Purification and characterization of mouse decidual calcyclin: a novel stimulator of mouse placental lactogen-II secretion

Blastocyst-Derived Lactic Acid May Regulate S100A6 Expression and Function in Mouse Decidualization via Stimulation of Uterine Epithelial Arachidonic Acid Secretion

(1) Background: Inflammatory responses are implicated in embryo implantation, decidualization, pregnancy maintenance and labor. Both embryo implantation and decidualization are essential to successful pregnancy in rodents and primates. S100A6 is involved in inflammation, tumor development, apoptosis and calcium homeostasis. S100A6 is strongly expressed in mouse decidua, but the underlying mechanisms of how S100A6 regulates implantation and decidualization are poorly defined. (2) Methods: Mouse endometrial stromal and epithelial cells are isolated from day 4 pseudopregnant mouse uteri. Both immunofluorescence and Western blotting are used to analyze the expression and localization of proteins. The molecular mechanism is verified in vitro by Western blotting and the quantitative polymerase chain reaction. (3) Results: From days 4 to 8 of pregnancy, S100A6 is specifically expressed in mouse subluminal stromal cells. Blastocyst-derived lactic acid induces AA secretion by activating the luminal epithelial p-cPLA2. The epithelial AA induces stromal S100A6 expression through the COX2/PGI2/PPAR δ pathway. Progesterone regulates S100A6 expression through the progesterone receptor (PR). S100A6/RAGE signaling can regulate decidualization via EGFR/ERK1/2 in vitro. (4) Conclusions: S100A6, as an inflammatory mediator, is important for mouse implantation and decidualization.

S100A6 Protein-Expression and Function in Norm and Pathology

S100A6, also known as calcyclin, is a calcium-binding protein belonging to the S100 protein family. It was first identified and purified more than 30 years ago. Initial structural studies, focused mostly on the mode and affinity of Ca²⁺ binding and resolution of the resultant conformational changes, were soon complemented by research on its expression, localization and identification of binding partners. With time, the use of biophysical methods helped to resolve the structure and versatility of S100A6 complexes with some of its ligands. Meanwhile, it became clear that S100A6 expression was altered in various pathological states and correlated with the stage/progression of many diseases, including cancers, indicative of its important, and possibly causative, role in some of these diseases. This, in turn, prompted researchers to look for the mechanism of S100A6 action and to identify the intermediary signaling pathways and effectors. After all these years, our knowledge on various aspects of S100A6 biology is robust but still incomplete. The list of S100A6 ligands is growing all the time, as is our understanding of the physiological importance of these interactions. The present review summarizes available data concerning S100A6 expression/localization, interaction with intracellular and extracellular targets, involvement in Ca²⁺-dependent cellular processes and association with various pathologies.

Lysozyme as the anti-proliferative agent to block the interaction between S100A6 and the RAGE V domain

In this report, using NMR and molecular modeling, we have studied the structure of lysozyme-S100A6 complex and the influence of tranilast [N-(3, 4-dimethoxycinnamoyl) anthranilic acid], an antiallergic drug which binds to lysozyme, on lysozyme-S100A6 and S100A6-RAGE complex formation and, finally, on cell proliferation. We have found that tranilast may block the S100A6-lysozyme interaction and enhance binding of S100A6 to RAGE. Using WST1 assay, we have found that lysozyme, most probably by blocking the interaction between S100A6 and RAGE, inhibits cell proliferation while tranilast may reverse this effect by binding to lysozyme. In conclusion, studies presented in this work, describing the protein-protein/-drug interactions, are of great importance for designing new therapies to treat diseases associated with cell proliferation such as cancers.

The dystrophin isoform Dp71eΔ71 is involved in neurite outgrowth and neuronal differentiation of PC12 cells

