Purinergic signaling in liver disease: calcium signaling and induction of inflammation.

Purinergic signaling regulates many metabolic functions and is implicated in liver physiology and pathophysiology. Liver functionality is modulated by ionotropic P2X and metabotropic P2Y receptors, specifically P2Y1, P2Y2, and P2Y6 subtypes, which physiologically exert their influence through calcium signaling, a key second messenger controlling glucose and fat metabolism in hepatocytes. Purinergic receptors, acting through calcium signaling, play an important role in a range of liver diseases. Ionotropic P2X receptors, such as the P2X7 subtype, and certain metabotropic P2Y receptors can induce aberrant intracellular calcium transients that impact normal hepatocyte function and initiate the activation of other liver cell types, including Kupffer and stellate cells. These P2Y- and P2X-dependent intracellular calcium increases are particularly relevant in hepatic disease states, where stellate and Kupffer cells respond with innate immune reactions to challenges, such as excess fat accumulation, chronic alcohol abuse, or infections, and can eventually lead to liver fibrosis. This review explores the consequences of excessive extracellular ATP accumulation, triggering calcium influx through P2X4 and P2X7 receptors, inflammasome activation, and programmed cell death. In addition, P2Y2 receptors contribute to hepatic steatosis and insulin resistance, while inhibiting the expression of P2Y6 receptors can alleviate alcoholic liver steatosis. Adenosine receptors may also contribute to fibrosis through extracellular matrix production by fibroblasts. Thus, pharmacological modulation of P1 and P2 receptors and downstream calcium signaling may open novel therapeutic avenues.

Impact of Matrix Gel Variations on Primary Culture of Microvascular Endothelial Cell Function.

OBJECTIVE: The endothelium regulates crucial aspects of vascular function, including hemostasis, vasomotor tone, proliferation, immune cell adhesion, and microvascular permeability. Endothelial cells (ECs), especially in arterioles, are pivotal for flow distribution and peripheral resistance regulation. Investigating vascular endothelium physiology, particularly in microvascular ECs, demands precise isolation and culturing techniques. METHODS: Freshly isolated ECs are vital for examining protein expression, ion channel behavior, and calcium dynamics. Establishing primary endothelial cell cultures is crucial for unraveling vascular functions and understanding intact microvessel endothelium roles. Despite the significance, detailed protocols and comparisons with intact vessels are scarce in microvascular research. We developed a reproducible method to isolate microvascular ECs, assessing substrate influence by cultivating cells on fibronectin and gelatin matrix gels. This comparative approach enhances our understanding of microvascular endothelial cell biology. RESULTS: Microvascular mesenteric ECs expressed key markers (VE-cadherin and eNOS) in both matrix gels, confirming cell culture purity. Under uncoated conditions, ECs were undetected, whereas proteins linked to smooth muscle cells and fibroblasts were evident. Examining endothelial cell (EC) physiological dynamics on distinct matrix substrates revealed comparable cell length, shape, and Ca2+ elevations in both male and female ECs on gelatin and fibronectin matrix gels. Gelatin-cultured ECs exhibited analogous membrane potential responses to acetylcholine (ACh) or adenosine triphosphate (ATP), contrasting with their fibronectin-cultured counterparts. In the absence of stimulation, fibronectin-cultured ECs displayed a more depolarized resting membrane potential than gelatin-cultured ECs. CONCLUSIONS: Gelatin-cultured ECs demonstrated electrical behaviors akin to intact endothelium from mouse mesenteric arteries, thus advancing our understanding of endothelial cell behavior within diverse microenvironments.

Short-term high-fat diet feeding of mice suppresses catecholamine-stimulated Ca2+ signalling in hepatocytes and intact liver.

