Intermittent hypoxia after transient focal ischemia induces hippocampal neurogenesis and c-Fos expression and reverses spatial memory deficits in rats

Delayed Chronic Acidic Postconditioning Improves Poststroke Motor Functional Recovery and Brain Tissue Repair by Activating Proton-Sensing TDAG8

Acidic postconditioning by transient CO2 inhalation applied within minutes after reperfusion has neuroprotective effects in the acute phase of stroke. However, the effects of delayed chronic acidic postconditioning (DCAPC) initiated during the subacute phase of stroke or other acute brain injuries are unknown. Mice received daily DCAPC by inhaling 5%/10%/20% CO2 for various durations (three cycles of 10- or 20-min CO2 inhalation/10-min break) at days 3-7, 7-21, or 3-21 after photothrombotic stroke. Grid-walk, cylinder, and gait tests were used to assess motor function. DCAPC with all CO2 concentrations significantly promoted motor functional recovery, even when DCAPC was delayed for 3-7 days. DCAPC enhanced the puncta density of GAP-43 (a marker of axon growth and regeneration) and synaptophysin (a marker of synaptogenesis) and reduced the amoeboid microglia number, glial scar thickness and mRNA expression of CD16 and CD32 (markers of proinflammatory M1 microglia) compared with those of the stroke group. Cerebral blood flow (CBF) increased in response to DCAPC. Furthermore, the mRNA expression of TDAG8 (a proton-activated G-protein-coupled receptor) was increased during the subacute phase of stroke, while DCAPC effects were blocked by systemic knockout of TDAG8, except for those on CBF. DCAPC reproduced the benefits by re-expressing TDAG8 in the peri-infarct cortex of TDAG8-/- mice infected with HBAAV2/9-CMV-TDAG8-3flag-ZsGreen. Taken together, we first showed that DCAPC promoted functional recovery and brain tissue repair after stroke with a wide therapeutic time window of at least 7 days after stroke. Brain-derived TDAG8 is a direct target of DCAPC that induces neuroreparative effects.

Hypoxia preconditioning increases Notch1 activity by regulating DNA methylation in vitro and in vivo

BACKGROUND

Our previous research has demonstrated that hypoxic preconditioning (HPC) can improve spatial learning and memory abilities in adult mice. Adult hippocampal neurogenesis has been associated with learning and memory. The Neurogenic locus notch homolog protein (Notch) was involved in adult hippocampal neurogenesis, as well as in learning and memory. It is currently unclear whether the Notch pathway regulates hippocampal neuroregeneration by modifying the DNA methylation status of the Notch gene following HPC.

METHOD

The HPC animal model and cell model were established through repeated hypoxia exposure using mice and the mouse hippocampal neuronal cell line HT22. Step-down test was conducted on HPC mice. Real-time PCR and Western blot analysis were used to assess the mRNA and protein expression levels of Notch1 and hairy and enhancer of split1 (HES1). The presence of BrdU-positive cells and Notch1 expression in the hippocampal dental gyrus (DG) were examined with confocal microscopy. The methylation status of the Notch1 was analyzed using methylation-specific PCR (MS-PCR). HT22 cells were employed to elucidate the impact of HPC on Notch1 in vitro.

RESULTS

HPC significantly improved the step-down test performance of mice with elevated levels of mRNA and protein expression of Notch1 and HES1 (P < 0.05). The intensities of the Notch1 signal in the control group, the H group and the HPC group were 2.62 ± 0.57 × 107, 2.87 ± 0.84 × 107, and 3.32 ± 0.14 × 107, respectively, and the number of BrdU (+) cells in the hippocampal DG were 1.83 ± 0.54, 3.71 ± 0.64, and 7.29 ± 0.68 respectively. Compared with that in C and H group, the intensity of the Notch1 signal and the number of BrdU (+) cells increased significantly in HPC group (P < 0.05). The methylation levels of the Notch1 promoter 0.82 ± 0.03, 0.65 ± 0.03, and 0.60 ± 0.02 in the C, H, and HPC groups, respectively. The methylation levels of Notch1 decreased significantly (P < 0.05). The effect of HPC on HT22 cells exhibited similarities to that observed in the hippocampus.

CONCLUSION

HPC may confer neuroprotection by activating the Notch1 signaling pathway and regulating its methylation level, resulting in the regeneration of hippocampal neurons.

Salvianolic acid A provides neuroprotective effects on cerebral ischemia-reperfusion injury in rats via PKA/CREB/c-Fos signaling pathway

BACKGROUND

Cerebral ischemia-reperfusion injury (CIRI) is a phenomenon that pathological injury of ischemic brain tissue is further aggravated after the restoration of blood supply. The complex pathological mechanism of CIRI has led to the failure of multiple neuroprotective agents in clinical studies. Salvianolic acid A (SAA) is a neuroprotective extract from Salvia miltiorrhiza Bge., with significant pharmacological activities in the treatment of brain injury. However, the neuroprotective mechanisms of SAA remain unclear.

PURPOSE

To explore the potential protective effect of SAA on CIRI and its mechanism, and to provide experimental basis for the research of new drugs for CIRI.

STUDY DESIGN

A model of transient middle cerebral artery occlusion (tMCAO) in rats was used to simulate clinical CIRI, and the neuroprotective effect of SAA on tMCAO rats was investigated within 14 days after reperfusion. The improvement effects of SAA on cognitive impairment of tMCAO rats were investigated by behavioral tests from days 7-14. Finally, the neuroprotective mechanism of SAA was investigated on day 14.

METHODS

The neuroprotective effects and mechanism of SAA were investigated by behavioral tests, HE and TUNEL staining, RNA sequence (RNA-seq) analysis and Western blot in tMCAO rats.

RESULTS

The brain protective effects of SAA were achieved by alleviating cerebral infarction, cerebral edema, cerebral atrophy and nerve injury in tMCAO rats. Meanwhile, SAA could effectively improve the cognitive impairment and pathological damage of hippocampal tissue, and inhibit cell apoptosis in tMCAO rats. Besides, SAA could provide neuroprotective effects by up-regulating the expression of Bcl-2, inhibiting the activation of Caspase 3, and regulating PKA/CREB/c-Fos signaling pathway.

CONCLUSION

SAA can significantly improve brain injury and cognitive impairment in CIRI rats, and this neuroprotective effect may be achieved through the anti-apoptotic effect and the regulation of PKA/CREB/c-Fos signaling pathway.

Neurogenesis, A Potential Target for Intermittent Hypoxia Leading to Cognitive Decline

As a sleep breathing disorder, characterized by intermittent hypoxia (IH) and Obstructive sleep apnea (OSA), is believed to decrease the cognitive function of patients. Many factors are thought to be responsible for cognitive decline in OSA patients. Neurogenesis, a process by which neural stem cells (NSCs) differentiate into new neurons in the brain, is a major determinant affecting cognitive function. However, there is no clear relationship between IH or OSA and neurogenesis. In recent years, increasing numbers of studies on IH and neurogenesis are documented. Therefore, this review summarizes the effects of IH on neurogenesis; then discusses the influencing factors that may cause these effects and the potential signaling pathways that may exist. Finally, based on this impact, we discuss potential methods and future directions for improving cognition.

