Apoptosis mitochondrial intermembrane space

In addition to cytochrome c, a second mitochondria-derived activator of caspases (SMAC) is also released from the mitochondrial intermembrane space. SMAC binds to an inhibitor of apoptosis protein (IAP) to prevent spontaneous caspase activation in a healthy cell. The IAP that binds to caspase-9 is called X-LAP. When SMAC binds to X-IAP, X-IAP is released from caspase-9. Autocleavage of the caspase-9 prodomain then exposes the catalytic site. Caspase-9 then activates caspase-3 which immediately cuts out the X-IAP binding domain from procaspase-9, preventing X-IAP inhibition and accelerating activation of both caspase-3 and caspase-9 (Fig. 13.11a), which digest all of the cell s proteins. The apoptotic cells also possess a caspase-activated DNase (CAD) that fragments chromosomal DNA. Other proteins from the mitochondrial intermembrane space can activate caspases by related mechanisms. The combination of mechanisms, or the overwhelming activation of any one of them, irreversibly propels a cell into apoptosis. 

Apoptosis-related cellular modification, induced by OA, such as mitochondrial transmembrane potential decrease, cytochrome c release from mitochondrial intermembrane space to the cytosol, activation of multiple caspases isoforms, decreased levels of monomeric Bcl-2 and Bax proteins, phosphorylation of Bcl-2, inhibition of protein synthesis and DNA fragmentation has been described in a large variety of cell lines.Finally, phosphorylation of histones, but not changes in their quantity, was reported to be an early step in DNA fragmentation, as observed in thymocytes apoptosis induced by... 

The Bcl-2 family of oncoproteins is known to play an important role in apoptosis through their ability to regulate cytochrome c release from mitochondria [11,15]. The antiapoptotic proteins Bcl-2 and Bc1-Xl prevent cytochrome c release, whereas the proapoptotic family members (e.g.. Bad, Bid, Bik, Bax) facilitate cytochrome c efflux or block the protective effects of Bcl-2 and Bc1-Xl. The mechanism involved is unclear the Bcl-2 family proteins may interact directly with the MTP (mitochondrial permeability transition) protein complex (the PTPC) or form independent ionic pores in the outer mitochondrial membrane (Fig. 3). Nonetheless, cytochrome c-depen-dent caspase-3 activation and changes in the expression or phosphorylation state of Bcl-2 family proteins are taken as indicative of mitochondria-dependent apoptotic pathways. It is important to remember that other apoptogenic proteins are also present in the mitochondrial intermembrane space, including smac/ DIABLO and flavoprotein (AIF) [10,11,60]. The release stimuli for the latter factors, which are currently being elucidated, may also involve the permeability transition or the Bcl-2 family proteins [37]. 

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. 

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],... 

Mitochondria release not only cytochrome c but also many pro-apoptotic factors (Table 17.1). They are normally localized in the intermembrane space of mitochondria. However, except for cytochrome c and the apoptosis inducing factor (AIF), their functions in the mitochondria have not been determined or they may have no function under normal conditions. Because they are larger than 5kD, they remain inside the mitochondrion. Once the mitochondrial outer membrane is permeabi-... 

We now know that release of cytochrome c and other proteins from the Intermembrane space of mitochondria Into the cytosol plays a major role In triggering apoptosis (Chapter 22). Certain members of the Bcl-2 family of apoptotic proteins and Ion channels localized In part to the outer mitochondrial membrane participate In this process. Yet much remains to be learned about the structure of these proteins In the mitochondrial membrane, their normal functions in cell metabolism, and the alterations that lead to apoptosis. 

Mitochondria and Apoptosis The loss of mitochondrial integrity is a major route initiating apoptosis (see Chapter 18, section V). The intermembrane space contains procaspases —2, —3, and 9, which are proteolytic enzymes that are in the zymogen form (i.e., must be proteolytically cleaved to be active). It also contains apoptosis initiating factor (AIF) and caspase-activated DNAase (CAD). Cytochrome c, which is loosely bound to the outer mitochondrial membrane, may also enter the intermembrane space when the electrochemical potential gradient is lost. The release of cytochrome c and the other proteins into the cytosol initiates apoptosis. 

A key event in preventing apoptosis is thus the retention of cytochrome c within mitochondria. The permeability transition pore complex is formed between the inner and outer mitochondrial membranes and is reported to control protein release from the intermembrane space. The permeabihty transition pore complex comprises the adenine nucleotide transporter, the voltage-dependent anion channel and possibly other proteins such as the benzodiazepine receptor and cyclophilin D [65]. Thus, cells possess specialised systems and processes for retaining cytochrome c within mitochondria to ensure survival, as well as systems that can rapidly mobilise this molecule when the apoptotic pathway is triggered. 

---