程序性细胞坏死(Necroptosis)是一种新的Programmed Cell Death，是一个国际前沿研究领域. MLKL是程序性细胞坏死的执行者(executioner). 死亡受体TNFR1通过RIPK3磷酸化修饰MLKL，引起MLKL多聚破坏细胞膜，导致程序性细胞坏死。但除了RIPK3的磷酸化，其它调控MLKL的机制还未知。本申请项目发现了调控MLKL功能的另一重要机制：caspase调控MLKL. Caspase-3在DQQD140↓A位点切割使MLKL降解，抑制程序性细胞坏死。决定TNFR1诱导的细胞死亡如何在Apoptosis和Necroptosis之间选择的机制还不清楚，本项目工作为此提供了重要线索。未来将进一步研究Caspase切割如何影响MLKL的多聚和细胞膜坏死能力，表达Caspase位点突变的MLKL，深入了解Caspase调控MLKL程序性细胞坏死功能的机制。

Necroptosis is a newly discovered form of programmed cell death (PCD). MLKL has a crucial role in TNFR1-activated necroptosis. Binding of TNF to TNFR1 induces RIPK3-mediated phosphorylation of MLKL at Thr357 and Ser358. These phosphorylations induce a conformational change in MLKL molecule and activate MLKL, leading to MLKL oligomerization and plasma membrane translocation. MLKL then disrupts cell membrane integrity and executes necrotic cell death. However, mechanisms other than RIPK3-mediated phosphorylation that control MLKL activity are currently unknown. In this proposal, we found an important mechanism that regulates MLKL function. We found MLKL is a caspase substrate. Caspase-3 cleaves MLKL at DQQD140↓A and inactivates the necrotic function of MLKL. This might be one of the important mechanisms determining the decision whether a cell will die by apoptosis or necroptosis after TNF stimulation. In future study, we will investigate how caspase-mediated cleavage affects the oligomerization and the necrotic function of MLKL. We will transfect cells with caspase-resistant MLKL mutant plasmid to see how cell fate decisions downstream of TNFR1 are regulated.