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Altered metabolism of the inositol sugars myo-inositol (MI) and D-chiro-inositol is implicated in diabetic complications. In animals, catabolism of MI and D-chiro-inositol depends on the enzyme MI oxygenase (MIOX), which catalyzes the first committed step of the glucuronate-xylulose pathway, and is found almost exclusively in the kidneys. The crystal structure of MIOX, in complex with MI, has been determined by multiwavelength anomalous diffraction methods and refined at 2.0-Å resolution (R ‫؍‬ 0.206, Rfree ‫؍‬ 0.253). The structure reveals a monomeric, single-domain protein with a mostly helical fold that is distantly related to the diverse HD domain superfamily. Five helices form the structural core and provide six ligands (four His and two Asp) for the di-iron center, in which the two iron atoms are bridged by a putative hydroxide ion and one of the Asp ligands, Asp-124. A key loop forms a lid over the MI substrate, which is coordinated in bidentate mode to one iron atom. It is proposed that this mode of iron coordination, and interaction with a key Lys residue, activate MI for bond cleavage. The structure also reveals the basis of substrate specificity and suggests routes for the development of specific MIOX inhibitors.D iabetes mellitus is a chronic disease characterized by hyperglycemia caused by defective action and͞or secretion of insulin. Hyperglycemia leads to complications such as nephropathy, neuropathy, retinopathy, and cataract. There is considerable evidence that both type-1 and type-2 diabetes are associated with altered inositol metabolism, particularly of myo-inositol (MI) and its less abundant epimeric form, D-chiro-inositol (DCI) (1-3). MI is an osmoregulator and a precursor for inositol-based second messengers, and both MI and DCI are known components of endogenous inositol phosphoglycans, which act as insulin mediators (4, 5), leading to the hypothesis that control of inositol levels may have therapeutic potential. Consistent with this hypothesis, administration of inositols, especially DCI and derivatives, lowers blood glucose in diabetes and enhances insulin action (5-7).The enzyme MI oxygenase (MIOX; EC 1.13.99.1) is a key regulator of inositol levels, catalyzing the first committed step in the glucuronate-xylulose pathway (8, 9), the only known pathway for MI catabolism (10). MIOX is almost exclusively expressed in the kidneys, where it is localized to the proximal tubular epithelial cells (11). Importantly, MIOX also acts on DCI (12) and potentially mediates its catabolism as well. MIOX expression is up-regulated in conditions of hyperosmotic stress (13-14), and in db͞db mice, a model for type-2 diabetes, increased MIOX activity correlates with increased hyperglycemia (13).MIOX is a 33-kDa nonheme iron protein that catalyzes the oxidative conversion of MI to D-glucuronic acid (15). This reaction (Fig. 1), in which the bond between C6 and C1 of MI is cleaved, involves a dioxygen-dependent four-electron oxidation that appears to be unique in biological systems (9). Recent reports indicat...

BACKGROUND AND PURPOSE Amylin (Amy) is an important glucoregulatory peptide and AMY receptors are clinical targets for diabetes and obesity. Human (h) AMY receptor subtypes are complexes of the calcitonin (CT) receptor with receptor activity‐modifying proteins (RAMPs); their rodent counterparts have not been characterized. To allow identification of the most clinically relevant receptor subtype, the elucidation of rat (r) AMY receptor pharmacology is necessary. EXPERIMENTAL APPROACH Receptors were transiently transfected into COS‐7 cells and cAMP responses measured in response to different agonists, with or without antagonists. Competition binding experiments were performed to determine rAmy affinity. KEY RESULTS rCT was the most potent agonist of rCT(a) receptors, whereas rAmy was most potent at rAMY1(a) and rAMY3(a) receptors. rAmy bound to these receptors with high affinity. Rat α‐calcitonin gene‐related peptide (CGRP) was equipotent to rAmy at both AMY receptors. Rat adrenomedullin (AM) and rAM2/intermedin activated all three receptors but were most effective at rAMY3(a). AC187, AC413 and sCT8‐32 were potent antagonists at all three receptors. rαCGRP8‐37 displayed selectivity for rAMY receptors over rCT(a) receptors. rAMY8‐37 was a weak antagonist but was more effective at rAMY1(a) than rAMY3(a). CONCLUSIONS AND IMPLICATIONS AMY receptors were generated by co‐expression of rCT(a) with rRAMP1 or 3, forming rAMY1(a) and rAMY3(a) receptors, respectively. CGRP was more potent at rAMY than at hAMY receptors. No antagonist tested was able to differentiate the rAMY receptor subtypes. The data emphasize the need for and provide a useful resource for developing new CT or AMY receptor ligands as pharmacological tools or potential clinical candidates. LINKED ARTICLES This article is part of a themed section on Secretin Family (Class B) G Protein‐Coupled Receptors. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.166.issue-1

