Ryanodine receptor subtype 2 encodes Ca2+ oscillations activated by acetylcholine via the M2 muscarinic receptor/cADP-ribose signalling pathway in duodenum myocytes

Foxp3-mediated blockage of ryanodine receptor 2 underlies contact-based suppression by regulatory T cells

The suppression mechanism of Tregs remains an intensely investigated topic. As our focus has shifted toward a model centered on indirect inhibition of DCs, a universally applicable effector mechanism controlled by the transcription factor forkhead box P3 (Foxp3) expression has not been found. Here, we report that Foxp3 blocked the transcription of ER Ca2+-release channel ryanodine receptor 2 (RyR2). Reduced RyR2 shut down basal Ca2+ oscillation in Tregs, which reduced m-calpain activities that are needed for T cells to disengage from DCs, suggesting a persistent blockage of DC antigen presentation. RyR2 deficiency rendered the CD4+ T cell pool immune suppressive and caused it to behave in the same manner as Foxp3+ Tregs in viral infection, asthma, hypersensitivity, colitis, and tumor development. In the absence of Foxp3, Ryr2-deficient CD4+ T cells rescued the systemic autoimmunity associated with scurfy mice. Therefore, Foxp3-mediated Ca2+ signaling inhibition may be a central effector mechanism of Treg immune suppression.

Estrogenic Modulation of Ionic Channels, Pumps and Exchangers in Airway Smooth Muscle

To preserve ionic homeostasis (primarily Ca²⁺, K⁺, Na⁺, and Cl^-), in the airway smooth muscle (ASM) numerous transporters (channels, exchangers, and pumps) regulate the influx and efflux of these ions. Many of intracellular processes depend on continuous ionic permeation, including exocytosis, contraction, metabolism, transcription, fecundation, proliferation, and apoptosis. These mechanisms are precisely regulated, for instance, through hormonal activity. The lipophilic nature of steroidal hormones allows their free transit into the cell where, in most cases, they occupy their cognate receptor to generate genomic actions. In the sense, estrogens can stimulate development, proliferation, migration, and survival of target cells, including in lung physiology. Non-genomic actions on the other hand do not imply estrogen's intracellular receptor occupation, nor do they initiate transcription and are mostly immediate to the stimulus. Among estrogen's non genomic responses regulation of calcium homeostasis and contraction and relaxation processes play paramount roles in ASM. On the other hand, disruption of calcium homeostasis has been closely associated with some ASM pathological mechanism. Thus, this paper intends to summarize the effects of estrogen on ionic handling proteins in ASM. The considerable diversity, range and power of estrogens regulates ionic homeostasis through genomic and non-genomic mechanisms.

Calcium-Dependent Regulation of the Neuronal Glycine Transporter GlyT2 by M2 Muscarinic Acetylcholine Receptors

The neuronal glycine transporter GlyT2 modulates inhibitory glycinergic neurotransmission and plays a key role in regulating nociceptive signal progression. The cholinergic system acting through muscarinic acetylcholine receptors (mAChRs) also mediates important regulations of nociceptive transmission being the M2 subtype the most abundantly expressed in the spinal cord. Here we studied the effect of M2 mAChRs stimulation on GlyT2 function co-expressed in a heterologous system with negligible levels of muscarinic receptor activity. We found GlyT2 is down-regulated by carbachol in a calcium-dependent manner. Different components involved in cell calcium homeostasis were analysed to establish a role in the mechanism of GlyT2 inhibition. GlyT2 down-regulation by carbachol was increased by thapsigargin and reduced by internal store depletion, although calcium release from endoplasmic reticulum or mitochondria had a minor role on GlyT2 inhibition. Our results are consistent with a GlyT2 sensitivity to intracellular calcium mobilized by M2 mAChRs in the subcortical area of the plasma membrane. A crucial role of the plasma membrane sodium calcium exchanger NCX is proposed.

Does Cyclic ADP-Ribose (cADPR) Activate the Non-selective Cation Channel TRPM2?

TRPM2 is a non-selective, Ca²⁺-permeable cation channel widely expressed in immune cells. It is firmly established that the channel can be activated by intracellular adenosine 5′-diphosphoribose (ADPR). Until recent cryo-EM structures have exhibited an additional nucleotide binding site in the N-terminus of the channel, this activation was thought to occur via binding to a C-terminal domain of the channel that is highly homologous to the ADPR pyrophosphatase NudT9. Over the years it has been controversially discussed whether the Ca²⁺ mobilizing second messenger cyclic ADP ribose (cADPR) might also directly activate Ca²⁺ entry via TRPM2. Here we will review the status of this discussion.

