Excitotoxic neuronal death in the immature brain is an apoptosis-necrosis morphological continuum

A 3D Tissue Model of Traumatic Brain Injury with Excitotoxicity That Is Inhibited by Chronic Exposure to Gabapentinoids

Injury progression associated with cerebral laceration is insidious. Following the initial trauma, brain tissues become hyperexcitable, begetting further damage that compounds the initial impact over time. Clinicians have adopted several strategies to mitigate the effects of secondary brain injury; however, higher throughput screening tools with modular flexibility are needed to expedite mechanistic studies and drug discovery that will contribute to the enhanced protection, repair, and even the regeneration of neural tissues. Here we present a novel bioengineered cortical brain model of traumatic brain injury (TBI) that displays characteristics of primary and secondary injury, including an outwardly radiating cell death phenotype and increased glutamate release with excitotoxic features. DNA content and tissue function were normalized by high-concentration, chronic administrations of gabapentinoids. Additional experiments suggested that the treatment effects were likely neuroprotective rather than regenerative, as evidenced by the drug-mediated decreases in cell excitability and an absence of drug-induced proliferation. We conclude that the present model of traumatic brain injury demonstrates validity and can serve as a customizable experimental platform to assess the individual contribution of cell types on TBI progression, as well as to screen anti-excitotoxic and pro-regenerative compounds.

Mean Diffusivity in Striatum Correlates With Acute Neuronal Death but Not Lesser Neuronal Injury in a Pilot Study of Neonatal Piglets With Encephalopathy

BACKGROUND

Diffusion MRI is routinely used to evaluate brain injury in neonatal encephalopathy. Although abnormal mean diffusivity (MD) is often attributed to cytotoxic edema, the specific contribution from neuronal pathology is unclear.

PURPOSE

To determine whether MD from high-resolution diffusion tensor imaging (DTI) can detect variable degrees of neuronal degeneration and pathology in piglets with brain injury induced by excitotoxicity or global hypoxia-ischemia (HI) with or without overt infarction.

STUDY TYPE

Prospective.

ANIMAL MODEL

Excitotoxic brain injury was induced in six neonatal piglets by intrastriatal stereotaxic injection of the glutamate receptor agonist quinolinic acid (QA). Three piglets underwent global HI or a sham procedure. Piglets recovered for 20-96 hours before undergoing MRI (n = 9).

FIELD STRENGTH/SEQUENCE

3.0T MRI with DTI, T₁ - and T₂ -weighted imaging.

ASSESSMENT

MD, fractional anisotropy (FA), and qualitative T₂ injury were assessed in the putamen and caudate. The cell bodies of normal neurons, degenerating neurons (excitotoxic necrosis, ischemic necrosis, or necrosis-apoptosis cell death continuum), and injured neurons with equivocal degeneration were counted by histopathology.

STATISTICAL TESTS

Spearman correlations were used to compare MD and FA to normal, degenerating, and injured neurons. T₂ injury and neuron counts were evaluated by descriptive analysis.

RESULTS

The QA insult generated titratable levels of neuronal pathology. In QA, HI, and sham piglets, lower MD correlated with higher ratios of degenerating-to-total neurons (P < 0.05), lower ratios of normal-to-total neurons (P < 0.05), and greater numbers of degenerating neurons (P < 0.05). MD did not correlate with abnormal neurons exhibiting nascent injury (P > 0.99). Neuron counts were not related to FA (P > 0.30) or to qualitative injury from T₂ -weighted MRI.

DATA CONCLUSION

MD is more accurate than FA for detecting neuronal degeneration and loss during acute recovery from neonatal excitotoxic and HI brain injury. MD does not reliably detect nonfulminant, nascent, and potentially reversible neuronal injury.

EVIDENCE LEVEL

1 TECHNICAL EFFICACY: Stage 2 J. Magn. Reson. Imaging 2020;52:1216-1226.

Hypoxia-Ischemia and Hypothermia Independently and Interactively Affect Neuronal Pathology in Neonatal Piglets with Short-Term Recovery

Therapeutic hypothermia is the standard of clinical care for moderate neonatal hypoxic-ischemic encephalopathy. We investigated the independent and interactive effects of hypoxia-ischemia (HI) and temperature on neuronal survival and injury in basal ganglia and cerebral cortex in neonatal piglets. Male piglets were randomized to receive HI injury or sham procedure followed by 29 h of normothermia, sustained hypothermia induced at 2 h, or hypothermia with rewarming during fentanyl-nitrous oxide anesthesia. Viable and injured neurons and apoptotic profiles were counted in the anterior putamen, posterior putamen, and motor cortex at 29 h after HI injury or sham procedure. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) identified genomic DNA fragmentation to confirm cell death. Though hypothermia after HI preserved viable neurons in the anterior and posterior putamen, hypothermia prevented neuronal injury in only the anterior putamen. Hypothermia initiated 2 h after injury did not protect against apoptotic cell death in either the putamen or motor cortex, and rewarming from hypothermia was associated with increased apoptosis in the motor cortex. In non-HI shams, sustained hypothermia during anesthesia was associated with neuronal injury and corresponding viable neuron loss in the anterior putamen and motor cortex. TUNEL confirmed increased neurodegeneration in the putamen of hypothermic shams. Anesthetized, normothermic shams did not show abnormal neuronal cytopathology in the putamen or motor cortex, thereby demonstrating minimal contribution of the anesthetic regimen to neuronal injury during normothermia. We conclude that the efficacy of hypothermic protection after HI is region specific and that hypothermia during anesthesia in the absence of HI may be associated with neuronal injury in the developing brain. Studies examining the potential interactions between hypothermia and anesthesia, as well as with longer durations of hypothermia, are needed.

Metallothionein-I + II Reduces Oxidative Damage and Apoptosis after Traumatic Spinal Cord Injury in Rats

After spinal cord injury (SCI), some self-destructive mechanisms start leading to irreversible neurological deficits. It is known that oxidative stress and apoptosis play a major role in increasing damage after SCI. Metallothioneins I and II (MT) are endogenous peptides with known antioxidant, neuroprotective capacities. Taking advantage of those capacities, we administered exogenous MT to rats after SCI in order to evaluate the protective effects of MT on the production of reactive oxygen species (ROS) and lipid peroxidation (LP), as markers of oxidative stress. The activities of caspases-9 and -3 and the number of annexin V and TUNEL-positive cells in the spinal cord tissue were also measured as markers of apoptosis. Rats were subjected to either sham surgery or SCI and received vehicle or two doses of MT (10 μg per rat) at 2 and 8 h after surgical procedure. The results showed a significant increase in levels of MT protein by effect of SCI and SCI plus treatment at 12 h, while at 24 h an increase of MT was observed only in the injury plus treatment group (p < 0.05). ROS production was decreased by effect of MT in lesioned tissue; likewise, we observed diminished LP levels by MT effect both in the sham group and in the group with SCI. Also, the results showed an increase in the activity of caspase-9 due to SCI, without changes by effect of MT, as compared to the sham group. Caspase-3 activity was increased by SCI, and again, MT treatment reduced this effect only at 24 h after injury. Finally, the results of the number of cells positive to annexin V and TUNEL showed a reduction due to MT treatment both at 24 and 72 h after the injury. With the findings of this work, we conclude that exogenously administered MT has antioxidant and antiapoptotic effects after SCI.

Atomoxetine, a selective norepinephrine reuptake inhibitor, improves short-term histological outcomes after hypoxic-ischemic brain injury in the neonatal male rat

BACKGROUND

Despite the recent progress of perinatal medicine, perinatal hypoxic-ischemic (HI) insult remains an important cause of brain injury in neonates, and is pathologically characterized by neuronal loss and the presence of microglia. Neurotransmitters, such as norepinephrine (NE) and glutamate, are involved in the pathogenesis of hypoxic-ischemic encephalopathy via the interaction between neurons and microglia. Although it is well known that the monoamine neurotransmitter NE acts as an anti-inflammatory agent in the brain under pathological conditions, its effects on perinatal HI insult remains elusive. Atomoxetine, a selective NE reuptake inhibitor, has been used clinically for the treatment of attention-deficit hyperactivity disorder in children. Here, we investigated whether the enhancement of endogenous NE by administration of atomoxetine could protect neonates against HI insult by using the neonatal male rat model. We also examined the involvement of microglia in this process.

METHODS

Unilateral HI brain injury was induced by the combination of left carotid artery dissection followed by ligation and hypoxia (8% O₂, 2 h) in postnatal day 7 (P7) male rat pups. The pups were randomized into three groups: the atomoxetine treatment immediately after HI insult, the atomoxetine treatment at 3 h after HI insult, or the vehicle treatment group. The pups were euthanized on P8 and P14, and the brain regions including the cortex, striatum, hippocampus, and thalamus were evaluated by immunohistochemistry.

