Browsing by Subject "secretion"

Now showing 1 - 6 of 6
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    An Acetate-Specific GPCR, FFAR2, Regulates Insulin Secretion
    (The Endocrine Society, 2015-07) Priyadarshini, Medha; Villa, Stephanie R.; Fuller, Miles; Wicksteed, Barton; Mackay, Charles R.; Alquier, Thierry; Poitout, Vincent; Mancebo, Helena; Mirmira, Raghavendra G.; Gilchrist, Annette; Layden, Brian T.; Department of Pediatrics, IU School of Medicine
    G protein-coupled receptors have been well described to contribute to the regulation of glucose-stimulated insulin secretion (GSIS). The short-chain fatty acid-sensing G protein-coupled receptor, free fatty acid receptor 2 (FFAR2), is expressed in pancreatic β-cells, and in rodents, its expression is altered during insulin resistance. Thus, we explored the role of FFAR2 in regulating GSIS. First, assessing the phenotype of wild-type and Ffar2(-/-) mice in vivo, we observed no differences with regard to glucose homeostasis on normal or high-fat diet, with a marginally significant defect in insulin secretion in Ffar2(-/-) mice during hyperglycemic clamps. In ex vivo insulin secretion studies, we observed diminished GSIS from Ffar2(-/-) islets relative to wild-type islets under high-glucose conditions. Further, in the presence of acetate, the primary endogenous ligand for FFAR2, we observed FFAR2-dependent potentiation of GSIS, whereas FFAR2-specific agonists resulted in either potentiation or inhibition of GSIS, which we found to result from selective signaling through either Gαq/11 or Gαi/o, respectively. Lastly, in ex vivo insulin secretion studies of human islets, we observed that acetate and FFAR2 agonists elicited different signaling properties at human FFAR2 than at mouse FFAR2. Taken together, our studies reveal that FFAR2 signaling occurs by divergent G protein pathways that can selectively potentiate or inhibit GSIS in mouse islets. Further, we have identified important differences in the response of mouse and human FFAR2 to selective agonists, and we suggest that these differences warrant consideration in the continued investigation of FFAR2 as a novel type 2 diabetes target.
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    Depressed basal hypothalamic neuronal activity in type-1 diabetic mice is correlated with proinflammatory secretion of HMBG1
    (Elsevier, 2016-02-26) Thinschmidt, Jeffrey S.; Colon-Perez, Luis M.; Febo, Marcelo; Caballero, Sergio; King, Michael A.; White, Fletcher A.; Grant, Maria B.; Department of Ophthalmology, School of Medicine
    We recently found indicators of hypothalamic inflammation and neurodegeneration linked to the loss of neuroprotective factors including insulin-like growth factor (IGF-1) and IGF binding protein-2 (IGFBP-3) in mice made diabetic using streptozotocin (STZ). In the current work, a genetic model of type-1 diabetes (Ins2(Akita) mouse) was used to evaluate changes in neuronal activity and concomitant changes in the proinflammatory mediator high-mobility group box-1 (HMBG1). We found basal hypothalamic neuronal activity as indicated by manganese-enhanced magnetic resonance imaging (MEMRI) was significantly decreased in 8 months old, but not 2 months old Ins2(Akita) diabetic mice compared to controls. In tissue from the same animals we evaluated the expression of HMBG1 using immunohistochemistry and confocal microscopy. We found decreased HMBG1 nuclear localization in the paraventricular nucleus of the hypothalamus (PVN) in 8 months old, but not 2 months old diabetic animals indicating nuclear release of the protein consistent with an inflammatory state. Adjacent thalamic regions showed little change in HMBG1 nuclear localization and neuronal activity as a result of diabetes. This work extends our previous findings demonstrating changes consistent with hypothalamic neuroinflammation in STZ treated animals, and shows active inflammatory processes are correlated with changes in basal hypothalamic neuronal activity in Ins2(Akita) mice.
