RIPK1 (Receptor-Interacting Kinase 1)

Overview

RIPK1 (Receptor-Interacting Serine/Threonine-Protein Kinase 1) is a 671 amino acid kinase and signaling adaptor that serves as a central decision point between cell survival and cell death. RIPK1 operates in three main contexts: survival signaling through NF-κB activation, apoptosis initiation through the ripoptosome (Complex II), and necroptosis initiation through the necrosome (RIPK1-RIPK3-MLKL complex) ^{1Necroptosis and RIPK1-mediated neuroinflammation in CNS diseases2019 · Nat Rev Neurosci · DOI 10.1038/s41583-018-0093-1Open reference}. Its activation state — governed by ubiquitination, phosphorylation, and caspase-8 cleavage — determines whether a cell responds to TNF-α signaling by surviving, dying by apoptosis, or dying by necroptosis. In the CNS, RIPK1 is activated in Alzheimer’s disease, Parkinson’s disease, ALS, and multiple sclerosis, where it drives both neuronal cell death and neuroinflammation through microglial activation. Multiple RIPK1 kinase inhibitors are in clinical development for neurodegenerative and inflammatory diseases ^{2Identification of RIP1 kinase as a specific cellular target of necrostatins2008 · Nat Chem Biol · DOI 10.1038/nchembio.83Open reference, 3RIPK1 inhibitor GSK2982772: results from first-in-human studies2021 · J Pharmacol Exp Ther · DOI 10.1124/jpet.120.000053Open reference}.

Pathway Diagram

flowchart TD
    RIPK1["RIPK1"]
    style RIPK1 fill:#006494,stroke:#4fc3f7,stroke-width:3px,color:#e0e0e0
    INFLAMMATION["INFLAMMATION"]
    RIPK1 -->|"inhibits"| INFLAMMATION
    Cell_Death["Cell Death"]
    RIPK1 -->|"mediates"| Cell_Death
    RIPK1 -->|"regulates"| Cell_Death
    Necroptosis["Necroptosis"]
    RIPK1 -->|"involved in"| Necroptosis
    Inflammation["Inflammation"]
    RIPK1 -->|"regulates"| Inflammation
    Inflammatory_Gene_Expression["Inflammatory Gene Expression"]
    RIPK1 -->|"upregulates"| Inflammatory_Gene_Expression
    neuroinflammation["neuroinflammation"]
    RIPK1 -->|"causes"| neuroinflammation
    APOPTOSIS["APOPTOSIS"]
    RIPK1 -->|"inhibits"| APOPTOSIS
    AMPK["AMPK"]
    AMPK -->|"phosphorylates"| RIPK1
    Necrostatin["Necrostatin"]
    Necrostatin -->|"inhibits"| RIPK1
    citronellol["citronellol"]
    citronellol -->|"activates"| RIPK1
    RIPK3["RIPK3"]
    RIPK3 -->|"regulates"| RIPK1
    INFLAMMATION -->|"activates"| RIPK1
    INFLAMMATION -->|"regulates"| RIPK1
    OPTN["OPTN"]
    OPTN -->|"inhibits"| RIPK1
    style INFLAMMATION fill:#006494,stroke:#888,color:#e0e0e0
    style Cell_Death fill:#006494,stroke:#888,color:#e0e0e0
    style Necroptosis fill:#006494,stroke:#888,color:#e0e0e0
    style Inflammation fill:#ef5350,stroke:#ef5350,color:#e0e0e0
    style Inflammatory_Gene_Expression fill:#006494,stroke:#888,color:#e0e0e0
    style neuroinflammation fill:#ef5350,stroke:#ff8a65,color:#e0e0e0
    style APOPTOSIS fill:#006494,stroke:#888,color:#e0e0e0
    style AMPK fill:#4a1a6b,stroke:#ce93d8,color:#e0e0e0
    style Necrostatin fill:#006494,stroke:#4fc3f7,color:#e0e0e0
    style citronellol fill:#006494,stroke:#4fc3f7,color:#e0e0e0
    style RIPK3 fill:#1b5e20,stroke:#81c784,color:#e0e0e0
    style OPTN fill:#1b5e20,stroke:#81c784,color:#e0e0e0

Structure

RIPK1 contains three functional domains that govern its dual role as a kinase and a signaling adaptor ^{2Identification of RIP1 kinase as a specific cellular target of necrostatins2008 · Nat Chem Biol · DOI 10.1038/nchembio.83Open reference}:

Protein Domains

1. Kinase domain (residues 1-294): Serine/threonine kinase with ATP-binding pocket. Kinase activity is required for necroptosis but not for survival/apoptosis signaling. Key regulatory sites:

   • Ser166 (activation loop): Autophosphorylation at this site correlates with kinase activation and necroptosis

