Splicing Factor SON Mediates Macrophage Susceptibility to Intracellular Pathogens

Abstract

We sought to identify host genes that contribute to macrophage death during infection with intracellular pathogens. Screening a custom-designed RNAi library for constructs that protect THP1 macrophages from killing by F. tularensis SchuS4 identified SON as a candidate susceptibility factor. SON knockdown also protected human primary macrophages from killing by F. tularensis LVS, B. anthracis and Y. pestis. SON encodes a regulator of transcription and splicing. To determine how SON expression contributes to killing of infected macrophages, we performed comparative proteomic analysis of normal and SON-knockdown macrophages, investigated expression of potential intermediates by qPCR, and tested the ability of specific inhibitors to mimic the effect of SON knockdown. SON expression is necessary for induction of numerous host-defense genes following infection with F. tularensis, or stimulation with LPS or IFNγ. This may partly be explained by reduced STAT family expression following SON knockdown. Autophagy maker LC3 accumulates in SON knockdown cells, apparently due to frustrated autophagic flux. Other proteins involved in intracellular vesicle transport are aberrantly expressed following SON knockdown, including the Golgi maintenance protein GBF1. Chemical inhibition of GBF1 with Golgicide A or Brefeldin A replicates many aspects of the SON knockdown phenotype, including LC3 accumulation, repression of STAT family members, host-defense gene induction, and, critically, improved survival following F. tularensis infection. We therefore propose that, by maintaining Golgi function, SON expression promotes death of infected macrophages. Funded by DTRA.