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Proliferation is one of the basic processes that control embryogenesis. To identify factors involved in the regulation of proliferation, we performed a zebrafish genetic screen in which we used proliferating cell nuclear antigen (PCNA) expression as a readout. Two mutants, hu418B and hu540A, show increased PCNA expression. Morphologically both mutants resembled the dre (dreumes), uki (ukkie), and lep (leprechaun) mutant class and both are shown to be additional uki alleles. Surprisingly, although an increased size is detected of multiple structures in these mutant embryos, adults become dwarfs. We show that these mutations disrupt repressors of the Hedgehog (Hh) signaling pathway. The dre, uki, and lep loci encode Su(fu) (suppressor of fused), Hip (Hedgehog interacting protein), and Ptc2 (Patched2) proteins, respectively. This class of mutants is therefore unique compared to previously described Hh mutants from zebrafish genetic screens, which mainly show loss of Hh signaling. Furthermore, su(fu) and ptc2 mutants have not been described in vertebrate model systems before. Inhibiting Hh activity by cyclopamine rescues uki and lep mutants and confirms the overactivation of the Hh signaling pathway in these mutants. Triple uki/dre/lep mutants show neither an additive increase in PCNA expression nor enhanced embryonic phenotypes, suggesting that other negative regulators, possibly Ptc1, prevent further activation of the Hh signaling pathway. The effects of increased Hh signaling resulting from the genetic alterations in the uki, dre, and lep mutants differ from phenotypes described as a result of Hh overexpression and therefore provide additional insight into the role of Hh signaling during vertebrate development. [ABSTRACT FROM AUTHOR]

TP53 is the most frequently mutated tumor suppressor gene in human cancer, with nearly 50% of all tumors exhibiting a loss-of-function mutation. To further elucidate the genetic pathways involving TPS3 and cancer, we have exploited the zebrafish, a powerful vertebrate model system that is amenable to whole- genome forward-genetic analysis and synthetic-lethal screens. Zebrafish lines harboring missense mutations in the tpS3 DNA- binding domain were identified by using a target-selected mutagenesis strategy. Homozygous mutant fish from two of these lines were viable and exhibited mutations similar to those found in human cancers (tp53N168K and tp53M214K. Although homozygous tp53N168K mutants were temperature-sensitive and suppressed radiation-induced apoptosis only at 37°C, cells in the tp53M214K embryos failed to undergo apoptosis in response to γ radiation at both 28 and 37°C. Unlike wild-type control embryos, irradiated tp53M214K embryos also failed to up-regulate p21 and did not arrest at the G1/S checkpoint. Beginning at 8.5 months of age, 28% of tp53M214K mutant fish developed malignant peripheral nerve sheath tumors. In addition to providing a model for studying the molecular pathogenic pathways of malignant peripheral nerve sheath tumors, these mutant zebrafish lines provide a unique platform for modifier screens to identify genetic mutations or small molecules that affect tp53-related pathways, including apoptosis, cell-cycle delay, and tumor suppression. [ABSTRACT FROM AUTHOR]

Discusses a method to do target-selected mutagenesis in zebrafish. Use of CEL-I-mediated heteroduplex cleavage (TILLING) and sequencing as a screening tool for mutations; Capability of the TILLING to detect the full spectrum of ENU-induced mutations in a vertebrate genome with the presence of many naturally occurring polymorphisms.

Truncation of the tumour suppressor adenomatous polyposis coli (Apc) constitutively activates the Wnt/ß-catenin signalling pathway. Apc has a role in development: for example, embryos of mice with truncated Apc do not complete gastrulation. To understand this role more fully, we examined the effect of truncated Apc on zebrafish development. Here we show that, in contrast to mice, zebrafish do complete gastrulation. However, mutant hearts fail to loop and form excessive endocardial cushions. Conversely, overexpression of Apc or Dickkopf 1 (Dkk1), a secreted Wnt inhibitor, blocks cushion formation. In wild-type hearts, nuclear ß-catenin, the hallmark of activated canonical Wnt signalling, accumulates only in valve-forming cells, where it can activate a Tcf reporter. In mutant hearts, all cells display nuclear ß-catenin and Tcf reporter activity, while valve markers are markedly upregulated. Concomitantly, proliferation and epithelial-mesenchymal transition, normally restricted to endocardial cushions, occur throughout the endocardium. Our findings identify a novel role for Wnt/ß-catenin signalling in determining endocardial cell fate. [ABSTRACT FROM AUTHOR]

The zebrafish has become a favorite organism for genetic analysis of vertebrate development, but methods for generating mutants by reverse genetic approaches have been lacking. We report a method to obtain stable mutants of a gene based on knowledge of the gene sequence only. Parental fish were mutagenized with N-ethyl-N-nitrosourea; in 2679 F, fish, the rag1 gene was analyzed for heterozygous mutations by resequencing. In total, we found 15 mutations: 9 resulted in amino acid substitutions and 1 resulted in a premature stop codon. This truncation mutant was found to be homozygous viable and defective in V(D)J joining. Although presumably immune deficient, these homozygous rag1 mutant fish are able to reach adulthood and are fertile. As sperm samples from all 2679 F[sub 1] fish were collected and cryopreserved, we have in principle generated a mutant library from which mutants of most zebrafish genes can be isolated. [ABSTRACT FROM AUTHOR]

MicroRNAs (miRNAs) are 20–23 nucleotide (nt) RNA molecules that regulate gene expression post-transcriptionally. A key step toward understanding the function of the hundreds of miRNAs identified in animals is to determine their expression during development. Here we performed a detailed analysis of conditions for in situ detection of miRNAs in the zebrafish embryo using locked nucleic acid (LNA)-modified DNA probes and report expression patterns for 15 miRNAs in the mouse embryo. [ABSTRACT FROM AUTHOR]

MicroRNAs (miRNAs) are small noncoding RNAs, about 21 nucleotides in length, that can regulate gene expression by base-pairing to partially complementary mRNAs. Regulation by miRNAs can play essential roles in embryonic development. We determined the temporal and spatial expression patterns of 115 conserved vertebrate miRNAs in zebrafish embryos by microarrays and by in situ hybridizations, using locked-nucleic acid-modified oligonucleotide probes. Most miRNAs were expressed in a highly tissue-specific manner during segmentation and later stages, but not early in development, which suggests that their role is not in tissue fate establishment but in differentiation or maintenance of tissue identity. [ABSTRACT FROM AUTHOR]

MicroRNAs (miRNAs) are produced by the Dicer1 enzyme; the role of Dicer1 in vertebrate development is unknown. Here we report target-selected inactivation of the dicer1 gene in zebrafish. We observed an initial build-up of miRNA levels, produced by maternal Dicer1, in homozygous dicer1 mutants, but miRNA accumulation stopped after a few days. This resulted in developmental arrest around day 10. These results indicate that miRNA-producing Dicer1 is essential for vertebrate development. [ABSTRACT FROM AUTHOR]