Hypoxia in the central nervous system

Recent Progress in the Understanding and Management of Acute Mountain Sickness: A Narrative Review

BACKGROUND

Individuals at higher altitudes may experience a decrease in blood oxygen levels, which can result in a variety of clinical illnesses, such as high-altitude pulmonary edema, high-altitude cerebral edema, and milder but more common acute mountain sickness (AMS).

OBJECTIVE

This study aims to review the current state of knowledge related to motion sickness, the risk of AMS, and pharmacological and non-pharmacological treatments for AMS.

METHODS

Several databases, including PubMed, Bentham Science, Elsevier, Springer, and Research Gate, were used to compile the data for the article following a thorough analysis of the various research findings connected to acute mountain sickness and motion sickness, along with treatments and prevention.

RESULTS

This article covers the research on mountain sickness as well as every imaginable form of conventional and alternative medicine. It contains ten medicinal plants that are useful in treating mountain sickness and various other remedies. Additionally, case studies are provided.

CONCLUSION

Therefore, the information in the paper will help travel medicine specialists better personalize their appropriate care for patients who travel to high-altitude locations. Additionally, all available antiemetic medications, serotonin agonists, nonsteroidal anti-inflammatory drugs, and herbal treatments for motion sickness were discussed. The prevention and consequences of acute mountain sickness are also covered in this study.

Environmental Enrichment Improved Cognitive Performance in Mice under Normoxia and Hypoxia

The mammalian brain modulates its microvascular network to accommodate tissue energy demand in a process referred to as angioplasticity. There is an aging effect on cognitive function and adaptive responses to hypoxia. Hypoxia-induced angiogenesis is delayed in the aging mouse brain. Additionally, it has been shown that environmental enrichment provides an environment that fosters increased physical activity and sensory stimulation for mice as compared to standard housing; this stimulation increases neuronal activity and consequently brain oxygen demand. In this study, we investigated the effect of environmental enrichment and chronic hypoxia on cognitive performance in the young (2-4 months old) and the aged mice (17-21 months old). Mice were placed in a non-enriched or an enriched environment for 4 weeks under normoxia followed by 3 weeks of hypobaric hypoxia (~0.4 atm, equivalent to 8% normobaric oxygen at sea level). Cognitive function was evaluated using the Y-maze and the novel object recognition tests in the enriched or non-enriched mice under normoxic or hypoxic conditions. In Y-maze, a high alternation rate is indicative of sustained cognition as the animals must remember which arm was entered last, so as not to re-enter it. Novel object recognition is based on the natural tendency of rodents to investigate a novel object instead of a familiar one; a higher novel object exploration rate is indicative of better cognitive function. The young mice showed a significantly higher alternation rate (%, 63 ± 7 vs. 48 ± 10, n = 8 and 10, respectively) in the Y-Maze test as compared to the aged mice. Under normoxia, the enriched mice showed an improved alternation rate (%, 63 ± 10, n = 10) in Y-Maze test and a higher novel object exploration rate (%, 68 ± 10 vs. 52 ± 10) compared to the non-enriched controls. Similar results were observed for both young and aged mice following hypoxic exposure. Our data suggests that environmental enrichment improved the cognitive performance in the young and aged mice under normoxic and hypoxic conditions.

Effect of 3-Day and 21-Day Hypoxic Preconditioning on Recovery Following Cerebral Ischemia in Rats

