RETINOID-X-RECEPTOR SIGNALING IN THE DEVELOPING SPINAL-CORD

Retinoids regulate gene expression through the action of retinoic acid receptors (RARs) and retinoid-X receptors (RXRs), which both belong t o the family of nuclear hormone receptors(1,2). Retinoids are of funda mental importance during development(2), but it has been difficult to assess the distribution of ligand-activated receptors in vivo. This is particularly the case for RXR, which is a critical unliganded auxilia ry protein for several nuclear receptors, including RAR(1), but its li gand-activated role in vivo remains uncertain, Here we describe an ass ay in transgenic mice, based on the expression of an effector fusion p rotein linking the ligand-binding domain of either RXR or RAR to the y east Gal4 DNA-binding domain, and the in situ detection of ligand-acti vated effector proteins by using an inducible transgenic lacZ reporter gene. We detect receptor activation in the spinal cord in a pattern t hat indicates that the receptor functions in the maturation of limb-in nervating motor neurons. Our results reveal a specific activation patt ern of Gal4-RXR which indicates that RXR is a critical bona fide recep tor in the developing spinal cord.