The presynaptic protein ␣-synuclein (␣-syn), particularly in its amyloid form, is widely recognized for its involvement in Parkinson disease (PD). Recent genetic studies reveal that mutations in the gene GBA are the most widespread genetic risk factor for parkinsonism identified to date. GBA encodes for glucocerebrosidase (GCase), the enzyme deficient in the lysosomal storage disorder, Gaucher disease (GD). In this work, we investigated the possibility of a physical linkage between ␣-syn and GCase, examining both wild type and the GD-related N370S mutant enzyme. Using fluorescence and nuclear magnetic resonance spectroscopy, we determined that ␣-syn and GCase interact selectively under lysosomal solution conditions (pH 5.5) and mapped the interaction site to the ␣-syn C-terminal residues, 118 -137. This ␣-syn-GCase complex does not form at pH 7.4 and is stabilized by electrostatics, with dissociation constants ranging from 1.2 to 22 M in the presence of 25 to 100 mM NaCl. Intriguingly, the N370S mutant form of GCase has a reduced affinity for ␣-syn, as does the inhibitor conduritol-␤-epoxidebound enzyme. Immunoprecipitation and immunofluorescence studies verified this interaction in human tissue and neuronal cell culture, respectively. Although our data do not preclude protein-protein interactions in other cellular milieux, we suggest that the ␣-syn-GCase association is favored in the lysosome, and that this noncovalent interaction provides the groundwork to explore molecular mechanisms linking PD with mutant GBA alleles.

Mutations in the gene encoding glucocerebrosidase (GBA), the enzyme deficient in the lysosomal storage disorder Gaucher disease, are associated with the development of Parkinson disease and other Lewy body disorders. In fact, GBA variants are currently the most common genetic risk factor associated with parkinsonism, and identified subjects with Parkinson disease are more than five times more likely to carry mutations in GBA. The mechanisms underlying this association are not known, but proposed theories include enhanced protein aggregation, alterations in lipid levels, and autophagy-lysosomal dysfunction promoting the retention of undegraded proteins. We review the genetic studies linking GBA to parkinsonism, as well as several of the mechanisms postulated to explain the association of GBA mutations and the synucleinopathies, which demonstrate how studies of a rare mendelian disease may provide insights into our understanding of a common complex disorder.

A major challenge in the field of Gaucher disease has been the development of new therapeutic strategies including molecular chaperones. All previously described chaperones of glucocerebrosidase are enzyme inhibitors, which complicates their clinical development, because their chaperone activity must be balanced against the functional inhibition of the enzyme. Using a novel high throughput screening methodology, we identified a chemical series that does not inhibit the enzyme, but can still facilitate its translocation to the lysosome as measured by immunostaining of glucocerebrosidase in patient fibroblasts. These compounds provide the basis for the development of a novel approach towards small molecule treatment for patients with Gaucher disease.