The Notch signaling pathway is an evolutionarily conserved intercellular signaling mechanism essential for embryonic development in mammals. Mutations in the human JAGGED1 ( JAG1 ) gene, which encodes a ligand for the Notch family of transmembrane receptors, cause the autosomal dominant disorder Alagille syndrome. We have examined the in vivo role of the mouse Jag1 gene by creating a null allele through gene targeting. Mice homozygous for the Jag1 mutation die from hemorrhage early during embryogenesis, exhibiting defects in remodeling of the embryonic and yolk sac vasculature. We mapped the Jag1 gene to mouse chromosome 2, in the vicinity of the Coloboma ( Cm ) deletion. Molecular and complementation analyses revealed that the Jag1 gene is functionally deleted in the Cm mutant allele. Mice heterozygous for the Jag1 null allele exhibit an eye dysmorphology similar to that of Cm /+ heterozygotes, but do not exhibit other phenotypes characteristic of Cm /+ mice or of humans with Alagille syndrome. These results establish the phenotype of Cm /+ mice as a contiguous gene deletion syndrome and demonstrate that Jag1 is essential for remodeling of the embryonic vasculature.

The Notch gene of Drosopbila encodes a large transmembrane protein involved in cell fate determination during embryonic and larval development. This gene is evolutionarily conserved, and Notch homologs have been cloned from several vertebrate species. To examine the in vivo role of the Notchl gene, a mouse homolog of Notch, a mutation was introduced by targeted disruption in embryonic stem cells, and these cells were used to generate mutant mice. Intercrosses of animals heterozygous for the Notchl mutation yielded no live-born homozygous mutant offspring. Homozygous mutant embryos died before 11.5 days of gestation. Morphological and histological analysis of the homozygous mutant embryos indicated that pattern formation through the first nine days of gestation appeared largely normal. However, histological analysis of mutant embryos subsequent to this stage revealed widespread cell death. Death of mutant embryos did not appear to be attributable to defects in placentation or vascularization. Examination of the RNA expression pattern of the Notch2 gene, another Notch gene family member, indicated that it partially overlapped the Notchl expression pattern. Genetic analysis of the Notchl mutation also demonstrated that it was not allelic to a mouse mutation described previously, Danforth's short tail (Sd). These results demonstrate that the Notchl gene plays a vital role during early postimplantation development in mice.

The mammalian cochlea contains an invariant mosaic of sensory hair cells and non-sensory supporting cells reminiscent of invertebrate structures such as the compound eye in Drosophila melanogaster. The sensory epithelium in the mammalian cochlea (the organ of Corti) contains four rows of mechanosensory hair cells: a single row of inner hair cells and three rows of outer hair cells. Each hair cell is separated from the next by an interceding supporting cell, forming an invariant and alternating mosaic that extends the length of the cochlear duct. Previous results suggest that determination of cell fates in the cochlear mosaic occurs via inhibitory interactions between adjacent progenitor cells (lateral inhibition). Cells populating the cochlear epithelium appear to constitute a developmental equivalence group in which developing hair cells suppress differentiation in their immediate neighbours through lateral inhibition. These interactions may be mediated through the Notch signalling pathway, a molecular mechanism that is involved in the determination of a variety of cell fates. Here we show that genes encoding the receptor protein Notch1 and its ligand, Jagged 2, are expressed in alternating cell types in the developing sensory epithelium. In addition, genetic deletion of Jag2 results in a significant increase in sensory hair cells, presumably as a result of a decrease in Notch activation. These results provide direct evidence for Notch-mediated lateral inhibition in a mammalian system and support a role for Notch in the development of the cochlear mosaic.

Here we report the isolation of a rat cDNA clone, Jagged, which we show encodes a ligand for vertebrate Notch. Our conclusion is based on three observations. First, sequence analysis reveals substantial homology between Jagged and invertebrate ligands for the LIN-12/Notch proteins. Second, in situ hybridization of rat embryos identifies both distinct and overlapping patterns of gene expression for Jagged with those for Notch1, Notch2, and Notch3. Finally, the biological activity of Jagged was tested using a cell culture assay in which Jagged activates rat Notch1 expressed in myoblasts and prevents muscle cell differentiation. Our data support the hypothesis that Notch-ligand interactions function in maintaining mammalian cells in an undifferentiated state.

The nature and identity of the pancreatic -cell precursor has remained elusive for many years. One model envisions an early multihormonal precursor that gives rise to both -and -cells and the other endocrine cell types. Alternatively, -cells have been suggested to arise late, directly from the GLUT2-and pancreatic duodenal homeobox factor-1 (PDX1)-expressing epithelium, which gives rise also to the acinar cells during this stage. In this study, we have identified a subset of the PDX1 + epithelial cells that are marked by expression of N e u r o g e n i n 3 (N g n 3 ). N g n 3 , a member of the basic helix-loop-helix (