We have previously reported that in a rat model of chronic hypoxia, HIF-1α and its target genes have significantly accumulated by 3 days of exposure, whereas no significant increase in capillary density has occurred; there is a significant increase in capillary density at 21 days of chronic hypoxic exposure. In this study we hypothesize that by utilizing 3 days and 21 days of hypoxic preconditioning, we would distinguish between the relative neuroprotective contributions of the accumulation of HIF-1α and its target genes and angiogenic adaptation in a rat middle cerebral artery occlusion (MCAO) model. Rats were randomly assigned to either hypoxic precondition groups (3-day and 21-day hypoxia) or normoxic control group. Hypoxic animals were kept in a hypobaric chamber at a constant pressure of 0.5 atmosphere (380 mmHg, equivalent to 10% normobaric oxygen at sea level) for either 3 or 21 days. Normoxic controls were housed in the same room next to the hypobaric chamber. Erythropoietin (EPO) was measured at 3 and 21 days of hypoxia using Western blotting analysis. Infarct volumes were measured following 24 hours of permanent MCAO. We found that EPO is upregulated at 3 days of hypoxia and returns to baseline by 21 days of hypoxia. The infarct volumes following 24-hour MCAO were significantly reduced with 3-day hypoxic preconditioning when compared to normoxic controls (%, 31.8 ± 5, n = 9 vs. 50.1 ± 10.9, n = 7). No significant differences in infarct volume were seen between the normoxic controls and 21-day hypoxic preconditioned rats. We have shown that a 3-day hypoxic preconditioning, but not 21-day hypoxic preconditioning, provides significant neuroprotection against focal ischemia in rats, supporting a larger role for the accumulations of HIF-1α and upregulation of its target genes in the neuroprotection against focal ischemia.

Convergent genomic signatures of high-altitude adaptation among domestic mammals

Abundant and diverse domestic mammals living on the Tibetan Plateau provide useful materials for investigating adaptive evolution and genetic convergence. Here, we used 327 genomes from horses, sheep, goats, cattle, pigs and dogs living at both high and low altitudes, including 73 genomes generated for this study, to disentangle the genetic mechanisms underlying local adaptation of domestic mammals. Although molecular convergence is comparatively rare at the DNA sequence level, we found convergent signature of positive selection at the gene level, particularly the EPAS1 gene in these Tibetan domestic mammals. We also reported a potential function in response to hypoxia for the gene C10orf67, which underwent positive selection in three of the domestic mammals. Our data provide an insight into adaptive evolution of high-altitude domestic mammals, and should facilitate the search for additional novel genes involved in the hypoxia response pathway.

NGF, TrkA-P and neuroprotection after a hypoxic event in the developing central nervous system

A decrease in the concentration of oxygen in the blood and tissues (hypoxia) produces important, sometimes irreversible, damages in the central nervous system (CNS) both during development and also postnatally. The present work aims at analyzing the expression of nerve growth factor (NGF) and p75 and the activation of TrkA in response to an acute normobaric hypoxic event and to evaluate the possible protective role of exogenous NGF. The developing chick optic tectum (OT), a recognized model of corticogenesis, was used as experimental system by means of in vivo and in vitro studies. Based on identification of the period of highest sensitivity of developmental programmed cell death (ED15) we show that hypoxia has a mild but reproducible effect that consist of a temporal increase of cell death 6 h after the end of a hypoxic treatment. Cell death was preceded by a significant early increase in the expression of Nerve Growth Factor (NGF) and its membrane receptor p75. In addition, we found a biphasic response of TrkA activation: a decrease during hypoxia followed by an increase -4 h later- that temporally coincide with the interval of NGF overexpression. To test the NGF - NGF receptors role in hypoxic cell death, we quantified, in primary neuronal cultures derived from ED15 OT, the levels of TrkA activation after an acute hypoxic treatment. A significant decline in the level of TrkA activation was observed during hypoxia followed, 24 h later, by significant cell death. Interestingly, this cell death can be reverted if TrkA inactivation during hypoxia is suppressed by the addition of NGF. Our results suggest that TrkA activation may play an important role in the survival of OT neurons subjected to acute hypoxia. The role of TrkA in neuronal survival after injury may be advantageously used for the generation of neuroprotective strategies to improve prenatal insult outcomes.

