Phosphatidylserine Efflux and Intercellular Fusion in a BeWo Model of Human Villous Cytotrophoblast

Diagnosis, Prevention, and Management of Fetal Growth Restriction (FGR)

Fetal growth restriction (FGR), or intrauterine growth restriction (IUGR), is still the second most common cause of perinatal mortality. The factors that contribute to fetal growth restriction can be categorized into three distinct groups: placental, fetal, and maternal. The prenatal application of various diagnostic methods can, in many cases, detect the deterioration of the fetal condition in time because the nature of the above disorder is thoroughly investigated by applying a combination of biophysical and biochemical methods, which determine the state of the embryo-placenta unit and assess the possible increased risk of perinatal failure outcome and potential for many later health problems. When considering the potential for therapeutic intervention, the key question is whether it can be utilized during pregnancy. Currently, there are no known treatment interventions that effectively enhance placental function and promote fetal weight development. Nevertheless, in cases with fetuses diagnosed with fetal growth restriction, immediate termination of pregnancy may have advantages not only in terms of minimizing perinatal mortality but primarily in terms of reducing long-term morbidity during childhood and maturity.

Apoptotic and non-apoptotic roles of caspases in placenta physiology and pathology

Caspases, a family of cysteine proteases, are pivotal regulators of apoptosis, the tightly controlled cell death process crucial for eliminating excessive or unnecessary cells during development, including placental development. Collecting research has unveiled the multifaceted roles of caspases in the placenta, extending beyond apoptosis. Apart from their involvement in placental tissue remodeling via apoptosis, caspases actively participate in essential regulatory processes, such as trophoblast fusion and differentiation, significantly influencing placental growth and functionality. In addition, growing evidence indicates an elevation in caspase activity under pathological conditions like pre-eclampsia (PE) and intrauterine growth restriction (IUGR), leading to excessive cell death as well as inflammation. Drawing from advancements in caspase research and placental development under both normal and abnormal conditions, we examine the significance of caspases in both cell death (apoptosis) and non-cell death-related processes within the placenta. We also discuss potential therapeutics targeting caspase-related pathways for placenta disorders.

Mechanisms of Cell Fusion in Cancer

Cell-cell fusion is a normal physiological mechanism that requires a well-orchestrated regulation of intracellular and extracellular factors. Dysregulation of this process could lead to diseases such as osteoporosis, malformation of muscles, difficulties in pregnancy, and cancer. Extensive literature demonstrates that fusion occurs between cancer cells and other cell types to potentially promote cancer progression and metastasis. However, the mechanisms governing this process in cancer initiation, promotion, and progression are less well-studied. Fusogens involved in normal physiological processes such as syncytins and associated factors such as phosphatidylserine and annexins have been observed to be critical in cancer cell fusion as well. Some of the extracellular factors associated with cancer cell fusion include chronic inflammation and inflammatory cytokines, hypoxia, and viral infection. The interaction between these extracellular factors and cell's intrinsic factors potentially modulates actin dynamics to drive the fusion of cancer cells. In this review, we have discussed the different mechanisms that have been identified or postulated to drive cancer cell fusion.

HTRA1 in Placental Cell Models: A Possible Role in Preeclampsia

The HtrA serine peptidase 1 (HTRA1) is a multidomain secretory protein with serine-protease activity involved in the regulation of many cellular processes in both physiological and pathological conditions. HTRA1 is normally expressed in the human placenta, and its expression is higher in the first trimester compared to the third trimester, suggesting an important role of this serine protease in the early phases of human placenta development. The aim of this study was to evaluate the functional role of HTRA1 in in vitro models of human placenta in order to define the role of this serine protease in preeclampsia (PE). BeWo and HTR8/SVneo cells expressing HTRA1 were used as syncytiotrophoblast and cytotrophoblast models, respectively. Oxidative stress was induced by treating BeWo and HTR8/SVneo cells with H₂O₂ to mimic PE conditions in order to evaluate its effect on HTRA1 expression. In addition, HTRA1 overexpression and silencing experiments were performed to evaluate the effects on syncytialization, cell mobility, and invasion processes. Our main data showed that oxidative stress significantly increased HTRA1 expression in both BeWo and HTR8/SVneo cells. In addition, we demonstrated that HTRA1 has a pivotal role in cell motility and invasion processes. In particular, HTRA1 overexpression increased while HTRA1 silencing decreased cell motility and invasion in HTR8/SVneo cell model. In conclusion, our results suggest an important role of HTRA1 in regulating extravillous cytotrophoblast invasion and motility during the early stage of placentation in the first trimester of gestation, suggesting a key role of this serine protease in PE onset.

Regulation of phospholipid distribution in the lipid bilayer by flippases and scramblases

Cellular membranes function as permeability barriers that separate cells from the external environment or partition cells into distinct compartments. These membranes are lipid bilayers composed of glycerophospholipids, sphingolipids and cholesterol, in which proteins are embedded. Glycerophospholipids and sphingolipids freely move laterally, whereas transverse movement between lipid bilayers is limited. Phospholipids are asymmetrically distributed between membrane leaflets but change their location in biological processes, serving as signalling molecules or enzyme activators. Designated proteins - flippases and scramblases - mediate this lipid movement between the bilayers. Flippases mediate the confined localization of specific phospholipids (phosphatidylserine (PtdSer) and phosphatidylethanolamine) to the cytoplasmic leaflet. Scramblases randomly scramble phospholipids between leaflets and facilitate the exposure of PtdSer on the cell surface, which serves as an important signalling molecule and as an 'eat me' signal for phagocytes. Defects in flippases and scramblases cause various human diseases. We herein review the recent research on the structure of flippases and scramblases and their physiological roles. Although still poorly understood, we address the mechanisms by which they translocate phospholipids between lipid bilayers and how defects cause human diseases.

Downregulation of argininosuccinate synthase 1 (ASS1) is associated with hypoxia in placental development

Argininosuccinate synthase (ASS1) is involved in nitric oxide production, which has a key role in placental development improving pregnancy outcomes. Syncytiotrophoblast and extravillous trophoblast differentiations are milestones of placental development and their impairment can cause pathologies, such as preeclampsia (PE) and fetal growth restriction (FGR). Immunohistochemistry and Western blotting were used to localize and quantify ASS1 in first trimester (8.2 ± 1.8 weeks), third trimester (38.6 ± 1.1 weeks), and PE (36.3 ± 1.5 weeks) placentas. In addition, cell cultures were used to evaluate ASS1 expression under hypoxic conditions and the syncytialization process. Our data showed that ASS1 is localized in the villous cytotrophoblast of first trimester, third trimester, and PE placentas, while the villous cytotrophoblast adjacent to the extravillous trophoblast of cell columns as well as the extravillous trophoblast were negative for ASS1 in first trimester placentas. In addition, ASS1 was decreased in third trimester compared to the first trimester placentas (p = 0.003) and no differences were detected between third trimester and PE placentas. Moreover, ASS1 expression was decreased in hypoxic conditions and syncytialized cells compared to those not syncytialized. In conclusion, we suggest that the expression of ASS1 in villous cytotrophoblast is related to maintaining proliferative phenotype, while ASS1 absence may be involved in promoting the differentiation of villous cytotrophoblast in extravillous cytotrophoblast of cell columns in first trimester placentas.

