Uncovering the molecular machinery driving caspase-2 activation

细胞凋亡是一种程序性细胞死亡的形式，其特征在于独特的形态学和生物化学特征，包括被称为半胱天冬酶的蛋白酶的激活，其被分类为起始半胱天冬酶和效应半胱天冬酶。 引发剂半胱天冬酶被募集到多聚体蛋白复合物中用于其二聚化和活化。半胱天冬酶-2是半胱天冬酶家族的独特成员，因为它表现出启动子和效应子半胱天冬酶的特征。 尽管是第一个克隆的哺乳动物胱天蛋白酶，胱天蛋白酶-2的作用仍然是谜，激活胱天蛋白酶-2作为起始剂的独特的凋亡刺激仍然是未知的。我们已经发现，caspase-2被激活作为一个启动子caspase在孔形成毒素（PFT）介导的细胞凋亡。PFT（例如α-溶血素和气溶素）是由病原菌分泌的主要细菌毒力因子，并且它们主要通过在宿主细胞膜上形成孔而引起细胞毒性作用。我们已经发现，半胱天冬酶-2是介导α-溶血素和气溶素介导的人上皮细胞凋亡所必需的。半胱天冬酶-2以PIDDosome独立的方式激活，并且通过采用高分辨率凝胶过滤色谱，我们已经确定半胱天冬酶-2在PFT处理的细胞中被募集到一种新的高分子量复合物（Castosome）。半胱天冬酶-2的招募，这种高分子量的复合物和它的激活依赖于PFT诱导的钾外流，从而揭示了钾离子在调节半胱天冬酶-2激活的关键作用。我们的目标是破译的组成部分，castosome复合物，并进一步阐明caspase-2激活背后的机制。这些结果将使我们能够设计合理的治疗药物来对抗几种细菌感染和癌症。

Apoptosis is a form of programmed cell death characterized by unique morphological and biochemical features including activation of proteases called caspases, which are classified into initiator and effector caspases. Initiator caspases are recruited to multimeric protein complexes for their dimerization and activation. Caspase-2 is a unique member of the caspase family as it exhibits features of both initiator and effector caspases. Despite being the first cloned mammalian caspase the role of caspase-2 remained enigmatic and the unique apoptotic stimuli that activate caspase-2 as an initiator caspase remained unknown. We have uncovered that caspase-2 is activated as an initiator caspase during Pore forming toxins (PFTs)-mediated apoptosis. PFTs (for instance alpha-hemolysin and aerolysin) are major bacterial virulence factors secreted by pathogenic bacteria and they elicit cytotoxic effects primarily by forming pores on the host cell membranes. We have sown that caspase-2 is required for mediating both alpha-hemolysin and aerolysin-mediated apoptosis in human epithelial cells. Caspase-2 is activated in a PIDDosome independent manner and by employing high-resolution gel filtration chromatography, we have identified that caspase-2 is recruited to a novel high molecular weight complex (Castosome) in PFT-treated cells. Recruitment of caspase-2 to this high molecular weight complex and its activation are dependent on PFT-induced potassium efflux thus revealing a crucial role for potassium ions in regulating caspase-2 activation. We aim to decipher the components of castosome complex and further elucidate mechanisms behind caspase-2 activation. These results will enable us to design rational therapeutic drugs to combat several bacterial infections and cancer.