Excess consumption of carbohydrates, fat and calories leads to non-alcoholic fatty liver disease (NAFLD) and hepatic insulin resistance; these are major factors in the pathogenesis of type II diabetes. Hormones and catecholamines acting through G-protein coupled receptors (GPCRs) linked to phospholipase C (PLC) and increases in cytosolic Ca2+ ([Ca2+ ]c ) regulate many metabolic functions of the liver. In the intact liver, catabolic hormones such as glucagon, catecholamines and vasopressin integrate and synergize to regulate the frequency and extent to which [Ca2+ ]c waves propagate across hepatic lobules to control metabolism. Dysregulation of hepatic Ca2+ homeostasis has been implicated in the development of metabolic disease, but changes in hepatic GPCR-dependent Ca2+ signalling have been largely unexplored in this context. We show that short-term, 1-week, high-fat diet (HFD) feeding of mice attenuates noradrenaline-stimulated Ca2+ signalling, reducing the number of cells responding and suppressing the frequency of [Ca2+ ]c oscillations in both isolated hepatocytes and intact liver. The 1-week HFD feeding paradigm did not change basal Ca2+ homeostasis; endoplasmic reticulum Ca2+ load, store-operated Ca2+ entry and plasma membrane Ca2+ pump activity were unchanged compared to low-fat diet (LFD)-fed controls. However, noradrenaline-induced inositol 1,4,5-trisphosphate production was significantly reduced after HFD feeding, demonstrating an effect of HFD on receptor-stimulated PLC activity. Thus, we have identified a lesion in the PLC signalling pathway induced by short-term HFD feeding, which interferes with hormonal Ca2+ signalling in isolated hepatocytes and the intact liver. These early events may drive adaptive changes in signalling, which lead to pathological consequences in fatty liver disease. KEY POINTS: Non-alcoholic fatty liver disease (NAFLD) is a growing epidemic. In healthy liver, the counteracting effects of catabolic and anabolic hormones regulate metabolism and energy storage as fat. Hormones and catecholamines promote catabolic metabolism via increases in cytosolic Ca2+ ([Ca2+ ]c ). We show that 1 week high-fat diet (HFD) feeding of mice attenuated the Ca2+ signals induced by physiological concentrations of noradrenaline. Specifically, HFD suppressed the normal pattern of periodic [Ca2+ ]c oscillations in isolated hepatocytes and disrupted the propagation of intralobular [Ca2+ ]c waves in the intact perfused liver. Short-term HFD inhibited noradrenaline-induced inositol 1,4,5-trisphosphate generation, but did not change basal endoplasmic reticulum Ca2+ load or plasma membrane Ca2+ fluxes. We propose that impaired Ca2+ signalling plays a key role in the earliest phases of the etiology of NAFLD, and is responsible for many of the ensuing metabolic and related dysfunctional outcomes at the cellular and whole tissue level.

Endothelial mechanisms for inactivation of inflammation-induced hyperpermeability.

Microvascular hyperpermeability is a hallmark of inflammation. Many negative effects of hyperpermeability are due to its persistence beyond what is required for preserving organ function. Therefore, we propose that targeted therapeutic approaches focusing on mechanisms that terminate hyperpermeability would avoid the negative effects of prolonged hyperpermeability while retaining its short-term beneficial effects. We tested the hypothesis that inflammatory agonist signaling leads to hyperpermeability and initiates a delayed cascade of cAMP-dependent pathways that causes inactivation of hyperpermeability. We applied platelet-activating factor (PAF) and vascular endothelial growth factor (VEGF) to induce hyperpermeability. We used an Epac1 agonist to selectively stimulate exchange protein activated by cAMP (Epac1) and promote inactivation of hyperpermeability. Stimulation of Epac1 inactivated agonist-induced hyperpermeability in the mouse cremaster muscle and in human microvascular endothelial cells (HMVECs). PAF induced nitric oxide (NO) production and hyperpermeability within 1 min and NO-dependent increased cAMP concentration in about 15-20 min in HMVECs. PAF triggered phosphorylation of vasodilator-stimulated phosphoprotein (VASP) in a NO-dependent manner. Epac1 stimulation promoted cytosol-to-membrane eNOS translocation in HMVECs and in myocardial microvascular endothelial (MyEnd) cells from wild-type mice, but not in MyEnd cells from VASP knockout mice. We demonstrate that PAF and VEGF cause hyperpermeability and stimulate the cAMP/Epac1 pathway to inactivate agonist-induced endothelial/microvascular hyperpermeability. Inactivation involves VASP-assisted translocation of eNOS from the cytosol to the endothelial cell membrane. We demonstrate that hyperpermeability is a self-limiting process, whose timed inactivation is an intrinsic property of the microvascular endothelium that maintains vascular homeostasis in response to inflammatory conditions.NEW & NOTEWORTHY Termination of microvascular hyperpermeability has been so far accepted to be a passive result of the removal of the applied proinflammatory agonists. We provide in vivo and in vitro evidence that 1) inactivation of hyperpermeability is an actively regulated process, 2) proinflammatory agonists (PAF and VEGF) stimulate microvascular hyperpermeability and initiate endothelial mechanisms that terminate hyperpermeability, and 3) eNOS location-translocation is critical in the activation-inactivation cascade of endothelial hyperpermeability.

