The Sarco(endo)plasmic Reticulum Ca2+-ATPases in the Cardiovascular System During Growth and Proliferation

An evidence for surface expression of an immunogenic epitope of sarcoplasmic/endoplasmic reticulum calcium-ATPase2a on antigen-presenting cells from naive mice in the mediation of autoimmune myocarditis

We recently reported identification of sarcoplasmic/endoplasmic reticulum calcium-ATPase2a (SERCA2a) 971-990, which induces atrial myocarditis by generating autoreactive T cells in A/J mice. However, it was unknown how antigen-sensitized T cells could recognize SERCA2a 971-990, since SERCA2a-expression is confined to an intracellular compartment. In this report, we present evidence that antigen-presenting cells (APCs) from lymphoid and non-lymphoid organs in naïve animals present SERCA2a 971-990 and stimulate antigen-specific T cells. Using major histocompatibility complex (MHC) class II dextramers for SERCA2a 971-990, we created a panel of T cell hybridomas and demonstrated that splenocytes from naïve A/J mice stimulated the hybridoma cells without exogenous supplementation of SERCA2a 971-990. We then recapitulated this phenomenon by using SERCA2a 971-990 -specific primary T cells, verifying that the T cell responses were MHC-restricted. Furthermore, SERCA2a 971-990 -sensitzed T cells exposed to APCs from naïve mice were found to produce the inflammatory cytokines interferon-γ, granulocyte macrophage colony stimulating factor, and interleukin-17A, which are implicated in the induction of myocarditis. Finally, while T cells exposed to mononuclear cells (MNCs) obtained from heart and liver also responded similarly to splenocytes, endothelial cells (ECs) generated from the corresponding organs displayed opposing effects, in that the proliferative responses were suppressed with the heart ECs, but not with the liver ECs. Taken together, our data suggest that the surface expression of SERCA2a 971-990 by naïve APCs can potentially trigger pathogenic autoreactive T cell responses under conditions of autoimmunity, which may have implications in endothelial dysfunction.

Ageing and activity: their effects on the functional reserve capacities of the heart and vascular smooth and skeletal muscles

During perinatal life striated muscles grow through the acquisition of more contractile cells (myocytes or fibres) followed by their postnatal enlargement (i.e. hypertrophy). In the ageing adult these events are reversed, with a progressive loss of myocytes that cannot be fully compensated despite the presence of cell renewal systems or reactive myocyte hypertrophy. Hence the functional reserve capacities of the heart and skeletal muscles decline with age. This is probably a consequence of physiological ageing and diminished levels of physical activity. As a result daily tasks once taken for granted become progressively more difficult, and eventually impossible, to perform. For example, sufficient coordinated absolute muscle force is required for an individual to rise from a chair or climb stairs, and the reserve capacity of the heart is a major determinant of an individual's ability to remain active and cope with daily stresses and illnesses. Long-term participation in endurance-based activities helps to preserve cardiac reserve, and has both direct and indirect beneficial effects on vascular smooth muscle and health preservation within the cardiovascular system. In contrast, this type of activity does little to protect skeletal muscles against the age-related losses of fast-twitch fibres, small motor units, overall muscle mass and power output. While resistance exercise promotes fibre hypertrophy in skeletal muscles, and to a lesser extent in myocytes of the heart, the explosive power of muscles still declines with age. Hence, while physical activity is important in attenuating age-related changes in muscle function and its reserve capacity, it delays rather than prevents the deleterious effects of ageing per se. Despite this, in a culture where inactivity has become an accepted part of life we still need to explore in greater detail the benefits of habitual physical activity, and use this information as a community-based educational tool to help prevent or delay cardiovascular disease, obesity, arthritis and the frailty associated with old age.

Interleukin-2 increases activity of sarcoplasmic reticulum Ca2+-ATPase, but decreases its sensitivity to calcium in rat cardiomyocytes

