Tailor-made Janus lectin with dual avidity assembles glycoconjugate multilayers and crosslinks protocells

Organization of an Artificial Multicellular System with a Tunable DNA Patch on a Membrane Surface

Coordinating multiple artificial cellular compartments into a well-organized artificial multicellular system (AMS) is of great interest in bottom-up synthetic biology. However, developing a facile strategy for fabricating an AMS with a controlled arrangement remains a challenge. Herein, utilizing in situ DNA hybridization chain reaction on the membrane surface, we developed a DNA patch-based strategy to direct the interconnection of vesicles. By tuning the DNA patch that generates heterotrophic adhesion for the attachment of vesicles, we could produce an AMS with higher-order structures straightforwardly and effectively. Furthermore, a hybrid AMS comprising live cells and vesicles was fabricated, and we found the hybrid AMS with higher-order structures arouses efficient molecular transportation from vesicles to living cells. In brief, our work provides a versatile strategy for modulating the self-assembly of AMSs, which could expand our capability to engineer synthetic biological systems and benefit synthetic cell research in programmable manipulation of intercellular communications.

Engineering Tissue-Scale Properties with Synthetic Cells: Forging One from Many

In metazoans, living cells achieve capabilities beyond individual cell functionality by assembling into multicellular tissue structures. These higher-order structures represent dynamic, heterogeneous, and responsive systems that have evolved to regenerate and coordinate their actions over large distances. Recent advances in constructing micrometer-sized vesicles, or synthetic cells, now point to a future where construction of synthetic tissue can be pursued, a boon to pressing material needs in biomedical implants, drug delivery systems, adhesives, filters, and storage devices, among others. To fully realize the potential of synthetic tissue, inspiration has been and will continue to be drawn from new molecular findings on its natural counterpart. In this review, we describe advances in introducing tissue-scale features into synthetic cell assemblies. Beyond mere complexation, synthetic cells have been fashioned with a variety of natural and engineered molecular components that serve as initial steps toward morphological control and patterning, intercellular communication, replication, and responsiveness in synthetic tissue. Particular attention has been paid to the dynamics, spatial constraints, and mechanical strengths of interactions that drive the synthesis of this next-generation material, describing how multiple synthetic cells can act as one.

Tuning specificity and topology of lectins through synthetic biology

Lectins are non-immunoglobulin and non-catalytic glycan binding proteins that are able to decipher the structure and function of complex glycans. They are widely used as biomarkers for following alteration of glycosylation state in many diseases and have application in therapeutics. Controlling and extending lectin specificity and topology is the key for obtaining better tools. Furthermore, lectins and other glycan binding proteins can be combined with additional domains, providing novel functionalities. We provide a view on the current strategy with a focus on synthetic biology approaches yielding to novel specificity, but other novel architectures with novel application in biotechnology or therapy.

A bispecific, crosslinking lectibody activates cytotoxic T cells and induces cancer cell death

BACKGROUND

Aberrant glycosylation patterns play a crucial role in the development of cancer cells as they promote tumor growth and aggressiveness. Lectins recognize carbohydrate antigens attached to proteins and lipids on cell surfaces and represent potential tools for application in cancer diagnostics and therapy. Among the emerging cancer therapies, immunotherapy has become a promising treatment modality for various hematological and solid malignancies. Here we present an approach to redirect the immune system into fighting cancer by targeting altered glycans at the surface of malignant cells. We developed a so-called "lectibody", a bispecific construct composed of a lectin linked to an antibody fragment. This lectibody is inspired by bispecific T cell engager (BiTEs) antibodies that recruit cytotoxic T lymphocytes (CTLs) while simultaneously binding to tumor-associated antigens (TAAs) on cancer cells. The tumor-related glycosphingolipid globotriaosylceramide (Gb3) represents the target of this proof-of-concept study. It is recognized with high selectivity by the B-subunit of the pathogen-derived Shiga toxin, presenting opportunities for clinical development.

