Involvement of caspase-3 in cell death after hypoxia-ischemia declines during brain maturation

Post-ischemic hypothermia-induced tissue protection and diminished apoptosis after neonatal cerebral hypoxia–ischemia

Hypothermia is possibly the single most effective method of neuroprotection developed to date. However, the mechanisms are not completely understood. The aim of this study was to investigate the effects of post-ischemic hypothermia on brain injury and apoptotic neuronal cell death as well as related biochemical changes after neonatal hypoxia-ischemia (HI). Seven-day-old rats were subjected to left common carotid artery ligation and hypoxia (7.8%) for 1 h. Systemic hypothermia was induced immediately after hypoxia-ischemia, and body temperature was maintained at 30 degrees C for 10 h. The normothermic group was kept at 36 degrees C. Brain infarct volumes and neuronal loss in the CA1 area of the hippocampus were significantly reduced at 72 h post-HI in the hypothermia group. Cytochrome c release and activation of caspase-3 and -2 at 24 h post-HI were significantly diminished by hypothermia. The numbers of cytochrome c- and TUNEL-positive cells in the cortex and dentate gyrus of the hippocampus were significantly reduced in the hypothermia group compared with the normothermia group at 72 h post-HI. These results indicate that hypothermia may, at least partially, act through inhibition of the intrinsic pathway of caspase activation in the neonatal brain, thereby preventing apoptotic cell death.

Inhibiting caspase-8 after injury reduces hypoxic–ischemic brain injury in the newborn rat

A broad spectrum caspase inhibitor reduces brain injury. Will a caspase-8 inhibitor provide protection? Seven-day-old rat pups had the right carotid artery ligated, then were subjected to 2.5 h of 8% oxygen. Caspase-8 activity in the right cortex was measured enzymatically. Caspase-8 activity was increased at 12 and 24 h after injury and IETD-CHO, (Ac-Ala-Ala-Val-Ala-Leu-Leu-Pro-Ala-Val-Leu-Leu-Ala-Pro-Ile-Glu-Thr-Asp-CHO, CHO is aldehyde) a cell permeable caspase-8 inhibitor, given by i.c.v. injection after the hypoxic period eliminated this increase with significant effect at 15 and 50 microg/pup (1.7 micromol/kg). Thirty pups were randomly assigned to receive 50 microg/pup of IETD-CHO or vehicle i.c.v. immediately after the hypoxic period. The loss of brain weight in the right hemisphere 22 days after injury was 29+/-5% in the vehicle-treated animals and 12+/-5% in the IETD-CHO-treated animals (P<0.05). Inhibiting caspase-8 activity after hypoxic-ischemic brain injury reduces brain injury.

Both apoptosis and necrosis occur early after intracerebral grafting of ventral mesencephalic tissue: a role for protease activation

Neural transplantation is an experimental treatment for Parkinson's disease. Widespread clinical application of the grafting technique is hampered by a relatively poor survival (around 10%) of implanted embryonic dopamine neurones. Earlier animal studies have indicated that a large proportion of the grafted cells die during graft tissue preparation and within the first few days after intracerebral implantation. The present study was designed to reveal the prevalence of cell death in rat intrastriatal grafts at 90 min, 1, 3, 6 and 42 days after implantation. We examined apoptotic cell death using semi-thin and paraffin sections stained with methylene blue and an antibody against activated caspase 3, respectively. We identified abundant apoptotic cell death up to 3 days after transplantation. In addition, we studied calpain activation using an antibody specific for calpain-cleaved fodrin. We report a peak in calpain activity 90 min after grafting. Surprisingly, we did not observe any significant difference in the number of dopaminergic neurones over time. The present results imply that grafted cells may be victims of either an early necrotic or a later apoptotic cell death and that there is substantial cell death as early as 90 min after implantation.

Apoptotic mitochondrial pathway in neurones and astrocytes after neonatal hypoxia-ischaemia in the rat brain

Neuronal apoptosis plays an essential role in early brain development and contributes to secondary neuronal loss after acute ischaemia. Recent studies have provided evidence that caspase-3 is an important downstream event after hypoxia-ischaemia in the immature brain, but a minor event in the adult brain. Our investigations have focused on cell populations that expressed apoptotic effectors in the enzymatic death pathway including cytochrome c, caspase-9 and caspase-3. Expression, activation and cellular localization of these proteins were studied using cleavage of fluorogenic substrate and immunohistochemistry in neonatal rat brain after unilateral focal ischaemia. Caspase-3 enzyme activity was elevated in brain homogenate between 6 and 48 h after reperfusion. This activation was preceded by that of caspase-9, between 3 and 24 h. Apoptotic cell death was finally accomplished by poly-ADP-ribose polymerase cleavage, an endogenous caspase-3 substrate. In addition, immunodetection demonstrated that cytochrome c and activated caspase-9 and caspase-3 were expressed not only in the neurones, the primarily affected cells, but also within the astrocytes, which constituted a dense network delineating the infarct. These results suggested that glial injury may promote the formation of cystic lesions such as those observed clinically in the newborn brain.

Ischemic neuronal death in the rat hippocampus: the calpain-calpastatin-caspase hypothesis

Inappropriate imbalances between proteases and protease inhibitors are known to occur under cerebral ischemia and neurodegenerative processes, and could be contributors to various diseases that are characterized by excessive (ischemia, AIDS) or inadequate (cancer, autoimmunity) cell death. For instance, calpain is activated in various necrotic and apoptotic conditions, whereas caspase-3 is only activated in neuronal apoptosis. Caspases and calpains are cysteine proteases that require proteolytic cleavage for activation. The substrates cleaved by caspases include cytoskeletal and associated proteins, kinases, members of the Bcl-2 family of apoptosis-related proteins, presenilins, and DNA-modulating enzymes. Calpain substrates include cytoskeletal and associated proteins, kinases and phosphatases, membrane receptors and transporters, and steroid receptors. This article provides a review of the properties of caspases and calpains, their roles in cell death pathways following cerebral ischemia, and the substrates upon which they act. Because calpain inhibitors and caspase inhibitors appear to protect brain tissue by distinct mechanisms in cerebral ischemia, the possible therapeutic interactions between these drugs in a well-defined rodent model of global ischemia are briefly discussed and documented.

Insulin-like growth factor (IGF)-1 suppresses oligodendrocyte caspase-3 activation and increases glial proliferation after ischemia in near-term fetal sheep

Insulin-like growth factor (IGF-1) markedly increases myelination and glial numbers in white matter after ischemia in near-term fetal sheep; however, it is unclear whether this is due to reduced cell loss or increased secondary proliferation. Brain injury was induced in near-term fetal sheep by 30 minutes of bilateral carotid artery occlusion. Ninety minutes after the occlusion, fetuses were given, intracerebroventricularly, either a single dose of IGF-1 (either 3 or 30 micro g), or 3 micro g followed by 3 micro g over 24 hours (3 + 3 micro g). White matter was assessed 4 days after reperfusion. Three micrograms, but not 30 micro g of IGF-1 prevented loss of oligodendrocytes and myelin basic protein density (P < 0.001) compared to the vehicle-treated ischemia controls. No additional effect was observed in the 3 + 3 micro g group. IGF-1 treatment was associated with reduced caspase-3 activation and increased glial proliferation in a similar dose-dependent manner. Caspase-3 was only expressed in oligodendrocytes that showed apoptotic morphology. Proliferating cell nuclear antigen co-localized with both oligodendrocytes and astrocytes and microglia. Thus, increased oligodendrocyte numbers after IGF-1 treatment is partly due to suppression of apoptosis, and partly to increased proliferation. In contrast, the increase in reactive glia was related only to proliferation. Speculatively, reactive glia may partly mediate IGF-1 white matter protection.

