Immunomodulation by maternal autoantibodies of the fetal serotoninergic 5-HT4receptor and its consequences in early BALB/c mouse embryonic development
© Kamel et al; licensee BioMed Central Ltd. 2007
Received: 05 July 2006
Accepted: 19 April 2007
Published: 19 April 2007
The presence of functional 5-HT4 receptors in human and its involvement in neonatal lupus erythematosus (NLE) have prompted us to study the receptor expression and role during embryogenesis. Earlier we managed to demonstrate that female BALB/c mice immunized against the second extracellular loop (SEL) of the 5-HT4 receptor gave birth to pups with heart block. To explain this phenomenon we investigated the expression of 5-HT4 receptors during mouse embryogenesis. At the same time we looked whether the consequence of 5-HT4 receptor immunomodulation observed earlier is in relation to receptor expression.
We studied the expression of 5-HT4 receptor at the mRNA level and its two isoforms 5-HT4(a) and 5-HT4(d) at the protein level in embryos from BALB/c mice, at 8th, 12th, 18th gestation days (GD) and 1 day post natal (DPN). Simultaneously the receptor activity was inhibited by rising antibodies, in female mice against SEL of the receptor. The mice were mated and embryos were collected at 8th, 12th, 18th GD and 1 DPN.
5-HT4 receptor mRNA increased in brain from 12th GD to 1 DPN. Its expression gradually decreased in heart and disappeared at birth. This was consistent with expression of the receptor isoforms 5-HT4(a) and (d). Abnormalities like decreased number of embryos, growth delay, spina bifida and sinus arrhythmia from 12th GD were documented in pups of mice showing anti-5-HT4 receptor antibodies.
serotoninergic 5-HT4 receptor plays an important role in mouse foetal development. In BALB/c mice there is a direct relation between the expression of receptor and the deleterious effect of maternal anti-5-HT4 receptor autoantibodies in early embryogenesis.
Serotoninergic 5-HT4 receptors belong to the family of 7-membrane spanning receptors coupled to Gs protein. Until now 10 different human isoforms have been cloned and sequenced (h5-HT4(a), (b), (c), (d), (e), (f), (g), (hb), (i) and (n)) [1–3] They are all coded by a complex gene (700 Kb, 38 exons), which generates 9 carboxy-terminal variants issued from alternative splicing. They have identical sequences up to Leu358, with the exception of h5-HT4(hb), which is characterized by a 14 amino acid residues insertion within the receptor second extracellular loop (SEL) of h5-HT4(b) . The presence of 5-HT4 receptor has been reported in different tissues. Receptor mRNA has been detected in brain, bladder, gastrointestinal tract, heart, kidney, pancreas and testis [3, 5, 6]. In postmortem human brain studies the receptor, ignoring its different isoforms, was detected in the basal ganglion (caudate nucleus, putamen, nucleus accumbens, globus pallidus and substantia nigra), in cortex and hippocampus (CA1 and subiculum) . In heart, mRNA for different h5-HT4 isoforms is mainly detected in atrium (5-HT4(a), (b), (c), (g), (i)) [3, 5]. The presence of four isoforms, mainly m5-HT4(a), (b), (e), (f) has been described in mouse [8, 9]. Although many studies have been performed to highlight the tissue distribution and functional activity of different isoforms of the receptor in adult human, rat and mouse, little is known about their expression and function during embryogenesis. Previously we have reported the involvement of 5-HT4 receptor in congenital heart block (CHB) associated to a systemic autoimmune response in the mother . In neonatal lupus erythematosus (NLE), autoantibodies against ribonucleoproteins 48-kDa SSB/La, 52-kDa SSA/Ro (Ro52) and 60-kDa SSA/Ro (Ro60) have received more attention, since they have been shown to be strongly associated with autoimmune responses involved in symptoms like CHB . It has been postulated that the transfer of maternal anti-Ro52 antibodies from mother to the fetus is responsible for the symptoms of NLE. In a previous study, we managed to induce NLE symptoms in a mouse model . This has prompted us to further explore the expression of different 5-HT4 receptor isoforms in BALB/c mice during embryogenesis. Meanwhile, we searched for the period when anti-receptor antibodies could induce the observed abnormalities.
