The Akt signaling pathway contributes to postconditioning's protection against stroke; the protection is associated with the MAPK and PKC pathways

NADPH oxidases as therapeutic targets in ischemic stroke

Reactive oxygen species (ROS) act physiologically as signaling molecules. In pathological conditions, such as ischemic stroke, ROS are released in excessive amounts and upon reperfusion exceed the body's antioxidant detoxifying capacity. This process leads to brain tissue damage during reoxygenation. Consequently, antioxidant strategies have long been suggested as a therapy for experimental stroke, but clinical trials have not yet been able to promote the translation of this concept into patient treatment regimens. As an evolution of this concept, recent studies have targeted the sources of ROS generation-rather than ROS themselves. In this context, NADPH oxidases have been identified as important generators of ROS in the cerebral vasculature under both physiological conditions in general and during ischemia/reoxygenation in particular. Inhibition of NADPH oxidases or genetic deletion of certain NADPH oxidase isoforms has been found to considerably reduce ischemic injury in experimental stroke. This review focuses on recent advances in the understanding of NADPH oxidase-mediated tissue injury in the cerebral vasculature, particularly at the level of the blood-brain barrier, and highlights promising inhibitory strategies that target the NADPH oxidases.

Activation of Akt/FoxO and inactivation of MEK/ERK pathways contribute to induction of neuroprotection against transient global cerebral ischemia by delayed hypoxic postconditioning in adult rats

Ischemic postconditioning, a series of mechanical interruptions of blood flow immediately after reperfusion, has been described in brain studies. However, hypoxic postconditioning (HPC) has never been reported in transient global cerebral ischemia (tGCI) adult rat model. The purpose of this study is to explore the effects of neuroprotection by delayed HPC against tGCI in adult rats and investigate underlying mechanisms involving the Akt/Forkhead transcription factor, class O (FoxO) and mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) pathways. Postconditioning with 60-120 min hypoxia significantly reduced cell death in hippocampal CA1 subregion after 10 min of tGCI. Postconditioning was effective only when applied 1-2 days after tGCI. Nevertheless, the combination of hypoxic preconditioning and postconditioning provided no additive protection. Additionally, postconditioning increased phosphorylation of Akt and FoxOs after tGCI. Inhibiting phosphorylation of Akt and FoxOs with LY294002 suppressed the postconditioning-induced neuroprotection. In addition, postconditioning blocked the increase of MEK and ERK phosphorylation after tGCI. Inhibiting phosphorylation of MEK and ERK with U0126 attenuated neuronal damage after tGCI. These results suggest that delayed HPC exerts neuroprotection against tGCI-induced injury in adult rats. The activation of Akt/FoxO and inactivation of MEK/ERK pathways by postconditioning contributed to the induction of neuroprotection against tGCI.

Lithium treatment reduces brain injury induced by focal ischemia with partial reperfusion and the protective mechanisms dispute the importance of akt activity

Lithium is a mood stabilizer shown to have neuroprotective effects against several chronic and acute neuronal injuries, including stroke. However, it is unknown whether lithium treatment protects against brain injury post-stroke in a rat model of permanent distal middle cerebral artery occlusion (MCAo) combined with transient bilateral common carotid artery occlusion (CCAo), a model that mimics human stroke with partial reperfusion. In addition, whether lithium treatment alters Akt activity as measured by the kinase activity assay has not been reported, although it is known to inhibit GSK3β activity. After stroke, Akt activity contributes to neuronal survival while GSK3β activity causes neuronal death. We report that a bolus of lithium injection at stroke onset robustly reduced infarct size measured by 2,3,5-triphenyltetrazolium chloride (TTC) staining at 48 h post-stroke and inhibited cell death in the ischemic penumbra, but not in the ischemic core, as shown by TUNEL staining performed 24 h post-stroke. However, lithium treatment did not alter the reduction in Akt activity as measured by Akt kinase assay. We further showed that lithium did not alter phosphorylated GSK3β protein levels, or the degradation of β-catenin, a substrate of GSK3β, which is consistent with previous findings that long-term treatment is required for lithium to alter GSK3β phosphorylation. In summary, we show innovative data that lithium protects against stroke in a focal ischemia model with partial reperfusion, however, our results dispute the importance of Akt activity in the protective effects of lithium.

Protective effect of delayed remote limb ischemic postconditioning: role of mitochondrial K(ATP) channels in a rat model of focal cerebral ischemic reperfusion injury

Delayed remote ischemic postconditioning (DRIPost) has been shown to protect the rat brain from ischemic injury. However, extremely short therapeutic time windows hinder its translational use and the mechanism of action remains elusive. Because opening of the mitochondria K(ATP) channel is crucial for cell apoptosis, we hypothesized that the neuroprotective effect of DRIPost may be associated with K(ATP) channels. In the present study, the neuroprotective effects of DRIPost were investigated using adult male Sprague-Dawley rats. Rats were exposed to 90 minutes of middle cerebral artery occlusion followed by 72 hours of reperfusion. Delayed remote ischemic postconditioning was performed with three cycles of bilateral femoral artery occlusion/reperfusion for 5 minutes at 3 or 6 hours after reperfusion. Neurologic deficit scores and infarct volumes were assessed, and cellular apoptosis was monitored by terminal deoxynucleotidyl transferase nick-end labeling. Our results showed that DRIPost applied at 6 hours after reperfusion exerted neuroprotective effects. The K(ATP) opener, diazoxide, protected rat brains from ischemic injury, while the K(ATP) blocker, 5-hydroxydecanote, reversed the neuroprotective effects of DRIPost. These findings indicate that DRIPost reduces focal cerebral ischemic injury and that the neuroprotective effects of DRIPost may be achieved through opening of K(ATP) channels.

