Dickkopf1 is required for embryonic head induction and limb morphogenesis in the mouse.

Dickkopf1 (Dkk1) is a secreted protein that acts as a Wnt inhibitor and, together with BMP inhibitors, is able to induce the formation of ectopic heads in Xenopus. Here, we show that Dkk1 null mutant embryos lack head structures anterior of the midbrain. Analysis of chimeric embryos implicates the requirement of Dkk1 in anterior axial mesendoderm but not in anterior visceral endoderm for head induction. In addition, mutant embryos show duplications and fusions of limb digits. Characterization of the limb phenotype strongly suggests a role for Dkk1 both in cell proliferation and in programmed cell death. Our data provide direct genetic evidence for the requirement of secreted Wnt antagonists during embryonic patterning and implicate Dkk1 as an essential inducer during anterior specification as well as a regulator during distal limb patterning.

LDL-receptor-related protein 6 is a receptor for Dickkopf proteins.

Wnt glycoproteins have been implicated in diverse processes during embryonic patterning in metazoa. They signal through frizzled-type seven-transmembrane-domain receptors to stabilize beta-catenin. Wnt signalling is antagonized by the extracellular Wnt inhibitor dickkopf1 (dkk1), which is a member of a multigene family. dkk1 was initially identified as a head inducer in Xenopus embryos but the mechanism by which it blocks Wnt signalling is unknown. LDL-receptor-related protein 6 (LRP6) is required during Wnt/beta-catenin signalling in Drosophila, Xenopus and mouse, possibly acting as a co-receptor for Wnt. Here we show that LRP6 (ref. 7) is a specific, high-affinity receptor for Dkk1 and Dkk2. Dkk1 blocks LRP6-mediated Wnt/beta-catenin signalling by interacting with domains that are distinct from those required for Wnt/Frizzled interaction. dkk1 and LRP6 interact antagonistically during embryonic head induction in Xenopus where LRP6 promotes the posteriorizing role of Wnt/beta-catenin signalling. Thus, DKKs inhibit Wnt co-receptor function, exemplifying the modulation of LRP signalling by antagonists.

Dickkopf1 and the Spemann-Mangold head organizer.

Work in amphibians indicates that inhibition of Wnt and BMP signals is essential for head development and that head induction by the Spemann-Mangold organizer may be mediated by secreted Wnt antagonists. Wnts are potent posteriorizing factors and antagonize the Spemann-Mangold organizer. Dickkopf1 (dkk1) encodes a secreted effector expressed in head organizing centers of Xenopus, mouse and zebrafish. It acts as a Wnt inhibitor and is able together with BMP inhibitors to induce the formation of ectopic embryonic heads in Xenopus. It anteriorizes both mesendoderm and neuroectoderm, promoting prechordal plate and forebrain fates. Injection of inhibitory antibodies leads to microcephaly and cyclopia. Dkk1 thus is an essential mediator of the vertebrate head organizer.

The role of Xenopus dickkopf1 in prechordal plate specification and neural patterning.

Dickkopf1 (dkk1) encodes a secreted WNT inhibitor expressed in Spemann's organizer, which has been implicated in head induction in Xenopus. Here we have analyzed the role of dkk1 in endomesoderm specification and neural patterning by gain- and loss-of-function approaches. We find that dkk1, unlike other WNT inhibitors, is able to induce functional prechordal plate, which explains its ability to induce secondary heads with bilateral eyes. This may be due to differential WNT inhibition since dkk1, unlike frzb, inhibits Wnt3a signalling. Injection of inhibitory antiDkk1 antibodies reveals that dkk1 is not only sufficient but also required for prechordal plate formation but not for notochord formation. In the neural plate dkk1 is required for anteroposterior and dorsoventral patterning between mes- and telencephalon, where dkk1 promotes anterior and ventral fates. Both the requirement of anterior explants for dkk1 function and their ability to respond to dkk1 terminate at late gastrula stage. Xenopus embryos posteriorized with bFGF, BMP4 and Smads are rescued by dkk1. dkk1 does not interfere with the ability of bFGF to induce its immediate early target gene Xbra, indicating that its effect is indirect. In contrast, there is cross-talk between BMP and WNT signalling, since induction of BMP target genes is sensitive to WNT inhibitors until the early gastrula stage. Embryos treated with retinoic acid (RA) are not rescued by dkk1 and RA affects the central nervous system (CNS) more posterior than dkk1, suggesting that WNTs and retinoids may act to pattern anterior and posterior CNS, respectively, during gastrulation.