Two types of type IV P-type ATPases independently re-establish the asymmetrical distribution of phosphatidylserine in plasma membranes

Phospholipids are asymmetrically distributed between the lipid bilayer of plasma membranes in which phosphatidylserine (PtdSer) is confined to the inner leaflet. ATP11A and ATP11C, type IV P-Type ATPases in plasma membranes, flip PtdSer from the outer to the inner leaflet, but involvement of other P4-ATPases is unclear. We herein demonstrated that once PtdSer was exposed on the cell surface of ATP11A^−/−ATP11C^−/− mouse T cell line (W3), its internalization to the inner leaflet of plasma membranes was negligible at 15 °C. However, ATP11A^−/−ATP11C^−/− cells internalized the exposed PtdSer at 37 °C, a temperature at which trafficking of intracellular membranes was active. In addition to ATP11A and 11C, W3 cells expressed ATP8A1, 8B2, 8B4, 9A, 9B, and 11B, with ATP8A1 and ATP11B being present at recycling endosomes. Cells deficient in four P4-ATPases (ATP8A1, 11A, 11B, and 11C) (QKO) did not constitutively expose PtdSer on the cell surface but lost the ability to re-establish PtdSer asymmetry within 1 hour, even at 37 °C. The expression of ATP11A or ATP11C conferred QKO cells with the ability to rapidly re-establish PtdSer asymmetry at 15 °C and 37 °C, while cells expressing ATP8A1 or ATP11B required a temperature of 37 °C to achieve this function, and a dynamin inhibitor blocked this process. These results revealed that mammalian cells are equipped with two independent mechanisms to re-establish its asymmetry: the first is a rapid process involving plasma membrane flippases, ATP11A and ATP11C, while the other is mediated by ATP8A1 and ATP11B, which require an endocytosis process.

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.

Inefficient development of syncytiotrophoblasts in the Atp11a-deficient mouse placenta

Plasma membranes are composed of a lipid bilayer in which phosphatidylserine (PtdSer) is confined to the inner leaflet by the action of flippase that translocates PtdSer from the outer to inner leaflets. Two P4-ATPases (ATP11A and ATP11C) work as flippase at plasma membranes. Here, we report that the mouse placenta expresses only ATP11A, and Atp11a-deficient mouse embryos die during embryogenesis due to inefficient formation of syncytiotrophoblasts in the placental labyrinth. The flippase-null mutation inactivates human choriocarcinoma BeWo cells to translocate PtdSer into the inner leaflet and undergo cell fusion. These findings highlight the importance of flippase to regulate the distribution of phospholipids for cell fusion, at least in trophoblast fusion.

The P4-ATPases ATP11A and ATP11C function as flippases at the plasma membrane to translocate phosphatidylserine from the outer to the inner leaflet. We herein demonstrated that Atp11a-deficient mouse embryos died at approximately E14.5 with thin-walled heart ventricles. However, the cardiomyocyte- or epiblast-specific Atp11a deletion did not affect mouse development or mortality. ATP11C may have compensated for the function of ATP11A in most of the cell types in the embryo. On the other hand, Atp11a, but not Atp11c, was expressed in the mouse placenta, and the Atp11a-null mutation caused poor development of the labyrinthine layer with an increased number of TUNEL-positive foci. Immunohistochemistry and electron microscopy revealed a disorganized labyrinthine layer with unfused trophoblasts in the Atp11a-null placenta. Human placenta-derived choriocarcinoma BeWo cells expressed the ATP11A and ATP11C genes. A lack of ATP11A and ATP11C eliminated the ability of BeWo cells to flip phosphatidylserine and fuse when treated with forskolin. These results indicate that flippases at the plasma membrane play an important role in the formation of syncytiotrophoblasts in placental development.

Apoptotic Features in Non-Apoptotic Processes

Apoptosis is the most thoroughly studied type of regulated cell death. Certain events, such as externalization of phosphatidylserine (PS) into the outer leaflet of plasma membrane, mitochondrial outer membrane permeabilization, caspase cascade activation, DNA fragmentation and blebbing, are widely considered to be hallmarks of apoptosis as well as being traditionally viewed as irreversible. This review shows that under particular circumstances these events can also participate in physiological processes not associated with initiation of apoptosis, such as cell differentiation, division, and motility, as well as non-apoptotic types of cell death. Moreover, these events may often be reversible. This review focuses on three processes: phosphatidylserine externalization, blebbing, and activation of apoptotic caspases. Mitochondrial outer membrane permeabilization and DNA fragmentation are not discussed.

The Fertilization Enigma: How Sperm and Egg Fuse

Fertilization is a multistep process that culminates in the fusion of sperm and egg, thus marking the beginning of a new organism in sexually reproducing species. Despite its importance for reproduction, the molecular mechanisms that regulate this singular event, particularly sperm-egg fusion, have remained mysterious for many decades. Here, we summarize our current molecular understanding of sperm-egg interaction, focusing mainly on mammalian fertilization. Given the fundamental importance of sperm-egg fusion yet the lack of knowledge of this process in vertebrates, we discuss hallmarks and emerging themes of cell fusion by drawing from well-studied examples such as viral entry, placenta formation, and muscle development. We conclude by identifying open questions and exciting avenues for future studies in gamete fusion. Expected final online publication date for the Annual Review of Cell and Developmental Biology, Volume 37 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Flagging fusion: Phosphatidylserine signaling in cell-cell fusion

Formations of myofibers, osteoclasts, syncytiotrophoblasts, and fertilized zygotes share a common step, cell–cell fusion. Recent years have brought about considerable progress in identifying some of the proteins involved in these and other cell-fusion processes. However, even for the best-characterized cell fusions, we still do not know the mechanisms that regulate the timing of cell-fusion events. Are they fully controlled by the expression of fusogenic proteins or do they also depend on some triggering signal that activates these proteins? The latter scenario would be analogous to the mechanisms that control the timing of exocytosis initiated by Ca²⁺ influx and virus-cell fusion initiated by low pH- or receptor interaction. Diverse cell fusions are accompanied by the nonapoptotic exposure of phosphatidylserine at the surface of fusing cells. Here we review data on the dependence of membrane remodeling in cell fusion on phosphatidylserine and phosphatidylserine-recognizing proteins and discuss the hypothesis that cell surface phosphatidylserine serves as a conserved “fuse me” signal regulating the time and place of cell-fusion processes.

