Role of mammalian homologue of Caenorhabditis elegans unc-13-1 (Munc13-1) in the recruitment of newcomer insulin granules in both first and second phases of glucose-stimulated insulin secretion in mouse islets

Pyruvate cycling has been implicated in glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells. The operation of some pyruvate cycling pathways is proposed to necessitate malate export from the mitochondria and NADP+-dependent decarboxylation of malate to pyruvate by cytosolic malic enzyme (ME1). Evidence in favor of and against a role of ME1 in GSIS has been presented by others using small interfering RNA-mediated suppression of ME1. ME1 was also proposed to account for methyl succinate-stimulated insulin secretion (MSSIS), which has been hypothesized to occur via succinate entry into the mitochondria in exchange for malate and subsequent malate conversion to pyruvate. In contrast to rat, mouse β-cells lack ME1 activity, which was suggested to explain their lack of MSSIS. However, this hypothesis was not tested. In this report, we demonstrate that although adenoviral-mediated overexpression of ME1 greatly augments GSIS in rat insulinoma INS-1 832/13 cells, it does not restore MSSIS, nor does it significantly affect GSIS in mouse islets. The increase in GSIS following ME1 overexpression in INS-1 832/13 cells did not alter the ATP-to-ADP ratio but was accompanied by increases in malate and citrate levels. Increased malate and citrate levels were also observed after INS-1 832/13 cells were treated with the malate-permeable analog dimethyl malate. These data suggest that although ME1 overexpression augments anaplerosis and GSIS in INS-1 832/13 cells, it is not likely involved in MSSIS and GSIS in pancreatic islets.

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Kisspeptin-Activated Autophagy Independently Suppresses Non-Glucose-Stimulated Insulin Secretion from Pancreatic β-Cells

Scientific Reports ◽

10.1038/s41598-019-53826-7 ◽

2019 ◽

Vol 9 (1) ◽

Cited By ~ 3

Author(s):

Chien Huang ◽

Hao-Yi Wang ◽

Mu-En Wang ◽

Meng-Chieh Hsu ◽

Yi-Hsieng Samuel Wu ◽

...

Keyword(s):

Insulin Secretion ◽

Β Cells ◽

Pancreatic Β Cells ◽

Insulin Granules ◽

Autophagic Degradation ◽

Basal Insulin Secretion ◽

Glucose Stimulated Insulin Secretion

AbstractPrevious studies have demonstrated the important role of kisspeptin in impaired glucose-stimulated insulin secretion (GSIS). In addition, it was reported that the activation of autophagy in pancreatic β-cells decreases insulin secretion by selectively degrading insulin granules. However, it is currently unknown whether kisspeptin suppresses GSIS in β-cells by activating autophagy. To investigate the involvement of autophagy in kisspeptin–regulated insulin secretion, we overexpressed Kiss1 in NIT-1 cells to mimic the long-term exposure of pancreatic β-cells to kisspeptin during type 2 diabetes (T2D). Interestingly, our data showed that although kisspeptin potently decreases the intracellular proinsulin and insulin ((pro)insulin) content and insulin secretion of NIT-1 cells, autophagy inhibition using bafilomycin A1 and Atg5 siRNAs only rescues basal insulin secretion, not kisspeptin-impaired GSIS. We also generated a novel in vivo model to investigate the long-term exposure of kisspeptin by osmotic pump. The in vivo data demonstrated that kisspeptin lowers GSIS and (pro)insulin levels and also activated pancreatic autophagy in mice. Collectively, our data demonstrated that kisspeptin suppresses both GSIS and non-glucose-stimulated insulin secretion of pancreatic β-cells, but only non-glucose-stimulated insulin secretion depends on activated autophagic degradation of (pro)insulin. Our study provides novel insights for the development of impaired insulin secretion during T2D progression.

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Syntaxin-4 mediates exocytosis of pre-docked and newcomer insulin granules underlying biphasic glucose-stimulated insulin secretion in human pancreatic beta cells

Diabetologia ◽

10.1007/s00125-015-3545-4 ◽

2015 ◽

Vol 58 (6) ◽

pp. 1250-1259 ◽

Cited By ~ 16

Author(s):

Li Xie ◽

Dan Zhu ◽

Subhankar Dolai ◽

Tao Liang ◽

Tairan Qin ◽

...

