Tight regulation of SpSoxB factors is required for patterning and morphogenesis in sea urchin embryos

Abstract

Previous studies in sea urchin embryos have demonstrated that nuclearization of beta-catenin is essential for initial steps in the specification of endoderm and mesenchyme, which are derived from vegetal blastomeres. This process begins at the 4th and extends through the 9th cleavage stage, an interval in which the SpSoxB1 transcription regulator is downregulated by beta-catenin-dependent gene products that include the transcription repressor SpKrl. These observations raise the possibility that SpSoxB1 removal is required to allow vegetal development to proceed. Here we show that elevated and ectopic expression of this factor suppresses differentiation of all vegetal cell types, a phenotype that is very similar to that caused by the suppression of beta-catenin nuclear function by cadherin overexpression. Suppression of vegetal fates involves interference at the protein-protein level because a mutation of SpSoxB1 that prevents its binding to DNA does not significantly reduce this activity. Reduction in SpSoxB1 level results in elevated TCF/Lef-beta-catenin-dependent expression of a luciferase reporter gene in vivo, indicating that in the normal embryo this protein suppresses the primary vegetal signaling mechanism that is required for specification of mesenchyme and endoderm. Surprisingly, normal expression of SpSoxB1 is required for gastrulation and endoderm differentiation, as shown by both morpholino-mediated translational interference and expression of a dominant negative protein. Similar gain-of-function and loss-of-function assays of a closely related factor, SpSoxB2, demonstrate that it, too, is required for gastrulation and that its overexpression can suppress vegetal development. However, significant phenotypic differences are apparent in the two perturbations, indicating that SpSoxB1 and SpSoxB2 have at least some distinct developmental functions. The results of all these studies support a model in which the concentration of SpSoxB factors must be tightly regulated along the animal-vegetal axis of the early sea urchin embryo to allow beta-catenin-dependent specification of endoderm and mesenchyme cell fates as well as to activate target genes required for gastrulation.

Direct development of neurons within foregut endoderm of sea urchin embryos

“…This appears to be the case, given that SoxB1 is successively down-regulated in all endomesoderm cell lineages except the foregut as they commit to specific fates: first skeletogenic mesenchyme (21), then nonskeletogenic mesoderm (21-23), and then midgut and hindgut (this study). This pattern of early ubiquitous SoxB1 expression followed by down-regulation through cell signaling is similar to that of the apparent SoxB1 ortholog Sox2 (19,21,24,25), whose role in maintaining pluripotency in mouse embryonic and induced pluripotent stem cells is now well established (26,27). The other SoxB1 class factors, Sox1 and Sox3, also maintain neural progenitors in an undifferentiated state, and their removal is also required for neural differentiation (17,18).…”

“…We propose that SoxB1 function supports retention of pluripotency in this region, at least in part by antagonizing canonical Wnt signaling that drives endomesoderm development. Previously, we showed that SoxB1 suppresses β-catenin activity in normal embryos during the period when early endoderm is specified and Six3 expression begins, and that misexpression of SoxB1 can completely block endomesoderm development (19). Thus, persistent SoxB1 expression specifically in the foregut could delay progression to a stable endodermal fate, which precludes neurogenesis.…”

Geometric control of ciliated band regulatory states in the sea urchin embryo

“…Two logical consequences follow from these observations. First, since SoxB1 is cleared from the endomesoderm during the blastula stage, which is a prerequisite for endomesodermal development to proceed (Kenny et al, 1999(Kenny et al, , 2003, its target genes can only be expressed at normal levels in ectodermal, CB, or apical domain cells. This provides a spatial constraint on the expression of SoxB1 target genes adjacent to CB subdomains 3 and 4, both of which abut the Veg1 endoderm.…”

The evolution of nervous system patterning: insights from sea urchin development

“…Many of the transcription factors expressed in the sea urchin embryo ANE (Kenny et al, 2003;Wei et al, 2009) are also expressed in the ectodermal layer of the cnidarian Nematostella vectensis [e.g. six3, hbn, rx, otx, soxb1, soxb2 (Marlow et al, 2009)].…”