Calcium-mediated cellular signals: a story of failures

Modeling Calcium Signaling in S. cerevisiae Highlights the Role and Regulation of the Calmodulin-Calcineurin Pathway in Response to Hypotonic Shock

Calcium homeostasis and signaling processes in Saccharomyces cerevisiae, as well as in any eukaryotic organism, depend on various transporters and channels located on both the plasma and intracellular membranes. The activity of these proteins is regulated by a number of feedback mechanisms that act through the calmodulin-calcineurin pathway. When exposed to hypotonic shock (HTS), yeast cells respond with an increased cytosolic calcium transient, which seems to be conditioned by the opening of stretch-activated channels. To better understand the role of each channel and transporter involved in the generation and recovery of the calcium transient—and of their feedback regulations—we defined and analyzed a mathematical model of the calcium signaling response to HTS in yeast cells. The model was validated by comparing the simulation outcomes with calcium concentration variations before and during the HTS response, which were observed experimentally in both wild-type and mutant strains. Our results show that calcium normally enters the cell through the High Affinity Calcium influx System and mechanosensitive channels. The increase of the plasma membrane tension, caused by HTS, boosts the opening probability of mechanosensitive channels. This event causes a sudden calcium pulse that is rapidly dissipated by the activity of the vacuolar transporter Pmc1. According to model simulations, the role of another vacuolar transporter, Vcx1, is instead marginal, unless calcineurin is inhibited or removed. Our results also suggest that the mechanosensitive channels are subject to a calcium-dependent feedback inhibition, possibly involving calmodulin. Noteworthy, the model predictions are in accordance with literature results concerning some aspects of calcium homeostasis and signaling that were not specifically addressed within the model itself, suggesting that it actually depicts all the main cellular components and interactions that constitute the HTS calcium pathway, and thus can correctly reproduce the shaping of the calcium signature by calmodulin- and calcineurin-dependent complex regulations. The model predictions also allowed to provide an interpretation of different regulatory schemes involved in calcium handling in both wild-type and mutants yeast strains. The model could be easily extended to represent different calcium signals in other eukaryotic cells.

L-theanine prevents progression of nonalcoholic hepatic steatosis by regulating hepatocyte lipid metabolic pathways via the CaMKKβ-AMPK signaling pathway

Background

L-theanine, a non-protein amino acid was found principally in the green tea, has been previously shown to exhibit potent anti-obesity property and hepatoprotective effect. Herein, we investigated the effects of L-theanine on alleviating nonalcoholic hepatic steatosis in vitro and in vivo, and explored the underlying molecular mechanism.

Methods

In vitro, HepG2 and AML12 cells were treated with 500 μM oleic acid (OA) or treated with OA accompanied by L-theanine. In vivo, C57BL/6J mice were fed with normal control diet (NCD), high‐fat diet (HFD), or HFD along with L-theanine for 16 weeks. The levels of triglycerides (TG), accumulation of lipid droplets and the expression of genes related to hepatocyte lipid metabolic pathways were detected in vitro and in vivo.

Results

Our data indicated that, in vivo, L-theanine significantly reduced body weight, hepatic steatosis, serum levels of alanine transaminase (ALT), aspartate transaminase (AST), TG and LDL cholesterol (LDL-C) in HFD-induced nonalcoholic fatty liver disease (NAFLD) mice. In vitro, L-theanine also significantly alleviated OA induced hepatocytes steatosis. Mechanic studies showed that L-theanine significantly inhibited the nucleus translocation of sterol regulatory element binding protein 1c (SREBP-1c) through AMPK-mTOR signaling pathway, thereby contributing to the reduction of fatty acid synthesis. We also identified that L-theanine enhanced fatty acid β-oxidation by increasing the expression of peroxisome proliferator–activated receptor α (PPARα) and carnitine palmitoyltransferase-1 A (CPT1A) through AMP-activated protein kinase (AMPK). Furthermore, our study indicated that L-theanine can active AMPK through its upstream kinase Calmodulin-dependent protein kinase kinase-β (CaMKKβ).

Conclusions

Taken together, our findings suggested that L-theanine alleviates nonalcoholic hepatic steatosis by regulating hepatocyte lipid metabolic pathways via the CaMKKβ-AMPK signaling pathway.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12986-022-00664-6.

Molecular Dysfunctions of Mitochondria-Associated Endoplasmic Reticulum Contacts in Atherosclerosis

Atherosclerosis is a chronic lipid-driven inflammatory disease that results in the formation of lipid-rich and immune cell-rich plaques in the arterial wall, which has high morbidity and mortality in the world. The mechanism of atherosclerosis is still unclear now. Potential hypotheses involved in atherosclerosis are chronic inflammation theory, lipid percolation theory, mononuclear-macrophage theory, endothelial cell (EC) injury theory, and smooth muscle cell (SMC) mutation theory. Changes of phospholipids, glucose, critical proteins, etc. on mitochondria-associated endoplasmic reticulum membrane (MAM) can cause the progress of atherosclerosis. This review describes the structural and functional interaction between mitochondria and endoplasmic reticulum (ER) and explains the role of critical molecules in the structure of MAM during atherosclerosis.

