The term "programmed cell death," and its synonym, "apoptosis," both invoke a vision of an orderly dismantling and disposal of the machinery of a living cell. Regulated by a cascade of caspase proteases, apoptosis is the dignified way for cells to die. In contrast, hear the word "necrosis" and the image that comes to mind is more like that of a train wreck. Necrosis is a far messier and unregulated affair where cells precipitously fall apart in the face of overwhelming insult. While necrosis and apoptosis both play important roles in the death of neurons—in stroke and neurodegenerative disease, for example—apoptosis has received the lion’s share of attention because it is considered a therapeutically tractable pathway (see Alzforum live discussion and Yuan et al., 2004). With the report of a new regulated cell death program, that may be about to change.
Junying Yuan and collaborators at Harvard Medical School have dubbed the caspase-independent, non-apoptotic, alternative death program “necroptosis,” because it shares many features with necrosis—including cell morphology changes, mitochondrial dysfunction, loss of plasma membrane integrity, and autophagic clearance of cell debris—and because it is triggered through the same family of death receptors that control apoptosis. A demonstration that a small molecule inhibitor of necroptosis can block neuron death in a mouse model of ischemia at once proves the physiological relevance of the alterative pathway and offers a new prospect for treating stroke. The availability of the inhibitor will also speed the task of determining the potential role of necroptosis in other pathologies, including Alzheimer disease, where death receptors have been implicated in amyloid-β toxicity (see ARF related news story and Li et al., 2004).
The work, reported in the May 29 online edition of Nature Chemical Biology, was stimulated by the observation that caspase inhibitors, despite their ability to potently and specifically block apoptosis, cannot always save cells from dying. Reports that activation of the Fas/tumor necrosis factor receptor (TNFR) family of death receptors caused necrosis even in the presence of caspase inhibitors led Yuan and coworkers to ask if there was another death program activated by the receptors. To investigate this, first author Alexei Degterev and his colleagues screened a chemical library of 15,000 compounds for inhibitors that could prevent the death of human U937 cells treated with TNF and a caspase inhibitor. From the screen, the authors isolated a compound they called necrostatin-1 (Nec-1), which inhibited necroptosis—cell death induced by death receptors but independent of caspase activation—in several cell types. The authors found that necrostatin could also prevent the death of cells lacking the signaling molecule FADD, which is essential for caspase activation. Nec-1 specifically prevented the mitochondrial dysfunction, loss of cellular ATP content and plasma membrane integrity, and late-stage autophagy response, which are all absent in apoptosis, but it had no effect on healthy cells or those undergoing caspase-dependent apoptosis.
To look for a physiologic role of necroptosis, the researchers turned to stroke, an injury where non-apoptotic cell death is known to be important. When Degterev and colleagues administered Nec-1 to mice, it significantly, and in a dose-dependent manner, reduced the infarct volume caused by subsequent blockage of the cerebral artery. Following the trauma, Nec-1-treated mice also had much more improvement in neurologic function compared to controls. Nec-1 did not block caspase activation, and its protective effect was additive with caspase inhibitors. However, unlike the latter, which must be given within a few hours of the trauma to have an effect, Nec-1 still worked when administration was delayed up to 6 hours, consistent with the observation of a delayed activation of necroptosis. For stroke, the extended time window for therapy would be a valuable asset.
The actions of Nec-1 in vitro and in vivo raise some immediate questions. First, what is the target of Nec-1? Structure-activity studies suggested that Nec-1 binds to a specific target, but the protein or pathway is so far unknown. Second, what are the triggers for necroptosis and what is its physiological role in relation to apoptosis? In non-neuronal cells, necroptosis may serve as a back-up mechanism to eliminate cells when apoptosis fails, speculates Yuan. But during brain ischemia in vivo, necroptosis appeared to play a primary role in neuronal death. “We hypothesize that intrinsic heterogeneity of neuronal populations and, potentially, conditions of stress might render some neuronal cells too ‘damaged’ to undergo ordered apoptosis, making them undergo necroptotic death instead,” Yuan explained. “We are currently in the process of characterizing induction of necroptosis in response to various neurodegenerative stimuli in vitro; however, already available in vivo data for the necrostatin-mediated neuroprotection following brain ischemia suggest that the necroptosis pathway is expressed in neurons and might play an important role in neuronal pathologic death, making necrostatins an interesting new class of neuroprotective agents.”—Pat McCaffrey
- Yuan J, Lipinski M, Degterev A. Diversity in the mechanisms of neuronal cell death. Neuron. 2003 Oct 9;40(2):401-13. PubMed.
- Li R, Yang L, Lindholm K, Konishi Y, Yue X, Hampel H, Zhang D, Shen Y. Tumor necrosis factor death receptor signaling cascade is required for amyloid-beta protein-induced neuron death. J Neurosci. 2004 Feb 18;24(7):1760-71. PubMed.
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- Degterev A, Huang Z, Boyce M, Li Y, Jagtap P, Mizushima N, Cuny GD, Mitchison TJ, Moskowitz MA, Yuan J. Chemical inhibitor of nonapoptotic cell death with therapeutic potential for ischemic brain injury. Nat Chem Biol. 2005 Jul;1(2):112-9. PubMed.