SAMHD1-mediated regulation of innate immunity during SARS-CoV-2 infection

Project Summary The COVID-19 pandemic caused by severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) remains a threat to public health, particularly in aging populations. The top priority for COVID-19 research is to improve fundamental knowledge of SARS-CoV-2 and viral pathogenesis, including studies to characterize the virus and to understand the host immunity. Our proposed study aims to better understand the molecular mechanisms underlying SARS-CoV-2 infection and innate immune regulation, which is important for controlling COVID-19 and potential future threats of other emerging coronaviruses. Sterile alpha motif (SAM) and HD-domain containing protein 1 (SAMHD1) is the unique mammalian dNTP hydrolase (dNTPase) that also regulates innate and adaptive immunity in hosts. Importantly, SAMHD1 phosphorylation negatively regulates its dNTPase activity and antiviral function. We found that SAMHD1 negatively regulates antiviral innate immune responses and inflammation through interacting with various key proteins in innate immune signaling in macrophages. SAMHD1 transcript is upregulated in SARS-CoV-2 infected primary human bronchial epithelial cells. Our preliminary data showed that SARS-CoV-2 infection of human lung epithelial cell lines or primary human airway epithelial (HAE) cultures significantly upregulated SAMHD1 phosphorylation. However, the function of SAMHD1 in regulating SARS-CoV-2 replication and innate immunity remains unclear. Our hypothesis is that SARS-CoV-2 infection increases inflammation in lung and airway epithelial cells by upregulating phosphorylation of SAMHD1, thereby reducing SAMHD1-mediated anti- inflammation effects. Our established primary HAE cultures provided a physiologically relevant in vitro model to study SARS-CoV-2 infection and cellular responses. We will also use an established mouse model of SARS-CoV-2 infection as a complementary in vivo approach in our studies. We designed two specific aims to test our hypothesis: Aim 1. To examine the role of SAMHD1 in SARS-CoV-2-induced inflammation of primary HAE cultures and SAMHD1- knockout mice; and Aim 2. To investigate the mechanisms of SARS-CoV-2-upregulated phosphorylation of human SAMHD1. Accomplishing our multidisciplinary studies will help define the mechanisms by which SAMHD1 regulates SARS-CoV-2 replication, inflammation, and antiviral innate immunity.