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T. Nakajima, R.D. Walkup, T.R. Shearer, M. Azuma; Characterization of Lacritin in Human Tears: Comparison to Other Species . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1933.
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© ARVO (1962-2015); The Authors (2016-present)
Tear fluid is important in maintaining ocular surfaces. Lacritin, an extracellular glycoprotein, was recently found to be expressed in the lacrimal gland and present in tear fluid. The functions of lacritin have not been fully elucidated. Lacritin might be an important factor in maintaining ocular surfaces since lacritin attenuated apoptosis in corneal epithelial cells. The purposes of the present experiments were 1) to characterize lacritin in human tears, 2) to compare human lacritin to lacritin in other species, and 3) to determine the tissues where lacritin was produced and secreted.
Diluted tear fluid was collected from human, monkey and rat after saline (10–50 µL) was instilled. Tear proteins were separated by SDS–PAGE. Lacritin was subjected to N–terminal protein sequencing and immunoblot analysis. Existence of carbohydrate chains on lacritin was determined by PAS staining. The expression of lacritin in tissues was determined by quantitative PCR and immunoblot analysis.
Lacritin migrated at 21 kDa on SDS–PAGE gels and was confirmed by N–terminal protein sequencing and immunoblotting. Lacritin comprised about 1.3% of total human tear proteins as determined by densitometric scanning of SDS–PAGE gels. PAS staining showed the lacritin band to be highly modified by carbohydrate chain. Carbohydrate chains were also associated with monkey lacritin. Monkey lacritin migrated to a similar position as human, and the amount of lacritin in monkey was higher than that in human. In monkey, lacritin was also highly expressed in lacrimal gland compared to other eye tissues examined. In contrast, no lacritin was observed at similar positions on SDS–PAGE in rat tears. These data agree with a search of the Genbank database, where no highly homologous lacritin protein was found for rat.
In higher animals, lacritin was produced in lacrimal gland and secreted into tear fluid. These results suggested that maintaining the ocular surface in higher animals required lacritin.
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