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Endocrinology. Jan 2009; 150(1): 452–462.
Published online Sep 25, 2008. doi:  10.1210/en.2008-0990
PMCID: PMC2630905

Severe Preeclampsia-Related Changes in Gene Expression at the Maternal-Fetal Interface Include Sialic Acid-Binding Immunoglobulin-Like Lectin-6 and Pappalysin-2

Abstract

Preeclampsia (PE), which affects 4–8% of human pregnancies, causes significant maternal and neonatal morbidity and mortality. Within the basal plate, placental cytotrophoblasts (CTBs) of fetal origin invade the uterus and extensively remodel the maternal vasculature. In PE, CTB invasion is often shallow, and vascular remodeling is rudimentary. To better understand possible causes, we conducted a global analysis of gene expression at the maternal-fetal interface in placental samples from women with PE (n = 12; 24–36 wk) vs. samples from women who delivered due to preterm labor with no evidence of infection (n = 11; 24–36 wk), a condition that our previous work showed is associated with normal CTB invasion. Using the HG-U133A&B Affymetrix GeneChip platform, and statistical significance set at log odds-ratio of B >0, 55 genes were differentially expressed in PE. They encoded proteins previously associated with PE [e.g. Flt-1 (vascular endothelial growth factor receptor-1), leptin, CRH, and inhibin] and novel molecules [e.g. sialic acid binding Ig-like lectin 6 (Siglec-6), a potential leptin receptor, and pappalysin-2 (PAPP-A2), a protease that cleaves IGF-binding proteins]. We used quantitative PCR to validate the expression patterns of a subset of the genes. At the protein level, we confirmed PE-related changes in the expression of Siglec-6 and PAPP-A2, which localized to invasive CTBs and syncytiotrophoblasts. Notably, Siglec-6 placental expression is uniquely human, as is spontaneous PE. The functional significance of these novel observations may provide new insights into the pathogenesis of PE, and assaying the circulating levels of these proteins could have clinical utility for predicting and/or diagnosing PE.

Preeclampsia (PE), a pregnancy complication, is manifested by the onset of hypertension and proteinuria in the second half of pregnancy. PE is relatively common (4–8% of pregnancies), with potentially deadly consequences for the mother and/or her offspring. Currently, the only definitive treatment for this condition is delivery of the placenta, and therefore the infant, accounting for 15% of all preterm births in the United States. Despite decades of research, a full understanding of the pathogenesis of PE remains elusive, hampering the development of better predictive tests, methods of diagnosis, and therapeutic interventions. Nevertheless, it is clear that the placenta plays a central role; the signs of PE can occur in molar pregnancies, which lack a fetus, and the disease resolves once the placenta is delivered. Interestingly, spontaneous PE does not occur in other animals, including nonhuman primates.

Formation of the human placenta entails differentiation of its specialized progenitor cells, termed cytotrophoblasts (CTBs), a process that establishes the architecture of the placenta and the maternal-fetal interface or basal plate (Fig. 11).). During CTB differentiation, progenitors assume one of two fates. In floating villi, they fuse to form multinucleate syncytiotrophoblasts (STBs), whose primary functions are transport and hormone production. In anchoring villi, mononuclear CTBs acquire tumor-like properties that enable them to invade the decidua, the endometrium of pregnancy, and the adjacent third of the myometrium (interstitial invasion). They also breach the small uterine vessels they encounter, completely replacing the resident maternal endothelial lining and intercalating within the muscular walls, a process that involves nearly the entire intrauterine course of arterioles, but only the termini of veins (endovascular invasion). As a result, high-resistance spiral arterioles are transformed into low-resistance, high-capacitance vessels that divert uterine blood flow to the floating villi. This process is most active during 10–20 wk of gestation, before the onset of the clinical symptoms of PE.

Figure 1
Diagram of the human maternal-fetal interface. A, Representation of the human placenta after delivery. The placental surface that was adjacent to the uterine wall is termed the basal plate. The boxed area denotes the region biopsied for these studies. ...

