Enhancement of macromolecular ligand binding by rabbit alveolar macrophages by mannose oligosaccharides and related compounds

Targeted gene delivery to hepatocytes with galactosylated amphiphilic cyclodextrins

Objectives

Achieving targeted delivery of gene medicines is desirable to maximise activity. Here, galactosylated amphiphilic cyclodextrins (CDs) are examined in terms of their ability to transfect asialoglycoprotein receptor-bearing HepG2 cells.

Methods

Cationic amphiphilic CDs were synthesised as well as amphiphilic CDs bearing galactose-targeting ligands with different linker lengths. Binding of galactosylated CDs to a galactose-specific lectin was examined by surface plasmon resonance. CDs were formulated with and without the helper lipid DOPE and complexed with plasmid DNA. Transfection was evaluated by luciferase assay. Intracellular trafficking was assessed by confocal microscopy.

Key findings

Binding of targeted CDs to a galactose-specific lectin was achieved. Binding decreased with linker length between the galactosyl group and the CD core. Contrary to the lectin binding results, transfection levels increased with an increase in linker length from 7 atoms to 15. Compared to non-targeted formulations, a significant increase in transfection was observed only in the presence of the helper lipid DOPE. Confocal microscopy revealed that DOPE caused a pronounced effect on cellular distribution.

Conclusions

The galactose-targeting ligand induced substantial increases in transfection over non-targeted formulations when DOPE was included, indicating the potential for targeted gene delivery using CD-based delivery systems.

Influence of Spacer Length on the Agglutination of Glycolipid‐Incorporated Liposomes by ConA as Model Membrane

Through a systematic investigation of the agglutination of long chain mannolipid and glucolipid incorporated liposomes by concanavalin A (ConA) it was found that the agglutination was dependent on different factors. The studied factors reported here are (1) spacer length and (2) ground lipid matrix. The threshold and the relative saturating ConA binding concentration (saturation point to attain the binding saturation condition) of glycosides with varying spacer length for agglutination are dependent on the spacer length of the glycolipid. These concentrations decrease with the increasing number of in-built ethyleneoxy spacer length in the glycolipid and find its minimum with 6 spacer units; it increases then more and more with increasing number of spacer units (>6 units). This is supposed to be due to the requirement of a proper distance of the hydrophilic determinant from the liposome surface for the response by ConA (response invoking distance), which may be most favorable in case of 6 spacer units. Further increase in number of spacer units (>6) results to an increasing probability of the bending of the spacer chain along with the terminal polar head group more and more towards the liposome surface; this leads to a reduction of the factual distance of the terminal hydrophilic head group from the liposome surface, weakening the response for ConA binding. The threshold concentration or saturation point decreases also with the rigidity of the ground lipid matrix. Increased rigidity of the ground matrix leads to a phase separation and localized 'Domain' formation with the glycolipid inside the ground matrix layer due to their immiscibility, invoking better response resulting to a reduction of required incorporated glycolipid concentration.

Influence of spacer length on interaction of mannosylated liposomes with human phagocytic cells

PURPOSE

To improve target specificity and uptake of liposomes by macrophages, one can improve high-affinity receptor binding to mannose determinants with their 175-kDa mannose receptor (MR), which is mainly influenced by the length and flexibility of the spacer between the carbohydrate head group and liposome surface. Liposomes containing alkylmannosides with hydrophilic spacers 0 to 8 ethyleneoxy units (EO) long (Man0...Man8) were used to investigate systematically the effects of spacer length on liposome-cell interactions.

METHODS

Concanavalin A (ConA)-induced liposome aggregation was studied by turbidity measurement and cell uptake using PMA-induced HL-60 cells or native human macrophages by determining 6-CF after cell lysis or NBD-fluorescence with flow cytometry. Detection of MR in native cell populations was carried out by an antibody assay using flow cytometry; MR-representing cells were selected analytically.

RESULTS

Liposomes containing mannosides with more than one EO spacer length were specifically aggregated by ConA, indicating accessibility of the carbohydrate ligands of these derivatives. Increase in EO spacer units of incorporated mannosides (two or more EO) led to suppression of cellular uptake of mannosylated liposomes by phagocytes lacking MR (HL60, U937). The extent of suppression increased with spacer length. Liposome uptake by native macrophages expressing MR was, on the contrary, improved, particularly by Man6 and Man8.

CONCLUSIONS

Uptake of liposomes modified with Man6 or Man8 by native cells was enhanced but did not reach an optimum. Thus, Man6, Man8, and mannosides with even longer spacer arms are of potential use in receptor-mediated targeting.

