Lysophospholipid acyltransferase-mediated formation of saturated glycerophospholipids maintained cell membrane integrity for hypoxic adaptation

Adaptation to hypoxia has attracted much public interest because of its clinical significance. However, hypoxic adaptation in the body is complicated and difficult to fully explore. To explore previously unknown conserved mechanisms and key proteins involved in hypoxic adaptation in different species, we first used a yeast model for mechanistic screening. Further multi-omics analyses in multiple species including yeast, zebrafish and mice revealed that glycerophospholipid metabolism was significantly involved in hypoxic adaptation with up-regulation of lysophospholipid acyltransferase (ALE1) in yeast, a key protein for the formation of dipalmitoyl phosphatidylcholine [DPPC (16:0/16:0)], which is a saturated phosphatidylcholine. Importantly, a mammalian homolog of ALE1, lysophosphatidylcholine acyltransferase 1 (LPCAT1), enhanced DPPC levels at the cell membrane and exhibited the same protective effect in mammalian cells under hypoxic conditions. DPPC supplementation effectively attenuated growth restriction, maintained cell membrane integrity and increased the expression of epidermal growth factor receptor under hypoxic conditions, but unsaturated phosphatidylcholine did not. In agreement with these findings, DPPC treatment could also repair hypoxic injury of intestinal mucosa in mice. Taken together, ALE1/LPCAT1-mediated DPPC formation, a key pathway of glycerophospholipid metabolism, is crucial for cell viability under hypoxic conditions. Moreover, we found that ALE1 was also involved in glycolysis to maintain sufficient survival conditions for yeast. The present study offers a novel approach to understanding lipid metabolism under hypoxia and provides new insights into treating hypoxia-related diseases.

Hypoxia and its effect on the cellular system

Eukaryotic cells utilize oxygen for different functions of cell organelles owing to cellular survival. A balanced oxygen homeostasis is an essential requirement to maintain the regulation of normal cellular systems. Any changes in the oxygen level are stressful and can alter the expression of different homeostasis regulatory genes and proteins. Lack of oxygen or hypoxia results in oxidative stress and formation of hypoxia inducible factors (HIF) and reactive oxygen species (ROS). Substantial cellular damages due to hypoxia have been reported to play a major role in various pathological conditions. There are different studies which demonstrated that the functions of cellular system are disrupted by hypoxia. Currently, study on cellular effects following hypoxia is an important field of research as it not only helps to decipher different signaling pathway modulation, but also helps to explore novel therapeutic strategies. On the basis of the beneficial effect of hypoxia preconditioning of cellular organelles, many therapeutic investigations are ongoing as a promising disease management strategy in near future. Hence, the present review discusses about the effects of hypoxia on different cellular organelles, mechanisms and their involvement in the progression of different diseases.

The Role of NO Synthase in the Infarct-Limiting Effect of Urgent and Chronic Adaptation to Normobaric Hypoxia

We studied the role of NO synthase in the infarct-limiting effect of short-term (SNH) and chronic continuous normobaric hypoxia (CNH). In male Wistar rats, SNH (6 sessions of 10-min hypoxia 8% O2 and 10-min reoxygenation) or CNH (12% O2 for 21 days) was modeled. In 30 min after SNH or 24 h after CNH, the rats were subjected to coronary artery occlusion (45 min) and reperfusion (2 h). The following drugs were administered to rats: non-selective NO synthase inhibitor L-NAME (10 mg/kg), inhibitor of inducible NO synthase S-methylthiourea (3 mg/kg), and inhibitor of neuronal NO-synthase 7-nitroindazole (50 mg/kg). NO donor diethylenetriamine was administered intravenously in a dose 2 mg/kg. It was found that L-NAME and S-methylthiourea abolished the infarct-limiting effect of SNH and CNH. Diethylenetriamine increased cardiac tolerance to ischemia/reperfusion. It is believed that inducible NO synthase plays an important role in the cardioprotective effect of normobaric hypoxia.

