Intracellular Ca2+ storage in health and disease: a dynamic equilibrium

Cigarette smoke-induced Ca2+ release leads to cystic fibrosis transmembrane conductance regulator (CFTR) dysfunction

Chronic obstructive pulmonary disease affects 64 million people and is currently the fourth leading cause of death worldwide. Chronic obstructive pulmonary disease includes both emphysema and chronic bronchitis, and in the case of chronic bronchitis represents an inflammatory response of the airways that is associated with mucus hypersecretion and obstruction of small airways. Recently, it has emerged that exposure to cigarette smoke (CS) leads to an inhibition of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel, causing airway surface liquid dehydration, which may play a role in the development of chronic bronchitis. CS rapidly clears CFTR from the plasma membrane and causes it to be deposited into aggresome-like compartments. However, little is known about the mechanism(s) responsible for the internalization of CFTR following CS exposure. Our studies revealed that CS triggered a rise in cytoplasmic Ca(2+) that may have emanated from lysosomes. Furthermore, chelation of cytoplasmic Ca(2+), but not inhibition of protein kinases/phosphatases, prevented CS-induced CFTR internalization. The macrolide antibiotic bafilomycin A1 inhibited CS-induced Ca(2+) release and prevented CFTR clearance from the plasma membrane, further linking cytoplasmic Ca(2+) and CFTR internalization. We hypothesize that CS-induced Ca(2+) release prevents normal sorting/degradation of CFTR and causes internalized CFTR to reroute to aggresomes. Our data provide mechanistic insight into the potentially deleterious effects of CS on airway epithelia and outline a hitherto unrecognized signaling event triggered by CS that may affect the long term transition of the lung into a hyper-inflammatory/dehydrated environment.

SERCA2 dysfunction in Darier disease causes endoplasmic reticulum stress and impaired cell-to-cell adhesion strength: rescue by Miglustat

Darier disease (DD) is a severe dominant genetic skin disorder characterized by the loss of cell-to-cell adhesion and abnormal keratinization. The defective gene, ATP2A2, encodes sarco/endoplasmic reticulum (ER) Ca2+ -ATPase isoform 2 (SERCA2), a Ca2+ -ATPase pump of the ER. Here we show that Darier keratinocytes (DKs) display biochemical and morphological hallmarks of constitutive ER stress with increased sensitivity to ER stressors. Desmosome and adherens junctions (AJs) displayed features of immature adhesion complexes: expression of desmosomal cadherins (desmoglein 3 (Dsg3) and desmocollin 3 (Dsc3)) and desmoplakin was impaired at the plasma membrane, as well as E-cadherin, β-, α-, and p120-catenin staining. Dsg3, Dsc3, and E-cadherin showed perinuclear staining and co-immunostaining with ER markers, indicative of ER retention. Consistent with these abnormalities, intercellular adhesion strength was reduced as shown by a dispase mechanical dissociation assay. Exposure of normal keratinocytes to the SERCA2 inhibitor thapsigargin recapitulated these abnormalities, supporting the role of loss of SERCA2 function in impaired desmosome and AJ formation. Remarkably, treatment of DKs with the orphan drug Miglustat, a pharmacological chaperone, restored mature AJ and desmosome formation, and improved adhesion strength. These results point to an important contribution of ER stress in DD pathogenesis and provide the basis for future clinical evaluation of Miglustat in Darier patients.

The yeast p5 type ATPase, spf1, regulates manganese transport into the endoplasmic reticulum

The endoplasmic reticulum (ER) is a large, multifunctional and essential organelle. Despite intense research, the function of more than a third of ER proteins remains unknown even in the well-studied model organism Saccharomyces cerevisiae. One such protein is Spf1, which is a highly conserved, ER localized, putative P-type ATPase. Deletion of SPF1 causes a wide variety of phenotypes including severe ER stress suggesting that this protein is essential for the normal function of the ER. The closest homologue of Spf1 is the vacuolar P-type ATPase Ypk9 that influences Mn(2+) homeostasis. However in vitro reconstitution assays with Spf1 have not yielded insight into its transport specificity. Here we took an in vivo approach to detect the direct and indirect effects of deleting SPF1. We found a specific reduction in the luminal concentration of Mn(2+) in ∆spf1 cells and an increase following it's overexpression. In agreement with the observed loss of luminal Mn(2+) we could observe concurrent reduction in many Mn(2+)-related process in the ER lumen. Conversely, cytosolic Mn(2+)-dependent processes were increased. Together, these data support a role for Spf1p in Mn(2+) transport in the cell. We also demonstrate that the human sequence homologue, ATP13A1, is a functionally conserved orthologue. Since ATP13A1 is highly expressed in developing neuronal tissues and in the brain, this should help in the study of Mn(2+)-dependent neurological disorders.

Are Orai1 and Orai3 channels more important than calcium influx for cell proliferation?

Transformed and tumoral cells share the characteristic of being able to proliferate even when external calcium concentration is very low. We have investigated whether Human Embryonic Kidney 293 cells, human hepatoma cell Huh-7 and HeLa cells were able to proliferate when kept 72h in complete culture medium without external calcium. Our data showed that cell proliferation rate was similar over a range of external calcium concentration (2μM to 1.8mM). Incubation in the absence of external calcium for 72h had no significant effect on endoplasmic reticulum (ER) Ca(2+) contents but resulted in a significant decrease in cytosolic free calcium concentration in all 3 cell types. Cell proliferation rates were dependent on Orai1 and Orai3 expression levels in HEK293 and HeLa cells. Silencing Orai1 or Orai3 resulted in a 50% reduction in cell proliferation rate. Flow cytometry analysis showed that Orai3 induced a small but significant increase in cell number in G2/M phase. RO-3306, a cdk-1 inhibitor, induced a 90% arrest in G2/M reversible in less than 15min. Our data showed that progression through G2/M phase after release from RO-3306-induced cell cycle arrest was slower in both Orai1 and Orai3 knock-downs. Overexpressing Orai1, Orai3 and the dominant negative non-permeant mutants E106Q-Orai1 and E81Q-Orai3 induced a 50% increase in cell proliferation rate in HEK293 cells. Our data clearly demonstrated that Orai1 and Orai3 proteins are more important than calcium influx to control cell proliferation in some cell lines and that this process is probably independent of ICRAC and Iarc.

