Molecular Mechanisms of High-Altitude Acclimatization

Altitude illnesses

Millions of people visit high-altitude regions annually and more than 80 million live permanently above 2,500 m. Acute high-altitude exposure can trigger high-altitude illnesses (HAIs), including acute mountain sickness (AMS), high-altitude cerebral oedema (HACE) and high-altitude pulmonary oedema (HAPE). Chronic mountain sickness (CMS) can affect high-altitude resident populations worldwide. The prevalence of acute HAIs varies according to acclimatization status, rate of ascent and individual susceptibility. AMS, characterized by headache, nausea, dizziness and fatigue, is usually benign and self-limiting, and has been linked to hypoxia-induced cerebral blood volume increases, inflammation and related trigeminovascular system activation. Disruption of the blood-brain barrier leads to HACE, characterized by altered mental status and ataxia, and increased pulmonary capillary pressure, and related stress failure induces HAPE, characterized by dyspnoea, cough and exercise intolerance. Both conditions are progressive and life-threatening, requiring immediate medical intervention. Treatment includes supplemental oxygen and descent with appropriate pharmacological therapy. Preventive measures include slow ascent, pre-acclimatization and, in some instances, medications. CMS is characterized by excessive erythrocytosis and related clinical symptoms. In severe CMS, temporary or permanent relocation to low altitude is recommended. Future research should focus on more objective diagnostic tools to enable prompt treatment, improved identification of individual susceptibilities and effective acclimatization and prevention options.

Baroreflex and chemoreflex interaction in high-altitude exposure: possible role on exercise performance

The hypoxic chemoreflex and the arterial baroreflex are implicated in the ventilatory response to exercise. It is well known that long-term exercise training increases parasympathetic and decreases sympathetic tone, both processes influenced by the arterial baroreflex and hypoxic chemoreflex function. Hypobaric hypoxia (i.e., high altitude [HA]) markedly reduces exercise capacity associated with autonomic reflexes. Indeed, a reduced exercise capacity has been found, paralleled by a baroreflex-related parasympathetic withdrawal and a pronounced chemoreflex potentiation. Additionally, it is well known that the baroreflex and chemoreflex interact, and during activation by hypoxia, the chemoreflex is predominant over the baroreflex. Thus, the baroreflex function impairment may likely facilitate the exercise deterioration through the reduction of parasympathetic tone following acute HA exposure, secondary to the chemoreflex activation. Therefore, the main goal of this review is to describe the main physiological mechanisms controlling baro- and chemoreflex function and their role in exercise capacity during HA exposure.

Novel insight into the genetic signatures of altitude adaptation related body composition in Tibetans

Background

The Tibetan population residing in high-altitude (HA) regions has adapted to extreme hypoxic environments. However, there is limited understanding of the genetic basis of body compositions in Tibetan population adapted to HA.

Methods

We performed a genome-wide association study (GWAS) to identify genetic variants associated with HA and HA-related body composition traits. A total of 755,731 single nucleotide polymorphisms (SNPs) were genotyped using the precision medicine diversity array from 996 Tibetan college students. T-tests and Pearson correlation analysis were used to estimate the association between body compositions and altitude. The mixed linear regression identified the SNPs significantly associated with HA and HA-related body compositions. LASSO regression was used to screen for important SNPs in HA and body compositions.

Results

Significant differences were observed in lean body mass (LBW), muscle mass (MM), total body water (TBW), standard weight (SBW), basal metabolic rate (BMR), total protein (TP), and total inorganic salt (Is) in different altitudes stratification. We identified three SNPs in EPAS1 (rs1562453, rs7589621 and rs7583392) that were significantly associated with HA (p < 5 × 10-7). GWAS analysis of 7 HA-related body composition traits, we identified 14 SNPs for LBM, 11 SNPs for TBW, 15 SNPs for MM, 16 SNPs for SBW, 9 SNPs for BMR, 12 SNPs for TP, and 26 SNPs for Is (p < 5.0 × 10-5).

Conclusion

These findings provide insight into the genetic basis of body composition in Tibetan college students adapted to HA, and lay the foundation for further investigation into the molecular mechanisms underlying HA adaptation.

