SUMMARYThe RNA-binding protein Bicaudal C is an important regulator of embryonic development in C. elegans, Drosophila and Xenopus. In mouse, bicaudal C (Bicc1) mutants are characterized by the formation of fluid-filled cysts in the kidney and by expansion of epithelial ducts in liver and pancreas. This phenotype is reminiscent of human forms of polycystic kidney disease (PKD). Here, we now provide data that Bicc1 functions by modulating the expression of polycystin 2 (Pkd2), a member of the transient receptor potential (TRP) superfamily. Molecular analyses demonstrate that Bicc1 acts as a post-transcriptional regulator upstream of Pkd2. It regulates the stability of Pkd2 mRNA and its translation efficiency. Bicc1 antagonized the repressive activity of the miR-17 microRNA family on the 3ЈUTR of Pkd2 mRNA. This was substantiated in Xenopus, in which the pronephric defects of bicc1 knockdowns were rescued by reducing miR-17 activity. At the cellular level, Bicc1 protein is localized to cytoplasmic foci that are positive for the Pbody markers GW182 and HEDLs. Based on these data, we propose that the kidney phenotype in Bicc1 -/-mutant mice is caused by dysregulation of a microRNA-based translational control mechanism.

Twisted gastrulation (Tsg) is a secreted protein that regulates Bmp signaling in the extracellular space through its direct interaction with Bmp/Dpp and Chordin (Chd)/Short gastrulation (Sog). The ternary complex of Tsg/Chd/Bmp is cleaved by the metalloprotease Tolloid (Tld)/Xolloid (Xld). Studies in Drosophila, Xenopus and zebrafish suggest that Tsg can act both as an anti-Bmp and as a pro-Bmp. We have analyzed Tsgloss-of-function in the mouse. Tsg homozygous mutants are viable but of smaller size and display mild vertebral abnormalities and osteoporosis. We provide evidence that Tsg interacts genetically with Bmp4. When only one copy of Bmp4 is present, a requirement of Tsgfor embryonic development is revealed. Tsg-/-;Bmp4+/- compound mutants die at birth and display holoprosencephaly, first branchial arch and eye defects. The results show that Tsg functions to promote Bmp4 signaling during mouse head development.

The vertebrate body plan follows stereotypical dorsal-ventral (D-V) tissue differentiation controlled by bone morphogenetic proteins (BMPs) and secreted BMP antagonists, such as Chordin. The three germ layers-ectoderm, mesoderm, and endoderm-are affected coordinately by the Chordin-BMP morphogen system. However, extracellular morphogen gradients of endogenous proteins have not been directly visualized in vertebrate embryos to date. In this study, we improved immunolocalization methods in Xenopus embryos and analyzed the distribution of endogenous Chordin using a specific antibody. Chordin protein secreted by the dorsal Spemann organizer was found to diffuse along a narrow region that separates the ectoderm from the anterior endoderm and mesoderm. This Fibronectin-rich extracellular matrix is called "Brachet's cleft" in the Xenopus gastrula and is present in all vertebrate embryos. Chordin protein formed a smooth gradient that encircled the embryo, reaching the ventral-most Brachet cleft. Depletion with morpholino oligos showed that this extracellular gradient was regulated by the Chordin protease Tolloid and its inhibitor Sizzled. The Chordin gradient, as well as the BMP signaling gradient, was self-regulating and, importantly, was able to rescale in dorsal half-embryos. Transplantation of Spemann organizer tissue showed that Chordin diffused over long distances along this signaling highway between the ectoderm and mesoderm. Chordin protein must reach very high concentrations in this narrow region. We suggest that as ectoderm and mesoderm undergo morphogenetic movements during gastrulation, cells in both germ layers read their positional information coordinately from a single morphogen gradient located in Brachet's cleft.