RESULTS

HI insult resulted in severe brain damage in the ipsilateral hemisphere at P14. Atomoxetine treatment immediately after HI insult significantly increased NE levels in the ipsilateral hemisphere at 1 h after HI insult and reduced the neuronal damage via the increased phosphorylation of cAMP response element-binding protein (pCREB) in all brain regions examined. In addition, the number of microglia was maintained under atomoxetine treatment compared with that of the vehicle treatment group. To determine the involvement of microglia in the process of neuronal loss by HI insult, we further examined the influence of hypoxia on rat primary cultured microglia by the quantitative real-time polymerase chain reaction. Hypoxia did not cause the upregulation of interleukin-1beta (IL-1β) mRNA expression, but decreased the microglial intrinsic nitric oxide synthase (iNOS)/arginase1 mRNA expression ratio. NE treatment further decreased the microglial iNOS/arginase1 mRNA expression ratio. In contrast, no significant neuroprotective effect was observed at P14 when atomoxetine was administered at 3 h after HI insult.

CONCLUSIONS

These findings suggested that the enhancement of intrinsic neurotransmitter NE signaling by a selective NE reuptake inhibitor, atomoxetine, reduced the perinatal HI insult brain injury. In addition, atomoxetine treatment was associated with changes of TUNEL, pCREB, and BDNF expression levels, and microglial numbers, morphology, and responses.

Neonatal Hypoxia Ischaemia: Mechanisms, Models, and Therapeutic Challenges

Neonatal hypoxia-ischaemia (HI) is the most common cause of death and disability in human neonates, and is often associated with persistent motor, sensory, and cognitive impairment. Improved intensive care technology has increased survival without preventing neurological disorder, increasing morbidity throughout the adult population. Early preventative or neuroprotective interventions have the potential to rescue brain development in neonates, yet only one therapeutic intervention is currently licensed for use in developed countries. Recent investigations of the transient cortical layer known as subplate, especially regarding subplate's secretory role, opens up a novel set of potential molecular modulators of neonatal HI injury. This review examines the biological mechanisms of human neonatal HI, discusses evidence for the relevance of subplate-secreted molecules to this condition, and evaluates available animal models. Neuroserpin, a neuronally released neuroprotective factor, is discussed as a case study for developing new potential pharmacological interventions for use post-ischaemic injury.

Endoplasmic reticulum pathology and stress response in neurons precede programmed necrosis after neonatal hypoxia-ischemia

The endoplasmic reticulum (ER) is tasked, among many other functions, with preventing excitotoxicity from killing neurons following neonatal hypoxia-ischemia (HI). With the search for delayed therapies to treat neonatal HI, the study of delayed ER responses becomes relevant. We hypothesized that ER stress is a prominent feature of delayed neuronal death via programmed necrosis after neonatal HI. Since necrostatin-1 (Nec-1), an inhibitor of programmed necrosis, provides delayed neuroprotection against neonatal HI in male mice, Nec-1 is an ideal tool to study delayed ER responses. C57B6 male mice were exposed to right carotid ligation followed by exposure to FiO2=0.08 for 45 min at p7. Mice were treated with vehicle or Nec-1 (0.1 μl of 8 μmol) intracerebroventricularly with age-matched littermates as controls. Biochemistry assays at 3 and 24h and electron microscopy (EM) and immunohistochemistry at 96 h after HI were performed. EM showed ER dilation and mitochondrial swelling as apparent early changes in neurons. With advanced neurodegeneration, large cytoplasmic fragments containing dilated ER "shed" into the surrounding neuropil and calreticulin immunoreactivity was lost concurrent with nuclear features suggestive of programmed necrosis. Nec-1 attenuated biochemical markers of ER stress after neonatal HI, including PERK and eIF2α phosphorylation, and unconventional XBP-1 splicing, consistent with the mitigation of later ER pathology. ER pathology may be an indicator of severity of neuronal injury and potential for recovery characterized by cytoplasmic shedding, distinct from apoptotic blebbing, that we term neuronal macrozeiosis. Therapies to attenuate ER stress applied at delayed stages may rescue stressed neurons after neonatal HI.

Mitochondrial Optic Atrophy (OPA) 1 Processing Is Altered in Response to Neonatal Hypoxic-Ischemic Brain Injury

Perturbation of mitochondrial function and subsequent induction of cell death pathways are key hallmarks in neonatal hypoxic-ischemic (HI) injury, both in animal models and in term infants. Mitoprotective therapies therefore offer a new avenue for intervention for the babies who suffer life-long disabilities as a result of birth asphyxia. Here we show that after oxygen-glucose deprivation in primary neurons or in a mouse model of HI, mitochondrial protein homeostasis is altered, manifesting as a change in mitochondrial morphology and functional impairment. Furthermore we find that the mitochondrial fusion and cristae regulatory protein, OPA1, is aberrantly cleaved to shorter forms. OPA1 cleavage is normally regulated by a balanced action of the proteases Yme1L and Oma1. However, in primary neurons or after HI in vivo, protein expression of YmelL is also reduced, whereas no change is observed in Oma1 expression. Our data strongly suggest that alterations in mitochondria-shaping proteins are an early event in the pathogenesis of neonatal HI injury.

Excitotoxic neuroprotection and vulnerability with CaMKII inhibition

Aberrant calcium signaling is a common feature of ischemia and multiple neurodegenerative diseases. While activation of calcium-calmodulin (CaM)-dependent protein kinase II (CaMKII) is a key event in calcium signaling, its role in excitotoxicity is controversial. Our findings demonstrate neuroprotection in neuronal cultures treated with the small molecule (KN-93) and peptide (tat-AIP and tat-CN21) inhibitors of CaMKII immediately prior to excitotoxic glutamate/glycine insult. Unlike KN-93 which blocks CaMKII activation, but not constitutively active forms of CaMKII, tat-CN21 and tat-AIP significantly reduced excitotoxicity in cultured neurons when applied post-insult. We observed that the neuroprotective effects of tat-CN21 are greatest when applied before the toxic glutamate challenge and diminish with time, with the neuroprotection associated with CaMKII inhibition diminishing back to control 3h post glutamate insult. Mechanistically, tat-CN21 inhibition of CaMKII resulted in an increase in CaMKII activity and the percentage of soluble αCaMKII observed in neuronal lysates 24h following glutamate stimulation. To address the impact of prolonged CaMKII inhibition prior to excitotoxic insult, neuronal cultures were treated with CaMKII inhibitors overnight and then subjected to a sub-maximal excitotoxic insult. In this model, CaMKII inhibition prior to insult exacerbated neuronal death, suggesting that a loss of CaMKII enhances neuronal vulnerability to glutamate. Although changes in αCaMKII or NR2B protein levels are not responsible for this enhanced glutamate vulnerability, this process is blocked by the protein translation inhibitor cycloheximide. In total, the neuroprotection afforded by CaMKII inhibition can be seen as neuroprotective immediately surrounding the excitotoxic insult, whereas sustained CaMKII inhibition produced by excitotoxicity leads to neuronal death by enhancing neuronal vulnerability to glutamate.

Voltage-dependent potassium channels are involved in glutamate-induced apoptosis of rat hippocampal neurons

The role of voltage-dependent potassium channel currents in glutamate-treated rat hippocampal neurons was investigated. Cell viability was evaluated by MTT reduction assay and morphological changes. Apoptosis was detected by Hoechst33342 staining with fluorescent microscopy and propidium iodide staining with flow cytometry. Membrane potassium channel currents were recorded with whole-cell patch clamp recordings. Results showed that after shortly exposed to glutamate, about 25 and 50% cells died in 3 h and 24 h, respectively. Meanwhile, the enhancement of IK was observed within 6 h after the glutamate insult. TEA selectively blocked IK and significantly reduced cell apoptosis. IA did not change in the insult though 4-AP, the blocker of this current, showed a protective effect against the injury. These data were in consistent with the hypothesis that K+ efflux contributed to glutamate-triggered apoptosis and IK channels might have a therapeutic effect on the treatment of cerebral ischemia.

Effect of Ketamine on Dendritic Arbor Development and Survival of Immature GABAergic Neurons In Vitro

Ketamine, a noncompetitive antagonist of the N-methyl-D-aspartate type of glutamate receptors, was reported to induce neuronal cell death when administered to produce anesthesia in young rodents and monkeys. Subanesthetic doses of ketamine, as adjuvant to postoperative sedation and pain control, are also frequently administered to young children. However, the effects of these low concentrations of ketamine on neuronal development remain unknown. The present study was designed to evaluate the effects of increasing concentrations (0.01-40 microg/ml) and durations (1-96 h) of ketamine exposure on the differentiation and survival of immature gamma-aminobutyric acidergic (GABAergic) interneurons in culture. In line with previous studies (Scallet et al., 2004), we found that a 1-h-long exposure to ketamine at concentrations > or = 10 microg/ml was sufficient to trigger cell death. At lower concentrations of ketamine, cell loss was only observed when this drug was chronically (> 48 h) present in the culture medium. Most importantly, we found that a single episode of 4-h-long treatment with 5 microg/ml ketamine induced long-term alterations in dendritic growth, including a significant (p < 0.05) reduction in total dendritic length and in the number of branching points compared to control groups. Finally, long-term exposure (> 24 h) of neurons to ketamine at concentrations as low as 0.01 microg/ml also severely impaired dendritic arbor development. These results suggest that, in addition to its dose-dependent ability to induce cell death, even very low concentrations of ketamine could interfere with dendritic arbor development of immature GABAergic neurons and thus could potentially interfere with the development neural networks.

