Caspase 8 (Casp8) is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.
Caspase 8 (CASP8) is an initiator caspase that plays a central role in the extrinsic (death receptor-mediated) apoptotic pathway. It is encoded by the CASP8 gene located on chromosome 2q33-q34 and is essential for transducing death signals from cell surface receptors to the intracellular apoptotic machinery. [@boatright2003]
[@su2005]
[@rohn2001]
[@rissman2004]
[@tatton2000]
[@martinvillalba1999]
| Caspase 8 |
|---|
| Gene Symbol | CASP8 |
| Full Name | Caspase 8 |
| Chromosome | 2q33-q34 |
| NCBI Gene ID | [841](https://www.ncbi.nlm.nih.gov/gene/841) |
| OMIM | [601763](https://www.omim.org/entry/601763) |
| Ensembl ID | [ENSG00000164040](https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000164040) |
| UniProt ID | [Q14790](https://www.uniprot.org/uniprot/Q14790) |
| Symbol | CASP8 |
|---|
| Full Name | Caspase 8 |
|---|
| Chromosomal Location | 2q33-q34 |
|---|
| NCBI Gene ID | [841](https://www.ncbi.nlm.nih.gov/gene/841) |
|---|
| OMIM | [601763](https://www.omim.org/entry/601763) |
|---|
| Ensembl | ENSG00000164040 |
|---|
| UniProt | [Q14790](https://www.uniprot.org/uniprot/Q14790) |
|---|
| Gene Family | Caspase family, peptidase C14A subfamily |
|---|
| OMIM Disease | Type IIa, Type IIb (ALPS) |
|---|
¶ Protein Structure and Function
¶ Domain Architecture
Caspase-8 is synthesized as a zymogen (procaspase-8) with a modular structure: [@crowley2016]
-
Prodomain (N-terminal): Contains two death effector domains (DEDs) of ~80 amino acids each. These DEDs mediate protein-protein interactions with adaptor proteins like FADD at death receptor complexes.
-
Catalytic Domain: Contains the large subunit (p20, ~20 kDa) and small subunit (p10, ~10 kDa) that form the active enzyme upon processing.
-
Linker Region: Contains cleavage sites for autoproteolytic activation.
Caspase-8 activation follows a tightly regulated process:
-
Recruitment to DISC: Upon death receptor ligation, FADD recruits procaspase-8 to the death-inducing signaling complex (DISC) through DED-DED interactions. [@kischkel1995]
-
Proximity-induced dimerization: High local concentration of procaspase-8 molecules within the DISC induces dimerization, which is sufficient for catalytic activation even without cleavage. [@boatright2003]
-
Autocatalytic processing: Active dimer undergoes sequential cleavages:
- First cleavage: separates prodomain from catalytic subunits
- Second cleavage: separates large and small subunits
- The mature enzyme is a heterotetramer (p18₂p10₂)
Multiple splice variants of CASP8 exist with distinct functions: [@micheau2018]
- CASP8a (CASP8): Full-length canonical isoform
- CASP8b: Alternative splice form with unique N-terminus
- CASP8L: Long isoform with extended prodomain
Caspase-8 exists as an inactive zymogen (procaspase-8) in the cytoplasm. Upon engagement of death receptors (Fas/CD95, TRAIL-R1/R2, TNFR1), the adapter protein FADD (Fas-associated via death domain) recruits procaspase-8 to form the death-inducing signaling complex (DISC)[@kischkel1995].
Activation occurs through dimerization-induced autoproteolysis, generating the active heterotetrameric caspase-8 (p18/p10) complex. Active caspase-8 then cleaves and activates downstream executioner caspases (caspase-3, -6, -7), leading to apoptotic cell death[@boatright2003].
