Imaging intraorganellar Ca2+ at subcellular resolution using CEPIA

Resveratrol-induced autophagy is dependent on IP3Rs and on cytosolic Ca2

Previous work revealed that intracellular Ca²⁺ signals and the inositol 1,4,5-trisphosphate (IP₃) receptors (IP₃R) are essential to increase autophagic flux in response to mTOR inhibition, induced by either nutrient starvation or rapamycin treatment. Here, we investigated whether autophagy induced by resveratrol, a polyphenolic phytochemical reported to trigger autophagy in a non-canonical way, also requires IP₃Rs and Ca²⁺ signaling. Resveratrol augmented autophagic flux in a time-dependent manner in HeLa cells. Importantly, autophagy induced by resveratrol (80μM, 2h) was completely abolished in the presence of 10μM BAPTA-AM, an intracellular Ca²⁺-chelating agent. To elucidate the IP₃R's role in this process, we employed the recently established HEK 3KO cells lacking all three IP₃R isoforms. In contrast to the HEK293 wt cells and to HEK 3KO cells re-expressing IP₃R1, autophagic responses in HEK 3KO cells exposed to resveratrol were severely impaired. These altered autophagic responses could not be attributed to alterations in the mTOR/p70S6K pathway, since resveratrol-induced inhibition of S6 phosphorylation was not abrogated by chelating cytosolic Ca²⁺ or by knocking out IP₃Rs. Finally, we investigated whether resveratrol by itself induced Ca²⁺ release. In permeabilized HeLa cells, resveratrol neither affected the sarco- and endoplasmic reticulum Ca²⁺ ATPase (SERCA) activity nor the IP₃-induced Ca²⁺ release nor the basal Ca²⁺ leak from the ER. Also, prolonged (4 h) treatment with 100μM resveratrol did not affect subsequent IP₃-induced Ca²⁺ release. However, in intact HeLa cells, although resveratrol did not elicit cytosolic Ca²⁺ signals by itself, it acutely decreased the ER Ca²⁺-store content irrespective of the presence or absence of IP₃Rs, leading to a dampened agonist-induced Ca²⁺ signaling. In conclusion, these results reveal that IP₃Rs and cytosolic Ca²⁺ signaling are fundamentally important for driving autophagic flux, not only in response to mTOR inhibition but also in response to non-canonical autophagy inducers like resveratrol. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Axonal Endoplasmic Reticulum Ca2+ Content Controls Release Probability in CNS Nerve Terminals

Although the endoplasmic reticulum (ER) extends throughout axons and axonal ER dysfunction is implicated in numerous neurological diseases, its role at nerve terminals is poorly understood. We developed novel genetically encoded ER-targeted low-affinity Ca²⁺ indicators optimized for examining axonal ER Ca²⁺. Our experiments revealed that presynaptic function is tightly controlled by ER Ca²⁺ content. We found that neuronal activity drives net Ca²⁺ uptake into presynaptic ER although this activity does not contribute significantly to shaping cytosolic Ca²⁺ except during prolonged repetitive firing. In contrast, we found that axonal ER acts as an actuator of plasma membrane (PM) function: [Ca²⁺]_ER controls STIM1 activation in presynaptic terminals, which results in the local modulation of presynaptic function, impacting activity-driven Ca²⁺ entry and release probability. These experiments reveal a critical role of presynaptic ER in the control of neurotransmitter release and will help frame future investigations into the molecular basis of ER-driven neuronal disease states.

DPB162-AE, an inhibitor of store-operated Ca2+ entry, can deplete the endoplasmic reticulum Ca2+ store

Store-operated Ca²⁺ entry (SOCE), an important Ca²⁺ signaling pathway in non-excitable cells, regulates a variety of cellular functions. To study its physiological role, pharmacological tools, like 2-aminoethyl diphenylborinate (2-APB), are used to impact SOCE. 2-APB is one of the best characterized SOCE inhibitors. However, 2-APB also activates SOCE at lower concentrations, while it inhibits inositol 1,4,5-trisphosphate receptors (IP₃Rs), sarco/endoplasmic reticulum Ca²⁺-ATPases (SERCAs) and other ion channels, like TRP channels. Because of this, 2-APB analogues that inhibit SOCE more potently and more selectively compared to 2-APB have been developed. The recently developed DPB162-AE is such a structural diphenylborinate isomer of 2-APB that selectively inhibits SOCE currents by blocking the functional coupling between STIM1 and Orai1. However, we observed an adverse effect of DPB162-AE on the ER Ca²⁺-store content at concentrations required for complete SOCE inhibition. DPB162-AE increased the cytosolic Ca²⁺ levels by reducing the ER Ca²⁺ store in cell lines as well as in primary cells. DPB162-AE did not affect SERCA activity, but provoked a Ca²⁺ leak from the ER, even after application of the SERCA inhibitor thapsigargin. IP₃Rs partly contributed to the DPB162-AE-induced Ca²⁺ leak, since pharmacologically and genetically inhibiting IP₃R function reduced, but not completely blocked, the effects of DPB162-AE on the ER store content. Our results indicate that, in some conditions, the SOCE inhibitor DPB162-AE can reduce the ER Ca²⁺-store content by inducing a Ca²⁺-leak pathway at concentrations needed for adequate SOCE inhibition.

Extensive Ca2+ leak through K4750Q cardiac ryanodine receptors caused by cytosolic and luminal Ca2+ hypersensitivity

The K4750Q mutation in ryanodine receptor 2 causes severe catecholaminergic polymorphic ventricular tachycardia. Uehara et al. reveal extensive Ca²⁺ leak through this mutant receptor and show it is caused by altered gating kinetics, increased Ca²⁺ sensitivity, and the absence of Ca²⁺-dependent inactivation.

Various ryanodine receptor 2 (RyR2) point mutations cause catecholamine-induced polymorphic ventricular tachycardia (CPVT), a life-threatening arrhythmia evoked by diastolic intracellular Ca²⁺ release dysfunction. These mutations occur in essential regions of RyR2 that regulate Ca²⁺ release. The molecular dysfunction caused by CPVT-associated RyR2 mutations as well as the functional consequences remain unresolved. Here, we study the most severe CPVT-associated RyR2 mutation (K4750Q) known to date. We define the molecular and cellular dysfunction generated by this mutation and detail how it alters RyR2 function, using Ca²⁺ imaging, ryanodine binding, and single-channel recordings. HEK293 cells and cardiac HL-1 cells expressing RyR2-K4750Q show greatly enhanced spontaneous Ca²⁺ oscillations. An endoplasmic reticulum–targeted Ca²⁺ sensor, R-CEPIA1er, revealed that RyR2-K4750Q mediates excessive diastolic Ca²⁺ leak, which dramatically reduces luminal [Ca²⁺]. We further show that the K4750Q mutation causes three RyR2 defects: hypersensitization to activation by cytosolic Ca²⁺, loss of cytosolic Ca²⁺/Mg²⁺-mediated inactivation, and hypersensitization to luminal Ca²⁺ activation. These defects combine to kinetically stabilize RyR2-K4750Q openings, thus explaining the extensive diastolic Ca²⁺ leak from the sarcoplasmic reticulum, frequent Ca²⁺ waves, and severe CPVT phenotype. As the multiple concurrent defects are induced by a single point mutation, the K4750 residue likely resides at a critical structural point at which cytosolic and luminal RyR2 control input converge.

