Dynamics of [Ca2+] in the endoplasmic reticulum and cytoplasm of intact HeLa cells. A comparative study

Using Fluorescent GAP Indicators to Monitor ER Ca2

The endoplasmic reticulum (ER) is the main reservoir of Ca2+ of the cell. Accurate and quantitative measuring of Ca2+ dynamics within the lumen of the ER has been challenging. In the last decade a few genetically encoded Ca2+ indicators have been developed, including a family of fluorescent Ca2+ indicators, dubbed GFP-Aequorin Proteins (GAPs). They are based on the fusion of two jellyfish proteins, the green fluorescent protein (GFP) and the Ca2+-binding protein aequorin. GAP Ca2+ indicators exhibit a combination of several features: they are excitation ratiometric indicators, with reciprocal changes in the fluorescence excited at 405 and 470 nm, which is advantageous for imaging experiments; they exhibit a Hill coefficient of 1, which facilitates the calibration of the fluorescent signal into Ca2+ concentrations; they are insensible to variations in the Mg2+ concentrations or pH variations (in the 6.5-8.5 range); and, due to the lack of mammalian homologues, these proteins have a favorable expression in transgenic animals. A low Ca2+ affinity version of GAP, GAP3 (KD ≅ 489 µM), has been engineered to conform with the estimated [Ca2+] in the ER. GAP3 targeted to the lumen of the ER (erGAP3) can be utilized for imaging intraluminal Ca2+. The ratiometric measurements provide a quantitative method to assess accurate [Ca2+]ER, both dynamically and at rest. In addition, erGAP3 can be combined with synthetic cytosolic Ca2+ indicators to simultaneously monitor ER and cytosolic Ca2+. Here, we provide detailed methods to assess erGAP3 expression and to perform Ca2+ imaging, either restricted to the ER lumen, or simultaneously in the ER and the cytosol. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Detection of erGAP3 in the ER by immunofluorescence Basic Protocol 2: Monitoring ER Ca2+ Basic Protocol 3: Monitoring ER- and cytosolic-Ca2+ Support Protocol: Generation of a stable cell line expressing erGAP3.

Optimized Aequorin Reconstitution Protocol to Visualize Calcium Ion Transients in the Heart of Transgenic Zebrafish Embryos In Vivo

We introduce how to image calcium ion levels in the heart of zebrafish embryos and larvae up to 5 days post-fertilization with the photoprotein green fluorescent protein (GFP)-aequorin (GA) in the transgenic line Tg(myl7:GA). Incubation of the embryos with CTZ to obtain the functional photoprotein yields few emission counts, suggesting that, when the heart is beating, the rate of aequorin consumption is faster than that of the reconstitution with CTZ. In this chapter, we present an improved aequorin reconstitution protocol. We further describe the experimental procedure as well as the bioluminescence data analysis and processing.

Quantal Ca2+ release mediated by very few IP3 receptors that rapidly inactivate allows graded responses to IP3

Inositol 1,4,5-trisphosphate receptors (IP₃Rs) are intracellular Ca²⁺ channels that link extracellular stimuli to Ca²⁺ signals. Ca²⁺ release from intracellular stores is "quantal": low IP₃ concentrations rapidly release a fraction of the stores. Ca²⁺ release then slows or terminates without compromising responses to further IP₃ additions. The mechanisms are unresolved. Here, we synthesize a high-affinity partial agonist of IP₃Rs and use it to demonstrate that quantal responses do not require heterogenous Ca²⁺ stores. IP₃Rs respond incrementally to IP₃ and close after the initial response to low IP₃ concentrations. Comparing functional responses with IP₃ binding shows that only a tiny fraction of a cell's IP₃Rs mediate incremental Ca²⁺ release; inactivation does not therefore affect most IP₃Rs. We conclude, and test by simulations, that Ca²⁺ signals evoked by IP₃ pulses arise from rapid activation and then inactivation of very few IP₃Rs. This allows IP₃Rs to behave as increment detectors mediating graded Ca²⁺ release.

splitGFP Technology Reveals Dose-Dependent ER-Mitochondria Interface Modulation by α-Synuclein A53T and A30P Mutants

Familial Parkinson’s disease (PD) is associated with duplication or mutations of α-synuclein gene, whose product is a presynaptic cytosolic protein also found in mitochondria and in mitochondrial-associated ER membranes. We have originally shown the role of α-syn as a modulator of the ER-mitochondria interface and mitochondrial Ca²⁺ transients, suggesting that, at mild levels of expression, α-syn sustains cell metabolism. Here, we investigated the possibility that α-syn action on ER-mitochondria tethering could be compromised by the presence of PD-related mutations. The clarification of this aspect could contribute to elucidate key mechanisms underlying PD. The findings reported so far are not consistent, possibly because of the different methods used to evaluate ER-mitochondria connectivity. Here, the effects of the PD-related α-syn mutations A53T and A30P on ER-mitochondria relationship were investigated in respect to Ca²⁺ handling and mitochondrial function using a newly generated SPLICS sensor and aequorin-based Ca²⁺measurements. We provided evidence that A53T and A30P amino acid substitution does not affect the ability of α-syn to enhance ER/mitochondria tethering and mitochondrial Ca²⁺ transients, but that this action was lost as soon as a high amount of TAT-delivered A53T and A30P α-syn mutants caused the redistribution of α-syn from cytoplasm to foci. Our results suggest a loss of function mechanism and highlight a possible connection between α-syn and ER-mitochondria Ca²⁺ cross-talk impairment to the pathogenesis of PD.

Caffeine chelates calcium in the lumen of the endoplasmic reticulum

Cytosolic Ca²⁺ signals are often amplified by massive calcium release from the endoplasmic reticulum (ER). This calcium-induced calcium release (CICR) occurs by activation of an ER Ca²⁺ channel, the ryanodine receptor (RyR), which is facilitated by both cytosolic- and ER Ca²⁺ levels. Caffeine sensitizes RyR to Ca²⁺ and promotes ER Ca²⁺ release at basal cytosolic Ca²⁺ levels. This outcome is frequently used as a readout for the presence of CICR. By monitoring ER luminal Ca²⁺ with the low-affinity genetic Ca²⁺ probe erGAP3, we find here that application of 50 mM caffeine rapidly reduces the Ca²⁺ content of the ER in HeLa cells by ∼50%. Interestingly, this apparent ER Ca²⁺ release does not go along with the expected cytosolic Ca²⁺ increase. These results can be explained by Ca²⁺ chelation by caffeine inside the ER. Ca²⁺-overloaded mitochondria also display a drop of the matrix Ca²⁺ concentration upon caffeine addition. In contrast, in the cytosol, with a low free Ca²⁺ concentration (10^-7 M), no chelation is observed. Expression of RyR3 sensitizes the responses to caffeine with effects both in the ER (increase in Ca²⁺ release) and in the cytosol (increase in Ca²⁺ peak) at low caffeine concentrations (0.3-1 mM) that have no effects in control cells. Our results illustrate the fact that simultaneous monitoring of both cytosolic- and ER Ca²⁺ are necessary to understand the action of caffeine and raise concerns against the use of high concentrations of caffeine as a readout of the presence of CICR.

