Involvement of Tsukushi in chick early embryogenesis

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Wnt signaling orchestrates multiple aspects of central nervous system development, including cell proliferation and cell fate choices. In chick retina, Wnt2b is expressed in both the surface ectoderm and the retinal pigmented epithelium at optic cup stage. At later stage, Wnt2b is expressed in the margin almost tip of the embryonic chick retina. Using a clonal assay in retinal re-aggregation cultures, along with overexpression studies, it was proposed that Wnt2b plays a role in the maintenance and the proliferation of retinal progenitor cells without cell differentiation (Kubo, et. al., 2005).We have identified a BMP antagonist, Tsukushi (TSK), which is a soluble molecule containing 12 leucine-rich repeats and belongs to the Small Leucine-Rich Proteoglycan family. TSK is expressed in the primitive streak and Hensen's node during chick gastrulation and involved in their formation (Ohta, et. al., 2004 ; Ohta, et. al., 2006). TSK is also involved in the neural crest specification of Xenopus embryo by regulating BMP and Delta-1 activities at the boundary between the neural and the non-neural ectoderm (Kuriyama, et. al., 2006).When Wnt2b was over-expressed into chick optic vesicle at stage 10, Wnt2b induced prolonged proliferation of retinal progenitor cells in vitro. However, when Wnt2b was co-over-expressed with TSK, the proliferative activity of Wnt2b was almost inhibited. We now show that TSK, which is expressed in the ciliary marginal zone (CMZ) of chick retina, inhibits the retinal progenitor cells proliferation by functioning as Wnt signaling inhibitor.