Different Evolutionary Trends of Galloanseres: Mitogenomics Analysis

The two existing clades of Galloanseres, orders Galliformes (landfowl) and Anseriformes (waterfowl), exhibit dramatically different evolutionary trends. Mitochondria serve as primary sites for energy production in organisms, and numerous studies have revealed their role in biological evolution and ecological adaptation. We assembled the complete mitogenome sequences of two species of the genus Aythya within Anseriformes: Aythya baeri and Aythya marila. A phylogenetic tree was constructed for 142 species within Galloanseres, and their divergence times were inferred. The divergence between Galliformes and Anseriformes occurred ~79.62 million years ago (Mya), followed by rapid evolution and diversification after the Middle Miocene (~13.82 Mya). The analysis of selective pressure indicated that the mitochondrial protein-coding genes (PCGs) of Galloanseres species have predominantly undergone purifying selection. The free-ratio model revealed that the evolutionary rates of COX1 and COX3 were lower than those of the other PCGs, whereas ND2 and ND6 had faster evolutionary rates. The CmC model also indicated that most PCGs in Anseriformes exhibited stronger selective constraints. Our study suggests that the distinct evolutionary trends and energy requirements of Galliformes and Anseriformes drive different evolutionary patterns in the mitogenome.

Immune consequences of exercise in hypoxia: A narrative review

Immune outcomes are key mediators of many health benefits of exercise and are determined by exercise type, dose (frequency/duration, intensity), and individual characteristics. Similarly, reduced availability of ambient oxygen (hypoxia) modulates immune functions depending on the hypoxic dose and the individual capacity to respond to hypoxia. How combined exercise and hypoxia (e.g., high-altitude training) sculpts immune responses is not well understood, although such combinations are becoming increasingly popular. Therefore, in this paper, we summarize the impact on immune responses of exercise and of hypoxia, both independently and together, with a focus on specialized cells in the innate and adaptive immune system. We review the regulation of the immune system by tissue oxygen levels and the overlapping and distinct immune responses related to exercise and hypoxia, then we discuss how they may be modulated by nutritional strategies. Mitochondrial, antioxidant, and anti-inflammatory mechanisms underlie many of the adaptations that can lead to improved cellular metabolism, resilience, and overall immune functions by regulating the survival, differentiation, activation, and migration of immune cells. This review shows that exercise and hypoxia can impair or complement/synergize with each other while regulating immune system functions. Appropriate acclimatization, training, and nutritional strategies can be used to avoid risks and tap into the synergistic potentials of the poorly studied immune consequences of exercising in a hypoxic state.

Rapid altitude displacement induce zebrafish appearing acute high altitude illness symptoms

Rapid ascent to high-altitude areas above 2500 m often leads to acute high altitude illness (AHAI), posing significant health risks. Current models for AHAI research are limited in their ability to accurately simulate the high-altitude environment for drug screening. Addressing this gap, a novel static self-assembled water vacuum transparent chamber was developed to induce AHAI in zebrafish. This study identified 6000 m for 2 h as the optimal condition for AHAI induction in zebrafish. Under these conditions, notable behavioral changes including slow movement, abnormal exploration behavior and static behavior in the Novel tank test. Furthermore, this model demonstrated changes in oxidative stress-related markers included increased levels of malondialdehyde, decreased levels of glutathione, decreased activities of superoxide dismutase and catalase, and increased levels of inflammatory markers IL-6, IL-1β and TNF-α, and inflammatory cell infiltration and mild edema in the gill tissue, mirroring the clinical pathophysiology observed in AHAI patients. This innovative zebrafish model not only offers a more accurate representation of the high-altitude environment but also provides a high-throughput platform for AHAI drug discovery and pathogenesis research.

Longitudinal multi-omics analysis uncovers the altered landscape of gut microbiota and plasma metabolome in response to high altitude

BACKGROUND

Gut microbiota is significantly influenced by altitude. However, the dynamics of gut microbiota in relation to altitude remains undisclosed.

METHODS

In this study, we investigated the microbiome profile of 610 healthy young men from three different places in China, grouped by altitude, duration of residence, and ethnicity. We conducted widely targeted metabolomic profiling and clinical testing to explore metabolic characteristics.

RESULTS

Our findings revealed that as the Han individuals migrated from low altitude to high latitude, the gut microbiota gradually converged towards that of the Tibetan populations but reversed upon returning to lower altitude. Across different cohorts, we identified 51 species specifically enriched during acclimatization and 57 species enriched during deacclimatization to high altitude. Notably, Prevotella copri was found to be the most enriched taxon in both Tibetan and Han populations after ascending to high altitude. Furthermore, significant variations in host plasma metabolome and clinical indices at high altitude could be largely explained by changes in gut microbiota composition. Similar to Tibetans, 41 plasma metabolites, such as lactic acid, sphingosine-1-phosphate, taurine, and inositol, were significantly elevated in Han populations after ascending to high altitude. Germ-free animal experiments demonstrated that certain species, such as Escherichia coli and Klebsiella pneumoniae, which exhibited altitude-dependent variations in human populations, might play crucial roles in host purine metabolism.

