Semin Cell Dev Biol
February 1, 2007;
Xenopus as a model system for vertebrate heart 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
Semin Cell Dev Biol
Xla Wt + myocd MO
(Fig 1. C)
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Fig. 1. MO inhibition of myocardin expression results in reduced cardiac gene expression and defective cardiac morphogenesis. (A) Ventral view of embryo injected with control MO and then assayed by in situ hybridization for MHC expression. The paired heart patches are clearly visible. Injected side is indicated with an arrow.
(B) Section through heart of embryo injected with control MO. Heart tissue has been stained for Nkx2–5 transcripts. At this stage the myocardium is a simple linear tube surrounding the endocardial precursor cells. (C) Ventral view of embryo injected with MO directed against myocardin and assayed for MHC expression. Note that no MHC expression is detected on the injected side of the embryo (arrow). (D) Section through heart of embryo injected with myocardin MO. Heart tissue has been stained for Nkx2–5 transcripts. Note that the morphology of the developing heart is disrupted on the injected side (arrow). Adapted from  with permission
Fig. 2. The Xenopus animal cap explant procedure. SyntheticmRNAencoding a
potential regulator of cardiac development is injected into the fertilized Xenopus
egg at the one cell stage. When the embryo has reached the blastula stage,
the animal cap (ectodermal tissue overlying the blastocoel cavity) is carefully
removed using fine forceps and it placed into explant culture. After an appropriate
period in culture, the cap tissue and is subjected to appropriate assays for gene
expression. Reprinted from  with permission from Elsevier.
Fig. 3. Mutational analysis of the ANF promoter using Xenopus transgenesis. (A) Alignment of the proximal promoter region of the human (top) and Xenopus
(bottom) promoter sequences, showing conservation of sequence, spacing and orientation of transcription regulatory sequences. Two conserved sequences that do
not correspond to binding sites for known transcription factors are indicated (Unknown 1 and 2). (B) Fluorescent image of the heart of Xenopus embryo expressing
GFP reporter under control of the wild type ANF promoter. Note that GFP expression is restricted to the atrial chambers (a). (C) Fluorescent image of the heart of
a Xenopus embryo expressing GFP under control of an ANF promoter in which the NKE (Nkx2–5 binding site) has been mutated. Note that mutation of the NKE
prevents restriction of expression to the atria, with prominent expression persisting in the ventricular myocardium. Part A is adapted from  with permission from
Elsevier, and Parts B and C are reprinted from  with permission from Elsevier.
Transgenic Xenopus laevis embryos can be generated using phiC31 integrase.