Ca2+-binding protein of the human placenta. Characterization, immunohistochemical localization and functional involvement in Ca2+ transport

Bone development and mineral homeostasis in the fetus and neonate: roles of the calciotropic and phosphotropic hormones

Mineral and bone metabolism are regulated differently in utero compared with the adult. The fetal kidneys, intestines, and skeleton are not dominant sources of mineral supply for the fetus. Instead, the placenta meets the fetal need for mineral by actively transporting calcium, phosphorus, and magnesium from the maternal circulation. These minerals are maintained in the fetal circulation at higher concentrations than in the mother and normal adult, and such high levels appear necessary for the developing skeleton to accrete a normal amount of mineral by term. Parathyroid hormone (PTH) and calcitriol circulate at low concentrations in the fetal circulation. Fetal bone development and the regulation of serum minerals are critically dependent on PTH and PTH-related protein, but not vitamin D/calcitriol, fibroblast growth factor-23, calcitonin, or the sex steroids. After birth, the serum calcium falls and phosphorus rises before gradually reaching adult values over the subsequent 24-48 h. The intestines are the main source of mineral for the neonate, while the kidneys reabsorb mineral, and bone turnover contributes mineral to the circulation. This switch in the regulation of mineral homeostasis is triggered by loss of the placenta and a postnatal fall in serum calcium, and is followed in sequence by a rise in PTH and then an increase in calcitriol. Intestinal calcium absorption is initially a passive process facilitated by lactose, but later becomes active and calcitriol-dependent. However, calcitriol's role can be bypassed by increasing the calcium content of the diet, or by parenteral administration of calcium.

Calcium Homeostasis in Human Placenta: Role of Calcium‐Handling Proteins

The human placenta is a transitory organ, representing during pregnancy the unique connection between the mother and her fetus. The syncytiotrophoblast represents the specialized unit in the placenta that is directly involved in fetal nutrition, mainly involving essential nutrients, such as lipids, amino acids, and calcium. This ion is of particular interest since it is actively transported by the placenta throughout pregnancy and is associated with many roles during intrauterine life. At term, the human fetus has accumulated about 25-30 g of calcium. This transfer allows adequate fetal growth and development, since calcium is vital for fetal skeleton mineralization and many cellular functions, such as signal transduction, neurotransmitter release, and cellular growth. Thus, there are many proteins involved in calcium homeostasis in the human placenta.

Cytotoxicant-induced trophoblast dysfunction and abnormal pregnancy outcomes: role of zinc and metallothionein

Normal trophoblast function, including implantation, hormone production, and formation of the selectively permeable maternofetal barrier, is essential for the establishment and maintenance of the fetoplacental unit and proper fetal development. Maternal cytotoxicant exposure causes the destruction of these cells, especially the terminally differentiated syncytiotrophoblasts, and results in a myriad of poor pregnancy outcomes. These outcomes range from intrauterine growth retardation and malformation to spontaneous abortion or stillbirth. There is recent evidence that the metal-binding protein, metallothionein, is involved in the protection of human trophoblastic cells from heavy metal-induced and severe oxidative stress-induced apoptosis. Metallothionein, with its unique biochemical structure, can both bind essential metal ions, such as the transcription modulator zinc, and yet allow their ready displacement by toxic nonessential metal ions or damaging free radicals. These properties suggest that metallothionein may be responsible not only for sequestering the cytotoxic agents, but also for altering signal transduction in the affected cells. Here, we review several identified causes of adverse pregnancy outcomes (specifically, prenatal exposure to cigarette smoke and alcohol, gestational infection, and exposure to environmental contaminants), discuss the role of zinc in modulating the cellular response to these toxic insults, and then propose how metallothionein may function to mediate this protective response.

