Structure of the human sarco/endoplasmic reticulum Ca2+-ATPase 3 gene. Promoter analysis and alternative splicing of the SERCA3 pre-mRNA

Thyrotropin inhibits osteogenic differentiation of human periodontal ligament stem cells

BACKGROUND AND OBJECTIVE

Periodontal ligament stem cells (PDLSCs) are derived from the periodontal ligament and have the characteristics of pluripotent differentiation, including osteogenesis, and are one of the important seed cells in oral tissue engineering. Thyrotropin (TSH) has been shown to regulate bone metabolism independently of thyroid hormone, including the fate of osteoblasts and osteoclasts, but whether it affects osteogenic differentiation of PDLSCs is unknown.

MATERIALS AND METHODS

PDLSCs were isolated and cultured from human periodontal ligament and grown in osteogenic medium (containing sodium β-glycerophosphate, ascorbic acid, and dexamethasone). Recombinant human TSH was added to the culture medium. Osteogenic differentiation of PDLSCs was assessed after 14 days by staining with alkaline phosphatase and alizarin red and by detection of osteogenic differentiation genes. Differentially expressed genes (DEGs) in PDLSCs under TSH were detected by high-throughput sequencing. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyzed the biological functions and signaling pathways involved in DEGs.

RESULTS

We found that osteogenic differentiation of PDLSCs was significantly inhibited in the presence of TSH: including decreased calcium nodule formation, decreased alkaline phosphatase levels, and decreased collagen synthesis. Using high-throughput sequencing, we found changes in the expression of some osteogenesis-related genes, which may be the reason that TSH inhibits osteogenic differentiation of PDLSCs.

CONCLUSION

Unless TSH is ≥10 mU/L, patients with subclinical hypothyroidism usually do not undergo thyroxine supplementation therapy. However, in this work, we found that elevated TSH inhibited the osteogenic differentiation of PDLSCs. Therefore, correction of TSH levels in patients with subclinical hypothyroidism may be beneficial to improve orthodontic, implant, and periodontitis outcomes in these patients.

Epigenetic regulation of the human ATP2A3 gene promoter in gastric and colon cancer cell lines

The knowledge about the role of calcium-regulated pathways in cancer cell growth and differentiation could be useful for the development of new therapeutic approaches to diminish its mortality. The ATP2A genes encode for SERCA pumps, which modulate cytosolic Ca2+ concentration, regulating various cellular processes including cell growth. ATP2A3 gene transcriptional down-regulation has been reported in gastric and colon cancer, but there is still a lack of understanding about the epigenetic processes regulating its transcription. In this work, we report that butyrate, trichostatin A, and 5-azacytidine treatments increase SERCA3 expression, increased apoptosis, and decreased cell viability of the KATO-III gastric carcinoma cell line. We analyzed the methylation profile of the ATP2A3 gene promoter CpG island, finding clones with methylated status through -280 to -135 promoter region, harboring Sp1 and AP-2 binding sites, which could have a role in transcriptional repression. Post-translational modifications of histones show a major role in the ATP2A3 transcriptional regulation, and our results show histones marks linked to transcriptional repression associated with the -262 to -135 region, this repressive context changed to transcriptional permissive through SERCA3 re-expressing conditions. These results suggest that the nucleotide sequence from -280 to -135 position is an ATP2A3 epigenetic regulatory CpG region in KATO-III cells. Analyses of online-databases show a decreased SERCA3 expression in gastric and colon tumors, as well as overall survival results, showed that high SERCA3 expression could serve as a favorable prognostic marker for colon and gastric cancer patients.

The SERCA2: A Gatekeeper of Neuronal Calcium Homeostasis in the Brain

Calcium (Ca2+) ions are prominent cell signaling regulators that carry information for a variety of cellular processes and are critical for neuronal survival and function. Furthermore, Ca2+ acts as a prominent second messenger that modulates divergent intracellular cascades in the nerve cells. Therefore, nerve cells have developed intricate Ca2+ signaling pathways to couple the Ca2+ signal to their biochemical machinery. Notably, intracellular Ca2+ homeostasis greatly relies on the rapid redistribution of Ca2+ ions into the diverse subcellular organelles which serve as Ca2+ stores, including the endoplasmic reticulum (ER). It is well established that Ca2+ released into the neuronal cytoplasm is pumped back into the ER by the sarco-/ER Ca2+ ATPase 2 (SERCA2), a P-type ion-motive ATPase that resides on the ER membrane. Even though the SERCA2 is constitutively expressed in nerve cells, its precise role in brain physiology and pathophysiology is not well-characterized. Intriguingly, SERCA2-dependent Ca2+ dysregulation has been implicated in several disorders that affect cognitive function, including Darier's disease, schizophrenia, Alzheimer's disease, and cerebral ischemia. The current review summarizes knowledge on the expression pattern of the different SERCA2 isoforms in the nervous system, and further discusses evidence of SERCA2 dysregulation in various neuropsychiatric disorders. To the best of our knowledge, this is the first literature review that specifically highlights the critical role of the SERCA2 in the brain. Advancing knowledge on the role of SERCA2 in maintaining neuronal Ca2+ homeostasis may ultimately lead to the development of safer and more effective pharmacotherapies to combat debilitating neuropsychiatric disorders.

Induction of cell differentiation activates transcription of the Sarco/Endoplasmic Reticulum calcium-ATPase 3 gene (ATP2A3) in gastric and colon cancer cells

The Sarco/Endoplasmic Reticulum Ca2+ -ATPases (SERCAs), pump Ca2+ into the endoplasmic reticulum lumen modulating cytosolic Ca2+ concentrations to regulate various cellular processes including cell growth. Previous studies have reported a downregulation of SERCA3 protein expression in gastric and colon cancer cell lines and showed that in vitro cell differentiation increases its expression. However, little is known about the transcriptional mechanisms and transcription factors that regulate SERCA3 expression in epithelial cancer cells. In this work, we demonstrate that SERCA3 mRNA is upregulated up to 45-fold in two epithelial cancer cell lines, KATO-III and Caco-2, induced to differentiate with histone deacetylase inhibitors (HDACi) and by cell confluence, respectively. To evaluate the transcriptional elements responding to the differentiation stimuli, we cloned the human ATP2A3 promoter, generated deletion constructs and transfected them into KATO-III cells. Basal and differentiation responsive DNA elements were located by functional analysis within the first -135 bp of the promoter region. Using site-directed mutagenesis and DNA-protein binding assays we found that Sp1, Sp3, and Klf-4 transcription factors bind to ATP2A3 proximal promoter elements and regulate basal gene expression. We showed that these factors participated in the increase of ATP2A3 expression during cancer cell differentiation. This study provides evidence for the first time that Sp1, Sp3, and Klf-4 transcriptionally modulate the expression of SERCA3 during induction of epithelial cancer cell differentiation. © 2016 Wiley Periodicals, Inc.

Calcium-ATPases: Gene disorders and dysregulation in cancer

Ca(2+)-ATPases belonging to the superfamily of P-type pumps play an important role in maintaining low, nanomolar cytoplasmic Ca(2+) levels at rest and priming organellar stores, including the endoplasmic reticulum, Golgi, and secretory vesicles with high levels of Ca(2+) for a wide range of signaling functions. In this review, we introduce the distinct subtypes of Ca(2+)-ATPases and their isoforms and splice variants and provide an overview of their specific cellular roles as they relate to genetic disorders and cancer, with a particular emphasis on recent findings on the secretory pathway Ca(2+)-ATPases (SPCA). Mutations in human ATP2A2, ATP2C1 genes, encoding housekeeping isoforms of the endoplasmic reticulum (SERCA2) and secretory pathway (SPCA1) pumps, respectively, confer autosomal dominant disorders of the skin, whereas mutations in other isoforms underlie various muscular, neurological, or developmental disorders. Emerging evidence points to an important function of dysregulated Ca(2+)-ATPase expression in cancers of the colon, lung, and breast where they may serve as markers of differentiation or novel targets for therapeutic intervention. We review the mechanisms underlying the link between calcium homeostasis and cancer and discuss the potential clinical relevance of these observations. This article is part of a Special Issue entitled: Calcium and Cell Fate. Guest Editors: Jacques Haiech, Claus Heizmann, Joachim Krebs, Thierry Capiod and Olivier Mignen.

