A model for receptor-regulated calcium entry

Alteration of STIM1/Orai1-Mediated SOCE in Skeletal Muscle: Impact in Genetic Muscle Diseases and Beyond

Intracellular Ca2+ ions represent a signaling mediator that plays a critical role in regulating different muscular cellular processes. Ca2+ homeostasis preservation is essential for maintaining skeletal muscle structure and function. Store-operated Ca2+ entry (SOCE), a Ca2+-entry process activated by depletion of intracellular stores contributing to the regulation of various function in many cell types, is pivotal to ensure a proper Ca2+ homeostasis in muscle fibers. It is coordinated by STIM1, the main Ca2+ sensor located in the sarcoplasmic reticulum, and ORAI1 protein, a Ca2+-permeable channel located on transverse tubules. It is commonly accepted that Ca2+ entry via SOCE has the crucial role in short- and long-term muscle function, regulating and adapting many cellular processes including muscle contractility, postnatal development, myofiber phenotype and plasticity. Lack or mutations of STIM1 and/or Orai1 and the consequent SOCE alteration have been associated with serious consequences for muscle function. Importantly, evidence suggests that SOCE alteration can trigger a change of intracellular Ca2+ signaling in skeletal muscle, participating in the pathogenesis of different progressive muscle diseases such as tubular aggregate myopathy, muscular dystrophy, cachexia, and sarcopenia. This review provides a brief overview of the molecular mechanisms underlying STIM1/Orai1-dependent SOCE in skeletal muscle, focusing on how SOCE alteration could contribute to skeletal muscle wasting disorders and on how SOCE components could represent pharmacological targets with high therapeutic potential.

Inhibition of IP3R/Ca2+ Dysregulation Protects Mice From Ventilator-Induced Lung Injury via Endoplasmic Reticulum and Mitochondrial Pathways

Rationale

Disruption of intracellular calcium (Ca2+) homeostasis is implicated in inflammatory responses. Here we investigated endoplasmic reticulum (ER) Ca2+ efflux through the Inositol 1,4,5-trisphosphate receptor (IP3R) as a potential mechanism of inflammatory pathophysiology in a ventilator-induced lung injury (VILI) mouse model.

Methods

C57BL/6 mice were exposed to mechanical ventilation using high tidal volume (HTV). Mice were pretreated with the IP3R agonist carbachol, IP3R inhibitor 2-aminoethoxydiphenyl borate (2-APB) or the Ca2+ chelator BAPTA-AM. Lung tissues and bronchoalveolar lavage fluid (BALF) were collected to measure Ca2+ concentrations, inflammatory responses and mRNA/protein expression associated with ER stress, NLRP3 inflammasome activation and inflammation. Analyses were conducted in concert with cultured murine lung cell lines.

Results

Lungs from mice subjected to HTV displayed upregulated IP3R expression in ER and mitochondrial-associated-membranes (MAMs), with enhanced formation of MAMs. Moreover, HTV disrupted Ca2+ homeostasis, with increased flux from the ER to the cytoplasm and mitochondria. Administration of carbachol aggravated HTV-induced lung injury and inflammation while pretreatment with 2-APB or BAPTA-AM largely prevented these effects. HTV activated the IRE1α and PERK arms of the ER stress signaling response and induced mitochondrial dysfunction-NLRP3 inflammasome activation in an IP3R-dependent manner. Similarly, disruption of IP3R/Ca2+ in MLE12 and RAW264.7 cells using carbachol lead to inflammatory responses, and stimulated ER stress and mitochondrial dysfunction.

Conclusion

Increase in IP3R-mediated Ca2+ release is involved in the inflammatory pathophysiology of VILI via ER stress and mitochondrial dysfunction. Antagonizing IP3R/Ca2+ and/or maintaining Ca2+ homeostasis in lung tissue represents a prospective treatment approach for VILI.

Resonance assignment of coiled-coil 3 (CC3) domain of human STIM1

The protein stromal interaction molecule 1 (STIM1) plays a pivotal role in mediating store-operated calcium entry (SOCE) into cells, which is essential for adaptive immunity. It acts as a calcium sensor in the endoplasmic reticulum (ER) and extends into the cytosol, where it changes from an inactive (tight) to an active (extended) oligomeric form upon calcium store depletion. NMR studies of this protein are challenging due to its membrane-spanning and aggregation properties. Therefore follow the divide-and-conquer approach, focusing on individual domains first is in order. The cytosolic part is predicted to have a large content of coiled-coil (CC) structure. We report the 1H, 13C, 15N chemical shift assignments of the CC3 domain. This domain is crucial for the stabilisation of the tight quiescent form of STIM1 as well as for activating the ORAI calcium channel by direct contact, in the extended active form.

Calcium dysregulation and compensation in cortical pyramidal neurons of the R6/2 mouse model of Huntington's disease

Huntington's disease (HD) is a fatal, hereditary neurodegenerative disorder that predominantly affects striatal medium-sized spiny neurons and cortical pyramidal neurons (CPNs). It has been proposed that perturbations in Ca²⁺ homeostasis could play a role in CPN alterations. To test this hypothesis, we used the R6/2 mouse model of juvenile HD at different stages of disease progression; presymptomatic, early symptomatic, and late symptomatic. We combined whole-cell patch-clamp recordings of layer 2/3 CPNs with two-photon laser scanning microscopy to image somatic and dendritic Ca²⁺ transients associated with evoked action potentials (APs). We found that the amplitude of AP-induced Ca²⁺ transients recorded at the somata of CPNs was significantly reduced in presymptomatic and late symptomatic R6/2 mice compared with wild-type (WT) littermates. However, reduced amplitudes were compensated by increases in decay times, so that Ca²⁺ transient areas were similar between genotypes. AP-induced Ca²⁺ transients in CPN proximal dendrites were variable and differences did not reach statistical significance, except for reduced areas in the late symptomatic group. In late symptomatic mice, a specific store-operated Ca²⁺ channel antagonist, EVP4593, reduced somatic Ca²⁺ transient amplitude similarly in WT and R6/2 CPNs. In contrast, dantrolene, a ryanodine receptor (RyR) antagonist, and nifedipine, an L-type Ca²⁺ channel blocker, significantly reduced both somatic Ca²⁺ transient amplitude and area in R6/2 but not WT CPNs. These findings demonstrate that perturbations of Ca²⁺ homeostasis and compensation occur in CPNs before and after the onset of overt symptoms, and suggest RyRs and L-type Ca²⁺ channels as potential targets for therapeutic intervention.NEW & NOTEWORTHY We used two-photon microscopy to examine calcium influx induced by action potentials in cortical pyramidal neurons from a mouse model of Huntington's disease (HD), the R6/2. The amplitude of somatic calcium transients was reduced in R6/2 mice compared with controls. This reduction was compensated by increased decay times, which could lead to reduced calcium buffering capacity. L-type calcium channel and ryanodine receptor blockers reduced calcium transient area in HD neurons, suggesting new therapeutic avenues.

The interplay between mitochondria and store-operated Ca2+ entry: Emerging insights into cardiac diseases

Store‐operated Ca²⁺ entry (SOCE) machinery, including Orai channels, TRPCs, and STIM1, is key to cellular calcium homeostasis. The following characteristics of mitochondria are involved in the physiological and pathological regulation of cells: mitochondria mediate calcium uptake through calcium uniporters; mitochondria are regulated by mitochondrial dynamic related proteins (OPA1, MFN1/2, and DRP1) and form mitochondrial networks through continuous fission and fusion; mitochondria supply NADH to the electron transport chain through the Krebs cycle to produce ATP; under stress, mitochondria will produce excessive reactive oxygen species to regulate mitochondria‐endoplasmic reticulum interactions and the related signalling pathways. Both SOCE and mitochondria play critical roles in mediating cardiac hypertrophy, diabetic cardiomyopathy, and cardiac ischaemia‐reperfusion injury. All the mitochondrial characteristics mentioned above are determinants of SOCE activity, and vice versa. Ca²⁺ signalling dictates the reciprocal regulation between mitochondria and SOCE under the specific pathological conditions of cardiomyocytes. The coupling of mitochondria and SOCE is essential for various pathophysiological processes in the heart. Herein, we review the research focussing on the reciprocal regulation between mitochondria and SOCE and provide potential interplay patterns in cardiac diseases.

