Co-ordinated regulation of the plasma membrane calcium pump and the sarco(endo)plasmic reticular calcium pump gene expression by Ca2+

Calcium signaling and regulation of ecdysteroidogenesis in crustacean Y-organs

Crustacean Y-organs secrete ecdysteroid molting hormones. Ecdysteroids are released in increased amount during premolt, circulate in hemolymph, and stimulate the events in target cells that lead to molting. During much of the molting cycle, ecdysteroid production is suppressed by molt-inhibiting hormone (MIH), a peptide neurohormone produced in the eyestalks. The suppressive effect of MIH is mediated by a cyclic nucleotide second messenger. A decrease in circulating MIH is associated with an increase in the hemolymphatic ecdysteroid titer during pre-molt. Nevertheless, it has long been hypothesized that a positive regulatory signal or stimulus is also involved in promoting ecdysteroidogenensis during premolt. Data reviewed here are consistent with the hypothesis that an intracellular Ca²⁺ signal provides that stimulus. Pharmacological agents that increase intracellular Ca²⁺ in Y-organs promote ecdysteroidogenesis, while agents that lower intracellular Ca²⁺ or disrupt Ca²⁺ signaling suppress ecdysteroidogenesis. Further, an increase in the hemolymphatic ecdysteroid titer after eyestalk ablation or during natural premolt is associated with an increase in intracellular free Ca²⁺ in Y-organ cells. Several lines of evidence suggest elevated intracellular calcium is linked to enhanced ecdysteroidogenesis through activation of Ca²⁺/calmodulin dependent cyclic nucleotide phosphodiesterase, thereby lowering intracellular cyclic nucleotide second messenger levels and promoting ecdysteroidogenesis. Results of transcriptomic studies show genes involved in Ca²⁺ signaling are well represented in Y-organs. Several recent studies have focused on Ca²⁺ transport proteins in Y-organs. Complementary DNAs encoding a plasma membrane Ca²⁺ ATPase (PMCA) and a sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase (SERCA) have been cloned from crab Y-organs. The relative abundance of PMCA and SERCA transcripts in Y-organs is elevated during premolt, a time when Ca²⁺ levels in Y-organs are likewise elevated. The results are consistent with the notion that these transport proteins act to maintain the Ca²⁺ gradient across the cell membrane and re-set the cell for future Ca²⁺ signals.

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

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

Keratinocyte differentiation promotes ER stress-dependent lysosome biogenesis

Keratinocytes maintain epidermal integrity through cellular differentiation. This process enhances intraorganelle digestion in keratinocytes to sustain nutritional and calcium-ionic stresses observed in upper skin layers. However, the molecular mechanisms governing keratinocyte differentiation and concomitant increase in lysosomal function is poorly understood. Here, by using primary neonatal human epidermal keratinocytes, we identified the molecular link between signaling pathways and cellular differentiation/lysosome biogenesis. Incubation of keratinocytes with CaCl₂ induces differentiation with increased cell size and early differentiation markers. Further, differentiated keratinocytes display enhanced lysosome biogenesis generated through ATF6-dependent ER stress signaling, but independent of mTOR-MiT/TFE pathway. In contrast, chemical inhibition of mTORC1 accelerates calcium-induced keratinocyte differentiation, suggesting that activation of autophagy promotes the differentiation process. Moreover, differentiation of keratinocytes results in lysosome dispersion and Golgi fragmentation, and the peripheral lysosomes showed colocalization with Golgi-tethering proteins, suggesting that these organelles possibly derived from Golgi. In line, inhibition of Golgi function, but not the depletion of Golgi-tethers or altered lysosomal acidity, abolishes keratinocyte differentiation and lysosome biogenesis. Thus, ER stress regulates lysosome biogenesis and keratinocyte differentiation to maintain epidermal homeostasis.

Lysosomes are the key digestive organelles of differentiated keratinocytes in the epidermis. Mahanty et al. show that ER stress but not mTOR-MiT/TFE factors promotes lysosome biogenesis during keratinocyte differentiation, which is critical for epidermal homeostasis.

Sarco(endo)plasmic reticulum ATPase is a molecular partner of Wolfram syndrome 1 protein, which negatively regulates its expression

Wolfram syndrome is an autosomal recessive disorder characterized by neurodegeneration and diabetes mellitus. The gene responsible for the syndrome (WFS1) encodes an endoplasmic reticulum (ER)-resident transmembrane protein that is involved in the regulation of the unfolded protein response (UPR), intracellular ion homeostasis, cyclic adenosine monophosphate production and regulation of insulin biosynthesis and secretion. In this study, single cell Ca(2+) imaging with fura-2 and direct measurements of free cytosolic ATP concentration ([ATP]CYT) with adenovirally expressed luciferase confirmed a reduced and delayed rise in cytosolic free Ca(2+) concentration ([Ca(2+)]CYT), and additionally, diminished [ATP]CYT rises in response to elevated glucose concentrations in WFS1-depleted MIN6 cells. We also observed that sarco(endo)plasmic reticulum ATPase (SERCA) expression was elevated in several WFS1-depleted cell models and primary islets. We demonstrated a novel interaction between WFS1 and SERCA by co-immunoprecipitation in Cos7 cells and with endogenous proteins in human neuroblastoma cells. This interaction was reduced when cells were treated with the ER stress inducer dithiothreitol. Treatment of WFS1-depleted neuroblastoma cells with the proteasome inhibitor MG132 resulted in reduced accumulation of SERCA levels compared with wild-type cells. Together these results reveal a role for WFS1 in the negative regulation of SERCA and provide further insights into the function of WFS1 in calcium homeostasis.

Diurnal and nutritional adjustments of intracellular Ca2+ release channels and Ca2+ ATPases associated with restricted feeding schedules in the rat liver

Background

Intracellular calcium is a biochemical messenger that regulates part of the metabolic adaptations in the daily fed-fast cycle. The aim of this study was to characterize the 24-h variations of the liver ryanodine and IP₃ receptors (RyR and IP₃R) as well as of the endoplasmic-reticulum and plasma-membrane Ca²⁺-ATPases (SERCA and PMCA) in daytime restricted feeding protocol.

Methods

A biochemical and immunohistochemical approach was implemented in this study: specific ligand-binding for RyR and IP₃R, enzymatic activity (SERCA and PMCA), and protein levels and zonational hepatic-distribution were determined by immunoblot and immunohistochemistry respectively under conditions of fasting, feeding, and temporal food-restriction.