The Dp71 protein is the most abundant dystrophin in the central nervous system (CNS). Several dystrophin Dp71 isoforms have been described and are classified into three groups, each with a different C-terminal end. However, the functions of Dp71 isoforms remain unknown. In the present study, we analysed the effect of Dp71e_Δ71 overexpression on neuronal differentiation of PC12 Tet-On cells. Overexpression of dystrophin Dp71e_Δ71 stimulates neuronal differentiation, increasing the percentage of cells with neurites and neurite length. According to 2-DE analysis, Dp71e_Δ71 overexpression modified the protein expression profile of rat pheochromocytoma PC12 Tet-On cells that had been treated with neuronal growth factor (NGF) for nine days. Interestingly, all differentially expressed proteins were up-regulated compared to the control. The proteomic analysis showed that Dp71e_Δ71 increases the expression of proteins with important roles in the differentiation process, such as HspB1, S100A6, and K8 proteins involved in the cytoskeletal structure and HCNP protein involved in neurotransmitter synthesis. The expression of neuronal marker TH was also up-regulated. Mass spectrometry data are available via ProteomeXchange with identifier PXD009114. SIGNIFICANCE: This study is the first to explore the role of the specific isoform Dp71e_Δ71. The results obtained here support the hypothesis that the dystrophin Dp71e_Δ71 isoform has an important role in the neurite outgrowth by regulating the levels of proteins involved in the cytoskeletal structure, such as HspB1, S100A6, and K8, and in neurotransmitter synthesis, such as HCNP and TH, biological processes required to stimulate neuronal differentiation.

S100A6 protein: functional roles

S100A6 protein belongs to the A group of the S100 protein family of Ca2+-binding proteins. It is expressed in a limited number of cell types in adult normal tissues and in several tumor cell types. As an intracellular protein, S100A6 has been implicated in the regulation of several cellular functions, such as proliferation, apoptosis, the cytoskeleton dynamics, and the cellular response to different stress factors. S100A6 can be secreted/released by certain cell types which points to extracellular effects of the protein. RAGE (receptor for advanced glycation endproducts) and integrin β1 transduce some extracellular S100A6's effects. Dosage of serum S100A6 might aid in diagnosis in oncology.

S100A6 is secreted from Wharton's jelly mesenchymal stem cells and interacts with integrin β1

S100A6 is a calcium binding protein belonging to the S100 family. In this work we examined the function of extracellular S100A6. Using mesenchymal stem cells isolated from Wharton's jelly of the umbilical cord (WJMS cells) we have shown that S100A6 is secreted by these cells, and when added to the medium, increases their adhesion and inhibits proliferation. The search for a potential target/receptor of S100A6 in the membrane fraction of WJMS cells allowed us to identify some proteins, among them integrin β1, which interacts with S100A6 in a calcium dependent manner. The interaction between S100A6 and integrin β1, was then confirmed by ELISA using purified proteins. Applying specific antibodies against integrin β1 reversed the effect on cell adhesion and proliferation observed in the presence of S100A6 which indicates that S100A6 exerts its function due to interaction with integrin β1. Since the data show the influence of extracellular S100A6 on cells isolated from Wharton's jelly, our results might help to establish molecular mechanisms leading to some pathologies characteristic for this tissue.

Interaction of the S100A6 mutant (C3S) with the V domain of the receptor for advanced glycation end products (RAGE)

S100A6 is involved in several vital biological functions, such as calcium sensing and cell proliferation. It is a homodimeric protein that belongs to the S100 protein family. The receptor for advanced glycation end products (RAGE) has been shown to play a role in the progression of various disease conditions, such as diabetes and immune/inflammatory disorders. Information regarding the association of RAGE with S100 proteins at a molecular level is useful to understand the diversity of the RAGE signaling pathways. In this report, biomolecular NMR techniques were utilized for the resonance assignment of the C3S mutation in human S100A6 and characterizing its interaction with the RAGE V domain. Further binding affinity between S100A6m and the RAGE V domain was determined by isothermal titration calorimetric studies. HADDOCK was used to generate a heterotetramer model of the S100A6m-RAGE V domain complex. This model provides an important insights into the S100-RAGE cellular signaling pathway.

Changes in cell cycle and extracellular matrix gene expression during placental development in deer mouse (Peromyscus) hybrids

Crosses between two species of the rodent genus Peromyscus produce defects in both growth and development. The defects are pronounced in the hybrid placentas. Peromyscuys maniculatus (strain BW) females mated to P. polionotus (strain PO) males produce placentas half the size of the parental species, as well as growth-retarded embryos. In contrast, PO females mated to BW males result in defective conceptuses that display embryonic and placental overgrowth. These ‘parent-of-origin’-dependent phenotypes are consistent with previous studies that demonstrated altered expression of imprinted genes and genetic linkage of the overgrowth phenotypes to imprinted domains. In the present study, we take a broader approach in assessing perturbations in hybrid placental gene expression through the use of Mus musculus cDNA microarrays. In verifying classes of genes identified in microarray screens differentially regulated during hybrid placental development, we focused on those influencing the cell cycle and extracellular matrix (ECM). Our work suggests that cell cycle regulators at the G1/S phase check-point are downregulated in the large hybrid placenta, whereas the small hybrid placenta is more variable. The ECM genes are typically downstream targets of cell cycle regulation and their misregulation is consistent with many of the dysmorphic phenotypes. Thus, these data suggest imbalances in proliferation and differentiation in hybrid placentation.

