Conditional specification of endomesoderm

Abstract

Early in animal development many cells are conditionally specified based on observations that those cells can be directed toward alternate fates. The endomesoderm is so named because early specification produces cells that often have been observed to simultaneously express both early endoderm and mesoderm transcription factors. Experiments with these cells demonstrate that their progeny can directed entirely toward endoderm or mesoderm, whereas normally they establish both germ layers. This review examines the mechanisms that initiate the conditional endomesoderm state, its metastability, and the mechanisms that resolve that state into definitive endoderm and mesoderm.

Sea anemone Frizzled receptors play partially redundant roles in the oral-aboral axis patterning

“…In echinoderms, the early β-catenin-dependent specification of the endomesodermal domain is followed by the segregation of the endoderm from the mesoderm, and the subsequent Wnt-dependent PA patterning. In the endoderm, β-catenin signaling remains strong, while in the mesoderm β-catenin signaling becomes suppressed (Lhomond et al, 2012; Logan et al, 1999; McCauley et al, 2015; McClay et al, 2021; Range et al, 2013; Sun et al, 2021; Wikramanayake et al, 1998; Wikramanayake et al, 2004). A similar sequence of events – the early β-catenin-dependent definition of the future endodermal domain, the formation of the boundary between the β-catenin-sensitive future oral ectoderm and the β- catenin-insensitive future endoderm, and the subsequent Wnt-dependent OA patterning of the ectoderm also occurs in Nematostella , and these events seem to follow the same regulatory logic as described for the sea urchin.…”

Evolutionarily conserved Wnt/Sp5 signaling is critical for anterior-posterior axis patterning in sea urchin embryos

“…We also assayed for EMGRN gene expression in Sp5 morphants compared to control morpholino injected embryos at the mesenchyme blastula stage when the endoderm and mesoderm GRNs have segregated from one another. 14 , 33 , 44 We again found downregulated expression of gata4/5/6 , blimp1 and eve but also foxa, which was unaffected at the 120-cell stage ( Figures 3 Ba–d and f–i, S3 Da–d, S3Df–I, and S4 A). Conversely, Sp5 morphant embryos showed otx upregulation at this stage ( Figures 3 Be, j, S3 De, j, and S4 A).…”

“…The sea urchin EMGRN is one of the best characterized GRNs of any organism. 32 , 33 , 34 , 35 , 36 Currently, it consists of a hierarchical network of 16 TFs and several signaling components (e.g., Wnt1, Wnt8) that drive the differentiation gene batteries responsible for larval endoderm and mesoderm cells. 14 , 37 , 38 During late cleavage and early blastula stages many early endoderm GRN TFs are broadly co-expressed throughout the newly specified endoderm territory and excluded from the mesoderm territory around the apex of the posterior/vegetal pole.…”

“…These results reinforce striking parallels among the Wnt signaling mechanisms used by sea urchins and other metazoan embryos to specify and pattern early germ layer GRNs along the AP axis. 1 , 2 , 3 , 33 , 61 For instance, Wnt/β-catenin-Sp5 signaling has been implicated in endomesoderm specification and patterning in several chordates. 25 , 27 , 29 , 59 , 62 In zebrafish embryos Wnt/β-catenin signaling directly activates Sp5 which subsequently patterns mesoderm and in mice Wnt/β-catenin-Sp5 signaling cassettes are necessary for paraxial mesoderm gene activation as well as hindgut formation.…”

An RNA interference approach for functional studies in the sea urchin and its use in analysis of Nodal signaling gradients

“…Nonetheless, labs leveraging the developing urchin embryo as a model have benefitted from perturbation approaches using reverse genetics. Those approaches uncovered an extensive gene regulatory network (GRN) governing early specification and morphogenetic events up to the larval stage (Davidson et al, 2002;Davidson and Erwin, 2006;McClay, 2011;Peter and Davidson, 2015;Martik et al, 2016;McClay et al, 2021;Massri et al, 2023). While much has been learned about GRNs and other governing events in development, the diversity of perturbing reagents available historically has been limited in this system.…”