Evaluation of cerebrospinal fluid heparan sulfate as a biomarker of neuropathology in a murine model of mucopolysaccharidosis type II using high-sensitivity LC/MS/MS.

Mucopolysaccharidosis type II (MPS II or Hunter syndrome) is a lysosomal storage disorder caused by a deficiency of iduronate-2-sulfatase (IDS), an enzyme that catabolizes glycosaminoglycans (GAGs) including heparan sulfate (HS) and dermatan sulfate (DS). GAG accumulation leads to severe neurological and somatic impairments. At present, the most common treatment for MPS II is intravenous enzyme replacement therapy; however, the inability of recombinant IDS to cross the blood-brain barrier (BBB) restricts therapeutic efficacy for neurological manifestations. We recently developed a BBB-penetrating IDS fusion protein, JR-141, and demonstrated its ability to reduce GAG accumulation in the brain of human transferrin receptor knock-in and Ids knock-out mice (TFRC-KI/Ids-KO), an animal model of MPS II, following intravenous administration. Given the impossibility of measuring GAG accumulation in the brains of human patients with MPS II, we hypothesized that GAG content in the cerebrospinal fluid (CSF) might serve as an indicator of brain GAG burden. To test this hypothesis, we optimized a high-sensitivity method for quantifying HS and DS in low-volume samples by combining acidic methanolysis and liquid chromatography-tandem mass spectrometry (LC/MS/MS). We employed this method to quantify HS and DS in samples from TFRC-KI/Ids-KO mice and revealed that HS but not DS accumulated in the central nerve system (CNS). Moreover, concentrations of HS in CSF correlated with those in brain. Finally, intravenous treatment with JR-141 reduced levels of HS in the CSF and brain in TFRC-KI/Ids-KO mice. These results suggest that CSF HS content may be a useful biomarker for evaluating the brain GAG accumulation and the therapeutic efficacy of drugs in patients with MPS II.

Actin dynamics regulated by the balance of neuronal Wiskott-Aldrich syndrome protein (N-WASP) and cofilin activities determines the biphasic response of glucose-induced insulin secretion.

Actin dynamics in pancreatic ß-cells is involved in insulin secretion. However, the molecular mechanisms of the regulation of actin dynamics by intracellular signals in pancreatic ß-cells and its role in phasic insulin secretion are largely unknown. In this study, we elucidate the regulation of actin dynamics by neuronal Wiskott-Aldrich syndrome protein (N-WASP) and cofilin in pancreatic ß-cells and demonstrate its role in glucose-induced insulin secretion (GIIS). N-WASP, which promotes actin polymerization through activation of the actin nucleation factor Arp2/3 complex, was found to be activated by glucose stimulation in insulin-secreting clonal pancreatic ß-cells (MIN6-K8 ß-cells). Introduction of a dominant-negative mutant of N-WASP, which lacks G-actin and Arp2/3 complex-binding region VCA, into MIN6-K8 ß-cells or knockdown of N-WASP suppressed GIIS, especially the second phase. We also found that cofilin, which severs F-actin in its dephosphorylated (active) form, is converted to the phosphorylated (inactive) form by glucose stimulation in MIN6-K8 ß-cells, thereby promoting F-actin remodeling. In addition, the dominant-negative mutant of cofilin, which inhibits activation of endogenous cofilin, or knockdown of cofilin reduced the second phase of GIIS. However, the first phase of GIIS occurs in the G-actin predominant state, in which cofilin activity predominates over N-WASP activity. Thus, actin dynamics regulated by the balance of N-WASP and cofilin activities determines the biphasic response of GIIS.

Mouse model of Prinzmetal angina by disruption of the inward rectifier Kir6.1.

The inwardly rectifying K(+) channel Kir6.1 forms K(+) channels by coupling with a sulfonylurea receptor in reconstituted systems, but the physiological roles of Kir6.1-containing K(+) channels have not been determined. We report here that mice lacking the gene encoding Kir6.1 (known as Kcnj8) have a high rate of sudden death associated with spontaneous ST elevation followed by atrioventricular block as seen on an electrocardiogram. The K(+) channel opener pinacidil did not induce K(+) currents in vascular smooth-muscle cells of Kir6.1-null mice, and there was no vasodilation response to pinacidil. The administration of methylergometrine, a vasoconstrictive agent, elicited ST elevation followed by cardiac death in Kir6.1-null mice but not in wild-type mice, indicating a phenotype characterized by hypercontractility of coronary arteries and resembling Prinzmetal (or variant) angina in humans. The Kir6.1-containing K(+) channel is critical in the regulation of vascular tonus, especially in the coronary arteries, and its disruption may cause Prinzmetal angina.