   • Ser321 (UFD motif): Phosphorylated by IKKα/β — inhibits kinase activity and promotes survival

   • Ser25 (N-terminal): Phosphorylated by TBK1 — promotes cell survival

2. Intermediate domain / RIP homotypic interaction motif (RHIM) (residues 295-550): The RHIM (residues 496-531) is critical for homotypic interactions with other RHIM-containing proteins:

   • RIPK3: The key necroptosis partner — RHIM-RHIM interaction drives necrosome assembly

   • ZBP1/DAI: Another RHIM protein that can activate RIPK3 independently of RIPK1

   • The intermediate region also contains the UFD (Ubiquitin Fn1-like) motif involved in ubiquitin binding

3. Death domain (DD) (residues 583-671): C-terminal death domain that mediates interactions with other DD-containing proteins:

   • TNFR1: RIPK1 is recruited to the activated TNFR1 complex via TRADD

   • FADD: In Complex II (ripoptosome), FADD recruits caspase-8

   • DAXX: Additional DD interactions modulate signaling outcomes

Regulatory Mechanisms

RIPK1 activity is controlled by:

• Ubiquitination: cIAP1/2 and LUBAC (HOIP/HOIL1/Sharpin) attach linear and K63-linked ubiquitin chains to RIPK1. Ubiquitinated RIPK1 in Complex I scaffolds NF-κB signaling.

• Deubiquitination: CYLD and A20 remove ubiquitin chains from RIPK1, shifting the balance toward cell death.

• Phosphorylation: Ser166 (autophosphorylation) activates kinase function for necroptosis; Ser321 (IKK phosphorylation) inhibits kinase function and promotes survival.

• Caspase-8 cleavage: Caspase-8 cleaves RIPK1 at Asp324, generating a truncated form that lacks the DD. This cleavage separates survival from death signaling and prevents necroptosis.

Normal Function

TNFR1 Signaling Complex I (Survival)

When TNF-α binds to TNFR1, a sequential assembly of signaling complexes determines the cell’s fate:

1. Complex I formation: TNFR1 recruits TRADD, RIPK1, TRAF2/5, and cIAP1/2 into a membrane-associated complex

2. Ubiquitination: cIAP1/2 conjugate K63-linked ubiquitin chains to RIPK1 and other components. LUBAC adds linear ubiquitin chains.

3. NF-κB activation: Ubiquitinated RIPK1 recruits the IKK complex (IKKα, IKKβ, NEMO) and TAB2/3. TAK1 phosphorylates IKKβ → IKKβ phosphorylates IκB → NF-κB translocates to nucleus → pro-survival and inflammatory gene expression (c-IAP1/2, c-FLIP, XIAP, Bcl-2 family, cytokines)

4. Cell survival: NF-κB target genes block both apoptosis and necroptosis pathways

Complex IIa (Ripoptosome / Apoptosis)

When cIAP activity is pharmacologically or genetically reduced, RIPK1 becomes deubiquitinated by CYLD and enters the cytosol:

1. Cytosolic Complex IIa: RIPK1, TRADD, FADD, and caspase-8 form a cytosolic complex

2. RIPK1-dependent apoptosis: In this complex, RIPK1 can activate caspase-8 directly

3. Bid cleavage: Caspase-8 cleaves Bid → tBid → mitochondrial apoptosis

4. Executioner caspase activation: Caspase-8 activates executioner caspases (3, 6, 7) → apoptotic cell death

Necrosome (Complex IIb) / Necroptosis

When caspase-8 is inhibited (genetically or pharmacologically), RIPK1 initiates necroptosis:

1. RIPK1 deubiquitination: CYLD removes ubiquitin from RIPK1

2. Necrosome assembly: RIPK1 recruits RIPK3 via RHIM-RHIM interaction

3. RIPK1-RIPK3 cross-phosphorylation: Autophosphorylation of both kinases (RIPK1 at Ser166, RIPK3 at Ser227)

4. MLKL recruitment: Phosphorylated RIPK3 recruits and phosphorylates MLKL (Mixed Lineage Kinase Domain-Like)

5. Plasma membrane translocation: Phosphorylated MLKL oligomerizes and translocates to the plasma membrane

6. Membrane disruption: MLKL oligomers permeabilize the plasma membrane → necroptotic cell death (lytic, inflammatory) ^{4Ars Moriendi; the art of dying well — new insights into the molecular pathways of necroptotic cell death2014 · EMBO Rep · DOI 10.15252/embr.201337970Open reference}

Physiological Roles

• Embryonic development: Ripk1 knockout mice die postnatally with hyperinflammatory features

• Immune regulation: RIPK1 balances survival and death in immune cell populations

