The pituitary gland has provided unique insight into molecular mechanisms and regulatory factors controlling both differentiation and gene transcription. We identified Tpit, a novel T box factor only present in the two pituitary POMC-expressing lineages, the corticotrophs and melanotrophs, and apparently in no other tissue, including hypothalamic POMC neurons. In pituitary cells, Tpit activation of POMC gene transcription requires cooperation with Pitx1, the two factors binding to contiguous sites within the same regulatory element. In gain-of-function experiments, Tpit induces POMC expression in undifferentiated pituitary cells, indicating that it can initiate differentiation into POMC-expressing lineages. TPIT gene mutations were found in patients with isolated deficiency of pituitary POMC-derived ACTH, in support of an essential role of Tpit for differentiation of the pituitary POMC lineage.

NeuroD1(BETA2) and Tpit are cell-specific activators of pituitary proopiomelanocortin (POMC) gene transcription. Expression of both factors slightly precedes that of POMC at embryonic d 12.5 of mouse pituitary development. We now report that NeuroD1(BETA2) is required for early corticotroph differentiation. In agreement with the transcriptional synergism observed between Tpit and basic helix-loop-helix dimers containing NeuroD1(BETA2), POMC expression is delayed in NeuroD1-deficient mice. However, this differentiation defect does not reflect a change of corticotroph commitment as revealed by Tpit expression. The delay of corticotroph terminal differentiation is transient and coincides with the developmental window of NeuroD1 expression in corticotrophs. In contrast to their requirement in other NeuroD1-expressing cells, the neurogenin genes do not appear to be necessary for corticotroph differentiation. Taken together with a similar requirement of Tpit for corticotroph differentiation but not for commitment, the present data indicate that the POMC promoter is a point of convergence for independent corticotroph differentiating signals.

To investigate the role of nucleotide excision repair (NER) in the cellular processing of carcinogenic DNA photoproducts induced by defined, environmentally relevant portions of the solar wavelength spectrum, we have determined the mutagenic specificity of simulated sunlight (310-1100 nm), UVA (350-400 nm), and UVB (290-320 nm), as well as of the "nonsolar" model mutagen 254-nm UVC, at the adenine phosphoribosyltransferase (aprt) locus in NERdeficient (ERCC1) Chinese hamster ovary (CHO) cells. The frequency distributions of mutational classes induced by UVB and by simulated sunlight in repair-deficient CHO cells were virtually identical, each showing a marked increase in tandem CC -> TT transitions relative to NER-proficient cells. A striking increase in CC --TT events was also previously documented for mutated p53 tumor-suppressor genes from nonmelanoma tumors of NER-deficient, skin cancer-prone xeroderma pigmentosum patients, compared to normal individuals. The data therefore indicate that the aprt gene in NER-deficient cultured rodent cells irradiated with artificial solar light generates the same distinctive "fingerprint" for sunlight mutagenesis as the p53 locus in NER-deficient humans exposed to natural sunlight in vivo. Moreover, in strong contrast to the situation for repair-competent CHO cells, where a significant role for UVA was previously noted, the mutagenic specificity of simulated sunlight in NER-deficient CHO cells and of natural sunlight in humans afflicted with xeroderma pigmentosum can be entirely accounted for by the UVB portion of the solar wavelength spectrum.

No abstract

Ptx1 is a member of the small bicoid family of homeobox-containing genes; it was isolated as a tissue-restricted transcription factor of the pro-opiomelanocortin gene. Its expression during mouse and chick embryogenesis was determined by in situ hybridization in order to delineate its putative role in development. In the head, Ptx1 expression is first detected in the ectoderm-derived stomodeal epithelium at E8.0. Initially, expression is only present in the stomodeum and in a few cells of the rostroventral foregut endoderm. A day later, Ptx1 mRNA is detected in the epithelium and in a streak of mesenchyme of the first branchial arch, but not in other arches. Ptx1 expression is maintained in all derivatives of these structures, including the epithelia of the tongue, palate, teeth and olfactory system, and in Rathke's pouch. Expression of Ptx1 in craniofacial structures is strikingly complementary to the pattern of goosecoid expression. In addition, Ptx1 is expressed early (E6.8) in posterior and extraembryonic mesoderm, and in structures that derive from these. The restriction of expression to the posterior lateral plate is later evidenced by exclusive labelling of the hindlimb but not forelimb mesenchyme. In the anterior domain of expression, the stomodeum was shown by fate mapping to derive from the anterior neural ridge (ANR) which represents the most anterior domain of the embryo. The concordance between these fate maps and the stomodeal pattern of Ptx1 expression supports the hypothesis that Ptx1 defines a stomodeal ectomere, which lies anteriorly to the neuromeres that have been suggested to constitute units of a segmented plan directing head formation.

The hypothalamus-pituitary-adrenal (HPA) axis plays a primordial role in the control of homeostasis. Pituitary corticotroph cells produce POMC that is cleaved into ACTH. ACTH is an important regulator of HPA which stimulates production of glucocorticoids by the adrenals. The pituitary differentiation process is under the combined action of tissue-and cell-restricted transcription factors. In corticotrophs, Tpit and NeuroD1 are both cell-specific activators of POMC expression. Their expression precedes closely that of POMC suggesting a role in differentiation of the lineage.We generated Tpit-/-mice in which most Tpit coding sequences were replaced by a b-galactosidase gene. These mice are deficient in POMC-expressing cells consistent with a positive action of Tpit on these lineages; Tpit was also shown to be a negative regulator of the gonadotroph fate. However, the developing anterior pituitary of Tpit-/-mice has a normal number of b-galactosidase and NeuroD1-positive cells, indicating that initiation of corticotroph differentiation is not prevented in the absence of Tpit. Corticotroph differentiation is delayed in NeuroD1-/-mice but this is not resulting from

No abstract