Dual role of phosphatidylserine and its receptors in osteoclastogenesis

Fusion and apoptosis share a breakdown of the membrane phospholipids asymmetry, modes of which are largely unknown in osteoclastogenesis. Here, we investigated the externalization of phosphatidylserine (PS) and its receptors, and their biological functions in osteoclastogenesis. Strong immunoreactivities in vivo for the PS receptors TIM4, BAI1, and STAB2 were observed in the TRAP-positive multinucleated cells in the alveolar bone that was being remodeled around the developing dental follicles in rats. These receptors were significantly upregulated during M-CSF/RANKL-induced in vitro osteoclastogenesis using mouse bone marrow-derived cells. PS externalization in preosteoclasts was increased by the M-CSF/RANKL treatment. Multinucleation of preosteoclasts was markedly inhibited by antibodies against PS and its receptors. Among the investigated lipid transporter proteins, floppases (Abcb4, Abcc5, and Abcg1) were upregulated, whereas flippases (Atp11c and Atp8a1) downregulated during osteoclastogenesis. Preosteoclast fusion was markedly blocked by the ATPase inhibitor Na₃VO₄ and siRNAs against Abcc5 and Abcg1, revealing the importance of these lipid transporters in PS externalization. Further, the levels of Cd47 and Cd31, don't-eat-me signal inducers, were increased or sustained in the early phase of osteoclastogenesis, whereas those of AnnexinI and Mfg-e8, eat-me signals inducers, were increased in the late apoptotic phase. In addition, Z-VAD-FMK, a pan caspase inhibitor, had no effect on preosteoclast fusion in the early phase of osteoclastogenesis, whereas Abs against PS, TIM4, and BAI1 decreased osteoclast apoptosis during the late phase. These results suggest that PS externalization is essential for the whole process of osteoclastogenesis and share PS receptors and transporters in the early stage fusion and late stage apoptosis. Therefore, modulation of PS and its receptors could be a useful strategy to develop anti-bone resorptive agents.

TMEM16F phospholipid scramblase mediates trophoblast fusion and placental development

Cell-cell fusion or syncytialization is fundamental to the reproduction, development, and homeostasis of multicellular organisms. In addition to various cell type-specific fusogenic proteins, cell surface externalization of phosphatidylserine (PS), a universal eat-me signal in apoptotic cells, has been observed in different cell fusion events. Nevertheless, the molecular underpinnings of PS externalization and cellular mechanisms of PS-facilitated cell-cell fusion are unclear. Here, we report that TMEM16F, a Ca²⁺-activated phospholipid scramblase (CaPLSase), plays an essential role in placental trophoblast fusion by translocating PS to cell surface independent of apoptosis. The placentas from the TMEM16F knockout mice exhibit deficiency in trophoblast syncytialization and placental development, which lead to perinatal lethality. We thus identified a new biological function of TMEM16F CaPLSase in trophoblast fusion and placental development. Our findings provide insight into understanding cell-cell fusion mechanism of other cell types and on mitigating pregnancy complications such as miscarriage, intrauterine growth restriction, and preeclampsia.

Stabilin Receptors: Role as Phosphatidylserine Receptors

Phosphatidylserine is a membrane phospholipid that is localized to the inner leaflet of the plasma membrane. Phosphatidylserine externalization to the outer leaflet of the plasma membrane is an important signal for various physiological processes, including apoptosis, platelet activation, cell fusion, lymphocyte activation, and regenerative axonal fusion. Stabilin-1 and stabilin-2 are membrane receptors that recognize phosphatidylserine on the cell surface. Here, we discuss the functions of Stabilin-1 and stabilin-2 as phosphatidylserine receptors in apoptotic cell clearance (efferocytosis) and cell fusion, and their ligand-recognition and signaling pathways.

Epigenome-wide DNA methylation in placentas from preterm infants: association with maternal socioeconomic status

This study evaluated the hypothesis that prenatal maternal socioeconomic status (SES) adversity is associated with DNA methylation in the placenta. SES adversity was defined by the presence of, as well as a summative count of, four factors: less than college education, single marital status, food and nutritional service assistance, and public health insurance. Epigenome-wide DNA methylation was assessed using the Illumina EPIC array in 426 placentas from a sample of infants born < 28 weeks of gestation from the Extremely Low Gestational Age Newborn cohort. Associations between SES adversity and DNA methylation were assessed with robust linear regressions adjusted for covariates and controlled the false discovery rate at < 10%. We also examined whether such associations were sex specific. Indicators of SES adversity were associated with differential methylation at 33 CpG sites. Of the 33 identified CpG sites, 19 (57.6%) displayed increased methylation, and 14 (42.4%) displayed decreased methylation in association with at least one of the SES adversity factors. Sex differences were observed in DNA methylation associated with summative SES score; in which placentas derived from female pregnancies showed more robust differential CpG methylation than placentas from male pregnancies. Maternal SES adversity was associated with differential methylation of genes with key role in gene transcription and placental function, potentially altering immunity and stress response. Further investigation is needed to evaluate the role of epigenetic differences in mediating the association between maternal socioeconomic status during pregnancy and later life health outcomes in children.

Myoblast fusion confusion: the resolution begins

The fusion of muscle precursor cells is a required event for proper skeletal muscle development and regeneration. Numerous proteins have been implicated to function in myoblast fusion; however, the majority are expressed in diverse tissues and regulate numerous cellular processes. How myoblast fusion is triggered and coordinated in a muscle-specific manner has remained a mystery for decades. Through the discovery of two muscle-specific fusion proteins, Myomaker and Myomerger–Minion, we are now primed to make significant advances in our knowledge of myoblast fusion. This article reviews the latest findings regarding the biology of Myomaker and Minion–Myomerger, places these findings in the context of known pathways in mammalian myoblast fusion, and highlights areas that require further investigation. As our understanding of myoblast fusion matures so does our potential ability to manipulate cell fusion for therapeutic purposes.

Cell-surface phosphatidylserine regulates osteoclast precursor fusion

Bone-resorbing multinucleated osteoclasts that play a central role in the maintenance and repair of our bones are formed from bone marrow myeloid progenitor cells by a complex differentiation process that culminates in fusion of mononuclear osteoclast precursors. In this study, we uncoupled the cell fusion step from both pre-fusion stages of osteoclastogenic differentiation and the post-fusion expansion of the nascent fusion connections. We accumulated ready-to-fuse cells in the presence of the fusion inhibitor lysophosphatidylcholine and then removed the inhibitor to study synchronized cell fusion. We found that osteoclast fusion required the dendrocyte-expressed seven transmembrane protein (DC-STAMP)-dependent non-apoptotic exposure of phosphatidylserine at the surface of fusion-committed cells. Fusion also depended on extracellular annexins, phosphatidylserine-binding proteins, which, along with annexin-binding protein S100A4, regulated fusogenic activity of syncytin 1. Thus, in contrast to fusion processes mediated by a single protein, such as epithelial cell fusion in Caenorhabditis elegans, the cell fusion step in osteoclastogenesis is controlled by phosphatidylserine-regulated activity of several proteins.

