A critical review of mechanisms regulating remote preconditioning-induced brain protection

Preparing the spinal cord - priming or preconditioning? A systematic review of experimental studies

Objectives. Paraplegia is devastating complication associated with thoracic and thoracoabdominal aortic aneurysm repair. Vast evidence has been gathered on pre-, peri- and postoperative protective adjuncts aiming to minimize spinal cord ischemia. This review focuses on the pretreatment phase of open surgical or endovascular aortic procedures and gathers the experimental data on the interventional preconditioning and priming methods that increase the spinal cord ischemic tolerance. Design. By the start of March 2021, a systematic review was performed in PubMed, Scopus and Web of Science core collection to identify the articles that reported (i) either an ischemic preconditioning, remote ischemic preconditioning or priming method prior to (ii) experimental spinal cord ischemia performed in endovascular or open surgical fashion mimicking either thoracic, abdominal or thoracoabdominal aortic aneurysm procedures. (iii) The outcomes were reported via neurological, motor-evoked potential, somatosensory-evoked potential, histopathological, immunohistochemical, physiological analysis, or in different combinations of these measurements. Results. The search yielded 7802 articles, and 57 articles were included in the systematic review. The articles were assessed by the evaluated species, the utilized pretreatment, the measured protective effects, and the suggested underlying mechanisms. Conclusions. The reviewed articles showed several possible mechanisms in ischemic and remote ischemic preconditioning for prevention of spinal cord ischemia. The main suggested method for priming was arteriogenetic stimulus. Future studies should confirm these hints of arteriogenetic stimulus with more precise quantification of the protective recruitment process.

Adaptation to oxidative stress at cellular and tissue level

Several in vitro and in vivo investigations have already proved that cells and tissues, when pre-exposed to low oxidative stress by different stimuli such as chemical, physical agents and environmental factors, display more resistance against subsequent stronger ischaemic injuries, resulting in an adaptive response known as ischaemic preconditioning (IPC). The aim of this review is to report the most recent knowledge about the complex adaptive mechanisms, including signalling transduction pathways, antioxidant systems, apoptotic and inflammation pathways, underlying cell protection against oxidative damage. In addition, an update about in vivo adaptation strategies in response to ischaemic/reperfusion episodes and brain trauma is also given.

A clinical perspective on adipose-derived cell therapy for enhancing microvascular health and function: Implications and applications for reconstructive surgery

Restoration of form and function requires apposition of tissues in the form of flaps to reconstitute local perfusion. Successful reconstruction relies on flap survival and its integration with the recipient bed. The flap's precariously perfused hypoxic areas undergo adaptive microvascular changes both internally and in connection with the recipient bed. A cell-mediated, coordinated response to hypoxia drives these adaptive processes, restoring a tissue's normoxic homeostasis via de novo vasculogenesis, sprouting angiogenesis, and stabilizing arterialization. As cells exert prolonged and coordinated effects on site, their use as biological agents merit translational consideration of sourcing angio-competent cells and delivering them to territories enduring microcirculatory acclimatization. Angio-competent cells abound in adipose tissue: a reliable, accessible, and expendable source of adipose-derived cells (ADC). When subject to enzymatic digestion and centrifugation, adipose tissue separates its various ADC: A subset of buoyant oil-dense adipocytes (the tissue's parenchymal component) accumulates on a supra-natant layer, whereas the mesenchymal component remains in the infra-natant sediment, containing the tissue's stromal vascular fraction (SVF), where angio-component cells abound. The SVF can be further manipulated, selected, or culture expanded into more specific stromal subsets (herein defined as adipose stromal cells, ASC). While promising clinical applications for ADC await clinical proof and regulatory authorization, basic science investigation is needed to elucidate the specific ADC mechanisms that influence microvascular growth, remodeling, and function following flap surgery. The objective of this article is to share the clinical perspectives of reconstructive plastic surgeons regarding the use of ADC-based therapies to help with flap tissue integration, revascularization, and wound healing. Specifically, the focus will be on considering the potential for ADC as therapeutic agents and how their clinical application motivates basic science opportunities.

