Caspases procaspase

NO may react with superoxide to yield the highly reactive peroxynitrite, ONOO-. Superoxide may also be converted into H202 and the reactive hydroxyl radical, OH. In this way excessive activation of glutamate receptors leads to oxidative damage. The calcium influx has a major effect on mitochondria and causes them to depolarize and swell. This leads to a pore being formed in the outer mitochondrial membrane, which allows the escape of cytochrome c and procaspases from the mitochondria into the cytosol. Cytochrome c activates the caspase cascade, which leads to apoptotic cell death (Ch. 35). 

Fig. 2. Activation mechanism of caspases. Receptors convey activating signals to adapters that facilitate oligomerization and subsequent autoactivation of long prodomain caspases (I). Caspases transactivate other procaspases upon activation (11).

An essential part of the apoptotic program is a caspase cascade. Apoptosis is initiated by proteolytic processing of intiator-procapsases under the influence of a variety of signals. The mature initiator caspase catalyzes the processing of a effector-procaspase to the active enzyme, which degrades specific substrates and/or activates further procaspases. In this way, caspases can be activated sequentially in a protease cascade. 

Activation of the caspases requires the help of a number of cofactors that are also known as activators or adaptors. Different cofactors are involved depending on the trigger mechanism of caspase activation. A central function of the cofactors is to bring about aggregation and thus activation of the procaspases. This occurs by specific protein-protein interactions with the help of common structural motives. Examples of such motives are the death domains (DD), death effector domains (DED) and the caspase recruitment domains (CARD), which all have a similar structure of six a-heh-ces. 

In this model, cellular stress mediates the release of cytochrome C from the mitochondrion. The proapoptotic proteins Bax and BH3 proteins support the release of cytochrome C, while the antiapoptotic Bcl2 protein has an inhibitory effect. Cytosolic cytochrome C binds to the cofactor Apaf 1, which then associates via its CARD motif with procaspase 9 to a complex termed apopto-some. In this complex, procaspase 9 is processed to the mature caspase which subsequently activates downstream effector caspases and commits the cell to death. 

Procaspase 8 functions as an initiator caspase in this system, since its activation is the signal for activation of the downstream caspase cascade. The DED motif of caspase 8 is localized in its large prodomain. Similar motives are foimd in other caspases with large prodomains (caspases 2, 8 and 9). 

Binding of the ligand of the Fas receptor triggers clustering of the receptor and association of the cofactor FADD (fas-assodated protein with death domain) which interacts with the receptor via its death domain (DD). Procaspase 8 binds to FADD via a common DED (death effector domain) motif and is thereby also recruited into the Fas-receptor associated complex. Due to the clustering of the proteins, proximity-induced cleavage of procaspase 8 to the mature initiator caspase 8 takes place. This activates the effector caspases and triggers cell death. 

Bcl-2 and related cytoplasmic proteins are key regulators of apoptosis [26], Anti-apoptotic proteins such as Bcl-2 and Bc1-Xl prevent apoptosis in response to numerous stimuli. During the apoptotic process, cytochrome c is released from mitochondria, but the release can be inhibited by the presence of Bcl-2 on the organelles [27]. The released cytochrome c forms an essential part of die apoptosome, which is composed of cytochrome c, Apaf-1, and procaspase-9 [28]. The complex formation results in activation of caspase-9, which leads to the stimulation of caspase-3. Bcl-XL has recently been reported to bind to Apaf-1 [29], It may inhibit the association of Apaf-1 with procaspase-9 and thereby prevent caspase activation. 

Caspase family. This scheme illustrates the domain structures, internal cleavage sites, preferred peptide substrate sequences, and biological function of caspases. Each procaspase consists of a large and small domain and may also possess DED (death effector domain) and CARD (caspase recruitment domain) (adapted from Hill etai, 2003). 

Effector caspases are activated by a transactivation mechanism, which is characterized by the catalytic action of a mature caspase on a procaspase (Thornberry et al., 1997 Earnshaw et al., 1999 Slee et al., 1999). Nevertheless, their activation can also occur by the action of other proteases. Granzyme B, a serine-protease, also has proteolytic specificity for aspartic acid residues. It is able to cleave and directly activate caspase 3 (Darmon et al., 1995). Cathepsin B, a lysosomal protease, cleaves and activates procaspase 11 (Schotte et al., 1998). 

Besides cytochrome c and procaspase 9, other important apoptogenic factors are also released from the mitochondria, such as SMAC/DIABLO, Omi/HrtA2, AIF, and endonuclease G. The function of SMAC/DIABLO and Omi/HrtA2 is to activate caspase by suppressing the caspase inhibitory activity of IAP (Du et al., 2000 Verhagen et al., 2000). The protein... 

The DISC-induced procaspase assembly results in the autoactivation of caspases 8 and 10. The DISC process of caspase activation seems to be analogous to the apoptosome process of caspase 9 activation. Caspase 8, in turn, cleaves and activates caspase 3, which is responsible for the apoptotic signal amplification with subsequent cell collapse. 

The apoptotic process is mediated by the caspase family of cysteine protease. Caspases are implicated both in the induction and execution of the death sentence. Apoptosis can be induced by (1) activation of death receptors, such as Fas, which recruits procaspase 8 via adaptor proteins and promotes its autocatalytic activation, and (2) the release of cytochrome C from mitochondria by DNA damaging drugs and other chemotherapeutic agents (Figure 7.11). 

The large prodomains of procaspases contain structural motifs that belong to the death domain (DD) superfamily involved in the transduction of the apoptotic signals. This superfamily consists of the DD, the death effector domain (DED), and the caspase recruitment domain (CARD). Each of these motifs interacts with other proteins by homotypic interactions. DED is found in procaspase-8 and -10, and CARD is found in procaspase-1, -2, -4, -5, -9, -11, and -12. DED and CARD are responsible for the recruitment of initiator caspases into death- or inflammation-inducing signaling complexes, resulting in proteolytic autoactivation of caspases that subsequently initiates inflammation and apoptosis [26, 29, 30],... 

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]. 

Mitochondrial pathway of caspase activation has been demonstrated in various experimental models and humans to contribute significantly to car-diomyocyte apoptosis in the heart [41-44], Studies suggest that mitochondrial-mediated apoptosis contributes to cardiomyocyte loss through intermembrane space proteins release, Bcl-2 protein involvement, and procaspase activation [41, 45, 46], Due to the energy required by heart muscle, mitochondria are particularly abundant in cardiomyocytes. Under physiological conditions, prosurvival mechanisms exist to protect the myocardium from inappropriately triggered apoptosis [3],... 

---