The emerging use of bioluminescence in medical research

Bioluminescence is the light produced by a living organism and is commonly emitted by sea life with Ca²⁺-regulated photoproteins being the most responsible for bioluminescence emission. Marine coelenterates provide important functions involved in essential purposes such as defense, feeding, and breeding. In this review, the main characteristics of marine photoproteins including aequorin, clytin, obelin, berovin, pholasin and symplectin from different marine organisms will be discussed. We will focused on the recent use of recombinant photoproteins in different biomedical research fields including the measurement of Ca²⁺ in different intracellular compartments of animal cells, as labels in the design and development of binding assays. This review will also outline how bioluminescent photoproteins have been used in a plethora of analytical methods including ultra-sensitive assays and in vivo imaging of cellular processes. Due to their unique properties including elective intracellular distribution, wide dynamic range, high signal-to-noise ratio and low Ca²⁺-buffering effect, recombinant photoproteins represent a promising future analytical tool in several in vitro and in vivo experiments.

Molecular physiology and pathophysiology of stromal interaction molecules

Ca²⁺ release from the endoplasmic reticulum is an important component of Ca²⁺ signal transduction that controls numerous physiological processes in eukaryotic cells. Release of Ca²⁺ from the endoplasmic reticulum is coupled to the activation of store-operated Ca²⁺ entry into cells. Store-operated Ca²⁺ entry provides Ca²⁺ for replenishing depleted endoplasmic reticulum Ca²⁺ stores and a Ca²⁺ signal that regulates Ca²⁺-dependent intracellular biochemical events. Central to connecting discharge of endoplasmic reticulum Ca²⁺ stores following G protein-coupled receptor activation with the induction of store-operated Ca²⁺ entry are stromal interaction molecules (STIM1 and STIM2). These highly homologous endoplasmic reticulum transmembrane proteins function as sensors of the Ca²⁺ concentration within the endoplasmic reticulum lumen and activators of Ca²⁺ release-activated Ca²⁺ channels. Emerging evidence indicates that in addition to their role in Ca²⁺ release-activated Ca²⁺ channel gating and store-operated Ca²⁺ entry, STIM1 and STIM2 regulate other cellular signaling events. Recent studies have shown that disruption of STIM expression and function is associated with the pathogenesis of several diseases including autoimmune disorders, cancer, cardiovascular disease, and myopathies. Here, we provide an overview of the latest developments in the molecular physiology and pathophysiology of STIM1 and STIM2. Impact statement Intracellular Ca²⁺ signaling is a fundamentally important regulator of cell physiology. Recent studies have revealed that Ca²⁺-binding stromal interaction molecules (Stim1 and Stim2) expressed in the membrane of the endoplasmic reticulum (ER) are essential components of eukaryote Ca²⁺ signal transduction that control the activity of ion channels and other signaling effectors present in the plasma membrane. This review summarizes the most recent information on the molecular physiology and pathophysiology of stromal interaction molecules. We anticipate that the work presented in our review will provide new insights into molecular interactions that participate in interorganelle signaling crosstalk, cell function, and the pathogenesis of human diseases.

Role of STIM2 in cell function and physiopathology

An endoplasmic reticulum (ER)-resident protein that regulates cytosolic and ER free-Ca²⁺ concentration by induction of store-operated calcium entry: that is the original definition of STIM2 and its function. While its activity strongly depends on the amount of calcium stored in the ER, its function goes further, to intracellular signalling and gene expression. Initially under-studied owing to the prominent function of STIM1, STIM2 came to be regarded as vital in mice, gradually emerging as an important player in the nervous system, and cooperating with STIM1 in the immune system. STIM2 has also been proposed as a relevant player in pathological conditions related to ageing, Alzheimer's and Huntington's diseases, autoimmune disorders and cancer. The discovery of additional functions, together with new splicing forms with opposite roles, has clarified existing controversies about STIM2 function in SOCE. With STIM2 being essential for life, but apparently not for development, newly available data demonstrate a complex and still intriguing behaviour that this review summarizes, updating current knowledge of STIM2 function.

Using aequorin probes to measure Ca2+ in intracellular organelles

Aequorins are excellent tools for measuring intra-organellar Ca²⁺ and assessing its role in physiological and pathological functions. Here we review targeting strategies to express aequorins in various organelles. We address critical topics such as probe affinity tuning as well as normalization and calibration of the signal. We also focus on bioluminescent Ca²⁺ imaging in nucleus or mitochondria of living cells. Finally, recent advances with a new chimeric GFP-aequorin protein (GAP), which can be used either as luminescent or fluorescent Ca²⁺ probe, are presented. GAP is robustly expressed in transgenic flies and mice, where it has proven to be a suitable Ca²⁺ indicator for monitoring physiological Ca²⁺ signaling ex vivo and in vivo.

Measuring Ca2+ inside intracellular organelles with luminescent and fluorescent aequorin-based sensors

GFP-Aequorin Protein (GAP) can be used to measure [Ca²⁺] inside intracellular organelles, both by luminescence and by fluorescence. The low-affinity variant GAP3 is adequate for ratiometric imaging in the endoplasmic reticulum and Golgi apparatus, and it can be combined with conventional synthetic indicators for simultaneous measurements of cytosolic Ca²⁺. GAP is bioorthogonal as it does not have mammalian homologues, and it is robust and functionally expressed in transgenic flies and mice, where it can be used for Ca²⁺ measurements ex vivo and in vivo to explore animal models of health and disease. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Kinetics on Demand Is a Simple Mathematical Solution that Fits Recorded Caffeine-Induced Luminal SR Ca2+ Changes in Smooth Muscle Cells

The process of Ca²⁺ release from sarcoplasmic reticulum (SR) comprises 4 phases in smooth muscle cells. Phase 1 is characterized by a large increase of the intracellular Ca²⁺ concentration ([Ca²⁺]_i) with a minimal reduction of the free luminal SR [Ca²⁺] ([Ca²⁺]_FSR). Importantly, active SR Ca²⁺ ATPases (SERCA pumps) are necessary for phase 1 to occur. This situation cannot be explained by the standard kinetics that involves a fixed amount of luminal Ca²⁺ binding sites. A new mathematical model was developed that assumes an increasing SR Ca²⁺ buffering capacity in response to an increase of the luminal SR [Ca²⁺] that is called Kinetics-on-Demand (KonD) model. This approach can explain both phase 1 and the refractory period associated with a recovered [Ca²⁺]_FSR. Additionally, our data suggest that active SERCA pumps are a requisite for KonD to be functional; otherwise luminal SR Ca²⁺ binding proteins switch to standard kinetics. The importance of KonD Ca²⁺ binding properties is twofold: a more efficient Ca²⁺ release process and that [Ca²⁺]_FSR and Ca²⁺-bound to SR proteins ([Ca²⁺]_BSR) can be regulated separately allowing for Ca²⁺ release to occur (provided by Ca²⁺-bound to luminal Ca²⁺ binding proteins) without an initial reduction of the [Ca²⁺]_FSR.