During the last several years, a clearer picture of the pathogenesis of PE has begun to emerge. A two-stage model has been proposed in which the initiating event, poor placentation, is thought to occur early in gestation (1). This concept is supported by several studies that document the association between reduced blood flow to the placenta before 20 wk gestation, as determined by color Doppler ultrasound evaluation of terine arterial blood flow, and a greatly increased risk of developing PE (2,3). Anatomic examination shows that the specific area of the placenta most affected by this syndrome is the basal plate, the site of CTB invasion. Interstitial CTB invasion is often shallow, and endovascular invasion does not proceed beyond the terminal portions of the spiral arterioles. Thus, the maternal vessels do not undergo the complete spectrum of physiological changes that normally occur (e.g. loss of endothelial lining and musculoelastic tissue); the mean external diameter of the myometrial vessels is less than half that of equivalent vessels from uncomplicated pregnancies (4,5,6). In addition, fewer vessels show evidence of CTB invasion (7). Thus, their architecture precludes an adequate response to gestation-related fetal demands for increased blood flow.

The second stage of PE is thought to be the maternal response to abnormal placentation. Systemic endothelial dysfunction appears to be an important common denominator (1,8,9). Recent data point to an imbalance in circulating factors with angiogenic/vasculogenic functions, such as soluble vascular endothelial growth factor receptor-1 (VEGFR-1, sFlt-1), placental growth factor, and the TGF-β receptor endoglin (10,11,12,13,14,15,16).

The studies reported here employed an unbiased approach that required no previous knowledge of the molecules involved to achieve a better understanding of PE pathogenesis. Specifically, we used a microarray platform to evaluate gene expression patterns at a global level in the basal plate region, the area of CTB invasion, of affected individuals compared with control subjects who were matched for gestational age at the time of delivery. The importance of controlling for gestational age is illustrated by our recent work on gene expression changes at the maternal-fetal interface that revealed dramatic differences between migestation and term (17). In this regard, we exploited our recent observation that preterm labor (PTL) without signs of inflammation is associated with normal CTB differentiation/invasion. We did this by using basal plate samples collected from patients who delivered due to this pregnancy complication as gestation-matched controls (18). The results of the current study revealed 55 differentially expressed genes, of which the majority were not known to be dysregulated in severe PE. The results of this work highlight the complex pathophysiology of this condition and the many pathways it impacts.

Materials and Methods

Tissue collection

The University of California, San Francisco (UCSF), Committee on Human Research approved this study. Informed consent was obtained from each parturient tissue donor before delivery. Basal plate biopsy specimens of the maternal-fetal interface were collected from individuals who developed severe PE or PTL without signs of infection. Pregnancies complicated by multiple gestations, fetal anomalies, premature rupture of the membranes, infection, diabetes, or other autoimmune diseases were excluded. Gestational age was determined by using standard dating criteria (19). The basal plate was dissected and processed from the placenta proper as previously described (17). All samples were processed within 2 h of delivery.

Total RNA extraction

RNA was isolated from snap-frozen basal plate specimens using a modified Trizol method as previously described (20). Aliquots were evaluated by using the Agilent RNA 6000 Nano LabChip kit (Agilent Technologies, Santa Clara, CA) on an Agilent Bioanalyzer 2100 system. Capillary electrophoresis data in commaseparated value files were analyzed by using the Degradometer version 1.41 software (available at http://www.dnaarrays.org) (21). Only RNA with a degradation factor of less than 11 was used in subsequent microarray experiments.

Microarray hybridization

The microarray platform was the high-density HG-U133A and HG-U133B GeneChips (Affymetrix, Santa Clara, CA). Sample processing and hybridization was accomplished by using the protocols devised by the UCSF Gladstone (National Heart, Lung, and Blood Institute) Genomics Core Facility (http://www.gladstone.ucsf.edu/gladstone/php/section) as previously described (17). Samples from individual basal plates were analyzed separately. Specifically, the HG-U133A and HG-U133B Affymetrix GeneChips were each hybridized with 15 μg cRNA and then washed, stained, and imaged at the Gladstone Genomics Core Facility by using standard Affymetrix protocols. Data files were deposited in the GEO (Gene Expression Omnibus) data repository (accession no. pending).

Data analysis

The raw image data were analyzed by using GeneChip Expression Analysis software (Affymetrix) to produce perfect match and mismatch values. Subsequently, quality control, preprocessing, and linear modeling were performed using Bioconductor (22), an open source and open development software project based on the R statistical package (http://www.r-project.org). Initial hybridization quality was assessed using the Bioconductor package affyPLM, and slight variations in quality were compensated for during the preprocessing stage, which was performed in two steps. First, we used a Probe Level robust linear model (23) to obtain separate normalized log intensities for each chip (i.e. background subtraction, quantile normalization, and probe set summarization). Second, we applied a global median normalization at the probe set level to all A and B GeneChips (n = 46) and then combined these data into a matrix of log2-based gene expression measures, in which columns corresponded to different cRNA samples, and rows corresponded to the different probe sets.

