Rapid turnover of calcium in the endoplasmic reticulum during signaling. Studies with cameleon calcium indicators

In vivo imaging of the dynamics of glucose uptake in the cytosol of COS-7 cells by fluorescent nanosensors

Glucose homeostasis is a function of glucose supply, transport across the plasma membrane, and metabolism. To monitor glucose dynamics in individual cells, a glucose nanosensor was developed by flanking the Escherichia coli periplasmic glucose/galactose-binding protein with two different green fluorescent protein variants. Upon binding of substrate the FLIPglu-170n sensor showed a concentration-dependent decrease in fluorescence resonance energy transfer between the attached chromophores with a binding affinity for glucose of 170 nm. Fluorescence resonance energy transfer measurements with different sugars indicated a broad selectivity for monosaccharides. An affinity mutant with a Kd of approximately 600 microM was generated, which showed higher substrate specificity, and thus allowed specific monitoring of reversible glucose dynamics in COS-7 cells in the physiological range. At external glucose concentrations between 0.5 and 10 mM, reflecting typical blood levels, free cytosolic glucose concentrations remained at approximately 50% of external levels. The removal of glucose lead to reduced glucose levels in the cell, demonstrating reversibility and visualizing homeostasis. Glucose levels dropped even in the presence of the transport inhibitor cytochalasin B, indicating rapid metabolism. Consistently, the addition of 2-deoxyglucose, which is not recognized by the sensor, affects glucose uptake and metabolism rates. Within the physiological range, glucose utilization, i.e. hexokinase activity, was not limiting. Furthermore, the results show that in COS-7 cells, cytosolic glucose concentrations can vary over at least two orders of magnitude. The glucose nanosensor provides a novel tool with numerous scientific, medical, and environmental applications.

Cell spreading controls endoplasmic and nuclear calcium: a physical gene regulation pathway from the cell surface to the nucleus

Cells attaching to an extracellular matrix through integrins flatten out (spread) on the matrix, eliciting cellular signals needed for survival. We show that the shape of the nucleus changes and the nuclear calcium level increases in spreading cells. Moreover, cell spreading and osmotic stretching of isolated nuclei cause release of perinuclear Ca(2+), and patch clamping of nuclei reveals stretch-activated Ca(2+) permeable channels. Gene expression assays with myocyte enhancer factor 2, which is activated by calmodulin-dependent kinase IV, indicate that the elevation in nuclear Ca(2+) is functionally significant. We propose a mechano-transduction pathway in which spreading-induced nuclear stretching releases Ca(2+) from the perinuclear space, and the resulting Ca(2+) elevation in the nucleus provokes changes in gene expression.

Identification of a new SERCA2 splice variant regulated during monocytic differentiation

Sarco/endoplasmic reticulum-type calcium transport ATPases (SERCA enzymes) pump calcium ions from the cytosol into the endoplasmic reticulum. We report that in addition to the ubiquitously expressed SERCA2b isoform, a new splice variant of SERCA2 can be detected (SERCA2c) that arises from the inclusion of a short intronic sequence located between exons 20 and 21 of the SERCA2a isoform. Sequence analysis revealed classical splice donor and acceptor sites, as well as a branch-point site. Due to the presence in the new exon of an in-frame stop codon that is preceded by a 17 bp coding sequence, this mRNA potentially codes for a protein with a truncated C-terminus containing a short unique C-terminal peptide stretch. SERCA2c message was detected in epithelial, mesenchymal, and hematopoietic cell lines, as well as in primary human monocytes. Moreover, we found that during monocytic differentiation total SERCA2 ATPase expression is induced on the protein and mRNA level and that the novel SERCA2c messenger is also up-regulated during this process. These data indicate that the alternative splicing pattern of the 3(') region of the SERCA2 primary transcript is more complex than that previously thought and that this enzyme may be involved in the process of monocyte differentiation.