The genomic structure, chromosome location, and analysis of the human DKK1 head inducer gene as a candidate for holoprosencephaly.

Holoprosencephaly (HPE) is the most common developmental defect of the brain and face in humans. Here we report the analysis of the human ortholog of dkk-1 as a candidate gene for HPE. We determined the genomic structure of the human gene DKK1 and mapped it to chromosome 10q11.2. Functional analysis of four missense mutations identified in HPE patients revealed preserved activity in head induction assays in frogs suggesting a limited role for this gene in HPE pathogenesis.

Phenotypic effects in Xenopus and zebrafish suggest that one-eyed pinhead functions as antagonist of BMP signalling.

Zebrafish one-eyed pinhead (oep) is essential for embryonic axis and dorsal midline formation by promoting Nodal signalling and is thought to act as a permissive factor. Here we describe that oep elicits profound phenotypic effects when overexpressed in Xenopus and zebrafish. In Xenopus, wild-type oep inhibits mesoderm induction, disrupts axis formation and neuralizes animal caps. A secreted Oep dorsoanteriorizes and neuralizes Xenopus embryos indicative of BMP inhibition. In zebrafish, misexpression of smad1 in oep mutant embryos also reveals an interaction of oep with BMP signalling. Furthermore, the phenotypic effect of nodal overexpression can be rescued by coexpression of oep both in Xenopus and zebrafish. Taken together, our results support an interaction between oep and nodal but they suggest also (1) that the role of oep in Nodal signalling may include negative as well as positive regulation, (2) that oep is able to function in an active fashion and (3) that oep exerts a regulatory effect on the BMP signalling pathway.

Dissecting GHRH- and pituitary adenylate cyclase activating polypeptide-mediated signalling in Xenopus.

The highly conserved neuropeptide pituitary adenylate cyclase activating polypeptide (PACAP) has been implicated in a broad variety of physiological processes. The PACAP precursor protein gives rise to three different peptides, the cryptic peptide, GHRH, and PACAP, respectively, and here we dissect their functional properties using Xenopus as model system. PACAP and GHRH but not the cryptic peptide directly neuralize animal caps. In contrast to GHRH, the neuralizing effect mediated by PACAP is independent of the PKA pathway. Moreover, PACAP but not GHRH behaves like a BMP-4 antagonist. Blastocoel injection of PACAP-38 but not of the closely related peptides PACAP-27 and VIP leads to strong anteriorization of the injected embryos suggesting the possible involvement of a novel PACAP-preferring receptor.

Mutual antagonism between dickkopf1 and dickkopf2 regulates Wnt/beta-catenin signalling.

Wnts are secreted glycoproteins implicated in diverse processes during embryonic patterning in metazoans. They signal through seven-transmembrane receptors of the Frizzled (Fz) family [1] to stabilise beta-catenin [2]. Wnts are antagonised by several extracellular inhibitors including the product of the dickkopf1 (dkk1) gene, which was identified in Xenopus embryos and is a member of a multigene family. The dkk1 gene acts upstream of the Wnt pathway component dishevelled but its mechanism of action is unknown [3]. Although the function of Dkk1 as a Wnt inhibitor in vertebrates is well established [3-6], the effect of other Dkks on the Wnt/beta-catenin pathway is unclear. Here, we report that a related family member, Dkk2, activates rather than inhibits the Wnt/beta-catenin signalling pathway in Xenopus embryos. Dkk2 strongly synergised with Wnt receptors of the Fz family to induce Wnt signalling responses. The study identifies Dkk2 as a secreted molecule that is able to activate Wnt/beta-catenin signalling. The results suggest that a coordinated interplay between inhibiting dkk1 and activating dkk2 can modulate Fz signalling.

