The Hippo kinases control inflammatory Hippo signaling and restrict bacterial infection in eukaryotic phagocytes

The Hippo kinases MST1 and MST2 initiate a highly conserved signaling cascade called the Hippo pathway that limits organ size and tumor formation in animals. Intriguingly, pathogens hijack this host pathway during infection, but the role of MST1/2 in innate immune cells against pathogens is unclear. In this study, we generated Mst1/2 knockout macrophages to investigate the regulatory activities of the Hippo kinases in immunity. Transcriptomic analyses identified differentially expressed genes (DEGs) that are enriched in biological pathways, such as systemic lupus erythematosus, tuberculosis, and apoptosis. Surprisingly, pharmacological inhibition of the downstream components LATS1/2 in the canonical Hippo pathway did not affect expression of a set of immune DEGs, suggesting that MST1/2 control these genes via alternative inflammatory Hippo signaling. Moreover, MST1/2 may affect immune communication by influencing the release of cytokines, such as TNF, CXCL10, and IL-1ra. Comparative analyses of the single- and double-knockout macrophages revealed that MST1 and MST2 differentially regulate TNF release and expression of the immune transcription factor, MAF, demonstrating that the two homologous Hippo kinases individually play a unique role in innate immunity. Notably, MST1 and MST2 are both required for macrophages to activate apoptosis. Lastly, we demonstrated that the Hippo kinases are critical factors in mammalian macrophages and single-cell amoebae to restrict infection by Legionella pneumophila, Escherichia coli, and Pseudomonas aeruginosa. Together, these results uncover non-canonical inflammatory Hippo signaling in macrophages and the evolutionarily conserved role of the Hippo kinases in anti-microbial defense of eukaryotic hosts.