DNA fragmentation follows delayed neuronal death in CA1 neurons exposed to transient global ischemia in the rat

Exacerbation of delayed cell injury after transient global ischemia in mutant mice with CuZn superoxide dismutase deficiency

BACKGROUND AND PURPOSE

We have demonstrated that copper-zinc superoxide dismutase (CuZn-SOD), a cytosolic isoenzyme of SODs, has a protective role in the pathogenesis of superoxide radical-mediated brain injury. Using mice bearing a disruption of the CuZn-SOD gene (Sod1), the present study was designed to clarify the role of superoxide anion in the pathogenesis of selective vulnerability after transient global ischemia.

METHODS

Sod1 knockout homozygous mutant mice (Sod1 -/-) with a complete absence of endogenous CuZn-SOD activity, heterozygous mutant mice (Sod1 +/-) with a 50% decrease in the activity, and littermate wild-type mice (male, 35 to 45 g) were subjected to global ischemia. Since the plasticity of the posterior communicating artery (PcomA) has been reported to influence the outcome of hippocampal injury, we assessed the relation between the plasticity of PcomAs and the decrease of regional cerebral blood flow in global ischemia.

RESULTS

The fluorescence intensity of hydroethidine oxidation, a measurement of ethidium fluorescence for superoxide radicals, was increased in mutant mice 1 day after both 5 and 10 minutes of global ischemia, compared with wild-type mice. Hippocampal injury in the PcomA hypoplastic brains showed significant exacerbation in mutant mice compared with wild-type littermates 3 days after 5 minutes of global ischemia, although a marked difference was not observed at 1 day.

CONCLUSIONS

These data suggest that superoxide radicals play an important role in the pathogenesis of delayed injury in the vulnerable hippocampal CA1 subregion after transient global ischemia.

The time-course of DNA fragmentation in the choroid plexus and the CA1 region following transient global ischemia in the rat brain. The effect of intra-ischemic hypothermia

The time-course of DNA fragmentation in the CA1 region of the hippocampus and the choroid plexus was studied following induction of transient forebrain ischemia under lethal normothermic (37 degrees C), or sublethal hypothermic (33 degrees C) conditions. Oligonucleosomal- and high-molecular-weight DNA fragmentation were analysed by conventional agarose gel electrophoresis and pulsed-field gel electrophoresis, respectively. DNA breaks were visualized by the terminal deoxynucleotidyl transferase-mediated biotin-deoxyuridinetriphosphate nick-end labeling method. At 48 h of recovery following normothermic ischemia, in situ labeling of DNA breaks were widespread in medial CA1 and high-molecular-weight DNA cleavage was seen. In contrast, at the same time-point in lateral CA1, many pyknotic but few cells displaying in situ labeling of DNA breaks were observed. Major oligonucleosomal DNA fragmentation was not seen until 72 h of recovery. Following hypothermic ischemia, DNA fragmentation was absent in CA1. DNA fragmentation was seen in the choroid plexus at 24 h of recovery following normothermic ischemia, which was diminished by 48 h of recovery. In conclusion, oligonucleosomal and high-molecular-weight DNA fragmentation at 10-50 kilobase pairs, occur in CA1 after morphological signs, and acidophilia signifying neurodegeneration appear. DNA fragmentation and cell death in the choroid plexus precede neuronal death in CA1 and may play a causative role.

DNA cleavage and proteolysis of microtubule-associated protein 2 after cerebral ischemia of different severity