Foci of Segmentally Contracted Sarcomeres in Trapezius Muscle Biopsy Specimens in Myalgic and Nonmyalgic Human Subjects: Preliminary Results

OBJECTIVE

The myofascial trigger point hypothesis postulates that there are small foci of contracted sarcomeres in resting skeletal muscle. Only one example, in canine muscle, has been published previously. This study evaluated human muscle biopsies for foci of contracted sarcomeres.

SETTING

The Departments of Rehabilitation Sciences and Physiotherapy at Ghent University, Ghent, Belgium.

SUBJECTS

Biopsies from 28 women with or without trapezius myalgia were evaluated, 14 in each group.

METHODS

Muscle biopsies were obtained from regions of taut bands in the trapezius muscle and processed for light and electron microscopy and for histochemical analysis. Examination of the biopsies was blinded as to group.

RESULTS

A small number of foci of segmentally contracted sarcomeres were identified. One fusiform segmental locus involved the entire muscle fiber in tissue from a myalgic subject. Several transition zones from normal to contracted sarcomeres were found in both myalgic and nonmyalgic subjects. The distance between Z-lines in contracted sarcomeres was about 25-45% of the same distance in normal sarcomeres. Z-lines were disrupted and smeared in the contracted sarcomeres.

CONCLUSIONS

A small number of foci of segmentally contracted sarcomeres were found in relaxed trapezius muscle in human subjects, a confirmation of the only other example of spontaneous segmental contraction of sarcomeres (in a canine muscle specimen), consistent with the hypothesis of trigger point formation and with the presence of trigger point end plate noise.

Calcium Hypothesis of Alzheimer's disease and brain aging: A framework for integrating new evidence into a comprehensive theory of pathogenesis

This article updates the Calcium Hypothesis of Alzheimer's disease and brain aging on the basis of emerging evidence since 1994 (The present article, with the subtitle "New evidence for a central role of Ca²⁺ in neurodegeneration," includes three appendices that provide context and further explanations for the rationale for the revisions in the updated hypothesis-the three appendices are as follows: Appendix I "Emerging concepts on potential pathogenic roles of [Ca²⁺]," Appendix II "Future studies to validate the central role of dysregulated [Ca²⁺] in neurodegeneration," and Appendix III "Epilogue: towards a comprehensive hypothesis.") (Marx J. Fresh evidence points to an old suspect: calcium. Science 2007; 318:384-385). The aim is not only to re-evaluate the original key claims of the hypothesis with a critical eye but also to identify gaps in knowledge required to validate relevant claims and delineate additional studies and/or data that are needed. Some of the key challenges for this effort included examination of questions regarding (1) the temporal and spatial relationships of molecular mechanisms that regulate neuronal calcium ion (Ca²⁺), (2) the role of changes in concentration of calcium ion [Ca²⁺] in various subcellular compartments of neurons, (3) how alterations in Ca²⁺ signaling affect the performance of neurons under various conditions, ranging from optimal functioning in a healthy state to conditions of decline and deterioration in performance during aging and in disease, and (4) new ideas about the contributions of aging, genetic, and environmental factors to the causal relationships between dysregulation of [Ca²⁺] and the functioning of neurons (see Appendices I and II). The updated Calcium Hypothesis also includes revised postulates that are intended to promote further crucial experiments to confirm or reject the various predictions of the hypothesis (see Appendix III).

Sildenafil (Viagra(®)) prevents and restores LPS-induced inflammation in astrocytes

Astrocytes are effectively involved in the pathophysiological processes in the central nervous system (CNS) and may contribute to or protect against development of inflammatory and degenerative diseases. Sildenafil is a potent and selective phosphodiesterase-5 (PDE-5) inhibitor, which induces cyclic GMP accumulation. However, the mechanisms of actions on glial cells are not clear. The aim of the present work is to evaluate the role of sildenafil in lipopolysaccharide (LPS)-stimulated astrocytes. The cytoskeleton integrity and Ca(2+) waves were assessed as indicators of inflammatory state. Two main groups were done: (A) one prevention and (B) one restoration. Each of these groups: A1: control; A2: LPS for 24h; A3: sildenafil 1 or 10μM for 4h and then sildenafil 1 or 10μM+LPS for 24h. B1: control; B2: LPS for 24h; B3: LPS for 24h and then LPS+sildenafil 1 or 10μM for 24h. Cytoskeleton integrity was analyzed through GFAP immunolabeling and actin labeling with an Alexa 488-conjugated phalloidin probe. Calcium responses were assessed through a Ca(2+)-sensitive fluorophore Fura-2/AM. The results show that both preventive and restorative treatments with sildenafil (in both concentrations) reduced the Ca(2+) responses in intensity and induced a more organized actin fiber pattern, compared to LPS treated cells. This work demonstrated for the first time that astrocytes are a key part of the sildenafil protective effects in the CNS.

