Differential modulation of Cav1.2 and Cav1.3-mediated glucose-stimulated insulin secretion by cAMP in INS-1 cells: distinct roles for exchange protein directly activated by cAMP 2 (Epac2) and protein kinase A

Noncanonical Regulation of cAMP-Dependent Insulin Secretion and Its Implications in Type 2 Diabetes

Impaired glucose tolerance (IGT) and β-cell dysfunction in insulin resistance associated with obesity lead to type 2 diabetes (T2D). Glucose-stimulated insulin secretion (GSIS) from β-cells occurs via a canonical pathway that involves glucose metabolism, ATP generation, inactivation of K ATP channels, plasma membrane depolarization, and increases in cytosolic concentrations of [Ca 2+ ] c . However, optimal insulin secretion requires amplification of GSIS by increases in cyclic adenosine monophosphate (cAMP) signaling. The cAMP effectors protein kinase A (PKA) and exchange factor activated by cyclic-AMP (Epac) regulate membrane depolarization, gene expression, and trafficking and fusion of insulin granules to the plasma membrane for amplifying GSIS. The widely recognized lipid signaling generated within β-cells by the β-isoform of Ca 2+ -independent phospholipase A 2 enzyme (iPLA 2 β) participates in cAMP-stimulated insulin secretion (cSIS). Recent work has identified the role of a G-protein coupled receptor (GPCR) activated signaling by the complement 1q like-3 (C1ql3) secreted protein in inhibiting cSIS. In the IGT state, cSIS is attenuated, and the β-cell function is reduced. Interestingly, while β-cell-specific deletion of iPLA 2 β reduces cAMP-mediated amplification of GSIS, the loss of iPLA 2 β in macrophages (MØ) confers protection against the development of glucose intolerance associated with diet-induced obesity (DIO). In this article, we discuss canonical (glucose and cAMP) and novel noncanonical (iPLA 2 β and C1ql3) pathways and how they may affect β-cell (dys)function in the context of impaired glucose intolerance associated with obesity and T2D. In conclusion, we provide a perspective that in IGT states, targeting noncanonical pathways along with canonical pathways could be a more comprehensive approach for restoring β-cell function in T2D. © 2023 American Physiological Society. Compr Physiol 13:5023-5049, 2023.

Syndromic forms of congenital hyperinsulinism

Congenital hyperinsulinism (CHI), also called hyperinsulinemic hypoglycemia (HH), is a very heterogeneous condition and represents the most common cause of severe and persistent hypoglycemia in infancy and childhood. The majority of cases in which a genetic cause can be identified have monogenic defects affecting pancreatic β-cells and their glucose-sensing system that regulates insulin secretion. However, CHI/HH has also been observed in a variety of syndromic disorders. The major categories of syndromes that have been found to be associated with CHI include overgrowth syndromes (e.g. Beckwith-Wiedemann and Sotos syndromes), chromosomal and monogenic developmental syndromes with postnatal growth failure (e.g. Turner, Kabuki, and Costello syndromes), congenital disorders of glycosylation, and syndromic channelopathies (e.g. Timothy syndrome). This article reviews syndromic conditions that have been asserted by the literature to be associated with CHI. We assess the evidence of the association, as well as the prevalence of CHI, its possible pathophysiology and its natural course in the respective conditions. In many of the CHI-associated syndromic conditions, the mechanism of dysregulation of glucose-sensing and insulin secretion is not completely understood and not directly related to known CHI genes. Moreover, in most of those syndromes the association seems to be inconsistent and the metabolic disturbance is transient. However, since neonatal hypoglycemia is an early sign of possible compromise in the newborn, which requires immediate diagnostic efforts and intervention, this symptom may be the first to bring a patient to medical attention. As a consequence, HH in a newborn or infant with associated congenital anomalies or additional medical issues remains a differential diagnostic challenge and may require a broad genetic workup.

