Neuropathogenesis of SARS-CoV-2 Infection in Mice

Although severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection in humans primarily causes respiratory disease; some patients develop symptoms of neurological disease, such as headache, loss of taste and smell, cognitive dysfunction, and impaired consciousness. In this study, we analyzed the tissue tropism, immune response, and pathology in human ACE2-expressing (hACE2) mice after SARS-CoV-2 infection. Intranasal infection of hACE2 mice with SARS-CoV-2 resulted in a lethal disease with high levels of virus replication in the brain tissue. SARS-CoV-2-infected mice exhibited encephalitis hallmarks characterized by production of cytokines and chemokines, leukocyte infiltration, hemorrhage, and neuronal cell death. SARS-CoV-2 was also found to productively infect cells within the nasal turbinate, eye, and olfactory bulb, suggesting SARS-CoV-2 entry into the brain by this route after intranasal infection. NanoString gene expression analysis was performed to identify differentially expressed genes (DEGs) in the brains of mice following SARS-CoV-2 infection. Genes that were upregulated after infection were mainly associated with toll-like receptor (TLR) signaling, RIG-I like receptor signaling (RLR) and cell death pathways, while down-regulated genes were associated with neurodegeneration and synaptic signaling pathways. Next, we generated primary neuronal cultures from hACE2 mice to investigate the effects of a SARS-CoV-2 infection. Our data show that neuronal cultures obtained from hACE2 mice are permissive to SARS-CoV-2 infection and support productive virus replication. Like the brain, SARS-CoV-2 infection upregulated the expression of genes involved in innate immunity, inflammation, and necroptosis in the neurons. Because age is a risk factor for COVID-19-related death in humans, we next evaluated the pathogenesis of SARS-CoV-2 infection in yong and old mice. Our data show that intranasal inoculation of SARS-CoV-2 in the older BALB/c mice resulted in severe disease, as evident by rapid loss of body weight, gross and microscopic pathology of lungs, as well as elevated mRNA levels of inflammatory cytokines and chemokines. In addition, our results indicate that a diminished interferon response in the old mice may play a role in severe disease outcome observed in these mice. Together, our data provide new insights into the pathogenesis of SARS-CoV-2 infection in mice