Role of Calpain-mediated Cleavage of Calcineurin in Excitotoxic Neurodegeneration

钙调磷酸酶（calcalineurin）和钙蛋白酶（calpain）分别是钙调蛋白依赖蛋白磷酸酶和钙调蛋白依赖半胱氨酸蛋白酶，它们通过独立的级联介导神经元细胞死亡。在本研究中，我们发现在神经兴奋毒性过程中，钙调神经磷酸酶A （calcalpain, CnA）在体内和体外均被钙蛋白酶直接裂解，导致该酶转化为活性形式。质谱分析鉴定了三个裂解位点，其中两个是组成活性形式。裂解的CnA过表达可诱导半胱天冬酶活性和神经元细胞死亡。钙蛋白酶抑制剂和膜渗透性钙pastatin肽不仅阻断了CnA的裂解，而且在体外和体内均对兴奋性毒性神经元细胞死亡具有保护作用。这些结果表明，CnA是钙蛋白酶的一个重要靶点，而钙蛋白酶介导的CnA激活会引发兴奋性神经变性。本研究建立了calpain和calcalineurin之间的分子联系，从而揭示了calpain在某些病理状态下对calcalineurin进行蛋白水解调控的新机制。

Calcineurin and calpain, a Ca2+/calmodulin-dependent protein phosphatase and a Ca2+-dependent cysteine protease, respectively, mediate neuronal cell death through independent cascades. In this research, we showed that during neuroexcitotoxicity, calcineurin A (CnA) is directly cleaved by calpain in vitro and in vivo, resulting in the enzyme being converted to an active form. Mass spectrometry identified three cleavage sites in CnA, two of which were constitutively active forms. Overexpression of the cleaved CnA induced caspase activity and neuronal cell death. Calpain inhibitors and membrane-permeable calpastatin peptides not only blocked the cleavage of CnA, but also protected against excitotoxic neuronal cell death in vitro and in vivo. These results indicate that CnA is a crucial target for calpain, and the calpain-mediated activation of CnA triggers excitotoxic neurodegeneration. This study established a molecular link between calpain and calcineurin, thereby demonstrating a new mechanism for proteolytical regulation of calcineurin by calpain in response to certain pathological states.