Evidence for the involvement of cAMP-GEF (Epac) pathway in amylase release from the rat parotid gland

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.

N 6-modified cAMP derivatives that activate protein kinase A also act as full agonists of murine HCN2 channels

cAMP acts as a second messenger in many cellular processes. Three protein types mainly mediate cAMP-induced effects: PKA, exchange protein directly activated by cAMP (Epac), and cyclic nucleotide-modulated channels (cyclic nucleotide-gated or hyperpolarization-activated and cyclic nucleotide-modulated (HCN) channels). Discrimination among these cAMP signaling pathways requires specific targeting of only one protein. Previously, cAMP modifications at position N ⁶ of the adenine ring (PKA) and position 2'-OH of the ribose (Epac) have been used to produce target-selective compounds. However, cyclic nucleotide-modulated ion channels were usually outside of the scope of these previous studies. These channels are widely distributed, so possible channel cross-activation by PKA- or Epac-selective agonists warrants serious consideration. Here we demonstrate the agonistic effects of three PKA-selective cAMP derivatives, N ⁶-phenyladenosine-3',5'-cyclic monophosphate (N ⁶-Phe-cAMP), N ⁶-benzyladenosine-3',5'-cyclic monophosphate (N ⁶-Bn-cAMP), and N ⁶-benzoyl-adenosine-3',5'-cyclic monophosphate (N ⁶-Bnz-cAMP), on murine HCN2 pacemaker channels. Electrophysiological characterization in Xenopus oocytes revealed that these derivatives differ in apparent affinities depending on the modification type but that their efficacy and effects on HCN2 activation kinetics are similar to those of cAMP. Docking experiments suggested a pivotal role of Arg-635 at the entrance of the binding pocket in HCN2, either causing stabilizing cation-π interactions with the aromatic ring in N ⁶-Phe-cAMP or N ⁶-Bn-cAMP or a steric clash with the aromatic ring in N ⁶-Bnz-cAMP. A reduced apparent affinity of N ⁶-Phe-cAMP toward the variants R635A and R635E strengthened that notion. We conclude that some PKA activators also effectively activate HCN2 channels. Hence, when studying PKA-mediated cAMP signaling with cAMP derivatives in a native environment, activation of HCN channels should be considered.

Analysis of Corticosterone and Testosterone Synthesis in Rat Salivary Gland Homogenates

Extra-adrenal steroid hormone production has been reported in several tissues, the biological role of which is interesting in terms of hormonal regulation of metabolism, growth, and behavior. In this report, we describe for the first time steroidogenesis in rat salivary glands. Enzyme activities associated with corticosterone and testosterone production were detected in rat salivary glands by LC-MS analysis. In tissue homogenates of rat salivary glands, progesterone was produced enzymatically in vitro from pregnenolone in the presence of NADPH and NADH. Deoxycorticosterone was produced from progesterone, corticosterone from deoxycorticosterone, and testosterone from androstenedione (but not pregnenolone from cholesterol) via enzymatic reactions using the same tissue homogenates. Immunoblotting analysis indicated the expression of 11β-hydroxylase (cytochrome P450 11β1; CYP11β1), which mediated the production of corticosterone from deoxycorticosterone. However, CYP family 11 subfamily A member 1 (CYP11A1)-mediated production of pregnenolone from cholesterol was not detected in the salivary glands by immunoblotting using a specific antibody. These results indicate that corticosterone and testosterone are produced from pregnenolone in rat salivary glands. The initial substrate in salivary steroidogenesis and the roles of salivary corticosterone and testosterone are discussed.