HIF-1α-induced upregulation of m6A reader IGF2BP1 facilitates peripheral nerve injury recovery by enhancing SLC7A11 mRNA stabilization

The recovery of peripheral nerve injury (PNI) is not ideal in clinic. Our previous study revealed that hypoxia treatment promoted PNI repair by inhibiting ferroptosis. The aim of this study was to investigate the underlying molecular mechanism of HIF-1α in hypoxia-PNI recovery. M6A dot blot was used to determine the total level of m6A modification. Besides, HIF-1α small interfering RNA (siRNA) or IGF2BP1 overexpression vector was transfected into dorsal root ganglion (DRG) neurons to alter the expression of HIF-1α and IGF2BP1. Subsequently, MeRIP-PCR analysis was applied to validate the m6A methylation level of SLC7A11. We demonstrated the hypoxia stimulated HIF-1α-dependent expression of IGF2BP1 and promoted the overall m6A methylation levels of DRG neurons. Overexpression of HIF-1α increased the expressions of neurotrophic factors including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and glial-derived neurotrophic factor (GDNF), which could be effectively reversed by siRNA knockdown of IGF2BP1. Moreover, upregulation of HIF-1α contributed to the m6A methylation level and mRNA stabilization of SLC7A11. This study revealed that the HIF-1α/IGF2BP1/SLC7A11 regulatory axis facilitated the recovery of injured DRG neurons. Our findings suggest a novel insight for the m6A methylation modification in PNI recovery.

Oxidative Stress Response Kinetics after 60 Minutes at Different Levels (10% or 15%) of Normobaric Hypoxia Exposure

In this study, the metabolic responses of hypoxic breathing for 1 h to inspired fractions of 10% and 15% oxygen were investigated. To this end, 14 healthy nonsmoking subjects (6 females and 8 males, age: 32.2 ± 13.3 years old (mean ± SD), height: 169.1 ± 9.9 cm, and weight: 61.6 ± 16.2 kg) volunteered for the study. Blood samples were taken before, and at 30 min, 2 h, 8 h, 24 h, and 48 h after a 1 h hypoxic exposure. The level of oxidative stress was evaluated by considering reactive oxygen species (ROS), nitric oxide metabolites (NOx), lipid peroxidation, and immune-inflammation by interleukin-6 (IL-6) and neopterin, while antioxidant systems were observed in terms of the total antioxidant capacity (TAC) and urates. Hypoxia abruptly and rapidly increased ROS, while TAC showed a U-shape pattern, with a nadir between 30 min and 2 h. The regulation of ROS and NOx could be explained by the antioxidant action of uric acid and creatinine. The kinetics of ROS allowed for the stimulation of the immune system translated by an increase in neopterin, IL-6, and NOx. This study provides insights into the mechanisms through which acute hypoxia affects various bodily functions and how the body sets up the protective mechanisms to maintain redox homeostasis in response to oxidative stress.

Mechanism of action of Daqinjiao decoction in treating cerebral small vessel disease explored using network pharmacology and molecular docking technology

BACKGROUND AND PURPOSE

Cerebral small vessel disease (CSVD) is a clinically commonly-seen slow-progressing cerebral vascular disease. As a classic Chinese formula for the treatment of stroke, Daqinjiao Decoction (DQJD) is now used to treat CSVD with desirable effect. Since the mechanism of action is still unclear, this article will explore the therapeutic effect and mechanism of action of the formula using network pharmacology technology.

METHODS

The major chemical components and potential target genes of DQJD were screened by bioinformatics. The key targets in CSVD were identified based on network modules. Gene Ontology (GO) functional annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis were performed. Pharmacodynamics of the decoction was evaluated by establishing a rat model with bilateral common carotid artery occlusion in the brain. Molecular docking, Western blot analysis and quantitative real-time polymerase chain reaction (QRT-PCR) were performed to confirm the effectiveness of targets in related pathways.

RESULTS

Network pharmacology showed that 16 targets and 30 pathways were involved in the DQJD-targeted pathway network. Results revealed that DQJD might play a role by targeting the key targets including Caspse3 and P53 and regulating the P53 signaling pathway. Cognitive function and neuronal cell changes of rats were evaluated using Morris water maze, open field test and HE staining. It was indicated that DQJD could keep the nerve cells intact and neatly arranged. The decoction could improve the memory and learning ability of rats compared with the model group. It decreased the protein and mRNA expression levels of Caspse3 and P53 significantly (p<0.01).

CONCLUSION

The study shows that baicalein, quercetin and wogonin, the effective components of DQJD, may regulate multiple signaling pathways by targeting the targets like Caspse3 and P53 and treat CSVD by reducing the damage to brain nerve cells.

Microglial phagocytosis and regulatory mechanisms after stroke

Stroke, including ischemic stroke and hemorrhagic stroke can cause massive neuronal death and disruption of brain structure, which is followed by secondary inflammatory injury initiated by pro-inflammatory molecules and cellular debris. Phagocytic clearance of cellular debris by microglia, the brain's scavenger cells, is pivotal for neuroinflammation resolution and neurorestoration. However, microglia can also exacerbate neuronal loss by phagocytosing stressed-but-viable neurons in the penumbra, thereby expanding the injury area and hindering neurofunctional recovery. Microglia constantly patrol the central nervous system using their processes to scour the cellular environment and start or cease the phagocytosis progress depending on the "eat me" or "don't eat me'' signals on cellular surface. An optimal immune response requires a delicate balance between different phenotypic states to regulate neuro-inflammation and facilitate reconstruction after stroke. Here, we examine the literature and discuss the molecular mechanisms and cellular pathways regulating microglial phagocytosis, their resulting effects in brain injury and neural regeneration, as well as the potential therapeutic targets that might modulate microglial phagocytic activity to improve neurological function after stroke.

Amitriptyline Improves Cognitive and Neuronal Function in a Rat Model that Mimics Dementia with Lewy Bodies

Dementia with Lewy bodies (DLB), a highly prevalent neurodegenerative disorder, causes motor and cognitive deficits. The main pathophysiologies of DLB are glutamate excitotoxicity and accumulation of Lewy bodies comprising α-synuclein (α-syn) and β-amyloid (Aβ). Amitriptyline (AMI) promotes expression of glutamate transporter-1 and glutamate reuptake. In this study, we measured the effects of AMI on behavioral and neuronal function in a DLB rat model. We used rivastigmine (RIVA) as a positive control. To establish the DLB rat model, male Wistar rats were stereotaxically injected with recombinant adenoassociated viral vector with the SNCA gene (10μg/10μL) and Aβ (5μg/2.5μL) into the left ventricle and prefrontal cortex, respectively. AMI (10mg/kg/day, i.p.), RIVA (2mg/kg/day, i.p.), or saline was injected intraperitoneally after surgery. From the 29^th day, behavioral tests were performed to evaluate the motor and cognitive functions of the rats. Immunohistochemical staining was used to assess neuronal changes. We measured the α-syn level, number of newborn cells, and neuronal density in the hippocampus and in the nigrostriatal dopaminergic system. The DLB group exhibited deficit in object recognition. Both the AMI and RIVA treatments reversed these deficits. Histologically, the DLB rats exhibited cell loss in the substantia nigra pars compacta and in the hippocampal CA1 area. AMI reduced this cell loss, but RIVA did not. In addition, the DLB rats exhibited a lower number of newborn cells and higher α-syn levels in the dentate gyrus (DG). AMI did not affect α-syn accumulation but recovered neurogenesis in the DG of the rats, whereas RIVA reversed the α-syn accumulation but did not affect neurogenesis in the rats. We suggest that AMI may have potential for use in the treatment of DLB.

Phagocytic microglia and macrophages in brain injury and repair

AIMS

Phagocytosis is the cellular digestion of extracellular particles, such as pathogens and dying cells, and is a key element in the evolution of central nervous system (CNS) disorders. Microglia and macrophages are the professional phagocytes of the CNS. By clearing toxic cellular debris and reshaping the extracellular matrix, microglia/macrophages help pilot the brain repair and functional recovery process. However, CNS resident and invading immune cells can also magnify tissue damage by igniting runaway inflammation and phagocytosing stressed-but viable-neurons.

