Cited by 304Single-cell and spatial transcriptomicsNeuroscience and Neuropharmacology ResearchNeurotransmitter Receptor Influence on Behavior

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Single-cell transcriptomes and whole-brain projections of serotonin neurons in the mouse dorsal and median raphe nuclei

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Abstract

Serotonin neurons of the dorsal and median raphe nuclei (DR, MR) collectively innervate the entire forebrain and midbrain, modulating diverse physiology and behavior. To gain a fundamental understanding of their molecular heterogeneity, we used plate-based single-cell RNA-sequencing to generate a comprehensive dataset comprising eleven transcriptomically distinct serotonin neuron clusters. Systematic in situ hybridization mapped specific clusters to the principal DR, caudal DR, or MR. These transcriptomic clusters differentially express a rich repertoire of neuropeptides, receptors, ion channels, and transcription factors. We generated novel intersectional viral-genetic tools to access specific subpopulations. Whole-brain axonal projection mapping revealed that DR serotonin neurons co-expressing vesicular glutamate transporter-3 preferentially innervate the cortex, whereas those co-expressing thyrotropin-releasing hormone innervate subcortical regions in particular the hypothalamus. Reconstruction of 50 individual DR serotonin neurons revealed diverse and segregated axonal projection patterns at the single-cell level. Together, these results provide a molecular foundation of the heterogenous serotonin neuronal phenotypes.

Overview

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TL;DR

Plate-based single-cell RNA-sequencing of 999 mouse dorsal and median raphe serotonin neurons identified 11 transcriptomic clusters with distinct spatial, molecular, and projection features. Intersectional viral tracing and single-neuron reconstruction showed that Vglut3+ and Trh+ DR serotonin neurons preferentially innervate cortical versus subcortical targets, while individual DR serotonin neurons have highly diverse whole-brain arborization patterns.

Key findings

  • scRNA-seq of 999 Tph2+ serotonin neurons from 14 adult mouse brains resolved 11 transcriptomic clusters: six principal DR clusters (DR-1–6), one caudal DR cluster (cDR), and four median raphe clusters (MR-1–4); HCR-smFISH with 16 markers mapped these clusters to pDR, cDR, or MR locations.
  • Co-transmitter and neuropeptide expression was heterogeneous: 65% of serotonin neurons expressed Vglut3, 35% expressed Gad1, Gad2, or both, 13% expressed Vglut3 plus Gad1/Gad2, and 14% expressed none of these glutamate/GABA markers; Trh was expressed in 14% and Crh in 7% of serotonin neurons.
  • Intersectional Sert-Flp with Vglut3-Cre or Trh-Cre showed that Vglut3+ pDR serotonin neurons preferentially innervate cortical regions, whereas Trh+ pDR serotonin neurons innervate subcortical regions including anterior/medial hypothalamus and thalamic nuclei; whole-brain mapping used n = 3 mice per genotype.
  • Reconstruction of 50 individual DR serotonin neurons revealed total axon lengths from 1.9 cm to 26.3 cm, with cortex-projecting neurons having the longest axons; 37/50 projections were unilateral, and 34/46 forebrain-projecting neurons had >80% of forebrain axon length in either cortical or subcortical regions.

Methods

  • Serotonin neurons were labeled using Sert-Cre;Ai14 mice, acutely dissociated from brain slices of postnatal day 40–45 mice (six females, eight males), FACS-sorted, and processed with Smart-seq2; 709 cells came from eight MR/pDR/cDR samples and 290 cells from six pDR-only samples, yielding 999 cells after QC.
  • Clustering used Seurat v3.0 with PCA and Louvain clustering; HCR-based smFISH against 16 cluster markers plus Tph2 was used to map transcriptomic clusters anatomically.
  • For projection mapping, Sert-Flp mice were crossed with Vglut3-Cre or Trh-Cre and injected with AAV-CreON/FlpON-mGFP; brains were cleared with AdipoClear, imaged by light-sheet microscopy, segmented with a 3D U-Net, and aligned to the Allen Common Coordinate Framework. Single-neuron reconstructions used dual-AAV sparse labeling and fMOST in 14 Sert-Cre mice.

Limitations

  • Stated: single-neuron reconstruction was limited in scope to 50 reconstructed cells out of approximately 9000 DR serotonin neurons, and larger-scale reconstruction integrated with molecular identity and function is needed.
  • Stated: whole-brain tracing showed inter-individual variability in total labeled axons and detailed projection patterns, and heterogeneity within Vglut3+ or Trh+ populations may contribute; larger experiments or single-cluster Cre lines are required.
  • Stated/evident: the Sert-Flp line can drive reporter expression outside raphe serotonin neurons due to transient developmental Sert expression; the paper also notes that whether Gad1/Gad2+ serotonin neurons actually release GABA remains to be determined, and MR cluster anatomical organization was less obvious with clusters more intermingled.

