Neuronopathic GBA1 L444P mutation accelerates Glucosylsphingosine levels and formation of hippocampal alpha-synuclein inclusions

The most common genetic risk factor for Parkinson’s disease (PD) is heterozygous mutations in the GBA1 gene which encodes for the lysosomal enzyme, glucocerebrosidase (GCase). GCase impairments are associated with an accumulation of abnormal α-synuclein (α-syn) called Lewy pathology, which characterizes PD. PD patients heterozygous for the GBA1 L444P mutation (GBA1+/L444P) have a 5.6-fold increased risk of cognitive impairments. In this study, we used GBA1+/L444P mice to determine the effects of this severe GBA1 mutation on lipid metabolism, expression of synaptic proteins, behavior, and α-syn inclusion formation. GBA1+/L444P mice showed reduced GCase activity in limbic brain regions and expressed lower levels of hippocampal vGLUT1 compared to wildtype (GBA1+/+) mice. GBA+/L444P mice also demonstrated impaired fear conditioning, but no motor deficits. We show, using mass spectrometry, that mutant GCase and age increased levels of glucosylsphingosine (GlcSph), but not glucosylceramide (GlcCer), in the brains and serum of GBA1+/L444P mice. Aged GBA1+/+ mice also showed increased levels of GlcSph, and decreased GlcCer. To model disease pathology, templated α-syn pathology was used. α-Syn inclusions were increased in the hippocampus of GBA1+/L444P mice compared to GBA1+/+ mice, but not in the cortex, or substantia nigra pars compacta (SNc). Pathologic α-syn did not cause a loss of dopamine neurons in the SNc. Treatment with a GlcCer synthase inhibitor prevented loss of cortical α-syn inclusions, but not loss of dopamine neurons. Overall, these data suggest the critical importance to evaluate the contribution of hippocampal pathologic α-syn and brain and serum glucosylsphingosine in synucleinopathies. SIGNIFICANCE STATEMENT Synucleinopathies, such as Parkinson’s disease (PD) and Dementia with Lewy bodies (DLB), are both pathologically characterized by abnormal α-synuclein (α-syn). Mutant GBA1 is a risk factor for both PD and DLB where a reduction of glucocerebrosidase (GCase) activity is seen. Collectively, this indicates the significance of evaluating mutant GCase in synucleinopathies. Our data suggest the critical importance to evaluate the contribution of hippocampal pathologic α-syn and brain and serum glucosylsphingosine (GlcSph) accumulation in synucleinopathies. Moreover, these pathologic outcomes may contribute to the nonmotor symptoms clinically observed in PD and DLB. Our findings highlight the importance of GlcSph as a relevant biomarker for future therapeutics. ### Competing Interest Statement The authors have declared no competing interest.