Blocking Orai1 constitutive activity inhibits B-cell cancer migration and synergistically acts with drugs to reduce B-CLL cell survival

Orai1 channel capacity to control store-operated Ca2+ entry (SOCE) and B-cell functions is poorly understood and more specifically in B-cell cancers, including human lymphoma and leukemia. As compared to normal B-cells, Orai1 is overexpressed in B-chronic lymphocytic leukemia (B-CLL) and contributes in resting B-CLL to mediate an elevated basal Ca2+ level through a constitutive Ca2+ entry, and in BCR-activated B-cell to regulate the Ca2+ signaling response. Such observations were confirmed in human B-cell lymphoma and leukemia lines, including RAMOS, JOK-1, MEC-1 and JVM-3 cells. Next, the use of pharmacological Orai1 inhibitors (GSK-7975 A and Synta66) blocks constitutive Ca2+ entry and in turn affects B-cell cancer (primary and cell lines) survival and migration, controls cell cycle, and induces apoptosis through a mitochondrial and caspase-3 independent pathway. Finally, the added value of Orai1 inhibitors in combination with B-CLL drugs (ibrutinib, idelalisib, rituximab, and venetoclax) on B-CLL survival was tested, showing an additive/synergistic effect including in the B-cell cancer lines. To conclude, this study highlights the pathophysiological role of the Ca2+ channel Orai1 in B-cell cancers, and pave the way for the use of ORAI1 modulators as a plausible therapeutic strategy.

Ip3r-Grp75-Vdac and relevant Ca2+ signaling regulate dietary palmitic acid-induced de novo lipogenesis by mitochondria-associated ER membrane (MAM) recruiting Seipin in yellow catfish

BACKGROUND

The mitochondria-associated endoplasmic reticulum membrane membranes (MAM), is the central hub for endoplasmic reticulum and mitochondria functional communicate. It plays a crucial role in hepatic lipid homeostasis. However, even though MAM has been acknowledged to be enriched in enzymes that contribute to lipid biosynthesis, no study has yet investigated the exact role of MAM on hepatic neutral lipid synthesis.

OBJECTIVES

To address these gaps, this study investigated the systemic control mechanisms for MAM in neutral lipids synthesis by recruiting Seipin, focusing on the role of the Ip3r-Grp75-Vdac complex and their relevant Ca2+ signaling in this process.

METHODS

To this end, a model animal for lipid metabolism, yellow catfish (Pelteobagrus fulvidraco), were fed six different diets containing a range of palmitic acid (PA) concentrations from 0 g/kg to 150 g/kg in vivo for 10 weeks. In vitro experiments were also conducted to intercept the MAM-mediated Ca2+ signaling in isolated hepatocytes by transfecting them with si-mitochondrial calcium uniporter (mcu). Since mcu was resided in the inner mitochondrial membrane (IMM), si-mcu cannot disrupt MAM's structural integrity.

RESULTS

1. Hepatocellular MAM sub-proteome analysis indicated excessive dietary PA intake enhanced hepatic MAM structural join by activating Ip3r-Grp75-voltage-dependent anion channel (Vdac) complexes. 2. Dietary PA intake induced hepatic neutral lipid accumulation through MAM recruiting Seipin, which activated lipid droplet biogenesis. Our findings also revealed a previously unidentified mechanism whereby MAM recruited Seipin and controlled hepatic lipid homeostasis, depending on Ip3r-Grp75-Vdac-controlled Ca2+ signaling but not only MAM's structural integrity.

CONCLUSIONS

These results offer a novel insight into the MAM-recruited Seipin in controlling hepatic lipid synthesis in a MAM structural integrity-dependent and Ca2+ signaling-dependent manner, highlighting the critical contribution of MAM in maintaining hepatic neutral lipid homeostasis.

Altered GM1 catabolism affects NMDAR-mediated Ca2+ signaling at ER-PM junctions and increases synaptic spine formation in a GM1-gangliosidosis model

Endoplasmic reticulum-plasma membrane (ER-PM) junctions mediate Ca2+ flux across neuronal membranes. The properties of these membrane contact sites are defined by their lipid content, but little attention has been given to glycosphingolipids (GSLs). Here, we show that GM1-ganglioside, an abundant GSL in neuronal membranes, is integral to ER-PM junctions; it interacts with synaptic proteins/receptors and regulates Ca2+ signaling. In a model of the neurodegenerative lysosomal storage disease, GM1-gangliosidosis, pathogenic accumulation of GM1 at ER-PM junctions due to β-galactosidase deficiency drastically alters neuronal Ca2+ homeostasis. Mechanistically, we show that GM1 interacts with the phosphorylated N-methyl D-aspartate receptor (NMDAR) Ca2+ channel, thereby increasing Ca2+ flux, activating extracellular signal-regulated kinase (ERK) signaling, and increasing the number of synaptic spines without increasing synaptic connectivity. Thus, GM1 clustering at ER-PM junctions alters synaptic plasticity and worsens the generalized neuronal cell death characteristic of GM1-gangliosidosis.

Docosahexaenoic and Eicosapentaenoic Acids Promote the Accumulation of Browning-Related Myokines via Calcium Signaling in Insulin-Resistant Mice

BACKGROUND

Myokines have a prominent effect on improving insulin resistance (IR) by inducing browning of white adipose tissue (WAT). Although docosahexaenoic acids (DHA) and eicosapentaenoic acids (EPA) play roles in improving IR and stimulating browning, whether they mediate myokines directly remains unknown.

OBJECTIVE

This study aims to investigate the effects of DHA and EPA on browning-related myokines under IR and clarify the mechanism via Ca2+ signaling.

METHODS

The expression and secretion levels of myokines in IR mice and IR myotubes were detected after DHA/EPA treatment. The crosstalk between myotubes and adipocytes was evaluated through a method in which IR adipocytes were treated with the culture medium supernatant of myotubes treated with DHA/EPA. The expression of browning markers in the WAT of IR mice and adipocytes was determined. A calcium chelator was used to determine whether DHA and EPA regulate myokine production through a calcium ion-dependent pathway.

RESULTS

In vivo experiments: 3:1 and 1:3 DHA/EPA promoted the mRNA levels of Irisin, IL-6, IL-15, and FGF21 in skeletal muscle, stimulated WAT browning, reduced lipid accumulation; 3:1 DHA/EPA upregulated the serum concentration of Irisin; 1:3 DHA/EPA upregulated the serum concentrations of Irisin, IL-6, and FGF21. In vitro experiments: the levels of Irisin and IL-6 in C2C12 myotubes and their medium supernatant were significantly elevated in the 3:1 and 1:3 groups and the upregulation of browning markers and reduction in fat accumulation were observed in adipocytes treated with the medium supernatant of C2C12 myotubes in the 3:1 and 1:3 groups. However, the above phenomena disappeared when Ca2+ signaling was inhibited.

CONCLUSIONS

Treatment with DHA and EPA at composition ratios of 3:1 and 1:3 induces browning of WAT in IR mice, which is likely related to the promotion of the accumulation of myokines, especially Irisin and IL-6, via Ca2+ signaling.

Calcium homeostasis and signaling in plant immunity

Calcium (Ca2+) signaling consists of three steps: (1) initiation of a change in cellular Ca2+ concentration in response to a stimulus, (2) recognition of the change through direct binding of Ca2+ by its sensors, (3) transduction of the signal to elicit downstream responses. Recent studies have uncovered a central role for Ca2+ signaling in both layers of immune responses initiated by plasma membrane (PM) and intracellular receptors, respectively. These advances in our understanding are attributed to several lines of research, including invention of genetically-encoded Ca2+ reporters for the recording of intracellular Ca2+ signals, identification of Ca2+ channels and their gating mechanisms, and functional analysis of Ca2+ binding proteins (Ca2+ sensors). This review analyzes the recent literature that illustrates the importance of Ca2+ homeostasis and signaling in plant innate immunity, featuring intricate Ca2+dependent positive and negative regulations.

Calcimycin mediates apoptosis in breast and cervical cancer cell lines by inducing intracellular calcium levels in a P2RX4-dependent manner

BACKGROUND

Calcimycin (A23187) is a polyether antibiotic and divalent cation ionophore, extracted from Streptomyces chartrecensis. With wide variety of antimicrobial activities, it also exhibits cytotoxicity of tumor cells. Calcimycin exhibit therapeutic potential against tumor cell growth; however, the molecular mechanism remains to be fully elucidated. Present study explores the mechanism of calcimycin-induced apoptosis cancer cell lines.

