Cerebral preconditioning and ischaemic tolerance

The role of microglia in ischemic preconditioning

Ischemic preconditioning (IPC) is an experimental phenomenon in which a brief ischemic stimulus confers protection against a subsequent prolonged ischemic event. Initially thought to be due to mechanistic changes in neurons, our understanding of IPC has evolved to encompass a global reprogramming of the Central Nervous System (CNS) after transient ischemia/reperfusion that requires innate immune signaling pathways including Toll-like receptors (TLRs) and Type I interferons. Microglia are the CNS resident neuroimmune cells that express these key innate immune receptors. Studies suggest that microglia are required for IPC-mediated neuronal and axonal protection. Multiple paradigms targeting TLRs have converged on a distinctive Type I interferon response in microglia that is critical for preconditioning-mediated protection against ischemia. These pathways can be targeted through administration of TLR agonists, cytokines including interferon-β, and pharmaceutical agents that induce preconditioning through cross-tolerance mechanisms. Transcriptomic analyses and single cell RNA studies point to specific gene expression signatures in microglia that functionally shift these mutable cells to an immunomodulatory or protective phenotype. Although there are technological challenges and gaps in knowledge to overcome, the targeting of specific molecular signaling pathways in microglia is a promising direction for development of novel and effective pharmacotherapies for stroke. Studies on preconditioning in animal models, including nonhuman primates, show promise as prophylactic preconditioning treatments for selected at risk patient populations. In addition, our growing understanding of the mechanisms of IPC-mediated protection is identifying novel cellular and molecular targets for therapeutic interventions that could apply broadly to both acute stroke and chronic vascular cognitive impairment patients.

Effects of resveratrol postconditioning on cerebral ischemia in mice: role of the sirtuin-1 pathway

Evidence has demonstrated that resveratrol preconditioning exhibits neuroprotection against cerebral ischemia-reperfusion (IR) injury. The current investigation aimed to explore whether pharmacological postconditioning, by administering resveratrol, after a sustained ischemia and prior to prolonged reperfusion abrogates cerebral IR injury. Cerebral IR-induced injury mice model was employed in this study to evaluate the neuroprotective effects of pharmacological postconditioning with resveratrol (30 mg/kg; i.p.) administered 5 min before reperfusion. We administered sirtinol, a SIRT1/2 selective inhibitor (10 mg/kg; i.p.) 10 min before ischemia (17 min) and reperfusion (24 h), to elucidate whether the neuroprotection with resveratrol postconditioning depends on SIRT1 activation. Various biochemical and behavioural parameters and histopathological changes were assessed to examine the effect of pharmacological postconditioning. Infarct size is estimated using TTC staining. It was established that resveratrol postconditioning abrogated the deleterious effects of IR injury expressed with regard to biochemical parameters of oxidative stress (TBARS, SOD, GSH), acetylcholinesterase activity, behavioural parameters (memory, motor coordination), infarct size, and histopathological changes. Sirtinol significantly reversed the effect of resveratrol postconditioning. We conclude that induced neuroprotective benefits of resveratrol postconditioning may be the consequence of SIRT1 activation and resveratrol can be considered, for further studies, as potential agent inducing pharmacological postconditioning in clinical situations.

Enolase1 Alleviates Cerebral Ischemia-Induced Neuronal Injury via Its Enzymatic Product Phosphoenolpyruvate

Stroke is a leading cause of disability and the second leading cause of death among adults worldwide, while the mechanisms underlying neuronal death and dysfunction remain poorly understood. Here, we investigated the differential proteomic profiles of mouse brain homogenate with 3 h of middle cerebral artery occlusion (MCAO) ischemia, or sham, using Coomassie Brilliant Blue staining, followed by mass spectrometry. We identified enolase1 (ENO1), a key glycolytic enzyme, as a potential mediator of neuronal injury in MCAO ischemic model. Reverse transcription polymerase chain reaction and western blotting data showed that ENO1 was ubiquitously expressed in various tissues, distinct regions of brain, and different postnatal age. Immunohistochemical analysis revealed that ENO1 is localized in neuronal cytoplasm and dendrites. Interestingly, the expression level of ENO1 was significantly increased in the early stage, but dramatically decreased in the late stage, of cerebral ischemia in vivo. This dynamic change was consistent with our finding in cultured hippocampal neurons treated with oxygen/glucose deprivation (OGD) in vitro. Importantly, ENO1 overexpression in cultured neurons alleviated dendritic and spinal loss caused by OGD treatment. Furthermore, the enzymatic product of ENO1, phosphoenolpyruvate (PEP), was also synchronously changed along with the dynamic ENO1 level. The neuronal injury caused by OGD treatment in vitro or ischemia in vivo was mitigated by the application of PEP. Taken together, our data revealed that ENO1 plays a novel and protective role in cerebral ischemia-induced neuronal injury, highlighting a potential of ENO1 as a therapeutic target of neuronal protection from cerebral ischemia.

Hypoxia tolerance in the Norrin-deficient retina and the chronically hypoxic brain studied at single-cell resolution

The mammalian CNS is capable of tolerating chronic hypoxia, but cell type-specific responses to this stress have not been systematically characterized. In the Norrin KO (Ndp ^KO ) mouse, a model of familial exudative vitreoretinopathy (FEVR), developmental hypovascularization of the retina produces chronic hypoxia of inner nuclear-layer (INL) neurons and Muller glia. We used single-cell RNA sequencing, untargeted metabolomics, and metabolite labeling from ¹³C-glucose to compare WT and Ndp ^KO retinas. In Ndp ^KO retinas, we observe gene expression responses consistent with hypoxia in Muller glia and retinal neurons, and we find a metabolic shift that combines reduced flux through the TCA cycle with increased synthesis of serine, glycine, and glutathione. We also used single-cell RNA sequencing to compare the responses of individual cell types in Ndp ^KO retinas with those in the hypoxic cerebral cortex of mice that were housed for 1 week in a reduced oxygen environment (7.5% oxygen). In the hypoxic cerebral cortex, glial transcriptome responses most closely resemble the response of Muller glia in the Ndp ^KO retina. In both retina and brain, vascular endothelial cells activate a previously dormant tip cell gene expression program, which likely underlies the adaptive neoangiogenic response to chronic hypoxia. These analyses of retina and brain transcriptomes at single-cell resolution reveal both shared and cell type-specific changes in gene expression in response to chronic hypoxia, implying both shared and distinct cell type-specific physiologic responses.

Interaction of neurotransmitters and neurochemicals with lymphocytes

Neurotransmitters and neurochemicals can act on lymphocytes by binding to receptors expressed by lymphocytes. This review describes lymphocyte expression of receptors for a selection of neurotransmitters and neurochemicals, the anatomical locations where lymphocytes can interact with neurotransmitters, and the effects of the neurotransmitters on lymphocyte function. Implications for health and disease are also discussed.

Dose of remote limb ischemic conditioning for enhancing learning in healthy young adults

Remote limb ischemic conditioning (RLIC) is a technique in which tissues distant from the target organ are exposed to brief, sub-lethal bouts of ischemia. The effects of remotely applied ischemic conditioning are systemically transferred to the target organ, and typically manifested as protection from subsequent ischemic injury. Previous studies in our lab have found and confirmed that RLIC enhances learning and retention during motor training on a balance task. The current study tested the effect of RLIC dose (number of cycles) on learning enhancement in young, healthy adults. Forty healthy participants age 18-40 years were randomized to receive 5 cycles of sham conditioning (n = 9), 3 cycles of RLIC (n = 11), 4 cycles of RLIC (n = 10), or 5 cycles of RLIC (n = 10) using a blood pressure cuff around the upper arm once a day for 7 consecutive weekdays (Days 1-7). Participants concurrently trained on a balance task, bimanual cup stacking task, and a discrete sequence production task on Days 3-7. Change in performance on each of the three tasks was compared across groups. Participants in all four groups improved their performance on each of the three tasks over time. However, RLIC at any dose did not enhance learning on any of the three tasks. While RLIC is safe, inexpensive, and clinically feasible, reproducibility may be challenged by unidentified factors, raising critical challenges to the straightforward translation of RLIC for improving rehabilitation outcomes in individuals recovering from neurological injury.

