Ischemic tolerance in the brain

Dual Preconditioning: A Novel Strategy to Withstand Mesenchymal Stem Cells against Harsh Microenvironments

Purpose: Poor survival rate of mesenchymal stem cells (MSCs) following their transplantation is one of the major challenges in their therapeutic application. Therefore, it is necessary to augment the viability of the MSCs in order to improve their therapeutic efficacy. Several strategies have been used to overcome this problem. Preconditioning of MSCs with oxidative stresses has gained a lot of attention. Therefore, in the present study, we investigated the effects of simultaneous preconditioning of MSCs with hydrogen peroxide and serum deprivation stresses on their survival and resistance to stressful conditions.

Methods: MSCs were isolated from human umbilical cord blood. To perform simultaneous preconditioning, the cells were cultured in DMEM medium containing 1, 2.5 and 5 percent FBS and different concentrations of H₂O₂ (5, 10, 15, 20, 25, 30, 35, 40, 50, 60, 80 and 100 µM) for 24 hrs. Then, the cells were cultured in recovery culture medium. Finally, one group of the cells was exposed to a lethal concentration of H₂O₂ (300µM), and the other cells were cultivated in FBS free DMEM medium as the lethal situation. In addition, the percentage of apoptotic cells was analyzed using Caspase 3 assay kit.

Results: Simultaneous preconditioning of the MSCs with 15µM H₂O₂ plus serum deprivation, 2.5% FBS, significantly increased the resistance of the cells to the toxicity induced following their cultivation in FBS free DMEM medium. It exerted the protective effect on the cells after treating with the lethal dose of H₂O₂ as well.

Conclusion: Simultaneous preconditioning of MSCs with oxidative and serum deprivation stresses enhances their survival against harsh conditions, which might increase the viability and stability of the MSCs following their transplantation.

Neuroprotection of hypoxic postconditioning against global cerebral ischemia through influencing posttranslational regulations of heat shock protein 27 in adult rats

We previously reported that hypoxic postconditioning (HPC) ameliorated hippocampal neuronal death induced by transient global cerebral ischemia (tGCI) in adult rats. However, the mechanism of HPC-induced neuroprotection is still elusive. Notably, heat shock protein 27 (Hsp27) has recently emerged as a potent neuroprotectant in cerebral ischemia. Although its robust protective effect on stroke has been recognized, the mechanism of Hsp27-mediated neuroprotection is largely unknown. Here, we investigated the potential molecular mechanism by which HPC modulates the posttranslational regulations of Hsp27 after tGCI. We found that HPC increased expression of Hsp27 in CA1 subregion after tGCI. Inhibition of Hsp27 expression with lentivirus-mediated short hairpin RNA (shRNA) abolished the neuroprotection induced by HPC in vivo. Furthermore, pretreatment with cycloheximide, a protein synthesis inhibitor, resulted in a significant decrease in the degradation rate of Hsp27 protein in postconditioned rats, suggesting that the increase in the expression of Hsp27 after HPC might result from its decreased degradation. Next, pretreatment with leupeptin, a lysosomal inhibitor, resulted in an accumulation of Hsp27 after tGCI, indicating that autophagic pathway may be responsible for the degradation of Hsp27. We further showed that the formation of LC3-II and autophagosomes increased after tGCI. Meanwhile, the degradation of Hsp27 was suppressed and neuronal damage was reduced when blocking autophagy with 3-Methyladenine, whereas activating autophagy with rapamycin showed an opposite tendency. Lastly, we confirmed that HPC increased the expression of phosphorylated MAPKAP kinase 2 (MK2) and Hsp27 after tGCI. Also, administration of SB203580, a p38 mitogen-activated protein kinase inhibitor, decreased the expressions of phosphorylated MK2 and Hsp27. Our results suggested that inhibition of Hsp27 degradation mediated by down-regulation of autophagy may induce ischemic tolerance after HPC. Additionally, phosphorylation of Hsp27 induced by MK2 might be associated with the neuroprotection of HPC.