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    Excessive Osteocytic Fgf23 Secretion Contributes to Pyrophosphate Accumulation and Mineralization Defect in Hyp Mice
    (Public Library of Science (PLoS), 2016-04) Murali, Sathish K.; Andrukhova, Olena; Clinkenbeard, Erica L.; White, Kenneth E.; Erben, Reinhold G.; Department of Medical & Molecular Genetics, IU School of Medicine
    X-linked hypophosphatemia (XLH) is the most frequent form of inherited rickets in humans caused by mutations in the phosphate-regulating gene with homologies to endopeptidases on the X-chromosome (PHEX). Hyp mice, a murine homologue of XLH, are characterized by hypophosphatemia, inappropriately low serum vitamin D levels, increased serum fibroblast growth factor-23 (Fgf23), and osteomalacia. Although Fgf23 is known to be responsible for hypophosphatemia and reduced vitamin D hormone levels in Hyp mice, its putative role as an auto-/paracrine osteomalacia-causing factor has not been explored. We recently reported that Fgf23 is a suppressor of tissue nonspecific alkaline phosphatase (Tnap) transcription via FGF receptor-3 (FGFR3) signaling, leading to inhibition of mineralization through accumulation of the TNAP substrate pyrophosphate. Here, we report that the pyrophosphate concentration is increased in Hyp bones, and that Tnap expression is decreased in Hyp-derived osteocyte-like cells but not in Hyp-derived osteoblasts ex vivo and in vitro. In situ mRNA expression profiling in bone cryosections revealed a ~70-fold up-regulation of Fgfr3 mRNA in osteocytes versus osteoblasts of Hyp mice. In addition, we show that blocking of increased Fgf23-FGFR3 signaling with anti-Fgf23 antibodies or an FGFR3 inhibitor partially restored the suppression of Tnap expression, phosphate production, and mineralization, and decreased pyrophosphate concentration in Hyp-derived osteocyte-like cells in vitro. In vivo, bone-specific deletion of Fgf23 in Hyp mice rescued the suppressed TNAP activity in osteocytes of Hyp mice. Moreover, treatment of wild-type osteoblasts or mice with recombinant FGF23 suppressed Tnap mRNA expression and increased pyrophosphate concentrations in the culture medium and in bone, respectively. In conclusion, we found that the cell autonomous increase in Fgf23 secretion in Hyp osteocytes drives the accumulation of pyrophosphate through auto-/paracrine suppression of TNAP. Hence, we have identified a novel mechanism contributing to the mineralization defect in Hyp mice.
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    Microinjections of acetaldehyde or salsolinol into the posterior ventral tegmental area increase dopamine release in the nucleus accumbens shell
    (Wiley Blackwell (Blackwell Publishing), 2013-05) Deehan, Gerald A.; Engleman, Eric A.; Ding, Zheng-Ming; McBride, William J.; Rodd, Zachary A.; Department of Psychiatry, IU School of Medicine
    BACKGROUND: Published findings indicate that acetaldehyde (ACD; the first metabolite of ethanol [EtOH]) and salsolinol (SAL; formed through the nonenzymatic condensation of ACD and dopamine [DA]) can be formed following EtOH consumption. Both ACD and SAL exhibit reinforcing properties within the posterior ventral tegmental area (pVTA) and both exhibit an inverted "U-shaped" dose-response curve. The current study was undertaken to examine the dose-response effects of microinjections of ACD or SAL into the pVTA on DA efflux in the nucleus accumbens shell (AcbSh). METHODS: For the first experiment, separate groups of male Wistar rats received pulse microinjections of artificial cerebrospinal fluid (aCSF) or 12-, 23-, or 90-μM ACD into the pVTA, while extracellular DA levels were concurrently measured in the AcbSh. The second experiment was similarly conducted, except rats were given microinjections of aCSF or 0.03-, 0.3-, 1.0-, or 3.0-μM SAL, while extracellular levels of DA were measured in the AcbSh. RESULTS: Both ACD and SAL produced a dose-dependent inverted "U-shaped" response on DA release in the AcbSh, with 23-μM ACD (200% baseline) and 0.3-μM SAL (300% baseline) producing maximal peak responses with higher concentrations of ACD (90 μM) and SAL (3.0 μM) producing significantly lower DA efflux. CONCLUSIONS: The findings from the current study indicate that local application of intermediate concentrations of ACD and SAL stimulated DA neurons in the pVTA, whereas higher concentrations may be having secondary effects within the pVTA that inhibit DA neuronal activity. The present results parallel the studies on the reinforcing effects of ACD and SAL in the pVTA and support the idea that the reinforcing effects of ACD and SAL within the pVTA are mediated by activating DA neurons.