Estrogenic endocrine disruptive components interfere with calcium handling and differentiation of human trophoblast cells

During development, calcium (Ca) is actively transported by placental trophoblasts to meet fetal nutritional and the skeletal mineralization needs. Maternal exposure to estrogenic pesticides, such as 1,1-bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT) and methoxychlor (MTC), has been shown to result in reproductive disorders and/or abnormal fetal development. In this study, we have examined the effects of exposure of trophoblastic cells to MTC and DTT, in comparison to 17beta-estradiol (E2) and diethylstilbestrol (DES), to test the hypothesis that cellular Ca handling is a target for these endocrine disruptive components. Treatment with DDT, MTC, DES, or E2 increased cellular Ca uptake, and the expression of trophoblast-specific human Ca binding protein (HCaBP) was down-regulated by both MTC and DDT. Treatment with MTC, DDT, and DES inhibited cell proliferation, induced apoptosis, and suppressed expression of several trophoblast differentiation marker genes. These effects were reversed by overexpression of metallothionein IIa, a gene highly responsive to cadmium and other metals. These results strongly suggest that trophoblast Ca handling functions are endocrinally modulated, and that their alteration by candidate endocrine disruptors, such as MTC and DDT, constitutes a possible pathway of the harmful effects of these components on fetal development.

Lead enters Rcho-1 trophoblastic cells by calcium transport mechanisms and complexes with cytosolic calcium-binding proteins

Within the placenta, a specialized Ca(2+) transport pathway develops in trophoblasts to promote growth of the fetus and hypothetically to enhance fetal uptake of Pb(2+). This hypothesis could not be tested until a method to monitor Pb(2+) influx by indo-1 fluorescence quench became available. We have applied this new method to cultured undifferentiated and differentiated Rcho-1 trophoblastic cells. Pb(2+) concentrations of 1 and 10 microM are equivalent to blood levels of 20 and 200 microg/dl in pregnant women. Over this range, Pb(2+) uptake increased with time and concentration in medium containing 1 mM Ca(2+) but was greater in Ca(2+)-omitted solutions. Activation of capacitative Ca(2+) entry (CCE) with thapsigargin, an endoplasmic reticulum (ER) Ca(2+) pump inhibitor, increased Pb(2+) uptake, while inhibition of CCE by La(3+) decreased influx. Parathyroid hormone-related peptide (PTHrP) stimulates the synthesis of Ca(2+)-binding proteins (CaBPs), as well as Ca(2+) transporters, during trophoblastic differentiation. Pretreatment for 72 h with PTHrP increased Pb(2+) uptake by undifferentiated Rcho-1 cells but had little effect on the quench in differentiated cells, probably due to their greater content of CaBPs which competed for Pb(2+)-binding with indo-1. This competition was most evident in differentiated cells when 1 microM Pb(2+) caused an initial quench, followed by a rise in fluorescence. This rise was not inhibited by thapsigargin, thereby ruling out sequestration into the ER and leaving complexation of Pb(2+) by CaBPs as the most plausible interpretation. We conclude that trophoblasts have the ability to clear Pb(2+) from the maternal circulation and deliver it to the fetus.

Calcium-binding proteins: distribution and implication in mammalian placenta

During gestation, transport by the placenta is solely responsible for nutrient supply to the developing fetus. In this context, calcium (Ca2+) transport machinery of the placenta thus represents the primary tissue site for regulating fetal Ca2+ homeostasis. In humans, the transplacental movements of Ca2+ increase dramatically during the last trimester of gestation, when fetal skeletal mineralization is at its highest. However, little is known about the exact mechanism of transport. Evidence suggests that some developmentally expressed cytosolic Ca(2+)-binding proteins (CaBPs) have an important role in regulating or shuttling cytosolic Ca2+ since they are endowed with a high affinity for Ca2+ (approximately 10(6) M(-1)). CaBPs belong to a large family of eukaryotic proteins containing a specific helix-loop-helix structure, referred to as the EF-hand motif, which counts more than 200 members. Several of these CaBPs were identified in the placenta: CaBP9k, CaBP28k, CaBP57k, oncomodulin, S-100P, S-100alpha, and S-100beta. This review discusses the current views in this field to guide future investigations into the localization and functions of CaBPs during Ca2+ intracellular homeostasis in the placenta.

