Steven H. Graham

Programmed Cell Death in Cerebral Ischemia

Programmed cell death (PCD) is an ordered and tightly controlled set of changes in gene expression and protein activity that results in neuronal cell death during brain development. This article reviews the molecular pathways by which PCD is executed in mammalian cells and the potential relation of these pathways to pathologic neuronal cell death. Whereas the classical patterns of apoptotic morphologic change often do not appear in the brain after ischemia, there is emerging biochemical and pharmacologic evidence suggesting a role for PCD in ischemic brain injury. The most convincing evidence for the induction of PCD after ischemia includes the altered expression and activity in the ischemic brain of deduced key death-regulatory genes. Furthermore, studies have shown that alterations in the activity of these gene products by peptide inhibitors, viral vector-mediated gene transfer, antisense oligonucleotides, or transgenic mouse techniques determine, at least in part, whether ischemic neurons live or die after stroke. These studies provide strong support for the hypothesis that PCD contributes to neuronal cell death caused by ischemic injury. However, many questions remain regarding the precise pathways that initiate, sense, and transmit cell death signals in ischemic neurons and the molecular mechanisms by which neuronal cell death is executed at different stages of ischemic injury. Elucidation of these pathways and mechanisms may lead to the development of novel therapeutic strategies for brain injury after stroke and related neurologic disorders.

Caspase‐3 Mediated Neuronal Death After Traumatic Brain Injury in Rats

Abstract: During programmed cell death, activation of caspase‐3 leads to proteolysis of DNA repair proteins, cytoskeletal proteins, and the inhibitor of caspase‐activated deoxyribonuclease, culminating in morphologic changes and DNA damage defining apoptosis. The participation of caspase‐3 activation in the evolution of neuronal death after traumatic brain injury in rats was examined. Cleavage of pro‐caspase‐3 in cytosolic cellular fractions and an increase in caspase‐3‐like enzyme activity were seen in injured brain versus control. Cleavage of the caspase‐3 substrates DNA‐dependent protein kinase and inhibitor of caspase‐activated deoxyribonuclease and co‐localization of cytosolic caspase‐3 in neurons with evidence of DNA fragmentation were also identified. Intracerebral administration of the caspase‐3 inhibitor N‐benzyloxycarbonyl‐Asp‐Glu‐Val‐Asp‐fluoromethyl ketone (480 ng) after trauma reduced caspase‐3‐like activity and DNA fragmentation in injured brain versus vehicle at 24 h. Treatment with N‐benzyloxycarbonyl‐Asp‐Glu‐Val‐Asp‐fluoromethyl ketone for 72 h (480 ng/day) reduced contusion size and ipsilateral dorsal hippocampal tissue loss at 3 weeks but had no effect on functional outcome versus vehicle. These data demonstrate that caspase‐3 activation contributes to brain tissue loss and downstream biochemical events that execute programmed cell death after traumatic brain injury. Caspase inhibition may prove efficacious in the treatment of certain types of brain injury where programmed cell death occurs.

