Growth factor-mediated mesodermal cell guidance and skeletogenesis during sea urchin gastrulation

Abstract

Growth factor signaling pathways provide essential cues to mesoderm cells during gastrulation in many metazoans. Recent studies have implicated the VEGF and FGF pathways in providing guidance and differentiation cues to primary mesenchyme cells (PMCs) during sea urchin gastrulation, although the relative contributions of these pathways and the cell behaviors they regulate are not fully understood. Here, we show that FGF and VEGF ligands are expressed in distinct domains in the embryonic ectoderm of Lytechinus variegatus. We find that PMC guidance is specifically disrupted in Lv-vegf3 morphants and these embryos fail to form skeletal elements. By contrast, PMC migration is unaffected in Lv-fgfa morphants, and well-patterned but shortened skeletal elements form. We use a VEGFR inhibitor, axitinib, to show that VEGF signaling is essential not only for the initial phase of PMC migration (subequatorial ring formation), but also for the second phase (migration towards the animal pole). VEGF signaling is not required, however, for PMC fusion. Inhibition of VEGF signaling after the completion of PMC migration causes significant defects in skeletogenesis, selectively blocking the elongation of skeletal rods that support the larval arms, but not rods that form in the dorsal region of the embryo. Nanostring nCounter analysis of ∼100 genes in the PMC gene regulatory network shows a decrease in the expression of many genes with proven or predicted roles in biomineralization in vegf3 morphants. Our studies lead to a better understanding of the roles played by growth factors in sea urchin gastrulation and skeletogenesis.

Neurogenesis in sea urchin embryos and the diversity of deuterostome neurogenic mechanisms

“…FGF and its role in ectodermal signaling, specifically to pattern mesoderm, have been examined in sea urchins, and there is evidence that FGF is expressed in ectodermal cells adjacent to postoral neural progenitors and the APD (Lapraz et al, 2006;Rottinger et al, 2008;Adomako and Ettensohn, 2013). In addition, FGF receptor 1 (FGFR1) is expressed in cells of the oral half of the embryo, including the APD, presumptive ciliary band, oral ectoderm, and what will become the aboral ectoderm adjacent to the ciliary band (Lapraz et al, 2006;Fig.…”

“…FGFA is expressed in restricted regions of the blastoderm (Rottinger et al, 2008) and FGFR2 is expressed in mesodermal lineages. To date, analysis of FGF function in sea urchin embryos has focused on roles in patterning mesoderm (Lapraz et al, 2006;Rottinger et al, 2008;Adomako and Ettensohn, 2013). However, a role for FGF in initiating neurogenesis in other metazoans suggests that this is a conserved function (Rentzsch et al, 2008;Sinigaglia et al, 2013).…”

microRNA-31 modulates skeletal patterning in the sea urchin embryos

“…Following ingression into the blastocoel, PMCs undergo reproducible patterning that is controlled by both PMC-derived factors and ectodermal positioning cues (Duloquin et al, 2007;Adomako-Ankomah and Ettensohn, 2013;McIntyre et al, 2013). The PMCs migrate along the sides of the blastocoel to form a subequatorial, ventrolateral ring, followed by the bilateral aggregation of PMCs to form the two ventrolateral clusters (VLCs), which are the sites of initial skeletogenic tri-radiate rudiments.…”

“…The PMCs migrate along the sides of the blastocoel to form a subequatorial, ventrolateral ring, followed by the bilateral aggregation of PMCs to form the two ventrolateral clusters (VLCs), which are the sites of initial skeletogenic tri-radiate rudiments. The VLCs are formed at the intersections between the border ectoderm (BE) and dorsal-ventral margin (DVM) domains (Duloquin et al, 2007;Adomako-Ankomah and Ettensohn, 2013;McIntyre et al, 2013). The BE is defined as a narrow boundary region between the endoderm and ectoderm and is established by short-range Wnt5 signaling from endodermal cells to the adjacent ectodermal cells (McIntyre et al, 2013).…”

“…The DVM is a region that separates the dorsal and ventral ectoderm and later becomes the ciliary band Saudemont et al, 2010;Yaguchi et al, 2010). Once the initial VLCs are established, the PMCs undergo another phase of directed migration anteriorly in response to Vegf and Fgf signaling pathways (Duloquin et al, 2007;Adomako-Ankomah and Ettensohn, 2013). Vegf3 is a key ligand expressed in the ectoderm that is crucial for proper PMC patterning.…”