Estimated log ratios (M value) between cases and controls were determined by using the limma software package in R (24). Then differentially expressed genes were selected by statistical analysis of LOD ratios, and B >0 was set as significant. The results showed that the expression of 55 genes (71 probe sets) was significantly modulated. Then the normalized intensity values for this data set were centered to the median intensity value for each probe set, after which the probe sets were ranked according to their M values (representing fold change) and depicted as a gene expression color map. Evaluation of these filtered probes in the context of both clinical and experimental variables was performed to assess confounding factors.

Quantitative PCR

Reverse transcription of RNA (total) samples isolated from basal plate biopsies was carried out as previously described (17). All templates were amplified with Assay-on-Demand kits (Applied Biosystems, Foster City, CA) or primer/probe sets designed by the UCSF Biomolecular Research Center (see supplemental Table SI, published as supplemental data on The Endocrine Society’s Journals Online web site at http://endo.endojournals.org). Quantitative PCR (Q-PCR) was carried out as previously described (17). Negative control RNA samples were not reverse transcribed or lacked template inputs. Relative quantification was determined by using the standard curve method (see Applied Biosystems User Bulletin no. 2; http://www3.appliedbiosystems.com). In preliminary experiments, we investigated the utility of 11 potential targets as endogenous controls (endogenous control plate; Applied Biosystems). The results showed that the 18S rRNA did not vary with gestational age or disease state. Accordingly, the levels of this transcript were used to obtain normalized values for the target amplicons. A 24-wk PTL sample, the earliest gestational age included in our analysis, was used as the calibrator for the relative quantification using the standard curve method. Results are reported as the relative fold mRNA levels ± sd for each basal plate specimen. The mean values of the PE and PTL samples were compared using a two-tailed Student’s t test (P < 0.05).

Sialic acid binding Ig-like lectin 6 (Siglec-6) immunohistochemistry

Serial sections (5 μm) from formalin-fixed tissues were deparaffinized and rehydrated. Staining was performed as previously published (25). The tissue sections were incubated overnight at 4 C with one of the primary antibodies [anti-Siglec-6 (26), 1:25, or anti-cytokeratin-7 (anti-CK-7) clone OV-TL 12/30 (Dako, Carpinteria, CA), diluted 1:50 in DakoCytomatin antibody diluent (Dako)]. Negative control tissue sections were incubated without primary antibody. Visualization was achieved by incubation with diaminobenzidine for 2 min following the manufacturer’s instructions (Vector Laboratories, Burlingame, CA), and nuclei were counterstained with hematoxylin QS (Vector Laboratories). The tissue sections were imaged by using a Nikon eclipse 80i microscope and photographed with a Q-imaging Retiga 2000R digital camera.

For immunofluorescence staining, frozen sections (5 μm) from OCT-embedded tissues were washed in PBS, and nonspecific reactivity was blocked by incubating with 3% BSA (wt/vol), 0.1% Triton X-100 (vol/vol), and 0.5% Tween 20 (vol/vol) in PBS for 30 min. Then sections were incubated with anti-Siglec-6 (1:100 in blocking buffer) for 1 h and washed in PBS. Negative controls were incubated without the primary antibody. Sections were then exposed to anti-CK-7 [clone 7D3 (27), 1:100] for 1 h and washed in PBS as described above. The sections were then incubated with Alexa Fluor 594-conjugated goat antimouse IgG (1:1000; Molecular Probes Inc., Eugene, OR) and fluorescein isothiocyanate-labeled donkey antirat IgG (1:200; Jackson ImmunoResearch Laboratories, West Grove, PA) antibodies for 30 min and washed in PBS. Tissue sections were mounted in Vectashield containing 4′,6-diamidino-2-phenylindole (Vector Laboratories), which allowed visualization of the nuclei. Imaging was done using a Leica DM 5000B fluorescence microscope equipped with a Leica DFC 350FX digital camera (Leica Microsystems, Bannockburn, IL; and Leica-Camera, Solms, Germany).