Transient Activation of GABAB Receptors Suppresses SK Channel Currents in Substantia Nigra Pars Compacta Dopaminergic Neurons

Dopaminergic (DA) neurons in the substantia nigra pars compacta (SNc) are richly innervated by GABAergic neurons. The postsynaptic effects of GABA on SNc DA neurons are mediated by a mixture of GABAA and GABAB receptors. Although activation of GABAA receptors inhibits spike generation, the consequences of GABAB receptor activation are less well characterized. To help fill this gap, perforated patch recordings were made from young adult mouse SNc DA neurons. Sustained stimulation of GABAB receptors hyperpolarized SNc DA neurons, as previously described. However, transient stimulation of GABAB receptors by optical uncaging of GABA did not; rather, it reduced the opening of small-conductance, calcium-activated K+ (SK) channels and increased the irregularity of spiking. This modulation was attributable to inhibition of adenylyl cyclase and protein kinase A. Thus, because suppression of SK channel activity increases the probability of burst spiking, transient co-activation of GABAA and GABAB receptors could promote a pause-burst pattern of spiking.

Exploring cells with targeted biosensors

Cellular signaling networks are composed of multiple pathways, often interconnected, that form complex networks with great potential for cross-talk. Signal decoding depends on the nature of the message as well as its amplitude, temporal pattern, and spatial distribution. In addition, the existence of membrane-bound organelles, which are both targets and generators of messages, add further complexity to the system. The availability of sensors that can localize to specific compartments in live cells and monitor their targets with high spatial and temporal resolution is thus crucial for a better understanding of cell pathophysiology. For this reason, over the last four decades, a variety of strategies have been developed, not only to generate novel and more sensitive probes for ions, metabolites, and enzymatic activity, but also to selectively deliver these sensors to specific intracellular compartments. In this review, we summarize the principles that have been used to target organic or protein sensors to different cellular compartments and their application to cellular signaling.

Calcium channel regulator Mid1 links TORC2-mediated changes in mitochondrial respiration to autophagy

Autophagy is a catabolic process that recycles cytoplasmic contents and is crucial for cell survival during stress. The target of rapamycin (TOR) kinase regulates autophagy as part of two distinct protein complexes, TORC1 and TORC2. TORC1 negatively regulates autophagy according to nitrogen availability. In contrast, TORC2 functions as a positive regulator of autophagy during amino acid starvation, via its target kinase Ypk1, by repressing the activity of the calcium-dependent phosphatase calcineurin and promoting the general amino acid control (GAAC) response. Precisely how TORC2-Ypk1 signaling regulates calcineurin within this pathway remains unknown. Here we demonstrate that activation of calcineurin requires Mid1, an endoplasmic reticulum-localized calcium channel regulatory protein implicated in the oxidative stress response. We find that normal mitochondrial respiration is perturbed in TORC2-Ypk1-deficient cells, which results in the accumulation of mitochondrial-derived reactive oxygen species that signal to Mid1 to activate calcineurin, thereby inhibiting the GAAC response and autophagy. These findings describe a novel pathway involving TORC2, mitochondrial oxidative stress, and calcium homeostasis for autophagy regulation.

Methods for monitoring signaling molecules in cellular compartments

Cells, irrespective of whether they are from multicellular or single-celled organisms, must communicate with the external environment through dynamic regulation of their internal metabolism, which are critical for their survival. Fluorescent and bioluminescent proteins, and related genetic engineering technologies, have provided new opportunities to investigate the molecular dynamics of cells and their internal compartments, with high spatio-temporal resolution. In this review article, since there is a sufficient number of previous reviews summarizing the history of their development and the techniques behind them, here we will focus on molecular features or technologies that have the potential to further open novel investigations of cellular and subcellular dynamics.

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.

Design and mechanistic insight into ultrafast calcium indicators for monitoring intracellular calcium dynamics

Calmodulin-based genetically encoded fluorescent calcium indicators (GCaMP-s) are powerful tools of imaging calcium dynamics from cells to freely moving animals. High affinity indicators with slow kinetics however distort the temporal profile of calcium transients. Here we report the development of reduced affinity ultrafast variants of GCaMP6s and GCaMP6f. We hypothesized that GCaMP-s have a common kinetic mechanism with a rate-limiting process in the interaction of the RS20 peptide and calcium-calmodulin. Therefore we targeted specific residues in the binding interface by rational design generating improved indicators with GCaMP6f_u displaying fluorescence rise and decay times (t_1/2) of 1 and 3 ms (37 °C) in vitro, 9 and 22-fold faster than GCaMP6f respectively. In HEK293T cells, GCaMP6f_u revealed a 4-fold faster decay of ATP-evoked intracellular calcium transients than GCaMP6f. Stimulation of hippocampal CA1 pyramidal neurons with five action potentials fired at 100 Hz resulted in a single dendritic calcium transient with a 2-fold faster rise and 7-fold faster decay time (t_1/2 of 40 ms) than GCaMP6f, indicating that tracking high frequency action potentials may be limited by calcium dynamics. We propose that the design strategy used for generating GCaMP6f_u is applicable for the acceleration of the response kinetics of GCaMP-type calcium indicators.

The selective Bcl-2 inhibitor venetoclax, a BH3 mimetic, does not dysregulate intracellular Ca2+ signaling

Anti-apoptotic B cell-lymphoma-2 (Bcl-2) proteins are emerging as therapeutic targets in a variety of cancers for precision medicines, like the BH3-mimetic drug venetoclax (ABT-199), which antagonizes the hydrophobic cleft of Bcl-2. However, the impact of venetoclax on intracellular Ca²⁺ homeostasis and dynamics in cell systems has not been characterized in detail. Here, we show that venetoclax did not affect Ca²⁺-transport systems from the endoplasmic reticulum (ER) in permeabilized cell systems. Venetoclax (1μM) did neither trigger Ca²⁺ release by itself nor affect agonist-induced Ca²⁺ release in a variety of intact cell models. Among the different cell types, we also studied two Bcl-2-dependent cancer cell models with a varying sensitivity towards venetoclax, namely SU-DHL-4 and OCI-LY-1, both diffuse large B-cell lymphoma cell lines. Acute application of venetoclax did also not dysregulate Ca²⁺ signaling in these Bcl-2-dependent cancer cells. Moreover, venetoclax-induced cell death was independent of intracellular Ca²⁺ overload, since Ca²⁺ buffering using BAPTA-AM did not suppress venetoclax-induced cell death. This study therefore shows that venetoclax does not dysregulate the intracellular Ca²⁺ homeostasis in a variety of cell types, which may underlie its limited toxicity in human patients. Furthermore, venetoclax-induced cell death in Bcl-2-dependent cancer cells is not mediated by intracellular Ca²⁺ overload. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Mag-Fluo4 in T cells: Imaging of intra-organelle free Ca2+ concentrations

