Differential regulation of Ca2+ release-activated Ca2+ influx by heterotrimeric G proteins

Bi-component HlgC/HlgB and HlgA/HlgB γ-hemolysins from S. aureus: Modulation of Ca2+ channels activity through a differential mechanism

Staphylococcal bi-component leukotoxins known as *pore-forming toxins* induce upon a specific binding to membrane receptors, two independent cellular events in human neutrophils. First, they provoke the opening of pre-existing specific ionic channels including Ca²⁺ channels. Then, they form membrane pores specific to monovalent cations leading to immune cells death. Among these leukotoxins, HlgC/HlgB and HlgA/HlgB γ-hemolysins do act in synergy to induce the opening of different types of Ca²⁺ channels in the absence as in the presence of extracellular Ca²⁺. Here, we investigate the mechanism underlying the modulation of Ca²⁺-independent Ca²⁺ channels in response to both active leukotoxins in human neutrophils. In the absence of extracellular Ca²⁺, the Mn²⁺ has been used as a Ca²⁺ surrogate to determine the activity of Ca²⁺-independent Ca²⁺ channels. Our findings provide new insights about different mechanisms involved in the staphylococcal γ-hemolysins activity to regulate three different types of Ca²⁺-independent Ca²⁺ channels. We conclude that (i) HlgC/HlgB stimulates the opening of La³⁺-sensitive Ca²⁺ channels, through a cholera toxin-sensitive G protein, (ii) HlgA/HlgB stimulates the opening of Ca²⁺ channels not sensitive to La³⁺, through a G protein-independent process, and (iii) unlike HlgA/HlgB, HlgC/HlgB toxins prevent the opening of a new type of Ca²⁺ channels by phosphorylation/de-phosphorylation-dependent mechanisms.

CD45-associated protein promotes the response of primary CD4 T cells to low-potency T-cell receptor (TCR) stimulation and facilitates CD45 association with CD3/TCR and lck

Although it is clear that the CD45 tyrosine phosphatase is required for efficient T-cell activation and T-cell development, the factors that regulate CD45 function remain uncertain. Previous data have indicated that there is an association of CD45 with CD4 and the T-cell receptor (TCR) complex controlled by the variable ectodomain of CD45 and, following activation, by high- and low-potency peptides. This suggests that controlling substrate access to CD45 may be an important regulatory mechanism during T-cell activation. In the present study we have examined the role of the transmembrane adapter-like molecule CD45-associated protein (CD45-AP) in regulating the association of CD45 with CD3/TCR and lck, and in regulating primary CD4(+) T-lymphocyte activation. In CD4(+) T cells from CD45-AP-deficient mice, coimmunoprecipitation of CD45 with the CD3/TCR complex, in addition to lck, is significantly reduced compared with wild-type T cells. Functionally, this correlates with a decreased proliferative response, a decrease in interleukin (IL)-2 production, and a decrease in calcium flux upon stimulation with a low-potency altered peptide ligand. However, the response of CD45-AP-deficient T cells to stimulation with a high-avidity agonist peptide was largely intact, except for a modest decrease in IL-2 production. These data suggest that CD45-AP promotes or stabilizes the association of CD45 with substrates and regulates the threshold of T-cell activation.

Priming of long-term potentiation mediated by ryanodine receptor activation in rat hippocampal slices

Administration of the Group 1 metabotropic glutamate receptor (mGluR) agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) facilitates ("primes") subsequent long-term potentiation (LTP) through a phospholipase C signaling cascade that may involve release of Ca2+ from the endoplasmic reticulum (ER). We investigated the intracellular calcium pathways involved in this priming effect, recording field potentials from area CA1 of rat hippocampal slices before and after high-frequency stimulation. The priming of LTP by DHPG was prevented by co-administration of cyclopiazonic acid, which depletes ER Ca2+ stores. The priming effect was also blocked by the ryanodine receptor (RYR) antagonist ryanodine (RYA, 100 microM). In contrast, a low dose of RYA (10 microM) which opens the RYR channel, by itself primed LTP. In addition to RYR activation, entry of extracellular calcium through store-operated channels appears necessary for priming, since diverse treatments known to impede store-operated channel activity completely blocked both RYA and DHPG priming effects. Thus, RYR activation plays a critical role in the priming of LTP by Group 1 mGluRs, and this effect is coupled to the entry of extracellular calcium, probably through store-operated calcium channels.