CONCLUSIONS

This study provides insights into the dynamics of gut microbiota and host plasma metabolome with respect to altitude changes, indicating that their dynamics may have implications for host health at high altitude and contribute to host adaptation. Video Abstract.

Recommendations for Women in Mountain Sports and Hypoxia Training/Conditioning

The (patho-)physiological responses to hypoxia are highly heterogeneous between individuals. In this review, we focused on the roles of sex differences, which emerge as important factors in the regulation of the body's reaction to hypoxia. Several aspects should be considered for future research on hypoxia-related sex differences, particularly altitude training and clinical applications of hypoxia, as these will affect the selection of the optimal dose regarding safety and efficiency. There are several implications, but there are no practical recommendations if/how women should behave differently from men to optimise the benefits or minimise the risks of these hypoxia-related practices. Here, we evaluate the scarce scientific evidence of distinct (patho)physiological responses and adaptations to high altitude/hypoxia, biomechanical/anatomical differences in uphill/downhill locomotion, which is highly relevant for exercising in mountainous environments, and potentially differential effects of altitude training in women. Based on these factors, we derive sex-specific recommendations for mountain sports and intermittent hypoxia conditioning: (1) Although higher vulnerabilities of women to acute mountain sickness have not been unambiguously shown, sex-dependent physiological reactions to hypoxia may contribute to an increased acute mountain sickness vulnerability in some women. Adequate acclimatisation, slow ascent speed and/or preventive medication (e.g. acetazolamide) are solutions. (2) Targeted training of the respiratory musculature could be a valuable preparation for altitude training in women. (3) Sex hormones influence hypoxia responses and hormonal-cycle and/or menstrual-cycle phases therefore may be factors in acclimatisation to altitude and efficiency of altitude training. As many of the recommendations or observations of the present work remain partly speculative, we join previous calls for further quality research on female athletes in sports to be extended to the field of altitude and hypoxia.

Human hypoxia models in aerospace medicine: Potential applications for human pharmacological research

Aerospace medicine required controlled terrestrial models to investigate influences of altered atmosphere conditions, such as hypoxia, on human health and performance. These models could potentially be expanded to encompass disease conditions or treatment targets regulated through hypoxia or hypercapnia. Hypoxia, a condition in which the body is deprived of adequate oxygen supply, profoundly affects human physiology at multiple levels and contributes to the pathogenesis of various diseases. Experimental exposure to hypoxic conditions has gained recognition as a model for studying diseases such as pulmonary hypertension, chronic obstructive pulmonary disease, obstructive sleep apnoea, migraine and kidney disease. This approach may be particularly useful in mechanism-oriented early-stage clinical studies. This review discusses the ability of hypoxia models from space medicine research to mimic or induce these conditions in a controlled laboratory setting as a tool for testing the efficacy and safety of new pharmaceutical interventions.

Mitochondrial uncoupling proteins protect human airway epithelial ciliated cells from oxidative damage

Apical cilia on epithelial cells defend the lung by propelling pathogens and particulates out of the respiratory airways. Ciliated cells produce ATP that powers cilia beating by densely grouping mitochondria just beneath the apical membrane. However, this efficient localization comes at a cost because electrons leaked during oxidative phosphorylation react with molecular oxygen to form superoxide, and thus, the cluster of mitochondria creates a hotspot for oxidant production. The relatively high oxygen concentration overlying airway epithelia further intensifies the risk of generating superoxide. Thus, airway ciliated cells face a unique challenge of producing harmful levels of oxidants. However, surprisingly, highly ciliated epithelia produce less reactive oxygen species (ROS) than epithelia with few ciliated cells. Compared to other airway cell types, ciliated cells express high levels of mitochondrial uncoupling proteins, UCP2 and UCP5. These proteins decrease mitochondrial protonmotive force and thereby reduce production of ROS. As a result, lipid peroxidation, a marker of oxidant injury, decreases. However, mitochondrial uncoupling proteins exact a price for decreasing oxidant production; they decrease the fraction of mitochondrial respiration that generates ATP. These findings indicate that ciliated cells sacrifice mitochondrial efficiency in exchange for safety from damaging oxidation. Employing uncoupling proteins to prevent oxidant production, instead of relying solely on antioxidants to decrease postproduction oxidant levels, may offer an advantage for targeting a local area of intense ROS generation.

