Retinoids, Homeobox Genes, and Toxicants

L.J.Gudas, A.L. Means, S.W. Chen, A. Langston, L. Ho, and J.R. Thompson
Pharmocolgy Dept.
Cornell University Medical College
New York, NY 10021
Fax: (212) 746-8858

Retinoids (vitamin A and its derivatives) influence the proliferation and differentiation of a wide variety of cell types. Retinoids also exhibit striking effects on vertebrate development. This laboratory has analyzed the mechanisms by which a number of RA-responsive genes are regulated by retinoic acid (RA) in both cultured embryonic cells and during the development of transgenic mice.

The murine homeobox gene Hoxa1 is a member of the vertebrate Hox complex and plays a role in defining the body plan during development. At day 8.0-9.0 post coitus, Hoxa1 transcripts are detected extensively throughout the embryo in the neural tube, adjacent mesenchyme, paraxial mesoderm, somites and gut epithelium; expression extends from most caudal region of the embryo to the rhombomere 3/4 border. This spatiotemporal expression of Hoxa1 mRNA is critical for normal embryonic development. We identified an RA-responsive enhancer 3' of the Hoxa1 (1.6) gene, and conserved enhancers were also found on the 3' of the human homolog of Hoxa1 and 3' of the paralogous Hoxb1 (2.9) genes of the chicken and mouse. All these enhancers share a conserved RA response element (RARE) and two other highly conserved blocks of homology (CE1 and CE2 elements). In cultured cells the function of the murine Hoxa1 (1.6) enhancer depends on the presence of the enhancer; Hoxa1 (1.6)-IacZ transgenes including the 3'enhancer reproduce the expression pattern and in vivo RA response of the endogenous gene in the developing embryo. Transgenes lacking the enhancer, but otherwise identical, are expressed in a distinct pattern, and are not RA-responsive.

The CRABP-I gene is expressed in a spatiotemporal pattern in neural and mesenchymal tissues at the onset of organogenesis. THe neural pattern of CRABP-I expression includes specific rhomberes of the hindbrain, neural crest cells and their derivatives, the optic stalk, and the central area of the neural retina. We have created transgenic mouse lines with CRAPB-15' and transcribed regions fused to the IacZ structural gene that recapitulate much of this neural pattern of expression. Sequences between -7.8 and-3.2 kb confer B-gal expression to specific rhomberes, migrating neural crest cells, trigeminal ganglion, the optic stalk, and the neural retina. A region located between axon 1 and axon 3 confers a portion of this pattern, including the mesencephalic projections of the trigeminal ganglion, the inner layer of the neural retina, and the peripheral layer of the posterior hindbrain. These molecular studies provide a basis for understanding actions of endogenous retinoids in embryogenesis and the teratogenic actions of excess retinoids on the developing organism.