The Role of cAMP in Beta Cell Stimulus-Secretion and Intercellular Coupling

Pancreatic beta cells secrete insulin in response to stimulation with glucose and other nutrients, and impaired insulin secretion plays a central role in development of diabetes mellitus. Pharmacological management of diabetes includes various antidiabetic drugs, including incretins. The incretin hormones, glucagon-like peptide-1 and gastric inhibitory polypeptide, potentiate glucose-stimulated insulin secretion by binding to G protein-coupled receptors, resulting in stimulation of adenylate cyclase and production of the secondary messenger cAMP, which exerts its intracellular effects through activation of protein kinase A or the guanine nucleotide exchange protein 2A. The molecular mechanisms behind these two downstream signaling arms are still not fully elucidated and involve many steps in the stimulus-secretion coupling cascade, ranging from the proximal regulation of ion channel activity to the central Ca2+ signal and the most distal exocytosis. In addition to modifying intracellular coupling, the effect of cAMP on insulin secretion could also be at least partly explained by the impact on intercellular coupling. In this review, we systematically describe the possible roles of cAMP at these intra- and inter-cellular signaling nodes, keeping in mind the relevance for the whole organism and translation to humans.

Regulation of cAMP accumulation and activity by distinct phosphodiesterase subtypes in INS-1 cells and human pancreatic β-cells

Pancreatic β-cells express multiple phosphodiesterase (PDE) subtypes, but the specific roles for each in β-cell function, particularly in humans, is not clear. We evaluated the cellular role of PDE1, PDE3, and PDE4 activity in the rat insulinoma cell line INS-1 and in primary human β-cells using subtype-selective PDE inhibitors. Using a genetically encoded, FRET-based cAMP sensor, we found that the PDE1 inhibitor 8MM-IBMX, elevated cAMP levels in the absence of glucose to a greater extent than either the PDE3 inhibitor cilostamide or the PDE4 inhibitor rolipram. In 18 mM glucose, PDE1 inhibition elevated cAMP levels to a greater extent than PDE3 inhibition in INS-1 cells, while PDE4 inhibition was without effect. Inhibition of PDE1 or PDE4, but not PDE3, potentiated glucose-stimulated insulin secretion in INS-1 cells. PDE1 inhibition, but not PDE3 or PDE4 inhibition, reduced palmitate-induced caspase-3/7 activation, and enhanced CREB phosphorylation in INS-1 cells. In human β-cells, only PDE3 or PDE4 inhibition increased cAMP levels in 1.7 mM glucose, but PDE1, PDE3, or PDE4 inhibition potentiated cAMP levels in 16.7 mM glucose. Inhibition of PDE1 or PDE4 increased cAMP levels to a greater extent in 16.7 mM glucose than in 1.7 mM glucose in human β-cells. In contrast, elevation of cAMP levels by PDE3 inhibition was not different at these glucose concentrations. PDE1 inhibition also potentiated insulin secretion from human islets, suggesting that the role of PDE1 may be conserved between INS-1 cells and human pancreatic β-cells. Our results suggest that inhibition of PDE1 may be a useful strategy to potentiate glucose-stimulated insulin secretion, and to protect β-cells from the toxic effects of excess fatty acids.

Ca2+ influx through L-type Ca2+ channels and Ca2+-induced Ca2+ release regulate cAMP accumulation and Epac1-dependent ERK 1/2 activation in INS-1 cells

We previously reported that INS-1 cells expressing the intracellular II-III loop of the L-type Ca(2+) channel Cav1.2 (Cav1.2/II-III cells) are deficient in Ca(2+)-induced Ca(2+) release (CICR). Here we show that glucose-stimulated ERK 1/2 phosphorylation (GSEP) is slowed and reduced in Cav1.2/II-III cells compared to INS-1 cells. This parallels a decrease in glucose-stimulated cAMP accumulation (GS-cAMP) in Cav1.2/II-III cells. Influx of Ca(2+) via L-type Ca(2+) channels and CICR play roles in both GSEP and GS-cAMP in INS-1 cells since both are inhibited by nicardipine or ryanodine. Further, the Epac1-selective inhibitor CE3F4 abolishes glucose-stimulated ERK activation in INS-1 cells, as measured using the FRET-based sensor EKAR. The non-selective Epac antagonist ESI-09 but not the Epac2-selective antagonist ESI-05 nor the PKA antagonist Rp-cAMPs inhibits GSEP in both INS-1 and Cav1.2/II-III cells. We conclude that L-type Ca(2+) channel-dependent cAMP accumulation, that's amplified by CICR, activates Epac1 and drives GSEP in INS-1 cells.

