Exploring the use of high-dose simvastatin as therapy for oxidative stress in disease models of neuroinflammation

Background: There is growing evidence that HMG-CoA reductase inhibitors, otherwise known as the statin family, can exert pleiotropic effects in many areas. Of these, potential neuroprotective effects have gained significant attention. It is well established that microglia, the brain’s resident phagocytes, play a pivotal role in the pathogenesis of neuroinflammation and neurodegeneration. This process is thought to be, in part, due to the presence of chronically activated microglia. In this study, we investigated the potential neuroprotective properties of HMG-CoA reductase using an in vitro model of inflammatory cell activation and in vivo models of posterior uveitis and multiple sclerosis. The focus of this study was to investigate the effect of simvastatin on the microglial cell and its activation products. Emphasis was placed on the production of reactive species released by this cell type and the subsequent damage these cause to biological macromolecules. Methods: The microglial cell line BV2 were treated with simvastatin (1μM; 2 to 120 h), in vitro before being activated for 48 hours with a pro-inflammatory mix of LPS, TNFa and IFNg. Supernatants were taken and nitric oxide levels measured using the Griess assay. The animal model of posterior uveitis, experimental autoimmune uveoretinitis (EAU), was established in wild type C57BL/6 mice through subcutaneous injection of IRBP1-20. Mice were treated orally with simvastatin at 50, 75 or 100 mg/kg. Fundus images were taken before and after treatment administration for evaluation of clinical ocular pathology. Retinal flat mounts were prepared from simvastatin treated mice to assess cellular infiltrates. Experimental autoimmune encephalomyelitis (EAE), an in vivo model of MS, was induced by rMOG subcutaneous immunisation. The effect of simvastatin treatment was assessed clinically and by immunohistochemical analysis of tissue sections to determine cellular infiltrates and levels of oxidative damage to biological macromolecules, consistent with those assessed in EAU. Results: The levels of nitric oxide produced by microglial cells were significantly reduced when exposed to a pre-incubation of simvastatin for 3 48 hours, compared to cells receiving the pro-inflammatory mix alone. In EAU, fundoscopic analysis revealed that high-dose simvastatin halts clinical disease progression in IRBP1-20 induced posterior uveitis. Retinal flat mounts prepared from these cohorts showed a significant decrease in the expression of the innate immune cell surface receptor CD11b. Additionally, histological examination of eye sections displayed a significant reduction in lipid peroxidation as revealed by the marker 4-Hydroxynonenal (4HNE), nitrosylated proteins, as measured by 3-nitrotyrosine and oxidised DNA/RNA as determined by 8-OHdG. In line with this study, results from our EAE model demonstrated an important role for microglial cell number in disease, whilst also providing evidence of simvastatin decreasing oxidative damage to macromolecules in areas of extensive pathology. Discussion: These data provide evidence to support the notion that microglial cell activation may contribute to the pathogenesis of neuroinflammatory disease and that statins may attenuate damage through their ability to inhibit the production of reactive species. Further to this, we provide therapeutic, chemical and physical evidence that simvastatin can provide protection against 1) nitric oxide production in an inflammatory environment 2) clinical disease attenuation in EAU and EAE and 3) reduction in peroxynitrite levels in vivo. Collectively, these data provide evidence that statins may attenuate microglial cell activation by ways of inhibiting the production of reactive species. Thus, the evidence presented in this thesis points to the importance and potential use of simvastatin therapy as a neuroprotective therapeutic agent