DISCUSSION

Microglia/macrophages help mediate intercellular communication and react quickly to the "find-me" signals expressed by dead/dying neurons. The activated microglia/macrophages then migrate to the injury site to initiate the phagocytic process upon encountering "eat-me" signals on the surfaces of endangered cells. Thus, healthy cells attempt to avoid inappropriate engulfment by expressing "do not-eat-me" signals. Microglia/macrophages also have the capacity to phagocytose immune cells that invade the injured brain (e.g., neutrophils) and to regulate their pro-inflammatory properties. During brain recovery, microglia/macrophages engulf myelin debris, initiate synaptogenesis and neurogenesis, and sculpt a favorable extracellular matrix to support network rewiring, among other favorable roles. Here, we review the multilayered nature of phagocytotic microglia/macrophages, including the molecular and cellular mechanisms that govern microglia/macrophage-induced phagocytosis in acute brain injury, and discuss strategies that tap into the therapeutic potential of this engulfment process.

CONCLUSION

Identification of biological targets that can temper neuroinflammation after brain injury without hindering the essential phagocytic functions of microglia/macrophages will expedite better medical management of the stroke recovery stage.

Angelica sinensis extract promotes neuronal survival by enhancing p38 MAPK-mediated hippocampal neurogenesis and dendritic growth in the chronic phase of transient global cerebral ischemia in rats

ETHNOPHARMACOLOGICAL RELEVANCE

Angelica sinensis (Oliv.) Diels (ASD), commonly known as Dang Gui, is a popular Chinese herb that has long been used to treat ischemic stroke. However, the effects of ASD in chronic cerebral ischemia and its underlying mechanisms still remain unclear.

AIM OF THE STUDY

This study aimed to determine the effects of the ASD extract on hippocampal neuronal survival at 28 d after transient global cerebral ischemia (GCI) and to investigate the precise mechanisms underlying the p38 mitogen-activated protein kinase (MAPK)-related signaling pathway's involvement in hippocampal neurogenesis.

MATERIALS AND METHODS

Rats underwent 25 min of four-vessel occlusion. The ASD extract was intragastrically administered at doses of 0.25 g/kg (ASD-0.25 g), 0.5 g/kg (ASD-0.5 g), 1 g/kg (ASD-1 g), 1 g/kg after dimethyl sulfoxide administration (D + ASD-1 g), or 1 g/kg after SB203580 (a p38 MAPK inhibitor) administration (SB + ASD-1 g) at 1, 3, 7, 10, 14, 17, 21, and 24 d after transient GCI.

RESULTS

ASD-0.5 g, ASD-1 g, and D + ASD-1 g treatments had the following effects: upregulation of bromodeoxyuridine (BrdU) and Ki67 expression, and BrdU/neuronal nuclei (NeuN) and Ki67/nestin co-expression in the hippocampal dentate gyrus (DG); upregulation of microtubule-associated protein 2/NeuN co-expression, and NeuN and glial fibrillary acidic protein (GFAP) expression, and downregulation of tumor necrosis factor-α/GFAP co-expression in the hippocampal CA1 region; upregulation of phospho-p38 MAPK (p-p38 MAPK), phospho-cAMP response element-binding protein (p-CREB), brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), and vascular endothelial growth factor A (VEGF-A) expression in the hippocampus. SB + ASD-1 g treatment abrogated the effects of ASD-1 g on the expression of these proteins.

CONCLUSIONS

ASD-0.5 g and ASD-1 g treatments promotes neuronal survival by enhancing hippocampal neurogenesis. The effects of the ASD extract on astrocyte-associated hippocampal neurogenesis and dendritic growth are caused by the activation of p38 MAPK-mediated CREB/BDNF, GDNF, and VEGF-A signaling pathways in the hippocampus at 28 d after transient GCI.

Functional Activation of Newborn Neurons Following Alcohol-Induced Reactive Neurogenesis

Abstinence after alcohol dependence leads to structural and functional recovery in many regions of the brain, especially the hippocampus. Significant increases in neural stem cell (NSC) proliferation and subsequent "reactive neurogenesis" coincides with structural recovery in hippocampal dentate gyrus (DG). However, whether these reactively born neurons are integrated appropriately into neural circuits remains unknown. Therefore, adult male rats were exposed to a binge model of alcohol dependence. On day 7 of abstinence, the peak of reactive NSC proliferation, rats were injected with bromodeoxyuridine (BrdU) to label dividing cells. After six weeks, rats underwent Morris Water Maze (MWM) training then were sacrificed ninety minutes after the final training session. Using fluorescent immunohistochemistry for c-Fos (neuronal activation), BrdU, and Neuronal Nuclei (NeuN), we investigated whether neurons born during reactive neurogenesis were incorporated into a newly learned MWM neuronal ensemble. Prior alcohol exposure increased the number of BrdU+ cells and newborn neurons (BrdU+/NeuN+ cells) in the DG versus controls. However, prior ethanol exposure had no significant impact on MWM-induced c-Fos expression. Despite increased BrdU+ neurons, no difference in the number of activated newborn neurons (BrdU+/c-Fos+/NeuN+) was observed. These data suggest that neurons born during alcohol-induced reactive neurogenesis are functionally integrated into hippocampal circuitry.

Stroke and immunotherapy: Potential mechanisms and its implications as immune-therapeutics

Ischemia or brain injuries are mostly associated with emergency admissions and huge mortality rates. Stroke is a fatal cerebrovascular malady and second top root of disability and death in both developing and developed countries with a projected rise of 24.9% (from 2010) by 2030. It's the most frequent cause of morbidities and systemic permanent morbidities due to its multi-organ systemic pathology. Brain edema or active immune response cause disturbed or abnormal systemic affects causing inflammatory damage leading to secondary infection and secondary immune response which leads to activation like pneumonia or urine tract infections. There are a variety of post stroke treatments available which claims their usefulness in reducing or inhibiting post stroke and recurrent stroke damage followed by heavy inflammatory actions. Stroke does change the quality of life and also ensures daily chronic rapid neurodegeneration and cognitive decline. The only approved therapies for stroke are alteplase and thrombectomy which is associated with adverse outcomes and are not a total cure for ischemic stroke. Stroke and immune response are reciprocal to the pathology and time of event and it progresses till untreated. The immune reaction during ischemia opens new doors for advanced targeted therapeutics. Nowadays stem cell therapy has shown better results in stroke-prone individuals. Few monoclonal antibodies like natalizumab have shown great impact on pre-clinical and clinical stroke trial studies. In this current review, we have explored an immunology of stroke, current therapeutic scenario and future potential targets as immunotherapeutic agents in stroke therapeutics.

Exploring How Low Oxygen Post Conditioning Improves Stroke-Induced Cognitive Impairment: A Consideration of Amyloid-Beta Loading and Other Mechanisms

Cognitive impairment is a common and disruptive outcome for stroke survivors, which is recognized to be notoriously difficult to treat. Previously, we have shown that low oxygen post-conditioning (LOPC) improves motor function and limits secondary neuronal loss in the thalamus after experimental stroke. There is also emerging evidence that LOPC may improve cognitive function post-stroke. In the current study we aimed to explore how exposure to LOPC may improve cognition post-stroke. Experimental stroke was induced using photothrombotic occlusion in adult, male C57BL/6 mice. At 72 h post-stroke animals were randomly assigned to either normal atmospheric air or to one of two low oxygen (11% O2) exposure groups (either 8 or 24 h/day for 14 days). Cognition was assessed during the treatment phase using a touchscreen based paired-associate learning assessment. At the end of treatment (17 days post-stroke) mice were euthanized and tissue was collected for subsequent histology and biochemical analysis. LOPC (both 8 and 24 h) enhanced learning and memory in the 2nd week post-stroke when compared with stroke animals exposed to atmospheric air. Additionally we observed LOPC was associated with lower levels of neuronal loss, the restoration of several vascular deficits, as well as a reduction in the severity of the amyloid-beta (Aβ) burden. These findings provide further insight into the pro-cognitive benefits of LOPC.

