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Depletion of retinoic acid receptors initiates a novel positive feedback mechanism that promotes teratogenic increases in retinoic acid.

D'Aniello E, Rydeen AB, Anderson JL, Mandal A, Waxman JS - PLoS Genet. (2013)

Bottom Line: Here, we report that zebrafish embryos deficient for RA receptor αb1 (RARαb1), a conserved RAR splice variant, have enlarged hearts with increased cardiomyocyte (CM) specification, which are surprisingly the consequence of increased RA signaling.Importantly, depletion of RARαb2 or concurrent depletion of RARαb1 and RARαb2 also results in increased RA signaling, suggesting this effect is a broader consequence of RAR depletion.Concurrent depletion of RARαb1 and Cyp26a1, an enzyme that facilitates degradation of RA, and employment of a novel transgenic RA sensor line support the hypothesis that the increases in RA signaling in RAR deficient embryos are the result of increased embryonic RA coupled with compensatory RAR expression.

View Article: PubMed Central - PubMed

Affiliation: The Heart Institute, Molecular Cardiovascular Biology and Developmental Biology Divisions, Cincinnati Children's Hospital Medical Center, Cincinnati, Ohio, United States of America.

ABSTRACT
Normal embryonic development and tissue homeostasis require precise levels of retinoic acid (RA) signaling. Despite the importance of appropriate embryonic RA signaling levels, the mechanisms underlying congenital defects due to perturbations of RA signaling are not completely understood. Here, we report that zebrafish embryos deficient for RA receptor αb1 (RARαb1), a conserved RAR splice variant, have enlarged hearts with increased cardiomyocyte (CM) specification, which are surprisingly the consequence of increased RA signaling. Importantly, depletion of RARαb2 or concurrent depletion of RARαb1 and RARαb2 also results in increased RA signaling, suggesting this effect is a broader consequence of RAR depletion. Concurrent depletion of RARαb1 and Cyp26a1, an enzyme that facilitates degradation of RA, and employment of a novel transgenic RA sensor line support the hypothesis that the increases in RA signaling in RAR deficient embryos are the result of increased embryonic RA coupled with compensatory RAR expression. Our results support an intriguing novel mechanism by which depletion of RARs elicits a previously unrecognized positive feedback loop that can result in developmental defects due to teratogenic increases in embryonic RA.

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Related in: MedlinePlus

Models of the effects of RA signaling on heart patterning and the RA feedback mechanism.(A) Model depicting the consequences on atrial and ventricular CM specification at different levels of RA signaling. (B) Model of the previously unrecognized feedback mechanism that triggers increased RA signaling when depleting RARs. ROL = retinol. RAL = retinal. Red and green arrows indicate the effects on gene expression. Blue arrow indicates the effect on RA levels.
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pgen.1003689-g007: Models of the effects of RA signaling on heart patterning and the RA feedback mechanism.(A) Model depicting the consequences on atrial and ventricular CM specification at different levels of RA signaling. (B) Model of the previously unrecognized feedback mechanism that triggers increased RA signaling when depleting RARs. ROL = retinol. RAL = retinal. Red and green arrows indicate the effects on gene expression. Blue arrow indicates the effect on RA levels.

Mentions: Together, our study supports a novel paradigm whereby RARαb depletion elicits a positive feedback mechanism that can result in teratogenic increases in RA signaling. Importantly, our work highlights that loss and gain of RA signaling can cause similar developmental defects. RA signaling is required to restrict CM specification 25,26, while high increases in RA signaling can eliminate CM specification (Figure 7A) 27. However, our present findings suggest that low increases in RA signaling, achieved when treating embryos with µM concentrations of RA or through RARαb depletion, can also promote increases in both atrial and ventricular CM specification (Figure 7A). As we found previously, modest, but slightly higher increases of RA signaling can promote atrial CM specification without significantly affecting ventricular CM specification 27, which is strikingly similar to what we found with concurrent depletion of the RARαb variants here (Figure 7A). Moreover, intermediate increases in RA signaling can inhibit ventricular CM specification, which is similar what we observed when concurrently depleting RARαb1 and Cyp26a1 (Figure 7A). It also appears that modulation of Hox activity downstream of both gain and loss RA signaling is at least partially responsible for the increases in CM specification, suggesting the hypothesis that the similar effects on CM number are actually due to opposite perturbations of anterior-posterior patterning within the ALPM. Therefore, our analysis corroborates and extends previous observations that there are differential effects on atrial and ventricular CM populations as there is a progressive increase from low to intermediate levels of RA signaling in the early embryo.