Apoptosis in perinatal hypoxic-ischemic brain injury: how important is it and should it be inhibited?

The discovery of safe and effective therapies for perinatal hypoxia-ischemia (HI) and stroke remains an unmet goal of perinatal medicine. Hypothermia and antioxidants such as allopurinol are currently under investigation as treatments for neonatal HI. Drugs targeting apoptotic mechanisms are currently being studied in adult diseases such as cancer, stroke, and trauma and have been proposed as potential therapies for perinatal HI and stroke. Before developing antiapoptosis therapies for perinatal brain injury, we must determine whether this form of cell death plays an important role in these injuries and if the inhibition of these pathways promotes more benefit than harm. This review summarizes current evidence for apoptotic mechanisms in perinatal brain injury and addresses issues pertinent to the development of antiapoptosis therapies for perinatal HI and stroke.

Simultaneous glutamate and GABA(A) receptor agonist administration increases calbindin levels and prevents hippocampal damage induced by either agent alone in a model of perinatal brain injury

Perinatal brain injury is associated with the release of amino acids, principally glutamate and GABA, resulting in massive increases in intracellular calcium and eventual cell death. We have previously demonstrated that independent administration of kainic acid (KA), an AMPA/kainate receptor agonist, or muscimol, a GABA(A) receptor agonist, to newborn rats results in hippocampal damage [Hilton, G.D., Ndubuizu, A., and McCarthy, M.M., 2004. Neuroprotective effects of estradiol in newborn female rat hippocampus. Dev. Brain Res. 150, 191-198; Hilton, G. D., Nunez, J.L. and McCarthy, M.M., 2003. Sex differences in response to kainic acid and estradiol in the hippocampus of newborn rats. Neuroscience. 116, 383-391; Nunez, J.L. and McCarthy, M.M., 2003. Estradiol exacerbates hippocampal damage in a model of preterm infant brain injury. Endocrinology. 144, 2350-2359; Nunez, J.L., Alt, J.J. and McCarthy, M.M., 2003. A new model for prenatal brain damage. I. GABA(A) receptor activation induces cell death in developing rat hippocampus. Exp. Neurol. 181, 258-269]. We now report that KA or muscimol alone administered to immature hippocampal neurons in culture induces significant cell death as evidenced by TUNEL assay. Surprisingly, simultaneous administration of equimolar quantities of these two agonists blocks the effect of either one alone. Moreover, treatment of newborn pups results in less damage compared to either muscimol or KA alone. We further observed that immunoreactivity for the calcium-binding protein, calbindin D(28K), is increased in the brains of pups simultaneously administered KA and muscimol as compared to either alone.

Biphasic changes in the levels of poly(ADP-ribose) polymerase-1 and caspase 3 in the immature brain following hypoxia-ischemia

Poly(ADP-ribose) polymerase-1 (PARP-1) is a DNA repair-associated enzyme that has multiple roles in cell death. This study examined the involvement of PARP-1 in ischemic brain injury in the 7-day old rat, 0.5-48 h after unilateral carotid artery ligation and 2 h of 7.8% oxygen. This experimental paradigm produced a mild to moderate injury; 40-67% of animals in the ligated groups had histological evidence of neuronal death. Ipsilateral cortical injury was seen at all survival times, while mild contralateral cortical injury was seen only at the 1h survival time. Hippocampal injury was delayed relative to the cortex and did not show a biphasic pattern. Immunohistochemical staining for PARP showed bilateral increased staining as early as 1 h post-hypoxia. PARP staining at early time periods was most intense in layer V of cortex, but did not demonstrate a pattern of cell clusters or columns. Ipsilateral PARP-1 levels quantified by western blotting showed a biphasic pattern of elevation with peaks at 0.5 and 12 h post-hypoxia. Contralateral PARP-1 levels were also elevated at 0.5 and 24 h. PARP activity as determined by immunoreactivity for poly(ADP-ribose) (PAR) was increased ipsilaterally at 0.5, 2 and 12 h survival times. Cortical caspase 3-activity was increased ipsilaterally at 6, 12, and 24 h and contralaterally at 0.5, 1, 2 and 6 h post-hypoxia. There are three main findings in this study. First, changes in the distribution and amount of cell death correlate well with measured PARP-1 levels after hypoxia-ischemia, and both display biphasic characteristics. Second, there are significant early, transient morphological and biochemical changes in the contralateral cortex after neonatal hypoxia-ischemia due to unilateral permanent occlusion of a carotid artery followed by 2 h of systemic hypoxia. Third, variability in the responses of individual pups to hypoxia-ischemia suggests the presence of unidentified confounding factors.

Molecular mediators of hypoxic-ischemic injury and implications for epilepsy in the developing brain

Perinatal hypoxia-ischemia (HI) is the most common cause of cerebral palsy, and an important consequence of perinatal HI is epilepsy. Epilepsy is a disorder in which the balance between cerebral excitability and inhibition is tipped toward uncontrolled excitability. Selected neuronal circuits as well as certain populations of glial cells die from the excitotoxicity triggered by HI. Excitotoxicity, a term referring to cell death caused by overstimulation of the excitatory glutamate neurotransmitter receptors, plays a critical role in brain injury caused by perinatal HI. Ample evidence suggests distinct differences between the immature and mature brain with respect to the pathology and consequences of hypoxic-ischemic brain injury. Thus, the intrinsic vulnerability of specific cell types and systems in the developing brain is particularly important in determining the final pattern of damage and functional disability caused by perinatal HI. These patterns of neuronal vulnerability are associated with clinical syndromes of neurologic disorders such as cerebral palsy, epilepsy, and seizures. Recent studies have uncovered important molecular and cellular aspects of hypoxic-ischemic brain injury. The cascade of biochemical and histopathological events initiated by HI can extend for days to weeks after the insult is triggered, which may provide a "therapeutic window" for intervening in the pathogenesis in the developing brain. Activation of apoptotic programs accounts for the majority of HI-induced pathophysiology in neonatal brain disorders. New experimental approaches to protecting brain tissue from the effects of neonatal HI include administration of neuronal growth factors and effective inhibition of the death effector pathways, such as caspase cascade, and their downstream targets, which execute apoptosis and/or induction of their regulatory cellular proteins. Our recent findings that a novel neuronal protein, neuronal pentraxin 1 (NP1), is induced following HI in neonatal brain and that NP1 gene silencing is neuroprotective suggest that NP1 could be a new molecular target in the central neurons for preventing HI injury in developing brain. Most importantly, the specific interactions between NP1 and the excitatory glutamate receptors and their colocalization further implicate a role for this novel neuronal protein in the excitotoxic cascade. Recent experimental work suggests that these approaches may be effective during a longer therapeutic window after the insult, as they are acting on events that are relatively delayed, creating the potential for therapeutic interventions for these lifelong neurological disabilities.

Quantification of sPLA2-induced early and late apoptosis changes in neuronal cell cultures using combined TUNEL and DAPI staining

The terminal deoxynucleotidyl transferase (TdT) dUTP nick end labeling (TUNEL) stain is in wide use for measuring apoptosis in neurons, as well as in other cell types. TUNEL may give false positive results due to variations in labeling technique as well as staining of cells that have undergone non-apoptotic DNA strand breaks. Therefore, in isolation, TUNEL is not a certain indicator of apoptosis. Recently, we have demonstrated the potent apoptotic effect of secreted phospholipase A2 from group III (sPLA2-III) on primary cortical neurons from rat. Here we describe a computer-assisted method for quantifying TUNEL-positive neurons after sPLA2-III induced apoptosis. Extent of TUNEL is normalized to total nuclear content using 4',6-diamidino-2-phenylindole (DAPI) staining. Furthermore, DAPI counterstaining allows for determination of a nuclear morphology indicator, based on nuclear size and roundness, which we call the nuclear area factor. We found that the nuclear area factor is an early indicator of cell death (significant after 4 h post treatment), while TUNEL staining is significant at later times (26 h). Thus, the independent staining techniques using TUNEL and DAPI complement each other, and with commercially available image analysis software, may be used to indicate early as well as delayed cell injury processes.