METHODS

Apoptotic induction in a dose-dependent manner were recorded with MTT assays, Phase contrast imaging, wound healing assay, fluorescence imaging by DAPI and AO/EB staining and FACS using cell line model. Mitochondrial potential was analyzed by TMRM assay as Ca2+ signaling is well known to be influenced and synchronized by mitochondria also.

RESULTS

Calcimycin induces apoptosis in dose dependent manner, also accompanied by increased intracellular calcium-level and expression of purinergic receptor-P2RX4, a ligand-gated ion channel.

CONCLUSION

Calcimycin tends to increase the intracellular calcium level, mRNA expression of ATP receptor P2RX4, and phosphorylation of p38. Blocking of either intracellular calcium by BAPTA-AM, P2RX4 expression by antagonist 5-BDBD, and phospho-p38 by SB203580, abrogated the apoptotic activity of calcimycin.

GENERAL SIGNIFICANCE

Taken together, these results show that calcimycin induces apoptosis in P2RX4 and ATP mediated intracellular Ca2+ and p38 MAPK mediated pathway in both the cancer cell lines. This study explored a new mode of action for calcimycin in cancer that could be potentially employed in future studies for cancer therapeutic research. This study disentangles that the calcimycin-induced apoptotic cell death is P2RX4 and ATP involved, intracellular Ca2+ and p38 MAPK mediated pathway.

The anthelmintic meclonazepam activates a schistosome transient receptor potential channel

Parasitic flatworms cause various clinical and veterinary infections that impart a huge burden worldwide. The most clinically impactful infection is schistosomiasis, a neglected tropical disease caused by parasitic blood flukes. Schistosomiasis is treated with praziquantel (PZQ), an old drug introduced over 40 years ago. New drugs are urgently needed, as while PZQ is broadly effective it suffers from several limitations including poor efficacy against juvenile worms, which may prevent it from being completely curative. An old compound that retains efficacy against juvenile worms is the benzodiazepine meclonazepam (MCLZ). However, host side effects caused by benzodiazepines preclude development of MCLZ as a drug and MCLZ lacks an identified parasite target to catalyze rational drug design for engineering out human host activity. Here, we identify a transient receptor potential ion channel of the melastatin subfamily, named TRPMMCLZ, as a parasite target of MCLZ. MCLZ potently activates Schistosoma mansoni TRPMMCLZ through engagement of a binding pocket within the voltage-sensor-like domain of the ion channel to cause worm paralysis, tissue depolarization, and surface damage. TRPMMCLZ reproduces all known features of MCLZ action on schistosomes, including a lower activity versus Schistosoma japonicum, which is explained by a polymorphism within this voltage-sensor-like domain-binding pocket. TRPMMCLZ is distinct from the TRP channel targeted by PZQ (TRPMPZQ), with both anthelmintic chemotypes targeting unique parasite TRPM paralogs. This advances TRPMMCLZ as a novel druggable target that could circumvent any target-based resistance emerging in response to current mass drug administration campaigns centered on PZQ.

Regulation of cardiac calcium signaling by newly identified calcium pump modulators

In cardiomyocytes, the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA2a) is a central component of intracellular Ca2+ regulation. Several heart diseases, including heart failure, are associated with reduced myocardial contraction due to SERCA2a downregulation. Therefore, the need for developing new drugs that could improve SERCA2a function is high. We have recently identified SERCA2a modulators (Compounds 6 and 8) from our screening campaigns and confirmed activation of biochemical SERCA2a ATPase activity and Ca2+ uptake activity. In this study, confocal microscopy and in-cell Ca2+ imaging were used to characterize the effects of these SERCA2a activators on Ca2+ regulation in mouse ventricular myocytes and endoplasmic reticulum (ER) Ca2+ uptake in a HEK293 cell expressing human SERCA2a. Analysis of cytosolic Ca2+ dynamics in cardiomyocytes revealed that both Compounds (6 and 8) increase the action potential-induced Ca2+ transients and sarcoplasmic reticulum (SR) Ca2+ load. While Compound 6 induced a negligible effect on Ca2+ transients invoked by the L-type Ca2+ channel (LTCC) current, Compound 8 increased Ca2+ transients during LTCC activation, suggesting an off-target protein interaction of Compound 8. Analysis of ER Ca2+ transport by human SERCA2a in HEK cells showed that only Compound 6 increased both ER Ca2+ uptake and ER Ca2+ load significantly, whereas Compound 8 had no effect on SERCA2a Ca2+ transport. This study revealed that Compound 6 exhibits promising characteristics that can improve intracellular Ca2+ dynamics by selectively enhancing SERCA2a Ca2+ uptake.

Calcium signaling in plant mineral nutrition: From uptake to transport

Plant mineral nutrition is essential for crop yields and human health. However, the uneven distribution of mineral elements over time and space leads to a lack or excess of available mineral elements in plants. Among the essential nutrients, calcium (Ca2+) stands out as a prominent second messenger that plays crucial roles in response to extracellular stimuli in all eukaryotes. Distinct Ca2+ signatures with unique parameters are induced by different stresses and deciphered by various Ca2+ sensors. Recent research on the participation of Ca2+ signaling in regulation of mineral elements has made great progress. In this review, we focus on the impact of Ca2+ signaling on plant mineral uptake and detoxification. Specifically, we emphasize the significance of Ca2+ signaling for regulation of plant mineral nutrition and delve into key points and novel avenues for future investigations, aiming to offer new insights into plant ion homeostasis.

SERCA2 phosphorylation at the heart of the disease

Gonnot et al. [1] thoroughly investigated the regulatory role of glycogen synthase kinase 3 beta (GSK3β) in modulating cardiac isoform 2 of sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA2) activity. They have found that in ischemic hearts of patients and mouse-GSK3β -mediated SERCA2 phosphorylation at serine 663 dampens the SERCA2 pump activity and induces Ca2+ overload which sensitizes towards myocardial ischemia-reperfusion (I/R) injury. The inhibition of serine 663 phosphorylation significantly increases SERCA2 activity and, by preventing cytosolic and mitochondrial Ca2+ overload, reduces cell death during reperfusion. Augmented SERCA2 activity also substantially improves excitation-contraction coupling in cardiomyocytes upon recovery from reperfusion injury. This study provides valuable insights into pathophysiological relevance of GSK3β -mediated SERCA2 phosphorylation in the context of heart diseases and paves the way for designing novel clinical therapeutic approaches to alleviate post infartion heart failure.

Human IP3 receptor triple knockout stem cells remain pluripotent despite altered mitochondrial metabolism

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are ER Ca2+-release channels that control a broad set of cellular processes. Animal models lacking IP3Rs in different combinations display severe developmental phenotypes. Given the importance of IP3Rs in human diseases, we investigated their role in human induced pluripotent stem cells (hiPSC) by developing single IP3R and triple IP3R knockouts (TKO). Genome edited TKO-hiPSC lacking all three IP3R isoforms, IP3R1, IP3R2, IP3R3, failed to generate Ca2+ signals in response to agonists activating GPCRs, but retained stemness and pluripotency. Steady state metabolite profiling and flux analysis of TKO-hiPSC indicated distinct alterations in tricarboxylic acid cycle metabolites consistent with a deficiency in their pyruvate utilization via pyruvate dehydrogenase, shifting towards pyruvate carboxylase pathway. These results demonstrate that IP3Rs are not essential for hiPSC identity and pluripotency but regulate mitochondrial metabolism. This set of knockout hiPSC is a valuable resource for investigating IP3Rs in human cell types of interest.