High altitude retinopathy: An overview and new insights

High altitude retinopathy (HAR) is a common ocular disorder that occurs on ascent to high altitude. There are many clinical symptoms, retinal vascular dilatation, retinal edema and hemorrhage are common. These usually do not or slightly affect vision; rarely, severe cases develop serious or permanent vision loss. At present, the research progress of HAR mainly focuses on hemodynamic changes, blood-retinal barrier damage, oxidative stress and inflammatory response. Although the related studies on HAR are limited, it shows that HAR still belongs to hypoxia, and hypobaric hypoxia plays an aggravating role in promoting the development of the disease. Various studies have demonstrated the correlation of HAR with acute mountain sickness (AMS) and high-altitude cerebral edema (HACE), so a deeper understanding of HAR is important. The slow ascent rates and ascent altitude are the key to preventing any altitude sickness. Research on traditional chinese medicine (TCM) and western medicine has been gradually carried out. Further exploration of the pathogenesis and prevention strategies of HAR will provide better guidance for doctors and high-altitude travelers.

A high altitude respiration and SpO2 dataset for assessing the human response to hypoxia

This report presents the Harespod dataset, an open dataset for high altitude hypoxia research, which includes respiration and SpO2 data. The dataset was collected from 15 college students aged 23-31 in a hypobaric oxygen chamber, during simulated altitude changes and induced hypoxia. Real-time physiological data, such as oxygen saturation waveforms, oxygen saturation, respiratory waveforms, heart rate, and pulse rate, were obtained at 100 Hz. Approximately 12 hours of valid data were collected from all participants. Researchers can easily identify the altitude corresponding to physiological signals based on their inherent patterns. Time markers were also recorded during altitude changes to facilitate realistic annotation of physiological signals and analysis of time-difference-of-arrival between various physiological signals for the same altitude change event. In high altitude scenarios, this dataset can be used to enhance the detection of human hypoxia states, predict respiratory waveforms, and develop related hardware devices. It will serve as a valuable and standardized resource for researchers in the field of high altitude hypoxia research, enabling comprehensive analysis and comparison.

Hypobaric hypoxia drives selection of altitude-associated adaptative alleles in the Himalayan population

Genetic variants play a crucial role in shaping the adaptive phenotypes associated with high-altitude populations. Nevertheless, a comprehensive understanding of the specific impacts of different environments associated with increasing altitudes on the natural selection of these genetic variants remains undetermined. Hence, this study aimed to identify genetic markers responsible for high-altitude adaptation with specific reference to different altitudes, majorly focussing on an altitude elevation range of ∼1500 m and a corresponding decrease of ≥5 % in ambient oxygen availability. We conducted a comprehensive genome-wide investigation (n = 192) followed by a validation study (n = 514) in low-altitude and three high-altitude populations (>2400 m) of Nubra village (NU) (3048 m), Sakti village (SKT) (3812 m), and Tso Moriri village (TK) (4522 m). Extensive genetic analysis identified 86 SNPs that showed significant associations with high-altitude adaptation. Frequency mapping of these SNPs revealed 38 adaptive alleles and specific haplotypes that exhibited a strong linear correlation with increasing altitude. Notably, these SNPs spanned crucial genes, such as ADH6 and NAPG along with the vastly studied genes like EGLN1 and EPAS1, involved in oxygen sensing, metabolism, and vascular homeostasis. Correlation analyses between these adaptive alleles and relevant clinical and biochemical markers provided evidence of their functional relevance in physiological adaptation to hypobaric hypoxia. High-altitude population showed a significant increase in plasma 8-isoPGF2α levels as compared to low-altitude population. Similar observation showcased increased blood pressure in NU as compared to TK (P < 0.0001). In silico analyses further confirmed that these alleles regulate gene expression of EGLN1, EPAS1, COQ7, NAPG, ADH6, DUOXA1 etc. This study provides genetic insights into the effects of hypobaric-hypoxia on the clinico-physiological characteristics of natives living in increasing high-altitude regions. Overall, our findings highlight the synergistic relationship between environment and evolutionary processes, showcasing physiological implications of genetic variants in oxygen sensing and metabolic pathway genes in increasing high-altitude environments.