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Crossref · 10.7554/elife.49424

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Figures

Figure 1. Single-cell transcriptomic profiling of serotonergic neurons. (A) Schematic representation of scRNA-seq pipeline used to analyze individual serotonin neurons. Tissue slices containing either principal dorsal raphe (pDR, n = 6 brains) or entire DR and MR (n = 8 brains) of Sert-Cre;Ai14 adult were dissociated to a single-cell suspension. tdTomato+ neurons were FACS-sorted in 96-well plates and processed for scRNA-seq using Smart-seq2 protocol. tSNE plot of all processed Tph2+ neurons colored by anatomical localization. cDR, caudal DR; MR, median raphe. (B) tSNE plot of 999 Tph2+ cells obtained from 14 brains and clustered by gene expression. Cells are colored-coded according to identified transcriptomic clusters. (C) Expression of genes defining distinct serotonin neuron populations. Cells are colored by log-normalized expression of each transcript, and the color legend reflects the expression values of ln(CPM+1). CPM, counts per million. (D) Violin plots of expression of marker genes across 11 clusters.
Figure 1. Single-cell transcriptomic profiling of serotonergic neurons. (A) Schematic representation of scRNA-seq pipeline used to analyze individual serotonin neurons. Tissue slices containing either principal dorsal raphe (pDR, n = 6 brains) or entire DR and MR (n = 8 brains) of Sert-Cre;Ai14 adult were dissociated to a single-cell suspension. tdTomato+ neurons were FACS-sorted in 96-well plates and processed for scRNA-seq using Smart-seq2 protocol. tSNE plot of all processed Tph2+ neurons colored by anatomical localization. cDR, caudal DR; MR, median raphe. (B) tSNE plot of 999 Tph2+ cells obtained from 14 brains and clustered by gene expression. Cells are colored-coded according to identified transcriptomic clusters. (C) Expression of genes defining distinct serotonin neuron populations. Cells are colored by log-normalized expression of each transcript, and the color legend reflects the expression values of ln(CPM+1). CPM, counts per million. (D) Violin plots of expression of marker genes across 11 clusters.CC BY · PubMed Central
Figure 1—figure supplement 1. Technical characteristics of scRNA-seq experiments. (A) Schematic illustration of the sampling strategy. Top, orange shadows indicate dissected regions in the coronal brainstem sections in the first set of experiments. These regions contain the entire MR and DR. Bottom, purple shadows indicate dissected regions in the coronal brainstem sections in the second set of experiments. These regions only contain the principal DR. aq, aqueduct. (B) Number of genes per cell and number of reads per cells mapping to exons across 11 clusters. (C) Expression of Fev, Tph2, Sert and Vmat2 across individual cells. Cells are colored by log-normalized expression of each transcript, and the color legend reflects the expression values of ln(CPM+1). (D,E) tSNE plots of 999 analyzed cells colored by sex (D) and batch (E).
Figure 1—figure supplement 1. Technical characteristics of scRNA-seq experiments. (A) Schematic illustration of the sampling strategy. Top, orange shadows indicate dissected regions in the coronal brainstem sections in the first set of experiments. These regions contain the entire MR and DR. Bottom, purple shadows indicate dissected regions in the coronal brainstem sections in the second set of experiments. These regions only contain the principal DR. aq, aqueduct. (B) Number of genes per cell and number of reads per cells mapping to exons across 11 clusters. (C) Expression of Fev, Tph2, Sert and Vmat2 across individual cells. Cells are colored by log-normalized expression of each transcript, and the color legend reflects the expression values of ln(CPM+1). (D,E) tSNE plots of 999 analyzed cells colored by sex (D) and batch (E).CC BY · PubMed Central
Figure 1—figure supplement 2. Heatmap of gene expression of top ten marker genes identified for each cluster. Expression values represent z-scores of ln(CPM+1).
Figure 1—figure supplement 2. Heatmap of gene expression of top ten marker genes identified for each cluster. Expression values represent z-scores of ln(CPM+1).CC BY · PubMed Central
Figure 1—figure supplement 3. Expression patterns of selected genes in DR and MR across individual serotonin neurons presented as tSNE plots (Part I). Cells are colored by log-normalized expression of each transcript, and the color legend reflects the expression values of ln(CPM+1).
Figure 1—figure supplement 3. Expression patterns of selected genes in DR and MR across individual serotonin neurons presented as tSNE plots (Part I). Cells are colored by log-normalized expression of each transcript, and the color legend reflects the expression values of ln(CPM+1).CC BY · PubMed Central
Figure 1—figure supplement 4. Expression patterns of selected genes in DR and MR across individual serotonin neurons presented as tSNE plots (Part II).
Figure 1—figure supplement 4. Expression patterns of selected genes in DR and MR across individual serotonin neurons presented as tSNE plots (Part II).CC BY · PubMed Central
Figure 1—figure supplement 5. Expression patterns of selected genes in DR and MR across individual serotonin neurons presented as tSNE plots (Part III).
