In their recent publication, Afouda et al. reported expression of a cardiomyocyte-specific marker, MHCα, in GATA4-GR-injected animal caps at stages 11 and 20 [see figure 3 in Afouda et al. (Afouda et al., 2008)]. This unusual result could have important implications for studies of early heart development in Xenopus and it deserves further attention. This is because the onset of MHCα expression in Xenopus embryos and, more generally, the beginning of cardiomyocyte differentiation, are thought to occur after stage 28 (Logan and Mohun, 1993), and because animal caps expressing GATA4-GR are thought to faithfully reproduce normal cardiomyocyte differentiation (Latinkic et al., 2003).
Our own RT-PCR expression analysis, using sequence-verified amplicons derived from the 5′, central and 3′ regions of the MHCα transcript, confirms that MHCα is only expressed from the onset of cardiomyocyte differentiation (stage 28). We also find, in contrast to Afouda et al., that GATA4-GR-injected animal caps only express MHCα on schedule, i.e. at the time the gene is expressed in sibling control embryos (see Fig. S1 in the supplementary material). We therefore consider GATA4-GR-expressing animal caps to be a valid model of cardiogenesis that recapitulates the normal course of cardiomyocyte differentiation.
The reasons for the discrepancy between our results and those of Afouda et al. are unclear at present, but are likely to include the details of experimental design. Irrespective of the causes of the differences between our results, we feel that it is important to clarify the utility of both the GATA4-GR-expressing animal cap model of cardiogenesis and of MHCα as a specific marker of cardiomyocyte differentiation.