Mitochondria and ischemic reperfusion damage in the adult and in the developing brain

The developing and the adult brain respond in similar ways to ischemia, but also display clear differences. For example, the relative contributions of necrosis and apoptosis to neuronal death may be different, such that apoptotic mechanisms would be more prevalent in the developing brain. During normal development, more than half of the neurons in some brain regions are removed through apoptosis, and effectors like caspase-3 are highly upregulated in the immature brain. Mitochondria are pivotal regulators of cell death through their role in energy production and calcium homeostasis, their capacity to release apoptogenic proteins and to produce reactive oxygen species. This review will summarize some of the current studies dealing with mitochondria-related mechanisms of ischemic brain damage, with special reference to developmental aspects.

Bcl-2 overexpression protects against neuron loss within the ischemic margin following experimental stroke and inhibits cytochrome c translocation and caspase-3 activity

Bcl-2 protects against both apoptotic and necrotic death induced by several cerebral insults. We and others have previously demonstrated that defective herpes simplex virus vectors expressing Bcl-2 protect against various insults in vitro and in vivo, including cerebral ischemia. Because the infarct margin may be a region that is most amenable to treatment, we first determined whether gene transfer to the infarct margin is possible using a focal ischemia model. Since ischemic injury with and without reperfusion may occur by different mechanisms, we also determined whether Bcl-2 protects against focal cerebral ischemic injury either with or without reperfusion in rats. Bax expression, cytochrome c translocation and activated caspase-3 expression were also assessed. Viral vectors overexpressing Bcl-2 were delivered to the infarct margin. Reperfusion resulted in larger infarcts than permanent occlusion. Bcl-2 overexpression significantly improved neuron survival in both ischemia models. Bcl-2 overexpression did not alter overall Bax expression, but inhibited cytosolic accumulation of cytochrome c and caspase-3 activation. Thus, we provide the first evidence that gene transfer to the infarct margin is feasible, that overexpression of Bcl-2 protects against damage to the infarct margin induced by ischemia with and without reperfusion, and that Bcl-2 overexpression using gene therapy attenuates apoptosis-related proteins. This suggests a potential therapeutic strategy for stroke.

Cross-talk between calpain and caspase proteolytic systems during neuronal apoptosis

Cross-talk between calpain and caspase proteolytic systems has complicated efforts to determine their distinct roles in apoptotic cell death. This study examined the effect of overexpressing calpastatin, the specific endogenous calpain inhibitor, on the activity of the two proteolytic systems following an apoptotic stimulus. Human SH-SY5Y neuroblastoma cells were stably transfected with full-length human calpastatin cDNA resulting in 20-fold overexpression based on Western blot and 5-fold greater calpain inhibitory activity in cell extracts. Wild type and calpastatin overexpressing (CST1) cells were neuronally differentiated and apoptosis-induced with staurosporine (0.1-1.0 microm). Calpastatin overexpression decreased calpain activation, increased caspase-3-like activity, and accelerated the appearance of apoptotic nuclear morphology. Following 0.1-0.2 microm staurosporine, plasma membrane integrity based on calcein-acetoxymethyl fluorescence was significantly greater at 24 h in differentiated CST1 compared with differentiated wild type cells. However, this protective effect was lost at higher staurosporine doses (0.5-1.0 microm), which resulted in pronounced caspase-mediated degradation of the overexpressed calpastatin. These results suggest a dual role for calpains during neuronal apoptosis. In the early execution phase, calpain down-regulates caspase-3-like activity and slows progression of apoptotic nuclear morphology. Subsequent calpain activity, facilitated by caspase-mediated degradation of calpastatin, contributes to plasma membrane disruption and secondary necrosis.

Postnatal brain development and neural cell differentiation modulate mitochondrial Bax and BH3 peptide-induced cytochrome c release

Bax mediates cytochrome c release and apoptosis during neurodevelopment. Brain mitochondria that were isolated from 8-day, 17-day, and adult rats displayed decreasing levels of mitochondrial Bax. The amount of cytochrome c released from brain mitochondria by a peptide containing the BH3 cell death domain decreased with increasing age. However, approximately 60% of cytochrome c in adult brain mitochondria could be released by the BH3 peptide in the presence of exogenous human recombinant Bax. Mitochondrial Bax was downregulated in PC12S neural cells differentiated with nerve growth factor, and mitochondria isolated from these cells demonstrated decreased sensitivity to BH3-peptide-induced cytochrome c release. These results demonstrate that immature brain mitochondria and mitochondria from undifferentiated neural cells are particularly sensitive to cytochrome c release mediated by endogenous Bax and a BH3 death domain peptide. Postnatal developmental changes in mitochondrial Bax levels may contribute to the increased susceptibility of neurons to pathological apoptosis in immature animals.

Effects of hyperthermia on hypoxic-ischemic brain damage in the immature rat: its influence on caspase-3-like protease

OBJECTIVE

Recent clinical studies suggested that intrapartum maternal fever is a strong independent risk factor for neonatal encephalopathy. With use of a well-studied rat model of neonatal hypoxic-ischemic encepalopathy, this study investigated the hypothesis that intraischemic hyperthermia accelerates and worsens brain injury in immature animals and examined whether apoptotic cell death machinery is involved in the underlying mechanisms.

STUDY DESIGN

Seven-day-old rats underwent a combination of left common carotid artery ligation and exposure to 8% oxygen for 15 minutes (n = 32 rats). During the 15-minute hypoxic insult, body temperature was elevated to 40 degrees C in 16 animals (hyperthermic hypoxic insult group), and was maintained at 37 degrees C in 16 animals (normothermic hypoxic insult group). Then both groups were placed in the same chamber in a water bath at 37 degrees C for 24 hours and finally returned to the mothers. Caspase-3-like activity was assessed 36 hours after the hypoxic-ischemic insult. One week later, microtubule-associated protein-2 immunostaining was used to examine neuronal damage.

RESULTS

Intraischemic hyperthermia was shown to activate the caspase-3 activity 36 hours after hypoxia-ischemia while caspase-3 was activated insignificantly in the normothermic hypoxic insult group at that time. The hyperthermic hypoxic insult group also showed a reduced microtubule-associated protein-2-positive area 7 days after hypoxia-ischemia compared with that in the normothermia group.

CONCLUSION

Hyperthermia during hypoxia-ischemia makes the immature brain inordinately susceptible to hypoxic-ischemic insult and causes brain injury, even if hypoxic-ischemic insult is so mild that it causes no or little injury by itself. This effect may be mediated by the escalation of the apoptotic cell death pathway in the immature animal.