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.

Expression and hormonal regulation of calcyclin-binding protein (CacyBP) in the mouse uterus during early pregnancy

Calcyclin-binding protein (Siah-1-Interacting Protein, CacyBP/SIP), is a calcium signaling protein involved in the degradation of beta-catenin, however, little is known about its role in reproductive biology. The present study was to character its temporospatial expression pattern and regulation in mouse uterus and to investigate whether it plays a role in the regulation of normal endometrial events. While prominently expressed in both luminal and glandular epithelia, CacyBP underwent dynamic changes during early pregnancy. CacyBP expression was observed weakly from days 1-4. An intense accumulation in luminal and glandular epithelia as well as decidua surrounding the embryo at later stages (days 5-7) was observed. Most notably, CacyBP accumulation in trophoblast was pronounced at day 7. Using ovariectomized and pseudopregnant mice, we found that progesterone (P(4)) and 17beta-estradiol (E(2)) led to increased expression of CacyBP gene and this could be abolished by Ru486 and tamoxifen, respectively. Antisense oligonucleotides (ODNs) against CacyBP significantly inhibited cultured endometrial stromal cells' (ESCs) apoptosis induced by UV irradiation. Injection of antisense ODNs into mouse uterine horn severely impaired the number of implanted blastocysts. Taken together, our results suggested that CacyBP expression was positively regulated by P(4) and E(2). CacyBP may be involved in the regulation of endometrial cell apoptosis during early pregnancy and play an important role in mouse endometrial events such as pregrancy establishment.

Calcium-regulated interaction of Sgt1 with S100A6 (calcyclin) and other S100 proteins

S100A6 (calcyclin), a small calcium-binding protein from the S100 family, interacts with several target proteins in a calcium-regulated manner. One target is Calcyclin-Binding Protein/Siah-1-Interacting Protein (CacyBP/SIP), a component of a novel pathway of beta-catenin ubiquitination. A recently discovered yeast homolog of CacyBP/SIP, Sgt1, associates with Skp1 and regulates its function in the Skp1/Cullin1/F-box complex ubiquitin ligase and in kinetochore complexes. S100A6-binding domain of CacyBP/SIP is in its C-terminal region, where the homology between CacyBP/SIP and Sgt1 is the greatest. Therefore, we hypothesized that Sgt1, through its C-terminal region, interacts with S100A6. We tested this hypothesis by performing affinity chromatography and chemical cross-linking experiments. Our results showed that Sgt1 binds to S100A6 in a calcium-regulated manner and that the S100A6-binding domain in Sgt1 is comprised of 71 C-terminal residues. Moreover, S100A6 does not influence Skp1-Sgt1 binding, a result suggesting that separate Sgt1 domains are responsible for interactions with S100A6 and Skp1. Sgt1 binds not only to S100A6 but also to S100B and S100P, other members of the S100 family. The interaction between S100A6 and Sgt1 is likely to be physiologically relevant because both proteins were co-immunoprecipitated from HEp-2 cell line extract using monoclonal anti-S100A6 antibody. Phosphorylation of the S100A6-binding domain of Sgt1 by casein kinase II was inhibited by S100A6, a result suggesting that the role of S100A6 binding is to regulate the phosphorylation of Sgt1. These findings suggest that protein ubiquitination via Sgt1-dependent pathway can be regulated by S100 proteins.