Ca²⁺ signaling is a major signal transduction pathway involved in T cell activation, but also in apoptosis of T cells. Since T cells make use of several Ca²⁺-mobilizing second messengers, such as nicotinic acid adenine dinucleotide phosphate, d-myo-inositol 1,4,5-trisphosphate, and cyclic ADP-ribose, we intended to analyze luminal Ca²⁺ concentration upon cell activation. Mag-Fluo4/AM, a low-affinity Ca²⁺ dye known to localize to the endoplasmic reticular lumen in many cell types, showed superior brightness and bleaching stability, but, surprisingly, co-localized with mito-tracker, but not with ER-tracker in Jurkat T cells. Thus, we used Mag-Fluo4/AM to monitor the free luminal mitochondrial Ca²⁺ concentration ([Ca²⁺]_mito) in these cells. Simultaneous analysis of the free cytosolic Ca²⁺ concentration ([Ca²⁺]_i) and [Ca²⁺]_mito upon cell stimulation revealed that Ca²⁺ signals in the majority of mitochondria were initiated at [Ca²⁺ ]_i≥approx. 400 to 550nM. In primary murine CD4⁺ T cells, Mag-Fluo4 showed two different localization patterns: either co-localization with mito-tracker, as in Jurkat T cells, or with ER-tracker. Thus, in single primary murine CD4⁺ T cells, either decreases of [Ca²⁺ ]_ER or increases of [Ca²⁺ ]_mito were observed upon cell stimulation. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

The Growing and Glowing Toolbox of Fluorescent and Photoactive Proteins

Over the past 20 years, protein engineering has been extensively used to improve and modify the fundamental properties of fluorescent proteins (FPs) with the goal of adapting them for a fantastic range of applications. FPs have been modified by a combination of rational design, structure-based mutagenesis, and countless cycles of directed evolution (gene diversification followed by selection of clones with desired properties) that have collectively pushed the properties to photophysical and biochemical extremes. In this review, we provide both a summary of the progress that has been made during the past two decades, and a broad overview of the current state of FP development and applications in mammalian systems.

A type 2 ryanodine receptor variant associated with reduced Ca2+ release and short-coupled torsades de pointes ventricular arrhythmia

BACKGROUND

Ventricular fibrillation may be caused by premature ventricular contractions (PVCs) whose coupling intervals are <300 ms, a characteristic of the short-coupled variant of torsades de pointes (scTdP).

OBJECTIVE

The purpose of this study was to analyze the underlying cardiac ryanodine receptor (RyR2) variants in patients with scTdP.

METHODS

Seven patients with scTdP (mean age 34 ± 12 years; 4 men and 3 women) were enrolled in this study. The RyR2 gene was screened by targeted gene sequencing methods; variant minor allele frequency was confirmed in 3 databases; and the pathogenicity was investigated in silico analysis using multiple tools. The activity of wild-type and mutant RyR2 channels was evaluated by monitoring Ca²⁺ signals of HEK293 cells with a [³H]ryanodine binding assay.

RESULTS

The mean coupling interval of PVCs was 282 ± 13 ms. The 12-lead electrocardiogram had no specific findings except PVCs with an extremely short-coupling interval. Genetic analysis revealed 3 novel RyR2 variants and 1 polymorphism, all located in the cytoplasmic region. p.Ser4938Phe was not detected in 3 databases, and in silico analysis indicated its pathogenicity. In functional analysis, p.Ser4938Phe demonstrated loss of function and impaired RyR2 channel Ca²⁺ release, while 2 other variants, p.Val1024Ile and p.Ala2673Val, had mild gain-of-function effects but were similar to the polymorphism p.Asn1551Ser.

CONCLUSION

We identified an RyR2 variant associated with reduced Ca²⁺ release and short-coupled torsades de pointes ventricular arrhythmia. The mechanisms of arrhythmogenesis remain unclear.

Genotype-Phenotype Correlations of Malignant Hyperthermia and Central Core Disease Mutations in the Central Region of the RYR1 Channel

Type 1 ryanodine receptor (RYR1) is a Ca²⁺ release channel in the sarcoplasmic reticulum of skeletal muscle and is mutated in some muscle diseases, including malignant hyperthermia (MH) and central core disease (CCD). Over 200 mutations associated with these diseases have been identified, and most mutations accelerate Ca²⁺ -induced Ca²⁺ release (CICR), resulting in abnormal Ca²⁺ homeostasis in skeletal muscle. However, it remains largely unknown how specific mutations cause different phenotypes. In this study, we investigated the CICR activity of 14 mutations at 10 different positions in the central region of RYR1 (10 MH and four MH/CCD mutations) using a heterologous expression system in HEK293 cells. In live-cell Ca²⁺ imaging, the mutant channels exhibited an enhanced sensitivity to caffeine, a reduced endoplasmic reticulum Ca²⁺ content, and an increased resting cytoplasmic Ca²⁺ level. The three parameters for CICR (Ca²⁺ sensitivity for activation, Ca²⁺ sensitivity for inactivation, and attainable maximum activity, i.e., gain) were obtained by [³ H]ryanodine binding and fitting analysis. The mutant channels showed increased gain and Ca²⁺ sensitivity for activation in a site-specific manner. Genotype-phenotype correlations were explained well by the near-atomic structure of RYR1. Our data suggest that divergent CICR activity may cause various disease phenotypes by specific mutations.

Organelle-Specific Sensors for Monitoring Ca2+ Dynamics in Neurons

Calcium (Ca²⁺) plays innumerable critical functions in neurons ranging from regulation of neurotransmitter release and synaptic plasticity to activity-dependent transcription. Therefore, more than any other cell types, neurons are critically dependent on spatially and temporally controlled Ca²⁺ dynamics. This is achieved through an exquisite level of compartmentalization of Ca²⁺ storage and release from various organelles. The function of these organelles in the regulation of Ca²⁺ dynamics has been studied for decades using electrophysiological and optical methods combined with pharmacological and genetic alterations. Mitochondria and the endoplasmic reticulum (ER) are among the organelles playing the most critical roles in Ca²⁺ dynamics in neurons. At presynaptic boutons, Ca²⁺ triggers neurotransmitter release and synaptic plasticity, and postsynaptically, Ca²⁺ mobilization mediates long-term synaptic plasticity. To explore Ca²⁺ dynamics in live cells and intact animals, various synthetic and genetically encoded fluorescent Ca²⁺ sensors were developed, and recently, many groups actively increased the sensitivity and diversity of genetically encoded Ca²⁺ indicators (GECIs). Following conjugation with various signal peptides, these improved GECIs can be targeted to specific subcellular compartments, allowing monitoring of organelle-specific Ca²⁺ dynamics. Here, we review recent findings unraveling novel roles for mitochondria- and ER-dependent Ca²⁺ dynamics in neurons and at synapses.

