NLRC5 affects diet-induced adiposity in female mice and co-regulates peroxisome proliferator-activated receptor PPARγ target genes.

Nucleotide-binding and oligomerization domain containing 5 (NLRC5) is the key transcriptional regulator of major histocompatibility (MHC) class I genes. Recent observations suggest a role for NLRC5 in metabolic traits and in transcriptional regulation beyond MHC class I genes. To understand the function of NLRC5 in metabolic disease, we subjected Nlrc5 -/- mice to high-fat diet (HFD) feeding. Female Nlrc5 -/- mice presented with higher weight gain and more adipose tissue (AT) compared to wild-type (WT) animals. Mechanistically, we demonstrate that NLRC5 enhanced the expression of peroxisome proliferator-activated receptor (PPAR) γ target genes in human cells. We identify Sin3A and negative elongation factor (NELF) B as two novel NLRC5 interaction partners and show that Sin3A partly modulates the synergistic transcriptional effect of NLRC5 on PPARγ. Collectively, we show that NLRC5 contributes to weight gain in mice, which involves transcriptional enhancement of PPARγ targets by NLRC5 that is co-regulated by Sin3A.

Bacterial subversion of NLR-mediated immune responses.

Members of the mammalian Nod-like receptor (NLR) protein family are important intracellular sensors for bacteria. Bacteria have evolved under the pressure of detection by host immune sensing systems, leading to adaptive subversion strategies to dampen immune responses for their benefits. These include modification of microbe-associated molecular patterns (MAMPs), interception of innate immune pathways by secreted effector proteins and sophisticated instruction of anti-inflammatory adaptive immune responses. Here, we summarise our current understanding of subversion strategies used by bacterial pathogens to manipulate NLR-mediated responses, focusing on the well-studied members NOD1/2, and the inflammasome forming NLRs NLRC4, and NLRP3. We discuss how bacterial pathogens and their products activate these NLRs to promote inflammation and disease and the range of mechanisms used by bacterial pathogens to evade detection by NLRs and to block or dampen NLR activation to ultimately interfere with the generation of host immunity. Moreover, we discuss how bacteria utilise NLRs to facilitate immunotolerance and persistence in the host and outline how various mechanisms used to attenuate innate immune responses towards bacterial pathogens can also aid the host by reducing immunopathologies. Finally, we describe the therapeutic potential of harnessing immune subversion strategies used by bacteria to treat chronic inflammatory conditions.

14-3-3 and erlin proteins differentially interact with RIPK2 complexes.

The receptor interacting serine/threonine kinase 2 (RIPK2) is essential for signal transduction induced by the pattern recognition receptors NOD1 and NOD2 (referred to collectively as NOD1/2). Upon NOD1/2 activation, RIPK2 forms complexes in the cytoplasm of human cells. Here, we identified the molecular composition of these complexes. Infection with Shigella flexneri to activate NOD1-RIPK2 revealed that RIPK2 formed dynamic interactions with several cellular proteins, including A20 (also known as TNFAIP3), erlin-1, erlin-2 and 14-3-3. Whereas interaction of RIPK2 with 14-3-3 proteins was strongly reduced upon infection with Shigella, erlin-1 and erlin-2 (erlin-1/2) specifically bound to RIPK2 complexes. The interaction of these proteins with RIPK2 was validated using protein binding assays and immunofluorescence staining. Beside bacterial activation of NOD1/2, depletion of the E3 ubiquitin ligase XIAP and treatment with RIPK2 inhibitors also led to the formation of RIPK2 cytosolic complexes. Although erlin-1/2 were recruited to RIPK2 complexes following XIAP inhibition, these proteins did not associate with RIPK2 structures induced by RIPK2 inhibitors. While the specific recruitment of erlin-1/2 to RIPK2 suggests a role in innate immune signaling, the biological response regulated by the erlin-1/2-RIPK2 association remains to be determined.

DDX3X Links NLRP11 to the Regulation of Type I Interferon Responses and NLRP3 Inflammasome Activation.

