To die or not to die for neurons in ischemia, traumatic brain injury and epilepsy: a review on the stress-activated signaling pathways and apoptotic pathways

Bim, Bad, and Bax: a deadly combination in epileptic seizures

Several Bcl-2 family members, including Bim, may contribute to programmed cell death by inducing mitochondrial cytochrome c release, which activates caspase-9 and then caspase-3, the "executioner" of the cell. In this issue of the JCI, Shinoda and collaborators show the key role of Bim in epileptic seizure-induced neuronal injury and identify the contribution of transcription factors responsible for seizure-induced Bim upregulation.

Bim regulation may determine hippocampal vulnerability after injurious seizures and in temporal lobe epilepsy

Programmed cell death pathways have been implicated in the mechanism by which neurons die following brief and prolonged seizures, but the significance of proapoptotic Bcl-2 family proteins in the process remains poorly defined. Expression of the death agonist Bcl-2-interacting mediator of cell death (Bim) is under the control of the forkhead in rhabdomyosarcoma (FKHR) transcription factors. This prompted us to examine the response of this pathway to experimental seizures and in hippocampi from patients with intractable temporal lobe epilepsy. A short period of status epilepticus in rats that damaged the hippocampus activated FKHR/FKHRL-1 and induced a significant increase in expression of Bim. Blocking of FKHR/FKHRL-1 dephosphorylation after seizures improved hippocampal neuronal survival in vivo, and Bim antisense oligonucleotides were neuroprotective against seizures in vitro. Inhibition of Akt increased the FKHR/Bim response and DNA fragmentation within the normally resistant cortex. Analysis of hippocampi from patients with intractable epilepsy revealed that Bim levels were significantly lower than in controls and FKHR was inhibited; we were able to reproduce these results experimentally in rats by evoking multiple brief, noninjurious electroshock seizures. We conclude that Bim expression may be a critical determinant of whether seizures damage the brain, and that its control may be neuroprotective in status epilepticus and epilepsy.

Subcellular distribution of Bcl-2 family proteins and 14-3-3 within the hippocampus during seizure-induced neuronal death in the rat

The molecular regulation of seizure-induced neuronal death may involve interactions between proteins of the Bcl-2 and 14-3-3 families. To further examine these pathways we performed subcellular fractionation on hippocampi obtained following a brief period of status epilepticus in the rat. Western blotting determined seizures induced caspase-8 cleavage and increased Bcl-w levels within the cytoplasm. Bax, Bad and Bid were largely present within the cytoplasm before and after seizures, although some Bax and, following seizures, truncated Bid was detected in mitochondria. Levels of 14-3-3 were significantly reduced in the cytoplasm and microsomal fractions. These data establish the expression and distribution profile of key Bcl-2 family proteins and the signaling chaperone 14-3-3 in the rat and provide additional evidence for the activation of programmed cell death pathways by seizures.

Latency to onset of status epilepticus determines molecular mechanisms of seizure-induced cell death

The molecular mechanisms mediating degeneration in response to neuronal insults, including damage evoked by prolonged seizure activity, show substantial variability across laboratories and injury models. Here we investigate the extent to which the proportion of cell death occurring by apoptotic vs. necrotic mechanisms may be shifted by changing the temporal parameters of the insult. In initial studies with continuous seizures (status epilepticus, SE), signs of apoptotic degeneration were most clearly observed when SE occurred following a long latency (>86 min) after injection of kainic acid as compared with a short-latency SE (<76 min). Therefore, in this study we directly compared short- with long-latency SE for the expression of molecular markers for apoptosis and necrosis in an especially vulnerable brain region (rhinal cortex). Molecular markers of apoptosis (DNA fragmentation, cleavage of ICAD, an inhibitor of "caspase-activated DNase" (CAD), and prevalence of a caspase-generated fragment of alpha-spectrin) were detected following long-latency SE. Short-latency SE resulted in expression of predominantly necrotic features of cell death, such as "non-ladder" pattern of genomic DNA degradation, prevalence of a calpain-generated alpha-spectrin fragment, and absence of ICAD cleavage. Silver staining revealed no significant difference in the extent and spatial distribution of degeneration between long- or short-latency SE. These data indicate that the latency to onset of SE determines the extent to which apoptotic or necrotic mechanisms contribute to the degeneration following SE. The presence of a long latency period, during which multiple brief seizure episodes may occur, favors the occurrence of apoptotic cell death. It is possible that the absence of such "preconditioning" period in short-latency SE favors predominantly necrotic profile.