[Regulatory mechanism of incretin secretion].

Incretin hormones GIP(glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) improve glycemic control by potentiating glucose-induced insulin secretion in pancreatic beta-cells and also have beneficial effects on appetite control and body weight. In response to food ingestion, GIP and GLP-1 are secreted from enteroendocrine K- and L-cells, respectively. In these cells, it is shown that a variety of molecular sensors are involved in the detection of carbohydrates, lipids, and proteins. In view of development of new incretin-related drugs, these sensors are attractive targets to enhance the endogenous pools of incretins.

Rab11 and its effector Rip11 participate in regulation of insulin granule exocytosis.

Rab GTPases and their effectors play important roles in membrane trafficking between cellular compartments in eukaryotic cells. In the present study, we examined the roles of Rab11B and its effectors in insulin secretion in pancreatic beta-cells. In the mouse insulin-secreting cell line MIN6, Rab11 was co-localized with insulin-containing granules, and over-expression of the GTP- or the GDP-bound form of Rab11B significantly inhibited regulated secretion, indicating involvement of Rab11B in regulated insulin secretion. To determine the downstream signal of Rab11-mediated insulin secretion, we examined the effects of various Rab11-interacting proteins on insulin secretion, and found that Rip11 is involved in cAMP-potentiated insulin secretion but not in glucose-induced insulin secretion. Analyses by immunocytochemistry and subcellular fractionation revealed Rip11 to be co-localized with insulin granules. The inhibitory effect of the Rip11 mutant was not altered in MIN6 cells lacking Epac2, which mediates protein kinase A (PKA)-independent potentiation of insulin secretion, compared with wild-type MIN6 cells. In addition, Rip11 was found to be phosphorylated by PKA in MIN6 cells. The present study shows that both Rab11 and its effector Rip11 participate in insulin granule exocytosis and that Rip11, as a substrate of PKA, regulates the potentiation of exocytosis by cAMP in pancreatic beta-cells.

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.

Pancreatic beta-cell signaling: toward better understanding of diabetes and its treatment.

Pancreatic beta-cells play a central role in the maintenance glucose homeostasis by secreting insulin, a key hormone that regulates blood glucose levels. Dysfunction of the beta-cells and/or a decrease in the beta-cell mass are associated closely with the pathogenesis and pathophysiology of diabetes mellitus, a major metabolic disease that is rapidly increasing worldwide. Clarification of the mechanisms of insulin secretion and beta-cell fate provides a basis for the understanding of diabetes and its better treatment. In this review, we discuss cell signaling critical for the insulin secretory function based on our recent studies.

Critical role of the N-terminal cyclic AMP-binding domain of Epac2 in its subcellular localization and function.

cAMP is a well-known regulator of exocytosis, and cAMP-GEFII (Epac2) is involved in the potentiation of cAMP-dependent, PKA-independent regulated exocytosis in secretory cells. However, the mechanisms of its action are not fully understood. In the course of our study of Epac2 knockout mice, we identified a novel splicing variant of Epac2, which we designate Epac2B, while renaming the previously identified Epac2 Epac2A. Epac2B, which lacks the first cAMP-binding domain A in the N-terminus but has the second cAMP-binding domain B of Epac2A, possesses GEF activity towards Rap1, as was found for Epac2A. Immunocytochemical analysis revealed that exogenously introduced Epac2A into insulin-secreting MIN6 cells was localized near the plasma membrane, while Epac2B was found primarily in the cytoplasm. Interestingly, cAMP-binding domain A alone introduced into MIN6 cells was also localized near the plasma membrane. In MIN6 cells, Epac2A was involved in triggering hormone secretion by stimulation with 5.6 mM glucose plus 1 mM 8-Bromo-cAMP, but Epac2B was not. The addition of a membrane-targeting signal to the N-terminus of Epac2B was able to mimic the effect of Epac2A on hormone secretion. Thus, the present study indicates that the N-terminal cAMP-binding domain A of Epac2A plays a critical role in determining its subcellular localization and potentiating insulin secretion by cAMP. J. Cell. Physiol. 219: 652-658, 2009. (c) 2009 Wiley-Liss, Inc.

