NOD2 is one the most well known genes associated with Crohn’s disease (CD). Initial research regarding its role in immunology revealed that it’s an internal pattern-recognition receptor (PRR) that allows cells to detect invading bacteria and initiate pro-inflammatory cascades. This discovery led to some confusion as the null variation of the gene was associated with CD patients. Many asked the question, “how can the loss of something pro-inflammatory lead to more inflammation in the intestinal tract?” A Nod2-/- mouse was created, and, surprisingly, it did not develop spontaneous colitis, which stumped researchers even more. Although much research has been performed since the knock-out mouse was developed in 2005, there is still a lot to be learned.
Nod2-/- Mice Have Increased Intestinal Regulatory T Cells
Amedola et al attempted to find out why Nod2-/- mice did not get spontaneous colitis. They found that the main reason appeared to an increase of regulatory T cells (Tregs) with latent transforming growth factor-β (TGFβ). The population of these cells seemed to be dependent on the intestinal flora and the increased intestinal permeability of the Nod2-/- mice. Their ability to modulate colitis susceptibility was confirmed with transfer experiments. In a nutshell, their results showed that, in actuality, Nod2-/- mice were actually more resistant to some intestinal insults than their wild type counterparts, including TNBS colitis and ethanol-induced colitis.
Pellino3, a Downstream Mediator of Nod2
Underneath it all, Nod2 is a receptor like any other that initiates a signaling pathway. Studies under the “cellular” hood are also extremely useful for learning about how this PRR functions. Yang et al has now revealed that one mediator, Pellino3, is particularly important to Nod2 signaling. Pellino3 was revealed to be an ubiquitin ligase and performed its job on RIP2. Ubiquitination of RIP2 is needed for the activation of NF-κB and MAPKs via Nod2 stimulation. Loss of Pellino3 reduced the ability of cells to produce cytokines after Nod2 stimulation. Interestingly, loss of Pellino3 was also noted in CD patients, suggesting a loss of NOD2 signaling in patients. Pellino3-/- mice were also more susceptible to TNBS and Citrobacter colitis than wild types, which combined well with the human data. However, some of you may note that this TNBS result is different than what one would expect after reading Amedola article mentioned above.
CARD9, Not Just Another CARD in the Pack
Another name for NOD2 is CARD15. CARD stands for caspase recruitment domain-containing protein. As some of you might know, caspases are enzymes needed to process large proteins into more active, small ones. A good example of this is caspase 1, which cleaves the interleukin-1β protein into its active form. Sokol et al looked at CARD9, which functions as an adaptor protein downstream of PRRs that sense fungi. Using a Card9-/- mouse, they found that Card9 was needed to induce pro-inflammatory cytokines during intestinal trauma caused by DSS-induced colitis. Contrary to what one might first think, this actually led to a more severe colitis and a slower recovery. In particular, these mice had defective Th17 responses and loss of innate lymphoid cells. This was further confirmed in the Th17-restricted Citrobacter-colitis model, where the mice were unable to shake the infection. The authors refreshingly suggest that the loss of effective Th17 is actually not helpful and may actually be the reason for increased intestinal inflammation.
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Amendola, A., Butera, A., Sanchez, M., Strober, W., & Boirivant, M. (2013). Nod2 deficiency is associated with an increased mucosal immunoregulatory response to commensal microorganisms. Mucosal Immunology. doi:10.1038/mi.2013.58
Sokol, H., Conway, K. L., Zhang, M., Choi, M., Morin, B., Cao, Z., et al. (2013). Card9 Mediates Intestinal Epithelial Cell Restitution, T-Helper 17 Responses, and Control of Bacterial Infection in Mice. Gastroenterology. doi:10.1053/j.gastro.2013.05.047
Yang, S., Wang, B., Humphries, F., Jackson, R., Healy, M. E., Bergin, R., et al. (2013). Pellino3 ubiquitinates RIP2 and mediates Nod2-induced signaling and protective effects in colitis. Nature immunology, 14(9