2000b) Complex regulation of calciumbinding protein D9k (Calbindin-D9k) in the mouse uterus during early pregnancy and at the site of embryo implantation

Establishment of receptive endometrium is essential for im-plantation. Our aim was to identify and characterize genes uniquely regulated at the sites of implantation in mouse uterus by RNA differential display polymerase chain reaction (DDPCR). One of the gene fragments identified was 86 % homologous to rat calcium-binding protein D9k (calbindin-D9k); the mouse counterpart had not then been cloned, but subsequently an mRNA sequence of mouse calbindin-D9k became available in GenBank (accession number: AF028071). This sequence is 99% homologous to the DDPCR-derived gene tag but has a shorter 39 end. Reverse transcription-polymerase chain reaction (RT-PCR) was performed using the sequence of 39 end of the DDPCR product and the 59 end of AF028071, and a full cDNA was ob-tained. This gene was primarily up-regulated by progesterone, but not by estrogen. It was further increased by the combination of the two steroids. Expression of calbindin-D9k was overall in-creased in the uterus during early pregnancy, but the level was significantly lower in implantation compared to interimplanta-tion sites on Days 4.5 and 5.5 of pregnancy, becoming barely detectable in both sites after Day 6.5. In situ hybridization lo-calized this mRNA predominantly in the luminal epithelium of the pregnant uterus. The complex regulation of calbindin-D9k in mouse uterus suggests an important role for this protein during pregnancy