New Insights into the Mechanism of Action of the Cyclopalladated Complex (CP2) in Leishmania: Calcium Dysregulation, Mitochondrial Dysfunction, and Cell Death.

The current treatment of leishmaniasis is based on a few drugs that present several drawbacks, such as high toxicity, difficult administration route, and low efficacy. These disadvantages raise the necessity to develop novel antileishmanial compounds allied with a comprehensive understanding of their mechanisms of action. Here, we elucidate the probable mechanism of action of the antileishmanial binuclear cyclopalladated complex [Pd(dmba)(µ-N3)]2 (CP2) in Leishmania amazonensis. CP2 causes oxidative stress in the parasite, resulting in disruption of mitochondrial Ca2+ homeostasis, cell cycle arrest at the S-phase, increasing the reactive oxygen species (ROS) production and overexpression of stress-related and cell detoxification proteins, and collapsing the Leishmania mitochondrial membrane potential, and promotes apoptotic-like features in promastigotes, leading to necrosis, or directs programmed cell death (PCD)-committed cells toward necrotic-like destruction. Moreover, CP2 reduces the parasite load in both liver and spleen in Leishmania infantum-infected hamsters when treated for 15 days with 1.5 mg/kg body weight/day CP2, expanding its potential application in addition to the already known effectiveness on cutaneous leishmaniasis for the treatment of visceral leishmaniasis, showing the broad spectrum of action of this cyclopalladated complex. The data presented here bring new insights into the CP2 molecular mechanisms of action, assisting the promotion of its rational modification to improve both safety and efficacy.

Targeting a Novel KRAS Binding Site: Application of One-Component Stapling of Small (5-6-mer) Peptides.

RAS proteins are central in the proliferation of many types of cancer, but a general approach toward the identification of pan-mutant RAS inhibitors has remained unresolved. In this work, we describe the application of a binding pharmacophore identified from analysis of known RAS binding peptides to the design of novel peptides. Using a chemically divergent approach, we generated a library of small stapled peptides from which we identified compounds with weak binding activity. Exploration of structure-activity relationships (SARs) and optimization of these early compounds led to low-micromolar binders of KRAS that block nucleotide exchange.

Receptor-specific Ca2+ oscillation patterns mediated by differential regulation of P2Y purinergic receptors in rat hepatocytes.

Extracellular agonists linked to inositol-1,4,5-trisphosphate (IP3) formation elicit cytosolic Ca2+ oscillations in many cell types, but despite a common signaling pathway, distinct agonist-specific Ca2+ spike patterns are observed. Using qPCR, we show that rat hepatocytes express multiple purinergic P2Y and P2X receptors (R). ADP acting through P2Y1R elicits narrow Ca2+ oscillations, whereas UTP acting through P2Y2R elicits broad Ca2+ oscillations, with composite patterns observed for ATP. P2XRs do not play a role at physiological agonist levels. The discrete Ca2+ signatures reflect differential effects of protein kinase C (PKC), which selectively modifies the falling phase of the Ca2+ spikes. Negative feedback by PKC limits the duration of P2Y1R-induced Ca2+ spikes in a manner that requires extracellular Ca2+. By contrast, P2Y2R is resistant to PKC negative feedback. Thus, the PKC leg of the bifurcated IP3 signaling pathway shapes unique Ca2+ oscillation patterns that allows for distinct cellular responses to different agonists.

Selective CRAF Inhibition Elicits Transactivation.

Discovering molecules that regulate closely related protein isoforms is challenging, and in many cases the consequences of isoform-specific pharmacological regulation remains unknown. RAF isoforms are commonly mutated oncogenes that serve as effector kinases in MAP kinase signaling. BRAF/CRAF heterodimers are believed to be the primary RAF signaling species, and many RAF inhibitors lead to a "paradoxical activation" of RAF kinase activity through transactivation of the CRAF protomer; this leads to resistance mechanisms and secondary tumors. It has been hypothesized that CRAF-selective inhibition might bypass paradoxical activation, but no CRAF-selective inhibitor has been reported and the consequences of pharmacologically inhibiting CRAF have remained unknown. Here, we use bio-orthogonal ligand tethering (BOLT) to selectively target inhibitors to CRAF. Our results suggest that selective CRAF inhibition promotes paradoxical activation and exemplify how BOLT may be used to triage potential targets for drug discovery before any target-selective small molecules are known.

