Caspase 5 (CASP5) is a member of the inflammatory caspase family of cysteine proteases that is involved in inflammation and cell death1–3. CASP5 shares the highest homology with inflammatory CASP4, but whereas CASP4 is essential for noncanonical inflammasome activation, CASP5 is dispensable4–6, and its function remains unknown. Here we show that CASP5 is restricted to the human intestinal epithelium and manifests as three isoforms—CASP5A, CASP5B and CASP5C—among which CASP5C uniquely promotes Wnt signalling, which is essential for epithelial development and regeneration7. We identified dishevelled, which bridges Wnt receptors to the β-catenin destruction complex8, as a prominent CASP5 binding partner in colonic epithelial cells. Dishevelled interacts with the CASP5 catalytic domain through its DEP (dishevelled, EGL-10 and pleckstrin) domain. Lacking the inhibitory caspase activation and recruitment domain (CARD) of CASP5A and CASP5B, CASP5C cleaves the central scaffold protein APC at Asp556 in the Armadillo repeat domain, destabilizing the β-catenin destruction complex and thereby enhancing Wnt signalling. CASP5C expression peaks in transit-amplifying cells, the Wnt-reliant progeny of intestinal stem cells7, whereas CASP5A and CASP5B predominate in mature enterocytes. Endogenous and ectopic CASP5C drive growth of colonic and small intestinal organoids, which is known to require proliferation of transit-amplifying cells9. Furthermore, CASP5C is selectively induced upon intestinal epithelial injury, and its expression is increased in inflammatory bowel disease. Thus, CASP5C is an enzymatic amplifier of Wnt signalling that cleaves APC to sustain proliferation of transit-amplifying cells amid a declining Wnt gradient, safeguarding epithelial renewal. These findings broaden the roles of inflammatory caspases beyond innate immunity, uncovering their contribution to tissue homeostasis. CASP5 expression is restricted to the human intestinal epithelium, and the CASP5C isoform has a key role in promoting Wnt signalling, which is required for epithelial homeostasis, through binding to dishevelled and cleavage of APC to regulate β-catenin turnover.
Jia et al. (Wed,) studied this question.