RIPK1-mediated regulated necrosis

Receptor-interacting serine/threonine-kinase protein 1 (RIPK1) and RIPK3-dependent necrosis is called necroptosis or programmed necrosis. The kinase activities of RIPK1 and RIPK3 are essential for the necroptotic cell death in human, mouse cell lines and genetic mice models (Cho YS et al. 2009; He S et al. 2009, 2011; Zhang DW et al. 2009; McQuade T et al. 2013; Newton et al. 2014). The initiation of necroptosis can be stimulated by the same death ligands that activate extrinsic apoptotic signaling pathway, such as tumor necrosis factor (TNF) alpha, Fas ligand (FasL), and TRAIL (TNF-related apoptosis-inducing ligand) or toll like receptors 3 and 4 ligands (Holler N et al. 2000; He S et al. 2009; Feoktistova M et al. 2011; Voigt S et al. 2014). In contrast to apoptosis, necroptosis represents a form of cell death that is optimally induced when caspases are inhibited (Holler N et al. 2000; Hopkins-Donaldson S et al. 2000; Sawai H 2014). Specific inhibitors of caspase-independent necrosis, necrostatins, have recently been identified (Degterev A et al. 2005, 2008). Necrostatins have been shown to inhibit the kinase activity of RIPK1 (Degterev A et al. 2008). Importantly, cell death of apoptotic morphology can be shifted to a necrotic phenotype when caspase 8 activity is compromised, otherwise active caspase 8 blocks necroptosis by the proteolytic cleavage of RIPK1 and RIPK3 (Kalai M et al. 2002; Degterev A et al. 2008; Lin Y et al. 1999; Feng S et al. 2007). When caspase activity is inhibited under certain pathophysiological conditions or by pharmacological agents, deubiquitinated RIPK1 is engaged in physical and functional interactions with the cognate kinase RIPK3 leading to formation of necrosome, a necroptosis-inducing complex consisting of RIPK1 and RIPK3 (Sawai H 2013; Moquin DM et al. 2013; Kalai M et al. 2002; Cho YS et al. 2009, He S et al. 2009, Zhang DW et al. 2009). Within the necrosome RIPK1 and RIPK3 bind to each other through their RIP homotypic interaction motif (RHIM) domains. The RHIMs can facilitate RIPK1:RIPK3 oligomerization, allowing them to form amyloid-like fibrillar structures (Li J et al. 2012; Mompean M et al. 2018). RIPK3 in turn interacts with mixed lineage kinase domain-like protein (MLKL) (Sun L et al. 2012; Zhao J et al. 2012; Murphy JM et al. 2013; Chen W et al. 2013). The precise mechanism of MLKL activation by RIPK3 is incompletely understood and may vary across species (Davies KA et al. 2020). Mouse MLKL activation relies on transient engagement of RIPK3 to facilitate phosphorylation of the pseudokinase domain (Murphy JM et al. 2013; Petrie EJ et al. 2019a), while it appears that stable recruitment of human MLKL by necrosomal RIPK3 is an additional crucial step in human MLKL activation (Davies KA et al. 2018; Petrie EJ et al. 2018, 2019b). RIPK3-mediated phosphorylation is thought to initiate MLKL oligomerization, membrane translocation and membrane disruption (Sun L et al. 2012; Wang H et al. 2014; Petrie EJ et al. 2020; Samson AL et al. 2020). Studies in human cell lines suggest that upon induction of necroptosis MLKL shifts to the plasma membrane and membranous organelles such as mitochondria, lysosome, endosome and ER (Wang H et al. 2014), but it is trafficking via a Golgi-microtubule-actin-dependent mechanism that facilitates plasma membrane translocation, where membrane disruption causes death (Samson AL et al. 2020). The mechanisms of necroptosis regulation and execution downstream of MLKL remain elusive. The precise oligomeric form of MLKL that mediates plasma membrane disruption has been highly debated (Cai Z et al. 2014; Chen X et al. 2014; Dondelinger Y et al. 2014; Wang H et al. 2014; Petrie EJ et al. 2017, 2018; Samson AL et al. 2020 ). However, microscopy data revealed that MLKL assembles into higher molecular weight species upon cytoplasmic necrosomes within human cells, and upon phosphorylation by RIPK3, MLKL is trafficked to the plasma membrane (Samson AL et al. 2020). At the plasma membrane, phospho-MLKL forms heterogeneous higher order assemblies, which are thought to permeabilize cells, leading to release of DAMPs to invoke inflammatory responses. MLKL also exerts non-necroptotic functions such as regulation of endosomal trafficking or MLKL-induced activation of the NLRP3 inflammasome (Yoon S et al. 2017; Shlomovitz I et al. 2020; Yoon S et al. 2022). While RIPK1, RIPK3 and MLKL are the core signaling components in the necroptosis pathway, many additional molecules have been proposed to positively and negatively tune the signaling pathway. Currently, this picture is evolving rapidly as new modulators continue to be discovered.<p>The Reactome module describes MLKL-mediated necroptotic events on the plasma membrane.

