Necroptosis in Neurodegeneration

Overview

Necroptosis is a caspase-independent programmed cell death pathway that has emerged as a significant contributor to neuronal loss in Alzheimer’s disease, Parkinson’s disease, amyotrophic lateral sclerosis, multiple sclerosis, and other neurodegenerative conditions. This pathway involves the RIPK1-RIPK3-MLKL signaling axis and is characterized by membrane rupture and release of damage-associated molecular patterns (DAMPs), triggering robust neuroinflammation^{1Chemical inhibitor of nonapoptotic cell death with therapeutic potential for ischemic brain injury2005 · Nature Chemical Biology · DOI 10.1038/nchembio711Open reference}.

The recognition of necroptosis as a major cell death mechanism in neurodegeneration has opened new therapeutic avenues, with several small molecule RIPK1 inhibitors advancing to clinical trials. This page provides a comprehensive analysis of necroptosis mechanisms in specific neurodegenerative diseases and the therapeutic implications^{2Necroptosis activation in Alzheimer's disease2017 · Nature Neuroscience · DOI 10.1038/nn.4608Open reference}.

Historical Context and Discovery

The discovery of necroptosis dates to the early 2000s when researchers observed a form of cell death that was morphologically necrotic but genetically programmed. Initial studies by Degterev and colleagues in 2005 identified necrostatin-1 (Nec-1) as a specific inhibitor of this novel cell death pathway, distinguishing it from apoptosis and necrosis^{3Identification of RIP1 kinase as a specific cellular target of necrostatins2008 · Nature Chemical Biology · DOI 10.1038/nchembio.83Open reference}.

Key historical milestones include:

• 2005: Discovery of necrostatin-1 as a necroptosis inhibitor

• 2008: Identification of RIPK3 as essential for necroptosis execution

• 2012: Discovery of MLKL as the downstream effector

• 2015: First clinical trials of RIPK1 inhibitors

• 2020-2024: Multiple failed Phase 2 trials but continued research

Molecular Mechanisms of Necroptosis

The RIPK1-RIPK3-MLKL Signaling Cascade

Necroptosis is activated when death receptor signaling fails to engage the apoptotic pathway, typically due to caspase-8 inhibition or deficiency^{4The RIP1/RIP3 necrosome forms a functional amyloid signaling complex2012 · Cell · DOI 10.1016/j.cell.2011.11.031Open reference}:

1. Initiation Phase: Death receptors (TNFR1, Fas, TRAIL-R) or pathogen recognition receptors (TLR3, TLR4, ZBP1) transmit death signals. When TNF-α binds TNFR1, complex I forms at the receptor containing TRADD, TRAF2, cIAP1/2, and RIPK1, normally promoting NF-κB-mediated survival signaling.

2. Necrosome Formation: When caspase-8 is inhibited, depleted, or overwhelmed, RIPK1 autophosphorylates at Ser166 and recruits RIPK3 through RHIM (RIP homotypic interaction motif) domain interactions. RIPK1 and RIPK3 form an amyloid-like signaling complex called the necrosome, characterized by amyloid fiber formation^{5Structure of the RIPK1-RIPK3 necrosome2020 · Nature · DOI 10.1038/s41586-020-0279-1Open reference}.

3. MLKL Phosphorylation: RIPK3 phosphorylates MLKL at Thr357 and Ser358 (human) or Ser345 (mouse), triggering a conformational change that exposes the N-terminal 4-helix bundle domain. This allows MLKL to translocate to cellular membranes^{6Mixed lineage kinase domain-like protein mediates necrosis signaling downstream of RIP3 kinase2012 · Cell · DOI 10.1016/j.cell.2012.09.024Open reference}.

4. Membrane Permeabilization: Phosphorylated MLKL oligomerizes and translocates to the plasma membrane, where it inserts into the lipid bilayer and forms pores, leading to ion influx (Ca²⁺, Na⁺), osmotic swelling, and membrane rupture. This results in the release of intracellular contents^{7Mixed lineage kinase domain-like protein MLKL causes necrotic membrane disruption2014 · Molecular Cell · DOI 10.1016/j.molcel.2014.01.003Open reference}.

5. DAMP Release: Ruptured cells release intracellular contents including HMGB1, ATP, IL-33, mitochondrial DNA, and other DAMPs that activate innate immune signaling through pattern recognition receptors on microglia, propagating inflammation.