Requirement for Xvent-1 and Xvent-2 gene function in dorsoventral patterning of Xenopus mesoderm.

Xvent-1 and Xvent-2 are members of a novel homeobox subfamily that have been implicated in dorsoventral patterning in Xenopus mesoderm and are thought to function in BMP signalling. Here we investigate the requirement for Xvent function by employing two dominant-negative strategies. Loss of Xvent function dorsalizes ventral mesoderm, induces secondary embryonic axes and directly neuralizes ectoderm. We further find that (1) Xvents act as transcriptional repressors, (2) Xvents function in an additive fashion and (3) a surprising number of genes are able to rescue dominant-negative Xvent phenotypes including Bmp-4, Smad-1 and wild-type Xvents and Xhox3, but not Xwnt-8. The results show that Xvent-1 and Xvent-2 are essential for ventral mesoderm formation and for preventing neural differentiation. A model is suggested to explain how Bmp-4 positional information is converted into distinct cellular responses.

Dickkopf-1 is a member of a new family of secreted proteins and functions in head induction.

The Spemann organizer in amphibian embryos is a tissue with potent head-inducing activity, the molecular nature of which is unresolved. Here we describe dickkopf-1 (dkk-1), which encodes Dkk-1, a secreted inducer of Spemann's organizer in Xenopus and a member of a new protein family. Injections of mRNA and antibody indicate that dkk-1 is sufficient and necessary to cause head induction. dkk-1 s a potent antagonist of Wnt signalling, suggesting that dkk genes encode a family of secreted Wnt inhibitors.

Head induction by simultaneous repression of Bmp and Wnt signalling in Xenopus.

The Spemann organizer of the amphibian embryo can be subdivided into two discrete activities, namely trunk organizer and head organizer. Several factors secreted from the organizer that are involved in trunk organization are thought to act by repressing Bmp signalling. With the exception of the secreted factor cerberus, little is known about head-organizer inducers. Here we show that co-expression of a dominant-negative Bmp receptor with inhibitors of the Wnt-signalling pathway in Xenopus leads to the induction of complete secondary axes, including a head. This induction does not require expression of the siamois marker of Nieuwkoop centre signalling, suggesting that cells are directly shifting to head-organizer fate. Furthermore, we find that cerberus is a potent inhibitor of Wnt signalling. Our results indicate that head-organizer activity results from the simultaneous repression of Bmp and Wnt signalling and they suggest a mechanism for region-specific induction by the organizer.

Combinatorial signalling by Xwnt-11 and Xnr3 in the organizer epithelium.

The epithelium of the Spemann organizer plays an important role in embryonic axis formation and transplantation experiments have shown that epithelial organizer cells have potent axis-inducing potential. Known axis-inducing molecules like noggin and chordin are not expressed in the epithelium and cannot account for its inductive properties. Xwnt-11 is expressed in the epithelium but has only poor dorsalizing activity. In an expression screen for genes that are able to functionally cooperate with Xwnt-11 we have identified a cDNA encoding Xenopus nodal-related 3 (XNR3), a member of the TGF-beta family, coexpressed with Xwnt-11 in the organizer epithelium. Xwnt-11 and Xnr3 act highly cooperatively in inducing secondary embryonic axes and dorsalizing ventral mesoderm. Xwnt-11/Xnr3 interfere with BMP signalling without themselves inducing chordin or noggin. The results indicate that induction by the organizer epithelium may result from the combinatorial action of instructive Xnr3 and permissive Xwnt-11 signalling.

