Purkinje neurons express the SERCA3 isoform of the organellar type Ca(2+)-transport ATPase

PDX1, a transcription factor essential for organ differentiation, regulates SERCA-dependent Ca2+ homeostasis in sensory neurons

Pancreatic and duodenal homeobox 1 (PDX1) is a transcription factor required for the development and differentiation of the pancreas. Previous studies indicated that PDX1 expression was restricted to the gastrointestinal tract. Using a cre-dependent reporter, we observed PDX1-dependent expression of tdtomato (PDX1-tom) in a subpopulation of sensory nerves. Many of these PDX1-tom afferents expressed the neurofilament 200 protein and projected to the skin. Tdtomato-labeled terminals were associated with hair follicles in the form of longitudinal and circumferential lanceolate endings suggesting a role in tactile and proprioceptive perception. To begin to examine the functional significance of PDX1 in afferents, we used Fura-2 imaging to examine calcium (Ca2+) handling under naïve and nerve injury conditions. Neuropathic injury is associated with increased intracellular Ca2+ signaling that in part results from dysregulation of the sarco/endoplasmic reticulum calcium transport ATPase (SERCA). Here we demonstrate that under naïve conditions, PDX1 regulates expression of the SERCA2B isoform in sensory neurons. In response to infraorbital nerve injury, a significant reduction of PDX1 and SERCA2B expression and dysregulation of Ca2+ handling occurs in PDX1-tom trigeminal ganglia neurons. The identification of PDX1 expression in the somatosensory system and its regulation of SERCA2B and Ca2+ handling provide a new mechanism to explain pathological changes in primary afferents that may contribute to pain associated with nerve injury.

Subcellular and regional localization of mRNA translation in midbrain dopamine neurons

Midbrain dopaminergic (mDA) neurons exhibit extensive dendritic and axonal arborizations, but local protein synthesis is not characterized in these neurons. Here, we investigate messenger RNA (mRNA) localization and translation in mDA neuronal axons and dendrites, both of which release dopamine (DA). Using highly sensitive ribosome-bound RNA sequencing and imaging approaches, we find no evidence for mRNA translation in mDA axons. In contrast, mDA neuronal dendrites in the substantia nigra pars reticulata (SNr) contain ribosomes and mRNAs encoding the major components of DA synthesis, release, and reuptake machinery. Surprisingly, we also observe dendritic localization of mRNAs encoding synaptic vesicle-related proteins, including those involved in exocytic fusion. Our results are consistent with a role for local translation in the regulation of DA release from dendrites, but not from axons. Our translatome data define a molecular signature of sparse mDA neurons in the SNr, including the enrichment of Atp2a3/SERCA3, an atypical ER calcium pump.

Presynaptic endoplasmic reticulum regulates short-term plasticity in hippocampal synapses

Short-term plasticity preserves a brief history of synaptic activity that is communicated to the postsynaptic neuron. This is primarily regulated by a calcium signal initiated by voltage dependent calcium channels in the presynaptic terminal. Imaging studies of CA3-CA1 synapses reveal the presence of another source of calcium, the endoplasmic reticulum (ER) in all presynaptic terminals. However, the precise role of the ER in modifying STP remains unexplored. We performed in-silico experiments in synaptic geometries based on reconstructions of the rat CA3-CA1 synapses to investigate the contribution of ER. Our model predicts that presynaptic ER is critical in generating the observed short-term plasticity profile of CA3-CA1 synapses and allows synapses with low release probability to operate more reliably. Blocking the ER lowers facilitation in a manner similar to what has been previously characterized in animal models of Alzheimer's disease and underscores the important role played by presynaptic stores in normal function.

Endoplasmic Reticulum Calcium Pumps and Tumor Cell Differentiation

Endoplasmic reticulum (ER) calcium homeostasis plays an essential role in cellular calcium signaling, intra-ER protein chaperoning and maturation, as well as in the interaction of the ER with other organelles. Calcium is accumulated in the ER by sarco/endoplasmic reticulum calcium ATPases (SERCA enzymes) that generate by active, ATP-dependent transport, a several thousand-fold calcium ion concentration gradient between the cytosol (low nanomolar) and the ER lumen (high micromolar). SERCA enzymes are coded by three genes that by alternative splicing give rise to several isoforms, which can display isoform-specific calcium transport characteristics. SERCA expression levels and isoenzyme composition vary according to cell type, and this constitutes a mechanism whereby ER calcium homeostasis is adapted to the signaling and metabolic needs of the cell, depending on its phenotype, its state of activation and differentiation. As reviewed here, in several normal epithelial cell types including bronchial, mammary, gastric, colonic and choroid plexus epithelium, as well as in mature cells of hematopoietic origin such as pumps are simultaneously expressed, whereas in corresponding tumors and leukemias SERCA3 expression is selectively down-regulated. SERCA3 expression is restored during the pharmacologically induced differentiation of various cancer and leukemia cell types. SERCA3 is a useful marker for the study of cell differentiation, and the loss of SERCA3 expression constitutes a previously unrecognized example of the remodeling of calcium homeostasis in tumors.