Keyword(s):

Insulin Secretion ◽

Beta Cells ◽

Pancreatic Beta Cells ◽

Insulin Granules ◽

Syntaxin 4 ◽

Glucose Stimulated Insulin Secretion ◽

Newcomer Insulin Granules

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Glucose but not KCl diminishes submembrane granule turnover in mouse beta-cells

Journal of Molecular Endocrinology ◽

10.1530/jme-17-0063 ◽

2017 ◽

Vol 59 (3) ◽

pp. 311-324 ◽

Cited By ~ 8

Author(s):

Dennis Brüning ◽

Kirstin Reckers ◽

Peter Drain ◽

Ingo Rustenbeck

Keyword(s):

Insulin Secretion ◽

Beta Cells ◽

Initial Response ◽

Secretory Response ◽

Insulin Granules ◽

Primary Mouse ◽

Mouse Islets ◽

Glucose Stimulated Insulin Secretion ◽

Secretion Rates ◽

Reversed Order

KCl depolarization is widely used to mimic the depolarization during glucose-stimulated insulin secretion. Consequently, the insulin secretion elicited by KCl is often regarded as the equivalent of the first phase of glucose-induced insulin secretion. Here, the effects of both stimuli were compared by measuring the secretion of perifused mouse islets, the cytosolic Ca2+ concentration of single beta-cells and the mobility of submembrane insulin granules by TIRF microscopy of primary mouse beta-cells. Two cargo-directed granule labels were used namely insulin-EGFP and C-peptide-emGFP. The granule behaviour common to both was used to compare the effect of sequential stimulation with 40 mM KCl and 30 mM glucose and sequential stimulation with the same stimuli in reversed order. At the level of the cell secretory response, the sequential pulse protocol showed marked differences depending on the order of the two stimuli. KCl produced higher maximal secretion rates and diminished the response to the subsequent glucose stimulus, whereas glucose enhanced the response to the subsequent KCl stimulus. At the level of granule behaviour, a difference developed during the first stimulation phase in that the total number of granules, the short-term resident granules and the arriving granules, which are all parameters of granule turnover, were significantly smaller for glucose than for KCl. These differences at both the level of the cell secretory response and granule behaviour in the submembrane space are incompatible with identical initial response mechanisms to KCl and glucose stimulation.

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Metabolic amplification of insulin secretion by glucose is independent of β-cell microtubules

AJP Cell Physiology ◽

10.1152/ajpcell.00329.2010 ◽

2011 ◽

Vol 300 (3) ◽

pp. C697-C706 ◽

Cited By ~ 21

Author(s):

Nizar I. Mourad ◽

Myriam Nenquin ◽

Jean-Claude Henquin

Keyword(s):

Insulin Secretion ◽

Microtubule Disruption ◽

Insulin Granules ◽

Priming Process ◽

Previous Proposal ◽

Mouse Islets ◽

Second Phases ◽

Channel Dependent ◽

Amplifying Pathway ◽

Small Inhibition

Glucose-induced insulin secretion (IS) by β-cells is controlled by two pathways. The triggering pathway involves ATP-sensitive potassium (KATP) channel-dependent depolarization, Ca2+ influx, and rise in the cytosolic Ca2+ concentration ([Ca2+]c), which triggers exocytosis of insulin granules. The metabolic amplifying pathway augments IS without further increasing [Ca2+]c. After exclusion of the contribution of actin microfilaments, we here tested whether amplification implicates microtubule-dependent granule mobilization. Mouse islets were treated with nocodazole or taxol, which completely depolymerized and polymerized tubulin. They were then perifused to measure [Ca2+]c and IS. Metabolic amplification was studied during imposed steady elevation of [Ca2+]c by tolbutamide or KCl or by comparing [Ca2+]c and IS responses to glucose and tolbutamide. Nocodazole did not alter [Ca2+]c or IS changes induced by the three secretagogues, whereas taxol caused a small inhibition of IS that is partly ascribed to a decrease in [Ca2+]c. When [Ca2+]c was elevated and controlled by KCl or tolbutamide, the amplifying action of glucose was unaffected by microtubule disruption or stabilization. Both phases of IS were larger in response to glucose than tolbutamide, although triggering [Ca2+]c was lower. This difference, due to amplification, persisted in nocodazole- or taxol-treated islets, even when IS was augmented fourfold by microfilament disruption with cytochalasin B or latrunculin B. In conclusion, metabolic amplification rapidly augments first and second phases of IS independently of insulin granule translocation along microtubules. We therefore extend our previous proposal that it does not implicate the cytoskeleton but corresponds to acceleration of the priming process conferring release competence to insulin granules.