Progesterone is neuroprotective against ischemic brain injury through its effects on the phosphoinositide 3-kinase/protein kinase B signaling pathway

We tested the hypothesis that the phosphoinositide 3-kinase/protein kinase B (PI3K/Akt) pathway mediates some of the neuroprotective effects of progesterone (PROG) after ischemic stroke. We examined whether PROG acting through the PI3K/Akt pathway could affect the expression of vascular endothelial growth factor (VEGF) and brain-derived neurotrophic factor (BDNF). Rats underwent permanent focal cerebral ischemia by electrocoagulation and received intraperitoneal injections of PROG (8 mg/kg) or vehicle at 1 h post-occlusion and subcutaneous injections at 6, 24, and 48 h. PAkt/Akt levels, apoptosis and apoptosis-related proteins (phosphorylated Bcl-2-associated death promoter (pBAD), BAD, caspase-3, and cleaved caspase-3) were analyzed by TUNEL assays, Western blotting and immunohistochemistry at 24 h post-pMCAO. VEGF and BDNF were analyzed at 24, 72 h and 14 days post-pMCAO with Western blots. Following pMCAO, PROG treatment significantly (P<0.05) reduced ischemic lesion size and edema. Treatment with PROG significantly (P<0.05) decreased VEGF at 24 and 72 h but increased VEGF expression 14 days after injury. The treatment also increased BDNF, and attenuated apoptosis by increasing Akt phosphorylation compared with vehicle alone. The selective PI3K inhibitor wortmannin compromised PROG-induced neuroprotective effects and reduced the elevation of pAkt levels in the ischemic penumbra. Our findings lead us to suggest that the PI3K/Akt pathway can play a role in mediating the neuroprotective effects of PROG after stroke by altering the expression of trophic factors in the brain.

The chronic protective effects of limb remote preconditioning and the underlying mechanisms involved in inflammatory factors in rat stroke

We recently demonstrated that limb remote preconditioning (LRP) protects against focal ischemia measured 2 days post-stroke. Here, we studied whether LRP provides long-term protection and improves neurological function. We also investigated whether LRP transmits its protective signaling via the afferent nerve pathways from the preconditioned limb to the ischemic brain and whether inflammatory factors are involved in LRP, including the novel galectin-9/Tim-3 inflammatory cell signaling pathway, which induces cell death in lymphocytes. LRP in the left hind femoral artery was performed immediately before stroke. LRP reduced brain injury size both at 2 days and 60 days post-stroke and improved behavioral outcomes for up to 2 months. The sensory nerve inhibitors capsaicin and hexamethonium, a ganglion blocker, abolished the protective effects of LRP. In addition, LRP inhibited edema formation and blood-brain barrier (BBB) permeability measured 2 days post-stroke. Western blot and immunostaining analysis showed that LRP inhibited protein expression of both galectin-9 and T-cell immunoglobulin domain and mucin domain 3 (Tim-3), which were increased after stroke. In addition, LRP decreased iNOS and nitrotyrosine protein expression after stroke. In conclusion, LRP executes long-term protective effects against stroke and may block brain injury by inhibiting activities of the galectin-9/Tim-3 pathway, iNOS, and nitrotyrosine.

Ischemic postconditioning attenuates liver warm ischemia-reperfusion injury through Akt-eNOS-NO-HIF pathway

BACKGROUND

Ischemic postconditioning (IPO) has been demonstrated to attenuate ischemia/reperfusion (I/R) injury in the heart and brain, its roles to liver remain to be defined. The study was undertaken to determine if IPO would attenuate liver warm I/R injury and its protective mechanism.

METHODS

Mice were divided into sham, I/R, IPO+I/R (occlusing the porta hepatis for 60 min, then treated for three cycles of 10 sec brief reperfusion consecutively, followed by a persistent reperfusion); L-NAME+ sham (L-NAME, 16 mg/kg, i.v., 5 min before repefusion); L-NAME+I/R; and L-NAME+ IPO. Blood flow of caudate and left lobe of the liver was blocked. Functional and morphologic changes of livers were evaluated. Contents of nitric oxide, eNOS and iNOS in serum were assayed. Concentration of eNOS, iNOS, malondialdehyde (MDA) and activity of superoxide dismutase (SOD) in hepatic tissue were also measured. Expressions of Akt, p-Akt and HIF-1α protein were determined by western blot. Expressions of TNF-α and ICAM-1 were measured by immunohistochemistry and RT-PCR.

RESULTS

IPO attenuated the dramatically functional and morphological injuries. The levels of ALT was significantly reduced in IPO+I/R group (p < 0.05). Contents of nitric oxide and eNOS in serum were increased in the IPO+I/R group (p < 0.05). IPO also up-regulated the concentration of eNOS, activity of SOD in hepatic tissue (p < 0.05), while reduced the concentration of MDA (p < 0.05). Moreover, protein expressions of HIF-1α and p-Akt were markedly enhanced in IPO+I/R group. Protein and mRNA expression of TNF-α and ICAM-1 were markedly suppressed by IPO (p < 0.05). These protective effects of IPO could be abolished by L-NAME.

CONCLUSIONS

We found that IPO increased the content of NO and attenuated the overproduction of ROS and I/R-induced inflammation. Increased NO contents may contribute to increasing HIF-1α level, and HIF-1α and NO would simultaneously protect liver from I/R injury. These findings suggested IPO may have the therapeutic potential through Akt-eNOS-NO-HIF pathway for the better management of liver I/R injury.

Rapid conditioning with oxygen oscillation: neuroprotection by intermittent normobaric hyperoxia after transient focal cerebral ischemia in rats

BACKGROUND AND PURPOSE

Normobaric hyperoxia (NBO) has been shown to exert neuroprotective effects against cerebral ischemia and to restore penumbral oxygenation. Inspired by recent reports on postconditioning with intermittent occlusions of cerebral artery, we tested the hypothesis that intermittent NBO (iNBO) may cause oscillation of cerebral oxygenation and thereby elicit repetitive interruptions to reperfusion, leading to attenuated ischemia/reperfusion damage after transient focal cerebral ischemia in rats.

METHODS

Rats were subjected to 90 minutes of middle cerebral artery occlusion. During ischemia, animals received air, iNBO (4 cycles of 3 minutes of NBO and 2 minutes of air), continuous NBO (cNBO; 75 minutes), short NBO (18 minutes), or a combination of iNBO and cNBO. Infarct volume and neurological score were evaluated at 24 and 72 hours after ischemia. Production of superoxide was assessed by the hydroethidine method, and the expression of Akt and phosphorylated Akt was examined by Western blot.

