Small angle neutron scattering as sensitive tool to detect ligand-dependent shape changes in a plant lectin with β-trefoil folding and their dependence on the nature of the solvent

Biophysical Approaches for the Characterization of Protein-Metabolite Interactions

With an estimate of hundred thousands of protein molecules per cell and the number of metabolites several orders of magnitude higher, protein-metabolite interactions are omnipresent. In vitro analyses are one of the main pillars on the way to establish a solid understanding of how these interactions contribute to maintaining cellular homeostasis. A repertoire of biophysical techniques is available by which protein-metabolite interactions can be quantitatively characterized in terms of affinity, specificity, and kinetics in a broad variety of solution environments. Several of those provide information on local or global conformational changes of the protein partner in response to ligand binding. This review chapter gives an overview of the state-of-the-art biophysical toolbox for the study of protein-metabolite interactions. It briefly introduces basic principles, highlights recent examples from the literature, and pinpoints promising future directions.

Tumour cell derived effects on monocyte/macrophage polarization and function and modulatory potential of Viscum album lipophilic extract in vitro

Background

Macrophages are highly versatile cells that play an important role in tumour microenvironment. Tumour associated macrophages (TAMs) have been linked to both, good or bad prognosis of several cancer types depending on their number, composition and polarization. Viscum album lipophilic extract (VALE) contains several pentacyclic triterpenes known to modulate the activity of monocytes and other immune cells and to exhibit anticancer properties. In our in vitro study, we investigated the effect of tumour cell lines on macrophage polarization and monocyte chemotactic transmigration and examined the modulatory potential of VALE and its predominant triterpene oleanolic acid (OA).

Methods

Human peripheral blood monocytes were differentiated into monocyte derived macrophages (MDM) using M-CSF and polarized into M1 by IFN-γ and LPS and into M2 macrophages by IL-4 and IL-13 or by co-culture with two different tumour cell lines. Polarized macrophages were subsequently treated with VALE or OA. Phenotypic markers and cytokines were assessed by flow cytometry and immunoanalysis. Migration of human peripheral blood monocytes induced by monocyte chemotactic protein-1 (MCP-1) or supernatants of different tumour cell lines under the influence of VALE or OA was measured in a chemotaxis transmigration assay.

Results

In vitro polarized M1 and M2 type macrophages revealed specific phenotypic patterns and tumour cell co-cultured MDM displayed ambiguous phenotypes with M1 as well as M2 associated markers. VALE and OA showed modest influence on cell surface marker profile and cytokine expression of tumour cell co-cultured macrophages. All tumour cell supernatants markedly enhanced the migratory activity of monocytes. VALE and OA significantly inhibited MCP-1 induced monocyte transmigration, whereas monocyte migration initiated by tumour cell derived supernatants was not affected.

Conclusions

In our study we reconfirmed that co-culture with different tumour cell lines can result in a mixed macrophage phenotype with M1 as well as M2 patterns, a finding that is important for a better understanding of tumour microenvironment functions. Moreover, we demonstrated that VALE shows slight immunomodulatory effects on tumour cell co-cultured macrophages and modulates monocyte chemotactic transmigration in vitro, indicating promising possibilities of triterpenes from Viscum album L. to contribute in a multimodal concept of anti-cancer therapy in future. Our data contribute to an understanding of monocyte function and macrophage polarization in vitro and of the possibility to influence their behaviour by triterpene containing mistletoe extracts.

Inhibitory potential of chemical substitutions at bioinspired sites of β-D-galactopyranose on neoglycoprotein/cell surface binding of two classes of medically relevant lectins

Galactose is the key contact site for plant AB-toxins and the human adhesion/growth-regulatory galectins. Natural anomeric extensions and 3'-substitutions enhance its reactivity, thus prompting us to test the potential of respective chemical substitutions of galactose in the quest to develop potent inhibitors. Biochemical screening of a respective glycoside library with 60 substances in a solid-phase assay was followed by examining the compounds' activity to protect cells from lectin binding. By testing 32 anomeric extensions, 18 compounds with additional 3'-substitution, three lactosides and two Lewis-type trisaccharides rather mild effects compared to the common haptenic inhibitor lactose were detected in both assays. When using trivalent glycoclusters marked enhancements with 6- to 8-fold increases were revealed for the toxin and three of four tested galectins. Since the most potent compound and also 3'-substituted thiogalactosides reduced cell growth of a human tumor line at millimolar concentrations, biocompatible substitutions and scaffolds will be required for further developments. The synthesis of suitable glycoclusters, presenting headgroups which exploit differences in ligand selection in interlectin comparison to reduce cross-reactivity, and the documented strategic combination of initial biochemical screening with cell assays are considered instrumental to advance inhibitor design.

Hydrodynamic properties of human adhesion/growth-regulatory galectins studied by fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy is applied on homologous human lectins (i.e., adhesion/growth-regulatory galectins) to detect influence of ligand binding and presence of the linker peptide in tandem-repeat-type proteins on hydrodynamic properties. Among five tested proteins, lactose binding increased the diffusion constant only in the cases of homodimeric galectin-1 and the linkerless variant of tandem-repeat-type galectin-4. To our knowledge, the close structural similarity among galectins does not translate into identical response to ligand binding. Kinetic measurements show association and dissociation rate constants in the order of 1 to 10(3) M(-1) s(-1) and 10(-4) s(-1), respectively. Presence of the linker peptide in tandem-repeat-type protein leads to anomalous scaling with molecular mass. These results provide what we believe to be new insights into lectin responses to glycan binding, detectable so far only by small angle neutron scattering, and the structural relevance of the linker peptide. Methodologically, fluorescence correlation spectroscopy is shown to be a rather simple technical tool to characterize hydrodynamic properties of these proteins at a high level of sensitivity.

Alpha-O-linked glycopeptide mimetics: synthesis, conformation analysis, and interactions with viscumin, a galactoside-binding model lectin

Efficient cycloaddition of a silylidene-protected galactal with a suitable heterodiene yielded the basis for a facile diastereoselective route to a glycopeptide-mimetic scaffold. Its carbohydrate part was further extended by beta1-3-linked galactosylation. The pyranose rings retain their (4)C(1) chair conformation, as shown by molecular modeling and NMR spectroscopy, and the typical exo-anomeric geometry was observed for the disaccharide. The expected bioactivity was ascertained by saturation-transfer-difference NMR spectroscopy by using the galactoside-specific plant toxin viscumin as a model lectin. The experimental part was complemented by molecular docking. The described synthetic route and the strategic combination of computational and experimental techniques to reveal conformational properties and bioactivity establish the prepared alpha-O-linked glycopeptide mimetics as promising candidates for further exploitation of this scaffold to give O-glycans for lectin blocking and vaccination.