Optimisation of recombinant production of active human cardiac SERCA2a ATPase

Methods for recombinant production of eukaryotic membrane proteins, yielding sufficient quantity and quality of protein for structural biology, remain a challenge. We describe here, expression and purification optimisation of the human SERCA2a cardiac isoform of Ca(2+) translocating ATPase, using Saccharomyces cerevisiae as the heterologous expression system of choice. Two different expression vectors were utilised, allowing expression of C-terminal fusion proteins with a biotinylation domain or a GFP- His8 tag. Solubilised membrane fractions containing the protein of interest were purified onto Streptavidin-Sepharose, Ni-NTA or Talon resin, depending on the fusion tag present. Biotinylated protein was detected using specific antibody directed against SERCA2 and, advantageously, GFP-His8 fusion protein was easily traced during the purification steps using in-gel fluorescence. Importantly, talon resin affinity purification proved more specific than Ni-NTA resin for the GFP-His8 tagged protein, providing better separation of oligomers present, during size exclusion chromatography. The optimised method for expression and purification of human cardiac SERCA2a reported herein, yields purified protein (> 90%) that displays a calcium-dependent thapsigargin-sensitive activity and is suitable for further biophysical, structural and physiological studies. This work provides support for the use of Saccharomyces cerevisiae as a suitable expression system for recombinant production of multi-domain eukaryotic membrane proteins.

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.

Endoplasmic reticulum Ca(2+) handling in excitable cells in health and disease

The endoplasmic reticulum (ER) is a morphologically and functionally diverse organelle capable of integrating multiple extracellular and internal signals and generating adaptive cellular responses. It plays fundamental roles in protein synthesis and folding and in cellular responses to metabolic and proteotoxic stress. In addition, the ER stores and releases Ca(2+) in sophisticated scenarios that regulate a range of processes in excitable cells throughout the body, including muscle contraction and relaxation, endocrine regulation of metabolism, learning and memory, and cell death. One or more Ca(2+) ATPases and two types of ER membrane Ca(2+) channels (inositol trisphosphate and ryanodine receptors) are the major proteins involved in ER Ca(2+) uptake and release, respectively. There are also direct and indirect interactions of ER Ca(2+) stores with plasma membrane and mitochondrial Ca(2+)-regulating systems. Pharmacological agents that selectively modify ER Ca(2+) release or uptake have enabled studies that revealed many different physiological roles for ER Ca(2+) signaling. Several inherited diseases are caused by mutations in ER Ca(2+)-regulating proteins, and perturbed ER Ca(2+) homeostasis is implicated in a range of acquired disorders. Preclinical investigations suggest a therapeutic potential for use of agents that target ER Ca(2+) handling systems of excitable cells in disorders ranging from cardiac arrhythmias and skeletal muscle myopathies to Alzheimer disease.

Factors controlling the activity of the SERCA2a pump in the normal and failing heart

Heart failure is the leading cause of death in western countries and is often associated with impaired Ca(2+) handling in the cardiomyocyte. In fact, cardiomyocyte relaxation and contraction are tightly controlled by the activity of the cardiac sarco(endo)plasmic reticulum (ER/SR) Ca(2+) pump SERCA2a, pumping Ca(2+) from the cytosol into the lumen of the ER/SR. This review addresses three important facets that control the SERCA2 activity in the heart. First, we focus on the alternative splicing of the SERCA2 messenger, which is strictly regulated in the developing heart. This splicing controls the formation of three SERCA2 splice variants with different enzymatic properties. Second, we will discuss the role and regulation of SERCA2a activity in the normal and failing heart. The two well-studied Ca(2+) affinity modulators phospholamban and sarcolipin control the activity of SERCA2a within a narrow window. An aberrantly high or low Ca(2+) affinity is often observed in and may even trigger cardiac failure. Correcting SERCA2a activity might therefore constitute a therapeutic approach to improve the contractility of the failing heart. Finally, we address the controversies and unanswered questions of other putative regulators of the cardiac Ca(2+) pump, such as sarcalumenin, HRC, S100A1, Bcl-2, HAX-1, calreticulin, calnexin, ERp57, IRS-1, and -2.

ATP2A3 gene is involved in cancer susceptibility

The sarco/endoplasmatic reticulum calcium-ATPase (SERCA) translocates Ca(2+) from cytosol to the lumen of the ER and thus regulates Ca(2+) homeostasis, perturbations of which have been suggested to contribute to cancer. We have previously detected an increased number of alterations in the ATP2A2 gene in various cancer types and in the ATP2A3 gene in head and neck squamous cell carcinoma. Here, we further analyzed the ATP2A3 gene in colon, lung, and CNS cancers. We identified a statistically significant increase of alterations in each (colon cancer, p=0.0052, lung cancer, p=0.0026, CNS tumors, p=0.0045) cancer type, and all 3 types together (p=0.0016). Epigenetic study of the ATP2A3 gene indicated an unchanged methylation status, whereas expression of the ATP2A3 gene was normal for exon 14 mutations and reduced in connection with a nucleotide change in intron VI in all studied cancer types. Identification of a significant number of alterations in cancer patients suggests that ATP2A3 is involved in increased cancer susceptibility in humans. The mostly normal expression and methylation status of the ATP2A3 gene, as well as the absence of somatic alterations, further suggest that the ATP2A3 gene may not act as a classical tumor suppressor gene, but rather haplo-insufficiency of this gene may be enough to change the cell and tissue environment in such a way to predispose to cancer development.

Chronic beta2-adrenoceptor stimulation impairs cardiac relaxation via reduced SR Ca2+-ATPase protein and activity

We determined the cardiovascular effects of chronic beta2-adrenoceptor (beta2-AR) stimulation in vivo and examined the mechanism for the previously observed prolonged diastolic relaxation. Rats (3 mo old; n = 6), instrumented with implantable radiotelemeters, received the selective beta2-AR agonist formoterol (25 microg.kg(-1).day(-1) ip) for 4 wk, with selected cardiovascular parameters measured daily throughout this period, and for a further 7 days after cessation of treatment. Chronic beta2-AR stimulation was associated with an increase in heart rate (HR) of 17% (days 1-14) and 5% (days 15-28); a 11% (days 1-14) and 6% (days 15-28) decrease in mean arterial blood pressure; and a 24% (days 1-14) increase in the rate of cardiac relaxation (-dP/dt) compared with initial values (P < 0.05). Cessation of beta2-AR stimulation resulted in an 8% decrease in HR and a 7% decrease in -dP/dt, compared with initial values (P < 0.05). The prolonged cardiac relaxation with chronic beta2-AR stimulation was associated with a 30% decrease in the maximal rate (Vmax) of sarco(endo)plasmic reticulum Ca2+ -ATPase (SERCA) activity, likely attributed to a 50% decrease in SERCA2a protein (P < 0.05). glycogen synthase kinase-3beta (GSK-3beta) has been implicated as a negative regulator of SERCA2 gene transcription, and we observed a approximately 60% decrease (P < 0.05) in phosphorylated GSK-3beta protein after chronic beta2-AR stimulation. Finally, we found a 40% decrease (P < 0.05) in the mRNA expression of the novel A kinase anchoring protein AKAP18, also implicated in beta2-AR-mediated cardiac relaxation. These findings highlight some detrimental cardiovascular effects of chronic beta2-AR agonist administration and identify concerns for their current and future use for treating asthma or for conditions where muscle wasting and weakness are indicated.

Deletion of one SERCA2 allele confers protection against bladder wall hypertrophy in a murine model of partial bladder outlet obstruction

The sarco(endo)plasmic reticulum Ca2+-ATPase2 (SERCA2) is downregulated in cardiac hypertrophy with decompensation. We sought to determine whether mice heterozygous for the SERCA2 allele would develop greater bladder hypertrophy and decompensation than their wild-type littermates following partial bladder outlet obstruction (pBOO). We found that following 4 wk of surgically created pBOO, SERCA2 heterozygous murine bladders showed significantly less hypertrophy, improved in vitro cystometry performance, diminished expression of the slow myosin isoform A analyzed by RT-PCR, a significant drop in nuclear translocation of nuclear factor of activated T cells by EMSA, and decreased cell proliferation within the smooth muscle layer following 5-bromo-2′-deoxyuridine labeling compared with their wild-type littermates. Thus, in contrast to cardiac muscle, deletion of a SERCA2 allele confers protection against bladder hypertrophy in a murine model of pBOO. Compensatory mechanisms in heterozygous mice seem to be related to the calcineurin pathway. Further studies are underway to better define the molecular basis of this observation, which has potential clinical applications.

