Role of Ins(1,4,5)P3, cADP-ribose and nicotinic acid-adenine dinucleotide phosphate in Ca2+ signalling in mouse submandibular acinar cells

VIP and muscarinic synergistic mucin secretion by salivary mucous cells is mediated by enhanced PKC activity via VIP-induced release of an intracellular Ca2+ pool

Mucin secretion by salivary mucous glands is mediated predominantly by parasympathetic acetylcholine activation of cholinergic muscarinic receptors via increased intracellular free calcium ([Ca²⁺]_i) and activation of conventional protein kinase C isozymes (cPKC). However, the parasympathetic co-neurotransmitter, vasoactive intestinal peptide (VIP), also initiates secretion, but to a lesser extent. In the present study, cross talk between VIP- and muscarinic-induced mucin secretion was investigated using isolated rat sublingual tubuloacini. VIP-induced secretion is mediated by cAMP-activated protein kinase A (PKA), independently of increased [Ca²⁺]_i. Synergistic secretion between VIP and the muscarinic agonist, carbachol, was demonstrated but only with submaximal carbachol. Carbachol has no effect on cAMP ± VIP. Instead, PKA activated by VIP releases Ca²⁺ from an intracellular pool maintained by the sarco/endoplasmic reticulum Ca²⁺-ATPase pump. Calcium release was independent of phospholipase C activity. The resultant sustained [Ca²⁺]_i increase is additive to submaximal, but not maximal carbachol-induced [Ca²⁺]_i. Synergistic mucin secretion was mimicked by VIP plus either phorbol 12-myristate 13-acetate or 0.01 μM thapsigargin, and blocked by the PKC inhibitor, Gö6976. VIP-induced Ca²⁺ release also promoted store-operated Ca²⁺ entry. Synergism is therefore driven by VIP-mediated [Ca²⁺]_i augmenting cPKC activity to enhance muscarinic mucin secretion. Additional data suggest ryanodine receptors control VIP/PKA-mediated Ca²⁺ release from a Ca²⁺ pool also responsive to maximal carbachol. A working model of muscarinic and VIP control of mucous cell exocrine secretion is presented. Results are discussed in relation to synergistic mechanisms in other secretory cells, and the physiological and therapeutic significance of VIP/muscarinic synergism controlling salivary mucous cell exocrine secretion.

Small Molecule Antagonists of NAADP-Induced Ca2+ Release in T-Lymphocytes Suggest Potential Therapeutic Agents for Autoimmune Disease

Nicotinic acid adenine dinucleotide phosphate (NAADP) is the most potent Ca²⁺-releasing second messenger known to date, but the precise NAADP/Ca²⁺ signalling mechanisms are still controversial. We report the synthesis of small-molecule inhibitors of NAADP-induced Ca²⁺ release based upon the nicotinic acid motif. Alkylation of nicotinic acid with a series of bromoacetamides generated a diverse compound library. However, many members were only weakly active or had poor physicochemical properties. Structural optimisation produced the best inhibitors that interact specifically with the NAADP/Ca²⁺ release mechanism, having no effect on Ca²⁺ mobilized by the other well-known second messengers D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃] or cyclic adenosine 5'-diphospho-ribose (cADPR). Lead compound (2) was an efficient antagonist of NAADP-evoked Ca²⁺ release in vitro in intact T lymphocytes and ameliorated clinical disease in vivo in a rat experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis. Compound (3) (also known as BZ194) was synthesized as its bromide salt, confirmed by crystallography, and was more membrane permeant than 2. The corresponding zwitterion (3a), was also prepared and studied by crystallography, but 3 had more desirable physicochemical properties. 3 Is potent in vitro and in vivo and has found widespread use as a tool to modulate NAADP effects in autoimmunity and cardiovascular applications. Taken together, data suggest that the NAADP/Ca²⁺ signalling mechanism may serve as a potential target for T cell- or cardiomyocyte-related diseases such as multiple sclerosis or arrhythmia. Further modification of these lead compounds may potentially result in drug candidates of clinical use.