A Role for SERCA Pumps in the Neurobiology of Neuropsychiatric and Neurodegenerative Disorders

Calcium (Ca²⁺) is a fundamental regulator of cell fate and intracellular Ca²⁺ homeostasis is crucial for proper function of the nerve cells. Given the complexity of neurons, a constellation of mechanisms finely tunes the intracellular Ca²⁺ signaling. We are focusing on the sarco/endoplasmic reticulum (SR/ER) calcium (Ca²⁺)-ATPase (SERCA) pump, an integral ER protein. SERCA's well established role is to preserve low cytosolic Ca²⁺ levels ([Ca²⁺]_cyt), by pumping free Ca²⁺ ions into the ER lumen, utilizing ATP hydrolysis. The SERCA pumps are encoded by three distinct genes, SERCA1-3, resulting in 12 known protein isoforms, with tissue-dependent expression patterns. Despite the well-established structure and function of the SERCA pumps, their role in the central nervous system is not clear yet. Interestingly, SERCA-mediated Ca²⁺ dyshomeostasis has been associated with neuropathological conditions, such as bipolar disorder, schizophrenia, Parkinson's disease and Alzheimer's disease. We summarize here current evidence suggesting a role for SERCA in the neurobiology of neuropsychiatric and neurodegenerative disorders, thus highlighting the importance of this pump in brain physiology and pathophysiology.

Aberrant SERCA3 expression during the colorectal adenoma-adenocarcinoma sequence

Sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) 3 is involved in calcium mobilization from the endoplasmic reticulum into the cytosol and is closely linked to metabolism, neuronal plasticity, gene transcription, cell growth, differentiation, apoptosis, protein folding and carcinogenesis. In order to elucidate the role of SERCA3 in colorectal carcinogenesis and subsequent progression, its expression was examined using immunohistochemistry and in situ hybridization (ISH) on tissue microarrays containing colorectal carcinomas, adjacent non-neoplastic mucosa (NNM) and adenoma, and metastatic carcinoma in lymph node and liver. Colorectal carcinoma tissue and cell lines were assessed for SERCA3 expression by western blotting or RT-PCR, respectively. SERCA3 was distinctively expressed in Colo201, Colo205, DLD-1, HCT-15, HCT-116, HT-29, KM-12, SW480, SW620 and WiDr cells at both the mRNA and protein levels. SERCA3 mRNA expression was low in carcinoma when compared to that in matched NNM by quantitative PCR (P<0.05), while the converse was true by ISH. Lower expression of SERCA3 was immunohistochemically observed in colorectal carcinoma when compared to that in NNM and adenoma (P<0.05). In contrast, primary carcinoma showed high SERCA3 expression when compared to that in metastatic carcinoma in lymph node or liver by IHC (P<0.05). Immunohistochemically, SERCA3 expression was negatively related to lymphatic invasion, but not with age, gender, depth of invasion, venous invasion, lymph node metastasis, distant metastasis, TNM stage, degree of differentiation or survival rate (P>0.05). There was a positive relationship between SERCA3 expression and serum CEA levels in the carcinoma patients (P<0.05). Cox's proportional hazards model indicated that depth of invasion and distant metastasis are independent prognostic factors for overall colorectal carcinomas (P<0.05). These findings suggest that aberrant SERCA3 expression is closely linked to the adenoma-adenocarcinoma sequence and progression of colorectal carcinomas.

Functional integration of calcium regulatory mechanisms at Purkinje neuron synapses

Cerebellar Purkinje neurons receive synaptic inputs from three different sources: the excitatory parallel fibre and climbing fibre synapses as well as the inhibitory synapses from molecular layer stellate and basket cells. These three synaptic systems use distinct mechanisms in order to generate Ca(2+) signals that are specialized for specific modes of neurotransmitter release and post-synaptic signal integration. In this review, we first describe the repertoire of Ca(2+) regulatory mechanisms that generate and regulate the amplitude and timing of Ca(2+) fluxes during synaptic transmission and then explore how these mechanisms interact to generate the unique functional properties of each of the Purkinje neuron synapses.