Intermittent hypoxia in sport nutrition, performance, health status and body composition

Introduction

Intermittent hypoxia refers to the discontinuous use of low oxygen levels in normobaric environment. These conditions can be reproduced in hypoxic tents or chambers while the individual is training in different physical activity protocols. Intermittent hypoxia can affect several body systems, impacting nutrition, physical performance, health status and body composition. Therefore, it is necessary to assess protocols, regarding time and frequency of exposure, passive exposure or training in hypoxia, and the simulated altitude. At the molecular level, the hypoxia-inducible factor-1α is the primary factor mediating induction of target genes, including vascular endothelial growth factor and erythropoietin. The goal of these molecular changes is to preserve oxygen supply for cardiac and neuronal function. In addition, hypoxia produces a sympathetic adrenal activation that can increase the resting metabolic rate. Altogether, these changes are instrumental in protocols designed to improve physical performance as well as functional parameters for certain pathological disorders. In addition, nutrition must adapt to the increased energy expenditure. In this last context, performing physical activity in intermittent hypoxia improves insulin sensitivity by increasing the presence of the glucose transporter GLUT-4 in muscle membranes. These changes could also be relevant for obesity and type 2 diabetes treatment. Also, the anorectic effect of intermittent hypoxia modulates serotonin and circulating leptin levels, which may contribute to regulate food intake and favor body weight adaptation for optimal sport performance and health. All these actions suggest that intermittent hypoxia can be a very effective tool in sports training as well as in certain clinical protocols.

Hypoxia Sensing and Responses in Parkinson's Disease

Parkinson's disease (PD) is associated with various deficits in sensing and responding to reductions in oxygen availability (hypoxia). Here we summarize the evidence pointing to a central role of hypoxia in PD, discuss the relation of hypoxia and oxygen dependence with pathological hallmarks of PD, including mitochondrial dysfunction, dopaminergic vulnerability, and alpha-synuclein-related pathology, and highlight the link with cellular and systemic oxygen sensing. We describe cases suggesting that hypoxia may trigger Parkinsonian symptoms but also emphasize that the endogenous systems that protect from hypoxia can be harnessed to protect from PD. Finally, we provide examples of preclinical and clinical research substantiating this potential.

High altitude hypoxia and oxidative stress: The new hope brought by free radical scavengers

Various strategies can be employed to prevent and manage altitude illnesses, including habituation, oxygenation, nutritional support, and medication. Nevertheless, the utilization of drugs for the prevention and treatment of hypoxia is accompanied by certain adverse effects. Consequently, the quest for medications that exhibit minimal side effects while demonstrating high efficacy remains a prominent area of research. In this context, it is noteworthy that free radical scavengers exhibit remarkable anti-hypoxia activity. These scavengers effectively eliminate excessive free radicals and mitigate the production of reactive oxygen species (ROS), thereby safeguarding the body against oxidative damage induced by plateau hypoxia. In this review, we aim to elucidate the pathogenesis of plateau diseases that are triggered by hypoxia-induced oxidative stress at high altitudes. Additionally, we present a range of free radical scavengers as potential therapeutic and preventive approaches to mitigate the occurrence of common diseases associated with hypoxia at high altitudes.

Altitude-Related Venous Thrombosis With Heart Disease: An Updated Review

Altitude-related venous thrombosis (ARVT) is a condition of growing concern among individuals engaged in high-altitude travel and activities. This updated review explores the epidemiology, pathophysiological mechanisms, clinical presentations, and management of ARVT based on a thematic analysis and synthesis of the existing literature. ARVT's multifactorial etiology involves the interplay of hypobaric hypoxia and endothelial dysfunction, creating a procoagulant state and increasing the risk of thrombosis. Common clinical manifestations include pain, swelling, and redness in the extremities, necessitating accurate and timely diagnosis, particularly in remote settings. Thromboprophylaxis during high-altitude travel and activities plays a crucial role in reducing the risk of ARVT, while anticoagulation remains the mainstay of management. Further research is needed to optimize preventive and treatment strategies, enhancing patient outcomes and safety in high-altitude environments.

Comprehensive viewpoints on heart rate variability at high altitude

OBJECTIVES

Hypoxia is a physiological state characterized by reduced oxygen levels in organs and tissues. It is a common clinicopathological process and a major cause of health problems in highland areas.  Heart rate variability (HRV) is a measure of the balance in autonomic innervation to the heart. It provides valuable information on the regulation of the cardiovascular system by neurohumoral factors, and changes in HRV reflect the complex interactions between multiple systems. In this review, we provide a comprehensive overview of the relationship between high-altitude hypoxia and HRV. We summarize the different mechanisms of diseases caused by hypoxia and explore the changes in HRV across various systems. Additionally, we discuss relevant pharmaceutical interventions. Overall, this review aims to provide research ideas and assistance for in-depth studies on HRV. By understanding the intricate relationship between high-altitude hypoxia and HRV, we can gain insights into the underlying mechanisms and potential therapeutic approaches to mitigate the effects of hypoxia on cardiovascular and other systems.

