G protein-dependent Ca2+ signaling complexes in polarized cells

Modulation of Type 5 Metabotropic Glutamate Receptor-Mediated Intracellular Calcium Mobilization by Regulator of G Protein Signaling 4 (RGS4) in Cultured Astrocytes

Acting as GTPase activating proteins promoting the silencing of activated G-proteins, regulators of G protein signaling (RGSs) are generally considered negative modulators of cell signaling. In the CNS, the expression of RGS4 is altered in diverse pathologies and its upregulation was reported in astrocytes exposed to an inflammatory environment. In a model of cultured cortical astrocytes, we herein investigate the influence of RGS4 on intracellular calcium signaling mediated by type 5 metabotropic glutamate receptor (mGluR5), which is known to support the bidirectional communication between neurons and glial cells. RGS4 activity was manipulated by exposure to the inhibitor CCG 63802 or by infecting the cells with lentiviruses designed to achieve the silencing or overexpression of RGS4. The pharmacological inhibition or silencing of RGS4 resulted in a decrease in the percentage of cells responding to the mGluR5 agonist DHPG and in the proportion of cells showing typical calcium oscillations. Conversely, RGS4-lentivirus infection increased the percentage of cells showing calcium oscillations. While the physiological implication of cytosolic calcium oscillations in astrocytes is still under investigation, the fine-tuning of calcium signaling likely determines the coding of diverse biological events. Indirect signaling modulators such as RGS4 inhibitors, used in combination with receptor ligands, could pave the way for new therapeutic approaches for diverse neurological disorders with improved efficacy and selectivity.

Label-Free Imaging of Dynamic and Transient Calcium Signaling in Single Cells

Cell signaling consists of diverse events that occur at various temporal and spatial scales, ranging from milliseconds to hours and from single biomolecules to cell populations. The pathway complexities require the development of new techniques that detect the overall signaling activities and are not limited to quantifying a single event. A plasmonic-based electrochemical impedance microscope (P-EIM) that can provide such data with excellent temporal and spatial resolution and does not require the addition of any labels for detection has now been developed. The highly dynamic and transient calcium signaling activities at the early stage of G-protein-coupled receptor (GPCR) stimulation were thus studied. It could be shown that a subpopulation of cells is more responsive towards agonist stimulation, and the heterogeneity of the local distributions and the transient activities of the ion channels during agonist-activated calcium flux in single HeLa cells were investigated.

Role of regulator of G protein signaling proteins in bone

Regulators of G protein signaling (RGS) proteins are a family with more than 30 proteins that all contain an RGS domain. In the past decade, increasing evidence has indicated that RGS proteins play crucial roles in the regulation of G protein coupling receptors (GPCR), G proteins, and calcium signaling during cell proliferation, migration, and differentiation in a variety of tissues. In bone, those proteins modulate bone development and remodeling by influencing various signaling pathways such as GPCR-G protein signaling, Wnt, calcium oscillations and PTH. This review summarizes the recent advances in the understanding of the regulation of RGS gene expression, as well as the functions and mechanisms of RGS proteins, especially in regulating GPCR-G protein signaling, Wnt signaling, calcium oscillations signaling and PTH signaling during bone development and remodeling. This review also highlights the regulation of different RGS proteins in osteoblasts, chondrocytes and osteoclasts. The knowledge from the recent advances of RGS study summarized in the review would provide the insights into new therapies for bone diseases.

Regulator of G protein signaling (RGS16) inhibits hepatic fatty acid oxidation in a carbohydrate response element-binding protein (ChREBP)-dependent manner

G protein-coupled receptor (GPCR) pathways control glucose and fatty acid metabolism and the onset of obesity and diabetes. Regulators of G protein signaling (RGS) are GTPase-activating proteins (GAPs) for G(i) and G(q) α-subunits that control the intensity and duration of GPCR signaling. Herein we determined the role of Rgs16 in GPCR regulation of liver metabolism. Rgs16 is expressed during the last few hours of the daily fast in periportal hepatocytes, the oxygen-rich zone of the liver where lipolysis and gluconeogenesis predominate. Rgs16 knock-out mice had elevated expression of fatty acid oxidation genes in liver, higher rates of fatty acid oxidation in liver extracts, and higher plasma β-ketone levels compared with wild type mice. By contrast, transgenic mice that overexpressed RGS16 protein specifically in liver exhibited reciprocal phenotypes as well as low blood glucose levels compared with wild type littermates and fatty liver after overnight fasting. The transcription factor carbohydrate response element-binding protein (ChREBP), which induces fatty acid synthesis genes in response to high carbohydrate feeding, was unexpectedly required during fasting for maximal Rgs16 transcription in liver and in cultured primary hepatocytes during gluconeogenesis. Thus, RGS16 provides a signaling mechanism for glucose production to inhibit GPCR-stimulated fatty acid oxidation in hepatocytes.

Regulator of G protein signaling proteins as drug targets: current state and future possibilities

Regulators of G protein signaling (RGS) proteins have emerged in the past two decades as novel drug targets in many areas of research. Their importance in regulating signaling via G protein-coupled receptors has become evident as numerous studies have been published on the structure and function of RGS proteins. A number of genetic models have also been developed, demonstrating the potential clinical importance of RGS proteins in various disease states, including central nervous system disorders, cardiovascular disease, diabetes, and several types of cancer. Apart from their classical mechanism of action as GTPase-activating proteins (GAPs), RGS proteins can also serve other noncanonical functions. This opens up a new approach to targeting RGS proteins in drug discovery as the view on the function of these proteins is constantly evolving. This chapter summarizes the latest development in RGS protein drug discovery with special emphasis on noncanonical functions and regulatory mechanisms of RGS protein expression. As more reports are being published on this group of proteins, it is becoming clear that modulation of GAP activity might not be the only way to therapeutically target RGS proteins.

