First phase of glucose-stimulated insulin secretion from MIN 6 cells does not always require extracellular calcium influx

Kinesin-1 mediates proper ER folding of the CaV1.2 channel and maintains mouse glucose homeostasis

Glucose-stimulated insulin secretion (GSIS) from pancreatic beta cells is a principal mechanism for systemic glucose homeostasis, of which regulatory mechanisms are still unclear. Here we show that kinesin molecular motor KIF5B is essential for GSIS through maintaining the voltage-gated calcium channel CaV1.2 levels, by facilitating an Hsp70-to-Hsp90 chaperone exchange to pass through the quality control in the endoplasmic reticulum (ER). Phenotypic analyses of KIF5B conditional knockout (cKO) mouse beta cells revealed significant abolishment of glucose-stimulated calcium transients, which altered the behaviors of insulin granules via abnormally stabilized cortical F-actin. KIF5B and Hsp90 colocalize to microdroplets on ER sheets, where CaV1.2 but not Kir6.2 is accumulated. In the absence of KIF5B, CaV1.2 fails to be transferred from Hsp70 to Hsp90 via STIP1, and is likely degraded via the proteasomal pathway. KIF5B and Hsc70 overexpression increased CaV1.2 expression via enhancing its chaperone binding. Thus, ER sheets may serve as the place of KIF5B- and Hsp90-dependent chaperone exchange, which predominantly facilitates CaV1.2 production in beta cells and properly enterprises GSIS against diabetes.

Engineering water exchange is a safe and effective method for magnetic resonance imaging in diverse cell types

Aquaporin-1 (Aqp1), a water channel, has garnered significant interest for cell-based medicine and in vivo synthetic biology due to its ability to be genetically encoded to produce magnetic resonance signals by increasing the rate of water diffusion in cells. However, concerns regarding the effects of Aqp1 overexpression and increased membrane diffusivity on cell physiology have limited its widespread use as a deep-tissue reporter. In this study, we present evidence that Aqp1 generates strong diffusion-based magnetic resonance signals without adversely affecting cell viability or morphology in diverse cell lines derived from mice and humans. Our findings indicate that Aqp1 overexpression does not induce ER stress, which is frequently associated with heterologous expression of membrane proteins. Furthermore, we observed that Aqp1 expression had no detrimental effects on native biological activities, such as phagocytosis, immune response, insulin secretion, and tumor cell migration in the analyzed cell lines. These findings should serve to alleviate any lingering safety concerns regarding the utilization of Aqp1 as a genetic reporter and should foster its broader application as a noninvasive reporter for in vivo studies.

Engineering water exchange is a safe and effective method for magnetic resonance imaging in diverse cell types

Aquaporin-1 (Aqp1), a water channel, has garnered significant interest for cell-based medicine and in vivo synthetic biology due to its ability to be genetically encoded to produce magnetic resonance signals by increasing the rate of water diffusion in cells. However, concerns regarding the effects of Aqp1 overexpression and increased membrane diffusivity on cell physiology have limited its widespread use as a deep-tissue reporter. In this study, we present evidence that Aqp1 generates strong diffusion-based magnetic resonance signals without adversely affecting cell viability or morphology in diverse cell lines derived from mice and humans. Our findings indicate that Aqp1 overexpression does not induce ER stress, which is frequently associated with heterologous expression of membrane proteins. Furthermore, we observed that Aqp1 expression had no detrimental effects on native biological activities, such as phagocytosis, immune response, insulin secretion, and tumor cell migration in the analyzed cell lines. These findings should serve to alleviate any lingering safety concerns regarding the utilization of Aqp1 as a genetic reporter and should foster its broader application as a noninvasive reporter for in vivo studies.

Calcium modulation of exocytosis-linked plasma membrane potential oscillations in INS-1 832/13 cells

Oscillations in plasma membrane potential initiated by substrate-dependent blockade of ATP-sensitive K+ channels in insulin-secreting INS-1 832/13 are differentially linked to distinct voltage-activated Ca2+ channels and drive exocytosis. Ca2+ feeds back to control oscillation frequency, amplitude and prevalence.