METHODS

The relevant literature was collected systematically from scientific database, including PubMed, Web of Science, China National Knowledge Infrastructure (CNKI), Baidu Scholar, as well as other literature sources, such as classic books of hypoxia.

RESULTS

There is a close relationship between heart rate variability and high-altitude hypoxia. Heart rate variability is an indicator that evaluates the impact of hypoxia on the cardiovascular system and other related systems. By improving the observation of HRV, we can estimate the progress of cardiovascular diseases and predict the impact on other systems related to cardiovascular health. At the same time, changes in heart rate variability can be used to observe the efficacy of preventive drugs for altitude related diseases.

CONCLUSIONS

HRV can be used to assess autonomic nervous function under various systemic conditions, and can be used to predict and monitor diseases caused by hypoxia at high altitude. Investigating the correlation between high altitude hypoxia and heart rate variability can help make HRV more rapid, accurate, and effective for the diagnosis of plateau-related diseases.

Phenotypic Alterations in Erythroid Nucleated Cells of Spleen and Bone Marrow in Acute Hypoxia

Hypoxia leads to metabolic changes at the cellular, tissue, and organismal levels. The molecular mechanisms for controlling physiological changes during hypoxia have not yet been fully studied. Erythroid cells are essential for adjusting the rate of erythropoiesis and can influence the development and differentiation of immune cells under normal and pathological conditions. We simulated high-altitude hypoxia conditions for mice and assessed the content of erythroid nucleated cells in the spleen and bone marrow under the existing microenvironment. For a pure population of CD71+ erythroid cells, we assessed the production of cytokines and the expression of genes that regulate the immune response. Our findings show changes in the cellular composition of the bone marrow and spleen during hypoxia, as well as changes in the composition of the erythroid cell subpopulations during acute hypoxic exposure in the form of a decrease in orthochromatophilic erythroid cells that are ready for rapid enucleation and the accumulation of their precursors. Cytokine production normally differs only between organs; this effect persists during hypoxia. In the bone marrow, during hypoxia, genes of the C-lectin pathway are activated. Thus, hypoxia triggers the activation of various adaptive and compensatory mechanisms in order to limit inflammatory processes and modify metabolism.

Hypoxia and the Aging Cardiovascular System

Older individuals represent a growing population, in industrialized countries, particularly those with cardiovascular diseases, which remain the leading cause of death in western societies. Aging constitutes one of the largest risks for cardiovascular diseases. On the other hand, oxygen consumption is the foundation of cardiorespiratory fitness, which in turn is linearly related to mortality, quality of life and numerous morbidities. Therefore, hypoxia is a stressor that induces beneficial or harmful adaptations, depending on the dose. While severe hypoxia can exert detrimental effects, such as high-altitude illnesses, moderate and controlled oxygen exposure can potentially be used therapeutically. It can improve numerous pathological conditions, including vascular abnormalities, and potentially slows down the progression of various age-related disorders. Hypoxia can exert beneficial effects on inflammation, oxidative stress, mitochondrial functions, and cell survival, which are all increased with age and have been discussed as main promotors of aging. This narrative review discusses specificities of the aging cardiovascular system in hypoxia. It draws upon an extensive literature search on the effects of hypoxia/altitude interventions (acute, prolonged, or intermittent exposure) on the cardiovascular system in older individuals (over 50 years old). Special attention is directed toward the use of hypoxia exposure to improve cardiovascular health in older individuals.

Environmental and behavioral regulation of HIF-mitochondria crosstalk

Reduced oxygen availability (hypoxia) can lead to cell and organ damage. Therefore, aerobic species depend on efficient mechanisms to counteract detrimental consequences of hypoxia. Hypoxia inducible factors (HIFs) and mitochondria are integral components of the cellular response to hypoxia and coordinate both distinct and highly intertwined adaptations. This leads to reduced dependence on oxygen, improved oxygen supply, maintained energy provision by metabolic remodeling and tapping into alternative pathways and increased resilience to hypoxic injuries. On one hand, many pathologies are associated with hypoxia and hypoxia can drive disease progression, for example in many cancer and neurological diseases. But on the other hand, controlled induction of hypoxia responses via HIFs and mitochondria can elicit profound health benefits and increase resilience. To tackle pathological hypoxia conditions or to apply health-promoting hypoxia exposures efficiently, cellular and systemic responses to hypoxia need to be well understood. Here we first summarize the well-established link between HIFs and mitochondria in orchestrating hypoxia-induced adaptations and then outline major environmental and behavioral modulators of their interaction that remain poorly understood.