Generation of antihuman pappalysin-2 (PAPP-A2) polyclonal serum

A fragment encoding Ser-234 to Gln-495 of PAPP-A2 was amplified by PCR using the plasmid pPA2 as the template (28). The forward primer was 5′-CGATAGATCTATCGAGGGTAGGAGTCCACCGGAGGAAAGCAAC-3′ (a BglII site is underlined, and a factor Xa recognition site is shown in bold). The reverse primer was 5′-CGATAAGCTTTCACTGCAAAGGCGACAGAATCTC-3′ (a HindIII site is underlined, and a stop codon is shown in bold). The 821-bp product was digested with BglII and HindIII, and cloned into the BamHI/HindIII sites of the Escherichia coli expression vector pT7H6UB (29). The resulting construct, pT7H6UBFX_P2_234–495, encoded a fusion protein with an N-terminal hexa-His tag followed by residues 2–76 of human ubiquitin, a factor Xa recognition site, and residues 234–495 of human PAPP-A2. E. coli strain BL21 (DE3), transformed with pT7H6UBFX_P2_234–495, was grown at 37 C to an OD600 of approximately 0.8. Expression was induced by the addition of isopropyl-β-d-thiogalactopyranoside to a final concentration of 1 mm. After 4 h at 37 C, the bacteria were harvested by centrifugation, resuspended in 20 mm Tris-HCl, 0.5 m NaCl (pH 8.0) (buffer A), and disrupted by sonication, and the lysate was centrifuged. The pellet containing inclusion bodies was washed three times with buffer A containing (2 m urea and 2% Triton X-100), and then redissolved in buffer A containing 8 m urea, and loaded onto a Ni-NTA column (GE Healthcare, Piscataway, NJ). Bound protein was eluted with buffer A containing 8 m urea and 20 mm EDTA and refolded by rapid dilution into 0.4 m l-arginine, 10 mm Tris-HCl (pH 8.0) (30). After concentration, centrifugation, and dialysis into 20 mm Tris-HCl, 100 mm NaCl (pH 8.0), the fusion protein was cleaved with factor Xa (Sigma Chemical Co., St. Louis, MO) to remove the ubiquitin domain. The recombinant PAPP-A2 domain was purified by ion-exchange chromatography on Q-Sepharose (GE Healthcare). Rabbits were immunized with antigen dissolved in Freund’s complete adjuvant (Sigma) essentially as described (31). Freund’s incomplete adjuvant (Sigma) was used as the vehicle in subsequent injections. Serum was collected at 6-wk intervals.

Immunoblot analysis

To assess the specificity of the anti-PAPP-A2 sera, reduced recombinant PAPP-A2 (0.1 ng), human term pregnancy serum (1 μl), purified PAPP-A/pro-major basic protein complex (0.1 μg) (32), and reduced recombinant PAPP-A (0.1 μg) (33) were separated on a 7% TA Novex gel under reducing conditions and transferred to a polyvinylidene difluoride (PVDF) membrane. Blocking was performed in 2% Tween 20 for 20 min. The membrane was incubated overnight at 4 C with either anti-PAPP-A2 serum or the preimmune serum (1:10,000 dissolved in blocking buffer). After washing, the membrane was incubated with a secondary antibody (antirabbit IgG Dako P217; 1:4000 dissolved in blocking buffer) for 1 h at room temperature. To evaluate placental expression, 10 μg of a protein lysate prepared in RIPA lysis buffer [50 mm Tris-HCl (pH 7.4), 150 mm NaCl, 0.5% Na deoxycholate (wt/vol), 0.1% sodium dodecyl sulfate (wt/vol), and 1% Nonidet P-40 (vol/vol)] supplemented with 1% protease inhibitor cocktail (Sigma) from basal plate specimens obtained from PE (n = 10) or PTL (n = 9) patients was separated on a 4–20% gradient SDS-PAGE gel and transferred to a PVDF membrane. After blocking nonspecific reactivity by incubating in 5% BSA in Tris-buffered saline with Tween 20 (10 mm Tris base; 150 mm NaCl; and 0.05% Tween 20, pH 8.4) for 1 h at room temperature, the membrane was incubated with anti-PAPP-A2 serum (1:10,000 diluted in blocking buffer) overnight at 4 C, washed with Tris-buffered saline with Tween 20, followed by incubation with antirabbit horseradish peroxidase-conjugated antibody (1:10,000 dissolved in blocking buffer) for 1 h at room temperature. Immunoreactive bands were visualized by using chemiluminescence (Pierce, Rockford, IL) and exposure to XR film (Kodak, Rochester, NY). To evaluate for protein loading, the PVDF membrane was stripped and incubated with anti-actin antibody (C4, 1:10,000; Sigma) for 1 h at room temperature and then processed as described above. Densitometry was performed using QuantityOne software (Bio-Rad, Hercules, CA).

