alpha-, beta-, gamma-catenin, and p120(CTN) expression during the terminal differentiation and fusion of human mononucleate cytotrophoblasts in vitro and in vivo

Regulators involved in trophoblast syncytialization in the placenta of intrauterine growth restriction

Placental dysfunction refers to the insufficiency of placental perfusion and chronic hypoxia during early pregnancy, which impairs placental function and causes inadequate supply of oxygen and nutrients to the fetus, affecting fetal development and health. Fetal intrauterine growth restriction, one of the most common outcomes of pregnancy-induced hypertensions, can be caused by placental dysfunction, resulting from deficient trophoblast syncytialization, inadequate trophoblast invasion and impaired vascular remodeling. During placental development, cytotrophoblasts fuse to form a multinucleated syncytia barrier, which supplies oxygen and nutrients to meet the metabolic demands for fetal growth. A reduction in the cell fusion index and the number of nuclei in the syncytiotrophoblast are found in the placentas of pregnancies complicated by IUGR, suggesting that the occurrence of IUGR may be related to inadequate trophoblast syncytialization. During the multiple processes of trophoblasts syncytialization, specific proteins and several signaling pathways are involved in coordinating these events and regulating placental function. In addition, epigenetic modifications, cell metabolism, senescence, and autophagy are also involved. Study findings have indicated several abnormally expressed syncytialization-related proteins and signaling pathways in the placentas of pregnancies complicated by IUGR, suggesting that these elements may play a crucial role in the occurrence of IUGR. In this review, we discuss the regulators of trophoblast syncytialization and their abnormal expression in the placentas of pregnancies complicated by IUGR.

How trophoblasts fuse: an in-depth look into placental syncytiotrophoblast formation

In humans, cell fusion is restricted to only a few cell types under normal conditions. In the placenta, cell fusion is a critical process for generating syncytiotrophoblast: the giant multinucleated trophoblast lineage containing billions of nuclei within an interconnected cytoplasm that forms the primary interface separating maternal blood from fetal tissue. The unique morphology of syncytiotrophoblast ensures that nutrients and gases can be efficiently transferred between maternal and fetal tissue while simultaneously restricting entry of potentially damaging substances and maternal immune cells through intercellular junctions. To maintain integrity of the syncytiotrophoblast layer, underlying cytotrophoblast progenitor cells terminate their capability for self-renewal, upregulate expression of genes needed for differentiation, and then fuse into the overlying syncytium. These processes are disrupted in a variety of obstetric complications, underscoring the importance of proper syncytiotrophoblast formation for pregnancy health. Herein, an overview of key mechanisms underlying human trophoblast fusion and syncytiotrophoblast development is discussed.

CDK1 inhibition facilitates formation of syncytiotrophoblasts and expression of human Chorionic Gonadotropin

AIMS

The human placental syncytiotrophoblast (STB) cells play essential roles in embryo implantation and nutrient exchange between the mother and the fetus. STBs are polyploid which are formed by fusion of diploid cytotrophoblast (CTB) cells. Abnormality in STBs formation can result in pregnancy-related disorders. While a number of genes have been associated with CTB fusion the initial events that trigger cell fusion are not well understood. Primary objective of this study was to enhance our understanding about the molecular mechanism of placental cell fusion.

METHODS

FACS and microscopic analysis was used to optimize Forskolin-induced fusion of BeWo cells (surrogate of CTBs) and subsequently, changes in the expression of different cell cycle regulator genes were analyzed through Western blotting and qPCR. Immunohistochemistry was performed on the first trimester placental tissue sections to validate the results in the context of placental tissue. Effect of Cyclin Dependent Kinase 1 (CDK1) inhibitor, RO3306, on BeWo cell fusion was studied by microscopy and FACS, and by monitoring the expression of human Chorionic Gonadotropin (hCG) by Western blotting and qPCR.

RESULTS

The data showed that the placental cell fusion was associated with down regulation of CDK1 and its associated cyclin B, and significant decrease in DNA replication. Moreover, inhibition of CDK1 by an exogenous inhibitor induced placental cell fusion and expression of hCG.

CONCLUSION

Here, we report that the placental cell fusion can be induced by inhibiting CDK1. This study has a high therapeutic significance to manage pregnancy related abnormalities.

