SERCA1 truncated proteins unable to pump calcium reduce the endoplasmic reticulum calcium concentration and induce apoptosis

The Meeting of Micropeptides with Major Ca2+ Pumps in Inner Membranes-Consideration of a New Player, SERCA1b

Calcium is a major signalling bivalent cation within the cell. Compartmentalization is essential for regulation of calcium mediated processes. A number of players contribute to intracellular handling of calcium, among them are the sarco/endoplasmic reticulum calcium ATP-ases (SERCAs). These molecules function in the membrane of ER/SR pumping Ca²⁺ from cytoplasm into the lumen of the internal store. Removal of calcium from the cytoplasm is essential for signalling and for relaxation of skeletal muscle and heart. There are three genes and over a dozen isoforms of SERCA in mammals. These can be potentially influenced by small membrane peptides, also called regulins. The discovery of micropeptides has increased in recent years, mostly because of the small ORFs found in long RNAs, annotated formerly as noncoding (lncRNAs). Several excellent works have analysed the mechanism of interaction of micropeptides with each other and with the best known SERCA1a (fast muscle) and SERCA2a (heart, slow muscle) isoforms. However, the array of tissue and developmental expressions of these potential regulators raises the question of interaction with other SERCAs. For example, the most abundant calcium pump in neonatal and regenerating skeletal muscle, SERCA1b has never been looked at with scrutiny to determine whether it is influenced by micropeptides. Further details might be interesting on the interaction of these peptides with the less studied SERCA1b isoform.

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

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

The MAMs Structure and Its Role in Cell Death

The maintenance of cellular homeostasis involves the participation of multiple organelles. These organelles are associated in space and time, and either cooperate or antagonize each other with regards to cell function. Crosstalk between organelles has become a significant topic in research over recent decades. We believe that signal transduction between organelles, especially the endoplasmic reticulum (ER) and mitochondria, is a factor that can influence the cell fate. As the cellular center for protein folding and modification, the endoplasmic reticulum can influence a range of physiological processes by regulating the quantity and quality of proteins. Mitochondria, as the cellular "energy factory," are also involved in cell death processes. Some researchers regard the ER as the sensor of cellular stress and the mitochondria as an important actuator of the stress response. The scientific community now believe that bidirectional communication between the ER and the mitochondria can influence cell death. Recent studies revealed that the death signals can shuttle between the two organelles. Mitochondria-associated membranes (MAMs) play a vital role in the complex crosstalk between the ER and mitochondria. MAMs are known to play an important role in lipid synthesis, the regulation of Ca²⁺ homeostasis, the coordination of ER-mitochondrial function, and the transduction of death signals between the ER and the mitochondria. Clarifying the structure and function of MAMs will provide new concepts for studying the pathological mechanisms associated with neurodegenerative diseases, aging, and cancers. Here, we review the recent studies of the structure and function of MAMs and its roles involved in cell death, especially in apoptosis.

Molecular Dysfunctions of Mitochondria-Associated Membranes (MAMs) in Alzheimer's Disease

Alzheimer’s disease (AD) is a multifactorial neurodegenerative pathology characterized by a progressive decline of cognitive functions. Alteration of various signaling cascades affecting distinct subcellular compartment functions and their communication likely contribute to AD progression. Among others, the alteration of the physical association between the endoplasmic reticulum (ER) and mitochondria, also referred as mitochondria-associated membranes (MAMs), impacts various cellular housekeeping functions such as phospholipids-, glucose-, cholesterol-, and fatty-acid-metabolism, as well as calcium signaling, which are all altered in AD. Our review describes the physical and functional proteome crosstalk between the ER and mitochondria and highlights the contribution of distinct molecular components of MAMs to mitochondrial and ER dysfunctions in AD progression. We also discuss potential strategies targeting MAMs to improve mitochondria and ER functions in AD.

Alterations of the Endoplasmic Reticulum (ER) Calcium Signaling Molecular Components in Alzheimer's Disease

Sustained imbalance in intracellular calcium (Ca²⁺) entry and clearance alters cellular integrity, ultimately leading to cellular homeostasis disequilibrium and cell death. Alzheimer’s disease (AD) is the most common cause of dementia. Beside the major pathological features associated with AD-linked toxic amyloid beta (Aβ) and hyperphosphorylated tau (p-tau), several studies suggested the contribution of altered Ca²⁺ handling in AD development. These studies documented physical or functional interactions of Aβ with several Ca²⁺ handling proteins located either at the plasma membrane or in intracellular organelles including the endoplasmic reticulum (ER), considered the major intracellular Ca²⁺ pool. In this review, we describe the cellular components of ER Ca²⁺ dysregulations likely responsible for AD. These include alterations of the inositol 1,4,5-trisphosphate receptors’ (IP₃Rs) and ryanodine receptors’ (RyRs) expression and function, dysfunction of the sarco-endoplasmic reticulum Ca²⁺ ATPase (SERCA) activity and upregulation of its truncated isoform (S1T), as well as presenilin (PS1, PS2)-mediated ER Ca²⁺ leak/ER Ca²⁺ release potentiation. Finally, we highlight the functional consequences of alterations of these ER Ca²⁺ components in AD pathology and unravel the potential benefit of targeting ER Ca²⁺ homeostasis as a tool to alleviate AD pathogenesis.

Ca2+ Fluxes and Cancer

Ca²⁺ ions are key second messengers in both excitable and non-excitable cells. Owing to the rather pleiotropic nature of Ca²⁺ transporters and other Ca²⁺-binding proteins, however, Ca²⁺ signaling has attracted limited attention as a potential target of anticancer therapy. Here, we discuss cancer-associated alterations of Ca²⁺ fluxes at specific organelles as we identify novel candidates for the development of drugs that selectively target Ca²⁺ signaling in malignant cells.

Role of Insulin-Like Growth Factor Receptor 2 across Muscle Homeostasis: Implications for Treating Muscular Dystrophy

The insulin-like growth factor 2 receptor (IGF2R) plays a major role in binding and regulating the circulating and tissue levels of the mitogenic peptide insulin-like growth factor 2 (IGF2). IGF2/IGF2R interaction influences cell growth, survival, and migration in normal tissue development, and the deregulation of IGF2R expression has been associated with growth-related disease and cancer. IGF2R overexpression has been implicated in heart and muscle disease progression. Recent research findings suggest novel approaches to target IGF2R action. This review highlights recent advances in the understanding of the IGF2R structure and pathways related to muscle homeostasis.

