X-ray reflectivity and grazing incidence diffraction studies of interaction between human adhesion/growth-regulatory galectin-1 and DPPE-GM1 lipid monolayer at an air/water interface

Structure of Galectin-3 bound to a model membrane containing ganglioside GM1

Galectin-3 (Gal-3) is a β-galactosidase-binding protein involved in various biological processes, including neuronal growth and adhesion. The pairing of Gal-3 with ganglioside GM1's pentasaccharide chain at the outer leaflet of the plasma membrane, which triggers downstream cell-signaling cascades, seems to be involved in these processes. A crucial feature of Gal-3 is its ability to form oligomers and supramolecular assemblies that connect various carbohydrate-decorated molecules. Although we know the atomistic structure of Gal-3 bound to small carbohydrate ligands, it remains unclear how Gal-3 binds GM1 in a membrane. Furthermore, the influence of this interaction on Gal-3's structure and oligomeric assembly has to be elucidated. In this study, we used X-ray reflectivity (XR) from a model membrane to determine the structure and surface coverage of Gal-3 bound to a membrane containing GM1. We observed that the carbohydrate recognition domain interacts with GM1's pentasaccharide, while the N-terminal domain is pointed away from the membrane, likely to facilitate protein-protein interactions. In a membrane containing 20 mol % GM1, Gal-3 covered ∼50% of the membrane surface with one Gal-3 molecule bound per 2130 Å². We used molecular dynamics simulations and Voronoi tessellation algorithms to build an atomistic model of membrane-bound Gal-3, which is supported by the XR results. Overall, this work provides structural information describing how Gal-3 can bind GM1's pentasaccharide chain, a prerequisite for triggering regulatory processes in neuronal growth and adhesion.

Influence of protein (human galectin-3) design on aspects of lectin activity

The concept of biomedical significance of the functional pairing between tissue lectins and their glycoconjugate counterreceptors has reached the mainstream of research on the flow of biological information. A major challenge now is to identify the principles of structure–activity relationships that underlie specificity of recognition and the ensuing post-binding processes. Toward this end, we focus on a distinct feature on the side of the lectin, i.e. its architecture to present the carbohydrate recognition domain (CRD). Working with a multifunctional human lectin, i.e. galectin-3, as model, its CRD is used in protein engineering to build variants with different modular assembly. Hereby, it becomes possible to compare activity features of the natural design, i.e. CRD attached to an N-terminal tail, with those of homo- and heterodimers and the tail-free protein. Thermodynamics of binding disaccharides proved full activity of all proteins at very similar affinity. The following glycan array testing revealed maintained preferential contact formation with N-acetyllactosamine oligomers and histo-blood group ABH epitopes irrespective of variant design. The study of carbohydrate-inhibitable binding of the test panel disclosed up to qualitative cell-type-dependent differences in sections of fixed murine epididymis and especially jejunum. By probing topological aspects of binding, the susceptibility to inhibition by a tetravalent glycocluster was markedly different for the wild-type vs the homodimeric variant proteins. The results teach the salient lesson that protein design matters: the type of CRD presentation can have a profound bearing on whether basically suited oligosaccharides, which for example tested positively in an array, will become binding partners in situ. When lectin-glycoconjugate aggregates (lattices) are formed, their structural organization will depend on this parameter. Further testing (ga)lectin variants will thus be instrumental (i) to define the full range of impact of altering protein assembly and (ii) to explain why certain types of design have been favored during the course of evolution, besides opening biomedical perspectives for potential applications of the novel galectin forms.

Glycan Chains of Gangliosides: Functional Ligands for Tissue Lectins (Siglecs/Galectins)

Molecular signals on the cell surface are responsible for adhesion and communication. Of relevance in this respect, their chemical properties endow carbohydrates with the capacity to store a maximum of information in a minimum of space. One way to present glycans on the cell surface is their covalent conjugation to a ceramide anchor. Among the resulting glycosphingolipids, gangliosides are special due to the presence of at least one sialic acid in the glycan chains. Their spatial accessibility and the dynamic regulation of their profile are factors that argue in favor of a role of glycans of gangliosides as ligands (counterreceptors) for carbohydrate-binding proteins (lectins). Indeed, as discovered first for a bacterial toxin, tissue lectins bind gangliosides and mediate contact formation (trans) and signaling (cis). While siglecs have a preference for higher sialylated glycans, certain galectins also target the monosialylated pentasaccharide of ganglioside GM1. Enzymatic interconversion of ganglioside glycans by sialidase action, relevant for neuroblastoma cell differentiation and growth control in vitro, for axonogenesis and axon regeneration, as well as for proper communication between effector and regulatory T cells, changes lectin-binding affinity profoundly. The GD1a-to-GM1 "editing" is recognized by such lectins, for example, myelin-associated glycoprotein (siglec-4) losing affinity and galectin-1 gaining reactivity, and then translated into postbinding signaling. Orchestrations of loss/gain of affinity, of ganglioside/lectin expression, and of lectin presence in a network offer ample opportunities for fine-tuning. Thus glycans of gangliosides such as GD1a and GM1 are functional counterreceptors by a pairing with tissue lectins, an emerging aspect of ganglioside and lectin functionality.