Placental calcification: a metastatic process?

Placental calcification commonly increases with gestational age. The mechanism of apatite mineralization probably involves one of three known mechanisms of tissue calcification: physiological (like bone), dystrophic (ischaemia-related) or metastatic (mineralization in a supersaturated environment). This study was designed to determine the mechanism of calcification by examining (1) the mineral content of placental calcifications in comparison to other physiological and pathological apatites, and (2) the expression of bone morphogenetic proteins (BMPs), which are important in physiological calcification, across gestational age. By energy-dispersive x-ray analysis (EDXA), the Ca/P weight ratio for apatitic mineral from mature calcifications was 2.00+/-0.05 (s.e.), which is similar to that for stones formed in a metastatic, supersaturated environment and lower than that observed in physiological calcification. Biologically active BMP, which was determined by bioassay, was demonstrated in mature and postmature placentae. The BMPs PLAB, PDF and related protein INSL-4 were identified by semiquantitative reverse transcriptase polymerase chain reaction (RT-PCR), but their mRNA expression was independent of gestational age (7-41 weeks of gestation). We conclude that (1) the identified BMPs were not related directly to placental calcification, which argues against physiological calcification, and (2) the chemical composition of the apatitic mineral was suggestive of rapid formation in a supersaturated environment, which is consistent with a metastatic mechanism of calcification.

Calcium transport across the dental enamel epithelium

Dental enamel is the most highly calcified tissue in mammals, and its formation is an issue of fundamental biomedical importance. The enamel-forming cells must somehow supply calcium in bulk yet avoid the cytotoxic effects of excess calcium. Disrupted calcium transport could contribute to a variety of developmental defects in enamel, and the underlying cellular machinery is a potential target for drugs to improve enamel quality. The mechanisms used to transport calcium remain unclear despite much progress in our understanding of enamel formation. Here, current knowledge of how enamel cells handle calcium is reviewed in the context of findings from other epithelial calcium-transport systems. In the past, most attention has focused on approaches to boost the poor diffusion of calcium in cytosol. Recent biochemical findings led to an alternative proposal that calcium is routed through high-capacity stores associated with the endoplasmic reticulum. Research areas needing further attention and a working model are also discussed. Calcium-handling mechanisms in enamel cells are more generally relevant to the understanding of epithelial calcium transport, biomineralization, and calcium toxicity avoidance.

Functional analysis of placental 57-kDa Ca(2+)-binding protein: overexpression and downregulation in a trophoblastic cell line

The placental trophoblastic epithelium functions to transport nutrients needed by the fetus, including calcium, which is required in the greatest amounts during the last third of pregnancy when the majority of fetal skeletal mineralization occurs. The mechanism of placental calcium transport and the developmental changes in the trophoblast that facilitate this process are currently incompletely understood. We have previously identified a 57-kDa, Ca(2+)-binding protein (CaBP) functionally implicated in placental calcium transport and trophoblast differentiation. In this study we have directly examined the role of CaBP in these processes by (1) recombinantly overexpressing CaBP in an inducible manner and (2) downregulating CaBP expression using antisense technology, using the rat choriocarcinoma cell line Rcho-1 as a trophoblastic cell model system. Our results show that overexpression of CaBP stimulates both cellular calcium uptake and vectorial calcium transport activities in Rcho-1 cells. Those cells stably expressing CaBP also exhibit higher levels of steady-state intracellular calcium and enhanced calcium-buffering ability. In addition, prolonged overexpression of CaBP in Rcho-1 cultures promotes trophoblast differentiation. Conversely, downregulation of CaBP expression had a negative effect on calcium uptake, calcium transport, and trophoblast differentiation in Rcho-1 cells. These data indicate that CaBP plays a direct role in placental calcium transport, functioning both as an intracellular calcium buffer and as a shuttle. These results also support a more direct role for CaBP in the trophoblast differentiation pathway.