New insights concerning the molecular basis for defective glucoregulation in soluble adenylyl cyclase knockout mice

Recently published findings indicate that a knockout (KO) of soluble adenylyl cyclase (sAC, also known as AC-10) gene expression in mice leads to defective glucoregulation that is characterized by reduced pancreatic insulin secretion and reduced intraperitoneal glucose tolerance. Summarized here are current concepts regarding the molecular basis for this phenotype, with special emphasis on the potential role of sAC as a determinant of glucose-stimulated insulin secretion. Highlighted is new evidence that in pancreatic beta cells, oxidative glucose metabolism stimulates mitochondrial CO₂production that in turn generates bicarbonate ion (HCO(3)(-)). Since HCO(3)(-) binds to and directly stimulates the activity of sAC, we propose that glucose-stimulated cAMP production in beta cells is mediated not simply by transmembrane adenylyl cyclases (TMACs), but also by sAC. Based on evidence that sAC is expressed in mitochondria, there exists the possibility that beta-cell glucose metabolism is linked to mitochondrial cAMP production with consequent facilitation of oxidative phosphorylation. Since sAC is also expressed in the cytoplasm, sAC catalyzed cAMP production may activate cAMP sensors such as PKA and Epac2 to control ion channel function, intracellular Ca²⁺ handling, and Ca²⁺-dependent exocytosis. Thus, we propose that the existence of sAC in beta cells provides a new and unexpected explanation for previously reported actions of glucose metabolism to stimulate cAMP production. It seems possible that alterations of sAC activity might be of importance when evaluating new strategies for the treatment of type 2 diabetes (T2DM), or when evaluating why glucose metabolism fails to stimulate insulin secretion in patients diagnosed with T2DM. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

Uncoupling of Cav1.2 from Ca(2+)-induced Ca(2+) release and SK channel regulation in pancreatic β-cells

We investigated the role of Cav1.2 in pancreatic β-cell function by expressing a Cav1.2 II-III loop/green fluorescent protein fusion in INS-1 cells (Cav1.2/II-III cells) to disrupt channel-protein interactions. Neither block of KATP channels nor stimulation of membrane depolarization by tolbutamide was different in INS-1 cells compared with Cav1.2/II-III cells, but whole-cell Cav current density was significantly increased in Cav1.2/II-III cells. Tolbutamide (200 μM) stimulated insulin secretion and Ca(2+) transients in INS-1 cells, and Cav1.2/II-III cells were completely blocked by nicardipine (2 μM), but thapsigargin (1 μM) blocked tolbutamide-stimulated secretion and Ca(2+) transients only in INS-1 cells. Tolbutamide-stimulated endoplasmic reticulum [Ca(2+)] decrease was reduced in Cav1.2/II-III cells compared with INS-1 cells. However, Ca(2+) transients in both INS-1 cells and Cav1.2/II-III cells were significantly potentiated by 8-pCPT-2'-O-Me-cAMP (5 μM), FPL-64176 (0.5 μM), or replacement of extracellular Ca(2+) with Sr(2+). Glucose (10 mM) + glucagon-like peptide-1 (10 nM) stimulated discrete spikes in [Ca(2+)]i in the presence of verapamil at a higher frequency in INS-1 cells than in Cav1.2/II-II cells. Glucose (18 mM) stimulated more frequent action potentials in Cav1.2/II-III cells and primary rat β-cells expressing the Cav1.2/II-II loop than in control cells. Further, apamin (1 μM) increased glucose-stimulated action potential frequency in INS-1 cells, but not Cav1.2/II-III cells, suggesting that SK channels were not activated under these conditions in Cav1.2/II-III loop-expressing cells. We propose the II-III loop of Cav1.2 as a key molecular determinant that couples the channel to Ca(2+)-induced Ca(2+) release and activation of SK channels in pancreatic β-cells.

