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Cell death following traumatic brain injury (TBI) is a major cause of neurological deficits and mortality. Understanding mechanisms of delayed post-traumatic cell loss may lead to new therapies that improve outcome. Although TBI induces changes in multiple cell types, mechanisms of neuronal cell death have been the predominant focus. Recent work has emphasized the diversity of neuronal death phenotypes, which have generally been defined by either morphological or molecular changes. This has lead to confusing and at times contradictory nomenclature. Here we review the historical basis of proposed definitions of neuronal cell death with the goal of clarifying critical research questions and implications for therapy in TBI. We believe that both morphological and molecular features must be used to clarify post-traumatic cell death and related therapeutic targets. Further, we underscore that the most effective neuroprotective strategies will need to target multiple pathways and reflect the regional and temporal changes underlying diverse neuronal cell death phenotypes.
Neuronal cell death is required for normal development of the CNS, as well as for removal of dysfunctional cells in pathological conditions including trauma. However, excessive neuronal cell loss underlies acute and chronic neurodegenerative disorders 1. Conceptually, trauma-induced neuronal cell loss has been characterized as primary or secondary; the former refers to immediate cell death related to physical disruption of membranes, whereas the latter reflects delayed cell death in surrounding or distant regions. Secondary neuronal cell death results from physiological and biochemical changes induced by the insult.
Although cell death itself can be readily defined as the irreversible loss of integrated cellular activities and/or terminal disruption of key cellular substructures, it has been more difficult to develop a generally acceptable taxonomy of cell death. Criteria such as type of inducer, specific molecular mechanisms, energy-dependence and morphological changes have all been used to classify cell death, leading to widespread confusion. Moreover, failure to cite critical prior work in other fields has resulted in both controversy and a tendency to over-simplify. For example, neuronal cell death has been commonly delineated into two distinct categories: necrosis, considered a passive process associated with loss of ionic homeostasis, failure of membrane integrity and organelle and cell swelling; and apoptosis, often reflecting an energy-dependent process characterized by cytoplasmic and nuclear condensation and fragmentation, diminished cell volume and relative preservation of organellar structure. The limitations of such nomenclature become readily apparent when “apoptotic-type” mechanisms are associated with “necrotic-type” morphology 2. In fact these two patterns of cell death often co-exist and intermediate morphological forms have been identified, leading some to propose the term “aponecrosis” 3.
The word “necrosis” originates from the ancient Greek word nekros, meaning dead body, and was used by the roman physician Galen 4. In the mid-19th century “necrosis” was mentioned by Rokitansky5 and Virchow6 - who used it to refer to macroscopic phenomena reflecting advanced tissue breakdown. By the end of the 19th century, use of microscopy techniques revealed the connection between macroscopic necrosis and cell death 4. Various changes were observed in the cellular nucleus after ischemia and/or injury and were described by terms such as “pyknosis” (chromatin condensation), “karyolysis” (nuclear disappearance), “karyorhexis” (nuclear fragmentation) and “chromatin margination” (chromatin condensation on the inside of the nuclear membrane) 7–10. Cell death was also observed under physiological conditions such as regressing ovarian follicles -with reference to pyknotic chromatin as well as cell fractionation into smaller bodies, a process called “chromatolysis” 11. Chromatolysis was also identified in the lactating mammary gland 12 and in breast cancer tissue 13. In the early 1950s, chromatolysis was rediscovered in physiological cell death during embryo development, with the key detail that mitochondria appeared largely unaffected 14.