Whole-Transcriptome Analysis of Yak and Cattle Heart Tissues Reveals Regulatory Pathways Associated With High-Altitude Adaptation

BACKGROUND

The yak (Bos grunniens) is an important livestock species that can survive the extremely cold, harsh, and oxygen-poor conditions of the Qinghai-Tibetan Plateau and provide meat, milk, and transportation for the Tibetans living there. However, the regulatory network that drive this hypoxic adaptation remain elusive.

RESULTS

The heart tissues from LeiRoqi (LWQY) yak and their related cattle (Bos Taurus) breeds, which are two native cattle breeds located in high altitude (HAC) and low altitude (LAC) regions, respectively, were collected for RNA sequencing. A total of 178 co-differentially expressed protein-coding transcripts (co-DETs) were discovered in each of the LAC-vs-LWQY and LAC-vs-HAC comparison groups, including NFATC2, NFATC1, ENPP2, ACSL4, BAD, and many other genes whose functions were reported to be associated with the immune-system, endocrine-system, and lipid metabolism. Two and 230 lncRNA transcripts were differentially expressed in the LAC-vs-LWQY and LAC-vs-HAC comparisons' respectively, but no lncRNA transcripts that were co-differentially expressed. Among the 58 miRNAs that were co-differentially expressed, 18 were up-regulated and 40 were down-regulated. In addition, 640 (501 up-regulated and 139 down-regulated) and 152 (152 up-regulated and one down-regulated) circRNAs showed differential expression in LAC-vs-LWQY and LAC-vs-HAC comparison groups, respectively, and 53 up-regulated co-differentially expressed circRNAs were shared. Multiple co-DETs, which are the targets of miRNAs/lncRNAs, are significantly enriched in high-altitude adaptation related processes, such as, T cell receptor signaling, VEGF signaling, and cAMP signaling. A competing endogenous RNA (ceRNA) network was constructed by integrating the competing relationships among co-differentially expressed mRNAs, miRNAs, lncRNAs and circRNAs. Furthermore, the hypoxic adaptation related ceRNA network was constructed, and the six mRNAs (MAPKAPK3, PXN, NFATC2, ATP7A, DIAPH1, and F2R), the eight miRNAs (including miR-195), and 15 circRNAs (including novel-circ-017096 and novel-circ-018073) are proposed as novel and promising candidates for regulation of hypoxic adaptation in the heart.

CONCLUSION

In conclusion, the data recorded in the present study provides new insights into the molecular network of high-altitude adaptation along with more detailed information of protein-coding transcripts and non-coding transcripts involved in this physiological process, the detailed mechanisms behind how these transcripts "crosstalk" with each other during the plateau adaptation are worthy of future research efforts.

Abundant Chitinous Structures in Chilostomella (Foraminifera, Rhizaria) and Their Potential Functions

Benthic foraminifera, members of Rhizaria, inhabit a broad range of marine environments and are particularly common in hypoxic sediments. The biology of benthic foraminifera is key to understanding benthic ecosystems and relevant biogeochemical cycles, especially in hypoxic environments. Chilostomella is a foraminiferal genus commonly found in hypoxic deep-sea sediments and has poorly understood ecological characteristics. For example, the carbon isotopic compositions of their lipids are substantially different from other co-occurring genera, probably reflecting unique features of its metabolism. Here, we investigated the cytoplasmic and ultrastructural features of Chilostomella ovoidea from bathyal sediments of Sagami Bay, Japan, based on serial semi-thin sections examined using an optical microscope followed by a three-dimensional reconstruction, combined with TEM observations of ultra-thin sections. Observations by TEM revealed the presence of abundant electron-dense structures dividing the cytoplasm. Based on histochemical staining, these structures are shown to be composed of chitin. Our 3D reconstruction revealed chitinous structures in the final seven chambers. These exhibited a plate-like morphology in the final chambers but became rolled up in earlier chambers (toward the proloculus). These chitinous, plate-like structures may function to partition the cytoplasm in a chamber to increase the surface/volume ratio and/or act as a reactive site for some metabolic functions.