Brain Tissue PO2 Measurement During Normoxia and Hypoxia Using Two-Photon Phosphorescence Lifetime Microscopy

Key to the understanding of the principles of physiological and structural acclimatization to changes in the balance between energy supply (represented by substrate and oxygen delivery, and mitochondrial oxidative phosphorylation) and energy demand (initiated by neuronal activity) is to determine the controlling variables, how they are sensed and the mechanisms initiated to maintain the balance. The mammalian brain depends completely on continuous delivery of oxygen to maintain its function. We hypothesized that tissue oxygen is the primary sensed variable. In this study two-photon phosphorescence lifetime microscopy (2PLM) was used to determine and define the tissue oxygen tension field within the cerebral cortex of mice to a cortical depth of between 200-250 μm under normoxia and acute hypoxia (FiO₂ = 0.10). High-resolution images can provide quantitative distributions of oxygen and intercapillary oxygen gradients. The data are best appreciated by quantifying the distribution histogram that can then be used for analysis. For example, in the brain cortex of a mouse, at a depth of 200 μm, tissue oxygen tension was mapped and the distribution histogram was compared under normoxic and mild hypoxic conditions. This powerful method can provide for the first time a description of the delivery and availability of brain oxygen in vivo.

Deferoxamine, Cerebrovascular Hemodynamics, and Vascular Aging: Potential Role for Hypoxia-Inducible Transcription Factor-1-Regulated Pathways

BACKGROUND AND PURPOSE

Iron chelation therapy is emerging as a novel neuroprotective strategy. The mechanisms of neuroprotection are diverse and include both neuronal and vascular pathways. We sought to examine the effect of iron chelation on cerebrovascular function in healthy aging and to explore whether hypoxia-inducible transcription factor-1 activation may be temporally correlated with vascular changes.

METHODS

We assessed cerebrovascular function (autoregulation, vasoreactivity, and neurovascular coupling) and serum concentrations of vascular endothelial growth factor and erythropoietin, as representative measures of hypoxia-inducible transcription factor-1 activation, during 6 hours of deferoxamine infusion in 24 young and 24 older healthy volunteers in a randomized, blinded, placebo-controlled cross-over study design. Cerebrovascular function was assessed using the transcranial Doppler ultrasound. Vascular endothelial growth factor and erythropoietin serum protein assays were conducted using the Meso Scale Discovery platform.

RESULTS

Deferoxamine elicited a strong age- and time-dependent increase in the plasma concentrations of erythropoietin and vascular endothelial growth factor, which persisted ≤3 hours post infusion (age effect P=0.04; treatment×time P<0.01). Deferoxamine infusion also resulted in a significant time- and age-dependent improvement in cerebral vasoreactivity (treatment×time P<0.01; age P<0.01) and cerebral autoregulation (gain: age×time×treatment P=0.04).

CONCLUSIONS

Deferoxamine infusion improved cerebrovascular function, particularly in older individuals. The temporal association between improved cerebrovascular function and increased serum vascular endothelial growth factor and erythropoietin concentrations is supportive of shared hypoxia-inducible transcription factor-1-regulated pathways. Therefore, pharmacological activation of hypoxia-inducible transcription factor-1 to enhance cerebrovascular function may be a promising neuroprotective strategy in acute and chronic ischemic syndromes, especially in elderly patients.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrials.gov. Unique identifier: NCT013655104.

Oxidative modulation of K+ channels in the central nervous system in neurodegenerative diseases and aging

SIGNIFICANCE

Oxidative stress and damage are well-established components of neurodegenerative diseases, contributing to neuronal death during disease progression. Here, we consider key K(+) channels as target proteins that can undergo oxidative modulation, describe what is understood about how this influences disease progression, and consider regulation of these channels by gasotransmitters as a means of cellular protection.

RECENT ADVANCES

Oxidative regulation of the delayed rectifier Kv2.1 and the Ca(2+)- and voltage-sensitive BK channel are established, but recent studies contest how their redox sensitivity contributes to altered excitability, progression of neurodegenerative diseases, and healthy aging.

