A non-canonical role of Caspase-1 in regulating bacterial antimicrobial resistance

Summary
Antimicrobial resistance (AMR) is a prominent healthcare threat with an estimated 4.95 million cases associated
with bacterial AMR in 2019, and poses a particularly difficult challenge with regard to intracellular pathogens that
have evolved to hijack host defenses for their own benefit. A deep view of host-pathogen interactions including
a greater understanding of how pathogens adapt and change in response to host cues is critical to gain insights
into the factors responsible for AMR. While the response of hosts to bacterial infection has been extensively
studied, the converse i.e. how pathogen gene expression and in turn pathogen physiology is modulated in
response to host cues has gathered little attention. Lack of robust technologies to sequence minute amounts of
bacterial RNA from infected cells has been one of the limiting factors. Using a recently developed technology -
Path-Seq, we reliably sequenced macrophage-resident Salmonella to discover a role for host Caspase-1 in
dampening AMR of intracellular Salmonella. This was mediated through inhibition of the bacterial two-component
signal transduction system – PhoPQ, a major contributor responsible for Salmonella's ability to resist host
cationic antimicrobial peptides (CAMPs) and the drug Polymyxin B, which is a last resort antibiotic against Gram-
negative pathogens. Interestingly, Caspase-1 which is conventionally recognized as a protease important for
inflammasome activation and pyroptosis, dampens CAMP resistance of Salmonella in a manner independent of
its catalytic/protease activity. In this proposal we will thoroughly investigate the mechanism by which host
Caspase-1 inhibits PhoPQ activation and CAMP resistance of intracellular Salmonella independent of its activity.
In Aim 1 we will conduct a molecular dissection of the bacterial pathway and the pathogen effectors downstream
of PhoPQ activation that are targeted by Caspase-1. In Aim 2 we will investigate how host Caspase-1 inhibits
CAMP resistance of intracellular Salmonella, either by directly acting on the bacterium or indirectly by regulating
host processes that in turn control PhoPQ activation and CAMP resistance. Targeting this non-canonical,
pyroptosis independent arm of Caspase-1 may be particularly useful in curbing AMR of pathogens such as
Salmonella that evade inflammasome activation and replicate intracellularly. Our findings will reveal a novel
activity-independent role for Caspase-1 in controlling bacterial signaling and AMR, and because PhoPQ
activation dictates multiple aspects of pathogen physiology such as intracellular replication and virulence, will
also form a framework for exploring the impact of host Caspase-1 on processes beyond AMR in intracellular
Gram-negative pathogens.

Grant Number: 5R21AI178424-02
NIH Institute/Center: NIH
Principal Investigator: Ajay Akhade