Long-chain GM1 gangliosides alter transmembrane domain registration through interdigitation

Extracellular and cytosolic leaflets in cellular membranes are distinctly different in lipid composition, yet they contribute together to signaling across the membranes. Here we consider a mechanism based on long-chain gangliosides for coupling the extracellular and cytosolic membrane leaflets together. Based on atomistic molecular dynamics simulations, we find that long-chain GM1 in the extracellular leaflet exhibits a strong tendency to protrude into the opposing bilayer leaflet. This interdigitation modulates the order in the cytosolic monolayer and thereby strengthens the interaction and coupling across a membrane. Coarse-grained simulations probing longer time scales in large membrane systems indicate that GM1 in the extracellular leaflet modulates the phase behavior in the cytosolic monolayer. While short-chain GM1 maintains phase-symmetric bilayers with a strong membrane registration effect, the situation is altered with long-chain GM1. Here, the significant interdigitation induced by long-chain GM1 modulates the behavior in the cytosolic GM1-free leaflet, weakening and slowing down the membrane registration process. The observed physical interaction mechanism provides a possible means to mediate or foster transmembrane communication associated with signal transduction.

Multivalent Carbohydrate-Lectin Interactions: How Synthetic Chemistry Enables Insights into Nanometric Recognition

Glycan recognition by sugar receptors (lectins) is intimately involved in many aspects of cell physiology. However, the factors explaining the exquisite selectivity of their functional pairing are not yet fully understood. Studies toward this aim will also help appraise the potential for lectin-directed drug design. With the network of adhesion/growth-regulatory galectins as therapeutic targets, the strategy to recruit synthetic chemistry to systematically elucidate structure-activity relationships is outlined, from monovalent compounds to glyco-clusters and glycodendrimers to biomimetic surfaces. The versatility of the synthetic procedures enables to take examining structural and spatial parameters, alone and in combination, to its limits, for example with the aim to produce inhibitors for distinct galectin(s) that exhibit minimal reactivity to other members of this group. Shaping spatial architectures similar to glycoconjugate aggregates, microdomains or vesicles provides attractive tools to disclose the often still hidden significance of nanometric aspects of the different modes of lectin design (sequence divergence at the lectin site, differences of spatial type of lectin-site presentation). Of note, testing the effectors alone or in combination simulating (patho)physiological conditions, is sure to bring about new insights into the cooperation between lectins and the regulation of their activity.

Sweet complementarity: the functional pairing of glycans with lectins

Carbohydrates establish the third alphabet of life. As part of cellular glycoconjugates, the glycans generate a multitude of signals in a minimum of space. The presence of distinct glycotopes and the glycome diversity are mapped by sugar receptors (antibodies and lectins). Endogenous (tissue) lectins can read the sugar-encoded information and translate it into functional aspects of cell sociology. Illustrated by instructive examples, each glycan has its own ligand properties. Lectins with different folds can converge to target the same epitope, while intrafamily diversification enables functional cooperation and antagonism. The emerging evidence for the concept of a network calls for a detailed fingerprinting. Due to the high degree of plasticity and dynamics of the display of genes for lectins the validity of extrapolations between different organisms of the phylogenetic tree yet is inevitably limited.

Exogenous and Endogeneous Disialosyl Ganglioside GD1b Induces Apoptosis of MCF-7 Human Breast Cancer Cells

