Technical advances in cell biology have revolutionized the field of cell biology. With new technology it is now possible to address scientific questions in cell biology at the molecular level. Single-cell RNA-sequencing can reveal transcriptomic information for single cells and spatially resolved transcriptomic technology can visualize thousands or millions of cells and transcripts for spatial molecular profiling. The work in this thesis describes the technological development from traditional in situ hybridization to the current state-of-the-art technology for spatial multiplexed gene expression analysis. This development has enabled RNA-based molecular characterization of cells and tissues with the spatial dimension maintained. The work included in the thesis highlights the potential and the advantages of padlock-probe-based technology for spatial RNA-based profiling of cells and tissues. Furthermore, it demonstrates the possibilities arising from the inherent ability of padlock probes to distinguish between transcripts based on differences in single nucleotides.

The study in paper I investigates the prevalence of Enterovirus species B in patients with Crohn’s disease by a chromogenic in situ hybridization assay combined with immunohistochemistry to detect viral RNA and proteins directly in tissue samples.

In paper II, padlock probes were used to study the spatial gene expression of gene homologs from the X and Y chromosome in human embryonic nervous tissue. Furthermore, a strategy was devised to visualize and evaluate spatial expression patterns.

The padlock probe-based approach for multiplexed spatial transcriptional profiling, in situ sequencing, was applied in paper III to study the regional and cell-type-specific dynamics of A-to-I RNA editing in the developing mouse brain.

In paper IV, a technical characterization of padlock probes was performed with the aim of determining how to design a padlock probe to obtain optimal detection efficiency.

The work in this thesis demonstrates the dramatic shift in how biological questions in cell and tissue biology can be addressed, enabled by the technological evolution of traditional in situ hybridization assays into high-throughput, multiplexed spatial transcription profiling.