Hypoxic Preconditioning Suppresses Glial Activation and Neuroinflammation in Neonatal Brain Insults

Hypoxic Preconditioning Averts Sporadic Alzheimer's Disease-Like Phenotype in Rats: A Focus on Mitochondria

Aims: Brief episodes of sublethal hypoxia reprogram brain response to face possible subsequent lethal stimuli by triggering adaptive and prosurvival events-a phenomenon denominated hypoxic preconditioning (HP). To date, the potential therapeutic implications of HP to forestall sporadic Alzheimer's disease (sAD) pathology remain unexplored. Using a well-established protocol of HP and focusing on hippocampus as a first brain region affected in AD, this study was undertaken to investigate the potential protective effects of HP in a sAD rat model induced by the intracerebroventricular (icv) administration of streptozotocin (STZ) and to uncover the mitochondrial adaptations underlying this nonpharmacological strategy. Results: HP prevented the memory and learning deficits as well as tau pathology in the icvSTZ rat model. HP also attenuated icvSTZ-related reactive astrogliosis, as noted by increased glial fibrillary acidic protein immunoreactivity and myo-inositol levels. Notably, HP abrogated the icvSTZ-related impaired energy metabolism and oxidative damage. Particularly, HP averted increased lactate, glutamate, and succinate levels, and decreased mitochondrial respiratory chain function and mitochondrial DNA content. Concerning mitochondrial adaptations underlying HP-triggered tolerance to icvSTZ, preconditioned hippocampal mitochondria displayed an enhanced complex II-energized mitochondrial respiration, which resulted from a coordinated interaction between mitochondrial biogenesis and fusion-fission. Mitochondrial biogenesis was stimulated immediately after HP, whereas in a latter phase mitochondrial fusion-fission events are modulated favoring the generation of elongated mitochondria. Innovation and Conclusion: Overall, these results demonstrate for the first time that HP prevents the sAD-like phenotype, in part, by targeting mitochondria emerging as a preventive strategy in the context of AD. Antioxid. Redox Signal. 37, 739-757.

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.

AD-16 Protects Against Hypoxic-Ischemic Brain Injury by Inhibiting Neuroinflammation

Neuroinflammation is a key contributor to the pathogenic cascades induced by hypoxic-ischemic (HI) insult in the neonatal brain. AD-16 is a novel anti-inflammatory compound, recently found to exert potent inhibition of the lipopolysaccharide-induced production of pro-inflammatory and neurotoxic mediators. In this study, we evaluated the effect of AD-16 on primary astrocytes and neurons under oxygen-glucose deprivation (OGD) in vitro and in mice with neonatal HI brain injury in vivo. We demonstrated that AD-16 protected against OGD-induced astrocytic and neuronal cell injury. Single dose post-treatment with AD-16 (1 mg/kg) improved the neurobehavioral outcome and reduced the infarct volume with a therapeutic window of up to 6 h. Chronic administration reduced the mortality rate and preserved whole-brain morphology following neonatal HI. The in vitro and in vivo effects suggest that AD-16 offers promising therapeutic efficacy in attenuating the progression of HI brain injury and protecting against the associated mortality and morbidity.

Gallic acid inhibits neuroinflammation and reduces neonatal hypoxic-ischemic brain damages

Neuroinflammation is a leading cause of secondary neuronal injury in neonatal hypoxic-ischemic encephalopathy (HIE). Regulation of neuroinflammation may be beneficial for treatment of HIE and its secondary complications. Gallic acid (GA) has been shown to have anti-inflammatory and antioxidant effects. In this report we found that oxygen-glucose deprivation and/reoxygenation (OGD/R)-induced cell death, and the generation of excessive reactive oxygen species (ROS) and inflammatory cytokines by microglia were inhibited by GA treatment. Furthermore, GA treatment reduced neuroinflammation and neuronal loss, and alleviated motor and cognitive impairments in rats with hypoxic-ischemic brain damage (HIBD). Together, our results reveal that GA is an effective regulator of neuroinflammation and has potential as a pharmaceutical intervention for HIE therapy.

