DWnt4 and wingless elicit similar cellular responses during imaginal development

Wnt genes encode evolutionarily conserved secreted proteins that provide critical functions during development. Although Wnt proteins share highly conserved features, they also show sequence divergence, which almost certainly contributes to the variety of their signaling activities. We previously reported that DWnt4 and wingless (wg), two divergent clustered Wnt genes, can have either antagonist or distinct functions during Drosophila embryogenesis. Here we provide evidence that both genes can elicit similar cellular responses during imaginal development. Ectopic expression of DWnt4 along the anterior/posterior (A/P) boundary of imaginal discs alters morphogenesis of adult appendages. In the wing disc, DWnt4 phenocopies ectopic Wg activity by inducing notum to wing transformation, suggesting similar signaling capabilities of both molecules. In support of this, we demonstrate that DWnt4 can rescue wg loss-of-function phenotypes in the antenna and haltere and is able to substitute for Wg in wing field specification. We also show that both genes are transcribed in overlapping domains in imaginal discs, suggesting that DWnt4 may cooperate with wg during limb patterning.

Molecular, genetic and physiological characterisation of dystrobrevin-like (dyb-1) mutants of Caenorhabditis elegans

Dystrobrevins are protein components of the dystrophin complex, whose disruption leads to Duchenne muscular dystrophy and related diseases. The Caenorhabditis elegans dystrobrevin gene (dyb-1) encodes a protein 38 % identical with its mammalian counterparts. The C. elegans dystrobrevin is expressed in muscles and neurons. We characterised C. elegans dyb-1 mutants and showed that: (1) their behavioural phenotype resembles that of dystrophin (dys-1) mutants; (2) the phenotype of dyb-1 dys-1 double mutants is not different from the single ones; (3) dyb-1 mutants are more sensitive than wild-type animals to reductions of acetylcholinesterase levels and have an increased response to acetylcholine; (4) dyb-1 mutations alone do not lead to muscle degeneration, but synergistically produce a progressive myopathy when combined with a mild MyoD/hlh-1 mutation. All together, these findings further substantiate the role of dystrobrevins in cholinergic transmission and as functional partners of dystrophin.

Genetic suppression of phenotypes arising from mutations in dystrophin-related genes in Caenorhabditis elegans

BACKGROUND

Dystrophin is the product of the gene that is mutated in Duchenne muscular dystrophy (DMD), a progressive neuromuscular disease for which no treatment is available. Mice carrying a mutation in the gene for dystrophin (mdx mice) display only a mild phenotype, but it is aggravated when combined with a mutation in the MyoD gene. The nematode worm Caenorhabditis elegans has a dystrophin homologue (dys-1), but null mutations in dys-1 do not result in muscle degeneration.

RESULTS

We generated worms carrying both the dys-1 null mutation cx18, and a weak mutation, cc561ts, of the C. elegans MyoD homologue hlh-1. The double mutants displayed a time-dependent impairment of locomotion and egg laying, a phenotype not seen in the single mutants, and extensive muscle degeneration. This result allowed us to look for genes that, when misexpressed, could suppress the dys-1; hlh-1 phenotype. When overexpressed, the dyc-1 gene - whose loss-of-function phenotype resembles that of dys-1 - partially suppressed the dys-1; hlh-1 phenotype. The dyc-1 gene encodes a novel protein sharing similarities with the mammalian neural nitric oxide synthase (nNOS)-binding protein CAPON, and is expressed in the muscles of the worm.

CONCLUSIONS

As a C. elegans model for dystrophin-dependent myopathy, the dys-1; hlh-1 worms should permit the identification of genes, and ultimately drugs, that would reverse the muscle degeneration in this model.

DWnt-4 and Wingless have distinct activities in the Drosophila dorsal epidermis

Wnt genes are often expressed in overlapping patterns, where they affect a wide array of developmental processes. To address the way in which various Wnt signals elicit distinct effects we compared the activities of two Wnt genes in Drosophila, DWnt-4, and wingless. We show that these Wnt signals produce distinct responses in cells of the dorsal embryonic epidermis. Whereas wingless acts independently of hedgehog signaling in these cells, we show that DWnt-4 requires Hh to elicit its effects. We also show that expression of Wg signal transduction components does not mimic expression of DWnt-4, suggesting that DWnt-4 signaling proceeds through a distinct pathway. The dorsal epidermis may therefore be useful in the identification of novel Wnt signaling components.

