Source of nuclear calcium signals

Microrobotic Swarms for Intracellular Measurement with Enhanced Signal-to-Noise Ratio

In cell biology, fluorescent dyes are routinely used for biochemical measurements. The traditional global dye treatment method suffers from low signal-to-noise ratios (SNR), especially when used for detecting a low concentration of ions, and increasing the concentration of fluorescent dyes causes more severe cytotoxicity. Here, we report a robotic technique that controls how a low amount of fluorescent-dye-coated magnetic nanoparticles accurately forms a swarm and increases the fluorescent dye concentration in a local region inside a cell for intracellular measurement. Different from existing magnetic micromanipulation systems that generate large swarms (several microns and above) or that cannot move the generated swarm to an arbitrary position, our system is capable of generating a small swarm (e.g., 1 μm) and accurately positioning the swarm inside a single cell (position control accuracy: 0.76 μm). In experiments, the generated swarm inside the cell showed an SNR 10 times higher than the traditional global dye treatment method. The high-SNR robotic swarm enabled intracellular measurements that had not been possible to achieve with traditional global dye treatment. The robotic swarm technique revealed an apparent pH gradient in a migrating cell and was used to measure the intracellular apparent pH in a single oocyte of living C. elegans. With the position control capability, the swarm was also applied to measure calcium changes at the perinuclear region of a cell before and after mechanical stimulation. The results showed a significant calcium increase after mechanical stimulation, and the calcium increase was regulated by the mechanically sensitive ion channel, PIEZO1.

The bitter end: T2R bitter receptor agonists elevate nuclear calcium and induce apoptosis in non-ciliated airway epithelial cells

Bitter taste receptors (T2Rs) localize to airway motile cilia and initiate innate immune responses in retaliation to bacterial quorum sensing molecules. Activation of cilia T2Rs leads to calcium-driven NO production that increases cilia beating and directly kills bacteria. Several diseases, including chronic rhinosinusitis, COPD, and cystic fibrosis, are characterized by loss of motile cilia and/or squamous metaplasia. To understand T2R function within the altered landscape of airway disease, we studied T2Rs in non-ciliated airway cell lines and primary cells. Several T2Rs localize to the nucleus in de-differentiated cells that typically localize to cilia in differentiated cells. As cilia and nuclear import utilize shared proteins, some T2Rs may target to the nucleus in the absence of motile cilia. T2R agonists selectively elevated nuclear and mitochondrial calcium through a G-protein-coupled receptor phospholipase C mechanism. Additionally, T2R agonists decreased nuclear cAMP, increased nitric oxide, and increased cGMP, consistent with T2R signaling. Furthermore, exposure to T2R agonists led to nuclear calcium-induced mitochondrial depolarization and caspase activation. T2R agonists induced apoptosis in primary bronchial and nasal cells differentiated at air-liquid interface but then induced to a squamous phenotype by apical submersion. Air-exposed well-differentiated cells did not die. This may be a last-resort defense against bacterial infection. However, it may also increase susceptibility of de-differentiated or remodeled epithelia to damage by bacterial metabolites. Moreover, the T2R-activated apoptosis pathway occurs in airway cancer cells. T2Rs may thus contribute to microbiome-tumor cell crosstalk in airway cancers. Targeting T2Rs may be useful for activating cancer cell apoptosis while sparing surrounding tissue.

TRPC6, a therapeutic target for pulmonary hypertension

Idiopathic pulmonary arterial hypertension (PAH) is a fatal and progressive disease. Sustained vasoconstriction due to pulmonary arterial smooth muscle cell (PASMC) contraction and concentric arterial remodeling due partially to PASMC proliferation are the major causes for increased pulmonary vascular resistance and increased pulmonary arterial pressure in patients with precapillary pulmonary hypertension (PH) including PAH and PH due to respiratory diseases or hypoxemia. We and others observed upregulation of TRPC6 channels in PASMCs from patients with PAH. A rise in cytosolic Ca2+ concentration ([Ca2+]cyt) in PASMC triggers PASMC contraction and vasoconstriction, while Ca2+-dependent activation of PI3K/AKT/mTOR pathway is a pivotal signaling cascade for cell proliferation and gene expression. Despite evidence supporting a pathological role of TRPC6, no selective and orally bioavailable TRPC6 antagonist has yet been developed and tested for treatment of PAH or PH. In this study, we sought to investigate whether block of receptor-operated Ca2+ channels using a nonselective blocker of cation channels, 2-aminoethyl diphenylborinate (2-APB, administered intraperitoneally) and a selective blocker of TRPC6, BI-749327 (administered orally) can reverse established PH in mice. The results from the study show that intrapulmonary application of 2-APB (40 µM) or BI-749327 (3-10 µM) significantly and reversibly inhibited acute alveolar hypoxia-induced pulmonary vasoconstriction. Intraperitoneal injection of 2-APB (1 mg/kg per day) significantly attenuated the development of PH and partially reversed established PH in mice. Oral gavage of BI-749327 (30 mg/kg, every day, for 2 wk) reversed established PH by ∼50% via regression of pulmonary vascular remodeling. Furthermore, 2-APB and BI-749327 both significantly inhibited PDGF- and serum-mediated phosphorylation of AKT and mTOR in PASMC. In summary, the receptor-operated and mechanosensitive TRPC6 channel is a good target for developing novel treatment for PAH/PH. BI-749327, a selective TRPC6 blocker, is potentially a novel and effective drug for treating PAH and PH due to respiratory diseases or hypoxemia.

Anti-Cancer Agents in Proliferation and Cell Death: The Calcium Connection

Calcium (Ca2+) signaling and the modulation of intracellular calcium ([Ca2+]i) levels play critical roles in several key processes that regulate cellular survival, growth, differentiation, metabolism, and death in normal cells. On the other hand, aberrant Ca2+-signaling and loss of [Ca2+]i homeostasis contributes to tumor initiation proliferation, angiogenesis, and other key processes that support tumor progression in several different cancers. Currently, chemically and functionally distinct drugs are used as chemotherapeutic agents in the treatment and management of cancer among which certain anti-cancer drugs reportedly suppress pro-survival signals and activate pro-apoptotic signaling through modulation of Ca2+-signaling-dependent mechanisms. Most importantly, the modulation of [Ca2+]i levels via the endoplasmic reticulum-mitochondrial axis and corresponding action of channels and pumps within the plasma membrane play an important role in the survival and death of cancer cells. The endoplasmic reticulum-mitochondrial axis is of prime importance when considering Ca2+-signaling-dependent anti-cancer drug targets. This review discusses how calcium signaling is targeted by anti-cancer drugs and highlights the role of calcium signaling in epigenetic modification and the Warburg effect in tumorigenesis.

