Apoptosis, Pyroptosis and Necrosis

Fink, S.L. and Cookson, B.T., Apoptosis, Pyroptosis, and Necrosis: Mechanistic Description of Dead and Dying Eukaryotic Cells, Infection and Immunity, Vol.73, No.4, April 2005, p.1907-1916

The purpose of this review article on apoptosis, pyroptosis and necrosis is to help increase our understanding of the different mechanisms of pathogenesis and why certain cells are more susceptible to pathogen-induced cell death. I believe this knowledge can be used to reveal new therapeutic targets for disease prevention. First, the authors provide a comprehensive summary of five different types of cell death. Apoptosis is active, programmed cell death with nuclear and cytoplasmic condensation and cellular fragmentation into apoptotic bodies. In apoptotic cells, caspases are the genetic bases for programmed cell death since initiator caspases activate effector caspases which cleave cellular substrates. Necrosis is described as passive, accidental cell death due to environmental changes and necrotic cells release inflammatory contents upon lysing. Oncosis is the prelethal pathway leading to cell death with cell swelling caused by the failure of ion pumps in the plasma membranes of ATP-depleted cells. Autophagy occurs when autophagic vacuoles degrade cellular components. Lastly, pyroptosis is the proinflammatory programmed cell death that is dependent on caspase-1 since this protease activates inflammatory cytokines which leads to cell lysis and the release of inflammatory contents.

However, the five types of cell death described are not independent and separate entities. In fact, many types of cell death can be observed at the same time in cell cultures exposed to the same stimulus. The authors suggest that the intensity of the stimulus might determine the pathway of cell death taken by the cells. For example, E.coli in small numbers inhibits polymorphonuclear neutrophil apoptosis while the same E.coli cells promote polymorphonuclear neutrophil death in larger numbers. Additionally, there are new forms of cell death that are being uncovered such as paraptosis and thus, it is important to continue conducting experiments to characterize and increase our understanding of novel forms of cell death.

Significance:
Many different experimental methods have been used to study cell death however these techniques must be used to identify one specific type of cell death that can be proven with certainty. For example, staining cells with fluoresecent dyes to be observed in light microscopes cannot be used to distinguish apoptotic from oncotic cells because both have condensed chromatin. Similarly, gel electrophoresis cannot be used to distinguish between apoptotic and necrotic cells because both cells have DNA fragmentation. It is also not possible to use annexin V to distinguish apoptotic cells from necrotic and oncotic cells because all three have loose phospholipid asymmetry due to membrane damage. Thus, the best methods to identify apoptotic cells are using knockout mice with RNA interference to determine cell death pathways or by using a Western blot with a colorimetric assay and staining cells with antibodies for caspases and then performing flow cytometry to determine caspase activity which is a genetic basis for apoptosis. Scientists need to be certain that the cells they are treating with a drug are dying due to the effects of the drug rather than external factors that are not inducing programmed cell death by using the most appropriate experimental methods. Furthermore, placing a greater emphasis on examining the type of cell deaths when conducting cell culture experiments will help increase our understanding of the influence of inflammation in cell death and also shed light on a variety of human disease pathways.