A New Synuclein-Transgenic Mouse Model for Early Parkinson's Reveals Molecular Features of Preclinical Disease

Abstract

Understanding Parkinson's disease (PD), in particular in its earliest phases, is important for diagnosis and treatment. However, human brain samples are collected post-mortem, reflecting mainly end-stage disease. Because brain samples of mouse models can be collected at any stage of the disease process, they are useful in investigating PD progression. Here, we compare ventral midbrain transcriptomics profiles from α-synuclein transgenic mice with a progressive, early PD-like striatal neurodegeneration across different ages using pathway, gene set, and network analysis methods. Our study uncovers statistically significant altered genes across ages and between genotypes with known, suspected, or unknown function in PD pathogenesis and key pathways associated with disease progression. Among those are genotype-dependent alterations associated with synaptic plasticity and neurotransmission, as well as mitochondria-related genes and dysregulation of lipid metabolism. Age-dependent changes were among others observed in neuronal and synaptic activity, calcium homeostasis, and membrane receptor signaling pathways, many of which linked to G-protein coupled receptors. Most importantly, most changes occurred before neurodegeneration was detected in this model, which points to a sequence of gene expression events that may be relevant for disease initiation and progression. It is tempting to speculate that molecular changes similar to those changes observed in our model happen in midbrain dopaminergic neurons before they start to degenerate. In other words, we believe we have uncovered molecular changes that accompany the progression from preclinical to early PD.

Keywords: Disease-stage; Network analysis; Parkinson’s disease; Pathway analysis; Transgenic mouse model; α-Synuclein.

Fig. 1

Data analysis workflow

Fig. 2

Alpha-synuclein expression in BAC-TG3(SNCA^E46K) “Line 3” mouse brains. RT-PCR measurements (a) showed: that SNCA was detected in all HET mice, that the levels of RNA for murine Snca were similar between HET mice and WT littermates, and that the levels of overexpression (measured by levels of total RNA for both SNCA and Snca) were about 1.8-fold (78%) in ventral midbrain of HET mice compared to that of WT littermates. ∗∗∗ p < 0.0001,∗p < 0.05 by unpaired Student’s T test. Fluorescent immunostaining showed that the transgene human alpha-synuclein (panels in left column) shows roughly the same regional distribution than the endogenous murine alpha-synuclein (panels in right column). Scale bar (b)= 1.4 mm

Fig. 3

Age-dependent degeneration in the dorsal striatum but not th e substantia nigra, in heterozygous (HET) BAC-Tg3(SNCA^E46K) mice compared with that in littermate wild-type (WT) controls. Measurements for Tyrosine-hydroxylase (TH, green channel) and dopamine transporter (DAT, red channel) were performed as described in “Material and Methods.” Two-way ANOVA followed by post hoc tests was used for statistical analysis. N = 6 mice per age group and genotype. ^∗ symbol for p < 0.05 in genotype comparisons, ## symbol for p < 0.01, and ### for p < 0.001 when comparing 9M HET and 13M HET mice, respectively, to 3M HET mice. Values for TH-positive neurons in the SN were normalized to WT controls for each age group. Different levels of the SN, from medial to lateral, are shown. Scale bars: 30 μm for immunostaining panels in columns A and B, and 100 μm for immunostaining panels in column C

Fig. 4

(a) Overlap of differentially expressed genes for the age-matched mice of different genotypes (upper panel), (b) Differential expression across ages for HET mice (lower panel,left), WT mice (lower panel, right), overlap intersections HET (323 genes), and WT (1091 genes) in previous figures (lower panel, middle)

Fig. 5

DEGs—brain cell type where the genes show the highest expression (extracted from the Brain RNAseq database (https://www.brainrnaseq.org/) [133]). (a) Genotype-dependent DEGs: left panel: 3M (30% same cellular source); right panel: 9M (40% same cellular source). (b) Age-dependent DEGs: upper left panel: type 1—DEGs increasing with age (40% same cellular source); upper right panel: type 2—DEGs decreasing with age (only 1 DEG, different cellular source between mouse and human); lower left panel: type 3a—DEGs with increased expression between 3M and 9M, and decreased expression between 9M and 13M (28% same cellular source); lower right panel: type 3b—DEGs with decreased expression between 3M and 9M, and increased expression between 9M and 13M (40% same cellular source)