Skeletal Muscle Fiber Type in Hypoxia: Adaptation to High-Altitude Exposure and Under Conditions of Pathological Hypoxia

Endothelial-specific telomerase inactivation causes telomere-independent cell senescence and multi-organ dysfunction characteristic of aging

It has remained unclear how aging of endothelial cells (EC) contributes to pathophysiology of individual organs. Cell senescence results in part from inactivation of telomerase (TERT). Here, we analyzed mice with Tert knockout specifically in EC. Tert loss in EC induced transcriptional changes indicative of senescence and tissue hypoxia in EC and in other cells. We demonstrate that EC-Tert-KO mice have leaky blood vessels. The blood-brain barrier of EC-Tert-KO mice is compromised, and their cognitive function is impaired. EC-Tert-KO mice display reduced muscle endurance and decreased expression of enzymes responsible for oxidative metabolism. Our data indicate that Tert-KO EC have reduced mitochondrial content and function, which results in increased dependence on glycolysis. Consistent with this, EC-Tert-KO mice have metabolism changes indicative of increased glucose utilization. In EC-Tert-KO mice, expedited telomere attrition is observed for EC of adipose tissue (AT), while brain and skeletal muscle EC have normal telomere length but still display features of senescence. Our data indicate that the loss of Tert causes EC senescence in part through a telomere length-independent mechanism undermining mitochondrial function. We conclude that EC-Tert-KO mice is a model of expedited vascular senescence recapitulating the hallmarks aging, which can be useful for developing revitalization therapies.

The Role of Hypoxia and Hypoxia Signaling in Skeletal Muscle Physiology

Hypoxia and hypoxia signaling play an integral role in regulating skeletal muscle physiology. Environmental hypoxia and tissue hypoxia in muscles cue for their appropriate physiological response and adaptation, and cause an array of cellular and metabolic changes. In addition, muscle stem cells (satellite cells), exist in a hypoxic state, and this intrinsic hypoxic state correlates with their quiescence and stemness. The mechanisms of hypoxia-mediated regulation of satellite cells and myogenesis are yet to be characterized, and their seemingly contradicting effects reported leave their exact roles somewhat perplexing. This review summarizes the recent findings on the effect of hypoxia and hypoxia signaling on the key aspects of muscle physiology, namely, stem cell maintenance and myogenesis with a particular attention given to distinguish the intrinsic versus local hypoxia in an attempt to better understand their respective regulatory roles and how their relationship affects the overall response. This review further describes their mechanistic links and their possible implications on the relevant pathologies and therapeutics.

The intersection of HIF-1α, O-GlcNAc, and skeletal muscle loss in chronic obstructive pulmonary disease

Sarcopenia, defined as the loss of muscle mass and strength, is a major cause of morbidity and mortality in COPD (chronic obstructive pulmonary disease) patients. However, the molecular mechanisms that cause sarcopenia remain to be determined. In this review, we will highlight the unique molecular and metabolic perturbations that occur in the skeletal muscle of COPD patients in response to hypoxia, and emphasize important areas of future research. In particular, the mechanisms related to the glycolytic shift that occurs in skeletal muscle in response to hypoxia may occur via a hypoxia-inducible factor 1-alpha (HIF-1α)-mediated mechanism. Upregulated glycolysis in skeletal muscle promotes a unique post-translational glycosylation of proteins known as O-GlcNAcylation, which further shifts metabolism toward glycolysis. Molecular changes in the skeletal muscle of COPD patients are associated with fiber-type shifting from Type I (oxidative) muscle fibers to Type II (glycolytic) muscle fibers. The metabolic shift toward glycolysis caused by HIF-1α and O-GlcNAc modified proteins suggests a potential cause for sarcopenia in COPD, which is an emerging area of future research.

Normobaric hypoxia shows enhanced FOXO1 signaling in obese mouse gastrocnemius muscle linked to metabolism and muscle structure and neuromuscular innervation

Skeletal muscle relies on mitochondria for sustainable ATP production, which may be impacted by reduced oxygen availability (hypoxia). Compared with long-term hypoxia, the mechanistic in vivo response to acute hypoxia remains elusive. Therefore, we aimed to provide an integrated description of the Musculus gastrocnemius response to acute hypoxia. Fasted male C57BL/6JOlaHsd mice, fed a 40en% fat diet for six weeks, were exposed to 12% O2 normobaric hypoxia or normoxia (20.9% O2) for six hours (n = 12 per group). Whole-body energy metabolism and the transcriptome response of the M. gastrocnemius were analyzed and confirmed by acylcarnitine determination and Q-PCR. At the whole-body level, six hours of hypoxia reduced energy expenditure, increased blood glucose and tended to decreased the respiratory exchange ratio (RER). Whole-genome transcriptome analysis revealed upregulation of forkhead box-O (FOXO) signalling, including an increased expression of tribbles pseudokinase 3 (Trib3). Trib3 positively correlated with blood glucose levels. Upregulated carnitine palmitoyltransferase 1A negatively correlated with the RER, but the significantly increased in tissue C14-1, C16-0 and C18-1 acylcarnitines supported that β-oxidation was not regulated. The hypoxia-induced FOXO activation could also be connected to altered gene expression related to fiber-type switching, extracellular matrix remodeling, muscle differentiation and neuromuscular junction denervation. Our results suggest that a six-hour exposure of obese mice to 12% O2 normobaric hypoxia impacts M. gastrocnemius via FOXO1, initiating alterations that may contribute to muscle remodeling of which denervation is novel and warrants further investigation. The findings support an early role of hypoxia in tissue alterations in hypoxia-associated conditions such as aging and obesity.

Protective effect of heme chloride on hypoxia-induced tissue injury in mice

OBJECTIVES

Heme chloride (Hemin) is an in vitro purified form of natural heme and an important raw material for anti-anemia and antitumor drugs. This study aims to analyze the protective effect of Hemin on tissue damage in low-pressure oxygen chamber simulated plateau hypoxic mice, and explore its role in anti-plateau hypoxia.