Identification and initial characterization of calcyclin and phospholipase A2 in equine conceptuses

For development to proceed normally, the appropriate genes must be expressed in the correct tissues and in the correct time frame. Knowledge of gene expression during development provides information about the changes taking place within the conceptus as well as possible reasons for pregnancy failure. However, little is known about gene expression during development in the equine conceptus. In this study, we examined differences in gene expression between day 12 and day 15 equine conceptuses by suppression subtractive hybridization. This technique was used to isolate transcripts that are more abundantly expressed in day 15 conceptuses compared to day 12 conceptuses. Between day 12 and 15 of pregnancy in horses, maternal recognition of pregnancy occurs, gastrulation is taking place, and mesoderm is beginning to form. Fifty cDNA clones were isolated, sequenced, and compared to known sequences in the GenBank database. Two cDNA clones identified that were of primary interest were calcyclin and phospholipase A2. Calcyclin is a calcium-binding protein of the S-100 protein family that has been found in mouse decidua and trophoblast. Calcyclin was found to be expressed in both day 12 and 15 equine conceptuses, with approximately a 30-fold increase in transcript abundance between days 12 and 15. Phospholipase A2 is an enzyme that cleaves phospholipids to release fatty acids and is involved in arachidonic acid release needed for prostaglandin, thromboxane, and leukotriene synthesis. Multiple forms of PLA2, that appear to be differentially regulated in day 12 and 15 conceptuses, were detected by northern blotting.

Roles of intracellular Ca2+ receptors in the pancreatic beta-cell in insulin secretion

Ca2+ is the central second messenger in the regulation of insulin release from the pancreatic beta-cell; and intracellular Ca2+ -binding proteins, classified into two groups, the EF hand proteins and the Ca2+/phospholipid binding proteins, are considered to mediate Ca2+ signaling. A number of Ca binding proteins have been suggested to participate in the secretory machinery in the beta-cell. Calmodulin, the ubiquitous EF hand protein, is the predominant intracellular Ca2+ receptor that modulates insulin release via the multiplicity of its binding to target proteins including protein kinases. Other Ca binding proteins such as calcyclin and the Ca2+/phospholipid binding proteins may also be suggested to be involved. Ca2+ influx from the extracellular space appears to be responsible for exocytosis of insulin via Ca2+ -dependent protein/protein interactions. On the other hand, intracellular Ca2+ mobilization resulting in secretory granule movement may be controlled by Ca2+/calmodulin-dependent protein phosphorylation. Thus, Ca2+ exerts versatile effects on the secretory cascade via binding to specific binding proteins in the pancreactic beta-cells.

Calcyclin in the mouse decidua: expression and effects on placental lactogen secretion

Placental lactogens are protein hormones produced by the placentas of many mammals. These hormones have diverse reproductive functions and are structurally similar to the pituitary hormones growth hormone and prolactin. Isolated decidual cells were previously shown to release a protein that stimulates mouse placental lactogen (mPL)-II release from mouse trophoblast cells in culture. Partial amino acid sequence data suggested that this protein shared sequence identity with mouse calcyclin. In the present study the sequence identity of this protein was determined by sequencing reverse transcription-polymerase chain reaction (RT-PCR) products. The sequence matched that of mouse calcyclin. The distribution of calcyclin message was determined in the conceptus by in situ hybridization, and a gestational profile of calcyclin mRNA was determined by Northern blot analysis. Calcyclin was localized primarily to cells that exhibited a uterine natural killer cell morphology within the decidua and to glycogen cells of the labyrinth and junctional zone. Although calcyclin was detectable by RT-PCR in midterm placentas, isolated trophoblast cells in culture did not contain detectable quantities of calcyclin by RT-PCR. Calcyclin stimulated secretion of mPL-II from isolated trophoblast cells in vitro but did not affect mPL-I secretion.

Characterization of chicken gizzard calcyclin and examination of its interaction with caldesmon

Using a procedure developed to purify calcyclin from mouse Ehrlich ascites tumor cells calcyclin was purified from smooth muscle of chicken gizzard. Chicken gizzard calcyclin bound to phenyl-Sepharose in a calcium dependent manner as did mouse EAT cells and rabbit lung calcyclin but appeared to be more acidic than its mammalian counterparts as revealed by ion exchange chromatography on Mono Q. Chicken gizzard calcyclin bound 45Ca2+ on nitrocellulose filters and exhibited a shift in electrophoretic mobility on urea-PAGE depending on Ca2+ concentration. Crosslinking experiments with BS3 showed that chicken gizzard calcyclin was able to form noncovalent dimers. As indicated by a decrease in maximum tryptophan fluorescence emission of caldesmon (about 14% at 1:1 molar ratio) and displacement of calmodulin from its complex with caldesmon, chicken gizzard calcyclin binds caldesmon. This binding was, however, much weaker than that of calmodulin and could not influence the interaction of caldesmon with actin. In consequence, calcyclin was unable to reverse the inhibitory effect of caldesmon on actin-activated Mg(2+)-ATPase activity of myosin in the presence of Ca2+.