The use of a juvenile hormone binding protein for the quantitative assay of juvenile hormone.

The suitability of the haemolymph juvenile hormone binding protein (JHBP) of Locusta migratoria for use in a competition assay for juvenile hormone (JH) III has been investigated, and a simple quantitative assay procedure using this protein has been developed. JHBP partially purified from haemolymph of precocene treated adult locusts gives rapid and stable binding of [3H]10R-JH III, and can be separated from the unbound hormone with hydroxylapatite (HAP). The sensitivity of the method is such that 0.15 pmol (40 pg) 10R-JH III gives 50% displacement of [3H]10R-JH III from the binding protein. Competition by JH II is about 5 times less and JH I about 10 times less than that by JH III, JH III diol and acid compete at least 1000 times less strongly. A procedure for extraction and assay of JH from 50 microliters haemolymph samples is described, the interference by non-specific haemolymph components is shown to be relatively small, and some data on JH III titres in maturing adult locusts are presented.

Roles of juvenile hormone, a brain factor and adipokinetic hormone in regulation of vitellogenin biosynthesis in locusta migratoria.

In maturing adult female migratory locusts, the rises in JH and Vg in the hemolymph are greatly accelerated by enriched feeding; hereafter the JH titer fluctuates with vitellogenic cycles, falling to a low level at the oviposition stage. In fat bodies incubated in vitro, the JH analog, methoprene, and brain extract from well-fed locusts (but not starved locusts) stimulated Vg synthesis synergistically. Repeated washing of fat bodies from oviposition stage locusts led to a rise in Vg synthesis after 4 h, which was prevented by addition of locust adipokinetic hormone (AKH). We conclude that at least three hormonal factors interact in the control of Vg synthesis in locus fat body: JH and a brain factor stimulate, reflecting development and nutrition, while AKH inhibits at the oviposition stage.

Differential regulation of protein biosynthesis at translational level in the fat body cells of adult Locusta migratoria.

Vitellogenin (Vg) and lipophorin (Lp) are synthesized by the fat body of adult locust (Locusta migratoria) females. We have shown by an immunohistochemical technique that both proteins are produced in the same cells of the fat body. The rate of Vg synthesis was measured with the use of double immunoprecipitation of labeled proteins at oviposition and 24 h later. It was found that the rate of Vg synthesis declined significantly by the time of oviposition; however, 24 h later, it was raised to the highest possible level. The rate of Lp synthesis remained constant at both indicated points. The similar postlaying increase in the Vg synthesis rate was observed in the fat bodies of females treated by alpha-amanitin immediately after oviposition. The data provide evidence that Vg biosynthesis in L. migratoria is regulated by selective periodical repression and derepression of Vg mRNAs in the fat body cells but not by total inhibition and stimulation of protein-synthesizing machinery.

[Glutaraldehyde and glyoxal fixation of ribonucleoproteins for their analysis by cesium chloride equilibrium gradient centrifugation]. / Fiksatsiia ribonukleoproteidov glutarovym al'degidom i glioksalem dlia ikh analiza ravnovesnym tsentriougirovaniem v gradiente plotnosti CsC1.

Conditions for fixation of different RNP (ribosomes, poliribosomes, informosomes) by glutaraldehyde and glyoxal for their subsequent analysis in CsCl density-gradient has been developed. Higher dialdehyde concentration and longer incubation time should be used for fixation of ribosomes and polyribosomes than for that of informosomes. For the fixation of all RNP studied their incubation with 0.01 M (0.1%) glutaraldehyde for several minutes is sufficient. Much higher concentration of the fixating agent (about 0.2-0.5 M i. e. 1-3%) and more prolonged time of incubation (in order of several 10 hours) are needed for the fixation of the RNP in the case of glyoxal. Conditions for selective aldehyde fixation of informosomes in the presence of ribosomes and polyribosomes has been developed.