Plasma Membrane and Organellar Targets of STIM1 for Intracellular Calcium Handling in Health and Neurodegenerative Diseases

Located at the level of the endoplasmic reticulum (ER) membrane, stromal interacting molecule 1 (STIM1) undergoes a complex conformational rearrangement after depletion of ER luminal Ca²⁺. Then, STIM1 translocates into discrete ER-plasma membrane (PM) junctions where it directly interacts with and activates plasma membrane Orai1 channels to refill ER with Ca²⁺. Furthermore, Ca²⁺ entry due to Orai1/STIM1 interaction may induce canonical transient receptor potential channel 1 (TRPC1) translocation to the plasma membrane, where it is activated by STIM1. All these events give rise to store-operated calcium entry (SOCE). Besides the main pathway underlying SOCE, which mainly involves Orai1 and TRPC1 activation, STIM1 modulates many other plasma membrane proteins in order to potentiate the influxof Ca²⁺. Furthermore, it is now clear that STIM1 may inhibit Ca²⁺ currents mediated by L-type Ca²⁺ channels. Interestingly, STIM1 also interacts with some intracellular channels and transporters, including nuclear and lysosomal ionic proteins, thus orchestrating organellar Ca²⁺ homeostasis. STIM1 and its partners/effectors are significantly modulated in diverse acute and chronic neurodegenerative conditions. This highlights the importance of further disclosing their cellular functions as they might represent promising molecular targets for neuroprotection.

Store-Operated Calcium Entry in Skeletal Muscle: What Makes It Different?

Current knowledge on store-operated Ca²⁺ entry (SOCE) regarding its localization, kinetics, and regulation is mostly derived from studies performed in non-excitable cells. After a long time of relative disinterest in skeletal muscle SOCE, this mechanism is now recognized as an essential contributor to muscle physiology, as highlighted by the muscle pathologies that are associated with mutations in the SOCE molecules STIM1 and Orai1. This review mainly focuses on the peculiar aspects of skeletal muscle SOCE that differentiate it from its counterpart found in non-excitable cells. This includes questions about SOCE localization and the movement of respective proteins in the highly organized skeletal muscle fibers, as well as the diversity of expressed STIM isoforms and their differential expression between muscle fiber types. The emerging evidence of a phasic SOCE, which is activated during EC coupling, and its physiological implication is described as well. The specific issues related to the use of SOCE modulators in skeletal muscles are discussed. This review highlights the complexity of SOCE activation and its regulation in skeletal muscle, with an emphasis on the most recent findings and the aim to reach a current picture of this mesmerizing phenomenon.

Mechanism of Endoplasmic Reticulum Stress in Cerebral Ischemia

Endoplasmic reticulum (ER) is the main organelle for protein synthesis, trafficking and maintaining intracellular Ca²⁺ homeostasis. The stress response of ER results from the disruption of ER homeostasis in neurological disorders. Among these disorders, cerebral ischemia is a prevalent reason of death and disability in the world. ER stress stemed from ischemic injury initiates unfolded protein response (UPR) regarded as a protection mechanism. Important, disruption of Ca²⁺ homeostasis resulted from cytosolic Ca²⁺ overload and depletion of Ca²⁺ in the lumen of the ER could be a trigger of ER stress and the misfolded protein synthesis. Brain cells including neurons, glial cells and endothelial cells are involved in the complex pathophysiology of ischemic stroke. This is generally important for protein underfolding, but even more for cytosolic Ca²⁺ overload. Mild ER stress promotes cells to break away from danger signals and enter the adaptive procedure with the activation of pro-survival mechanism to rescue ischemic injury, while chronic ER stress generally serves as a detrimental role on nerve cells via triggering diverse pro-apoptotic mechanism. What’s more, the determination of some proteins in UPR during cerebral ischemia to cell fate may have two diametrically opposed results which involves in a specialized set of inflammatory and apoptotic signaling pathways. A reasonable understanding and exploration of the underlying molecular mechanism related to ER stress and cerebral ischemia is a prerequisite for a major breakthrough in stroke treatment in the future. This review focuses on recent findings of the ER stress as well as the progress research of mechanism in ischemic stroke prognosis provide a new treatment idea for recovery of cerebral ischemia.

Stromal interaction molecule 1 (STIM1) knock down attenuates invasion and proliferation and enhances the expression of thyroid-specific proteins in human follicular thyroid cancer cells

Stromal interaction molecule 1 (STIM1) and the ORAI1 calcium channel mediate store-operated calcium entry (SOCE) and regulate a multitude of cellular functions. The identity and function of these proteins in thyroid cancer remain elusive. We show that STIM1 and ORAI1 expression is elevated in thyroid cancer cell lines, compared to primary thyroid cells. Knock-down of STIM1 or ORAI1 attenuated SOCE, reduced invasion, and the expression of promigratory sphingosine 1-phosphate and vascular endothelial growth factor-2 receptors in thyroid cancer ML-1 cells. Cell proliferation was attenuated in these knock-down cells due to increased G1 phase of the cell cycle and enhanced expression of cyclin-dependent kinase inhibitory proteins p21 and p27. STIM1 protein was upregulated in thyroid cancer tissue, compared to normal tissue. Downregulation of STIM1 restored expression of thyroid stimulating hormone receptor, thyroid specific proteins and increased iodine uptake. STIM1 knockdown ML-1 cells were more susceptible to chemotherapeutic drugs, and significantly reduced tumor growth in Zebrafish. Furthermore, STIM1-siRNA-loaded mesoporous polydopamine nanoparticles attenuated invasion and proliferation of ML-1 cells. Taken together, our data suggest that STIM1 is a potential diagnostic and therapeutic target for treatment of thyroid cancer.

TRPM4 links calcium signaling to membrane potential in pancreatic acinar cells

Transient receptor potential cation channel subfamily M member 4 (TRPM4) is a Ca²⁺-activated nonselective cation channel that mediates membrane depolarization. Although, a current with the hallmarks of a TRPM4-mediated current has been previously reported in pancreatic acinar cells (PACs), the role of TRPM4 in the regulation of acinar cell function has not yet been explored. In the present study, we identify this TRPM4 current and describe its role in context of Ca²⁺ signaling of PACs using pharmacological tools and TRPM4-deficient mice. We found a significant Ca²⁺-activated cation current in PACs that was sensitive to the TRPM4 inhibitors 9-phenanthrol and 4-chloro-2-[[2-(2-chlorophenoxy)acetyl]amino]benzoic acid (CBA). We demonstrated that the CBA-sensitive current was responsible for a Ca²⁺-dependent depolarization of PACs from a resting membrane potential of −44.4 ± 2.9 to −27.7 ± 3 mV. Furthermore, we showed that Ca²⁺ influx was higher in the TRPM4 KO- and CBA-treated PACs than in control cells. As hormone-induced repetitive Ca²⁺ transients partially rely on Ca²⁺ influx in PACs, the role of TRPM4 was also assessed on Ca²⁺ oscillations elicited by physiologically relevant concentrations of the cholecystokinin analog cerulein. These data show that the amplitude of Ca²⁺ signals was significantly higher in TRPM4 KO than in control PACs. Our results suggest that PACs are depolarized by TRPM4 currents to an extent that results in a significant reduction of the inward driving force for Ca²⁺. In conclusion, TRPM4 links intracellular Ca²⁺ signaling to membrane potential as a negative feedback regulator of Ca²⁺ entry in PACs.

Investigation of cytotoxic effect of the bufanolide steroid compound cinobufagin and its related underlying mechanism in brain cell models

Cinobufagin, a bufadienolide of toad venom of Bufo bufo gargarizans, is used as a cardiotonic, central nervous system (CNS) respiratory agent, as well as an analgesic and anesthetic. However, several research showed that bufadienolide has a few side effects on the CNS, such as breathlessness or coma. Although cinobufagin was shown to display pharmacological effects in various models, the toxic effect of cinobufagin is elusive in brain cell models. The aim of this study was to explore whether cinobufagin affected viability, Ca²⁺ homeostasis, and reactive oxygen species (ROS) production in Gibco® Human Astrocyte (GHA) and HCN-2 neuronal cell line. In GHA cells but not in HCN-2 cells, cinobufagin (20-60 μM) induced [Ca²⁺ ]_i rises. In terms of cell viability, chelation of cytosolic Ca²⁺ with 1,2-bis(2-aminophenoxy)ethane-N,N,N'N'-tetraacetic acid reduced cinobufagin-induced cytotoxicity in GHA cells. In GHA cells, cinobufagin-induced Ca²⁺ entry was inhibited by 2-aminoethoxydiphenyl borate or SKF96365. In a Ca²⁺ -free medium, treatment with thapsigargin or U73122 abolished cinobufagin-evoked [Ca²⁺ ]_i rises. Furthermore, treatment with N-acetylcysteine reversed ROS production and cytotoxicity in cinobufagin-treated GHA cells. Together, in GHA cells but not in HCN-2 cells cinobufagin caused cytotoxicity that was linked to preceding [Ca²⁺ ]_i rises by Ca²⁺ influx via store-operated Ca²⁺ entry and phospholipase C-dependent Ca²⁺ release from the endoplasmic reticulum. Moreover, cinobufagin induced ROS-associated cytotoxicity.