Contribution of NAADP to Glutamate-Evoked Changes in Ca2+ Homeostasis in Mouse Hippocampal Neurons

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a second messenger that evokes calcium release from intracellular organelles by the engagement of calcium release channels, including members of the Transient Receptor Potential (TRP) family, such as TRPML1, the (structurally) related Two Pore Channel type 1 (TPC1) and TPC2 channels as well as Ryanodine Receptors type 1 (RYR1; Guse, 2012). NAADP evokes calcium release from acidic calcium stores of many cell types (Guse, 2012), and NAADP-sensitive Ca²⁺ stores have been described in hippocampal neurons of the rat (Bak et al., 1999; McGuinness et al., 2007). Glutamate triggers Ca²⁺-mediated neuronal excitotoxicity in inflammation-induced neurodegenerative pathologies such as Multiple Sclerosis (MS; Friese et al., 2014), and when applied extracellularly to neurons glutamate can elevate NAADP levels in these cells. Accordingly, glutamate-evoked Ca²⁺ signals from intracellular organelles were inhibited by preventing organelle acidification (Pandey et al., 2009). Analysis of reported RNA sequencing experiments of cultured hippocampal neurons revealed the abundance of Mcoln1 (encoding TRPML1), Tpcn1, and Tpcn2 (encoding TPC1 and TPC2, respectively) as potential NAADP target channels in these cells. Transcripts encoding Ryr1 were not found in contrast to Ryr2 and Ryr3. To study the contribution of NAADP signaling to glutamate-evoked calcium transients in murine hippocampal neurons we used the NAADP antagonists Ned-19 (Naylor et al., 2009) and BZ194 (Dammermann et al., 2009). Our results show that both NAADP antagonists significantly reduce glutamate-evoked calcium transients. In addition to extracellular glutamate application, we studied synchronized calcium oscillations in the cells of the neuronal cultures evoked by addition of the GABA_A receptor antagonist bicuculline. Pretreatment with Ned-19 (50 μM) or BZ194 (100 μM) led to an increase in the frequency of bicuculline-induced calcium oscillations at the cost of calcium transient amplitudes. Interestingly, Ned-19 triggered a rise in intracellular calcium concentrations 25 min after bicuculline stimulation, leading to the question whether NAADP acts as a neuroprotective messenger in hippocampal neurons. Taken together, our results are in agreement with the concept that NAADP signaling significantly contributes to glutamate evoked Ca²⁺ rise in hippocampal neurons and to the amplitude and frequency of synchronized Ca²⁺ oscillations triggered by spontaneous glutamate release events.

Portal vein gas and pneumatosis intestinalis: A case of intestinal necrosis caused by acute organophosphorus pesticide poisoning?

Acute organophosphorus pesticide poisoning (AOPP) is fairly common in rural areas of Asia. The symptoms of AOPP are mainly caused by acetylcholine accumulation. According to the clinical characteristics, AOPP symptoms can fall into the following three categories: muscarinic, nicotinic, and central. Death from fatal poisoning is caused by respiratory paralysis, and neurological complications are common. However, no case of intestinal necrosis caused by AOPP has been reported. Hepatic portal vein gas and pneumatosis intestinalis are considered typical and early imaging manifestations of intestinal necrosis. In this article, we describe a very rare case of computed tomography imaging-proven intestinal necrosis caused by AOPP.

Calcium mobilizing second messengers derived from NAD

Nicotinamide adenine dinucleotide (NAD) has been known since a long period of time as co-factor of oxidoreductases. However, in the past couple of decades further roles have been assigned to NAD. Here, metabolism of NAD to the Ca²⁺ mobilizing second messengers cyclic adenosine diphosphoribose, nicotinic acid adenine dinucleotide phosphate and adenosine diphosphoribose is reviewed. Moreover, the mechanisms of Ca²⁺ mobilization by these adenine nucleotides and their putative target Ca²⁺ channels, ryanodine receptors and transient receptor potential channels are discussed. This article is part of a Special Issue entitled: Cofactor-dependent proteins: evolution, chemical diversity and bio-applications.

MicroRNA-708 regulates CD38 expression through signaling pathways JNK MAP kinase and PTEN/AKT in human airway smooth muscle cells

Background

The cell-surface protein CD38 mediates airway smooth muscle (ASM) contractility by generating cyclic ADP-ribose, a calcium-mobilizing molecule. In human ASM cells, TNF-α augments CD38 expression transcriptionally by NF-κB and AP-1 activation and involving MAPK and PI3K signaling. CD38^−/− mice develop attenuated airway hyperresponsiveness following allergen or cytokine challenge. The post-transcriptional regulation of CD38 expression in ASM is relatively less understood. In ASM, microRNAs (miRNAs) regulate inflammation, contractility, and hyperproliferation. The 3’ Untranslated Region (3’UTR) of CD38 has multiple miRNA binding sites, including a site for miR-708. MiR-708 is known to regulate PI3K/AKT signaling and hyperproliferation of other cell types. We investigated miR-708 expression, its regulation of CD38 expression and the underlying mechanisms involved in such regulation in human ASM cells.

Methods

Growth-arrested human ASM cells from asthmatic and non-asthmatic donors were used. MiRNA and mRNA expression were measured by quantitative real-time PCR. CD38 enzymatic activity was measured by a reverse cyclase assay. Total and phosphorylated MAPKs and PI3K/AKT as well as enzymes that regulate their activation were determined by Western blot analysis of cell lysates following miRNA transfection and TNF-α stimulation. Dual luciferase reporter assays were performed to determine whether miR-708 binds directly to CD38 3’UTR to alter gene expression.

Results

Using target prediction algorithms, we identified several miRNAs with potential CD38 3’UTR target sites and determined miR-708 as a potential candidate for regulation of CD38 expression based on its expression and regulation by TNF-α. TNF-α caused a decrease in miR-708 expression in cells from non-asthmatics while it increased its expression in cells from asthmatics. Dual luciferase reporter assays in NIH-3 T3 cells revealed regulation of expression by direct binding of miR-708 to CD38 3’UTR. In ASM cells, miR-708 decreased CD38 expression by decreasing phosphorylation of JNK MAPK and AKT. These effects were associated with increased expression of MKP-1, a MAP kinase phosphatase and PTEN, a phosphatase that terminates PI3 kinase signaling.

