# Probing the activity of cysteine cathepsins in inflammatory bowel diseases

## TL;DR

This study explores the role of cathepsin S in inflammatory bowel diseases and finds it contributes to some symptoms, but more effective inhibition methods are needed.

## Contribution

The study identifies cathepsin S as a potential therapeutic target in colitis, with insights into its role and limitations of current inhibition strategies.

## Key findings

- Fecal cathepsin S levels are significantly higher in ulcerative colitis patients compared to healthy controls.
- Pharmacologic inhibition of cathepsin S led to increased mucosal protease activity and worsened histological scores in mice.
- Cathepsin S deficiency in mice reduced rectal bleeding and splenomegaly but did not significantly affect other colitis indicators.

## Abstract

Cathepsin S is a cysteine protease that has been implicated in inflammatory bowel diseases (IBD) for its ability to promote visceral pain. Given its pro-inflammatory roles, we hypothesized that cathepsin S would drive other symptoms associated with IBD. Using activity-based probes, we investigated cysteine cathepsin activation in human and murine colitis. We observed a significant increase in fecal cathepsin S in patients with ulcerative colitis compared to healthy controls, while cathepsin S in mucosal biopsies was unchanged. Mice with experimental colitis exhibited a modest increase in mucosal activity of both cathepsin S and X compared to naïve mice. Luminal secretion of cathepsin S was dramatically increased upon colitis induction, although differences between mouse colonies were observed. To investigate the contribution of cathepsin S and cathepsin X to colitis, we induced colitis in cathepsin-deficient mice. Cathepsin X-deficient mice exhibited no clear differences in disease indicators compared to wild-type mice. While cathepsin S-deficient mice exhibited less rectal bleeding, less splenomegaly and marginally improved histological scores, weight loss, diarrhea, colon shortening, and myeloperoxidase activity were not significantly different from wild-type mice. To determine whether pharmacologic inhibition of cathepsin S activity would ameliorate symptoms of colitis, a reversible inhibitor LY3000328 was administered to mice at the initiation of colitis. LY3000328 provoked a clear upregulation of cathepsin S and L activity in the mucosa, most likely through a compensatory mechanism. This increase in protease activity was associated with exacerbated histological scores and slight splenomegaly. Collectively, these results suggest that cathepsin S, but not cathepsin X, may contribute to some of the symptoms of experimental colitis. While cathepsin S has potential to be a therapeutic target in colitis, improved strategies to sustain its inhibition are required in future.

The online version contains supplementary material available at 10.1038/s41598-025-32489-7.