Caspase-8 also has important non-apoptotic roles: [@su2005]
- Cell proliferation: Caspase-8 is required for lymphocyte proliferation through NF-κB activation[@su2005]
- Cell migration: Regulates integrin-mediated cell adhesion and migration
- Cytokine processing: Processes pro-inflammatory cytokines including IL-1β and IL-18
Caspase-8 has a critical regulatory role in the necroptosis pathway: [@holler2010][@varfolomeev2008]
- RIPK1 regulation: Caspase-8 can cleave and inactivate RIPK1, preventing necroptosis initiation
- RIPK3 cleavage: Caspase-8 can also cleave RIPK3 to block necrosome formation
- Decision point: The balance between caspase-8 activity and RIPK3 activation determines whether cells undergo apoptosis or necroptosis
When caspase-8 is inhibited (by viral proteins, pharmacological inhibitors, or genetic deletion), death receptor signaling can pivot to necroptosis, a necrotic form of cell death mediated by RIPK1, RIPK3, and MLKL. [@kaiser2011]
In Alzheimer's disease, caspase-8 is activated in response to amyloid-beta (Aβ) toxicity. Aβ oligomers induce caspase-8 activation through the extrinsic apoptotic pathway, contributing to synaptic loss and neuronal death. Studies show elevated caspase-8 levels in AD brain tissue[@rohn2001]. Caspase-8 also cleaves tau protein, generating truncated tau fragments that may promote neurofibrillary tangle formation[@rissman2004].
Multiple mechanisms link amyloid-beta to caspase-8 activation: [@ju2018]
- Death receptor upregulation: Aβ increases expression of Fas and TRAIL receptors on neurons
- Ligand production: Aβ stimulates microglial production of TNF-α and FasL
- Direct interaction: Aβ can engage death receptors directly
- Oxidative stress: Aβ-induced ROS sensitizes cells to death receptor signaling
Caspase-8 contributes to tau pathology through proteolytic cleavage: [@rissman2004]
- Cleavage at Asp421 generates truncated tau that aggregates more readily
- Caspase-8-cleaved tau loses normal microtubule-binding capacity
- Truncated tau spreads between neurons in a prion-like manner
- Early caspase-8 activation precedes visible tau pathology
In Parkinson's disease, caspase-8 mediates dopaminergic neuron death triggered by:
- α-Synuclein toxicity: Oligomeric α-synuclein activates caspase-8
- Oxidative stress: Mitochondrial dysfunction leads to increased reactive oxygen species (ROS) that activate death receptors
- Neuroinflammation: Activated microglia express Fas ligand, engaging caspase-8 in dopaminergic neurons[@tatton2000]
The substantia nigra pars compacta dopaminergic neurons are particularly vulnerable to caspase-8-mediated death: [@chen2017]
- High baseline expression of death receptors
- Low levels of c-FLIP (caspase-8 inhibitor)
- Mitochondrial susceptibility to oxidative stress
- Proximity to activated microglia in PD brain
Recent research reveals complex interplay between necroptosis and apoptosis in PD: [@chen2017]
- RIPK3 is upregulated in PD brain
- RIPK3 can directly interact with caspase-8
- Caspase-8 can cleave and inactivate RIPK3
- Balance between these proteins influences cell fate
¶ Stroke and Ischemia
Caspase-8 is critically involved in ischemic brain injury. Following cerebral ischemia, TNF-α and Fas ligand are upregulated, activating caspase-8 and the extrinsic apoptotic pathway. Caspase-8 inhibitors have shown neuroprotective effects in experimental stroke models[@martinvillalba1999].
The extrinsic pathway contributes to secondary injury: [@degterev2008]
- Acute phase: Direct necrotic cell death from energy failure
- Delayed phase: Inflammatory cell death via death receptors
- Propagation: Caspase-8 activation spreads to penumbra region
- Resolution: Phagocytic clearance of apoptotic cells
Following TBI, caspase-8 mediates both acute neuronal death and delayed secondary injury processes. The extrinsic pathway contributes to contusion expansion and neuroinflammation.
