CELL DEATH: APOPTOSIS, NECROSIS & AUTOPHAGY
Cell death can be classified according to its morphological appearance (which may be apoptotic, necrotic, autophagic or associated with mitosis), enzymological criteria (with and without the involvement of nucleases or of distinct classes of proteases, such as caspases, calpains, cathepsins and transglutaminases), functional aspects (programmed or accidental, physiological or pathological) or immunological characteristics (immunogenic or non-immunogenic)
Apoptosis, being a highly complex and sophisticated process, involves a series of biochemical events leading to a spatiotemporal sequence of morphological changes, such as nuclear condensation and fragmentation, as well as plasma membrane blebbing. This series of dramatic perturbations in cellular architecture will contribute not only to cell death, but also prepare future cell removal by phagocytes and prevent unwanted immune responses.
Apoptosis can occur via extrinsic and intrinsic pathways, which are initiated either by extracellular death receptors, such as FAS ligand (FASLG), TNF-α, and TRAIL receptors, or by heterogenous intracellular stimuli, such as DNA damage, hypoxia, and nutrient deprivation. After activation of the signaling pathways, a subset of highly specific endoproteases, caspases present as inactive zymogens in healthy cells, will be activated through a cascade of proteolytic reactions. Among the different caspases (Casps), some play an initiator role (caspases-2, -9 for intrinsic, caspases -8, -10 for extrinsic pathway). Others act as effectors (caspase-3, -6 and -7) that will orchestrate the dismantling of diverse cell structures through cleavage of specific substrates. This process is tightly regulated by different proteins such as apoptosis inducing factor (AIFM1) or GDF15. Among the cleaved substrates figure type I intermediate filaments of the cytoskeleton such as cytokeratin-18 (KRT18). The subsequent release of CK-18 fragments into the extracellular space occurs during cell death and can be monitored. The full process of apoptosis can be measured by fluorometric caspase 3/apopain activity monitoring in full cell extracts.
Necrosis has long been described as a consequence of physicochemical stress and thus considered as an accidental and uncontrolled process. Recently, it is becoming clear that necrotic cell death may be finely tuned up by a set of signal transduction pathways and catabolic mechanisms.
In the necrosis pathway, the activation of TNF-α/FASLG binds to their receptors TNFR1/FAS to activate the kinases RIPK1 (RIP) and RIPK3 (RIP3) which in turn will promote the activation of plasma membrane permeabilization (PMP) and then triggers the necrosis.
Morphologically, necrosis is characterized by a gain in cell volume, swelling of organelles, plasma membrane rupture and subsequent loss of intracellular contents that can be precisely monitored and measured in a time dependent manner (free cell DNA, LDH activity,… ).
Autophagy is an essential and conserved catabolic process, which is initiated by membrane nucleation, followed by vesicle expansion to form the autophagosome and fuse with the lysosome to degrade cellular components.
Autophagic cell death is mediated by autophagy and autophagy-related proteins and that is characterized by AMPK activation and mTOR suppression. During autophagy initiation and autophagosome formation, Beclin 1 (BECN1) binds microtubule-associated protein-1 light chain 3 (MAP1LC3A) that is converted to its membrane-bound form (MAP1LC3B) and interacts with the ubiquitin-binding protein p62/sequestosome 1 (SQSTM1). Thus, autophagic clearance of protein aggregates requires p62 polymerization and binding to NBR-1.