Vascular development depends on the highly coordinated actions of a variety of angiogenic regulators, most of which apparently act downstream of vascular endothelial growth factor (VEGF). One potential such regulator is delta-like 4 ligand (Dll4), a recently identified partner for the Notch receptors. We generated mice in which the Dll4 gene was replaced with a reporter gene, and found that Dll4 expression is initially restricted to large arteries in the embryo, whereas in adult mice and tumor models, Dll4 is specifically expressed in smaller arteries and microvessels, with a striking break in expression just as capillaries merge into venules. Consistent with these arterial-specific expression patterns, heterozygous deletion of Dll4 resulted in prominent albeit variable defects in arterial development (reminiscent of those in Notch knockouts), including abnormal stenosis and atresia of the aorta, defective arterial branching from the aorta, and even arterial regression, with occasional extension of the defects to the venous circulation; also noted was gross enlargement of the pericardial sac and failure to remodel the yolk sac vasculature. These striking phenotypes resulting from heterozygous deletion of Dll4 indicate that vascular development may be as sensitive to subtle changes in Dll4 dosage as it is to subtle changes in VEGF dosage, because VEGF accounts for the only other example of haploid insufficiency, resulting in obvious vascular abnormalities. In summary, Dll4 appears to be a major trigger of Notch receptor activities previously implicated in arterial and vascular development, and it may represent a new opportunity for pro-and anti-angiogenic therapies.

One of the most effective approaches for determining gene function involves engineering mice with mutations or deletions in endogenous genes of interest. Historically, this approach has been limited by the difficulty and time required to generate such mice. We describe the development of a high-throughput and largely automated process, termed VelociGene, that uses targeting vectors based on bacterial artificial chromosomes (BACs). VelociGene permits genetic alteration with nucleotide precision, is not limited by the size of desired deletions, does not depend on isogenicity or on positive-negative selection, and can precisely replace the gene of interest with a reporter that allows for high-resolution localization of target-gene expression. We describe custom genetic alterations for hundreds of genes, corresponding to about 0.5-1.0% of the entire genome. We also provide dozens of informative expression patterns involving cells in the nervous system, immune system, vasculature, skeleton, fat and other tissues.

Transforming growth factor (TGF-)β is secreted as a latent complex in which the mature growth factor remains associated with its propeptide. In order to elicit a biological response, the cytokine must be released from the latent complex, a process termed latent TGF-β activation or TGF-β formation. Although latent TGF-β activation is a critical step in the regulation of its activity, little is known about the molecular mechanisms that lead to the production of active TGF-β. In this article, we present an overview of the data available on this topic, and we propose a tentative model for the mechanism of TGF-β formation based upon the observations with different cell systems and on recent findings on the structure of the latent TGF-β complex.

The accession numbers supplied by the authors in Table 1, p. 555, were incorrect. The correct ones are given below.

Rifkin DB, Mazzieri R, Munger JS, Noguera I & Sung J. Proteolytic control of growth factor availability. APMIS 1999;107:80-5.Most growth factors are released from cells in a form that does not permit immediate interaction with their high affinity receptors. An important mechanism for presentation of these released latent growth factors is activation by the plasminogen activatorplasmin system. The involvement of this system in the biology of Transforming Growth Factor-P (TGF-P) is reviewed.