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. 2018 Mar 1;102(3):427-446.
doi: 10.1016/j.ajhg.2018.02.001.

Single-Cell RNA-Seq of Mouse Dopaminergic Neurons Informs Candidate Gene Selection for Sporadic Parkinson Disease

Paul W Hook  1 , Sarah A McClymont  1 , Gabrielle H Cannon  1 , William D Law  1 , A Jennifer Morton  2 , Loyal A Goff  3 , Andrew S McCallion  4
Affiliations

Single-Cell RNA-Seq of Mouse Dopaminergic Neurons Informs Candidate Gene Selection for Sporadic Parkinson Disease

Paul W Hook et al. Am J Hum Genet. .

Abstract

Genetic variation modulating risk of sporadic Parkinson disease (PD) has been primarily explored through genome-wide association studies (GWASs). However, like many other common genetic diseases, the impacted genes remain largely unknown. Here, we used single-cell RNA-seq to characterize dopaminergic (DA) neuron populations in the mouse brain at embryonic and early postnatal time points. These data facilitated unbiased identification of DA neuron subpopulations through their unique transcriptional profiles, including a postnatal neuroblast population and substantia nigra (SN) DA neurons. We use these population-specific data to develop a scoring system to prioritize candidate genes in all 49 GWAS intervals implicated in PD risk, including genes with known PD associations and many with extensive supporting literature. As proof of principle, we confirm that the nigrostriatal pathway is compromised in Cplx1-null mice. Ultimately, this systematic approach establishes biologically pertinent candidates and testable hypotheses for sporadic PD, informing a new era of PD genetic research.

Keywords: Complexin 1; Parkinson disease; dopaminergic neurons; gene regulatory networks; genome-wide association studies; mouse; single-cell RNA sequencing; substantia nigra.