Regulation of Store-Operated Ca2+ Entry by SARAF

Calcium (Ca²⁺) signaling plays a dichotomous role in cellular biology, controlling cell survival and proliferation on the one hand and cellular toxicity and cell death on the other. Store-operated Ca²⁺ entry (SOCE) by CRAC channels represents a major pathway for Ca²⁺ entry in non-excitable cells. The CRAC channel has two key components, the endoplasmic reticulum Ca²⁺ sensor stromal interaction molecule (STIM) and the plasma-membrane Ca²⁺ channel Orai. Physical coupling between STIM and Orai opens the CRAC channel and the resulting Ca²⁺ flux is regulated by a negative feedback mechanism of slow Ca²⁺ dependent inactivation (SCDI). The identification of the SOCE-associated regulatory factor (SARAF) and investigations of its role in SCDI have led to new functional and molecular insights into how SOCE is controlled. In this review, we provide an overview of the functional and molecular mechanisms underlying SCDI and discuss how the interaction between SARAF, STIM1, and Orai1 shapes Ca²⁺ signaling in cells.

Altered Ca2+ Handling and Oxidative Stress Underlie Mitochondrial Damage and Skeletal Muscle Dysfunction in Aging and Disease

Skeletal muscle contraction relies on both high-fidelity calcium (Ca²⁺) signals and robust capacity for adenosine triphosphate (ATP) generation. Ca²⁺ release units (CRUs) are highly organized junctions between the terminal cisternae of the sarcoplasmic reticulum (SR) and the transverse tubule (T-tubule). CRUs provide the structural framework for rapid elevations in myoplasmic Ca²⁺ during excitation-contraction (EC) coupling, the process whereby depolarization of the T-tubule membrane triggers SR Ca²⁺ release through ryanodine receptor-1 (RyR1) channels. Under conditions of local or global depletion of SR Ca²⁺ stores, store-operated Ca²⁺ entry (SOCE) provides an additional source of Ca²⁺ that originates from the extracellular space. In addition to Ca²⁺, skeletal muscle also requires ATP to both produce force and to replenish SR Ca²⁺ stores. Mitochondria are the principal intracellular organelles responsible for ATP production via aerobic respiration. This review provides a broad overview of the literature supporting a role for impaired Ca²⁺ handling, dysfunctional Ca²⁺-dependent production of reactive oxygen/nitrogen species (ROS/RNS), and structural/functional alterations in CRUs and mitochondria in the loss of muscle mass, reduction in muscle contractility, and increase in muscle damage in sarcopenia and a wide range of muscle disorders including muscular dystrophy, rhabdomyolysis, central core disease, and disuse atrophy. Understanding the impact of these processes on normal muscle function will provide important insights into potential therapeutic targets designed to prevent or reverse muscle dysfunction during aging and disease.

A viroporin-like 2B protein of duck hepatitis A virus 1 that induces incomplete autophagy in DEF cells

Duck hepatitis A virus 1 (DHAV-1) can cause high morbidity and fatal acute infectious hepatitis in ducklings, which seriously endangers animal husbandry. Viroporin is a small molecular weight hydrophobic transmembrane protein encoded by the virus, that has been suggested to induce autophagy in host cells by increasing the membrane permeability through disturbing the ion balance. In this study, we aimed to investigate whether the DHAV-1 2B protein can induce autophagy in DEF cells with a viroporin-like function. Bioinformatics analysis has indicated that the 2B protein is characterized by a viroporin domain, which is consistent with the type IA viroporin transmembrane protein. We experimentally confirmed that the 2B protein disturbed the Ca2+ balance of infected cells by elevating the intracellular Ca2+ concentration. Eukaryotic expression of the 2B protein upregulates the expression of microtubule-associated protein 1 light chain 3 II (LC3-II) and the number of autophagosomes in the cell. Interestingly, the Western Blot (WB) results showed that 2B protein expression induced less protein degradation of the autophagic substrate sequestosome 1 (SQSTM1/p62) than the positive control, while microscopy observations showed that the autophagosomes did not colocalize with the lysosomes. In summary, 2B protein expression induced autophagy in host cells, but the autophagic flow was incomplete. The results of this experiment are expected to provide reference scientific data for elucidating the infective and pathogenic mechanism of DHAV-1.

Improper Remodeling of Organelles Deputed to Ca2+ Handling and Aerobic ATP Production Underlies Muscle Dysfunction in Ageing

Proper skeletal muscle function is controlled by intracellular Ca2+ concentration and by efficient production of energy (ATP), which, in turn, depend on: (a) the release and re-uptake of Ca2+ from sarcoplasmic-reticulum (SR) during excitation-contraction (EC) coupling, which controls the contraction and relaxation of sarcomeres; (b) the uptake of Ca2+ into the mitochondrial matrix, which stimulates aerobic ATP production; and finally (c) the entry of Ca2+ from the extracellular space via store-operated Ca2+ entry (SOCE), a mechanism that is important to limit/delay muscle fatigue. Abnormalities in Ca2+ handling underlie many physio-pathological conditions, including dysfunction in ageing. The specific focus of this review is to discuss the importance of the proper architecture of organelles and membrane systems involved in the mechanisms introduced above for the correct skeletal muscle function. We reviewed the existing literature about EC coupling, mitochondrial Ca2+ uptake, SOCE and about the structural membranes and organelles deputed to those functions and finally, we summarized the data collected in different, but complementary, projects studying changes caused by denervation and ageing to the structure and positioning of those organelles: a. denervation of muscle fibers-an event that contributes, to some degree, to muscle loss in ageing (known as sarcopenia)-causes misplacement and damage: (i) of membrane structures involved in EC coupling (calcium release units, CRUs) and (ii) of the mitochondrial network; b. sedentary ageing causes partial disarray/damage of CRUs and of calcium entry units (CEUs, structures involved in SOCE) and loss/misplacement of mitochondria; c. functional electrical stimulation (FES) and regular exercise promote the rescue/maintenance of the proper architecture of CRUs, CEUs, and of mitochondria in both denervation and ageing. All these structural changes were accompanied by related functional changes, i.e., loss/decay in function caused by denervation and ageing, and improved function following FES or exercise. These data suggest that the integrity and proper disposition of intracellular organelles deputed to Ca2+ handling and aerobic generation of ATP is challenged by inactivity (or reduced activity); modifications in the architecture of these intracellular membrane systems may contribute to muscle dysfunction in ageing and sarcopenia.

Cross-Talk Between the Adenylyl Cyclase/cAMP Pathway and Ca2+ Homeostasis

Cyclic AMP and Ca²⁺ are the first second or intracellular messengers identified, unveiling the cellular mechanisms activated by a plethora of extracellular signals, including hormones. Cyclic AMP generation is catalyzed by adenylyl cyclases (ACs), which convert ATP into cAMP and pyrophosphate. By the way, Ca²⁺, as energy, can neither be created nor be destroyed; Ca²⁺ can only be transported, from one compartment to another, or chelated by a variety of Ca²⁺-binding molecules. The fine regulation of cytosolic concentrations of cAMP and free Ca²⁺ is crucial in cell function and there is an intimate cross-talk between both messengers to fine-tune the cellular responses. Cancer is a multifactorial disease resulting from a combination of genetic and environmental factors. Frequent cases of cAMP and/or Ca²⁺ homeostasis remodeling have been described in cancer cells. In those tumoral cells, cAMP and Ca²⁺ signaling plays a crucial role in the development of hallmarks of cancer, including enhanced proliferation and migration, invasion, apoptosis resistance, or angiogenesis. This review summarizes the cross-talk between the ACs/cAMP and Ca²⁺ intracellular pathways with special attention to the functional and reciprocal regulation between Orai1 and AC8 in normal and cancer cells.