A new low-Ca²⁺ affinity GAP indicator to monitor high Ca²⁺ in organelles by luminescence

We have recently described a new class of genetically encoded Ca(2+) indicators composed of two jellyfish proteins, a variant of green fluorescent protein (GFP) and the calcium binding protein apoaequorin, named GAP (Rodriguez-García et al., 2014). GAP is a unique dual-mode Ca(2+) indicator, able to function either as a fluorescent or a luminescent probe, depending on whether the photoprotein aequorin is in its apo-state or reconstituted with its cofactor coelenterazine. We describe here a novel application of GAP as a low affinity bioluminescent indicator, suitable for measurements of [Ca(2+)] in ER or in Golgi apparatus. We used the low affinity variant, GAP1, which carries mutations in two EF-hands of aequorin, reconstituted with coelenterazine n. In comparison to previous bioluminescent aequorin fusions, the decay rate of GAP1 was decreased 8 fold and the affinity for Ca(2+) was lowered one order of magnitude. This improvement allows long-term measurements in high Ca(2+) environments avoiding fast aequorin consumption. GAP1 was targeted to the ER of various cell types, where it monitored resting Ca(2+) concentrations in the range from 400 to 600 μM. ER could be emptied of calcium by stimulation with ATP, carbachol or histamine in intact cells, and by challenge with inositol tris-phosphate in permeabilized cells. GAP1 was also targeted to the Golgi apparatus where it was able to precisely monitor long-term calcium dynamics. GAP1 provides a novel and robust indicator applicable to bioluminescent high-throughput quantitative assays.

Differential calcium handling by the cis and trans regions of the Golgi apparatus

High Ca2+ content in the Golgi apparatus (Go) is essential for protein processing and sorting. In addition, the Go can shape the cytosolic Ca2+ signals by releasing or sequestering Ca2+. We generated two new aequorin-based Ca2+ probes to specifically measure Ca2+ in the cis/cis-to-medial-Go (cGo) or the trans-Go (tGo). Ca2+ homoeostasis in these compartments and in the endoplasmic reticulum (ER) has been studied and compared. Moreover, the relative size of each subcompartment was estimated from aequorin consumption. We found that the cGo accumulates Ca2+ to high concentrations (150-300 μM) through the sarco plasmic/endoplasmic reticulum Ca2+-ATPase (SERCA). The tGo, in turn, is divided into two subcompartments: tGo1 and tGo2. The subcompartment tGo1 contains 20% of the aequorin and has a high internal [Ca2+]; Ca2+ is accumulated in this subcompartment via the secretory pathway Ca2+-ATPase 1 (SPCA-1) at a very high affinity (K50=30 nM). The subcompartment tGo2 contains 80% of aequorin, has a lower [Ca2+] and no SPCA-1 activity; Ca2+ uptake happens through SERCA and is slower than in tGo1. The two tGo subcompartments, tGo1 and tGo2, are diffusionally isolated. Inositol trisphosphate mobilizes Ca2+ from the cGo and tGo2, but not from tGo1, whereas caffeine releases Ca2+ from all the Golgi regions, and nicotinic acid dinucleotide phosphate and cADP ribose from none.

Methods to measure intracellular Ca(2+) fluxes with organelle-targeted aequorin-based probes

The photoprotein aequorin generates blue light upon binding of Ca(2+) ions. Together with its very low Ca(2+)-buffering capacity and the possibility to add specific targeting sequences, this property has rendered aequorin particularly suitable to monitor Ca(2+) concentrations in specific subcellular compartments. Recently, a new generation of genetically encoded Ca(2+) probes has been developed by fusing Ca(2+)-responsive elements with the green fluorescent protein (GFP). Aequorin has also been employed to this aim, resulting in an aequorin-GFP chimera with the Ca(2+) sensitivity of aequorin and the fluorescent properties of GFP. This setup has actually solved the major limitation of aequorin, for example, its poor ability to emit light, which rendered it inappropriate for the monitoring of Ca(2+) waves at the single-cell level by imaging. In spite of the numerous genetically encoded Ca(2+) indicators that are currently available, aequorin-based probes remain the method of election when an accurate quantification of Ca(2+) levels is required. Here, we describe currently available aequorin variants and their use for monitoring Ca(2+) waves in specific subcellular compartments. Among various applications, this method is relevant for the study of the alterations of Ca(2+) homeostasis that accompany oncogenesis, tumor progression, and response to therapy.

Microscopic heat pulse-induced calcium dynamics in single WI-38 fibroblasts

Temperature-sensitive Ca²⁺ dynamics occur primarily through transient receptor potential channels, but also by means of Ca²⁺ channels and pumps on the endoplasmic reticulum membrane. As such, cytoplasmic Ca²⁺ concentration ([Ca²⁺]_cyt) is re-equilibrated by changes in ambient temperature. The present study investigated the effects of heat pulses (heating duration: 2 s or 150 s) on [Ca²⁺]_cyt in single WI-38 fibroblasts, which are considered as normal cells. We found that Ca²⁺ burst occurred immediately after short (2 s) heat pulse, which is similar to our previous report on HeLa cells, but with less thermosensitivity. The heat pulses originated from a focused 1455-nm infrared laser light were applied in the vicinity of cells under the optical microscope. Ca²⁺ bursts induced by the heat pulse were suppressed by treating cells with inhibitors for sarco/endoplasmic reticulum Ca²⁺ ATPase (SERCA) or inositol trisphosphate receptor (IP₃R). Long (150 s) heat pulses also induced Ca²⁺ bursts after the onset of heating and immediately after re-cooling. Cells were more thermosensitive at physiological (37°C) than at room (25°C) temperature; however, at 37°C, cells were responsive at a higher temperature (ambient temperature+heat pulse). These results strongly suggest that the heat pulse-induced Ca²⁺ burst is caused by a transient imbalance in Ca²⁺ flow between SERCA and IP₃R, and offer a potential new method for thermally controlling Ca²⁺-regulated cellular functions.