METHODS

Thirty male BALB/c mice were randomly divided into a blank group, a positive drug group (acetazolomide, 200 mg/kg), a Hemin low-dose group (15 mg/kg), a Hemin medium-dose group (30 mg/kg), and a Hemin high-dose group (60 mg/kg) with intraperitoneal injection. The anti-hypoxic activity of Hemin was explored by atmospheric closed hypoxia experiment and the optimal dose was screened. Thirty-six male BALB/c mice were randomly divided into a blank group, a hypoxia group, a positive drug group, and a Hemin high-dose group. The plasma inflammatory factor levels and oxidative stress indicators malondialdehyde (MDA), glutataione (GSH), and superoxide dismutase (SOD) levels of myocardium, brain, lung, and liver tissues were measured in different groups with hypoxia for 24 h. The degree of histopathological damage of mice was observed with HE staining. The degree of protection of Hemin against tissue hypoxia injury was detected with the hypoxia probe piperidazole.

RESULTS

Compared with the blank group, the survival time of mice in the positive drug group, the Hemin medium-dose group, and high-dose group was significantly extended (all P<0.05), with the highest prolongation rate in the Hemin high-dose group. Compared with the hypoxia group, mice in the Hemin high-dose group showed a significant increase in SOD level and GSH content of brain tissue, and a significant decrease in MDA content of lung tissue (all P<0.05). The results of HE staining and hypoxia probe showed that Hemin had a significant protective effect on the damage of liver, heart, brain and lung tissues of mice with hypoxia, and the most obvious effect on that of the brain tissue.

CONCLUSIONS

Hemin has an effect of improvement on oxidative stress and inflammatory response caused by hypoxia, and has obvious protective effect on tissue damage caused by hypoxia.

High Altitude

With ascent to high altitude, barometric pressure declines, leading to a reduction in the partial pressure of oxygen at every point along the oxygen transport chain from the ambient air to tissue mitochondria. This leads, in turn, to a series of changes over varying time frames across multiple organ systems that serve to maintain tissue oxygen delivery at levels sufficient to prevent acute altitude illness and preserve cognitive and locomotor function. This review focuses primarily on the physiological adjustments and acclimatization processes that occur in the lungs of healthy individuals, including alterations in control of breathing, ventilation, gas exchange, lung mechanics and dynamics, and pulmonary vascular physiology. Because other organ systems, including the cardiovascular, hematologic and renal systems, contribute to acclimatization, the responses seen in these systems, as well as changes in common activities such as sleep and exercise, are also addressed. While the pattern of the responses highlighted in this review are similar across individuals, the magnitude of such responses often demonstrates significant interindividual variability which accounts for subsequent differences in tolerance of the low oxygen conditions in this environment.

iPSCs ameliorate hypoxia-induced autophagy and atrophy in C2C12 myotubes via the AMPK/ULK1 pathway

BACKGROUND

Duchenne muscular dystrophy (DMD) is an X-linked lethal genetic disorder for which there is no effective treatment. Previous studies have shown that stem cell transplantation into mdx mice can promote muscle regeneration and improve muscle function, however, the specific molecular mechanisms remain unclear. DMD suffers varying degrees of hypoxic damage during disease progression. This study aimed to investigate whether induced pluripotent stem cells (iPSCs) have protective effects against hypoxia-induced skeletal muscle injury.

RESULTS

In this study, we co-cultured iPSCs with C2C12 myoblasts using a Transwell nested system and placed them in a DG250 anaerobic workstation for oxygen deprivation for 24 h. We found that iPSCs reduced the levels of lactate dehydrogenase and reactive oxygen species and downregulated the mRNA and protein levels of BAX/BCL2 and LC3II/LC3I in hypoxia-induced C2C12 myoblasts. Meanwhile, iPSCs decreased the mRNA and protein levels of atrogin-1 and MuRF-1 and increased myotube width. Furthermore, iPSCs downregulated the phosphorylation of AMPKα and ULK1 in C2C12 myotubes exposed to hypoxic damage.

CONCLUSIONS

Our study showed that iPSCs enhanced the resistance of C2C12 myoblasts to hypoxia and inhibited apoptosis and autophagy in the presence of oxidative stress. Further, iPSCs improved hypoxia-induced autophagy and atrophy of C2C12 myotubes through the AMPK/ULK1 pathway. This study may provide a new theoretical basis for the treatment of muscular dystrophy in stem cells.

Effect of hypobaric hypoxia on the fiber type transition of skeletal muscle: a synergistic therapy of exercise preconditioning with a nanocurcumin formulation

Hypobaric hypoxia (HH) leads to various adverse effects on skeletal muscles, including atrophy and reduced oxidative work capacity. However, the effects of HH on muscle fatigue resistance and myofiber remodeling are largely unexplored. Therefore, the present study aimed to explore the impact of HH on slow-oxidative fibers and to evaluate the ameliorative potential of exercise preconditioning and nanocurcumin formulation on muscle anti-fatigue ability. C2C12 cells (murine myoblasts) were used to assess the effect of hypoxia (0.5%, 24 h) with and without the nanocurcumin formulation (NCF) on myofiber phenotypic conversion. To further validate this hypothesis, male Sprague Dawley rats were exposed to a simulated HH (7620 m) for 7 days, along with NCF administration and/or exercise training. Both in vitro and in vivo studies revealed a significant reduction in slow-oxidative fibers (p < 0.01, 61% vs. normoxia control) under hypoxia. There was also a marked decrease in exhaustion time (p < 0.01, 65% vs. normoxia) in hypoxia control rats, indicating a reduced work capacity. Exercise preconditioning along with NCF supplementation significantly increased the slow-oxidative fiber proportion and exhaustion time while maintaining mitochondrial homeostasis. These findings suggest that HH leads to an increased transition of slow-oxidative fibers to fast glycolytic fibers and increased muscular fatigue. Administration of NCF in combination with exercise preconditioning restored this myofiber remodeling and improved muscle anti-fatigue ability.