Potentiation of sulfonylurea action by an EPAC-selective cAMP analog in INS-1 cells: comparison of tolbutamide and gliclazide and a potential role for EPAC activation of a 2-APB-sensitive Ca2+ influx

Tolbutamide and gliclazide block the K(ATP) channel K(ir)6.2/Sur1, causing membrane depolarization and stimulating insulin secretion in pancreatic beta cells. We examined the ability of the EPAC-selective cAMP analog 8-pCPT-2'-O-Me-cAMP-AM to potentiate the action of these drugs and the mechanism that might account for it. Insulin secretion stimulated by both 200 μM tolbutamide and 20 μM gliclazide, concentrations that had equivalent effects on membrane potential, was inhibited by thapsigargin (1 μM) or the L-type Ca(2+) channel blocker nicardipine (2 μM) and was potentiated by 8-pCPT-2'-O-Me-cAMP-AM at concentrations ≥2 μM in INS-1 cells. Ca(2+) transients stimulated by either tolbutamide or gliclazide were inhibited by thapsigargin or nicardipine and were significantly potentiated by 8-pCPT-2'-O-Me-cAMP-AM at 5 μM but not 1 μM. Both tolbutamide and gliclazide stimulated phospholipase C activity; however, only gliclazide did so independently of its activity at K(ATP) channels, and this activity was partially inhibited by pertussis toxin. 8-pCPT-2'-O-Me-cAMP-AM alone (5 μM) did not stimulate insulin secretion, but did increase intracellular Ca(2+) concentration significantly, and this activity was inhibited by 25 μM 2-aminoethoxydiphenylborate (2-APB) or the removal of extracellular Ca(2+). 8-pCPT-2'-O-Me-cAMP-AM potentiation of insulin secretion stimulated by tolbutamide was markedly inhibited by 2-APB (25 μM) and enhanced by the PKC inhibitor bisindolylmaleimide I (1 μM). Our data demonstrate that the actions of both tolbutamide and gliclazide are strongly potentiated by 8-pCPT-2'-O-Me-cAMP-AM, that gliclazide can stimulate phospholipase C activity via a partially pertussis toxin-sensitive mechanism, and that 8-pCPT-2'-O-Me-cAMP-AM potentiation of tolbutamide action may involve activation of a 2-APB-sensitive Ca(2+) influx.

Phospholipase C-ε links Epac2 activation to the potentiation of glucose-stimulated insulin secretion from mouse islets of Langerhans

Glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells is potentiated by cAMP-elevating agents, such as the incretin hormone glucagon-like peptide-1 (GLP-1), and cAMP exerts its insulin secretagogue action by activating both protein kinase A (PKA) and the cAMP-regulated guanine nucleotide exchange factor designated as Epac2. Although prior studies of mouse islets demonstrated that Epac2 acts via Rap1 GTPase to potentiate GSIS, it is not understood which downstream targets of Rap1 promote the exocytosis of insulin. Here, we measured insulin secretion stimulated by a cAMP analog that is a selective activator of Epac proteins in order to demonstrate that a Rap1-regulated phospholipase C-epsilon (PLC-ε) links Epac2 activation to the potentiation of GSIS. Our analysis demonstrates that the Epac activator 8-pCPT-2'-O-Me-cAMP-AM potentiates GSIS from the islets of wild-type (WT) mice, whereas it has a greatly reduced insulin secretagogue action in the islets of Epac2 (-/-) and PLC-ε (-/-) knockout (KO) mice. Importantly, the insulin secretagogue action of 8-pCPT-2'-O-Me-cAMP-AM in WT mouse islets cannot be explained by an unexpected action of this cAMP analog to activate PKA, as verified through the use of a FRET-based A-kinase activity reporter (AKAR3) that reports PKA activation. Since the KO of PLC-ε disrupts the ability of 8-pCPT-2'-O-Me-cAMP-AM to potentiate GSIS, while also disrupting its ability to stimulate an increase of β-cell [Ca2+]i, the available evidence indicates that it is a Rap1-regulated PLC-ε that links Epac2 activation to Ca2+-dependent exocytosis of insulin.

