Hand1 regulates cardiomyocyte proliferation versus differentiation in the developing heart

Differentially expressed genes in embryonic cardiac tissues of mice lacking Folr1 gene activity

BACKGROUND

Heart anomalies are the most frequently observed among all human congenital defects. As with the situation for neural tube defects (NTDs), it has been demonstrated that women who use multivitamins containing folic acid peri-conceptionally have a reduced risk for delivering offspring with conotruncal heart defects 123. Cellular folate transport is mediated by a receptor or binding protein and by an anionic transporter protein system. Defective function of the Folr1 (also known as Folbp1; homologue of human FRalpha) gene in mice results in inadequate transport, accumulation, or metabolism of folate during cardiovascular morphogenesis.

RESULTS

We have observed cardiovascular abnormalities including outflow tract and aortic arch arterial defects in genetically compromised Folr1 knockout mice. In order to investigate the molecular mechanisms underlying the failure to complete development of outflow tract and aortic arch arteries in the Folr1 knockout mouse model, we examined tissue-specific gene expression difference between Folr1 nullizygous embryos and morphologically normal heterozygous embryos during early cardiac development (14-somite stage), heart tube looping (28-somite stage), and outflow track septation (38-somite stage). Microarray analysis was performed as a primary screening, followed by investigation using quantitative real-time PCR assays. Gene ontology analysis highlighted the following ontology groups: cell migration, cell motility and localization of cells, structural constituent of cytoskeleton, cell-cell adhesion, oxidoreductase, protein folding and mRNA processing. This study provided preliminary data and suggested potential candidate genes for further description and investigation.

CONCLUSION

The results suggested that Folr1 gene ablation and abnormal folate homeostasis altered gene expression in developing heart and conotruncal tissues. These changes affected normal cytoskeleton structures, cell migration and motility as well as cellular redox status, which may contribute to cardiovascular abnormalities in mouse embryos lacking Folr1 gene activity.

Hand transcription factors cooperatively regulate development of the distal midline mesenchyme

Hand proteins are evolutionally conserved basic helix-loop-helix (bHLH) transcription factors implicated in development of neural crest-derived tissues, heart and limb. Hand1 is expressed in the distal (ventral) zone of the branchial arches, whereas the Hand2 expression domain extends ventrolaterally to occupy two-thirds of the mandibular arch. To circumvent the early embryonic lethality of Hand1 or Hand2-null embryos and to examine their roles in neural crest development, we generated mice with neural crest-specific deletion of Hand1 and various combinations of mutant alleles of Hand2. Ablation of Hand1 alone in neural crest cells did not affect embryonic development, however, further removing one Hand2 allele or deleting the ventrolateral branchial arch expression of Hand2 led to a novel phenotype presumably due to impaired growth of the distal midline mesenchyme. Although we failed to detect changes in proliferation or apoptosis between the distal mandibular arch of wild-type and Hand1/Hand2 compound mutants at embryonic day (E)10.5, dysregulation of Pax9, Msx2 and Prx2 was observed in the distal mesenchyme at E12.5. In addition, the inter-dental mesenchyme and distal symphysis of Meckel's cartilage became hypoplastic, resulting in the formation of a single fused lower incisor within the hypoplastic fused mandible. These findings demonstrate the importance of Hand transcription factors in the transcriptional circuitry of craniofacial and tooth development.

Msx1 and Msx2 regulate survival of secondary heart field precursors and post-migratory proliferation of cardiac neural crest in the outflow tract

Msx1 and Msx2 are highly conserved, Nk-related homeodomain transcription factors that are essential for a variety of tissue-tissue interactions during vertebrate organogenesis. Here we show that combined deficiencies of Msx1 and Msx2 cause conotruncal anomalies associated with malalignment of the cardiac outflow tract (OFT). Msx1 and Msx2 play dual roles in outflow tract morphogenesis by both protecting secondary heart field (SHF) precursors against apoptosis and inhibiting excessive proliferation of cardiac neural crest, endothelial and myocardial cells in the conotruncal cushions. During incorporation of SHF precursors into the OFT myocardium, ectopic apoptosis in the Msx1-/-; Msx2-/- mutant SHF is associated with reduced expression of Hand1 and Hand2, which from work on Hand1 and Hand2 mutants may be functionally important in the inhibition of apoptosis in Msx1/2 mutants. Later during aorticopulmonary septation, excessive proliferation in the OFT cushion mesenchyme and myocardium of Msx1-/-; Msx2-/- mutants is associated with premature down-regulation of p27(KIP1), an inhibitor of cyclin-dependent kinases. Diminished accretion of SHF precursors to the elongating OFT myocardium and excessive accumulation of mesenchymal cells in the conotruncal cushions may work together to perturb the rotation of the truncus arteriosus, leading to OFT malalignment defects including double-outlet right ventricle, overriding aorta and pulmonary stenosis.

Cardiac progenitors and the embryonic cell cycle

Despite the critical importance of proper cell cycle regulation in establishing the correct morphology of organs and tissues during development, relatively little is known about how cell proliferation is regulated in a tissue-specific manner. The control of cell proliferation within the developing heart is of considerable interest, given the high prevalence of congenital cardiac abnormalities among humans, and recent interest in the isolation of cardiac progenitor populations. We therefore review studies exploring the contribution of cell proliferation to overall cardiac morphology and the molecular mechanisms regulating this process. In addition, we also review recent studies that have identified progenitor cell populations within the adult myocardium, as well as those exploring the capability of differentiated myocardial cells to proliferate post-natally. Thus, the exploration of cardiomyocyte cell cycle regulation, both during development as well as in the adult heart, promises to yield many exciting and important discoveries over the coming years.

Combinatorial signaling in the heart orchestrates cardiac induction, lineage specification and chamber formation

The complexity of mammalian cardiogenesis is compounded, as the heart must function in the embryo whilst it is still being formed. Great advances have been made recently as additional cardiac progenitor cell populations have been identified. The induction and maintenance of these progenitors, and their deployment to the developing heart relies on combinatorial molecular signalling, a feature also of cardiac chamber formation. Many forms of congenital heart disease in humans are likely to arise from defects in the early stages of heart development; therefore it is important to understand the molecular pathways that underlie some of the key events that shape the heart during the early stages of it development.