Reduction of TRPC1/TRPC3 mediated Ca2+-signaling protects oxidative stress-induced COPD

Oxidative stress is a predisposing factor in Chronic Obstructive Pulmonary Disease (COPD). Specifically, pulmonary epithelial (PE) cells reduce antioxidant capacity during COPD because of the continuous production of reactive oxygen species (ROS). However, the molecular pathogenesis that governs such ROS activity is unclear. Here we show that the dysregulation of intracellular calcium concentration ([Ca2+]i) in PE cells from COPD patients, compared to the healthy PE cells, is associated with the robust functional expressions of Transient Receptor Potential Canonical (TRPC)1 and TRPC3 channels, and Ca2+ entry (SOCE) components, Stromal Interaction Molecule 1 (STIM1) and ORAI1 channels. Additionally, the elevated expression levels of fibrotic, inflammatory, oxidative, and apoptotic markers in cells from COPD patients suggest detrimental pathway activation, thereby reducing the ability of lung remodeling. To further delineate the mechanism, we used human lung epithelial cell line, A549, since the behavior of SOCE and the expression patterns of TRPC1/C3, STIM1, and ORAI1 were much like PE cells. Notably, the knockdown of TRPC1/C3 in A549 cells substantially reduced the SOCE-induced [Ca2+]i rise, and reversed the ROS-mediated oxidative, fibrotic, inflammatory, and apoptotic responses, thus confirming the role of TRPC1/C3 in SOCE driven COPD-like condition. Higher TRPC1/C3, STIM1, and ORAI1 expressions, along with a greater Ca2+ entry, via SOCE in ROS-induced A549 cells, led to the rise in oxidative, fibrotic, inflammatory, and apoptotic gene expression, specifically through the extracellular signal-regulated kinase (ERK) pathway. Abatement of TRPC1 and/or TRPC3 reduced the mobilization of [Ca2+]i and reversed apoptotic gene expression and ERK activation, signifying the involvement of TRPC1/C3. Together these data suggest that TRPC1/C3 and SOCE facilitate the COPD condition through ROS-mediated cell death, thus implicating their likely roles as potential therapeutic targets for COPD. SUMMARY: Alterations in Ca2+ signaling modalities in normal pulmonary epithelial cells exhibit COPD through oxidative stress and cellular injury, compromising repair, which was alleviated through inhibition of store-operated calcium entry. SUBJECT AREA: Calcium, ROS, Cellular signaling, lung disease.

MICU1 deficiency alters mitochondrial morphology and cytochrome C release

The mitochondrial inner boundary membrane harbors a protein called MICU1, which is sensitive to Ca²⁺ and binds to the MICOS components Mic60 and CHCHD2. Changes in the mitochondrial cristae junction structure and organization in MICU1^-/- cells lead to increased cytochrome c release, membrane potential rearrangement, and changes in mitochondrial Ca²⁺ uptake dynamics. These findings shed new light on the multifaceted role of MICU1, highlighting its involvement not only as an interaction partner and regulator of the MCU complex but also as a crucial determinant of mitochondrial ultrastructure and, thus, an essential player in processes initiating apoptosis.

Calcium signaling in chemotherapy-induced neuropathy

Alterations in calcium (Ca²⁺) signaling is a major mechanism in the development of chemotherapy-induced peripheral neuropathy (CIPN), a side effect caused by multiple chemotherapy regimens. CIPN is associated with numbness and incessant tingling in hands and feet which diminishes quality of life during treatment. In up to 50% of survivors, CIPN is essentially irreversible. There are no approved, disease-modifying treatments for CIPN. The only recourse for oncologists is to modify the chemotherapy dose, a situation that can compromise optimal chemotherapy and impact patient outcomes. Here we focus on taxanes and other chemotherapeutic agents that work by altering microtubule assemblies to kill cancer cells, but also have off-target toxicities. There have been many molecular mechanisms proposed to explain the effects of microtubule-disrupting drugs. In neurons, an initiating step in the off-target effects of treatment by taxane is binding to neuronal calcium sensor 1 (NCS1), a sensitive Ca²⁺ sensor protein that maintains the resting Ca²⁺ concentration and dynamically enhances responses to cellular stimuli. The taxane/NCS1 interaction causes a Ca²⁺ surge that starts a pathophysiological cascade of consequences. This same mechanism contributes to other conditions including chemotherapy-induced cognitive impairment. Strategies to prevent the Ca²⁺ surge are the foundation of current work.

Cyanidin inhibits adipogenesis in 3T3-L1 preadipocytes by activating the PLC-IP3 pathway

Cyanidin is the most abundant anthocyanin found in red-purple plants and possesses anti-obesity properties. However, its mechanism of action in adipocytes remains unknown. The objective of this study was to elucidate how cyanidin inhibits adipocyte formation in 3T3-L1 preadipocytes. Cells were cultured in adipogenic differentiation medium supplemented with cyanidin and examined for adipogenesis, cell viability, and adipocyte gene expression using Oil Red O staining, MTT assay, and RT-qPCR. Real-time Ca²⁺ imaging analysis was performed in living cells to elucidate cyanidin's mechanism of action. The results demonstrated that cyanidin (1-50 μM) supplementation to the adipogenic medium inhibited adipogenesis by downregulating adipogenic marker gene expression (PPARγ, C/EBPα, adiponectin, and aP2) without affecting cell viability after 4 days of treatment. Stimulation of cells with cyanidin (30-100 μM) increased intracellular Ca²⁺ in a concentration dependent manner with peak calcium increases at 50 μM. Pretreatment of cells with the phospholipase C (PLC) inhibitor U73122, inositol triphosphate (IP₃) receptor blocker 2-APB, and depletion of endoplasmic reticulum Ca²⁺ stores by thapsigargin abolished the Ca²⁺ increases by cyanidin. These findings suggested that cyanidin inhibits adipocyte formation by activating the PLC-IP₃ pathway and intracellular Ca²⁺ signaling. Our study is the first report describing the mechanism underlying the anti-obesity effect of cyanidin.

Anti-inflammatory role of extracellular l-arginine through calcium sensing receptor in human renal proximal tubular epithelial (HK-2) cells

Renal tubular epithelial cells are capable of synthesizing interleukins (IL) in response to a variety of proinflammatory cytokines. Moreover, elevated urinary levels of IL have been shown in patients with various forms of nephritic diseases. However, the underlying intracellular signaling mechanism is unclear. Here we show the immunological signaling role of l-Arginine (l-Arg) through Ca2+-sensing receptor (CaSR) in human kidney 2 (HK-2) renal proximal tubular epithelial cells, using Ca2+ imaging and patch clamp techniques and its mechanistic link to the downstream cellular function. Both pharmacological and siRNA inhibitors support the activation CaSR by extracellular l-Arg to induced Ca2+ entry via a Transient receptor potential canonical (TRPC) channel in HK-2 cells mainly through the receptor operated Ca2+ entry (ROCE). Activation of CaSR by l-Arg led to the rise in p-p38/p38 expression suggesting [Ca2+]i as a regulator for p38-signaling pathways. Notably, l-Arg activated CaSR-induced Ca2+ signaling reduced the expressions of key fibrotic, inflammatory, and apoptotic genes, suggesting its nephroprotective role via Ca2+ signaling through CaSR in HK-2 cells. Since we found that the IL-6 expressions were inversely proportional to the increasing concentrations of l-Arg in HK-2 cells, we measured the release of IL-6, which steadily decreased as the concentrations of l-Arg were elevated. Taken together, extracellular l-Arg is a negative regulator for IL-6-induced inflammatory process, through the activation of CaSR and TRPC channel by ROCE pathway and can have a potential to alleviate inflammatory renal diseases.

The endoplasmic reticulum stress and unfolded protein response in Alzheimer's disease: a calcium dyshomeostasis perspective

Protein misfolding is prominent in early cellular pathology of Alzheimer's disease (AD), implicating pathophysiological significance of endoplasmic reticulum stress/unfolded protein response (ER stress/UPR) and highlighting it as a target for drug development. Experimental data from animal AD models and observations on human specimens are, however, inconsistent. ER stress and associated UPR are readily observed in in vitro AD cellular models and in some AD model animals. In the human brain, components and markers of ER stress as well as UPR transducers are observed at Braak stages III-VI associated with severe neuropathology and neuronal death. The picture, however, is further complicated by the brain region- and cell type-specificity of the AD-related pathology. Terms 'disturbed' or 'non-canonical' ER stress/UPR were used to describe the discrepancies between experimental data and the classic ER stress/UPR cascade. Here we discuss possible 'disturbing' or 'interfering' factors which may modify ER stress/UPR in the early AD pathogenesis. We focus on the dysregulation of the ER Ca²⁺ homeostasis, store-operated Ca²⁺ entry, and the interaction between the ER and mitochondria. We suggest that a detailed study of the CNS cell type-specific alterations of Ca²⁺ homeostasis in early AD may deepen our understanding of AD-related dysproteostasis.

Structure, biochemical function, and signaling mechanism of plant NLRs

To counter pathogen invasion, plants have evolved a large number of immune receptors, including membrane-resident pattern recognition receptors (PRRs) and intracellular nucleotide-binding and leucine-rich repeat receptors (NLRs). Our knowledge about PRR and NLR signaling mechanisms has expanded significantly over the past few years. Plant NLRs form multi-protein complexes called resistosomes in response to pathogen effectors, and the signaling mediated by NLR resistosomes converges on Ca²⁺-permeable channels. Ca²⁺-permeable channels important for PRR signaling have also been identified. These findings highlight a crucial role of Ca²⁺ in triggering plant immune signaling. In this review, we first discuss the structural and biochemical mechanisms of non-canonical NLR Ca²⁺ channels and then summarize our knowledge about immune-related Ca²⁺-permeable channels and their roles in PRR and NLR signaling. We also discuss the potential role of Ca²⁺ in the intricate interaction between PRR and NLR signaling.