cAMP-dependent recruitment of acidic organelles for Ca2+ signaling in the salivary gland

Autonomic neural activation of intracellular Ca²⁺ release in parotid acinar cells induces the secretion of the fluid and protein components of primary saliva critical for maintaining overall oral homeostasis. In the current study, we profiled the role of acidic organelles in shaping the Ca²⁺ signals of parotid acini using a variety of imaging and pharmacological approaches. Results demonstrate that zymogen granules predominate as an apically polarized population of acidic organelles that contributes to the initial Ca²⁺ release. Moreover, we provide evidence that indicates a role for the intracellular messenger NAADP in the release of Ca²⁺ from acidic organelles following elevation of cAMP. Our data are consistent with the "trigger" hypothesis where localized release of Ca²⁺ sensitizes canonical intracellular Ca²⁺ channels to enhance signals from the endoplasmic reticulum. Release from acidic stores may be important for initiating saliva secretion at low levels of stimulation and a potential therapeutic target to augment secretory activity in hypofunctioning salivary glands.

Generation of cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate by CD38 for Ca2+ signaling in interleukin-8-treated lymphokine-activated killer cells

We have previously demonstrated that cyclic ADP-ribose (cADPR) is a calcium signaling messenger in interleukin 8 (IL-8)-induced lymphokine-activated killer (LAK) cells. In this study we examined the possibility that IL-8 activates CD38 to produce another messenger, nicotinic acid adenine dinucleotide phosphate (NAADP), in LAK cells, and we showed that IL-8 induced NAADP formation after cADPR production. These calcium signaling messengers were not produced when LAK cells prepared from CD38 knock-out mice were treated with IL-8, indicating that the synthesis of both NAADP and cADPR is catalyzed by CD38 in LAK cells. Application of cADPR to LAK cells induced NAADP production, whereas NAADP failed to increase intracellular cADPR levels, confirming that the production of cADPR precedes that of NAADP in IL-8-treated LAK cells. Moreover, NAADP increased intracellular Ca(2+) signaling as well as cell migration, which was completely blocked by bafilomycin A1, suggesting that NAADP is generated in lysosome-related organelles after cADPR production. IL-8 or exogenous cADPR, but not NAADP, increased intracellular cAMP levels. cGMP analog, 8-(4-chlorophenylthio)-guanosine 3',5'-cyclic monophosphate, increased both cADPR and NAADP production, whereas the cAMP analog, 8-(4-chlorophenylthio)-cAMP, increased only NAADP production, suggesting that cAMP is essential for IL-8-induced NAADP formation. Furthermore, activation of Rap1, a downstream molecule of Epac, was required for IL-8-induced NAADP formation in LAK cells. Taken together, our data suggest that IL-8-induced NAADP production is mediated by CD38 activation through the actions of cAMP/Epac/protein kinase A/Rap1 in LAK cells and that NAADP plays a key role in Ca(2+) signaling of IL-8-induced LAK cell migration.

Inducible nitric oxide synthase increases secretion from inflamed salivary glands

Objective. Salivary gland secretion is dependent on cholinergic stimulation via autonomic nerves and calcium signalling in acinar cells. Secretory dysfunction associated with SS may be partly caused by the damaging effects of increased glandular concentrations of nitric oxide (NO) derived from up-regulation of inducible NO synthase (iNOS) that accompanies glandular inflammation. The present study examines the effects of increased iNOS expression on salivary gland secretory function.

Methods. The inflammogen lipopolysaccharide (LPS) was introduced intraductally into rat submandibular glands, and glandular responsiveness to cholinergic stimulation was determined.

Results. LPS provoked a rapid, long-lasting inflammation, increasing gland weight (by almost 20%) and inflammatory cell infiltration at 3 and 24 h. Immunoblotting of glandular homogenates indicated that iNOS expression was increased ∼4-fold, and immunohistochemistry of frozen tissue sections showed increased iNOS expression in acinar cells. Salivary secretion from inflamed glands was significantly increased in response to low doses of methacholine and accompanied by increased acinar cell calcium signalling in vitro. Prior administration of the iNOS inhibitors, aminoguanidine or l-NIL [l-N6-(1-iminoethyl)-lysine dihydrochloride] abolished increased secretion and acinar cell calcium signalling.