Anoctamins/TMEM16 Proteins: Chloride Channels Flirting with Lipids and Extracellular Vesicles

Anoctamin (ANO)/TMEM16 proteins exhibit diverse functions in cells throughout the body and are implicated in several human diseases. Although the founding members ANO1 (TMEM16A) and ANO2 (TMEM16B) are Ca²⁺-activated Cl^- channels, most ANO paralogs are Ca²⁺-dependent phospholipid scramblases that serve as channels facilitating the movement (scrambling) of phospholipids between leaflets of the membrane bilayer. Phospholipid scrambling significantly alters the physical properties of the membrane and its landscape and has vast downstream signaling consequences. In particular, phosphatidylserine exposed on the external leaflet of the plasma membrane functions as a ligand for receptors vital for cell-cell communication. A major consequence of Ca²⁺-dependent scrambling is the release of extracellular vesicles that function as intercellular messengers by delivering signaling proteins and noncoding RNAs to alter target cell function. We discuss the physiological implications of Ca²⁺-dependent phospholipid scrambling, the extracellular vesicles associated with this activity, and the roles of ANOs in these processes.

Over-expression of stomatin causes syncytium formation in nonfusogenic JEG-3 choriocarcinoma placental cells

Placental trophoblast differentiation involves the continuous fusion of mononuclear cytotrophoblasts. However, except for syncytin, little is known about the detailed mechanisms underlying trophoblast fusion. A previous study indicated that lipid rafts play an important role in HTLV-1 syncytium formation. To identify proteins that may be involved in placental trophoblast differentiation, we examined stomatin, an important lipid-raft protein that localizes to detergent-resistant membrane domains. The syncytium and human chorionic gonadotropin (β-hCG; a marker of placental trophoblast differentiation) were visualized by immunofluorescence staining. We found that overexpression of stomatin in the nonfusogenic JEG-3 cell line caused syncytium formation and increased the fusion index of cells. Treating these cells with N(6) ,2'-O-dibutyryladenosine 3',5'-cyclic monophosphate further increased cell fusion by stomatin. β-hCG was found in a few JEG-3 cells overexpressing stomatin at 48 h, and its levels increased dramatically at 72 h along with the formation of the multinuclear syncytium. RNA interference was used to decrease stomatin expression in BeWo cells, a fusogenic human choriocarcinoma cell line. After knockdown for 72 h, stomatin levels decreased by almost 95%. The fusion indexes of control and stomatin-knockdown cells at 72 h were 9.4 and 6.5%, respectively. Our data indicated that stomatin could trigger syncytium formation and upregulate β-hCG for cell fusion in nonfusogenic JEG-3 cells. Downregulation of stomatin slightly inhibited the fusion index of fusogenic BeWo cells. Thus, these data suggested that stomatin plays an important role in trophoblast differentiation.

Stabilin-2 modulates the efficiency of myoblast fusion during myogenic differentiation and muscle regeneration

Myoblast fusion is essential for the formation of skeletal muscle myofibres. Studies have shown that phosphatidylserine is necessary for myoblast fusion, but the underlying mechanism is not known. Here we show that the phosphatidylserine receptor stabilin-2 acts as a membrane protein for myoblast fusion during myogenic differentiation and muscle regeneration. Stabilin-2 expression is induced during myogenic differentiation, and is regulated by calcineurin/NFAT signalling in myoblasts. Forced expression of stabilin-2 in myoblasts is associated with increased myotube formation, whereas deficiency of stabilin-2 results in the formation of small, thin myotubes. Stab2-deficient mice have myofibres with small cross-sectional area and few myonuclei and impaired muscle regeneration after injury. Importantly, myoblasts lacking stabilin-2 have reduced phosphatidylserine-dependent fusion. Collectively, our results show that stabilin-2 contributes to phosphatidylserine-dependent myoblast fusion and provide new insights into the molecular mechanism by which phosphatidylserine mediates myoblast fusion during muscle growth and regeneration.

Phosphatidylserine and its receptors are associated with cell-cell fusion. Here, the authors show the phosphatidylserine receptor stabilin-2 is expressed by muscle cells and plays a vital role in myoblast fusion and post-injury muscle regeneration in mice.

Pleiotropic actions of forskolin result in phosphatidylserine exposure in primary trophoblasts

Forskolin is an extract of the Coleus forskholii plant that is widely used in cell physiology to raise intracellular cAMP levels. In the field of trophoblast biology, forskolin is one of the primary treatments used to induce trophoblastic cellular fusion. The syncytiotrophoblast (ST) is a continuous multinucleated cell in the human placenta that separates maternal from fetal circulations and can only expand by fusion with its stem cell, the cytotrophoblast (CT). Functional investigation of any aspect of ST physiology requires in vitro differentiation of CT and de novo ST formation, thus selecting the most appropriate differentiation agent for the hypothesis being investigated is necessary as well as addressing potential off-target effects. Previous studies, using forskolin to induce fusion in trophoblastic cell lines, identified phosphatidylserine (PS) externalization to be essential for trophoblast fusion and showed that widespread PS externalization is present even after fusion has been achieved. PS is a membrane phospholipid that is primarily localized to the inner-membrane leaflet. Externalization of PS is a hallmark of early apoptosis and is involved in cellular fusion of myocytes and macrophages. We were interested to examine whether PS externalization was also involved in primary trophoblast fusion. We show widespread PS externalization occurs after 72 hours when fusion was stimulated with forskolin, but not when stimulated with the cell permeant cAMP analog Br-cAMP. Using a forskolin analog, 1,9-dideoxyforskolin, which stimulates membrane transporters but not adenylate cyclase, we found that widespread PS externalization required both increased intracellular cAMP levels and stimulation of membrane transporters. Treatment of primary trophoblasts with Br-cAMP alone did not result in widespread PS externalization despite high levels of cellular fusion. Thus, we concluded that widespread PS externalization is independent of trophoblast fusion and, importantly, provide evidence that the common differentiation agent forskolin has previously unappreciated pleiotropic effects on trophoblastic cells.