Dissection of local Ca(2+) signals inside cytosol by ER-targeted Ca(2+) indicator

Calcium (Ca(2+)) is a versatile intracellular second messenger that operates in various signaling pathways leading to multiple biological outputs. The diversity of spatiotemporal patterns of Ca(2+) signals, generated by the coordination of Ca(2+) influx from the extracellular space and Ca(2+) release from the intracellular Ca(2+) store the endoplasmic reticulum (ER), is considered to underlie the diversity of biological outputs caused by a single signaling molecule. However, such Ca(2+) signaling diversity has not been well described because of technical limitations. Here, we describe a new method to report Ca(2+) signals at subcellular resolution. We report that OER-GCaMP6f, a genetically encoded Ca(2+) indicator (GECI) targeted to the outer ER membrane, can monitor Ca(2+) release from the ER at higher spatiotemporal resolution than conventional GCaMP6f. OER-GCaMP6f was used for in vivo Ca(2+) imaging of C. elegans. We also found that the spontaneous Ca(2+) elevation in cultured astrocytes reported by OER-GCaMP6f showed a distinct spatiotemporal pattern from that monitored by plasma membrane-targeted GCaMP6f (Lck-GCaMP6f); less frequent Ca(2+) signal was detected by OER-GCaMP6f, in spite of the fact that Ca(2+) release from the ER plays important roles in astrocytes. These findings suggest that targeting of GECIs to the ER outer membrane enables sensitive detection of Ca(2+) release from the ER at subcellular resolution, avoiding the diffusion of GECI and Ca(2+). Our results indicate that Ca(2+) imaging with OER-GCaMP6f in combination with Lck-GCaMP6f can contribute to describing the diversity of Ca(2+) signals, by enabling dissection of Ca(2+) signals at subcellular resolution.

Off-target effect of the cPLA2α inhibitor pyrrophenone: Inhibition of calcium release from the endoplasmic reticulum

Cytosolic phospholipase A2α (cPLA2α) mediates agonist-induced release of arachidonic acid from membrane phospholipid for production of eicosanoids. The activation of cPLA2α involves increases in intracellular calcium, which binds to the C2 domain and promotes cPLA2α translocation from the cytosol to membrane to access substrate. The cell permeable pyrrolidine-containing cPLA2α inhibitors including pyrrophenone have been useful to understand cPLA2α function. Although this serine hydrolase inhibitor does not inhibit other PLA2s or downstream enzymes that metabolize arachidonic acid, we reported that it blocks increases in mitochondrial calcium and cell death in lung fibroblasts. In this study we used the calcium indicators G-CEPIA1er and CEPIA2mt to compare the effect of pyrrophenone in regulating calcium levels in the endoplasmic reticulum (ER) and mitochondria in response to A23187 and receptor stimulation. Pyrrophenone blocked calcium release from the ER and concomitant increases in mitochondrial calcium in response to stimulation by ATP, serum and A23187. In contrast, ER calcium release induced by the sarco/endoplasmic reticulum Ca(2+)-ATPase inhibitor thapsigargin was not blocked by pyrrophenone suggesting specificity for the calcium release pathway. As a consequence of blocking calcium mobilization, pyrrophenone inhibited serum-stimulated translocation of the cPLA2α C2 domain to Golgi. The ability of pyrrophenone to block ER calcium release is an off-target effect since it occurs in fibroblasts lacking cPLA2α. The results implicate a serine hydrolase in regulating ER calcium release and highlight the importance of careful dose-response studies with pyrrophenone to study cPLA2α function.

In Vivo Gene Transfer to Schwann Cells in the Rodent Sciatic Nerve by Electroporation

The formation of the myelin sheath by Schwann cells (SCs) is essential for rapid conduction of nerve impulses along axons in the peripheral nervous system. SC-selective genetic manipulation in living animals is a powerful technique for studying the molecular and cellular mechanisms of SC myelination and demyelination in vivo. While knockout/knockin and transgenic mice are powerful tools for studying SC biology, these methods are costly and time consuming. Viral vector-mediated transgene introduction into the sciatic nerve is a simpler and less laborious method. However, viral methods have limitations, such as toxicity, transgene size constraints, and infectivity restricted to certain developmental stages. Here, we describe a new method that allows selective transfection of myelinating SCs in the rodent sciatic nerve using electroporation. By applying electric pulses to the sciatic nerve at the site of plasmid DNA injection, genes of interest can be easily silenced or overexpressed in SCs in both neonatal and more mature animals. Furthermore, this in vivo electroporation method allows for highly efficient simultaneous expression of multiple transgenes. Our novel technique should enable researchers to efficiently manipulate SC gene expression, and facilitate studies on SC development and function.

Characterization of the ER-Targeted Low Affinity Ca(2+) Probe D4ER

Calcium ion (Ca²⁺) is a ubiquitous intracellular messenger and changes in its concentration impact on nearly every aspect of cell life. Endoplasmic reticulum (ER) represents the major intracellular Ca²⁺ store and the free Ca²⁺ concentration ([Ca²⁺]) within its lumen ([Ca²⁺]_ER) can reach levels higher than 1 mM. Several genetically-encoded ER-targeted Ca²⁺ sensors have been developed over the last years. However, most of them are non-ratiometric and, thus, their signal is difficult to calibrate in live cells and is affected by shifts in the focal plane and artifactual movements of the sample. On the other hand, existing ratiometric Ca²⁺ probes are plagued by different drawbacks, such as a double dissociation constant (K_d) for Ca²⁺, low dynamic range, and an affinity for the cation that is too high for the levels of [Ca²⁺] in the ER lumen. Here, we report the characterization of a recently generated ER-targeted, Förster resonance energy transfer (FRET)-based, Cameleon probe, named D4ER, characterized by suitable Ca²⁺ affinity and dynamic range for monitoring [Ca²⁺] variations within the ER. As an example, resting [Ca²⁺]_ER have been evaluated in a known paradigm of altered ER Ca²⁺ homeostasis, i.e., in cells expressing a mutated form of the familial Alzheimer’s Disease-linked protein Presenilin 2 (PS2). The lower Ca²⁺ affinity of the D4ER probe, compared to that of the previously generated D1ER, allowed the detection of a conspicuous, more clear-cut, reduction in ER Ca²⁺ content in cells expressing mutated PS2, compared to controls.

Fine tuning of cytosolic Ca (2+) oscillations

Ca ²⁺ oscillations, a widespread mode of cell signaling, were reported in non-excitable cells for the first time more than 25 years ago. Their fundamental mechanism, based on the periodic Ca ²⁺ exchange between the endoplasmic reticulum and the cytoplasm, has been well characterized. However, how the kinetics of cytosolic Ca ²⁺ changes are related to the extent of a physiological response remains poorly understood. Here, we review data suggesting that the downstream targets of Ca ²⁺ are controlled not only by the frequency of Ca ²⁺ oscillations but also by the detailed characteristics of the oscillations, such as their duration, shape, or baseline level. Involvement of non-endoplasmic reticulum Ca ²⁺ stores, mainly mitochondria and the extracellular medium, participates in this fine tuning of Ca ²⁺ oscillations. The main characteristics of the Ca ²⁺ exchange fluxes with these compartments are also reviewed.

Genetically Encoded Fluorescent Indicators for Organellar Calcium Imaging

Optical Ca(2+) indicators are powerful tools for investigating intracellular Ca(2+) signals in living cells. Although a variety of Ca(2+) indicators have been developed, deciphering the physiological functions and spatiotemporal dynamics of Ca(2+) in intracellular organelles remains challenging. Genetically encoded Ca(2+) indicators (GECIs) using fluorescent proteins are promising tools for organellar Ca(2+) imaging, and much effort has been devoted to their development. In this review, we first discuss the key points of organellar Ca(2+) imaging and summarize the requirements for optimal organellar Ca(2+) indicators. Then, we highlight some of the recent advances in the engineering of fluorescent GECIs targeted to specific organelles. Finally, we discuss the limitations of currently available GECIs and the requirements for advancing the research on intraorganellar Ca(2+) signaling.