Conclusions. Up-regulation of glandular iNOS expression can increase cholinergically evoked salivary secretion and appears to offset any secretory hypofunction linked with glandular inflammation. It seems unlikely that increased glandular levels of NO are responsible for the secretory hypofunction that accompanies SS.

A role for nitric oxide-mediated glandular hypofunction in a non-apoptotic model for Sjogren's syndrome

OBJECTIVE

To investigate a role for the inflammatory mediator, nitric oxide (NO) in SS, an autoimmune condition characterized by salivary and lacrimal gland hypofunction resulting from failure of acinar cells to secrete.

METHODS

FURA-2 microfluorimetry was used to measure agonist-evoked changes of [Ca(2+)](i) in isolated mouse and human salivary acinar cells following exposure to NO donors.

RESULTS

NO had a biphasic effect on salivary acinar function. Acute exposure to NO (2 min) caused a cyclic guanosine monophosphate (GMP)-dependent, 1-H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-sensitive increase in the Ca(2+) signal elicited in response to acetylcholine (ACh) stimulation, consistent with stimulation of ryanodine receptors by cyclic adenosine diphosphate ribose. Prolonged exposure to NO (>40 min) significantly reduced the ACh-evoked Ca(2+) signal by a mechanism independent of cyclic GMP. We found no differences between the responses of human and mouse acinar cells.

CONCLUSION

Our data show that chronic exposure to NO, which is known to be elevated in SS, could have a role in salivary gland hypofunction. We note a similarity in the response to stimulation of salivary acinar exposed to NO and that which we have previously reported in salivary acinar cells isolated from patients with SS. We speculate that NO-mediated nitrosylation of one or more elements of the signal transduction pathway could underlie down-regulation of salivary function in SS.

Synergistic regulation of endogenous TRPM2 channels by adenine dinucleotides in primary human neutrophils

The Ca(2+)-permeable TRPM2 channel is a dual function protein that is activated by intracellular ADPR through its enzymatic pyrophosphatase domain with Ca(2+) acting as a co-factor. Other TRPM2 regulators include cADPR, NAADP and H(2)O(2), which synergize with ADPR to potentiate TRPM2 activation. Although TRPM2 has been thoroughly characterized in overexpression or cell-line systems, little is known about the features of TRPM2 in primary cells. We here characterize the regulation of TRPM2 activation in human neutrophils and report that ADPR activates TRPM2 with an effective half-maximal concentration (EC(50)) of 1microM. Potentiation by Ca(2+) is dose-dependent with an EC(50) of 300nM. Both cADPR and NAADP activate TRPM2, albeit with lower efficacy than in the presence of subthreshold levels of ADPR (100nM), which significantly shifts the EC(50) for cADPR from 44 to 3muM and for NAADP from 95 to 1microM. TRPM2 activation by ADPR can be suppressed by AMP with an IC(50) of 10microM and cADPR-induced activation can be blocked by 8-Bromo-cADPR. We further show that 100microM H(2)O(2) enables subthreshold concentrations of ADPR (100nM) to activate TRPM2. We conclude that agonistic and antagonistic characteristics of TRPM2 as seen in overexpression systems are largely compatible with the functional properties of TRPM2 currents measured in human neutrophils, but the potencies of agonists in primary cells are significantly higher.

Rat salivary gland ligation causes reversible secretory hypofunction

AIM

To determine the influence of inflammation on salivary secretion. Secretion by salivary glands involves interactions between nerves, blood vessels and salivary cells. The present study investigated the effects of inflammation on rat submandibular gland function following acute ductal obstruction.

METHODS

Under recovery anaesthesia a metal clip was placed on the main duct of the submandibular gland. After 24 h salivary secretion was evoked by nerve and methacholine stimulation. For recovery experiments the clip was removed after 24 h and the animal left to recover for 3 days when salivary function was again assessed.