Pretreatment with Korean red ginseng or dimethyl fumarate attenuates reactive gliosis and confers sustained neuroprotection against cerebral hypoxic-ischemic damage by an Nrf2-dependent mechanism

The transcriptional factor Nrf2, a master regulator of oxidative stress and inflammation that are tightly linked to the development and progression of cerebral ischemia pathology, plays a vital role in inducing the endogenous neuroprotective process. Here, hypoxic-ischemia (HI) was performed in adult Nrf2 knockout and wildtype mice that were orally pretreated either with standardized Korean red ginseng extract (Ginseng) or dimethyl fumarate (DMF), two candidate Nrf2 inducers, to determine whether the putative protection was through an Nrf2-dependent mechanism involving the attenuation of reactive gliosis. Results show that Nrf2 target cytoprotective genes were distinctly elevated following HI. Pretreatment with Ginseng or DMF elicited robust neuroprotection against the deterioration of acute cerebral ischemia damage in an Nrf2-dependent manner as revealed by the reductions of neurological deficits score, infarct volume and brain edema, as well as enhanced expression levels of Nrf2 target antioxidant proteins and anti-inflammation mediators. In both ischemic striatum and cortex, the dynamic pattern of attenuated reactive gliosis in astrocytes and microglia, including affected astrocytic dysfunction in glutamate metabolism and water homeostasis, correlated well with the Nrf2-dependent neuroprotection by Ginseng or DMF. Furthermore, such neuroprotective benefits extended to the late phase of ischemic brain damage after HI, as evidenced by improvements in neurobehavioral outcomes, infarct volume and brain edema. Overall, pretreatment with Ginseng or DMF identically attenuates reactive gliosis and confers long-lasting neuroprotective efficacy against ischemic brain damage through an Nrf2-dependent mechanism. This study also provides new insight into the profitable contribution of reactive gliosis in the Nrf2-dependent neuroprotection in acute brain injury.

rt-PA with remote ischemic postconditioning for acute ischemic stroke

Objective

To investigate the feasibility and safety of remote ischemic postconditioning (RIPC) in acute ischemic stroke patients after intravenous recombinant tissue plasminogen activator (rt‐PA) thrombolysis (IVT).

Methods

We performed a pilot randomized trial involving acute ischemic stroke patients with IVT. The patients were randomized 1:1 to receive RIPC or standard medical therapy. In the RIPC group, the participants underwent instant RIPC within 2 h of IVT, followed by repeated RIPC therapy for 7 days. The feasibility end point was the completion of RIPC and time from the first RIPC to finishing IVT in the RIPC group. The safety end point included tissue and neurovascular injury resulting from RIPC, changes in vital signs, level of plasma myoglobin, any hemorrhagic transformation, and other adverse events.

Results

Thirty patients (15 RIPC and 15 Control) were recruited after IVT. The mean age was 65.7 ± 10.2 years, with a National Institutes of Health Stroke Scale (NIHSS) score of 6.5 (4.0–10.0). The completion rate for RIPC was 97.0%. The mean time from first RIPC to completing IVT was 66.0 (25.0–75.0) min in the RIPC group. One case of hemorrhagic transformation was observed in the RIPC group. No significant difference was found in the level of myoglobin between the two groups (P > 0.05).

Interpretation

RIPC is effective and safe for AIS patients after intravenous rt‐PA thrombolysis.

IRAK-M Deficiency Exacerbates Ischemic Neurovascular Injuries in Experimental Stroke Mice

Background: Innate immune response to neuronal death is one of the key events of the pathogenesis of ischemic brain injury. Interleukin-1 receptor-associated kinase (IRAK)-M, encoded by gene Irak3, negatively regulates toll-like receptor signaling by interacting with the MyD88–IRAK-4–IRAK-1 complex and blocking the phosphorylation and dissociation of IRAK-1. Its function in the ischemic stroke is unknown.

Objective: This study aims to investigate whether IRAK-M deficiency could exacerbate neuroinflammation and neurovascular injuries during cerebral ischemia and reperfusion.

Methods: Male C57BL/6 mice and Irak3 knockout mice were subjected to 45 min of middle cerebral artery occlusion and 4 or 24 h of reperfusion. Transcription of Irak3 gene was evaluated by quantitative real-time PCR (qRT-PCR). Then, infarct volume, neurological score, brain water content, and Evans blue leakage were compared between knock-out and wild-type mice after reperfusion. Through the observation of gross brain specimen after cerebral ischemia, the incidence of hemorrhage transformation was compared between KO and WT mice. To explore underlying signaling pathways involved in IRAK-M deficiency, major proinflammatory cytokines and NF-κB signaling were measured by qRT-PCR and Western blot.

Results: The expression of IRAK-M peaked at 1 h after reperfusion, and then gradually decreased within the first 24 h, which was abolished by blocking the expression of hypoxia induced factor 1α. IRAK-M deficiency increased infarct volume, brain edema, the incidence of hemorrhage transformation, and the permeability of blood–brain barrier. In addition, the NF-κB-mediated expressions of proinflammatory cytokines and the activation of microglia in the ipsilateral brain from knock-out mice were much higher than those in wild-type littermates.

Conclusion: IRAK-M deletion exacerbates neurovascular damages which are related to the pronounced activation of NF-κB signaling and neuroinflammatory responses during cerebral ischemia-reperfusion in mice. Our study indicates that IRAK-M has neuroprotective effect and has potential to facilitate the development of new pharmaceuticals that reduce neurovascular complications.

Non-canonical activation of CREB mediates neuroprotection in a Caenorhabditis elegans model of excitotoxic necrosis

Excitotoxicity, caused by exaggerated neuronal stimulation by Glutamate (Glu), is a major cause of neurodegeneration in brain ischemia. While we know that neurodegeneration is triggered by overstimulation of Glu-receptors (GluRs), the subsequent mechanisms that lead to cellular demise remain controversial. Surprisingly, signaling downstream of GluRs can also activate neuroprotective pathways. The strongest evidence involves activation of the transcription factor cAMP response element-binding protein (CREB), widely recognized for its importance in synaptic plasticity. Canonical views describe CREB as a phosphorylation-triggered transcription factor, where transcriptional activation involves CREB phosphorylation and association with CREB-binding protein. However, given CREB's ubiquitous cross-tissue expression, the multitude of cascades leading to CREB phosphorylation, and its ability to regulate thousands of genes, it remains unclear how CREB exerts closely tailored, differential neuroprotective responses in excitotoxicity. A non-canonical, alternative cascade for activation of CREB-mediated transcription involves the CREB co-factor cAMP-regulated transcriptional co-activator (CRTC), and may be independent of CREB phosphorylation. To identify cascades that activate CREB in excitotoxicity we used a Caenorhabditis elegans model of neurodegeneration by excitotoxic necrosis. We demonstrated that CREB's neuroprotective effect was conserved, and seemed most effective in neurons with moderate Glu exposure. We found that factors mediating canonical CREB activation were not involved. Instead, phosphorylation-independent CREB activation in nematode excitotoxic necrosis hinged on CRTC. CREB-mediated transcription that depends on CRTC, but not on CREB phosphorylation, might lead to expression of a specific subset of neuroprotective genes. Elucidating conserved mechanisms of excitotoxicity-specific CREB activation can help us focus on core neuroprotective programs in excitotoxicity. Cover Image for this issue: doi: 10.1111/jnc.14494.

Very Delayed Remote Ischemic Post-conditioning Induces Sustained Neurological Recovery by Mechanisms Involving Enhanced Angioneurogenesis and Peripheral Immunosuppression Reversal

Ischemic conditioning is defined as a transient and subcritical period of ischemia integrated in an experimental paradigm that involves a stimulus of injurious ischemia, activating endogenous tissue repair mechanisms that lead to cellular protection under pathological conditions like stroke. Whereas ischemic pre-conditioning is irrelevant for stroke treatment, ischemic post-conditioning, and especially non-invasive remote ischemic post-conditioning (rPostC) is an innovative and potential strategy for stroke treatment. Although rPostC has been shown to induce neuroprotection in stroke models before, resulting in some clinical trials on the way, fundamental questions with regard to its therapeutic time frame and its underlying mechanisms remain elusive. Hence, we herein used a model of non-invasive rPostC of hind limbs after cerebral ischemia in male C57BL6 mice, studying the optimal timing for the application of rPostC and its underlying mechanisms for up to 3 months. Mice undergoing rPostC underwent three different paradigms, starting with the first cycle of rPostC 12 h, 24 h, or 5 days after stroke induction, which is a very delayed time point of rPostC that has not been studied elsewhere. rPostC as applied within 24 h post-stroke induces reduction of infarct volume on day three. On the contrary, very delayed rPostC does not yield reduction of infarct volume on day seven when first applied on day five, albeit long-term brain injury is significantly reduced. Likewise, very delayed rPostC yields sustained neurological recovery, whereas early rPostC (i.e., <24 h) results in transient neuroprotection only. The latter is mediated via heat shock protein 70 that is a well-known signaling protein involved in the pathophysiological cellular cascade of cerebral ischemia, leading to decreased proteasomal activity and decreased post-stroke inflammation. Very delayed rPostC on day five, however, induces a pleiotropic effect, among which a stimulation of angioneurogenesis, a modulation of the ischemic extracellular milieu, and a reversal of the stroke-induced immunosuppression occur. As such, very delayed rPostC appears to be an attractive tool for future adjuvant stroke treatment that deserves further preclinical attention before large clinical trials are in order, which so far have predominantly focused on early rPostC only.