We report temporal profiles of cytoplasmic proteolysis and genomic DNA cleavage after cerebral ischemia of different severity in gerbils. Global forebrain ischemia by bilateral common carotid artery occlusion for 5 min with reperfusion, severe unilateral hemispheric ischemia by unilateral common carotid artery occlusion for 30 min with reperfusion, and complete ischemia by decapitation were used. The hippocampus was examined for proteolysis by using immunohistochemistry for microtubule-associated protein 2, DNA cleavage by using in situ nick-end labelling, and nuclear morphology by Hematoxylin staining. During evolution of delayed neuronal death after transient forebrain ischemia, loss of the immunoreaction for microtubule-associated protein 2 occurred almost in parallel with DNA cleavage in the CA1 region. In contrast, disappearance of the immunoreaction for microtubule-associated protein 2 was much faster than genomic DNA cleavage after unilateral hemispheric ischemia and reperfusion. The microtubule-associated protein 2 immunoreactivity was completely lost before development of changes in nuclear morphology or DNA cleavage after complete ischemia. The present study demonstrated the differences between necrosis and delayed neuronal death, but the nuclear morphology in the latter was not exactly the same as seen in apoptosis. Some elements of both necrotic and apoptotic machineries may work following transient ischemia, and the degree of ischemic insult may determine the character of cell death process.

Changes in expression of the DNA repair protein complex DNA-dependent protein kinase after ischemia and reperfusion

Reperfusion of ischemic tissue causes an immediate increase in DNA damage, including base lesions and strand breaks. Damage is reversible in surviving regions indicating that repair mechanisms are operable. DNA strand breaks are repaired by nonhomologous end joining in mammalian cells. This process requires DNA-dependent protein kinase (DNA-PK), composed of heterodimeric Ku antigen and a 460,000 Da catalytic subunit (DNA-PKcs). In this study, a rabbit spinal cord model of reversible ischemia was used to demonstrate the effect of acute CNS injury on the activity and expression of DNA-dependent protein kinase. The DNA-binding activity of Ku antigen, analyzed by an electrophoretic mobility shift assay, increased during reperfusion after a short ischemic insult (15 min of occlusion), from which the animals recover neurological function. After severe ischemic injury (60 min of occlusion) and reperfusion that results in permanent paraplegia, Ku DNA binding was reduced. Protein levels of the DNA-PK components-Ku70, Ku80, and DNA-PKcs-were monitored by immunoblotting. After 60 min of occlusion, the amount of DNA-PKcs and the enzyme poly(ADP-ribose) polymerase (PARP) decreased with the same time course during reperfusion. Concurrently 150 and 120 kDa fragments were immunostained by an anti-DNA-PKcs monoclonal antibody. This antibody was shown to cross-react with alpha-fodrin breakdown products. The 120 kDa fodrin peptide is associated with caspase-3 activation during apoptosis. Both DNA-PKcs and PARP are also substrates for caspase-3-like activities. The results are consistent with a model in which after a short ischemic insult, DNA repair proteins such as DNA-PK are activated. After severe ischemic injury, DNA damage overwhelms repair capabilities, and cell death programs are initiated.

Electron microscopic evidence against apoptosis as the mechanism of neuronal death in global ischemia

It has been repeatedly claimed that neuronal death in the hippocampal CA1 sector after untreated global ischemia occurs via apoptosis. This is based largely on DNA laddering, nick end labeling, and light microscopy. Delineation of apoptosis requires fine structural examination to detect morphological events of cell death. We studied the light and ultrastructural characteristics of CA1 injury after 5 min of untreated global ischemia in gerbils. To increase the likelihood of apoptosis, some ischemic gerbils were subjected to delayed postischemic hypothermia, a treatment that mitigates injury and delays the death of some neurons. In these gerbils, 2 d of mild hypothermia was initiated 1, 6, or 12 hr after ischemia, and gerbils were killed 4, 14, or 60 d later. Ischemia without subsequent cooling killed 96% of CA1 neurons by day 4, whereas all hypothermia-treated groups had significantly reduced injury at all survival times (2-67% loss). Electron microscopy of ischemic neurons with or without postischemic hypothermia revealed features of necrotic, not apoptotic, neuronal death even in cells that died 2 months after ischemia. Dilated organelles and intranuclear vacuoles preceded necrosis. Unique to the hypothermia-treated ischemic groups, some salvaged neurons were persistently abnormal and showed accumulation of unusual, morphologically complex secondary lysosomes. These indicate selective mitochondrial injury, because they were closely associated with normal and degenerate mitochondria, and transitional forms between mitochondria and lysosomes occurred. The results show that untreated global ischemic injury has necrotic, not apoptotic, morphology but do not rule out programmed biochemical events of the apoptotic pathway occurring before neuronal necrosis.