Cyclic AMP signaling in pancreatic islets

Cyclic 3'5'AMP (cAMP) is an important physiological amplifier of glucose-induced insulin secretion by the pancreatic islet beta-cell, where it is formed by the activity of adenylyl cyclases, which are stimulated by glucose, through elevation in intracellular calcium concentrations, and by the incretin hormones (GLP-1 and GIP). cAMP is rapidly degraded in the pancreatic islet beta-cell by various cyclic nucleotide phosphodiesterase (PDE) enzymes. Many steps involved in glucose-induced insulin secretion are modulated by cAMP, which is also important in regulating pancreatic islet beta-cell differentiation, growth and survival. This chapter discusses the formation, destruction and actions of cAMP in the islets with particular emphasis on the beta-cell.

Ca(2+) channels on the move

The versatility of Ca(2+) as an intracellular messenger derives largely from the spatial organization of cytosolic Ca(2+) signals, most of which are generated by regulated openings of Ca(2+)-permeable channels. Most Ca(2+) channels are expressed in the plasma membrane (PM). Others, including the almost ubiquitous inositol 1,4,5-trisphosphate receptors (IP(3)R) and their relatives, the ryanodine receptors (RyR), are predominantly expressed in membranes of the sarcoplasmic or endoplasmic reticulum (ER). Targeting of these channels to appropriate destinations underpins their ability to generate spatially organized Ca(2+) signals. All Ca(2+) channels begin life in the cytosol, and the vast majority are then functionally assembled in the ER, where they may either remain or be dispatched to other membranes. Here, by means of selective examples, we review two issues related to this trafficking of Ca(2+) channels via the ER. How do cells avoid wayward activity of Ca(2+) channels in transit as they pass from the ER via other membranes to their final destination? How and why do some cells express small numbers of the archetypal intracellular Ca(2+) channels, IP(3)R and RyR, in the PM?

Glucose-dependent potentiation of mouse islet insulin secretion by Epac activator 8-pCPT-2'-O-Me-cAMP-AM

Epac2 is a cAMP-regulated guanine nucleotide exchange factor (cAMP-GEF) that is proposed to mediate stimulatory actions of the second messenger cAMP on mouse islet insulin secretion. Here we have used methods of islet perifusion to demonstrate that the acetoxymethyl ester (AM-ester) of an Epac-selective cAMP analog (ESCA) penetrates into mouse islets and is capable of potentiating both first and second phases of glucose-stimulated insulin secretion (GSIS). When used at low concentrations (1-10 μM), 8-pCPT-2'-O-Me-cAMP-AM activates Rap1 GTPase but exhibits little or no ability to activate protein kinase A (PKA), as validated in assays of in vitro PKA activity (phosphorylation of Kemptide), Ser (133) CREB phosphorylation status, RIP1-CRE-Luc reporter gene activity, and PKA-dependent AKAR3 biosensor activation. Since quantitative PCR demonstrates Epac2 mRNA to be expressed at levels ca. 5.3-fold greater than that of Epac1, available evidence indicates that Epac2 does in fact mediate stimulatory actions of cAMP on mouse islet GSIS.