Amarcord: I remember

I have tried to offer a historical account of a success story, as I saw it develop from the early times when it interested only a few aficionados to the present times when it has pervaded most of cell biochemistry and physiology. It is of course the story of calcium signaling. It became my topic of work when I was a young postdoctoral fellow at The Johns Hopkins University. I entered it through a side door, that of mitochondria, which had been my area of work during my earlier days in Italy. The 1960s and 1970s were glorious times for mitochondrial calcium signaling, but the golden period was not going to last. As I have discussed below, mitochondrial calcium gradually lost appeal, entering a long period of oblivion. Its fading happened as the general area of calcium signaling was instead experiencing a phase of explosive growth, with landmark discoveries at the molecular and cellular levels. These discoveries established that calcium signaling was one of the most important areas of cell biology. However, mitochondria as calcium partners were not dead; they were only dormant. In the 1990s, they were rescued from their state of neglect to the central position of the regulation of cellular calcium signaling, which they had once rightly occupied. Meanwhile, it had also become clear that calcium is an ambivalent messenger. Hardly anything important occurs in cells without the participation of the calcium message, but calcium must be controlled with absolute precision. This is an imperative necessity, which becomes unfortunately impaired in a number of disease conditions that transform calcium into a messenger of death.

The plasma membrane calcium pump in health and disease

The Ca(2+) ATPases of the plasma membrane (PMCA pumps) export Ca(2+) from all eukaryotic cells. In mammals they are the products of four separate genes. PMCA types 1 and 4 are distributed ubiquitously; PMCA types 2 and 3 are restricted to some tissues, the most important being the nervous system. Alternative splicing at two sites greatly increases the number of pump isoforms. The two ubiquitous isoforms are no longer considered as only housekeeping pumps as they also perform tissue-specific functions. The PMCAs are classical P-type pumps, their reaction cycle repeating that of all other pumps of the family. Their 3D structure has not been solved, but molecular modeling on SERCA pump templates shows the essential structural pattern of the latter. PMCAs are regulated by calmodulin, which interacts with high affinity with their cytosolic C-terminal tail. A second calmodulin-binding domain with lower affinity is present in some splicing variants of the pump. The PMCAs are essential to the regulation of cellular Ca(2+), but the all-important Ca(2+) signal is ambivalent: defects in its control generate various pathologies, the most thoroughly studied being those of genetic origin. Genetic defects of PMCA function produce disease phenotypes: the best characterized is a form of deafness in mice and in humans linked to PMCA2 mutations. A cerebellar X-linked human ataxia has recently been found to be caused by a mutation in the calmodulin-binding domain of PMCA3.

Fluorescence lifetime readouts of Troponin-C-based calcium FRET sensors: a quantitative comparison of CFP and mTFP1 as donor fluorophores

We have compared the performance of two Troponin-C-based calcium FRET sensors using fluorescence lifetime read-outs. The first sensor, TN-L15, consists of a Troponin-C fragment inserted between CFP and Citrine while the second sensor, called mTFP-TnC-Cit, was realized by replacing CFP in TN-L15 with monomeric Teal Fluorescent Protein (mTFP1). Using cytosol preparations of transiently transfected mammalian cells, we have measured the fluorescence decay profiles of these sensors at controlled concentrations of calcium using time-correlated single photon counting. These data were fitted to discrete exponential decay models using global analysis to determine the FRET efficiency, fraction of donor molecules undergoing FRET and calcium affinity of these sensors. We have also studied the decay profiles of the donor fluorescent proteins alone and determined the sensitivity of the donor lifetime to temperature and emission wavelength. Live-cell fluorescence lifetime imaging (FLIM) of HEK293T cells expressing each of these sensors was also undertaken. We confirmed that donor fluorescence of mTFP-TnC-Cit fits well to a two-component decay model, while the TN-L15 lifetime data was best fitted to a constrained four-component model, which was supported by phasor analysis of the measured lifetime data. If the constrained global fitting is employed, the TN-L15 sensor can provide a larger dynamic range of lifetime readout than the mTFP-TnC-Cit sensor but the CFP donor is significantly more sensitive to changes in temperature and emission wavelength compared to mTFP and, while the mTFP-TnC-Cit solution phase data broadly agreed with measurements in live cells, this was not the case for the TN-L15 sensor. Our titration experiment also indicates that a similar precision in determination of calcium concentration can be achieved with both FRET biosensors when fitting a single exponential donor fluorescence decay model to the fluorescence decay profiles. We therefore suggest that mTFP-based probes are more suitable for FLIM experiments than CFP-based probes.