Annexin-A5 organized in 2D-network at the plasmalemma eases human trophoblast fusion

Only a limited number of human cells can fuse to form a multinucleated syncytium. Cell fusion occurs as part of the differentiation of some cell types, including myotubes in muscle and osteoclasts in remodeling bone. In the differentiation of the human placenta, mononuclear cytotrophoblasts aggregate and fuse to form endocrinologically active, non-proliferative, multinucleated syncytia. These syncytia allow the exchange of nutrients and gases between the maternal and fetal circulation. Alteration of syncytial formation during pregnancy affects fetal growth and the outcome of the pregnancy. Here, we demonstrate the role of annexin A5 (AnxA5) in syncytial formation by cellular delivery of recombinant AnxA5 and RNA interference. By a variety of co-immunoprecipitation, immunolocalization and proximity experiments, we show that a pool of AnxA5 organizes at the inner-leaflet of the plasma membrane in the vicinity of a molecular complex that includes E-Cadherin, α-Catenin and β-Catenin, three proteins previously shown to form adherens junctions implicated in cell fusion. A combination of knockdown and reconstitution experiments with AnxA5, with or without the ability to self-assemble in 2D-arrays, demonstrate that this AnxA5 2D-network mediates E-Cadherin mobility in the plasmalemma that triggers human trophoblasts aggregation and thereby cell fusion.

Review: An overview of molecular events occurring in human trophoblast fusion

During human placentation, mononuclear cytotrophoblasts fuse to form a multinucleated syncytia ensuring hormonal production and nutrient exchanges between the maternal and fetal circulation. Syncytia formation is essential for the maintenance of pregnancy and for fetal growth. The trophoblast cell fusion process first requires the acquisition of cell fusion properties, then cells set up plasma membrane protein macrocomplexes and fusogen machinery that trigger cell-cell fusion. Numerous proteins have been shown to be directly involved in the initiation of trophoblast cell fusion. These proteins must expressed at the right time and in the right place to trigger cell-cell fusion. In this review, we describe the role of certain fusogenic protein macrocomplexes that form the scaffold for the fusogen machinery underlying human trophoblastic-lipid mixing and merging of cell contents that lead to cell fusion in physiological conditions.

Involvement of transcription factor NR2F2 in human trophoblast differentiation

Background

During the in vitro differentiation of human villous cytotrophoblast (CTB) cells to a syncytiotrophoblast (STB) phenotype, mRNA levels for the nuclear hormone receptor NR2F2 (ARP-1, COUP-TFII) increase rapidly, reaching a peak at day 1 of differentiation that is 8.8-fold greater than that in undifferentiated CTB cells. To examine whether NR2F2 is involved in the regulation of villous CTB cell differentiation, studies were performed to determine whether NR2F2 regulates the expression of TFAP2A (AP-2α), a transcription factor that is critical for the terminal differentiation of these cells to a STB phenotype.

Methodology/Primary Findings

Overexpression of NR2F2 in primary cultures of human CTB cells and JEG-3 human choriocarcinoma cells induced dose-dependent increases in TFAP2A promoter activity. Conversely, siRNA mediated silencing of the NR2F2 gene in villous CTB undergoing spontaneous differentiation blocked the induction of the mRNAs for TFAP2A and several STB cell specific marker genes, including human placental lactogen (hPL), pregnancy specific glycoprotein 1 (PSG1) and corticotropin releasing hormone (CRH) by 51–59%. The induction of TFAP2A promoter activity by NR2F2 was potentiated by the nuclear hormone receptors retinoic acid receptor alpha (RARA) and retinoid X receptor alpha (RXRA).

Conclusions/Significance

Taken together, these results strongly suggest that NR2F2 is involved in villous CTB cell differentiation and that NR2F2 acts, at least in part, by directly activating TFAP2A gene expression and by potentiating the transactivation of TFAP2A by RARA and RXRA.

Integrin-linked kinase can facilitate syncytialization and hormonal differentiation of the human trophoblast-derived BeWo cell line

BACKGROUND

In the fusion pathway of trophoblast differentiation, stem villous cytotrophoblast cells proliferate and daughter cells differentiate and fuse with existing syncytiotrophoblast to maintain the multi-nucleated layer. Integrin-linked kinase (ILK) is highly expressed in 1st and 2nd trimester villous cytotrophoblast cells, yet barely detectable in syncytiotrophoblast, thus we examined the potential role of ILK in aiding trophoblast fusion.

