Glucocerebrosidase deficiency promotes release of α-synuclein fibrils from cultured neurons

Mutations in the GBA gene, which encodes the lysosomal enzyme glucocerebrosidase (GCase), are the most important genetic risk factor for Parkinson disease (PD). GCase activity is also decreased in sporadic PD brains and with normal ageing. Loss of GCase activity impairs the autophagy lysosomal pathway resulting in increased alpha-synuclein (alpha-syn) levels. Furthermore, elevated alpha-syn results in decreased GCase activity. Although the role of alpha-syn in PD remains unclear, evidence indicates that aggregated alpha-syn fibrils are a pathogenic species in PD, passing between neurons and inducing endogenous native alpha-syn to aggregate; spreading pathology through the brain. We have investigated if preformed alpha-syn fibrils (PFFs) impair GCase activity in mouse cortical neurons and differentiated dopaminergic cells, and whether GCase deficiency in these models increased the transfer of alpha-syn pathology to naive cells. Neurons treated with PFFs induced endogenous alpha-syn to become insoluble and phosphorylated at Ser129 to a greater extent than monomeric alpha-syn-treatment. PFFs, but not monomeric alpha-syn, inhibited lysosomal GCase activity in these cells and induced the unfolded protein response. Neurons in which GCase was inhibited by conduritol beta-epoxide did not increase the amount of insoluble monomeric alpha-syn or its phosphorylation status. Instead the release of alpha-syn fibrils from GCase deficient cells was significantly increased. Co-culture studies showed that the transfer of alpha-syn pathology to naive cells was greater from GCase deficient cells. This study suggests that GCase deficiency increases the spread of alpha-syn pathology and likely contributes to the earlier age of onset and increased cognitive decline associated with GBA-PD.