Cerebral endothelial cell apoptosis after ischemia-reperfusion: role of PARP activation and AIF translocation

Plasticity and injury in the developing brain

The child's brain is more malleable or plastic than that of adults and this accounts for the ability of children to learn new skills quickly or recovery from brain injuries. Several mechanisms contribute to this ability including overproduction and deletion of neurons and synapses, and activity-dependent stabilization of synapses. The molecular mechanisms for activity-dependent synaptic plasticity are being discovered and this is leading to a better understanding of the pathogenesis of several disorders including neurofibromatosis, tuberous sclerosis, Fragile X syndrome and Rett syndrome. Many of the same pathways involved in synaptic plasticity, such as glutamate-mediated excitation, can also mediate brain injury when the brain is exposed to stress or energy failure such as hypoxia-ischemia. Recent evidence indicates that cell death pathways activated by injury differ between males and females. This new information about the molecular pathways involved in brain plasticity and injury are leading to insights that will provide better therapies for pediatric neurological disorders.

Cell death in the injured brain: roles of metallothioneins

In traumatic brain injury (TBI), the primary, irreversible damage associated with the moment of impact consists of cells dying from necrosis. This contributes to fuelling a chronic central nervous system (CNS) inflammation with increased formation of proinflammatory cytokines, enzymes and reactive oxygen species (ROS). ROS promote oxidative stress, which leads to neurodegeneration and ultimately results in programmed cell death (secondary injury). Since this delayed, secondary tissue loss occurs days to months following the primary injury it provides a therapeutic window where potential neuroprotective treatment could alleviate ongoing neurodegeneration, cell death and neurological impairment following TBI. Various neuroprotective drug candidates have been described, tested and proven effective in pre-clinical studies, including glutamate receptor antagonists, calcium-channel blockers, and caspase inhibitors. However, most of the scientific efforts have failed in translating the experimental results into clinical trials. Despite intensive research, effective neuroprotective therapies are lacking in the clinic, and TBI continues to be a major cause of morbidity and mortality. This paper provides an overview of the TBI pathophysiology leading to cell death and neurological impairment. We also discuss endogenously expressed neuroprotectants and drug candidates, which at this stage may still hold the potential for treating brain injured patients.

Neuroprotective effect of erythropoietin after experimental cold injury-induced vasogenic brain edema in rats

BACKGROUND

The aims of this study were to evaluate the efficiency of EPO in the treatment of cold injury-induced brain edema, apoptosis, and inflammation and to compare its effectiveness with DSP.

METHODS

One hundred fifteen adult male Sprague-Dawley rats weighing between 280 and 300 g were used for the study. Rats were divided into 5 groups. Controls received craniotomy only. The injury group underwent cold injury and had no medication. In the EPO group, a single dose of 1000 IU/kg body weight of EPO was administered. The DSP group received 0.2 mg/kg body weight of DSP. The vehicle group received a vehicle solution containing human serum albumin, which is the solvent for EPO. Brain edema was formed by cold injury using metal sterile rods with a diameter of 4 mm that were previously cooled at -80 degrees C. Twenty-four hours after the injury, animals were decapitated and brain tissues were investigated for brain edema, tissue MPO and caspase-3 levels, and ultrastructure.

RESULTS

A significant increase in brain water content was revealed in injury group of rats at 24 hours after cold injury. Injury significantly increased tissue MPO and caspase-3 levels and resulted in ultrastructural damage. Both EPO and DSP markedly decreased tissue MPO and caspase-3 levels and preserved ultrastructure of the injured brain cortex.

CONCLUSIONS

Erythropoietin and DSP were found to be neuroprotective in cold injury-induced brain edema model in rats via anti-apoptotic and anti-inflammatory actions.

Viral caspase inhibitor p35, but not crmA, is neuroprotective in the ischemic penumbra following experimental stroke

Apoptosis, a predominant cause of neuronal death after stroke, can be executed in a caspase-dependent or apoptosis inducing factor (AIF)-dependent manner. Herpes simplex virus (HSV) vectors expressing caspase inhibitors p35 and crmA have been shown to be neuroprotective against various excitotoxic insults. Here we further evaluated the possible neuroprotective role of p35 and crmA in a rat stroke model. Overexpression of p35, but not crmA, significantly increased neuronal survival. Results of double immunofluorescence staining indicate that compared with neurons infected with crmA or control vectors, p35-infected neurons had less active caspase-3 expression, cytosolic cytochrome c and nuclear AIF translocation.

