Caspase-6 Products Available By Family
The Enigmatic Caspase-6: Elucidating Function
Caspase-6 is a member of the cysteine-aspartic acid protease (caspase) family that is best known for orchestrating apoptosis. The apoptotic caspases are categorized as upstream initiators (caspases 2, 8 and 9) or downstream executioners (caspases 3, 6, and 7). Historically, it has been the most ambiguous and difficult to categorize as caspase-6 is only weakly apoptotic. However, over-expression does result in apoptosis. It is interesting to note that caspase-6 has been reported to act upstream of the initiator caspase-8, which would actually place it outside of the traditional executioner category. The ambiguousness as well as apparent complexity of caspase-6 continues to perk the interest of researchers worldwide.
Caspase-6 and Neurodegenerative Diseases
It is now fairly certain that caspase-6 plays important roles in neurodegenerative diseases including:
- Alzheimer’s disease (AD). Caspase-6 cleaves the amyloid precursor protein (APP) at position D664, leading to the production of the toxic APP-C31 fragment found in AD. Blocking the D664 cleavage site in mice has a protective effective, implicating a causal role for caspase-6 activity in AD pathology.
- Huntington’s disease (HD). Caspase-6 cleaves the pathological polyglutamine-expanded Huntingtin protein characteristic of HD. It is interesting to note that the cleavage site is unique at D586; this site is not recognized by caspase-3. Caspase-6 cleavage of Huntingtin is thought to be required for the development of HD associated behavioral and neurological pathology. Blocking the cleavage site protects mice from neural dysfunction implicating a causal relationship between caspase-6 activity and HD.
Phosphorylation inactivates Caspase-6
Phosphorylation, a post-translational modification, is increasingly being recognized as an essential form of caspase regulation. It was first reported to inactivate caspase-9 in 1998, but has now been found to also regulate caspase-2, 6, 7, and 8 activity. Caspase-6 has a single phosphorylation site located at serine 257. Phosphorylated S257 (pS257) by the kinase ARK5 results in inactivation, which is sufficient to suppress Akt dependent cell death mediated by the death receptor complex.
Prevention of Caspase-6 Auto-processing Blocks Non-apoptotic Roles
It is interesting that auto-processing or self-activation of caspase-6 does not result in apoptosis. This is in contrast to a number of other caspases where auto-processing contributes to the apoptotic cascade. pS257 can prevent caspase-6 auto-processing. Hence, any non-apoptotic caspase-6 dependent activity that relies on auto-processing will be blocked by pS257. This includes cleavage of APP and Huntingtin proteins. This implicates that caspase-6 phosphorylation events may have a protective role in neurodegenerative diseases.
Caspase-6 as a Model System for Studying Phosphorylation Events
Collectively, phosphorylation occurs in all structural domains of caspases and is thought to block activity by preventing formation of the active, cleaved caspase. However, the structural details remain to be fully elucidated. Caspase-6 provides a unique opportunity to study structural mechanisms because it apparently has only one phosphorylation site, S257. Current research suggests that pS257 inhibits both active caspase-6 and the processing of procaspase-6. Apparently steric clash between pS257 and proline 201 inhibits its activity. In essence, phosphorylation of S257 causes misalignment of the four substrate-binding grove loops, preventing substrate binding and activation. Loop misalignment by phosphorylation as a mechanism of inhibiting activation has now been predicted for other caspases.
Caspase-6 S257 as a Drug Target
Since pS257 can inhibit both apoptotic and neurodegenerative functions, it is under investigation as a potential therapeutic target for inhibiting disease-causing caspase-6 activity . One goal would be to lock caspase-6 into the pS257 conformation to prevent activation. This paradigm is thought to have particular promise for treatment of neurodegenerative diseases in which caspase-6 plays a role.
Caspase 6 Factoids
-Caspase-6 orthologs have been identified in a number of mammals. Orthologs are genes in different species that evolved from a common ancestral gene by speciation, and typically retain the same function throughout the course of evolution. Hence caspase-6 would be expected to have the same function among the various orthologs. Unique orthologs are present in birds, lizards, amphibians, and teleosts.
-Caspase-6 is cleaved and activated by caspases 7, 8, and 10.
-Caspase-6 can also undergo self-processing, that is self cleavage and activation. However, this mechanism does not apparently contribute to apoptotic processes.
-The IAP family of inhibitors plays a key role regulating caspases. XIAP is the most well studied and inhibits caspases 3 and 7, caspase-6’s closest homologs, by binding to their active sites. XIAP inhibits caspase-9 by blocking self dimerization rather than active-site binding. It is interesting to note that caspase-6 is the only executioner caspase resistant to XIAP-mediated inhibition at either the active site or dimer interface.
-Caspase-6 mediates axon pruning through the DR6 death receptor pathway during neural development.
-The caspase-6 gene maps on chromosome 4, at 4q25. Alternative splicing of the gene results in transcript variants that encode different isoforms. However, the number splice variants remains to be fully elucidated and reports range from two to at least eight splice variants.
Caspase-6 antibodies are available for studying caspase-6 expression and role in apoptosis and neurodegenerative disease.
1. Phosphorylation regulates assembly of the caspase-6 substrate-binding groove. Velazquez EM and JA Hardy. Structure 20:742-751 (2012).
2. Inhibitory mechanism of RIPK-dependent necrosis and it regulation by caspases: A mystery in five acts. Green DR, A Oberst, CP Dillon, R Weinlich, GS Salvesen. 44:9-116 (2011).