Integrin β1 is bound to galectin-1 in human trophoblast

The functional roles of protein glycosylation in human maternal-fetal crosstalk

BACKGROUND

The establishment of maternal-fetal crosstalk is vital to a successful pregnancy. Glycosylation is a post-translational modification in which glycans (monosaccharide chains) are attached to an organic molecule. Glycans are involved in many physiological and pathological processes. Human endometrial epithelium, endometrial gland secretions, decidual immune cells, and trophoblasts are highly enriched with glycoconjugates and glycan-binding molecules important for a healthy pregnancy. Aberrant glycosylation in the placenta and uterus has been linked to repeated implantation failure and various pregnancy complications, but there is no recent review summarizing the functional roles of glycosylation at the maternal-fetal interface and their associations with pathological processes.

OBJECTIVE AND RATIONALE

This review aims to summarize recent findings on glycosylation, glycosyltransferases, and glycan-binding receptors at the maternal-fetal interface, and their involvement in regulating the biology and pathological conditions associated with endometrial receptivity, placentation and maternal-fetal immunotolerance. Current knowledge limitations and future insights into the study of glycobiology in reproduction are discussed.

SEARCH METHODS

A comprehensive PubMed search was conducted using the following keywords: glycosylation, glycosyltransferases, glycan-binding proteins, endometrium, trophoblasts, maternal-fetal immunotolerance, siglec, selectin, galectin, repeated implantation failure, early pregnancy loss, recurrent pregnancy loss, preeclampsia, and fetal growth restriction. Relevant reports published between 1980 and 2023 and studies related to these reports were retrieved and reviewed. Only publications written in English were included.

OUTCOMES

The application of ultrasensitive mass spectrometry tools and lectin-based glycan profiling has enabled characterization of glycans present at the maternal-fetal interface and in maternal serum. The endometrial luminal epithelium is covered with highly glycosylated mucin that regulates blastocyst adhesion during implantation. In the placenta, fucose and sialic acid residues are abundantly presented on the villous membrane and are essential for proper placentation and establishment of maternal-fetal immunotolerance. Glycan-binding receptors, including selectins, sialic-acid-binding immunoglobulin-like lectins (siglecs) and galectins, also modulate implantation, trophoblast functions and maternal-fetal immunotolerance. Aberrant glycosylation is associated with repeated implantation failure, early pregnancy loss and various pregnancy complications. The current limitation in the field is that most glycobiological research relies on association studies, with few studies revealing the specific functions of glycans. Technological advancements in analytic, synthetic and functional glycobiology have laid the groundwork for further exploration of glycans in reproductive biology under both physiological and pathological conditions.

WIDER IMPLICATIONS

A deep understanding of the functions of glycan structures would provide insights into the molecular mechanisms underlying their involvement in the physiological and pathological regulation of early pregnancy. Glycans may also potentially serve as novel early predictive markers and therapeutic targets for repeated implantation failure, pregnancy loss, and other pregnancy complications.

The universe of galectin-binding partners and their functions in health and disease

Galectins, a family of evolutionarily conserved glycan-binding proteins, play key roles in diverse biological processes including tissue repair, adipogenesis, immune cell homeostasis, angiogenesis, and pathogen recognition. Dysregulation of galectins and their ligands has been observed in a wide range of pathologic conditions including cancer, autoimmune inflammation, infection, fibrosis, and metabolic disorders. Through protein-glycan or protein-protein interactions, these endogenous lectins can shape the initiation, perpetuation, and resolution of these processes, suggesting their potential roles in disease monitoring and treatment. However, despite considerable progress, a full understanding of the biology and therapeutic potential of galectins has not been reached due to their diversity, multiplicity of cell targets, and receptor promiscuity. In this article, we discuss the multiple galectin-binding partners present in different cell types, focusing on their contributions to selected physiologic and pathologic settings. Understanding the molecular bases of galectin-ligand interactions, particularly their glycan-dependency, the biochemical nature of selected receptors, and underlying signaling events, might contribute to designing rational therapeutic strategies to control a broad range of pathologic conditions.