Stress Proteins and Tolerance to Focal Cerebral Ischemia

Stress proteins are induced after a variety of neuronal injuries. The inducible 72-kDa heat shock protein (hsp70) is a stress protein that protects neurons from glutamate toxicity in vitro. Hsp70 has also been proposed to underlie the phenomenon of ischemic tolerance whereby brief sublethal intervals of global ischemia protect the hippocampus from subsequent lethal prolonged ischemia. To determine if the phenomenon of tolerance occurs in cortex after focal ischemia, the rat middle cerebral artery (MCA) was occluded by the suture method. Three 10-min intervals of transient ischemia (3 × 10-isc) separated by 45-min periods of reperfusion made up the most effective paradigm of preconditioning ischemia studied, and substantially reduced the volume of infarction 72 h after subsequent 100-min MCA occlusion. This approach induced protection if the interval between the 3 × 10-isc and the 100-min ischemia was 2, 3, or 5 days but not 1 or 7 days. Three 10-min intervals of transient ischemia alone produced minimal histological changes in the cortex at 72 h. Moreover, there were no significant changes in regional cerebral blood flow in the tolerant regions at 72 h after 3 × 10-isc before or during MCA occlusion. To explore the role of stress proteins in the induction of tolerance, expression of hsp70 and the glucose-regulated proteins grp75 and grp78 were studied. Samples from tolerant regions of the brain that had undergone preconditioning ischemia were evaluated at 1, 2, 3, 5, 7, and 14 days after 3 × 10-isc by Western blot analysis. The time course of hsp70 expression most closely correlated with tolerance. Hsp70 protein expression increased during times when tolerance was present (at 2–5 days) but did not increase thereafter (at 7 and 14 days). However, hsp70 was also increased before tolerance was present (at 1 day). Immunocytochemistry showed that hsp70 protein was expressed in neurons in the tolerant regions 24 h after 3 × 10-isc and was expressed in both neurons and glia after 72 h. Although immunocytochemistry suggested that there was increased neuronal expression of grp75 and grp78, no significant differences were found in protein expression as determined by Western blot before (at 1 day), during (at 2–5 days), and after (at 7 days and thereafter) tolerance. Thus, the time course of grp75 and grp78 expression did not correlate with that of tolerance. This model of ischemic tolerance is a useful method by which mechanisms of endogenous neuroprotection may be explored.

Critical Role of Calpain I in Mitochondrial Release of Apoptosis-Inducing Factor in Ischemic Neuronal Injury

Loss of mitochondrial membrane integrity and release of apoptogenic factors are a key step in the signaling cascade leading to neuronal cell death in various neurological disorders, including ischemic injury. Emerging evidence has suggested that the intramitochondrial protein apoptosis-inducing factor (AIF) translocates to the nucleus and promotes caspase-independent cell death induced by glutamate toxicity, oxidative stress, hypoxia, or ischemia. However, the mechanism by which AIF is released from mitochondria after neuronal injury is not fully understood. In this study, we identified calpain I as a direct activator of AIF release in neuronal cultures challenged with oxygen–glucose deprivation and in the rat model of transient global ischemia. Normally residing in both neuronal cytosol and mitochondrial intermembrane space, calpain I was found to be activated in neurons after ischemia and to cleave intramitochondrial AIF near its N terminus. The truncation of AIF by calpain activity appeared to be essential for its translocation from mitochondria to the nucleus, because neuronal transfection of the mutant AIF resistant to calpain cleavage was not released after oxygen–glucose deprivation. Adeno-associated virus-mediated overexpression of calpastatin, a specific calpain-inhibitory protein, or small interfering RNA-mediated knockdown of calpain I expression in neurons prevented ischemia-induced AIF translocation. Moreover, overexpression of calpastatin or knockdown of AIF expression conferred neuroprotection against cell death in neuronal cultures and in hippocampal CA1 neurons after transient global ischemia. Together, these results define calpain I-dependent AIF release as a novel signaling pathway that mediates neuronal cell death after cerebral ischemia.

Intra-mitochondrial poly-ADP-ribosylation contributes to NAD+ depletion and cell death induced by oxidative stress

Poly(ADP-ribosylation), primarily via poly(ADP-ribose) polymerase-1 (PARP-1), is a pluripotent cellular process important for maintenance of genomic integrity and RNA transcription in cells. However, during conditions of oxidative stress and energy depletion, poly(ADP-ribosylation) paradoxically contributes to mitochondrial failure and cell death. Although it has been presumed that poly(ADP-ribosylation) within the nucleus mediates this pathologic process, PARP-1 and other poly(ADP-ribosyltransferases) are also localized within mitochondria. To this end, the presence of PARP-1 and poly(ADP-ribosylation) were verified within mitochondrial fractions from primary cortical neurons and fibroblasts. Inhibition of poly(ADP-ribosylation) within the mitochondrial compartment preserved transmembrane potential (ΔΨ m ), NAD+content, and cellular respiration, prevented release of apoptosis-inducing factor, and reduced neuronal cell death triggered by oxidative stress. Treatment with liposomal NAD+ also preserved ΔΨ m and cellular respiration during oxidative stress. Furthermore, inhibition of poly(ADP-ribosylation) prevented intranuclear localization of apoptosis-inducing factor and protected neurons from excitotoxic injury; and PARP-1 null fibroblasts were protected from oxidative stress-induced cell death. Collectively these data suggest that poly(ADP-ribosylation) compartmentalized to the mitochondria can be converted from a homeostatic process to a mechanism of cell death when oxidative stress is accompanied by energy depletion. These data implicate intra-mitochondrial poly(ADP-ribosylation) as an important therapeutic target for central nervous system and other diseases associated with oxidative stress and energy failure.