Main Pathogenic Mechanisms and Recent Advances in COPD Peripheral Skeletal Muscle Wasting

Chronic obstructive pulmonary disease (COPD) is a worldwide prevalent respiratory disease mainly caused by tobacco smoke exposure. COPD is now considered as a systemic disease with several comorbidities. Among them, skeletal muscle dysfunction affects around 20% of COPD patients and is associated with higher morbidity and mortality. Although the histological alterations are well characterized, including myofiber atrophy, a decreased proportion of slow-twitch myofibers, and a decreased capillarization and oxidative phosphorylation capacity, the molecular basis for muscle atrophy is complex and remains partly unknown. Major difficulties lie in patient heterogeneity, accessing patients' samples, and complex multifactorial process including extrinsic mechanisms, such as tobacco smoke or disuse, and intrinsic mechanisms, such as oxidative stress, hypoxia, or systemic inflammation. Muscle wasting is also a highly dynamic process whose investigation is hampered by the differential protein regulation according to the stage of atrophy. In this review, we report and discuss recent data regarding the molecular alterations in COPD leading to impaired muscle mass, including inflammation, hypoxia and hypercapnia, mitochondrial dysfunction, diverse metabolic changes such as oxidative and nitrosative stress and genetic and epigenetic modifications, all leading to an impaired anabolic/catabolic balance in the myocyte. We recapitulate data concerning skeletal muscle dysfunction obtained in the different rodent models of COPD. Finally, we propose several pathways that should be investigated in COPD skeletal muscle dysfunction in the future.

Experimental evidence and clinical implications of Warburg effect in the skeletal muscle of Fabry disease

Skeletal muscle (SM) pain and fatigue are common in Fabry disease (FD). Here, we undertook the investigation of the energetic mechanisms related to FD-SM phenotype. A reduced tolerance to aerobic activity and lactate accumulation occurred in FD-mice and patients. Accordingly, in murine FD-SM we detected an increase in fast/glycolytic fibers, mirrored by glycolysis upregulation. In FD-patients, we confirmed a high glycolytic rate and the underutilization of lipids as fuel. In the quest for a tentative mechanism, we found HIF-1 upregulated in FD-mice and patients. This finding goes with miR-17 upregulation that is responsible for metabolic remodeling and HIF-1 accumulation. Accordingly, miR-17 antagomir inhibited HIF-1 accumulation, reverting the metabolic-remodeling in FD-cells. Our findings unveil a Warburg effect in FD, an anaerobic-glycolytic switch under normoxia induced by miR-17-mediated HIF-1 upregulation. Exercise-intolerance, blood-lactate increase, and the underlying miR-17/HIF-1 pathway may become useful therapeutic targets and diagnostic/monitoring tools in FD.

Oxygen-generating microparticles downregulate HIF-1α expression, increase cardiac contractility, and mitigate ischemic injury

Myocardial hypoxia is the low oxygen tension in the heart tissue implicated in many diseases, including ischemia, cardiac dysfunction, or after heart procurement for transplantation. Oxygen-generating microparticles have recently emerged as a potential strategy for supplying oxygen to sustain cell survival, growth, and tissue functionality in hypoxia. Here, we prepared oxygen-generating microparticles with poly D,L-lactic-co-glycolic acid, and calcium peroxide (CPO), which yielded a continuous morphology capable of sustained oxygen release for up to 24 h. We demonstrated that CPO microparticles increased primary rat cardiomyocyte metabolic activity while not affecting cell viability during hypoxia. Moreover, hypoxia-inducible factor (HIF)-1α, which is upregulated during hypoxia, can be downregulated by delivering oxygen using CPO microparticles. Single-cell traction force microscopy data demonstrated that the reduced energy generated by hypoxic cells could be restored using CPO microparticles. We engineered cardiac tissues that showed higher contractility in the presence of CPO microparticles compared to hypoxic cells. Finally, we observed reduced myocardial injuries in ex vivo rabbit hearts treated with CPO microparticles. In contrast, an acute early myocardial injury was observed for the hearts treated with control saline solution in hypoxia. In conclusion, CPO microparticles improved cell and tissue contractility and gene expression while reducing hypoxia-induced myocardial injuries in the heart. STATEMENT OF SIGNIFICANCE: Oxygen-releasing microparticles can reduce myocardial ischemia, allograft rejection, or irregular heartbeats after heart transplantation. Here we present biodegradable oxygen-releasing microparticles that are capable of sustained oxygen release for more than 24 hrs. We then studied the impact of sustained oxygen release from microparticles on gene expresseion and cardiac cell and tissue function. Previous studies have not measured cardiac tissue or cell mechanics during hypoxia, which is important for understanding proper cardiac function and beating. Using traction force microscopy and an engineered tissue-on-a-chip, we demonstrated that our oxygen-releasing microparticles improve cell and tissue contractility during hypoxia while downregulating the HIF-1α expression level. Finally, using the microparticles, we showed reduced myocardial injuries in rabbit heart tissue, confirming the potential of the particles to be used for organ transplantation or tissue engineering.

Whole transcriptome analyses and comparison reveal the metabolic differences between oxidative and glycolytic skeletal muscles of yak

Mammalian skeletal muscle is composed of various muscle fibers that exhibit different physiological and metabolic features. Muscle fiber type composition has significant influences on the meat quality of livestock. In this study, we comprehensively analyzed the whole transcriptome profiles of the oxidative muscle biceps femoris (BF) and the glycolytic muscle obliquus externus abdominis (OEA) of yak. A total of 1436 mRNAs, 1172 lncRNAs, and 218 circRNAs were differentially expressed in the oxidative muscles compared with the glycolytic muscles. KEGG annotation showed that differentially expressed mRNAs regulated by lncRNA and circRNA were mainly involved in PPAR signaling pathway, citrate cycle (TCA cycle), and PI3K-Akt signaling pathway, which reflect the different metabolic properties between oxidative and glycolytic muscles. In addition, regulatory networks associated with muscle fiber type conversion and mitochondria energy metabolism in muscles were constructed. Our study provides new evidence for a better understanding of the molecular mechanisms underlying skeletal muscle fiber determination and meat quality traits of yak.

