Neuronal overexpression of insulin receptor substrate 2 leads to increased fat mass, insulin resistance, and glucose intolerance during aging.

The insulin receptor substrates (IRS) are adapter proteins mediating insulin's and IGF1's intracellular effects. Recent data suggest that IRS2 in the central nervous system (CNS) is involved in regulating fuel metabolism as well as memory formation. The present study aims to specifically define the role of chronically increased IRS2-mediated signal transduction in the CNS. We generated transgenic mice overexpressing IRS2 specifically in neurons (nIRS2 (tg)) and analyzed these in respect to energy metabolism, learning, and memory. Western blot (WB) analysis of nIRS2 (tg) brain lysates revealed increased IRS2 downstream signaling. Histopathological investigation of nIRS2 (tg) mice proved unaltered brain development and structure. Interestingly, nIRS2 (tg) mice showed decreased voluntary locomotoric activity during dark phase accompanied with decreased energy expenditure (EE) leading to increased fat mass. Accordingly, nIRS2 (tg) mice develop insulin resistance and glucose intolerance during aging. Exploratory behavior, motor function as well as food and water intake were unchanged in nIRS2 (tg) mice. Surprisingly, increased IRS2-mediated signals did not change spatial working memory in the T-maze task. Since FoxO1 is a key mediator of IRS2-transmitted signals, we additionally generated mice expressing a dominant negative mutant of FoxO1 (FoxO1DN) specifically in neurons. This mutant mimics the effect of increased IRS2 signaling on FoxO-mediated transcription. Interestingly, the phenotype observed in nIRS2 (tg) mice was not present in FoxO1DN mice. Therefore, increased neuronal IRS2 signaling causes decreased locomotoric activity in the presence of unaltered exploratory behavior and motor coordination that might lead to increased fat mass, insulin resistance, and glucose intolerance during aging independent of FoxO1-mediated transcription.

Complete failure of insulin-transmitted signaling, but not obesity-induced insulin resistance, impairs respiratory chain function in muscle.

The role of mitochondrial dysfunction in the development of insulin resistance and type 2 diabetes remains controversial. In order to specifically define the relationship between insulin receptor (InsR) signaling, insulin resistance, hyperglycemia, hyperlipidemia and mitochondrial function, we analyzed mitochondrial performance of insulin-sensitive, slow-oxidative muscle in four different mouse models. In obese but normoglycemic ob/ob mice as well as in obese but diabetic mice under high-fat diet, mitochondrial performance remained unchanged even though intramyocellular diacylglycerols (DAGs), triacylglycerols (TAGs), and ceramides accumulated. In contrast, in muscle-specific InsR knockout (MIRKO) and streptozotocin (STZ)-treated hypoinsulinemic, hyperglycemic mice, levels of mitochondrial respiratory chain complexes and mitochondrial function were markedly reduced. In STZ, but not in MIRKO mice, this was caused by reduced transcription of mitochondrial genes mediated via decreased PGC-1α expression. We conclude that mitochondrial dysfunction is not causally involved in the pathogenesis of obesity-associated insulin resistance under normoglycemic conditions. However, obesity-associated type 2 diabetes and accumulation of DAGs or TAGs is not associated with impaired mitochondrial function. In contrast, chronic hypoinsulinemia and hyperglycemia as seen in STZ-treated mice as well as InsR deficiency in muscle of MIRKO mice lead to mitochondrial dysfunction. We postulate that decreased mitochondrial mass and/or performance in skeletal muscle of non-diabetic, obese or type 2 diabetic, obese patients observed in clinical studies must be explained by genetic predisposition, physical inactivity, or other still unknown factors.

Central FoxO3a and FoxO6 expression is down-regulated in obesity induced diabetes but not in aging.

BACKGROUND: Recent data suggest that insulin-like growth factor (IGF)-1 resistance in neurons prolongs longevity. In C. elegans this effect is mediated via DAF-16 the ortholog of the mammalian FoxO transcription factors. 3 different FoxO transcription factors (FoxOs) are expressed in rodent CNS: FoxO1, FoxO3a and FoxO6. METHODS: To define whether the different FoxOs are region-, sex- and age-specifically expressed, we analyzed FoxO mRNA levels in different brain regions from 6, 16, 60 and 100 weeks old mice using realtime-PCR. In addition, we fed mice a high fat diet (HFD) to experimentally induce obesity and diabetes and analyzed FoxO mRNA in the different brain regions. RESULTS: Interestingly, FoxO1 was predominantly expressed in the hippocampus whereas FoxO3a was quantitatively the most abundant FoxO in the neocortex. During aging, FoxO1 expression peaked in all brain regions at 16 weeks and FoxO6 showed its highest expression at 60 weeks in the parietal and occipital cortex. In 6 weeks old mice FoxO6 expression was higher in male compared to female mice in the hippocampus and all cortical regions. Surprisingly, in HFD animals FoxO3a was significantly less expressed in the cerebellum and all cortical regions compared to control animals. Even more dramatic, FoxO6 expression dropped about 80% in all brain regions in response to HFD. CONCLUSION: Thus, FoxOs in the CNS showed a highly distinct expression, which in addition was age- and sex-dependent. In contrast to FoxO1, FoxO3a and FoxO6 were specifically diminished in the CNS of HFD animals possibly contributing to the reduced lifespan observed in these animals.