Trophoblast Cell Function in the Antiphospholipid Syndrome

Antiphospholipid syndrome (APS) is a complex thrombo-inflammatory autoimmune disease characterized by the presence of antiphospholipid antibodies (aPL). Women with APS are at high risk of recurrent early pregnancy loss as well as late obstetrical complications-premature birth due to placental insufficiency or severe preeclampsia. Accumulating evidence implies that vascular thrombosis is not the only pathogenic mechanism in obstetric APS, and that the direct negative effect of aPL on the placental cells, trophoblast, plays a major role. In this review, we summarize the current findings regarding the potential mechanisms involved in aPL-induced trophoblast dysfunction. Introduction on the APS and aPL is followed by an overview of the effects of aPL on trophoblast-survival, cell function and aPL internalization. Finally, the implication of several non-coding RNAs in pathogenesis of obstetric APS is discussed, with special emphasis of their possible role in trophoblast dysfunction and the associated mechanisms.

Roles of N-linked glycosylation and glycan-binding proteins in placentation: trophoblast infiltration, immunomodulation, angiogenesis, and pathophysiology

Protein N-linked glycosylation is a structurally diverse post-translational modification that stores biological information in a larger order of magnitude than other post-translational modifications such as phosphorylation, ubiquitination and acetylation. This gives N-glycosylated proteins a diverse range of properties and allows glyco-codes (glycan-related information) to be deciphered by glycan-binding proteins (GBPs). The intervillous space of the placenta is richly populated with membrane-bound and secreted glycoproteins. Evidence exists to suggest that altering the structural nature of their N-glycans can impact several trophoblast functions, which include those related to interactions with decidual cells. This review summarizes trophoblast-related activities influenced by N-glycan-GBP recognition, exploring how different subtypes of trophoblasts actively adapt to characteristics of the decidualized endometrium through cell-specific expression of N-glycosylated proteins, and how these cells receive decidua-derived signals via N-glycan-GBP interactions. We highlight work on how changes in N-glycosylation relates to the success of trophoblast infiltration, interactions of immunomodulators, and uterine angiogenesis. We also discuss studies that suggest aberrant N-glycosylation of trophoblasts may contribute to the pathogenesis of pregnancy complications (e.g. pre-eclampsia, early spontaneous miscarriages and hydatidiform mole). We propose that a more in-depth understanding of how N-glycosylation shapes trophoblast phenotype during early pregnancy has the potential to improve our approach to predicting, diagnosing and alleviating poor maternal/fetal outcomes associated with placental dysfunction.

Axonal Regeneration Mediated by a Novel Axonal Guidance Pair, Galectin-1 and Secernin-1

Galectin-1 (Gal-1), a member of the Galectin family, is expressed in various tissues and responsible for multiple biological activities. Previous studies reported that extracellular Gal-1 participated in axonal growth and repair, and Gal-1 knockout mice exhibited memory impairment. However, no study has demonstrated the direct contribution of intracellular Gal-1 upregulation in neurons to promoting axonal regeneration in the brain and recovering memory function. In the present study, we found that axonal growth is promoted by overexpression of Gal-1 via adeno-associated virus serotype 9 delivery in primary cultured hippocampal neurons. Moreover, Gal-1 was expressed on the membranes of growth cones in hippocampal neurons and interacted with a novel axonal guidance molecule, Secernin-1, which was secreted from prefrontal cortex (PFC) neurons. Gal-1-overexpression-driven axonal growth was enhanced when recombinant (extracellular) Secernin-1 was treated to the axonal site in a neuron device chamber. Direct binding of extracellular Secernin-1 with Gal-1 was detected through immunoprecipitation and immunocytochemistry, demonstrating that Gal-1 possibly works as an axonal guidance receptor for Secernin-1 in hippocampal neurons. In the PFC, the expression of Gal-1 in axonal shafts and terminals of hippocampal neurons was decreased in the 5XFAD mouse model of Alzheimer's disease (AD). Overexpression of Gal-1 in hippocampal neurons recovered memory deficits and induced axonal regeneration toward the PFC in 5XFAD mice. This study suggests that the enhanced interaction of Secernin-1 and Gal-1 can be harnessed as a therapeutic strategy for long-distance and direction-specific axonal regeneration in AD.

