Calcineurin controls the transcription of Na+/Ca2+ exchanger isoforms in developing cerebellar neurons

The expression of the PDZ protein MALS-1/velis is regulated by calcium and calcineurin in cerebellar granule cells

Activity-dependent gene expression is thought to be important in shaping neuronal development and in modifying the protein content of neurons. Ca(2+) entry into neurons appears to be one of the key effectors of activity-dependent gene expression. Among the possible downstream targets of calcium, the protein phosphatase calcineurin represents a prime candidate. We hereby report that in cultured cerebellar granule cells the activation of the Ca(2+)/calcineurin pathway via either voltage- or ligand- operated Ca(2+) channels regulates MALS-1 and MALS-2 expression at the transcriptional level. These proteins are integral parts of the post-synaptic density and are also involved in receptor trafficking. MALS regulation is not at the level of mRNA stability and does not require de novo protein synthesis, thereby suggesting a direct pathway. These data suggest that Ca(2+) entry by means of calcineurin is capable of controlling the structure of the post-synaptic density by controlling the expression of key components at the transcriptional level.

A study of the activity of the plasma membrane Na/Ca exchanger in the cellular environment

The Na/Ca exchanger is a large-capacity system that is supposed to be predominant in the ejection of calcium in excitable tissues. Its activity is normally evaluated in electro-physiological experiments or using Na-loaded cells or plasma membrane vesicles in experiments in which the activity is estimated by measuring the uptake of isotopic calcium. Here the activity of the exchanger has been evaluated in model cells that coexpress the exchanger together with the (protein) Ca indicator aequorin targeted to the different subcellular compartments. Exchanger isoform 1 (NCX1) has been used for experiments in which the effect of its overexpression has been evaluated on the Ca homeostasis in the cytoplasm at large, in the endoplasmic reticulum, in mitochondria, and in the layer of cytoplasm underneath the plasma membrane. The experiments have shown that, despite its very low calcium affinity, the exchanger was active in cells at rest in which calcium is around 100 nM. They have also shown that expressed NCX1 significantly reduced the cytosolic and mitochondrial calcium transients produced by the emptying of the ER calcium stores with InsP3-linked agonists. The effect of the expressed NCX1 on the calcium homeostasis in the domain underneath the plasma membrane was more complex: The exchanger apparently catalyzed a reverse operation, leading to increased penetration of calcium.

Brain distribution of the Na+/Ca2+ exchanger-encoding genes NCX1, NCX2, and NCX3 and their related proteins in the central nervous system

In the central nervous system, the Na(+)/Ca(2+) exchanger plays a fundamental role in controlling changes in the intracellular concentrations of Na(+) and Ca(2+) ions that occur in physiologic conditions such as neurotransmitter release, cell migration and differentiation, gene expression, as well as neuro-degenerative processes. Three genes, NCX1, NCX2, and NCX3, encoding for Na(+)/Ca(2+) exchanger isoforms have been cloned. In this review, by using non-radioactive in situ hybridization and light immunohistochemistry with NCX isoform-specific riboprobes and antibodies, respectively, a systematic brain mapping for both transcripts and proteins encoded by all three NCX genes is described. Intense expression of NCX transcripts and proteins was detected in the cerebral cortex, hippocampus, thalamus, metathalamus, hypothalamus, brainstem, spinal cord, and cerebellum. In these areas, NCX transcripts and proteins were often found with an overlapping distribution pattern, although specific brain areas displaying a peculiar expression of each exchanger isoform were also found. Furthermore, immunoelectron and confocal microscopy revealed the expression of the NCX1 isoform of the exchanger at both pre- and postsynaptic sites as well as in association with membranes of the endoplasmic reticulum. Collectively, these data suggest that the different isoforms of the Na(+)/Ca(2+) exchanger appear to be selectively expressed in several CNS regions where they might underlie different functional roles.