Adipokinetic hormones control amylase activity in the cockroach (Periplaneta americana) gut

This study examined the biochemical characteristics of α-amylase and hormonal (adipokinetic hormone: AKH) stimulation of α-amylase activity in the cockroach (Periplaneta americana) midgut. We applied two AKHs in vivo and in vitro, then measured resultant amylase activity and gene expression, as well as the expression of AKH receptor (AKHR). The results revealed that optimal amylase activity is characterized by the following: pH: 5.7, temperature: 38.4 °C, K_m (Michaelis-Menten constant): 2.54 mg starch/mL, and V_max (maximum reaction velocity): 0.185 μmol maltose/mL/min. In vivo application of AKHs resulted in significant increase of amylase activity: by two-fold in the gastric caeca and 4-7 fold in the rest of the midgut. In vitro experiments supported results seen in vivo: a 24-h incubation with the hormones resulted in the increase of amylase activity by 1.4 times in the caeca and 4-9 times in the midgut. Further, gene expression analyses reveal that AKHR is expressed in both the caeca and the rest of the midgut, although expression levels in the former were 23 times higher than levels in the latter. A similar pattern was found for the amylase (AMY) gene. Hormonal treatment did not affect the expression of either gene. This study is the first to provide evidence indicating direct AKH stimulation of digestive enzyme activity in the insect midgut, supported by specific AKHR gene expression in this organ.

Fusion of lysosomes with secretory organelles leads to uncontrolled exocytosis in the lysosomal storage disease mucolipidosis type IV

Mutations in TRPML1 cause the lysosomal storage disease mucolipidosis type IV (MLIV). The role of TRPML1 in cell function and how the mutations cause the disease are not well understood. Most studies focus on the role of TRPML1 in constitutive membrane trafficking to and from the lysosomes. However, this cannot explain impaired neuromuscular and secretory cells' functions that mediate regulated exocytosis. Here, we analyzed several forms of regulated exocytosis in a mouse model of MLIV and, opposite to expectations, we found enhanced exocytosis in secretory glands due to enlargement of secretory granules in part due to fusion with lysosomes. Preliminary exploration of synaptic vesicle size, spontaneous mEPSCs, and glutamate secretion in neurons provided further evidence for enhanced exocytosis that was rescued by re-expression of TRPML1 in neurons. These features were not observed in Niemann-Pick type C1. These findings suggest that TRPML1 may guard against pathological fusion of lysosomes with secretory organelles and suggest a new approach toward developing treatment for MLIV.

Ca²⁺-regulated secretory granule exocytosis in pancreatic and parotid acinar cells

Protein secretion from acinar cells of the pancreas and parotid glands is controlled by G-protein coupled receptor activation and generation of the cellular messengers Ca(2+), diacylglycerol and cAMP. Secretory granule (SG) exocytosis shares some common characteristics with nerve, neuroendocrine and endocrine cells which are regulated mainly by elevated cell Ca(2+). However, in addition to diverse signaling pathways, acinar cells have large ∼1 μm diameter SGs (∼30 fold larger diameter than synaptic vesicles), respond to stimulation at slower rates (seconds versus milliseconds), demonstrate significant constitutive secretion, and in isolated acini, undergo sequential compound SG-SG exocytosis at the apical membrane. Exocytosis proceeds as an initial rapid phase that peaks and declines over 3 min followed by a prolonged phase that decays to near basal levels over 20-30 min. Studies indicate the early phase is triggered by Ca(2+) and involves the SG proteins VAMP2 (vesicle associated membrane protein2), Ca(2+)-sensing protein synatotagmin 1 (syt1) and the accessory protein complexin 2. The molecular details for regulation of VAMP8-mediated SG exocytosis and the prolonged phase of secretion are still emerging. Here we review the known regulatory molecules that impact the sequential exocytic process of SG tethering, docking, priming and fusion in acinar cells.

Adenylyl cyclases in the digestive system

Adenylyl cyclases (ACs) are a group of widely distributed enzymes whose functions are very diverse. There are nine known transmembrane AC isoforms activated by Gαs. Each has its own pattern of expression in the digestive system and differential regulation of function by Ca(2+) and other intracellular signals. In addition to the transmembrane isoforms, one AC is soluble and exhibits distinct regulation. In this review, the basic structure, regulation and physiological roles of ACs in the digestive system are discussed.