Results

Binding assays and immunoblots for IP₃R1 and 2 showed a peak at the light/dark transition in the ad-libitum (AL) group, whereas in the restricted-feeding (RF) group the peak shifted towards the food-access time. In the case of RyR binding experiments, both AL and RF groups showed a modest elevation during the dark period, with the RF rats exhibiting increased binding in response to feeding. The AL group showed 24-h rhythmicity in SERCA level; in contrast, RF group showed a pronounced amplitude elevation and a peak phase-shift during the light-period in SERCA level and activity. The activity of PMCA was constant along day in both groups; PMCA1 levels showed a 24-h rhythmicity in the RF rats (with a peak in the light period), meanwhile PMCA4 protein levels showed rhythmicity in both groups. The fasted condition promoted an increase in IP₃R binding and protein level; re-feeding increased the amount of RyR; neither the activity nor expression of SERCA and PMCA protein was affected by fasting–re-feeding conditions. Histochemical experiments showed that the distribution of the Ca²⁺-handling proteins, between periportal and pericentral zones of the liver, varied with the time of day and the feeding protocol.

Conclusions

Our findings show that RF influences mainly the phase and amplitude of hepatic IP₃R and SERCA rhythms as well as discrete zonational distribution for RyR, IP₃Rs, SERCA, and PMCA within the liver acinus, suggesting that intracellular calcium dynamics could be part of the rheostatic adaptation of the liver due to diurnal meal entrainment/food entrained oscillator expression.

Transcriptional analysis implicates endoplasmic reticulum stress in bovine spongiform encephalopathy

Bovine spongiform encephalopathy (BSE) is a fatal, transmissible, neurodegenerative disease of cattle. To date, the disease process is still poorly understood. In this study, brain tissue samples from animals naturally infected with BSE were analysed to identify differentially regulated genes using Affymetrix GeneChip Bovine Genome Arrays. A total of 230 genes were shown to be differentially regulated and many of these genes encode proteins involved in immune response, apoptosis, cell adhesion, stress response and transcription. Seventeen genes are associated with the endoplasmic reticulum (ER) and 10 of these 17 genes are involved in stress related responses including ER chaperones, Grp94 and Grp170. Western blotting analysis showed that another ER chaperone, Grp78, was up-regulated in BSE. Up-regulation of these three chaperones strongly suggests the presence of ER stress and the activation of the unfolded protein response (UPR) in BSE. The occurrence of ER stress was also supported by changes in gene expression for cytosolic proteins, such as the chaperone pair of Hsp70 and DnaJ. Many genes associated with the ubiquitin-proteasome pathway and the autophagy-lysosome system were differentially regulated, indicating that both pathways might be activated in response to ER stress. A model is presented to explain the mechanisms of prion neurotoxicity using these ER stress related responses. Clustering analysis showed that the differently regulated genes found from the naturally infected BSE cases could be used to predict the infectious status of the samples experimentally infected with BSE from the previous study and vice versa. Proof-of-principle gene expression biomarkers were found to represent BSE using 10 genes with 94% sensitivity and 87% specificity.

Infrared A radiation influences the skin fibroblast transcriptome: mechanisms and consequences

Infrared A (IRA) radiation (760-1440 nm) is a major component of solar radiation and, similar to UVR, causes photoaging of human skin by increasing the expression of matrix metalloproteinase-1 in human skin fibroblasts. In this study, we assessed the IRA-induced transcriptome in primary human skin fibroblasts. Microarray analysis revealed 599 IRA-regulated transcripts. The IRA-induced transcriptome differed from changes known to be induced by UV. IRA-responsive genes include the categories extracellular matrix, calcium homeostasis, stress signaling, and apoptosis. Selected results were confirmed by real-time PCR experiments analyzing 13 genes representing these four categories. By means of chemical inhibitors of known signaling pathways, we showed that ERK1/2, the p38-, JNK-, PI3K/AKT-, STAT3-, and IL-6 as well as the calcium-mediated signaling pathways, are functionally involved in the IRA gene response and that a major part of it is triggered by mitochondrial and, to a lesser extent, non-mitochondrial production of reactive oxygen species. Our results identify IRA as an environmental factor with relevance for skin homeostasis and photoaging.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

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.

Heterologous expression of polycystin-1 inhibits endoplasmic reticulum calcium leak in stably transfected MDCK cells

We previously found that polycystin-1 accelerated the decay of ligand-activated cytoplasmic calcium transients through enhanced reuptake of calcium into the endoplasmic reticulum (ER; Hooper KM, Boletta A, Germino GG, Hu Q, Ziegelstein RC, Sutters M. Am J Physiol Renal Physiol 289: F521-F530, 2005). Calcium flux across the ER membrane is determined by the balance of active uptake and passive leak. In the present study, we show that polycystin-1 inhibited calcium leak across the ER membrane, an effect that would explain the capacity of this protein to accelerate clearance of calcium from the cytoplasm following a calcium release response. Calcium leak was detected by measurement of the accumulation of calcium in the cytoplasm following treatment with thapsigargin. Heterologous polycystin-1, stably expressed in Madin-Darby canine kidney cells, attenuated the thapsigargin-induced calcium peak with no effect on basal calcium stores, mitochondrial calcium uptake, or extrusion of calcium across the plasma membrane. The capacity of polycystin-1 to limit the rate of decay of ER luminal calcium following inhibition of the pump was shown indirectly using the calcium ionophore ionomycin, and directly by loading the ER with a low-affinity calcium indicator. We conclude that disruption of ER luminal calcium homeostasis may contribute to the cyst phenotype in autosomal dominant polycystic kidney disease.

Age-dependent changes in Ca2+ homeostasis in peripheral neurones: implications for changes in function

Calcium ions represent universal second messengers within neuronal cells integrating multiple cellular functions, such as release of neurotransmitters, gene expression, proliferation, excitability, and regulation of cell death or apoptotic pathways. The magnitude, duration and shape of stimulation-evoked intracellular calcium ([Ca2+]i) transients are determined by a complex interplay of mechanisms that modulate stimulation-evoked rises in [Ca2+]i that occur with normal neuronal function. Disruption of any of these mechanisms may have implications for the function and health of peripheral neurones during the aging process. This review focuses on the impact of advancing age on the overall function of peripheral adrenergic neurones and how these changes in function may be linked to age-related changes in modulation of [Ca2+]i regulation. The data in this review suggest that normal aging in peripheral autonomic neurones is a subtle process and does not always result in dramatic deterioration in their function. We present studies that support the idea that in order to maintain cell viability peripheral neurones are able to compensate for an age-related decline in the function of at least one of the neuronal calcium-buffering systems, smooth endoplasmic reticulum calcium ATPases, by increased function of other calcium-buffering systems, namely, the mitochondria and plasmalemma calcium extrusion. Increased mitochondrial calcium uptake may represent a 'weak point' in cellular compensation as this over time may contribute to cell death. In addition, we present more recent studies on [Ca2+]i regulation in the form of the modulation of release of calcium from smooth endoplasmic reticulum calcium stores. These studies suggest that the contribution of the release of calcium from smooth endoplasmic reticulum calcium stores is altered with age through a combination of altered ryanodine receptor levels and modulation of these receptors by neuronal nitric oxide containing neurones.