The Orai1-AC8 Interplay: How Breast Cancer Cells Escape from Orai1 Channel Inactivation

The interplay between the Ca2+-sensitive adenylyl cyclase 8 (AC8) and Orai1 channels plays an important role both in the activation of the cAMP/PKA signaling and the modulation of Orai1-dependent Ca2+ signals. AC8 interacts with a N-terminal region that is exclusive to the Orai1 long variant, Orai1α. The interaction between both proteins allows the Ca2+ that enters the cell through Orai1α to activate the generation of cAMP by AC8. Subsequent PKA activation results in Orai1α inactivation by phosphorylation at serine-34, thus shaping Orai1-mediated cellular functions. In breast cancer cells, AC8 plays a relevant role supporting a variety of cancer hallmarks, including proliferation and migration. Breast cancer cells overexpress AC8, which shifts the AC8-Orai1 stoichiometry in favor of the former and leads to the impairment of PKA-dependent Orai1α inactivation. This mechanism contributes to the enhanced SOCE observed in triple-negative breast cancer cells. This review summarizes the functional interaction between AC8 and Orai1α in normal and breast cancer cells and its relevance for different cancer features.

Calcium Channel Protein ORAI1 Mediates TGF-β Induced Epithelial-to-Mesenchymal Transition in Colorectal Cancer Cells

Accumulating evidence suggested that calcium release-activated calcium modulator 1(ORAI1), a key calcium channel pore-forming protein-mediated store-operated Ca2+ entry (SOCE), is associated with human cancer. However, its role in colorectal cancer (CRC) progression has not been well studied. Epithelial-mesenchymal transition (EMT) is a multistep process that occurs during the progression of cancers and is necessary for metastasis of epithelial cancer. Transforming growth factor-β (TGF-β) is a pleiotropic cytokine that has been shown to induce EMT. In this study, we are aimed at exploring the effects of ORAI1 on TGF-β1-induced EMT process in CRC cells. Herein, we confirmed ORAI1 expression was higher in CRC tissues than in adjacent non-cancerous tissues by using immunohistochemical staining and Western blot analysis. Higher ORAI1 expression was associated with more advanced clinical stage, higher incidence of metastasis and shorter overall survival. We compared ORAI1 expression in SW480 and SW620 cells, two CRC cell lines with the same genetic background, but different metastatic potential. We found ORAI1 expression was significantly higher in SW620 cells which exhibited higher EMT characteristics. Furthermore, knockdown of ORAI1 suppressed the EMT of SW620 Cells. After induced the EMT process in SW480 cells with TGF-β1, we found treatment of TGF-β1 showed a significant increase in cell migration along with the loss of E-cadherin and an increase in N-cadherin and Vimentin protein levels. Also, TGF-β1 treatment increased ORAI1 expression and was closely associated with the increase of SOCE. Silencing ORAI1 significantly suppressed Ca2+ entry, reversed the changes of EMT-relevant marks expression induced by TGF-β1, and inhibited TGF-β1-mediated calpain activation and cell migration. Finally, we blocked SOCE with 2-APB (2-Aminoethyl diphenylborinate), a pharmacological inhibitor. Interestingly, 2-APB and sh-ORAI1 both exhibited similar inhibition effects to the SW480 cells. In conclusion, our results demonstrated that ORAI1 could mediate TGF-β-Induced EMT by promoting Ca2+ entry and calpain activity in Colorectal Cancer Cells.

Store-Operated Calcium Channels as Drug Target in Gastroesophageal Cancers

Gastroesophageal cancers, including tumors occurring in esophagus and stomach, usually have poor prognosis and lack effective chemotherapeutic drugs for treatment. The association between dysregulated store-operated calcium entry (SOCE), a key intracellular Ca2+ signaling pathway and gastroesophageal cancers are emerging. This review summarizes the recent advances in understanding the contribution of SOCE-mediated intracellular Ca2+ signaling to gastroesophageal cancers. It assesses the pathophysiological role of each component in SOCE machinery, such as Orais and STIMs in the cancer cell proliferation, migration, and invasion as well as stemness maintenance. Lastly, it discusses efforts towards development of more specific and potent SOCE inhibitors, which may be a new set of chemotherapeutic drugs appearing at the horizon, to provide either targeted therapy or adjuvant treatment to overcome drug resistance for gastroesophageal cancers.

Mechanism of action of a diterpene alkaloid hypaconitine on cytotoxicity and inhibitory effect of BAPTA-AM in HCN-2 neuronal cells

Hypaconitine, a neuromuscular blocker, is a diterpene alkaloid found in the root of Aconitum carmichaelii. Although hypaconitine was shown to affect various physiological responses in neurological models, the effect of hypaconitine on cell viability and the mechanism of its action of Ca2+ handling is elusive in cortical neurons. This study examined whether hypaconitine altered viability and Ca2+ signalling in HCN-2 neuronal cell lines. Cell viability was measured by the cell proliferation reagent (WST-1). Cytosolic Ca2+ concentrations [Ca2+ ]i was measured by the Ca2+ -sensitive fluorescent dye fura-2. In HCN-2 cells, hypaconitine (10-50 μmol/L) induced cytotoxicity and [Ca2+ ]i rises in a concentration-dependent manner. Removal of extracellular Ca2+ partially reduced the hypaconitine's effect on [Ca2+ ]i rises. Furthermore, chelation of cytosolic Ca2+ with BAPTA-AM reduced hypaconitine's cytotoxicity. In Ca2+ -containing medium, hypaconitine-induced Ca2+ entry was inhibited by modulators (2-APB and SKF96365) of store-operated Ca2+ channels and a protein kinase C (PKC) inhibitor (GF109203X). Hypaconitine induced Mn2+ influx indirectly suggesting that hypaconitine evoked Ca2+ entry. In Ca2+ -free medium, treatment with the endoplasmic reticulum Ca2+ pump inhibitor thapsigargin abolished hypaconitine-induced [Ca2+ ]i rises. Conversely, treatment with hypaconitine inhibited thapsigargin-induced [Ca2+ ]i rises. However, inhibition of phospholipase C (PLC) with U73122 did not inhibit hypaconitine-induced [Ca2+ ]i rises. Together, hypaconitine caused cytotoxicity that was linked to preceding [Ca2+ ]i rises by Ca2+ influx via store-operated Ca2+ entry involved PKC regulation and evoking PLC-independent Ca2+ release from the endoplasmic reticulum. Because BAPTA-AM loading only partially reversed hypaconitine-induced cell death, it suggests that hypaconitine induced a second Ca2+ -independent cytotoxicity in HCN-2 cells.

Endogenous Mas-related G-protein-coupled receptor X1 activates and sensitizes TRPA1 in a human model of peripheral nerves

Mas-related G-protein-coupled receptor X1 (MrgprX1) is a human-specific Mrgpr and its expression is restricted to primary sensory neurons. However, its role in nociception and pain signaling pathways is largely unknown. This study aims to investigate a role for MrgprX1 in nociception via interaction with the pain receptor, Transient Receptor Potential Ankyrin 1 (TRPA1), using in-vitro and in-vivo human neuronal models. MrgprX1 protein expression in human trigeminal nociceptors was investigated by the immunolabeling of the dental pulp and cultured peripheral neuronal equivalent (PNE) cells. MrgprX1 receptor signaling was monitored by Fura-2-based Ca²⁺ imaging using PNEs and membrane potential responses were measured using FluoVolt^TM . Immunofluorescent staining revealed MrgprX1 expression in-vivo in dental afferents, which was more intense in inflamed compared to healthy dental pulps. Endogenous MrgprX1 protein expression was confirmed in the in-vitro human PNE model. MrgprX1 receptor signaling and the mechanisms through which it couples to TRPA1 were studied by Ca²⁺ imaging. Results showed that MrgprX1 activates TRPA1 and induces membrane depolarization in a TRPA1 dependent manner. In addition, MrgprX1 sensitizes TRPA1 to agonist stimulation via Protein Kinase C (PKC). The activation and sensitization of TRPA1 by MrgprX1 in a model of human nerves suggests an important role for this receptor in the modulation of nociception.