The beta2-adrenoceptor agonist clenbuterol modulates Bcl-2, Bcl-xl and Bax protein expression following transient forebrain ischemia

It is well known that proteins encoded by the Bcl-2 gene family play a major role in the regulation of apoptosis. We have demonstrated previously that neuronal apoptosis can be induced in the hippocampus and striatum after global ischemia. Clenbuterol, a beta2-adrenoceptor agonist, showed considerable activity against neuronal apoptosis. In the present study, we attempted to find out whether the members of the Bcl-2 family are induced after ischemia, and whether expression of these genes could be altered by clenbuterol. Transient forebrain ischemia was performed in male Wistar rats by clamping both common carotid arteries and reducing the blood pressure to 40 mmHg for 10 min. Clenbuterol (0.5 mg/kg, i.p.) or vehicle were injected 3 h before onset of ischemia or in non-ischemic rats. The hippocampus and striatum were taken from non-ischemic rats 3, 6 and 24 h after injection of clenbuterol, as well as from drug-treated and untreated rats 6 and 24 h after ischemia. Eighty micrograms/lane total protein were loaded on a 15% sodium dodecyl sulfate-polyacrylamide gel for western blotting. Bcl-2, Bax and Bcl-xl proteins were detectable in the non-ischemic hippocampus and the striatum. Clenbuterol up-regulated the expression of Bcl-2 protein at 3, 6 and 24 h after administration. Enhanced Bcl-xl signals were found in the non-ischemic striatum 3, 6 and 24 h after clenbuterol treatment, but no change of Bcl-xl expression by clenbuterol was seen in the non-ischemic hippocampus. Bax expression was not altered by clenbuterol in the non-ischemic hippocampus and striatum. Bcl-2 was up-regulated in both detected regions at 24 h after ischemia, while the increase in Bax and Bcl-xl protein expression had appeared already at 6 h and also 24 h after ischemia. Clenbuterol further increased the expression of Bcl-2 at 6 and 24 h after ischemia. In contrast, Bax protein level was down-regulated by clenbuterol at 6 and 24 h after ischemia. Clenbuterol also increased Bcl-xl level in the ischemic striatum. The results suggest that global ischemia induces proto-oncogenes which are associated with apoptosis. Clenbuterol not only increased Bcl-2 expression in the non-ischemic hippocampus and striatum, but also up-regulated Bcl-2 and down-regulated Bax expression in the ischemic hippocampus and striatum. The increase in the ratio of Bcl-2 and Bax may contribute to the anti-apoptotic effect of clenbuterol. The present study indicates that pharmacological modulation of Bcl-2 family member expression could become a new strategy to interfere with neuronal damage.

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.

Lithium-pilocarpine-induced status epilepticus produces necrotic neurons with internucleosomal DNA fragmentation in adult rats

Prolonged and continuous epileptic seizures [status epilepticus (SE)] produce a widespread pattern of neuronal death, primarily in limbic brain regions. Because it has been suggested that seizure-induced neuronal death may be apoptotic in nature, we tested the hypothesis that lithium-pilocarpine-induced status epilepticus (LPCSE) produces apoptotic neurons. LPCSE lasting 3 h was induced in male Wistar rats which were allowed to recover for 24 or 72 h before perfusion-fixation. Neuronal death was assessed by light microscopy with the haematoxylin-and-eosin stain (H&E), with in situ DNA nick-end labelling (TUNEL stain), by electron microscopy, and by agarose gel electrophoresis of DNA extracted from vulnerable brain regions. Ultrastructurally, acidophilic neurons identified with H&E were dark, shrunken and necrotic in appearance, exhibiting pyknotic nuclei, irregular, dispersed chromatin clumps and cytoplasmic vacuolization. No cells with apoptotic features were seen. Acidophilic neurons were found in 21 out of 23 brain regions examined, and comprised 26-45% of the total number of neurons examined. A subset of these neurons (< 10% of the total number of neurons) were TUNEL-positive at 72 h, but not 24 h, after SE. Internucleosomal DNA cleavage (DNA 'laddering') was found in the six brain regions examined ultrastructurally 24 and 72 h after SE. These results indicate that, in adult rats, LPCSE produces neuronal injury with the appearance of necrosis rather than apoptosis. The necrotic neurons show nuclear pyknosis, chromatin condensation and internucleosomal DNA fragmentation, confirming the nonspecificity of these nuclear changes. Internucleosomal DNA cleavage and other programmed cell death mechanisms can be activated by SE in neurons which become necrotic.