Metabolomic Profiling Reveals That Reprogramming of Cerebral Glucose Metabolism Is Involved in Ischemic Preconditioning-Induced Neuroprotection in a Rodent Model of Ischemic Stroke

Ischemic tolerance renders the brain resistant to ischemia-reperfusion (I/R) injury as a result of the activation of endogenous adaptive responses triggered by various types of preconditioning. The complex underlying metabolic mechanisms responsible for the neuroprotection of cerebral ischemic preconditioning (IPC) remain elusive. Herein, gas chromatography-mass spectrometry (GC-MS) technique was applied to delineate the dynamic changes of brain metabolome in a rodent model of ischemic stroke (transient occlusion of the middle cerebral artery, tMCAO), alone or after pretreatment with nonlethal ischemic tolerance induction (transient occlusion of the bilateral common carotid arteries, tBCCAO). Metabolomic analysis showed that accumulation of glucose (concentration increased more than 4 fold) and glycolytic intermediates is the prominent feature of brain I/R-induced metabolic disturbance. IPC attenuated brain I/R damage by subduing postischemic hyperglycolysis, increasing the pentose phosphate pathway (PPP) flux and promoting the utilization of β-hydroxybutyrate. The expression analysis of pivotal genes and proteins involved in relevant metabolic pathways revealed that the downregulation of AMP-activated protein kinase (AMPK)-mediated glucose transporter-1 (GLUT-1) and 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase-3 (PFKFB3) and reduced mRNA levels of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) subunits were associated with IPC-induced metabolic flexibility, which allows the brain to be more capable of withstanding severe I/R insults. The present study provided mechanistic insights into the metabolic signature of IPC and indicated that adaptively modulating brain glucose metabolism could be an effective approach for the therapeutic intervention of ischemic stroke.

Models and methods for conditioning the ischemic brain

BACKGROUND

In the last decades the need to find new neuroprotective targets has addressed the researchers to investigate the endogenous molecular mechanisms that brain activates when exposed to a conditioning stimulus. Indeed, conditioning is an adaptive biological process activated by those interventions able to confer resistance to a deleterious brain event through the exposure to a sub-threshold insult. Specifically, preconditioning and postconditioning are realized when the conditioning stimulus is applied before or after, respectively, the harmul ischemia.

AIMS AND RESULTS

The present review will describe the most common methods to induce brain conditioning, with particular regards to surgical, physical exercise, temperature-induced and pharmacological approaches. It has been well recognized that when the subliminal stimulus is delivered after the ischemic insult, the achieved neuroprotection is comparable to that observed in models of ischemic preconditioning. In addition, subjecting the brain to both preconditioning as well as postconditioning did not cause greater protection than each treatment alone.

CONCLUSIONS

The last decades have provided fascinating insights into the mechanisms and potential application of strategies to induce brain conditioning. Since the identification of intrinsic cell-survival pathways should provide more direct opportunities for translational neuroprotection trials, an accurate examination of the different models of preconditioning and postconditioning is mandatory before starting any new project.

Prophylactic Zinc and Therapeutic Selenium Administration Increases the Antioxidant Enzyme Activity in the Rat Temporoparietal Cortex and Improves Memory after a Transient Hypoxia-Ischemia

In the cerebral hypoxia-ischemia rat model, the prophylactic administration of zinc can cause either cytotoxicity or preconditioning effect, whereas the therapeutic administration of selenium decreases the ischemic damage. Herein, we aimed to explore whether supplementation of low doses of prophylactic zinc and therapeutic selenium could protect from a transient hypoxic-ischemic event. We administrated zinc (0.2 mg/kg of body weight; ip) daily for 14 days before a 10 min common carotid artery occlusion (CCAO). After CCAO, we administrated sodium selenite (6 μg/kg of body weight; ip) daily for 7 days. In the temporoparietal cerebral cortex, we determined nitrites by the Griess method and lipid peroxidation by the Gerard-Monnier assay. qPCR was used to measure mRNA of nitric oxide synthases, antioxidant enzymes, chemokines, and their receptors. We measured the enzymatic activity of SOD and GPx and protein levels of chemokines and their receptors by ELISA. We evaluated long-term memory using the Morris-Water maze test. Our results showed that prophylactic administration of zinc caused a preconditioning effect, decreasing nitrosative/oxidative stress and increasing GPx and SOD expression and activity, as well as eNOS expression. The therapeutic administration of selenium maintained this preconditioning effect up to the late phase of hypoxia-ischemia. Ccl2, Ccr2, Cxcl12, and Cxcr4 were upregulated, and long-term memory was improved. Pyknotic cells were decreased suggesting prevention of neuronal cell death. Our results show that the prophylactic zinc and therapeutic selenium administration induces effective neuroprotection in the early and late phases after CCAO.

Hypoxic preconditioning attenuates necroptotic neuronal death induced by global cerebral ischemia via Drp1-dependent signaling pathway mediated by CaMKIIα inactivation in adult rats

Hypoxic preconditioning (HPC) alleviates the selective and delayed neuronal death in the hippocampal CA1 region induced by transient global cerebral ischemia (tGCI). This type of cell death may include different programmed cell death mechanisms, namely, apoptosis and necroptosis. Although apoptotic signaling is well defined, the mechanisms that underlie neuronal necroptosis are yet to be fully elucidated. In this study, we investigated whether HPC protects neurons from cerebral ischemia-induced necroptosis. We observed that tGCI up-regulated the expression of receptor-interacting protein (RIP) 3 and increased the interaction of RIP1-RIP3 in CA1 at the early stage of reperfusion. The pretreatment with HPC or necrostatin-1 decreased the expression of RIP3 and the formation of RIP1-RIP3 after tGCI. We also found that HPC decreased the expression and the activity of caspase-8 in CA1 after tGCI, and notably, the pretreatment with Z-VAD-FMK, a pan-caspase inhibitor, did not trigger necroptosis but attenuated the tGCI-induced neuronal damage. Furthermore, we demonstrated that HPC decreased the activation of calcium-calmodulin kinase (CaMK) IIα and the interaction of RIP1 and CaMKIIα induced by tGCI. Intriguingly, the pretreatment with a CaMKs inhibitor KN-93 before tGCI resulted in significantly reduced RIP1-3 interaction and tGCI-induced neuronal damage. Finally, we ascertained that HPC prevented the dephosphorylation of dynamin-related protein 1 (Drp1)-Ser637 (serine 637) and inhibited the translocation of Drp1 to mitochondria induced by tGCI. Importantly, the treatment with a Drp1 inhibitor Mdivi-1 or necrostatin-1 before tGCI also abolished Drp1 dephosphorylation at Ser637 and mitochondrial translocation. Taken together, our results highlight that HPC attenuates necroptotic neuronal death induced by tGCI via Drp1-dependent mitochondrial signaling pathways mediated by CaMKIIα inactivation.-Zhan, L., Lu, Z., Zhu, X., Xu, W., Li, L., Li, X., Chen, S., Sun, W., Xu, E. Hypoxic preconditioning attenuates necroptotic neuronal death induced by global cerebral ischemia via Drp1-dependent signaling pathway mediated by CaMKIIα inactivation in adult rats.

Brain ischemic preconditioning protects against moderate, not severe, transient global cerebral ischemic injury

Ischemic preconditioning (IPC) in the brain increases ischemic tolerance to subsequent ischemic insults. In this study, we examined whether IPC protects neurons and attenuates microgliosis or not in the hippocampus following severe transient global cerebral ischemia (TCI) in gerbils. Gerbils were assigned to 8 groups; 5- and 15-min sham operated groups, 5-min and 15-min TCI operated groups, IPC plus 5- and 15-min sham operated groups, and IPC plus 5- and 15-min TCI operated groups. IPC was induced by subjecting animals to 2-min transient ischemia 1 day before 5-min TCI for a typical transient ischemia and 15-min TCI for severe transient ischemia. Neuronal damage was examined by cresyl violet staining and Fluoro-Jade B histofluorescence staining. In addition, microglial activation was examined using immunohistochemistry for Iba-1 (a marker for microglia). Delayed neuronal death and microgliosis was found in the CA1 alone in the 5-min TCI operated group at 5 days post-ischemia, and, in the 15-min TCI operated group, neuronal death and microgliosis was shown in all CA areas (CA1-3) and the dentate gyrus. IPC displayed neuroprotection and attenuated microglial activation in the 5-min TCI operated group. However, in the 15-min TCI operated group, IPC did not show neuroprotection and not attenuate microglial activation. Our present findings indicate that IPC hardly protect against severe transient cerebral ischemic injury.

Brain ischemic preconditioning protects against ischemic injury and preserves the blood-brain barrier via oxidative signaling and Nrf2 activation

Brain ischemic preconditioning (IPC) with mild ischemic episodes is well known to protect the brain against subsequent ischemic challenges. However, the underlying mechanisms are poorly understood. Here we demonstrate the critical role of the master redox transcription factor, nuclear factor (erythroid-derived 2)-like 2 (Nrf2), in IPC-mediated neuroprotection and blood-brain barrier (BBB) preservation. We report that IPC causes generation of endogenous lipid electrophiles, including 4-hydroxy-2-nonenal (4-HNE), which release Nrf2 from inhibition by Keap1 (via Keap1-C288) and inhibition by glycogen synthase kinase 3β (via GSK3β-C199). Nrf2 then induces expression of its target genes, including a new target, cadherin 5, a key component of adherens junctions of the BBB. These effects culminate in mitigation of BBB leakage and of neurological deficits after stroke. Collectively, these studies are the first to demonstrate that IPC protects the BBB against ischemic injury by generation of endogenous electrophiles and activation of the Nrf2 pathway through inhibition of Keap1- and GSK3β-dependent Nrf2 degradation.

