Engagement of the macrophage integrated stress response in Salmonella infection

Engagement of the macrophage integrated stress response in Salmonella infection Project Summary Infection of mammalian host cells by bacterial pathogens can be sensed through perturbations of homeostatic cellular function. The integrated stress response (ISR) pathway is a central signaling module that transforms information about cellular perturbation into programmatic remodeling of many cellular processes, including protein translation, metabolism and inflammatory signaling. The ISR consists of four sensor kinases that can phosphorylate a common target, eukaryotic initiation factor-2α (eIF2α). Phosphorylated eIF2α suppresses bulk protein translation but allows preferential translation of key transcripts, notably the transcription factor, ATF4, a hallmark of the ISR. The mechanisms by which the ISR controls the inflammatory response to bacterial infection is poorly defined. Our preliminary data demonstrate that in murine macrophages the ISR kinase, GCN2, a sensor of amino acid depletion, triggers the ISR early during infection by the maladapted Salmonella enterica serovar Typhi (STy), but not by the closely related pathogen, S. Typhimurium (STm). GCN2 potentiated induction of pro-inflammatory cytokines and anti-microbial defenses in response to STy, but not STm. Addition of exogenous essential amino acids suppressed GCN2- dependent production of ATF4 by STy. Curiously, ATF4 is activated at time points with minimal bacterial replication, suggesting that depletion of host amino acids by bacterial competition may not mechanistically explain activation of GCN2. However, we found that only live STy triggered ATF4 production, pointing to the involvement of a bacterially-driven process. Together, these findings lead to our central hypothesis that STy infection actively triggers GCN2 kinase activation promoting development of macrophage effector mechanisms and inflammatory cytokines that control STy intracellular survival and replication. We will test this hypothesis with the following specific aims: (1) define STy-specific GCN2 activating signals and (2) determine the requirement for GCN2 and the ISR in modulating macrophage metabolism, inflammation and anti-microbial function. These studies will yield mechanistic insight into infection-induced triggers of the ISR and elucidate therapeutic opportunities to modulate inflammation and infection.