Neuropathology of seizures in the immature rabbit

Acute morphologic changes of brain due to chemically induced seizures are studied in developing rabbits. Accordingly, rabbits of postnatal days 6 and 7 (p6-7) and p10-12 are injected with a single dose of 1-6 mg/kg kainic acid (KA) intraperitoneally (i.p.) or injected with a single dose of 200-300 mg/kg pilocarpine subcutaneously (s.c.). Many animals developed seizures of varying severity and length. Histologic examination of brain 2 days following injection showed that KA-induced seizures did not cause neuronal death. Pilocarpine-induced seizures resulted in neuronal death mainly involving the CA1 region of hippocampus. In the p6-7 group, only a small number of brains were involved, lesions were mild and limited to CA1. In the p10-12 group, majority of the brains were damaged, lesions were relatively severe, and in some brains extended beyond the CA1 region involving the subiculum, CA3, cortex, and amygdala. Measurements of physiologic parameters indicate that these changes were not secondary to hypoxemia during seizures. However, there was hypotension and hyperthermia, both of which may contribute to brain damage during seizures. The findings suggest that pilocarpine-induced seizures during the second postnatal week in rabbits is a useful model to study the morphologic changes of brain due to seizure in the developing animal and also to assess the systemic physiologic alterations during 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.

Caspase inhibition switches the mode of cell death induced by cyanide by enhancing reactive oxygen species generation and PARP-1 activation

Execution of apoptosis can involve activation of the caspase family of proteases. Recent studies show that caspase inhibition can switch the morphology of cell death from apoptotic to necrotic without altering the level of death among cell populations. In the present study, the effect of caspase inhibition on cortical (CX) cell death induced by cyanide was investigated. In primary cultured CX cells exposed to cyanide (400 microM), death was primarily apoptotic as indicated by positive TUNEL staining. Reactive oxygen species (ROS) generation and subsequent caspase activation mediated the apoptosis. Inhibition of the caspase cascade with zVAD-fmk switched the apoptotic response to necrotic cell death, as assessed by increased cellular efflux of LDH and propidium iodide uptake by the cells. The change in death mode was accompanied by a marked increase in poly (ADP-ribose) polymerase-1 (PARP-1) activity, reactive oxygen species (ROS) generation, a reduction in the mitochondrial membrane potential (Delta psi(m)), and reduced cellular ATP. Prior treatment of cells with 3-aminobenzamide (3-AB), a PARP-1 inhibitor, prevented the cells from undergoing necrosis and preserved intracellular ATP levels. These findings indicate that apoptosis and necrosis share common initiation pathways and caspase inhibition can switch the apoptotic response to necrosis. Inhibition of PARP-1 preserves cellular ATP levels and in turn blocks execution of the necrotic death pathway.

Apoptosis of auditory neurons following central process injury

Although apoptotic changes in auditory neurons induced by injury to peripheral processes (dendrites) have been intensively studied, apoptotic changes in auditory neurons induced by injury to central processes (axons of spiral ganglion cells, SGCs) have not been reported previously, probably due to lack of an experimental model. The present study reports for the first time the appearance, extent, and time course of SGC apoptosis following injury to the central processes. Apoptosis was studied in a rat model that consisted of compression of the auditory nerve in the cerebellopontine (CP) angle cistern with intraoperative recordings of auditory nerve compound action potentials (CAPs) to ensure highly reproducible results. Rats were killed between day 0 and day 14 after compression and apoptosis of SGCs was evaluated quantitatively as well as qualitatively by terminal deoxynucleotidyl transferase (TdT)-mediated deoxyuridine triphosphate nick-end labeling (TUNEL) staining, anti-activated caspase-3 immunostaining, Hoechst 33342 staining, and electron microscopy. The average number of TUNEL-positive apoptotic SGCs in each cochlear turn increased from day 1 to day 5 and then decreased gradually to an undetectable level on day 14 after compression. The average proportion of apoptotic SGCs identified in any cochlear turn on any day was always lower than 10%. The results of our present study should be useful in determining the therapeutic time window for rescuing auditory neurons undergoing apoptosis due to injury during surgery in the CP angle.

Rotenone induces non‐specific central nervous system and systemic toxicity

We investigated the dopaminergic (DA) neuronal degeneration in animals subjected to systemic treatment of rotenone via subcutaneous delivery. Behavioral observations revealed a hypokinetic period in rats sacrificed at 3 and 5 days, and dystonic episodes in animals sacrificed at 8 days. Less than 20% of the total number of animals given rotenone depicted brain lesions after 8 days of treatment, as demonstrated by a significant loss of DA fibers in the striatum, but not of DA nigral neurons. Tyrosine hydroxylase-negative striatal territories were characterized by post-synaptic toxicity as demonstrated by a decreased number of interneurons labeled for choline acetyltransferase, NADPH-diaphorase, parvalbumin, and projection neurons labeled for calbindin and nerve growth factor inducible-B (NGFI-B). Post-synaptic neurodegeneration was demonstrated further by abundant striatal staining for Fluoro-Jade. Decrease in the nuclear orphan receptor Nurr1 expression was the only significant change observed at the level of the substantia nigra. Autopsy reports confirmed that animals suffered from severe digestion problems. These data suggest that hypokinesia observed between 3 and 5 days is the result of general health problems rather than a specific motor deficit associated to Parkinson's disease (PD) symptoms. Overall, the effects of rotenone toxicity are widespread, and subcutaneous administration of this toxin does not provide the neuropathological and behavioral basis for a relevant and reliable PD model.

Resiniferatoxin-induced loss of plasma membrane in vanilloid receptor expressing cells

Resiniferatoxin (RTX), a potent analog of capsaicin, was evaluated electrophysiologically in dorsal root ganglion (DRG) cells and cell lines ectopically expressing the vanilloid receptor type 1 (VR1) to determine if cell phenotype influenced RTXs neurotoxic properties. Furthermore, capsaicin and heat activation of VR1 were evaluated in these cells to determine if cellular damage was unique to RTX activation of the receptors. RTX application to DRG cells identified as type 1, 2 or 5, cell types known to express VR1, induced large inward currents. RTX did not induce currents in DRG cells that do not express the receptor (type 4 cells). In cell lines ectopically expressing VR1, RTX-induced similar currents. RTX produced no effect in non-transfected cells. After exposure to RTX both DRG cells and transfected cells failed to respond to subsequent applications of the agonist. In addition, whole cell capacitance was reduced up to 70%. The decrease in capacitance was associated with the loss of plasma membrane, as determined by confocal microscopy. Cell phenotype, other than VR1 expression, did not influence the response to RTX. Interestingly, capsaicin and heat activation of vanilloid receptors also decreased cell capacitance, but the loss of membrane was not as great as with RTX and responses to these stimuli were not lost after the initial exposure. The loss of cell membrane required elevated intracellular levels of Ca2+. From these data it was concluded that the loss of cell membrane was dependent on the presence of both VR1 and intracellular Ca2+ accumulation, but not on cell phenotype.

Insulin-like growth factor-1 and post-ischemic brain injury

Insulin-like growth factor-1 (IGF-1) is a naturally occurring neurotrophic factor that plays an important role in promoting cell proliferation and differentiation during normal brain development and maturation. The present review examines recent evidence that endogenous IGF-1 also plays a significant role in recovery from insults such as hypoxia-ischemia and that giving additional exogenous IGF-1 can actively ameliorate damage. It is now well established that neurons and other cell types die many hours or even days after initial injury due to activation of programmed cell death pathways. IGF-1 and its binding proteins and receptors are intensely induced within damaged brain regions following brain injury, suggesting a possible a role for IGF-1 in brain recovery. Exogenous administration of IGF-1 within a few hours after brain injury is now known to be protective in both gray and white matter and leads to improved somatic function. In contrast, pre-treatment is ineffective, likely reflecting limited intracerebral penetration of IGF-1 into the uninjured brain. The neuroprotective effects of IGF-1 are mediated by IGF-1 receptors and its binding proteins and are specific to particular cellular phenotypes and brain regions. The window of opportunity for treatment with IGF-1 is limited to a few hours after normothermic brain injury, reflecting its specific actions on early, intracellular events in the apoptotic cascade. However, injury-associated mild post-hypoxic hypothermia, which delays the development of cell death, can shift and dramatically extend the window of opportunity for delayed treatment with IGF-1. Such a combined approach is likely to be essential for any clinical treatment.

