During necrosis, the levels of both reactive oxygen species and intracellular calcium increase reviewed in [ 20 ]. It is important to consider that the internal cell environment is highly regulated, so certain stimuli are able to alter cell membrane permeability and thus produce an imbalance among different ions, such as potassium, sodium, and calcium. Calcium is regulated by the endoplasmic reticulum, and a loss of calcium homeostasis can lead to several intracellular alterations.
In contrast, increased calcium levels can affect diverse mitochondrial functions and result in alterations of the production of reactive oxygen species. When high calcium levels are sustained over time, they disrupt mitochondrial inner membrane integrity and cause a loss of the ability to generate ATP [ 21 ] and, eventually, necrotic cell death Figure 9.
In addition to their effects inside the mitochondria, altered cytosolic calcium levels can activate different types of proteases, including calpains. Calpains are intracellular cysteine proteases present in inactive form, that may be activated by increased cytosolic calcium [ 22 , 23 ].
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Once activated, they can disrupt the lysosomal membrane with the resulting release of cathepsines B and L [ 24 ]. This group of reactions causes destabilization of the final membrane system. Together, these alterations cause the cell to lose its membranes such that the cellular contents are released into the extracellular space. Biochemical cellular changes during necrosis cell death. Sustained high calcium levels alter cell permeability of the membranes, leading to the dysfunction and ruptureof membranes.
During necrosis, the ATP depletion is conducted. The molecular hallmark of necrosis is drastic ATP depletion, which is believed to be the underlying cause of cell death. There is a metabolic disruption accompanied by energy depletion and loss of ATP that leads to cellular edema, while the mitochondria become round and swollen, the endoplasmic reticulum dilates, the lysosomes are disrupted, and the formation of plasma membrane protrusions called blebs is apparent [ 25 ].
Necrosis cell death occurs due to a failure in plasma membrane permeability that disrupts the plasma membrane and releases cellular components. This cell death process is associated with the indiscriminate extracellular release of soluble intracellular constituents through the permeabilized plasma membrane.
The way in which necrosis occurs allows us to determine that it is not merely accidental. Necrosis is present during the longitudinal growth of bones in young animals [ 26 ], and is also found in intestinal epithelial cells in adults [ 27 ]. On the other hand, during caspase inhibition, necrosis may be activated as an alternative route of cell elimination, suggesting that it is not simply an accidental process, but that, under certain conditions could function as a programmed event called necroptosis.
The necrosis process has long been conceived as an accidental, passive event; however, recent detailed observations were able to identify dying cells by the expression of different proteins and the intracellular disposition of several proteins that may be active during the event. As mentioned above, programmed cell death, or apoptosis, can be activated by two routes, one of which entails the participation of receptors present in the cytoplasmic membrane.
This activation of the caspase system triggers execution of the apoptotic process. Diverse findings indicate that the receptors involved in apoptotic cell death may also participate in the occurrence of a different type of cell death under distinct conditions of molecular resource availability. For cells that do not express caspase-8, it was predicted that they could not respond to an apoptotic stimuli directed by FasL induction. However, those results were surprising because they included the multimerization of FADD in the absence of caspase activation.
The morphology of apoptosis was not present, but ultrastructural analyses of those dying cells revealed necrotic morphological changes [ 29 ]. All these observations suggest that the receptor regulators of apoptosis were involved not only in that process, but also in the activation of a different signaling pathway that allows the formation of protein complexes which lead the cell toward a death process with necrotic features.
These developments led to the emergence of a new concept of programmed cell death called necroptosis, whose morphological characteristics are similar to those of accidental necrosis, although the molecular events that occur indicate that it is a coordinated process. Necroptosis has been found under special conditions, where pro-apoptotic enzymes were absent or limited. In experimental embryo models, interdigital membrane regression in mouse embryos was effectuated by necrosis triggered by either caspase inhibition or drugs [ 30 ].
