Characterization of Brain Lysosomal Activities in GBA-Related and Sporadic Parkinson's Disease and Dementia with Lewy Bodies

Abstract

Mutations in the GBA gene, encoding the lysosomal hydrolase glucocerebrosidase (GCase), are the most common known genetic risk factor for Parkinson's disease (PD) and dementia with Lewy bodies (DLB). The present study aims to gain more insight into changes in lysosomal activity in different brain regions of sporadic PD and DLB patients, screened for GBA variants. Enzymatic activities of GCase, β-hexosaminidase, and cathepsin D were measured in the frontal cortex, putamen, and substantia nigra (SN) of a cohort of patients with advanced PD and DLB as well as age-matched non-demented controls (n = 15/group) using fluorometric assays. Decreased activity of GCase (- 21%) and of cathepsin D (- 15%) was found in the SN and frontal cortex of patients with PD and DLB compared to controls, respectively. Population stratification was applied based on GBA genotype, showing substantially lower GCase activity (~ - 40%) in GBA variant carriers in all regions. GCase activity was further significantly decreased in the SN of PD and DLB patients without GBA variants in comparison to controls without GBA variants. Our results show decreased GCase activity in brains of PD and DLB patients with and without GBA variants, most pronounced in the SN. The results of our study confirm findings from previous studies, suggesting a role for GCase in GBA-associated as well as sporadic PD and DLB.

Keywords: Autophagy-lysosomal pathway; Cathepsin D; GBA variants; β-hexosaminidase.

Fig. 1

Lysosomal enzymatic activities in different regions of the human postmortem brain. a Cluster analysis of average enzymatic activities across patients for GCase, β-Hex, and CathD in the different studied regions reveals that GCase activity was highest in the frontal cortex, while CathD and β-Hex activities were higher in the putamen compared to the other studied regions. b, c, and d Correlation matrices for lysosomal enzymatic activities and PMD for frontal cortex (b), putamen (c), and SN (d). High correlations were sometimes observed between activities of different lysosomal enzymes, but not with PMD. Values represent Spearman’s correlation coefficients. *p < 0.05; **p < 0.01; ***p < 0.001

Fig. 2

Lysosomal enzyme activities in the brain of PD and DLB patients compared to controls. Group comparison of measured specific activities for β-Hex (a), GCase (b), and CathD (c) expressed in pmol/min/mg of proteins. Graphs show the average specific activity and standard deviation for non-demented controls (blue), PD patients (green), and DLB patients (blue). d. Overview of mean enzyme activities ± standard deviations. The p value indicates the between-group difference per brain region. GCase activity was significantly decreased in the SN of PD patients compared to controls, while lower CathD activity was found in the frontal cortex of DLB patients compared to controls *p < 0.05

Fig. 3

mRNA expression levels for GBA, CTSD, and other ALP components. Correlation matrices showing association between mRNA expression levels of GBA and genes encoding CTSD, GCase’s protein interactors LIMP-2 (SCARB2) and saposin C (PSAP), and the lysosomal membrane-associated protein LAMP-1 in the frontal cortex (a) and SN (b). Normalized mRNA expression levels for GBA (c), CTSD (d), and SCARB2 (e) in PD (green) and DLB (red) patients as well as non-demented controls (blue). Expression levels are expressed as percentage of the average expression level of non-demented controls. f Schematic outline of the interplay between the measured gene products. LIMP-2 functions as a transporter molecule for GCase from the endoplasmatic reticulum, while the interaction of GCase with SapC is essential for its proper function [31]. LAMP-1 and LAMP-2a are lysosome-associated membrane proteins, while transcription factor EB (TFEB) is the master regulator of biogenesis and function of lysosomes, by activating transcription of autophagy-related genes (ATGs). *p < 0.05; **p < 0.01; ***p < 0.001

Fig. 4

Population stratification based on GBA genotype. Specific enzymatic activities for GCase (a), β-Hex (c), and CathD (d) for non-demented controls without GBA variants (blue) and with GBA risk factors (orange), for PD and DLB patients without GBA variants (green), with GBA risk factors (red), and with pathogenic GBA variants (purple). b Measured GCase activities in the SN per GBA variant for controls (blue), PD patients (green), and DLB patients (red). d Overview of mean enzyme activities ± standard deviations in different GBA genetic subgroups. p values indicate the between-group difference per brain region. e GBA mRNA expression levels for controls, PD and DLB patients without GBA variants, with GBA risk factors, and pathogenic GBA variants. *p < 0.05; **p < 0.01; ***p < 0.001; ****p < 0.0001