Functional importance of PMCA isoforms in growth and development of PC12 cells

Intracellular Ca2+ in neuronal cells is an essential regulatory ion responsible for excitability, synaptic plasticity, and neurite outgrowth. Plasma membrane calcium ATPase (PMCA) is the most sensitive enzyme in decreasing of the Ca2+ concentration. The diverse PMCA isoforms composition in the membranes suggests their specific function in the cell, and whereas PMCA1 and 4 appear to be ubiquitous, PMCA2 and 3 are characteristic isoforms for excitable cells. The aim of our study was to elucidate if and how the elimination of neuron-specific isoforms affects the pattern of cell growth and development. We have obtained stable-transfected PC12 cell lines with a suppressed expression of PMCA2, PMCA3, or both neuron-specific isoforms. The modified profile of PMCA generated considerable changes in morphology of examined PC12 lines, suggesting the activation of a differentiation process to pseudoneuronal phenotype. Experiments with Fura-2/AM-loaded cells revealed an increased cytosolic Ca2+ concentration in the cell lines with blocked PMCA2 isoform. The suppression of PMCA2 concomitantly altered expression of sarco/endoplasmic Ca2+-ATPase 2 isoform (SERCA2) at the protein level. Comparative flow cytometry analysis, using Annexin V/PI conjugate, showed the difference in the mean percentage of apoptotic cells in modified PC12 lines. Our data suggest that specific PMCA isoforms presence can regulate the intact cell development; however, it may involve multiple unidentified yet signaling pathways.

Molecular characterization of the sarcoplasmic calcium-binding protein (SCP) from crayfish Procambarus clarkii

Sarcoplasmic Calcium-binding Protein (SCP) is believed to function as the invertebrate equivalent of vertebrate parvalbumin, namely to "buffer" cytosolic Ca2+. We have cloned and characterized a novel SCP from axial abdominal muscle of crayfish Procambarus clarkii (referred to as pcSCP1), and have examined tissue specific distribution and expression as a function of molting stage in non-epithelial and epithelial tissues. The complete sequence of pcSCP1 consists of 1,052 bp with a 579 bp open reading frame, coding for 193 amino acid residues (molecular mass of 21.8 kDa). There is a 387 bp 3' terminal non-coding region with a poly (A) tail. The deduced pcSCP1 protein sequence matched most closely with published SCP sequences from another crayfish Astacus leptodactylus (92.8%) and from shrimp (78.6-81.2%) and fruit fly (53%). Real-time PCR analysis confirmed that pcSCP1 is ubiquitously expressed in all tissues tested (gill, hepatopancreas, intestine, antennal gland, muscle); however it is most abundant in muscle particularly in the axial abdominal muscle. The real-time PCR analysis revealed that pcSCP1 expression is downregulated in pre- and postmolt stages compared with intermolt. Epithelial (hepatopancreas and antennal gland) SCP expression exhibited a more dramatic decrease than that observed in muscle. Expression trends for pcSCP1 paralleled published trends for sarco/endoplasmic reticular calcium ATPase (SERCA), suggesting that their cellular function in regulating intracellular Ca2+ is linked.

Antisense-mediated Inhibition of the Plasma Membrane Calcium-ATPase Suppresses Proliferation of MCF-7 Cells

Alterations in Ca2+ signaling may contribute to tumorigenesis and the mechanism of action of some anti-cancer drugs. The plasma membrane calcium-ATPase (PMCA) is a crucial controller of intracellular Ca2+ signaling. Altered PMCA expression occurs in the mammary gland during lactation and in breast cancer cell lines. Despite this, the consequences of PMCA inhibition in breast cancer cell lines have not been investigated. In this work, we used Tet-off PMCA antisense-expressing MCF-7 cells to assess the effects of PMCA inhibition in a human breast cancer cell line. At a level of PMCA inhibition that did not completely prevent PMCA-mediated Ca2+ efflux and did not induce cell death, a dramatic inhibition of cellular proliferation was observed. Fluorescence-activated cell sorting analysis indicated that PMCA antisense involves changes in cell cycle kinetics but not cell cycle arrest. We concluded that modulation of PMCA has important effects in regulating the proliferation of human breast cancer MCF-7 cells.

Ca2+ clearance in smooth muscle: lessons from gene-altered mice

The regulation of intracellular [Ca(2+)](i) is important for all cells, but in particular for smooth muscle, as [Ca(2+)](i) is a key second messenger leading to contraction. Mechanisms for the cellular clearance of [Ca(2+)](i) form one side of Ca(2+) homeostasis and include: Plasma Membrane Ca(2+) ATPases (PMCA), Sarcoplasmic/Endoplasmic Reticulum Ca(2+) ATPases (SERCA), Na(+)-Ca(2+)-exchangers (NCX) when coupled to the Na(+)-K(+) ATPases (NKA) and in some cases mitochondria. The nature and relative contribution of these various components of cytosolic Ca(2+) clearance have long been an important topic for study in smooth muscle, particularly as related to regulation of contractility. These studies have largely depended on inhibition of the various components. Recently advances in gene-targeting and transgenesis have made it possible to add or delete individual components, and importantly specific isoforms from the cell. In this brief review, we will focus on new information on Ca(2+) clearance in smooth muscle gained from studies on gene-altered mice models. These provide a deeper understanding of distinct functional roles for individual isoforms and the interactions between various components.

Characterization and expression of plasma membrane Ca2+ ATPase (PMCA3) in the crayfish Procambarus clarkii antennal gland during molting

The discontinuous pattern of crustacean cuticular mineralization (the molting cycle) has emerged as a model system to study the spatial and temporal regulation of genes that code for Ca2+-transporting proteins including pumps, channels and exchangers. The plasma membrane Ca2+-ATPase (PMCA) is potentially of significant interest due to its role in the active transport of Ca2+ across the basolateral membrane, which is required for routine maintenance of intracellular Ca2+ as well as unidirectional Ca2+ influx. Prior research has suggested that PMCA expression is upregulated during periods of elevated Ca2+ influx associated with postmolt cuticular mineralization. This paper describes the cloning, sequencing and functional characterization of a novel PMCA3 gene from the antennal gland (kidney) of the crayfish Procambarus clarkii. The complete sequence, the first obtained from a non-genetic invertebrate species, was obtained through reverse transcription-polymerase chain reaction (RTPCR) and rapid amplification of cDNA ends (RACE) techniques. Crayfish PMCA3 consists of 4148 bp with a 3546 bp open reading frame coding for 1182 amino acid residues with a molecular mass of 130 kDa. It exhibits 77.5-80.9% identity at the mRNA level and 85.3-86.9% identity at the protein level with PMCA3 from human, mouse and rat. Membrane topography was typical of published mammalian PMCAs. Northern blot analysis of total RNA from crayfish gill, antennal gland, cardiac muscle and axial abdominal muscle revealed that a 7.5 kb species was ubiquitous. The level of PMCA3 mRNA expression in all tissues (transporting epithelia and muscle) increased significantly in pre/postmolt stages compared with relatively low abundance in intermolt. Western analysis confirmed corresponding changes in PMCA protein expression (130 kDa).