Cerebral ischemia produces laddered DNA fragments distinct from cardiac ischemia and archetypal apoptosis

The electrophoretic pattern of laddered DNA fragments which has been observed after cerebral ischemia is considered to indicate that neurons are dying by apoptosis. Herein the authors directly demonstrate using ligation-mediated polymerase chain reaction methods that 99% of the DNA fragments produced after either global or focal ischemia in adult rats, or produced after hypoxia-ischemia in neonatal rats, have staggered ends with a 3' recess of approximately 8 to 10 nucleotides. This is in contrast to archetypal apoptosis in which the DNA fragments are blunt ended as seen during developmental programmed cell death in dying cortical neurons, neuroblastoma, or thymic lymphocytes. It is not simply ischemia that results in staggered ends in DNA fragments because ischemic myocardium is similar to archetypal apoptosis with a vast majority of blunt-ended fragments. It is concluded that the endonucleases that produce this staggered fragmentation of the DNA backbone in ischemic brain must be different than those of classic or type I apoptosis.

The tumor suppressor p53 and its response gene p21WAF1/Cip1 are not markers of neuronal death following transient global cerebral ischemia

The tumor suppressor protein p53 is implicated in cell cycle arrest and DNA repair as well as in apoptosis. In the CNS, p53 has been associated with neuronal cell death following various insults, including cerebral ischemia. We investigated the expression of p53 messenger RNA and protein, and the messenger RNA expression of the p53-responsive gene p21(WAF1/CiP1, in specific hippocampal regions following 15 min of normothermic and neuroprotective hypothermic (33 degrees C) global forebrain ischemia in the rat. Both p53 and p21WAF1/Cip1 messenger RNAs were transiently induced in ischemia resistant regions following normo- and hypothermic ischemia. In the ischemia sensitive CA1 region, p53 and p21WAF1/Cip1 messenger RNAs were up-regulated throughout reperfusion following the normothermic insult. The p53 protein levels increased following the insult, most markedly in ischemia-resistant CA3 neurons after normothermic ischemia, and in the CA1 neurons following hypothermic ischemia. Concomitantly, the protein was translocated to nuclei. These findings indicate that p53 and p21WAF1/Cip1 are not markers of neuronal death following global cerebral ischemia. Their rapid and transient induction correlates with cell survival, and suggests a possible role in DNA repair.

Continuing postischemic neuronal death in CA1: influence of ischemia duration and cytoprotective doses of NBQX and SNX-111 in rats

BACKGROUND AND PURPOSE

Transient forebrain ischemia results in a 24- to 72-hour delayed loss of CA1 neurons. Previous work has not assessed whether insult durations can vary the degree and maturation rate of CA1 injury and whether there are different ultrastructural features of death after brief or severe ischemia. We also tested whether known cytoprotective drugs achieve permanent or transient neuroprotection.

METHODS

In the first experiment, ischemia was induced for 5, 15, or 30 minutes with the use of the 4-vessel occlusion rat model with 1- to 28-day survival. Others subjected to 5 or 15 minutes of ischemia and allowed to survive for 14 or 7 days, respectively, were examined with electron microscopy. Finally, we determined whether NBQX (30 mg/kg x3 at 0 or 6 hours after ischemia), an AMPA antagonist, and SNX-111 (5 mg/kg at 6 hours after ischemia), an N-type Ca2+ channel antagonist, provided enduring CA1 protection against 10 minutes of ischemia.