R-CEPIA1er as a new tool to directly measure sarcoplasmic reticulum [Ca] in ventricular myocytes

In cardiomyocytes, [Ca] within the sarcoplasmic reticulum (SR; [Ca]SR) partially determines the amplitude of cytosolic Ca transient that, in turn, governs myocardial contraction. Therefore, it is critical to understand the molecular mechanisms that regulate [Ca]SR handling. Until recently, the best approach available to directly measure [Ca]SR was to use low-affinity Ca indicators (e.g., Fluo-5N). However, this approach presents several limitations, including nonspecific cellular localization, dye extrusion, and species limitation. Recently a new genetically encoded family of Ca indicators has been generated, named Ca-measuring organelle-entrapped protein indicators (CEPIA). Here, we tested the red fluorescence SR-targeted Ca sensor (R-CEPIA1er) as a tool to directly measure [Ca]SR dynamics in ventricular myocytes. Infection of rabbit and rat ventricular myocytes with an adenovirus expressing the R-CEPIA1er gene displayed prominent localization in the SR and nuclear envelope. Calibration of R-CEPIA1er in myocytes resulted in a Kd of 609 μM, suggesting that this sensor is sensitive in the whole physiological range of [Ca]SR [Ca]SR dynamics measured with R-CEPIA1er were compared with [Ca]SR measured with Fluo5-N. We found that both the time course of the [Ca]SR depletion and fractional SR Ca release induced by an action potential were similar between these two Ca sensors. R-CEPIA1er fluorescence did not decline during experiments, indicating lack of dye extrusion or photobleaching. Furthermore, measurement of [Ca]SR with R-CEPIA1er can be combined with cytosolic [Ca] measurements (with Fluo-4) to obtain more detailed information regarding Ca handling in cardiac myocytes. In conclusion, R-CEPIA1er is a promising tool that can be used to measure [Ca]SR dynamics in myocytes from different animal species.

T-Type voltage-sensitive calcium channels mediate mechanically-induced intracellular calcium oscillations in osteocytes by regulating endoplasmic reticulum calcium dynamics

One of the earliest responses of bone cells to mechanical stimuli is a rise in intracellular calcium (Ca(2+)), and osteocytes in particular exhibit robust oscillations in Ca(2+) when subjected to loading. Previous studies implicate roles for both the endoplasmic reticulum (ER) and T-Type voltage-sensitive calcium channels (VSCC) in these responses, but their interactions or relative contributions have not been studied. By observing Ca(2+) dynamics in the cytosol (Ca(2+)cyt) and the ER (Ca(2+)ER), the focus of this study was to explore the role of the ER and T-Type channels in Ca(2+) signaling in bone cells. We demonstrate that inhibition of T-Type VSCC in osteocytes significantly reduces the number of Ca(2+)cyt responses and affects Ca(2+)ER depletion dynamics. Simultaneous observation of Ca(2+) exchange among these spaces revealed high synchrony between rises in Ca(2+)cyt and depressions in Ca(2+)ER, and this synchrony was significantly reduced by challenging T-Type VSCC. We further confirmed that this effect was mediated directly through the ER and not through store-operated Ca(2+) entry (SOCE) pathways. Taken together, our data suggests that T-Type VSCC facilitate the recovery of Ca(2+)ER in osteocytes to sustain mechanically-induced Ca(2+) oscillations, uncovering a new mechanism underlying the behavior of osteocytes as mechanosensors.

GFP-Aequorin Protein Sensor for Ex Vivo and In Vivo Imaging of Ca(2+) Dynamics in High-Ca(2+) Organelles

Proper functioning of organelles such as the ER or the Golgi apparatus requires luminal accumulation of Ca(2+) at high concentrations. Here we describe a ratiometric low-affinity Ca(2+) sensor of the GFP-aequorin protein (GAP) family optimized for measurements in high-Ca(2+) concentration environments. Transgenic animals expressing the ER-targeted sensor allowed monitoring of Ca(2+) signals inside the organelle. The use of the sensor was demonstrated under three experimental paradigms: (1) ER Ca(2+) oscillations in cultured astrocytes, (2) ex vivo functional mapping of cholinergic receptors triggering ER Ca(2+) release in acute hippocampal slices from transgenic mice, and (3) in vivo sarcoplasmic reticulum Ca(2+) dynamics in the muscle of transgenic flies. Our results provide proof of the suitability of the new biosensors to monitor Ca(2+) dynamics inside intracellular organelles under physiological conditions and open an avenue to explore complex Ca(2+) signaling in animal models of health and disease.

Bioinspired design of a polymer gel sensor for the realization of extracellular Ca(2+) imaging

Although the role of extracellular Ca²⁺ draws increasing attention as a messenger in intercellular communications, there is currently no tool available for imaging Ca²⁺ dynamics in extracellular regions. Here we report the first solid-state fluorescent Ca²⁺ sensor that fulfills the essential requirements for realizing extracellular Ca²⁺ imaging. Inspired by natural extracellular Ca²⁺-sensing receptors, we designed a particular type of chemically-crosslinked polyacrylic acid gel, which can undergo single-chain aggregation in the presence of Ca²⁺. By attaching aggregation-induced emission luminogen to the polyacrylic acid as a pendant, the conformational state of the main chain at a given Ca²⁺ concentration is successfully translated into fluorescence property. The Ca²⁺ sensor has a millimolar-order apparent dissociation constant compatible with extracellular Ca²⁺ concentrations, and exhibits sufficient dynamic range and excellent selectivity in the presence of physiological concentrations of biologically relevant ions, thus enabling monitoring of submillimolar fluctuations of Ca²⁺ in flowing analytes containing millimolar Ca²⁺ concentrations.

Ca2+ dialogue between acidic vesicles and ER

Extracellular stimuli evoke the synthesis of intracellular second messengers, several of which couple to the release of Ca2+ from Ca2+-storing organelles via activation of cognate organellar Ca2+-channel complexes. The archetype is the inositol 1,4,5-trisphosphate (IP3) and IP3 receptor (IP3R) on the endoplasmic reticulum (ER). A less understood, parallel Ca2+ signalling cascade is that involving the messenger nicotinic acid adenine dinucleotide phosphate (NAADP) that couples to Ca2+ release from acidic Ca2+ stores [e.g. endo-lysosomes, secretory vesicles, lysosome-related organelles (LROs)]. NAADP-induced Ca2+ release absolutely requires organellar TPCs (two-pore channels). This review discusses how ER and acidic Ca2+ stores physically and functionally interact to generate and shape global and local Ca2+ signals, with particular emphasis on the two-way dialogue between these two organelles.

Probing the Complexities of Astrocyte Calcium Signaling

Astrocytes are abundant glial cells that tile the entire central nervous system and mediate well-established functions for neurons, blood vessels, and other glia. These ubiquitous cells display intracellular Ca(2+) signals, which have been intensely studied for 25 years. Recently, the use of improved methods has unearthed the panoply of astrocyte Ca(2+) signals and a variable landscape of basal Ca(2+) levels. In vivo studies have started to reveal the settings under which astrocytes display behaviorally relevant Ca(2+) signaling. Studies in mice have emphasized how astrocyte Ca(2+) signaling is altered in distinct neurodegenerative diseases. Progress in the past few years, fueled by methodological advances, has thus reignited interest in astrocyte Ca(2+) signaling for brain function and dysfunction.