Aberrant SERCA3 expression is closely linked to pathogenesis, invasion, metastasis, and prognosis of gastric carcinomas

Sarco (endo)plasmic reticulum Ca(2+)-ATPase (SERCAs) 3 is involved in calcium mobilization from endoplasmic reticulum into cytosol and closely links to metabolism, neuronal plasticity, gene transcription, cell growth, differentiation, apoptosis, protein folding, and carcinogenesis. To clarify the role of SERCA3 in gastric carcinogenesis and subsequent progression, its expression was examined by immunohistochemistry and in situ hybridization (ISH) on tissue microarrays containing gastric carcinomas, adjacent non-neoplastic mucosa (NNM), and metastatic lymph node. SERCA3 expression was studied in gastric carcinoma tissue and cell lines by Western blot, reverse transcriptase-polymerase chain reaction, or immunofluorescence. The results demonstrated that SERCA3 was distinctively expressed in GES-1, AGS, BGC-823, GT-3TKB, HGC-27, KATO-III, MGC-803, MKN28, MKN45, SCH, SGC-7901, and STKM-2 at both mRNA and protein levels. The carcinomas showed higher SERCA3 mRNA expression than the matched NNM by real-time PCR and ISH (P > 0.05). Immunohistochemically, SERCA3 expression was decreased from gastric NNM, primary to metastatic carcinoma (P > 0.05). SERCA3 expression was negatively related to depth of invasion, distant metastasis, and tumor node metastasis (TNM) staging (P > 0.05), but not to age, sex, lymphatic or venous invasion, or lymph node metastasis (P > 0.05). Kaplan-Meier analysis indicated that SERCA3 expression was positively associated with favorable prognosis of the patients with gastric carcinoma (P > 0.05). Cox's proportional hazard model indicated that venous invasion, distant metastasis and TNM staging (P > 0.05) were independent prognostic factors for gastric carcinomas. It was suggested that downregulated SERCA3 expression is closely linked to pathogenesis, invasion, metastasis, and prognosis of gastric carcinomas. It might be employed to indicate the pathobiological behaviors and prognosis of gastric carcinomas.

The Ca2+ pumps of the endoplasmic reticulum and Golgi apparatus

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

How many Ca2+ATPase isoforms are expressed in a cell type? A growing family of membrane proteins illustrated by studies in platelets

Ca(2+) signaling plays a key role in normal and abnormal platelet functions. Understanding platelet Ca(2+) signaling requires the knowledge of proteins involved in this process. Among these proteins are Ca(2+)ATPases or Ca(2+) pumps that deplete the cytosol of Ca(2+) ions. Here, we will particularly focus on two Ca(2+) pump families: the plasma membrane Ca(2+)ATPases (PMCAs) that extrude cytosolic Ca(2+) towards the extracellular medium and the sarco/endoplasmic reticulum Ca(2+)ATPases (SERCAs) that pump Ca(2+) into the endoplasmic reticulum (ER). In the present review, we will summarize data on platelet Ca(2+)ATPases including their identification and biogenesis. First of all, we will present the Ca(2+)ATPase genes and their isoforms expressed in platelets. We will especially focus on a member of the SERCA family, SERCA3, recently found to give rise to a number of species-specific isoforms. Next, we will describe the differences in Ca(2+)ATPase patterns observed in human and rat platelets. Last, we will analyze how the expression of Ca(2+)ATPase isoforms changes during megakaryocytic maturation and show that megakaryocytopoiesis is associated with a profound reorganization of the expression and/or activity of Ca(2+)ATPases. Taken together, these data provide new aspects of investigations to better understand normal and abnormal platelet Ca(2+) signaling.

Determination of apparent calcium affinity for endogenously expressed human sarco(endo)plasmic reticulum calcium-ATPase isoform SERCA3

