Retinoid signaling is required to complete the vertebrate cardiac left/right asymmetry pathway

Vitamin A-deficient (VAD) quail embryos have severe abnormalities, includin g a high incidence of reversed cardiac situs. Using this model we examined in vivo the physiological function of vitamin A in the left/right (L/R) car diac asymmetry pathway. Molecular analysis reveals the expression of early asymmetry genes activin receptor IIa, sonic hedgehog, Caronte, Lefty-1, and Fgf8 to be unaffected by the lack of retinoids, while expression of the do wnstream genes nodal-related, snail-related (cSnR), and Pitx2 is altered. I n VAD embryos nodal expression in left lateral plate mesoderm (LPM) is seve rely downregulated and the expression domain altered during neurulation. Si milarly, the expression of cSnR in the right LPM and of Pitx2 in the left s ide posterior heart-forming region (HFR) is downregulated in the VAD embryo s. The lack of retinoids does not cause randomization or ectopic expression of nodal, cSnR, or Pitx2. At the six- to eight-somite stage nodal is expre ssed transiently in the left posterior HFR of normal quail embryos; this ex pression is missing in VAD embryos and may be linked to the loss of Pitx2 e xpression in this region of VAD quail embryos. Administration of retinoids to VAD embryos prior to the six-somite stage rescues the expression of noda l, cSnR, and Pitx2 as well as the randomized VAD cardiac phenotype. There i s an absolute requirement for retinoids at the four- to five-somite develop mental window for cardiogenesis and cardiac L/R specification to proceed no rmally. We conclude that retinoids do not regulate the left/right-specific sidedness assignments for expression of genes on the vertebrate cardiac asy mmetry pathway, but are required during neurulation for the maintenance of adequate levels of their expression and for the development of the posterio r heart tube and a loopable heart. Cardiac asymmetry may be but one of seve ral critical events regulated by retinoid signaling in the retinoid-sensiti ve developmental window. (C) 2000 Academic Press.