Figure 1—figure supplement 5. Expression patterns of selected genes in DR and MR across individual serotonin neurons presented as tSNE plots (Part III).CC BY · PubMed Central
Figure 1—figure supplement 6. Expression patterns of selected genes in DR and MR across individual serotonin neurons presented as tSNE plots (Part IV).
Figure 1—figure supplement 6. Expression patterns of selected genes in DR and MR across individual serotonin neurons presented as tSNE plots (Part IV).CC BY · PubMed Central
Figure 2. Anatomical location of serotonin neuron clusters determined by hybridization-chain reaction-based single-molecule fluorescence in situ hybridization (HCR-smFISH) of 16 cluster markers. (A) Positions of double-positive neurons (Tph2 and each of 16 marker genes color-coded on the top) are shown on four schematics representing coronal slices 250 µm apart. Red line around the aqueduct represents the average boundary drawn from raw data for each slice. Scale bar, 500 µm. Schematics summarizing HCR-smFISH data for individual genes can be found in Figure 2—figure supplements 1 and 2. (B) Representative images for each of the genes schematized in (A). All green cells are Tph2-positive, red cells express the indicated marker gene. Each image corresponds to one of the four numbered schematics in (A) and is located immediately ventral to the aqueduct unless otherwise noted as MR, lateral wing (lw), or cDR. Cyan box highlights the individual color zoom region at right. White arrowheads mark examples of double-positive neurons. Scale bars, 200 µm main panels, 20 µm zoom panels.
Figure 2. Anatomical location of serotonin neuron clusters determined by hybridization-chain reaction-based single-molecule fluorescence in situ hybridization (HCR-smFISH) of 16 cluster markers. (A) Positions of double-positive neurons (Tph2 and each of 16 marker genes color-coded on the top) are shown on four schematics representing coronal slices 250 µm apart. Red line around the aqueduct represents the average boundary drawn from raw data for each slice. Scale bar, 500 µm. Schematics summarizing HCR-smFISH data for individual genes can be found in Figure 2—figure supplements 1 and 2. (B) Representative images for each of the genes schematized in (A). All green cells are Tph2-positive, red cells express the indicated marker gene. Each image corresponds to one of the four numbered schematics in (A) and is located immediately ventral to the aqueduct unless otherwise noted as MR, lateral wing (lw), or cDR. Cyan box highlights the individual color zoom region at right. White arrowheads mark examples of double-positive neurons. Scale bars, 200 µm main panels, 20 µm zoom panels.CC BY · PubMed Central
Figure 2—figure supplement 1. Location of cluster marker genes determined by HCR-smFISH (Part I). Schematics as described in Figure 2, broken out to show double-positive cells for Tph2 and one marker gene at a time.
Figure 2—figure supplement 1. Location of cluster marker genes determined by HCR-smFISH (Part I). Schematics as described in Figure 2, broken out to show double-positive cells for Tph2 and one marker gene at a time.CC BY · PubMed Central
Figure 2—figure supplement 2. Location of cluster marker genes determined by HCR-smFISH (Part II).
Figure 2—figure supplement 2. Location of cluster marker genes determined by HCR-smFISH (Part II).CC BY · PubMed Central
Figure 3. Summary of spatial distribution of transcriptomic clusters of serotonin neurons. (A) Purple dots represent distribution of Clusters 1–6 markers listed in panel (B); green dots represent the distribution Irx2 and Piezo2 cells that are Cluster 9–11 markers; and cyan dots present distribution of Met+ and Tacr3+ cells, which are co-expressed in Cluster 7, but is additionally expressed in Cluster 10 (Met) or Cluster 11 (Tacr3). (B) Collectively, scRNA-seq and HCR-smFISH experiments support the model that Clusters 1–6 from Figure 1B correspond to pDR serotonin neurons (renamed DR-1–6), Cluster 7 corresponds to cDR serotonin neurons, and Clusters 8–11 correspond to MR serotonin neurons (renamed MR-1–4).
Figure 3. Summary of spatial distribution of transcriptomic clusters of serotonin neurons. (A) Purple dots represent distribution of Clusters 1–6 markers listed in panel (B); green dots represent the distribution Irx2 and Piezo2 cells that are Cluster 9–11 markers; and cyan dots present distribution of Met+ and Tacr3+ cells, which are co-expressed in Cluster 7, but is additionally expressed in Cluster 10 (Met) or Cluster 11 (Tacr3). (B) Collectively, scRNA-seq and HCR-smFISH experiments support the model that Clusters 1–6 from Figure 1B correspond to pDR serotonin neurons (renamed DR-1–6), Cluster 7 corresponds to cDR serotonin neurons, and Clusters 8–11 correspond to MR serotonin neurons (renamed MR-1–4).CC BY · PubMed Central
Figure 3—figure supplement 1. Expression of marker genes across 11 clusters (Part I). Expression levels denote for log-transformed expression [ln(counts per million+1)] of each transcript.
Figure 3—figure supplement 1. Expression of marker genes across 11 clusters (Part I). Expression levels denote for log-transformed expression [ln(counts per million+1)] of each transcript.CC BY · PubMed Central
Figure 3—figure supplement 2. Expression of marker genes across 11 clusters (Part II).
Figure 3—figure supplement 2. Expression of marker genes across 11 clusters (Part II).CC BY · PubMed Central
Figure 3—figure supplement 3. Expression of marker genes across 11 clusters (Part III).
