The critical role of Nramp1 in degrading α-synuclein oligomers in microglia under iron overload condition

Oligomeric α-synuclein is a key mediator in the pathogenesis of Parkinson's disease (PD) and is mainly cleared by autophagy-lysosomal pathway, whose dysfunction results in the accumulation and cell-to-cell transmission of α-synuclein. In this study, concomitant with the accumulation of iron and oligomeric α-synuclein, higher expression of a lysosomal iron transporter, natural resistance-associated macrophage protein-1 (Nramp1), was observed in microglia in post-mortem striatum of sporadic PD patients. Using Nramp1-deficient macrophage (RAW264.7) and microglial (BV-2) cells as in-vitro models, iron exposure significantly reduced the degradation rate of the administered human α-synuclein oligomers, which can be restored by the expression of the wild-type, but not mutant (D543N), Nramp1. Likewise, under iron overload condition, mice with functional Nramp1 (DBA/2 and C57BL/6 congenic mice carrying functional Nramp1) had a better ability to degrade infused human α-synuclein oligomers than mice with nonfunctional Nramp1 (C57BL/6) in the brain and microglia. The interplay between iron and Nramp1 exhibited parallel effects on the clearance of α-synuclein and the activity of lysosomal cathepsin D in vitro and in vivo. Collectively, these findings suggest that the function of Nramp1 contributes to microglial degradation of oligomeric α-synuclein under iron overload condition and may be implicated in the pathogenesis of PD.