Small RNA sequencing reveals miRNAs important for hypoxic adaptation in the Tibetan chicken

1. The Tibetan chicken, which is an indigenous breed living on the Tibetan Plateau, exhibits hypoxic adaptations to its high-altitude environment. However, the molecular mechanism behind this hypoxic adaptation is still unclear. This study aimed to investigate differentially expressed miRNAs involved in hypoxic adaptation through high-throughput RNA sequencing. 2. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) was used to verify the differentially expressed miRNAs and their target genes in chicken embryonic heart tissues and fibroblasts. Luciferase reporter assays were performed to confirm the relationship between miRNAs and target genes. 3. The study identified 37 differentially expressed miRNAs in Tibetan chicken embryonic heart tissues, including 20 up- and 17 down-regulated miRNAs, compared to those found in lowland chickens. Differentially expressed miRNAs were mainly involved in biological processes, such as cell cycle arrest, toll-like receptor signalling pathways, and I-kappa B kinase/NF-kappa B signalling. The data showed that gga-miR-34 c-5p was significantly upregulated in Tibetan chicken tissues and hypoxic fibroblasts, while EHHADH, a target gene of gga-miR-34 c-5p, was downregulated. Moreover, gga-miR-34 c-5p dramatically decreased the luciferase activity of the wild EHHADH, whereas no effect on the mutational EHHADH was found. 4. This study identified miRNA expression profiles in the Tibetan chicken and suggested that miR-34 c-5p acts as a novel miRNA associated with hypoxic adaptation. This facilitates the understanding of molecular mechanisms that underlie long-term exposure to hypoxia.

HIF-1α contributes to hypoxia adaptation of the naked mole rat

Background/Aims

Naked mole rats (NMRs) spend their lives in burrow systems containing very low levels of oxygen, indicating long-term hypoxic exposure, and suggesting that pathological changes caused by hypoxia are attenuated or absent in this hypoxia-tolerant species. The mechanisms underlying NMRs hypoxia tolerance remain poorly understood. In this study, we explored whether hypoxia inducible factor 1α (HIF-1α), and vascular endothelial growth factor A (VEGFA) play a role in NMRs adaption to hypoxia.

Methods

Primary hepatic stellate cells (HSCs) isolated from NMRs and mice were treated with 50 μM YC-1, 50 μM KC7F2 or VEGFA siRNA. HIF-1α or VEGFA expression was detected by Western blot and real-time PCR. Apoptosis was determined by flow cytometry. The expression of autophagy markers (LC3 and p62) was detected by Western blot.

Results

Our results showed that HIF-1α and VEGFA expression in NMRs was significantly higher than in hypoxia-sensitive mice. Inhibition of HIF-1α expression induced apoptosis in both NMR and mouse HSCs following hypoxia. However, blocking VEGFA transcription results in a significant increase of apoptosis in both NMR and mouse HSCs before and after hypoxia. In addition, NMR HSCs displayed higher levels of autophagy (ratio of LC3ΙΙ/LC3Ι = 9.6) than mouse HSCs (relative ratio of LC3ΙΙ/ LC3Ι = 4.9) under hypoxic conditions.

Conclusion

We conclude that HIF-1α activation may be an important mechanism for hypoxia adaption. However, high expression of VEGFA follows HIF-1α activation in NMRs.

