<p>Brain accumulation of toxic soluble α-synuclein (α-syn) oligomers represents a prodromal marker of synucleinopathies in Parkinson’s disease (PD), contributing to progressive nigrostriatal neurodegeneration. Dysfunction in beta-glucocerebrosidase (GCase) and leucine-rich repeat kinase 2 (LRRK2) mutation are genetic risks for developing synucleinopathies. However, whether pharmacological GCase activation ameliorated synucleinopathies in LRRK2-PD was unexplored. Here, we showed that long-term treatment of ambroxol (ABX), a brain-penetrant GCase activator, reduced α-syn oligomer accumulation in aged mutant LRRK2^R1441G mouse striatum. Acute ABX treatment (50 µM) increased cellular GCase enzymatic activity and reduced Ser129-α-syn phosphorylation in human SH-SY5Y cells and mutant LRRK2 mouse fibroblasts, independent to LRRK2 kinase activity. Real-time DQ-BSA assay revealed lysosomal dysfunction in mutant MEFs, which was partially attenuated by ABX treatment. Lysosomal stress by bafilomycin-A1 induced endogenous GCase activity in wildtype (WT) MEFs, which was not observed in the LRRK2 mutant. Single gavage of ABX (400 mg/kg) in aged mice achieved peak drug level in serum and brain within 6 h post-administration. Ad libitum feeding of ABX (in food pellets) over 18 weeks (average dose: 45.9 mg/kg/day) elevated brain GCase activity in both WT and mutant striatum without affecting body weight. This regimen significantly reduced α-syn oligomer level in mutant striatum to a comparable physiological level in age-matched WT without altering total α-syn and Ser129-phosphorylation levels. This is the first study demonstrating reduced α-syn oligomer accumulation by chronic treatment of GCase activator in aged mouse brains vulnerable to PD, suggesting early intervention to alter progression of synucleinopathies as a key determinant of clinical outcomes of PD.</p>