Reperfusion differentially induces caspase-3 activation in ischemic core and penumbra after stroke in immature brain

BACKGROUND AND PURPOSE

Different strategies for neuroprotection of neonatal stroke may be required because the developing brain responds differently to hypoxia-ischemia than the mature brain. This study was designed to determine the role of caspase-dependent injury in the pathophysiology of pure focal cerebral ischemia in the immature brain.

METHODS

Postnatal day 7 rats were subjected to permanent or transient middle cerebral artery (MCA) occlusion. Diffusion-weighted MRI was used during occlusion to noninvasively map the evolving ischemic core. The time course of caspase-3 activation in ischemic brain tissue was determined with the use of an Asp-Glu-Val-Asp-aminomethylcoumarin cleavage assay. The anatomy of caspase-3 activation in the ischemic core and penumbra was mapped immunohistochemically with an anti-activated caspase-3 antibody in coronal sections that matched the imaging planes on diffusion-weighted MRI.

RESULTS

A marked increase in caspase-3 activity occurred within 24 hours of reperfusion after transient MCA occlusion. In contrast, caspase-3 activity remained significantly lower within 24 hours of permanent MCA occlusion. Cells with activated caspase-3 were prominent in the penumbra beginning at 3 hours after reperfusion, while a more delayed but marked caspase-3 activation was observed in the ischemic core by 24 hours after reperfusion.

CONCLUSIONS

In the neonate, caspase-3 activation is likely to contribute substantially to cell death not only in the penumbra but also in the core after ischemia with reperfusion. Furthermore, persistent perfusion deficits result in less caspase-3 activation and appear to favor caspase-independent injury.

Pathogenesis of hypoxic-ischemic cerebral injury in the term infant: current concepts

Multiple, biochemical cascades contribute to the pathogenesis of neonatal hypoxic-ischemic brain injury. This article summarizes experimental evidence that supports the role of excitatory amino acids, calcium, free radicals, nitric oxide, proinflammatory cytokines, and bioactive lipids. Specific vulnerabilities that distinguish the response of the immature brain from that of the mature brain are highlighted. These include increased susceptibility to excitotoxicity and free radical injury, greater tendency to apoptotic death, and heightened vulnerability of developing oligodendrocytes. Available supportive evidence from human studies is also included. Implications for clinical neuroprotective strategies are discussed.

Caspase inhibitors reduce the apoptotic but not necrotic component of kainate injury in primary murine cortical neuronal cultures

Excitotoxicity has been demonstrated to play a major role in ischemic neuronal injury. While the necrotic component of excitotoxicity has been well demonstrated, apoptosis has also been shown to play a role. We sought to quantitate and modulate the apoptotic component of kainate-induced injury. Experiments were performed in mouse primary cortical neuronal cultures after three or 10 days in vitro. Cell death was assessed by Hoechst/propidium iodide staining and cell counting. Apoptosis was further confirmed with inhibition by caspase inhibitors. Exposure of three-day old neurons to 100 microM kainate produced an injury in which 56% +/- 0.9% of cells showed apoptotic nuclei and 13.5% +/- 2.0% showed necrotic nuclei. After 10 days in vitro neurons were more easily injured by kainate, but the cell death had primarily necrotic characteristics. Inhibition of both caspases 1 and 3 significantly reduced the apoptotic injury of 3-day old neurons. Neither reduced the necrotic component. Inhibition of protein synthesis with cycloheximide was also effective in reducing the apoptotic injury without affecting the necrotic injury. Kainate injury causes both apoptosis and necrosis, with the injury depending on both the dose of kainate and the age of the culture. The apoptotic component can be selectively reduced by caspase inhibition or cycloheximide.

Comparison of calpain and caspase activities in the adult rat brain after transient forebrain ischemia

The role of calpain and caspase family proteases in postischemic neuronal death remains controversial. This study compared the timing, location, and relative activity of calpains and caspases in the adult rat brain following 10 min of transient forebrain ischemia. Western blots of cortical, striatal, and hippocampal homogenates demonstrated a alpha-spectrin cleavage pattern indicative of predominant calpain activity, which peaked between 24 and 48 h after reperfusion. However, immunohistochemical evidence of both caspase 3 activation and caspase-mediated substrate cleavage was detected as early as 1 h and as late as 7 days after reperfusion in circumscribed neuronal populations. Simultaneous or sequential caspase and calpain activation was also observed suggesting the potential for interaction of these protease systems. The complex spatiotemporal pattern of calpain and caspase activity observed in this study provides important insights for the development and evaluation of therapeutic strategies to reduce protease-mediated injury following global brain ischemia.

Caspase activation and neuroprotection in caspase-3- deficient mice after in vivo cerebral ischemia and in vitro oxygen glucose deprivation

Caspase-3 is a major cell death effector protease in the adult and neonatal nervous system. We found a greater number and higher density of cells in the cortex of caspase-3(-/-) adult mice, consistent with a defect in developmental cell death. Caspase-3(-/-) mice were also more resistant to ischemic stress both in vivo and in vitro. After 2 h of ischemia and 48 h of reperfusion, cortical infarct volume was reduced by 55%, and the density of terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling-positive cells was decreased by 36% compared with wild type. When subjected to oxygen-glucose deprivation (2 h), cortical neurons cultured from mice deficient in caspase-3 expression were also more resistant to cell death by 59%. Mutant brains showed caspase-specific poly(ADP-ribose) polymerase cleavage product (85-kDa fragment) in vivo and in vitro, suggesting redundant mechanisms and persistence of caspase-mediated cell death. In the present study, we found that caspase-8 mediated poly(ADP-ribose) polymerase cleavage in caspase-3(-/-) neurons in vivo and in vitro. In addition, mutant neurons showed no evidence of compensatory activation by caspase-6 or caspase-7 after ischemia. Taken together, these data extend the pharmacological evidence supporting an important role for caspase-3 and caspase-8 as cell death mediators in mammalian cortex and indicate the potential advantages of targeting more than a single caspase family member to treat ischemic cell injury.

Age-dependent resistance to lethal alphavirus encephalitis in mice: analysis of gene expression in the central nervous system and identification of a novel interferon-inducible protective gene, mouse ISG12

Several different mammalian neurotropic viruses produce an age-dependent encephalitis characterized by more severe disease in younger hosts. To elucidate potential factors that contribute to age-dependent resistance to lethal viral encephalitis, we compared central nervous system (CNS) gene expression in neonatal and weanling mice that were either mock infected or infected intracerebrally with a recombinant strain, dsTE12Q, of the prototype alphavirus Sindbis virus. In 1-day-old mice, infection with dsTE12Q resulted in rapidly fatal disease associated with high CNS viral titers and extensive CNS apoptosis, whereas in 4-week-old mice, dsTE12Q infection resulted in asymptomatic infection with lower CNS virus titers and undetectable CNS apoptosis. GeneChip expression comparisons of mock-infected neonatal and weanling mouse brains revealed developmental regulation of the mRNA expression of numerous genes, including some apoptosis regulatory genes, such as the proapoptotic molecules caspase-3 and TRAF4, which are downregulated during development, and the neuroprotective chemokine, fractalkine, which is upregulated during postnatal development. In parallel with increased neurovirulence and increased viral replication, Sindbis virus infection in 1-day-old mice resulted in both a greater number of host inflammatory genes with altered expression and greater changes in levels of host inflammatory gene expression than infection in 4-week-old mice. Only one inflammatory response gene, an expressed sequence tag similar to human ISG12, increased by a greater magnitude in infected 4-week-old mouse brains than in infected 1-day-old mouse brains. Furthermore, we found that enforced neuronal ISG12 expression results in a significant delay in Sindbis virus-induced death in neonatal mice. Together, our data identify genes that are developmentally regulated in the CNS and genes that are differentially regulated in the brains of different aged mice in response to Sindbis virus infection.