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What Is the Metabolic Amplification of Insulin Secretion and Is It (Still) Relevant?

Metabolites ◽

10.3390/metabo11060355 ◽

2021 ◽

Vol 11 (6) ◽

pp. 355

Author(s):

Ingo Rustenbeck ◽

Torben Schulze ◽

Mai Morsi ◽

Mohammed Alshafei ◽

Uwe Panten

Keyword(s):

Insulin Secretion ◽

Beta Cell ◽

Adenine Nucleotides ◽

Pancreatic Beta Cell ◽

Secretory Response ◽

Mitochondrial Activity ◽

Insulin Synthesis ◽

Sequence Of Events ◽

Insulin Granules

The pancreatic beta-cell transduces the availability of nutrients into the secretion of insulin. While this process is extensively modified by hormones and neurotransmitters, it is the availability of nutrients, above all glucose, which sets the process of insulin synthesis and secretion in motion. The central role of the mitochondria in this process was identified decades ago, but how changes in mitochondrial activity are coupled to the exocytosis of insulin granules is still incompletely understood. The identification of ATP-sensitive K+-channels provided the link between the level of adenine nucleotides and the electrical activity of the beta cell, but the depolarization-induced Ca2+-influx into the beta cells, although necessary for stimulated secretion, is not sufficient to generate the secretion pattern as produced by glucose and other nutrient secretagogues. The metabolic amplification of insulin secretion is thus the sequence of events that enables the secretory response to a nutrient secretagogue to exceed the secretory response to a purely depolarizing stimulus and is thus of prime importance. Since the cataplerotic export of mitochondrial metabolites is involved in this signaling, an orienting overview on the topic of nutrient secretagogues beyond glucose is included. Their judicious use may help to define better the nature of the signals and their mechanism of action.

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mTORC1 and mTORC2 regulate insulin secretion through Akt in INS-1 cells

Journal of Endocrinology ◽

10.1530/joe-12-0351 ◽

2012 ◽

Vol 216 (1) ◽

pp. 21-29 ◽

Cited By ~ 21

Author(s):

Olivier Le Bacquer ◽

Gurvan Queniat ◽

Valery Gmyr ◽

Julie Kerr-Conte ◽

Bruno Lefebvre ◽

...

Keyword(s):

Insulin Secretion ◽

Cell Function ◽

Antagonistic Effect ◽

Dominant Negative ◽

Insulin Content ◽

Insulin Biosynthesis ◽

Direct Role ◽

Β Cell Function ◽

Glucose Stimulated Insulin Secretion

Regulated associated protein of mTOR (Raptor) and rapamycin-insensitive companion of mTOR (rictor) are two proteins that delineate two different mTOR complexes, mTORC1 and mTORC2 respectively. Recent studies demonstrated the role of rictor in the development and function of β-cells. mTORC1 has long been known to impact β-cell function and development. However, most of the studies evaluating its role used either drug treatment (i.e. rapamycin) or modification of expression of proteins known to modulate its activity, and the direct role of raptor in insulin secretion is unclear. In this study, using siRNA, we investigated the role of raptor and rictor in insulin secretion and production in INS-1 cells and the possible cross talk between their respective complexes, mTORC1 and mTORC2. Reduced expression of raptor is associated with increased glucose-stimulated insulin secretion and intracellular insulin content. Downregulation of rictor expression leads to impaired insulin secretion without affecting insulin content and is able to correct the increased insulin secretion mediated by raptor siRNA. Using dominant-negative or constitutively active forms of Akt, we demonstrate that the effect of both raptor and rictor is mediated through alteration of Akt signaling. Our finding shed new light on the mechanism of control of insulin secretion and production by the mTOR, and they provide evidence for antagonistic effect of raptor and rictor on insulin secretion in response to glucose by modulating the activity of Akt, whereas only raptor is able to control insulin biosynthesis.

