Redox biology in retinal degeneration

Retinitis pigmentosa (RP) involves a group of hereditary diseases that cause progressive and severe visual impairment, with a prevalence of about 1 in 4,000 individuals worldwide. Unfortunately, there is currently no cure for RP and most patients become legally blind by age 40 due to the loss of retinal photoreceptors. The difficulty in finding a treatment relies on the complexity of its genetics. Although more than 3,000 mutations have been described to cause it, about 40-50% of RP cases still correspond to unknown patterns. In most cases, mutations affecting rods, which degenerate first, subsequently produce the death of the remaining photoreceptor cells, the cones. Several therapeutic approaches have been studied during the last decades. In 2011, our group discovered the neuroprotective properties of ‘Norgestrel’, a synthetic progestin used in the female oral contraceptive pill, in the retina. Norgestrel was shown to protect against retinal cell death in three different models: in vitro, in vivo and ex vivo, using retinal explants. In fact, two mouse models were used in order to demonstrate such protection, the balb/c induced light damage model and the genetic rd10 (Pde6b rd10– /rd10– ) model of RP. Since then, some components of its mechanism of action have been elucidated, as is the case of the receptor through which it works, the progesterone receptor membrane component 1 (PGRMC1); the neurotrophic factor basic fibroblast growth factor (bFGF) and its ability to reduce inflammation and gliosis in the diseased retina. Reactive oxygen species (ROS), have been traditionally associated with cellular damage, and have been discovered to participate in signalling responses, including cellular responses that are protetective. The number of studies about their protective properties of ROS have increased in the last decades. Nevertheless, the accumulation of ROS and/or their persistance during time within cells have detrimental consequences. Antioxidant machinery is the defense mechanism that cells possess against harmful ROS. However, this system is not infallible. Worsening of several diseases including RP is known to be produced by the disregulation of intracellular ROS levels, which is known as ‘oxidative stress’. In 2016, we demonstrated that Norgestrel effectively reduces the damaging ROS levels in the balb/c light damage mouse model. In the current study, using the 661W cone photoreceptor-like cell line and retinal explants from rd10 mice, we demonstrated that ROS are used by Norgestrel to enhance cell survival. We found that such stimulation of pro-survival ROS levels occurs very rapidly, and is both PGRMC1 and bFGF-dependent (Chapter 3). Interestingly, we demonstrated that treatment with some antioxidants that prevent the up-regulation of ROS molecules, abrogates the Norgestrel-mediated neuroprotection and therefore, indicates that ROS are a crucial part of this survival response. However, little was known about the subsequent downstream mechanism in the Norgestrel-mediated signalling response that prevents cell death and thus, this study aimed to elucidate other processes that could be implicated in such a response (Chapter 4). Using the genetic rd10 mouse model of RP we additionally demonstrated that Norgestrel was able to reduce the levels of damaging ROS due to its antioxidant properties via stimulating the transcription factor nuclear factor erythroid 2 (NF-E2)-related factor-2 (Nrf2) and its effector protein, superoxide dismutase 2 (SOD2) (Chapter 5). Taken together, this study highlights a dual nature of reactive oxygen species and we have demonstrated for the first time the implication of redox biology in the Norgestrel-mediated neuroprotection against retinal degeneration