METHODS

The lectibody was realized by conjugating an anti-CD3 single-chain antibody fragment to the B-subunit of Shiga toxin to target Gb3⁺ cancer cells. The reactive non-canonical amino acid azidolysine (AzK) was inserted at predefined single positions in both proteins. The azido groups were functionalized by bioorthogonal conjugation with individual linkers that facilitated selective coupling via an alternative bioorthogonal click chemistry reaction. In vitro cell-based assays were conducted to evaluate the antitumoral activity of the lectibody. CTLs, Burkitt´s lymphoma-derived cells and colorectal adenocarcinoma cell lines were screened in flow cytometry and cytotoxicity assays for activation and lysis, respectively.

RESULTS

This proof-of-concept study demonstrates that the lectibody activates T cells for their cytotoxic signaling, redirecting CTLs´ cytotoxicity in a highly selective manner and resulting in nearly complete tumor cell lysis-up to 93%-of Gb3⁺ tumor cells in vitro.

CONCLUSIONS

This research highlights the potential of lectins in targeting certain tumors, with an opportunity for new cancer treatments. When considering a combinatorial strategy, lectin-based platforms of this type offer the possibility to target glycan epitopes on tumor cells and boost the efficacy of current therapies, providing an additional strategy for tumor eradication and improving patient outcomes.

Building an Artificial Plant Cell Wall on a Lipid Bilayer by Assembling Polysaccharides and Engineered Proteins

The cell wall constitutes a fundamental structural component of plant cells, providing them with mechanical resistance and flexibility. Mimicking this wall is a critical step in the conception of an experimental model of the plant cell. The assembly of cellulose/hemicellulose in the form of cellulose nanocrystals and xyloglucans as a representative model of the plant cell wall has already been mastered; however, these models lacked the pectin component. In this work, we used an engineered chimeric protein designed for bridging pectin to the cellulose/hemicellulose network, therefore achieving the assembly of complete cell wall mimics. We first engineered a carbohydrate-binding module from Ruminococcus flavefaciens able to bind oligogalacturonan, resulting in high-affinity polygalacturonan receptors with K_d in the micromolar range. A Janus protein, with cell wall gluing property, was then designed by assembling this carbohydrate-binding module with a Ralstonia solanacearum lectin specific for fucosylated xyloglucans. The resulting supramolecular architecture is able to bind fucose-containing xyloglucans and homogalacturonan, ensuring high affinity for both. A two-dimensional assembly of an artificial plant cell wall was then built first on synthetic polymer and then on the supported lipid bilayer. Such an artificial cell wall can serve as a basis for the development of plant cell mechanical models and thus deepen the understanding of the principles underlying various aspects of plant cells and tissues.

Multivalent glycocyclopeptides: conjugation methods and biological applications

Click chemistry was extensively used to decorate synthetic multivalent scaffolds with glycans to mimic the cell surface glycocalyx and to develop applications in glycosciences. Conjugation methods such as oxime ligation, copper(I)-catalyzed alkyne-azide cycloaddition, thiol-ene coupling, squaramide coupling or Lansbury aspartylation proved particularly suitable to achieve this purpose. This review summarizes the synthetic strategies that can be used either in a stepwise manner or in an orthogonal one-pot approach, to conjugate multiple copies of identical or different glycans to cyclopeptide scaffolds (namely multivalent glycocyclopeptides) having different size, valency, geometry and molecular composition. The second part of this review will describe the potential of these structures to interact with various carbohydrate binding proteins or to stimulate immunity against tumor cells.