Enhancing and Extending Biological Performance and Resilience

Human performance, endurance, and resilience have biological limits that are genetically and epigenetically predetermined but perhaps not yet optimized. There are few systematic, rigorous studies on how to raise these limits and reach the true maxima. Achieving this goal might accelerate translation of the theoretical concepts of conditioning, hormesis, and stress adaptation into technological advancements. In 2017, an Air Force-sponsored conference was held at the University of Massachusetts for discipline experts to display data showing that the amplitude and duration of biological performance might be magnified and to discuss whether there might be harmful consequences of exceeding typical maxima. The charge of the workshop was "to examine and discuss and, if possible, recommend approaches to control and exploit endogenous defense mechanisms to enhance the structure and function of biological tissues." The goal of this white paper is to fulfill and extend this workshop charge. First, a few of the established methods to exploit endogenous defense mechanisms are described, based on workshop presentations. Next, the white paper accomplishes the following goals to provide: (1) synthesis and critical analysis of concepts across some of the published work on endogenous defenses, (2) generation of new ideas on augmenting biological performance and resilience, and (3) specific recommendations for researchers to not only examine a wider range of stimulus doses but to also systematically modify the temporal dimension in stimulus inputs (timing, number, frequency, and duration of exposures) and in measurement outputs (interval until assay end point, and lifespan). Thus, a path forward is proposed for researchers hoping to optimize protocols that support human health and longevity, whether in civilians, soldiers, athletes, or the elderly patients. The long-term goal of these specific recommendations is to accelerate the discovery of practical methods to conquer what were once considered intractable constraints on performance maxima.

Tumor necrosis factor receptor 2 is required for ischemic preconditioning-mediated neuroprotection in the hippocampus following a subsequent longer transient cerebral ischemia

Tumor Necrosis Factor-α (TNF-α) is a proinflammatory cytokine implicated in neuronal damage in response to cerebral ischemia. Ischemic preconditioning (IPC) provides neuroprotection against a subsequent severer or longer transient ischemia by ischemic tolerance. Here, we focused on the role of TNF-α in IPC-mediated neuroprotection against neuronal death following a subsequent longer transient cerebral ischemia (TCI). Gerbils used in this study were randomly assigned to eight groups; sham group, TCI operated group, IPC plus (+) sham group, IPC + TCI operated group, sham + etanercept (an inhibitor of TNF-a) group, TCI + etanercept group, IPC + sham + etanercept group, and IPC + TCI + etanercept group. IPC was induced by a 2-min sublethal transient ischemia, which was operated 1 day prior to a longer (5-min) TCI. A significant death of neurons was found in the stratum pyramidale (SP) in the CA1 area (CA1) of the hippocampus 5 days after TCI; however, IPC protected SP neurons from TCI. We found that TNF-α immunoreactivity was significantly increased in CA1 pyramidal neurons in the TCI and IPC + TCI groups compared to the sham group. TNF-R1 expression in CA1 pyramidal neurons of the TCI group was also increased 1 and 2 days after TCI; however, in the IPC + TCI group, TNF-R1 expression was significantly lower than that in the TCI group. On the other hand, we did not detect TNF-R2 immunoreactivity in CA1 pyramidal neurons 1 and 2 days after TCI; meanwhile, in the IPC + TCI group, TNF-R2 expression was significantly increased compared to TNF-R2 expression at 1 and 2 days after TCI. In addition, in this group, TNF-R2 was newly expressed in pericytes, which are important cells in the blood brain barrier, from 1 day after TCI. When we treated etanercept to the IPC + TCI group, IPC-induced neuroprotection was significantly weakened. In brief, this study indicates that IPC confers neuroprotection against TCI by TNF-α signaling through TNF-R2 and suggests that the enhancement of TNF-R2 expression by IPC may be a legitimate strategy for a therapeutic intervention of TCI.

Remote Ischemic Conditioning in Cerebral Diseases and Neurointerventional Procedures: Recent Research Progress

Cerebral ischemia and stroke are increasing in prevalence and are among the leading causes of morbidity and mortality in both developed and developing countries. Despite the progress in endovascular treatment, ischemia/reperfusion (IR) injury is an important contributor to post-surgical mortality and morbidity affecting a wide range of neurointerventional procedures. However, pharmacological recruitment of effective cerebral protective signaling has been largely disappointing to date. In remote ischemic conditioning (RIC), repetitive transient mechanical obstruction of vessels at a limb remote from the IR injury site protects vital organs from IR injury and confers infarction size reduction following prolonged arterial occlusion. Results of pharmacologic agents appear to be species specific, while RIC is based on the neuroprotective influences of phosphorylated protein kinase B, signaling proteins, nitric oxide, and transcriptional activators, the benefits of which have been confirmed in many species. Inducing RIC protection in patients undergoing cerebral vascular surgery or those who are at high risk of brain injury has been the subject of research and has been enacted in clinical settings. Its simplicity and non-invasive nature, as well as the flexibility of the timing of RIC stimulus, also makes it feasible to apply alongside neurointerventional procedures. Furthermore, despite nonuniform RIC protocols, emerging literature demonstrates improved clinical outcomes. The aims of this article are to summarize the potential mechanisms underlying different forms of conditioning, to explore the current translation of this paradigm from laboratory to neurovascular diseases, and to outline applications for patient care.

Role of Phosphatidylinositol-3 Kinase Pathway in NMDA Preconditioning: Different Mechanisms for Seizures and Hippocampal Neuronal Degeneration Induced by Quinolinic Acid

N-methyl D-aspartate (NMDA) preconditioning is evoked by the administration of a subtoxic dose of NMDA and is protective against neuronal excitotoxicity. This effect may involve a diversity of targets and cell signaling cascades associated to neuroprotection. Phosphatidylinositol-3 kinase/protein kinase B (PI3K/Akt) and mitogen-activated protein kinases (MAPKs) such as extracellular regulated protein kinase 1/2 (ERK1/2) and p38^MAPK pathways play a major role in neuroprotective mechanisms. However, their involvement in NMDA preconditioning was not yet fully investigated. The present study aimed to evaluate the effect of NMDA preconditioning on PI3K/Akt, ERK1/2, and p38^MAPK pathways in the hippocampus of mice and characterize the involvement of PI3K on NMDA preconditioning-evoked prevention of seizures and hippocampal cell damage induced by quinolinic acid (QA). Thus, mice received wortmannin (a PI3K inhibitor) and 15 min later a subconvulsant dose of NMDA (preconditioning) or saline. After 24 h of this treatment, an intracerebroventricular QA infusion was administered. Phosphorylation levels and total content of Akt, glycogen synthase protein kinase-3β (GSK-3β), ERK1/2, and p38^MAPK were not altered after 24 h of NMDA preconditioning with or without wortmmanin pretreatment. Moreover, after QA administration, behavioral seizures, hippocampal neuronal degeneration, and Akt activation were evaluated. Inhibition of PI3K pathway was effective in abolishing the protective effect of NMDA preconditioning against QA-induced seizures, but did not modify neuronal protection promoted by preconditioning as evaluated by Fluoro-Jade B staining. The study confirms that PI3K participates in the mechanism of protection induced by NMDA preconditioning against QA-induced seizures. Conversely, NMDA preconditioning-evoked protection against neuronal degeneration is not altered by PI3K signaling pathway inhibition. These results point to differential mechanisms regarding protection against a behavioral and cellular manifestation of neural damage.

Correction to: Neuroimmune Response in Ischemic Preconditioning

Ischemic preconditioning (IPC) is a robust neuroprotective phenomenon in which a brief period of cerebral ischemia confers transient tolerance to subsequent ischemic challenge. Research on IPC has implicated cellular, molecular, and systemic elements of the immune response in this phenomenon. Potent molecular mediators of IPC include innate immune signaling pathways such as Toll-like receptors and type 1 interferons. Brain ischemia results in release of pro- and anti-inflammatory cytokines and chemokines that orchestrate the neuroinflammatory response, resolution of inflammation, and transition to neurological recovery and regeneration. Cellular mediators of IPC include microglia, the resident central nervous system immune cells, astrocytes, and neurons. All of these cell types engage in cross-talk with each other using a multitude of signaling pathways that modulate activation/suppression of each of the other cell types in response to ischemia. As the postischemic neuroimmune response evolves over time there is a shift in function toward provision of trophic support and neuroprotection. Peripheral immune cells infiltrate the central nervous system en masse after stroke and are largely detrimental, with a few subtypes having beneficial, protective effects, though the role of these immune cells in IPC is largely unknown. The role of neural progenitor cells in IPC-mediated neuroprotection is another active area of investigation as is the role of microglial proliferation in this setting. A mechanistic understanding of these molecular and cellular mediators of IPC may not only facilitate more effective direct application of IPC to specific clinical scenarios, but also, more broadly, reveal novel targets for therapeutic intervention in stroke.

