Complex modulation of cytokine-induced α-synuclein aggregation by glypican-1-derived heparan sulfate in neural cells

In Parkinson's disease (PD), there is accumulation of α-synuclein (SYN) aggregates in neurons, which is promoted by neuroinflammation. The cytokines TNF-α, IL-1β and IL-6 induce accumulation of degradation products of the amyloid precursor protein (APP) combined with heparan sulfate (HS) chains released from glypican-1 (Gpc-1) by NO-dependent cleavage. We have investigated the effects of the cytokines and HS on SYN aggregation and secretion in dividing human neuroblastoma (SH-SY5Y) and inducible neural progenitor cells (NPC) by using immunofluorescence microscopy, vesicle isolation and slot blotting with antibodies recognizing SYN monomers and aggregates, Gpc-1, the released HS, endosomes, and autophagosomes. In SH-SY5Y cells, the capacity to release HS was fully utilized, while NPC displayed dormant capacity. TNF-α induced increased formation of SYN aggregates and clustering of HS in SH-SY5Y cells. When the supply of NO was simultaneously increased, SYN and HS accumulation disappeared. When NO formation was inhibited, SYN and HS aggregation also disappeared, but there was now a 4-fold increase in SYN secretion. In NPC, IL-6 induced increased aggregation of SYN and stimulated HS release from Gpc-1. Both SYN and HS co-localized with autophagosome marker. When HS-deficient Gpc-1 was simultaneously generated, by using a cyanobacterial neurotoxin, accumulation diminished and there was massive secretion of SYN. We suggest that the cytokines increase APP processing, which initiates NO-dependent release of HS from Gpc-1. The APP degradation products also trigger SYN aggregation. As HS can inhibit APP processing, HS- or NO-deficiency may result in autophagosomal dysfunction and both APP degradation products and SYN are secreted.