Dynamics of neural induction at the single cell level in chordates – NEUCECHO

The first step in the formation of the vertebrate central nervous system (CNS) during embryonic development is called neural induction (NI). It is the instructive process by which naive ectodermal cells are committed to a neural fate. This concept was established by Mangold and Spemann who showed that the dorsal blastopore lip of a newt gastrula (i.e. organizer), when grafted to the ventral side of a host gastrula, was able to induce the formation of a Siamese twin embryo in which part of the secondary CNS developed from the host and not from the graft. Concerning the nature of neural inductive signals coming from the organizer contrasting views exist and no unifying scheme can be drawn. Several cellular signals appear to be important in this process. Thus, while inhibition of BMP signaling is necessary for NI in vertebrates, the emerging view is that NI requires a sequence of signals in a specific order. Several signals seem to play a role in this process such as FGF, Wnt and Notch. An additional important question concerns the evolutionary origin of the NI process, in other words, how this process appeared and evolved in the chordate lineage.
Chordates represent a group of deuterostome metazoans including the vertebrates but also invertebrate groups such as the cephalochordates (i.e. amphioxus), and the urochordates (i.e. ascidians). The morphological and developmental characteristics of cephalochordates show many similarities with vertebrates, and it is considered that amphioxus is vertebrate-like but simpler. On the other hand, urochordates show some developmental modalities and genetic characteristics that are derived. Concerning NI, urochordates have lost the organizer, the inhibition of BMP does not appear to play a major role, and the endogenous neural inducer is FGF9/16/20. However, in amphioxus, we have shown that there is a functional organizer which is able to induce a complete secondary axis when grafted onto a host embryo, and which is able to induce neural fate in an ectodermic explant that acquires epidermal fate by default in absence of the organizer. We have also shown that i) the activation of the BMP signal ventralises the embryo and induces epidermal fate in the whole ectoderm, in a similar way to vertebrates; ii) inhibition of BMP generates an undifferentiated ectoderm, and that therefore a neural inducing signal is necessary; iii) the FGF signal does not seem to play an essential role in neural induction; iv) the neural inducing signal is Nodal/Activin, and v) using an ATAC-seq approach we have been able to construct a virtual GRN controlling NI in amphioxus downstream of Nodal/Activin.
Therefore, the main objective of this project is to understand the different steps that lead an undifferentiated ectodermal cell to become a neural cell during the early development of chordates. Our goal is focused on both a cellular and a molecular level. By using a single cell RNA-seq (scRNA-seq) approach, we intend to decipher in a dynamic way how some undifferentiated ectodermal cells of the cephalochordate (i. e. amphioxus) embryo become neural, and also how the different genes implicated in the neural induction gene regulatory network (GRN) are expressed in each of these cells during early developmental stages. Insights into the temporal and spatial factors essential for neural induction will be crucial to understand how such a complex process occurs. In addition, our study on amphioxus embryos will allow us to perform comparisons with known data in vertebrates and will contribute to our understanding of the evolution of this developmental process.