Glucocerebrosidase (GBA1) gene mutations increase the risk of Parkinson disease (PD). While the cellular mechanisms associating GBA1 mutations and PD are unknown, loss of the glucocerebrosidase enzyme (GCase) activity, inhibition of autophagy and increased α-synuclein levels have been implicated. Here we show that autophagy lysosomal reformation (ALR) is compromised in cells lacking functional GCase. ALR is a cellular process controlled by mTOR which regenerates functional lysosomes from autolysosomes formed during macroautophagy. A decrease in phopho-S6K levels, a marker of mTOR activity, was observed in models of GCase deficiency, including primary mouse neurons and the PD patient derived fibroblasts with GBA1 mutations, suggesting that ALR is compromised. Importantly Rab7, a GTPase crucial for endosome-lysosome trafficking and ALR, accumulated in GCase deficient cells, supporting the notion that lysosomal recycling is impaired. Recombinant GCase treatment reversed ALR inhibition and lysosomal dysfunction. Moreover, ALR dysfunction was accompanied by impairment of macroautophagy and chaperone-mediated autophagy, increased levels of total and phosphorylated (S129) monomeric α-synuclein, evidence of amyloid oligomers and increased α-synuclein release. Concurrently, we found increased cholesterol and altered glucosylceramide homeostasis which could compromise ALR. We propose that GCase deficiency in PD inhibits lysosomal recycling. Consequently neurons are unable to maintain the pool of mature and functional lysosomes required for the autophagic clearance of α-synuclein, leading to the accumulation and spread of pathogenic α-synuclein species in the brain. Since GCase deficiency and lysosomal dysfunction occur with ageing and sporadic PD pathology, the decrease in lysosomal reformation may be a common feature in PD.

Parkinson disease (PD) is the second most common neurodegenerative disorder after Alzheimer disease, whereas Gaucher disease (GD) is the most frequent lysosomal storage disorder caused by homozygous mutations in the glucocerebrosidase (GBA1) gene. Increased risk of developing PD has been observed in both GD patients and carriers. It has been estimated that GBA1 mutations confer a 20‐ to 30‐fold increased risk for the development of PD, and that at least 7–10% of PD patients have a GBA1 mutation. To date, mutations in the GBA1 gene constitute numerically the most important risk factor for PD. The type of PD associated with GBA1 mutations (PD‐GBA1) is almost identical to idiopathic PD, except for a slightly younger age of onset and a tendency to more cognitive impairment. Importantly, the pathology of PD‐GBA1 is identical to idiopathic PD, with nigral dopamine cell loss, Lewy bodies, and neurites containing alpha‐synuclein. The mechanism by which GBA1 mutations increase the risk for PD is still unknown. However, given that clinical manifestation and pathological findings in PD‐GBA1 patients are almost identical to those in idiopathic PD individuals, it is likely that, as in idiopathic PD, alpha‐synuclein accumulation, mitochondrial dysfunction, autophagic impairment, oxidative and endoplasmic reticulum stress may contribute to the development and progression of PD‐GBA1. Here, we review the GBA1 gene, its role in GD, and its link with PD. image The impact of glucocerebrosidase 1 (GBA1) mutations on functioning of endoplasmic reticulum (ER), lysosomes, and mitochondria. GBA1 mutations resulting in production of misfolded glucocerebrosidase (GCase) significantly affect the ER functioning. Misfolded GCase trapped in the ER leads to both an increase in the ubiquitin–proteasome system (UPS) and the ER stress. The presence of ER stress triggers the unfolded protein response (UPR) and/or endoplasmic reticulum‐associated degradation (ERAD). The prolonged activation of UPR and ERAD subsequently leads to increased apoptosis. The presence of misfolded GCase in the lysosomes together with a reduction in wild‐type GCase levels lead to a retardation of alpha‐synuclein degradation via chaperone‐mediated autophagy (CMA), which subsequently results in alpha‐synuclein accumulation and aggregation. Impaired lysosomal functioning also causes a decrease in the clearance of autophagosomes, and so their accumulation. GBA1 mutations perturb normal mitochondria functioning by increasing generation of free radical species (ROS) and decreasing adenosine triphosphate (ATP) production, oxygen consumption, and membrane potential. GBA1 mutations also lead to accumulation of dysfunctional and fragmented mitochondria.This article is part of a special issue on Parkinson disease.

