Feedback mechanisms regulate retinoic acid production and degradation in the zebrafish embryo

Abstract

Retinoic acid (RA) signaling in vertebrate embryos occurs in a distinct physical and temporal pattern. Regulating this spatial distribution is crucial to the development of the embryo, as RA in excess or in inappropriate tissues is teratogenic. In order to understand how RA availability is determined in zebrafish we have investigated the expression of cyp26a1, an enzyme that inactivates RA, and its relationship to raldh2, one of the enzymes that produce RA from retinal. cyp26a1 expression follows three phases: in presumptive anterior neurectoderm and in a circumblastoporal ring during gastrulation, in the tailbud throughout somitogenesis, and in multiple specific tissue types beginning at mid-somitogenesis and continuing through 48 h postfertilization (hpf). This expression was either adjacent or opposite to those tissues expressing raldh2. We then investigated how RA production might regulate these relationships. Endogenous RA produced by raldhs did not play a role in setting cyp26a1 expression in most tissues. However, exogenous RA regulates expression of both enzymes. cyp26a1 is up regulated in the embryo in a time, concentration, and tissue-dependent manner. Conversely, raldh2 expression is reduced with RA treatment. Tests of the raldh2 promoter in cell transfections proved that RA directly represses its activity. These data demonstrate that the feedback mechanisms regulating production and degradation of RA must be considered in any experiments altering levels of RA in the developing vertebrate embryo.

RAR-mediated epigenetic control of the cytochrome P450 Cyp26a1 in embryocarcinoma cells

“…Concomitantly with Cyp26a1 repression, cells are less sensitive to the growth-suppressive activity of RA and lose neuron differentiation potential. This evidence supports the supposition that Cyp26a1, which is critical for P19 differentiation (Sonneveld et al, 1999), may act as an epigenetic switch capable of controlling differentiation by regulating the intracellular level of RA and its catabolites (Dobbs-McAuliffe et al, 2004). Further work is required to establish whether and how RA catabolism can affect RA-induced differentiation of embryocarcinoma cells.…”

Effects of acute exposure to polybrominated diphenyl ethers on retinoid signaling in zebrafish larvae

“…These genes have been used to indirectly evaluate toxicant-induced impacts on RA homeostasis in zebrafish and mammals (Alsop et al, 2007;Dobbs-McAuliffe et al, 2004). In the present study, gene transcription of raldh2 was upregulated significantly.…”

“…The retinol is catalyzed via a two-step conversion to retinoic acid (RA). Initially, retinol is converted to retinal by retinol dehydrogenase (RDH) in a rate-limiting reaction, and then retinal is converted to the bioactive metabolite RA by retinal dehydrogenase (RALDH), via an irreversible oxidation step (Dobbs-McAuliffe et al, 2004).…”

“…In target cells, after being transported by cellular retinoic acid binding protein (CRABP) into the nucleus, RA binds to RA receptors (RARs) and retinoid X receptors (RXRs), which initiates the transcription of specific genes that contribute to developmental patterning of the hindbrain, spinal cord, heart, and eye (Novák et al, 2008). However, RA in non-target cells is catalyzed to oxidized derivatives by an RA-inducible gene, cyp26a, in order to regulate RA activity and avoid RA-induced teratogenesis (Dobbs-McAuliffe et al, 2004;White et al, 1996). Meanwhile, in the eyes, the chromophore retinal binds covalently to photoreceptor opsins to form visual pigments during photoexcitation to initiates the visual transduction cascade.…”

Retinoic acid down‐regulates Tbx1 expression in vivo and in vitro

“…RA also represses AP-1-mediated gene activation, partly by disrupting c-fos/ c-jun interaction, and this action may account for much indirect ligand-induced repressive activity (Zhou et al, 1999). In some cases, RA repression may be mediated directly; RA significantly represses the activity of zebrafish raldh2-promoter within 2 hr in luciferase reporter constructs and in the embryo (Dobbs-McAuliffe et al, 2004). RA also represses the expression of Bmp-4 in both a cell line model and through bead delivery to the chick otocyst-experiments reminiscent of those conducted here.…”