Primary Active Ca2+ Transport Systems in Health and Disease

Calcium ions (Ca²⁺) are prominent cell signaling effectors that regulate a wide variety of cellular processes. Among the different players in Ca²⁺ homeostasis, primary active Ca²⁺ transporters are responsible for keeping low basal Ca²⁺ levels in the cytosol while establishing steep Ca²⁺ gradients across intracellular membranes or the plasma membrane. This review summarizes our current knowledge on the three types of primary active Ca²⁺-ATPases: the sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) pumps, the secretory pathway Ca²⁺- ATPase (SPCA) isoforms, and the plasma membrane Ca²⁺-ATPase (PMCA) Ca²⁺-transporters. We first discuss the Ca²⁺ transport mechanism of SERCA1a, which serves as a reference to describe the Ca²⁺ transport of other Ca²⁺ pumps. We further highlight the common and unique features of each isoform and review their structure-function relationship, expression pattern, regulatory mechanisms, and specific physiological roles. Finally, we discuss the increasing genetic and in vivo evidence that links the dysfunction of specific Ca²⁺-ATPase isoforms to a broad range of human pathologies, and highlight emerging therapeutic strategies that target Ca²⁺ pumps.

Upregulation of the Sarco-Endoplasmic Reticulum Calcium ATPase 1 Truncated Isoform Plays a Pathogenic Role in Alzheimer's Disease

Dysregulation of the Endoplasmic Reticulum (ER) Ca²⁺ homeostasis and subsequent ER stress activation occur in Alzheimer Disease (AD). We studied the contribution of the human truncated isoform of the sarco-endoplasmic reticulum Ca²⁺ ATPase 1 (S1T) to AD. We examined S1T expression in human AD-affected brains and its functional consequences in cellular and transgenic mice AD models. S1T expression is increased in sporadic AD brains and correlates with amyloid β (Aβ) and ER stress chaperone protein levels. Increased S1T expression was also observed in human neuroblastoma cells expressing Swedish-mutated β-amyloid precursor protein (βAPP) or treated with Aβ oligomers. Lentiviral overexpression of S1T enhances in return the production of APP C-terminal fragments and Aβ through specific increases of β-secretase expression and activity, and triggers neuroinflammation. We describe a molecular interplay between S1T-dependent ER Ca²⁺ leak, ER stress and βAPP-derived fragments that could contribute to AD setting and/or progression.

ER stress-linked autophagy stabilizes apoptosis effector PERP and triggers its co-localization with SERCA2b at ER-plasma membrane junctions

Specific molecular interactions that underpin the switch between ER stress-triggered autophagy-mediated cellular repair and cellular death by apoptosis are not characterized. This study reports the unexpected interaction elicited by ER stress between the plasma membrane (PM)-localized apoptosis effector PERP and the ER Ca²⁺ pump SERCA2b. We show that the p53 effector PERP, which specifically induces apoptosis when expressed above a threshold level, has a heterogeneous distribution across the PM of un-stressed cells and is actively turned over by the lysosome. PERP is upregulated following sustained starvation-induced autophagy, which precedes the onset of apoptosis indicating that PERP protein levels are controlled by a lysosomal pathway that is sensitive to cellular physiological state. Furthermore, ER stress stabilizes PERP at the PM and induces its increasing co-localization with SERCA2b at ER–PM junctions. The findings highlight a novel crosstalk between pro-survival autophagy and pro-death apoptosis pathways and identify, for the first time, accumulation of an apoptosis effector to ER–PM junctions in response to ER stress.

Identification of a pathogenic mutation in ATP2A1 via in silico analysis of exome data for cryptic aberrant splice sites

BACKGROUND

Pathogenic mutations causing aberrant splicing are often difficult to detect. Standard variant analysis of next-generation sequence (NGS) data focuses on canonical splice sites. Noncanonical splice sites are more difficult to ascertain.

METHODS

We developed a bioinformatics pipeline that screens existing NGS data for potentially aberrant novel essential splice sites (PANESS) and performed a pilot study on a family with a myotonic disorder. Further analyses were performed via qRT-PCR, immunoblotting, and immunohistochemistry. RNAi knockdown studies were performed in Drosophila to model the gene deficiency.

RESULTS

The PANESS pipeline identified a homozygous ATP2A1 variant (NC_000016.9:g.28905928G>A; NM_004320.4:c.1287G>A:p.(Glu429=)) that was predicted to cause the omission of exon 11. Aberrant splicing of ATP2A1 was confirmed via qRT-PCR, and abnormal expression of the protein product sarcoplasmic/endoplasmic reticulum Ca++ ATPase 1 (SERCA1) was demonstrated in quadriceps femoris tissue from the proband. Ubiquitous knockdown of SERCA led to lethality in Drosophila, as did knockdown targeting differentiating or fusing myoblasts.

CONCLUSIONS

This study confirms the potential of novel in silico algorithms to detect cryptic mutations in existing NGS data; expands the phenotypic spectrum of ATP2A1 mutations beyond classic Brody myopathy; and suggests that genetic testing of ATP2A1 should be considered in patients with clinical myotonia.

Epicardial adipose tissue and atrial fibrillation: pathophysiological mechanisms, clinical implications, and potential therapies

BACKGROUND

Atrial fibrillation (AF) is the most common arrhythmia in clinical practice and is associated with increased cardiovascular morbidity and mortality. Epicardial adipose tissue (EAT) serves as a biologically active organ with important endocrine and inflammatory function. Review An accumulating body of evidence suggests that EAT is associated with the initiation, perpetuation, and recurrence of AF, but the precise role of EAT in AF pathogenesis is not completely elucidated. Pathophysiological mechanisms involve adipocyte infiltration, profibrotic and pro-inflammatory paracrine effects, oxidative stress, neural mechanisms, and genetic factors.

CONCLUSIONS

Notably, EAT accumulation seems to be associated with stroke and adverse cardiovascular outcomes in AF. Weight loss, specific medications and ablation of ganglionated plexi (GP) seem to be potential therapies in this setting.

Altered sarco(endo)plasmic reticulum calcium adenosine triphosphatase 2a content: Targets for heart failure therapy

Sarco(endo)plasmic reticulum calcium adenosine triphosphatase is responsible for transporting cytosolic calcium into the sarcoplasmic reticulum and endoplasmic reticulum to maintain calcium homeostasis. Sarco(endo)plasmic reticulum calcium adenosine triphosphatase is the dominant isoform expressed in cardiac tissue, which is regulated by endogenous protein inhibitors, post-translational modifications, hormones as well as microRNAs. Dysfunction of sarco(endo)plasmic reticulum calcium adenosine triphosphatase is associated with heart failure, which makes sarco(endo)plasmic reticulum calcium adenosine triphosphatase a promising target for heart failure therapy. This review summarizes current approaches to ameliorate sarco(endo)plasmic reticulum calcium adenosine triphosphatase function and focuses on phospholamban, an endogenous inhibitor of sarco(endo)plasmic reticulum calcium adenosine triphosphatase, pharmacological tools and gene therapies.

The Unfolded Protein Response and Membrane Contact Sites: Tethering as a Matter of Life and Death?