Use of optical tweezers to probe epithelial mechanosensation

Cellular mechanosensation mechanisms have been implicated in a variety of disease states. Specifically in renal tubules, the primary cilium and associated mechanosensitive ion channels are hypothesized to play a role in water and salt homeostasis, with relevant disease states including polycystic kidney disease and hypertension. Previous experiments investigating ciliary-mediated cellular mechanosensation have used either fluid flow chambers or micropipetting to elicit a biological response. The interpretation of these experiments in terms of the "ciliary hypothesis" has been difficult due the spatially distributed nature of the mechanical disturbance-several competing hypotheses regarding possible roles of primary cilium, glycocalyx, microvilli, cell junctions, and actin cytoskeleton exist. I report initial data using optical tweezers to manipulate individual primary cilia in an attempt to elicit a mechanotransduction response-specifically, the release of intracellular calcium. The advantage of using laser tweezers over previous work is that the applied disturbance is highly localized. I find that stimulation of a primary cilium elicits a response, while stimulation of the apical surface membrane does not. These results lend support to the hypothesis that the primary cilium mediates transduction of mechanical strain into a biochemical response in renal epithelia.

Functional organization of TRPC-Ca2+ channels and regulation of calcium microdomains

TRP family of proteins are components of unique cation channels that are activated in response to diverse stimuli ranging from growth factor and neurotransmitter stimulation of plasma membrane receptors to a variety of chemical and sensory signals. This review will focus on members of the TRPC sub-family (TRPC1-TRPC7) which currently appear to be the strongest candidates for the enigmatic Ca(2+) influx channels that are activated in response to stimulation of plasma membrane receptors which result in phosphatidyl inositol-(4,5)-bisphosphate (PIP(2)) hydrolysis, generation of IP(3) and DAG, and IP(3)-induced Ca(2+) release from the intracellular Ca(2+) store via inositol trisphosphate receptor (IP(3)R). Homomeric or selective heteromeric interactions between TRPC monomers generate distinct channels that contribute to store-operated as well as store-independent Ca(2+) entry mechanisms. The former is regulated by the emptying/refilling of internal Ca(2+) store(s) while the latter depends on PIP(2) hydrolysis (due to changes in PIP(2) per se or an increase in diacylglycerol, DAG). Although the exact physiological function of TRPC channels and how they are regulated has not yet been conclusively established, it is clear that a variety of cellular functions are controlled by Ca(2+) entry via these channels. Thus, it is critical to understand how cells coordinate the regulation of diverse TRPC channels to elicit specific physiological functions. It is now well established that segregation of TRPC channels mediated by interactions with signaling and scaffolding proteins, determines their localization and regulation in functionally distinct cellular domains. Furthermore, both protein and lipid components of intracellular and plasma membranes contribute to the organization of these microdomains. Such organization serves as a platform for the generation of spatially and temporally dictated [Ca(2+)](i) signals which are critical for precise control of downstream cellular functions.

Signaling proteins in raft-like microdomains are essential for Ca2+ wave propagation in glial cells

The hypothesis that calcium signaling proteins segregate into lipid raft-like microdomains was tested in isolated membranes of rat oligodendrocyte progenitor (OP) cells and astrocytes using Triton X-100 solubilization and density gradient centrifugation. Western blot analysis of gradient fractions showed co-localization of caveolin-1 with proteins involved in the Ca2+ signaling cascade. These included agonist receptors, P2Y1, and M1, TRPC1, IP3R2, ryanodine receptor, as well as the G protein Galphaq and Homer. Membranes isolated from agonist-stimulated astrocytes showed an enhanced recruitment of phospholipase C (PLCbeta1), IP3R2 and protein kinase C (PKC-alpha) into lipid raft fractions. IP3R2, TRPC1 and Homer co-immunoprecipitated, suggesting protein-protein interactions. Disruption of rafts by cholesterol depletion using methyl-beta-cyclodextrin (beta-MCD) altered the distribution of caveolin-1 and GM1 to non-raft fractions with higher densities. beta-MCD-induced disruption of rafts inhibited agonist-evoked Ca2+ wave propagation in astrocytes and attenuated wave speeds. These results indicate that in glial cells, Ca2+ signaling proteins might exist in organized membrane microdomains, and these complexes may include proteins from different cellular membrane systems. Such an organization is essential for Ca2+ wave propagation.

New roles for Galpha and RGS proteins: communication continues despite pulling sisters apart

Large G protein alpha subunits and their attendant regulators of G-protein signaling (RGS) proteins control both intercellular signaling and asymmetric cell divisions by distinct pathways. The classical pathway, found throughout higher eukaryotic organisms, mediates intercellular communication via hormone binding to G-protein-coupled receptors (GPCRs). Recent studies have led to the discovery of GPCR-independent activation of Galpha subunits by the guanine nucleotide exchange factor RIC-8 in both asymmetric cell division and synaptic vesicle priming in metazoan organisms. Protein-protein interactions and protein function in each pathway are driven through the cycle of GTP binding and hydrolysis by the Galpha subunit. This review builds a conceptual framework for understanding RIC-8-mediated pathways by comparison with the mechanism of classical G-protein activation and inhibition in GPCR signaling.