Mechanism of bradykinin-induced Ca2+ mobilization in murine proximal tubule epithelial cells

Despite the recognized physiological role of bradykinin (BK) in the kidney in maintaining glomerular and tubule function and its role in pathological states such as endotoxemia, diabetes, and other diseases, relatively little is known about the mechanisms by which BK can impact kidney function. Furthermore, the signaling of BK receptors in the murine nephron has not been fully characterized. The present studies were undertaken to examine BK-stimulated Ca(2+) signaling using Fura-2 in the murine proximal tubule epithelial cell line TKPTS. BK produced a concentration-dependent rise in intracellular Ca(2+) ([Ca(2+)])(i) (pEC(50) = 8.39 +/- 0.04). Selective antagonists showed the rise in [Ca(2+)](i) was mediated through B2 receptors. The rise in [Ca(2+)](i) was rapid and reversible and was maximally stimulated at 1 microM (697 +/- 70 nM above basal level of 115 +/- 6 nM). Studies with thapsigargin and EGTA showed Ca(2+) mobilization was dependent on two events: release and influx. Both U73122 (1-[6-[[17-beta-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione) [a phospholipase C (PLC) inhibitor] and genistein (a tyrosine kinase inhibitor) partially inhibited BK-stimulated rise in [Ca(2+)](i). When combined, both agents produced a further decrease, suggesting multiple pathways for PLC activation may be involved. The ability of Ni(2+) to inhibit influx indicated the activation of a Ca(2+) release-activated channel (CRAC). Ca(2+) mobilization did not seem to be affected by cyclic nucleotides or protein kinase C. In summary, the TKPTS murine proximal tubule cell line expresses functional B2 receptors linked to Ca(2+) mobilization that is dependent on phospholipase C and activation of CRAC.

CD45: a critical regulator of signaling thresholds in immune cells

Regulation of tyrosine phosphorylation is a critical control point for integration of environmental signals into cellular responses. This regulation is mediated by the reciprocal actions of protein tyrosine kinases and phosphatases. CD45, the first and prototypic receptor-like protein tyrosine phosphatase, is expressed on all nucleated hematopoietic cells and plays a central role in this process. Studies of CD45 mutant cell lines, CD45-deficient mice, and CD45-deficient humans initially demonstrated the essential role of CD45 in antigen receptor signal transduction and lymphocyte development. It is now known that CD45 also modulates signals emanating from integrin and cytokine receptors. Recent work has focused on regulation of CD45 expression and alternative splicing, isoform-specific differences in signal transduction, and regulation of phosphatase activity. From these studies, a model is emerging in which CD45 affects cellular responses by controlling the relative threshold of sensitivity to external stimuli. Perturbation of this function may contribute to autoimmunity, immunodeficiency, and malignancy. Moreover, recent advances suggest that modulation of CD45 function can have therapeutic benefit in many disease states.

Ca(2+) channels activated by endothelin-1 in CHO cells expressing endothelin-A or endothelin-B receptors

We compared the Ca(2+) channels activated by endothelin-1 (ET-1) in Chinese hamster ovary (CHO) cells stably expressing endothelin type A (ET(A)) or endothelin type B (ET(B)) receptors using the Ca(2+) channel blockers LOE-908 and SK&F-96365. In both CHO-ET(A) and CHO-ET(B), ET-1 at 0.1 nM activated the Ca(2+)-permeable nonselective cation channel-1 (NSCC-1), which was sensitive to LOE-908 and resistant to SK&F-96365. ET-1 at 1 nM activated NSCC-2 in addition to NSCC-1; NSCC-2 was sensitive to both LOE-908 and SK&F-96365. ET-1 at 10 nM activated the same channels as 1 nM ET-1 in both cell types, but in CHO-ET(A), it additionally activated the store-operated Ca(2+) channel (SOCC), which was resistant to LOE-908 and sensitive to SK&F-96365. Up to 1 nM ET-1, the level of the formation of inositol phosphates (IPs) was low and similar in both cell types, but, at 10 nM ET-1, it was far greater in CHO-ET(A) than in CHO-ET(B). These results show that, in CHO-ET(A) and CHO-ET(B), ET-1 up to 10 nM activated the same Ca(2+) entry channels: 0.1 nM ET-1 activated NSCC-1, and ET-1 > or = 1 nM activated NSCC-1 and NSCC-2. Notably, in CHO-ET(A), 10 nM ET-1 activated SOCCs because of the higher formation of IPs.

Involvement of calmodulin in 1alpha,25-dihydroxyvitamin D3 stimulation of store-operated Ca2+ influx in skeletal muscle cells