Figure 3—figure supplement 3. Expression of marker genes across 11 clusters (Part III).CC BY · PubMed Central
Figure 3—figure supplement 4. Comparison of Marker Genes for Subtypes of MR Serotonin Neurons. Ten marker genes that are enriched in R1–R3-derived serotonin neurons from the Okaty et al. (2015) study (data taken from its Figure 2D, Figure S2, and Figure S5C) and MR-1–MR-4 transcriptomic clusters from this study. ‘+” indicate enriched compared to ‘–” in respective studies. The first eight markers support the correspondence between R1 and MR-1 (blue), R2 and MR-4, R3 and MR-2/3. The last two markers are not fully consistent.
Figure 3—figure supplement 4. Comparison of Marker Genes for Subtypes of MR Serotonin Neurons. Ten marker genes that are enriched in R1–R3-derived serotonin neurons from the Okaty et al. (2015) study (data taken from its Figure 2D, Figure S2, and Figure S5C) and MR-1–MR-4 transcriptomic clusters from this study. ‘+” indicate enriched compared to ‘–” in respective studies. The first eight markers support the correspondence between R1 and MR-1 (blue), R2 and MR-4, R3 and MR-2/3. The last two markers are not fully consistent.CC BY · PubMed Central
Figure 4. Molecular and functional characteristics of distinct serotonin neuron clusters. (A) Venn diagram showing the number of cells co-expressing genes associated with markers for different neurotransmitter systems: Gad1/2, Vmat2, Vgat, and Vglut3 (left) and neuropeptides: Trh, Crh, Sst, Pdyn (right). We consider a gene to be expressed if it has at least one read mapping to it and is detected in at least 3 cells (Materials and methods). (B) Expression of the most variable neuropeptides, receptors, and transcription factors across molecularly distinct serotonin neuron clusters. (C) Network representation of co-expressed genes that belong to one of the functional gene categories that organize transcriptional regulation, synaptic connectivity, and neuronal communications. Networks were constructed based on Pearson correlation coefficient (rp) of gene expression across all 999 cells, and identified networks that are centered on pDR- and MR+cDR-specific genes. Genes appear connected if rp >0.4. Line width represents rp values as indicated. Nodes are colored according to functional gene categories.
Figure 4. Molecular and functional characteristics of distinct serotonin neuron clusters. (A) Venn diagram showing the number of cells co-expressing genes associated with markers for different neurotransmitter systems: Gad1/2, Vmat2, Vgat, and Vglut3 (left) and neuropeptides: Trh, Crh, Sst, Pdyn (right). We consider a gene to be expressed if it has at least one read mapping to it and is detected in at least 3 cells (Materials and methods). (B) Expression of the most variable neuropeptides, receptors, and transcription factors across molecularly distinct serotonin neuron clusters. (C) Network representation of co-expressed genes that belong to one of the functional gene categories that organize transcriptional regulation, synaptic connectivity, and neuronal communications. Networks were constructed based on Pearson correlation coefficient (rp) of gene expression across all 999 cells, and identified networks that are centered on pDR- and MR+cDR-specific genes. Genes appear connected if rp >0.4. Line width represents rp values as indicated. Nodes are colored according to functional gene categories.CC BY · PubMed Central
Figure 4—figure supplement 1. Expression of the most variable genes among the listed functional categories across 11 molecularly distinct Tph2+ clusters.
Figure 4—figure supplement 1. Expression of the most variable genes among the listed functional categories across 11 molecularly distinct Tph2+ clusters.CC BY · PubMed Central
Figure 4—figure supplement 2. Sexually dimorphic genes consistently detected across the majority of serotonin cell subtypes. Expression levels denote for log-transformed expression [ln(CPM+1)] of each transcript.
Figure 4—figure supplement 2. Sexually dimorphic genes consistently detected across the majority of serotonin cell subtypes. Expression levels denote for log-transformed expression [ln(CPM+1)] of each transcript.CC BY · PubMed Central
Figure 5. Intersectional strategy to genetically access specific serotonin neuron populations. (A) Schematic representing cells dually gated by Cre- and Flp-expression. (B) Sert-Flp mice were crossed with H11-CAG-FRT-STOP-FRT-EGFP (STOPFRT-GFP) mice. Anti-Tph2 staining (red) was performed on consecutive coronal sections containing DR. 98.5% GFP+ neurons are Tph2+ and 100% Tph2+ neurons are GFP+ (n = 3 mice). Insets: magnified images showing the neurons indicated with arrowheads in individual channels. Scale, 25 µm. (C) In mice triple transgenic for Vglut3-Cre, Sert-Flp, and the IS reporter (Rosa-CAG-loxP-stop-loxP-FRT-tdTomato-FRT-EGFP), EGFP+ (Flp+Cre+) cells are referentially found in ventral pDR and in cDR (arrowheads). Coronal sections containing DR are shown, counterstained with Anti-Tph2 (red). (D) In mice triple transgenic for Gad2-Cre, Sert-Flp, and the IS reporter, EGFP+ (Flp+Cre+) cells are referentially found in dorsal pDR (arrows). Coronal sections containing DR are shown, counterstained with Anti-Tph2 (red). Scale, 100 µm.
