Cystatin C Induces Insulin Resistance in Hippocampal Neurons and Promotes Cognitive Dysfunction in Rodents
Cystatin C Induces Insulin Resistance in Hippocampal Neurons and Promotes Cognitive Dysfunction in Rodents
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DOI：
10.1007/s12264-018-0226-6
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发表时间：
2018-04
影响因子：
5.6
通讯作者：
Lan Luo;Jinyu Ma;Yue Li;Zongkang Hu;Chengfeng Jiang;H. Cai;Cheng Sun
中科院分区：
文献类型：
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作者：
Lan Luo;Jinyu Ma;Yue Li;Zongkang Hu;Chengfeng Jiang;H. Cai;Cheng Sun
认知障碍是阿尔茨海默病 (AD) 和帕金森病等神经退行性疾病的标志。越来越多的证据表明认知障碍与胰岛素抵抗密切相关。例如，糖尿病患者患AD的风险增加[1]。此外，认知障碍者表现出高水平的空腹血糖和空腹胰岛素、体内平衡模型评估升高、胰岛素对葡萄糖的反应受损以及糖尿病发生频率增加。与此相一致，鼻内胰岛素给药极大地改善了遗忘性轻度认知障碍或AD患者的延迟记忆和护理者评定的功能能力[2]。胱抑素 C (CysC) 是一种普遍表达的蛋白质，也是半胱氨酸蛋白酶的抑制剂。现已证明血浆CysC是中心性肥胖和胰岛素抵抗的阳性指标[3]。最近，一项临床调查表明，较高滴度的血浆 CysC 会损害 1 型糖尿病患者的胰岛素敏感性 [4]。这一证据表明 CysC 是胰岛素抵抗的诱导剂。事实上，我们之前表明 CysC 通过诱导内质网 (ER) 应激损害肝细胞中的胰岛素信号通路 [5]。鉴于 CysC 是胰岛素抵抗的一个致病因素，因此可以合理地预测 CysC 水平升高会通过促进胰岛素抵抗而导致认知障碍。为了研究 CysC 是否影响胰岛素的作用，我们用不同剂量的重组 CysC 处理大鼠原代海马神经元。如图 S1A 所示，这些神经元中的胰岛素刺激使磷酸化 Akt (p-Akt) 和 GSK3b (p-GSK3b) 显着增加。然而，CysC 以剂量依赖性方式降低了这种增加（图 S1A、B）。为了验证这些发现，我们还测量了初级海马神经元中胰岛素刺激的葡萄糖摄取。正如预期的那样，我们发现胰岛素的应用极大地刺激了葡萄糖的摄取，而这种刺激在 CysC 存在的情况下很大程度上被削弱了（图 S1C）。这些数据表明，原代海马神经元中CysC阻断胰岛素信号转导。接下来，我们探讨了CysC介导的原代海马神经元胰岛素信号转导阻断的机制。研究表明，CysC 通过诱导 ER 应激来减弱肝细胞中的胰岛素信号转导 [5]。因此，我们测量了内质网应激相关基因Grp78、Chop和Erdj4，结果显示CysC对其表达没有影响（数据未显示）。细胞因子信号传导抑制因子 1/3 (SOCS1/3) 是胰岛素信号转导的负调节因子 [6]。因此，我们测量了SOCS1/3的表达，发现SOCS1的mRNA水平被CysC显着上调，而SOCS3没有变化（图S2A）。 CysC 也增强了 SOCS1 的蛋白水平（图 S2B、C）。
Cognitive impairment is a hallmark of neurodegenerative disorders such as Alzheimer’s disease (AD) and Parkinson’s disease. Growing evidence has demonstrated that cognitive impairment is closely associated with insulin resistance. For instance, the risk of AD is increased in diabetic patients [1]. Moreover, cognitively impaired persons exhibit high levels of fasting glucose and fasting insulin, elevated Homeostasis Model Assessment, an impaired insulin response to glucose, and increased frequency of diabetes. In accord with this, intranasal insulin administration greatly improves the delayed memory and caregiver-rated functional ability of patients with amnestic mild cognitive impairment or AD [2]. Cystatin C (CysC) is a ubiquitously-expressed protein and an inhibitor of cysteine protease. It has been demonstrated that plasma CysC is a positive indicator of central adiposity and insulin resistance [3]. Recently, a clinical survey indicated that a higher titer of plasma CysC impairs insulin sensitivity in patients with type 1 diabetes [4]. This evidence implies that CysC serves as an inducer of insulin resistance. Indeed, we previously showed that CysC impairs the insulin signal pathway in hepatocytes by inducing endoplasmic reticulum (ER) stress [5]. Given that CysC is a causative factor for insulin resistance, it is reasonable to predict that elevated CysC levels lead to cognitive impairment by promoting insulin resistance. To investigate whether CysC affects the actions of insulin, we treated rat primary hippocampal neurons with recombinant CysC at different dosages. As shown in Fig. S1A, phosphorylated Akt (p-Akt) and GSK3b (p-GSK3b) were dramatically increased by insulin stimulation in these neurons. However, the increases were decreased by CysC in a dose-dependent manner (Fig. S1A, B). To verify these findings, we also measured insulin-stimulated glucose uptake in primary hippocampal neurons. As expected, we found that the application of insulin greatly stimulated glucose uptake and this stimulation was largely blunted in the presence of CysC (Fig. S1C). These data showed that insulin signal transduction is blocked by CysC in primary hippocampal neurons.Next, we explored the mechanism underlying the CysC-mediated blockade of insulin signal transduction in primary hippocampal neurons. It has been shown that CysC attenuates insulin signal transduction in hepatocytes by inducing ER stress [5]. Therefore, we measured the ER stress-related genes Grp78, Chop, and Erdj4 and the results showed that CysC had no effect on their expression (data not shown). Suppressor of cytokine signaling 1/3 (SOCS1/3) are negative regulators of insulin signal transduction [6]. Thus, we measured SOCS1/3 expression and found that the mRNA level of SOCS1 was significantly up-regulated by CysC, while SOCS3 did not change (Fig. S2A). The protein level of SOCS1 was also enhanced by CysC (Fig. S2B, C).