Fertilization induces a transient exposure of phosphatidylserine in mouse eggs

Phosphatidylserine (PS) is normally localized to the inner leaflet of the plasma membrane and the requirement of PS translocation to the outer leaflet in cellular processes other than apoptosis has been demonstrated recently. In this work we investigated the occurrence of PS mobilization in mouse eggs, which express flippase Atp8a1 and scramblases Plscr1 and 3, as determined by RT-PCR; these enzyme are responsible for PS distribution in cell membranes. We find a dramatic increase in binding of flouresceinated-Annexin-V, which specifically binds to PS, following fertilization or parthenogenetic activation induced by SrCl₂ treatment. This increase was not observed when eggs were first treated with BAPTA-AM, indicating that an increase in intracellular Ca²⁺ concentration was required for PS exposure. Fluorescence was observed over the entire egg surface with the exception of the regions overlying the meiotic spindle and sperm entry site. PS exposure was also observed in activated eggs obtained from CaMKIIγ null females, which are unable to exit metaphase II arrest despite displaying Ca²⁺ spikes. In contrast, PS exposure was not observed in TPEN-activated eggs, which exit metaphase II arrest in the absence of Ca²⁺ release. PS exposure was also observed when eggs were activated with ethanol but not with a Ca²⁺ ionophore, suggesting that the Ca²⁺ source and concentration are relevant for PS exposure. Last, treatment with cytochalasin D, which disrupts microfilaments, or jasplakinolide, which stabilizes microfilaments, prior to egg activation showed that PS externalization is an actin-dependent process. Thus, the Ca²⁺ rise during egg activation results in a transient exposure of PS in fertilized eggs that is not associated with apoptosis.

Increased resistance to apoptosis during differentiation and syncytialization of BeWo choriocarcinoma cells

Transition from mononuclear villous cytotrophoblast into multinuclear syncytiotrophoblast in the human placenta is accompanied by changes in apoptosis-related proteins and an apparent increased resistance to induced apoptosis. We investigated the specific nature and timing of changes in Bcl-2, Bax, p53, and caspases 3 and 8 in forskolin-treated BeWo choriocarcinoma cells, a model for villous cytotrophoblast differentiation. BeWo cells were treated with forskolin or vehicle alone for up to 72 h and evaluated at 24 h intervals for syncytialization and quantitative expression specific apoptosis-related proteins and mRNAs. Syncytialization was quantified using fluorescent staining of intercellular membranes and enumeration of the percentage of nuclei in multinucleate cells, and differential localization of apoptosis-related proteins to multinuclear or mononuclear cells was determined by quantitative immunofluorescence. Forskolin treatment for up to 72 h resulted in 80% syncytialization, increased expression of Bcl-2 protein (P < 0.01) and mRNA (P < 0.05), and significantly decreased expression of protein and mRNA for Bax, p53, and caspases 3 and 8. Syncytialized cells expressed higher levels of Bcl-2 protein concurrent with increased resistance to cisplatin-induced apoptosis. Thus, syncytialization of BeWo cells was accompanied by altered transcription of apoptotic-related proteins characteristic of increased apoptosis resistance secondary to increased expression of the anti-apoptotic protein Bcl-2 and diminish expression of pro-apoptotic proteins.

Involvement of membrane GRP78 in trophoblastic cell fusion

BACKGROUND

Glucose-regulated protein 78 (GRP78) is highly expressed in first trimester cytrophoblastic cells (CTBs), especially in syncytiotrophoblast (STB). However, the role of GRP78 in these cells has never been investigated.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we have examined the role of GRP78 in trophoblast fusion using the Bewo choriocarcinoma cell line as a model of cytotrophoblast fusion. Down regulation of GRP78 by siRNA or chemical inhibitors and use of antibodies against GRP78 in culture medium significantly decreased forskolin-induced fusion capacity of Bewo cells suggesting the involvement of membrane GRP78 in trophoblast fusion. GRP78 expression was also studied in preeclamptic (PE) CTBs which are known to have lower fusion capacity compared to control CTBs. Interestingly, despite the increase of GRP78 mRNA in PE CTBs, membrane GRP78 is significantly decreased in PE CTBs compared to control CTBs, suggesting that relocation of GRP78 from the endoplasmic reticulum to cell surface is probably altered in PE CTBs.

CONCLUSIONS

Our results imply that membrane GRP78 could play an important role in syncytialisation. They also suggest that deregulation of GRP78 expression or relocation at cell surface might be involved in pregnancy complication associated with defective syncytialisation, such as preeclampsia.

The contribution of apoptosis-inducing factor (AIF) to villous trophoblast differentiation

Apoptosis is postulated to be a delayed but important part of the differentiation of placental villous cytotrophoblasts (CT) into functional syncytiotrophoblast (ST). This hypothesis is based on the observation that the externalization of phosphatidylserine and the activation of caspase 8 are required for trophoblast differentiation. In contradiction to this hypothesis we have previously found that differentiation occurs in the presence of both broad spectrum and caspase 8 specific inhibitors. Apoptosis-inducing factor (AIF) is a mitochondria-associated protein which is known to translocate to the nucleus and induce caspase-independent nuclear condensation, phosphatidylserine externalization and cell death. Thus AIF nuclear translocation may result in the apoptotic-like features associated with trophoblast differentiation and may be an obligatory event for differentiation to proceed. AIF translocation was assessed in isolated primary trophoblasts by optical section microscopy of antibody stained cells. We found AIF to be strongly expressed in the villous trophoblast and that small amounts of AIF were localized to the nucleus of the cells. Significantly, inhibitors of AIF translocation (calpeptin and zFA-fmk) blocked translocation but not differentiation of the cells. We conclude that AIF translocation is not involved in CT differentiation in isolated cell culture.

The P2X7 receptor and Pannexin-1 are both required for the promotion of multinucleated macrophages by the inflammatory cytokine GM-CSF

The P2X(7) receptor (P2X(7)R), an ATP-gated ion channel, has been implicated in the process of cell-to-cell fusion into multinucleated macrophages (MA), but its contribution to MA fusion driven by physiological/pathological stimuli is not clearly established. Based on several lines of evidence, we demonstrate that P2X(7)R is critical for the induction of multinucleated MA by the inflammatory cytokine GM-CSF: 1) pharmacological inhibition of P2X(7)R with oxidized ATP (oATP), KN-62, and the selective antagonist A740003 abrogated GM-CSF action on rat alveolar MA and murine peritoneal MA; 2) a murine J774 P2X(7) low MA clone, selected for defective P2X(7)R function, was unresponsive; 3) MA from mice lacking P2X(7)R failed to respond to GM-CSF, in contrast to wild-type. GM-CSF also stimulated ATP-induced membrane permeabilization in J774 P2X(7) high MA and rat alveolar MA, an effect absent in the P2X(7) low MA clone and inhibited by the P2X(7) blockers oATP and KN-62. Notably, the stimulatory effects of GM-CSF on pore formation and MA fusion were both inhibited by blocking functional Pannexin-1 (Panx-1), and GM-CSF failed to stimulate MA fusion in cells from Panx-1 knockout mice. We provide further evidence that extracellular ATP release from peritoneal MA is dependent on P2X(7) but not on Panx-1 expression and that its metabolism to adenosine mediates P2X(7)-dependent MA fusion. These data demonstrate that both P2X(7) and Panx-1 are required for GM-CSF promotion of MA fusion but likely act independently through different signaling pathway(s).