Objective: Gaucher disease is caused by mutations in the glucocerebrosidase 1 gene that result in deficiency of the lysosomal enzyme glucocerebrosidase. Both homozygous and heterozygous glucocerebrosidase 1 mutations confer an increased risk for developing Parkinson disease. Current estimates indicate that 10 to 25% of Parkinson patients carry glucocerebrosidase 1 mutations. Ambroxol is a small molecule chaperone that has been shown to increase glucocerebrosidase activity in vitro. This study investigated the effect of ambroxol treatment on glucocerebrosidase activity and on α‐synuclein and phosphorylated α‐synuclein protein levels in mice.Methods: Mice were treated with ambroxol for 12 days. After the treatment, glucocerebrosidase activity was measured in the mouse brain lysates. The brain lysates were also analyzed for α‐synuclein and phosphorylated α‐synuclein protein levels.Results: Ambroxol treatment resulted in increased brain glucocerebrosidase activity in (1) wild‐type mice, (2) transgenic mice expressing the heterozygous L444P mutation in the murine glucocerebrosidase 1 gene, and (3) transgenic mice overexpressing human α‐synuclein. Furthermore, in the mice overexpressing human α‐synuclein, ambroxol treatment decreased both α‐synuclein and phosphorylated α‐synuclein protein levels.Interpretation: Our work supports the proposition that ambroxol should be further investigated as a potential novel disease‐modifying therapy for treatment of Parkinson disease and neuronopathic Gaucher disease to increase glucocerebrosidase activity and decrease α‐synuclein and phosphorylated α‐synuclein protein levels. Ann Neurol 2016;80:766–775

Mutations in the glucocerebrosidase 1 (GBA1) gene are related to both Parkinson disease (PD) and Gaucher disease (GD). In both cases, the condition is associated with deficiency of glucocerebrosidase (GCase), the enzyme encoded by GBA1. Ambroxol is a small molecule chaperone that has been shown in mice to cross the blood‐brain barrier, increase GCase activity and reduce alpha‐synuclein protein levels. In this study, we analyze the effect of ambroxol treatment on GCase activity in healthy nonhuman primates. We show that daily administration of ambroxol results in increased brain GCase activity. Our work further indicates that ambroxol should be investigated as a novel therapy for both PD and neuronopathic GD in humans.

Glucocerebrosidase (GBA1) mutations are the major genetic risk factor for Parkinson’s Disease (PD). The pathogenic mechanism is still unclear, but alterations in lysosomal-autophagy processes are implicated due to reduction of mutated glucocerebrosidase (GCase) in lysosomes. Wild-type GCase activity is also decreased in sporadic PD brains. Small molecule chaperones that increase lysosomal GCase activity have potential to be disease-modifying therapies for GBA1-associated and sporadic PD. Therefore we have used mouse cortical neurons to explore the effects of the chaperone ambroxol. This chaperone increased wild-type GCase mRNA, protein levels and activity, as well as increasing other lysosomal enzymes and LIMP2, the GCase transporter. Transcription factor EB (TFEB), the master regulator of the CLEAR pathway involved in lysosomal biogenesis was also increased upon ambroxol treatment. Moreover, we found macroautophagy flux blocked and exocytosis increased in neurons treated with ambroxol. We suggest that ambroxol is blocking autophagy and driving cargo towards the secretory pathway. Mitochondria content was also found to be increased by ambroxol via peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1-α). Our data suggest that ambroxol, besides being a GCase chaperone, also acts on other pathways, such as mitochondria, lysosomal biogenesis, and the secretory pathway.

Mutations in glucocerebrosidase 1 (GBA1) represent the most prevalent risk factor for Parkinson’s disease. The molecular mechanisms underlying the link between GBA1 mutations and Parkinson’s disease are incompletely understood. We analysed two aged (24-month-old) Gba1 mouse models, one carrying a knock-out mutation and the other a L444P knock-in mutation. A significant reduction of glucocerebrosidase activity was associated with increased total alpha-synuclein accumulation in both these models. Gba1 mutations alone did not alter the number of nigral dopaminergic neurons nor striatal dopamine levels. We then investigated the effect of overexpression of human alpha-synuclein in the substantia nigra of aged (18 to 21-month-old) L444P Gba1 mice. Following intraparenchymal injections of human alpha-synuclein carrying viral vectors, pathological accumulation of phosphorylated alpha-synuclein occurred within the transduced neurons. Stereological counts of nigral dopaminergic neurons revealed a significantly greater cell loss in Gba1-mutant than wild-type mice. These results indicate that Gba1 deficiency enhances neuronal vulnerability to neurodegenerative processes triggered by increased alpha-synuclein expression.