Subunit composition, molecular environment, and activation of native TRPC channels encoded by their interactomes

In the mammalian brain TRPC channels, a family of Ca²⁺-permeable cation channels, are involved in a variety of processes from neuronal growth and synapse formation to transmitter release, synaptic transmission and plasticity. The molecular appearance and operation of native TRPC channels, however, remained poorly understood. Here, we used high-resolution proteomics to show that TRPC channels in the rodent brain are macro-molecular complexes of more than 1 MDa in size that result from the co-assembly of the tetrameric channel core with an ensemble of interacting proteins (interactome). The core(s) of TRPC1-, C4-, and C5-containing channels are mostly heteromers with defined stoichiometries for each subtype, whereas TRPC3, C6, and C7 preferentially form homomers. In addition, TRPC1/C4/C5 channels may co-assemble with the metabotropic glutamate receptor mGluR1, thus guaranteeing both specificity and reliability of channel activation via the phospholipase-Ca²⁺ pathway. Our results unveil the subunit composition of native TRPC channels and resolve the molecular details underlying their activation.

Synthesis and biological evaluation of novel photo-clickable adenosine and cyclic ADP-ribose analogs: 8-N3-2'-O-propargyladenosine and 8-N3-2'-O-propargyl-cADPR

A photo-clickable analog of adenosine was devised and synthesized in which the photoactive functional group (8-azidoadenosine) and the click moiety (2'-O-propargyl-ether) were compactly combined within the structure of the adenosine nucleoside itself. We synthesized 8-N3-2'-O-propargyl adenosine in four steps starting from adenosine. This photo-clickable adenosine was 5'-phosphorylated and coupled to nicotinamide mononucleotide to form the NAD analog 8-N3-2'-O-propargyl-NAD. This NAD analog was recognized by Aplysia californica ADP-ribosyl cyclase and enzymatically cyclized producing 8-N3-2'-O-propargyl cyclic ADP-ribose. Photo-clickable cyclic-ADP-ribose analog was envisioned as a probe to label cyclic ADP-ribose binding proteins. The monofunctional 8-N3-cADPR has previously been shown to be an antagonist of cADPR-induced calcium release [T.F. Walseth et. al., J. Biol. Chem (1993) 268, 26686-26691]. 2'-O-propargyl-cADPR was recognized as an agonist which elicited Ca2+ release when added at low concentration to sea urchin egg homogenates. The bifunctional 8-N3-2'-O-propargyl cyclic ADP-ribose did not elicit Ca2+ release at low concentration or impact cyclic ADP-ribose mediated Ca2+ release either when added to sea urchin egg homogenates or when microinjected into cultured human U2OS cells. The photo-clickable adenosine will none-the-less be a useful scaffold for synthesizing photo-clickable probes for identifying proteins that interact with a variety of adenosine nucleotides.

The T-type calcium channel CaV3.2 regulates insulin secretion in the pancreatic β-cell

Voltage-gated Ca²⁺ (Ca_V) channel dysfunction leads to impaired glucose-stimulated insulin secretion in pancreatic β-cells and contributes to the development of type-2 diabetes (T2D). The role of the low-voltage gated T-type Ca_V channels in β-cells remains obscure. Here we have measured the global expression of T-type Ca_V3.2 channels in human islets and found that gene expression of CACNA1H, encoding Ca_V3.2, is negatively correlated with HbA1c in human donors, and positively correlated with islet insulin gene expression as well as secretion capacity in isolated human islets. Silencing or pharmacological blockade of Ca_V3.2 attenuates glucose-stimulated cytosolic Ca²⁺ signaling, membrane potential, and insulin release. Moreover, the endoplasmic reticulum (ER) Ca²⁺ store depletion is also impaired in Ca_V3.2-silenced β-cells. The linkage between T-type (Ca_V3.2) and L-type Ca_V channels is further identified by the finding that the intracellular Ca²⁺ signaling conducted by Ca_V3.2 is highly dependent on the activation of L-type Ca_V channels. In addition, CACNA1H expression is significantly associated with the islet predominant L-type CACNA1C (Ca_V1.2) and CACNA1D (Ca_V1.3) genes in human pancreatic islets. In conclusion, our data suggest the essential functions of the T-type Ca_V3.2 subunit as a mediator of β-cell Ca²⁺ signaling and membrane potential needed for insulin secretion, and in connection with L-type Ca_V channels.

The mitochondrial sodium/calcium exchanger NCLX (Slc8b1) in B lymphocytes

Antigen receptor stimulation triggers cytosolic Ca2+ signals, which activate transcriptional and metabolic programs critical for immune function. B-cell receptor (BCR) engagement causes rapid cytosolic Ca2+ rise through the ubiquitous store-operated calcium entry (SOCE) pathway. Slc8b1, which encodes the mitochondrial Na+/Ca2+ exchanger (NCLX), extrudes Ca2+ out of the mitochondria and maintains optimal SOCE activity. Inhibition of NCLX in DT40 and A20 B lymphocyte lines was recently shown to impair cytosolic Ca2+ transients in response to antigen-receptor stimulation, however the downstream functional consequences of this impairment remain unclear. Here, we generated Slc8b1 knockout A20 B-cell lines using CRISPR/Cas9 technology and B-cell specific Slc8b1 knockout mice. Surprisingly, while loss of Slc8b1 in B lymphocytes led to reduction in SOCE, it had a marginal effect on mitochondrial Ca2+ extrusion, suggesting that NCLX is not the major mitochondrial Ca2+ extrusion mechanism in B cells. Furthermore, endoplasmic reticulum (ER) Ca2+ content and rates of ER depletion and refilling remained unaltered in Slc8b1 knockout B cells. Slc8b1 deficiency increased mitochondrial production of oxidants, reduced mitochondrial bioenergetics and altered mitochondrial ultrastructure. B-cell specific Slc8b1 knockout mice showed reduced germinal center B cell responses following foreign antigen and pathogen driven immune responses. Our studies provide novel insights into the function of Slc8b1 in germinal center B cells and its contribution to B-cell signaling and effector function.

Involvement of NO and Ca2+ in the enhancement of cold tolerance induced by melatonin in winter turnip rape (Brassica rapa L.)

As a multifunctional phytohormone, melatonin (Mel) plays pivotal roles in plant responses to multiple stresses. However, its mechanism of action remains elusive. In the present study, we evaluated the role of NO and Ca²⁺ signaling in Mel enhanced cold tolerance in winter turnip rape. The results showed that the NO content and concentration of intracellular free Ca²⁺ ([Ca²⁺]_cyt) increased by 35.42% and 30.87%, respectively, in the leaves of rape seedlings exposed to cold stress. Compared with those of the seedlings in cold stress alone, the NO content and concentration of [Ca²⁺]_cyt in rape seedlings pretreated with Mel increased further. In addition, the Mel-mediated improvement of cold tolerance was inhibited by L-NAME (a NO synthase inhibitor), tungstate (a nitrate reductase inhibitor), LaCl₃ (a Ca²⁺ channel blocker), and EGTA (a Ca²⁺ chelator), and this finding was mainly reflected in the increase in ROS content and the decrease in osmoregulatory capacity, photosynthetic efficiency and antioxidant enzyme activities, and expression levels of antioxidant enzyme genes. These findings suggest that NO and Ca²⁺ are necessary for Mel to improve cold tolerance and function synergistically downstream of Mel. Notably, the co-treatment of Mel with L-NAME, tungstate, LaCl₃, or EGTA also inhibited the Mel-induced expression of MAPK3/6 under cold stress. In conclusion, NO and Ca²⁺ are involved in the enhancement of cold tolerance induced by Mel through activating the MAPK cascades in rape seedlings, and a crosstalk may exist between NO and Ca²⁺ signaling.