Placental and trophoblastic in vitro models to study preventive and therapeutic agents for preeclampsia

In the field of preeclampsia, enormous efforts are ongoing to identify biomarkers predicting the syndrome already in the first trimester of pregnancy. At the same time, there is the need for in vitro models to test such biomarkers prior to their use in clinical trials. In addition, in vitro models may accelerate the development and evaluation of the benefit of any putative therapeutics. Therefore, in vitro systems have been established to evaluate the release of biomarkers and measure the effect of putative therapeutics using placental villous explants as well as the choriocarcinoma cell line BeWo. For explants, a cryogenic method to freeze, transport and thaw villous explants was developed to use such tissues for a multi-site tissue culture evaluation. Here we focus on three out of many in vitro models that have been established for human placental trophoblast. (1) Choriocarcinoma cell lines such as BeWo, Jeg-3 and Jar cells (2) isolated primary trophoblast cells, and (2) villous explants from normal placentas delivered at term. Cell lines were used to assess the effect of differentiation and fusion on the expression and release of a preeclampsia marker (placental protein 13; PP13) and beta-hCG. Moreover, cell lines were used to study the effect of putative preeclampsia therapeutics such as vitamins C and E, heparin and aspirin on marker release and viability. Cryopreservation of villous explants enabled shipment to a remote laboratory and testing of parameters in different countries using explants from one and the same placenta. Recently published data make it tempting to speculate that the choriocarcinoma cell line BeWo as well as fresh and cryogenically stored placental villous explants may well serve as in vitro models to study preventive and therapeutic agents in the field of preeclampsia.

Trophoblast fusion

The villous trophoblast of the human placenta is the epithelial cover of the fetal chorionic villi floating in maternal blood. This epithelial cover is organized in two distinct layers, the multinucleated syncytiotrophoblast directly facing maternal blood and a second layer of mononucleated cytotrophoblasts. During pregnancy single cytotrophoblasts continuously fuse with the overlying syncytiotrophoblast to preserve this end-differentiated layer until delivery. Syncytial fusion continuously supplies the syncytiotrophoblast with compounds of fusing cytotrophoblasts such as proteins, nucleic acids and lipids as well as organelles. At the same time the input of cytotrophoblastic components is counterbalanced by a continuous release of apoptotic material from the syncytiotrophoblast into maternal blood. Fusion is an essential step in maintaining the syncytiotrophoblast. Trophoblast fusion was shown to be dependant on and regulated by multiple factors such as fusion proteins, proteases and cytoskeletal proteins as well as cytokines, hormones and transcription factors. In this chapter we focus on factors that may be involved in the fusion process of trophoblast directly or that may prepare the cytotrophoblast to fuse.

Caspase activation is not required for villous cytotrophoblast fusion into syncytiotrophoblasts

The villous trophoblast renews itself by fusion of individual stem cells (cytotrophoblasts, CT) with a functional syncytium (syncytiotrophoblast, ST). The literature indicates that fusion occurs with limited activation (proteolytic cleavage) of caspase-8 in CT and is inhibited either by blocking caspase-8 synthesis or inhibiting activation with a caspase-8-specific inhibitor, zIETD. We challenge part of this evidence: inhibition of differentiation with caspase-8 inhibitors. Br-cAMP-stimulated differentiation of isolated CT into multinucleated syncytia in culture is not blocked with three different low molecular weight inhibitors of caspase-8: broad caspase inhibitors zVAD-fmk and qVD-OPh and the caspase-8-specific inhibitor zIETD-fmk. Syncytialization was determined by desmoplakin staining of intracellular boundaries surrounding >2 nuclei and by diffusion within fused cells of long-lived cytoplasmic staining from half of original CT to the unstained half. Differentiation of isolated CT into hCGβ-secreting syncytiotrophoblast was also not blocked by the inhibitors nor was upregulation of hCGβ secretion blocked in ST-stripped and regenerated 5 day explant cultures. The ratio of CT to ST nuclei present was also not changed in explant cultures by caspase inhibitors. The effectiveness of caspase inhibitors was demonstrated by their ability to completely block TNFα-induced apoptosis. We conclude that activation of caspases in general, and caspase-8 in particular, is not required for villous CT differentiation into ST. However, another role of intact caspase-8 (proform) in CT differentiation remains possible.

Downregulation of caveolin-1 enhances fusion of human BeWo choriocarcinoma cells

Background

Fusion of placental villous cytotrophoblasts with the overlying syncytiotrophoblast is essential for the maintenance of successful pregnancy, and disturbances in this process have been implicated in pathological conditions such as pre-eclampsia and intra-uterine growth retardation. Caveolin-1 has been shown to be expressed in human villous cytotrophoblast and to be downregulated during fusion into syncytiotrophoblast but it is unclear whether it plays a role in this process.

Methodology/Principal Findings

We used RNA interference to determine whether caveolin-1 plays a role in differentiation and fusion in the BeWo choriocarcinoma cell line, a model of villous cytotrophoblast fusion. Assessment of cell fusion by desmosomal protein immunostaining revealed that cells transfected with caveolin-1 siRNA showed significantly enhanced fusion in response to treatment with dibutyryl cyclic AMP compared with cells transfected with a non-silencing control. Furthermore, caveolin-1 knockdown alone was sufficient to promote spontaneous fusion. In addition, biochemical differentiation, assessed by expression of placental alkaline phosphatase, was upregulated in caveolin-1 siRNA-transfected cells, with or without dbcAMP treatment. Assessment of Akt phosphorylation showed that caveolin-1 knockdown resulted in a significant reduction in phosphorylation at Thr³⁰⁸.

Conclusions/Significance

Taken together, these results suggest that caveolin-1 regulates BeWo cell differentiation and fusion, possibly through a mechanism involving modulation of Akt activity.

The paradox of caspase 8 in human villous trophoblast fusion

Differentiation and subsequent fusion of villous cytotrophoblasts with the overlying syncytiotrophoblast is an essential process for growth and maintenance of the villous trophoblast layer in the human placenta. The understanding of intrinsic mechanisms behind this process is in its infancy, while the list of suggested factors, involved in intercellular fusion of trophoblasts, rapidly increased in the recent past and promises progress on this issue. The early stages of the apoptosis cascade, in particular caspase 8, was suggested to trigger differentiation of cytotrophoblasts, priming them for upcoming fusion. This may sound paradoxical, especially for those who still associate caspase activity with apoptosis only. Here, we summarize data on caspase 8 in the villous trophoblast layer, with a specific focus on localization of pro- and active forms, the sites of its activation and deactivation, and its role and regulation during fusion. Moreover, we revisit the knowledge on fusogens in the villous trophoblast, compare in vitro models for trophoblast fusion and discuss methods to quantify fusion.

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.