Isoproterenol stimulates transient SNAP23-VAMP2 interaction in rat parotid glands

The exocytosis of salivary proteins is mainly regulated by cAMP, although soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), which mediate cAMP-dependent exocytic membrane fusion, have remained unidentified. Here we examined the effect of isoproterenol (ISO) and cytochalasin D (CyD) on the level of SNARE complexes in rat parotid glands. When SNARE complexes were immunoprecipitated by anti-SNAP23, the coprecipitation of VAMP2 was significantly increased in response to ISO and/or CyD, although the coprecipitation of VAMP8 or syntaxin 4 was scarcely augmented. These results suggest that the SNAP23-VAMP2 interaction plays a key role in cAMP-mediated exocytosis from parotid glands.

Multiple roles for the actin cytoskeleton during regulated exocytosis

Regulated exocytosis is the main mechanism utilized by specialized secretory cells to deliver molecules to the cell surface by virtue of membranous containers (i.e., secretory vesicles). The process involves a series of highly coordinated and sequential steps, which include the biogenesis of the vesicles, their delivery to the cell periphery, their fusion with the plasma membrane, and the release of their content into the extracellular space. Each of these steps is regulated by the actin cytoskeleton. In this review, we summarize the current knowledge regarding the involvement of actin and its associated molecules during each of the exocytic steps in vertebrates, and suggest that the overall role of the actin cytoskeleton during regulated exocytosis is linked to the architecture and the physiology of the secretory cells under examination. Specifically, in neurons, neuroendocrine, endocrine, and hematopoietic cells, which contain small secretory vesicles that undergo rapid exocytosis (on the order of milliseconds), the actin cytoskeleton plays a role in pre-fusion events, where it acts primarily as a functional barrier and facilitates docking. In exocrine and other secretory cells, which contain large secretory vesicles that undergo slow exocytosis (seconds to minutes), the actin cytoskeleton plays a role in post-fusion events, where it regulates the dynamics of the fusion pore, facilitates the integration of the vesicles into the plasma membrane, provides structural support, and promotes the expulsion of large cargo molecules.

Exocyst subunits are involved in isoproterenol-induced amylase release from rat parotid acinar cells

Exocytosis of secretory granules in parotid acinar cells requires multiple events: tethering, docking, priming, and fusion with a luminal plasma membrane. The exocyst complex, which is composed of eight subunits (Sec3, Sec5, Sec6, Sec8, Sec10, Sec15, Exo70, and Exo84) that are conserved in yeast and mammalian cells, is thought to participate in the exocytotic pathway. However, to date, no exocyst subunit has been identified in salivary glands. In the present study, we investigated the expression and function of exocyst subunits in rat parotid acinar cells. The expression of mRNA for all eight exocyst subunits was detected in parotid acinar cells by RT-PCR, and Sec6 and Sec8 proteins were localized on the luminal plasma membrane. Sec6 interacted with Sec8 after 5 min of stimulation with isoproterenol. In addition, antibodies to-Sec6 and Sec8 inhibited isoproterenol-induced amylase release from streptolysin O-permeabilized parotid acinar cells. These results suggest that an exocyst complex of eight subunits is required for amylase release from parotid acinar cells.

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.

