Reduction of mucin-1 expression during the receptive phase in the rat uterus

Mucin-1 (Muc1), an integral membrane mucin, is expressed on the apical surface of uterine epithelial cells (UE) of various species. Loss of Mud is believed to be necessary for embryo attachment. Mud expression is markedly reduced in luminal epithelia during the receptive phase in mice, baboons, and pigs. in the present study, we examined Mud expression during the rat estrous cycle and at Day 5 of pregnancy, the time of embryo attachment In contrast to findings in the mouse, indirect immunofluorescence revealed that uterine Mud protein expression was unaltered during the estrous cycle. However, similar to what is observed in the mouse and other species, Muc1 protein decreased at Day 5 of pregnancy in luminal UE. The decrease in Mud expression was specific to luminal UE and did not occur in glandular UE. A partial cDNA corresponding to the cytoplasmic tail region of rat Muc1 was generated by a reverse transcription-polymerase chain reaction (RT-PCR) strategy. This cDNA sequence is 89% and 91% identical to the corresponding region of mouse Mud at the nucleotide and amino acid levels, respectively. The predicted sequence of rat Muc1 protein has 70-90% identity to the Mud protein sequence obtained in other species. Semiquantitative RT-PCR experiments indicated that the mRNA encoding rat Mud decreased 57% at Day 5 as compared with the levels found at estrus. This value included mRNA from both luminal and glandular UE and so may underestimate the relative decrease in mRNA in the luminal compartment. In conclusion, we have determined that the levels of rat Muc1 protein and mRNA decrease in the luminal UE at the time of implantation, a pattern similar to that seen in the mouse, baboon, and pig. This supports the general theory that reduction of Muc1 expression is necessary for embryo implantation.