Maternal regulation of zerknüllt: a homoeobox gene controlling differentiation of dorsal tissues in Drosophila

The homoeobox gene zerknüllt (zen) plays an important role in the differentiation of dorsal tissues during Drosophila development. zen- embryos show transformations in the dorsal-most regions of the fate map, and lack several tissues that normally derive from these regions, including the amnioserosa and optic lobe. zen displays a simple dorsal on/ventral off pattern as early as cleavage cycle 10-11 (ref. 2). We have prepared a polyclonal antibody against a full-length zen protein, and used this to examine its pattern of expression in mutants that disrupt dorsal-ventral polarity. Most or all of the maternally expressed genes that are involved in this process have been previously identified and fall into two classes, so called 'dorsalizers' and 'ventralizers' (see refs 4-7, reviewed in ref. 8). On the basis of our analysis of zen expression in each of these maternal mutants we propose that one or more of the dorsalizing genes encodes a repressor which inhibits the expression of zen in ventral regions of developing embryos. The ventralizing gene cactus might play an important role in restricting the activity of this repressor to ventral regions, thereby permitting the activation of zen in those dorsal tissues where its function is critically required.     

BMP inhibition-driven regulation of six-3 underlies induction of newt lens regeneration

Lens regeneration in adult newts is a classic example of how cells can faithfully regenerate a complete organ through the process of transdifferentiation. After lens removal, the pigment epithelial cells of the dorsal, but not the ventral, iris dedifferentiate and then differentiate to form a new lens. Understanding how this process is regulated might provide clues about why lens regeneration does not occur in higher vertebrates. The genes six-3 and pax-6 are known to induce ectopic lenses during embryogenesis. Here we tested these genes, as well as members of the bone morphogenetic protein (BMP) pathway that regulate establishment of the dorsal-ventral axis in embryos, for their ability to induce lens regeneration. We show that the lens can be regenerated from the ventral iris when the BMP pathway is inhibited and when the iris is transfected with six-3 and treated with retinoic acid. In intact irises, six-3 is expressed at higher levels in the ventral than in the dorsal iris. During regeneration, however, only expression in the dorsal iris is significantly increased. Such an increase is seen in ventral irises only when they are induced to transdifferentiate by six-3 and retinoic acid or by BMP inhibitors. These data suggest that lens regeneration can be achieved in noncompetent adult tissues and that this regeneration occurs through a gene regulatory mechanism that is more complex than the dorsal expression of lens regeneration-specific genes.     

A gene required for the specification of dorsal-ventral pattern in Drosophila appears to encode a serine protease

The maternal effect gene snake is required for the establishment of the dorsal-ventral axis during the embryonic development of Drosophila. The molecular cloning of the gene and analysis of a complementary DNA sequence suggest that the gene encodes a serine protease which is structurally similar to proteases involved in blood clotting, peptide processing, and complement fixation pathways.     

Information for the dorsal--ventral pattern of the Drosophila embryo is stored as maternal mRNA

Maternal-effect mutations in 10 loci in Drosophila produce totally 'dorsalized' embryos. Injection of RNA isolated from wild-type embryos into mutants at six loci partially restores dorsal-ventral polarity. For the mutant snake, injection of poly(A)+ RNA restores a complete dorsal-ventral pattern.     

Cloning and roles of goldfish maternal factor β-Catenin cDNA in embryonic development

Interaction between nucleus and cytoplasm has been focused in the field of animal embryonic development, in which study of maternal factors is required positively. β-Catenin, an important maternal factor in early embryogenesis, has been analyzed in its expression pattern and functions in this paper. We have cloned goldfish β-Catenin cDNA gene and compared it with zebrafish β-Catenin cDNA. High homology was found in cDNA and in amino acid sequences between them, 93% (2227/2384 bp) and 98.5% (768/780 aa) respectively. The expression pattern of β-Catenin by in situ hybridization and the roles of β-Catenin on embryonic development by co-injection of anti-sense RNA and reporter gene, EGFP have been investigated in the whole process of goldfish embryonic development. The results suggest that β-Catenin presents dynamic distribution, mainly locates at body axis, dorsal tissues, head and tail structures after being fertilized. The loss of β-Catenin activity would cause serious destruction of embryo in dorsal tissues and in anteroposterior axes, and leads embryos to die before larva get hatched.     