Regulatory Mechanisms

Protein	Function	Regulation
RIPK1	Kinase, scaffold	Ubiquitylation, phosphorylation
RIPK3	Kinase, MLKL activator	Phosphorylation, oligomerization
MLKL	Effector pore formation	Phosphorylation, oligomerization
caspase-8	Inhibitor	Cleavage of RIPK1/RIPK3
CYLD	Deubiquitinase	Promotes necroptosis
TAK1	Inhibitor	Blocks RIPK1 activation

Non-Canonical Necroptosis Pathways

Necroptosis can also be activated independently of RIPK1 through several alternative pathways^{8Z-nucleic-acid sensing triggers ZBP1-dependent necroptosis2020 · Nature · DOI 10.1038/s41586-020-2570-8Open reference}:

• ZBP1 (DAI)-dependent necroptosis: ZBP1 detects Z-form nucleic acids (Z-DNA/Z-RNA) and directly activates RIPK3 via RHIM domain interaction. This pathway is relevant in viral infections and may contribute to neurodegeneration through endogenous retroelement activation.

• TRIF-dependent pathway: The TLR3/TLR4 adaptor protein TRIF can directly engage RIPK3 via its RHIM domain, linking innate immune sensing to necroptosis without requiring RIPK1.

• TLR-induced necroptosis: In macrophages and other immune cells, TLR signaling can directly induce necroptosis under certain conditions.

Necroptosis in Alzheimer’s Disease

Evidence for necroptosis in Alzheimer’s disease has grown substantially over the past decade^{9The necroptosis cell death pathway drives neurodegeneration2024 · Acta Neuropathologica Communications · DOI 10.1186/s40478-024-01745-6Open reference}:

Postmortem Brain Studies

• Elevated RIPK1, RIPK3, MLKL: Protein levels are elevated 2-3 fold in hippocampi of AD patients compared to age-matched controls.

• Colocalization studies: RIPK1 colocalizes with RIPK3 and MLKL in neurons with high levels of phosphorylated tau, and expression levels correlate with Braak staging.

• Neuronal localization: Phosphorylated MLKL is predominantly neuronal, not glial, in AD brain.

• Regional specificity: The hippocampus and entorhinal cortex show the highest necroptosis marker levels.

Molecular Interactions with AD Pathology

• Amyloid-β interaction: Amyloid-β oligomers activate TNF-α signaling in neurons and microglia, promoting RIPK1-dependent necroptosis. Amyloid plaques are surrounded by dystrophic neurites with elevated RIPK1 and phospho-MLKL.

• TNF-α-dependent pathway: In AD, TNF-α released by activated microglia creates a pro-necroptotic environment. Neurons with low caspase-8 activity become vulnerable to RIPK1-mediated death.

• Tau pathology correlation: The severity of tau pathology correlates with necroptosis marker levels, suggesting a relationship between tau and necroptotic cell death.

• Microglial activation: Activated microglia surrounding plaques express high levels of RIPK1, contributing to both inflammation and potential necroptosis.

Therapeutic Implications

• RIPK1 inhibitors: May protect neurons from amyloid-β-induced necroptosis.

• Exercise benefits: Exercise has been shown to decrease MLKL expression and phosphorylation of RIPK1 and RIPK3, suggesting potential neuroprotective strategies^{10Exercise modulates neuronal necroptosis in Alzheimer's disease2025 · Frontiers in Aging Neuroscience · DOI 10.3389/fnagi.2025.1499871Open reference}.

• Combination approaches: Targeting both necroptosis and amyloid pathology may provide synergistic benefits.

Necroptosis in Parkinson’s Disease

In Parkinson’s disease, necroptosis contributes to dopaminergic neuron loss in the substantia nigra pars compacta^{2Necroptosis activation in Alzheimer's disease2017 · Nature Neuroscience · DOI 10.1038/nn.4608Open reference}0:

Evidence from Human Studies

• Postmortem studies: Elevated RIPK1, RIPK3, and phospho-MLKL in substantia nigra of PD patients.

• Correlation with disease severity: Necroptosis marker levels correlate with disease duration and severity.

• Dopaminergic specificity: Changes are most pronounced in vulnerable dopaminergic neurons.

Evidence from Animal Models

• MPTP model: In the MPTP mouse model of PD, RIPK1, RIPK3, and MLKL are upregulated in the substantia nigra. RIPK3 knockout or MLKL inhibition attenuates dopaminergic neuron death^{2Necroptosis activation in Alzheimer's disease2017 · Nature Neuroscience · DOI 10.1038/nn.4608Open reference}1.

