Chen, J. N., van Eeden, F. J. M., Warren, K. S., Chin, A., Nüsslein-Volhard, C., Haffter, P. and Fishman, M. C. (1997). Left-right pattern of cardiac BMP4 may drive asymmetry of the heart in zebrafish. Development 124, 4373-82.
The first evident break in left-right symmetry of the primitive zebrafish heart tube is the shift in pattern of BMP4 expression from radially symmetric to left-predominant. The midline heart tube then 'jogs' to the left and subsequently loops to the right. We examined 279 mutations, affecting more than 200 genes, and found 21 mutations that perturb this process. Some cause BMP4 to remain radially symmetric. Others randomize the asymmetric BMP4 pattern. Retention of BMP4 symmetry is associated with failure to jog: right-predominance of the BMP4 pattern is associated with reversal of the direction of jogging and looping. Raising BMP4 diffusely throughout the heart, via sonic hedgehog injection, or the blocking of its action by injection of a dominant negative BMP4 receptor, prevent directional jogging or looping. The genes crucial to directing cardiac asymmetry include a subset of those needed for patterning the dorsoventral axis and for notochord and ventral spinal cord development. Thus, the pattern of cardiac BMP4 appears to be in the pathway by which the heart interprets lateralizing signals from the midline.
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Ferrandon, D., Koch, I., Westhof, E. and Nüsslein-Volhard, C. (1997). RNA-RNA interaction is required for the formation of specific bicoid mRNA 3' UTR-STAUFEN ribonucleoprotein particles. EMBO J 16, 1751-8.
The formation of the anterior pattern of the Drosophila embryo is dependent on the localization of the mRNA of the morphogen Bicoid (bcd) to the anterior pole of the egg cell. Staufen protein (STAU) is required in a late step of the localization to anchor the bcd mRNA in the anterior cytoplasm. We have shown previously that endogenous STAU associates specifically with injected bcd mRNA 3'-untranslated region (UTR), resulting in the formation of characteristic RNA-protein particles that are transported along microtubules of the mitotic spindles in a directed manner. The regions recognized by STAU in this in vivo assay are predicted to form three stem-loop structures involving large double-stranded stretches. Here, we show that the STAU interaction requires a double-stranded conformation of the stems within the RNA localization signal. In addition, base pairing between two single-stranded loops plays a major role in particle formation. This loop-loop interaction is intermolecular, not intramolecular; thus dimers or multimers of the RNA localization signal must be associated with STAU in these particles. The bcd mRNA 3' UTR can also dimerize in vitro in the absence of STAU. Thus, in addition to RNA-protein interactions, RNA-RNA interaction might be involved in the formation of ribonucleoprotein particles for transport and localization.
Heisenberg, C. P. and Nüsslein-Volhard, C. (1997). The function of silberblick in the positioning of the eye anlage in the zebrafish embryo. Dev Biol 184, 85-94.
In zebrafish, as in other vertebrates, an initially singular eye field within the neural plate has to split during morphogenesis to allow the development of two separated eyes. It has been suggested that anterior progression of midline tissue within the neural plate is involved in the bilateralization of the eye field. Mutations in the recently identified silberblick (slb) gene cause an incomplete separation of the eyes. During gastrulation and early somitogenesis, the ventral midline of the central nervous system (CNS) together with the underlying axial mesendoderm is shortened and broadened in slb embryos. While in wild-type embryos the ventral CNS midline extends to the anterior limit of the neural plate at the end of gastrulation, there is a gap between the anterior tip of the ventral CNS midline and the anterior edge of the neural plate in slb. To investigate the cause for the shortening of the ventral CNS midline in slb we determined the fate of labeled ventral CNS midline cells in wild-type and slb embryos at different stages of development. In slb, anterior migration of ventral CNS midline cells is impaired, which indicates that migration of these cells is needed for elongation of the ventral CNS midline. The anterior shortening of the ventral CNS midline in slb leads to medial instead of bilateral induction of optic stalks followed by a partial fusion of the eyes at later developmental stages. The analysis of the slb phenotype indicates that anterior migration of midline cells within the neural plate is required for proper induction and subsequent bilateralization of an initially singular eye field. These findings may therefore provide a starting point in elucidating the role of neural plate morphogenesis in positioning of the eye.
Kishimoto, Y., Lee, K.H., Zon, L., Hammerschmidt, M., and Schulte-Merker, S. (1997). The molecular nature of zebrafish swirl: BMP2 function is essential during early dorsoventral patterning. Development 124, 4457-66.
