Alpha-synuclein fine-tunes neuronal response to pro-inflammatory cytokines

Pro-inflammatory cytokines are emerging as neuroinflammatory mediators in Parkinson's disease (PD) due to their ability to act through neuronal cytokine receptors. Critical questions persist regarding the role of cytokines in neuronal dysfunction and their contribution to PD pathology. Specifically, the potential synergy of the hallmark PD protein alpha-synuclein (α-syn) with cytokines is of interest. We therefore investigated the direct impact of pro-inflammatory cytokines on neurons and hypothesized that α-syn pathology exacerbates cytokine-induced neuronal deficits in PD. iPSC-derived cortical neurons (CNs) from healthy controls and patients with α-syn gene locus duplication (SNCA dupl) were stimulated with IL-17A, TNF-α, IFN-γ, or a combination thereof. For rescue experiments, CNs were pre-treated with α-syn anti-oligomerisation compound NPT100-18A prior to IL-17A stimulation. Cytokine receptor expression, microtubule cytoskeleton, axonal transport and neuronal activity were assessed. SNCA dupl CNs displayed an increased IL-17A receptor expression and impaired IL-17A-mediated cytokine receptor regulation. Cytokines exacerbated the altered distribution of tubulin post-translational modifications in SNCA dupl neurites, with SNCA dupl-specific IL-17A effects. Tau pathology in SNCA dupl CNs was also aggravated by IL-17A and cytokine mix. Cytokines slowed down mitochondrial axonal transport, with IL-17A-mediated retrograde slowing in SNCA dupl only. The pre-treatment of SNCA dupl CNs with NPT100-18A prevented the IL-17A-induced functional impairments in axonal transport and neural activity. Our work elucidates the detrimental effects of pro-inflammatory cytokines, particularly IL-17A, on human neuronal structure and function in the context of α-syn pathology, suggesting that cytokine-mediated inflammation represents a second hit to neurons in PD which is amenable to disease modifying therapies that are currently in clinical trials.