The steroid hormone 1alpha,25-dihydroxyvitamin D(3) (1, 25-(OH)(2)D(3)) rapidly modulates Ca(2+) homeostasis in avian skeletal muscle cells by driving a complex signal transduction mechanism, which promotes Ca(2+) release from inner stores and cation influx from the outside through both L-type and store-operated Ca(2+) (SOC) channels. In the present work, we evaluated the involvement of calmodulin (CAM) in 1,25-(OH)(2)D(3) regulation of SOC influx in chick skeletal muscle cells. Treatment with 10(-9) m 1,25-(OH)(2)D(3) in Ca(2+)-free medium resulted in a rapid but transient Ca(2+) rise correlated with the sterol-induced inositol 1,4,5-trisphosphate (IP(3)) production. The SOC influx stimulated by the hormone was insensitive to both CAM antagonists (fluphenazine, trifluoperazine, chlorpromazine, compound 48/80) and the CAM-dependent protein kinase II (CAMKII) inhibitor KN-62 when added after the sterol-dependent Ca(2+) transient, but it was completely abolished when added prior to the IP(3)-induced mobilization of Ca(2+) from endogenous stores. Moreover, in cells microinjected with antisense oligonucleotides directed against the CAM mRNA the sterol-stimulated SOC influx was reduced up to 60% respect to uninjected cells. The present results suggest that the 1, 25-(OH)(2)D(3)-induced (IP(3)-mediated) cytosolic Ca(2+) transient is required for CAM, activation which in turn activates SOC influx in a mechanism that seems to include CAMKII.

Nitric oxide acts independently of cGMP to modulate capacitative Ca(2+) entry in mouse parotid acini

Carbachol- and thapsigargin-induced changes in cGMP accumulation were highly dependent on extracellular Ca(2+) in mouse parotid acini. Inhibition of nitric oxide synthase (NOS) and soluble guanylate cyclase (sGC) resulted in complete inhibition of agonist-induced cGMP levels. NOS inhibitors reduced agonist-induced Ca(2+) release and capacitative Ca(2+) entry, whereas the inhibition of sGC had no effect. The effects of NOS inhibition were not reversed by 8-bromo-cGMP. The NO donor GEA-3162 increased cGMP levels blocked by the inhibition of sGC. GEA-3162-induced increases in Ca(2+) release from ryanodine-sensitive stores and enhanced capacitative Ca(2+) entry, both of which were unaffected by inhibitors of sGC but reduced by NOS inhibitors. Results support a role for NO, independent of cGMP, in agonist-mediated Ca(2+) release and Ca(2+) entry. Data suggest that agonist-induced Ca(2+) influx activates a Ca(2+)-dependent NOS, leading to the production of NO and the release of Ca(2+) from ryanodine-sensitive stores, providing a feedback loop by which store-depleted Ca(2+) channels are activated.

Agonist-dependent phosphorylation of the inositol 1,4,5-trisphosphate receptor: A possible mechanism for agonist-specific calcium oscillations in pancreatic acinar cells

The properties of inositol 1,4,5-trisphosphate (IP3)-dependent intracellular calcium oscillations in pancreatic acinar cells depend crucially on the agonist used to stimulate them. Acetylcholine or carbachol (CCh) cause high-frequency (10-12-s period) calcium oscillations that are superimposed on a raised baseline, while cholecystokinin (CCK) causes long-period (>100-s period) baseline spiking. We show that physiological concentrations of CCK induce rapid phosphorylation of the IP3 receptor, which is not true of physiological concentrations of CCh. Based on this and other experimental data, we construct a mathematical model of agonist-specific intracellular calcium oscillations in pancreatic acinar cells. Model simulations agree with previous experimental work on the rates of activation and inactivation of the IP3 receptor by calcium (DuFour, J.-F., I.M. Arias, and T.J. Turner. 1997. J. Biol. Chem. 272:2675-2681), and reproduce both short-period, raised baseline oscillations, and long-period baseline spiking. The steady state open probability curve of the model IP3 receptor is an increasing function of calcium concentration, as found for type-III IP3 receptors by Hagar et al. (Hagar, R.E., A.D. Burgstahler, M.H. Nathanson, and B.E. Ehrlich. 1998. Nature. 396:81-84). We use the model to predict the effect of the removal of external calcium, and this prediction is confirmed experimentally. We also predict that, for type-III IP3 receptors, the steady state open probability curve will shift to lower calcium concentrations as the background IP3 concentration increases. We conclude that the differences between CCh- and CCK-induced calcium oscillations in pancreatic acinar cells can be explained by two principal mechanisms: (a) CCK causes more phosphorylation of the IP3 receptor than does CCh, and the phosphorylated receptor cannot pass calcium current; and (b) the rate of calcium ATPase pumping and the rate of calcium influx from the outside the cell are greater in the presence of CCh than in the presence of CCK.