The choanoflagellate pore-forming lectin SaroL-1 punches holes in cancer cells by targeting the tumor-related glycosphingolipid Gb3

Choanoflagellates are primitive protozoa used as models for animal evolution. They express a large variety of multi-domain proteins contributing to adhesion and cell communication, thereby providing a rich repertoire of molecules for biotechnology. Adhesion often involves proteins adopting a β-trefoil fold with carbohydrate-binding properties therefore classified as lectins. Sequence database screening with a dedicated method resulted in TrefLec, a database of 44714 β-trefoil candidate lectins across 4497 species. TrefLec was searched for original domain combinations, which led to single out SaroL-1 in the choanoflagellate Salpingoeca rosetta, that contains both β-trefoil and aerolysin-like pore-forming domains. Recombinant SaroL-1 is shown to bind galactose and derivatives, with a stronger affinity for cancer-related α-galactosylated epitopes such as the glycosphingolipid Gb3, when embedded in giant unilamellar vesicles or cell membranes. Crystal structures of complexes with Gb3 trisaccharide and GalNAc provided the basis for building a model of the oligomeric pore. Finally, recognition of the αGal epitope on glycolipids required for hemolysis of rabbit erythrocytes suggests that toxicity on cancer cells is achieved through carbohydrate-dependent pore-formation.

A curated lectin database, structural characterization, and in vitro assays show that choanoflagellate lectin SaroL-1 binds to cancer-related α-galactosylated epitopes and can be toxic to cancer cells through a carbohydrate-dependent pore-formation mechanism.

One, two, many: Strategies to alter the number of carbohydrate binding sites of lectins

Carbohydrates are more than an energy-storage. They are ubiquitously found on cells and most proteins, where they encode biological information. Lectins bind these carbohydrates and are essential for translating the encoded information into biological functions and processes. Hundreds of lectins are known, and they are found in all domains of life. For half a century, researchers have been preparing variants of lectins in which the binding sites are varied. In this way, the traits of the lectins such as the affinity, avidity and specificity towards their ligands as well as their biological efficacy were changed. These efforts helped to unravel the biological importance of lectins and resulted in improved variants for biotechnological exploitation and potential medical applications. This review gives an overview on the methods for the preparation of artificial lectins and complexes thereof and how reducing or increasing the number of binding sites affects their function.

Self-Assembling Lectin Nano-Block Oligomers Enhance Binding Avidity to Glycans

Lectins, carbohydrate-binding proteins, are attractive biomolecules for medical and biotechnological applications. Many lectins have multiple carbohydrate recognition domains (CRDs) and strongly bind to specific glycans through multivalent binding effect. In our previous study, protein nano-building blocks (PN-blocks) were developed to construct self-assembling supramolecular nanostructures by linking two oligomeric proteins. A PN-block, WA20-foldon, constructed by fusing a dimeric four-helix bundle de novo protein WA20 to a trimeric foldon domain of T4 phage fibritin, self-assembled into several types of polyhedral nanoarchitectures in multiples of 6-mer. Another PN-block, the extender PN-block (ePN-block), constructed by tandemly joining two copies of WA20, self-assembled into cyclized and extended chain-type nanostructures. This study developed novel functional protein nano-building blocks (lectin nano-blocks) by fusing WA20 to a dimeric lectin, Agrocybe cylindracea galectin (ACG). The lectin nano-blocks self-assembled into various oligomers in multiples of 2-mer (dimer, tetramer, hexamer, octamer, etc.). The mass fractions of each oligomer were changed by the length of the linkers between WA20 and ACG. The binding avidity of the lectin nano-block oligomers to glycans was significantly increased through multivalent effects compared with that of the original ACG dimer. Lectin nano-blocks with high avidity will be useful for various applications, such as specific cell labeling.

Homo- and Heterovalent Neoglycoproteins as Ligands for Bacterial Lectins

Click chemistry gives access to unlimited set of multivalent glycoconjugates to explore carbohydrate-protein interactions and discover high affinity ligands. In this study, we have created supramolecular systems based on a carrier protein that was grafted by Cu(I)-catalyzed azide-alkyne cycloaddition with tetravalent glycodendrons presenting αGal, βGal and/or αFuc. Binding studies of the homo- (4 a-c) and heterovalent (5) neoglycoproteins (neoGPs) with the LecA and LecB lectins from P. aeruginosa has first confirmed the interest of the multivalent presentation of glycodendrons by the carrier protein (IC50 up to 2.8 nM). Moreover, these studies have shown that the heterovalent display of glycans (5) allows the interaction with both lectins (IC50 of 10 nM) despite the presence of unspecific moieties, and even with similar efficiency for LecB. These results demonstrate the potential of multivalent and multispecific neoGPs as a promising strategy to fight against resistant pathogens.