Isoproterenol and cAMP block ERK phosphorylation and enhance [Ca2+]i increases and oxygen consumption by muscarinic receptor stimulation in rat parotid and submandibular acinar cells

Salivary glands are innervated by sympathetic and parasympathetic neurons, which release neurotransmitters that promote fluid secretion and exocytosis when they bind to muscarinic and beta-adrenergic receptors, respectively. Signaling pathways downstream of these receptors are mainly distinct, but there is cross-talk that affects receptor-dependent events. Here we report that the beta-adrenergic ligand isoproterenol blocks increases in extracellular signal-related kinase (ERK) phosphorylation, a protein kinase C-dependent event promoted by the muscarinic receptor ligand carbachol in freshly dispersed rat parotid acinar cells. The inhibitory action of isoproterenol was reproduced by cAMP stimuli (forskolin) and mimetics (dibutyryl-cAMP, 8-(4-chlorophenylthio)-cAMP), including one highly selective for protein kinase A (N(6)-benzoyl-cAMP). In contrast, Epac (exchange proteins directly activated by cAMP)-selective activators did not mimic the blockade of ERK by isoproterenol, suggesting that inhibition involved protein kinase A. Isoproterenol also blocked ERK downstream of phorbol 12-myristate 13-acetate and the P2X(7) and epidermal growth factor receptors. Isoproterenol and forskolin blocked MEK phosphorylation, reduced RAF phosphorylation on a stimulatory site (Ser-338), and increased RAF phosphorylation on an inhibitory site (Ser-259). Inhibitory effects on ERK were also observed in freshly dispersed rat submandibular acinar cells but not in three immortalized/cancer salivary cell lines (Par-C10, HSY, HSG), indicating significant differences between native cells and cell lines. Notably, in native parotid cells isoproterenol enhanced the carbachol-promoted increases in [Ca(2+)](i) and oxygen consumption, events that initiate and accompany, respectively, the stimulation of fluid secretion by muscarinic ligands. Thus, isoproterenol produces opposite effects on prominent events downstream of the muscarinic receptor second messengers diacylglycerol (decrease in ERK phosphorylation) and inositol trisphosphate (increase in [Ca(2+)](i) and fluid secretion).

The contribution of AKAP5 in amylase secretion from mouse parotid acini

A-kinase (PKA) anchoring proteins (AKAPs) are essential for targeting type II PKA to specific locales in the cell to control function. In the present study, AKAP5 (formerly AKAP150) and AKAP6 were identified in mouse parotid acini by type II PKA regulatory subunit (RII) overlay assay and Western blot analysis of mouse parotid cellular fractions, and the role of AKAP5 in mouse parotid acinar cell secretion was determined. Mice were euthanized with CO(2). Immunofluorescence staining of acinar cells localized AKAP5 to the basolateral membrane, whereas AKAP6 was associated with the perinuclear region. In functional studies, amylase secretion from acinar cells of AKAP5 mutant [knockout (KO)] mice treated with the beta-adrenergic agonist, isoproterenol, was reduced overall by 30-40% compared with wild-type (WT) mice. In contrast, amylase secretion in response to the adenylyl cyclase (AC) activator, forskolin, and the cAMP-dependent protein kinase (PKA) activator, N(6)-phenyl-cAMP, was not statistically different in acini from WT and AKAP5 KO mice. Treatment of acini with isoproterenol mimicked the effect of the Epac activator, 8-(4-methoxyphenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate (8-pMeOPT-2'-O-Me-cAMP), in stimulating Rap1. However, in contrast to isoproterenol, treatment of acini with 8-pMeOPT-2'-O-Me-cAMP resulted in stimulation of amylase secretion from both AKAP5 KO and WT acinar cells. As a scaffolding protein, AKAP5 was found to coimmunoprecipitate with AC6, but not AC8. Data suggest that isoproterenol-stimulated amylase secretion occurs via both an AKAP5/AC6/PKA complex and a PKA-independent, Epac pathway in mouse parotid acini.

