Involvement of Tsukushi in chick early embryogenesis

Wnt信号调控中枢神经系统发育的多个方面，包括细胞增殖和细胞命运选择。在鸡视网膜中，Wnt2b在视杯期的外胚层和视网膜色素上皮中均有表达。在后期，Wnt2b在胚胎鸡视网膜的边缘几乎顶端表达。通过视网膜再聚集培养的克隆试验和过表达研究，研究人员提出Wnt2b在视网膜祖细胞的维持和增殖中发挥作用，但没有细胞分化（Kubo等，2005）。我们已经鉴定出一种BMP拮抗剂Tsukushi (TSK)，它是一种可溶性分子，含有12个富含亮氨酸的重复序列，属于富含亮氨酸的小蛋白多糖家族。TSK在鸡原肠胚期的原始条纹和Hensen's结中表达，并参与其形成（Ohta, et al., 2004; Ohta, et al., 2006）。TSK还通过调节神经外胚层和非神经外胚层交界处的BMP和Delta-1活性，参与爪蟾胚胎神经嵴的形成（Kuriyama等，2006）。当Wnt2b在10期过表达到鸡视神经囊泡时，Wnt2b诱导视网膜祖细胞在体外延长增殖。然而，当Wnt2b与TSK共过表达时，Wnt2b的增殖活性几乎被抑制。我们现在发现，在鸡视网膜纤毛边缘区（CMZ）表达的TSK可以作为Wnt信号抑制剂抑制视网膜祖细胞的增殖。

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.