Hypoxic preconditioning protects human brain endothelium from ischemic apoptosis by Akt-dependent survivin activation

Preconditioning-induced ischemic tolerance is well documented in the brain, but cell-specific responses and mechanisms require further elucidation. The aim of this study was to develop an in vitro model of ischemic tolerance in human brain microvascular endothelial cells (HBMECs) and to examine the roles of phosphatidylinositol 3-kinase (PI3-kinase)/Akt and the inhibitor-of- apoptosis protein, survivin, in the ability of hypoxic preconditioning (HP) to protect endothelium from apoptotic cell death. Cultured HBMECs were subjected to HP, followed 16 h later by complete oxygen and glucose deprivation (OGD) for 8 h; cell viability was quantified at 20 h of reoxygenation (RO) by the 3-(4,5-dimethylthiazol)-2,5-diphenyltetrazolium bromide assay. HBMECs were examined at various times after HP or OGD/RO using immunoblotting and confocal laser scanning immunofluorescence microscopy for appearance of apoptotic markers and expression of phosphorylated (p)-Akt and p-survivin. Causal evidence for the participation of the PI3-kinase/Akt pathway in HP-induced protection and p-survivin upregulation was assessed by the PI3-kinase inhibitor LY-294002. HP significantly reduced OGD/RO-induced injury by 50% and also significantly reduced the OGD-induced translocation of apoptosis-inducing factor (AIF) from mitochondria to nucleus and the concomitant cleavage of poly(ADP-ribose) polymerase-1 (PARP-1). PI3-kinase inhibition blocked HP-induced increases in Akt phosphorylation, reversed the effects of HP on OGD-induced AIF translocation and PARP-1 cleavage, blocked HP-induced survivin phosphorylation, and ultimately attenuated HP-induced protection of HBMECs from OGD. Thus HP promotes an antiapoptotic phenotype in HBMECs, in part by activating survivin via the PI3-kinase/Akt pathway. Survivin and other phosphorylation products of p-Akt may be therapeutic targets to protect cerebrovascular endothelium from apoptotic injury following cerebral ischemia.

Molecular targets in cerebral ischemia for developing novel therapeutics

Cerebral ischemia (stroke) triggers a complex series of biochemical and molecular mechanisms that impairs the neurologic functions through breakdown of cellular integrity mediated by excitotoxic glutamatergic signalling, ionic imbalance, free-radical reactions, etc. These intricate processes lead to activation of signalling mechanisms involving calcium/calmodulin-dependent kinases (CaMKs) and mitogen-activated protein kinases (MAPKs) such as extracellular signal-regulated kinase (ERK), p38, and c-Jun N-terminal kinase (JNK). The distribution of these transducers bring them in contact with appropriate molecular targets leading to altered gene expression, e.g. ERK and JNK mediated early gene induction, responsible for activation of cell survival/damaging mechanisms. Moreover, inflammatory reactions initiated at the neurovascular interface and alterations in the dynamic communication between the endothelial cells, astrocytes and neurons are thought to substantially contribute to the pathogenesis of the disease. The damaging mechanisms may proceed through rapid nonspecific cell lysis (necrosis) or by active form of cell demise (apoptosis or necroptosis), depending upon the severity and duration of the ischemic insult. A systematic understanding of these molecular mechanisms with prospect of modulating the chain of events leading to cellular survival/damage may help to generate the potential strategies for neuroprotection. This review briefly covers the current status on the molecular mechanisms of stroke pathophysiology with an endeavour to identify potential molecular targets such as targeting postsynaptic density-95 (PSD-95)/N-methyl-d-aspartate (NMDA) receptor interaction, certain key proteins involved in oxidative stress, CaMKs and MAPKs (ERK, p38 and JNK) signalling, inflammation (cytokines, adhesion molecules, etc.) and cell death pathways (caspases, Bcl-2 family proteins, poly (ADP-ribose) polymerase-1 (PARP-1), apoptosis-inducing factor (AIF), inhibitors of apoptosis proteins (IAPs), heat shock protein 70 (HSP70), receptor interacting protein (RIP), etc., besides targeting directly the genes itself. However, selecting promising targets from various signalling cascades, for drug discovery and development is very challenging, nevertheless such novel approaches may lead to the emergence of new avenues for therapeutic intervention in cerebral ischemia.

Mechanisms for the magnolol-induced cell death of CGTH W-2 thyroid carcinoma cells

Magnolol, a substance purified from the bark of Magnolia officialis, inhibits cell proliferation and induces apoptosis in a variety of cancer cells. The aim of this study was to study the effects of magnolol on CGTH W-2 thyroid carcinoma cells. After 24 h treatment with 80 microM magnolol in serum-containing medium, about 50% of the cells exhibited apoptotic features and 20% necrotic features. Cytochrome-c staining was diffused in the cytoplasm of the apoptotic cells, but restricted to the mitochondria in control cells. Western blot analyses showed an increase in levels of activated caspases (caspase-3 and -7) and of cleaved poly (ADP-ribose) polymerase (PARP) by magnolol. Concomitantly, immunostaining for apoptosis inducing factor (AIF) showed a time-dependent translocation from the mitochondria to the nucleus. Inhibition of either PARP or caspase activity blocked magnolol-induced apoptosis, supporting the involvement of the caspases and PARP. In addition, magnolol activated phosphatase and tensin homolog deleted on chromosome 10 (PTEN) and inactivated Akt by decreasing levels of phosphorylated PTEN and phosphorylated Akt. These data suggest that magnolol promoted apoptosis probably by alleviating the inhibitory effect of Akt on caspase 9. Furthermore, inhibition of PARP activity, but not of caspase activity, completely prevented magnolol-induced necrosis, suggesting the notion that it might be caused by depletion of intracellular ATP levels due to PARP activation. These results show that magnolol initiates apoptosis via the cytochrome-c/caspase 3/PARP/AIF and PTEN/Akt/caspase 9/PARP pathways and necrosis via PARP activation.

