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BOR - Papers in Press, published online ahead of print June 25, 2003.
Biol Reprod 2003, 10.1095/biolreprod.103.016360
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BIOLOGY OF REPRODUCTION 69, 1472–1480 (2003)
DOI: 10.1095/biolreprod.103.016360
© 2003 by the Society for the Study of Reproduction, Inc.

Pregnancy

Requirement of Gap Junctional Intercellular Communication for Human Villous Trophoblast Differentiation1

Laurent Cronier3, Jean-Louis Frendo4, Norah Defamie5, Guillaume Pidoux4, Gladys Bertin4, Jean Guibourdenche4, Georges Pointis5, and André Malassiné2,4

Laboratoire de Biomembranes et Signalisation cellulaire,3 Université de Poitiers, 86022 Poitiers cedex, France INSERM, unité 427,4 Faculté de Pharmacie, 75270 Paris cedex 6, France INSERM, EMI 00-09,5 Faculté de médecine, 06107 Nice cedex, France


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
During pregnancy, the villous trophoblast develops from the fusion of cytotrophoblastic cells (CT) into a syncytiotrophoblast (ST), supporting the main physiological functions of the human placenta. Connexin43 (Cx43) is demonstrated in situ and in vitro in the villous trophoblast between CT and between CT and ST. Moreover, the presence of a gap junctional intercellular communication (GJIC) during in vitro trophoblast differentiation was previously demonstrated. Because the exchange of molecules through gap junctions is considered to play a major role in the control of cell and tissue differentiation, we studied the effects of a gap junctional uncoupler, heptanol, on morphological and functional trophoblast differentiation and on GJIC measured by the fluorescence recovery after photobleaching method. We found that when the GJIC was interrupted, CT still aggregated but fused poorly. This morphological effect was associated with a significant decrease of trophoblastic-specific gene expression (ß human chorionic gonadotropin and human chorionic somatomammotropin). This blocking action was reversible as demonstrated by recovery of GJIC and trophoblast differentiation process after heptanol removal. Moreover, the inhibition of the trophoblast differentiation did not affect Cx43 transcript expression and Cx43 protein expression. These data suggest that the molecular exchanges through gap junctions preceding cellular fusion are essential for trophoblast differentiation generating the multifunctional syncytiotrophoblast.

human chorionic gonadotropin, placenta, signal transduction, syncytiotrophoblast, trophoblast


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
One of the major processes in human placental development is the fusion of mononucleated cytotrophoblastic cells (CTs) to generate multinucleated syncytiotrophoblast (ST) [1, 2]. Indeed, the ST that covers the chorionic villi is bathed with maternal blood in the intervillous space. Given its frontier position, ST plays a major role in numerous placental functions, including exchanges, metabolism, and synthesis of steroid and peptide hormones required for fetal growth and development [3, 4]. This process of cell fusion can be reproduced in vitro [5]. Isolated mononucleated CTs make initial contacts by pseudopodia with neighboring cells, transform into cellular aggregates, and then fuse to form a syncytium. This in vitro differentiation mimics the activities of normal CTs during their in vivo maturation. Indeed, the morphological differentiation is associated, both in vivo and in vitro, with an increase in the production of specific hormones like hCG and human chorionic somatomammotropin (hCS) and with specific gene expression [6, 7].

Gap junctions are clusters of transmembrane channels composed of hexamers of connexins (Cx) providing a pathway for the diffusion of ions and small molecules such as cAMP, cGMP, inositol triphosphate, and Ca2+. Connexins represent a family of closely related membrane proteins that are encoded by a multigene family of at least 20 members in humans [8]. It has been suggested that molecular exchanges through gap junctions are involved in the control of cell proliferation, cell and tissue differentiation, metabolic cooperation, and spatial compartmentalization during embryonic development [9–12]. Furthermore, gap junctional intercellular communications (GJICs) have been described to be directly involved in the fusion of mononucleated myoblast to generate multinucleated myotube [13, 14]. We have previously demonstrated, both in situ and in vitro, the expression of Cx43 mRNA and the presence of Cx43 protein localized between CTs and between CTs and STs, whereas Cx26, Cx32, Cx33, and Cx40 were not detected in the villous trophoblast [15]. Furthermore, using fluorescence recovery after photobleaching method (gap-FRAP), we demonstrated the presence of a functional intertrophoblastic communication via gap junctions preceding trophoblastic fusion [15, 16].

