Fibroblasts from idiopathic Parkinson's disease exhibit deficiency of lysosomal glucocerebrosidase activity associated with reduced levels of the trafficking receptor LIMP2.

Lysosomal dysfunction is a central pathway associated with Parkinson's disease (PD) pathogenesis. Haploinsufficiency of the lysosomal hydrolase GBA (encoding glucocerebrosidase (GCase)) is one of the largest genetic risk factors for developing PD. Deficiencies in the activity of the GCase enzyme have been observed in human tissues from both genetic (harboring mutations in the GBA gene) and idiopathic forms of the disease. To understand the mechanisms behind the deficits of lysosomal GCase enzyme activity in idiopathic PD, this study utilized a large cohort of fibroblast cells from control subjects and PD patients with and without mutations in the GBA gene (N370S mutation) (control, n = 15; idiopathic PD, n = 31; PD with GBA N370S mutation, n = 6). The current data demonstrates that idiopathic PD fibroblasts devoid of any mutations in the GBA gene also exhibit reduction in lysosomal GCase activity, similar to those with the GBA N370S mutation. This reduced GCase enzyme activity in idiopathic PD cells was accompanied by decreased expression of the GBA trafficking receptor, LIMP2, and increased ER retention of the GBA protein in these cells. Importantly, in idiopathic PD fibroblasts LIMP2 protein levels correlated significantly with GCase activity, which was not the case in control subjects or in genetic PD GBA N370S cells. In conclusion, idiopathic PD fibroblasts have decreased GCase activity primarily driven by altered LIMP2-mediated transport of GBA to lysosome and the reduced GCase activity exhibited by  the genetic GBA N370S derived PD fibroblasts occurs through a different mechanism.