Modulation of Ca2+ signaling by antiapoptotic Bcl-2 versus Bcl-xL: From molecular mechanisms to relevance for cancer cell survival

Members of the Bcl-2-protein family are key controllers of apoptotic cell death. The family is divided into antiapoptotic (including Bcl-2 itself, Bcl-xL, Mcl-1, etc.) and proapoptotic members (Bax, Bak, Bim, Bim, Puma, Noxa, Bad, etc.). These proteins are well known for their canonical role in the mitochondria, where they control mitochondrial outer membrane permeabilization and subsequent apoptosis. However, several proteins are recognized as modulators of intracellular Ca²⁺ signals that originate from the endoplasmic reticulum (ER), the major intracellular Ca²⁺-storage organelle. More than 25 years ago, Bcl-2, the founding member of the family, was reported to control apoptosis through Ca²⁺ signaling. Further work elucidated that Bcl-2 directly targets and inhibits inositol 1,4,5-trisphosphate receptors (IP₃Rs), thereby suppressing proapoptotic Ca²⁺ signaling. In addition to Bcl-2, Bcl-xL was also shown to impact cell survival by sensitizing IP₃R function, thereby promoting prosurvival oscillatory Ca²⁺ release. However, new work challenges this model and demonstrates that Bcl-2 and Bcl-xL can both function as inhibitors of IP₃Rs. This suggests that, depending on the cell context, Bcl-xL could support very distinct Ca²⁺ patterns. This not only raises several questions but also opens new possibilities for the treatment of Bcl-xL-dependent cancers. In this review, we will discuss the similarities and divergences between Bcl-2 and Bcl-xL regarding Ca²⁺ homeostasis and IP₃R modulation from both a molecular and a functional point of view, with particular emphasis on cancer cell death resistance mechanisms.

Effects of Isoprenaline on ventricular myocyte shortening and Ca2+ transport in the Zucker rat

Obesity is an important risk factor for diabetes mellitus (DM) which is a major global health problem. Electro-mechanical dysfunction has been extensively described in diabetic heart and cardiovascular complications are an important cause of mortality and morbidity in diabetic patients.

OBJECTIVES

To examine the effects of Isoprenaline (ISO) in obesity and diabesity on ventricular myocyte shortening and Ca²⁺ transport in Zucker fatty (ZF), Zucker diabetic fatty (ZDF) in comparison to Zucker lean (ZL) rats.

METHODS

Myocyte shortening and intracellular Ca²⁺ were investigated with video imaging and fluorescence photometry, respectively.

RESULTS

The amplitude of Isoprenaline stimulated shortening was significantly (p < 0.05) decreased in ZDF and ZF compared to ZL myocytes. The amplitude of Isoprenaline stimulated Ca²⁺ transient was also significantly (p < 0.05) reduced in ZF compared to ZL and modestly reduced in ZDF compared to ZL myocytes. Mean Isoprenaline stimulated time to peak along with time to half relaxation of shortening were unchanged in ZDF and ZF compared to ZL myocytes. Mean Isoprenaline stimulated time to peak Ca²⁺ transient was significantly shortened in ZF compared to ZL myocytes. Time to half decay of the Ca²⁺ transient was considerably prolonged in ZDF compared to ZL myocytes. Amplitude of Isoprenaline stimulated caffeine-evoked Ca²⁺ transients were significantly reduced in ZDF and ZF in comparison to ZL myocytes.

CONCLUSION

Isoprenaline was less effective at generating an increase in the amplitude of shortening in ZDF and ZF in comparison to ZL myocytes and defects in Ca²⁺ signaling, and in particular SR Ca²⁺ transport, might partly underlie these abnormalities.

Transcriptional regulatory network of plant cold-stress responses

Recent studies have revealed the complex and flexible transcriptional regulatory network involved in cold-stress responses. Focusing on two major signaling pathways that respond to cold stress, we outline current knowledge of the transcriptional regulatory network and the post-translational regulation of transcription factors in the network. Cold-stress signaling pathways are closely associated with other signaling pathways such as those related to the circadian clock, and large amounts of data on their crosstalk and tradeoffs are available. However, it remains unknown how plants sense and transmit cold-stress signals to regulate gene expression. We discuss recent reports on cold-stress sensing and associated signaling pathways that regulate the network. We also emphasize future directions for developing abiotic stress-tolerant crop plants.

Ca2+ signaling-mediated low-intensity pulsed ultrasound-induced proliferation and activation of motor neuron cells

Motor neuron diseases (MND) including amyotrophic lateral sclerosis and Parkinson disease are commonly neurodegenerative, causing a gradual loss of nerve cells and affecting the mechanisms underlying changes in calcium (Ca²⁺)-regulated dendritic growth. In this study, the NSC-34 cell line, a population of hybridomas generated using mouse spinal cord cells with neuroblastoma, was used to investigate the effect of low-intensity pulsed ultrasound (LIPUS) as part of an MND treatment model. After NSC-34 cells were seeded for 24 h, LIPUS stimulation was performed on the cells at days 1 and 3 using a non-focused transducer at 1.15 MHz for 8 min. NSC-34 cell proliferation and morphological changes were observed at various LIPUS intensities and different combinations of Ca²⁺ channel blockers. The nuclear translocation of Ca²⁺-dependent transcription factors was also examined. We observed that the neurite outgrowth and cell number of NSC-34 significantly increased with LIPUS stimulation at days 2 and 4, which may be associated with the treatment's positive effect on the activation of Ca²⁺-dependent transcription factors, such as nuclear factor of activated T cells and nuclear factor-kappa B. Our findings suggest that the LIPUS-induced Ca²⁺ signaling and transcription factor activation facilitate the morphological maturation and proliferation of NSC-34 cells, presenting a promising noninvasive method to improve stimulation therapy for MNDs in the future.

Assessment of mitochondrial dysfunction and implications in cardiovascular disorders

Mitochondria play a pivotal role in cellular function, not only acting as the powerhouse of the cell, but also regulating ATP synthesis, reactive oxygen species (ROS) production, intracellular Ca²⁺ cycling, and apoptosis. During the past decade, extensive progress has been made in the technology to assess mitochondrial functions and accumulating evidences have shown that mitochondrial dysfunction is a key pathophysiological mechanism for many diseases including cardiovascular disorders, such as ischemic heart disease, cardiomyopathy, hypertension, atherosclerosis, and hemorrhagic shock. The advances in methodology have been accelerating our understanding of mitochondrial molecular structure and function, biogenesis and ROS and energy production, which facilitates new drug target identification and therapeutic strategy development for mitochondrial dysfunction-related disorders. This review will focus on the assessment of methodologies currently used for mitochondrial research and discuss their advantages, limitations and the implications of mitochondrial dysfunction in cardiovascular disorders.

Biological function of calcium-sensing receptor (CAS) and its coupling calcium signaling in plants

The calcium-sensing receptor (CAS), as a chloroplast thylakoid membrane protein, is involved in the process of external Ca2+-induced cytosolic Ca2+ increase in plants. However, the underlying mechanism regulating this process is lacking. Furthermore, recent evidence suggests that CAS may perform additional roles in plants. Here, we provided an update covering the multiple roles of CAS in stomatal movement regulation and Ca2+ signaling in plants. We also analyzed the possible phosphorylation mechanism of CAS by light and discuss the role of CAS in abiotic stress (drought, salt stress) and biotic stresses (plant immune signaling). Finally, we proposed a perspective for future experiments that are required to fill gaps in our understanding of the biological function of CAS in plants.

Functional coupling of TRPM2 and extrasynaptic NMDARs exacerbates excitotoxicity in ischemic brain injury

Excitotoxicity induced by NMDA receptor (NMDAR) activation is a major cause of neuronal death in ischemic stroke. However, past efforts of directly targeting NMDARs have unfortunately failed in clinical trials. Here, we reveal an unexpected mechanism underlying NMDAR-mediated neurotoxicity, which leads to the identification of a novel target and development of an effective therapeutic peptide for ischemic stroke. We show that NMDAR-induced excitotoxicity is enhanced by physical and functional coupling of NMDAR to an ion channel TRPM2 upon ischemic insults. TRPM2-NMDAR association promotes the surface expression of extrasynaptic NMDARs, leading to enhanced NMDAR activity and increased neuronal death. We identified a specific NMDAR-interacting motif on TRPM2 and designed a membrane-permeable peptide to uncouple the TRPM2-NMDAR interaction. This disrupting peptide protects neurons against ischemic injury in vitro and protects mice against ischemic stroke in vivo. These findings provide an unconventional strategy to mitigate excitotoxic neuronal death without directly targeting NMDARs.

