Tip Cells in Angiogenesis

Abstract

In angiogenesis, the process in which blood vessel sprouts grow out from a pre‐existing vascular network, the so‐called endothelial tip cells play an essential role. Tip cells are the leading cells of the sprouts; they guide following endothelial cells and sense their environment for guidance cues. Because of this essential role, the tip cells are a potential therapeutic target for anti‐angiogenic therapies, which need to be developed for diseases such as cancer and major eye diseases. The potential of anti‐tip cell therapies is now widely recognised, and the surge in research this has caused has led to improved insights in the function and regulation of tip cells, as well as the development of novel in vitro and in silico models. These new models in particular will help understand essential mechanisms in tip cell biology and may eventually lead to new or improved therapies to prevent blindness or cancer spread.

Key Concepts

  • Sprouting angiogenesis is led by tip cells, which can sense their environment and direct the sprouting process.
  • Tip cells are followed by stalk cells, which have a more proliferative phenotype.
  • The number of tip cells is regulated by lateral inhibition between the tip cells and the stalk cells; the tip cells prevent the adjacent cells from becoming tip cells and thereby optimise the number of tip cells.
  • The tip cell and stalk cell phenotype are definite, and stalk cells can overtake tip cells through stochastic cellular and environmental differences, the stalk cells then become tip cells and force the former tip cells to become stalk cells.
  • Tip cells exist in human umbilical vein endothelial cell cultures and can therefore be studied in vitro.

Keywords: angiogenesis; tip cell; stalk cell; anti‐angiogenic therapy; VEGF

Figure 1. (a) Tip cells in the retina of a mouse (white arrow) at the top of newly formed capillaries during angiogenesis at 5 days after birth (P5). The capillaries are stained with Isolectin B4 (green). (b) Higher magnification of the edge of an area in the mouse retina where tip cells at the top of newly formed capillaries are shown to have filopodia (arrow heads). Bars are 250 µm.
Figure 2. Overview of the key regulatory pathways for tip cell selection. Tip cells are represented in red, stalk cells in blue and the phalanx cells in orange. (a) VEGF: VEGF‐A is produced upon hypoxia; tip cells express VEGFR2 and ‐3, two receptors that exert the pro‐angiogenic effects of VEGF. Soluble VEGFR1 is produced by stalk cells and acts as a sink for VEGF to prevent signalling in stalk cells. (b) Neuropilins: NRP‐1 acts as a co‐receptor for VEGFR2 to enhance VEGF signalling. (c) Notch1‐Dll4: Dll4 is expressed by tip cells, Notch1 by stalk cells. Upon binding, Notch1 is cleaved, and the intracellular domain (NICD) is translocated to the nucleus where it recruits transcription factors to replace expression of tip cell genes by expression of stalk cell genes. (d) Angiopoietins and Tie2: Ang1 is expressed in mature vessels and its main function is vessel stabilisation. (e) Ang2 can bind Tie2 to inhibit Ang1‐mediated phosphorylation of Tie2; it also binds integrins on tip cells to enhance sprouting. (f) BMPs and SMADs: Pro‐ and anti‐angiogenic BMPs bind to their receptor and phosphorylate SMAD1/5/8. In tip cells, the SMAD1/5/8 complex induces polarisation and migration. (g) In stalk cells, the SMAD1/5/8 complex forms a complex with NICD to promote the stalk cell phenotype.
Figure 3. In vivo, in vitro and in silico models for angiogenesis. (a) Fli1a‐eGFP transgenic zebrafish, at 24‐h post fertilisation. At this stage, sprouting occurs in the intersegmental vessels (white arrows) and tip cells are present on each sprouting vessel. (b) CD34+ cell in a human umbilical vein endothelial cell (HUVEC) culture. HUVECs were grown on coverslips coated with gelatin and stained for F‐actin (red), CD34 (green) and DAPI (blue). (c) In silico vascular network formed from a spheroid of endothelial cells by a mechanism of cell–cell contact‐inhibited chemotaxis (Merks et al., ). Tip cells are indicated in red.
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Dallinga, Marchien G, Boas, Sonja EM, Klaassen, Ingeborg, Merks, Roeland HM, van Noorden, Cornelis JF, and Schlingemann, Reinier O(May 2015) Tip Cells in Angiogenesis. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0025977]