A novel gene’s role in an ancient mechanism: secreted Frizzled-related protein 1 is a critical component in the anterior–posterior Wnt signaling network that governs the establishment of the anterior neuroectoderm in sea urchin embryos

Abstract

The anterior neuroectoderm (ANE) in many deuterostome embryos (echinoderms, hemichordates, urochordates, cephalochordates, and vertebrates) is progressively restricted along the anterior–posterior axis to a domain around the anterior pole. In the sea urchin embryo, three integrated Wnt signaling branches (Wnt/β-catenin, Wnt/JNK, and Wnt/PKC) govern this progressive restriction process, which begins around the 32- to 60-cell stage and terminates by the early gastrula stage. We previously have established that several secreted Wnt modulators of the Dickkopf and secreted Frizzled-related protein families (Dkk1, Dkk3, and sFRP-1/5) are expressed within the ANE and play important roles in modulating the Wnt signaling network during this process. In this study, we use morpholino and dominant-negative interference approaches to characterize the function of a novel Frizzled-related protein, secreted Frizzled-related protein 1 (sFRP-1), during ANE restriction. sFRP-1 appears to be related to a secreted Wnt modulator, sFRP3/4, that is essential to block Wnt signaling and establish the ANE in vertebrates. Here, we show that the sea urchin sFRP3/4 orthologue is not expressed during ANE restriction in the sea urchin embryo. Instead, our results indicate that ubiquitously expressed maternal sFRP-1 and Fzl1/2/7 signaling act together as early as the 32- to 60-cell stage to antagonize the ANE restriction mechanism mediated by Wnt/β-catenin and Wnt/JNK signaling. Then, starting from the blastula stage, Fzl5/8 signaling activates zygotic sFRP-1 within the ANE territory, where it works with the secreted Wnt antagonist Dkk1 (also activated by Fzl5/8 signaling) to antagonize Wnt1/Wnt8–Fzl5/8–JNK signaling in a negative feedback mechanism that defines the outer ANE territory boundary. Together, these data indicate that maternal and zygotic sFRP-1 protects the ANE territory by antagonizing the Wnt1/Wnt8–Fzl5/8–JNK signaling pathway throughout ANE restriction, providing precise spatiotemporal control of the mechanism responsible for the establishment of the ANE territory around the anterior pole of the sea urchin embryo.Electronic supplementary materialThe online version of this article (10.1186/s13227-017-0089-3) contains supplementary material, which is available to authorized users.

A dual role for Axin in establishing the anterior-posterior axis in the early sea urchin embryo

“…Strikingly, analysis of spatial expression of ANE markers in Axin-overexpressing embryos showed that ANE markers were broadly expressed throughout the anteriorized embryos (Range et al, 2013). Since Axin is a strong negative regulator of cWnt signaling, this result was consistent with a body of evidence that cWnt pathway-dependent signals from posterior cells restrict the ANE domain to the anterior pole of the embryo (Range et al, 2013;Range, 2014;Range, 2018;). However, the Range et al (2013) study did not carry out any molecular analyses on the effects of Axin overexpression on cWnt activation and endomesoderm specification and hence, we examined these processes in the Axin-mediated anteriorized embryos.…”

“…In addition to the role of cWnt signaling in specifying endomesoderm in vegetal cells, there is very good evidence that cWnt-dependent signaling from these cells initiates a signaling cascade that restricts anterior neural ectoderm (ANE) cell fates to the most anterior end of the embryo (Range, 2014;Range et al, 2013;Yaguchi et al, 2008). Several studies have shown that secreted Wnt inhibitors expressed in embryos around the time of gastrulation protect the ANE territory from posterior Wnt signaling similar to what has been shown in other bilaterian taxa (Khadka et al, 2018;Range, 2014;Range et al, 2013;Range and Wei, 2016). While many aspects of the ontogeny of cWnt-mediated AP axis patterning in the sea urchin embryo are similar to what is observed in other embryos it is not clear if there is a repression of cWnt signaling in the early embryo analogous to what has been seen in vertebrates and Tribolium.…”

cis‐Regulatory analysis for later phase of anterior neuroectoderm‐specific foxQ2 expression in sea urchin embryos

“…It might be better to suppose that the spatial expression of foxQ2 at the later stage is likely maintained by the combination of global activators including Meis and unknown factors expressed specifically in the anterior neuroectoderm (and possibly a global repressor for foxQ2b ), that is, its expression is not caused by global activation and repression in the nonanterior region. A previous study examining S. purpuratus has reported that sequential Wnt signaling restricts foxQ2 expression into the future anterior neuroectoderm (Khadka, Martínez‐Bartolomé, Burr, & Range, ; Range et al, ). In the later phase of foxQ2 expression, Fzl5/8‐JNK noncanonical Wnt signaling represses foxQ2 expression in the nonanterior neuroectoderm.…”

Wnt lipidation: Roles in trafficking, modulation, and function

“…The sFRPs play an important role in blocking Wnt signaling activity during embryogenesis. Indeed, the antagonizing activity of sFRP1 on Wnt1/Wnt8–Fzl5/8–JNK signaling was necessary for anterior neuroectoderm formation in sea urchin embryos (Khadka, Martínez‐Bartolomé, Burr, & Range, ). sFRP was previously reported to have an inhibitory effect on cancer cell growth in colorectal cancer and breast cancer cells through blocking the Wnt signaling pathway (Lavergne et al, ).…”