Structural and Mechanistic Bases of Nuclear Calcium Signaling in Human Pluripotent Stem Cell-Derived Ventricular Cardiomyocytes

The loss of nonregenerative, terminally differentiated cardiomyocytes (CMs) due to aging or diseases is generally considered irreversible. Human pluripotent stem cells (hPSCs) can self-renew while maintaining their pluripotency to differentiate into all cell types, including ventricular (V) cardiomyocytes (CMs), to provide a potential unlimited ex vivo source of CMs for heart disease modeling, drug/cardiotoxicity screening, and cell-based therapies. In the human heart, cytosolic Ca²⁺ signals are well characterized but the contribution of nuclear Ca²⁺ is essentially unexplored. The present study investigated nuclear Ca²⁺ signaling in hPSC-VCMs. Calcium transient or sparks in hPSC-VCMs were measured by line scanning using a spinning disc confocal microscope. We observed that nuclear Ca²⁺, which stems from unitary sparks due to the diffusion of cytosolic Ca²⁺ that are mediated by RyRs on the nuclear reticulum, is functional. Parvalbumin- (PV-) mediated Ca²⁺ buffering successfully manipulated Ca²⁺ transient and stimuli-induced apoptosis in hPSC-VCMs. We also investigated the effect of Ca²⁺ on gene transcription in hPSC-VCMs, and the involvement of nuclear factor of activated T-cell (NFAT) pathway was identified. The overexpression of Ca²⁺-sensitive, nuclear localized Ca²⁺/calmodulin-dependent protein kinase II δ_B (CaMKIIδ_B) induced cardiac hypertrophy through nuclear Ca²⁺/CaMKIIδB/HDAC4/MEF2 pathway. These findings provide insights into nuclear Ca²⁺ signal in hPSC-VCMs, which may lead to novel strategies for maturation as well as improved systems for disease modeling, drug discovery, and cell-based therapies.

Combined Menin and EGFR Inhibitors Synergize to Suppress Colorectal Cancer via EGFR-Independent and Calcium-Mediated Repression of SKP2 Transcription

Menin is a nuclear epigenetic regulator that can both promote and suppress tumor growth in a highly tissue-specific manner. The role of menin in colorectal cancer, however, remains unclear. Here, we demonstrate that menin was overexpressed in colorectal cancer and that inhibition of menin synergized with small-molecule inhibitors of EGFR (iEGFR) to suppress colorectal cancer cells and tumor xenografts in vivo in an EGFR-independent manner. Mechanistically, menin bound the promoter of SKP2, a pro-oncogenic gene crucial for colorectal cancer growth, and promoted its expression. Moreover, the iEGFR gefitinib activated endoplasmic reticulum calcium channel inositol trisphosphate receptor 3 (IP3R3)-mediated release of calcium, which directly bound menin. Combined inhibition of menin and iEGFR-induced calcium release synergistically suppressed menin-mediated expression of SKP2 and growth of colorectal cancer. Together, these findings uncover a molecular convergence of menin and the iEGFR-induced, IP3R3-mediated calcium release on SKP2 transcription and reveal opportunities to enhance iEGFR efficacy to improve treatments for colorectal cancer. SIGNIFICANCE: Menin acts as a calcium-responsive regulator of SKP2 expression, and small molecule EGFR inhibitors, which induce calcium release, synergize with Menin inhibition to reduce SKP2 expression and suppress colorectal cancer.

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.

Cardiac inositol 1,4,5-trisphosphate receptors

Calcium is a second messenger that regulates almost all cellular functions. In cardiomyocytes, calcium plays an integral role in many functions including muscle contraction, gene expression, and cell death. Inositol 1,4,5-trisphosphate receptors (IP₃Rs) are a family of calcium channels that are ubiquitously expressed in all tissues. In the heart, IP₃Rs have been associated with regulation of cardiomyocyte function in response to a variety of neurohormonal agonists, including those implicated in cardiac disease. Notably, IP₃R activity is thought to be essential for mediating the hypertrophic response to multiple stimuli including endothelin-1 and angiotensin II. In this review, we will explore the functional implications of IP₃R activity in the heart in health and disease.

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.

Brief low [Mg(2+)]o-induced Ca(2+) spikes inhibit subsequent prolonged exposure-induced excitotoxicity in cultured rat hippocampal neurons

Reducing [Mg²⁺]_o to 0.1 mM can evoke repetitive [Ca²⁺]_i spikes and seizure activity, which induces neuronal cell death in a process called excitotoxicity. We examined the issue of whether cultured rat hippocampal neurons preconditioned by a brief exposure to 0.1 mM [Mg²⁺]_o are rendered resistant to excitotoxicity induced by a subsequent prolonged exposure and whether Ca²⁺ spikes are involved in this process. Preconditioning by an exposure to 0.1 mM [Mg²⁺]_o for 5 min inhibited significantly subsequent 24 h exposure-induced cell death 24 h later (tolerance). Such tolerance was prevented by both the NMDA receptor antagonist D-AP5 and the L-type Ca²⁺ channel antagonist nimodipine, which blocked 0.1 mM [Mg²⁺]_o-induced [Ca²⁺]_i spikes. The AMPA receptor antagonist NBQX significantly inhibited both the tolerance and the [Ca²⁺]_i spikes. The intracellular Ca²⁺ chelator BAPTA-AM significantly prevented the tolerance. The nonspecific PKC inhibitor staurosporin inhibited the tolerance without affecting the [Ca²⁺]_i spikes. While Gö6976, a specific inhibitor of PKCα had no effect on the tolerance, both the PKCε translocation inhibitor and the PKCζ pseudosubstrate inhibitor significantly inhibited the tolerance without affecting the [Ca²⁺]_i spikes. Furthermore, JAK-2 inhibitor AG490, MAPK kinase inhibitor PD98059, and CaMKII inhibitor KN-62 inhibited the tolerance, but PI-3 kinase inhibitor LY294,002 did not. The protein synthesis inhibitor cycloheximide significantly inhibited the tolerance. Collectively, these results suggest that low [Mg²⁺]_o preconditioning induced excitotoxic tolerance was directly or indirectly mediated through the [Ca²⁺]_i spike-induced activation of PKCε and PKCξ, JAK-2, MAPK kinase, CaMKII and the de novo synthesis of proteins.

