Caspase-8 (CASP8)

Caspase-8 (CASP8)

Overview

Caspase-8 (encoded by the CASP8 gene) is the initiator caspase of the extrinsic apoptosis pathway, activated at death receptor complexes. As a member of the cysteine-aspartic protease family, caspase-8 plays a central role in transducing extracellular death signals into intracellular proteolytic events that execute programmed cell death [@muzio1996]. Its activation at the death-inducing signaling complex (DISC) triggers both the extrinsic apoptotic pathway (direct activation of executioner caspases) and the intrinsic mitochondrial pathway (via Bid cleavage). Beyond apoptosis, caspase-8 regulates necroptosis (by cleaving RIPK1 to prevent kinase-driven necrosis), influences cell proliferation and differentiation, and modulates immune cell activation and inflammation. Dysregulated caspase-8 activity contributes to cancer (insufficient activity allows tumor cell survival) and to neurodegenerative diseases (excessive activation causes neuronal death) [@kelley2001].

Caspase-8 (CASP8)

Overview

Caspase-8 (encoded by the CASP8 gene) is the initiator caspase of the extrinsic apoptosis pathway, activated at death receptor complexes. As a member of the cysteine-aspartic protease family, caspase-8 plays a central role in transducing extracellular death signals into intracellular proteolytic events that execute programmed cell death [@muzio1996]. Its activation at the death-inducing signaling complex (DISC) triggers both the extrinsic apoptotic pathway (direct activation of executioner caspases) and the intrinsic mitochondrial pathway (via Bid cleavage). Beyond apoptosis, caspase-8 regulates necroptosis (by cleaving RIPK1 to prevent kinase-driven necrosis), influences cell proliferation and differentiation, and modulates immune cell activation and inflammation. Dysregulated caspase-8 activity contributes to cancer (insufficient activity allows tumor cell survival) and to neurodegenerative diseases (excessive activation causes neuronal death) [@kelley2001].

<div class="infobox infobox-protein">
<table>
<tr><th colspan="2">Caspase-8 Protein</th></tr>
<tr><td>Protein Name</td><td>Caspase-8 (FLICE, MACH)</td></tr>
<tr><td>Gene</td><td>[CASP8](/genes/casp8)</td></tr>
<tr><td>UniProt ID</td><td>[Q14790](https://www.uniprot.org/uniprot/Q14790)</td></tr>
<tr><td>Molecular Weight</td><td>~55 kDa (procaspase-8), ~43 kDa (active p18/p10)</td></tr>
<tr><td>Subcellular Localization</td><td>Cytoplasm, plasma membrane (DISC)</td></tr>
<tr><td>Protein Family</td><td>Caspase family (cysteine protease)</td></tr>
<tr>
<td class="label">Associated Diseases</td>
<td><a href="/wiki/aging" style="color:#ef9a9a">Aging</a>, <a href="/wiki/als" style="color:#ef9a9a">Als</a>, <a href="/wiki/alzheimer" style="color:#ef9a9a">Alzheimer</a>, <a href="/wiki/autoimmune" style="color:#ef9a9a">Autoimmune</a>, <a href="/wiki/cancer" style="color:#ef9a9a">Cancer</a></td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">153 edges</a></td>
</tr>
</table>
</div>

Structure

Caspase-8 is synthesized as a zymogen (procaspase-8) of 479 amino acids with a characteristic domain architecture:

Protein Domains

Activation Mechanism

Procaspase-8 activation involves:

Caspase-8 also exists in a stable (p43)1 form, generated by cleavage at Asp210, which remains associated with the DISC and may have non-apoptotic signaling functions.

Normal Function

Death Receptor Signaling Pathway

Caspase-8 is the central mediator of death receptor-initiated apoptosis [@tummers2007]:

Substrate Specificity

Caspase-8 has a preference for the tetrapeptide sequence (I/V/L)X(E/D)X(D/I) and cleaves many substrates:

• Executioner caspases (caspase-3, -6, -7): Direct cleavage activates these effector caspases, which execute the apoptotic program
• Bid: Cleavage of the BH3-only protein Bid generates tBid (truncated Bid), which translocates to mitochondria and induces cytochrome c release — this links extrinsic signaling to the intrinsic mitochondrial pathway
• PARP: Cleavage of poly(ADP-ribose) polymerase inactivates DNA repair, committing the cell to death
• Structural proteins: Lamin A/C, actin, beta-catenin — dismantling of cellular architecture
• RIPK1: Cleavage separates the kinase domain from the death domain, preventing necroptosis

Regulation of Necroptosis

Caspase-8 is a critical suppressor of necroptosis [@liang2020]:

• RIPK1 cleavage: Caspase-8 cleaves RIPK1 at Asp324, preventing the assembly of the necrosome (RIPK1-RIPK3-MLKL complex)
• Switch function: Caspase-8 acts as a molecular switch between apoptosis (sufficient activity) and necroptosis (caspase-8 inhibited or absent)
• Necroptosis in neurodegeneration: In some contexts, caspase-8 inhibition leads to necroptotic neuronal death, which is particularly inflammatory and damaging