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.

Vitamin D3upregulates plasma membrane Ca2+-ATPase expression and potentiates apico-basal Ca2+flux in MDCK cells

Plasma membrane Ca2+-ATPases (PMCAs) are a ubiquitous system for the expulsion of Ca2+from eukaryotic cells. In tight monolayers of polarized Madin-Darby canine kidney (MDCK) cells representing a distal kidney tubule model, PMCAs are responsible for about one-third of the vectorial Ca2+transport under resting conditions, with the remainder being provided by the Na+/Ca2+exchanger. Vitamin D3(VitD) is known to increase PMCA expression and activity in Ca2+-transporting tissues such as the intestine, as well as in osteoblasts and Madin-Darby bovine kidney epithelial cells. We found that VitD upregulated the expression of the PMCAs (mainly PMCA4b) in MDCK cell lysates at the RNA and protein level in a time- and dose-dependent manner. Interestingly, VitD caused a decrease of the PMCAs in the apical plasma membrane fraction and a concomitant increase of the pumps in the basolateral membrane. Functional studies demonstrated that transcellular45Ca2+flux from the apical-to-basolateral compartment was significantly enhanced by VitD. These findings demonstrate that VitD is a positive regulator of the PMCAs in MDCK epithelial cells. The correlation of decreased apical/increased basolateral expression of the PMCAs with an increase in transcellular Ca2+flux from the apical (urine) toward the basolateral (blood) compartment indicates the physiological relevance of VitD function in kidney tubular Ca2+reabsorption.

Neuronal ageing from an intraneuronal perspective: roles of endoplasmic reticulum and mitochondria

The nature of brain ageing and the age-dependent decline in cognitive functions remains poorly understood. Physiological brain ageing is characterised by mild mental dysfunctions, whereas age-dependent neurodegeneration, as illustrated by Alzheimer disease (AD), results rapidly in severe dementia. These two states of the aged brain, the physiological and the pathological, are fundamentally different as the latter stems from significant neuronal loss, whereas the former develops without significant neuronal demise. In this paper, we review the changes in neuronal Ca(2+) homeostasis that occur during brain ageing, and conclude that normal, physiological ageing is characterised mainly by a decrease of neuronal homeostatic reserve, defined as the capacity to respond effectively to functional and metabolic stressors, but does not reach the trigger required to induce neuronal death. In contrast, during neurodegenerative states, Ca(2+) homeostasis is affected early during the pathological process and result in significant neuronal demise. We also review recent evidence suggesting that the endoplasmic reticulum (ER) might play an important role in controlling the balance between healthy and pathological neuronal ageing.

Plasma membrane Ca2+-ATPase mRNA expression in murine hepatocarcinoma and regenerating liver cells

The plasma membrane calcium ATPase (PMCA) is an ubiquitous enzyme that extrudes calcium from the cytoplasm to the extracellular space. Four PMCA genes through alternative splicing produce a large diversity of isoforms of this enzyme. We reported previously that the PMCA contained in AS-30D hepatocarcinoma cells showed significant differences in activity in comparison to normal and regenerating liver. In the present study we investigate if the difference in PMCA activity could be related to differential expression of mRNAs encoding different isoforms of PMCA. Using RT-PCR we found that variants 1b, 1x, and 4b are expressed in all liver samples. The hepatoma AS-30 and liver at 2 days of regeneration express low amounts of isoforms 2w, 4b and 4x, and do not express isoforms 4a, 4d and 4z. Fetal and neonatal liver do not express variants 4a and 4d, but they do express variants 4x and 4z. Immunoblot analysis showed a higher ratio ATPase/total protein in the hepatoma AS-30D in comparison to normal liver. Our results suggest that the Ca2+-ATPase kinetic pattern previously observed by us in the AS-30D cells, could be at least partially explained by changes in the mRNA expression of several of the PMCA isoforms expressed in the liver.

Interleukin-1beta induces calcium transients and enhances basal and store operated calcium entry in human myometrial smooth muscle

We have previously reported increased protein expression of sarcoplasmic reticulum calcium ATPase (SERCA) 2b in myometrium from women in labor at term, but the stimulus for this change is unknown. Proinflammatory cytokines have been implicated in the cascade of events leading to preterm and term labor, and we hypothesize that interleukin (IL)-1beta may induce changes in key calcium homeostatic mechanisms and, in turn, augment myometrial contractility before labor. The aim of the present study was to investigate the long-term effects of IL-1beta on SERCA 2b protein expression, calcium mobilization from intracellular stores, and store-operated calcium entry. Myometrial biopsies were obtained, with consent, from women undergoing elective cesarean section at term. Primary-cultured human myometrial smooth muscle (HMSM) cells were exposed to IL-1beta (10 ng/ml) for 24 h or to culture medium alone (control). Cells were subsequently used in Western blot studies or loaded with fura-2 to assess calcium dynamics using fluorescent digital imaging. The present study clearly demonstrated that IL-1beta significantly increased SERCA 2b protein expression in HMSM cells. Cyclopiazonic acid-induced calcium transients were also augmented, predominantly by activation of lanthanum-sensitive, store-operated calcium entry. HMSM cell excitability was enhanced, as evidenced by increased basal calcium entry and the initiation of spontaneous calcium transients in 37% of IL-1beta-treated cells. IL-1beta modulation of calcium mobilization may be an important mechanism in the cascade of events preparing the pregnant uterus for labor.

Regulation of sarco/endoplasmic Ca2+ -ATPase expression by calcium in human lens cells

Maintenance of cellular calcium levels is critical to cell function. Loss of calcium homeostasis might be a contributing factor to the development of cataract in the lens. In lens epithelium, calcium is involved in cell signaling and its precise regulation is vital. In this study, we investigated the regulation of sarco/endoplasmic reticulum Ca(2+)-ATPase2b (SERCA2b) and SERCA3 isoform expression in cultured epithelial B-3 cells from human lenses. Both mRNA and membrane proteins samples were collected for semi quantitative RT-PCR using GAPDH as a control. Western blot analyses were performed on membrane samples.Thapsigargin, a SERCA isoform inhibitor which causes increased cytosolic levels elicited dose-and time-dependent up-regulation of SERCA3 at both mRNA and protein levels; SERCA2b expression was unaffected. Both EGTA and actinomycin partially inhibited the thapsigargin-induced SERCA3 up-regulation. These results indicate that the up-regulation of SERCA3 by thapsigargin is dependent on a calcium-mediated pathway that is likely to occur at the transcription level.

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.

