Endogenous retroviral syncytin: compilation of experimental research on syncytin and its possible role in normal and disturbed human placentogenesis

Abstract

Placental syncytin was first described in the year 2000 as a fusogenic glycoprotein originally derived from a human endogenous retroviral envelope gene. Although the presence of stable integrated retroviral elements within the human genome has been known for many years, their biological significance is still obscure and has usually been designated as irrelevant or even harmful. Syncytin, however, demonstrates tissue-specific expression and distinctive receptor interaction during trophoblast cell differentiation and syncytium formation. These findings indicate an involvement of syncytin in the development of the human placenta. Disturbances in placental architecture leading to severe placental dysfunction, such as pre-eclampsia, may therefore be discussed as a consequence of an altered syncytin system. We evaluate the hypothesis that syncytin is essential for human placenta formation and may also have played an important role in human placental evolution.

ASCT2, human endogenous retrovirus, placenta, syncytin, syncytiotrophoblast

Introduction

The human placenta is essential not only for intrauterine development but may be involved in programming health in later life. This organ is destined to perish after birth but is an exciting tool for studying fundamental developmental processes. This article focuses on experimental studies on expression and effects of the recently discovered placental glycoprotein syncytin and its presumed physiological role during human placenta formation.

The syncytiotrophoblast forms in the boundary layer between the fetal and maternal compartment in the placenta and is of vital importance for the nurture and protection of the growing fetus. In human pregnancy, fetal cytotrophoblast cells form the multinucleated syncytiotrophoblast layer by cell–cell interaction, differentiation and fusion (Figure 1). After its discovery in the year 2000, syncytin, a captive retroviral protein in placental cells, was characterized as a fusogenic glycoprotein, mediating trophoblast cell fusion processes and syncytia formation (Blond et al., 2000; Mi et al., 2000).

Retroviral integrants have colonized the human germ line from our earliest development and are inherited as endogenous retroviral particles. A great diversity of endogenous retroviral elements is found in the genomes of eukaryotes, where the different types are, to some extent, species-specific (Harris, 1998). About 8% of the genetic material in the human genome is originally retrovirus-derived and is found widely distributed on the chromosomes (Gifford and Tristem, 2003). The integrated retroviral sequences are mostly transcriptionally inactive and consist of highly repetitive sequences, but certain integrants are still capable of expression and replication. Some retroviral particles have already been investigated in the context of autoimmune or neoplastic diseases, with ambiguous results (Boller et al., 1997; Herbst et al., 1999; Knerr et al., 1999; Depil et al., 2002; Christensen et al., 2003; Wang-Johanning et al., 2003).

The human endogenous retrovirus (HERV)-derived syncytin may have played a role in the evolution of the human placenta due to its niche function in human placentogenesis, albeit in a highly specific manner, as the placentas in different mammalian species exhibit structural discrepancies (Stoye and Coffn, 2000).

Placental HERV and syncytin

In 1999, a newly discovered human endogenous retrovirus family, the HERV-W family, was described at a molecular and phylogenetic level (Blond et al., 1999). Due to its characteristics at the primer binding site, the use of a tRNA^Trp, the name HERV-W was approved (W being the one-letter symbol for the amino acid tryptophan). HERV-W is not detectable as a complete provirus in the human genome, but its envelope is expressed in the placenta and in trophoblast-derived cell lines. Thereafter, the envelope gene of HERV-W was further characterized as being encoded on chromosome 7 (map position 7q21-7q22, OMIM 604659) and expressed as two major transcripts with a length of 4 and 8 kb (Blond et al., 2000; Mi et al., 2000). The mature envelope protein consists of 518 amino acids and is presumed to be a membrane protein (Blond et al., 2000; Mi et al., 2000). Due to its interaction with a specific receptor, known to function as a retrovirus receptor and as an amino acid transporter, and due to the stimulation of cell–cell fusion processes, the protein was designated as syncytin.