RESULTS

iNBO and cNBO had similar effects in reducing infarct volume and neurological deficit at 24 hours after ischemia, whereas at 72 hours the neuroprotection exerted by iNBO was greater than cNBO. Combining iNBO and cNBO produced no greater protection, and short NBO failed to provide neuroprotection. Both iNBO and cNBO attenuated superoxide production. Importantly, prolonged activation of Akt was observed in the iNBO group, and neuroprotection by iNBO was partly eliminated by inhibition of Akt activation.

CONCLUSIONS

iNBO may represent a novel form of postconditioning, and this neuroprotection is likely mediated by attenuating superoxide generation and activation of the Akt pathway.

miR-181 regulates GRP78 and influences outcome from cerebral ischemia in vitro and in vivo

MicroRNAs (miRNA) are short (~22nt) single stranded RNAs that downregulate gene expression. Although recent studies indicate extensive miRNA changes in response to ischemic brain injury, there is currently little information on the roles of specific miRNAs in this setting. Heat shock proteins (HSP) of the HSP70 family have been extensively studied for their multiple roles in cellular protection, but there is little information on their regulation by miRNAs. We used bioinformatics to identify miR-181 as a possible regulator of several HSP70 family members. We validated GRP78/BIP as a target by dual luciferase assay. In response to stroke in the mouse we find that miR-181 increases in the core, where cells die, but decreases in the penumbra, where cells survive. Increased levels of miR-181a are associated with decreased GRP78 protein levels, but increased levels of mRNA, implicating translational arrest. We manipulated levels of miR-181a using plasmid overexpression of pri-miR-181ab or mimic to increase, and antagomir or inhibitor to reduce levels. Increased miR-181a exacerbated injury both in vitro and in the mouse stroke model. Conversely, reduced levels were associated with reduced injury and increased GRP78 protein levels. Studies in C6 cells show that if GRP78 levels are maintained miR-181a no longer exerts a toxic effect. These data demonstrate that miR-181 levels change in response to stroke and inversely correlate with levels of GRP78. Importantly, reducing or blocking miR-181a protects the brain from stroke.

Limb remote postconditioning alleviates cerebral reperfusion injury through reactive oxygen species-mediated inhibition of delta protein kinase C in rats

BACKGROUND

Remote ischemic postconditioning (RPostC) is an emerging concept for cerebral infarction protection, and its potential protective mechanisms have not been well established. We attempted to investigate the implications of reactive oxygen species (ROS) and δ protein kinase C (δPKC) in neuroprotection induced by RPostC in a rat model of focal cerebral ischemia, and also to explore a possible relationship between ROS and εPKC.

METHODS

Focal cerebral ischemia was induced by middle cerebral artery occlusion using the intraluminal filament technique in male rats. RPostC was generated by 3 10-minute cycles of femoral artery occlusion/reperfusion on the right limb at the onset of middle cerebral artery reperfusion. RPostC was performed alone or with pretreatment of N-acetylcysteine, a ROS scavenger. In separate group, TAT-δV1-1, a δPKC-selective peptide inhibitor, was administered at the onset of reperfusion. Brain ischemic injury was evaluated by neurologic scores, infarction volumes, and TUNEL staining. Moreover, the activation of δPKC in the ischemic penumbra was investigated by Western blot after reperfusion.

RESULTS

RPostC improved neurologic outcome, reduced infarct size, and inhibited neuronal apoptosis as well as suppressed the activation of δPKC after reperfusion. Moreover, systemic delivery of TAT-δV1-1 conferred neuroprotection against cerebral reperfusion injury at the onset of reperfusion. Pretreatment with N-acetylcysteine not only completely prevented all aspects of RPostC-induced neuroprotection, but also reversed RPostC-induced inhibition of δPKC activation after reperfusion.

CONCLUSION

These findings suggested that RPostC performed in one limb alleviated reperfusion injury after focal cerebral ischemia through ROS-mediated inhibition of endogenous δPKC activation signaling cascade in an in vivo rat model of focal cerebral ischemia.

Delayed ischemic postconditioning protects hippocampal CA1 neurons by preserving mitochondrial integrity via Akt/GSK3β signaling

Delayed ischemic postconditioning (Post C), which involves a brief ischemia followed by reperfusion 2 days after 8-10min global cerebral ischemia (GCI), has been shown to exert a remarkable protection of the vulnerable hippocampal CA1 region of the brain and attenuation of behavioral deficits, although the mechanisms remain poorly understood. The purpose of the current study was to explore the effect of Post C upon mitochondrial integrity, cytochrome c release and Bax translocation as a potential key mechanism for Post C protection of the critical hippocampal CA1 region neurons. The results of the study revealed that ischemic Post C (3min) administered 2 days after 8-min GCI exerted a robust preservation from GCI injury, as evidenced by the increase of NeuN-positive and the decrease of TUNEL-positive cells, as well as morphological features of mitochondrial integrity in the hippocampal CA1 region. We also found that Post C significantly blocked inner mitochondrial membrane potential depolarization, as shown by JC-1 staining, and attenuates cytochrome c release and Bax translocation induced by GCI. Pre-treatment of the PI3K inhibitor LY294002, 20min prior to Post C, significantly attenuated Post C-induced elevation of p-Akt and p-GSK3β, as well as prevented Post C enhancement of mitochondrial integrity and Post C neuroprotection. The results suggest that phosphorylation of Akt and subsequent inactivation of GSK3β signaling is critical in mediating Post C beneficial effects upon mitochondrial integrity, function and neuroprotection following GCI injury.

GSK-3 as a Target for Lithium-Induced Neuroprotection Against Excitotoxicity in Neuronal Cultures and Animal Models of Ischemic Stroke