The Zucker diabetic fatty (ZDF) rat is a commonly used animal model of type 2 diabetes yet complete descriptions of insulin resistance in this model are limited. We present a full characterisation of in vivo insulin resistance in obese (fa/fa) animals compared to lean (+/?) littermates. Anaesthetised, ten-week old, obese ZDF rats and their lean littermates underwent a hyperinsulinaemic euglycaemic glucose clamp. Compared with lean littermates, obese ZDF rats required an 89% lower glucose infusion rate to maintain euglycaemia and showed a 35% decrease in peripheral glucose disposal. Insulin-stimulated glucose uptake (R(g')) in obese animals was also significantly less in all skeletal muscles studied. R(g') in cardiac and white adipose tissue was not different between the two groups. Total glycogen content in skeletal and cardiac muscle was significantly less in obese animals, while total glycogen content in the liver was significantly greater than in lean littermates. Glycogen synthesis was also decreased in skeletal muscle of obese animals. Compared with lean animals, total triglyceride content was significantly greater in skeletal muscle, heart and liver of obese ZDF rats. Obese animals also showed significantly increased glucose incorporation into lipid in all of these tissues, indicating an increase in lipogenesis. Collectively, these results provide an integrated characterisation of in vivo insulin resistance in obese ZDF rats and a direct comparison with lean littermates.

There is a significant correlation between the occurrence of pancreatic islet amyloid and beta-cell failure in advanced type II diabetes mellitus. Islet amyloid is composed primarily of the fibrillar form of the pancreatic hormone, amylin. Using thioflavin-T fluorescence binding and radioprecipitation assays, we investigated whether or not a series of small tricyclic compounds, tetracycline or Congo Red could interfere with the conversion of synthetic human amylin into its insoluble amyloid form. Of the compounds investigated, incubation of human amylin with a 20-fold molar excess of either Congo Red or Acridine Orange resulted in significant inhibition in the rate of amyloid formation. With Congo Red, maximal inhibition effectively occurred at a 1:1 molar ratio or greater over human amylin, whereas inhibition by Acridine Orange was dose-dependent. A 20-fold molar excess of the compound tetracycline also decreased insoluble amyloid content after extended incubation periods of approx. 20 h. Amyloid fibril morphology in the presence of tetracycline, as measured by transmission electron microscopy, was characterized by short fragmented fibrils compared with the longer and denser appearance of fibrils formed by amylin alone. These findings show that polycyclic compounds can suppress the formation of amyloid by human amylin, providing support for an alternative approach to peptide-based strategies by which islet amyloid formation could be modulated.

