Figure 2.

The Wnt signaling pathway regulated by SFRPs. In the inactive state of Wnt, SFRPs sequestered Wnt proteins via either CRD or NTR domain and prevent them from binding to the frizzled receptor. SFRPs can also bind to Frizzled receptor via CRD domain to block Wnt signal transduction (left). In the active state of Wnt, SFRPs form inactive complexes by their own and Wnt proteins directly bind to Frizzled receptor; or SFRPs form complex with Wnts and interact with Frizzled receptor to promote Wnt signal transduction (right). The Wnt proteins then activate at least three different downstream signaling pathways: the canonical Wnt-β-catenin, the non-canonical planar cell polarity (PCP) and the Wnt-Ca²⁺ pathways.