Moderate and severe traumatic brain injury: epidemiologic, imaging and neuropathologic perspectives

This article examines 3 contexts in which moderate or severe traumatic brain injury can be approached. The epidemiologic background of moderate and severe traumatic brain injury is presented, with particular attention paid to new findings from the study of a national hospital inpatient database. We review aspects of neuroimaging and how new imaging modalities can reveal fine detail about traumatic brain injury. Finally we examine the current state of neuropathologic evaluation of, and recent developments in, understanding of the neural disruptions that occur following traumatic brain injury, together with cellular reactions to these disruptions.

Regional ischemia after head injury

PURPOSE OF REVIEW

To examine the evidence of regional cerebral ischemia after traumatic brain injury.

RECENT FINDINGS

This review describes the mechanisms responsible for secondary brain injury and the similarities between traumatic and ischemic neuronal cell death. Cerebral ischemia is defined, and the difficulties of quantifying the burden of cerebral ischemia in the context of clinical head injury are presented. Recent clinical data obtained from monitoring brain tissue oxygenation, tissue metabolites using microdialysis, and cerebral blood flow, blood volume, oxygen metabolism, and oxygen extraction fraction using oxygen-15 positron emission tomography are discussed. These data highlight that significant episodes of regional ischemia occur within the acute phase after injury and are associated with poor outcome. Although various monitoring tools are capable of detecting significant episodes of regional ischemia, each of the currently available techniques is limited in its clinical application.

SUMMARY

There is increasing evidence to suggest that a small but significant volume of brain tissue is at risk of ischemic injury after trauma. Future studies should examine the pathophysiology underlying such ischemia and how monitoring techniques can be used to direct appropriate therapy and influence outcome.

Development of a model of seizure-induced hippocampal injury with features of programmed cell death in the BALB/c mouse

Although mice are amenable to gene knockout, they have not been exploited in the setting of seizure-induced neurodegeneration due to the resistance to injury of key mouse strains. We refined and developed models of seizure-induced neuronal death in the C57BL/6 and BALB/c strains by focally evoking seizures using intra-amygdala kainic acid. Seizures in adult male BALB/c mice, or C57BL/6 mice as reference, caused ipsilateral death of CA1 and CA3 neurons within the hippocampus. Termination of seizures by lorazepam was more effective than diazepam in both strains, largely restricting neuronal loss to the CA3 sector. Electroencephalography (EEG) recordings defined injurious and non-injurious seizure patterns, which could not be separated adequately by behavioral observation alone. Degenerating neurons in the hippocampus were positive for DNA fragmentation and approximately a third of these exhibited morphologic features of programmed cell death. Western blot analysis revealed the cleavage of caspase-8 after seizures in both strains. These data refine our C57BL/6 model and establish a companion model of focally evoked limbic seizures in the BALB/c mouse that provides further evidence for activation of programmed cell death after seizures.

Death-associated protein kinase expression in human temporal lobe epilepsy

Experimental and human data suggest programmed (active) cell death may contribute to the progressive hippocampal atrophy seen in patients with refractory temporal lobe epilepsy. Death-associated protein (DAP) kinase is a novel calcium/calmodulin-activated kinase that functions in apoptosis mediated by death receptors. Because seizure-induced neuronal death involves both death receptor activation and calcium, we examined DAP kinase expression, localization, and interactions in hippocampal resections from patients with intractable temporal lobe epilepsy (n = 10) and autopsy controls (n = 6). Expression and phosphorylation of DAP kinase was significantly increased in epilepsy brain compared with control. DAP kinase and DAP kinase-interacting protein 1 (DIP-1) localized to mitochondria in control brain, whereas levels of both were increased in the cytoplasm and microsomal (endoplasmic reticulum) fraction in epilepsy samples. Coimmunoprecipitation analysis showed increased DAP kinase binding to calmodulin, DIP-1, and the Fas-associated protein with death domain (FADD) in epilepsy samples. Finally, immunohistochemistry determined DAP kinase was coexpressed with DIP-1 in neurons. This study provides the first description of DAP kinase and DIP-1 in human brain and suggests DAP kinase is a novel molecular regulator of neuronal death in epilepsy.

