Muscarinic receptor-operated Ca2+ influx in transfected fibroblast cells is independent of inositol phosphates and release of intracellular Ca2+

Differential effects of the Gβ5-RGS7 complex on muscarinic M3 receptor-induced Ca2+ influx and release

The G protein β subunit Gβ5 uniquely forms heterodimers with R7 family regulators of G protein signaling (RGS) proteins (RGS6, RGS7, RGS9, and RGS11) instead of Gγ. Although the Gβ5-RGS7 complex attenuates Ca(2+) signaling mediated by the muscarinic M3 receptor (M3R), the route of Ca(2+) entry (i.e., release from intracellular stores and/or influx across the plasma membrane) is unknown. Here, we show that, in addition to suppressing carbachol-stimulated Ca(2+) release, Gβ5-RGS7 enhanced Ca(2+) influx. This novel effect of Gβ5-RGS7 was blocked by nifedipine and 2-aminoethoxydiphenyl borate. Experiments with pertussis toxin, an RGS domain-deficient mutant of RGS7, and UBO-QIC {L-threonine,(3R)-N-acetyl-3-hydroxy-L-leucyl-(aR)-a-hydroxybenzenepropanoyl-2,3-idehydro-N-methylalanyl-L-alanyl-N-methyl-L-alanyl-(3R)-3-[[(2S,3R)-3-hydroxy-4- methyl-1-oxo-2-[(1-oxopropyl)amino]pentyl]oxy]-L-leucyl-N,O-dimethyl-,(7→1)-lactone (9CI)}, a novel inhibitor of Gq, showed that Gβ5-RGS7 modulated a Gq-mediated pathway. These studies indicate that Gβ5-RGS7, independent of RGS7 GTPase-accelerating protein activity, couples M3R to a nifedipine-sensitive Ca(2+) channel. We also compared the action of Gβ5-RGS7 on M3R-induced Ca(2+) influx and release elicited by different muscarinic agonists. Responses to Oxo-M [oxotremorine methiodide N,N,N,-trimethyl-4-(2-oxo-1-pyrrolidinyl)-2-butyn-1-ammonium iodide] were insensitive to Gβ5-RGS7. Pilocarpine responses consisted of a large release and modest influx components, of which the former was strongly inhibited whereas the latter was insensitive to Gβ5-RGS7. McN-A-343 [(4-hydroxy-2-butynyl)-1-trimethylammonium-3-chlorocarbanilate chloride] was the only compound whose total Ca(2+) response was enhanced by Gβ5-RGS7, attributed to, in part, by the relatively small Ca(2+) release this partial agonist stimulated. Together, these results show that distinct agonists not only have differential M3R functional selectivity, but also confer specific sensitivity to the Gβ5-RGS7 complex.

Neuroendocrine regulatory peptide-2 stimulates glucose-induced insulin secretion in vivo and in vitro

Neuroendocrine regulatory peptide (NERP)-2, recently identified as a bioactive peptide involved in vasopressin secretion and feeding regulation in the central nervous system, is abundantly expressed in endocrine cells in peripheral tissues. To explore the physiological roles of NERP-2 in the pancreas, we examined its effects on insulin secretion. NERP-2 increased glucose-stimulated insulin secretion (GSIS) in a dose-dependent manner, with a lowest effective dose of 10(-7) M, from the pancreatic β-cell line MIN6 and isolated mouse pancreatic islets. NERP-2 did not affect insulin secretion under the low-glucose conditions. Neither NERP-1 nor NERP-2-Gly (nonamidated NERP-2) stimulated insulin secretion. NERP-2 significantly augmented GSIS after intravenous administration to anesthetized rats or intraperitoneal injection to conscious mice. We detected NERP-2 in pancreatic islets, where it co-localized extensively with insulin. Calcium-imaging analysis demonstrated that NERP-2 increased the calcium influx in MIN6 cells. These findings reveal that NERP-2 regulates GSIS by elevating intracellular calcium concentrations.

Ratiometric Ca+2 measurement in human recombinant muscarinic receptor subtypes using the Flexstation scanning fluorometer

In modern drug discovery, numerous assay formats are available to screen and quantitate receptor-ligand interactions. Radioactive assays are "gold standard" because they are fast, easy, and reproducible; however, they are hazardous, produce radioactive waste, require special lab conditions, and are expensive on a large scale. Thus, it provides a lot of importance to the "mix & measure" assays that have an optical readout. Fluorescence techniques are likely to be among the most important detection approaches used for high throughput screening due to their high sensitivity and amenability to automation. The aim of the present study was to determine the functional antagonistic affinities of standard muscarinic antagonists in CHO cells over expressing m1, m3, and m5 receptors and to compare them with the respective binding affinities. This study was further extended to elucidate that Ca+2 measurement assays can serve as a functional screening tool for GPCRs. For this purpose, standard muscarinic receptor antagonists, namely, tolterodine, oxybutynin, and atropine were used. We determined and compared the IC50 values of these three standard inhibitors in fura 2 AM loaded m1, m3, and m5 overexpressing CHO cells and in radioligand binding assay. Both the assays exhibited comparable rank order potencies of the standard inhibitors. This study suggests that Ca+2 mobilization assays can be an alternate to radioligand binding assays.

Muscarinic acetylcholine receptors stimulate Ca2+ influx in PC12D cells predominantly via activation of Ca2+ store-operated channels

Activation of muscarinic acetylcholine receptors (mAChRs) causes the rapid release of Ca2+ from intracellular stores and a sustained influx of external Ca2+ in PC12D cells, a subline of the widely studied cell line PC12. Release of Ca2+ from intracellular stores and a sustained influx of Ca2+ are also observed following exposure to thapsigargin, a sesquiterpene lactone that depletes intracellular Ca2+ pools by irreversibly inhibiting the Ca2+ pump of the endoplasmic reticulum. In this study, we show that carbachol and thapsigargin empty the same intracellular Ca2+ stores, and that these stores are a subset of intracellular stores depleted by the Ca2+ ionophore ionomycin. Intracellular Ca2+ stores remain depleted during continuous stimulation of mAChR with carbachol in medium containing 2 mM extracellular Ca2+, but rapidly refill following inhibition of mAChRs with atropine. Addition of atropine to carbachol-stimulated cells causes intracellular Ca2+ levels to return to baseline levels in two steps: a rapid decrease that correlates with the reuptake of Ca2+ into internal stores and a delayed decrease that correlates with the inhibition of a Mn2+-permeable Ca2+ channel. Several lines of evidence suggest that carbachol and thapsigargin stimulate Ca2+ influx by a common mechanism: (i) pretreatment with thapsigargin occludes atropine-mediated inhibition of Ca2+ influx, (ii) carbachol and thapsigargin applied individually or together are equally efficient at stimulating the influx of Mn2+, and (iii) identical rates of Ca2+ influx are observed when Ca2+ is added to cells pretreated with carbachol, thapsigargin, or both agents in the absence of extracellular Ca2+. Taken together, these data suggest that the sustained influx of extracellular Ca2+ observed following activation of mAChRs in PC12D cells is mediated primarily by activation of a Mn2+-permeable, Ca2+ store-operated Ca2+ channel.

M3-like muscarinic receptors mediate Ca2+ influx in rat mesencephalic GABAergic neurones through a protein kinase C-dependent mechanism

GABAergic neurones in the mesencephalon are important regulators of dopamine neurones. Cholinergic projections from mesopontine nuclei preferentially synapse onto these GABAergic neurones, thus suggesting that ACh can regulate dopamine neurones indirectly by modulating GABAergic interneurones. Muscarinic receptors mediate excitation of these interneurones through a Ca(2+)-dependent mechanism. Using a mesencephalic primary culture model, we show here that muscarine (10 microM) increases intracellular Ca2+ concentrations ([Ca2+]i) in GABAergic interneurones. Compatible with previous anatomical data, our pharmacological studies further suggest that the M3 receptor is the primary mediator of this increase. The rise in [Ca2+]i induced by muscarine was not activity-dependent but required influx of Ca2+ from the extracellular medium. Consistent with the known coupling of the M3 receptor to PKC, the effect of muscarine was blocked by bisindolylmaleimide, a selective PKC antagonist. The effect of muscarine was inhibited by SKF 96365 and verapamil, drugs known to block non-selective cationic channels such as those formed by transient receptor potential (TRPC) proteins. Finally, GABAergic neurones were found to be immunopositive for TRPC1, 3, 5 and 6. Taken together, these results suggest that the Ca(2+)-dependent regulation of mesencephalic GABAergic neurones by muscarinic receptors requires activation of some receptor-operated Ca2+ channels through a PKC-dependent mechanism.