Early dorsoventral pattern formation in vertebrate embryos is regulated by opposing activities of ventralizing bone morphogenetic proteins (BMPs) and dorsal-specific BMP antagonists such as Chordin, Noggin and Follistatin. Specific defects in early dorsoventral patterning have been recently found in a number of zebrafish mutants, which exhibit either a ventralized or dorsalized phenotype. One of these, the ventralized mutant chordino (originally called dino) is caused by a mutation in the zebrafish chordin homologue and interacts genetically with the dorsalized mutant swirl. In swirl mutant embryos, dorsal structures such as notochord and somites are expanded while ventral structures such as blood and nephros are missing. Here we demonstrate that the swirl phenotype is caused by mutations in the zebrafish bmp2 gene (zbmp2). While injection of mRNAs encoded by the mutant alleles has no ventralizing effect, injection of wild-type zbmp2 mRNA leads to a complete rescue of the swirl mutant phenotype. Fertile adult mutant fish were obtained, showing that development after gastrulation is not dependent on zbmp2 function. In addition zBMP2 has no maternal role in mesoderm induction. Our analysis shows that swirl/BMP2, unlike mouse BMP2 but like mouse BMP4, is required for early dorsoventral patterning of the zebrafish embryo.
Rauch, G.-J., Granato, M. and Haffter, P. (1997). A polymorphic zebrafish line for genetic mapping using SSLPs on high-percentage agarose gels. Technical Tips Online T01208.
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Rauch, G.-J., Hammerschmidt, M., Blader, P., Schauerte, H. E., Strähle, U., Ingham, P. W., McMahon, A. P. and Haffter, P. (1997a). WNT5 is required for tail formation in the zebrafish embryo. Cold Spring Harbor Symp. Quant. Biol. 62, 227-33.
Intercellular signaling molecules, such as those encoded by the Wnt gene family, have a fundamental role in various aspects of pattern formation in the developing embryo. The zebrafish wnt5 gene encodes a member of a subfamily of Wnt molecules thought to be involved in modulating cell behavior during vertebrate development. Here, we show that the zebrafish pipetail gene is identical to wnt5. The pipetail mutant phenotype is characterized by defects in tail formation and impaired maturation of cells that contribute to cartilaginous elements of the head skeleton. This suggests a major role for wnt5 in morphogenetic processes underlying tail outgrowth and cartilaginous differentiation in the head. To investigate the function of maternally derived wnt5 mRNA, we generated females that were homozygous for pipetail. The lack of a maternal effect phenotype in the progeny of these females suggests that no obvious function for the maternal wnt5 expression can be deduced.
Schulte-Merker, S., Lee, K. J., McMahon, A. P. and Hammerschmidt, M. (1997). The zebrafish organizer requires chordino (Scientific Correspondence). Nature 387, 862.
Wilsch-Bräuninger, M., Schwarz, H. and Nüsslein-Volhard, C. (1997). A sponge-like structure involved in the association of maternal products during Drosophila oogenesis. J Cell Biol 139, 817-29.
Localization of maternally provided RNAs during oogenesis is required for formation of the antero-posterior axis of the Drosophila embryo. Here we describe a subcellular structure in nurse cells and oocytes which may function as an intracellular compartment for assembly and transport of maternal products involved in RNA localization. This structure, which we have termed ''sponge body,'' consists of ER-like cisternae, embedded in an amorphous electron-dense mass. It lacks a surrounding membrane and is frequently associated with mitochondria. The sponge bodies are not identical to the Golgi complexes. We suggest that the sponge bodies are homologous to the mitochondrial cloud in Xenopus oocytes, a granulo-fibrillar structure that contains RNAs involved in patterning of the embryo.
Exuperantia protein, the earliest factor known to be required for the localization of bicoid mRNA to the anterior pole of the Drosophila oocyte, is highly enriched in the sponge bodies but not an essential structural component of these. RNA staining indicates that sponge bodies contain RNA. However, neither the intensity of this staining nor the accumulation of Exuperantia in the sponge bodies is dependent on the amount of bicoid mRNA present in the ovaries. Sponge bodies surround nuage, a possible polar granule precursor. Microtubules and microfilaments are not present in sponge bodies, although transport of the sponge bodies through the cells is implied by their presence in cytoplasmic bridges. We propose that the sponge bodies are structures that, by assembly and transport of included molecules or associated structures, are involved in localization of mRNAs in Drosophila oocytes.