SEPARATE CIS-ACTING DNA ELEMENTS OF THE MOUSE PRO-ALPHA-1(I) COLLAGENPROMOTER DIRECT EXPRESSION OF REPORTER GENES TO DIFFERENT TYPE-I COLLAGEN-PRODUCING CELLS IN TRANSGENIC MICE

The genes coding for the two type I collagen chains, which are active selectively in osteoblasts, odontoblasts, fibroblasts, and some mesenc hymal cells, constitute good models for studying the mechanisms respon sible for the cell-specific activity of genes which are expressed in a small number of discrete cell types. To test whether separate genetic elements could direct the activity of the mouse pro-alpha 1(I) collag en gene to different cell types in which it is expressed, transgenic m ice were generated harboring various fragments of the proximal promote r of this gene cloned upstream of the Escherichia coli beta-galactosid ase gene. During embryonic development, X-gal staining allows for the precise identification of the different cell types in which the beta-g alactosidase gene is active. Transgenic mice harboring 900 bp of the p ro-alpha 1(I) proximal promoter expressed the transgene at relatively low levels almost exclusively in skin. In mice containing 2.3 kb of th is proximal promoter, the transgene was also expressed at high levels in osteoblasts and odontoblasts, but not in other type I collagen-prod ucing cells, Transgenic mice additional high level expression of the t ransgene in tendon and fascia fibroblasts. The pattern of expression o f the lacZ transgene directed by the 0.9- and 2.3-kb pro-alpha 1(I) pr oximal promoters was confirmed by using the firefly luciferase gene as a reporter gene. The pattern of expression of this transgene, which c an be detected even when it is active at very low levels, paralleled t hat of the beta-galactosidase gene. These data strongly suggest a modu lar arrangement of separate cell-specific cis-acting elements that can activate the mouse pro-alpha(I) collagen gene in different type I col lagen-producing cells. At least three different types of cell-specific elements would be located in the first 3.2 kb of the promoter: (a) an element that confers low level expression in dermal fibroblasts; (b) a second that mediates high level expression in osteoblasts and odonto blasts; and (c) one responsible for high level expression in tendon an d fascia fibroblasts. Our data also imply that other cis-acting cell-s pecific elements which direct activity of the gene to still other type I collagen-producing cells remain to be identified.