Microgliosis and aberrant interferon response in Adar Mavs brain are rescued by PKR removal

Mutations in the human ADAR gene, encoding adenosine deaminase acting on RNA 1 (ADAR1), cause Aicardi-Goutières syndrome 6, which is a severe auto-inflammatory encephalopathy with aberrant interferon (IFN) induction. AdarΔ2-13 null mutant mouse embryos lacking ADAR1 protein die with high levels of IFN-stimulated gene (ISG) transcripts. In Adar Mavs double mutants also lacking the Mitochondrial antiviral signalling (MAVS) adaptor, the aberrant IFN induction is prevented. Live pups are born and survive for 2 weeks, allowing ADAR1 function to be investigated. We have shown that early death of Adar Mavs mutants is rescued by the deletion of the Eif2ak2 gene encoding the antiviral dsRNA sensor protein kinase R (PKR). Here, we focused on characterizing the brain defects in Adar Mavs mutants and their dependencies on PKR. Mouse brains were collected on postnatal Days 8 and 14, then analysed by mass spectrometry, immunohistochemistry and RT-qPCR. The proteomic analyses showed upregulation of ISG-encoded proteins in the Adar Mavs double mutant, and the morphological analyses confirmed aberrant microgliosis in the brains. Both are prevented in Adar Mavs Eif2ak2 triple mutants, indicating the key role of aberrant PKR activation; PKR expression is also increased by IFN signalling. Altered expression levels of transcripts encoding differentially expressed proteins and of ADAR-edited transcripts were confirmed by RT-qPCR. Analysis of the expression levels of transcripts in the brains of mutants expressing a catalytically inactive ADAR E861A protein revealed that the levels of some but not all altered transcripts are restored. A further group of proteins, downregulated in Adar Mavs double mutants, are not rescued by removal of PKR and might result from effects of loss of the widespread ADAR1 RNA editing known to occur in brain transcripts. This group includes several motor proteins, some of which have been reported to be encoded by ADAR-edited transcripts. In this study, we show that Adar Mavs double mutants exhibit an aberrant IFN response in the brain, probably attributable to reactive microglia and astrocytes. Microgliosis, which is rescued in the triple mutant, is mostly dependent on aberrant PKR activation and is partly dependent on RNA editing.