Therefore, the aim of the present study was to investigate the involvement of gap junction–mediated cell-cell communication in trophoblast differentiation by interrupting the exchange of molecules with heptanol, a junctional uncoupler [17]. We demonstrated that heptanol-treated CTs aggregated but fused very poorly with an associated decrease of ßhCG secretion and hCS expression compared to untreated cells. The uncoupling action totally abolished GJIC, whereas Cx43 mRNA expression, protein levels, and tissue localization were not affected by the heptanol treatment. Our data show that interruption of the molecular exchanges by gap junctions during human trophoblast differentiation impaired cellular fusion, suggesting the requirement of a GJIC for the syncytiotrophoblast formation.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Trophoblastic Cell Culture

Human placentas were obtained after cesarian sections from mothers with uncomplicated pregnancies. CTs were isolated using a method previously described [18]. After several sequential trypsin/DNase digestions followed by Percoll gradient centrifugation, the cells were further purified by means of a negative selection procedure to obtain a trophoblast preparation without contamination of other cells according to a published method [19]. Monoclonal anti-human leukocytic antigen-A, -B, and -C antibody (W6–32HL, Sera Lab, Crawley Down, UK) reacts with most cell types (e.g., macrophages, fibroblasts, extravillous trophoblast) but not with villous cyto- or syncytiotrophoblast. Briefly, the isolated cells were transferred to plastic culture dishes coated with the monoclonal antibody. After 15 min at 37°C, nonadherent cells were recovered by gently rocking the dishes and removing them with a pipette. CTs were diluted to a final concentration of 0.5 x 106/ml in minimum essential medium (MEM) containing 10% fetal calf serum (FCS), 25 mM glucose, and 50 µg/ml gentamicin. Cells were plated in 35-mm plastic dishes (Nunclon, Nunc, Roskilde, Denmark) and incubated at 37°C in 5% CO2. For long-term uncoupler treatment, heptanol (1.5 mM) or octanol (0.4 mM) purchased from Sigma (St. Louis, MO) was added to the culture medium after CT adherence (4 h after cell seeding). The culture medium was renewed every 8 h to avoid the evaporation of alcohol. Cytokeratin 07 immunocytochemistry was performed after each purification to confirm the cytotrophoblastic nature of the attached cells, and about 95% of the cells were positively stained.

Immunocytochemistry

To detect desmoplakin, cultured cells were rinsed with PBS solution and fixed in methanol at -20°C for 25 min. A monoclonal antidesmoplakin antibody (Sigma), diluted 1:400, was applied, followed by fluorescein isothiocyanate (FITC)-labeled goat anti-mouse immunoglobulin (Ig)G (Sigma) diluted 1:100 as previously described [20]. After washing, samples were mounted in medium with 4',6'-diamino-2-phenylindole (DAPI) for nuclear staining (Vector Laboratories, Burlingame, CA).

For Cx43 detection, cultured cells were fixed for 10 min in methanol at -20°C. They were then washed three times in PBS and processed using a method previously described [15]. After incubation in a blocking solution consisting of 2% BSA and 1% Triton X-100 in PBS for 30 min at room temperature, specimens were incubated overnight in PBS 1%BSA containing a monoclonal anti-cx43 primary antibody (1/200; Transduction Laboratories, Lexington, KY). After five further washes in PBS, the secondary antibody conjugated to FITC (1/200; Jackson Immunoresearch Laboratories, West Grove, PA) was applied for 45 min at room temperature. After washing, samples were mounted in medium with DAPI for nuclear staining. All controls performed by omitting the primary antibody or using a nonspecific IgG of the same isotype (IgG1; Immunotech, Marseille, France) were negative.