Regulation and critical role of potassium homeostasis in apoptosis

Programmed cell death or apoptosis is broadly responsible for the normal homeostatic removal of cells and has been increasingly implicated in mediating pathological cell loss in many disease states. As the molecular mechanisms of apoptosis have been extensively investigated a critical role for ionic homeostasis in apoptosis has been recently endorsed. In contrast to the ionic mechanism of necrosis that involves Ca(2+) influx and intracellular Ca(2+) accumulation, compelling evidence now indicates that excessive K(+) efflux and intracellular K(+) depletion are key early steps in apoptosis. Physiological concentration of intracellular K(+) acts as a repressor of apoptotic effectors. A huge loss of cellular K(+), likely a common event in apoptosis of many cell types, may serve as a disaster signal allowing the execution of the suicide program by activating key events in the apoptotic cascade including caspase cleavage, cytochrome c release, and endonuclease activation. The pro-apoptotic disruption of K(+) homeostasis can be mediated by over-activated K(+) channels or ionotropic glutamate receptor channels, and most likely, accompanied by reduced K(+) uptake due to dysfunction of Na(+), K(+)-ATPase. Recent studies indicate that, in addition to the K(+) channels in the plasma membrane, mitochondrial K(+) channels and K(+) homeostasis also play important roles in apoptosis. Investigations on the K(+) regulation of apoptosis have provided a more comprehensive understanding of the apoptotic mechanism and may afford novel therapeutic strategies for apoptosis-related diseases.

Neonatal monosodium glutamate treatment modifies glutamic acid decarboxylase activity during rat brain postnatal development

Monosodium glutamate (MSG) produces neurodegeneration in several brain regions when it is administered to neonatal rats. From an early embryonic age to adulthood, GABA neurons appear to have functional glutamatergic receptors, which could convert them in an important target for excitotoxic neurodegeneration. Changes in the activity of the GABA synthesizing enzyme, glutamic acid decarboxylase (GAD), have been shown after different neuronal insults. Therefore, this work evaluates the effect of neonatal MSG treatment on GAD activity and kinetics in the cerebral cortex, striatum, hippocampus and cerebellum of the rat brain during postnatal development. Neonatal MSG treatment decreased GAD activity in the cerebral cortex at 21 and 60 postnatal days (PD), mainly due to a reduction in the enzyme affinity (K(m)). In striatum, the GAD activity and the enzyme maximum velocity (V(max)) were increased at PD 60 after neonatal MSG treatment. Finally, in the hippocampus and cerebellum, the GAD activity and V(max) were increased, but the K(m) was found to be lower in the experimental group. The results could be related to compensatory mechanisms from the surviving GABAergic neurons, and suggest a putative adjustment in the GAD isoform expression throughout the development of the postnatal brain, since this enzyme is regulated by the synaptic activity under physiological and/or pathophysiological conditions.

Cell death in models of spinal cord injury

Current treatments for acute spinal cord injury are based on animal models of human spinal cord injury (SCI). These models have shown that the initial traumatic injury to cord tissue is followed by a long period of secondary injury that includes a number of cellular and biochemical cascades. These secondary injury processes are potential targets for therapies. Continued refinement of rat and mouse models of SCI, along with more detailed analyses of the biology of the lesion in these models, points to both necrotic and apoptotic mechanisms of cell death after SCI. In this chapter, we review recent evidence for long-term apoptotic death of oligodendrocytes in long tracts undergoing Wallerian degeneration following SCI. This process appears to be related closely to activation of microglial cells. It is has been thought that microglial cells might be the source of cytotoxic cytokines, such as tumor necrosis factor-alpha (TNF-alpha), that kill oligodendrocytes. However, more recent evidence in vivo suggests that TNF-alpha by itself may not induce necrosis or apoptosis in oligodendrocytes. We review data that suggests other possible pathways for apoptosis, such as the neurotrophin receptor p75 which is expressed in both neurons and oligodendrocytes after SCI in rats and mice. In addition, it appears that microglial activation and TNF-alpha may be important in acute SCI. Ninety minutes after a moderate contusion lesion, microglia are activated and surround dying neurons. In an 'atraumatic' model of SCI, we have now shown that TNF-alpha appears to greatly potentiate cell death mediated by glutamate receptors. These studies emphasize that multiple mechanisms and interactions contribute to secondary injury after SCI. Continued study of both contusion models and other new approaches to studying these mechanisms will be needed to maximize strategies for acute and chronic therapies, and for neural repair.

Multiparametric flow cytometric analysis of biochemical and functional events associated with apoptosis and oncosis using the 7-aminoactinomycin D assay

Apoptosis and primary necrosis are the two modes of cell death induced by a lethal injury. The majority of structural and biochemical events occurring during cell death can be analysed by flow cytometry. The 7-aminoactinomycin D (7-AAD) assay can be used to detect the loss of membrane integrity during apoptosis of murine thymocytes and human peripheral lymphocytes. We describe here new applications of the 7-AAD assay. It can be applied to a variety of cell lines of different origins, including adherent cell lines, and it allows the co-detection of lipidic antigens such as phosphatidylserine (PS) residues, and biochemical processes linked to apoptosis, such as the loss of mitochondrial transmembrane potential, cardiolipin peroxidation, the expression of the 7A6 mitochondrial antigen and DNA fragmentation. Thus, this assay is a noninvasive method particularly adapted to the analysis of biochemical events associated with cell death. Finally, we show that this assay is not specific for apoptosis since it detects oncosis, the early stage of primary necrosis.

A murine photochemical stroke model with histologic correlates of apoptotic and nonapoptotic mechanisms

INTRODUCTION

The neuronal cell death that occurs after ischemia-induced cerebral infarction (stroke) contains elements of apoptosis and necrosis, an intermediary form of the two, and a distinct excitotoxic process. We previously developed a photochemical model of stroke in the rat. We have now adapted this model for use in the mouse. The present manuscript describes the mouse model.

METHODS

Minimal beam intensity (0.1 W/cm(2)) cold white light (8 min exposure) was used to evoke discrete infarcts in the parietal lobes of 11 mice sensitized by the administration of fresh Rose Bengal (10 mg/kg by rapid iv infusion).

RESULTS

At 2 h, five out of five mice and at 6 h, six out of six mice demonstrated light microscopic histologic features like those in the rat model. These included a superior ischemic zone with shrunken and pyknotic nuclei, a middle transition zone of edematous vacuolated neuropil but normal neurons with open chromatin and retained Nissl granules, and an inferior zone with normal neurons. There was widespread nuclear terminal deoxynucleotidyl-transferase-mediated dUTP nick end labeling (TUNEL) in the superior infarct zone in 11/11 mice. However, in the edematous vacuolated transition zone, 11/11 mice had TUNEL positive and negative nuclei randomly mixed. Light microscopic analysis of that same transition zone showed no pyknosis or chromatin bodies in the TUNEL positive or negative cells.

DISCUSSION

In mice, photoactivation of Rose Bengal evoked similar infarct and transition zone patterns found previously in rats, with TUNEL evidence of apoptotic and nonapoptotic events. Thus, it will be possible to use this model for further quantitative study of apoptotic and excitotoxic events in wild and transgenic mice.

Maternal deprivation increases cell death in the infant rat brain

Prolonged separation from the mother can interfere with normal growth and development and is a significant risk factor for adult psychopathology. In rodents, separation of a pup from its mother increases the behavioral and endocrine responses to stress for the lifetime of the animal. Here we investigated whether maternal deprivation could affect brain development of infant rats via changes in the rate of cell death as measured by labeling the 3' end of DNA fragments using terminal transferase (ApopTag). At postnatal day 12 (P12), the number of cells undergoing cell death approximately doubled in the cerebral cortex, cerebellar cortex and in several white matter tracts following 24 h of maternal deprivation. Deprivation strongly increased the number of ApopTag-labeled cells at P6 but not at P20. Stroking the infant rats only partially reversed the effects of maternal deprivation. Increased cell death in white matter tracts correlated with an induction of nerve growth factor which has been previously associated with oligodendrocyte cell death. Cell birth was either unchanged or decreased in response to deprivation. These results indicate that maternal deprivation can alter normal brain development by increasing cell death of neurons and glia, and provides a potential mechanism by which early environmental stressors may influence subsequent behavior.

Differential expression of active, phosphorylation-dependent MAP kinases, MAPK/ERK, SAPK/JNK and p38, and specific transcription factor substrates following quinolinic acid excitotoxicity in the rat

Excitotoxicity is considered a major cell death inductor in neurodegeneration. Yet mechanisms involved in cell death and cell survival following excitotoxic insults are poorly understood. Expression of active, phosphorylation-dependent mitogen-activated extracellular signal-regulated kinases (MAPK/ERKs), stress activated c-Jun N-terminal kinases (SAPK/JNKs) and p38 kinases, as well as their putative active specific transcriptional factor substrates CREB, Elk-1, ATF-2, c-Myc and c-Jun, have been examined following intracortical injection of the glutamate analogue quinolinic acid (QA). Increased JNK(P) and p38(P) immunoreactivity has been found in the core at 1 h following QA injection, whereas increased MAPK(P) immunoreactivity occurs in neurons and glial cells localised around the lesion and in neurons in remote cortical regions. This is accompanied by strong phosphorylated Ser63 c-Jun (c-Jun(P)) immunoreactivity in the core at 3 h, and by strong phosphorylated CREB, Elk-1 and ATF-2 (CREB(P), Elk-1(P) and ATF-2(P)) immunoreactivity mainly in neurons around the core at 24 h following QA injection. Examination with the method of in situ end-labelling of nuclear DNA fragmentation has revealed large numbers of positive cells with no apoptotic morphology in the core at 24 h, thus indicating that JNK(P), p38(P) and c-Jun(P) over-expression precedes cell death. In contrast, MAPK(P), CREB(P), Elk-1(P) and ATF-2(P), but not phosphorylated c-Myc (c-Myc(P)), over-expression correlates with cell survival. Examination of cleaved, active caspase-3 has shown specific immunoreactivity restricted to a few hematogenous cells in the area of injection. Since cleaved caspase-3 is not expressed by dying cells in the present paradigm, JNK(P), p38(P) and c-Jun(P) expression is not associated with caspase-3 activation. The present results demonstrate selective activation of specific MAPK signals which are involved either in cell death or cell survival triggered by excitotoxic insult.