Integrative Analysis Reveals the Landscape of Hypoxia-Inducible Factor (HIF) Family Genes in Pan-Cancer

Inside the cancer microenvironment, reduced O₂ concentration, termed as hypoxia, is a common phenotype and leads to cancer progression. However, little is known about how and when those HIF members are dysregulated in distinct cancers. Here, by integrating a full range of data of thousands of patients, we comprehensively analyzed the genetics, epigenetics, and transcriptomic level of HIF genes and further defined pathways triggered by disrupted hypoxia-inducible factors. We reveal the expression landscape of HIF family genes and further demonstrate that copy number variations underlie such dysregulation. Further analysis indicates that HIF genes associate with cancer hallmarks such as cell cycle and DNA damage response. Drug resistance analysis showed that HIF globally impacts drug effectiveness such as docetaxel. In summary, the overall analysis reveals the landscape of HIF genes in pan-cancer and may assist mechanism research about hypoxia.

Characterizing Ischaemic Tolerance in Rat Pheochromocytoma (PC12) Cells and Primary Rat Neurons

Preconditioning tissue with sublethal ischaemia or hypoxia can confer tolerance (protection) against subsequent ischaemic challenge. In vitro ischaemic preconditioning (IPC) is typically achieved through oxygen glucose deprivation (OGD), whereas hypoxic preconditioning (HPC) involves oxygen deprivation (OD) alone. Here, we report the effects of preconditioning of OGD, OD or glucose deprivation (GD) in ischaemic tolerance models with PC12 cells and primary rat neurons. PC12 cells preconditioned (4 h) with GD or OGD, but not OD, prior to reperfusion (24 h) then ischaemic challenge (OGD 6 h), showed greater mitochondrial activity, reduced cytotoxicity and decreased apoptosis, compared to sham preconditioned PC12 cells. Furthermore, 4 h preconditioning with reduced glucose (0.565 g/L, reduced from 4.5 g/L) conferred protective effects, but not for higher concentrations (1.125 or 2.25 g/L). Preconditioning (4 h) with OGD, but not OD or GD, induced stabilization of hypoxia inducible factor 1α (HIF1α) and upregulation of HIF1 downstream genes (Vegf, Glut1, Pfkfb3 and Ldha). In primary rat neurons, only OGD preconditioning (4 h) conferred neuroprotection. OGD preconditioning (4 h) induced stabilization of HIF1α and upregulation of HIF1 downstream genes (Vegf, Phd2 and Bnip3). In conclusion, OGD preconditioning (4 h) followed by 24 h reperfusion induced ischaemic tolerance (against OGD, 6 h) in both PC12 cells and primary rat neurons. The OGD preconditioning protection is associated with HIF1α stabilization and upregulation of HIF1 downstream gene expression. GD preconditioning (4 h) leads to protection in PC12 cells, but not in neurons. This GD preconditioning-induced protection was not associated with HIF1α stabilization.

Donepezil attenuates injury following ischaemic stroke by stimulation of neurogenesis, angiogenesis, and inhibition of inflammation and apoptosis

Donepezil has proven to be an effective drug to reduce neuronal death and subsequently injury in neurodegenerative diseases. The current study evaluated the neuroprotective effects of donepezil in a rat model of ischaemic stroke and explored possible mechanisms which by this drug may reduce cell death. Temporary middle cerebral artery occlusion (tMCAO) was exerted for 45 min to induce ischaemic stroke. The animals were assigned into five groups: sham, control, and three groups treated with different doses of donepezil. Donepezil was intraperitoneally (IP) injected 4 h after reperfusion for 10 consecutive days. Infarct size was determined using TTC staining. The expression of proteins was evaluated using immunohistochemistry assays. Compared with the control group, infarct size was significantly reduced in tMCAO rats treated with different doses of donepezil. Moreover, our results showed significant decreased expression levels of apoptotic markers and pro-inflammatory mediators after treatment with different doses of donepezil for 10 days (P < 0.05). Likewise, significant increase of brain-derived neurotrophic factor (BDNF) and peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) proteins were found in tMCAO rats treated with donepezil compared with the control group (P < 0.05). Collectively, our findings show the validity of donepezil as a new therapeutic agent for attenuation of injury following ischaemic stroke through attenuation of inflammation and improvement of mitochondrial function, neurogenesis, and angiogenesis.