Gap-FRAP Method

The cell-to-cell diffusion of a fluorescent dye was measured by the gap-FRAP method [21], using an interactive laser cytometer (ACAS 570, Meridian Instruments, Okemos, MI), which allows for convenient digital video imaging and analysis. After washing, cultured trophoblastic cells were loaded for 10 min at room temperature in saline solution containing the membrane-permeant molecule 6-carboxyfluorescein diacetate (Sigma: 7 µg/ml in 0.25% dimethylsulfoxide). This lipophilic compound is hydrolyzed by cytoplasmic esterases to 6-carboxyfluorescein, a hydrophilic derivative that accumulates inside the cells. After washing off the excess extracellular fluorogenic ester to avoid further loading, the fluorescence of some selected cells adjacent to others cells was photobleached by applying strong light pulses (8 mW; 2 sec) from an argon laser set at 488 nm. The fluorescence intensity was recorded in the bleached cells before and after photobleaching during 12 min (each time period = 2 min). In each experiment, one labeled isolated cell, left unbleached, served as a reference for the loss of fluorescence because of repeated scanning and dye leakage, and an isolated bleached cell served as a control (see Fig. 4). The return of fluorescence following a fast step-like course, reaching at least 90% of the final steady state in less than 30 sec after photobleaching (see Fig. 4B), indicated that the diffusion of dye was neither prevented by cell membrane nor limited by the presence of gap junctions. It was inferred that fusion of cell membranes had been completed and that the cellular elements were part of a true syncytium. When the bleached cells were interconnected by open gap junctional channels to unbleached contiguous cells, a fluorescence recovery following a slow exponential time course was measured (see Fig. 4A). Therefore, the analysis of the kinetic of fluorescence recovery allows to discriminate between aggregated cytotrophoblastic cells and syncytiotrophoblast. In our experimental conditions, GJIC was investigated (coupled cells or not) in a population of cells in contact with results expressed as a percentage of coupled cells.



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  		FIG. 4. Gap-FRAP analysis. Top, Typical computer-generated images of fluorescence intensities in cultured trophoblast measured during FRAP experiments. White represents the highest intensity. Areas 2 and 3 closely corresponding to cell territories (syncytiotrophoblasts) were selected in the prebleach scan and photobleached by laser pulses. Area 1 serves as an unbleached control cell to measure the loss of fluorescence because of repeated scanning and weak dye leakage. Immediately after photobleaching, the scan at time 0 shows the reduction of fluorescence emission in area 2 and 3. After 12 min, a fluorescence recovery had occurred in area 2, whereas the fluorescence intensity remains weak in area 3. Bottom, Evolution of fluorescence intensities versus time in isolated cells in cells connected by gap junctions and in fused cells. A) Graphic display of gap-FRAP experiment shown in upper row. Fluorescence recovery in cell 2 () follows a closely exponential time course, reflecting the presence of gap junctional communication. In contrast, the fluorescence intensity remains unchanged in cell 3 ({square}). B) Fast fluorescence recovery following a step-like course, when the membrane fusion is achieved between two cells (). It was previously demonstrated that, in this case, heptanol (3 mM), a known junctional uncoupler, had no effect or fluorescence recovery, confirming the absence of gap junctional communication. The fluorescence intensities remained unchanged in the reference unbleached cell ({block}) and isolated bleached cell ({square}). (Reproduced from [43] with permission of the publisher, Wiley, Inc)

Immunoblotting

To detect Cx43 and hCS, cell extracts were prepared as previously described [20]. Solubilized proteins were immunoblotted using either a polyclonal antibody directed against hCS (1/250, Dako, Glostrup, Denmark) or a monoclonal antibody directed against Cx43 (1/1000, Transduction Laboratories). The corresponding antigens were detected after incubation with alkaline phosphatase-coupled secondary antibodies (1/7500 for anti-rabbit IgG and 1/10 000 for anti-mouse IgG). Densitometric analyses of hCS or Cx43 signals were performed by means of image-master software (Pharmacia Biotech, Orsay, France).

Reverse Transcription-Polymerase Chain Reaction

Total RNA was isolated by RNeasy method (Quiagen, Hilden, Germany) from isolated highly purified CT cells. The total RNA was treated with Range-free DNase I (Life Technologies, Cergy Pontoise, France) at 22°C for 5 min.

For Cx43 transcripts, total RNA (5 µg) was transcribed into cDNA using oligo(dT)12–18 as primer and superScriptII reverse transcriptase (Boehringer Mannheim, Mannheim, Germany). One fifth of the reaction mixture was amplified with Taq polymerase (Life Technologies) in a final volume of 50 µl. For the semiquantitative polymerase chain reaction (PCR), each cycle consisted of denaturation at 94°C for 1 min, primer annealing at 60°C for 1 min, and primer extension at 72°C for 1 min. To quantify amplification in exponential phase, primers for glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were added during cycle 8 and PCR products (10 µl) were obtained in cycles 23, 24, 25, 26, and 27. Primers were designed from previously published sequence data: the human GAPDH 5' sense CTG-CAC-CAC-CAA-CTG-CTT-AG-3' and 5' antisense AGG-TCC-ACC-ACT-GAC-ACG-TT-3'; human Cx43 5' sense AGT-CTA-TCT-TTG-AGG-TGG-CC-3' and 5' antisense GGC-TGT-AAT-TCA-TGT-CCA-GC-3'. Reaction products were resolved on an agarose gel (1.5%) and stained with ethidium bromide. Sizes of the expected amplification products are 1158 base pairs (bp) for Cx43 and 275 bp for GAPDH.