Results and Discussion
1. Production of anti-peptide antibodies
2. Expression of receptor isoforms
3. In vivo effect of anti-5-HT4antibodies
In order to highlight the role of 5-HT4 receptor on normal development of the embryos, we sought to immunomodulate the activity of the receptor in BALB/c mice embryos, through the passage of maternal anti-receptor antibodies induced with a peptide corresponding to the second extracellular loop of the 5-HT4 receptor. Pregnant mice at 8th, 12th, 18th GD and one DPN were sacrificed and the embryos were counted and collected for morphological studies.
The outcome of mated mice immunised with G21V peptide
1 growth delay
1 spina bifida
1 arrhythmia and 1 growth delay
1 arrhythmia and 1 growth delay
Peptide sequences derived from human 5-HT4 receptors
Up to now there is no information on the distribution of the 5-HT4 receptor isoforms at the protein level. The fact that there are at least 10 different 5-HT4 receptor isoforms and mRNA expression in both central and peripheral organs, point to the importance of this family of receptors in the maintenance of normal cellular activity. Although 5-HT4 receptor has been reported to be involved in memory and learning as well as gastrointestinal function, less or almost nothing is known about its role in embryogenesis.
The importance of the embryonic serotoninergic system in central nervous and cardiovascular functions has been largely described [15–20]. In early mouse embryogenesis, maternal serotonin (5-HT) activates different 5-HT receptors to control gene expression, migration and proliferation of neuronal crest and neuronal-crest derived cells [21–25]. Although there is enough evidence fortifying the 5-HT hypothesis in embryogenesis, no major abnormalities were found in 5-HT4 receptor knockout mice. One has just observed an attenuated response to stress and novelty as well as hypersensitivity to seizures . There are many knockout and transgenic mouse models with such phenotypes. For instance null mutation of α4 subunit nicotinic cholinergic receptor increases the susceptibility to proconvulsant-induced seizures . This is also the case for 5-HT1A receptor, prion protein, superoxide dismutase, tumor necrosis factor-alpha [32–34] and many others. Blocking the expression
of 5-HT receptors using traditional knockout method gives rise to compensatory mechanism that occur during the constitutive deletion of the receptor throughout the lifespan of the animal. Furthermore, the inherent complexity in the pattern of expression and in the function of some genes and gene products contributes more to compensation and hidden developmental roles . On the contrary by using alternative methods like blocking the expression of the receptor in restricted periods of time one circumvents the compensations . The latter is also possible by modulating the protein activity at the cell membrane rather than blocking the function of the gene. Given the pathophysiological complexity of these conditions and involvement of many other factors one cannot explain these abnormal conditions with the lack of 5-HT4 receptors. One plausible explanation to the latter is the expanded variety of the 5-HT receptor family. Loosing one receptor subtype or isoform probably leads to a compensation mechanism conducted by other members of the family here probably 5-HT6 serotoninergic receptor. The only 5-HT receptor knockout with pathophysiological consequences is the 5-HT2(b) receptor. 5-HT2(b) receptor knockout mice embryos developed cardiac hypoplasia . The receptor's importance in regulating cardiac activity was reported two years later . Our group has shown in pups of BALB/c mice that maternal anti-5-HT4 antibodies caused fetal cardiac hyperplasia and arrhythmia . In the central nervous system, 5-HT4 receptor isoform mRNAs were found in almost all brain structures. However, up to now neither the receptor expression nor its function during the fetal period was studied.
In the first part of this study, we demonstrated the specificity of the anti-peptide antibodies for their corresponding peptide. Anti-5HT4(a) was specific for its isoform as it did not cross react with any of the other receptor isoforms tested, i.e. 5-HT4(b), (c) and (d). On the contrary, 5-HT4(d) antibodies, besides binding its corresponding peptide could also recognize all the other isoforms including 5-HT4(a). This was not due to a cross reactivity but to their specificity, since the C- terminal end of 5-HT4(d) is common between 5-HT4(a), (b), (c) and (d). We report here the presence of cardiac isoforms 5-HT4(a) in embryos from BALB/c mice at GD 8th (data not shown), 12th, 18th and one day postnatal. Since the anti-5-HT4(d) antibodies are able to recognize other already mentioned isoforms, we can only be sure about the expression of 5-HT4(a) in the heart and brain. Whether there are other isoforms expressed during embryogenesis remains elusive. However 5-HT4(a) receptor was found both in fetal brain and heart. Both 5-HT4(a) and 5-HT4(d) receptor isoforms were expressed in brain, at 1 DP, but we could not detect any signal in the pups' heart for the two explored isoforms. One remains, however, puzzled of the receptor's disappearance at 1 DP. This could be explained by the progressive appearance of adrenergic receptors. Indeed, both α- and β-adrenergic receptors are already detected at 12th GD, and, contrary to 5-HT4 receptor, activity of β-adrenergic receptor in mouse heart increases significantly until birth .