A BRET-based homogeneous insulin assay using interacting domains in the primary binding site of the insulin receptor

A new homogeneous insulin assay requiring no chemical modification of an insulin recognition domain, which can be applied to continuous monitoring of the time-dependent cellular response in vitro, was developed. The carboxy-terminal α-chain (αCT) segment and first leucine-rich-repeat (L1) domain in the primary binding site on the insulin receptor were genetically fused with a bioluminescent protein (Nanoluc, Nluc) and a fluorescent protein (yellow fluorescent protein, YPet) to produce the insulin-sensing probe proteins Nluc-αCT and L1-YPet. The BRET signal was observed on simple mixing of insulin with these protein probes, in a so-called homogeneous assay. The BRET signal was proportional to the insulin concentration, and the lower detection limit was 0.8 μM. Time-dependent insulin secretion from drug-stimulated MIN6 cells was also successfully monitored continuously with the probe proteins. This BRET-based homogeneous insulin assay method is thus expected to be applicable to drug development by high-throughput screening.

OATP1B3 is expressed in pancreatic β-islet cells and enhances the insulinotropic effect of the sulfonylurea derivative glibenclamide

Organic anion transporting polypeptide OATP1B3 is a membrane-bound drug transporter that facilitates cellular entry of a variety of substrates. Most of the previous studies focused on its hepatic expression and function in hepatic drug elimination. In this study, we report expression of OATP1B3 in human pancreatic tissue, with the abundance of the transporter localized in the islets of Langerhans. Transport studies using OATP1B3-overexpressing MDCKII cells revealed significant inhibition of the cellular uptake of the known substrate cholecystokinin-8 in the presence of the insulinotropic antidiabetes compounds tolbutamide, glibenclamide, glimepiride, and nateglinide and identified glibenclamide as a novel substrate of OATP1B3. Sulfonylurea derivatives exert their insulinotropic effect by binding to the SUR1 subunit of the KATP channels inducing insulin secretion in β-cells. Here, we show that transient overexpression of human OATP1B3 in a murine β-cell line (MIN6)-which exhibits glucose and glibenclamide-sensitive insulin secretion-significantly enhances the insulinotropic effect of glibenclamide without affecting glucose-stimulated insulin secretion. Taken together, our data provide evidence that the drug transporter OATP1B3 functions as a determinant of the insulinotropic effect of glibenclamide on the tissue level. Changes in transport activity based on drug-drug interactions or genetic variability may therefore influence glibenclamide efficacy.

Transglutaminase 2 transamidation activity during first-phase insulin secretion: natural substrates in INS-1E

Transglutaminase 2 (TG2) is a multifunctional protein with Ca(2+)-dependent transamidating and G protein activity. Previously, we reported that tgm2 -/- mice have an impaired insulin secretion and that naturally occurring TG2 mutations associated with familial, early-onset type 2 diabetes, show a defective transamidating activity. Aim of this study was to get a better insight into the role of TG2 in insulin secretion by identifying substrates of TG2 transamidating activity in the pancreatic beta cell line INS-1E. To this end, we labeled INS-1E that are capable of secreting insulin upon glucose stimulation in the physiologic range, with an artificial acyl acceptor (biotinamido-pentylamine) or donor (biotinylated peptide), in basal condition and after stimulus with glucose for 2, 5, and 8 min. Biotinylated proteins were analyzed by two-dimensional electrophoresis and mass spectrometry. In addition, subcellular localization of TG2 in human endocrine pancreas was studied by electron microscopy. Among several TG2's transamidating substrates in INS-1E, mass spectrometry identified cytoplasmic actin (a result confirmed in human pancreatic islet), tropomyosin, and molecules that participate in insulin granule structure (e.g., GAPDH), glucose metabolism, or [Ca(2+)] sensing (e.g., calreticulin). Physical interaction between TG2 and cytoplasmic actin during glucose-stimulated first-phase insulin secretion was confirmed by co-immunoprecipitation. Electron microscopy revealed that TG2 is localized close to insulin and glucagon granules in human pancreatic islet. We propose that TG2's role in insulin secretion may involve cytoplasmic actin remodeling and may have a regulative action on other proteins during granule movement. A similar role of TG2 in glucagon secretion is also suggested.