The receptor, or at least the first known functional syncytin binding site, is a sodium-dependent neutral amino acid transporter, which transports alanine, serine and cysteine, known by the acronym ASCT2 (Sommerfelt et al., 1990; Rasko et al., 1999; Kudo and Boyd, 2002b). It has been shown, using transfected cells overexpressing the protein, that syncytin is a highly fusogenic membrane glycoprotein with syncytium-inducing properties (Mi et al., 2000). Pronounced syncytin expression is followed by further cell differentiation and generation of the syncytium, the formation of gap junctions and an increase in β-hCG secretion (Frendo et al., 2003b). The effects of syncytin can be blocked in vitro by antibodies directed against syncytin or by the use of syncytin antisense strategies (Mi et al., 2000; Frendo et al., 2003b). In addition to the in vitro studies it could be demonstrated recently that the syncytin locus is strongly preserved in a large cohort of individuals, including the long terminal repeat (LTR) elements involved in regulation of gene transcription (Mallet et al., 2004). Apart from these findings, syncytin orthologous loci have been identified in the genomes of great apes (Mallet et al., 2004) and, beyond that, expression of syncytin has been described in the rhesus monkey endometrium where it may play a role in decidualization or receptivity (Okulitz and Ace, 2003).

Receptor affinities

The receptor attributed to syncytin is the well-known amino acid transporter ASCT2. This is a sodium-dependent amino acid transporter for neutral amino acids such as alanine, serine and cysteine, product of the SLC1A5 gene on chromosome 19 (gene map locus 19q13.3). It is also named amino acid transporter B0 or type-D retrovirus receptor (RDR) and was identified using different cDNA library approaches (Sommerfelt et al., 1990; Kekuda et al., 1996; Rasko et al., 1999; Tailor et al., 1999). Various amino acid transport systems exist in the human placenta, the products of ≥18 genes, and their functional activities depend on ion gradients, cellular localization and also on the developmental stage of the fetal–placental unit (Kudo and Boyd, 2002b). The expression of ASCT2 has been demonstrated predominantly at the basal membrane of the human syncytiotrophoblast (Furesz et al., 1993; Moe, 1995; Kudo and Boyd, 2002a; Nelson et al., 2003) and the basal membrane of the syncytiotrophoblast may therefore function as the interaction site between syncytin and its receptor. A modification of the dual functioning ASCT2 (neutral amino acid transporter—retrovirus receptor) is possible in vitro via the translational generation of truncated isoforms or via the glycosylation and deglycosylation processes of its active sites (Marin et al., 2003). During cell differentiation, as mimicked in forskolin-stimulated BeWo cells, the levels of amino acid transporter ASCT2 mRNA decrease together with the receptor-mediated influx of amino acids in vitro, whereas the mRNA of syncytin increases (Kudo and Boyd, 2002a). The observation that, during cell syncytialization, syncytin and ASCT2 correlate reversely could be partially explained by the phenomenon that retroviruses may cause a down-regulation of their receptors (Tailor et al., 2001). This can be seen after the infection of cells by wild-type viruses of the type-D interference group which impairs neutral amino acid transport (Rasko et al., 1999).

It has been previously reported that an alteration of particular placental amino acid transporters is associated with intrauterine growth restriction (IUGR) and distinctive anthropometric features in the fetus, especially for system A, which transports neutral amino acids (Glazier et al., 1997; Norberg et al., 1998; Harrington et al., 1999). System A, in contrast to the ASCT2 transporter, is down-regulated in the case of oxygen deficiency and up-regulated in the case of excess oxygen (Nelson et al., 2003), which may contribute to fetal IUGR respectively. Other transport systems, e.g. for arginine, have been mentioned in the context of pre-eclamptic placentas (Speake et al., 2003). Whether or not a disturbed interaction between syncytin and ASCT2 contributes to IUGR has not yet been investigated.