Characterization and comparison of raft-like membranes isolated by two different methods from rat submandibular gland cells

Lipid rafts are defined as cholesterol and sphingolipid enriched domains in biological membranes. Their role in signalling and other cellular processes is widely accepted but the methodology used for their biochemical isolation and characterization remains controversial. Raft-like membranes from rat submandibular glands were isolated by two different protocols commonly described in the literature; one protocol was based on selective solubilization by Triton X-100 at low temperature and the other protocol consisted in extensive sonication. In both cases a low density vesicular fraction was obtained after ultracentrifugation in a sucrose density gradient. These fractions contained about 20% of total cholesterol but less than 8% of total proteins, and were more rigid than bulk membranes. Fatty acid analyses revealed a similar composition of raft-like membranes isolated by the two different methods, which was characterized by an enrichment in saturated fatty acids in detriment of polyunsaturated acids when compared with the whole cell membranes. Protein profile of detergent resistant membranes or raft-like membranes prepared by sonication was assessed by silver staining after SDS-PAGE and by MALDI-TOF. Both analyses provided evidence of a different protein composition of the Triton X-100 and sonication preparations. Immunoblot experiments revealed that raft-like membranes prepared by detergent extraction or sonication were free of Golgi apparatus or endoplasmic reticulum protein markers (beta-COP and calnexin, respectively) and that they were not substantially contaminated by transferrin receptor (a non-raft protein). While caveolin-1 was highly enriched in raft-like membranes prepared by the two methods, the P2X(7) receptor was enriched in raft-like membrane fractions prepared by sonication, but almost undetectable in the detergent resistant membranes. It can be concluded that both methods can be used to obtain raft-like membranes, but that detergent may affect protein interactions responsible for their association with different membrane domains.

Regulation of zymogen granule exocytosis by Ca2+, cAMP, and PKC in pancreatic acinar cells

The effect of cAMP and PKC on zymogen granule exocytosis was investigated by simultaneously measuring cytosolic Ca2+ concentration ([Ca2+]c) and individual zymogen granule exocytosis in isolated mouse pancreatic acini. When acinar cells were stimulated with acetylcholine (ACh, 10 microM), exocytic events were detected through granule-attached apical membranes with [Ca2+]c rise. Application of secretin, forskolin (an adenylate cyclase activator), or PMA (a PKC activator) alone did not elicit any [Ca2+]c rise or zymogen granule exocytosis, but co-stimulation with ACh led to exocytosis in that the total number of secreted granules increased markedly without a significant difference in [Ca2+]c rises. When we evoked exocytosis by [Ca2+]c ramps, pretreatment with forskolin or PMA elicited exocytosis at lower [Ca2+]c levels. These results indicate that PKC or cAMP alone could not directly elicit zymogen granule exocytosis, but that they increase the total releasable pool by rendering zymogen granules more sensitive to Ca2+.

Apical Localization of a Functional TRPC3/TRPC6-Ca2+-Signaling Complex in Polarized Epithelial Cells

Receptor-coupled [Ca2+]i increase is initiated in the apical region of epithelial cells and has been associated with apically localized Ca2+-signaling proteins. However, localization of Ca2+ channels that are regulated by such Ca2+-signaling events has not yet been established. This study examines the localization of TRPC channels in polarized epithelial cells and demonstrates a role for TRPC3 in apical Ca2+ uptake. Endogenously and exogenously expressed TRPC3 was localized apically in polarized Madin-Darby canine kidney cells (MDCK) and salivary gland epithelial cells. In contrast, TRPC1 was localized basolaterally, whereas TRPC6 was detected in both locations. Localization of Galpha(q/11), inositol 1,4,5-trisphosphate receptor-3, and phospholipase Cbeta1 and -beta2 was also predominantly apical. TRPC3 co-immunoprecipitated with endogenous TRPC6, phospholipase Cbetas, Galpha(q/11), inositol 1,4,5-trisphosphate receptor-3, and syntaxin 3 but not with TRPC1. Furthermore, 1-oleoyl-2-acetyl-sn-glycerol (OAG)-stimulated apical 45Ca2+ uptake was higher in TRPC3-MDCK cells compared with control (MDCK) cells. Bradykinin-stimulated apical 45Ca2+ uptake and transepithelial 45Ca2+ flux were also higher in TRPC3-expressing cells. Consistent with this, OAG induced [Ca2+]i increase in the apical, but not basal, region of TRPC3-MDCK cells that was blocked by EGTA addition to the apical medium. Most importantly, (i) TRPC3 was detected in the apical region of rat submandibular gland ducts, whereas TRPC6 was present in apical as well as basolateral regions of ducts and acini; and (ii) OAG stimulated Ca2+ influx into dispersed ductal cells. These data demonstrate functional localization of TRPC3/TRPC6 channels in the apical region of polarized epithelial cells. In salivary gland ducts this could contribute to the regulation of salivary [Ca2+] and secretion.