Wnt信号通路与干细胞定向分化

Wnt信号通路在胚胎发育过程中参与背腹轴的形成、细胞极性的建立以及决定细胞命运.利用干细胞定向分化模型可在体外培养条件下探索Wnt信号通路在哺乳动物早期胚胎发育过程中的分子机理,了解其信号网络对干细胞定向分化中各个事件的调控机制.本文通过综合近期Wnt信号通路的研究,阐述经典Wnt信号通路(Canonical Wnt signaling pathway)在干细胞定向分化中的作用.     

Centrosomes direct cell polarity independently of microtubule assembly in C. elegans embryos

Polarity establishment requires a symmetry-breaking event, resulting in an axis along which determinants are segregated. In Caenorhabditis elegans, oocytes are apolar and are triggered to polarize rapidly along one axis after fertilization. The establishment of this first polarity axis is revealed by the asymmetric distribution of PAR proteins and cortical activity in the one-celled embryo. Current evidence suggests that the centrosome-pronucleus complex contributed by the sperm is involved in defining the polarization axis. Here we directly assess the contribution of the centrosome to polarity establishment by laser ablating the centrosome before and during polarization. We find that the centrosome is required to initiate polarity but not to maintain it. Initiation of polarity coincides with the proximity of the centrosome to the cortex and the assembly of pericentriolar material on the immature sperm centrosome. Depletion of microtubules or the microtubule nucleator gamma-tubulin did not affect polarity establishment. These results demonstrate that the centrosome provides an initiating signal that polarizes C. elegans embryos and indicate that this signalling event might be independent of the role of the centrosome as a microtubule nucleator.     

A role for Drosophila LKB1 in anterior-posterior axis formation and epithelial polarity

The PAR-4 and PAR-1 kinases are necessary for the formation of the anterior-posterior (A-P) axis in Caenorhabditis elegans. PAR-1 is also required for A-P axis determination in Drosophila. Here we show that the Drosophila par-4 homologue, lkb1, is required for the early A-P polarity of the oocyte, and for the repolarization of the oocyte cytoskeleton that defines the embryonic A-P axis. LKB1 is phosphorylated by PAR-1 in vitro, and overexpression of LKB1 partially rescues the par-1 phenotype. These two kinases therefore function in a conserved pathway for axis formation in flies and worms. lkb1 mutant clones also disrupt apical-basal epithelial polarity, suggesting a general role in cell polarization. The human homologue, LKB1, is mutated in Peutz-Jeghers syndrome and is regulated by prenylation and by phosphorylation by protein kinase A. We show that protein kinase A phosphorylates Drosophila LKB1 on a conserved site that is important for its activity. Thus, Drosophila and human LKB1 may be functional homologues, suggesting that loss of cell polarity may contribute to tumour formation in individuals with Peutz-Jeghers syndrome.     

Frizzled-7 signalling controls tissue separation during Xenopus gastrulation.

Cell signalling through Frizzled receptors has evolved to considerable complexity within the metazoans. The Frizzled-dependent signalling cascade comprises several branches, whose differential activation depends on specific Wnt ligands, Frizzled receptor isoforms and the cellular context. In Xenopus laevis embryos, the canonical beta-catenin pathway contributes to the establishment of the dorsal-ventral axis. A different branch, referred to as the planar cell polarity pathway, is essential for cell polarization during elongation of the axial mesoderm by convergent extension. Here we demonstrate that a third branch of the cascade is independent of Dishevelled function and involves signalling through trimeric G proteins and protein kinase C (PKC). During gastrulation, Frizzled-7 (Fz7)-dependent PKC signalling controls cell-sorting behaviour in the mesoderm. Loss of zygotic Fz7 function results in the inability of involuted anterior mesoderm to separate from the ectoderm, which leads to severe gastrulation defects. This result provides a developmentally relevant in vivo function for the Fz/PKC pathway in vertebrates.     