Hormone Assay

The hCG concentrations were determined by means of an enzyme-linked fluorescence assay (Vidas system, Biomérieux, Marcy-l'étoile, France) in cell culture media at various times as previously described [22]. Assay sensitivity was 2 mIU/ml. The values are the means ± SEM of triplicate determinations. Significant differences were identified using the Student t test.

Analysis of Syncytium Formation

Syncytium formation was monitored by analyzing the distribution of desmoplakin and nuclei in fixed cells, as described by Keryer et al. [23]. Desmoplakin staining of aggregated cells gradually disappears at the boundaries with the syncytium formation [24]. From a random point near the middle of the coverslip, the nuclei of 100 desmoplakin-delimited cellular elements were counted, distributed in class, and the results expressed as the percentage of cellular elements versus number of nuclei per syncytium. Data are the mean ± SEM of three separate experiments.

3-[4,5-Dimethylthiazol-2-yl]-2,5-Diphenyltetrazolium Bromide Assay

Heptanol toxicity was estimated by a 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) (Sigma) test. After 3 days of culture in control or heptanol conditions, trophoblastic cells were rinsed in Earles balanced saline solution, and MTT was added to the culture medium at 0.5 mg/ml. At the end of a 3-h incubation period at 37°C, the formazan formation caused by active mitochondrial dehydrogenases was measured spectrophotometrically at a wavelength of 570 nm with background subtraction at 630 nm.

Each determination was performed in triplicate. All statistics are given as mean ± SEM.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Effects of Long-Term Heptanol Treatment During Trophoblast Differentiation

When purified CTs were cultured in the presence of 10% FCS, after adhesion and flattening, cells make initial contacts by pseudopodia with neighboring cells, transform into cellular aggregates, and then fuse to form a large multinucleated ST [5, 16]. This morphological differentiation is currently followed by staining cells with antidesmoplakin antibodies to reveal cell boundaries. Indeed, the staining of desmoplakin present at the intercellular boundaries in aggregated cells progressively disappears with the syncytium formation (Fig. 1, A and B). After 48 h of culture, mononuclear CTs mainly differentiated into aggregated cells and ST. This is illustrated in Figure 1A by a gathering of numerous nuclei in large cytoplasmic mass of the ST. In contrast, CT incubated with 1.5 mM heptanol aggregated but did not fuse or fused poorly (Fig. 1, C and D). To further investigate the influence of heptanol on cell fusion and ST formation, we estimated the number of DAPI-stained nuclei per desmoplakin-defined ST. As illustrated in Figure 2A, after 72 h of culture, 73.3% ± 1.8% of examined STs contained more than four nuclei. In contrast, ST formation was impaired in the presence of heptanol because only small STs with two or three nuclei being observed (Fig. 2B). Interestingly, the heptanol removal after 48 h of culture induced an increase of trophoblastic fusion as indicated by the presence of 38.7% ± 2.0% of STs with more than four nuclei (Fig. 2C). Furthermore, statistical analysis using a chi-square test showed a significant difference in the nuclei distribution between trophoblastic cells cultured 72 h in control conditions and in the presence of heptanol. The difference was also significant when heptanol removal was compared with heptanol treatment alone. This reappearance of cellular fusion process illustrated the reversibility of heptanol action on trophoblast differentiation. It should be noted that the presence of heptanol in the culture medium did not prevent migration or cell-to-cell adhesion as shown in Figure 1D, leading to a majority of aggregated CT.



