The role of caspases in cellular apoptosis (programmed cell death)

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Caspases are a family of cysteine proteases that are present within the cells of normal healthy tissue in inactive (procaspase) forms (1,2). The procaspase configuration of a caspase is quiescent and in order for a caspase to become active it requires cleavage of its prodomain. After the prodomain has been cleaved from the procaspase molecule, the caspase molecule is activated and can now act upon and cleave a specific tetrapeptide sequence (this sequence varies for each member of the caspase family) in targeted proteins, which can include both cytoskeletal, and nuclear proteins of an affected cell (3).  There are currently fourteen known members of the caspase family of proteases (i.e. Caspases-1 to -14), which have been conserved throughout evolution (2).  The first member of the family of caspases (i.e. caspase-1) was originally identified during genetic studies of neural development in the nematode, C. elegans (1).   Not all members of the caspase family participate in programmed cell death (apoptosis) of cells, for example caspases-1 and -11 function mainly in the processing of cytokines, e.g. interleukin 1 beta (2). However, many of the members of the caspase family of proteases (e.g. caspases-2, -3 and -6 to -10) have been proven to participate in the regulation and execution of apoptosis of affected cells, which can be either the result of a normal developmental event (e.g. neuronal competition for trophic support from a target tissue) or a cell’s response to a high level of internal injury (e.g. membrane lipid peroxidation) that results from exposure to oxidative stress (4-6).  The members of the caspase family that participate in apoptosis can be separated into two general groups: 1) initiator caspases (e.g. caspases-8 and -9) (5); and 2) effector caspases (e.g. caspases-3, -6 and -7) (4).  Initiator caspases can be activated by either an “Extrinsic” Cell Death Pathway (e.g. Fas ligand-Fas receptor/procaspase-8 interactions) or an “Intrinsic” Cell Death Pathway (e.g. MAPK/mitochondria/cytochrome-c/procaspase-9 interactions) as presented in figure 1.  Initiator caspases (e.g. caspase-9) then activate downstream effector caspases (e.g. caspase-3) which in turn act as executioners in the process of apoptosis of an affected cell (see figure 1) (5).  The “Extrinsic” cell death pathway involves the binding and dimerization of cell death receptors from members of the tumor necrosis factor (TNF) family of receptors and ligands (e.g. Fas ligand/Fas) which brings procaspase-8 molecules into close apposition initiating dimerization of the procaspase-8 molecules and thereby causing catalytic cleavage of the prodomains resulting in actived caspase-8 molecules (2).  Activated caspase-8 can then participate in either the “Extrinsic” (high caspase-8 concentration) cell death pathway with caspase-8 directly activating quiescent effector caspases such as procaspase-3 or the “Intrinsic” (low caspase-8 concentration) cell death pathway by cleaving BID which then acts on the mitochondrial cell death pathway by facilitating the release of cytochrome-c thereby indirectly participating in the activation of another quiescent initiator caspase, i.e. procaspase-9.  Activation of the “Intrinsic” cell death pathway generally involves an insult that generates oxidative stress with the creation of reactive oxygen species (ROS) and other free radicals (e.g. OH ^.) that damage the cell’s organelles and internal membranes resulting in mitochondrial membrane damage and loss of membrane potential. This results in a release of cytochrome c (Cyto C) from the damaged mitochondria into the cell’s cytoplasm where it combines with apoptotic protease-activating factor 1 (APAF-1), dATP, and procaspase-9 to form the Apoptosome (also known as the Aposome) to generate activated caspase-9 (2,5,7).  Smac/Diablo is a small mitochondrial protein that is thought to be required for activation of a competence to die reaction within an affected cell by inhibiting some of the damaged cell’s naturally occurring caspase inhibitory molecules (e.g. NIAP inhibitiory protein in neurons) (8-10).  Once procaspase-9 has been activated its downstream targets are the effector caspases, e.g. caspases-3, -6, -7 (4). The cell’s naturally occuring apoptosis inhibitory proteins (IAPs) are thought to target activated effector caspases such as caspases-3 and -7 (3,11).  However these IAPs are themselves natural targets of the effector caspases (3).  The effector caspases interact with a large number of targets (i.e. >280) within an affected cell to bring about its destruction (3).  Some of the cellular molecules targeted by effector caspases  are: PARP-1 ( a DNA repair enzyme); DNA; nuclear lamins A, B and C; DNA fragmentation factor 45; inhibitor of caspase-activated DNase; receptor interacting protein (RIP); topoisomerase; signal transducer and activator of transcription-1; Rb; X-linked inhibitor of apoptosis (XIAP); U1 small nucleoprotein; fodrin; vimentin; and procaspases-2 , -6 and -10.