Myocardium-Specific Deletion of Rac1 Causes Ventricular Noncompaction and Outflow Tract Defects

Background: Left ventricular noncompaction (LVNC) is a cardiomyopathy that can lead to arrhythmias, embolic events and heart failure. Despite our current knowledge of cardiac development, the mechanisms underlying noncompaction of the ventricular myocardium are still poorly understood. The small GTPase Rac1 acts as a crucial regulator of numerous developmental events. The present study aimed to investigate the cardiomyocyte specific role of Rac1 in embryonic heart development. Methods and Results: The Nkx2.5-Cre transgenic mice were crossed with Rac1(f/f) mice to generate mice with a cardiomyocyte specific deletion of Rac1 (Rac1(Nkx2.5)) during heart development. Embryonic Rac1(Nkx2.5) hearts at E12.5–E18.5 were collected for histological analysis. Overall, Rac1(Nkx2.5) hearts displayed a bifid apex, along with hypertrabeculation and a thin compact myocardium. Rac1(Nkx2.5) hearts also exhibited ventricular septal defects (VSDs) and double outlet right ventricle (DORV) or overriding aorta. Cardiomyocytes had a rounded morphology and were highly disorganized, and the myocardial expression of Scrib, a planar cell polarity protein, was reduced in Rac1(Nkx2.5) hearts. In addition, cell proliferation rate was significantly decreased in the Rac1(Nkx2.5) ventricular myocardium at E9.5. Conclusions: Rac1 deficiency in the myocardium impairs cardiomyocyte elongation and organization, and proliferative growth of the heart. A spectrum of CHDs arises in Rac1(Nkx2.5) hearts, implicating Rac1 signaling in the ventricular myocardium as a crucial regulator of OFT alignment, along with compact myocardium growth and development.