Prevention of apoptosis-inducing factor translocation is a possible mechanism for protective effects of hepatocyte growth factor against neuronal cell death in the hippocampus after transient forebrain ischemia

Hepatocyte growth factor (HGF) is one of the prospective agents for therapy against a variety of neurologic and neurodegenerative disorders, although the precise mechanisms for the effect of HGF remain to be elucidated. We showed that treatment with HGF protected hippocampal cornu ammonis (CA) subregion 1 neurons from apoptotic cell death after transient forebrain ischemia. Accumulating evidence indicates that ischemia-induced neuronal damage occurs via caspase-independent pathways. In the present study, we focused on the localization of apoptosis-inducing factor (AIF), which is an important protein in the signal-transduction system through caspase-independent pathways, to investigate the possible mechanism for the protective effect of HGF after transient forebrain ischemia. Hepatocyte growth factor attenuated the increase in the expression of AIF protein in the nucleus after transient forebrain ischemia. We further explored the upstream components of AIF translocation. Primary DNA damage induced by Ca(2+) influx and subsequent NO formation are thought to be the initial events for AIF translocation, which results in the subsequent DNA damage by AIF. Hepatocyte growth factor prevented the primary oxidative DNA damage, as was estimated by using anti-8-OHdG (8-hydroxy-2'-deoxyguanosine) antibody. Oxidative DNA damage after ischemia is known to lead to the activation of poly(ADP-ribose) polymerase (PARP) and p53, resulting in AIF translocation. Marked increases in the PAR polymer formation and the expression of p53 protein after ischemia were effectively prevented by HGF treatment. In the present study, we first showed that HGF was capable of preventing neuronal cell death by inhibiting the primary oxidative DNA damage and then preventing the activation of the PARP/p53/AIF pathway.

Neurovascular and neuronal protection by E64d after focal cerebral ischemia in rats

Calpains and cathepsins are two families of proteases that play an important role in ischemic cell death. In this study, we investigated the effect of E64d, a mu-calpain and cathepsin B inhibitor, in the prevention of neuronal and endothelial apoptotic cell death after focal cerebral ischemia in rats. Rats underwent 2 hr of transient focal ischemia from middle cerebral artery occlusion (MCAO) and were sacrificed 24 hr later. E64d (5 mg/ kg intraperitoneally) was administered 30 min before MCAO. Assessment included neurological function, infarction volume, brain water content, blood-brain barrier permeability, histology, and immunohistochemistry. The E64d-treated rats had significant brain protection against ischemic damage. We observed a reduction of infarction volume, brain edema, and improved neurological scores in E64d-treated rats compared with the nontreated control. Furthermore, there was a remarkable reduction in both proteases and caspase-3 activation and apoptotic changes in both neurons and endothelial cells in E64d-treated rats. These results suggest that E64d protects the brain against ischemic/reperfusion injury by attenuating neuronal and endothelial apoptosis.

Anti-inflammatory effects of PJ34, a poly(ADP-ribose) polymerase inhibitor, in transient focal cerebral ischemia in mice

BACKGROUND AND PURPOSE

Activation of poly(ADP-ribose) polymerase (PARP) is deleterious during cerebral ischemia. We assessed the influence of PARP activation induced by cerebral ischemia on the synthesis of proinflammatory mediators including the cytokines, tumour necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) and the adhesion molecules, E-selectin and intercellular adhesion molecule-1 (ICAM-1).

EXPERIMENTAL APPROACH

Ischemia was induced by intravascular occlusion of the left middle cerebral artery for 1 h in male Swiss mice anaesthetized with ketamine and xylazine. The PARP inhibitor PJ34 (1.25-25 mg kg(-1)) was administered intraperitoneally 15 min before and 4 hours after, the onset of ischemia. Animals were killed 6 h or 24 h after ischemia and cerebral tissue removed for analysis.

KEY RESULTS

Ischemia increased TNF-alpha protein in cerebral tissue at 6 and 24 h after ischemia. All doses of PJ34 blocked the increase in TNF-alpha at 6 h and 25 mg kg(-1) PJ34 had a sustained effect for up to 24 h. Quantitative real time polymerase chain reaction showed that PJ34 (25 mg kg(-1)) reduced the increase in TNF-alpha mRNA by 70% at 6 h. PJ34 also prevented the increase in mRNAs encoding IL-6 (-41%), E-selectin (-81%) and ICAM-1 (-54%). PJ34 (25 mg kg(-1)) reduced the infarct volume (-26%) and improved neurological deficit, 24 h after ischemia.

CONCLUSIONS AND IMPLICATIONS

PJ34 inhibited the increase in the mRNAs of four inflammatory mediators, caused by cerebral ischemia. The contribution of this effect of PJ34 to neuroprotection remains to be clarified.