Alpha-synuclein modulates retinal iron homeostasis by facilitating the uptake of transferrin-bound iron: Implications for visual manifestations of Parkinson's disease

Abstract

Aggregation of α-synuclein (α-syn) in neurons of the substantia nigra is diagnostic of Parkinson’s disease (PD), a neuro-motor disorder with prominent visual symptoms. Here, we demonstrate that α-syn, the principal protein involved in the pathogenesis of PD, is expressed widely in the neuroretina, and facilitates the uptake of transferrin-bound iron (Tf-Fe) by retinal pigment epithelial (RPE) cells that form the outer blood-retinal barrier. Absence of α-syn in knock-out mice (α-syn−/−) resulted in down-regulation of ferritin in the neuroretina, indicating depletion of cellular iron stores. A similar phenotype of iron deficiency was observed in the spleen, femur, and brain tissue of α-syn−/− mice, organs that utilize mainly Tf-Fe for their metabolic needs. The liver and kidney, organs that take up significant amounts of non-Tf-bound iron (NTBI), showed minimal change. Evaluation of the underlying mechanism in the human RPE47 cell line suggested a prominent role of α-syn in the uptake of Tf-Fe by modulating the endocytosis and recycling of transferrin (Tf)/transferrin-receptor (TfR) complex. Down-regulation of α-syn in RPE cells by RNAi resulted in the accumulation of Tf/TfR complex in common recycling endosomes (CREs), indicating disruption of recycling to the plasma membrane. Over-expression of exogenous α-syn in RPE cells, on the other hand, up-regulated ferritin and TfR expression. Interestingly, exposure to exogenous iron increased membrane association and co-localization of α-syn with TfR, supporting its role in iron uptake by the Tf/TfR complex. Together with our observations indicating basolateral expression of α-syn and TfR on RPE cells in vivo, this study reveals a novel function of α-syn in the uptake of Tf-Fe by the neuroretina. It is likely that retinal iron dyshomeostasis due to impaired or altered function of α-syn contributes to the visual symptoms associated with PD.

Prion protein modulates iron transport in the anterior segment: Implications for ocular iron homeostasis and prion transmission

“…Since the corneal endothelial cells of PrP −/− mice express significantly less ferritin than PrP +/+ controls, it is likely that PrP C facilitates uptake of iron by the Tf/TfR pathway as in other cell types (Baksi et al, 2016). This is an important unanswered question with significant clinical implications since a breach of these monolayers is likely to result in corneal and lens opacities with eventual blindness.…”

“…Infected animals were euthanized when clinical signs of disease appeared after ~141 days post inoculation, and the cornea, CB, and retina isolated from enucleated eyes were lysed and processed for differential centrifugation and Western blotting as reported (Baksi et al, 2016). Uninoculated control transgenic mice were analyzed in parallel.…”

Transport of Non-Transferrin Bound Iron to the Brain: Implications for Alzheimer’s Disease

“…SDS-PAGE and western blotting [34], native gel electrophoresis [35], Perls’ staining [36], immunostaining [37], and quantitative RT-PCR were performed essentially as described [38, 39]. …”

Aggregation and conformation of alpha-synuclein: effects of ligand binding and phosphomimetics

“…It is also thought to have role in iron homeostasis, as its 5' promotor region is known to contain an iron response element (Rogers et al, 2011). The protein itself is often found localised to membranes, suggesting there may be a role in endo and exocytosis of this 18 element (Baksi et al, 2016;Snead and Eliezer, 2014). αSyn is also known to have ferrireductase activity, and it has been demonstrated to bind to and utilise the redox cycling ability of copper ions to remove electrons from Fe(III), reducing it to Fe(II).…”