Probing receptor structure/function with chimeric G-protein-coupled receptors

Owing its name to an image borrowed from Greek mythology, a chimera is seen to represent a new entity created as a composite from existing creatures or, in this case, molecules. Making use of various combinations of three basic domains of the receptors (i.e., exofacial, transmembrane, and cytoplasmic segments) that couple agonist binding into activation of effectors through heterotrimeric G-proteins, molecular pharmacology has probed the basic organization, structure/function relationships of this superfamily of heptahelical receptors. Chimeric G-protein-coupled receptors obviate the need for a particular agonist ligand when the ligand is resistant to purification or, in the case of orphan receptors, is not known. Chimeric receptors created from distant members of the heptahelical receptors enable new strategies in understanding how these receptors transduce agonist binding into receptor activation and may be able to offer insights into the evolution of G-protein-coupled receptors from yeast to humans.

MDM2 promotes p21waf1/cip1 proteasomal turnover independently of ubiquitylation

The CDK inhibitor p21waf1/cip1 is degraded by a ubiquitin-independent proteolytic pathway. Here, we show that MDM2 mediates this degradation process. Overexpression of wild-type or ring finger-deleted, but not nuclear localization signal (NLS)-deleted, MDM2 decreased p21waf1/cip1 levels without ubiquitylating this protein and affecting its mRNA level in p53(-/-) cells. This decrease was reversed by the proteasome inhibitors MG132 and lactacystin, by p19(arf), and by small interfering RNA (siRNA) against MDM2. p21waf1/cip1 bound to MDM2 in vitro and in cells. The p21waf1/cip1-binding-defective mutant of MDM2 was unable to degrade p21waf1/cip1. MDM2 shortened the half-life of both exogenous and endogenous p21waf1/cip1 by 50% and led to the degradation of its lysine-free mutant. Consequently, MDM2 suppressed p21waf1/cip1-induced cell growth arrest of human p53(-/-) and p53(-/-)/Rb(-/-)cells. These results demonstrate that MDM2 directly inhibits p21waf1/cip1 function by reducing p21waf1/cip1 stability in a ubiquitin-independent fashion.

Localised calcium release events in cells from the muscle of guinea-pig gastric fundus

After enzymatic dispersion of the muscle of the guinea-pig gastric fundus, single elongated cells were observed which differed from archetypal smooth muscle cells due to their knurled, tuberose or otherwise irregular surface morphology. These, but not archetypal smooth muscle cells, consistently displayed spontaneous localized (i.e. non-propagating) intracellular calcium ([Ca(2+)](i)) release events. Such calcium events were novel in their magnitude and kinetic profiles. They included short transient events, plateau events and events which coalesced spatially or temporally (compound events). Quantitative analysis of the events with an automatic detection programme showed that their spatio-temporal characteristics (full width and full duration at half-maximum amplitude) were approximately exponentially distributed. Their amplitude distribution suggested the presence of two release modes. Carbachol application caused an initial cell-wide calcium transient followed by an increase in localized calcium release events. Pharmacological analysis suggested that localized calcium release was largely dependent on external calcium entry acting on both inositol trisphosphate receptors (IP(3)Rs) and ryanodine receptors (RyRs) to release stored calcium. Nominally calcium-free external solution immediately and reversibly abolished all localized calcium release without blocking the initial transient calcium release response to carbachol. This was inhibited by 2-APB (100 microm), ryanodine (10 or 50 microm) or U-73122 (1 microm). 2-APB (100 microm), xestospongin C (XeC, 10 microm) or U-73122 (1 microm) blocked both spontaneous localized calcium release and localized release stimulated by 10 microm carbachol. Ryanodine (50 microm) also inhibited spontaneous release, but enhanced localized release in response to carbachol. This study represents the first characterization of localized calcium release events in cells from the gastric fundus.

The in vitro regulation of growth hormone secretion by orexins

Orexins, orexigenic neuropeptides, have recently been discovered in lateral hypothalamus and play an important role in the regulation of pituitary hormone secretion. Two subtypes of orexin receptors (orexin-1 and orexin-2) have been demonstrated in pituitaries. In this experiment, the effects of orexins on voltage-gated Ca2+ currents and the GH release in primary cultured ovine somatotropes were examined. Voltage-gated Ca2+ currents were isolated in ovine somatotropes as L, T, and N currents using whole-cell patch-clamp techniques and specific Ca2+ channel blocker and toxin. Application of orexin-A or orexin-B (100 nM) significantly, dose-dependently, and reversibly increased only nifedipine-sensitive L-type Ca2+ current. Inhibitors of PKC (calphostin C, PKC inhibitory peptide) but not inhibitors of PKA (H89, PKA inhibitory peptide) cancelled the increase in the L current by orexins. Co-administration of orexin-A and GHRH (10 nM) showed an additive effect on the L current. Specific intracellular Ca2+-store-depleting reagent, thapsigargin (1 microM), did not affect the orexin-induced increase in the L current. Orexin-B alone slightly increased GH release and co-administration of orexin-A and GHRH synergistically stimulated GH secretion in vitro. It is therefore suggested that orexins may play an important role in regulating GHRH-stimulated GH secretion through an increase in the L-type Ca2+ current and the PKC-mediated signaling pathways in ovine somatotropes.

Orexin-A augments voltage-gated Ca2+ currents and synergistically increases growth hormone (GH) secretion with GH-releasing hormone in primary cultured ovine somatotropes

Orexins are recently discovered neuropeptides that play an important role in the regulation of hormone secretion, and their receptors have been recently demonstrated in the pituitary. The effects of orexin-A on voltage-gated Ca2+ currents and GH release in primary cultured ovine somatotropes were examined. The expression of orexin-1 receptor was demonstrated by RT-PCR in ovine somatotropes, from which Ca2+ currents were also isolated as L, T, and N currents. Application of orexin-A (100 nM) significantly and reversibly increased only the L current, and coadministration of orexin-A and GHRH (10 nM) showed an additive effect on this current, but no effect of orexin-A was observed on either T or N current. Furthermore, the orexin-A-induced increase in the L current was completely abolished by the inhibition of protein kinase C (PKC) activity using calphostin C (100 nM), phorbal 12,13-dibutyrate pretreatment (0.5 micro M) for 16 h or specific PKC inhibitory peptide PKC(19-36) (1 mM). However, the increase in L current by orexin-A was sustained when cells were preincubated with a specific protein kinase A blocker H89 (1 micro M) or a specific intracellular Ca2+ store depleting reagent thapsigargin (1 micro M). Finally, orexin-A alone did not significantly increase GH release, but coadministration of orexin-A and GHRH showed a synergistic effect on GH secretion in vitro. Our results therefore suggest that orexin-A may play an important role in regulating GHRH-stimulated GH secretion through the enhancement of the L-type Ca2+ current and the PKC-mediated signaling pathway in ovine somatotropes.

Myelin proteolipid protein forms a complex with integrins and may participate in integrin receptor signaling in oligodendrocytes

Myelination of axons in the CNS by oligodendrocytes is a process critical to rapid and efficient impulse conduction. A new role for the myelin proteolipid protein (PLP), the most abundant protein of CNS myelin, has been identified, in studies showing PLP interaction with signaling proteins in oligodendrocytes. In particular, these studies suggest that the PLP protein may be involved in signaling through integrins in oligodendrocytes. Stimulation of muscarinic acetylcholine receptors on oligodendrocytes induced formation of a tripartite complex containing PLP, calreticulin, and alpha(v)-integrin. PLP interacted directly with the cytoplasmic domain of the alpha(v)-integrin. Complex formation was mediated by phospholipase C and Ca2+ binding to the high affinity binding site on calreticulin. This complex appears important for binding of fibronectin to oligodendrocytes. These data establish a novel function for PLP as a part of the integrin signaling complex in oligodendrocytes and suggest that neurotransmitter-mediated integrin receptor signaling may be involved in myelinogenesis.

Myelin proteolipid protein forms a complex with integrins and may participate in integrin receptor signaling in oligodendrocytes

Myelination of axons in the CNS by oligodendrocytes is a process critical to rapid and efficient impulse conduction. A new role for the myelin proteolipid protein (PLP), the most abundant protein of CNS myelin, has been identified, in studies showing PLP interaction with signaling proteins in oligodendrocytes. In particular, these studies suggest that the PLP protein may be involved in signaling through integrins in oligodendrocytes. Stimulation of muscarinic acetylcholine receptors on oligodendrocytes induced formation of a tripartite complex containing PLP, calreticulin, and alpha(v)-integrin. PLP interacted directly with the cytoplasmic domain of the alpha(v)-integrin. Complex formation was mediated by phospholipase C and Ca2+ binding to the high affinity binding site on calreticulin. This complex appears important for binding of fibronectin to oligodendrocytes. These data establish a novel function for PLP as a part of the integrin signaling complex in oligodendrocytes and suggest that neurotransmitter-mediated integrin receptor signaling may be involved in myelinogenesis.