Ethanol Disrupts Hormone-Induced Calcium Signaling in Liver.

Receptor-coupled phospholipase C (PLC) is an important target for the actions of ethanol. In the ex vivo perfused rat liver, concentrations of ethanol >100 mM were required to induce a rise in cytosolic calcium (Ca2+) suggesting that these responses may only occur after binge ethanol consumption. Conversely, pharmacologically achievable concentrations of ethanol (≤30 mM) decreased the frequency and magnitude of hormone-stimulated cytosolic and nuclear Ca2+ oscillations and the parallel translocation of protein kinase C-ß to the membrane. Ethanol also inhibited gap junction communication resulting in the loss of coordinated and spatially organized intercellular Ca2+ waves in hepatic lobules. Increasing the hormone concentration overcame the effects of ethanol on the frequency of Ca2+ oscillations and amplitude of the individual Ca2+ transients; however, the Ca2+ responses in the intact liver remained disorganized at the intercellular level, suggesting that gap junctions were still inhibited. Pretreating hepatocytes with an alcohol dehydrogenase inhibitor suppressed the effects of ethanol on hormone-induced Ca2+ increases, whereas inhibiting aldehyde dehydrogenase potentiated the inhibitory actions of ethanol, suggesting that acetaldehyde is the underlying mediator. Acute ethanol intoxication inhibited the rate of rise and the magnitude of hormone-stimulated production of inositol 1,4,5-trisphosphate (IP3), but had no effect on the size of Ca2+ spikes induced by photolysis of caged IP3. These findings suggest that ethanol inhibits PLC activity, but does not affect IP3 receptor function. We propose that by suppressing hormone-stimulated PLC activity, ethanol interferes with the dynamic modulation of [IP3] that is required to generate large, amplitude Ca2+ oscillations.

A Tale of two receptors.

Calcium (Ca2+) oscillations in hepatocytes have a wide dynamic range. In particular, recent experimental evidence shows that agonist stimulation of the P2Y family of receptors leads to qualitatively diverse Ca2+ oscillations. We present a new model of Ca2+ oscillations in hepatocytes based on these experiments to investigate the mechanisms controlling P2Y-activated Ca2+ oscillations. The model accounts for Ca2+ regulation of the IP3 receptor (IP3R), the positive feedback from Ca2+ on phospholipase C (PLC) and the P2Y receptor phosphorylation by protein kinase C (PKC). Furthermore, PKC is shown to control multiple cellular substrates. Utilising the model, we suggest the activity and intensity of PLC and PKC necessary to explain the qualitatively diverse Ca2+ oscillations in response to P2Y receptor activation.

The genetic Ca2+ sensor GCaMP3 reveals multiple Ca2+ stores differentially coupled to Ca2+ entry in the human malaria parasite Plasmodium falciparum.

Cytosolic Ca2+ regulates multiple steps in the host-cell invasion, growth, proliferation, and egress of blood-stage Plasmodium falciparum, yet our understanding of Ca2+ signaling in this endemic malaria parasite is incomplete. By using a newly generated transgenic line of P. falciparum (PfGCaMP3) that expresses constitutively the genetically encoded Ca2+ indicator GCaMP3, we have investigated the dynamics of Ca2+ release and influx elicited by inhibitors of the sarcoplasmic/endoplasmic reticulum Ca2+-ATPase pumps, cyclopiazonic acid (CPA), and thapsigargin (Thg). Here we show that in isolated trophozoite phase parasites: (i) both CPA and Thg release Ca2+ from intracellular stores in P. falciparum parasites; (ii) Thg is able to induce Ca2+ release from an intracellular compartment insensitive to CPA; (iii) only Thg is able to activate Ca2+ influx from extracellular media, through a mechanism resembling store-operated Ca2+ entry, typical of mammalian cells; and (iv) the Thg-sensitive Ca2+ pool is unaffected by collapsing the mitochondria membrane potential with the uncoupler carbonyl cyanide m-chlorophenyl hydrazone or the release of acidic Ca2+ stores with nigericin. These data suggest the presence of two Ca2+ pools in P. falciparum with differential sensitivity to the sarcoplasmic/endoplasmic reticulum Ca2+-ATPase pump inhibitors, and only the release of the Thg-sensitive Ca2+ store induces Ca2+ influx. Activation of the store-operated Ca2+ entry-like Ca2+ influx may be relevant for controlling processes such as parasite invasion, egress, and development mediated by kinases, phosphatases, and proteases that rely on Ca2+ levels for their activation.

