The activity and localization patterns of cathepsins B and X in cells of the mouse gastrointestinal tract differ along its length

Cysteine cathepsins are expressed in most tissues, including the gastrointestinal tract. We demonstrated an involvement of mouse intestinal cathepsin B in extracellular matrix remodeling for regeneration from trauma. The present study aimed at elucidating roles of cysteine cathepsins in the non-traumatized gastrointestinal tract of mice. Thus we investigated expression and localization patterns of cathepsin B and its closest relative, cathepsin X, along the length of the gastrointestinal tract, and determined the effects of their absence. Cathepsin B showed the highest protein levels in the anterior segments of the gastrointestinal tract, whereas the highest activity was observed in the jejunum, as revealed by cathepsin B-specific activity-based probe labeling. Cathepsin X was most abundant in the jejunum and protein levels were elevated in duodenum and colon of Ctsb(-/-) mice. The segmental pattern of cathepsin expression was reflected by a compartmentalized distribution of junction proteins and basal lamina constituents, changes in tissue architecture and altered activities of the brush border enzyme aminopeptidase N. In conclusion, we observed different compensatory effects and activity levels of cysteine peptidases along the length of the small and large intestines in a segment-specific manner suggesting specific in situ functions of these enzymes in particular parts of the gastrointestinal tract.