In the second part of our study, we observed that all female BALB/c mice immunized with peptide derived from the second extracellular loop of 5-HT4 receptor developed anti-peptide antibodies that could recognize membranes from 5-HT4(g) transfected CHO cells. Although the mice were all fertile, they had to be mated several times before developing a successful pregnancy. All mice in the control group gave birth 21 days post mating. In immunized mice, the number of pups for the first pregnancy was between 5 and 7, which was similar to the control group. However, the number of fetuses of immunized mice decreased from 12th to 18th GD (7 to 2, respectively). This was in accordance with the titer of circulating antibodies in the mothers. The latter was also observed in two littermates, where one mouse gave birth to 2 and the other to 6 pups. Here we observed again that the titer of circulating antibodies for the mother with just two pups was 7 times higher than the mouse with 6 pups.
In this study, we demonstrated that 5-HT4 receptors are expressed already at the 8th GD (in ectoderm), at least for the 5-HT4(a) isoform. We also provided evidence for the functionality of the anti-receptor antibodies, since 1) they decreased fertility as shown by the increased number of mating for a successful litter, 2) they decreased the number of fetuses and pups in relation with the titer of circulating antibodies, 3) they induced morphological and functional abnormalities both in peripheral and central organs. In our earlier study  we demonstrated that anti-5-HT4 antibodies passively transferred from mother to the pups could induce atrioventricular block type I and II (AVB), tremor and skin lesions. Here we confirmed the presence of malformation in some cases and could correlate its incidence with the expression of the receptor and titre of the anti-receptor antibodies. This means that at high concentration of transferred anti-receptor antibodies the chance of malformation is considerably higher, since 21-days post mating either the number of pups decreased or no labour was issued. The effect of anti-5-HT4 antibodies could be through their binding to at least 5-HT4(a) receptor isoform, since it is already expressed from 8th GD. It has been shown that antibodies against the second extracellular loop of the receptor block the effect of serotonin in human cardiomyocytes [10, 28]. In embryonic heart muscle expression of vascular α-actin has been demonstrated. Alpha-actin acts as a precursor in immature contractile structures, when the embryonic heart just starts to beat, and might only gradually be replaced by the mature cardiac isoform [35–37]. For this reason we used α-smooth muscle actin as a marker for heart development. Our observation that myocardium of mouse fetus with circulating maternal anti-5-HT4 antibodies has less or no immunoreactivity for α-actin smooth muscle could be due to delayed maturation of fetal myocardium in immunized animals. However although the presences of anti-Ro52 autoantibodies cross reacting with SEL of 5-HT4 receptors shows a good correlation with the prevalence for neonatal lupus their pathogenic role cannot be generalized. We have shown this in our previous work . This could be due to at one hand that the tested sera were from pretreated pregnant women with lupus. In no studies yet published the blood samples are collected following the same period of pregnancy. Beside the important fluctuation of circulating autoantibodies makes the exploration even harder. On the other hand we don't have any idea about the expression of the 5-HT4 receptor during human embryogenesis. Therefore we cannot draw any general conclusion. In mice (BALB/c) with no known autoimmune genetic background, therefore requiring no treatment, only the presence of 5-HT4 receptor antibodies induced anomalies in fetus, which coincide with the receptor expression. Here we could postulate that the SEL of 5-HT4 receptor could be a pathogenic target for autoantibodies
Although the effect of the antibodies seems to be through the inhibition of the serotonin signal we however have no evidence for its cellular mechanism.
Finally, we also show that immunomodulation of 5-HT4 receptor activity, during embryogenesis, is an honorable alternative to the gene knockout technique. Since the receptor is already expressed and functional, modulating or inhibiting its activity will not activate any genetically compensatory mechanism. Therefore, its activity and physiological role can be explored independently.
