Initial retinoid requirement for early avian development coincides with retinoid receptor coexpression in the precardiac fields and induction of normal cardiovascular development

Abstract

Vitamin A requirement for early embryonic development is clearly evident in the gross cardiovascular and central nervous system abnormalities and an early death of the vitamin A-deficient quail embryo. This retinoid knockout model system was used to examine the biological activity of various natural retinoids in early cardiovascular development. We demonstrate that all-trans-, 9-cis-, 4-oxo-, and didehydroretinoic acids, and didehydroretinol and all-trans-retinol induce and maintain normal cardiovascular development as well as induce expression of the retinoic acid receptor beta2 in the vitamin A-deficient quail embryo. The expression of RARbeta2 is at the same level and at the same sites where it is expressed in the normal embryo. Retinoids provided to the vitamin A-deficient embryo up to the 5-somite stage of development, but not later, completely rescue embryonic development, suggesting the 5-somite stage as a critical retinoid-sensitive time point during early avian embryogenesis. Retinoid receptors RARalpha, RARgamma, and RXRalpha are expressed in both the precardiac endoderm and mesoderm in the normal and the vitamin A-deficient quail embryo, while the expression of RXRgamma is restricted to precardiac endoderm. Vitamin A deficiency downregulates the expression of RARalpha and RARbeta. Our studies provide strong evidence for a narrow retinoid-requiring developmental window during early embryogenesis, in which the presence of bioactive retinoids and their receptors is essential for a subsequent normal embryonic development.

Retinoic acid deficiency alters second heart field formation

“…We show that endogenous RA is required to prevent Fgf expression in the posterior lateral mesoderm (including in the posterior region of the SHF). Not only did RA deficiency lead to expanded Fgf8 and Fgf10 (Mlc1v-nlacZ-24-enhancer trap) expression, this expansion occurred at approximately the five-somite stage, when cardiac defects become irreversible in the quail vitamin A-deficiency model (33). Conversely, we saw that excess exogenous RA reduces SHF Fgf8 levels, similar to what has been observed when endogenous RA is increased by blocking the activity of its metabolizing CYP26 enzymes (39).…”

“…Because restoring Hoxa1 expression in RAdeficient amphioxus embryos can rescue patterning defects (32), we postulate that the lack of Hoxa1 expression in the cardiac field may result in cardiac perturbations found in Raldh2 mutants (4). We found that the earliest events of cardiac RA deficiency were the caudal expansions of SHF gene expression, occurring at a stage when cardiac defects become irreversible in an avian vitamin A-deficiency model (33). We propose that these initial molecular alterations in SHF expression are why RA-deficient heart formation cannot be restored after this time.…”

Retinoic acid administration is associated with changes in the extracellular matrix and cardiac mesenchyme within the endocardial cushion

“…RAR␤ mRNA is enhanced by retinoids in mouse (Jiang et al, 1994) and chicken (Lu et al, 1997) embryos. The expression of RAR␣ is downregulated in vitamin A deficient quail embryo and returns to normal levels when RA levels are restored before the 5-somite stage (Kostetskii et al, 1998). Excess RA in this project may turn on or increase certain RA receptor expression or turn off or decrease other receptors, therefore, changing the secretion of the particulate matrix from the myocardium.…”

“…These include the injection of large amounts of RA to the yolk, application of RA to the vitelline membrane, and implanting of RA-soaked beads (Eichele et al, 1984;Bouman et al, 1998;Kostetskii et al, 1998). Implanting RA beads was chosen for this study because the beads allows for local delivery of RA to the heart, avoided the toxic effect of large amounts of RA seen in other delivery systems and avoided significant degradation of RA before it reached the intended site of action.…”

“…To date there is only limited information available about the spatiotemporal distribution of RA receptors in the cardiogenic area. Examination of quail embryos at the 5-somite stage indicated that RAR␣, RAR␥ and RXR␣ are expressed in both the precardiac endoderm and mesoderm whereas the expression of RXR␥ is restricted to precardiac endoderm (Kostetskii et al, 1998). In older stages, RAR␥ was found expressed in the endocardial cushions (Lyons et al, 1992).…”