Engagement of NOD2 has a dual effect on proIL‐1β mRNA transcription and secretion of bioactive IL‐1β

Abstract

Synthesis and release of pro-inflammatory cytokines, such as IL-1b, play a crucial role in the intestinal inflammation that characterizes Crohn's disease. Mutations in the nucleotide oligomerization domain 2 (NOD2) gene are associated with an increased risk of Crohn's disease. Although it is known that NOD2 mediates cytokine responses to muramyl dipeptide (MDP), it is yet unclear whether NOD2 stimulation mediates only transcription of pro-IL-1b mRNA, or whether NOD2 is also involved in the activation of caspase-1 and release of active IL-1b. By investigating the response of MNC from Crohn's disease patients homozygous for the 3020insC NOD2 mutation, we were able to show that NOD2 signaling after stimulation with MDP has a dual effect by activating proIL-1b mRNA transcription and inducing release of bioactive IL-1b. Because NOD2 engagement amplifies TLR stimulation, we investigated whether activation of caspase-1 by MDP is involved in the NOD2/TLR synergism. The synergy in IL-1b production between NOD2 and TLR is mediated at post-translational level in a caspase-1-dependent manner, which indirectly suggests that NOD2 also induces caspase-1 activation. In contrast, the synergy in TNF-a production after stimulation with MDP and LPS is induced at transcriptional level. This demonstrates that both caspase-1-dependent and -independent mechanisms are involved in the synergy between NOD2 and TLR.Introduction NOD-like receptors (NLR) are intracellular receptors for bacterial peptidoglycans, which complement the recognition of pathogen-associated molecular patterns (PAMP) by membrane-bound TLR [1,2]. Nucleotide oligomerization domain 2 (NOD2) is a member of the NACHT-LRR (NLR) receptor family, which recognizes muramyl dipeptide (MDP), the minimal motif of peptidoglycan of both Gram-positive and Gram-negative bacteria [3]. Mutations in the NOD2 gene are associated with Crohn's disease [4,5], but how NOD2 exactly acts in the pathogenesis of this auto-inflammatory disease is unclear [6][7][8]. Therefore, a better understanding of the intracellular events induced by the interaction between NOD2 and peptidoglycan is crucial for both the insight into recognition of Gram-positive pathogens by the innate immune system, and for the pathogenesis of the inflammatory reactions in Crohn's disease. Activation of human mononuclear cells (MNC) by MDP leads to production of pro-inflammatory cytokines, especially IL-1b [7,9]. IL-1b is produced as pro-IL-1b a 31-34-kDa inactive form of the cytokine, which is later cleaved by caspase-1 to the bioactive 17-kDa . This is followed by IL-1b excretion in microvesicles into the extracellular environment [11]. Apparently, MDP is capable of inducing all three steps, but it is unclear whether NOD2 alone or other receptors are involved in one or more of these steps of IL-1b production. It has been proposed that several of the NLR family members are able to recognize MDP, most notably NOD2, NALP3, and NALP1, and that they execute different functions necessary for cytokine production. In this concept, ...

A NOD2–NALP1 complex mediates caspase-1-dependent IL-1β secretion in response to Bacillus anthracis infection and muramyl dipeptide

“…Our results strongly suggest that, although NOD2 can signal to NF-B (11), its more critical function in the context of an actual bacterial infection is activation of caspase-1. This conclusion is consistent with a recent report that demonstrates that NOD2 plays dual roles in pro-IL-1␤ synthesis and caspase-1-dependent IL-1␤ maturation in human monocytes (30). Indeed, the absence of NOD2 prevents B. anthracis-induced IL-1␤ secretion but has little effect on pro-IL-1␤ synthesis in infected macrophages.…”

ATP is released by monocytes stimulated with pathogen-sensing receptor ligands and induces IL-1β and IL-18 secretion in an autocrine way

“…In agreement with a previous study (42), the simultaneous stimulation of different PRRs provides addictive effects, thus allowing the innate immune system to modulate the intensity of its response according to characteristics of the inflammatory trigger (virulence and amounts of the pathogens, variety, and relative abundance of DAMPs). The entire set of PRRs is likely redundant, with extensive cross-talk and possibly positive or negative feedback controls (3).…”

Cloning of two rainbow trout nucleotide-binding oligomerization domain containing 2 (NOD2) splice variants and functional characterization of the NOD2 effector domains

“…However, mammalian NOD2 was also shown to induce caspase-1 activation [44,45] but the present study found no effect of trout NOD2 on caspase-1 expression. Whilst further study is needed to determine whether fish NOD2 could activate caspase-1 at the post-translational level, it should also be noted that the role of caspase-1 in the fish inflammasome is less clear with no obvious ICE cute site present in IL-1b for example [28].…”