To further explore the role of interleukin-2 (IL-2) in cardiac function, we investigated its effects on the intracellular calcium transient and the activity of sarcoplasmic reticulum (SR) Ca2+-ATPase in rat cardiomyocytes. IL-2 (200 U/ml) decreased the amplitude of electrically stimulated and caffeine-induced intracellular Ca2+ transients in ventricular myocytes, but increased the end-diastolic calcium level. IL-2 did not affect the sarcolemmal L-type Ca2+ channel activity. The activity of SR Ca2+-ATPase from IL-2-treated hearts increased in a dose-dependent manner, but the sarcolemmal Ca2+-ATPase activity did not change. After incubation of SR with ATP, the activity of SR Ca2+-ATPase from IL-2-treated hearts increased much more than that in the control group. The responsiveness of SR Ca2+-ATPase from IL-2-perfused hearts to the free calcium concentration was inhibited. The Ca2+ uptake and Ca2+ content were reduced in the SR vesicles prepared from IL-2-treated rat heart. Pretreatment with the kappa-opioid receptor antagonist nor-binaltorphimine (10 nM) attenuated the effect of IL-2 on the SR Ca2+-ATPase activity, SR Ca2+ uptake, and Ca2+ content. The activity of Ca2+-ATPase in SR isolated from untreated hearts did not change when IL-2 and SR were coincubated. Thus, we conclude that the decreased calcium transient induced by IL-2 results from reduced SR calcium release, which is due to decreased SR Ca2+ uptake mediated by cardiac kappa-opioid receptors, but not from reduced activity of the sarcolemmal L-type calcium channel.

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

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

Deleterious effects of nifedipine on smooth muscle cells implies alterations of intracellular calcium signaling

Nifedipine (NIF), a calcium channel blocker (CCB) from the first generation of dihydropyridines, induces detrimental effects on patients with cardiovascular diseases. We designed experiments to study, at cellular and molecular level, the mechanisms involved in the induction of deleterious effects by this drug. To this purpose, cultured human smooth muscle cells (HSMC) were used. The effect of NIF and two other CCB (FEL, AML) and inhibitors of intracellular signaling pathways (RR, TG, CAF and GEN) on intracellular calcium [Ca(2+)]I was determined by spectrofluorimetry using Fura 2 AM assay. The results showed that: (i) 10 microM NIF induced the increase of [Ca(2+)]I above the basal values (202.77 +/- 23.98 nM vs. 48.68 +/- 6.45 nM), an effect that was prevented by RR (50.45 +/- 13.9 nM) and was not induced by the two other CCB; (ii) NIF had a thapsigargin-like effect, because it induced the same release of intracellular calcium as TG (212.1 +/- 25.62 nM); (iii) The response to NIF was reduced by 40% after the inhibition of IP3 receptor (121.21 +/- 26.01 nM) and by 50% after the inhibition of tyrosine kinase (101.91 +/- 7.76 nM). Together, these data demonstrate that NIF produces a deregulation of intracellular calcium homeostasis. The abnormal increase of [Ca(2+)]I is due to the activation of store operated channels from the plasma membrane responsible for capacitative calcium entry, a process modulated by the activity of tyrosine kinase and the Ca(2+)-ATPase pump from the sarcoplasmic reticulum.

Clinical implications of physiological changes in the aging heart

Elderly individuals experience a disproportionate burden from cardiovascular disease. Global changes in aging will have a significant impact on the future of medical practice. However, most physicians have little formal training in geriatric medicine and sometimes fail to distinguish disease states from normal aging. Increasingly, it is recognised that a sedentary lifestyle may be responsible for a large fraction of the so-called 'age-related' changes in the cardiovascular system. Nonetheless, well characterised changes do occur in most individuals with aging. Loss of myocytes with subsequent hypertrophy of the remaining cells is usually observed. Calcification involving the conduction and valvular apparatus is seen in most elderly individuals and may predispose to the common arrhythmias of old age. Age-related loss of arterial compliance contributes to isolated systolic hypertension and left ventricular hypertrophy. Despite these changes, for the majority of healthy older adults, cardiac output is well maintained in the basal state through use of the Frank-Starling principle, in the setting of reduced early diastolic filling. Myocardial relaxation is slowed in part due to age-related changes in the sarcoplasmic reticulum Ca2+ ATPase pump. Elevated blood levels of catecholamines contribute to desensitisation to noradrenergic stimulation and this is associated with an age-related decline in maximum achievable heart rate. Changes in the baroreceptor reflex function and decreased sodium conservation may predispose some individuals to orthostatic and postprandial hypotension. The aetiology of cardiovascular aging is under intense study. The most likely mechanisms involve the result of cumulative damage mediated through a variety of insults. Oxidative stress, non-enzymatic glycation, inflammation and changes in cardiovascular gene expression all seem to influence cardiovascular aging. The benefits of exercise continue to be discovered. Endurance-type training has been shown to have a dramatic impact on parameters of cardiovascular aging. Favourable effects are seen in maximum oxygen consumption, diastolic filling, relaxation and arterial stiffness. Some changes such as the maximum heart rate response do not appear to change with conditioning. Pharmacotherapy may afford the opportunity to influence the aging process. Drugs that can reduce age-associated arterial stiffness, cardiac fibrosis and ventricular hypertrophy should prove useful. Antioxidants continue to be a topic of great interest and require more study. Despite some well described changes with aging, most elderly individuals maintain the opportunity for improved cardiovascular function through conditioning. Early recognition and treatment of diseases that are distinguishable from normal aging, including hypertension and atherosclerosis, together with preventive efforts, should reduce the predicted trends in cardiovascular morbidity and mortality among the aged.