CRITICAL ISSUES

Both Kv2.1 and BK channels have recently been established as target proteins for regulation by the gasotransmitters carbon monoxide and hydrogen sulfide. Establishing the molecular basis of such regulation, and exactly how this influences excitability and vulnerability to apoptotic cell death will determine whether such regulation can be exploited for therapeutic benefit.

FUTURE DIRECTIONS

Developing a more comprehensive picture of the oxidative modulation of K(+) channels (and, indeed, other ion channels) within the central nervous system in health and disease will enable us to better understand processes associated with healthy aging as well as distinct processes underlying progression of neurodegenerative diseases. Advances in the growing understanding of how gasotransmitters can regulate ion channels, including redox-sensitive K(+) channels, are a research priority for this field, and will establish their usefulness in design of future approaches for the treatment of such diseases.

Cerebral neovascularization in diabetes: implications for stroke recovery and beyond

Neovascularization is an innate physiologic response by which tissues respond to various stimuli through collateral remodeling (arteriogenesis) and new vessel formation from existing vessels (angiogenesis) or from endothelial progenitor cells (vasculogenesis). Diabetes has a major impact on the neovascularization process but the response varies between different organ systems. While excessive angiogenesis complicates diabetic retinopathy, impaired neovascularization contributes to coronary and peripheral complications of diabetes. How diabetes influences cerebral neovascularization remained unresolved until recently. Diabetes is also a major risk factor for stroke and poor recovery after stroke. In this review, we discuss the impact of diabetes, stroke, and diabetic stroke on cerebral neovascularization, explore potential mechanisms involved in diabetes-mediated neovascularization as well as the effects of the diabetic milieu on poststroke neovascularization and recovery, and finally discuss the clinical implications of these effects.

Evolution of the couple cytochrome c and cytochrome c oxidase in primates

Mitochondrial energy metabolism has been affected by a broad set of ancient and recent evolutionary events. The oldest example is the endosymbiosis theory that led to mitochondria and a recently proposed example is adaptation to cold climate by anatomically modern human lineages. Mitochondrial energy metabolism has also been associated with an important area in anthropology and evolutionary biology, brain enlargement in human evolution. Indeed, several studies have pointed to the need for a major metabolic rearrangement to supply a sufficient amount of energy for brain development in primates.The genes encoding for the coupled cytochrome c (Cyt c) and cytochrome c oxidase (COX, complex IV, EC 1.9.3.1) seem to have an exceptional pattern of evolution in the anthropoid lineage. It has been proposed that this evolution was linked to the rearrangement of energy metabolism needed for brain enlargement. This hypothesis is reinforced by the fact that the COX enzyme was proposed to have a large role in control of the respiratory chain and thereby global energy production.After summarizing major events that occurred during the evolution of COX and cytochrome c on the primate lineage, we review the different evolutionary forces that could have influenced primate COX evolution and discuss the probable causes and consequences of this evolution. Finally, we discuss and review the co-occurring primate phenotypic evolution.

Neuroprotection by hypoxic preconditioning involves upregulation of hypoxia-inducible factor-1 in a prenatal model of acute hypoxia

The molecular pathways underlying the neuroprotective effects of preconditioning are promising, potentially drugable targets to promote cell survival. However, these pathways are complex and are not yet fully understood. In this study we have established a paradigm of hypoxic preconditioning based on a chick embryo model of normobaric acute hypoxia previously developed by our group. With this model, we analyzed the role of hypoxia-inducible factor-1α (HIF-1α) stabilization during preconditioning in HIF-1 signaling after the hypoxic injury and in the development of a neuroprotective effect against the insult. To this end, we used a pharmacological approach, based on the in vivo administration of positive (Fe(2+), ascorbate) and negative (CoCl(2)) modulators of the activity of HIF-prolyl hydroxylases (PHDs), the main regulators of HIF-1. We have found that preconditioning has a reinforcing effect on HIF-1 accumulation during the subsequent hypoxic injury. In addition, we have also demonstrated that HIF-1 induction during hypoxic preconditioning is necessary to obtain an enhancement in HIF-1 accumulation and to develop a tolerance against a subsequent hypoxic injury. We provide in vivo evidence that administration of Fe(2+) and ascorbate modulates HIF accumulation, suggesting that PHDs might be targets for neuroprotection in the CNS.

