Abstract

Cysteinyl aspartate specific proteases (caspases) play an important role in development, differentiation, apoptosis and inflammation in metazoa. The 12 known human caspases, members of peptidase family MEROPS:C14, can be classified in 4 groups based on their function and the length of their prodomain. Group I caspases are inflammatory caspases with a large prodomain and includes caspase-1, -4, -5, and -12. Caspases-2, -5, -8, -9, and -10 belong to group II and have also a large prodomain, but initiate apoptosis. Caspase-3, -6, and -7 constitute group III and are effector caspases with a short (20-30 aa) prodomain that execute the apoptotic program by cleavage of various proteins. Caspase-14 is involved in keratinocyte differentiation (Lavrik IN et al. 2005 PMID:16200200, Pop C et al. 2009 PMID:19473994). A general characteristic of caspases is their high specificity to cleave C-terminal after aspartate (Stennicke HR et al. 2000 PMID:10947972). This primary specificity for aspartate is unique to the granzyme B and caspase families of proteases (Harris JL et al. 2000 PMID:10869434). The amino acids N-terminal of the aspartate, mainly the first four, determine the caspase’s specificity. Under normal conditions caspases are present as inactive enzyme precursors (zymogens), the procaspases. They consist of an N-terminal prodomain, the large subunit (p20), an optional linker sequence, and the small subunit (p10). The structure of all caspases is a heterotetramer formed by head-tail organised heterodimers that are composed of the small and the large subunit (Fu G et al. 2008 PMID:18780184, Chai J et al. 2001 PMID:11701129). The caspases’ substrate is stabilized by amino acids from both subunits, whereas the catalytic dyad is localised within the large subunit and consists of a cysteine and a histidine (Wilson KP et al. 1994 PMID:8035875). In vivo active initiator caspase-8, -9, and -10 and the lymphocyte-specific serine protease granzyme B perform proteolytic activation of the caspase-3 and -7 zymogen dimer by cleavage of the prodomain and the inhibiting linker. This activation can occur by two different mechanisms: the extrinsic and the intrinsic pathway. In the extrinsic or death receptor-mediated pathway death receptor ligands induce the oligomerization of death receptor (CD95 or TRAIL-R1/R2) resulting in the formation of the death-inducing signalling complex (DISC). Caspase-8 and -10 are activated by DISC and cleave caspase-3 and -7. The intrinsic or mitochondria-mediated pathway is induced by stimuli such as DNA damage, cytotoxic stress, and heat shock leading to the release of cytochrome C from the mitochondria and the formation of the apoptosome. After its activation by the apoptosome caspase-9 processes caspase-3 and -7 (Jiang X and Wang X, 2000 PMID:10940292). Executor caspases-3 and -7 cleave a variety of downstream proteins resulting in membrane blebbing, pyknotic nuclei, cell rounding, formation of apoptotic vesicles, and finally in apoptotic cell death. Non-apoptotic activities of caspases including involvement in immune response (Zhang J et al. 1998b PMID:9422780), proliferation (Woo et al. 2003), differentiation (Zermati Y et al. 2001 PMID:11208865, Carlile GW et al. 2004 PMID:14976035), and cell motility (Barnhart et al. 2004 PMID:15272306) are also described. However little is known about this, particularly the control and regulation of specific caspase cleavage. Regulation of caspases’ non-apoptotic activities presumably occurs by post-translational modification of the caspases and/or the substrates, subcellular compartmentalisation of caspases, protection of potential substrates by scaffold proteins or protein complexes, activation of anti-apoptotic factors, and recruitment of antagonistic proteins at the level of caspase activation complexes (Launay S et al. 2005 PMID:16079910, Yi CH and Yuan J. 2009 PMID:19154716). Due to their ability to induce apoptotic cell death, the activation of caspases and active caspases are modulated and/or inhibited by a number of regulatory mechanisms. The activation of caspase-8 at the DISC can be modulated by cellular FLICE-inhibiting protein (cFLIP), a member of the DISC (Krueger A et al. 2001 PMID:11713262). Inhibition of apoptosis protein (IAP) family inhibits the enzymatic activities of caspases using baculoviral IAP repeats (BIR LIG_BIR_II_1, LIG_BIR_III_1, LIG_BIR_III_2, LIG_BIR_III_3, LIG_BIR_III_4) (Deveraux QL et al. 1999 PMID:9990849). The most prominent IAP XIAP inhibits caspase-3, -7 and -9. It interacts with the N-terminal of the small caspase subunit and shields the catalytic side of caspase-3 and -7 by reverse binding (Eckelman BP et al. 2006 PMID:17016456). Two other natural, viral pan-caspase inhibitors are known: p35 (Xu G et al. 2001 PMID:11260720) and CrmA (Renatus M et al. 2000 PMID:10903953).