Structure of zebrafish NLRP3 reveals a novel mode of inflammasome activation

NLRP3 is an innate immune sensor of a broad range of stimuli, which upon activation forms a multiprotein inflammasome complex triggering caspase-1 activation, IL-1{beta} and IL-18 maturation, and inflammatory cell death. The canonical NLRP3 activation pathway has been well characterized from a structural perspective. It involves the association of NLRP3 with membranes in the form of inactive oligomeric "cage" complexes, which, upon activation, convert to an active oligomeric NLRP3 disc. NLRP3 structural rearrangements during non-classical NLRP3 activation pathways, however, remain unknown. Here, we report a novel mode of NLRP3 activation utilized by the NLRP3 homolog from zebrafish. The cryo-EM structure of zebrafish NLRP3 shows that, unlike human NLRP3, it forms disc-shaped heptamers that undergo further trimerization, resulting in a 21-mer oligomeric arrangement. Surprisingly, a single zebrafish NLRP3 heptamer cannot arrange its PYD domains into a PYD helix and therefore requires a trimer of heptamers to form a PYD filament that enables ASC oligomerization. Furthermore, zebrafish NLRP3 does not associate with the Golgi network, nor does it form inactive "cage" oligomers or interact with NEK7. Thus, our data demonstrate an ancestral non-canonical structural mechanism of NLRP3 activation, which may shed light on alternative NLRP3 activation pathways present in humans.