Glucagon receptor inactivation leads to α-cell hyperplasia in zebrafish.

Glucagon antagonism is a potential treatment for diabetes. One potential side effect is α-cell hyperplasia, which has been noted in several approaches to antagonize glucagon action. To investigate the molecular mechanism of the α-cell hyperplasia and to identify the responsible factor, we created a zebrafish model in which glucagon receptor (gcgr) signaling has been interrupted. The genetically and chemically tractable zebrafish, which provides a robust discovery platform, has two gcgr genes (gcgra and gcgrb) in its genome. Sequence, phylogenetic, and synteny analyses suggest that these are co-orthologs of the human GCGR. Similar to its mammalian counterparts, gcgra and gcgrb are mainly expressed in the liver. We inactivated the zebrafish gcgra and gcgrb using transcription activator-like effector nuclease (TALEN) first individually and then both genes, and assessed the number of α-cells using an α-cell reporter line, Tg(gcga:GFP). Compared to WT fish at 7 days postfertilization, there were more α-cells in gcgra-/-, gcgrb-/-, and gcgra-/-;gcgrb-/- fish and there was an increased rate of α-cell proliferation in the gcgra-/-;gcgrb-/- fish. Glucagon levels were higher but free glucose levels were lower in gcgra-/-, gcgrb-/-, and gcgra-/-;gcgrb-/- fish, similar to Gcgr-/- mice. These results indicate that the compensatory α-cell hyperplasia in response to interruption of glucagon signaling is conserved in zebrafish. The robust α-cell hyperplasia in gcgra-/-;gcgrb-/- larvae provides a platform to screen for chemical and genetic suppressors, and ultimately to identify the stimulus of α-cell hyperplasia and its signaling mechanism.

Comparison of the physiological relevance of systemic vs. portal insulin delivery to evaluate whole body glucose flux during an insulin clamp.

To understand the underlying pathology of metabolic diseases, such as diabetes, an accurate determination of whole body glucose flux needs to be made by a method that maintains key physiological features. One such feature is a positive differential in insulin concentration between the portal venous and systemic arterial circulation (P/S-IG). P/S-IG during the determination of the relative contribution of liver and extra-liver tissues/organs to whole body glucose flux during an insulin clamp with either systemic (SID) or portal (PID) insulin delivery was examined with insulin infusion rates of 1, 2, and 5 mU·kg(-1)·min(-1) under either euglycemic or hyperglycemic conditions in 6-h-fasted conscious normal rats. A P/S-IG was initially determined with endogenous insulin secretion to exist with a value of 2.07. During an insulin clamp, while inhibiting endogenous insulin secretion by somatostatin, P/S-IG remained at 2.2 with PID, whereas, P/S-IG disappeared completely with SID, which exhibited higher arterial and lower portal insulin levels compared with PID. Consequently, glucose disappearance rates and muscle glycogen synthetic rates were higher, but suppression of endogenous glucose production and liver glycogen synthetic rates were lower with SID compared with PID. When the insulin clamp was performed with SID at 2 and 5 mU·kg(-1)·min(-1) without managing endogenous insulin secretion under euglycemic but not hyperglycemic conditions, endogenous insulin secretion was completely suppressed with SID, and the P/S-IG disappeared. Thus, compared with PID, an insulin clamp with SID underestimates the contribution of liver in response to insulin to whole body glucose flux.

Assessment of human pancreatic islet architecture and composition by laser scanning confocal microscopy.

The recent success of pancreatic islet transplantation has generated considerable enthusiasm. To better understand the quality and characteristics of human islets used for transplantation, we performed detailed analysis of islet architecture and composition using confocal laser scanning microscopy. Human islets from six separate isolations provided by three different islet isolation centers were compared with isolated mouse and non-human primate islets. As expected from histological sections of murine pancreas, in isolated murine islets alpha and delta cells resided at the periphery of the beta-cell core. However, human islets were markedly different in that alpha, beta, and delta cells were dispersed throughout the islet. This pattern of cell distribution was present in all human islet preparations and islets of various sizes and was also seen in histological sections of human pancreas. The architecture of isolated non-human primate islets was very similar to that of human islets. Using an image analysis program, we calculated the volume of alpha, beta, and delta cells. In contrast to murine islets, we found that populations of islet cell types varied considerably in human islets. The results indicate that human islets not only are quite heterogeneous in terms of cell composition but also have a substantially different architecture from widely studied murine islets.

Assessment of pancreatic islet mass after islet transplantation using in vivo bioluminescence imaging.

