Hand2 ensures an appropriate environment for cardiac fusion by limiting Fibronectin function

Abstract

SUMMARYHeart formation requires the fusion of bilateral cardiomyocyte populations as they move towards the embryonic midline. The bHLH transcription factor Hand2 is essential for cardiac fusion; however, the effector genes that execute this function of Hand2 are unknown. Here, we provide in zebrafish the first evidence for a downstream component of the Hand2 pathway that mediates cardiac morphogenesis. Although hand2 is expressed in cardiomyocytes, mosaic analysis demonstrates that it plays a non-autonomous role in regulating cardiomyocyte movement. Gene expression profiles reveal heightened expression of fibronectin 1 (fn1) in hand2 mutant embryos. Reciprocally, overexpression of hand2 leads to decreased Fibronectin levels. Furthermore, reduction of fn1 function enables rescue of cardiac fusion in hand2 mutants: bilateral cardiomyocyte populations merge and exhibit improved tissue architecture, albeit without major changes in apicobasal polarity. Together, our data provide a novel example of a tissue creating a favorable environment for its morphogenesis: the Hand2 pathway establishes an appropriate environment for cardiac fusion through negative modulation of Fn1 levels.

Myocardium and BMP signaling are required for endocardial differentiation

“…The midline expression of fli1a, kdrl, cdh5 and etv2:GFP in endocardial progenitors was largely unaffected in hand2 mutants at the 17-somite stage (Fig. 1I-L), in agreement with previous studies (Garavito-Aguilar et al, 2010). To visualize endocardial development during the stages of the cone formation, we analyzed endocardial and myocardial formation in double-transgenic etv2: GFP; myl7:mCherry embryos at 22-36 hpf.…”

“…Previous studies have demonstrated that endocardial precursors in hand2 mutants migrate to the midline and form the endocardial sheet similar to their wild-type siblings but exhibit defective antero-posterior spreading (Garavito-Aguilar et al, 2010). However, endocardial differentiation has not been previously analyzed in hand2 mutants.…”

“…However, the identity of all fn1-expressing cells in the ALPM region has not been clear. Interestingly, increased fn1 expression has been previously reported in hand2 mutants (Garavito-Aguilar et al, 2010). To analyze further the effect of the hand2 mutation on endocardial morphogenesis, we performed in situ hybridization and fn1:mCherry-NTR reporter analysis (Wang et al, 2013).…”

The novel transmembrane protein Tmem2 is essential for coordination of myocardial and endocardial morphogenesis

“…Similarly, Tmem2 could act to limit the activity of inductive signals that specify AVC endocardium; for example, the similarities between frv mutants and apc mutants suggest a possible role of Tmem2 in restricting Wnt signaling (Hurlstone et al, 2003;Verhoeven et al, 2011). During cardiac fusion, Tmem2 might facilitate the transduction of an unknown myocardial-to-endocardial motility cue; however, this scenario seems unlikely because the endocardium can move to the midline in hand2 mutants, which possess very few cardiomyocytes (Garavito-Aguilar et al, 2010). Alternatively, rather than influencing specific signaling pathways, Tmem2 might impact myocardial and endocardial morphogenesis by modulating the extracellular environment.…”

“…Additionally, either diminished or excessive deposition of extracellular matrix (ECM) can hinder myocardial and endocardial movement (e.g. Arrington and Yost, 2009;Garavito-Aguilar et al, 2010;Trinh and Stainier, 2004). Furthermore, the myocardium and endocardium may impact the behavior of one another.…”