Placental development, regulators of differentiation and syncytium formation

Early development of the human conceptus requires a succession of predetermined processes, of which proliferation and differentiation of trophoblast cells are among the most basic events. During placentation, cytotrophoblast cells give rise either to the floating villi and form the syncytial layer, or they contribute to cell columns at anchoring villi that adhere to the uterine wall. At the latter site, extravillous trophoblast cells invade the maternal uterine wall, which can be understood as an alternative pathway of cellular differentiation (Janatpour et al., 1999). Undifferentiated cytotrophoblast stem cells persist throughout pregnancy, can differentiate into mature trophoblast cells and give rise to the syncytiotrophoblast layer if required.

Only a few cell types in humans differentiate naturally into multinucleated syncytia via cell fusion. The syncytiotrophoblast is such a syncytium, and it maintains a polarity with an apical microvillous membrane adjoining the maternal bloodstream and a basal membrane adjacent to cytrotrophoblast cells. These structural features are important for the multiple tasks of the syncytiotrophoblast such as gas exchange and transport activities, in addition to metabolic and endocrine functions. The cascade of cytotrophoblast fusion regulators is still rather mysterious. A variety of key molecules and some distinct pathways regulating placental differentiation are known from knockout models, involving many transcription factors, cytokines, matrix factors and other regulators, but differences between the species must also be considered (Copp, 1995; Rinkenberger et al., 1997; Morrish et al., 1998; Janatpour et al., 1999; Cross, 2000; Morrish et al., 2001; Rossant and Cross, 2001; Cross et al., 2002). Local oxygen tension as well as regulators of mitotic events and of controlled cell death also play an important role in the differentiation and fusion of trophoblast cells and the preservation of the syncytiotrophoblast (Genbacev et al., 1997; Huppertz et al., 1998; Ratts et al., 2000; Yusuf et al., 2002; Black et al., 2004).

Trophoblast differentiation and fusion are orchestrated by a variety of transcription factors such as the human Mash-2 homologue Hash-2 and other basic helix-loop-helix proteins and the Id family (inhibitor of DNA binding proteins) and GCMa (GCM1, glial cell missing gene), as well as by ligands of the nuclear receptor superfamily of proteins (Janatpour et al., 1999, 2000; Genbacev et al., 2000; Cross et al., 2002; Schild et al., 2002). Obviously, the potential key regulators differ depending on the trophoblast subtype such as interstitial or endovascular cytotrophoblast, villous cytotrophoblast, or syncytiotrophoblast (Loregger et al., 2003). The differentiation processes of human and murine cytotrophoblast cells share distinctive regulatory elements (such as Hash-2/Mash-2) but differ in others (e.g. Hand-1 which is almost undetectable in human cytotrophoblast cells) possibly due to a redundant mechanism (Scott et al., 2000; Rossant and Cross, 2001).

In particular, GCMa has been suggested as an important regulator for syncytium formation not only in mice, but also in humans, since the human analogue of the Drosophila melanogaster neural transcription factor GCM is expressed in placenta in the cytotrophoblast and syncytiotrophoblast, especially at stages of differentiation just before and after syncytial fusion (Nait-Oumesmar et al., 2000; Yamada et al., 2000). In the murine system, GCMa seems to be crucial for transforming undifferentiated cytotrophoblast cells into the differentiated syncytiotrophoblast (Cross et al., 2002). GCMa was recently described as being involved in the regulation of syncytin gene expression via two GCMa binding sites at the 5′-flanking region of the syncytin gene (Yu et al., 2002). Additionally, GCMa is able to regulate syncytin gene expression in BeWo and JEG3 cells and stimulates syncytin-mediated cell fusion in vitro (Yu et al., 2002). Human GCMa influences the activity of a placental enhancer of the aromatase gene CYP19 found in the syncytiotrophoblast layer, so there could be a link to estrogen synthesis and further metabolic pathways (Cross et al., 2002; Loregger et al., 2003). The regulation of the syncytin expression must be possible via cell-specific factors but has not yet been proven. This is of paramount importance to the cells, as the overexpression of syncytin causes cell death in vitro after a phase of extensive cell fusion (Yu et al., 2002). However, the cascade of molecular mechanisms and the master genes in human syncytiotrophoblast formation have not yet been elucidated. There is already evidence for an interaction between the regulatory networks of the transcription factors, i.e. between the basic helix-loop-helix transcription factors and GCMa, and also for an adaptive response to milieu conditions or hypoxia during early placental development (Baczyk et al., 2004), e.g. via the hypoxia inducible factor 1α (HIF-1α), a member of the basic helix-loop-helix family which is essential for the adaptive cellular response to changes in oxygen availability (Janatpour et al., 1999; Ivan et al., 2001).