Oxygen Gradient Induced in Microfluidic Chips Can Be Used as a Model for Liver Zonation

Availability of oxygen plays an important role in tissue organization and cell-type specific metabolism. It is, however, difficult to analyze hypoxia-related adaptations in vitro because of inherent limitations of experimental model systems. In this study, we establish a microfluidic tissue culture protocol to generate hypoxic gradients in vitro, mimicking the conditions found in the liver acinus. To accomplish this, four microfluidic chips, each containing two chambers, were serially connected to obtain eight interconnected chambers. HepG2 hepatocytes were uniformly seeded in each chamber and cultivated under a constant media flow of 50 µL/h for 72 h. HepG2 oxygen consumption under flowing media conditions established a normoxia to hypoxia gradient within the chambers, which was confirmed by oxygen sensors located at the inlet and outlet of the connected microfluidic chips. Expression of Hif1α mRNA and protein was used to indicate hypoxic conditions in the cells and albumin mRNA and protein expression served as a marker for liver acinus-like zonation. Oxygen measurements performed over 72 h showed a change from 17.5% to 15.9% of atmospheric oxygen, which corresponded with a 9.2% oxygen reduction in the medium between chamber1 (inlet) and 8 (outlet) in the connected microfluidic chips after 72 h. Analysis of Hif1α expression and nuclear translocation in HepG2 cells additionally confirmed the hypoxic gradient from chamber1 to chamber8. Moreover, albumin mRNA and protein levels were significantly reduced from chamber1 to chamber8, indicating liver acinus zonation along the oxygen gradient. Taken together, microfluidic cultivation in interconnected chambers provides a new model for analyzing cells in a normoxic to hypoxic gradient in vitro. By using a well-characterized cancer cell line as a homogenous hepatocyte population, we also demonstrate that an approximate 10% reduction in oxygen triggers translocation of Hif1α to the nucleus and reduces albumin production.

Comparative transcriptomic and metabolomic analysis reveals pectoralis highland adaptation across altitudinal songbirds

Pectoralis phenotypic variation plays a fundamental role in locomotion and thermogenesis in highland birds. However, its regulatory and metabolic mechanisms remain enigmatic to date. Here, we integrated phenomic, transcriptomic, and metabolomic approaches to determine muscle variation and its underpinning mechanisms across altitudinal songbirds. Phenomics confirmed that all highland birds had considerable increases in muscle oxidative capacity, capillarity, and mitochondrial abundance in our study. Correspondingly, transcriptomic analyses found that differentially expressed genes in phenotype-associated modules enriched for blood vessel, muscle structure development, and mitochondrial organization. Despite similar traits and functional enrichments across highland birds, different mechanisms drove their occurrence in high-altitude tree sparrow and 2 snow finches. Importantly, a metabolic feature shared by all the 3 highland birds is the improvement in insulin sensitivity and glucose utilization through activating insulin signaling pathway, which is vital to increase muscle oxidative capacity and maintain metabolic homeostasis. Nevertheless, fatty acid biosynthesis and oxidation are enhanced in only 2 snow finches which had a long evolutionary history on the high plateau, also differing from ketone body metabolism in recently introduced colonizer of the tree sparrow of the high plateau. Our study represents a vital contribution to reveal the regulatory and metabolic basis of pectoralis variation across altitudinal songbirds.

Adaptive Responses to Hypoxia and/or Hyperoxia in Humans

Significance: Oxygen is indispensable for aerobic life, but its utilization exposes cells and tissues to oxidative stress; thus, tight regulation of cellular, tissue, and systemic oxygen concentrations is crucial. Here, we review the current understanding of how the human organism (mal-)adapts to low (hypoxia) and high (hyperoxia) oxygen levels and how these adaptations may be harnessed as therapeutic or performance enhancing strategies at the systemic level. Recent Advances: Hyperbaric oxygen therapy is already a cornerstone of modern medicine, and the application of mild hypoxia, that is, hypoxia conditioning (HC), to strengthen the resilience of organs or the whole body to severe hypoxic insults is an important preparation for high-altitude sojourns or to protect the cardiovascular system from hypoxic/ischemic damage. Many other applications of adaptations to hypo- and/or hyperoxia are only just emerging. HC-sometimes in combination with hyperoxic interventions-is gaining traction for the treatment of chronic diseases, including numerous neurological disorders, and for performance enhancement. Critical Issues: The dose- and intensity-dependent effects of varying oxygen concentrations render hypoxia- and/or hyperoxia-based interventions potentially highly beneficial, yet hazardous, although the risks versus benefits are as yet ill-defined. Future Directions: The field of low and high oxygen conditioning is expanding rapidly, and novel applications are increasingly recognized, for example, the modulation of aging processes, mood disorders, or metabolic diseases. To advance hypoxia/hyperoxia conditioning to clinical applications, more research on the effects of the intensity, duration, and frequency of altered oxygen concentrations, as well as on individual vulnerabilities to such interventions, is paramount. Antioxid. Redox Signal. 37, 887-912.

Training at moderate altitude improves submaximal but not maximal performance-related parameters in elite rowers

Maximal oxygen consumption (V̇O_2max), physiological thresholds, and hemoglobin mass are strong predictors of endurance performance. High values of V̇O_2max, maximal aerobic power (MAP), and power output at anaerobic thresholds are key variables in elite rowers. Endurance athletes often use altitude training as a strategy to improve performance. However, no clear evidence exists that training at natural altitude enhances sea-level performance in elite rowers. This study aimed to evaluate the effect of altitude training on rowing-performance parameters at sea level. The study was conducted on eleven rowers (Six females, five males) from the Chilean National Team during a 3-week moderate altitude training (∼2,900 m. a.s.l.) under the live high-train high (LHTH) model. It included a rowing ergometer maximal incremental test and blood analysis (pre and post-altitude). Gas exchange analysis was performed to measure V̇O_2max, ventilatory thresholds (VTs) and rowing economy/efficiency (ECR/GE%). LHTL training improves performance-related variables at sea level (V̇E_max: 3.3% (95% CI, 1.2–5.5); hemoglobin concentration ([Hb]): 4.3% (95% CI, 1.7–6.9); hematocrit (%): 4.5% (95% CI, 0.9–8.2); RBC (red blood cells) count: 5.3% (95% CI, 2.3–8.2); power at VT2: 6.9% (95% CI, 1.7–12.1), V̇E_VT2: 6.4% (95% CI, 0.4–12.4); power at VT1: 7.3% (95% CI, 1.3–13.3), V̇E_VT1: 8.7% (95% CI, 1.6–15.8)) and economy/efficiency-related variables (ECR_VT2: 5.3% (95% CI, −0.6 to −10.0); GE(%): 5.8% (95% CI, 0.8–10.7)). The LHTH training decreased breathing economy at MAP (−2.8% (95% CI, 0.1–5.6)), pVT2 (−9.3% (95% CI, −5.9 to −12.7)), and pVT1 (−9.3% (95% CI, −4.1 to −14.4)). Non-significant changes were found for V̇O_2max and MAP. This study describes the effects of a 3-week moderate altitude (LHTH training) on performance and economy/efficiency-related variables in elite rowers, suggesting that it is an excellent option to induce positive adaptations related to endurance performance.