BACKGROUND: Pancreatic islet transplantation is an emerging therapy for type 1 diabetes, but it is difficult to assess islets after transplantation and thus to design interventions to improve islet survival. METHODS: To image and quantify islets, the authors transplanted luciferase-expressing murine or human islets (by adenovirus-mediated gene transfer) into the liver or beneath the renal capsule of immunodeficient mice and quantified the in vivo bioluminescence imaging (BLI) of mice using a cooled charge-coupled device camera and digital photon-counting image analysis. To account for variables that are independent of islet mass such as transplant site, animal positioning, and wound healing, the BLI of transplanted islets was calibrated against measurement of luminescence of an implanted bead emitting a constant light intensity. RESULTS: BLI of mice bearing islet transplants was seen in the expected anatomic location, was stable for more than 8 weeks after transplantation, and correlated with the number of islets transplanted into the liver or kidney. BLI of the luminescent bead and of transplanted islets in the kidney was approximately four times greater than when transplanted in the liver, indicating that photon emission is dependent on optical absorption of generated light and thus light source location. CONCLUSION: In vivo BLI allows for quantitative, serial measurements of pancreatic islet mass after transplantation and should be useful in assessing interventions to sustain or increase islet survival of transplanted islets.

Gene expression phenotyping of an ACTH-producing small cell lung cancer line.

DNA microarray techniques were used to compare gene expression in an adrenocorticotropin (ACTH)-producing human small cell lung carcinoma line (DMS-79) with six other small cell lung cancer (SCLC) lines that do not produce ACTH. Twelve genes were expressed at more than five-fold higher levels in DMS-79 cells. Two transcription factors were the genes that exhibited the most remarkable over-expression: T-box 3 mRNA was detected at levels 19.37 +/- 3.78 times those observed in the SCLCs. Thyroid transcription factor (TTF-1, T/ebp, Nkx2.1) was expressed at 14.24 +/- 3.41-fold higher in DMS-79 cells. Seven genes were identified whose expression levels were at least five-fold lower in the ACTH-producing cell line. Variation in culture medium formulation did not significantly affect the gene expression profile of DMS-79 cells and expression data observed in microarray experiments were corroborated by northern blot analysis of RNA from the same cell lines. These experiments reveal new candidate genes that could be involved in the dysregulation of POMC gene expression manifested by ACTH-producing nonpituitary tumors.

A novel mutation in the preprovasopressin gene identified in a kindred with autosomal dominant neurohypophyseal diabetes insipidus.

Autosomal dominant neurohypophyseal diabetes insipidus (ADNDI) is a defect in free water conservation caused by mutations in the single gene that encodes both vasopressin (VP) and its binding protein, neurophysin II (NP II). Most of the human mutations in this gene have been in the portion encoding the NP molecule; the resultant abnormal gene products are believed to cause cellular toxicity as improperly folded precursor molecules accumulate in the endoplasmic reticulum. We identified a new American kindred with ADNDI and found a novel mutation in the VP molecule. A 78-yr-old man was noted to have hypotonic polyuria and plasma hyperosmolarity; the urinary concentration defect was reversed by administration of VP. His symptomatology dated to childhood, and his family history was consistent with autosomal transmission of the polyuric syndrome, with affected members in three generations, including several females. Affected individuals were found to be heterozygous for a 3-bp deletion in exon 1 of arginine VP (AVP)-NP II, predicting a deletion of phenylalanine 3 (known to be critical for receptor binding) in the VP nonapeptide. Neuro 2A cells stably transfected with the mutant AVP-NP construct showed increased rates of apoptosis as assessed by flow cytometric methods. These observations support the concept that cellular toxicity of abnormal AVP-NP gene products underlies the development of ADNDI, and the data further demonstrate that mutations affecting the AVP moiety can result in initiation of these pathological processes.

Glucocorticoids enhance activation of the human type II 3beta-hydroxysteroid dehydrogenase/Delta5-Delta4 isomerase gene.