The role of apoptotic processes and their regulation in the syncytium has been further characterized and it has been concluded by Huppertz and co-workers that syncytium formation by cytotrophoblast is an effect of initial apoptotic steps which are then delayed and impeded within the syncytium (Katsuragawa et al., 1997; Huppertz et al., 1998; Ratts et al., 2000). Early steps of the apoptosis cascade such as initiator caspase 8 are important for the differentiation and fusion of cytotrophoblast cells (Black et al., 2004). Changes in membrane composition and stability in the course of apoptotic events contribute to the physicochemical properties. However, whether or not syncytin interacts with these membrane rearrangements or whether it induces cell fusion more directly as an exogenous retrovirus (LaBranche et al., 2001) has yet to be elucidated. Finally, the continuous fusion of cytotrophoblast into syncytiotrophoblast is balanced by a continuous extrusion of late apoptotic material into the maternal bloodstream (Huppertz et al., 1998). Due to occasional damage of the syncytiotrophoblast, gaps and fibrin deposits may trigger the full apoptosis cascade even in cytotrophoblast cells (Nelson, 1996).

Phosphatidylserine is a phospholipid normally restricted to the inner layer of the plasma membrane. It translocates to the outer membrane layer (phosphatidylserine flip) as an early apoptotic event which then facilitates cell fusion of the trophoblast cells. When creating the syncytiotrophoblast out of cytotrophoblast cells, an intimate contact with neighbouring cells, maternal tissue and the extracellular matrix (e.g. collagen) is fundamental (Castellucci et al., 1990). Several effectors have been identified which stimulate cytotrophoblast cell differentiation, for example epidermal growth factors, estradiol, glucocorticoids and gonadotrophic hormone, depending also on local milieu conditions and the stage of development (Morrish et al., 1998; Dakour et al., 1999; Frendo et al., 2003b; Yang et al., 2003).

Cell–cell communication requires gap junctions for the flux of small signal molecules such as cyclic adenosine monophosphate (cAMP), Ca^2 +and other substrates. Gap junctions are clusters of channels which connect the cytoplasm of adjacent cells and are composed of transmembrane proteins called connexins. Although the expression pattern of connexins is not placenta-specific, it is typically related to the stage of placental differentiation (Cronier et al., 1997, 2003; Winterhager et al., 2000). Gap junction connexins such as connexin 40 and 43 (Cx43) are expressed in a characteristic spatial and temporal manner and play an important role during trophoblast cell formation and differentiation (Larsen and Wert, 1988; Cronier et al., 1994; Winterhager et al., 1999, 2000). In general, connexins are expressed in a variety of cells in a characteristic manner, facilitating metabolic cooperation and electric conductivity between cells. They are localized between cytotrophoblast cells and between cytotrophoblast and the syncytiotrophoblast layer (Cronier et al., 2002; Frendo et al., 2003a). In particular, the gap junction protein Cx43 is a candidate channel protein for sufficient syncytia formation, as it can be regulated by cAMP, by hCG, and by estrogen, and as it is expressed in trophoblast cell populations (Schreiber et al., 1993; Cronier et al., 1994, 1997; Winterhager et al., 2000). Recent experimental work indicates that a direct involvement of Cx43 in the formation of syncytiotrophoblast is likely (Cronier et al., 2003; Frendo et al., 2003a), further strengthened by the observation that stimulation of trophoblast cells with cAMP agonistic agents in vitro leads to a further increase of syncytin mRNA and protein along with cell differentiation and fusion (Kudo and Boyd, 2002a; Frendo et al., 2003b).

