Human conjunctival goblet cells express the membrane associated mucin MUC16: Localization to mucin granules

MUC16 is an extraordinarily large 22,152 amino acid membrane spanning mucin that has been shown to be present in the glycocalyx of the apical cells of the human cornea and conjunctiva where it interfaces with the tear film. The ectodomain of the molecule has been demonstrated in tears, where it has been presumed to be from surface epithelial cells. Data presented here from multiple assays, including immunohistochemistry, immunoelectron microscopy, in situ hybridization, and RT-PCR of RNA isolated from goblet cells isolated by laser capture microdissection, demonstrate that the membrane tethered mucin is also expressed by conjunctival goblet cells both in humans and in mice. The mucin is present in mucin granules and appears to be localized to the mucin granule membrane. Correlation analyses of the amounts of the goblet cell secreted mucin MUC5AC and the amounts of MUC16 and of MUC1 another membrane tethered mucin ectodomain found in human tear samples demonstrated that MUC5AC amounts correlated to the amounts of MUC16 but not to MUC1. These data suggest that goblet cells are a second source of the mucin in tears. The function of the membrane tethered mucin in the mucin granule remains to be determined.