Secretory pathway Ca2+-ATPase SPCA2 regulates mitochondrial respiration and DNA damage response through store-independent calcium entry

A complex interplay between the extracellular space, cytoplasm and individual organelles modulates Ca2+ signaling to impact all aspects of cell fate and function. In recent years, the molecular machinery linking endoplasmic reticulum stores to plasma membrane Ca2+ entry has been defined. However, the mechanism and pathophysiological relevance of store-independent modes of Ca2+ entry remain poorly understood. Here, we describe how the secretory pathway Ca2+-ATPase SPCA2 promotes cell cycle progression and survival by activating store-independent Ca2+ entry through plasma membrane Orai1 channels in mammary epithelial cells. Silencing SPCA2 expression or briefly removing extracellular Ca2+ increased mitochondrial ROS production, DNA damage and activation of the ATM/ATR-p53 axis leading to G0/G1 phase cell cycle arrest and apoptosis. Consistent with these findings, SPCA2 knockdown confers redox stress and chemosensitivity to DNA damaging agents. Unexpectedly, SPCA2-mediated Ca2+ entry into mitochondria is required for optimal cellular respiration and the generation of mitochondrial membrane potential. In hormone receptor positive (ER+/PR+) breast cancer subtypes, SPCA2 levels are high and correlate with poor survival prognosis. We suggest that elevated SPCA2 expression could drive pro-survival and chemotherapy resistance in cancer cells, and drugs that target store-independent Ca2+ entry pathways may have therapeutic potential in treating cancer.

Involvement of Ca2+ and ROS signals in nickel-impaired human sperm function

As one of the main environmental pollutants and occupational hazards, nickel has been reported to have mutagenic, carcinogenic, and teratogenic properties, as well as reproductive toxicity. However, how nickel affects human reproduction is still unclear. In this study, the toxicity of nickel on human sperm and the underlying mechanisms were evaluated in vitro. We found that NiCl₂ (10, 50, and 250 μM) impaired sperm total motility and progressive motility in a dose- and time-dependent manner. In addition, sperm hyperactivation and the ability of human sperm to penetrate a viscous medium were found to be compromised after nickel exposure. Mechanically, NiCl₂ significantly inhibited the basal intracellular Ca²⁺ signaling. Besides, reactive oxygen species (ROS), superoxide, and malondialdehyde levels were increased in human sperm after exposure to different concentrations of NiCl₂. Consistently, eliminating excess ROS by N-acetyl-L-cysteine or tocopherol significantly alleviated nickel-impaired sperm motility. Taken together, these results revealed that nickel could compromise sperm functions by interfering with Ca²⁺ signaling and inducing excessive oxidative stress. These findings suggest that, in the high and occupational nickel exposure environments, the contribution of nickel toxicity to the males who wish to preserve their fertility is worthy of careful evaluation.

A junctional cAMP compartment regulates rapid Ca2+ signaling in atrial myocytes

Axial tubule junctions with the sarcoplasmic reticulum control the rapid intracellular Ca²⁺-induced Ca²⁺ release that initiates atrial contraction. In atrial myocytes we previously identified a constitutively increased ryanodine receptor (RyR2) phosphorylation at junctional Ca²⁺ release sites, whereas non-junctional RyR2 clusters were phosphorylated acutely following β-adrenergic stimulation. Here, we hypothesized that the baseline synthesis of 3',5'-cyclic adenosine monophosphate (cAMP) is constitutively augmented in the axial tubule junctional compartments of atrial myocytes. Confocal immunofluorescence imaging of atrial myocytes revealed that junctin, binding to RyR2 in the sarcoplasmic reticulum, was densely clustered at axial tubule junctions. Interestingly, a new transgenic junctin-targeted FRET cAMP biosensor was exclusively co-clustered in the junctional compartment, and hence allowed to monitor cAMP selectively in the vicinity of junctional RyR2 channels. To dissect local cAMP levels at axial tubule junctions versus subsurface Ca²⁺ release sites, we developed a confocal FRET imaging technique for living atrial myocytes. A constitutively high adenylyl cyclase activity sustained increased local cAMP levels at axial tubule junctions, whereas β-adrenergic stimulation overcame this cAMP compartmentation resulting in additional phosphorylation of non-junctional RyR2 clusters. Adenylyl cyclase inhibition, however, abolished the junctional RyR2 phosphorylation and decreased L-type Ca²⁺ channel currents, while FRET imaging showed a rapid cAMP decrease. In conclusion, FRET biosensor imaging identified compartmentalized, constitutively augmented cAMP levels in junctional dyads, driving both the locally increased phosphorylation of RyR2 clusters and larger L-type Ca²⁺ current density in atrial myocytes. This cell-specific cAMP nanodomain is maintained by a constitutively increased adenylyl cyclase activity, contributing to the rapid junctional Ca²⁺-induced Ca²⁺ release, whereas β-adrenergic stimulation overcomes the junctional cAMP compartmentation through cell-wide activation of non-junctional RyR2 clusters.

C2cd6-encoded CatSperτ targets sperm calcium channel to Ca2+ signaling domains in the flagellar membrane

In mammalian sperm cells, regulation of spatiotemporal Ca²⁺ signaling relies on the quadrilinear Ca²⁺ signaling nanodomains in the flagellar membrane. The sperm-specific, multi-subunit CatSper Ca²⁺ channel, which is crucial for sperm hyperactivated motility and male fertility, organizes the nanodomains. Here, we report CatSperτ, the C2cd6-encoded membrane-associating C2 domain protein, can independently migrate to the flagella and serve as a major targeting component of the CatSper channel complex. CatSperτ loss of function in mice demonstrates that it is essential for sperm hyperactivated motility and male fertility. CatSpertτ targets the CatSper channel into the quadrilinear nanodomains in the flagella of developing spermatids, whereas it is dispensable for functional channel assembly. CatSperτ interacts with ciliary trafficking machinery in a C2-dependent manner. These findings provide insights into the CatSper channel trafficking to the Ca²⁺ signaling nanodomains and the shared molecular mechanisms of ciliary and flagellar membrane targeting.

Hwang et al. report that the C2 domain protein CatSperτ targets the sperm CatSper Ca²⁺ channel to linear domains of the sperm flagellum during development. The findings provide fundamental insights into CatSper trafficking and the shared molecular mechanisms among ciliary and flagellar membrane targeting.

Calcium signaling and regulation of ecdysteroidogenesis in crustacean Y-organs

Crustacean Y-organs secrete ecdysteroid molting hormones. Ecdysteroids are released in increased amount during premolt, circulate in hemolymph, and stimulate the events in target cells that lead to molting. During much of the molting cycle, ecdysteroid production is suppressed by molt-inhibiting hormone (MIH), a peptide neurohormone produced in the eyestalks. The suppressive effect of MIH is mediated by a cyclic nucleotide second messenger. A decrease in circulating MIH is associated with an increase in the hemolymphatic ecdysteroid titer during pre-molt. Nevertheless, it has long been hypothesized that a positive regulatory signal or stimulus is also involved in promoting ecdysteroidogenensis during premolt. Data reviewed here are consistent with the hypothesis that an intracellular Ca²⁺ signal provides that stimulus. Pharmacological agents that increase intracellular Ca²⁺ in Y-organs promote ecdysteroidogenesis, while agents that lower intracellular Ca²⁺ or disrupt Ca²⁺ signaling suppress ecdysteroidogenesis. Further, an increase in the hemolymphatic ecdysteroid titer after eyestalk ablation or during natural premolt is associated with an increase in intracellular free Ca²⁺ in Y-organ cells. Several lines of evidence suggest elevated intracellular calcium is linked to enhanced ecdysteroidogenesis through activation of Ca²⁺/calmodulin dependent cyclic nucleotide phosphodiesterase, thereby lowering intracellular cyclic nucleotide second messenger levels and promoting ecdysteroidogenesis. Results of transcriptomic studies show genes involved in Ca²⁺ signaling are well represented in Y-organs. Several recent studies have focused on Ca²⁺ transport proteins in Y-organs. Complementary DNAs encoding a plasma membrane Ca²⁺ ATPase (PMCA) and a sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase (SERCA) have been cloned from crab Y-organs. The relative abundance of PMCA and SERCA transcripts in Y-organs is elevated during premolt, a time when Ca²⁺ levels in Y-organs are likewise elevated. The results are consistent with the notion that these transport proteins act to maintain the Ca²⁺ gradient across the cell membrane and re-set the cell for future Ca²⁺ signals.

Targeting T-type channels in cancer: What is on and what is off?

Over the past 20 years, various studies have demonstrated a pivotal role of T-type calcium channels (TTCCs) in tumor progression. Cytotoxic effects of TTCC pharmacological blockers have been reported in vitro and in preclinical models. However, their roles in cancer physiology are only beginning to be understood. In this review, we discuss evidence for the signaling pathways and cellular processes stemming from TTCC activity, mainly inferred by inverse reasoning from pharmacological blocks and, only in a few studies, by gene silencing or channel activation. A thorough analysis indicates that drug-induced cytotoxicity is partially an off-target effect. Dissection of on/off-target activity is paramount to elucidate the physiological roles of TTCCs, and to deliver efficacious therapies suited to different cancer types and stages.