Two episodes of remote ischemia preconditioning improve motor and sensory function of hind limbs after spinal cord ischemic injury

Objectives: To investigate the effect of one and two remote ischemia preconditioning episodes (1-RIPC or 2-RIPC, respectively) on neuro-protection after spinal cord ischemic injury (SCI) in rats. Design: Experimental animal study. Setting: College of Medicine, King Khalid University, Abha, KSA. Interventions: Male rats (n = 10/group) were divided into control, sham, SCIRI, 1-RIPC + SCIRI, and 2-RIPC + SCIRI. SCI was induced by aortic ligation for 45 min and each RIPC episode was induced by 3 cycles of 10 min ischemia/10 min perfusion. The two preconditioning procedures were separated by 24 h. Outcome measures: after 48 h of RIPC procedure, Tarlov's test, withdrawal from the painful stimulus and placing/stepping reflex (SPR) were used to evaluate the hind limbs neurological function. SC homogenates were used to measure various biochemical parameters. Results: Motor and sensory function of hind limbs were significantly improved and levels of MDA, AOPPs, PGE2, TNF-α, and IL-6, as well as the activity of SOD, was significantly decreased in SC tissue in either 1 or 2 episodes of RIPC intervention. Concomitantly, levels of total nitrate/nitrite and eNOS activity were significantly increased in both groups. Interestingly, except for activity of SOD, eNOS and levels of nitrate/nitrite, the improvements in all neurological biochemical endpoint were more profound in 2-RIPC + SCIRI compared with 1-RIPC + SCIRI. Conclusion: applying two preconditioning episodes of 3 cycles of 10 min ischemia/10 min perfusion, separated by 24 h, boost the neuro-protection effect of RIPC maneuver in rats after ischemic induced SCI in rats.

Genetically Encoded Tools for Research of Cell Signaling and Metabolism under Brain Hypoxia

Hypoxia is characterized by low oxygen content in the tissues. The central nervous system (CNS) is highly vulnerable to a lack of oxygen. Prolonged hypoxia leads to the death of brain cells, which underlies the development of many pathological conditions. Despite the relevance of the topic, different approaches used to study the molecular mechanisms of hypoxia have many limitations. One promising lead is the use of various genetically encoded tools that allow for the observation of intracellular parameters in living systems. In the first part of this review, we provide the classification of oxygen/hypoxia reporters as well as describe other genetically encoded reporters for various metabolic and redox parameters that could be implemented in hypoxia studies. In the second part, we discuss the advantages and disadvantages of the primary hypoxia model systems and highlight inspiring examples of research in which these experimental settings were combined with genetically encoded reporters.

Hypoxia-Inducible Factor 1α and 2α Have Beneficial Effects in Remote Ischemic Preconditioning Against Stroke by Modulating Inflammatory Responses in Aged Rats

