Kate G. Storey

The homeobox gene goosecoid and the origin of organizer cells in the early chick blastoderm

The chick homeobox gene goosecoid (gsc) is first expressed in a barely noticeable cell population near the posterior margin (Kollers sickle) of the unincubated egg. Then it is detected in Hensens node, traditionally considered the chick organizer. Later, gsc-expressing cells leave the node with the prechordal plate. Fate mapping indicates that these three regions are related by cell lineage, and transplantation experiments suggest that they all have inducing activity. Quail posterior margin and anterior primitive streak grafts (gsc expressing) induce gsc transcription in neighboring chick host cells. We propose that development of the chick organizer starts earlier than previously thought and that gsc marks this changing cell population.

Negative-feedback regulation of FGF signalling by DUSP6/MKP-3 is driven by ERK1/2 and mediated by Ets factor binding to a conserved site within the DUSP6/MKP-3 gene promoter.

DUSP6 (dual-specificity phosphatase 6), also known as MKP-3 [MAPK (mitogen-activated protein kinase) phosphatase-3] specifically inactivates ERK1/2 (extracellular-signal-regulated kinase 1/2) in vitro and in vivo. DUSP6/MKP-3 is inducible by FGF (fibroblast growth factor) signalling and acts as a negative regulator of ERK activity in key and discrete signalling centres that direct outgrowth and patterning in early vertebrate embryos. However, the molecular mechanism by which FGFs induce DUSP6/MKP-3 expression and hence help to set ERK1/2 signalling levels is unknown. In the present study, we demonstrate, using pharmacological inhibitors and analysis of the murine DUSP6/MKP-3 gene promoter, that the ERK pathway is critical for FGF-induced DUSP6/MKP-3 transcription. Furthermore, we show that this response is mediated by a conserved binding site for the Ets (E twenty-six) family of transcriptional regulators and that the Ets2 protein, a known target of ERK signalling, binds to the endogenous DUSP6/MKP-3 promoter. Finally, the murine DUSP6/MKP-3 promoter coupled to EGFP (enhanced green fluorescent protein) recapitulates the specific pattern of endogenous DUSP6/MKP-3 mRNA expression in the chicken neural plate, where its activity depends on FGFR (FGF receptor) and MAPK signalling and an intact Ets-binding site. These findings identify a conserved Ets-factor-dependent mechanism by which ERK signalling activates DUSP6/MKP-3 transcription to deliver ERK1/2-specific negative-feedback control of FGF signalling.

Specification and maintenance of the spinal cord stem zone

Epiblast cells adjacent to the regressing primitive streak behave as a stem zone that progressively generates the entire spinal cord and also contributes to paraxial mesoderm. Despite this fundamental task, this cell population is poorly characterised, and the tissue interactions and signalling pathways that specify this unique region are unknown. Fibroblast growth factor (FGF) is implicated but it is unclear whether it is sufficient and/or directly required for stem zone specification. It is also not understood how establishment of the stem zone relates to the acquisition of spinal cord identity as indicated by expression of caudal Hox genes. Here, we show that many cells in the chick stem zone express both early neural and mesodermal genes; however, stem zone-specific gene expression can be induced by signals from underlying paraxial mesoderm without concomitant induction of an ambivalent neural/mesodermal cell state. The stem zone is a site of FGF/MAPK signalling and we show that although FGF alone does not mimic paraxial mesoderm signals, it is directly required in epiblast cells for stem zone specification and maintenance. We further demonstrate that caudal Hox gene expression in the stem zone also depends on FGF and that neither stem zone specification nor caudal Hox gene onset requires retinoid signalling. These findings thus support a two step model for spinal cord generation - FGF-dependent establishment of the stem zone in which progressively more caudal Hox genes are expressed, followed by the retinoid-dependent assignment of spinal cord identity.

A region of the vertebrate neural plate in which neighbouring cells can adopt neural or epidermal fates

Cells in the neurogenic region of the fly, Drosophila melanogaster, become either neural stem cells or epidermis and the selection of the former requires the activity of the proneural genes [1]. In contrast, it is commonly thought that all cells in the vertebrate neural plate contribute to the neural tube and that consequently there is no need for the selection of individual neural precursors (e.g., [2]). Here we present a detailed fate map of the chick caudal neural plate (CNP), a cell population that generates the posterior hindbrain and spinal cord. We show that this is a unique region of the neural plate where neighbouring cells can contribute to neural tube or epidermis. Further, neural tube precursors leave the CNP in an approximate rostro-caudal order and give rise to discrete portions of the neural tube where they or their progeny behave as neural stem cells [3]. Our data suggest that neural and epidermal cell fates are acquired on a cell-by-cell basis within the CNP and thus in a manner strikingly similar to that in the fly. Indeed, the assignment of neural cell fate in this region may prove to be mediated by the functional homologue of the fly proneural genes (chick achaete-scute homologue 4, cash4), which is expressed heterogeneously within this cell population [4].

Markers in vertebrate neurogenesis

Embryologists have long used morphological characteristics, and more recently marker genes, to identify neural tissue and to test the neural-inducing activity of specific cell populations and signalling molecules. These markers are also used to assess the function(s) of neural genes themselves. Progression from neural induction to terminal differentiation of neurons is a multistep process, and each step involves the activation and/or repression of genes that can be used as molecular markers for these different events. Here we briefly review these key steps in neurogenesis within the vertebrate central nervous system, and evaluate the markers used to define them. We emphasize the importance of cellular context and an understanding of gene function for interpreting the significance of marker genes.

Negative feedback predominates over cross-regulation to control ERK MAPK activity in response to FGF signalling in embryos

Expression of the gene encoding the MKP‐3/Pyst1 protein phosphatase, which inactivates ERK MAPK, is induced by FGF. However, which intracellular signalling pathway mediates this expression is unclear, with essential roles proposed for both ERK and PI(3)K in chick embryonic limb. Here, we report that MKP‐3/Pyst1 expression is sensitive to inhibition of ERK or MAPKK, that endogenous MKP‐3/Pyst1 co‐localizes with activated ERK, and expression of MKP‐3/Pyst1 in mice lacking PDK1, an essential mediator of PI(3)K signalling. We conclude that MKP‐3/Pyst1 expression is mediated by ERK activation and that negative feedback control predominates in limiting the extent of FGF‐induced ERK activity.

c-Irx2 expression reveals an early subdivision of the neural plate in the chick embryo

We have cloned c-Irx2, a chick homologue of the Xiro2 and mIrx2 genes and a new member of the Iroquois family of homeodomain-containing transcription factors. Strikingly, c-Irx2 expression reveals an early subdivision of the neural plate at late primitive streak stages which later transiently resolves to a single stripe within the developing hindbrain corresponding to rhombomere 1.

Brain or Brawn: How FGF Signaling Gives Us Both

How does FGF (fibroblast growth factor) signaling induce both neural and mesodermal cell fates in the early embryo? Two papers address this fundamental question in this issue of Cell. Bertrand et al. show in the ascidian that a GATA factor determines the neural response of animal cells to FGF signaling, while in the chick, Sheng et al. demonstrate that the slow induction by FGF of a new transcription factor (Churchill) in the neural plate in turn induces expression of Sip1 (Smad interacting protein-1), which inhibits mesodermal genes and sensitizes cells to later neural inducing factors.

The chick embryo revealed

Abstract The Atlas of the Chick Development by Ruth Bellairs and Mark Osmon Academic Press, 1998. £75.00 hbk (xii+323 pages) ISBN 0 12 084790 6