Complex regulation of calcium-binding protein D9k (Calbindin-D-9k) in the mouse uterus during early pregnancy and at the site of embryo implantation

Establishment of receptive endometrium is essential for implantation. Our a im was to identify and characterize genes uniquely regulated at the sites o f 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-D-9k); the mouse counterpart had not then been cloned, but subsequently an mRNA sequence of mouse calbin din-D-9k became available in GenBank (accession number: AF028071), This seq uence is 99% homologous to the DDPCR-derived gene tag but has a shorter 3' end. Reverse transcription-polymerase chain reaction (RT-PCR) was performed using the sequence of 3' end of the DDPCR product and the 5' end of AF0280 71, and a full cDNA was obtained. This gene was primarily up-regulated by p rogesterone, but not by estrogen, It was further increased by the combinati on of the two steroids. Expression of calbindin-D-9k was overall increased in the uterus during early pregnancy, but the level was significantly lower in implantation compared to interimplantation sites on Days 4.5 and 5.5 of pregnancy, becoming barely detectable in both sites after Day 6.5, In situ hybridization localized this mRNA predominantly in the luminal epithelium of the pregnant uterus, The complex regulation of calbindin-D-9k in mouse u terus suggests an important role for this protein during pregnancy.