Study On The Mechanism Of Placental Calcification In Mice With Xpr1 Knock Out

The placenta is a temporary organ formed during embryonic development in mammals and is involved in the exchange of gases,nutrients and metabolic wastes between the mother and fetus,which is essential to maintain the normal growth and development of the fetus.Placental calcification refers to the deposition of calcium phosphate in placental tissue.Severe placental calcification will affect the development and function of the placenta and increase the risk of fetal birth abnormalities.However,there are few studies on the mechanism of placental calcification.XPR1 encodes a xenotropic and polytropic murine leukemia virus receptor(XPR1),which is expressed on the cell membrane.Further studies have found that it has the function of transporting intracellular inorganic phosphate(Pi)to the extracellular.It is the only Pi exporter found in mammals and is widely expressed in tissues.Recent studies have shown that XPR1 is one of the genes responsible for human idiopathic basal ganglia calcification(IBGC).However,in the process of using Xpr1 knockout(Xpr1-KO)mice as model to study the pathogenesis of IBGC,our team found that the heterozygosis of Xpr1 knockout(Xpr1^+/-)mice presented a low birthrate and no similar phenotype of IBGC,while the homozygosis of Xpr1 knockout(Xpr1^-/-)mice could not be born.Further studies showed that in the late pregnancy,the weight of Xpr1^-/-and Xpr1^+/-fetal mice decreased obviously,and the Xpr1-deficient placenta had a severe calcification phenotype.How does Xpr1 gene knock out cause placental calcification in mice?In this study,the spatiotemporal expression and localization of Xpr1 in mouse placenta was firstly analyzed by in situ hybridization and real-time quantitative PCR.It was found that Xpr1was widely expressed in wide-type(WT)placenta at embryonic(E)18.5,and the expression increased with the development of placenta.Secondly,the placental histopathology showed that the calcified nodules of Xpr1^-/-and Xpr1^+/-placentas were mainly deposited in the complex labyrinth of placenta.The silver staining demonstrates that initial placental calcification occurs in trophoblast and endothelial cells,and is an intracellular calcification.Radioactive ³²P was used to detect the Pi transport capacity of the placenta,and it was found that the ability of Xpr1^-/-and Xpr1^+/-placentas to transport the Pi from maternal blood to fetal blood was significantly reduced at E15.5 compared with the WT placentas.However,the Pi transport capacity of Xpr1^-/-and Xpr1^+/-placentas was significantly increased at E18.5(possibly related to severe placental calcification during this period),indicating that the Pi transport function of Xpr1-deficient placentas was abnormal.This result indicates that Xpr1 deficiency damages the placental Pi homeostasis,which may be the cause of placental calcification.It is known that fetal Pi is actively transported from maternal blood mainly through the placenta.Therefore,it is speculated that during fetal development,placental Pi transport dysfunction may affect fetal Pi homeostasis and thus affect fetal growth and development.Through detecting the Pi concentration in the amniotic fluid and serum of fetal mice,it was found that the Pi concentration in amniotic fluid and serum of Xpr1^-/-and Xpr1^+/-fetuses was significantly lower than that of WT fetuses.Analysis of skeletal development in fetal mice showed that compared with WT fetuses,the bone mineralization of Xpr1^-/-and Xpr1^+/-fetuses was decreased.These results indicated that Xpr1 deficiency disrupted the phosphorus homeostasis of the fetus.This may be related to the Pi transport dysfunction of the Xpr1-deficient placenta.In order to further explore the molecular mechanism of placental calcification caused by Xpr1 deficiency,transcriptomic techniques were used to analyze differentially expressed genes in WT and Xpr1^+/-placentas.Compared with WT placenta,the expression of granzymes and perforin in Xpr1^+/-heterozygous placenta was up-regulated,which may be involved in the formation of placental calcification.In summary,this study is the first to report the phenotype of placental calcification in Xpr1-deficient mice,which initially occurs in cells and may be related to the imbalance of placental Pi homeostasis.In addition,the placental Pi transport dysfunction may disrupt the fetal Pi homeostasis,resulting in fetal growth and development constraints and birth abnormalities.These data are significant to better understand the biological function of XPR1 and the mechanism of placental calcification.