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: 9.92). 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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    • "Fgf8 expression is initiated upon limb but outgrowth in spite of the inhibitory role of RA, and the simulations predict that receptor binding limits diffusion of RA from the flank initially, once RA signalling enhances the expression of RA receptors, as indeed observed in experiments (Noji et al., 1991; Tabin, 1991). According to the model, receptor saturation eventually permits RA to diffuse further distally and to form a gradient that could regulate aspects of proximal-distal limb bud development (Figure 4C) and that could define the proximal part (stylopod -> humerus) of the proximal-distal axis as suggested by two recent studies (Cooper et al., 2011a; Roselló-Díez et al., 2011) and challenged by the Duester group (Cunningham et al., 2013; Zhao et al., 2009). "
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    • "The nomenclature used throughout this work when referring to wing digits of different identities will reflect these findings. Endogenous retinoids are required for the establishment of the proximal part of the forelimb field (Stratford et al. 1996; Niederreither et al. 2002; Mic et al. 2004; Yashiro et al. 2004; Zhao et al. 2009; Cunningham et al. 2013). The ability of topical retinoid application to induce duplication and posteriorisation of digits is thought to result from their ability to induce Hand2 and shh expression (Riddle et al. 1993; Fernandez-Teran et al. 2000) and does not reflect a direct endogenous role for retinoids in antero-posterior patterning of the digits. "
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