Conclusions

In human ASM cells, TNF-α-induced CD38 expression is regulated by miR-708 directly binding to 3’UTR and indirectly by regulating JNK MAPK and PI3K/AKT signaling and has the potential to control airway inflammation, ASM contractility and proliferation.

Acetylcholine promotes Ca2+ and NO-oscillations in adipocytes implicating Ca2+→NO→cGMP→cADP-ribose→Ca2+ positive feedback loop--modulatory effects of norepinephrine and atrial natriuretic peptide

PURPOSE

This study investigated possible mechanisms of autoregulation of Ca(2+) signalling pathways in adipocytes responsible for Ca(2+) and NO oscillations and switching phenomena promoted by acetylcholine (ACh), norepinephrine (NE) and atrial natriuretic peptide (ANP).

METHODS

Fluorescent microscopy was used to detect changes in Ca(2+) and NO in cultures of rodent white adipocytes. Agonists and inhibitors were applied to characterize the involvement of various enzymes and Ca(2+)-channels in Ca(2+) signalling pathways.

RESULTS

ACh activating M3-muscarinic receptors and Gβγ protein dependent phosphatidylinositol 3 kinase induces Ca(2+) and NO oscillations in adipocytes. At low concentrations of ACh which are insufficient to induce oscillations, NE or α1, α2-adrenergic agonists act by amplifying the effect of ACh to promote Ca(2+) oscillations or switching phenomena. SNAP, 8-Br-cAMP, NAD and ANP may also produce similar set of dynamic regimes. These regimes arise from activation of the ryanodine receptor (RyR) with the implication of a long positive feedback loop (PFL): Ca(2+)→NO→cGMP→cADPR→Ca(2+), which determines periodic or steady operation of a short PFL based on Ca(2+)-induced Ca(2+) release via RyR by generating cADPR, a coagonist of Ca(2+) at the RyR. Interplay between these two loops may be responsible for the observed effects. Several other PFLs, based on activation of endothelial nitric oxide synthase or of protein kinase B by Ca(2+)-dependent kinases, may reinforce functioning of main PFL and enhance reliability. All observed regimes are independent of operation of the phospholipase C/Ca(2+)-signalling axis, which may be switched off due to negative feedback arising from phosphorylation of the inositol-3-phosphate receptor by protein kinase G.

CONCLUSIONS

This study presents a kinetic model of Ca(2+)-signalling system operating in adipocytes and integrating signals from various agonists, which describes it as multivariable multi feedback network with a family of nested positive feedback.

Altered CD38/Cyclic ADP-Ribose Signaling Contributes to the Asthmatic Phenotype

CD38 is a transmembrane glycoprotein expressed in airway smooth muscle cells. The enzymatic activity of CD38 generates cyclic ADP-ribose from β-NAD. Cyclic ADP-ribose mobilizes intracellular calcium during activation of airway smooth muscle cells by G-protein-coupled receptors through activation of ryanodine receptor channels in the sarcoplasmic reticulum. Inflammatory cytokines that are implicated in asthma upregulate CD38 expression and increase the calcium responses to contractile agonists in airway smooth muscle cells. The augmented intracellular calcium responses following cytokine exposure of airway smooth muscle cells are inhibited by an antagonist of cyclic ADP-ribose. Airway smooth muscle cells from CD38 knockout mice exhibit attenuated intracellular calcium responses to agonists, and these mice have reduced airway response to inhaled methacholine. CD38 also contributes to airway hyperresponsiveness as shown in mouse models of allergen or cytokine-induced inflammatory airway disease. In airway smooth muscle cells obtained from asthmatics, the cytokine-induced CD38 expression is significantly enhanced compared to expression in cells from nonasthmatics. This differential induction of CD38 expression in asthmatic airway smooth muscle cells stems from increased activation of MAP kinases and transcription through NF-κB, and altered post-transcriptional regulation through microRNAs. We propose that increased capacity for CD38 signaling in airway smooth muscle in asthma contributes to airway hyperresponsiveness.

Role of M2 and M3 muscarinic acetylcholine receptor subtypes in activation of bladder afferent pathways in spinal cord injured rats

OBJECTIVE

To evaluate the role of M2 and M3 muscarinic acetylcholine receptor (mAChR) subtypes in the activation of bladder afferent pathways in rats with chronic spinal cord injury (SCI).

METHODS

Adult female Sprague-Dawley rats were spinalized at the T9 level. Continuous cystometry was performed under awake conditions 2 or 4 weeks after SCI. The effects of intravesical administration of an mAChR agonist (oxotremorine-methiodide), a nonselective antagonist (atropine), an M2-selective antagonist (methoctramine), and an M3-selective antagonist (darifenacin) were examined. After cystometry, the bladder was removed and separated into the mucosa and detrusor, and the M2 and M3 mAChR mRNA expression in the mucosa was determined using real-time quantitative polymerase chain reaction.

RESULTS

At 2 and 4 weeks after SCI, intravesical administration of a nonselective mAChR agonist (25 μM oxotremorine-methiodide) increased the area under the curve of nonvoiding contractions, although the intercontraction interval of voiding contractions and maximal voiding pressure did not change. This effect was blocked by atropine and methoctramine (10 μM) but not by darifenacin (50 μM). However, mAChR antagonists alone (10-50 μM) had no effect on cystometric parameters. M2 mAChR mRNA expression was increased in the mucosa of SCI rats compared with that in normal rats.

