The role of PGAM5 in regulating viral infection and the pathogenesis of intestinal inflammation
Viral infections trigger host innate immune responses, characterized by the production of type-I interferons (IFN) including IFNβ. IFNβ induces cellular antiviral defense mechanisms by expressing IFN-stimulated genes (ISG), thereby contributing to pathogen clearance. Accumulating evidence suggests that mitochondria constitute a crucial platform for the induction of antiviral immunity and cell death. The mitochondrial protein phosphoglycerate mutase family member 5 (PGAM5) has been implicated in a broad range of biological processes including certain cell death pathways and NLRP3 inflammasome activation. The hypothesis of this thesis was that PGAM5 is involved in regulating cellular immune defense and cell death in the gut. Thus, this thesis aimed to investigate functional roles of PGAM5 via in vitro and in vivo models. Initially, poly(I:C) was used to mimic RNA virus infection in HeLa cells and the presence of intracellular RNA leads to PGAM5 multimer formation and co-localization at aggregated mitochondria. Furthermore, this thesis showed that PGAM5 deficiency specifically attenuated IFNβ expression induced by intracellular poly(I:C) but not when poly(I:C) was added into the medium. Decreased phosphorylation levels of IRF3 and TBK1 in PGAM5 deficient cells further confirmed these finding. On the molecular level, a direct interaction of PGAM5 with the mitochondrial antiviral-signaling protein (MAVS) was demonstrated. Finally, this thesis verified the functional role of PGAM5 in the process of viral infection. PGAM5 deficient cells, upon infection with vesicular stomatitis virus (VSV), revealed diminished Ifnβ expression and increased VSV replication. In addition, this thesis also demonstrated that PGAM5 is important for regulating ISG responses. PGAM5 deficient cells exhibited decreased phosphorylation levels of STAT1 and expression of ISG when challenged with three different types of IFNs. Mechanistically, PGAM5 deficiency significantly up-regulated SOCS3 expression. These in vitro data supported the initial hypothesis about the functional role of PGAM5 in cellular immune defense. However, the hypothesis about cell death in gut could not be proved in vivo. PGAM5 is dispensable for poly(I:C)-induced small intestinal inflammation and DSS-induced colitis. Collectively, this thesis identified PGAM5 as an important regulator in antiviral responses.