Placental 57-kDa Ca(2+)-binding protein: regulation of expression and function in trophoblast calcium transport

During gestation, transport by placental trophoblasts is solely responsible for nutrient supply to the developing fetus. The calcium (Ca) transport machinery of the placenta thus represents the primary tissue site for regulating fetal Ca homeostasis. The exact mechanism of trophoblast Ca transport is not known. However, there is evidence suggesting that a developmentally expressed cytosolic, trophoblast-specific, high M(r) 57-kDa Ca-binding protein (CaBP) plays an important role in regulating and/or shuttling cytosolic Ca. We report here the cloning of a full-length cDNA of the mouse CaBP which shows significant homology with calreticulin, an endoplasmic reticulum-associated Ca binding protein. The functional role of CaBP in cellular Ca handling was investigated using a trophoblastic cell line, Rcho-1, derived from a rat choriocarcinoma. Upon differentiation, Rcho-1 cells exhibit enhanced Ca uptake compared to undifferentiated Rcho-1 stem cells, and CaBP expression is upregulated. To analyze the regulation of CaBP expression, placenta organ cultures and Rcho-1 cells were treated for 48 h in vitro with a series of agents implicated in Ca homeostasis. In both placenta organ cultures and undifferentiated as well as differentiated Rcho-1 cells, treatment with 1,25-dihydroxy vitamin D3, estrogen, parathyroid hormone (PTH), parathyroid hormone-related protein (PTHrP 1-34), and Ca had no effect on CaBP mRNA and protein levels, which were significantly stimulated by PTHrP 67-84. PTHrP 67-84-treated Rcho-1 cells also exhibited higher Ca uptake activity than untreated control cells. The upregulation of CaBP expression during and/or following the differentiation of Rcho-1 cells into trophoblastic giant cells supports the importance of CaBP in trophoblast maturation and the validity of the Rcho-1 rat model cell system. In addition, the action of PTHrP on placental trophoblast Ca transport is likely to involve the regulation of CaBP expression to handle the increasing Ca requirements of the developing fetus.

Identification of L-type calcium channels associated with kappa opioid receptors in human placenta

Transduction pathways of kappa receptor activation are not fully understood. Human placenta at term expresses only this type of opioid receptors and therefore offers a unique advantage for such investigations. It has previously been postulated that kappa receptors-mediated modulation of acetylcholine and placental lactogen release from human placentas require the influx of extracellular calcium and into the cells, possibly via voltage-dependent channels. We report here that another opioid-regulated placental function, the release of human chorionic gonadotropin (hCG), depends on extracellular calcium and the modality of its influx via L-type channels. Data presented demonstrated that the stimulation of hCG secretion by the kappa-selective agonist U69,593 was abolished in presence of either EGTA or the calcium channel blocker nifedipine. Results obtained on the combined effect of opioids and dihydropyridines indicated that placental kappa opioid receptors could be directly coupled to L-type calcium channels. The identification of the latter in villus membrane preparations, reported here for the first time, further contributes to the hypothesis that, in human placenta, kappa receptors-linked transduction mechanisms involve calcium and its conductance across villus membranes.

