16 17 ǂ Corresponding authors: Christos Samakovlis (christos.samakovlis@scilifelab.se), Alexandros 18 Sountoulidis (alexandros.sountoulidis@scilifelab.se) 19 20 Keywords: SCRINSHOT, padlock-probe, multiplex RNA FISH, spatial cell-type map, lung 21 22 2 23 Abstract 24 Changes in cell identities and positions underlie tissue development and disease progression. 25 Although, single-cell mRNA sequencing (scRNA-Seq) methods rapidly generate extensive lists of 26 cell-states, spatially resolved single-cell mapping presents a challenging task. We developed 27 SCRINSHOT (Single Cell Resolution IN Situ Hybridization On Tissues), a sensitive, multiplex 28 RNA mapping approach. Direct hybridization of padlock probes on mRNA is followed by 29 circularization with SplintR ligase and rolling circle amplification (RCA) of the hybridized padlock 30 probes. Sequential detection of RCA-products using fluorophore-labeled oligonucleotides profiles 31 thousands of cells in tissue sections. We evaluated SCRINSHOT specificity and sensitivity on 32 murine and human organs. SCRINSHOT quantification of marker gene expression shows high 33 correlation with published scRNA-Seq data over a broad range of gene expression levels. We 34 demonstrate the utility of SCRISHOT by mapping the locations of abundant and rare cell types 35 along the murine airways. The amenability, multiplexity and quantitative qualities of SCRINSHOT 36 facilitate single cell mRNA profiling of cell-state alterations in tissues under a variety of native and 37 experimental conditions. 38 39 40 41 42 43 44 45 65addressed by the sequential fluorescence in situ hybridization (seqFISH) (14, 15) and the 66 multiplexed error-robust FISH (MERFISH) (4). These approaches utilize sequential rounds of 67 hybridization of FISH probes or barcode-based primary probes to detect multiple RNA species.
68The outstanding throughput of these methods makes them strong candidates for generation of 69 spatial transcriptome maps in tissues. However, since the principle of these techniques is similar 70 to smFISH, they require large number of gene-specific probes, confocal or super-resolution 4 71 microscopy to deconvolve the signals and complicated algorithms for both probe design and 72 analysis. Nevertheless, the low signal-to-noise ratio still remains a major technical challenge of 73 these methods, especially for tissue sections with strong auto-fluorescence from structural 74 extracellular matrix components like collagen and elastin (16). New strategies for signal 75 amplifications, such as branched-DNA amplification (RNAScope) (17) and hybridization chain 76 reaction (18, 19), have been recently combined with sophisticated probe design (AmpFISH) (20) 77 to increase sensitivity and specificity of smFISH. 78 Padlock probes have been successfully used to detect RNA species (21). They are linear DNA 79 molecules, with complementary arms to the target mRNA sequence and a common "backbone". 80 Upon hybridization with the target sequence, they can be ligated, creating circular single-stranded 81...