Ca2+ pools and cell growth. Evidence for sarcoendoplasmic Ca2+-ATPases 2B involvement in human prostate cancer cell growth control

The present study demonstrates for the first time that intracellular calcium-ATPases and calcium pool content are closely associated with prostate cancer LNCaP cell growth. Cell growth was modulated by changing the amount of epidermal growth factor, serum, and androgene in culture media. Using the microspectrofluorimetric method with Fura-2 and Mag Fura-2 as probes, we show that in these cells, the growth rate is correlated with intracellular calcium pool content. Indeed, an increased growth rate is correlated with an increase in the calcium pool filling state, whereas growth-inhibited cells show a reduced calcium pool load. Using Western blotting and immunocytochemistry, we show that endoplasmic reticulum calcium pump expression is closely linked to LNCaP cell growth, and are a common target of physiological stimuli that control cell growth. Moreover, we clearly demonstrate that inhibition of these pumps, using thapsigargin, inhibits LNCaP cell growth and prevents growth factor from stimulating cell proliferation. Our results thus provide evidence for the essential role of functional endoplasmic reticulum calcium pumps and calcium pool in control of prostate cancer LNCaP cell growth, raising the prospect of new targets for the treatment of prostate cancer.

Expression of endoplasmic reticulum stress proteins during skeletal muscle disuse atrophy

Disuse atrophy of skeletal muscle leads to an upregulation of genes encoding sarcoplasmic reticulum (SR) calcium-handling proteins. Because many of the proteins that are induced with endoplasmic reticulum (ER) stress are ER calcium-handling proteins, we sought to determine whether soleus muscle atrophy was associated with a prototypical ER stress response. Seven days of rat hindlimb unloading did not alter expression of ubiquitous ER stress proteins such as Grp78, calreticulin, and CHOP/GADD-153, nor other proteins that have been shown to be activated by ER stressors such as vinculin, the type I D-myo-inositol 1,4,5-trisphosphate receptor, or protein kinase R, a eukaryotic initiation factor 2 alpha kinase. On the other hand, expression of heme oxygenase-1 (HO-1), an antioxidant ER stress protein, was significantly increased 2.2-fold. In addition, unloading led to an increase in calsequestrin, the muscle-specific SR calcium-binding protein, at both the mRNA (68%) and protein (24%) levels. Although disuse atrophy is associated with a significant remodeling of muscle-specific proteins controlling SR calcium flux, it is not characterized by a prototypical ER stress response. However, the upregulation of HO-1 may indicate ER adaptation to oxidative stress during muscle unloading.

Plasma membrane calcium ATPase plays a role in reducing Ca(2+)-mediated cytotoxicity in PC12 cells

In many cell types, cell death induced by a variety of insults is accompanied by an increase in intracellular calcium. The Ca(2+) homeostatic mechanisms affected by such insults, however, have not been fully determined. Recent evidence indicates that kainic acid-induced seizures alter plasma membrane calcium ATPase mRNA expression within vulnerable hippocampal cell populations before the onset of cell death. We examined the effects of altering plasma membrane calcium ATPase expression on cell vulnerability in rat pheochromocytoma 12 cells. Pheochromocytoma 12 cells are vulnerable to Ca(2+) overload induced by the Ca(2+) ionophore A23187. Reverse transcriptase-PCR and Western blot data indicated that plasma membrane calcium ATPase isoform 4b constitutes a major calcium pump isoform in the pheochromocytoma 12 cells. Therefore, permanently transfected pheochromocytoma 12-derived cell lines were established that either over-expressed plasma membrane calcium ATPase isoform 4b, or suppressed the expression of the endogenous plasma membrane calcium ATPase isoform 4. Over-expressing clones were less vulnerable to Ca(2+)-mediated cell death induced by A23187 whereas "antisense" clones were considerably more susceptible. These data indicate that regulation of plasma membrane calcium ATPase expression may be critical to cellular survival when cells are exposed to pathological increases in intracellular calcium.

Upregulation of the SERCA-type Ca2+ pump activity in response to endoplasmic reticulum stress in PC12 cells

BACKGROUND

Ca2+-ATPases of endoplasmic reticulum (SERCAs) are responsible for maintenance of the micro- to millimolar Ca2+ ion concentrations within the endoplasmic reticulum (ER) of eukaryotic cells. This intralumenal Ca2+ storage is important for the generation of Ca2+ signals as well as for the correct folding and posttranslational processing of proteins entering ER after synthesis. ER perturbations such as depletion of Ca2+ or abolishing the oxidative potential, inhibition of glycosylation, or block of secretory pathway, activate the Unfolded Protein Response, consisting of an upregulation of a number of ER-resident chaperones/stress proteins in an effort to boost the impaired folding capacity.

RESULTS

We show here that in PC12 cells, depletion of ER Ca2+ by EGTA, as well as inhibition of disulphide bridge formation within the ER by dithiotreitol or inhibition of N-glycosylation by tunicamycin, led to a 2- to 3-fold increase of the SERCA-mediated 45Ca2+ transport to microsomes isolated from cells exposed to these stress agents. The time course of this response corresponded to that for transcriptional upregulation of ER stress proteins, as well as to the increase in the SERCA2b mRNA, as we recently observed in an independent study.

CONCLUSIONS

These findings provide the first functional evidence for the increase of SERCA pumping capacity in cells subjected to the ER stress. Since at least three different and unrelated mechanisms of eliciting the ER stress response were found to cause this functional upregulation of Ca2+ transport into the ER, these results support the existence of a coupling between the induction of the UPR pathway in general, and the regulation of expression of at least one of the SERCA pump isoforms.

Regulation of SERCA Ca2+ pump expression by cytoplasmic Ca2+ in vascular smooth muscle cells

Vascular smooth muscle cells (VSMC) express three isoforms of the sarcoplasmic or endoplasmic reticulum Ca2+-ATPase (SERCA) pump; SERCA2b predominates (91%), whereas SERCA2a (6%) and SERCA3 (3%) are present in much smaller amounts. Treatment with thapsigargin (Tg) or A-23187 increased the level of mRNA encoding SERCA2b four- to fivefold; SERCA3 increased about 10-fold, whereas SERCA2a was unchanged. Ca2+ chelation prevented the Tg-induced SERCA2b increase, whereas Ca2+ elevation itself increased SERCA2b expression. These responses were discordant with those of 78-kDa glucose-regulated protein/immunoglobulin-binding protein (grp78/BiP), an endoplasmic reticulum stress-response protein. SERCA2b mRNA elevation was much larger than could be accounted for by the observed increase in message stability. The induction of SERCA2b by Tg did not require protein synthesis, nor was it affected by inhibitors of calcineurin, protein kinase C, Ca2+/calmodulin-dependent protein kinase, or tyrosine protein kinases. Treatment with the nonselective protein kinase inhibitor H-7 prevented Tg-induced SERCA2b expression from occurring, whereas another nonselective inhibitor, staurosporine, was without effect. We conclude that changes in cytosolic Ca2+ control the expression of SERCA2b in VSMC via a mechanism involving a currently uncharacterized, H-7-sensitive but staurosporine-insensitive, protein kinase.