Cloning, characterization, and expression patterns of three sarco/endoplasmic reticulum Ca2+-ATPase isoforms from pearl oyster (Pinctada fucata)

A large amount of calcium is required for mollusk biomineralization. Sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) is a well-known protein with the function of sustaining the calcium homeostasis. How does it possibly function in the process of pearl oyster biomineralization? Three SERCA isoforms, namely PSERA, PSERB, and PSERC were cloned from the pearl oyster, Pinctada fucata. The cDNAs of the three isoforms were isolated by reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends. PSERA consisted of 3568 bp encoding 1007 amino acids, PSERB included 3953 bp encoding 1024 amino acids, and PSERC comprised of 3450 bp encoding 1000 amino acids. The three isoforms showed high homology (65%-87%) with SERCAs from other species. Consistent with the results from other invertebrates, Southern blot analysis revealed that the three isoforms originated from a single gene that was also related to SERCA1, SERCA2, and SERCA3 of vertebrates. The splicing mechanism of the three isoforms was similar to that of isoforms of vertebrate SERCA3. Semiquantitative RT-PCR was carried out to study the expression patterns of the three isoforms. The results showed that PSERB was ubiquitously expressed in all tested tissues and was a potential "housekeeping" SERCA isoform; PSERA was expressed in the adductor muscle and foot and was likely to be a muscle-specific isoform, and PSERC was expressed in the other tissues except the adductor muscle or foot with the highest expression levels in the gill and mantle, indicating that it was a non-muscle-specific isoform and might be involved in calcium homeostasis during pearl oyster biomineralization.

Suppression of cytokine response by GATA inhibitor K-7174 via unfolded protein response

K-7174, a GATA-specific inhibitor, is a putative anti-inflammatory agent that attenuates effects of inflammatory cytokines in certain cell types. However, molecular mechanisms involved have not been elucidated. We found that, in glomerular podocytes, induction of monocyte chemoattractant protein 1 (MCP-1) and inducible nitric oxide synthase (iNOS) by TNF-alpha was abrogated by K-7174. It was correlated with unexpected induction of unfolded protein response (UPR) evidenced by: (1) induction of endogenous indicators 78 kDa glucose-regulated protein and CCAAT/enhancer-binding protein-homologous protein, and (2) suppression of an exogenous indicator, endoplasmic reticulum stress-repressive alkaline phosphatase. In podocytes, induction of UPR by either tunicamycin, thapsigargin, A23187 or AB5 subtilase cytotoxin completely reproduced the suppressive effect of K-7174. Furthermore, K-7174-elicited UPR abrogated induction of MCP-1 and iNOS not only by TNF-alpha but also by medium conditioned by activated macrophages. These results suggested a novel, UPR-dependent mechanism underlying the anti-inflammatory potential of K-7174.

Sequence, annotation and developmental expression of the sea urchin Ca2+-ATPase family

Whole genome sequence data permit the study of protein families regulating cellular homeostasis during development. Here we present a study of the sea urchin Ca(2+)-ATPases made possible by the Sea Urchin Genome Sequencing Project. This is of potential interest because adult sea urchins, their gametes and embryos live in the relatively high Ca(2+) concentration of 10 mM. Three Ca(2+)-ATPases regulate Ca(2+) levels in animal cells: plasma membrane Ca(2+)-ATPase (PMCA), sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase (SERCA) and secretory pathway Ca(2+)-ATPase (SPCA). The primary structures of Sp-PMCA and Sp-SERCA in the sea urchin, Strongylocentrotus purpuratus (Sp), have been published. Here, we present the primary structure of Sp-SPCA, which is 912 amino acids and has 66% identity and 80% similarity to human SPCA1. Southern blots and genome analysis show that Sp-SPCA is a single copy gene. Each Sp Ca(2+)-ATPase is highly conserved when compared to its human ortholog, indicating that human and sea urchin share structurally similar energy driven Ca(2+) homeostasis mechanisms that have been maintained throughout the course of deuterostome evolution. Annotation using the assembled sea urchin genome reveals that Sp-SPCA, Sp-PMCA and Sp-SERCA have 23, 17 and 24 exons. RT-Q-PCR shows that transcripts of Sp-SPCA are at low levels compared to Sp-PMCA and Sp-SERCA. Gradual increases in Sp-PMCA and Sp-SERCA mRNA begin at the 18 hour hatched blastula stage and peak 4-5-fold higher by 25 h at the mid to late blastulae stage.

SERCA pumps and human diseases

Sarco(endo)plasmic reticulum (SER) Ca2+ ATPases represent a highly conserved family of Ca2+ pumps which actively transport Ca2+ from the cytosol to the SER against a large concentration gradient. In humans, 3 genes (ATP2A1-3) generate multiple isoforms (SERCAla,b, SERCA2a-c, SECA3a-f) by developmental or tissue-specific alternative splicing. These pumps differ by their regulatory and kinetic properties, allowing for optimized function in the tissue where they are expressed. They play a central role in calcium signalling through regenerating SER Ca2+ stores, maintaining appropriate Ca2+ levels in this organelle and shaping cytosolic and nuclear Ca2+ variations which govern cell response. Defects in ATP2A1 encoding SERCA1 cause recessive Brody myopathy, mutations in ATP2A2 coding for SERCA2 underlie a dominant skin disease, Darier disease and its clinical variants. SERCA2a expression is reduced in heart failure in human and in mice models. Gene-targeting studies in mouse confirmed the expected function of these isoforms in some cases, but also resulted in unexpected phenotypes: SERCA1 null mutants die from respiratory failure, SERCA2 heterozygous mutant mice develop skin cancer with age and SERCA3 null mice display no diabetes. These unique phenotypes have provided invaluable information on the role of these pumps in specific tissues and species, and have improved our understanding of Ca2+ regulated processes in muscles, the heart and the skin in human and in mice. Although the understanding of the pathogenesis of these diseases is still incomplete, these recent advances hold the promise of improved knowledge on the disease processes and the identification of new targets for therapeutic interventions.

Control of protein expression through mRNA stability in calcium signalling

Specific sequences (cis-acting elements) in the 3'-untranslated region (UTR) of RNA, together with stabilizing and destabilizing proteins (trans-acting factors), determine the mRNA stability, and consequently, the level of expression of several proteins. Such interactions were discovered initially for short-lived mRNAs encoding cytokines and early genes like c-jun and c-myc. However, they may also determine the fate of more stable mRNAs in a tissue and disease-dependent manner. The interactions between the cis-acting elements and the trans-acting factors may also be modulated by Ca(2+) either directly or via a control of the phosphorylation status of the trans-acting factors. We focus initially on the basic concepts in mRNA stability with the trans-acting factors AUF1 (destabilizing) and HuR (stabilizing). Sarco/endoplasmic reticulum Ca(2+) pumps, SERCA2a (cardiac and slow twitch muscles) and SERCA2b (most cells including smooth muscle cells), are pivotal in Ca(2+) mobilization during signal transduction. SERCA2a and SERCA2b proteins are encoded by relatively stable mRNAs that contain cis-acting stability determinants in their 3'-regions. We present several pathways where 3'-UTR mediated mRNA decay is key to Ca(2+) signalling: SERCA2a and beta-adrenergic receptors in heart failure, renin-angiotensin system, and parathyroid hormones. Other examples discussed include cytokines vascular endothelial growth factor, endothelin and endothelial nitric oxide synthase. Roles of Ca(2+) and Ca(2+)-binding proteins in mRNA stability are also discussed. We anticipate that these novel modes of control of protein expression will form an emerging area of research that may explore the central role of Ca(2+) in cell function during development and in disease.

Comprehensive analysis of expression and function of 51 sarco(endo)plasmic reticulum Ca2+-ATPase mutants associated with Darier disease

We examined possible defects of sarco(endo)plasmic reticulum Ca2+-ATPase 2b (SERCA2b) associated with its 51 mutations found in Darier disease (DD) pedigrees, i.e. most of the substitution and deletion mutations of residues reported so far. COS-1 cells were transfected with each of the mutant cDNAs, and the expression and function of the SERCA2b protein was analyzed with microsomes prepared from the cells and compared with those of the wild type. Fifteen mutants showed markedly reduced expression. Among the other 36, 29 mutants exhibited completely abolished or strongly inhibited Ca2+-ATPase activity, whereas the other seven possessed fairly high or normal ATPase activity. In four of the aforementioned seven mutants, Ca2+ transport activity was significantly reduced or almost completely lost, therefore uncoupled from ATP hydrolysis. The other three were exceptional cases as they were seemingly normal in protein expression and Ca2+ transport function, but were found to have abnormalities in the kinetic properties altered by the three mutations, which happened to be in the three DD pedigrees found by us previously (Sato, K., Yamasaki, K., Daiho, T., Miyauchi, Y., Takahashi, H., Ishida-Yamamoto, A., Nakamura, S., Iizuka, H., and Suzuki, H. (2004) J. Biol. Chem. 279, 35595-35603). Collectively, our results indicated that in most cases (48 of 51) DD mutations cause severe disruption of Ca2+ homeostasis by the defects in protein expression and/or transport function and hence DD, but even a slight disturbance of the homeostasis will result in the disease. Our results also provided further insight into the structure-function relationship of SERCAs and revealed critical regions and residues of the enzyme.