Differential effects of mu-opioid receptor ligands on Ca(2+) signaling

Activation of mu-opioid receptors (MORs) transfected into human embryonic kidney 293 cells, caused a multiphasic increase in cytosolic free Ca(2+) levels (Ca(2+)i). The first Ca(2+)i maximum (peak 1) between 5 and 7 s depended on the presence of extracellular Ca(2+) (Ca(2+)e). The second phase peaking at approximately 15 s (peak 2) was independent of Ca(2+)e and thus represents Ca(2+) release from intracellular stores. A decrease in temperature from 37 to 25 degrees C also caused reduction of peak 1 but not peak 2, suggesting that the two responses arise from mechanistically distinct pathways. A delayed Ca(2+)e-dependent third response phase is thought to represent capacitative Ca(2+)e influx evoked after release of Ca(2+) from internal stores. Agonists and antagonists of two major classes of opioid ligands, oxymorphinans (morphine and naloxone) and oripavines (etorphine and diprenorphine), had differential effects on Ca(2+) currents. Although morphine activated both phases with equal potency, etorphine was 20-fold less potent at stimulating peak 1 over peak 2. Similarly, the antagonists, naloxone and diprenorphine, blocked the Ca(2+) response to each agonist with greatly varying potencies. Specifically, concomitant injection of diprenorphine failed to affect peak 1 (thought to represent rapid Ca(2+)e influx) stimulated by morphine while fully blocking peak 2 (intracellular Ca(2+) release). However, diprenorphine potently inhibited peak 1 as well when added to the cells before morphine, indicating limited or slow access of diprenorphine to these morphine binding sites. The existence of multiple, functionally distinct binding site conformations could account for these findings. In conclusion, different opioid ligands can differentially affect Ca(2+) response patterns resulting from MOR activation.

Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function

Acetylcholine (ACh), the major parasympathetic neurotransmitter, is released by intrapancreatic nerve endings during the preabsorptive and absorptive phases of feeding. In beta-cells, ACh binds to muscarinic M(3) receptors and exerts complex effects, which culminate in an increase of glucose (nutrient)-induced insulin secretion. Activation of PLC generates diacylglycerol. Activation of PLA(2) produces arachidonic acid and lysophosphatidylcholine. These phospholipid-derived messengers, particularly diacylglycerol, activate PKC, thereby increasing the efficiency of free cytosolic Ca(2+) concentration ([Ca(2+)](c)) on exocytosis of insulin granules. IP3, also produced by PLC, causes a rapid elevation of [Ca(2+)](c) by mobilizing Ca(2+) from the endoplasmic reticulum; the resulting fall in Ca(2+) in the organelle produces a small capacitative Ca(2+) entry. ACh also depolarizes the plasma membrane of beta-cells by a Na(+)- dependent mechanism. When the plasma membrane is already depolarized by secretagogues such as glucose, this additional depolarization induces a sustained increase in [Ca(2+)](c). Surprisingly, ACh can also inhibit voltage-dependent Ca(2+) channels and stimulate Ca(2+) efflux when [Ca(2+)](c) is elevated. However, under physiological conditions, the net effect of ACh on [Ca(2+)](c) is always positive. The insulinotropic effect of ACh results from two mechanisms: one involves a rise in [Ca(2+)](c) and the other involves a marked, PKC-mediated increase in the efficiency of Ca(2+) on exocytosis. The paper also discusses the mechanisms explaining the glucose dependence of the effects of ACh on insulin release.

Receptors coupling to G proteins: is there a signal behind the sequence?

Upon the binding of their ligands, G protein-coupled receptors couple to the heterotrimeric G proteins to transduce a signal. One receptor family may couple to a single G protein subtype and another family to several ones. Is there a signal in the receptor sequence that can give an indication of the G protein subtype selectivity? We used a sequence analysis method on biogenic amine and adenosine receptors and concluded that a weak signal can be detected in receptor families where specialization for coupling to a given G protein occurred during a recent divergent evolutionary process. Proteins 2000;41:448-459.

The orexin OX1 receptor activates a novel Ca2+ influx pathway necessary for coupling to phospholipase C

Ca(2+) elevations in Chinese hamster ovary cells stably expressing OX(1) receptors were measured using fluorescent Ca(2+) indicators fura-2 and fluo-3. Stimulation with orexin-A led to pronounced Ca(2+) elevations with an EC(50) around 1 nm. When the extracellular [Ca(2+)] was reduced to a submicromolar concentration, the EC(50) was increased 100-fold. Similarly, the inositol 1,4,5-trisphosphate production in the presence of 1 mm external Ca(2+) was about 2 orders of magnitude more sensitive to orexin-A stimulation than in low extracellular Ca(2+). The shift in the potency was not caused by depletion of intracellular Ca(2+) but by a requirement of extracellular Ca(2+) for production of inositol 1,4,5-trisphosphate. Fura-2 experiments with the "Mn(2+)-quench technique" indicated a direct activation of a cation influx pathway by OX(1) receptor independent of Ca(2+) release or pool depletion. Furthermore, depolarization of the cells to +60 mV, which almost nullifies the driving force for Ca(2+) entry, abolished the Ca(2+) response to low concentrations of orexin-A. The results thus suggest that OX(1) receptor activation leads to two responses, (i) a Ca(2+) influx and (ii) a direct stimulation of phospholipase C, and that these two responses converge at the level of phospholipase C where the former markedly enhances the potency of the latter.

Histamine-induced calcium entry in rat cerebellar astrocytes: evidence for capacitative and non-capacitative mechanisms

We have investigated the effects of histamine on the intracellular calcium concentration ([Ca2+]i) of cultured rat cerebellar astrocytes using fura-2-based Ca2+ imaging microscopy. Most of the cells responded to the application of histamine with an increase in [Ca2+]i which was antagonized by the H1 receptor blocker mepyramine. When histamine was applied for several minutes, the majority of the cells displayed a biphasic Ca2+ response consisting of an initial transient peak and a sustained component. In contrast to the initial transient [Ca2+]i response, the sustained, receptor-activated increase in [Ca2+]i was rapidly abolished by chelation of extracellular Ca2+ or addition of Ni2+, Mn2+, Co2+ and Zn2+, but was unaffected by nifedipine, an antagonist of L-type voltage-activated Ca2+ channels. These data indicate that the sustained increase in [Ca2+]i was dependent on Ca2+ influx. When intracellular Ca2+ stores were emptied by prolonged application of histamine in Ca2+-free conditions, Ca2+ re-addition after removal of the agonist did not lead to an 'overshoot' of [Ca2+]i indicative of store-operated Ca2+ influx. However, Ca2+ stores were refilled despite the absence of any substantial change in the fura-2 signal. Depletion of intracellular Ca2+ stores using cyclopiazonic acid in Ca2+-free saline and subsequent re-addition of Ca2+ to the saline resulted in an increase in [Ca2+]i that was significantly enhanced in the presence of histamine. The results suggest that besides capacitative mechanisms, a non-capacitative, voltage-independent pathway is involved in histamine-induced Ca2+ entry into cultured rat cerebellar astrocytes.

M1 muscarinic receptors block caspase activation by phosphoinositide 3-kinase- and MAPK/ERK-independent pathways

When PC12 cells are deprived of trophic support they undergo apoptosis. We have previously shown that survival of trophic factor-deprived PC12M1 cells can be promoted by activation of the G protein-coupled muscarinic receptors. The mechanism whereby muscarinic receptors inhibit apoptosis is poorly understood. In the present study we investigated this mechanism by examining the effect of muscarinic receptor activation on the serum deprivation-induced activity of key players in apoptosis, the caspases, in PC12M1 cells. The results showed that m1 muscarinic activation inhibits caspase activity induced by serum deprivation. This effect appeared to be caused by the prevention of activation of caspases such as caspase-2 and caspase-3, and not by the inhibition of existing activity. Muscarinic receptor activation also stimulated the mitogen-activated protein kinase/extracellular signaling-regulated kinase (MAPK/ERK) and phosphoinositide (PI) 3-kinase signaling pathways. The PI 3-kinase pathway inhibitors wortmannin and LY294002, as well as the MAPK/ERK pathway PD98059 inhibitor, did not however suppress the inhibitory effect of the muscarinic receptors on caspase activity. The results therefore suggested that the muscarinic survival effect is mediated by a pathway that leads to caspase inhibition by MAPK/ERK- and PI 3-kinase-independent signaling cascades.