Kerr, Wyllie and colleagues, in seminal papers 15–18, described a conserved cell death process occurring in development, physiological tissue turnover and various pathological conditions; it was characterized by ultrastructural features such as condensation of the cytoplasm and chromatin, cell shrinkage, formation of chromatin balls, normal organelles and fragmentation of cells by budding of membrane enclosed fragments (i.e. apoptotic bodies) and the absence of an inflammatory response. Initially, they called this form of cell death “shrinkage necrosis” but later coined the name “apoptosis” from Greek words apo (from) and ptosis (falling) to reflect the falling leaves of autumn. Although “apoptosis” and ‘chromatolysis” were evidently different names for a similar process, the latter authors were apparently not aware of the prior literature. Kerr, Wyllie and colleagues 19 also described a “necrotic cell death” phenotype, which was present in severely injured tissues and suggested the dichotomy of apoptotic cell death and necrotic cell death.
Schweichel and Merker 20 proposed a classification of developmental cell death based on lysosomes as a key ultrastructural feature: type 1 involving lysosomes of phagocytic cells (heterophagocytosis); type 2, involving the dying cells’ own lysosomes (autophagocytosis); and type 3, showing no lysosomal involvement. Clarke 21 revised the Schweichel and Merker classification to include not only lysosomes but also ultrastructural changes affecting the nuclei/chromatin, cytoplasm, organelles and cell budding/blebbing. Each distinct cell death type shared a conserved set of ultrastructural features and was thought to reflect distinct mechanisms of cell demise. Type 1 is essentially “apoptosis” where the apoptotic bodies are engulfed/destroyed by lysosomes of neighboring cells and/or macrophages. Type 2 is autophagic degeneration, marked by presence of “autophagic vacuoles” which destroy the cytosol and organelles of the cell. Unlike apoptosis, type 2 in general does not feature prominent nuclear pyknosis, shows dilation of organelles and is generally observed in regions undergoing massive degeneration20, (although exception are known in each case)21. Most developmental cell death can be classified as type 1 or 2 in this nomenclature. Type 3A (non-lysosomal vesiculate degradation features prominent swelling of the organelles, presence of lacunar spaces in the cytosol and cellular disintegration without phagocytosis. Type 3B (cytoplasmic degeneration) starts with dilation of organelles, vacuolation of the cytosol followed by nuclear degeneration by karyolysis, but the cell is phagocytosed without fragmentation. Although less common, some forms of developmental cell death reflect features of more than one type or lack a critical component 21. Type 3B developmental cell death shows ultrastructural characteristics (mitochondrial swelling, dilation of organelles, late karyolysis) similar to the “necrotic cell death” phenotype reported in severely injured tissues19, further blurring the separation between physiological and pathological cell death. Majno and Joris also disagree with the apoptosis vs. necrosis dichotomy 4 and argue the importance of identifying the point of “no return” in cell death. In their view a the cell should be considered dead past this stage, even if active processes such as cell dismantling continue. Others prefer a later threshold for death and define it as the irreversible loss of integrated cellular activity22, which may correspond to the rupture of cell membranes and lysis of the cell, cell fragmentation in separate bodies or cell engulfment by other cells23. Majno and Joris interpret necrosis as the presence of dead tissue surrounded by living tissue. For them necrosis is not a form of cell death, but instead reflects the secondary changes that occur after cell death, being cellular equivalent of post-mortem decay 24. Using ischemia as a cell death inducer, the authors observe that cell swelling is one of the earliest observed changes and propose the term “oncosis” to describe these pre-lethal processes leading to cell death, i.e. point of no return24. Yet oncosis (from the Greek word onkos-to swell) was in fact already used much earlier to describe the osteocyte cell death during rickets, as well as during cartilage growth 25. Oncosis and type 3 cell death are virtually identical in their defining elements of cellular morphology. Majno and Joris, as well as others 26, propose limiting necrosis to denote the late phases of cell degradation irrespective of the mechanisms of the cell death. Levin and colleagues 27 suggest adding a descriptor to mark the type of necrosis, for example apoptotic necrosis or oncotic necrosis. Furthermore, they argue that only morphologic/ultrastructural analysis can inform the cell death type and observe that other frequently used criteria are nonspecific; for example, apoptosis can affect large masses of cells and be accompanied by inflammation28 and alternatively that oncosis can occur in single cells.