Taken together, our results demonstrate that 5-HT4 receptors are crucial for embryonic development of the cardiovascular and central nervous systems in BALB/c mice. Coincidence of anti-receptor antibodies and expression of 5-HT4 receptor results in anatomical abnormalities. Further studies are needed to highlight the function of different receptor isoforms during embryogenesis.
Female adult rabbits (n = 4), female (n = 15) and male (n = 16) BALB/c mice were purchased from Janvier (Tours, France). The animals were kept on a 12:12-h light-dark cycle (lights on at 6:00 AM) at 21 ± 1°C ambient temperature. Water and food were available ad libitum throughout the experiments.
A peptide corresponding to the second extracellular loop of h5-HT4 receptor (G21V) [10, 9], two other peptides corresponding to C- terminal ends of the 5-HT4 isoforms h5-HT4(a) (Y23F) and h5-HT4(d) (C7F) were synthesized as previously described (Table 2) [10, 29]. Their structure and purity were checked by mass-spectrometry and HPLC.
3. Induction of anti-peptide antibodies
Eight female adult rabbits (2.5–2.7 kg) were immunized with either Y23F or C7F peptides. Four female rabbits were immunized with 200 μg of peptide Y23F with 1 mg of mBSA. Prime injections were given in CFA. The peptide C7F was conjugated to BSA using SBA prior to administration to other four rabbits. The peptide was coupled according to the following procedure. All sulphydryl groups on BSA (4 mg) were blocked with 0.5 M iodoacetamide in a coupling buffer (pH = 7) composed of K2HPO4 (100 mM), EDTA (1 mM) and sodium azide (0.02%). The mixture was dialyzed 3 times 1 liter, against the coupling buffer without sodium azide. Thereafter 30 mg (0.13 mmol) of SBA was added to 4 mg of saturated BSA in the presence of 0.25 ml of dimethylformamide and diluted with coupling buffer to the final volume of 3 ml. SBA conjugated BSA was separated from the unconjugated ones on a G-25 Sephadex column (2.2 × 27 cm) and dialyzed against coupling buffer (3 times 1 liter). Finally 3.4 mg/ml of activated BSA molecule was mixed with 1.6 mg of C7F peptide. It was then dialyzed against the coupling buffer and stored at -20°C. Rabbits were immunized with an equivalent of 200 μg of BSA conjugated C7F in presence of CFA. Thereafter rabbits received 3 booster injections with three weeks interval where CFA was replaced with Incomplete Freund Adjuvant (IFA). They were bled and the presence of anti-peptide antibodies was checked.
Female BALB/c mice (n = 5) were immunized with 15 μg/mouse of peptide G21V together with 100 μg/mouse mBSA supplemented with PBS/CFA (50% v/v). Two booster injections were thereafter administered with 3 weeks intervals, where CFA was replaced with IFA. Control mice (n = 5) received just CFA and IFA. Mice were bled one week after each booster injection and sera were tested for the presence of the anti-peptide antibodies.
4. Affinity purification
Y23F was fixed on carboxyl activated ECH Sepharose beads (Amersham Pharmacia, Sweden), and C7F or G21V peptides were fixed on thiol activated Sepharose beads (Amersham Pharmacia, Sweden) according to the standard procedures. Immunoglobulins from the immunized rabbit sera as well as lupus patient plasmaphereses with positive activity on G21V peptide in ELISA, were precipitated with 33% saturated ammonium sulfate and extensively dialyzed over night against PBS. Immunoglobulins were thereafter diluted 10–15 times with filtered PBS, before passing through the affinity columns at 4°C for 3 h. After wash out, the anti-peptide polyclonal antibodies were eluted with 3 M KSCN and dialyzed immediately against 6 liters PBS at 4°C overnight.