MFN2 deficiency promotes cardiac response to hypobaric hypoxia by reprogramming cardiomyocyte metabolism

AIM

Under hypobaric hypoxia (HH), the heart triggers various defense mechanisms including metabolic remodeling against lack of oxygen. Mitofusin 2 (MFN2), located at the mitochondrial outer membrane, is closely involved in the regulation of mitochondrial fusion and cell metabolism. To date, however, the role of MFN2 in cardiac response to HH has not been explored.

METHODS

Loss- and gain-of-function approaches were used to investigate the role of MFN2 in cardiac response to HH. In vitro, the function of MFN2 in the contraction of primary neonatal rat cardiomyocytes under hypoxia was examined. Non-targeted metabolomics and mitochondrial respiration analyses, as well as functional experiments were performed to explore underlying molecular mechanisms.

RESULTS

Our data demonstrated that, following 4 weeks of HH, cardiac-specific MFN2 knockout (MFN2 cKO) mice exhibited significantly better cardiac function than control mice. Moreover, restoring the expression of MFN2 clearly inhibited the cardiac response to HH in MFN2 cKO mice. Importantly, MFN2 knockout significantly improved cardiac metabolic reprogramming during HH, resulting in reduced capacity for fatty acid oxidation (FAO) and oxidative phosphorylation, and increased glycolysis and ATP production. In vitro data showed that down-regulation of MFN2 promoted cardiomyocyte contractility under hypoxia. Interestingly, increased FAO through palmitate treatment decreased contractility of cardiomyocyte with MFN2 knockdown under hypoxia. Furthermore, treatment with mdivi-1, an inhibitor of mitochondrial fission, disrupted HH-induced metabolic reprogramming and subsequently promoted cardiac dysfunction in MFN2-knockout hearts.

CONCLUSION

Our findings provide the first evidence that down-regulation of MFN2 preserves cardiac function in chronic HH by promoting cardiac metabolic reprogramming.

High-altitude cerebral hypoxia promotes mitochondrial dysfunction and apoptosis of mouse neurons

Introduction

Neuronal cell death is an important factor in the pathogenesis of acute high-altitude cerebral hypoxia; however, the underlying molecular mechanism remains unclear. In this study, we tested if high-altitude hypoxia (HAH) causes neuronal death and mitochondrial dysfunction using various in vivo and in vitro approaches.

Methods

Acute high-altitude cerebral hypoxia was induced by hypobaric hypoxia chamber in male mice. we explored the mechanisms of neuronal cell death using immunofluorescence, western blotting, transmission electron microscopy, and flow cytometry. Next, mitochondrial function and morphology were observed using Jc-1 staining, seahorse assay, western blotting, MitoTracker staining, and transmission electron microscopy. Moreover, open field test, elevated plus test, and Morris water maze were applied for animal behavior.

Results

Results revealed that HAH disrupted mitochondrial function and promoted neuronal apoptosis and necroptosis both in HT-22 cells and in mouse hippocampal neurons. Moreover, the mitochondrial membrane potential and adenosine triphosphate production decreased in neurons after HAH, while oxidative stress and mitochondrial fission increased. Behavioral studies suggested that HAH induced anxiety-like behavior and impaired spatial memory, while it had no effect on athletic ability.

Discussion

These findings demonstrated that HAH promotes mitochondrial dysfunction and apoptosis of mouse neurons, thus providing new insights into the role of mitochondrial function and neuronal cell death in acute high-altitude cerebral hypoxia.

The Brain at High Altitude: From Molecular Signaling to Cognitive Performance

The brain requires over one-fifth of the total body oxygen demand for normal functioning. At high altitude (HA), the lower atmospheric oxygen pressure inevitably challenges the brain, affecting voluntary spatial attention, cognitive processing, and attention speed after short-term, long-term, or lifespan exposure. Molecular responses to HA are controlled mainly by hypoxia-inducible factors. This review aims to summarize the cellular, metabolic, and functional alterations in the brain at HA with a focus on the role of hypoxia-inducible factors in controlling the hypoxic ventilatory response, neuronal survival, metabolism, neurogenesis, synaptogenesis, and plasticity.