RESULTS

CA1 damage was not detected at 24 hours. Thirty minutes of ischemia produced 47% and 84% CA1 damage at 2 and 3 days, respectively. A 15-minute occlusion yielded 11%, 74%, and 86% loss at 2, 3, and 7 days, respectively. Five minutes of ischemia produced an even slower progression with 24%, 52%, and 59% loss at 3, 7, and 14 days, respectively. Ultrastructural examination after 5 and 15 minutes of ischemia revealed necrosis with no morphological evidence of apoptosis. Both NBQX (P<0.021) and SNX-111 (P<0.001) significantly reduced CA1 death at 7 days (</=35%) but not at 28 days (>/=80%) compared with saline treatment ( approximately 79%).

CONCLUSIONS

Brief forebrain ischemia results in a slower progression of CA1 loss than more severe insults. Nonetheless, neuronal injury had necrotic, not apoptotic, morphology. NBQX and SNX-111 only postponed CA1 injury.

Delayed neuronal cell death in the rat hippocampus is mediated by the mitogen-activated protein kinase signal transduction pathway

Transient global ischemia caused by 5 min of cardiac arrest induced delayed neuronal cell death (apoptosis) in the CA1 region of the rat hippocampus. To characterize the molecular mechanisms that regulate apoptosis in vivo, the contributions to cell death of mitogen-activated protein kinase family members were examined in the hippocampal region after brain ischemia-reperfusion. Ischemia-reperfusion led to a strong activation of the JNK/SAPK (c-Jun NH2-terminal protein kinase/stress activated protein kinase), ERK (extracellular signal-regulated kinase), and p38 enzymes. These results with other previous studies suggest that the activation of JNK/SAPK in accordance with p38 contributes to the induction of apoptosis in CA1 neurons.

Blockade of tetrahydrobiopterin synthesis protects neurons after transient forebrain ischemia in rat: a novel role for the cofactor

The generation of nitric oxide (NO) aggravates neuronal injury. (6R)-5,6,7,8-Tetrahydro-L-biopterin (BH4) is an essential cofactor in the synthesis of NO by nitric oxide synthase (NOS). We attempted to attenuate neuron degeneration by blocking the synthesis of the cofactor BH4 using N-acetyl-3-O-methyldopamine (NAMDA). In vitro data demonstrate that NAMDA inhibited GTP cyclohydrolase I, the rate-limiting enzyme for BH4 biosynthesis, and reduced nitrite accumulation, an oxidative metabolite of NO, without directly inhibiting NOS activity. Animals exposed to transient forebrain ischemia and treated with NAMDA demonstrated marked reductions in ischemia-induced BH4 levels, NADPH-diaphorase activity, and caspase-3 gene expression in the CA1 hippocampus. Moreover, delayed neuronal injury in the CA1 hippocampal region was significantly attenuated by NAMDA. For the first time, these data demonstrate that a cofactor, BH4, plays a significant role in the generation of ischemic neuronal death, and that blockade of BH4 biosynthesis may provide novel strategies for neuroprotection.

Effect of NGF treatment on outcome measures in a rat model of middle cerebral artery occlusion

Ischemic insults to the brain result in a time-dependent increase in neuronal death that is responsible for some of the functional deficits associated with stroke. Our working hypothesis is that ischemia results in a prompt depletion of high energy phosphate species resulting in decreased pH and glutathione levels in brain in a temporal and spatial pattern that disrupts nerve growth factor homeostasis and increases neuronal apoptosis. Here we show hemispheric depletion of active phosphate species after ischemia. Also, we observed that the striatum is an early target for oxidative stress that is followed by energy metabolic impairment and altered neurotrophin levels that were detected by noninvasive magnetic resonance imaging (MRI) measurements of cytotoxicity and conventional biochemical determinations of apoptosis, glutathione, and nerve growth factor (NGF) protein levels in a pattern distinct from that observed in the hippocampus. Furthermore, early assessment of intracellular pH by 31P-magnetic resonance spectroscopy (31P-MRS) was a predictor of later infarct development as determined by MRI. We also show that pretreatment with pharmacological doses of NGF did not have overall significant beneficial consequences on irreversible ischemia in an intraluminal unilateral irreversible model of stroke in rat brain.