Remarkably, few papers provide precise descriptions of cell death based on ultrastructural changes and distinguish between pre-lethal cell death processes (i.e. apoptosis, autophagy or oncosis) vs. post-lethal degradation (i.e. necrosis). In part, this may reflect the smaller number of papers using electron microcopy based criteria, leading many authors to use the simplistic apoptosis vs. necrosis dichotomy, or focusing on mechanisms rather than morphology to define cell death. Kroemer suggests that authors refrain from making definitive pronouncements regarding cell death type and instead provide descriptive terms when presenting results (e.g. “caspase-3-positive cell death” instead of “apoptosis”) 23. Unfortunately, this useful suggestion has largely been ignored; thus, the term “necrosis” is commonly used to describe both type 3/oncotic cell death and the uncontrolled cell lysis often associated with tissue inflammation 29. Yet, type 1/apoptosis and type 3/oncosis can be initiated by the same triggering events; with low intensity of the insult favoring apoptosis and high intensity of the insult pushing cells toward oncosis30; or with high intracellular ATP levels favoring initiation of apoptosis and low intracellular ATP levels associated with non-apoptotic cell death 31. Moreover, data suggest that inhibition of one cell death pathway may redirect the cell toward other mechanisms and phenotypes of cell death32.
Another key concept is that of Programmed Cell Death (PCD), which is not equivalent to apoptosis. PCD was first described in a developmental context, even before apoptosis, as cell death that occurs at a predetermined time as a result of a genetic clock 33. Others view PCD as a cell death process that follows a fixed molecular pathway 34. As all developmental cell death meets one or both criteria, they can all be considered versions of PCD. A more useful definition of PCD was inspired in part by the work of R. Sloviter 35 and suggests that PCD encompasses all “active cell deaths” where cellular processes are required for pushing the cell to and beyond the point of “no return”. In this interpretation PCD requires time to develop and involves cellular mechanisms that are potentially open to therapeutic intervention. In opposition to PCD, “passive cell death” results from overwhelming cell injuries (e.g. catastrophic cell damage) that instantaneously push the cell past the point of “no return” without involvement of active cell processes36. Unfortunately, necrosis is often considered the paradigm of passive cell death, which incorrectly implies as untreatable many forms PCD with “necrotic-type” morphology, such as type-3/oncotic cell death.
The study of neuronal cell death in the CNS is particularly challenging because in brightfield microscopy most dying neurons appear shrunken, eosinophilic and with pyknotic nuclei - regardless of the type of death 27. Even at the ultrastructural level dying neurons do not well fit the cell death descriptions established in other tissues and/or during development 31. Thus, distinct types of neuronal cell death such as paraptosis 37 have been described that feature a requirement for gene expression, non-apoptotic morphology marked by vacuolization, and absence of caspases activation. Although it has some partial similarities with type 3B cell death 37, paraptosis is one of the several forms of neuronal cell death with non-classical morphological characteristics and biochemical markers that have been identified.
To attempt to clarify neuronal cell death phenotypes, studies led by J.W. Olney and D.G. Fujikawa have used ultrastructural features. Olney and colleagues identified two types of neuronal cell death in the brain 38, termed “physiologic” cell death and “excitotoxic” cell death (ECD). Physiologic cell death was initially characterized as a developmental death of neurons in the mammalian brain; its description is similar to type 1/apoptosis 39 although with some features that differ from apoptosis in other tissues38. For example, in the CNS, although organelles initially appear normal except for mild mitochondrial swelling 39, mitochondrial degeneration and endoplasmic reticulum vacuolization become prominent after rupture of the nuclear membrane 38. This pattern of orderly morphological changes is found in neurons from various brain regions during development 38 or in response to diverse injuries such as ethanol or TBI 40. In contrast, ECD is characterized by rapid cytoplasmic changes including marked cell swelling, as well as dilation of mitochondria and endoplasmic reticulum, followed by their rupture. Later, chromatin condensation in the nucleus results in small chromatin clumps that eventually consolidate to form a large irregular mass at the center of the nucleus. Subsequently, the cell membrane ruptures but the nuclear membrane remains intact 40. ECD closely resembles type 3b/oncosis cell death, except that the former shows more intense and longer-lasting masses of clumped chromatin. Although at the ultrastructural level ECD is very different from apoptosis, these two cell death processes are more difficult to distinguish under typical light microscopy 39. Following TBI, ECD is detected in the first hours, mostly at the local lesion site, whereas PCD is seen later, particularly at a distance 38. These authors conclude that physiological cell death and ECD are two fundamentally distinct neuronal cell death types and the activation of one path is at the exclusion of the other. Notably, his interpretation conflicts with the view that these death paradigms are extremes of the same process-the continuum theory 41.