Oligomerization and Ca2+/calmodulin control binding of the ER Ca2+-sensors STIM1 and STIM2 to plasma membrane lipids

Ca²⁺ (calcium) homoeostasis and signalling rely on physical contacts between Ca²⁺ sensors in the ER (endoplasmic reticulum) and Ca²⁺ channels in the PM (plasma membrane). STIM1 (stromal interaction molecule 1) and STIM2 Ca²⁺ sensors oligomerize upon Ca²⁺ depletion in the ER lumen, contact phosphoinositides at the PM via their cytosolic lysine (K)-rich domains, and activate Ca²⁺ channels. Differential sensitivities of STIM1 and STIM2 towards ER luminal Ca²⁺ have been studied but responses towards elevated cytosolic Ca²⁺ concentration and the mechanism of lipid binding remain unclear. We found that tetramerization of the STIM1 K-rich domain is necessary for efficient binding to PI(4,5)P₂-containing PM-like liposomes consistent with an oligomerization-driven STIM1 activation. In contrast, dimerization of STIM2 K-rich domain was sufficient for lipid binding. Furthermore, the K-rich domain of STIM2, but not of STIM1, forms an amphipathic α-helix. These distinct features of the STIM2 K-rich domain cause an increased affinity for PI(4,5)P₂, consistent with the lower activation threshold of STIM2 and a function as regulator of basal Ca²⁺ levels. Concomitant with higher affinity for PM lipids, binding of CaM (calmodulin) inhibited the interaction of the STIM2 K-rich domain with liposomes in a Ca²⁺ and PI(4,5)P₂ concentration-dependent manner. Therefore we suggest that elevated cytosolic Ca²⁺ concentration down-regulates STIM2-mediated ER–PM contacts via CaM binding.

Regulation of inositol 1,4,5-trisphosphate receptors during endoplasmic reticulum stress

The endoplasmic reticulum (ER) performs multiple functions in the cell: it is the major site of protein and lipid synthesis as well as the most important intracellular Ca(2+) reservoir. Adverse conditions, including a decrease in the ER Ca(2+) level or an increase in oxidative stress, impair the formation of new proteins, resulting in ER stress. The subsequent unfolded protein response (UPR) is a cellular attempt to lower the burden on the ER and to restore ER homeostasis by imposing a general arrest in protein synthesis, upregulating chaperone proteins and degrading misfolded proteins. This response can also lead to autophagy and, if the stress can not be alleviated, to apoptosis. The inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) and IP3-induced Ca(2+) signaling are important players in these processes. Not only is the IP3R activity modulated in a dual way during ER stress, but also other key proteins involved in Ca(2+) signaling are modulated. Changes also occur at the structural level with a strengthening of the contacts between the ER and the mitochondria, which are important determinants of mitochondrial Ca(2+) uptake. The resulting cytoplasmic and mitochondrial Ca(2+) signals will control cellular decisions that either promote cell survival or cause their elimination via apoptosis. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

Enhanced parkin levels favor ER-mitochondria crosstalk and guarantee Ca(2+) transfer to sustain cell bioenergetics

Loss-of-function mutations in PINK1 or parkin genes are associated with juvenile-onset autosomal recessive forms of Parkinson disease. Numerous studies have established that PINK1 and parkin participate in a common mitochondrial-quality control pathway, promoting the selective degradation of dysfunctional mitochondria by mitophagy. Upregulation of parkin mRNA and protein levels has been proposed as protective mechanism against mitochondrial and endoplasmic reticulum (ER) stress. To better understand how parkin could exert protective function we considered the possibility that it could modulate the ER-mitochondria inter-organelles cross talk. To verify this hypothesis we investigated the effects of parkin overexpression on ER-mitochondria crosstalk with respect to the regulation of two key cellular parameters: Ca(2+) homeostasis and ATP production. Our results indicate that parkin overexpression in model cells physically and functionally enhanced ER-mitochondria coupling, favored Ca(2+) transfer from the ER to the mitochondria following cells stimulation with an 1,4,5 inositol trisphosphate (InsP(3)) generating agonist and increased the agonist-induced ATP production. The overexpression of a parkin mutant lacking the first 79 residues (ΔUbl) failed to enhance the mitochondrial Ca(2+) transients, thus highlighting the importance of the N-terminal ubiquitin like domain for the observed phenotype. siRNA-mediated parkin silencing caused mitochondrial fragmentation, impaired mitochondrial Ca(2+) handling and reduced the ER-mitochondria tethering. These data support a novel role for parkin in the regulation of mitochondrial homeostasis, Ca(2+) signaling and energy metabolism under physiological conditions.

Measurements of Ca²⁺ concentration with recombinant targeted luminescent probes

In the last two decades the study of Ca(2+) homeostasis in living cells has been enhanced by the explosive development of genetically encoded Ca(2+)-indicators. The cloning of the Ca(2+)-sensitive photoprotein aequorin and of the green fluorescent protein (GFP) from the jellyfish Aequorea victoria has been enormously advantageous. As polypeptides, aequorin and GFP allow their endogenous production in cell systems as diverse as bacteria, yeast, slime molds, plants, and mammalian cells. Moreover, it is possible to specifically localize them within the cell by including defined targeting signals in the amino acid sequence. These two proteins have been extensively engineered to obtain several recombinant probes for different biological parameters, among which Ca(2+) concentration reporters are probably the most relevant. The GFP-based Ca(2+) probes and aequorin are widely employed in the study of intracellular Ca(2+) homeostasis. The new generation of bioluminescent probes that couple the Ca(2+) sensitivity of aequorin to GFP fluorescence emission allows real-time measurements of subcellular Ca(2+) changes in single cell imaging experiments and the video-imaging of Ca(2+) concentrations changes in live transgenic animals that express GFP-aequorin bifunctional probes.

Mitochondrial free [Ca(2+)] dynamics measured with a novel low-Ca(2+) affinity aequorin probe

Mitochondria have a very large capacity to accumulate Ca(2+) during cell stimulation driven by the mitochondrial membrane potential. Under these conditions, [Ca(2+)](M) (mitochondrial [Ca(2+)]) may well reach millimolar levels in a few seconds. Measuring the dynamics of [Ca(2+)](M) during prolonged stimulation has been previously precluded by the high Ca(2+) affinity of the probes available. We have now developed a mitochondrially targeted double-mutated form of the photoprotein aequorin which is able to measure [Ca(2+)] in the millimolar range for long periods of time without problems derived from aequorin consumption. We show in the present study that addition of Ca(2+) to permeabilized HeLa cells triggers an increase in [Ca(2+)](M) up to an steady state of approximately 2-3 mM in the absence of phosphate and 0.5-1 mM in the presence of phosphate, suggesting buffering or precipitation of calcium phosphate when the free [Ca(2+)] reaches 0.5-1 mM. Mitochondrial pH acidification partially re-dissolved these complexes. These millimolar [Ca(2+)](M) levels were stable for long periods of time provided the mitochondrial membrane potential was not collapsed. Silencing of the mitochondrial Ca(2+) uniporter largely reduced the rate of [Ca(2+)](M) increase, but the final steady-state [Ca(2+)](M) reached was similar. In intact cells, the new probe allows monitoring of agonist-induced increases of [Ca(2+)](M) without problems derived from aequorin consumption.