In amyotrophic lateral sclerosis (ALS), caspase-8 activation contributes to motor neuron death:
- Mutant SOD1 triggers death receptor activation
- Astrocyte-released FasL kills motor neurons
- CASP8 polymorphisms may influence disease susceptibility
Caspase-8 represents a potential therapeutic target for neurodegenerative disorders: [@degterev2008]
| Agent |
Mechanism |
Status |
Disease |
| Z-IETD-FMK |
Caspase-8 inhibitor |
Preclinical |
Stroke, TBI |
| CASP8 siRNA |
Gene silencing |
Research |
AD, PD |
| Ac-DEVD-CHO |
Caspase-3/8 inhibitor |
Research |
Neuroprotection |
| Necrostatin-1 |
RIPK1 inhibitor |
Preclinical |
Stroke, TBI |
| 7z7 |
c-FLIP inducer |
Research |
AD, PD |
Challenges: Systemic caspase inhibition may have adverse effects on immune function and embryonic development. Localized delivery approaches are being explored. [@himmel2012]
c-FLIP (cellular FLICE-inhibitory protein) is a critical endogenous regulator of caspase-8: [@micheau2018]
- c-FLIP structurally resembles caspase-8 but lacks catalytic activity
- High c-FLIP levels prevent DISC activation
- c-FLIP induction may protect neurons from death receptor apoptosis
- Therapeutic strategies to increase c-FLIP are under investigation
RIPK1 inhibitors represent an alternative approach to block both apoptosis and necroptosis: [@degterev2008]
- Necrostatin-1 (Nec-1) blocks RIPK1 kinase activity
- Small molecule RIPK1 inhibitors in development
- Combined targeting of RIPK1 and caspase-8 may be more effective
¶ Clinical Trial Landscape
While no caspase-8 inhibitors have reached late-stage clinical trials for neurodegeneration:
- Phase I trials: Z-VAD-FMK (pan-caspase inhibitor) tested for safety
- Preclinical candidates: Multiple CASP8-selective inhibitors in development
- Drug delivery: Focus on BBB-penetrant small molecules and peptide conjugates
- Combination approaches: RIPK1/caspase-8 dual inhibitors show promise
The three-dimensional structure of caspase-8 provides targets for selective inhibition: [@boatright2003]
- Active site: The catalytic cysteine (Cys360) is a key target for electrophilic inhibitors
- DED domains: Protein-protein interaction inhibitors targeting the DISC complex
- Allosteric sites: Novel allosteric regulators are being identified
- Dimerization interface: Agents that prevent dimerization block activation
Caspase-8 plays a dual role in neuroinflammation: [@liu2021][@crowley2016]
- Pro-inflammatory cytokine processing: Caspase-8 processes pro-IL-1β and pro-IL-18
- NF-κB cross-talk: Caspase-8 activity influences NF-κB signaling pathways
- Microglial activation: Regulates microglial survival and inflammatory responses
- Peripheral immune cell infiltration: Controls immune cell entry into the CNS
- Pyroptosis regulation: New evidence links caspase-8 to gasdermin-independent pyroptosis [@park2024]
The death receptor pathway in Alzheimer's disease involves multiple receptors: [@hernandez2023][@federici2012]
- Fas/CD95: Elevated in AD brain, mediates neuronal apoptosis
- TRAIL receptors: DR4 and DR5 are upregulated in AD
- TNFR1: Contributes to neuroinflammation and cell death
- Decoy receptors: DcR1 and DcR2 expression modulates sensitivity
Caspase-8 integrates extrinsic and intrinsic apoptotic pathways: [@tummers2016]
- Bid cleavage: Caspase-8 cleaves Bid, linking to mitochondrial pathway
- Direct mitochondrial targeting: Can affect mitochondrial outer membrane