Dual mechanisms of Ca2+ oscillations in hepatocytes.

Calcium (Ca2+) oscillations in hepatocytes control many critical cellular functions, including glucose metabolism and bile secretion. The mechanisms underlying repetitive Ca2+ oscillations and how these mechanisms regulate these oscillations is not fully understood. Recent experimental evidence has shown that both Ca2+ regulation of the inositol 1,4,5-trisphosphate (IP3) receptor and IP3 metabolism generate Ca2+ oscillations and co-exist in hepatocytes. To investigate the effects of these feedback mechanisms on the Ca2+ response, we construct a mathematical model of the Ca2+ signalling network in hepatocytes. The model accounts for the biphasic regulation of Ca2+ on the IP3 receptor (IP3R) and the positive feedback from Ca2+ on IP3 metabolism, via activation of phospholipase C (PLC) by agonist and Ca2+. Model simulations show that Ca2+ oscillations exist for both constant [IP3] and for [IP3] changing dynamically. We show, both experimentally and in the model, that as agonist concentration increases, Ca2+ oscillations transition between simple narrow-spike oscillations and complex broad-spike oscillations. The model predicts that narrow-spike oscillations persist when Ca2+ transport across the plasma membrane is blocked. This prediction has been experimentally validated. In contrast, broad-spike oscillations are terminated when plasma membrane transport is blocked. We conclude that multiple feedback mechanisms participate in regulating Ca2+ oscillations in hepatocytes.

IP3-Dependent Ca2+ Oscillations Switch into a Dual Oscillator Mechanism in the Presence of PLC-Linked Hormones.

Ca2+ oscillations that depend on inositol-1,4,5-trisphosphate (IP3) have been ascribed to biphasic Ca2+ regulation of the IP3 receptor (IP3R) or feedback mechanisms controlling IP3 levels in different cell types. IP3 uncaging in hepatocytes elicits Ca2+ transients that are often localized at the subcellular level and increase in magnitude with stimulus strength. However, this does not reproduce the broad baseline-separated global Ca2+ oscillations elicited by vasopressin. Addition of hormone to cells activated by IP3 uncaging initiates a qualitative transition from high-frequency spatially disorganized Ca2+ transients, to low-frequency, oscillatory Ca2+ waves that propagate throughout the cell. A mathematical model with dual coupled oscillators that integrates Ca2+-induced Ca2+ release at the IP3R and mutual feedback mechanisms of cross-coupling between Ca2+ and IP3 reproduces this behavior. Thus, multiple Ca2+ oscillation modes can coexist in the same cell, and hormonal stimulation can switch from the simpler to the more complex to yield robust signaling.

Intercellular calcium waves integrate hormonal control of glucose output in the intact liver.

KEY POINTS: Sympathetic outflow and circulating glucogenic hormones both regulate liver function by increasing cytosolic calcium, although how these calcium signals are integrated at the tissue level is currently unknown. We show that stimulation of hepatic nerve fibres or perfusing the liver with physiological concentrations of vasopressin only will evoke localized cytosolic calcium oscillations and modest increases in hepatic glucose production. The combination of these stimuli acted synergistically to convert localized and asynchronous calcium responses into co-ordinated intercellular calcium waves that spread throughout the liver lobule and elicited a synergistic increase in hepatic glucose production. The results obtained in the present study demonstrate that subthreshold levels of one hormone can create an excitable medium across the liver lobule, which allows global propagation of calcium signals in response to local sympathetic innervation and integration of metabolic regulation by multiple hormones. This enables the liver lobules to respond as functional units to produce full-strength metabolic output at physiological levels of hormone. ABSTRACT: Glucogenic hormones, including catecholamines and vasopressin, induce frequency-modulated cytosolic Ca2+ oscillations in hepatocytes, and these propagate as intercellular Ca2+ waves via gap junctions in the intact liver. We investigated the role of co-ordinated Ca2+ waves as a mechanism for integrating multiple endocrine and neuroendocrine inputs to control hepatic glucose production in perfused rat liver. Sympathetic nerve stimulation elicited localized Ca2+ increases that were restricted to hepatocytes in the periportal zone. During perfusion with subthreshold vasopressin, sympathetic stimulation converted asynchronous Ca2+ signals in a limited number of hepatocytes into co-ordinated intercellular Ca2+ waves that propagated across entire lobules. A similar synergism was observed between physiological concentrations of glucagon and vasopressin, where glucagon also facilitated the recruitment of hepatocytes into a Ca2+ wave. Hepatic glucose production was significantly higher with intralobular Ca2+ waves. We propose that inositol 1,4,5-trisphosphate (IP3 )-dependent Ca2+ signalling gives rise to an excitable medium across the functional syncytium of the hepatic lobule, co-ordinating and amplifying the metabolic responses to multiple hormonal inputs.