Syncytin and human placentogenesis

Syncytin cannot be regarded as an exclusive mediator for cell fusion processes in mammalian placentogenesis, since it is derived from a human endogenous retrovirus lacking in other mammals apart from great apes, but it can be considered a representative of an evolutionary strategy which might have led to benefits in human placentogenesis.

Only the precise modulation of local factors can enable the development of the proliferative cytotrophoblast into the syncytiotrophoblast. As distinctive key regulators of mammalian placentogenesis are restricted in their species distribution, syncytin is obviously replaceable by other mechanisms inducing syncytialization in other mammals. Although there is still some obscurity following the discovery of syncytin (Huppertz et al., 2002a), the probable biological role of the protein seems obvious.

As already pointed out, the hypothesis that syncytin is essential for human placental formation and that it has also played an important role in human placental evolution is supported by a number of observations. For instance, syncytin expression is restricted to the placenta apart from minor expression within the testis (Mi et al., 2000). Inside the placenta it is expressed preferentially in and near the syncytiotrophoblast. Also, syncytin has an intact open reading frame which has been preserved over thousands of years, whereas the open reading frames for other HERV-W genomic elements are defective (Voisset et al., 2000), and the syncytin gene is directed by a functional promoter (Cheng et al., 2003). The syncytin promoter is localized in the 5′ LTR region of the HERV-W gene. It exhibits several binding sites for transcriptional regulation which are involved in the control of proliferation and differentiation steps or in the expression of trophoblast-specific genes, e.g. for the transcription factors activating protein AP-1 and AP-2, specificity protein Sp1 and Oct which is binding to a DNA sequence termed the octamer motif (Peters et al., 2000; Sharma and Richards, 2000; Cheng et al., 2003; Loregger et al., 2003; Vasicek et al., 2003; Wang et al., 2004). However, transcription factor AP-2 is not considered a key regulator for early syncytialization processes of cytotrophoblasts or the expression of syncytin (Cheng et al., 2004).

One could speculate that the provirus HERV-W underwent several mutations leading to the reproductive silence of potentially harmful sequences and to the selective preservation of the syncytin gene. It has also been demonstrated that syncytin interacts with the cell surface receptor/amino acid transporter ASCT2 and exerts specific effects on cell differentiation and syncytialization in vitro (Lavillette et al., 2002; Kudo and Boyd, 2002a). However, the receptor-mediated pathways after the binding of syncytin are still indistinct. At present there are no data available on the mechanisms regulating syncytin gene transcription and on the availability of syncytin to cellular requirements. One possibility may be alternative splicing (Smallwood et al., 2003). We do not know at present whether acquired mutations in the syncytin gene may lead to either premature termination of gene transcription or generation of an altered protein causing failed placentation.

The possible role of syncytin 2, a second fusiogenic endogenous retroviral envelope, in human placentogenesis also needs to be investigated (Blaise et al., 2003).

