CD34 marks angiogenic tip cells in human vascular endothelial cell cultures: a new model to study mechanisms of ocular angiogenesis

Purpose The functional shift of quiescent endothelial cells into tip cells that migrate and stalk cells that proliferate is a key event during sprouting angiogenesis. Unfortunately, a model of tip cells in vascular endothelial cell cultures is lacking. Methods We employed the sialomucin CD34 to isolate a small subset of elongated endothelial cells with filopodia from endothelial cell cultures and tested if these cells had properties similar to tip cells in vivo at the functional and molecular level. Results As predicted by our hypothesis, CD34+ endothelial cells had low proliferation activity. The CD34+ phenotype was upregulated by VEGF‐A and downregulated by TNF‐alpha and DLL4, three mechanisms known to regulate the tip cell phenotype in vivo. Real‐time qPCR and microarray data analysis of the CD34+ cells identified increased expression of all known genes previously associated with tip cells in vivo. Genome‐wide mRNA profiling analysis of CD34+ cells demonstrated enrichment for biological functions related to angiogenesis and migration, whereas CD34‐negative cells were enriched for functions related to proliferation Conclusion Our findings suggest that cells with virtually all known properties of tip cells are present in vascular endothelial cell cultures, and that they can be isolated based on expression of CD34. In addition, we characterized the transcriptome of these cells and identified many novel genes with potential significance for angiogenesis. This novel strategy may open alternative avenues of research that may help to understand the molecular processes and functions in angiogenesis in general and of the specialized endothelial tip cell in particular.