Haptoglobin expression and release by rabbit oviduct and endometrium, its localization in blastocyst extra-embryonic matrix and fluid during preimplantation time.

Evidence is emerging that haptoglobin, an acute phase protein with immunomodulatory properties, is expressed by the endometrium of various species. The present study describes an in-depth investigation of haptoglobin expression and release in the rabbit reproductive tract and in preimplantation embryos. The full-length cDNA sequence of rabbit haptoglobin was determined by rapid amplification of cDNA ends PCR. Haptoglobin expression was studied in the oviductal ampull, and isthmus, endometrium and embryos from the time of ovulation up to adhesion. These results were completed by western blot analysis of reproductive tract secretions and embryonic tissues. cDNA sequencing showed a high homology between rabbit and human haptoglobin (84.1%). In oviductal tissues haptoglobin mRNA is clearly expressed from 6 h post-conception (p.c.) to day 3, and in the uterus on days 5 and 6. In the oviductal fluid highest haptoglobin protein content was found between 6 h p.c and day 2, and in the uterine fluid on days 5 and 6 p.c. Embryos do not express haptoglobin mRNA during preimplantation development. However, considerable amounts of maternal haptoglobin protein were detected in the blastocyst coverings and in blastocyst fluid. Already during periovulatory time and oviductal passage, high amounts of haptoglobin are present in the microenvironment surrounding the oocyte/embryo. Two days before implantation, again, high haptoglobin levels are detectable in the embryo's environment. The incorporation of haptoglobin into the extra-embryonic matrix may be of particular functional significance.