LOCALIZATION OF PREGNANCY-ASSOCIATED PLASMA PROTEIN-A AND COLOCALIZATION OF PREGNANCY-ASSOCIATED PLASMA PROTEIN-A MESSENGER-RIBONUCLEIC-ACID AND EOSINOPHIL GRANULE MAJOR BASIC-PROTEIN MESSENGER-RIBONUCLEIC-ACID IN PLACENTA

BACKGROUND: The human eosinophil granule major basic protein (MBP), a 13.8 kilodalton cationic polypeptide constituting the core of the eosi nophil granule, is cytotoxic to parasites and numerous mammalian cells . Concentrations of a molecule immunochemically similar to eosinophil granule MBP are present in maternal plasma, and MBP mRNA has been loca lized to placental X cells by in situ hybridization. Eosinophil granul e MBP is initially translated as a nontoxic precursor (proMBP), contai ning a 9.9 kilodalton acidic pro-portion that is believed to neutraliz e MBP toxicity. Recent analyses of sera from pregnant women have revea led that pregnancy-associated plasma protein-A (PAPP-A), previously th ought to be a homotetramer of PAPP-A subunits, is actually composed of PAPP-A subunits bound by disulfide bonds to equimolar amounts of proM BP molecules to form a complex, PAPP-A/proMBP. In addition, the PAPP-A subunit nucleotide and deduced amino acid sequence have been determin ed from cloned cDNA. The PAPP-A monomer found in plasma contains 1547 amino acid residues. EXPERIMENTAL DESIGN: Because of the new evidence that PAPP-A is complexed with proMBP, previous studies on the localiza tion of PAPP-A using antibodies to PAPP-A must be questioned, To deter mine the localization of the PAPP-A subunit, immunofluorescence was pe rformed on normal placental tissues using proMBP absorbed anti-PAPP-A antibody. Furthermore, the expression of PAPP-A mRNA was investigated by in situ hybridization. RESULTS: Immunofluorescence staining with pr oMBP absorbed anti-PAPP-A antibody showed that PAPP-A is localized to placental septa, anchoring villi, and the syncytia of chorionic villi, whereas MBP is localized only to septa and anchoring villi. By in sit u hybridization, PAPP-A mRNA is detected in placental X cells and sync ytiotrophoblasts, but MBP mRNA is localized only to placental X cells. CONCLUSIONS: The presence of PAPP-A mRNA and PAPP-A subunit protein i n placental X cells and syncytiotrophoblasts indicates that both X cel ls and syncytiotrophoblasts synthesize the PAPP-A subunit, whereas onl y X cells synthesize proMBP.