Rabies virus-based barcoded neuroanatomy resolved by single-cell RNA and in situ sequencing

Abstract

Mapping the connectivity of diverse neuronal types provides the foundation for understanding the structure and function of neural circuits. High-throughput and low-cost neuroanatomical techniques based on RNA barcode sequencing have the potential to map circuits at cellular resolution and a brain-wide scale, but existing Sindbis virus-based techniques can only map long-range projections using anterograde tracing approaches. Rabies virus can complement anterograde tracing approaches by enabling either retrograde labeling of projection neurons or monosynaptic tracing of direct inputs to genetically targeted postsynaptic neurons. However, barcoded rabies virus has so far been only used to map non-neuronal cellular interactions in vivo and synaptic connectivity of cultured neurons. Here we combine barcoded rabies virus with single-cell and in situ sequencing to perform retrograde labeling and transsynaptic labeling in the mouse brain. We sequenced 96 retrogradely labeled cells and 295 transsynaptically labeled cells using single-cell RNA-seq, and 4,130 retrogradely labeled cells and 2,914 transsynaptically labeled cells in situ. We found that the transcriptomic identities of rabies virus-infected cells can be robustly identified using both single-cell RNA-seq and in situ sequencing. By associating gene expression with connectivity inferred from barcode sequencing, we distinguished long-range projecting cortical cell types from multiple cortical areas and identified cell types with converging or diverging synaptic connectivity. Combining in situ sequencing with barcoded rabies virus complements existing sequencing-based neuroanatomical techniques and provides a potential path for mapping synaptic connectivity of neuronal types at scale.

Figure 1.. Models of multiplexed retrograde labeling and monosynaptic tracing using barcoded rabies virus.

(A) In multiplexed retrograde labeling, rabies viruses carrying different barcodes are injected into different brain regions, and retrogradely labeled neurons can be distinguished based on the barcodes they carry. Numbers and colors indicate different barcodes injected into each area. (B) In multiplexed monosynaptic tracing, potential source cells are first labeled by helper AAV viruses expressing TVA and rabies glycoprotein (yellow cells, left). These cells can be infected by barcoded rabies virus (barcodes are indicated by numbers, middle). These source cells can then pass the barcodes to presynaptic neurons (numbered gray cells, right). Both rabies barcodes and endogenous mRNAs can be read out to infer cell type connectivity. However, if multiple source cells share the same barcode, they may obscure single-cell connectivity mapping and must be filtered out (crossed out cells, right).

Figure 2.. High-quality cell typing of rabies virus-barcoded neurons using single-cell RNA-seq.

(A) Illustration of the design of barcoded rabies virus libraries. (B) Barcode distribution in the CCS and non-CCS libraries. The y axis indicates the count of barcodes, which are sorted in descending order. The rank of the barcode is shown on the x axis. (C) An outline of the experiments. The two libraries are injected into VISal and LGd. After a week, VISp is dissected, and the mCherry-expressing cells are FACS-enriched and processed for single-cell RNA-seq. (D) Histogram showing the cluster mapping confidence of rabies-labeled cells from this study and AAV-labeled cells from Graybuck et al. (2021). (E) The expression of select marker gene in non-infected cells from Tasic et al. (2018) and in rabies-infected cells of matching cell types. Dot size indicates the proportion of cells with non-zero marker expression, and colors indicate relative gene expression levels scaled per row. (F) UMAP plot of gene expression patterns of cells infected with rabies virus (black) overlaid on non-infected cells from (Tasic et al. 2018). The non-infected cells are color-coded by cluster identities and cluster names are indicated. (G) The expression of rabies-encoded genes in the sequenced cells. Columns indicate cells and rows indicate genes. Colors indicate the log transformed count for each rabies-encoded gene after scaling by the sum of all reads that mapped to viral constructs multiplied by 10,000. The bar on top indicates donor animals. (H) Example barcode sequences in three sequenced cells. Letter heights indicate probabilities at each position. Gray boxes indicate the barcode region. (I) Distribution of cell types of retrogradely labeled cells. Colors indicate cell types and match those in (F), and dot size indicates the number of cells. VISp, primary visual cortex; VISal, anterolateral visual cortex; LGd, dorsal lateral geniculate nucleus.

Figure 3.. In situ sequencing identifies transcriptomic types of neurons infected with barcoded rabies virus.

