The sugar code: letters and vocabulary, writers, editors and readers and biosignificance of functional glycan-lectin pairing

Cysteine Oxidation in Human Galectin-1 Occurs Sequentially via a Folded Intermediate to a Fully Oxidized Unfolded Form

Galectins are multifunctional effectors in cellular homeostasis and dysregulation. Oxidation of human galectin-1 (Gal-1) with its six sulfhydryls produces a disulfide-bridged oxidized form that lacks normal lectin activity yet gains new glycan-independent functionality. Nevertheless, the mechanistic details as to how Gal-1 oxidation occurs remain unclear. Here, we used 15N and 13C HSQC NMR spectroscopy to gain structural insight into the CuSO4-mediated path of Gal-1 oxidation and identified a minimum two-stage conversion process. During the first phase, disulfide bridges form slowly between C16-C88 and/or C42-C66 to produce a partially oxidized, conformationally flexible intermediate that retains the ability to bind lactose. Site-directed mutagenesis of C16 to S16 impedes the onset of this overall slow process. During the second phase, increased motional dynamics of the intermediate enable the relatively distant C2 and C130 residues to form the third and final disulfide bond, leading to an unfolded state and consequent dimer dissociation. This fully oxidized end state loses the ability to bind lactose, as shown by the hemagglutination assay. Consistent with this model, we observed that the Gal-1 C2S mutant maintains intermediate-state structural features with a free sulfhydryl group at C130. Incubation with dithiothreitol reduces all disulfide bonds and allows the lectin to revert to its native state. Thus, the sequential, non-random formation of three disulfide bridges in Gal-1 in an oxidative environment acts as a molecular switch for fundamental changes to its functionality. These data inspire detailed bioactivity analysis of the structurally defined oxidized intermediate in, e.g., acute and chronic inflammation.

Microbial lectins as a potential therapeutics for the prevention of certain human diseases

Lectins are protein or glycoprotein molecules with a specific ability to bind to carbohydrates. From viruses to mammals, they are found in various organisms and exhibit remarkable diverse structures and functions. They are significant contributors to defense mechanisms against microbial attacks in plants. They are also involved in functions such as controlling lymphocyte migration, regulating glycoprotein biosynthesis, cell-cell recognition, and embryonic development in animals. In addition, lectins serve as invaluable molecular tools in various biological and medical disciplines due to their reversible binding ability and enable the monitoring of cell membrane changes in physiological and pathological contexts. Microbial lectins, often referred to as adhesins, play an important role in microbial colonization, pathogenicity, and interactions among microorganisms. Viral lectins are located in the bilayered viral membrane, whereas bacterial lectins are found intracellularly and on the bacterial cell surface. Microfungal lectins are typically intracellular and have various functions in host-parasite interaction, and in fungal growth and morphogenesis. Although microbial lectin studies are less extensive than those of plants and animals, they provide insights into the infection mechanisms and potential interventions. Glycan specificity, essential functions in infectious diseases, and applications in the diagnosis and treatment of viral and bacterial infections are critical aspects of microbial lectin research. In this review, we will discuss the application and therapeutic potential of viral, bacterial and microfungal lectins.

Human brain glycoform coregulation network and glycan modification alterations in Alzheimer's disease

Despite the importance of protein glycosylation to brain health, current knowledge of glycosylated proteoforms or glycoforms in human brain and their alterations in Alzheimer's disease (AD) is limited. Here, we report a proteome-wide glycoform profiling study of human AD and control brains using intact glycopeptide-based quantitative glycoproteomics coupled with systems biology. Our study identified more than 10,000 human brain N-glycoforms from nearly 1200 glycoproteins and uncovered disease signatures of altered glycoforms and glycan modifications, including reduced sialylation and N-glycan branching and elongation as well as elevated mannosylation and N-glycan truncation in AD. Network analyses revealed a higher-order organization of brain glycoproteome into networks of coregulated glycoforms and glycans and discovered glycoform and glycan modules associated with AD clinical phenotype, amyloid-β accumulation, and tau pathology. Our findings provide valuable insights into disease pathogenesis and a rich resource of glycoform and glycan changes in AD and pave the way forward for developing glycosylation-based therapies and biomarkers for AD.

Galectins in epithelial-mesenchymal transition: roles and mechanisms contributing to tissue repair, fibrosis and cancer metastasis

Galectins are soluble glycan-binding proteins that interact with a wide range of glycoproteins and glycolipids and modulate a broad spectrum of physiological and pathological processes. The expression and subcellular localization of different galectins vary among tissues and cell types and change during processes of tissue repair, fibrosis and cancer where epithelial cells loss differentiation while acquiring migratory mesenchymal phenotypes. The epithelial-mesenchymal transition (EMT) that occurs in the context of these processes can include modifications of glycosylation patterns of glycolipids and glycoproteins affecting their interactions with galectins. Moreover, overexpression of certain galectins has been involved in the development and different outcomes of EMT. This review focuses on the roles and mechanisms of Galectin-1 (Gal-1), Gal-3, Gal-4, Gal-7 and Gal-8, which have been involved in physiologic and pathogenic EMT contexts.

Nuclear magnetic resonance-based structural elucidation of novel marine glycans and derived oligosaccharides

Marine glycans of defined structures are unique representatives among all kinds of structurally complex glycans endowed with important biological actions. Besides their unique biological properties, these marine sugars also enable advanced structure-activity relationship (SAR) studies given their distinct and defined structures. However, the natural high molecular weights (MWs) of these marine polysaccharides, sometimes even bigger than 100 kDa, pose a problem in many biophysical and analytical studies. Hence, the preparation of low MW oligosaccharides becomes a strategy to overcome the problem. Regardless of the polymeric or oligomeric lengths of these molecules, structural elucidation is mandatory for SAR studies. For this, nuclear magnetic resonance (NMR) spectroscopy plays a pivotal role. Here, we revisit the NMR-based structural elucidation of a series of marine sulfated poly/oligosaccharides discovered in our laboratory within the last 2 years. This set of structures includes the α-glucan extracted from the bivalve Marcia hiantina; the two sulfated galactans extracted from the red alga Botryocladia occidentalis; the fucosylated chondroitin sulfate isolated from the sea cucumber Pentacta pygmaea; the oligosaccharides produced from the fucosylated chondroitin sulfates from this sea cucumber species and from another species, Holothuria floridana; and the sulfated fucan from this later species. Specific 1H and 13C chemical shifts, generated by various 1D and 2D homonuclear and heteronuclear NMR spectra, are exploited as the primary source of information in the structural elucidation of these marine glycans.