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Figures

Figure 1
Figure 1
scRNA-Seq Analysis of Isolated Cells Allows Their Separation by Developmental Time (A) Diagram of scRNA-seq experimental procedures for isolating and sequencing EGFP+ cells. Timeline adapted from Barallobre et al. (B) Principal component analysis (PCA) on all cells collected using genes with highly variant transcriptional profiles. The cells that were included are those that passed quality control measures. The greatest source of variation (PC1) is explained by the time point at which the cells were collected, not the region from which the cells were collected. (C) The top ten Gene Ontology (GO) gene sets enriched in genes with positive (red) and negative (green) PC1 loadings from the PCA plot in (B). Gene sets are arranged by normalized enrichment scores (NES) and all gene sets displayed had a false discovery rate (FDR) q value ≤ 0.05. (D) A t-distributed stochastic neighbor embedding (t-SNE) plot of all collected cells that passed quality control measures colored by regional identity. E15.5 cells cluster together while P7 cells cluster primarily by regional identity. (E) A t-SNE plot of all collected cells colored by subset cluster identity. Through iterative analysis, time point-regions collected can be separated into multiple subpopulations (13 in total). Abbreviations: midbrain, MB; forebrain, FB; olfactory bulb, OB; fluorescence activated cell sorting, FACS.
Figure 2
Figure 2
Subclusters of P7 Th+ Neurons Are Identified Based on Marker Gene Analyses (A) A t-SNE plot of all P7 neurons collected colored by subset cluster identity. The neurons mostly cluster by regional identity. (B) t-SNE plot of P7 FB neurons. P7 FB neurons cluster into two distinct populations. (C) t-SNE plot of P7 OB neurons. P7 OB neurons cluster into three populations. These populations represent a trajectory of Th+ OB maturation (Table S3) as indicated by the red arrow. (D) A t-SNE plot of P7 MB neurons. P7 MB neurons cluster into four clusters: the substantia nigra (SN), the ventral tegmental area (VTA), the periaqueductal gray area (PAG), and a neuroblast-like population.
Figure 3
Figure 3
Multiplex, smFISH Confirms the Existence of a Putative Postnatal Neuroblast Population (A) Boxplots displaying the expression of four genes (Th, Slc6a3, Lhx9, and Ldb2) across all subclusters identified. E15.MB.1 and P7.MB.2 labels are bold due to similar expression profile of displayed genes (Tables S2 and S3). ±1.5× interquartile range is represented by the whiskers on the boxplots. Data points beyond 1.5× interquartile range are considered as outliers and plotted as black points. (B) Representative image of multiplex single molecule fluorescent in situ hybridization (smFISH) for Th, Slc6a3, and Lhx9 in the mouse ventral midbrain. Zoomed-in panels represent cell populations observed. Scale bar, 50 μm. (C) Representative image of multiplex smFISH for Th, Slc6a3, and Ldb2, in the mouse ventral midbrain. Zoomed-in panels represent cell populations observed. Scale bar, 50 μm. (D) Diagram of ventral midbrain summarizing the results of smFISH. Th+/Slc6a3−/Lhx9+ and Th+/Slc6a3−/Ldb2+ cells are both found in the dorsal SN. Abbreviations: NB, neuroblast; SN, substantia nigra; VTA, ventral tegmental area; IPN, interpeduncular nucleus.
Figure 4
Figure 4
Genetic Markers and Gene Modules Reveal Context-Specific SN DA Biology (A) Reference Atlas diagram from the Allen Brain Atlas (ABA) of the P56 mouse ventral midbrain. Important abbreviations include: VTA, ventral tegmental area; SNc, substantia nigra pars compacta; SNr, substantia nigra pars reticulata. (B) Confirmation of SN DA neuron marker genes through the use of ABA in situ hybridization data. Coronal, P56 mouse in situ data were explored in order to confirm the expression of 25 previously uncharacterized SN markers. Th expression in P56 mice was used as an anatomical reference during analysis. (C) Correlation heatmap of the Pearson correlation between module eigengenes and P7 Th+ subset cluster identity. Modules are represented by their assigned colors at the bottom of the matrix. Modules that had a positive correlation with a subset cluster and had a correlation p value less than the Bonferroni corrected significance level (p < 3.5 × 10−4) contain an asterisk. SN cluster (P7.MB.4) identity is denoted by a black rectangle. Modules (“green” and “brown”) that were enriched for the “Parkinson’s Disease” KEGG gene set are labeled with “PD.” (D) The eigengene value for each P7 neuron in the seven WGCNA modules shown to be significantly positively associated with a subset cluster overlaid on the t-SNE plot of all P7 neurons (Figure 2A). Plotting of eigengenes confirms strict spatial restriction of module association. Only the “lightcyan” module does not seem to show robust spatial restriction.
Figure 5
Figure 5
Context-Specific SN DA Data Allow for the Prioritization of Genes in PD GWAS Loci (A) A locus plot displaying 4-megabase regions in the human genome (hg38) centered on PD GWAS SNPs in six loci. Genes are displayed as boxes on their appropriate strand. Genes are shaded by their prioritization score and gene names are displayed for genes with a score of 3 or higher in each locus. (B) In situ hybridization from the Allen Brain Atlas (ABA) of five prioritized genes along with Th for an anatomical reference. Coronal, P56 mouse in situ data were used. (C) Boxplots displaying expression of prioritized genes from the MAPT locus (Figure 5A; Table 2). ±1.5× interquartile range is represented by the whiskers on the boxplots. Data points beyond 1.5× interquartile range are considered as outliers and plotted as black points. (D) Representative light microscopy images of Th+ innervation density in the striatum of WT and Cplx1 knockout (KO) mice. Scale bar, 1 mm. (E) Boxplots comparing the level of Th+ striatum innervation between WT and Cplx1 KO mice. DAB staining density was measured in 35 μm, horizontal sections in WT mice (mice = 3, sections = 16) and Cplx1 KO mice (mice = 8, sections = 40). Each point in the boxplot represents the average signal from a stained, 35 μm section. Statistical analyses were performed between conditions with section averages in order to preserve observed variability (WT n = 16, Cplx1 KO n = 40). A two-sample t test revealed that Th+ innervation density was significantly lower in Cplx1 KO mice (t = 6.4395, df = 54, p = 3.386 × 10−8). Data points outside of 1.5× interquartile range, represented by the whiskers on the boxplots, are considered as outliers and plotted as black points.
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