CONCLUSION

Our results suggest that the M2 mAChR subtype plays an important role in bladder afferent activation that enhances detrusor overactivity in SCI rats. However, because mAChR antagonists alone did not affect any cystometric parameters, the muscarinic mechanism controlling bladder afferent activity might not be involved in the emergence of detrusor overactivity in SCI.

Differential roles of M2 and M3 muscarinic receptor subtypes in modulation of bladder afferent activity in rats

OBJECTIVES

To investigate the effects of various muscarinic acetylcholine receptor (mAChR) antagonists, including selective M2 and M3 mAChR antagonists, on bladder overactivity. It has been proposed that the urothelium modulates the activity of bladder afferent pathways. However, the differential roles of mAChR subtypes in local bladder afferent activation remain unclear.

METHODS

Cystometry was performed in urethane-anesthetized female rats. We examined the effects of intravesical administration of antimuscarinic agents (nonselective mAChR antagonists: atropine sulfate, tolterodine tartrate, and propiverine hydrochloride; M2-selective antagonists: dimethindene maleate and methoctramine hemihydrate; M3-selective antagonists: darifenacin hydrobromide and 4-DAMP) on bladder overactivity induced by oxotremorine-M (oxo-M; nonselective mAChR agonist).

RESULTS

Intravesical administration of oxo-M (200 microM) elicited bladder overactivity as evidenced by decreased intercontraction interval, bladder capacity, and pressure threshold. These effects were blocked by intravesical administration of nonselective or M2-selective antagonists (30-60 microM), whereas M3-selective antagonists (150 microM) did not suppress the overactivity. When instilled intravesically by itself, none of the antimuscarinic agents (nonselective, M2-selective or M3-selective antagonists) affected any cystometric parameters.

CONCLUSIONS

The M2 mAChR subtype plays an important role in the local cholinergic modulation of bladder afferent activity that contributes to bladder overactivity in normal rats. Therefore, it is expected that antimuscarinic agents that have antagonistic activity against M2 mAChR can be more beneficial for the treatment of patients with overactive bladder if enhanced acetylcholine mechanisms are involved in pathogenesis of overactive bladder.

Comparison between gentamycin and exon skipping treatments to restore ryanodine receptor subtype 2 functions in mdx mouse duodenum myocytes

In Duchenne muscular dystrophy, a stop-codon mutation in the dystrophin gene induces an impairment of skeletal and smooth muscles contraction. In duodenum from mdx mouse, the disease model, the decrease of contractility was linked with the decrease of calcium signals encoded by ryanodine receptor subtype 2. Aminoglycoside and antisense oligonucleotide strategies were investigated to restore calcium signalling in the mdx mouse. Mdx mice were treated by intraperitoneal injection of gentamycin or 2-O-methyl antisense ribonucleotide directed against exon 23 of dystrophin for 2 weeks. The efficiency of both therapeutic strategies was determined by the level of dystrophin protein expression. The physiological effects of both treatments on ryanodine receptor expression and function were followed by RT-PCR, western blot and calcium measurements. Fourteen days after injection of gentamycin or anti-dystrophin antisense, the expression of dystrophin was recovered in skeletal muscle from treated mdx mice. In duodenum cells, RT-PCR and western blot indicated that the expression of ryanodine receptor subtype 2 was similar in treated mice than in control mice in association with the recovery of caffeine-induced Ca(2+) response. No significant difference was observed in the ryanodine subtype 3-dependent spontaneous Ca(2+) oscillations in untreated and treated mice. Conclusions - these results may help to explain the efficiency of aminoglycoside and anti-dystrophin antisense treatments in smooth muscle. Both treatments could be an interesting therapeutic option to restore smooth muscle contraction in patients with Duchenne muscular dystrophy.

High spatial resolution studies of muscarinic neuroeffector junctions in mouse isolated vas deferens

It is acknowledged that neurotransmission in the mouse vas deferens is predominantly mediated by ATP and noradrenaline (NA) released from sympathetic nerves while cholinergic transmission in the rodent vas deferens is often overlooked despite early literature. Recently we have characterized a cholinergic component of neurogenic contraction of mouse isolated vas deferens. In the present paper, by confocal imaging of Ca(2+) dynamics we detected acetylcholine (ACh) action at muscarinic cholinergic neuroeffector junctions at high-resolution. Experiments were carried out in the presence of prazosin (100 nM) and alpha,beta methylene ATP (alpha,beta-MeATP) (1 microM) to inhibit responses to NA and ATP respectively. Exogenous ACh (10 microM) elicited Ca(2+) transients, an effect blocked by the muscarinic receptor antagonist, cyclopentolate (1 microM). Ca(2+) transients were evoked by electrical stimulation of intrinsic nerves in the presence of the cholinesterase inhibitor neostigmine (10 microM). Stimulation produced a marked increase in the frequency and number of Ca(2+) transients. Cyclopentolate reduced the frequency of occurrence of spontaneous and evoked events to control levels. The alpha(2)-adrenoceptor antagonist yohimbine (300 nM) did not affect the spontaneous Ca(2+) transients, but increased the frequency of occurrence of evoked transients, an effect inhibited by cyclopentolate. The postjunctional effects of neuronally-released ACh are limited by the action of cholinesterase. Release of ACh appears to be tonically inhibited by NA released from sympathetic nerve terminals through action at prejunctional alpha(2)-adrenoceptors. Tetrodotoxin (TTX, 300 nM) abolished the nerve-evoked Ca(2+) events, with no effect on Ca(2+) transients elicited by exogenous ACh. In conclusion, the presence of spontaneous and evoked cholinergic Ca(2+) transients in smooth muscle cells of the mouse isolated vas deferens has been revealed. These events are mediated by ACh acting at M(3) muscarinic receptors. This action stands in marked contrast to the lack of effect of neuronally-released NA on smooth muscle Ca(2+) dynamics in this tissue.