Intrauterine growth restriction, human placental development and trophoblast cell death

Intrauterine growth restriction (IUGR) is a failure to achieve the growth potential of a fetus that is promised by the genetic constitution and environmental influences endogenous to the pregnancy. Optimal placental development and the ability of the placenta to compensate for stimulus-induced injury are central in promotion of normal fetal growth. In this review, we will overview placental development with a focus on how villous structure relates to function. We will also describe the differentiation and turnover of villous trophoblast while highlighting selected features of microscopic placental injury. Histopathological studies of the placenta in IUGR indicate that abnormalities of the maternal spiral arterioles, dysregulated villous vasculogenesis, and abundant fibrin deposition are characteristic of the injuries associated with this condition. We identify selected insults, including oxidative stress and complement activation, and key pathways that regulate apoptosis in villous trophoblast, including increased p53 activity, altered translation of AKT and mTOR proteins, and the stress response of the endoplasmic reticulum. We surmise that trophoblast dysregulation at a subcellular level and loss of functional mass of villous trophoblast via cell death pathways are key contributors to the suboptimal placental performance that yields IUGR. We predict that a better understanding of placental dysfunction in IUGR will lead to targeted therapeutic options for this important clinical condition.

Disseminated intravascular coagulation in obstetric disorders and its acute haematological management

As activation of the coagulation pathway is a physiological response to injury, the development of disseminated intravascular coagulation (DIC) is a warning signal to the clinician that the primary pathological disease state is decompensating. In pregnancy, DIC can occur in several settings, which include emergencies such as placental abruption and amniotic fluid embolism as well as complications such as pre-eclampsia. Whilst the acuteness of the event and the proportionality in the coagulant and fibrinolytic responses may vary between these different conditions, a common theme for pregnancy-associated DIC is the pivotal role played by the placenta. Removal of the placenta is the linchpin to treatment in most cases but appropriate blood product support is also key to management. This is necessary because DIC itself can have pathological consequences that translate clinically into a worse prognosis for affected patients. This article will describe how pregnancy-associated DIC can be diagnosed promptly and how treatment should be managed strategically. It also discusses the latest developments in our understanding of haemostatic mechanisms within the placenta and how these may have relevance to new diagnostic approaches as well as novel therapeutic modalities.

Evidence for transcriptional activity in the syncytiotrophoblast of the human placenta

The aim was to test for evidence of transcriptional activity within the nuclei of the syncytiotrophoblast of the human placenta. The syncytiotrophoblast forms the epithelial covering of the villous tree, and is a multinucleated, terminally-differentiated syncytium generated through fusion of the underlying progenitor cytotrophoblast cells. Its nuclei are heterogeneous with respect to chromatin condensation, and previous functional studies of 3H-uridine uptake in vitro have indicated that they are transcriptionally inactive. This observation is surprising given the key roles this tissue plays in active transport, hormone synthesis and metabolic regulation, and has widespread implications for trophoblast physiology and pathophysiology. We used three different approaches to look for evidence of transcriptional activity. First, immunofluorescence staining was performed on paraffin-embedded early pregnancy and term placental villi, using an antibody directed specifically against the actively transcribing form of RNA polymerase II. Second, a nucleoside incorporation assay was applied to placental villi maintained in short-term culture, with and without the transcription blocker α-amanitin. Third, histone modifications associated with active chromatin were identified by immunohistochemistry and immunofluorescence. Each of these methods showed transcription to be occurring in a proportion of syncytiotrophoblast nuclei, with qualitative evidence for transcription being more abundant in the first trimester than at term. These findings correlated with electron microscopical observations of prominent nucleoli within the nuclei, particularly during early pregnancy, signifying transcription of ribosomal RNA. Contrary to previous findings, these results confirm that a proportion of syncytiotrophoblast nuclei actively produce mRNA transcripts.

Placenta trophoblast fusion

It has been known for more than 150 years that syncytial fusion is a normal feature in biological systems. In humans there are two larger syncytial tissues: skeletal muscles fibers and placental syncytiotrophoblast. Other fusion events take place as well from fertilization of the oocyte to infection of human cells by enveloped viruses (however, the latter does not necessarily lead to syncytium formation).Although knowledge of the fusion process is incomplete, it is clear that membranes do not fuse easily; specific proteins and other factors are required and are selectively activated. In this chapter, we describe the classic proteins, such as the syncytins, assumed to be involved in the fusion process. We also describe other factors that may play roles in the fusion process or in the preparation of the cells to fuse, such as charged phospholipids, divalent cations, and intracellular proteases. Finally, we speculate on why trophoblast cells fuse in vitro and deal with in vitro models of trophoblast fusion and how their fusion rates can be quantified.

Fusiogenic endogenous-retroviral syncytin-1 exerts anti-apoptotic functions in staurosporine-challenged CHO cells

Fusiogenic glycoprotein syncytin-1, expressed in human placenta, is a promising candidate for acquiring a basic knowledge of placental syncytialization. However, its cellular mode of action is unidentified. We investigated whether syncytin-1 may exert influence on apoptotic processes. Therefore, we incubated CHO cells after stable transfection with syncytin-1 (CHO-52) in the presence or absence of staurosporine (STS), a kinase inhibitor well characterized to induce apoptosis. When testing the phenotype of CHO-52 cells, we could demonstrate that the induction of apoptosis by STS was delayed over a period of up to 24 h. Furthermore, the cell death rate was decreased by approx 75% following transfection of syncytin-1 in CHO-52 compared to mock-treated cells. In detail, after 18h of incubation with 500 nM STS, 64 +/- 2% of CHO-52 cells were viable compared to 16 +/- 1% of CHO-mocks, after 24 h 43 +/- 3% vs 5 +/- 2%, respectively. CHO-52 cells exhibited a lower expression of active caspase 3 and anti-apoptotic Bcl-2 was found to be increased in CHO-52 cells at baseline and following STS treatment. Our study provides first evidence that syncytin-1 serves anti-apoptotic function under certain conditions. A lessened activation of caspase 3 and an increased expression of Bcl-2 are possible mechanisms.

The Xenobiotic Transporter ABCG2 Plays a Novel Role in Differentiation of Trophoblast-like BeWo Cells

Trophoblast cells undergo loss of plasma membrane lipid asymmetry during cell fusion without further progression to terminal phases of apoptosis. The nature of the anti-apoptotic mechanisms providing cell survival during this process is unknown. Using a BeWo cell model, we explored the role of the xenobiotic/lipid transporter ABCG2 in promoting cell survival during forskolin-induced differentiation. Suppression of ABCG2 expression by siRNA led to a marked increase in phosphatidylserine externalisation followed by accumulation of ceramides and increased apoptosis. Expression of markers of syncytial formation (beta-hCG and HERV-W) was decreased by ABCG2 silencing, although fusion was unaffected. These findings suggest that ABCG2 protects cells during the period of transient membrane instability associated with cell differentiation and fusion, highlighting a novel, previously unrecognised role of ABCG2 as a survival factor during the formation of the placental syncytium.

