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The Dual Role of Mitochondria

Powering life and causing death

Mitochondria as mechanisms of apoptosis

Mitochondria are famous for being the “powerhouse of cells” and producing ATP for respiration by being the site for the Krebs cycle, the electron transport chain and the location of electron carriers.However, one thing mitochondria are not known for is mediating programmed cell death, or apoptosis. This is a tightly controlled process within a cell to prevent the growth of cancer cells.

One way apoptosis occurs is through the mitochondria initiating protein activation in the cytosol (a part of the cytoplasm). Proteins such as cytochrome c activate caspases by binding to them, causing cell death. Caspases are enzymes that degrade cellular components so they can be removed by phagocytes.Mitochondrial apoptosis is also controlled by the B cell lymphoma 2 (BCL-2) family of proteins. They are split into pro-apoptotic and pro-survival proteins, so the correct balance of these two types of BCL-2 proteins is important in cellular life and death.

Regulation and initiation of mitochondrial apoptosis

Mitochondrial apoptosis can be regulated by the BCL-2 family of proteins. They can be activated due to things such as transcriptional upregulation or post-translational modification. Transcriptional upregulation is when the production of RNA from a gene is increased. Post-translational modification is when chemical groups (such as acetyl groups and methyl groups) are added to proteins after they have been translated from RNA. This can change the structure and interactions of proteins.

After one of these processes, BAX and BAK (some examples of pro-apoptotic BCL-2 proteins) are activated. They form pores in the mitochondrial outer membrane in a process called mitochondrial outer membrane permeabilisation (MOMP). This allows pro-apoptotic proteins to be released into the cytosol, leading to apoptosis.

Therapeutic uses of mitochondria

Dysregulation of mitochondrial apoptosis can lead to many neurological and infectious diseases, such as neurodegenerative diseases and autoimmune disorders, as well as cancer. Therefore, mitochondria can act as important drug targets, providing therapeutic opportunities. Some peptides and proteins are known as mitochondriotoxins or mitocans, and they are able to trigger apoptosis. Their use has been investigated for cancer treatment. 

One example of a mitochondriotoxin is melittin, the main component in bee venom. This compound works by incorporating into plasma membranes and interfering with the organisation of the bilayer by forming pores, which stops membrane proteins from functioning. 

Drugs consisting of melittin have been used as treatments for conditions such as rheumatoid arthritis and multiple sclerosis. It has also been investigated as a potential treatment for cancer, and it induced apoptosis in certain types of leukaemia cells. This resulted in the downregulation of BCL-2 proteins, meaning there was decreased expression and activity.The result of the melittin-induced apoptosis is a preclinical finding, and more research is needed for clinical applications. This shows that mechanisms of mitochondrial apoptosis can be harnessed to create novel therapeutics for diseases such as cancer. 

It is evident that mitochondria are essential for respiration but also involved in apoptosis. Moreover, mitochondria are regulated by the activation of proteins like BCL-2, BAX and BAK. With further research, scientists can develop more targeted and effective drugs to treat various diseases associated with mitochondria.

Written by Naoshin Haque

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