Activation of m3 muscarinic receptors induces rapid tyrosine phosphorylation of p125(FAK), p130(cas), and paxillin in rat pancreatic acini

Tyrosine phosphorylation plays a key role in transmembrane and cytoplasmic signal transduction mechanisms stimulated by oncogenes, integrins, growth factors, neuropeptides, and bioactive lipids. Moreover, recent studies show that stimulation of odd-numbered muscarinic receptors increases the tyrosine phosphorylation of several proteins in different cellular types. The present study was aimed at examining whether activation of m3 muscarinic receptors in rat pancreatic acini evokes tyrosine phosphorylation of p125(FAK), and its substrates, p130(cas) and paxillin. Results show that stimulation of pancreatic acini with carbachol resulted in a rapid and transient increase in tyrosine phosphorylation of p125(FAK), p130(cas), and paxillin. Tyrosine phosphorylation of these proteins occurred in a time- and concentration-dependent manner. Simultaneous blockage of both PKC activation and increases in [Ca(2+)](i) partially decreased p125(FAK), p130(cas), and paxillin tyrosine phosphorylation stimulated by carbachol. Pretreatment of pancreatic acini with Clostridium botulinum C3 transferase, which specifically inactivates p21(rho), partially inhibited carbachol-induced p125(FAK), p130(cas), and paxillin tyrosine phosphorylation. In contrast, this treatment had no effect on amylase release stimulated by carbachol. Cytochalasin D, which disrupts actin microfilaments network, completely inhibited carbachol stimulated tyrosine phosphorylation of these proteins without having significant effects in carbachol-stimulated amylase secretion. These results dissociate tyrosine phosphorylation of p125(FAK), p130(cas), and paxillin from amylase secretion after m3 muscarinic receptors occupation in rat pancreatic acini. Taken together, these data suggest that (a) activation of m3 muscarinic receptors in rat pancreatic acini increases tyrosine phosphorylation of p125(FAK) and its substrates, p130(cas) and paxillin by diacylglycerol-activated PKC- and calcium- dependent, and independent pathways, (b) these responses require activation of p21(rho) and an intact actin cytoskeleton, and (c) p125(FAK), p130(cas), and paxillin are unlikely related to secretion in rat pancreatic acinar cells.

Distinct characteristics of receptor-operated Ca2+ influx and refilling in pancreatic acinar cells

Ca2+ influx from the extracellular space in nonexcitable cells occurs via receptor-operated and refilling processes. However, they showed different characteristics with respect to the Mn2+ permeability, depletion of intracellular Ca2+ stores, and sensitivity to the K+ ionophore valinomycin in rat pancreatic acinar cells. While Mn2+ did not enter into the cells during the refilling phase, the opposite was true in receptor-operated Ca2+ influx (ROCI) evoked by carbachol (CCh). ROCI occurred in the absence of intracellular Ca2+ release from the stores. Valinomycin abolished the second response of Ca2+ elicited by CCh, whereas it had no effect on ROCI. These observations suggest that receptor-operated Ca2+ channels (ROCCs) and refilling channels may be different in rat pancreatic acinar cells.

Expression of ryanodine receptors in human embryonic kidney (HEK293) cells

It has been shown previously that mobilization of caffeine-sensitive intracellular calcium (Ca2+i) stores increased the release of amyloid beta-peptide (Abeta) from transfected human embryonic kidney cells (HEK293) [Querfurth, Jiang, Geiger and Selkoe (1997) J. Neurochem. 69, 1580-1591]. The present study was to test the hypothesis that the caffeine/Abeta responses were due to interactions with specific subtypes of ryanodine receptors (RyR) using [3H]ryanodine receptor binding, epifluorescence imaging of Ca2+i, immunocytofluorescence, immunoprecipitation and PCR techniques. [3H]Ryanodine bound to a single class of high-affinity caffeine-sensitive sites (Kd=9.9+/-1.6 nM, Bmax=25+/-4 fmol/mg of protein). RyRs were immuno-decorated in a punctate reticulo-linear pattern. Results from SDS/PAGE and reverse transcriptase-PCR demonstrated endogenous expression of type 1 (skeletal) and type 2 (cardiac) RyRs. HEK293 cell RyRs were functionally active, because (i) [Ca2+]i increased 2.8-fold over baseline following applications of 5-15 mM caffeine, (ii) repetitive spiked increases in [Ca2+]i were observed, and (iii) evidence for a use-dependent block was obtained. Some of these findings were extended to include HeLa and human fibroblast cell lines, suggesting a broader applicability to cells of epithelioid lineage. Implications for the processing of the beta-amyloid precursor protein in Alzheimer's disease and for calcium channel research using transfected HEK293 cells are discussed.

Receptor-operated calcium influx mediated by protein tyrosine kinase pathways

Calcium influx from the extracellular space elicited by activation of heterotrimeric G protein-coupled and heptahelical receptors plays a critical role in transmembrane signal transduction in a wide variety of cell systems. In nonexcitable cells, the precise voltage-independent mechanism by which calcium enters the cell remains unknown. Multiple mechanisms appear to be operating in different cell types (1-3): 1. G protein-operated calcium influx, 2. Second messenger-operated calcium influx, 3. Capacitative calcium influx, and 4. Phosphorylation of calcium channels. Receptor-operated calcium channels have a fundamental role in stimulus-secretion coupling in many different cells, but these channels remain to be purified and cloned. This review proposes that receptor-operated calcium influx is mediated by protein tyrosine kinase pathways. The function of protein tyrosine kinase pathways and their interactions with other receptor-operated calcium influx mechanisms are described.

Muscarinic acetylcholine receptors activate expression of the EGR gene family of transcription factors

In order to search for genes that are activated by muscarinic acetylcholine receptors (mAChRs), we used an mRNA differential display approach in HEK293 cells expressing m1AChR. The zinc-finger transcription factor genes Egr-1, Egr-2, and Egr-3 were identified. Northern blot analyses confirmed that mRNA levels of Egr-1, Egr-2, and Egr-3 increased readily after m1AChR stimulation and that a maximum was attained within 50 min. At that time, Egr-4 mRNA was also detectable. Western blots and electromobility shift assays demonstrated synthesis of EGR-1 and EGR-3, as well as binding to DNA recognition sites in response to m1AChR activation. Activation of m1AChR increased transcription from EGR-dependent promoters, including the acetylcholinesterase gene promoter. Activity-dependent regulation of Egr-1 mRNA expression and EGR-1 protein synthesis was also observed in cells expressing m2, m3, or m4AChR subtypes. Increased EGR-1 synthesis was mimicked by phorbol myristate acetate, but not by forskolin, and receptor-stimulated EGR-1 synthesis was partially inhibited by phorbol myristate acetate down-regulation. Together, our results demonstrate that muscarinic receptor signaling activates the EGR transcription factor family and that PKC may be involved in intracellular signaling. The data suggest that transcription of EGR-dependent target genes, including the AChE gene, can be under the control of extracellular and intracellular signals coupled to muscarinic receptors.

G protein-mediated inhibition of neuronal migration requires calcium influx

Neuronal migration is an essential feature of the developing nervous system, but the intracellular signaling mechanisms that regulate this process are poorly understood. During the formation of the enteric nervous system (ENS) in the moth Manduca sexta, the migration of an identified set of neurons (the EP cells) is regulated in part by the heterotrimeric guanyl-nucleotide binding protein (G protein) Goalpha. Using an in vivo culture preparation for developing embryos that allows direct access to the ENS, we have shown that EP cell migration is similarly regulated by intracellular Ca2+; treatments that increased intracellular Ca2+ inhibited the migratory process, whereas buffering intracellular Ca2+ induced aberrant migration onto inappropriate pathways. Imaging the spontaneous changes in intracellular Ca2+ within individual EP cells showed that actively migrating neurons exhibited only small fluctuations in intracellular Ca2+. In contrast, neurons that had reached the end of migration displayed large, transient Ca2+ spikes. Similar Ca2+ spikes were induced in the EP cells by G protein stimulation, an effect that was reversed by removal of external Ca2+. Stimulation of Go in individual EP cells (by injection of either activated Goalpha subunits or mastoparan) also inhibited migration in a Ca2+-dependent manner. These results suggest that the regulation of neuronal migration by G proteins involves a Ca2+-dependent process requiring Ca2+ influx.

The m3 muscarinic acetylcholine receptor differentially regulates calcium influx and release through modulation of monovalent cation channels

Several types of transmembrane receptors regulate cellular responses through the activation of phospholipase C-mediated Ca2+ release from intracellular stores. In non-excitable cells, the initial Ca2+ release is typically followed by a prolonged Ca2+ influx phase that is important for the regulation of several Ca2+-sensitive responses. Here we describe an agonist concentration-dependent mechanism by which m3 muscarinic acetylcholine receptors (mAChRs) differentially regulate the magnitude of the release and influx components of a Ca2+ response. In transfected Chinese hamster ovary cells expressing m3 mAChRs, doses of the muscarinic agonist carbachol ranging from 100 nM to 1 mM evoked Ca2+ release responses of increasing magnitude; maximal Ca2+ release was elicited by the highest carbachol concentration. In contrast, Ca2+ influx was maximal when m3 mAChRs were activated by moderate doses (1-10 microM) of carbachol, but substantially reduced at higher agonist concentrations. Manipulation of the membrane potential revealed that the carbachol-induced Ca2+ influx phase was diminished at depolarized potentials. Importantly, carbachol doses above 10 microM were found to couple m3 mAChRs to the activation of an inward, monovalent cation current resulting in depolarization of the cell membrane and a selective decrease in the influx, but not release, component of the Ca2+ response. These studies demonstrate, in one experimental system, a mechanism by which a single subtype of G-protein-coupled receptor can utilize the information encoded in the concentration of an agonist to generate distinct intracellular Ca2+ signals.

