RAT AND MOUSE PROOPIOMELANOCORTIN GENE-SEQUENCES TARGET TISSUE-SPECIFIC EXPRESSION TO THE PITUITARY-GLAND BUT NOT TO THE HYPOTHALAMUS OF TRANSGENIC MICE

The proopiomelanocortin (POMC) gene is expressed predominantly in corticotrophs of the pituitary anterior lobe, melanotrophs of the intermediate lobe and neurons of the arcuate nucleus of the hypothalamus. The different ontogeny of POMC mRNA as well as the complicated hormonal regulation of POMC gene expression in the three different cell types suggests a concerted interaction between several cis-acting elements in the POMC gene and transcription factors located in each of the three cell. types. To investigate cell-specific elements in the POMC gene we tested two different constructs in transgenic mice. The construct -4000rP0MCLacZ, carrying 4 kb of the rat POMC promoter fused to the Escherichia coli P-galactosidase gene, showed appropriate expression in melanotrophs in 50% of the mice analyzed. P-Galactosidase activity was less evident in corticotrophs under basal environmental conditions. In brain, 7 out of 15 independently derived transgenic founders had ectopic expression of the transgene in different areas; however, none of the animals analyzed expressed P-galactosidase in neurons of the arcuate nucleus. The construct HAL*, a 'tagged' 10.2-kb mouse genomic fragment, was more efficiently targeted to the pituitary. Using in situ hybridization, we detected uniform expression of HAL* in melanotrophs in 100% of the 6 pedigrees analyzed and transgenic mRNA levels paralleled those of the endogenous POMC mRNA. In corticotrophs, basal expression was low but after adrenalectomy HAL* mRNA levels were comparable to those of POMC. None of the 6 pedigrees had appropriate expression of HAL* in the brain; however, 2 lines had ectopic expression in the dentate gyrus of the hippocampus. These data, together with previously reported studies, suggest that accurate neuronal expression of the POMC gene requires DNA elements in addition to the sequences that are sufficient for expression in pituitary melanotrophs and corticotrophs. The consistently lower level of transgene expression in corticotrophs compared to melanotrophs under basal conditions indicates that these two pituitary cell types may also differ in their requirements for POMC gene regulatory elements.