Evidence for cross-hemispheric preconditioning in experimental Parkinson's disease

Dopamine loss and motor deficits in Parkinson's disease typically commence unilaterally and remain asymmetric for many years, raising the possibility that endogenous defenses slow the cross-hemispheric transmission of pathology. It is well-established that the biological response to subtoxic stress prepares cells to survive subsequent toxic challenges, a phenomenon known as preconditioning, tolerance, or stress adaptation. Here we demonstrate that unilateral striatal infusions of the oxidative toxicant 6-hydroxydopamine (6-OHDA) precondition the contralateral nigrostriatal pathway against the toxicity of a second 6-OHDA infusion in the opposite hemisphere. 6-OHDA-induced loss of dopaminergic terminals in the contralateral striatum was ablated by cross-hemispheric preconditioning, as shown by two independent markers of the dopaminergic phenotype, each measured by two blinded observers. Similarly, loss of dopaminergic somata in the contralateral substantia nigra was also abolished, according to two blinded measurements. Motor asymmetries in floor landings, forelimb contacts with a wall, and spontaneous turning behavior were consistent with these histological observations. Unilateral 6-OHDA infusions increased phosphorylation of the kinase ERK2 and expression of the antioxidant enzyme CuZn superoxide dismutase in both striata, consistent with our previous mechanistic work showing that these two proteins mediate preconditioning in dopaminergic cells. These findings support the existence of cross-hemispheric preconditioning in Parkinson's disease and suggest that dopaminergic neurons mount impressive natural defenses, despite their reputation as being vulnerable to oxidative injury. If these results generalize to humans, Parkinson's pathology may progress slowly and asymmetrically because exposure to a disease-precipitating insult induces bilateral upregulation of endogenous defenses and elicits cross-hemispheric preconditioning.

STAT3 precedes HIF1α transcriptional responses to oxygen and oxygen and glucose deprivation in human brain pericytes

Brain pericytes are important to maintain vascular integrity of the neurovascular unit under both physiological and ischemic conditions. Ischemic stroke is known to induce an inflammatory and hypoxic response due to the lack of oxygen and glucose in the brain tissue. How this early response to ischemia is molecularly regulated in pericytes is largely unknown and may be of importance for future therapeutic targets. Here we evaluate the transcriptional responses in in vitro cultured human brain pericytes after oxygen and/or glucose deprivation. Hypoxia has been widely known to stabilise the transcription factor hypoxia inducible factor 1-alpha (HIF1α) and mediate the induction of hypoxic transcriptional programs after ischemia. However, we find that the transcription factors Jun Proto-Oncogene (c-JUN), Nuclear Factor Of Kappa Light Polypeptide Gene Enhancer In B-Cells (NFκB) and signal transducer and activator of transcription 3 (STAT3) bind genes regulated after 2hours (hs) of omitted glucose and oxygen before HIF1α. Potent HIF1α responses require 6hs of hypoxia to substantiate transcriptional regulation comparable to either c-JUN or STAT3. Phosphorylated STAT3 protein is at its highest after 5 min of oxygen and glucose (OGD) deprivation, whereas maximum HIF1α stabilisation requires 120 min. We show that STAT3 regulates angiogenic and metabolic pathways before HIF1α, suggesting that HIF1α is not the initiating trans-acting factor in the response of pericytes to ischemia.

Sevoflurane Blocks the Induction of Long-term Potentiation When Present during, but Not When Present Only before, the High-frequency Stimulation

Background

This study tests the hypothesis that sevoflurane blocks long-term potentiation only if it is present during the high-frequency stimulation that induces long-term potentiation.

Methods

Long-term potentiation, an electrophysiologic correlate of memory, was induced by high-frequency stimulation and measured as a persistent increase in the field excitatory postsynaptic potential slope in the CA1 region.

Results

Long-term potentiation was induced in the no sevoflurane group (171 ± 58% vs. 96 ± 11%; n = 13, mean ± SD); when sevoflurane (4%) was present during the high-frequency stimulation, long-term potentiation was blocked (92 ± 22% vs. 99 ± 7%, n = 6). While sevoflurane reduced the size of the field excitatory postsynaptic potential to single test stimuli by 59 ± 17%, it did not significantly reduce the size of the field excitatory postsynaptic potentials during the 100 Hz high-frequency stimulation. If sevoflurane was removed from the artificial cerebrospinal fluid superfusing the slices 10 min before the high-frequency stimulation, then long-term potentiation was induced (185 ± 48%, n = 7); this was not different from long-term potentiation in the no sevoflurane slices (171 ± 58). Sevoflurane before, but not during, ⊖-burst stimulation, a physiologic stimulus, did not block the induction of long-term potentiation (151 ± 37% vs. 161 ± 34%, n = 7).

Conclusions

Sevoflurane blocks long-term potentiation formation if present during the high-frequency stimulation; this blockage of long-term potentiation does not persist if sevoflurane is discontinued before the high-frequency stimulation. These results may explain why short periods of insufficient sevoflurane anesthesia may lead to recall of painful or traumatic events during surgery.

Remote Limb Ischemic Conditioning during Cerebral Ischemia Reduces Infarct Size through Enhanced Collateral Circulation in Murine Focal Cerebral Ischemia

BACKGROUND

Remote ischemic conditioning (RIC) induces protection in focal cerebral ischemia. The conditioning is divided into pre-, per-, and postconditioning. However, the mechanisms of RIC remain unknown.

OBJECTIVES

This study aimed to determine the most effective subtype of RIC. We also examined involvement of collateral circulation on RIC.

METHODS

Transient middle cerebral artery occlusion (MCAO) was performed with nylon sutures in adult C57BL/6 mice under the monitoring of cerebral blood flow (CBF). Fifty mice were divided into 5 groups: MCAO control group, delayed pre-RIC group (RIC 24 hours before MCAO), early pre-RIC group (RIC 5 minutes before MCAO), per-RIC group (RIC during MCAO), and post-RIC group (RIC 5 minutes after MCAO). In other middle cerebral artery (MCA) control and per-RIC groups, collateral circulation was visualized with latex compound perfusion.

RESULTS

After MCAO, CBF was reduced by 80% in all groups. At the end of MCAO, relative increase in CBF in per-RIC group was significantly greater than that in MCA control, whereas the infarct volume in per-RIC group was significantly smaller than that in other groups. The diameter of leptomeningeal anastomosis was larger in the per-RIC group than that in the control group.

CONCLUSIONS

Among the 4 RIC procedures, only the per-RIC group showed clear brain protection. Enhancement of collateral circulation could play a role in the protective effect of per-RIC.

The MDM2-p53 pathway is involved in preconditioning-induced neuronal tolerance to ischemia

Brain preconditioning (PC) refers to a state of transient tolerance against a lethal insult that can be evoked by a prior mild event. It is thought that PC may induce different pathways responsible for neuroprotection, which may involve the attenuation of cell damage pathways, including the apoptotic cell death. In this context, p53 is a stress sensor that accumulates during brain ischemia leading to neuronal death. The murine double minute 2 gene (MDM2), a p53-specific E3 ubiquitin ligase, is the main cellular antagonist of p53, mediating its degradation by the proteasome. Here, we study the role of MDM2-p53 pathway on PC-induced neuroprotection both in cultured neurons (in vitro) and rat brain (in vivo). Our results show that PC increased neuronal MDM2 protein levels, which prevented ischemia-induced p53 stabilization and neuronal death. Indeed, PC attenuated ischemia-induced activation of the p53/PUMA/caspase-3 signaling pathway. Pharmacological inhibition of MDM2-p53 interaction in neurons abrogated PC-induced neuroprotection against ischemia. Finally, the relevance of the MDM2-p53 pathway was confirmed in rat brain using a PC model in vivo. These findings demonstrate the key role of the MDM2-p53 pathway in PC-induced neuroprotection against a subsequent ischemic insult and poses MDM2 as an essential target in ischemic tolerance.

Administration of 5-methoxyindole-2-carboxylic acid that potentially targets mitochondrial dihydrolipoamide dehydrogenase confers cerebral preconditioning against ischemic stroke injury

The objective of this study was to investigate a possible role of mitochondrial dihydrolipoamide dehydrogenase (DLDH) as a chemical preconditioning target for neuroprotection against ischemic injury. We used 5-methoxyindole-2-carboxylic acid (MICA), a reportedly reversible DLDH inhibitor, as the preconditioning agent and administered MICA to rats mainly via dietary intake. Upon completion of 4 week's MICA treatment, rats underwent 1h transient ischemia and 24h reperfusion followed by tissue collection. Our results show that MICA protected the brain against ischemic stroke injury as the infarction volume of the brain from the MICA-treated group was significantly smaller than that from the control group. Data were then collected without or with stroke surgery following MICA feeding. It was found that in the absence of stroke following MICA feeding, DLDH activity was lower in the MICA treated group than in the control group, and this decreased activity could be partly due to DLDH protein sulfenation. Moreover, DLDH inhibition by MICA was also found to upregulate the expression of NAD(P)H-ubiquinone oxidoreductase 1(NQO1) via the Nrf2 signaling pathway. In the presence of stroke following MICA feeding, decreased DLDH activity and increased Nrf2 signaling were also observed along with increased NQO1 activity, decreased oxidative stress, decreased cell death, and increased mitochondrial ATP output. We also found that MICA had a delayed preconditioning effect four weeks post MICA treatment. Our study indicates that administration of MICA confers chemical preconditioning and neuroprotection against ischemic stroke injury.