Cholesterol depletion perturbs calcium handling by rat submandibular glands

The sensitivity to cholesterol depletion of calcium handling by rat submandibular glands was investigated. The glands were digested with collagenase. After homogenization, the lysate was extracted at 4 degrees C with 0.5% Triton X-100 and the extract was submitted to an ultracentrifugation in a sucrose discontinuous gradient. A population of detergent-resistant membranes (DRM) was collected at the 5%-35% interface. The DRM had a higher content of cholesterol, saturated and long-chain fatty acids. Caveolin-1 and alpha(q/11) were located in these membranes. They were more ordered than vesicles from total cellular lysate as determined by anisotropy measurement. They disappeared after cholesterol extraction with methyl-beta-cyclodextrin (MCD). Exposure of the cellular suspension with MCD nearly abolished the response to carbachol, epinephrine, and substance P and inhibited the activation of phospholipase C (PLC) by these agonists and by sodium fluoride. MCD did not affect the mobilization of intracellular pools of calcium by thapsigargin. It increased the uptake of extracellular calcium or barium and did not inhibit the uptake of calcium after depletion of the intracellular stores of this ion. From these results, it is concluded that Triton X-100 can extract a fraction of membrane resistant to detergents. Treatment of the cells with MCD disrupts these membranes. The coupling between the heterotrimeric GTP-binding protein G(q/11) and poly-phosphoinositide-specific PLC is affected by disruption of these membrane fractions. At the opposite, the store-operated calcium channel (SOCC) is not affected by DRM-disruption.

Role of Regulator of G Protein Signaling 2 (RGS2) in Ca2+ Oscillations and Adaptation of Ca2+ Signaling to Reduce Excitability of RGS2–/– Cells

Regulators of G protein signaling (RGS) proteins accelerate the GTPase activity of Galpha subunits to determine the duration of the stimulated state and control G protein-coupled receptor-mediated cell signaling. RGS2 is an RGS protein that shows preference toward Galpha(q). To better understand the role of RGS2 in Ca(2+) signaling and Ca(2+) oscillations, we characterized Ca(2+) signaling in cells derived from RGS2(-/-) mice. Deletion of RGS2 modified the kinetic of inositol 1,4,5-trisphosphate (IP(3)) production without affecting the peak level of IP(3), but rather increased the steady-state level of IP(3) at all agonist concentrations. The increased steady-state level of IP(3) led to an increased frequency of [Ca(2+)](i) oscillations. The cells were adapted to deletion of RGS2 by reducing Ca(2+) signaling excitability. Reduced excitability was achieved by adaptation of all transporters to reduce Ca(2+) influx into the cytosol. Thus, IP(3) receptor 1 was down-regulated and IP(3) receptor 3 was up-regulated in RGS2(-/-) cells to reduce the sensitivity for IP(3) to release Ca(2+) from the endoplasmic reticulum to the cytosol. Sarco/endoplasmic reticulum Ca(2+) ATPase 2b was up-regulated to more rapidly remove Ca(2+) from the cytosol of RGS2(-/-) cells. Agonist-stimulated Ca(2+) influx was reduced, and Ca(2+) efflux by plasma membrane Ca(2+) was up-regulated in RGS2(-/-) cells. The result of these adaptive mechanisms was the reduced excitability of Ca(2+) signaling, as reflected by the markedly reduced response of RGS2(-/-) cells to changes in the endoplasmic reticulum Ca(2+) load and to an increase in extracellular Ca(2+). These findings highlight the central role of RGS proteins in [Ca(2+)](i) oscillations and reveal a prominent plasticity and adaptability of the Ca(2+) signaling apparatus.

Cellular domains that contribute to Ca2+ entry events

Stimulation of cell surface receptors that increase phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis leads to intracellular Ca2+ release and activation of plasma membrane Ca2+ entry channels. Ca2+ entry via these channels regulates a wide array of physiological functions. The molecular composition of these channels and the mechanisms that activate or inactivate them have not yet been elucidated. Members of the TRPC subfamily of the TRP (transient receptor potential) family of proteins have been recently suggested as molecular components of these channels. In addition, Ca2+ signaling proteins and the signals they generate are compartmentalized and spatiotemporally regulated. Thus, the mechanisms involved in the assembly and trafficking of Ca2+ signaling proteins, including TRPC channels, will determine the regulation of Ca2+ entry and its effect on cellular function.

Morphological and functional changes of dissociated single pancreatic acinar cells: testing the suitability of the single cell as a model for exocytosis and calcium signaling

Isolated single pancreatic acinar cells have long been used as a model for studying many kinds of signaling processes due to their structural and functional polarities, but without significant validation. In this study, we examined the morphological and functional changes of dissociated single pancreatic acinar cells. Acutely isolated single cells showed a collapsed membrane potential and a much reduced secretion of zymogen granules in response to acetylcholine (ACh) stimulation, whereas clustered cells showed a much more negative membrane potential and potent exocytotic secretion. The isolated single cells became vertically flattened due to the loss of supporting adhesions with nearby cells, and the granule-attached luminal membrane was severely reduced versus that of clustered cells. However, polarized Ca(2+) signals and mitochondrial localizations were relatively well preserved in the isolated single cells, in that Ca(2+) release by ACh commenced at the indented luminal membrane. In clusters, the Ca(2+) release site was closest to the lumen where more than three cells met or at the tips of conical regions of the luminal membrane. These findings suggest that the dissociated single pancreatic acinar cells preserve an intact Ca(2+) signaling machinery but alter in shape and have impaired exocytotic functions and resting membrane potentials.