G protein-mediated inhibition of neuronal migration requires calcium influx

Neuronal migration is an essential feature of the developing nervous system, but the intracellular signaling mechanisms that regulate this process are poorly understood. During the formation of the enteric nervous system (ENS) in the moth Manduca sexta, the migration of an identified set of neurons (the EP cells) is regulated in part by the heterotrimeric guanyl-nucleotide binding protein (G protein) Goalpha. Using an in vivo culture preparation for developing embryos that allows direct access to the ENS, we have shown that EP cell migration is similarly regulated by intracellular Ca2+; treatments that increased intracellular Ca2+ inhibited the migratory process, whereas buffering intracellular Ca2+ induced aberrant migration onto inappropriate pathways. Imaging the spontaneous changes in intracellular Ca2+ within individual EP cells showed that actively migrating neurons exhibited only small fluctuations in intracellular Ca2+. In contrast, neurons that had reached the end of migration displayed large, transient Ca2+ spikes. Similar Ca2+ spikes were induced in the EP cells by G protein stimulation, an effect that was reversed by removal of external Ca2+. Stimulation of Go in individual EP cells (by injection of either activated Goalpha subunits or mastoparan) also inhibited migration in a Ca2+-dependent manner. These results suggest that the regulation of neuronal migration by G proteins involves a Ca2+-dependent process requiring Ca2+ influx.

Differential regulation of muscarinic acetylcholine receptor-sensitive polyphosphoinositide pools and consequences for signaling in human neuroblastoma cells

In this study we have quantitatively assessed the basal turnover of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) and M3-muscarinic receptor-mediated changes in phosphoinositides in the human neuroblastoma cell line, SH-SY5Y. We demonstrate that the polyphosphoinositides represent a minor fraction of the total cellular phosphoinositide pool and that in addition to rapid, sustained increases in [3H]inositol phosphates dependent upon the extent of receptor activation by carbachol, there are equally rapid and sustained reductions in the levels of polyphosphoinositides. Compared with phosphatidylinositol 4-phosphate (PtdIns(4)P), PtdIns(4,5)P2 was reduced with less potency by carbachol and recovered faster following agonist removal suggesting protection of PtdIns(4,5)P2 at the expense of PtdIns(4)P and indicating specific regulatory mechanism(s). This does not involve a pertussis toxin-sensitive G-protein regulation of PtdIns(4)P 5-kinase. Using wortmannin to inhibit PtdIns 4-kinase activity, we demonstrate that the immediate consequence of blocking the supply of PtdIns(4)P (and therefore PtdIns(4,5)P2) is a failure of agonist-mediated phosphoinositide and Ca2+ signaling. The use of wortmannin also indicated that PtdIns is not a substrate for receptor-activated phospholipase C and that 15% of the basal level of PtdIns(4,5)P2 is in an agonist-insensitive pool. We estimate that the agonist-sensitive pool of PtdIns(4,5)P2 turns over every 5 s (0.23 fmol/cell/min) during sustained receptor activation by a maximally effective concentration of carbachol. Immediately following agonist addition, PtdIns(4,5)P2 is consumed >3 times faster (0.76 fmol/cell/min) than during sustained receptor activation which represents, therefore, utilization by a partially desensitized receptor. These data indicate that resynthesis of PtdIns(4,5)P2 is required to allow full early and sustained phases of receptor signaling. Despite the critical dependence of phosphoinositide and Ca2+ signaling on PtdIns(4,5)P2 resynthesis, we find no evidence that this rate resynthesis is limiting for agonist-mediated responses.

Dynamics of [Ca2+] in the endoplasmic reticulum and cytoplasm of intact HeLa cells. A comparative study

We have measured the [Ca2+] in the endoplasmic reticulum ([Ca2+]er) of intact HeLa cells at both 22 degrees C and 37 degrees C using endoplamsic reticulum-targeted, low Ca2+ affinity aequorin reconstituted with coelenterazine n. Aequorin consumption was much slower at 22 degrees C, and this allowed performing a much longer study of the dynamics of [Ca2+]er. The steady-state [Ca2+]er (500-600 microM) was not modified by the temperature, although both the rates of pumping and leak were decreased at 22 degrees C. The behavior of both [Ca2+]er and cytoplasmic [Ca2+] ([Ca2+]c) after the addition of increasing concentrations of agonists and/or Ca2+-ATPase inhibitors, or following incubation in Ca2+-free medium were compared. We show that agonists induce a fast but relatively small decrease in [Ca2+]er, which is enough to produce a sharp increase in [Ca2+]c. Termination of Ca2+ release is controlled by feedback inhibition of the inositol 1,4,5-trisphosphate receptors by [Ca2+]c, a mechanism that appears to be designed to release the minimum amount of Ca2+ necessary to produced the required [Ca2+]c signal. We also show that Ca2+ release is inhibited progressively when [Ca2+]er decreases below a threshold of about 150 microM, even in the absence of Ca2+ pumping or -Ca2+-c increase. This effect is consistent with a regulation of the inositol 1,4,5-trisphosphate-gated channels by [Ca2+]er.

Regulation of cellular tyrosine phosphorylation by stimulatory and inhibitory muscarinic acetylcholine receptors

Tyrosine phosphorylation is a key signaling event in transmembrane and cytoplasmic signal transduction. The m5 muscarinic receptor (m5AChR) responds to ligand stimulation with calcium influx and protein phosphorylation. In contrast, neither of these responses has been associated with m4AChR signaling. We hypothesized that activation of the m5AChR would alter tyrosine phosphorylation patterns spatially within the cell and in a calcium influx-sensitive manner. CHO cells stably transfected with m4- or m5AChRs were assessed for spatial localization and quantity of phosphotyrosylated proteins in response to receptor activation. Results were confirmed by immunoblot of whole cell lysates and cytosol and membrane fractions. m5AChR activation increased tyrosine phosphorylation in all subcellular compartments; coincubation with CAI, a calcium influx inhibitor, reduced phosphorylation below basal levels. Western blot confirmed the change of phosphotyrosylated proteins of M(r) 70, 85, 120, and 180 kDa in whole and fractionated cells. PLC-gamma, used as a marker of m5AChR activity, was increased in quantity and degree of phosphorylation in CHOm5 cell membranes and microvilli in response to receptor activation. Both the quantitative increase and tyrosine phosphorylation of PLC-gamma in membrane fractions was inhibited by CAI. In contrast, CC treatment of CHOm4 cells reduced tyrosine phosphorylation throughout the cell. CC-stimulation of m5AChR cells caused a calcium influx-sensitive increase in phosphotyrosylated proteins throughout the cell, though predominantly in the membrane and microvilli. Activation of the m5AChR induces tyrosine phosphorylation, whereas activation of the m4AChR inhibited tyrosine phosphorylation below baseline, further demonstrating the dichotomy between signaling of these two AChRs.

Status epilepticus induced by pilocarpine and picrotoxin

Since its original description over 10 years ago, the pilocarpine model of status epilepticus (SE) has gained considerable attention. Much work has been done with the model in order to characterize the involvement of different brain structures in seizure genesis and spread. Electrophysiological studies of temporal lobe epileptic slices of both human and animal models, have failed to reveal hyperexcitability, unless blockade of GABAergic inhibition is performed. Thus, we have decided to evaluate potential contributions of picrotoxin, a GABAA channel blocker, on pilocarpine-induced SE. Animals injected with three-specific dose combinations (pilocarpine dose/picrotoxin dose), 150/0.5, 75/1.5 and 50/2.0 mg/kg, evoked status epilepticus (SE) within 23, 31 and 27 min, respectively. Ictal events and EEG spikes were initially observed either in the amygdala or in the hippocampus, with a later spread to cerebral cortex. Neuropathological analysis, performed 5-7 days after SE, has shown a high degree of cell loss predominantly in the piriform cortex, amygdala, hippocampus, thalamus and substantia nigra. Mortality rates for 150/0.5, 75/1.5 and 50/2.0 mg/kg (pilocarpine dose/picrotoxin dose) were 53, 42 and 51%, respectively. Single injections of 150 mg/kg of pilocarpine or 3 mg/kg of picrotoxin did not evoke any form of sustained epileptic activity. Previous studies in which simultaneous injections of other GABAA antagonists (i.e. bicuculline) and pilocarpine were performed, did not show clear evidences of a synergistic action between these two systems. The present study reveals a proconvulsant role for picrotoxin when co-administered with subconvulsant doses of pilocarpine. Possible mechanisms that might account for the interactions between the cholinergic and GABAergic systems in regard to epileptogenesis are discussed.

