Purpose: Mucins are important structural and functional components of the precorneal tear film, yet little is known of their composition and synthesis. The mRNAs of MUC1, MUC4, and MUC5AC have previously been identified in human conjunctiva. Of these, only MUC5AC mRNA appears to be associated with goblet cells. The purpose of the this study was to quantify MUC5AC transcript levels, to identify MUC5AC protein in conjunctiva, tears, and goblet cells and to determine whether this mucin is secreted in response to the calcium ionophore ionomycin.
Methods: MUC5AC mRNA from normal human conjunctiva was identified, quantified, and compared with beta2-microglobulin levels using a competitive reverse transcription-polymerase chain reaction (RT-PCR) method. An antibody to a MUC5AC peptide was used to localize this mucin in conjunctival sections by immunohistochemistry. Anti-MUC5AC antiserum was used to label western blot analysis of conjunctiva and tears. Conjunctival tissues were incubated with ionomycin, and secreted mucins were detected with Helix pomatia agglutinin conjugated to horseradish peroxidase and with anti-MUC5AC antiserum.
Results: MUC5AC and beta2-microglobulin transcripts were expressed at a ratio of approximately 1:500. Immunochemical labeling showed that MUC5AC was localized in conjunctival goblet cells and at the apical surface of the conjunctival epithelium. MUC5AC protein was present in conjunctiva and in the tear film. Ionomycin stimulation of conjunctival secretion resulted in a fourfold increase in total mucin secretion and in a corresponding increase in secreted MUC5AC.
Conclusions: MUC5AC is synthesized by goblet cells of the normal human conjunctiva, and this mucin is a component of conjunctival secretions and of normal human tears.