The mood stabilizer lithium inhibits glycogen synthase kinase-3 (GSK-3) directly or indirectly by enhancing serine phosphorylation of both α and β isoforms. Lithium robustly protected primary brain neurons from glutamate-induced excitotoxicity; these actions were mimicked by other GSK-3 inhibitors or silencing/inhibiting GSK-3α and/or β isoforms. Lithium rapidly activated Akt to enhance GSK-3 serine phosphorylation and to block glutamate-induced Akt inactivation. Lithium also up-regulated Bcl-2 and suppressed glutamate-induced p53 and Bax. Induction of brain-derived neurotrophic factor (BDNF) was required for lithium’s neuroprotection to occur. BDNF promoter IV was activated by GSK-3 inhibition using lithium or other drugs, or through gene silencing/inactivation of either isoform. Further, lithium’s neuroprotective effects were associated with inhibition of NMDA receptor-mediated calcium influx and down-stream signaling. In rodent ischemic models, post-insult treatment with lithium decreased infarct volume, ameliorated neurological deficits, and improved functional recovery. Up-regulation of heat-shock protein 70 and Bcl-2 as well as down-regulation of p53 likely contributed to lithium’s protective effects. Delayed treatment with lithium improved functional MRI responses, which was accompanied by enhanced angiogenesis. Two GSK-3-regulated pro-angiogenic factors, matrix metalloproteinase-9 (MMP-9) and vascular endothelial growth factor were induced by lithium. Finally, lithium promoted migration of mesenchymal stem cells (MSCs) by up-regulation of MMP-9 through GSK-3β inhibition. Notably, transplantation of lithium-primed MSCs into ischemic rats enhanced MSC migration to the injured brain regions and improved the neurological performance. Several other GSK-3 inhibitors have also been reported to be beneficial in rodent ischemic models. Together, GSK-3 inhibition is a rational strategy to combat ischemic stroke and other excitotoxicity-related brain disorders.

Kainate postconditioning restores LTP in ischemic hippocampal CA1: onset-dependent second pathophysiological stress

Postconditioning can be induced by a broad range of stimuli within minutes to days after an ischemic cerebral insult. A special form is elicited by pharmacological intervention called second pathophysiological stress. The present study aimed to evaluate the effects of low-dose (5 mg/kg) kainate postconditioning with onsets 0, 24 and 48 h after the ischemic insult on the hippocampal synaptic plasticity in a 2-vessel occlusion model in rat. The hippocampal function was tested by LTP measurements of Schaffer collateral-CA1 pyramidal cell synapses in acute slices and the changes in density of Golgi-Cox-stained apical dendritic spines. Postconditioning 0 and 24 h after ischemia was not protective, whereas 48-h-onset postconditioning resulted in the reappearance of a normal spine density (>100,000 spines) 3 days after ischemia, in parallel with the long-term restoration of the damaged LTP function. Similar, but somewhat less effects were observed after 10 days. Our data clearly demonstrate the onset dependence of postconditioning elicited by a subconvulsant dose of kainate treatment in global ischemia, with restoration of the structural plasticity and hippocampal function.

Protective conditioning of the brain: expressway or roadblock?

The brain responds to noxious stimulation with protective signalling. Over the last decades, a number of experimental strategies have been established to study endogenous brain protection. Pre-, per-, post- and remote 'conditioning' are now widely used to unravel the underlying mechanisms of endogenous neuroprotection. Some of these strategies are currently being tested in clinical trials to protect the human brain against anticipated damage or to boost protective responses during or after injury. Here we summarize the principles of 'conditioning' research and current efforts to translate this knowledge into effective treatment of patients. Conditioning to induce protected brain states provides an experimental window into endogenous brain protection and can lead to the discovery of drugs mimicking the effects of conditioning. Mechanisms of endogenous brain tolerance can be activated through a wide variety of stimuli that signal 'danger' to the brain. These danger signals lead to the induction of regulator and effector mechanisms, which suppress death and induce survival pathways, decrease metabolism, as well as increase substrate delivery. We conclude that preclinical research on endogenous brain protection has greatly benefited from conditioning strategies, but that clinical applications are challenging, and that we should not prematurely rush into ill-designed and underpowered clinical trials.

Protein kinase B (Akt) and mitogen-activated protein kinase p38α in retinal ischemic post-conditioning

In previous studies, it was shown that post-conditioning, a transient period of brief ischemia following prolonged severe ischemia in the retina, could provide significant improvement in post-ischemic recovery, attenuation of cell loss, and decreased apoptosis. However, the mechanisms of post-conditioning in the retina have not been elucidated. We hypothesized that two kinases, mitogen-activated protein kinase p38α and protein kinase B (Akt), were involved in the mechanism of post-conditioning. Ischemia was induced in rat retina in vivo. Recovery after ischemia followed by 8 min of post-conditioning early in the reperfusion period after prolonged ischemia was assessed functionally (electroretinography) and histologically at 7 days after ischemia. We examined the role of p38α and Akt subtypes 1-3 in post-conditioning by intravitreal injection of interfering RNA 6 h prior to ischemia and post-conditioning and compared the results to injection of non-silencing interfering RNA sequence. The blockade of p38α significantly decreased the recovery after ischemia and post-conditioning, and enhanced cell loss and disorganization of the retina. Blockade of Akt1, and to a lesser degree, Akt2, significantly decreased the recovery after ischemia and enhanced cell loss and disorganization. These differences in the effects of blockade of Akt subtypes were not explainable by distribution of Akt subtypes in the retina, which were similar. In conclusion, both p38 and Akt are essential components of the neuroprotection induced by post-ischemic conditioning in the retina.

Hepatocyte growth factor-induced c-Src-phosphatidylinositol 3-kinase-AKT-mammalian target of rapamycin pathway inhibits dendritic cell activation by blocking IκB kinase activity

Hepatocyte growth factor modulates activation and antigen-presenting cell function of dendritic cells. However, the molecular basis for immunoregulation of dendritic cells by hepatocyte growth factor is undefined. In the current study, we demonstrate that hepatocyte growth factor exhibits inhibitory effect on dendritic cell activation by blocking IκB kinase activity and subsequent nuclear factor-κB activation. Inhibition of IκB kinase is mediated by hepatocyte growth factor-induced activation of c-Src. Proximal signaling events induced in dendritic cells by hepatocyte growth factor include a physical association of c-Src with the hepatocyte growth factor receptor c-MET and concomitant activation of c-Src. Activation of c-Src in turn establishes a complex consisting of phosphatidylinositol 3-kinase and c-MET, and promotes downstream activation of the phosphatidylinositol 3-kinase/AKT pathway and mammalian target of rapamycin. Blocking activation of c-Src, phosphatidylinositol 3-kinase and mammalian target of rapamycin prevents hepatocyte growth factor-induced inhibition of IκB kinase, nuclear factor-κB and dendritic cell activation. Notably, hepatocyte growth factor-stimulated c-Src activation results in induction of phosphatidylinositol 3-kinase complexes p85α/p110α and p85α/p110δ, which is required for activation of mammalian target of rapamycin, and consequent inhibition of IκB kinase and nuclear factor-κB activation. Our findings, for the first time, have identified the c-Src-phosphatidylinositol 3-kinase-AKT-mammalian target of rapamycin pathway that plays a pivotal role in mediating the inhibitory effects of hepatocyte growth factor on dendritic cell activation by blocking nuclear factor-κB signaling.