Insight into the Effects of High-Altitude Hypoxic Exposure on Learning and Memory

The earth land area is heterogeneous in terms of elevation; about 45% of its land area belongs to higher elevation with altitude above 500 meters compared to sea level. In most cases, oxygen concentration decreases as altitude increases. Thus, high-altitude hypoxic stress is commonly faced by residents in areas with an average elevation exceeding 2500 meters and those who have just entered the plateau. High-altitude hypoxia significantly affects advanced neurobehaviors including learning and memory (L&M). Hippocampus, the integration center of L&M, could be the most crucial target affected by high-altitude hypoxia exposure. Based on these points, this review thoroughly discussed the relationship between high-altitude hypoxia and L&M impairment, in terms of hippocampal neuron apoptosis and dysfunction, neuronal oxidative stress disorder, neurotransmitters and related receptors, and nerve cell energy metabolism disorder, which is of great significance to find potential targets for medical intervention. Studies illustrate that the mechanism of L&M damaged by high-altitude hypoxia should be further investigated based on the entire review of issues related to this topic.

Red cell distribution width, anemia, and lower-extremity physical function among rural-dwelling older adults

BACKGROUND

Elevated red cell distribution width (RDW) has been associated with degenerative conditions in aging.

AIMS

We aimed to evaluate the associations of RDW and anemia with lower-extremity physical function among rural-dwelling older adults.

METHODS

This population-based cross-sectional study included 5093 rural residents (age ≥ 60 years, 57.3% women) who participated in the MIND-China Study in Shandong. Data were collected via face-to-face interviews, clinical examinations, and laboratory tests. RDW was categorized according to quartiles and the lower-extremity physical function was assessed using the Short Physical Performance Battery (SPPB), RESULTS: Multiple linear regression analyses suggested that the fourth quartile of RDW (vs. first quartile) was associated with lower SPPB summary score (β-coefficient - 0.38; 95% CI - 0.58 to - 0.18) and lower scores in balance test (- 0.09; - 0.17 to - 0.01), chair stand test (- 0.17; - 0.27 to - 0.07), and walking speed test (- 0.12; - 0.19 to - 0.05). Anemia was associated with a multiple-adjusted β-coefficient of - 0.34 (- 0.52 to - 0.16) for SPPB summary score. Stratified analysis by anemia showed that there was a linear association between RDW and SPPB in individuals without anemia but a J-shaped association in individuals with anemia.

DISCUSSION

This large-scale population-based study revealed the associations of high RDW and anemia with poor lower-extremity physical function among rural-dwelling Chinese older adults. These findings suggest that an elevated RDW might be a biochemical marker for poor lower-extremity physical function among older adults.

CONCLUSIONS

Anemia and an elevated RDW are associated with poor performance in lower-extremity physical function among rural-dwelling Chinese older adults.

L-theanine induces skeletal muscle fiber type transformation by activation of prox1/CaN signaling pathway in C2C12 myotubes

The aim of this study was to investigate the effect and mechanism of L-theanine (LT) on muscle fiber type transformation in C2C12 myotubes. Our data showed that LT exhibited significantly higher slow oxidative muscle fiber expression and lower glycolytic fibers expression. In addition, LT significantly increased the activities of malate dehydrogenase (MDH) and succinic dehydrogenase (SDH), and decreased lactate dehydrogenase (LDH) activity, the calcineurin (CaN) activity and the protein expressions of nuclear factor of activated T cell 1 (NFATc1), prospero-related homeobox1 (prox1), and calcineurin A (CnA) were significantly increased. However, inhibition of CaN activity by cyclosporine A (CsA) abolished LT-induced increase of slow oxidative muscle fiber expression and decrease of glycolytic fibers expression. Moreover, inhibition of prox1 expression by prox1-siRNA disrupted LT-induced activation of CaN signaling pathway and muscle fiber type transformation. Taken together, these results indicated that LT could promote skeletal muscle fiber type transformation from type II to type I via activation of prox1/CaN signaling pathway.

A Nanocurcumin and Pyrroloquinoline Quinone Formulation Prevents Hypobaric Hypoxia-Induced Skeletal Muscle Atrophy by Modulating NF-κB Signaling Pathway