The endoplasmic reticulum (ER) is the most extensive organelle of the eukaryotic cell and constitutes the major site of protein and lipid synthesis and regulation of intracellular Ca2+ levels. To exert these functions properly, the ER network is shaped in structurally and functionally distinct domains that dynamically remodel in response to intrinsic and extrinsic cues. Moreover, the ER establishes a tight communication with virtually all organelles of the cell through specific subdomains called membrane contact sites. These contact sites allow preferential, nonvesicular channeling of key biological mediators including lipids and Ca2+ between organelles and are harnessed by the ER to interface with and coregulate a variety of organellar functions that are vital to maintain homeostasis. When ER homeostasis is lost, a condition that triggers the activation of an evolutionarily conserved pathway called the unfolded protein response (UPR), the ER undergoes rapid remodeling. These dynamic changes in ER morphology are functionally coupled to the modulation or formation of contact sites with key organelles, such as mitochondria and the plasma membrane, which critically regulate cell fate decisions of the ER-stressed cells. Certain components of the UPR have been shown to facilitate the formation of contact sites through various mechanisms including remodeling of the actin cytoskeleton. In this review, we discuss old and emerging evidence linking the UPR machinery to contact site formation in mammalian cells and discuss their important role in cellular homeostasis.

Emerging molecular mechanisms in chemotherapy: Ca2+ signaling at the mitochondria-associated endoplasmic reticulum membranes

Inter-organellar communication often takes the form of Ca2+ signals. These Ca2+ signals originate from the endoplasmic reticulum (ER) and regulate different cellular processes like metabolism, fertilization, migration, and cell fate. A prime target for Ca2+ signals are the mitochondria. ER-mitochondrial Ca2+ transfer is possible through the existence of mitochondria-associated ER membranes (MAMs), ER structures that are in the proximity of the mitochondria. This creates a micro-domain in which the Ca2+ concentrations are manifold higher than in the cytosol, allowing for rapid mitochondrial Ca2+ uptake. In the mitochondria, the Ca2+ signal is decoded differentially depending on its spatiotemporal characteristics. While Ca2+ oscillations stimulate metabolism and constitute pro-survival signaling, mitochondrial Ca2+ overload results in apoptosis. Many chemotherapeutics depend on efficient ER-mitochondrial Ca2+ signaling to exert their function. However, several oncogenes and tumor suppressors present in the MAMs can alter Ca2+ signaling in cancer cells, rendering chemotherapeutics ineffective. In this review, we will discuss recent studies that connect ER-mitochondrial Ca2+ transfer, tumor suppressors and oncogenes at the MAMs, and chemotherapy.

Endoplasmic reticulum stress in the pathogenesis of fibrotic disease

Eukaryotic cells contain an elegant protein quality control system that is crucial in maintaining cellular homeostasis; however, dysfunction of this system results in endoplasmic reticulum (ER) stress and activation of the unfolded protein response (UPR). Severe or prolonged ER stress is associated with the development of degenerative and fibrotic disorders in multiple organs, as evidenced by the identification of disease-causing mutations in epithelial-restricted genes that lead to protein misfolding or mistrafficking in familial fibrotic diseases. Emerging evidence implicates ER stress and UPR signaling in a variety of profibrotic mechanisms in individual cell types. In epithelial cells, ER stress can induce apoptosis, inflammatory signaling, and epithelial-mesenchymal transition. In other cell types, ER stress is linked to myofibroblast activation, macrophage polarization, and T cell differentiation. ER stress-targeted therapies have begun to emerge using approaches that range from global enhancement of chaperone function to selective targeting of activated ER stress sensors and other downstream mediators. As the complex regulatory mechanisms of this system are further clarified, there are opportunities to develop new disease-modifying therapeutic strategies in a wide range of chronic fibrotic diseases.

An Update on Sec61 Channel Functions, Mechanisms, and Related Diseases

The membrane of the endoplasmic reticulum (ER) of nucleated human cells harbors the protein translocon, which facilitates membrane integration or translocation of almost every newly synthesized polypeptide targeted to organelles of the endo- and exocytotic pathway. The translocon comprises the polypeptide-conducting Sec61 channel and several additional proteins and complexes that are permanently or transiently associated with the heterotrimeric Sec61 complex. This ensemble of proteins facilitates ER targeting of precursor polypeptides, modification of precursor polypeptides in transit through the Sec61 complex, and Sec61 channel gating, i.e., dynamic regulation of the pore forming subunit to mediate precursor transport and calcium efflux. Recently, cryoelectron tomography of translocons in native ER membrane vesicles, derived from human cell lines or patient fibroblasts, and even intact cells has given unprecedented insights into the architecture and dynamics of the native translocon and the Sec61 channel. These structural data are discussed in light of different Sec61 channel activities including ribosome receptor function, membrane insertion, and translocation of newly synthesized polypeptides as well as the putative physiological roles of the Sec61 channel as a passive ER calcium leak channel. Furthermore, the structural insights into the Sec61 channel are incorporated into an overview and update on Sec61 channel-related diseases—the Sec61 channelopathies—and novel therapeutic concepts for their treatment.

Calcium signaling at the endoplasmic reticulum: fine-tuning stress responses

Endoplasmic reticulum (ER) calcium signaling is implicated in a myriad of coordinated cellular processes. The ER calcium content is tightly regulated as it allows a favorable environment for protein folding, in addition to operate as a major reservoir for fast and specific release of calcium. Altered ER homeostasis impacts protein folding, activating the unfolded protein response (UPR) as a rescue mechanism to restore proteostasis. ER calcium release impacts mitochondrial metabolism and also fine-tunes the threshold to undergo apoptosis under chronic stress. The global coordination between UPR signaling and energetic demands takes place at mitochondrial associated membranes (MAMs), specialized subdomains mediating interorganelle communication. Here we discuss current models explaining the functional relationship between ER homeostasis and various cellular responses to coordinate proteostasis and metabolic maintenance.

Proteomic analyses of nucleus laminaris identified candidate targets of the fragile X mental retardation protein

The avian nucleus laminaris (NL) is a brainstem nucleus necessary for binaural processing, analogous in structure and function to the mammalian medial superior olive. In chickens (Gallus gallus), NL is a well-studied model system for activity-dependent neural plasticity. Its neurons have bipolar extension of dendrites, which receive segregated inputs from two ears and display rapid and compartment-specific reorganization in response to unilateral changes in auditory input. More recently, fragile X mental retardation protein (FMRP), an RNA-binding protein that regulates local protein translation, has been shown to be enriched in NL dendrites, suggesting its potential role in the structural dynamics of these dendrites. To explore the molecular role of FMRP in this nucleus, we performed proteomic analysis of NL, using micro laser capture and liquid chromatography tandem mass spectrometry. We identified 657 proteins, greatly represented in pathways involved in mitochondria, translation and metabolism, consistent with high levels of activity of NL neurons. Of these, 94 are potential FMRP targets, by comparative analysis with previously proposed FMRP targets in mammals. These proteins are enriched in pathways involved in cellular growth, cellular trafficking and transmembrane transport. Immunocytochemistry verified the dendritic localization of several proteins in NL. Furthermore, we confirmed the direct interaction of FMRP with one candidate, RhoC, by in vitro RNA binding assays. In summary, we provide a database of highly expressed proteins in NL and in particular a list of potential FMRP targets, with the goal of facilitating molecular characterization of FMRP signaling in future studies.