Nitric oxide is involved in ischemia-induced apoptosis in brain: a study in neuronal nitric oxide synthase null mice

Nitric oxide can promote or inhibit apoptosis depending on the cell type and coexisting metabolic or experimental conditions. We examined the impact of nitric oxide on development of apoptosis 6, 24, and 72 h after permanent middle cerebral artery occlusion in mutant mice that lack the ability to generate nitric oxide from neuronal nitric oxide synthase. Adjacent coronal sections passing through the anterior commissure were stained with hematoxylin and eosin or terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL). Immunoblotting was used to identify changes in the anti- and proapoptotic proteins Bcl-2 and Bax, respectively. Activation of caspases was assessed by appearance of actin cleavage products using a novel antiserum directed against 32-kDa actin fragment (fractin). In the neuronal nitric oxide synthase mutant mouse, infarct size and TUNEL positive apoptotic neurons were reduced compared to the wild-type controls. At 6 h, Bcl-2 levels in the ischemic hemisphere were increased in mutants but decreased in the wild-type strain. Bax levels did not change significantly. Caspase-mediated actin cleavage appeared in the ischemic hemisphere at this time point, and was significantly less in mutant brains at 72 h compared to the wild-type. The reduction in the number of TUNEL and fractin positive apoptotic cells appears far greater than anticipated based on the smaller lesion size in mutant mice.Hence, from these data we suggest that a deficiency in neuronal nitric oxide production slows the development of apoptotic cell death after ischemic injury and is associated with preserved Bcl-2 levels and delayed activation of effector caspases.

Subcellular distribution of calcium and ultrastructural changes after cerebral hypoxia-ischemia in immature rats

Recent data imply that mitochondrial regulation of calcium is critical in the process leading to hypoxic-ischemic brain injury. The aim was to study the subcellular distribution of calcium in correlation with ultrastructural changes after hypoxia-ischemia in neonatal rats. Seven-day-old rats were subjected to permanent unilateral carotid artery ligation and exposure to hypoxia (7.7% oxygen in nitrogen) for 90 min. Animals were perfusion-fixed after 30 min, 3 h or 24 h of reperfusion. Sections were sampled for light microscopy and electron microscopy combined with the oxalate-pyroantimonate technique. At 30 min and 3 h of reflow, a progressive accumulation of calcium was detected in the endoplasmic reticulum, cytoplasm, nucleus and, most markedly, in the mitochondrial matrix of neurons in the gray matter in the core area of injury. Some mitochondria developed a considerable degree of swelling reaching a diameter of several microm at 3 h of reflow whereas the majority of mitochondria appeared moderately affected. Chromatin condensation was observed in nuclei of many cells with severely swollen mitochondria with calcium deposits. A whole spectrum of morphological features ranging from necrosis to apoptosis was seen in degenerating cells. After 24 h, there was extensive injury in the cerebral cortex as judged by breaks of mitochondrial and plasma membranes, and a general decrease of cellular electron density. In the white matter of the core area of injury, the axonal elements exhibited varicosity-like swellings filled with calcium-pyroantimonate deposits. Furthermore, the thin myelin sheaths were loaded with calcium. Numerous oligodendroglia-like cells displayed apoptotic morphology with shrunken cytoplasm and chromatin condensation, whereas astroglial necrosis was not seen. In conclusion, markedly swollen 'giant' mitochondria with large amounts of calcium were found at 3 h of reperfusion often in neuronal cells with condensation of the nuclear chromatin. The results are discussed in relation to mitochondrial permeability transition and activation of apoptotic processes.

Cyclosporin A targets involved in protection against glutamate excitotoxicity

The toxicity of glutamate in neuronal cultures has been attributed in part to a mitochondrial dysfunction involving the permeability transition pore. The participation of the permeability transition pore in this process has been pharmacologically demonstrated by the use of cyclosporin A, which inhibits pore opening by interaction with mitochondrial cyclophilin and, thus, prevents cell death and upstream events. Since cyclosporin A also acts on calcineurin, we have investigated which of the targets of cyclosporin A was responsible for the inhibition of glutamate-excitotoxicity in cerebrocortical primary neuronal cultures. Reactive oxygen species production and early (30 min to 2 h) drop in ATP levels are initial events in glutamate excitotoxicity taking place before neuronal death. Cyclosporin A did not inhibit reactive oxygen species production, but reduced the drop in ATP levels and subsequent neuronal death. However, cyclosporin derivatives that do not bind to calcineurin had smaller effect on survival than cyclosporin A, (regardless of whether they were able to bind cyclophilin), indicating that cyclosporin A protects against glutamate toxicity also through calcineurin-related mechanisms. Consistent with this view, ATP loss appears to result from nitric oxide synthase (NOS) activation (including calcineurin-dependent dephosphorylation) and nitric oxide (NO)/peroxinitrite-dependent increase in poly (ADP-ribose) polymerase activity, since it was reduced by inhibitors of these activities. Collectively, these results suggest that cyclosporin A exerts its protective effects through calcineurin-dependent and independent mechanisms.

Kainic acid-induced seizures produce necrotic, not apoptotic, neurons with internucleosomal DNA cleavage: implications for programmed cell death mechanisms

Prolonged seizures (status epilepticus) induced by kainic acid activate programmed cell death mechanisms, and it is believed that kainic acid-induced status epilepticus induces neuronal apoptosis. In order to test this hypothesis, adult rats were subjected to 3-h kainic acid-induced seizures, with 24- or 72-h recovery periods. Neuronal death was assessed by light microscopy with the Hematoxylin and Eosin stain and with in situ terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL stain), by electron microscopy, and by agarose gel electrophoresis of DNA extracted from five vulnerable brain regions. Spontaneous and MK-801-induced apoptotic neurons from retrosplenial cortex of neonatal rats, evaluated by light and electron microscopy, were used as positive controls for apoptosis. Surprisingly, the large chromatin clumps of apoptotic neurons were TUNEL negative, whereas the cytoplasm showed light-to-moderate TUNEL staining, consistent with a lack of identifiable nuclear membranes ultrastructurally, and with intermingling of nuclear and cytoplasmic contents. Ultrastructurally, the acidophilic neurons produced by kainic acid-induced status epilepticus, identified with Hematoxylin and Eosin stain, were dark, shrunken and necrotic, with pyknotic nuclei containing small, dispersed chromatin clumps, and with cytoplasmic vacuoles, some of which were swollen, disrupted mitochondria. No apoptotic cells were seen. Acidophilic neurons were found in up to 20 of 23 brain regions examined and comprised 10-25% of the total number of neurons examined. A subset of these neurons (<10% of the total number of neurons in five of 23 regions) had TUNEL-positive nuclei 72h but not 24h after status epilepticus. Internucleosomal DNA cleavage (DNA "laddering") occurred in the four most damaged brain regions examined by electron microscopy 24h after SE and the three most damaged regions 72h after status epilepticus. Our results demonstrate that kainic acid-induced status epilepticus produces neuronal necrosis and not apoptosis in adult rats. The necrotic neurons show nuclear pyknosis, chromatin condensation and DNA laddering. Programmed cell death mechanisms activated by kainic acid-induced status epilepticus occur in neurons which become necrotic and could contribute to necrotic, as well as apoptotic, neuronal death.

