Is modulation of glucocerebrosidase a viable new treatment for Parkinson's disease?

Background: Parkinson's disease is the second most prevalent neurodegenerative disorder. There are currently no treatments that cure Parkinson’s disease (PD) or slow its progression. There is therefore an urgent need for novel treatments. It was recently discovered that patients with mutations in GBA1 are more likely to develop PD. GBA1 encodes the glucocerebrosidase protein (GCase). GCase is a lysosomal enzyme that breaks down glucosylceramide. Several studies suggest that a decrease in GCase activity and the accompanying increase in upstream glycosphingolipids cause an increase in levels of α-synuclein and its propensity for aggregation. We investigate if there is a molecular link between GCase activity and α-synuclein. Furthermore, we study the endogenous GCase activator saposin C. Saposin C is a sphingolipid activator protein that acts as a co-factor for glucosylceramide degradation by enabling GCase access to the lipid membrane.

Aims: We aim to investigate how GBA1 mutations predispose to PD. Additionally we attempt to increase GCase activity using saposin C, and whether activation of GCase with saposin C is protective in PD.

Methods and results: We studied the effect of GCase inhibition or genetic manipulation in SH-SY5Ycells. We establish that knockdown using RNA interference (RNAi) of GCase increases levels of α-synuclein. However, using a potent chemical inhibitor of GCase does not alter levels of α-synuclein. This finding suggests that GCase has an activity independent function that relies on the protein but not its enzymatic action to influence levels of α-synuclein. We are the first group to show that genetic and chemical inhibition produce different results. The increase in α-synuclein can be returned to physiological levels by addition of active recombinant GCase. Our data suggests that using GCase chaperones to aid folding and trafficking could be protective in PD patients with GBA1 mutations. To assess whether saposin C has a protective role in PD we generated short peptide fragments of different regions of saposin C. We confirmed that the GCase activating sequence is encoded in amino acids 41-68. We synthesized a cell-permeable peptide but were unable to achieve increase in GCase activity. We therefore, created a cell line overexpressing full length prosaposin. We could show that this leads to a natural increase in GCase activity and decrease in levels of α-synuclein. We show that the decrease in α-synuclein is independent of GCase activity as it persists in CBE inhibited cells. We also provide preliminary evidence that saposin C can detach α-synuclein from artificial lipid membranes that act as a primary site for α-synuclein aggregation. Saposin C is therefore an interesting target for further studies.

List of scientific papers

I. Zurbruegg, M., Chan, M. Y., & Svenningsson, P. (2019). GBA RNAi but not catalytic inhibition of glucocerebrosidase with Conduritol-β-epoxide increases levels of total α-synuclein in SH-SY5Y cells. Neuroscience Letters.
https://doi.org/10.1016/j.neulet.2019.05.027

II. Mark Zurbruegg, Wojciech Paslawski, Per Svenningsson. (2019). Saposin C an endogenous activator of glucocerebrosidase reduces levels of α-synuclein. [Manuscript]