Effects of cadmium on trophoblast calcium transport

Maternal exposure to cadmium (Cd) during pregnancy has been linked to low fetal birthweight, which may be attributed to placental damage and/or dysfunction in nutrient transport. Previous studies have suggested that Cd is accumulated in the placenta, and that placental transport of calcium (Ca) and zinc (Zn) is perturbed by Cd. To investigate the mechanism of Cd perturbation of Ca transport, we used JEG-3, a human choriocarcinoma cell line which exhibits trophoblastic properties, to analyse Cd effects in vitro. Treatment with Cd at low, physiologically relevant concentrations (e.g. 0.04 microM) did not result in obvious changes in cell morphology or integrity, whereas higher concentrations (> or = 0.16 microM) affected cell integrity. With lower concentrations of Cd treatment for 24 h, activities of cellular Ca uptake and transport, and Ca2+ binding were decreased, and intracellular [Ca2+] ([Ca2+]i) profile was also altered; however, membrane-associated Ca(2+)-activated ATPase activity remained relatively unchanged. Interestingly, cellular Ca uptake activity was unaffected by short-term (30 min) Cd pretreatment. The 24-h Cd treatment also resulted in elevated expression of the metal-binding protein, metallothionein, whereas the expression of a trophoblast-specific cytosolic Ca(2+)-binding protein (HCaBP) was drastically reduced. These results strongly suggest that Cd exposure significantly compromises the Ca handling ability of trophoblastic cells; this effect is probably not due to perturbations in Ca channel or membrane Ca pump activities, but rather a consequence of alterations in subcellular, cytosolic Ca2+ binding activities.

A biochemical-morphological study on microvillus plasma membrane development

The microvillus plasma membrane of the human placental syncytiotrophoblast at term has been extensively studied, while little is known about the characteristics of its development. The aim of the present work was to compare functional and structural properties of this membrane at early and term gestational age. Ten normal term placentas (40 weeks) and ten placentas at 10 weeks of gestational age were studied. The Na+/K+-ATPase activity is significantly decreased in the syncytiotrophoblast plasma membrane obtained from term placentas as compared to the early ones, with significant variation of maximum velocity (Vmax). The microviscosity, evaluated by the P parameter of DPH and Sn parameters of 5- and 16-NS, is increased in the term placentas compared to the early placentas. This alteration is accompanied by an increased cholesterol to phospholipids ratio in term placentas, while there is a decreased unsaturated to saturated fatty acid ratio. As follows from morphological studies, an increased mean diameter in the E face was observed in the term placenta with respect to the early placenta. The distribution factor DF, which indicates the particle aggregation state, decreased in the E face in the term placenta as compared to the early one. The present biochemical morphological study shows that a deep modification of the membrane is at the basis of the syncytiotrophoblast plasma membrane development.

ATP independent calcium transport and binding by basal plasma membrane of human placenta

The transport of large amounts of Ca2+ by the plasma membranes of human placental syncytiotrophoblast is essential to the mineralization of the growing fetal skeleton. We have investigated transport by the basal (fetal-facing) plasma membrane (BPM). Ca2+ was taken up by purified BPM vesicles in a time-dependent manner and equilibrium attained in approximately 60 min. The apparent equilibrium space was many fold higher than that determined using other substrates (e.g. leucine), suggesting that Ca2+ is concentrated or bound within the vesicles. The more rapid uptake and exit in the presence of A23187 indicates that membrane transport is rate limiting and that Ca2+ is internalized within the membrane space. The initial rate of uptake was approximately by measurement during the first 2 s of incubation. Concentration dependence data were fit to a Michaelis-Menten model with one saturable site and diffusion (Km = 12 microM; Vmax = 4 nmol/min/mg; KD = 39 nmol/min/mg/mM). Saturable Ca2+ binding (Kd = 16 microM; Bmax = 3.4 nmol/mg) was of lower capacity than previously observed for microvillous membrane.

Changes in membrane fluidity and Na+/K(+)-ATPase activity during human trophoblast cell culture

The human placenta plays an essential role in embryo development, in particular regulating the transport of ions, nutrients and immunoglobulins from the maternal to the fetal circulation. Trophoblast organization into a syncytial layer involves structural and functional steps that may be monitored and elucidated by in vitro studies. The structural stages by which the syncytial trophoblast is formed are not yet understood. In order to clarify the mechanism of trophoblast development, we studied the morphological characteristics of the syncytial trophoblast formation in culture and the functional changes (transport properties and membrane microviscosity) accompanying the structural modifications. By using both 5-nitroxystearate and 16-nitroxystearate as spin labels, we observed an initial increase in membrane order over 0-24 h of culture, which can be associated with two events: recovery of cell membranes from trypsin and initial aggregation of cytotrophoblasts. The similar behaviour of the order parameters determined with both probes indicates that membrane order changes both inside and in the outer part of the lipid bilayer. The subsequent decrease in membrane order observed at 36-48 h might be related to the process of cellular fusion. The increase in sodium/potassium pump activity in the first 24 h of culture might be an expression of cell recovery following trypsin treatment. The subsequent decrease might represent an adaptive mechanism by which metabolic energy is mainly used for morphogenetic changes.

