| 1. |
- Gandasi, Nikhil, et al.
(författare)
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GLP-1 metabolite GLP-1(9-36) is a systemic inhibitor of mouse and human pancreatic islet glucagon secretion
- 2023
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Ingår i: DIABETOLOGIA. - 0012-186X .- 1432-0428.
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Tidskriftsartikel (refereegranskat)abstract
- Aims/hypothesis Diabetes mellitus is associated with impaired insulin secretion, often aggravated by oversecretion of glucagon. Therapeutic interventions should ideally correct both defects. Glucagon-like peptide 1 (GLP-1) has this capability but exactly how it exerts its glucagonostatic effect remains obscure. Following its release GLP-1 is rapidly degraded from GLP-1(7-36) to GLP-1(9-36). We hypothesised that the metabolite GLP-1(9-36) (previously believed to be biologically inactive) exerts a direct inhibitory effect on glucagon secretion and that this mechanism becomes impaired in diabetes.Methods We used a combination of glucagon secretion measurements in mouse and human islets (including islets from donors with type 2 diabetes), total internal reflection fluorescence microscopy imaging of secretory granule dynamics, recordings of cytoplasmic Ca2+ and measurements of protein kinase A activity, immunocytochemistry, in vivo physiology and GTP-binding protein dissociation studies to explore how GLP-1 exerts its inhibitory effect on glucagon secretion and the role of the metabolite GLP-1(9-36).Results GLP-1(7-36) inhibited glucagon secretion in isolated islets with an IC50 of 2.5 pmol/l. The effect was particularly strong at low glucose concentrations. The degradation product GLP-1(9-36) shared this capacity. GLP-1(9-36) retained its glucagonostatic effects after genetic/pharmacological inactivation of the GLP-1 receptor. GLP-1(9-36) also potently inhibited glucagon secretion evoked by beta-adrenergic stimulation, amino acids and membrane depolarisation. In islet alpha cells, GLP-1(9-36) led to inhibition of Ca2+ entry via voltage-gated Ca2+ channels sensitive to omega-agatoxin, with consequential pertussis-toxin-sensitive depletion of the docked pool of secretory granules, effects that were prevented by the glucagon receptor antagonists REMD2.59 and L-168049. The capacity of GLP-1(9-36) to inhibit glucagon secretion and reduce the number of docked granules was lost in alpha cells from human donors with type 2 diabetes. In vivo, high exogenous concentrations of GLP-1(9-36) (>100 pmol/l) resulted in a small (30%) lowering of circulating glucagon during insulin-induced hypoglycaemia. This effect was abolished by REMD2.59, which promptly increased circulating glucagon by >225% (adjusted for the change in plasma glucose) without affecting pancreatic glucagon content.Conclusions/interpretation We conclude that the GLP-1 metabolite GLP-1(9-36) is a systemic inhibitor of glucagon secretion. We propose that the increase in circulating glucagon observed following genetic/pharmacological inactivation of glucagon signalling in mice and in people with type 2 diabetes reflects the removal of GLP-1(9-36)'s glucagonostatic action.
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| 2. |
- MacDonald, Patrick E., et al.
(författare)
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A K-ATP channel-dependent pathway within alpha cells regulates glucagon release from both rodent and human islets of langerhans
- 2007
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Ingår i: PLoS Biology. - : Public Library of Science (PLoS). - 1545-7885. ; 5:6, s. 1236-1247
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Tidskriftsartikel (refereegranskat)abstract
- Glucagon, secreted from pancreatic islet a cells, stimulates gluconeogenesis and liver glycogen breakdown. The mechanism regulating glucagon release is debated, and variously attributed to neuronal control, paracrine control by neighbouring beta cells, or to an intrinsic glucose sensing by the a cells themselves. We examined hormone secretion and Ca2+ responses of a and b cells within intact rodent and human islets. Glucose-dependent suppression of glucagon release persisted when paracrine GABA or Zn (2+) signalling was blocked, but was reversed by low concentrations (1-20 mu M) of the ATP-sensitive K+ (K-ATP) channel opener diazoxide, which had no effect on insulin release or b cell responses. This effect was prevented by the K-ATP channel blocker tolbutamide (100 mu M). Higher diazoxide concentrations (>= 30 mu M) decreased glucagon and insulin secretion, and alpha-and beta-cell Ca2+ responses, in parallel. In the absence of glucose, tolbutamide at low concentrations (< 1 mu M) stimulated glucagon secretion, whereas high concentrations (> 10 mu M) were inhibitory. In the presence of a maximally inhibitory concentration of tolbutamide (0.5 mM), glucose had no additional suppressive effect. Downstream of the K-ATP channel, inhibition of voltage-gated Na+ (TTX) and N-type Ca2+ channels (omega-conotoxin), but not L-type Ca2+ channels (nifedipine), prevented glucagon secretion. Both the N-type Ca2+ channels and alpha-cell exocytosis were inactivated at depolarised membrane potentials. Rodent and human glucagon secretion is regulated by an a-cell K-ATP channel-dependent mechanism. We propose that elevated glucose reduces electrical activity and exocytosis via depolarisation-induced inactivation of ion channels involved in action potential firing and secretion.
