Project Summary
Inflammasomes are mega-Dalton protein complexes that initiate inflammation responses and play important
roles in the innate immune system. Upon activation, inflammasomes recruit and activate the effector protein
caspase-1. Caspase-1 in turn cleaves the Gasdermin D (GSDMD) protein to release its N-terminal domain, which
inserts into the cell membrane to punch holes on the cell surface. As a result, the host cell will undergo pyroptotic
cell death (pyroptosis) and release the cell contents into the extracellular environment. Pro-inflammatory
cytokines IL-1¿ and IL-18, both activated by caspase-1, will also be released to activate the downstream
inflammation reactions.
NAIP is a family of cytosolic immunological receptors that activate the NAIP/NLRC4 inflammasomes in
response to Gram-negative bacterial infections. There are seven NAIP proteins in mouse, each sense a specific
ligand such as Flagellin, Needle protein or Inner Rod protein in the type III secretion system. In previous studies,
we have shown active NAIPs activate NLRC4 through the nucleated polymerization mechanism. However, it is
still largely unknown about how NAIPs remain inactive in the resting cells, and how is the ligand specificity is
determined among different NAIPs. Unlike in mouse, humans only have one NAIP and it was shown to be
activated by all three bacterial ligands. This leads to many questions including what is the structural basis of the
broad detection of bacterial ligands by human NAIP and what does it tell us about the human inflammasomal
bacterial detection mechanism? To address these questions, we propose to combine electron cryo-microscopy
(cryoEM) with biochemical and functional studies to elucidate the structural mechanisms underlying the signal
transduction by NAIP proteins. Dysregulation of NAIP/NLRC4 inflammasomes causes macrophage activation
syndrome (MAS) and autoinflammation, and mutations of NAIP protein are highly correlated with spinal muscular
atrophy (SMA). The successful execution of this work will broadly advance the development of effective therapies
to treat infectious diseases, autoinflammatory diseases, SMA, and cancer through targeting the inflammasome
pathway.