α-Synuclein controls mitochondrial calcium homeostasis by enhancing endoplasmic reticulum-mitochondria interactions

α-Synuclein has a central role in Parkinson disease, but its physiological function and the mechanism leading to neuronal degeneration remain unknown. Because recent studies have highlighted a role for α-synuclein in regulating mitochondrial morphology and autophagic clearance, we investigated the effect of α-synuclein in HeLa cells on mitochondrial signaling properties focusing on Ca(2+) homeostasis, which controls essential bioenergetic functions. By using organelle-targeted Ca(2+)-sensitive aequorin probes, we demonstrated that α-synuclein positively affects Ca(2+) transfer from the endoplasmic reticulum to the mitochondria, augmenting the mitochondrial Ca(2+) transients elicited by agonists that induce endoplasmic reticulum Ca(2+) release. This effect is not dependent on the intrinsic Ca(2+) uptake capacity of mitochondria, as measured in permeabilized cells, but correlates with an increase in the number of endoplasmic reticulum-mitochondria interactions. This action specifically requires the presence of the C-terminal α-synuclein domain. Conversely, α-synuclein siRNA silencing markedly reduces mitochondrial Ca(2+) uptake, causing profound alterations in organelle morphology. The enhanced accumulation of α-synuclein into the cells causes the redistribution of α-synuclein to localized foci and, similarly to the silencing of α-synuclein, reduces the ability of mitochondria to accumulate Ca(2+). The absence of efficient Ca(2+) transfer from endoplasmic reticulum to mitochondria results in augmented autophagy that, in the long range, could compromise cellular bioenergetics. Overall, these findings demonstrate a key role for α-synuclein in the regulation of mitochondrial homeostasis in physiological conditions. Elevated α-synuclein expression and/or eventually alteration of the aggregation properties cause the redistribution of the protein within the cell and the loss of modulation on mitochondrial function.

Unraveling STIM2 function

The discovery of molecular players in capacitative calcium (Ca(2+)) entry, also referred to as store-operated Ca(2+) entry (SOCE), supposed a great advance in the knowledge of cellular mechanisms of Ca(2+) entry, which are essential for a broad range of cellular functions. The identification of STIM1 and STIM2 proteins as the sensors of Ca(2+) stored in the endoplasmic reticulum unraveled the mechanism by which depletion of intracellular Ca(2+) stores is communicated to store-operated Ca(2+) channels located in the plasma membrane, triggering the activation of SOCE and intracellular Ca(2+)-dependent signaling cascades. Initial studies suggested a dominant function of STIM1 in SOCE and SOCE-dependent cellular functions compared to STIM2, especially those that participate in immune responses. Consequently, most of the subsequent studies focused on STIM1. However, during the last years, STIM2 has been demonstrated to play a more relevant and complex function than initially reported, being even important to sustain normal life in mice. These studies have led to reconsider the role of STIM2 in SOCE and its relevance in cellular physiology. This review is intended to summarize and provide an overview of the current data available about this exciting isoform, STIM2, and its actual position together with STIM1 in the mechanism of SOCE.

Ca2+-activated nucleotidase 1, a novel target gene for the transcriptional repressor DREAM (downstream regulatory element antagonist modulator), is involved in protein folding and degradation

DREAM is a Ca(2+)-dependent transcriptional repressor highly expressed in neuronal cells. A number of genes have already been identified as the target of its regulation. Targeted analysis performed on cerebella from transgenic mice expressing a dominant active DREAM mutant (daDREAM) showed a drastic reduction of the amount of transcript of Ca(2+)-activated nucleotidase 1 (CANT1), an endoplasmic reticulum (ER)-Golgi resident Ca(2+)-dependent nucleoside diphosphatase that has been suggested to have a role in glucosylation reactions related to the quality control of proteins in the ER and the Golgi apparatus. CANT1 down-regulation was also found in neuroblastoma SH-SY5Y cells stably overexpressing wild type (wt) DREAM or daDREAM, thus providing a simple cell model to investigate the protein maturation pathway. Pulse-chase experiments demonstrated that the down-regulation of CANT1 is associated with reduced protein secretion and increased degradation rates. Importantly, overexpression of wtDREAM or daDREAM augmented the expression of the EDEM1 gene, which encodes a key component of the ER-associated degradation pathway, suggesting an alternative pathway to enhanced protein degradation. Restoring CANT1 levels in neuroblastoma clones recovered the phenotype, thus confirming a key role of CANT1, and of the regulation of its gene by DREAM, in the control of protein synthesis and degradation.

Calcium homoeostasis modulator 1 (CALHM1) reduces the calcium content of the endoplasmic reticulum (ER) and triggers ER stress

CALHM1 (calcium homoeostasis modulator 1), a membrane protein with similarity to NMDA (N-methyl-D-aspartate) receptor channels that localizes in the plasma membrane and the ER (endoplasmic reticulum) of neurons, has been shown to generate a plasma-membrane Ca2+ conductance and has been proposed to influence Alzheimer's disease risk. In the present study we have investigated the effects of CALHM1 on intracellular Ca2+ handling in HEK-293T [HEK (human embryonic kidney)-293 cells expressing the large T-antigen of SV40 (simian virus 40)] cells by using targeted aequorins for selective monitorization of Ca2+ transport by organelles. We find that CALHM1 increases Ca2+ leak from the ER and, more importantly, reduces ER Ca2+ uptake by decreasing both the transport capacity and the Ca2+ affinity of SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase). As a result, the Ca2+ content of the ER is drastically decreased. This reduction in the Ca2+ content of the ER triggered the UPR (unfolded protein response) with induction of several ER stress markers, such as CHOP [C/EBP (CCAAT/enhancer-binding protein)-homologous protein], ERdj4, GRP78 (glucose-regulated protein of 78 kDa) and XBP1 (X-box-binding protein 1). Thus CALHM1 might provide a relevant link between Ca2+ homoeostasis disruption, ER stress and cell damage in the pathogenesis of neurodegenerative diseases