Delayed post-ischemic conditioning significantly improves the outcome after retinal ischemia

In previous studies, it was shown that post-conditioning, a transient period of brief ischemia following prolonged severe ischemia in the retina, could provide significant improvement in post-ischemic recovery, attenuation of cell loss, and decreased apoptosis. These studies showed that post-conditioning effectively prevented damage after retinal ischemia when it was instituted early (within 1 h) in the post-ischemic period. While post-ischemic conditioning holds high promise of clinical translation, patients often present late after the onset of retinal ischemia and therefore immediate application of this anti-ischemic maneuver is generally not feasible. In this study, we examined the hypothesis that application of a post-conditioning stimulus at 24 h or greater following the end of prolonged ischemia would decrease the extent of ischemic injury. Ischemia was induced in rat retina in vivo. Recovery after ischemia followed by 5 min of post-conditioning brief ischemia 24 or 48 h after prolonged ischemia was assessed functionally (electroretinography) and histologically at 7 days after ischemia and post-conditioning or sham post-conditioning. We found that the brief ischemic stimulus applied 24, but not 48 h after prolonged ischemia significantly improved functional recovery and decreased histological damage induced by prolonged ischemia. We conclude that within a defined time window, delayed post-ischemic conditioning ameliorated post-ischemic injury in rats. Compared to earlier studies, the present work demonstrates for the first time the novel ability of a significantly delayed ischemic stimulus to provide robust neuroprotection in the retina following ischemia.

Neuroprotective effects of ischemic postconditioning on global brain ischemia in rats through upregulation of hippocampal glutamine synthetase

Brain ischemic postconditioning is the induction of brief periods of ischemia-reperfusion during the early stages following ischemia, and it has been shown to produce neuroprotective effects. The mechanisms underlying these neuroprotective effects are poorly understood. Glutamate excitotoxicity is one cause of postischemic neuronal death. Glutamine synthetase (GS) is an enzyme that is expressed in glial cells and may affect glutamate excitotoxicity. We induced global ischemia in rats and performed postconditioning with 6 cycles of 10 seconds reperfusion and 10 seconds reocclusion before final reperfusion. Hematoxylin and eosin staining revealed extensive neuronal loss (44.0 ± 2.8% cell survival) in the hippocampal CA1 region. Ischemic postconditioning decreased neuronal death (82.0 ± 5.6% cell survival; p<0.05). Western blotting revealed significantly increased GS expression in the hippocampus for the ischemia-reperfusion group over time compared with the sham group (p<0.05). Ischemic postconditioning resulted in significantly increased (p<0.05) GS expression compared with both the sham and ischemia-reperfusion groups, suggesting that upregulation of GS expression after ischemia constitutes a neuroprotective mechanism.

Volatile anesthetic post-treatment induces protection via inhibition of glycogen synthase kinase 3β in human neuron-like cells

Application of the volatile anesthetic isoflurane during the early phase of reperfusion reduces ischemic heart and brain injury (anesthetic post-conditioning). We hypothesize that inhibition of glycogen synthase kinase 3β (GSK3β), a protein whose activation can lead to cell death, participates in anesthetic post-conditioning-induced neuroprotection. SH-SY5Y cells, a human neuroblastoma cell line, were induced by retinoic acid to differentiate into terminal neuron-like cells. The cells were then subjected to a 1-h oxygen-glucose deprivation (OGD), a condition to simulate ischemia in vitro, and a 20-h simulated reperfusion. Isoflurane, sevoflurane or desflurane, three commonly used volatile anesthetics, were applied for 1 h during the early phase of simulated reperfusion. Cell injury was quantified by lactate dehydrogenase (LDH) release. Phospho-GSK3β at Ser9 and total GSK3β were quantified at 1 or 3 h after the OGD. OGD increased LDH release, suggesting that OGD induced cell injury. Post-treatment with isoflurane, sevoflurane or desflurane reduced this cell injury. This protection was apparent when 2% isoflurane was applied within 1 h after the onset of reperfusion. Isoflurane post-treatment also significantly increased the phosphorylation of GSK3β at Ser9 at 1 h after the OGD. GSK3β inhibitors reduced OGD and simulated reperfusion-induced LDH release. The combination of GSK3β inhibitors and isoflurane post-conditioning did not cause a greater protection than isoflurane post-conditioning alone. These results suggest that volatile anesthetic post-conditioning reduces OGD and simulated reperfusion-induced cell injury. Since phospho-GSK3β at Ser9 decreases GSK3β activity, our results suggest that volatile anesthetic post-conditioning in human neuron-like cells may be mediated by GSK3β inhibition.

Preconditioning and postconditioning for neuroprotection: the most recent evidence

Stroke is a leading cause of morbidity and mortality, with perioperative stroke being an important complication in the practice of anaesthesia. Unfortunately, pharmacological treatment options are very limited and often not applicable in the perioperative period. The notion of applying a subtoxic stimulus prior to an otherwise lethal event is termed preconditioning. The main focus of the article is on describing the different concepts of preconditioning, including remote ischaemic preconditioning and anaesthetic preconditioning, as well as postconditioning and summarizing the most recent discoveries in this exciting field.

Involvement of Glutamate transporter-1 in neuroprotection against global brain ischemia-reperfusion injury induced by postconditioning in rats

Ischemic postconditioning refers to several transient reperfusion and ischemia cycles after an ischemic event and before a long duration of reperfusion. The procedure produces neuroprotective effects. The mechanisms underlying these neuroprotective effects are poorly understood. In this study, we found that most neurons in the CA1 region died after 10 minutes of ischemia and is followed by 72 hours of reperfusion. However, brain ischemic postconditioning (six cycles of 10 s/10 s reperfusion/re-occlusion) significantly reduced neuronal death. Significant up-regulation of Glutamate transporter-1 was found after 3, 6, 24, 72 hours of reperfusion. The present study showed that ischemic postconditioning decreases cell death and that upregulation of GLT-1 expression may play an important role on this effect.