Differentiation induces up-regulation of plasma membrane Ca(2+)-ATPase and concomitant increase in Ca(2+) efflux in human neuroblastoma cell line IMR-32

Precise regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)) is achieved by the coordinated function of Ca(2+) channels and Ca(2+) buffers. Neuronal differentiation induces up-regulation of Ca(2+) channels. However, little is known about the effects of differentiation on the expression of the plasma membrane Ca(2+)-ATPase (PMCA), the principal Ca(2+) extrusion mechanism in neurons. In this study, we examined the regulation of PMCA expression during differentiation of the human neuroblastoma cell line IMR-32. [Ca(2+)](i) was monitored in single cells using indo-1 microfluorimetry. When the Ca(2+)-ATPase of the endoplasmic reticulum was blocked by cyclopiazonic acid, [Ca(2+)](i) recovery after small depolarization-induced Ca(2+) loads was governed primarily by PMCAs. [Ca(2+)](i) returned to baseline by a process described by a monoexponential function in undifferentiated cells (tau = 52 +/- 4 s; n = 25). After differentiation for 12-16 days, the [Ca(2+)](i) recovery rate increased by more than threefold (tau = 17 +/- 1 s; n = 31). Western blots showed a pronounced increase in expression of three major PMCA isoforms in IMR-32 cells during differentiation, including PMCA2, PMCA3 and PMCA4. These results demonstrate up-regulation of PMCAs on the functional and protein level during neuronal differentiation in vitro. Parallel amplification of Ca(2+) influx and efflux pathways may enable differentiated neurons to precisely localize Ca(2+) signals in time and space.

Interspecies differences in hepatic Ca(2+)-ATPase activity and the effect of cold preservation on porcine liver Ca(2+)-ATPase function

The accumulation of intracellular calcium ([Ca(2+)](i)) caused by ischemia-reperfusion during liver transplantation has been implicated as a factor leading to primary graft nonfunction. Plasma membrane (PM) and endoplasmic reticulum (ER) Ca(2+)-adenosinetriphosphatases (ATPases) are the primary transporters that maintain [Ca(2+)](i) homeostasis in the liver. We hypothesized that the porcine liver is better than the rat liver as a model for the study of human liver Ca(2+)-ATPase activity. We also hypothesized that cold preservation would depress Ca(2+)-ATPase activity in the porcine liver. Pig and rat livers were harvested, and human liver samples were obtained from surgical resection specimens. All were preserved with University of Wisconsin solution, and porcine livers were also preserved on ice for 2 to 18 hours. Ca(2+)-ATPase activity was measured after incubation with (45)Ca(2+) and adenosine triphosphate in the presence of specific Ca(2+)-ATPase inhibitors. Porcine PM and ER Ca(2+)-ATPase activities were 0.47 +/- 0.03 and 1.57 +/- 0.10 nmol of Ca(2+)/mg of protein/min, respectively. This was not significantly different from human liver, whereas rat liver was significantly greater at 2.60 +/- 0.03 and 9.2 +/- 0.9 nmol of Ca(2+)/mg of protein/min, respectively. We conclude that the Ca(2+)-ATPase activity in the pig liver is equivalent to that of human liver, and thus, the pig liver is a better model than the rat liver. Cold preservation studies showed a significant decrease in porcine hepatic PM Ca(2+)-ATPase activity after 4 hours of storage and near-total inhibition after 12 hours. Porcine hepatic ER Ca(2+)-ATPase activity showed a 45% decrease in activity by 12 hours and a 69% decrease by 18 hours. We conclude that cold ischemia at clinically relevant times depresses PM Ca(2+)-ATPase more than ER Ca(2+)-ATPase activity in pig liver homogenates.

Role of alternative splicing in generating isoform diversity among plasma membrane calcium pumps

Calcium pumps of the plasma membrane (also known as plasma membrane Ca(2+)-ATPases or PMCAs) are responsible for the expulsion of Ca(2+) from the cytosol of all eukaryotic cells. Together with Na(+)/Ca(2+) exchangers, they are the major plasma membrane transport system responsible for the long-term regulation of the resting intracellular Ca(2+) concentration. Like the Ca(2+) pumps of the sarco/endoplasmic reticulum (SERCAs), which pump Ca(2+) from the cytosol into the endoplasmic reticulum, the PMCAs belong to the family of P-type primary ion transport ATPases characterized by the formation of an aspartyl phosphate intermediate during the reaction cycle. Mammalian PMCAs are encoded by four separate genes, and additional isoform variants are generated via alternative RNA splicing of the primary gene transcripts. The expression of different PMCA isoforms and splice variants is regulated in a developmental, tissue- and cell type-specific manner, suggesting that these pumps are functionally adapted to the physiological needs of particular cells and tissues. PMCAs 1 and 4 are found in virtually all tissues in the adult, whereas PMCAs 2 and 3 are primarily expressed in excitable cells of the nervous system and muscles. During mouse embryonic development, PMCA1 is ubiquitously detected from the earliest time points, and all isoforms show spatially overlapping but distinct expression patterns with dynamic temporal changes occurring during late fetal development. Alternative splicing affects two major locations in the plasma membrane Ca(2+) pump protein: the first intracellular loop and the COOH-terminal tail. These two regions correspond to major regulatory domains of the pumps. In the first cytosolic loop, the affected region is embedded between a putative G protein binding sequence and the site of phospholipid sensitivity, and in the COOH-terminal tail, splicing affects pump regulation by calmodulin, phosphorylation, and differential interaction with PDZ domain-containing anchoring and signaling proteins. Recent evidence demonstrating differential distribution, dynamic regulation of expression, and major functional differences between alternative splice variants suggests that these transporters play a more dynamic role than hitherto assumed in the spatial and temporal control of Ca(2+) signaling. The identification of mice carrying PMCA mutations that lead to diseases such as hearing loss and ataxia, as well as the corresponding phenotypes of genetically engineered PMCA "knockout" mice further support the concept of specific, nonredundant roles for each Ca(2+) pump isoform in cellular Ca(2+) regulation.

Expression of Ca(2+) Transport Genes in Platelets and Endothelial Cells in Hypertension

-Altered Ca(2+) handling is observed in different cells in essential hypertension. We investigated the expression of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) and inositol 1,4,5-trisphosphate receptor (IP(3)R) isoforms in platelets and aortic endothelial cells (EC) isolated from spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats by ratio reverse-transcriptase-polymerase chain reaction (RT-PCR) analysis and Western blotting. SERCA2b and SERCA3 were assessed at mRNA (EC and platelets) and at protein level (platelets). IP(3)R1, IP(3)R2, and IP(3)R3 mRNAs were demonstrated in both cell types, but only IP(3)R1 and IP(3)R2 proteins were detected in platelets. Compared with WKY, SHR EC and platelets showed higher SERCA3 and IP(3)R2 expression and lower IP(3)R1 expression. We then investigated the effect of lisinopril (20 mg. kg(-)(1). d(-)(1); 10-week treatment of 4-week-old rats or 2-week treatment of adult rats) and captopril (100 mg. kg(-)(1). d(-)(1); 2-week treatment of adult rats). Consequently, expression patterns of SERCAs and IP(3)Rs were significantly modified. Except for SERCAs mRNA in platelets, all differences between SHR and WKY disappeared. However, SERCA3 remained the predominant isoform. Both EC and platelets demonstrated a high equal expression of IP(3)R2 mRNA. IP(3)R1 was the predominant platelet protein isoform, as it was in untreated WKY. mRNA was also isolated from pancreatic islets of WKY and SHR, but no effect of either rat strain or of lisinopril treatment was observed on the expression of the studied genes. We hypothesize that the identical expression pattern of SERCAs and IP(3)Rs after treatment with ACE inhibitors represents a different nonhypertensive configuration, which, through changes in intracellular Ca(2+) handling, improves endothelial and platelet dysfunction in SHR but has no effect in WKY.