Mitochondrial and endoplasmic reticulum calcium homeostasis and cell death

The endoplasmic reticulum (ER) and mitochondria cannot be considered as static structures, as they intimately communicate, forming very dynamic platforms termed mitochondria-associated membranes (MAMs). In particular, the ER transmits proper Ca²⁺ signals to mitochondria, which decode them into specific inputs to regulate essential functions, including metabolism, energy production and apoptosis. Here, we will describe the different molecular players involved in the transfer of Ca²⁺ ions from the ER lumen to the mitochondrial matrix and how modifications in both ER-mitochondria contact sites and Ca²⁺ signaling can alter the cell death execution program.

Molecular Mechanisms of Increased Heart Rate in Shenxianshengmai-treated Bradycardia Rabbits

Background:

The molecular mechanisms of Shenxianshengmai (SXSM), a traditional Chinese medicine, on bradycardia have been incompletely understood. The study tried to investigate the gene expression profile and proteomics of bradycardia rabbits’ hearts after SXSM treatment.

Methods:

Twenty-four adult rabbits were randomly assigned in four groups: sham, model, model plus SXSM treatment, and sham plus SXSM treatment groups. Heart rate was recorded in all rabbits. Then, total RNA of atria and proteins of ventricle were isolated and quantified, respectively. Gene expression profiling was conducted by gene expression chip, and quantitative real-time reverse transcription-polymerase chain reaction (RT-PCR) was performed to confirm the results of gene expression chip. We used isobaric tags for elative and absolute quantitation and Western blotting to identify altered proteins after SXSM treatment.

Results:

There was a constant decrease in the mean heart rate (32%, from 238 ± 6 beats/min to 149 ± 12 beats/min) after six weeks in model compared with that in sham group. This effect was partially reversed by 4-week SXSM treatment. Complementary DNA microarray demonstrated that the increased acetylcholinesterase and reduced nicotinic receptor were take responsibility for the increased heart rate. In addition, proteins involved in calcium handling and signaling were affected by SXSM treatment. Real-time RT-PCR verified the results from gene chip. Results from proteomics demonstrated that SXSM enhanced oxidative phosphorylation and tricarboxylic acid (TCA) cycle in ventricular myocardium to improve ATP generation.

Conclusions:

Long-term SXSM stimulates sympathetic transmission by increasing the expression of acetylcholinesterase and reduces the expression of nicotinic receptor to increase heart rate. SXSM also restored the calcium handling genes and altered genes involved in signaling. In addition, SXSM improves the ATP supply of ventricular myocardium by increasing proteins involved in TCA cycle and oxidation-respiratory chain.

Regulation of Calcium Homeostasis by ER Redox: A Close-Up of the ER/Mitochondria Connection

Calcium signaling plays an important role in cell survival by influencing mitochondria-related processes such as energy production and apoptosis. The endoplasmic reticulum (ER) is the main storage compartment for cell calcium (Ca²⁺; ~60-500μM), and the Ca²⁺ released by the ER has a prompt effect on the homeostasis of the juxtaposed mitochondria. Recent findings have highlighted a close connection between ER redox and Ca²⁺ signaling that is mediated by Ca²⁺-handling proteins. This paper describes the redox-regulated mediators and mechanisms that orchestrate Ca²⁺ signals from the ER to mitochondria.

Human epicardial adipose tissue has a specific transcriptomic signature depending on its anatomical peri-atrial, peri-ventricular, or peri-coronary location

AIMS

Human epicardial adipose tissue (EAT) is a visceral and perivascular fat that has been shown to act locally on myocardium, atria, and coronary arteries. Its abundance has been linked to coronary artery disease (CAD) and atrial fibrillation. However, its physiological function remains highly debated. The aim of this study was to determine a specific EAT transcriptomic signature, depending on its anatomical peri-atrial (PA), peri-ventricular (PV), or peri-coronary location.

METHODS AND RESULTS

Samples of EAT and thoracic subcutaneous fat, obtained from 41 patients paired for cardiovascular risk factors, CAD, and atrial fibrillation were analysed using a pangenomic approach. We found 2728 significantly up-regulated genes in the EAT vs. subcutaneous fat with 400 genes being common between PA, PV, and peri-coronary EAT. These common genes were related to extracellular matrix remodelling, inflammation, infection, and thrombosis pathways. Omentin (ITLN1) was the most up-regulated gene and secreted adipokine in EAT (fold-change >12, P < 0.0001). Among EAT-enriched genes, we observed different patterns depending on adipose tissue location. A beige expression phenotype was found in EAT but PV EAT highly expressed uncoupled protein 1 (P = 0.01). Genes overexpressed in peri-coronary EAT were implicated in proliferation, O-N glycan biosynthesis, and sphingolipid metabolism. PA EAT displayed an atypical pattern with genes implicated in cardiac muscle contraction and intracellular calcium signalling pathway.

CONCLUSION

This study opens new perspectives in understanding the physiology of human EAT and its local interaction with neighbouring structures.

Store-operated calcium entry compensates fast ER calcium loss in resting hippocampal neurons

The endoplasmic reticulum (ER) acts as a dynamic calcium store and is involved in the generation of specific patterns of calcium signals in neurons. Calcium is mobilized from the ER store by multiple signaling cascades, and neuronal activity is known to regulate ER calcium levels. We asked how neurons regulate ER calcium levels in the resting state. Direct ER calcium imaging showed that ER calcium was lost quite rapidly from the somatic and dendritic ER when resting neurons were transiently kept under calcium-free conditions. Interestingly, free ER and free cytosolic calcium was lost continuously across the plasma membrane and was not held back in the cytosol, implying the presence of a prominent calcium influx mechanism to maintain ER calcium levels at rest. When neurons were treated acutely with inhibitors of store-operated calcium entry (SOCE), an immediate decline in ER calcium levels was observed. This continuous SOCE-like calcium entry did not require the activation of a signaling cascade, but was rather a steady-state phenomenon. The SOCE-like mechanism maintains medium-high ER calcium levels at rest and is essential for balanced resting calcium levels in the ER and cytosol.