Understanding doxorubicin associated calcium remodeling during triple-negative breast cancer treatment: an in silico study

Aim:

Triple-negative breast cancer (TNBC) is the most malignant subtype of breast cancer with high heterogeneity, rapid progression, and paucity of treatment options. The most effective chemotherapeutic drug used to treat TNBC is doxorubicin (Doxo) which is an anthracycline antibiotic. However, Doxo treatment alters cytosolic calcium dynamics leading to drug-resistance condition. The aim of this study is to capture the alterations in the activity of various calcium channels and pumps during Doxo treatment and their consequences on cytosolic calcium dynamics that ultimately result in drug resistance.

Methods:

In the present study, a mathematical model is proposed to capture the complex dynamical landscape of intracellular calcium during Doxo treatment. This study provides an insight into Doxo remodeling of calcium dynamics and associated drug-resistance effect. The model was first analyzed analytically and then explored through numerical simulation using techniques like global sensitivity analysis, parameter recalibration, etc.

Results:

The model is used to predict the potential combination therapy for Doxo that can overcome Doxo associated drug resistance. The results show targeting the dysregulated Ca²⁺ channels and pumps might provide efficient chemotherapy in TNBC. It was also observed that the indispensability of calcium influx rate is paramount in the Doxo drug resistance. Finally, three drugs were identified from existing literature that could be used as a combination therapy along with Doxo.

Conclusions:

The investigation highlights the importance of integrating the calcium signaling of various calcium regulating compounds for their effective anti-tumor effects deliverance along with chemotherapeutic agents. The results from this study might provide a new direction to the experimental biologists to explore different combination therapies with Doxo to enhance its anti-tumor effect.

1,2,4-Oxadiazole-Bearing Pyrazoles as Metabolically Stable Modulators of Store-Operated Calcium Entry

Store-operated calcium entry (SOCE) is a pivotal mechanism in calcium homeostasis, and, despite still being under investigation, its dysregulation is known to be associated with severe human disorders. SOCE modulators are therefore needed both as chemical probes and as therapeutic agents. While many small molecules have been described so far, their poor properties in terms of drug-likeness have limited their translation into the clinical practice. In this work, we describe the bioisosteric replacement of the ester moiety in pyrazole derivatives with a 1,2,4-oxadiazole ring as a means to afford a class of modulators with high metabolic stability. Moreover, among our derivatives, a compound able to increase the calcium entry was identified, further enriching the library of available SOCE activators.

ORAI1 Ca2+ Channel as a Therapeutic Target in Pathological Vascular Remodelling

In the adult, vascular smooth muscle cells (VSMC) are normally physiologically quiescent, arranged circumferentially in one or more layers within blood vessel walls. Remodelling of native VSMC to a proliferative state for vascular development, adaptation or repair is driven by platelet-derived growth factor (PDGF). A key effector downstream of PDGF receptors is store-operated calcium entry (SOCE) mediated through the plasma membrane calcium ion channel, ORAI1, which is activated by the endoplasmic reticulum (ER) calcium store sensor, stromal interaction molecule-1 (STIM1). This SOCE was shown to play fundamental roles in the pathological remodelling of VSMC. Exciting transgenic lineage-tracing studies have revealed that the contribution of the phenotypically-modulated VSMC in atherosclerotic plaque formation is more significant than previously appreciated, and growing evidence supports the relevance of ORAI1 signalling in this pathologic remodelling. ORAI1 has also emerged as an attractive potential therapeutic target as it is accessible to extracellular compound inhibition. This is further supported by the progression of several ORAI1 inhibitors into clinical trials. Here we discuss the current knowledge of ORAI1-mediated signalling in pathologic vascular remodelling, particularly in the settings of atherosclerotic cardiovascular diseases (CVDs) and neointimal hyperplasia, and the recent developments in our understanding of the mechanisms by which ORAI1 coordinates VSMC phenotypic remodelling, through the activation of key transcription factor, nuclear factor of activated T-cell (NFAT). In addition, we discuss advances in therapeutic strategies aimed at the ORAI1 target.

Store operated calcium channels in cancer progression

In recent decades cancer emerged as one of the leading causes of death in the developed countries, with some types of cancer contributing to the top 10 causes of death on the list of the World Health Organization. Carcinogenesis, a malignant transformation causing formation of tumors in normal tissues, is associated with changes in the cell cycle caused by suppression of signaling pathways leading to cell death and facilitation of those enhancing proliferation. Further progression of cancer, during which benign tumors acquire more aggressive phenotypes, is characterized by metastatic dissemination through the body driven by augmented motility and invasiveness of cancer cells. All these processes are associated with alterations in calcium homeostasis in cancer cells, which promote their proliferation, motility and invasion, and dissuade cell death or cell cycle arrest. Remodeling of store-operated calcium entry (SOCE), one of the major pathways regulating intracellular Ca²⁺ concentration ([Ca²⁺]_i), manifests a key event in many of these processes. This review systematizes current knowledge on the mechanisms recruiting SOCE-related proteins in carcinogenesis and cancer progression.

Cellular and mitochondrial calcium communication in obstructive lung disorders

Calcium (Ca²⁺) signalling is well known to dictate cellular functioning and fate. In recent years, the accumulation of Ca²⁺ in the mitochondria has emerged as an important factor in Chronic Respiratory Diseases (CRD) such as Asthma and Chronic Obstructive Pulmonary Disease (COPD). Various reports underline an aberrant increase in the intracellular Ca²⁺, leading to mitochondrial ROS generation, and further activation of the apoptotic pathway in these diseases. Mitochondria contribute to Ca²⁺ buffering which in turn regulates mitochondrial metabolism and ATP production. Disruption of this Ca²⁺ balance leads to impaired cellular processes like apoptosis or necrosis and thus contributes to the pathophysiology of airway diseases. This review highlights the key role of cytoplasmic and mitochondrial Ca²⁺ signalling in regulating CRD, such as asthma and COPD. A better understanding of the dysregulation of mitochondrial Ca²⁺ homeostasis in these diseases could provide cues for the development of advanced therapeutic interventions in these diseases.

Novel ORAI1 Mutation Disrupts Channel Trafficking Resulting in Combined Immunodeficiency

Store-operated Ca²⁺ entry (SOCE) represents a predominant Ca²⁺ influx pathway in non-excitable cells. SOCE is required for immune cell activation and is mediated by the plasma membrane (PM) channel ORAI1 and the endoplasmic reticulum (ER) Ca²⁺ sensor STIM1. Mutations in the Orai1 or STIM1 genes abolish SOCE leading to combined immunodeficiency (CID), muscular hypotonia, and anhidrotic ectodermal dysplasia. Here, we identify a novel autosomal recessive mutation in ORAI1 in a child with CID. The patient is homozygous for p.C126R mutation in the second transmembrane domain (TM2) of ORAI1, a region with no previous loss-of-function mutations. SOCE is suppressed in the patient’s lymphocytes, which is associated with impaired T cell proliferation and cytokine production. Functional analyses demonstrate that the p.C126R mutation does not alter protein expression but disrupts ORAI1 trafficking. Orai1-C126R does not insert properly into the bilayer resulting in ER retention. Insertion of an Arg on the opposite face of TM2 (L135R) also results in defective folding and trafficking. We conclude that positive side chains within ORAI1 TM2 are not tolerated and result in misfolding, defective bilayer insertion, and channel trafficking thus abolishing SOCE and resulting in CID.

SOCE in the cardiomyocyte: the secret is in the chambers

Store-operated Ca²⁺ entry (SOCE) is an ancient and ubiquitous Ca²⁺ signaling pathway that is present in virtually every cell type. Over the last two decades, many studies have implicated this non-voltage dependent Ca²⁺ entry pathway in cardiac physiology. The relevance of the SOCE pathway in cardiomyocytes is often questioned given the well-established role for excitation contraction coupling. In this review, we consider the evidence that STIM1 and SOCE contribute to Ca²⁺ dynamics in cardiomyocytes. We discuss the relevance of this pathway to cardiac growth in response to developmental and pathologic cues. We also address whether STIM1 contributes to Ca²⁺ store refilling that likely impacts cardiac pacemaking and arrhythmogenesis in cardiomyocytes.