Galectins in Early Pregnancy and Pregnancy-Associated Pathologies

Galectins are a family of conserved soluble proteins defined by an affinity for β-galactoside structures present on various glycoconjugates. Over the past few decades, galectins have been recognized as important factors for successful implantation and maintenance of pregnancy. An increasing number of studies have demonstrated their involvement in trophoblast cell function and placental development. In addition, several lines of evidence suggest their important roles in feto-maternal immune tolerance regulation and angiogenesis. Changed or dysregulated galectin expression is also described in pregnancy-related disorders. Although the data regarding galectins' clinical relevance are still at an early stage, evidence suggests that some galectin family members are promising candidates for better understanding pregnancy-related pathologies, as well as predicting biomarkers. In this review, we aim to summarize current knowledge of galectins in early pregnancy as well as in pregnancy-related pathologies.

The role of decidual natural killer cell derived soluble factors in early pregnancy

Decidual natural killer cell (dNK), the predominant decidual lymphocytes in early pregnancy, are primarily identified based on their CD56^bright CD16^- phenotype and play an important role in maintaining immune tolerance at the maternal-fetal interface. dNK dysfunction reportedly leads to pathological pregnancy. Indeed, various dNK-derived soluble factors are involved in a series of key processes related to pregnancy outcomes. In this review, we summarize the roles of these dNK-derived factors in immune tolerance and embryonic development to improve the current understanding regarding the physiological and pathological mechanisms that occur during pregnancy, while potentially informing the development of effective therapeutics.

Proteomimetic surface fragments distinguish targets by function

The fragment-centric design promises a means to develop complex xenobiotic protein surface mimetics, but it is challenging to find locally biomimetic structures. To address this issue, foldameric local surface mimetic (LSM) libraries were constructed. Protein affinity patterns, ligand promiscuity and protein druggability were evaluated using pull-down data for targets with various interaction tendencies and levels of homology. LSM probes based on H14 helices exhibited sufficient binding affinities for the detection of both orthosteric and non-orthosteric spots, and overall binding tendencies correlated with the magnitude of the target interactome. Binding was driven by two proteinogenic side chains and LSM probes could distinguish structurally similar proteins with different functions, indicating limited promiscuity. Binding patterns displayed similar side chain enrichment values to those for native protein-protein interfaces implying locally biomimetic behavior. These analyses suggest that in a fragment-centric approach foldameric LSMs can serve as useful probes and building blocks for undruggable protein interfaces.

Role of galectin-glycan circuits in reproduction: from healthy pregnancy to preterm birth (PTB)

Growing evidence suggests that galectins, an evolutionarily conserved family of glycan-binding proteins, fulfill key roles in pregnancy including blastocyst implantation, maternal-fetal immune tolerance, placental development, and maternal vascular expansion, thereby establishing a healthy environment for the growing fetus. In this review, we comprehensively present the function of galectins in shaping cellular circuits that characterize a healthy pregnancy. We describe the current understanding of galectins in term and preterm labor and discuss how the galectin-glycan circuits contribute to key immunological pathways sustaining maternal tolerance and preventing microbial infections. A deeper understanding of the glycoimmune pathways regulating early events in preterm birth could offer the broader translational potential for the treatment of this devastating syndrome.

Collagen at the maternal-fetal interface in human pregnancy

The survival and development of a semi-allogenic fetus during pregnancy require special immune tolerance microenvironment at the maternal fetal interface. During the establishment of a successful pregnancy, the endometrium undergoes a series of changes, and the extracellular matrix (ECM) breaks down and remodels. Collagen is one of the most abundant ECM. Emerging evidence has shown that collagen and its fragment are expressed at the maternal fetal interface. The regulation of expression of collagen is quite complex, and this process involves a multitude of factors. Collagen exerts a critical role during the successful pregnancy. In addition, the abnormal expressions of collagen and its fragments are associated with certain pathological states associated with pregnancy, including recurrent miscarriage, diabetes mellitus with pregnancy, preeclampsia and so on. In this review, the expression and potential roles of collagen under conditions of physiological and pathological pregnancy are systematically discussed.

