Convergence and extension movements mediate the specification and fate maintenance of zebrafish slow muscle precursors

During vertebrate gastrulation, concurrent inductive events and cell movements fashion the body plan. Convergence and extension (C&E) gastrulation movements narrow the vertebrate embryonic body mediolaterally while elongating it rostrocaudally. Segmented somites are shaped and positioned by C&E alongside the notochord and differentiate into skeleton, fast, and slow muscles during somitogenesis. In zebrafish, simultaneous inactivation of non-canonical Wnt signaling components Knypek and Trilobite strongly impairs C&E gastrulation movements. Here we show that knypek;trilobite double mutants exhibit a severe deficit in slow muscles and their precursor, adaxial cells, revealing essential roles of C&E movements in adaxial cell development. Adaxial cells become distinguishable in the presomitic mesoderm during late gastrulation by their expression of myogenic factors and axial-adjacent position. Using cell tracing analyses and genetic manipulations, we demonstrate that C&E movements regulate the number of prospective adaxial cells specified during gastrulation by determining the size of the interface between the inductive axial and target presomitic tissues. During segmentation, when the range of Hedgehog signaling from the axial tissue declines, tight apposition of prospective adaxial cells to the notochord, which is achieved by convergence movements, is necessary for their continuous Hedgehog reception and fate maintenance. We provide direct evidence to show that the deficiency of adaxial cells in knypek;trilobite double mutants is due to impaired C&E movements, rather than an alteration in Hedgehog signal and its reception, or a cell-autonomous requirement for Knypek and Trilobite in adaxial cell development. Our results underscore the significance of precise coordination between cell movements and inductive tissue interactions during cell fate specification.     

Synthesis and pharmacological assessment of derivatives of isoxazolo[4,5-d]pyrimidine

A series of new 5-alkyl and 5-arylisoxazolo[4,5-d]pyrimidinones (5a-g, 6-8) were prepared from 4-amino-3-oxo-isoxazolidine-5-carboxylic acid amide. Some of the aryl derivatives of isoxazolo[4,5-d]pyrimidine were tested pharmacologically in comparison with Diazepam. Compounds 5b-d and 7 demonstrated interesting anxiolytic activity.     

Axis formation--beta-catenin catches a Wnt

In this issue of Cell, the Heasman group implicates Wnt11 as a component of the canonical Wnt signaling pathway that specifies Xenopus laevis axis formation (Tao et al., 2005). This important work not only identifies a long-sought-after dorsalizing factor but also highlights the pivotal role of extracellular cofactors in specifying the activation of either canonical or noncanonical Wnt pathways.     

Hedgehog signaling induces arterial endothelial cell formation by repressing venous cell fate

In vertebrate embryos, the dorsal aorta and the posterior cardinal vein form in the trunk to comprise the original circulatory loop. Previous studies implicate Hedgehog (Hh) signaling in the development of the dorsal aorta. However, the mechanism controlling specification of artery versus vein remains unclear. Here, we investigated the cell-autonomous mechanism of Hh signaling in angioblasts (endothelial progenitor cells) during arterial-venous specification utilizing zebrafish mutations in Smoothened (Smo), a G protein-coupled receptor essential for Hh signaling. smo mutants exhibit an absence of the dorsal aorta accompanied by a reciprocal expansion of the posterior cardinal vein. The increased number of venous cells is equivalent to the loss of arterial cells in embryos with loss of Smo function. Activation of Hh signaling expands the arterial cell population at the expense of venous cell fate. Time-lapse imaging reveals two sequential waves of migrating progenitor cells that contribute to the dorsal aorta and the posterior cardinal vein, respectively. Angioblasts deficient in Hh signaling fail to contribute to the arterial wave; instead, they all migrate medially as a single population to form the venous wave. Cell transplantation analyses demonstrate that Smo plays a cell-autonomous role in specifying angioblasts to become arterial cells, and Hh signaling-depleted angioblasts differentiate into venous cells instead. Collectively, these studies suggest that arterial endothelial cells are specified and formed via repressing venous cell fate at the lateral plate mesoderm by Hh signaling during vasculogenesis.     

Six3 cooperates with Hedgehog signaling to specify ventral telencephalon by promoting early expression of Foxg1a and repressing Wnt signaling

Six3 exerts multiple functions in the development of anterior neural tissue of vertebrate embryos. Whereas complete loss of Six3 function in the mouse results in failure of forebrain formation, its hypomorphic mutations in human and mouse can promote holoprosencephaly (HPE), a forebrain malformation that results, at least in part, from abnormal telencephalon development. However, the roles of Six3 in telencephalon patterning and differentiation are not well understood. To address the role of Six3 in telencephalon development, we analyzed zebrafish embryos deficient in two out of three Six3-related genes, six3b and six7, representing a partial loss of Six3 function. We found that telencephalon forms in six3b;six7-deficient embryos; however, ventral telencephalic domains are smaller and dorsal domains are larger. Decreased cell proliferation or excess apoptosis cannot account for the ventral deficiency. Instead, six3b and six7 are required during early segmentation for specification of ventral progenitors, similar to the role of Hedgehog (Hh) signaling in telencephalon development. Unlike in mice, we observe that Hh signaling is not disrupted in embryos with reduced Six3 function. Furthermore, six3b overexpression is sufficient to compensate for loss of Hh signaling in isl1- but not nkx2.1b-positive cells, suggesting a novel Hh-independent role for Six3 in telencephalon patterning. We further find that Six3 promotes ventral telencephalic fates through transient regulation of foxg1a expression and repression of the Wnt/β-catenin pathway.