5. Enzyme- linked immunosorbent assay (ELISA)
Microtitre plates (Falcon, Canada) were coated with 5 μg/ml of peptide (50μl/well) in coating buffer (carbonate buffer 0.1 M, pH = 9.6) for 1 h at 37°C. After saturation with 1% BSA in PBS supplemented with 0.1% Tween 20 (blocking buffer), the plates were incubated with increasing dilutions (10-2 to 10-6) of sera from immunized and control rabbits for 1 h at 37°C. After several washes with PBS-Tween, the plates were allowed to react with affinity-purified goat anti-rabbit IgG, Fc fragment specific antibodies conjugated to horse-radish peroxidase for 1 h and revealed with 3,3',5,5'-tetramethyl benzidine in the presence of H2O2. Reaction was stopped with 1 N HCl and absorbance was read at 450 nm in a microplate reader. Inhibition immunoassay was performed as previously described . Before using affinity purified rabbit polyclonal anti-5-HT4a and d antibodies in ELISA as described above, the antibodies were diluted 1/12800 in blocking buffer and incubated with decreasing concentrations (10 - 0.04 μg/ml) of either Y23F or C7F respectively for 1 h at 37°C.
6. Cell culture and transfection
Non-transfected CHO cells were cultivated in Dulbecco's Modified Eagle's Medium (DMEM) (Sigma Aldrich) supplemented with 10% bovine fetal serum (BFS), 100 U/ml penicillin, 100 μg/ml streptomycin. The cells were incubated in a humidified incubator at 37°C under an atmosphere of 5% CO2. Semi-confluent CHO cells were transfected as previously described by . Briefly, transfection was performed using a mixture of the vector polyethyleneimine (PEI) and plasmids corresponding to h5-HT4 receptor isoforms (a) or (d) at a ratio of 20 nM PEI/mg of DNA. The cells were incubated for 5 h at 37°C with serum free DMEM supplemented with 100 U/ml penicillin, 100 μg/ml streptomycin. This was followed by incubation in BFS supplemented medium for 24 h.
CHO cells expressing the 5-HT4 receptor isoforms (a) and (d) as well as non-transfected cells were deposited on Superfrost Plus slides and fixed for 2 min in paraformaldehyde 4%. They were washed with PBS-Tween 20 (0.1%) and saturated with PBS supplemented with 0.1% Tween 20 and 0.1% BSA for 1 h at room temperature. They were incubated with 1/200 affinity purified anti-peptide antibodies corresponding to each isoform for 2 h at room temperature followed by 1 μg/ml 4',6-diamidino-2-phenylindole (DAPI) (Sigma Aldrich) and Alexa Fluor 594 goat anti-rabbit IgG (H+L) (Molecular Probes, Oregon, USA) diluted 1/500 for 1 h at the same temperature. The slides were mounted using DAKO Fluorescent mounting medium (DAKO CORPORATION, Carpinteria).
8. Flow cytometry analyses
Stably transfected 5-HT4(g) and M2 muscarinic CHO cells were cultured and growth to confluence separately in DMEM medium supplemented with 10% heat inactivated BFS. Twenty-four hours before experiment, cells were harvested with chilled Trypsin/EDTA washed and dispatched at a concentration of 105 cells/tube in pretreated FACS tubes, in order to prevent cell attachment on the tube wall. Affinity purified rabbit polyclonal anti-G21V (1/500), mouse sera (1/100) and affinity purified anti-G21V (1/50) from a lupus patient were incubated with the cells for 1 h in normal culture condition. Cells were washed 3 times with DMEM. They were then incubated with either FITC-conjugated goat anti-rabbit or FITC-conjugated goat anti-mouse or biotinylated goat anti-human antibodies in DMEM supplemented with 1% goat sera for 1 h in normal culture conditions. After 3 washes only tubes treated with anti-human antibodies were further incubated with FITC- conjugated streptavidin for 30 min in normal culture conditions.
Cells were analyzed by flow cytometry with a FACSCalibur® . At least 10,000 events were acquired for each experiment using the CellQuest 3.3 software (Becton Dickinson, Pont de Claix, France). The data were processed with the same program using analyses option.