Cav1.2 and Cav1.3 are differentially coupled to glucagon-like peptide-1 potentiation of glucose-stimulated insulin secretion in the pancreatic beta-cell line INS-1

The incretin peptides, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), potentiate glucose-stimulated insulin secretion (GSIS) and beta-cell proliferation and differentiation. Ca(2+) influx via voltage-gated L-type Ca(2+) channels is required for GLP-1 and GIP potentiation of GSIS. We investigated the role of the L-type Ca(2+) channels Ca(v)1.2 and Ca(v)1.3 in mediating GLP-1- and GIP-stimulated events in INS-1 cells and INS-1 cell lines expressing dihydropyridine-insensitive (DHPi) mutants of either Ca(v)1.2 or Ca(v)1.3. Ca(v)1.3/DHPi channels supported full potentiation of GSIS by GLP-1 (50 nM) compared with untransfected INS-1 cells. However, GLP-1-potentiated GSIS mediated by Ca(v)1.2/DHPi channels was markedly reduced compared with untransfected INS-1 cells. In contrast, GIP (10 nM) potentiation of GSIS mediated by both Ca(v)1.2/DHPi and Ca(v)1.3/DHPi channels was similar to that observed in untransfected INS-1 cells. Disruption of intracellular Ca(2+) release with thapsigargin, ryanodine, or 2-aminoethyldiphenylborate and inhibition of protein kinase A (PKA) or protein kinase C (PKC) significantly reduced GLP-1 potentiation of GSIS by Ca(v)1.3/DHPi channels and by endogenous L-type channels in INS-1 cells, but not by Ca(v)1.2/DHPi channels. Inhibition of glucose-stimulated phospholipase C activity with 1-(6-((17b-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione (U73122) did not inhibit potentiation of GSIS by GLP-1 in INS-1 cells. In contrast, wortmannin, an inhibitor of phosphatidylinositol 3-kinase, and 2'-amino-3'-methoxyflavone (PD98059), an inhibitor of mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) kinase, both markedly inhibited GLP-1 potentiation of GSIS by endogenous channels in INS-1 cells and Ca(v)1.3/DHPi channels, but not by Ca(v)1.2/DHPi channels. Thus, Ca(v)1.3 is preferentially coupled to GLP-1 potentiation of GSIS in INS-1 cells via a mechanism that requires intact intracellular Ca(2+) stores, PKA and PKC activity, and activation of ERK1/2.

The intracellular II-III loops of Cav1.2 and Cav1.3 uncouple L-type voltage-gated Ca2+ channels from glucagon-like peptide-1 potentiation of insulin secretion in INS-1 cells via displacement from lipid rafts

L-type Ca(2+) channels play a key role in the integration of physiological signals regulating insulin secretion that probably requires their localization to specific subdomains of the plasma membrane. We investigated the role of the intracellular II-III loop domains of the L-type channels Ca(v)1.2 and 1.3 in coupling of Ca(2+) influx with glucose-stimulated insulin secretion (GSIS) potentiated by the incretin hormone glucagon-like peptide (GLP)-1. In INS-1 cell lines expressing the Ca(v)1.2/II-III or Ca(v)1.3/II-III peptides, GLP-1 potentiation of GSIS was inhibited markedly, coincident with a decrease in GLP-1-stimulated cAMP accumulation and the redistribution of Ca(v)1.2 and Ca(v)1.3 out of lipid rafts. Neither the Ca(v)1.2/II-III nor the Ca(v)1.3/II-III peptide decreased L-type current density compared with untransfected INS-1 cells. GLP-1 potentiation of GSIS was restored by the L-type channel agonist 2,5-dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylic acid methyl ester (FPL-64176). In contrast, potentiation of GSIS by 8-bromo-cAMP (8-Br-cAMP) was inhibited in Ca(v)1.2/II-III but not Ca(v)1.3/II-III cells. These differences may involve unique protein-protein interactions because the Ca(v)1.2/II-III peptide, but not the Ca(v)1.3/II-III peptide, immunoprecipitates Rab3-interacting molecule (RIM) 2 from INS-1 cell lysates. RIM2, and its binding partner Piccolo, localize to lipid rafts, and they may serve as anchors for Ca(v)1.2 localization to lipid rafts in INS-1 cells. These findings suggest that the II-III interdomain loops of Ca(v)1.2, and possibly Ca(v)1.3, direct these channels to membrane microdomains in which the proteins that mediate potentiation of GSIS by GLP-1 and 8-Br-cAMP assemble.