The Two Sweet Sides of Janus Lectin Drive Crosslinking of Liposomes to Cancer Cells and Material Uptake

A chimeric, bispecific Janus lectin has recently been engineered with different, rationally oriented recognition sites. It can bind simultaneously to sialylated and fucosylated glycoconjugates. Because of its multivalent architecture, this lectin reaches nanomolar avidities for sialic acid and fucose. The lectin was designed to detect hypersialylation—a dysregulation in physiological glycosylation patterns, which promotes the tumor growth and progression of several cancer types. In this study, the characteristic properties of this bispecific Janus lectin were investigated on human cells by flow cytometry and confocal microscopy in order to understand the fundamentals of its interactions. We evaluated its potential in targeted drug delivery, precisely leading to the cellular uptake of liposomal content in human epithelial cancer cells. We successfully demonstrated that Janus lectin mediates crosslinking of glyco-decorated giant unilamellar vesicles (GUVs) and H1299 lung epithelial cells. Strikingly, the Janus lectin induced the internalization of liposomal lipids and also of complete GUVs. Our findings serve as a solid proof of concept for lectin-mediated targeted drug delivery using glyco-decorated liposomes as possible drug carriers to cells of interest. The use of Janus lectin for tumor recognition certainly broadens the possibilities for engineering diverse tailor-made lectin constructs, specifically targeting extracellular structures of high significance in pathological conditions.

Segregated Protein-Cucurbit[7]uril Crystalline Architectures via Modulatory Peptide Tectons

One approach to protein assembly involves water-soluble supramolecular receptors that act like glues. Bionanoarchitectures directed by these scaffolds are often system-specific, with few studies investigating their customization. Herein, the modulation of cucurbituril-mediated protein assemblies through the inclusion of peptide tectons is described. Three peptides of varying length and structural order were N-terminally appended to RSL, a β-propeller building block. Each fusion protein was incorporated into crystalline architectures mediated by cucurbit[7]uril (Q7). A trimeric coiled-coil served as a spacer within a Q7-directed sheet assembly of RSL, giving rise to a layered material of varying porosity. Within the spacer layers, the coiled-coils were dynamic. This result prompted consideration of intrinsically disordered peptides (IDPs) as modulatory tectons. Similar to the coiled-coil, a mussel adhesion peptide (Mefp) also acted as a spacer between protein-Q7 sheets. In contrast, the fusion of a nucleoporin peptide (Nup) to RSL did not recapitulate the sheet assembly. Instead, a Q7-directed cage was adopted, within which disordered Nup peptides were partially "captured" by Q7 receptors. IDP capture occurred by macrocycle recognition of an intrapeptide Phe-Gly motif in which the benzyl group was encapsulated by Q7. The modularity of these protein-cucurbituril architectures adds a new dimension to macrocycle-mediated protein assembly. Segregated protein crystals, with alternating layers of high and low porosity, could provide a basis for new types of materials.