PAPP-A2 immunohistochemistry

Serial sections (5 μm) were deparaffinized and rehydrated before antigen retrieval in citrate buffer (pH 6.8) for 20 min at 95–100 C. Endogenous peroxidase activity was quenched as described above. Nonspecific reactivity was blocked by incubating the tissue sections with preimmune serum (ABC Elite kits; Vector Laboratories) according to the manufacturer’s instructions before overnight exposure at 4 C to a primary antibody dissolved in blocking buffer at the concentrations indicated [anti-PAPP-A2, 1:30,000; anti-PAPP-A (34), 1:50; anti-CK-7, 1:100; and anti-HLA-G, 1:50 (35)]. Binding of the primary antibody was detected by incubating the tissue sections in a solution of the species-appropriate secondary antibodies provided in the ABC Elite kits (Vector Laboratories). Control sections were incubated without primary antibody. Visualization, counterstaining, and imaging were performed as described above.

Results

Using a microarray approach to analyze the basal plate regions of placentas obtained from women who experienced PTL (n = 11) and women whose pregnancies were complicated by PE (n = 12), we identified 55 genes (71 probe sets) that were differentially expressed (Fig. 22).). Inclusion criteria for the PTL patients included a singleton pregnancy, no evidence of infection (clinical or histological), absence of premature rupture of membranes, and no significant maternal disease. Approximately one third of the women delivered due to a diagnosis of cervical insufficiency. The PE patients fulfilled the criteria for PE as defined in a 2002 American College of Obstetrics and Gynecology bulletin (36). Clinical characteristics of the two groups are compared in Table 11.. The majority of PE patients (10 of 12) experienced labor (with or without a cesarean section). Although more patients with PE (n = 6) than with PTL (n = 2) delivered by cesarean section, post hoc statistical analyses of the data showed that the method of delivery was not a confounding factor. The characteristics that defined the PE group are shown in Table 22.. Among the most highly up-regulated genes were those that encode molecules whose expression was previously reported to be elevated in PE: leptin, VEGFR-1 (the gene that is alternatively spliced to produce sFlt-1), and CRH. In addition, this analysis revealed more than 40 novel transcripts, which to our knowledge have not previously been reported to be differentially expressed in PE or associated with the pathogenesis of this syndrome.

Figure 2
Heat map of differentially expressed genes in basal plate of PE placentas compared with controls. The normalized log intensity values for 71 differentially expressed probe sets were centered to the median value of each probe set and colored on a range ...
Table 1
Clinical characteristics of study groups
Table 2
Characteristics of patients with severe PE (n = 12)

To rule out the possibility that clinical and/or experimental variables accounted for the observed expression patterns, we searched for possible correlations. We found no association with gestational age, parity, labor, maternal age, or fetal sex. Additional variables that were considered included time between betamethasone injection and delivery, other common medications that are used in these patient populations, mode of delivery, criteria for diagnosing PE, RNA degradation value, and tissue-processing time. Furthermore, we evaluated the level of the CTB-specific molecule CK-7, which is expressed by only trophoblasts, to determine that differences did not simply reflect different ratios of fetal to maternal tissue contribution to the samples. CK-7 levels were not different between the PE and PTL samples or with gestational age.

Next, we used a Q-PCR approach to validate the expression patterns of nine genes. They included those genes whose normal expression patterns are known to be dysregulated in PE, which served as positive controls, and a number of novel molecules over a range of expression. The results of these experiments confirmed the expression patterns observed in the microarray analysis; all were statistically significant except hydroxysteroid (17-β) dehydrogenase 1, which showed the same trend (Fig. 33).). For comparison, the microarray data are plotted as insets to the individual bar graphs. In accordance with many published studies that have profiled the expression patterns of individual molecules in PE (37,38), considerable variation among individual samples was apparent. This may be a reflection of the heterogeneity of the fetal and maternal responses to PE.

Figure 3
Q-PCR confirmation of a subset of differentially expressed genes. Total RNA isolated from basal plate biopsies obtained from PTL (24–36 wk; n = 10) or PE (24–36 wk; n = 10) was analyzed using TaqMan primer/probe sets. Relative ...

