VEGF-A induced hyperpermeability of blood-retinal barrier endothelium in vivo is predominantly associated with pinocytotic vesicular transport and not with formation of fenestrations

Purpose. In tissues outside the brain, vascular endothelial growth factor-A (VEGF) causes vascular hyper-permeability by opening of inter-endothelial junctions and induction of fenestrations and vesiculo-vacuolar organelles ( VVOs). In preliminary studies, we observed that in blood-retinal barrier en dothelium, other cellular mechanisms may underlie increased permeability ca used by VEGF. This was further investigated in material of an in in vivo ex perimental model of VEGF-induced retinopathy. Methods. Two monkeys received 4 intravitreal injections of 0.5 mug VEGF in one eye and PBS in the other eye prior to sacrifice at day 9. One monkey re ceived 12 injections of 1.25 mug VEGF in one eye and PBS in the other eye p rior to sacrifice at day 24. As a control, an untreated eye of a fourth mon key was studied. Results. In the high-dose VEGF-injected eye, fluorescein angiography showed intense retinal micro-vascular leakage. This leakage was also demonstrated by immunohistochemistry demonstrating extravasation of endogenous fibrinog en and IgG. However, in these leaky blood vessels the number of pinocytotic vesicles (caveolae) at the endothelial luminal membrane were significantly higher and, only in the VEGF-injected eyes, these pinocytotic vesicles tra nsported plasma IgG. By electron microscopy, no fenestrations or VVOs were found in the endothelial cells of the VEGF-injected eyes. Conclusion. We conclude that increased vascular permeability for plasma pro teins induced by VEGF in blood-retinal barrier endothelium is predominantly caused by a mechanism involving active trans-endothelial transport via pin ocytotic vesicles and not by formation of endothelial fenestrations or VVOs .