Stage-dependent effects of intermittent hypoxia influence the outcome of hippocampal adult neurogenesis

Over one billion adults worldwide are estimated to suffer from sleep apnea, a condition with wide-reaching effects on brain health. Sleep apnea causes cognitive decline and is a risk factor for neurodegenerative conditions such as Alzheimer’s disease. Rodents exposed to intermittent hypoxia (IH), a hallmark of sleep apnea, exhibit spatial memory deficits associated with impaired hippocampal neurophysiology and dysregulated adult neurogenesis. We demonstrate that IH creates a pro-oxidant condition that reduces the Tbr2⁺ neural progenitor pool early in the process, while also suppressing terminal differentiation of adult born neurons during late adult neurogenesis. We further show that IH-dependent cell-autonomous hypoxia inducible factor 1-alpha (HIF1a) signaling is activated in early neuroprogenitors and enhances the generation of adult born neurons upon termination of IH. Our findings indicate that oscillations in oxygen homeostasis, such as those found in sleep apnea, have complex stage-dependent influence over hippocampal adult neurogenesis.

Endothelial Dysfunction in a Cell Culture Model Exposed to Various Intermittent Hypoxia Modes

Wang, Juan, Jiahui Wang, Xin Li, Wanju Hou, Jie Cao, and Jing Feng. Endothelial dysfunction in a cell culture model exposed to various intermittent hypoxia modes. High Alt Med Biol. 21:388-395, 2020. Objective: To construct an in vitro model of endothelial cells exposed to various intermittent hypoxia (IH) modes, and determine whether different frequencies and degrees can cause different effects on endothelial cells. Methods: EA.hy926 cells were used to set up the cell model. A program-controlled gas delivery system was designed to regulate the flow of premixed air into the cell culture chamber. The cells were divided into eight groups exposed to various IH modes: standard cell culture group, intermittent normoxia (IN) group (21% O₂ 15 seconds/21% O₂ 3 minutes 45 seconds for 12 cycles/h), IH1 group (1.5% O₂ 15 seconds/21% O₂ 8 minutes 15 seconds for 6.32 cycles/h), IH2 group (1.5% O₂ 15 seconds/21% O₂ 5 minutes 15 seconds for 9.23 cycles/h), IH3 group (1.5% O₂ 15 seconds/21% O₂ 3 minutes 45 seconds for 12 cycles/h), IH4 group (1.5% O₂ 15 seconds/21% O₂ 1 minute 45 seconds for 20 cycles/h), IH5 group (1.5% O₂ 15 seconds/21% O₂ 15 seconds for 40 cycles/h), and IH6 group (10% O₂ 15 seconds/21% O₂ 3 minutes 45 seconds for 12 cycles/h). Results: Nuclear factor κB (NFκB) p65, c-fos, tumor necrosis factor-alpha (TNFα), malondialdehyde (MDA), and endothelin-1 (ET-1) were higher in the IH3 group or IH6 group than those in IN group, and they were much higher in IH3 group than those in IH6 group. Superoxide dismutase (SOD) and nitric oxide (NO) were the opposite results. In IH1, IH2, IH3, IH4, and IH5 groups, the frequencies increased gradually. NFκB p65, TNFα, and c-fos were the highest in IH3 group. MDA and ET-1 were the highest in IH2 group. SOD and NO were the lowest in IH2 group. Conclusions: Different IH frequencies and degrees could cause different effects on endothelial cells. The endothelial responses varied with the duration of reoxygenation. So, the duration of reoxygenation was the key phase for endothelial dysfunction.

Hypoxic preconditioning ameliorated neuronal injury after middle cerebral artery occlusion by promoting neurogenesis

OBJECTIVES

Sequelae of stroke were mainly caused by neuronal injury. Oxygen is a key factor affecting the microenvironment of neural stem cells (NSCs), and oxygen levels are used to promote NSC neurogenesis. In this study, effects of intermittent hypoxic preconditioning (HPC) on neurogenesis were investigated in a rat model of middle cerebral artery occlusion (MCAO).

METHODS

SD rats were used to establish the MCAO model. Nissl staining and Golgi staining were used to confirm the neuronal injury status in the MCAO model. Immunofluorescence, transmission electron microscopy, Western blot, and qPCR were used to observe the effects of HPC on neurogenesis. At the same time, the hypothesis that HPC could affect proliferation, apoptosis, differentiation, and migration of NSC was verified in vitro.

RESULTS

Hypoxic preconditioning significantly ameliorated the neuronal injury induced by MCAO. Compared with MCAO group, the dendrites, Edu⁺ /SOX2⁺ , Edu⁺ /DCX⁺ , Edu⁺ /NeuN⁺ , Edu⁺ /GFAP⁺ , and Edu⁺ /Tubulin⁺ positive cells in the HPC + MCAO group exhibited significantly difference. Similarly, axonal and other neuronal injuries in the HPC + MCAO group were also ameliorated. In the in vitro experiments, mild HPC significantly enhanced the viability of NSCs, promoted the migration of differentiated cells, and reduced apoptosis.

CONCLUSIONS

Our results showed that HPC significantly promotes neurogenesis after MCAO and ameliorates neuronal injury.

Activated Mesenchymal Stem Cells Induce Recovery Following Stroke Via Regulation of Inflammation and Oligodendrogenesis

Background Brain repair mechanisms fail to promote recovery after stroke, and approaches to induce brain regeneration are scarce. Mesenchymal stem cells (MSC) are thought to be a promising therapeutic option. However, their efficacy is not fully elucidated, and the mechanism underlying their effect is not known. Methods and Results The middle cerebral artery occlusion model was utilized to determine the efficacy of interferon-γ-activated mesenchymal stem cells (aMSCγ) as an acute therapy for stroke. Here we show that treatment with aMSCγ is a more potent therapy for stroke than naive MSC. aMSCγ treatment results in significant functional recovery assessed by the modified neurological severity score and open-field analysis compared with vehicle-treated animals. aMSCγ-treated animals showed significant reductions in infarct size and inhibition of microglial activation. The aMSCγ treatment suppressed the hypoxia-induced microglial proinflammatory phenotype more effectively than treatment with naive MSC. Importantly, treatment with aMSCγ induced recruitment and differentiation of oligodendrocyte progenitor cells to myelin-producing oligodendrocytes in vivo. To elucidate the mechanism underlying high efficacy of aMSCγ therapy, we examined the secretome of aMSCγ and compared it to that of naive MSC. Intriguingly, we found that aMSCγ but not nMSC upregulated neuron-glia antigen 2, an important extracellular signal and a hallmark protein of oligodendrocyte progenitor cells. Conclusions These results suggest that activation of MSC with interferon-γ induces a potent proregenerative, promyelinating, and anti-inflammatory phenotype of these cells, which increases the potency of aMSCγ as an effective therapy for ischemic stroke.

Age-related difference in protective effect of early post-conditioning on ischemic brain injury: possible involvement of MAP-2/Synaptophysin role

Brain Ischemia/Reperfusion (I/R) injury leads to the failure of the microtubules function and neuronal death. Ischemic post-conditioning is defined as a series of rapid alternating interruptions of blood flow in the first seconds of reperfusion. In the present study, the caspase-3, Microtubule-Associated Protein-2 (MAP-2), Protein Kinase C α (PKCα), c-fos, and synaptophysin were evaluated in the hippocampus of focal I/R post-conditioning model in a time -dependent study in aged and young rats. Adult and aged rats were subjected to right MCAO for 30 min and post-conditioned (10 s) for 3 cycles. Sensory-motor tests were performed, and locomotion and anxiety-like behavior were evaluated. Molecular tests were done by detection kit, RT-PCR, and Western blotting techniques. Ninety-six hours after I/R post-conditioning, neurological signs, locomotion, anxiety-like behavior, and ischemic area were improved in young rats compared to 6 h after I/R post-conditioning (P < 0.001). Caspase-3 activity declined in the hippocampus and cortex of I/R post-conditioned young rats in 96 h after I/R post-conditioning compared with 6 h after I/R post-conditioning (P < 0.001). Also, MAP-2 mRNA, MAP-2 protein level, PKCα, c-fos and synaptophysin protein levels were enhanced during post-conditioning in young rats in 96 h after I/R post-conditioning compared with 6 h after induction of I/R post-conditioning. The results of the present study suggested that, early post-conditioning might be considered as a candidate for therapeutic methods against I/R in the adult animals not aged rats. Moreover, inhibition of cell death in post-conditioned ischemic rats was found to be regulated by some neuroprotective molecules as well as MAP-2 and c-fos in young rats. Graphical abstract Graphical abstract representing the post-conditioning (PC) treatment timeline in adult and old rats.