Mutations in the dystrophin-like dys-1 gene of Caenorhabditis elegans result in reduced acetylcholinesterase activity

Mutations of the Caenorhabditis elegans dystrophin/utrophin-like dys-1 gene lead to hyperactivity and hypercontraction of the animals. In addition dys-1 mutants are hypersensitive to acetylcholine and acetylcholinesterase inhibitors. We investigated this phenotype further by assaying acetylcholinesterase activity. Total extracts from three different dys-1 alleles showed significantly less acetylcholinesterase-specific activity than wild-type controls. In addition, double mutants carrying a mutation in the dys-1 gene plus a mutation in either of the two major acetylcholinesterase genes (ace-1 and ace-2) display locomotor defects consistent with a strong reduction of acetylcholinesterases, whereas none of the single mutants does. Therefore, in C. elegans, disruption of the dystrophin/utrophin-like dys-1 gene affects acetylcholinesterase activity.

In vitro interactions of Caenorhabditis elegans dystrophin with dystrobrevin and syntrophin

Dystrophin, the product of the gene mutated in Duchenne muscular dystrophy (DMD) is bound by its C-terminus to a protein complex including the related protein dystrobrevin. Both proteins contain a putative coiled-coil domain consisting of two alpha-helices. It has been reported that the two proteins bind to each other by the first one of the two alpha-helices. We have revisited this question using the Caenorhabditis elegans homologs of dystrophin and dystrobrevin. In vitro interaction occurs through the more conserved second helix. We propose a new model of dystrophin interactions with associated proteins.

Antagonist activity of DWnt-4 and wingless in the Drosophila embryonic ventral ectoderm and in heterologous Xenopus assays

Wnt genes encode secreted signalling molecules involved in a number of basic developmental processes. In Drosophila, wingless and DWnt-4 are two physically clustered Wnt genes, which are transcribed in overlapping patterns during embryogenesis and, in several instances, are controlled by the same regulatory molecules. To address the question of the functional relationship of wingless and DWnt-4, we analysed how embryonic cells respond when they are exposed, simultaneously or not, to the encoded Wnt signals. We show that DWnt-4 has the capacity to antagonise Wingless signalling both in the Drosophila ventral epidermis and in a heterologous system, the Xenopus embryo. We provide evidence that DWnt-4 inhibits the Wingless/Wnt-1 signalling pathway upstream of the activation of transcriptional targets. This is the first report that antagonising Wnt signals exist in Drosophila.

Dystrobrevin- and dystrophin-like mutants display similar phenotypes in the nematode Caenorhabditis elegans

Dystrophin, the protein disrupted in Duchenne muscular dystrophy, forms a transmembrane complex with dystrophin-associated proteins. Dystrobrevins, proteins showing homology to the C-terminal end of dystrophin, and whose function is unknown, are part of the dystrophin complex. We report here that, in the nematode Caenorhabditis elegans, animals carrying mutations in either the dystrophin-like gene dys-1 or the dystrobrevin-like gene dyb-1 display similar behavioral and pharmacological phenotypes consistent with an alteration of cholinergic signalling. These findings suggest that: (1) dystrobrevin and dystrophin are functionally related and (2) their disruption impairs cholinergic signalling.

wingless and DWnt4, 2 Drosophila Wnt genes, have related expression, regulation and function during the embryonic development

The multigenic Wnt family encode secreted signalling molecules with important regulatory functions in various developmental processes. This paper reports an analysis of the relationships, in terms of structure, expression and function, that exist between the Drosophila genes wingless, the orthologue of the mammalian Wnt1 proto-oncogene, and DWnt4, a new member of the Wnt family. The 2 genes are physically clustered, are transcribed in overlapping embryonic territories under the control of the same regulatory molecules. Co-expression and co-regulation suggest first, that the close physical linkage results from the sharing of cis-control elements and second, that the 2 Wnt signals cooperate in developmental patterning events. Antisense RNA experiments revealed that signalling by DWnt4 is essential for cells from the anterior compartment of each parasegment to adopt a denticled fate. We propose that wingless and DWnt4 achieve opposite, but complementary functions in intrasegmental cell patterning of the embryonic ectoderm.

DWnt-4, a novel Drosophila Wnt gene acts downstream of homeotic complex genes in the visceral mesoderm

Wnt genes encode putative cell signalling proteins which play crucial roles during development. From a library of DNA fragments associated, in vivo, with Ultrabithorax proteins, we isolated a novel Drosophila Wnt gene, DWnt-4. Neither a paralog nor an ortholog of the gene exist in the current repertoire of full-length Wnt sequences. DWnt-4 maps close (30 kb) to wingless, suggesting that the two Wnt genes derive from a duplication that occurred early in evolution, since they are significantly diverged in sequence and structure. Developmental expression of DWnt-4 partially overlaps that of wingless. The gene is transcribed following a segment polarity-like pattern in the posterior-most cells of each parasegment of the ectoderm, and at two locations that correspond to parasegments 4 and 8 of the visceral mesoderm. The control of DWnt-4 expression in the visceral mesoderm involves a network of regulatory molecules that includes Ultrabithorax and other proteins from the homeotic complex (HOM-C), as well as the TGF-beta decapentaplegic gene product.