Fujikawa and colleagues 42 also contest the existence of a cell death continuum. Based on ultrastructural cell changes, they describe two unique neuronal cell death phenotypes: one with features very similar to apoptosis; the other, ECD, shows ultrastructural changes similar to type 3b/oncosis. However, unlike non-neuronal type 3b/oncotic cell death, neuronal ECD shows a condensed cytoplasm and nucleus (pyknosis) 43. Fujikawa et al also emphasizes that few if any studies demonstrate typical ultrastructural features of neuronal apoptosis outside of neonatal animals 44. Moreover, they conclude that widely held concepts about the apoptosis-specificity of mechanisms/markers- such as TUNEL, internucleosomal DNA cleavage and caspases activation-have no basis, as they can be detected and are required for execution of certain ECD. The latter observations indicate an active cell participation (PCD) in excitotoxicity. Therefore, characterization of cell death should be based on morphology- which reflects the summation of participating factors.
PCD is a form of cell death executed by activation/inactivation of various molecular pathways, with multiple death phenotypes. Many varieties of PCD and associated molecular mechanisms have been identified in the CNS as described in several excellent and comprehensive reviews 36, 45. PCD has been confirmed as a major cause of post-traumatic neuronal cell death and is associated with poorer prognosis in patients after TBI 46. Morphologically defined PCD includes apoptosis, autophagy, paraptosis, calcium-dependent death, and oncosis. The cell death mechanisms that mediate the specific PCD processes include caspases and pro-apoptotic members of Bcl-2 family (apoptosis), JNK and ATG orthologs (autophagy), ERK2 (paraptosis, a cell death associated with trophotoxicity), PARP/AIF (PARP/AIF-dependent death), calpains/cathepsis (calcium-dependent death) and JNK (oncosis)36, 45 among many others Importantly, few or none of these mechanisms are irreplaceable (necessary) for taking a cell past the point of no return. Most often, multiple mechanisms are simultaneously and redundantly activated, and in response to blocking any individual mechanism others can serve to execute the cell death. Several significant developments involving cell death mechanisms will be discussed.
Mature neurons are post-mitotic cells and as such where believed to be unable to re-enter the cell cycle. However, cell cycle events (CCE) can be induced in mature differentiated neurons, where they lead to neuronal cell death47. As shown in several human chronic neurodegenerative disorders, markers of cell cycle re-entry can be detected long before neuronal death- suggesting that CCE may be upstream of the initiation of neuronal cell death execution 47. Acute CNS injuries such as TBI also cause activation of the cell cycle in neurons, leading to cell death 47. TBI is associated with increased expression of cell cycle activation markers such as Cyclin D1, CDK4, E2F5, c-myc and PCNA, as well as down-regulation of various endogenous cell cycle inhibitors, in neurons initiating molecular pathways of apoptosis such as caspase activation48, 49. Moreover, blocking cell cycle activation pathways using pharmacological inhibitors of CDKs attenuate neuronal cell death and significantly improves outcome after TBI in rodent models 48–50.