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.

Intracellular Ca(2+) handling in vascular smooth muscle cells is affected by proliferation

Despite intensive interest in the dedifferentiation process of vascular smooth muscle cells, very little data are available on intracellular Ca(2+) signaling. The present study was designed to investigate the evolution of the intracellular Ca(2+) pools when rat aortic smooth muscle cells (RASMCs) proliferate and to define the mechanisms involved in the functional alterations. RASMCs were cultured in different conditions, and [Ca(2+)](i) was measured by use of fura 2. Expression of the sarco(endo)plasmic reticulum Ca(2+) pumps (SERCA2a and SERCA2b), Ca(2+) channels, the ryanodine receptor (RyR), and the inositol trisphosphate receptor (IP3R) was studied by reverse transcription-polymerase chain reaction and immunofluorescence. Antibodies specific for myosin heavy chain isoforms were used as indicators of the differentiation state of the cell, whereas an anti-proliferating cell nuclear antigen antibody was a marker of proliferation. SERCA2a, SERCA2b, RyR3, and IP3R-1 mainly were present in the aorta in situ and in freshly isolated RASMCs. These cells used the 2 types of Ca(2+) channels to release Ca(2+) from a common thapsigargin-sensitive store. Proliferation of RASMCs, induced by serum or by platelet-derived growth factor-BB, resulted in the disappearance of RyR and SERCA2a mRNAs and proteins and in the loss of the caffeine- and ryanodine-sensitive pool. The differentiated nonproliferative phenotype was maintained in low serum or in cells cultured at high density. In these conditions, RyR and SERCA2a were also present in RASMCs. Thus, expression of RyR and SERCA2a is repressed by cell proliferation, inducing loss of the corresponding Ca(2+) pool. In arterial smooth muscle, Ca(2+) release through RyRs is involved in vasodilation, and suppression of the ryanodine-sensitive pool might thus alter the control of vascular tone.

The expression of SR calcium transport ATPase and the Na(+)/Ca(2+)Exchanger are antithetically regulated during mouse cardiac development and in Hypo/hyperthyroidism

The mouse has been used extensively for generating transgenic animal models to study cardiovascular disease. Recently, a number of transgenic mouse models have been created to investigate the importance of sarcoplasmic reticulum (SR) Ca(2+)transport proteins in cardiac pathophysiology. However, the expression and regulation of cardiac SR Ca(2+)ATPase and other Ca(2+)transport proteins have not been studied in detail in the mouse. In this study, we used multiplex RNase mapping analysis to determine SERCA2, phospholamban (PLB), and Na(+)/Ca(2+)-exchanger (NCX-1) gene expression throughout mouse heart development and in hypo/hyperthyroid animals. Our results demonstrate that the expression of SERCA2 and PLB mRNA increase eight-fold from fetal to adult stages, indicating that SR function increases with heart development. In contrast, the expression of the Na(+)/Ca(2+)-exchanger gene is two-fold higher in fetal heart compared to adult. Our study also makes the important observation that in hypothyroidic hearts the NCX-1 mRNA and protein levels were upregulated, whereas the SERCA2 mRNA/protein levels were downregulated. In hyperthyroidic hearts, however, an opposite response was identified. These findings are important and point out that the expression of NCX-1 is regulated antithetically to that of SERCA2 during heart development and in response to alterations in thyroid hormone levels.

Platelet Ca(2+)ATPases : a plural, species-specific, and multiple hypertension-regulated expression system