Abnormal intracellular calcium homeostasis in sympathetic neurons from young prehypertensive rats

Hypertension is associated with cardiac noradrenergic hyperactivity, although it is not clear whether this precedes or follows the development of hypertension itself. We hypothesized that Ca(2+) homeostasis in postganglionic sympathetic neurons is impaired in spontaneously hypertensive rats (SHRs) and may occur before the development of hypertension. The depolarization-induced rise in intracellular free calcium concentration ([Ca(2+)](i); measured using fura-2-acetoxymethyl ester) was significantly larger in cultured sympathetic neurons from prehypertensive SHRs than in age matched normotensive Wistar-Kyoto rats. The decay of the [Ca(2+)](i) transient was also faster in SHRs. The endoplasmic reticulum Ca(2+) content and caffeine-induced [Ca(2+)](i) amplitude were significantly greater in the young SHRs. Lower protein levels of phospholamban and more copies of ryanodine receptor mRNA were also observed in the young SHRs. Depleting the endoplasmic reticulum Ca(2+) store did not alter the difference of the evoked [Ca(2+)](i) transient and decay time between young SHRs and Wistar-Kyoto rats. However, removing mitochondrial Ca(2+) buffering abolished these differences. A lower mitochondrial membrane potential was also observed in young SHR sympathetic neurons. This resulted in impaired mitochondrial Ca(2+) uptake and release, which might partly be responsible for the increased [Ca(2+)](i) transient and faster decay in SHR sympathetic neurons. This Ca(2+) phenotype seen in early development in cardiac stellate and superior cervical ganglion neurons may contribute to the sympathetic hyperresponsiveness that precedes the onset of hypertension.

Ca(2+) signaling: an outlook on the characterization of Ca(2+) channels and their importance in cellular functions

Calcium (Ca(2+)) is essential in regulating a plethora of cellular functions that includes cell proliferation and differentiation, axonal guidance and cell migration, neuro/enzyme secretion and exocytosis, development/maintenance of neural circuits, cell death and many more. Since Ca(2+) regulates so many fundamental processes, it could be anticipated that numerous Ca(2+) channels and transporters will assist in regulating Ca(2+) entry across the plasma membrane. Towards this several Ca(2+) channels such as voltage-gated channels, store-operated Ca(2+) entry (SOCE) channels, NMDA, AMPA and other ligand gated channels have been identified. In recent years research focus has been targeted towards identification of the precise function of these essential channels. Furthermore, characterization of these individual Ca(2+) channels has also gained much attention, since specific Ca(2+) channels have been shown to influence a particular cellular response. Moreover, perturbations in these Ca(2+) channels have also been implicated in a spectrum of pathological conditions. Hence, understanding the precise involvement of these Ca(2+) channels in disease conditions would presumably unveil avenues for plausible therapeutic interventions. We thus review the role of Ca(2+) signaling in select -disease conditions and also provide experimental evidence as how they can be characterized in a given cell.

Mitochondrial permeability transition in Ca(2+)-dependent apoptosis and necrosis

A variety of stimuli utilize an increase of cytosolic free Ca(2+) concentration as a second messenger to transmit signals, through Ca(2+) release from the endoplasmic reticulum or opening of plasma membrane Ca(2+) channels. Mitochondria contribute to the tight spatiotemporal control of this process by accumulating Ca(2+), thus shaping the return of cytosolic Ca(2+) to resting levels. The rise of mitochondrial matrix free Ca(2+) concentration stimulates oxidative metabolism; yet, in the presence of a variety of sensitizing factors of pathophysiological relevance, the matrix Ca(2+) increase can also lead to opening of the permeability transition pore (PTP), a high conductance inner membrane channel. While transient openings may serve the purpose of providing a fast Ca(2+) release mechanism, persistent PTP opening is followed by deregulated release of matrix Ca(2+), termination of oxidative phosphorylation, matrix swelling with inner membrane unfolding and eventually outer membrane rupture with release of apoptogenic proteins and cell death. Thus, a rise in mitochondrial Ca(2+) can convey both apoptotic and necrotic death signals by inducing opening of the PTP. Understanding the signalling networks that govern changes in mitochondrial free Ca(2+) concentration, their interplay with Ca(2+) signalling in other subcellular compartments, and regulation of PTP has important implications in the fine comprehension of the main biological routines of the cell and in disease pathogenesis.

Genome-wide meta-analysis for serum calcium identifies significantly associated SNPs near the calcium-sensing receptor (CASR) gene

Calcium has a pivotal role in biological functions, and serum calcium levels have been associated with numerous disorders of bone and mineral metabolism, as well as with cardiovascular mortality. Here we report results from a genome-wide association study of serum calcium, integrating data from four independent cohorts including a total of 12,865 individuals of European and Indian Asian descent. Our meta-analysis shows that serum calcium is associated with SNPs in or near the calcium-sensing receptor (CASR) gene on 3q13. The top hit with a p-value of 6.3×10^-37 is rs1801725, a missense variant, explaining 1.26% of the variance in serum calcium. This SNP had the strongest association in individuals of European descent, while for individuals of Indian Asian descent the top hit was rs17251221 (p = 1.1×10^-21), a SNP in strong linkage disequilibrium with rs1801725. The strongest locus in CASR was shown to replicate in an independent Icelandic cohort of 4,126 individuals (p = 1.02×10^-4). This genome-wide meta-analysis shows that common CASR variants modulate serum calcium levels in the adult general population, which confirms previous results in some candidate gene studies of the CASR locus. This study highlights the key role of CASR in calcium regulation.