The sarco(endo)plasmic reticulum Ca2+-ATPases (SERCAs) play a crucial role in regulating free cytosolic Ca2+ concentration in diverse cell types. It has been shown that recombinant SERCA3, when measured in heterologous systems, exhibits low apparent affinity for Ca2+; however, Ca2+ affinity of native SERCA3 in an endogenous setting has not been examined. Such a measurement is complicated, because SERCA3 is always coexpressed with the housekeeping isoform SERCA2b. We used a fluorescence-based assay for monitoring continuous Ca2+ uptake into microsomes to examine the properties of endogenous human SERCA3 and SERCA2b. The kinetic parameters were derived using a cooperative two-component uptake model for Ca2+ activation, and the values assigned to SERCA3 were confirmed using the highly specific human SERCA3 inhibitory antibody PL/IM430. First, using recombinant human SERCA3 and SERCA2b proteins transiently expressed in HEK-293 cells, we confirmed the previously observed low apparent Ca2+ affinity for SERCA3 compared with SERCA2b (1.10 ± 0.04 vs. 0.26 ± 0.01 μM), and using mixtures of recombinant protein isoforms, we validated the two-component uptake model. Then we determined apparent Ca2+ affinity for SERCA proteins present endogenously in cultured Jurkat T lymphocytes and freshly isolated human tonsil lymphocytes. The apparent Ca2+ affinity in these two preparations was 1.04 ± 0.07 and 1.1 ± 0.2 μM for SERCA3 and 0.27 ± 0.02 and 0.26 ± 0.01 μM for SERCA2b, respectively. Our data demonstrate, for the first time, that affinity for Ca2+ is inherently lower for SERCA3 expressed in situ than for other SERCA isoforms.

Activation of the androgen receptor alters the intracellular calcium response to glutamate in primary hippocampal neurons and modulates sarco/endoplasmic reticulum calcium ATPase 2 transcription

Androgens have been shown to have a number of effects on hippocampal function. Although androgen receptors (AR) are found at high levels in hippocampal neurons, the intracellular mechanisms responsible for androgen's actions are unknown. If androgens were capable of altering internal calcium concentration ([Ca(2+)](i)), they could influence a variety of intracellular signaling pathways, maintain neuronal homeostasis and Ca(2+) induced excitotoxicity. In the present study, calcium imaging was used to measure the [Ca(2+)](i) in rat primary hippocampal neurons treated with either the AR agonist dihydrotestosterone (DHT), DHT+flutamide (AR antagonist), flutamide alone, or vehicle for 24 h and subsequently presented with an excitatory glutamate stimulus. In the absence of glutamate stimulation, DHT treatment caused a significant upward shift in baseline [Ca(2+)](i) when compared with neurons from all other groups. Glutamate had a greater effect on [Ca(2+)](i) in DHT-treated neurons and DHT-treated neurons returned to baseline levels significantly faster than all other groups. Cyclopiazonic acid, an inhibitor of sarco/endoplasmic reticulum calcium ATPase (SERCA) had a larger response in DHT-treated neurons compared with controls, suggesting increased Ca(2+) stores in DHT-treated neurons. In all cases the effects of DHT were blocked by treatment with flutamide indicating an AR-mediated mechanism. To determine a possible mechanism by which AR activation could be influencing [Ca(2+)](i), SERCA2 mRNA levels were measured in primary hippocampal neurons. SERCA2 is inserted into the endoplasmic reticulum (ER) membrane and functions to rapidly pump [Ca(2+)](i) into the ER. Following treatment of primary hippocampal neurons with DHT, SERCA2 mRNA was increased, an effect that was blocked in the presence of flutamide. Taken together these results indicate that DHT, working through AR, causes an up-regulation of SERCA2, which increases the sequestering of [Ca(2+)](i) in the endoplasmic reticulum of hippocampal neurons. Such changes may allow the neurons to respond more robustly to a stimulus and recover more quickly following a highly stimulatory challenge.

A developmental profile of the levels of calcium pumps in chick cerebellum

The functional expression and distribution of intracellular ATPase (sarco(endo)plasmic reticulum Ca(2+)-ATPase: SERCA) and plasma membrane Ca(2+)-ATPase (PMCA) was analyzed in the developing chick cerebellum. The activity and Ca(2+) uptake increase with development for both ATPases. However, the protein content increases with the stage of development only for SERCA, remaining constant for PMCA. Immunohistochemical assays showed that the ontogenesis of these ATPases goes along with definite stages of cerebellum histogenesis, and is complete at hatching. The SERCA is mainly distributed in Purkinje neurons, whereas the PMCA seems to be expressed initially in climbing fibers, shifting to soma and spiny branchlets of Purkinje cells at late embryonic stages. Granule cells express both ATPases according to their degree of maturity, whereas only PMCA is present in cerebellar glomeruli. These pumps are present in deep nuclei and the choroid plexus, although in this latter tissue their expression declines with development. The spatio-temporal distribution of SERCA and PMCA must be closely related to their association with the development of specific cells and processes of the chick cerebellum.