Regulation of the sarcoplasmic reticulum Ca2+-ATPase expression in the hypertrophic and failing heartThis paper is part of a series in the Journal's “Made in Canada” section. The paper has undergone peer review

The sarcoplasmic reticulum (SR) plays a central role in the contraction and relaxation coupling in the myocardium. The SR Ca2+-ATPase (SERCA2) transports Ca2+ inside the SR lumen during relaxation of the cardiac myocyte. It is well known that diminished contractility of the hypertrophic cardiac myocyte is the main factor of ventricular dysfunction in the failing heart. A key feature of the failing heart is a decreased content and activity of SERCA2, which is the cause of some of the physiological defects observed in the hypertrophic cardiomyocyte performance that are important during transition of compensated hypertrophy to heart failure. In this review different possible mechanisms responsible for decreased transcriptional regulation of the SERCA2 gene are examined, which appear to be the primary cause for decreased SERCA2 expression in heart failure. The experimental evidence suggests that several signalling pathways are involved in the downregulation of SERCA2 expression in the hypertrophic and failing cardiomyocyte. Therapeutic upregulation of SERCA2 expression using replication deficient adenoviral expression vectors, pharmacological interventions using thyroid hormone analogues, β-adrenergic receptor antagonists, and novel metabolically active compounds are currently under investigation for the treatment of uncompensated cardiac hypertrophy and heart failure.

Ca2+-ATPases in non-failing and failing heart: evidence for a novel cardiac sarco/endoplasmic reticulum Ca2+-ATPase 2 isoform (SERCA2c)

We recently documented the expression of a novel human mRNA variant encoding a yet uncharacterized SERCA [SR (sarcoplasmic reticulum)/ER (endoplasmic reticulum) Ca2+-ATPase] protein, SERCA2c [Gélébart, Martin, Enouf and Papp (2003) Biochem. Biophys. Res. Commun. 303, 676-684]. In the present study, we have analysed the expression and functional characteristics of SERCA2c relative to SERCA2a and SERCA2b isoforms upon their stable heterologous expression in HEK-293 cells (human embryonic kidney 293 cells). All SERCA2 proteins induced an increased Ca2+ content in the ER of intact transfected cells. In microsomes prepared from transfected cells, SERCA2c showed a lower apparent affinity for cytosolic Ca2+ than SERCA2a and a catalytic turnover rate similar to SERCA2b. We further demonstrated the expression of the endogenous SERCA2c protein in protein lysates isolated from heart left ventricles using a newly generated SERCA2c-specific antibody. Relative to the known uniform distribution of SERCA2a and SERCA2b in cardiomyocytes of the left ventricle tissue, SERCA2c was only detected in a confined area of cardiomyocytes, in close proximity to the sarcolemma. This finding led us to explore the expression of the presently known cardiac Ca2+-ATPase isoforms in heart failure. Comparative expression of SERCAs and PMCAs (plasma-membrane Ca2+-ATPases) was performed in four nonfailing hearts and five failing hearts displaying mixed cardiomyopathy and idiopathic dilated cardiomyopathies. Relative to normal subjects, cardiomyopathic patients express more PMCAs than SERCA2 proteins. Interestingly, SERCA2c expression was significantly increased (166+/-26%) in one patient. Taken together, these results demonstrate the expression of the novel SERCA2c isoform in the heart and may point to a still unrecognized role of PMCAs in cardiomyopathies.

Biotin supplementation decreases the expression of the SERCA3 gene (ATP2A3) in Jurkat cells, thus, triggering unfolded protein response

Protein folding in the endoplasmic reticulum (ER) depends on Ca(2+); uptake of Ca(2+) into the ER is mediated by sarco/endoplasmic reticulum Ca(2+)-ATPase 3 (SERCA3). The 5'-flanking region of the SERCA3 gene (ATP2A3) contains numerous binding sites for the transcription factors Sp1 and Sp3. Biotin affects the nuclear abundance of Sp1 and Sp3, which may act as transcriptional activators or repressors. Here we determined whether biotin affects the expression of the SERCA3 gene and, thus, protein folding in human lymphoid cells. Jurkat cells were cultured in media containing 0.025 nmol/L biotin (denoted "deficient") or 10 nmol/L biotin ("supplemented"). The transcriptional activity of the full-length human SERCA3 promoter was 50% lower in biotin-supplemented cells compared to biotin-deficient cells. Biotin-dependent repressors bind to elements located 731-1312 bp upstream from the transcription start site in the SERCA3 gene. The following suggest that low expression of SERCA3 in biotin-supplemented cells impaired folding of secretory proteins in the ER, triggering unfolded protein response: (i) sequestration of Ca(2+) in the ER decreased by 14-24% in response to biotin supplementation; (ii) secretion of interleukin-2 into the extracellular space decreased by 75% in response to biotin supplementation; (iii) the nuclear abundance of stress-induced transcription factors increased in response to biotin supplementation; and (iv) the abundance of stress-related proteins such ubiquitin activating enzyme 1, growth arrest and DNA damage 153 gene, X-box binding protein 1 and phosphorylated eukaryotic translation initiation factor 2alpha increased in response to biotin supplementation. Collectively, this study suggests that supplements containing pharmacological doses of biotin may cause cell stress by impairing protein folding in the ER.

Transcription of the sarcoplasmic/endoplasmic reticulum Ca2+-ATPase type 3 gene, ATP2A3, is regulated by the calcineurin/NFAT pathway in endothelial cells

Histamine, known to induce Ca2+ oscillations in endothelial cells, was used to alter Ca2+ cycling. Treatment of HUVEC (human umbilical-vein endothelial cell)-derived EA.hy926 cells with histamine for 1-3 days increased the levels of SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase) 3, but not of SERCA 2b, transcripts and proteins. Promoter-reporter gene assays demonstrated that this increase in expression was due to activation of SERCA 3 gene transcription. The effect of histamine was abolished by mepyramine, but not by cimetidine, indicating that the H1 receptor, but not the H2 receptor, was involved. The histamine-induced up-regulation of SERCA 3 was abolished by cyclosporin A and by VIVIT, a peptide that prevents calcineurin and NFAT (nuclear factor of activated T-cells) from interacting, indicating involvement of the calcineurin/NFAT pathway. Histamine also induced the nuclear translocation of NFAT. NFAT did not directly bind to the SERCA 3 promoter, but activated Ets-1 (E twenty-six-1), which drives the expression of the SERCA 3 gene. Finally, cells treated with histamine and loaded with fura 2 exhibited an improved capacity in eliminating high cytosolic Ca2+ concentrations, in accordance with an increase in activity of a low-affinity Ca2+-ATPase, like SERCA 3. Thus chronic treatment of endothelial cells with histamine up-regulates SERCA 3 transcription. The effect of histamine is mediated by the H1R (histamine 1 receptor) and involves activation of the calcineurin/NFAT pathway. By increasing the rate of Ca2+ sequestration, up-regulation of SERCA 3 counteracts the cytosolic increase in Ca2+ concentration.