Dual Functions of Microglia in Ischemic Stroke

Ischemic stroke is a leading cause of morbidity and mortality worldwide. Resident microglia are the principal immune cells of the brain, and the first to respond to the pathophysiological changes induced by ischemic stroke. Traditionally, it has been thought that microglial activation is deleterious in ischemic stroke, and therapies to suppress it have been intensively explored. However, increasing evidence suggests that microglial activation is also critical for neurogenesis, angiogenesis, and synaptic remodeling, thereby promoting functional recovery after cerebral ischemia. Here, we comprehensively review the dual role of microglia during the different phases of ischemic stroke, and the possible mechanisms controlling the post-ischemic activity of microglia. In addition, we discuss the dynamic interactions between microglia and other cells, such as neurons, astrocytes, oligodendrocytes, and endothelial cells within the brain parenchyma and the neurovascular unit.

Low oxygen post conditioning prevents thalamic secondary neuronal loss caused by excitotoxicity after cortical stroke

In the current study, we were interested in investigating whether Low oxygen post-conditioning (LOPC) was capable of limiting the severity of stroke-induced secondary neurodegeneration (SND). To investigate the effect of LOPC we exposed adult male C57/BL6 mice to photothrombotic occlusion (PTO) of the motor and somatosensory cortex. This is known to induce progressive neurodegeneration in the thalamus within two weeks of infarction. Two days after PTO induction mice were randomly assigned to one of four groups: (i) LOPC-15 day exposure group; (ii) a LOPC 15 day exposure followed by a 15 day exposure to normal atmosphere; (iii) normal atmosphere for 15 days and (iv) normal atmosphere for 30 days (n = 20/group). We observed that LOPC reduced the extent of neuronal loss, as indicated by assessment of both area of loss and NeuN⁺ cell counts, within the thalamus. Additionally, we identified that LOPC reduced microglial activity and decreased activity within the excitotoxic signalling pathway of the NMDAR axis. Together, these findings suggest that LOPC limits neuronal death caused by excitotoxicity in sites of secondary damage and promotes neuronal survival. In conclusion, this work supports the potential of utilising LOPC to intervene in the sub-acute phase post-stroke to restrict the severity of SND.

Abolition of aberrant neurogenesis ameliorates cognitive impairment after stroke in mice

Poststroke cognitive impairment is considered one of the main complications during the chronic phase of ischemic stroke. In the adult brain, the hippocampus regulates both encoding and retrieval of new information through adult neurogenesis. Nevertheless, the lack of predictive models and studies based on the forgetting processes hinders the understanding of memory alterations after stroke. Our aim was to explore whether poststroke neurogenesis participates in the development of long-term memory impairment. Here, we show a hippocampal neurogenesis burst that persisted 1 month after stroke and that correlated with an impaired contextual and spatial memory performance. Furthermore, we demonstrate that the enhancement of hippocampal neurogenesis after stroke by physical activity or memantine treatment weakened existing memories. More importantly, stroke-induced newborn neurons promoted an aberrant hippocampal circuitry remodeling with differential features at ipsi- and contralesional levels. Strikingly, inhibition of stroke-induced hippocampal neurogenesis by temozolomide treatment or using a genetic approach (Nestin-CreERT2/NSE-DTA mice) impeded the forgetting of old memories. These results suggest that hippocampal neurogenesis modulation could be considered as a potential approach for treatment of poststroke cognitive impairment.

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.

Intermittent Hypoxia Activates Duration-Dependent Protective and Injurious Mechanisms in Mouse Lung Endothelial Cells

Intermittent hypoxia is a major factor in clinical conditions like the obstructive sleep apnea syndrome or the cyclic recruitment and derecruitment of atelectasis in acute respiratory distress syndrome and positive pressure mechanical ventilation. In vivo investigations of the direct impact of intermittent hypoxia are frequently hampered by multiple co-morbidities of patients. Therefore, cell culture experiments are important model systems to elucidate molecular mechanisms that are involved in the cellular response to alternating oxygen conditions and could represent future targets for tailored therapies. In this study, we focused on mouse lung endothelial cells as a first frontier to encounter altered oxygen due to disturbances in airway or lung function, that play an important role in the development of secondary diseases like vascular disease and pulmonary hypertension. We analyzed key markers for endothelial function including cell adhesion molecules, molecules involved in regulation of fibrinolysis, hemostasis, redox balance, and regulators of gene expression like miRNAs. Results show that short-time exposure to intermittent hypoxia has little impact on vitality and health of cells. At early timepoints and up to 24 h, many endothelial markers are unchanged in their expression and some indicators of injury are even downregulated. However, in the long-term, multiple signaling pathways are activated, that ultimately result in cellular inflammation, oxidative stress, and apoptosis.

Acquired Resilience: An Evolved System of Tissue Protection in Mammals

This review brings together observations on the stress-induced regulation of resilience mechanisms in body tissues. It is argued that the stresses that induce tissue resilience in mammals arise from everyday sources: sunlight, food, lack of food, hypoxia and physical stresses. At low levels, these stresses induce an organised protective response in probably all tissues; and, at some higher level, cause tissue destruction. This pattern of response to stress is well known to toxicologists, who have termed it hormesis. The phenotypes of resilience are diverse and reports of stress-induced resilience are to be found in journals of neuroscience, sports medicine, cancer, healthy ageing, dementia, parkinsonism, ophthalmology and more. This diversity makes the proposing of a general concept of induced resilience a significant task, which this review attempts. We suggest that a system of stress-induced tissue resilience has evolved to enhance the survival of animals. By analogy with acquired immunity, we term this system 'acquired resilience'. Evidence is reviewed that acquired resilience, like acquired immunity, fades with age. This fading is, we suggest, a major component of ageing. Understanding of acquired resilience may, we argue, open pathways for the maintenance of good health in the later decades of human life.

Daily Intermittent Normobaric Hypoxia Over 2 Weeks Reduces BDNF Plasma Levels in Young Adults - A Randomized Controlled Feasibility Study

Background: The results from animal and human research indicate that acute intermittent hypoxia can enhance brain-derived neurotrophic factor (BDNF) plasma levels and gene expression. As BDNF is known to promote the differentiation of new neurons and the formation of synapses, it has been proposed to mediate adult neuroplasticity. Thus, the present study aimed to analyze the long-term effects of daily intermittent exposure to normobaric hypoxia (simulating high altitude exposure at approximately 4000–5000 m) over 2 weeks on BDNF levels in young adults.

Methods: Twenty-eight young adults (age: 19–33 years) were randomized into a hypoxic intervention group (N = 14) or the control group (N = 14). Participants in the intervention group breathed intermittent normobaric hypoxic air at resting conditions (5 min intervals, 80–85% SpO₂ measured via a finger pulse oximeter, 12 sessions for 60 min/day for 2 weeks) via a hypoxic generator. BDNF plasma and serum levels were determined at baseline and at 2 weeks after intervention using sandwich ELISAs.

Results: After 2 weeks of daily intermittent hypoxic treatment (IHT), we found a significant group x time interaction effect for BDNF plasma levels based on a significant decrease in BDNF levels in the hypoxia group.