Recombinant expression of the plasma membrane Na(+)/Ca(2+) exchanger affects local and global Ca(2+) homeostasis in Chinese hamster ovary cells

The cardiac type Na(+)/Ca(2+) exchanger (NCX1) has been transiently expressed in Chinese hamster ovary cells, which do not contain an endogenous exchanger, together with aequorin chimeras that are targeted to different intracellular compartments to investigate intracellular Ca(2+) homeostasis. The expression of NCX decreased the endoplasmic reticulum Ca(2+) concentration, [Ca(2+)](er), in resting cells, showing that the exchanger was operative under these conditions. It induced a greater reduction in the height of the mitochondrial and cytosolic Ca(2+) transients in agonist-stimulated cells than would have been expected from the [Ca(2+)](er) decrease. It also had a major effect on the sub-plasma membrane Ca(2+) concentration, [Ca(2+)](pm): after a transient [Ca(2+)](pm) rise induced by the activation of capacitative Ca(2+) influx, [Ca(2+)](pm) settled to a value about 3-fold higher than in controls. The sustained [Ca(2+)](pm) increase after the transient was due to the operation of the exchanger, either directly by operating in the Ca(2+) entry mode, or indirectly by removing the Ca(2+) inhibition on the capacitative Ca(2+) influx channels.

Distinct N-terminal regulatory domains of Ca(2+)/H(+) antiporters

The regulation of intracellular Ca(2+) levels is achieved in part by high-capacity vacuolar Ca(2+)/H(+) antiporters. An N-terminal regulatory region (NRR) on the Arabidopsis Ca(2+)/H(+) antiporter CAX1 (cation exchanger 1) has been shown previously to regulate Ca(2+) transport by a mechanism of N-terminal auto-inhibition. Here, we examine the regulation of other CAX transporters, both within Arabidopsis and from another plant, mung bean (Vigna radiata), to ascertain if this mechanism is commonly used among Ca(2+)/H(+) antiporters. Biochemical analysis of mung bean VCAX1 expressed in yeast (Saccharomyces cerevisiae) showed that N-terminal truncated VCAX1 had approximately 70% greater antiport activity compared with full-length VCAX1. A synthetic peptide corresponding to the NRR of CAX1, which can strongly inhibit Ca(2+) transport by CAX1, could not dramatically inhibit Ca(2+) transport by truncated VCAX1. The N terminus of Arabidopsis CAX3 was also shown to contain an NRR. Additions of either the CAX3 or VCAX1 regulatory regions to the N terminus of an N-terminal truncated CAX1 failed to inhibit CAX1 activity. When fused to N-terminal truncated CAX1, both the CAX3 and VCAX1 regulatory regions could only auto-inhibit CAX1 after mutagenesis of specific amino acids within this NRR region. These findings demonstrate that N-terminal regulation is present in other plant CAX transporters, and suggest distinct regulatory features among these transporters.

Cell-specific expression of plasma membrane calcium ATPase isoforms in retinal neurons

Ca(2+) extrusion by high-affinity plasma membrane calcium ATPases (PMCAs) is a principal mechanism for the clearance of Ca(2+) from the cytosol. The PMCA family consists of four isoforms (PMCA1-4). Little is known about the selective expression of these isoforms in brain tissues or about the physiological function conferred upon neurons by any given isoform. We investigated the cellular and subcellular distribution of PMCA isoforms in a mammalian retina. Mouse photoreceptors, cone bipolar cells and horizontal cells, which respond to light with a graded polarization, express isoform 1 (PMCA1) of the PMCA family. PMCA2 is localized to rod bipolar cells, horizontal cells, amacrine cells, and ganglion cells, and PMCA3 is predominantly expressed in spiking neurons, including both amacrine and ganglion cells but is also found in horizontal cells. PMCA4 was found to be selectively expressed in both synaptic layers. Optical measurements of Ca(2+) clearance showed that PMCAs mediate Ca(2+) extrusion in both rod and cone bipolar cells. In addition, we found that rod bipolar cells, but not cone bipolar cells possess a prominent Na(+)/Ca(2+) exchange mechanism. We conclude that PMCA isoforms are selectively expressed in retinal neurons and that processes of Ca(2+) clearance are different in rod and cone bipolar cells.