A Review of Anesthetic Effects on Renal Function: Potential Organ Protection

BACKGROUND

Renal protection is a critical concept for anesthesiologists, nephrologists, and urologists, since anesthesia and renal function are highly interconnected and can potentially interfere with one another. Therefore, a comprehensive understanding of anesthetic drugs and their effects on renal function remains fundamental to the success of renal surgeries, especially transplant procedures. Some experimental studies have shown that some anesthetics provide protection against renal ischemia/reperfusion (IR) injury, but there is limited clinical evidence.

SUMMARY

The effects of anesthetic drugs on renal failure are particularly important in the context of kidney transplantation, since the conditions of preservation following removal profoundly influence the recovery of organ function. Currently, preservation procedures are typically based on the usage of a cold-storage solution. Some anesthetic drugs induce anti-inflammatory, anti-necrotic, and anti-apoptotic effects. A more thorough understanding of anesthetic effects on renal function can present a novel approach for developing organ-protective strategies. The aim of this review is to discuss the effects of different anesthetic drugs on renal function, with particular focus on IR injury. Many studies have demonstrated the organ-protective effects of some anesthetic drugs, specifically propofol, which indicate the potential of some anesthetics to introduce novel organ protective targets. This is not surprising, since lipid emulsions are major components of propofol, which accumulating data show provide organ protective effects against IR injury. Key Messages: Thorough understanding of the interaction between anesthetic drugs and renal function remains fundamental to the delivery of safe perioperative care and to optimizing outcomes after renal surgeries, particularly transplant procedures. Anesthetics can be repurposed for organ protection with more information about their effects, especially during transplant procedures. Here, we review the effects of different anesthetic drugs - specifically those that contain lipids in their structure, with special reference to IR injury.

Effect of Hyperhomocysteinemia on Redox Balance and Redox Defence Enzymes in Ischemia-Reperfusion Injury and/or After Ischemic Preconditioning in Rats

Increased level of homocysteine (hHcy) in plasma is an accompanying phenomenon of many diseases, including a brain stroke. This study determines whether hyperhomocysteinemia (which is a risk factor of brain ischemia) itself or in combination with ischemic preconditioning affects the ischemia-induced neurodegenerative changes, generation of reactive oxygen species (ROS), lipoperoxidation, protein oxidation, and activity of antioxidant enzymes in the rat brain cortex. The hHcy was induced by subcutaneous administration of homocysteine (0.45 μmol/g body weight) twice a day in 8 h intervals for 14 days. Rats were preconditioned by 5 min ischemia. Two days later, 15 min of global forebrain ischemia was induced by four vessel's occlusion. The study demonstrates that in the cerebral cortex, hHcy alone induces progressive neuronal cell death and morphological changes. Neuronal damage was associated with the pro-oxidative effect of hHcy, which leads to increased ROS formation, peroxidation of lipids and oxidative alterations of cortical proteins. Ischemic reperfusion injury activates degeneration processes and de-regulates redox balance which is aggravated under hHcy conditions and leads to the augmented lipoperoxidation and protein oxidation. If combined with hHcy, ischemic preconditioning could preserve the neuronal tissue from lethal ischemic effect and initiates suppression of lipoperoxidation, protein oxidation, and alterations of redox enzymes with the most significant effect observed after prolonged reperfusion. Increased prevalence of hyperhomocysteinemia in the Western population and crucial role of elevated Hcy level in the pathogenesis of neuronal disorders makes this amino acid as an interesting target for future research. Understanding the multiple etiological mechanisms and recognition of the co-morbid risk factors that lead to the ischemic/reperfusion injury and ischemic tolerance is therefore important for developing therapeutic strategies in human brain stroke associated with the elevated level of Hcy.

Integrity of Cerebellar Fastigial Nucleus Intrinsic Neurons Is Critical for the Global Ischemic Preconditioning

Excitation of intrinsic neurons of cerebellar fastigial nucleus (FN) renders brain tolerant to local and global ischemia. This effect reaches a maximum 72 h after the stimulation and lasts over 10 days. Comparable neuroprotection is observed following sublethal global brain ischemia, a phenomenon known as preconditioning. We hypothesized that FN may participate in the mechanisms of ischemic preconditioning as a part of the intrinsic neuroprotective mechanism. To explore potential significance of FN neurons in brain ischemic tolerance we lesioned intrinsic FN neurons with excitotoxin ibotenic acid five days before exposure to 20 min four-vessel occlusion (4-VO) global ischemia while analyzing neuronal damage in Cornu Ammoni area 1 (CA1) hippocampal area one week later. In FN-lesioned animals, loss of CA1 cells was higher by 22% compared to control (phosphate buffered saline (PBS)-injected) animals. Moreover, lesion of FN neurons increased morbidity following global ischemia by 50%. Ablation of FN neurons also reversed salvaging effects of five-minute ischemic preconditioning on CA1 neurons and morbidity, while ablation of cerebellar dentate nucleus neurons did not change effect of ischemic preconditioning. We conclude that FN is an important part of intrinsic neuroprotective system, which participates in ischemic preconditioning and may participate in naturally occurring neuroprotection, such as "diving response".

Renal Protection Mediated by Hypoxia Inducible Factor-1α Depends on Proangiogenesis Function of miR-21 by Targeting Thrombospondin 1

Background

Angiogenesis contributes to the repair process after renal ischemia/reperfusion (I/R) injury. In the present study, we tested the role of miR-21 in the angiogenesis induced by hypoxia inducible factor (HIF)-1α through inhibiting a predicted target gene thrombospondin 1 (TSP-1).

Methods

To stabilize HIF-1α, hypoxia (1% O₂ for 24 hours) was performed in human umbilical vein endothelial cells and cobalt chloride (CoCl₂) was pretreated intraperitoneally 24 hours before renal I/R in mice. Locked nucleic acid modified anti-miR-21 and scrambled control was transfected with hypoxic cells or delivered into the mice via tail vein 1 hour before CoCl₂ injection. The kidneys and blood were collected at 24 hours after reperfusion.

Results

HIF-1α induced by hypoxia and CoCl₂ upregulated vascular endothelial growth factor and miR-21, and increased angiogenesis. It was found that expression of TSP-1 was inversely related with miR-21 in vitro and in vivo. Targeting of TSP-1 by miR-21 was further confirmed in vitro. Furthermore, HIF-1α improved renal function, accompanied with increased angiogenesis after I/R injury in mice. The protective effect of HIF-1α was attenuated by inhibition of miR-21.

Conclusions

HIF-1α induced angiogenesis by upregulating not only vascular endothelial growth factor but also miR-21 via inhibiting a novel target gene TSP-1. Both of them may contribute to the protective effect of HIF-1α on renal I/R injury.

Hypoxia induces HIF-1α which upregulates not only VEGF but also miR-21, and this last one inhibits a novel target gene, thrombospondin 1. Angiogenesis induced by hypoxia depends at least partially on production of VEGF and inhibition of thrombospondin 1 through miR-21.

The prolyl 4-hydroxylase inhibitor GSK360A decreases post-stroke brain injury and sensory, motor, and cognitive behavioral deficits

There is interest in pharmacologic preconditioning for end-organ protection by targeting the HIF system. This can be accomplished by inhibition of prolyl 4-hydroxylase (PHD). GSK360A is an orally active PHD inhibitor that has been previously shown to protect the failing heart. We hypothesized that PHD inhibition can also protect the brain from injuries and resulting behavioral deficits that can occur as a result of surgery. Thus, our goal was to investigate the effect of pre-stroke surgery brain protection using a verified GSK360A PHD inhibition paradigm on post-stroke surgery outcomes. Vehicle or an established protective dose (30 mg/kg, p.o.) of GSK360A was administered to male Sprague-Dawley rats. Initially, GSK360A pharmacokinetics and organ distribution were determined, and then PHD-HIF pharmacodynamic markers were measured (i.e., to validate the pharmacological effects of the GSK360A administration regimen). Results obtained using this validated PHD dose-regimen indicated significant improvement by GSK360A (30mg/kg); administered at 18 and 5 hours prior to transient middle cerebral artery occlusion (stroke). GSK360A exposure and plasma, kidney and brain HIF-PHD pharmacodynamics endpoints (e.g., erythropoietin; EPO and Vascular Endothelial Growth Factor; VEGF) were measured. GSK360A provided rapid exposure in plasma (7734 ng/ml), kidney (45–52% of plasma level) and brain (1–4% of plasma level), and increased kidney EPO mRNA (80-fold) and brain VEGF mRNA (2-fold). We also observed that GSK360A increased plasma EPO (300-fold) and VEGF (2-fold). Further assessments indicated that GSK360A reduced post-stroke surgery neurological deficits (47–64%), cognitive dysfunction (60–75%) and brain infarction (30%) 4 weeks later. Thus, PHD inhibition using GSK360A pretreatment produced long-term post-stroke brain protection and improved behavioral functioning. These data support PHD inhibition, specifically by GSK360A, as a potential strategy for pre-surgical use to reduce brain injury and functional decline due to surgery-related cerebral injury.