An integrated mechanism of cardiomyocyte nuclear Ca(2+) signaling

In cardiomyocytes, Ca(2+) plays a central role in governing both contraction and signaling events that regulate gene expression. Current evidence indicates that discrimination between these two critical functions is achieved by segregating Ca(2+) within subcellular microdomains: transcription is regulated by Ca(2+) release within nuclear microdomains, and excitation-contraction coupling is regulated by cytosolic Ca(2+). Accordingly, a variety of agonists that control cardiomyocyte gene expression, such as endothelin-1, angiotensin-II or insulin-like growth factor-1, share the feature of triggering nuclear Ca(2+) signals. However, signaling pathways coupling surface receptor activation to nuclear Ca(2+) release, and the phenotypic responses to such signals, differ between agonists. According to earlier hypotheses, the selective control of nuclear Ca(2+) signals by activation of plasma membrane receptors relies on the strategic localization of inositol trisphosphate receptors at the nuclear envelope. There, they mediate Ca(2+) release from perinuclear Ca(2+) stores upon binding of inositol trisphosphate generated in the cytosol, which diffuses into the nucleus. More recently, identification of such receptors at nuclear membranes or perinuclear sarcolemmal invaginations has uncovered novel mechanisms whereby agonists control nuclear Ca(2+) release. In this review, we discuss mechanisms for the selective control of nuclear Ca(2+) signals with special focus on emerging models of agonist receptor activation.

Mucosal reactive oxygen species are required for antiviral response: role of Duox in influenza a virus infection

AIMS

Influenza A virus (IAV), a major airborne pathogen, is closely associated with significant morbidity and mortality. The primary target for influenza virus replication is the respiratory epithelium, which reacts to infection by mounting a multifaceted antiviral response. A part of this mucosal host defense is the generation of reactive oxygen species (ROS) by NADPH oxidases. Duox1 and Duox2 are the main ROS-producing enzymes in the airway epithelium, but their contribution to mammalian host defense is still ill defined.

RESULTS

To gain a better understanding of Duox function in respiratory tract infections, human differentiated lung epithelial cells and an animal model were used to monitor the effect of epithelial ROS on IAV propagation. IAV infection led to coordinated up-regulation of Duox2 and Duox-mediated ROS generation. Interference with H2O2 production and ROS signaling by oxidase inhibition or H2O2 decomposition augmented IAV replication. A nuclear pool of Duox enzymes participated in the regulation of the spliceosome, which is critical for alternative splicing of viral transcripts and controls the assembly of viable virions. In vivo silencing of Duox increased the viral load on intranasal infection with 2009 pandemic H1N1 influenza virus.

INNOVATION

This is the first study conclusively linking Duox NADPH oxidases with the antiviral mammalian immune response. Further, ROS generated by Duox enzymes localized adjacent to nuclear speckles altered the splicing of viral genes.

CONCLUSION

Duox-derived ROS are host protective and essential for counteracting IAV replication.

Detection of differentially regulated subsarcolemmal calcium signals activated by vasoactive agonists in rat pulmonary artery smooth muscle cells

Intracellular calcium (Ca(2+)) plays pivotal roles in distinct cellular functions through global and local signaling in various subcellular compartments, and subcellular Ca(2+) signal is the key factor for independent regulation of different cellular functions. In vascular smooth muscle cells, subsarcolemmal Ca(2+) is an important regulator of excitation-contraction coupling, and nucleoplasmic Ca(2+) is crucial for excitation-transcription coupling. However, information on Ca(2+) signals in these subcellular compartments is limited. To study the regulation of the subcellular Ca(2+) signals, genetically encoded Ca(2+) indicators (cameleon), D3cpv, targeting the plasma membrane (PM), cytoplasm, and nucleoplasm were transfected into rat pulmonary arterial smooth muscle cells (PASMCs) and Ca(2+) signals were monitored using laser scanning confocal microscopy. In situ calibration showed that the Kd for Ca(2+) of D3cpv was comparable in the cytoplasm and nucleoplasm, but it was slightly higher in the PM. Stimulation of digitonin-permeabilized cells with 1,4,5-trisphosphate (IP3) elicited a transient elevation of Ca(2+) concentration with similar amplitude and kinetics in the nucleoplasm and cytoplasm. Activation of G protein-coupled receptors by endothelin-1 and angiotensin II preferentially elevated the subsarcolemmal Ca(2+) signal with higher amplitude in the PM region than the nucleoplasm and cytoplasm. In contrast, the receptor tyrosine kinase activator, platelet-derived growth factor, elicited Ca(2+) signals with similar amplitudes in all three regions, except that the rise-time and decay-time were slightly slower in the PM region. These data clearly revealed compartmentalization of Ca(2+) signals in the subsarcolemmal regions and provide the basis for further investigations of differential regulation of subcellular Ca(2+) signals in PASMCs.

Activation of intracellular calcium by multiple Wnt ligands and translocation of β-catenin into the nucleus: a convergent model of Wnt/Ca2+ and Wnt/β-catenin pathways

Ca²⁺ and β-catenin, a 92-kDa negatively charged transcription factor, transduce Wnt signaling via the non-canonical, Wnt/Ca²⁺ and canonical, Wnt/β-catenin pathways independently. The nuclear envelope is a barrier to large protein entry, and this process is regulated by intracellular calcium [Ca²⁺]_i and trans-nuclear potential. How β-catenin traverses the nuclear envelope is not well known. We hypothesized that Wnt/Ca²⁺ and Wnt/β-catenin pathways act in a coordinated manner and that [Ca²⁺]_i release facilitates β-catenin entry into the nucleus in mammalian cells. In a live assay using calcium dyes in PC3 prostate cancer cells, six Wnt peptides (3A, 4, 5A, 7A, 9B, and 10B) mobilized [Ca²⁺]_i but Wnt11 did not. Based upon dwell time (range = 15–30 s) of the calcium waveform, these Wnts could be classified into three classes: short, 3A and 5A; long, 7A and 10B; and very long, 4 and 9B. Wnt-activated [Ca²⁺]_i release was followed by an increase in intranuclear calcium and the depolarization of both the cell and nuclear membranes, determined by using FM4-64. In cells treated with Wnts 5A, 9B, and 10B, paradigm substrates for each Wnt class, increased [Ca²⁺]_i was followed by β-catenin translocation into the nucleus in PC3, MCF7, and 253J, prostate, breast, and bladder cancer cell lines; both the increase in Wnt 5A, 9B, and 10B induced [Ca²⁺]_i release and β-catenin translocation are suppressed by thapsigargin in PC3 cell line. We propose a convergent model of Wnt signaling network where Ca²⁺ and β-catenin pathways may act in a coordinated, interdependent, rather than independent, manner.