9. Reverse Transcription – Polymerase Chain Reaction (RT-PCR) and southern blot
Balb/c mice (n = 8) were mated and the fetuses were collected at 8th, 12th, 16th 18th, 20th GD and 1 DPN. From the same litter, half of fetus was used for RT-PCR and the other half was frozen down for immunohistochemistry (see below). Total RNA was prepared from mouse tissues using the Trizol RNA purification system (Life Technologies Inc.). After purification, RNA was treated with DNase I (Life Technologies Inc.) and 5 μg of total RNA was then hybridized with oligo(dT) primer and reverse transcribed using Superscript reverse transcriptase II (Life Technologies Inc.). The resulting single strand cDNAs were used as templates in PCR reactions using a specific primer pair for the m5-HT4R (5'-CCTGTGCTGTATTTCCCTGG-3'; 5'-GAATGGGGGGTCTTTTGTAG-3'). This primer couple selective of the common part of mouse 5-HT4 receptor isoform allows the amplification of a single DNA band migrating at 688 bp (base pair). The sequence of β-actin primers has been previously described . PCR reactions were performed using the following cycle conditions: denaturation for 1 min at 94°C, annealing for 30 s at 55°C, and extension for 1 min 30 sec at 72°C with a final extension step for 8 min at 72°C. The PCR products were electrophoresed on 1.5% agarose gel containing 0.01% ethidium bromide and photographed under U.V. irradiation. For Southern blot, PCR products were transferred to Zetaprobe membrane (Biorad). The following primer 5'-AGGATGATGAGGAAGGCACG-3' was labeled with 32P using the random primer DNA labeling kit (Life Technologies Inc.), and used as an internal probe for hybridization. Hybridization was performed at 57°C for 12 h in a Church buffer (0.5 M NaHPO4 pH 7.2, 1 mM EDTA, 7% SDS) containing 100 μg/ml of denatured salmon sperm DNA. The membrane was revealed using a phosphoimager (Storm, Molecular Dynamics).
10. Immunofluorescence and histochemistry
Frozen fetuses 12th 18th GD, pups and adult brains were subjected to serial cuts at 5-μm thickness, using a Leica cryostat. After fixing the tissues with 1% paraformaldehyde for 5 min at room temperature, they were permeabilized in PBS at pH 7.4, supplemented with 0.1% Triton-X for 3 min. The tissue was saturated with blocking buffer (PBS, 1% BSA, 0.1%Tween) for 1 h at room temperature. Affinity purified anti-peptide antibodies corresponding to each isoform were incubated for 2 h (1/200). After three washes in PBS, the tissues were incubated for 1 h at room temperature with Alexa Fluor 594 goat anti-rabbit IgG (H+L) (1/500) and visualized under a Leica epifluorescence microscope and photographed.
Fetuses at 12th GD were collected and fixed overnight in 4% formaldehyde/PBS solution. They were then dehydrated according to the standard procedures. Paraffin embedded fetuses were cut on the sagital axis at 4 μm mounted on superfrost plus slides. They were then dried over night at 57°C. Sections were rehydrated and saturated for 1 h in TBS-Tween 0.1% supplemented with 1% goat sera. Slides were thereafter incubated with anti- Actin, α smooth muscle (α-ASM) mouse monoclonal (Sigma-Aldrich St Louis). After three washes Vectastain ABC KIT goat anti-mouse immunoglobulins (Vector laboratories, Burlingame, CA) was used according to the manufacturer recommendations. Finally after extensive washes (5 times in TBS Tween) the bound antibodies were revealed using DAB chromogen KIT (DakoCystomation, Carpinteria, CA). The slides were counter stained with hematoxylin for 20 sec and washed for 15 min under demineralized water. They were then dehydrated and mounted according to the standard procedures. The immunoreactivity in fetal heart was quantified in gray scale using MacBas densitometry program on 5 serial slices and 10 different regions for each slide.
Fetuses at 12th GD were collected and fixed over night in 4% formaldehyde/PBS solution. They were then dehydrated according to the standard procedures. Paraffin embedded fetuses were cut on the sagital axis at 4 μm and subjected to standard hematoxilin/eosin coloration.
List of Abbreviations
Chinese Hamster Ovary
Complete Freund's Adjuvant
congenital heart block
Dulbecco's Modified Eagle's Medium
day post natal
Enzyme- Linked immunosorbent assay
human 5-HT4 receptor
Incomplete Freund Adjuvant
mouse 5-HT4 receptor
methylated bovine serum albumin
neonatal lupus erythematosus
one day postnatal
Reverse Transcription – Polymerase Chain Reaction
second extracellular loop.
RK has a fellowship of the Egyptian government. Part of this work was financed by the Centre National de la Recherche Scientifique, France. The authors would like to thank Dr M. Decossas for her help in histology and improvement of image quality.
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