Gaining insight into nonmuscle Ca(2+) signaling requires basic knowledge of the major structures involved. We investigated the expression of platelet Ca(2+)ATPases in normal and hypertension-associated abnormal Ca(2+) signaling. First, overall identification of normotensive Wistar-Kyoto rat Ca(2+)ATPases was attempted by looking for newly described human platelet 3'-end alternatively spliced sarco/endoplasmic reticulum Ca(2+)ATPases (SERCA) 3b mRNA and plasma membrane Ca(2+)ATPase (PMCA) 1b and 4b proteins, in addition to SERCA2b and SERCA3a isoforms. For SERCAs, comparative analyses of human and Wistar-Kyoto rat SERCA3 platelet mRNA by reverse transcription-polymerase chain reaction (RT-PCR) followed by sequencing established that human platelets coexpressed SERCA3b and a third SERCA3c, while rat cells were devoid of them but expressed a still unknown splice variant that we termed rSERCA3b/3c. Its identification using 3'-end SERCA3 gene and rapid amplification of cDNA ends (RACE)-PCR studies showed that it results from an additional SERCA3 alternative splicing process, which uses a second alternative polyadenylation site located in the last intron. For PMCAs, with the use of gene-specific RT-PCR followed by sequencing and Western blotting using 5F10 monoclonal antibody, expression of human and rat platelet PMCA1b and PMCA4b was similar. Second, comparative analysis of these newly identified Ca(2+)ATPases and SERCA3a in age-matched spontaneously hypertensive rat platelets demonstrated (1) a marked downregulation of rSERCA3b/3c, which became null, and a 1.71-fold increase in SERCA3a and (2) an opposite regulation of the 2 PMCAs, namely, a 3.3-fold decrease in PMCA1b mRNA and a 3.7-fold increase in PMCA4b mRNA. Hence, platelets coexpress multiple, diverse, and species-specific Ca(2+)ATPases, including a novel fourth SERCA3. Moreover, expression of PMCA (1b and 4b), SERCA3a, and rSERCA3b/3c was modulated in rat hypertension. Hence, Ca(2+)ATPases should be regarded as constituting a new rational basis for the understanding of nonmuscle cell Ca(2+) signaling.

Study of calcium signaling in non-excitable cells

The fundamental importance of calcium signaling in the control of cellular physiology is widely recognized. A dramatic illustration of this is the fact that a Medline search for review articles containing the word "calcium" in the title reveals 4,629 hits, whereas the whole body of calcium signaling literature (approximately 2 x 10(6) pages) is more than enough to fill a decent-sized library. Most of this literature deals with calcium signaling in excitable cells types (mainly neurons and muscle cells), but non-excitable cell types are capable of calcium signaling as well. Although calcium fluxes in the latter cell types have attracted much less interest, the literature involved is still vast. Nevertheless, in this review article we hope to contribute some valuable insights to the field. First we shall discuss the experimental techniques available to the researcher interested in calcium signaling in non-excitable cell types with special attention to patch clamp electrophysiology. Subsequently, we shall review some of the results obtained with these techniques by focussing on the calcium-regulating mechanisms in non-excitable cells and discussing the importance of these mechanisms for physiology.

Pregnant rat myometrial cells show heterogeneous ryanodine- and caffeine-sensitive calcium stores

Intracellular Ca(2+) release channels such as ryanodine receptors play crucial roles in the Ca(2+)-mediated signaling that triggers excitation-contraction coupling in muscles. Although the existence and the role of these channels are well characterized in skeletal and cardiac muscles, their existence in smooth muscles, and more particularly in the myometrium, is very controversial. We have now clearly demonstrated the expression of ryanodine receptor Ca(2+) release channels in rat myometrial smooth muscle, and for the first time, intracellular Ca(2+) concentration experiments with indo 1 on single myometrial cells have revealed the existence of a functional ryanodine- and caffeine-sensitive Ca(2+) release mechanism in 30% of rat myometrial cells. RT-PCR and RNase protection assay on whole myometrial smooth muscle demonstrate the existence of all three ryr mRNAs in the myometrium: ryr3 mRNA is the predominant subtype, with much lower levels of expression for ryr1 and ryr2 mRNAs, suggesting that the ryanodine Ca(2+) release mechanism in rat myometrium is largely encoded by ryr3. Moreover, using intracellular Ca(2+) concentration measurements and RNase protection assays, we have demonstrated that the expression, the percentage of cells responding to ryanodine, and the function of these channels are not modified during pregnancy.

Characterization of the 3' end of the mouse SERCA 3 gene and tissue distribution of mRNA spliced variants

The sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) type 1 and 2 genes are alternatively spliced at their 3' end. We hypothesized that similar mechanism may occur for SERCA 3. Two spliced variants were identified by RNase protection analysis. We then isolated and sequenced the 3' end portion of the mouse SERCA 3 gene, and confirmed the presence of an alternative mRNA transcript by sequencing a cDNA fragment obtained by RT-PCR. Tissue distribution of the alternatively spliced mRNAs was studied by RT-PCR: SERCA 3b was the only isoform expressed in endothelial cells from aorta and heart and also was the major isoform in lung and kidney whereas SERCA 3a and 3b were coexpressed in trachea, intestine, thymus, spleen, and fetal liver.