A caspase-8 substrate in T cell activation

描述（由申请人提供）：Caspase-8最初被确定为连接死亡受体与凋亡过程的一系列蛋白酶中的第一个。现在从几项研究中可以清楚地看到，caspase-8对于包括T淋巴细胞在内的几种细胞类型的增殖起始也是必需的。然而，这一过程中的caspase-8底物多年来一直难以捉摸。我们现在提出caspase-8同源物c-FLIPL既是caspase-8的激活剂，也是T细胞激活后caspase-8的初始底物。我们的初步研究结果支持一种新的模型，即T细胞受体（TCR）连接启动caspase-8与c-FLIPL的异源二聚化，通过c-FLIPL c端已知的激活环激活caspase-8。c-FLIPL随后在Asp376处被caspase-8快速切割，得到缺乏caspase-8激活环的p43FLIP。然后，p43FLIP能够招募促进NF-kB活化的适配蛋白RIP1和TRAF2 （c-FLIPL不能招募）。因此，我们假设通过转染和转基因表达p43FLIP将消除在T细胞NF-kB激活和增殖中对caspase-8活性的需要，并且还将限制可能导致T细胞过早死亡的持续caspase-8激活。不可切割的D376A-FLIPL突变体则相反，即不能招募RIP1和TRAF2，并会促进过度的caspase-8激活和T细胞过早死亡。该模型将在Aim 1的体外信号研究和Aim 2的体内研究中进行测试，以检查对效应T细胞和记忆T细胞产生的影响。公共卫生相关性：T淋巴细胞活化和细胞死亡的协调是感染、自身免疫性疾病和淋巴瘤发生期间免疫反应的基础。涉及半胱天冬酶的信号通路以前被认为只与细胞死亡有关。现在很清楚，caspase-8对T细胞活化也是至关重要的，但是caspase-8在这个功能中的底物几年来仍然难以捉摸。我们提出c-FLIPL既是caspase的激活剂，也是T细胞活化中关键的caspase-8底物。

DESCRIPTION (provided by applicant): Caspase-8 was originally identified as the first in a series of proteases that link death receptors to the apoptotic process. It is now clear from several studies that caspase-8 is also required for the initiation of proliferation for several cell types, including T lymphocytes. However, the caspase-8 substrate in this process has remained elusive for years. We now propose that the caspase-8 homologue, c-FLIPL, is both the activator of caspase-8 and its initial substrate following T cell activation. Our preliminary findings support a novel model in which T cell receptor (TCR) ligation initiates heterodimerization of caspase-8 with c-FLIPL, which activates caspase-8 through a known activation loop in the C-terminus of c-FLIPL. c-FLIPL is then rapidly cleaved by caspase-8 at Asp376, yielding p43FLIP, which lacks the activation loop for caspase-8. p43FLIP is then able to recruit the adaptor proteins RIP1 and TRAF2 (that c-FLIPL cannot recruit) that promote activation of NF-kB. We thus hypothesize that expression of p43FLIP by transfection and transgenesis will obviate the need for caspase-8 activity in T cell activation of NF-kB and proliferation, and will also limit continual caspase-8 activation that might lead to premature T cell death. A non-cleavable D376A-FLIPL mutant is expected to do the opposite, namely be unable to recruit RIP1 and TRAF2, and will promote excessive caspase-8 activation and premature T cell death. This model will be test by both in vitro signaling studies in Aim 1 and in vivo in Aim 2 to examine the effects on the generation of effector and memory T cells. Public Health Relevance: The coordination of T lymphocyte activation and cell death is fundamental to the immune responses during infection, autoimmune diseases, and lymphomagenesis. A signal pathway involving caspases was previously felt to be involved only with cell death. Now it is clear that caspase-8 is also critical for T cell activation, but the caspase-8 substrate in this function has remained elusive for several years. We propose c-FLIPL as both the activator of caspase and the critical caspase-8 substrate in T cell activation.