Quantification of nanoscale forces in lectin-mediated bacterial attachment and uptake into giant liposomes

Interactions of the bacterial lectin LecA with the host cells glycosphingolipid Gb3 have been shown to be crucial for the cellular uptake of the bacterium Pseudomonas aeruginosa. LecA-induced Gb3 clustering, referred to as lipid zipper mechanism, leads to full membrane engulfment of the bacterium. Here, we aim for a nanoscale force characterization of this mechanism using two complementary force probing techniques, atomic force microscopy (AFM) and optical tweezers (OT). The LecA-Gb3 interactions are reconstituted using giant unilamellar vesicles (GUVs), a well-controlled minimal system mimicking the plasma membrane and nanoscale forces between either bacteria (PAO1 wild-type and LecA-deletion mutant strains) or LecA-coated probes (as minimal, synthetic bacterial model) and vesicles are measured. LecA-Gb3 interactions strengthen the bacterial attachment to the membrane (1.5-8-fold) depending on the membrane tension and the applied technique. Moreover, significantly less energy (reduction up to 80%) is required for the full uptake of LecA-coated beads into Gb3-functionalized vesicles. This quantitative approach highlights that lectin-glycolipid interactions provide adequate forces and energies to drive bacterial attachment and uptake.

Facile Fabrication of Protein-Macrocycle Frameworks

Precisely defined protein aggregates, as exemplified by crystals, have applications in functional materials. Consequently, engineered protein assembly is a rapidly growing field. Anionic calix[n]arenes are useful scaffolds that can mold to cationic proteins and induce oligomerization and assembly. Here, we describe protein-calixarene composites obtained via cocrystallization of commercially available sulfonato-calix[8]arene (sclx₈) with the symmetric and “neutral” protein RSL. Cocrystallization occurred across a wide range of conditions and protein charge states, from pH 2.2–9.5, resulting in three crystal forms. Cationization of the protein surface at pH ∼ 4 drives calixarene complexation and yielded two types of porous frameworks with pore diameters >3 nm. Both types of framework provide evidence of protein encapsulation by the calixarene. Calixarene-masked proteins act as nodes within the frameworks, displaying octahedral-type coordination in one case. The other framework formed millimeter-scale crystals within hours, without the need for precipitants or specialized equipment. NMR experiments revealed macrocycle-modulated side chain pK_a values and suggested a mechanism for pH-triggered assembly. The same low pH framework was generated at high pH with a permanently cationic arginine-enriched RSL variant. Finally, in addition to protein framework fabrication, sclx₈ enables de novo structure determination.

Carbohydrate supramolecular chemistry: beyond the multivalent effect

(Hetero)multivalency acts as a multichannel switch that shapes the supramolecular properties of carbohydrates in an intrinsically multifactorial biological context.

Structure and engineering of tandem repeat lectins

Through their ability to bind complex glycoconjugates, lectins have unique specificity and potential for biomedical and biotechnological applications. In particular, lectins with short repeated peptides forming carbohydrate-binding domains are not only of high interest for understanding protein evolution but can also be used as scaffold for engineering novel receptors. Synthetic glycobiology now provides the tools for engineering the specificity of lectins as well as their structure, multivalency and topologies. This review focuses on the structure and diversity of two families of tandem-repeat lectins, that is, β-trefoils and β-propellers, demonstrated as the most promising scaffold for engineering novel lectins.

The challenges of glycan recognition with natural and artificial receptors

Glycans - simple or complex carbohydrates - play key roles as recognition determinants and modulators of numerous physiological and pathological processes. Thus, many biotechnological, diagnostic and therapeutic opportunities abound for molecular recognition entities that can bind glycans with high selectivity and affinity. This review begins with an overview of the current biologically and synthetically derived glycan-binding scaffolds that include antibodies, lectins, aptamers and boronic acid-based entities. It is followed by a more detailed discussion on various aspects of their generation, structure and recognition properties. It serves as the basis for highlighting recent key developments and technical challenges that must be overcome in order to fully deal with the specific recognition of a highly diverse and complex range of glycan structures.