Caspases are cysteinyl aspartic acid-proteases activated by proteolytic cleavage. The cellular morphological changes that are hallmarks of apoptosis, such as membrane budding, chromatin condensation and nuclear fragmentation require caspase-dependent cleavage of specific substrates. Caspase-3 activation can occur through the extrinsic pathway involving TNF and FAS receptors, or the intrinsic pathway that involves mitochondrial outer membrane permeabilization (MOMP) followed by release of cytochrome c from mitochondria inter-membrane space to the cytosol. There, cytochrome c forms an ATP-dependent complex with the apoptosis-inducing factor (Apaf-1) to activate in turn caspase-9 and caspase-3 51. A recently proposed neuronal death concept involves “dependence receptors”, a group of trophic receptors that promote neuronal survival in the presence of their ligands but in the absence of their ligands can bind and activate caspases, thus becoming caspase substrates that contribute to cell death 45.
Caspase-3 appears to be the major member of the group of effector or executioner caspases, which also include caspases -6 and -7. Caspase-3 plays a major role in injury-induced neuronal loss after TBI 51. Neuronal apoptosis associated with activation of caspases has been shown after both human TBI and in various animal models 52–55. Treatment with structurally different caspase inhibitors improves outcome after experimental TBI, and shows a prolonged therapeutic window 54, 56, 57.
The endoplasmic reticulum (ER) can also serve as the origin of a cell death pathway involving caspase-1258. Caspase-12 can be induced in the brain after TBI, suggesting that the ER apoptotic pathway may play a role in injury-dependent neuronal death 55.
Caspase activity can be regulated by several classes of molecules. Among these, the Bcl-2 family plays an important role,. It includes Bcl-2 and Bcl-xL, which antagonize mitochondria permeabilization, as well as proteins that have the opposite effect. This latter class includes three subtypes; one including Bax and Bak that can directly permeabilize the mitochondria; another including Bid and Bim that activate the first subtype; and one including proteins such as Bad, Puma and Noxa that can target and inactivate Bcl-2 and Bcl-xL 45.The balance between the activities of these two antagonistic types of Bcl-2 family proteins is a major determinant of apostat, the probability of apoptotic death and is reflected in the neuronal cell death after TBI. Increased Bcl-2 and Bcl-xL expression in the brain after TBI is associated with attenuation of cell death and a more favorable prognosis59 whereas increased post-TBI expression of Bax, Bad or Bim may promote cell death 46.
Caspase-independent mechanisms are important mediators of neuronal cell death 60. Some of the most significant findings of the last decade were that the mitochondria inter-membrane space contains other pro-apoptotic molecules in addition to cytochome c. Following MOMP, these proteins -including Smac/DIABLO, Omi/HtrA2, AIF, and endonuclease G- may be released into the cytosol and modulate cell death61–64.
AIF is a phylogenetically ancient flavoprotein NADH oxidase resident in the mitochondrial intermembrane space, where its oxidoreductase activities are required for oxidative phosphorylation 65. Following MOMP, AIF is released into the cytosol and then translocates to the nucleus, where it binds chromatin and causes peripheral chromatin condensation and high molecular weight DNA fragmentation 61. Neuronal cell death following TBI involves AIF nuclear translocation 66. Most studies support a model where AIF-mediated cell death is cytochrome c-, Apaf-1- and caspase-independent 67, 68. Some of the key regulators of AIF release from mitochondria and translocation to the nucleus include PARP-1, Cyclophilin A and HSP-70. There is experimental support for the hypothesis that AIF release is mediated by activation of PARP-1 69. Moreover, inhibition of PARP-1 activity has neuroprotective effects after TBI 70. The mechanisms proposed to explain PARP-1 dependent release of AIF from the mitochondria include depletion of cytosolic NAD+ which causes mitochondrial dysfunction and MOMP71, 72 and poly(ADP-ribose) (PAR) polymer, a product of PARP-1 activity having direct or calpain-mediated effects on mitochondria 73.
