Morphologic Evidence for a Preferential Storage of Tissue Plasminogen Activator (t-PA) in Perivascular Axons of the Rat Uvea

The uveal layer is thought to hold the largest stores of tissue plasminogen activator (t-PA) within the eye. However, the uveal cell types that contain and could release t-PA to contiguous tissues and fluids have not been clearly identified. In the present study the general distribution pattern of t-PA antigen in fresh rat iris and choroid tissue was determined by immunofluorescence in preliminary light microscopic (LM) cryosections. Transmission electron microscopic (TEM) immunogold localization was then used to detect specific cellular and subcellular sites of t-PA antigen. The primary antibody was rabbit anti-mouse t-PA IgG. The immunofluorescence in preliminary LM cryosections of both tissues was most intense over discrete linear and cross-sectioned structures that resembled the contours of axon bundles. This impression was strengthened when silver impregnation highlighted similar structures in separate sections of the same tissue samples. TEM immunogold labeling of thin sections then confirmed that the t-PA antigen was confined to the axoplasm of both myelinated and unmyelinated perivascular nerve fibers in both the iris and choroid. Gold particles were not observed over axonal membranes, myelin sheaths, Schwann cells, retinal pigment epithelium or vascular endothelial cells. Ultrathin TEM cryosections of the iris showed a localization of some particles over structures that resembled tubules and vesicles within the axoplasm, but not over mitochondria. The axonal location of t-PA was shown by the co-localization of t-PA with an antibody against rat neurofilaments. The typical axon morphology that enclosed the t-PA particle markers in all TEM sections also indicated an axonal location. Separate TEM sections were processed with conventional fixatives and stains to highlight the typical uveal axon morphology, which also confirmed the identity of perivascular axons as the sites of t-PA localization. Affinity of the primary antibody for rat t-PA was shown by an inhibition ELISA against rat uveal tissue extracts and by the inhibition of t-PA activity in aqueous humor. An amidolytic assay was used to quantify t-PA activity. Possible explanations for the preferential immunolocalization of t-PA antigen to the axoplasm of uveal nerve terminals and the need for additional functional studies to confirm a putative neural t-PA synthesis are discussed.