Conclusion: Our results demonstrate that daily intermittent administration of hypoxic air has a significant effect on BDNF regulation in healthy young adults. Contrary to other results reporting an increase in BDNF levels under hypoxic conditions, the present data suggest that hypoxic treatment using intensive IHT can reduce BDNF plasma levels for at least 2 weeks. This finding indicates that the daily application of hypoxic air is too frequent for the aimed physiological response, namely, an increase in BDNF levels.

Low Oxygen Post Conditioning as an Efficient Non-pharmacological Strategy to Promote Motor Function After Stroke

Low oxygen post conditioning (LOPC) has shown promising results in terms of neuroprotection after stroke, but the effects on motor function have not been considered. Cortical stroke targeting the motor and sensory cortex was induced by photothrombotic occlusion and after 48 h allocated to LOPC (11% O₂) for 2 weeks. Motor impairment was assessed using the cylinder and grid walk tests during the exposure period and for two further weeks upon completion of the intervention. Neuroprotection was evaluated by histological and molecular analysis at two time points. Two weeks of LOPC was sufficient to significantly reduce motor deficits and tissue loss after stroke. This functional improvement was associated with increased capillary density, enhanced levels of BDNF, decreased neuronal loss and decreased microglia activation. These improvements, in most instances, were maintained up to 2 weeks after the end of the treatment. To our knowledge, this is the first study to demonstrate that LOPC induces a persistent improvement in motor function and neuroprotection after stroke, and in doing so provides evidence to support a case for considering taking LOPC forward to early stage clinical research.

Perinatal Hypoxia and Ischemia in Animal Models of Schizophrenia

Intrauterine or perinatal complications constitute a major risk for psychiatric diseases. Infants who suffered from hypoxia-ischemia (HI) are at twofold risk to develop schizophrenia in later life. Several animal models attempt to reproduce these complications to study the yet unknown steps between an insult in early life and outbreak of the disease decades later. However, it is very challenging to find the right type and severity of insult leading to a disease-like phenotype in the animal, but not causing necrosis and focal neurological deficits. By contrast, too mild, repetitive insults may even be protective via conditioning effects. Thus, it is not surprising that animal models of hypoxia lead to mixed results. To achieve clinically translatable findings, better protocols are urgently needed. Therefore, we compare widely used models of hypoxia and HI and propose future directions for the field.

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.

Medical gases for stroke therapy: summary of progress 2015-2016

Stroke is a cerebrovascular disease with high mortality and morbidity. Despite extensive research, there are only a very limited number of therapeutic approaches suitable for treatment of stroke patients as yet. Mounting evidence has demonstrated that such gases as oxygen, hydrogen and hydrogen sulfide are able to provide neuroprotection after stroke. In this paper, we will focus on the recent two years’ progress in the development of gas therapies of stroke and in understanding the molecular mechanisms underlying protection induced by medical gases. We will also discuss the advantages and challenges of these approaches and provide information for future study.

Impact of obstructive sleep apnoea and intermittent hypoxia on cardiovascular and cerebrovascular regulation

NEW FINDINGS

What is the topic of this review? This review examines the notion that obstructive sleep apnoea (OSA) and intermittent hypoxia (IH) have hormetic effects on vascular health. What advances does it highlight? Clinical (OSA patient) and experimental animal and human models report that IH is detrimental to vascular regulation. However, mild IH and, by extension, mild OSA also have physiological and clinical benefits. This review highlights clinical and experimental animal and human data linking OSA and IH to vascular disease and discusses how hormetic effects of OSA and IH relate to OSA severity, IH intensity and duration, and patient/subject age. Obstructive sleep apnoea (OSA) is associated with increased risk of cardiovascular and cerebrovascular disease, a consequence attributed in part to chronic intermittent hypoxia (IH) resulting from repetitive apnoeas during sleep. Although findings from experimental animal, and human, models have shown that IH is detrimental to vascular regulation, the severity of IH used in many of these animal studies [e.g. inspired fraction of oxygen (FI,O2) = 2-3%; oxygen desaturation index = 120 events h^-1 ] is considerably greater than that observed in the majority of patients with OSA. This may also explain disparities between animal and recently developed human models of IH, where IH severity is, by necessity, less severe (e.g. FI,O2 = 10-12%; oxygen desaturation index = 15-30 events h^-1 ). In this review, we highlight the current knowledge regarding the impact of OSA and IH on cardiovascular and cerebrovascular regulation. In addition, we critically discuss the recent notion that OSA and IH may have hormetic effects on vascular health depending on conditions such as OSA severity, IH intensity and duration, and age. In general, data support an independent causal link between OSA and vascular disease, particularly for patients with severe OSA. However, the data are equivocal for older OSA patients and patients with mild OSA, because advanced age and short-duration, low-intensity IH have been reported to provide a degree of protection against IH and ischaemic events such as myocardial infarction and stroke, respectively. Overall, additional studies are needed to investigate the beneficial/detrimental effects of mild OSA on the various vascular beds.

Mechanisms of microglial activation in models of inflammation and hypoxia: Implications for chronic intermittent hypoxia

Chronic intermittent hypoxia (CIH) is a hallmark of sleep apnoea, a condition associated with diverse clinical disorders. CIH and sleep apnoea are characterized by increased reactive oxygen species formation, peripheral and CNS inflammation, neuronal death and neurocognitive deficits. Few studies have examined the role of microglia, the resident CNS immune cells, in models of CIH. Thus, little is known concerning their direct contributions to neuropathology or the cellular mechanisms regulating their activities during or following pathological CIH. In this review, we identify gaps in knowledge regarding CIH-induced microglial activation, and propose mechanisms based on data from related models of hypoxia and/or hypoxia-reoxygenation. CIH may directly affect microglia, or may have indirect effects via the periphery or other CNS cells. Peripheral inflammation may indirectly activate microglia via entry of pro-inflammatory molecules into the CNS, and/or activation of vagal afferents that trigger CNS inflammation. CIH-induced release of damage-associated molecular patterns from injured CNS cells may also activate microglia via interactions with pattern recognition receptors expressed on microglia. For example, Toll-like receptors activate mitogen-activated protein kinase/transcription factor pathways required for microglial inflammatory gene expression. Although epigenetic effects from CIH have not yet been studied in microglia, potential epigenetic mechanisms in microglial regulation are discussed, including microRNAs, histone modifications and DNA methylation. Epigenetic effects can occur during CIH, or long after it has ended. A better understanding of CIH effects on microglial activities may be important to reverse CIH-induced neuropathology in patients with sleep disordered breathing.

In Vivo Expression of Reprogramming Factors Increases Hippocampal Neurogenesis and Synaptic Plasticity in Chronic Hypoxic-Ischemic Brain Injury

Neurogenesis and synaptic plasticity can be stimulated in vivo in the brain. In this study, we hypothesized that in vivo expression of reprogramming factors such as Klf4, Sox2, Oct4, and c-Myc would facilitate endogenous neurogenesis and functional recovery. CD-1® mice were induced at 1 week of age by unilaterally carotid artery ligation and exposure to hypoxia. At 6 weeks of age, mice were injected GFP only or both four reprogramming factors and GFP into lateral ventricle. Passive avoidance task and open field test were performed to evaluate neurobehavioral function. Neurogenesis and synaptic activity in the hippocampus were evaluated using immunohistochemistry, qRT-PCR, and/or western blot analyses. Whereas BrdU⁺GFAP⁺ cells in the subgranular zone of the hippocampus were not significantly different, the numbers of BrdU⁺βIII-tubulin⁺ and BrdU⁺NeuN⁺ cells were significantly higher in treatment group than control group. Expressions of synaptophysin and PSD-95 were also higher in treatment group than control group. Importantly, passive avoidance task and open field test showed improvement in long-term memory and decreased anxiety in treatment group. In conclusion, in vivo expression of reprogramming factors improved behavioral functions in chronic hypoxic-ischemic brain injury. The mechanisms underlying these repair processes included endogenous neurogenesis and synaptic plasticity in the hippocampus.

