The ability to generate specific genetic modifications in mice provides a powerful approach to assess gene function. When genetic modifications have been generated in the germ line, however, the resulting phenotype often only reflects the first time a gene has an influence on - or is necessary for - a particular biological process. Therefore, systems allowing conditional genetic modification have been developed (for a review, see [1]); for example, inducible forms of the Cre recombinase from P1 phage have been generated that can catalyse intramolecular recombination between target recognition sequences (loxP sites) in response to ligand [2] [3] [4] [5]. Here, we assessed whether a tamoxifen-inducible form of Cre recombinase (Cre-ERTM) could be used to modify gene activity in the mouse embryo in utero. Using the enhancer of the Wnt1 gene to restrict the expression of Cre-ERTM to the embryonic neural tube, we found that a single injection of tamoxifen into pregnant mice induced Cre-mediated recombination within the embryonic central nervous system, thereby activating expression of a reporter gene. Induction was ligand dependent, rapid and efficient. The results demonstrate that tamoxifen-inducible recombination can be used to effectively modify gene function in the mouse embryo.

A number of proteins have been rendered functionally oestrogen-dependent by fusion with the hormone-binding domain of the oestrogen receptor. There are, however, several significant disadvantages with such fusion proteins. First, their use in cells in vitro requires phenol red-free medium and laborious stripping of steroid hormones from serum in order to avoid constitutive activation. Secondly, control of oestrogen receptor fusion proteins in vivo is precluded by high endogenous levels of circulating oestrogens. Thirdly, the hormone-binding domain of the oestrogen receptor functions as a hormone-dependent transcriptional activation domain making interpretation of fusions with transcription factors problematical. In order to overcome these drawbacks we have used a transcriptionally inactive mutant of the murine oestrogen receptor which is unable to bind oestrogen yet retains normal affinity for the synthetic ligand, 4-hydroxytamoxifen. When the hormone-binding domain of this mutant oestrogen receptor is fused to the C-terminus of the c-Myc protein, Myc-induced proliferation and apoptosis in fibroblasts becomes dependent on 4-hydroxytamoxifen, but remains refractory to 17 beta-oestradiol.

Shh signaling is not required for initiating hair follicle development. Shh signaling is essential, however, for controlling ingrowth and morphogenesis of the hair follicle.

The oestrogen receptor stimulates transcription by means of at least two distinct transcriptional activation domains, TAF‐1 in the N‐terminal domain and TAF‐2 in the hormone binding domain. Here we show that TAF‐2 activity requires a region in the C‐terminus of the hormone binding domain between residues 538 and 552 in the mouse oestrogen receptor which is conserved among many nuclear hormone receptors. Point mutagenesis of conserved hydrophobic and charged residues significantly reduced ligand dependent transcriptional activation but had no effect on steroid or DNA binding. Mutation of the corresponding residues in the glucocorticoid receptor also abolished transcriptional activation. We therefore propose that the conserved region may be essential for ligand dependent transcriptional activation by other members of the nuclear receptor family.

Mutation of the RB-1 tumour suppressor occurs in one third of all human tumours and is particularly associated with retinoblastoma and osteosarcoma1. Numerous functions have been ascribed to the product of the human RB-1 gene, pRB. The best known is pRB’s ability to promote cell cycle exit through inhibition of the E2F transcription factors and the transcriptional repression of genes encoding cell cycle regulators1. In addition, pRB has been shown in vitro to regulate several transcription factors that are master differentiation inducers2. Depending on the differentiation factor and cellular context, pRB can either suppress or promote their transcriptional activity. For example, pRB binds to Runx2 and potentiates its ability to promote osteogenic differentiation program in vitro3. In contrast, pRB acts together with E2F to suppress PPARγ, the master activator of adipogenesis4,5. Since osteoblasts and adipocytes can both arise from mesenchymal stem cells, these observations suggest that pRB might play a role in the choice between these two fates. However, to date, there is no evidence for this in vivo. Here we use mouse models to address this hypothesis in the context of mesenchymal tissue development and tumorigenesis. Our data show that Rb status plays a key role in establishing fate choice between bone and brown adipose tissue in vivo.