Elucidation of abnormality of plasma membrane microdomain in type 2 diabetes mellitus

富含神经节苷脂的膜微区被认为是胰岛素信号正确区隔的关键。质膜相关唾液酸酶NEU3是神经节苷脂水解酶的关键酶。我们之前曾报道，过度表达NEU3的小鼠患上了严重的胰岛素抵抗糖尿病。为了研究NEU3在体内对胰岛素敏感性和糖耐量的可能贡献，用腺病毒载体在标准和高脂饲料喂养的C57BL/6小鼠和标准饲料喂养的胰岛素抵抗KKAy小鼠的肝脏中表达了NEU3。肝脏NEU3过表达显著改善了饲喂标准饲料的C57BL/6小鼠的糖耐量和胰岛素敏感性，并显著改善了饲喂高脂饲料的C57BL/6小鼠和KKAy小鼠的葡萄糖耐量。在C57BL/6小鼠和KKAy小鼠中，NEU3过表达增加了肝糖原沉积和甘油三酯蓄积，并增加了肝脏PPARg和胎球蛋白的表达。TLC分析显示，肝脏NEU3过表达的小鼠(NEU3小鼠)肝脏中GM1水平增加，GM3水平显著降低。NEU3大鼠肝脏胰岛素受体底物1(IRS1)的基础酪氨酸磷酸化和胰岛素刺激下的酪氨酸磷酸化显著增加，但胰岛素受体和IRS2的酪氨酸磷酸化没有变化。在NEU3小鼠的脂肪组织中，胰岛素刺激的胰岛素受体酪氨酸磷酸化增加。这些结果表明，肝脏NEU3过表达通过改变神经节苷脂组成和PPARγ信号通路来改善胰岛素敏感性和糖耐量。我们的发现还提供了进一步的证据，证明NEU3是胰岛素敏感性和葡萄糖耐量的重要调节因子。

Membrane microdomains rich in gangliosides are recognized as being critical for proper compartmentalization of insulin signaling. Plasma membrane-associated sialidase, NEU3, is a key enzyme for ganglioside hydrolysis. We previously reported that the mice overexpressing NEU3 developed severe insulin-resistant diabetes. To examine possible the contributions of NEU3 to in vivo insulin sensitivity and glucose tolerance, NEU3 was expressed using adenoviral vectors in the livers of C57BL/6 mice on standard and high-fat diets, and insulin-resistant KKAy mice on standard diets. Hepatic NEU3 overexpression significantly improved glucose tolerance and insulin sensitivity in the C57BL/6 mice fed standard diets, and glucose tolerance in the C57BL/6 mice fed high-fat diets and in KKAy mice. Hepatic NEU3 overexpression increased hepatic glycogen deposition and triglyceride accumulation, and enhanced the hepatic PPARg and fetuin expression in the C57BL/6 mice on standard and high-fat diets, and in KKAy mice. TLC analysis demonstrated increased levels of GM1 and markedly reduced GM3 in the livers of mice with hepatic NEU3 overexpression (NEU3 mice). Basal and insulin-stimulated tyrosine phosphorylations of insulin receptor substrate 1 (IRS1) were significantly increased, but tyrosine phosphorylations of the insulin receptor and IRS2 in the NEU3 liver were unchanged. Insulin-stimulated tyrosine phosphorylations of the insulin receptor were increased in adipose tissues of NEU3 mice. These results suggest that hepatic NEU3 overexpression improves insulin sensitivity and glucose tolerance through modification of ganglioside composition and PPARγ signaling. Our findings also provide further evidence that NEU3 is an important regulator of insulin sensitivity and glucose tolerance.