Heterotypic interactions among NACHT domains: implications for regulation of innate immune responses

Abstract

Proteins of the NACHT [NAIP (neuronal apoptosis inhibitory protein), CIITA (MHC class II transcription activator), HET-E (incompatibility locus protein from Podospora anserina) and TP1 (telomerase-associated protein)] family may serve as critical pathogen-sensing and signal-transducing molecules within the innate immune system. In the present paper, we show that CLAN [CARD (caspase-recruitment domain), LRR (leucine-rich repeat) and NACHT domain-containing protein], a NACHT-containing protein originally demonstrated to bind and activate pro-caspase 1, is also capable of influencing the functions of other members of the NACHT family. Through heterotypic NACHT-domain interactions, CLAN was found to associate with Nod1, Nod2 and NAC [nucleotide-binding domain and CARD-containing protein; NALP1 (NACHT, LRR and PYRIN protein 1)] when coexpressed in HEK-293T (human embryonic kidney) cells. NF-κB (nuclear factor κB) reporter assays demonstrated that co-expression of either full-length CLAN or the NACHT domain of CLAN significantly inhibited NF-κB activation induced by Nod1 or Nod2 overexpression. In addition, co-expression of CLAN or the NACHT domain of CLAN with Nod1 or Nod2 inhibited the ability of these proteins to generate active IL-1β (interleukin 1β) through their association with pro-caspase 1. The NACHT domain of CLAN was demonstrated by co-immunoprecipitation experiments to bind all NACHT domains that were tested, including the NACHT domains from CLAN itself, Nod1, Nod2, cryopyrin, NAC, PAN2 {PAAD [pyrin, AIM (absentin-melanoma), ASC (apoptosis-associated speck-like protein containing a CARD) and death-domain-like]-and NACHT-containing protein} and NAIP (neuronal apoptosis inhibitory protein). Finally, monocyte-expressed CLAN was found to associate with Nod2 following exposure to bacterial peptidoglycan, implying a regulatory role for interaction of these NACHT proteins in the innate immune response. These studies suggest that by mediating hetero-oligomerization, NACHT domains provide a means by which various NACHT-containing proteins may interact, creating protein-interaction networks that potentially modulate immune responses to invading pathogens.

Systematic expression profiling of innate immune genes defines a complex pattern of immunosenescence in peripheral and intestinal leukocytes

“…26 NALP4 directly associates with IKK-a and the NLR family member IPAF. 27 Although the exact ligand and signaling pathways of NALP4 have not been described, the data suggest that NALP4 may play a critical role in controlling inflammatory responses and Figure 3 mRNA expression of pattern-recognition receptors (PRRs) as well as their adapter molecules decreases with age in colonic biopsies. The results represent mRNA expression levels of 24 colonic biopsy samples (30 to o50 years, n ¼ 5; 50 to o70 years, n ¼ 9; 70 to o90 years, n ¼ 10).…”

Synergistic enhancement of Toll-like receptor responses by NOD1 activation

“…Recent data suggests that activation of the homologous NOD2 protein leads to ubiquitinylation of NEMO, a key component of the NF-jB signalling complex [25]. However, NOD proteins have also been shown to interact with other proteins including TAK [26], CLAN [27], caspase-1 [11] and stimulate the NALP3/Cryopyrin inflammasome [28]. Our current understanding of the (perhaps multiple) signalling mechanisms of NOD1 is limited and further research is necessary, in particular the response to different ligand concentrations and TLR co-activation should be an aim of future studies.…”

Pyrin-Only Protein 2 Modulates NF-κB and Disrupts ASC:CLR Interactions

“…CLR activation of NF-B appears to correlate with an ability to interact with ASC and speck formation. Conversely, the PYD of PAN2/PYPAF4 that inhibits NF-B fails to form a speck in the presence of ASC (4,22,35). In contrast, POP1 interacts with ASC, but inhibits NF-B (27).…”