Is Low Alveolar Type II Cell SOD3 in the Lungs of Elderly Linked to the Observed Severity of COVID-19?

Abstract

Human lungs single-cell RNA sequencing data from healthy donors (elderly and young; GEO accession no. GSE122960) were analyzed to isolate and specifically study gene expression in alveolar type II cells. Colocalization of angiotensin-converting enzyme 2 (ACE2) and TMPRSS2 enables severe acute respiratory syndrome coronavirus 2 (SARS-CoV 2) to enter the cells. Expression levels of these genes in the alveolar type II cells of elderly and young patients were comparable and, therefore, do not seem to be responsible for worse outcomes observed in coronavirus disease 2019 (COVID-19) affected elderly. In cells from the elderly, 263 genes were downregulated and 95 upregulated. Superoxide dismutase 3 (SOD3) was identified as the top-ranked gene that was most downregulated in the elderly. Other redox-active genes that were also downregulated in cells from the elderly included activating transcription factor 4 (ATF4) and metallothionein 2A (M2TA). ATF4 is an endoplasmic reticulum stress sensor that defends lungs via induction of heme oxygenase 1. The study of downstream factors known to be induced by ATF4, according to Ingenuity Pathway Analysis™, identified 24 candidates. Twenty-one of these were significantly downregulated in the cells from the elderly. These downregulated candidates were subjected to enrichment using the Reactome Database identifying that in the elderly, the ability to respond to heme deficiency and the ATF4-dependent ability to respond to endoplasmic reticulum stress is significantly compromised. SOD3-based therapeutic strategies have provided beneficial results in treating lung disorders including fibrosis. The findings of this study propose the hypotheses that lung-specific delivery of SOD3/ATF4-related antioxidants will work in synergy with promising antiviral drugs such as remdesivir to further improve COVID-19 outcomes in the elderly.

Keywords: ATF4; COVID19; ROS; SOD3; lung; single-cell RNA sequencing.

FIG. 1.

Identification of 14 distinct clusters within the lung tissue with unique markers for each. (A) t-SNE projection of the filtered data (18,284 cells; 8501 young; 9783 old). Each cell is represented as a dot. (B) t-SNE clustering of the epithelial alveolar type II cells, cells showing clear separation between the cells from young and old groups. (C) Expression level of ACE2 (top) and TMPRSS2 (bottom) in alveolar type II cells. (D) Heatmap of the top differentially expressed genes between young and old lung alveolar type II cells (log fold change ±0.4). Rows represent genes and columns represent cells. ACE2, angiotensin-converting enzyme 2; t-SNE, t-distributed stochastic neighbor embedding.

FIG. 2.

Downregulation of SOD3 and ATF4 in alveolar type II cells of lungs from elderly donors. (A) t-SNE plots showing SOD3 and (B) ATF4 expression level between young and old age groups in (top) all the 14 clusters and (bottom) in alveolar type II cells. (C) Heatmap of the downstream targets of ATF4. Rows represent genes and columns represent cells. (D) Pathways enrichment for ATF4 downstream targets that are downregulated in elderly. Circles represent genes and diamonds represent the enriched terms. ATF4, activating transcription factor 4; SOD3, superoxide dismutase 3.