The Wnt family of growth factors regulate many different aspects of embryonic development. Assembly of the complete mouse and human genome sequences, plus expressed sequence tag surveys have established the existence of 19 Wnt genes in mammalian genomes. However, despite the importance of model vertebrates for studies in developmental biology, the complete complement of Wnt genes has not been established for nonmammalian genomes. Using genome sequences for chicken (Gallus gallus), frog (Xenopus tropicalis), and fish (Danio rerio and Tetraodon nigroviridis), we have analyzed gene synteny to identify the orthologues of all 19 human Wnt genes in these species. We find that, in addition to the 19 Wnts observed in humans, chicken contained an additional Wnt gene, Wnt11b, which is orthologous to frog and zebrafish Wnt11 (silberblick). Frog and fish genomes contained orthologues of the 19 mammalian Wnt genes, plus Wnt11b and several duplicated Wnt genes. Specifically, the Xenopus tropicalis genome contained 24 Wnt genes, including additional copies of Wnt7-related genes (Wnt7c) and 3 recent Wnt duplications (Wnt3, Wnt9b, and Wnt11). The Danio rerio genome contained 27 Wnt genes with additional copies of Wnt2, Wnt2b, Wnt4b, Wnt6, Wnt7a, and Wnt8a. The presence of the additional Wnt11 sequence (Wnt11b) in the genomes of all ancestral vertebrates suggests that this gene has been lost during mammalian evolution. Through these studies, we identified the frog orthologues of the previously uncharacterized Wnt2, Wnt3, Wnt9a, Wnt9b, Wnt10a, and Wnt16 genes and their expression has been characterized during early Xenopus development.

The African clawed frog, Xenopus laevis, is a valuable model system for studies of vertebrate heart development. In the following review, we describe a range of embryological and molecular methodologies that are used in Xenopus research and discuss key discoveries relating to heart development that have been made using this model system. We also discuss how the sequence of the Xenopus tropicalis genome provides a valuable tool for identification of orthologous genes and for identification of evolutionarily conserved promoter elements. Finally, both forward and reverse genetic approaches are currently being applied to Xenopus for the study of vertebrate heart development.

Myocardin is a cardiac- and smooth muscle-specific cofactor for the ubiquitous transcription factor serum response factor (SRF). Using gain-of-function approaches in the Xenopus embryo, we show that myocardin is sufficient to activate transcription of a wide range of cardiac and smooth muscle differentiation markers in non-muscle cell types. We also demonstrate that, for the myosin light chain 2 gene (MLC2), myocardin cooperates with the zinc-finger transcription factor Gata4 to activate expression. Inhibition of myocardin activity in Xenopus embryos using morpholino knockdown methods results in inhibition of cardiac development and the absence of expression of cardiac differentiation markers and severe disruption of cardiac morphological processes. We conclude that myocardin is an essential component of the regulatory pathway for myocardial differentiation.

The mammalian heart expresses two myosin heavy chain genes (Myh6 and Myh7), which are major components of the thick filaments of the sarcomere. We have determined that a third myosin heavy chain, MYH7B, is also expressed in the myocardium. Developmental analysis shows Myh7b expression in cardiac and skeletal muscle of Xenopus, chick and mouse embryos, and in smooth muscle tissues during later stages of mouse embryogenesis. Myh7b is also expressed in the adult human heart. The promoter region of the Myh7b gene shows remarkable similarity between diverse species, suggesting that transcriptional control mechanisms have been conserved. Using luciferase reporter analysis in rat cardiomyocytes, it can be shown that MEF2, GATA and E-box regulatory elements are essential for efficient expression of the Myh7b gene. In addition two conserved elements that do not correspond to consensus binding sites for known transcription factors are also essential for full transcriptional activity of the Myh7b reporter. Finally, the Myh7b gene shows a transcriptional response similar to Myh6 in response to cardiac hypertrophy.

Myf5 ͉ serum response factor ͉ Xenopus