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Necroptosis is thus a form of programmed cell death that has been demonstrated under experimental conditions, when apoptosis is inhibited. Several studies have succeeded in discerning the molecular events that occur during necroptosis, and it is those events that differentiate between necrosis an accidental process and necroptosis a programmed process.
Necroptosis has been observed in several pathological cell death events, such as ischemic brain injury, myocardial infarction, exotoxicity, and chemotherapy-induced cell death [ 31 ]. Necroptosis is morphologically characterized by several cytoplasmic changes. In fact, it is sometimes possible to distinguish the different degrees of advance of this process as the organelles swell, the cell membrane fragments, and cytoplasmic and nuclear disintegration become evident.
During necroptosis, the nuclei remain intact and there is no massive caspase activation, chromatin condensation, spillage of cell contents, phagocytosis by macropinocytosis, lysosomal leakage, or oxidative bursts [ 32 ]. The term necroptosis has been introduced to identify a process of cell death with morphological characteristics distinct from those of apoptosis. Necroptosis is a programmed event that ends with the delivery of the cytoplasmic contents into the extracellular space.
Membrane destabilization is a consequence of different intracellular mechanisms that generate osmotic changes by damaging the ion balance. When DNA damage occurred due to reactive oxygen species, the PARP protein was activated and began the reparation process; however, this process consumed abundant ATP and that reduction initiated a sequence of events that led to a deficient cellular efflux of calcium. The breakdown of the cellular membrane, in turn, released several signals that activated the immune system.
These soluble signals were proteins with pro-inflammatory properties that stimulate the recruitment of neutrophils to the site of cell death [ 34 ]. Activation of TNF receptors by their ligands triggers different responses that involve pro-survival or pro-cell-death processes. However, once this complex becomes established it is able to recruit FADD and procaspase-8, which produces a complex that could initiate either apoptosis or necroptosis.
Biochemical aspects of necroptosis. The RIP3 is implicated in necroptosis during inflammatory responses to virus infections [ 37 ], and during cellular necrosis in response to the TNF-alpha family of death-inducing cytokines [ 38 ].
Ischemic Cell Death in Brain Neurons
Activation of ligands associated with apoptotic cell death, such as Fas, in conditions that are unfavorable for apoptosis, that is, when caspases are absent or inhibited, allows RIP activation, which leads to death by necrosis [ 36 , 39 ]. The biochemical process of necroptosis is a new and active field, so not all the routes of activation of this event have been determined. Diverse studies concur that kinase RIP is involved, as we have mentioned.
Cell death is a normal event that controls tissue homeostasis.
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Today, we know that cells can be eliminated by means of different pathways that involve programmed or accidental mechanisms. Apoptotic cell death has been considered the major factor in physiological cell death, but recent evidence demonstrates that other routes of cell elimination — such as autophagy — also play important roles in maintaining homeostasis. A third route of cell death is necrosis, which was long considered an accidental form, characterized by general membrane swelling and ATP depletion. More recently, however, a new concept has been introduced: necroptosis.
Necroptosis has been proposed as a kind of programmed cell death that is distinct from necrosis and apoptosis; one in which several signals involved in apoptosis participate significantly to initiate the process. It is important to note that necroptosis is an event that can be activated and regulated by such receptors as TNF or Fas — both of which are involved in the extrinsic route of apoptosis activation — when the pro-apoptotic signals are not available or are inhibited.
Activation of death receptors triggers a signaling cascade that includes activation of kinase RIP1, which in turn generates diverse intracellular reactions that lead the cell toward energy failure and conclude with the loss of intracellular homeostasis and the rupture of the membranes that, finally, generates an immunological response. The authors kindly thank Paul C. Kersey Johnson for reviewing the English word usage and grammar. Licensee IntechOpen.
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Luisa Escobar, Olga M. Downloaded: Abstract The process of cell death is the mechanism through which organisms eliminate useless cells. Keywords Apoptosis autophagy cell death necrosis necroptosis. Introduction The tissular environment includes a series of signals that maintain the rates of cell proliferation and cell death so as to conserve structural integrity and functionality.