Novel biomarkers of mitochondrial dysfunction in Long COVID patients

Abstract

Coronavirus disease 2019 (COVID-19) can lead to severe acute respiratory syndrome, and while most individuals recover within weeks, approximately 30-40% experience persistent symptoms collectively known as Long COVID, post-COVID-19 syndrome, or post-acute sequelae of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection (PASC). These enduring symptoms, including fatigue, respiratory difficulties, body pain, short-term memory loss, concentration issues, and sleep disturbances, can persist for months. According to recent studies, SARS-CoV-2 infection causes prolonged disruptions in mitochondrial function, significantly altering cellular energy metabolism. Our research employed transmission electron microscopy to reveal distinct mitochondrial structural abnormalities in Long COVID patients, notably including significant swelling, disrupted cristae, and an overall irregular morphology, which collectively indicates severe mitochondrial distress. We noted increased levels of superoxide dismutase 1 which signals oxidative stress and elevated autophagy-related 4B cysteine peptidase levels, indicating disruptions in mitophagy. Importantly, our analysis also identified reduced levels of circulating cell-free mitochondrial DNA (ccf-mtDNA) in these patients, serving as a novel biomarker for the condition. These findings underscore the crucial role of persistent mitochondrial dysfunction in the pathogenesis of Long COVID. Further exploration of the cellular and molecular mechanisms underlying post-viral mitochondrial dysfunction is critical, particularly to understand the roles of autoimmune reactions and the reactivation of latent viruses in perpetuating these conditions. This comprehensive understanding could pave the way for targeted therapeutic interventions designed to alleviate the chronic impacts of Long COVID. By utilizing circulating ccf-mtDNA and other novel mitochondrial biomarkers, we can enhance our diagnostic capabilities and improve the management of this complex syndrome.

Keywords: Mitochondria; Mitophagy; Oxidative damage; Post-COVID; mtDNA.

Fig. 1

Analysis of mitochondrial morphology and expression of specific proteins related to mitochondrial function in patients with post-COVID-19 (PC) syndrome and control participants by TEM. Mitochondrial morphology and immunodetection of proteins associated with mitochondrial function in patients (first column) and control (C) participants (second column). Protein markers analyzed include A ATG4B; B SOD1, DRP1, and MFN2; and C LDH, MFN1, and FIS1. In the third column, violin plots quantitatively present the immunodetection results corresponding to the protein markers listed in the same row. Statistical significance between PC and C samples is denoted by asterisks: *p < 0.05, ***p < 0.001. “ns” indicates no significant differences (p > 0.05). D Quantitatively presents the analysis of mitochondrial morphology and copy number differences in PC patients

Fig. 2

Quantitative analysis of ccf-mtDNA content in patients with post-COVID-19 (PC) syndrome and control participants. A Heatmap displaying the levels of ccf-mtDNA for five mitochondrial genes (MTATP6, MTCYTB, MTND1, MTND4, MTND5) in post-COVID-19 (PC, blue) and control (C, red) individuals. B Violin plots (first and third rows) showing the distribution of ccf-mtDNA levels for each mitochondrial gene, alongside receiver operating characteristic (ROC) curves (second and fourth rows) which evaluate the diagnostic potential of ccf-mtDNA measurements in distinguishing between the PC and C groups

Fig. 3

Mechanisms and consequences of mitochondrial damage and dysfunction in the pathogenesis of Long COVID. This schematic illustrates the cascade of events leading from initial SARS-CoV-2 infection to persistent mitochondrial dysfunction and its systemic effects. The diagram highlights key steps: (1) initial mitochondrial damage through direct viral interaction and immune-mediated responses; (2) activation of mitophagy in an attempt to clear damaged mitochondria; (3) persistent mitochondrial dysfunction due to incomplete removal of damaged mitochondria, evidenced by reduced ccf-mtDNA levels; (4) resultant systemic effects contributing to the symptomatology of Long COVID; (5) utilization of ccf-mtDNA as a diagnostic and monitoring tool to assess the extent of mitochondrial dysfunction. Each component integrates findings from the current study, emphasizing the role of mitochondrial damage in the pathogenesis of Long COVID