Secretory Pathway Ca2+-ATPase Research Articles

Crosstalk among Calcium ATPases: PMCA, SERCA and SPCA in Mental Diseases.

Calcium in mammalian neurons is essential for developmental processes, neurotransmitter release, apoptosis, and signal transduction. Incorrectly processed Ca2+ signal is well-known to trigger a cascade of events leading to altered response to variety of stimuli and persistent accumulation of pathological changes at the molecular level. To counterbalance potentially detrimental consequences of Ca2+, neurons are equipped with sophisticated mechanisms that function to keep its concentration in a tightly regulated range. Calcium pumps belonging to the P-type family of ATPases: plasma membrane Ca2+-ATPase (PMCA), sarco/endoplasmic Ca2+-ATPase (SERCA) and secretory pathway Ca2+-ATPase (SPCA) are considered efficient line of defense against abnormal Ca2+ rises. However, their role is not limited only to Ca2+ transport, as they present tissue-specific functionality and unique sensitive to the regulation by the main calcium signal decoding protein—calmodulin (CaM). Based on the available literature, in this review we analyze the contribution of these three types of Ca2+-ATPases to neuropathology, with a special emphasis on mental diseases.

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.

Secretory Pathway Calcium ATPases in Breast Cancer

Secretory Pathway Ca2+-ATPases (SPCA) are P-type pumps that localize to the Golgi and post-Golgi vesicular compartments into which they sequester Ca2+ and Mn2+ ions required for protein processing, glycosylation and sorting. There are two isoforms in human that play distinct roles in breast cancer, a leading cause of death in women worldwide. Previously, we showed that SPCA2, but not SPCA1, interacts with the Ca2+ channel Orai1 to initiate store-independent Ca2+ entry, which promotes tumorigenesis. We sought to understand the roles of SPCA1 and SPCA2 in breast cancer and how they correlate with the different subtypes and with epithelial-mesenchymal transition (EMT). SPCA2 expression is greatly amplified in luminal subtypes but has a lower expression in basal subtypes, with opposite expression patterns for SPCA1. We present analysis of patient survival and microarray data for both SPCA1 and SPCA2 expression, and evaluate isoform-specific roles in two different breast cancer cell lines, which represent epithelial (MCF7) and post-EMT (MDA-MB-231) subtypes. Using knockdown and overexpression approaches, we correlate isoform expression with cell proliferation, tumorsphere formation, and plasma membrane trafficking of a cell adhesion protein critical for tumorigenesis, E-cadherin. From these studies, we conclude that SPCA2 promotes oncogenesis by regulating E-cadherin and that SPCA2 down regulation may be critical for metastasis.

Bis(2-hydroxy-3-tert-butyl-5-methyl-phenyl)-methane (bis-phenol) is a potent and selective inhibitor of the secretory pathway Ca2+ ATPase (SPCA1)

The secretory pathway Ca2+ ATPase (SPCA) provides the Golgi apparatus with a Ca2+ supply essential for Ca2+-dependent enzymes involved in the post-translational modification of proteins in transit through the secretory pathway. Ca2+ in the Golgi apparatus is also agonist-releasable and plays a role in hormone-induced Ca2+ transients. Although the Ca2+ ATPase inhibitors thapsigargin is more selective for the sarcoplasmic–endoplasmic reticulum Ca2+ ATPase (SERCA) than for SPCA, no inhibitor has been characterised that selectively inhibits SPCA.A number of inhibitors were assessed for their selectivity to the human SPCA1d compared to the more ubiquitous human SERCA2b. Each isoform was over-expressed in COS-7 cells and the Ca2+-dependent ATPase activity measured in their microsomal membranes. Both bis(2-hydroxy-3-tert-butyl-5-methyl-phenyl)methane(bis-phenol) and 2-aminoethoxydiphenylborate (2-APB) selectively inhibited SPCA1d (with IC50 values of 0.13μM and 0.18mM, respectively), which were of 62- and 8.3-fold greater potency than the values for hSERCA2b (IC50 values; 8.1μM and 1.5mM, respectively). Other inhibitors tested such as bis-phenol-A, tetrabromobisphenol-A and trifluoperazine inhibited both Ca2+ ATPases similarly. Furthermore, bis-phenol was able to mobilize Ca2+ in cells that had been pre-treated with thapsigargin. Therefore we conclude that given the potency and selectivity of bis-phenol it may prove a valuable tool in further understanding the role of SPCA in cellular processes.