Determinants of postsynaptic Ca2+ signaling in Purkinje neurons

Neuronal integration in Purkinje neurons involves many forms of Ca2+ signaling. Two afferent synaptic inputs, the parallel and the climbing fibers, provide a major drive for these signals. These two excitatory synaptic inputs are both glutamatergic. Postsynaptically they activate alpha-amino-3-hydroxy-5-methyl-4-propionic acid (AMPA) receptors (AMPARs) and metabotropic glutamate receptors (mGluRs). Unlike most other types of central neurons, Purkinje neurons do not express NMDA (N-methyl-D-aspartate) receptors (NMDARs). AMPARs in Purkinje neurons are characterized by a low permeability for Ca2+ ions. AMPAR-mediated synaptic depolarization may activate voltage-gated Ca2+ channels, mostly of the P/Q-type. The resulting intracellular Ca2+ signals are shaped by the Ca2+ buffers calbindin and parvalbumin. Ca2+ clearance from the cytosol is brought about by Ca2+-ATPases in the plasma membrane and the endoplasmic reticulum, as well as the Na+-Ca2+-exchanger. Binding of glutamate to mGluRs induces postsynaptic Ca2+-transients through two G protein-dependent pathways: involving (1) the release of Ca2+ ions from intracellular Ca2+ stores and (2) the opening of the cation channel TRPC1. Homer proteins appear to play an important role in postsynaptic Ca2+ signaling by providing a direct link between the plasma membrane-resident elements (mGluRs and TRPC1) and their intracellular partners, including the IP3Rs.

Identification of candidate Purkinje cell-specific markers by gene expression profiling in wild-type and pcd(3J) mice

The identification of mRNAs that have restricted expression patterns in the brain represents powerful tools with which to characterize and manipulate the nervous system. Here, we describe a strategy using microarray technology (Affymetrix Mouse Genome 430 2.0 Arrays) to identify mRNA transcripts that are candidate markers of cerebellar Purkinje neurons. Initially, gene expression profiles were compared between cerebella of 4-month-old Purkinje cell degeneration (pcd(3J)) mice, in which most Purkinje cells had already degenerated and wild-type littermates with a normal complement of Purkinje neurons. Of 14,563 probe sets expressed in wild-type cerebellum, 797 showed a significant (p<0.0001) reduction in pcd(3J) mice. These probes could represent transcripts with varying levels of specificity for Purkinje cells as well as transcripts in other cell types that decline as a secondary consequence of Purkinje cell loss. Ranking of the probe signals revealed that well-known Purkinje cell-specific transcripts such as calbindin and L7/pcp2 clustered in a group that was <33% of wild-type levels. Therefore, to identify potentially new Purkinje cell-specific transcripts that cluster with the known markers, more stringent selection criteria were applied (<33% of wild-type signal and p<0.0001). With these criteria, 55 independent transcripts were identified of which 33 were annotated genes and 22 were ESTs and RIKEN cDNAs. A literature search revealed that 25 of the 33 annotated genes were expressed in Purkinje cells, with no data being available on the other 8. Thus, the additional 8 annotated and 22 un-annotated genes are clustered with many genes expressed in Purkinje cells making them candidate markers. To confirm the microarray data, eight representative annotated genes were selected including five reported to be in Purkinje neurons and three for which no data was available. Semi-quantitative RT-PCR demonstrated reduced expression of all eight transcripts in cerebella from pcd(3J) mice. The promoters of genes expressed selectively in subsets of neurons can be used to direct heterologous gene expression in transgenic mice and the more restricted the expression pattern the greater their utility. Therefore, microarray analysis was used to assess expression levels of all 55 transcripts in cerebral cortex, striatum, substantia nigra and ventral tegmental area. This permitted the identification of a set of genes whose promoters might have utility for selectively targeting gene expression to cerebellar Purkinje cells.

Localization of intracellular and plasma membrane Ca2+-ATPases in the cerebellum

The sarco-endoplasmic reticulum Ca2+-ATPase and the plasma membrane Ca2+-ATPase contribute to the regulation of the intracellular Ca2+ concentration. These proteins transport Ca2+ ions into the endoplasmic reticulum and to the extracellular medium, respectively. A different localization of the two families of Ca2+-ATPases has been shown in concrete subcellular areas of Purkinje cells and in other neuronal elements from cerebellum. In the light of the actual knowledge of Ca2+-ATPases, this strict distribution suggests the existence of different demands on Ca2+ homeostasis in these cerebellar and cellular subregions.

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.