Preconditioning increases brain resistance against acute brain injury via neuroinflammation modulation

Acute brain injury (ABI) is a broad concept mainly comprised of sudden parenchymal brain injury. Acute brain injury outcomes are dependent not only on the severity of the primary injury, but the delayed secondary injury that subsequently follows as well. These are both taken into consideration when determining the patient's prognosis. Growing clinical and experimental evidence demonstrates that "preconditioning," a prophylactic approach in which the brain is exposed to various pre-injury stressors, can induce varying degrees of "tolerance" against the impact of the ABI by modulating neuroinflammation. In this review, we will summarize the pathophysiology of ABI, and discuss the involved mechanisms of neuroinflammation in ABI, as well as existing experimental and clinical studies demonstrating the efficacy of preconditioning methods in various types of ABI by modulating neuroinflammation.

Early changes of NLRP3 inflammasome activation after hypoxic-ischemic brain injury in neonatal rats

The pathogenesis of neonatal hypoxic-ischemic (HI) brain injury may involve activation of the NOD-like receptor family pyrin domain-containing-3 (NLRP3) inflammasome and its downstream effectors, caspase-1 and interleukin (IL)-1β. The start time of therapy is associated with adverse neurodevelopmental outcome following HI injury. We performed this study investigating early dynamic changes in NLRP3, caspase-1, and IL-1β expression during the first 24 h following HI brain injury in an animal model, in order to optimize selection of treatment time after injury. Rats were randomized to an HI group (n=40) and sham group (n=40). Rats in the HI group were subjected to right common carotid artery ligation and then exposed to hypoxia (8% O₂) for 2 h, and divided into 5 subgroups with 8 cases in each group at 5 postoperative time points (0, 4, 8, 12, 24 h). Brain injury during the first 24 h after surgery/hypoxia was evaluated by cranial ultrasonography. RT-PCR, western blot, and immunohistochemistry were applied to determine protein and mRNA expressions. In the HI group, ultrasonography revealed accelerated right vertebrobasilar artery flow at 4 h, enhanced brain parenchyma echogenicity at 24 h, and blood stealing from the vertebrobasilar artery at 24 h. In the HI group, immunohistochemistry demonstrated elevated expressions of NLRP3 and IL-1β at 4, 8, 12, and 24 h and enhanced expression of caspase-1 at 8 and 12 h (all P < 0.01). Western blot and RT-PCR revealed that, compared with the sham group, the HI group exhibited elevated expression of NLRP3 at 4, 8, and 24 h, caspase-1 at 12 h, and IL-1β at 8 h (all P < 0.05). In summary, the present results suggested that activation of NLRP3/caspase-1/IL-1β signaling occurs within 4 h of HI brain injury in the neonatal rat.

N-acetylserotonin Derivative Exerts a Neuroprotective Effect by Inhibiting the NLRP3 Inflammasome and Activating the PI3K/Akt/Nrf2 Pathway in the Model of Hypoxic-Ischemic Brain Damage

Neonatal hypoxic-ischemic encephalopathy (HIE) is one of the main causes of neonatal disability and death. As a derivative of N-acetylserotonin, N-[2-(5-hydroxy-1H-indol-3-yl) ethyl]-2-oxopiperidine-3-carboxamide (HIOC) can easily cross the blood-brain barrier and have a long half-life in the brain. In this study, the hypothesis was verified that HIOC plays a neuroprotective role in the HIE model and its potential mechanism was evaluated. Firstly, an HIE rat model was established to deliver HIOC, revealing that it can reduce cerebral infarction volume, cerebral edema, and neuronal apoptosis. The results of immunofluorescence staining, Western blots and RT-PCR further showed that HIOC could inhibit the activation of the NLRP3 inflammasome and the expression of related proteins. Finally, the activation of the phosphatidylinositol-3-kinase (PI3K)/Akt/nuclear factor erythroid 2-related factor 2 (Nrf2) pathway by HIOC was verified in vitro and in vivo. It was discovered that HIOC could increase the nuclear translocation of Nrf2, and that this induction can be reversed by the PI3K/Akt pathway inhibitor LY294002. In general terms, the neuroprotective effect of HIOC was confirmed in the HIE model, which is related to the activation of the Pi3k/Akt/Nrf2 signal pathway and the inhibition of the NLRP3 inflammasome.