Limb remote ischemic preconditioning (RIPC) has been proven to alleviate stroke injury in young rats, but its protective effect and its mechanism in aged rats are still unclear. Hypoxia-inducible factor (HIF) is one of the important markers of stroke, and its high expression plays an important role in the pathogenesis of stroke. In this study, we tested the hypothesis that RIPC could regulate the expression of HIF, leading to reduced inflammatory responses in aged rats. Stroke was induced by transient middle cerebral artery occlusion (MCAo) in aged rats, and RIPC was conducted in both hind limbs. The HIF-1α and HIF-2α mRNA and protein were examined by real-time RT-PCR and western blotting (WB). Inflammatory cytokines in the peripheral blood and brain were measured using AimPlex multiplex immunoassays. The protein levels of p-Akt, Akt, p-ERK, and ERK were examined by WB. We investigated that RIPC reduced the infarct size, improved neurological functions, and decreased the expression of HIF-1α and HIF-2α in the ischemic brain. RIPC reduced the levels of IL-1β, IL-6 and IFN-γ in the peripheral blood and the levels of IL-1β and IFN-γ in the ischemic brain 48 h post-stroke. Moreover, intraperitoneal injection of the HIF inhibitor, acriflavine hydrochloride (ACF), abolished the protection of RIPC with respect to infarct size and neurological functions and neutralized the downregulation of pro-inflammatory IL-1β, IL-6 and IFN-γ. ACF also reversed the activation of the Akt signaling pathway induced by RIPC following stroke. HIF may play a key role in RIPC, which was likely mediated by the Akt signaling pathway and systemic modulation of the inflammatory response in aged rats.

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.

Ischaemic and hypoxic conditioning: potential for protection of vital organs

NEW FINDINGS

What is the topic of this review? Remote ischaemic preconditioning (RIPC) and hypoxic preconditioning as novel therapeutic approaches for cardiac and neuroprotection. What advances does it highlight? There is improved understanding of mechanisms and signalling pathways associated with ischaemic and hypoxic preconditioning, and potential pitfalls with application of these therapies to clinical trials have been identified. Novel adaptations of preconditioning paradigms have also been developed, including intermittent hypoxia training, RIPC training and RIPC-exercise, extending their utility to chronic settings.

ABSTRACT

Myocardial infarction and stroke remain leading causes of death worldwide, despite extensive resources directed towards developing effective treatments. In this Symposium Report we highlight the potential applications of intermittent ischaemic and hypoxic conditioning protocols to combat the deleterious consequences of heart and brain ischaemia. Insights into mechanisms underlying the protective effects of intermittent hypoxia training are discussed, including the activation of hypoxia-inducible factor-1 and Nrf2 transcription factors, synthesis of antioxidant and ATP-generating enzymes, and a shift in microglia from pro- to anti-inflammatory phenotypes. Although there is little argument regarding the efficacy of remote ischaemic preconditioning (RIPC) in pre-clinical models, this strategy has not consistently translated into the clinical arena. This lack of translation may be related to the patient populations targeted thus far, and the anaesthetic regimen used in two of the major RIPC clinical trials. Additionally, we do not fully understand the mechanism through which RIPC protects the vital organs, and co-morbidities (e.g. hypercholesterolemia, diabetes) may interfere with its efficacy. Finally, novel adaptations have been made to extend RIPC to more chronic settings. One adaptation is RIPC-exercise (RIPC-X), an innovative paradigm that applies cyclical RIPC to blood flow restriction exercise (BFRE). Recent findings suggest that this novel exercise modality attenuates the exaggerated haemodynamic responses that may limit the use of conventional BFRE in some clinical settings. Collectively, intermittent ischaemic and hypoxic conditioning paradigms remain an exciting frontier for the protection against ischaemic injuries.

Splenic responses play an important role in remote ischemic preconditioning-mediated neuroprotection against stroke

Background

Remote ischemic preconditioning (RIPC) of a limb has been reported to protect against ischemic stroke. Our previous results demonstrated that the RIPC-mediated neuroprotection is associated with alterations in circulating immune cell populations. Here, we evaluated the effect of the spleen, the largest reservoir of immune cells, on RIPC-mediated neuroprotection against stroke.

