In This Issue of Diabetes

A method to quantify both basal and insulin-stimulated glucose uptake in the same mouse is described by Cutler et al. (p. 359). Promising to substantially cut the numbers of animals used in experiments, the approach addresses the need for a separate group of animals to measure basal glucose uptake, which can be problematic when trying to quantify insulin action in individual tissues. Called the Dual Tracer Test, the approach involves sequential retroorbital injections of two versions of radiolabeled 2-deoxyglucose separated by 40 min with a maximal dose of insulin given with the second injection. This approach, the authors highlight, means paired basal and insulin-stimulated measures of glucose can be collected and thus used to adjust values to account for the effect of basal glucose on insulin-stimulated glucose uptake. In initial pilot experiments, the authors found the approach was feasible, obtaining expected trajectories of glucose following anesthesia in the mice and injection of the different glucose tracers and insulin. They then validated the method in a classic diet-induced model of insulin resistance and a novel transgenic mouse model with reduced GLUT4 expression. “In the rapidly advancing field of metabolism and the use of mouse models, there is a pressing need for simpler, more robust measures of tissue-specific insulin action in this model,” said author David E. James. “Our goal is to introduce a novel method that could empower laboratories to enhance their metabolic phenotyping capability beyond current methods.” While highlighting several strengths of the approach, the authors stress there are still limitations that future users should account for, along with recommended minimum data requirements. Nevertheless, according to James, the approach can be expanded into different mouse strains to look at the impact of genetic backgrounds on phenotypes. “We are particularly excited about the potential of the method for studying how genes and the environment interact to determine metabolic phenotypes like insulin sensitivity,” he added.

Protein tyrosine phosphatase N2, or PTPN2, appears to play a role in β-cell fitness and susceptibility to stressors that lead to failure and type 1 diabetes, according to Kim et al. (p. 434). Specifically, PTPN2 appears to modulate mitochondrial function and, with it, insulin secretion, and this finding raises the prospect that any deficiency in activity may make β-cells more susceptible to inflammation. The findings come from an investigation centered on a mouse model with β-cell–specific deletion of Ptpn2 that was specifically aimed to test whether PTPN2 contributed to β-cell dysfunction when exposed to an inflammatory environment. After initially characterizing the mouse model and finding no particular indication of glucose intolerance compared with controls, the authors employed RNA sequencing to probe pathways affected by the β-cell–specific deletion of Ptpn2. Only a small number of genes were differentially expressed compared with the control, and there were no gene expression changes related to insulin, endocrine hormones, or islet transcription. However, a disproportionate number of dysregulated genes were related to mitochondrial function. Imaging studies and experiments with proinflammatory cytokine treatment revealed subtle mitochondrial defects and impaired β-cell metabolic responses. Subsequent in vitro experiments revealed yet more differentially expressed genes related to the deletion that largely matched the phenotypic changes seen in vivo, including key indicators of mitochondrial dysfunction. The authors also identified a series of proteins that may interact with PTPN2. Many were involved in regulation of glycolysis, the tricarboxylic acid cycle, and gluconeogenesis. However, one protein, ATP-citrate synthase, was significantly increased in the absence of PTPN2, suggesting it is an important substrate that contributes to the dysregulation of mitochondrial function. “Since PTPN2 was identified as a type 1 diabetes susceptibility gene, our studies suggest that individuals carrying mutations in PTPN2 could have compromised β-cell function in addition to autoimmune propensity,” said author Lori Sussel. “This would explain why the β-cell is especially sensitive to autoimmune conditions.”

The relationship between albuminuria and subclinical signs of cardiovascular disease in type 2 diabetes is explored by Rasmussen et al. (p. 490). They suggest an underlying mechanism exists that connects cardiovascular and kidney diseases and potentially identifies the initial stages of cardiorenal syndrome. However, further studies will likely be needed to validate the findings, not least because of the explorative nature of the study and a variety of limitations. Using novel noninvasive imaging in a cross-sectional study design, the authors evaluated a series of measures of subclinical coronary artery pathology in individuals with type 2 diabetes but without overt cardiovascular disease. The cohort consisted of 90 individuals, and they were stratified according to whether they had normoalbuminuria (n = 30) or historic or current albuminuria (n = 60), defined as a urine albumin creatinine ratio ≥30 mg/g. They found that the group with any albuminuria had higher levels of a measure of microcalcification and a lower level of myocardial flow reserve compared with those with normoalbuminuria. The differences, however, did not remain after adjustment for confounding, with blood pressure (among other factors) helping to explain the associations. A series of measures of cardiac inflammation were similar in the groups, while left ventricular ejection fraction was lower in the group with albuminuria after adjustment. The authors explain that the loss of significance and remaining trends were likely due to limitations such as high levels of treatment received by the patients and potentially also selection bias, misclassification, limited sample size, and measurement issues. They note that this likely weakened their ability to detect “true” differences between the groups. Nevertheless, they suggest the findings indicate albuminuria is related to a more severe phenotype of coronary artery pathology, which, if validated, would presumably point toward early preventative measures or treatments in those that need them.

Higher blood plasma levels of diet- and gut microbiota–derived branched short-chain fatty acids appear to be associated with a lower prevalence of dysglycemia and improved glucose homeostasis, according to Aslamy et al. (p. 385). Based on the findings, the authors propose that further investigation is now warranted into whether branched short-chain fatty acids can be used as targets for the prevention or treatment of diabetes. The findings come from further analysis of the Microbiome and Insulin Longitudinal Evaluation Study (MILES) and involved 345 participants who underwent oral glucose tolerance tests and provided blood samples for glucose and insulin measurements and determination of different short-chain fatty acids. They found that branched versions of short-chain fatty acids exhibited a bimodal distribution where 25 participants (termed the H-BSCFA group) had markedly higher concentrations than the rest of the cohort (termed the L-BSCFA group; n = 320). Notably, in the H-BSCFA group, 12% had prediabetes and 4% had diabetes. In contrast, in the much larger L-BSCFA group, 41% had prediabetes and 8% had diabetes. This translated to an odds ratio of dysglycemia (i.e., diabetes and prediabetes) in the H-BSCFA group of 0.20 (95% CI 0.066–0.59, P = 0.0036) compared with the L-BSCFA group. The association remained after multiple adjustments, including for nonbranched short-chain fatty acids. Finally, the H-BSCFA group had lower levels of various measures of glucose homeostasis and C-peptide, but there was no difference in insulin sensitivity or secretion between the groups. The authors note that the study design largely precludes their ability to assess whether branched short-chain fatty acids are causal for dysglycemia. “While there have been many studies of abundant short-chain fatty acids, particularly butyrate, for a role in diabetes, there have been very few studies focused on branched short-chain fatty acids,” said author Mark O. Goodarzi. “We hope that our study will encourage others to examine whether these fatty acids are associated with glucose homeostasis in additional independent cohorts.”