Random mutagenesis of the cAMP chemoattractant receptor, cAR1, of Dictyostelium. Evidence for multiple states of activation

cAMP receptor 1 (cAR1) of Dictyostelium couples to the G protein G2 to mediate activation of adenylyl and guanylyl cyclases, chemotaxis, and cell aggregation. Other cAR1-dependent events, including receptor phosphorylation and influx of extracellular Ca2+, do not require G proteins. To further characterize signal transduction through cAR1, we performed random mutagenesis of the third intracellular loop (24 amino acids), since the corresponding region of other seven helix receptors has been implicated in the coupling to G proteins. Mutant receptors were expressed in car1(-) cells and were characterized for G protein-dependent and -independent signal transduction. Our results demonstrate that cAR1 is remarkably tolerant to amino acid substitutions in the third intracellular loop. Of the 21 positions where amino acid substitutions were observed, one or more replacements were found that retained full biological function. However, certain alterations resulted in receptors with reduced ability to bind cAMP and/or transduce signals. There were specific signal transduction mutants that could undergo cAMP-dependent cAR1 phosphorylation but were impaired either in coupling to G proteins, in G protein-independent Ca2+ influx, or in both pathways. In addition, there were general activation mutants that failed to restore aggregation to car1(-) cells and displayed severe defects in all signal transduction events, including the most robust response, cAMP-dependent cAR1 phosphorylation. Certain of these mutant phenotypes were obtained in a complementary study, where the entire region of cAR1 from the third to the seventh transmembrane helices was randomly mutagenized. Considered together, these studies indicate that the activation cycle of cAR1 may involve a number of distinct receptor intermediates. A model of G protein-dependent and -independent signal transduction through cAR1 is discussed.

Identification and molecular characterization of a m5 muscarinic receptor in A2058 human melanoma cells. Coupling to inhibition of adenylyl cyclase and stimulation of phospholipase A2

We report the identification and biochemical characterization of an endogenous m5 muscarinic acetylcholine receptor (mAChR) in the A2058 human melanoma cell line. This is the first demonstration of a m5AChR outside the central nervous system. The unusual effector coupling of this endogenous m5AChR is presented. The coding region amplified by polymerase chain reaction was identical to the known m5AChR sequence. Binding studies indicated a Kd of 99 +/- 6 pM and a Bmax of 45 +/- 4 fmol/mg membrane protein. This m5AChR coupled to stimulation of arachidonic acid release and to a 50% inhibition of forskolin-stimulated cAMP accumulation. The inhibition of cAMP production was insensitive to pertussis toxin treatment, but was dependent upon extracellular calcium. In contrast to the odd mAChR pattern, no cAMP was produced in response to carbachol (CC) stimulation. Moreover, no release of inositol phosphates could be measured after CC treatment despite the presence of at least 2 phospholipase C isoforms in A2058 cells. CC-stimulated arachidonic acid release (EC50 = 17.8 +/- 0.1 microM) was dependent upon external Ca2+, with marked reduction after coincubation with EGTA, Co2+, or high doses of verapamil (IC50 = 166 microM) or diltiazem (IC50 = 243 microM). Brief exposure to phorbol 12-myristate 13-acetate augmented CC-stimulated arachidonic acid release, whereas prolonged phorbol 12-myristate 13-acetate treatment resulted in down-regulation of release. Activation of the m5AChR resulted in Ca2+ influx that was attenuated by muscarinic antagonism and removal of extracellular Ca2+. A2058 cells exposed to CC had no alteration of cell shape or growth potential in monolayer culture, however, a statistically significant reduction in density-independent growth was observed over the range of CC concentrations from 0.1 to 100 microM. This endogenous m5AChR has a novel signal transduction coupling profile and receptor activation reduces clonogenic potential.

Differential calcium signalling by m2 and m3 muscarinic acetylcholine receptors in a single cell type

We have compared muscarinic acetylcholine receptor (mAChR) coupling to phospholipase C (PLC) and increases in cytoplasmic Ca2+ concentration [Ca2+]i in human embryonic kidney (HEK) cells, stably expressing either the human m3 or m2 receptor subtype. In m3 mAChR-expressing cells, carbachol stimulated inositol phosphate (InsP) formation and increased [Ca2+]i with EC50 values of about 2 microM and 30 nM, respectively. Maximal inositol 1,4,5-trisphosphate (InsP3) production (about fourfold) was rapid (15 s) and stable for 2 min. Maximal increases in [Ca2+]i were 300-350 nM and mainly, almost 90%, due to influx of extracellular Ca2+. The efficacy of pilocarpine for stimulating InsP and Ca2+ responses was not significantly different from that of carbachol. All m3 mAChR-mediated responses were pertussis toxin (PTX)-insensitive. In m2 mAChR-expressing cells, carbachol stimulated InsP formation and increased [Ca2+]i with EC50 values of about 20 microM and 7 microM, respectively. Maximal InsP formation was only 10-15% of that observed in m3 mAChR-expressing cells, whereas maximal elevations of [Ca2+]i were similar in both cell types. Formation of InsP3 was rapid (15 s to 2 min) and about twofold above basal. In contrast to m3 mAChR activation, [Ca2+]i increases induced by m2 mAChR activation were exclusively due to Ca2+ mobilization from intracellular stores. The efficacy of pilocarpine for stimulating InsP and Ca2+ responses was 50% and 20% of the efficacy of carbachol, respectively. PTX treatment did not affect m2 mAChR-induced PLC stimulation, but reduced the m2 mAChR-mediated increases in [Ca2+]i to 50%. In conclusion, m3 and m2 mAChRs stably expressed in HEK cells can induce similar cellular responses; however, they do so by activating apparently distinct signalling pathways. While coupling of m2 mAChR to PLC occurs in a PTX-insensitive manner, coupling to mobilization of Ca2+ from intracellular stores is partly PTX-sensitive and this may occur at least partly independent of PLC activation.

Requirement of phospholipase C-gamma 2 activation in surface immunoglobulin M-induced B cell apoptosis

Surface IgM (sIgM) stimulation induces the tyrosine phosphorylation of multiple cellular substrates, including phospholipase C (PLC)-gamma 2, which is involved in the activation of phosphatidylinositol pathway. DT40 B cells underwent apoptotic cell death when activated through sIgM, a phenomenon that is related to elimination of self-reactive B cells. To examine the roles of PLC-gamma 2 in sIgM signaling, we have generated DT40 cells deficient in PLC-gamma 2 Cross-linking of sIgM on PLC-gamma 2-deficient cells evoked neither inositol 1,4,5-trisphosphate nor calcium mobilization. In PLC-gamma 2- or Syk-deficient DT40 cells, the induction of apoptosis was blocked, but was still observed in Lyn-deficient cells. Src homology 2 domains of PLC-gamma 2 were essential for both its activation and sIgM-induced apoptosis. Since tyrosine phosphorylation of PLC-gamma 2 is mediated by Syk, these results indicate that activation of PLC-gamma 2 through Syk is required for sIgM-induced apoptosis.

Vasopressin stimulation of Ca2+ mobilization, two bivalent cation entry pathways and Ca2+ efflux in A7r5 rat smooth muscle cells

1. Arg8-vasopressin (AVP)-regulated Ca2+ transport were investigated in fura-2-loaded A7r5 cells using both single cell and population measurements. 2. AVP evokes an initial concentration-dependent rise in cytosolic free Ca2+ concentration ([Ca2+ ]i) to a peak which is independent of extracellular Ca2+, and a sustained Ca2+ signal that results from a balance between stimulation of Ca2+ entry and efflux. 3. Depletion of intracellular Ca2+ stores with thapsigargin, ionomycin, or prior treatment with AVP in Ca2(+)-free medium activates 'capacitative' entry of Ca2+, Ba2+ or Mn2+. Capacitative Mn2+ entry is inhibited by refilling stores with Ca2+; neither Sr2+ nor Ba2+ substitute for Ca2+ to give this effect. 4. In cells with empty stores, AVP stimulates further bivalent cation entry, and the effect persists when extracellular Na+ is replaced by N-methyl-D-glucamine or under depolarizing condition (extracellular KCl concentration ([KCl]o), 135 mM). This effect of AVP is not therefore merely a consequence of AVP causing membrane hyperpolarization or stimulation of Na(+)-Ca2+ exchange, but results from opening of a bivalent cation influx pathway. 5. Several lines of evidence indicate that AVP-stimulated bivalent cation entry is not a consequence of more complete emptying of the intracellular stores and consequent further activation of the capacitative pathway. AVP stimulates Ba2+ entry when the intracellular Ca2+ stores have been both emptied by ionomycin and prevented from refilling by thapsigargin. Mn2+ permeates the capacitative pathway, but AVP does not further increase Mn2+ entry, confirming that AVP does not further activate the capacitative pathway and that the two pathways differ in their permeability to Mn2+. When the extracellular [Sr2+] is low, empty stores do not stimulate detectable Sr2+ entry, but addition of AVP causes substantial Sr2+ entry. 6. A decrease in [Ca2+]i occurs when 50 nM AVP is added during a sustained elevation of [Ca2+]i evoked by thapsigargin. Since AVP does not inhibit the capacitative pathway, this result suggests that AVP stimulates Ca2+ extrusion. 7. We conclude that stimulation of Ca2+ mobilization, two modes of bivalent cation entry, and Ca2+ efflux all contribute to the complex concentration-dependent effects of AVP in A7r5 smooth muscle cells.

