The retinoic acid-inactivating enzyme CYP26 is essential for establishing an uneven distribution of retinoic acid along the anterio-posterior axis within the mouse embryo

Abstract

Retinoic acid (RA), a derivative of vitamin A, plays a pivotal role in vertebrate development. The level of RA may be determined by the balance between its synthesis and degradation. We have examined the role of CYP26, a P450 enzyme that may degrade RA, by generating mutant mice that lack CYP26. CYP26 −/− mice exhibited anomalies, including caudal agenesis, similar to those induced by administration of excess RA. The concentration of endogenous RA, as revealed by marker gene activity, was markedly increased in the tailbud of the mutant animals, in which CYP26 is normally expressed. Expression of T (Brachyury) and Wnt3a in the tailbud was down-regulated in CYP26 −/− mice, which may underlie the caudal truncation. The lack of CYP26 also resulted in homeotic transformation of vertebrae as well as in misspecification of the rostral hindbrain associated with anterior expansion of RA-positive domains. These results suggest that local degradation of RA by CYP26 is required for establishing an uneven distribution of RA along the anterio-posterior axis, which is essential for patterning the hindbrain, vertebrae, and tailbud.

Retinoid metabolism: a balancing act

“…Studies of Cyp26-deficient mice have largely supported the conclusion that 4-hydroxy-retinoic acid and 4-oxoretinoic acid are inactive degradation products of the all-trans isomer. Accordingly, embryos from these mice have abnormalities that mimic all-trans retinoic acid-induced teratogenicity 9,10 . It is possible, however, that some of the abnormalities observed in Cyp26 -/-mice result from a deficiency in Cyp26-generated metabolites that have active biological roles in vivo.…”

Expression of key retinoic acid modulating genes suggests active regulation during development and regeneration of the amphibian limb

“…Mice treated with R115866, an inhibitor of Cyp26 activity, have fused cervical vertebrae (Laue et al, 2008). Cyp26a is also seen in small anterior and posterior stripes flanking the position of the forming proximal phalanx in digits II-V. Cyp26a mutant mice exhibit sirenomelia and axial skeletal defects, but forelimbs develop normally (AbuAbed et al, 2001;Sakai et al, 2001).…”

Fate of retinoic acid–activated embryonic cell lineages

“…Conditions of RA excess through maternal administration (Marshall et al, 1992;Simeone et al, 1995, and references therein) or Cyp26 knock-down Sakai et al, 2001;Emoto et al, 2005;Hernandez et al, 2007), and of RA deficiency through mutation of Rdh10 or Raldh2 (Niederreither et al, 2000;Sandell et al, 2007), have severe effects on hindbrain development and can lead to changes in rhombomeric molecular identities. Several models have been proposed to account for the effects of endogenous RA in controlling hindbrain development, mainly relying on the establishment of a posterior-toanterior RA diffusion gradient, and/or on the sequential action of CYP26 enzymes to generate RA-depleted areas in specific rhombomere territories (Gavalas, 2002;Maden, 2002;Maves and Kimmel, 2005;Sirbu et al, 2005;Hernandez et al, 2007;White et al, 2007;White and Schilling, 2008).…”

“…The RARE-hsp68-lacZ transgene has been widely used in subsequent studies (e.g., Malpel et al, 2000;Mic et al, 2002;Niederreither et al, 2002b;Fan et al, 2003;Matt et al, 2003Matt et al, , 2005Yashiro et al, 2004;Sirbu et al, 2005;Molotkov et al, 2006;Sirbu and Duester, 2006;Ribes et al, 2009;Zhao et al, 2009). Three lines of evidence have validated this transgene as a reliable indicator of RA activity: (1) there is a close match between the patterns of lacZ activity and the expression patterns of RA-synthesizing enzymes (Rdh10, Raldhs) at early embryonic stages, (2) activity of the lacZ reporter is severely down-regulated in knockout mice for the above enzymes (Niederreither et al, 1999;Mic et al, 2002;Matt et al, 2005;Molotkov et al, 2006;Sandell et al, 2007), (3) on the other hand, the reporter is almost ubiquitously activated within 6 h after administration of a bolus dose of RA to the pregnant mothers (Rossant et al, 1991), and is ectopically activated when RA-metabolizing enzymes are absent Sakai et al, 2001;Uehara et al, 2009). RARE-containing reporters are available in other species such as in zebrafish where transgenic lines have been generated (Perz-Edwards et al, 2001).…”