Browsing by Author "Stull, Natalie D."

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    An early, reversible cholesterolgenic etiology of diet-induced insulin resistance
    (Elsevier, 2023) Covert, Jacob D.; Grice, Brian A.; Thornburg, Matthew G.; Kaur, Manpreet; Ryan, Andrew P.; Tackett, Lixuan; Bhamidipati, Theja; Stull, Natalie D.; Kim, Teayoun; Habegger, Kirk M.; McClain, Donald A.; Brozinick, Joseph T.; Elmendorf, Jeffrey S.; Anatomy, Cell Biology and Physiology, School of Medicine
    Objective: A buildup of skeletal muscle plasma membrane (PM) cholesterol content in mice occurs within 1 week of a Western-style high-fat diet and causes insulin resistance. The mechanism driving this cholesterol accumulation and insulin resistance is not known. Promising cell data implicate that the hexosamine biosynthesis pathway (HBP) triggers a cholesterolgenic response via increasing the transcriptional activity of Sp1. In this study we aimed to determine whether increased HBP/Sp1 activity represented a preventable cause of insulin resistance. Methods: C57BL/6NJ mice were fed either a low-fat (LF, 10% kcal) or high-fat (HF, 45% kcal) diet for 1 week. During this 1-week diet the mice were treated daily with either saline or mithramycin-A (MTM), a specific Sp1/DNA-binding inhibitor. A series of metabolic and tissue analyses were then performed on these mice, as well as on mice with targeted skeletal muscle overexpression of the rate-limiting HBP enzyme glutamine-fructose-6-phosphate-amidotransferase (GFAT) that were maintained on a regular chow diet. Results: Saline-treated mice fed this HF diet for 1 week did not have an increase in adiposity, lean mass, or body mass while displaying early insulin resistance. Consistent with an HBP/Sp1 cholesterolgenic response, Sp1 displayed increased O-GlcNAcylation and binding to the HMGCR promoter that increased HMGCR expression in skeletal muscle from saline-treated HF-fed mice. Skeletal muscle from these saline-treated HF-fed mice also showed a resultant elevation of PM cholesterol with an accompanying loss of cortical filamentous actin (F-actin) that is essential for insulin-stimulated glucose transport. Treating these mice daily with MTM during the 1-week HF diet fully prevented the diet-induced Sp1 cholesterolgenic response, loss of cortical F-actin, and development of insulin resistance. Similarly, increases in HMGCR expression and cholesterol were measured in muscle from GFAT transgenic mice compared to age- and weight-match wildtype littermate control mice. In the GFAT Tg mice we found that these increases were alleviated by MTM. Conclusions: These data identify increased HBP/Sp1 activity as an early mechanism of diet-induced insulin resistance. Therapies targeting this mechanism may decelerate T2D development.
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    Divergent compensatory responses to high-fat diet between C57BL6/J and C57BLKS/J inbred mouse strains
    (American Physiological Society (APS), 2013-12-15) Sims, Emily K.; Hatanaka, Masayuki; Morris, David L.; Tersey, Sarah A.; Kono, Tatsuyoshi; Chaudry, Zunaira Z.; Day, Kathleen H.; Moss, Dan R.; Stull, Natalie D.; Mirmira, Raghavendra G.; Evans-Molina, Carmella; Department of Medicine, IU School of Medicine
    Impaired glucose tolerance (IGT) and type 2 diabetes (T2DM) are polygenic disorders with complex pathophysiologies; recapitulating them with mouse models is challenging. Despite 70% genetic homology, C57BL/6J (BL6) and C57BLKS/J (BLKS) inbred mouse strains differ in response to diet- and genetic-induced obesity. We hypothesized these differences would yield insight into IGT and T2DM susceptibility and response to pharmacological therapies. To this end, male 8-wk-old BL6 and BLKS mice were fed normal chow (18% kcal from fat), high-fat diet (HFD; 42% kcal from fat), or HFD supplemented with the PPARγ agonist pioglitazone (PIO; 140 mg PIO/kg diet) for 16 wk. Assessments of body composition, glucose homeostasis, insulin production, and energy metabolism, as well as histological analyses of pancreata were undertaken. BL6 mice gained weight and adiposity in response to HFD, leading to peripheral insulin resistance that was met with increased β-cell proliferation and insulin production. By contrast, BLKS mice responded to HFD by restricting food intake and increasing activity. These behavioral responses limited weight gain and protected against HFD-induced glucose intolerance, which in this strain was primarily due to β-cell dysfunction. PIO treatment did not affect HFD-induced weight gain in BL6 mice, and decreased visceral fat mass, whereas in BLKS mice PIO increased total fat mass without improving visceral fat mass. Differences in these responses to HFD and effects of PIO reflect divergent human responses to a Western lifestyle and underscore the careful consideration needed when choosing mouse models of diet-induced obesity and diabetes treatment.
