Bonno M, Oxvig C, Kephart G M, Wagner J M, Kristensen T, Sottrup-Jensen L, Gleich G J
Department of Immunology, Mayo Clinic, Rochester, Minnesota.
Lab Invest. 1994 Oct;71(4):560-6.
The human eosinophil granule major basic protein (MBP), a 13.8 kilodalton cationic polypeptide constituting the core of the eosinophil 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 localized to placental X cells by in situ hybridization. Eosinophil granule MBP is initially translated as a nontoxic precursor (proMBP), containing a 9.9 kilodalton acidic pro-portion that is believed to neutralize MBP toxicity. Recent analyses of sera from pregnant women have revealed that pregnancy-associated plasma protein-A (PAPP-A), previously thought to be a homotetramer of PAPP-A subunits, is actually composed of PAPP-A subunits bound by disulfide bonds to equimolar amounts of proMBP molecules to form a complex, PAPP-A/proMBP. In addition, the PAPP-A subunit nucleotide and deduced amino acid sequence have been determined from cloned cDNA. The PAPP-A monomer found in plasma contains 1547 amino acid residues.
Because of the new evidence that PAPP-A is complexed with proMBP, previous studies on the localization of PAPP-A using antibodies to PAPP-A must be questioned. To determine the localization of the PAPP-A subunit, immunofluorescence was performed on normal placental tissues using proMBP absorbed anti-PAPP-A antibody. Furthermore, the expression of PAPP-A mRNA was investigated by in situ hybridization.
Immunofluorescence staining with proMBP 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 situ hybridization, PAPP-A mRNA is detected in placental X cells and syncytiotrophoblasts, but MBP mRNA is localized only to placental X cells.
The presence of PAPP-A mRNA and PAPP-A subunit protein in placental X cells and syncytiotrophoblasts indicates that both X cells and syncytiotrophoblasts synthesize the PAPP-A subunit, whereas only X cells synthesize proMBP.
人类嗜酸性粒细胞颗粒主要碱性蛋白（MBP）是一种13.8千道尔顿的阳离子多肽，构成嗜酸性粒细胞颗粒的核心，对寄生虫和许多哺乳动物细胞具有细胞毒性。在母体血浆中存在免疫化学上与嗜酸性粒细胞颗粒MBP相似的分子浓度，并且通过原位杂交已将MBP mRNA定位到胎盘X细胞。嗜酸性粒细胞颗粒MBP最初被翻译为无毒前体（proMBP），其含有一个9.9千道尔顿的酸性部分，据信该部分可中和MBP的毒性。最近对孕妇血清的分析表明，妊娠相关血浆蛋白A（PAPP-A），以前被认为是PAPP-A亚基的同四聚体，实际上是由通过二硫键与等摩尔量的proMBP分子结合形成复合物PAPP-A/proMBP的PAPP-A亚基组成。此外，已从克隆的cDNA中确定了PAPP-A亚基的核苷酸和推导的氨基酸序列。血浆中发现的PAPP-A单体含有1547个氨基酸残基。
由于有新证据表明PAPP-A与proMBP复合，因此以前使用抗PAPP-A抗体对PAPP-A进行定位的研究必须受到质疑。为了确定PAPP-A亚基的定位，使用经proMBP吸收的抗PAPP-A抗体对正常胎盘组织进行免疫荧光检测。此外，通过原位杂交研究PAPP-A mRNA的表达。
用经proMBP吸收的抗PAPP-A抗体进行免疫荧光染色显示，PAPP-A定位于胎盘隔、固定绒毛和绒毛膜绒毛的合体滋养层，而MBP仅定位于胎盘隔和固定绒毛。通过原位杂交，在胎盘X细胞和合体滋养层中检测到PAPP-A mRNA，但MBP mRNA仅定位于胎盘X细胞。
胎盘X细胞和合体滋养层中存在PAPP-A mRNA和PAPP-A亚基蛋白，表明X细胞和合体滋养层均合成PAPP-A亚基，而只有X细胞合成proMBP。