Visualization of Ca2+ Filling Mechanisms upon Synaptic Inputs in the Endoplasmic Reticulum of Cerebellar Purkinje Cells

The endoplasmic reticulum (ER) plays crucial roles in intracellular Ca(2+) signaling, serving as both a source and sink of Ca(2+), and regulating a variety of physiological and pathophysiological events in neurons in the brain. However, spatiotemporal Ca(2+) dynamics within the ER in central neurons remain to be characterized. In this study, we visualized synaptic activity-dependent ER Ca(2+) dynamics in mouse cerebellar Purkinje cells (PCs) using an ER-targeted genetically encoded Ca(2+) indicator, G-CEPIA1er. We used brief parallel fiber stimulation to induce a local decrease in the ER luminal Ca(2+) concentration ([Ca(2+)]ER) in dendrites and spines. In this experimental system, the recovery of [Ca(2+)]ER takes several seconds, and recovery half-time depends on the extent of ER Ca(2+) depletion. By combining imaging analysis and numerical simulation, we show that the intraluminal diffusion of Ca(2+), rather than Ca(2+) reuptake, is the dominant mechanism for the replenishment of the local [Ca(2+)]ER depletion immediately following the stimulation. In spines, the ER filled almost simultaneously with parent dendrites, suggesting that the ER within the spine neck does not represent a significant barrier to Ca(2+) diffusion. Furthermore, we found that repetitive climbing fiber stimulation, which induces cytosolic Ca(2+) spikes in PCs, cumulatively increased [Ca(2+)]ER. These results indicate that the neuronal ER functions both as an intracellular tunnel to redistribute stored Ca(2+) within the neurons, and as a leaky integrator of Ca(2+) spike-inducing synaptic inputs.

Interplay Between Intracellular Ca(2+) Oscillations and Ca(2+)-stimulated Mitochondrial Metabolism

Oscillations of cytosolic Ca(2+) concentration are a widespread mode of signalling. Oscillatory spikes rely on repetitive exchanges of Ca(2+) between the endoplasmic reticulum (ER) and the cytosol, due to the regulation of inositol 1,4,5-trisphosphate receptors. Mitochondria also sequester and release Ca(2+), thus affecting Ca(2+) signalling. Mitochondrial Ca(2+) activates key enzymes involved in ATP synthesis. We propose a new integrative model for Ca(2+) signalling and mitochondrial metabolism in electrically non-excitable cells. The model accounts for (1) the phase relationship of the Ca(2+) changes in the cytosol, the ER and mitochondria, (2) the dynamics of mitochondrial metabolites in response to cytosolic Ca(2+) changes, and (3) the impacts of cytosol/mitochondria Ca(2+) exchanges and of mitochondrial metabolism on Ca(2+) oscillations. Simulations predict that as expected, oscillations are slowed down by decreasing the rate of Ca(2+) efflux from mitochondria, but also by decreasing the rate of Ca(2+) influx through the mitochondrial Ca(2+) uniporter (MCU). These predictions were experimentally validated by inhibiting MCU expression. Despite the highly non-linear character of Ca(2+) dynamics and mitochondrial metabolism, bioenergetics were found to be robust with respect to changes in frequency and amplitude of Ca(2+) oscillations.

Use of genetically-encoded calcium indicators for live cell calcium imaging and localization in virus-infected cells

Calcium signaling is a ubiquitous and versatile process involved in nearly every cellular process, and exploitation of host calcium signals is a common strategy used by viruses to facilitate replication and cause disease. Small molecule fluorescent calcium dyes have been used by many to examine changes in host cell calcium signaling and calcium channel activation during virus infections, but disadvantages of these dyes, including poor loading and poor long-term retention, complicate analysis of calcium imaging in virus-infected cells due to changes in cell physiology and membrane integrity. The recent expansion of genetically-encoded calcium indicators (GECIs), including blue and red-shifted color variants and variants with calcium affinities appropriate for calcium storage organelles like the endoplasmic reticulum (ER), make the use of GECIs an attractive alternative for calcium imaging in the context of virus infections. Here we describe the development and testing of cell lines stably expressing both green cytoplasmic (GCaMP5G and GCaMP6s) and red ER-targeted (RCEPIAer) GECIs. Using three viruses (rotavirus, poliovirus and respiratory syncytial virus) previously shown to disrupt host calcium homeostasis, we show the GECI cell lines can be used to detect simultaneous cytoplasmic and ER calcium signals. Further, we demonstrate the GECI expression has sufficient stability to enable long-term confocal imaging of both cytoplasmic and ER calcium during the course of virus infections.

Fast-Response Calmodulin-Based Fluorescent Indicators Reveal Rapid Intracellular Calcium Dynamics

Faithful reporting of temporal patterns of intracellular Ca²⁺ dynamics requires the working range of indicators to match the signals. Current genetically encoded calmodulin-based fluorescent indicators are likely to distort fast Ca²⁺ signals by apparent saturation and integration due to their limiting fluorescence rise and decay kinetics. A series of probes was engineered with a range of Ca²⁺ affinities and accelerated kinetics by weakening the Ca²⁺-calmodulin-peptide interactions. At 37 °C, the GCaMP3-derived probe termed GCaMP3_fast is 40-fold faster than GCaMP3 with Ca²⁺ decay and rise times, t_1/2, of 3.3 ms and 0.9 ms, respectively, making it the fastest to-date. GCaMP3_fast revealed discreet transients with significantly faster Ca²⁺ dynamics in neonatal cardiac myocytes than GCaMP6f. With 5-fold increased two-photon fluorescence cross-section for Ca²⁺ at 940 nm, GCaMP3_fast is suitable for deep tissue studies. The green fluorescent protein serves as a reporter providing important novel insights into the kinetic mechanism of target recognition by calmodulin. Our strategy to match the probe to the signal by tuning the affinity and hence the Ca²⁺ kinetics of the indicator is applicable to the emerging new generations of calmodulin-based probes.

BAX inhibitor-1 is a Ca(2+) channel critically important for immune cell function and survival

The endoplasmic reticulum (ER) serves as the major intracellular Ca(2+) store and has a role in the synthesis and folding of proteins. BAX (BCL2-associated X protein) inhibitor-1 (BI-1) is a Ca(2+) leak channel also implicated in the response against protein misfolding, thereby connecting the Ca(2+) store and protein-folding functions of the ER. We found that BI-1-deficient mice suffer from leukopenia and erythrocytosis, have an increased number of splenic marginal zone B cells and higher abundance and nuclear translocation of NF-κB (nuclear factor-κ light-chain enhancer of activated B cells) proteins, correlating with increased cytosolic and ER Ca(2+) levels. When put into culture, purified knockout T cells and even more so B cells die spontaneously. This is preceded by increased activity of the mitochondrial initiator caspase-9 and correlated with a significant surge in mitochondrial Ca(2+) levels, suggesting an exhausted mitochondrial Ca(2+) buffer capacity as the underlying cause for cell death in vitro. In vivo, T-cell-dependent experimental autoimmune encephalomyelitis and B-cell-dependent antibody production are attenuated, corroborating the ex vivo results. These results suggest that BI-1 has a major role in the functioning of the adaptive immune system by regulating intracellular Ca(2+) homeostasis in lymphocytes.