Na(+)/Ca(2+) exchanger expression in the developing rat cortex

The Na(+)/Ca(2+) exchanger (NCX) participates in the regulation of neuronal Ca(2+) homeostasis and is also believed to be involved in the neuronal responses to hypoxia. However, there are very limited data on how NCX mRNA and protein expression are regulated during brain development. In the present study, we sought to elucidate the developmental expression of NCX1 and NCX2 in the rat cortex from late fetal to adult stages using reverse transcription-polymerase chain reaction and western blot assays. The primers for NCX1 mRNA targeted the alternative splicing domain to allow differentiation between NCX1 splice variants. Our results show that: (1) only two NCX1 mRNA splice variants (NCX1.5 and NCX1.4) are present in the cortex and their expression is age-dependent; (2) total NCX1 mRNA levels are low in fetal tissue, reach maximum density at postnatal day 8 and substantially decline with further maturation; (3) NCX2 mRNA density is significantly greater than total NCX1 mRNA for all ages and increases markedly during maturation from fetus/neonate to adult; and (4) NCX1 protein expression is lowest in late fetal cortex and reaches maximum levels after 2 weeks postnatally, even though expression levels are not significantly different between newborn and adult animals. Also, we found a similar NCX1 protein trend in the subcortical and cerebellar regions during development. From these data we suggest that NCX1 and NCX2 are differentially expressed in the cortex with a predominance of NCX2 levels during postnatal development. We speculate that the developmental increase in NCX2 expression is responsible for the overall increase in Na(+)/Ca(2+) exchange capacity during maturation.

Preferential expression of plasmalemmal K-dependent Na+/Ca2+ exchangers in neurons versus astrocytes

Numerous isoforms of plasmalemmal K-dependent (NCKX) and K-independent (NCX) Na+/Ca2+ exchangers are expressed in the brain. The physiological functions of each isoform are presently unknown. Therefore, in this study, we compared expression of NCKX and NCX transcripts between primary cultures of cerebellar granule cells, and astrocytes. Northern blot analysis showed that granule cells expressed NCKX2, NCKX3, NCKX4 and NCX3, whereas astrocytes expressed primarily NCX1. Consistent with this molecular characterization, a significant fraction of 45Ca2+ accumulation in Na-loaded granule cells, but not in astrocytes, depended on external K+. This is the first demonstration of native NCKX activity in neurons derived from the central nervous system. Our data suggest that NCKX isoform expression may correspond to the unique Ca2+ homeostasis requirements of neurons.

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.

Instrumental role of Na+ in NMDA excitotoxicity in glucose-deprived and depolarized cerebellar granule cells

In glucose-deprived cerebellar granule cells, substitution of extracellular Na+ with Li+ or Cs+ prevented N-methyl-D-aspartate (NMDA)-induced excitotoxicity. NMDA stimulated 45Ca2+ accumulation and ATP depletion in a Na-dependent manner, and caused neuronal death, even if applied while Na,K-ATPase was inhibited by 1 mM ouabain. The cells treated with NMDA in the presence of ouabain accumulated sizable 45Ca2+ load but most of them failed to elevate cytosolic [Ca2+] upon mitochondrial depolarization. Na/Ca exchange inhibitor, KB-R7943, inhibited Na-dependent and NMDA-induced 45Ca2+ accumulation but only if Na,K-ATPase activity was compromised by ouabain. In cells energized by glucose and exposed to NMDA without ouabain, KB-R7943 reduced NMDA-elicited ionic currents by 19% but failed to inhibit 45Ca2+ accumulation. It appears that a large part of NMDA-induced Ca2+ influx in depolarized and glucose-deprived cells is mediated by reverse Na/Ca exchange. A high level of reverse Na/Ca exchange operation is maintained by a sustained Na+ influx via NMDA channels and depolarization of the plasma membrane. In cells energized by glucose, however, most Ca2+ enters directly via NMDA channels because Na,K-ATPase regenerating Na+ and K+ concentration gradients prevents Na/Ca exchange reversal. Since under these conditions Na/Ca exchange extrudes Ca2+, its inhibition destabilizes Ca2+ homeostasis.