Kushwaha, Asha D., and Deepika Saraswat. A nanocurcumin and pyrroloquinoline quinone formulation prevents hypobaric hypoxia-induced skeletal muscle atrophy by modulating NF-κB signaling pathway. High Alt Med Biol 00:000-000, 2022. Background: Hypobaric hypoxia (HH)-induced deleterious skeletal muscle damage depends on exposure time and availability of oxygen at cellular level, which eventually can limit human work performance at high altitude (HA). Despite the advancements made in pharmacological (performance enhancer, antioxidants) and nonpharmacological therapeutics (acclimatization strategies), only partial success has been achieved in improving physical performance at HA. A distinctive combination of nanocurcumin (NC) and pyrroloquinoline quinone (PQQ) has been formulated (named NCF [nanocurcumin formulation], Indian patent No. 302877) in our laboratory, and has proven very promising in improving cardiomyocyte adaptation to chronic HH. We hypothesized that NCF might improve skeletal muscle adaptation and could be a performance enhancer at HA. Material and Methods: Adult Sprague-Dawley rats (220 ± 10 g) were divided into five groups (n = 6/group): normoxia vehicle control, hypoxia vehicle control, hypoxia NCF, hypoxia NC, and hypoxia PQQ. All the animals (except those in normoxia) were exposed to simulated HH in a chamber at temperature 22°C ± 2°C, humidity 50% ± 5%, altitude 25,000 ft for 1, 3, or 7 days. After completion of the stipulated exposure time, gastrocnemius and soleus muscles were excised from animals for further analysis. Results: Greater lengths of hypoxic exposure caused progressively increased muscle ring finger-1 (MuRF-1; p < 0.01) expression and calpain activation (0.56 ± 0.05 vs. 0.13 ± 0.02 and 0.44 ± 0.03 vs. 0.12 ± 0.021) by day 7, respectively in the gastrocnemius and soleus muscles. Myosin heavy chain type I (slow oxidative) fibers significantly (p > 0.01) decreased in gastrocnemius (>50%) and soleus (>46%) muscles by the seventh day of exposure. NCF supplementation showed (p ≤ 0.05) tremendous improvement in skeletal muscle acclimatization through effective alleviation of oxidative damage, and changes in calpain activity and atrophic markers at HA compared with hypoxia control or treatment alone with NC/PQQ. Conclusion: Thus, NCF-mediated anti-oxidative, anti-inflammatory effects lead to decreased proteolysis resulting in mitigated skeletal muscle atrophy under HH.

The Acute Effects of Normobaric Hypoxia on Strength, Muscular Endurance and Cognitive Function: Influence of Dose and Sex

The aim of this study was to examine the acute effects of different levels of hypoxia on maximal strength, muscular endurance, and cognitive function in males and females. In total, 13 males (mean ± SD: age, 23.6 ± 2.8 years; height, 176.6 ± 3.9 cm; body mass, 76.6 ± 2.1 kg) and 13 females (mean ± SD: age, 22.8 ± 1.4 years; height, 166.4 ± 1.9 cm; body mass, 61.6 ± 3.4 kg) volunteered for a randomized, double-blind, crossover study. Participants completed a one repetition strength and muscular endurance test (60% of one repetition maximum to failure) for squat and bench press following four conditions; (i) normoxia (900 m altitude; FiO2: 21%); (ii) low dose hypoxia (2000 m altitude; FiO2: 16%); (iii) moderate dose hypoxia (3000 m altitude; FiO2: 14%); and (iv) high dose hypoxia (4000 m altitude; FiO2: 12%). Heart rate, blood lactate, rating of perceived exertion, and cognitive function was also determined during each condition. The one repetition maximum squat (p = 0.33) and bench press (p = 0.68) did not differ between conditions or sexes. Furthermore, squat endurance did not differ between conditions (p = 0.34). There was a significant decrease in bench press endurance following moderate (p = 0.02; p = 0.04) and high (p = 0.01; p = 0.01) doses of hypoxia in both males and females compared to normoxia and low dose hypoxia, respectively. Cognitive function, ratings of perceived exertion, and lactate were also significantly different in high and moderate dose hypoxia conditions compared to normoxia (p < 0.05). Heart rate was not different between the conditions (p = 0.30). In conclusion, high and moderate doses of acute normobaric hypoxia decrease upper body muscular endurance and cognitive performance regardless of sex; however, lower body muscular endurance and maximal strength are not altered.

Human and rat skeletal muscle single-nuclei multi-omic integrative analyses nominate causal cell types, regulatory elements, and SNPs for complex traits

Skeletal muscle accounts for the largest proportion of human body mass, on average, and is a key tissue in complex diseases and mobility. It is composed of several different cell and muscle fiber types. Here, we optimize single-nucleus ATAC-seq (snATAC-seq) to map skeletal muscle cell-specific chromatin accessibility landscapes in frozen human and rat samples, and single-nucleus RNA-seq (snRNA-seq) to map cell-specific transcriptomes in human. We additionally perform multi-omics profiling (gene expression and chromatin accessibility) on human and rat muscle samples. We capture type I and type II muscle fiber signatures, which are generally missed by existing single-cell RNA-seq methods. We perform cross-modality and cross-species integrative analyses on 33,862 nuclei and identify seven cell types ranging in abundance from 59.6% to 1.0% of all nuclei. We introduce a regression-based approach to infer cell types by comparing transcription start site-distal ATAC-seq peaks to reference enhancer maps and show consistency with RNA-based marker gene cell type assignments. We find heterogeneity in enrichment of genetic variants linked to complex phenotypes from the UK Biobank and diabetes genome-wide association studies in cell-specific ATAC-seq peaks, with the most striking enrichment patterns in muscle mesenchymal stem cells (∼3.5% of nuclei). Finally, we overlay these chromatin accessibility maps on GWAS data to nominate causal cell types, SNPs, transcription factor motifs, and target genes for type 2 diabetes signals. These chromatin accessibility profiles for human and rat skeletal muscle cell types are a useful resource for nominating causal GWAS SNPs and cell types.

Diverse energy metabolism patterns in females in Neodon fuscus, Lasiopodomys brandtii, and Mus musculus revealed by comparative transcriptomics under hypoxic conditions

The effects of global warming and anthropogenic disturbance force animals to migrate from lower to higher elevations to find suitable new habitats. As such migrations increase hypoxic stress on the animals, it is important to understand how plateau- and plain-dwelling animals respond to low-oxygen environments. We used comparative transcriptomics to explore the response of Neodon fuscus, Lasiopodomys brandtii, and Mus musculus skeletal muscle tissues to hypoxic conditions. Results indicate that these species have adopted different oxygen transport and energy metabolism strategies for dealing with a hypoxic environment. N. fuscus promotes oxygen transport by increasing hemoglobin synthesis and reduces the risk of thrombosis through cooperative regulation of genes, including Fga, Fgb, Alb, and Ttr; genes such as Acs16, Gpat4, and Ndufb7 are involved in regulating lipid synthesis, fatty acid β-oxidation, hemoglobin synthesis, and electron-linked transmission, thereby maintaining a normal energy supply in hypoxic conditions. In contrast, the oxygen-carrying capacity and angiogenesis of red blood cells in L. brandtii are promoted by genes in the CYP and COL families; this species maintains its bodily energy supply by enhancing the pentose phosphate pathway and mitochondrial fatty acid synthesis pathway. However, under hypoxia, M. musculus cannot effectively transport additional oxygen; thus, its cell cycle, proliferation, and migration are somewhat affected. Given its lack of hypoxic tolerance experience, M. musculus also shows significantly reduced oxidative phosphorylation levels under hypoxic conditions. Our results suggest that the glucose capacity of M. musculus skeletal muscle does not provide sufficient energy during hypoxia; thus, we hypothesize that it supplements its bodily energy by synthesizing ketone bodies. For the first time, we describe the energy metabolism pathways of N. fuscus and L. brandtii skeletal muscle tissues under hypoxic conditions. Our findings, therefore, improve our understanding of how vertebrates thrive in high altitude and plain habitats when faced with hypoxic conditions.