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    Profound defects in β-cell function in screen-detected type 2 diabetes are not improved with glucose-lowering treatment in the Early Diabetes Intervention Program (EDIP)
    (Wiley-Blackwell, 2014-11) Hannon, Tamara S.; Kirkman, M. S.; Patel, Yash R.; Considine, Robert V.; Mather, Kieren J.; Department of Pediatrics, IU School of Medicine
    BACKGROUND: Few studies have measured the ability of interventions to affect declining β-cell function in screen-detected type 2 diabetes. The Early Diabetes Intervention Programme (ClinicalTrials.gov NCT01470937) was a randomized study based on the hypothesis that improving postprandial glucose excursions with acarbose would slow the progression of fasting hyperglycaemia in screen-detected type 2 diabetes. In the Early Diabetes Intervention Programme, the effect of acarbose plus lifestyle advice on progression of fasting hyperglycaemia over a 5-year period was not greater than that of placebo. However, there was an early glucose-lowering effect of the trial. The objective of the current secondary analysis was to describe β-cell function changes in response to glucose lowering. METHODS: Participants were overweight adult subjects with screen-detected type 2 diabetes. β-cell function was measured using hyperglycaemic clamps and oral glucose tolerance testing. The primary outcome was the change in β-cell function from baseline to year 1, the time point where the maximal glucose-lowering effect was seen. RESULTS: At baseline, participants exhibited markedly impaired first-phase insulin response. Despite significant reductions in weight, fasting plasma glucose (PG) and 2-h PG, there was no clinically significant improvement in the first-phase insulin response. Late-phase insulin responses declined despite beneficial glycaemic effects of interventions. CONCLUSIONS: Insulin secretion is already severely impaired in early, screen-detected type 2 diabetes. Effective glucose-lowering intervention with acarbose was not sufficient to improve insulin secretion or halt the decline of β-cell function.
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    VAV2, a guanine nucleotide exchange factor for Rac1, regulates glucose-stimulated insulin secretion in pancreatic beta cells
    (Springer-Verlag, 2015-11) Veluthakal, Rajakrishnan; Tunduguru, Ragadeepthi; Arora, Daleep Kumar; Sidarala, Vaibhav; Syeda, Khadija; Vlaar, Cornelis P.; Thurmond, Debbie C.; Kowluru, Anjaneyulu; Department of Pediatrics, IU School of Medicine
    AIMS/HYPOTHESIS: Rho GTPases (Ras-related C3 botulinum toxin substrate 1 [Rac1] and cell division cycle 42 [Cdc42]) have been shown to regulate glucose-stimulated insulin secretion (GSIS) via cytoskeletal remodelling, trafficking and fusion of insulin-secretory granules with the plasma membrane. GTP loading of these G proteins, which is facilitated by GDP/GTP exchange factors, is a requisite step in the regulation of downstream effector proteins. Guanine nucleotide exchange factor VAV2 (VAV2), a member of the Dbl family of proteins, has been identified as one of the GDP/GTP exchange factors for Rac1. Despite recent evidence on the regulatory roles of VAV2 in different cell types, roles of this guanine nucleotide exchange factor in the signalling events leading to GSIS remain undefined. Using immunological, short interfering RNA (siRNA), pharmacological and microscopic approaches we investigated the role of VAV2 in GSIS from islet beta cells. METHODS: Co-localisation of Rac1 and VAV2 was determined by Triton X-114 phase partition and confocal microscopy. Glucose-induced actin remodelling was quantified by live cell imaging using the LifeAct-GFP fluorescent biosensor. Rac1 activation was determined by G protein linked immunosorbent assay (G-LISA). RESULTS: Western blotting indicated that VAV2 is expressed in INS-1 832/13 beta cells, normal rat islets and human islets. Vav2 siRNA markedly attenuated GSIS in INS-1 832/13 cells. Ehop-016, a newly discovered small molecule inhibitor of the VAV2-Rac1 interaction, or siRNA-mediated knockdown of VAV2 markedly attenuated glucose-induced Rac1 activation and GSIS in INS-1 832/13 cells. Pharmacological findings were recapitulated in primary rat islets. A high glucose concentration promoted co-localisation of Rac1 and VAV2. Real-time imaging in live cells indicated a significant inhibition of glucose-induced cortical actin remodelling by Ehop-016. CONCLUSIONS/INTERPRETATION: Our data provide the first evidence to implicate VAV2 in glucose-induced Rac1 activation, actin remodelling and GSIS in pancreatic beta cells.