Heteroglycoclusters With Dual Nanomolar Affinities for the Lectins LecA and LecB From Pseudomonas aeruginosa

Multivalent structures displaying different instead of similar sugar units, namely heteroglycoclusters (hGCs), are stimulating the efforts of glycochemists for developing compounds with new biological properties. Here we report a four-step strategy to synthesize hexadecavalent hGCs displaying eight copies of αFuc and βGal. These compounds were tested for the binding to lectins LecA and LecB from Pseudomonas aeruginosa. While parent fucosylated (19) and galactosylated (20) homoclusters present nanomolar affinity with LecB and LecA, respectively, we observed that hGCs combining these sugars (11 and 13) maintain their binding potency with both lectins despite the presence of an unspecific sugar. The added multivalency is therefore not a barrier for efficient recognition by bacterial receptors and it opens the route for adding different sugars that can be selected for their immunomodulatory properties.

Synthetic glycoscapes: addressing the structural and functional complexity of the glycocalyx

The glycocalyx is an information-dense network of biomacromolecules extensively modified through glycosylation that populates the cellular boundary. The glycocalyx regulates biological events ranging from cellular protection and adhesion to signalling and differentiation. Owing to the characteristically weak interactions between individual glycans and their protein binding partners, multivalency of glycan presentation is required for the high-avidity interactions needed to trigger cellular responses. As such, biological recognition at the glycocalyx interface is determined by both the structure of glycans that are present as well as their spatial distribution. While genetic and biochemical approaches have proven powerful in controlling glycan composition, modulating the three-dimensional complexity of the cell-surface 'glycoscape' at the sub-micrometre scale remains a considerable challenge in the field. This focused review highlights recent advances in glycocalyx engineering using synthetic nanoscale glycomaterials, which allows for controlled de novo assembly of complexity with precision not accessible with traditional molecular biology tools. We discuss several exciting new studies in the field that demonstrate the power of precision glycocalyx editing in living cells in revealing and controlling the complex mechanisms by which the glycocalyx regulates biological processes.

'Clickable lectins': bioorthogonal reactive handles facilitate the directed conjugation of lectins in a modular fashion

Lectins are carbohydrate-binding proteins with specificity for their target ligands. They play diverse roles in cellular recognition and signalling processes, as well as in infections and cancer metastasis. Owing to their specificity, lectins find application in biotechnology and medicine, e.g. for blood group typing, purification of glycoproteins or lipids and as markers that target cancer cells. For some applications, lectins are immobilized on a solid support, or they are conjugated with other molecules. Classical protein conjugation reactions at nucleophilic amino acids such as cysteine or lysine are often non-selective, and the site of conjugation is difficult to pre-define. Random conjugation, however, can interfere with protein function. Therefore, we sought to equip lectins with a unique reactive handle, which can be conjugated with other molecules in a pre-defined manner. We site-specifically introduced non-canonical amino acids carrying bioorthogonal reactive groups into several lectins. As a proof of principle, we conjugated these 'clickable lectins' with small molecules. Furthermore, we conjugated lectins with different ligand specificities with one another to produce superlectins. The 'clickable lectins' might be useful for any process where lectins shall be conjugated with another module in a selective, pre-defined and site-specific manner.

Glycan-decorated protocells: novel features for rebuilding cellular processes

In synthetic biology approaches, lipid vesicles are widely used as protocell models. While many compounds have been encapsulated in vesicles (e.g. DNA, cytoskeleton and enzymes), the incorporation of glycocalyx components in the lipid bilayer has attracted much less attention so far. In recent years, glycoconjugates have been integrated in the membrane of giant unilamellar vesicles (GUVs). These minimal membrane systems have largely contributed to shed light on the molecular mechanisms of cellular processes. In this review, we first introduce several preparation and biophysical characterization methods of GUVs. Then, we highlight specific applications of protocells investigating glycolipid-mediated endocytosis of toxins, viruses and bacteria. In addition, we delineate how prototissues have been assembled from glycan-decorated protocells by using lectin-mediated cross-linking of opposed glycoreceptors (e.g. glycolipids and glycopeptides). In future applications, glycan-decorated protocells might be useful for investigating cell-cell interactions (e.g. adhesion and communication). We also speculate about the implication of lectin-glycoreceptor interactions in membrane fusion processes.