The Jagged2 gene maps to chromosome 12 and is a candidate for the lgl and sm mutations

Loss of distinct arterial and venous boundaries in mice lacking endoglin, a vascular-specific TGFbeta coreceptor

Several characteristic morphological and functional differences distinguish arteries from veins. It was thought that hemodynamic forces shaped these differences; however, increasing evidence suggests that morphogenetic programs play a central role in blood vessel differentiation. Hereditary hemorrhagic telangiectasia (HHT) is a vascular dysplasia characterized by the inappropriate fusion of arterioles with venules. The genes implicated in this disease, ALK1 and endoglin, may be involved in defining the fundamental boundaries between arteries and veins. We previously showed that mice lacking Alk1 lost structural, molecular, and functional distinctions between arteries and veins. Here, we report that mice lacking endoglin develop arterial-venous malformations and fail to confine intraembryonic hematopoiesis to arteries. In contrast to Alk1 mutants, endoglin mutants do not show profound vessel dilation or downregulation of arterial ephrinB2. Finally, our data indicate that a failure in cardiac cushion formation observed in both strains may be secondary to the peripheral vasculature defect. The phenotypic similarities, yet reduced severity, implicates endoglin as an accessory coreceptor that specifically modulates Alk1 signaling. We propose that endoglin and Alk1 are necessary for the maintenance of distinct arterial-venous vascular beds and that attenuation of the Alk1 signaling pathway is the precipitating event in the etiology of HHT.

Evidence that a single replication fork proceeds from early to late replicating domains in the IgH locus in a non-B cell line

In non-B cell lines, like the murine erythroleukemia cell line (MEL), the most distal IgH constant region gene, C alpha, replicates early in S; other heavy chain constant region genes, joining and diversity segments, and the most proximal Vh gene replicate successively later in S in a 3' to 5' direction proportional to their distance from C alpha. In MEL, replication forks detected in the IgH locus also proceed in the same 3' to 5' direction for approximately 400 kb, beginning downstream of the IgH 3' regulatory region and continuing to the D region, as well as within the Vh81X gene. Downstream of the initiation region is an early replicating domain, and upstream of Vh81X is a late replicating domain. Hence, the gradual transition between early and late replicated domains can be achieved by a single replication fork.

The loss of ventral ectoderm identity correlates with the inability to form an AER in the legless hindlimb bud

We have characterized the early stages of murine hindlimb morphogenesis in the legless (lgl)mutant and non-mutant littermates. Initially the entire ventral ectoderm expresses many genetic markers characteristic of the AER (en-1, fgf-8, msx-2, dlx-2, cd44, and cx-43). Subsequently, the expression domain of most of these genes is restricted to the thickened ectoderm of the disto-ventral limb margin prior to forming an AER. In lgl, the expression of these genes is initiated but not maintained and the disto-ventral marginal ectoderm does not thicken. In contrast, Wnt7a expression is initiated and maintained in the dorsal ectoderm. The limb mesenchyme of lgl and non-mutant embryos initially expresses lmx-1b and fgf-10 uniformly. As the ventro-distal marginal ectoderm thickens, lmx-1b is progressively dorsally restricted in non-mutants but continues to be expressed ventrally in lgl hindlimb buds. These data suggest that establishment of a dorso-ventral ectodermal interface is not sufficient for AER formation and that restriction of lmx-1b to the dorsal mesenchyme is coordinately linked to AER formation.

Defects in limb, craniofacial, and thymic development in Jagged2 mutant mice

The Notch signaling pathway is a conserved intercellular signaling mechanism that is essential for proper embryonic development in numerous metazoan organisms. We have examined the in vivo role of the Jagged2 (Jag2) gene, which encodes a ligand for the Notch family of transmembrane receptors, by making a targeted mutation that removes a domain of the Jagged2 protein required for receptor interaction. Mice homozygous for this deletion die perinatally because of defects in craniofacial morphogenesis. The mutant homozygotes exhibit cleft palate and fusion of the tongue with the palatal shelves. The mutant mice also exhibit syndactyly (digit fusions) of the fore- and hindlimbs. The apical ectodermal ridge (AER) of the limb buds of the mutant homozygotes is hyperplastic, and we observe an expanded domain of Fgf8 expression in the AER. In the foot plates of the mutant homozygotes, both Bmp2 and Bmp7 expression and apoptotic interdigital cell death are reduced. Mutant homozygotes also display defects in thymic development, exhibiting altered thymic morphology and impaired differentiation of gamma delta lineage T cells. These results demonstrate that Notch signaling mediated by Jag2 plays an essential role during limb, craniofacial, and thymic development in mice.