Nanospaces between endoplasmic reticulum and mitochondria as control centres of pancreatic β-cell metabolism and survival

Nanometre-scale spaces between organelles represent focused nodes for signal transduction and the control of cellular decisions. The endoplasmic reticulum (ER) and the mitochondria form dynamic quasi-synaptic interaction nanodomains in all cell types examined, but the functional role of these junctions in cellular metabolism and cell survival remains to be fully understood. In this paper, we review recent evidence that ER Ca(2+) channels, such as the RyR and IP(3)R, can signal specifically across this nanodomain to the adjacent mitochondria to pace basal metabolism, with focus on the pancreatic β-cell. Blocking these signals in the basal state leads to a form of programmed cell death associated with reduced ATP and the induction of calpain-10 and hypoxia-inducible factors. On the other hand, the hyperactivity of this signalling domain plays a deleterious role during classical forms of apoptosis. Thus, the nanospace between ER and mitochondria represents a critical rheostat controlling both metabolism and programmed cell death. Many aspects of the mechanisms underlying this control system remain to be uncovered, and new nanotechnologies are required understand these domains at a molecular level.

Differential phosphorylation of RhoGDI mediates the distinct cycling of Cdc42 and Rac1 to regulate second-phase insulin secretion

Cdc42 cycling through GTP/GDP states is critical for its function in the second/granule mobilization phase of insulin granule exocytosis in pancreatic islet beta cells, although the identities of the Cdc42 cycling proteins involved remain incomplete. Using a tandem affinity purification-based mass spectrometry screen for Cdc42 cycling factors in beta cells, RhoGDI was identified. RNA interference-mediated depletion of RhoGDI from isolated islets selectively amplified the second phase of insulin release, consistent with the role of RhoGDI as a Cdc42 cycling factor. Replenishment of RhoGDI to RNA interference-depleted cells normalized secretion, confirming the action of RhoGDI to be that of a negative regulator of Cdc42 activation. Given that RhoGDI also regulates Rac1 activation in beta cells, and that Rac1 activation occurs in a Cdc42-dependent manner, the question as to how the beta cell utilized RhoGDI for differential Cdc42 and Rac1 cycling was explored. Co-immunoprecipitation was used to determine that RhoGDI-Cdc42 complexes dissociated upon stimulation of beta cells with glucose for 3 min, correlating with the timing of glucose-induced Cdc42 activation and the onset of RhoGDI tyrosine phosphorylation. Glucose-induced disruption of RhoGDI-Rac1 complexes occurred subsequent to this, coincident with Rac1 activation, which followed the onset of RhoGDI serine phosphorylation. RhoGDI-Cdc42 complex dissociation was blocked by mutation of RhoGDI residue Tyr-156, whereas RhoGDI-Rac1 dissociation was blocked by RhoGDI mutations Y156F and S101A/S174A. Finally, expression of a triple Y156F/S101A/S174A-RhoGDI mutant specifically inhibited only the second/granule mobilization phase of glucose-stimulated insulin secretion, overall supporting the integration of RhoGDI into the activation cycling mechanism of glucose-responsive small GTPases.