HumanLectome, an update of UniLectin for the annotation and prediction of human lectins

The UniLectin portal (https://unilectin.unige.ch/) was designed in 2019 with the goal of centralising curated and predicted data on carbohydrate-binding proteins known as lectins. UniLectin is also intended as a support for the study of lectomes (full lectin set) of organisms or tissues. The present update describes the inclusion of several new modules and details the latest (https://unilectin.unige.ch/humanLectome/), covering our knowledge of the human lectome and comprising 215 unevenly characterised lectins, particularly in terms of structural information. Each HumanLectome entry is protein-centric and compiles evidence of carbohydrate recognition domain(s), specificity, 3D-structure, tissue-based expression and related genomic data. Other recent improvements regarding interoperability and accessibility are outlined.

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.

Glycosort: A Computational Solution to Post-process Quantitative Large-Scale Intact Glycopeptide Analyses

Protein glycosylation is a post-translational modification involving the addition of carbohydrates to proteins and plays a crucial role in protein folding and various biological processes such as cell recognition, differentiation, and immune response. The vast array of natural sugars available allows the generation of plenty of unique glycan structures in proteins, adding complexity to the regulation and biological functions of glycans. The diversity is further increased by enzymatic site preferences and stereochemical conjugation, leading to an immense amount of different glycan structures. Understanding glycosylation heterogeneity is vital for unraveling the impact of glycans on different biological functions. Evaluating site occupancies and structural heterogeneity aids in comprehending glycan-related alterations in biological processes. Several software tools are available for large-scale glycoproteomics studies; however, integrating identification and quantitative data to assess heterogeneity complexity often requires extensive manual data processing. To address this challenge, we present a python script that automates the integration of Byonic and MaxQuant outputs for glycoproteomic data analysis. The script enables the calculation of site occupancy percentages by glycans and facilitates the comparison of glycan structures and site occupancies between two groups. This automated tool offers researchers a means to organize and interpret their high-throughput quantitative glycoproteomic data effectively.

Human brain glycoform co-regulation network and glycan modification alterations in Alzheimer's disease

Despite the importance of protein glycosylation to brain health, current knowledge of glycosylated proteoforms or glycoforms in human brain and their alterations in Alzheimer's disease (AD) is limited. Here, we present a new paradigm of proteome-wide glycoform profiling study of human AD and control brains using intact glycopeptide-based quantitative glycoproteomics coupled with systems biology. Our study identified over 10,000 human brain N-glycoforms from nearly 1200 glycoproteins and uncovered disease signatures of altered glycoforms and glycan modifications, including reduced sialylation and N-glycan branching as well as elevated mannosylation and N-glycan truncation in AD. Network analyses revealed a higher-order organization of brain glycoproteome into networks of co-regulated glycoforms and glycans and discovered glycoform and glycan modules associated with AD clinical phenotype, amyloid-β accumulation, and tau pathology. Our findings provide novel insights and a rich resource of glycoform and glycan changes in AD and pave the way forward for developing glycosylation-based therapies and biomarkers for AD.

Differential cellular localization of lectins in the testes of dromedary camel (Camelus dromedarius) during active and inactive breeding seasons

The reproductive activity of the male dromedary camel (Camelus dromedarius) as a seasonal breeder is affected by various seasonal changes that reflect on the reproductive performance. In the current study, we explored a differential cellular localization of lectins in eight dromedary camel testes utilizing lectin histochemistry (LHC). The glycoconjugates' localizations were detected within the testicular tissue utilizing 13 biotin-labeled lectins (PNA, ConA, LCA, RCA120, GS IB4, WGA, BPL, DBA, ECA, PHA-E4, UEA-1, PTL-II, and SBA) distributed into six sets. The cellular structures revealed diverse lectins distribution that may reflect various glycoproteins' structures and their compositional modifications during spermatogenesis. Some of the investigated lectins were restricted to acrosomes of spermatids that will help study different stages during the spermatogenic cycle of dromedary camel, particularly PNA, and ECA. The statistical analysis showed a marked positive correlation between the response intensity of various lectins and the breeding season (P < 0.05). We can conclude that lectins have a fundamental role during camel spermatogenesis and are associated with the reproductive activity of dromedary camel.

Nothing in evolution makes sense except in the light of code biology

This article highlights the potential contribution of biological codes to the course and dynamics of evolution. The concept of organic codes, developed by Marcello Barbieri, has fundamentally changed our view of how living systems function. The notion that molecular interactions built on adaptors that arbitrarily link molecules from different "worlds" in a conventional, i.e., rule-based way, departs significantly from the law-based constraints imposed on livening things by physical and chemical mechanisms. In other words, living and non-living things behave like rules and laws, respectively, but this important distinction is rarely considered in current evolutionary theory. The many known codes allow quantification of codes that relate to a cell, or comparisons between different biological systems and may pave the way to a quantitative and empirical research agenda in code biology. A starting point for such an endeavour is the introduction of a simple dichotomous classification of structural and regulatory codes. This classification can be used as a tool to analyse and quantify key organising principles of the living world, such as modularity, hierarchy, and robustness, based on organic codes. The implications for evolutionary research are related to the unique dynamics of codes, or ´Eigendynamics´ (self-momentum) and how they determine the behaviour of biological systems from within, whereas physical constraints are imposed mainly from without. A speculation on the drivers of macroevolution in light of codes is followed by the conclusion that a meaningful and comprehensive understanding of evolution depends including codes into the equation of life.

Step-Growth Glycopolymers with a Defined Tacticity for Selective Carbohydrate-Lectin Recognition

Glycopolymers are potent candidates for biomedical applications by exploiting multivalent carbohydrate-lectin interactions. Owing to their specific recognition capabilities, glycosylated polymers can be utilized for targeted drug delivery to certain cell types bearing the corresponding lectin receptors. A fundamental challenge in glycopolymer research, however, is the specificity of recognition to receptors binding to the same sugar unit (e.g., mannose). Variation of polymer backbone chirality has emerged as an effective method to distinguish between lectins on a molecular level. Herein, we present a facile route toward producing glycopolymers with a defined tacticity based on a step-growth polymerization technique using click chemistry. A set of polymers have been fabricated and further functionalized with mannose moieties to enable lectin binding to receptors relevant to the immune system (mannose-binding lectin, dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin, and dendritic and thymic epithelial cell-205). Surface plasmon resonance spectrometry was employed to determine the kinetic parameters of the step-growth glycopolymers. The results highlight the importance of structural complexity in advancing glycopolymer synthesis, yet multivalency remains a main driving force in lectin recognition.