Methods

Noninvasive RIPC was achieved by four repeated cycles of 5-min blood flow constriction in the hindlimbs using a tourniquet. The blood and spleens were collected before and 1 h and 3 days after preconditioning to analyze the effect of RIPC on the spleen and the correlation between splenic and peripheral lymphocytes. Moreover, spleen weight and splenic lymphocytes were compared in stroke rats with or without RIPC. Finally, splenectomy was made 1 day or 2 weeks before RIPC and 90-min middle cerebral artery occlusion (MCAO). The infarct areas and deficits were assessed. Blood was collected 1 h after RIPC and 3 days after MCAO to explore the impact of splenectomy on RIPC-induced neuroprotection and immune changes. The contralateral and ipsilateral hemispheres were collected 3 days after MCAO to detect the infiltration of immune cells after RIPC and splenectomy.

Results

Flow cytometry analysis demonstrated that the RIPC promptly increased the percentages of CD3⁺CD8⁺ cytotoxic T (Tc) cells in the spleen with a relatively delayed elevation in CD3⁺CD161⁺ natural killer T (NKT) and CD3⁻CD45RA⁺ B lymphocytes. The percentages of circulating lymphocytes are positively correlated with the percentages of splenic lymphocytes in normal rats. Interestingly, RIPC resulted in negative correlations between the percentages of splenic and circulating T lymphocytes, while the correlation between splenic and circulating B lymphocytes remained positive. For animals subjected to RIPC followed by MCAO, RIPC increased splenic volume with an expansion of splenic lymphocytes 3 days after MCAO. Furthermore, the removal of the spleen 1 day or 2 weeks before RIPC and MCAO reduced the protective effect of RIPC on ischemic brain injury and reversed the effects of RIPC on circulating immune cell composition. RIPC significantly reduced brain infiltration of Tc and NKT cells. Prior splenectomy showed no effect on immune cell infiltration after RIPC and stroke.

Conclusion

These results reveal an immunomodulatory effect of the spleen, effecting mainly the spleen-derived lymphocytes, during RIPC-afforded neuroprotection against cerebral ischemia.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1190-9) contains supplementary material, which is available to authorized users.

Hypoxic conditioning in blood vessels and smooth muscle tissues: effects on function, mechanisms, and unknowns

Hypoxic preconditioning, the protective effect of brief, intermittent hypoxic or ischemic episodes on subsequent more severe hypoxic episodes, has been known for 30 yr from studies on cardiac muscle. The concept of hypoxic preconditioning has expanded; excitingly, organs beyond the heart, including the brain, liver, and kidney, also benefit. Preconditioning of vascular and visceral smooth muscles has received less attention despite their obvious importance to health. In addition, there has been no attempt to synthesize the literature in this field. Therefore, in addition to overviewing the current understanding of hypoxic conditioning, in the present review, we consider the role of blood vessels in conditioning and explore evidence for conditioning in other smooth muscles. Where possible, we have distinguished effects on myocytes from other cell types in the visceral organs. We found evidence of a pivotal role for blood vessels in conditioning and for conditioning in other smooth muscle, including the bladder, vascular myocytes, and gastrointestinal tract, and a novel response in the uterus of a hypoxic-induced force increase, which helps maintain contractions during labor. To date, however, there are insufficient data to provide a comprehensive or unifying mechanism for smooth muscles or visceral organs and the effects of conditioning on their function. This also means that no firm conclusions can be drawn as to how differences between smooth muscles in metabolic and contractile activity may contribute to conditioning. Therefore, we have suggested what may be general mechanisms of conditioning occurring in all smooth muscles and tabulated tissue-specific mechanistic findings and suggested ideas for further progress.

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.