The neonatal sarcoplasmic reticulum Ca2+-ATPase gives a clue to development and pathology in human muscles

The sarcoplasmic/endoplasmic reticulum calcium ATPase 1 (SERCA1) has two muscle specific splice isoforms; SERCA1a in fast-type adult and SERCA1b in neonatal and regenerating skeletal muscles. At the protein level the only difference between these two isoforms is that SERCA1a has C-terminal glycine while SERCA1b has an octapeptide tail instead. This makes the generation of a SERCA1a specific antibody not feasible. The switch between the two isoforms is a hallmark of differentiation so we describe here a method based on the signal ratios of the SERCA1b specific and pan SERCA1 antibodies to estimate the SERCA1b/SERCA1a dominance on immunoblot of human muscles. Using this method we showed that unlike in mouse and rat, SERCA1b was only expressed in pre-matured infant leg and arm muscles; it was replaced by SERCA1a in more matured neonatal muscles and was completely absent in human foetal and neonatal diaphragms. Interestingly, only SERCA1a and no SERCA1b were detected in muscles of 7-12 years old boys with Duchenne, a degenerative-regenerative muscular dystrophy. However, in adult patients with myotonic dystrophy type 2 (DM2), the SERCA1b dominated over SERCA1a. Thus the human SERCA1b has a different expression pattern from that of rodents and it is associated with DM2.

Calcium pumps: why so many?

Ca(2+)-ATPases (pumps) are key to the regulation of Ca(2+) in eukaryotic cells: nine are known today, belonging to three multigene families. The three endo(sarco)plasmic reticulum (SERCA) and the four plasma membrane (PMCA) pumps have been known for decades, the two Secretory Pathway Ca(2+) ATPase (SPCA) pumps have only become known recently. The number of pump isoforms is further increased by alternative splicing processes. The three pump types share the basic features of the catalytic mechanism, but differ in a number of properties related to tissue distribution, regulation, and role in the cellular homeostasis of Ca(2+). The molecular understanding of the function of all pumps has received great impetus from the solution of the three-dimensional (3D) structure of one of them, the SERCA pump. This landmark structural advance has been accompanied by the emergence and rapid expansion of the area of pump malfunction. Most of the pump defects described so far are genetic and produce subtler, often tissue and isoform specific, disturbances that affect individual components of the Ca(2+)-controlling and/or processing machinery, compellingly indicating a specialized role for each Ca(2+) pump type and/or isoform.

Benefit of SERCA2a gene transfer to vascular endothelial and smooth muscle cells: a new aspect in therapy of cardiovascular diseases

Despite the great progress in cardiovascular health and clinical care along with marked decline in morbidity and mortality, cardiovascular diseases remain the leading causes of death and disability in the developed world. New therapeutic approaches, targeting not only systematic but also causal dysfunction, are ultimately needed to provide a valuable alternative for treatment of complex cardiovascular diseases. In heart failure, there are currently a number of trials that have been either completed or are ongoing targeting the sarcoplasmic reticulum calcium ATPase pump (SERCA2a) gene transfer in the context of heart failure. Recently, a phase 2 trial was completed, demonstrating safety and suggested benefit of adeno-associated virus type 1/SERCA2a gene transfer in advanced heart failure, supporting larger confirmatory trials. The experimental and clinical data suggest that, when administrated through perfusion, virus vector carrying SERCA2a can also transduce vascular endothelial and smooth muscle cells (EC and SMC) thereby improving the clinical benefit of gene therapy. Indeed, recent advances in understanding the molecular basis of vascular dysfunction point towards a reduction of sarcoplasmic reticulum Ca2+ uptake and an impairment of Ca2+ cycling in vascular EC and SMC from patients and preclinical models with cardiac diseases or with cardiovascular risk factors such as diabetes, hypercholesterolemia, coronary artery diseases, as well as other conditions such as pulmonary hypertension. In recent years, several studies have established that SERCA2a gene-based therapy could be an efficient option to treat vascular dysfunction. This review focuses on the recent finding showing the beneficial effects of SERCA2a gene transfer in vascular EC and SMC.

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.

Structural lesions and changing pattern of expression of genes encoding cardiac muscle proteins are associated with ventricular myocyte dysfunction in type 2 diabetic Goto-Kakizaki rats fed a high-fat diet

Given the clinical prevalence of type 2 diabetes and obesity and their association with high mortality linked to cardiovascular disease, the aim of the study was to investigate the effects of feeding type 2 diabetic Goto-Kakizaki (GK) rats either high- or low-fat diets on cardiomyocyte structure and function. The GK rats were fed either a high-fat diet (HFD) or a low-fat diet (LFD) from the age of 2 months for a period of 7 months. The GK-HFD rats gained more weight, ate less food and drank less water compared with GK-LFD rats. At 7 months, non-fasting blood glucose was higher in GK-LFD (334 ± 35 mg dl(-1)) compared with GK-HFD rats (235 ± 26 mg dl(-1)). Feeding GK rats with a HFD had no significant effect on glucose clearance following a glucose challenge. Time-to-peak (t(peak)) shortening was reduced in myocytes from GK-HFD (131.8 ± 2.1 ms) compared with GK-LFD rats (144.5 ± 3.0 ms), and time-to-half (t(1/2)) relaxation of shortening was also reduced in myocytes from GK-HFD (71.7 ± 6.9 ms) compared with GK-LFD rats (86.1 ± 3.6 ms). The HFD had no significant effect on the amplitude of shortening. The HFD had no significant effect on t(peak), t(1/2) decay, amplitude of the Ca(2+) transient, myofilament sensitivity to Ca(2+), sarcoplasmic reticulum Ca(2+) content, fractional release of Ca(2+) and the rate of Ca(2+) uptake. Structurally, ventricular myocytes from GK-HFD rats showed extensive mitochondrial lesions, including swelling, loss of cristae, and loss of inner and outer membranes, resulting in gross vacuolarization and deformation of ventricular mitochondria with a subsequent reduction in mitochondrial density. Expression of genes encoding various L-type Ca(2+) channel proteins (Cacnb2) and cardiac muscle proteins (Myl2 and Atp2a1) were downregulated in GK-HFD compared with GK-LFD rats. Structural lesions and changed expression of genes encoding various cardiac muscle proteins might partly underlie the altered time course of myocyte shortening and relaxation in myocytes from GK-HFD compared with GK-LFD rats.

The Ca2+ pumps of the endoplasmic reticulum and Golgi apparatus

The various splice variants of the three SERCA- and the two SPCA-pump genes in higher vertebrates encode P-type ATPases of the P(2A) group found respectively in the membranes of the endoplasmic reticulum and the secretory pathway. Of these, SERCA2b and SPCA1a represent the housekeeping isoforms. The SERCA2b form is characterized by a luminal carboxy terminus imposing a higher affinity for cytosolic Ca(2+) compared to the other SERCAs. This is mediated by intramembrane and luminal interactions of this extension with the pump. Other known affinity modulators like phospholamban and sarcolipin decrease the affinity for Ca(2+). The number of proteins reported to interact with SERCA is rapidly growing. Here, we limit the discussion to those for which the interaction site with the ATPase is specified: HAX-1, calumenin, histidine-rich Ca(2+)-binding protein, and indirectly calreticulin, calnexin, and ERp57. The role of the phylogenetically older and structurally simpler SPCAs as transporters of Ca(2+), but also of Mn(2+), is also addressed.