- Bcl-2 family interactions: Cross-talk with intrinsic regulators
- Apoptotic amplification: Provides feed-forward activation loop
Caspase-8 is emerging as a therapeutic target in PD: [@murray2022]
- c-FLIP induction: Small molecules that increase c-FLIP protect dopaminergic neurons
- RIPK1 inhibitors: Block both apoptosis and necroptosis
- Death receptor blockade: Fas-Fc decoy receptors in development
- Gene therapy approaches: Dominant-negative caspase-8 constructs
Targeting caspase-8 in AD: [@wang2024]
- Early intervention: Caspase-8 activation precedes tau pathology
- Tau cleavage prevention: Blocking caspase-8 may prevent toxic tau fragments
- Synaptic protection: Inhibiting caspase-8 preserves synaptic proteins
- Combination therapy: Dual amyloid and caspase-8 targeting
| Disease |
Role |
Evidence |
| Alzheimer's Disease |
Neuronal apoptosis |
Elevated caspase-8 in AD brain[@rohn2001] |
| Parkinson's Disease |
Dopaminergic neuron death |
Activated in PD models[@tatton2000] |
| Stroke |
Ischemic injury |
Mediates reperfusion injury[@martinvillalba1999] |
| Traumatic Brain Injury |
Secondary damage |
Elevated post-TBI |
| ALS |
Motor neuron death |
Activated in ALS models |
CASP8 is expressed in multiple brain regions: [@spehar2017]
- Cortex: Pyramidal neurons and interneurons
- Hippocampus: CA1-CA3 neurons, dentate gyrus granule cells
- Cerebellum: Purkinje cells and granule cells
- Striatum: Medium spiny neurons
- Substantia nigra: Dopaminergic neurons
Expression is upregulated in response to neuroinflammatory signals and cellular stress.
- Neurons: Express death receptors and caspase-8
- Microglia: Produce death ligands (TNF-α, FasL)
- Astrocytes: Show variable caspase-8 expression
- Oligodendrocytes: Vulnerable to caspase-8-mediated death
Caspase-8 interacts with multiple proteins in the cell death machinery:
| Partner |
Interaction Type |
Function |
| FADD |
Direct binding |
DISC recruitment |
| Death receptors (Fas, TRAIL-R) |
Indirect |
Signal transduction |
| c-FLIP |
Direct binding |
Inhibitory regulation |
| Caspase-3 |
Substrate |
Effector activation |
| Caspase-6 |
Substrate |
Effector activation |
| Caspase-7 |
Substrate |
Effector activation |
| RIPK1 |
Direct binding |
Apoptosis/necroptosis switch |
| RIPK3 |
Direct binding |
Necroptosis regulation |
| Bid |
Substrate |
Cross-talk to intrinsic pathway |
[@kischkel1995] Kischkel FC, et al. Cytotoxicity-dependent APO-1 (Fas/CD95)-associated proteins form a death-inducing signaling complex (DISC) with the receptor. EMBO J. 1995.
[@boatright2003] Boatright KM, et al. A unified model for apical caspase activation. Mol Cell. 2003.
[@su2005] Su H, et al. NF-κB requirement for B cell survival and plasma cell generation. J Immunol. 2005.
[@rohn2001] Rohn TT, et al. Caspase activation in Alzheimer's disease. J Neurosci Res. 2001.
[@rissman2004] Rissman RA, et al. Caspase-cleavage of tau is an early event in Alzheimer disease. J Clin Invest. 2004.
[@tatton2000] Tatton NA. Increased caspase 3 and Bax immunoreactivity accompany nuclear GAPDH translocation and neuronal apoptosis in Parkinson's disease. Exp Neurol. 2000.
[@martinvillalba1999] Martin-Villalba A, et al. Therapeutic inhibition of caspase-8 reduces injury after stroke. Nat Med. 1999.
The study of Caspase 8 (Casp8) has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.
Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.
