The amount of MUC5B mucin in cervical mucus peaks at midcycle

The physical character and amount of mucus secreted by the endocervix chang es dramatically during the menstrual cycle to facilitate sperm migration at the time of midcycle ovulation. Mucins are highly glycosylated, high-molec ular-weight proteins, which are the major structural components of the prot ective mucus gel covering all wet-surfaced epithelia, including that of the endocervix. We have previously demonstrated that the endocervical epitheli um expresses messenger RNA (mRNA) of three of the large gel-forming mucins, designated MUC5AC, MUC5B, and MUC6, with mRNA of MUC5B predominating. Beca use mucin protein levels may be regulated posttranscriptionally, measuremen t of MUC5B protein levels with cycle are needed for correlation to mRNA lev els. Measurement of specific mucin gene products within mucus secretions ha s been limited by availability of specific, well-characterized antibodies a nd by volume requirements of the isolation protocols for mucins, which incl ude CsCl density centrifugation and fraction isolation. To measure MUC5B pr otein within the cervical mucus through the hormone cycle, we developed a p olyclonal antibody specific to the mucin. The antibody, designated no. 799, is to a synthetic peptide mimicking a 19-amino-acid segment of an intercys teine-rich region within the D4 domain in the 3' region of the MUC5B protei n. It recognizes native as well as denatured MUC5B on immunoblot, is preads orbable with its peptide, and binds to apical secretory vesicles of epithel ia expressing MUC5B. We used the MUC5B antibody along with a cervical mucin standard cervical mucin isolate in enzyme-linked immunosorbent assay to de termine the relative amount of MUC5B mucin in samples of human cervical muc us taken through the menstrual cycle. We demonstrate a peak of MUC5B mucin in human cervical mucus collected at midcycle, compared with mucus from ear ly or late in the cycle. This peak in MUC5B content coincides with the chan ge in mucus character that occurs at midcycle, suggesting that this large m ucin species may be important to sperm transit to the uterus.