Ischemic postconditioning protects brain from ischemia/reperfusion injury by attenuating endoplasmic reticulum stress-induced apoptosis through PI3K-Akt pathway

Endoplasmic reticulum (ER) stress has been implicated in the pathology of cerebral ischemia. During prolonged period of stress or when the adaptive response fails, apoptotic cell death ensues. Cerebral ischemic postconditioning (Postcond) has been shown to reduce cerebral ischemia/reperfusion (I/R) injury in both focal and global cerebral ischemia model. However, the mechanism remains to be understood. This study aimed to elucidate whether Postcond attenuates brain I/R damage by suppressing ER stress-induced apoptosis and if the phosphatidylinositol-3kinase/Akt (PI3K/Akt) pathway is involved. A focal cerebral ischemia rat model was used in the study. Rat brain infarct size and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) positive cells in ischemic penumbra were assessed after reperfusion of the brain. The expressions of C/EBP-homologous protein (CHOP), caspase-12, glucose-regulated protein 78 (GRP78) and the phosphorylation of Akt (Ser473) in ischemic penumbra were measured after reperfusion. Our results showed that Postcond significantly attenuated brain I/R injury, as shown by reduction in infarct size, cell apoptosis, CHOP expression, caspase-12 activation and increase in GRP78 expression. LY294002, a phosphoinositide 3-kinase inhibitor, increased the number of TUNEL-positive cells suppressed by Postcond in penumbra. In addition, LY294002 diminished the effect of Postcond on the activation of CHOP, caspase-12 and GRP78. These results suggest that Postcond protects brain from I/R injury by suppressing ER stress-induced apoptosis and PI3K/Akt pathway is involved.

Neuroprotection induced in vitro by ischemic preconditioning and postconditioning: modulation of apoptosis and PI3K-Akt pathways

Preconditioning and postconditioning are mild ischemic exposures before or after severe injurious ischemia, respectively, that elicit endogenous neuroprotective responses. Molecular mechanisms of neuroprotection through preconditioning and postconditioning are not completely understood. Here we optimized the in vitro oxygen and glucose deprivation (OGD) models of preconditioning and postconditioning in primary cortical neuron cultures that allow the studies of the corresponding molecular mechanisms of neuroprotection. We found that the cortical cells preconditioned with a single 45-min OGD treatment administered 24 h prior to injurious 2 h OGD were robustly protected after both 3 h and 16 h of reperfusion. For the postconditioning treatment, we found that three cycles of 15 min OGD followed by 15 min reperfusion, applied immediately after injurious 2 h OGD and prior to complete reperfusion, resulted in effective neuroprotection at both 3 h and 16 h of reperfusion. Using real-time RT-PCR arrays focused on genes of the apoptosis and PI3K-Akt pathways, we found that injurious OGD mainly induced apoptosis-related and repressed PI3K-Akt pathway-related genes after either 3 h or 16 h of reperfusion. Preconditioning treatment resulted in the activation of both pro-survival and anti-apoptotic pathways after 3 h of reperfusion and mainly anti-apoptotic pathway after 16 h of reperfusion. In contrast, the activation of PI3K-Akt pathway mainly contributed to the neuroprotective effect by the postconditioning treatment after 3 h of reperfusion, but differential gene expression likely contributed minimally, if at all, to the neuroprotection observed after 16 h of reperfusion. Among the novel markers of neuroprotection, Nol3 gene upregulation was observed after 3 h of reperfusion following either preconditioning or postconditioning treatments and after 16 h of reperfusion following preconditioning treatment.

p120 catenin/αN-catenin are molecular targets in the neuroprotection and neuronal plasticity mediated by atorvastatin after focal cerebral ischemia

Atorvastatin (ATV), a 3-hydroxy 3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor, exerts beneficial effects on stroke through several pleiotropic mechanisms. However, its role following cerebral ischemia is not completely understood yet. We evaluated the effect of ATV treatment on the synaptic adhesion proteins after a transient middle cerebral artery occlusion (t-MCAO) model in rats. Ischemic male Wistar rats were treated with 10 mg/kg ATV. The first dose was 6 hr after reperfusion, then every 24 hr for 3days. Our findings showed that ATV treatment produced an increase in pAkt ser473 and a decrease in pMAPK 44/42 protein levels 12 and 24 hr postischemia in the cerebral cortex and the hippocampus. However, p120 catenin and αN-catenin became drastically increased throughout the temporal course of postischemia treatment (12-72 hr), mainly in the hippocampus. Neurological recovery was observed at 48 and 72 hr, supported by a significant reduction of infarct volume, neuronal loss, and glial hyperreactivity after 72 hr of postischemia treatment with ATV. ATV treatment also up-regulated the association of p120(ctn) , αN-catenin to PSD-95, accompanied by a reduction of RhoA activation and the recovery of MAP2 immunoreactivity, these being significantly affected by the focal cerebral ischemia. Our findings suggested that p120(ctn) and αN-catenin synaptic adhesion proteins are crucial molecular targets in ATV-mediated neuroprotection and neuronal plasticity after focal cerebral ischemia.

ASIC1a contributes to neuroprotection elicited by ischemic preconditioning and postconditioning

Acid-sensing ion channels, ASICs, are proton-gated cation channels widely expressed in peripheral sensory neurons and in neurons of the central nervous system that play an important role in a variety of physiological and pathological processes. To further confirm the role played by ASIC1a in cerebral ischemia, here we examined the involvement of this channel in two endogenous recently characterized neuroprotective strategies: brain ischemic preconditioning and postconditioning. The main aim of this study was to elucidate whether ASIC1a might take part as effector in the neuroprotection evoked by brain ischemic preconditioning and postconditioning. For this purpose we investigated the effect of ischemic preconditioning and postconditioning on (1) ASIC1a mRNA and protein expression in the temporoparietal cortex of rats at different time intervals; and (2) the effect of p-AKT inhibition on ASIC1a expression during ischemic preconditioning and postconditioning. Ischemic preconditioning and postconditioning were experimentally induced in adult male rats by subjecting them to different protocols of middle cerebral artery occlusion and reperfusion. ASIC1a expression was dramatically reduced in both the neuroprotective processes. These changes in ASIC expression were p-AKT mediated, since LY-294002, a specific p-AKT inhibitor, was able to prevent variations in ASIC1a expression. The results of the present study support the idea that the downregulation of ASIC1a expression and activity might be a reasonable strategy to reduce the infarct extension after stroke.

