Global transcriptomics analyses reveal specialized roles for splicing regulatory proteins in the macrophage innate immune response

Abstract

Pathogen sensing via pattern recognition receptors triggers massive reprogramming of macro-phage gene expression. While the signaling cascades and transcription factors that activate these responses are well-known, the role of post-transcriptional RNA processing in modulating innate immune gene expression remains understudied. Recent phosphoproteomics analyses revealed that members of the SR and hnRNP families of splicing regulatory proteins are dynamically post-translationally modified in infected macrophages. To begin to test if these splicing factors play a privileged role in controlling the innate immune transcriptome, we analyzed steady state gene expression and alternatively spliced isoform production in ten SR/hnRNP knockdown RAW 264.7 macrophage cell lines following infection with the bacterial pathogen Salmonella enterica serovar Typhimurium (Salmonella). We identified thousands of transcripts whose abundance was increased or decreased by SR/hnRNP knockdown in macrophages. We observed that different SR/hnRNPs control the expression of distinct gene regulons in uninfected and Salmonella-infected macrophages, with several key innate immune genes (Nos2, Mx1, Il1a) relying on multiple SR/hnRNPs to maintain proper induction and/or repression. Knockdown of SR/hnRNPs promoted differential isoform usage (DIU) for a number of key immune sensors and signaling molecules and many of these splicing changes were again, distinct in uninfected and Salmonella-infected macrophages. Finally, after observing a surprising degree of similarity between the DEGs and DIUs in hnRNP K and U knockdown macrophages, we found that these cells are better able to restrict vesicular stomatitis virus replication than control cells, supporting a role for these hnRNPs in controlling infection outcomes in macrophages ex vivo. Based on these findings, we conclude that many innate immune genes have evolved to rely on one or more splicing regulatory factors to ensure the proper timing and magnitude of their induction, bolstering a model wherein pre-mRNA splicing is a critical regulatory node in the innate immune response.