Role of mitochondria in apoptosis

Mitochondria play an important role in the regulation of cell death. They contain many pro-apoptotic proteins such as Apoptosis Inducing Factor (AIF), Smac/DIABLO and cytochrome C. These factors are released from the mitochondria following the formation of a pore in the mitochondrial membrane called the Permeability Transition pore, or PT pore. These pores are thought to form through the action of the pro-apoptotic members of the bcl-2 family of proteins, which in turn are activated by apoptotic signals such as cell stress, free radical damage or growth factor deprivation. Mitochondria also play an important role in amplifying the apoptotic signalling from the death receptors, with receptor recruited caspase 8 activating the pro-apoptotic bcl-2 protein, Bid.

Role of Bcl-2 proteins

The bcl-2 proteins are a family of proteins involved in the response to apoptosis. Some of these proteins (such as bcl-2 and bcl-XL) are anti-apoptotic, while others (such as Bad, Bax or Bid) are pro-apoptotic. The sensitivity of cells to apoptotic stimuli can depend on the balance of pro- and anti-apoptotic bcl-2 proteins. When there is an excess of pro-apoptotic proteins the cells are more sensitive to apoptosis, when there is an excess of anti-apoptotic proteins the cells will tend to be more resistant. An excess of pro-apoptotic bcl-2 proteins at the surface of the mitochondria is thought to be important in the formation of the PT pore.

An animation illustrating the general principles is shown below.


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The pro-apoptotic bcl-2 proteins are often found in the cytosol where they act as sensors of cellular damage or stress. Following cellular stress they relocate to the surface of the mitochondria where the anti-apoptotic proteins are located. This interaction between pro- and anti-apoptotic proteins disrupts the normal function of the anti-apoptotic bcl-2 proteins and can lead to the formation of pores in the mitochondria and the release of cytochrome C and other pro-apoptotic molecules from the intermembrane space. This in turn leads to the formation of the apoptosome and the activation of the caspase cascade.

The release of cytochrome C from the mitochondria is a particularly important event in the induction of apoptosis. Once cytochrome C has been released into the cytosol it is able to interact with a protein called Apaf-1. This leads to the recruitment of pro-caspase 9 into a multi-protein complex with cytochrome C and Apaf-1 called the apoptosome. Formation of the apoptosome leads to activation of caspase 9 and the induction of apoptosis.

The role of mitochondria in the induction of apoptosis is summarised in the figure below.


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