Targeting Ca2+ signaling: A new arsenal against cancer

The drug resistance of cancer cells is a major concern in medical oncology, resulting in the failure of chemotherapy. Ca²⁺ plays a pivotal role in inducing multidrug resistance in cancer cells. Calcium signaling is a critical regulator of many cancer hallmarks, such as angiogenesis, invasiveness, and migration. In this review, we describe the involvement of Ca²⁺ signaling and associated proteins in cancer progression and in the development of multidrug resistance in cancer cells. We also highlight the possibilities and challenges of targeting the Ca²⁺ channels, transporters, and pumps involved in Ca²⁺ signaling in cancer cells through structure-based drug design. This work will open a new therapeutic window to be used against cancer in upcoming years.

Quantal Ca2+ release mediated by very few IP3 receptors that rapidly inactivate allows graded responses to IP3

Inositol 1,4,5-trisphosphate receptors (IP₃Rs) are intracellular Ca²⁺ channels that link extracellular stimuli to Ca²⁺ signals. Ca²⁺ release from intracellular stores is "quantal": low IP₃ concentrations rapidly release a fraction of the stores. Ca²⁺ release then slows or terminates without compromising responses to further IP₃ additions. The mechanisms are unresolved. Here, we synthesize a high-affinity partial agonist of IP₃Rs and use it to demonstrate that quantal responses do not require heterogenous Ca²⁺ stores. IP₃Rs respond incrementally to IP₃ and close after the initial response to low IP₃ concentrations. Comparing functional responses with IP₃ binding shows that only a tiny fraction of a cell's IP₃Rs mediate incremental Ca²⁺ release; inactivation does not therefore affect most IP₃Rs. We conclude, and test by simulations, that Ca²⁺ signals evoked by IP₃ pulses arise from rapid activation and then inactivation of very few IP₃Rs. This allows IP₃Rs to behave as increment detectors mediating graded Ca²⁺ release.

Estrogen depletion on In vivo osteocyte calcium signaling responses to mechanical loading

Microstructural adaptation of bone in response to mechanical stimuli is diminished with estrogen deprivation. Here we tested in vivo whether ovariectomy (OVX) alters the acute response of osteocytes, the principal mechanosensory cells of bone, to mechanical loading in mice. We also used super resolution microscopy (Structured Illumination microscopy or SIM) in conjunction with immunohistochemistry to assess changes in the number and organization of "osteocyte mechanosomes" - complexes of Panx1 channels, P2X7 receptors and CaV3 voltage-gated Ca²⁺ channels clustered around α_vβ₃ integrin foci on osteocyte processes. Third metatarsals bones of mice expressing an osteocyte-targeted genetically encoded Ca²⁺ indicator (DMP1-GCaMP3) were cyclically loaded in vivo to strains from 250 to 3000 με and osteocyte intracellular Ca²⁺ signaling responses were assessed in mid-diaphyses using multiphoton microscopy. The number of Ca²⁺ signaling osteocytes in control mice increase monotonically with applied strain magnitude for the physiological range of strains. The relationship between the number of Ca²⁺ signaling osteocytes and loading was unchanged at 2 days post-OVX. However, it was altered markedly at 28 days post-OVX. At loads up to 1000 με, there was a dramatic reduction in number of responding (i.e. Ca²⁺ signaling) osteocytes; however, at higher strains the numbers of Ca²⁺ signaling osteocytes were similar to control mice. OVX significantly altered the abundance, make-up and organization of osteocyte mechanosome complexes on dendritic processes. Numbers of α_vβ₃ foci also staining with either Panx 1, P2X7R or CaV3 declined by nearly half after OVX, pointing to a loss of osteocyte mechanosomes on the dendritic processes with estrogen depletion. At the same time, the areas of the remaining foci that stained for α_vβ₃ and channel proteins increased significantly, a redistribution of mechanosome components suggesting a potential compensatory response. These results demonstrate that the deleterious effects of estrogen depletion on skeletal mechanical adaptation appear at the level of mechanosensation; osteocytes lose the ability to sense small (physiological) mechanical stimuli. This decline may result at least partly from changes in the structure and organization of osteocyte mechanosomes, which contribute to the distinctive sensitivity of osteocytes (particularly their dendritic processes) to mechanical stimulation.

Inhibition of the pesticide rotenone-induced Ca2+ signaling, cytotoxicity and oxidative stress in HCN-2 neuronal cells by the phenolic compound hydroxytyrosol

Rotenone, a plant-derived pesticide belonging to genera Derris and Lonchorcarpus, is an inhibitor of NADH dehydrogenase complex. Studies have shown that rotenone was applied as a neurotoxic agent in various neuronal models. Hydroxytyrosol [2-(3,4-dihydroxyphenyl)-ethanol] is a natural phenolic compound found in the olive (Olea europaea L.). Studies of hydroxytyrosol have dramatically increased because this compound may contribute to the prevention of neurodegenerative diseases. Although hydroxytyrosol has received increasing attention due to its multiple pharmacological activities, it is not explored whether hydroxytyrosol inhibited rotenone-induced cytotoxicity in the neuronal cell model. The aim of this study was to explore whether hydroxytyrosol prevented rotenone-induced Ca²⁺ signaling, cytotoxicity and oxidative stress in HCN-2 neuronal cell line. In HCN-2 cells, rotenone (5-30 μM) concentration-dependently induced cytosolic Ca²⁺ concentrations ([Ca²⁺]_i) rises and cytotoxicity. Treatment with hydroxytyrosol (30 μM) reversed rotenone (20 μM)-induced cytotoxic responses. In Ca²⁺-containing medium, rotenone-induced Ca²⁺ entry was inhibited by 2-APB (a store-operated Ca²⁺ channel modulator) or hydroxytyrosol. In Ca²⁺-free medium, treatment with thapsigargin (an endoplasmic reticulum Ca²⁺ pump inhibitor) or hydroxytyrosol significantly inhibited rotenone-induced [Ca²⁺]_i rises. Furthermore, treatment with hydroxytyrosol reversed ROS levels, cytotoxic responses, and antioxidant enzyme activities (SOD, GPX and CAT) in rotenone-treated cells. Together, in HCN-2 cells, rotenone induced Ca²⁺ influx via store-operated Ca²⁺ entry and Ca²⁺ release from the endoplasmic reticulum and caused oxidative stress. Moreover, hydroxytyrosol ameliorated Ca²⁺ or ROS-associated cytotoxicity. It suggests that hydroxytyrosol might have a protective effect on rotenone-induced neurotoxicity in human neuronal cells.

WNT5B in cellular signaling pathways

The Wnt signaling ligand WNT5B is implicated in various developmental pathways, both in normal and pathological physiology. Most of the research on WNT5B has been associated with expression analysis and disease states, leaving the signaling pathways underexplored. Here, we review the current understandings of WNT5B's regulation of signal transduction, from receptors to downstream mediators and transcription factors. We also describe its roles in β-catenin-dependent and β-catenin-independent (Planar Cell Polarity and Wnt/Ca²⁺) Wnt signaling.

A potassium-sensing niche in Arabidopsis roots orchestrates signaling and adaptation responses to maintain nutrient homeostasis

Organismal homeostasis of the essential ion K⁺ requires sensing of its availability, efficient uptake, and defined distribution. Understanding plant K⁺ nutrition is essential to advance sustainable agriculture, but the mechanisms underlying K⁺ sensing and the orchestration of downstream responses have remained largely elusive. Here, we report where plants sense K⁺ deprivation and how this translates into spatially defined ROS signals to govern specific downstream responses. We define the organ-scale K⁺ pattern of roots and identify a postmeristematic K⁺-sensing niche (KSN) where rapid K⁺ decline and Ca²⁺ signals coincide. Moreover, we outline a bifurcating low-K⁺-signaling axis of CIF peptide-activated SGN3-LKS4/SGN1 receptor complexes that convey low-K⁺-triggered phosphorylation of the NADPH oxidases RBOHC, RBOHD, and RBOHF. The resulting ROS signals simultaneously convey HAK5 K⁺ uptake-transporter induction and accelerated Casparian strip maturation. Collectively, these mechanisms synchronize developmental differentiation and transcriptome reprogramming for maintaining K⁺ homeostasis and optimizing nutrient foraging by roots.