Neuronal Ganglioside and Glycosphingolipid (GSL) Metabolism and Disease : Cascades of Secondary Metabolic Errors Can Generate Complex Pathologies (in LSDs)

Glycosphingolipids (GSLs) are a diverse group of membrane components occurring mainly on the surfaces of mammalian cells. They and their metabolites have a role in intercellular communication, serving as versatile biochemical signals (Kaltner et al, Biochem J 476(18):2623-2655, 2019) and in many cellular pathways. Anionic GSLs, the sialic acid containing gangliosides (GGs), are essential constituents of neuronal cell surfaces, whereas anionic sulfatides are key components of myelin and myelin forming oligodendrocytes. The stepwise biosynthetic pathways of GSLs occur at and lead along the membranes of organellar surfaces of the secretory pathway. After formation of the hydrophobic ceramide membrane anchor of GSLs at the ER, membrane-spanning glycosyltransferases (GTs) of the Golgi and Trans-Golgi network generate cell type-specific GSL patterns for cellular surfaces. GSLs of the cellular plasma membrane can reach intra-lysosomal, i.e. luminal, vesicles (ILVs) by endocytic pathways for degradation. Soluble glycoproteins, the glycosidases, lipid binding and transfer proteins and acid ceramidase are needed for the lysosomal catabolism of GSLs at ILV-membrane surfaces. Inherited mutations triggering a functional loss of glycosylated lysosomal hydrolases and lipid binding proteins involved in GSL degradation cause a primary lysosomal accumulation of their non-degradable GSL substrates in lysosomal storage diseases (LSDs). Lipid binding proteins, the SAPs, and the various lipids of the ILV-membranes regulate GSL catabolism, but also primary storage compounds such as sphingomyelin (SM), cholesterol (Chol.), or chondroitin sulfate can effectively inhibit catabolic lysosomal pathways of GSLs. This causes cascades of metabolic errors, accumulating secondary lysosomal GSL- and GG- storage that can trigger a complex pathology (Breiden and Sandhoff, Int J Mol Sci 21(7):2566, 2020).

Carbon-based glyco-nanoplatforms: towards the next generation of glycan-based multivalent probes

Cell surface carbohydrates mediate a wide range of carbohydrate-protein interactions key to healthy and disease mechanisms. Many of such interactions are multivalent in nature and in order to study these processes at a molecular level, many glycan-presenting platforms have been developed over the years. Among those, carbon nanoforms such as graphene and their derivatives, carbon nanotubes, carbon dots and fullerenes, have become very attractive as biocompatible platforms that can mimic the multivalent presentation of biologically relevant glycosides. The most recent examples of carbon-based nanoplatforms and their applications developed over the last few years to study carbohydrate-mediate interactions in the context of cancer, bacterial and viral infections, among others, are highlighted in this review.

Targeting osteoarthritis-associated galectins and an induced effector class by a ditopic bifunctional reagent: Impact of its glycan part on binding measured in the tissue context

Pairing glycans with tissue lectins controls multiple effector pathways in (patho)physiology. A clinically relevant example is the prodegradative activity of galectins-1 and -3 (Gal-1 and -3) in the progression of osteoarthritis (OA) via matrix metalloproteinases (MMPs), especially MMP-13. The design of heterobifunctional inhibitors that can block galectin binding and MMPs both directly and by preventing their galectin-dependent induction selectively offers a perspective to dissect the roles of lectins and proteolytic enzymes. We describe the synthesis of such a reagent with a bivalent galectin ligand connected to an MMP inhibitor and of two tetravalent glycoclusters with a subtle change in headgroup presentation for further elucidation of influence on ligand binding. Testing was performed on clinical material with mixtures of galectins as occurring in vivo, using sections of fixed tissue. Two-colour fluorescence microscopy monitored binding to the cellular glycome after optimization of experimental parameters. In the presence of the inhibitor, galectin binding to OA specimens was significantly reduced. These results open the perspective to examine the inhibitory capacity of custom-made ditopic compounds on binding of lectins in mixtures using sections of clinical material with known impact of galectins and MMPs on disease progression.

Exploring the In situ pairing of human galectins toward synthetic O-mannosylated core M1 glycopeptides of α-dystroglycan

Dystroglycan (DG), which constitutes a part of the dystrophin-glycoprotein complex, connects the extracellular matrix to the cytoskeleton. The matriglycans presented by the extracellular α-DG serve as a contact point with extracellular matrix proteins (ECM) containing laminin G-like domains, providing cellular stability. However, it remains unknown whether core M1 (GlcNAcβ1-2Man) structures can serve as ligands among the various O-Mannosylated glycans. Therefore, based on the presence of N-acetylLactosamine (LacNAc) in this glycan following the core extension, the binding interactions with adhesion/growth-regulatory galectins were explored. To elucidate this process, the interaction between galectin (Gal)-1, -3, -4 and -9 with α-DG fragment ³⁷²TRGAIIQTPTLGPIQPTRV³⁹⁰ core M1-based glycopeptide library were profiled, using glycan microarray and nuclear magnetic resonance studies. The binding of galectins was revealed irrespective of its modular architecture, adding galectins to the list of possible binding partners of α-DG core M1 glycoconjugates by cis-binding (via peptide- and carbohydrate-protein interactions), which can be abrogated by α2,3-sialylation of the LacNAc units. The LacNAc-terminated α-DG glycopeptide interact simultaneously with both the S- and F-faces of Gal-1, thereby inducing oligomerization. Furthermore, Gal-1 can trans-bridge α-DG core M1 structures and laminins, which proposed a possible mechanism by which Gal-1 ameliorates muscular dystrophies; however, this proposal warrants further investigation.

Galectin-9 supports primary T cell transendothelial migration in a glycan and integrin dependent manner

Adaptive immunity relies on the efficient recruitment of T cells from the blood into peripheral tissues. However, the current understanding of factor(s) coordinating these events is incomplete. Previous studies on galectin-9 (Gal-9), have proposed a functionally significant role for this lectin in mediating leukocyte adhesion and transmigration. However, very little is known about its function in T cell migration. Here, we have investigated the role of the Gal-9 on the migration behaviour of both human primary CD4⁺ and CD8⁺ T cells. Our data indicate that Gal-9 supports both CD4⁺ and CD8⁺ T cell adhesion and transmigration in a glycan dependent manner, inducing L-selectin shedding and upregulation of LFA-1 and CXCR4 expression. Additionally, when immobilized, Gal-9 promoted capture and firm adhesion of T cells under flow, in a glycan and integrin-dependent manner. Using an in vivo model, dorsal air pouch, we found that Gal-9 deficient mice display impaired leukocyte trafficking, with a reduction in pro-inflammatory cytokines/chemokines generated locally. Furthermore, we also demonstrate that Gal-9 inhibits the chemotactic function of CXCL12 through direct binding. In conclusion, our study characterises, for the first time, the capture, adhesion, and migration behaviour of CD4⁺ and CD8⁺ T cells to immobilised /endothelial presented Gal-9, under static and physiological flow conditions. We also demonstrate the differential binding characteristics of Gal-9 to T cell subtypes, which could be of potential therapeutic significance, particularly in the treatment of inflammatory-based diseases, given Gal-9 ability to promote apoptosis in pathogenic T cell subsets.

Proteomics and glycoproteomics of beer and wine

Beer and wine are fermented beverages that contain abundant proteins released from barley or grapes, and secreted from yeast. These proteins are associated with many quality attributes including turbidity, foamability, effervescence, flavour and colour. Many grape proteins and secreted yeast proteins are glycosylated, and barley proteins can be glycated under the high temperatures in the beer making process. The emergence of high-resolution mass spectrometry has allowed proteomic and glycoproteomic analyses of these complex mixtures of proteins towards understanding their role in determining beer and wine attributes. In this review, we summarise recent studies of proteomic and glycoproteomic analyses of beer and wine including their strategies for mass spectrometry (MS)-based identification, quantification and characterisation of the glyco/proteomes of fermented beverages to control product quality.

