Glucocerebrosidase Deficiency Dysregulates Human Astrocyte Lipid Metabolism

Deficiency in the lysosomal enzyme, glucocerebrosidase (GCase), caused by mutations in the GBA1 gene, is the most common genetic risk factor for Parkinson's disease (PD). However, the consequence of reduced enzyme activity within neural cell sub-types remains ambiguous. Thus, the purpose of this study was to define the effect of GCase deficiency specifically in human astrocytes and test their non-cell autonomous influence upon dopaminergic neurons in a midbrain organoid model of PD.

Wild-type (GBA+/+), N370S mutant (GBA+/N370), and GBA1 knockout (GBA-/-) astrocytes were rapidly and directly induced from human pluripotent stem cells (hPSCs) via transcription factor-based differentiation. These astrocytes were extensively characterized for GCase-dependent phenotypes using immunocytochemistry, organoid coculture, enzymatic assays, lipid tracers, transcriptomics, and lipidomics.

hPSC lines were rapidly induced into astrocytes and enzymatic assays confirmed that GBA-/- astrocytes completely lacked GCase activity, while GBA+/N370 preserved partial activity. GBA-/-, but not GBA+/N370S, exhibited lysosomal alterations, with enlarged lysosomes and glucosylceramide (GlcCer) accumulation. GCase deficiency also exacerbated TNF-alpha-induced secretion of the inflammatory biomarker, CCL2. In midbrain organoids, GCase activity did not modulate the ability of astrocytes to support dopamine neuron production and survival. Lipidomics revealed a GBA-/--specific increase in sphingomyelin, and a decrease of triglycerides. Direct rescue of GCase activity with GBA1 mRNA treatment reduced GlcCer accumulation. Astrocytes exhibited a relatively high uptake and storage of fatty acid analogs as lipid droplets, in comparison to neurons, and this process was impaired in GBA-/- astrocytes. Lastly, GBA-/- astrocytes accumulate neuronal membrane-derived GlcCer. These findings highlight the critical role of astrocytic GCase in lipid metabolism and its neuronal influence.

GCase deficiency does not inhibit human astrocyte differentiation nor cause a non-cell autonomous neurotoxic effect upon dopaminergic neurons within midbrain organoids. However, it does elicit enhanced inflammatory reactivity, accumulation of GlcCer, and a distinct lipidomic profile, indicating impaired lipid metabolism in astrocytes that can dysregulate neuron-astrocyte intercellular signaling. Overall, these insights underscore dysfunctional astrocyte lipid metabolism as a high priority therapeutic target in Parkinson's disease and related neurodegenerative disorders.