Gene regulatory networks and developmental plasticity in the early sea urchin embryo: alternative deployment of the skeletogenic gene regulatory network

Abstract

Cell fates in the sea urchin embryo are remarkably labile, despite the fact that maternal polarity and zygotic programs of differential gene expression pattern the embryo from the earliest stages. Recent work has focused on transcriptional gene regulatory networks (GRNs) deployed in specific embryonic territories during early development. The micromere-primary mesenchyme cell (PMC) GRN drives the development of the embryonic skeleton. Although normally deployed only by presumptive PMCs, every lineage of the early embryo has the potential to activate this pathway. Here, we focus on one striking example of regulative activation of the skeletogenic GRN; the transfating of non-skeletogenic mesoderm (NSM) cells to a PMC fate during gastrulation. We show that transfating is accompanied by the de novo expression of terminal, biomineralization-related genes in the PMC GRN, as well as genes encoding two upstream transcription factors, Lvalx1 and Lvtbr. We report that Lvalx1, a key component of the skeletogenic GRN in the PMC lineage, plays an essential role in the regulative pathway both in NSM cells and in animal blastomeres. MAPK signaling is required for the expression of Lvalx1 and downstream skeletogenic genes in NSM cells, mirroring its role in the PMC lineage. We also demonstrate that Lvalx1 regulates the signal from PMCs that normally suppresses NSM transfating. Significantly, misexpression of Lvalx1 in macromeres (the progenitors of NSM cells) is sufficient to activate the skeletogenic GRN. We suggest that NSM cells normally deploy a basal mesodermal pathway and require only an Lvalx1-mediated sub-program to express a PMC fate. Finally, we provide evidence that, in contrast to the normal pathway, activation of the skeletogenic GRN in NSM cells is independent of Lvpmar1. Our studies reveal that, although most features of the micromere-PMC GRN are recapitulated in transfating NSM cells, different inputs activate this GRN during normal and regulative development.

The control offoxN2/3expression in sea urchin embryos and its function in the skeletogenic gene regulatory network

“…This result implies both that FoxN2/3 is an indirect input into expression of the VEGFR, and that the transfating mechanism employs a network that no longer requires FoxN2/3. Indeed, it was shown earlier that transfated PMCs also do not require Pmar1 (Ettensohn et al, 2007). As we show that foxN2/3 expression is downregulated at ingression, yet in normal PMCs the suite of differentiation genes continues to be expressed, including VEGFR, apparently transfated PMCs are able to drive expression of those differentiation genes by somehow bypassing FoxN2/3.…”

“…The sea urchin micromere gene regulatory network (GRN) is one of the best understood of all metazoan embryonic networks (Ettensohn et al, 2007;Oliveri et al, 2002;Oliveri et al, 2003;Oliveri et al, 2008). It specifies the large micromeres toward a skeletogenic fate.…”

Larval mesenchyme cell specification in the primitive echinoid occurs independently of the double-negative gate

“…The functions of alx1 and tbr were perturbed by gene-specific MOs. To estimate the ets1 function, the MEK inhibitor U0126 was used because it blocks the mitogen-activated protein kinase (MAPK) pathway that is required for ets1 activation in euechinoids (Röttinger et al, 2004;Ettensohn et al, 2007).…”

Early developmental gene regulation in Strongylocentrotus purpuratus embryos in response to elevated CO2 seawater conditions

“…Alx1 is an upstream PMC GRN regulatory gene that is essential for PMC ingression and their movement within the blastocoel (Ettensohn et al, 2007). The genes chosen for this study were in pathways that either regulate or are regulated by Alx1.…”