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  		FIG. 1. Effects of long-term heptanol treatment on morphological trophoblast differentiation. A, B) Double immunofluorescence for nuclear staining with DAPI (A) and desmoplakin (B) in villous CT cells cultured for 48 h in control conditions. A strong desmoplakin staining is observed at the intercellular boundaries between CT and ST cells and between CT cells but disappears in fused STs. Scale bars = 50 µm. C, D) In the presence of heptanol for 48 h, aggregated CT cells were mainly observed, indicating that the trophoblast differentiation is delayed. Scale bars = 50 µm. These pictures are representative of three experiments



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  		FIG. 2. Effects of heptanol on cell fusion index. Human CT cells were cultured in MEM for 72 h (A) or in the presence of 1.5 mM heptanol for 72 h (B) or 1.5 mM heptanol (48 h) and then in MEM for 24 h (C). Data show the distribution of syncytia as a function of the number of nuclei per syncytium and are the mean ± SEM of three separate experiments. Statistical analysis using a chi-square test revealed a significant difference between trophoblastic cells cultured in control conditions for 72 h and in the presence of heptanol for 72 h (chi-square = 92.26, df = 9, P < 0.0001). The difference is also significant between trophoblastic cells cultured in heptanol for 72 h and in heptanol for 48 h and then in MEM for 24 h (chi-square = 28.46; df = 6; P < 0.0001)

Given the potential nonspecific effects of heptanol, the action of octanol, another gap junctional inhibitor, was investigated. In this experiment, after 72 h of culture in control conditions, 66% ± 3.1% of examined STs contained more than four nuclei, and when the cells were cultured in the presence of 0.4 mM octanol for the same period, only 11% ± 1.5% of the analyzed STs contained more than four nuclei. Furthermore, octanol removal after 48 h of culture induced an increase in the number of STs, with more than four nuclei (34.3% ± 3.5%) observed 24 h later.

Effects of Long-Term Heptanol Treatment on Trophoblastic Hormonal Productions

As previously reported, the ST formation by fusion of CT is associated with a significant increase in ßhCG secretion [5] and hCS expression [25]. Figure 3A shows that cells treated with heptanol for 72 h exhibited a significant decrease in ßhCG secretion, compared with cells in control conditions. Interestingly, when heptanol was removed after the first 48-h period, ßhCG rose to a level similar to the control at 72 h, showing the reversibility of the action. Moreover, Western blotting analyses of hCS production showed that in the presence of heptanol for 2 or 3 days, expressions represent 26.8% ± 2.3% and 52.8% ± 2.9% of the control value, respectively (Fig. 3C).



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  		FIG. 3. Effects of long-term heptanol treatment on specific hormonal productions. A) Third day ßhCG secretion in control conditions (MEM) in the presence of 1.5 mM heptanol for 72 h or in the presence of heptanol for a 48-h period followed by a 24-h period in control conditions. Values represent mean of data ± SEM of triplicate determinations. Values of ßhCG secretion in three independent experiments are shown in the table, and graph is representative of experiment 1. **P < 0.01; Student t test. B) Immunoblotting of hCS after 2 days (lanes 1 and 2) and 3 days (lanes 3 and 4). Lanes 1 and 3 represent hCS expression in trophoblastic cells cultured in control conditions and lanes 2 and 4 in trophoblastic cells cultured in the presence of 1.5 mM heptanol. The figure illustrates one of six of a representative experiment. C) Normalized densitometric analyses of hCS protein performed on six independent experiments. The hCS expressions are significantly different at P < 0.01 from MEM (Student t test for grouped data)

Effects of Long-Term Heptanol Treatment on GJIC and Connexin Expression During Trophoblast Differentiation