Hypoxia therapy--a new hope for the treatment of mitochondrial dysfunctions

Mitochondrial dysfunctions are characteristic features of numerous diseases and play a critical role in disease pathogenesis. Despite intensive research in this area, there are no approved therapies that directly target mitochondria. Recently a study by Jain et al. from Massachusetts General Hospital, USA reported the effectiveness of hypoxia for treatment of mitochondrial disease in mice. In this commentary, we summarized the potential mechanisms underlying the therapeutic effects of hypoxia on mitochondrial dysfunction, and clinical limitations of hypoxia as a therapy for human patients. We hope that our concerns will be helpful for further clinical studies addressing moderate hypoxia in mitochondrial dysfunction.

Emerging co-morbidities of obstructive sleep apnea: cognition, kidney disease, and cancer

Obstructive sleep apnea (OSA) causes daytime fatigue and sleepiness, and has an established relationship with cardiovascular and metabolic disease. Recent years have seen the emergence of an evidence base linking OSA with an increased risk of degenerative neurological disease and associated cognitive impairment, an accelerated rate of decline in kidney function with an increased risk of clinically significant chronic kidney disease (CKD), and with a significantly higher rate of cancer incidence and death. This review evaluates the evidence base linking OSA with these seemingly unrelated co-morbidities, and explores potential mechanistic links underpinning their development in patients with OSA, including intermittent hypoxia (IH), sleep fragmentation, sympathetic excitation, and immune dysregulation.

Activation of PPARγ Ameliorates Spatial Cognitive Deficits through Restoring Expression of AMPA Receptors in Seipin Knock-Out Mice

A characteristic phenotype of congenital generalized lipodystrophy 2 (CGL2) that is caused by loss-of-function of seipin gene is mental retardation. Here, we show that seipin deficiency in hippocampal CA1 pyramidal cells caused the reduction of peroxisome proliferator-activated receptor gamma (PPARγ). Twelve-week-old systemic seipin knock-out mice and neuronal seipin knock-out (seipin-nKO) mice, but not adipose seipin knock-out mice, exhibited spatial cognitive deficits as assessed by the Morris water maze and Y-maze, which were ameliorated by the treatment with the PPARγ agonist rosiglitazone (rosi). In addition, seipin-nKO mice showed the synaptic dysfunction and the impairment of NMDA receptor-dependent LTP in hippocampal CA1 regions. The density of AMPA-induced current (IAMPA) in CA1 pyramidal cells and GluR1/GluR2 expression were significantly reduced in seipin-nKO mice, whereas the NMDA-induced current (INMDA) and NR1/NR2 expression were not altered. Rosi treatment in seipin-nKO mice could correct the decrease in expression and activity of AMPA receptor (AMPAR) and was accompanied by recovered synaptic function and LTP induction. Furthermore, hippocampal ERK2 and CREB phosphorylation in seipin-nKO mice were reduced and this could be rescued by rosi treatment. Rosi treatment in seipin-nKO mice elevated BDNF concentration. The MEK inhibitor U0126 blocked rosi-restored AMPAR expression and LTP induction in seipin-nKO mice, but the Trk family inhibitor K252a did not. These findings indicate that the neuronal seipin deficiency selectively suppresses AMPAR expression through reducing ERK-CREB activities, leading to the impairment of LTP and spatial memory, which can be rescued by PPARγ activation.

SIGNIFICANCE STATEMENT

Congenital generalized lipodystrophy 2 (CGL2), caused by loss-of-function mutation of seipin gene, is characterized by mental retardation. By the generation of systemic or neuronal seipin knock-out mice, the present study provides in vivo evidence that neuronal seipin deficiency causes deficits in spatial memory and hippocampal LTP induction. Neuronal seipin deficiency selectively suppresses AMPA receptor expression, ERK-CREB phosphorylation with the decline of PPARγ. The PPARγ agonist rosiglitazone can ameliorate spatial cognitive deficits and rescue the LTP induction in seipin knock-out mice by restoring AMPA receptor expression and ERK-CREB activities.

Intermittent hypoxia initiated plasticity in humans: A multipronged therapeutic approach to treat sleep apnea and overlapping co-morbidities

Over the past three decades exposure to intermittent hypoxia (IH) has generally been considered a stimulus associated with a number of detrimental outcomes. However, there is sufficient evidence to link IH to many beneficial outcomes but they have largely been ignored, particularly in the field of sleep medicine in the United States. Recent reviews have postulated that this apparent contradiction is related to the severity and duration of exposure to IH; mild forms of IH initiate beneficial outcomes while severe forms of IH are coupled to detrimental consequences. In the present review we explore the role that IH has in initiating respiratory plasticity and the potential this form of plasticity has to mitigate obstructive sleep apnea (OSA) in humans. In taking this approach, we address the possibility that IH could serve as an adjunct therapy coupled with continuous positive airway pressure (CPAP) to treat OSA. Our working hypothesis is that exposure to mild IH leads to respiratory plasticity that manifests in increased stability of the upper airway, which could ultimately reduce the CPAP required to treat OSA. In turn, this reduction could increase CPAP compliance and extend the length of treatment each night, which might improve the magnitude of outcome measures. Improved treatment compliance coupled with the direct effect that IH has on numerous overlapping conditions (i.e. asthma, chronic obstructive pulmonary disease, spinal cord injury) may well lead to substantial improvements that exceed outcomes following treatment with CPAP alone. Overall, this review will consider evidence from the published literature which suggests that IH could serve as an effective multipronged therapeutic approach to treat sleep apnea and its overlapping co-morbidities.

Chronic intermittent hypoxia and obstructive sleep apnea: an experimental and clinical approach

Obstructive sleep apnea (OSA) is a prevalent sleep disorder considered as an independent risk factor for cardiovascular consequences, such as systemic arterial hypertension, ischemic heart disease, cardiac arrhythmias, metabolic disorders, and cognitive dysfunction. The pathogenesis of OSA-related consequence is assumed to be chronic intermittent hypoxia (IH) inducing alterations at the molecular level, oxidative stress, persistent systemic inflammation, oxygen sensor activation, and increase of sympathetic activity. Overall, these mechanisms have an effect on vessel permeability and are considered to be important factors for explaining vascular, metabolic, and cognitive OSA-related consequences. The present review attempts to examine together the research paradigms and clinical studies on the effect of acute and chronic IH and the potential link with OSA. We firstly describe the literature data on the mechanisms activated by acute and chronic IH at the experimental level, which are very helpful and beneficial to explaining OSA consequences. Then, we describe in detail the effect of IH in patients with OSA that we can consider "the human model" of chronic IH. In this way, we can better understand the specific pathophysiological mechanisms proposed to explain the consequences of IH in OSA.

The biphasic function of microglia in ischemic stroke

Microglia are brain resident macrophages originated from primitive progenitor cells in the yolk sac. Microglia can be activated within hours and recruited to the lesion site. Traditionally, microglia activation is considered to play a deleterious role in ischemic stroke, as inhibition of microglia activation attenuates ischemia induced brain injury. However, increasing evidence show that microglia activation is critical for attenuating neuronal apoptosis, enhancing neurogenesis, and promoting functional recovery after cerebral ischemia. Differential polarization of microglia could likely explain the biphasic role of microglia in ischemia. We comprehensively reviewed the mechanisms involved in regulating microglia activation and polarization. The latest discoveries of microRNAs in modulating microglia function are discussed. In addition, the interaction between microglia and other cells including neurons, astrocytes, oligodendrocytes, and stem cells were also reviewed. Future therapies targeting microglia may not exclusively aim at suppressing microglia activation, but also at modulating microglia polarization at different stages of ischemic stroke. More work is needed to elucidate the cellular and molecular mechanisms of microglia polarization under ischemic environment. The roles of microRNAs and transplanted stem cells in mediating microglia activation and polarization during brain ischemia also need to be further studied.