Bax Inhibitor-1-mediated Ca2+ leak is decreased by cytosolic acidosis

Bax Inhibitor-1 (BI-1) is an evolutionarily conserved six-transmembrane domain endoplasmic reticulum (ER)-localized protein that protects against ER stress-induced apoptotic cell death. This function is closely connected to its ability to lower steady-state ER Ca2+ levels. Recently, we elucidated BI-1's Ca(2+)-channel pore in the C-terminal part of the protein and identified the critical amino acids of its pore. Based on these insights, a Ca(2+)-channel pore-dead mutant BI-1 (BI-1(D213R)) was developed. We determined whether BI-1 behaves as a bona fide H+/Ca2+ antiporter or as an ER Ca(2+)-leak channel by investigating the effect of pH on unidirectional Ca(2+)-efflux rates. At pH 6.8, wild-type BI-1 expression in BI-1(-/-) cells increased the ER Ca(2+)-leak rate, correlating with its localization in the ER compartment. In contrast, BI-1(D231R) expression in BI-1(-/-), despite its ER localization, did not increase the ER Ca(2+)-leak rate. However, at pH < 6.8, the BI-1-mediated ER Ca2+ leak was blocked. Finally, a peptide representing the Ca(2+)-channel pore of BI-1 promoting Ca2+ flux from the ER was used. Lowering the pH from 6.8 to 6.0 completely abolished the ability of the BI-1 peptide to mediate Ca2+ flux from the ER. We propose that this pH dependence is due to two aspartic acid residues critical for the function of the Ca(2+)-channel pore and located in the ER membrane-dipping domain, which facilitates the protonation of these residues.

Wolfram Syndrome protein, Miner1, regulates sulphydryl redox status, the unfolded protein response, and Ca2+ homeostasis

Miner1 is a redox-active 2Fe2S cluster protein. Mutations in Miner1 result in Wolfram Syndrome, a metabolic disease associated with diabetes, blindness, deafness, and a shortened lifespan. Embryonic fibroblasts from Miner1(-/-) mice displayed ER stress and showed hallmarks of the unfolded protein response. In addition, loss of Miner1 caused a depletion of ER Ca(2+) stores, a dramatic increase in mitochondrial Ca(2+) load, increased reactive oxygen and nitrogen species, an increase in the GSSG/GSH and NAD(+)/NADH ratios, and an increase in the ADP/ATP ratio consistent with enhanced ATP utilization. Furthermore, mitochondria in fibroblasts lacking Miner1 displayed ultrastructural alterations, such as increased cristae density and punctate morphology, and an increase in O2 consumption. Treatment with the sulphydryl anti-oxidant N-acetylcysteine reversed the abnormalities in the Miner1 deficient cells, suggesting that sulphydryl reducing agents should be explored as a treatment for this rare genetic disease.

Curcumin affects proprotein convertase activity: elucidation of the molecular and subcellular mechanism

Proprotein convertases (PCs) form a group of serine endoproteases that are essential for the activation of proproteins into their active form. Some PCs have been proposed to be potential therapeutic targets for cancer intervention because elevated PC activity has been observed in many different cancer types and because many of the PC substrates, such as pro-IGF-1R, pro-TGF-beta, pro-VEGF, are involved in signaling pathways related to tumor development. Curcumin, reported to possess anticancer activity, also affects many of these pathways. We therefore investigated the effect of curcumin on PC activity. Our results show that curcumin inhibits PC activity in a cell lysate-based assay but not in vitro. PC zymogen maturation in the endoplasmic reticulum appears to be inhibited by curcumin. Treating cells with thapsigargin or cyclopiazonic acid, two structurally unrelated inhibitors of the sarco- and endoplasmic reticulum Ca(2+)ATPase (SERCA), also hampered both the PC zymogen maturation and the PC activity. Importantly, curcumin, like the SERCA inhibitors, impaired ATP-driven (45)Ca(2+) uptake in the endoplasmic reticulum. These results indicate that curcumin likely restrains PC activity by inhibiting SERCA-mediated Ca(2+)-uptake activity. Experiments in three colon cancer cell lines confirm that curcumin inhibits both the (45)Ca(2+) uptake and PC activity, notably the processing of pro-IGF-1R. Both curcumin and thapsigargin inhibit the anchorage-independent growth of these three colon carcinoma cell lines. In conclusion, our findings indicate that curcumin inhibits PC zymogen maturation and consequently PC activity and that its inhibitory effect on Ca(2+) uptake into the ER allows and is sufficient to explain this phenomenon.

mTOR-Controlled Autophagy Requires Intracellular Ca(2+) Signaling

Autophagy is a lysosomal degradation pathway important for cellular homeostasis and survival. Inhibition of the mammalian target of rapamycin (mTOR) is the best known trigger for autophagy stimulation. In addition, intracellular Ca²⁺ regulates autophagy, but its exact role remains ambiguous. Here, we report that the mTOR inhibitor rapamycin, while enhancing autophagy, also remodeled the intracellular Ca²⁺-signaling machinery. These alterations include a) an increase in the endoplasmic-reticulum (ER) Ca²⁺-store content, b) a decrease in the ER Ca²⁺-leak rate, and c) an increased Ca²⁺ release through the inositol 1,4,5-trisphosphate receptors (IP₃Rs), the main ER-resident Ca²⁺-release channels. Importantly, buffering cytosolic Ca²⁺ with BAPTA impeded rapamycin-induced autophagy. These results reveal intracellular Ca²⁺ signaling as a crucial component in the canonical mTOR-dependent autophagy pathway.

The selective BH4-domain biology of Bcl-2-family members: IP3Rs and beyond

Anti-apoptotic Bcl-2-family members not only neutralize pro-apoptotic proteins but also directly regulate intracellular Ca(2+) signaling from the endoplasmic reticulum (ER), critically controlling cellular health, survival, and death initiation. Furthermore, distinct Bcl-2-family members may selectively regulate inositol 1,4,5-trisphosphate receptor (IP3R): Bcl-2 likely acts as an endogenous inhibitor of the IP3R, preventing pro-apoptotic Ca(2+) transients, while Bcl-XL likely acts as an endogenous IP3R-sensitizing protein promoting pro-survival Ca(2+) oscillations. Furthermore, distinct functional domains in Bcl-2 and Bcl-XL may underlie the divergence in IP3R regulation. The Bcl-2 homology (BH) 4 domain, which targets the central modulatory domain of the IP3R, is likely to be Bcl-2's determining factor. In contrast, the hydrophobic cleft targets the C-terminal Ca(2+)-channel tail and might be more crucial for Bcl-XL's function. Furthermore, one amino acid critically different in the sequence of Bcl-2's and Bcl-XL's BH4 domains underpins their selective effect on Ca(2+) signaling and distinct biological properties of Bcl-2 versus Bcl-XL. This difference is evolutionary conserved across five classes of vertebrates and may represent a fundamental divergence in their biological function. Moreover, these insights open novel avenues to selectively suppress malignant Bcl-2 function in cancer cells by targeting its BH4 domain, while maintaining essential Bcl-XL functions in normal cells. Thus, IP3R-derived molecules that mimic the BH4 domain's binding site on the IP3R may function synergistically with BH3-mimetic molecules selectivity suppressing Bcl-2's proto-oncogenic activity. Finally, a more general role for the BH4 domain on IP3Rs, rather than solely anti-apoptotic, may not be excluded as part of a complex network of molecular interactions.

