Studies of intestinal morphology and cathepsin B expression in a transgenic mouse aiming at intestine-specific expression of Cath B-EGFP

Upregulation of gut cathepsin L during Eimeria tenella infection

Coccidiosis is a disease caused by Eimeria, which represents the first parasitic disease in poultry farming. Among them, E. tenella is a virulent species which specifically colonizes the caecum. The inflammatory response to infection is associated to numerous host proteases including cysteine cathepsins that can be deleterious for tissue and innate immunity integrity. Here, germ-free and conventional chickens were used as models to find out whether the microbiota could modify the intestinal expression of host cysteine cathepsins during coccidiosis. The basal caecal peptidase activity primarily relies on host proteases rather than proteases from the commensal flora. While mRNA levels of E. tenella cathepsins B and L remained unchanged in germ-free and conventional broilers, an overall increase in endopeptidase activity of cysteine cathepsins was found in E. tenella-infected caeca in both experimental models (P < 0.005). A significant decrease in avian cystatin C transcription was also observed in infected conventional, but not in infected germ-free broilers. Despite an unchanged mRNA level of avian cathepsin L (CatL), its protein expression raised following infection, in parallel with an increased transcription of antimicrobial β-defensins (AvBD1, AvBD2, AvBD4, AvBD6, and AvBD7). Taken together, data support that host CatL is post-translationally upregulated during E. tenella infection, and thus may be involved in the alteration of the gut proteolytic balance. Furthermore, CatL may participate to inflammation occurring during coccidiosis through its known ability to proteolytically inactivates up-regulated avian β-defensins that are key molecules of innate immunity.

The Unusual Resistance of Avian Defensin AvBD7 to Proteolytic Enzymes Preserves Its Antibacterial Activity

Defensins are frontline peptides of mucosal immunity in the animal kingdom, including birds. Their resistance to proteolysis and their ensuing ability to maintain antimicrobial potential remains questionable and was therefore investigated. We have shown by bottom-up mass spectrometry analysis of protein extracts that both avian beta-defensins AvBD2 and AvBD7 were ubiquitously distributed along the chicken gut. Cathepsin B was found by immunoblotting in jejunum, ileum, caecum, and caecal tonsils, while cathepsins K, L, and S were merely identified in caecal tonsils. Hydrolysis product of AvBD2 and AvBD7 incubated with a panel of proteases was analysed by RP-HPLC, mass spectrometry and antimicrobial assays. AvBD2 and AvBD7 were resistant to serine proteases and to cathepsins D and H. Conversely cysteine cathepsins B, K, L, and S degraded AvBD2 and abolished its antibacterial activity. Only cathepsin K cleaved AvBD7 and released Ile4-AvBD7, a N-terminal truncated natural peptidoform of AvBD7 that displayed antibacterial activity. Besides the 3-stranded antiparallel beta-sheet typical of beta-defensins, structural analysis of AvBD7 by two-dimensional NMR spectroscopy highlighted the restricted accessibility of the C-terminus embedded by the N-terminal region and gave a formal evidence of a salt bridge (Asp9-Arg12) that could account for proteolysis resistance. The differential susceptibility of avian defensins to proteolysis opens intriguing questions about a distinctive role in the mucosal immunity against pathogen invasion.

Effects of cathepsin K deficiency on intercellular junction proteins, luminal mucus layers, and extracellular matrix constituents in the mouse colon

Cathepsin K has been shown to exhibit antimicrobial and anti-inflammatory activities in the mouse colon. To further elucidate its role, we used Ctsk-/- mice and demonstrated that the absence of cathepsin K was accompanied by elevated protein levels of related cysteine cathepsins (cathepsins B, L, and X) in the colon. In principle, such changes could result in altered subcellular localization; however, the trafficking of cysteine cathepsins was not affected in the colon of Ctsk-/- mice. However, cathepsin K deficiency affected the extracellular matrix constituents, as higher amounts of collagen IV and laminin were observed. Moreover, the localization pattern of the intercellular junction proteins E-cadherin and occludin was altered in the colon of Ctsk-/- mice, suggesting potential impairment of the barrier function. Thus, we used an ex vivo method for assessing the mucus layers and showed that the absence of cathepsin K had no influence on mucus organization and growth. The data of this study support the notion that cathepsin K contributes to intestinal homeostasis and tissue architecture, but the lack of cathepsin K activity is not expected to affect the mucus-depending barrier functions of the mouse colon. These results are important with regard to oral administration of cathepsin K inhibitors that are currently under investigation in clinical trials.