Rac1 Signaling Is Critical to Cardiomyocyte Polarity and Embryonic Heart Development

Abstract

BackgroundDefects in cardiac septation are the most common form of congenital heart disease, but the mechanisms underlying these defects are still poorly understood. The small GTPase Rac1 is implicated in planar cell polarity of epithelial cells in Drosophila; however, its role in mammalian cardiomyocyte polarity is not clear. We tested the hypothesis that Rac1 signaling in the second heart field regulates cardiomyocyte polarity, chamber septation, and right ventricle development during embryonic heart development.Methods and ResultsMice with second heart field–specific deficiency of Rac1 (Rac1SHF) exhibited ventricular and atrial septal defects, a thinner right ventricle myocardium, and a bifid cardiac apex. Fate‐mapping analysis showed that second heart field contribution to the interventricular septum and right ventricle was deficient in Rac1SHF hearts. Notably, cardiomyocytes had a spherical shape with disrupted F‐actin filaments in Rac1SHF compared with elongated and well‐aligned cardiomyocytes in littermate controls. Expression of Scrib, a core protein in planar cell polarity, was lost in Rac1SHF hearts with decreased expression of WAVE and Arp2/3, leading to decreased migratory ability. In addition, Rac1‐deficient neonatal cardiomyocytes displayed defects in cell projections, lamellipodia formation, and cell elongation. Furthermore, apoptosis was increased and the expression of Gata4, Tbx5, Nkx2.5, and Hand2 transcription factors was decreased in the Rac1SHF right ventricle myocardium.ConclusionsDeficiency of Rac1 in the second heart field impairs elongation and cytoskeleton organization of cardiomyocytes and results in congenital septal defects, thin right ventricle myocardium, and a bifid cardiac apex. Our study suggests that Rac1 signaling is critical to cardiomyocyte polarity and embryonic heart development.

hace1 Influences zebrafish cardiac development via ROS‐dependent mechanisms

“…However, several murine studies have highlighted the importance of Rac1 in the heart. Rac1 SHF neonatal mice exhibited severe heart defects similar to common human congenital heart defects (CHDs), including atrial and ventricular septal defects, ventricular bifurcation, and poorly developed right ventricular myocardium (Leung et al, 2014). This is consistent with our finding of increased cardiomyocyte-specific ROS production (Fig.…”

“…This is not surprising since global knockout of Rac1 in mice is embryonically lethal. Second, mice deficient in Rac1 solely in the second heart field (SHF) have recently been developed (Rac1 SHF ) (Leung et al, 2014). However, several murine studies have highlighted the importance of Rac1 in the heart.…”

Rac1 Signaling Is Required for Anterior Second Heart Field Cellular Organization and Cardiac Outflow Tract Development

“…The anterior SHF‐specific Mef2c‐Cre transgenic mice were rederived from embryos obtained from the Mutant Mouse Regional Resource Centers (Chapel Hill, NC) . A breeding program to generate Mef2c‐Cre;Rac1 f/f ( Rac1 SHF ) mice was carried out and genotyping was performed, as described previously . All mouse experiments and procedures were approved by the animal use subcommittee at the University of Western Ontario in accordance with the guidelines of the Canadian Council of Animal Care.…”

“…Neonatal and embryonic samples were fixed overnight in 4% paraformaldehyde at 4°C, dehydrated, and embedded in paraffin. Embryos were serially sectioned as 5‐μm‐thick sections with a Leica RM2255 microtome, and sections were mounted onto positively charged albumin/glycerin‐coated slides, as described previously . Slides underwent a dewaxing process and were stained with hematoxylin and eosin or picrosirius red for morphological analysis.…”

“…Specifically, the Cre recombinase sequence in the Mef2c‐Cre transgenic mouse is under regulatory control of an anterior heart field–specific enhancer and promoter, which was first discovered by Dodou et al . To study the role of Rac1 in OFT formation, we generated an anterior SHF‐specific deletion of Rac1 using the Mef2c‐Cre transgenic mouse, as described previously . We showed that Rac1 signaling in the anterior SHF is critical for progenitor cellular organization in the splanchnic mesoderm and that disruptions in Rac1 signaling result in a spectrum of OFT defects, along with aortic valve defects.…”