Intravital imaging of 3 different microvascular beds in SARS-CoV-2-infected mice.

SARS-CoV-2 enters the respiratory tract where it infects the alveoli epithelial lining. However, patients have sequelae that extend well beyond the alveoli into the pulmonary vasculature and perhaps beyond to brain and other organs. Because of the dynamic events within blood vessels, histology fails to report on platelet and neutrophil dynamics. Because of the rapid non-transcriptional behaviour of these cells neither single-cell RNAseq nor proteomics report robustly on their critical behaviours. We used intravital-microscopy in level-3 containment to examine the pathogenesis of SARS-CoV-2 within three organs in mice expressing human-ACE-2 ubiquitously (CAG-AC-70) or on epithelium (K18-promoter). Using a neon-green-SARS-CoV-2, we observed both epithelium and endothelium infected in AC70 mice but only the epithelium in K18 mice. There was increased neutrophils in the microcirculation but not in the alveoli of the AC70 lungs. Platelets formed large aggregates in the pulmonary capillaries. Despite only neurons being infected within the brain, profound neutrophil adhesion forming the nidus of large platelet aggregates were observed in the brain microcirculation with many non-perfused microvessels. Neutrophils were seen breaching the brain endothelial layer associated with a significant disruption of the blood-brain-barrier. Despite ubiquitous ACE-2 expression, CAG-AC-70 mice had very small increases in blood cytokine, no increase in thrombin, no infected circulating cells and no liver involvement suggesting limited systemic effects. In summary, our imaging of SARS-CoV-2-infected mice gave direct evidence that there is a significant perturbation locally in the lung and brain microcirculation induced by local viral infection leading to increased inflammation and thrombosis in these organs.

Nsp1 proteins of human coronaviruses HCoV-OC43 and SARS-CoV2 inhibit stress granule formation.

Stress granules (SGs) are cytoplasmic condensates that often form as part of the cellular antiviral response. Despite the growing interest in understanding the interplay between SGs and other biological condensates and viral replication, the role of SG formation during coronavirus infection remains poorly understood. Several proteins from different coronaviruses have been shown to suppress SG formation upon overexpression, but there are only a handful of studies analyzing SG formation in coronavirus-infected cells. To better understand SG inhibition by coronaviruses, we analyzed SG formation during infection with the human common cold coronavirus OC43 (HCoV-OC43) and the pandemic SARS-CoV2. We did not observe SG induction in infected cells and both viruses inhibited eukaryotic translation initiation factor 2α (eIF2α) phosphorylation and SG formation induced by exogenous stress. Furthermore, in SARS-CoV2 infected cells we observed a sharp decrease in the levels of SG-nucleating protein G3BP1. Ectopic overexpression of nucleocapsid (N) and non-structural protein 1 (Nsp1) from both HCoV-OC43 and SARS-CoV2 inhibited SG formation. The Nsp1 proteins of both viruses inhibited arsenite-induced eIF2α phosphorylation, and the Nsp1 of SARS-CoV2 alone was sufficient to cause a decrease in G3BP1 levels. This phenotype was dependent on the depletion of cytoplasmic mRNA mediated by Nsp1 and associated with nuclear accumulation of the SG-nucleating protein TIAR. To test the role of G3BP1 in coronavirus replication, we infected cells overexpressing EGFP-tagged G3BP1 with HCoV-OC43 and observed a significant decrease in virus replication compared to control cells expressing EGFP. The antiviral role of G3BP1 and the existence of multiple SG suppression mechanisms that are conserved between HCoV-OC43 and SARS-CoV2 suggest that SG formation may represent an important antiviral host defense that coronaviruses target to ensure efficient replication.

Thiopurines inhibit coronavirus Spike protein processing and incorporation into progeny virions.