Calcium levels in blood serum play an important role in many biological processes. The regulation of serum calcium is under strong genetic control. This study describes the first meta-analysis of a genome-wide association study from four cohorts totaling 12,865 participants of European and Indian Asian descent. Confirming previous results in some candidate gene studies, we find that common polymorphisms at the calcium-sensing receptor (CASR) gene locus are associated with serum calcium concentrations. We show that CASR variants give rise to the strongest signals associated with serum calcium levels in both European and Indian Asian populations, while no other locus reaches genome-wide significance. Our results show that CASR is a key player in genetic regulation of serum calcium in the adult general population.

Calcium Pumps in Health and Disease

Ca2+-ATPases (pumps) are key actors in the regulation of Ca2+ in eukaryotic cells and are thus essential to the correct functioning of the cell machinery. They have high affinity for Ca2+ and can efficiently regulate it down to very low concentration levels. Two of the pumps have been known for decades (the SERCA and PMCA pumps); one (the SPCA pump) has only become known recently. Each pump is the product of a multigene family, the number of isoforms being further increased by alternative splicing of the primary transcripts. The three pumps share the basic features of the catalytic mechanism but differ in a number of properties related to tissue distribution, regulation, and role in the cellular homeostasis of Ca2+. The molecular understanding of the function of the pumps has received great impetus from the solution of the three-dimensional structure of one of them, the SERCA pump. These spectacular advances in the structure and molecular mechanism of the pumps have been accompanied by the emergence and rapid expansion of the topic of pump malfunction, which has paralleled the rapid expansion of knowledge in the topic of Ca2+-signaling dysfunction. Most of the pump defects described so far are genetic: when they are very severe, they produce gross and global disturbances of Ca2+ homeostasis that are incompatible with cell life. However, pump defects may also be of a type that produce subtler, often tissue-specific disturbances that affect individual components of the Ca2+-controlling and/or processing machinery. They do not bring cells to immediate death but seriously compromise their normal functioning.

cADPR stimulates SERCA activity in Xenopus oocytes

The intracellular second messenger cyclic ADP-ribose (cADPR) induces Ca(2+) release through the activation of ryanodine receptors (RyRs). Moreover, it has been suggested that cADPR may serve an additional role to modulate sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) pump activity, but studies have been complicated by concurrent actions on RyR. Here, we explore the actions of cADPR in Xenopus oocytes, which lack RyRs. We examined the effects of cADPR on the sequestration of cytosolic Ca(2+) following Ca(2+) transients evoked by photoreleased inositol 1,4,5-trisphosphate (InsP(3)), and by Ca(2+) influx through expressed nicotinic acetylcholine receptors (nAChR) in the oocytes membrane. In both cases the decay of the Ca(2+) transients was accelerated by intracellular injection of a non-metabolizable analogue of cADPR, 3-Deaza-cADPR, and photorelease of cADPR from a caged precursor demonstrated that this action is rapid (a few s). The acceleration was abolished by pre-treatment with thapsigargin to block SERCA activity, and was inhibited by two specific antagonists of cADPR, 8-NH(2)-cADPR and 8-br-cADPR. We conclude that cADPR serves to modulate Ca(2+) sequestration by enhancing SERCA pump activity, in addition to its well-established action on RyRs to liberate Ca(2+).

Alteration of the cortical actin cytoskeleton deregulates Ca2+ signaling, monospermic fertilization, and sperm entry

BACKGROUND

When preparing for fertilization, oocytes undergo meiotic maturation during which structural changes occur in the endoplasmic reticulum (ER) that lead to a more efficient calcium response. During meiotic maturation and subsequent fertilization, the actin cytoskeleton also undergoes dramatic restructuring. We have recently observed that rearrangements of the actin cytoskeleton induced by actin-depolymerizing agents, or by actin-binding proteins, strongly modulate intracellular calcium (Ca2+) signals during the maturation process. However, the significance of the dynamic changes in F-actin within the fertilized egg has been largely unclear.

METHODOLOGY/PRINCIPAL FINDINGS

We have measured changes in intracellular Ca2+ signals and F-actin structures during fertilization. We also report the unexpected observation that the conventional antagonist of the InsP(3) receptor, heparin, hyperpolymerizes the cortical actin cytoskeleton in postmeiotic eggs. Using heparin and other pharmacological agents that either hypo- or hyperpolymerize the cortical actin, we demonstrate that nearly all aspects of the fertilization process are profoundly affected by the dynamic restructuring of the egg cortical actin cytoskeleton.

CONCLUSIONS/SIGNIFICANCE

Our findings identify important roles for subplasmalemmal actin fibers in the process of sperm-egg interaction and in the subsequent events related to fertilization: the generation of Ca2+ signals, sperm penetration, cortical granule exocytosis, and the block to polyspermy.

Synthesis, photochromic properties, and light-controlled metal complexation of a naphthopyran derivative

A light-controlled reversible binding switch based on photochromic 3H-naphtho[2,1-b]pyran is under development for studying cellular oscillatory calcium signals. The binding affinities of the closed and open forms of substituted naphthopyran 1 for Ca(2+), Mg(2+), and Sr(2+) in buffer were determined. The photochemically ring-opened form of the receptor exhibited increased affinity compared to the thermally stable closed form of the receptor. The binding affinity difference for Ca(2+) was approximately 77-fold at pH 7.6.