When the purified CTS were cultured for 48 h, the GJIC was analyzed between contiguous CTs, between contiguous STs, and between contiguous CTs and STs. Functionality of the intercellular channels was demonstrated by means of gap-FRAP. Typical changes in the fluorescence intensity of selected cells within a field before and after the photobleaching procedure as well as the corresponding fluorescence recovery curves are depicted in Figure 4. The slow exponential fluorescence recovery (Fig. 4A) characteristic of GJIC was recorded between CT cells, between CT cells and STs, and between contiguous STs. It was demonstrated that a functional coupling occurred in 2.6% and 6% of tested cells after, respectively, 1 or 2 days. The presence of 1.5 mM heptanol in the culture medium during these periods totally abolished the number of coupled cells (Fig. 5). These results indicated that the culture conditions used for this study maintained the inhibitory effects of the junctional uncoupler. Moreover, this inhibitory action was simply reversed by washing off heptanol, leading to the reappearance of a functional coupling. This fact demonstrated the reversibility of the heptanol action. To firmly establish that heptanol was not cytotoxic, cell viability was also evaluated with the tetrazolium colorimetric assay. After 3 days of culture, the optical densities (OD) were, respectively, 0.445 ± 0.039 and 0.382 ± 0.063 in untreated and heptanol-treated trophoblastic cells (mean ± SEM). These OD570–630 were not significantly different (Student t test), reflecting a similar number of living cells. To determine a possible action of heptanol on Cx43 gene and protein expressions, Cx43 mRNA was analyzed by reverse transcription (RT)-PCR and Cx43 protein by Western blotting and immunostaining. Figure 6A shows that Cx43 PCR products were not affected by the presence of heptanol, compared with control conditions. Furthermore, Western blotting analysis showed no significant difference between heptanol-treated cells and control cells (Fig. 6, B and C). Moreover, as illustrated in Figure 7, A and B, immunofluorescence (IF) localization of Cx43 protein in term villous trophoblastic cells revealed a punctuate immunostaining at the borders of contiguous cells (mainly between CTs and STs) and around the nuclei. Addition of heptanol in the culture medium did not significantly decrease Cx43 punctuate IF in the aggregated trophoblastic cells (Fig. 7, C and D).



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  		FIG. 5. Effects of heptanol on GJIC. Percentage of coupled cells between contiguous villous trophoblastic cells after 24 or 48 h in MEM (control conditions) or in the presence of heptanol 1.5 mM or after a 24-h period in the presence of heptanol followed by a 24-h period in control conditions. Coupled cells were characterized by the exponential time course of fluorescence recovery into a photobleached cell as shown in Figure 1A. Number of intercellular contacts analyzed is indicated on top of the bars



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  		FIG. 6. Effects of heptanol on Cx43 mRNA and protein expressions. A) Expression of Cx43 mRNA in villous trophoblastic cells. The expected size of the PCR products is 1143 bp for Cx43 (top) and 275 bp for GAPDH (bottom). Lane 1 represents extracts from cytotrophoblastic cells cultured for 2 days in MEM and lane 3 from cytotrophoblastic cells cultured for 2 days in the presence of 1.5 mM heptanol. Lanes 2 and 4 represent respective controls without RT. A 100-bp DNA ladder was used for size analysis. B) Immunoblotting of Cx43 after 2 days (lanes 2 and 3) and 3 days (lanes 4 through 6). Lane 1 represents positive control for Cx43 (rat brain). A, B) Representative of three separate experiments. C) Normalized densitometric analyses of Cx43 protein. Bars = mean ± SEM of values from three independent experiments. Statistical analysis reveals that Cx43 expression is not significantly different in all conditions tested (Student t test for grouped data)



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  		FIG. 7. A, B) Double IF for nuclear staining with DAPI (A) and for Cx43 (B) in villous CT cells cultured for 48 h in control conditions. Cx43 punctuate IF is observed around the cluster of nuclei of a ST and at the border of a cytotrophoblastic cell that is still in contact with the ST. Scale bars = 10 µm. C, D) Double immunofluorescence for nuclear staining with DAPI (C) and for Cx43 (D) in villous CT cells cultured for 48 h in the presence of 1.5 mM heptanol. In the presence of uncoupler for 48 h, only aggregated CT cells were observed, but the Cx43 localization seems unchanged with a Cx43 punctuate IF at intercellular boundaries and around the nuclei. Scale bars = 10 µm


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In the present study, we found that an agent blocking GJIC impaired trophoblastic cell fusion and differentiation, as revealed by morphological and functional analyses. This blocking action is reversible as demonstrated by the recovery of GJIC and trophoblast differentiation process after heptanol removal.

Only a few human cell types can fuse together, differentiate, and form syncytia:osteoclast [26, 27], myoblast [14, 28], and CT cells. The process of syncytium formation is poorly understood. In the human trophoblast, a phosphatidylserine (PS) flip has been involved in the cell-cell fusion process [29]. According to Huppertz et al. [30], this PS flip is a consequence of activation of initiator caspase (e.g., caspase 8), leading to the concept that the molecular machinery of early apoptosis is involved in the fusion process. Implication of endogenous retrovirus genome in trophoblastic fusion has been suggested, and recently a role of endogenous retroviral envelope glycoprotein encoded by the HERV-W (syncytin) was demonstrated [31, 6].

