Mutations in GBA are known to be the most common genetic risk factor for developing Parkinson's disease. With up to 30% of Parkinson's disease patients carrying GBA mutations, the need for a targeted therapy for these patients is increasing. GBA is encoding the lysosomal enzyme glucocerebrosidase, which is implicated in lipid metabolism and α-synuclein degradation. When mutated, glucocerebrosidase is commonly misfolded and redirected to the proteasome for degradation. At cellular levels this results in lysosomal dysfunction, α-synuclein accumulation and ultimately to death of the dopaminergic neurons. The E3-ubiquitin ligase Parkin, encoded by PRKN, has been proposed to be implicated in the degradation of mutated glucocerebrosidase.

In our study, we investigated the relationship between glucocerebrosidase and Parkin in iPSC-derived neurons from Parkinson's disease patients harbouring mutations in GBA, PRKN or both genes. Pharmacological rescue of glucocerebrosidase via Ambroxol, genetic correction of GBA via CRISPR-Cas9 and modulation of Parkin levels were used to dissect the specific contribution of each gene to the cellular phenotype.

We evaluated α-synuclein homeostasis in patient-derived neurons harbouring the heterozygous N370S mutation in GBA under different levels of expression of Parkin. The overexpression of Parkin resulted in a decrease of glucocerebrosidase levels and consequently to an increase of α-synuclein levels. On the other hand, when silencing Parkin, the levels of glucocerebrosidase were enhanced, resulting in a decrease of intracellular α-synuclein. To deepen our understanding of this relationship in a more physiological model, we evaluated α-synuclein levels in iPSC-derived neurons from a Parkinson's disease patient harbouring both a loss of Parkin and the heterozygous N370S mutation in GBA. These neurons presented conserved levels of glucocerebrosidase while intracellular α-synuclein levels were reduced compared to control cells. When rescuing glucocerebrosidase via pharmacological or genetic strategies, the intracellular levels of α-synuclein were increasing.

Our results show that Parkin modulates glucocerebrosidase levels which subsequently impacts α-synuclein levels. Therefore, the inhibition of the ubiquitination of glucocerebrosidase in order to reduce α-synuclein levels may be a potential novel pharmacological target for the treatment of GBA-associated Parkinson's disease.