Synthesis and metal complexation properties of bisbenzospiropyran chelators in water

As a step towards developing a light-controlled reversible binding switch based on photochromic bisbenzospiropyran for investigating intracellular calcium signaling, substituted bisbenzospiropyran and phenolic chelators were synthesized and examined for metal binding strength. The complexation of these compounds with alkaline earth and zinc ions in methanol and buffer was characterized using NMR and luminescence spectroscopy. An increased length of convergent ligands on the rigid scaffold maintained binding affinity for Ca(2+) but decreased selectivity for Ca(2+) over Mg(2+). Results indicate that at least three carboxylate ligands are required for significant binding, and increased length of the ligands will result in a fully water-soluble photoswitch that exhibits two states with approximately 300-fold difference in binding affinity for Ca(2+).

Calcium dynamics: analyzing the Ca2+ regulatory network in intact cells

Calcium signaling is critical for all cells. As a free ion (Ca(2+)), calcium links many physiological stimuli to their intracellular effectors by interacting with binding proteins whose occupancy determines the cellular effect of stimulation. Because binding site occupancy depends on the history of Ca(2+) concentration ([Ca(2+)]), Ca(2+) dynamics are critical. Calcium dynamics depend on the functional interplay between Ca(2+) transport and buffering systems whose activities depend nonlinearly on [Ca(2+)]. Thus, understanding Ca(2+) dynamics requires detailed information about these Ca(2+) handling systems and their regulation in intact cells. However, effective methods for measuring and characterizing intracellular Ca(2+) handling have not been available until recently. Using concepts relating voltage-gated ion-channel activity to membrane potential dynamics, we developed such methods to analyze Ca(2+) fluxes in intact cells. Here we describe this approach and applications to understanding depolarization-induced Ca(2+) responses in sympathetic neurons.

Trifluoperazine protects brain plasma membrane Ca(2+)-ATPase from oxidative damaging

In the central nervous system (CNS), a number of different pathological processes such as necrosis, Parkinson's and Alzheimer's diseases are related to disturbance in calcium homeostasis associated with oxidative stress. Here we compare the susceptibility of rat brain plasma membrane Ca(2+)-ATPase (PMCA) and sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA) isoforms to in vitro oxidative stress, and investigate a putative role of trifluoperazine (TFP), an antipsychotic drug that is also a powerful inhibitor of Ca(2+)-transporter proteins, in protecting these enzymes. It is shown that, in rat brain, PMCA is very sensitive to the damage induced by preincubation with Fe(2+)-ascorbate, or Fe(2+)-ascorbate plus H2O2, while SERCA is resistant. Inhibition of PMCA activity promoted by Fe(2+)/ascorbate medium is fully prevented by the presence of microM concentrations of either butylated hydroxytoluene (BHT) or TFP, but only partially protected, or reversed, by dithiothreitol (DTT), pointing to some protein cysteine(s) as one of the main targets for a lipid peroxidation-dependent damaging mechanism. However, when 0.5-1 mM H2O2 is added together with Fe(2+)/ascorbate, both BHT and TFP only partially prevent ATPase activity inhibition, and DTT does not confer any protection, suggesting two possible additional mechanisms involving both lipid peroxidation and direct damage to PMCA at amino acid residues other than cysteines. A possible use of micromolar concentrations of TFP as a direct antioxidant protector for PMCA under oxidative stress conditions is discussed.

Calcium and cell death

Calcium signalling system controls majority of cellular reactions. Calcium signals occurring within tightly regulated temporal and spatial domains, govern a host of Ca2(+)-dependent enzymes, which in turn determine specified cellular responses. Generation of Ca2+ signals is achieved through coordinated activity of several families of Ca2+ channels and transporters differentially distributed between intracellular compartments. Cell damage induced by environmental insults or by overstimulation of physiological pathways results in pathological Ca2+ signals, which trigger necrotic or apoptotic cellular death.

The complex regulatory function of the ligand-binding domain of the inositol 1,4,5-trisphosphate receptor

The inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R) can be divided in three functionally distinct regions: a ligand-binding domain, a modulatory domain and a channel domain. Numerous regulatory mechanisms including inter- and intra-molecular protein-protein interactions and phosphorylation events act via these domains to regulate the function of the IP(3)R. Regulation at the level of the ligand-binding domain primarily affects the affinity for IP(3). The extent of IP(3)-induced Ca(2+) release (IICR) is, however, not only determined by the affinity for IP(3) but also by the effectiveness of the coupling between ligand binding and channel opening. As a result, regulation as well as malfunction of IICR may be affected by both steps in the activation mechanism. The 3D structures of the two subdomains of the ligand-binding domain have recently been determined by X-ray diffraction analysis. This allows a more detailed molecular explanation of the regulatory events situated at the ligand-binding domain of the IP(3)R. In this review, we will focus on recent structural and functional data on the ligand-binding domain that have extended and clarified the view on the molecular mechanisms of IP(3)R regulation.