Postconditioning and protection from reperfusion injury: where do we stand? Position paper from the Working Group of Cellular Biology of the Heart of the European Society of Cardiology

Ischaemic postconditioning (brief periods of ischaemia alternating with brief periods of reflow applied at the onset of reperfusion following sustained ischaemia) effectively reduces myocardial infarct size in all species tested so far, including humans. Ischaemic postconditioning is a simple and safe manoeuvre, but because reperfusion injury is initiated within minutes of reflow, postconditioning must be applied at the onset of reperfusion. The mechanisms of protection by postconditioning include: formation and release of several autacoids and cytokines; maintained acidosis during early reperfusion; activation of protein kinases; preservation of mitochondrial function, most strikingly the attenuation of opening of the mitochondrial permeability transition pore (MPTP). Exogenous recruitment of some of the identified signalling steps can induce cardioprotection when applied at the time of reperfusion in animal experiments, but more recently cardioprotection was also observed in a proof-of-concept clinical trial. Indeed, studies in patients with an acute myocardial infarction showed a reduction of infarct size and improved left ventricular function when they underwent ischaemic postconditioning or pharmacological inhibition of MPTP opening during interventional reperfusion. Further animal studies and large-scale human studies are needed to determine whether patients with different co-morbidities and co-medications respond equally to protection by postconditioning. Also, our understanding of the underlying mechanisms must be improved to develop new therapeutic strategies to be applied at reperfusion with the ultimate aim of limiting the burden of ischaemic heart disease and potentially providing protection for other organs at risk of reperfusion injury, such as brain and kidney.

Limb remote ischemic postconditioning protects against focal ischemia in rats

Remote ischemic postconditioning (RIP) refers to an ischemia conducted in a distant organ that protects against a prior ischemia in another organ. We tested whether RIP protects against focal ischemia in the rat brain. Stroke was generated by a permanent occlusion of the left distal middle cerebral artery combined with a 30-min occlusion of the bilateral common carotid arteries (CCA) in male rats. After CCA release, RIP was generated by three cycles of 15-min occlusion/15-min release of the left-hind femoral artery. The results showed that rapid RIP performed immediately after CCA release reduced infarction by 67% measured at 2 days after stroke. In addition, delayed RIP initiated as late as 3 h, but not 6 h, still robustly reduced infarction by 43% 2 days after stroke. RIP's protective effect was abolished by injecting the protein synthesis inhibitor, cycloheximide, and the afferent nerve blocker, capsaicin, suggesting that RIP blocks ischemic injury by modulating protein synthesis and nerve activity. Nevertheless, rapid RIP did not reduce infarction size 2 months after stroke while it ameliorated the outcome of the behavioral test. In conclusion, RIP attenuates brain injury after focal ischemia.

Delayed hypoxic postconditioning protects against cerebral ischemia in the mouse

BACKGROUND AND PURPOSE

Inspired from preconditioning studies, ischemic postconditioning, consisting of the application of intermittent interruptions of blood flow shortly after reperfusion, has been described in cardiac ischemia and recently in stroke. It is well known that ischemic tolerance can be achieved in the brain not only by ischemic preconditioning, but also by hypoxic preconditioning. However, the existence of hypoxic postconditioning has never been reported in cerebral ischemia.

METHODS

Adult mice subjected to transient middle cerebral artery occlusion underwent chronic intermittent hypoxia starting either 1 or 5 days after ischemia and brain damage was assessed by T2-weighted MRI at 43 days. In addition, we investigated the potential neuroprotective effect of hypoxia applied after oxygen glucose deprivation in primary neuronal cultures.

RESULTS

The present study shows for the first time that a late application of hypoxia (5 days) after ischemia reduced delayed thalamic atrophy. Furthermore, hypoxia performed 14 hours after oxygen glucose deprivation induced neuroprotection in primary neuronal cultures. We found that hypoxia-inducible factor-1alpha expression as well as those of its target genes erythropoietin and adrenomedullin is increased by hypoxic postconditioning. Further studies with pharmacological inhibitors or recombinant proteins for erythropoietin and adrenomedullin revealed that these molecules participate in this hypoxia postconditioning-induced neuroprotection.

CONCLUSIONS

Altogether, this study demonstrates for the first time the existence of a delayed hypoxic postconditioning in cerebral ischemia and in vitro studies highlight hypoxia-inducible factor-1alpha and its target genes, erythropoietin and adrenomedullin, as potential effectors of postconditioning.

Role of glycogen synthase kinase-3beta in cardioprotection

Limitation of infarct size by ischemic/pharmacological pre- and postconditioning involves activation of a complex set of cell-signaling pathways. Multiple lines of evidence implicate the mitochondrial permeability transition pore (mPTP) as a key end effector of ischemic/pharmacological pre- and postconditioning. Increasing the ROS threshold for mPTP induction enhances the resistance of cardiomyocytes to oxidant stress and results in infarct size reduction. Here, we survey and synthesize the present knowledge about the role of glycogen synthase kinase (GSK)-3beta in cardioprotection, including pre- and postconditioning. Activation of a wide spectrum of cardioprotective signaling pathways is associated with phosphorylation and inhibition of a discrete pool of GSK-3beta relevant to mitochondrial signaling. Therefore, GSK-3beta has emerged as the integration point of many of these pathways and plays a central role in transferring protective signals downstream to target(s) that act at or in proximity to the mPTP. Bcl-2 family proteins and mPTP-regulatory elements, such as adenine nucleotide translocator and cyclophilin D (possibly voltage-dependent anion channel), may be the functional downstream target(s) of GSK-3beta. Gaining a better understanding of these interactions to control and prevent mPTP induction when appropriate will enable us to decrease the negative impact of the reperfusion-induced ROS burst on the fate of mitochondria and perhaps allow us to limit propagation of damage throughout and between cells and consequently, to better limit infarct size.