Enhanced Rap1 activation and insulin secretagogue properties of an acetoxymethyl ester of an Epac-selective cyclic AMP analog in rat INS-1 cells: studies with 8-pCPT-2'-O-Me-cAMP-AM

To ascertain the identities of cyclic nucleotide-binding proteins that mediate the insulin secretagogue action of cAMP, the possible contributions of the exchange protein directly activated by cAMP (Epac) and protein kinase A (PKA) were evaluated in a pancreatic beta cell line (rat INS-1 cells). Assays of Rap1 activation, CREB phosphorylation, and PKA-dependent gene expression were performed in combination with live cell imaging and high throughput screening of a fluorescence resonance energy transfer-based cAMP sensor (Epac1-camps) to validate the selectivity with which acetoxymethyl esters (AM-esters) of cAMP analogs preferentially activate Epac or PKA. Selective activation of Epac or PKA was achieved following exposure of INS-1 cells to 8-pCPT-2'-O-Me-cAMP-AM or Bt(2)cAMP-AM, respectively. Both cAMP analogs exerted dose-dependent and glucose metabolism-dependent actions to stimulate insulin secretion, and when each was co-administered with the other, a supra-additive effect was observed. Because 2.4-fold more insulin was secreted in response to a saturating concentration (10 microm) of Bt(2)cAMP-AM as compared with 8-pCPT-2'-O-Me-cAMP-AM, and because the action of Bt(2)cAMP-AM but not 8-pCPT-2'-O-Me-cAMP-AM was nearly abrogated by treatment with 3 microm of the PKA inhibitor H-89, it is concluded that for INS-1 cells, it is PKA that acts as the dominant cAMP-binding protein in support of insulin secretion. Unexpectedly, 10-100 microm of the non-AM-ester of 8-pCPT-2'-O-Me-cAMP failed to stimulate insulin secretion and was a weak activator of Rap1 in INS-1 cells. Moreover, 10 microm of the AM-ester of 8-pCPT-2'-O-Me-cAMP stimulated insulin secretion from mouse islets, whereas the non-AM-ester did not. Thus, the membrane permeability of 8-pCPT-2'-O-Me-cAMP in insulin-secreting cells is so low as to limit its biological activity. It is concluded that prior reports documenting the failure of 8-pCPT-2'-O-Me-cAMP to act in beta cells, or other cell types, need to be re-evaluated through the use of the AM-ester of this cAMP analog.

Epac-selective cAMP analogs: new tools with which to evaluate the signal transduction properties of cAMP-regulated guanine nucleotide exchange factors

The identification of 2'-O-methyl substituted adenosine-3',5'-cyclic monophosphate (cAMP) analogs that activate the Epac family of cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs, also known as Epac1 and Epac2), has ushered in a new era of cyclic nucleotide research in which previously unrecognized signalling properties of the second messenger cAMP have been revealed. These Epac-Selective Cyclic AMP Analogs (ESCAs) incorporate a 2'-O-methyl substitution on the ribose ring of cAMP, a modification that impairs their ability to activate protein kinase A (PKA), while leaving intact their ability to activate Epac (the Exchange Protein directly Activated by Cyclic AMP). One such ESCA in wide-spread use is 8-pCPT-2'-O-Me-cAMP. It is a cell-permeant derivative of 2'-O-Me-cAMP, and it is a super activator of Epac. A wealth of newly published studies demonstrate that 8-pCPT-2'-O-Me-cAMP is a unique tool with which to asses atypical actions of cAMP that are PKA-independent. Particularly intriguing are recent reports demonstrating that ESCAs reproduce the PKA-independent actions of ligands known to stimulate Class I (Family A) and Class II (Family B) GTP-binding protein-coupled receptors (GPCRs). This topical review summarizes the current state of knowledge regarding the molecular pharmacology and signal transduction properties of Epac-selective cAMP analogs. Special attention is focused on the rational drug design of ESCAs in order to improve their Epac selectivity, membrane permeability, and stability. Also emphasized is the usefulness of ESCAs as new tools with which to assess the role of Epac as a determinant of intracellular Ca2+ signalling, ion channel function, neurotransmitter release, and hormone secretion.