Calcium signaling: a tale for all seasons

An experiment performed in London nearly 120 years ago, which by today's standards would be considered unacceptably sloppy, marked the beginning of the calcium (Ca(2+)) signaling saga. Sidney Ringer [Ringer, S. (1883) J. Physiol. 4, 29-43] was studying the contraction of isolated rat hearts. In earlier experiments, Ringer had suspended them in a saline medium for which he admitted to having used London tap water, which is hard: The hearts contracted beautifully. When he proceeded to replace the tap water with distilled water, he made a startling finding: The beating of the hearts became progressively weaker, and stopped altogether after about 20 min. To maintain contraction, he found it necessary to add Ca(2+) salts to the suspension medium. Thus, Ringer had serendipitously discovered that Ca(2+), hitherto exclusively considered as a structural element, was active in a tissue that has nothing to do with bone or teeth, and performed there a completely novel function: It carried the signal that initiated heart contraction. It was a landmark observation, which should have immediately aroused wide interest. Unexpectedly, however, for decades it attracted no particular attention. Occasionally, farsighted pioneers argued forcefully for a messenger role of Ca(2+), offering compelling experimental evidence. Among them, one could quote L. V. Heilbrunn [Heilbrunn, L. V. (1940) Physiol. Zool. 13, 88-94], who contracted frog muscle fibers by applying Ca(2+) salts to their cut ends, but not to their surfaces. Heilbrunn correctly concluded that Ca(2+) had diffused from the cut ends to the internal contractile elements to elicit their contraction. One could also quote K. Bailey [Bailey, K. (1942) Biochem. J. 36, 121-139], who showed that the ATPase activity of myosin was strongly activated by Ca(2+) (but not by Mg(2+)), and concluded that the liberation of Ca(2+) in the neighborhood of the myosin controlled muscle contraction. Clearly, enough evidence was there, but only a handful of people had the vision to see it and to foresee its far-reaching implications. Perhaps no better example of clairvoyance can be offered than the quip by O. Loewy in 1959: "Ja Kalzium, das ist alles!"

Calcium and oxidative stress: from cell signaling to cell death

Reactive oxygen and nitrogen species can be used as a messengers in normal cell functions. However, at oxidative stress levels they can disrupt normal physiological pathways and cause cell death. Such a switch is largely mediated through Ca(2+) signaling. Oxidative stress causes Ca(2+) influx into the cytoplasm from the extracellular environment and from the endoplasmic reticulum or sarcoplasmic reticulum (ER/SR) through the cell membrane and the ER/SR channels, respectively. Rising Ca(2+) concentration in the cytoplasm causes Ca(2+) influx into mitochondria and nuclei. In mitochondria Ca(2+) accelerates and disrupts normal metabolism leading to cell death. In nuclei Ca(2+) modulates gene transcription and nucleases that control cell apoptosis. Both in nuclei and cytoplasm Ca(2+) can regulate phosphorylation/dephosphorylation of proteins and can modulate signal transduction pathways as a result. Since oxidative stress is associated with many diseases and the aging process, understanding how oxidants alter Ca(2+) signaling can help to understand process of aging and disease, and may lead to new strategies for their prevention.