Gangliosides have been known to play a role in the regulation of apoptosis in cancer cells. This study has employed disialyl-ganglioside GD1b to apoptosis in human breast cancer MCF-7 cells using exogenous treatment of the cells with GD1b and endogenous expression of GD1b in MCF-7 cells. First, apoptosis in MCF-7 cells was observed after treatment of GD1b. Treatment of MCF-7 cells with GD1b reduced cell growth rates in a dose and time dependent manner during GD1b treatment, as determined by XTT assay. Among the various gangliosides, GD1b specifically induced apoptosis of the MCF-7 cells. Flow cytometry and immunofluorescence assays showed that GD1b specifically induces apoptosis in the MCF-7 cells with Annexin V binding for apoptotic actions in early stage and propidium iodide (PI) staining the nucleus of the MCF-7 cells. Treatment of MCF-7 cells with GD1b activated apoptotic molecules such as processed forms of caspase-8, -7 and PARP (Poly(ADP-ribose) polymerase), without any change in the expression of mitochondria-mediated apoptosis molecules such as Bax and Bcl-2. Second, to investigate the effect of endogenously produced GD1b on the regulation of cell function, UDP-gal: β1,3-galactosyltransferase-2 (GD1b synthase, Gal-T2) gene has been transfected into the MCF-7 cells. Using the GD1b synthase-transfectants, apoptosis-related signal proteins linked to phenotype changes were examined. Similar to the exogenous GD1b treatment, the cell growth of the GD1b synthase gene-transfectants was significantly suppressed compared with the vector-transfectant cell lines and transfection activated the apoptotic molecules such as processed forms of caspase-8, -7 and PARP, but not the levels of expression of Bax and Bcl-2. GD1b-induced apoptosis was blocked by caspase inhibitor, Z-VAD. Therefore, taken together, it was concluded that GD1b could play an important role in the regulation of breast cancer apoptosis.

Functional interplay between ganglioside GM1 and cross-linking galectin-1 induces axon-like neuritogenesis via integrin-based signaling and TRPC5-dependent Ca²⁺ influx

Axon‐like neuritogenesis in neuroblastoma (NG108‐15) cells and primary cerebellar granular neurons is furthered by the presence of ganglioside GM1. We describe here that galectin‐1 (Gal‐1), a homobivalent endogenous lectin, is an effector by cross‐linking the ganglioside and its associated glycoprotein α₅β₁‐integrin. The thereby triggered signaling cascade involves autophosphorylation of focal adhesion kinase and activation of phospholipase Cγ and phosphoinositide‐3 kinase. This leads to a transient increase in the intracellular Ca²⁺ concentration by opening of TRPC5 channels, which belong to the signal transduction‐gated cation channels. Controls with GM1‐defective cells (NG‐CR72 and neurons from ganglio‐series KO mice) were retarded in axonal growth, underscoring the relevance of GM1 as functional counterreceptor for Gal‐1. The lectin's presence was detected in the NG108‐15 cells, suggesting an autocrine mechanism of action, and in astrocytes in situ. Gal‐1, as cross‐linking lectin, can thus translate metabolic conversion of ganglioside GD1a to GM1 by neuraminidase action into axon growth.

Galectin‐1 (Gal‐1) was shown an effector of axonogenesis in cerebellar granule neurons (CGNs) and NG108‐15 cells by cross‐linking GM1 ganglioside and its associated glycoprotein α₅β₁‐integrin. The resulting signaling led to a transient increase in intracellular Ca²⁺ by opening TRPC5 channels. CGNs deficient in GM1 showed retarded axonogenesis, underscoring the relevance of GM1 as functional counterreceptor for Gal‐1 in this process. This Gal‐1/GM1‐induced signaling was manifest only at the earliest, initiating stage of axon development.

Network monitoring of adhesion/growth-regulatory galectins: localization of the five canonical chicken proteins in embryonic and maturing bone and cartilage and their introduction as histochemical tools

Divergence from an ancestral gene leads to a family of homologous proteins. Whether they are physiologically distinct, similar, or even redundant is an open question in each case. Defining profiles of tissue localization is a step toward giving diversity a functional meaning. Due to the significance of endogenous sugar receptors (lectins) as effectors for a wide range of cellular activities we have focused on galectins. The comparatively low level of network complexity constituted by only five canonical proteins makes chicken galectins (CGs) an attractive choice to perform comprehensive analysis, here studied on bone/cartilage as organ system. Galectin expression was monitored by Western blotting and immunohistochemistry using non-cross-reactive antibodies. Overall, three galectins (CG-1B, CG-3, CG-8) were present with individual expression patterns, one was found exclusively in the mesenchyme (CG-1A), the fifth (CG-2) not being detectable. The documented extents of separation are a sign for functional divergence; in cases with overlapping stainings, as for example in the osteoprogenitor layer or periosteum, cooperation may also be possible. Recombinant production enabled the introduction of the endogenous lectins as tools for binding-site localization. Their testing revealed developmental regulation and cell-type-specific staining. Of relevance for research on mammalian galectins, this study illustrates that certain cell types can express more than one galectin, letting functional interrelationships appear likely. Thus, complete network analysis irrespective of its degree of complexity is mandatory.