A Low Affinity GCaMP3 Variant (GCaMPer) for Imaging the Endoplasmic Reticulum Calcium Store

Endoplasmic reticulum calcium homeostasis is critical for cellular functions and is disrupted in diverse pathologies including neurodegeneration and cardiovascular disease. Owing to the high concentration of calcium within the ER, studying this subcellular compartment requires tools that are optimized for these conditions. To develop a single-fluorophore genetically encoded calcium indicator for this organelle, we targeted a low affinity variant of GCaMP3 to the ER lumen (GCaMPer (10.19)). A set of viral vectors was constructed to express GCaMPer in human neuroblastoma cells, rat primary cortical neurons, and human induced pluripotent stem cell-derived cardiomyocytes. We observed dynamic changes in GCaMPer (10.19) fluorescence in response to pharmacologic manipulations of the ER calcium store. Additionally, periodic calcium efflux from the ER was observed during spontaneous beating of cardiomyocytes. GCaMPer (10.19) has utility in imaging ER calcium in living cells and providing insight into luminal calcium dynamics under physiologic and pathologic states.

Neuronal Regulation of Schwann Cell Mitochondrial Ca(2+) Signaling during Myelination

Schwann cells (SCs) myelinate peripheral neurons to promote the rapid conduction of action potentials, and the process of myelination is known to be regulated by signals from axons to SCs. Given that SC mitochondria are one of the potential regulators of myelination, we investigated whether SC mitochondria are regulated by axonal signaling. Here, we show a purinergic mechanism that sends information from neurons to SC mitochondria during myelination. Our results show that electrical stimulation of rat sciatic nerve increases extracellular ATP levels enough to activate purinergic receptors. Indeed, electrical stimulation of sciatic nerves induces Ca(2+) increases in the cytosol and the mitochondrial matrix of surrounding SCs via purinergic receptor activation. Chronic suppression of this pathway during active myelination suppressed the longitudinal and radial development of myelinating SCs and caused hypomyelination. These results demonstrate a neuron-to-SC mitochondria signaling, which is likely to have an important role in proper myelination.

Monitoring Endoplasmic Reticulum Calcium Homeostasis Using a Gaussia Luciferase SERCaMP

The endoplasmic reticulum (ER) contains the highest level of intracellular calcium, with concentrations approximately 5,000-fold greater than cytoplasmic levels. Tight control over ER calcium is imperative for protein folding, modification and trafficking. Perturbations to ER calcium can result in the activation of the unfolded protein response, a three-prong ER stress response mechanism, and contribute to pathogenesis in a variety of diseases. The ability to monitor ER calcium alterations during disease onset and progression is important in principle, yet challenging in practice. Currently available methods for monitoring ER calcium, such as calcium-dependent fluorescent dyes and proteins, have provided insight into ER calcium dynamics in cells, however these tools are not well suited for in vivo studies. Our lab has demonstrated that a modification to the carboxy-terminus of Gaussia luciferase confers secretion of the reporter in response to ER calcium depletion. The methods for using a luciferase based, secreted ER calcium monitoring protein (SERCaMP) for in vitro and in vivo applications are described herein. This video highlights hepatic injections, pharmacological manipulation of GLuc-SERCaMP, blood collection and processing, and assay parameters for longitudinal monitoring of ER calcium.

Downregulation of STIM2 improves neuronal survival after traumatic brain injury by alleviating calcium overload and mitochondrial dysfunction

Although store-operated calcium entry (SOCE) has been implicated in several neurological disorders, the exact mechanism for its role in traumatic brain injury (TBI) has not been elucidated. In this study, we found that TBI upregulated the expression of a calcium sensor protein called stromal interactive molecule 2 (STIM2); however, the levels of its homologue, STIM1, were unaffected. Both STIM1 and STIM2 are crucial components of SOCE, both in vivo and in vitro. Using shRNA, we discovered that downregulation of STIM2, but not STIM1, significantly improved neuronal survival in both an in vitro and in vivo model of TBI, decreasing neuronal apoptosis, and preserving neurological function. This neuroprotection was associated with alleviating TBI-induced calcium overload and preserving mitochondrial function. Additionally, downregulation of STIM2 not only inhibited Ca(2+) release from the endoplasmic reticulum (ER), but also reduced SOCE-mediated Ca(2+) influx, decreased mitochondrial Ca(2+), restored mitochondrial morphology and improved mitochondrial function, including MMP maintenance, ROS production and ATP synthesis. These results indicate that inhibition of STIM2 can protect neurons from TBI by inhibiting calcium overload and preserving mitochondrial function. This suggests that STIM2 might be an effective interventional target for TBI.

Divergent Activity Profiles of Type 1 Ryanodine Receptor Channels Carrying Malignant Hyperthermia and Central Core Disease Mutations in the Amino-Terminal Region

The type 1 ryanodine receptor (RyR1) is a Ca²⁺ release channel in the sarcoplasmic reticulum of skeletal muscle and is mutated in several diseases, including malignant hyperthermia (MH) and central core disease (CCD). Most MH and CCD mutations cause accelerated Ca²⁺ release, resulting in abnormal Ca²⁺ homeostasis in skeletal muscle. However, how specific mutations affect the channel to produce different phenotypes is not well understood. In this study, we have investigated 11 mutations at 7 different positions in the amino (N)-terminal region of RyR1 (9 MH and 2 MH/CCD mutations) using a heterologous expression system in HEK293 cells. In live-cell Ca²⁺ imaging at room temperature (~25 °C), cells expressing mutant channels exhibited alterations in Ca²⁺ homeostasis, i.e., an enhanced sensitivity to caffeine, a depletion of Ca²⁺ in the ER and an increase in resting cytoplasmic Ca²⁺. RyR1 channel activity was quantitatively evaluated by [³H]ryanodine binding and three parameters (sensitivity to activating Ca²⁺, sensitivity to inactivating Ca²⁺ and attainable maximum activity, i.e., gain) were obtained by fitting analysis. The mutations increased the gain and the sensitivity to activating Ca²⁺ in a site-specific manner. The gain was consistently higher in both MH and MH/CCD mutations. Sensitivity to activating Ca²⁺ was markedly enhanced in MH/CCD mutations. The channel activity estimated from the three parameters provides a reasonable explanation to the pathological phenotype assessed by Ca²⁺ homeostasis. These properties were also observed at higher temperatures (~37 °C). Our data suggest that divergent activity profiles may cause varied disease phenotypes by specific mutations. This approach should be useful for diagnosis and treatment of diseases with mutations in RyR1.

Microtubule-Associated Protein EB3 Regulates IP3 Receptor Clustering and Ca(2+) Signaling in Endothelial Cells

The mechanisms by which the microtubule cytoskeleton regulates the permeability of endothelial barrier are not well understood. Here, we demonstrate that microtubule-associated end-binding protein 3 (EB3), a core component of the microtubule plus-end protein complex, binds to inositol 1,4,5-trisphosphate receptors (IP3Rs) through an S/TxIP EB-binding motif. In endothelial cells, α-thrombin, a pro-inflammatory mediator that stimulates phospholipase Cβ, increases the cytosolic Ca(2+) concentration and elicits clustering of IP3R3s. These responses, and the resulting Ca(2+)-dependent phosphorylation of myosin light chain, are prevented by depletion of either EB3 or mutation of the TxIP motif of IP3R3 responsible for mediating its binding to EB3. We also show that selective EB3 gene deletion in endothelial cells of mice abrogates α-thrombin-induced increase in endothelial permeability. We conclude that the EB3-mediated interaction of IP3Rs with microtubules controls the assembly of IP3Rs into effective Ca(2+) signaling clusters, which thereby regulate microtubule-dependent endothelial permeability.

