PROGRAMMING GENE-EXPRESSION IN DEVELOPING EPIDERMIS

As the major proteins of adult keratinocytes, keratins provide biochem ical markers for exploring mouse epidermal embryogenesis. Here, we use d a modified method of whole-mount in situ hybridization to track skin -specific expression of endogenous keratin mRNAs throughout embryogene sis. To monitor transcriptional regulation, we coupled this with beta- galactosidase expression of a human epidermal keratin promoter-driven transgene. These studies have radically changed our perception of how the program of gene expression becomes established during epidermal de velopment. Specifically, we have discovered that (1) basal keratin (K5 and K14) genes are first detected at E9.5 in a highly regional fashio n, and surprisingly as early as the single layered ectodermal stage; ( 2) the early patterns do not correlate with morphogenesis per se, but rather with regional variations in the embryonic origin of underlying mesenchyme, supporting morphogenetic criteria that early inductive cue s are mesenchymal; (3) epidermal keratin genes are expressed in peride rm, supporting the notion that this layer arises from ectodermal strat ification, even though it is simple epithelial-like in morphology and is subsequently sloughed during development; (4) later embryonic patte rns of K5 and K14 gene expression parallel proliferative capacity and not stratification; and (5) K1 and K10 mRNAs are first detected as ear ly as E13.5, and their patterns correlate with differentiation and not stratification. These patterns of epidermal gene expression led us to explore whether potential transcriptional regulators of these genes a re expressed similarly. We show that AP2 (but not Sp1) cRNAs hybridize in a pattern similar to, but preceding that of basal keratin cRNAs. F inally, using gene expression in cultured cells, we demonstrate that A P2 has a strong inductive effect on basal keratin expression in a cell ular environment that does not normally possess AP2 activity.