The ASC Speck and NLRP3 Inflammasome Function Are Spatially and Temporally Distinct.

ABSTRACT

Although considered the ternary inflammasome structure, whether the singular, perinuclear NLRP3ASC speck is synonymous with the NLRP3 inflammasome is unclear. Herein, we report that the NLRP3ASC speck is not required for nigericin-induced inflammasome activation but facilitates and maximizes IL-1ß processing. Furthermore, the NLRP3 agonists H2O2 and MSU elicited IL-1ß maturation without inducing specks. Notably, caspase-1 activity is spatially distinct from the speck, occurring at multiple cytoplasmic sites. Additionally, caspase-1 activity negatively regulates speck frequency and speck size, while speck numbers and IL-1ß processing are negatively correlated, cyclical and can be uncoupled by NLRP3 mutations or inhibiting microtubule polymerization. Finally, when specks are present, caspase-1 is likely activated after leaving the speck structure. Thus, the speck is not the NLRP3 inflammasome itself, but is instead a dynamic structure which may amplify the NLRP3 response to weak stimuli by facilitating the formation and release of small NLRP3ASC complexes which in turn activate caspase-1.