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Short answer
Necroptosis is a caspase-independent form of regulated cell death. A death receptor or pattern-recognition receptor triggers assembly of a RIPK1-RIPK3 complex (the necrosome), RIPK3 phosphorylates the pseudokinase MLKL, and MLKL permeabilises the plasma membrane, so the cell contents spill out and drive inflammation. The most specific bench readout is phosphorylated MLKL, not TUNEL or cleaved caspase-3.
Definition and scope
Necroptosis is classed as a regulated, caspase-independent mode of cell death implicated in many inflammatory and infectious diseases (Kelepouras et al., 2024). It is one of the forms of regulated cell death covered by the Nomenclature Committee on Cell Death recommendations (Galluzzi et al., 2018), the usual naming reference for papers and methods sections. It differs from accidental necrosis in that it runs through defined molecular machinery that can be blocked genetically, for example in Ripk3 or Mlkl knockout mice.
The RIPK3-MLKL pathway, step by step
- Trigger. Death receptors (TNFR1, FAS/CD95, TRAIL-R1/DR4, TRAIL-R2/DR5) or pattern-recognition receptors including TLR3 and TLR4.
- Necrosome assembly. RIPK1 and RIPK3 associate through their RIP homotypic interaction motif (RHIM) domains. The cytosolic nucleic-acid sensor ZBP1 can also associate with RIPK3, but the paper reports this only in the absence of RIPK1 or its RHIM.
- MLKL phosphorylation. RIPK3 phosphorylates MLKL at threonine 357/serine 358 in human and serine 345 in mouse.
- Execution. MLKL oligomerises, accumulates at the plasma membrane and destabilises it, releasing cellular contents.
- Consequence. Dying cells release TNF, type I interferon, IL-6, IL-33, IL-36, HMGB1 and IL-1α, which can start a wave of inflammation.
Source: Kelepouras et al., Cell Death & Differentiation 2024;31:897-909, open access (CC BY 4.0), read 2026-11-07.
Necroptosis vs apoptosis, pyroptosis and ferroptosis
The defining contrast with apoptosis is caspase dependence: necroptosis is caspase-independent, and its executioner is MLKL rather than the caspase cascade. For the gasdermin-driven pathway, see the CASRAI guide on pyroptosis, which compares it with apoptosis and necroptosis; for iron-dependent lipid peroxidation, see ferroptosis. For apoptosis itself, see what causes apoptosis.
Checklist: which readouts point to necroptosis
Kelepouras et al. state that there is currently no reliable, reproducible way to address necroptosis in situ in tissues from different animal models, and that cleaved caspase-3 and TUNEL are not specific to necroptosis. The table summarises what that paper says about common readouts.
| Readout | What it indicates | Specific to necroptosis? |
|---|---|---|
| Phospho-MLKL, human T357/S358 | RIPK3-mediated MLKL activation; widely used as a conclusive marker | Yes, as the executioner phosphorylation |
| Phospho-MLKL, mouse S345 | Same event in mouse; the paper validated pMLKL-S345 immunohistochemistry and immunofluorescence in tissue | Yes, as the executioner phosphorylation |
| Cleaved caspase-3 | Apoptosis readout in vivo | No |
| TUNEL | Hallmark of apoptosis, but also labels necrosis, necroptosis, pyroptosis and parthanatos | No |
| Ripk3 or Mlkl knockout | Genetic test of whether a phenotype depends on the pathway | Yes, as a loss-of-function test |
Practical reading. The paper used pMLKL-S345, cleaved caspase-3 and TUNEL together to decide which pathway was operating. In SARS-CoV-infected mouse lung it found more TUNEL-positive cells that were cleaved-caspase-3-negative, pointing to a death mode other than classical apoptosis. Species matters when choosing antibodies: the human and mouse phosphosites are different residues.
Where it matters for labs
Cell-death assays are routinely reported in biomedical papers, and the choice of readout determines which claim a result supports. A TUNEL-only or caspase-3-only figure cannot distinguish necroptosis from other modes. Guidelines for cardiac research (Mishra et al., 2019) likewise treat necroptosis as one of six major regulated death forms in cardiac pathology and discuss the methods used to evaluate each, with their limitations. For techniques used alongside these readouts, see the guides on Annexin V/PI flow cytometry and interpreting western blot results.
Common misunderstandings
- Necroptosis is just necrosis. It is regulated and depends on RIPK3 and MLKL.
- TUNEL-positive means apoptotic. TUNEL also labels necrosis, necroptosis, pyroptosis and parthanatos.
- Caspase-3-negative means necroptosis. It only argues against classical apoptosis; other modes remain possible.
- Mouse and human markers are interchangeable. The phosphorylated residues differ (S345 in mouse; T357/S358 in human).
FAQ
What does necroptosis mean?
It names a regulated, caspase-independent form of cell death in which RIPK3 and MLKL execute plasma membrane rupture.
What is the necroptosis pathway?
Receptor trigger, RIPK1-RIPK3 necrosome, RIPK3 phosphorylation of MLKL, then MLKL oligomerisation and plasma membrane permeabilisation, as set out above.
Necroptosis vs apoptosis: what is the key difference?
Apoptosis relies on caspases; necroptosis is caspase-independent and executed by phosphorylated MLKL.
Sources
- Kelepouras K, et al. Cell Death Differ 2024;31(7):897-909. doi:10.1038/s41418-024-01313-6 (PMC11239901). Read 2026-11-07.
- Galluzzi L, et al. Molecular mechanisms of cell death: recommendations of the Nomenclature Committee on Cell Death 2018. Cell Death Differ 2018;25(3):486-541. doi:10.1038/s41418-017-0012-4.
- Mishra PK, et al. Guidelines for evaluating myocardial cell death. Am J Physiol Heart Circ Physiol 2019;317(5):H891-H922 (PMC6879915), abstract read 2026-11-07.
