Immunohistochemical localization of pregnancy-associated plasma protein A (PAPP-A) in placentae from normal and pre-eclamptic pregnancies

Pregnancy-associated plasma protein A (PAPP-A) and preeclampsia

Pregnancy-associated plasma protein A (PAPP-A) is a key regulator of insulin-like growth factor bioavailability essential for normal fetal development. In maternal blood, this protein increases with gestational age and then rapidly decreases after delivery. It is routinely used for Down syndrome screening in the first trimester of pregnancy, and its decrease compared to a normal pregnancy indicates an increased risk for both chromosomal anomalies and adverse pregnancy outcomes. It belongs to a group of biomarkers that predict later preeclampsia development, primarily early onset preeclampsia; however, it should be combined with a Doppler ultrasonography of the uterine artery (pulsatile index) and other biochemical and maternal factors to achieve a higher detection rate with an acceptable false positivity rate. Some studies have demonstrated an even more pronounced decrease of PAPP-A in the early second trimester of pregnancy in women who subsequently develop preeclampsia compared with women who do not develop preeclampsia. Conversely, during the last trimester of pregnancy, its concentration increases even more in patients with preeclampsia than in patients without. It is also detectable at very low levels in nonpregnant individuals, and a higher concentration indicates an adverse effect in patients with acute coronary syndromes or stable atherosclerotic disease and in patients with end-stage renal disease who are being treated with hemodialysis.

Differential regulation of pregnancy associated plasma protein (PAPP)-A during pregnancy in human and mouse

Serum IGFBP-4 proteolytic activity increases dramatically during human pregnancy and is mainly attributed to the pregnancy-associated plasma protein-A (PAPP-A). To understand the regulation and actions of PAPP-A in vivo, we evaluated the utility of a mouse model system. Serum from day-9 and day-17 pregnant mice and age-matched controls was tested for IGFBP-4 proteolytic activity using recombinant mouse IGFBP-4 as the substrate. Surprisingly, IGFBP-4 proteolytic activity in mouse pregnancy serum (mPS) was not significantly different from that of non-pregnancy serum (mNPS). Addition of IGF-II to mPS or mNPS at a dose sufficient to increase IGFBP-4 proteolysis by human PS failed to enhance IGFBP-4 proteolysis. PAPP-A neutralization antibody did not inhibit IGFBP-4 proteolysis by mPS or mNPS, but completely blocked IGFBP-4 proteolytic activity in human PS (hPS, mouse osteoblast conditioned medium, and mouse amniotic fluid). To determine whether the lack of PAPP-A activity in mPS was due to low expression of PAPP-A in the placenta, we cloned a mouse genomic DNA, which contained 1 kb of the entire exon 2 coding sequence and 2.5 kb of the flanking intron sequences. The exon 2-coded mouse and human PAPP-A shared 86% amino acid sequence identity. RT-PCR analysis revealed that the PAPP-A mRNA level in mouse placenta was lower compared to that in human placenta by at least two orders of magnitude. PAPP-A expression was also lower in mouse placenta compared to those in mouse kidney, osteoblasts, and bone marrow stromal cells.

CONCLUSIONS

(1) Serum IGFBP-4 proteolytic activity is differentially regulated by pregnancy in human and mouse. (2) The lack of an increase in serum IGFBP-4 proteolytic activity during mouse pregnancy is due to the low level of PAPP-A expression in the placenta.

Identification and regulation of the IGFBP-4 protease and its physiological inhibitor in human trophoblasts and endometrial stroma: evidence for paracrine regulation of IGF-II bioavailability in the placental bed during human implantation

The IGF family plays an important role in implantation and placental physiology. IGF-II is abundantly expressed by placental trophoblasts, and IGF binding protein (IGFBP)-4, a potent inhibitor of IGF actions, is the second most abundant IGFBP in the placental bed, expressed exclusively by the maternal decidua. Proteolysis of IGFBP-4 results in decreased affinity for IGF peptides, thereby enhancing IGF actions. In the current study, we have identified the IGFBP-4 protease and its inhibitor in human trophoblast and decidualized endometrial stromal cell cultures, and we have investigated their regulation in an effort to understand control of IGF-II bioavailability at the placental-decidual interface in human implantation. IGFBP-4 protease activity was detected in conditioned media (CM) from human trophoblasts and decidualized endometrial stromal cells using (125)I-IGFBP-4 substrate. Identification of the IGFBP-4 protease as pregnancy-associated plasma protein-A (PAPP-A) was confirmed by specific immunoinhibition and immunodepletion of the IGFBP-4 protease activity with specific PAPP-A antibodies. The IGFBP-4 protease activity was IGF-II-dependent in trophoblast CM. In decidualized stromal CM, PAPP-A/IGFBP-4 protease activity was also IGF-II-dependent, but was evident only when IGF-II was added in molar excess of the predominant IGFBP in decidualized stromal cell CM, IGFBP-1, supporting bioavailable IGF-II as a key cofactor of IGFBP-4 proteolysis by PAPP-A. Cultured first and second trimester human trophoblasts (n = 5) secreted PAPP-A into CM with mean +/- SEM levels of 172.4 +/- 32.8 mIU/liter.10(5) cells, determined by specific ELISA. PAPP-A in trophoblast CM (n = 3) and did not change in the presence of IGF-II (1-100 ng/ml). Cultured human endometrial stromal cells (n = 4) secreted low levels of PAPP-A (6.25 +/- 3.6 mIU/liter.10(5) cells). A physiological inhibitor of PAPP-A, the proform of eosinophil major basic protein (proMBP), was detected in trophoblast CM at levels of 1853 +/- 308 mIU/liter.10(5) cells, determined by specific ELISA, and was nearly undetectable in CM of human endometrial stromal cells. Upon in vitro decidualization of endometrial stromal cells with progesterone, PAPP-A levels in CM increased nearly 9-fold without a concomitant change in proMBP. In contrast to the experiments with trophoblasts, IGF-II and the IGF analogues, Leu(27) IGF-II, and Des (1-6) IGF-II, resulted in a dose-dependent decrease of PAPP-A levels in decidualized endometrial stromal CM by 70-90%, and a dose-dependent increase in proMBP of 14- to 41-fold. The data demonstrate conclusively that the IGF-II-dependent IGFBP-4 protease of human trophoblast and decidual origin is PAPP-A. Furthermore, the differential regulation of decidual PAPP-A and proMBP by insulin-like peptides supports a role for trophoblast-derived IGF-II as a paracrine regulator of these maternal decidual products that have the potential to regulate IGF-II bioavailability at the trophoblast-decidual interface. Overall, the data underscore potential roles for a complex family of enzyme (PAPP-A), substrate (IGFBP-4), inhibitor (proMBP), and cofactor (IGF-II) in the placental bed during human implantation.