Calcium signaling dysfunction in schizophrenia: a unifying approach

The present paper demonstrates a remarkable pervasiveness of underlying Ca(2+) signaling motifs among the available biochemical findings in schizophrenic patients and among the major molecular hypotheses of this disease. In addition, the paper reviews the findings suggesting that Ca(2+) is capable of inducing structural and cognitive deficits seen in schizophrenia. The evidence of the ability of antipsychotic drugs to affect Ca(2+) signaling is also presented. Based on these data, it is proposed that altered Ca(2+) signaling may constitute the central unifying molecular pathology in schizophrenia. According to this hypothesis schizophrenia can result from alterations in multiple proteins and other molecules as long as these alterations lead to abnormalities in certain key aspects of intracellular Ca(2+) signaling cascades.

Subcellular distribution of Homer 1b/c in relation to endoplasmic reticulum and plasma membrane proteins in Purkinje neurons

The subcellular distribution of endoplasmic reticulum proteins (IP3R1 and RYR), plasma membrane (PM) proteins (mGluR1 and PMCA Ca(2+)-pump), and scaffolding proteins, such as Homer 1b/c, was assessed by laser scanning confocal microscopy of rat cerebellum parasagittal sections. There appeared to be two classes of Ca2+ stores, nonjunctional Ca2+ stores and junctional Ca2+ stores, possibly referable to central cisternae/tubules and sub-PM cisternae, respectively, in soma, dendrites, and dendritic spines. Only some IP3R1s appeared to be part of multimeric, junctional Ca2+ signaling networks, whose composition is shown to include PMCA, mGluR1, Homer 1b/c and, not always, RYR1.

Histamine potentiates IP(3)-mediated Ca(2+) release via thapsigargin-sensitive Ca(2+) pumps

We have studied the histamine-induced potentiation of inositol 1,4,5-trisphosphate (IP(3))-mediated Ca(2+) release in HeLa cells. Intracellular IP(3) levels were increased by IP(3) dialysis with the whole-cell configuration of the patch-clamp technique (cell dialysis of IP(3)). Low concentrations of extracellular histamine (1 microM) accelerated the rate of IP(3)-mediated Ca(2+) release, an effect that required the coincidence of both histamine signalling and the increase in IP(3) levels. Our data suggest that the potentiation effect of histamine cannot be explained simply by agonist-induced increase in IP(3) levels. Disordering microfilaments with cytochalasin D and microtubules with colchicine caused a decrease in the histamine-induced Ca(2+) response. Furthermore, both cytochalasin D and colchicine diminished the rate of IP(3)-mediated Ca(2+) release, while only the former reduced slightly the histamine-induced potentiation effect. Remarkably, rapid inhibition of SERCA pumps with thapsigargin to avoid the depletion of internal Ca(2+) stores diminished the histamine-induced potentiation of IP(3)-mediated Ca(2+) release, without affecting the rate of IP(3)-mediated Ca(2+) release. These data indicate that histamine-induced potentiation of Ca(2+) release in HeLa cells requires active SERCA pumps and suggest that SERCA pumps are an important factor in determining the efficiency of agonist-induced Ca(2+) release.

Effects of Helicobacter pylori on intracellular Ca2+ signaling in normal human gastric mucous epithelial cells

In stomach, Helicobacter pylori (Hp) adheres to gastric mucous epithelial cells (GMEC) and initiates several different signal transduction events. Alteration of intracellular Ca2+ concentration ([Ca2+]i) is an important signaling mechanism in numerous bacteria-host model systems. Changes in [Ca2+]i induced by Hp in normal human GMEC have not yet been described; therefore, we examined effects of Hp on [Ca2+]i in normal human GMEC and a nontransformed GMEC line (HFE-145). Cultured cells were grown on glass slides, porous filters, or 96-well plates and loaded with fura 2 or fluo 4. Hp wild-type strain 60190 and vacA-, cagA-, and picB-/cagE- isogenic mutants were incubated with cells. Changes in [Ca2+]i were recorded with a fluorimeter or fluorescence plate reader. Wild-type Hp produced dose-dependent biphasic transient [Ca2+]i peak and plateau changes in both cell lines. Hp vacA- isogenic mutant produced changes in [Ca2+]i similar to those produced by wild type. Compared with wild type, cagA- and picB-/cagE- isogenic mutants produced lower peak changes and did not generate a plateau change. Preloading cultures with intracellular Ca2+ chelator BAPTA blocked all Hp-induced [Ca2+]i changes. Thapsigargin pretreatment of cultures to release Ca2+ from internal stores reduced peak change. Extracellular Ca2+ removal reduced plateau response. Hp-induced peak response was sensitive to G proteins and PLC inhibitors. Hp-induced plateau change was sensitive to G protein inhibitors, src kinases, and PLA2. These findings are the first to show that H. pylori alters [Ca2+]i in normal GMEC through a Ca2+ release/influx mechanism that depends on expression of cagA and picB/cagE genes.

