Kathleen M. Scully

During mammalian pituitary gland development, distinct cell types emerge from a common primordium. Appearance of specific cell types occurs in response to opposing signaling gradients that emanate from distinct organizing centers. These signals induce expression of interacting transcriptional regulators, including DNA binding-dependent activators and DNA binding-independent transrepressors, in temporally and spatially overlapping patterns. Together they synergistically regulate precursor proliferation and induction of distinct cell types. Terminal cell type differentiation requires selective gene activation strategies and long-term active repression, mediated by cell type-specific and promoter-specific recruitment of coregulatory complexes. These mechanisms imply the potential for flexibility in the ultimate identity of differentiated cell types.

A pituitary LIM homeodomain factor, P-Lim, is expressed as Rathke's pouch forms and as specific pituitary cell phenotypes are established, suggesting functional roles throughout pituitary development. While selectively expressed in both anterior and intermediate pituitary in mature mice, P-Lim is also transiently expressed in the developing ventral neural cord and brainstem. P-Lim binds to and activates the promoter of the alpha-glycoprotein subunit gene, a marker of early pituitary development, and synergizes with Pit-1 in transcriptional activation of genes encoding terminal differentiation markers. The LIM domain of P-Lim specifically interacts with the Pit-1 POU domain and is required for synergistic interactions with Pit-1, but not for basal transcriptional activation events.

We have identified and characterized a new member of the leucine zipper (bZIP) gene family of transcription factors, thyrotroph embryonic factor (TEF). Analysis of the ontogeny of TEF gene expression reveals the presence of TEF transcripts, beginning on embryonic day 14, only in the region of the rat anterior pituitary gland in which thyrotrophs arise. This pattern of gene expression corresponds temporally and spatially to the onset of thyroid-stimulating hormone (TSH beta) gene expression, which defines the thyrotroph phenotype. Coupled with this observation, we find that TEF can bind to and trans-activate the TSH beta promoter. In contrast to this restricted pattern of expression during embryogenesis, TEF transcripts appear in several tissues in the mature organism. We propose that TEF belongs to a new class of bZIP proteins on the basis of the unique homology between TEF and another member of the bZIP gene family, the albumin D box-binding protein (DBP). TEF and DBP transcripts are coexpressed in a pituitary cell line, and these two proteins can readily form heterodimers. The DNA-binding and dimerization domains of TEF correspond to those found in other bZIP proteins. We have however, identified a cluster of basic amino acids, found only in TEF and DBP, that is necessary for the proper DNA-binding site specificity of TEF. A major trans-activation domain of TEF resides outside the region of homology to other bZIP proteins. These data are consistent with a role for a member of a new class of bZIP transcription factors in activating gene expression in the developing thyrotroph.

Reciprocal gene activation and restriction during cell type differentiation from a common lineage is a hallmark of mammalian organogenesis. A key question, then, is whether a critical transcriptional activator of cell type-specific gene targets can also restrict expression of the same genes in other cell types. Here, we show that whereas the pituitary-specific POU domain factor Pit-1 activates growth hormone gene expression in one cell type, the somatotrope, it restricts its expression from a second cell type, the lactotrope. This distinction depends on a two-base pair spacing in accommodation of the bipartite POU domains on a conserved growth hormone promoter site. The allosteric effect on Pit-1, in combination with other DNA binding factors, results in the recruitment of a corepressor complex, including nuclear receptor corepressor N-CoR, which, unexpectedly, is required for active long-term repression of the growth hormone gene in lactotropes.