Increases in Bcl-2 and cleavage of caspase-1 and caspase-3 in human brain after head injury

The bcl‐2 and caspase families are important regulators of programmed cell death in experimental models of ischemic, excitotoxic, and traumatic brain injury. The Bcl‐2 family members Bcl‐2 and Bcl‐xL suppress programmed cell death, whereas Bax promotes programmed cell death. Activated caspase‐1 (interleukin‐1β converting enzyme) and caspase‐3 (Yama/Apopain/Cpp32) cleave proteins that are important in maintaining cytoskeletal integrity and DNA repair, and activate deoxyribonucleases, producing cell death with morphological features of apoptosis. To address the question of whether these Bcl‐2 and caspase family members participate in the process of delayed neuronal death in humans, we examined brain tissue samples removed from adult patients during surgical decompression for intracranial hypertension in the acute phase after traumatic brain injury (n=8) and compared these samples to brain tissue obtained at autopsy from non‐trauma patients (n=6). An increase in Bcl‐2 but not Bcl‐xL or Bax, cleavage of caspase‐1, up‐regulation and cleavage of caspase‐3, and evidence for DNA fragmentation with both apoptotic and necrotic morphologies were found in tissue from traumatic brain injury patients compared with controls. These findings are the first to demonstrate that programmed cell death occurs in human brain after acute injury, and identify potential pharmacological and molecular targets for the treatment of human head injury.—Clark, R. S. B., Kochanek, P. M., Chen, M., Watkins, S. C., Marion, D. W., Chen, J., Hamilton, R. L., Loeffert, J. E., Graham, S. H. Increases in Bcl‐2 and cleavage of caspase‐1 and caspase‐3 in human brain after head injury. FASEB J. 13, 813–821 (1999)

Expression of the apoptosis-effector gene, Bax, is up-regulated in vulnerable hippocampal CA1 neurons following global ischemia

Abstract: The observation that delayed death of CA1 neurons after global ischemia is inhibited by protein synthesis inhibitors suggests that the delayed death of these neurons is an active process that requires new gene expression. Delayed death in CA1 has some of the characteristics of apoptotic death; however, candidate proapoptotic proteins have not been identified in the CA1 after ischemia. We studied the expression of Bax protein and mRNA, a member of the bcl‐2 family that is an effector of apoptotic cell death, after global ischemia in the four‐vessel global ischemia model in the rat and compared these results with the expression of the antiapoptotic gene bcl‐2. Bax mRNA and protein are both expressed in CA1 before delayed death, whereas bcl‐2 protein is not expressed. Bcl‐2 protein expression, but not that of Bax, is increased in CA3, a region that is ischemic but less susceptible to ischemic injury. In the dentate gyrus, both Bax and bcl‐2 proteins are expressed. The selective expression of Bax in CA1 supports the hypothesis that Bax could contribute to delayed neuronal death in these vulnerable neurons by an independent mechanism or by forming heterodimers with gene family members other than bcl‐2.

