In the development of the vertebrate body plan, Wnt3a is thought to promote the formation of paraxial mesodermal progenitors (PMPs) of the trunk region while suppressing neural specification. Recent lineage-tracing experiments have demonstrated that these trunk neural progenitors and PMPs derive from a common multipotent progenitor called the neuromesodermal progenitor (NMP). NMPs are known to reside in the anterior primitive streak (PS) region; however, the extent to which NMPs populate the PS and contribute to the vertebrate body plan, and the precise role that Wnt3a plays in regulating NMP self-renewal and differentiation are unclear. To address this, we used cell-specific markers (Sox2 and T) and tamoxifen-induced Cre recombinase-based lineage tracing to locate putative NMPs in vivo. We provide functional evidence for NMP location primarily in the epithelial PS, and to a lesser degree in the ingressed PS. Lineage-tracing studies in Wnt3a/β-catenin signaling pathway mutants provide genetic evidence that trunk progenitors normally fated to enter the mesodermal germ layer can be redirected towards the neural lineage. These data, combined with previous PS lineage-tracing studies, support a model that epithelial anterior PS cells are Sox2 + T + multipotent NMPs and form the bulk of neural progenitors and PMPs of the posterior trunk region. Finally, we find that Wnt3a/β-catenin signaling directs trunk progenitors towards PMP fates; however, our data also suggest that Wnt3a positively supports a progenitor state for both mesodermal and neural progenitors.

Wnt11 is a secreted protein that signals through the non-canonical planar cell polarity pathway and is a potent modulator of cell behavior and movement. In human, mouse, and chicken, there is a single Wnt11 gene, but in zebrafish and Xenopus, there are two genes related to Wnt11. The originally characterized Xenopus Wnt11 gene is expressed during early embryonic development and has a critical role in regulation of gastrulation movements. We have identified a second Xenopus Wnt11-Related gene (Wnt11-R) that is expressed after gastrulation. Sequence comparison suggests that Xenopus Wnt11-R, not Wnt11, is the ortholog of mammalian and chicken Wnt11. Xenopus Wnt11-R is expressed in neural tissue, dorsal mesenchyme derived from the dermatome region of the somites, the brachial arches, and the muscle layer of the heart, similar to the expression patterns reported for mouse and chicken Wnt11. Xenopus Wnt11-R exhibits biological properties similar to those previously described for Xenopus Wnt11, in particular the ability to activate Jun-N-terminal kinase (JNK) and to induce myocardial marker expression in ventral marginal zone (VMZ) explants. Morpholino inhibition experiments demonstrate, however, that Wnt11-R is not required for cardiac differentiation, but functions in regulation of cardiac morphogenesis. Embryos with reduced Wnt11-R activity exhibit aberrant cell-cell contacts within the myocardial wall and defects in fusion of the nascent heart tube.

Neuromesodermal (NM) stem cells generate neural and paraxial presomitic mesoderm (PSM) cells, which are the respective progenitors of the spinal cord and musculoskeleton of the trunk and tail. The Wnt-regulated basic helix-loop-helix (bHLH) transcription factor mesogenin 1 (Msgn1) has been implicated as a cooperative regulator working in concert with T-box genes to control PSM formation in zebrafish, although the mechanism is unknown. We show here that, in mice, Msgn1 alone controls PSM differentiation by directly activating the transcriptional programs that define PSM identity, epithelialmesenchymal transition, motility and segmentation. Forced expression of Msgn1 in NM stem cells in vivo reduced the contribution of their progeny to the neural tube, and dramatically expanded the unsegmented mesenchymal PSM while blocking somitogenesis and notochord differentiation. Expression of Msgn1 was sufficient to partially rescue PSM differentiation in Wnt3a −/− embryos, demonstrating that Msgn1 functions downstream of Wnt3a as the master regulator of PSM differentiation. Our data provide new insights into how cell fate decisions are imposed by the expression of a single transcriptional regulator.

SUMMARY The coronary vessels and epicardium arise from an extracardiac rudiment called the proepicardium. Failed fusion of the proepicardium to the heart results in severe coronary and heart defects. However, it is unknown how the proepicardium protrudes toward and attaches to the looping heart tube. Here we show that ectopic expression of BMP ligands in the embryonic myocardium can cause proepicardial cells to target aberrant regions of the heart. Additionally, misexpression of a BMP antagonist, Noggin, suppresses proepicardium protrusion and contact with the heart. Finally, proepicardium explant preferentially expands toward a co-cultured heart segment. This preference can be mimicked by BMP2/4 and suppressed by Noggin. These results support a model in which myocardium-derived BMP signals regulate the entry of coronary progenitors to the specific site of the heart by directing their morphogenetic movement.

The Wnt family of growth factors regulate many different aspects of embryonic development. Assembly of the complete mouse and human genome sequences, plus expressed sequence tag surveys have established the existence of 19 Wnt genes in mammalian genomes. However, despite the importance of model vertebrates for studies in developmental biology, the complete complement of Wnt genes has not been established for nonmammalian genomes. Using genome sequences for chicken (Gallus gallus), frog (Xenopus tropicalis), and fish (Danio rerio and Tetraodon nigroviridis), we have analyzed gene synteny to identify the orthologues of all 19 human Wnt genes in these species. We find that, in addition to the 19 Wnts observed in humans, chicken contained an additional Wnt gene, Wnt11b, which is orthologous to frog and zebrafish Wnt11 (silberblick). Frog and fish genomes contained orthologues of the 19 mammalian Wnt genes, plus Wnt11b and several duplicated Wnt genes. Specifically, the Xenopus tropicalis genome contained 24 Wnt genes, including additional copies of Wnt7-related genes (Wnt7c) and 3 recent Wnt duplications (Wnt3, Wnt9b, and Wnt11). The Danio rerio genome contained 27 Wnt genes with additional copies of Wnt2, Wnt2b, Wnt4b, Wnt6, Wnt7a, and Wnt8a. The presence of the additional Wnt11 sequence (Wnt11b) in the genomes of all ancestral vertebrates suggests that this gene has been lost during mammalian evolution. Through these studies, we identified the frog orthologues of the previously uncharacterized Wnt2, Wnt3, Wnt9a, Wnt9b, Wnt10a, and Wnt16 genes and their expression has been characterized during early Xenopus development.