Rapid actions of calcitriol and its side chain analogues CB1093 and GS1500 on intracellular calcium levels in skeletal muscle cells: a comparative study

1. The ability of synthetic analogues of the secosteroid hormone 1alpha,25-dihydroxy-vitamin-D3 [calcitriol, CT; 1,25(OH)2D3] to exert non-genomic (rapid) effects on target cells has been scarcely studied. To evaluate the pharmacological potential of the CT side-chain analogues CB1093 and GS1500, we compared their fast effects on intracellular calcium concentration ([Ca2+]i) in chick skeletal muscle cells with those elicited by the natural hormone. 2. Both analogues, similarly to CT, specifically induced rapid (30-60 s) and sustained rises in [Ca2+]i levels. CB1093 and GS1500 were more potent than the natural hormone at concentrations as low as 10(-13) M (4.5 fold stimulation) and 10(-12) M (2.5 fold), respectively, whereas higher concentrations (10(-9)- 10(-8) M) of CT were more effective than the analogues in elevating [Ca2+]i. Cyclic AMP was markedly increased by both analogues pointing for a role of this messenger in the fast actions of the synthetic compounds. 3. In Ca2+ free medium CT and analogues elicited a transient elevation in [Ca2+]i. The PLC inhibitors U73122 (2 microM) and neomycin (0.5 mM), as well as depletion of intracellular stores with thapsigargin (1 microM), completely prevented CB1093/GS1500-dependent changes in [Ca2+]i suggesting that, similarly to CT, these analogues mobilized Ca2+ from an IP3/thapsigargin-sensitive store. 4. The voltage-dependent calcium channel (VDCC) blocker nifedipine (2 microM) reduced by 50-60% the influx phase of the [Ca2+]i response to CB1093 and GS1500, indicating that VDCC contributed partially to Ca2+ entry. The Ca2+ readdition protocol suggested that analogue-dependent activation of a SOC entry pathway accounted, to the same extent as for CT, for the remaining non-VDCC mediated Ca2+ influx.

Endothelin-B receptors activate Galpha13

Endothelin (ET) receptors activate heterotrimeric G proteins that are members of the Gi, Gq, and Gs families but may also activate members of other families such as Galpha12/13. Galpha13 has multiple complex cellular effects that are similar to those of ET. We studied the ability of ET receptors to activate Galpha13 using an assay for G protein alpha-chain activation that is based on the fact that an activated (GTP-bound) alpha-chain is resistant to trypsinization compared with an inactive (GDP-bound) alpha-chain. Nonhydrolyzable guanine nucleotides and AlMgF protected Galpha13 from degradation by trypsin. In membranes from human embryonic kidney 293 cells that coexpress ETB receptors and alpha13, ET-3 and 5'-guanylylimidodiphosphate [Gpp(NH)p] increased the protection of alpha13 compared with Gpp(NH)p alone. The specificity of ETB receptor-alpha13 coupling was documented by showing that beta2 receptors and isoproterenol or ETA receptors and ET-1 did not activate alpha13 and that a specific antagonist for ETB receptors blocked ET-3-dependent activation of alpha13.

Modulation of pancreatic acinar cell to cell coupling during ACh-evoked changes in cytosolic Ca2+

The temporal changes in cytosolic free Ca2+ ([Ca2+]i), Ca2+-dependent membrane currents (Im), and gap junctional current (Ij) elicited by acetylcholine (ACh) were measured in rat pancreatic acinar cells using digital imaging and dual perforated patch-clamp recording. ACh (50 nM-5 microM) increased [Ca2+]i and evoked Im currents without altering Ij in 19 of 37 acinar cell pairs. Although [Ca2+]i rose asynchronously in cells comprising a cluster, the delay of the [Ca2+]i responses decreased with increasing ACh concentrations. Perfusion of inositol 1,4,5-trisphosphate (IP3) into one cell of a cluster resulted in [Ca2+]i responses in neighboring cells that were not necessarily in direct contact with the stimulated one. This suggests that extensive coupling between acinar cells provides a pathway for cell-to-cell diffusion of Ca2+-releasing signals. Strikingly, maximal (1-5 microM) ACh concentrations reduced Ij by 69 +/- 15% (n = 9) in 25% of the cell pairs subjected to dual patch-clamping. This decrease occurred shortly after the Im peak and was prevented by incubating acinar cells in a Ca2+-free medium, suggesting that uncoupling was subsequent to the initiation of the Ca2+-mobilizing responses. Depletion of Ca2+-sequestering stores by thapsigargin resulted in a reduction of intercellular communication similar to that observed with ACh. In addition, ACh-induced uncoupling was prevented by blocking nitric oxide production with L-nitro-arginine and restored by exposing acinar cells to dibutyryl cGMP. The results suggest that ACh-induced uncoupling and capacitative Ca2+ entry are regulated concurrently. Closure of gap junction channels may occur to functionally isolate nearby cells differing in their intrinsic sensitivity to ACh and thereby to allow for sustained activity of groups of secreting cells.