Gender differences in hypoxic acclimatization in cyclooxygenase-2-deficient mice

The aim of this study was to determine the effect of cyclooxygenase‐2 (COX‐2) gene deletion on the adaptive responses during prolonged moderate hypobaric hypoxia. Wild‐type (WT) and COX‐2 knockout (KO) mice of both genders (3 months old) were exposed to hypobaric hypoxia (~0.4 ATM) or normoxia for 21 days and brain capillary densities were determined. Hematocrit was measured at different time intervals; brain hypoxia‐inducible factor ‐1α (HIF‐1α), angiopoietin 2 (Ang‐2), brain erythropoietin (EPO), and kidney EPO were measured under normoxic and hypoxic conditions. There were no gender differences in hypoxic acclimatization in the WT mice and similar adaptive responses were observed in the female KO mice. However, the male KO mice exhibited progressive vulnerability to prolonged hypoxia. Compared to the WT and female KO mice, the male COX‐2 KO mice had significantly lower survival rate and decreased erythropoietic and polycythemic responses, diminished cerebral angiogenesis, decreased brain accumulation of HIF‐1α, and attenuated upregulation of VEGF, EPO, and Ang‐2 during hypoxia. Our data suggest that there are physiologically important gender differences in hypoxic acclimatization in COX‐2‐deficient mice. The COX‐2 signaling pathway appears to be required for acclimatization in oxygen‐limiting environments only in males, whereas female COX‐2‐deficient mice may be able to access COX‐2‐independent mechanisms to achieve hypoxic acclimatization.

Cloning and polymorphisms of yak lactate dehydrogenase B gene

The main objective of this work was to study the unique polymorphisms of the lactate dehydrogenase-1 (LDH1) gene in yak (Bos grunniens). Native polyacrylamide gel electrophoresis revealed three phenotypes of LDH1 (a tetramer of H subunit) in yak heart and longissimus muscle extracts. The corresponding gene, ldhb, encoding H subunits of three LDH1 phenotypes was obtained by RT-PCR. A total of six nucleotide differences were detected in yak ldhb compared with that of cattle, of which five mutations cause amino acid substitutions. Sequence analysis shows that the G896A and C689A, mutations of ldhb gene, result in alterations of differently charged amino acids, and create the three phenotypes (F, M, and S) of yak LDH1. Molecular modeling of the H subunit of LDH indicates that the substituted amino acids are not located within NAD+ or substrate binding sites. PCR-RFLP examination of G896A mutation demonstrated that most LDH1-F samples are actually heterozygote at this site. These results help to elucidate the molecular basis and genetic characteristic of the three unique LDH1 phenotypes in yak.

Identification of a Tibetan-specific mutation in the hypoxic gene EGLN1 and its contribution to high-altitude adaptation

Tibetans are well adapted to high-altitude hypoxic conditions, and in recent genome-wide scans, many candidate genes have been reported involved in the physiological response to hypoxic conditions. However, the limited sequence variations analyzed in previous studies would not be sufficient to identify causal mutations. Here we conducted resequencing of the entire genomic region (59.4 kb) of the hypoxic gene EGLN1 (one of the top candidates from the genome-wide scans) in Tibetans and identified 185 sequence variations, including 13 novel variations (12 substitutions and 1 insertion or deletion). There is a nonsynonymous mutation (rs186996510, D4E) showing surprisingly deep divergence between Tibetans and lowlander populations (Fst = 0.709 between Tibetans and Han Chinese). It is highly prevalent in Tibetans (70.9% on average) but extremely rare in Han Chinese, Japanese, Europeans, and Africans (0.56-2.27%), suggesting that it might be the causal mutation of EGLN1 contributing to high-altitude hypoxic adaptation. Neutrality test confirmed the signal of Darwinian positive selection on EGLN1 in Tibetans. Haplotype network analysis revealed a Tibetan-specific haplotype, which is absent in other world populations. The estimated selective intensity (0.029 for the C allele of rs186996510) puts EGLN1 among the known genes that have undergone the strongest selection in human populations, and the onset of selection was estimated to have started at the early Neolithic (∼8,400 years ago). Finally, we detected a significant association between rs186996510 and hemoglobin levels in Tibetans, suggesting that EGLN1 contributes to the adaptively low hemoglobin level of Tibetans compared with acclimatized lowlanders at high altitude.