Calcium and cell death: the mitochondrial connection

Physiological stimuli causing an increase of cytosolic free Ca2+ [Ca2+], or the release of Ca2+ from the endoplasmic reticulum invariably induce mitochondrial Ca2+ uptake, with a rise of mitochondrial matrix free [Ca2+] ([Ca2+]m). The [Ca2+]m rise occurs despite the low affinity of the mitochondrial Ca2+ uptake systems measured in vitro and the often limited amplitude of the cytoplasmic [Ca2+]c increases. The [Ca2+]m increase is typically in the 0.2-3 microM range, which allows the activation of Ca2(+)-regulated enzymes of the Krebs cycle; and it rapidly returns to the resting level if the [Ca2+], rise recedes due to activation of mitochondrial efflux mechanisms and matrix Ca2+ buffering. Mitochondria thus accumulate Ca2+ and efficiently control the spatial and temporal shape of cellular Ca2+ signals, yet this situation exposes them to the hazards of Ca2+ overload. Indeed, mitochondrial Ca2+, which is so important for metabolic regulation, can become a death factor by inducing opening of the permeability transition pore (PTP), a high conductance inner membrane channel. Persistent PTP opening is followed by depolarization with Ca2+ release, cessation of oxidative phosphorylation, matrix swelling with inner'membrane remodeling and eventually outer membrane rupture with release of cytochrome c and other apoptogenic proteins. Understanding the mechanisms through which the Ca2+ signal can be shifted from a physiological signal into a pathological effector is an unresolved problem of modern pathophysiology that holds great promise for disease treatment.

Ca2+ regulation in the Na+/Ca2+ exchanger involves two markedly different Ca2+ sensors

The plasma membrane Na+/Ca2+ exchanger (NCX) is almost certainly the major Ca2+ extrusion mechanism in cardiac myocytes. Binding of Na+ and Ca2+ ions to its large cytosolic loop regulates ion transport of the exchanger. We determined the solution structures of two Ca2+ binding domains (CBD1 and CBD2) that, together with an alpha-catenin-like domain (CLD), form the regulatory exchanger loop. CBD1 and CBD2 are very similar in the Ca2+ bound state and describe the Calx-beta motif. Strikingly, in the absence of Ca2+, the upper half of CBD1 unfolds while CBD2 maintains its structural integrity. Together with a 7-fold higher affinity for Ca2+, this suggests that CBD1 is the primary Ca2+ sensor. Specific point mutations in either domain largely allow the interchange of their functionality and uncover the mechanism underlying Ca2+ sensing in NCX.

Cyclic GMP modulates store-operated calcium entry inducing phosphatidylserine translocation at the surface of megakaryocytic cells

When subjected to stimulation, cells from the vascular compartment show a spontaneous collapse of the plasma membrane phospholipid asymmetry and phosphatidylserine is exposed at the external leaflet. Thus, phosphatidylserine externalization is essential for normal hemostasis and phagocytosis. The mechanism governing the migration of phosphatidylserine to the exoplasmic leaflet is not yet fully understood. We have proposed that store-operated calcium entry (SOCE) constitutes a key step of this process. Here, interaction of [Ca(2+)](i), cAMP and cGMP pathways and phosphatidylserine exposure was examined in human megakaryocytic cells. The membrane permeable cAMP and cGMP analogues, pCPT-cAMP and pCPT-cGMP, enhanced the Ca(2+) signal induced by ionophore and SOCE. Responses to pCPT-cAMP and pCPT-cGMP were independent of protein kinase A, protein kinase G (PKG) or ERK pathways. Inhibition of small G-proteins reduced or abolished the increase of [Ca(2+)](i) induced by pCPT-cAMP or pCPT-cGMP, respectively. pCPT-cGMP but not pCPT-cAMP enhanced the ability of cells to expose phosphatidylserine. This effect was not prevented by the inhibition of PKG or small G-proteins. These results show the differential role of cyclic nucleotides in the Ca(2+)-dependent membrane remodeling. Hence, pCPT-cGMP is another regulatory element for the completion of SOCE-induced phosphatidylserine transmembrane redistribution in HEL cells through a mechanism implicating small G-proteins.

Correlation ofvisinin-like-protein-1 expression with clinicopathological features in squamous cell carcinoma of the esophagus

EF-hand Ca(2+)-sensor proteins are key molecules for transducing Ca(2+) signals into physiological answers and changes in cytosolic Ca(2+) concentration control a variety of cellular responses, including proliferation, migration, and differentiation, which are relevant for tumor progression. The Ca(2+)-sensor visinin-like protein-1 (VILIP-1) has recently attracted major interest due to its putative tumor suppressor function. Whereas VILIP-1 is expressed in normal skin, it is downregulated in skin tumors in a murine tumor model. The aim of this study was to investigate the expression of the Ca(2+)-sensor VILIP-1 in squamous cell carcinoma of the esophagus and to correlate expression levels with clinicopathological features of the tumor. We examined VILIP-1 expression in 54 specimens of esophageal squamous cell carcinomas and 24 normal esophagus tissues, with immunohistochemical staining and immunofluorescence co-staining techniques. VILIP-1 expression was completely lost or significantly reduced in esophageal tumor tissue compared with normal squamous epithelium. Correlation with clinicopathological features indicated that there was significantly less VILIP-1 expression in lymph node positive (N = 1) versus lymph node negative (N = 0) tumors (P = 0.002). Although there was no significant difference between highly (G(1)), moderately (G(2)) and poorly differentiated (G(3)) tumors (P = 0.177), VILIP-1 expression in tumors is significantly correlated with the depth of tumor invasion (P = 0.028 between T1, T2, T3, and T4). In contrast, co-staining with the proliferation marker Ki-67 indicated no significant correlation with proliferation rates in tumors (Ki-67 index of the tumor). In summary, the expression of the Ca(2+)-sensor VILIP-1 was found to be lost during development of squamous cell carcinoma of the esophagus. The protein expression level significantly correlates with invasive features, such as depth of tumor invasion and local lymph node metastasis, but not with proliferation rate of tumor cells.