Figure 5. Intersectional strategy to genetically access specific serotonin neuron populations. (A) Schematic representing cells dually gated by Cre- and Flp-expression. (B) Sert-Flp mice were crossed with H11-CAG-FRT-STOP-FRT-EGFP (STOPFRT-GFP) mice. Anti-Tph2 staining (red) was performed on consecutive coronal sections containing DR. 98.5% GFP+ neurons are Tph2+ and 100% Tph2+ neurons are GFP+ (n = 3 mice). Insets: magnified images showing the neurons indicated with arrowheads in individual channels. Scale, 25 µm. (C) In mice triple transgenic for Vglut3-Cre, Sert-Flp, and the IS reporter (Rosa-CAG-loxP-stop-loxP-FRT-tdTomato-FRT-EGFP), EGFP+ (Flp+Cre+) cells are referentially found in ventral pDR and in cDR (arrowheads). Coronal sections containing DR are shown, counterstained with Anti-Tph2 (red). (D) In mice triple transgenic for Gad2-Cre, Sert-Flp, and the IS reporter, EGFP+ (Flp+Cre+) cells are referentially found in dorsal pDR (arrows). Coronal sections containing DR are shown, counterstained with Anti-Tph2 (red). Scale, 100 µm.CC BY · PubMed Central
Figure 6. Dually gated serotonin neuron and axonal terminal labeling by viral-genetic intersection. (A) Schematics representing of the AAV-CreON/FlpON-mGFP dual labeling design. (B) mGFP expression (green) and Tph2 immunoreactivity (red) after injection of AAV-CreON/FlpON-mGFP into the DR of Sert-Flp;Vglut3-Cre mice. mGFP is mostly restricted to ventral pDR. (C) mGFP expression (green) and Tph2 immunoreactivity (red) after injection of AAV-CreON/FlpON-mGFP into the DR of Sert-Flp;Trh-Cre mice. mGFP is mostly restricted to dorsal pDR; the left-right asymmetry was likely due to AAV injection being biased towards the right hemisphere. (D) Axonal terminal expression of mGFP in different brain regions of Sert-Flp;Vglut3-Cre mice and Sert-Flp;Trh-Cre mice. The left and right images represent comparable brain regions cropped from serial coronal sections. Scale, 100 µm.
Figure 6. Dually gated serotonin neuron and axonal terminal labeling by viral-genetic intersection. (A) Schematics representing of the AAV-CreON/FlpON-mGFP dual labeling design. (B) mGFP expression (green) and Tph2 immunoreactivity (red) after injection of AAV-CreON/FlpON-mGFP into the DR of Sert-Flp;Vglut3-Cre mice. mGFP is mostly restricted to ventral pDR. (C) mGFP expression (green) and Tph2 immunoreactivity (red) after injection of AAV-CreON/FlpON-mGFP into the DR of Sert-Flp;Trh-Cre mice. mGFP is mostly restricted to dorsal pDR; the left-right asymmetry was likely due to AAV injection being biased towards the right hemisphere. (D) Axonal terminal expression of mGFP in different brain regions of Sert-Flp;Vglut3-Cre mice and Sert-Flp;Trh-Cre mice. The left and right images represent comparable brain regions cropped from serial coronal sections. Scale, 100 µm.CC BY · PubMed Central
Figure 7. Whole-brain projectome of Trh+ and Vglut3+ serotonin neuron populations. (A) Experimental schematic outlining the intersectional viral strategy, brain clearing, automated 3D axon segmentation, and alignment to the Allen Brain Institute Common Coordinate Framework. (B) Axonal innervation in a 3D view of the left hemisphere of one representative brain each from the intersection of Sert-Flp and either Vglut3-Cre or Trh-Cre. (C) Coronal Z-projection (500 µm of depth) showing axonal innervation patterns of 6 aligned brains. The schematic reference image is one 5 µm thick plane in the middle of the 500 µm stack. (D) Coronal heatmaps of axonal innervation patterns at 12 positions along the rostral–caudal axis for the same six brains as seen in (C). Weightings for individual voxels represent axonal content within a radius of 225 µm. (E) Top, bar plot shows the quantification of total axonal content in each of 6 brains prior to normalization. Bottom, heatmap breaks out the total content into each of 282 individual brain regions using boundaries from the Allen Institute CCF. Values are normalized to both target region volume and total axon content per brain. Display order is grouped by mean normalized prevalence of axons in each genotype and ordered by the second principal component. P-values for individual t-tests are uncorrected; those that survive FDR-testing at 10% are indicated with a black bar. See Supplementary file 5 for full list of regions. EP, Endopiriform nucleus; M1, Primary motor area; S1-B, Primary somatosensory area, barrel field; CLI, Central linear nucleus raphe; AHN, Anterior hypothalamic nucleus; DMH, Dorsomedial nucleus of the hypothalamus; MG, Medial geniculate nucleus; ZI, Zona incerta.
Figure 7. Whole-brain projectome of Trh+ and Vglut3+ serotonin neuron populations. (A) Experimental schematic outlining the intersectional viral strategy, brain clearing, automated 3D axon segmentation, and alignment to the Allen Brain Institute Common Coordinate Framework. (B) Axonal innervation in a 3D view of the left hemisphere of one representative brain each from the intersection of Sert-Flp and either Vglut3-Cre or Trh-Cre. (C) Coronal Z-projection (500 µm of depth) showing axonal innervation patterns of 6 aligned brains. The schematic reference image is one 5 µm thick plane in the middle of the 500 µm stack. (D) Coronal heatmaps of axonal innervation patterns at 12 positions along the rostral–caudal axis for the same six brains as seen in (C). Weightings for individual voxels represent axonal content within a radius of 225 µm. (E) Top, bar plot shows the quantification of total axonal content in each of 6 brains prior to normalization. Bottom, heatmap breaks out the total content into each of 282 individual brain regions using boundaries from the Allen Institute CCF. Values are normalized to both target region volume and total axon content per brain. Display order is grouped by mean normalized prevalence of axons in each genotype and ordered by the second principal component. P-values for individual t-tests are uncorrected; those that survive FDR-testing at 10% are indicated with a black bar. See Supplementary file 5 for full list of regions. EP, Endopiriform nucleus; M1, Primary motor area; S1-B, Primary somatosensory area, barrel field; CLI, Central linear nucleus raphe; AHN, Anterior hypothalamic nucleus; DMH, Dorsomedial nucleus of the hypothalamus; MG, Medial geniculate nucleus; ZI, Zona incerta.CC BY · PubMed Central
