The intracellular innate immune response is triggered by the recognition of specific pathogen-associated molecular patterns (PAMPs) of host cells to restrict and remove these foreign species. RNA PAMPs often come from viruses, but can also come from cellular RNAs that are improperly capped or have other foreign RNA features, including the presence or absence of specific RNA modifications. Without pathogens, innate immunity can be also activated under a diverse set of stress conditions (e.g., ischemia reperfusion and nutrient deprivation) with significant impact on the tissue injury and regeneration. However, the underlying mechanisms remains largely unknown. Here, we propose to investigate the role of a stress-induced protein MESH1 to trigger innate immunity by regulating the levels of 3'- dephosphorylated-CoA (dpCoA) and dpCoA-capped RNAs. MESH1 is a metazoan homologue of the bacterial (p)ppGpp hydrolase SpoT that regulates bacterial stringent response triggered by various nutrient deprivations and stresses. Because neither a homologue of the (p)ppGpp synthetase nor (p)ppGpp itself has been found in metazoa, the substrates and functions of MESH1 remain unknown. We have found that MESH1 is a CoA phosphatase which is induced by ischemia-reperfusion and nutrient deprivations. Over-expression of MESH1 dramatically increases the level of dpCoA and triggers dramatic induction of the interferons and inflammatory cytokines. Therefore, we hypothesize that the elevated dpCoA upon MESH1 induction results in the appearance of dpCoA-capped RNA that is sensed as foreign RNA by RNA-sensing pathways of innate immunity. We will test this hypothesis in two specific aims. First, we will characterize the enzymatic and functional activities of MESH1 as the first mammalian CoA phosphatase and its regulation of dpCoA and CoA levels under various MESH1-inducing stress conditions. Next, we will employ thio-reactive chemistry to capture and sequence dpCoA- capped RNA species to identify functional or sequence enrichment. In addition, we will define the effects of dpCoA cap on the translation, stability and other function of the RNA. Finally, we will determine the functional role of dpCoA-capped mRNA in triggering the RNA-sensing pathways of the innate immune response upon MESH1 induction. Together, the completion of the proposed experiments will elucidate the novel enzymatic activities of CoA phosphatase in regulating the levels of the dpCoA and dpCoA-RNA. These knowledge will enhance our understanding the aberrant activation of innate immunity under various stress conditions in various human diseases.
Innate immunity and inflammatory response is often triggered by various tissue injuries with important roles in the tissue repair and regeneration. The proposed study will investigate a new mechanism by which these innate immunity is triggered through a new enzyme and a new form of RNA modification.
