The Notch pathway is a highly conserved signaling system that controls a diversity of growth, differentiation, and patterning processes. In growing blood vessels, sprouting of endothelial tip cells is inhibited by Notch signaling, which is activated by binding of the Notch receptor to its ligand Delta-like 4 (Dll4). Here, we show that the Notch ligand Jagged1 is a potent proangiogenic regulator in mice that antagonizes Dll4-Notch signaling in cells expressing Fringe family glycosyltransferases. Upon glycosylation of Notch, Dll4-Notch signaling is enhanced, whereas Jagged1 has weak signaling capacity and competes with Dll4. Our findings establish that the equilibrium between two Notch ligands with distinct spatial expression patterns and opposing functional roles regulates angiogenesis, a mechanism that might also apply to other Notch-controlled biological processes.

The mammalian spinal cord does not regenerate motor neurons that are lost as a result of injury or disease. Here we demonstrate that adult zebrafish, which show functional spinal cord regeneration, are capable of motor neuron regeneration. After a spinal lesion, the ventricular zone shows a widespread increase in proliferation, including slowly proliferating olig2-positive (olig2 ϩ ) ependymo-radial glial progenitor cells. Lineage tracing in olig2:green fluorescent protein transgenic fish indicates that these cells switch from a gliogenic phenotype to motor neuron production. Numbers of undifferentiated small HB9 ϩ and islet-1 ϩ motor neurons, which are double labeled with the proliferation marker 5-bromo-2-deoxyuridine (BrdU), are transiently strongly increased in the lesioned spinal cord. Large differentiated motor neurons, which are lost after a lesion, reappear at 6 -8 weeks after lesion, and we detected ChAT ϩ /BrdU ϩ motor neurons that were covered by contacts immunopositive for the synaptic marker SV2. These observations suggest that, after a lesion, plasticity of olig2 ϩ progenitor cells may allow them to generate motor neurons, some of which exhibit markers for terminal differentiation and integration into the existing adult spinal circuitry.

Notch signaling has been shown to regulate various aspects of vascular development. However, a specific role of the ligand Delta-like 1 (DLL1) has not been shown thus far. Here, we demonstrate that during fetal development, DLL1 is an essential Notch ligand in the vascular endothelium of large arteries to activate Notch1 and maintain arterial identity. DLL1 was detected in fetal arterial endothelial cells beginning at embryonic day 13.5. While DLL4-mediated activation has been shown to suppress vascular endothelial growth factor (VEGF) pathway components in growing capillary beds, DLL1-Notch signaling was required for VEGF receptor expression in fetal arteries. In the absence of DLL1 function, VEGF receptor 2 (VEGFR2) and its coreceptor, neuropilin-1 (NRP1), were down-regulated in mutant arteries, which was followed by up-regulation of chicken ovalbumin upstream promoter-transcription factor II (COUP- TFII IntroductionA functional cardiovascular system with precisely formed and connected arteries and veins is established early during mammalian embryogenesis and is essential for subsequent development and survival. First, endothelial precursors, the angioblasts, differentiate into endothelial cells and form a primitive vascular network in a process called vasculogenesis. These first vessels are simple tubes composed entirely of endothelial cells. Subsequently, the simple vascular network is remodeled into a hierarchical network of veins, arteries and capillaries in a process called angiogenesis. 1 Angiogenesis is a precisely orchestrated multistep process that involves localized cell proliferation and migration, the extension of endothelial sprouts and their conversion into patent tubules, as well as the selective pruning of some vessel connections. During angiogenesis, the division into morphologically, functionally, and molecularly unique arteries and veins is established and supporting cell types, such as smooth muscle cells and pericytes, are recruited to form mature and fully functional vessels. 2 Vascular endothelial growth factor (VEGF) signaling is essential for establishment of the embryonic vasculature as is indicated by absence or severe malformations of vessels in embryos lacking VEGF or its receptors (reviewed in Rossant and Howard 3 ). Early during angiogenesis, ie, before development of a functional circulation, the distinction between arteries and veins is established in a process termed arteriovenous differentiation. Accordingly, certain markers and signaling molecules, such as the transmembrane protein ephrinB2 (EFNB2), are specifically expressed in the endothelium of developing arteries. Conversely, the expression of the receptor tyrosine kinase EPHB4, an interaction partner of ephrinB2, and other gene products are confined to the venous endothelium. Loss of either EphrinB2 or EPHB4 leads to very similar defects in vascular remodeling, suggesting that they mediate bidirectional signaling essential for angiogenesis and the establishment of boundaries between venous and arterial domains. [4][5][6...