Linda Karlsson

Dll4 signalling through Notch1 regulates formation of tip cells during angiogenesis

In sprouting angiogenesis, specialized endothelial tip cells lead the outgrowth of blood-vessel sprouts towards gradients of vascular endothelial growth factor (VEGF)-A. VEGF-A is also essential for the induction of endothelial tip cells, but it is not known how single tip cells are selected to lead each vessel sprout, and how tip-cell numbers are determined. Here we present evidence that delta-like 4 (Dll4)–Notch1 signalling regulates the formation of appropriate numbers of tip cells to control vessel sprouting and branching in the mouse retina. We show that inhibition of Notch signalling using γ-secretase inhibitors, genetic inactivation of one allele of the endothelial Notch ligand Dll4, or endothelial-specific genetic deletion of Notch1, all promote increased numbers of tip cells. Conversely, activation of Notch by a soluble jagged1 peptide leads to fewer tip cells and vessel branches. Dll4 and reporters of Notch signalling are distributed in a mosaic pattern among endothelial cells of actively sprouting retinal vessels. At this location, Notch1-deleted endothelial cells preferentially assume tip-cell characteristics. Together, our results suggest that Dll4–Notch1 signalling between the endothelial cells within the angiogenic sprout serves to restrict tip-cell formation in response to VEGF, thereby establishing the adequate ratio between tip and stalk cells required for correct sprouting and branching patterns. This model offers an explanation for the dose-dependency and haploinsufficiency of the Dll4 gene, and indicates that modulators of Dll4 or Notch signalling, such as γ-secretase inhibitors developed for Alzheimer’s disease, might find usage as pharmacological regulators of angiogenesis.

PDGF-C is a new protease-activated ligand for the PDGF alpha-receptor.

Platelet-derived growth factors (PDGFs) are important in many types of mesenchymal cell. Here we identify a new PDGF, PDGF-C, which binds to and activates the PDGF α-receptor. PDGF-C is activated by proteolysis and induces proliferation of fibroblasts when overexpressed in transgenic mice. In situ hybridization analysis in the murine embryonic kidney shows preferential expression of PDGF-C messenger RNA in the metanephric mesenchyme during epithelial conversion. Analysis of kidneys lacking the PDGF α-receptor shows selective loss of mesenchymal cells adjacent to sites of expression of PDGF-C mRNA; this is not found in kidneys from animals lacking PDGF-A or both PDGF-A and PDGF-B, indicating that PDGF-C may have a unique function.

Role of platelet-derived growth factors in angiogenesis and alveogenesis.

Platelet-derived growth factors (PGDFs) are 30-kDa dimeric proteins that exert their functions by binding to and activating PDGF receptors in the cell membrane [12, 22]. Two different PDGF monomers exist; the A chain and the B chain, and these may assemble into AA and BB homodimers as well as AB heterodimers. The two known PDGF-receptor proteins, the PDGF-α receptor (PDGF-Rα) and the β receptor (PDGF-Rβ) are both receptor tyrosine kinases and interact differentially with the PDGF molecules; PDGF-Rβ binds only the PDGF B chain with high affinity, whereas PDGF-Rα binds both chains with high affinity. Accordingly, the different PDGF dimers may bind to, dimerize and signal through different receptor pairs. Dimerization of the receptors is a prerequisite for signaling, since it allows for tyrosine phosphorylation of the intracellular part of the receptor molecules [11]. The resulting phosphotyrosine residues constitute binding sites for molecules carrying src-homology-2 (SH2) and other phosphotyrosine-binding domains. Their association with the PDGF receptor is a critical step in downstream signaling [5, 13].