SPECIFIC EXPRESSION OF HAPTOGLOBIN MESSENGER-RNA IN IMPLANTATION-STAGE RABBIT UTERINE EPITHELIUM

A glycoprotein, termed GP42, was previously identified in uterine flui d obtained from peri-implantation-stage rabbits. N-terminus amino acid sequencing of purified GP42 demonstrated identity through the first 1 3 amino acids with the beta subunit of liver haptoglobin. The present study was undertaken to determine ii GP42 is indeed identical to hapto globin and, if so, to determine whether it is expressed in the uterus as opposed to being present as a transudate from plasma. Reverse trans cription-PCR amplification of poly(A)(+) RNA prepared from implantatio n-stage rabbit endometrium with GP42- and haptoglobin-specific primers yielded a predicted 667 bp cDNA product. Sequence analysis of the clo ned cDNA confirmed the identity of GP42 with beta-haptoglobin. Norther n blot analysis demonstrated the specific expression of haptoglobin mR NA in the peri-implantation-stage endometrium and the absence of its e xpression in the estrous or day 4 pseudopregnant endometrium. Nonisoto pic in situ hybridization revealed that the haptoglobin mRNA was restr icted to the epithelium lining the luminal surface and mucosal folds o f day 6 3/4 pregnant or pseudopregnant uteri and that no haptoglobin m RNA was detectable in the epithelium of the deep glands or cells of th e stroma or myometrium. Similarly, in situ hybridization revealed no e xpression of haptoglobin mRNA in any cell types of the estrous uterus. These data establish the identity of GP42 with beta-haptoglobin and d emonstrate that it is expressed in a stage-specific manner just prior to implantation, correlating with uterine receptivity to blastocyst im plantation. Endometrial GP42 mRNA expression is not dependent on the p resence of blastocysts.