Irx4 forms an inhibitory complex with the vitamin D and retinoic X receptors to regulate cardiac chamber-specific slow MyHC3 expression

Developmental pattern of ANF gene expression reveals a strict localization of cardiac chamber formation in chicken

In mouse, atrial natriuretic factor (ANF) gene expression was shown to be a marker for chamber formation within the embryonic heart. To gain insight into the process of chamber formation in the chicken embryonic heart, we analyzed the expression pattern of cANF during development. We found cANF to be specifically expressed in the myocardium of the morphologically distinguishable atrial and ventricular chambers, similar to ANF in mouse. cANF expression was never detected in the myocardium of the atrioventricular canal (AVC), inner curvature, and outflow tract (OFT), which is lined by endocardial cushions. Expression was strictly excluded from the interventricular myocardium and most proximal part of the bundle branches, as identified by the expression of Msx-2, whereas the rest of the bundle branches, trabeculae, and surrounding working myocardium did express cANF. The myocardium that forms de novo within the cushions after looping did not express cANF. At HH9 cANF expression was first observed in a subset of cardiomyocytes, which was localized ventrally in the fused heart tube and laterally in the unfused cardiac sheets. Together, these results show that cANF expression can be used to distinguish differentiated chamber (working) myocardium, including the peripheral ventricular conduction system, from embryonic myocardium. We conclude that differentiation of chamber myocardium takes place already at HH9 at the ventral side of the linear heart tube, possibly preceded by latero-medial signals in the unfused cardiac sheets.

Regulation of heart development and function through combinatorial interactions of transcription factors

Understanding the molecular mechanisms controlling cardiac-specific gene transcription requires the dissection of the cis-elements that govern the complex spatio-temporal expression of these genes. The four-chambered vertebrate heart is formed during the late phases of fetal development following a series of complex morphogenetic events that require the functional presence of different proteins. The gradient-like expression of some genes, as well as the chamber-specific expression of others, is tightly regulated by combinatorial interactions of several transcription factors and their cofactors. Chamber- and stage-specific cardiac myocyte cultures have been invaluable for identifying transcription factor binding sites involved in basal, chamber-specific, and inducible expression of many cardiac promoters; these studies, which were largely confirmed in vivo in transgenic mouse models, led to the isolation of key regulators of heart development. In addition, the use of pluripotent embryonic stem cells helped elucidate the early molecular events controlling cardiomyocyte differentiation. Together, these studies point to a major role for GATA transcription factors and their interacting partners in transcriptional control of heart development. In addition, members of the T-box family of transcription factors and homeodomain containing proteins, together with chamber-restricted transcriptional repressors and co-repressors play critical roles in heart septation and chamber specification. These fine-tuned cooperative interactions between different classes of proteins are at the basis of normal cardiac function, and alteration in their expression level or function leads to cardiac pathologies.

Retinoid signaling and cardiac anteroposterior segmentation

Establishment of anterior-posterior polarity is one of the earliest decisions in cardiogenesis. Specification of anterior (outflow) and posterior (inflow) structures ensures proper connections between venous system and inflow tract and between arterial tree and outflow tract. The last few years have witnessed remarkable progress in our understanding of cardiac anteroposterior patterning. Molecular cloning and subsequent studies on RALDH2, the key embryonic retinaldehyde dehydrogenase in retinoic acid (RA) synthesis, provided the missing link between teratogenic studies on RA deficiency and excess and normal chamber morphogenesis. We discuss work establishing the foundations of our current understanding of the mechanisms of cardiac anteroposterior segmentation, the reasons why early evidence pointing to the role of RA in anteroposterior segmentation was overlooked, and the key experiments unraveling the role of RA in cardiac anteroposterior segmentation. We have also integrated recent experiments in a model of cardiac anteroposterior patterning in which RALDH2 expression determines anteroposterior boundaries in the heart field.