Necroptosis

Necroptosis (from the Greek nekros, "corpse," and ptosis, "falling") is a genetically programmed, regulated form of lytic, caspase-independent inflammatory cell death executed by the Receptor-Interacting Serine/Threonine-Protein Kinase 1 and 3 (RIPK1–RIPK3) signaling complex—termed the necrosome—and its terminal effector, the pore-forming pseudokinase Mixed Lineage Kinase Domain-Like protein (MLKL). Historically, cell biology contrasted quiet, caspase-driven programmed apoptosis against accidental, unprogrammed necrosis caused by acute physical trauma, freeze–thaw injury, or ATP depletion. In 2005, Junying Yuan and ** Alexei Degterev** demonstrated that necrosis triggered by Tumor Necrosis Factor-α (TNF-α) when caspases are inhibited is strictly regulated by the kinase activity of RIPK1 and can be pharmacologically blocked by a small-molecule allosteric inhibitor termed Necrostatin-1 (Nec-1)—coining the term necroptosis. Morphologically, necroptotic cells undergo organelle swelling (oncosis), plasma membrane permeabilization, and cellular lysis, releasing intracellular damage-associated molecular patterns (DAMPs) that alert the innate immune system when intracellular pathogens attempt to block host apoptosis.

Necroptosis can be initiated by three convergent families of innate immune receptors, each of which signals through a conserved ~16–20 amino-acid amyloid-forming motif known as the RIP Homotypic Interaction Motif (RHIM):

1. Death Receptors (TNFR1, Fas/CD95, TRAIL-R1/R2): Binding of trimeric TNF-α to Tumor Necrosis Factor Receptor 1 (TNFR1) recruits the adaptor TRADD, the E3 ubiquitin ligases cIAP1/cIAP2, the Linear Ubiquitin Chain Assembly Complex (LUBAC), and RIPK1 into membrane-associated Complex I. So long as RIPK1 remains decorated with K63-linked and M1-linear polyubiquitin chains and phosphorylated by the IKKα/β and TBK1 kinases, Complex I activates canonical NF-κB pro-survival transcription.
2. Deubiquitination and Complex IIa (Apoptosis) vs. Complex IIb (Necrosome): When cIAP1/2 are depleted (e.g., by SMAC mimetics) or the deubiquitinase CYLD removes polyubiquitin chains from RIPK1, RIPK1 dissociates into the cytosol to nucleate Complex II:
   • Active Caspase-8 (Complex IIa → Apoptosis): Under baseline physiological conditions, the FADD–Caspase-8–cFLIP_L heterodimer proteolytic complex cleaves RIPK1 at residue Asp324 (Asp325 in mice) and RIPK3 at Asp328, dismantling the necrosome and routing the cell toward non-inflammatory apoptosis (or survival). This proteolytic brake explains why germline deletion of Casp8 or Fadd in mice causes lethal embryonic vascular necroptosis at E10.5 that is completely rescued by co-deletion of Ripk3 or Mlkl.
   • Inhibited Caspase-8 (Complex IIb → Necroptosis): When Caspase-8 is genetically absent, pharmacologically inhibited (by pan-caspase inhibitors such as z-VAD-fmk), or blocked by viral caspase inhibitors, intact RIPK1 autophosphorylates at Ser166 and recruits RIPK3 via their shared RHIM domains to assemble the functional necrosome.
3. RIPK1-Independent RHIM Sensors (TRIF and ZBP1): Two additional innate immune sensors contain endogenous RHIM domains that can directly nucleate RIPK3 amyloid polymerization without requiring RIPK1:
   • Toll-Like Receptors TLR3 and TLR4: Sensing of viral double-stranded RNA (via endosomal TLR3) or bacterial lipopolysaccharide (via TLR4) recruits the RHIM-containing adaptor TRIF (TICAM-1) directly to RIPK3.
   • Z-DNA Binding Protein 1 (ZBP1 / DAI): Sensing of left-handed Z-RNA or Z-DNA duplexes generated during influenza A virus, orthopoxvirus, or herpesvirus replication triggers ZBP1–RIPK3 necrosome assembly directly inside the nucleus or cytoplasm.

