A novel glucosylceramide synthase inhibitor attenuates alpha synuclein pathology and lysosomal dysfunction in preclinical models of synucleinopathy

Mutations in the lysosomal enzyme glucocerebrosidase (GCase, GBA1 gene) are the most common genetic risk factor for developing Parkinson's disease (PD). GCase metabolizes the glycosphingolipids glucosylceramide (GlcCer) and glucosylsphingosine (GlcSph). Mutations in GBA1 reduce enzyme activity and the resulting accumulation of glycosphingolipids may contribute to the underlying pathology of PD, possibly via altering lysosomal function. While reduction of GCase activity exacerbates α-synuclein (α-syn) aggregation, it has not been determined that this effect is the result of altered glycosphingolipid levels and lysosome function or some other effect of altering GCase. The glycosphingolipid GlcCer is synthesized by a single enzyme, glucosylceramide synthase (GCS), and small molecule inhibitors (GCSi) reduce cellular glycosphingolipid levels. In the present studies, we utilize a preformed fibril (PFF) rodent primary neuron in vitro model of α-syn pathology to investigate the relationship between glycosphingolipid levels, α-syn pathology, and lysosomal function. In primary cultures, pharmacological inhibition of GCase and D409V GBA1 mutation enhanced accumulation of glycosphingolipids and insoluble phosphorylated α-syn. Administration of a novel small molecule GCSi, benzoxazole 1 (BZ1), significantly decreased glycosphingolipid concentrations in rodent primary neurons and reduced α-syn pathology. BZ1 rescued lysosomal deficits associated with the D409V GBA1 mutation and α-syn PFF administration, and attenuated α-syn induced neurodegeneration of dopamine neurons. In vivo studies revealed BZ1 had pharmacological activity and reduced glycosphingolipids in the mouse brain to a similar extent observed in neuronal cultures. These data support the hypothesis that reduction of glycosphingolipids through GCS inhibition may impact progression of synucleinopathy and BZ1 is useful tool to further examine this important biology.

Functionalization of the TMEM175 p.M393T variant as a risk factor for Parkinson disease

Multiple genome-wide association studies (GWAS) in Parkinson disease (PD) have identified a signal at chromosome 4p16.3; however, the causal variant has not been established for this locus. Deep investigation of the region resulted in one identified variant, the rs34311866 missense SNP (p.M393T) in TMEM175, which is 20 orders of magnitude more significant than any other SNP in the region. Because TMEM175 is a lysosomal gene that has been shown to influence α-synuclein phosphorylation and autophagy, the p.M393T variant is an attractive candidate, and we have examined its effect on TMEM175 protein and PD-related biology. After knocking down each of the genes located under the GWAS peak via multiple shRNAs, only TMEM175 was found to consistently influence accumulation of phosphorylated α-synuclein (p-α-syn). Examination of the p.M393T variant showed effects on TMEM175 function that were intermediate between the wild-type (WT) and knockout phenotypes, with reduced regulation of lysosomal pH in response to starvation and minor changes in clearance of autophagy substrates, reduced lysosomal localization, and increased accumulation of p-α-syn. Finally, overexpression of WT TMEM175 protein reduced p-α-syn, while overexpression of the p.M393T variant resulted in no change in α-synuclein phosphorylation. These results suggest that the main signal in the chromosome 4p16.3 PD risk locus is driven by the TMEM175 p.M393T variant. Modulation of TMEM175 may impact α-synuclein biology and therefore may be a rational therapeutic strategy for PD.