Glucose-induced mixed [Ca2+]c oscillations in mouse beta-cells are controlled by the membrane potential and the SERCA3 Ca2+-ATPase of the endoplasmic reticulum

Stimulatory concentrations of glucose induce two patterns of cytosolic Ca2+ concentration ([Ca2+]c) oscillations in mouse islets: simple or mixed. In the mixed pattern, rapid oscillations are superimposed on slow ones. In the present study, we examined the role of the membrane potential in the mixed pattern and the impact of this pattern on insulin release. Simultaneous measurement of [Ca2+]c and insulin release from single islets revealed that mixed [Ca2+]c oscillations triggered synchronous oscillations of insulin secretion. Simultaneous recordings of membrane potential in a single beta-cell within an islet and of [Ca2+]c in the whole islet demonstrated that the mixed pattern resulted from compound bursting (i.e., clusters of membrane potential oscillations separated by prolonged silent intervals) that was synchronized in most beta-cells of the islet. Each slow [Ca2+]c increase during mixed oscillations was due to a progressive summation of rapid oscillations. Digital image analysis confirmed the good synchrony between subregions of an islet. By contrast, islets from sarco(endo)plasmic reticulum Ca2+-ATPase isoform 3 (SERCA3)-knockout mice did not display typical mixed [Ca2+]c oscillations in response to glucose. This results from a lack of progressive summation of rapid oscillations and from altered spontaneous electrical activity, i.e., lack of compound bursting, and membrane potential oscillations characterized by lower-frequency but larger-depolarization phases than observed in SERCA3+/+ beta-cells. We conclude that glucose-induced mixed [Ca2+]c oscillations result from compound bursting in all beta-cells of the islet. Disruption of SERCA3 abolishes mixed [Ca2+]c oscillations and augments beta-cell depolarization. This latter observation indicates that the endoplasmic reticulum participates in the control of the beta-cell membrane potential during glucose stimulation.

Cloning of a sea urchin sarco/endoplasmic reticulum Ca2+ ATPase

Sarco/endoplasmic reticulum Ca2+ ATPase (SERCA), a vesicular integral membrane protein, is the best-characterized member of the P-type ion translocating ATPase superfamily. Here we describe the cloning and structural analysis of a sea urchin SERCA (suSERCA) cloned from testis cDNA. The approximately 112 kDa suSERCA is 1022 amino acids with approximately 70% identity and 80% similarity to all known mammalian SERCA isoforms. suSERCA shares all the structural features of mammalian SERCAs, including domains: A, actuator; N, nucleotide-binding; and P, phosphorylation, and also 10 transmembrane helices. Like human SERCA2, the suSERCA has a possible 11th transmembrane segment in its extreme C-terminus. The alignment of three sequences (suSERCA, human SERCA2, and rabbit SERCA1a) shows that the Ca2+ binding residues and kinks (required to form the ion-binding pocket) are 100% conserved. The annotated suSERCA gene consists of 24 exons separated by 23 introns and is approximately 30 kb. Western blots show that suSERCA is present in sea urchin eggs and testis, but not in mature spermatozoa. Treatment of live sperm with SERCA inhibitors has no effect on intracellular calcium, suggesting the absence of SERCA in sea urchin spermatozoa.

Uptake, localization, and noncarboxylase roles of biotin

Evidence is emerging that biotin participates in processes other than classical carboxylation reactions. Specifically, novel roles for biotin in cell signaling, gene expression, and chromatin structure have been identified in recent years. Human cells accumulate biotin by using both the sodium-dependent multivitamin transporter and monocarboxylate transporter 1. These transporters and other biotin-binding proteins partition biotin to compartments involved in biotin signaling: cytoplasm, mitochondria, and nuclei. The activity of cell signals such as biotinyl-AMP, Sp1 and Sp3, nuclear factor (NF)-kappaB, and receptor tyrosine kinases depends on biotin supply. Consistent with a role for biotin and its catabolites in modulating these cell signals, greater than 2000 biotin-dependent genes have been identified in various human tissues. Many biotin-dependent gene products play roles in signal transduction and localize to the cell nucleus, consistent with a role for biotin in cell signaling. Posttranscriptional events related to ribosomal activity and protein folding may further contribute to effects of biotin on gene expression. Finally, research has shown that biotinidase and holocarboxylase synthetase mediate covalent binding of biotin to histones (DNA-binding proteins), affecting chromatin structure; at least seven biotinylation sites have been identified in human histones. Biotinylation of histones appears to play a role in cell proliferation, gene silencing, and the cellular response to DNA repair. Roles for biotin in cell signaling and chromatin structure are consistent with the notion that biotin has a unique significance in cell biology.

The loss of sarco/endoplasmic reticulum calcium transport ATPase 3 expression is an early event during the multistep process of colon carcinogenesis

Calcium accumulation in the endoplasmic reticulum is accomplished by sarco/endoplasmic reticulum calcium transport ATPases (SERCA enzymes). To better characterize the role of SERCA3 in colon carcinogenesis, its expression has been investigated in colonic epithelium, benign lesions, adenomas, and adenocarcinomas. In addition, the regulation of SERCA3 expression was analyzed in the context of the adenomatous polyposis coli/beta-catenin/T-cell factor 4 (TCF4) pathway and of specificity protein 1 (Sp1)-like factor-dependent transcription. We report that SERCA3 expression increased along the crypts as cells differentiated in normal colonic mucosa and in hyperplastic polyps, was moderately and heterogeneously expressed in colonic adenomas with expression levels inversely correlated with the degree of dysplasia, was barely detectable in well and moderately differentiated adenocarcinomas, and was absent in poorly differentiated tumors. Inhibition of Sp1-like factor-dependent transcription blocked SERCA3 expression during cell differentiation, and SERCA3 expression was induced by the expression of dominant-negative TCF4 in colon cancer cells. These data link SERCA3 expression to the state of differentiation of colonic epithelial cells, and relate SERCA3 expression, already decreased in adenomas, to enhanced adenomatous polyposis coli/beta-catenin/TCF4-dependent signaling and deficient Sp1-like factor-dependent transcription. In conclusion, intracellular calcium homeostasis becomes progressively anomalous during colon carcinogenesis as reflected by deficient SERCA3 expression.

Phenotypes of SERCA and PMCA knockout mice

P-type Ca2+-ATPases of the sarco(endo)plasmic reticulum (SERCAs) and plasma membrane (PMCAs) are responsible for maintaining the Ca2+ gradients across cellular membranes that are required for regulation of Ca2+-mediated signaling and other biological processes. Gene-targeting studies of SERCA isoforms 1, 2, and 3 and PMCA isoforms 1, 2, and 4 have confirmed some of the general functions proposed for these pumps, such as a major role in excitation-contraction coupling for SERCA1 and SERCA2 and housekeeping functions for PMCA1 and SERCA2, but have also revealed some unexpected phenotypes. These include squamous cell cancer and plasticity in the regulation of Ca2+-mediated exocytosis in SERCA2 heterozygous mutant mice, modulation of Ca2+ signaling in SERCA3-deficient mice, deafness and balance disorders in PMCA2 null mice, and male infertility in PMCA4 null mice. These unique phenotypes provide new information about the cellular functions of these pumps, the requirement of their activities for higher order physiological processes, and the pathophysiological consequences of pump dysfunction.

Endoplasmic reticulum calcium transport ATPase expression during differentiation of colon cancer and leukaemia cells

The calcium homeostasis of the endoplasmic reticulum (ER) is connected to a multitude of cell functions involved in intracellular signal transduction, control of proliferation, programmed cell death, or the synthesis of mature proteins. Calcium is accumulated in the ER by various biochemically distinct sarco/endoplasmic reticulum calcium transport ATPase isoenzymes (SERCA isoforms). Experimental data indicate that the SERCA composition of some carcinoma and leukaemia cell types undergoes significant changes during differentiation, and that this is accompanied by modifications of SERCA-dependent calcium accumulation in the ER. Because ER calcium homeostasis can also influence cell differentiation, we propose that the modulation of the expression of various SERCA isoforms, and in particular, the induction of the expression of SERCA3-type proteins, is an integral part of the differentiation program of some cancer and leukaemia cell types. The SERCA content of the ER may constitute a new parameter by which the calcium homeostatic characteristics of the organelle are adjusted. The cross-talk between ER calcium homeostasis and cell differentiation may have some implications for the better understanding of the signalling defects involved in the acquisition and maintenance of the malignant phenotype.

The nuclear abundance of transcription factors Sp1 and Sp3 depends on biotin in Jurkat cells

Biotin affects gene expression in mammals; however, the signaling pathways leading to biotin-dependent transcriptional activation and inactivation of genes are largely unknown. Members of the Sp/Krüppel-like factor family of transcription factors (e.g., the ubiquitous Sp1 and Sp3) play important roles in the expression of numerous mammalian genes. We tested the hypothesis that the nuclear abundance of Sp1 and Sp3 depends on biotin in human T cells (Jurkat cells) mediating biotin-dependent gene expression. Jurkat cells were cultured in biotin-deficient (0.025 nmol/L) and biotin-supplemented (10 nmol/L) media for 5 wk prior to transcription factor analysis. The association of Sp1 and Sp3 with DNA-binding sites (GC box and CACCC box) was 76-149% greater in nuclear extracts from biotin-supplemented cells compared with biotin-deficient cells, as determined by electrophoretic mobility shift assays. The increased DNA-binding activity observed in biotin-supplemented cells was caused by increased transcription of genes encoding Sp1 and Sp3, as shown by mRNA levels and reporter-gene activities; increased transcription of Sp1 and Sp3 genes was associated with the increased abundance of Sp1 and Sp3 protein in nuclei. Notwithstanding the important role for phosphorylation of Sp1 and Sp3 in regulating DNA-binding activity, the present study suggests that the effects of biotin on phosphorylation of Sp1 and Sp3 are minor. The increased nuclear abundance of Sp1 and Sp3 in biotin-supplemented cells was associated with increased transcriptional activity of 5'-flanking regions in Sp1/Sp3-dependent genes in reporter-gene assays. This study provides evidence that some effects of biotin on gene expression might be mediated by the nuclear abundance of Sp1 and Sp3.

