Identification of Vascular Endothelial Side Population Cells in the Choroidal Vessels and Their Potential Role in Age-Related Macular Degeneration

PURPOSE. The neovascular form of age-related macular degeneration (AMD) is characterized by the growth of abnormal new blood vessels from the choroid, termed choroidal neovascularization (CNV). The origin of the new vessels in CNV, however, has not been elucidated fully to our knowledge. The purpose of this study is to identify vascular endothelial side population (SP) cells in the preexisting choroidal vessels, and investigate their potential role in AMD. METHODS. We made single cell suspensions of freshly isolated mouse choroidal, retinal, and brain tissue by enzymatic digestion. Vascular endothelial SP cells were isolated using flow cytometry based on the ability to efflux the DNA-binding dye, Hoechst 33342, via ATP-binding cassette (ABC) transporters. RESULTS. In the choroid, 2.8% of CD31(+)CD45 vascular endothelial cells (ECs) showed a typical SP staining pattern. They were not bone marrow-derived and possessed high colony-forming capacity in vitro. They proliferated during laser-induced CNV in vivo. In contrast, stereotypic SP staining pattern was not observed in retinal and brain ECs. Retinal and brain EC-SP cells included increased SP populations with less colony-forming capacity within the SP compartment, because they contained cells with and without proliferative potential. The latter still could efflux the dye due to high levels of ABC transporters, such as ABCB1a, ABCC4, and ABCC6. CONCLUSIONS. The EC-SP cells in the choroid may represent vessel-residing endothelial stem/progenitor cells contributing mainly to angiogenesis, and may be useful for augmenting vascular regeneration or for developing new antiangiogenic therapy in AMD.