Mitochondrial calcium signalling in cell death

The development of targeted probes (based on the molecular engineering of luminescent or fluorescent proteins) has allowed the specific measurement of [Ca2+] in intracellular organelles or cytoplasmic subdomains. This approach gave novel information on different aspects of cellular Ca2+ homeostasis. Regarding mitochondria, it was possible to demonstrate that, upon physiological stimulation of cells, Ca2+ is rapidly accumulated in the matrix. We will discuss the basic characteristics of this process, its role in modulating physiological and pathological events, such as the regulation of aerobic metabolism and the induction of cell death, and new insight into the regulatory mechanisms operating in vivo.

Calcium signalling: Past, present and future

Ca2+ is a universal second messenger controlling a wide variety of cellular reactions and adaptive responses. The initial appreciation of Ca2+ as a universal signalling molecule was based on the work of Sydney Ringer and Lewis Heilbrunn. More recent developments in this field were critically influenced by the invention of the patch clamp technique and the generation of fluorescent Ca2+ indicators. Currently the molecular Ca2+ signalling mechanisms are being worked out and we are beginning to assemble a reasonably complete picture of overall Ca2+ homeostasis. Furthermore, investigations of organellar Ca2+ homeostasis have added complexity to our understanding of Ca2+ signalling. The future of the Ca2+ signalling field lies with detailed investigations of the integrative function in vivo and clarification of the pathology associated with malfunctions of Ca2+ signalling cascades.

Ca2+/calmodulin-dependent protein kinase kinase-beta acts upstream of AMP-activated protein kinase in mammalian cells

AMP-activated protein kinase (AMPK) is the downstream component of a kinase cascade that plays a pivotal role in energy homeostasis. Activation of AMPK requires phosphorylation of threonine 172 (T172) within the T loop region of the catalytic alpha subunit. Recently, LKB1 was shown to activate AMPK. Here we show that AMPK is also activated by Ca(2+)/calmodulin-dependent protein kinase kinase (CaMKK). Overexpression of CaMKKbeta in mammalian cells increases AMPK activity, whereas pharmacological inhibition of CaMKK, or downregulation of CaMKKbeta using RNA interference, almost completely abolishes AMPK activation. CaMKKbeta isolated from rat brain or expressed in E. coli phosphorylates and activates AMPK in vitro. In yeast, CaMKKbeta expression rescues a mutant strain lacking the three kinases upstream of Snf1, the yeast homolog of AMPK. These results demonstrate that AMPK is regulated by at least two upstream kinases and suggest that AMPK may play a role in Ca(2+)-mediated signal transduction pathways.

Calcium - a universal carrier of biological signals

Calcium is the most universal carrier of signals to cells. Chosen by evolution because of its peculiar flexibility as a ligand, it now regulates all important aspects of cell activity, beginning with the creation of new life at fertilization and ending with the dramatic event of apoptotic suicide at the end of the life cycle. The process of signal transduction by Ca2+ displays a number of properties that make it unique among all other carriers of signals: for instance, the ability to perform both a first messenger and a second messenger function, or the frequent activation of autoregulatory mechanisms. The aspect that distinguishes the Ca2+ signaling function most dramatically is ambivalence. Cells have an absolute dependence on the messenger function of Ca2+ in order to function properly and must control its homeostasis with precision to maintain its free concentration in their interior at the appropriate low level. Catastrophy, however, invariably follows whenever protracted failures of the control mechanisms lead to sustained Ca2+ overload.

The intriguing Ca2+ requirement of calpain activation

Mammalian ubiquitous micro- and m-calpains, as well as their Drosophila homologs, Calpain A and Calpain B, are Ca(2+)-activated cytoplasmic proteases that act by limited proteolysis of target proteins. Calpains are thought to be part of many cellular signaling pathways. These enzymes, however, require such high Ca(2+) concentration for half-maximal activation in vitro, [Ca(2+)](0.5), that hardly ever occurs in intact cells. This major dilemma has pervaded the literature on calpains for decades. In this paper several considerations are put forward that challenge the orthodox view and envisage mechanisms that may govern calpain action in vivo. The "unphysiologically" high Ca(2+) demand for activation may turn out to be an evolutionarily adjusted safety device.

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.