Non-Apoptotic Functions

• Cell proliferation: Caspase-8 activates NF-κB via the IKK complex, promoting cell survival and proliferation
• T cell activation: Required for activation-induced cell death (AICD) in T lymphocytes
• Embryonic development: Casp8 knockout mice die in utero with cardiac defects and impaired vascular development
• Immune regulation: Modulates macrophage polarization and cytokine production

Role in Neurodegeneration

Alzheimer's Disease

Caspase-8 activation contributes to neuronal apoptosis in AD:

• Aβ-induced activation: Amyloid-beta oligomers activate caspase-8 in cultured neurons and in AD brain tissue
• Death receptor involvement: Fas/FasL signaling is upregulated in AD brain, contributing to caspase-8 activation
• Synaptic damage: Caspase-8 cleavage of synaptic proteins (Arc, PSD95) may contribute to early synaptic dysfunction
• Cross-talk with tau: Caspase-8 can activate executioner caspases that promote tau cleavage and aggregation
• Therapeutic targeting: Caspase-8 inhibitors (z-IETD-fmk, specific peptides) reduce neuronal death in AD models

Parkinson's Disease

In PD, caspase-8 is activated in dopaminergic neurons undergoing degeneration:

• Death receptor activation: Fas, TNF-R1, and DR4/DR5 are expressed on dopaminergic neurons and can activate caspase-8
• Mitochondrial links: The Bid cleavage pathway connects extrinsic signaling to mitochondrial dysfunction, a hallmark of PD
• Endoplasmic reticulum stress: ER stress triggers caspase-8 activation in dopaminergic neurons via mechanisms that remain partially characterized
• Neuroinflammation: Microglial-derived TNF-α and FasL can activate caspase-8 in adjacent neurons

Glaucoma and Optic Neuropathy

Caspase-8 is activated in retinal ganglion cells (RGCs) during glaucomatous neurodegeneration [@booth2008]:

• Axonal insult: Elevated intraocular pressure causes axonal transport disruption, leading to caspase-8 activation in RGCs
• ProBDNF signaling: The p75NTR proBDNF pathway activates caspase-8 in retinal neurons
• Therapeutic potential: Caspase-8 inhibition is neuroprotective in mouse models of glaucoma

Stroke and Ischemia

Ischemic brain injury prominently activates caspase-8:

• Excitotoxicity: Glutamate-induced overactivation of NMDA receptors triggers caspase-8 activation
• Fas/FasL upregulation: Ischemia induces Fas expression on neurons and FasL on endothelial cells and microglia
• Dual pathways: Both direct caspase-8 activation (extrinsic) and Bid-mediated mitochondrial amplification (intrinsic) contribute to post-ischemic neuronal death

Huntington's Disease

Caspase-8 is activated in models of Huntington's disease:

• Mutant huntingtin aggregation: Can trigger extrinsic apoptosis
• Death receptor upregulation: Fas and TRAIL-R are increased in HD models
• Bid cleavage: Contributes to mitochondrial dysfunction
• Therapeutic targeting: Caspase-8 inhibition is protective in some models

Amyotrophic Lateral Sclerosis

Caspase-8 is activated in ALS models and patient tissue:

• SOD1 mutations: Trigger caspase-8 activation in motor neurons
• TDP-43 pathology: Associated with caspase-8 activation
• Death receptor involvement: Fas signaling contributes to motor neuron death
• Astrocyte contributions: Mutant astrocytes trigger caspase-8 in co-cultured neurons

Multiple Sclerosis

Caspase-8 has dual roles in autoimmune demyelination:

• Oligodendrocyte death: Caspase-8 contributes to myelin loss
• T cell activation: Required for activation-induced cell death (AICD)
• Remyelination failure: Caspase-8 inhibition may protect oligodendrocyte progenitors

COVID-19 Neurological Complications

Emerging evidence links COVID-19 to caspase-8 activation:

• Systemic inflammation: Cytokine storm includes TNF-α, activating caspase-8
• Blood-brain barrier: FasL can disrupt BBB
• Microglial activation: Caspase-8 in activated microglia

Molecular Mechanisms in Disease

Death Receptor Regulation in Neurodegeneration

The extrinsic pathway is regulated at multiple levels:

Receptor Expression

Death receptor expression is dynamic in disease:

• Fas (CD95): Upregulated in AD, PD, HD, and stroke
• TNF-R1: Increased in neuroinflammation
• DR4/DR5 (TRAIL-R): Variable changes

Ligand Expression

• FasL: Expressed on activated microglia, T cells, and some neurons
• TNF-α: Released by activated microglia and astrocytes
• TRAIL: Expressed in immune cells

Decoy Receptors

• DcR1/DcR2: Decoy receptors that sequester ligands
• Soluble Fas: Alternative splicing generates soluble Fas
• Viral proteins: Some viruses express decoyDeath

c-FLIP Regulation

c-FLIP (CFLAR) is a master regulator of caspase-8:

• c-FLIPL: Long isoform, catalytically inactive
• c-FLIPS: Short isoform, dominant-negative
• Regulation: c-FLIP is downregulated in some neurodegenerative conditions
• Therapeutic potential: c-FLIP overexpression protects neurons

Bid Regulation in Neurodegeneration

Bid connects extrinsic and intrinsic pathways:

• Full-length Bid: Inactive until cleaved by caspase-8
• tBid: Truncated form, traffics to mitochondria
• Mitochondrial amplification: tBid triggers cytochrome c release
• Therapeutic targeting: Bid inhibitors block amplification

Non-Caspase Substrates

Caspase-8 cleaves substrates beyond executioner caspases:

| Substrate | Cleavage Consequence |
|-----------|---------------------|
| RIPK1 | Prevents necroptosis (protective) |
| ROCK1 | Alters cytoskeleton |
| MALT1 | Modulates NF-κB |
| p75NTR | Generates pro-apoptotic fragment |
| Presenilins | May affect γ-secretase |

Necroptosis Suppression

Caspase-8 is a critical suppressor of necroptosis:

• RIPK1 cleavage: Prevents necrosome formation
• RIPK3 binding: Blocks MLKL activation
• Switch function: Low caspase-8 → necroptosis
• Therapeutic implications: Caspase-8 inhibition can cause necroptosis

Therapeutic Targeting

Caspase-8 Inhibitors

| Inhibitor | Type | Specificity | Status |
|-----------|------|-------------|--------|
| z-IETD-fmk | Peptide mimetic | Caspase-8 specific | Research tool only |
| Ac-IETD-CHO | Peptide aldehyde | Caspase-8 specific | Research tool only |
| CrmA (SerpB) | Viral serpin | Caspase-8 and granzyme B | Research tool only |
| AEVD-fmk | Peptide | Broader caspase inhibitor | Research tool only |

Clinical challenge: Broad caspase inhibition risks immunosuppression and tumor promotion. Selective targeting of neuronal caspase-8 activation is difficult.

Alternative Approaches

• Decoy receptors: Soluble Fas/Fc proteins that sequester FasL before it reaches the receptor
• Death receptor blocking antibodies: Anti-Fas or anti-DR4/DR5 antibodies can block ligand binding
• Upstream modulation: Reducing TNF-α or FasL production (anti-inflammatory approaches)
• Bid inhibitors: Preventing Bid cleavage blocks the extrinsic-to-intrinsic amplification
• RIPK1 inhibitors (Necrostatin-1, GSK'872): Indirectly prevent caspase-8-mediated necroptosis suppression

Neuroprotective Strategies

Rather than globally inhibiting caspase-8, more targeted approaches include:

• Substrate-specific inhibitors: Blocking only harmful cleavages while preserving necessary functions
• Cell-penetrating peptides: Targeting the DISC or specific neuronal populations
• Gene therapy: siRNA or shRNA targeting CASP8 specifically in neurons
• Combination: Low-dose caspase-8 inhibition + anti-inflammatory + anti-excitotoxic approaches

Protein Interactions

| Partner | Interaction Type | Functional Consequence |
|---------|----------------|----------------------|
| FADD | DED-DED binding | DISC recruitment, activation |
| Fas (CD95) | Indirect (via FADD) | Death receptor signaling |
| TNF-R1 | Indirect (via FADD) | TNF-α signaling |
| DR4/DR5 | Indirect (via FADD) | TRAIL signaling |
| RIPK1 | Substrate cleavage | Necroptosis suppression |
| RIPK3 | Protein interaction | Blocks necrosome formation |
| Caspase-3 | Substrate cleavage | Executioner caspase activation |
| Caspase-7 | Substrate cleavage | Executioner caspase activation |
| Bid | Substrate cleavage | Links to mitochondrial pathway |
| c-FLIP (CFLAR) | Heterodimerization | Catalytic inhibition (dominant-negative) |
| IKK complex | Substrate cleavage | NF-κB activation (non-apoptotic) |
| PARP1 | Substrate cleavage | DNA repair inactivation |
| p75NTR | Receptor | proBDNF-mediated activation |

See Also

• [CASP8 Gene](/genes/casp8)
• [Extrinsic Apoptosis Pathway](/mechanisms/extrinsic-apoptosis)
• [Death-Inducing Signaling Complex (DISC)](/mechanisms/disc)
• [Apoptosis in Neurodegeneration](/mechanisms/apoptosis-neurodegeneration)
• [FADD Protein](/proteins/fadd-protein)
• [Bid and Neurodegeneration](/mechanisms/bid-signaling)
• [Alzheimer's Disease](/diseases/alzheimers-disease)
• [Parkinson's Disease](/diseases/parkinsons-disease)
• [Glaucoma](/diseases/glaucoma)
• [Necroptosis](/mechanisms/necroptosis)

References