In the developing organism, cells differentiate, divide and die as part of groups of hundreds or thousands of cells called 'morphogenetic fields'. Fields have the remarkable property of self-regulation: for example, if the forelimb field is bisected, each half can give rise to a complete limb after transplantation, as discovered by Ross Harrison in 1918. Therefore, cells in the morphogenetic field are capable of long-range communication with each other in order to ascertain their position [1]. This positional information is relayed in the extracellular space in the form of concentration gradients of specific classes of extracellular molecules called 'morphogens' that trigger cellular responses by binding and activating cell surface receptors. Here, we focus on a family of morphogens called 'Bone Morphogenetic Proteins' (BMPs), which has provided a new paradigm for signaling regulation in the extracellular space.

Crossveinless-2 (Cv2), Twisted Gastrulation (Tsg) and Chordin (Chd) are components of an extracellular biochemical pathway that regulates Bone Morphogenetic Protein (BMP) activity during dorso-ventral patterning of Drosophila and Xenopus embryos, the formation of the fly wing, and mouse skeletogenesis. Because the nature of their genetic interactions remained untested in the mouse, we generated a null allele for Cv2 which was crossed to Tsg and Chd mutants to obtain Cv2;Tsg and Cv2;Chd compound mutants. We found that Cv2 is essential for skeletogenesis as its mutation caused the loss of multiple bone structures and posterior homeotic transformation of the last thoracic vertebra. During early vertebral development, Smad1 phosphorylation in the intervertebral region was decreased in the Cv2 mutant, even though CV2 protein is normally located in the future vertebral bodies. Because Cv2 mutation affects BMP signaling at a distance, this suggested that CV2 is involved in the localization of the BMP morphogenetic signal. Cv2 and Chd mutations did not interact significantly. However, mutation of Tsg was epistatic to all CV2 phenotypes. We propose a model in which CV2 and Tsg participate in the generation of a BMP signaling morphogenetic field during vertebral formation in which CV2 serves to concentrate diffusible Tsg/BMP4 complexes in the vertebral body cartilage.

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Sirenomelia or mermaid-like phenotype is one of the principal human congenital malformations that can be traced back to the stage of gastrulation. Sirenomelia is characterized by the fusion of the two hindlimbs into a single one. In the mouse, sirens have been observed in crosses between specific strains and as the consequence of mutations that increase retinoic acid levels. We report that the loss of bone morphogenetic protein 7 (Bmp7) in combination with a half dose or complete loss of twisted gastrulation (Tsg)causes sirenomelia in the mouse. Tsg is a Bmp- and chordin-binding protein that has multiple effects on Bmp metabolism in the extracellular space; Bmp7 is one of many Bmps and is shown here to bind to Tsg. In Xenopus,co-injection of Tsg and Bmp7 morpholino oligonucleotides (MO) has a synergistic effect, greatly inhibiting formation of ventral mesoderm and ventral fin tissue. In the mouse, molecular marker studies indicate that the sirenomelia phenotype is associated with a defect in the formation of ventroposterior mesoderm. These experiments demonstrate that dorsoventral patterning of the mouse posterior mesoderm is regulated by Bmp signaling, as is the case in other vertebrates. Sirens result from a fusion of the hindlimb buds caused by a defect in the formation of ventral mesoderm.

We have identified a novel mouse member of the Wnt family, Wnt13. Among mouse Wnt genes, Wnt13 is most closely related to Wnt2. Sequence comparisons and chromosomal localization strongly suggest that Wnt13, rather than Wnt2, is the mouse orthologue of both the human WNT13 and Xenopus XWnt2 genes. Wnt13 is expressed in the embryonic mesoderm during gastrulation. At later stages, transcripts are detected in the dorsal midline of the diencephalon and mesencephalon, the heart primordia, the periphery of the lung bud and the otic and optic vesicles. These data suggest that Wnt13 function might partially overlap with those of other Wnt genes in the cell signaling mechanisms controlling mesoderm specification during gastrulation and some aspects of brain, heart and lung formation.