Redistribution of small GTP-binding protein, Rab27B, in rat parotid acinar cells after stimulation with isoproterenol

Small GTP-binding protein, Rab27, has been implicated in the regulation of different types of membrane trafficking, including melanosome transport in melanocytes and regulated secretion events in a wide variety of secretory cells. We have previously shown that Rab27 is involved in the control of isoproterenol (IPR)-induced amylase release from rat parotid acinar cells. Although Rab27 is predominantly localized on secretory granules under resting conditions, changes to its intracellular localization after beta-stimulation have never been elucidated. The present study investigated IPR-induced redistribution of Rab27B in the parotid acinar cells, revealing translocation from secretory granules to the subapical region after 5 min of IPR treatment and then diffusion into the cytosol after 30 min of IPR treatment. Dissociation of Rab27B from the apical plasma membrane is probably mediated through the Rab GDP dissociation inhibitor (GDI) in the cytosol extracting GDP-bound Rab protein from membranes, as a dramatic increase in the amount of the Rab27B-GDI complex in the cytosol was observed 30 min after stimulation with IPR. These results indicate that, in parotid acinar cells, Rab27B is translocated, in a time-dependent manner, from secretory granules into the apical plasma membrane as a result of exposure to IPR, and then into the cytosol through binding with the GDI.

Phosphorylation of myristoylated alanine-rich C kinase substrate is involved in the cAMP-dependent amylase release in parotid acinar cells

Myristoylated alanine-rich C kinase substrate (MARCKS) is known as a major cellular substrate for protein kinase C (PKC). MARCKS has been implicated in the regulation of brain development and postnatal survival, cellular migration and adhesion, as well as phagocytosis, endocytosis, and exocytosis. The involvement of MARCKS phosphorylation in secretory function has been reported in Ca(2+)-mediated exocytosis. In rat parotid acinar cells, the activation of beta-adrenergic receptors provokes exocytotic amylase release via accumulation of intracellular cAMP levels. Here, we studied the involvement of MARCKS phosphorylation in the cAMP-dependent amylase release in rat parotid acinar cells. MARCKS protein was detected in rat parotid acinar cells by Western blotting. The beta-adrenergic agonist isoproterenol (IPR) induced MARCKS phosphorylation in a time-dependent manner. Translocation of a part of phosphorylated MARCKS from the membrane to the cytosol and enhancement of MARCKS phosphorylation at the apical membrane site induced by IPR were observed by immunohistochemistry. H89, a cAMP-dependent protein kinase (PKA) inhibitor, inhibited the IPR-induced MARCKS phosphorylation. The PKCdelta inhibitor rottlerin inhibited the IPR-induced MARCKS phosphorylation and amylase release. IPR activated PKCdelta, and the effects of IPR were inhibited by the PKA inhibitors. A MARCKS-related peptide partially inhibited the IPR-induced amylase release. These findings suggest that MARCKS phosphorylation via the activation of PKCdelta, which is downstream of PKA activation, is involved in the cAMP-dependent amylase release in parotid acinar cells.

Rap1 activation plays a regulatory role in pancreatic amylase secretion

Rap1 is a member of the Ras superfamily of small GTP-binding proteins and is localized on pancreatic zymogen granules. The current study was designed to determine whether GTP-Rap1 is involved in the regulation of amylase secretion. Rap1A/B and the two Rap1 guanine nucleotide exchange factors, Epac1 and CalDAG-GEF III, were identified in mouse pancreatic acini. A fraction of both Rap1 and Epac1 colocalized with amylase in zymogen granules, but only Rap1 was integral to the zymogen granule membranes. Stimulation with cholecystokinin (CCK), carbachol, and vasoactive intestinal peptide all induced Rap1 activation, as did calcium ionophore A23187, phorbol ester, forskolin, 8-bromo-cyclic AMP, and the Epac-specific cAMP analog 8-pCPT-2'-O-Me-cAMP. The phospholipase C inhibitor U-73122 abolished carbachol- but not forskolin-induced Rap1 activation. Co-stimulation with carbachol and 8-pCPT-2'-O-Me-cAMP led to an additive effect on Rap1 activation, whereas a synergistic effect was seen on amylase release. Although the protein kinase A inhibitor H-89 abolished forskolin-stimulated CREB phosphorylation, it did not modify forskolin-induced GTP-Rap1 levels, excluding PKA participation. Overexpression of Rap1 GTPase-activating protein, which blocked Rap1 activation, reduced the effect of 8-bromo-cyclic AMP, 8-pCPT-2'-O-Me-cAMP, and vasoactive intestinal peptide on amylase release by 60% and reduced CCK- as well as carbachol-stimulated pancreatic amylase release by 40%. These findings indicate that GTP-Rap1 is required for pancreatic amylase release. Rap1 activation not only mediates the cAMP-evoked response via Epac1 but is also involved in CCK- and carbachol-induced amylase release, with their action most likely mediated by CalDAG-GEF III.