Interactions Between Secretory Pathway Ca2+-ATPase 2 with TRP and Orai Channels Mediate Store Independent Ca2+ Entry in Lactation

Secretory pathway Ca2+ ATPases (SPCAs) are important in sequestering Ca2+ and Mn2+ from the cytoplasm into the Golgi and post-Golgi vesicles for important post-translational modifications to a multitude of enzymes. The two isoforms, SPCA1 and SPCA2 share high homology but have distinct localization and distribution. Whereas SPCA1 is a ubiquitous and essential protein with conventional Golgi localization, SPCA2 is expressed in highly secretory or absorptive epithelia where it can traffic to the plasma membrane. We showed that SPCA2 interacts with Orai1 to activate store-independent Ca2+ entry in breast cancer (Feng, et al, 2010). However, the normal physiological explanation for these interactions remains to be elucidated. Clinical studies have suggested a possible preventive role for lactation with respect to breast cancer (Neubauer, et al, 1994). In this study, we examine the interaction of SPCA2 with Orai1 and TRP channels throughout lactation. We show that: i) SPCA2 interacts with Orai1 and TRP channels, ii) these interactions are involved in the early stages of lactation and iii) these interactions may play a role in the sequestration of Ca2+ from the blood (2mM Ca2+ ) to the milk (40-80mM Ca2+).

An Unconventional Role in Store-Independent Constitutive Calcium Signaling by the Secretory Pathway Calcium - Atpases in Mammary Tumors

Constitutive calcium signaling in cancer cells drives tumor proliferation and metastasis. Secretory Pathway Ca2+-ATPases (SPCA) were highly upregulated in breast cancer derived cell lines and human breast tumors. Depletion of SPCA in human breast adenocarcinoma cells attenuated basal Ca2+ levels and downstream cell proliferation, anchorage-independent growth and tumor formation in mice. Contrary to its known role in Golgi Ca2+ sequestration, SPCA overexpression increased cytosolic Ca2+ by activation of the store-operated Ca2+ channels. However, SPCA mediated Ca2+ influx was independent of Ca2+ stores or sensors and not dependent on its transport ATPase activity, revealing a new signaling paradigm.

CMIP and ATP2C2 Modulate Phonological Short-Term Memory in Language Impairment

Specific language impairment (SLI) is a common developmental disorder characterized by difficulties in language acquisition despite otherwise normal development and in the absence of any obvious explanatory factors. We performed a high-density screen of SLI1, a region of chromosome 16q that shows highly significant and consistent linkage to nonword repetition, a measure of phonological short-term memory that is commonly impaired in SLI. Using two independent language-impaired samples, one family-based (211 families) and another selected from a population cohort on the basis of extreme language measures (490 cases), we detected association to two genes in the SLI1 region: that encoding c-maf-inducing protein (CMIP, minP = 5.5 × 10−7 at rs6564903) and that encoding calcium-transporting ATPase, type2C, member2 (ATP2C2, minP = 2.0 × 10−5 at rs11860694). Regression modeling indicated that each of these loci exerts an independent effect upon nonword repetition ability. Despite the consistent findings in language-impaired samples, investigation in a large unselected cohort (n = 3612) did not detect association. We therefore propose that variants in CMIP and ATP2C2 act to modulate phonological short-term memory primarily in the context of language impairment. As such, this investigation supports the hypothesis that some causes of language impairment are distinct from factors that influence normal language variation. This work therefore implicates CMIP and ATP2C2 in the etiology of SLI and provides molecular evidence for the importance of phonological short-term memory in language acquisition.

Contribution of intracellular Ca 2+ stores to Ca 2+ signaling during chemokinesis of human neutrophil granulocytes

Extracellular agonists increase the cytosolic free Ca 2+ concentration ([Ca 2+] c) by Ca 2+ influx and by stimulating Ca 2+ release from intracellular stores, mainly the endoplasmic reticulum and to a lesser extent also later compartments of the secretory pathway, particularly the Golgi. The Golgi takes up Ca 2+ via Sarco/ Endoplasmic Reticulum Ca 2+ ATPases (SERCAs) and the Secretory- Pathway Ca 2+ ATPases (SPCAs). The endogenous expression of SERCAs and SPCAs neutrophils was demonstrated by Western blotting and immunocytochemistry. Up till now, all cytosolic Ca 2+ transients due to intracellular Ca 2+ release have been found to originate from SERCA-dependent stores. We found that human neutrophils also present Ca 2+ release from a SERCA-independent store. Changes in [Ca 2+] c of neutrophils were investigated during chemokinesis induced by chemotactic factors in Ca 2+-free solution with and without the SERCA-specific inhibitor thapsigargin. Using N-formyl-methionyl-leucyl-phenylalanine or interleukin-8 as agonists, Ca 2+ release from intracellular stores was observed in respectively about 40% and 20% of the neutrophils pre-treated with Ca 2+-free solution and thapsigargin. In the latter condition, 20–30% of the cells preserved migratory behaviour. These results indicate that both SERCA-dependent and SERCA-independent (presumably SPCA-dependent) intracellular Ca 2+ stores contribute to Ca 2+ signaling during chemokinesis of human neutrophil granulocytes.