Free fatty acid induces endoplasmic reticulum stress and apoptosis of β-cells by Ca2+/calpain-2 pathways

Dysfunction of β-cells is a major characteristic in the pathogenesis of type 2 diabetes mellitus (T2DM). The combination of obesity and T2DM is associated with elevated plasma free fatty acids (FFAs). However, molecular mechanisms linking FFAs to β-cell dysfunction remain poorly understood. In the present study, we identified that the major endoplasmic reticulum stress (ERS) marker, Grp78 and ERS-induced apoptotic factor, CHOP, were time-dependently increased by exposure of β-TC3 cells to FFA. The expression of ATF6 and the phosphorylation levels of PERK and IRE1, which trigger ERS signaling, markedly increased after FFA treatments. FFA treatments increased cell apoptosis by inducing ERS in β-TC3 cells. We also found that FFA-induced ERS was mediated by the store-operated Ca²⁺ entry through promoting the association of STIM1 and Orai1. Moreover, calpain-2 was required for FFA-induced expression of CHOP and activation of caspase-12 and caspase-3, thus promoting cell apoptosis in β-TC3 cells. Together, these results reveal pivotal roles for Ca²⁺/calpain-2 pathways in modulating FFA-induced β-TC3 cell ERS and apoptosis.

Fragmented inositol 1,4,5-trisphosphate receptors retain tetrameric architecture and form functional Ca2+ release channels

Inositol 1,4,5-trisphosphate receptor isoforms are a family of ubiquitously expressed ligand-gated channels encoded by three individual genes. The proteins are localized to membranes of intracellular Ca(2+) stores and play pivotal roles in Ca(2+) homeostasis. Previous studies have demonstrated that IP3R1 is cleaved by the intracellular proteases calpain and caspase both in vivo and in vitro. However, the resultant cleavage products are poorly defined, and the functional consequences of these proteolytic events are not fully understood. We demonstrate that IP3R1 is cleaved during staurosporine-induced apoptosis, yielding N-terminal fragments encompassing the ligand-binding domain and the majority of the central modulatory domain together with a C-terminal fragment containing the channel domain and cytosolic tail. Notably, these fragments remain associated with the membrane after initiation of apoptotic cleavage. Furthermore, when recombinant IP3R1 fragments, corresponding to those predicted to be generated by caspase or calpain cleavage, are stably coexpressed in cells, they physically associate and form functional channels. These data provide novel insights regarding the regulation of IP3R1 during proteolysis and provide direct evidence that polypeptide continuity is not required for IP3R activation and Ca(2+) release.

Characterization of functional TRPV1 channels in the sarcoplasmic reticulum of mouse skeletal muscle

TRPV1 represents a non-selective cation channel activated by capsaicin, acidosis and high temperature. In the central nervous system where TRPV1 is highly expressed, its physiological role in nociception is clearly identified. In skeletal muscle, TRPV1 appears implicated in energy metabolism and exercise endurance. However, how as a Ca(2+) channel, it contributes to intracellular calcium concentration ([Ca(2+)]i) maintenance and muscle contraction remains unknown. Here, as in rats, we report that TRPV1 is functionally expressed in mouse skeletal muscle. In contrast to earlier reports, our analysis show TRPV1 presence only at the sarcoplasmic reticulum (SR) membrane (preferably at the longitudinal part) in the proximity of SERCA1 pumps. Using intracellular Ca(2+) imaging, we directly accessed to the channel functionality in intact FDB mouse fibers. Capsaicin and resiniferatoxin, both agonists as well as high temperature (45°C) elicited an increase in [Ca(2+)]i. TRPV1-inhibition by capsazepine resulted in a strong inhibition of TRPV1-mediated functional responses and abolished channel activation. Blocking the SR release (with ryanodine or dantrolene) led to a reduced capsaicin-induced Ca(2+) elevation suggesting that TRPV1 may participate to a secondary SR Ca(2+) liberation of greater amplitude. In conclusion, our experiments point out that TRPV1 is a functional SR Ca(2+) leak channel and may crosstalk with RyR1 in adult mouse muscle fibers.

The contribution of systematic approaches to characterizing the proteins and functions of the endoplasmic reticulum

The endoplasmic reticulum (ER) is a complex organelle responsible for a range of functions including protein folding and secretion, lipid biosynthesis, and ion homeostasis. Despite its central and essential roles in eukaryotic cells during development, growth, and disease, many ER proteins are poorly characterized. Moreover, the range of biochemical reactions that occur within the ER membranes, let alone how these different activities are coordinated, is not yet defined. In recent years, focused studies on specific ER functions have been complemented by systematic approaches and innovative technologies for high-throughput analysis of the location, levels, and biological impact of given components. This article focuses on the recent progress of these efforts, largely pioneered in the budding yeast Saccharomyces cerevisiae, and also addresses how future systematic studies can be geared to uncover the "dark matter" of uncharted ER functions.

Endoplasmic reticulum membrane reorganization is regulated by ionic homeostasis

Recently we described a new, evolutionarily conserved cellular stress response characterized by a reversible reorganization of endoplasmic reticulum (ER) membranes that is distinct from canonical ER stress and the unfolded protein response (UPR). Apogossypol, a putative broad spectrum BCL-2 family antagonist, was the prototype compound used to induce this ER membrane reorganization. Following microarray analysis of cells treated with apogossypol, we used connectivity mapping to identify a wide range of structurally diverse chemicals from different pharmacological classes and established their ability to induce ER membrane reorganization. Such structural diversity suggests that the mechanisms initiating ER membrane reorganization are also diverse and a major objective of the present study was to identify potentially common features of these mechanisms. In order to explore this, we used hierarchical clustering of transcription profiles for a number of chemicals that induce membrane reorganization and discovered two distinct clusters. One cluster contained chemicals with known effects on Ca²⁺ homeostasis. Support for this was provided by the findings that ER membrane reorganization was induced by agents that either deplete ER Ca²⁺ (thapsigargin) or cause an alteration in cellular Ca²⁺ handling (calmodulin antagonists). Furthermore, overexpression of the ER luminal Ca²⁺ sensor, STIM1, also evoked ER membrane reorganization. Although perturbation of Ca²⁺ homeostasis was clearly one mechanism by which some agents induced ER membrane reorganization, influx of extracellular Na⁺ but not Ca²⁺ was required for ER membrane reorganization induced by apogossypol and the related BCL-2 family antagonist, TW37, in both human and yeast cells. Not only is this novel, non-canonical ER stress response evolutionary conserved but so also are aspects of the mechanism of formation of ER membrane aggregates. Thus perturbation of ionic homeostasis is important in the regulation of ER membrane reorganization.

