Defining the differential roles of HOIL1 and the Linear Ubiquitin Chain Assembly Complex in interferon induction by MDA5 and RIG-I during viral infection.

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PROJECT SUMMARY Despite the significant burden that viral infections continue to impose on human health, effective therapeutics and vaccines are still unavailable for many viral pathogens. A stronger understanding of the mechanisms by which the immune system senses and controls viral infections will enable the development of new strategies for their prevention and treatment. Interferons (IFN) are critical mediators of the immune response to viral infections, but excessive amounts can lead to chronic inflammation and autoimmunity. The production of IFN is therefore tightly regulated. Replicating viruses produce RNAs that are detected within cells by RNA sensors, RIG-I and MDA5. RIG-I detects viruses such as Sendai virus (SeV) and vesicular stomatitis virus (VSV), whereas MDA5 is essential for IFN induction to control picornaviruses such as Theiler’s murine encephalomyelitis virus (TMEV), and the Calicivirus, murine norovirus (MNoV). Once activated, these sensors initiate a common signaling cascade that leads to the transcriptional induction of IFN and establishes an anti-viral environment. Although the Linear Ubiquitin Chain Assembly Complex (LUBAC) has been shown previously to inhibit IFN induction by RIG-I, our studies have shown that LUBAC subunit, HOIL1, is essential for IFN induction by MDA5 in murine dendritic cells and fibroblasts, and to control MNoV infection in vivo. Furthermore, we have found that the E3 ubiquitin ligase activity of HOIL1 is essential for IFN induction during TMEV infection of fibroblasts. However, the mechanism by which HOIL1 E3 ligase activity regulates MDA5-dependent IFN induction, and the biological consequences of ubiquitination by HOIL1 are unknown. We hypothesize that HOIL1 and LUBAC are recruited selectively to the MDA5 signaling pathway, wherein HOIL1 catalyzes ubiquitination of one or more signaling molecules in the pathway to facilitate signal transduction and transcription of IFN genes. First, we will determine the location of the blockade in the MDA5 signaling cascade in HOIL1 E3 ligase mutant cells, and identify other LUBAC protein domains that regulate MDA5 signaling and IFN induction. Second, we will use both candidate and unbiased biochemical approaches to identify HOIL1-interactors and ubiquitinated proteins during MDA5 signaling. Last, we will test whether HOIL1 E3 ligase activity differentially regulates IFN induction and viral pathogenesis in vivo during MNoV, SeV, VSV and TMEV infection of mice, and will use cell type-specific knock-out mice to identify the cell types that require HOIL1 to induce IFN and control MNoV persistent infection in vivo. We expect our studies to identify a novel mechanism of IFN regulation, thereby revealing potential therapeutic targets for viral and IFN-mediated autoimmune diseases.