TLR5 Activation through NF-κB Is a Neuroprotective Mechanism of Postconditioning after Cerebral Ischemia in Mice

Postconditioning has been shown to protect the mouse brain from ischemic injury. However, the neuroprotective mechanisms of postconditioning remain elusive. We have found that toll-like receptor 5 (TLR5) plays an integral role in postconditioning-induced neuroprotection through Akt/nuclear factor kappa B (NF-κB) activation in cerebral ischemia. Compared to animals that received 30 min of transient middle cerebral artery occlusion (tMCAO) group, animals that also underwent postconditioning showed a significant reduction of up to 60.51% in infarct volume. Postconditioning increased phospho-Akt (p-Akt) levels and NF-κB translocation to the nucleus as early as 1 h after tMCAO and oxygen-glucose deprivation. Furthermore, inhibition of Akt by Akt inhibitor IV decreased NF-κB promoter activity after postconditioning. Immunoprecipitation showed that interactions between TLR5, MyD88, and p-Akt were increased from postconditioning both in vivo and in vitro. Similar to postconditioning, flagellin, an agonist of TLR5, increased NF-κB nuclear translocation and Akt phosphorylation. Our results suggest that postconditioning has neuroprotective effects by activating NF-κB and Akt survival pathways via TLR5 after cerebral ischemia. Additionally, the TLR5 agonist flagellin can simulate the neuroprotective mechanism of postconditioning in cerebral ischemia.

Global ablation of the mitochondrial calcium uniporter increases glycolysis in cortical neurons subjected to energetic stressors

The effects of global mitochondrial calcium (Ca²⁺) uniporter (MCU) deficiency on hypoxic-ischemic (HI) brain injury, neuronal Ca²⁺ handling, bioenergetics and hypoxic preconditioning (HPC) were examined. Forebrain mitochondria isolated from global MCU nulls displayed markedly reduced Ca²⁺ uptake and Ca²⁺-induced opening of the membrane permeability transition pore. Despite evidence that these effects should be neuroprotective, global MCU nulls and wild-type (WT) mice suffered comparable HI brain damage. Energetic stress enhanced glycolysis and depressed Complex I activity in global MCU null, relative to WT, cortical neurons. HI reduced forebrain NADH levels more in global MCU nulls than WT mice suggesting that increased glycolytic consumption of NADH suppressed Complex I activity. Compared to WT neurons, pyruvate dehydrogenase (PDH) was hyper-phosphorylated in MCU nulls at several sites that lower the supply of substrates for the tricarboxylic acid cycle. Elevation of cytosolic Ca²⁺ with glutamate or ionomycin decreased PDH phosphorylation in MCU null neurons suggesting the use of alternative mitochondrial Ca²⁺ transport. Under basal conditions, global MCU nulls showed similar increases of Ca²⁺ handling genes in the hippocampus as WT mice subjected to HPC. We propose that long-term adaptations, common to HPC, in global MCU nulls compromise resistance to HI brain injury and disrupt HPC.

Prior transient ischemic attacks may have a neuroprotective effect in patients with ischemic stroke

INTRODUCTION

Although functional recovery and survival after ischemic infarction seem to improve in patients with prior transient ischemic attack (TIA), little is known about the role of characteristics of prior TIA in subsequent cerebral infarction. Thus, the objective of this study was to explore how the characteristics of prior TIA have a neuroprotective effect on patients with ischemic stroke.

MATERIAL AND METHODS

A total of 221 patients admitted consecutively to a primary care center for first-ever ischemic stroke were divided into two groups on the basis of the presence or absence of prior TIAs. The initial NIHSS modified Rankin Scale was used to measure the severity and disability after the stroke. Subgroups were based on the TIA duration (< 10 min, 10 to 60 min, and > 60 min), TIA frequency (1 time, 2-3 times, more than 3 times), and the interval of stroke (< 1 week, 1-4 weeks, > 4 weeks). The severity of the neurologic picture on admission and functional disability after stroke were compared between patients with and without TIAs and subgroups as well.

RESULTS

A total of 132 (59.73%) of the 221 patients had prior TIAs before stroke. Risk factors and the initial clinical picture did not differ between patients with or without TIAs. Patients with prior TIA had a more favorable outcome than those without TIA (59.09% vs. 43.82%), and a significant difference between the two groups was observed (χ² = 4.976, p = 0.026). Furthermore, neurological outcome in patients with prior TIA lasting for 60 min, less than 3 times and shorter intervals within 4 weeks was significantly different from that in the non-TIA group (p < 0.05).

CONCLUSIONS

Prior transient ischemic attacks may have a neuroprotective effect on the subsequent ischemic stroke, and this effect might be affected by the characteristics of TIAs. Patients with TIAs of low frequency, short duration and short interval are considered to have better neurological outcomes.

Neurological outcomes of patients with history of obstructive sleep apnea after a cardiac arrest

BACKGROUND

Cardiac arrest survivors may have disabilities due to hypoxic brain injury. Patients with obstructive sleep apnea are exposed to intermittent hypoxemia that may lead to ischemic preconditioning. We have hypothesized that patients with obstructive sleep apnea have better neurological outcomes following a cardiac arrest due to preconditioning of the brain.

METHODS

We retrospectively analyzed all the survivors of in-hospital cardiac arrest from January 2006 to September 2016. Patients with confirmed or suspected obstructive sleep apnea were selected for further analysis and those without were used as comparison. Primary outcome was neurological functionality on hospital discharge by the Cerebral Performance Category.

RESULTS

A total of 739 patients had cardiac arrest within the study period. The immediate mortality rate was 59% (N=43) in patients with obstructive sleep apnea and 94% (N=623) in those without (p<0.001). Approximately 10% (N=73) were discharged alive and these were selected for further analysis. Patients without obstructive sleep apnea had more frequently "Poor" outcomes compared to those with obstructive sleep apnea (OR 2.91; 95% CI, 1.11-7.66; p=0.03). After adjusting in a multivariate analysis, obstructive sleep apnea was "protective" of "Poor" neurological outcomes: adjusted OR 0.21; 95% CI, 0.06-0.64; p=0.01.

CONCLUSION

Patients with obstructive sleep apnea had better unadjusted survival rates, and favorable adjusted neurological outcomes at discharge compared to those without obstructive sleep apnea. These results suggest that obstructive sleep apnea patients may tolerate better acute brain ischemia due to preconditioning.

Hypoxic conditioning and the central nervous system: A new therapeutic opportunity for brain and spinal cord injuries?

Central nervous system diseases are among the most disabling in the world. Neuroprotection and brain recovery from either acute or chronic neurodegeneration still represent a challenge in neurology and neurorehabilitation as pharmacology treatments are often insufficiently effective. Conditioning the central nervous system has been proposed as a potential non-pharmacological neuro-therapeutic. Conditioning refers to a procedure by which a potentially deleterious stimulus is applied near to but below the threshold of damage to the organism to increase resistance to the same or even different noxious stimuli given above the threshold of damage. Hypoxic conditioning has been investigated in several cellular and preclinical models and is now recognized as inducing endogenous mechanisms of neuroprotection. Ischemic, traumatic, or chronic neurodegenerative diseases can benefit from hypoxic conditioning strategies aiming at preventing the deleterious consequences or reducing the severity of the pathological condition (preconditioning) or aiming at inducing neuroplasticity and recovery (postconditioning) following central nervous system injury. Hypoxic conditioning can consist in single (sustained) or cyclical (intermittent, interspersed by short period of normoxia) hypoxia stimuli which duration range from few minutes to several hours and that can be repeated over several days or weeks. This mini-review addresses the existing evidence regarding the use of hypoxic conditioning as a potential innovating neuro-therapeutic modality to induce neuroprotection, neuroplasticity and brain recovery. This mini-review also emphasizes issues which remain to be clarified and future researches to be performed in the field. Impact statement Neuroprotection and brain recovery from either acute or chronic neurodegeneration still represent a challenge in neurology and neurorehabilitation. Hypoxic conditioning may represent a harmless and efficient non-pharmacological new therapeutic modality in the field of neuroprotection and neuroplasticity, as supported by many preclinical data. Animal studies provide clear evidence for neuroprotection and neuroplasticity induced by hypoxic conditioning in several models of neurological disorders. These studies show improved functional outcomes when hypoxic conditioning is applied and provides important information to translate this intervention to clinical practice. Some studies in humans provide encouraging data regarding the tolerance and therapeutic effects of hypoxic conditioning strategies. The main issues to address in future research include the definition of the appropriate hypoxic dose and pattern of exposure, the determination of relevant physiological biomarkers to assess the effects of the treatment and the evaluation of combined strategies involving hypoxic conditioning and other pharmacological or non-pharmacological treatments.

The role of Toll-like receptor 4 (TLR4) in cardiac ischaemic-reperfusion injury, cardioprotection and preconditioning

Cardiac ischaemic-reperfusion injury (IRI) remains the primary cause of mortality throughout the developed world. Molecular mechanisms underlying IRI are complex and are often interlinked with each other driving a synergistic response. Toll-like receptor 4 (TLR4), an immunosurveillance receptor, is known to enhance tissue injury during IRI by enhancing the inflammatory response. The release of endogenous components during IRI bind onto TLR4 leading to the activation of multiple signalling kinases. Once this event occurs these proteins are defined as danger associated molecular patterns molecules (DAMPs) or alarmins. Examples include heat shock proteins, high mobility group box one (HMGB1) and extracellular matrix proteins, all of which are involved in IRI. However, literature in the last two decades suggests that transient stimulation of TLR4 may suppress IRI and thus improve cardiac recovery. Furthermore, it remains to be seen what role TLR4 plays during ischaemic-preconditioning where acute bouts of ischaemia, preceding a harmful bout of ischaemic-reperfusion, is cardioprotective. The other question which also needs to be considered is that if transient TLR4 signalling drives a preconditioning response then what are the ligands which drive this? Hence the second part of this review explores the possible TLR4 ligands which may promote cardioprotection against IRI.