Distinct types of abnormality in kinetic properties of three Darier disease-causing sarco(endo)plasmic reticulum Ca2+-ATPase mutants that exhibit normal expression and high Ca2+ transport activity

The possible functional abnormalities in three different Darier disease-causing Ca(2+)-ATPase (SERCA2b) mutants, Ile(274) --> Val at the lumenal end of M3, Leu(321) --> Phe on the cytoplasmic part of M4, and Met(719) --> Ile in P domain, were explored, because they exhibited nearly normal expression and localization in COS-1 cells and the high ATPase and coupled Ca(2+) transport activities that were essentially identical (L321F) or slightly lower (I274V by approximately 35% and M719I by approximately 30%) as compared with those of the wild type. These mutations happened to be in Japanese patients found previously by us. Kinetic analyses revealed that each of the mutants possesses distinct types of abnormalities; M719I and L321F possess the 2-3-fold reduced affinity for cytoplasmic Ca(2+), whereas I274V possesses the normal high affinity. L321F exhibited also the remarkably reduced sensitivity to the feedback inhibition of the transport cycle by accumulated lumenal Ca(2+), as demonstrated with the effect of Ca(2+) ionophore on ATPase activity and more specifically with the effects of Ca(2+) (up to 50 mm) on the decay of phosphoenzyme intermediates. The results on I274V and M719I suggest that the physiological requirement for Ca(2+) homeostasis in keratinocytes to avoid haploinsufficiency is very strict, probably much more than considered previously. The insensitivity to lumenal Ca(2+) in L321F likely brings the lumenal Ca(2+) to an abnormally elevated level. The three mutants with their distinctively altered kinetic properties will thus likely cause different types of perturbation of intracellular Ca(2+) homeostasis, but nevertheless all types of perturbation result in Darier disease. It might be possible that the observed unique feature of L321F could possibly be associated with the specific symptoms in the pedigree with this mutation, neuropsychiatric disorder, and behavior problems. The results also provided further insight into the global nature of conformational changes of SERCAs for ATP-driven Ca(2+) transport.

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.

Sarcoendoplasmic reticulum Ca2+ ATPase type 2 downregulated in human oral squamous cell carcinoma

Mice with a heterozygous deletion of the Atp2a2 gene (Atp2a2(+/-)) encoding SERCA2 spontaneously develop SCCs of the skin and upper digestive tract, including the oral cavity. To elucidate the contribution of ATP2A2 to human oral carcinogenesis, we analyzed genetic and epigenetic changes as well as mRNA and protein expression in primary OSCCs and OPLs. With the exception of one OSCC-derived cell line showing a 12 bp deletion of ATP2A2, we found no mutations in the coding sequence of the gene in primary OSCCs (n = 52), OPLs (n = 32) and cell lines (n = 8). In immunohistochemistry, however, high frequencies of ATP2A2 downregulation were evident not only in primary OSCCs (42%, 42/100) but also in OPLs (31%, 10/32). Real-time quantitative RT-PCR data were consistent with the protein expression status. Aberrant DNA methylation within ATP2A2 also was detected in 9 of 30 ATP2A2-downregulated OSCCs. Moreover, restoration or elevated expression of the ATP2A2 protein was induced in most of the cell lines showing ATP2A2 methylation after treatment with 5-aza-2'-dC, a DNA demethylating agent. These results suggest that inactivation of the ATP2A2 gene is a frequent and early event during oral carcinogenesis and that loss of expression may be regulated partly by an epigenetic mechanism.

Dissection of the functional differences between sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) 1 and 2 isoforms and characterization of Darier disease (SERCA2) mutants by steady-state and transient kinetic analyses

Steady-state and rapid kinetic studies were conducted to functionally characterize the overall and partial reactions of the Ca2+ transport cycle mediated by the human sarco(endo)plasmic reticulum Ca2+-ATPase 2 (SERCA2) isoforms, SERCA2a and SERCA2b, and 10 Darier disease (DD) mutants upon heterologous expression in HEK-293 cells. SERCA2b displayed a 10-fold decrease in the rate of Ca2+ dissociation from E1Ca2 relative to SERCA2a (i.e. SERCA2b enzyme manifests true high affinity at cytosolic Ca2+ sites) and a lower rate of dephosphorylation. These fundamental kinetic differences explain the increased apparent affinity for activation by cytosolic Ca2+ and the reduced catalytic turnover rate in SERCA2b. Relative to SERCA1a, both SERCA2 isoforms displayed a 2-fold decrease of the rate of E2 to E1Ca2 transition. Furthermore, seven DD mutants were expressed at similar levels as wild type. The expression level was 2-fold reduced for Gly23 --> Glu and Ser920 --> Tyr and 10-fold reduced for Gly749 --> Arg. Uncoupling between Ca2+ translocation and ATP hydrolysis and/or changes in the rates of partial reactions account for lack of function for 7 of 10 mutants: Gly23 --> Glu (uncoupling), Ser186 --> Phe, Pro602 --> Leu, and Asp702 --> Asn (block of E1 approximately P(Ca2) to E2-P transition), Cys318 --> Arg (uncoupling and 3-fold reduction of E2-P to E2 transition rate), and Thr357 --> Lys and Gly769 --> Arg (lack of phosphorylation). A 2-fold decrease in the E1 approximately P(Ca2) to E2-P transition rate is responsible for the 2-fold decrease in activity for Pro895 --> Leu. Ser920 --> Tyr is a unique DD mutant showing an enhanced molecular Ca2+ transport activity relative to wild-type SERCA2b. In this case, the disease may be a consequence of the low expression level and/or reduction of Ca2+ affinity and sensitivity to inhibition by lumenal Ca2+.

Mutations in the sarcoplasmic/endoplasmic reticulum Ca2+ ATPase isoform cause Darier's disease

Darier's disease is an autosomal dominantly inherited skin disorder, characterized by loss of adhesion between epidermal cells and abnormal keratinization. ATP2A2 encoding the sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA)2 has been identified as the defective gene in Darier's disease. All mutations previously reported occur in the region of ATP2A2 encoding both SERCA2a and SERCA2b isoforms. These isoforms result from alternative splicing of exon 20, with SERCA2b being the major isoform expressed in the epidermis. In this report, we studied a family affected with Darier's disease and identified a deletion (2993delTG) in a region of exon 20 of ATP2A2, which is specific for SERCA2b. This heterozygous mutation predicts a frameshift with a premature termination codon (PTC+32aa) in the eleventh transmembrane domain of SERCA2b. It segregates with the disease phenotype in the family members tested, and functional analysis shows a drastic reduction of the expression of the mutated protein in comparison with the wild-type SERCA2b. Our result suggests that the mutated allele causes the disease phenotype through loss of function of SERCA2b isoform. This finding indicates that SERCA2b plays a key role in the biology of the epidermis, and its defects are sufficient to cause Darier's disease.

Effect of Hailey-Hailey Disease mutations on the function of a new variant of human secretory pathway Ca2+/Mn2+-ATPase (hSPCA1)

ATP2C1, encoding the human secretory pathway Ca2+/Mn2+ ATPase (hSPCA1), was recently identified as the defective gene in Hailey-Hailey Disease (HHD), an autosomal dominant skin disorder characterized by persistent blisters and erosions. To investigate the underlying cause of HHD, we have analyzed the changes in expression level and function of hSPCA1 caused by mutations found in HHD patients. Mutations were introduced into hSPCA1d, a novel splice variant expressed in keratinocytes, described here for the first time. Encoded by the full-length of optional exons 27 and 28, hSPCA1d was longer than previously identified splice variants. The protein competitively transported Ca2+ and Mn2+ with equally high affinity into the Golgi of COS-1 cells. Ca2+- and Mn2+-dependent phosphoenzyme intermediate formation in forward (ATP-fuelled) and reverse (Pi-fuelled) directions was also demonstrated. HHD mutant proteins L341P, C344Y, C411R, T570I, and G789R showed low levels of expression, despite normal levels of mRNA and correct targeting to the Golgi, suggesting instability or abnormal folding of the mutated hSPCA1 polypeptides. P201L had little effect on the enzymatic cycle, whereas I580V caused a block in the E1 approximately P --> E2-P conformational transition. D742Y and G309C were devoid of Ca2+- and Mn2+-dependent phosphoenzyme formation from ATP. The capacity to phosphorylate from Pi was retained in these mutants but with a loss of sensitivity to both Ca2+ and Mn2+ in D742Y and a preferential loss of sensitivity to Mn2+ in G309C. These results highlight the crucial role played by Asp-742 in the architecture of the hSPCA1 ion-binding site and reveal a role for Gly-309 in Mn2+ transport selectivity.