Multiple oxygen tension environments reveal diverse patterns of transcriptional regulation in primary astrocytes

The central nervous system normally functions at O₂ levels which would be regarded as hypoxic by most other tissues. However, most in vitro studies of neurons and astrocytes are conducted under hyperoxic conditions without consideration of O₂-dependent cellular adaptation. We analyzed the reactivity of astrocytes to 1, 4 and 9% O₂ tensions compared to the cell culture standard of 20% O₂, to investigate their ability to sense and translate this O₂ information to transcriptional activity. Variance of ambient O₂ tension for rat astrocytes resulted in profound changes in ribosomal activity, cytoskeletal and energy-regulatory mechanisms and cytokine-related signaling. Clustering of transcriptional regulation patterns revealed four distinct response pattern groups that directionally pivoted around the 4% O₂ tension, or demonstrated coherent ascending/decreasing gene expression patterns in response to diverse oxygen tensions. Immune response and cell cycle/cancer-related signaling pathway transcriptomic subsets were significantly activated with increasing hypoxia, whilst hemostatic and cardiovascular signaling mechanisms were attenuated with increasing hypoxia. Our data indicate that variant O₂ tensions induce specific and physiologically-focused transcript regulation patterns that may underpin important physiological mechanisms that connect higher neurological activity to astrocytic function and ambient oxygen environments. These strongly defined patterns demonstrate a strong bias for physiological transcript programs to pivot around the 4% O₂ tension, while uni-modal programs that do not, appear more related to pathological actions. The functional interaction of these transcriptional ‘programs’ may serve to regulate the dynamic vascular responsivity of the central nervous system during periods of stress or heightened activity.

Local cell proliferation upon enucleation in Direct Retinal Brain Targets in the Visual system of the Adult Mouse

In this study we used incorporation of the DNA synthesis marker 5-bromo-2′-deoxyuridine or BrdU to visualize cell proliferation in the visual system of the adult mouse as a response to monocular enucleation. We detected new BrdU-labeled cells in different subcortical retinal target regions and we established a specific time frame in which this cell proliferation occurred. By performing immunofluorescent double stainings for BrdU and different vascular (glucose transporter type 1, collagen type IV), glial (thymosin β4, glial fibrillary acidic protein) and neuronal (Neuronal Nuclei, doublecortin) markers, we identified these proliferating cells as activated microglia. Additional immunohistochemical stainings for thymosin β4 and glial fibrillary acidic protein also revealed reactive astrocytes in the different retinorecipient nuclei and allowed us to delineate a time frame for microglial and astroglial activation. A PCR array experiment further showed increased levels of cytokines, chemokines, growth factors and enzymes that play an important role in microglial-astroglial communication during the glial activation process in response to the deafferentation.

Impaired expression of neuroprotective molecules in the HIF-1alpha pathway following traumatic brain injury in aged mice