Reduction in intracellular calcium levels induces injury in developing neurons

The neurotransmitter glutamate influences intracellular Ca(2+) levels and plays an essential role in maintaining neuronal viability during early development. Blockade of NMDA receptors induces cell death in the neonatal forebrain via mechanisms that are not understood. Other neuromodulators that can influence intracellular Ca(2+) levels include the nucleoside adenosine, which acts via A(1) adenosine receptors subtypes (A(1)ARs). Because A(1)AR activation inhibits glutamate release and action, A(1)AR activation may also contribute to neonatal brain injury. To examine this possibility, we treated primary neuronal cultures with the A(1)AR agonist CPA, the NMDAR antagonist MK801, or CPA + MK801. Combined MK801 + CPA treatment resulted in profound cellular injury, exceeding that seen in other groups. In keeping with the hypothesis that altered Ca(2+) signaling mediates CPA + MK801 injury, reduction of Ca(2+) levels with EGTA, thapsigargin, or BAPTA-AM enhanced CPA + MK801-induced neuronal damage. In contrast, increasing intracellular Ca(2+) using ionomycin reversed CPA + MK801 toxicity. Direct visualization of intracellular Ca(2+) by confocal microscopy revealed that CPA + MK801 inhibited KCl-evoked increases in intracellular Ca(2+). Supporting the concept that A(1)AR activation and NMDAR blockade results in brain injury, neonatal rats injected with A(1)AR agonists + MK801 showed widespread apoptosis in many brain regions. These observations show that A(1)AR activation and NMDAR blockade lead to early postnatal cell injury by mechanisms that involve inhibition of intracellular Ca(2+) signaling.

Downregulation of matrix metalloproteinase-9 and attenuation of edema via inhibition of ERK mitogen activated protein kinase in traumatic brain injury

Emerging data suggest that matrix metalloproteinase-9 (MMP-9) plays a critical role in the pathophysiology of brain injury. However, the regulatory mechanisms involved in vivo remain unclear. In this study, we focus on a mitogen activated protein kinase (MAPK) pathway that may trigger MMP-9 after traumatic brain injury. We aim to show that inhibition of the extracellular signal regulated kinase (ERK) would attenuate MMP-9 levels, reduce blood-brain barrier damage, and attenuate edema after trauma induced by controlled cortical impact in mouse brain. Western blots showed that phospho-ERK was rapidly upregulated after trauma. Treatment with U0126, which inhibits MEK, the kinase upstream of ERK, effectively prevented the activation of ERK. After trauma, gelatin zymography showed an increase in MMP-9. U0126 significantly reduced trauma-induced MMP-9 levels. Correspondingly, U0126 ameliorated the degradation of the tight junction protein ZO-1, which is an MMP-9 substrate, and significantly attenuated tissue edema. At 7 days after trauma, traumatic lesion volumes were significantly reduced by U0126 compared with saline-treated controls. These data indicate that the ERK MAPK pathway triggers the upregulation in MMP-9 after trauma, and further suggest that targeting the upstream signaling mechanisms that regulate deleterious MMP-9 activity may reveal new therapeutic opportunities for traumatic brain injury.

Neuroprotection by selective nitric oxide synthase inhibition at 24 hours after perinatal hypoxia-ischemia

BACKGROUND AND PURPOSE

Perinatal hypoxia-ischemia is a major cause of neonatal morbidity and mortality. Until now no established neuroprotective intervention after perinatal hypoxia-ischemia has been available. The delay in cell death after perinatal hypoxia-ischemia creates possibilities for therapeutic intervention after the initial insult. Excessive nitric oxide and reactive oxygen species generated on hypoxia-ischemia and reperfusion play a key role in the neurotoxic cascade. The present study examines the neuroprotective properties of neuronal and inducible but not endothelial nitric oxide synthase inhibition by 2-iminobiotin in a piglet model of perinatal hypoxia-ischemia.

METHODS

Twenty-three newborn piglets were subjected to 60 minutes of hypoxia-ischemia, followed by 24 hours of reperfusion and reoxygenation. Five additional piglets served as sham-operated controls. On reperfusion, piglets were randomly treated with either vehicle (n=12) or 2-iminobiotin (n=11). At 24 hours after hypoxia-ischemia, the cerebral energy state, presence of vasogenic edema, amount of apparently normal neuronal cells, caspase-3 activity, amount of terminal deoxynucleotidyl transferase-mediated dUTP-biotin in situ nick end labeling (TUNEL)-positive cells, and degree of tyrosine nitration were assessed.

RESULTS

A 90% improvement in cerebral energy state, 90% reduction in vasogenic edema, and 60% to 80% reduction in apoptosis-related neuronal cell death were demonstrated in 2-iminobiotin-treated piglets at 24 hours after hypoxia- ischemia. A significant reduction in tyrosine nitration in the cerebral cortex was observed in 2-iminobiotin-treated piglets, indicating decreased formation of reactive nitrogen species.

CONCLUSIONS

Simultaneous and selective inhibition of neuronal and inducible nitric oxide synthase by 2-iminobiotin is a promising strategy for neuroprotection after perinatal hypoxia-ischemia.

Effect of hypoxia on caspase-3, -8, and -9 activity and expression in the cerebral cortex of newborn piglets

Caspases play an important role in programmed cell death. Caspase-3 is a key executioner of apoptosis, whose activation is mediated by the initiator caspases, caspase-8 and caspase-9. The present study tested the hypothesis that cerebral hypoxia results in increased activation and expression of caspases-3, -8, and -9 in the cytosolic fraction of the cerebral cortex of newborn piglets. To test this hypothesis the activity and expression of caspases-3, -8, and -9 were determined in newborn piglets divided into normoxic and hypoxic groups. Caspase activity was determined spectrofluorometrically using enzyme specific substrates. The expression of caspase protein was assessed by Western blot analysis using enzyme specific antibody. Caspases-3, -8, and -9 activity and expression was significantly higher in the hypoxic group than in the normoxic group. These results demonstrate that hypoxia induces activation and increased expression of both the initiator caspases and the executioner caspase in the cerebral cortex of newborn piglets. We conclude that hypoxia results in stimulation of both the pathways of caspase-3 activation.