Regulation of inositol 1,4,5-trisphosphate receptors during endoplasmic reticulum stress

The endoplasmic reticulum (ER) performs multiple functions in the cell: it is the major site of protein and lipid synthesis as well as the most important intracellular Ca(2+) reservoir. Adverse conditions, including a decrease in the ER Ca(2+) level or an increase in oxidative stress, impair the formation of new proteins, resulting in ER stress. The subsequent unfolded protein response (UPR) is a cellular attempt to lower the burden on the ER and to restore ER homeostasis by imposing a general arrest in protein synthesis, upregulating chaperone proteins and degrading misfolded proteins. This response can also lead to autophagy and, if the stress can not be alleviated, to apoptosis. The inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) and IP3-induced Ca(2+) signaling are important players in these processes. Not only is the IP3R activity modulated in a dual way during ER stress, but also other key proteins involved in Ca(2+) signaling are modulated. Changes also occur at the structural level with a strengthening of the contacts between the ER and the mitochondria, which are important determinants of mitochondrial Ca(2+) uptake. The resulting cytoplasmic and mitochondrial Ca(2+) signals will control cellular decisions that either promote cell survival or cause their elimination via apoptosis. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

IP3, a small molecule with a powerful message

Research conducted over the past two decades has provided convincing evidence that cell death, and more specifically apoptosis, can exceed single cell boundaries and can be strongly influenced by intercellular communication networks. We recently reported that gap junctions (i.e. channels directly connecting the cytoplasm of neighboring cells) composed of connexin43 or connexin26 provide a direct pathway to promote and expand cell death, and that inositol 1,4,5-trisphosphate (IP3) diffusion via these channels is crucial to provoke apoptosis in adjacent healthy cells. However, IP3 itself is not sufficient to induce cell death and additional factors appear to be necessary to create conditions in which IP3 will exert proapoptotic effects. Although IP3-evoked Ca(2+) signaling is known to be required for normal cell survival, it is also actively involved in apoptosis induction and progression. As such, it is evident that an accurate fine-tuning of this signaling mechanism is crucial for normal cell physiology, while a malfunction can lead to cell death. Here, we review the role of IP3 as an intracellular and intercellular cell death messenger, focusing on the endoplasmic reticulum-mitochondrial synapse, followed by a discussion of plausible elements that can convert IP3 from a physiological molecule to a killer substance. Finally, we highlight several pathological conditions in which anomalous intercellular IP3/Ca(2+) signaling might play a role. This article is part of a Special Issue entitled:12th European Symposium on Calcium.

Acute effect of ethanol on hepatic reticular G6Pase and Ca2+ pool

BACKGROUND

Hydrolysis of glucose 6-phosphate (G6P) via glucose 6-phosphatase (G6Pase) enlarges the reticular Ca(2+) pool of the hepatocyte. Exposure of liver cells to ethanol (EtOH) impairs reticular Ca(2+) homeostasis. The present study investigated the effect of acute EtOH administration on G6P-supported Ca(2+) accumulation in liver cells.

METHODS

Total microsomes were isolated from rat livers acutely perfused with varying doses of EtOH (0.01, 0.1, or 1% v/v) for 8 minutes. Calcium uptake was assessed by (45) Ca redistribution. Inorganic phosphate (Pi) formation was measured as an indicator of G6Pase hydrolytic activity.

RESULTS

G6P-supported Ca(2+) uptake decreased in a manner directly proportional to the dose of EtOH infused in the liver, whereas Ca(2+) uptake via SERCA pumps was decreased by ~25% only at the highest dose of alcohol administered. The reduced accumulation of Ca(2+) within the microsomes resulted in a smaller inositol 1,4,5-trisphosphate (IP(3))-induced Ca(2+) release. Kinetic assessment of IP(3) and passive Ca(2+) release indicated a faster mobilization in microsomes from EtOH-treated livers, suggesting alcohol-induced alteration of Ca(2+) releasing mechanisms. Pretreatment of livers with chloromethiazole (CMZ) or dithiothreitol (DTT), but not 4-methyl-pyrazole prevented the inhibitory effect of EtOH on G6Pase activity and Ca(2+) homeostasis.

CONCLUSIONS

Liver G6Pase activity and IP(3) -mediated Ca(2+) release are rapidly inhibited following acute (8 minutes) exposure to EtOH, thus compromising the ability of the endoplasmic reticulum to dynamically modulate Ca(2+) homeostasis in the hepatocyte. The protective effect of CMZ and DTT suggests that the inhibitory effect of EtOH is mediated through its metabolism via reticular cyP4502E1 and consequent free radicals formation.

Altered Ca(2+) signaling in cancer cells: proto-oncogenes and tumor suppressors targeting IP3 receptors

Proto-oncogenes and tumor suppressors critically control cell-fate decisions like cell survival, adaptation and death. These processes are regulated by Ca(2+) signals arising from the endoplasmic reticulum, which at distinct sites is in close proximity to the mitochondria. These organelles are linked by different mechanisms, including Ca(2+)-transport mechanisms involving the inositol 1,4,5-trisphosphate receptor (IP3R) and the voltage-dependent anion channel (VDAC). The amount of Ca(2+) transfer from the endoplasmic reticulum to mitochondria determines the susceptibility of cells to apoptotic stimuli. Suppressing the transfer of Ca(2+) from the endoplasmic reticulum to the mitochondria increases the apoptotic resistance of cells and may decrease the cellular responsiveness to apoptotic signaling in response to cellular damage or alterations. This can result in the survival, growth and proliferation of cells with oncogenic features. Clearly, proper maintenance of endoplasmic reticulum Ca(2+) homeostasis and dynamics including its links with the mitochondrial network is essential to detect and eliminate altered cells with oncogenic features through the apoptotic pathway. Proto-oncogenes and tumor suppressors exploit the central role of Ca(2+) signaling by targeting the IP3R. There are an increasing number of reports showing that activation of proto-oncogenes or inactivation of tumor suppressors directly affects IP3R function and endoplasmic reticulum Ca(2+) homeostasis, thereby decreasing mitochondrial Ca(2+) uptake and mitochondrial outer membrane permeabilization. In this review, we provide an overview of the current knowledge on the proto-oncogenes and tumor suppressors identified as IP3R-regulatory proteins and how they affect endoplasmic reticulum Ca(2+) homeostasis and dynamics.

Models of calcium dynamics in cerebellar granule cells

Intracellular calcium dynamics is critical for many functions of cerebellar granule cells (GrCs) including membrane excitability, synaptic plasticity, apoptosis, and regulation of gene transcription. Recent measurements of calcium responses in GrCs to depolarization and synaptic stimulation reveal spatial compartmentalization and heterogeneity within dendrites of these cells. However, the main determinants of local calcium dynamics in GrCs are still poorly understood. One reason is that there have been few published studies of calcium dynamics in intact GrCs in their native environment. In the absence of complete information, biophysically realistic models are useful for testing whether specific Ca(2+) handling mechanisms may account for existing experimental observations. Simulation results can be used to identify critical measurements that would discriminate between different models. In this review, we briefly describe experimental studies and phenomenological models of Ca(2+) signaling in GrC, and then discuss a particular biophysical model, with a special emphasis on an approach for obtaining information regarding the distribution of Ca(2+) handling systems under conditions of incomplete experimental data. Use of this approach suggests that Ca(2+) channels and fixed endogenous Ca(2+) buffers are highly heterogeneously distributed in GrCs. Research avenues for investigating calcium dynamics in GrCs by a combination of experimental and modeling studies are proposed.