To further validate our findings, we performed a more extensive, protein-level analysis of novel molecules with PE-associated alterations in their expression patterns that potentially could play important functions in the pathogenesis of this syndrome. In particular, we focused on Siglec-6, which was originally cloned on the basis of its ability to bind leptin (26). Interestingly, leptin expression, known to be dysregulated in PE (39,40,41,42,43,44), was also markedly increased in our microarray data set (Fig. 22),), but the classical leptin receptors (e.g. ObR) were not. By applying an immunohistochemical approach, we showed that Siglec-6 expression was in general more pronounced in the basal plate of PE samples (n = 14) than in the PTL controls (n = 11, Fig. 4A4A).). Additionally, we found that PE was associated with a dramatic increase in STB expression of Siglec-6, particularly earlier in gestation (<33 wk), when there was no immunoreactivity in the PTL samples and clear immunoreactivity in PE.

Figure 4
Siglec-6 expression in basal plate of PE as compared with gestation age-matched, control PTL samples. A, Serial sections of formalin-fixed, paraffin-embedded biopsy specimens of the placental basal plate (BP) region along with floating chorionic villi ...

To identify the cells in the basal plate region that displayed Siglec-6 immunoreactivity, we costained tissue sections from PE (n = 5) and from PTL controls (n = 4) with anti-CK-7, which identifies all subsets of fetal trophoblasts, and with anti-Siglec-6. The results are shown in Fig. 4B4B.. At 24 wk gestation, the PTL tissue exhibited no immunoreactivity. At the same gestational age, STBs and a subset of the maternal cells stained with anti-Siglec-6 in the PE specimens. By 30 wk, the PTL samples showed STB and rare maternal cell staining, reminiscent of the pattern in PE at 24 wk. The fact that Siglec-6 was detected by immunofluorescence at this gestational age and not by immunohistochemistry likely reflects differences in the sensitivity of the methods. In PE tissue at 30 wk, a subset of invasive CTBs also expressed the Siglec-6 antigen. At 36 wk, PTL samples displayed invasive CTB Siglec-6 staining for the first time and continued to have more STB expression. However, the PE sample at 36 wk still showed more Siglec-6 staining overall. This staining pattern may reflect some aspects of premature placental aging or degenerative changes in PE.

We also validated PAPP-A2 expression at the protein level because of its known role in IGF biology, which plays important roles in implantation and placentation. Because antibodies that specifically reacted with PAPP-A2 were not commercially available, we produced a fragment of the PAPP-A2 protein in E. coli and used it to immunize rabbits for the purpose of producing polyclonal antibodies. The PAPP-A2 serum, which specifically recognized PAPP-A2, did not cross-react with PAPP-A, the only homolog of PAPP-A2 encoded by the human genome (Fig. 55).). In subsequent experiments, we used this antibody for immunoblot analyses, which confirmed increased levels of PAPP-A2 in the PE basal plate compared with PTL (Fig. 66).). Densitometry of PAPP-A2 normalized to actin showed roughly a 4.5-fold-higher mean expression in PE compared with PTL (31.35 ± 11.5 vs. 7.05 ± 3.53; P = 0.00001).

Figure 5
Immunoblotting showed that the anti-PAPP-A2 polyclonal rabbit antiserum reacted with a single band of the appropriate molecular weight. A, The samples were reduced and separated on a 7% TA Novex gel: lane 1, supernatant from mock-transfected HEK293 cells; ...
Figure 6
PAPP-A2 protein levels were increased in the basal plate portion of placentas from PE pregnancies. Protein lysates from basal plate biopsy samples (10 μg) of PTL (n = 9) and PE (n = 9) placentas were separated by SDS-PAGE (4–20% ...

We used an immunolocalization approach to assess PAPP-A2 expression at the maternal-fetal interface. Serial tissue sections of the human basal plate region from PE (n = 6) and gestational age-matched controls (n = 6) were stained for CK-7 to identify trophoblasts, for HLA-G to identify invasive CTBs, or for PAPP-A2. In both PTL and PE samples, invasive CTBs stained with anti-PAPP-A2. However, at least for the earliest gestational ages, the intensity appeared stronger in the PE samples. For all gestational ages, the PAPP-A2 immunoreactivity of STBs was much more prominent in the PE samples (Fig. 77).). Of note, this pattern was distinct from that obtained for PAPP-A expression, which was observed in association with STBs but not invasive CTBs (data not shown).

Figure 7
PE is associated with enhanced staining of invasive CTBs and STBs for PAPP-A2. Serial sections of formalin-fixed, paraffin-embedded basal plate biopsy specimens of placentas from PTL (A–C and G–I) and PE (D–F and J–L) at ...

