Necrosis is considered to be one of the main caspase-independent cell death types and morphologically distinct from apoptosis. Among the major features of necrosis are the extensive swelling of the cell and various cellular organelles, the random degradation and clumping of nuclear DNA, the formation of small, tightly wrapped membrane whorls, the rupture of the plasma membrane and the appearance of autophagosomes (Edinger and Thompson 2004). The word necrosis is derived from the Greek expression "necros," standing for "dead" and was traditionally considered as the chaotic breakdown of the cell. In humans, necrotic cell death accompanies prolonged hypoxia, ischemia, hypoglycemia, toxin exposure, exposure to reactive oxygen metabolites, extreme changes in temperature, and nutrient deprivation (Nicotera et al. 1999). Necrosis is also involved in neurodegenerative diseases such as Alzheimer's disease, Huntington's disease, Parkinson's disease, amyotrophic lateral sclerosis and epilepsy (Stefanis 2005). Necrotic cell death, unlike apoptosis, was thought to be a passive process, not requiring energy, synthesis of new proteins and specific regulatory mechanisms. However, recent findings in Drosophila and C. elegans have forced a shift of this simplistic view (Syntichaki and Tavernarakis 2002; Kourtis and Tavernarakis 2007).
In the nematode necrotic neuronal death can be triggered by a great variety of extrinsic and intrinsic signals, mainly by the expression of ion channels bearing a hyperactive mutation (Fig. 2.3; Syntichaki and Tavernarakis 2003). The most thoroughly studied case of necrotic cell death is the one induced by hyperactive
deg-1 (d) (degenerin) and mec-4(d) (mechanosensory) both carrying dominant mutations and causing necrosis in special neurons of C. elegans: Gain of function mutations in deg-1 induce necrosis in a group of interneurons of the posterior touch sensory circuit (Chalfie and Wolinsky 1990). mec-4 gain of function mutations cause similar effects in the six touch receptor neurons, which are required for the sensation of gentle touch of the body (Syntichaki and Tavernarakis 2004). Both genes belong to the family of degenerins, which induce cell degeneration when mutated to a hyperactive form. Dying cells exhibit the typical morphological characteristics of necrotic cell death. Degenerins are similar in sequence to the subunits of the amiloride-sensitive epithelial Na+ channel (ENaC) in mammals (Tavernarakis and Driscoll 2001). Large side chain substitutions of amino acids close to the pore forming region of degenerins enhance sodium and calcium conductivity leading to necrotic cell death (Syntichaki and Tavernarakis 2004). Ultimately, extensive ion influx disrupts cellular Ca2+ homeostasis (Syntichaki and Tavernarakis 2003). Calcium imbalance caused by mutated ion channels triggers further release of Ca2+ from the endoplasmic reticulum (ER)via the ryanodine (RyR) and inositol-1,4,5-triphosphate receptors (Ins(1,4,5)P3PR).
The ionic imbalance and subsequent cell death induced by mutant degenerins resembles excitotoxicity in vertebrates, where the collapse of presynaptic neuron membrane potential due to energy depletion results in the release of high amounts of the excitatory neurotransmitter glutamate into the synaptic cleft (Olney 1994). Accumulation of glutamate at the synapse causes hyper-excitation and necrotic cell death of postsynaptic neurons. Excitotoxicity is the prominent mechanism of neuronal loss during stroke, when nutrient and energy supply to neuronal cells is disrupted by blockage of the blood flow. Degenerin-induced neuronal death in C. elegans is an attractive model of excitotoxicity that renders the nematode a suitable and powerful tool for dissecting the molecular mechanisms of neurodegeneration.
In addition to mutant degenerins, several other triggers of nonprogrammed cell death in C. elegans have been described. Constitutive activation of the GTP-binding protein Gas, chemical inhibitors of the respiratory chain (e.g., NaN3), hypoxic treatment, toxins, polyglutamine repeat proteins and macromolecular damage caused by radiation are potent inducers of cell death (Kourtis and Tavernarakis 2007). These inducers have been exploited in genetic and molecular studies that have elucidated key facets of necrotic cell death mechanisms (Artal-Sanz and Tavernarakis 2005).
Null mutations in calreticulin and knock-down of calnexin, which are calcium-binding chaperones, suppress necrotic cell death in C. elegans neurons triggered by mec-4(d). Also the blockage of Ca2+ release from the ER, either by mutations in the calcium release channels encoded by unc-68 (RyR) and itr-1 (Ins(1,4,5)P3PR) or by pharmacological treatment results in similar suppression. These findings indicate that Ca2+ release from the ER plays an essential role in necrotic cell death (Xu et al. 2001).
The cytoplasmic protease calpain, which is activated by calcium and functions in several signaling and metabolic pathways, also plays a role in necrosis. High levels of calcium activate calpains which then localize to lysosomes and cause disintegration of the lysosomal membrane. Subsequent release of lysosomal aspartyl proteases and cathepsins into the cytoplasm causes the breakdown of the cell and rupturing of the plasma membrane. Detailed studies of cell death following brain ischemia in monkeys have led to the formulation of the "calpain-cathepsin" hypothesis for the execution of necrosis (Yamashima 2000, 2004). Genetic studies in C. elegans support the involvement of a calpain-cathepsin axis during neurodegeneration. Downregulation of the calpains CLP-1 and TRA-3 and cathepsins ASP-3 and ASP-4 by RNAi ameliorates neurodegeneration in the nematode (Syntichaki et al. 2002). The proteolytic action of cathepsins in the cytoplasm is further enhanced by the drop of pH in the cell, mediated by the vacuolar H+-ATPase, which acidifies lysosomes and other cell organelles. Alkalization of those organelles prevents necrosis in C. elegans, supporting the involvement of cyto-plasmic acidification in the process (Syntichaki et al. 2005).
The active involvement of lysosomes in necrotic, caspase-independent cell death mechanisms is corroborated by observations in mutant nematodes, defective in lysosomal function (Artal-Sanz et al. 2006). cup-5(lf) mutants, which show increased number of enlarged lysosomes (Hersh et al. 2002) are significantly more sensitive to necrotic cell death inducing insults. Visualization of lysosomal morphology during necrosis reveals aggregation of lysosomes around a swollen nucleus and ultimately lysosomal rupture, consistent with the calpain-cathepsin hypothesis (Artal-Sanz et al. 2006).
In Drosophila, a similar model of excitotoxicity has been utilized to gain insight into the mechanisms of neurodegeneration. The excitatory amino acid transporters (EAATs) are high-affinity transporters for L-glutamate (Glu) involved in clearing Glu from the synaptic cleft and preventing over-excitation of the postsynaptic neuron (Beart and O'Shea 2007). Downregulation of Drosophila dEAAT1, which is expressed in glia, reduces Glu uptake and clearing, which leads to degeneration of neuropil. Similarly to excitotoxicity, degeneration is accompanied by the formation of vacuoles, electron-dense material, and swollen mitochondria (Rival et al. 2004), which are typical features of necrotic cell death.
Was this article helpful?