Nucleoplasmic calcium signaling and cell proliferation: calcium signaling in the nucleus

Calcium (Ca²⁺) is an essential signal transduction element involved in the regulation of several cellular activities and it is required at various key stages of the cell cycle. Intracellular Ca²⁺ is crucial for the orderly cell cycle progression and plays a vital role in the regulation of cell proliferation. Recently, it was demonstrated by in vitro and in vivo studies that nucleoplasmic Ca²⁺ regulates cell growth. Even though the mechanism by which nuclear Ca²⁺ regulates cell proliferation is not completely understood, there are reports demonstrating that activation of tyrosine kinase receptors (RTKs) leads to translocation of RTKs to the nucleus to generate localized nuclear Ca²⁺ signaling which are believed to modulate cell proliferation. Moreover, nuclear Ca²⁺ regulates the expression of genes involved in cell growth. This review will describe the nuclear Ca²⁺ signaling machinery and its role in cell proliferation. Additionally, the potential role of nuclear Ca²⁺ as a target in cancer therapy will be discussed.

Intracellular organelles in the saga of Ca2+ homeostasis: different molecules for different purposes?

An increase in the concentration of cytosolic free Ca(2+) is a key component regulating different cellular processes ranging from egg fertilization, active secretion and movement, to cell differentiation and death. The multitude of phenomena modulated by Ca(2+), however, do not simply rely on increases/decreases in its concentration, but also on specific timing, shape and sub-cellular localization of its signals that, combined together, provide a huge versatility in Ca(2+) signaling. Intracellular organelles and their Ca(2+) handling machineries exert key roles in this complex and precise mechanism, and this review will try to depict a map of Ca(2+) routes inside cells, highlighting the uniqueness of the different Ca(2+) toolkit components and the complexity of the interactions between them.

Role of the nuclear envelope in calcium signalling

The endoplasmic reticulum (ER) is the major Ca(2+) store inside the cell. Its organisation in specialised subdomains allows the local delivery of Ca(2+) to specific cell areas on stimulation. The nuclear envelope (NE), which is continuous with the ER, has a double role: it insulates the nucleoplasm from the cytoplasm and it stores Ca(2+) around the nucleus. Furthermore, all the constituents of the signalling cascade leading to Ca(2+) mobilisation are found in the NE; this allows the nuclear Ca(2+) to be regulated autonomously. On the other hand, cytosolic Ca(2+) transients can propagate within the nucleus via the nuclear pore complex. The variations in nuclear Ca(2+) concentration are important for controlling gene transcription and progression in the cell cycle. Recent data suggest that invaginations of the NE modify the morphology of the nucleus and may affect Ca(2+) dynamics in the nucleus and regulate transcriptional activity.

Nuclear calcium signaling: an emerging topic in plants

The calcium ion is probably one of the most studied second messenger both in plant and animal fields. A large number of reviews have browsed the diversity of cytosolic calcium signatures and evaluated their pleiotropic roles in plant and animal cells. In the recent years, an increasing number of reviews has focused on nuclear calcium, especially on the possible roles of nuclear calcium concentration variations on nuclear activities. Experiments initially performed on animal cells gave conflicting results that brought about a controversy about the ability of the nucleus to generate its own calcium signals and to regulate its calcium level. But in plant cells, several converging scientific pieces of evidence support the hypothesis of nucleus autonomy. The present review briefly summarizes data supporting this hypothesis and tries to put forward some possible roles for these nucleus-generated calcium signals in controlling nuclear activity.

Phenotypic changes in mouse pancreatic stellate cell Ca2+ signaling events following activation in culture and in a disease model of pancreatitis

The specific characteristics of intracellular Ca 2+ signaling and the downstream consequences of these events were investigated in mouse pancreatic stellate cells (PSC) in culture and in situ using multiphoton microscopy in pancreatic lobules. PSC undergo a phenotypic transformation from a quiescent state to a myofibroblast-like phenotype in culture. This is believed to parallel the induction of an activated state observed in pancreatic disease such as chronic pancreatitis and pancreatic cancer. By day 7 in culture, the complement of cell surface receptors coupled to intracellular Ca 2+ signaling was shown to be markedly altered. Specifically, protease-activated receptors (PAR) 1 and 2, responsive to thrombin and trypsin, respectively, and platelet-derived growth factor (PDGF) receptors were expressed only in activated PSC (aPSC). PAR-1, ATP, and PDGF receptor activation resulted in prominent nuclear Ca 2+ signals. Nuclear Ca 2+ signals and aPSC proliferation were abolished by expression of parvalbumin targeted to the nucleus. In pancreatic lobules, PSC responded to agonists consistent with the presence of only quiescent PSC. aPSC were observed following induction of experimental pancreatitis. In contrast, in a mouse model of pancreatic disease harboring elevated K-Ras activity in acinar cells, aPSC were present under control conditions and their number greatly increased following induction of pancreatitis. These data are consistent with nuclear Ca 2+ signaling generated by agents such as trypsin and thrombin, likely present in the pancreas in disease states, resulting in proliferation of "primed" aPSC to contribute to the severity of pancreatic disease.

Single-cell analysis of phosphoinositide 3-kinase and phosphatase and tensin homolog activation

A single-cell assay was developed to measure the activation of phosphoinositide 3-kinase (PI3K) using microanalytical chemical separations and a fluorescently labeled lipid substrate. Phosphatidyl-inositol 4,5 bisphosphate labeled on its acyl chain with Bodipy fluorescein (Bodipy Fl PIP(2)) was utilized as a substrate for both in vitro and cell-based assays. Detection limits for the substrate and product of the PI3K reaction were 10 to 20 zeptomol. In vitro assays with PI3K with and without pharmacologic inhibitors demonstrated that Bodipy Fl PIP(2) was converted to phosphatidyl-inositol 3,4,5 trisphosphate (Bodipy Fl PIP(3)). Bodipy Fl PIP(3) could be back converted to Bodipy Fl PIP(2) by the phosphatase PTEN. When Bodipy Fl PIP(2) was added to a cell lysate, 1.4 fmol of the Bodipy Fl PIP(3) were produced per ng of protein in the cytoplasmic extract in 10 min. Addition of Bodipy Fl PIP(3) to a cell lysate yielded 3 fmol of Bodipy Fl PIP(2) per ng of protein in 8 min. Both Bodipy Fl PIP(2) and Bodipy Fl PIP(3) were measureable in single cells and the two species could be inter-converted. Under the appropriate conditions, a fluorescent diacylglycerol was also detected in single cells. When the FcepsilonR 1 receptor on the cells loaded with the fluorescent lipid was cross-linked, the amount of Bodipy Fl PIP(3) generated per cell increased 4-fold over that of unstimulated cells. This production of Bodipy Fl PIP(3) was blocked by wortmannin. Chemical cytometry utilizing the fluorescent lipids will be of value in understanding lipid metabolism at the single-cell level.

