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

Abhinit Nagar,Tabassum Rahman,Jonathan A Harton

doi:10.3389/fimmu.2021.752482

Abstract

Although considered the ternary inflammasome structure, whether the singular, perinuclear NLRP3:ASC speck is synonymous with the NLRP3 inflammasome is unclear. Herein, we report that the NLRP3:ASC speck is not required for nigericin-induced inflammasome activation but facilitates and maximizes IL-1β processing. Furthermore, the NLRP3 agonists H2O2 and MSU elicited IL-1β 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-1β 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 NLRP3:ASC complexes which in turn activate caspase-1.

Highlights

Inflammasomes are intracellular, multiprotein complexes that assemble and activate caspase-1 following stimuli of microbial, host, and environmental origin [1, 2]
We considered that higher concentrations of nigericin might activate the inflammasome in the absence of the speck, but whether colchicine blockade of NLRP3:ASC speck assembly is overcome under these conditions is untested
IL-1b processing can occur in the absence of specks, and speck formation may not be essential for the NLRP3 inflammasome response

Summary

Introduction

Inflammasomes are intracellular, multiprotein complexes that assemble and activate caspase-1 following stimuli of microbial, host, and environmental origin [1, 2]. Most inflammasomes comprise members of the NLRP family of intracellular pathogen receptors that associate with the ASC adaptor protein to recruit caspase-1 [1, 3, 4]. ASC-dependent inflammasome activation is accompanied by rapid relocation of the NLR and ASC into a singular, perinuclear, punctate “speck” structure of approximately 1 μm [6, 7]. Speck formation is rapid causing a concomitant drop in cytosolic ASC concentration, approximately 200fold, to submicromolar concentrations within 100 s [2], and almost all available NLRP3 and ASC

Methods

Results

Conclusion