VENTRICULAR MUSCLE-RESTRICTED TARGETING OF THE RXR-ALPHA GENE REVEALSA 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 ve ntricular chambers and consequent cardiac failure. In this study, we a ddress the issue of whether the RXR alpha: gene is required in the car diomyocyte lineage by generating mice that harbor a ventricular restri cted deficiency in RXR alpha at the earliest stages of ventricular cha mber specification. We first created a conditional ('floxed') allele o f RXR alpha by flanking a required exon of the gene with loxP recombin ation sequences. To achieve ventricular myocardium-specific gene targe ting, 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 ex pressed exclusively in ventricular cardiomyocytes at the earliest stag es of ventricular specification. Crossing the MLC2v-cre allele with th e 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-/- e mbryos, and normal adult viability. We conclude that the RXR alpha mut ant phenotype is not cell autonomous for the cardiomyocyte lineage, an d suggest that RXR alpha functions in a neighboring compartment of the developing heart to generate a signal that is required for ventricula r cardiomyocyte development and chamber maturation.