Analysis of functional roles of Ca^<2+>-dependent protease calpain in neuronal cell death

阿尔茨海默病（AD）的病因涉及Aβ在脑中的积聚。迄今为止，大多数治疗策略都集中在发病机制上游的Aβ代谢。然而，研究位于发病机制下游的执行器也很重要，该执行器诱导神经元功能障碍或细胞死亡。我们专注于钙蛋白酶-钙蛋白酶抑制蛋白系统在大脑中，并调查钙蛋白酶过度激活AD发病机制的相关性。钙蛋白酶是一种胞质半胱氨酸蛋白酶，它被细胞内Ca 2+内流激活，其蛋白酶活性受内源性抑制蛋白钙蛋白酶抑制蛋白（calpastatin）调节。然而，目前尚不清楚内源性蛋白质的钙蛋白酶依赖性降解是否是神经元功能障碍或细胞死亡的原因，或者可能是哪些蛋白质。因此，我们试图通过蛋白质组学的方法，使用野生型和calpastain缺陷的小鼠原代神经元，这是代谢标记，然后用谷氨酸刺激，以确定底物蛋白的钙蛋白酶。钙蛋白酶的候选蛋白底物通过比较野生型和钙蛋白酶抑制剂缺陷神经元之间的相应蛋白质的降解速率的半衰期来确定，基于2D电泳凝胶上的斑点的放射性。因此，我们已经确定了几个重要的蛋白质作为候选底物的钙蛋白酶，这可能是负责神经元功能障碍或细胞死亡。我们还将钙蛋白酶抑制蛋白缺陷小鼠与AD模型小鼠杂交（即，淀粉样前体蛋白转基因小鼠），发现神经元钙蛋白酶不仅被脑中Aβ积累激活，而且还介导体内Aβ诱导的神经炎性解体和tau蛋白磷酸化。这些结果表明，钙蛋白酶抑制剂可能是一个有前途的新的药物治疗AD。我们目前正在探索的病理学相关性的降解新发现的底物蛋白与钙蛋白酶激活AD模型小鼠。

The etiology of Alzheimer's disease (AD) involves Aβ accumulation in the brain. Most therapeutic strategies to date have focused on Aβ metabolism upstream of pathogenesis. However, it is also important to investigate an executor that is located downstream of the pathogenesis, which induces neuronal dysfunction or cell death. We focused on a calpain-calpastatin system in the brain and investigated the relevance of calpain hyperactivation to AD pathogenesis. Calpain is a cytoplasmic cysteine protease, which is activated by intracellular Ca2+ influx, and its protease activity is regulated by the endogenous inhibitory protein calpastatin. However, it is not yet known whether calpain-dependent degradation of endogenous proteins is responsible for neuronal dysfunction or cell death, or which proteins those might be. We therefore attempted to identify substrate proteins of calpain by a proteomics approach using wild-type and calpastain-deficient mouse primary neurons, which were metabolically labeled and then stimulated with glutamate. Candidate protein substrates of calpain were determined by comparing half-lives of the degradation rate of the corresponding proteins between wild-type and calpastatin-deficient neurons based on the radioactivity of spots on 2D-electrophoresis gels. We have thus identified several important proteins as candidate substrates of calpain, which could be responsible for neuronal dysfunction or cell death. We also crossbred calpastatin-deficient mice with AD-model mice (i.e., amyloid precursor protein-transgenic mice) and found that neuronal calpain is not only activated by Aβ accumulation in the brains, but it also mediates Aβ-induced neuritic disorganization and tau phosphorylation in vivo. These results suggest that a calpain inhibitor may be a promising new medication for AD. We are currently exploring the pathological relevance of degradation of the newly identified substrate proteins with calpain activation using AD model mice.