Fjx1: a notch-inducible secreted ligand with specific binding sites in developing mouse embryos and adult brain.

The mouse fjx1 gene was identified as a homologue to the Drosophila gene four-jointed (fj). Fj encodes a transmembrane type II glycoprotein that is partially secreted. The gene was found to be a downstream target of the Notch signaling pathway in leg segmentation and planar cell polarity processes during eye development of Drosophila. Here, we show that fjx1 is not only conserved in vertebrates, but we also identified the murine fjx1 gene as a direct target of Notch signaling. In addition to the previously described expression of fjx1 in mouse brain, we show here that fjx1 is expressed in the peripheral nervous system, epithelial cells of multiple organs, and during limb development. The protein is processed and secreted as a presumptive ligand. Through the use of an fjx1-AP fusion protein, we could visualize fjx1 binding sites at complementary locations, supporting the notion that fjx1 may function as a novel signaling molecule. Developmental Dynamics 234:602-612, 2005. (c) 2005 Wiley-Liss, Inc.