Systemic Proteasome Inhibition Induces Sustained Post-stroke Neurological Recovery and Neuroprotection via Mechanisms Involving Reversal of Peripheral Immunosuppression and Preservation of Blood-Brain-Barrier Integrity

In view of its profound effect on cell survival and function, the modulation of the ubiquitin-proteasome-system has recently been shown to promote neurological recovery and brain remodeling after focal cerebral ischemia. Hitherto, local intracerebral delivery strategies were used, which can hardly be translated to human patients. We herein analyzed effects of systemic intraperitoneal delivery of the proteasome inhibitor BSc2118 on neurological recovery, brain injury, peripheral and cerebral immune responses, neurovascular integrity, as well as cerebral neurogenesis and angiogenesis in a mouse model of transient intraluminal middle cerebral artery occlusion. Systemic delivery of BSc2118 induced acute neuroprotection reflected by reduced infarct volume when delivered up to 9 h post-stroke. The latter was associated with reduced brain edema and stabilization of blood-brain-barrier integrity, albeit cerebral proteasome activity was only mildly reduced. Neuronal survival persisted in the post-acute stroke phase up to 28 days post-stroke and was associated with improved neurological recovery when the proteasome inhibitor was continuously delivered over 7 days. Systemic proteasome inhibition prevented stroke-induced acute leukocytosis in peripheral blood and reversed the subsequent immunosuppression, namely, the reduction of blood lymphocyte and granulocyte counts. On the contrary, post-ischemic brain inflammation, cerebral HIF-1α abundance, cell proliferation, neurogenesis, and angiogenesis were not influenced by the proteasome inhibitor. The modulation of peripheral immune responses might thus represent an attractive target for the clinical translation of proteasome inhibitors.

Reduced Cerebral Oxygen Content in the DG and SVZ In Situ Promotes Neurogenesis in the Adult Rat Brain In Vivo

Neurogenesis in the adult brain occurs mainly within two neurogenic structures, the dentate gyrus (DG) of the hippocampus and the sub-ventricular zone (SVZ) of the forebrain. It has been reported that mild hypoxia promoted the proliferation of Neural Stem Cells (NSCs)in vitro. Our previous study further demonstrated that an external hypoxic environment stimulated neurogenesis in the adult rat brain in vivo. However, it remains unknown how external hypoxic environments affect the oxygen content in the brain and result in neurogenesis. Here we use an optical fiber luminescent oxygen sensor to detect the oxygen content in the adult rat brain in situ under normoxia and hypoxia. We found that the distribution of oxygen in cerebral regions is spatiotemporally heterogeneous. The Po2 values in the ventricles (45∼50 Torr) and DG (approximately 10 Torr) were much higher than those of other parts of the brain, such as the cortex and thalamus (approximately 2 Torr). Interestingly, our in vivo studies showed that an external hypoxic environment could change the intrinsic oxygen content in brain tissues, notably reducing oxygen levels in both the DG and SVZ, the major sites of adult neurogenesis. Furthermore, the hypoxic environment also increased the expression of HIF-1α and VEGF, two factors that have been reported to regulate neurogenesis, within the DG and SVZ. Thus, we have demonstrated that reducing the oxygen content of the external environment decreased Po2 levels in the DG and SVZ. This reduced oxygen level in the DG and SVZ might be the main mechanism triggering neurogenesis in the adult brain. More importantly, we speculate that varying oxygen levels may be the physiological basis of the regionally restricted neurogenesis in the adult brain.

The impact of sleep and hypoxia on the brain: potential mechanisms for the effects of obstructive sleep apnea

PURPOSE OF REVIEW

Obstructive sleep apnea (OSA) is a chronic, highly prevalent, multisystem disease, which is still largely underdiagnosed. Its most prominent risk factors, obesity and older age, are on the rise, and its prevalence is expected to grow further. The last few years have seen an exponential increase in studies to determine the impact of OSA on the central nervous system. OSA-induced brain injury is now a recognized clinical entity, although its possible dual relationship with several other neuropsychiatric and neurodegenerative disorders is debated. The putative neuromechanisms behind some of the effects of OSA on the central nervous system are discussed in this review, focusing on the nocturnal intermittent hypoxia and sleep fragmentation.

RECENT FINDINGS

Recent preclinical and clinical findings suggest that neurogenic ischemic preconditioning occurs in some OSA patients, and that it may partly explain variability in clinical findings to date. However, the distinct parameters of the interplay between ischemic preconditioning, neuroinflammation, sleep fragmentation and cerebrovascular changes in OSA-induced brain injury are still largely unclear, and more research is required.

SUMMARY

Early diagnosis and intervention in patients with OSA is of paramount importance. Future clinical studies should utilize multimodal investigative approaches to enable more reliable referencing for the acuity of the pathological process, as well as its reversibility following the treatment.

Enhanced Retinal Ganglion Cell Survival in Glaucoma by Hypoxic Postconditioning After Disease Onset

The neuroprotective efficacy of adaptive epigenetics, wherein beneficial gene expression changes are induced by nonharmful "conditioning" stimuli, is now well established in several acute, preclinical central nervous system injury models. Recently, in a mouse model of glaucoma, we demonstrated retinal ganglion cell (RGC) protection by repetitively "preconditioning" with hypoxia prior to disease onset, indicating an epigenetic approach may also yield benefits in chronic neurodegenerative disease. Herein, we determined whether presenting the repetitive hypoxic stimulus after disease initiation [repetitive hypoxic "postconditioning" (RH-Post)] could afford similar functional and morphologic protection against glaucomatous RGC injury. Chronic elevations in intraocular pressure (IOP) were induced unilaterally in adult male C57BL/6 mice by episcleral vein ligation. Mice were randomized to an RH-Post [1 h of systemic hypoxia (11% oxygen) every other day, starting 4 days after IOP elevation] or an untreated control group. After 3 weeks of experimental glaucoma, the 21-27% reduction and 5-25% prolongation in flash visual-evoked potential amplitudes and latencies, respectively, and the 30% impairment in visual acuity were robustly improved in RH-Post-treated mice, as was the 17% loss in RGC soma number and 20% reduction in axon integrity. These protective effects were observed without RH-Post affecting IOP. The present findings demonstrate that functional and morphologic protection of RGCs can be realized by stimulating epigenetic responses during the early stages of disease, and thus constitute a new conceptual approach to glaucoma therapeutics.

Hypoxic Conditioning as a New Therapeutic Modality

Preconditioning refers to a procedure by which a single noxious stimulus below the threshold of damage is applied to the tissue in order to increase resistance to the same or even different noxious stimuli given above the threshold of damage. Hypoxic preconditioning relies on complex and active defenses that organisms have developed to counter the adverse consequences of oxygen deprivation. The protection it confers against ischemic attack for instance as well as the underlying biological mechanisms have been extensively investigated in animal models. Based on these data, hypoxic conditioning (consisting in recurrent exposure to hypoxia) has been suggested a potential non-pharmacological therapeutic intervention to enhance some physiological functions in individuals in whom acute or chronic pathological events are anticipated or existing. In addition to healthy subjects, some benefits have been reported in patients with cardiovascular and pulmonary diseases as well as in overweight and obese individuals. Hypoxic conditioning consisting in sessions of intermittent exposure to moderate hypoxia repeated over several weeks may induce hematological, vascular, metabolic, and neurological effects. This review addresses the existing evidence regarding the use of hypoxic conditioning as a potential therapeutic modality, and emphasizes on many remaining issues to clarify and future researches to be performed in the field.