Editors' ChoiceCell Biology

Stressed Out About Crohn’s Disease

Sci. Signal.  04 Mar 2014:
Vol. 7, Issue 315, pp. ec66
DOI: 10.1126/scisignal.2005232

A single-nucleotide polymorphism (SNP) that is strongly associated with increased susceptibility to developing Crohn’s disease, a type of inflammatory bowel disease, is a Thr300→Ala (T300A) variant of the autophagy complex adaptor protein ATG16L1 (ATG16L1T300A). Murthy et al. identified a caspase cleavage site at residues 296 to 299 in human ATG16L1 and demonstrated that ATG16L1T300A was more sensitive than wild-type ATG16L1 (ATG16L1WT) to cleavage by caspase 3 in vitro and in cultured cells. Tumor necrosis factor–α (TNF-α) stimulates apoptosis by activating caspase 3, and cleavage of ATG16L1 was increased in TNF-α­–stimulated macrophages isolated from homozygous Atg16l1T300A human donors compared with those from homozygous Atg16l1WT human donors. These results were recapitulated in macrophages isolated from mice harboring a knock-in allele of the equivalent mutation (Atg16LlT316A). Mutating the caspase cleavage site abolished cleavage of ATG16L1WT, and there was no difference in the cleavage of wild-type ATG16L1 and the T300A and T316 variants of ATG16L1 in the absence of caspase 3. Nutrient starvation, which stimulates autophagy and activates apoptotic caspases, reduced the abundance of ATG16L1 and reduced autophagosome formation in Atg16l1T316A knock-in macrophages compared with wild-type macrophages. Autophagy is also important in the clearance of intracellular pathogens such as Yersinia enterolitica. Y. enterolitica infection stimulated caspase 3 activity in mouse macrophages, and the bacteria were cleared less effectively from human and mouse macrophages harboring the T300A and T316A variants, respectively. Deletion of Atg16l1 in immortalized mouse bone marrow progenitor cells impaired bacterial clearance, which was rescued by expression of a transgene encoding a form of ATG16L1 with a mutated caspase cleavage site, but not ATG16LT316A. Infected Atg16l1T316A knock-in macrophages secreted more TNF-α than wild-type macrophages, and oral delivery of Y. enterolitica caused intestinal inflammation, increased the abundance of inflammatory cytokines in the serum, and increased the abundance of transcripts encoding inflammatory cytokines in mesenteric lymph nodes. The authors propose that basal caspase 3 activity is responsible for the normal turnover of ATG16L1 and that increased caspase 3 activity in response to infection or apoptotic stress stimuli impairs autophagy and promotes the production of inflammatory cytokines. Because the T300A variant of ATG16L1 is more susceptible to caspase 3–dependent cleavage and degradation than wild-type ATG16L1, this Crohn’s-associated SNP increases the likelihood of developing a chronic inflammatory state after stress or infection. Although there are other pathologies associated with impaired or deregulated autophagy, ATG16L1T300A is associated only with Crohn’s disease, suggesting that this allele’s effect on autophagy is specific to intestinal cells. Commentary by Kaser and Blumberg puts these findings into the broader context of Crohn’s disease pathology.

A. Murthy, Y. Li, I. Peng, M. Reichelt, A. K. Katakam, R. Noubade, M. Roose-Girma, J. DeVoss, L. Diehl, R. R. Graham, M. van Lookeren Campagne, A Crohn's disease variant in Atg16l1 enhances its degradation by caspase 3. Nature 506, 456–462 (2014). [PubMed]

A. Kaser, R. S. Blumberg, Stressful genetics in Crohn's disease. Nature 506, 441–442 (2014). [PubMed]

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