Depletion of retinoic acid receptors initiates a novel positive feedback mechanism that promotes teratogenic increases in retinoic acid.

D'Aniello E, Rydeen AB, Anderson JL, Mandal A, Waxman JS - PLoS Genet. (2013)

Models of the effects of RA signaling on heart patterning and the RA feedback mechanism.(A) Model depicting the consequences on atrial and ventricular CM specification at different levels of RA signaling. (B) Model of the previously unrecognized feedback mechanism that triggers increased RA signaling when depleting RARs. ROL = retinol. RAL = retinal. Red and green arrows indicate the effects on gene expression. Blue arrow indicates the effect on RA levels.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3750112&req=5

pgen.1003689-g007: Models of the effects of RA signaling on heart patterning and the RA feedback mechanism.(A) Model depicting the consequences on atrial and ventricular CM specification at different levels of RA signaling. (B) Model of the previously unrecognized feedback mechanism that triggers increased RA signaling when depleting RARs. ROL = retinol. RAL = retinal. Red and green arrows indicate the effects on gene expression. Blue arrow indicates the effect on RA levels.
Mentions: Together, our study supports a novel paradigm whereby RARαb depletion elicits a positive feedback mechanism that can result in teratogenic increases in RA signaling. Importantly, our work highlights that loss and gain of RA signaling can cause similar developmental defects. RA signaling is required to restrict CM specification 25,26, while high increases in RA signaling can eliminate CM specification (Figure 7A) 27. However, our present findings suggest that low increases in RA signaling, achieved when treating embryos with µM concentrations of RA or through RARαb depletion, can also promote increases in both atrial and ventricular CM specification (Figure 7A). As we found previously, modest, but slightly higher increases of RA signaling can promote atrial CM specification without significantly affecting ventricular CM specification 27, which is strikingly similar to what we found with concurrent depletion of the RARαb variants here (Figure 7A). Moreover, intermediate increases in RA signaling can inhibit ventricular CM specification, which is similar what we observed when concurrently depleting RARαb1 and Cyp26a1 (Figure 7A). It also appears that modulation of Hox activity downstream of both gain and loss RA signaling is at least partially responsible for the increases in CM specification, suggesting the hypothesis that the similar effects on CM number are actually due to opposite perturbations of anterior-posterior patterning within the ALPM. Therefore, our analysis corroborates and extends previous observations that there are differential effects on atrial and ventricular CM populations as there is a progressive increase from low to intermediate levels of RA signaling in the early embryo.

Bottom Line: Here, we report that zebrafish embryos deficient for RA receptor αb1 (RARαb1), a conserved RAR splice variant, have enlarged hearts with increased cardiomyocyte (CM) specification, which are surprisingly the consequence of increased RA signaling.Importantly, depletion of RARαb2 or concurrent depletion of RARαb1 and RARαb2 also results in increased RA signaling, suggesting this effect is a broader consequence of RAR depletion.Concurrent depletion of RARαb1 and Cyp26a1, an enzyme that facilitates degradation of RA, and employment of a novel transgenic RA sensor line support the hypothesis that the increases in RA signaling in RAR deficient embryos are the result of increased embryonic RA coupled with compensatory RAR expression.

View Article: PubMed Central - PubMed

Affiliation: The Heart Institute, Molecular Cardiovascular Biology and Developmental Biology Divisions, Cincinnati Children's Hospital Medical Center, Cincinnati, Ohio, United States of America.

ABSTRACT
Normal embryonic development and tissue homeostasis require precise levels of retinoic acid (RA) signaling. Despite the importance of appropriate embryonic RA signaling levels, the mechanisms underlying congenital defects due to perturbations of RA signaling are not completely understood. Here, we report that zebrafish embryos deficient for RA receptor αb1 (RARαb1), a conserved RAR splice variant, have enlarged hearts with increased cardiomyocyte (CM) specification, which are surprisingly the consequence of increased RA signaling. Importantly, depletion of RARαb2 or concurrent depletion of RARαb1 and RARαb2 also results in increased RA signaling, suggesting this effect is a broader consequence of RAR depletion. Concurrent depletion of RARαb1 and Cyp26a1, an enzyme that facilitates degradation of RA, and employment of a novel transgenic RA sensor line support the hypothesis that the increases in RA signaling in RAR deficient embryos are the result of increased embryonic RA coupled with compensatory RAR expression. Our results support an intriguing novel mechanism by which depletion of RARs elicits a previously unrecognized positive feedback loop that can result in developmental defects due to teratogenic increases in embryonic RA.

Show MeSH
Related in: MedlinePlus