A Myc-Slug (Snail2)/Twist regulatory circuit directs vascular development

Myc-deficient mice fail to develop normal vascular networks and Myc-deficient embryonic stem cells fail to provoke a tumor angiogenic response when injected into immune compromised mice. However, the molecular underpinnings of these defects are poorly understood. To assess whether Myc indeed contributes to embryonic vasculogenesis we evaluated Myc function in Xenopus laevis embryogenesis. Here, we report that Xc-Myc is required for the normal assembly of endothelial cells into patent vessels during both angiogenesis and lymphangiogenesis. Accordingly, the specific knockdown of Xc-Myc provokes massive embryonic edema and hemorrhage. Conversely, Xc-Myc overexpression triggers the formation of ectopic vascular beds in embryos. Myc is required for normal expression of Slug/Snail2 and Twist, and either XSlug/Snail2 or XTwist could compensate for defects manifest by Xc-Myc knockdown. Importantly, knockdown of Xc-Myc, XSlug/Snail2 or XTwist within the lateral plate mesoderm, but not the neural crest, provoked embryonic edema and hemorrhage. Collectively, these findings support a model in which Myc, Twist and Slug/Snail2 function in a regulatory circuit within lateral plate mesoderm that directs normal vessel formation in both the vascular and lymphatic systems.     

Insights from a sea lamprey into the evolution of neural crest gene regulatory network

The neural crest is a vertebrate innovation that forms at the embryonic neural plate border, transforms from epithelial to mesenchymal, migrates extensively throughout the embryo along well-defined pathways, and differentiates into a plethora of derivatives that include elements of peripheral nervous system, craniofacial skeleton, melanocytes, etc. The complex process of neural crest formation is guided by multiple regulatory modules that define neural crest gene regulatory network (NC GRN), which allows the neural crest to progressively acquire all of its defining characteristics. The molecular study of neural crest formation in lamprey, a basal extant vertebrate, consisting in identification and functional tests of molecular elements at each regulatory level of this network, has helped address the question of the timing of emergence of NC GRN and define its basal state. The results have revealed striking conservation in deployment of upstream factors and regulatory modules, suggesting that proximal portions of the network arose early in vertebrate evolution and have been tightly conserved for more than 500 million years. In contrast, certain differences were observed in deployment of some neural crest specifier and downstream effector genes expected to confer species-specific migratory and differentiation properties.     

Neural crest induction by paraxial mesoderm in Xenopus embryos requires FGF signals

At the border of the neural plate, the induction of the neural crest can be achieved by interactions with the epidermis, or with the underlying mesoderm. Wnt signals are required for the inducing activity of the epidermis in chick and amphibian embryos. Here, we analyze the molecular mechanisms of neural crest induction by the mesoderm in Xenopus embryos. Using a recombination assay, we show that prospective paraxial mesoderm induces a panel of neural crest markers (Slug, FoxD3, Zic5 and Sox9), whereas the future axial mesoderm only induces a subset of these genes. This induction is blocked by a dominant negative (dn) form of FGFR1. However, neither dnFGFR4a nor inhibition of Wnt signaling prevents neural crest induction in this system. Among the FGFs, FGF8 is strongly expressed by the paraxial mesoderm. FGF8 is sufficient to induce the neural crest markers FoxD3, Sox9 and Zic5 transiently in the animal cap assay. In vivo, FGF8 injections also expand the Slug expression domain. This suggests that FGF8 can initiate neural crest formation and cooperates with other DLMZ-derived factors to maintain and complete neural crest induction. In contrast to Wnts, eFGF or bFGF, FGF8 elicits neural crest induction in the absence of mesoderm induction and without a requirement for BMP antagonists. In vivo, it is difficult to dissociate the roles of FGF and WNT factors in mesoderm induction and neural patterning. We show that, in most cases, effects on neural crest formation were parallel to altered mesoderm or neural development. However, neural and neural crest patterning can be dissociated experimentally using different dominant-negative manipulations: while Nfz8 blocks both posterior neural plate formation and neural crest formation, dnFGFR4a blocks neural patterning without blocking neural crest formation. These results suggest that different signal transduction mechanisms may be used in neural crest induction, and anteroposterior neural patterning.