Retinal ischemia: mechanisms of damage and potential therapeutic strategies

Retinal ischemia is a common cause of visual impairment and blindness. At the cellular level, ischemic retinal injury consists of a self-reinforcing destructive cascade involving neuronal depolarisation, calcium influx and oxidative stress initiated by energy failure and increased glutamatergic stimulation. There is a cell-specific sensitivity to ischemic injury which may reflect variability in the balance of excitatory and inhibitory neurotransmitter receptors on a given cell. A number of animal models and analytical techniques have been used to study retinal ischemia, and an increasing number of treatments have been shown to interrupt the "ischemic cascade" and attenuate the detrimental effects of retinal ischemia. Thus far, however, success in the laboratory has not been translated to the clinic. Difficulties with the route of administration, dosage, and adverse effects may render certain experimental treatments clinically unusable. Furthermore, neuroprotection-based treatment strategies for stroke have so far been disappointing. However, compared to the brain, the retina exhibits a remarkable natural resistance to ischemic injury, which may reflect its peculiar metabolism and unique environment. Given the increasing understanding of the events involved in ischemic neuronal injury it is hoped that clinically effective treatments for retinal ischemia will soon be available.

Pathways of survival induced by NGF and extracellular ATP after growth factor deprivation

In a previous work we demonstrated that extracellular adenosine-5'-triphosphate (ATP), acting on P2 receptors, exerts neuritogenic and trophic effects on the phaeochromocytoma PC12 cell line. These actions are comparable to those sustained by nerve growth factor (NGF) and involve several overlapping pathways. In this work, we describe some of the mechanisms recruited by ATP and NGF in maintaining PC12 cell survival after serum deprivation. We show that both ATP and NGF upregulate the expression of the stress-induced heat shock protein HSP70 and HSP90, whilst glucose-response protein GRP75 and GRP78 are not affected. In parallel with NGF, ATP prevents the cleavage and activation of caspase-2 and inhibits the release of cytochrome c from mitochondria into the cytoplasm. Finally, neither NGF, nor ATP directly modulate the expression of P2 receptors in the induction of cell survival. Our data contribute to dissect the biological mechanisms activated by extracellular purines exerting trophic actions and to establish that survival and neurite outgrowth lie on different mechanistic pathways.

Time window of fibroblast growth factor-18-mediated neuroprotection after occlusion of the middle cerebral artery in rats

To assess the time window for fibroblast growth factor-18 (FGF18)-mediated neuroprotection, FGF18 was administered by intravenous infusion at various times after transient occlusion of the middle cerebral artery (MCAO) in rats. Vehicle or FGF18 (100 microg x kg(-1) x h(-1)) was infused at 0.25, 0.5, 1.0, 2.0, 4.0, or 8.0 hours after MCAO with infarct volumes and behavioral deficits measured at 24.0 hours after MCAO. A separate group of animals received the infusions 24 hours after MCAO with endpoints measured at 48 hours after MCAO. Infusion of FGF18 reduced infarct volumes and improved scores in tests of reference and working memory, motor ability, and exploratory behavior. FGF18 was most efficacious when infused within 2 hours after MCAO. Significant reductions in infarct volumes and reductions in deficits of reference memory and motor activity were also observed with FGF18 infused 24 hours after MCAO. Measurements taken at infusion times before 2 hours after MCAO showed that regional cerebral blood flow was increased by FGF18. Administration of vehicle or FGF18 had no significant effect on mean arterial blood pressure, heart rate, brain temperature, blood pH, Pco2, or Po2. These results demonstrate that FGF18 is an effective neuroprotective agent when administered early after transient MCAO in rats. Efficacy observed with infusions at later times suggests an expanded time window for FGF18-mediated neuroprotection.

Discrete gene loci regulate neurodegeneration, lymphocyte infiltration, and major histocompatibility complex class II expression in the CNS

Neurodegeneration and inflammation are fundamental aspects of many neurological diseases. A genome-wide scan of the response to ventral root avulsion (VRA) in a rat F2 cross discloses specific gene regions that regulate these processes. Two gene loci displayed linkage to neurodegeneration and T cell infiltration, respectively, and a single locus displayed extreme linkage to VRA-induced major histocompatibility complex class II expression on microglia. The demonstration that polymorphic genes in different loci control neurodegeneration and CNS inflammation has implications for various experimental rodent nervous system paradigms and potentially for genetically regulated susceptibility to a variety of human CNS diseases.