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Glucagonotropic and Glucagonostatic Effects of KATP Channel Closure and Potassium Depolarization

Endocrinology ◽

10.1210/endocr/bqaa136 ◽

2020 ◽

Vol 162 (1) ◽

Author(s):

Eike Früh ◽

Christin Elgert ◽

Frank Eggert ◽

Stephan Scherneck ◽

Ingo Rustenbeck

Keyword(s):

Insulin Secretion ◽

Beta Cells ◽

Katp Channel ◽

Glucagon Secretion ◽

Alpha Cells ◽

High Potassium ◽

Intrinsic Signal ◽

Mouse Islets ◽

Glucagon And Insulin

Abstract The role of depolarization in the inverse glucose-dependence of glucagon secretion was investigated by comparing the effects of KATP channel block and of high potassium. The secretion of glucagon and insulin by perifused mouse islets was simultaneously measured. Lowering glucose raised glucagon secretion before it decreased insulin secretion, suggesting an alpha cell–intrinsic signal recognition. Raising glucose affected glucagon and insulin secretion at the same time. However, depolarization by tolbutamide, gliclazide, or 15 mM KCl increased insulin secretion before the glucagon secretion receded. In contrast to the robust depolarizing effect of arginine and KCl (15 and 40 mM) on single alpha cells, tolbutamide was of variable efficacy. Only when applied before other depolarizing agents had tolbutamide a consistent depolarizing effect and regularly increased the cytosolic Ca2+ concentration. When tested on inside-out patches tolbutamide was as effective on alpha cells as on beta cells. In the presence of 1 µM clonidine, to separate insulinotropic from glucagonotropic effects, both 500 µM tolbutamide and 30 µM gliclazide increased glucagon secretion significantly, but transiently. The additional presence of 15 or 40 mM KCl in contrast led to a marked and lasting increase of the glucagon secretion. The glucagon secretion by SUR1 knockout islets was not increased by tolbutamide, whereas 40 mM KCl was of unchanged efficiency. In conclusion a strong and sustained depolarization is compatible with a marked and lasting glucagon secretion. KATP channel closure in alpha cells is less readily achieved than in beta cells, which may explain the moderate and transient glucagonotropic effect.

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SPARC promotes insulin secretion through down-regulation of RGS4 protein in pancreatic β cells

Scientific Reports ◽

10.1038/s41598-020-74593-w ◽

2020 ◽

Vol 10 (1) ◽

Author(s):

Li Hu ◽

Fengli He ◽

Meifeng Huang ◽

Qian Zhao ◽

Lamei Cheng ◽

...

Keyword(s):

Insulin Secretion ◽

Underlying Mechanism ◽

Negative Regulator ◽

Β Cells ◽

Pancreatic Β Cells ◽

Promoting Effect ◽

Down Regulation ◽

Mouse Islets ◽

Glucose Stimulated Insulin Secretion ◽

Phosphoinositide 3 Kinase

Abstract SPARC-deficient mice have been shown to exhibit impaired glucose tolerance and insulin secretion, but the underlying mechanism remains unknown. Here, we showed that SPARC enhanced the promoting effect of Muscarinic receptor agonist oxotremorine-M on insulin secretion in cultured mouse islets. Overexpression of SPARC down-regulated RGS4, a negative regulator of β-cell M3 muscarinic receptors. Conversely, knockdown of SPARC up-regulated RGS4 in Min6 cells. RGS4 was up-regulated in islets from sparc −/− mice, which correlated with decreased glucose-stimulated insulin secretion (GSIS). Furthermore, inhibition of RGS4 restored GSIS in the islets from sparc −/− mice, and knockdown of RGS4 partially decreased the promoting effect of SPARC on oxotremorine-M-stimulated insulin secretion. Phosphoinositide 3-kinase (PI3K) inhibitor LY-294002 abolished SPARC-induced down-regulation of RGS4. Taken together, our data revealed that SPARC promoted GSIS by inhibiting RGS4 in pancreatic β cells.

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