1. pH-dependence of glucose- dependent activity of beta cell networks in acute mouse pancreatic tissue sliceSandra Postić, Marko Gosak, Wen-Hao Tsai, Johannes Pfabe, Srdjan Sarikas, Andraž Stožer, Dean Korošak, Shi-Bing Yang, Marjan Rupnik, 2022, original scientific article Abstract: Extracellular pH has the potential to affect various aspects of the pancreatic beta cell function. To explain this effect, a number of mechanisms was proposed involving both extracellular and intracellular targets and pathways. Here, we focus on reassessing the influence of extracellular pH on glucose-dependent beta cell activation and collective activity in physiological conditions. To this end we employed mouse pancreatic tissue slices to perform high-temporally resolved functional imaging of cytosolic Ca2+ oscillations. We investigated the effect of either physiological H+ excess or depletion on the activation properties as well as on the collective activity of beta cell in an islet. Our results indicate that lowered pH invokes activation of a subset of beta cells in substimulatory glucose concentrations, enhances the average activity of beta cells, and alters the beta cell network properties in an islet. The enhanced average activity of beta cells was determined indirectly utilizing cytosolic Ca2+ imaging, while direct measuring of insulin secretion confirmed that this enhanced activity is accompanied by a higher insulin release. Furthermore, reduced functional connectivity and higher functional segregation at lower pH, both signs of a reduced intercellular communication, do not necessary result in an impaired insulin release.
Keywords: insulin secretion, membrane excitability, potassium channels, beta cell network, collective activity, calcium waves, pancreatic islets, pH-dependence
Published in DKUM: 01.07.2024; Views: 149; Downloads: 6
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2. Lipotoxicity in a vicious cycle of pancreatic beta cell exhaustionVladimir Grubelnik, Jan Zmazek, Matej Završnik, Marko Marhl, 2022, original scientific article Abstract: Hyperlipidemia is a common metabolic disorder in modern society and may precede hyperglycemia and diabetes by several years. Exactly how disorders of lipid and glucose metabolism are related is still a mystery in many respects. We analyze the effects of hyperlipidemia, particularly free fatty acids, on pancreatic beta cells and insulin secretion. We have developed a computational model to quantitatively estimate the effects of specific metabolic pathways on insulin secretion and to assess the effects of short- and long-term exposure of beta cells to elevated concentrations of free fatty acids. We show that the major trigger for insulin secretion is the anaplerotic pathway via the phosphoenolpyruvate cycle, which is affected by free fatty acids via uncoupling protein 2 and proton leak and is particularly destructive in long-term chronic exposure to free fatty acids, leading to increased insulin secretion at low blood glucose and inadequate insulin secretion at high blood glucose. This results in beta cells remaining highly active in the “resting” state at low glucose and being unable to respond to anaplerotic signals at high pyruvate levels, as is the case with high blood glucose. The observed fatty-acid-induced disruption of anaplerotic pathways makes sense in the context of the physiological role of insulin as one of the major anabolic hormones.
Keywords: diabetes, insulin secretion, lipids, PEP cycle, uncoupling proteins, mitochondrial dysfunction
Published in DKUM: 20.05.2024; Views: 133; Downloads: 15
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3. SNAP-25b-deficiency increases insulin secretion and changes spatiotemporal profile of $Ca^{2+}$ oscillations in $\beta$ cell networksTeresa Daraio, Lidija Križančić Bombek, Marko Gosak, Ismael Valladolid-Acebes, Maša Skelin, Essam Refai, Per-Olof Berggren, Kerstin Brismar, Marjan Rupnik, Christina Bark, 2017, original scientific article Abstract: SNAP-25 is a protein of the core SNARE complex mediating stimulus-dependent release of insulin from pancreatic $\beta$ cells. The protein exists as two alternatively spliced isoforms, SNAP-25a and SNAP-25b, differing in 9 out of 206 amino acids, yet their specific roles in pancreatic $\beta$ cells remain unclear. We explored the effect of SNAP-25b-deficiency on glucose-stimulated insulin release in islets and found increased secretion both in vivo and in vitro. However, slow photo-release of caged $Ca^{2+}$ in $\beta$ cells within pancreatic slices showed no significant differences in $Ca^{2+}$-sensitivity, amplitude or rate of exocytosis between SNAP-25b-deficient and wild-type littermates. Therefore, we next investigated if $Ca^{2+}$ handling was affected in glucose-stimulated [beta] cells using intracellular $Ca^{2+}$-imaging and found premature activation and delayed termination of [$Ca^{2+}$] i elevations. These findings were accompanied by less synchronized $Ca^{2+}$-oscillations and hence more segregated functional $\beta$ cell networks in SNAP-25b-deficient mice. Islet gross morphology and architecture were maintained in mutant mice, although sex specific compensatory changes were observed. Thus, our study proposes that SNAP-25b in pancreatic [beta] cells, except for participating in the core SNARE complex, is necessary for accurate regulation of $Ca^{2+}$-dynamics.
Keywords: insulin secretion, pre-diabetes
Published in DKUM: 23.08.2017; Views: 1504; Downloads: 218
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4. Intracellular serotonin modulates insulin secretion from pancreatic ß-cells by protein serotonylationNils Paulmann, Maik Grohmann, Jörg-Peter Voigt, Bettina Bert, Jakob Vowinckel, Michael Bader, Maša Skelin, Marko Jevšek, Heidrun Fink, Marjan Rupnik, Diego Walther, 2009, original scientific article Abstract: While serotonin (5-HT) co-localization with insulin in granules of pancreatic ß-cells was demonstrated more than three decades ago, its physiological role in the etiology of diabetes is stili unclear. We combined biochemical and electrophysiological analyses of mice selectively deficient in peripheral tryptophan hydroxylase (Tph1-/-) and 5-HT to show that intracellular 5-HT regulates insulin secretion. We found that these mice are diabetic and have an impaired insulin secretion due to the lack of 5-HT in the pancreas. The pharmacological restoration of peripheral 5-HT levels rescued the impaired insulin secretion in vivo. These findings were further evidenced by patch clamp experiments with isolated Tph1-/- ß-cells, which clearly showed that the secretory defect is downstream of Ca2+ -signaling and can be rescued by direct intracellular application of 5-HT via the clamp pipette. In elucidating the underlying mechanism further, we demonstrate the covalent coupling of 5-HT by transglutaminases during insulin exocytosis to two key players in insulin secretion, the small GTPases Rab3a and Rab27a. This renders them constitutively active in a receptor-independent signaling mechanism we have recently termed serotonylation. Concordantly, an inhibition of such activating serotonylation in ß-cells abates insulin secretion. We also observed inactivation of serotonylated Rab3a by enhanced proteasomal degradation, which is in line with the inactivation of other serotonylated GTPases. Our results demonstrate that 5-HT regulates insulin secretion by serotonylation of GTPases within pancreatic ß-cells and suggest that intracellular 5-HT functions in various microenvironments via this mechanism in concert with the known receptor-mediated signaling.
Keywords: insulin secretion, serotonin, insulin, glucose, diabetes mellitus, guanosine triphosphatase, exocytosis, pancreas
Published in DKUM: 16.06.2017; Views: 2195; Downloads: 207
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