SERCA function declines with age in adrenergic nerves from the superior cervical ganglion

1. Intracellular calcium is a universal second messenger integrating numerous cellular pathways. An age-related breakdown in the mechanisms controlling [Ca2+]i homeostasis could contribute to neuronal degeneration. One component of neuronal calcium regulation believed to decline with age is the function of sarco/endoplasmic reticulum calcium ATPase (SERCA) pumps. 2. Therefore we investigated the impact of age on the capacity of SERCA pumps to control high (68 mM) [K+]-evoked [Ca2+]i-transients in acutely dissociated superior cervical ganglion (SCG) cells from 6- and 20-month-old Fisher-344 rats. Calcium transients were measured by fura-2 microfluorometry in the presence of vanadate (0.1 microM) to selectively block plasma membrane calcium ATPase (PMCA) pumps, dinitrophenol (100 microM) to block mitochondrial calcium uptake and extracellular sodium replaced with tetraethylammonium to block Na+/Ca2+-exchanger, thus forcing the neuronal cells to rely on SERCA uptake to control [Ca2+]i homeostasis. 3. In the presence of these calcium buffering blockers, the rate of recovery of [Ca2+]i was significantly slower and time to recover to approximately 90% of resting [Ca2+]i was significantly greater in SCG cells from old (20 months) compared with young (6 months) animals. 4. This age-related change in the recovery phase of [K+]-evoked [Ca2+]i-transients could not be explained by differences in the sensitivity of SCG cells to the calcium buffering blockers, as no age-related difference in basal [Ca2+]i was observed. 5. These studies illustrate that when rat SCG cells are forced to rely on SERCAs to buffer [K+]-evoked [Ca2+]i-transients, an age-related decline in SERCA function is revealed. Such age-related declines in calcium regulation coupled with neuronal sensitivity to calcium overload underscore the importance of understanding the components of [Ca2+]i homeostasis and the functional compensation that may occur with advancing age.

Calcium homeostatic pathways change with gestation in human myometrium

A rise in intracellular calcium is the primary trigger for contractile activity in pregnant human myometrium. It is hypothesized that key proteins involved in myometrial calcium homeostasis are gestationally regulated and play an important role in the preparation for labor. The aims of the study were to investigate the role of sarcoplasmic reticulum Ca ATPases (SERCAs) in regulating spontaneous contractile activity in myometrium, and to determine the expression of SERCA isoforms 2a and 2b, and the plasma membrane Ca ATPase (PMCA), at term and during labor. Western blot analysis demonstrated that the expression of SERCA 2a and 2b significantly increased in myometrium of women in labor compared with those not in labor. The augmentation of contractile activity in laboring myometrium in the presence of a SERCA 2 inhibitor, cyclopiazonic acid (CPA), demonstrated the functional significance of this observation. It is interesting that the application of CPA in the presence of a calcium-activated potassium channel inhibitor to term nonlabor myometrium mimicked the response of myometrium from women in active labor to CPA alone. We conclude that the activity of SERCA isoforms becomes increasingly important in the maintenance of regular contractile activity during labor and may compensate for the functional loss of other calcium control pathways at term.

Identification of weight-bearing-responsive elements in the skeletal muscle sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA1) gene

The skeletal muscle sarco(endo)plasmic reticulum calcium ATPase (SERCA1) gene is transactivated as early as 2 days after the removal of weight-bearing (Peters, D. G., Mitchell-Felton, H., and Kandarian, S. C. (1999) Am. J. Physiol. 276, C1218-C1225), but the transcriptional mechanisms are elusive. Here, the rat SERCA1 5' flank and promoter region (-3636 to +172 base pairs) was comprehensively examined using in vivo somatic gene transfer into rat soleus muscles (n = 804) to identify region(s) that are both necessary and sufficient for sensitivity to weight-bearing. In all, 40 different SERCA1 reporter plasmids were constructed and tested. Several different regions of the SERCA1 5' flank were sufficient to confer a transcriptional response to 7 days of muscle unloading when placed upstream of a heterologous promoter. Two of these regions were analyzed further because they were necessary for the unloading response of -3636 to +172, as demonstrated using internal deletion constructs. Deletion analysis of these regions (-1373 to -1158 and -330 to +172) suggested that unloading responsiveness corresponded to CACC sites and E-boxes. Mutagenesis of cis-elements in the first region showed that a specific CACC box (-1262) was involved in SERCA1 transactivation and a nearby E-box (-1248) was also implicated. Constructs containing trimerized CACC sites and E-boxes showed that the presence of both elements is required to activate transcription. This is the first identification of specific cis-elements required for the regulation of a Ca(2+) handling gene by changes in muscle loading condition.

The sarco/endoplasmic reticulum calcium-ATPase 2b is an endoplasmic reticulum stress-inducible protein

The sarco/endoplasmic reticulum calcium-ATPase (SERCA) translocates Ca(2+) from the cytosol to the lumen of the endoplasmic reticulum. This Ca(2+) storage is important for cellular processes such as calcium signaling and endoplasmic reticulum (ER)-associated posttranslational protein modifications. We investigated the expression of the SERCA2 and SERCA3 isozymes in PC12 cells exposed to agents interfering with different aspects of the posttranslational protein processing within the ER, thereby activating the ER stress-induced unfolded protein response (UPR). All agents increased the SERCA2b mRNA level 3-4-fold, in parallel with increasing mRNA levels for the ER stress marker proteins BiP/GRP78 and CHOP/GADD153. In contrast, SERCA3 mRNA levels did not change. SERCA2b mRNA stability was not changed, indicating that the mechanism of its up-regulation was transcriptional, in accordance with the presence of ER stress response elements in the promoter region of the SERCA2 gene. SERCA2b was also increased at the protein level upon ER stress treatments. Induction of ER stress by tunicamycin, dithiothreitol, or l-azetidine 2-carboxylic acid did not result in depletion of ER calcium, showing that such depletion was not necessary for up-regulation of SERCA2b expression or UPR activation in general. We conclude that the SERCA2b expression can be controlled by the UPR pathway independently of ER Ca(2+) depletion.