Interaction of calcyclin and its cyanogen bromide fragments with annexin II and glyceraldehyde 3-phosphate dehydrogenase

The structural properties of calcyclin protein are quite well characterized but its function remains obscure. To help elucidate the biological role of calcyclin we have performed the in vitro studies of the Ca(2+)-dependent interaction of Ehrlich ascites tumor cells calcyclin and its cyanogen bromide fragments with two potential calcyclin targets: annexin II and glyceraldehyde 3-phosphate dehydrogenase (GAPDH). The binding of annexin II, evidenced by the reaction with 125I-calcyclin, was found to be very weak and occurred only for intact calcyclin. On the other hand the interaction between calcyclin and GAPDH was of high affinity and could be assigned to the N-terminal region of calcyclin. Intact calcyclin and its N-terminal fragment bound to GAPDH in the gel overlay and affinity chromatography assay. When examined in the presence of a crosslinking agent the interaction resulted in the formation of 46K covalent adduct between calcyclin monomer and GAPDH subunit. Fluorescence of 5-iodoacetamido-fluorescein-labelled calcyclin was efficiently quenched by GAPDH in the presence of Ca2+. Titration experiments revealed the stoichiometry of one calcyclin monomer binding to each of GAPDH subunits with a binding constant of 10(8) M-1. The results of this work suggest that the binding between calcyclin and GAPDH may have bearing on calcyclin function.

Mouse uterus peptidylarginine deiminase is expressed in decidual cells during pregnancy

Peptidylarginine deiminase is localized in the cytosol of the luminal and glandular epithelia of the nonpregnant murine uterus and its expression is regulated by sex hormones [Takahara et al., [1989]: J Biol Chem 264, 13361-13368; Takahara et al. [1992]: J Biol Chem 267,520-525]. Here, we demonstrate that changes occur in the enzyme level in the mouse uterus during pregnancy and parturition. After a rapid decrease in enzymatic activity from day 1 to day 5 of pregnancy, the activity sharply increased during the middle stage of pregnancy (day 8 to day 10) and then gradually decreased during late pregnancy. Expression of the enzyme occurred only in the decidual cells that had differentiated from endometrial stroma cells surrounding the implantation site. The immunochemical properties of the enzyme expressed in the decidualized cells was indistinguishable from those in the uterine epithelia. These results suggest that peptidylarginine deiminase has important roles in decidual cells and not just in the epithelia of the nonpregnant uterus. Moreover, the level of enzyme activity increased slightly just before parturition (day 17), and then decreased during the 12 h period after parturition. The tissue localization of the enzyme expressed around the time of parturition changed from decidua to the luminal and glandular epithelia. Semiquantitative analyses of the enzyme mRNA content in the pregnant uteri showed a remarkable increase from day 7 leading to the onset of the enzyme synthesis in the decidual cells. After reaching the maximal level at day 12, small peaks in the mRNA level were observed at two times during late pregnancy. Since these serial changes in the mRNA level did not correlate with changes in sex hormones, the expression of decidual peptidylarginine deiminase seemed to be controlled by factors other than sex hormones.

Gene expression and protein localisation of calcyclin, a calcium-binding protein of the S-100 family in fresh neuroblastomas

Calcyclin gene, a Ca(2+)-binding protein with homology to S-100, has been found to be expressed at different levels in leukaemic cells and in other tumour cells. We recently reported the expression of the gene in human neuroblastoma (NB) cell lines, and suggested a possible role of calcyclin in cell differentiation. To extend our findings, we investigated the expression of the gene in NB cells induced to differentiate by retinoic acid (RA), using the reverse transcriptase-polymerase chain reaction (RT-PCR) technique. Time-course experiments employing LA-N-5 cells showed that calcyclin mRNA appeared 2 h after RA treatment, long before the cells were blocked in the G1 cell-cycle phase and before the neurite-like structures outgrew from the cell bodies. This suggests the involvement of the gene in the early phase of cell differentiation. Furthermore, we investigated mRNA expression in a series of fresh neuroblastomas. NB tumours showed a heterogeneous pattern of calcyclin expression, although calcyclin seemed to be expressed more frequently in cases with a favourable Shimada histology. We also studied the expression of the protein in formalin fixed and paraffin embedded tissues, by using a specific anticalcyclin antibody. The protein was detected in stromal cells which characterise a more mature histological type, and in nerve sheaths, whereas neuroblasts were negative. The tissue that expressed calcyclin protein showed a Schwann-like differentiation and, unlike S-100 protein, calcyclin was expressed in the perineurium.