Neuroprotection of hypoxic/ischemic preconditioning in neonatal brain with hypoxic-ischemic injury

The neonatal brain is susceptible to hypoxic-ischemic injury due to its developmental characteristics. Hypoxia-ischemia means a decreased perfusion of oxygen and glucose, which can lead to severe encephalopathy. Although early initiation of therapeutic hypothermia was reported to provide neuroprotection for infants after HI, hypothermia administered alone after the acute insult cannot reverse the severe damage that already has occurred or improve the prognosis of severe hypoxic-ischemic encephalopathy. Therefore, exploring new protective mechanisms for treating hypoxic-ischemic brain damage are imperative. Until now, many studies reported the neuroprotective mechanisms of hypoxic/ischemic preconditioning in protecting the hypoxic-ischemic newborn brains. After hypoxia and ischemia, hypoxia-inducible factor signaling pathway is involved in the transcriptional regulation of many genes and is also play a number of different roles in protecting brains during hypoxic/ischemic preconditioning. Hypoxic/ischemic preconditioning could protect neonatal brain by several mechanisms, including vascular regulation, anti-apoptosis, anti-oxidation, suppression of excitotoxicity, immune regulation, hormone levels regulation, and promote cell proliferation. This review focused on the protective mechanisms underlying hypoxic/ischemic preconditioning for neonatal brain after hypoxia-ischemia and emphasized on the important roles of hypoxia inducible factor 1 signaling pathway.

Molecular Basis of Sex Difference in Neuroprotection induced by Hypoxia Preconditioning in Zebrafish

Hypoxia, the major cause of ischemic injury, leads to debilitating disease in infants via birth asphyxia and cerebral palsy, whereas in adults via heart attack and stroke. A widespread, natural protective phenomenon termed 'hypoxic preconditioning' (PH) occurs when prior exposures to hypoxia eventually result in robust hypoxia resistance. Accordingly, we have developed and optimized a novel model of hypoxic preconditioning in adult zebrafish to mimic the tolerance of mini stroke(s) in human, which appears to protect against the severe damage inflicted by a major stroke event. Here, we observed a remarkable difference in the progression pattern of neuroprotection between preconditioning hypoxia followed by acute hypoxia (PH) group, and acute hypoxia (AH) only group, with noticeable sex difference when compared with normoxia behaviour upon recovery. Since gender difference has been reported in stroke risk factors and disease history, it was pertinent to investigate whether any such sex difference also exists in PH's protective mechanism against acute ischemic stroke. In order to elucidate the neural molecular mechanisms behind sex difference in neuroprotection induced by PH, a high throughput proteomics approach utilizing iTRAQ was performed, followed by protein enrichment analysis using ingenuity pathway analysis (IPA) tool. Out of thousands of significantly altered proteins in zebrafish brain, the ones having critical role either in neuroglial proliferation/differentiation or neurotrophic functions were validated by analyzing their expression levels in preconditioned (PH), acute hypoxia (AH), and normoxia groups. The data indicate that female zebrafish brains are more protected against the severity of AH when exposed to the hypoxic preconditioning. The study also sheds light on the involvement of many signalling pathways underlying sex difference in preconditioning-induced neuroprotective mechanism, which can be further validated for the therapeutic approach.

Response to Hypoxic Preconditioning of Glial Cells from the Roof of the Fourth Ventricle

