Abstract
Preclinical and clinical studies support a strong association between mutations in the GBA1 gene that encodes beta-glucocerebrosidase (GCase) (EC 3.2.1.45; glucosylceramidase beta) and Parkinson's disease (PD). Alpha-synuclein (AS), a key player in PD pathogenesis, and GBA1 mutations may independently and synergistically cause lysosomal dysfunction and thus, embody clinically well-validated targets of the neurodegenerative disease process in PD. However, in vivo models, recapitulating pathological features of PD that can be used to dissect the nature of the complex relationship between GCase and AS on the nigrostriatal axis, the region particularly vulnerable in PD, are direly needed. To address this, we implemented a bidirectional approach in mice to examine the effects of: 1) GCase overexpression (wild-type and mutant N370S GBA) on endogenous AS levels and 2) downregulation of endogenous GCase (Gba) combined with AS overexpression. Striatal delivery of viral-mediated GCase overexpression revealed minimal effects on cortical and nigrostriatal AS tissue levels and no significant effect on dopaminergic system integrity. On the other hand, microRNA (miR)-mediated Gba1 downregulation (miR Gba), combined with virus-mediated human AS overexpression (+AS), yields decreased GCase activity in the cortex, mimicking levels seen in GBA1 heterozygous carriers (30–40%), increased astrogliosis and microgliosis, decreased striatal dopamine levels (50% compared to controls) and loss of nigral dopaminergic neurons (~33%)- effects that were all reversible with miR rescue. Most importantly, the synergistic neurodegeneration of miR Gba + AS correlated with augmented AS accumulation and extracellular release in the striatum. Collectively, our results suggest that GCase downregulation alone is not sufficient to recapitulate key pathological features of PD in vivo, but its synergistic interplay with AS, via increased AS levels and extracellular release, drives nigrostriatal neurodegeneration. Furthermore, we report a novel double-hit GBA-AS model that can be used to identify putative mechanisms driving PD pathophysiology and can be subsequently used to test novel therapeutic approaches.
Highlights
Preclinical and clinical studies support a strong association between mutations in the GBA1 gene that encodes beta-glucocerebrosidase (GCase) (EC 3.2.1.45; glucosylceramidase beta) and Parkinson’s disease (PD)
Wild-type GBA (WT GBA) GCase overexpressing HEK293 cells demonstrated enhanced GCase enzymatic activity levels compared to the control group (p
Unilateral striatal injections of green fluorescent protein (GFP), WT GBA or N370S associated viral vectors (AAVs) in mice led to efficient delivery of transgenes to the striatum, expression in the cortex, and retrograde transport to the substantia nigra at 8 weeks post-injection (WPI) (SF2)
Summary
Preclinical and clinical studies support a strong association between mutations in the GBA1 gene that encodes beta-glucocerebrosidase (GCase) (EC 3.2.1.45; glucosylceramidase beta) and Parkinson’s disease (PD). Conclusion: Collectively, our results suggest that GCase downregulation alone is not sufficient to recapitulate key pathological features of PD in vivo, but its synergistic interplay with AS, via increased AS levels and release, drives nigrostriatal neurodegeneration. Heterozygous mutant GBA1 carriers demonstrate increased post-mortem levels of AS, while sporadic cases of PD have decreased brain GCase activity associated with early accumulation of AS, inflammation, dysregulation of chaperonemediated autophagy and lipid metabolism (7–9). The central role of AS accumulation in GBA-PD is emphasized by the reciprocal relationship between GCase activity and AS demonstrated in SH-SY5Y cell cultures, neuronal cultures, conduritol-B-epoxide (CBE)-treated mice and transgenic GBA mouse models (10– 16,16–18). Cell and mouse models have demonstrated that increased AS causes a decrease in GCase activity (13,20)
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