Multivalent benzene polyphosphate derivatives are non-Ca2+-mobilizing Ins(1,4,5)P3 receptor antagonists

Inositol 1,4,5-trisphosphate [Ins(1,4,5)P31] mobilizes intracellular Ca2+ through the Ins(1,4,5)P3 receptor [InsP3R]. Although some progress has been made in the design of synthetic InsP3R partial agonists and antagonists, there are still few examples of useful small molecule competitive antagonists. A "multivalent" approach is explored and new dimeric polyphosphorylated aromatic derivatives were designed, synthesized and biologically evaluated. The established weak InsP3R ligand benzene 1,2,4-trisphosphate [Bz(1,2,4)P32] is dimerized through its 5-position in two different ways, first directly as the biphenyl derivative biphenyl 2,2',4,4',5,5'-hexakisphosphate, [BiPh(2,2',4,4',5,5')P68] and with its regioisomeric biphenyl 3,3',4,4',5,5'-hexakisphosphate [BiPh(3,3',4,4',5,5')P611]. Secondly, a linker motif is introduced in a flexible ethylene-bridged dimer (9) with its corresponding 1,2-bisphosphate dimer (10), both loosely analogous to the very weak antagonist 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA 7). In permeabilized L15 fibroblasts overexpressing type 1 InsP3R, BiPh(2,2',4,4',5,5')P6 (8) inhibits Ins(1,4,5)P3-induced Ca2+ release in a apparently competitive fashion [IC50 187 nM] and the Bz(1,2,4)P3 dimer (9) is only slightly weaker [IC50 380 nM]. Compounds were also evaluated against type I Ins(1,4,5)P3 5-phosphatase. All compounds are resistant to dephosphorylation, with BiPh(2,2',4,4',5,5')P6 (8), being the most effective inhibitor of any biphenyl derivative synthesized to date [IC50 480 nM] and the Bz(1,2,4)P3 ethylene dimer (9) weaker [IC50 3.55 μM]. BiPh(3,3',4,4',5,5')P6 (11) also inhibits 5-phosphatase [IC50 730 nM] and exhibits unexpected Ca2+ releasing activity [EC50 800 nM]. Thus, relocation of only a single mirrored phenyl phosphate group in (11) from that of antagonist (8) does not markedly change enzyme inhibitory activity, but elicits a dramatic switch in Ca2+-releasing activity. Such new agents demonstrate the power of the multivalent approach and may be useful to investigate the chemical biology of signaling through InsP3R and as templates for further design.

Trafficking of bdelloid rotifer late embryogenesis abundant proteins

The bdelloid rotifer Adineta ricciae is an asexual microinvertebrate that can survive desiccation by entering an ametabolic state known as anhydrobiosis. Two late embryogenesis abundant (LEA) proteins, ArLEA1A and ArLEA1B, have been hypothesized to contribute to desiccation tolerance in these organisms, since in vitro assays suggest that ArLEA1A and ArLEA1B stabilize desiccation-sensitive proteins and membranes, respectively. To examine their functions in vivo, it is important to analyse the cellular distribution of the bdelloid LEA proteins. Bioinformatics predicted their translocation into the endoplasmic reticulum (ER) via an N-terminal ER translocation signal and persistence in the same compartment via a variant C-terminal retention signal sequence ATEL. We assessed the localization of LEA proteins in bdelloids and in a mammalian cell model. The function of the N-terminal sequence of ArLEA1A and ArLEA1B in mediating ER translocation was verified, but our data showed that, unlike classical ER-retention signals, ATEL allows progression from the ER to the Golgi and limited secretion of the proteins into the extracellular medium. These results suggest that the N-terminal ER translocation signal and C-terminal ATEL sequence act together to regulate the distribution of rotifer LEA proteins within intracellular vesicular compartments, as well as the extracellular space. We speculate that this mechanism allows a small number of LEA proteins to offer protection to a large number of desiccation-sensitive molecules and structures both inside and outside cells in the bdelloid rotifer.

Polycystins and cellular Ca2+ signaling

The cystic phenotype in autosomal dominant polycystic kidney disease is characterized by a profound dysfunction of many cellular signaling patterns, ultimately leading to an increase in both cell proliferation and apoptotic cell death. Disturbance of normal cellular Ca²⁺ signaling seems to be a primary event and is clearly involved in many pathways that may lead to both types of cellular responses. In this review, we summarize the current knowledge about the molecular and functional interactions between polycystins and multiple components of the cellular Ca²⁺-signaling machinery. In addition, we discuss the relevant downstream responses of the changed Ca²⁺ signaling that ultimately lead to increased proliferation and increased apoptosis as observed in many cystic cell types.

Observations of calcium dynamics in cortical secretory vesicles

Calcium (Ca(2+)) dynamics were evaluated in fluorescently labeled sea urchin secretory vesicles using confocal microscopy. 71% of the vesicles examined exhibited one or more transient increases in the fluorescence signal that was damped in time. The detection of transient increases in signal was dependent upon the affinity of the fluorescence indicator; the free Ca(2+) concentration in the secretory vesicles was estimated to be in the range of ∼10 to 100 μM. Non-linear stochastic analysis revealed the presence of extra variance in the Ca(2+) dependent fluorescence signal. This noise process increased linearly with the amplitude of the Ca(2+) signal. Both the magnitude and spatial properties of this noise process were dependent upon the activity of vesicle p-type (Ca(v)2.1) Ca(2+) channels. Blocking the p-type Ca(2+) channels with ω-agatoxin decreased signal variance, and altered the spatial noise pattern within the vesicle. These fluorescence signal properties are consistent with vesicle Ca(2+) dynamics and not simply due to obvious physical properties such as gross movement artifacts or pH driven changes in Ca(2+) indicator fluorescence. The results suggest that the free Ca(2+) content of cortical secretory vesicles is dynamic; this property may modulate the exocytotic fusion process.

The impact of the unfolded protein response on human disease

A central function of the endoplasmic reticulum (ER) is to coordinate protein biosynthetic and secretory activities in the cell. Alterations in ER homeostasis cause accumulation of misfolded/unfolded proteins in the ER. To maintain ER homeostasis, eukaryotic cells have evolved the unfolded protein response (UPR), an essential adaptive intracellular signaling pathway that responds to metabolic, oxidative stress, and inflammatory response pathways. The UPR has been implicated in a variety of diseases including metabolic disease, neurodegenerative disease, inflammatory disease, and cancer. Signaling components of the UPR are emerging as potential targets for intervention and treatment of human disease.