Transport and binding in calcium uptake by microvillous membrane of human placenta

Calcium entry across the microvillous membrane of the human placental syncytiotrophoblast is the first step in the transfer of this important nutrient to the fetus. Calcium uptake by isolated microvillous membranes was time dependent. Equilibrium uptake was very much greater than could be explained by equilibration of the vesicle space with medium, indicating that calcium is bound to internal sites. Addition of the ionophore A23187 greatly increased the rates of influx and efflux, indicating that transport across the plasma membrane is rate limiting in entry. Concentration dependence data for calcium transport at 4 s fit well to a Michaelis-Menten equation having two saturable sites and diffusion. Calcium entry by both transporters was unaffected by calcium channel blockers but was strongly inhibited by the group II metals. The distinct inhibition constant values for strontium inhibition provided additional evidence for two transporters. Calcium binding fit well to a single-site model saturable in the micromolar range. In vivo, the saturable transport processes may mediate calcium entry into the syncytiotrophoblast and binding may regulate concentration within the placental microvilli.

Calciotropic hormones during reproduction

This review summarizes the reported effects of the menstrual cycle, pregnancy and lactation on serum concentration of the calciotropic hormones PTH and 1,25(OH)2D. A midcycle rise in PTH and 1,25(OH)2D has been observed, but in the majority of studies there was no change in PTH and 1,25(OH)2D concentrations throughout the menstrual cycle. Both total and free 1,25(OH)2D levels are increased during pregnancy. The renal 1,25(OH)2D production is stimulated, and there is some evidence of 1,25(OH)2D production by decidua/placenta and fetal kidney in vitro; the decidual/placental production should not be overestimated in vivo. The increased renal 1 alpha-hydroxylase activity is possibly mediated by estrogens and PTH, although the effect of pregnancy on PTH remains uncertain. Increased serum 1,25(OH)2D concentrations probably result in a rise of intestinal calcium absorption during pregnancy. There is a postdelivery drop in PTH and 1,25(OH)2D levels, but they are increased when lactation is prolonged, or in mothers nursing twins. The l alpha-hydroxylase activity during lactation may be stimulated by PTH, but also by prolactin.

Localization of mRNA for the oncotrophoblastic protein oncomodulin during implantation and early placentation in the rat

The mRNA for the oncodevelopmental calcium-binding protein oncomodulin (MW 11,700) has been detected in tissues of the rat conceptus by in situ hybridization using biotinylated RNA probes. Oncomodulin mRNA was detected in the basal zone and labyrinth of rat placenta, following a similar distribution to that shown for oncomodulin by immunohistochemistry. Oncomodulin mRNA was also detected in rat ectoplacental cone at ten days and in amnion and PYS, but not VYS from 11 days onward. Previously oncomodulin was not detected embryonically from day 14 to birth, but in the present study of oncomodulin mRNA and protein, both were detected in implantation stages from blastula through egg cylinder. Staining was also present on decidual tissue. The suggestion is made that the oncomodulin gene is initially active in all cell types, but later its activity is confined to extraembryonic tissues.