Inhibition of human SERCA3 by PL/IM430. Molecular analysis of the interaction

The monoclonal antibody PL/IM430 has previously been reported to uncouple Ca(2+) transport from ATP hydrolysis in platelet membranes (Hack, N., Wilkinson, J. M., and Crawford, N. (1988) Biochem. J. 250, 355-361). More recently, we have demonstrated that this antibody is specific for human SERCA3 (Poch, E., Leach, S., Snape, S., Cacic, T., MacLennan, D. H., and Lytton, J. (1998) Am. J. Physiol. 275, C1449-C1458). In this paper, we have extended the analysis of the PL/IM430-SERCA3 interaction. Using HEK293 cells to express human SERCA3a, we were able to measure both ATP-mediated, oxalate-dependent (45)Ca(2+) uptake and Ca(2+)-dependent ATP hydrolysis activities due exclusively to SERCA3. Treatment with PL/IM430 inhibited both activities almost identically, with a maximal inhibition of 81 and 73% and a half-maximal concentration of 8.3 and 5.9 microg/ml, for Ca(2+) uptake and ATP hydrolysis, respectively. We conclude that PL/IM430 does inhibit SERCA3 activity but does not uncouple Ca(2+) transport from ATP hydrolysis. Using a combination of partial proteolysis, GST fusion protein expression, and mutation of residues that differ between rat and human SERCA3, we have identified human SERCA3 amino acids Pro(8) and Glu(192) as essential to forming the PL/IM430 epitope. PL/IM430 thus recognizes a linearly noncontiguous set of amino acids within the actuator domain of human SERCA3. We propose that PL/IM430 inhibits SERCA3 activity by sterically preventing movement of the actuator domain into a catalytically critical position in the E2 conformation of the enzyme.

Identification of a new SERCA2 splice variant regulated during monocytic differentiation

Sarco/endoplasmic reticulum-type calcium transport ATPases (SERCA enzymes) pump calcium ions from the cytosol into the endoplasmic reticulum. We report that in addition to the ubiquitously expressed SERCA2b isoform, a new splice variant of SERCA2 can be detected (SERCA2c) that arises from the inclusion of a short intronic sequence located between exons 20 and 21 of the SERCA2a isoform. Sequence analysis revealed classical splice donor and acceptor sites, as well as a branch-point site. Due to the presence in the new exon of an in-frame stop codon that is preceded by a 17 bp coding sequence, this mRNA potentially codes for a protein with a truncated C-terminus containing a short unique C-terminal peptide stretch. SERCA2c message was detected in epithelial, mesenchymal, and hematopoietic cell lines, as well as in primary human monocytes. Moreover, we found that during monocytic differentiation total SERCA2 ATPase expression is induced on the protein and mRNA level and that the novel SERCA2c messenger is also up-regulated during this process. These data indicate that the alternative splicing pattern of the 3(') region of the SERCA2 primary transcript is more complex than that previously thought and that this enzyme may be involved in the process of monocyte differentiation.

Insulin receptor signaling and sarco/endoplasmic reticulum calcium ATPase in beta-cells

Glucose is the main physiological secretagogue for insulin secretion by pancreatic beta-cells, and the major biochemical mechanisms involved have been elucidated. In particular, an increase in intracellular calcium is important for insulin exocytosis. More recently, it has become apparent that the beta-cell also has many of the elements of the insulin receptor signal transduction pathway, including the insulin receptor and insulin receptor substrate (IRS) proteins 1 and 2. Studies with transgenic models have shown that the beta-cell-selective insulin receptor knockout and the IRS-1 knockout lead to reduced glucose-induced insulin secretion. Overexpression of the insulin receptor and IRS-1 in beta-cells results in increased insulin secretion and increased cytosolic Ca(2+). We have thus postulated the existence of a novel autocrine-positive feedback loop of insulin on its own secretion involving interaction with the insulin receptor signal transduction pathway and regulation of intracellular calcium homeostasis. Our current working hypothesis is that this glucose-dependent interaction occurs at the level of IRS-1 and the sarco(endo)plasmic reticulum calcium ATPase, the calcium pump of the endoplasmic reticulum.

Dissection of the functional differences between sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) 1 and 3 isoforms by steady-state and transient kinetic analyses

Steady-state and transient-kinetic studies were conducted to characterize the overall and partial reactions of the Ca(2+)-transport cycle mediated by the human sarco(endo)plasmic reticulum Ca(2+)-ATPase 3 (SERCA3) isoforms: SERCA3a, SERCA3b, and SERCA3c. Relative to SERCA1a, all three human SERCA3 enzymes displayed a reduced apparent affinity for cytosolic Ca(2+) in activation of the overall reaction due to a decreased E(2) to E(1)Ca(2) transition rate and an increased rate of Ca(2+) dissociation from E(1)Ca(2). At neutral pH, the ATPase activity of the SERCA3 enzymes was not significantly enhanced upon permeabilization of the microsomal vesicles with calcium ionophore, indicating a difference from SERCA1a with respect to regulation of the lumenal Ca(2+) level (either an enhanced efflux of lumenal Ca(2+) through the pump in E(2) form or insensitivity to inhibition by lumenal Ca(2+)). Other differences from SERCA1a with respect to the overall ATPase reaction were an alkaline shift of the pH optimum, increased catalytic turnover rate at pH optimum (highest for SERCA3b, the isoform with the longest C terminus), and an increased sensitivity to inhibition by vanadate that disappeared under equilibrium conditions in the absence of Ca(2+) and ATP. The transient-kinetic analysis traced several of the differences from SERCA1a to an enhancement of the rate of dephosphorylation of the E(2)P phosphoenzyme intermediate, which was most pronounced at alkaline pH and increased with the length of the alternatively spliced C terminus.

Molecular physiology of the SERCA and SPCA pumps

Intracellular Ca(2+)-transport ATPases exert a pivotal role in the endoplasmic reticulum and in the compartments of the cellular secretory pathway by maintaining a sufficiently high lumenal Ca(2+) (and Mn(2+)) concentration in these compartments required for an impressive number of vastly different cell functions. At the same time this lumenal Ca(2+) represents a store of releasable activator Ca(2+) controlling an equally impressive number of cytosolic functions. This review mainly focuses on the different Ca(2+)-transport ATPases found in the intracellular compartments of mainly animal non-muscle cells: the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) pumps. Although it is not our intention to treat the ATPases of the specialized sarcoplasmic reticulum in depth, we can hardly ignore the SERCA1 pump of fast-twitch skeletal muscle since its structure and function is by far the best understood and it can serve as a guide to understand the other members of the family. In a second part of this review we describe the relatively novel family of secretory pathway Ca(2+)/Mn(2+) ATPases (SPCA), which in eukaryotic cells are primarily found in the Golgi compartment.

