ABSTRACT
As the coronavirus disease 2019 (COVID-19) pandemic continues to wreak havoc, researchers around the globe are working together to understand how the responsible agent - severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) damages the respiratory system and other organs. Macroautophagy/autophagy is an innate immune response against viral infection and is known to be manipulated by positive-strand RNA viruses, including SARS-CoV-2. Nevertheless, the link between autophagic subversion and cell death or inflammation in COVID-19 remains unclear. Emerging evidence suggests that SARS-CoV-2 could trigger pyroptosis, a form of inflammatory programmed cell death characterized by the activation of inflammasomes and CASP1 (caspase 1) and the formation of transmembrane pores by GSDMD (gasdermin D). In this connection, autophagic flux impairment is a known activator of inflammasomes. This prompted us to investigate if SARS-CoV-2 could target autophagy to induce inflammasome-dependent pyroptosis in lung epithelial cells.Abbreviations: ATP6AP1: ATPase H+ transporting accessory protein 1; CASP1: caspase 1; COVID-19: coronavirus disease 2019; GSDMD: gasdermin D; IL1B: interleukin 1 beta; IL18: interleukin 18; KRT 18: keratin 18; NLRP3: NLR family pyrin domain containing 3; NOD: nucleotide oligomerization domain; NSP6: non-structural protein 6; TFEB: transcription factor EB; SARS-CoV-2: severe acute respiratory syndrome coronavirus 2.
Subject(s)
COVID-19 , Vacuolar Proton-Translocating ATPases , Autophagy , Caspase 1/metabolism , Humans , Inflammasomes/metabolism , Interleukin-1beta/metabolism , Lung/metabolism , Lysosomes/metabolism , NLR Family, Pyrin Domain-Containing 3 Protein/metabolism , Pyroptosis , SARS-CoV-2 , Vacuolar Proton-Translocating ATPases/metabolismABSTRACT
A recent mutation analysis suggested that Non-Structural Protein 6 (NSP6) of the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) is a key determinant of the viral pathogenicity. Here, by transcriptome analysis, we demonstrated that the inflammasome-related NOD-like receptor signaling was activated in SARS-CoV-2-infected lung epithelial cells and Coronavirus Disease 2019 (COVID-19) patients' lung tissues. The induction of inflammasomes/pyroptosis in patients with severe COVID-19 was confirmed by serological markers. Overexpression of NSP6 triggered NLRP3/ASC-dependent caspase-1 activation, interleukin-1ß/18 maturation, and pyroptosis of lung epithelial cells. Upstream, NSP6 impaired lysosome acidification to inhibit autophagic flux, whose restoration by 1α,25-dihydroxyvitamin D3, metformin or polydatin abrogated NSP6-induced pyroptosis. NSP6 directly interacted with ATP6AP1, a vacuolar ATPase proton pump component, and inhibited its cleavage-mediated activation. L37F NSP6 variant, which was associated with asymptomatic COVID-19, exhibited reduced binding to ATP6AP1 and weakened ability to impair lysosome acidification to induce pyroptosis. Consistently, infection of cultured lung epithelial cells with live SARS-CoV-2 resulted in autophagic flux stagnation, inflammasome activation, and pyroptosis. Overall, this work supports that NSP6 of SARS-CoV-2 could induce inflammatory cell death in lung epithelial cells, through which pharmacological rectification of autophagic flux might be therapeutically exploited.
Subject(s)
COVID-19 , Coronavirus Nucleocapsid Proteins , NLR Family, Pyrin Domain-Containing 3 Protein , SARS-CoV-2 , Vacuolar Proton-Translocating ATPases , COVID-19/metabolism , COVID-19/virology , Coronavirus Nucleocapsid Proteins/genetics , Coronavirus Nucleocapsid Proteins/metabolism , Humans , Inflammasomes/metabolism , Interleukin-1beta/metabolism , NLR Family, Pyrin Domain-Containing 3 Protein/genetics , NLR Family, Pyrin Domain-Containing 3 Protein/metabolism , Pyroptosis , SARS-CoV-2/genetics , SARS-CoV-2/metabolism , SARS-CoV-2/pathogenicity , Vacuolar Proton-Translocating ATPases/metabolismABSTRACT
Coronavirus disease 2019 (COVID-19) has spread rapidly worldwide, causing significant mortality. There is a mechanistic relationship between intracellular coronavirus replication and deregulated autophagosome-lysosome system. We performed transcriptome analysis of peripheral blood mononuclear cells (PBMCs) from COVID-19 patients and identified the aberrant upregulation of genes in the lysosome pathway. We further determined the capability of two circulating markers, namely microtubule-associated proteins 1A/1B light chain 3B (LC3B) and (p62/SQSTM1) p62, both of which depend on lysosome for degradation, in predicting the emergence of moderate-to-severe disease in COVID-19 patients requiring hospitalization for supplemental oxygen therapy. Logistic regression analyses showed that LC3B was associated with moderate-to-severe COVID-19, independent of age, sex and clinical risk score. A decrease in LC3B concentration <5.5 ng/ml increased the risk of oxygen and ventilatory requirement (adjusted odds ratio: 4.6; 95% CI: 1.1-22.0; P = 0.04). Serum concentrations of p62 in the moderate-to-severe group were significantly lower in patients aged 50 or below. In conclusion, lysosome function is deregulated in PBMCs isolated from COVID-19 patients, and the related biomarker LC3B may serve as a novel tool for stratifying patients with moderate-to-severe COVID-19 from those with asymptomatic or mild disease. COVID-19 patients with a decrease in LC3B concentration <5.5 ng/ml will require early hospital admission for supplemental oxygen therapy and other respiratory support.