In a recent study posted to bioRxiv*, researchers reported that dysregulation in energy metabolism and inflammation could contribute to long-term coronavirus disease 2019 (COVID-19) syndrome (LTCS).
Numerous studies have uncovered the role of metabolites in COVID-19. Specifically, peripheral blood mononuclear cells (PBMCs) show dysregulation of glycolysis and oxidative phosphorylation, with increased lactate and reduced glucose levels in COVID-19 patients. Therefore, identifying metabolites in blood would provide a picture of the pathophysiological state of the disease.
Several COVID-19 patients develop LTCS following the acute phase, with symptoms like dyspnea, chronic fatigue, and brain fog, among others. Mitochondrial dysfunction, impaired interleukin (IL)-10 production, and fatty acid metabolism have been reported in LTCS individuals. Therefore, the role of inflammation and metabolism during LTCS progression warrants further investigation.
About the study
In the present study, researchers performed an integrated analysis of blood serum/plasma using the in vitro diagnostics research nuclear magnetic resonance (NMR) analytical platform and cytokine measurements in COVID-19 patients, recovered individuals, LTCS patients, and healthy controls. All participants were aged 18 or older. LTCS subjects were recruited only if samples were collected more than 28 days after testing positive for COVID-19.
Initially, samples were collected for DNA isolation for whole genomic and epigenetic analyses, and leftover surplus blood was used for analysis in the current study. Plasma was isolated from blood samples and tested for inflammatory cytokine levels using a 13-plex flow cytometry-based assay, which permits simultaneous measurement of 13 cytokines and chemokines. Data were analyzed automatically and manually by standard curve detection.
In the automatic gating strategy, two sets of beads were used, each with a unique size that can be identified by the side (SSC) and forward (FSC) scatter profiles. Raw NMR spectra were recorded for blood samples. Five one-dimensional 1H-NMR experiments were performed for each sample. Nuclear Overhauser effect spectroscopy (NOESY) was performed to show NMR spectrum quality and enable estimation of metabolites and high molecular weight compounds.
In total, 125 persons were recruited. Controls were significantly younger than recovered individuals, while the age of LTCS, COVID-19 patients, and recovered individuals was not significantly different. Males and females were equally represented in controls and LTCS patients. Samples were collected after a median of 152 and 128 days after infection from LTCS patients and recovered individuals, respectively.
LTCS patients reported fatigue, dyspnea, dizziness, anosmia, ageusia, anxiety, myalgia, neuropathy, and headache. Creatine (amino acid) and 3-hydroxybutyrate (a ketone body) were the highest in the plasma of COVID-19 patients; histidine and citrate were the highest in LTCS samples, and lactate and pyruvate were the highest in the recovered subjects. Furthermore, LTCS subjects had higher acetone, citrate, and formate levels than recovered individuals.
The authors mostly focused on comparisons between LTCS subjects and controls, as samples from COVID-19 patients and recovered subjects were limited. As such, a supervised classification model was built to differentiate between controls and LTCS individuals. This revealed elevated levels of lactate, pyruvate, alanine, and methionine in LTCS patients relative to controls.
In LTCS subjects, 18 and 38 lipoproteins were increased and decreased, respectively, relative to controls. Blood triglycerides were higher in COVID-19 patients, recovered, and LTCS subjects than in controls. Very low-density lipoprotein (VLDL) phospholipids were also higher in LTCS and recovered subjects. COVID-19 patient samples showed the highest levels of cytokines and chemokines than other groups.
Most cytokines and chemokines were in lower levels among LTCS and recovered individuals than in controls, except IL-18, which was higher in LTCS and recovered subjects. IL-6, IL-10, and IL-18 were abundant in COVID-19 patient samples. Correlation analyzes revealed that COVID-19 patients had the highest levels of cytokines, whereas LTCS subjects had low levels of cytokines and chemokines.
Histidine was highly enriched in LTCS samples. Histidine, phenylalanine, and ketone bodies increased with the deterioration of health. COVID-19 patients had a strong positive correlation to glycerides, 2-oxoglutaric acid, succinate, acetoacetate, 3-hydroxybutyrate, and a negative correlation towards high-density lipoprotein (HDL) free cholesterol and phospholipids.
In summary, the researchers identified dysregulation of several metabolites, cytokines, and lipoproteins in LTCS patients than in other groups. Notably, relative to controls, pyruvate, and lactate were highly elevated in LTCS individuals. Glycine, phenylalanine, creatinine, glutamine, and acetate were downregulated in LTCS samples. Blood levels of phenylalanine, formate, creatine, pyruvate, and ketone bodies may indicate increased LTCS severity. Together, the findings provided novel insights into metabolomic, lipoprotein, and inflammation changes in LTCS subjects.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.