The cerebellum harbors a specialized area on the roof of the fourth ventricle that is composed of glial cells and neurons that interface with the cerebrospinal fluid. This region includes the so-called ventromedial cord (VMC), which is composed of cells that are glial fibrillary acidic protein (GFAP)-positive and nestin-positive and distributes along the midline in association with blood vessels. We hypothesized that these cells should compare to GFAP and nestin-positive cells that are known to exist in other areas of the brain, which undergo proliferation and differentiation under hypoxic conditions. Thus, we tested whether cells of the VMC would display a similar reaction to hypoxic preconditioning (HPC). Indeed, we found that the VMC does respond to HPC by reorganizing its cellular components before it gradually returns to its basal state after about a week. This response we documented by monitoring global changes in the expression of GFAP-EGFP in transgenic mice, using light-sheet fluorescence microscopy (LSFM) revealed a dramatic loss of EGFP upon HPC, and was paralleled by retraction of Bergmann glial cell processes. This EGFP loss was supported by western blot analysis, which also showed a loss in the astrocyte-markers GFAP and ALDH1L1. On the other hand, other cell-markers appeared to be upregulated in the blots (including nestin, NeuN, and Iba1). Finally, we found that HPC does not remarkably affect the incorporation of BrdU into cells on the cerebellum, but strongly augments BrdU incorporation into periventricular cells on the floor of the fourth ventricle over the adjacent medulla.

Hypoxic-ischemic-related cerebrovascular changes and potential therapeutic strategies in the neonatal brain

Perinatal hypoxic-ischemic (HI)-related brain injury is an important cause of morbidity and long-standing disability in newborns. The only currently approved therapeutic strategy available to reduce brain injury in the newborn is hypothermia. Therapeutic hypothermia can only be used to treat HI encephalopathy in full-term infants and survivors remain at high risk for a wide spectrum of neurodevelopmental abnormalities as a result of residual brain injury. Therefore, there is an urgent need for adjunctive therapeutic strategies. Inflammation and neurovascular damage are important factors that contribute to the pathophysiology of HI-related brain injury and represent exciting potential targets for therapeutic intervention. In this review, we address the role of each component of the neurovascular unit (NVU) in the pathophysiology of HI-related injury in the neonatal brain. Disruption of the blood-brain barrier (BBB) observed in the early hours after an HI-related event is associated with a response at the basal lamina level, which comprises astrocytes, pericytes, and immune cells, all of which could affect BBB function to further exacerbate parenchymal injury. Future research is required to determine potential drugs that could prevent or attenuate neurovascular damage and/or augment repair. However, some studies have reported beneficial effects of hypothermia, erythropoietin, stem cell therapy, anti-cytokine therapy and metformin in ameliorating several different facets of damage to the NVU after HI-related brain injury in the perinatal period.

The expression of glucose transporters and mitochondrial division and fusion proteins in rats exposed to hypoxic preconditioning to attenuate propofol neurotoxicity

Purpose: Evidence has shown that propofol may cause widespread apoptotic neurodegeneration. Hypoxic preconditioning has been demonstrated to provide neuroprotection and brain recovery from both acute and chronic neurodegeneration in several cellular and animal models. However, the mechanism has not been well elucidated. Therefore, the present study was designed to investigate the expression of glucose transporters (GLUT1 and GLUT3) and mitochondrial division and fusion (Drp1 and Mfn2) proteins in rats exposed to hypoxic preconditioning to attenuate propofol neurotoxicity.Methods: Propofol (100 mg/kg) was given to 7-day-old Sprague-Dawley rats; in some rats, hypoxic preconditioning was administered before intraperitoneal propofol injection by subjecting rats to five cycles of 10 min of hypoxia (8% O₂₎ and 10 min of normoxia (21% O₂). Then, the rats were allowed to breathe room air for 2 h. Neuronal mitochondrial morphology was observed by transmission electron microscopy. ATP content was detected using an ATP assay kit. The expression levels of GLUT1, GLUT3, pDrp1, Drp1 and Mfn2 were detected by Western blot, and the expression levels of GLUT1 and GLUT3 were further examined by immunohistochemistry.Results: Propofol damaged mitochondria, and decreased ATP content and GLUT3 and pDrp1 protein expression. However, our results suggested that hypoxic preconditioning could attenuate propofol neurotoxicity by reducing mitochondrial damage and increasing ATP content and pDrp1, GLUT1 and GLUT3 protein expression.Conclusion: Hypoxic preconditioning reduced propofol-induced damage in the hippocampus of neonatal rats by attenuating the increase in mitochondrial division and decrease in GLUT3 expression.