Mitochondrial metabolism and calcium homeostasis in the development of NAFLD leading to hepatocellular carcinoma

Non-alcoholic fatty liver disease (NAFLD) is a metabolic syndrome characterized by excessive accumulation of hepatic lipid droplets. The disease progresses with steatosis as the premise for hepatocytic damage and tissue scarring, often culminating in hepatocellular carcinoma (HCC). Perturbations in mitochondrial metabolism and energetics were found to be associated with, and often instrumental in various stages of this progression. Functional impairment of the mitochondria affects all aspects of cellular functioning and a particularly important one is calcium signalling. Changes in mitochondrial calcium specifically in hepatocytes of a fatty liver, is reflected by alterations in calcium signalling as well as calcium transporter activities. This deranged Ca²⁺ homeostasis aids in even more uptake of lipids into the mitochondria and a shift in equilibrium, both metabolically as well as in terms of energy production, leading to completely altered cellular states. These alterations have been reviewed as a perspective to understand the disease progression through NAFLD leading to HCC.

Endoplasmic Reticulum-Plasma Membrane Contact Sites: Regulators, Mechanisms, and Physiological Functions

The endoplasmic reticulum (ER) forms direct membrane contact sites with the plasma membrane (PM) in eukaryotic cells. These ER-PM contact sites play essential roles in lipid homeostasis, ion dynamics, and cell signaling, which are carried out by protein-protein or protein-lipid interactions. Distinct tethering factors dynamically control the architecture of ER-PM junctions in response to intracellular signals or external stimuli. The physiological roles of ER-PM contact sites are dependent on a variety of regulators that individually or cooperatively perform functions in diverse cellular processes. This review focuses on proteins functioning at ER-PM contact sites and highlights the recent progress in their mechanisms and physiological roles.

Assisted oocyte activation effects on the morphokinetic pattern of derived embryos

OBJECTIVE

Assisted oocyte activation (AOA) can restore fertilization rates after IVF/ICSI cycles with fertilization failure. AOA is an experimental technique, and its downstream effects remain poorly characterized. Clarifying the relationship between AOA and embryo, morphokinetics could offer complementary insights into the quality and viability of the embryos obtained with this technique. The aim of this study is to compare the preimplantation morphokinetic development of embryos derived from ICSI-AOA (experimental group) vs. ICSI cycles (control group).

METHODS

A retrospective cohort study was carried out with 141 embryos from fresh oocyte donation cycles performed between 2013 and 2017; 41 embryos were derived from 7 ICSI-AOA cycles and 100 embryos from 18 ICSI cycles. Morphokinetic development of all embryos was followed using a time-lapse system.

RESULTS

We show that embryos from both groups develop similarly for most milestones, with the exception of the time of second polar body extrusion (tPB2) and the time to second cell division (t3).

CONCLUSIONS

We conclude that ionomycin mediated AOA does not seem to affect the morphokinetic pattern of preimplantation embryo development, despite the alterations found in tPB2 and t3, which could directly reflect the use of a Ca2+ ionophore as a transient and quick non-physiologic increase of free intracytoplasmic Ca2+.

Interhelical interactions within the STIM1 CC1 domain modulate CRAC channel activation

The calcium release activated calcium channel is activated by the endoplasmic reticulum-resident calcium sensor protein STIM1. On activation, STIM1 C terminus changes from an inactive, tight to an active, extended conformation. A coiled-coil clamp involving the CC1 and CC3 domains is essential in controlling STIM1 activation, with CC1 as the key entity. The nuclear magnetic resonance-derived solution structure of the CC1 domain represents a three-helix bundle stabilized by interhelical contacts, which are absent in the Stormorken disease-related STIM1 R304W mutant. Two interhelical sites between the CC1α1 and CC1α2 helices are key in controlling STIM1 activation, affecting the balance between tight and extended conformations. Nuclear magnetic resonance-directed mutations within these interhelical interactions restore the physiological, store-dependent activation behavior of the gain-of-function STIM1 R304W mutant. This study reveals the functional impact of interhelical interactions within the CC1 domain for modifying the CC1-CC3 clamp strength to control the activation of STIM1.

Mechanism of a methylxanthine drug theophylline-induced Ca2+ signaling and cytotoxicity in AML12 mouse hepatocytes

Theophylline is a methylxanthine drug used in therapy for respiratory diseases. However, the impact of theophylline on Ca²⁺ signaling has not been explored in liver cells. This study examined whether theophylline affected Ca²⁺ homeostasis and its related cytotoxicity in AML12 mouse hepatocytes. Cell viability was measured by the cell viability reagent (WST-1). Cytosolic Ca²⁺ concentration ([Ca²⁺]_i) was measured by the Ca²⁺-sensitive fluorescent dye fura-2. Theophylline (25-125 μM) induced [Ca²⁺]_i rises and cause cytotoxicity in AML12 cells. This cytotoxic response was reversed by chelation of cytosolic Ca²⁺ with BAPTA/AM. In Ca²⁺-free medium, treatment with the endoplasmic reticulum Ca²⁺ pump inhibitor thapsigargin abolished theophylline-induced [Ca²⁺]_i rises. Conversely, treatment with theophylline also abolished thapsigargin-induced [Ca²⁺]_i rises. However, inhibition of PLC failed to alter theophylline-evoked [Ca²⁺]_i rises. In Ca²⁺-containing medium, modulators of store-operated Ca²⁺ channels inhibited 30% of the [Ca²⁺]_i rises, whereas the PKC modulators had no effect. Furthermore, theophylline-induced Ca²⁺ influx was confirmed by Mn²⁺-induced quench of fura-2 fluorescence. Together, in AML12 cells, theophylline caused Ca²⁺-associated cytotoxicity and induced Ca²⁺ entry through PLC-independent Ca²⁺ release from the endoplasmic reticulum and PKC-insensitive store-operated Ca²⁺ channels. BAPTA-AM with its protective effects may be a potential compound for prevention of theophylline-induced cytotoxicity.

Relevance of Membrane Contact Sites in Cancer Progression

Membrane contact sites (MCS) are typically defined as areas of proximity between heterologous or homologous membranes characterized by specific proteins. The study of MCS is considered as an emergent field that shows how crucial organelle interactions are in cell physiology. MCS regulate a myriad of physiological processes such as apoptosis, calcium, and lipid signaling, just to name a few. The membranal interactions between the endoplasmic reticulum (ER)–mitochondria, the ER–plasma membrane, and the vesicular traffic have received special attention in recent years, particularly in cancer research, in which it has been proposed that MCS regulate tumor metabolism and fate, contributing to their progression. However, as the therapeutic or diagnostic potential of MCS has not been fully revisited, in this review, we provide recent information on MCS relevance on calcium and lipid signaling in cancer cells and on its role in tumor progression. We also describe some proteins associated with MCS, like CERT, STIM1, VDAC, and Orai, that impact on cancer progression and that could be a possible diagnostic marker. Overall, these information might contribute to the understanding of the complex biology of cancer cells.

Long-Term Exercise Reduces Formation of Tubular Aggregates and Promotes Maintenance of Ca2+ Entry Units in Aged Muscle

Tubular aggregates (TAs) in skeletal muscle fibers are unusual accumulation of sarcoplasmic reticulum (SR) tubes that are found in different disorders including TA myopathy (TAM). TAM is a muscular disease characterized by muscle pain, cramping, and weakness that has been recently linked to mutations in STIM1 and ORAI1. STIM1 and ORAI1 are the two main proteins mediating store-operated Ca2+ entry (SOCE), a mechanism activated by depletion of intracellular Ca2+ stores (e.g., SR) that allows recovery of Ca2+ from the extracellular space during repetitive muscle activity. We have recently shown that exercise triggers the formation of unique intracellular junctions between SR and transverse tubules named Ca 2+ entry units (CEUs). CEUs promote colocalization of STIM1 with ORAI1 and improve muscle function in presence of external Ca2+. TAs virtually identical to those of TAM patients are also found in fast-twitch fibers of aging male mice. Here, we used a combination of electron and confocal microscopy, Western blotting, and ex vivo stimulation protocols (in presence or absence of external Ca2+) to evaluate the presence of TAs, STIM1-ORAI1 localization and expression and fatigue resistance of intact extensor digitorum longus (EDL) muscles in wild-type male adult (4-month-old) and aged (24-month-old) mice and in mice trained in wheel cages for 15 months (from 9 to 24 months of age). The results collected indicate that (i) aging causes STIM1 and ORAI1 to accumulate in TAs and (ii) long-term exercise significantly reduced formation of TAs. In addition, (iii) EDL muscles from aged mice exhibited a faster decay of contractile force than adult muscles, likely caused by their inability to refill intracellular Ca2+ stores, and (iv) exercise in wheel cages restored the capability of aged EDL muscles to use external Ca2+ by promoting maintenance of CEUs. In conclusion, exercise prevented improper accumulation of STIM1 and ORAI1 in TAs during aging, maintaining the capability of aged muscle to refill intracellular Ca2+ stores via SOCE.