Mechanisms of calcium decay kinetics in hippocampal spines: role of spine calcium pumps and calcium diffusion through the spine neck in biochemical compartmentalization

Dendritic spines receive most excitatory inputs in the CNS and compartmentalize calcium. Although the mechanisms of calcium influx into spines have been explored, it is unknown what determines the calcium decay kinetics in spines. With two-photon microscopy we investigate action potential-induced calcium dynamics in spines from rat CA1 pyramidal neurons in slices. The [Ca(2+)](i) in most spines shows two decay kinetics: an initial fast component, during which [Ca(2+)](i) in spines decays to dendritic levels, followed by a slower decay phase in which the spine follows dendritic kinetics. The correlation between [Ca(2+)](i) in spine and dendrite at the breakpoint of the decay kinetics demonstrates diffusional equilibration between spine and dendrite during the slower component. To explain the faster initial decay, we rule out saturation or kinetic effects of endogenous or exogenous buffers and focus instead on (1) active calcium extrusion and (2) buffered diffusion of calcium from spine to dendrite. The presence of an undershoot in most spines indicates that extrusion mechanisms can be intrinsic to the spine. Supporting the two mechanisms, pharmacological blockade of smooth endoplasmic reticulum calcium (SERCA) pumps and the length of the spine neck affect spine decay kinetics. Using a mathematical model, we find that the contribution of calcium pumps and diffusion varies from spine to spine. We conclude that dendritic spines have calcium pumps and that their density and kinetics, together with the morphology of the spine neck, determine the time during which the spine compartmentalizes calcium.

Selective decrease of mRNAs encoding plasma membrane calcium pump isoforms 2 and 3 in rat kidney

BACKGROUND

Although the existence of multiple isoforms of plasma membrane calcium ATPase (PMCA) is now well documented, their biological functions are not yet known. In this study, we set out to investigate the potential role of PMCA isoforms, previously identified in renal cortical tissue, in tubular reabsorption of calcium (Ca2+).

METHODS

With use of reverse transcription-polymerase chain reaction analysis, we determined levels of mRNAs encoding isoforms of PMCA1 through PMCA4 in renal cortex, liver, and brain of rats with hypercalciuria induced by feeding with a low-phosphate diet (LPD) as compared with Ca2+-retaining rats that were fed a high-phosphate diet (HPD).

RESULTS

We observed that in hypercalciuric LPD-fed rats, the mRNAs encoding isoforms PMCA2b and PMCA3(a + c) are significantly lower (Delta approximately-50%) than in HPD-fed hypocalciuric rats, whereas no changes in mRNAs encoding isoforms PMCA1b and PMCA4 were observed, and mRNA encoding calbindin 28 kDa was increased. On the other hand, the content of mRNAs encoding PMCA2b and PMCA3(a + c) in liver and brain, respectively, was not changed.

CONCLUSION

These findings are evidence that expression of PMCA isoforms in the kidney can be selectively modulated in response to pathophysiologic stimuli. The association of a decrease in mRNA encoding PMCA2b and PMCA3(a + c) with hypercalciuria suggests that the two PMCA isoforms may be operant in tubular reabsorption of Ca2+ and its regulation.

Developmental expression of the four plasma membrane calcium ATPase (Pmca) genes in the mouse

The plasma membrane calcium ATPases are critical components in the regulation of cellular calcium homeostasis and signaling. In mammals, there are 4 Pmca genes, and information on the cellular and tissue distribution of their expression during development will provide insight into the regulation and possible function of each Pmca isoform. Using specific probes and in situ hybridization, we found that the four Pmca genes are expressed in spatially overlapping but distinct patterns in the mouse embryo. The dynamic temporal patterns of expression indicate that the individual isoforms are subject to both positive and negative regulation. The differential and restricted expression of Pmca genes supports the notion that they play unique functional roles in mammalian development.

Identification of a calmodulin-regulated Ca2+-ATPase in the endoplasmic reticulum

A unique subfamily of calmodulin-dependent Ca2+-ATPases was recently identified in plants. In contrast to the most closely related pumps in animals, plasma membrane-type Ca2+-ATPases, members of this new subfamily are distinguished by a calmodulin-regulated autoinhibitor located at the N-terminal instead of a C-terminal end. In addition, at least some isoforms appear to reside in non-plasma membrane locations. To begin delineating their functions, we investigated the subcellular localization of isoform ACA2p (Arabidopsis Ca2+-ATPase, isoform 2 protein) in Arabidopsis. Here we provide evidence that ACA2p resides in the endoplasmic reticulum (ER). In buoyant density sucrose gradients performed with and without Mg2+, ACA2p cofractionated with an ER membrane marker and a typical "ER-type" Ca2+-ATPase, ACA3p/ECA1p. To visualize its subcellular localization, ACA2p was tagged with a green fluorescence protein at its C terminus (ACA2-GFPp) and expressed in transgenic Arabidopsis. We collected fluorescence images from live root cells using confocal and computational optical-sectioning microscopy. ACA2-GFPp appeared as a fluorescent reticulum, consistent with an ER location. In addition, we observed strong fluorescence around the nuclei of mature epidermal cells, which is consistent with the hypothesis that ACA2p may also function in the nuclear envelope. An ER location makes ACA2p distinct from all other calmodulin-regulated pumps identified in plants or animals.

The plasma membrane calcium pump, its role and regulation: new complexities and possibilities

Significant progress has been achieved in elucidating the role of the plasma membrane Ca2(+)-ATPase in cellular Ca2+ homeostasis and physiology since the enzyme was first purified and physiology since the enzyme was first purified and cloned a number of years ago. The simple notion that the PM Ca2(+)-ATPase controls resting levels of [Ca2+]CYT has been challenged by the complexity arising from the finding of four major isoforms and splice variants of the Ca2+ pump, and the finding that these are differentially localized in various organs and subcellular regions. Furthermore, the isoforms exhibit differential sensitivities to Ca2+, calmodulin, ATP, and kinase-mediated phosphorylation. The latter pathways of regulation can give rise to activation or inhibition of the Ca2+ pump activity, depending on the kinase and the particular Ca2+ pump isoform. Significant progress is being made in elucidating subtle and more profound roles of the PM Ca2(+)-ATPase in the control of cellular function. Further understanding of these roles awaits new studies in both transfected cells and intact organelles, a process that will be greatly aided by the development of new and selective Ca2+ pump inhibitors.

The endoplasmic reticulum Ca2+ store: a view from the lumen

The endoplasmic reticulum (ER) and its specialized subcompartments such as the sarcoplasmic reticulum, is the main dynamic Ca2+ storage compartment of the cell. Key cellular functions are regulated, either directly or indirectly, by the free Ca2+ concentration in the ER. This article discusses the properties of Ca2+ storage in the ER and considers the functions that appear to be regulated by the Ca2+ stores within the ER, both in and around the ER and at a distance from it.