Immunolocalization of occludin and claudin-1 to tight junctions in intact CNS vessels of mammalian retina

The distributions of occludin and claudin-1, two tight junction-associated integral membrane proteins were investigated by immunohistochemical analysi s of whole-mount preparations of the blood vessels in the myelinated streak of the rabbit retina. Light microscopy revealed that occludin and claudin- 1 immunoreactivities were abundant along the interface of adjacent endothel ial cells of all blood vessels. Electron microscopy revealed that both prot eins were distributed in a regular pattern (at regular intervals of approxi mately 80 nm) along the length of tight junctions, probably in the regions of tight junction strands. No other structures or cell types expressed eith er of these two proteins in the myelinated streak. Whereas occludin immunor eactivity was concentrated only at the tight junction interface, claudin-1 immunoreactivity also extended into the cytoplasm of the endothelial cells, suggesting a different structural role for claudin-1 than for occludin at tight junctions. Retinal pigment epithelial cells expressed occludin around their entire circumference, consistent with the function of these cells as a barrier separating the retina from the leaky vessels of the choroid. Als o consistent with the association of occludin expression with vessels that exhibit functional tight junctions, this protein was expressed at only a lo w level in, and showed an irregular distribution along, the vessels of the choroid, a vascular bed that lacks blood-barrier properties. Further, the d istribution of occludin was examined during formation and remodelling of th e rat retinal vasculature. Occludin expression was evident at the leading e dge of vessel formation and was found on all vessels in both the inner and outer vascular plexus. Numerous vascular segments at the early stage of vas cular formation and regression lost occludin expression. The biological sig nificance of this transient loss of occludin expression in terms of barrier function remains to be elucidated.