Tight regulation of inflammatory cytokine and interferon (IFN) production in innate immunity is pivotal for optimal control of pathogens and avoidance of immunopathology. The human Nod-like receptor (NLR) NLRP11 has been shown to regulate type I IFN and pro-inflammatory cytokine responses. Here, we identified the ATP-dependent RNA helicase DDX3X as a novel binding partner of NLRP11, using co-immunoprecipitation and LC-MS/MS. DDX3X is known to enhance type I IFN responses and NLRP3 inflammasome activation. We demonstrate that NLRP11 can abolish IKKϵ-mediated phosphorylation of DDX3X, resulting in lower type I IFN induction upon viral infection. These effects were dependent on the LRR domain of NLRP11 that we mapped as the interaction domain for DDX3X. In addition, NLRP11 also suppressed NLRP3-mediated caspase-1 activation in an LRR domain-dependent manner, suggesting that NLRP11 might sequester DDX3X and prevent it from promoting NLRP3-induced inflammasome activation. Taken together, our data revealed DDX3X as a central target of NLRP11, which can mediate the effects of NLRP11 on type I IFN induction as well as NLRP3 inflammasome activation. This expands our knowledge of the molecular mechanisms underlying NLRP11 function in innate immunity and suggests that both NLRP11 and DDX3X might be promising targets for modulation of innate immune responses.

Role of NLRs in the Regulation of Type I Interferon Signaling, Host Defense and Tolerance to Inflammation.

Type I interferon signaling contributes to the development of innate and adaptive immune responses to either viruses, fungi, or bacteria. However, amplitude and timing of the interferon response is of utmost importance for preventing an underwhelming outcome, or tissue damage. While several pathogens evolved strategies for disturbing the quality of interferon signaling, there is growing evidence that this pathway can be regulated by several members of the Nod-like receptor (NLR) family, although the precise mechanism for most of these remains elusive. NLRs consist of a family of about 20 proteins in mammals, which are capable of sensing microbial products as well as endogenous signals related to tissue injury. Here we provide an overview of our current understanding of the function of those NLRs in type I interferon responses with a focus on viral infections. We discuss how NLR-mediated type I interferon regulation can influence the development of auto-immunity and the immune response to infection.

XIAP controls RIPK2 signaling by preventing its deposition in speck-like structures.

The receptor interacting serine/threonine kinase 2 (RIPK2) is essential for linking activation of the pattern recognition receptors NOD1 and NOD2 to cellular signaling events. Recently, it was shown that RIPK2 can form higher order molecular structures in vitro. Here, we demonstrate that RIPK2 forms detergent insoluble complexes in the cytosol of host cells upon infection with invasive enteropathogenic bacteria. Formation of these structures occurred after NF-κB activation and depended on the caspase activation and recruitment domain of NOD1 or NOD2. Complex formation upon activation required RIPK2 autophosphorylation at Y474 and was influenced by phosphorylation at S176. We found that the E3 ligase X-linked inhibitor of apoptosis (XIAP) counteracts complex formation of RIPK2, accordingly mutation of the XIAP ubiquitylation sites in RIPK2 enhanced complex formation. Taken together, our work reveals novel roles of XIAP in the regulation of RIPK2 and expands our knowledge on the function of RIPK2 posttranslational modifications in NOD1/2 signaling.

The NLR family pyrin domain-containing 11 protein contributes to the regulation of inflammatory signaling.

Mammalian Nod-like receptor (NLR) proteins contribute to the regulation and induction of innate and adaptive immunity in mammals, although the function of about half of the currently identified NLR proteins remains poorly characterized. Here we analyzed the function of the primate-specific NLRP11 gene product. We show that NLRP11 is highly expressed in immune cells, including myeloid cells, B cells, and some B cell lymphoma lines. Overexpression of NLRP11 in human cells did not trigger key innate immune signaling pathways, including NF-κB and type I interferon responses. NLRP11 harbors a pyrin domain, which is responsible for inflammasome formation in related NLR proteins. However, NLRP11 did not interact with the inflammasome adaptor protein ASC, and it did not trigger caspase-1 activation. By contrast, expression of NLRP11 specifically repressed NF-κB and type I interferon responses, two key innate immune pathways involved in inflammation. This effect was independent of the pyrin domain and ATPase activity of NLRP11. siRNA-mediated knockdown of NLRP11 in human myeloid THP1 cells validated these findings and revealed enhanced lipopolysaccharide and Sendai virus-induced cytokine and interferon responses, respectively, in cells with reduced NLRP11 expression. In summary, our work identifies a novel role of NLRP11 in the regulation of inflammatory responses in human cells.