Monitoring of free calcium in the neuronal endoplasmic reticulum: an overview of modern approaches

The concentration of free calcium within the lumen of the endoplasmic reticulum ([Ca2+]L) fluctuates between 100 and 1000 microM. High [Ca2+]L provides an electro-driving force for Ca2+ release and supports high Ca2+ diffusion rate within the endoplasmic reticulum lumen. Fluctuations in [Ca2+]L also regulate numerous chaperones, responsible for postranslational protein processing. Thus, [Ca2+]L integrates various signalling events and establishes a link between fast signalling, associated with the endoplasmic reticulum Ca2+release/uptake, and long-lasting adaptive responses relying primarily on the regulation of protein synthesis. This paper overviews modern approaches for the direct monitoring of [Ca2+]L which rely on three classes of low-affinity Ca2+ probes: ER-targeted aequorin, synthetic fluorescent Ca2+ dyes and GFP-based ER-targeted Ca2+ probes. These techniques, especially as applied to neurones, may substantially widen our appreciation of the endoplasmic reticulum as a universal signalling organelle.

The endoplasmic reticulum and neuronal calcium signalling

The endoplasmic reticulum (ER) is a multifunctional signalling organelle regulating a wide range of neuronal functional responses. The ER is intimately involved in intracellular Ca(2+) signalling, producing local or global cytosolic calcium fluctuations via Ca(2+)-induced Ca(2+) release (CICR) or inositol-1,4,5-trisphosphate-induced Ca(2+) release (IICR). The CICR and IICR are controlled by two subsets of Ca(2+) release channels residing in the ER membrane, the Ca(2+)-gated Ca(2+) release channels, generally known as ryanodine receptors (RyRs) and InsP(3)-gated Ca(2+) release channels, referred to as InsP(3)-receptors (InsP(3)Rs). Both types of Ca(2+) release channels are expressed abundantly in nerve cells and their activation triggers cytoplasmic Ca(2+) signals important for synaptic transmission and plasticity. The RyRs and InsP(3)Rs show heterogeneous localisation in distinct cellular sub-compartments, conferring thus specificity in local Ca(2+) signals. At the same time, the ER Ca(2+) store emerges as a single interconnected pool fenced by the endomembrane. The continuity of the ER Ca(2+) store could play an important role in various aspects of neuronal signalling. For example, Ca(2+) ions may diffuse within the ER lumen with comparative ease, endowing this organelle with the capacity for "Ca(2+) tunnelling". Thus, continuous intra-ER Ca(2+) highways may be very important for the rapid replenishment of parts of the pool subjected to excessive stimulation (e.g. in small compartments within dendritic spines), the facilitated removal of localised Ca(2+) loads, and finally in conveying Ca(2+) signals from the site of entry towards the cell interior and nucleus.

Expression of endomembrane calcium pumps in colon and gastric cancer cells. Induction of SERCA3 expression during differentiation

Calcium mobilization from the endoplasmic reticulum (ER) into the cytosol is a key component of several signaling networks controlling tumor cell growth, differentiation, or apoptosis. Sarco/endoplasmic reticulum calcium transport ATPases (SERCA-type calcium pumps), enzymes that accumulate calcium in the ER, play an important role in these phenomena. We report that SERCA3 expression is significantly reduced or lost in colon carcinomas when compared with normal colonic epithelial cells, which express this enzyme at a high level. To study the involvement of SERCA enzymes in differentiation, in this work differentiation of colon and gastric cancer cell lines was initiated, and the change in the expression of SERCA isoenzymes as well as intracellular calcium levels were investigated. Treatment of the tumor cells with butyrate or other established differentiation inducing agents resulted in a marked and specific induction of the expression of SERCA3, whereas the expression of the ubiquitous SERCA2 enzymes did not change significantly or was reduced. A similar marked increase in SERCA3 expression was found during spontaneous differentiation of post-confluent Caco-2 cells, and this closely correlated with the induction of other known markers of differentiation. Analysis of the expression of the SERCA3 alternative splice isoforms revealed induction of all three known iso-SERCA3 variants (3a, 3b, and 3c). Butyrate treatment of the KATO-III gastric cancer cells led to higher resting cytosolic calcium concentrations and, in accordance with the lower calcium affinity of SERCA3, to diminished ER calcium content. These data taken together indicate a defect in SERCA3 expression in colon cancers as compared with normal colonic epithelium, show that the calcium homeostasis of the endoplasmic reticulum may be remodeled during cellular differentiation, and indicate that SERCA3 constitutes an interesting new differentiation marker that may prove useful for the analysis of the phenotype of gastrointestinal adenocarcinomas.

