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Caspase-6 is an executioner cysteine-aspartate protease encoded by the CASP6 gene. It is synthesized as a 34 kDa inactive zymogen (procaspase-6) that requires proteolytic processing to generate the active p18/p11 heterodimer. Caspase-6 occupies a unique position among executioner caspases due to its substrate specificity for neurodegeneration-critical proteins: it cleaves tau at Asp421, generating neurotoxic tau fragments found in Alzheimer's disease tangles, and cleaves huntingtin at Asp586, producing the toxic N-terminal fragments essential for Huntington's disease pathogenesis.
Procaspase-6 (293 amino acids) comprises:
- Prodomain (residues 1-23): Short N-terminal peptide removed during activation. Unlike initiator caspases, caspase-6 has a short prodomain without CARD or DED motifs.
- Large subunit (p18) (residues 24-179): Contains the catalytic cysteine (Cys163) in the conserved QACQG pentapeptide motif
- Intersubunit linker (residues 180-193): Removed during activation; contains the processing site Asp193
- Small subunit (p11) (residues 194-293): Completes the active site and contributes to substrate binding
The mature enzyme is a homodimer of p18/p11 heterodimers (p18₂p11₂):
- Two active sites face the same direction in the dimer
- Each active site is formed at the interface between p18 and p11 subunits
- Crystal structures (PDB: 3OD5, 4FXO) reveal the typical caspase fold with a central β-sheet flanked by α-helices
- Substrate specificity pocket accommodates VEID/VEHD sequences (P4-P1 positions)
Caspase-6 has several distinguishing structural properties:
- Self-activation capability: Unlike caspase-3/7, procaspase-6 can undergo intramolecular self-cleavage at Asp179, enabling activation without upstream caspases
- Zinc inhibition site: An allosteric zinc-binding site (His121, Cys163, Cys263) allows physiological regulation by zinc ions
- Conformational flexibility: The active-site loop (L2') adopts an extended conformation not seen in other executioner caspases, contributing to distinct substrate specificity
Caspase-6 prefers the recognition sequence VEID↓X (Val-Glu-Ile-Asp):
- Optimal P4: hydrophobic (Val, Leu)
- P3: Glu (distinguishes from caspase-3 preference for Asp at P3)
- P2: Ile or hydrophobic
- P1: Asp (absolutely required)
- This sequence differs from caspase-3 (DEVD) and caspase-7 (DEVD), conferring distinct substrate selectivity
¶ Function and Signaling
In the classical apoptotic cascade:
- Intrinsic pathway: Cytochrome c → apoptosome → caspase-9 → caspase-3 → caspase-6
- Extrinsic pathway: Death receptors → caspase-8 → caspase-6 (direct) or → caspase-3 → caspase-6
- Caspase-1 can directly activate caspase-6, linking inflammation to apoptosis
Caspase-6 creates a positive feedback loop by cleaving and activating caspase-8, which in turn activates more caspase-6. This caspase-6/caspase-8 amplification circuit is particularly active in neurons and contributes to the irreversibility of the apoptotic program.
Tau cleavage (D421):
- Caspase-6 cleaves tau at Asp421, removing the C-terminal 20 amino acids
- Truncated tau (Tau-ΔC, 1-421) has:
- Enhanced aggregation propensity (~3-fold faster fibril formation)
- Increased toxicity to cultured neurons
- Greater ability to seed tau aggregation
- Reduced microtubule binding affinity
- Caspase-6-cleaved tau is one of the earliest post-translational modifications detected in AD brains
Huntingtin cleavage (D586):
- Caspase-6 cleaves huntingtin at Asp586, generating:
- N-terminal fragment (1-586) that is toxic when it contains expanded polyQ
- The caspase-6 cleavage fragment is the predominant toxic species in HD models
- YAC128 mice with D586A-resistant huntingtin are completely protected from neurodegeneration
Other neuronal substrates:
- Lamin A/C: Nuclear envelope collapse
- α-Tubulin: Microtubule destabilization and axonal transport failure
- Presenilin-1: Modulation of γ-secretase and amyloid-β production
- VDAC1: Mitochondrial permeabilization
- DJ-1: Oxidative stress response impairment
Caspase-6 has important roles beyond cell death:
- Axon pruning: Mediates selective axon degeneration during development via localized activation
- Synaptic depression: Low-level caspase-6 activity at synapses participates in AMPA receptor internalization and long-term depression (LTD)
- Inflammasome signaling: Activates the NLRP3 inflammasome complex
- B cell development: Required for normal B lymphocyte activation and differentiation
Active caspase-6 is intimately linked to AD pathology:
- Immunohistochemistry reveals active caspase-6 in neuritic plaques, neuropil threads, and neurofibrillary tangles
- Active caspase-6 is detected in mild cognitive impairment (MCI) cases before clinical AD diagnosis
- Caspase-6-cleaved tau (TauC6 neoepitope) colocalizes with PHF-1 (phospho-tau) in tangles
- Higher caspase-6 activity in hippocampal lysates predicts lower cognitive performance
- Caspase-6 activation correlates with APOE ε4 carrier status
- Caspase-6 cleavage of mutant huntingtin at D586 is required for disease onset in mice
- Active caspase-6 is elevated in presymptomatic and symptomatic HD striatum
- Preventing caspase-6 cleavage of huntingtin is sufficient to prevent motor deficits, cognitive decline, and striatal atrophy in YAC128 mice
- This has made caspase-6-resistant huntingtin a therapeutic strategy for HD
| Partner |
Interaction Type |
Consequence |
| Tau |
Substrate (D421) |
Toxic truncated tau |
| Huntingtin |
Substrate (D586) |
Toxic N-terminal fragments |
| Caspase-8 |
Substrate + activator |
Feedback amplification |
| Caspase-3 |
Upstream activator |
Apoptotic cascade |
| Caspase-1 |
Upstream activator |
Inflammation-apoptosis link |
| Lamin A/C |
Substrate |
Nuclear breakdown |
| XIAP |
Inhibitor |
Negative regulation |
| Zinc (Zn²⁺) |
Allosteric inhibitor |
Physiological regulation |