Essential role of Epac2/Rap1 signaling in regulation of insulin granule dynamics by cAMP

cAMP is well known to regulate exocytosis in various secretory cells, but the precise mechanism of its action remains unknown. Here, we examine the role of cAMP signaling in the exocytotic process of insulin granules in pancreatic beta cells. Although activation of cAMP signaling alone does not cause fusion of the granules to the plasma membrane, it clearly potentiates both the first phase (a prompt, marked, and transient increase) and the second phase (a moderate and sustained increase) of glucose-induced fusion events. Interestingly, all granules responsible for this potentiation are newly recruited and immediately fused to the plasma membrane without docking (restless newcomer). Importantly, cAMP-potentiated fusion events in the first phase of glucose-induced exocytosis are markedly reduced in mice lacking the cAMP-binding protein Epac2 (Epac2(ko/ko)). In addition, the small GTPase Rap1, which is activated by cAMP specifically through Epac2 in pancreatic beta cells, mediates cAMP-induced insulin secretion in a protein kinase A-independent manner. We also have developed a simulation model of insulin granule movement in which potentiation of the first phase is associated with an increase in the insulin granule density near the plasma membrane. Taken together, these data indicate that Epac2/Rap1 signaling is essential in regulation of insulin granule dynamics by cAMP, most likely by controlling granule density near the plasma membrane.

Activation mechanisms for the cystic fibrosis transmembrane conductance regulator protein involve direct binding of cAMP

The CFTR [CF (cystic fibrosis) transmembrane conductance regulator] chloride channel is activated by cyclic nucleotide-dependent phosphorylation and ATP binding, but also by non-phosphorylation-dependent mechanisms. Other CFTR functions such as regulation of exocytotic protein secretion are also activated by cyclic nucleotide elevating agents. A soluble protein comprising the first NBD (nucleotide-binding domain) and R-domain of CFTR (NBD1-R) was synthesized to determine directly whether CFTR binds cAMP. An equilibrium radioligand-binding assay was developed, firstly to show that, as for full-length CFTR, the NBD1-R protein bound ATP. Half-maximal displacement of [3H]ATP by non-radioactive ATP at 3.5 microM and 3.1 mM was demonstrated. [3H]cAMP bound to the protein with different affinities from ATP (half-maximal displacement by cAMP at 2.6 and 167 microM). Introduction of a mutation (T421A) in a motif predicted to be important for cyclic nucleotide binding decreased the higher affinity binding of cAMP to 9.2 microM. The anti-CFTR antibody (MPNB) that inhibits CFTR-mediated protein secretion also inhibited cAMP binding. Thus binding of cAMP to CFTR is consistent with a role in activation of protein secretion, a process defective in CF gland cells. Furthermore, the binding site may be important in the mechanism by which drugs activate mutant CFTR and correct defective DeltaF508-CFTR trafficking.

