| Alzheimer’s disease (AD) is the most common form of dementia that is characterized by progressive cognitive and functional impairment. Though huge efforts are being made by scientists around the world, there is still no efficient strategy that can cure or halt the progression of this devasting disease.Defective brain insulin signaling has been observed in AD brain, and is believed to be one of the etiologies for sporadic AD. Studies have shown that patients with AD have defective insulin receptor (IR) expression and IR binding. Furthermore, amyloid beta (AP) oligomers can conversely disrupt insulin signaling through reducing surface expression of IRs in neurons or competing with insulin. Several small-scale clinical trials have shown that intranasal insulin improved memory and attention in healthy participants, as well as in patients with mild cognitive impairment (MCI) and AD. However, the underlying molecular mechanism by which insulin exerts its effects on AD brain and improves cognition is poorly understood.We recently found that intranasal administration of insulin for 7 days can restore insulin signaling, increase synaptic proteins, and reduces Aβ level and microglia activation in the brains of 9-month-old 3xTg-AD mice. In order to further explore the function of intranasal insulin on early AD pathologies. We treated 4.5-month-old APPswe/PS1dE9 (APP/PS1) mice, a widely used transgenic mouse model of AD, with intranasal insulin (1U) for 6 weeks, and investigated the effects of intranasal insulin on AD by behavioral and biochemical methods. Some major research results are as follows:1. During the treatment of intranasal insulin, the body weights of the mice were recorded every week. There was no significant difference in body weight among wild type treated with normal saline (WT-veh), APP/PS1-veh and APP/PS1 treated with insulin (APP/PS1-ins) group.2. After consecutive treatment for 6 weeks, we conducted open field test to evaluate the anxiety level of these mice and Morris water maze (MWM) test was used to assess cognitive function. Compared with WT mice, APP/PS1 mice showed increased anxiety level. However, insulin treatment alleviated the anxious state to a level comparable to that of wild type controls. In standard MWM test, no significant group differences in mean escape latency or path length to platform were found during acquisition phase. During probe trial, we didn’t find any significant differences in the time spent in the former platform quadrant either. These results indicate that 6-month-old APP/PS1 mice are indistinguishable from WT control in spatial learning and memory assessed by MWM task. However, in the acquisition phase of reversal MWM (rMWM) task, the learning curves indicated that APP/PS1 mice took longer time and travelled longer distance than the WT controls, while intranasal insulin treatment caused an overall decrease in escape latency and path length traveled. These data indicate that these transgenic mice do show impairment in memory flexibility, and intranasal insulin treatment can alleviate this impairment.3. By using Western blot analyses, our results indicated that intranasal insulin ameliorated defective brain insulin signaling in APP/PS1 mice brain. Furthermore, c-Jun N-terminal kinase (JNK) activation, which plays a pivotal role in insulin resistance and AD pathologies, was significantly decreased by intranasal insulin.4. There were obvious amyloid βplaque (Aβ plaque) deposits in the brain of APP/PS1 mice by immunohistochemical analyses. Intranasal insulin has a potential to decrease the number of plaque deposits as well as the area occupied by Aβ plaque in these mice. Insulin were also found to decrease the level of soluble Aβoligomers as well as soluble Aβ40 and Aβ42 in the hippocampi of APP/PS1 mice using dot blot and ELISA analyses respectively.5. In order to further investigate the mechanisms underlying the inhibitory effect in Aβ level and plaque deposits of intranasal insulin, we evaluated whether insulin altered APP processing in APP/PS1mice. Intranasal insulin significantly decreased the production of β-secretase cleaved secreted sAPPβ and increased the level of α-secretase cleaved sAPPa in APP/PS1 mice. The ratio of CTFβ/CTFa was also decreased by intranasal insulin.6. As to the proteins involved in APP processing and Aβ metabolism, APP/PS1 mice had increased cerebral levels of BACE1 (the major β secretase) and apolipoprotein E (ApoE) proteins, and intranasal insulin restored the elevated level of BACE1 and APOE protein. Insulin also increased the level of ADAM 10 (the major a secretase) in the brain of APP/PS1 mice. But the levels of insulin degrading enzyme (IDE) and LRP1 were not significantly different among the three groups.7. The astrocytes and microglia were found to activate in the brain of APP/PS1 mice compared with WT controls. Intranasal insulin was not able to reverse these changes. Synapse associated proteins synaptophysin, PSD95 and synapsinl in the hippocampi were not differently expressed among the three groups.8. We did not find any elevated levels of phosphorylated tau proteins as well as tau kinases in 6-month-old APP/PS1 mice brain.9. Intranasal insulin had a potential to improve neurogenesis as indicated by increased level of doublecortin, which is a marker of neurogenesis.In summary,6-month-old APP/PS1 transgenic mice did not exhibit obvious impairments in cognitive function as assessed by MWM test, but did show impaired spatial memory plasticity. Intranasal insulin treatment was able to improve the memory plasticity of these mice. Intranasal insulin also substantially reduced Aβ production and plaque deposits. The alleviation of amyloid pathology by intranasal insulin resulted mainly from enhanced nonamyloidogenic processing and compromised amyloidogenic processing of APP, and from a reduction in APOE protein which is involved in Aβ metabolism. Besides, intranasal insulin effectively promoted hippocampal neurogenesis in the brain of APP/PS1 mice. All these effects may contribute to the behavioral improvement of these mice. |
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