Selective, reversible caspase-3 inhibitor is neuroprotective and reveals distinct pathways of cell death after neonatal hypoxic-ischemic brain injury

Hypoxia-ischemia (H-I) in the developing brain results in brain injury with prominent features of both apoptosis and necrosis. A peptide-based pan-caspase inhibitor is neuroprotective against neonatal H-I brain injury, suggesting a central role of caspases in brain injury. Because previously studied peptide-based caspase inhibitors are not potent and are only partially selective, the exact contribution of specific caspases and other proteases to injury after H-I is not clear. In this study, we explored the neuroprotective effects of a small, reversible caspase-3 inhibitor M826. M826 selectively and potently inhibited both caspase-3 enzymatic activity and apoptosis in cultured cells in vitro. In a rat model of neonatal H-I, M826 blocked caspase-3 activation and cleavage of its substrates, which begins 6 h and peaks 24 h after H-I. Although M826 significantly reduced DNA fragmentation and brain tissue loss, it did not prevent calpain activation in the cortex. This activation, which is associated with excitotoxic/necrotic cell injury, occurred within 30 min to 2 h after H-I even in the presence of M826. Similar to calpain activation, we found evidence of caspase-2 processing within 30 min to 2 h after H-I that was not affected by M826. Caspase-2 processing appeared to be secondary to calpain-mediated cleavage and was not associated with caspase-2 activation. These data suggest that caspase-3 specifically contributes to delayed cell death and brain injury after neonatal H-I and that calpain activation is associated with and likely a marker for the early component of excitotoxic/necrotic brain injury previously demonstrated in this model.

Mechanisms of hypoxic neurodegeneration in the developing brain

Asphyxia and other insults to the developing brain are responsible for several human neurodevelopmental disorders. The pattern of neonatal brain injury differs from that seen in the adult nervous system, and there are wide differences in regional vulnerability. Recent evidence suggests that two events that contribute to this pattern of selective vulnerability are developmental changes in excitatory glutamate-containing neurotransmitter circuits and the propensity for immature neurons to die by apoptosis rather than necrosis. Developmental up-regulation of NMDA receptors with enhanced function and increased expression of caspase-3 at critical periods in development are linked to these mechanisms. Although these molecular changes enhance the developing brain's capacity for plasticity by helping to prune redundant synapses and neurons, they can become "Achilles heels" in the face of a brain energy crisis.

Secretion of matrix metalloproteinase-2 and -9 after mechanical trauma injury in rat cortical cultures and involvement of MAP kinase

Matrix metalloproteinases (MMP) are involved in the pathophysiology of brain injury. We recently showed that knockout mice deficient in MMP-9 expression were protected against traumatic brain injury. However, the cellular sources of MMP activity after trauma remain to be fully defined. In this study, we investigated the hypothesis that resident brain cells secrete MMP after mechanical trauma injury in vitro, and mitogen-activated protein (MAP) kinase signal transduction pathways are involved in this response. Rat primary cortical neurons, astrocytes, and co-cultures were subjected to needle scratch mechanical injury, and levels of MMP-2 and MMP-9 in conditioned media were assayed by zymography. MMP-2 and MMP-9 were increased in cortical astrocytes and co-cultures, whereas only MMP-2 was increased in neurons. Western blots showed that phosphorylated extracellular signal regulated kinase (ERK1/2) and p38 were rapidly upregulated in co-cultures after mechanical injury. No change in phosphorylated c-jun N-terminal kinase (JNK) was observed. In-gel kinase assays confirmed this lack of response in the JNK pathway. Treatment with either 10 microM of U0126 (a MAP kinase/ERK1/2 kinase inhibitor) or 10 microM of SB203580 (a p38 inhibitor) had no detectable effect on MMP-2 and MMP-9 levels after mechanical injury. However, combination treatment with both inhibitors significantly reduced secretion of MMP-9. Herein, we demonstrate that (1) resident brain cells secrete MMP after mechanical injury, (2) astrocytes are the main source of MMP-9 activity, and (3) ERK and p38 MAP kinases are upregulated after mechanical injury, and mediate the secretion of MMP-9.

Role of caspase-3 activation in cerebral ischemia-induced neurodegeneration in adult and neonatal brain

These studies have addressed the role of caspase-3 activation in neuronal death after cerebral ischemia in different animal models. The authors were unable to show activation of procaspase-3 measured as an induction of DEVDase (Asp-Glu-Val-Asp) activity after focal or transient forebrain ischemia in rats. DEVDase activity could not be induced in the cytosolic fraction of the brain tissue obtained from these animals by exogenous cytochrome c/dATP and Ca2+. However, the addition of granzyme B to these cytosolic fractions resulted in a significant activation of DEVDase, confirming that the conditions were permissive to analyze proteolytic cleavage of the DEVD-AMC (7-amino-4-methyl-coumarin) substrate. Consistent with these findings, zVal-Ala-Asp-fluoromethylketone administered after focal ischemia did not have a neuroprotective effect. In contrast to these findings, a large increase in DEVDase activity was detected in a model of hypoxic-ischemia in postnatal-day-7 rats. Furthermore, in postnatal-day-7 animals treated with MK-801, in which it has been suggested that excessive apoptosis is induced, the authors were unable to detect activation of DEVDase activity but were able to induce it in vitro by the addition of cytochrome c/dATP and Ca2+ to the cytosolic fraction. Analysis of cytochrome c distribution did not provide definitive evidence for selective cytochrome c release in the permanent focal ischemia model, whereas in the transient model a small but consistent amount of cytochrome c was found in the cytosolic fraction. However, in both models the majority of cytochrome c remained associated with the mitochondrial fraction. In conclusion, the authors were unable to substantiate a role of mitochondrially derived cytochrome c and procaspase-3 activation in ischemia-induced cell death in adult brain, but did see a clear induction of caspase-3 in neonatal hypoxia.

Involvement of caspase-3 and p38 mitogen-activated protein kinase in cobalt chloride-induced apoptosis in PC12 cells

Our previous study showed that cobalt chloride (CoCl2) could induce PC12 cell apoptosis and that the CoCl2-treated PC12 cells may serve as a simple in vitro model for the study of the mechanism of hypoxia-linked neuronal disorders. The aim of this study is to elucidate the mechanism of CoCl2-induced apoptosis in PC12 cells. Caspases are known to be involved in the apoptosis induced by various stimuli in many cell types. To investigate the involvement of caspases in CoCl2-induced apoptosis in PC12 cells, we generated PC12 cells that stably express the viral caspases inhibitor gene p35 and analyzed the effect of p35 on the process of apoptosis induced by CoCl2. We also examined the effect of cell-permeable peptide inhibitors of caspases. The results showed that the baculovirus p35 gene and the general caspases inhibitor Z-VAD-FMK significantly block apoptosis induced by CoCl2, confirming that caspase is involved in CoCl2-induced apoptosis. Further investigation showed that in this process the caspase-3-like activity is increased, as indicated by the cells' ability to cleave the fluorogenic peptide substrate Ac-Asp-Glu-Val-Asp-7-AMC and to degrade the DNA-repairing enzyme poly-(ADP-ribose) polymerase (PARP), an endogenous caspase-3 substrate. At the same time, caspase-3-specific inhibitors, namely, the peptide Ac-DEVD-CHO, Ac-DEVD-FMK, partially inhibit CoCl2-induced apoptosis. These findings suggested that caspase-3 or caspase-3-like proteases are involved in the apoptosis induced by CoCl2 in PC12 cells. Additionally, we have observed that another apoptotic marker, p38 mitogen-activated protein kinase (MAPK), is significantly activated in this process in a time-dependent manner and that a selective p38 MAPK inhibitor, SB203580, partially inhibits this cell death. The addition of SB203580 also partially suppresses caspase-3-like activity. All these results confirm that the CoCl2-treated PC12 cell is a useful in vitro model with which to study hypoxia-linked neuronal disorders. Furthermore, the results showing that the baculovirus p35 gene and caspase inhibitors possess a remarkable ability to rescue PC12 cells from CoCl2-induced cell death may have implications for future neuroprotective therapeutic approaches for the hypoxia-associated disorders.

