Adiponectin has recently been reported to generate a negative energy balance by increasing energy expenditure. However, it is unclear whether such effects require the presence and direct action of the adiponectin protein in the central nervous system. In this study, neither radiolabeled nonglycosylated nor glycosylated globular adiponectin crossed the blood-brain barrier (BBB) in mice. In addition, adiponectin was not detectable in human cerebrospinal fluid using various established methods. Using murine cerebral microvessels, we demonstrated expression of adiponectin receptors, which are upregulated during fasting, in brain endothelium. Interestingly, treatment with adiponectin reduced secretion of the centrally active interleukin-6 from brain endothelial cells, a phenomenon that was paralleled by a similar trend of other proinflammatory cytokines. In summary, our data suggest that direct effects of endogenous adiponectin on central nervous system pathways are unlikely to exist. However, the identification of adiponectin receptors on brain endothelial cells and the finding of a modified secretion pattern of centrally active substances from BBB cells provides an alternate explanation as to how adiponectin may evoke effects on energy metabolism. Diabetes 55: [141][142][143][144][145][146][147] 2006 A diponectin is an adipocyte-specific protein, and its structure consists of an NH 2 -terminal collagenous domain and a COOH-terminal globular domain (1-6). Various studies have associated adiponectin with insulin sensitivity (7-10). In epidemiological studies, high levels of adiponectin were associated with a reduced diabetes and coronary heart disease risk (11-13).A growing body of evidence suggests that adiponectin directly affects energy balance by increasing thermogenesis (14). Recent studies in C57BL6 mice demonstrated that globular and full-length adiponectin decreases body weight after central or peripheral administration by increasing energy expenditure. Comparable effects were observed in leptin-deficient ob/ob mice, while central treatment had no effects in agouti yellow mice (A y /a), suggesting melanocortin but not leptin receptor activation as an essential prerequisite for adiponectin-induced weight loss (15). In another study, however, peripheral administration of full-length adiponectin in A y /a mice increased energy expenditure, while central application was again without effect (16). Increased energy expenditure in adiponectintreated mice therefore might in part be mediated via peripheral adiponectin receptors, including those located at the luminal surface of the blood-brain barrier (BBB).However, systemic adiponectin levels increase after weight reduction, while that physiological state is clearly associated with reduced energy expenditure (17-19). Thus, the energy expenditure-increasing effects of adiponectin seem unlikely to play a major physiological role. The picture becomes even more confusing since the latest results indicate that mice with increased circulating adiponectin levels due to e...

We have previously identified transcription factor B1 mitochondrial (TFB1M) as a type 2 diabetes (T2D) risk gene, using human and mouse genetics. To further understand the function of TFB1M and how it is associated with T2D, we created a β-cell-specific knockout of Tfb1m, which gradually developed diabetes. Prior to the onset of diabetes, β-Tfb1m(-/-) mice exhibited retarded glucose clearance owing to impaired insulin secretion. β-Tfb1m(-/-) islets released less insulin in response to fuels, contained less insulin and secretory granules and displayed reduced β-cell mass. Moreover, mitochondria in Tfb1m-deficient β-cells were more abundant with disrupted architecture. TFB1M is known to control mitochondrial protein translation by adenine dimethylation of 12S ribosomal RNA (rRNA). Here, we found that the levels of TFB1M and mitochondrial-encoded proteins, mitochondrial 12S rRNA methylation, ATP production and oxygen consumption were reduced in β-Tfb1m(-/-) islets. Furthermore, the levels of reactive oxygen species (ROS) in response to cellular stress were increased whereas induction of defense mechanisms was attenuated. We also show increased apoptosis and necrosis as well as infiltration of macrophages and CD4(+) cells in the islets. Taken together, our findings demonstrate that Tfb1m-deficiency in β-cells caused mitochondrial dysfunction and subsequently diabetes owing to combined loss of β-cell function and mass. These observations reflect pathogenetic processes in human islets: using RNA sequencing, we found that the TFB1M risk variant exhibited a negative gene-dosage effect on islet TFB1M mRNA levels, as well as insulin secretion. Our findings highlight the role of mitochondrial dysfunction in impairments of β-cell function and mass, the hallmarks of T2D.

As models for β-cell metabolism, rat islets are, to some extent, a, heterogeneous cell population stressed by the islet isolation procedure, whereas rat-derived clonal β-cells exhibit a tumor-like phenotype. To describe to what extent either of these models reflect normal cellular metabolism, we compared metabolite profiles and gene expression in rat islets and the INS-1 832/13 line, a widely used clonal β-cell model. We found that insulin secretion and metabolic regulation provoked by glucose were qualitatively similar in these β-cell models. However, rat islets exhibited a more pronounced glucose-provoked increase of glutamate, glycerol-3-phosphate, succinate, and lactate levels, whereas INS-1 832/13 cells showed a higher glucose-elicited increase in glucose-6-phosphate, alanine, isocitrate, and α-ketoglutarate levels. Glucose induced a decrease in levels of γ-aminobutyrate (GABA) and aspartate in rat islets and INS-1 832/13 cells, respectively. Genes with cellular functions related to proliferation and the cell cycle were more highly expressed in the INS-1 832/13 cells. Most metabolic pathways that were differentially expressed included GABA metabolism, in line with altered glucose responsiveness of GABA. Also, lactate dehydrogenase A, which is normally expressed at low levels in mature β-cells, was more abundant in rat islets than in INS-1 832/13 cells, confirming the finding of elevated glucose-provoked lactate production in the rat islets. Overall, our results suggest that metabolism in rat islets and INS-1 832/13 cells is qualitatively similar, albeit with quantitative differences. Differences may be accounted for by cellular heterogeneity of islets and proliferation of the INS-1 832/13 cells.

Background: Elevated glucose may cause ␤-cell dysfunction in type 2 diabetes, i.e., glucotoxicity. Results: Elevated glucose, but not pyruvate, perturbed insulin secretion and content, plasma and mitochondrial membrane potentials, and proton leak, while increasing glycolytic metabolites in ␤-cells. Conclusion: Early metabolism of glucose exerts a toxic effect on clonal insulin-producing cells. Significance: Unraveling these mechanisms may provide protection of ␤-cells in diabetes.