Intracellular Ca2+ Signaling in Protozoan Parasites: An Overview with a Focus on Mitochondria

Ca²⁺ signaling has been involved in controling critical cellular functions such as activation of proteases, cell death, and cell cycle control. The endoplasmatic reticulum plays a significant role in Ca²⁺ storage inside the cell, but mitochondria have long been recognized as a fundamental Ca²⁺ pool. Protozoan parasites such as Plasmodium falciparum, Toxoplasma gondii, and Trypanosoma cruzi display a Ca²⁺ signaling toolkit with similarities to higher eukaryotes, including the participation of mitochondria in Ca²⁺-dependent signaling events. This review summarizes the most recent knowledge in mitochondrial Ca²⁺ signaling in protozoan parasites, focusing on the mechanism involved in mitochondrial Ca²⁺ uptake by pathogenic protists.

A Novel Modulator of STIM2-Dependent Store-Operated Ca2+ Channel Activity

Store-operated Ca2+ entry is one of the main pathways of calcium influx into non-excitable cells, which entails the initiation of many intracellular processes. The endoplasmic reticulum Ca2+ sensors STIM1 and STIM2 are the key components of store-operated Ca2+ entry in mammalian cells. Under physiological conditions, STIM proteins are responsible for store-operated Ca2+ entry activation. The STIM1 and STIM2 proteins differ in their potency for activating different store-operated channels. At the moment, there are no selective modulators of the STIM protein activity. We screened a library of small molecules and found the 4-MPTC compound, which selectively inhibited STIM2-dependent store-operated Ca2+ entry (IC50 = 1 μM) and had almost no effect on the STIM1-dependent activation of store-operated channels.

Cellular and molecular effects of hyperglycemia on ion channels in vascular smooth muscle

Diabetes affects millions of people worldwide. This devastating disease dramatically increases the risk of developing cardiovascular disorders. A hallmark metabolic abnormality in diabetes is hyperglycemia, which contributes to the pathogenesis of cardiovascular complications. These cardiovascular complications are, at least in part, related to hyperglycemia-induced molecular and cellular changes in the cells making up blood vessels. Whereas the mechanisms mediating endothelial dysfunction during hyperglycemia have been extensively examined, much less is known about how hyperglycemia impacts vascular smooth muscle function. Vascular smooth muscle function is exquisitely regulated by many ion channels, including several members of the potassium (K+) channel superfamily and voltage-gated L-type Ca2+ channels. Modulation of vascular smooth muscle ion channels function by hyperglycemia is emerging as a key contributor to vascular dysfunction in diabetes. In this review, we summarize the current understanding of how diabetic hyperglycemia modulates the activity of these ion channels in vascular smooth muscle. We examine underlying mechanisms, general properties, and physiological relevance in the context of myogenic tone and vascular reactivity.

Physiological and Pathological Relevance of Selective and Nonselective Ca2+ Channels in Skeletal and Cardiac Muscle

Contraction of the striated muscle is fundamental for human existence. The action of voluntary skeletal muscle enables activities such as breathing, establishing body posture, and diverse body movements. Additionally, highly precise motion empowers communication, artistic expression, and other activities that define everyday human life. The involuntary contraction of striated muscle is the core function of the heart and is essential for blood flow. Several ion channels are important in the transduction of action potentials to cytosolic Ca2+ signals that enable muscle contraction; however, other ion channels are involved in the progression of muscle pathologies that can impair normal life or threaten it. This chapter describes types of selective and nonselective Ca2+ permeable ion channels expressed in the striated muscle, their participation in different aspects of muscle excitation and contraction, and their relevance to the progression of some pathological states.

STIM Proteins: An Ever-Expanding Family

Stromal interaction molecules (STIM) are a distinct class of ubiquitously expressed single-pass transmembrane proteins in the endoplasmic reticulum (ER) membrane. Together with Orai ion channels in the plasma membrane (PM), they form the molecular basis of the calcium release-activated calcium (CRAC) channel. An intracellular signaling pathway known as store-operated calcium entry (SOCE) is critically dependent on the CRAC channel. The SOCE pathway is activated by the ligand-induced depletion of the ER calcium store. STIM proteins, acting as calcium sensors, subsequently sense this depletion and activate Orai ion channels via direct physical interaction to allow the influx of calcium ions for store refilling and downstream signaling processes. This review article is dedicated to the latest advances in the field of STIM proteins. New results of ongoing investigations based on the recently published functional data as well as structural data from nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations are reported and complemented with a discussion of the latest developments in the research of STIM protein isoforms and their differential functions in regulating SOCE.

A calcium optimum for cytotoxic T lymphocyte and natural killer cell cytotoxicity

Cytotoxic T lymphocytes (CTL) and natural killer (NK) cells are required for host defense. They destroy malignant target cells like cancer cells. Among metal cations, Ca²⁺ plays a prescinded role for CTL and NK cytotoxicity as it is the only cation used as ubiquitous second messenger. Measuring intracellular Ca²⁺ concentrations [Ca²⁺]_int in single cells has greatly changed our understanding of Ca²⁺ signaling. Yet, comparing the role of Ca²⁺ in the pre-[Ca²⁺]_int and [Ca²⁺]_int measurement era reveals that even in the pre-[Ca²⁺]_int measurement era (before 1980), the functions of Ca²⁺ and some other metal cations for the cytotoxic immune response were well established. It was even shown that Ca²⁺ influx across the plasma membrane but not Ca²⁺ release from intracellular sources is relevant for lymphocyte cytotoxicity and that very little Ca²⁺ is needed for efficient lymphocyte cytotoxicity against cancer cells. In the [Ca²⁺]_int measurement era after 1980, many of the important findings were better and more quantitatively refined and in addition the molecules important for Ca²⁺ transport were defined. The unexpected finding that there is a Ca²⁺ optimum of CTL and NK cell cytotoxicity deserves some attention and may be important for anti-cancer therapy.

Altered Ca2+ Homeostasis in Immune Cells during Aging: Role of Ion Channels

Aging is an unstoppable process and begins shortly after birth. Each cell of the organism is affected by the irreversible process, not only with equal density but also at varying ages and with different speed. Therefore, aging can also be understood as an adaptation to a continually changing cellular environment. One of these very prominent changes in age affects Ca²⁺ signaling. Especially immune cells highly rely on Ca²⁺-dependent processes and a strictly regulated Ca²⁺ homeostasis. The intricate patterns of impaired immune cell function may represent a deficit or compensatory mechanisms. Besides, altered immune function through Ca²⁺ signaling can profoundly affect the development of age-related disease. This review attempts to summarize changes in Ca²⁺ signaling due to channels and receptors in T cells and beyond in the context of aging.

Dysregulation of Neuronal Calcium Signaling via Store-Operated Channels in Huntington's Disease

Huntington's disease (HD) is a progressive neurodegenerative disorder that is characterized by motor, cognitive, and psychiatric problems. It is caused by a polyglutamine expansion in the huntingtin protein that leads to striatal degeneration via the transcriptional dysregulation of several genes, including genes that are involved in the calcium (Ca²⁺) signalosome. Recent research has shown that one of the major Ca²⁺ signaling pathways, store-operated Ca²⁺ entry (SOCE), is significantly elevated in HD. SOCE refers to Ca²⁺ flow into cells in response to the depletion of endoplasmic reticulum Ca²⁺ stores. The dysregulation of Ca²⁺ homeostasis is postulated to be a cause of HD progression because the SOCE pathway is indirectly and abnormally activated by mutant huntingtin (HTT) in γ-aminobutyric acid (GABA)ergic medium spiny neurons (MSNs) from the striatum in HD models before the first symptoms of the disease appear. The present review summarizes recent studies that revealed a relationship between HD pathology and elevations of SOCE in different models of HD, including YAC128 mice (a transgenic model of HD), cellular HD models, and induced pluripotent stem cell (iPSC)-based GABAergic medium spiny neurons (MSNs) that are obtained from adult HD patient fibroblasts. SOCE in MSNs was shown to be mediated by currents through at least two different channel groups, Ca²⁺ release-activated Ca²⁺ current (I_CRAC) and store-operated Ca²⁺ current (I_SOC), which are composed of stromal interaction molecule (STIM) proteins and Orai or transient receptor potential channel (TRPC) channels. Their role under physiological and pathological conditions in HD are discussed. The role of Huntingtin-associated protein 1 isoform A in elevations of SOCE in HD MSNs and potential compounds that may stabilize elevations of SOCE in HD are also summarized. Evidence is presented that shows that the dysregulation of molecular components of SOCE or pathways upstream of SOCE in HD MSN neurons is a hallmark of HD, and these changes could lead to HD pathology, making them potential therapeutic targets.