Hypoxic preconditioning-induced mitochondrial protection is not disrupted in a cell model of mtDNA T8993G mutation-induced F1F0-ATP synthase defect: the role of mitochondrial permeability transition

Transient opening of the mitochondrial permeability transition pore plays a crucial role in hypoxic preconditioning-induced protection. Recently, the cyclophilin-D component of the mitochondrial permeability transition pore has been shown to interact with and regulate the F1F0-ATP synthase. However, the precise role of the F1F0-ATP synthase and the interaction between cyclophilin-D and F1F0-ATP synthase in the mitochondrial permeability transition pore and hypoxic preconditioning remain uncertain. Here we found that a 1-h hypoxic preconditioning delayed apoptosis and improved cell survival after stimulation with various apoptotic inducers including H2O2, ionomycin, and arachidonic acid in mitochondrial DNA T8993G mutation (NARP) osteosarcoma 143B cybrids, an F1F0-ATP synthase defect cell model. This hypoxic preconditioning protected NARP cybrid cells against focal laser irradiation-induced oxidative stress by suppressing reactive oxygen species formation and preventing the depletion of cardiolipin. Furthermore, the protective functions of transient opening of the mitochondrial permeability transition pore in both NARP cybrids and wild-type 143B cells can be augmented by hypoxic preconditioning. Disruption of the interaction between cyclophilin-D and F1F0-ATP synthase by cyclosporin A attenuated the mitochondrial protection induced by hypoxic preconditioning in both NARP cybrids and wild-type 143B cells. Our results demonstrate that the interaction between cyclophilin-D and F1F0-ATP synthase is important in the hypoxic preconditioning-induced cell protection. This finding improves our understanding of the mechanism of mitochondrial permeability transition pore opening in cells in response to hypoxic preconditioning, and will be helpful in further developing new pharmacological agents targeting hypoxia-reoxygenation injury and mitochondria-mediated cell death.

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.

Experimental models for assaying microvascular endothelial cell pathophysiology in stroke

It is important to understand the molecular mechanisms underlying neuron death following stroke in order to develop effective neuroprotective strategies. Since studies on human stroke are extremely limited due to the difficulty in collecting post-mortem tissue at different time points after the onset of stroke, brain ischaemia research focuses on information derived from in-vitro models of neuronal death through ischaemic injury [1]. This review aims to provide an update on the different in-vitro stroke models with brain microvascular endothelial cells that are currently being used. These models provide a physiologically relevant tool to screen potential neuroprotective drugs in stroke and to study the molecular mechanisms involved in brain ischaemia.

Mechanisms and prospects of ischemic tolerance induced by cerebral preconditioning

In the brain, brief episodes of ischemia induce tolerance against a subsequent severe episode of ischemia. This phenomenon of endogenous neuroprotection is known as preconditioning-induced ischemic tolerance. The purpose of this review is to summarize the current state of knowledge about mechanisms and potential applications of cerebral preconditioning and ischemic tolerance. Articles related to the terms ischemic preconditioning and ischemic tolerance were systematically searched via MEDLINE/PubMed, and articles published in English related to the nervous system were selected and analyzed. The past two decades have provided interesting insights into the molecular mechanisms of this neuroprotective phenomenon. Although both rapid and delayed types of tolerance have been documented in experimental settings, the delayed type has been found to be more prominent in the case of neuronal ischemic tolerance. Many intracellular signaling pathways have been implicated regarding ischemic preconditioning. Most of these are associated with membrane receptors, kinase cascades, and transcription factors. Moreover, ischemic tolerance can be induced by exposing animals or cells to diverse types of endogenous and exogenous stimuli that are not necessarily hypoxic or ischemic in nature. These cross-tolerances raise the hope that, in the future, it will be possible to pharmacologically activate or mimic ischemic tolerance in the human brain. Another promising approach is remote preconditioning in which preconditioning of one organ or system leads to the protection of a different (remote) organ that is difficult to target, such as the brain. The preconditioning strategy and related interventions can confer neuroprotection in experimental ischemia, and, thus, have promise for practical applications in cases of vascular neurosurgery and endo-vascular therapy.

Activation of Akt/FoxO signaling pathway contributes to induction of neuroprotection against transient global cerebral ischemia by hypoxic pre-conditioning in adult rats

It is well established that pre-conditioning protects neuronal injury against ischemia. However, the molecular mechanisms underlying ischemic tolerance are not completely understood. The purpose of the present study was to investigate the role of Akt/forkhead transcription factor, class O (FoxO) pathway in hypoxic pre-conditioning (HPC) using a newly developed HPC to transient global cerebral ischemia (tGCI) model in adult rats. HPC for 30-120 min significantly reduced cell death in the CA1 subregion after 10 min of tGCI. HPC was effective only when applied 1-4 days before ischemia. The maximum protection was observed with 30 min of hypoxia and 1 day interval between hypoxia and ischemia. The phosphorylated Akt and FoxOs measured by western blot and immunohistochemistry were significantly increased after hypoxia-ischemia except for a transient decrease in the HPC group. Lateral ventricular infusion of LY294002 before HPC blocked the increase in phosphorylated Akt and FoxOs and increased neuronal damage in HPC animals. These results suggest that pre-exposure to hypoxia induces protection against tGCI in adult rats. Activation of Akt results in the inactivation of FoxOs which may mediate ischemic tolerance after HPC.

