Background and Objective: Type 2 Diabetes (T2D) patients are more prone to develop Alzheimer’s Disease (AD). We have previously shown that Glucagon-like peptide-1 receptor agonist exendin-4 (Ex-4) reduces tau hyperphosphorylation in T2D animals through upregulating insulin signaling, and peripheral injected Ex-4 increases insulin levels in the T2D brain. This study aims to further clarify whether the elevated insulin in the brain is produced by nerve cells under the action of Ex-4.

Methods: The neuronal cell line-HT22 was treated with Ex-4 under high glucose or normal cultivation, and the number of insulin-positive cells as well as the expression levels of insulin synthesis-related genes were examined. The db/db mice were treated with the peripheral injection of Ex-4 and/or IntraCerebroVentricular (ICV) injection of siRNA to inhibit the expression of insulin synthesis- related genes and the behavior tests were carried on. Finally, plasma glucose, Cerebrospinal Fluid (CSF) glucose, CSF insulin, phosphorylation of tau, phosphorylation of AKT and GSK-3β of db/db mice were detected.

Results: We found that Ex-4 promoted the expression of insulin synthesis-related genes and induced an obvious increase of insulin-positive HT-22 neuronal cells in a high glucose environment. Peripheral injection of Ex-4 improved the cognitive function of db/db mice and increased brain insulin levels which activated brain insulin signaling and subsequently alleviated tau hyperphosphorylation. However, when siRNA-neurod1 was injected to block insulin synthesis, the cognitive function of db/db mice was not improved under the action of Ex-4 anymore. Moreover, the brain insulin levels dropped to an extremely low level, and the phosphorylation level of tau increased significantly.

Conclusion: This study demonstrated that Ex-4 improved cognition function by promoting brain insulin synthesis followed by the activation of brain insulin signaling and alleviation of tau hyperphosphorylation.

背景和目的：2 型糖尿病 (T2D) 患者更容易患上阿尔茨海默病 (AD)。我们之前已经表明，胰高血糖素样肽-1 受体激动剂 exendin-4 (Ex-4) 通过上调胰岛素信号传导降低 T2D 动物的 tau 过度磷酸化，外周注射的 Ex-4 增加 T2D 大脑中的胰岛素水平。本研究旨在进一步阐明大脑中升高的胰岛素是否是由神经细胞在Ex-4的作用下产生的。

方法：在高糖或正常培养条件下，用Ex-4处理神经元细胞株-HT22，检测胰岛素阳性细胞数及胰岛素合成相关基因的表达水平。db/db小鼠外周注射Ex-4和/或脑室内(ICV)注射siRNA以抑制胰岛素合成相关基因的表达并进行行为测试。最后，检测db/db小鼠的血浆葡萄糖、脑脊液(CSF)葡萄糖、CSF胰岛素、tau磷酸化、AKT和GSK-3β的磷酸化。

结果：我们发现Ex-4促进了胰岛素合成相关基因的表达，并在高糖环境中诱导了胰岛素阳性HT-22神经元细胞的明显增加。外周注射 Ex-4 可改善 db/db 小鼠的认知功能并增加脑胰岛素水平，从而激活脑胰岛素信号传导并随后缓解 tau 过度磷酸化。然而，当注射siRNA-neurod1阻断胰岛素合成时，db/db小鼠的认知功能在Ex-4的作用下并没有得到改善。此外，脑内胰岛素水平降至极低水平，tau 磷酸化水平显着升高。

结论：本研究表明，Ex-4 通过促进脑胰岛素合成、随后激活脑胰岛素信号传导和减轻 tau 过度磷酸化来改善认知功能。