Nkx2.5-negative myocardium of the posterior heart field and its correlation with podoplanin expression in cells from the developing cardiac pacemaking and conduction system

Epicardial development in lamprey supports an evolutionary origin of the vertebrate epicardium from an ancestral pronephric external glomerulus

The epicardium is the outer layer of the vertebrate heart. Both the embryonic epicardium and its derived mesenchyme are critical to heart development, contributing to the coronary vasculature and modulating the proliferation of the ventricular myocardium. The embryonic epicardium arises from an extracardiac, originally paired progenitor tissue called the proepicardium, a proliferation of coelomic cells found at the limit between the liver and the sinus venosus. Proepicardial cells attach to and spread over the cardiac surface giving rise to the epicardium. Invertebrate hearts always lack of epicardium, and no hypothesis has been proposed about the origin of this tissue and its proepicardial progenitor in vertebrates. We herein describe the epicardial development in a representative of the most basal living lineage of vertebrates, the agnathan Petromyzon marinus (lamprey). The epicardium in lampreys develops by migration of coelomic cells clustered in a paired structure at the roof of the coelomic cavity, between the pronephros and the gut. Later on, these outgrowths differentiate into the pronephric external glomerulus (PEG), a structure composed of capillary networks, mesangial cells, and podocytes. This observation is consistent with the conclusion that the primordia of the most anterior pair of PEG in agnathans have been retained and transformed into the proepicardium in gnathostomes. Glomerular progenitor cells are highly vasculogenic and probably allowed for the vascularization of a cardiac tube primarily devoid of coronary vessels. This new hypothesis accounts for the striking epicardial expression of Wt1 and Pod1, two transcription factors essential for development of the excretory system.

Targeted mutation reveals essential functions of the homeodomain transcription factor Shox2 in sinoatrial and pacemaking development

BACKGROUND

Identifying molecular pathways regulating the development of pacemaking and coordinated heartbeat is crucial for a comprehensive mechanistic understanding of arrhythmia-related diseases. Elucidation of these pathways has been complicated mainly by an insufficient definition of the developmental structures involved in these processes and the unavailability of animal models specifically targeting the relevant tissues. Here, we report on a highly restricted expression pattern of the homeodomain transcription factor Shox2 in the sinus venosus myocardium, including the sinoatrial nodal region and the venous valves.

METHODS AND RESULTS

To investigate its function in vivo, we have generated mouse lines carrying a targeted mutation of the Shox2 gene. Although heterozygous animals did not exhibit obvious defects, homozygosity of the targeted allele led to embryonic lethality at 11.5 to 13.5 dpc. Shox2-/- embryos exhibited severe hypoplasia of the sinus venosus myocardium in the posterior heart field, including the sinoatrial nodal region and venous valves. We furthermore demonstrate aberrant expression of connexin 40 and connexin 43 and the transcription factor Nkx2.5 in vivo specifically within the sinoatrial nodal region and show that Shox2 deficiency interferes with pacemaking function in zebrafish embryos.

CONCLUSIONS

From these results, we postulate a critical function of Shox2 in the recruitment of sinus venosus myocardium comprising the sinoatrial nodal region.