied in high-fat-diet or methionine–choline-deficient diet induced C57BL/6J mice models and in palmitic acid (PA) induced L02 human liver cell model. Calcium fluorescence experiments, computational virtual docking analysis and biochemical assays were used in identifying the locus of Mat.

Results

The results showed that Mat-treated mice were more resistant to steatosis in the liver than vehicle-treated mice and that Mat significantly reduced hepatic inflammation, lipid peroxides. The beneficial effect of Mat was associated with suppressing ER stress and restoring mitochondrial dysfunction. Additionally, Mat decreased the PA-induced lipid accumulation, ER stress and cytosolic calcium level ([Ca²⁺]_c) in hepatocyte cell lines in low and middle dose. However, the high dose Mat did not show satisfactory results in cell model. Calcium fluorescence experiments showed that Mat was able to regulate [Ca²⁺]_c. By computational virtual docking analysis and biochemical assays, Mat was shown to influence [Ca²⁺]_c via direct inhibition of SERCA.

Conclusions

The results showed that the bi-directional regulation of Mat to endoplasmic reticulum at different doses was based on the inhibition of SERCA. In addition, the results also provide a theoretical basis for Mat as a potential therapeutic strategy in NAFLD/NASH.

Electronic supplementary material

The online version of this article (10.1186/s12967-018-1685-2) contains supplementary material, which is available to authorized users.

Endoplasmic Reticulum-Mitochondrial Ca2+ Fluxes Underlying Cancer Cell Survival

Calcium ions (Ca²⁺) are crucial, ubiquitous, intracellular second messengers required for functional mitochondrial metabolism during uncontrolled proliferation of cancer cells. The mitochondria and the endoplasmic reticulum (ER) are connected via “mitochondria-associated ER membranes” (MAMs) where ER–mitochondria Ca²⁺ transfer occurs, impacting the mitochondrial biology related to several aspects of cellular survival, autophagy, metabolism, cell death sensitivity, and metastasis, all cancer hallmarks. Cancer cells appear addicted to these constitutive ER–mitochondrial Ca²⁺ fluxes for their survival, since they drive the tricarboxylic acid cycle and the production of mitochondrial substrates needed for nucleoside synthesis and proper cell cycle progression. In addition to this, the mitochondrial Ca²⁺ uniporter and mitochondrial Ca²⁺ have been linked to hypoxia-inducible factor 1α signaling, enabling metastasis and invasion processes, but they can also contribute to cellular senescence induced by oncogenes and replication. Finally, proper ER–mitochondrial Ca²⁺ transfer seems to be a key event in the cell death response of cancer cells exposed to chemotherapeutics. In this review, we discuss the emerging role of ER–mitochondrial Ca²⁺ fluxes underlying these cancer-related features.