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    A high-fat diet catalyzes progression to hyperglycemia in mice with selective impairment of insulin action in Glut4-expressing tissues
    (Elsevier, 2022-01) Reilly, Austin M.; Yan, Shijun; Huang, Menghao; Abhyankar, Surabhi D.; Conley, Jason M.; Bone, Robert N.; Stull, Natalie D.; Horan, Daniel J.; Roh, Hyun C.; Robling, Alexander G.; Ericsson, Aaron C.; Dong, Xiaocheng C.; Evans-Molina, Carmella; Ren, Hongxia; Pediatrics, School of Medicine
    Insulin resistance impairs postprandial glucose uptake through glucose transporter type 4 (GLUT4) and is the primary defect preceding type 2 diabetes. We previously generated an insulin-resistant mouse model with human GLUT4 promoter-driven insulin receptor knockout (GIRKO) in the muscle, adipose, and neuronal subpopulations. However, the rate of diabetes in GIRKO mice remained low prior to 6 months of age on normal chow diet (NCD), suggesting that additional factors/mechanisms are responsible for adverse metabolic effects driving the ultimate progression of overt diabetes. In this study, we characterized the metabolic phenotypes of the adult GIRKO mice acutely switched to high-fat diet (HFD) feeding in order to identify additional metabolic challenges required for disease progression. Distinct from other diet-induced obesity (DIO) and genetic models (e.g., db/db mice), GIRKO mice remained leaner on HFD feeding, but developed other cardinal features of insulin resistance syndrome. GIRKO mice rapidly developed hyperglycemia despite compensatory increases in β-cell mass and hyperinsulinemia. Furthermore, GIRKO mice also had impaired oral glucose tolerance and a limited glucose-lowering benefit from exendin-4, suggesting that the blunted incretin effect contributed to hyperglycemia. Secondly, GIRKO mice manifested severe dyslipidemia while on HFD due to elevated hepatic lipid secretion, serum triglyceride concentration, and lipid droplet accumulation in hepatocytes. Thirdly, GIRKO mice on HFD had increased inflammatory cues in the gut, which were associated with the HFD-induced microbiome alterations and increased serum lipopolysaccharide (LPS). In conclusion, our studies identified important gene/diet interactions contributing to diabetes progression, which might be leveraged to develop more efficacious therapies.
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    Intestinal Gpr17 deficiency improves glucose metabolism by promoting GLP-1 secretion
    (Elsevier, 2022-01) Yan, Shijun; Conley, Jason M.; Reilly, Austin M.; Stull, Natalie D.; Abhyankar, Surabhi D.; Ericsson, Aaron C.; Kono, Tatsuyoshi; Molosh, Andrei I.; Kubal, Chandrashekhar A.; Evans-Molina, Carmella; Ren, Hongxia; Medicine, School of Medicine
    G protein-coupled receptors (GPCRs) in intestinal enteroendocrine cells (EECs) respond to nutritional, neural, and microbial cues and modulate the release of gut hormones. Here we show that Gpr17, an orphan GPCR, is co-expressed in glucagon-like peptide-1 (GLP-1)-expressing EECs in human and rodent intestinal epithelium. Acute genetic ablation of Gpr17 in intestinal epithelium improves glucose tolerance and glucose-stimulated insulin secretion (GSIS). Importantly, inducible knockout (iKO) mice and Gpr17 null intestinal organoids respond to glucose or lipid ingestion with increased secretion of GLP-1, but not the other incretin glucose-dependent insulinotropic polypeptide (GIP). In an in vitro EEC model, overexpression or agonism of Gpr17 reduces voltage-gated calcium currents and decreases cyclic AMP (cAMP) production, and these are two critical factors regulating GLP-1 secretion. Together, our work shows that intestinal Gpr17 signaling functions as an inhibitory pathway for GLP-1 secretion in EECs, suggesting intestinal GPR17 is a potential target for diabetes and obesity intervention.