Calcium Transport Mechanisms of PC12 Cells

Many studies of Ca2+ signaling use PC12 cells, yet the balance of Ca2+ clearance mechanisms in these cells is unknown. We used pharmacological inhibition of Ca2+ transporters to characterize Ca2+ clearance after depolarizations in both undifferentiated and nerve growth factor-differentiated PC12 cells. Sarco-endoplasmic reticulum Ca2+ ATPase (SERCA), plasma membrane Ca2+ ATPase (PMCA), and Na+/Ca2+ exchanger (NCX) account for almost all Ca2+ clearance in both cell states, with NCX and PMCA making the greatest contributions. Any contribution of mitochondrial uniporters is small. The ATP pool in differentiated cells was much more labile than that of undifferentiated cells in the presence of agents that dissipated mitochondrial proton gradients. Differentiated PC12 cells have a small component of Ca2+ clearance possessing pharmacological characteristics consistent with secretory pathway Ca2+ ATPase (SPCA), potentially residing on Golgi and/or secretory granules. Undifferentiated and differentiated cells are similar in overall Ca2+ transport and in the small transport due to SERCA, but they differ in the fraction of transport by PMCA and NCX. Transport in neurites of differentiated PC12 cells was qualitatively similar to that in the somata, except that the ER stores in neurites sometimes released Ca2+ instead of clearing it after depolarization. We formulated a mathematical model to simulate the observed Ca2+ clearance and to describe the differences between these undifferentiated and NGF-differentiated states quantitatively. The model required a value for the endogenous Ca2+ binding ratio of PC12 cell cytoplasm, which we measured to be 268 ± 85. Our results indicate that Ca2+ transport in undifferentiated PC12 cells is quite unlike transport in adrenal chromaffin cells, for which they often are considered models. Transport in both cell states more closely resembles that of sympathetic neurons, for which differentiated PC12 cells often are considered models. Comparison with other cell types shows that different cells emphasize different Ca2+ transport mechanisms.

Loss of the Atp2c1 Secretory Pathway Ca2+-ATPase (SPCA1) in Mice Causes Golgi Stress, Apoptosis, and Midgestational Death in Homozygous Embryos and Squamous Cell Tumors in Adult Heterozygotes

Loss of one copy of the human ATP2C1 gene, encoding SPCA1 (secretory pathway Ca(2+)-ATPase isoform 1), causes Hailey-Hailey disease, a skin disorder. We performed targeted mutagenesis of the Atp2c1 gene in mice to analyze the functions of this Golgi membrane Ca(2+) pump. Breeding of heterozygous mutants yielded a normal Mendelian ratio among embryos on gestation day 9.5; however, null mutant (Spca1(-/-)) embryos exhibited growth retardation and did not survive beyond gestation day 10.5. Spca1(-/-) embryos had an open rostral neural tube, but hematopoiesis and cardiovascular development were ostensibly normal. Golgi membranes of Spca1(-/-) embryos were dilated, had fewer stacked leaflets, and were expanded in amount, consistent with increased Golgi biogenesis. The number of Golgi-associated vesicles was also increased, and rough endoplasmic reticulum had fewer ribosomes. Coated pits, junctional complexes, desmosomes, and basement membranes appeared normal in mutant embryos, indicating that processing and trafficking of proteins in the secretory pathway was not massively impaired. However, apoptosis was increased, possibly the result of secretory pathway stress, and a large increase in cytoplasmic lipid was observed in mutant embryos, consistent with impaired handling of lipid by the Golgi. Adult heterozygous mice appeared normal and exhibited no evidence of Hailey-Hailey disease; however, aged heterozygotes had an increased incidence of squamous cell tumors of keratinized epithelial cells of the skin and esophagus. These data show that loss of the Golgi Ca(2+) pump causes Golgi stress, expansion of the Golgi, increased apoptosis, and embryonic lethality and demonstrates that SPCA1 haploinsufficiency causes a genetic predisposition to cancer.