The endoplasmic reticulum as an integrating signalling organelle: from neuronal signalling to neuronal death

The endoplasmic reticulum is one of the largest intracellular organelles represented by continuous network of cisternae and tubules, which occupies the substantial part of neuronal somatas and extends into finest neuronal processes. The endoplasmic reticulum controls protein synthesis as well as their post-translational processing, and generates variety of nucleus-targeted signals through Ca(2+)-binding chaperones. The normal functioning of the endoplasmic reticulum signalling cascades requires high concentrations of free calcium ions within the endoplasmic reticulum lumen ([Ca(2+)](L)), and severe alterations in [Ca(2+)](L) trigger endoplasmic reticulum stress response, manifested by either unfolded protein response (UPR) or endoplasmic reticulum overload response (EOR). At the same time, the endoplasmic reticulum is critically involved in fast neuronal signalling, by producing local or global cytosolic calcium signals via Ca(2+)-induced Ca(2+) release (CICR) or inositol-1,4,5-trisphosphate-induced Ca(2+) release (IICR). Both CICR and IICR are important for synaptic transmission and synaptic plasticity. Several special techniques allowing real-time [Ca(2+)](L) monitoring were developed recently. Video-imaging of [Ca(2+)](L) in neurones demonstrates that physiological signalling triggers minor decreases in overall intraluminal Ca(2+) concentration due to strong activation of Ca(2+) uptake, which prevents severe [Ca(2+)](L) alterations. The endoplasmic reticulum lumen also serves as a "tunnel" which allows rapid transport of Ca(2+) ions within highly polarised nerve cells. Fluctuations of intraluminal free Ca(2+) concentration represent a universal mechanism, which integrates physiological cellular signalling with protein synthesis and processing. In pathological conditions, fluctuations in [Ca(2+)](L) may initiate either adaptive or fatal stress responses.

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.

Dynamic imaging of endoplasmic reticulum Ca2+ concentration in insulin-secreting MIN6 Cells using recombinant targeted cameleons: roles of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA)-2 and ryanodine receptors

The endoplasmic reticulum (ER) plays a pivotal role in the regulation of cytosolic Ca(2+) concentrations ([Ca(2+)](cyt)) and hence in insulin secretion from pancreatic beta-cells. However, the molecular mechanisms involved in both the uptake and release of Ca(2+) from the ER are only partially defined in these cells, and the presence and regulation of ER ryanodine receptors are a matter of particular controversy. To monitor Ca(2+) fluxes across the ER membrane in single live MIN6 beta-cells, we have imaged changes in the ER intralumenal free Ca(2+) concentration ([Ca(2+)](ER)) using ER-targeted cameleons. Resting [Ca(2+)](ER) (approximately 250 micromol/l) was markedly reduced after suppression (by approximately 40%) of the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)-2b but not the SERCA3 isoform by microinjection of antisense oligonucleotides, implicating SERCA2b as the principle ER Ca(2+)-ATPase in this cell type. Nutrient secretagogues that elevated [Ca(2+)](cyt) also increased [Ca(2+)](ER), an effect most marked at the cell periphery, whereas inositol 1,4,5-trisphosphate-generating agents caused a marked and homogenous lowering of [Ca(2+)](ER). Demonstrating the likely presence of ryanodine receptors (RyRs), caffeine and 4-chloro-3-ethylphenol both caused an almost complete emptying of ER Ca(2+) and marked increases in [Ca(2+)](cyt). Furthermore, photolysis of caged cyclic ADP ribose increased [Ca(2+)](cyt), and this effect was largely abolished by emptying ER/Golgi stores with thapsigargin. Expression of RyR protein in living MIN6, INS-1, and primary mouse beta-cells was also confirmed by the specific binding of cell-permeate BODIPY TR-X ryanodine. RyR channels are likely to play an important part in the regulation of intracellular free Ca(2+) changes in the beta-cell and thus in the regulation of insulin secretion.