Aqueous extract of Abutilon indicum Sweet inhibits glucose absorption and stimulates insulin secretion in rodents

The objective of this study was to evaluate the antidiabetic effects of the aqueous extract derived from the Thai Abutilon indicum Sweet plant and to explore its effects on intestinal glucose absorption and insulin secretion. The authors hypothesized that the plasma glucose level could be reduced through the inhibition of glucose absorption and/or the enhancement of insulin secretion. Administration of the extract (0.5 and 1 g/kg body weight) in an oral glucose tolerance test led to a significant reduction in plasma glucose levels in 30 minutes after the administration in moderately diabetic rats, as compared with untreated rats (P < .05), and this was at a faster rate than the use of an antidiabetic drug, glibenclamide. The inhibition of glucose absorption through the small intestine was investigated using an everted intestinal sac. The results showed that the extract at concentrations of 0.156 to 5 mg/mL caused a reduction of glucose absorption in a dose response manner. The maximum response was noted at a dose of 2.5 mg/mL. The promotion of the extract on insulin secretion was confirmed by incubating beta cell of pancreatic islets and INS-1E insulinoma cells with the extract at 1 to 1000 microg/mL. These observations suggest that the aqueous extract from the A indicum plant has antidiabetic properties, which inhibited glucose absorption and stimulated insulin secretion. Phytochemical screening also revealed that the extract contained alkaloids, flavonoids, tannins, glycosides, and saponins that could account for the observed pharmacologic effects of the plant extract.

Low, but physiological, concentration of GLP-1 stimulates insulin secretion independent of the cAMP-dependent protein kinase pathway

Glucagon-like peptide-1 (GLP-1) induces pancreatic insulin secretion via the cAMP-dependent protein kinase (PKA) pathway. However, the GLP-1 concentration used in the previous in vitro experiments was far from the in vivo concentrations. Alteration of plasma GLP-1 concentration at pM order lowers blood glucose concentration. In this study, we examined the GLP-1 action mechanism at a physiological concentration on insulin secretion. A high concentration of GLP-1 (10 nM) stimulated intracellular cAMP accumulation and insulin secretion was significantly inhibited by KT5720, a selective inhibitor of PKA. Low GLP-1 concentrations (1 pM) also increased insulin secretion without significant accumulation of intracellular cAMP, and KT5720 did not affect insulin secretion. Insulin secretion stimulated by 1 pM GLP-1 was reduced by inhibitors of calcium action, including verapamil, dantrolene, and BAPTA. Thus, we concluded that relatively low GLP-1 concentrations-comparable to in vivo blood concentrations-promoted insulin secretion independent of the cAMP-PKA pathway. This effect was dependent on intracellular Ca2+ concentration. The results of the present study may further the understanding of the dose-dependent response of GLP-1 signal transducing pathways and the complicated mechanism of insulin secretion. Studies of GLP-1 at physiologic concentrations may lead to new developments in studies of pancreatic beta-cell function.

Nateglinide and Mitiglinide, but Not Sulfonylureas, Induce Insulin Secretion through a Mechanism Mediated by Calcium Release from Endoplasmic Reticulum

Nateglinide and mitiglinide (glinides) are characterized as rapid-onset and short-acting insulinotropic agents. Although both compounds do not have a sulfonylurea structure, it has been postulated that insulin secretion is preceded by their binding to Kir6.2/SUR1 complex, and a mechanism of insulin secretion of glinides has been accounted for by this pathway. However, we hypothesized the involvement of additional mechanisms of insulin secretion enhanced by glinides, and we analyzed the pattern of time course of insulin secretion from MIN6 cells with the existence of agents that have specific pharmacologic actions. Dose-dependent effects of tolbutamide, glibenclamide, nateglinide, and mitiglinide were observed. Insulin secretion induced by 3 microM tolbutamide and 1 nM glibenclamide was completely inhibited by 10 microM diazoxide and 3 microM verapamil, although the latter half-component of insulin secretion profile induced by 3 microM nateglinide or 30 nM mitiglinide remained with the existence of those agents. Glinides enhanced insulin secretion even in Ca2+-depleted medium, and its pattern of secretion was same as the pattern with existence of verapamil. The latter half was suppressed by 1 microM dantrolene, and concomitant addition of verapamil and dantrolene completely suppressed the entire pattern of insulin secretion enhanced by nateglinide. Thus, we conclude that glinide action is demonstrated through two pathways, dependently and independently, from the pathway through K(ATP) channels. We also demonstrated that the latter pathway involves the intracellular calcium release from endoplasmic reticulum via ryanodine receptor activation.