Localization of endoplasmic reticulum and plasma membrane Ca2+-ATPases in subcellular fractions and sections of pig cerebellum

Subcellular fractions and sections of the cerebellum were analysed to evaluate the relative activity and distribution of organellar and plasma membrane Ca2+-ATPases (SERCA and PMCA). Western blot analysis of the fractions with IID8 or Y/1F4 SERCA-specific antibodies or else with 5F10 or pbPMCA antibodies, specific to PMCA pump, revealed a major content of SERCA protein in microsomes and of PMCA protein in plasma membrane vesicles. The Ca2+-ATPase activity of microsomes was more sensitive to thapsigargin, a SERCA-specific inhibitor, whereas the activity of the plasma membrane vesicle fraction was inhibited more by vanadate, a blocker of PMCA activity. The SERCA and PMCA distribution analysed in cerebellar sections revealed IID8 antibody reactions in Purkinje cell cytoplasm, granule cells and cerebellar glomeruli. Y/1F4 gave immunostaining in Purkinje cells, molecular layer interneurons (basket and stellate cells) and glomeruli, but granule cells were not labelled. The 5F10 antibody reacted with Purkinje cells, including their dendritic spines, as well as cerebellar glomeruli, whereas the pbPMCA antibody labelled several processes in all three layers and some synaptic interaction sites. The differential content and localization of the two types of Ca2+ pumps in specific neuronal areas of pig cerebellum indicate precise Ca2+ requirements of specific cellular regions.

'New' functions for 'old' proteins: the role of the calcium-binding proteins calbindin D-28k, calretinin and parvalbumin, in cerebellar physiology. Studies with knockout mice

Calretinin (CR), calbindin D-28k (CB) and parvalbumin (PV) belong to the large family of EF-hand calcium-binding proteins, which comprises more than 200 members in man. Structurally these proteins are characterized by the presence of a variable number of evolutionary well-conserved helix-loop-helix motives, which bind Ca2+ ions with high affinity. Functionally, they fall into two groups: by interaction with target proteins, calcium sensors translate calcium concentrations into signaling cascades, whereas calcium buffers are thought to modify the spatiotemporal aspects of calcium transients. Although CR, CB and PV are currently being considered calcium buffers, this may change as we learn more about their biology. Remarkable differences in their biophysical properties have led to the distinction of fast and slow buffers and suggested functional specificity of individual calcium buffers. Evaluation of the physiological roles of CR, CB and PV has been facilitated by the recent generation of mouse strains deficient in these proteins. Here, we review the biology of these calcium-binding proteins with distinct reference to the cerebellum, since they are particularly enriched in specific cerebellar neurons. CR is principally expressed in granule cells and their parallel fibres, while PV and CB are present throughout the axon, soma, dendrites and spines of Purkinje cells. PV is additionally found in a subpopulation of inhibitory interneurons, the stellate and basket cells. Studies on deficient mice together with in vitro work and their unique cell type-specific distribution in the cerebellum suggest that these calcium-binding proteins have evolved as functionally distinct, physiologically relevant modulators of intracellular calcium transients. Analysis of different brain regions suggests that these proteins are involved in regulating calcium pools critical for synaptic plasticity. Surprisingly, a major role of any of these three calcium-binding proteins as an endogenous neuroprotectant is not generally supported.

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.

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.

Altered calcium homeostasis in cerebellar Purkinje cells of leaner mutant mice

The leaner (tg(la)) mouse mutation occurs in the gene encoding the voltage-activated Ca(2+) channel alpha(1A) subunit, the pore-forming subunit of P/Q-type Ca(2+) channels. This mutation results in dramatic reductions in P-type Ca(2+) channel function in cerebellar Purkinje neurons of tg(la)/tg(la) mice that could affect intracellular Ca(2+) signaling. We combined whole cell patch-clamp electrophysiology with fura-2 microfluorimetry to examine aspects of Ca(2+) homeostasis in acutely dissociated tg(la)/tg(la) Purkinje cells. There was no difference between resting somatic Ca(2+) concentrations in tg(la)/tg(la) cells and in wild-type (+/+) cells. However, by quantifying the relationship between intracellular Ca(2+) elevations and depolarization-induced Ca(2+) influx, we detected marked alterations in rapid calcium buffering between the two genotypes. Calcium buffering values (ratio of bound/free ions) were significantly reduced in tg(la)/tg(la) (584 +/- 52) Purkinje cells relative to +/+ (1,221 +/- 80) cells. By blocking the endoplasmic reticulum (ER) Ca(2+)-ATPases with thapsigargin, we observed that the ER had a profound impact on rapid Ca(2+) buffering that was also differential between tg(la)/tg(la) and +/+ Purkinje cells. Diminished Ca(2+) uptake by the ER apparently contributes to the reduced buffering ability of mutant cells. This report constitutes one of the few instances in which the ER has been implicated in rapid Ca(2+) buffering. Concomitant with this reduced buffering, in situ hybridization with calbindin D28k and parvalbumin antisense oligonucleotides revealed significant reductions in mRNA levels for these Ca(2+)-binding proteins (CaBPs) in tg(la)/tg(la) Purkinje cells. All of these results suggest that alterations of Ca(2+) homeostasis in tg(la)/tg(la) mouse Purkinje cells may serve as a mechanism whereby reduced P-type Ca(2+) channel function contributes to the mutant phenotype.