Evidence for amylase release by cGMP via cAMP-dependent protein kinase in rat parotid acinar cells

Amylase release from the rat parotid gland is primarily mediated by a cAMP-dependent protein kinase (PKA). We previously reported that cGMP/cGMP-dependent protein kinase (PKG) signaling evokes amylase release. In the present study, we investigated whether cGMP-mediated amylase release might be due to cGMP/PKA signaling, as well as cGMP/PKG pathway. Activation of PKA by cGMP was required 100-1000-fold greater concentration than activation by cAMP in a parotid cytosol fraction. Synergistic activation of PKA by the combination of physiological cAMP and low concentration of cGMP was observed. Amylase release from intact acinar cells was synergistically stimulated by the combination of diBu-cAMP and 8-pCPT-cGMP. cGMP dose-dependently stimulated amylase release from saponin-permeabilized parotid acinar cells. Phosphorylation by cGMP produced phosphorylated proteins of the same size as those produced by cAMP. Phosphorylation by cGMP was inhibited by the addition of PKA inhibitor, H-89. These results suggest that cGMP activates both PKG and PKA. Thus, it appears that both cGMP/PKG and cGMP/PKA pathways mediate amylase release from rat parotid acinar cells.

Cell physiology of cAMP sensor Epac

Epac is an acronym for the exchange proteins activated directly by cyclic AMP, a family of cAMP-regulated guanine nucleotide exchange factors (cAMPGEFs) that mediate protein kinase A (PKA)-independent signal transduction properties of the second messenger cAMP. Two variants of Epac exist (Epac1 and Epac2), both of which couple cAMP production to the activation of Rap, a small molecular weight GTPase of the Ras family. By activating Rap in an Epac-mediated manner, cAMP influences diverse cellular processes that include integrin-mediated cell adhesion, vascular endothelial cell barrier formation, and cardiac myocyte gap junction formation. Recently, the identification of previously unrecognized physiological processes regulated by Epac has been made possible by the development of Epac-selective cyclic AMP analogues (ESCAs). These cell-permeant analogues of cAMP activate both Epac1 and Epac2, whereas they fail to activate PKA when used at low concentrations. ESCAs such as 8-pCPT-2'-O-Me-cAMP and 8-pMeOPT-2'-O-Me-cAMP are reported to alter Na(+), K(+), Ca(2+) and Cl(-) channel function, intracellular [Ca(2+)], and Na(+)-H(+) transporter activity in multiple cell types. Moreover, new studies examining the actions of ESCAs on neurons, pancreatic beta cells, pituitary cells and sperm demonstrate a major role for Epac in the stimulation of exocytosis by cAMP. This topical review provides an update concerning novel PKA-independent features of cAMP signal transduction that are likely to be Epac-mediated. Emphasized is the emerging role of Epac in the cAMP-dependent regulation of ion channel function, intracellular Ca(2+) signalling, ion transporter activity and exocytosis.

Epac1 and cAMP-dependent protein kinase holoenzyme have similar cAMP affinity, but their cAMP domains have distinct structural features and cyclic nucleotide recognition

The cAMP-dependent protein kinase (PKA I and II) and the cAMP-stimulated GDP exchange factors (Epac1 and -2) are major cAMP effectors. The cAMP affinity of the PKA holoenzyme has not been determined previously. We found that cAMP bound to PKA I with a K(d) value (2.9 microM) similar to that of Epac1. In contrast, the free regulatory subunit of PKA type I (RI) had K(d) values in the low nanomolar range. The cAMP sites of RI therefore appear engineered to respond to physiological cAMP concentrations only when in the holoenzyme form, whereas Epac can respond in its free form. Epac is phylogenetically younger than PKA, and its functional cAMP site has presumably evolved from site B of PKA. A striking feature is the replacement of a conserved Glu in PKA by Gln (Epac1) or Lys (Epac2). We found that such a switch (E326Q) in site B of human RIalpha led to a 280-fold decreased cAMP affinity. A similar single switch early in Epac evolution could therefore have decreased the high cAMP affinity of the free regulatory subunit sufficiently to allow Epac to respond to physiologically relevant cAMP levels. Molecular dynamics simulations and cAMP analog mapping indicated that the E326Q switch led to flipping of Tyr-373, which normally stacks with the adenine ring of cAMP. Combined molecular dynamics simulation, GRID analysis, and cAMP analog mapping of wild-type and mutated BI and Epac1 revealed additional differences, independent of the Glu/Gln switch, between the binding sites, regarding space (roominess), hydrophobicity/polarity, and side chain flexibility. This helped explain the specificity of current cAMP analogs and, more importantly, lays a foundation for the generation of even more discriminative analogs.

