Secreted frizzled-related proteins (sFRPS) function as modulators of Wnt signaling through direct interaction with Wnts []. They have a role in regulating cell growth and differentiation in specific cell types.SFRP1 is an antagonist of the Wnt-Frizzled pathway and is involved in regulating many processes, such as vascular cell proliferation [], heart morphogenesis [], bone trabecular formation []and lung morphogenesis [].
The cysteine-rich domain (CRD) is an essential part of the secreted frizzled-related protein 1 (SFRP1), which is an antagonist of the Wnt-Frizzled pathway and is involved in regulating many processes, such as vascular cell proliferation [], heart morphogenesis [], bone trabecular formation []and lung morphogenesis []. SFRP1 is expressed in many tissues and is involved in the regulation of Wnt signaling in osteoblasts, leading to enhanced trabecular bone formation in adults; it has also been shown to control the growth of retinal ganglion cell axons and the elongation of the antero-posterior axis [, ].In general, SFRPs antagonize the activation of Wnt signaling by binding to the CRD domains of frizzled (Fz) proteins, thereby preventing Wnt proteins from binding to these receptors. SFRPs are also known to have functions unrelated to Wnt, as enhancers of procollagen cleavage by the TLD proteinases []. SFRPs and Fz proteins both contain CRD domains, but SFRPs lack the seven-pass transmembrane domain which is an integral part of Fzs [, ].