Functional and immunocytochemical evidence for the expression and localization of the secretory pathway Ca2+‐ATPase isoform 1 (SPCA1) in cerebellum relative to other Ca2+ pumps

Membrane fractions of pig cerebellum show Ca2+-ATPase activity and Ca2+ transport due to the presence of the secretory pathway Ca2+-ATPase (SPCA). The SPCA1 isoform shows a wide distribution in the neurons of pig cerebellum, where it is found in the Golgi complex of the soma of Purkinje, stellate, basket and granule cells, and also in more distal components of the secretory pathway associated with a synaptic localization such as in cerebellar glomeruli. The SPCA1 may be involved in loading the Golgi complex and the secretory vesicles of these specific neuronal cell types with Ca2+ and also Mn2+. This study of the cellular and subcellular localization of SPCA1 pumps relative to the sarco(endo) plasmic reticulum Ca2+-ATPase and plasma membrane Ca2+-ATPase pumps hints to a possible specific role of SPCA1 in controlling the luminal secretory pathway Ca2+ (or Mn2+) levels as well as the local cytosolic Ca2+ levels. In addition, it helps to specify the zones that are most vulnerable to Ca2+ and/or Mn2+ dyshomeostasis, a condition that is held responsible of an increasing number of neurological disorders.

The Role of the Calcium Sensing Receptor in Regulating Intracellular Calcium Handling in Human Epidermal Keratinocytes

Calcium is critical for controlling the balance of proliferation and differentiation in epidermal keratinocytes. We previously reported that the calcium sensing receptor (CaR) is required for mediating Ca2+ signaling and extracellular Ca2+ (Ca2+(o))-induced differentiation. In this study, we investigated the mechanism by which CaR regulates intracellular Ca2+ (Ca2+(i)) and its role in differentiation. Membrane fractionation, fluorescence immunolocalization, and co-immunoprecipitation studies were performed to assess potential interactions between CaR and other regulators of Ca2+ stores and channels. We found that the glycosylated form of CaR forms a complex with phospholipase C gamma1, IP3 receptor (IP3R), and the Golgi Ca2+-ATPase, secretory pathway Ca2+-ATPase 1, in the trans-Golgi. Inactivation of the endogenous CaR gene by adenoviral expression of a CaR antisense cDNA inhibited Ca2+(i) response to Ca2+(o), decreased Ca2+(i) stores, decreased Ca2+(o)-induced differentiation, but augmented store-operated channel activity and Ca2+ uptake by intracellular organelles. Our results indicate that CaR regulates keratinocyte differentiation in part by modulating Ca2+(i) stores via interactions with Ca2+ pumps and channels that regulate those stores.

Sequence, annotation and developmental expression of the sea urchin Ca 2+-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.

Calcium in the Golgi apparatus

The secretory-pathway Ca2+-ATPases (SPCAs) represent a recently recognized family of phosphorylation-type ATPases that supply the lumen of the Golgi apparatus with Ca2+ and Mn2+ needed for the normal functioning of this structure. Mutations of the human SPCA1 gene (ATP2C1) cause Hailey–Hailey disease, an autosomal dominant skin disorder in which keratinocytes in the suprabasal layer of the epidermis detach. We will first review the physiology of the SPCAs and then discuss how mutated SPCA1 proteins can lead to an epidermal disorder.

Calcium and magnesium competitively influence the growth of a PMR1 deficient Saccharomyces cerevisiae strain

PMR1, the Ca 2+/Mn 2+ ATPase of the secretory pathway in Saccharomyces cerevisiae was the first member of the secretory pathway Ca 2+ ATPases (SPCA) to be characterized. In the past few years, pmr1Δ yeast have received more attention due to the recognition that the human homologue of this protein, hSPCA1 is defective in chronic benign pemphigus or Hailey–Hailey disease (HHD). Recent publications have described pmr1Δ S. cerevisiae as a useful model organism for studying the molecular pathology of HHD. Some observations indicated that the high Ca 2+ sensitive phenotype of PMR1 defective yeast strains may be the most relevant in this respect. Here we show that the total cellular calcium response of a pmr1Δ S. cerevisiae upon extracellular Ca 2+ challenge is decreased compared to the wild type strain similarly as observed in keratinocytes. Additionally, the novel magnesium sensitivity of PMR1 defective yeast is revealed, which appears to be a result of competition for uptake between Ca 2+ and Mg 2+ at the plasma membrane level. Our findings indicate that extracellular Ca 2+ and Mg 2+ competitively influence the intracellular Ca 2+ homeostasis of S. cerevisiae. These observations may further our understanding of HHD.