Syncytin and immune tolerance

Another important issue is the possible role of syncytin in maternal immune tolerance to the fetus. Exogenous retroviral envelope proteins can inhibit the immune responses of leukocytes by means of a highly consistent amino acid sequence (Cianciolo et al., 1985). This sequence discovered in the syncytin protein may therefore be attributed to a specific immune modulating mechanism within the developing placenta (Mi et al., 2000). Further relationships to, or interactions with, other mechanisms such as the MHC class I molecules, in particular HLA-G which is thought to produce immune tolerance (Blaschitz et al., 2001), or the immunoregulatory cytokines such as the immunosuppressive interleukin-10 (Bennett et al., 1999), are not clear at present. However, it is possible that syncytin-stimulated syncytiotrophoblast formation may subsequently assure syncytiotrophoblast survival by additional interaction with immune or apoptotic mechanisms. Whether, or to what extent, syncytin contributes to the maternal immune tolerance of the fetus or may interact with other mechanisms such as apoptotic events in the maternal–fetal interface, is unclear at present.

Role in health and disease

In normal human placenta the expression of syncytin at the mRNA level is correlated positively with gestational age and with increased placental weight (Figure 2, our own unpublished data). However, the amount of protein may be stable or even decline during pregnancy due to competition of the two syncytin mRNA species (Smallwood et al., 2003), or other post-transcriptional regulation processes, depending on physiological necessities. The protein is distributed in the basal, but also in the apical, membrane of human syncytiotrophoblast in placentas of various ages (Lee et al., 2001; Frendo et al., 2003b). The question remains as to how syncytin communicates its message from the syncytiotrophoblast to the cytotrophoblast.

There are alterations of syncytin exposition or its regulatory systems in cases with disturbed placental morphology and function, such as in pre-eclampsia.

About 6–8% of human pregnancies are complicated by pre-eclampsia, a multisystem disorder of heterogeneous and still uncertain aetiology (Myers and Baker, 2002; Sibai et al., 2003). Pre-eclampsia is unique to human pregnancy and is still one of the main causes of perinatal morbidity and even mortality. It is clinically defined by arterial hypertension (blood pressure of ≥140 mmHg after the 20th week of pregnancy) and proteinuria (>300 mg over 24 h or++ proteinuria on a reagent strip in at least two spontaneous urine samples) (Davey and MacGillivray, 1988; Myers and Baker, 2002). Manifestation of severe and early onset pre-eclampsia is associated with adverse perinatal outcome and higher rates of preterm deliveries (Sibai et al., 2003).

This is in line with an increase in morphological and pathophysiological anomalies, such as arrested placental cell differentiation, changes in the vascular mediators, altered extracellular matrix conditions, immune dysfunction, inflammatory response and oxidative stress (Genbacev et al., 1996; Granger et al., 2001; Goldman-Wohl and Yagel, 2002; Rein et al., 2003). In vitro experiments have revealed that severe hypoxia increases necrotic rather than apoptotic shedding of placental syncytiotrophoblast, which may contribute to maternal systemic reactions in pre-eclampsia (Huppertz et al., 2003).

It has also been reported that experimental hypoxia reduces the amount of syncytin expression in BeWo cells by 80% in vitro and also in isolated placental cotyledons ex vivo (Knerr et al., 2003). Hypoxia also impairs forskolin-induced BeWo cell fusion in vitro, whereas ASCT2 is mostly preserved in a hypoxic environment (Knerr et al., 2003; Kudo et al., 2003; Nelson et al., 2003).

We have recently shown, along with other authors, that syncytin expression is markedly reduced in placental specimens of women with pre-eclampsia (Figure 3) or with HELLP syndrome, characterized by haemolysis, elevated liver enzymes and low platelets (Lee et al., 2001; Keith et al., 2002; Knerr et al., 2002a).

Syncytin may display a deregulated protein distribution in placentas of pre-eclamptic women. It has been found that syncytin, which is usually predominantly located at the basal syncytiotrophoblast membrane, is found mainly on the apical syncytiotrophoblast membrane of pre-eclamptic placentas (Lee et al., 2001). However, syncytin can also be found at the apical syncytiotrophoblast membrane in normal placentas (Frendo et al., 2003b). There is no explanation as to why these irregularities of syncytin protein distribution occur and how they may affect placental villi differentiation and syncytiotrophoblast growth.