Galectin-1-Related Modulation of Trophoblast Endothelial Interactions by Integrins α1 and β1

During normal trophoblast invasion, integrins α6β4 are downregulated, and α1β1 are upregulated in invasive cytotrophoblast cells. In preeclampsia both interstitial and endovascular invasion are shallow and cytotrophoblasts fail to upregulate α1β1 and downregulate α6β4. This study aims to investigate the role of integrins α1β1 and α6β4 on cellular pathways influencing trophoblast integration into endothelial cellular networks in vitro. Red fluorescent-labeled human uterine myometrial microvascular endothelial cells (UtMVECs) were seeded on Matrigel to form endothelial networks. Green fluorescent-labeled trophoblastic HTR-8/SVneo cells pre-incubated with 20 μg/ml of neutralizing antibodies (anti-α1, β1, α6, β4, α1 + β1, or α6 + β4) for 1 h were then co-cultured with endothelial networks with the neutralizing antibodies for 24 h. Fluorescent images were captured, and quantified utilizing Image J. Cells were retrieved to analyze mRNA expression of galectin-1, TIMP-1, and PAI-1 by quantitative PCR. MMP-2, MMP-9, free sFlt-1, and PlGF from conditioned media were measured by ELISA. The integration of trophoblast cells into endothelial cellular networks was inhibited by anti-β1(- 28 ± 3%, p < 0.0001), and increased by anti-α6(+ 19 ± 5%, p < 0.01). Galectin-1 mRNA expression was decreased by anti-α1(- 35 ± 7%, p < 0.001), anti-β1(- 23 ± 5%, p < 0.05), and anti-α1+β1(- 35 ± 5%, p < 0.001). The mRNA expression of TIMP-1 was inhibited by anti-α1(- 59 ± 9%, p < 0.01) and anti-β1(- 63 ± 7%, p < 0.001) while PAI-1 mRNA expression was increased by anti-α1 + β1(+ 285 ± 70%, p < 0.0001). In the conditioned medium, anti-α1 reduced MMP-2(-28 ± 1%, p < 0.001), MMP-9(-27 ± 8%, p < 0.01), and sFlt-1(-27 ± 5%, p < 0.001) production. Anti-β1 reduced MMP-2(- 15 ± 2%, p < 0.05) production. There were no changes in PlGF. Appropriate integrins α1β1 modulate trophoblast cell integration into endothelial cellular networks in vitro through invasive pathways including galectin-1, TIMP-1, PAI-1, MMP-2, and MMP-9 production.

Pregnancy Galectinology: Insights Into a Complex Network of Glycan Binding Proteins

Galectins are a phylogenetically conserved family of soluble β-galactoside binding proteins, consisting of 15 different types, each with a specific function. Galectins contribute to placentation by regulating trophoblast development, migration, and invasion during early pregnancy. In addition, galectins are critical players regulating maternal immune tolerance to the embedded embryo. Recently, the role of galectins in angiogenesis during decidualization and in placenta formation has gained attention. Altered expression of galectins is associated with abnormal pregnancies and infertility. This review focuses on the role of galectins in pregnancy-associated processes and discusses the relevance of galectin-glycan interactions as potential therapeutic targets in pregnancy disorders.

Loss of Stromal Galectin-1 Enhances Multiple Myeloma Development: Emphasis on a Role in Osteoclasts

Multiple myeloma osteolytic disease is caused by an uncoupled bone-remodelling process with an increased osteoclast activity. Disease development relies on interactions between myeloma cells and bone marrow stromal cells. Recent findings suggest a role for glycan-binding proteins in myeloma microenvironment. Here, we investigated lectins involved in osteoclastogenesis and their role in myeloma bone disease. Microarray data analysis showed a lower expression of galectin-1 (gal-1) in mature osteoclasts compared to monocytic progenitor cells, confirmed at the RNA and protein levels in osteoclast cultures. Confocal microscopy showed that gal-1 localised predominantly in the sealing zone of mature osteoclasts. Although equal differentiated-osteoclast numbers, gal-1^−/− osteoclasts showed a higher resorption activity compared to wild-type controls. Micro-computed tomography showed an aberrant bone phenotype with decreased bone densities in gal-1^−/− mice. In vivo, tumour progression was faster in gal-1^−/− mice and associated with a marked bone loss. Additionally, myeloma cells were found to decrease gal-1 expression in osteoclasts. Our results demonstrate that galectin-1 regulates osteoclast activity with an increased resorption by gal-1^−/− osteoclasts and decreased bone densities in gal-1^−/− mice. We observed an enhanced tumour development in gal-1^−/− mice compared to wild-type mice, suggesting that galectin-1 has a functional role in stromal cells in myeloma microenvironment.