受受体激酶蛋白1 (RIPK1) 和RIPK3依赖性坏死称为坏死性凋亡或程序性坏死。RIPK1和RIPK3的激酶活性对于人类、小鼠细胞系和遗传小鼠模型中的坏死性细胞死亡至关重要（Cho YS等，2009；He S等，2009，2011；张德武等，2009；McQuade T等，2013；Newton等，2014）。坏死性凋亡的启动可以被激活外在凋亡信号通路的相同死亡配体所刺激，例如肿瘤坏死因子α (TNFα)、Fas配体 (FasL) 和TNF相关凋亡诱导配体 (TRAIL) 或Toll样受体3和4配体（Holler N等，2000；He S等，2009；Feoktistova M等，2011；Voigt S等，2014）。与凋亡不同，坏死性凋亡代表了一种细胞死亡形式，当caspases被抑制时，其诱导尤为有效（Holler N等，2000；Hopkins-Donaldson S等，2000；Sawai H，2014）。最近已鉴定出特定的caspase非依赖性坏死抑制剂，即坏死抑制剂（necrostatins）（Degterev A等，2005，2008）。研究表明，坏死抑制剂可抑制RIPK1的激酶活性（Degterev A等，2008）。值得注意的是，当caspase 8活性受损时，凋亡形态的细胞死亡可以转变为坏死表型，否则活性caspase 8通过蛋白酶裂解RIPK1和RIPK3来阻断坏死性凋亡（Kalai M等，2002；Degterev A等，2008；Lin Y等，1999；Feng S等，2007）。在某些病理生理条件下或通过药物抑制caspase活性时，去泛素化的RIPK1会与相应的激酶RIPK3相互作用，从而形成坏死体，这是一种由RIPK1和RIPK3组成的诱导坏死性凋亡的复合物（Sawai H，2013；Moquin DM等，2013；Kalai M等，2002；Cho YS等，2009；He S等，2009，张德武等，2009）。在坏死体中，RIPK1和RIPK3通过其RIP同源相互作用基序 (RHIM) 结合。RHIMs可以促进RIPK1:RIPK3寡聚化，使其能够形成类似淀粉样纤维的结构（Li J等，2012；Mompean M等，2018）。RIPK3随后与混合谱系激酶结构域样蛋白 (MLKL) 相互作用（Sun L等，2012；Zhao J等，2012；Murphy JM等，2013；Chen W等，2013）。RIPK3激活MLKL的确切机制尚不完全清楚，并且可能在不同物种中有所不同（Davies KA等，2020）。小鼠MLKL的激活依赖于RIPK3的短暂结合以促进伪激酶结构域的磷酸化（Murphy JM等，2013；Petrie EJ等，2019a），而人类MLKL的稳定募集似乎是人类MLKL激活的另一个关键步骤（Davies KA等，2018；Petrie EJ等，2018，2019b）。RIPK3介导的磷酸化被认为启动MLKL寡聚化、膜转位和膜破坏（Sun L等，2012；Wang H等，2014；Petrie EJ等，2020；Samson AL等，2020）。人类细胞系的研究表明，在诱导坏死性凋亡后，MLKL会转移到质膜和膜性细胞器，如线粒体、溶酶体、内体和内质网（Wang H等，2014），但其通过高尔基体-微管-肌动蛋白依赖性机制进行转运，促进质膜转位，其中膜破坏导致死亡（Samson AL等，2020）。MLKL下游坏死性凋亡调节和执行的机制仍然神秘莫测。介导质膜破坏的MLKL的确切寡聚形式一直是高度争议的（Cai Z等，2014；Chen X等，2014；Dondelinger Y等，2014；Wang H等，2014；Petrie EJ等，2017，2018；Samson AL等，2020）。然而，显微镜数据揭示了MLKL在人类细胞中的细胞质坏死体内组装成高分子量物种，并且RIPK3磷酸化后，MLKL会被转运到质膜上（Samson AL等，2020）。在质膜上，磷酸化MLKL形成异质的高阶组装，这些组装被认为可以透化细胞，导致危险信号的释放，从而引发炎症反应。MLKL还发挥非坏死性凋亡功能，如调节内体转运或MLKL诱导的NLRP3炎症小体的激活（Yoon S等，2017；Shlomovitz I等，2020；Yoon S等，2022）。虽然RIPK1、RIPK3和MLKL是坏死性凋亡通路中的核心信号分子，但许多其他分子已被提出可以正负调节信号通路。目前，这一图景正在迅速演变，因为新的调节剂不断被发现。《Reactome模块描述了MLKL介导的质膜上坏死性凋亡事件。》