• Barrier function: RIPK1 maintains intestinal and epidermal barrier integrity

• Host defense: Necroptosis limits pathogen spread

Role in Neurodegeneration

Alzheimer’s Disease

RIPK1 is activated in AD brain and drives both neuronal loss and neuroinflammation ^{5RIPK1 mediates a disease-associated microglial response in Alzheimer's disease2017 · Proc Natl Acad Sci U S A · PMID 28923949Open reference}:

Microglial RIPK1:

• RIPK1 is activated in microglia surrounding amyloid plaques

• Activated microglia release inflammatory cytokines (TNF-α, IL-1β) via RIPK1-dependent mechanisms

• RIPK1+ microglia are found in postmortem AD brain — the proportion correlates with disease severity

Neuronal RIPK1:

• Aβ oligomers activate RIPK1 in neurons via TNF-α signaling

• RIPK1-dependent necroptosis contributes to neuronal loss in AD models

• RIPK1 activation may also be triggered by DAMPs from dead cells

Therapeutic evidence:

• RIPK1 inhibitors (necrostatin-1s, GSK2982772) reduce Aβ pathology and improve cognition in AD mouse models

• Genetic deletion of Ripk1 in microglia reduces neuroinflammation in 5xFAD mice

Parkinson’s Disease

RIPK1 contributes to dopaminergic neuron death and neuroinflammation in PD ^{6Pharmacological inhibition of necroptosis protects from dopaminergic neuronal cell death in Parkinson's disease models2018 · Cell Death Dis · PMID 29449548Open reference}:

• Elevated RIPK1 in SNc: RIPK1 expression and activation are elevated in PD substantia nigra

• α-Synuclein activates RIPK1: Fibrillar α-synuclein triggers microglial TNF-α production that activates neuronal RIPK1

• Dopaminergic neuron sensitivity: The substantia nigra pars compacta microenvironment is enriched with activated microglia capable of driving RIPK1-mediated neuronal death

Protection by RIPK1 inhibition:

• Necrostatin-1s protects dopaminergic neurons in MPTP and 6-OHDA mouse models

• RIPK1 knockdown or knockout reduces neuronal death in PD models

• DNL747 (brain-penetrant RIPK1 inhibitor) is in clinical trials for ALS and AD

ALS

Necroptosis markers (p-RIPK1, p-RIPK3, p-MLKL) are elevated in ALS spinal cord:

• Motor neuron death: RIPK1/RIPK3/MLKL axis contributes to motor neuron degeneration in SOD1, TDP-43, and FUS models

• TDP-43 pathology: Cytoplasmic TDP-43 aggregates may trigger RIPK1 activation through ER stress pathways

• SOD1 mutations: Mutant SOD1 directly interacts with RIPK1 and RIPK3, promoting necroptosis

• Non-cell autonomous toxicity: Microglial RIPK1 drives neurotoxic inflammation

Clinical development:

• DNL747 (Denali): Brain-penetrant RIPK1 inhibitor, entered Phase 1b/2a for ALS and AD

• Other RIPK1 inhibitors in development for ALS and related conditions

Multiple Sclerosis (MS) and EAE

• Oligodendrocyte necroptosis: RIPK1 activation in oligodendrocytes contributes to demyelination

• Axonal loss: Necroptosis of neurons and glia contributes to axonal damage in MS lesions

• Neuroinflammation: RIPK1 sustains inflammatory microglial activation in the CNS

• EAE model: RIPK1 inhibitors (necrostatin-1s, GSK’772) reduce disease severity, demyelination, and axonal loss

Therapeutic Targeting

RIPK1 Kinase Inhibitors

GSK2982772 showed dose-proportional pharmacokinetics, good tolerability, and target engagement in healthy volunteers and patients with rheumatoid arthritis. It is in Phase 2 trials for inflammatory diseases and is being evaluated for CNS indications.

DNL747 was specifically developed for brain penetration and CNS indications. Phase 1 data showed good safety and CNS penetration. It is the most advanced RIPK1 inhibitor for neurodegeneration.

Therapeutic Strategy Considerations

• Partial inhibition: May be preferable to avoid immunosuppression (complete RIPK1 inhibition impairs host defense)

• Early intervention: RIPK1 inhibition is likely most effective when given before extensive cell death

• Combination therapy: RIPK1 inhibition may synergize with anti-inflammatory or neuroprotective agents

• Cell type targeting: Microglial vs. neuronal RIPK1 may have distinct roles — tissue selectivity is desirable

• Necroptosis vs. apoptosis: RIPK1 kinase activity is specifically required for necroptosis — not for apoptosis (which can be driven by RIPK1 scaffolding function)

Protein Interactions

See Also

• RIPK3 Protein

• MLKL Protein

• Necroptosis

• Alzheimer’s Disease

• Parkinson’s Disease

• ALS

• Multiple Sclerosis

• TNFR1 Signaling

• Apoptosis in Neurodegeneration