Epithelial Na+ channel and the glycocalyx: a sweet and salty relationship for arterial shear stress sensing

PURPOSE OF REVIEW

The ability of endothelial cells to sense mechanical force, and shear stress in particular, is crucial for normal vascular function. This relies on an intact endothelial glycocalyx that facilitates the production of nitric oxide (NO). An emerging arterial shear stress sensor is the epithelial Na+ channel (ENaC). This review highlights existing and new evidence for the interdependent activity of the glycocalyx and ENaC and its implications for vascular function.

RECENT FINDINGS

New evidence suggests that the glycocalyx and ENaC are physically connected and that this is important for shear stress sensing. The connection relies on N-glycans attached to glycosylated asparagines of α-ENaC. Removal of specific N-glycans reduced ENaC's shear stress response. Similar effects were observed following degradation of the glycocalyx. Endothelial specific viral transduction of α-ENaC increased blood pressure (∼40 mmHg). This increase was attenuated in animals transduced with an α-ENaC version lacking N-glycans.

SUMMARY

These observations indicate that ENaC is connected to the glycocalyx and their activity is interdependent to facilitate arterial shear stress sensation. Future research focusing on how N-glycans mediate this interaction can provide new insights for the understanding of vascular function in health and disease.

Computer Simulation to Rationalize “Rational” Engineering of Glycoside Hydrolases and Glycosyltransferases

Glycoside hydrolases and glycosyltransferases are the main classes of enzymes that synthesize and degrade carbohydrates, molecules essential to life that are a challenge for classical chemistry. As such, considerable efforts have been made to engineer these enzymes and make them pliable to human needs, ranging from directed evolution to rational design, including mechanism engineering. Such endeavors fall short and are unreported in numerous cases, while even success is a necessary but not sufficient proof that the chemical rationale behind the design is correct. Here we review some of the recent work in CAZyme mechanism engineering, showing that computational simulations are instrumental to rationalize experimental data, providing mechanistic insight into how native and engineered CAZymes catalyze chemical reactions. We illustrate this with two recent studies in which (i) a glycoside hydrolase is converted into a glycoside phosphorylase and (ii) substrate specificity of a glycosyltransferase is engineered toward forming O-, N-, or S-glycosidic bonds.

Introducing 77Se NMR Spectroscopy to Analyzing Galectin -Ligand Interaction

Their emerging nature as multifunctional effectors explains the large interest to monitor glycan binding to galectins and to define bound-state conformer(s) of their ligands in solution. Basically, NMR spectroscopy facilitates respective experiments. Towards developing new and even better approaches for these purposes, extending the range of exploitable isotopes beyond ¹H, ¹³C, and ¹⁵N offers promising perspectives. Having therefore prepared selenodigalactoside and revealed its bioactivity as galectin ligand, monitoring of its binding by ⁷⁷Se NMR spectroscopy at a practical level becomes possible by setting up a 2D ¹H, ⁷⁷Se CPMG-HSQBMC experiment including CPMG-INEPT long-range transfer. This first step into applying ⁷⁷Se as sensor for galectin binding substantiates its potential for screening relative to inhibitory potencies in compound mixtures and for achieving sophisticated epitope mapping. The documented strategic combination of synthetic carbohydrate chemistry and NMR spectroscopy prompts to envision to work with isotopically pure ⁷⁷Se-containing β-galactosides and to build on the gained experience with ⁷⁷Se by adding ¹⁹F as second sensor in doubly labeled glycosides.

What Happens If a Human Galectin Enters the Endoplasmic Reticulum?

Mammalian galectins have no signal peptide, and it is not known what would happen if a galectin is directed to take the classical export route. The corresponding engineering of galectin-specific cDNA will answer questions on the fate of a signal peptide-bearing protein variant after its entry into the endoplasmic reticulum (ER). Affinity chromatography and mass-spectrometric analysis of occupancy of potential N-glycosylation sites for the galectin, binding and functional assays with cells as well as subcellular fractionation by density gradient ultracentrifugation and immunocytochemical colocalization with ER/Golgi markers report on aspects of the consequences of letting a galectin enter new territory. Applying these methods will help to clarify why galectins are leaderless and thus produced by free ribosomes.

Examining Galectin Gene Regulation by Reporter Assays

Matching their role as potent and versatile effectors in cellular homeostasis and disease processes, galectins are subject to a fine-tuned transcriptional regulation of gene expression. It can apparently even involve coregulation with certain elements of the enzymatic machinery for glycan biosynthesis/remodeling and/or functional carriers of galectin-binding glycans such as the α₅β₁-integrin. All this suggests not yet fully known combinatorial processes to reach the desired outcome. Identification of transcription start point(s), cloning of upstream promoter region, and the design of plasmids for luciferase-based reporter assays establish the platform to initiate a systematic search of regulatory sequences. Their elucidation is also a step toward rationally manipulating expression of galectin genes in pathogenesis.

Exploring the Galectin Network by Light and Fluorescence Microscopy

Dynamic changes of a cell's glycophenotype are increasingly interpreted as shifts in the capacity to interact with tissue (endogenous) lectins. The status of glycan branching or chain length (e.g., core 1 vs core 2 mucin-type O-glycans and polyLacNAc additions) as well as of sialylation/sulfation has been delineated to convey signals. They are "read" by galectins, for example regulating lattice formation on the membrane and cell growth. Owing to the discovery of the possibility that these effectors act in networks physiologically resulting in functional antagonism or cooperation, their detection and distribution profiling need to be expanded from an individual (single) protein to the-at best-entire family. How to work with non-cross-reactive antibodies and with the labeled tissue-derived proteins (used as probes) is exemplarily documented for chicken and human galectins including typical activity and specificity controls. This description intends to inspire the systematic (network) study of members of a lectin family and also the application of tissue proteins beyond a single lectin category in lectin histochemistry.

Structural Characterization of Rat Galectin-5, an N-Tailed Monomeric Proto-Type-like Galectin

Galectins are multi-purpose effectors acting via interactions with distinct counterreceptors based on protein-glycan/protein recognition. These processes are emerging to involve several regions on the protein so that the availability of a detailed structural characterization of a full-length galectin is essential. We report here the first crystallographic information on the N-terminal extension of the carbohydrate recognition domain of rat galectin-5, which is precisely described as an N-tailed proto-type-like galectin. In the ligand-free protein, the three amino-acid stretch from Ser2 to Ser5 is revealed to form an extra β-strand (F0), and the residues from Thr6 to Asn12 are part of a loop protruding from strands S1 and F0. In the ligand-bound structure, amino acids Ser2–Tyr10 switch position and are aligned to the edge of the β-sandwich. Interestingly, the signal profile in our glycan array screening shows the sugar-binding site to preferentially accommodate the histo-blood-group B (type 2) tetrasaccharide and N-acetyllactosamine-based di- and oligomers. The crystal structures revealed the characteristically preformed structural organization around the central Trp77 of the CRD with involvement of the sequence signature’s amino acids in binding. Ligand binding was also characterized calorimetrically. The presented data shows that the N-terminal extension can adopt an ordered structure and shapes the hypothesis that a ligand-induced shift in the equilibrium between flexible and ordered conformers potentially acts as a molecular switch, enabling new contacts in this region.