Effects of ischemic preconditioning on PDGF-BB expression in the gerbil hippocampal CA1 region following transient cerebral ischemia

Ischemic preconditioning (IPC) is induced by exposure to brief durations of transient ischemia, which results in ischemic tolerance to a subsequent longer or lethal period of ischemia. In the present study, the effects of IPC (2 min of transient cerebral ischemia) were examined on immunoreactivity of platelet‑derived growth factor (PDGF)‑BB and on neuroprotection in the gerbil hippocampal CA1 region following lethal transient cerebral ischemia (LTCI; 5 min of transient cerebral ischemia). IPC was subjected to a 2‑min sublethal ischemia and a LTCI was given 5‑min transient ischemia. The animals in all of the groups were given recovery times of 1, 2 and 5 days and change in PDGF‑BB immunoreactivity was examined as was the neuronal damage/death in the hippocampus induced by LTCI. LTCI induced a significant loss of pyramidal neurons in the hippocampal CA1 region 5 days after LTCI, and significantly decreased PDGF‑BB immunoreactivity in the CA1 pyramidal neurons from day 1 after LTCI. Conversely, IPC effectively protected the CA1 pyramidal neurons from LTCI and increased PDGF‑BB immunoreactivity in the CA1 pyramidal neurons post‑LTCI. In conclusion, the results demonstrated that LTCI significantly altered PDGF‑BB immunoreactivity in pyramidal neurons in the hippocampal CA1 region, whereas IPC increased the immunoreactivity. These findings indicated that PDGF‑BB may be associated with IPC‑mediated neuroprotection.

Vulnerability to a Metabolic Challenge Following Perinatal Asphyxia Evaluated by Organotypic Cultures: Neonatal Nicotinamide Treatment

The hypothesis of enhanced vulnerability following perinatal asphyxia was investigated with a protocol combining in vivo and in vitro experiments. Asphyxia-exposed (AS) (by 21 min water immersion of foetuses containing uterine horns) and caesarean-delivered control (CS) rat neonates were used at P2-3 for preparing triple organotypic cultures (substantia nigra, neostriatum and neocortex). At DIV 18, cultures were exposed to different concentrations of H₂O₂ (0.25-45 mM), added to the culture medium for 18 h. After a 48-h recovery period, the cultures were either assessed for cell viability or for neurochemical phenotype by confocal microscopy. Energy metabolism (ADP/ATP ratio), oxidative stress (GSH/GSSG) and a modified ferric reducing/antioxidant power assay were applied to homogenates of parallel culture series. In CS cultures, the number of dying cells was similar in substantia nigra, neostriatum and neocortex, but it was several times increased in AS cultures evaluated under the same conditions. A H₂O₂ challenge led to a concentration-dependent increase in cell death (>fourfold after 0.25 mM of H₂O₂) in CS cultures. In AS cultures, a significant increase in cell death was only observed after 0.5 mM of H₂O₂. At higher than 1 mM of H₂O₂ (up to 45 mM), cell death increased several times in all cultures, but the effect was still more prominent in CS than in AS cultures. The cell phenotype of dying/alive cells was investigated in formalin-fixed cultures exposed to 0 or 1 mM of H₂O₂, co-labelling for TUNEL (apoptosis), MAP-2 (neuronal phenotype), GFAP (astroglial phenotype) and TH (tyrosine hydroxylase; for dopamine phenotype), counterstaining for DAPI (nuclear staining), also evaluating the effect of a single dose of nicotinamide (0.8 nmol/kg, i.p. injected in 100 μL, 60 min after delivery). Perinatal asphyxia produced a significant increase in the number of DAPI/TUNEL cells/mm³, in substantia nigra and neostriatum. One millimolar of H₂0₂ increased the number of DAPI/TUNEL cells/mm³ by ≈twofold in all regions of CS and AS cultures, an effect that was prevented by neonatal nicotinamide treatment. In substantia nigra, the number of MAP-2/TH-positive cells/mm³ was decreased in AS compared to CS cultures, also by 1 mM of H₂0₂, both in CS and AS cultures, prevented by nicotinamide. In agreement, the number of MAP-2/TUNEL-positive cells/mm³ was increased by 1 mM H₂O₂, both in CS (twofold) and AS (threefold) cultures, prevented by nicotinamide. The number of MAP-2/TH/TUNEL-positive cells/mm³ was only increased in CS (>threefold), but not in AS (1.3-fold) cultures. No TH labelling was observed in neostriatum, but 1 mM of H₂O₂ produced a strong increase in the number of MAP-2/TUNEL-positive cells/mm³, both in CS (>2.9-fold) and AS (>fourfold), decreased by nicotinamide. In neocortex, H₂O₂ increased the number of MAP-2/TUNEL-positive cells/mm³, both in CS and AS cultures (≈threefold), decreased by nicotinamide. The ADP/ATP ratio was increased in AS culture homogenates (>sixfold), compared to CS homogenates, increased by 1 mM of H₂0₂, both in CS and AS homogenates. The GSH/GSSG ratio was significantly decreased in AS, compared to CS cultures. One millimolar of H₂O₂ decreased that ratio in CS and AS homogenates. The present results demonstrate that perinatal asphyxia induces long-term changes in metabolic pathways related to energy and oxidative stress, priming cell vulnerability with both neuronal and glial phenotype. The observed effects were region dependent, being the substantia nigra particularly prone to cell death. Nicotinamide administration in vivo prevented the deleterious effects observed after perinatal asphyxia in vitro, a suitable pharmacological strategy against the deleterious consequences of perinatal asphyxia.

Remote ischemic postconditioning confers neuroprotective effects via inhibition of the BID-mediated mitochondrial apoptotic pathway

Ischemic postconditioning has been demonstrated to alleviate brain ischemia/reperfusion-induced neuronal apoptosis; however, the protective mechanisms underlying the improved and more convenient method of remote ischemic postconditioning (RIPostC) are only recently beginning to be elucidated. Mitochondria are important in the regulation of cell apoptosis, and the B-cell lymphoma 2 (Bcl-2) homology 3 interacting-domain death agonist (BID) promotes the insertion/oligomerization of Bcl-2-associated X protein into the mitochondrial outer membrane, leading to the release of proapoptotic proteins from the mitochondria. The present study hypothesized that RIPostC targets the BID-mediated mitochondrial apoptotic pathway to exert neuroprotective effects, and the optimal time window for RIPostC application was investigated. RIPostC was conducted as follows: Three 10-min cycles of bilateral femoral artery occlusion with intervals of 10 min reperfusion after 0, 10 or 30 min of brain reperfusion. The results revealed that reperfusion induced significant activation of BID, via proteolytic cleavage and translocation to the mitochondria, as determined using western blot analysis and immunofluorescence staining. Mitochondrial release of cytochrome c was additionally detected during BID activation, all of which were inhibited by the application of RIPostC. When RIPostC was applied during reperfusion, it demonstrated a significant protective effect. Furthermore, the infarct volume, neurological function and the degree of neuronal apoptosis were improved with application of RIPostC. These results suggested that the protective mechanisms of RIPostC may be associated with inhibition of the BID-mediated mitochondrial apoptotic pathway, which may act as a potential molecular target for therapeutic intervention in the future.

Microglial Interferon Signaling and White Matter

Microglia, the resident immune cells of the CNS, are primary regulators of the neuroimmune response to injury. Type I interferons (IFNs), including the IFNαs and IFNβ, are key cytokines in the innate immune system. Their activity is implicated in the regulation of microglial function both during development and in response to neuroinflammation, ischemia, and neurodegeneration. Data from numerous studies in multiple sclerosis (MS) and stroke suggest that type I IFNs can modulate the microglial phenotype, influence the overall neuroimmune milieu, regulate phagocytosis, and affect blood-brain barrier integrity. All of these IFN-induced effects result in numerous downstream consequences on white matter pathology and microglial reactivity. Dysregulation of IFN signaling in mouse models with genetic deficiency in ubiquitin specific protease 18 (USP18) leads to a severe neurological phenotype and neuropathological changes that include white matter microgliosis and pro-inflammatory gene expression in dystrophic microglia. A class of genetic disorders in humans, referred to as pseudo-TORCH syndrome (PTS) for the clinical resemblance to infection-induced TORCH syndrome, also show dysregulation of IFN signaling, which leads to severe neurological developmental disease. In these disorders, the excessive activation of IFN signaling during CNS development results in a destructive interferonopathy with similar induction of microglial dysfunction as seen in USP18 deficient mice. Other recent studies implicate "microgliopathies" more broadly in neurological disorders including Alzheimer's disease (AD) and MS, suggesting that microglia are a potential therapeutic target for disease prevention and/or treatment, with interferon signaling playing a key role in regulating the microglial phenotype.