SERCA3 ablation does not impair insulin secretion but suggests distinct roles of different sarcoendoplasmic reticulum Ca(2+) pumps for Ca(2+) homeostasis in pancreatic beta-cells

Two sarcoendoplasmic reticulum Ca(2+)-ATPases, SERCA3 and SERCA2b, are expressed in pancreatic islets. Immunocytochemistry showed that SERCA3 is restricted to beta-cells in the mouse pancreas. Control and SERCA3-deficient mice were used to evaluate the role of SERCA3 in beta-cell cytosolic-free Ca(2+) concentration ([Ca(2+)](c)) regulation, insulin secretion, and glucose homeostasis. Basal [Ca(2+)](c) was not increased by SERCA3 ablation. Stimulation with glucose induced a transient drop in basal [Ca(2+)](c) that was suppressed by inhibition of all SERCAs with thapsigargin (TG) but unaffected by selective SERCA3 ablation. Ca(2+) mobilization by acetylcholine was normal in SERCA3-deficient beta-cells. In contrast, [Ca(2+)](c) oscillations resulting from intermittent glucose-stimulated Ca(2+) influx and [Ca(2+)](c) transients induced by pulses of high K(+) were similarly affected by SERCA3 ablation or TG pretreatment of control islets; their amplitude was increased and their slow descending phase suppressed. This suggests that, during the decay of each oscillation, the endoplasmic reticulum releases Ca(2+) that was pumped by SERCA3 during the upstroke phase. SERCA3 ablation increased the insulin response of islets to 15 mmol/l glucose. However, basal and postprandial plasma glucose and insulin concentrations in SERCA3-deficient mice were normal. In conclusion, SERCA2b, but not SERCA3, is involved in basal [Ca(2+)](c) regulation in beta-cells. SERCA3 becomes operative when [Ca(2+)](c) rises and is required for normal [Ca(2+)](c) oscillations in response to glucose. However, a lack of SERCA3 is insufficient in itself to alter glucose homeostasis or impair insulin secretion in mice.

Basal transcription of the mouse sarco(endo)plasmic reticulum Ca2+-ATPase type 3 gene in endothelial cells is controlled by Ets-1 and Sp1

We reported previously that the sarco(endo)plasmic reticulum Ca(2+)-ATPase type 3 (SERCA3) gene is expressed in many tissues and in a subset of cells such as endothelial, epithelial, and lymphoid lineages. Here we analyzed the mechanisms involved in the regulation of transcription of the SERCA3 gene in endothelial cells. The promoter of the murine SERCA3 gene was isolated, and a single transcription initiation site located 301 bp upstream of the translation initiation site was identified. Analysis of the transcriptional activity of fragments of the SERCA3 promoter showed the existence of a minimal promoter region located between bases -97 and +153 that contains one ETS-binding site (EBS) and two Sp1 elements that are essential for basal transcription. Mutation of the EBS or of the Sp1 sites abolished the basal activity of the promoter. We identified Ets-1 and Sp1 among endothelial nuclear factors that recognize the EBS and Sp1 sites on the promoter. Furthermore, transactivation of the -97/+301 promoter fragment by Ets-1 requires the presence of both the EBS and Sp1 sites, suggesting an interaction of the transcription factors on the gene promoter. Finally, overexpression of Ets-1 induced the expression of SERCA3 in endothelial cells and in fibroblasts.

Expression of a P-type Ca(2+)-transport ATPase in Bacillus subtilis during sporulation

The open reading frame designated yloB in the genomic sequence of Bacillus subtilis encodes a putative protein that is most similar to the typically eukaryotic type IIA family of P-type ion-motive ATPases, including the endo(sarco)plasmic reticulum (SERCA) and PMR1 Ca(2+)-transporters, located respectively in the SERCA and the Golgi apparatus. The overall amino acid sequence is more similar to that of the Pmr1s than to the SERCAs, whereas the inverse is seen for the 10 amino acids that form the two Ca(2+)-binding sites in SERCA. Sporulating but not vegetative B. subtilis cells express the predicted protein, as shown by Western blotting and by the formation of a Ca(2+)-dependent phosphorylated intermediate. Half-maximal activation of phosphointermediate formation occurred at 2.5 microM Ca(2+). Insertion mutation of the yloB gene did not affect the growth of vegetative cells, did not prevent the formation of viable spores, and did not significantly affect 45Ca accumulation during sporulation. However, spores from knockouts were less resistant to heat and showed a slower rate of germination. It is concluded that the P-type Ca(2+)-transport ATPase from B. subtilis is not essential for survival, but assists in the formation of resistant spores. The evolutionary relationship of the transporter to the eukaryotic P-type Ca(2+)-transport ATPases is discussed.

Expression of endomembrane calcium pumps in colon and gastric cancer cells. Induction of SERCA3 expression during differentiation

Calcium mobilization from the endoplasmic reticulum (ER) into the cytosol is a key component of several signaling networks controlling tumor cell growth, differentiation, or apoptosis. Sarco/endoplasmic reticulum calcium transport ATPases (SERCA-type calcium pumps), enzymes that accumulate calcium in the ER, play an important role in these phenomena. We report that SERCA3 expression is significantly reduced or lost in colon carcinomas when compared with normal colonic epithelial cells, which express this enzyme at a high level. To study the involvement of SERCA enzymes in differentiation, in this work differentiation of colon and gastric cancer cell lines was initiated, and the change in the expression of SERCA isoenzymes as well as intracellular calcium levels were investigated. Treatment of the tumor cells with butyrate or other established differentiation inducing agents resulted in a marked and specific induction of the expression of SERCA3, whereas the expression of the ubiquitous SERCA2 enzymes did not change significantly or was reduced. A similar marked increase in SERCA3 expression was found during spontaneous differentiation of post-confluent Caco-2 cells, and this closely correlated with the induction of other known markers of differentiation. Analysis of the expression of the SERCA3 alternative splice isoforms revealed induction of all three known iso-SERCA3 variants (3a, 3b, and 3c). Butyrate treatment of the KATO-III gastric cancer cells led to higher resting cytosolic calcium concentrations and, in accordance with the lower calcium affinity of SERCA3, to diminished ER calcium content. These data taken together indicate a defect in SERCA3 expression in colon cancers as compared with normal colonic epithelium, show that the calcium homeostasis of the endoplasmic reticulum may be remodeled during cellular differentiation, and indicate that SERCA3 constitutes an interesting new differentiation marker that may prove useful for the analysis of the phenotype of gastrointestinal adenocarcinomas.

Three novel sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) 3 isoforms. Expression, regulation, and function of the membranes of the SERCA3 family

Sarco/endoplasmic reticulum Ca2+-ATPases (SERCAs) pump Ca2+ into the endoplasmic reticulum. Recently, three human SERCA3 (h3a-c) proteins and a previously unknown rat SERCA3 (r3b/c) mRNA have been described. Here, we (i) document two novel human SERCA3 splice variants h3d and h3e, (ii) provide data for the expression and mechanisms regulating the expression of all known SERCA3 variants (r3a, r3b/c, and h3a-e), and (iii) show functional characteristics of the SERCA3 isoforms. h3d and h3e are issued from the insertion of an additional penultimate exon 22 resulting in different carboxyl termini for these variants. Distinct distribution patterns of the SERCA3 gene products were observed in a series of cell lines of hematopoietic, epithelial, embryonic origin, and several cancerous types, as well as in panels of rat and human tissues. Hypertension and protein kinase C, calcineurin, or retinoic acid receptor signaling pathways were found to differently control rat and human splice variant expression, respectively. Stable overexpression of each variant was performed in human embryonic kidney 293 cells, and the SERCA3 isoforms were fully characterized. All SERCA3 isoforms were found to pump Ca2+ with similar affinities. However, they modulated the cytosolic Ca2+ concentration ([Ca2+]c) and the endoplasmic reticulum Ca2+ content ([Ca2+]er) in different manners. A newly generated polyclonal antibody and a pan-SERCA3 antibody proved the endogenous expression of the three novel SERCA3 proteins, h3d, h3e, and r3b/c. All these data suggest that the SERCA3 gene products have a more widespread role in cellular Ca2+ signaling than previously appreciated.

Regulation of the endoplasmic reticulum calcium storage during the unfolded protein response--significance in tissue ischemia?

Endoplasmic reticulum (ER) is an organelle intimately involved in control of cell activities through Ca(2+) signaling, as well as in post-translational protein folding and maturation. Ca(2+) storage within the ER is required for both of these functions. Several of the ER-resident proteins essential for the protein folding pathway require Ca(2+) binding for their activity. A number of factors, including Ca(2+) depletion, may interfere with the folding pathway within the ER, with a potential for cell injury through an accumulation of malfolded protein aggregates. The Unfolded Protein Response involves a transcriptional upregulation of a number of the ER-resident folding helper proteins and becomes triggered when the folding pathway is blocked. To be effective, these upregulated proteins require a sufficient supply of Ca(2+) cofactor within the ER lumen. In tissue ischemia, where the availablity of this cofactor may be compromised, the newly described ability of the cell to boost the ER Ca(2+)-loading capacity by upregulating the ER Ca(2+) pump may be of particular importance for limiting cell injury and promoting survival. The novel focus on the pathophysiological significance of ER Ca(2+)depletion extends the scope of disturbed Ca(2+) homeostasis following ischemia beyond the consequences of the cytosolic calcium overload.