Galectin network in osteoarthritis: galectin-4 programs a pathogenic signature of gene and effector expression in human chondrocytes in vitro

Galectin-4 (Gal-4) is a member of the galectin family, which have been identified as galactose-binding proteins. Gal-4 possesses two tandem repeat carbohydrate recognition domains and acts as a cross-linking bridge in sulfatide-dependent glycoprotein routing. We herein document its upregulation in osteoarthritis (OA) in correlation with the extent of cartilage degradation in vivo. Primary human OA chondrocytes in vitro respond to carbohydrate-inhibitable Gal-4 binding with the upregulation of pro-degradative/-inflammatory proteins such as interleukin-1β (IL-1β) and matrix metalloproteinase-13 (MMP-13), as documented by RT-qPCR-based mRNA profiling and transcriptome data processing. Activation of p65 by phosphorylation of Ser536 within the NF-κB pathway and the effect of three p65 inhibitors on Gal-4 activity support downstream involvement of such signaling. In 3D (pellet) cultures, Gal-4 presence causes morphological and biochemical signs of degradation. Taken together, our findings strongly support the concept of galectins acting as a network in OA pathogenesis and suggest that blocking their activity in disease progression may become clinically relevant in the future.

The marriage of chemokines and galectins as functional heterodimers

Trafficking of leukocytes and their local activity profile are of pivotal importance for many (patho)physiological processes. Fittingly, microenvironments are complex by nature, with multiple mediators originating from diverse cell types and playing roles in an intimately regulated manner. To dissect aspects of this complexity, effectors are initially identified and structurally characterized, thus prompting familial classification and establishing foci of research activity. In this regard, chemokines present themselves as role models to illustrate the diversification and fine-tuning of inflammatory processes. This in turn discloses the interplay among chemokines, their cell receptors and cognate glycosaminoglycans, as well as their capacity to engage in new molecular interactions that form hetero-oligomers between themselves and other classes of effector molecules. The growing realization of versatility of adhesion/growth-regulatory galectins that bind to glycans and proteins and their presence at sites of inflammation led to testing the hypothesis that chemokines and galectins can interact with each other by protein-protein interactions. In this review, we present some background on chemokines and galectins, as well as experimental validation of this chemokine-galectin heterodimer concept exemplified with CXCL12 and galectin-3 as proof-of-principle, as well as sketch out some emerging perspectives in this arena.

What is the Sugar Code?

A code is defined by the nature of the symbols, which are used to generate information‐storing combinations (e. g. oligo‐ and polymers). Like nucleic acids and proteins, oligo‐ and polysaccharides are ubiquitous, and they are a biochemical platform for establishing molecular messages. Of note, the letters of the sugar code system (third alphabet of life) excel in coding capacity by making an unsurpassed versatility for isomer (code word) formation possible by variability in anomery and linkage position of the glycosidic bond, ring size and branching. The enzymatic machinery for glycan biosynthesis (writers) realizes this enormous potential for building a large vocabulary. It includes possibilities for dynamic editing/erasing as known from nucleic acids and proteins. Matching the glycome diversity, a large panel of sugar receptors (lectins) has developed based on more than a dozen folds. Lectins ‘read’ the glycan‐encoded information. Hydrogen/coordination bonding and ionic pairing together with stacking and C−H/π‐interactions as well as modes of spatial glycan presentation underlie the selectivity and specificity of glycan‐lectin recognition. Modular design of lectins together with glycan display and the nature of the cognate glycoconjugate account for the large number of post‐binding events. They give an entry to the glycan vocabulary its functional, often context‐dependent meaning(s), hereby building the dictionary of the sugar code.

Simulating cellular galectin networks by mixing galectins in vitro reveals synergistic activity

Background

Even though members of the family of adhesion/growth-regulatory galectins are increasingly detected to be co-expressed, they are still being routinely tested separately. The recent discovery of heterodimer formation among galectins-1, -3, and -7 in mixtures prompts further study of their functional activities in mixtures.

Methods

Cell agglutination, galectin binding to cells, as well as effects on cell proliferation, onset of apoptosis and migration were determined in assays using various cell types and mixtures of galectins-1, -3, and -7.

Results

Evidence for a more than additive increases of experimental parameters was consistently obtained.

Conclusion

Testing galectins in mixtures simulates the situation of co-expression in situ and reveals unsuspected over-additive activities. This new insight is relevant for analyzing galectin functionality in (patho)physiological conditions.

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Galectins-1, -3, and -7 form heterodimers in solution.

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Mixtures of galectins simulates galectin co-expression in situ.

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Mixtures display synergistic activities in vitro.

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Cell agglutination, apoptosis, proliferation, migration affected.

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Findings are relevant for galectin functionality in vivo.

The expression and role of glycans at the feto-maternal interface in humans

During pregnancy, both the maternal endometrium and the blastocyst have highly glycosylated proteins with glycosylations controlled in a specific manner. Carbohydrates play a fundamental role in cell-cell and cell-matrix recognition and are involved in defining the structure and integrity of tissues. The uterus' secretions, which are rich in glycoproteins and glycogen and the presence of a functional glycocalyx on the uterine epithelium, establish a favourable milieu, which is essential for the correct implantation and subsequent development of the blastocyst. Likewise, carbohydrate residues such as fucose and sialic acid present at the placental level are determinant in creating an immuno-environment, which supports the mother's tolerance towards the fetal antigens. In this review, we explore the literature concerning the role of important glycan-epitopes at the feto-maternal interface in the human species. Moreover, we also show some unpublished interesting results on changes of glycan residues in human placenta tissues from the first trimester of pregnancy.

The glycan alphabet is not universal: a hypothesis

Several monosaccharides constitute naturally occurring glycans, but it is uncertain whether they constitute a universal set like the alphabets of proteins and DNA. Based on the available experimental observations, it is hypothesized herein that the glycan alphabet is not universal. Data on the presence/absence of pathways for the biosynthesis of 55 monosaccharides in 12 939 completely sequenced archaeal and bacterial genomes are presented in support of this hypothesis. Pathways were identified by searching for homologues of biosynthesis pathway enzymes. Substantial variations were observed in the set of monosaccharides used by organisms belonging to the same phylum, genera and even species. Monosaccharides were grouped as common, less common and rare based on their prevalence in Archaea and Bacteria. It was observed that fewer enzymes are sufficient to biosynthesize monosaccharides in the common group. It appears that the common group originated before the formation of the three domains of life. In contrast, the rare group is confined to a few species in a few phyla, suggesting that these monosaccharides evolved much later. Fold conservation, as observed in aminotransferases and SDR (short-chain dehydrogenase reductase) superfamily members involved in monosaccharide biosynthesis, suggests neo- and sub-functionalization of genes led to the formation of the rare group monosaccharides. The non-universality of the glycan alphabet begets questions about the role of different monosaccharides in determining an organism’s fitness.