Other evidences point to an important role of gap junctions and GJIC in the control of cell and tissue differentiation processes [9, 10, 32]. The most clear-cut examples of GJIC intervention in differentiation and development processes were given by interruption of molecular exchanges with anti-connexin antibody [33] or analysis of connexin knockout mice [34, 35]. In these studies, the uncoupling induced developmental abnormalities and cardiac or bone malformations. The role of gap junctions in differentiation can be also studied by chemically inhibiting GJIC. A variety of substances such as aliphatic alcohols, halothane, glycyrrhetinic acid, and oleamide can uncouple cells. Heptanol is the most common gap junctional inhibitor used. Long-term incubation with heptanol considerably reduces the fusion of myoblasts [13, 14] and inhibits the fibroblast-populated collagen lattices [36], osteoclastic bone resorption [27], and actin organization and calcium propagation in astrocytes [37]. Although its exact mechanism of action is not known, it seems related to a decreased fluidity of membranous cholesterol-reach domains [17, 38], leading to a decrease of the open probability of junctional channels. Furthermore, in cultured neonatal rat cardiomyocytes, heptanol did not decrease the number of gap junctional channels, and in pancreatic acinar cells, there is a cessation of GJIC, although gap junctions remain structurally intact [39]. In the same way, in our study, heptanol totally abolished GJIC as measured by gap-FRAP without affecting Cx43 transcripts production and Cx43 protein expression and localization. It must be pointed out that in our experimental conditions, heptanol action was not due to a cytotoxic effect because cell adherence and aggregation were not affected, an MTT test demonstrated no effects of heptanol on cell viability, and heptanol removal restored gap junctional communication and functional activities of the trophoblast (hCG and hCS production).

These results suggest that the diffusion of molecules between aggregated trophoblastic cells might regulate the fusion process. Indeed, according to a recent review [40], the initiation of the syncytial fusion seems require a concerted action of tissue-specific recognition molecules like HERV-W retroviral glycoprotein and phospholipids at the outer cellular surface. It is conceivable that second messengers like Ca2+, inositol triphosphate3, and cAMP would control various cellular effectors involved in fusion and in the transcription of syncytiotrophoblast-specific genes [7]. For instance, it has been recently demonstrated that the stimulation of trophoblastic cell fusion and differentiation by cAMP is associated with a concomitant increase in HERV-W env. mRNA and protein expression [41]. These intercellular messengers may also cross-talk with gap junction channels, which are regulated by cAMP and Ca2+. Furthermore, the fusion process coincides with a decrease in basal Ca2+ activity [20] and with an increase in cellular cAMP levels [42]. It was also demonstrated that trophoblastic villous differentiation occurs earlier when cell-cell communication and Cx43 expression were enhanced (cAMP analogues, hCG, glucocorticoids). On the contrary, differentiation process is delayed when this communication is decreased by transforming growth factorß1 or endothelin [16, 20, 43, 44].

Recently gene knock-out approaches have been demonstrated that Cx26 and Cx31 deficiencies disturbed placental development in mice, but these molecules are not expressed in the human placenta [45, 46]. Therefore, given the striking species diversity of placental structures and endocrine functions, care must be taken when extrapolating mouse gene inhibition/disruption data to humans. For evident ethical reason, studies of the human placenta require carefully designed approaches, and our in vitro study demonstrated the involvement of GJIC in the trophoblastic fusion and consequently in human placental development.


   ACKNOWLEDGMENTS
 
We are grateful for the Clinique du Fief de Grimoire and the Department of Obstetrics and Gynecology at St. Vincent de Paul Hospital for providing us the placentas. We would like to thank Dr. F. Gaillard for his expertise in statistical analysis and J. Habrioux for his technical assistance.


   FOOTNOTES
 
1 Partly supported by grants from the Langlois Foundation. Back

2 Correspondence: A. Malassiné, INSERM U427, Faculté de pharmacie, 4 Av. de l'observatoire, 75270 Paris cedex 6, France. FAX: 33 1 44 07 39 92; amalassi{at}pharmacie.univ-paris5.fr Back

Received: 13 February 2003.

First decision: 7 March 2003.

Accepted: 9 June 2003.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

  1. Richard R. Studies of placental morphogenesis I. Radioautographic studies of human placenta utilizing tritiated thymidine. Proc Soc Exp Biol Med 1961 106:829-831
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