Morphology and glycoprotein synthesis of uterine glandular epithelium in human basal plate at term: an ultrastructural and autoradiographic study

This study describes the morphologic features of uterine glandular epithelium in human basal plate at term and identifies this epithelium as an active site of glycoprotein synthesis. Wedge biopsies were obtained from the basal plate at the time of repeat cesarean section from 11 normal pregnant patients at term. Biopsy specimens were either processed immediately for microscopic examination or incubated in vitro with 25 microCi/cc of 3H-galactose or 3H-leucine. Tissues incubated with tritiated compounds (2-hour pulse +/- 3-hour chase in nonradioactive medium) were either processed for light microscopic autoradiographic analysis or extracted for determination of trichloroacetic-acid-precipitable tritiated macromolecules in tissues and medium. Profiles of cuboidal-columnar glandular epithelium were identified in the decidual component of the basal plate region adjacent to spiral arterioles and perpendicular to the inner layers of myometrial muscle. Autoradiographic and biochemical studies identified the glandular epithelium, as well as large decidual cells, to be major sites of incorporation of both 3H-galactose and 3H-leucine and to be prime sources for secretion of tritiated macromolecules that appeared in the medium during in vitro incubation of basal plate tissue. Ultrastructurally, the glandular epithelium was noted to rest on a basal lamina, to have lateral cell membranes with numerous desmosomes, and to exhibit an apical surface with microvilli projecting into a luminal space. Cytologic features of the cells included abundant profiles of rough endoplasmic reticulum, multiple mitochondria with lamellar cristae, a well-developed perinuclear but nonpolarized Golgi apparatus,and nuclei containing predominantly euchromatin.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of different antibody preparations against pregnancy-associated plasma protein-A (PAPP-A) for use in localization and immunoassay studies

Four antibody preparations against pregnancy-associated plasma protein (PAPP-A) were compared in order to find an explanation for the contradictory results published on tissue localization, clinical usefulness and biological function of PAPP-A. One of the preparations studied was a rabbit anti-PAPP-A antiserum which has been offered for general scientific use (Bischof et al. 1979). Only the IgG fraction of anti-PAPP-A antisera which appeared to be monospecific and had been further absorbed with fetal connective tissue gave specific uniform staining of the cytoplasm of the syncytiotrophoblast exclusively. Circulating PAPP-A could not be detected by RIA employing this IgG preparation in the non-pregnant state, or before 18 days after conception. Circulating PAPP-A could be detected in all seven pregnant women studied within 4 weeks after conception. Identical results were obtained with a commercially available IgG fraction against PAPP-A.

Immunohistochemical aspects of immunological cross-reaction and masking of epitopes for localization studies on pregnancy-associated plasma protein A

The influence of antibody absorption procedures and proteolytic pre-treatment of formaldehyde-fixed placental tissue on the localization of pregnancy-associated plasma protein A by immunoperoxidase technique was examined. Apparently monospecific IgG fraction of the anti-plasma protein applied directly on fixed tissue resulted in staining of connective tissue and a thin apical rim of the syncytiotrophoblast. Further absorption of the antibody with foetal connective tissue abolished this staining reaction. Pre-treatment of the fixed placental tissue with trypsin prior to application of the antibody, which had been absorbed with connective tissue, resulted in staining within the cytoplasm of the syncytiotrophoblast exclusively. Identical staining was seen when this IgG preparation was used directly on frozen placental tissue. The results point to the importance of the specificity of the antibody preparations and of proteolytic unmasking of epitopes when fixed tissues are used for localization studies of pregnancy-associated plasma protein A by immunoperoxidase technique.

An immunohistological comparison of the secretory capacity of villous and extravillous trophoblast in the human placenta

Using an immunoperoxidase technique the localization of hCG, hPL, SP1, PAPP-A and PP5 within villous and extravillous trophoblast has been studied. The villous syncytiotrophoblast appears to be the sole significant source of hCG, SP1, PAPP-A and PP5 but hPL is also present in the infiltrating extravillous trophoblast. Within the interstitial extravillous trophoblast the proportion of cells staining positively for hPL increases progressively as the cells extend deeper into the uterine tissues but all the vascular extravillous trophoblast within the lumina of the spiral arteries stains positively for hPL at all levels within the decidua and inner myometrium. It is not clear why the synthetic capacity of the infiltrating trophoblast is limited to the production of hPL. We suggest that this might be indicative of selectivity of a specific subpopulation of cytotrophoblastic cells. It is unlikely that the physiological effects of infiltrating trophoblast are mediated by hPL, this substance being probably only a marker of a cell population with a particular invasive capacity.