Glycans in autophagy, endocytosis and lysosomal functions

Glycans have been shown to function as versatile molecular signals in cells. This prompted us to look at their roles in endocytosis, endolysosomal system and autophagy. We start by introducing the cell biological aspects of these pathways, the concept of the sugar code, and provide an overview on the role of glycans in the targeting of lysosomal proteins and in lysosomal functions. Moreover, we review evidence on the regulation of endocytosis and autophagy by glycans. Finally, we discuss the emerging concept that cytosolic exposure of luminal glycans, and their detection by endogenous lectins, provides a mechanism for the surveillance of the integrity of the endolysosomal compartments, and serves their eventual repair or disposal.

What Molecular Recognition Systems Do Mesenchymal Stem Cells/Medicinal Signaling Cells (MSC) Use to Facilitate Cell-Cell and Cell Matrix Interactions? A Review of Evidence and Options

Mesenchymal stem cells, also called medicinal signaling cells (MSC), have been studied regarding their potential to facilitate tissue repair for >30 years. Such cells, derived from multiple tissues and species, are capable of differentiation to a number of lineages (chondrocytes, adipocytes, bone cells). However, MSC are believed to be quite heterogeneous with regard to several characteristics, and the large number of studies performed thus far have met with limited or restricted success. Thus, there is more to understand about these cells, including the molecular recognition systems that are used by these cells to perform their functions, to enhance the realization of their potential to effect tissue repair. This perspective article reviews what is known regarding the recognition systems available to MSC, the possible systems that could be looked for, and alternatives to enhance their localization to specific injury sites and increase their subsequent facilitation of tissue repair. MSC are reported to express recognition molecules of the integrin family. However, there are a number of other recognition molecules that also could be involved such as lectins, inducible lectins, or even a MSC-specific family of molecules unique to these cells. Finally, it may be possible to engineer expression of recognition molecules on the surface of MSC to enhance their function in vivo artificially. Thus, improved understanding of recognition molecules on MSC could further their success in fostering tissue repair.

Neural Tissue Homeostasis and Repair Is Regulated via CS and DS Proteoglycan Motifs

Chondroitin sulfate (CS) is the most abundant and widely distributed glycosaminoglycan (GAG) in the human body. As a component of proteoglycans (PGs) it has numerous roles in matrix stabilization and cellular regulation. This chapter highlights the roles of CS and CS-PGs in the central and peripheral nervous systems (CNS/PNS). CS has specific cell regulatory roles that control tissue function and homeostasis. The CNS/PNS contains a diverse range of CS-PGs which direct the development of embryonic neural axonal networks, and the responses of neural cell populations in mature tissues to traumatic injury. Following brain trauma and spinal cord injury, a stabilizing CS-PG-rich scar tissue is laid down at the defect site to protect neural tissues, which are amongst the softest tissues of the human body. Unfortunately, the CS concentrated in gliotic scars also inhibits neural outgrowth and functional recovery. CS has well known inhibitory properties over neural behavior, and animal models of CNS/PNS injury have demonstrated that selective degradation of CS using chondroitinase improves neuronal functional recovery. CS-PGs are present diffusely in the CNS but also form denser regions of extracellular matrix termed perineuronal nets which surround neurons. Hyaluronan is immobilized in hyalectan CS-PG aggregates in these perineural structures, which provide neural protection, synapse, and neural plasticity, and have roles in memory and cognitive learning. Despite the generally inhibitory cues delivered by CS-A and CS-C, some CS-PGs containing highly charged CS disaccharides (CS-D, CS-E) or dermatan sulfate (DS) disaccharides that promote neural outgrowth and functional recovery. CS/DS thus has varied cell regulatory properties and structural ECM supportive roles in the CNS/PNS depending on the glycoform present and its location in tissue niches and specific cellular contexts. Studies on the fruit fly, Drosophila melanogaster and the nematode Caenorhabditis elegans have provided insightful information on neural interconnectivity and the role of the ECM and its PGs in neural development and in tissue morphogenesis in a whole organism environment.

Recent advances in multivalent carbon nanoform-based glycoconjugates

Multivalent carbohydrate-mediated interactions are fundamental to many biological processes, including disease mechanisms. To study these significant glycan-mediated interactions at a molecular level, carbon nanoforms such as fullerenes, carbon nanotubes, or graphene and their derivatives have been identified as promising biocompatible scaffolds that can mimic the multivalent presentation of biologically relevant glycans. In this minireview, we will summarize the most relevant examples of the last few years in the context of their applications.

3'-Sulfo-TF Antigen Determined by GAL3ST2/ST3GAL1 Is Essential for Antitumor Activity of Fungal Galectin AAL/AAGL

Many lectins have been reported to have antitumor activities; identifying the glycan ligands in tumor cells of lectins is crucial for lectin clinical application. An edible mushroom galectin, Agrocybe aegerita lectin (AAL/AAGL), that has a high antitumor activity has been reported. In this paper, based on the glycan array data, it is showed that the Thomsen-Friedenreich antigen (TF antigen)-related O-glycans were found to be highly correlated with the antitumor activity of AAL/AAGL. Further glycosyltransferase quantification suggested that the ratio between GAL3ST2 and ST3GAL1 (GAL3ST2/ST3GAL1), which determined the 3'-sulfo-TF expression level, was highly correlated with the antitumor activity of AAL/AAGL. Overexpressing the enzyme of GAL3ST2 in HL60 and HeLa cell lines could increase the growth inhibition ratio of AAL/AAGL from 22.7 to 43.9% and 27.8 to 39.1%, respectively. However, ST3GAL1 in Jurkat cells could decrease the growth inhibition ratio from 44.7 to 35.6%. All the data suggested that the 3'-sulfo-TF antigen is one of the main glycan ligands that AAL/AAGL recognizes in tumor cells. AAL/AAGL may potentially serve as a reagent for cancer diagnosis and a targeted therapy for the 3'-sulfo-TF antigen.

