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-2a (eIF2a). Phosphorylated eIF2a 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.