Transport activity and surface expression of the Na+-Ca2+ exchanger NCX1 are inhibited by the immunosuppressive agent cyclosporin A and by the nonimmunosuppressive agent PSC833

Cyclosporin A (CsA) treatment of HEK 293 cells expressing the rat heart RHE-1 (NCX1.1, EMBL accession number ) or the rat brain RBE-2 (NCX1.5, GenBank(TM) accession number ) Na(+)-Ca(2+) exchanger inhibited their transport activity in a concentration-dependent manner. The inhibition was detectable at 2 microm CsA, and exposure of the cells to 20 microm CsA resulted in a decrease of the Na(+)-dependent Ca(2+) uptake to about 20% relative to that of untreated cells. Determination of the surface expression of the exchanger protein revealed a parallel concentration-dependent reduction in the amount of the immunoreactive protein. No reduction was detected in the amount of total immunoreactive exchanger protein in CsA-treated cells relative to untreated ones. Among the different drugs tested, only PSC833, an analog of cyclosporin D, mimicked the effects of CsA. Exposure of the transfected cells to the chemically related cyclosporin D and macrolide drugs (FK506 or rapamycin) had no effect on the transport activity or the surface expression of the Na(+)-Ca(2+) exchanger. Co-expression of the human multidrug transporter P-glycoprotein (of which both drugs are modulators) with the cloned Na(+)-Ca(2+) exchanger revealed that transport activity and surface expression of each transporter in the co-transfected system were similar to those of each transporter alone in both the presence and absence of CsA or PSC833. CsA and PSC833 inhibited the surface expression of the NCX1 protein but did not alter the surface expression of P-glycoprotein. Unlike some P-glycoprotein endoplasmic reticulum-retained mutants (Loo, T. W., and Clarke, D. M. (1997) J. Biol. Chem. 272, 709-712), CsA did not rescue RBE-2/F913-->Stop, an endoplasmic reticulum-retained function-competent mutant of the Na(+)-Ca(2+) exchanger (Kasir, J., Ren, X., Furman, I., and Rahamimoff, H. (1999) J. Biol. Chem. 274, 24873-24880) and did not induce its kinesis to the surface membrane, further demonstrating molecular differences between P-glycoprotein and NCX1 mutants for interaction with CsA.

Nups, Kaps, NXFs and others: making sense in the Babel of nuclear transport

The spatio-temporal aspects of signal transduction, and the molecular basis of nucleocytoplasmic transport, have attracted much attention recently. How they might be linked was the theme of a recent conference*. Many aspects of nuclear transport were discussed and novel results presented, either in talks or posters, by the approximately 80 participants. (*EMBO workshop on 'Signal-transduction-mediated regulation of nuclear transport'; 11-14 August 2001; Strasbourg, France. Organized by A.N. Malviya.)

Na+-Ca2+ exchanger remodeling in pressure overload cardiac hypertrophy

Perturbations of Ca(2+) metabolism are central to the pathogenesis of cardiac hypertrophy. The electrogenic Na(+)-Ca(2+) exchanger mediates a substantial component of transmembrane Ca(2+) movement in cardiac myocytes and is up-regulated in heart failure. However, the role of the exchanger in the pathogenesis of cardiac hypertrophy is poorly understood. Thoracic aortic banding in mice induced 50-60% increases in heart mass and cardiomyocyte size. Despite the absence of myocardial dysfunction, steady-state NCX1 transcript and protein levels were increased to an extent similar to that reported in heart failure. As recent studies indicate that calcineurin is critical to the expression of Na(+)-Ca(2+) exchanger genes, we inhibited calcineurin with cyclosporin. Calcineurin inhibition blunted the increases in NCX1 transcript and protein levels and eliminated the increases in heart mass and cell volume normally associated with pressure overload. To examine the functional significance of these changes, we measured Na(+)-Ca(2+) exchanger current in two independent ways. Surprisingly, exchanger current density was decreased in hypertrophied myocytes, and this down-regulation was eliminated by calcineurin inhibition. Together, these data reveal a role for Na(+)-Ca(2+) exchanger current in the electrical remodeling of hypertrophy and implicate calcineurin signaling therein. In addition, these data suggest the Na(+)-Ca(2+) exchanger is functionally regulated in hypertrophy.

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.