STIM2 Contributes to Enhanced Store-operated Ca Entry in Pulmonary Artery Smooth Muscle Cells from Patients with Idiopathic Pulmonary Arterial Hypertension

Pulmonary vasoconstriction and vascular remodeling are two major causes for elevated pulmonary vascular resistance and pulmonary arterial pressure in patients with idiopathic pulmonary arterial hypertension (IPAH). An increase in cytosolic free Ca²⁺concentration ([Ca²⁺]_cyt) in pulmonary artery smooth muscle cells (PASMC) is a major trigger for pulmonary vasoconstriction and an important stimulus for PASMC proliferation, which causes pulmonary vascular remodeling. Store-operated Ca²⁺ entry (SOCE), induced by depletion of stored Ca²⁺ in the sarcoplasmic reticulum (SR), can increase [Ca²⁺]_cyt in PASMC, independent of other means of Ca²⁺ entry. Stromal interaction molecule (STIM) proteins, STIM1 and STIM2, were both recently identified as sensors for store depletion and also signaling molecules to open store-operated Ca²⁺ channels. We previously reported that SOCE was significantly enhanced in PASMC from IPAH patients compared to PASMC from normotensive control subjects. Enhanced SOCE plays an important role in the pathophysiological changes in PASMC associated with pulmonary arterial hypertension. In this study, we examine whether the expression levels of STIM1 and STIM2 are altered in IPAH-PASMC compared to control PASMC, and whether these putative changes in the STIM1 and STIM2 expression levels are responsible for enhanced SOCE and proliferation in IPAH-PASMC. Compared to control PASMC, the protein expression level of STIM2 was significantly increased in IPAH-PASMC, whereas STIM1 protein expression was not significantly changed. In IPAH-PASMC, the small interfering RNA (siRNA)-mediated knockdown of STIM2 decreased SOCE and proliferation, while knockdown of STIM2 in control PASMC had no effect on either SOCE or proliferation. Overexpression of STIM2 in the control PASMC failed to enhance SOCE or proliferation. These data indicate that enhanced protein expression of STIM2 is necessary, but not sufficient, for enhanced SOCE and proliferation of IPAH-PASMC.

Patho-, physiological roles of voltage-dependent K+ channels in pulmonary arterial smooth muscle cells

In this review, we demonstrate the basic properties, modulation of, and pathological changes in voltage-dependent K+ (Kv) channels that are expressed in pulmonary arterial smooth muscle cells (PASMCs). Pulmonary Kv channels are thought to play a crucial role in the maintenance of resting membrane potentials, and therefore the vascular tone of the pulmonary arteries. Although the molecular identity of pulmonary Kv channels is not clear, Kv1.1, Kv1.2, Kv1.5, Kv2.1, Kv9.3, and Kv3.1 subtypes are expressed in PASMCs. In addition, resistant PASMCs contain greater amount of Kv channels as compared to conduit PASMCs. This heterogenetic expression of Kv channels is consistent with regional differences in the contractile response to hypoxia. Similar to other K+ channels, pulmonary Kv channels can also be modulated by several vasoconstrictors concomitant with the activation of protein kinase C (PKC). Alterations in Kv channel function have several additional and interrelated consequences, including the regulation of cell proliferation and apoptosis, which ultimately lead to pulmonary vascular remodeling. Increased pulmonary vasoconstriction in pulmonary arterial hypertension is attributable to decreased expression and activity of Kv channels in smooth muscle cells. Kv channels play a central role in the maintenance of cellular homeostasis and ion channels, and consequential signaling cascades. Therefore, Kv channels are potential therapeutic targets for the treatment of pulmonary vascular disease.

Identification of source of calcium in HeLa cells by femtosecond laser excitation

Calcium is an important messenger in cells and whose store and diffusion dynamics at the subcellular level remain unclear. By inducing a controlled slow subcellular Ca2+ release through femtosecond laser irradiation in HeLa cells immersed in different media, cytoplasm is identified to be the major intracellular Ca2+ store, with the nucleus being the minor store and the extracellular Ca2+ also contributing to the total cellular Ca2+ level. Furthermore, Ca2+ released in either the cytoplasm or nucleus diffuses into the nucleus or cytoplasm, respectively, at different rates and influences the Ca2+ release in those regions.

Ca2+ hot spots on the mitochondrial surface are generated by Ca2+ mobilization from stores, but not by activation of store-operated Ca2+ channels

Although it is widely accepted that mitochondria in living cells can efficiently uptake Ca(2+) during stimulation because of their vicinity to microdomains of high [Ca(2+)], the direct proof of Ca(2+) hot spots' existence is still lacking. Thanks to a GFP-based Ca(2+) probe localized on the cytosolic surface of the outer mitochondrial membrane, we demonstrate that, upon Ca(2+) mobilization, the [Ca(2+)] in small regions of the mitochondrial surface reaches levels 5- to 10-fold higher than in the bulk cytosol. We also show that the [Ca(2+)] to which mitochondria are exposed during capacitative Ca(2+) influx is similar between near plasma membrane mitochondria and organelles deeply located in the cytoplasm, whereas it is 2- to 3-fold higher in subplasma membrane mitochondria upon activation of voltage-gated Ca(2+) channels. These results demonstrate that mitochondria are exposed to Ca(2+) hot spots close to the ER but are excluded from the regions where capacitative Ca(2+) influx occurs.

THE ROLE OF INTRACELLULAR SODIUM (Na) IN THE REGULATION OF CALCIUM (Ca)-MEDIATED SIGNALING AND TOXICITY

It is known that activated N-methyl-D-aspartate receptors (NMDARs) are a major route of excessive calcium ion (Ca(2+)) entry in central neurons, which may activate degradative processes and thereby cause cell death. Therefore, NMDARs are now recognized to play a key role in the development of many diseases associated with injuries to the central nervous system (CNS). However, it remains a mystery how NMDAR activity is recruited in the cellular processes leading to excitotoxicity and how NMDAR activity can be controlled at a physiological level. The sodium ion (Na(+)) is the major cation in extracellular space. With its entry into the cell, Na(+) can act as a critical intracellular second messenger that regulates many cellular functions. Recent data have shown that intracellular Na(+) can be an important signaling factor underlying the up-regulation of NMDARs. While Ca(2+) influx during the activation of NMDARs down-regulates NMDAR activity, Na(+) influx provides an essential positive feedback mechanism to overcome Ca(2+)-induced inhibition and thereby potentiate both NMDAR activity and inward Ca(2+) flow. Extensive investigations have been conducted to clarify mechanisms underlying Ca(2+)-mediated signaling. This review focuses on the roles of Na(+) in the regulation of Ca(2+)-mediated NMDAR signaling and toxicity.