Identification and characterization of a novel member of the ATP-sensitive K+ channel subunit family, Kir6.3, in zebrafish.

ATP-sensitive K+ (KATP) channels play a crucial role in coupling cellular metabolism to membrane potential. In addition to the orthologs corresponding to Kir6.1 and Kir6.2 of mammals, we have identified a novel member, designated Kir6.3 (zKir6.3), of the inward rectifier K+ channel subfamily Kir6.x in zebrafish. zKir6.3 is a protein of 432 amino acids that shares 66% identity with mammalian Kir6.2 but differs considerably from mammalian Kir6.1 and Kir6.2 in the COOH terminus, which contain an Arg-Lys-Arg (RKR) motif, an endoplasmic reticulum (ER) retention signal. Single-channel recordings of reconstituted channels show that zKir6.3 requires the sulfonylurea receptor 1 (SUR1) subunit to produce KATP channel currents with single-channel conductance of 57.5 pS. Confocal microscopic analysis shows that zebrafish Kir6.3 requires the SUR1 subunit for its trafficking to the plasma membrane. Analyses of chimeric protein between human Kir6.2 and zKir6.3 and a COOH-terminal deletion of zKir6.3 indicate that interaction between the COOH terminus of zKir6.3 and SUR1 is critical for both channel activity and trafficking to the plasma membrane. We also identified zebrafish orthologs corresponding to mammalian SUR1 (zSUR1) and SUR2 (zSUR2) by the genomic database. Both Kir6.3 and SUR1 are expressed in embryonic brain of zebrafish, as assessed by whole mount in situ hybridization. These data indicate that Kir6.3 and SUR1 form functional KATP channels at the plasma membrane in zebrafish through a mechanism independent from ER retention by the RKR motif.

Structure and functional roles of Epac2 (Rapgef4).

Epac (exchange protein activated by cyclic-AMP) 2 is a direct target of 3'-5'-cyclic adenosine monophosphate (cAMP) and is involved in cAMP-mediated signal transduction through activation of the Ras-like small GTPase Rap. Crystallographic analyses revealed that activation of Epac2 by cAMP is accompanied by dynamic structural changes. Epac2 is expressed mainly in brain, neuroendocrine and endocrine tissues, and is involved in diverse cellular functions in the tissues. In this review, we summarize the structure and function of Epac2. We also discuss the physiological and pathophysiological roles of Epac2, and the possibility of Epac2 as a therapeutic target.

Integration of ATP, cAMP, and Ca2+ signals in insulin granule exocytosis.

Intracellular ATP, cAMP, and Ca2+ are major signals involved in the regulation of insulin secretion in the pancreatic beta-cell. We recently found that the ATP-sensitive K+ channel (KATP channel) as an ATP sensor, cAMP-GEFII as a cAMP sensor, Piccolo as a Ca2+ sensor, and L-type voltage-dependent Ca2+ channel (VDCC) can interact with each other. In the present study, we examined the effects of cAMP and ATP on the interaction of cAMP-GEFII and sulfonylurea receptor-1 (SUR1). Interaction of cAMP-GEFII with SUR1 was inhibited by the cAMP analog 8-bromo-cAMP but not by ATP, and the inhibition by 8-bromo-cAMP persisted in the presence of ATP. In addition, SUR1, cAMP-GEFII, and Piccolo could form a complex. Piccolo also interacted with the alpha1 1.2 subunit of VDCC in a Ca2+-independent manner. These data suggest that the interactions of the KATP channel, cAMP-GEFII, Piccolo, and L-type VDCC are regulated by intracellular signals such as cAMP and Ca2+ and that the ATP, cAMP, and Ca2+ signals are integrated at a specialized region of pancreatic beta-cells.