Figure 7—figure supplement 1. Individual projectome variability of the Trh+ serotonin population. Coronal slices of data along the rostral–caudal axis, at spacing of 500 µm. The three left panels are 500 µm Z-projections of individual brains, color-coded by Z-depth. Merge image shows the same volume of the three slices overlaid and colored individually by brain. Lower right, 3D view of axonal content for each of the three brains.
Figure 7—figure supplement 1. Individual projectome variability of the Trh+ serotonin population. Coronal slices of data along the rostral–caudal axis, at spacing of 500 µm. The three left panels are 500 µm Z-projections of individual brains, color-coded by Z-depth. Merge image shows the same volume of the three slices overlaid and colored individually by brain. Lower right, 3D view of axonal content for each of the three brains.CC BY · PubMed Central
Figure 7—figure supplement 2. Individual projectome variability of the Vglut3+ serotonin population. Coronal slices of data along the rostral–caudal axis, at spacing of 500 µm. The three left panels are 500 µm Z-projections of individual brains, color-coded by Z-depth. Merge image shows the same volume of the three slices overlaid and colored individually by brain. Lower right, 3D view of axonal content for each of the three brains.
Figure 7—figure supplement 2. Individual projectome variability of the Vglut3+ serotonin population. Coronal slices of data along the rostral–caudal axis, at spacing of 500 µm. The three left panels are 500 µm Z-projections of individual brains, color-coded by Z-depth. Merge image shows the same volume of the three slices overlaid and colored individually by brain. Lower right, 3D view of axonal content for each of the three brains.CC BY · PubMed Central
Figure 7—figure supplement 3. The cell body location and initial axonal segments of the Trh+ and Vglut3+ serotonin neuron subpopulations. (A) Representative Z projections covering the extent of labeled cell bodies located at the injection site in DR for brains used in iDISCO-based whole brain imaging of axons. Dashed lines mark the ventral boundary of the aqueduct in the sagittal view and the ventral (Vglut3-Cre) and lateral wing (Trh-Cre) locations of labeled Sert-Flp neurons. Scale bars 500 µm. (B) 3D volumes of DR and initial axon branches projecting anterior and ventral into the median forebrain bundle. Volumes are from one brain each representing either Vglut3-Cre or Trh-Cre labeled Sert-Flp neurons. Merge at right highlights overlap. (C) Dots represent the central location of cells labeled by viral injection for each of the six brains. Axis labels: A, anterior; P, posterior, D, dorsal; V, ventral; M, medial; L, lateral.
Figure 7—figure supplement 3. The cell body location and initial axonal segments of the Trh+ and Vglut3+ serotonin neuron subpopulations. (A) Representative Z projections covering the extent of labeled cell bodies located at the injection site in DR for brains used in iDISCO-based whole brain imaging of axons. Dashed lines mark the ventral boundary of the aqueduct in the sagittal view and the ventral (Vglut3-Cre) and lateral wing (Trh-Cre) locations of labeled Sert-Flp neurons. Scale bars 500 µm. (B) 3D volumes of DR and initial axon branches projecting anterior and ventral into the median forebrain bundle. Volumes are from one brain each representing either Vglut3-Cre or Trh-Cre labeled Sert-Flp neurons. Merge at right highlights overlap. (C) Dots represent the central location of cells labeled by viral injection for each of the six brains. Axis labels: A, anterior; P, posterior, D, dorsal; V, ventral; M, medial; L, lateral.CC BY · PubMed Central
Figure 8. Whole-brain axonal arborization patterns of individual serotonin neurons. (A) Schematic of single-neuron reconstruction pipeline. (B) Four examples of cortex-projecting DR serotonin neurons, projecting primarily to olfactory cortex and olfactory bulb (1st), dorsal cortex (2nd), prefrontal cortex (3rd), and entorhinal cortex (4th). (C) Two examples of amygdala-projecting DR serotonin neurons. (D) A striatum-projecting DR serotonin neuron. (E) A brainstem-projecting DR serotonin neuron. (F) Merged example neurons from panels B–E. (G) Schematic diagram illustrating the major projection targets of 8 sample neurons in panel F. (H) Histogram showing the distribution of cell numbers according to the total axon length.