Cryo-electron microscopy and solid-state NMR spectroscopy (Mompeán et al., 2018) have revealed that the core of the RIPK1–RIPK3 necrosome is a heterotypic functional amyloid fibril:

• Cross-β RHIM Amyloid Core: The conserved tetrapeptide core of the RHIM domain (I-Q-I-G in RIPK1 and V-Q-V-G in RIPK3) interdigitates into two antiparallel β-sheets stacked at a 4.7 Å vertical spacing, forming an ultra-stable steric zipper scaffold from which the globular kinase domains protrude radially.
• RIPK3 Autophosphorylation: Proximity inside the RHIM amyloid filament induces trans-autophosphorylation of human RIPK3 at Ser227 (Thr231/Ser232 in mice), creating a high-affinity docking surface for the pseudokinase domain of MLKL.
• MLKL Conformational Unlatching and Plasma Membrane Permeabilization: Inactive monomeric MLKL consists of an N-terminal four-helix bundle (4HB) executioner domain tethered by a two-helix brace to a C-terminal pseudokinase domain (which binds ATP but lacks the catalytic Asp residue required for phosphotransfer). When RIPK3 phosphorylates the activation loop of human MLKL at Thr357 and Ser358, a conformational switch unlatches the autoinhibited N-terminal 4HB domain, driving MLKL assembly into disulfide-stabilized tetramers and octamers.
• Phosphoinositide Binding and Osmotic Lysis: Oligomerized MLKL translocates via Golgi–actin-microtubule vesicular trafficking to the inner leaflet of the plasma membrane, where basic positively charged patches on the 4HB domain bind specifically to phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] and inositol hexakisphosphate (IP₆). Insertion of the 4HB helices into the lipid bilayer forms cation-permeable pores (permeabilizing Na⁺, Ca²⁺, and Mg²⁺), causing water influx, dissipation of membrane potential, cell swelling, and explosive plasma-membrane rupture (counteracted transiently by ESCRT-III membrane-budding repair vesicles).

Immunologists distinguish necroptosis from apoptosis, pyroptosis, and ferroptosis by its initiating sensors, executioner pores, caspase dependence, and immunological output:

Why did vertebrates evolve necroptosis alongside apoptosis? Because apoptotic cell death requires several hours to dismantle viral replication factories, large DNA viruses—including cytomegaloviruses (MCMV/HCMV), herpes simplex viruses (HSV-1/HSV-2), and poxviruses (Vaccinia / Cowpox)—evolved potent viral caspase-8 inhibitors (such as the cowpox serpin CrmA and the cytomegalovirus protein vICA / UL36) to keep their host cell alive. Necroptosis evolved as a fail-safe "trapdoor" defense: the moment a viral protein blocks host Caspase-8, the proteolytic brake on RIPK1 and RIPK3 is released, triggering immediate necroptotic cell lysis before mature virions can assemble and flooding the tissue with alarmins (HMGB1, IL-1α, IL-33, S100 proteins, and uric acid) to recruit neutrophils and dendritic cells.

In counter-adaptation, viruses evolved specialized viral RHIM decoys and MLKL antagonists:

• Viral RHIM Inhibitors (vIRA): Murine cytomegalovirus encodes M45 (viral Inhibitor of RIP Activation), and HSV-1/HSV-2 encode the ribonucleotide reductase large subunit ICP6 / ICP10, which carry viral RHIM domains that intercalate into host RIPK1/RIPK3/ZBP1 filaments and cap amyloid polymerization.
• Ischemia-Reperfusion and Neurodegenerative Disease: Conversely, sterile activation of RIPK1–RIPK3–MLKL necroptosis drives pathological tissue destruction during myocardial infarction, ischemic stroke, acute kidney injury, inflammatory bowel disease (Crohn's disease), and amyotrophic lateral sclerosis (ALS), motivating clinical trials of brain-penetrant allosteric RIPK1 kinase inhibitors (such as DNL747 and SAR443820).

1. Degterev, A., Hitomi, J., Germscheid, M., Ch'en, I. L., Korkina, O., Teng, X., ... & Yuan, J. (2008). Identification of RIP1 kinase as a specific cellular target of necrostatins. Nature Chemical Biology, 4(5), 313–321. https://doi.org/10.1038/nchembio.83
2. Sun, L., Wang, H., Wang, Z., He, S., Chen, S., Liao, D., ... & Wang, X. (2012). Mixed lineage kinase domain-like protein mediates necrosis signaling downstream of RIP3 kinase. Cell, 148(1–2), 213–227. https://doi.org/10.1016/j.cell.2011.11.031
3. Mompeán, M., Li, W., Li, J., Laage, S., Siemer, A. B., Bozkurt, G., ... & McDermott, A. E. (2018). The structure of the necrosome RIPK1-RIPK3 core, a human hetero-amyloid signaling complex. Cell, 173(5), 1244–1253. https://doi.org/10.1016/j.cell.2018.03.032
4. Weinlich, R., Oberst, A., Beere, H. M., & Green, D. R. (2017). Necroptosis in development, inflammation and disease. Nature Reviews Molecular Cell Biology, 18(2), 127–136. https://doi.org/10.1038/nrm.2016.149