1alpha,25-dihydroxy-vitamin-D3-induced store-operated Ca2+ influx in skeletal muscle cells. Modulation by phospholipase c, protein kinase c, and tyrosine kinases

In skeletal muscle cells the steroid hormone 1alpha, 25-dihydroxy-vitamin-D3 (1,25(OH)2D3) nongenomically promotes Ca2+ release from intracellular stores and cation influx through both L-type and store-operated Ca2+ (SOC) channels. In the present work we evaluated the regulation and kinetics of the 1, 25(OH)2D3-stimulated SOC influx in chick muscle cells. Stimulation with 10(-9) M 1,25(OH)2D3 in Ca2+-free medium resulted in a rapid (40-60 s) but transient [Ca2+]i rise, which correlated with sterol-dependent inositol 1,4,5-trisphosphate production. The SOC influx stimulated by the hormone was insensitive to both L-type channel antagonists and polyphosphoinositide-specific phospholipase C (PPI-PLC) inhibitors but was fully inhibitable by La3+ and Ni2+. PPI-PLC blockade prior to 1,25(OH)2D3 stimulation suppressed both the [Ca2+]i transient and the SOC influx. 1,25(OH)2D3-induced SOC entry was markedly increased after 3 min of treatment (30% above basal) and then rapidly reached a steady-state level. The sterol-stimulated SOC influx was prevented by protein kinase C and tyrosine kinase inhibitors but unaffected by blockade of the protein kinase A pathway. None of these inhibitors altered the thapsigargin-induced SOC entry, suggesting the operation of a signaling mechanism different from that for sterol-dependent SOC influx. The present results indicate that 1,25(OH)2D3-induced activation of PPI-PLC is upstream to Ca2+ influx through SOC channels and point for a role of both protein kinase C and tyrosine kinases but not protein kinase A in the regulation of the sterol-dependent SOCE pathway.

Promiscuous coupling of receptors to Gq class alpha subunits and effector proteins in pancreatic and submandibular gland cells

Mice with deficiencies in one or more Gq class alpha subunit genes were used to examine the role of the alpha subunit in regulating Ca2+ signaling in pancreatic and submandibular gland cells. Western blot analysis showed that these cells express three of the four Gq class subunits, Galphaq, Galpha11, and Galpha14 but not Galpha15. Surprisingly, all parameters of Ca2+ signaling were identical in cells from wild type and four lines of mutant mice: 1) Galpha11-/-, 2) Galpha11-/-/Galpha14-/-, 3) Galpha14-/-/Galpha15-/-, and 4) Galphaq-/-/Galpha15-/-. These parameters included the Kapp for several Gq class coupled receptors, induction of [Ca2+]i oscillations by weak stimulation, and a biphasic [Ca2+]i response by strong stimulation. Furthermore, Ca2+ release from internal stores and Ca2+ entry were not affected in cells from any of the mutant mice. We conclude that Galphaq, Galpha11, and Galpha14 promiscuously couple several receptors (m3 muscarinic, bombesin, cholecystokinin, and alpha1 adrenergic) to effector proteins that activate both Ca2+ release from internal stores and Ca2+ entry.

Use of replication deficient adenoviruses to investigate the role of G proteins in Ca2+ signalling in epithelial cells

Here we report on the feasibility of using replication deficient adenoviruses to modify signal transduction systems in epithelia. We constructed two viruses, one expressing a dominant negative mutant of the alpha-subunit of Gq (Ad-EF1-dnG alpha q) and the other expressing the wild-type alpha-subunit of Gq (Ad-EF1-wtG alpha q). We used an adenovirus expressing green fluorescent protein (Ad-EF1-GFP20) to show that infection of cultured cells with an adenovirus results in at least 95% expression of the transgene in both HSG and HT29 cells. We also used an adenovirus that expresses no transgene (Ad-MX17) to demonstrate that adenoviral infection itself does not affect the resting concentration of cytosolic Ca2+ ([Ca2+]i) or the carbachol responses in these cells. We further show that Ad-EF1-dnG alpha q inhibits the increase in [Ca2+]i produced by muscarinic receptor activation in both the cell lines we studied. This inhibitory effect is not shared by Ad-EF1-wtG alpha q, which indicates that in both HSG and HT29 cells, the increase in [Ca2+]i produced by muscarinic receptor activation is largely mediated by activation of Gq. Neither virus affected the resting level of [Ca2+]i in these cells. Our findings confirm the feasibility of using replication deficient adenoviruses expressing dominant negative mutants to investigate the role of G proteins in signal transduction systems.