Identification and functional analysis of the rat caspase-3 gene promoter

Caspase-3 is the major effector in apoptosis triggered by various stimuli. Previous studies demonstrated a significant increase in transcriptional activity of the caspase-3 gene during neuronal apoptosis. Recent findings suggest that differential expression of the caspase-3 gene may underlie the regulation of apoptotic susceptibility during brain development and after acute injury to the mature brain. We identified and cloned the rat caspase-3 gene promoter, determined its structure, and examined its regulation during a course of apoptosis in PC12 cells. Results demonstrate that this promoter lacks a TATA-box and contains a cluster of Sp1 elements and multiple transcription start sites. The first identified transcription start site is located 87-bp upstream from the first splicing site. A role of Sp1 elements in the regulation of caspase-3 promoter activity is demonstrated by the inhibition of Sp1 binding using mithramycin A. Results of deletion analysis show that an Ets-1-like element located between nucleotides -1646 and -1632 relative to the most extended transcription start site is necessary to achieve sustained transcriptional activity. Homology analysis revealed that the 5'-flanking region of the human caspase-3 gene exhibits significant similarity to a regulatory region of the rat gene.

Excitotoxicity in neonatal hypoxia

Hypoxic-ischemic encephalopathy (HIE) in neonates is a disorder of excessive neuronal excitation that includes seizures, abnormal EEG activity, and delayed failure of oxidative metabolism with elevated levels of lactic acid in the brain. Evidence from experimental models and clinical investigation indicates that HIE is triggered by a profound disruption in the function of glutamate synapses so that re-uptake of glutamate from the synapse is impaired and post-synaptic membranes containing glutamate receptors are depolarized. Severe hypoxemia preferentially depolarizes neuronal membranes, while ischemia probably has greater impact on the activity of glial glutamate re-uptake. Together, severe hypoxia and ischemia trigger a delayed cascade of events that may result in cell death by necrosis and/or apoptosis. Apoptosis is far more prominent in the neonate than in the adult and activation of cysteine proteases such as caspase-3 is a very important pathway in excitotoxic neonatal injury. Understanding the complex molecular networks triggered by an excitotoxic insult in the neonate provides insight into patterns of selective neuronal vulnerability and potential therapeutic strategies.

Traumatic brain injury: developmental differences in glutamate receptor response and the impact on treatment

Perinatal brain injury following trauma, hypoxia, and/or ischemia represents a substantial cause of pediatric disabilities including mental retardation. Such injuries lead to neuronal cell death through either necrosis or apoptosis. Numerous in vivo and in vitro studies implicate ionotropic (iGluRs) and metabotropic (mGluRs) glutamate receptors in the modulation of such cell death. Expression of glutamate receptors changes as a function of developmental age, with substantial implications for understanding mechanisms of post-injury cell death and its potential treatment. Recent findings suggest that the developing brain is more susceptible to apoptosis after injury and that such caspase mediated cell death may be exacerbated by treatment with N-methyl-D-aspartate receptor antagonists. Moreover, group I metabotropic glutamate receptors appear to have opposite effects on necrotic and apoptotic cell death. Understanding the relative roles of glutamate receptors in post-traumatic or post-ischemic cell death as a function of developmental age may lead to novel targeted approaches to the treatment of pediatric brain injury.

Perinatal asphyxia induced neuronal loss by apoptosis in the neonatal rat striatum: a combined TUNEL and stereological study

Perinatal asphyxia can lead to cell damage in various regions of the brain, such as the neostriatum. In this study, we investigated the mechanism of cell death that leads to neuron loss in the neostriatum of rat pups. Asphyxia was induced by immersing fetus-containing uterus horns in a water bath at 37 degrees C for 20 min. This led to an increase in mortality rate (+/- 40%) compared to control pups (0%). TUNEL-positive cell profiles were visible in all groups at postnatal day (P) 2, P8, and P15, peaking at P8. A significant increase of 40% at P8 and 45% at P15 in the number of TUNEL-positive cell profiles was observed in asphyctic rats compared to control rats. Nuclear condensation and fragmentation was visible with the DNA stain Hoechst 33342. Furthermore, laser-scanning confocal microscopy showed multiple DNA fragments in TUNEL-positive cell profiles. We found a decrease of 16% in the total number of striatal neurons in the asphyctic pups compared to the control pups at 21 days postasphyxia using stereology. These data show that asphyxia causes exaggerated apoptotic cell death during the first week of life and as a consequence a small amount of neuron loss in the neostriatum.

Effects of neonatal hypoxic-ischemic brain injury on skilled motor tasks and brainstem function in adult rats

In an attempt to establish more sensitive long-term neurofunctional measurements for neonatal hypoxic-ischemic brain injury, we examined skilled motor task and brainstem functions in adult rats after neonatal cerebral hypoxia-ischemia (H-I), using a staircase test and auditory brainstem response (ABR), respectively. Seven-day-old rats underwent a combination of left common carotid artery ligation and exposure to 8% O(2) for 1 h (n=16). The control animals only received sham operation (n=16). At 3 months of age, the staircase test and ABR were performed. In the staircase test, H-I animals showed marked impairment of skilled forelimb use in the side contralateral to the occluded artery, and the degree of brain damage correlated significantly to skilled forelimb use. In the ABR, H-I animals showed brainstem dysfunction assessed by measuring interpeak latencies for waves III-V and I-V. We also examined the brainstem with antibodies specific for activated caspase-3, a protein involved in initiation of apoptosis, and observed that caspase-3 was activated in the ipsilateral inferior colliculus at 24 h after H-I. The present study shows that both the staircase test and ABR are sensitive and objective long-term neurofunctional measurements that can be used in future studies to assess therapeutic intervention in this neonatal cerebral H-I model.

Effects of hypothermia on neonatal hypoxic-ischemic brain injury in the rat: phosphorylation of Akt, activation of caspase-3-like protease

Neuroprotective mechanisms of hypothermia have not been clearly established especially in the immature brain. To investigate the effect of hypothermia on cell death and cell survival signal pathways, we studied caspase-3-like activity and activation of Akt in a rat model of neonatal hypoxic-ischemic (H-I) brain injury. Seven-day-old rats underwent a combination of left common carotid artery ligation and exposure to 8% O(2) for 1-h (n=32). During recovery, the body temperature was reduced to 30 degrees C for 24 h in 16 animals, but was kept at 37 degrees C in 16 animals. Post-ischemic hypothermia was shown to diminish the caspase-3-like activity compared to normothermia at 6 and 24 h after H-I. Phospho-Akt was increased during the early reperfusion period after H-I in the normothermia group, but hypothermia rather decreased this enhanced phosphorylation of Akt following H-I. These results indicated that hypothermia may have some depressant effects on both cell death and cell survival signal pathways, and that Akt conceivably may not play a major role in the neuroprotective effect of hypothermia in the immature brain.