Elderly traumatic brain injury (TBI) patients have higher rates of mortality and worse functional outcome than non-elderly TBI patients. The mechanisms involved in poor outcomes in the elderly are not well understood. Hypoxia-inducible factor-1 alpha (HIF-1alpha) is a basic helix-loop-helix transcription factor that modulates expression of key genes involved in neuroprotection. In this study, we studied the expression of HIF-1alpha and its target survival genes, heme oxygenase-1 (HO-1), vascular endothelial growth factor (VEGF), and erythropoietin (EPO) in the brains of adult versus aged mice following controlled cortical impact (CCI) injury. Adult (5-6 months) and aged (23-24 months) C57Bl/6 mice were injured using a CCI device. At 72 h post-injury, mice were sacrificed and the injured cortex was used for mRNA and protein analysis using real-time reverse transcription--polymerase chain reaction (RT-PCR) and Western blotting protocols. Following injury, HIF-1alpha, HO-1, and VEGF showed upregulation in both the young and aged mice, but in the aged animals the increase in HIF-1alpha and VEGF in response to injury was much lower than in the adult injured animals. EPO was upregulated in the adult injured brain, but not in the aged injured brain. These results support the hypothesis that reduced expression of genes in the HIF-1alpha neuroprotective pathway in aging may contribute to poor prognosis in the elderly following TBI.

Neurons express hemoglobin alpha- and beta-chains in rat and human brains

Hemoglobin is the oxygen carrier in vertebrate blood erythrocytes. Here we report that hemoglobin chains are expressed in mammalian brain neurons and are regulated by a mitochondrial toxin. Transcriptome analyses of laser-capture microdissected nigral dopaminergic neurons in rats and striatal neurons in mice revealed the presence of hemoglobin alpha, adult chain 2 (Hba-a2) and hemoglobin beta (Hbb) transcripts, whereas other erythroid markers were not detected. Quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) analysis confirmed the expression of Hba-a2 and Hbb in nigral dopaminergic neurons, striatal gamma-aminobutyric acid (GABA)ergic neurons, and cortical pyramidal neurons in rats. Combined in situ hybridization histochemistry and immunohistochemistry with the neuronal marker neuronal nuclear antigen (NeuN) in rat brain further confirmed the presence of hemoglobin mRNAs in neurons. Immunohistochemistry identified hemoglobin alpha- and beta-chains in both rat and human brains, and hemoglobin proteins were detected by Western blotting in whole rat brain tissue as well as in cultures of mesencephalic neurons, further excluding the possibility of blood contamination. Systemic administration of the mitochondrial inhibitor rotenone (2 mg/kg/d, 7d, s.c.) induced a marked decrease in Hba-a2 and Hbb but not neuroglobin or cytoglobin mRNA in transcriptome analyses of nigral dopaminergic neurons. Quantitative RT-PCR confirmed the transcriptional downregulation of Hba-a2 and Hbb in nigral, striatal, and cortical neurons. Thus, hemoglobin chains are expressed in neurons and are regulated by treatments that affect mitochondria, opening up the possibility that they may play a novel role in neuronal function and response to injury.

Endogenous neuroprotection: mitochondria as gateways to cerebral preconditioning?

From single to multicellular organisms, protective mechanisms have evolved against endogenous and exogenous noxious stimuli. Preconditioning paradigms, in which stimulation below the threshold of injury results in subsequent protection of the brain, have played an important role in elucidating such endogenous protective mechanisms. Consequently, over the past decades numerous signaling pathways have been discovered by which the brain senses and reacts to such insults as neurotoxins, substrate deprivation, or inflammation. Research on preconditioning is aimed at understanding endogenous neuroprotection to boost it, or to supplement its effectors therapeutically once damage to the brain has occurred, such as after stroke or brain trauma. Another goal of establishing preconditioning protocols is to induce endogenous neuroprotection in anticipation of incipient brain damage. Currently several endogenous neuroprotectants are being investigated in controlled clinical trials. In the present review we will give a short overview on the signals, sensors, transducers, and effectors of endogenous neuroprotection. We will first focus on common mechanisms, on which pathways of endogenous neuroprotection converge, and in particular on mitochondria, which may be considered master integrators of endogenous neuroprotection. We will then discuss various applications of preconditioning, including pharmacological and anesthetic preconditioning, as well as postconditioning, and explore the prospects of endogenous neuroprotective therapeutic approaches.