The decrease of expression of ryanodine receptor sub-type 2 is reversed by gentamycin sulphate in vascular myocytes from mdx mice

The mdx mouse, a model of the human Duchenne muscular dystrophy, displays impaired contractile function in skeletal, cardiac and smooth muscles. We explored the possibility that ryanodine receptor (RYR) expression could be altered in vascular muscle. The three RYR sub-types were expressed in portal vein myocytes. As observed through mRNA and protein levels, RYR2 expression was strongly decreased in mdx myocytes, whereas RYR3 and RYR1 expression were unaltered. The use of antisense oligonucleotide directed against RYR sub-types indicated that caffeine-induced Ca²⁺ response and Ca²⁺ spark frequency depended on RYR2 and RYR1. In mdx mice, caffeine-induced Ca²⁺ responses were decreased in both amplitude and maximal rate of rise, and the frequency of Ca²⁺ sparks was also strongly decreased. The gentamycin treatment was able to increase both the expression of RYR2 and the caffeine-induced Ca²⁺ response to the same level as that observed in wild-type mice. Taken together, these results confirm that both RYR1 and RYR2 are required for vascular Ca²⁺ signalling and indicate that inhibition of RYR2 expression may account for the decreased Ca²⁺ release from the SR in mdx vascular myocytes. Finally, we suggest that gentamycin can restore the Ca²⁺ signalling in smooth muscle from mdx mice by increasing RYR2 and dystrophin expression. These results may help explain the reduced efficacy of contraction in vascular myocytes of mdx mice and Duchenne muscular dystrophy–afflicted patients. Gentamycin treatment could be a good therapeutic tool to restore the vascular function.

Acetylcholine evokes an InsP3R1-dependent transient Ca2+ signal in rat duodenum myocytes

In smooth muscle myocytes, agonist-activated release of calcium ions (Ca2+) stored in the sarcoplasmic reticulum (SR) occurs via different but overlapping transduction pathways. Hence, to fully study how SR Ca2+ channels are activated, the simultaneous activation of different Ca2+ signals should be separated. In rat duodenum myocytes, we have previously characterized that acetylcholine (ACh) induces Ca2+ oscillations by binding to its M2 muscarinic receptor and activating the ryanodine receptor subtype 2. Here, we show that ACh simultaneously evokes a Ca2+ signal dependent on activation of inositol 1,4,5-trisphosphate (InsP3) receptor subtype 1. A pharmacologic approach, the use of antisense oligonucleotides directed against InsP3R1, and the expression of a specific biosensor derived from green-fluorescent protein coupled to the pleckstrin homology domain of phospholipase C, suggested that the InsP3R1-dependent Ca2+ signal is transient and due to a transient synthesis of InsP3 via M3 muscarinic receptor. Moreover, we suggest that both M2 and M3 signalling pathways are modulating phosphatidylinositol 4,5-bisphosphate and InsP3 concentration, thus describing closely interacting pathways activated by ACh in duodenum myocytes.

Formation and function of ceramide-enriched membrane platforms with CD38 during M1-receptor stimulation in bovine coronary arterial myocytes

CD38 contains an ADP ribosylcyclase domain that mediates intracellular Ca(2+) signaling by the production of cyclic ADP-ribose (cADPR), but the mechanisms by which the agonists activate this enzyme remain unclear. The present study tested a hypothesis that a special lipid-raft (LR) form, ceramide-enriched lipid platform, contributes to CD38 activation to produce cADPR in response to muscarinic type 1 (M(1)) receptor stimulation in bovine coronary arterial myocytes (CAMs). By confocal microscopic analysis, oxotremorine (Oxo), an M(1) receptor agonist, was found to increase LR clustering on the membrane with the formation of a complex of CD38 and LR components such as GM(1), acid sphingomyelinase (ASMase), and ceramide, a typical ceramide-enriched macrodomain. At 80 microM, Oxo increased LR clustering by 78.8%, which was abolished by LR disruptors, methyl-beta-cyclodextrin (MCD), or filipin. With the use of a fluorescence resonance energy transfer (FRET) technique, 15.5+/-1.9% energy transfer rate (vs. 5.3+/-0.9% of control) between CD38 and LR component, ganglioside M(1) was detected, further confirming the proximity of both molecules. In the presence of MCD or filipin, there were no FRET signals detected. In floated detergent-resistant membrane fractions, CD38 significantly increased in LR fractions of CAMs treated by Oxo. Moreover, MCD or filipin attenuated Oxo-induced production of cADPR via CD38. Functionally, Oxo-induced intracellular Ca(2+) release and coronary artery constriction via cADPR were also blocked by LR disruption or ASMase inhibition. These results provide the first evidence that the formation of ceramide-enriched lipid macrodomains is crucial for Oxo-induced activation of CD38 to produce cADPR in CAMs, and these lipid macrodomains mediate transmembrane signaling of M(1) receptor activation to produce second messenger cADPR.