Discussion

We characterized gene expression profiles at a global level in the basal plate portion of human placentas from pregnancies complicated by preterm PE compared with PTL controls. We were particularly interested in this region because it contains the CTB subpopulations that carry out interstitial and endovascular invasion, the processes that go awry in PE with very dramatic effects associated with the severest forms of this syndrome. Using LOD ratios of B >0, we found that 55 genes were differentially expressed in severe PE. This list included molecules that were previously reported to be present at higher than normal levels in maternal serum, chorionic villi, and/or cord blood in pregnancies complicated by PE, a finding that gives added confidence to the novel genes that we identified as similarly regulated. However, even for these previously reported molecules, in most cases, this is the first description of their increased expression in the basal plate region of the placenta. At the RNA level, we used a Q-PCR approach to confirm the expression patterns of nine of the differentially regulated genes; two of the most interesting novel observations, enhanced Siglec-6 and PAPP-A2 expression, were also validated at the protein level. In toto, these results suggest fundamental alterations in important biological processes including pathways that are regulated by leptin and IGF signals.

Other investigators have characterized (human) placental gene expression patterns in PE compared with normal pregnancy. For example, Reimer et al. (45) demonstrated a large increase in leptin production by chorionic villi. A small-scale study with the same design found a PE-associated up-regulation of the gene encoding the muscle subunit of glycogen phosphorylase (46); an increase in the expression of the soluble form of the disintegrin, ADAM 12, was discovered in the same way (47). The fact that we did not identify the latter two molecules is likely attributable to differences in the cellular composition of our samples, which contained a much smaller proportion of chorionic villi and, consequently, STBs that release a myriad of substances into maternal blood. Other relevant considerations include the fact that the causes of PE are complex. For example, in some instances, poor placentation leads to intrauterine growth restriction without PE (7). Conversely, abnormal maternal responses may occur even if placentation is normal (48,49). In this case, the metabolic, vascular, and immunological demands of pregnancy may synergize with the effects of preexisting medical conditions, such as arterial disease, hypertension, obesity, and diabetes, which in turn strongly predispose women to develop PE. Interestingly, many of these disease processes are associated with altered leptin levels. Numerous investigators have reported a PE-associated increase in circulating levels of leptin (39,40,41,42,43,44,50,51,52), and a leptin gene polymorphism has been linked to an increased risk of developing this pregnancy complication (53). However, a clear picture of how an increase in leptin expression is linked to the pathophysiology of PE has yet to emerge.

Our data demonstrated increased leptin expression in the basal plate of PE placentas compared with control tissue. Interestingly, although the classic leptin receptors were not differentially expressed, we observed elevated levels of the mRNA that encodes Siglec-6, a transmembrane protein that also binds leptin. These findings suggest that this molecule may play an important role as a placental leptin receptor and that increased Siglec-6 levels could contribute to the pathogenesis of PE. Although the cloning strategy for Siglec-6 was based on its ability to interact with leptin, the other Siglec family members bind sialic acid-containing glycans. Some family members have exquisite ligand specificity, whereas others have less rigid structural requirements (54). For example, Siglec-6 has binding specificity for the sialyl-Tn epitope (Siaα2–6Gal-NAcα1-O-R, where R is a serine or threonine). Published data suggest that in the placenta, leptin is a Siglec-6 ligand, but the endogenous binding partners have yet to be identified (26). Additionally, Siglec-6 expression has other interesting features. For example, in humans, it is restricted to the placenta and B lymphocytes. In other species, including nonhuman primates, placental cells lack Siglec-6 expression, whereas the B cells exhibit it (25). The fact that Siglec-6 is expressed only in human placentas and not in nonhuman primate placentas (25) is extremely intriguing, because PE is thought to be a uniquely human disease; spontaneous PE has not been reported in other animals, even nonhuman primates (55).

The function(s) of Siglec-6 in any biological system, including B cells, have yet to be determined. A recent study suggested a role for Siglec-6 in term labor (25). Interestingly, Siglec-6 expression was not observed in our earliest gestation PTL samples but did increase as pregnancy advanced. Siglec-6 has conserved immunoreceptor tyrosine-based inhibitory motif (ITIM) and ITIM-like domains, suggesting that ligand binding results in tyrosine phosphorylation and recruitment of Src homology domain-containing adapters with inhibitory effects. The effect of leptin has been investigated in mice and bats (56,57). In both species, leptin appears to have a stimulatory effect on trophoblast invasion in vitro, but its mechanism of action is not known. Determining whether leptin and Siglec-6 function during human CTB invasion is a critical first step in understanding whether these molecules play a functional role in PE pathogenesis.

