Citrullination of cathepsin H promotes NETs formation and exacerbates IBD via PI3K-Akt pathway.

Peptidyl arginine deiminase 4 (PAD4) is previously known for its role in inflammatory bowel disease (IBD) through its facilitation of neutrophil extracellular traps (NETs) by citrullinating histones. However, the specific citrullinated substrates have not been fully elucidated, especially non-histones. By performing citrullination mapping, we unraveled the involvement of cathepsin H (CTSH), a novel substrate of PAD4, which undergoes citrullination at the R315 site. Then the citrullination of CTSH obviously altered its molecular conformation and subsequently reduced its enzymatic activity, which significantly boosted the formation of NETs. Consistently, CTSH knockout mice demonstrated exacerbated colonic inflammation and higher levels of NETs, which might be achieved by activating the phosphatidylinositol 3-kinase/protein kinase B (PI3K-Akt) signaling pathway. These findings underscore the critical role of CTSH in IBD pathogenesis and position it as a potential therapeutic target, highlighting the complex interplay among PAD4, CTSH, and IBD.