The fetus at the tipping point: modifying the outcome of fetal asphyxia

Brain injury around birth is associated with nearly half of all cases of cerebral palsy. Although brain injury is multifactorial, particularly after preterm birth, acute hypoxia-ischaemia is a major contributor to injury. It is now well established that the severity of injury after hypoxia-ischaemia is determined by a dynamic balance between injurious and protective processes. In addition, mothers who are at risk of premature delivery have high rates of diabetes and antepartum infection/inflammation and are almost universally given treatments such as antenatal glucocorticoids and magnesium sulphate to reduce the risk of death and complications after preterm birth. We review evidence that these common factors affect responses to fetal asphyxia, often in unexpected ways. For example, glucocorticoid exposure dramatically increases delayed cell loss after acute hypoxia-ischaemia, largely through secondary hyperglycaemia. This critical new information is important to understand the effects of clinical treatments of women whose fetuses are at risk of perinatal asphyxia.

Hypoxic preconditioning reduces propofol-induced neuroapoptosis via regulation of Bcl-2 and Bax and downregulation of activated caspase-3 in the hippocampus of neonatal rats

OBJECTIVE

 Evidence has shown that propofol may cause widespread apoptotic neurodegeneration. Hypoxic preconditioning (HPC) was previously demonstrated to provide neuroprotection and brain recovery from either acute or chronic neurodegeneration in several cellular and animal models. Therefore, the present study was designed to investigate the protective effects of hypoxic preconditioning on apoptosis caused by propofol in neonatal rats.

METHODS

Propofol (100 mg/kg) was given to 7-day-old (P7) Sprague Dawley pups. Before the propofol injection, hypoxic preconditioning was administered by subjecting rats to five cycles of 10 min of hypoxia (8% O2) and 10 min of normoxia (21% O2), then 2 h of room air. We detected neuronal structure changes and apoptosis by hematoxylin and eosin (HE) staining and TUNEL assay, respectively. Bcl-2, Bax and cleaved-caspase-3 levels were quantified using Western blotting and immunohistochemistry.

RESULT

 After treatment with propofol, Bcl-2 levels decreased and Bax and cleaved-caspase-3 levels increased. However, our results suggest that hypoxic preconditioning could reverse this change. Conclusion: Our results indicate that pretreatment with hypoxic preconditioning prevents propofol-induced neuroapoptosis by increasing the levels of Bcl-2 and decreasing the levels of Bax and cleaved-caspase-3.

Brain-immune interactions in perinatal hypoxic-ischemic brain injury

Perinatal hypoxia-ischemia remains the primary cause of acute neonatal brain injury, leading to a high mortality rate and long-term neurological deficits, such as behavioral, social, attentional, cognitive and functional motor deficits. An ever-increasing body of evidence shows that the immune response to acute cerebral hypoxia-ischemia is a major contributor to the pathophysiology of neonatal brain injury. Hypoxia-ischemia provokes an intravascular inflammatory cascade that is further augmented by the activation of resident immune cells and the cerebral infiltration of peripheral immune cells response to cellular damages in the brain parenchyma. This prolonged and/or inappropriate neuroinflammation leads to secondary brain tissue injury. Yet, the long-term effects of immune activation, especially the adaptive immune response, on the hypoxic-ischemic brain still remain unclear. The focus of this review is to summarize recent advances in the understanding of post-hypoxic-ischemic neuroinflammation triggered by the innate and adaptive immune responses and to discuss how these mechanisms modulate the brain vulnerability to injury. A greater understanding of the reciprocal interactions between the hypoxic-ischemic brain and the immune system will open new avenues for potential immunomodulatory therapy in the treatment of neonatal brain injury.

Metformin treatment after the hypoxia-ischemia attenuates brain injury in newborn rats

Neonatal hypoxic-ischemic (HI) brain injury is a devastating disease that often leads to death and detrimental neurological deficits. The present study was designed to evaluate the ability of metformin to provide neuroprotection in a model of neonatal hypoxic-ischemic brain injury and to study the associated molecular mechanisms behind these protective effects. Here, we found that metformin treatment remarkably attenuated brain infarct volumes and brain edema at 24 h after HI injury, and the neuroprotection of metformin was associated with inhibition of neuronal apoptosis, suppression of the neuroinflammation and amelioration of the blood brain barrier breakdown. Additionally, metformin treatment conferred long-term protective against brain damage at 7 d after HI injury. Our study indicates that metformin treatment protects against neonatal hypoxic-ischemic brain injury and thus has potential as a therapy for this disease.