The NAD+-Dependent Family of Sirtuins in Cerebral Ischemia and Preconditioning

SIGNIFICANCE

Sirtuins are an evolutionarily conserved family of NAD+-dependent lysine deacylases and ADP ribosylases. Their requirement for NAD+ as a cosubstrate allows them to act as metabolic sensors that couple changes in the energy status of the cell to changes in cellular physiological processes. NAD+ levels are affected by several NAD+-producing and NAD+-consuming pathways as well as by cellular respiration. Thus their intracellular levels are highly dynamic and are misregulated in a spectrum of metabolic disorders including cerebral ischemia. This, in turn, compromises several NAD+-dependent processes that may ultimately lead to cell death. Recent Advances: A number of efforts have been made to replenish NAD+ in cerebral ischemic injuries as well as to understand the functions of one its important mediators, the sirtuin family of proteins through the use of pharmacological modulators or genetic manipulation approaches either before or after the insult. Critical Issues and Future Directions: The results of these studies have regarded the sirtuins as promising therapeutic targets for cerebral ischemia. Yet, additional efforts are needed to understand the role of some of the less characterized members and to address the sex-specific effects observed with some members. Sirtuins also exhibit cell-type-specific expression in the brain as well as distinct subcellular and regional localizations. As such, they are involved in diverse and sometimes opposing cellular processes that can either promote neuroprotection or further contribute to the injury; which also stresses the need for the development and use of sirtuin-specific pharmacological modulators. Antioxid. Redox Signal. 28, 691-710.

A Systematic Review of Neuroprotective Strategies during Hypovolemia and Hemorrhagic Shock

Severe trauma constitutes a major cause of death and disability, especially in younger patients. The cerebral autoregulatory capacity only protects the brain to a certain extent in states of hypovolemia; thereafter, neurological deficits and apoptosis occurs. We therefore set out to investigate neuroprotective strategies during haemorrhagic shock. This review was performed in accordance to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines. Before the start of the search, a review protocol was entered into the PROSPERO database. A systematic literature search of Pubmed, Web of Science and CENTRAL was performed in August 2017. Results were screened and evaluated by two researchers based on a previously prepared inclusion protocol. Risk of bias was determined by use of SYRCLE's risk of bias tool. The retrieved results were qualitatively analysed. Of 9093 results, 119 were assessed in full-text form, 16 of them ultimately adhered to the inclusion criteria and were qualitatively analyzed. We identified three subsets of results: (1) hypothermia; (2) fluid therapy and/or vasopressors; and (3) other neuroprotective strategies (piracetam, NHE1-inhibition, aprotinin, human mesenchymal stem cells, remote ischemic preconditioning and sevoflurane). Overall, risk of bias according to SYRCLE's tool was medium; generally, animal experimental models require more rigorous adherence to the reporting of bias-free study design (randomization, etc.). While the individual study results are promising, the retrieved neuroprotective strategies have to be evaluated within the current scientific context-by doing so, it becomes clear that specific promising neuroprotective strategies during states of haemorrhagic shock remain sparse. This important topic therefore requires more in-depth research.

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".

Exploring effects of remote ischemic preconditioning in a pig model of hypothermic circulatory arrest

OBJECTIVES

During aortic and cardiac surgery, risks for mortality and morbidity are inevitable. Surgical setups involving deep hypothermic circulatory arrest (DHCA) are effective to achieve organ protection against ischemic injury. The aim of this study was to identify humoural factors mediating additive protective effects of remote ischemic preconditioning (RIPC) in a porcine model of DHCA.

DESIGN

Twenty-two pigs were randomized into the RIPC group (n = 11) and the control group (n = 11). The RIPC group underwent four 5-minute hind limb ischemia-reperfusion cycles prior to cardiopulmonary bypass and DHCA. All animals underwent identical surgical procedures including 60 min DHCA at 18 °C. Blood samples were collected from vena cava and sagittal sinus at several time points. After the 8-hour follow-up period, the brain, heart, and kidney tissue samples were collected for tissue analyses.

RESULTS

Serum levels of brain damage marker S100B recovered faster in the RIPC group, after 4 hours of the arrest, (p < .05). Systemic lactate levels were lower and cardiac index was higher in the RIPC group postoperatively. Immunohistochemical cerebellum regional scores of antioxidant response regulator Nrf2 were better in the RIPC group (mean: 1.1, IQR: 0.0-2.5) compared with the control group (mean: 0.0, IQR: 0.0-0.0), reaching borderline statistical significance (p = .064). RIPC induced detectable modulations of plasma proteome and metabolites.