Combining genetics and cell biology to crack the code of plant cell calcium signaling

Plant hormones, light receptors, pathogens, and abiotic signals trigger elevations in the cytosolic calcium concentration, which mediate physiological and developmental responses. Recent studies are reviewed here that reveal how specific genetic mutations impair or modify stimulus-induced calcium elevations in plant cells. These studies provide genetic evidence for the importance of calcium as a second messenger in plant signal transduction. A fundamental question arises: How can different stimuli use the same second messenger, calcium, to mediate different responses? Recent research and models are reviewed that suggest that several important mechanisms contribute to specificity in calcium signaling in plant cells. These mechanisms include (i) activation of different calcium channels in the plasma membrane and organellar membranes, (ii) stimulus-specific calcium oscillation parameters, (iii) cell type-specific responses, and (iv) intracellular localization of calcium gradients and calcium elevations in plant cells.

Genetically encoded optical sensors of neuronal activity and cellular function

Fluorescent proteins (FPs) have been engineered to produce an optical report in response to cellular signals. FP fluorescence can be made directly sensitive to the chemical environment, via specific mutations of or around the chromophore. Alternatively, FPs can be made indirectly sensitive to cellular signals by their fusion to 'detector' proteins that respond to specific cellular signals with structural rearrangements that act on the FP to alter fluorescence. These optical sensors of membrane voltage, neurotransmitter release, and intracellular messengers, including powerful new sensors of Ca(2+), cyclic nucleotides and nitric oxide, are likely to provide new insights into the workings of cellular signals and of information processing in neural circuits.

All three splice variants of the human sarco/endoplasmic reticulum Ca2+-ATPase 3 gene are translated to proteins: a study of their co-expression in platelets and lymphoid cells

The molecular cloning of two previously unknown human sarco/endoplasmic reticulum Ca(2+)-ATPase 3 (SERCA3) 3'-end transcripts, 3b and 3c, has been recently published. Data were lacking, however, for the presence of these SERCA3 variants in different tissue or cell types at the protein level. Here we report the co-expression of three human SERCA3 protein isoforms in platelets and T lymphoid Jurkat cells. Isoform-specific polyclonal anti-peptide antibodies have been generated that recognize specifically the SERCA3a, 3b or 3c splice variants at their C-termini, and this has been confirmed by peptide-competition experiments as well. None of these antibodies cross-reacted with the housekeeping SERCA2b isoform co-expressed endogenously with SERCA3 proteins in non-muscle cells. Although all three SERCA3 isoforms could be detected in platelets, the 3a form was the most abundantly expressed species. Its size matched the apparent size of SERCA3a over-expressed in HEK-293 cells. Immunoprecipitation of the SERCA3 variants from platelet membranes using a PL/IM 430-affinity matrix provided evidence that the putative pan-anti-SERCA3 antibody, PL/IM 430, recognizes all SERCA3 protein isoforms. The epitope for the PL/IM 430 antibody could be localized in a 40 kDa N-terminal tryptic fragment common to all three SERCA3 variants. Comparative Western-blot analysis showed that the expression level of the SERCA3a, 3b and 3c isoforms was more than 10 times lower in Jurkat cells than in platelets, whereas expression of the ubiquitous SERCA2b was nearly identical. This work highlights new Ca(2+)-transporting proteins of haematopoietic cells and provides specific antibodies for their detection.