Mechanisms involved in the cellular calcium homeostasis in vascular smooth muscle: calcium pumps

The regulation of cytosolic Ca2+ homeostasis is essential for cells, and particularly for vascular smooth muscle cells. In this regulation, there is a participation of different factors and mechanisms situated at different levels in the cell, among them Ca2+ pumps play an important role. Thus, Ca2+ pump, to extrude Ca2+; Na+/Ca2+ exchanger; and different Ca2+ channels for Ca2+ entry are placed in the plasma membrane. In addition, the inner and outer surfaces of the plasmalemma possess the ability to bind Ca2+ that can be released by different agonists. The sarcoplasmic reticulum has an active role in this Ca2+ regulation; its membrane has a Ca2+ pump that facilitates luminal Ca2+ accumulation, thus reducing the cytosolic free Ca2+ concentration. This pump can be inhibited by different agents. Physiologically, its activity is regulated by the protein phospholamban; thus, when it is in its unphosphorylated state such a Ca2+ pump is inhibited. The sarcoplasmic reticulum membrane also possesses receptors for 1,4,5-inositol trisphosphate and ryanodine, which upon activation facilitates Ca2+ release from this store. The sarcoplasmic reticulum and the plasmalemma form the superficial buffer barrier that is considered as an effective barrier for Ca2+ influx. The cytosol possesses different proteins and several inorganic compounds with a Ca2+ buffering capacity. The hypothesis of capacitative Ca2+ entry into smooth muscle across the plasma membrane after intracellular store depletion and its mechanisms of inhibition and activation is also commented.

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

Human chromosome 17-specific genomic clones extending over 90 kilobases (kb) of DNA and coding for sarco/endoplasmic reticulum Ca2+-ATPase 3 (SERCA3) were isolated. The presence of the D17S1828 genetic marker in the cosmid contig enabled us to map the SERCA3 gene (ATP2A3) 11 centimorgans from the top of the short arm p of chromosome 17, in the vicinity of the cystinosis gene locus. The SERCA3 gene contains 22 exons spread over 50 kb of genomic DNA. The exon/intron boundaries are well conserved between human SERCA3 and SERCA1 genes, except for the junction between exons 8 and 9 which is found in the SERCA1 gene but not in SERCA3 and SERCA2 genes. The transcription start site (+1) is located 152 nucleotides (nt) upstream of the AUG codon. The 5'-flanking region, including exon 1, is embedded in a 1.5-kb CpG island and is characterized by the absence of a TATA box and by the presence of 14 putative Sp1 sites, 11 CACCC boxes, 5 AP-2-binding motifs, 3 GGCTGGGG motifs, 3 CANNTG boxes, a GATA motif, as well as single sites for Ets-1, c-Myc, and TFIIIc. Functional promoter analysis indicated that the GC-rich region (87% G + C) from -135 to -31 is of critical importance in initiating SERCA3 gene transcription in Jurkat cells. Exon 21 (human, 101 base pairs; mouse, 86 base pairs) can be alternatively excluded, partially included, or totally included, thus generating, respectively, SERCA3a (human and mouse, 999 amino acids (aa)), SERCA3b (human, 1043 aa; mouse, 1038 aa), or SERCA3c (human, 1024 aa; mouse, 1021 aa) isoforms with different C termini. Expression of the mouse SERCA3 isoforms in COS-1 cells demonstrated their ability to function as active pumps, although with different apparent affinities for Ca2+.

A tool coming of age: thapsigargin as an inhibitor of sarco-endoplasmic reticulum Ca(2+)-ATPases

Thapsigargin is the most widely used inhibitor of the ubiquitous sarco-endoplasmic reticulum Ca(2+)-ATPases in mammalian cells. Over the past ten years, this guaianolide compound of plant origin has become a popular tool in a host of studies directed at elucidating the mechanisms of intracellular Ca2+ signalling. Its remarkable potency and selectivity have been instrumental in widening our view of the function of intracellular Ca2+ stores to include such key aspects as store-operated Ca2+ entry or the involvement of the stores in protein synthesis or cell growth. In this article Marek Treiman, Casper Caspersen and Søren Brøgger Christensen review the key pharmacological features of thapsigargin action; they also discuss some of the ways in which its unique properties have shown to be important for obtaining new insights into the biology of Ca2+ stores, and how these properties might encompass a therapeutic potential. In parallel, attention is drawn to some of the limitations and pitfalls encountered when working with thapsigargin.

