Our current research efforts are directed at understanding the changes in retinal vessels that occur in diabetes, in particular the increased microvessel permeability that occurs early in diabetes. This research may be more broadly characterized as understanding the mechanisms by which receptor tyrosine kinases regulate cell/cell interactions and tissue permeability. The blood vessels of the retina, like other tissues with strong permeability barriers, are characterized by the formation of tight junctions. These junctions seal cells together forming a strong tissue barrier to macromolecules such as proteins and lipids as wells as water and ions. Diabetes causes an increased expression of a number of growth factors in the retina including VEGF and PDGF. VEGF is a well-characterized vascular permeabilizing agent and we have found PDGF can also effect barrier permeability.

We have recently identified three alterations in tight junction proteins that appear to mediate the changes in permeability induced by receptor tyrosine kinases. First, VEGF causes rapid changes in tight junction protein phosphorylation state. ZO-1 is a tight junction protein that contains a number of binding domains and as such is believed to organize tight junctions. VEGF treatment causes an increase in ZO-1 protein tyrosine phosphorylation. Additionally, we have observed the tight junction specific, transmembrane protein occludin is rapidly phosphorylated in response to VEGF. This is, in fact, the first demonstration of phosphorylation of this transmembrane protein associated with increased permeability. Second, we have observed the tight junction proteins ZO-1 and occludin co-migrate away from the cell membrane into the cytoplasm in response to VEGF. This migration appears to occur through the process of membrane invagination and vesicle formation. Finally, we have observed a decrease in tight junction protein content with chronic exposure to VEGF such that 6 h of VEGF treatment caused a 50% reduction in occludin content. Additionally, 3 months of experimentally induced diabetes in rats caused a 35% decrease in occludin content commensurate with an increase in albumin permeability.

Our goals for the future are to determine the roles for tight junction protein phosphorylation and movement in the regulation of barrier permeability. Furthermore, we hope to understand the mechanism of tight junction protein degradation. We hope that by elucidating the underlying alterations that cause vascular permeability we will provide targets for drug development leading to novel treatments for diabetic retinopathy as well as other pathologies that lead to vascular permeability such as tumors.