An update on nuclear calcium signalling

Over the past 15 years or so, numerous studies have sought to characterise how nuclear calcium (Ca2+) signals are generated and reversed, and to understand how events that occur in the nucleoplasm influence cellular Ca2+ activity, and vice versa. In this Commentary, we describe mechanisms of nuclear Ca2+ signalling and discuss what is known about the origin and physiological significance of nuclear Ca2+ transients. In particular, we focus on the idea that the nucleus has an autonomous Ca2+ signalling system that can generate its own Ca2+ transients that modulate processes such as gene transcription. We also discuss the role of nuclear pores and the nuclear envelope in controlling ion flux into the nucleoplasm.

Indo-1 derivatives for local calcium sensing

The role of calcium in signal transduction relies on the precise spatial and temporal control of its concentration. The existing means to detect fluctuations in Ca2+ concentrations with adequate temporal and spatial resolution are limited. We introduce here a method to measure Ca2+ concentrations in defined locations in living cells that is based on linking the Ca2+-sensitive dye Indo-1 to SNAP-tag fusion proteins. Fluorescence spectroscopy of SNAP-Indo-1 conjugates in vitro showed that the conjugates retained the Ca2+-sensing ability of Indo-1. In a proof-of-principle experiment, local Ca2+ sensing was demonstrated in single cells dissociated from muscle of adult mice expressing a nucleus-localized SNAP-tag fusion. Ca2+ concentrations inside nuclei of resting cells were measured by shifted excitation and emission ratioing of confocal microscopic images of fluorescence. After permeabilizing the plasma membrane, changes in the bathing solution induced corresponding changes in nuclear [Ca2+] that were readily detected and used for a preliminary calibration of the technique. This work thus demonstrates the synthesis and application of SNAP-tag-based Ca2+ indicators that combine the spatial specificity of genetically encoded calcium indicators with the advantageous spectroscopic properties of synthetic indicators.

Sensitivity of NFAT cycling to cytosolic calcium concentration: implications for hypertrophic signals in cardiac myocytes

The nuclear factor of activated T-cell (NFAT) transcription factors play an important role in many biological processes, including pathological cardiac hypertrophy. Stimulated by calcium signals, NFAT is translocated to the nucleus where it can regulate hypertrophic genes (excitation-transcription coupling). In excitable cells, such as myocytes, calcium is a key second messenger for multiple signaling events, including excitation-contraction coupling. Whether the calcium signals due to excitation-contraction and excitation-transcription coupling coincide or how they can be differentiated is currently unclear. Here we construct a mathematical model of NFAT cycling fitted to skeletal myocyte and baby hamster kidney cell data. The model replicates key behavior with respect to sensitivity to calcineurin overexpression and to calcium oscillations. Finally, we measure the sensitivity of the system to a simulated hypertrophic calcium signal, against a background excitation-contraction coupling calcium oscillation. We find that NFAT cycling is sensitive to excitation-transcription coupling even when both calcium signals are in the same cellular compartment, thus showing that separation of the signals may not be necessary in vitro.

Nuclear calcium signaling: a cell within a cell

Calcium (Ca2+) is a versatile second messenger that regulates a wide range of cellular functions. Although it is not established how a single second messenger coordinates diverse effects within a cell, there is increasing evidence that the spatial patterns of Ca2+ signals may determine their specificity. Ca2+ signaling patterns can vary in different regions of the cell and Ca2+ signals in nuclear and cytoplasmic compartments have been reported to occur independently. No general paradigm has been established yet to explain whether, how, or when Ca2+ signals are initiated within the nucleus or their function. Here we highlight that receptor tyrosine kinases rapidly translocate to the nucleus. Ca2+ signals that are induced by growth factors result from phosphatidylinositol 4,5-bisphosphate hydrolysis and inositol 1,4,5-trisphosphate formation within the nucleus rather than within the cytoplasm. This novel signaling mechanism may be responsible for growth factor effects on cell proliferation.

Role of Yersinia enterocolitica heat-stable enterotoxin (Y-STa) on differential regulation of nuclear and cytosolic calcium signaling in rat intestinal epithelial cells

The heat-stable enterotoxin (Y-STa) produced by the pathogenic strains of Yersinia enterocolitica is a causative agent of secretory diarrhea. We have reported earlier that Y-STa-induced inositol trisphosphate-mediated cytosolic calcium rise occurs in rat intestinal epithelial cells. In the present communication, the involvement of a nuclear calcium store in the action mechanism of Y-STa in rat intestinal epithelial cells has been shown. Calcium imaging with time series confocal microscopy shows that Y-STa stimulates both the nuclear and cytosolic calcium levels in rat intestinal epithelial cells where a rise in nuclear calcium precedes the cytosolic events. Moreover, Y-STa stimulates both cytosolic and nuclear inositol trisphosphate (IP(3)) levels in a time-dependent manner. Western blot and immunocytochemical analysis reveal a higher density of IP(3) receptor type II in the nuclear membrane compared to the cytosol, which may be the cause of an early rise of the nuclear calcium level. Therefore, it is suggested that Y-STa regulates the nuclear and cytosolic calcium signals in a distinct temporal manner in rat intestinal epithelial cells.

Calcium signal transmission in chick sensory neurones is diffusion based

In many cells, the cytosol is an excitable medium through which calcium waves propagate by calcium induced calcium release (CICR). Many labs. have reported CICR in neurones subsequent to calcium influx through voltage gated channels. However, these have used long depolarizations. We have imaged calcium within chick sensory neurones following 50 ms depolarizations. Calcium signals travelled rapidly throughout the cell, such that changes at the cell centre were delayed by 24 ms compared to regions 3 microm from the plasma membrane. The nuclear envelope imposed a delay of 9 ms. A simple diffusion model with few unknowns gave good fits to the measured data, indicating that passive diffusion is responsible for signal transmission in these neurones. Simulations run without indicator dye did not reveal markedly different spatiotemporal dynamics, although concentration changes were larger. Simulations of calcium changes during action potentials revealed that large calcium transients occurring in the cytosol close to the nucleus are significantly attenuated by the nuclear envelope. Our results indicate that for the brief depolarisations that neurones will experience during normal signal processing calcium signals are transmitted by passive diffusion only. Diffusion is perfectly capable of transmitting the calcium signal into the interior of nerve cell bodies, and into the nucleoplasm.