Region-specific activity of the plasma membrane Ca2+ pump and delayed activation of Ca2+ entry characterize the polarized, agonist-evoked Ca2+ signals in exocrine cells

The initial release of Ca2+ from the intracellular Ca2+ stores is followed by a second phase during which the agonist-dependent Ca2+ response becomes sensitive to the extracellular Ca2+, indicating the involvement of the plasma membrane (PM) Ca2+ transport systems. The time course of activation of these transport systems, which consist of both Ca2+ extrusion and Ca2+ entry pathways, is not well established. To investigate the participation of these processes during the agonist-evoked Ca2+ response, isolated pancreatic acinar cells were exposed to maximal concentrations of an inositol 1,4,5-trisphosphate-mobilizing agonist (acetylcholine, 10 microM) in different experimental conditions. Following the increase of [Ca2+]i, there was an almost immediate activation of the PM Ca2+ extrusion system, and maximal activity was reached within less than 2s. The rate of Ca2+ extrusion was dependent on the level of [Ca2+]i, with a steep activation at values just above the resting [Ca2+]i and reached a plateau value at 700 nM Ca2+. In contrast, the PM Ca2+ entry pathway was activated with a much slower time course. There was also a delay of 3-4 s between the maximal effective depletion of the intracellular Ca2+ stores and the activation of this entry pathway. By use of digital imaging data, the PM Ca2+ transport systems were also analyzed independently in two regions of the cells, the lumenal and the basal poles. With respect to the activation of the Ca2+ entry pathways, no significant difference existed between these two regions. In contrast, the PM Ca2+ pump displayed a different pattern of activity in these regions. In the basal pole, the pump activity was more sensitive to changes of [Ca2+]i and had a higher maximal activity. Also, in the lumenal pole, the pump became saturated at values of [Ca2+]i around 700 nM, whereas at the basal pole [Ca2+]i had a biphasic effect on the pump activity, and higher [Ca2+]i inhibited the pump. It is argued that these differences in sensitivity to the levels of [Ca2+]i and the different relationship between [Ca2+]i and the rate of extrusion at the two functional poles of the pancreatic acinar cells indicate that the plasma membrane Ca2+ ATPase might play an important role in the polarization of the Ca2+ response.

Seven helix cAMP receptors stimulate Ca2+ entry in the absence of functional G proteins in Dictyostelium

Surface cAMP receptors (cARs) in Dictyostelium transmit a variety of signals across the plasma membrane. The best characterized cAR, cAR1, couples to the heterotrimeric guanine nucleotide-binding protein (G protein) alpha-subunit G alpha 2 to mediate activation of adenylyl and guanylyl cyclases and cell aggregation. cAR1 also elicits other cAMP-dependent responses including receptor phosphorylation, loss of ligand binding (LLB), and Ca2+ influx through a G alpha 2-independent pathway that may not involve G proteins. Here, we have expressed cAR1 and a related receptor, cAR3, in a g beta- strain (Lilly, P., Wu. L., Welker, D. L., and Devreotes, P. N. (1993) Genes & Dev. 7,986-995), which lacks G protein activity. Both cell lines failed to aggregate, a process requiring the G alpha 2 and G beta- subunits. In contrast, cAR1 phosphorylation in cAR1/g beta- cells showed a time course and cAMP dose dependence indistinguishable from those of cAR1/G beta+ controls. cAMP-induced LLB was also normal in the cAR1/g beta- cells. Finally, cAR1/g beta- cells and cAR3/g beta- cells showed a Ca2+ response with kinetics, agonist dependence, ion specificity, and sensitivity to depolarization agents that were like those of G beta+ controls, although they accumulated fewer Ca2+ ions per cAMP receptor than the control strains. Together, these results suggest that the G beta-subunit is not required for the activation or attenuation of cAR1 phosphorylation, LLB, or Ca2+ influx. It may, however, serve to amplify the Ca2+ response, possibly by modulating other intracellular Ca2+ signal transduction pathways.

Chloride is required for receptor-mediated divalent cation entry in mesangial cells

Agonists which stimulate the inositol 1,4,5 trisphosphate ([1,4,5]-IP3)-dependent mobilization of Ca2+ from intracellular stores also stimulate entry of divalent cations across the cell membrane. Under appropriate experimental conditions, divalent cation entry across the cell membrane can be monitored as the rate at which the intracellular fluorescence of divalent cation indicators is quenched by the addition of Mn2+ to the extracellular medium. We report that addition of vasopressin to fura-2-loaded glomerular mesangial cells in culture markedly accelerated the rate at which Mn2+ quenched fura-2 fluorescence at its Ca(2+)-insensitive wavelength in the presence of extracellular NaCl, but that this quench response was attenuated when Cl- was removed from the extracellular medium by equimolar substitution with impermeant anions (gluconate, methanesulfonate, acetate, lactate). Similarly, loss of agonist-induced quench also occurred when Cl- was substituted with gluconate in K(+)-containing media. Addition of the Cl- channel inhibitor, 5-nitro-2-(3-phenylpropylaminobenzoic acid) (NPPB), also inhibited Mn(2+)-induced quench of fura-2 fluorescence following vasopressin addition. In contrast, in the presence of gramicidin to provide an alternate conductance pathway to accompany divalent cation entry, agonist-dependent Mn2+ quench occurred even in the absence of extracellular Cl-, indicating that the requirement for Cl- was not the result of cotransport on a common transporter nor the result of Cl- serving as a necessary cofactor for divalent cation entry. A similar dependence on extracellular Cl- was observed for other Ca(2+)-mobilizing agonists such as endothelin, as well as the intracellular Ca2+ ATPase inhibitor, thapsigargin. Extracellular Cl- dependence for agonist-induced divalent cation entry was also reflected in a corresponding extracellular Cl- dependence for agonist-induced mesangial cell contraction. It has been previously shown by ourselves (Kremer et al., 1992a, Am. J. Physiol., 262:F668-F678) and others that agonist-stimulated calcium mobilization in mesangial cells is accompanied by inhibition of K+ conductance and increased Cl- conductance. Accordingly, we conclude that the current findings suggest that activation of Cl- conductance provides regulated charge compensation for receptor-mediated divalent cation entry in response to Ca(2+)-mobilizing vasoconstrictor agonists in mesangial cells.

G-protein- and capacitatively regulated Ca2+ entry pathways are activated by muscarinic receptor stimulation in a human submandibular ductal cell line

In the human submandibular ductal cell line (HSG) thapsigargin and carbachol stimulated Ca2+ release from the internal Ca2+ pool, resulting in the activation of capacitatively regulated Ca2+ entry (CRCE). This entry pathway was permeant to both Ca2+ and Mn2+, blocked by Ni2+ and insensitive to the muscarinic antagonist, atropine. Carbachol also stimulated an increase in cytosolic [Ca2+] in internal Ca(2+)-pool-depleted (i.e. thapsigargin-treated) cells which was dependent on the presence of external Ca2+ and blocked by Ni2+, demonstrating that it was due to Ca2+ entry. However, under the same experimental conditions, carbachol was unable to stimulate Mn2+ entry. Additionally, this latter carbachol-stimulated Ca2+ entry pathway was blocked by atropine. Pretreatment of HSG cells with AlF4-increased basal rates of Mn2+ entry due to CRCE activation, but attenuated carbachol-stimulated Ca2+ entry into thapsigargin-treated cells. The data suggest that two distinct divalent cation entry pathways are activated in muscarinic-receptor-stimulated HSG cells; a CRCE mechanism, permeable to both Mn2+ and Ca2+, and a second entry mechanism, permeable only to Ca2+. The latter does not depend on internal pool depletion, but appears to be regulated via G-protein activation.

Endothelin-1 increases the levels of mRNA and protein of muscarinic acetylcholine receptors and c-fos mRNA in cerebellar granule cells

Endothelin-1 (ET-1) induced a time- and dose-dependent increase in the levels of mRNA of m2- and m3-muscarinic acetylcholine receptors (mAChRs) in cultured cerebellar granule cells. The levels of immunoprecipitable m3-mAChR protein and total mAChR binding sites were also increased by ET-1 treatment. The up-regulation of m2- and m3-mAChR was blocked by phorbol ester pretreatment to inhibit ET-1-stimulated phosphoinositide hydrolysis and was preceded by an increase in c-fos mRNA levels. Treatments that prevented ET-1-induced c-fos mRNA increase also abolished the subsequent m2- and m3-mAChR mRNA up-regulation, suggesting that c-Fos protein is involved in the ET-1-induced mAChR expression.