Heterogeneity of muscarinic receptor-mediated Ca2+ responses in cultured urothelial cells from rat

Muscarinic receptors (mAChRs) have been identified in the urothelium, a tissue that may be involved in bladder sensory mechanisms. This study investigates the expression and function of mAChRs using cultured urothelial cells from the rat. RT-PCR established the expression of all five mAChR subtypes. Muscarinic agonists acetylcholine (ACh; 10 microM), muscarine (Musc; 20 microM), and oxotremorine methiodide (OxoM; 0.001-20 microM) elicited transient repeatable increases in the intracellular calcium concentration ([Ca(2+)](i)) in approximately 50% of cells. These effects were blocked by the mAChR antagonist atropine methyl nitrate (10 microM). The sources of [Ca(2+)](i) changes included influx from external milieu in 63% of cells and influx from external milieu plus release from internal stores in 27% of cells. The use of specific agonists and antagonists (10 microM M(1) agonist McN-A-343; 10 microM M(2), M(3) antagonists AF-DX 116, 4-DAMP) revealed that M(1), M(2), M(3) subtypes were involved in [Ca(2+)](i) changes. The PLC inhibitor U-73122 (10 microM) abolished OxoM-elicited Ca(2+) responses in the presence of the M(2) antagonist AF-DX 116, suggesting that M(1), M(3), or M(5) mediates [Ca(2+)](i) increases via PLC pathway. ACh (0.1 microM), Musc (10 microM), oxotremorine sesquifumarate (20 microM), and McN-A-343 (1 muM) acting on M(1), M(2), and M(3) mAChR subtypes stimulated ATP release from cultured urothelial cells. In summary, cultured urothelial cells express functional M(1), M(2), and M(3) mAChR subtypes whose activation results in ATP release, possibly through mechanisms involving [Ca(2+)](i) changes.

Oscillatory membrane currents paradoxically induced via NO-activated pathways in detrusor cells

Oscillatory inward membrane currents (I(oscil-in)) reflecting intracellular Ca(2+) ([Ca(2+)](i)) activity in detrusor cells, are thought to play an important role in producing tonic bladder contractions during micturition. The present patch clamp study revealed a new activation mechanism: sodium nitroprusside (SNP), a nitric oxide (NO) donor induced I(oscil-in) in a subpopulation of detrusor cells. The inhibitory effect of niflumic acid on SNP-induced I(oscil-in) suggests that Ca(2+)-activated Cl(-) channels are responsible for this current. In addition, SNP-induced I(oscil-in) required the cooperation of Ca(2+) influx through SK&F96365-sensitive channels and intracellular Ca(2+) release channels sensitive to ryanodine but insensitive to xestospongin C (XeC). This is also true for muscarinic agonist (carbachol: CCh)-induced I(oscil-in). However, 1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a guanylyl cyclase inhibitor, suppressed SNP-induced I(oscil-in) but not CCh-induced I(oscil-in). The results suggest that a subpopulation of detrusor cells employ the NO/cGMP cascade to potentiate bladder contraction. Mechanisms underlying NO-induced I(oscil-in) are likely to contribute not only to the physiology but also to the pathophysiology of the lower urinary tract.

Full length ryanodine receptor subtype 3 encodes spontaneous calcium oscillations in native duodenal smooth muscle cells

Two isoforms of the ryanodine receptor subtype 3 (RYR3) have been described in smooth muscle. The RYR3 short isoform (RYR3S) negatively regulates the calcium-induced calcium release mechanism encoded by the RYR2, whereas the role of the full length isoform of RYR3 (RYR3L) was still unclear. Here, we describe RYR-dependent spontaneous Ca(2+) oscillations measured in 10% of native duodenum myocytes. We investigated the role of RYR3 isoforms in these spontaneous Ca(2+) signals. Inhibition of RYR3S expression by antisense oligonucleotides revealed that both RYR2 and RYR3L were able to propagate spontaneous Ca(2+) waves that were distinguishable by frequency analysis. When RYR3L expression was inhibited, the spontaneous Ca(2+) oscillations were never observed, indicating that RYR3S inhibited the function of RYR2. RYR2 expression inhibition led to Ca(2+) oscillations identical to those observed in control cells suggesting that RYR3S did not functionally interact with RYR3L. The presence and frequency of RYR3L-dependent Ca(2+) oscillations were dependent on sarcoplasmic reticulum Ca(2+) content as revealed by long-term changes of the extracellular Ca(2+) concentration. Our study shows that, in native duodenal myocytes, the spontaneous Ca(2+) waves are encoded by the RYR3L alone, which activity is regulated by sarcoplasmic reticulum Ca(2+) loading.