• α-Synuclein models: In models of α-synuclein overexpression, necroptosis markers are elevated.

• 6-OHDA model: RIPK1 inhibition protects against 6-hydroxydopamine-induced toxicity.

Molecular Mechanisms

• α-Synuclein connection: Aggregated α-synuclein activates microglial TLR2, triggering TNF-α release and subsequent RIPK1-dependent necroptosis in neighboring neurons.

• Exercise-mediated protection: Rotarod training in MPTP-treated mice significantly decreases MLKL expression and phosphorylation of RIPK1 and RIPK3, suggesting exercise may be neuroprotective partly through necroptosis suppression.

• LRRK2 mutations: Studies suggest that LRRK2 G2019S mutation may sensitize neurons to necroptotic cell death through alterations in inflammatory signaling pathways.

• PINK1/PARKIN: Mitochondrial dysfunction in PINK1/PARKIN models may intersect with necroptosis pathways.

Necroptosis in Amyotrophic Lateral Sclerosis

ALS shows significant involvement of the necroptosis pathway^{2Necroptosis activation in Alzheimer's disease2017 · Nature Neuroscience · DOI 10.1038/nn.4608Open reference}2:

Human Evidence

• Postmortem tissue: Elevated RIPK1 and phospho-MLKL in motor cortex and spinal cord.

• SOD1 patients: Levels correlate with disease duration and progression.

• TDP-43 pathology: Strong correlation between TDP-43 inclusions and necroptosis markers.

Genetic Models

• SOD1 mutant models: In SOD1-G93A transgenic mice, RIPK1 and RIPK3 are progressively upregulated in the spinal cord, and RIPK1 inhibition delays disease onset and extends survival.

• Optineurin mutations: Loss-of-function mutations in optineurin (OPTN), a cause of familial ALS, sensitize motor neurons to TNF-α-induced necroptosis by impairing RIPK1 ubiquitylation.

• C9orf72 models: Dipeptide repeat proteins from C9orf72 expansions may promote necroptosis.

TDP-43 Pathology

• Mechanism: TDP-43 pathology promotes microglial activation and TNF-α release, creating a pro-necroptotic environment in the motor cortex and spinal cord.

• Feedback loop: Necroptotic neurons release TDP-43, which may spread pathology.

Clinical Trials

• SAR443820 (DNL788): Sanofi/Denali tested the brain-penetrant RIPK1 inhibitor SAR443820 in a Phase 2 ALS trial, but it did not meet the primary endpoint of change in ALSFRS-R score.

Necroptosis in Multiple Sclerosis

In multiple sclerosis, necroptosis drives both inflammatory demyelination and axonal damage^{2Necroptosis activation in Alzheimer's disease2017 · Nature Neuroscience · DOI 10.1038/nn.4608Open reference}3:

Evidence in MS

• Oligodendrocyte death: TNF-α released by infiltrating immune cells triggers necroptotic death of oligodendrocytes, contributing to demyelination.

• Microglial necroptosis: Necroptotic microglia release pro-inflammatory DAMPs that recruit additional immune cells, amplifying CNS inflammation.

• Active lesions: Active MS lesions show elevated RIPK1 and phospho-MLKL expression in both neurons and oligodendrocytes.

Experimental Models

• EAE model: RIPK1 and RIPK3 are elevated in experimental autoimmune encephalomyelitis.

• Oligodendrocyte-specific effects: Cultured oligodendrocytes are highly sensitive to TNF-α-induced necroptosis.

Clinical Trial

• SAR443820: Sanofi tested SAR443820 in a Phase 2 trial for relapsing and progressive MS (K2 study) but discontinued the trial in October 2024 after it failed to meet primary endpoints.

Necroptosis in Other Neurodegenerative Diseases

Huntington’s Disease

• Mutant huntingtin effects: Mutant huntingtin sensitizes striatal neurons to TNF-α-induced necroptosis.

• Postmortem studies: RIPK1 and RIPK3 are upregulated in the caudate nucleus of HD patients.

• Therapeutic potential: RIPK1 inhibitors protect striatal neurons in HD models.

Frontotemporal Dementia

• Progranulin: GRN (progranulin) haploinsufficiency increases microglial production of TNF-α and sensitizes neurons to RIPK1-dependent cell death.

• TDP-43: TDP-43 pathology in FTD may promote necroptosis.

