In three papers in eLife, the Schier lab provides surprising insights into the regulation of early vertebrate embryogenesis. Focusing on the roles of the TGF-b Nodal signaling pathway, the lab reports that:
(3) Nodal regulates the transcription of a G-protein coupled receptor, which in turn promotes the movement of mesodermal cells (Norris et al. 2017) .
Collectively, these papers revise the current textbook view of early embryonic signaling.
In the first paper, Montague and Schier revisit the molecular nature of the signals that induce mesoderm and endoderm. It has been thought for almost 20 years that Nodal signals are necessary and sufficient for mesendoderm induction. However, two other TGF-b signals, Activin and Vg1, were initially candidate mesendoderm inducers: Activin can ectopically induce mesendoderm and Vg1 is expressed at the right time and place to act as a mesendoderm inducer. Nevertheless, subsequent experiments revealed that Activin is not expressed during normal mesendoderm induction, while antisense blocking and misexpression experiments did not support a role for Vg1 in mesendoderm induction. It has therefore been assumed that Nodal is the sole mesendoderm inducer.
By mutating Vg1 in zebrafish, Montague and Schier found that Vg1 is as important as Nodal for mesendoderm induction. Embryos without Vg1 lack heart, blood, gut, kidney and other internal organs, much like embryos without Nodal.
How do Nodal and Vg1 work together to induce mesendoderm? Biochemical and imaging experiments suggest that Vg1 is present in the embryo as a precursor that gets cleaved and activated only upon binding to Nodal. The resulting heteromeric complex of cleaved Nodal and Vg1 then binds and activates receptors on target cells.
Why would the embryo rely on two signals instead of simply one? Vg1 is present in the early embryo before Nodal. Montague and Schier suggest that as soon as Nodal is synthesized it “hits the ground running” by binding to existing Vg1 and immediately initiating signaling. In this model, Vg1 prepares the embryo for the arrival of Nodal. This system might not only apply to the early embryo but might be a more general phenomenon: It has been shown that wherever Nodal is expressed, Vg1 or its orthologues are already present.
In the second paper, Rogers, Lord et al. ask what feedback inhibition is good for. Feedback inhibition is a broadly encountered regulatory motif in which an activator turns on its own inhibitor. This regulatory interaction can lead to the restriction of pathway activation to a small spatial domain or a short time window. To test this idea in the context of Nodal signaling, the authors studied the TGF-b superfamily member Lefty, which is activated by and inhibits Nodal signaling. As expected for an inhibitor, absence of Lefty leads to too much Nodal signaling. But contrary to the proposed role of feedback, Lefty can be replaced by treating embryos with a low concentration of Nodal inhibitor drug. The resulting inhibition without feedback produces viable and fertile fish.
Why then is inhibition normally regulated by feedback? Rogers, Lord et al. find that wild-type embryos can correct for reduced or increased Nodal signaling by reducing or increasing inhibitor expression, respectively, and thus develop normally. In contrast, feedback-impaired embryos cannot respond to alterations in Nodal levels and develop abnormally. These results reveal that embryogenesis without inhibitory feedback is functional but fragile
In the third paper, Norris et al. address the relationship of Nodal signaling with another signaling pathway active in the early embryo – the Toddler/Apelin receptor pathway. It has been shown that this GPCR pathway is needed for the normal movement of mesendodermal cells (Pauli et al. Science 2014), but its site of action and relationship to Nodal signaling have been unclear and controversial. Norris et al. show that Nodal signaling activates the transcription of the Toddler/Apelin GPCR receptor and therefore acts upstream of Toddler. Toddler signaling then promotes the migration of mesodermal cells during gastrulation. These results reveal a linear pathway from Nodal inducing mesodermal gene expression to Toddler promoting mesodermal migration.
Taken together, the three papers overturn previous assumptions about the regulation of Nodal signaling and suggest that more surprises are in store for our understanding of embryogenesis.