Nkx genes are essential for maintenance of ventricular identity

Abstract

SUMMARYEstablishment of specific characteristics of each embryonic cardiac chamber is crucial for development of a fully functional adult heart. Despite the importance of defining and maintaining unique features in ventricular and atrial cardiomyocytes, the regulatory mechanisms guiding these processes are poorly understood. Here, we show that the homeodomain transcription factors Nkx2.5 and Nkx2.7 are necessary to sustain ventricular chamber attributes through repression of atrial chamber identity. Mutation of nkx2.5 in zebrafish yields embryos with diminutive ventricular and bulbous atrial chambers. These chamber deformities emerge gradually during development, with a severe collapse in the number of ventricular cardiomyocytes and an accumulation of excess atrial cardiomyocytes as the heart matures. Removal of nkx2.7 function from nkx2.5 mutants exacerbates the loss of ventricular cells and the gain of atrial cells. Moreover, in these Nkx-deficient embryos, expression of vmhc, a ventricular gene, fades, whereas expression of amhc, an atrial gene, expands. Cell-labeling experiments suggest that ventricular cardiomyocytes can transform into atrial cardiomyocytes in the absence of Nkx gene function. Through suggestion of transdifferentiation from ventricular to atrial fate, our data reveal a pivotal role for Nkx genes in maintaining ventricular identity and highlight remarkable plasticity in differentiated myocardium. Thus, our results are relevant to the etiologies of fetal and neonatal cardiac pathology and could direct future innovations in cardiac regenerative medicine.

FGF signaling enforces cardiac chamber identity in the developing ventricle

“…Thus, in addition to preventing ectopic amhc expression in the ventricle, FGF signaling also plays an important role in preserving the relative proportions of the ventricle and the atrium. Intriguingly, the gradual and reciprocal shift in chamber proportions observed in Tg(hsp70:dnfgfr1) embryos echoes the phenotype seen in Nkx-deficient embryos, in which cardiomyocytes appear to undergo a ventricular-to-atrial transformation (Targoff et al, 2013). This similarity suggests that FGF signaling, like Nkx genes, promotes the maintenance of chamber proportions, possibly by preventing a conversion from ventricular to atrial identity.…”

“…(Here, we will refer to these as 'ectopic amhc-expressing cells'.) This scenario is reminiscent of the phenotype of nkx2.5 mutants, which initially exhibit normal expression patterns of amhc and vmhc, but then display ectopic amhc-expressing cells at later stages (George et al, 2015;Targoff et al, 2013). We therefore explored whether this aspect of the fgf8a mutant phenotype reflected a later role for FGF signaling in preventing inappropriate amhc expression, in addition to its earlier influence on the initial production of ventricular cells.…”

“…Bearing in mind the notion that a shift in chamber proportions could result from a ventricular-to-atrial fate transformation (Targoff et al, 2013), we wondered whether the appearance of ectopic amhcexpressing cells in embryos lacking FGF signaling might also reflect a defect in maintaining ventricular identity. Does aberrant amhc expression arise within resident ventricular cardiomyocytes, or do the ectopic amhc-expressing cells originate elsewhere and then enter the ventricle?…”

“…Our recent work in zebrafish has indicated that Nkx transcription factors play important roles in controlling regionalized expression of Irx4 and Hey2 and in maintaining ventricular identity (Targoff et al, 2013). Both irx4a and hey2 are downregulated in nkx2.5; nkx2.7 double mutants; furthermore, loss of Nkx gene function leads to gradual reduction of vmhc expression and simultaneous gain of amhc expression within the ventricle, ultimately yielding an expanded atrium, a diminished ventricle, and ectopic amhcexpressing cells within the ventricular remnant.…”

Role of regeneration in tissue repairing and therapies

“…There are many promising treatment approaches which have shown enormous potential in regenerative medicine for tissue engineering and somatic nuclear transfer (SNT) (Figure 7) [154]. However, mammalian adipose tissue derived stem cells are used for repairing tissue defects.…”