B-Raf is required for normal murine cardiac development and function

Developing cardiomyocytes grow by proliferative and hypertrophic pathways and can be mediated by the rapidly accelerated fibrosarcoma (RAF)/mitogen-activated protein kinase kinase (MEK)/extracellular regulated kinase (ERK) [mitogen-activated protein kinase (MAPK)] pathway. Dysregulation of Raf-1 in late development causes murine myocardial hypertrophy; however, less is known about B-Raf. We hypothesized that the loss of B-Raf (conditional knockout) would lead to reduced neonatal cardiomyocyte proliferation and accelerate maturation, resulting in impaired function in adulthood. We also determined sex differences to ensure complete interrogation of the model. Murine neonatal hearts undergo a proliferative to terminal differentiation switch that is complete by 2 wk of age and thus studied hearts at four time points in that period (days 1, 3, 8, and 14). Primary cultures of knockout (KO) hearts revealed reduced B-Raf and phosphorylated ERK1/2 by postnatal day 3, even when stimulated with insulin growth factor 1. Histology revealed increased cardiomyocyte volume in KO hearts by 3d and elevated markers of hypertrophy and maturation by 8d [mammalian target of rapamycin (mTOR) and SERC2A]. There were also elevated cell cycle inhibitors p21, p27, and p53, accompanied by increased cyclin levels. Heart weight to body weight ratio was greater in 8d KO compared with age-matched wild-type (WT) animals as well as other KO ages, whereas there was no difference among WT ages. The earliest signs of altered myocardium via echocardiographs were detected by 3 mo. KO mice exhibited dilated cardiomyopathy, thinned myocardial walls, and enlarged left chamber volume. Deletion of B-Raf led to adaptive remodeling of male KO hearts. This adaptation maintains cardiac function; however, future studies will interrogate changes in the face of physiological stress.NEW & NOTEWORTHY Raf-1 and B-Raf are upstream kinases that modulate the activation of ERK1/2. We show that conditional deletion of B-Raf is not compensated for by an increase of Raf-1. This suggests that the two Raf isoforms have distinct functions during cardiac development. In this report, we show early molecular and cellular changes that result in functional changes by adulthood. Developing murine cardiomyocytes are sensitive to loss of B-Raf, resulting in dilated cardiomyopathy in adulthood.