Initiator caspases activation mechanisms

BH3 domain) of the BH3-only proteins binds to other Bcl-2 family members thereby influencing their conformation. This interaction facilitates the release of cytochrome C and other mitochondrial proteins from the intermembrane space of mitochondria. Despite much effort the exact biochemical mechanism which governs this release is not yet fully understood. The release of cytochrome C facilitates the formation of the apoptosome, the second platform for apoptosis initiation besides the DISC. At the apoptosome which is also a multi-protein complex the initiator caspase-9 is activated. At this point the two pathways converge. 

When a cell is infected with a virus, the latter utilises the metabolic machinery within the host cell to generate viral proteins, RNA and DNA to produce more virus particles which then escape to infect other cells. The process is stopped by death of the host cells so that generation of new viruses is halted. The major mechanism that results in death of the host cell is apoptosis. The cells that are responsible for the death of the infected cells are either cytotoxic lymphocytes or natural killer cells. Death is caused either by release of toxic biochemicals and/or proteolytic enzymes or by binding to a death receptor, which is present on many cells. The entry of proteolytic enzymes or binding to the death receptor results in activation of initiator caspases. These activate effector caspases that cause damage to the cell which results in death due to apoptosis (Chapter 17 Figures 17.28, 29 and 30). 

Tlie initiator caspases receive proapoptotic signals and initiate the activation of a caspase cascade. Tliey are activated by an interaction with a transmembrane receptor or by cytotoxic influences. A complex is thus formed known as the apoptosome (see 15.4). The effector caspases are activated by an upstream caspase via a cascade mechanism. They are the component that executes apoptosis, initiates degradation of central proteins and directs the cell to death. 

Based on the triggering stimulus and the nature of the components involved, at least two apoptotic signaling pathways can be differentiated that lead to activation of the effector caspases. On the one hand, receptor systems may be involved on the other hand, activation may be triggered by cytotoxic stress. The two pathways differ in the mechanism of activation of the initiator caspase but use the same effector caspase at least partially. 

Intrinsic (mitochondrial) pathway of caspase activation is initiated by the permeabilization of the mitochondrial outer membrane by proapoptotic members of the Bcl-2 family, resulting in a release of cytochrome c and other proteins from the intermembrane space of mitochondria into the cytosol. Cytochrome c translocation to the cytosol may follow a number of possible mechanisms. However, once in the cytosol, cytochrome c binds to apoptosis protease activating factor (Apaf-1) and in the presence of dATP or ATP facilitates Apaf-1 oligomerization and the recruitment of procaspase-9. The formation of this caspase-activating complex, termed the apoptosome, results in the activation of procaspase-9, and this in turn cleaves and activates the effector caspase-3 and -7. Activated effector caspases cleave key substrates in the cell and produce the cellular and biochemical events characteristic for apoptosis [33-35]. 

The activation of the inflammatory caspases uses a mechanism resembling that of the initiator caspases. The presence of a complex, known as the inflammasome (Martinon et al., 2002), is required for activation of this set of proteases. The recruitment of caspases into this complex results in their activation. For caspase-1, the adaptor ASC (apoptosis-associated specklike protein containing a CARD) is critical in inflammasome formation in response to a variety of stimuli, whereas involvement of the adaptors Ipaf (ICE-protease-activating factor) and NALP3 is stimulus-dependent (Mariathasan, 2007). 

Activation of caspases is irreversible, because it involves peptide-bond cleav e. This is unlike most other protein modifications which play a role in cellular regulation. Therefore, proteolysis is involved only in unidirectional, irreversible processes, such as the cell cycle and cell death. But, the possibilities to regulate irreversible reactions are rather limited. In a cascade of proteolytic reactions, the first enzyme in the chain is the most likely point of control. This is the initiator caspase. The signals controlling initiator caspases vary, there are both external and internal signals (Fig. 13.5). Several mechanisms control the irreversible activation of caspases, including phosphorylation, separation, and compartmentalization of pro-caspases and positive and n ative regulators. 

The XIAP-BIR3 domain is responsible for inhibition of the initiator caspase-9, but it functions via a completely different mechanism. The BIR3 domain is an allosteric inhibitor of caspase-9 it binds to the dimer interface and prevents dimerization and subsequent activation of the enzyme (9) (Fig. 4). Caspase-9 is at the apex of the apoptotic cascade that leads to the activation of executioner cascades. As such, BIR3 can provide an extra level of regulation by sequestering monomers in a catalytically inactive conformation and ensuring that no unwanted caspase-9 activity occurs. 

Proteolytic activation of procaspases uses two mechanisms, depending on whether the caspase functions as an effector caspase (caspases-3, -6 and -7) or as an initiator caspase (caspases-8 and -9). Initiator caspases are the first to be activated in response to a proapoptotic stimulus and are responsible for activating the effector caspases by limited proteolysis. The effector caspases are thought to be responsible for most of the substrate proteolysis observed during apoptosis. By digesting central proteins, the effector caspases direct the cell to death. 

The two main pathways of apoptosis, the death receptor pathway and the mitochondrial pathway, differ in the mechanism of activation of the initiator caspase but use the same effector caspase at least partially. 

Snipas SJ, Drag M, Stennice HR, Salversen GS. Activation mechanism and substrate specificity of the drosophila initiator caspase DRONC. Cell Death Differ. 2008 15 938-45. 

Apoptosis (programmed cell death) is involved in the development and elimination of the damaged cells. Deregulation of apoptosis can cause diseases such as cancers. Ubiquitin and cytochrome c are important in the mechanism of cell death being involved in the first steps of apoptosis. Apoptosis is executed by a subfamily of cysteine proteases known as caspases. In mammalian cells, a major caspase activation is the cytochrome c initiated pathway [31]. On the other hand, ubiquitin is used by cells as a covalent modifier of other proteins both to activate their function and to target them for degradation [32]. 

Figure 2. Mechanisms and signalings of neuronal death. Death can be initiated at the membrane by activation of death domain receptors (DDR), or by intracellular signalings through oxidative stress (and the production of reactive oxigen species, ROS), perturbed calcium homeostasis, mitochondrial dysfunction (release of cytochrome c, cytC), activation of caspases, as well as reactivation of cell cycle genes such as the transcription factor E2F (see text). Interconnections have been demonstrated (dotted lines) depending on the apoptotic context...

Figure 20.35 Mechanisms by which external or internal stress leads to cell damage resulting in apoptosis. The stress leads to activation of initiator proteolytic enzymes (caspases) that initiate activation of effector caspases. These enzymes cause proteolytic damage to the cytoskeleton, plasma membrane and DNA. The activation of DNAases in the nucleus results in cleavage of DNA chains between histones that produces a specific pattern of DNA damage which, upon electrophoresis, gives a specific pattern of DNA fragments. The major endproduct of apoptosis are the apoptolic bodies which are removed by the phagocytes.

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