Innate immune response is the first line of defense against infections and damages and is triggered by dedicated receptors activated by pathogen-associated molecular patterns (PAMPs) and/or damage-associated molecular patterns (DAMPs). Among them, NACHT, leucine-rich repeat (LRR), and pyrin domain (PYD) -containing 3 (NLRP3) assembled the inflammasome signaling complex that controls the secretion of IL-1β and IL-18 and cell death by pyroptosis. Dysregulation of NLRP3 inflammasome is involved in many diseases. The post-transcriptional modifications (PTMs) are essential for NLRP3 inflammasome activation, in particular deubiquitination of NLRP3 LRR by BRCC3. Our goal is to better understand the regulation mechanism of PTMs in NLRP3 LRR domain.
Using mass spectrometry, we identified 3 ubiquitinated lysines and 3 phosphorylated serines. The IL-1β and IL-18 secretion were significantly decreased in reconstituted human and mouse cell lines expressing NLRP3 mutants bearing substitutions of the ubiquitinated lysines, supporting that these sites are important for NLRP3 inflammasome activation. The impact of these ubiquitinations will need to be confirmed in primary cells and in vivo. Similarly, one of the phosphorylated serines is critical for inflammasome assembly in reconstituted cell lines, as well as in primary bone marrow derived macrophages from in-house generated knock-in mice bearing a phosphomimetic substitution of this serine. Mechanistically, S/D substitution leads to NLRP3 hyper-ubiquitination and suppresses NLRP3 inflammasome activity via impairing BRCC3 recruitment. Therefore, we discovered an additional checkpoint upstream of BRCC3 recruitment. Besides, we identified 4 kinases as candidates to target this serine and identified by siRNA screen phosphatases regulating the NLRP3 inflammasome pathway. In the future, targeting this phosphorylation could be used as a novel therapeutical target for anti-inflammatory drugs.
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