REDUCTION OF MUCIN-1 EXPRESSION DURING THE RECEPTIVE PHASE IN THE RATUTERUS

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 expres sion during the rat estrous cycle and at Day 5 of pregnancy, the time of embryo attachment In contrast to findings in the mouse, indirect im munofluorescence revealed that uterine Mud protein expression was unal tered during the estrous cycle. However, similar to what is observed i n the mouse and other species, Muc1 protein decreased at Day 5 of preg nancy in luminal UE. The decrease in Mud expression was specific to lu minal UE and did not occur in glandular UE. A partial cDNA correspondi ng to the cytoplasmic tail region of rat Muc1 was generated by a rever se transcription-polymerase chain reaction (RT-PCR) strategy. This cDN A sequence is 89% and 91% identical to the corresponding region of mou se Mud at the nucleotide and amino acid levels, respectively. The pred icted sequence of rat Muc1 protein has 70-90% identity to the Mud prot ein sequence obtained in other species. Semiquantitative RT-PCR experi ments 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 relati ve decrease in mRNA in the luminal compartment. In conclusion, we have determined that the levels of rat Muc1 protein and mRNA decrease in t he luminal UE at the time of implantation, a pattern similar to that s een in the mouse, baboon, and pig. This supports the general theory th at reduction of Muc1 expression is necessary for embryo implantation.