Purpose: To determine whether the epithelium of the human nasolacrimal ducts contains aquaporins (AQPs), a family of membrane proteins that function as selective pores and are able to transport water, glycerol, and other small solutes across the cell plasma membrane.
Methods: Expression of AQPs 0 and 1-10 in human nasolacrimal duct tissue was determined by reverse transcription polymerase chain reaction (RT-PCR). Positive PCR amplification products were verified by direct cDNA sequencing. Western blot analysis was used to detect AQPs 3-5. Antisera specific for AQPs were used in immunohistochemical analysis to determine the presence and distribution of ten AQPs (AQP 0 and 1-9) in epithelia and subepithelial glands of the nasolacrimal ducts. All samples investigated originated from human postmortem tissue.
Results: In human nasolacrimal duct samples, AQPs 1, 3, 4, 5, 7, 8, 9, and 10 were identified by RT-PCR. No RT-PCR products were detected for AQPs 0, 2, and 6. All AQPs, which were detected by RT-PCR, were also confirmed by direct sequencing of the cDNA. Immunohistochemical analyses revealed AQPs 1, 3, 5, 7, 8, and 9 in human nasolacrimal duct epithelium and were detected in different cells. Expression of AQP 4 could not be detected immunohistochemically but by Western blot analysis. Protein detection of AQP 10 could not be performed due to the unavailability of an appropriate antibody.
Conclusions: The results suggest specific roles for AQPs in water transport through the epithelium of the nasolacrimal ducts and underline the presumption that tear fluid components are selectively absorbed in the nasolacrimal passage.