Ultrastructural identification of dentate granule cell death from pilocarpine-induced seizures

Cell loss in the hippocampal formation is a common event in patients with temporal lobe epilepsy. The belief that dentate granule neurons are relatively resistant to excitotoxic injury has recently been challenged both, in epileptic patients and in animal models of temporal lobe epilepsy. The nature of dentate granule cell damage in epilepsy has been reported as either apoptotic, necrotic or both. The lack of a consensus on this topic stems from use of different animal models and different experimental techniques for characterizing the apoptotic/necrotic process. Using electron microscopy for defining the, nature of cell loss and one of the main animal models of status epilepticus (SE) we have focussed on the nature of the degenerative process in dentate granule cells. Ultrastructural morphological changes of these cells were evaluated 2.5-48 h after pilocarpine-induced status epilepticus. A variety of morphologies ranging from apoptosis to necrosis, could be seen at 2.5 h after SE onset and continued at least over the following 48 h. Some cells displayed coalescence of chromatin against nuclear membranes. In such cases however, chromatin did not have well-defined edges (as it should, if it were apoptosis). Condensation of cytoplasm. present in both processes was also frequently found. Neither obvious apoptotic budding-off of cytoplasm nor typical membrane-bound apoptotic bodies were found. Our results indicate that in the dentate granule cell layer pilocarpine-induced SE promotes a degenerative process in which apoptotic and necrotic features overlap.

Flupirtine and retigabine prevent L-glutamate toxicity in rat pheochromocytoma PC 12 cells

Flupirtine is an analgesic drug thought to have NMDA receptor antagonistic and antiapoptotic effects. We investigated the effects of Ethyl-2-amino-6-(4-(4-fluorbenzyl)amino)-pyridine-3-carbamamic+ ++ acid, maleate (flupirtine) and the related compound N-(2-amino-4-(4-fluorobenzylamino)-phenyl)-carbamic acid, ethyl ester) (retigabine) (Desaza-flupirtine) on the toxicity of L-glutamate and L-3,4-dihydroxyphenylalanine (L-DOPA) in rat pheochromocytoma PC 12 cells in vitro. Both drugs (10 microM) markedly decreased nonreceptor-mediated necrotic cell death in PC 12 cultures treated with L-glutamate (10 mM) for 72 h. In contrast, apoptosis induced by L-DOPA (250 microM) after 48 h was not affected by either substance. While L-DOPA elicited massive generation of reactive oxygen intermediates, L-glutamate-induced cell death was accompanied by only slightly increased levels of reactive oxygen intermediates. Flupirtine and retigabine exerted anti-oxidative effects in PC 12 cultures independent of their ability to prevent cell death. Further examination of the protective action of flupirtine and retigabine against L-glutamate toxicity showed that it had no influence on monoamine oxidase (monoamine: oxygen oxidoreductase (deaminating), EC 1.4.3.4., MAO) activity. Thus, flupirtine and retigabine provided protection against cystine deprivation and L-glutamate toxicity but did not protect against L-glutamate under cystine-free conditions indicating that both compounds are sufficiently effective to compensate the oxidative stress elicited by cystine deprivation but not excessive activity of monoamine oxidase after L-glutamate treatment.

Status epilepticus-induced neuronal damage in the rat amygdaloid complex: distribution, time-course and mechanisms

The present study was designed to elucidate the distribution, time-course and mechanism(s) of status epilepticus-induced neuronal damage in the rat amygdaloid complex. Status epilepticus was induced with kainate (9 mg/kg, i.p.), and the behavioral and electrographic seizure activity of each rat was monitored via cortical electrodes attached to a continuous video electrocorticogram system. Rats were subsequently perfused 1, 2, 4, 8, 16, 24 or 48 h after kainate injection. The first signs of amygdaloid damage were seen in rats perfused 4 h after kainate injection, though the severity and temporal appearance of damage varied substantially between the different amygdaloid nuclei and their subdivisions. Second, terminal transferase dUTP nick-end labeling (TUNEL)-positive nuclei and laddering of DNA in gel electrophoresis appeared in the amygdala 8 and 16 h after kainate, respectively. The distribution and density of TUNEL-positive nuclei in the different amygdaloid nuclei correlated with the distribution of neuronal damage in Thionin- and silver-stained sections. Third, the immunoreactivity of Bax protein, a promoter of apoptotic neuronal death, increased in the vulnerable medial division of the lateral nucleus prior to the appearance of argyrophilic neurons and TUNEL-positive nuclei. Fourth, the severity of neuronal damage progressed in some, but not all, amygdaloid regions throughout the 48-h follow-up, even though the occurrence of high-amplitude and frequency discharges, which are typically associated with behavioral seizure activity, extinguished after 7 h. These data show that status epilepticus-induced neuronal damage in the amygdala is a dynamic region-specific process, the severity of which depends on the duration of seizure activity. At least one mechanism underlying the damage involves apoptosis, which continues long after the behavioral and electrographic seizures have subsided.

Immunolocalization of caspase proteolysis in situ: evidence for widespread caspase-mediated apoptosis of neurons and glia in the postnatal rat brain

Activation of a member of the caspase family of cysteine proteases is thought to be required for the execution of apoptosis in neurons and other cell types. We describe here an antibody (Ab127) reactive with a neoantigenic site on caspase substrate proteins degraded during apoptosis, and its characterization as a biochemical and histochemical probe for apoptosis-associated proteolysis in growth factor-deprived neural cells in vitro and the developing postnatal rat brain. Neuronally differentiated PC12 cells became strongly Ab127 immunoreactive only during apoptosis following nerve growth factor withdrawal. Apoptosis-associated caspase proteolysis was detectable on western blots as markedly increased immunoreactivity of a approximately 46,000 mol. wt polypeptide, a product also generated by caspase-3 treatment of cell-free extracts. In the postnatal rat brain, intense immunoreactivity indicative of caspase activation was exhibited by small proportions of neurons and glia in distinct regional and temporal patterns. The degenerating nature of these cells was confirmed by their argyrophilia, cytoplasmic immunoreactivity for c-jun and fragmented processes. Combined immunofluorescence and Hoechst 33342 staining demonstrated that cells immunopositive for caspase activation have apoptotic nuclear morphologies. Caspase proteolysis was observed throughout the neuraxis in a minority of progenitor cells in germinal zones, postmitotic neurons in the parenchyma, and glia in the corpus callosum and other white matter tracts, but was observed rarely in the adult brain. These data characterize a new approach for evaluating apoptosis in physiological and pathological neurodegeneration, and demonstrate that caspase-associated apoptosis is a widespread mechanism for the programmed death of neurons and glia in the postnatal rat brain.

Astrocytic demise precedes delayed neuronal death in focal ischemic rat brain

Active neuronal-glial interaction is important in the maintenance of brain homeostasis and is vital for neuronal survival following brain injury. The time course of post-ischemic astroglial dysfunction and neuronal death was studied in the spontaneously hypertensive rat (SHR) brain following permanent middle cerebral artery occlusion (MCAO). In situ hybridization with 35S-labeled riboprobes for GFAP and GLUT3 was used to monitor mRNA expression in glia and neurons. Astrocytic proteins GFAP, vimentin, S100, Glutathione-S-Transferase Yb (GST Yb) and neuronal protein TG2 were detected by immunofluorescence. Cells were co-stained with in situ end labeling (ISEL) to detect DNA fragmentation, a hallmark of cell death. GFAP mRNA expression declined rapidly in the ischemic region of the cortex and was almost absent by 12 h. Immunohistochemical studies revealed a parallel decline in the corresponding protein: a reduction in GFAP staining was apparent in the infarct after 3 h and by 24 h, there was essentially no remaining GFAP. Three other glial proteins (vimentin, S100 and GST Yb) disappeared from infarct over a similar time course. A few ISEL positive cells were observed in the infarct at 6 h, but maximal detection was not seen until 24-48 h. Most of the ISEL-positive cells were neurons, identified by co-staining with the neuronal marker TG2. Few cells expressing GFAP or other glial markers were positive at any time point. Neuronal GLUT3 mRNA declined more slowly than GFAP mRNA in the ischemic core and disappeared during the period of neuronal death. Concurrent with the loss of GFAP mRNA and protein expression in the infarct, there was a rapid rise in GFAP mRNA in the peri-infarct region of ipsilateral hemisphere and proximal region of the contralateral hemisphere. This was followed by the enhanced GFAP protein expression characteristic of reactive astrocytes, but over a significantly slower time course. These studies show that MCAO leads to a rapid decline of GFAP mRNA and glial proteins, which appears to precede the decline in neuronal mRNA and neuronal death within the infarct. Early astroglial dysfunction may play a critical role in determining the outcome of acute hypoxic-ischemic injury by compromising neuronal-glial interactions.

Neuronal death and survival in two models of hypoxic-ischemic brain damage

Two unilateral hypoxic-ischemia (HI) models (moderate and severe) in immature rat brain have been used to investigate the role of various transcription factors and related proteins in delayed neuronal death and survival. The moderate HI model results in an apoptotic-like neuronal death in selectively vulnerable regions of the brain while the more severe HI injury consistently produces widespread necrosis resulting in infarction, with some necrosis resistant cell populations showing evidence of an apoptotic type death. In susceptible regions undergoing an apoptotic-like death there was not only a prolonged induction of the immediate early genes, c-jun, c-fos and nur77, but also of possible target genes amyloid precursor protein (APP751) and CPP32. In contrast, increased levels of BDNF, phosphorylated CREB and PGHS-2 were found in cells resistant to the moderate HI insult suggesting that these proteins either alone or in combination may be of importance in the process of neuroprotection. An additional feature of both the moderate and severe brain insults was the rapid activation and/or proliferation of glial cells (microglia and astrocytes) in and around the site of damage. The glial response following HI was associated with an upregulation of both the CCAAT-enhancer binding protein alpha (microglia only) and NFkappaB transcription factors.