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    Mouse and human islets survive and function after coating by biosilicification
    (American Physiological Society, 2013-11-15) Jaroch, David B.; Lu, Jing; Madangopal, Rajtarun; Stull, Natalie D.; Stensberg, Matthew; Shi, Jin; Kahn, Jennifer L.; Herrera-Perez, Ruth; Zeitchek, Michael; Sturgis, Jennifer; Robinson, J. Paul; Yoder, Mervin C.; Porterfield, D. Marshall; Mirmira, Raghavendra; Rickus, Jenna L.; Medicine, School of Medicine
    Inorganic materials have properties that can be advantageous in bioencapsulation for cell transplantation. Our aim was to engineer a hybrid inorganic/soft tissue construct by inducing pancreatic islets to grow an inorganic shell. We created pancreatic islets surrounded by porous silica, which has potential application in the immunoprotection of islets in transplantation therapies for type 1 diabetes. The new method takes advantage of the islet capsule surface as a template for silica formation. Mouse and human islets were exposed to medium containing saturating silicic acid levels for 9-15 min. The resulting tissue constructs were then cultured for up to 4 wk under normal conditions. Scanning electron microscopy and energy dispersive X-ray spectroscopy was used to monitor the morphology and elemental composition of the material at the islet surface. A cytokine assay was used to assess biocompatibility with macrophages. Islet survival and function were assessed by confocal microscopy, glucose-stimulated insulin release assays, oxygen flux at the islet surface, expression of key genes by RT-PCR, and syngeneic transplant into diabetic mice.
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    Syntaxin 4 up-regulation increases efficiency of insulin release in pancreatic islets from humans with and without type 2 diabetes mellitus
    (The Endocrine Society, 2014-05) Oh, Eunjin; Stull, Natalie D.; Mirmira, Raghavendra G.; Thurmond, Debbie C.; Department of Pediatrics, IU School of Medicine
    CONTEXT: Evidence suggests that dysfunctional β-cell insulin release precedes type 1 and type 2 diabetes (T1D and T2D, respectively) and that enhancing the efficiency of insulin release from pancreatic islet β-cells may delay/prevent these diseases. We took advantage of the rare opportunity to test this paradigm using islets from human type 2 diabetic individuals. OBJECTIVES: Insulin release capacity is limited by the abundance of fusogenic soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. Because enrichment of Syntaxin 4, a plasma membrane soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein, enhances β-cell function in mice, we investigated its potential to restore functional insulin secretion to human diabetic islets. DESIGN: Human islets from type 2 diabetic and healthy individuals transduced to overexpress Syntaxin 4 were examined by perifusion analysis. Streptozotocin-induced diabetic recipient mice transplanted with Syntaxin 4-enriched or normal islets were assessed for rescue of diabetes in vivo. RESULTS: Syntaxin 4 up-regulation in human islets enhanced β-cell function by approximately 2-fold in each phase of secretion. Syntaxin 4 abundance in type 2 diabetes islets was approximately 70% reduced, and replenishment significantly improved insulin secretion. Islets from Syntaxin 4 overexpressing transgenic mice more effectively attenuated streptozotocin-induced diabetes than did control islets. CONCLUSIONS: These data show that the addition of just Syntaxin 4 is sufficient to significantly improve insulin secretory function to human type 2 diabetes islets retaining low levels of residual function and provide proof of concept that by building a more efficient β-cell with up-regulated Syntaxin 4, fewer islets may be required per patient, clearing a major barrier in transplantation therapy.CONTEXT: Evidence suggests that dysfunctional β-cell insulin release precedes type 1 and type 2 diabetes (T1D and T2D, respectively) and that enhancing the efficiency of insulin release from pancreatic islet β-cells may delay/prevent these diseases. We took advantage of the rare opportunity to test this paradigm using islets from human type 2 diabetic individuals. OBJECTIVES: Insulin release capacity is limited by the abundance of fusogenic soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. Because enrichment of Syntaxin 4, a plasma membrane soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein, enhances β-cell function in mice, we investigated its potential to restore functional insulin secretion to human diabetic islets. DESIGN: Human islets from type 2 diabetic and healthy individuals transduced to overexpress Syntaxin 4 were examined by perifusion analysis. Streptozotocin-induced diabetic recipient mice transplanted with Syntaxin 4-enriched or normal islets were assessed for rescue of diabetes in vivo. RESULTS: Syntaxin 4 up-regulation in human islets enhanced β-cell function by approximately 2-fold in each phase of secretion. Syntaxin 4 abundance in type 2 diabetes islets was approximately 70% reduced, and replenishment significantly improved insulin secretion. Islets from Syntaxin 4 overexpressing transgenic mice more effectively attenuated streptozotocin-induced diabetes than did control islets. CONCLUSIONS: These data show that the addition of just Syntaxin 4 is sufficient to significantly improve insulin secretory function to human type 2 diabetes islets retaining low levels of residual function and provide proof of concept that by building a more efficient β-cell with up-regulated Syntaxin 4, fewer islets may be required per patient, clearing a major barrier in transplantation therapy.