Differential expression of apoptotic protease-activating factor-1 and caspase-3 genes and susceptibility to apoptosis during brain development and after traumatic brain injury

Neuronal apoptosis plays an essential role in early brain development and contributes to secondary neuronal loss after acute brain injury. Recent studies have provided evidence that neuronal susceptibility to apoptosis induced by traumatic or ischemic injury decreases during brain development. However, the molecular mechanisms responsible for this age-dependent phenomenon remain unclear. Here we demonstrate that, during brain maturation, the potential of the intrinsic apoptotic pathway is progressively reduced and that such repression is associated with downregulation of apoptotic protease-activating factor-1 (Apaf-1) and caspase-3 gene expression. A similar decline in apoptotic susceptibility associated with downregulation of Apaf-1 expression as a function of developmental age was also found in cultured primary rat cortical neurons. Injury-induced cytochrome c-specific cleavage of caspase-9 followed by activation of caspase-3 in mature brain correlated with marked increases in Apaf-1 and caspase-3 mRNA and protein expression. These results suggest that differential expression of Apaf-1 and caspase-3 genes may underlie regulation of apoptotic susceptibility during brain development, as well as after acute injury to mature brain, through the intrinsic pathway of caspase activation.

Post-ischemic hypothermia blocks caspase-3 activation in the newborn rat brain after hypoxia-ischemia

The effects of hypothermia on caspase-3 activation were investigated in the newborn rat brain after hypoxia-ischemia (HI). Intense caspase-3 activation was observed in the control brains after HI, but this activation was significantly reduced by postischemic hypothermia. These findings suggest that the inhibition of caspase-3 activation may be an interventional point underlying the neuroprotective effect of hypothermia in neonates.

Caspase-dependent apoptotic pathways in CNS injury

Recent studies have suggested a role for neuronal apoptosis in cell loss following acute CNS injury as well as in chronic neurodegeneration. Caspases are a family of cysteine requiring aspartate proteases with sequence similarity to Ced-3 protein of Caenorhabditis elegans. These proteases have been found to contribute significantly to the morphological and biochemical manifestations of apoptotic cell death. Caspases are translated as inactive zymogens and become active after specific cleavage. Of the 14 identified caspases, caspase-3 appears to be the major effector of neuronal apoptosis induced by a variety of stimuli. A role for caspase-3 in injury-induced neuronal cell death has been established using semispecific peptide caspase inhibitors. This article reviews the current literature relating to pathways regulating caspase activation in apoptosis associated with acute and chronic neurodegeneration, and suggests that identification of critical upstream caspase regulatory mechanisms may permit more effective treatment of such disorders.

Caspase-3 activation after neonatal rat cerebral hypoxia-ischemia

Caspase-3 is a major effector protease in several apoptotic pathways, but its role in hypoxic-ischemic (HI) brain injury is incompletely understood. Cerebral HI was induced in 7-day-old rats by unilateral carotid artery ligation and exposure to 7.7% oxygen for 55 min. Caspase-3-like activity was significantly increased at 1 h (208%), peaked at 24 h (2,563%) and was still increased 6 days after HI (169%) in the ipsilateral cerebral cortex. Concomitantly, cleavage of the caspase-3 proform (31/33 kD) was detected on immunoblots, producing 29- and 17-kD fragments. Furthermore, significant degradation of the endogenous caspase-3 substrates inhibitor of caspase-activated DNase (DNA fragmentation factor 45), poly(ADP-ribose) polymerase and fodrin occurred. In conclusion, caspase-3 is activated extensively in the immature brain after HI. The subsequent cleavage of proteins involved in cellular homeostasis and repair may contribute to the process of brain injury.

Is caspase-3 inhibition a valid therapeutic strategy in cerebral ischemia?

Neurodegenerative diseases are characterized by progressive impairment of brain function as a consequence of ongoing neuronal cell death. Apoptotic mechanisms have been implicated in this process and a major involvement of caspase-3, a typical pro-apoptotic executioner protease, has been claimed. In this review, the role of caspase-3 in neuronal cell loss in animal models of stroke is discussed and critically evaluated. In summary, it is concluded that the biochemical evidence favoring caspase-3 as a therapeutic target in cerebral ischemia is not convincing, and the development of selective caspase-3 inhibitors for the treatment of human stroke must be viewed as high risk.

Status epilepticus causes necrotic damage in the mediodorsal nucleus of the thalamus in immature rats

Status epilepticus (StE) in immature rats causes long-term functional impairment. Whether this is associated with structural alterations remains controversial. The present study was designed to test the hypothesis that StE at an early age results in neuronal loss. StE was induced with lithium-pilocarpine in 12-d-old rats, and the presence of neuronal damage was investigated in the brain from 12 hr up to 1 week later using silver and Fluoro-Jade B staining techniques. Analysis of the sections indicated consistent neuronal damage in the central and lateral segments of the mediodorsal nucleus of the thalamus, which was confirmed using adjacent cresyl violet-stained preparations. The mechanism of thalamic damage (necrosis vs apoptosis) was investigated further using TUNEL, immunohistochemistry for caspase-3 and cytochrome c, and electron microscopy. Activated microglia were detected using OX-42 immunohistochemistry. The presence of silver and Fluoro-Jade B-positive degenerating neurons in the mediodorsal thalamic nucleus was associated with the appearance of OX-42-immunopositive activated microglia but not with the expression of markers of programmed cell death, caspase-3, or cytochrome c. Electron microscopy revealed necrosis of the ultrastructure of damaged neurons, providing further evidence that the mechanism of StE-induced damage in the mediodorsal thalamic nucleus at postnatal day 12 is necrosis rather than apoptosis. Finally, these data together with previously described functions of the medial and lateral segments of the mediodorsal thalamic nucleus suggest that some functions, such as adaptation to novelty, might become compromised after StE early in development.

Neurobiology of hypoxic-ischemic injury in the developing brain

Hypoxic ischemia is a common cause of damage to the fetal and neonatal brain. Although systemic and cerebrovascular physiologic factors play an important role in the initial phases of hypoxic-ischemic injuries, the intrinsic vulnerability of specific cell types and systems in the developing brain may be more important in determining the final pattern of damage and functional disability. Excitotoxicity, a term applied to the death of neurons and certain other cells caused by overstimulation of excitatory, mainly glutamate, neurotransmitter receptors, plays a critical role in these processes. Selected neuronal circuits as well as certain populations of glia such as immature periventricular oligodendroglia may die from excitotoxicity triggered by hypoxic ischemia. These patterns of neuropathologic vulnerability are associated with clinical syndromes of neurologic disability such as the extrapyramidal and spastic diplegia forms of cerebral palsy. The cascade of biochemical and histopathologic events triggered by hypoxic ischemia can extend for days to weeks after the insult is triggered, creating the potential for therapeutic interventions.