Evidence for a new allele at the SERCA1 locus affecting pork meat quality in part through the imbalance of Ca2+ homeostasis

Sarcoendoplasmic reticulum Ca2+-ATPase 1 (SERCA1) as a Ca2+ release channel plays a key role in the relaxation of skeletal muscle through pumping cytosolic Ca2+ into the SR (sarcoplasmic reticulum). In this study, a novel single nucleotide polymorphism (SNP) in exon 8 (C > T) was detected by tetra-primer ARMS-PCR and the tissue expression pattern of SERCA1 was analyzed in eleven tissues. A model of primary skeletal muscle cells in vitro exposed to dexamethasone (DEX, a synthetic corticosteroid) was also employed to determine whether stress hormones cause an increase in intracellular Ca2+ concentration that is associated with alteration in SERCA1 and in turn subsequently affect meat quality. The results showed that the CC genotype has lower content intramuscular fat and higher water than pig carrying the genotype CT and CC. In addition, the additive effects were both significantly (P < 0.05) and allele T seemed to be associate with increase in intramuscular fat, while decrease in water content. Accompanied with previous studies, the high abundance of porcine SERCA1 was found in skeletal muscle tissue. DEX markedly down-regulated the expression of SERCA1, leading to Ca2+ overload. Furthermore, the imbalance of Ca2+ homeostasis up-regulated the transcription level of Calpain1. Taken together, we demonstrated a novel mechanism that the changes in expression of SERCA1 potential disturb the normal Ca2+ channel as well as the balance of Ca2+ homeostasis and which in turn finally activated Ca2+-dependent proteases such as Calpain1 which could affect meat quality.

Mitochondria mediated cell death in diabetes

Mitochondrial dysfunction plays a role in the pathogenesis of a wide range of diseases that involve disordered cellular fuel metabolism and survival/death pathways, including neurodegenerative diseases, cancer and diabetes. Cytokine, virus recognition and cellular stress pathways converging on mitochondria cause apoptotic and/or necrotic cell death of beta-cells in type-1 diabetes. Moreover, since mitochondria generate crucial metabolic signals for glucose stimulated insulin secretion (GSIS), mitochondrial dysfunction underlies both the functional derangement of GSIS and (over-nutrition) stress-induced apoptotic/necrotic beta-cell death, hallmarks of type-2 diabetes. The apparently distinct mechanisms governing beta-cell life/death decisions during the development of diabetes provide a remarkable example where remote metabolic, immune and stress signalling meet with mitochondria mediated apoptotic/necrotic death pathways to determine the fate of the beta-cell. We summarize the main findings supporting such a pivotal role of mitochondria in beta-cell death in the context of current trends in diabetes research.

Calcium Pumps in Health and Disease

Ca2+-ATPases (pumps) are key actors in the regulation of Ca2+ in eukaryotic cells and are thus essential to the correct functioning of the cell machinery. They have high affinity for Ca2+ and can efficiently regulate it down to very low concentration levels. Two of the pumps have been known for decades (the SERCA and PMCA pumps); one (the SPCA pump) has only become known recently. Each pump is the product of a multigene family, the number of isoforms being further increased by alternative splicing of the primary transcripts. The three pumps share the basic features of the catalytic mechanism but differ in a number of properties related to tissue distribution, regulation, and role in the cellular homeostasis of Ca2+. The molecular understanding of the function of the pumps has received great impetus from the solution of the three-dimensional structure of one of them, the SERCA pump. These spectacular advances in the structure and molecular mechanism of the pumps have been accompanied by the emergence and rapid expansion of the topic of pump malfunction, which has paralleled the rapid expansion of knowledge in the topic of Ca2+-signaling dysfunction. Most of the pump defects described so far are genetic: when they are very severe, they produce gross and global disturbances of Ca2+ homeostasis that are incompatible with cell life. However, pump defects may also be of a type that produce subtler, often tissue-specific disturbances that affect individual components of the Ca2+-controlling and/or processing machinery. They do not bring cells to immediate death but seriously compromise their normal functioning.

Interactions between the endoplasmic reticulum, mitochondria, plasma membrane and other subcellular organelles

Several recent works show structurally and functionally dynamic contacts between mitochondria, the plasma membrane, the endoplasmic reticulum, and other subcellular organelles. Many cellular processes require proper cooperation between the plasma membrane, the nucleus and subcellular vesicular/tubular networks such as mitochondria and the endoplasmic reticulum. It has been suggested that such contacts are crucial for the synthesis and intracellular transport of phospholipids as well as for intracellular Ca(2+) homeostasis, controlling fundamental processes like motility and contraction, secretion, cell growth, proliferation and apoptosis. Close contacts between smooth sub-domains of the endoplasmic reticulum and mitochondria have been shown to be required also for maintaining mitochondrial structure. The overall distance between the associating organelle membranes as quantified by electron microscopy is small enough to allow contact formation by proteins present on their surfaces, allowing and regulating their interactions. In this review we give a historical overview of studies on organelle interactions, and summarize the present knowledge and hypotheses concerning their regulation and (patho)physiological consequences.

Role of SERCA1 truncated isoform in the proapoptotic calcium transfer from ER to mitochondria during ER stress

Among the new players at the endoplasmic reticulum (ER)-mitochondria interface regulating interorganelle calcium signaling, those specifically involved during ER stress are not known at present. We report here that the truncated variant of the sarcoendoplasmic reticulum Ca(2+)-ATPase 1 (S1T) amplifies ER stress through the PERK-eIF2alpha-ATF4-CHOP pathway. S1T, which is localized in the ER-mitochondria microdomains, determines ER Ca(2+) depletion due to increased Ca(2+) leak, an increased number of ER-mitochondria contact sites, and inhibition of mitochondria movements. This leads to increased Ca(2+) transfer to mitochondria in both resting and stimulated conditions and activation of the mitochondrial apoptotic pathway. Interestingly, S1T knockdown was shown to prevent ER stress, mitochondrial Ca(2+) overload, and subsequent apoptosis. Thus, by bridging ER stress to apoptosis through increased ER-mitochondria Ca(2+) transfer, S1T acts as an essential determinant of cellular fate.