Vitamin D metabolism in pregnant and pseudopregnant rats: identification of 25-hydroxycholecalciferol-1-hydroxylase in decidual tissue

Elevated levels of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) are found in late pregnancy but the factors responsible for this are not known. To determine if the maternal-fetal calcium flux or the presence of a previously described extrarenal 25-hydroxycholecalciferol-1-hydroxylase (25(OH)-D3-1-hydroxylase) play a role, serum calcium and 1,25(OH)2D3 were measured in pregnant, nonpregnant, and decidua-bearing pseudopregnant rats. Serum calcium was 8.74 +/- 0.26 mg/dl (mean +/- SEM) in nonpregnant rats. In pregnant rats, serum calcium was not significantly different from nonpregnant controls on day 12 and only slightly higher on day 15. Pseudopregnant rats were significantly hypercalcemic on days 12 (11.93 +/- 0.19 mg/dl) and 15 (11.45 +/- 0.23 mg/dl) compared with nonpregnant rats (P less than 0.001). In nonpregnant controls the serum level of 1,25(OH)2D3 was 44.6 +/- 6.3 pg/ml. Levels in pregnant rats were not significantly different on days 12 or 15 but tended to be higher by day 15 (75.2 +/- 19.7 pg/ml). Pseudopregnant rats had levels of 72.6 +/- 13.5 pg/ml on day 12 and 102.8 +/- 10.9 pg/ml on day 15, the latter of which was significantly higher than nonpregnant values (P less than 0.05). 25(OH)D3-1-hydroxylase activity was determined in whole tissue homogenates of placenta and decidua. Placenta from pregnant rats and decidua from pregnant and pseudopregnant rats both formed putative 1,25(OH)2D3 in short-term incubation with 25(OH)D3 as identified by comigration with authentic 1,25(OH)2D3 on high pressure liquid chromatography (HPLC).(ABSTRACT TRUNCATED AT 250 WORDS)

Mouse placental 57-kDa calcium-binding protein: II. Localization of mRNA in mouse and human placentae by in situ cDNA hybridization

Using a cloned cDNA to the mouse placental calcium-binding protein (MCaBP) [Tuan and Kirwin, preceding paper], mRNA transcripts of the MCaBP have been localized in the mouse placenta by in situ cDNA-RNA hybridization. The procedure involved the use of a nonradioactive, biotinylated cDNA probe. Paraffin sections of the mouse chorioallantoic placenta were processed for in situ hybridization using a procedure modified from that of Lo [8]. The results demonstrated that MCaBP mRNA is localized specifically to the fetal trophoblast of the mouse placenta, and is expressed in a development-dependent manner. The MCaBP cDNA also hybridized strongly in situ to sections of human term placenta, giving rise to signals localized specifically to the syncytiotrophoblastic layer of the chorionic villi.

Mouse placental 57-kDa calcium-binding protein: I. Cloning of cDNA and characterization of developmental expression

A cDNA clone to the mouse placental 57-kDa calcium-binding protein (MCaBP) [29] was isolated by immunoscreening a mouse placenta cDNA library constructed in the expression phage vector, lambda gt 11. The MCaBP cDNA was 0.7 kb in size, with restriction sites for StuI and Bg/II, and its identity to the MCaBP was confirmed by mRNA hybrid selection. RNA blot hybridization revealed a predominant, 3.9-kb transcript of the MCaBP in day-18 mouse placenta. The expression of the MCaBP during development was analyzed with respect to the levels of protein activity, translatable MCaBP mRNA, and total MCaBP transcript.

Detection of oncomodulin, an oncodevelopmental protein in human placenta and choriocarcinoma cell lines

Oncomodulin was detected by immunocytochemical means in human placenta and found to occur in the cytotrophoblastic shell during implantation, in Langhans type villar cytotrophoblastic cells, and in extravillar cytotrophoblast or intermediate trophoblast. The presence of oncomodulin during first and early second trimester was confirmed by immunoradiometric assay (IRMA). In term placenta oncomodulin appeared in intermediate trophoblast cells, and was largely absent from the villi which lack cytotrophoblast. Furthermore, oncomodulin was abundant in the choriocarcinoma cell line BeWo, and detectable in JAR but not JEG-3. The function of this oncodevelopmental calcium-binding protein in normal development or in neoplasia is not known.