Differential routes of Ca2+ influx in Swiss 3T3 fibroblasts in response to receptor stimulation

Ca2+ influx into cells in response to stimulation of various receptors was studied with Swiss 3T3 fibroblasts. The mechanisms involved were found to be so diverse that they were classified into four groups, Type I to IV. Type-I influx occurred, via pertussis toxin-susceptible G-proteins, immediately after internal Ca2+ mobilization by bradykinin, thrombin, endothelin, vasopressin or angiotensin II. Type-II influx induced by bombesin differed from Type I in its insusceptibility to pertussis toxin treatment. Ca2+ influx induced by prostaglandin E1, referred to as Type-III influx, was unique in that phospholipase C was apparently not activated without extracellular Ca2+, strongly suggesting that the Ca2+ influx preceded and was responsible for InsP3 generation and internal Ca2+ mobilization. More Ca2+ entered the cells more slowly via the Type-IV route opened by platelet-derived and other growth factors. These types of Ca2+ influx could be differentiated by their different susceptibilities to protein kinase C maximally activated by 1 h of exposure of cells to PMA, which inhibited phospholipase Cbeta coupled to receptors involved in Type-I and -II influx but did not inhibit growth-factor-receptor-coupled phospholipase Cgamma. Type-I and -II Ca2+ influxes, together with store-operated influx induced by thapsigargin, were not directly inhibited by exposure of cells to PMA, but Type-III and -IV influxes were completely inhibited. In addition, stimulation of receptors involved in Type-I and -IV Ca2+ influx, but not Type-II and -III influx, led to phospholipase A2 activation in the presence of extracellular Ca2+. Inhibition of Type-I and -IV Ca2+ influxes by their respective inhibitors, diltiazem and nifedipine, resulted in abolition of phospholipase A2 activation induced by the respective receptor agonists, in agreement with the notion that Ca2+ influx via these routes is responsible for receptor-mediated phospholipase A2 activation.

Evidence that a low-molecular-mass GTP-binding protein is required for store-activated Ca2+ inflow in hepatocytes

The roles of a monomeric GTP-binding regulatory protein in the activation of store-activated plasma membrane Ca2+ channels and in the release of Ca2+ from the smooth endoplasmic reticulum (SER) in rat liver parenchymal cells were investigated with the use of freshly isolated rat hepatocytes and rat liver microsomes. A low concentration (approx. 130 microM intracellular) of guanosine 5'-[gamma-thio]triphosphate (GTP[S]) activated Ca2+ inflow in intact hepatocytes in the absence of an agonist, whereas a high concentration (approx. 530 microM intracellular) of GTP-S- or guanosine 5'-[betagamma-imido]triphosphate (p[NH]ppG) inhibited the Ca2+ inflow induced by inhibitors of the activity of the endoplasmic-reticulum Ca2+-ATPase (SERCA) and by vasopressin. GTP (530 microM) prevented the inhibition of Ca2+ inflow by GTP-S- and p[NH]ppG. Brefeldin A and the peptide human Arf-1-(2-17), which inhibit many functions of ADP ribosylation factor (Arf) proteins, inhibited the Ca2+ inflow induced by SERCA inhibitors and vasopressin, and altered the profile of Ca2+ release from the SER. These effects were observed at concentrations of Brefeldin A and Arf-1-(2-17) comparable with those that inhibit the functions of Arf proteins in other systems. Succinylated Arf-1-(2-17) had a negligible effect on Ca2+ inflow. GTP[S] and Arf-1-(2-17) completely inhibited the synergistic action of GTP and Ins(1,4,5)P3 in releasing 45Ca2+ from rat liver microsomes loaded with 45Ca2+. AlF4(-) (under conditions expected to activate trimeric G-proteins) and succinylated Arf-1-(2-17) had no effect on GTP/Ins(1,4,5))3-induced 45Ca2+ release, and a mastoparan analogue caused partial inhibition. Arf-1-(2-17) did not inhibit 45Ca2+ release induced by either thapsigargin or ionomycin. It is concluded that a low-molecular-mass G-protein, most probably a member of the Arf protein family, is required for store-activated Ca2+ inflow in rat hepatocytes. The idea that the role of this G-protein is to maintain a region of the SER in the correct intracellular location is discussed briefly.

Stimulation of Ca2+ release-activated Ca2+ channels as a potential mechanism involved in non-genomic 1,25(OH)2-vitamin D3-induced Ca2+ entry in skeletal muscle cells