A PE-associated increase in the expression of PAPP-A2 was another novel observation that emerged from our work. This is the first report of PAPP-A2 expression at the protein level in the placenta, experiments that were made possible by the generation of rabbit polyclonal antibodies that specifically reacted with this molecule. PAPP-A2, which has 46% sequence identity with PAPP-A, is a metalloproteinase that cleaves IGF-binding protein-5 (IGFBP-5) (28). Depending on the cellular context, IGFBPs either stimulate or inhibit IGF-I and -II actions. For example, in smooth muscle cells, IGFBP-5 binds to extracellular matrix components, including thrombospondin-1 and osteopontin, and potentiates the cells’ response to IGF-I. In contrast, IGFBP-4, a PAPP-A substrate, largely inhibits IGF actions (58). In a fibroblast model, an increase in IGFBP-5 proteolysis attenuates its stimulatory effects on cell migration (59). If CTBs respond in an analogous manner, then the observed PE-associated increase in PAPP-A2 levels could inhibit CTB invasion by mechanisms that include an increase in IGFBP-5 proteolysis.

Data suggest that there is cross talk between the (classical) leptin and insulin signaling pathways (60). Specifically, suppressor of cytokine signaling (SOCS-3) attenuates both pathways (61). Furthermore, related Siglec family members’ (CD33/Siglec-3 and Siglec-7) responses are blocked by SOCS-3 via accelerated proteasomal degradation (62,63). Given that IGF and insulin signal through the same pathways, it is possible that alterations in leptin, Siglec-6, and PAPP-A2 levels may be working in concert to exert inhibitory effects on CTB invasion. Understanding the pregnancy-related functions of these molecules and the other genes that were differentially expressed in PE will likely lead to a better understanding of the pathogenesis of this human-specific condition, the crucial first step in the rational design of treatments (both preventative and therapeutic) that address the causes, rather than the consequences, of this pregnancy complication. Additionally, we envision that PE-associated alterations in the expression of gene products that are secreted in maternal blood can be used to predict a woman’s risk of developing this syndrome and/or enable diagnosis during the early stages of PE when the signs are sometimes ambiguous.

Supplementary Material

[Supplemental Data]

Acknowledgments

We thank Ms. Jean Perry, the study research nurse coordinator, who assisted in tissue collection, as did the nurses, residents, and faculty at the UCSF Birth Center. Dr. Chris Barker, Dr. Chandi Griffin, and Ms. Jennifer Gregg, members of the UCSF Gladstone (National Heart, Lung, and Blood Institute) Shared Microarray Facility, contributed invaluable technical and intellectual expertise. We also thank Dr. David Erle and Michael Salazar, members of the Sandler Genomics Core Facility, and Nissi Varki (University of California, San Diego) for helpful discussions. We are grateful to Mr. Evan Messenger and Dr. Kathy Ivanetich, UCSF Biomolecular Research Center, for technical assistance in performing the Q-PCR experiments. We thank Ms. Mary McKenney for critical review of the manuscript.

Footnotes

This work was supported by National Institutes of Health Grants R01 HL 64597 (to S.J.F.), R01 GM 32373 (to A.V.), 5-MO1-RR 00083 (General Clinical Research Center, San Francisco General Hospital), HL 072301 (UCSF, National Heart, Lung, and Blood Institute Shared Microarray Facility), and the Danish Medical Research Council (to C.O.). V.D.W. is a March of Dimes Reproductive Scientists Development Program scholar (National Institute of Child Health and Human Development 5K12HD00849) and an American Board of Obstetrics and Gynecology/American Association of Obstetricians and Gynecologists Foundation scholar.

Disclosure Statement: M.G., A.G., K.K.-S., A.K., K.R., R.-F.Y., M.T.O., and A.V. have nothing to declare. V.D.W., R.H.-K., and S.J.F. have a patent filed for the array data. V.D.W., C.O., and S.J.F. have research grants as listed above.

First Published Online September 25, 2008

Abbreviations: CTB, Cytotrophoblast; IGFBP, IGF-binding protein; PAPP-A2, pappalysin-2; PE, preeclampsia; PTL, preterm labor; PVDF, polyvinylidene difluoride; Q-PCR, quantitative PCR; Siglec-6, sialic acid binding Ig-like lectin 6; SOCS, suppressor of cytokine signaling; STB, syncytiotrophoblast; VEGFR, vascular endothelial growth factor receptor.

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