Newborn blood gas derangements of children born extremely preterm and neurocognitive dysfunctions at age 10 years

Among 740 children born extremely preterm, we evaluated the relationship between the highest and lowest quartiles of the distributions of PaO₂ and PaCO₂, as well as the lowest quartile of pH on one day, and separately on two days, and the risk of neurocognitive, language, and behavioral dysfunctions at age 10 years. Children who had hypoxemia, hyperoxemia, hypocapnia, hypercapnia, and acidemia, sometimes on only one day, and sometimes on two or more days, were more likely than others to have a high illness severity score (within the first 12 postnatal hours), and 10 years later to have multiple dysfunctions. The tendency of blood gas derangements to be associated with high illness severity scores and with multiple dysfunctions 10 years later is compatible with the possibility that blood gas derangements are indicators of physiologic instability/vulnerability/immaturity rather than contributors to brain damage.

Oxidative stress and mitochondrial dysfunction-linked neurodegenerative disorders

Reactive species play an important role in physiological functions. Overproduction of reactive species, notably reactive oxygen (ROS) and nitrogen (RNS) species along with the failure of balance by the body's antioxidant enzyme systems results in destruction of cellular structures, lipids, proteins, and genetic materials such as DNA and RNA. Moreover, the effects of reactive species on mitochondria and their metabolic processes eventually cause a rise in ROS/RNS levels, leading to oxidation of mitochondrial proteins, lipids, and DNA. Oxidative stress has been considered to be linked to the etiology of many diseases, including neurodegenerative diseases (NDDs) such as Alzheimer diseases, Amyotrophic lateral sclerosis, Friedreich's ataxia, Huntington's disease, Multiple sclerosis, and Parkinson's diseases. In addition, oxidative stress causing protein misfold may turn to other NDDs include Creutzfeldt-Jakob disease, Bovine Spongiform Encephalopathy, Kuru, Gerstmann-Straussler-Scheinker syndrome, and Fatal Familial Insomnia. An overview of the oxidative stress and mitochondrial dysfunction-linked NDDs has been summarized in this review.

Re-exposure to the hypobaric hypoxic brain injury of high altitude: plasma S100B levels and the possible effect of acclimatisation on blood-brain barrier dysfunction

Hypobaric hypoxic brain injury results in elevated peripheral S100B levels which may relate to blood-brain barrier (BBB) dysfunction. A period of acclimatisation or dexamethasone prevents altitude-related illnesses and this may involve attenuation of BBB compromise. We hypothesised that both treatments would diminish the S100B response (a measure of BBB dysfunction) on re-ascent to the hypobaric hypoxia of high altitude, in comparison to an identical ascent completed 48 h earlier by the same group. Twelve healthy volunteers, six of which were prescribed dexamethasone, ascended Mt Fuji (summit 3700 m) and serial plasma S100B levels measured. The S100B values reduced from a baseline 0.183 µg/l (95 % CI 0.083-0.283) to 0.145 µg/l (95 % CI 0.088-0.202) at high altitude for the dexamethasone group (n = 6) and from 0.147 µg/l (95 % CI 0.022-0.272) to 0.133 µg/l (95 % CI 0.085-0.182) for the non-treated group (n = 6) [not statistically significant (p = 0.43 and p = 0.82) for the treated and non-treated groups respectively]. [These results contrasted with the statistically significant increase during the first ascent, S100B increasing from 0.108 µg/l (95 % CI 0.092-0.125) to 0.216 µg/l (95 % CI 0.165-0.267) at high altitude]. In conclusion, an increase in plasma S100B was not observed in the second ascent and this may relate to the effect of acclimatisation (or hypoxic pre-conditioning) on the BBB. An exercise stimulated elevation of plasma S100B levels was also not observed during the second ascent. The small sample size and wide confidence intervals, however, precludes any statistically significant conclusions and a larger study would be required to confirm these findings.