Elucidation of the Role of Methylarginine Metabolism in Regulation of Nitric Oxide Production and Inflammation

Abstract

Atherosclerosis is a major global health issue, and inflammation is important in its pathogenesis. Many atherosclerosis risk factors lead to reduced nitric oxide (NO) bioavailability. Asymmetric dimethylarginine (ADMA), an independent cardiovascular disease risk factor and NO synthase inhibitor, is metabolised by dimethylarginine dimethylaminohydrolase (DDAH). DDAH2 is the isoform present in the immune system. A deeper understanding of ADMA metabolism could help reveal new therapies for atherosclerosis. However, it is debated if DDAH2 hydrolyses ADMA. There is evidence that DDAH2 has NO-independent cellular functions, and research in our group showed that DDAH2 regulates macrophage functions. This thesis initially aimed to investigate the role of DDAH2 in regulating inflammation in atherosclerosis models. However, this was derailed by limitations imposed by the Covid-19 pandemic. Therefore, models of inflammation were used. Genes and mechanisms associated with inflammation and atherosclerosis were investigated. RAW 264.7 murine macrophage cell line and bone marrow-derived macrophages (BMDM) were validated for suitability to study the DDAH-ADMA-NOS pathway. To better understand the functions of DDAH2, a macrophage-specific Ddah2 null mouse model was re-derived and validated. RNA sequencing data previously generated by our group from peritoneal macrophages of the same model was re-analysed and revealed almost 5,000 genes to be DDAH-dependent and required for normal immune response. More than 200 Reactome pathways appeared enriched, with apoptosis being the most enriched. The in silico data was validated in vitro in DDAH2-knockout peritoneal macrophages from the macrophage-specific Ddah2 null mouse model. Inferred hypotheses were investigated in DDAH2-Knockout BMDMs from the macrophage-specific Ddah2 null mouse model with confirmatory studies on C57BL/6J BMDMs using ADMA. The in vitro analysis in the BMDMs showed no conclusive evidence supporting the in silico data that DDAH2 regulates the investigated genes (except Il17a), nor did ADMA alter the gene response to LPS. Il17a was shown by the in silico analysis to be regulated by DDAH2 and was validated in vitro in peritoneal macrophages by both RT-qPCR and ELISA. Given the significant role of IL17A in inflammation and its existing use in treating systemic inflammatory conditions such as psoriasis, this thesis proposes DDAH2 as a potential therapeutic target for inflammatory diseases in general and atherosclerosis in particular.