An immunosuppressant, FK506, protects against neuronal dysfunction and death but has no effect on electrographic and behavioral activities induced by systemic kainate

Kainate is a potent agonist of an excitatory amino acid receptor subtype in the central nervous system, and causes neuronal death in several regions of the brain. Neurons are preferentially killed in the hippocampus, especially in the CA1 region, by systemic administration of kainate. It is speculated that functional alterations occur in the neurons preceding death. We examined the effect of FK506 on kainate-induced neuronal death and functional alterations in the rat hippocampal CA1 region. FK506 had no effect on electrographic and behavioral seizure activities induced by kainate; however, it prevented neuronal death measured seven days after administration. Although neither death nor morphological alterations of neurons were observed in the CA1 region 24 h after administration, the neurons exhibited decreased excitatory postsynaptic potentials and enhanced long-term potentiation. This functional alteration was not detected in the rats administered FK506 prior to kainate. Taken together, these observations indicate that functional alteration precedes neuronal death in rats systemically administered kainate and that FK506 prevents both. It is suggested that FK506 exerts its neuroprotective effect not by attenuating electrographic and behavioral seizure activities, but by protecting neurons from kainate-induced functional disorders.

Occipital cortex ablation in adult rat causes retrograde neuronal death in the lateral geniculate nucleus that resembles apoptosis

The mechanisms of retrograde neurodegeneration following axotomy and target deprivation in the adult central nervous system remain poorly understood. We used a unilateral occipital cortex ablation model in adult rats to test the hypothesis that retrograde neurodegeneration in the dorsal lateral geniculate nucleus resembles apoptosis. Using the retrograde tracer Fluorogold, combined with nuclear dyes or the terminal transferase-mediated deoxyuridine triphosphate-biotin nick end labeling method for detecting nuclear DNA fragmentation, apoptotic geniculocortical projection neurons were identified at approximately. six to seven days postlesion. Degeneration of dorsal lateral geniculate neurons was characterized by aberrant accumulation of perikaryal non-phosphorylated neurofilaments and, ultrastructurally, by early vacuolation and subsequent swelling of dendrites. Ultrastructural alterations in the perikaryon of dying dorsal lateral geniculate neurons included the classic chromatolytic response, with redistribution of the rough endoplasmic reticulum and dispersion of free ribosomes followed by fragmentation of the rough endoplasmic reticulum, as well as dilatation and vesiculation of the Golgi, and accumulation of intact mitochondria. Subcellular alterations evolved into classic apoptotic changes, including progressive cytoplasmic and nuclear condensation with chromatin compaction into uniformly large round clumps, while the morphological integrity of mitochondria was preserved until late in the progression of neuronal death. Cytoplasmic and then nuclear fragments budded into the surrounding neuropil and were engulfed by oligodendrocytes. We conclude that the retrograde neurodegeneration of geniculocortical neurons in adult brain results in neuronal death which has a phenotype that closely resembles apoptosis. The morphological changes that occur during this process progress from chromatolysis through consecutive stages associated with apoptosis.

Lipopolysaccharide and Ceramide Use Divergent Signaling Pathways to Induce Cell Death in Murine Macrophages

Ceramide is a well-known apoptotic agent that has been implicated in LPS signaling. Therefore, we examined whether LPS-induced macrophage cytotoxicity is mediated by mimicking ceramide. Both LPS and the cell-permeable ceramide analogue, C2 ceramide, induced significant cell death in IFN-γ-activated, thioglycollate-elicited peritoneal macrophages after 48 and 24 h, respectively. Ceramide-induced cell death was neither accompanied by DNA fragmentation nor phosphatidyl serine externalization, characteristics of apoptosis. In contrast, LPS induced a significant fraction of cells to undergo apoptosis, as demonstrated by DNA fragmentation and quantified by DNA analysis on FACS, yet the majority of the cells died in a necrotic fashion. C3H/HeJ Lpsd macrophages were resistant to LPS-induced cell death and less sensitive to C2 ceramide-evoked cytotoxicity, when compared with Lpsn macrophages. C2 ceramide plus IFN-γ failed to activate release of nitric oxide (NO·), whereas LPS-induced cell death, but not C2-induced cytotoxicity, was blocked by an inhibitor of inducible NO· synthase (iNOS), NG-monomethyl-l-arginine. Macrophages from IFN regulatory factor-1 (−/−) mice shown previously to respond marginally to LPS plus IFN-γ to express iNOS mRNA and NO·, were refractory to LPS plus IFN-γ-induced cytotoxicity and apoptosis. These data suggest that although LPS may mimic certain ceramide effects, signal transduction events that lead to cytotoxicity, as well as the downstream mediators, diverge.

The role of neuronal growth factors in neurodegenerative disorders of the human brain

Recent evidence suggests that neurotrophic factors that promote the survival or differentiation of developing neurons may also protect mature neurons from neuronal atrophy in the degenerating human brain. Furthermore, it has been proposed that the pathogenesis of human neurodegenerative disorders may be due to an alteration in neurotrophic factor and/or trk receptor levels. The use of neurotrophic factors as therapeutic agents is a novel approach aimed at restoring and maintaining neuronal function in the central nervous system (CNS). Research is currently being undertaken to determine potential mechanisms to deliver neurotrophic factors to selectively vulnerable regions of the CNS. However, while there is widespread interest in the use of neurotrophic factors to prevent and/or reduce the neuronal cell loss and atrophy observed in neurodegenerative disorders, little research has been performed examining the expression and functional role of these factors in the normal and diseased human brain. This review will discuss recent studies and examine the role members of the nerve growth factor family (NGF, BDNF and NT-3) and trk receptors as well as additional growth factors (GDNF, TGF-alpha and IGF-I) may play in neurodegenerative disorders of the human brain.

Cell death and associated c-jun induction in perinatal hypoxia-ischemia. Effect of the neuroprotective drug dexamethasone

Previous studies in a model of unilateral hypoxia-ischemia in the developing rat brain have shown induction of the mRNAs of c-fos and c-jun and presence of apoptotic DNA fragmentation. In this same model, dexamethasone confers neuroprotection if given before the insult. Since c-fos and c-jun have been involved in several models of cell death, we investigated whether the neuroprotective effect of dexamethasone could be associated with changes in expression of these genes. Rat pups, pre-treated with either 0.5 mg/kg dexamethasone or vehicle 48 h, 24 h and immediately before the injury, were subjected to ligation of the left common carotid artery followed by 3 h hypoxia. Analysis of c-fos and c-jun expression at 2 h, by means of in situ hybridization, revealed diminished induction in dexamethasone-treated animals. Jun immunoreactivity, but not Fos, and DNA fragmentation, assessed by in situ end-labeling of fragmented DNA, were present at 24 h only in vehicle-injected animals. Electrophoresis of brain extracted DNA revealed a ladder pattern in all the animals. Our results show a relationship between Jun overexpression and cell-death in the hypoxic-ischemic developing brain and suggest that dexamethasone exerts its protective effect anteceding immediate early gene induction, at some early point in post-ischemic signal transduction.

Strong c-Jun/AP-1 immunoreactivity is restricted to apoptotic cells following intracerebral ibotenic acid injection in developing rats

Strong c-Jun immunoreactivity, as revealed with the antibody c-Jun/activator protein 1 (AP-1) which is raised against the amino acids 91-105 mapping with the amino terminal domain of mouse c-Jun p39, is observed in apoptotic cells, but not in necrotic cells, following intracerebral injection of ibotenic acid in the developing rat brain processed for immunohistochemistry. Immunostaining occurs in the cytoplasm and dendrites, thus suggesting impaired nuclear translocation of c-Jun in apoptotic cells. Western blotting of total brain homogenates, using the same antibody, shows a band at p39 which is more marked in treated animals than in age-matched controls. In addition, increased c-Jun N-terminal kinase 1 (JNK-1) expression, as revealed on Western blots, is found in rats treated with ibotenic acid when compared with controls. In contrast, apoptotic cells are not stained with antibodies to Jun B and Jun D. These results give further support to previous studies showing strong c-Jun expression in apoptotic cells at determinate stages of development, and emphasize that intracellular distribution of c-Jun, possible post-translational modifications of c-Jun due to phosphorylation at specific transactivation sites, and lack of associated Jun B and Jun D expression may differentiate the Jun response in apoptotic cells from other forms of cellular response involving c-Jun which are not associated with cell death.