β-amyloid protein precursor expression in lacrimal glands and tear fluid

Purpose. Proteases in tear fluid play important roles in the regulation of corneal wound healing. Inhibitors of proteolytic activity are major modulators of the associated events. Although it is known that various enzyme inhibitors exist in tear fluid, it is not known whether certain isoforms of the β-amyloid protein precursor (β-APP), a potent inhibitor of serine proteases, are present in tear fluid. The purpose of this study was to investigate whether β-APP can be detected in human tear fluid and, if so, to determine the isoform composition and cellular origin. Methods. Tear fluid was collected from healthy volunteers. The β-APP was identified and characterized immunoblotting using antibodies specific for domains of the β-APP. The protein was characterized further by ion exchange chromatography. Expression of the β-APP gene was studied using in situ hybridization and RNA-RNA solution hybridization assay. Results. β-APP with protease inhibitory properties was identified in all samples of human tear fluid. Immunologic analysis revealed that it had been processed proteolytically before secretion. Gene expression studies showed that the β-APP gene was expressed in lacrimal glands, particularly in acinar cells. The gene transcript almost exclusively correspond to β-APP containing the protease inhibitor insert. Conclusions. β-APP is expressed in lacrimal glands and subsequently is secreted into tear fluid. Because the bulk of the β-APP contained the protease inhibitor insert, the authors propose that β- APP is an important regulation of proteolysis in tear fluid and that possibly it plays a role in the events associated with corneal wound healing. This suggests a novel physiological function of β-APP in addition to those previously described-regulation of blood coagulation and cell growth.