Frizzled5/8 is required in secondary mesenchyme cells to initiate archenteron invagination during sea urchin development

Abstract

Wnt signaling pathways play key roles in numerous developmental processes both in vertebrates and invertebrates. Their signals are transduced by Frizzled proteins, the cognate receptors of the Wnt ligands. This study focuses on the role of a member of the Frizzled family, Fz5/8, during sea urchin embryogenesis. During development, Fz5/8 displays restricted expression, beginning at the 60-cell stage in the animal domain and then from mesenchyme blastula stage, in both the animal domain and a subset of secondary mesenchyme cells (SMCs). Loss-of-function analyses in whole embryos and chimeras reveal that Fz5/8 is not involved in the specification of the main embryonic territories. Rather, it appears to be required in SMCs for primary invagination of the archenteron, maintenance of endodermal marker expression and apical localization of Notch receptors in endodermal cells. Furthermore, among the three known Wnt pathways, Fz5/8 appears to signal via the planar cell polarity pathway. Taken together, the results suggest that Fz5/8 plays a crucial role specifically in SMCs to control primary invagination during sea urchin gastrulation.

Specific functions of the Wnt signaling system in gene regulatory networks throughout the early sea urchin embryo

“…Similarly, early expression of a fzd gene, fzd9/10, terminates after 15 h, and only one fzd gene, fzd5/8, is transcribed after 21 h in a subset of mesodermal cells located in the oral portion of the veg2 mesoderm. This finding is consistent with earlier results (22). No sfrp or dkk genes are expressed in Veg2 mesodermal cells after 15 h. Thus, with the exception of oral mesodermal cells, most mesodermal cells express neither wnt nor fzd genes from 18 h to gastrulation and are not likely to send or receive Wnt signaling.…”

A global view of gene expression in lithium and zinc treated sea urchin embryos: new components of gene regulatory networks

“…First, apical pole genes, which are those that are expressed at the animal most ectodermal region (such as Fz5/8 [15] and SpNK2.1 [30]), are eliminated. As shown in detail in Table 2, the expression ratios in lithium-treated embryos for newly discovered apical plate markers such as FoxQ2 , Zfhpf4 (Figure 6), and Dp-Hbn (WISH database [22]) are 0.15, 0.01, and 0.05, respectively, which correspond to 6-fold, 100-fold, and 20-fold downregulation, respectively (as determined by Q-PCR).…”

“…As a control, we measured Spec2a expression in zinc-treated embryos and find that it is about tenfold upregulated (Table 2). One transcription factor that is expressed in the aboral ectoderm but that is also expressed in other territories is the T-box gene Tbx2 / 3 [15,33]. This gene was found to be significantly downregulated (Table 2; namely, clone RUDIREA_28I11, which is downregulated by a factor of 0.20; P = 3.50 e-12 ) in lithium-treated embryos and is upregulated in zinc-treated embryos.…”

“…As result of the vegetalization, the endomesodermal domain is expanded at the expense of ectodermal territories. A recent study suggested that lithium chloride treatment induces an increase in endoderm at the expense of the ectoderm, but without alterating the mesodermal territories, because the expression domain of Frizzled5 / 8 at the animal pole is eliminated whereas its expression at the secondary mesenchyme cells (SMCs) is not affected [15]. Furthermore, recent evidence based on study of Nodal suggests that lithium chloride also intervenes with the oral-aboral axis of the embryo, because the region expressing the oral marker Nodal is reduced and shifted to the animal side [16], which is consistent with the conversion of part of the ectoderm to endoderm.…”

A homologue of snail is expressed transiently in subsets of mesenchyme cells in the sea urchin embryo and is down‐regulated in axis‐deficient embryos

“…SMC derivatives include pigment cells, which ingress at various times during gastrulation in different species (Gibson and Burke, 1985;Takata and Kominami, 2004), blastocoelar cells (Tamboline and Burke, 1992), esophageal muscle precursors (Ishimoda-Takagi et al, 1984;Wessel et al, 1990;Venuti et al, 1993), and cells expressing the 5-hydroxytryptamine receptor (Katow et al, 2004) At least some of the SMC derivatives that express Lv-snail mRNA and protein are likely to be pigment cells, because some Lv-snailexpressing cells are embedded in the ectoderm, and because in nickeltreated embryos, which largely lack pigment (Hardin et al, 1990;Takata and Kominami, 2004), Lv-snail is down-regulated. The small number of SMCs that express Lv-snail compared with other more general markers for SMCs, such as Frizzled5/8 (Croce et al, 2006) and most SMC markers identified using macroarray approaches (Calestani et al, 2003) suggests that Lv-snail is only expressed in a subset of SMCs. Based on our situ hybridization results, Lv-snail is detectable mainly in SMCs that ingress late in gastrulation, a population that has been suggested to be a reserve population of cells capable of "converting" to the skeletogenic fate (Ettensohn and .…”