Editors' ChoiceHost-Pathogen Interactions

Pathogen exploits Wnt receptors

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Sci. Signal.  25 Oct 2016:
Vol. 9, Issue 451, pp. ec247
DOI: 10.1126/scisignal.aal2552

Clostridium difficile is an opportunistic pathogen that causes diarrhea and colonic inflammation that can be life threatening. C. difficile toxin B (TcdB) is one of two exotoxins produced by this bacterium that enter colonic epithelial cells through receptor-mediated endocytosis and induce cell death. TcdB is sufficient to elicit physiological symptoms and mortality in animal models. Although TcdB can enter many different types of cells by binding to chondroitin sulfate proteoglycan 4 (CSPG4), this integral membrane protein is not present in colonic epithelia. Tao et al. found that TcdB-induced death of HeLa cells required FZD2, a member of the Frizzled family of Wnt receptors. TcdB also bound to FZD1 and FZD7 on HeLa cells, and cells lacking endogenous FZD1, -2, and -7 were resistant to TcdB-induced cell death. TcdB bound to the cysteine-rich domain (CRD) of FZD2, and TcdB binding involved a region of the exotoxin distinct from that used to interact with CSPG4. Pre-incubating TcdB with a soluble, recombinant form of the FZD2 CRD prevented TcdB from entering human colonic HT-29 and Caco-2 cells. Both full-length TcdB and a form of TcdB lacking the domain that mediates the interaction with CSPG4 reduced the survival of cells in colonic organoids generated from wild-type mice. Compared with colonic organoids from wild-type mice, organoids from FZD7–/– mice exhibited reduced TcdB-induced cell death, and knocking down FZD1 and FZD2 in these mutant organoids further reduced TcdB-induced cell death. TcdB competed with Wnts for binding to FZDs in organoids and inhibited expression of a Wnt-responsive reporter in 293T cells in a dose-dependent manner. Injecting TcdB into the colonic lumen of wild-type mice disrupted the colonic epithelium and caused colonic inflammation, but these pathologies were reduced in FZD7–/– mice. By identifying FZDs as physiologically relevant receptors that mediate the toxicity of TcdB, these findings not only suggest that targeting FZDs might be of therapeutic benefit for treating C. difficile infection, they also suggest that recombinant forms of TcdB might be used to inhibit aberrant Wnt signaling in the colon, which contributes to tumorigenesis.

L. Tao, J. Zhang, P. Meraner, A. Tovaglieri, X. Wu, R. Gerhard, X. Zhang, W. B. Stallcup, J. Miao, Xi He, J. G. Hurdle, D. T. Breault, A. L. Brass, M. Dong, Frizzled proteins are colonic epithelial receptors for C. difficile toxin B. Nature 538, 350–355 (2016). [PubMed]