Effects of temperature on the locomotor performance and contraction properties of skeletal muscle from two Phrynocephalus lizards at high and low altitude

Locomotor performance and skeletal muscle contraction are critical for animals and are susceptible to changes in the external thermal environment, especially for ectotherms. Phrynocephalus erythrurus, which is endemic to the Qinghai-Tibetan plateau, is known for living at the highest elevation among all reptiles in the world (4500-5300 m). In this study, which compares P. erythrurus with the lowland Phrynocephalus przewalskii, we evaluated the locomotor performance at different body temperatures, the effects of temperature and oxygen partial pressure (PO₂) on the contractile properties of iliofibularis (IF) muscle in vitro, ATPase activity of IF muscle at different temperatures, and the fiber types of IF muscle. Lowland P. przewalskii runs significantly faster than highland P. erythrurus at all test body temperatures. Almost all contractile properties of the IF muscle of P. przewalskii were better than that of P. erythrurus under all test temperatures and PO₂. However, P. erythrurus could achieve both optimal isometric (e.g., dP_o/dt) and optimal isotonic (e.g., V_max) contraction at a lower temperature compared with P. przewalskii. Multi-factor analysis further revealed that temperature has a significant effect on the contractile properties of IF muscle for both species. Although the proportion of fibers types and ATPase activities of IF muscle have no significant interspecies difference, the changing pattern of ATPase activities with temperature is consistent with certain contractile properties and locomotor performance. The interspecies differences in locomotor ability and contractile properties of skeletal muscle in high- and low-altitude lizards may be the results of long-term adaptation to the local environment.

Hypoxia and Hypoxia-Inducible Factor Signaling in Muscular Dystrophies: Cause and Consequences

Muscular dystrophies (MDs) are a group of inherited degenerative muscle disorders characterized by a progressive skeletal muscle wasting. Respiratory impairments and subsequent hypoxemia are encountered in a significant subgroup of patients in almost all MD forms. In response to hypoxic stress, compensatory mechanisms are activated especially through Hypoxia-Inducible Factor 1 α (HIF-1α). In healthy muscle, hypoxia and HIF-1α activation are known to affect oxidative stress balance and metabolism. Recent evidence has also highlighted HIF-1α as a regulator of myogenesis and satellite cell function. However, the impact of HIF-1α pathway modifications in MDs remains to be investigated. Multifactorial pathological mechanisms could lead to HIF-1α activation in patient skeletal muscles. In addition to the genetic defect per se, respiratory failure or blood vessel alterations could modify hypoxia response pathways. Here, we will discuss the current knowledge about the hypoxia response pathway alterations in MDs and address whether such changes could influence MD pathophysiology.

Transcriptomic diversity in longissimus thoracis muscles of Barbari and Changthangi goat breeds of India

The present study is an attempt to examine the differential expression of genes in longissimus thoracis muscles between meat and wool type Indian goat breeds. Barbari goat is considered the best meat breed while Changthangi is famous for its fine fibre quality. RNA sequencing data was generated from four biological replicates of longissimus thoracis muscles of Barbari and Changthangi goats. A clear demarcation could be observed between the breeds in terms of expression of genes associated with lipid metabolism (FASN, SCD, THRSP, DGAT2 and FABP3). Most significant genes with high connectivity identified by gene co-expression network analysis were associated with triacylglycerol biosynthesis pathway in Barbari goat. Highly interactive genes identified in Changthangi goat were mainly associated with muscle fibre type. This study provides an insight into the differential expression of genes in longissimus thoracis muscles between Barbari and Changthangi goats that are adapted to and reared in different agro-climatic regions.

Physiological and genetic convergence supports hypoxia resistance in high-altitude songbirds

Skeletal muscle plays a central role in regulating glucose uptake and body metabolism; however, highland hypoxia is a severe challenge to aerobic metabolism in small endotherms. Therefore, understanding the physiological and genetic convergence of muscle hypoxia tolerance has a potential broad range of medical implications. Here we report and experimentally validate a common physiological mechanism across multiple high-altitude songbirds that improvement in insulin sensitivity contributes to glucose homeostasis, low oxygen consumption, and relative activity, and thus increases body weight. By contrast, low-altitude songbirds exhibit muscle loss, glucose intolerance, and increase energy expenditures under hypoxia. This adaptive mechanism is attributable to convergent missense mutations in the BNIP3L gene, and METTL8 gene that activates MEF2C expression in highlanders, which in turn increases hypoxia tolerance. Together, our findings from wild high-altitude songbirds suggest convergent physiological and genetic mechanisms of skeletal muscle in hypoxia resistance, which highlights the potentially medical implications of hypoxia-related metabolic diseases.

Changes in energy system contributions to the Wingate anaerobic test in climbers after a high altitude expedition

PURPOSE

The Wingate anaerobic test measures the maximum anaerobic capacity of the lower limbs. The energy sources of Wingate test are dominated by anaerobic metabolism (~ 80%). Chronic high altitude exposure induces adaptations on skeletal muscle function and metabolism. Therefore, the study aim was to investigate possible changes in the energy system contribution to Wingate test before and after a high-altitude sojourn.