The endoplasmic reticulum in cardiovascular health and disease

The endoplasmic reticulum has an intricate network of pathways built to deal with the secretory and integral membrane protein synthesis demands of the cell, as well as adaptive responses set up for the endoplasmic reticulum to rely on when stressed. These pathways are both essential and complex, and because of these 2 factors, several situations can lead to a dysfunctional endoplasmic reticulum and result in a dysfunctional cell with the potential to contribute to the progression of disease. The endoplasmic reticulum has been implicated in several metabolic, neurodegenerative, inflammatory, autoimmune, and renal diseases and disorders, and in particular, cardiovascular diseases. The role of the endoplasmic reticulum in cardiovascular disease shows how the change in function of a particular microscopic organelle can lead to macroscopic changes in the form of disease.

Ins(1,4,5)P3 receptor-mediated Ca2+ signaling and autophagy induction are interrelated

The role of intracellular Ca2+ signaling in starvation-induced autophagy remains unclear. Here, we examined Ca2+ dynamics during starvation-induced autophagy and the underlying molecular mechanisms. Tightly correlating with autophagy stimulation, we observed a remodeling of the Ca2+ signalosome. First, short periods of starvation (1 to 3 h) caused a prominent increase of the ER Ca2+-store content and enhanced agonist-induced Ca2+ release. The mechanism involved the upregulation of intralumenal ER Ca2+-binding proteins, calreticulin and Grp78/BiP, which increased the ER Ca2+-buffering capacity and reduced the ER Ca2+ leak. Second, starvation led to Ins(1,4,5)P3R sensitization. Immunoprecipitation experiments showed that during starvation Beclin 1, released from Bcl-2, first bound with increasing efficiency to Ins(1,4,5)P3Rs; after reaching a maximal binding after 3 h, binding, however, decreased again. The interaction site of Beclin 1 was determined to be present in the N-terminal Ins(1,4,5)P3-binding domain of the Ins(1,4,5)P3R. The starvation-induced Ins(1,4,5)P3R sensitization was abolished in cells treated with BECN1 siRNA, but not with ATG5 siRNA, pointing toward an essential role of Beclin 1 in this process. Moreover, recombinant Beclin 1 sensitized Ins(1,4,5)P3Rs in 45Ca2+-flux assays, indicating a direct regulation of Ins(1,4,5)P3R activity by Beclin 1. Finally, we found that Ins(1,4,5)P3R-mediated Ca2+ signaling was critical for starvation-induced autophagy stimulation, since the Ca2+ chelator BAPTA-AM as well as the Ins(1,4,5)P3R inhibitor xestospongin B abolished the increase in LC3 lipidation and GFP-LC3-puncta formation. Hence, our results indicate a tight and essential interrelation between intracellular Ca2+ signaling and autophagy stimulation as a proximal event in response to starvation.

The Molecular Basis for Ca2+ Signalling by NAADP: Two-Pore Channels in a Complex?

NAADP is a potent Ca²⁺ mobilizing messenger in a variety of cells but its molecular mechanism of action is incompletely understood. Accumulating evidence indicates that the poorly characterized two-pore channels (TPCs) in animals are NAADP sensitive Ca²⁺-permeable channels. TPCs localize to the endo-lysosomal system but are functionally coupled to the better characterized endoplasmic reticulum Ca²⁺ channels to generate physiologically relevant complex Ca²⁺ signals. Whether TPCs directly bind NAADP is not clear. Here we discuss the idea based on recent studies that TPCs are the pore-forming subunits of a protein complex that includes tightly associated, low molecular weight NAADP-binding proteins.

Triggering of Ca2+ signals by NAADP-gated two-pore channels: a role for membrane contact sites?

NAADP (nicotinic acid-adenine dinucleotide phosphate) is a potent Ca2+-mobilizing messenger implicated in many Ca2+-dependent cellular processes. It is highly unusual in that it appears to trigger Ca2+ release from acidic organelles such as lysosomes. These signals are often amplified by archetypal Ca2+ channels located in the endoplasmic reticulum. Recent studies have converged on the TPCs (two-pore channels) which localize to the endolysosomal system as the likely primary targets through which NAADP mediates its effects. 'Chatter' between TPCs and endoplasmic reticulum Ca2+ channels is disrupted when TPCs are directed away from the endolysosomal system. This suggests that intracellular Ca2+ release channels may be closely apposed, possibly at specific membrane contact sites between acidic organelles and the endoplasmic reticulum.

CD147 induces UPR to inhibit apoptosis and chemosensitivity by increasing the transcription of Bip in hepatocellular carcinoma

The unfolded protein response (UPR) is generally activated in solid tumors and results in tumor cell anti-apoptosis and drug resistance. However, tumor-specific UPR transducers are largely unknown. In the present study, we identified CD147, a cancer biomarker, as an UPR inducer in hepatocellular carcinoma (HCC). The expression of the major UPR target, Bip, was found to be positively associated with CD147 in human hepatoma tissues. By phosphorylating FAK and Src, CD147-enhanced TFII-I tyrosine phosphorylation at Tyr248. CD147 also induced p-TFII-I nuclear localization and binding to the Bip promoter where endoplasmic reticulum (ER) stress response element 1 (ERSE1) (-82/-50) is the most efficient target of the three ERSEs, thus increasing transcription of Bip. Furthermore, by inducing UPR, CD147 inhibited HCC cell apoptosis and decreased cell Adriamycin chemosensitivity, thus decreasing the survival rate of hepatoma-bearing nude mice. Together, these results reveal pivotal roles for CD147 in modulating the UPR in HCC and raise the possibility that CD147 is a target that promotes HCC cell apoptosis and increases the sensitivity of tumors to anti-cancer drugs. Therefore, CD147 inhibition provides an opportunity to enhance the efficacy of existing agents and represents a novel target for HCC treatment.

Intracellular organelles in the saga of Ca2+ homeostasis: different molecules for different purposes?

An increase in the concentration of cytosolic free Ca(2+) is a key component regulating different cellular processes ranging from egg fertilization, active secretion and movement, to cell differentiation and death. The multitude of phenomena modulated by Ca(2+), however, do not simply rely on increases/decreases in its concentration, but also on specific timing, shape and sub-cellular localization of its signals that, combined together, provide a huge versatility in Ca(2+) signaling. Intracellular organelles and their Ca(2+) handling machineries exert key roles in this complex and precise mechanism, and this review will try to depict a map of Ca(2+) routes inside cells, highlighting the uniqueness of the different Ca(2+) toolkit components and the complexity of the interactions between them.

Ablation of junctin or triadin is associated with increased cardiac injury following ischaemia/reperfusion

AIMS

Junctin and triadin are calsequestrin-binding proteins that regulate sarcoplasmic reticulum (SR) Ca(2+) release by interacting with the ryanodine receptor. The levels of these proteins are significantly down-regulated in failing human hearts. However, the significance of such decreases is currently unknown. Here, we addressed the functional role of these accessory proteins in the heart's responses to ischaemia/reperfusion (I/R) injury.