Epac-mediated Ca(2+) mobilization and exocytosis in inner medullary collecting duct

PKA has traditionally been thought as the binding protein of cAMP for mediating arginine vasopressin (AVP)-regulated osmotic water permeability in kidney collecting duct. It is now known that cAMP also exerts its effects via Epac (exchange protein directly activated by cAMP) and that intracellular Ca(2+) mobilization is necessary for AVP-induced apical exocytosis in inner medullary collecting duct (IMCD). The role of Epac as an effector of cAMP action in addition to PKA was investigated using confocal fluorescence microscopy in perfused IMCD. PKA inhibitors (1 microM H-89 or 10 microM KT-5720) at concentrations known to inhibit aquaporin-2 (AQP2) phosphorylation did not prevent AVP-induced Ca(2+) mobilization and oscillations. Epac-selective cAMP agonist (8-pCPT-2'-O-Me-cAMP) mimicked AVP in triggering Ca(2+) mobilization and oscillations, which was blocked by ryanodine but not by Rp-cAMP (a competitive antagonist of cAMP binding to PKA). 8-pCPT-2'-O-Me-cAMP also triggered apical exocytosis in the presence of a PKA inhibitor. Immunolocalization of AQP2 in perfused IMCD demonstrated that 8-pCPT-2'-O-Me-cAMP induces apical targeting of AQP2 and that AQP2 is abundant in junctional regions of basolateral membrane. Immunofluorescence study also confirmed the presence of Epac (isoform I) in IMCD. These results indicate that activation of Epac by an exogenous cAMP analog triggers intracellular Ca(2+) mobilization and apical exocytotic insertion of AQP2 in IMCD.

Calcium-induced acrosomal exocytosis requires cAMP acting through a protein kinase A-independent, Epac-mediated pathway

Epac, a guanine nucleotide exchange factor for the small GTPase Rap, binds to and is activated by the second messenger cAMP. In sperm, there are a number of signaling pathways required to achieve egg-fertilizing ability that depend upon an intracellular rise of cAMP. Most of these processes were thought to be mediated by cAMP-dependent protein kinases. Here we report a new dependence for the cAMP-induced acrosome reaction involving Epac. The acrosome reaction is a specialized type of regulated exocytosis leading to a massive fusion between the outer acrosomal and the plasma membranes of sperm cells. Ca2+ is the archetypical trigger of regulated exocytosis, and we show here that its effects on acrosomal release are fully mediated by cAMP. Ca2+ failed to trigger acrosomal exocytosis when intracellular cAMP was depleted by an exogenously added phosphodiesterase or when Epac was sequestered by specific blocking antibodies. The nondiscriminating dibutyryl-cAMP and the Epac-selective 8-(p-chlorophenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate analogues triggered the acrosome reaction in the effective absence of extracellular Ca2+. This indicates that cAMP, via Epac activation, has the ability to drive the whole cascade of events necessary to bring exocytosis to completion, including tethering and docking of the acrosome to the plasma membrane, priming of the fusion machinery, mobilization of intravesicular Ca2+, and ultimately, bilayer mixing and fusion. cAMP-elicited exocytosis was sensitive to anti-alpha-SNAP, anti-NSF, and anti-Rab3A antibodies, to intra-acrosomal Ca2+ chelators, and to botulinum toxins but was resistant to cAMP-dependent protein kinase blockers. These experiments thus identify Epac in human sperm and evince its indispensable role downstream of Ca2+ in exocytosis.