Involvement of the purinergic P2X7 receptor in the formation of multinucleated giant cells

Multinucleated giant cells (MGC), a hallmark of chronic inflammatory reactions, remain an enigma of cell biology. There is evidence implicating the purinergic P2X7 receptor in the fusion process leading to MGC. To investigate this, we used HEK 293 cells stably transfected with either 1) the full-length rat P2X7 receptor (P2X7 cells), 2) a rat P2X7 receptor lacking the C-terminal domain (P2X7TC), or 3) a mock vector, and rat alveolar macrophages (MA) expressing the native receptor. P2X7 cells cultured in serum-free medium formed increased numbers of MGC and displayed a higher fusion index compared with mock transfectants. Stimulation of P2X7 pore-forming activity in P2X7 cells by polymyxin B (PMB) further increased significantly the formation of MGC. Conversely, blockers of P2X-receptors including oxidized ATP, brilliant blue G, and pyridoxal phosphate-6-azophenyl-2'-4'-disulfonic acid inhibited significantly MGC formation in both unstimulated and PMB-stimulated P2X7-transfected cells. In contrast, cells transfected with the truncated P2X7TC were devoid of pore-forming activity, did not respond to PMB stimulation, and failed to form enhanced numbers of MGC, thus behaving as mock transfectants. As found for P2X7-transfected cells, PMB also potentiated dose-dependently the formation of multinucleated MA by rat alveolar MA. Pretreatment with oxidized ATP abrogated the PMB stimulatory effects. Together, these data demonstrate unequivocally the participation of P2X7 receptor in the process of MGC formation. Our study also provides evidence suggesting that stimulation of the P2X7 receptor pathway in MA may mediate increased formation of MGC during chronic inflammatory reactions.

Caspase-14: a new player in cytotrophoblast differentiation

The human placenta is responsible for the exchange of nutrients, gas and wastes through the trophoblast maternal-fetal barrier, which is formed by the fusion of villous cytotrophoblasts to form the continuous multinucleated syncytiotrophoblast separating the maternal and fetal circulations. Caspase-14 is a seemingly non-apoptotic caspase involved in keratinocyte differentiation and cornification. It is proposed that caspase-14 has a conserved role in cellular differentiation and a role in differentiation and fusion in the trophoblast. The human choriocarcinoma BeWo cell line was treated with staurosporine and forskolin to induce apoptosis and differentiation respectively. Staurosporine initiated apoptosis within 3 h of treatment, while apoptosis was completed following 6 h treatment. Caspase-14 gene and protein expression was unchanged throughout this process. During BeWo differentiation, caspase-14 mRNA was elevated after 48 h forskolin treatment, while its protein was increased after 24 h. Therefore, caspase-14 is up-regulated during trophoblast differentiation, as represented by the BeWo cell line. Moreover, caspase-14 may interact with other signalling molecules to facilitate differentiation. This new data confirms the potential for the BeWo cell line in the functional dissection of this unusual caspase and its prospective role in trophoblast differentiation.

Proteomic analysis of hypoxia-induced responses in the syncytialization of human placental cell line BeWo

Syncytiotrophoblast formation is affected by a number of pathological conditions and suppressed syncytiotrophoblast formation due to hypoxia may play a role in the pathogenesis of preeclampsia. However, the molecular basis of hypoxia-inhibited trophoblast syncytialization is poorly understood. To determine the effect of hypoxia on trophoblast syncytialization, a proteomic analysis was performed in the human cytotrophoblast cell line BeWo using two-dimensional electrophoresis and MALDI-TOF-TOF-MS. Hypoxia induced marked inhibition of BeWo cell fusion and differentiation. The proteomic profiling was established under hypoxia in BeWo cell syncytialization. The results showed that twenty proteins were significantly up-or down-regulated under hypoxia, compared with cells under normoxia. In response to hypoxia, three antioxidants, peroxiredoxin 1, peroxiredoxin 2 and 1-Cys peroxiredoxin, were down-regulated, two proteins involved in glycolysis pathway (malate dehydrogenase and enolase) were up-regulated. The expression of two members of the annexin family (annexin A2 and annexin A5) increased. We also found a decreased expression of 14-3-3 tau protein in hypoxia treated cells. Proteins implied in protein degradation and folding were also identified. The expression of two cytoskeleton components (keratin 1 and beta-actin) was found to be down-regulated. In addition, galectin-3 was up-regulated. These proteins have been implicated in regulating cellular oxidative stress, glycolysis, signal transduction, protein folding and degradation, cell mobility and cytoskeletal structure formation. Western blot analysis revealed that the levels of peroxiredoxin 1 and 14-3-3 tau decreased, whereas the levels of annexin A5 and annexin A2 increased in BeWo cells under hypoxia. These findings provided new insights into the molecular mechanisms in mediating cellular response to hypoxia in trophoblast syncytialization.

Arginine residues are important in determining the binding of human monoclonal antiphospholipid antibodies to clinically relevant antigens

In the antiphospholipid syndrome (APS), antiphospholipid Abs (aPL) bind to anionic phospholipids (PL) and various associated proteins, especially beta(2)-glycoprotein I (beta2GPI) and prothrombin. In the present study, we show that altering specific Arg residues in the H chain of a human pathogenic beta2GPI-dependent aPL, IS4, has major effects on its ability to bind these clinically important Ags. We expressed whole human IgG in vitro by stable transfection of Chinese hamster ovary cells with expression plasmids containing different V(H) and V(L) sequences. V(H) sequences were derived from IS4 by altering the number of Arg residues in CDR3. V(L) sequences were those of IS4, B3 (anti-nucleosome Ab), and UK4 (beta2GPI-independent aPL). Binding of the expressed H/L chain combinations to a range of anionic, neutral, and zwitterionic PL, as well as prothrombin, beta2GPI, dsDNA, and chicken OVA, was determined by ELISA. Of four Arg residues in IS4VH CDR3 substituted to Ser, two at positions 100 and 100g, reduced binding to all Ags, while two at positions 96 and 97 reduced binding to beta2GPI but increased or decreased binding to different PL. Eleven of 14 H/L chain combinations displayed weak binding to OVA with Arg to Ser replacements of all four Arg residues enhancing binding to this Ag. Only one H/L chain combination bound neutral PL and none bound dsDNA; hence, these effects are particularly relevant to Ags important in antiphospholipid syndrome. We hypothesize that these four Arg residues have developed as a result of somatic mutations driven by an Ag containing both PL and beta2GPI.

The role of human endogenous retroviruses in trophoblast differentiation and placental development

A major portion of the human genome appears to be of retroviral origin. These endogenous retroviral elements are expressed in a variety of normal tissues and during disease states, such as autoimmune and malignant conditions. Recently, potential roles have been described for endogenous retroviral envelope proteins in normal differentiation of human villous cytotrophoblast into syncytiotrophoblast. This article provides a brief critical review of the current state of knowledge concerning the expression of the env regions of three endogenous retroviral elements: ERV-3, HERV-W, and HERV-FRD. A testable model of villous cytotrophoblast differentiation is constructed, in which a complementary expression of endogenous retroviral envelope proteins initiates hCG production, decreased cell proliferation, and intercellular fusion.