METHODS AND RESULTS

Isolated mouse hearts were subjected to global I/R, and contractile parameters were assessed in wild-type (WT), junctin-knockout (JKO), and triadin-knockout (TKO) hearts. Both JKO and TKO were associated with significantly depressed post-I/R contractile recovery. However, ablation of triadin resulted in the most severe post-I/R phenotype. The additional contractile impairment of TKO hearts was not related to a mitochondrial death pathway, but attributed to endoplasmic reticulum (ER) stress-mediated apoptosis. Activation of the X-box-binding protein-1 and transcriptional up-regulation of C/EBP-homologous protein (CHOP) provided a molecular mechanism of caspase-12-dependent apoptosis in myocytes. In addition, elevation of cytosolic Ca(2+) during reperfusion was associated with the activation of calpain proteases and troponin I breakdown. Accordingly, treatment with the calpain inhibitor MDL-28170 significantly ameliorated post-I/R impairment of contractile recovery in intact hearts.

CONCLUSION

These findings indicate that deficiency of either junctin or triadin impairs the contractile recovery in post-ischaemic hearts, which appears to be primarily attributed to increased ER stress and activation of calpain.

Calcium oscillations and pacemaking

Calcium plays important role in biological systems where it is involved in diverse mechanisms such as signaling, muscle contraction and neuromodulation. Action potentials are generated by dynamic interaction of ionic channels located on the plasma-membrane and these drive the rhythmic activity of biological systems such as the smooth muscle and the heart. However, ionic channels are not the only pacemakers; an intimate interaction between intracellular Ca(2+) stores and ionic channels underlie rhythmic activity. In this review we will focus on the role of Ca(2+) stores in regulation of rhythmical behavior.

Spatio-temporal expression analysis of the calcium-binding protein calumenin in the rodent brain

Calumenin is a Ca²⁺-binding protein that belongs to the CREC superfamily. It contains six EF-hand domains that exhibit a low affinity for Ca²⁺ as well as an endoplasmic reticulum retention signal. Calumenin exhibits a broad and relatively high expression in various brain regions during development as demonstrated by in situ hybridization. Signal intensity of calumenin is highest during the early development and then declines over time to reach a relatively low expression in adult animals. Immunohistochemistry indicates that at the P0 stage, calumenin expression is most abundant in migrating neurons in the zones around the lateral ventricle. In the brain of adult animals, it is expressed in various glial and neuronal cell types, including immature neurons in subgranular zone of hippocampal dentate gyrus. At the subcellular level, calumenin is identified in punctuate and diffuse distribution mostly in somatic regions where it co-localizes with endoplasmic reticulum (ER) and partially Golgi apparatus. Upon subcellular fractionation, calumenin is enriched in fractions containing membranes and is only weakly present in soluble fractions. This study points to a possible important role of calumenin in migration and differentiation of neurons, and/or in Ca²⁺ signaling between glial cells and neurons.

The C terminus of Bax inhibitor-1 forms a Ca2+-permeable channel pore

Bax inhibitor-1 (BI-1) is a multitransmembrane domain-spanning endoplasmic reticulum (ER)-located protein that is evolutionarily conserved and protects against apoptosis and ER stress. Furthermore, BI-1 is proposed to modulate ER Ca(2+) homeostasis by acting as a Ca(2+)-leak channel. Based on experimental determination of the BI-1 topology, we propose that its C terminus forms a Ca(2+) pore responsible for its Ca(2+)-leak properties. We utilized a set of C-terminal peptides to screen for Ca(2+) leak activity in unidirectional (45)Ca(2+)-flux experiments and identified an α-helical 20-amino acid peptide causing Ca(2+) leak from the ER. The Ca(2+) leak was independent of endogenous ER Ca(2+)-release channels or other Ca(2+)-leak mechanisms, namely translocons and presenilins. The Ca(2+)-permeating property of the peptide was confirmed in lipid-bilayer experiments. Using mutant peptides, we identified critical residues responsible for the Ca(2+)-leak properties of this BI-1 peptide, including a series of critical negatively charged aspartate residues. Using peptides corresponding to the equivalent BI-1 domain from various organisms, we found that the Ca(2+)-leak properties were conserved among animal, but not plant and yeast orthologs. By mutating one of the critical aspartate residues in the proposed Ca(2+)-channel pore in full-length BI-1, we found that Asp-213 was essential for BI-1-dependent ER Ca(2+) leak. Thus, we elucidated residues critically important for BI-1-mediated Ca(2+) leak and its potential channel pore. Remarkably, one of these residues was not conserved among plant and yeast BI-1 orthologs, indicating that the ER Ca(2+)-leak properties of BI-1 are an added function during evolution.

Transfer of IP₃ through gap junctions is critical, but not sufficient, for the spread of apoptosis

Decades of research have indicated that gap junction channels contribute to the propagation of apoptosis between neighboring cells. Inositol 1,4,5-trisphosphate (IP₃) has been proposed as the responsible molecule conveying the apoptotic message, although conclusive results are still missing. We investigated the role of IP₃ in a model of gap junction-mediated spreading of cytochrome C-induced apoptosis. We used targeted loading of high-molecular-weight agents interfering with the IP₃ signaling cascade in the apoptosis trigger zone and cell death communication zone of C6-glioma cells heterologously expressing connexin (Cx)43 or Cx26. Blocking IP₃ receptors or stimulating IP₃ degradation both diminished the propagation of apoptosis. Apoptosis spread was also reduced in cells expressing mutant Cx26, which forms gap junctions with an impaired IP₃ permeability. However, IP₃ by itself was not able to induce cell death, but only potentiated cell death propagation when the apoptosis trigger was applied. We conclude that IP₃ is a key necessary messenger for communicating apoptotic cell death via gap junctions, but needs to team up with other factors to become a fully pro-apoptotic messenger.

Arterial calcification in chronic kidney disease: key roles for calcium and phosphate

Vascular calcification contributes to the high risk of cardiovascular mortality in chronic kidney disease (CKD) patients. Dysregulation of calcium (Ca) and phosphate (P) metabolism is common in CKD patients and drives vascular calcification. In this article, we review the physiological regulatory mechanisms for Ca and P homeostasis and the basis for their dysregulation in CKD. In addition, we highlight recent findings indicating that elevated Ca and P have direct effects on vascular smooth muscle cells (VSMCs) that promote vascular calcification, including stimulation of osteogenic/chondrogenic differentiation, vesicle release, apoptosis, loss of inhibitors, and extracellular matrix degradation. These studies suggest a major role for elevated P in promoting osteogenic/chondrogenic differentiation of VSMC, whereas elevated Ca has a predominant role in promoting VSMC apoptosis and vesicle release. Furthermore, the effects of elevated Ca and P are synergistic, providing a major stimulus for vascular calcification in CKD. Unraveling the complex regulatory pathways that mediate the effects of both Ca and P on VSMCs will ultimately provide novel targets and therapies to limit the destructive effects of vascular calcification in CKD patients.