Ryanodine receptors are targeted by anti-apoptotic Bcl-XL involving its BH4 domain and Lys87 from its BH3 domain

Anti-apoptotic B-cell lymphoma 2 (Bcl-2) family members target several intracellular Ca(2+)-transport systems. Bcl-2, via its N-terminal Bcl-2 homology (BH) 4 domain, inhibits both inositol 1,4,5-trisphosphate receptors (IP3Rs) and ryanodine receptors (RyRs), while Bcl-XL, likely independently of its BH4 domain, sensitizes IP3Rs. It remains elusive whether Bcl-XL can also target and modulate RyRs. Here, Bcl-XL co-immunoprecipitated with RyR3 expressed in HEK293 cells. Mammalian protein-protein interaction trap (MAPPIT) and surface plasmon resonance (SPR) showed that Bcl-XL bound to the central domain of RyR3 via its BH4 domain, although to a lesser extent compared to the BH4 domain of Bcl-2. Consistent with the ability of the BH4 domain of Bcl-XL to bind to RyRs, loading the BH4-Bcl-XL peptide into RyR3-overexpressing HEK293 cells or in rat hippocampal neurons suppressed RyR-mediated Ca(2+) release. In silico superposition of the 3D-structures of Bcl-2 and Bcl-XL indicated that Lys87 of the BH3 domain of Bcl-XL could be important for interacting with RyRs. In contrast to Bcl-XL, the Bcl-XL(K87D) mutant displayed lower binding affinity for RyR3 and a reduced inhibition of RyR-mediated Ca(2+) release. These data suggest that Bcl-XL binds to RyR channels via its BH4 domain, but also its BH3 domain, more specific Lys87, contributes to the interaction.

Fluorescence methods for analysis of interactions between Ca(2+) signaling, lysosomes, and endoplasmic reticulum

The endoplasmic reticulum (ER) is both the major source of intracellular Ca(2+) for cell signaling and the organelle that forms the most extensive contacts with the plasma membrane and other organelles. Lysosomes fulfill important roles in degrading cellular materials and in cholesterol handling, but they also contribute to Ca(2+) signaling by both releasing and sequestering Ca(2+). Interactions between ER and other Ca(2+)-transporting membranes, notably mitochondria and the plasma membrane, often occur at sites where the two membranes are closely apposed, allowing local Ca(2+) signaling between them. These interactions are often facilitated by scaffold proteins. Recent evidence suggests similar local interactions between ER and lysosomes. We describe simple fluorescence-based methods that allow the interplay between Ca(2+) signals, the ER, and lysosomes to be examined.

Rapid recycling of Ca2+ between IP3-sensitive stores and lysosomes

Inositol 1,4,5-trisphosphate (IP₃) evokes release of Ca²⁺ from the endoplasmic reticulum (ER), but the resulting Ca²⁺ signals are shaped by interactions with additional intracellular organelles. Bafilomycin A₁, which prevents lysosomal Ca²⁺ uptake by inhibiting H⁺ pumping into lysosomes, increased the amplitude of the initial Ca²⁺ signals evoked by carbachol in human embryonic kidney (HEK) cells. Carbachol alone and carbachol in combination with parathyroid hormone (PTH) evoke Ca²⁺ release from distinct IP₃-sensitive Ca²⁺ stores in HEK cells stably expressing human type 1 PTH receptors. Bafilomycin A₁ similarly exaggerated the Ca²⁺ signals evoked by carbachol or carbachol with PTH, indicating that Ca²⁺ released from distinct IP₃-sensitive Ca²⁺ stores is sequestered by lysosomes. The Ca²⁺ signals resulting from store-operated Ca²⁺ entry, whether evoked by thapsigargin or carbachol, were unaffected by bafilomycin A₁. Using Gd³⁺ (1 mM) to inhibit both Ca²⁺ entry and Ca²⁺ extrusion, HEK cells were repetitively stimulated with carbachol to assess the effectiveness of Ca²⁺ recycling to the ER after IP₃-evoked Ca²⁺ release. Blocking lysosomal Ca²⁺ uptake with bafilomycin A₁ increased the amplitude of each carbachol-evoked Ca²⁺ signal without affecting the rate of Ca²⁺ recycling to the ER. This suggests that Ca²⁺ accumulated by lysosomes is rapidly returned to the ER. We conclude that lysosomes rapidly, reversibly and selectively accumulate the Ca²⁺ released by IP₃ receptors residing within distinct Ca²⁺ stores, but not the Ca²⁺ entering cells via receptor-regulated, store-operated Ca²⁺ entry pathways.

Calcium signalling in diabetes

Molecular cascades responsible for Ca(2+) homeostasis and Ca(2+) signalling could be assembled in highly plastic toolkits that define physiological adaptation of cells to the environment and which are intimately involved in all types of cellular pathology. Control over Ca(2+) concentration in different cellular compartments is intimately linked to cell metabolism, because (i) ATP production requires low Ca(2+), (ii) Ca(2+) homeostatic systems consume ATP and (iii) Ca(2+) signals in mitochondria stimulate ATP synthesis being an essential part of excitation-metabolic coupling. The communication between the ER and mitochondria plays an important role in this metabolic fine tuning. In the insulin resistance state and diabetes this communication has been impaired leading to different disorders, for instance, diminished insulin production by pancreatic β cells, reduced heart and skeletal muscle contractility, reduced NO production by endothelial cells, increased glucose production by liver, increased lipolysis by adipose cells, reduced immune responses, reduced cognitive functions, among others. All these processes eventually trigger degenerative events resulting in overt diabetes due to reduction of pancreatic β cell mass, and different complications of diabetes, such as retinopathy, nephropathy, neuropathy, and different cardiovascular diseases.

Endoplasmic reticulum stress in insulin resistance and diabetes

The endoplasmic reticulum is the main intracellular Ca(2+) store for Ca(2+) release during cell signaling. There are different strategies to avoid ER Ca(2+) depletion. Release channels utilize first Ca(2+)-bound to proteins and this minimizes the reduction of the free luminal [Ca(2+)]. However, if release channels stay open after exhaustion of Ca(2+)-bound to proteins, then the reduction of the free luminal ER [Ca(2+)] (via STIM proteins) activates Ca(2+) entry at the plasma membrane to restore the ER Ca(2+) load, which will work provided that SERCA pump is active. Nevertheless, there are several noxious conditions that result in decreased activity of the SERCA pump such as oxidative stress, inflammatory cytokines, and saturated fatty acids, among others. These conditions result in a deficient restoration of the ER [Ca(2+)] and lead to the ER stress response that should facilitate recovery of the ER. However, if the stressful condition persists then ER stress ends up triggering cell death and the ensuing degenerative process leads to diverse pathologies; particularly insulin resistance, diabetes and several of the complications associated with diabetes. This scenario suggests that limiting ER stress should decrease the incidence of diabetes and the mobility and mortality associated with this illness.