Glucocorticoids indirectly alter adrenocortical steroid output through the inhibition of ACTH secretion by the anterior pituitary. However, previous studies suggest that glucocorticoids can directly affect adrenocortical steroid production. Therefore, we have investigated the ability of glucocorticoids to affect transcription of adrenocortical steroid biosynthetic enzymes. One potential target of glucocorticoid action in the adrenal is an enzyme critical for adrenocortical steroid production: 3beta-hydroxysteroid dehydrogenase/Delta5-Delta4 isomerase (3beta-HSD). Treatment of the adrenocortical cell line (H295R) with the glucocorticoid agonist dexamethasone (DEX) increased cortisol production and 3beta-HSD mRNA levels alone or in conjunction with phorbol ester. This increase in 3beta-HSD mRNA was paralleled by increases in Steroidogenic Acute Regulatory Protein (StAR) mRNA levels. The human type II 3beta-HSD promoter lacks a consensus palindromic glucocorticoid response element (GRE) but does contain a Stat5 response element (Stat5RE) suggesting that glucocorticoids could affect type II 3beta-HSD transcription via interaction with Stat5. Transfection experiments show enhancement of human type II 3beta-HSD promoter activity by coexpression of the glucocorticoid receptor (GR) and Stat5A and treatment with 100nM dexamethasone. Furthermore, removal of the Stat5RE either by truncation of the 5' flanking sequence in the promoter or introduction of point mutations to the Stat5RE abolished the ability of DEX to enhance 3beta-HSD promoter activity. These studies demonstrate the ability of glucocorticoids to directly enhance the expression of an adrenal steroidogenic enzyme gene albeit independent of a consensus palindromic glucocorticoid response element.

Pancreatic islet beta-cells transiently metabolize pyruvate.

Pancreatic beta-cell metabolism was followed during glucose and pyruvate stimulation of pancreatic islets using quantitative two-photon NAD(P)H imaging. The observed redox changes, spatially separated between the cytoplasm and mitochondria, were compared with whole islet insulin secretion. As expected, both NAD(P)H and insulin secretion showed sustained increases in response to glucose stimulation. In contrast, pyruvate caused a much lower NAD(P)H response and did not generate insulin secretion. Low pyruvate concentrations decreased cytoplasmic NAD(P)H without affecting mitochondrial NAD(P)H, whereas higher concentrations increased cytoplasmic and mitochondrial levels. However, the pyruvate-stimulated mitochondrial increase was transient and equilibrated to near-base-line levels. Inhibitors of the mitochondrial pyruvate-transporter and malate-aspartate shuttle were utilized to resolve the glucose- and pyruvate-stimulated NAD(P)H response mechanisms. These data showed that glucose-stimulated mitochondrial NAD(P)H and insulin secretion are independent of pyruvate transport but dependent on NAD(P)H shuttling. In contrast, the pyruvate-stimulated cytoplasmic NAD(P)H response was enhanced by both inhibitors. Surprisingly the malate-aspartate shuttle inhibitor enabled pyruvate-stimulated insulin secretion. These data support a model in which glycolysis plays a dominant role in glucose-stimulated insulin secretion. Based on these data, we propose a mechanism for glucose-stimulated insulin secretion that includes allosteric inhibition of tricarboxylic acid cycle enzymes and pH dependence of mitochondrial pyruvate transport.

Reduction in pancreatic transcription factor PDX-1 impairs glucose-stimulated insulin secretion.

Complete lack of transcription factor PDX-1 leads to pancreatic agenesis, whereas heterozygosity for PDX-1 mutations has been recently noted in some individuals with maturity-onset diabetes of the young (MODY) and in some individuals with type 2 diabetes. To determine how alterations in PDX-1 affect islet function, we examined insulin secretion and islet physiology in mice with one PDX-1 allele inactivated. PDX-1(+/-) mice had a normal fasting blood glucose and pancreatic insulin content but had impaired glucose tolerance and secreted less insulin during glucose tolerance testing. The expression of PDX-1 and glucose transporter 2 in islets from PDX-1(+/-) mice was reduced to 68 and 55%, respectively, whereas glucokinase expression was not significantly altered. NAD(P)H generation in response to glucose was reduced by 30% in PDX-1(+/-) mice. The in situ perfused pancreas of PDX-1(+/-) mice secreted about 45% less insulin when stimulated with 16.7 mm glucose. The K(m) for insulin release was similar in wild type and PDX-1(+/-) mice. Insulin secretion in response to 20 mm arginine was unchanged; the response to 10 nm glucagon-like peptide-1 was slightly increased. However, insulin secretory responses to 10 mm 2-ketoisocaproate and 20 mm KCl were significantly reduced (by 61 and 66%, respectively). These results indicate that a modest reduction in PDX-1 impairs several events in glucose-stimulated insulin secretion (such as NAD(P)H generation, mitochondrial function, and/or mobilization of intracellular Ca(2+)) and that PDX-1 is important for normal function of adult pancreatic islets.