METHODS

The temporal/spatial expression and activity of ILK were determined in BeWo cells undergoing syncytialization by immunoblot and immunofluorescence analyses. BeWo cells were also transfected with pEGFP expression vectors containing wildtype or two mutant ILK cDNA constructs. The incidence of cell fusion in transfected cells grown under syncytialization conditions was then scored by the presence or absence of E-cadherin immunostaining. Beta-hCG expression in transfected cells, a marker of syncytiotrophoblast hormonal differentiation, was also similarly assessed.

RESULTS

ILK catalytic activity increased and ILK began to increasingly localize to BeWo cell nuclei during syncytialization in correlation with increased pAkt and Snail protein expression. Syncytialization was also significantly elevated (p < 0.05) in BeWo cells expressing constitutively active (ca)-ILK vs cells containing empty vector or dn-ILK. Furthermore, cytoplasmic Beta-hCG expression markedly increased (p < 0.05) in cells expressing wt- and ca-ILK.

CONCLUSION

ILK-facilitated syncytialization is dependent, at least in part, on ILK catalytic activity while hormonal differentiation appears dependent on both ILK-associated protein interactions and catalytic activity. This study demonstrates that ILK plays a novel role in BeWo syncytialization and differentiation, perhaps through an ILK-Akt-Snail pathway, and implicates ILK in the same process in villous cytotrophoblasts in vivo.

Nuclear beta-catenin and Ki-67 expression in choriocarcinoma and its pre-malignant form

OBJECTIVE

To study the expression of nuclear beta-catenin and Ki-67 in patients with normal gestation products (NGP), complete hydatidiform moles (CHM), and choriocarcinoma to elucidate their roles in carcinogenesis and their interrelations.

METHODS

Expression of nuclear beta-catenin and Ki-67 was studied by immunohistochemistry using paraffin embedded blocks. Sixty NGP, 60 CHM, and 10 choriocarcinomas were analysed. In addition, approximately 400 trophoblasts each in 40 NGP, 40 CHM, and 10 choriocarcinomas from the same batch of samples were microdissected for quantitative reverse transcriptase polymerase chain reaction (Q-RT-PCR) to compare beta-catenin mRNA concentration among them.

RESULTS

In the chorionic villi of NGP, beta-catenin was consistently expressed in the nuclei of cytotrophoblasts but not syncytiotrophoblasts. Nuclear beta-catenin expression was comparatively reduced in CHM trophoblasts and was absent in choriocarcinoma. By contrast, Ki-67 expression was increased from cytotrophoblasts but not in syncytiotrophoblasts in the chorionic villi of NGP to CHM trophoblasts and choriocarcinoma. Using Q-RT-PCR, beta-catenin mRNA was detected in 10 NGP, 13 CHM, and three choriocarcinoma specimens, with median copy numbers of 43,230, 18,229, and 17,334 per 400 trophoblasts, respectively. A housekeeping gene glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA was detected as a control in the NGP, CHM, and choriocarcinoma specimens, with median copy numbers of 51,300, 54,270, and 97,150 per 400 trophoblasts, respectively. Thus median beta-catenin mRNA values after normalisation were 0.85 in NGP (n = 10), 0.31 in CHM (n = 13), and 0.16 in choriocarcinoma (n = 3).

CONCLUSIONS

Decreased nuclear beta-catenin expression and increased Ki-67 expression may be involved in choriocarcinoma carcinogenesis. The findings also suggest that nuclear beta-catenin may play a role in trophoblast differentiation during normal placental development.

Integrin-linked kinase (ILK) is highly expressed in first trimester human chorionic villi and regulates migration of a human cytotrophoblast-derived cell line

