Effects of high-fat diet on nutrient metabolism and cognitive functions in young APPKINL-G-F/NL-G-F mice.

AIM: Type 2 diabetes mellitus (T2DM) is an increased risk factor for Alzheimer's disease (AD); however, the relationship between the 2 conditions is controversial. High-fat diet (HFD) causes cognitive impairment with/without Aß accumulation in middle-aged or aged transgenic (Tg) and knock-in (KI) AD mouse models, except for metabolic disorders, which commonly occur in all mice types. Alternatively, whether HFD in early life has an impact on nutrient metabolism and neurological phenotypes in young AD mouse models is not known. In the present study, we examined the effects of HFD on young APPKINL-G-F/NL-G-F mice, one of the novel KI-AD mouse models. METHODS: The mice were categorized by diet into 2 experimental groups, normal diet (ND) and HFD. Four-week-old wild-type (WT) and APPKINL-G-F/NL-G-F mice were fed ND or HFD for 9 weeks. Both types of mice on ND and HFD were examined during young adulthood. RESULTS: HFD caused T2DM-related metabolic disturbances in both young WT and APPKINL-G-F/NL-G-F mice, whereas impaired thermoregulation and shortage of alternative energy sources specifically occurred in young APPKINL-G-F/NL-G-F mice. However, HFD had no impact on the cognitive function, Aß levels, and phosphorylation of hippocampal insulin receptor substrate 1 (IRS1) at all the 3 Ser sites in both types of mice. CONCLUSION: HFD is effective in causing metabolic disturbances in young WT and APPKINL-G-F/NL-G-F mice but is ineffective in inducing neurological disorders in both types of mice, suggesting that the aging effects, along with long-term HFD, facilitate neurological alterations.

Irs2 deficiency alters hippocampus-associated behaviors during young adulthood.

Type 2 diabetes mellitus (T2DM), characterized by hyperglycemia and insulin resistance, has been recognized as a risk factor for cognitive impairment and dementia, including Alzheimer's disease (AD). Insulin receptor substrate2 (IRS2) is a major component of the insulin/insulin-like growth factor-1 signaling pathway. Irs2 deletion leads to life-threatening T2DM, promoting premature death in male mice regardless of their genetic background. Here, we showed for the first time that young adult male mice lacking Irs2 on a C57BL/6J genetic background (Irs2-/-/6J) survived in different experimental environments and displayed hippocampus-associated behavioral alterations. Young adult male Irs2-/-/6J mice also exhibit aberrant alterations in energy and nutrient sensors, such as AMP-activated protein kinase (AMPK) and glucose transporter3 (GLUT3), and reduced core body temperature accompanied by abnormal change in the temperature sensor in the brain. These results suggest that Irs2 deficiency-induced impairments of brain energy metabolism and thermoregulation contribute to hippocampus-associated behavioral changes in young adult male mice.

Serine Phosphorylation of IRS1 Correlates with Aß-Unrelated Memory Deficits and Elevation in Aß Level Prior to the Onset of Memory Decline in AD.

The biological effects of insulin signaling are regulated by the phosphorylation of insulin receptor substrate 1 (IRS1) at serine (Ser) residues. In the brain, phosphorylation of IRS1 at specific Ser sites increases in patients with Alzheimer's disease (AD) and its animal models. However, whether the activation of Ser sites on neural IRS1 is related to any type of memory decline remains unclear. Here, we show the modifications of IRS1 through its phosphorylation at etiology-specific Ser sites in various animal models of memory decline, such as diabetic, aged, and amyloid precursor protein (APP) knock-in NL-G-F (APPKINL-G-F) mice. Substantial phosphorylation of IRS1 at specific Ser sites occurs in type 2 diabetes- or age-related memory deficits independently of amyloid-ß (Aß). Furthermore, we present the first evidence that, in APPKINL-G-F mice showing Aß42 elevation, the increased phosphorylation of IRS1 at multiple Ser sites occurs without memory impairment. Our findings suggest that the phosphorylation of IRS1 at specific Ser sites is a potential marker of Aß-unrelated memory deficits caused by type 2 diabetes and aging; however, in Aß-related memory decline, the modifications of IRS1 may be a marker of early detection of Aß42 elevation prior to the onset of memory decline in AD.