Synthesis and Characterization of Store-Operated Calcium Entry Inhibitors Active in the Submicromolar Range

The store-operated calcium entry, better known as SOCE, forms the main Ca²⁺ influx pathway in non-excitable cells, especially in leukocytes, where it is required for cell activation and the immune response. During the past decades, several inhibitors were developed, but they lack specificity or efficacy. From the non-specific SOCE inhibitor 2-aminoethyl diphenylborinate (2-APB), we synthetized 16 new analogues by replacing/modifying the phenyl groups. Among them, our compound P11 showed the best inhibitory capacity with a K_i ≈ 75 nM. Furthermore, below 1 µM, P11 was devoid of any inhibitory activity on the two other main cellular targets of 2-APB, the IP₃ receptors, and the SERCA pumps. Interestingly, Jurkat T cells secrete interleukin-2 under phytohemagglutinin stimulation but undergo cell death and stop IL-2 synthesis when stimulated in the presence of increasing P11 concentrations. Thus, P11 could represent the first member of a new and potent family of immunosuppressors.

Store-Operated Calcium Entry as a Therapeutic Target in Acute Pancreatitis: Discovery and Development of Drug-Like SOCE Inhibitors

Store-operated calcium entry (SOCE) is important in the maintenance of calcium homeostasis and alterations in this mechanism are responsible for several pathological conditions, including acute pancreatitis. Since the discovery of SOCE, many inhibitors have been identified and extensively used as chemical probes to better elucidate the role played by this cellular mechanism. Nevertheless, only a few have demonstrated drug-like properties so far. Here, we report a class of biphenyl triazoles among which stands out a lead compound, 34, that is endowed with an inhibitory activity at nanomolar concentrations, suitable pharmacokinetic properties, and in vivo efficacy in a mouse model of acute pancreatitis.

Store-operated calcium entry: Pivotal roles in renal physiology and pathophysiology

Research conducted over the last two decades has dramatically advanced the understanding of store-operated calcium channels (SOCC) and their impact on renal function. Kidneys contain many types of cells, including those specialized for glomerular filtration (fenestrated capillary endothelium, podocytes), water and solute transport (tubular epithelium), and regulation of glomerular filtration and renal blood flow (vascular smooth muscle cells, mesangial cells). The highly integrated function of these myriad cells effects renal control of blood pressure, extracellular fluid volume and osmolality, electrolyte balance, and acid-base homeostasis. Many of these cells are regulated by Ca²⁺ signaling. Recent evidence demonstrates that SOCCs are major Ca²⁺ entry portals in several renal cell types. SOCC is activated by depletion of Ca²⁺ stores in the sarco/endoplasmic reticulum, which communicates with plasma membrane SOCC via the Ca²⁺ sensor Stromal Interaction Molecule 1 (STIM1). Orai1 is recognized as the main pore-forming subunit of SOCC in the plasma membrane. Orai proteins alone can form highly Ca²⁺ selective SOCC channels. Also, members of the Transient Receptor Potential Canonical (TRPC) channel family are proposed to form heteromeric complexes with Orai1 subunits, forming SOCC with low Ca²⁺ selectivity. Recently, Ca²⁺ entry through SOCC, known as store-operated Ca²⁺ entry (SOCE), was identified in glomerular mesangial cells, tubular epithelium, and renovascular smooth muscle cells. The physiological and pathological relevance and the characterization of SOCC complexes in those cells are still unclear. In this review, we summarize the current knowledge of SOCC and their roles in renal glomerular, tubular and vascular cells, including studies from our laboratory, emphasizing SOCE regulation of fibrotic protein deposition. Understanding the diverse roles of SOCE in different renal cell types is essential, as SOCC and its signaling pathways are emerging targets for treatment of SOCE-related diseases.

Store-Operated Calcium Channels in Physiological and Pathological States of the Nervous System

Store-operated calcium channels (SOCs) are widely expressed in excitatory and non-excitatory cells where they mediate significant store-operated calcium entry (SOCE), an important pathway for calcium signaling throughout the body. While the activity of SOCs has been well studied in non-excitable cells, attention has turned to their role in neurons and glia in recent years. In particular, the role of SOCs in the nervous system has been extensively investigated, with links to their dysregulation found in a wide variety of neurological diseases from Alzheimer’s disease (AD) to pain. In this review, we provide an overview of their molecular components, expression, and physiological role in the nervous system and describe how the dysregulation of those roles could potentially lead to various neurological disorders. Although further studies are still needed to understand how SOCs are activated under physiological conditions and how they are linked to pathological states, growing evidence indicates that SOCs are important players in neurological disorders and could be potential new targets for therapies. While the role of SOCE in the nervous system continues to be multifaceted and controversial, the study of SOCs provides a potentially fruitful avenue into better understanding the nervous system and its pathologies.

Triclosan disrupts immune cell function by depressing Ca2+ influx following acidification of the cytoplasm

Triclosan (TCS) is an antimicrobial agent that was effectively banned by the FDA from hand soaps in 2016, hospital soaps in 2017, and hand sanitizers in 2019; however, TCS can still be found in a few products. At consumer-relevant, non-cytotoxic doses, TCS inhibits the functions of both mitochondria and mast cells, a ubiquitous cell type. Via the store-operated Ca2+ entry mechanism utilized by many immune cells, mast cells undergo antigen-stimulated Ca2+ influx into the cytosol, for proper function. Previous work showed that TCS inhibits Ca2+ dynamics in mast cells, and here we show that TCS also inhibits Ca2+ mobilization in human Jurkat T cells. However, the biochemical mechanism behind the Ca2+ dampening has yet to be elucidated. Three-dimensional super-resolution microscopy reveals that TCS induces mitochondrial swelling, in line with and extending the previous finding of TCS inhibition of mitochondrial membrane potential via its proton ionophoric activity. Inhibition of plasma membrane potential (PMP) by the canonical depolarizer gramicidin can inhibit mast cell function. However, use of the genetically encoded voltage indicators (GEVIs) ArcLight (pH-sensitive) and ASAP2 (pH-insensitive), indicates that TCS does not disrupt PMP. In conjunction with data from a plasma membrane-localized, pH-sensitive reporter, these results indicate that TCS, instead, induces cytosolic acidification in mast cells and T cells. Acidification of the cytosol likely inhibits Ca2+ influx by uncoupling the STIM1/ORAI1 interaction that is required for opening of plasma membrane Ca2+ channels. These results provide a mechanistic explanation of TCS disruption of Ca2+ influx and, thus, of immune cell function.

Calmodulin Binding Proteins and Alzheimer's Disease: Biomarkers, Regulatory Enzymes and Receptors That Are Regulated by Calmodulin

The integral role of calmodulin in the amyloid pathway and neurofibrillary tangle formation in Alzheimer’s disease was first established leading to the “Calmodulin Hypothesis”. Continued research has extended our insight into the central function of the small calcium sensor and effector calmodulin and its target proteins in a multitude of other events associated with the onset and progression of this devastating neurodegenerative disease. Calmodulin’s involvement in the contrasting roles of calcium/CaM-dependent kinase II (CaMKII) and calcineurin (CaN) in long term potentiation and depression, respectively, and memory impairment and neurodegeneration are updated. The functions of the proposed neuronal biomarker neurogranin, a calmodulin binding protein also involved in long term potentiation and depression, is detailed. In addition, new discoveries into calmodulin’s role in regulating glutamate receptors (mGluR, NMDAR) are overviewed. The interplay between calmodulin and amyloid beta in the regulation of PMCA and ryanodine receptors are prime examples of how the buildup of classic biomarkers can underly the signs and symptoms of Alzheimer’s. The role of calmodulin in the function of stromal interaction molecule 2 (STIM2) and adenosine A2A receptor, two other proteins linked to neurodegenerative events, is discussed. Prior to concluding, an analysis of how targeting calmodulin and its binding proteins are viable routes for Alzheimer’s therapy is presented. In total, calmodulin and its binding proteins are further revealed to be central to the onset and progression of Alzheimer’s disease.