The G2019S variant of leucine-rich repeat kinase 2 (LRRK2) alters endolysosomal trafficking by impairing the function of the GTPase RAB8A.

Mutations in the gene encoding for leucine-rich repeat kinase 2 (LRRK2) are a common cause of hereditary Parkinson's disease. LRRK2 regulates various intracellular vesicular trafficking pathways, including endolysosomal degradative events such as epidermal growth factor receptor (EGFR) degradation. Recent studies have revealed that a subset of RAB proteins involved in secretory and endocytic recycling are LRRK2 kinase substrates in vivo However, the effects of LRRK2-mediated phosphorylation of these substrates on membrane trafficking remain unknown. Here, using an array of immunofluorescence and pulldown assays, we report that expression of active or phosphodeficient RAB8A variants rescues the G2019S LRRK2-mediated effects on endolysosomal membrane trafficking. Similarly, up-regulation of the RAB11-Rabin8-RAB8A cascade, which activates RAB8A, also reverted these trafficking deficits. Loss of RAB8A mimicked the effects of G2019S LRRK2 on endolysosomal trafficking and decreased RAB7A activity. Expression of pathogenic G2019S LRRK2 or loss of RAB8A interfered with EGFR degradation by causing its accumulation in a RAB4-positive endocytic compartment, which was accompanied by a deficit in EGFR recycling and was rescued upon expression of active RAB7A. Dominant-negative RAB7A expression resulted in similar deficits in EGF degradation, accumulation in a RAB4 compartment, and deficits in EGFR recycling, which were all rescued upon expression of active RAB8A. Taken together, these findings suggest that, by impairing RAB8A function, pathogenic G2019S LRRK2 deregulates endolysosomal transport and endocytic recycling events.

InsP3 Signaling in Apicomplexan Parasites.

BACKGROUND: Phosphoinositides (PIs) and their derivatives are essential cellular components that form the building blocks for cell membranes and regulate numerous cell functions. Specifically, the ability to generate myo-inositol 1,4,5-trisphosphate (InsP3) via phospholipase C (PLC) dependent hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) to InsP3 and diacylglycerol (DAG) initiates intracellular calcium signaling events representing a fundamental signaling mechanism dependent on PIs. InsP3 produced by PI turnover as a second messenger causes intracellular calcium release, especially from endoplasmic reticulum, by binding to the InsP3 receptor (InsP3R). Various PIs and the enzymes, such as phosphatidylinositol synthase and phosphatidylinositol 4-kinase, necessary for their turnover have been characterized in Apicomplexa, a large phylum of mostly commensal organisms that also includes several clinically relevant parasites. However, InsP3Rs have not been identified in genomes of apicomplexans, despite evidence that these parasites produce InsP3 that mediates intracellular Ca2+ signaling. CONCLUSION: Evidence to supporting IP3-dependent signaling cascades in apicomplexans suggests that they may harbor a primitive or non-canonical InsP3R. Understanding these pathways may be informative about early branching eukaryotes, where such signaling pathways also diverge from animal systems, thus identifying potential novel and essential targets for therapeutic intervention.

Differential Regulation of Multiple Steps in Inositol 1,4,5-Trisphosphate Signaling by Protein Kinase C Shapes Hormone-stimulated Ca2+ Oscillations.

