Hearts of mice lacking Isl1, a LIM homeodomain transcription factor, are completely missing the outflow tract, right ventricle, and much of the atria. isl1 expression and lineage tracing of isl1-expressing progenitors demonstrate that Isl1 is a marker for a distinct population of undifferentiated cardiac progenitors that give rise to the cardiac segments missing in isl1 mutants. Isl1 function is required for these progenitors to contribute to the heart. In isl1 mutants, isl1-expressing progenitors are progressively reduced in number, and FGF and BMP growth factors are downregulated. Our studies define two sets of cardiogenic precursors, one of which expresses and requires Isl1 and the other of which does not. Our results have implications for the development of specific cardiac lineages, left-right asymmetry, cardiac evolution, and isolation of cardiac progenitor cells.

Transcriptional readout downstream of canonical Wnt signaling is known to be mediated by β-catenin activation of well-described targets, but potential transcriptional readout in response to noncanonical Wnt signaling remains poorly understood. Here, we define a transcriptional pathway important in noncanonical Wnt signaling. We have found that Wnt11 is a direct target of a canonical β-catenin pathway in developing heart and that Wnt11 mutants show cardiac outflow tract defects. We provide genetic and biochemical evidence that Wnt11 signaling affects Reprints and permissions information is available online at

In these studies, we have taken advantage of a transient transgenic strategy in Xenopus embryos to demonstrate that BMP signaling is required in vivo for heart formation in vertebrates. Ectopic expression of dominant negative Type I (tALK3) or Type II (tBMPRII) BMP receptors in developing Xenopus embryos results in reduction or absence of heart formation. Additionally, blocking BMP signaling in this manner downregulates expression of XNkx2-5, a homeobox gene required for cardiac specification, prior to differentiation. Notably, however, initial expression of XNkx2-5 is not affected. Mutant phenotypes can be rescued by co-injection of mutant with wild-type receptors or co-injection of mutant receptors with XSmad1, a downstream mediator of BMP signaling. Whole-mount in situ analyses indicate that ALK3 and XSmad1 are coexpressed in cardiogenic regions. Together, our results demonstrate that BMP signaling is required for maintenance of XNkx2-5 expression and heart formation and suggest that ALK3, BMPRII, and XSmad1 may mediate this signaling.

HCN4 is a hyperpolarization-activated nucleotide-gated cation channel involved in the generation of the I f current that drives cardiac pacemaker activity. Previous studies have demonstrated that HCN4 is highly expressed in a restricted manner in adult sinoatrial (SA) node [Eur. J. Biochem. 268 (2001) 1646]. However, its developmental expression pattern is unknown. We have examined expression of HCN4 mRNA during mouse heart development. HCN4 mRNA was first detected in the cardiac crescent at embryonic day (ED) 7.5. At ED 8 it was symmetrically located in the most caudal portion of the heart tube, the sinus venosus where pacemaker activity has previously been reported [Am. J. Physiol. 212 (1967) 407]. With further development, HCN4 expression became asymmetrically distributed, occupying the dorsal wall of the right atria, and was progressively restricted to the junction of the right atrial appendage and the superior vena cava. The site of HCN4 expression in late embryonic heart coincided with the location of the SA node in postnatal and adult heart [Cardiovasc. Res. 52 (2001) 51]. Our results suggest that HCN4 may be a unique marker of the developing SA node KeywordsHyperpolarization-activated cyclic nucleotide-gated cation channels; Sinus venosus; Development; Pacemaker Pacemaker activity in the heart sinoatrial (SA) node is controlled by a depolarizing, Na + /K + current named I f or I h (Accili et al., 2002;DiFrancesco, 1993DiFrancesco, , 1995. Although the pacemaker current has been known for more than 20 years, cloning of the pacemaker channels responsible for the I f current occurred only recently (Ludwig et al., 1998;Santoro et al., 1998). Today, four members of the HCN (hyperpolarization-activated cyclic nucleotide-gated cation channels) family have been reported, including HCN1, HCN2, HCN3 and HCN4.Both HCN2 and HCN4 are expressed in the murine adult SA node. HCN2 shows a moderate level of expression within the node, and a broader distribution that includes ventricular myocardium; HCN4 is restricted to the node with a high level of expression, constituting a good molecular marker for the adult cardiac pacemaker (Moosmang et al., 2001). HHS Public Access Author Manuscript Author ManuscriptAuthor Manuscript Author ManuscriptThe earliest stage at which cardiac pacemaker activity has been recorded is in the nonbeating heart of an 8 (sp) chick embryo (which would correspond to a 5-10 (sp) mouse embryo (Van Mierop, 1967). At this stage, an impulse generated in the prospective SA region is conducted to the bulboventricular portion of the heart. Later on in development, the impulse that triggers the heartbeat likewise originates from the caudalmost region of the heart tube, the sinus venosus.Based on the previously reported specific expression of the pacemaker channel HCN4 in the SA node in adult mouse heart (Moosmang et al., 2001) we have studied its pattern of expression during development. Our studies reveal an intriguing early appearance of HCN4 mRNA in the cardiac crescent, and also suggest tha...