As in other vitamin D target cells, activation of voltage-dependent Ca2+ channels (VDCC) mediates the fast, non-genomic, 1,25(OH)2D3 stimulation of Ca2+ influx in skeletal muscle cells (SMC). 1,25(OH)2D3 has also been shown to rapidly induce the release of Ins(1,4,5)P3 in SMC. Experiments were performed to investigate whether Ca2+ release-activated Ca2+ channels (CRAC) also participate in the mechanism by which 1,25(OH)2D3 regulates Ca2+ entry into these cells. In cultured chick SMC loaded with Fura-2/AM the hormone (10(-12) - 10(-8) M) induced a rapid (30 sec) followed by a sustained (up to 5 min) increase in intracellular Ca2+ concentration ([Ca2+]i) associated to Ca2+ mobilization from internal stores and influx of extracellular Ca2+, respectively. Thus, the initial, transient, 1,25(OH)2D3-dependent increment in [Ca2+]i could be observed in Ca2+-free medium and was abolished by the PLC inhibitor U73122. Readdition of Ca2+ to cells that had undergone the initial 1,25(OH)2D3-induced [Ca2+]i rise in Ca2+ free medium resulted in a fast increment in [Ca2+]i indicating the existence of a hormone-activated CRAC entry pathway. The sustained phase of the Ca2+ response to 1,25(OH)2D3 was only partially (60%) suppressed by nifedipine, whereas lanthanum (10 microM) completely abolished the hormone effects. Accordingly, depletion of intracellular Ca2+ stores by thapsigargin reproduced 1,25(OH)2D3-induced Ca2+ influx, inhibiting any further response to the sterol. 1,25(OH)2D3 increased the rate of quenching of Fura-2 fluorescence by Mn2+, indicating activation of Mn2+ permeable channels. Altogether, these results provide the first evidence involving CRAC channels in the rapid modulation of Ca2+ entry in animal cells by 1,25(OH)2D3.

nNOS and Ca2+ influx in rat pancreatic acinar and submandibular salivary gland cells

Regulation of agonist-activated Ca2+ influx by the NOS pathway through generation of cGMP is being found in an increasing number of cell types. In the present work, we examined the role of the NOS pathway in agonist-evoked [Ca2+]i oscillations and attempted to identify the NOS isoform most likely to regulate Ca2+ influx. For this, we first show that two Ca(2+)-mobilizing agonists acting on pancreatic acinar cells, bombesin (BS) and the cholecystokinin analog CCK-JMV-180 (CCKJ), evokes different type of [Ca2+]i oscillations. The BS-evoked [Ca2+]i oscillations rapidly became acutely dependent on the presence of extracellular Ca2+, whereas the CCKJ-evoked oscillations continue for long periods of time in the absence of Ca2+ influx. This differential behavior allowed us to isolate Ca2+ influx and study its regulation while controlling for non specific effects on all other Ca2+ transporting events involved in generating [Ca2+]i oscillations. Inhibitors of selective steps in the NOS pathway inhibited agonist-induced cGMP production. The inhibitors were then used to show that scavenging NO with reduced hemoglobin, inhibition of guanylyl cyclase with 1H-[1,2,4] oxadiazolo[4,3-a] quinoxaline-1-one (ODQ) and inhibition of protein kinase G with Rp-8-pCPT-cGMPS inhibited [Ca2+]i oscillations evoked by BS but not those evoked by CCKJ. These findings were extended to duct and acinar cells of the SMG. In these cells, Ca(2+)-mobilizing agonists stimulate large Ca2+ influx, which was inhibited by all inhibitors of the NOS pathway. Western blot analysis and immunolocalization revealed that the cells did not express iNOS, eNOS was expressed only in blood vessels and capillaries whereas nNOS was expressed at high levels next to the plasma membrane of all cells. Accordingly, the nNOS inhibitor 7-nitroindazole (7-NI) inhibited BS- but not CCKJ-evoked [Ca2+]i oscillations and Ca2+ influx into SMG acinar and duct cells. Thus, together, our findings favor nNOS as the isoform activated by the Ca2+ released from internal stores to generate cGMP and regulate Ca2+ influx.

An evaluation of strategies available for the identification of GTP-binding proteins required in intracellular signalling pathways

Strategies which can be used to elucidate the nature of a GTP-binding regulatory protein (G-protein) involved in an intracellular pathway of interest in the complex environment of the cell are described and evaluated. A desirable strategy is considered to be one in which the first stage indicates a requirement for one or more G-proteins, provides information on whether a monomeric, trimeric or other type of G-protein is involved, and gives some idea of the G-protein sub-class. In the second stage the specific G-protein involved is identified. Approaches available for investigations in the first stage include the use of analogues of GTP and GDP, AlF4-, inhibitors of G-protein isoprenylation, bacterial toxins which covalently modify G-proteins, and the introduction of a purified GDP dissociation inhibitor, GDP exchange and/or GTP-ase activating protein. Identification of the specific G-protein in the second stage can be achieved using anti G-protein antibodies, G-protein-or receptor-derived peptides, antisense G-protein RNA and over-expressed, constitutively-active or dominant-negative G-protein mutants. The correct interpretation of results obtained with GTP and GDP analogues and AlF4- in the first stage is complex and often difficult, and requires a thorough understanding of the functions and mechanisms of activation of G-proteins. Nevertheless, it is important to reach the correct conclusion at this stage since considerable time and expense are usually required for investigations in the second stage.