Remote ischemic preconditioning improves spatial memory and sleep of young males during acute high-altitude exposure

OBJECTIVE

The high-altitude hypoxia environment will cause poor acclimatization in a portion of the population. Remote ischemic preconditioning（RIPC）has been demonstrated to prevent cardiovascular and cerebrovascular diseases under ischemic or hypoxic conditions. However, its role in improving acclimatization and preventing acute mountain sickness (AMS) at high altitude has been undetermined. This study aims to estimate the effect of RIPC on acclimatization of individuals exposed to high altitude.

METHODS

The project was designed as a randomized controlled trial with 82 healthy young males, who received RIPC training once a day for 7 consecutive days. Then they were transported by aircraft to a high altitude (3680 m) and examined for 6 days. Lake Louise Score(LLS) of AMS, physiological index, self-reported sleep pattern, and Pittsburgh Sleep Quality Index（PSQI）score were applied to assess the acclimatization to the high altitude. Five neurobehavioral tests were conducted to assess cognitive function.

RESULTS

The result showed that the RIPC group had a significantly lower AMSscore than the control group (2.43 ± 1.58 vs 3.29 ± 2.03, respectively; adjusted mean difference-0.84, 95% confidence interval-1.61 to -0.06, P = 0.036). and there was no significant difference in AMS incidence between the two groups (25.0% vs 28.57%, P = 0.555). The RIPC group performed better than the control group in spatial memory span score (11[9-12] vs 10[7.5-11], P=0.025) and the passing digit (7[6-7.5] vs 6[5-7], P= 0.001). Spatial memory was significantly higher in the high-altitude RIPC group than in the low-altitude RIPC group (P＜0.01). And the RIPC group obtained significantly lower self-reported sleep quality score (P = 0.024) and PSQI score (P = 0.031).

CONCLUSIONS

The RIPC treatment improved spatial memory and sleep quality in subjects exposed to acute hypoxic exposure and this may lead to improved performance at high altitude.

Cardio-Respiratory and Muscle Oxygenation Responses to Submaximal and Maximal Exercise in Normobaric Hypoxia: Comparison between Children and Adults

As differential physiological responses to hypoxic exercise between adults and children remain poorly understood, we aimed to comprehensively characterise cardiorespiratory and muscle oxygenation responses to submaximal and maximal exercise in normobaric hypoxia between the two groups. Following familiarisation, fifteen children (Age = 9 ± 1 years) and fifteen adults (Age = 22 ± 2 years) completed two graded cycling exercise sessions to exhaustion in a randomized and single-blind manner in normoxia (NOR; FiO2 = 20.9) and normobaric hypoxia (HYP; FiO2 = 13.0) exercises conditions. Age-specific workload increments were 25 W·3 min−1 for children and 40 W·3 min−1 for adults. Gas exchange and vastus lateralis oxygenation parameters were measured continuously via metabolic cart and near-infrared spectroscopy, respectively. Hypoxia provoked significant decreases in maximal power output PMAX (children = 29%; adults 16% (F = 39.3; p < 0.01)) and power output at the gas exchange threshold (children = 10%; adults:18% (F = 8.08; p = 0.01)) in both groups. Comparable changes were noted in most respiratory and gas exchange parameters at similar power outputs between groups. Children, however, demonstrated, lower PETCO2 throughout the test at similar power outputs and during the maintenance of V˙CO2 at the maximal power output. These data indicate that, while most cardiorespiratory responses to acute hypoxic exercise are comparable between children and adults, there exist age-related differential responses in select respiratory and muscle oxygenation parameters.

The Oxygen Cascade from Atmosphere to Mitochondria as a Tool to Understand the (Mal)adaptation to Hypoxia

Hypoxia is a life-threatening challenge for about 1% of the world population, as well as a contributor to high morbidity and mortality scores in patients affected by various cardiopulmonary, hematological, and circulatory diseases. However, the adaptation to hypoxia represents a failure for a relevant portion of the cases as the pathways of potential adaptation often conflict with well-being and generate diseases that in certain areas of the world still afflict up to one-third of the populations living at altitude. To help understand the mechanisms of adaptation and maladaptation, this review examines the various steps of the oxygen cascade from the atmosphere to the mitochondria distinguishing the patterns related to physiological (i.e., due to altitude) and pathological (i.e., due to a pre-existing disease) hypoxia. The aim is to assess the ability of humans to adapt to hypoxia in a multidisciplinary approach that correlates the function of genes, molecules, and cells with the physiologic and pathological outcomes. We conclude that, in most cases, it is not hypoxia by itself that generates diseases, but rather the attempts to adapt to the hypoxia condition. This underlies the paradigm shift that when adaptation to hypoxia becomes excessive, it translates into maladaptation.