Galphaq and Galpha11 proteins mediate endothelin-1 signaling in neural crest-derived pharyngeal arch mesenchyme

Endothelin-A (ET(A)) is a G-protein-coupled receptor expressed in the neural crest-derived mesenchyme of the pharyngeal arches during craniofacial development. Targeted deletion of the ET(A) receptor or its ligand endothelin-1 (ET-1) causes cleft palate and hypoplasia of the mandible, otic cup, and tympanic ring. Previously we showed that Galpha(q)/Galpha(11)-null mice die around E11.0, whereas Galpha(q)((-/-))Galpha(11)((+/-)) mice survive to birth with hypomorphic phenotypes similar to, but less severe than, ET(A) or ET-1-null mice. To determine whether ET-1 signaling is transduced by Galpha(q)/Galpha(11) proteins, we examined the expression patterns of several ET-1 dependent and independent transcription factors in Galpha(q)/Galpha(11)-deficient embryos. Expression of genes encoding the ET-1-dependent transcription factors Dlx3, Dlx6, dHAND, and eHAND was specifically downregulated in the pharyngeal arches of Galpha(q)/Galpha(11)-deficient mice. In contrast, pharyngeal arch expression of the homeobox gene Msx1, which is not regulated by ET-1 signaling, was maintained in these embryos. We conclude that the Galpha(q) and Galpha(11) proteins serve as the intracellular mediators of ET-1 signaling in the pharyngeal arch mesenchyme.

Interaction with neuronal calcium sensor NCS-1 mediates desensitization of the D2 dopamine receptor

Dopaminergic transmission within limbic regions of the brain is highly dependent on the regulation of D2 receptor activity. Here we show that the neuronal calcium sensor-1 (NCS-1) can mediate desensitization of D2 dopamine receptors. Analysis of D2 receptors expressed in human embryonic kidney 293 cells indicates that NCS-1 attenuates agonist-induced receptor internalization via a mechanism that involves a reduction in D2 receptor phosphorylation. This effect of NCS-1 was accompanied by an increase in D2 receptor-mediated cAMP inhibition after dopamine stimulation. The ability of NCS-1 to modulate D2 receptor signaling was abolished after a single amino acid mutation in NCS-1 that has been shown to impair the calcium-binding properties of NCS-1. Coimmunoprecipitation experiments from striatal neurons reveal that NCS-1 is found in association with both the D2 receptor and G-protein-coupled receptor kinase 2, a regulator of D2 receptor desensitization. Colocalization of NCS-1 and D2 receptors was examined in both primate and rodent brain. In striatum, NCS-1 and D2 receptors were found to colocalize within sites of synaptic transmission and in close proximity to intracellular calcium stores. NCS-1-D2 receptor interaction may serve to couple dopamine and calcium signaling pathways, thereby providing a critical component in the regulation of dopaminergic signaling in normal and diseased brain.

Mutation analysis of the ATP2A2 gene in Taiwanese patients with Darier's disease

BACKGROUND

Darier's disease (DD) is an autosomal dominant skin disorder characterized by abnormal keratinization and acantholysis. Pathogenic mutations in the ATP2A2 gene encoding SERCA2, a calcium pump of the sarco/endoplasmic reticulum, have recently been identified.

OBJECTIVES

To identify mutations of the ATP2A2 gene in Taiwanese patients with DD.

METHODS

Mutation analysis of genomic DNA was performed on five families with DD and two sporadic cases. All 21 exons and the flanking intron boundaries were amplified and followed by direct sequencing. Restriction fragment analysis or direct sequencing in each family and in normal controls further verified the mutations.

RESULTS

Mutations in the functional domains of the ATP2A2 gene were identified and verified in all seven pedigrees. They consisted of four mis-sense mutations (R131Q, P680L, G703S, G807R), one altered splice-site mutation (2980 + 5insA) and one frameshift deletion mutation (1457-1458delAG). Of these, R131Q, which was reported twice previously, was detected in two unrelated families. The remaining five were novel mutations.

CONCLUSIONS

Six pathogenic mutations in the ATP2A2 gene were identified in seven Taiwanese DD pedigrees. The results confirmed that most mutations in the ATP2A2 gene are private and of the mis-sense type.

Fluorescent histochemical techniques for analysis of intracellular signaling

Intracellular signaling relies on the orchestrated cooperation of signaling proteins and modules, their intracellular localization, and membrane trafficking. Recently, a repertoire of fluorescence-based techniques, which significantly increases our potential for detailed studies of the involved mechanisms, has been introduced. Microscopic techniques with increased resolution have been combined with improved techniques for detection of signaling proteins. Transfections of fluorescently tagged proteins have allowed in vivo microscopy of their trafficking and interactions with other proteins and intracellular structures. We present an overview of general signaling principles and a description of techniques based on fluorescent microscopy suited for studies of signaling mechanisms.

Receptor biology and signal transduction

This year has witnessed substantial advances in receptor biology and signal transduction that are relevant to the function and regulation of the healthy pancreas and to the pathogenesis and potential therapy of pancreatitis and pancreatic carcinoma. There has been an expansion in the cast of pancreatic regulatory molecules, now including protease-activated receptors, chemokines, and chemokine receptors. There have been new insights into the cellular distribution and signaling initiated at the classic pancreatic receptors. There have also been dramatic advances in insights into the structure of G protein-coupled receptors, with the first solution of a crystal structure of a member of this superfamily, and into the molecular basis of ligand binding and activation of these important molecules. This will clearly improve the opportunities for the rational design and refinement of receptor-active drugs. In addition to these fundamental advances, there has been renewed attention to the expression, function, and regulation of receptors and signaling pathways in pancreatic cells present in the setting of pancreatitis and pancreatic carcinoma. It is hoped that this will contribute toward earlier diagnosis, more successful therapy, and new chemopreventive strategies for these illnesses.