Methyl tert-butyl ether (MTBE) induced Ca(2+)-dependent cytotoxicity in isolated rabbit tracheal epithelial cells

As a volatile synthetic organic chemical, methyl tert-butyl ether (MTBE) was the most common gasoline additive. The increasing use of MTBE raised concern over its health safety. Inhalation was the principle route of exposure for the general population. This study used a model of rabbit tracheal epithelial cells (RTEs) in primary culture to investigate the cytotoxic effects induced by MTBE and the potential mechanism. RTEs were incubated with medium alone (control), 0.5, 50, 5000ppm MTBE respectively. MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazo liumbromide) assay, staining with fluorescein diacetate, propidium iodide and lactate dehydrogenase leakage ratio were used to assess MTBE cytotoxicity on cells. We also observed a significant elevation in cytosolic Ca2+ by fluorescence probe Fluo-3AM at 3, 6 and 12h following exposure to MTBE. Loss of mitochondrial membrane potential (MMP) was detected following 12 and 24h treatment of NP and assessment by rhodamine 123 (Rh123) staining. Activity changes of the Ca(2+)-ATPase, Ca(2+)-Mg(2+)-ATPase following MTBE treatment displayed a similar trend, suggesting an initial elevation before 6h and subsequent dramatic decrease at 12h. Our results demonstrated that induction of cell injury, associated with mitochondrial dysfunction, and alterations in cytosolic Ca2+ in RTEs represent key mechanisms by which MTBE exerts its cytotoxic effects.

Gamma-radiation induces apoptosis via sarcoplasmatic reticulum in guinea pig ileum smooth muscle cells

We investigated the effects of gamma-radiation on cells isolated from the longitudinal smooth muscle layer of the guinea pig ileum, a relatively radioresistant tissue. Single doses (up to 50 Gy) reduced the amount of sarcoplasmatic reticulum and condensed the myofibrils, as shown by electron microscopy 3 days post-irradiation. After that, contractility of smooth muscle strips was reduced. Ca(2+) handling was altered after irradiation, as shown in fura-2 loaded cells, with elevated basal intracellular Ca(2+), reduced amount of intrareticular Ca(2+), and reduced capacitive Ca(2+) entry. Radiation also induced apoptosis, judged from flow cytometry of cells loaded with proprium iodide. Electron microscopy showed that radiation caused condensation of chromatin in dense masses around the nuclear envelope, the presence of apoptotic bodies, fragmentation of the nucleus, detachment of cells from their neighbors, and reductions in cell volume. Radiation also caused activation of caspase 12. Apoptosis was reduced by the administration of the caspase inhibitor Z-Val-Ala-Asp-fluoromethyl-ketone methyl ester (Z-VAD-FMK) during the 3 day period after irradiation, and by the chelator of intracellular Ca(2+), 1,2-bis(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA), from 1 h before until 2 h after irradiation. BAPTA also reduced the effects of radiation on contractility, basal intracellular Ca(2+), amount of intrareticular Ca(2+), capacitative Ca(2+) entry, and apoptosis. In conclusion, the effects of gamma radiation on contractility, Ca(2+) handling, and apoptosis appear due to a toxic action of intracellular Ca(2+). Ca(2+)-induced damage to the sarcoplasmatic reticulum seems a key event in impaired Ca(2+) handling and apoptosis induced by gamma-radiation.

Transcriptomic analysis reveals early signs of liver toxicity in female MRL +/+ mice exposed to the acylating chemicals dichloroacetyl chloride and dichloroacetic anhydride

Dichloroacetyl chloride (DCAC) is a reactive metabolite of trichloroethene (TCE). TCE and its metabolites have been implicated in the induction of organ-specific and systemic autoimmunity, in the acceleration of autoimmune responses, and in the development of liver toxicity and hepatocellular carcinoma. In humans, effects of environmental toxicants are often multifactorial and detected only after long-term exposure. Therefore, we developed a mouse model to determine mechanisms by which DCAC and related acylating agents affect the liver. Autoimmune-prone female MRL +/+ mice were injected intraperitoneally with 0.2 mmol/kg of DCAC or dichloroacetic anhydride (DCAA) in corn oil twice weekly for six weeks. No overt liver pathology was detectable. Using microarray gene expression analysis, we detected changes in the liver transcriptome consistent with inflammatory processes. Both acylating toxicants up-regulated the expression of acute phase response and inflammatory genes. Furthermore, metallothionein genes were strongly up-regulated, indicating effects of the toxicants on zinc ion homeostasis and stress responses. In addition, DCAC and DCAA induced the up-regulation of several genes indicative of tumorigenesis. Our data provide novel insight into early mechanisms for the induction of liver disease by acylating agents. The data also demonstrate the power of microarray analysis in detecting early changes in liver function following exposure to environmental toxicants.

The human corneal endothelium: new insights into electrophysiology and ion channels

The corneal endothelium is a monolayer that mediates the flux of solutes and water across the posterior corneal surface. Thereby, it plays an essential role to maintain the transparency of the cornea. Unlike the epithelium, the human endothelium is an amitotic cell layer with a critical cell density and the risk of corneal decompensation. The number of endothelial cells subsequently decreases with age. Moreover, the endothelial cell loss is accelerated after various impairments such as surgical trauma (e.g. cataract extraction) and following corneal transplantation. This cell loss is associated with programmed cell death (apoptosis) and changed ion channel activity. However, little is known about the electrophysiology and ion channel expression (in particular Ca2+ channels) in corneal endothelial cells. This article reviews our current knowledge about the electrophysiology of the corneal endothelium. It highlights ion channel expression, which may have a major role in corneal cell physiology and pathological events. A better understanding of the (electro)physiological function of the cornea may lead to the development of clinical relevant new therapeutic and preventive measures.

Calcium signalling and cancer cell growth

Cancer is caused by defects in the mechanisms underlying cell proliferation and cell death. Calcium ions are central to both phenomena, serving as major signalling agents with spatial localization, magnitude and temporal characteristics of calcium signals ultimately determining cell's fate. There are four primary compartments: extracellular space, cytoplasm, endoplasmic reticulum and mitochondria that participate in the cellular Ca2+ circulation. They are separated by own membranes incorporating divers Ca2(+)-handling proteins whose concerted action provides for Ca2+ signals with the spatial and temporal characteristics necessary to account for specific cellular response. The transformation of a normal cell into a cancer cell is associated with a major re-arrangement of Ca2+ pumps, Na/Ca exchangers and Ca2+ channels, which leads to the enhanced proliferation and impaired ability to die. In the present chapter we examine what changes in Ca+ signalling and the mechanisms that support it underlie the passage from normal to pathological cell growth and death control. Understanding this changes and identifying molecular players involved provides new prospects for cancers treatment.

Expression des pompes calciques de type SERCA au cours de la différenciation cellulaire et de la tumorigenèse: application à la carcinogenèse colique

Calcium homeostasis of the endoplasmic reticulum (ER) is involved in intracellular signaling pathways and is implicated in major cell functions such as cell growth, differentiation, protein synthesis and apoptosis. The accumulation of calcium in the ER is performed by specific sarco/endoplasmic reticulum calcium transport ATPases (SERCA iso-enzymes). The expression of biochemically distinct SERCA isoforms is cell type dependent and developmentally regulated. This review summarizes pertinent data about the modulation of the expression of SERCA enzymes during the differentiation of normal and tumor cells. These data support the implication of SERCA pumps and especially SERCA3 in the differentiation program of cancer and leukemia cells. During the multi-step process of colon carcinogenesis, the decrease of SERCA3 expression seems to be linked to enhanced APC/ss-catenin/TCF4 signaling and deficient Sp1-like factor-dependent transcription.

