ERM, ER81 and PEA3 are three highly related transcription factors belonging to the ETS family. Together they form the PEA3 group within this family. Little data is yet available regarding the roles of these three genes during embryonic development. A prerequisite to investigations in this ®eld is to obtain an accurate spatio-temporal expression map for the erm, er81 and pea3 genes. To this end, we have used in situ hybridization to compare their expression patterns during critical stages of murine embryogenesis. We report that all three genes are expressed in numerous developing organs coming from dierent embryonic tissues. The three genes appeared co-expressed in dierent organs but presented speci®c sites of expression, so that the resultant expression pattern could in fact reveal several distinct functions depending upon isolated and/or various combinations of the PEA3 member expression. These results suggest that erm, er81 and pea3 genes are dierentially regulated, probably to serve important functions as cell proliferation control, tissue interaction mediator or cell dierentiation, all over successive steps of the mouse organogenesis.

The Erg gene belongs to the Ets family encoding a class of transcription factors. To gain new insight on the in vivo functional specificity of the Erg gene within the wide Ets family, we used in situ hybridization to determine its expression pattern during murine embryogenesis. We found that the Erg gene expression predominates in mesodermal tissues, including the endothelial, precartilaginous and urogenital areas. A specific Erg gene expression was also identified in migrating neural crest cells. A comparison with Fli-1, the most closely Erg-related gene, revealed that both gene expressions partially overlap, suggesting that they may contribute to related functions in these tissues. Like other Ets family genes, Erg seems involved in several fundamental developmental steps in murine embryogenesis, including epithelio-mesenchymal transition, cell migration, settlement and differentiation.

The ets gene superfamily encodes a class of transcription factors that bind to a purine rich sequence through a 85 amino-acid ETS domain. Among them, the human erg gene has been found to be involved in Ewing's sarcoma, primitive neurectodermal tumour of childhood and acute myeloid leukaemia. Nevertheless, little is known about human erg expression. Northern blot analyses have shown a human erg expression restricted to few cell lines and thymus, but the status concerning expression during development remains unknown probably because no homologue of this gene has yet been isolated and studied in other vertebrates. We thus choose to clone the chicken erg gene (ck-erg) and to study its expression during chicken development. We obtained a bona fide clone of ck-erg and defined the transcriptional modulating properties of its product. The ck-Erg protein acts as a transcriptional activator through a conventional consensus ETS binding site. Northern blot studies on various chicken tissues, in situ analyses and comparison with the well-characterised c-ets-1 expression show that ck-erg is expressed in mesoderm- and, to a lesser extent, in ectoderm-derived tissues. During chicken development, two salient features could be observed. From stage E1 to E3.5, ck-erg expression was widely distributed in mesodermal derivatives and neural crest, resembling c-ets-1 expression. However, by E6, the expression of ck-erg exhibited, unlike c-ets-1, a drastically new and strong signal in precartilaginous condensation zones and cartilaginous skeletal primordia. These stages are the first steps of bone formation during skeletal elaboration. Our results show for the first time a possible specific involvement of ck-erg in cartilage morphogenesis.

We studied the expression of the estrogen-related receptor 1 (ERR-1) during mouse embryonic development. ERR-1 is expressed at very early stages in ES cells and at E8.5 in the mesodermal cells of the visceral yolk sac. ERR-1 continues to be expressed later in mesodermal tissues and particularly in heart and in skeletal muscles. This expression persists during all the embryonic development and in adult stage. ERR-1 transcripts level increases during muscle differentiation. Accordingly, we show that ERR-1 expression increases during the myoblast to myotube transition in differentiating C2 myoblastic cells. ERR-1 has also been detected in the nervous system during embryonic development. At E10.5, a high level of ERR-1 transcripts can be observed in differentiated cells of the intermediate zone of the spinal cord which also suggests a role of ERR-1 in the differentiation of the nervous system. The same is observed in the telencephalon vesicules at E13.5. Later, at E15.5 and E17.5, expression persists in the spinal cord but decreases dramatically in the central nervous system. Moreover, ERR-1 expression increases during skin formation and is detected in the stratum spinosum which contains differentiated Malpighian cells. Finally, we also observed ERR-1 in endodermal derivatives such as the epithelium of intestine and urogenital system. The DNA target of ERR-1 has been identified to be the SF-1/FTZ-F1 responsive element (SFRE) and we show in this paper that SF-1/FTZ-F1 and ERR-1 bind to and activate transcription independently through the SFRE element. Our study suggests that ERR-1 may be implicated in numerous physiological or developmental functions, particularly in the muscle, the central and peripheral nervous system and the epidermis. Interestingly, in these various systems ERR-1 expression is correlated with post-mitotic cells stage, suggesting that ERR-1 may play a role in the differentiation process.

The genetic program that controls reciprocal tissue interactions during epithelial organogenesis is still poorly understood. Erm, Er81 and Pea3 are three highly related transcription factors belonging to the Ets family, within which they form the PEA3 group. Little information is yet available regarding the function of these transcription factors. We have previously used in situ hybridization to compare their expression pattern during critical stages of murine embryogenesis [Oncogene 15 (1997), 937; Mech. Dev. 108 (2001), 191]. In this study, we have examined the expression of PEA3 group members during organogenesis of the lung, salivary gland, kidney, and mammary gland. In all of these developmental settings, we observed a tight correlation between branching morphogenesis and the expression of specific members of the PEA3 group. To assess the functional relevance of these findings, Erm and Pea3 were overexpressed in the TAC-2.1 mammary epithelial cell line, which has the ability to form branching duct-like structures when grown in collagen gels. We found that overexpression of Erm and Pea3 markedly enhances branching tubulogenesis of TAC-2.1 cells and also promotes their invasion into a collagen matrix. Collectively, these findings suggest that the differential expression of PEA3 group transcription factors has an important role in the regulation of branching morphogenesis and raise the question of their implication in branching signaling.