Recent developments in non-excitable cell calcium entry

Influx of calcium into cells following stimulation of cell surface receptors is a key process controlling cellular activity. However, despite intensive research, there is still no consensus on precisely how calcium entry is controlled in electrically no n-excitable cells. In particular, the regulation of depletion-activated or 'capacitative' calcium entry continues to be a focus of debate. Work published in the last 2 years has lent new impetus to the so-called 'conformational coupling' theory, although evidence for the existence of soluble messengers between the ER and the plasma membrane also continues to appear. In addition, there remains disagreement on whether intra-store [Ca(2+)] has to fall below a threshold before Ca(2+)entry is activated. A further major question is the identity of the putative depletion-operated Ca(2+)channel or channels. Here discussion has largely focussed on whether homologue(s) of the Drosophila TRP ('Transient Receptor Potential') protein is/are the elusive channel, or at least a part of it. Finally, it remains possible that Ca(2+)entry mechanisms other than depletion-activated channels may be important in agonist-evoked Ca(2+)influx. This commentary summarizes recent developments in the field, and highlights both current debates and critical unsolved questions.

Specific Ca2+ signaling evoked by cholecystokinin and acetylcholine: the roles of NAADP, cADPR, and IP3

In order to control cell functions, hormones and neurotransmitters generate an amazing diversity of Ca2+ signals such as local and global Ca2+ elevations and also Ca2+ oscillations. In pancreatic acinar cells, cholecystokinin (CCK) stimulates secretion of digestive enzyme and promotes cell growth, whereas acetylcholine (ACh) essentially triggers enzyme secretion. Pancreatic acinar cells are a classic model for the study of CCK- and ACh-evoked specific Ca2+ signals. In addition to inositol 1,4,5 trisphosphate (IP3), recent studies have shown that cyclic ADPribose (cADPr) and nicotinic acid adenine dinucleotide phosphate (NAADP) release Ca2+ in pancreatic acinar cells. Moreover, it has also been shown that both ACh and CCK trigger Ca2+ spikes by co-activation of IP3 and ryanodine receptors but by different means. ACh uses IP3 and Ca2+, whereas CCK uses cADPr and NAADP. In addition, CCK activates phospholipase A2 and D. The concept emerging from these studies is that agonist-specific Ca2+ signals in a single target cell are generated by combination of different intracellular messengers.

The organisation and functions of local Ca2+ signals

Calcium (Ca2+) is a ubiquitous intracellular messenger, controlling a diverse range of cellular processes, such as gene transcription, muscle contraction and cell proliferation. The ability of a simple ion such as Ca2+ to play a pivotal role in cell biology results from the facility that cells have to shape Ca2+ signals in space, time and amplitude. To generate and interpret the variety of observed Ca2+ signals, different cell types employ components selected from a Ca2+ signalling ‘toolkit’, which comprises an array of homeostatic and sensory mechanisms. By mixing and matching components from the toolkit, cells can obtain Ca2+ signals that suit their physiology. Recent studies have demonstrated the importance of local Ca2+ signals in defining the specificity of the interaction of Ca2+ with its targets. Furthermore, local Ca2+ signals are the triggers and building blocks for larger global signals that propagate throughout cells.

Plasticity and adaptation of Ca2+ signaling and Ca2+-dependent exocytosis in SERCA2(+/-) mice

Darier's disease (DD) is a high penetrance, autosomal dominant mutation in the ATP2A2 gene, which encodes the SERCA2 Ca2+ pump. Here we have used a mouse model of DD, a SERCA2(+/-) mouse, to define the adaptation of Ca2+ signaling and Ca2+-dependent exocytosis to a deletion of one copy of the SERCA2 gene. The [Ca2+]i transient evoked by maximal agonist stimulation was shorter in cells from SERCA2(+/-) mice, due to an up-regulation of specific plasma membrane Ca2+ pump isoforms. The change in cellular Ca2+ handling caused approximately 50% reduction in [Ca2+]i oscillation frequency. Nonetheless, agonist-stimulated exocytosis was identical in cells from wild-type and SERCA2(+/-) mice. This was due to adaptation in the levels of the Ca2+ sensors for exocytosis synaptotagmins I and III in cells from SERCA2(+/-) mice. Accordingly, exocytosis was approximately 10-fold more sensitive to Ca2+ in cells from SERCA2(+/-) mice. These findings reveal a remarkable plasticity and adaptability of Ca2+ signaling and Ca2+-dependent cellular functions in vivo, and can explain the normal function of most physiological systems in DD patients.

New thoughts on the role of the beta-gamma subunit in G-protein signal transduction

Heterotrimeric G proteins are involved in numerous biological processes, where they mediate signal transduction from agonist-bound G-protein-coupled receptors to a variety of intracellular effector molecules and ion channels. G proteins consist of two signaling moieties: a GTP-bound alpha subunit and a beta-gamma heterodimer. The beta-gamma dimer, recently credited as a significant modulator of G-protein-mediated cellular responses, is postulated to be a major determinant of signaling fidelity between G-protein-coupled receptors and downstream effectors. In this review we have focused on the role of beta-gamma signaling and have included examples to demonstrate the heterogeneity in the heterodimer composition and its implications in signaling fidelity. We also present an overview of some of the effectors regulated by beta-gamma and draw attention to the fact that, although G proteins and their associated receptors play an instrumental role in development, there is rather limited information on beta-gamma signaling in embryogenesis.

