VITAMIN-A-DEFICIENCY AND MUTATIONS OF RXR-ALPHA, RXR-BETA AND RAR-ALPHA LEAD TO EARLY DIFFERENTIATION OF EMBRYONIC VENTRICULAR CARDIOMYOCYTES

Knock-out of the mouse RXR alpha gene was previously shown to result i n a hypoplastic heart ventricular wall, histologically detectable in 1 2.5 dpc fetuses, We show here that a precocious differentiation can be detected as early as 8.5 dpc in ventricular cardiomyocytes of RXR alp ha(-/-) mutants, This precocious differentiation, which is characteriz ed by the presence of striated myofibrils, sarcoplasmic reticulum and intercalated disks, is found after 9.5 dpc in about 50% of RXR alpha(- /-) subepicardial myocytes. In contrast, wild-type subepicardial myocy tes remain morphologically undifferentiated up to at least 16.5 dpc. A similar precocious differentiation was observed in 9.5 dpc subepicard ial myocytes of several RXR beta(-/-) and RAR alpha(-/-) mutants, as w ell as in vitamin A-deficient embryos, The proportion of differentiate d subepicardial myocytes almost reached 100% in RXR alpha/RXR beta dou ble null mutants, indicating a partial functional redundancy between R XR alpha and RXR beta. This differentiation defect was always parallel ed by a decrease in the mitotic index, In addition, subepicardial myoc ytes of RXR alpha(-/-), RXR alpha(-/-)/RXR beta(-/-) or vitamin A defi cient, but not of RXR beta(-/-) and RAR alpha(-/-) embryos, were often flattened and more loosely connected to one another than those of WT embryos, Thus, retinoids are required at early stages of cardiac devel opment to prevent differentiation, support cell proliferation and cont rol the shape of ventricular myocytes, and both RXRs and RARs particip ate in the mediation of these functions.