Endothelial Progenitor Cell Subpopulation Profiling Reveals a Critical Role for Endoglin in Retinal Neovascularization

Abstract

The field of research into endothelial progenitor cells is complex and difficult to interpret. There are a number different investigators working on specific subpopulations that are isolated in unique ways using unique or semi-unique cell surface antigens. Each population is, additionally, assayed in a specialized way corresponding to a certain tissue or disease process. All of these variables make it difficult to determine what cell populations might be important in a particular tissue or disease state, and greatly complicate comparisons between studies and cell populations. These studies sought to develop methods for: (1) analyzing definitive EPC populations and (2) comparing them to one another in the context of ocular neovascularization. The first aim of this work was to develop quantum dot-coded EPC subpopulations and to assess their recruitment to neovascular tufts. This was done using quantum dot nanocrystals conjugated to acetylated low density lipoproteins. Labeled endothelial progenitor cells were then tracked for incorporation into a model of laser-induced choroidal neovascularization. The second aim was to develop high throughput, in vitro methods to analyze the angiogenic capacity of EPCs using quantum dot coded subpopulations. This analytical tool aids in separating the individual aspects of EPC functions: (1) homing to angiogenic sites, (2) incorporating into and forming capillary tubes, and (3) proliferating into neovascular lesions. A unique system of using a parallel plate flow chamber was developed to assess EPC homing capabilities. Using this system, the EPC subpopulation CD133+/CD34+ was shown to be highly capable in terms of all three EPC functions. The third aim was to determine the role of endoglin in oxygen-induced retinopathy (OIR) using the methods developed to analyze the angiogenic capacity of EPCs. Endoglin was studied in the context of retinal neovascularization, and determined to have a pro-angiogenic role, largely through its control of cell proliferation in EPCs. Using an antibody to block endoglin in an OIR model resulted in the dose dependent inhibition of neovascular area, which inhibited additively with VEGF-directed treatments.