NMDA receptors elicit flux-independent intracellular Ca2+ signals via metabotropic glutamate receptors and flux-dependent nitric oxide release in human brain microvascular endothelial cells

The excitatory neurotransmitter glutamate gates post-synaptic N-methyl-d-aspartate (NMDA) receptors (NMDARs) to mediate extracellular Ca²⁺ entry and stimulate neuronal nitric oxide (NO) synthase to release NO and trigger neurovascular coupling (NVC). Neuronal and glial NMDARs may also operate in a flux-independent manner, although it is unclear whether their non-ionotropic mode of action is involved in NVC. Recently, endothelial NMDARs were found to trigger Ca²⁺-dependent NO production and induce NVC, but the underlying mode of signaling remains elusive. Herein, we report that GluN1 protein, as well as GluN2C and GluN3B transcripts and proteins, were expressed and that NMDA did not elicit inward currents, but induced a dose-dependent increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) in the human brain microvascular endothelial cell line, hCMEC/D3. A multidisciplinary approach, including live cell imaging, whole-cell patch-clamp recordings, pharmacological manipulation and gene targeting, revealed that NMDARs increase the [Ca²⁺]_i in a flux-independent manner in hCMEC/D3 cells. The Ca²⁺ response to NMDA was triggered by endogenous Ca²⁺ release from the endoplasmic reticulum and the lysosomal Ca²⁺ stores and sustained by store-operated Ca²⁺ entry. Unexpectedly, pharmacological and genetic blockade of mGluR1 and mGluR5 dramatically impaired NMDARs-mediated Ca²⁺ signals. These findings indicate that NMDARs may increase the endothelial [Ca²⁺]_i in a flux-independent manner via group 1 mGluRs. However, imaging of DAF-FM fluorescence revealed that NMDARs may also induce Ca²⁺-dependent NO release by signaling in a flux-dependent manner. These findings, therefore, shed novel light on the mechanisms whereby brain microvascular endothelium decodes glutamatergic signaling and regulates NVC.

The sigma 1 receptor: A local media influencer

The endoplasmic reticulum (ER) Ca2+ store contains many rapidly differentiable subdomains with specialized signaling properties. Recent work highlights how an integral ER membrane protein - the sigma 1 receptor (S1R) - nucleates local formation of cholesterol-rich ER subdomains. Biophysical approaches cast new light on S1Rs and how their dynamics is impacted by drugs and disease states.

TRPM5 Negatively Regulates Calcium-Dependent Responses in Lipopolysaccharide-Stimulated B Lymphocytes

B cells produce high amounts of cytokines and immunoglobulins in response to lipopolysaccharide (LPS) stimulation. Calcium signaling cascades are critically involved in cytokine production of T cells, and the cytosolic calcium concentration is regulated by calcium-activated monovalent cation channels (CAMs). Calcium signaling is also implicated in B cell activation; however, its involvement in the cytokine production of LPS-stimulated B cells remains less well characterized. Here, we show that the transient receptor potential melastatin 5 channel (TRPM5), which is one of the CAMs, negatively modulates calcium signaling, thereby regulating LPS-induced proliferative and inflammatory responses by B cells. LPS-stimulated B cells of Trpm5-deficient mice exhibit an increased cytosolic calcium concentration, leading to enhanced proliferation and the production of the inflammatory cytokines interleukin-6 and CXCL10. Furthermore, Trpm5-deficient mice show an exacerbation of endotoxic shock with high mortality. Our findings demonstrate the importance of TRPM5-dependent regulatory mechanisms in LPS-induced calcium signaling of splenic B cells.

Ca2+-mediated coupling between neuromuscular junction and mitochondria in skeletal muscle

The volitional movement of skeletal is controlled by the motor neuron at the site of neuromuscular junction (NMJ) where the retrograde signals are also passed back from muscle to the motor neuron. As the normal function of muscle largely depends on mitochondria that determine the fate of a skeletal muscle myofiber, there must exist a fine-controlled functional coupling between NMJ and mitochondria in myofibers. This mini-review discusses recent publications that reveal how spatiotemporal profiles of intracellular free Ca²⁺ could couple mitochondrial function with the activity of NMJ in skeletal muscle myofibers.

The first junior European Calcium Society meeting: calcium research across scales, Kingdoms and countries

The first junior European Calcium Society online meeting, held October 20-21, 2020, aimed to promote junior researchers in the Ca²⁺ community. The meeting included four scientific sessions, covering Ca²⁺ research from molecular detail to whole organisms. Each session featured one invited speaker and three speakers selected based on submitted abstracts, with the overall aim of actively involving early-career researchers. Consequently, the meeting underlined the diversity of Ca²⁺ physiology, by showcasing research across scales and Kingdoms, as presented by a correspondingly diverse speaker panel across career stages and countries. In this meeting report, we introduce the visions of the junior European Calcium Society board and summarize the meeting content.

Balancing ER-Mitochondrial Ca2+ Fluxes in Health and Disease

Organelles cooperate with each other to control cellular homeostasis and cell functions by forming close connections through membrane contact sites. Important contacts are present between the endoplasmic reticulum (ER), the main intracellular Ca²⁺-storage organelle, and the mitochondria, the organelle responsible not only for the majority of cellular ATP production but also for switching on cell death processes. Several Ca²⁺-transport systems focalize at these contact sites, thereby enabling the efficient transmission of Ca²⁺ signals from the ER toward mitochondria. This provides tight control of mitochondrial functions at the microdomain level. Here, we discuss how ER-mitochondrial Ca²⁺ transfers support cell function and how their dysregulation underlies, drives, or contributes to pathogenesis and pathophysiology, with a major focus on cancer and neurodegeneration but also with attention to other diseases such as diabetes and rare genetic diseases.

2-Methoxyestradiol and its derivatives inhibit store-operated Ca2+ entry in T cells: Identification of a new and potent inhibitor

T cell activation starts with formation of second messengers that release Ca²⁺ from the endoplasmic reticulum (ER) and thereby activate store-operated Ca²⁺ entry (SOCE), one of the essential signals for T cell activation. Recently, the steroidal 2-methoxyestradiol was shown to inhibit nuclear translocation of the nuclear factor of activated T cells (NFAT). We therefore investigated 2-methoxyestradiol for inhibition of Ca²⁺ entry in T cells, screened a library of 2-methoxyestradiol analogues, and characterized the derivative 2-ethyl-3-sulfamoyloxy-17β-cyanomethylestra-1,3,5(10)-triene (STX564) as a novel, potent and specific SOCE inhibitor. STX564 inhibits Ca²⁺ entry via SOCE without affecting other ion channels and pumps involved in Ca²⁺ signaling in T cells. Downstream effects such as cytokine expression and cell proliferation were also inhibited by both 2-methoxyestradiol and STX564, which has potential as a new chemical biology tool.

Emerging roles of the CBL-CIPK calcium signaling network as key regulatory hub in plant nutrition

Plant physiology and development essentially depend on sufficient uptake of various essential nutritive ions via their roots and their appropriate transport and distribution within the organism. Many of these essential nutrients are heterogeneously distributed in the soil or are available in fluctuating concentrations. This natural situation requires constant regulatory adjustment and balancing of nutrient uptake and homeostasis. Here, we review recent findings on the role of Ca²⁺ signals and Ca²⁺-dependent regulation via the CBL-CIPK Ca²⁺ sensor-protein kinase network in these processes. We put special emphasis on Ca²⁺ controlled processes that contribute to establishing the homeostasis of macro-nutrients like potassium (K⁺), nitrogen (N), and magnesium (Mg²⁺) and on the micro-nutrient iron (Fe). Increasing experimental evidence indicates the occurrence of nutrient-specific, spatially and temporally defined cytoplasmic Ca²⁺ elevations as early responses to nutrient fluctuations. Specific CBL-CIPK complexes translate these signals into phosphorylation regulation of important channels and transporters like AKT1, NPF6.3/NRT1.1, AMT1, SLAC1, TPK1 and IRT1. We discuss a crucial and coordinating role for these Ca²⁺ signaling mechanisms in regulating the sensing, uptake, distribution and storage of various ions. Finally, we reflect on the emerging multifaceted and potentially integrating role of the "nutrient" kinase CIPK23 in regulating multiple nutrient responses. From this inventory, we finally deduce potential mechanisms that can convey the coordinated regulation of distinct steps in the transport of one individual ion and mechanisms that can bring about the integration of adaptive responses to fluctuations of different ions to establish a faithfully balanced plant nutrient homeostasis.