CONCLUSIONS

The faster recovery of S100B, lower systemic lactate levels and favourable regional antioxidant response suggest possible neuronal cellular and mitochondrial protection by RIPC, whereas better cardiac index underlines functional effects of RIPC. The exact humoural factor remains unclear.

Sleep Is Critical for Remote Preconditioning-Induced Neuroprotection

STUDY OBJECTIVES

Episodes of brief limb ischemia (remote preconditioning) in mice induce tolerance to modeled ischemic stroke (focal brain ischemia). Since stroke outcomes are in part dependent on sleep-wake history, we sought to determine if sleep is critical for the neuroprotective effect of limb ischemia.

METHODS

EEG/EMG recording electrodes were implanted in mice. After a 24 h baseline recording, limb ischemia was induced by tightening an elastic band around the left quadriceps for 10 minutes followed by 10 minutes of release for two cycles. Two days following remote preconditioning, a second 24 h EEG/EMG recording was completed and was immediately followed by a 60-minute suture occlusion of the middle cerebral artery (modeled ischemic stroke). This experiment was then repeated in a model of circadian and sleep abnormalities (Bmal1 knockout [KO] mice sleep 2 h more than wild-type littermates). Brain infarction was determined by vital dye staining, and sleep was assessed by trained identification of EEG/EMG recordings.

RESULTS

Two days after limb ischemia, wild-type mice slept an additional 2.4 h. This additional sleep was primarily comprised of non-rapid eye movement (NREM) sleep during the middle of the light-phase (i.e., naps). Repeating the experiment but preventing increases in sleep after limb ischemia abolished tolerance to ischemic stroke. In Bmal1 knockout mice, remote preconditioning did not increase daily sleep nor provide tolerance to subsequent focal ischemia.

CONCLUSIONS

These results suggest that sleep induced by remote preconditioning is both sufficient and necessary for its neuroprotective effects on stroke outcome.

Remote Limb Ischemic Conditioning at Two Cuff Inflation Pressures Yields Learning Enhancements in Healthy Adults

The authors tested whether 2 doses of remote limb ischemic conditioning (RLIC), induced via blood pressure cuff inflation, enhanced motor and cognitive learning to an equal extent, and explored a panel of blood biomarkers of RLIC. Thirty-two young adults were randomized to 3 groups and underwent a 7-day protocol of RLIC/sham followed by motor and cognitive training, with follow-up. Both RLIC groups had greater motor learning and a trend toward greater cognitive learning compared with the sham group. RLIC at the lower inflation pressure was as effective as RLIC with the higher inflation pressure. No significant candidate blood biomarkers were found. RLIC could be a well-tolerated method to enhance learning and improve rehabilitation outcomes in people with neurological conditions.

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 neuroinflammtory 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.

Review of remote ischemic preconditioning: from laboratory studies to clinical trials

In remote ischemic preconditioning (RIPC) short periods of non-lethal ischemia followed by reperfusion of tissue or organ prepare remote tissue or organ to resist a subsequent more severe ischemia-reperfusion injury. The signaling mechanism of RIPC can be humoral communication, neuronal stimulation, systemic modification of circulating immune cells, and activation of hypoxia inducible genes. Despite promising evidence from experimental studies, the clinical effects of RIPC have been controversial. Heterogeneity of inclusion and exclusion criteria and confounding factors such as comedication, anesthesia, comorbidities, and other risk factors may have influenced the efficacy of RIPC. Although the cardioprotective pathways of RIPC are more widely studied, there is also evidence of benefits in CNS, kidney and liver protection. Future research should explore the potential of RIPC, not only in cardiac protection, but also in patients with threatening ischemia of the brain, organ transplantation of the heart, liver and kidney and extensive cardiovascular surgery. RIPC is generally well-tolerated, safe, effective, and easily feasible. It has a great prospect for ischemic protection of the heart and other organs.

Long-term window of ischemic tolerance: An evolutionarily conserved form of metabolic plasticity regulated by epigenetic modifications?