Cyclophilins are a subgroup of peptidylprolyl cis/trans isomerases 74. Among these, cyclophilin A (cyclophilin-18, CypA) appears to be both required for the translocation of AIF from the cytosol to the nucleus as well as for the chromatolytic effects of AIF 75. Cyp A knockout animals demonstrate reduced infarct volume after cerebral hypoxia-ischemia suggesting a significant role for the AIF-CypA axis in neuronal cell death in this model 76.
The heat shock proteins of the HSP70 family have multiple functions including serving as ATP-dependent chaperones, assisting the folding of newly synthesized proteins as well as cytoprotection when up-regulated in response to cellular stress 77. Increased levels of HSP70 provide neuroprotection against brain ischemia 78, whereas the absence of HSP70 results in larger lesions after ischemic injury 79. The mechanisms responsible for HSP70-dependent neuroprotection include binding of Apaf-1 and AIF, thereby neutralizing their pro-apoptotic function by blocking the formation of the apoptosome 80 and attenuating nuclear translocation of AIF81, respectively. HSP70 over-expression attenuates ischemic brain injury by sequestering AIF 82 and by reducing caspase-dependent apoptosis 77. Conversely, decreased HSP70 results in increased release of cytochrome c and activation of caspase-3 and associated cell death after cerebral ischemia 79.
In contrast to caspase-mediated cell death, AIF-mediated cell death can proceed under compromised bioenergetic conditions, and is prominent in the central areas of the lesion after cerebral ischemia 67. In fact, in PARP-1-dependent cell death, mitochondria release AIF as well as cytochrome C, but caspases fail be activated because of the associated ATP depletion 71, 73. We hypothesize that unlike caspase-mediated cell death, AIF may play a larger pathophysiological role after more severe brain injuries, which are expected to result in significant bioenergetic declines.
Autophagy is a process that involves lysosomal degradation of proteins and organelles, controlled by genes from the Atg family and under physiological conditions can have a protective role by generating amino acids and energy for the cell 45. Although autophagy has been often seen in dying cells the challenge has been to determine when autophagy serves as a causal factor for cell death (autophagic PCD) and not simply a secondary event 45. Interestingly, in some cell death models inhibition of the mitochondria permeabilization and/or caspases activation creates conditions where autophagy and the Atg genes are required for PCD45, suggesting that autophagy might either be a process parallel but secondary to apoptosis or act as a compensatory mechanism initiated by inhibition of apoptosis45.
Inhibition of caspase-dependent cell death pathways may not provide substantial neuroprotection because of the associated activation of caspase-independent PCD pathways 83. In fact, there is growing support for the hypothesis that in every stress-induced model of cell death there may be multiple pathways involved 84 and the slower/less intense mechanisms become apparent primarily when the dominant mechanisms are attenuated 45. A key role is likely played by the cellular bioenergetic status. When it is preserved, caspases are dominant and AIF-dependent cell death becomes important only after caspase activation has been blocked. However, under bioenergetic deficient conditions, AIF may be the more predominant mechanism. An important question is whether inhibition of the dominant pathway may actually initiate/enhance alternative pathways 3, 85–88. Whether inhibition of the AIF pathway causes an enhanced caspase-dependent response has not been fully determined 89. However, there is solid evidence that AIF and caspases act through parallel pathways, and that strategies that target both have potentially additive therapeutic effects 90. Autophagic and caspase-dependent PCD might also show a similar relationship 45.
