By Christiana Popovic
Cell death is essential for an organism to maintain tissue homeostasis; the removal of unwanted or damaged cells plays vital roles in cell growth and division. Understanding the mechanisms behind such a tightly controlled process is important as abnormal degrees of apoptosis can lead to the development of fatal diseases such as AIDS, Huntington’s disease and cancer (Elmore, 2007). Although there are several ways in which a cell may die, there are two main types of cell death: necrosis and apoptosis.
A brief comparison of these two types of cell death; apoptosis is a highly regulated form of programmed cell death in which the target cell is self-destructed by first decreasing in size and then splitting up into multiple membrane bound apoptotic bodies. This process requires a signal transduction pathway, while necrosis on the other hand undergoes a loss of cell membrane integrity. This is where the cell swells outwards, to the point that cellular contents start to leak into the surrounding tissue. Necrosis is considered to be a type of premature cell death as it is usually triggered by external stress such as high temperatures, infection from pathogens, and hypoxia.
Apoptosis is made up of two pathways: the intrinsic and extrinsic pathways. The intrinsic pathway, otherwise known as the mitochondrial pathway, is initiated by an apoptotic stimulus that occurs within the target cell. Stimuli that could initiate intrinsic apoptosis include DNA damage, protein stress, ER stress, alongside the release of reactive oxygen species. Intrinsic apoptosis in which proapoptotic proteins, BAX and BAK, form pores in the mitochondrial outer membrane results in the subsequent increase in mitochondrial outer membrane permeability, allowing for the diffusion of cytochrome c from the intermembrane space of the mitochondria into the cytosol of the cell (Wang and Youle, 2009). Cytochrome c binds both to a protein called Apaf-1 and the initiator procaspase-9 in the presence of ATP, thus forming a multi protein complex called an apoptosome. This apoptosome activates the inactive procaspase-9, thereby converting it into caspase-9 (Bratton and Salvesen, 2010).
Caspase-9 is part of a large group of caspases which are the ultimate drivers of apoptosis. As protease enzymes, they contain a cysteine residue in their active sites and have capabilities in selectively cleaving proteins downstream of an aspartate residue (Seaman et al., 2016). Once activated, the initiator caspase-9 cleaves the effector caspases-3 and 7, a process that ultimately leads to cell death. Although the exact function of the effectors are not clearly defined, they do however have two particular functions; the first being to actively kill the cell by cleaving their target proteins, and the other to create a positive feedback loop to further amplify the cascade by activating more caspases (Brentnall et al., 2013).
The BCL2 family of proteins play a key role in the regulation of cell death through the mitochondrial pathway. These proteins can be subdivided into classes of proteins based on their structure and function. Examples such as the pro-apoptotic, BH3-only, and pro-survival proteins are key for such regulation. The proapoptotic proteins include Bax and Bak which increase mitochondrial outer membrane permeability. Similarly, the BH3-only proteins promote cell death through being activated by specific stimuli, however they perform a more regulatory role in mitochondrial apoptosis. They may directly activate Bax and Bak, or alternatively, they may act to neutralise the pro-survival BCL2 family proteins (Wang and Youle, 2009). Both mechanisms are likely to take place, yet their relative importance may differ depending on the context. The pro-survivals on the other hand can act via two modes to inhibit both the pro-apoptotic and BH3-only proteins.
Other regulatory proteins include SMAC and the IAPs. Just as cytochrome c is released from the mitochondria, a protein called SMAC, otherwise known as DIABLO, is also released into the cytosol upon mitochondrial outer membrane permeability (Adrain et al., 2001). SMAC is a proapoptotic protein that deactivates antiapoptotic proteins, such as IAPs. In a normal functioning cell IAPs work by inhibiting all downstream caspase activation pathways to prevent them from carrying out apoptosis (Schimmer, 2004).
Intrinsic apoptosis takes on a complex, highly regulated mechanism, relying on multiple proteins and protein complexes. A lack of regulation of apoptosis can be fatal and is observed in many diseases where there are abnormal levels of apoptosis. However, if correctly controlled and with tight regulation, the body is able to use this mechanism to remove damaged or unwanted cells in a highly regulated way.
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