SUMMARYThe availability of animal models of epileptic seizures provides opportunities to identify novel anticonvulsants for the treatment of people with epilepsy. We found that exposure of 2-day-old zebrafish embryos to the convulsant agent pentylenetetrazole (PTZ) rapidly induces the expression of synaptic-activity-regulated genes in the CNS, and elicited vigorous episodes of calcium (Ca2+) flux in muscle cells as well as intense locomotor activity. We then screened a library of ∼2000 known bioactive small molecules and identified 46 compounds that suppressed PTZ-inducedtranscription of the synaptic-activity-regulated gene fos in 2-day-old (2 dpf) zebrafish embryos. Further analysis of a subset of these compounds, which included compounds with known and newly identified anticonvulsant properties, revealed that they exhibited concentration-dependent inhibition of both locomotor activity and PTZ-induced fos transcription, confirming their anticonvulsant characteristics. We conclude that this in situ hybridisation assay for fos transcription in the zebrafish embryonic CNS is a robust, high-throughput in vivo indicator of the neural response to convulsant treatment and lends itself well to chemical screening applications. Moreover, our results demonstrate that suppression of PTZ-induced fos expression provides a sensitive means of identifying compounds with anticonvulsant activities.

Myoblast fusion follows a defined sequence of events that is strikingly similar in vertebrates and invertebrates. Genetic analysis in Drosophila has identified many of the molecules that mediate the different steps in the fusion process; by contrast, the molecular basis of myoblast fusion during vertebrate embryogenesis remains poorly characterised. A key component of the intracellular fusion pathway in Drosophila is the protein encoded by the myoblast city (mbc) gene, a close homologue of the vertebrate protein dedicator of cytokinesis 1 (DOCK1, formerly DOCK180). Using morpholino antisense-oligonucleotide-mediated knockdown of gene activity in the zebrafish embryo, we show that the fusion of embryonic fast-twitch myoblasts requires the activities of Dock1 and the closely related Dock5 protein. In addition, we show that the adaptor proteins Crk and Crk-like (Crkl), with which Dock proteins are known to interact physically, are also required for myoblast fusion.

SUMMARYSox6 has been proposed to play a conserved role in vertebrate skeletal muscle fibre type specification. In zebrafish, sox6 transcription is repressed in slow-twitch progenitors by the Prdm1a transcription factor. Here we identify sox6 cis-regulatory sequences that drive fast-twitch-specific expression in a Prdm1a-dependent manner. We show that sox6 transcription subsequently becomes derepressed in slow-twitch fibres, whereas Sox6 protein remains restricted to fast-twitch fibres. We find that translational repression of sox6 is mediated by miR-499, the slow-twitch-specific expression of which is in turn controlled by Prdm1a, forming a regulatory loop that initiates and maintains the slow-twitch muscle lineage.

Hedgehog (Hh) proteins are members of a family of secreted signaling factors that orchestrate the development of many organs and tissues including those of the gastrointestinal (GI) tract. The requirement for Hh activity is not limited to early development but underlies the homeostasis of a number of tissues, and abnormal activity of the Hh pathway is associated with several GI malignancies. Understanding the roles and mechanisms of action of Hh signaling both in development and postnatally should thus give novel insights into potential treatments for these diseases. Here we focus on the Hh signaling pathway and its role in GI tract development and maintenance and consider the diseases resulting from aberrant Hh activity.

Using sapje zebrafish which lack dystrophin, we have assessed both the quantitation of muscle damage in dystrophic fish, and the efficacy of the proteasomal inhibitor MG132 in reducing the dystrophic symptoms. Fourier analysis of birefringence patterns in normal and dystrophic fish was found to be a simple and reliable quantitative measure of muscle damage. MG132, as in mdx mouse, was found to be effective in reducing muscle damage with an EC50 of 0.4µM. This study adds further to the utility of zebrafish as a model of choice for testing muscular dystrophy therapeutics.

The Hedgehog (Hh) family of signalling molecules is essential for a wide range of developmental processes. Mammalian studies have implicated the Hedgehog pathway in the aetiology of anorectal malformations (ARMs), relatively common congenital anomalies caused by failures in the development of the cloaca. In this study we demonstrate that Hh signalling is absolutely required for the formation of the zebrafish cloaca and that the severity of the posterior gut abnormalities induced by a reduction in Hh activity is dependent on the levels of Hh signal transduction. The complete loss of all Hh activity results in the most severe defects and the critical period for Hh activity is between 34 and 74 hours post fertilisation. Using a range of mutant genotypes that cause notochord and floorplate abnormalities, we show that the source of the Hh signals required for posterior gut formation is the endoderm and not the notochord, as previously postulated in mammalian models of ARMs. We show that Adriamycin, a drug known to cause ARMs in rat, but not chick embryos, has no effect on the development of the zebrafish gastrointestinal tract. These studies establish the zebrafish as a model for ARMs, and for the elucidation of other pathways involved in hindgut developmental processes.

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