SIALOMUCIN COMPLEX AT THE RAT OCULAR SURFACE - A NEW MODEL FOR OCULARSURFACE PROTECTION

The ocular surface, which is among the most accessible and vulnerable tissues in mammals, is protected by a complex tear film composed of li pid, aqueous and mucin layers. In spite of its importance, the molecul ar nature of the mucin contribution remains uncertain. Since membrane mucins have been implicated in the protection of other epithelia, we h ave analysed rat corneal and conjunctival tissues for sialomucin compl ex (SMC), a membrane mucin found at the apical epithelial cell surface s in the airway and uterus. Using Northern and Western blot analyses, SMC expression was found in both ocular tissues, being particularly ab undant in the cornea. In contrast with the other known membrane mucin, MUC1, SMC was localized more heavily towards the apical surface of th e epithelial cells. SMC in ocular surface epithelia was produced in bo th soluble and membrane forms, the latter being found predominantly in the most superficial cells and at apical surfaces. The soluble form w as found loosely adsorbed to apical cell surfaces, particularly of the cornea, as indicated by a mild rinsing protocol. Finally, the tear fl uid contained substantial amounts of SMC. From these results, we propo se a new model for tear mucin components in which SMC is expressed at the apical ocular surface in both membrane-bound and adsorbed soluble forms to provide a direct protective barrier. SMC secreted into the te ar fluid may also participate in maintaining the stability of the preo cular tear film by acting with other secreted mucins to determine the physical properties and protective behaviour of the tear film.