OBJECTIVEAggregation of human amylin/islet amyloid polypeptide (hA/hIAPP) into small soluble β-sheet–containing oligomers is linked to islet β-cell degeneration and the pathogenesis of type 2 diabetes. Here, we used tetracycline, which modifies hA/hIAPP oligomerization, to probe mechanisms whereby hA/hIAPP causes diabetes in hemizygous hA/hIAPP-transgenic mice.RESEARCH DESIGN AND METHODSWe chronically treated hemizygous hA/hIAPP transgenic mice with oral tetracycline to determine its effects on rates of diabetes initiation, progression, and survival.RESULTSHomozygous mice developed severe spontaneous diabetes due to islet β-cell loss. Hemizygous transgenic animals also developed spontaneous diabetes, although severity was less and progression rates slower. Pathogenesis was characterized by initial islet β-cell dysfunction followed by progressive β-cell loss. Islet amyloid was absent from hemizygous animals with early-onset diabetes and correlated positively with longevity. Some long-lived nondiabetic hemizygous animals also had large islet-amyloid areas, showing that amyloid itself was not intrinsically cytotoxic. Administration of tetracycline dose-dependently ameliorated hyperglycemia and polydipsia, delayed rates of diabetes initiation and progression, and increased longevity compared with water-treated controls.CONCLUSIONSThis is the first report to show that treating hA/hIAPP transgenic mice with a modifier of hA/hIAPP misfolding can ameliorate their diabetic phenotype. Fibrillar amyloid was neither necessary nor sufficient to cause diabetes and indeed was positively correlated with longevity therein, whereas early- to mid-stage diabetes was associated with islet β-cell dysfunction followed by β-cell loss. Interventions capable of suppressing misfolding in soluble hA/hIAPP oligomers rather than mature fibrils may have potential for treating or preventing type 2 diabetes.

Misfolding of the islet β-cell peptide hA (human amylin) into β-sheet-containing oligomers is linked to β-cell apoptosis and the pathogenesis of T2DM (Type 2 diabetes mellitus). In the present study, we have investigated the possible effects on hA misfolding of the chaperones HSP (heat-shock protein) 70, GRP78/BiP (glucose-regulated protein of 78 kDa/immunoglobulin heavy-chain-binding protein) and HSP40/DnaJ. We demonstrate that hA underwent spontaneous time-dependent β-sheet formation and aggregation by thioflavin-T fluorescence in solution, whereas rA (rat amylin) did not. HSP70, GRP78/BiP and HSP40/DnaJ each independently suppressed hA misfolding. Maximal molar protein/hA ratios at which chaperone activity was detected were 1:200 (HSP70, HSP40/DnaJ and GRP78/BiP). By contrast, none of the chaperones modified the secondary structure of rA. hA, but not rA, was co-precipitated independently with HSP70 and GRP78/BiP by anti-amylin antibodies. As these effects occur at molar ratios consistent with chaperone binding to relatively rare misfolded hA species, we conclude that HSP70 and GRP78/BiP can detect and bind misfolded hA oligomers, thereby effectively protecting hA against bulk misfolding and irreversible aggregation. Defective β-cell chaperone biology could contribute to hA misfolding and initiation of apoptosis in T2DM.

Alpha-calcitonin gene-related peptide (alphaCGRP) is a neuropeptide that is expressed in motor and sensory neurons. It is a powerful vasodilator and has been implicated in diverse metabolic roles. However, its precise physiological function remains unclear. In this study, we investigated the role of alphaCGRP in lipid metabolism by chronically challenging alphaCGRP-specific knockout (alphaCGRP(-/-)) and control mice with high-fat diet regimens. At the start of the study, both animal groups displayed similar body weights, serum lipid markers, and insulin sensitivity. However, alphaCGRP(-/-) mice displayed higher core temperatures, increased energy expenditures, and a relative daytime (nonactive) depression in respiratory quotients, which indicated increased beta-oxidation. In response to fat feeding, alphaCGRP(-/-) mice were comparatively protected against diet-induced obesity with an attenuated body weight gain and an overall reduction in adiposity across all the three diets examined. AlphaCGRP(-/-) mice also displayed improved glucose handling and insulin sensitivity, lower im and hepatic lipid accumulation, and improved overall metabolic health. These findings define a new role for alphaCGRP as a mediator of energy metabolism and opens up therapeutic opportunities to target CGRP action in obesity.