PROTEASE NEXIN-1 IS EXPRESSED AT THE MOUSE MET- MESENCEPHALIC JUNCTION AND FGF SIGNALING REGULATES ITS PROMOTER ACTIVITY IN PRIMARY MET-/MESENCEPHALIC CELLS/

The expression pattern of the serine protease inhibitor gene Protease nexin-1 (PN-1) has been analyzed by in situ hybridization during embry onic nervous system development. PN-1 was found to be specifically exp ressed at the junction between the mes- and metencephalon (mid- and hi ndbrain), Transgenic embryos expressing the bacterial lacZ gene under the control of different fragments of the PN-1 upstream regulatory reg ion were used to demarcate an enhancer sufficient for expression at th is putative segmental border. Primary cell cultures derived from the e mbryonic neural tube at the level of the met-/mesencephalic junction w ere used to demonstrate a specific effect on transcriptional activity by basic fibroblast growth factor and Engrailed transcription factors, providing evidence that PN-1 is a target gene of these factors. The r esults of this study place PN-1 into the signaling cascade(s) consider ed to be important for the development of this junction and suggest a role for PN-1 in the establishment or maintenance of the differences b etween these adjacent neuromeres.