The Secretory Pathway Ca2+/Mn2+-ATPase 2 Is a Golgi-localized Pump with High Affinity for Ca2+ Ions

Accumulation of Ca(2+) into the Golgi apparatus is mediated by sarco(endo)plasmic reticulum Ca(2+)-ATPases (SERCAs) and by secretory pathway Ca(2+)-ATPases (SPCAs). Mammals and birds express in addition to the housekeeping SPCA1 (human gene name ATP2C1, cytogenetic position 3q22.1) a homologous SPCA2 isoform (human gene name ATP2C2, cytogenetic position 16q24.1). We show here that both genes present an identical exon/intron layout. We confirmed that hSPCA2 has the ability to transport Ca(2+), demonstrated its Mn(2+)-transporting activity, showed its Ca(2+)- and Mn(2+)-dependent phosphoprotein intermediate formation, and documented the insensitivity of these functional activities to thapsigargin inhibition. The mRNA encoding hSPCA2 showed a limited tissue expression pattern mainly confined to the gastrointestinal and respiratory tract, prostate, thyroid, salivary, and mammary glands. Immunocytochemical localization in human colon sections presented a typical apical juxtanuclear Golgi-like staining. The expression in COS-1 cells allowed the direct demonstration of (45)Ca(2+) (K(0.5) = 0.27 microm) or (54)Mn(2+) transport into an A23187-releasable compartment.

SPCA1 pumps and Hailey–Hailey disease

Both the endoplasmic reticulum and the Golgi apparatus are agonist-sensitive intracellular Ca 2+ stores. The Golgi apparatus has Ca 2+-release channels and a Ca 2+-uptake mechanism consisting of sarco(endo)plasmic-reticulum Ca 2+-ATPases (SERCA) and secretory-pathway Ca 2+-ATPases (SPCA). SPCA1 has been shown to transport both Ca 2+ and Mn 2+ in the Golgi lumen and therefore plays an important role in the cytosolic and intra-Golgi Ca 2+ and Mn 2+ homeostasis. Human genetic studies have provided new information on the physiological role of SPCA1. Loss of one functional copy of the SPCA1 ( ATP2C1) gene causes Hailey–Hailey disease, a skin disorder arising in the adult age with recurrent vesicles and erosions in the flexural areas. Here, we review recent experimental evidence showing that the Golgi apparatus plays a much more important role in intracellular ion homeostasis than previously anticipated.

Characterization of Cos-7 cells overexpressing the rat secretory pathway Ca2+-ATPase.

On the basis of sequence similarities to the yeast PMR1 and hSPCA gene, the rat alternatively spliced mRNA has been suggested to be a Golgi secretory pathway Ca2+-ATPase (SPCA). Data in this report lend further support for this hypothesis in that sucrose gradient fractionation of rat liver microsomes resulted in SPCA comigrating with the Golgi calcium binding protein CALNUC, which was well resolved from the endoplasmic reticulum marker calreticulin. Also, in PC-12 cells, antibody to SPCA colocalized with an antibody to the Golgi marker alpha-mannosidase II. To study the biological effects of SPCA expression, we performed stable overexpression of SPCA in COS-7 cells. Seven clones were selected for further comparison with COS-7 cells containing an empty expression vector. Overexpression of SPCA resulted in a significant reduction of plasma membrane Ca2+-ATPase, sarco(endo)plasmic reticulum Ca2+-ATPase, and calreticulin expression in these clones. In contrast, the expression of the Golgi calcium-binding protein CALNUC increased significantly. The phosphoenzyme intermediate formed using membranes from clone G11/5 was calcium dependent, significantly more intense than in COS-7 cells, and not affected by La3+ treatment. Calcium uptake by G11/5 microsomes was ATP dependent and significantly greater than in microsomes from parent COS-7 cells. The overexpression of SPCA significantly increased the growth rate of these cells compared with COS-7 cells containing only the empty vector. These data demonstrate that overexpression of the rat SPCA results in significant changes in the expression of calcium transport and storage proteins in COS-7 cells.

Gene knockout studies of Ca2+-transporting ATPases.

The biochemical functions of intracellular and plasma membrane Ca2+-transporting ATPases in the control of cytosolic and organellar Ca2+ levels are well established, but the physiological roles of specific isoforms are less well understood. There appear to be three different types of Ca2+ pumps in mammalian tissues: the sarco(endo)plasmic reticulum Ca2+-ATPases (SERCAs), which sequester Ca2+ within the endoplasmic or sarcoplasmic reticulum, the plasma membrane Ca2+-ATPases (PMCAs), which extrude Ca2+ from the cell, and the putative secretory pathway Ca2+-ATPase (SPCA), the function of which is poorly understood. This review describes the results of recent analyses of mouse models with null mutations in the genes encoding SERCA and PMCA isoforms and genetic studies of SERCA and SPCA dysfunction in both humans and model organisms. These studies are yielding important insights regarding the physiological functions of individual Ca2+-transporting ATPases in vivo.