Insulin and IGF1 signaling are important for adipose tissue development and function; however, their role in mature adipocytes is unclear. Mice with a tamoxifen-inducible knockout of insulin and/or IGF1 receptors (IR/IGF1R) demonstrate a rapid loss of white and brown fat due to increased lipolysis and adipocyte apoptosis. This results in insulin resistance, glucose intolerance, hepatosteatosis, islet hyperplasia with hyperinsulinemia, and cold intolerance. This phenotype, however, resolves over 10-30 days due to a proliferation of preadipocytes and rapid regeneration of both brown and white adipocytes as identified by mTmG lineage tracing. This cycle can be repeated with a second round of receptor inactivation. Leptin administration prior to tamoxifen treatment blocks development of the metabolic syndrome without affecting adipocyte loss or regeneration. Thus, IR is critical in adipocyte maintenance, and this loss of adipose tissue stimulates regeneration of brown/white fat and reversal of metabolic syndrome associated with fat loss.

Insulin and IGF-1 receptors (IR and IGF1R) are major regulators of metabolism and cell growth throughout the body, however, their roles in the intestine remain controversial. Here we show that genetic ablation of the IR or IGF1R in intestinal epithelial cells of mice does not impair intestinal growth or development or the composition of the gut microbiome. However, loss of IR alters intestinal epithelial gene expression, especially in pathways related to glucose uptake and metabolism. More importantly, loss of IR reduces intestinal glucose uptake. As a result, mice lacking the IR in intestinal epithelium retain normal glucose tolerance during aging as compared to controls, which show an age-dependent decline in glucose tolerance. Loss of the insulin receptor also results in a reduction of GIP expression from K-cells and decreased GIP release in vivo following glucose ingestion, but has no effect on GLP-1 expression or secretion. Thus, the IR in the intestinal epithelium plays important roles in intestinal gene expression, glucose uptake and GIP production, which may contribute to pathophysiological changes in diabetes, metabolic syndrome and other insulin resistant states.

Adipose tissue is a major site of energy storage and has a role in the regulation of metabolism through the release of adipokines. Here we show that mice with an adipose-tissue-specific knockout of the microRNA (miRNA)-processing enzyme Dicer (ADicerKO), as well as humans with lipodystrophy, exhibit a substantial decrease in levels of circulating exosomal miRNAs. Transplantation of both white and brown adipose tissue-brown especially-into ADicerKO mice restores the level of numerous circulating miRNAs that are associated with an improvement in glucose tolerance and a reduction in hepatic Fgf21 mRNA and circulating FGF21. This gene regulation can be mimicked by the administration of normal, but not ADicerKO, serum exosomes. Expression of a human-specific miRNA in the brown adipose tissue of one mouse in vivo can also regulate its 3' UTR reporter in the liver of another mouse through serum exosomal transfer. Thus, adipose tissue constitutes an important source of circulating exosomal miRNAs, which can regulate gene expression in distant tissues and thereby serve as a previously undescribed form of adipokine.

Insulin resistance, a critical component of type 2 diabetes (T2D), precedes and predicts T2D onset. T2D is also associated with mitochondrial dysfunction. To define the cause-effect relationship between insulin resistance and mitochondrial dysfunction, we compared mitochondrial metabolism in induced pluripotent stem cells (iPSC) from 5 healthy individuals and 4 patients with genetic insulin resistance due to insulin receptor mutations. Insulin-resistant iPSC had increased mitochondrial number and decreased mitochondrial size. Mitochondrial oxidative function was impaired, with decreased citrate synthase activity and spare respiratory capacity. Simultaneously, expression of multiple glycolytic enzymes was decreased, while lactate production increased 80%. These perturbations were accompanied by an increase in ADP/ATP ratio and 3-fold increase in AMPK activity, indicating energetic stress. Insulin-resistant iPSC also showed reduced catalase activity and increased susceptibility to oxidative stress. Thus, insulin resistance can lead to mitochondrial dysfunction with reduced mitochondrial size, oxidative activity, and energy production.

Scientific evidence has established several links between metabolic and neurocognitive dysfunction, and epidemiologic evidence has revealed an increased risk of Alzheimer's disease and vascular dementia in patients with diabetes. In July 2015, the National Institute of Diabetes, Digestive, and Kidney Diseases gathered experts from multiple clinical and scientific disciplines, in a workshop entitled "The Intersection of Metabolic and Neurocognitive Dysfunction", to clarify the state-of-the-science on the mechanisms linking metabolic dysfunction, and insulin resistance and diabetes in particular, to neurocognitive impairment and dementia. This perspective is intended to serve as a summary of the opinions expressed at this meeting, which focused on identifying gaps and opportunities to advance research in this emerging area with important public health relevance.