G protein heterotrimer Galpha13beta1gamma3 couples the angiotensin AT1A receptor to increases in cytoplasmic Ca2+ in rat portal vein myocytes

The subunit composition of angiotensin AT1 receptor-activated G protein was identified by using antisense oligonucleotide injection into the nucleus of rat portal vein myocytes. In these cells, we have previously shown that increases in the cytoplasmic calcium concentration ([Ca2+]i) induced by activation of angiotensin AT1 receptors were dependent on extracellular Ca2+ entry by L-type Ca2+ channels and subsequent Ca2+-induced Ca2+ release from the intracellular stores. The angiotensin AT1 receptor-activated increases in [Ca2+]i were selectively inhibited by injection of antisense oligonucleotides directed against the mRNAs coding for the alpha13, beta1, and gamma3 subunits. A correlating reduction in Galpha13, Gbeta1, and Ggamma3 protein expression was confirmed by immunocytochemistry. In addition, anti-alpha13 antibody and synthetic peptide corresponding to the carboxyl terminus of the Galpha13 subunit inhibited, in a concentration-dependent manner, the angiotensin AT1 receptor-mediated Ca2+ response. Reverse transcription-polymerase chain reaction analysis showed that only the angiotensin AT1A receptor was expressed in rat portal vein smooth muscle. Furthermore, injection of anti-AT1A oligonucleotides selectively inhibited the angiotensin II-induced increase in [Ca2+]i. We conclude that the receptor-activated signal leading to increases in [Ca2+]i is transduced by the heterotrimeric G13 protein composed of alpha13/beta1/gamma3 subunits and that the carboxyl terminus of the Galpha13 subunit interacts with the angiotensin AT1A receptor.

Distinct functions of Gq and G11 proteins in coupling alpha1-adrenoreceptors to Ca2+ release and Ca2+ entry in rat portal vein myocytes

In this study, we identified the subunit composition of Gq and G11 proteins coupling alpha1-adrenoreceptors to increase in cytoplasmic Ca2+ concentration ([Ca2+]i) in rat portal vein myocytes maintained in short-term primary culture. We used intranuclear antisense oligonucleotide injection to inhibit selectively the expression of subunits of G protein. Increases in [Ca2+]i were measured in response to activation of alpha1-adrenoreceptors, angiotensin AT1 receptors, and caffeine. Antisense oligonucleotides directed against the mRNAs coding for alphaq, alpha11, beta1, beta3, gamma2, and gamma3 subunits selectively inhibited the increase in [Ca2+]i activated by alpha1-adrenoreceptors. A corresponding reduction of the expression of these G protein subunits was immunochemically confirmed. In experiments performed in Ca2+-free solution only cells injected with anti-alphaq antisense oligonucleotides displayed a reduction of the alpha1-adrenoreceptor-induced Ca2+ release. In contrast, in Ca2+-containing solution, injection of anti-alpha11 antisense oligonucleotides suppressed the alpha1-adrenoreceptor-induced stimulation of the store-operated Ca2+ influx. Agents that specifically bound Gbetagamma subunits (anti-betacom antibody and overexpression of a beta-adrenergic receptor kinase carboxyl-terminal fragment) had no effect on the alpha1-adrenoreceptor-induced signal transduction. Taken together, these results suggest that alpha1-adrenoreceptors utilize two different Galpha subunits to increase [Ca2+]i. Galphaq may activate phosphatidylinositol 4,5-bisphosphate hydrolysis and induce release of Ca2+ from intracellular stores. Galpha11 may enhance the Ca2+-activated Ca2+ influx that replenishes intracellular Ca2+ stores.

Induction of inducible nitric-oxide synthase by the heterotrimeric G protein Galpha13

While the functions of several G protein alpha subunits such as alpha(s( and alpha(q) are relatively well understood, the action of others such as alpha13 remain largely undefined. Because of recent interest in regulation of nitric-oxide synthase (NOS) by G protein-coupled signaling systems and findings that receptors for two proinflammatory substances, thrombin and thromboxane couple to alpha13, we studied the effect of alpha13 on NOS activity in a renal epithelial cell line. We found that stable overexpression of alpha13 or its GTPase-deficient mutant, alpha13Q226L, in a continuous renal epithelial cell line (MCT) increased NOS activity. The increased NOS activity was due to increased expression of the macrophage-inducible form of NOS (iNOS). iNOS protein and activity were not increased in similar cells expressing an activated alpha(s) (alpha(s)Q227L) or were minimally increased in cells expressing activated alpha(i1) (alpha-i1Q204L) and alpha(q) (alpha(q)Q209L), members of the three other G protein alpha chain families. Transient co-expression of alpha13 or alpha13Q226L increased the activity of an iNOS promoter-CAT construct demonstrating that alpha13 increases iNOS expression through transcription. Consequently, alpha13 induces iNOS through a novel mechanism that is distinct from that of other G protein alpha chains and that may mediate the actions of G protein-dependent proinflammatory agents.