Calcium signaling in airway smooth muscle

Contractility of airway smooth muscle requires elevation of intracellular calcium concentration. Under resting conditions, airway smooth muscle cells maintain a relatively low intracellular calcium concentration, and activation of the surface receptors by contractile agonists results in an elevation of intracellular calcium, culminating in contraction of the cell. The pattern of elevation of intracellular calcium brought about by agonists is a dynamic process and involves the coordinated activities of ion channels located in the plasma membrane and the sarcoplasmic reticulum. Among the signaling molecules involved in this dynamic calcium regulation in airway smooth muscle cells are inositol 1,4,5-trisphosphate and cyclic ADP-ribose, which mobilize calcium from the sarcoplasmic reticulum by acting via the inositol 1,4,5-trisphosphate and ryanodine receptors, respectively. In addition, calcium influx from the extracellular space is critical for the repletion of the intracellular calcium stores during activation of the cells by agonists. Calcium influx can occur via voltage- and receptor-gated channels in the plasma membrane, as well as by influx that is triggered by depletion of the intracellular stores (i.e., store-operated calcium entry mechanism). Transient receptor potential proteins appear to mediate the calcium influx via receptor- and store-operated channels. Recent studies have shown that proinflammatory cytokines regulate the expression and activity of the pathways involved in intracellular calcium regulation, thereby contributing to airway smooth muscle cell hyperresponsiveness. In this review, we will discuss the specific roles of cyclic ADP-ribose/ryanodine receptor channels and transient receptor potential channels in the regulation of intracellular calcium in airway smooth muscle cells.

Acetylcholine-induced Ca2+ oscillations are modulated by a Ca2+ regulation of InsP3R2 in rat portal vein myocytes

Oscillations of cytosolic Ca2+ levels are believed to have important roles in various metabolic and signalling processes in many cell types. Previously, we have demonstrated that acetylcholine (ACh) evokes Ca2+ oscillations in vascular myocytes expressing InsP3R1 and InsP3R2, whereas transient responses are activated in vascular myocytes expressing InsP3R1 alone. The molecular mechanisms underlying oscillations remain to be described in these native smooth muscle cells. Two major hypotheses are proposed to explain this crucial signalling activity: (1) Ca2+ oscillations are activated by InsP3 oscillations; and (2) Ca2+ oscillations depend on the regulation of the InsP3R by both InsP3 and Ca2+. In the present study, we used a fluorescent InsP3 biosensor and revealed that ACh induced a transient InsP3 production in all myocytes. Moreover, steady concentrations of 3F-InsP3, a poorly hydrolysable analogue of InsP3, and pharmacological activation of PLC evoked Ca2+ oscillations. Increasing cytosolic Ca2+ inhibited the ACh-induced calcium oscillations but not the transient responses and strongly reduced the 3F-InsP3-evoked Ca2+ response in oscillating cells but not in non-oscillating cells. These results suggest that, in native vascular myocytes, ACh-induced InsP3 production is transient and Ca2+ oscillations depend on a Ca2+ modulation of InsP3R2.

RyR1-specific requirement for depolarization-induced Ca2+ sparks in urinary bladder smooth muscle

Ryanodine receptor subtype 1 (RyR1) has been primarily characterized in skeletal muscle but several studies have revealed its expression in smooth muscle. Here, we used Ryr1-null mice to investigate the role of this isoform in Ca2+ signaling in urinary bladder smooth muscle. We show that RyR1 is required for depolarization-induced Ca2+ sparks, whereas RyR2 and RyR3 are sufficient for spontaneous or caffeine-induced Ca2+ sparks. Immunostaining revealed specific subcellular localization of RyR1 in the superficial sarcoplasmic reticulum; by contrast, RyR2 and RyR3 are mainly expressed in the deep sarcoplasmic reticulum. Paradoxically, lack of depolarization-induced Ca2+ sparks in Ryr1–/– myocytes was accompanied by an increased number of cells displaying spontaneous or depolarization-induced Ca2+ waves. Investigation of protein expression showed that FK506-binding protein (FKBP) 12 and FKBP12.6 (both of which are RyR-associated proteins) are downregulated in Ryr1–/– myocytes, whereas expression of RyR2 and RyR3 are unchanged. Moreover, treatment with rapamycin, which uncouples FKBPs from RyR, led to an increase of RyR-dependent Ca2+ signaling in wild-type urinary bladder myocytes but not in Ryr1–/– myocytes.

In conclusion, although decreased amounts of FKBP increase Ca2+ signals in Ryr1–/– urinary bladder myocytes the depolarization-induced Ca2+ sparks are specifically lost, demonstrating that RyR1 is required for depolarization-induced Ca2+ sparks and suggesting that the intracellular localization of RyR1 fine-tunes Ca2+ signals in smooth muscle.

Role of RYR3 splice variants in calcium signaling in mouse nonpregnant and pregnant myometrium

Alternative splicing of ryanodine receptor subtype 3 (RYR3) may generate a short isoform (RYR3S) without channel function and a functional full-length isoform (RYR3L). The RYR3S isoform has been shown to negatively regulate the native RYR2 subtype in smooth muscle cells as well as the RYR3L isoform when both isoforms were coexpressed in HEK-293 cells. Mouse myometrium expresses only the RYR3 subtype, but the role of RYR3 isoforms obtained by alternative splicing and their activation by cADP-ribose during pregnancy have never been investigated. Here, we show that both RYR3S and RYR3L isoforms are differentially expressed in nonpregnant and pregnant mouse myometrium. The use of antisense oligonucleotides directed against each isoform indicated that only RYR3L was activated by caffeine and cADP-ribose in nonpregnant myometrium. These RYR3L-mediated Ca(2+) releases were negatively regulated by RYR3S expression. At the end of pregnancy, the relative expression of RYR3L versus RYR3S and its ability to respond to cADP-ribose were increased. Therefore, our results suggest that physiological regulation of RYR3 alternative splicing may play an essential role at the end of pregnancy.