Reduction of apurinic/apyrimidinic endonuclease expression after transient global cerebral ischemia in rats: implication of the failure of DNA repair in neuronal apoptosis

BACKGROUND AND PURPOSE

To clarify the relationship between apurinic/apyrimidinic endonuclease (APE/Ref-1), a multifunctional protein in the DNA base excision repair pathway, and delayed neuronal cell death associated with apoptosis, we examined the expression of APE/Ref-1 before and after transient global ischemia in rats.

METHODS

Global ischemia was induced by bilateral common carotid artery occlusion and hypotension. Expression of the APE/Ref-1 protein was evaluated by Western blot and immunohistochemical analyses. Apoptosis after global ischemia was observed by DNA electrophoresis and terminal deoxynucleotidyl transferase-mediated uridine 5'-triphosphate-biotin nick end labeling (TUNEL) staining.

RESULTS

Immunohistochemistry showed the nuclear expression of APE/Ref-1 in the control brains. Nuclear immunoreactivity of APE/Ref-1 was significantly decreased 2 days after 10 minutes of ischemia in the hippocampal CA1 subregion. Western blot analysis of a sample from the normal brains showed a characteristic 37-kDa band, which was reduced in the hippocampal CA1 subregion after ischemia. A significant amount of DNA fragmentation was observed at 3 days but not at 1 day after ischemia. Double staining with APE/Ref-1 and TUNEL clearly showed that the neurons that lost APE/Ref-1 immunoreactivity became TUNEL positive.

CONCLUSIONS

Our data provide evidence that APE/Ref-1 decreased in hippocampal CA1 neurons after transient global ischemia and that this reduction precedes DNA fragmentation, which is destined to cause apoptosis. Our results suggest the possibility that a decrease of APE/Ref-1 activity and the failure of DNA repair may underlie the mechanism of apoptosis after transient focal ischemia.

Novel non-apoptotic morphological changes in neurons of the mouse hippocampus following transient hypoxic-ischemia

Apoptosis has been recently implicated in the dying process of neurons under several pathological conditions including ischemia. However, although apoptosis was originally defined on the basis of its unique ultrastructural features (Kerr et al., 1972. Br. J. Cancer 26, 239-257; Wyllie et al., 1980. Int. Rev. Cytol. 68, 251-306), unambiguous ultrastructural evidence of apoptosis has been rarely demonstrated in the adult brain. In this study, we examined ultrastructural changes in mouse hippocampal neurons after transient hypoxic-ischemia. A small population of dentate granule cells showed typical apoptotic ultrastructures that could be used as internal morphological standards of apoptosis, whereas most other hippocampal neurons consistently showed a distinct form of cellular disintegration. Nuclei of the latter cells shrank and became TUNEL-positive but were distinguishable from apoptotic nuclei by both the presence of characteristic reticular-formed chromatin condensation and the absence of nuclear fragmentation. Perikarya of degenerating neurons also shrank as in apoptosis, but apoptotic bodies were not observed. Although organelles other than mitochondria disappeared almost completely from the perikarya, neither plasma nor mitochondrial membranes were disrupted, indicating that these changes were also different from typical necrosis. The presence of a novel form of cell death suggests the necessity of morphological re-examination of neuronal death, particularly in mature neurons in vivo.

Amelioration of hippocampal neuronal damage after global ischemia by neuronal overexpression of BCL-2 in transgenic mice

BACKGROUND AND PURPOSE

Reports suggesting the involvement of apoptosis in ischemic neuronal damage have been accumulating, and protection against apoptotic death by BCL-2 has been shown in many types of cells. Overexpression of BCL-2 has been shown to reduce infarct size after focal ischemia. The purpose of the present study was to assess whether BCL-2 exerted its effect on selective neuronal vulnerability after transient global ischemia.

