PROJECT SUMMARY Mutations in the GBA gene that encodes glucocerebrosidase 1 (GCase1) have been identified as the most common genetic risk factor for Parkinson's disease (PD) and Dementia with Lewy Bodies (DLB). Homozygous loss-of-function GBA mutations causes Gaucher's disease (GD). Notably, PD and DLB patients with GBA mutations exhibit ?-synuclein pathology strongly suggesting a link between GCase1 and ?-synuclein (gene:SNCA). However, the underlying mechanisms linking these two proteins and diseases remain unclear. GCase1 is a lysosomal enzyme which hydrolyzes glucosylceramide (GlcCer) to ceramide and glucose. GD patients accumulate not only the primary substrate GlcCer, but also downstream bioactive lipids such as glucosylsphingosine (GlcSph) due to alterations in glycosphingolipid homeostasis. We recently showed that GlcSph triggers the formation of highly self-templating ?-synuclein oligomers (Taguchi et al., 2017). Using new long-lived mouse models of GD crossed to ?-synuclein transgenics (Gba/SNCA mice), we showed that GlcSph levels increase in the brains of young mice, months prior to increases in GlcCer levels and that glycosphingolipid accumulations are co-localized with ?-synuclein pathology (Taguchi et al., 2017). Our hypothesis for the proposed research is that GlcSph is an initial driver of ?-synuclein pathology in GBA-linked PD and DLB. In this application, we will examine in detail the spatial and temporal profiles of glycosphingolipids and phospholipids in brains of Gba/SNCA mice. We will also test whether genetically altering GlcSph generating enzymes will ameliorate ?-synuclein pathology in Gba/SNCA mice as well as GBA-linked PD and DLB patient iPSC derived neurons. These studies will determine whether therapeutic reductions of elevated GlcSph is beneficial in GBA- linked PD and DLB. Achieving these goals is important for human health, given the massive personal and societal burden imposed by the elderly population suffering from PD and DLB.