Choroidal endothelial cell activation in age-related macular degeneration

Age-related macular degeneration (AMD) is a devastating ocular disease affecting one third of the elderly population in the western world. Some cases of AMD develop neovascular membranes, which are characterized by the pathologic growth of new blood vessels into the retina. This pathology may be initiated by proteins capable of activating endothelial cells to become angiogenic or inflammatory, later causing them to grow abnormally. This investigation aimed to determine the causes of pathologic blood vessel growth in AMD.

Human eyes with AMD have been shown by us and others to have abnormal activities of angiogenin, complement component C5 anaphylatoxin (C5a), and/or elastin fragments. We therefore employed methods including PCR, immunoblotting, immunohistochemistry, morphometrics, tissue culture, ultrastructural observations, and functional assays to determine the effects angiogenin, C5a, and elastin fragments on the angiogenic and inflammatory changes of choroidal endothelial cells in vitro and in vivo.

It was shown that choroidal endothelial cells express the surface receptor for C5a. Also, these cells increase their expression of ICAM-1, a surface protein that mediates leukocyte trafficking, in response to elevated levels of C5a in organ culture. This indicates that increased levels of C5a associated with AMD increase the inflammatory behavior of choroidal endothelial cells. It was demonstrated that choroidal endothelial cells are able to internalize angiogenin, a potent inducer of angiogenesis. Although cells from the choroid did not increase their angiogenic responses to this protein, their ability to internalize it indicates that they may respond to it by a different mechanism. Elevated levels of elastin fragments, however, did increase the migratory response of choroidal endothelial cells in culture, which is a key event in angiogenesis. Elevated levels of elastin fragents also increased the amount of collagen IV deposition within Bruch's membrane in a mouse model. This is relevant to AMD pathology as deposits within Bruch's membrane are common manifestations associated with AMD.

This body of work has provided new insights into the roles of angiogenin, C5a, and elastin fragments in activating choroidal endothelial cells to becoming inflammatory or angiogenic. These endothelial cell behaviors are common characteristics found in neovascular AMD. These new findings will help aid in the further understanding of the pathobiology of this disease in hopes to provide improved treatments in the future.