Figure 8. Whole-brain axonal arborization patterns of individual serotonin neurons. (A) Schematic of single-neuron reconstruction pipeline. (B) Four examples of cortex-projecting DR serotonin neurons, projecting primarily to olfactory cortex and olfactory bulb (1st), dorsal cortex (2nd), prefrontal cortex (3rd), and entorhinal cortex (4th). (C) Two examples of amygdala-projecting DR serotonin neurons. (D) A striatum-projecting DR serotonin neuron. (E) A brainstem-projecting DR serotonin neuron. (F) Merged example neurons from panels B–E. (G) Schematic diagram illustrating the major projection targets of 8 sample neurons in panel F. (H) Histogram showing the distribution of cell numbers according to the total axon length.CC BY · PubMed Central
Figure 8—figure supplement 1. Single-cell reconstruction of DR serotonin neurons. (A) Design of dual-AAV sparse labeling system. (B) Left, an example neuron labeled by the sparse labeling system. Right, summary of the cell body position of 50 reconstructed neurons color-coded by their projection types. (C) Whole-brain quantification of the axon process length of the 50 reconstructed DR serotonin neurons. The 50 brains are ordered (from top to bottom) according to their projection patterns described in the text, and are the same as the numbers in . F.T., fiber tracts; Others refer to projections in regions unannotated by Allen Reference Atlas, or in ventricles due to errors in registration; for other abbreviations, see Materials and methods.
Figure 8—figure supplement 1. Single-cell reconstruction of DR serotonin neurons. (A) Design of dual-AAV sparse labeling system. (B) Left, an example neuron labeled by the sparse labeling system. Right, summary of the cell body position of 50 reconstructed neurons color-coded by their projection types. (C) Whole-brain quantification of the axon process length of the 50 reconstructed DR serotonin neurons. The 50 brains are ordered (from top to bottom) according to their projection patterns described in the text, and are the same as the numbers in . F.T., fiber tracts; Others refer to projections in regions unannotated by Allen Reference Atlas, or in ventricles due to errors in registration; for other abbreviations, see Materials and methods.CC BY · PubMed Central
Figure 8—figure supplement 2. Individually reconstructed DR serotonin neurons (part I). (A, A’) Whole brain projection pattern and the cell body locations of 17 cortex-projection DR serotonin neurons. (B, B’) Whole brain projection pattern and the cell body locations of 5 striatum-projecting serotonin neurons. (C, C’) Whole brain projection pattern and the cell body locations of 6 hypothalamus-projection DR serotonin neurons. For each neuron, the left image is a horizonal view, whereas the right image is a sagittal view (ventral to the left, dorsal to the right).
Figure 8—figure supplement 2. Individually reconstructed DR serotonin neurons (part I). (A, A’) Whole brain projection pattern and the cell body locations of 17 cortex-projection DR serotonin neurons. (B, B’) Whole brain projection pattern and the cell body locations of 5 striatum-projecting serotonin neurons. (C, C’) Whole brain projection pattern and the cell body locations of 6 hypothalamus-projection DR serotonin neurons. For each neuron, the left image is a horizonal view, whereas the right image is a sagittal view (ventral to the left, dorsal to the right).CC BY · PubMed Central
Figure 8—figure supplement 3. Individual reconstructed DR serotonin neurons (part II). Horizontal (left), sagittal (right) view and the cell body locations of 4 amygdala-projecting (A, A'), one thalamus-projecting (B, B’), and 17 caudal brainstem and cerebellum-projecting (D, D’) DR serotonin neurons.
Figure 8—figure supplement 3. Individual reconstructed DR serotonin neurons (part II). Horizontal (left), sagittal (right) view and the cell body locations of 4 amygdala-projecting (A, A'), one thalamus-projecting (B, B’), and 17 caudal brainstem and cerebellum-projecting (D, D’) DR serotonin neurons.CC BY · PubMed Central
Figure 8—figure supplement 4. Detailed projection patterns of example DR serotonin neurons. (A) Serotonin terminal arborization in the CoA (A1) and basal medial amygdala (BMA; A2). (A3) Examples of axon arbors of an amygdala-projecting serotonin neuron (yellow) that terminates in the BMA (yellow) and another one (blue) that terminates in the nearby CoA. (B) Two examples of serotonin terminal arborization in the thalamus. Top, one neuron projects to the medial dorsal nucleus of the thalamus (MD) without branches in the cortex. Bottom, one neuron has bilateral projections to the submedial nucleus of the thalamus (SMT) and also project to the cortex (not shown). (C) Two examples of striatum-projecting neurons differentially arborize at the core and shell of the nucleus accumbens (NAc).
Figure 8—figure supplement 4. Detailed projection patterns of example DR serotonin neurons. (A) Serotonin terminal arborization in the CoA (A1) and basal medial amygdala (BMA; A2). (A3) Examples of axon arbors of an amygdala-projecting serotonin neuron (yellow) that terminates in the BMA (yellow) and another one (blue) that terminates in the nearby CoA. (B) Two examples of serotonin terminal arborization in the thalamus. Top, one neuron projects to the medial dorsal nucleus of the thalamus (MD) without branches in the cortex. Bottom, one neuron has bilateral projections to the submedial nucleus of the thalamus (SMT) and also project to the cortex (not shown). (C) Two examples of striatum-projecting neurons differentially arborize at the core and shell of the nucleus accumbens (NAc).CC BY · PubMed Central

