DEVELOPMENTAL EXPRESSION OF 2 MEMBERS OF A NEW CLASS OF TRANSCRIPTIONFACTORS .2. EXPRESSION OF ARYL-HYDROCARBON RECEPTOR NUCLEAR TRANSLOCATOR IN THE C57BL 6N MOUSE EMBRYO/

The aryl hydrocarbon receptor (AhR) and the AhR nuclear translocator p rotein (ARNT) are basic-helix-loop-helix (bHLH) proteins involved in t ranscriptional regulation. The AhR is a ligand-activated partner of th e ARNT protein. Both proteins are required to transcriptionally regula te gene expression, ARNT must be complexed to AhR to permit binding to the regulatory DNA sequence. The AhR-ligand complex is known to media te a range of biological responses, such as developmental toxicity, in duction of cleft palate, and hydronephrosis. AhR and ARNT are expresse d in human embryonic palatal cells and AhR was recently shown to have a specific developmental pattern of expression in the mouse embryo. In the present study, expression of ARNT is characterized in C57BL/6N mo use embryos from gestation day (GD) 10-16 using immunohistochemistry a nd in situ hybridization, An affinity purified antibody against human ARNT (1.1 mu g/ml) was detected with an avidinbiotin-peroxidase comple x. ARNT mRNA was localized with a S-35-RNA probe from pBM5/NEO-M1-1. S pecific spatial and temporal patterns of ARNT expression emerged and m RNA and protein expression correlated. The GD 10-11 embryos showed hig hest levels of ARNT in neuroepithelial cells of the neural tube, visce ral arches, otic and optic placodes, and preganglionic complexes, The heart also had significant expression of ARNT with strong nuclear loca lization, After GD11, expression in heart and brain declined. In GD 12 -13 embryos expression was highest in the liver where expression incre ased from GD 12-16. At GD 15-16 the highest levels of ARNT occurred in adrenal gland and liver, although ARNT was also detected in submandib ular gland, ectoderm, tongue, bone, and muscle. In all of these tissue s ARNT was cytoplasmic as well as nuclear, except in some of the corti cal adrenal cells in which ARNT was strongly cytoplasmic with little o r no nuclear localization. These specific patterns of ARNT expression, which differ in certain tissues from the expression of AhR, suggest t hat ARNT may have additional roles in normal embryonic development. (C ) 1995 Wiley-Liss, Inc.