Preconditioning-induced ischemic tolerance: a window into endogenous gearing for cerebroprotection

Ischemic tolerance defines transient resistance to lethal ischemia gained by a prior sublethal noxious stimulus (i.e., preconditioning). This adaptive response is thought to be an evolutionarily conserved defense mechanism, observed in a wide variety of species. Preconditioning confers ischemic tolerance if not in all, in most organ systems, including the heart, kidney, liver, and small intestine. Since the first landmark experimental demonstration of ischemic tolerance in the gerbil brain in early 1990's, basic scientific knowledge on the mechanisms of cerebral ischemic tolerance increased substantially. Various noxious stimuli can precondition the brain, presumably through a common mechanism, genomic reprogramming. Ischemic tolerance occurs in two temporally distinct windows. Early tolerance can be achieved within minutes, but wanes also rapidly, within hours. Delayed tolerance develops in hours and lasts for days. The main mechanism involved in early tolerance is adaptation of membrane receptors, whereas gene activation with subsequent de novo protein synthesis dominates delayed tolerance. Ischemic preconditioning is associated with robust cerebroprotection in animals. In humans, transient ischemic attacks may be the clinical correlate of preconditioning leading to ischemic tolerance. Mimicking the mechanisms of this unique endogenous protection process is therefore a potential strategy for stroke prevention. Perhaps new remedies for stroke are very close, right in our cells.

Ischemic preconditioning targets the respiration of synaptic mitochondria via protein kinase C epsilon

In the brain, ischemic preconditioning (IPC) diminishes mitochondrial dysfunction after ischemia and confers neuroprotection. Activation of epsilon protein kinase C (epsilonPKC) has been proposed to be a key neuroprotective pathway during IPC. We tested the hypothesis that IPC increases the levels of epsilonPKC in synaptosomes from rat hippocampus, resulting in improved synaptic mitochondrial respiration. Preconditioning significantly increased the level of hippocampal synaptosomal epsilonPKC to 152% of sham-operated animals at 2 d of reperfusion, the time of peak neuroprotection. We tested the effect of epsilonPKC activation on hippocampal synaptic mitochondrial respiration 2 d after preconditioning. Treatment with the specific epsilonPKC activating peptide, tat-psiepsilonRACK (tat-psiepsilon-receptor for activated C kinase), increased the rate of oxygen consumption in the presence of substrates for complexes I, II, and IV to 157, 153, and 131% of control (tat peptide alone). In parallel, we found that epsilonPKC activation in synaptosomes from preconditioned animals resulted in altered levels of phosphorylated mitochondrial respiratory chain proteins: increased serine and tyrosine phosphorylation of 18 kDa subunit of complex I, decreased serine phosphorylation of FeS protein in complex III, increased threonine phosphorylation of COX IV (cytochrome oxidase IV), increased mitochondrial membrane potential, and decreased H2O2 production. In brief, ischemic preconditioning promoted significant increases in the level of synaptosomal epsilonPKC. Activation of epsilonPKC increased synaptosomal mitochondrial respiration and phosphorylation of mitochondrial respiratory chain proteins. We propose that, at 48 h of reperfusion after ischemic preconditioning, epsilonPKC is poised at synaptic mitochondria to respond to ischemia either by direct phosphorylation or activation of the epsilonPKC signaling pathway.

Anesthetic Preconditioning with Sevoflurane Does Not Protect the Spinal Cord After an Ischemic-Reperfusion Injury in the Rat

Anesthetic preconditioning (APC) is a protective mechanism, whereby exposure to a volatile anesthetic renders a tissue resistant to a subsequent ischemic insult. We hypothesized that APC of the rat spinal cord with sevoflurane would reduce neurologic deficit after an ischemic-reperfusion injury. Rats were randomly assigned to 1 of 5 groups. The ischemic preconditioning (IPC) group (n = 14) had 3 min of IPC, 30 min of reperfusion, and 12 min of ischemia. The chronic APC (cSEVO) group (n = 14) had 1 h of APC with 3.5% sevoflurane on each of 2 days before ischemia. The acute APC (aSEVO) group (n = 14) had 1 h of APC with 3.5% sevoflurane followed by a 1-h washout period before the induction of ischemia. The controls (n = 14) underwent no preconditioning before ischemia. IPC attenuated the ischemia-reperfusion injury, whereas aSEVO and cSEVO groups were no better than control animals. Histologic evaluation of the spinal cord showed severe neurologic damage in all groups except for the IPC group and sham-operated rats. APC with sevoflurane did not reduce neurologic injury in a rat model of spinal cord ischemia. Traditional ischemic preconditioning had a strong protective benefit on neurologic outcome.