Early retinoic acid signaling organizes the body axis and defines domains for the forelimb and eye

Abstract

All-trans RA (ATRA) is a small molecule derived from retinol (vitamin A) that directly controls gene expression at the transcriptional level by serving as a ligand for nuclear ATRA receptors. ATRA is produced by ATRA-generating enzymes that convert retinol to retinaldehyde (retinol dehydrogenase; RDH10) followed by conversion of retinaldehyde to ATRA (retinaldehyde dehydrogenase; ALDH1A1, ALDH1A2, or ALDH1A3). Determining what ATRA normally does during vertebrate development has been challenging as studies employing ATRA gain-of-function (RA treatment) often do not agree with genetic loss-of-function studies that remove ATRA via knockouts of ATRA-generating enzymes. In mouse embryos, ATRA is first generated at stage E7.5 by ATRA-generating enzymes whose genes are first expressed at that stage. This article focuses upon what ATRA normally does at early stages based upon these knockout studies. It has been observed that early-generated ATRA performs three essential functions: (1) activation of genes that control hindbrain and spinal cord patterning; (2) repression of Fgf8 in the heart field and caudal progenitors to provide an FGF8-free region in the trunk essential for somitogenesis, heart morphogenesis, and initiation of forelimb fields; and (3) actions that stimulate invagination of the optic vesicle to form the optic cup.

Keywords: Body axis formation; Eye morphogenesis; Forelimb initiation; Neural patterning; Retinoic acid.

Fig. 1

ATRA signaling during early embryogenesis. The diagram shows a 9-somite (E8.5) mouse embryo with anterior at the top and posterior (caudal) at the bottom. At this stage, ATRA signaling activity (shown in blue) is limited to the developing trunk and the eye (optic vesicle). The anterior border of trunk ATRA activity occurs in the posterior portion of hindbrain, and the posterior border of ATRA activity occurs at the border between the trunk and caudal epiblast where neuromesodermal progenitors (NMPs) exist that differentiate into either spinal cord or somites as the body axis extends in the anterior to posterior direction. One major function of ATRA signaling is to repress FGF8 signaling (shown in red) produced by the caudal (A) epiblast and the anterior heart field (B). ATRA repression of the Fgf8 gene ensures that the developing trunk (A) and posterior heart (B) are free of FGF8 signaling, which is essential for several processes including spinal cord neurogenesis, somitogenesis, heart patterning, and initiation of forelimb budding (C).

Fig. 2

ATRA role in limb development deciphered by analysis of Aldh1a2 and Rdh10 knockout mice. ATRA is required to initiate forelimb budding by providing a permissive environment for expression of the forelimb initiation gene Tbx5 in a specific region of trunk lateral plate mesoderm. ATRA performs this function by repressing Fgf8 in the forelimb field that lies between the heart and tail bud which allows forelimb Tbx5 expression (also see Fig. 1). ATRA also functions instructively to initiate Meis2 expression in trunk lateral plate mesoderm as it emerges from the tail bud progenitors during body axis extension. However, ATRA is not required to maintain Meis2 expression in lateral plate mesoderm as the body axis extends or to initiate Meis2 expression in the proximal region of forelimbs and hindlimbs. Thus, although Meis2 is required for limb proximodistal patterning, ATRA is not required for this downstream function of Meis2. Instead, studies on Aldh1a2 and Rdh10 knockout mice support a one-signal model for limb proximodistal patterning in which expression of Fgf8 in the distal limb represses Meis2 and limits its expression to a proximal limb domain.