Calcium in cell injury and death

Loss of Ca(2+) homeostasis, often in the form of cytoplasmic increases, leads to cell injury. Depending upon cell type and the intensity of Ca(2+) toxicity, the ensuing pathology can be reversible or irreversible. Although multiple destructive processes are activated by Ca(2+), lethal outcomes are determined largely by Ca(2+)-induced mitochondrial permeability transition. This form of damage is primarily dependent upon mitochondrial Ca(2+) accumulation, which is regulated by the mitochondrial membrane potential. Retention of the mitochondrial membrane potential during Ca(2+) increases favors mitochondrial Ca(2+) uptake and overload, resulting in mitochondrial permeability transition and cell death. In contrast, dissipation of mitochondrial membrane potential reduces mitochondrial Ca(2+) uptake, retards mitochondrial permeability transition, and delays death, even in cells with large Ca(2+) increases. The rates of mitochondrial membrane potential dissipation and mitochondrial Ca(2+) uptake may determine cellular sensitivity to Ca(2+) toxicity under pathological conditions, including ischemic injury.

Regulation of nuclear calcium concentration

Transient increases in nuclear calcium concentration have been shown to activate gene expression and other nuclear processes. It has been suggested that nuclear calcium signals are controlled by a mechanism that is independent of calcium signalling in the cytosol. This would be possible if calcium diffusion is slow and a separate calcium release mechanism is localized to the nuclear region. Alternatively, the nuclear envelope could act as a diffusion barrier for calcium ions released either inside or outside the nucleus. It has also been proposed that inositol 1,4,5-trisphosphate (InsP3) can be generated inside the nucleus and that there are calcium release channels in the inner membrane of the nuclear envelope. Most of the experimental evidence supporting these hypotheses is based on the calibration of nuclear and cytosolic calcium concentrations. However, recent studies suggest that the local calibration of calcium indicators may not be accurate. We propose that nuclear calcium signals can be investigated by a different approach that does not rely on accurate calibration of indicators. We have developed calcium indicators that minimize facilitated calcium diffusion and are localized to either the nucleus or the cytosol. Using the diffusion coefficient of calcium ions, and measuring the delay between cytosolic and nuclear calcium increases, we show that the nuclear envelope is not a substantial barrier for calcium ions in PC12 (phaeochromocytoma) cells. This suggests that nuclear and cytosolic calcium signals equilibrate rapidly in these cells.

Nucleoplasmic calcium is required for cell proliferation

Ca(2+) signals regulate cell proliferation, but the spatial and temporal specificity of these signals is unknown. Here we use selective buffers of nucleoplasmic or cytoplasmic Ca(2+) to determine that cell proliferation depends upon Ca(2+) signals within the nucleus rather than in the cytoplasm. Nuclear Ca(2+) signals stimulate cell growth rather than inhibit apoptosis and specifically permit cells to advance through early prophase. Selective buffering of nuclear but not cytoplasmic Ca(2+) signals also impairs growth of tumors in vivo. These findings reveal a major physiological and potential pathophysiological role for nucleoplasmic Ca(2+) signals and suggest that this information can be used to design novel therapeutic strategies to regulate conditions of abnormal cell growth.

Involvement of transient receptor potential canonical 1 (TRPC1) in angiotensin II-induced vascular smooth muscle cell hypertrophy

Angiotensin II (Ang II) induces vascular smooth muscle cell (VSMC) hypertrophy as one of the major events leading to atherosclerosis. Increased Ca(2+) entry is an important stimulus for VSMC hypertrophy, but the association with Ang II remains to be determined. Transient receptor potential canonical 1 (TRPC1) forms store-operated Ca(2+) (SOC) channels that are involved in Ca(2+) homeostasis. Our aim was to ascertain the potential involvement of TRPC1 in Ang II-induced VSMC hypertrophy. For this purpose, we used cultured human coronary artery smooth muscle cells (hCASMCs). Store-operated Ca(2+) entry (SOCE) increased in the Ang II-induced hypertrophied cells, and SOC channel blocker inhibited the Ang II-induced hypertrophic response. Although hCASMCs constitutively expressed TRPC1, C3, C4, C5, and C6, only TRPC1 increased in response to Ang II stimulation. TRPC1 siRNA decreased SOCE and prevented Ang II-induced hypertrophy. We found NF-kappaB binding sites in the 5'-regulatory region of the human TRPC1 gene. An electrophoretic mobility shift assay showed that Ang II increased the TRPC1 promoter's NF-kappaB binding activity. Co-treatment with NF-kappaB decoy oligonucleotides not only reduced TRPC1 expression, but also inhibited the hypertrophic responses. In conclusion, our data suggest that Ang II and subsequent NF-kappaB activation induces hCASMC hypertrophy through an enhancement of TRPC1 expression.

Broadening cell selection criteria with micropallet arrays of adherent cells

A host of technologies exists for the separation of living, nonadherent cells, with separation decisions typically based on fluorescence or immunolabeling of cells. Methods to separate adherent cells as well as to broaden the range of possible sorting criteria would be of high value and complementary to existing strategies. Cells were cultured on arrays of releasable pallets. The arrays were screened and individual cell(s)/pallets were released and collected. Conventional fluorescence and immunolabeling of cells were compatible with the pallet arrays, as were separations based on gene expression. By varying the size of the pallet and the number of cells cultured on the array, single cells or clonal colonies of cells were isolated from a heterogeneous population. Since cells remained adherent throughout the isolation process, separations based on morphologic characteristics, for example cell shape, were feasible. Repeated measurements of each cell in an array were performed permitting the selection of cells based on their temporal behavior, e.g. growth rate. The pallet array system provides the flexibility to select and collect adherent cells based on phenotypic and temporal criteria and other characteristics not accessible by alternative methods.

Calcium signaling in the nucleus

Cytosolic Ca(2+) is a versatile secondary messenger that regulates a wide range of cellular activities. In the past decade, evidence has accumulated that free Ca(2+) within the nucleus also plays an important messenger function. Here we review the mechanisms and effects of Ca(2+) signals within the nucleus. In particular, evidence is reviewed that the nucleus contains the machinery necessary for production of inositol 1,4,5-trisphosphate and for inositol 1,4,5-trisphosphate receptor-mediated Ca(2+) release. The role of Ca(2+) signals within the nucleus is discussed including regulation of such critical cell functions as gene expression, activation of kinases, and permeability of nuclear pores.

Chemical labeling strategies for cell biology

Methods to visualize, track, measure and perturb proteins in living cells are central to biomedicine's efforts to characterize and understand the spatial and temporal underpinnings of life inside cells. Although fluorescent proteins have revolutionized such studies, they have shortcomings, which have spurred the creation of alternative approaches to chemically label proteins in live cells. In this review we highlight research questions that can be addressed using site-specific chemical labeling and present a comparison of the various labeling techniques that have been developed. We also provide a 'roadmap' for selection of appropriate labeling techniques(s) and outline generalized strategies to validate and troubleshoot chemical labeling experiments.

