The toxic conformer of amyloid β-protein (Aβ) ending at 42 (Aβ42), which contains a unique turn conformation at amino acid residue positions 22 and 23 and tends to form oligomers that are neurotoxic, was reported to play a critical role in the pathomechanisms of Alzheimer's disease (AD), in which diabetes mellitus (DM)-like mechanisms are also suggested to be operative. It remains to be established whether the attenuation of insulin signaling is involved in an increase of toxic Aβ42 conformer levels. The present study investigated the association between impaired insulin metabolism and formation of toxic Aβ42 conformers in the brains of an AD mouse model. In particular, we studied whether insulin deficiency or resistance affected the formation of toxic Aβ42 conformers in vivo. We induced insulin deficiency and resistance in 3xTg-AD mice, a mouse AD model harboring two familial AD-mutant APP (KM670/671NL) and PS1 (M146 V) genes and a mutant TAU (P301L) gene, by streptozotocin (STZ) injection and a high fructose diet (HFuD), respectively. Cognitive impairment was significantly worsened by STZ injection but not by HFuD. Dot blot analysis revealed significant increases in total Aβ42 levels and the ratio of toxic Aβ42 conformer/total Aβ42 in STZ-treated mice compared with control and HFuD-fed mice. Immunostaining showed the accumulation of toxic Aβ42 conformers and hyper-phosphorylated tau protein (p-tau), which was more prominent in the cortical and hippocampal neurons of STZ-treated mice compared with HFuD-fed and control mice. HFuD-fed mice showed only a mild-to-moderate increase of these proteins compared with controls. Toxic Aβ42 conformers were co-localized with p-tau oligomers (Pearson's correlation coefficient = 0.62) in the hippocampus, indicating their co-aggregation. Toxic Aβ42 conformer levels were inversely correlated with pancreatic insulin secretion capacity as shown by fasting immunoreactive insulin levels in STZ-treated mice (correlation coefficient = -0.5879, p = .04441), but not HFuD-fed mice, suggesting a decrease in serum insulin levels correlates with toxic Aβ42 conformer formation. Levels of p-Akt and phosphorylated glycogen synthase kinase-3β measured by a homogeneous time-resolved fluorescence assay were significantly lower in STZ-treated mice than in HFuD-fed mice, suggesting a greater inhibition of brain insulin signaling by STZ than HFuD, although both levels were significantly decreased in these groups compared with controls. Iba1-positive and NOS2-positive areas in the cortex and hippocampus were significantly increased in STZ-treated mice and to a lesser extent in HFuD-fed mice compared with controls. These findings suggest that insulin deficiency rather than insulin resistance and the resultant impairment of brain insulin signaling facilitates the formation of toxic Aβ42 conformer and its co-aggregation with p-tau oligomers, and that insulin deficiency is an important pathogenic factor in the progression of AD.

中文翻译：

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在阿尔茨海默氏病模型中，胰岛素缺乏促进与超磷酸化tau寡聚物共聚集的有毒淀粉样β42构象异构体的形成。

据报道，以42（Aβ42）结尾的淀粉样β蛋白（Aβ）的毒性构象异构体在第22和23位氨基酸残基处具有独特的转向构象，并倾向于形成具有神经毒性的寡聚体，在该过程中起着至关重要的作用。阿尔茨海默氏病（AD）的病理机制，其中类似糖尿病（DM）的机制也被认为是有效的。胰岛素信号的减弱是否与毒性Aβ42构象子水平的增加有关尚待确定。本研究调查了AD小鼠模型的大脑中胰岛素代谢受损与毒性Aβ42构象体形成之间的关系。特别地，我们研究了胰岛素缺乏或抗药性是否影响体内毒性Aβ42构象异构体的形成。我们在3xTg-AD小鼠中诱导了胰岛素缺乏和抵抗力，分别通过链脲佐菌素（STZ）和高果糖饮食（HFuD）注射的小鼠AD模型，其具有两个家族AD突变APP（KM670 / 671NL）和PS1（M146 V）基因以及一个突变TAU（P301L）基因。注射STZ可使认知障碍显着恶化，而HFuD则不会。点印迹分析显示，与对照组和HFuD喂养的小鼠相比，STZ治疗的小鼠的总Aβ42水平和有毒Aβ42构象异构体/总Aβ42比率显着增加。免疫染色显示有毒的Aβ42构象异构体和高磷酸化的tau蛋白（p-tau）积累，与HFuD喂养和对照小鼠相比，在STZ处理的小鼠的皮层和海马神经元中更为突出。与对照组相比，用HFuD喂养的小鼠仅显示这些蛋白质的轻度至中度增加。有毒的Aβ42构象体与p-tau低聚物（皮尔森相关系数= 0.62）共定位在海马体中，表明它们是共聚集的。有毒的Aβ42构象水平与胰腺胰岛素分泌能力呈负相关，如STZ处理的小鼠（相关系数= -0.5879，p = .04441）而非HFuD喂养的小鼠禁食的免疫反应性胰岛素水平所示，表明血清胰岛素减少水平与毒性Aβ42构象异构体形成有关。通过均相时间分辨荧光测定法测量的ST-Z小鼠的p-Akt和磷酸化糖原合酶激酶3β的水平显着低于HFuD喂养的小鼠，这表明STZ比HFuD对STZ对脑胰岛素信号的抑制作用更大。与对照组相比，这些组的两个水平均显着降低。与对照组相比，在STZ处理的小鼠中，皮质和海马中的Iba1阳性和NOS2阳性区域显着增加，而在HFuD喂养的小鼠中，Iba1阳性和NOS2阳性区域显着增加。这些发现表明，胰岛素缺乏而不是胰岛素抵抗以及由此导致的脑胰岛素信号传导障碍促进了毒性Aβ42构象异构体的形成及其与p-tau低聚物的共聚集，并且胰岛素缺乏是AD进展的重要致病因素。 。