The placenta represents a critically important fetal-maternal interaction. Trophoblast migration and invasion into the uterine wall is a precisely controlled process and aberrations in these processes are implicated in diseases such as preeclampsia. Integrin-linked kinase (ILK) is a multifunctional, cytoplasmic, serine/threonine kinase that has been implicated in regulating processes such as cell proliferation, survival, migration, and invasion; yet the temporal and spatial pattern of expression of ILK in human chorionic villi and its role in early human placental development are completely unknown. We hypothesized that ILK would be expressed in trophoblast subtypes of human chorionic villi during early placental development and that it would regulate trophoblast migration. Immunoblot analysis revealed that ILK protein was highly detectable in placental tissue samples throughout gestation. In floating branches of chorionic villi, from 6 to 15 wk of gestation immunofluorescence analysis of ILK expression in placental tissue sections demonstrated that ILK was highly detectable in the cytoplasm and membranes of villous cytotrophoblast cells and in stromal mesenchyme, whereas it was barely detectable in the syncytiotrophoblast layer. In anchoring branches of villi, ILK was highly localized to plasma membranes of extravillous trophoblast cells. Transient expression of dominant negative E359K-ILK in the villous explant-derived trophoblast cell line HTR8-SVneo dramatically reduced migration into wounds compared to cells expressing wild-type ILK or empty vector. Therefore, our work has demonstrated that ILK is highly expressed in trophoblast subtypes of human chorionic villi during the first trimester of pregnancy and is a likely mediator of trophoblast migration during this period of development.

Src family kinases play multiple roles in differentiation of trophoblasts from human term placenta

Tyrosine phosphorylation plays a major role in controlling many biological processes in different cell types. Src family kinases (SFKs) are one of the most studied groups of tyrosine kinases and can mediate a variety of signalling pathways. However, little is known about the expression of SFKs in human term placenta and their implication in trophoblast differentiation. Therefore, we examined the expression profile of SFK members over time in culture and their implication in differentiation. In vitro, freshly isolated cytotrophoblast cells, cultured in 10% fetal bovine serum (FBS), spontaneously aggregate and fuse to form multinucleated cells that resemble phenotypically mature syncytiotrophoblasts, that concomitantly produce human chorionic gonadotropin (hCG) and human placental lactogen (hPL). In this study, we showed that trophoblasts expressed all SFK members and some of them are expressed as different splice variants. Moreover, using real-time PCR, this study showed two different expression profiles of SFKs in human trophoblasts during culture. In addition, the protein level and phosphorylation status of Src were evaluated using specific antibodies. Src was rapidly phosphorylated at Tyr-416 and dephosphorylated at Tyr-527 after FBS addition. Surprisingly, inhibition of SFKs by 4-amino-5-(4-chlorophenyl)-7-(t-butyl) pyrazolo[3,4-d] pyrimidine (PP2) or herbimycin A had different effects on trophoblast differentiation. While herbimycin A inhibited morphological and hormonal differentiation, PP2 stimulated hormonal differentiation and inhibited cell adhesion and spreading with no effect on cell fusion. In summary, this study showed that SFKs play different roles in trophoblast differentiation, probably depending on SFK members activated. Thus, this study increases our knowledge and understanding of pathology related to impaired trophoblast differentiation such as pre-eclampsia and trophoblast neoplasm.

Cadherin-11 modulates the terminal differentiation and fusion of human trophoblastic cells in vitro

E-cadherin and cadherin-11 are two members of the cadherin gene family of cell adhesion molecules that are differentially expressed during the aggregation, differentiation, and fusion of trophoblasts isolated from the human term placenta. E-cadherin expression is highest in cytotrophoblasts and decreases as these mononucleate cells undergo terminal differentiation and fusion. In contrast, cadherin-11 expression increases during the formation of multinucleated syncytium in these primary cultures. To define the role(s) of cadherin-11 in this developmental process, we examined the effects of ectopic cadherin-11 expression on the differentiation and fusion of JEG-3 choriocarcinoma cells, a mononucleate trophoblastic cell line. Cadherin-11 expression, but not the ectopic expression of the related cadherin subtype, cadherin-6, resulted in the formation of multinucleated syncytium in the transfected JEG-3 cell cultures. Multinucleated syncytium formation in the JEG-3 cells transfected with cadherin-11 was associated with a reduction in E-cadherin, alpha-, beta-, gamma-catenin, and p120(ctn) expression. Cadherin-11 also reduced cell proliferation and increased the levels of the mRNA transcript encoding the beta subunit of human chorionic gonadotropin, a biochemical marker of trophoblast differentiation, in these cultures. Furthermore, primary cytotrophoblasts cultured in the presence of antisense oligonucleotides specific for cadherin-11 maintained E-cadherin expression and did not undergo terminal differentiation and fusion with time in culture. Collectively, these observations demonstrate that cadherin-11 contributes to the morphological and functional differentiation of cultured mononucleate trophoblastic cells in a highly specific manner.