Relationship between RNA synthesis and the Ca2+-filled state of the nuclear envelope store

RNA synthesis and ATP-dependent (45)Ca(2+) uptake were measured simultaneously in isolated nuclear fraction of rat liver nuclei. Maximal level of RNA synthesis was obtained under ATP-dependent (45)Ca(2+)-uptake conditions (1 microM free [Ca(2+)] and 1 mM ATP in the bathing solution). This experimental condition was defined as "stimulated nuclei" condition. ATP-dependent (45)Ca(2+) uptake was inhibited using different strategies including: (a) eliminating Ca(2+) (1 mM EGTA); (b) lowering the ATP concentration; (c) modifying nuclear envelope membranes Ca(2+) permeability (Ca(2+) ionophores); or (d) inhibiting the nuclear Ca(2+) pump (thapsigargin and 3',3'',5',5''-tetraiodophenolsulfonephthalein). Under all the above conditions, RNA synthesis was lower than in "stimulated nuclei" condition. In the presence of ionomycin, RNA synthesis was significantly higher at 500 nM free [Ca(2+)], as compared with RNA synthesis in a Ca(2+)-free medium or at 1muM free [Ca(2+)]. However, even in such condition (500 nM free [Ca(2+)]), RNA synthesis was lower than RNA synthesis obtained in "stimulated nuclei" condition. We suggest two components for the effect of Ca(2+) on RNA synthesis: (A) a direct effect of nucleoplasmic [Ca(2+)]; and (B) an effect dependent on the accumulation of Ca(2+) in the nuclear envelope store mediated by the SERCA nuclear Ca(2+) pump.

Amplitude modulation of nuclear Ca2+ signals in human skeletal myotubes: A possible role for nuclear Ca2+ buffering

Video-rate confocal microscopy of Indo-1-loaded human skeletal myotubes was used to assess the relationship between the changes in sarcoplasmic ([Ca(2+)](S)) and nuclear ([Ca(2+)](N)) Ca(2+) concentration during low- and high-frequency electrostimulation. A single stimulus of 10 ms duration transiently increased [Ca(2+)] in both compartments with the same time of onset. Rate and amplitude of the [Ca(2+)] rise were significantly lower in the nucleus (4.0- and 2.5-fold, respectively). Similarly, [Ca(2+)](N) decayed more slowly than [Ca(2+)](S) (mono-exponential time constants of 6.1 and 2.5 s, respectively). After return of [Ca(2+)] to the prestimulatory level, a train of 10 stimuli was applied at a frequency of 1 Hz. The amplitude of the first [Ca(2+)](S) transient was 25% lower than that of the preceding single transient. Thereafter, [Ca(2+)](S) increased stepwise to a maximum that equalled that of the single transient. Similarly, the amplitude of the first [Ca(2+)](N) transient was 20% lower than that of the preceding single transient. In contrast to [Ca(2+)](S), [Ca(2+)](N) then increased to a maximum that was 2.3-fold higher than that of the single transient and equalled that of [Ca(2+)](S). In the nucleus, and to a lesser extent in the sarcoplasm, [Ca(2+)] decreased faster at the end of the stimulus train than after the preceding single stimulus (time constants of 3.3 and 2.1 s, respectively). To gain insight into the molecular principles underlying the shaping of the nuclear Ca(2+) signal, a 3-D mathematical model was constructed. Intriguingly, quantitative modelling required the inclusion of a satiable nuclear Ca(2+) buffer. Alterations in the concentration of this putative buffer had dramatic effects on the kinetics of the nuclear Ca(2+) signal. This finding unveils a possible mechanism by which the skeletal muscle can adapt to changes in physiological demand.

Biologic effects of calcium-containing crystals

PURPOSE OF REVIEW

Calcium-containing crystals can cause the degeneration of articular tissues in two separate pathways. In the direct pathway, crystals directly induce synoviocytes to proliferate and produce metalloproteinases and prostaglandins. The other pathway, the paracrine pathway, involves the interaction between crystals and macrophages/monocytes, which leads to the synthesis and release of cytokines, which can reinforce the action of crystals on synoviocytes and/or induce chondrocytes to secrete enzymes and which eventually causes the degeneration of articular tissues. The purpose of this review is to highlight the recent findings of the biologic effect of these crystals.

RECENT FINDINGS

In the past few years, major advances in the understanding of the biologic effect of crystals and the signal transduction pathway of crystal-induced cell activation offer a unique opportunity to examine the role of crystal in osteoarthritis and cartilage degeneration.

SUMMARY

Evidence for a causal role of crystals in cartilage degeneration in osteoarthritis is primarily inferential and is based on correlative data. Clinical observations indicate that exaggerated and uniquely distributed cartilage degeneration is associated with these deposits. Measurements of putative markers of cartilage breakdown suggest that these crystals magnify the degenerative process. Studies have shown two potential mechanisms by which crystals cause degeneration. These involve the stimulation of mitogenesis in synovial fibroblasts and the secretion of metalloproteinases by cells that subject these crystals to phagocytosis. New information on how crystals form and how they exert their biologic effects will help in the design of an effective therapeutic approach.

Anti-proliferating effect of iptakalim, a novel KATP channel opener, in cultured rabbit pulmonary arterial smooth muscle cells

ATP-sensitive potassium (K(ATP)) channels of pulmonary arterial smooth muscle cells (SMCs) have been implicated in pulmonary hypertension. Iptakalim, designed and synthesized by ourselves, is a newly selective K(ATP) channel opener. Here, we explored the effects of iptakalim on the rise of cytoplasmic free Ca(2+) concentration ([Ca(2+)](cyt)) induced by endothelin-1 (ET-1) and on the proliferation of cultured rabbit pulmonary arterial SMCs. The results showed that iptakalim inhibited the [Ca(2+)](cyt) increase. The enhanced [(3)H]thymidine incorporation was inhibited and the transition of cells from static phase (G(0)/G(1)) to DNA synthesis (S) and mitotic phase (G(2)/M) was held back by iptakalim in a concentration-dependent manner. Glyburide abolished the inhibitory effect of iptakalim. In conclusion, we have shown that iptakalim had an inhibitory effect on [Ca(2+)](cyt) increase and the proliferation of pulmonary arterial SMCs induced by endothelin-1 through activation of K(ATP) channels. These findings suggest that iptakalim might be a promising candidate for the treatment of pulmonary arterial remodeling in pulmonary hypertension.