On the basis of current data, we propose that an altered syncytin expression should be considered a major risk factor for diminished trophoblast differentiation and impaired syncytiotrophoblast formation. This could be linked to an alteration of vascular tone and perfusion, exemplified by reduced expression of vasodilatory adrenomedullin in pre-eclamptic placentas described earlier by our group (Knerr et al., 2002b), since a recent study has provided evidence for an impaired syncytialization accompanying a reduction of adrenomedullin production in pre-eclamptic tissues (Li et al., 2003). The pathogenesis of pre-eclampsia as a disease of the maternal–fetal interface is multi-factorial, involving a battery of molecular factors (Lachmeijer et al., 2002), and there is obviously no single genetic or epigenetic factor which leads to pre-eclampsia.

In general, the importance of endogenous retroviral particles in the human placenta has not been fully understood, but there is now increasing evidence that retrovirus-derived syncytin contributes to human placentogenesis. It seems plausible that syncytin also influences maternal immunosuppression, for example in the syncytiotrophoblast, being the most extensive contact zone between mother and fetus. Local changes in oxygen tension appear to be one major factor in disturbed placental syncytialization, possibly due to the down-regulation of syncytin gene expression, further diminishing cytotrophoblast cell differentiation.

The captured endogenous retroviral protein syncytin may be a newly discovered physiological mechanism in human placentogenesis. However, the regulatory processes need further investigation to elucidate the syncytium formation processes during human placentogenesis.

Conclusion

Syncytin, a fusogenic glycoprotein of endogenous retroviral origin, is considered to be an important stimulus for the differentiation and fusion of trophoblast cells and may also fulfil other important functions (such as the maternal immunotolerance to fetal tissues and the preservation of the syncytiotrophoblast layer). Although conspicuously absent in other mammalian species and therefore, in principal, replaceable by other mechanisms contributing to cell syncytialization, it may facilitate placental function in human pregnancy and contribute to pregnancy-associated disturbances such as pre-eclampsia or IUGR through disturbed gene expression or altered protein function. Since pre-eclampsia is unique to humans and since syncytin is derived from the HERV-W family exclusively found in humans and higher primates, it is an interesting candidate gene for research into pre-eclampsia on the basis of the published in vitro studies. Further molecular and immunological investigations are mandatory to clarify the role of syncytin in trophoblast cell fusion and syncytiotrophoblast preservation. Gene knockout mice will not provide a conclusive answer, although cross-species comparison of fundamental mechanisms will provide more insight into the diversity of niche components facilitating placental morphogenesis and function.

Figure 1.

Simplified drawing of the human placental villous structure, showing the cytotrophoblast at different stages of differentiation, syncytial fusion, syncytiotrophoblast differentiation and shedding. (From Huppertz et al., 2002b, reprinted with permission.)

Figure 2.

Syncytin gene expression normalized to glyceraldehyde-3-phosphate amounts in normal human placental villi specimens between 18 and 41 weeks of gestation. Linear regression analysis was performed and is presented together with the 95% confidence intervals; our own data, measurements according to methods published earlier (Knerr et al., 1999, 2002a).

Figure 3.

Syncytin mRNA/β-actin mRNA ratios (mean±SEM) in placental villi specimens of patients with pre-eclampsia (n=16) compared to healthy controls (n=30), *P<0.05, **P<0.01 after Mann–Whitney test; redrawn from our own published data (Knerr et al., 2002a).

The editing of the manuscript by Patricia Schmid is greatly appreciated. I. Knerr is supported by a grant of the University of Erlangen (ELAN program). We apologize for not citing all studies contributing to the topic on human trophoblast cell syncytialization.

Author notes

1University Children's Hospital and 2Department of Obstetrics and Gynaecology, University of Erlangen-Nuremberg, Erlangen and 3Department of Anatomy, University Hospital, Aachen, Germany