The Drosophila posterior-group gene nanos functions by repressing hunchback activity

The development of the body plan in the Drosophila embryo depends on the activity of maternal determinants localized at the anterior and posterior of the egg. These activities define both the polarity of the anterior-posterior (AP) axis and the spatial domains of expression of the zygotic gap genes, which in turn control the subsequent steps in segmentation. The nature and mode of action of one anterior determinant, the bicoid(bcd) gene product, has recently been defined, but the posterior determinants are less well characterized. At least seven maternally acting genes are required for posterior development. Mutations in these maternal posterior-group genes result in embryos lacking all abdominal segments. Cytoplasmic transplantation studies indicate that the maternally encoded product of the nanos(nos) gene may act as an abdominal determinant, whereas the other maternal posterior-group genes appear to be required for the appropriate localization and stabilization of this signal. Here we show that the lack of the nos gene product can be compensated for by eliminating the maternal activity of the gap gene hunchback (hb). Embryos lacking both of these maternally derived gene products are viable and can survive as fertile adults. These results suggest that the nos gene product functions by repressing the activity of the maternal hb products in the posterior of the egg.     

Maternal Rnf12/RLIM is required for imprinted X-chromosome inactivation in mice

Two forms of X-chromosome inactivation (XCI) ensure the selective silencing of female sex chromosomes during mouse embryogenesis. Imprinted XCI begins with the detection of Xist RNA expression on the paternal X?chromosome (Xp) at about the four-cell stage of embryonic development. In the embryonic tissues of the inner cell mass, a random form of XCI occurs in blastocysts that inactivates either Xp or the maternal X?chromosome (Xm). Both forms of XCI require the non-coding Xist RNA that coats the inactive X?chromosome from which it is expressed. Xist has crucial functions in the silencing of X-linked genes, including Rnf12 (refs 3, 4) encoding the ubiquitin ligase RLIM (RING finger LIM-domain-interacting protein). Here we show, by targeting a conditional knockout of Rnf12 to oocytes where RLIM accumulates to high levels, that the maternal transmission of the mutant X?chromosome (Δm) leads to lethality in female embryos as a result of defective imprinted XCI. We provide evidence that in Δm female embryos the initial formation of Xist clouds and Xp silencing are inhibited. In contrast, embryonic stem cells lacking RLIM are able to form Xist clouds and silence at least some X-linked genes during random XCI. These results assign crucial functions to the maternal deposit of Rnf12/RLIM for the initiation of imprinted XCI.     

cdx4 mutants fail to specify blood progenitors and can be rescued by multiple hox genes

Organogenesis is dependent on the formation of distinct cell types within the embryo. Important to this process are the hox genes, which are believed to confer positional identities to cells along the anteroposterior axis. Here, we have identified the caudal-related gene cdx4 as the locus mutated in kugelig (kgg), a zebrafish mutant with an early defect in haematopoiesis that is associated with abnormal anteroposterior patterning and aberrant hox gene expression. The blood deficiency in kgg embryos can be rescued by overexpressing hoxb7a or hoxa9a but not hoxb8a, indicating that the haematopoietic defect results from perturbations in specific hox genes. Furthermore, the haematopoietic defect in kgg mutants is not rescued by scl overexpression, suggesting that cdx4 and hox genes act to make the posterior mesoderm competent for blood development. Overexpression of cdx4 during zebrafish development or in mouse embryonic stem cells induces blood formation and alters hox gene expression. Taken together, these findings demonstrate that cdx4 regulates hox genes and is necessary for the specification of haematopoietic cell fate during vertebrate embryogenesis.     

Antero-posterior tissue polarity links mesoderm convergent extension to axial patterning

Remodelling its shape, or morphogenesis, is a fundamental property of living tissue. It underlies much of embryonic development and numerous pathologies. Convergent extension (CE) of the axial mesoderm of vertebrates is an intensively studied model for morphogenetic processes that rely on cell rearrangement. It involves the intercalation of polarized cells perpendicular to the antero-posterior (AP) axis, which narrows and lengthens the tissue. Several genes have been identified that regulate cell behaviour underlying CE in zebrafish and Xenopus. Many of these are homologues of genes that control epithelial planar cell polarity in Drosophila. However, elongation of axial mesoderm must be also coordinated with the pattern of AP tissue specification to generate a normal larval morphology. At present, the long-range control that orients CE with respect to embryonic axes is not understood. Here we show that the chordamesoderm of Xenopus possesses an intrinsic AP polarity that is necessary for CE, functions in parallel to Wnt/planar cell polarity signalling, and determines the direction of tissue elongation. The mechanism that establishes AP polarity involves graded activin-like signalling and directly links mesoderm AP patterning to CE.