STIM1, but not STIM2, is required for proper agonist-induced Ca2+ signaling

The stromal interaction molecules STIM1 and STIM2 sense a decreasing Ca(2+) concentration in the lumen of the endoplasmic reticulum and activate Ca(2+) channels in the plasma membrane. In addition, at least 2 reports suggested that STIM1 may also interact with the inositol 1,4,5-trisphosphate (IP(3)) receptor. Using embryonic fibroblasts from Stim1(-/-), Stim2(-/-) and wild-type mice, we now tested the hypothesis that STIM1 and STIM2 would also regulate the IP(3) receptor. We investigated whether STIM1 or STIM2 would be the luminal Ca(2+) sensor that controls the loading dependence of the IP(3)-induced Ca(2+) release. Partial emptying of the stores in plasma-membrane permeabilized cells resulted in an increased EC(50) and a decreased Hill coefficient for IP(3)-induced Ca(2+) release. This effect occurred both in the presence and absence of STIM proteins, indicating that these proteins were not the luminal Ca(2+) sensor for the IP(3) receptor. Although Stim1(-/-) cells displayed a normal IP(3)-receptor function, agonist-induced Ca(2+) release was reduced. This finding suggests that the presence of STIM1 is required for proper agonist-induced Ca(2+) signaling. Our data do not provide experimental evidence for the suggestion that STIM proteins would directly control the function of the IP(3) receptor.

Two distinct calcium pools in the endoplasmic reticulum of HEK-293T cells

Agonist-sensitive intracellular Ca2+ stores may be heterogeneous and exhibit distinct functional features. We have studied the properties of intracellular Ca2+ stores using targeted aequorins for selective measurements in different subcellular compartments. Both, HEK-293T [HEK (human embryonic kidney)-293 cells expressing the large T-antigen of SV40 (simian virus 40)] and HeLa cells accumulated Ca2+ into the ER (endoplasmic reticulum) to near millimolar concentrations and the IP3-generating agonists, carbachol and ATP, mobilized this Ca2+ pool. We find in HEK-293T, but not in HeLa cells, a distinct agonist-releasable Ca2+ pool insensitive to the SERCA (sarco/endoplasmic reticulum Ca2+ ATPase) inhibitor TBH [2,5-di-(t-butyl)-benzohydroquinone]. TG (thapsigargin) and CPA (cyclopiazonic acid) completely emptied this pool, whereas lysosomal disruption or manoeuvres collapsing endomembrane pH gradients did not. Our results indicate that SERCA3d is important for filling the TBH-resistant store as: (i) SERCA3d is more abundant in HEK-293T than in HeLa cells; (ii) the SERCA 3 ATPase activity of HEK-293T cells is not fully blocked by TBH; and (iii) the expression of SERCA3d in HeLa cells generated a TBH-resistant agonist-mobilizable compartment in the ER. Therefore the distribution of SERCA isoforms may originate the heterogeneity of the ER Ca2+ stores and this may be the basis for store specialization in diverse functions. This adds to recent evidence indicating that SERCA3 isoforms may subserve important physiological and pathophysiological mechanisms.

Calcium entry-calcium refilling (CECR) coupling between store-operated Ca(2+) entry and sarco/endoplasmic reticulum Ca(2+)-ATPase

Cross-talk between subcellular organelles is essential for cellular Ca(2+) homeostasis. We have studied the effects of knocking down STIM1, the Ca(2+) sensor of the endoplasmic reticulum (ER), on several homeostatic Ca(2+)-handling mechanisms, including plasma membrane Ca(2+) entry and transport by ER, mitochondria and nucleus. We have used targeted aequorins to selectively measure calcium fluxes in different organelles. Actions of STIM1 were extremely selective, restricted to store operated Ca(2+) channels (SOC) and Ca(2+) uptake by the ER. No interactions with uptake or release of Ca(2+) by mitochondria or nucleus were detected. Ca(2+) exit from the ER, including passive leak, release via inositol 1,4,5-trisphosphate and ryanodine receptors, was unaffected. STIM1 knock-down inhibited ER Ca(2+) uptake in intact but not in permeabilized cells, suggesting a privileged calcium entry-calcium refilling (CECR) coupling between plasma membrane SOC and ER calcium pump in the intact cell. As a result a large part of the entering Ca(2+) is taken up into the ER without reaching the bulk cytosol. The tightness of CECR, as measured by the slope of the stimulus-signal strength function, was comparable to classic excitation-response coupling mechanisms, such as excitation-contraction, excitation-secretion or excitation-transcription coupling.

Calcium signalling mediated through α7 and non-α7 nAChR stimulation is differentially regulated in bovine chromaffin cells to induce catecholamine release

BACKGROUND AND PURPOSE

Ca(2+) signalling and exocytosis mediated by nicotinic receptor (nAChR) subtypes, especially the α7 nAChR, in bovine chromaffin cells are still matters of debate.

EXPERIMENTAL APPROACH

We have used chromaffin cell cultures loaded with Fluo-4 or transfected with aequorins directed to the cytosol or mitochondria, several nAChR agonists (nicotine, 5-iodo-A-85380, PNU282987 and choline), and the α7 nAChR allosteric modulator PNU120596.

KEY RESULTS

Minimal [Ca(2+) ](c) transients, induced by low concentrations of selective α7 nAChR agonists and nicotine, were markedly increased by the α7 nAChR allosteric modulator PNU120596. These potentiated responses were completely blocked by the α7 nAChR antagonist α-bungarotoxin (α7-modulated-response). Conversely, high concentrations of the α7 nAChR agonists, nicotine or 5-iodo-A-85380 induced larger [Ca(2+) ](c) transients, that were blocked by mecamylamine but were unaffected by α-bungarotoxin (non-α7 response). [Ca(2+) ](c) increases mediated by α7 nAChR were related to Ca(2+) entry through non-L-type Ca(2+) channels, whereas non-α7 nAChR-mediated signals were related to L-type Ca(2+) channels; Ca(2+) -induced Ca(2+) -release contributed to both responses. Mitochondrial involvement in the control of [Ca(2+) ](c) transients, mediated by either receptor, was minimal. Catecholamine release coupled to α7 nAChRs was more efficient in terms of catecholamine released/[Ca(2+) ](c) .

CONCLUSIONS AND IMPLICATIONS

[Ca(2+) ](c) and catecholamine release mediated by α7 nAChRs required an allosteric modulator and low doses of the agonist. At higher agonist concentrations, the α7 nAChR response was lost and the non-α7 nAChRs were activated. Catecholamine release might therefore be regulated by different nAChR subtypes, depending on agonist concentrations and the presence of allosteric modulators of α7 nAChRs.