METHODS

Seven male climbers performed a Wingate test before and after a 43-day expedition in the Himalaya (23 days above 5.000 m). Mechanical parameters included: peak power (PP), average power (AP), minimum power (MP) and fatigue index (FI). The metabolic equivalents were calculated as aerobic contribution from O₂ uptake during the 30-s exercise phase (W_VO2), lactic and alactic anaerobic energy sources were determined from net lactate production (W_La) and the fast component of the kinetics of post-exercise oxygen uptake (W_PCr), respectively. The total metabolic work (W_TOT) was calculated as the sum of the three energy sources.

RESULTS

PP and AP decreased from 7.3 ± 1.1 to 6.7 ± 1.1 W/kg and from 5.9 ± 0.7 to 5.4 ± 0.8 W/kg, respectively, while FI was unchanged. W_TOT declined from 103.9 ± 28.7 to 83.8 ± 17.8 kJ. Relative aerobic contribution remained unchanged (19.9 ± 4.8% vs 18.3 ± 2.3%), while anaerobic lactic and alactic contributions decreased from 48.3 ± 11.7 to 43.1 ± 8.9% and increased from 31.8 ± 14.5 to 38.6 ± 7.4%, respectively.

CONCLUSION

Chronic high altitude exposure induced a reduction in both mechanical and metabolic parameters of Wingate test. The anaerobic alactic relative contribution increased while the anaerobic lactic decreased, leaving unaffected the overall relative anaerobic contribution to Wingate test.

The secretome of human dental pulp stem cells protects myoblasts from hypoxia‑induced injury via the Wnt/β‑catenin pathway

Human dental pulp stem cells (hDPSCs) present several advantages, including their ability to be non-invasively harvested without ethical concern. The secretome of hDPSCs can promote the functional recovery of various tissue injuries. However, the protective effects on hypoxia-induced skeletal muscle injury remain to be explored. The present study demonstrated that C2C12 myoblast coculture with hDPSCs attenuated CoCl₂-induced hypoxic injury compared with C2C12 alone. The hDPSC secretome increased cell viability and differentiation and decreased G2/M cell cycle arrest under hypoxic conditions. These results were further verified using hDPSC-conditioned medium (hDPSC-CM). The present data revealed that the protective effects of hDPSC-CM depend on the concentration ratio of the CM. In terms of the underlying molecular mechanism, hDPSC-CM activated the Wnt/β-catenin pathway, which increased the protein levels of Wnt1, phosphorylated-glycogen synthase kinase-3β and β-catenin and the mRNA levels of Wnt target genes. By contrast, an inhibitor (XAV939) of Wnt/β-catenin diminished the protective effects of hDPSC-CM. Taken together, the findings of the present study demonstrated that the hDPSC secretome alleviated the hypoxia-induced myoblast injury potentially through regulating the Wnt/β-catenin pathway. These findings may provide new insight into a therapeutic alternative using the hDPSC secretome in skeletal muscle hypoxia-related diseases.

HIF-hypoxia signaling in skeletal muscle physiology and fibrosis

Hypoxia refers to the decrease in oxygen tension in the tissues, and the central effector of the hypoxic response is the transcription factor Hypoxia-Inducible Factor α (HIF1-α). Transient hypoxia in acute events, such as exercising or regeneration after damage, play an important role in skeletal muscle physiology and homeostasis. However, sustained activation of hypoxic signaling is a feature of skeletal muscle injury and disease, which can be a consequence of chronic damage but can also increase the severity of the pathology and worsen its outcome. Here, we review evidence that supports the idea that hypoxia and HIF-1α can contribute to the establishment of fibrosis in skeletal muscle through its crosstalk with other profibrotic factors, such as Transforming growth factor β (TGF-β), the induction of profibrotic cytokines expression, as is the case of Connective Tissue Growth Factor (CTGF/CCN2), or being the target of the Renin-angiotensin system (RAS).

Anti-Hypoxic Molecular Mechanisms of Rhodiola crenulata Extract in Zebrafish as Revealed by Metabonomics

The health supplement of Rhodiola crenulata (RC) is well known for its effective properties against hypoxia. However, the mechanisms of its anti-hypoxic action were still unclear. The objective of this work was to evaluate the molecular mechanisms of RC extract against hypoxia in a hypoxic zebrafish model through metabonomics and network pharmacology analysis. The hypoxic zebrafish model in the environment with low concentration (3%) of oxygen was constructed and used to explore the anti-hypoxic effects of RC extract, followed by detecting the changes of the metabolome in the brain through liquid chromatography–high resolution mass spectrometry. An in silico network for metabolite-protein interactions was further established to examine the potential mechanisms of RC extract, and the mRNA expression levels of the key nodes were validated by real-time quantitative PCR. As results, RC extract could keep zebrafish survive after 72-h hypoxia via improving lactate dehydrogenase, citrate synthase, and hypoxia-induced factor-1α in brains. One hundred and forty-two differential metabolites were screened in the metabonomics, and sphingolipid metabolism pathway was significantly regulated after RC treatment. The constructed protein-metabolites network indicated that the HIF-related signals were recovered, and the mRNA level of AMPK was elevated. In conclusion, RC extract had markedly anti-hypoxic effects in zebrafish via changing sphingolipid metabolism, HIF-related and AMPK signaling pathways.

Physical Activity and Brain Health

Physical activity (PA) has been central in the life of our species for most of its history, and thus shaped our physiology during evolution. However, only recently the health consequences of a sedentary lifestyle, and of highly energetic diets, are becoming clear. It has been also acknowledged that lifestyle and diet can induce epigenetic modifications which modify chromatin structure and gene expression, thus causing even heritable metabolic outcomes. Many studies have shown that PA can reverse at least some of the unwanted effects of sedentary lifestyle, and can also contribute in delaying brain aging and degenerative pathologies such as Alzheimer’s Disease, diabetes, and multiple sclerosis. Most importantly, PA improves cognitive processes and memory, has analgesic and antidepressant effects, and even induces a sense of wellbeing, giving strength to the ancient principle of “mens sana in corpore sano” (i.e., a sound mind in a sound body). In this review we will discuss the potential mechanisms underlying the effects of PA on brain health, focusing on hormones, neurotrophins, and neurotransmitters, the release of which is modulated by PA, as well as on the intra- and extra-cellular pathways that regulate the expression of some of the genes involved.