Stroke and Brain Injury

• Ischemic stroke: Necroptosis contributes to secondary brain injury after stroke.

• Traumatic brain injury: RIPK1 activation is observed following TBI.

Necroptosis and Neuroinflammation: A Vicious Cycle

A critical feature of necroptosis in neurodegeneration is the feed-forward cycle between cell death and neuroinflammation^{2Necroptosis activation in Alzheimer's disease2017 · Nature Neuroscience · DOI 10.1038/nn.4608Open reference}4:

1. Disease triggers: Aβ, α-synuclein, mutant SOD1, or other pathological proteins activate microglia, which release TNF-α and other pro-inflammatory cytokines.

2. TNF-α signaling: In neurons, when caspase-8 is insufficient, TNF-α activates RIPK1-RIPK3-MLKL-dependent necroptosis.

3. Necroptotic death: Neurons rupture and release DAMPs (HMGB1, ATP, IL-33, mitochondrial DNA).

4. DAMP signaling: These DAMPs activate additional microglia through pattern recognition receptors.

5. Amplification: This creates a self-perpetuating cycle of cell death and inflammation.

This intersection of necroptosis and inflammation makes it an attractive therapeutic target, as inhibiting this pathway may break both the cell death and inflammatory components of neurodegeneration.

Therapeutic Approaches

RIPK1 Inhibitors

Multiple RIPK1 inhibitors have been developed and tested in clinical trials^{2Necroptosis activation in Alzheimer's disease2017 · Nature Neuroscience · DOI 10.1038/nn.4608Open reference}5:

Drug	Company	Stage	Indications
SAR443820 (DNL788)	Sanofi/Denali	Phase 2 (discontinued)	ALS, MS, AD
SAR443122 (DNL758)	Sanofi/Denali	Phase 2	Cutaneous lupus, UC
GSK2982772	GSK	Phase 2a	Psoriasis, UC, RA
ABBV-0403	AbbVie	Phase 1	Inflammatory conditions
SIR-2446	Sirocco Therapeutics	Phase 1	Inflammatory conditions

SAR443820 is a brain-penetrant, orally bioavailable RIPK1 inhibitor that was well-tolerated in healthy volunteers and showed target engagement in the CNS. Despite the setbacks in ALS and MS Phase 2 trials, the RIPK1 inhibitor pipeline continues to expand.

RIPK3 and MLKL Inhibitors

• RIPK3 inhibitors: Selective RIPK3 kinase inhibitors are in preclinical development. A challenge is that RIPK3 inhibition can paradoxically trigger apoptosis in some cellular contexts.

• MLKL inhibitors: Necrosulfonamide (NSA) binds human MLKL and blocks necroptosis execution. Selective, drug-like MLKL inhibitors are being developed for CNS applications.

Anti-TNF-α Therapy

Anti-TNF biologics (infliximab, adalimumab) are highly effective in autoimmune diseases but have limited blood-brain-barrier penetration. Brain-penetrant anti-TNF approaches (nanobodies, receptor decoys) are in preclinical development for neurodegenerative indications.

Combination Approaches

Given the convergence of multiple cell death pathways in neurodegeneration, combination strategies targeting necroptosis alongside ferroptosis or excitotoxicity may provide synergistic neuroprotection.

Biomarkers

Genetic Biomarkers

• RIPK1 polymorphisms: Some variants may affect necroptosis susceptibility.

• RIPK3 variants: May influence disease progression.

Protein Biomarkers

• Phospho-MLKL: Detectable in blood and CSF.

• RIPK1 activity: Measures of kinase activity.

• DAMPs: HMGB1, mitochondrial DNA in circulation.

Imaging

• PET tracers: RIPK1-targeted imaging agents under development.

Future Directions

Despite clinical trial setbacks, research continues to advance:

• Biomarker development: Patient stratification for necroptosis-targeted therapy.

• Combination approaches: Targeting multiple cell death pathways.

• Delivery methods: Improving CNS penetration of inhibitors.

• Timing: Identifying optimal intervention windows.

See Also

• Cell Death Pathways - Overview of cell death mechanisms in neurodegeneration

• Neuroinflammation - Role of inflammation in neurodegenerative diseases

• Apoptosis - Caspase-dependent cell death

• Ferroptosis - Iron-dependent cell death

• TNF Signaling - TNF-α signaling in neurodegeneration

• Microglia - Role of microglia in neuroinflammation

• RIPK1 Gene - Key signaling protein

• RIPK3 Gene - Key signaling protein