Tsukushi functions as a Wnt signaling inhibitor by competing with Wnt2b for binding to transmembrane protein Frizzled4

Abstract

The Wnt signaling pathway is essential for the development of diverse tissues during embryogenesis. Signal transduction is activated by the binding of Wnt proteins to the type I receptor lowdensity lipoprotein receptor-related protein 5/6 and the seven-pass transmembrane protein Frizzled (Fzd), which contains a Wntbinding site in the form of a cysteine-rich domain. Known extracellular antagonists of the Wnt signaling pathway can be subdivided into two broad classes depending on whether they bind primarily to Wnt or to low-density lipoprotein receptor-related protein 5/6. We show that the secreted protein Tsukushi (TSK) functions as a Wnt signaling inhibitor by binding directly to the cysteine-rich domain of Fzd4 with an affinity of 2.3 × 10 −10 M and competing with Wnt2b. In the developing chick eye, TSK is expressed in the ciliary/iris epithelium, whereas Wnt2b is expressed in the adjacent anterior rim of the optic vesicle, where it controls the differentiation of peripheral eye structures, such as the ciliary body and iris. TSK overexpression effectively antagonizes Wnt2b signaling in chicken embryonic retinal cells both in vivo and in vitro and represses Wnt-dependent specification of peripheral eye fates. Conversely, targeted inactivation of the TSK gene in mice causes expansion of the ciliary body and upregulation of Wnt2b and Fzd4 expression in the developing peripheral eye. Thus, we uncover a crucial role for TSK as a Wnt signaling inhibitor that regulates peripheral eye formation. eye development | signaling modulator | small leucine-rich proteoglycan

Small leucine rich proteoglycan family regulates multiple signalling pathways in neural development and maintenance

“…In TSK −/− mice, both the ventral and dorsal CB appeared larger than in wild type mice (Ohta et al . ). The canonical Wnt pathway regulates retinal stem cell proliferation (Kubo & Nakagawa ).…”

“…These observations indicate that TSK regulates neural stem cells as a niche molecule through its role as a Wnt signaling inhibitor (Ohta et al . , ).…”

“…; Ohta et al . ). The interacting proteins of the SLRP family and their regulated pathways are summarized in Table .…”

Tsukushi controls the hair cycle by regulating TGF-β1 signaling

“…TSK functions as an organizer inducer by inhibiting BMP signaling in cooperation with chordin (Ohta et al, 2004) and interacts with Vg1 to induce the primitive streak and Hensen's node formation in the chick embryo (Ohta et al, 2006). Recently, our group reported that TSK functions as a Wnt signaling inhibitor by competing with Wnt2b for binding to the transmembrane protein Frizzled4 (Ohta et al, 2011). Thus, TSK is a key coordinator of multiple pathways outside the cells through the regulation of an extracellular signaling network (Dellett et al, 2012).…”

RPE specification in the chick is mediated by surface ectoderm-derived BMP and Wnt signalling

“…Overexpression of Wnt2b induces Bmp7 expression and results in thinning of the NR (Ohta et al, 2011), whereas BMP application at ov stages induces Wnt2b and Wnt8b expression Teraoka et al, 2009). By contrast, NR specification requires suppression of Wnt8b in the mouse (Liu et al, 2010).…”