Postnatal Hypoxia Could Cause Pulmonary Hypertension Due to Low Expression of METTL3
Pulmonary hypertension (PH) is a type of high blood pressure that affects the blood vessels in the lungs, as well as the right side of the heart. Arteries in the lung become damaged as a result of an imbalance between vasodilator and vasoconstrictor signals. This condition also causes a thickening and narrowing of the arteries, resulting in less oxygen flow in the blood. Primary causes of PH include heart disease, liver disease, and genetics.
Recent studies have shown that early-life hypoxia could result in the development of PH later on in life. How this oxygen deficiency affects pulmonary maturation and function in adulthood is not understood. However, it seems that an epigenetic mechanism called m6Acould play a role in the progression of PH.
N6-methyladenosine(m6A) is a common RNA modification found in eukaryotes. It plays an important role in various biological processes like RNA stability and mRNA translation. The m6A modification is catalyzed by the METTL3/METTL14enzymes, which adds a methyl group to the nitrogen-6 position of adenosine residues. This reaction can be reversed by RNA demethylaseproteins FTO and ALKBH5, which removes the methyl group from the adenosine.
A team of researchers led by Dr. Shanshan Xu from the Zhejiang University School of Medicine in China wanted to investigate both the short-term and long-term lung effects of early-life hypoxia. Particularly, they wanted to determine how m6A contributes to PH development. Previous research has indicated that METTL3 is involved in lung cell growth, making it an important enzyme to consider in this study.
To begin, the research team placed a group of 24 newborn (24 hrs old) rat pups in a hypoxia chamber for 2 weeks and then returned them to normal conditions. The control group was made up of 16 newborn rats that lived under normal oxygen conditions. After 2 weeks, 12 hypoxia and 8 control rats were sacrificed. At 9 weeks, the remaining rats from both groups were sacrificed.
The team measured the pressure in the right ventricle (RVP) of the 2-week-old rats and the pulmonary arterial pressure (mPAP) in the 9-week-old rats before euthanizing and harvesting their lungs. This procedure was done to determine if the hypoxic conditions had any physical effect on lung growth and function immediately after birth and later in life.
Both the mPAP and RVP were found to be considerably higher in the hypoxic groups compared to the control groups, suggesting that hypoxia plays a role in remodeling lung and heart tissues, leading to PH.
To determine the total m6A levels in the lung tissues, they used the EpiQuik m6A RNA Methylation Quantification Kit (Colorimetric). Total m6A levels in the lung tissue were found not to be noticeably impacted by the hypoxic conditions. The team shifted their focus to five m6A methyltransferase(METTL3, METTL14, and WTAP) and demethylase(FTO and ALKBH5) proteins, which are also involved in lung tissue development.
Of the methyltransferases, METTL3 and METTL14 had decreased in the hypoxic group, whereas WTAP was unaffected. Interestingly, the expressions of demethylases FTO and ALKBH5 were also found to be drastically reduced—so the impact that postnatal hypoxia remained unclear.
Expression of m6A-related proteins after postnatal hypoxia.a, b.Western blot images showing the expression of m6A methyltransferase and demethylase proteins. c–f.Quantitative analysis of the western blot experiments. g.The total RNA m6A level in the lung tissue.
Next, they conducted a MeRIPanalysis on genes related to respiratory tube development, Notch signal pathways, and endothelial cell activation to help paint a better picture.
The MeRIP analysis revealed 21 hyper-methylated and 5-hypomethlated regions in the 2-week-old hypoxic rats. In the 9-week-old rats, they found 7 hyper-methylated peaks and 13 hypo-methylated peaks, indicating that postnatal hypoxia affects lung function early in life and could still impact its function down the line leading to PH.
m6A methylation following postnatal hypoxia in 9-week-old rats. a. Expression of METTL3 in adult rats. b. Distinct m6A peaks by peak comparison between the two groups
The data gathered in this study suggests that postnatal hypoxia can indeed cause PH in adulthood. Though its role on global m6A levels remains unclear, the team believes that low levels of METTL3 experienced in the hypoxic group affected PH-related genes, leading to the development of PH.
It's hoped that future studies can clarify the impact that postnatal hypoxic conditions have on m6A, as well as determine if those conditions affect oxygen uptake, physical lung capacity, and function, like when exercising.
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