METHODS

Transgenic mice overexpressing BCL-2 in neurons and their littermates were subjected to transient forebrain ischemia for 12 minutes, and the hippocampus was examined 7 days later with conventional histology, immunohistochemistry, and in situ terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling of fragmented DNA.

RESULTS

Although both types of mice showed a similar degree of ischemic insult, transgenic mice showed a lesser degree of neuronal death together with DNA fragmentation in the hippocampus than their littermates.

CONCLUSIONS

Overexpression of BCL-2 in neurons mitigates selective neuronal vulnerability in the hippocampus of transgenic mice after transient global ischemia.

Overexpression of SOD1 in transgenic rats protects vulnerable neurons against ischemic damage after global cerebral ischemia and reperfusion

Transient global cerebral ischemia resulting from cardiac arrest is known to cause selective death in vulnerable neurons, including hippocampal CA1 pyramidal neurons. It is postulated that oxygen radicals, superoxide in particular, are involved in cell death processes. To test this hypothesis, we first used in situ imaging of superoxide radical distribution by hydroethidine oxidation in vulnerable neurons. We then generated SOD1 transgenic (Tg) rats with a five-fold increase in copper zinc superoxide dismutase activity. The Tg rats and their non-Tg wild-type littermates were subjected to 10 min of global ischemia followed by 1 and 3 d of reperfusion. Neuronal damage, as assessed by cresyl violet staining and DNA fragmentation analysis, was significantly reduced in the hippocampal CA1 region, cortex, striatum, and thalamus in SOD1 Tg rats at 3 d, as compared with the non-Tg littermates. There were no changes in the hippocampal CA3 subregion and dentate gyrus, resistant areas in both SOD1 Tg and non-Tg rats. Quantitative analysis of the damaged CA1 subregion showed marked neuroprotection against transient global cerebral ischemia in SOD1 Tg rats. These results suggest that superoxide radicals play a role in the delayed ischemic death of hippocampal CA1 neurons. Our data also indicate that SOD1 Tg rats are useful tools for studying the role of oxygen radicals in the pathogenesis of neuronal death after transient global cerebral ischemia.

Transport of fragmented DNA in apical dendrites of gerbil CA1 pyramidal neurons following transient forebrain ischemia

Transport of fragmented DNA in apical dendrites of the CA1 pyramidal neurons of gerbil hippocampus is observed in the apoptotic process following transient forebrain ischemia. The time-course of specific DNA fragmentation was examined after the ischemic insult by in situ nick-end-labeling method and fluorescence detection technique by DAPI. Although the role of the fragmented DNA movement is unclear, the transport mechanism of fragmented DNA is still active in the late phase of apoptotic process.

DNA fragmentation and HSP72 gene expression by adenovirus-mediated gene transfer in postischemic gerbil hippocampus and ventricle

A replication defective adenoviral vector containing the E. coli lacZ gene (AdCMVnLacZ) was directly injected into right hippocampus and lateral ventricle immediately after 5 min of transient global ischemia in gerbils. The relations between the lacZ gene expression and DNA fragmentation or heat shock protein 72 (HSP72) immunoreactivity were examined up to 21 days post ischemia. The lacZ gene was transiently expressed at 1 day in the hippocampus except around the CA1 region, while a large number of the periventricular cells strongly expressed the lacZ gene from 8 h to 7 days. In CA1 layer, terminal deoxynucleotidyl dUTP nick end labeling (TUNEL) positive cells, which were present only adjacent to the needle track at 8 h to 1 day, became more extensive in the whole CA1 layer at 3 to 7 days. TUNEL-positive cells were also detected around the DG at 1 day, around the needle track at 8 h to 3 days, and in the choroid plexus cells at 7 days. HSP72 staining was detected in the subiculum at 1 to 3 days, the dentate granule cells at 8 h to 1 day, and in the CA3 or CA4 pyramidal cells at 1 to 3 days. Some lacZ expressing cells were double-positive with HSP72 in DG, while the majority of those were distinguished from the TUNEL-positive cells. Pyramidal neurons were almost completely lost in the CA1 sector at 7 days after the ischemia. The present study demonstrates the successful LacZ gene transfer into the hippocampus and ventricle of postischemic gerbil brain except in the vulnerable CA1 layer by adenoviral vector injection. However, adenovirus-mediated gene transfer may induce indirect apoptotic cell death in the DG and ventricle, in addition to direct traumatic injury around the needle track.