There is an outstanding need for broadly acting antiviral drugs to combat emerging viral diseases. Here, we report that thiopurines inhibit the replication of the betacoronaviruses HCoV-OC43 and SARS-CoV-2. 6-Thioguanine (6-TG) disrupted early stages of infection, limiting accumulation of full-length viral genomes, subgenomic RNAs and structural proteins. In ectopic expression models, we observed that 6-TG increased the electrophoretic mobility of Spike from diverse betacoronaviruses, matching the effects of enzymatic removal of N-linked oligosaccharides from Spike in vitro. SARS-CoV-2 virus-like particles (VLPs) harvested from 6-TG-treated cells were deficient in Spike. 6-TG treatment had a similar effect on production of lentiviruses pseudotyped with SARS-CoV-2 Spike, yielding pseudoviruses deficient in Spike and unable to infect ACE2-expressing cells. Together, these findings from complementary ectopic expression and infection models strongly indicate that defective Spike trafficking and processing is an outcome of 6-TG treatment. Using biochemical and genetic approaches we demonstrated that 6-TG is a pro-drug that must be converted to the nucleotide form by hypoxanthine phosphoribosyltransferase 1 (HPRT1) to achieve antiviral activity. This nucleotide form has been shown to inhibit small GTPases Rac1, RhoA, and CDC42; however, we observed that selective chemical inhibitors of these GTPases had no effect on Spike processing or accumulation. By contrast, the broad GTPase agonist ML099 countered the effects of 6-TG, suggesting that the antiviral activity of 6-TG requires the targeting of an unknown GTPase. Overall, these findings suggest that small GTPases are promising targets for host-targeted antivirals.

Human coronaviruses disassemble processing bodies.

A dysregulated proinflammatory cytokine response is characteristic of severe coronavirus infections caused by SARS-CoV-2, yet our understanding of the underlying mechanism responsible for this imbalanced immune response remains incomplete. Processing bodies (PBs) are cytoplasmic membraneless ribonucleoprotein granules that control innate immune responses by mediating the constitutive decay or suppression of mRNA transcripts, including many that encode proinflammatory cytokines. PB formation promotes turnover or suppression of cytokine RNAs, whereas PB disassembly corresponds with the increased stability and/or translation of these cytokine RNAs. Many viruses cause PB disassembly, an event that can be viewed as a switch that rapidly relieves cytokine RNA repression and permits the infected cell to respond to viral infection. Prior to this submission, no information was known about how human coronaviruses (CoVs) impacted PBs. Here, we show SARS-CoV-2 and the common cold CoVs, OC43 and 229E, induced PB loss. We screened a SARS-CoV-2 gene library and identified that expression of the viral nucleocapsid (N) protein from SARS-CoV-2 was sufficient to mediate PB disassembly. RNA fluorescent in situ hybridization revealed that transcripts encoding TNF and IL-6 localized to PBs in control cells. PB loss correlated with the increased cytoplasmic localization of these transcripts in SARS-CoV-2 N protein-expressing cells. Ectopic expression of the N proteins from five other human coronaviruses (OC43, MERS, 229E, NL63 and SARS-CoV) did not cause significant PB disassembly, suggesting that this feature is unique to SARS-CoV-2 N protein. These data suggest that SARS-CoV-2-mediated PB disassembly contributes to the dysregulation of proinflammatory cytokine production observed during severe SARS-CoV-2 infection.

Infecting kidney organoids with a cDNA reporter clone of SARS-CoV-2.

Induced pluripotent stem cell (iPSC)-derived kidney organoids can be used for disease modeling and drug testing. Here, we describe a protocol to prepare stocks of an infectious clone of SARS-CoV-2 expressing a stable mNeonGreen reporter (icSARS-CoV-2-mNG). We demonstrate the infection of kidney organoids, primarily at the proximal tubular cells, with icSARS-CoV-2-mNG. Using a TCID50 (tissue culture infectious dose 50) assay and confocal microscopy, we show the quantification of SARS-CoV-2-mNG signal in proximal tubular cells of the kidney organoids. For complete details on the use and execution of this protocol, please refer to Rahmani et al. (2022).

Attenuation of SARS-CoV-2 infection by losartan in human kidney organoids.

COVID-19-associated acute kidney injury (COVID-AKI) is a common complication of SARS-CoV-2 infection in hospitalized patients. The susceptibility of human kidneys to direct SARS-CoV-2 infection and modulation of the renin-angiotensin II signaling (RAS) pathway by viral infection remain poorly characterized. Using induced pluripotent stem cell-derived kidney organoids, SARS-CoV-1, SARS-CoV-2, and MERS-CoV tropism, defined by the paired expression of a host receptor (ACE2, NRP1 or DPP4) and protease (TMPRSS2, TMPRSS4, FURIN, CTSB or CTSL), was identified primarily among proximal tubule cells. Losartan, an angiotensin II receptor blocker being tested in patients with COVID-19, inhibited angiotensin II-mediated internalization of ACE2, upregulated interferon-stimulated genes (IFITM1 and BST2) known to restrict viral entry, and attenuated the infection of proximal tubule cells by SARS-CoV-2. Our work highlights the susceptibility of proximal tubule cells to SARS-CoV-2 and reveals a putative protective role for RAS inhibitors during SARS-CoV-2 infection.