Molecular Regulation of Endothelial Cell Tight Junctions and the Blood-Retinal Barrier

The Retinal Vascular Barrier

The inner retina, including the ganglion cell layer, is supported by the retinal vascular system that emanates from the central retinal artery in the optic nerve and radiates to the four retinal quadrants [1, 3]. These four branches form three capillary plexuses, one in the nerve fiber layer and ganglion cell layer and two outer capillary plexuses that border the inner nuclear layer termed the shallow inner nuclear layer and deep inner nuclear layer capillary beds [5]. The inner capillary bed that resides within the nerve fiber layer and ganglion cell layer can separate to two additional beds or appear as one capillary bed. The arterioles and venules are restricted to the ganglion cell layer, and nerve fiber layer with only capillaries developed from angiogenesis, extending deeper into the retina. Primates have an avascular region known as the macula that includes the fovea, which is highly enriched with cones necessary for the high-contrast central vision [6].

The BRB controls the flux of blood-borne solutes and fluid into the retina and maintains the proper retinal environment for normal neural conduction. Low number of vesicles and fenestrae, expression of multidrug-resistance genes, and well-developed junctional complex in both the retinal vasculature and RPE combine to provide the necessary defined neural environment for proper retinal function.

Multiple cell types in the retina contribute to endothelial junctional complex formation and regulation. Investigators have demonstrated the ability of glial cells to induce vascular barrier properties in a variety of systems for both brain and retinal glia. Both astrocytes [7] and Müller cells [8] are capable of inducing barrier properties in endothelial cells, and injection of astrocytes or Müller cells into the anterior chamber of the rat eye leads to vascularization and formation of vessels with elevated barrier properties. Conversely, transplanting the avascular neural tissue of stage 13 quail brains into the coelomic cavity of 3-day chick embryos caused the invading capillaries to take on blood-brain barrier characteristics including reduced permeability to circulating dye [9].

Recent studies have identified a signal transduction adaptor molecule that promotes production of probarrier factors from astrocytes. Src-suppressed C kinase substrate or SSECKS in rodents, also termed gravin in humans, or AKAP12, coordinates signal transduction pathways by binding and organizing signaling molecules such as protein kinase C, protein kinase A, calmodulin, cyclins, and b (beta)-adrenergic receptors. In brain, SSECKS colocalizes with GFAP, indicating a glial expression pattern, and a recent report demonstrated that expression of SSECKS contributes to astrocytic induction of the blood-brain barrier [10]. Overexpression of SSECKS reduces expression of vascular endothelial growth factor (VEGF) apparently through a reduction of c-Jun and AP1 signaling and promotes angiopoietin 1 production.

In addition to astrocytes, pericytes also contribute to barrier formation by secreting an angiopoietin 1 complex, which induces occludin expression [11]. Angiopoietin 1 is a ligand for the Tie2 receptor and both stabilizes blood vessels and protects them from VEGFinduced permeability [12, 13]. Together, these studies demonstrate that glia and pericytes contribute an important role in the induction of the blood-brain and BRB. Indeed, coculture of astrocytes and pericytes with endothelial cells induces barrier properties to a greater extent than either cell type alone [14]. An understanding of the molecular mechanisms by which this differentiation proceeds is only beginning to be elucidated.