Apoptotic and anti-apoptotic mechanisms following spinal cord injury

A number of studies have provided evidence that cell death from moderate traumatic spinal cord injury (SCI) is regulated, in part, by apoptosis that involves the caspase family of cysteine proteases. However, little or no information is available about anti-apoptotic mechanisms mediated by the inhibitors of apoptosis (IAP) family of proteins that inhibit cell death pathways. In the present study, we examined caspase and IAP expression in spinal cords of rats subjected to moderate traumatic injury. Within 6 h after injury, caspase-8 and-9 (2 initiators of apoptosis) were predominantly present in gray matter neurons within the lesion epicenter. By 3 days following spinal cord injury (SCI), caspase-8 and-9 immunoreactivity was localized to gray and white matter cells, and by 7 days following SCI, both upstream caspases were expressed in cells within white matter or within foamy macrophages in gray matter. Caspase-3, an effector caspase, was evident in a few fragmented cells in gray matter at 24 h following injury and then localized to white matter in later stages. Thus, distinct patterns of caspase expression can be found in the spinal cord following injury. XIAP, cIAP-1, and cIAP-2, members of the IAP family, were constitutively expressed in the cord. Immunoblots of spinal cord extracts revealed that the processed forms of caspases-8 and-9 and cleavage of PARP are present as early as 6 h following trauma. The expression of caspases corresponded with the detection of cleavage of XIAP into 2 fragments following injury. cIAP-1 and cIAP-2 expression remained constant during early periods following SCI but demonstrated alterations by 7 days following SCI. Our data are consistent with the idea that XIAP may have a protective role within the spinal cord, and that alteration in cleavage of XIAP may regulate cell death following SCI.

Both caspase-dependent and caspase-independent pathways may be involved in hippocampal CA1 neuronal death because of loss of cytochrome c From mitochondria in a rat forebrain ischemia model

In a rat forebrain ischemia model, the authors examined whether loss of cytochrome c from mitochondria correlates with ischemic hippocampal CA1 neuronal death and how cytochrome c release may shape neuronal death. Forebrain ischemia was induced by bilateral common carotid artery occlusion with simultaneous hypotension for 10 minutes. After reperfusion, an early rapid depletion of mitochondrial cytochrome c and a late phase of diffuse redistribution of cytochrome c occurred in the hippocampal CA1 region, but not in the dentate gyrus and CA3 regions. Intracerebroventricular administration of Z-DEVD-FMK, a relatively selective caspase-3 inhibitor, provided limited but significant protection against ischemic neuronal damage on day 7 after reperfusion. Treatment with 3 minutes of ischemia (ischemic preconditioning) 48 hours before the 10-minute ischemia attenuated both the early and late phases of cytochrome c redistribution. In another subset of animals treated with cycloheximide, a general protein synthesis inhibitor, the late phase of cytochrome c redistribution was inhibited, whereas most hippocampal CA1 neurons never regained mitochondrial cytochrome c. Examination of neuronal survival revealed that ischemic preconditioning prevents, whereas cycloheximide only delays, ischemic hippocampal CA1 neuronal death. DNA fragmentation detected by terminal deoxytransferase-mediated dUTP-nick end labeling (TUNEL) in situ was largely attenuated by ischemic preconditioning and moderately reduced by cycloheximide. These results indicate that the loss of cytochrome c from mitochondria correlates with hippocampal CA1 neuronal death after transient cerebral ischemia in relation to both caspase-dependent and -independent pathways. The amount of mitochondrial cytochrome c regained may determine whether ischemic hippocampal CA1 neurons survive or succumb to late-phase death.

Molecular and biochemical mechanisms of perinatal brain injury

Hypoxic-ischemic injury to the prenatal and perinatal brain is a major contributor to morbidity and mortality to infants, often leading to mental retardation, seizures, and cerebral palsy. The susceptibility of the immature CNS to hypoxia-ischemia is largely dependent on the temporal and regional status of critical developmental processes, as well as on the regulation of cerebral blood flow and metabolism. The molecular and biochemical mechanisms of acute injury to the neonatal brain in experimental rodent and murine models of hypoxic-ischemic and ischemic injury, including disturbances of intracellular homeostasis, role of glutamate receptors, free radicals and transitional ions, as well as the modifying role of gene expression to cell death/survival will be reviewed in this chapter.

Synergistic activation of caspase-3 by m-calpain after neonatal hypoxia-ischemia: a mechanism of "pathological apoptosis"?

The relative contributions of apoptosis and necrosis in brain injury have been a matter of much debate. Caspase-3 has been identified as a key protease in the execution of apoptosis, whereas calpains have mainly been implicated in excitotoxic neuronal injury. In a model of unilateral hypoxia-ischemia in 7-day-old rats, caspase-3-like activity increased 16-fold 24 h postinsult, coinciding with cleavage of the caspase-3 proenzyme and endogenous caspase-3 substrates. This activation was significantly decreased by pharmacological calpain inhibition, using CX295, a calpain inhibitor that did not inhibit purified caspase-3 in vitro. Activation of caspase-3 by m-calpain, but not mu-calpain, was facilitated in a dose-dependent manner in vitro by incubating cytosolic fractions, containing caspase-3 proform, with calpains. This facilitation required the presence of some active caspase-3 and could be abolished by including the specific calpain inhibitor calpastatin. This indicates that initial cleavage of caspase-3 by m-calpain, producing a 29-kDa fragment, facilitates the subsequent cleavage into active forms. This is the first report to our knowledge suggesting a direct link between the early, excitotoxic, calcium-mediated activation of calpain after cerebral hypoxia-ischemia and the subsequent activation of caspase-3, thus representing a tentative pathway of "pathological apoptosis."

Impairment of mitochondrial respiration after cerebral hypoxia-ischemia in immature rats: relationship to activation of caspase-3 and neuronal injury

Mitochondrial damage may play a key role in the development of necrotic and apoptotic hypoxic-ischemic (HI) brain damage. It has previously been shown that mitochondrial respiration is depressed in the cerebral cortex after HI in neonatal animals. The aim of the present study was to further characterize the time course of the mitochondrial impairment during reperfusion and the correlation between the respiratory control ratio and brain injury and activation of caspase-3. Rat pups were subjected to unilateral carotid artery ligation and exposed to hypoxia (7.7% oxygen). Mitochondrial respiration was measured 0-72 h after HI in a mitochondrial fraction isolated from cerebral cortex. Microtubule associated protein-2 (MAP2) and caspase-3 were analyzed with immunoblotting in cerebral cortex homogenates. In addition, the time course of caspase-3 activation was measured as DEVD cleavage. The mitochondrial respiratory control ratio in cerebral cortex decreased immediately after HI followed by a partial recovery at 3-8 h. Thereafter, a secondary drop occurred with a minimum reached at 24 h of reperfusion. The secondary loss of respiratory function was accompanied by depletion of MAP2, cleavage of caspase-3 and an increased caspase-3 -like activity at 3-24 h after the insult. In conclusion, the primary phase of mitochondrial dysfunction was paralleled by a moderate decrease of MAP2 and a limited activation of caspase-3. The secondary mitochondrial impairment was associated with neuronal injury and pronounced activation of caspase-3.