Imitating evolution's tinkering by protein engineering reveals extension of human galectin-7 activity

Wild-type lectins have distinct types of modular design. As a step to explain the physiological importance of their special status, hypothesis-driven protein engineering is used to generate variants. Concerning adhesion/growth-regulatory galectins, non-covalently associated homodimers are commonly encountered in vertebrates. The homodimeric galectin-7 (Gal-7) is a multifunctional context-dependent modulator. Since the possibility of conversion from the homodimer to hybrids with other galectin domains, i.e. from Gal-1 and Gal-3, has recently been discovered, we designed Gal-7-based constructs, i.e. stable (covalently linked) homo- and heterodimers. They were produced and purified by affinity chromatography, and the sugar-binding activity of each lectin unit proven by calorimetry. Inspection of profiles of binding of labeled galectins to an array-like platform with various cell types, i.e. sections of murine epididymis and jejunum, and impact on neuroblastoma cell proliferation revealed no major difference between natural and artificial (stable) homodimers. When analyzing heterodimers, acquisition of altered properties was seen. Remarkably, binding properties and activity as effector can depend on the order of arrangement of lectin domains (from N- to C-termini) and on the linker length. After dissociation of the homodimer, the Gal-7 domain can build new functionally active hybrids with other partners. This study provides a clear direction for research on defining the full range of Gal-7 functionality and offers the perspective of testing applications for engineered heterodimers.

Differential Glycosite Profiling-A Versatile Method to Compare Membrane Glycoproteomes

Glycosylation is the most prevalent and varied form of post-translational protein modifications. Protein glycosylation regulates multiple cellular functions, including protein folding, cell adhesion, molecular trafficking and clearance, receptor activation, signal transduction, and endocytosis. In particular, membrane proteins are frequently highly glycosylated, which is both linked to physiological processes and of high relevance in various disease mechanisms. The cellular glycome is increasingly considered to be a therapeutic target. Here we describe a new strategy to compare membrane glycoproteomes, thereby identifying proteins with altered glycan structures and the respective glycosites. The workflow started with an optimized procedure for the digestion of membrane proteins followed by the lectin-based isolation of glycopeptides. Since alterations in the glycan part of a glycopeptide cause mass alterations, analytical size exclusion chromatography was applied to detect these mass shifts. N-glycosidase treatment combined with nanoUPLC-coupled mass spectrometry identified the altered glycoproteins and respective glycosites. The methodology was established using the colon cancer cell line CX1, which was treated with 2-deoxy-glucose-a modulator of N-glycosylation. The described methodology is not restricted to cell culture, as it can also be adapted to tissue samples or body fluids. Altogether, it is a useful module in various experimental settings that target glycan functions.

Detection of N,N-diacetyllactosamine (LacdiNAc) containing free prostate-specific antigen for early stage prostate cancer diagnostics and for identification of castration-resistant prostate cancer patients

Prostate cancer (PCa) is one of the most common cancer types among men and also acommon cause of death globally. With an increasing incidence, there is aneed for low-cost, reliable biomarkers present in samples, which could be provided non-invasively (without a need to perform prostate biopsy). Glycosylation changes of free-PSA (fPSA) are considered cancer-specific, while the level of different PSA forms can increase under other than cancerous conditions. In the present study, we investigated the role ofN,N-diacetyllactosamine (LacdiNAc) epitope of fPSA (i.e. glycoprofile of fPSA or gPSA) in combination with total-PSA (tPSA), prostate volume, and tPSA density (tPSA level divided by prostate volume i.e. PSAd) as biomarkers for monitoring of PCa development and progression in 105 men. Furthermore, we applied an genetic (evolutionary) algorithm to identify any suspicious individuals in abenign cohort having benign prostatic hyperplasia (BPH). We identified 3 suspicious men originally diagnosed with BPH using gPSA analysis. In thefollow-up we found out that two men should not be considered as BPH patients since multiparametric magnetic resonance imaging (mpMRI) identified one man with clinically significant PCa via Prostate Imaging - Reporting and Data System (PI RADS v2 = 4) and the second man was with High-gradeprostatic intraepithelial neoplasia (HG PIN), commonly described as apre-cancerous stage. Moreover, in the study we described for the first time that changed LacdiNAc on PSA can be applied to identify prostatitis patients and most importantly this is the first study suggesting that changed glycosylation on PSA can be applied to identify castration-resistant prostate cancer (CRPCa) patients.

Characterizing ligand-induced conformational changes in clinically relevant galectin-1 by HN/H2O (D2O) exchange

Glycans of cellular glycoconjugates serve as biochemical signals for a multitude of (patho)physiological processes via binding to their receptors (e.g. lectins). In the case of human adhesion/growth-regulatory galectin-1 (Gal-1), small angle neutron scattering and fluorescence correlation spectroscopy have revealed a significant decrease of its gyration radius and increase of its diffusion coefficient upon binding lactose, posing the pertinent question on the nature and region(s) involved in the underlying structural alterations. Requiring neither a neutron source nor labeling, diffusion measurements by ¹H NMR spectroscopy are shown here to be sufficiently sensitive to detect this ligand-induced change. In order to figure out which region(s) of Gal-1 is (are) affected at the level of peptides, we first explored the use of H/D exchange mass spectrometry (HDX MS). Hereby, we found a reduction in proton exchange kinetics beyond the lactose-binding site. The measurement of fast H^N/H₂O exchange by phase-modulated NMR clean chemical exchange (CLEANEX) NMR on ¹⁵N-labeled Gal-1 then increased the spatial resolution to the level of individual amino acids. The mapped regions with increased protection from H^N/H₂O (D₂O) exchange that include the reduction of solvent exposure around the interface can underlie the protein's compaction. These structural changes have potential to modulate this galectin's role in lattice formation on the cell surface and its interaction(s) with protein(s) at the F-face.

N-Glycan profiling of chondrocytes and fibroblast-like synoviocytes: Towards functional glycomics in osteoarthritis

PURPOSE

N-Glycan profiling provides an indicator of the cellular potential for functional pairing with tissue lectins. Following the discovery of galectin expression by chondrocytes as a factor in osteoarthritis pathobiology, mapping of N-glycans upon their phenotypic dedifferentiation in culture and in fibroblast-like synoviocytes is a step to better understand glycobiological contributions to disease progression.

EXPERIMENTAL DESIGN

The profiles of cellular N-glycans of human osteoarthritic chondrocytes and fibroblast-like synoviocytes were characterized by mass spectrometry. RT-qPCR experiments determined mRNA levels of 16 glycosyltransferases. Responsiveness of cells to galectins was quantified by measuring the mRNA level for interleukin-1β.

RESULTS

The shift of chondrocytes to a fibroblastic phenotype (dedifferentiation) is associated with changes in N-glycosylation. The N-glycan profile of chondrocytes at passage 4 reflects characteristics of synoviocytes. Galectins-1 and -3 enhance expression of interleukin-1β mRNA in both cell types, most pronounced in primary culture. Presence of interleukin-1β leads to changes in sialylation in synoviocytes that favor galectin binding.

CONCLUSIONS AND CLINICAL RELEVANCE

N-Glycosylation reflects phenotypic changes of osteoarthritic cells in vitro. Like chondrocytes, fibroblast-like synoviocytes express N-glycans that are suited to bind galectins, and these proteins serve as inducers of pro-inflammatory markers in these cells. Synoviocytes can thus contribute to disease progression in osteoarthritis in situ.