Intranuclear localization of apoptosis‐inducing factor (AIF) and large scale dna fragmentation after traumatic brain injury in rats and in neuronal cultures exposed to peroxynitrite

Programmed cell death occurs after ischemic, excitotoxic, and traumatic brain injury (TBI). Recently, a caspase‐independent pathway involving intranuclear translocation of mitochondrial apoptosis‐inducing factor (AIF) has been reported in vitro; but whether this occurs after acute brain injury was unknown. To address this question adult rats were sacrificed at various times after TBI. Western blot analysis on subcellular protein fractions demonstrated intranuclear localization of AIF in ipsilateral cortex and hippocampus at 2–72 h. Immunocytochemical analysis showed AIF labeling in neuronal nuclei with DNA fragmentation in the ipsilateral cortex and hippocampus. Immunoelectronmicroscopy verified intranuclear localization of AIF in hippocampal neurons after TBI, primarily in regions of euchromatin. Large‐scale DNA fragmentation (∼50 kbp), a signature event in AIF‐mediated cell death, was detected in ipsilateral cortex and hippocampi by 6 h. Neuron‐enriched cultures exposed to peroxynitrite also demonstrated intranuclear AIF and large‐scale DNA fragmentation concurrent with impaired mitochondrial respiration and cell death, events that are inhibited by treatment with a peroxynitrite decomposition catalyst. Intranuclear localization of AIF and large‐scale DNA fragmentation occurs after TBI and in neurons under conditions of oxidative/nitrosative stress, providing the first evidence of this alternative mechanism by which programmed cell death may proceed in neurons after brain injury.

Mild Intraischemic Hypothermia Reduces Postischemic Hyperperfusion, Delayed Postischemic Hypoperfusion, Blood-Brain Barrier Disruption, Brain Edema, and Neuronal Damage Volume after Temporary Focal Cerebral Ischemia in Rats

Mild to moderate hypothermia (30–33°C) reduces brain injury after brief (<2-h) periods of focal ischemia, but its effectiveness in prolonged temporary ischemia is not fully understood. Thirty-two Sprague–Dawley rats anesthetized with 1.5% isoflurane underwent 3 h of middle cerebral artery occlusion under hypothermic (33°C) or normothermic (37°C) conditions followed by 3 or 21 h of reperfusion under normothermic conditions (n = 8/group). Laser–Doppler estimates of cortical blood flow showed that intraischemic hypothermia reduced both postischemic hyperperfusion (p ≤ 0.01) and postischemic delayed hypoperfusion (p ≤ 0.01). Hypothermia reduced the extent of blood-brain barrier (BBB) disruption as estimated from the extravasation of Evans blue dye at 6 h after the onset of ischemia (p ≤ 0.01). Hypothermia also reduced the volume of both brain edema (p ≤ 0.01) and neuronal damage (p ≤ 0.01) as estimated from Nissl-stained slides at both 6 and 24 h after the onset of ischemia. These results demonstrate that mild intraischemic hypothermia reduces tissue injury after prolonged temporary ischemia, possibly by attenuating postischemic blood flow disturbances and by reducing vasogenic edema resulting from BBB disruption.

Oxidative stress following traumatic brain injury in rats: Quantitation of biomarkers and detection of free radical intermediates

Abstract: Oxidative stress may contribute to many pathophysiologic changes that occur after traumatic brain injury. In the current study, contemporary methods of detecting oxidative stress were used in a rodent model of traumatic brain injury. The level of the stable product derived from peroxidation of arachidonyl residues in phospholipids, 8‐epi‐prostaglandin F2α, was increased at 6 and 24 h after traumatic brain injury. Furthermore, relative amounts of fluorescent end products of lipid peroxidation in brain extracts were increased at 6 and 24 h after trauma compared with sham‐operated controls. The total antioxidant reserves of brain homogenates and water‐soluble antioxidant reserves as well as tissue concentrations of ascorbate, GSH, and protein sulfhydryls were reduced after traumatic brain injury. A selective inhibitor of cyclooxygenase‐2, SC 58125, prevented depletion of ascorbate and thiols, the two major water‐soluble antioxidants in traumatized brain. Electron paramagnetic resonance (EPR) spectroscopy of rat cortex homogenates failed to detect any radical adducts with a spin trap, 5,5‐dimethyl‐1‐pyrroline N‐oxide, but did detect ascorbate radical signals. The ascorbate radical EPR signals increased in brain homogenates derived from traumatized brain samples compared with sham‐operated controls. These results along with detailed model experiments in vitro indicate that ascorbate is a major antioxidant in brain and that the EPR assay of ascorbate radicals may be used to monitor production of free radicals in brain tissue after traumatic brain injury.