Gut microbial DL-endopeptidase alleviates Crohn's disease via the NOD2 pathway

The pattern-recognition receptor NOD2 senses bacterial muropeptides to regulate host immunity and maintain homeostasis. Loss-of-function mutations in NOD2 are associated with Crohn's disease (CD), but how the variations in microbial factors influence NOD2 signaling and host pathology is elusive. We demonstrate that the Firmicutes peptidoglycan remodeling enzyme, DL-endopeptidase, increased the NOD2 ligand level in the gut and impacted colitis outcomes. Metagenomic analyses of global cohorts (n = 857) revealed that DL-endopeptidase gene abundance decreased globally in CD patients and negatively correlated with colitis. Fecal microbiota from CD patients with low DL-endopeptidase activity predisposed mice to colitis. Administering DL-endopeptidase, but not an active site mutant, alleviated colitis via the NOD2 pathway. Therapeutically restoring NOD2 ligands with a DL-endopeptidase-producing Lactobacillus salivarius strain or mifamurtide, a clinical analog of muramyl dipeptide, exerted potent anti-colitis effects. Our study suggests that the depletion of DL-endopeptidase contributes to CD pathogenesis through NOD2 signaling, providing a therapeutically modifiable target.

DL-endopeptidases function as both cell wall hydrolases and poly-γ-glutamic acid hydrolases

Biopolymers on the cell surface are very important for protecting microorganisms from environmental stresses, as well as storing nutrients and minerals. Synthesis of biopolymers is well studied, while studies on the modification and degradation processes of biopolymers are limited. One of these biopolymers, poly-γ-glutamic acid (γ-PGA), is produced by Bacillus species. Bacillus subtilis PgdS, possessing three NlpC/P60 domains, hydrolyses γ-PGA. Here, we have demonstrated that several dl-endopeptidases with an NlpC/P60 domain (LytE, LytF, CwlS, CwlO, and CwlT) in B. subtilis digest not only an amide bond of d-γ-glutamyl-diaminopimelic acid in peptidoglycans but also linkages of γ-PGA produced by B. subtilis. The hydrolase activity of dl-endopeptidases towards γ-PGA was inhibited by IseA, which also inhibits their hydrolase activity towards peptidoglycans, while the hydrolysis of PgdS towards γ-PGA was not inhibited. PgdS hydrolysed only the d-/l-Glu‒d-Glu linkages of d-Glu-rich γ-PGA (d-Glu:l-Glu=7 : 3) and l-Glu-rich γ-PGA (d-Glu:l-Glu=1 : 9), indicating that PgdS can hydrolyse only restricted substrates. On the other hand, the dl-endopeptidases in B. subtilis cleaved d-/l-Glu‒d-/l-Glu linkages of d-Glu-rich γ-PGA (d-Glu:l-Glu=7 : 3), indicating that these enzymes show different substrate specificities. Thus, the dl-endopeptidases digest γ-PGA more flexibly than PgdS, even though they are annotated as "dl-endopeptidase, digesting the d-γ-glutamyl-diaminopimelic acid linkage (d‒l amino acid bond)".