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| 3. |
- Rosengren, Anders, et al.
(författare)
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Reduced Insulin Exocytosis in Human Pancreatic β-cells With Gene Variants Linked to Type 2 Diabetes.
- 2012
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Ingår i: Diabetes. - : American Diabetes Association. - 1939-327X .- 0012-1797. ; 61:7, s. 1726-1733
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Tidskriftsartikel (refereegranskat)abstract
- The majority of genetic risk variants for type 2 diabetes (T2D) affect insulin secretion, but the mechanisms through which they influence pancreatic islet function remain largely unknown. We functionally characterized human islets to determine secretory, biophysical, and ultrastructural features in relation to genetic risk profiles in diabetic and nondiabetic donors. Islets from donors with T2D exhibited impaired insulin secretion, which was more pronounced in lean than obese diabetic donors. We assessed the impact of 14 disease susceptibility variants on measures of glucose sensing, exocytosis, and structure. Variants near TCF7L2 and ADRA2A were associated with reduced glucose-induced insulin secretion, whereas susceptibility variants near ADRA2A, KCNJ11, KCNQ1, and TCF7L2 were associated with reduced depolarization-evoked insulin exocytosis. KCNQ1, ADRA2A, KCNJ11, HHEX/IDE, and SLC2A2 variants affected granule docking. We combined our results to create a novel genetic risk score for β-cell dysfunction that includes aberrant granule docking, decreased Ca(2+) sensitivity of exocytosis, and reduced insulin release. Individuals with a high risk score displayed an impaired response to intravenous glucose and deteriorating insulin secretion over time. Our results underscore the importance of defects in β-cell exocytosis in T2D and demonstrate the potential of cellular phenotypic characterization in the elucidation of complex genetic disorders.
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| 4. |
- Zhang, Quan, et al.
(författare)
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R-type Ca2+-channel-evoked CICR regulates glucose-induced somatostatin secretion
- 2007
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Ingår i: Nature Cell Biology. - : Springer Science and Business Media LLC. - 1465-7392 .- 1476-4679. ; 9:4, s. 171-453
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Tidskriftsartikel (refereegranskat)abstract
- Pancreatic islets have a central role in blood glucose homeostasis. In addition to insulin-producing beta-cells and glucagon-secreting alpha-cells, the islets contain somatostatin-releasing delta-cells(1). Somatostatin is a powerful inhibitor of insulin and glucagon secretion(2). It is normally secreted in response to glucose(3) and there is evidence suggesting its release becomes perturbed in diabetes(4). Little is known about the control of somatostatin release. Closure of ATP-regulated K+-channels (K-ATP-channels)(5) and a depolarization-evoked increase in cytoplasmic free Ca2+ concentration ([Ca2+](i))(6-8) have been proposed to be essential. Here, we report that somatostatin release evoked by high glucose (>= 10 mM) is unaffected by the K-ATP-channel activator diazoxide and proceeds normally in K-ATP-channel-deficient islets. Glucose-induced somatostatin secretion is instead primarily dependent on Ca2+-induced Ca2+-release (CICR). This constitutes a novel mechanism for K-ATP-channel-independent metabolic control of pancreatic hormone secretion.
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