Monitoring mitochondrial [Ca(2+)] dynamics with rhod-2, ratiometric pericam and aequorin

The dynamics of mitochondrial [Ca(2+)] ([Ca(2+)](M)) plays a key role in a variety of cellular processes. The most important methods available to monitor [Ca(2+)](M) are fluorescent dyes such as rhod-2 and specifically targeted proteins such as aequorin and pericam. However, significant discrepancies, both quantitative and qualitative, exist in the literature between the results obtained with different methods. We have made here a systematic comparison of the response of several fluorescent dyes, rhod-2 and rhod-FF, and two Ca(2+)-sensitive proteins, aequorin and pericam. Our results show that measurements obtained with aequorin and pericam are consistent in terms of dynamic Ca(2+) changes. Instead, fluorescent dyes failed to follow Ca(2+) changes adequately, especially during repetitive stimulation. In particular, measures obtained with rhod-2 or rhod-FF evidenced the previously reported Ca(2+)-dependent inhibition of mitochondrial Ca(2+) uptake, but data obtained with aequorin or pericam under the same conditions did not. The reason for the loss of response of fluorescent dyes is unclear. Loading with these dyes produced changes in mitochondrial morphology and membrane potential, which were small and reversible at low concentrations (1-2 microM), but produced large and prolonged damage at higher concentrations. In addition, cells loaded with low concentrations of rhod-2 suffered large changes in mitochondrial morphology after light excitation. Our results suggest that [Ca(2+)](M) data obtained with these dyes should be taken with care.

The dynamics of mitochondrial Ca2+ fluxes

We have investigated the kinetics of mitochondrial Ca(2+) influx and efflux and their dependence on cytosolic [Ca(2+)] and [Na(+)] using low-Ca(2+)-affinity aequorin. The rate of Ca(2+) release from mitochondria increased linearly with mitochondrial [Ca(2+)] ([Ca(2+)](M)). Na(+)-dependent Ca(2+) release was predominant al low [Ca(2+)](M) but saturated at [Ca(2+)](M) around 400muM, while Na(+)-independent Ca(2+) release was very slow at [Ca(2+)](M) below 200muM, and then increased at higher [Ca(2+)](M), perhaps through the opening of a new pathway. Half-maximal activation of Na(+)-dependent Ca(2+) release occurred at 5-10mM [Na(+)], within the physiological range of cytosolic [Na(+)]. Ca(2+) entry rates were comparable in size to Ca(2+) exit rates at cytosolic [Ca(2+)] ([Ca(2+)](c)) below 7muM, but the rate of uptake was dramatically accelerated at higher [Ca(2+)](c). As a consequence, the presence of [Na(+)] considerably reduced the rate of [Ca(2+)](M) increase at [Ca(2+)](c) below 7muM, but its effect was hardly appreciable at 10muM [Ca(2+)](c). Exit rates were more dependent on the temperature than uptake rates, thus making the [Ca(2+)](M) transients to be much more prolonged at lower temperature. Our kinetic data suggest that mitochondria have little high affinity Ca(2+) buffering, and comparison of our results with data on total mitochondrial Ca(2+) fluxes indicate that the mitochondrial Ca(2+) bound/Ca(2+) free ratio is around 10- to 100-fold for most of the observed [Ca(2+)](M) range and suggest that massive phosphate precipitation can only occur when [Ca(2+)](M) reaches the millimolar range.

Sarcoplasmic reticulum function in smooth muscle

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

The endoplasmic reticulum of dorsal root ganglion neurons contains functional TRPV1 channels

Transient receptor potential vanilloid type 1 (TRPV1) is a plasma membrane Ca(2+) channel involved in transduction of painful stimuli. Dorsal root ganglion (DRG) neurons express ectopic but functional TRPV1 channels in the endoplasmic reticulum (ER) (TRPV1(ER)). We have studied the properties of TRPV1(ER) in DRG neurons and HEK293T cells expressing TRPV1. Activation of TRPV1(ER) with capsaicin or other vanilloids produced an increase of cytosolic Ca(2+) due to Ca(2+) release from the ER. The decrease of [Ca(2+)](ER) was directly revealed by an ER-targeted aequorin Ca(2+) probe, expressed in DRG neurons using a herpes amplicon virus. The sensitivity of TRPV1(ER) to capsaicin was smaller than the sensitivity of the plasma membrane TRPV1 channels. The low affinity of TRPV1(ER) was not related to protein kinase A- or C-mediated phosphorylations, but it was due to inactivation by cytosolic Ca(2+) because the sensitivity to capsaicin was increased by loading the cells with the Ca(2+) chelator BAPTA. Decreasing [Ca(2+)](ER) did not affect the sensitivity of TRPV1(ER) to capsaicin. Disruption of the TRPV1 calmodulin-binding domains at either the C terminus (Delta35AA) or the N terminus (K155A) increased 10-fold the affinity of TRPV1(ER) for capsaicin, suggesting that calmodulin is involved in the inactivation. The lack of TRPV1 sensitizers, such as phosphatidylinositol 4,5-bisphosphate, in the ER could contribute to decrease the affinity for capsaicin. The low sensitivity of TRPV1(ER) to agonists may be critical for neuron health, because otherwise Ca(2+) depletion of ER could lead to ER stress, unfolding protein response, and cell death.

A small component of the endoplasmic reticulum is required for store-operated Ca2+ channel activation in liver cells: evidence from studies using TRPV1 and taurodeoxycholic acid

The question of whether the activation of SOCs (store-operated Ca(2+) channels) requires the whole or part of the ER (endoplasmic reticulum) has not been fully resolved. The role of a putative sub-compartment of the ER in SOC activation in liver cells was investigated using ectopically expressed TRPV1 (transient receptor potential vanilloid 1), a non-selective cation channel, and TDCA (taurodeoxycholic acid), an activator of SOCs, to release Ca(2+) from different regions of the ER. TRPV1 was expressed in the ER and in the plasma membrane. The amount of Ca(2+) released from the ER by a TRPV1 agonist, measured using fura-2, was the same as that released by a SERCA (sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase) inhibitor, indicating that TRPV1 agonist-sensitive stores substantially overlap with SERCA inhibitor-sensitive stores. In contrast with SERCA inhibitors, TRPV1 agonists did not activate store-operated Ca(2+) entry. These findings were confirmed by patch-clamp recording. Using FFP-18, it was shown that SERCA inhibitors release Ca(2+) from the ER located closer to the plasma membrane than the region from which TRPV1 agonists release Ca(2+). In contrast with SERCA inhibitors, TRPV1 agonists did not induce a redistribution of STIM1 (stromal interaction molecule 1). TDCA caused the release of Ca(2+) from the ER, which was detected by FFP-18 but not by fura-2, and a redistribution of STIM1 to puncta similar to that caused by SERCA inhibitors. It is concluded that in liver cells, Ca(2+) release from a small component of the ER located near the plasma membrane is required to induce STIM1 redistribution and SOC activation.