(A) Illustration of probe designs and amplification approach for in situ sequencing of rabies barcodes. (B) Illustration of the experiments. The two libraries were injected in VISal and RSP/SC, and coronal sections from the boxed area were sequenced. Neurons that project to the two injection sites would be labeled with different sets of barcodes, as indicated by different colors. (C) Left, example image of a coronal section (outlined by a dashed line) during the hybridization cycle. The final sequencing cycles for genes and barcodes in the boxed area are shown on the right. The gene or nucleotide that corresponds to each color is indicated. Scale bars = 50 μm. See Table S3 for a list of genes interrogated. (D)(E) All sequenced cells shown on a UMAP plot of their gene expression patterns (D) or on a representative coronal section (E). Colors indicate subclass-level cluster labels as shown in the legend. (F) Cluster matching between BARseq subclass-level clusters and subclasses from scRNA-seq (Tasic et al. 2018). Dot size and colors indicate the frequency that neurons from a BARseq cluster are assigned to a reference scRNA-seq cluster. (G) The number of cells (y axis) with the indicated number of barcodes per cell (x axis). (H) The count of the primary barcode (x axis) and the count of the second most abundant barcode (y axis) in each barcoded cell. Cells above the dotted line are considered as having more than one barcode per cell. (I) Counts of the most dominant barcode (x axis) and the remaining barcodes (y axis) in each cell. (J) Summary of the number of cells with more than one barcodes (blue) and/or cells in which the primary barcodes accounted for less than half of all barcode reads (red). The shapes are not drawn to scale. (K) UMAP plot as plotted in (D), color coded by whether the cells had barcodes. (L)(M) Histograms showing the distribution of endogenous mRNA reads per cell and unique gene counts per cell in all cells (L) or the QC-filtered cells (M). In (L), the dotted vertical lines indicate QC thresholds. BC, Barcoded cells; VISal, anterolateral visual cortex; RSP, retrosplenial cortex; SC, superior colliculus.

Figure 4.. Multiplexed retrograde labeling recapitulates known cortical projections.

(A) Histograms showing the distribution of the minimum Hamming distance between each barcode and all other barcodes for barcodes in the VISal library (blue), the SC/RSP library (red), and random barcodes (dashed). (B) The distribution of mismatch between each sequenced barcode and the closest barcode in the CCS library (y axis) or in the non-CCS library (x axis). Colors indicate which library each barcode was mapped to. Gray cells did not match to either library, likely because the libraries were not sequenced to completion. (C) The frequencies of barcodes in the libraries (x axis) are plotted against the number of sequenced cells carrying those barcodes (y axis). Jitter is added on the y-axis to help visualize overlapping dots. (D) Two representative slices, one from each brain showing all sequenced cells on each slice. Barcoded cells are color-coded by projections and non-barcoded cells are color-coded by brain regions. (E)(F) The number of neurons from each cortical area and each subclass that projected to either VISal (E) or SC/RSP (F). Dot size and colors indicate the number of cells. (G) The distribution of the number of projecting cells (y axis) for each subclass (x axis). (H)-(J) The fractions of the indicated L4/5 IT types in each cortical area (H) and the fraction of those that project to SC/RSP (I) or VISal (J). (K)(L) Fractions of variance in the probability of projections explained by combinations of the compositional profiles of cell types (S: subclass, T: type, -: cell type was not used) with or without considering cortical areas (Area considered) for L4/5 IT neurons (K) or for all excitatory neurons (L). *p =6×10⁻⁷⁰, **p =6×10⁻⁵⁸, ***p =7×10^–78, ****p =6×10^–37 comparing the means to 100 iterations of shuffled controls using two-tailed t-tests.

Figure 5.. Multiplexed transsynaptic labeling by sequencing rabies barcodes in situ.

(A) Five possible types of barcode-sharing networks in a barcoded transsynaptic tracing experiment using rabies virus. Whether each network is compatible with monosynaptic tracing and/or mapping synaptic convergence is indicated below. *see text for considerations regarding connected-source networks. (B) Summary of the transsynaptic labeling experiment using barcoded rabies virus and in situ sequencing. (C) The image of a representative coronal section during sequencing. Images of the first gene sequencing cycle, the hybridization cycle, and the first barcode sequencing cycle of the boxed area are shown on the right. Scale bars = 50 μm. (D) First nine barcode sequencing images of example neurons that were bisected onto two adjacent sections. Scale bars = 10 μm. (E)(F) The distribution of endogenous mRNA reads per cell (E) and unique gene counts per cell (F) in non-barcoded cells (gray), barcoded cells (blue), and source cells (red). Dashed lines indicate quality control thresholds for gene expression. (G)(H) UMAP plots of the gene expression patterns of all barcoded cells color-coded by the cluster label at the subclass level (G) or by whether the cell is a potential source cell or presynaptic cell (H). (I) Locations and cell types of two source cells (red cross) and presynaptic cells (dots) that shared the same barcodes. Colors of dots indicate transcriptomic types of presynaptic neurons. Transcriptomic types of source cells are indicated below each plot. All other cells from the coronal sections that the source cells were on were plotted in gray. (J) Estimated numbers of barcode/source cell combinations that belonged to each of the three networks with source cells. (K) The probability (y axis) of the number of independent infection events (x axis) to generate the same number of barcodes found in the source cells in the experiment. (L) Histogram showing the number of cells that shared each barcode that was not found in a source cell. (M) Histogram showing the distribution of the maximum barcode frequency in the virus library that ensures single infection for 95% of barcodes across 10,000 simulations. (N) The ratios between the observed number of converging outputs and the expected number from random connectivity between cortical subclasses of neurons. Colors correspond to log10 of ratios, and the ratios are indicated in the plot. Only values with false positive rate (FPR) < 0.05 are shown (see Methods M14). As seen from the blue squares associated with L6 IT cells, these neurons were less likely to synapse onto the same post-synaptic neurons with other neuronal types (in particular L6 CT neurons, with only 50% of converging connections compared to those expected from random connectivity).