RESULTS

By 24 h of obstruction an inflammatory infiltrate had developed within the obstructed gland and stimulated salivary flows were just 20% of the normal secretion, whilst protein secretion and ion reabsorption were also severely impaired. If ductal obstruction was removed after 24 h the salivary function returned to normal after 3 days of recovery. In vitro analysis of cells from 24-h ligated glands revealed normal changes in intracellular calcium (the main secondary messenger involved in fluid secretion) in response to methacholine stimulation. Protein secretion from isolated cells indicated some changes in particular to methacholine-induced protein secretion although a significant protein secretion was still seen in response to isoprenaline - the main stimulus for protein secretion.

CONCLUSION

This report demonstrates reversible salivary inhibition associated with an inflammatory infiltrate within the salivary gland.

Regulation of salivary gland function by autonomic nerves

Oral homeostasis is dependent upon saliva and its content of proteins. Reflex salivary flow occurs at a low 'resting' rate and for short periods of the day more intense taste or chewing stimuli evoke up to ten fold increases in salivation. The secretion of salivary fluid and proteins is controlled by autonomic nerves. All salivary glands are supplied by cholinergic parasympathetic nerves which release acetylcholine that binds to M3 and (to a lesser extent) M1 muscarinic receptors, evoking the secretion of saliva by acinar cells in the endpieces of the salivary gland ductal tree. Most salivary glands also receive a variable innervation from sympathetic nerves which released noradrenaline from which tends to evoke greater release of stored proteins, mostly from acinar cells but also ductal cells. There is some 'cross-talk' between the calcium and cyclic AMP intracellular pathways coupling autonomic stimulation to secretion and salivary protein secretion is augmented during combined stimulation. Other non-adrenergic, non-cholinergic neuropeptides released from autonomic nerves evoke salivary gland secretion and parasympathetically derived vasointestinal peptide, acting through endothelial cell derived nitric oxide, plays a role in the reflex vasodilatation that accompanies secretion. Neuronal type, calcium-activated, soluble nitric oxide within salivary cells appears to play a role in mediating salivary protein secretion in response to autonomimetics. Fluid secretion by salivary glands involves aquaporin 5 and the extent to which the expression of aquaporin 5 on apical acinar cell membranes is upregulated by cholinomimetics remains uncertain. Extended periods of autonomic denervation, liquid diet feeding (reduced reflex stimulation) or duct ligation cause salivary gland atrophy. The latter two are reversible, demonstrating that glands can regenerate provided that the autonomic innervation remains intact. The mechanisms by which nerves integrate with salivary cells during regeneration or during salivary gland development remain to be elucidated.

Mechanism of butyrate-induced hyperpolarization of cultured rat myenteric neurones

Short-chain fatty acids produced by the bacterial fermentation of carbohydrates are present in high concentrations within the colonic lumen and have been shown to alter the excitability of enteric neurones. The present study was designed to investigate the mechanisms of butyrate-induced changes in membrane potential of myenteric neurones. Myenteric neurones from 4-10-day-old rats were isolated from the small and large intestine by an enzymatic digestion with collagenase and kept in culture. Membrane potential was measured with the whole-cell patch-clamp technique and the intracellular Ca2+ concentration was measured with the fura-2 method. The short-chain fatty acid butyrate (10-100 mmol L(-1)) induced a reversible and concentration-dependent hyperpolarization of the membrane with a half-maximal effect at 30 mmol L(-1). The hyperpolarization evoked by butyrate (50 mmol L(-1)) was strongly inhibited by charybdotoxin (10(-7) mol L(-1)), a specific blocker of Ca2+ -dependent K+ channels. The butyrate-induced hyperpolarization was resistant against blockade of phospholipase C by U-73122 (10(-5) mol L(-1)), and resistant against inclusion of heparin (6 x 10(-6) mol L(-1)), an inositol-1,4,5-trisphosphate receptor antagonist, in the patch-pipette. In contrast, ruthenium red (3 x 10(-5) mol L(-1)), an inhibitor of ryanodine receptors, significantly reduced both the hyperpolarization of the membrane as well as the increase in the intracellular Ca2+ concentration evoked by butyrate. Even in neurones permeabilized with saponin (10 mg L(-1)), butyrate was able to stimulate a release of stored intracellular Ca2+ suggesting a direct action of the short-chain fatty acid at the stores without mediation of a soluble intracellular second messenger.

