Investigation of the potential role of RORα in Parkinson’s disease: novel evidence for neuroprotection using post-mortem human tissues and in vitro studies

Abstract

Parkinson’s disease (PD) is characterised by the loss of dopaminergic neurons from the midbrain substantia nigra pars compacta (SNpc). Current therapies aimed at replacing dopamine do not halt disease progression and long-term use is associated with reduced efficacy and debilitating side effects, hence there is an urgent need for a better understanding of disease mechanisms leading to novel therapeutic approaches. Interestingly, being male is a major risk factor after aging for developing PD, with the neuroprotective capacity of estrogens contributing to male/female differences. The retinoic acid receptor-related orphan receptor alpha (RORα) has numerous neuronal functions, including transcriptional regulation of central aromatase, the enzyme responsible for estradiol synthesis. RORα has also been noted to have neuroprotective effects against oxidative stress, a fundamental mechanism contributing to the loss of dopaminergic neurons in PD. Using qRT-PCR and western blot, we have identified a higher expression of RORα in the SNpc of female human post-mortem control brains compared with males. In PD subjects RORα expression in the males, but not female, SNpc was significantly increased compared with controls. Accordingly, we hypothesized that RORα is a sex-specific neuroprotective factor in PD, with its greater innate expression in control females contributing to their relative protection, whereas its up-regulation in male PD may represent a response to combat the pathogenic process. Therefore, further studies aimed to investigate the hypothesis that RORα is a potential therapeutic target in PD. In order to investigate directly the neuroprotective potential of RORα, we developed an in vitro PD model using a dopaminergic neuron-like cell line (N27) and an astrocytic cell line (C6), which were challenged with the dopaminergic-selective neurotoxin, 6-hydroxy dopamine; cell viability after challenge was assessed using both cell survival (MTS) and cell death (LDH) assays. Pharmacological analysis using a selective RORα/γ agonist (SR1078) and antagonist (SR1001) demonstrated that RORα activation protects dopaminergic neuronal cells against oxidative stress. We further characterised the underlying mechanisms and we found that SR1078 mediates neuroprotection in our model by blocking mitochondrial Reactive Oxygen Spcies (ROS) production and inhibition of apoptosis. In summary, our human and in vitro experimental studies point to a novel role for RORα in PD, which may not only contribute to mechanisms underlying sex differences in the condition, but also suggests that up-regulation of RORα offers novel therapeutic potential in PD.