Challenges to successful treatment of TBI-induced neuronal cell death include the presence of a multitude of cell death pathways, which have both overlapping and distinct molecular mechanisms, and the short therapeutic windows for some types of neuronal cell death 91. Furthermore, in addition to neuron-specific pathways there is also neutoxicity secondary to microglia-initiated inflammatory responses 92. These facts might explain, in part, failure of clinical neuroprotection trials in TBI, which too often have only targeted a single cell death pathway or modulated mechanisms with a relatively short therapeutic window 91. In pre-clinical studies, improved levels of neuroprotection have been obtained using therapeutic agents with multifuctional activities 93, 94 such as small cyclized dipeptides95, progesterone 96, statins and erythropoietin, among others. Treatment with various cyclic dipeptides significantly improved motor and cognitive recovery after TBI in both rat and mouse models and attenuated apoptotic as well as oncotic cell death in primary neurons97. These agents limit mitochondria changes associated with cytochrome c release, decrease expression of secondary injury pathways such as cell cycle proteins and cathepsins/calpains, and increase expression of neuroprotective molecules including brain-derived neurotrophic factor (BDNF) and heat shock proteins97. In TBI models, progesterone treatment attenuates edema and inflammatory cytokines, and limits neuronal loss by preventing mitochondrial changes, with improved functional outcomes96. Erythropoietin provides neuroprotection after experimental TBI, likely reflecting its ability to stabilize mitochondrial function and to reduce inflammation and oxidative stress 98, 99. Statins have also been shown to significantly improve outcomes after experimental TBI with effects likely due, at least in part, to their ability to activate Akt-dependent mitochondrial sparring pathways as well as to attenuate microglial activation100, 101. What links these therapeutic strategies is the fact that the drugs each show pleiotropic actions, reflecting effects on multiple secondary injury pathways that are likely synergistic98. A different strategy is to focus on therapies that specifically and/or simultaneously target multiple PCD mechanisms. In this paradigm multifunctional effects can be generated by multi-drug combinations and/or by targeting single factors that modulate multiple secondary injury cascades - such as activation of the cell cycle 48–50, PARP-1 70, 102, 103, calpains 104 or HSP70 among others. Targeting processes such as autophagy represents a more complex issue because of studies that indicate both cell death and neuroprotective activity 105, 106. Figure 1 illustrates several key mechanisms that mediate neuronal cell death and shows both more mechanism-specific modulators (green), and pluripotential agents with diverse but less clearly defined mechanisms of action (yellow).
We suggest that improved therapeutic effects may result from treatment strategies that are directed at multiple specific targets/mechanisms of cell death, using either a combination of therapeutic agents and/or multifunctional (pluripotential) drug strategies. Optimal targets should include both caspase-dependent and caspase-independent PCD. Because activation of these pathways occurs in parallel and peak 1–3 days after injury, such strategies should be both additive/synergistic and show a relatively wide therapeutic window. Also promising are therapies that target not only neuron-specific cell death mechanisms, but also block microglial-dependent neurotoxicity92, and /or have actions on other cell types such as oligodendroglia 47, astroglia 47 or endothelial cells 107.
The creation of a comprehensive and rational taxonomy of cell death is difficult because of the diversity of cell death types and the numerous molecular mechanisms involved. Moreover, a given phenotype-even with an ultrastructural requirement- may reflect different mechanisms, or the same mechanism, albeit with different expression level or intensity, may lead to different phenotypes. Nonetheless, we believe that the consistent application of several key concepts and strategies should help clarify the process.
A determination of a cell death phenotype should ideally require ultrastructural support. In the absence of ultrastructural details, a descriptive mode should be employed that includes the known morphological and mechanistic data (e.g. caspse-3 positive apoptosis). Morphology alone may be unable to address the question of PCD vs. passive cell death, considering that both forms of cell death share type 3/oncotic morphology. Furthermore, morphology as the final integrator of all molecular pathways involved in the cell death tends to reflect the dominant pathways and may mask the potential role of additional contributing processes. The latter issue can best be addressed by detailed mechanistic studies. It is increasingly evident that any PCD process may involve multiple, inter-dependent mechanisms. For these reasons we suggest that the optimal therapeutic strategy to limit cell post-traumatic cell death is the use of combination or multipotential treatments that target multiple cell death pathways.
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