Ischemic postconditioning as a novel avenue to protect against brain injury after stroke

Ischemic postconditioning initially referred to a stuttering reperfusion performed immediately after reperfusion, for preventing ischemia/reperfusion injury in both myocardial and cerebral infarction. It has evolved into a concept that can be induced by a broad range of stimuli or triggers, and may even be performed as late as 6 h after focal ischemia and 2 days after transient global ischemia. The concept is thought to be derived from ischemic preconditioning or partial/gradual reperfusion, but in fact the first experiment for postconditioning was carried out much earlier than that of preconditioning or partial/gradual reperfusion, in the research on myocardial ischemia. This review first examines the protective effects and parameters of postconditioning in various cerebral ischemic models. Thereafter, it provides insights into the protective mechanisms of postconditioning associated with reperfusion injury and the Akt, mitogen-activated protein kinase (MAPK), protein kinase C (PKC), and ATP-sensitive K+ (K(ATP)) channel cell signaling pathways. Finally, some open issues and future challenges regarding clinical translation of postconditioning are discussed.

Delayed postconditioning protects against focal ischemic brain injury in rats

Background

We and others have reported that rapid ischemic postconditioning, interrupting early reperfusion after stroke, reduces infarction in rats. However, its extremely short therapeutic time windows, from a few seconds to minutes after reperfusion, may hinder its clinical translation. Thus, in this study we explored if delayed postconditioning, which is conducted a few hours after reperfusion, offers protection against stroke.

Methods and Results

Focal ischemia was generated by 30 min occlusion of bilateral common carotid artery (CCA) combined with permanent occlusion of middle cerebral artery (MCA); delayed postconditioning was performed by repetitive, brief occlusion and release of the bilateral CCAs, or of the ipsilateral CCA alone. As a result, delayed postconditioning performed at 3h and 6h after stroke robustly reduced infarct size, with the strongest protection achieved by delayed postconditioning with 6 cycles of 15 min occlusion/15 min release of the ipsilateral CCA executed from 6h. We found that this delayed postconditioning provided long-term protection for up to two months by reducing infarction and improving outcomes of the behavioral tests; it also attenuated reduction in 2-[¹⁸F]-fluoro-2-deoxy-D-glucose (FDG)-uptake therefore improving metabolism, and reduced edema and blood brain barrier leakage. Reperfusion in ischemic stroke patients is usually achieved by tissue plasminogen activator (tPA) application, however, t-PA's side effect may worsen ischemic injury. Thus, we tested whether delayed postconditioning counteracts the exacerbating effect of t-PA. The results showed that delayed postconditioning mitigated the worsening effect of t-PA on infarction.

Conclusion

Delayed postconditioning reduced ischemic injury after focal ischemia, which opens a new research avenue for stroke therapy and its underlying protective mechanisms.

Protective effects of ischemic postconditioning compared with gradual reperfusion or preconditioning

We examined the temporal factors of postconditioning, assessed whether gradual reperfusion reduces infarcts, and compared postconditioning's protection with that of both rapid and delayed preconditioning. Focal ischemia was generated by permanent occlusion of the left distal middle cerebral artery (dMCA) combined with 30 min of occlusion of both common carotid arteries (CCA) in rats. Postconditioning was performed by repetitive brief release and occlusion of CCA after 30 min of CCA occlusion. Gradual reperfusion was generated by controlled release of the bilateral CCA. We confirmed that postconditioning disrupted the early reperfusion but improved cerebral blood flow (CBF) thereafter. Postconditioning with three cycles, but not with 10 cycles, of 30 sec CCA release and 10 sec CCA occlusion (30s/10s) reduced infarction measured at 2 days after stroke. In addition, postconditioning with 10 cycles, but not with three cycles, of 10s/10s reduced infarction but it lost protection when initiated at 3 min after reperfusion. In addition, gradual reperfusion also reduced infarction. Moreover, both rapid and delayed preconditioning conducted 60 min and 3 days before stroke reduced infarct sizes. However, no additional protection was detected when postconditioning was combined with either rapid or delayed preconditioning. In conclusion, gradual reperfusion reduced infarction; postconditioning's protection depended on the number of cycles and duration of each cycle of reperfusion and occlusion and the onset time of postconditioning; postconditioning's protection was comparable to that of rapid preconditioning but not as robust as that of delayed preconditioning.

Ischaemic and pharmacological preconditionings protect liver via adenosine and redox status following hepatic ischaemia/reperfusion in rats

Although IPC (ischaemic preconditioning) is considered as a protective strategy in HI/R (hepatic ischaemia/reperfusion), the mechanisms for this effect have not been fully elucidated. In the present study we investigate whether PPC (pharmacological preconditioning) by transient activation of A(1)R (adenosine A(1) receptor) protects against long-term HI/R and whether the protective effects of IPC depend on A(1)R activation and whether both preconditionings affect remote organs. Wistar rats underwent IPC and long-term HI/R. Another set of animals were pharmacologically preconditioned with the A(1)R-agonist CCPA [2-chloro-N(6)-cyclopentyladenosine; 0.1 mg/kg of body weight, i.p. (intraperitoneally)] 24 h before HI/R. In other groups, rats received an A(1)R-antagonist, DPCPX (1,3-dipropyl-8-cyclopentylxanthine; 0.1 mg/kg of body weight, i.p.) 24 h before HI/R. Hepatic damage was evaluated by transaminase [AST (aspartate transaminase), ALT (alanine transaminase)] release; inflammation was assessed by hepatic MPO (myeloperoxidase) and serum TNFalpha (tumour necrosis factor alpha) and NO; oxidative stress was estimated by MDA (malondialdehyde) and 4-HDA (4-hydroxyalkenals), SOD (superoxide dismutase) activity, GSH and ADA (adenosine deaminase) as adenosine metabolism. Both preconditionings protected liver and lung against HI/R as indicated by the reduction in transaminases, MPO, MDA+4-HDA, NO, TNFalpha and ADA activity as compared with HI/R (P<0.05). However, pre-treatment with DPCPX abolished the protective effects of IPC and PPC. Preconditionings induced a significant increase in hepatic MnSOD (manganese SOD) activity and NO generation compared with the sham group, and this activity was abolished by DPCPX pre-treatment. A(1)R activation induced hepatic delayed preconditioning and blockade of A(1)R abolished hepatic IPC. IPC, as well as PPC, were able to prevent lung damage. These protective effects are associated with a reduction in oxidative stress, inflammation and endogenous antioxidant preservation.