Recent developments in non-excitable cell calcium entry

Influx of calcium into cells following stimulation of cell surface receptors is a key process controlling cellular activity. However, despite intensive research, there is still no consensus on precisely how calcium entry is controlled in electrically no n-excitable cells. In particular, the regulation of depletion-activated or 'capacitative' calcium entry continues to be a focus of debate. Work published in the last 2 years has lent new impetus to the so-called 'conformational coupling' theory, although evidence for the existence of soluble messengers between the ER and the plasma membrane also continues to appear. In addition, there remains disagreement on whether intra-store [Ca(2+)] has to fall below a threshold before Ca(2+)entry is activated. A further major question is the identity of the putative depletion-operated Ca(2+)channel or channels. Here discussion has largely focussed on whether homologue(s) of the Drosophila TRP ('Transient Receptor Potential') protein is/are the elusive channel, or at least a part of it. Finally, it remains possible that Ca(2+)entry mechanisms other than depletion-activated channels may be important in agonist-evoked Ca(2+)influx. This commentary summarizes recent developments in the field, and highlights both current debates and critical unsolved questions.

Mitochondria recycle Ca(2+) to the endoplasmic reticulum and prevent the depletion of neighboring endoplasmic reticulum regions

To study Ca(2+) fluxes between mitochondria and the endoplasmic reticulum (ER), we used "cameleon" indicators targeted to the cytosol, the ER lumen, and the mitochondrial matrix. High affinity mitochondrial probes saturated in approximately 20% of mitochondria during histamine stimulation of HeLa cells, whereas a low affinity probe reported averaged peak values of 106 +/- 5 microm, indicating that Ca(2+) transients reach high levels in a fraction of mitochondria. In concurrent ER measurements, [Ca(2+)](ER) averaged 371 +/- 21 microm at rest and decreased to 133 +/- 14 microm and 59 +/- 5 microm upon stimulation with histamine and thapsigargin, respectively, indicating that substantial ER refilling occur during agonist stimulation. A larger ER depletion was observed when mitochondrial Ca(2+) uptake was prevented by oligomycin and rotenone or when Ca(2+) efflux from mitochondria was blocked by CGP 37157, indicating that some of the Ca(2+) taken up by mitochondria is re-used for ER refilling. Accordingly, ER regions close to mitochondria released less Ca(2+) than ER regions lacking mitochondria. The ER heterogeneity was abolished by thapsigargin, oligomycin/rotenone, or CGP 37157, indicating that mitochondrial Ca(2+) uptake locally modulate ER refilling. These observations indicate that some mitochondria are very close to the sites of Ca(2+) release and recycle a substantial portion of the captured Ca(2+) back to vicinal ER domains. The distance between the two organelles thus determines both the amplitude of mitochondrial Ca(2+) signals and the filling state of neighboring ER regions.

Glycoprotein folding in the endoplasmic reticulum

Our understanding of eukaryotic protein folding in the endoplasmic reticulum has increased enormously over the last 5 years. In this review, we summarize some of the major research themes that have captivated researchers in this field during the last years of the 20th century. We follow the path of a typical protein as it emerges from the ribosome and enters the reticular environment. While many of these events are shared between different polypeptide chains, we highlight some of the numerous differences between proteins, between cell types, and between the chaperones utilized by different ER glycoproteins. Finally, we consider the likely advances in this field as the new century unfolds and we address the prospect of a unified understanding of how protein folding, degradation, and translation are coordinated within a cell.

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.

A high signal-to-noise Ca(2+) probe composed of a single green fluorescent protein

Recently, several groups have developed green fluorescent protein (GFP)-based Ca(2+) probes. When applied in cells, however, these probes are difficult to use because of a low signal-to-noise ratio. Here we report the development of a high-affinity Ca(2+) probe composed of a single GFP (named G-CaMP). G-CaMP showed an apparent K(d) for Ca(2+) of 235 nM. Association kinetics of Ca(2+) binding were faster at higher Ca(2+) concentrations, with time constants decreasing from 230 ms at 0.2 microM Ca(2+) to 2.5 ms at 1 microM Ca(2+). Dissociation kinetics (tau approximately 200 ms) are independent of Ca(2+) concentrations. In HEK-293 cells and mouse myotubes expressing G-CaMP, large fluorescent changes were observed in response to application of drugs or electrical stimulations. G-CaMP will be a useful tool for visualizing intracellular Ca2+ in living cells. Mutational analysis, together with previous structural information, suggests the residues that may alter the fluorescence of GFP.