Aurintricarboxylic acid prevents GLUR2 mRNA down-regulation and delayed neurodegeneration in hippocampal CA1 neurons of gerbil after global ischemia

Aurintricarboxylic acid (ATA), an inhibitor of endonuclease activity and other protein-nucleic acid interactions, blocks apoptosis in several cell types and prevents delayed death of hippocampal pyramidal CA1 neurons induced by transient global ischemia. Global ischemia in rats and gerbils induces down-regulation of GluR2 mRNA and increased alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-induced Ca2+ influx in CA1 before neurodegeneration. This result and neuroprotection by antagonists of AMPA receptors suggests that formation of AMPA receptors lacking GluR2, and therefore Ca2+ permeable, leads to excessive Ca2+ influx in response to endogenous glutamate; the resulting delayed neuronal death in CA1 exhibits many characteristics of apoptosis. In this study, we examined the effects of ATA on expression of mRNAs encoding glutamate receptor subunits in gerbil hippocampus after global ischemia. Administration of ATA by injection into the right cerebral ventricle 1 h before (but not 6 h after) bilateral carotid occlusion prevented the ischemia-induced decrease in GluR2 mRNA expression and the delayed neurodegeneration. These findings suggest that ATA is neuroprotective in ischemia by blocking the transcriptional changes leading to down-regulation of GluR2, rather than by simply blocking endonucleases, which presumably act later after Ca2+ influx initiates apoptosis. Maintaining formation of Ca2+ impermeable, GluR2 containing AMPA receptors could prevent delayed death of CA1 neurons after transient global ischemia, and block of GluR2 down-regulation may provide a further strategy for neuroprotection.

Apoptosis of motor neurons with induction of caspases in the spinal cord after ischemia

BACKGROUND AND PURPOSE

Some neuronal subpopulations are especially vulnerable to ischemic injury. In the spinal cord, large motor neurons are vulnerable to ischemia and are selectively lost after transient ischemia. However, the mechanisms of the neuronal loss have been uncertain. We hypothesized that spinal motor neurons might be lost by apoptosis and investigated a possible mechanism of neuronal death by detection of double-strand breaks in genomic DNA and immunohistochemical analysis for caspases, ie, interleukin-1beta converting enzyme (ICE), Nedd-2, and CPP32.

METHODS

We used a rabbit spinal cord ischemia model created with a balloon catheter. The spinal cord was removed at 8 hours, 1, 2, or 7 days after 15 minutes of transient ischemia, and histological changes were studied with hematoxylin-eosin staining. To detect double-strand breaks in DNA, a staining with terminal deoxynucleotidyl transferase-mediated dUTP-biotin in situ nick end labeling (TUNEL) was performed. Furthermore, expression of ICE, Nedd-2, and CPP32 was investigated by Western blotting and immunohistochemical analysis.

RESULTS

Motor neurons were selectively lost at 7 days after transient ischemia. TUNEL study demonstrated that no cells were positively labeled until 1 day after ischemia, but nuclei of some motor neurons were positively labeled at 2 days. Western blot analysis revealed no immunoreactivity for ICE and slight immunoreactivities for Nedd-2 and CPP32 in the sham-operated spinal cords. However, immunoreactivity became apparent at 8 hours after transient ischemia, decreased at 1 day, and returned to baseline level at 2 days. Immunohistochemical analysis demonstrated that motor neurons were responsible for induction of those caspases.

CONCLUSIONS

Double-strand breaks in genomic DNA and induction of three caspases were demonstrated. These results indicate that motor neuron death in the spinal cord after transient ischemia is profoundly associated with activation of apoptotic processes.