How Ca(2+) oscillations are generated and fine-tuned to yield versatile downstream responses remains to be elucidated. In hepatocytes, G protein-coupled receptor-linked Ca(2+) oscillations report signal strength via frequency, whereas Ca(2+) spike amplitude and wave velocity remain constant. IP3 uncaging also triggers oscillatory Ca(2+) release, but, in contrast to hormones, Ca(2+) spike amplitude, width, and wave velocity were dependent on [IP3] and were not perturbed by phospholipase C (PLC) inhibition. These data indicate that oscillations elicited by IP3 uncaging are driven by the biphasic regulation of the IP3 receptor by Ca(2+), and, unlike hormone-dependent responses, do not require PLC. Removal of extracellular Ca(2+) did not perturb Ca(2+) oscillations elicited by IP3 uncaging, indicating that reloading of endoplasmic reticulum stores via plasma membrane Ca(2+) influx does not entrain the signal. Activation and inhibition of PKC attenuated hormone-induced Ca(2+) oscillations but had no effect on Ca(2+) increases induced by uncaging IP3. Importantly, PKC activation and inhibition differentially affected Ca(2+) spike frequencies and kinetics. PKC activation amplifies negative feedback loops at the level of G protein-coupled receptor PLC activity and/or IP3 metabolism to attenuate IP3 levels and suppress the generation of Ca(2+) oscillations. Inhibition of PKC relieves negative feedback regulation of IP3 accumulation and, thereby, shifts Ca(2+) oscillations toward sustained responses or dramatically prolonged spikes. PKC down-regulation attenuates phenylephrine-induced Ca(2+) wave velocity, whereas responses to IP3 uncaging are enhanced. The ability to assess Ca(2+) responses in the absence of PLC activity indicates that IP3 receptor modulation by PKC regulates Ca(2+) release and wave velocity.

Hormone-induced calcium oscillations depend on cross-coupling with inositol 1,4,5-trisphosphate oscillations.

Receptor-mediated oscillations in cytosolic Ca(2+) concentration ([Ca(2+)]i) could originate either directly from an autonomous Ca(2+) feedback oscillator at the inositol 1,4,5-trisphosphate (IP3) receptor or as a secondary consequence of IP3 oscillations driven by Ca(2+) feedback on IP3 metabolism. It is challenging to discriminate these alternatives, because IP3 fluctuations could drive Ca(2+) oscillations or could just be a secondary response to the [Ca(2+)]i spikes. To investigate this problem, we constructed a recombinant IP3 buffer using type-I IP3 receptor ligand-binding domain fused to GFP (GFP-LBD), which buffers IP3 in the physiological range. This IP3 buffer slows hormone-induced [IP3] dynamics without changing steady-state [IP3]. GFP-LBD perturbed [Ca(2+)]i oscillations in a dose-dependent manner: it decreased both the rate of [Ca(2+)]i rise and the speed of Ca(2+) wave propagation and, at high levels, abolished [Ca(2+)]i oscillations completely. These data, together with computational modeling, demonstrate that IP3 dynamics play a fundamental role in generating [Ca(2+)]i oscillations and waves.

Reliable encoding of stimulus intensities within random sequences of intracellular Ca2+ spikes.

Ca(2+) is a ubiquitous intracellular messenger that regulates diverse cellular activities. Extracellular stimuli often evoke sequences of intracellular Ca(2+) spikes, and spike frequency may encode stimulus intensity. However, the timing of spikes within a cell is random because each interspike interval has a large stochastic component. In human embryonic kidney (HEK) 293 cells and rat primary hepatocytes, we found that the average interspike interval also varied between individual cells. To evaluate how individual cells reliably encoded stimuli when Ca(2+) spikes exhibited such unpredictability, we combined Ca(2+) imaging of single cells with mathematical analyses of the Ca(2+) spikes evoked by receptors that stimulate formation of inositol 1,4,5-trisphosphate (IP3). This analysis revealed that signal-to-noise ratios were improved by slow recovery from feedback inhibition of Ca(2+) spiking operating at the whole-cell level and that they were robust against perturbations of the signaling pathway. Despite variability in the frequency of Ca(2+) spikes between cells, steps in stimulus intensity caused the stochastic period of the interspike interval to change by the same factor in all cells. These fold changes reliably encoded changes in stimulus intensity, and they resulted in an exponential dependence of average interspike interval on stimulation strength. We conclude that Ca(2+) spikes enable reliable signaling in a cell population despite randomness and cell-to-cell variability, because global feedback reduces noise, and changes in stimulus intensity are represented by fold changes in the stochastic period of the interspike interval.