Role of Nod2 in the Response of Macrophages to Toll-Like Receptor Agonists

Abstract

Nod2 (CARD15) is a macrophage-specific protein containing two CARD domains, a large nucleotide binding domain and leucine-rich repeats. Human genetic studies have linked mutations in NOD2/CARD15 with Crohn's disease, although the mechanisms involved are unknown. However, Nod2 has been proposed to directly bind bacterial lipopolysaccharide (LPS) and subsequently act as an activator of NF-B via the association of the CARD domains with Rip2/RICK/CARDIAK. This is hypothesized to constitute a pathogen recognition pathway distinct from Toll-like receptor 4-mediated recognition of LPS. Using targeted mutagenesis, we introduced a mutation to delete the CARD domains of mouse Nod2. Mice lacking Nod2 were indistinguishable from controls and showed no signs of intestinal pathology. Macrophages responded normally to multiple Toll-like receptor agonists in terms of NF-B target activation, mitogen-activated protein kinase activation, and cytokine secretion. However, Nod2؊/؊ mice were significantly protected in endotoxin challenge experiments, and Nod2 ؊/؊ macrophages were refractory to muramyl dipeptide stimulation. These results argue that Nod2 does not play an essential, nonredundant role in the response of macrophages to bacterial products but rather plays unexpected roles in regulating systemic responses to pathogens.

Paths to understanding the genetic basis of autoimmune disease

“…This perhaps is not surprising given the complexity of the human disease; no single genetic or environmental factor is expected to be necessary or sufficient to cause the disease. These Nod2 -/-mice also did not show differential susceptibility to chemically induced gastrointestinal inflammation, but they did seem to be more susceptible to oral challenge with Gram-positive bacteria 48,49 . The results from knocking-in the human frameshift mutation into mice (Nod2 2939iC ), however, seem to suggest a very different mechanism: as opposed to the lack of response to MDP in the Nod2 -/-mice, the Nod2 2939iC mice have an elevated response to MDP and a heightened intestinal inflammatory response to chemical injury 50 .…”

Nucleotide-binding Oligomerization Domain Proteins Are Innate Immune Receptors for Internalized Streptococcus pneumoniae

“…A recently published paper describes macrophages from Nod2 knockout mice as responding normally to different pathogen-associated molecules, leading these authors to challenge the role of Nod2 in host defenses (30). In this study, cells were stimulated with known TLR ligands from the extracellular side, and no viable pathogens were used.…”

“…The experiments were performed twice with independent samples. able help to further elucidate the precise role of these proteins in host defense (12,13,30).…”

Inflammatory bowel disease: Genetic and epidemiologic considerations

“…As each of the three major mutations demonstrates decreased function in primary human cells [39,[44][45][46][47][48] , Nod2-deficient murine models provide an important model for human disease. The absence of intestinal inflammation in Nod2-deficient mice further highlights the insufficiency of this pathway alone to induce CD [42,53] . Since the discovery of the CD-association with Nod2, a number of studies have added to understanding of the Nod2 functional pathway.…”