Expression of the P2X7 receptor increases the Ca2+ content of the endoplasmic reticulum, activates NFATc1, and protects from apoptosis

The P2X(7) receptor is known for the cytotoxic activity because of its ability to cause opening of non-selective pores in the plasma membrane and activate apoptotic caspases. A key factor of P2X(7)-dependent cytotoxicity is the massive intracellular Ca(2+) increase triggered by its activation. Here we show that P2X(7) transfection increased the ability of the endoplasmic reticulum to accumulate, store, and release Ca(2+). This caused a larger agonist-stimulated increase in cytosol and mitochondrial Ca(2+) in P2X(7) transfectants than in mock transfected cells. P2X(7) transfectants survived and even proliferated in serum-free conditions and were resistant to apoptosis triggered by ceramide, staurosporin, or intracellular Zn(2+) chelation. Finally, the nuclear factor of activated T cells complex 1 (NFATc1) was strongly activated in the P2X(7) transfectants. These observations support our previous finding that the P2X(7) receptor under tonic conditions of stimulation, i.e. those observed in response to basal ATP release, has an anti-apoptotic or even growth promoting rather than cytotoxic activity.

Calcium dynamics in bovine adrenal medulla chromaffin cell secretory granules

The secretory granules constitute one of the less well-known compartments in terms of Ca2+ dynamics. They contain large amounts of total Ca2+, but the free intragranular [Ca2+] ([Ca2+]SG), the mechanisms for Ca2+ uptake and release from the granules and their physiological significance regarding exocytosis are still matters of debate. We used in the present work an aequorin chimera targeted to the granules to investigate [Ca2+]SG homeostasis in bovine adrenal chromaffin cells. We found that most of the intracellular aequorin chimera is present in a compartment with 50-100 microM Ca2+. Ca2+ accumulation into this compartment takes place mainly through an ATP-dependent mechanism, namely, a thapsigargin-sensitive Ca2+-ATPase. In addition, fast Ca2+ release was observed in permeabilized cells after addition of inositol 1,4,5-trisphosphate (InsP3) or caffeine, suggesting the presence of InsP3 and ryanodine receptors in the vesicular membrane. Stimulation of intact cells with the InsP3-producing agonist histamine or with caffeine also induced Ca2+ release from the vesicles, whereas acetylcholine or high-[K+] depolarization induced biphasic changes in vesicular[Ca2+], suggesting heterogeneous responses of different vesicle populations, some of them releasing and some taking up Ca2+during stimulation. In conclusion, our data show that chromaffin cell secretory granules have the machinery required for rapid uptake and release of Ca2+, and this strongly supports the hypothesis that granular Ca2+ may contribute to its own secretion.

The mitochondrial antioxidants MitoE(2) and MitoQ(10) increase mitochondrial Ca(2+) load upon cell stimulation by inhibiting Ca(2+) efflux from the organelle

Mitochondrial reactive oxygen species (ROS) production is recognized as a major pathogenic event in a number of human diseases, and mitochondrial scavenging of ROS appears a promising therapeutic approach. Recently, two mitochondrial antioxidants have been developed; conjugating alpha-tocopherol and the ubiquinol moiety of coenzyme Q to the lipophilic triphenylphosphonium cation (TPP+), denominated MitoE(2) and MitoQ(10), respectively. We have investigated the effect of these compounds on mitochondrial Ca(2+) homeostasis, which controls processes as diverse as activation of mitochondrial dehydrogenases and pro-apoptotic morphological changes of the organelle. We demonstrate that treatment of HeLa cells with both MitoE(2) and MitoQ(10) induces (albeit with different efficacy) a major enhancement of the increase in matrix Ca(2+) concentration triggered by cell stimulation with the inositol 1,4,5-trisphosphate-generating agonist histamine. The effect is a result of the inhibition of Ca(2+) efflux from the organelle and depends on the TPP+ moiety of these compounds. Overall, the data identify an effect independent of their antioxidant activity, that on the one hand may be useful in addressing disorders in which mitochondrial Ca(2+) handling is impaired (e.g., mitochondrial diseases) and on the other may favor mitochondrial Ca(2+) overload and thus increase cell sensitivity to apoptosis (thus possibly counteracting the benefits of the antioxidant activity).

Mitochondrial free [Ca2+] levels and the permeability transition

Mitochondrial Ca(2+) activates many processes, from mitochondrial metabolism to opening of the permeability transition pore (PTP) and apoptosis. However, there is considerable controversy regarding the free mitochondrial [Ca(2+)] ([Ca(2+)](M)) levels that can be attained during cell activation or even in mitochondrial preparations. Studies using fluorescent dyes (rhod-2 or similar), have reported that phosphate precipitation precludes [Ca(2+)](M) from increasing above 2-3 microM. Instead, using low-Ca(2+)-affinity aequorin probes, we have measured [Ca(2+)](M) values more than two orders of magnitude higher. We confirm here these values by making a direct in situ calibration of mitochondrial aequorin, and we show that a prolonged increase in [Ca(2+)](M) to levels of 0.5-1mM was actually observed at any phosphate concentration (0-10mM) during continuous perfusion of 3.5-100 microM Ca(2+)-buffers. In spite of this high and maintained (>10 min) [Ca(2+)](M), mitochondria retained functionality and the [Ca(2+)](M) drop induced by a protonophore was fully reversible. In addition, this high [Ca(2+)](M) did not induce PTP opening unless additional activators (phenyl arsine oxide, PAO) were present. PAO induced a rapid, concentration-dependent and irreversible drop in [Ca(2+)](M). In conclusion [Ca(2+)](M) levels of 0.5-1mM can be reached and maintained for prolonged periods (>10 min) in phosphate-containing medium, and massive opening of PTP requires additional pore activators.

A novel mode of translocation for cytolethal distending toxin

Thermal instability in the toxin catalytic subunit may be a common property of toxins that exit the endoplasmic reticulum (ER) by exploiting the mechanism of ER-associated degradation (ERAD). The Haemophilus ducreyi cytolethal distending toxin (HdCDT) does not utilize ERAD to exit the ER, so we predicted the structural properties of its catalytic subunit (HdCdtB) would differ from other ER-translocating toxins. Here, we document the heat-stable properties of HdCdtB which distinguish it from other ER-translocating toxins. Cell-based assays further suggested that HdCdtB does not unfold before exiting the ER and that it may move directly from the ER lumen to the nucleoplasm. These observations suggest a novel mode of ER exit for HdCdtB.