Altered mRNA splicing of the skeletal muscle ryanodine receptor and sarcoplasmic/endoplasmic reticulum Ca2+-ATPase in myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is a debilitating multisystemic disorder caused by a CTG repeat expansion in the DMPK gene. Aberrant splicing of several genes has been reported to contribute to some symptoms of DM1, but the cause of muscle weakness in DM1 and elevated Ca2+ concentrations in cultured DM muscle cells is unknown. Here, we investigated the alternative splicing of mRNAs of two major proteins of the sarcoplasmic reticulum, the ryanodine receptor 1 (RyR1) and sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA) 1 or 2. The fetal variants, ASI(-) of RyR1 which lacks residue 3481-3485, and SERCA1b which differs at the C-terminal were significantly increased in skeletal muscles from DM1 patients and the transgenic mouse model of DM1 (HSA(LR)). In addition, a novel variant of SERCA2 was significantly decreased in DM1 patients. The total amount of mRNA for RyR1, SERCA1 and SERCA2 in DM1 and the expression levels of their proteins in HSA(LR) mice were not significantly different. However, heterologous expression of ASI(-) in cultured cells showed decreased affinity for [3H]ryanodine but similar Ca2+ dependency, and decreased channel activity in single-channel recording when compared with wild-type (WT) RyR1. In support of this, RyR1-knockout myotubes expressing ASI(-) exhibited a decreased incidence of Ca2+ oscillations during caffeine exposure compared with that observed for myotubes expressing WT-RyR1. We suggest that aberrant splicing of RyR1 and SERCA1 mRNAs might contribute to impaired Ca2+ homeostasis in DM1 muscle.

Hepatitis C virus core triggers apoptosis in liver cells by inducing ER stress and ER calcium depletion

Hepatitis C virus (HCV) core, known to be involved in liver carcinogenesis, is processed in the endoplasmic reticulum (ER). We thus investigated the impact of three HCV core isolates on ER stress, ER calcium signalling and apoptosis. We show that HCV core constructs trigger hyperexpression of Grp78/BiP, Grp 94, calreticulin and sarco/endoplasmic reticulum calcium ATPase, inducing ER stress. By using the ER-targeted aequorin calcium probe, we found that ER calcium depletion follows ER stress in core-expressing cells. HCV core induces apoptosis through overexpression of the CHOP/GADD153 proapoptotic factor, Bax translocation to mitochondria, mitochondrial membrane depolarization, cytochrome c release, caspase-3 and PARP cleavage. Furthermore, reversion of HCV core-induced ER calcium depletion (by transfection of SERCA2) completely abolished mitochondrial membrane depolarization, suggesting that both ER stress (through CHOP overexpression) and calcium signalling play a major role in the HCV core-mediated control of apoptosis. ER stress and apoptosis were also found in a proportion of HCV-full-length replicon-expressing cells and in the liver of HCV core transgenic mice. In conclusion, our data demonstrate that HCV core deregulates the control of apoptosis by inducing ER stress and ER calcium depletion providing new elements to understand the mechanisms involved in HCV-related liver chronic diseases.

Redox active calcium ion channels and cell death

Calcium plays a key role in both apoptotic and necrotic cell death. Emptying of intracellular calcium stores and/or alteration in intracellular calcium levels can modulate cell death in almost all cell types. These calcium fluxes are determined by the activity of membrane channels normally under tight control. The channels may be ligand activated or voltage dependent as well as being under the control of affector molecules such as calmodulin. It has become increasingly apparent that many calcium channels are affected by reactive oxygen or reactive nitrogen species; ROS/RNS. This may be part of the normal signaling pathways in the cell or by the action of exogenously generated ROS or RNS often by toxins. This review covers the recent literature on the activity of these redox active channels as related to cell death.

Identification, Expression, Function, and Localization of a Novel (Sixth) Isoform of the Human Sarco/Endoplasmic Reticulum Ca2+ATPase 3 Gene

Understanding of Ca(2+) signaling requires the knowledge of proteins involved in this process. Among these proteins are sarco/endoplasmic reticulum Ca(2+)-ATPases (SERCAs) that pump Ca(2+) into the endoplasmic reticulum (ER). Recently, the human SERCA3 gene was shown to give rise to five isoforms (SERCA3a-e (h3a-h3e)). Here we demonstrate the existence of an additional new member, termed SERCA3f (h3f). By reverse transcriptase-PCR using monocytic U937 cell RNA, h3f mRNA was found to exclude the antepenultimate exon 21. h3f mRNA expression appeared as a human-specific splice variant. It was not found in rats or mice. h3f mRNA gave rise to an h3f protein differing in its C terminus from h3a-h3e. Of particular interest, h3f diverged in the first amino acids after the first splice site but presented the same last 21 amino acids as h3b. Consequently, we further investigated the structure-function-location relationships of the h3b and h3f isoforms. Comparative functional study of h3b and h3f recombinant proteins in intact HEK-293 cells and in fractionated membranes showed the following distinct characteristics: (i) resting cytosolic Ca(2+) concentration ([Ca(2+)](c)) and (ii) ER Ca(2+) content ([Ca(2+)](er)); similar characteristics were shown for the following: (i) the effects of the SERCA inhibitor, thapsigargin, on Ca(2+) release ([Ca(2+)](Tg)) and subsequent Ca(2+) entry ([Ca(2+)](e)) and (ii) the low apparent Ca(2+) affinity and the enhanced rate of dephosphorylation of the E(2)P phosphoenzyme intermediate. Subcellular location of h3b and h3f by immunofluorescence and/or confocal microscopy using the h3b- and h3f-specific polyclonal and the pan-h3 monoclonal (PL/IM430) antibodies suggested overlapping but distinct ER location. The endogenous expression of h3f protein was also proved in U937 cells. Altogether these data suggest that the SERCA3 isoforms have a more widespread role in cellular Ca(2+) signaling than previously appreciated.

Isoform-specific distribution of the plasma membrane Ca2+ ATPase in the rat brain

Regulation of cytoplasmic calcium is crucial both for proper neuronal function and cell survival. The concentration of Ca2+ in cytoplasm of a neuron at rest is 10,000 times lower than in the extracellular space, pointing to the importance of the transporters that extrude intracellular Ca2+. The family of plasma membrane calcium-dependent ATPases (PMCAs) represent a major component of the Ca2+ regulatory system. However, little information is available on the regional and cellular distribution of these calcium pumps. We used immunohistochemistry to investigate the distribution of each of the four PMCA isoforms (PMCA1-4) in the rat brain. Each isoform exhibited a remarkably precise and distinct pattern of distribution. In many cases, PMCA isoforms in a single brain structure were differentially expressed within different classes of neurons, and within different subcellular compartments. These data show that each isoform is independently organized and suggest that PMCAs may play a more complex role in calcium homeostasis than generally recognized.