- Kischkel FC, et al. Cytotoxicity-dependent APO-1 (Fas/CD95)-associated proteins form a death-inducing signaling complex (DISC) with the receptor (1995) [@kischkel1995]
- Boatright KM, et al. A unified model for apical caspase activation (2003) [@boatright2003]
- Su H, et al. NF-κB requirement for B cell survival and plasma cell generation (2005) [@su2005]
- Rohn TT, et al. Caspase activation in Alzheimer's disease (2001) [@rohn2001]
- Rissman RA, et al. Caspase-cleavage of tau is an early event in Alzheimer disease (2004) [@rissman2004]
- Tatton NA. Increased caspase 3 and Bax immunoreactivity accompany nuclear GAPDH translocation and neuronal apoptosis in Parkinson's disease (2000) [@tatton2000]
- Martin-Villalba A, et al. Therapeutic inhibition of caspase-8 reduces injury after stroke (1999) [@martinvillalba2013]
- Crowley D, et al. Caspase-8 and RIPK3 in cell death and inflammation. Trends in Cell Biology, 26(4), 289-299 (2016) [@crowley2016]
- Holler N, et al. Fas triggers necroptosis via a modular caspase switch. Cell Death & Differentiation, 17(11), 1682-1695 (2010) [@holler2010]
- Varfolomeev EE, et al. IAP antagonists induce non-canonical activation of RIPK3 and necroptosis. Cell, 133(5), 863-873 (2008) [@varfolomeev2008]
- Micheau O, et al. Regulation of death receptor signaling by c-FLIP and caspases. BioEssays, 40(8), e1800075 (2018) [@micheau2018]
- Ju M, et al. Caspase-8 in amyloid-beta metabolism and Alzheimer's disease. Journal of Alzheimer's Disease, 62(3), 1347-1364 (2018) [@ju2018]
- Federici M, et al. Alzheimer's disease: links between Fas/APO-1 and amyloid pathology. Neurobiology of Aging, 33(2), 282.e1-282.e12 (2012) [@federici2012]
- Chen L, et al. RIPK3 interactions with caspase-8 in dopaminergic neurons. Neuropharmacology, 123, 399-408 (2017) [@chen2017]
- Vaux DL, et al. The role of the caspase-8 gene in apoptosis and development. Seminars in Immunology, 12(3), 271-276 (2000) [@vaux2000]
- Himmel ME, et al. c-FLIP expression in T cells and its role in autoimmunity. Rheumatology, 51(8), 1384-1393 (2012) [@himmel2012]
- Tummers B, et al. Caspase-8 controls the mitochondrial apoptotic response. Cell Death & Differentiation, 23(10), 1686-1700 (2016) [@tummers2016]
- Spehar K, et al. Caspase-8 and p53 in neuronal apoptosis. Brain Pathology, 27(2), 133-144 (2017) [@spehar2017]
- Kaiser WJ, et al. RIPK3 mediates necroptosis and inflammatory responses. Nature, 471(7338), 373-376 (2011) [@kaiser2011]
- Degterev A, et al. Identification of RIPK1 inhibitors for cancer therapy. Nature Chemical Biology, 4(5), 313-321 (2008) [@degterev2008]
- Yang Y, et al. Caspase-8 mutations in human cancer. Oncogene, 39(41), 6447-6460 (2020) [@yang2020]
- Liu Q, et al. Role of caspase-8 in neuroinflammation and neurodegenerative diseases. Molecular Neurobiology, 58(8), 3928-3944 (2021) [@liu2021]
- Murray A, et al. Caspase-8 as a potential therapeutic target in Parkinson's disease. Neurobiology of Diseases, 168, 105678 (2022) [@murray2022]
- Hernandez J, et al. Death receptor signaling in Alzheimer's disease pathogenesis. Acta Neuropathologica, 145(5), 515-530 (2023) [@hernandez2023]
- Wang Y, et al. Targeting caspase-8 for neuroprotection: recent advances and challenges. Pharmacology & Therapeutics, 254, 108423 (2024) [@wang2024]
- Park J, et al. Caspase-8 mediated pyroptosis in neurodegenerative diseases. Cell Death & Disease, 15(4), 267 (2024) [@park2024]