Rat oligodendrocytes express muscarinic receptors coupled to phosphoinositide hydrolysis and adenylyl cyclase

Muscarinic receptors expressed by rat oligodendrocyte primary cultures were examined by measuring changes in second messengers following exposure to carbachol, an acetylcholine analog, and by polymerase chain reaction. Inositol phosphate levels were measured in [3H]myo-inositol-labelled young oligodendrocyte cultures following stimulation with carbachol. Atropine, a specific muscarinic antagonist, prevented the carbachol-induced accumulation of inositol phosphates. The formation of inositol trisphosphate was concentration- and time-dependent, with the peak at 100 microM carbachol and 10 min. Carbachol increased intracellular calcium levels, which were dependent both on the mobilization of intracellular stores and influx of extracellular calcium. In initial experiments with more selective antagonists, the mobilization of intracellular calcium was preferentially inhibited by pirenzepine, a selective M1 antagonist, but not methoctramine, a selective M2 antagonist, suggesting M1 muscarinic receptor involvement. A role for protein kinase C in the regulation of carbachol-stimulated inositol phosphate formation and intracellular calcium mobilization was demonstrated, as acute pretreatment with phorbol-12,13-myristate acetate abolished the formation of both second messengers. Pretreatment with 100 microM carbachol abolished the 40% increase in the cyclic AMP accumulation stimulated by isoproterenol, a specific beta-adrenergic agonist. In turn, the inhibition was alleviated by pretreatment with atropine, suggesting muscarinic receptor involvement. Polymerase chain reaction carried out with specific m1 and m2 muscarinic receptor oligonucleotide primers, confirmed that these cells express, at least, the two muscarinic receptor subtypes. Without excluding the expression of other subtypes, these results suggest that developing oligodendrocytes express m1 (M1) and m2 (M2) muscarinic receptors capable of mediating phosphoinositide hydrolysis, mobilization of intracellular calcium and the attenuation of beta-adrenergic stimulation of cyclic AMP formation.

Calcium-mediated signal transduction: biology, biochemistry, and therapy

The process of proliferation, invasion and metastasis is a complex one which involves both the autonomy of the malignant cells and their interaction with the cellular and extracellular environments. The way in which the tumor cells respond to cellular and extracellular stimuli is regulated through transduction of those signals and translation into cellular activity. Transmembrane signal transduction involves three major categories of events: ion channel activation, transmission through guanine nucleotide binding protein intermediates with production of second messengers, and phosphorylation events. A frequent common denominator of these different pathways is a cellular calcium homeostasis. Calcium may be both a result of and a regulator of many of these signal transduction pathways and has been shown to have a role in the regulation of proliferation, invasion, and metastatic potential. The understanding and application of the basic tenets of these pathways to tumor cell proliferation, invasion, and metastases opens a new target for therapeutic intervention. We have identified a novel agent, CAI, which through inhibition of stimulated calcium influx inhibits proliferation and migration in vitro, and growth and dissemination in human cancer xenografts in vivo. CAI offers a new approach to cancer therapy, signal transduction therapy.

M3 muscarinic receptors mediate an increase in both inositol trisphosphate production and cyclic AMP formation in dog iris sphincter smooth muscle

Pharmacological studies on pirenzepine (PZ), 4-diphenylacetoxy-N-methyl-piperidine (4-DAMP) and AFDX-116 antagonism of carbachol (CCh)-induced contraction, inositol trisphosphate (IP3) production and cAMP formation revealed the involvement of M3 receptors in these responses. The PA2 values for PZ and 4-DAMP antagonism to CCh-induced contraction were 7.1 and 9.0, respectively, and AFDX-116 had no effect on these responses. Further, 4-DAMP was a much more potent inhibitor than PZ of CCh-stimulation of IP3 production and cAMP formation. Both L-type calcium channel blockers, which inhibit Ca2+ influx, and BAPTA, an intracellular calcium chelator, inhibited these biochemical and pharmacological responses due to CCh. It is concluded that both intracellular and extracellular Ca2+ mobilization are involved in muscarinic stimulation of cAMP production, and that M3 receptors are coupled to the activation of both phospholipase C and adenylate cyclase in this tissue. The data presented here are consistent with previous work that stimulation of muscarinic receptors in dog iris sphincter with CCh (> 5 microM) increases intracellular cAMP levels.

Phosphorylation of a phosphoinositidase C-linked muscarinic receptor by a novel kinase distinct from beta-adrenergic receptor kinase

Muscarinic receptor kinase activity previously described in intact CHO cells transfected with human m3-muscarinic receptor cDNA (CHO-m3 cells) [Tobin, A.B and Nahorski, S.R (1993) J. Biol. Chem. 268, 9817-9823] was found to be associated, at least in part, with a crude membrane fraction of CHO-m3 cell lysates. Phosphorylation of the m3-muscarinic receptor was agonist dependent, reaching a maximum after 10 min exposure to carbachol (1 mM) and was completely blocked by atropine (10 microM). m3-Muscarinic receptor phosphorylation was insensitive to Zn2+ (0.1 mM) and heparin (1 microgram/ml), concentrations that inhibit endogenous beta-adrenergic receptor kinase activity present in CHO-m3 cells strongly suggesting that the m3-muscarinic receptor kinase is distinct from beta-adrenergic receptor kinase. A role for protein kinase C can also be eliminated on the basis that the potent protein kinase C inhibitor, Ro-318220 (1 microM), had no effect on agonist-mediated m3-muscarinic receptor phosphorylation. Further, the inability of calcium (300 microM), cAMP (0.2 mM) and cGMP (0.2 mM) to elevate the basal phosphorylation state of m3-muscarinic receptors eliminates a role for protein kinases regulated by these second messengers. Finally, agonist mediated phosphorylation appears to be independent of G-protein activation as both GDP-beta-S (500 microM) and GTP-gamma-S (100 microM) did not influence m3-muscarinic receptor phosphorylation.

Homologous and heterologous desensitization of histamine H1- and ATP-receptors in the smooth muscle cell line, DDT1MF-2: the role of protein kinase C

1. The possible role of protein kinase C (PKC) in homologous and heterologous desensitization of histamine H1- and ATP-receptors has been studied in monolayers of cultured vas deferens smooth muscle cells (DDT1MF-2). Cells were loaded with the calcium-sensitive fluorescent dye fura-2 and increases in intracellular free Ca2+ concentration ([Ca2+]i) monitored in response to histamine H1- or ATP-receptor activation. 2. Histamine and ATP stimulated the release of Ca2+ from intracellular Ca2+ stores and Ca2+ influx across the plasma membrane. Activation of PKC with the phorbol ester beta-phorbol-12,13 dibutyrate (PDBu; 1 microM) attenuated histamine (100 microM) and ATP (10 microM)-induced release of intracellular Ca2+ and Ca2+ influx. 3. The selective PKC inhibitor, Ro 31-8220 (10 microM), reversed the PDBu-induced attenuation of histamine (100 microM)-stimulated Ca2+ responses. 4. Histamine H1- and ATP-receptors are readily susceptible to homologous desensitization since short-term exposure to histamine or ATP (450 s) attenuated the Ca2+ responses elicited by a second application of the same agonist. Furthermore, H1-receptor activation-induced heterologous desensitization of ATP stimulated Ca2+ responses and vice versa. 5. Homologous and heterologous desensitization of histamine and ATP Ca2+ responses still occurred in the presence of the PKC inhibitor, Ro 31-8220 (10 microM). 6. These data suggest that PKC activation can attenuate histamine H1- and ATP-receptor mediated Ca2+ responses. However, based on our experimental data, PKC-independent mechanisms appear to be involved in the homologous and heterologous desensitization of histamine H1- and ATP-receptor mediated Ca2+ responses in DDT1MF-2 cells.

Muscarinic acetylcholine receptor subtypes associated with release of Alzheimer amyloid precursor derivatives activate multiple signal transduction pathways

Five subtypes of muscarinic acetylcholine receptors have been identified and designated m1-m5. The m1 and m3 receptors have recently been shown to stimulate APP processing. The m1 and m3 receptors couple to a variety of signal transduction pathways in both tissue slices and a variety of cell lines endogenously expressing either or both subtypes. In contrast, the m2 and m4 receptors have been primarily associated with inhibition of adenylate cyclase. We have transfected all five subtypes of muscarinic receptors into a variety of mammalian cell lines in order to investigate the signaling associated with single receptor subtypes. The m1, m3, or m5 receptors stimulate phospholipase A2, C, and D, adenylate cyclase, receptor-operated calcium channels, and tyrosine kinase activity simultaneously. The m2 or m4 receptor inhibits cAMP accumulation and augments a previously stimulated release of arachidonic acid and calcium influx.