This article describes the pathways of necroptosis cell death. Necroptosis can be described as a programmed form of necrosis that depends on the activation of RIPK3 (receptor-interacting kinase 3)1 as well as the MLKL (mixed lineage kinase domain-like) pseudokinase2.
Morphologically different to apoptosis, this type of cell death of necrosis involves discharge of cytoplasmic contents and membrane rupture. Death receptor activation triggers necroptosis, but this can also be triggered by certain death receptor-independent pathways.
In the majority of contexts, proapoptotic caspase 83–5 inhibits necroptosis; some intracellular pathogens inhibit caspase 8, suppressing apoptosis, and necroptosis serves as a back-up to remove infected cells6.
Inflammatory conditions such as amyotrophic lateral sclerosis (ALS), multiple sclerosis7, Crohn’s disease, and ischemia-reperfusion injury have been associated with necroptosis. However, it is not known whether necroptosis is a secondary consequence or a driving factor for these medical conditions8.
Death receptor-dependent pathway
Necroptosis is triggered as a result of ligand binding to death receptors such as tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) receptors, tumor necrosis factor receptor 1 (TNFR1), and Fas9,10, but TNFR1-mediated necroptosis is the most researched topic. In addition, these death receptors play a role in prosurvival signaling and apoptosis: the path taken by the cell is dictated by the type of complex formed due to ligand binding, as shown in Figure 1.
Figure 1. Necroptosis signaling: the intersection of prosurvival and apoptotic pathways.
After ligand binding, prosurvival complex I, containing RIPK1, TNF-receptor-associated death domain (TRADD), and a number of ubiquin E3 ligases, is recruited by the cytosolic death domain of TNFR1.
RIPK1 is polyubiquitinated in complex I, but subsequent deubiquitination of RIPK1 causes dissociation and formation of one of two complexes: while complex IIa mediates the activation of caspase 8 and triggers apoptosis, complex IIb – also referred to as the necrosome – assembles upon the inhibition of caspase 8 and initiates necroptosis1.
The necrosome and downstream signaling
In complex IIb, RIPK3 is recruited by RIPK1 by way of rip homeotypic interaction motifs (RHIMs) present within both proteins, which leads to their mutual phosphorylation. Phosphorylation of RIPK3 leads to oligomerization of RIPK3 which is required for its activation. After RIPK3 is activated, it phosphorylates MLKL at serine 3582 and threonine 357.
The phosphorylated MLKL then multimerizes and moves to the plasma membrane – a process that is key for necroptotic cell death11. It is still unknown for certain how cell death is triggered by MLKL12, but according to some reports MLKL directly permeabilizes the membrane by binding to phosphatidylinositol lipids and cardiolipin.13,14.
Death receptor-independent pathway
Aside from death receptor pathways, engagement of viral infection, Toll-like receptor TLR3 and TLR415, viral expression of RHIM-containing proteins, and type 1 and 2 interferons can all promote the induction of necroptosis.
Part of the innate immune system, Toll-like receptors are proteins that can sense cellular stress, infection, and damage. When activated, the adapter-inducing, TIR-domain-containing interferon-β (TRIF) adaptor protein forms a complex with RIPK315. Although TLR activation-dependent necroptosis depends on MLKL and RIPK3, it can continue without RIPK1.
Activation of DAI (DNA-dependent activator of IFN regulatory factors) also stimulates RIPK3-dependent necroptosis. Double-strand viral DNA is recognized by DAI, which includes a RHIM domain for recruitment of RIPK3 and formation of the necrosome.
Figure 2. Pathway of viral DNA activation of necroptosis
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- Sun, L. et al. Mixed lineage kinase domain-like protein mediates necrosis signaling downstream of RIP3 kinase. Cell 148, 213–227 (2012).
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- Mocarski, E. S., Guo, H. & Kaiser, W. J. Necroptosis: The Trojan horse in cell autonomous antiviral host defense. Virology 479–480, 160–166 (2015).
- Ofengeim, D. et al. Activation of necroptosis in multiple sclerosis. Cell Rep 10, 1836–49 (2015).
- Conrad, M., Angeli, J. P. F., Vandenabeele, P. & Stockwell, B. R. Regulated necrosis: disease relevance and therapeutic opportunities. Nat Rev Drug Discovery 15, 348–366 (2016).
- Degterev, A. et al. Chemical inhibitor of nonapoptotic cell death with therapeutic potential for ischemic brain injury. Nat Chem Biol 1, 112–119 (2005).
- Holler, N. et al. Fas triggers an alternative, caspase-8–independent cell death pathway using the kinase RIP as effector molecule. Nat Immunol 1, 489–495 (2000).
- Cai, Z. et al. Plasma membrane translocation of trimerized MLKL protein is required for TNF-induced necroptosis. Nat Cell Biol 16, 55–65 (2014).
- Galluzzi, L., Kepp, O. & Kroemer, G. MLKL regulates necrotic plasma membrane permeabilization. Cell Res. 24, 139–40 (2014).
- Dondelinger, Y. et al. MLKL compromises plasma membrane integrity by binding to phosphatidylinositol phosphates. Cell Rep 7, 971–81 (2014).
- Wang, H. et al. Mixed Lineage Kinase Domain-like Protein MLKL Causes Necrotic Membrane Disruption upon Phosphorylation by RIP3. Mol Cell 54, 133–146 (2014).
- He, S., Liang, Y., Shao, F. & Wang, X. Toll-like receptors activate programmed necrosis in macrophages through a receptor-interacting kinase-3-mediated pathway. Proc Natl Acad Sci 108, 20054–20059 (2011).
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