MUC5AC mucin is a component of the human precorneal tear film

PURPOSE. Mucins are important structural and functional components of the p recorneal tear film, yet little is known of their composition and synthesis . The mRNAs of MUC1, MUC4, and MUC5AC have previously been identified in hu man conjunctiva. Of these, only MUC5AC mRNA appears to be associated with g oblet cells. The purpose of the this study was to quantify MUC5AC transcrip t levels, to identify MUC5AC protein in conjunctiva, tears, and goblet cell s and to determine whether this mucin is secreted in response to the calciu m ionophore ionomycin. METHODs. MUC5AC mRNA from normal human conjunctiva was identified, quantifi ed, and compared with beta 2-microglobulin levels using a competitive rever se transcription-polymerase chain reaction (RT-PCR) method. An antibody to a MUC5AC peptide was used to localize this mucin in conjunctival sections b y immunohistochemistry. Anti-MUC5AC antiserum was used to label western blo t analysis of conjunctiva and tears. Conjunctival tissues were incubated wi th ionomycin, and secreted mucins were detected with Helix pomatia agglutin in conjugated to horseradish peroxidase and with anti-MUC5AC antiserum. RESULTS. MUC5AC and beta 2-microglobulin transcripts were expressed at a ra tio of approximately 1:500. Immunochemical labeling showed that MUC5AC was localized in conjunctival goblet cells and at the apical surface of the con junctival epithelium. MUC5AC protein was present in conjunctiva and in the tear him. Ionomycin stimulation of conjunctival secretion resulted in a fou rfold increase in total mucin secretion and in a corresponding increase in secreted MUC5AC. CONCLUSIONS. MUC5AC is synthesized by goblet cells of the normal human conj unctiva, and this mucin is a component of conjunctival secretions and of no rmal human tears.