Giant and binucleate trophoblast cells of mammals

The cellular origin, structure, and function of trophoblastic giant cells (GC) and binucleate cells (BNC) are reviewed. Mammals in which these cells have received the greatest attention include rodents, rabbits, and humans (GCs), and ruminants and equids (BNCs). In almost all cases these cells arise from the cytotrophoblast. All are large cells and contain either two diploid nuclei (BNCs), multiple nuclei (human placental bed GCs), or single nuclei with amplified DNA content (rodent and rabbit GCs). Giant and binucleate cells typically exhibit the capacity for migration or invasion, although the degree of migratory activity varies between species. While most end up within, or at the interface with, endometrial tissue, in some instances the GCs or BNCs contribute directly to the interhemal membrane of the placenta. Hormone production is a property which most GC-BNC populations have in common. Lactogen or gonadotropin has been documented in almost all cells of this type examined to date, and in some animals they are also steroidogenic (e.g., rats and sheep). In spite of some common features, both structural and functional differences remain and it is suggested that use of terms such as mononuclear giant cells, multinucleate giant cells, and binucleate cells be continued rather than assuming that these cells are all members of a single trophoblastic subtype.

Characterization of calcyclin fragments obtained by CNBr-cleavage

1. Two calcyclin fragments were obtained by CNBr-cleavage. 2. One fragment represented N-terminal end of a molecule (residues 1-56), and another one a C-terminal end (residues 57-89). 3. Properties of intact calcyclin such as binding of calcium, binding to hydrophobic resins and interaction with calcyclin specific antibodies were not retained by these fragments. 4. However, both fragments were able to form dimers and higher forms of aggregates as seen for uncleaved calcyclin. 5. This indicates that both halves of the molecule contain the regions responsible for non-covalent interaction which might participate in dimer formation.

The gene encoding the calcium binding protein calcyclin is expressed at sites of exocytosis in the mouse

Calcyclin is a member of the S100 family of calcium binding proteins. We have found by in situ hybridization that calcyclin transcripts are restricted to specific cell types within a limited number of mouse organs. High levels of expression in the epithelia lining the gastrointestinal, respiratory and urinary tracts, and specific localization of the transcripts to the goblet cells in the small intestine, lead us to suggest a role for calcyclin in the process of mucus secretion. In addition, calcyclin expression was detected in the corpus luteum, placenta and nerves within the gut wall, which are all sites of regulated exocytosis. We propose that this S100-like protein may be part of a calcium signalling pathway utilized in the secretion of various products by different cell types.

Regulated temporal and spatial expression of the calcium-binding proteins calcyclin and OPN (osteopontin) in mouse tissues during pregnancy

In situ hybridization and northern/slot blot analyses were used to quantify the expression of calcyclin (2A9, 5B10), osteopontin (opn, secreted phosphoprotein, 2ar) and calmodulin mRNAs in mouse tissues that support pregnancy. High-to-moderate levels of the mRNAs of all three genes were detected at discrete locations in the uterus, decidua and placenta as a function of gestation time. Calmodulin expression was constant in these tissues; calcyclin mRNA was high during early pregnancy and declined after day 8-9 of gestation; and opn mRNA was undetectable before day 7, with maximal levels on days 9-12 in each of these tissues. Calcyclin, but not opn, expression was also observed in the chorioamnion after day 12. Calcyclin was expressed throughout the decidua on day 8 but became restricted to the primary (antimesometrial) decidual zone and decidua lateralis on day 9, and the decidua capsularis after day 9. By contrast, opn mRNA was localized on day 9 to the mesometrial triangle, which contains a large population of granulated metrial gland cells, and to the decidua basalis. These two genes may serve as markers for the two types of decidual tissue. We suggest that one function of OPN, which may be an indicator of cells in the decidua that have a bone marrow genealogy, is to mediate the flux of calcium from the maternal circulation to the developing embryo.