Retinoic acid and mouse development: Identification of retinoic acid receptor target genes involved in axial patterning

Retinoic acid (RA), the major biologically active form of vitamin A, is required for normal development of the mouse embryo. In particular, RA is necessary for the correct anteroposterior specification of the embryonic axis. Exposure to RA at certain stages of development leads to premature truncation of the vertebral column, accompanied by spina bifida. In contrast, embryos that are homozygous null for RARγ do not exhibit these defects, indicating that this receptor specifically mediates this teratogenic effect. Differential display PCR and suppression subtractive hybridization were employed using the RARγ null embryos in an attempt to identify downstream targets that may be involved in the formation of these abnormalities. As a result of this process, a full-length cDNA molecule encoding a novel member of the aldo-keto reductase family (AKR1A4) was cloned. Although RA does not regulate the expression of this gene, its developmental expression pattern and substrate specificity suggests a potential role for this enzyme in the protection of certain rapidly growing embryonic structures from harmful metabolites.; A precise level of RA signaling is also required for proper specification of vertebral identity. Exposure to excess RA during the early stages of gastrulation results in posterior homeotic transformations of several vertebrae. These transformations are correlated with shifts in the anterior boundaries of Hox expression. In contrast, the loss of functional RARs leads to anterior vertebral transformations. Although retinoic acid response elements have been identified in Hox promoters, it is likely that RA regulates the expression of some Hox genes through intermediary factors. Cdx1 is a homeobox-containing transcription factor that influences vertebral patterning in a manner similar to RARs, and is directly regulated by RA in the mouse embryo. Analysis of an allelic series of RARγ/Cdx1 null mutant mice demonstrated that Cdx1 and RARγ act synergistically to pattern certain cervical vertebrae. In addition, Cdx1 is required for the full effect of RA treatment on the vertebral column. However, Cdx1 does not mediate all of the effects of RARγ as the incidence of a thoracic to cervical vertebral transformation is significantly higher in RARγ/Cdx1 double mutants as compared to either single null mouse.