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    The heterogeneity of DR and MR serotonin neurons must be reflected at the molecular level.

    “Ultimately, the heterogeneity of DR and MR serotonin neurons must be reflected at the molecular level.”
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    The DR contains approximately 9000 serotonin neurons.

    “our study is still limited by the scope (50 reconstructed cells out of 9000 DR serotonin neurons).”
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  1. Çiçek, 2016, Medical Image Computing and Computer-Assisted Intervention – MICCAI 2016. MICCAI 2016. Lecture Notes in Computer Science, 9901

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Metadata

Authors
Ren, J., Isakova, A., Friedmann, D., Zeng, J., Grutzner, S. M., Pun, A., Zhao, G. Q., Kolluru, S. S., Wang, R., Lin, R., Li, P., Li, A., Raymond, J. L., Luo, Q., Luo, M., Quake, S. R., & Luo, L.
Affiliations
  • Howard Hughes Medical Institute (US)
  • Stanford University (US)
  • National Institute of Biological Sciences, Beijing (CN)
  • Stanford Medicine (US)
  • Wuhan National Laboratory for Optoelectronics (CN)
  • Chan Zuckerberg Biohub San Francisco (US)
Corresponding author
  • Ren, Jing
  • Quake, Stephen R
  • Luo, Liqun
Venue
eLife
Published
2019-10-24
Volume
8
ISSN
2050-084X (ISSN-L)
OpenAlex
W2982239238
Cited by
304
References
97
Citations by year
2026: 32 · 2025: 52 · 2024: 50 · 2023: 59 · 2022: 42 · 2021: 41 · 2020: 25 · 2019: 2
FWCI
9.10
Type
journal article
Language
en
Open access
Locations
  • eLife (publishedVersion, cc-by)
  • bioRxiv (Cold Spring Harbor Laboratory) (acceptedVersion)
  • PubMed Central (publishedVersion, cc-by)
  • PubMed (publishedVersion)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
Funders
  • National Institutes of Health (R01 NS104698, NS057488)
  • National Science Foundation (NeuroNex)
  • Ministry of Science and Technology of the People's Republic of China (2015BAI08B02)
  • National Natural Science Foundation of China (91432114, 91632302)
  • Beijing Municipal Government
  • NSFC (61721092, 81827901)
  • Swiss National Science Foundation (PostDoc Mobility Fellowship)
Topics
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Single-cell and spatial transcriptomics
  • Life Sciences › Neuroscience › Cellular and Molecular Neuroscience › Neuroscience and Neuropharmacology Research
  • Life Sciences › Neuroscience › Cellular and Molecular Neuroscience › Neurotransmitter Receptor Influence on Behavior
Keywords
serotonergic neurons, dorsal raphe nucleus, median raphe nucleus, single-cell transcriptomics, neuronal heterogeneity, VGLUT3, thyrotropin-releasing hormone, hypothalamus, neuropeptides, single-neuron reconstruction, mice
MeSH terms
Animals, Brain Mapping, Neural Pathways, Raphe Nuclei, Sequence Analysis, RNA, Mice, Single-Cell Analysis, Serotonergic Neurons, Transcriptome

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