Calcium-activated ATPase of the human placenta: identification, characterization, and functional involvement in calcium transport

A specific, membrane-bound, Ca2+-activated and Mg2+-dependent adenosine triphosphatase (ATPase) activity is present in the human term placenta. The enzyme activity is fractionated electrophoretically into two distinct forms which correspond to molecular weights of 120,000 and 145,000. Cytohistochemistry localized the Ca2+-ATPase to the chorionic villi of the placental labyrinth, and specific staining was primarily associated with the syncytio- and cytotrophoblast layers as well as the perivascular cells. The enzyme activity is inhibited by phenothiazin and erythrosin B which also significantly inhibit active calcium in vitro by placental microsomal membrane vesicles.

Effect of Ca2+ on structure and fluidity of microvillus membranes of human placenta

This investigation shows the effect of a Ca2+ addition on the structural and physiochemical properties of microvillus plasma membranes obtained from human placenta. Ca2+ addition induces an increase in microviscosity, as shown by the increase of order parameter and rotational correlation time of 5- and 16-doxylsterate derivatives and by the increase of fluorescence polarization of diphenylhexatriene. All the effects were obtained in a wide temperature range. The morphometric analysis of the ultrastructural images shows that the vesicle profiles of syncytiotrophoblast membranes decrease both area and form factor (FF) in the presence of Ca2+ with respect to the controls. The freeze-fracture results also show that Ca2+ induces an enhanced tendency to IMP clusterization. The Ca2+-induced changes were observed in both E and P faces. Our results underline the important role of Ca2+ in the cell membrane structure per se and in modulating interactions between cytoplasmic and extracellular microenvironments. The results of morphometric analysis of the ultrastructural images agree with biochemical data showing an increased stability induced by calcium on plasma membranes.

Identification and characterization of a calcium-binding protein in the mouse chorioallantoic placenta

Mouse chorioallantoic placenta contains a specific calcium-binding protein (MCaBP). A procedure involving gel filtration and ion-exchange chromatography was developed to purify the MCaBP. The MCaBP activity increased as a function of embryonic gestation and was highly specific for Ca2+. The MCaBP is a monomeric protein of Mr 57000, with pI 4.7. Specific antibodies were prepared against the MCaBP and were used to localize the MCaBP to syncytiotrophoblasts of the chorionic villi of mouse chorioallantoic placenta. These properties suggest that the MCaBP may be involved in transplacental calcium transport.

Localization and synthesis of the tumor protein oncomodulin in extraembryonic tissues of the fetal rat

The calcium-binding protein oncomodulin, previously found only in tumors, has been detected during rat development. Specific antisera to purified rat hepatoma oncomodulin (MW 11,500) were used to detect oncomodulin by radioimmunoassay (RIA) and by avidin-biotin-peroxidase complex (ABC) immunohistochemistry. Using RIA, oncomodulin was found to increase in placenta from below the limits of detection (2 ng/mg protein) on Day 13 to approximately 25 ng/mg on Day 16 of pregnancy, and to remain high through to the end of gestation. Determinations on separated inner and outer placenta showed the increase to be greater in the outer placenta (basal zone and decidua) than in the inner placenta (labyrinth). The ABC technique on paraffin sections produced positive staining for oncomodulin throughout the placenta, with the most intense staining occurring in the outer placenta (cytotrophoblast and giant cells of the basal zone). Parietal and visceral yolk sac, and amnion also stained positively, while fetal organs did not. Oncomodulin synthesis measured by [35S]methionine incorporation into immunoprecipitates occurred in isolated inner and outer placenta, whole placenta, the separated trophectoderm and endoderm of the parietal yolk sac, and amnion. No oncomodulin synthesis could be measured in visceral yolk sac, fetal liver, or 16-day embryo. This occurrence in developing and transformed tissues demonstrates that oncomodulin is an oncodevelopmental protein.