The Role of Inflammatory Pathways in the Progression of Retinal Degenerative Diseases

Age-related macular degeneration (AMD) is a chronic disease of the central retina, that is characterised by the focal degeneration of the retinal pigmented epithelium (RPE) and the overlying light-sensitive photoreceptor cells, resulting in progressive and irreversible blindness1. For the more prevalent non-executive form of AMD, geographic atrophy, which comprises 90% of all cases, there are no available treatments2. In addition, to environmental, lifestyle and genetic risk factors3, it is becoming increasingly clear that inflammation is a key pathological contributor to the development and progression of AMD4,5, particularly in regards to photoreceptor cell death. This thesis will examine the role and regulation of inflammatory pathways in photoreceptor degenerations with the aim to elucidate novel therapeutic targets for slowing progressive photoreceptor cell death, as seen in AMD. In this thesis I explore the role of Glutathione S-transferase omega 1-1 (GSTO1-1) a thiol transferase protein that acts at the interface between oxidative stress and inflammatory pathways, in the healthy and degenerating retina. Results from this published work6 demonstrate the damaging role that GSTO1-1 has on photoreceptor survivability, with mice deficient in GSTO1-1 displaying preserved retinal function and reduced levels of oxidative stress and inflammation compared to wild type (WT) controls. This study provides valuable insight into the role that GSTO1-1 plays in modulating oxidative stress and inflammation in the degenerating retina, and indicates that targeting GSTO1-1 may provide therapeutic benefit in retinal degenerations such as AMD. This thesis also explores the role of the inflammasome, a central mediator of innate immunity in the progression of retinal degenerations. The inflammasome is a multi-complex oligomer, which act in the first line of defence1,7,8 sensing 'danger signals' such as infection or cellular stress8, and is heavily reported to be involved in AMD pathogenesis9-17. However, research to date investigating NLRP3, the most widely characterised inflammasome sensor, has been largely cell culture based and focused on the RPE, with the lack of in vivo studies preventing the exploration of direct causal links between NLRP3 activation and disease onset or development. Further, little has been investigated on the potential contribution of other inflammasome components in mediating retinal cell death. This thesis therefore investigates the role and therapeutic targeting of multiple inflammasome components in the development of AMD, following on from previous studies from this lab that demonstrate the pathogenic association between inflammasome-derived pro-inflammatory cytokine IL-1B and photoreceptor cell death18. Published results from this work19 demonstrate that while therapeutic inhibition of NLRP3 did not confer any retinal protection against photo-oxidative damage induced retinal degeneration, Caspase-1 and Gasdermin D were shown to play key roles in mediating retinal cell death, potentially via the increased secretion of small-to-medium sized extracellular vesicles (s-mEV) including exosomes from immune cell populations. Finally, concomitant with increased levels of oxidative stress and inflammation, the activation and recruitment of microglia/macrophage immune populations in the retina is a characteristic pathogenic feature of retinal degenerative diseases including AMD. However, how widespread local and systemic immune responses are initiated following retinal insult or in disease is unclear. This thesis presents published20 novel findings that retinal-derived s-mEV may modulate the immune response in retinal degenerations via the selective incorporation and cell-to-cell transfer of small gene regulators called miRNA. Taken together this thesis uncovers novel inflammasome targets for therapeutic development, and highlights the potential use of s-mEV as gene therapy agents