In the absence of effective neuroprotective agents in the clinic, ischemic and pharmacological preconditioning are gaining increased interest in the field of cerebral ischemia. Our lab recently reported that resveratrol preconditioning affords tolerance against a focal cerebral ischemic insult in mice that can last for at least 14 days in vivo making it the longest window of ischemic tolerance discovered to date by a single administration of a pharmacological agent. The mechanism behind this novel extended window of ischemic tolerance remains elusive. In the below commentary we discuss potential mechanisms that could explain this novel extended window of ischemic tolerance in the context of previously identified windows and the known mechanisms behind them. We also draw parallels from the fields of hibernation and hypoxia-tolerance, which are chronic adaptations to severe conditions of hypoxia and ischemia known to be mediated by a form of metabolic depression. We also briefly discuss the importance of epigenetic modifications in maintaining this depressed state of metabolism.

Remote limb preconditioning protects against ischemia-induced neuronal death through ameliorating neuronal oxidative DNA damage and parthanatos

Remote limb preconditioning (RPC) ameliorates ischemia-induced cerebral infarction and promotes neurological function recovery; however, the mechanism of RPC hasn't been fully understood, which limits its clinical application. The present study aimed at exploring the underlying mechanism of RPC through testing its effects on neuronal oxidative DNA damage and parthanatos in a rat focal cerebral ischemia model. Infarct volume was investigated by 2, 3, 5-triphenyltetrazolium chloride (TTC) staining, and neuronal survival was evaluated by Nissl staining. Oxidative DNA damage was investigated via analyzing the expression of 8-hydroxy-2'-deoxyguanosine (8-OHdG). Besides, terminal deoxynucleotidyl transferase-mediated biotinylated-dUTP nick-end labeling (TUNEL) and DNA laddering were utilized to evaluate neuronal DNA fragmentation. Moreover, we tested whether RPC regulated poly(ADP-ribose) polymer (PAR) and apoptosis inducing factor (AIF) pathway; thus, PAR expression, AIF translocation and AIF/histone H2AX (H2AX) interaction were investigated. The results showed that RPC exerted neuroprotective effects by ameliorating oxidative DNA damage and neuronal parthanatos; additionally, RPC suppressed PAR/AIF pathway through reducing AIF translocation and AIF/H2AX interaction. The present study further exposed neuroprotective mechanism of RPC, and provided new evidence for the research on RPC and ICS.

Current insights into the molecular mechanisms of hypoxic pre- and postconditioning using hypobaric hypoxia

Exposure of organisms to repetitive mild hypoxia results in development of brain hypoxic/ischemic tolerance and cross-tolerance to injurious factors of a psycho-emotional nature. Such preconditioning by mild hypobaric hypoxia functions as a “warning” signal which prepares an organism, and in particular the brain, to subsequent more harmful conditions. The endogenous defense processes which are mobilized by hypoxic preconditioning and result in development of brain tolerance are based on evolutionarily acquired gene-determined mechanisms of adaptation and neuroprotection. They involve an activation of intracellular cascades including kinases, transcription factors and changes in expression of multiple regulatory proteins in susceptible areas of the brain. On the other hand they lead to multilevel modifications of the hypothalamic-pituitary-adrenal endocrine axis regulating various functions in the organism. All these components are engaged sequentially in the initiation, induction and expression of hypoxia-induced tolerance. A special role belongs to the epigenetic regulation of gene expression, in particular of histone acetylation leading to changes in chromatin structure which ensure access of pro-adaptive transcription factors activated by preconditioning to the promoters of target genes. Mechanisms of another, relatively novel, neuroprotective phenomenon termed hypoxic postconditioning (an application of mild hypoxic episodes after severe insults) are still largely unknown but according to recent data they involve apoptosis-related proteins, hypoxia-inducible factor and neurotrophins. The fundamental data accumulated to date and discussed in this review open new avenues for elaboration of the effective therapeutic applications of hypoxic pre- and postconditioning.