By exploiting the experimental benefits of the zebrafish model, we have shown that ventricular cardiomyocytes transdifferentiate into atrial cardiomyocytes in the absence of nkx gene function, highlighting the malleable nature of differentiated myocardium. We propose that the newly revealed functions of nkx genes in preserving chamber-specific traits will help to explain the molecular, cellular, and electrophysiological phenotypes in models of nkx2-5 deficiency. In the long-term, these studies have potential to shed light on etiologies of fetal and neonatal cardiac pathology and to drive innovations in regenerative medicine.
Maintenance of Cardiac Chamber Identity by nkx Genes
With novel loss-of-function mutants for nkx2.5 and nkx2.7, analysis through in situ hybridization, immunostaining, cell counting, tracking and labeling experiments have revealed a crucial role for nkx genes in the preservation of unique ventricular cardiomyocyte characteristics. We aim to uncover the cellular and molecular mechanisms mediating the roles of nkx genes in chamber-specific identity maintenance. Specifically, we are interested in identifying downstream target genes through RNA-Seq and we are also employing technologies to capture transcriptomes and epigenetic marks in earlier cardiogenic lineages.
Regulation of nkx Genes in the Secondary Heart Field
The function of nkx2.5 and nkx2.7 in the secondary heart field has yet to be fully elucidated, yet this late-differentiating cardiac lineage plays an important role in the development of conotruncal and atrial abnormalities. Recent data from our lab suggest that nkx genes are required for the addition of late-differentiating cells at the arterial pole and are necessary to restrict contribution of this population to the venous pole. Insights gained from these studies will shed light on the molecular and cellular basis underlying the patterning of the inflow and outflow regions of the heart and have the potential to uncover novel findings regarding the development of the conduction system.
Temporal Regulation of nkx2.5
In this project, we aim to dissect the distinct roles of nkx genes at different developmental stages in the early- and late-differentiating cardiomyocyte populations. Using our novel transgenic line, Tg(hsp70:nkx2.5-GFP), we can systematically evaluate the timing of nkx gene function in cardiac chamber formation and preservation of identity. Recent data demonstrate that nkx2.5 is necessary from mid-late somitogenesis in maintenance of ventricular and atrial chamber morphology and cellular traits. This innovative idea broadens our appreciation of the initial roles of nkx genes to include chamber identity maintenance, and it also couples an early necessity with a later function. Furthermore, we show that overexpression of nkx genes during this critical window maintains a functional cardiac rescue through adulthood, thus, suggesting that exposure at an early stage mandates cellular “memory” during later development.