Ca2+ microdomains in smooth muscle

In smooth muscle, Ca(2+) controls diverse activities including cell division, contraction and cell death. Of particular significance in enabling Ca(2+) to perform these multiple functions is the cell's ability to localize Ca(2+) signals to certain regions by creating high local concentrations of Ca(2+) (microdomains), which differ from the cytoplasmic average. Microdomains arise from Ca(2+) influx across the plasma membrane or release from the sarcoplasmic reticulum (SR) Ca(2+) store. A single Ca(2+) channel can create a microdomain of several micromolar near (approximately 200 nm) the channel. This concentration declines quickly with peak rates of several thousand micromolar per second when influx ends. The high [Ca(2+)] and the rapid rates of decline target Ca(2+) signals to effectors in the microdomain with rapid kinetics and enable the selective activation of cellular processes. Several elements within the cell combine to enable microdomains to develop. These include the brief open time of ion channels, localization of Ca(2+) by buffering, the clustering of ion channels to certain regions of the cell and the presence of membrane barriers, which restrict the free diffusion of Ca(2+). In this review, the generation of microdomains arising from Ca(2+) influx across the plasma membrane and the release of the ion from the SR Ca(2+) store will be discussed and the contribution of mitochondria and the Golgi apparatus as well as endogenous modulators (e.g. cADPR and channel binding proteins) will be considered.

Excitation-contraction coupling in airway smooth muscle

Excitation-contraction (EC) coupling in striated muscles is mediated by the cardiac or skeletal muscle isoform of voltage-dependent L-type Ca(2+) channel (Ca(v)1.2 and Ca(v)1.1, respectively) that senses a depolarization of the cell membrane, and in response, activates its corresponding isoform of intracellular Ca(2+) release channel/ryanodine receptor (RyR) to release stored Ca(2+), thereby initiating muscle contraction. Specifically, in cardiac muscle following cell membrane depolarization, Ca(v)1.2 activates cardiac RyR (RyR2) through an influx of extracellular Ca(2+). In contrast, in skeletal muscle, Ca(v)1.1 activates skeletal muscle RyR (RyR1) through a direct physical coupling that negates the need for extracellular Ca(2+). Since airway smooth muscle (ASM) expresses Ca(v)1.2 and all three RyR isoforms, we examined whether a cardiac muscle type of EC coupling also mediates contraction in this tissue. We found that the sustained contractions of rat ASM preparations induced by depolarization with KCl were indeed partially reversed ( approximately 40%) by 200 mum ryanodine, thus indicating a functional coupling of L-type channels and RyRs in ASM. However, KCl still caused transient ASM contractions and stored Ca(2+) release in cultured ASM cells without extracellular Ca(2+). Further analyses of rat ASM indicated that this tissue expresses as many as four L-type channel isoforms, including Ca(v)1.1. Moreover, Ca(v)1.1 and RyR1 in rat ASM cells have a similar distribution near the cell membrane in rat ASM cells and thus may be directly coupled as in skeletal muscle. Collectively, our data implicate that EC-coupling mechanisms in striated muscles may also broadly transduce diverse smooth muscle functions.

Modulation of calcium signalling by dominant negative splice variant of ryanodine receptor subtype 3 in native smooth muscle cells

The ryanodine receptor subtype 3 (RYR3) is expressed ubiquitously but its physiological function varies from cell to cell. Here, we investigated the role of a dominant negative RYR3 isoform in Ca2+ signalling in native smooth muscle cells. We used intranuclear injection of antisense oligonucleotides to specifically inhibit endogenous RYR3 isoform expression. In mouse duodenum myocytes expressing RYR2 subtype and both spliced and non-spliced RYR3 isoforms, RYR2 and non-spliced RYR3 were activated by caffeine whereas the spliced RYR3 was not. Only RYR2 was responsible for the Ca2+-induced Ca2+ release mechanism that amplified Ca2+ influx- or inositol 1,4,5-trisphosphate-induced Ca2+ signals. However, the spliced RYR3 negatively regulated RYR2 leading to the decrease of amplitude and upstroke velocity of Ca2+ signals. Immunostaining in injected cells showed that the spliced RYR3 was principally expressed near the plasma membrane whilst the non-spliced isoform was revealed around the nucleus. This study shows for the first time that the short isoform of RYR3 controls Ca2+ release through RYR2 in native smooth muscle cells.

Acetylcholine stimulates cyclic ADP-ribose formation via M1 muscarinic receptors in rat superior cervical ganglion

The role of cyclic ADP-ribose (cADPR) as the downstream signal of neuronal muscarinic acetylcholine receptors (mAChRs) and the enzyme responsible for its synthesis, ADP-ribosyl cyclase, were examined in the rat superior cervical ganglion (SCG). Application of acetylcholine or other mAChR agonists increased the ADP-ribosyl cyclase activity by about 250-300% in crude membrane fractions from the SCG of 14-day-old rats. This effect was inhibited by atropine or by the M1-mAChR antagonist, pirenzepine, and was mimicked by GTP. These results indicate that the M1 mAChRs couple to the membrane-bound form of ADP-ribosyl cyclase and suggest that cADPR is a second messenger of M1 mAChR signaling in nervous tissue.