Manning J, Abad-Rodríguez J, Gabius HJ. What Cyto- and Histochemistry Can Do to Crack the Sugar Code

As letters form the vocabulary of a language, biochemical 'symbols' (the building blocks of oligo- and polymers) make writing molecular messages possible. Compared to nucleotides and amino acids, sugars have chemical properties that facilitate to reach an unsurpassed level of oligomer diversity. These glycans are a part of the ubiquitous cellular glycoconjugates. Cyto- and histochemically, the glycans' structural complexity is mapped by glycophenotyping of cells and tissues using receptors ('readers', thus called lectins), hereby revealing its dynamic spatiotemporal regulation: these data support the concept of a sugar code. When proceeding from work with plant (haem)agglutinins as such tools to the discovery of endogenous (tissue) lectins, it became clear that a broad panel of biological meanings can indeed be derived from the sugar-based vocabulary (the natural glycome incl. post-synthetic modifications) by glycan-lectin recognition in situ. As consequence, the immunocyto- and histochemical analysis of lectin expression is building a solid basis for the steps toward tracking down functional correlations, for example in processes leading to cell adhesion, apoptosis, autophagy or growth regulation as well as targeted delivery of glycoproteins. Introduction of labeled tissue lectins to glycan profiling assists this endeavor by detecting counterreceptor(s) in situ. Combining these tools and their applications strategically will help to take the trip toward the following long-range aim: to compile a dictionary for the glycan vocabulary that translates each message (oligosaccharide) into its bioresponse(s), that is to crack the sugar code.

Enhanced Shear Force Responsiveness of Epithelial Na+ Channel's (ENaC) δ Subunit Following the Insertion of N-Glycosylation Motifs Relies on the Extracellular Matrix

Members of the Degenerin/epithelial Na+ channel (ENaC) protein family and the extracellular cell matrix (ECM) form a mechanosensitive complex. A core feature of this complex are tethers, which connect the channel with the ECM, however, knowledge about the nature of these tethers is scarce. N-glycans of α ENaC were recently identified as potential tethers but whether N-glycans serve as a ubiquitous feature for mechanosensation processes remains unresolved. The purpose of this study was to reveal whether the addition of N-glycans to δ ENaC-which is less responsive to shear force (SF)-increases its SF-responsiveness and whether this relies on a linkage to the ECM. Therefore, N-glycosylation motifs were introduced via site-directed mutagenesis, the resulting proteins expressed with β and γ ENaC in Xenopus oocytes, and SF-activated currents measured by two-electrode voltage-clamp. The insertion of N-glycosylation motifs increases δ ENaC's SF responsiveness. The inclusion of a glycosylated asparagine (N) at position 487 did increase the molecular mass and provided a channel whose SF response was abolished following ECM degradation via hyaluronidase. This indicates that the addition of N-glycans improves SF-responsiveness and that this effect relies on an intact ECM. These findings further support the role of N-glycans as tethers for mechanotransduction.

The Dysregulated Galectin Network Activates NF-κB to Induce Disease Markers and Matrix Degeneration in 3D Pellet Cultures of Osteoarthritic Chondrocytes

This work aimed to study the dysregulated network of galectins in OA chondrocyte pellets, and to assess whether their recently discovered activity as molecular switches of functional biomarkers results in degradation of extracellular matrix in vitro. Scaffold-free 3D pellet cultures were established of human OA chondrocytes. Expression and secretion of galectin(Gal)-1, -3, and -8 were monitored relative to 2D cultures or clinical tissue sections by RT-qPCR, immunohistochemistry and ELISAs. Exposure of 2D and 3D cultures to an in vivo-like galectin mixture (Gal-1 and Gal-8: 5 µg/ml, Gal-3: 1 µg/ml) was followed by the assessment of pellet size, immunohistochemical matrix staining, and/or quantification of MMP-1, -3, and -13. Application of inhibitors of NF-κB activation probed into the potential of intervening with galectin-induced matrix degradation. Galectin profiling revealed maintained dysregulation of Gal-1, -3, and -8 in pellet cultures, resembling the OA situation in situ. The presence of the galectin mixture promoted marked reduction of pellet size and loss of collagen type II-rich extracellular matrix, accompanied by the upregulation of MMP-1, -3, and -13. Inhibition of p65-phosphorylation by caffeic acid phenethyl ester effectively alleviated the detrimental effects of galectins, resulting in downregulated MMP secretion, reduced matrix breakdown and augmented pellet size. This study suggests that the dysregulated galectin network in OA cartilage leads to extracellular matrix breakdown, and provides encouraging evidence of the feasible inhibition of galectin-triggered activities. OA chondrocyte pellets have the potential to serve as in vitro disease model for further studies on galectins in OA onset and progression.

Calorimetric Analysis of the Interplay between Synthetic Tn Antigen-Presenting MUC1 Glycopeptides and Human Macrophage Galactose-Type Lectin

Human macrophage galactose-type lectin (hMGL, HML, CD301, CLEC10A), a C-type lectin expressed by dendritic cells and macrophages, is a receptor for N-acetylgalactosamine α-linked to serine/threonine residues (Tn antigen, CD175) and its α2,6-sialylated derivative (sTn, CD175s). Because these two epitopes are among malignant cell glycan displays, particularly when presented by mucin-1 (MUC1), assessing the influence of the site and frequency of glycosylation on lectin recognition will identify determinants governing this interplay. Thus, chemical synthesis of the tandem-repeat O-glycan acceptor region of MUC1 and site-specific threonine glycosylation in all permutations were carried out. Isothermal titration calorimetry (ITC) analysis of the binding of hMGL to this library of MUC1 glycopeptides revealed an enthalpy-driven process and an affinity enhancement of an order of magnitude with an increasing glycan count from 6-8 μM for monoglycosylated peptides to 0.6 μM for triglycosylated peptide. ITC measurements performed in D2O permitted further exploration of the solvation dynamics during binding. A shift in enthalpy-entropy compensation and contact position-specific effects with the likely involvement of the peptide surroundings were detected. KinITC analysis revealed a prolonged lifetime of the lectin-glycan complex with increasing glycan valency and with a change in the solvent to D2O.

40 years of glyco-polyacrylamide in glycobiology

Polyacrylamide conjugates of glycans have long been widely used in many research areas of glycobiology, mainly for immobilizing glycans in solid-phase assays and as multivalent inhibitors. Pending biotin tag allows immobilizing Glyc-PAA quantitatively on any surface, and acts as a tracer for detection of carbohydrate-binding proteins. However, the scope of already realized capabilities of these probes is immeasurably richer than those listed above. This review is not so much about routine as about less common, but not less significant applications. Also, the data on the glycopolymers themselves, their molecular weight, size and polymer chain flexibility are presented, as well as the methods of synthesis, clusterisation and entropy factor in their interaction with proteins.