Functional analysis of transcriptional repressor Otx3/Dmbx1.

Otx3/Dmbx1 is a member of paired class homeodomain transcription factors. In this study, we found that Otx3/Dmbx1 represses the Otx2-mediated transactivation by forming heterodimer with Otx2 on the P3C (TAATCCGATTA) sequence in vitro. The 156 amino acid region (residues 1-156) of Otx3/Dmbx1 is required for its repressor activity, and interacts directly with Otx2. Co-localization of Otx3/Dmbx1 and Otx2 in brain sections was confirmed by in situ hybridization. These data suggest that Otx3/Dmbx1 represses Otx2-mediated transcription in the developing brain. We also identified the consensus binding sequence [TAATCCGATTA and TAATCC(N2-4)TAATCC] of Otx3/Dmbx1.

Physical and functional interaction of noc2/rab3 in exocytosis.

Rab, monomeric small Ras-like GTPase, regulates intracellular membrane trafficking in eukaryotic cells. Rab3 is involved in the exocytotic process in a variety of secretory cells including neuronal, neuroendocrine, endocrine, and exocrine cells. Noc2, originally identified as a molecule homologous to Rabphilin-3, is a putative effector of Rab3. Noc2 interacts with the active (GTP-bound) form of Rab3 and regulates hormone secretion in neuroendocrine and endocrine cells and enzyme release in exocrine cells. This chapter describes two kinds of interaction assay by which the association of Noc2 with Rab3 is analyzed: a yeast two-hybrid assay to detect the interaction of Noc2 with the active form of Rab3 in intact cells and a pull-down assay using GST-fused Noc2 protein to ascertain the physical interaction of Noc2 and Rab3 in vitro. Thus, the Noc2 knockout mouse is a useful model for studying the functional consequences of disruption of the interaction.

Role of Epac2A/Rap1 signaling in interplay between incretin and sulfonylurea in insulin secretion.

Incretin-related drugs and sulfonylureas are currently used worldwide for the treatment of type 2 diabetes. We recently found that Epac2A, a cAMP binding protein having guanine nucleotide exchange activity toward Rap, is a target of both incretin and sulfonylurea. This suggests the possibility of interplay between incretin and sulfonylurea through Epac2A/Rap1 signaling in insulin secretion. In this study, we examined the combinatorial effects of incretin and various sulfonylureas on insulin secretion and activation of Epac2A/Rap1 signaling. A strong augmentation of insulin secretion by combination of GLP-1 and glibenclamide or glimepiride, which was found in Epac2A(+/+) mice, was markedly reduced in Epac2A(-/-) mice. In contrast, the combinatorial effect of GLP-1 and gliclazide was rather mild, and the effect was not altered by Epac2A ablation. Activation of Rap1 was enhanced by the combination of an Epac-selective cAMP analog with glibenclamide or glimepiride but not gliclazide. In diet-induced obese mice, ablation of Epac2A reduced the insulin secretory response to coadministration of the GLP-1 receptor agonist liraglutide and glimepiride. These findings clarify the critical role of Epac2A/Rap1 signaling in the augmenting effect of incretin and sulfonylurea on insulin secretion and provide the basis for the effects of combination therapies of incretin-related drugs and sulfonylureas.

Glutamate acts as a key signal linking glucose metabolism to incretin/cAMP action to amplify insulin secretion.

Incretins, hormones released by the gut after meal ingestion, are essential for maintaining systemic glucose homeostasis by stimulating insulin secretion. The effect of incretins on insulin secretion occurs only at elevated glucose concentrations and is mediated by cAMP signaling, but the mechanism linking glucose metabolism and cAMP action in insulin secretion is unknown. We show here, using a metabolomics-based approach, that cytosolic glutamate derived from the malate-aspartate shuttle upon glucose stimulation underlies the stimulatory effect of incretins and that glutamate uptake into insulin granules mediated by cAMP/PKA signaling amplifies insulin release. Glutamate production is diminished in an incretin-unresponsive, insulin-secreting ß cell line and pancreatic islets of animal models of human diabetes and obesity. Conversely, a membrane-permeable glutamate precursor restores amplification of insulin secretion in these models. Thus, cytosolic glutamate represents the elusive link between glucose metabolism and cAMP action in incretin-induced insulin secretion.

