Article

Retinoic Acid Promotes Limb Induction through Effects on Body Axis Extension but Is Unnecessary for Limb Patterning

Development and Aging Program, Burnham Institute for Medical Research, 10901 North Torrey Pines Road, La Jolla, CA 92037, USA.
Current biology: CB (Impact Factor: 9.57). 07/2009; 19(12):1050-7. DOI: 10.1016/j.cub.2009.04.059
Source: PubMed

ABSTRACT

Retinoic acid (RA) is thought to be a key signaling molecule involved in limb bud patterning along the proximodistal or anteroposterior axes functioning through induction of Meis2 and Shh, respectively. Here, we utilize Raldh2-/- and Raldh3-/- mouse embryos lacking RA synthesis to demonstrate that RA signaling is not required for limb expression of Shh and Meis2. We demonstrate that RA action is required outside of the limb field in the body axis during forelimb induction but that RA is unnecessary at later stages when hindlimb budding and patterning occur. We provide evidence for a model of trunk mesodermal RA action in which forelimb induction requires RA repression of Fgf8 in the developing trunk similar to how RA controls somitogenesis and heart development. We demonstrate that pectoral fin development in RA-deficient zebrafish embryos can be rescued by an FGF receptor antagonist SU5402. In addition, embryo ChIP assays demonstrate that RA receptors bind the Fgf8 promoter in vivo. Our findings suggest that RA signaling is not required for limb proximodistal or anteroposterior patterning but that RA inhibition of FGF8 signaling during the early stages of body axis extension provides an environment permissive for induction of forelimb buds.

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    • "In wild type embryos the LPM exposed to RA signalling is not restricted to the limb forming regions [61]. Therefore, it is likely that RA functions as a permissive factor that confers cells in a broad region of the LPM with limb forming potential. "
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    ABSTRACT: The limbs are a significant evolutionary innovation that enabled vertebrates to diversify and colonise new environments. Tetrapods have two pairs of limbs, forelimbs in the upper body and hindlimbs in the lower body. The morphologies of the forelimbs and hindlimbs are distinct, reflecting their specific locomotory functions although they share many common signaling networks that regulate their development. The paired appendages in vertebrates form at fixed positions along the rostral-caudal axis and this occurs as a consequence of earlier subdivision of the lateral plate mesoderm (LPM) into regions with distinct limb forming potential. In this review, we discuss the molecular mechanisms that confer a broad region of the flank with limb-forming potential and its subsequent refinement into distinct forelimb-forming, hindlimb-forming and interlimb territories.
    Full-text · Article · Nov 2015 · Seminars in Cell and Developmental Biology
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    • "As Raldh3 is expressed in the mesonephros adjoining the hindlimb buds, similar rescue experiments were performed with Raldh2/Raldh3 double mutants . The hindimb buds of the rescued Raldh2/Raldh3 mutants are normal, which has been interpreted as demonstrating that RA is not required for hindlimb outgrowth (Zhao et al., 2009). This interpretation has been contested on the basis that it is Cell Reports 12, 879–891, August 4, 2015 ª2015 The Authors 879 difficult to exclude the possibility that RA administered to the mother has not had some impact on limb formation in the embryos (Roselló -Díez et al., 2014). "
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    ABSTRACT: The retinoic acid (RA)- and β-catenin-signaling pathways regulate limb bud induction and initiation; however, their mechanisms of action are not understood and have been disputed. We demonstrate that both pathways are essential and that RA and β-catenin/TCF/LEF signaling act cooperatively with Hox gene inputs to directly regulate Tbx5 expression. Furthermore, in contrast to previous models, we show that Tbx5 and Tbx4 expression in forelimb and hindlimb, respectively, are not sufficient for limb outgrowth and that input from RA is required. Collectively, our data indicate that RA signaling and Tbx genes act in a coherent feed-forward loop to regulate Fgf10 expression and, as a result, establish a positive feedback loop of FGF signaling between the limb mesenchyme and ectoderm. Our results incorporate RA-, β-catenin/TCF/LEF-, and FGF-signaling pathways into a regulatory network acting to recruit cells of the embryo flank to become limb precursors. Copyright © 2015 The Authors. Published by Elsevier Inc. All rights reserved.
    Full-text · Article · Jul 2015 · Cell Reports
    • "For the mouse Aldh1a2 KOs, although the cardiac phenotypes were initially interpreted as supporting the atrial-ventricular patterning model from cardiac morphology, revisiting the analysis of the cardiac defects with additional cardiac progenitor markers revealed that these mutants also display a posterior expansion of the cardiac progenitors (Ryckebusch et al., 2008; Sirbu et al., 2008). Studies initially in zebrafish and later in mice have suggested that the posterior expansion of the cardiac progenitors is potentially at the expense of neighboring forelimbs progenitors (Waxman et al., 2008; Zhao et al., 2009; Sorrell and Waxman, 2011; Cunningham et al., 2013). Despite the genetic data supporting a conserved requirement for RA in restricting cardiomyocyte specification, there are differences in interpretation as to whether or not there is strictly an expansion of the FHF, the earlier differentiating population of cardiomyocytes , and/or the second SHF, a later differentiating population of cardiomyocytes (Ryckebusch et al., 2008; Sirbu et al., 2008). "
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    ABSTRACT: Appropriate levels of retinoic acid (RA) signaling are critical for normal heart development in vertebrates. A fascinating property of RA signaling is the thoroughness by which positive and negative feedback are employed to promote proper embryonic RA levels. In the present short review, we first cover the advancement of hypotheses regarding the impact of RA signaling on cardiac specification. We then discuss our current understanding of RA signaling feedback mechanisms and the implications of recent studies, which have indicated improperly maintained RA signaling feedback can be a contributing factor to developmental malformations. Developmental Dynamics 244:513-523, 2015. © 2014 Wiley Periodicals, Inc.
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