Mitochondria role in apoptosis regulation

Cell death can occur in various ways. Classically the cell death pathways were classified by morphological changes. 

Apoptotic cells display cytoplasmic and nuclear shrinkage; the plasma membrane remains intact and cellular contents are not typically released. Autophagic cells show vacuolisation of the cytoplasm. Necrotic cells on the other hand display neither autophagic nor apoptotic characteristic features. Instead, necrosis is characterised by plasma membrane rupture and release of cellular contents.

Mitochondria play different roles in apoptosis and necrosis. In apoptosis the key event is mitochondrial outer membrane permeabilisation (MOMP), while in necrosis, it is opening of the mitochondrial permeability transition pore (mPTP). 

Apoptotic signalling is largely mediated by proteins in the Bcl-2 family such as Bad and Bax. This family also contains anti-apoptotic members which bind directly to apoptotic effectors (Leber et al 2007). BH3-only proteins in the family are the major sensors for cellular stress and BH3-only Bid can promote Bak and Bax oligomerisation when activated. 

Bak and Bax oligomers disrupt the mitochondrial membrane, initially thought to do so by insertion into the membrane as a hairpin (Annis et al 2005), although recent evidence suggests only shallow insertion (Westphal et al 2014). 

When the mitochondrial membrane is permeabilised, mitochondrial proteins such as cytochrome c are released into the cytosol via mitochondrial apoptosis-induced channel (MAC). MAC itself can be regulated by Bcl-2 proteins since MAC was undetectable in Bcl-2-overexpressing cells (Pavlov et al 2001). This constitutes intrinsic activation of caspases, cysteine proteases which target aspartic acid-containing substrates. 

Caspases can also be activated extrinsically, by triggering ligation of death receptors, a family of receptors that includes the TNF receptor family. This recruits and activates caspase 8 via Fas-associated death domains. Activated caspase-9 directly activates executioner caspases.

Necrosis most often occurs in response to external stimuli such as toxins, infection or ischaemia-reperfusion injury (Galuzzi et al 2009). It occurs via mPTP opening.

Conditions that favour mPTP opening include divalent cations, especially raised Ca2+ concentration; ATP depletion and charnge in mitochondrial membrane potential or membrane depolarisation. 

When open, the mPTP allows passive diffusion of solutes out of the mitochondria, leading to mitochondrial swelling due to the high protein concentration in the matrix, causing high colloidal pressure. ATP is depleted in an attempt to restore membrane potential and direct electron transfer to molecular oxygen leads to reactive oxygen species formation. Structural changes caused by mPTP opening could prompt mitophagy.

Crosstalk between the two cell death pathways. mPTP opening allows leakage of pro-apoptotic factors such as cytochrome c.

Mitochondria-mediated cell death can be mediated by interactions with the ER, since mitochondria are in direct contact with the ER, as shown by co-sedimentation and electron microscopy (Mannella et al 1998) 

DNA damage can induce mitochondrial biogenesis and function.

DNA interaction with ROS can cause DNA damage, which stabilises p53. The increased activity of p53 triggers Bax and PUMA expression, both members of the Bcl-2 family, which lead to MOMP. Mitochondria deficient in cytochrome c may then produce more ROS, amplifying the initial insult. 

ATM is a protein that normally initiates checkpoint arrest and DNA repair in response to DNA double-stranded breaks. ATM exists as an inactive dimer/tetramer that can be recruited to the site of double-stranded DNA breaks. Once recruited, it must be phosphorylated to be able to carry out DNA repair. 

Mutations in ATM result in ataxia-telangiectasia (A-T), an autosomal recessive disorder with a complex phenotype including neuronal degeneration, ocular telangiectasias and increased susceptibility to cancer. A-T cells have abnormal mitochondrial structure, more mitochondria with lower membrane potential and increased basal expression of oxidative damage-responsive genes such as MnSOD. ATM appears to metabolically maintain mitochondria homeostasis.

mtDNA is a particular target for damage by reactive oxygen species since it is in close proximity to the electron transport chain and is unprotected by histones.

Complex I inhibition is also linked to resistance to cisplatin, a common anti-cancer drug (Yao and Jones, 2012). This was not correlated with mtDNA content or oxidative function. However, expression of the other respiratory complexes was upregulated in cisplatin-resistant cells, and this was mediated by PGC-1a.

#cell-biology