Distribution and isoform diversity of the organellar Ca2+ pumps in the brain

The gene family of organellar-type Ca2+ transport ATPases consists of three members. SERCA1 is expressed exclusively in fast skeletal muscle; SERCA2 is ubiquitously expressed, whereas SERCA3 is considered to be mainly expressed in cells of the hematopoietic lineage and in some epithelial cells. In the brain, the organellar-type Ca2+ transport ATPases are almost exclusively transcribed from the SERCA2 gene. Four different SERCA2 mRNAs have been described (classes 1-4). However, unlike in nonneuronal cells, which express the class 1, 2, and 3 splice variants, the main SERCA2 mRNA in the brain is the class 4 messenger. Similar to classes 2 and 3, the class 4 codes for the ubiquitously expressed SERCA2b protein. Recently, we have reported the distribution of the SERCA isoforms in the brain (Baba-Aissa et al., 1996a,b). SERCA2b was present in most neurons of all investigated brain regions. The highest levels were found in the Purkinje neurons of the cerebellum and in the pyramidal cells of the hippocampus. Interestingly, SERCA3 and SERCA2a are coexpressed along with SERCA2b in the Purkinje neurons, but are weakly expressed in the other brain regions if present at all. Since these three protein isoforms have a different affinity for Ca2+, their possible roles in relation to Ca2+ stores in neurons are discussed.

Regulation of the sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) 2 gene transcript in neuronal cells

The sarco/endoplasmic reticulum Ca(2+)-transport ATPase (SERCA2) pre-mRNA is alternatively processed in a tissue-specific manner. At its 3' end, two 5' splice donor sites compete for the same 3' acceptor splice site (3'A). While the upstream 5' donor splice site (5'D1) is used in muscle cells giving rise to the class 1 mRNA, the downstream one (5'D2) is exclusively used in neuronal cells generating the class 4 mRNA. Using a neuroblastoma cell line and a minigene containing the 3' end of the SERCA2 gene, we have investigated the regulation of the neuronal-type of splicing. We have shown that a strong 3'A is required for splicing because exchanging it for a weaker one abolishes splicing. A second region spanning the entire exon 25 downstream of the 3'A is also necessary for the repression of the muscle-specific splicing in neuronal cells. In addition the tissue-specific (muscle/neuron) selection of the appropriate 5' donor splice site seems to be determined by at least two distinct but adjacent negative cis-active elements located in the last 237 nt of the optional exon 24. The upstream negative element controls the neuronal splicing while the downstream one represses the muscle-specific splicing in neuronal cells. It is suggested that the cis-active elements in the gene transcript are the target of trans-acting factors that are responsible for the repression of neuronal- or muscle-specific splicing in a tissue-specific manner.

Distribution of the organellar Ca2+ transport ATPase SERCA2 isoforms in the cat brain

Of the three genes encoding the Ca2+ transport ATPases of the endoplasmic reticulum, the SERCA2 gene is the major isoform expressed in the mammalian brain. The SERCA2 transcript is alternatively processed generating two protein isoforms: SERCA2a which is expressed in cardiac and slow-skeletal muscle, and SERCA2b, the house-keeping isoform which is ubiquitously expressed. We have studied the expression of SERCA2 in the cat brain, and at a less refined level also in the rat brain, using antibodies specific for either SERCA2a or SERCA2b. The SERCA2a staining was very restricted. The SERCA2a antibody clearly labeled the cell body of the Purkinje neurons and weakly stained the giant cells of the gigantocellular reticular nuclei. In contrast, the SERCA2b isoform was found in most regions of the brain. It appeared to be largely confined to neuronal cells. Neuroglial cells were negative. The antibody stained the cell body. In heavily labeled cells such as the pyramidal cells of the hippocampus and of the cerebral cortex, it also stained the proximal portion of the dendrites. The most intense labeling was observed in the Purkinje neurons, which were stained all over the cell including the distal ramifications of the dendritic tree. Remarkably the SERCA2b labeling in neuronal cells of the hypothalamic area and the substantia nigra was very weak. The possible physiological significance of these results is discussed.