Antidiabetic sulfonylureas and cAMP cooperatively activate Epac2A.

Sulfonylureas are widely used drugs for treating insulin deficiency in patients with type 2 diabetes. Sulfonylureas bind to the regulatory subunit of the pancreatic ß cell potassium channel that controls insulin secretion. Sulfonylureas also bind to and activate Epac2A, a member of the Epac family of cyclic adenosine monophosphate (cAMP)-binding proteins that promote insulin secretion through activation of the Ras-like guanosine triphosphatase Rap1. Using molecular docking simulation, we identified amino acid residues in one of two cyclic nucleotide-binding domains, cNBD-A, in Epac2A predicted to mediate the interaction with sulfonylureas. We confirmed the importance of the identified residues by site-directed mutagenesis and analysis of the response of the mutants to sulfonylureas using two assays: changes in fluorescence resonance energy transfer (FRET) of an Epac2A-FRET biosensor and direct sulfonylurea-binding experiments. These residues were also required for the sulfonylurea-dependent Rap1 activation by Epac2A. Binding of sulfonylureas to Epac2A depended on the concentration of cAMP and the structures of the drugs. Sulfonylureas and cAMP cooperatively activated Epac2A through binding to cNBD-A and cNBD-B, respectively. Our data suggest that sulfonylureas stabilize Epac2A in its open, active state and provide insight for the development of drugs that target Epac2A.

GLP-1 receptor activation and Epac2 link atrial natriuretic peptide secretion to control of blood pressure.

Glucagon-like peptide-1 receptor (GLP-1R) agonists exert antihypertensive actions through incompletely understood mechanisms. Here we demonstrate that cardiac Glp1r expression is localized to cardiac atria and that GLP-1R activation promotes the secretion of atrial natriuretic peptide (ANP) and a reduction of blood pressure. Consistent with an indirect ANP-dependent mechanism for the antihypertensive effects of GLP-1R activation, the GLP-1R agonist liraglutide did not directly increase the amount of cyclic GMP (cGMP) or relax preconstricted aortic rings; however, conditioned medium from liraglutide-treated hearts relaxed aortic rings in an endothelium-independent, GLP-1R-dependent manner. Liraglutide did not induce ANP secretion, vasorelaxation or lower blood pressure in Glp1r(-/-) or Nppa(-/-) mice. Cardiomyocyte GLP-1R activation promoted the translocation of the Rap guanine nucleotide exchange factor Epac2 (also known as Rapgef4) to the membrane, whereas Epac2 deficiency eliminated GLP-1R-dependent stimulation of ANP secretion. Plasma ANP concentrations were increased after refeeding in wild-type but not Glp1r(-/-) mice, and liraglutide increased urine sodium excretion in wild-type but not Nppa(-/-) mice. These findings define a gut-heart GLP-1R-dependent and ANP-dependent axis that regulates blood pressure.

Dynamics of insulin secretion and the clinical implications for obesity and diabetes.

Insulin secretion is a highly dynamic process regulated by various factors including nutrients, hormones, and neuronal inputs. The dynamics of insulin secretion can be studied at different levels: the single ß cell, pancreatic islet, whole pancreas, and the intact organism. Studies have begun to analyze cellular and molecular mechanisms underlying dynamics of insulin secretion. This review focuses on our current understanding of the dynamics of insulin secretion in vitro and in vivo and discusses their clinical relevance.

Sulfonylurea action re-revisited.

Sulfonylureas (SU), commonly used in the treatment of type 2 diabetes mellitus (T2DM), stimulate insulin secretion by inhibiting adenosine triphosphate (ATP)-sensitive K(+) (KATP) channels in pancreatic ß-cells. SU are now known to also activate cyclic adenosine monophosphate (cAMP) sensor Epac2 (cAMP-GEFII) to Rap1 signaling, which promotes insulin secretion. The different effects of various SU on Epac2/Rap1 signaling, as well as KATP channels in different tissues, underlie the diverse pancreatic and extra-pancreatic actions of SU. (J Diabetes Invest, doi: 10.1111/j.2040-1124.2010.00014.x, 2010).