Essential requirement for Wnt signaling in proliferation of adult small intestine and colon revealed by adenoviral expression of Dickkopf-1

Abstract

Whereas the adult gastrointestinal epithelium undergoes tremendous self-renewal through active proliferation in crypt stem cell compartments, the responsible growth factors regulating this continuous proliferation have not been defined. The exploration of physiologic functions of Wnt proteins in adult organisms has been hampered by functional redundancy and the necessity for conditional inactivation strategies. Dickkopf-1 (Dkk1) is a potent secreted Wnt antagonist that interacts with Wnt coreceptors of the LRP family. To address the contribution of Wnt signaling to gastrointestinal epithelial proliferation, adenoviral expression of Dkk1 was used to achieve stringent, conditional, and reversible Wnt inhibition in adult animals. Adenovirus Dkk1 (Ad Dkk1) treatment of adult mice repressed expression of the Wnt target genes CD44 and EphB2 within 2 days in both small intestine and colon, indicating an extremely broad role for Wnt signaling in the maintenance of adult gastrointestinal gene expression. In parallel, Ad Dkk1 markedly inhibited proliferation in small intestine and colon, accompanied by progressive architectural degeneration with the loss of crypts, villi, and glandular structure by 7 days. Whereas decreased Dkk1 expression at later time points (>10 days) was followed by crypt and villus regeneration, which was consistent with a reversible process, substantial mortality ensued from colitis and systemic infection. These results indicate the efficacy of systemic expression of secreted Wnt antagonists as a general strategy for conditional inactivation of Wnt signaling in adult organisms and illustrate a striking reliance on a single growth factor pathway for the maintenance of the architecture of the adult small intestine and colon.

Wnt signaling in disease and in development

“…In the colon, loss of TCF4 or Dkk over-expression promotes loss of stem cells in the colon crypt, indicating that Wnt signaling is required for maintenance of the stem cell compartment [40][41][42]. Wnt signaling may therefore be a fundamental regulator of stem cell choices to proliferate or self-renew.…”

Reg I-knockout mice reveal its role in regulation of cell growth that is required in generation and maintenance of the villous structure of small intestine

“…The residual growth activity observed in the Reg I-knockout mice could be mediated by another family of growth-promoting factors. One such candidate family is the Wnt family (Pinto et al, 2003;Kuhnert et al, 2004;Walters, 2005). To date, the only information available on secreted factors that enhance proliferation of intestinal cells pertains to the Wnts (Pinto et al, 2003;Kuhnert et al, 2004;Walters, 2005).…”

“…One such candidate family is the Wnt family (Pinto et al, 2003;Kuhnert et al, 2004;Walters, 2005). To date, the only information available on secreted factors that enhance proliferation of intestinal cells pertains to the Wnts (Pinto et al, 2003;Kuhnert et al, 2004;Walters, 2005). Therefore, our identification of Reg as an additional class of growth stimulators is of considerable importance.…”

Beyond tumorigenesis: cancer stem cells in metastasis

“…These signals exist as a delicate "Yin & Yang" balance between positive and negative regulators. Using genetic models, key factors of the Wnt signaling pathway have been shown to be essential for promoting stem cell self-renewal in various systems [14][15][16][17][18][19][20]. Notch and hedgehog signaling pathways have also been demonstrated to play crucial regulatory roles for self-renewal regulation of HSCs [21][22][23][24].…”