Cell proliferation, calcium influx and calcium channels

Both increases in the basal cytosolic calcium concentration ([Ca(2+)](cyt)) and [Ca(2+)](cyt) transients play major roles in cell cycle progression, cell proliferation and division. Calcium transients are observed at various stages of cell cycle and more specifically during late G(1) phase, before and during mitosis. These calcium transients are mainly due to calcium release and reuptake by the endoplasmic reticulum (ER) and are observed over periods of hours in oocytes and mammalian cells. Calcium entry sustains the ER Ca(2+) load and thereby helps to maintain these calcium transients for such a long period. Calcium influx also controls cell growth and proliferation in several cell types. Various calcium channels are involved in this process and the tight relation between the expression and activity of cyclins and calcium channels also suggests that calcium entry may be needed only at particular stages of the cell cycle. Consistent with this idea, the expression of l-type and T-type calcium channels and SOCE amplitude fluctuate along the cell cycle. But, as calcium influx regulates several other transduction pathways, the presence of a specific connection to trigger activation of proliferation and cell division in mammalian cells will be discussed in this review.

Conjugated linoleic acid-induced apoptosis in mouse mammary tumor cells is mediated by both G protein coupled receptor-dependent activation of the AMP-activated protein kinase pathway and by oxidative stress

Conjugated linoleic acid (CLA) has shown chemopreventive activity in several tumorigenesis models, in part through induction of apoptosis. We previously demonstrated that the t10,c12 isomer of CLA induced apoptosis of TM4t mouse mammary tumor cells through both mitochondrial and endoplasmic reticulum (ER) stress pathways, and that the AMP-activated protein kinase (AMPK) played a critical role in the apoptotic effect. In the current study, we focused on the upstream pathways by which AMPK was activated, and additionally evaluated the contributing role of oxidative stress to apoptosis. CLA-induced activation of AMPK and/or induction of apoptosis were inhibited by infection of TM4t cells with an adenovirus expressing a peptide which blocks the interaction between the G protein coupled receptor (GPCR) and Gα(q), by the phospholipase C (PLC) inhibitor U73122, by the inositol trisphosphate (IP(3)) receptor inhibitor 2-APB, by the calcium/calmodulin-dependent protein kinase kinase α (CaMKK) inhibitor STO-609 and by the intracellular Ca(2+) chelator BAPTA-AM. This suggests that t10,c12-CLA may exert its apoptotic effect by stimulating GPCR through Gα(q) signaling, activation of phosphatidylinositol-PLC, followed by binding of the PLC-generated IP(3) to its receptor on the ER, triggering Ca(2+) release from the ER and finally stimulating the CaMKK-AMPK pathway. t10,c12-CLA also increased oxidative stress and lipid peroxidation, and antioxidants blocked its apoptotic effect, as well as the CLA-induced activation of p38 MAPK, a downstream effector of AMPK. Together these data elucidate two major pathways by which t10,c12-CLA induces apoptosis, and suggest a point of intersection of the two pathways both upstream and downstream of AMPK.

Calcium homoeostasis modulator 1 (CALHM1) reduces the calcium content of the endoplasmic reticulum (ER) and triggers ER stress

CALHM1 (calcium homoeostasis modulator 1), a membrane protein with similarity to NMDA (N-methyl-D-aspartate) receptor channels that localizes in the plasma membrane and the ER (endoplasmic reticulum) of neurons, has been shown to generate a plasma-membrane Ca2+ conductance and has been proposed to influence Alzheimer's disease risk. In the present study we have investigated the effects of CALHM1 on intracellular Ca2+ handling in HEK-293T [HEK (human embryonic kidney)-293 cells expressing the large T-antigen of SV40 (simian virus 40)] cells by using targeted aequorins for selective monitorization of Ca2+ transport by organelles. We find that CALHM1 increases Ca2+ leak from the ER and, more importantly, reduces ER Ca2+ uptake by decreasing both the transport capacity and the Ca2+ affinity of SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase). As a result, the Ca2+ content of the ER is drastically decreased. This reduction in the Ca2+ content of the ER triggered the UPR (unfolded protein response) with induction of several ER stress markers, such as CHOP [C/EBP (CCAAT/enhancer-binding protein)-homologous protein], ERdj4, GRP78 (glucose-regulated protein of 78 kDa) and XBP1 (X-box-binding protein 1). Thus CALHM1 might provide a relevant link between Ca2+ homoeostasis disruption, ER stress and cell damage in the pathogenesis of neurodegenerative diseases

Endoplasmic-reticulum calcium depletion and disease

The endoplasmic reticulum (ER) as an intracellular Ca(2+) store not only sets up cytosolic Ca(2+) signals, but, among other functions, also assembles and folds newly synthesized proteins. Alterations in ER homeostasis, including severe Ca(2+) depletion, are an upstream event in the pathophysiology of many diseases. On the one hand, insufficient release of activator Ca(2+) may no longer sustain essential cell functions. On the other hand, loss of luminal Ca(2+) causes ER stress and activates an unfolded protein response, which, depending on the duration and severity of the stress, can reestablish normal ER function or lead to cell death. We will review these various diseases by mainly focusing on the mechanisms that cause ER Ca(2+) depletion.

Endoplasmic reticulum calcium pumps and cancer

Endoplasmic reticulum calcium homeostasis is involved in a multitude of signaling, as well as "house-keeping" functions that control cell growth, differentiation or apoptosis in every human/eukaryotic cell. Calcium is actively accumulated in the endoplasmic reticulum by Sarco/Endoplasmic Reticulum Calcium transport ATPases (SERCA enzymes). SERCA-dependent calcium transport is the only calcium uptake mechanism in this organelle, and therefore the regulation of SERCA function by the cell constitutes a key mechanism to adjust calcium homeostasis in the endoplasmic reticulum depending on the cell type and its state of differentiation. The direct pharmacological modulation of SERCA activity affects cell differentiation and survival. SERCA expression levels can undergo significant changes during cell differentiation or tumorigenesis, leading to modified endoplasmic reticulum calcium storage. In several cell types such as cells of hematopoietic origin or various epithelial cells, two SERCA genes (SERCA2 and SERCA3) are simultaneously expressed. Expression levels of SERCA3, a lower calcium affinity calcium pump are highly variable. In several cell systems SERCA3 expression is selectively induced during differentiation, whereas during tumorigenesis and blastic transformation SERCA3 expression is decreased. These observations point at the existence of a cross-talk, via the regulation of SERCA3 levels, between endoplasmic reticulum calcium homeostasis and the control of cell differentiation, and show that endoplasmic reticulum calcium homeostasis itself can undergo remodeling during differentiation. The investigation of the anomalies of endoplasmic reticulum differentiation in tumor and leukemia cells may be useful for a better understanding of the contribution of calcium signaling to the establishment of malignant phenotypes.