Targeted phosphorylation of inositol 1,4,5-trisphosphate receptors selectively inhibits localized Ca2+ release and shapes oscillatory Ca2+ signals

The current study provides biochemical and functional evidence that the targeting of protein kinase A (PKA) to sites of localized Ca(2+) release confers rapid, specific phosphoregulation of Ca(2+) signaling in pancreatic acinar cells. Regulatory control of Ca(2+) release by PKA-dependent phosphorylation of inositol 1,4, 5-trisphosphate (InsP(3)) receptors was investigated by monitoring Ca(2+) dynamics in pancreatic acinar cells evoked by the flash photolysis of caged InsP(3) prior to and following PKA activation. Ca(2+) dynamics were imaged with high temporal resolution by digital imaging and electrophysiological methods. The whole cell patch clamp technique was used to introduce caged compounds and to record the activity of a Ca(2+)-activated Cl(-) current. Photolysis of low concentrations of caged InsP(3) evoked Cl(-) currents that were inhibited by treatment with dibutryl-cAMP or forskolin. In contrast, PKA activators had no significant inhibitory effect on the activation of Cl(-) current evoked by uncaging Ca(2+) or by the photolytic release of higher concentrations of InsP(3). Treatment with Rp-adenosine-3',5'-cyclic monophoshorothioate, a selective inhibitor of PKA, or with Ht31, a peptide known to disrupt the targeting of PKA, largely abolished forskolin-induced inhibition of Ca(2+) release. Further evidence for the targeting of PKA to the sites of Ca(2+) mobilization was revealed using immunocytochemical methods demonstrating that the R(IIbeta) subunit of PKA was localized to the apical regions of acinar cells and co-immunoprecipitated with the type III but not the type I or type II InsP(3) receptors. Finally, we demonstrate that the pattern of signaling evoked by acetylcholine can be converted to one that is more "CCK-like" by raising cAMP levels. Our data provide a simple mechanism by which distinct oscillatory Ca(2+) patterns can be shaped.

The mammalian Sec6/8 complex interacts with Ca(2+) signaling complexes and regulates their activity

The localization of various Ca(2+) transport and signaling proteins in secretory cells is highly restricted, resulting in polarized agonist-stimulated Ca(2+) waves. In the present work, we examined the possible roles of the Sec6/8 complex or the exocyst in polarized Ca(2+) signaling in pancreatic acinar cells. Immunolocalization by confocal microscopy showed that the Sec6/8 complex is excluded from tight junctions and secretory granules in these cells. The Sec6/8 complex was found in at least two cellular compartments, part of the complex showed similar, but not identical, localization with the Golgi apparatus and part of the complex associated with Ca(2+) signaling proteins next to the plasma membrane at the apical pole. Accordingly, immunoprecipitation (IP) of Sec8 did not coimmunoprecipitate betaCOP, Golgi 58K protein, or mannosidase II, all Golgi-resident proteins. By contrast, IP of Sec8 coimmunoprecipitates Sec6, type 3 inositol 1,4,5-trisphosphate receptors (IP(3)R3), and the Gbetagamma subunit of G proteins from pancreatic acinar cell extracts. Furthermore, the anti-Sec8 antibodies coimmunoprecipitate actin, Sec6, the plasma membrane Ca(2+) pump, the G protein subunits Galphaq and Gbetagamma, the beta1 isoform of phospholipase C, and the ER resident IP(3)R1 from brain microsomal extracts. Antibodies against the various signaling and Ca(2+) transport proteins coimmunoprecipitate Sec8 and the other signaling proteins. Dissociation of actin filaments in the immunoprecipitate had no effect on the interaction between Sec6 and Sec8, but released the actin and dissociated the interaction between the Sec6/8 complex and Ca(2+) signaling proteins. Hence, the interaction between the Sec6/8 and Ca(2+) signaling complexes is likely mediated by the actin cytoskeleton. The anti-Sec6 and anti-Sec8 antibodies inhibited Ca(2+) signaling at a step upstream of Ca(2+) release by IP(3). Disruption of the actin cytoskeleton with latrunculin B in intact cells resulted in partial translocation of Sec6 and Sec8 from membranes to the cytosol and interfered with propagation of agonist-evoked Ca(2+) waves. Our results suggest that the Sec6/8 complex has multiple roles in secretory cells including governing the polarized expression of Ca(2+) signaling complexes and regulation of their activity.

Regulators of G-protein signaling in receptor complexes

G protein signaling pathways regulate heart development and adult cardiac function. G protein activity is controlled by the interplay between receptor-catalyzed activation and the inhibitory regulators of G protein signaling (RGS) proteins. Most RGS proteins are GTPase accelerating proteins (GAPs) for Gi and Gq class G protein alpha subunits, and thereby terminate signaling. However, RGS proteins also provide scaffolding properties to help assemble or maintain signaling complexes. Thus, RGS proteins are kinetic regulators that may sharpen both signal activation and termination. The five subfamilies of mammalian RGS proteins contain a characteristic RGS domain and distinct flanking domains that convey lipid and/or protein interactions within receptor complexes. The RGS domain provides GAP activity and additional interactions with the receptor complex. Distantly related RGS-like (RGL) proteins provide other regulatory and effector functions in G protein signaling pathways. RGS and RGL proteins provide exciting new therapeutic targets to combat cardiovascular diseases.