Zebrafish in the study of early cardiac development

Heart development is a complex process that involves cell specification and differentiation, as well as elaborate tissue morphogenesis and remodeling, to generate a functional organ. The zebrafish has emerged as a powerful model system to unravel the basic genetic, molecular, and cellular mechanisms of cardiac development and function. We summarize and discuss recent discoveries on early cardiac specification and the identification of the second heart field in zebrafish. In addition to the inductive signals regulating cardiac specification, these studies have shown that heart development also requires a repressive mechanism imposed by retinoic acid signaling to select cardiac progenitors from a multipotent population. Another recent advance in the study of early zebrafish cardiac development is the identification of the second heart field. These studies suggest that the molecular mechanisms that regulate the second heart field development are conserved between zebrafish and other vertebrates including mammals and provide insight into the evolution of the second heart field and its derivatives.     

From the Cover: Conditional control of gene function by an invertible gene trap in zebrafish

Conditional mutations are essential for determining the stage- and tissue-specific functions of genes. Here we achieve conditional mutagenesis in zebrafish using FT1, a gene-trap cassette that can be stably inverted by both Cre and Flp recombinases. We demonstrate that intronic insertions in the gene-trapping orientation severely disrupt the expression of the host gene, whereas intronic insertions in the neutral orientation do not significantly affect host gene expression. Cre- and Flp-mediated recombination switches the orientation of the gene-trap cassette, permitting conditional rescue in one orientation and conditional knockout in the other. To illustrate the utility of this system we analyzed the functional consequence of intronic FT1 insertion in supv3l1, a gene encoding a mitochondrial RNA helicase. Global supv311 mutants have impaired mitochondrial function, embryonic lethality, and agenesis of the liver. Conditional rescue of supv311 expression in hepatocytes specifically corrected the liver defects. To test whether the liver function of supv311 is required for viability we used Flp-mediated recombination in the germline to generate a neutral allele at the locus. Subsequently, tissue-specific expression of Cre conditionally inactivated the targeted locus. Hepatocyte-specific inactivation of supv311 caused liver degeneration, growth retardation, and juvenile lethality, a phenotype that was less severe than the global disruption of supv311. Thus, supv311 is required in multiple tissues for organismal viability. Our mutagenesis approach is very efficient and could be used to generate conditional alleles throughout the zebrafish genome. Furthermore, because FT1 is based on the promiscuous Tol2 transposon, it should be applicable to many organisms.High throughput functional genomic and informatic methods have been developed to interrogate the genome and extract functional predictions about many genes at a time. However, careful phenotypic analysis of genetic mutants remains the sine qua non of reductionist biological science. In most experimental organisms, random mutagenesis is the preferred or only mutagenic technique available. DNA alkylating agents, transposable elements, or retroviruses are traditionally used in these organisms. A major limitation of these traditional genetic methods is that they reveal only the earliest and/or most prominent function of a gene as later functions are masked by the earlier phenotype, which is often lethality. To assess later functions, for example in metabolism, aging, or behavior, conditional alleles are required.The development of conditional alleles has proven a boon to studying gene function in temporally or spatially restricted contexts. Traditional conditional alleles disrupt gene function by changing the environment, for example by increasing the temperature. Engineered conditional alleles disrupt gene function by activating a recombination-mediated molecular switch that ablates gene function in one state, but has no functional consequences in the other state (1, 2). In the mouse, engineered conditional alleles can be generated by homologous recombination to insert the molecular switch at defined loci or by retroviral-mediated random insertion of the molecular switch (3, 4). The second approach leverages the orientation-dependent gene disruption of a gene trap and the ability of Flp/Cre recombinases to stably invert the gene trap. By strategically arranging dimers of heterotypical flp- and cre-recombinase binding sites flanking the gene trap, stable inversion is achieved in cis by recombinase-mediated Flip and Excision (FlEx) (5). However, this conditional gene-trap mutagen has not been validated at the organismal level.A distinct advantage of FlEx-based conditional gene-trap mutations is the possibility of stage- and tissue-specific rescue or knockout of the mutated genes. In zebrafish, several gene-trap mutagenesis methods have been developed (6, 7), including the “gene-break” (6, 8) and “FlipTrap” (9) technologies. We set out to test whether the FlEx-based conditional gene-trap mutagenesis approach functions at the organismal level in zebrafish. We show here that a highly mutagenic transposable element can be used for conditional analysis of essential genes.