Ventricular muscle-restricted targeting of the RXRα gene reveals a non-cell-autonomous requirement in cardiac chamber morphogenesis

Mouse embryos lacking the retinoic acid receptor gene RXR alpha die in midgestation from hypoplastic development of the myocardium of the ventricular chambers and consequent cardiac failure. In this study, we address the issue of whether the RXR alpha: gene is required in the cardiomyocyte lineage by generating mice that harbor a ventricular restricted deficiency in RXR alpha at the earliest stages of ventricular chamber specification. We first created a conditional ('floxed') allele of RXR alpha by flanking a required exon of the gene with loxP recombination sequences. To achieve ventricular myocardium-specific gene targeting, and to avoid potential transgenic artifacts, we employed a knock-in strategy to place cre recombinase coding sequences into the myosin light chain 2v (MLC2v) genomic locus, a gene which in the heart is expressed exclusively in ventricular cardiomyocytes at the earliest stages of ventricular specification. Crossing the MLC2v-cre allele with the flexed RXR alpha gene resulted in embryos in which approximately 80% of the ventricular cardiomyocytes lacked RXR alpha function, and Set which displayed a completely normal phenotype, without evidence of the wide spectrum of congenital heart disease phenotype seen in RXRa-/- embryos, and normal adult viability. We conclude that the RXR alpha mutant phenotype is not cell autonomous for the cardiomyocyte lineage, and suggest that RXR alpha functions in a neighboring compartment of the developing heart to generate a signal that is required for ventricular cardiomyocyte development and chamber maturation.