Distinct Steps of Neural Induction Revealed by Asterix, Obelix and TrkC, Genes Induced by Different Signals from the Organizer

Abstract

The amniote organizer (Hensen's node) can induce a complete nervous system when grafted into a peripheral region of a host embryo. Although BMP inhibition has been implicated in neural induction, non-neural cells cannot respond to BMP antagonists unless previously exposed to a node graft for at least 5 hours before BMP inhibitors. To define signals and responses during the first 5 hours of node signals, a differential screen was conducted. Here we describe three early response genes: two of them, Asterix and Obelix, encode previously undescribed proteins of unknown function but Obelix appears to be a nuclear RNA-binding protein. The third is TrkC, a neurotrophin receptor. All three genes are induced by a node graft within 4–5 hours but they differ in the extent to which they are inducible by FGF: FGF is both necessary and sufficient to induce Asterix, sufficient but not necessary to induce Obelix and neither sufficient nor necessary for induction of TrkC. These genes are also not induced by retinoic acid, Noggin, Chordin, Dkk1, Cerberus, HGF/SF, Somatostatin or ionomycin-mediated Calcium entry. Comparison of the expression and regulation of these genes with other early neural markers reveals three distinct “epochs”, or temporal waves, of gene expression accompanying neural induction by a grafted organizer, which are mirrored by specific stages of normal neural plate development. The results are consistent with neural induction being a cascade of responses elicited by different signals, culminating in the formation of a patterned nervous system.

A strategy to discover new organizers identifies a putative heart organizer

“…These results suggest that other signals are required, possibly SHH and FGF8, but also considerable complexity, not unlike other organizers, which are believed to emit different signals at different times to account for their various inducing and patterning functions353637. Accordingly, some gene expression profiles change over time as the organizer changes its properties.…”

Calreticulin is a secreted BMP antagonist, expressed in Hensen's node during neural induction

“…When a graft of Hensen's node is placed into the proximal anterior area opaca, it can induce a full range of neural and border markers within about 12 h (Dias and Schoenwolf, 1990, Gallera, 1971, Linker et al, 2009, Linker and Stern, 2004, Pinho et al, 2011, Storey et al, 1992, Streit et al, 1998, Streit et al, 1997, Streit and Stern, 1999). To test whether Calreticulin misexpression can induce the competent epiblast of the area opaca to acquire expression of neural markers, pCAβ-CALR-IRES-GFP was electroporated into a discrete domain within the inner third of area opaca at stage 3 + , just anterior to the level of Hensen's node (Fig.…”

The hypoblast (visceral endoderm): an evo-devo perspective

“…However, the requirement for the AVE in brain formation is not due to direct induction of either forebrain or neural character on naive ectoderm by the hypoblast/AVE. The early and transient expression of specific proteins has been proposed to represent a 'pre-neural/pre-forebrain' state, which requires REVIEW Development 139 (6) REVIEW Development 139 (6) stabilization by signals from other tissues, including the organizer (anterior primitive streak and Hensen's node), for cells to acquire either neural or forebrain identity (Pinho et al, 2011;Stern, 2001;Stern et al, 2006;Streit et al, 2000). However, it is puzzling that mouse embryos lacking functional Nodal signaling in the AVE, and the AVE itself, show generalized pre-neural gene expression, rather than lack thereof (Camus et al, 2006).…”