Coordination of Ca2+ signalling by NAADP

Nicotinic acid adenine dinucleotide phosphate (NAADP) mobilizes intracellular Ca2+ stores in several cell types. Ample evidence suggests that NAADP activates intracellular Ca2+ channels distinct from those that are sensitive to inositol trisphosphate and ryanodine/cyclic ADP-ribose. Recent studies in intact cells have demonstrated functional coupling ('channel chatter') between Ca2+ release pathways mediated by NAADP, inositol trisphosphate and cyclic ADP-ribose. Thus, NAADP is probably an important determinant in shaping cytosolic Ca2+ signals.

NAADP receptors are present and functional in the heart

Alongside the well-studied inositol 1,4,5 trisphosphate and ryanodine receptors, evidence is gathering that a new intracellular release mechanism, gated by the pyridine nucleotide nicotinic acid adenine dinucleotide phosphate (NAADP), is present in numerous organisms, ranging from plant to mammalian cells (reviewed in [1]). Most cells have been shown to express at least two Ca(2+)-release mechanisms controlled by different messengers, and this can lead to redundancy, convergence, or divergence of responses. One exception appears to be muscle and heart contractile tissues. Here, it is thought that the dominant intracellular channel is the ryanodine receptor, while IP(3) receptors are poorly expressed and their role appears to be negligible. We now report that NAADP receptors are functional and abundant in cardiac microsomes. NAADP binds specifically and with high affinity (130 pM and 4 nM) to two sites on cardiac microsomes and releases Ca(2+) with an apparent EC(50) of 323 +/- 14 nM. Furthermore, binding experiments show that this receptor displays both positive and negative cooperativity, a peculiarity unique among intracellular Ca(2+) channels. Therefore, we show that the heart possesses multiple mechanisms to increase the complexity of Ca(2+) signaling and that NAADP may be integral in the functioning of this organ.

Acetylcholine-evoked calcium mobilization and ion channel activation in human labial gland acinar cells from patients with primary Sjögren's syndrome

Recent evidence has indicated that the salivary gland dysfunction associated with Sjögren's syndrome (SjS) is not necessarily due to immune-mediated destruction of acinar tissue. SjS sufferers may possess substantial reserves of acinar tissue but nevertheless be incapable of maintaining salivary flow rates in the normal range. We have investigated the ability of isolated labial gland acinar cells from SjS patients to fluid secrete by measuring agonist-evoked changes in intracellular Ca(2+) ([Ca(2+)](i)) using fura-2 microfluorimetry and activation of K(+) and Cl(-) channels using the patch-clamp whole cell technique. We can confirm that stimulation with a super-maximal dose of acetylcholine (ACh) increased [Ca(2+)]i equally in both control acinar cells and those derived from SjS patients. However, at submaximal concentrations, the dose-response curve for ACh was shifted to the right by approximately one order of magnitude in acinar cells from SjS patients compared to control acinar cells. Patch-clamp measurements consistent with the presence of Ca(2+)-activated K(+) and Cl(-) conductances were obtained from both control acinar cells and those obtained from SjS patients. Dose-dependent activation of the ion channels by acetylcholine was also right-shifted in acinar cells from SjS patients compared to control cells. Our data show that labial gland acinar cells from SjS patients were capable of responding to agonist stimulation by mobilizing [Ca(2+)](i) and activating K(+) and Cl(-) channels consistent with the requirements of fluid secretion. However, the persistent loss of sensitivity to ACh observed in from SjS patients may account for the lack of saliva production observed in these patients in vivo.