SNAREing voltage-gated K+ and ATP-sensitive K+ channels: tuning beta-cell excitability with syntaxin-1A and other exocytotic proteins

The three SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins, syntaxin, SNAP25 (synaptosome-associated protein of 25 kDa), and synaptobrevin, constitute the minimal machinery for exocytosis in secretory cells such as neurons and neuroendocrine cells by forming a series of complexes prior to and during vesicle fusion. It was subsequently found that these SNARE proteins not only participate in vesicle fusion, but also tether with voltage-dependent Ca(2+) channels to form an excitosome that precisely regulates calcium entry at the site of exocytosis. In pancreatic islet beta-cells, ATP-sensitive K(+) (K(ATP)) channel closure by high ATP concentration leads to membrane depolarization, voltage-dependent Ca(2+) channel opening, and insulin secretion, whereas subsequent opening of voltage-gated K(+) (Kv) channels repolarizes the cell to terminate exocytosis. We have obtained evidence that syntaxin-1A physically interacts with Kv2.1 (the predominant Kv in beta-cells) and the sulfonylurea receptor subunit of beta-cell K(ATP) channel to modify their gating behaviors. A model has proposed that the conformational changes of syntaxin-1A during exocytosis induce distinct functional modulations of K(ATP) and Kv2.1 channels in a manner that optimally regulates cell excitability and insulin secretion. Other proteins involved in exocytosis, such as Munc-13, tomosyn, rab3a-interacting molecule, and guanyl nucleotide exchange factor II, have also been implicated in direct or indirect regulation of beta-cell ion channel activities and excitability. This review discusses this interesting aspect that exocytotic proteins not only promote secretion per se, but also fine-tune beta-cell excitability via modulation of ion channel gating.

Functional interaction of neuronal Cav1.3 L-type calcium channel with ryanodine receptor type 2 in the rat hippocampus

Neuronal L-type Ca(2+) channels do not support synaptic transmission, but they play an essential role in synaptic activity-dependent gene expression. Ca(v)1.2 and Ca(v)1.3 are the two most widely expressed L-type Ca(2+) channels in neurons and have different biophysical and subcellular distributions. The function of the Ca(v) 1.3 L-type Ca(2+) channel and its cellular mechanisms in the central nervous system are poorly understood. In this study, using a yeast two-hybrid assay, we found that the N terminus of the rat Ca(v)1.3 alpha(1) subunit interacts with a partial N-terminal amino acid sequence of ryanodine receptor type 2 (RyR2). Reverse transcription-PCR and Western blot assays revealed high expression of both Ca(v)1.3 and RyR2 in the rat hippocampus. We also demonstrate a physical association of Ca(v)1.3 with RyR2 using co-immunoprecipitation assays. Moreover, immunocytochemistry revealed prominent co-localization between Ca(v)1.3 and RyR2 in hippocampal neurons. Depolarizing cells by an acute treatment of a high concentration of KCl (high-K, 60 mm) showed that the activation of L-type Ca(2+) channels induced RyR opening and led to RyR-dependent Ca(2+) release, even in the absence of extracellular Ca(2+). Furthermore, we found that RyR2 mRNA itself is increased by long term treatment of high-K via activation of L-type Ca(2+) channels. These acute and long term effects of high-K on RyRs were selectively blocked by small interfering RNA-mediated silencing of Ca(v)1.3. These results suggest a physical and functional interaction between Ca(v)1.3 and RyR2 and important implications of Ca(v)1.3/RyR2 clusters in translating synaptic activity into alterations in gene expression.

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.