Oligolysine-based saccharide clusters: synthesis and specificity

In search of specific and highly selective sugar clusters for cell receptors, such as membrane lectins, various disaccharides were coupled to small peptide cores through an amide bond. In a first step, the reducing disaccharides, i.e. lactose and three different dimannoses, were converted into glycosyl-pyroglutamyl-beta-alanine derivatives. The free carboxylic group of these conjugates was then coupled to the alpha and epsilon amino groups of the core peptide (Lys( n )-Ala-Cys-NH2) with n =1 to 5, with complete substitution leading to homogeneous glycoclusters. The thiol group of the cysteine residue was used to tag the glycosylated oligolysines upon reaction with fluorescein iodoacetamide. The affinity of these glycoclusters towards two plant lectins was assessed by surface plasmon resonance. The selectivity of their cell uptake was investigated by flow cytometry using two types of cells: a human hepatoma cell line (HepG2 cells) expressing the plasma membrane galactose-specific lectin, and monocyte-derived dendritic cells expressing the plasma membrane mannose-specific lectin. The glycoclusters containing four or five disaccharides were shown to bind plant lectins and cell surface membrane lectins with a narrow selectivity and with a high affinity.

Synthesis of clustered glycoside-antigen conjugates by two one-pot, orthogonal, chemoselective ligation reactions: scope and limitations

Major histocompatibility class II antigens have been bound to clustered glycosides for selective targeting of the dendritic cell mannose receptor. Di-, tetra-, and octavalent glycoside-antigen conjugates have been obtained after two, orthogonal, hydrazone/thioether ligations, performed by using thio derivatives of D-mannose, D-galactose, or D(-)-quinic acid, glyoxylyl (or hydrazino)-N-chloroacetylated lysinyl trees, and N-terminal hydrazino (or glyoxylyl) peptide antigens. Successful one-pot condensations have been developed to account for the nature of the antigens and the valency of the trees.

Lysine-based cluster mannosides that inhibit ligand binding to the human mannose receptor at nanomolar concentration

In search of synthetic high affinity ligands for the mannose receptor, we synthesized a series of lysine-based oligomannosides containing two (M2L) to six (M6L5) terminal alpha-D-mannose groups that are connected with the backbone by flexible elongated spacers (16 A). The synthesized cluster mannosides were all able to displace binding of biotinylated ribonuclease B and tissue-type plasminogen activator to isolated human mannose receptor. The affinity of these cluster mannosides for the mannose receptor was continuously enhanced from 18-23 microM to 0.5-2.6 nM, with mannose valencies increasing from two to six. On average, expansion of the cluster mannoside with an additional alpha-D-mannose group resulted in a 10-fold increase in its affinity for the mannose receptor. M3L2 to M6L5 displayed negative cooperative inhibition of ligand binding to the mannose receptor, suggesting that binding of these mannosides involves multiple binding sites. The nanomolar affinity of the most potent ligand, the hexamannoside M6L5 makes it the most potent synthetic cluster mannoside for the mannose receptor yet developed. As a result of its high affinity and accessible synthesis, M6L5 not only is a powerful tool to study the mechanism of ligand binding by the mannose receptor, but it is also a promising targeting device to accomplish cell-specific delivery of genes and drugs to liver endothelial cells or macrophages in bone marrow, lungs, spleen, and atherosclerotic plaques.

Macrophage membrane glycoprotein binding of Griffonia simplicifolia I-B4 induces TNF-alpha production and a tumoricidal response

Thioglycollate-elicited macrophages (m phi), upon binding the lectin Griffonia simplicifolia IB4 (GSIB4) at the plasma membrane, are induced to secrete several low molecular weight proteins. In this investigation, results from specific ELISA and immunoprecipitation analysis of these molecules confirmed that the cytokine, tumor necrosis factor-alpha (TNF-alpha), belongs to the group of elicited proteins. This specific m phi response is directly influenced by the dose of GSIB4 used and the time in contact with the cells. At 40 micrograms/ml GSIB4, the maximum dose of lectin used, the m phi activity was equal to that achieved when the cells were incubated with an interferon-gamma/lipopolysaccharide (IFN/LPS) stimulus alone. Moreover, the data showed that TNF-mediated tumoricidal activity was significantly influenced by GSIB4 binding to the m phi membrane. When the lectin was incubated alone or in sequence with IFN/LPS, this ligand-receptor binding promoted the lysis of WEHI 164 tumor target cells. However, concurrent incubation of both IFN/LPS and GSIB4 with m phi significantly diminished the tumoricidal response. This suggested that one of the metabolic pathways utilized subsequent to receptor-ligand binding was altered by these interactions. When cyclic AMP (cAMP) and inositol triphosphate (IP3) levels were examined, the results showed that the concentration of cAMP was unchanged despite the fact that IP3 levels were significantly enhanced upon m phi-GSIB4 binding. Collectively, the data show that GSIB4 binding to specific glycoproteins in the m phi membrane induces TNF-alpha production and facilitates TNF-alpha dependent tumoricidal responses. It also appears that the transduction of the signal, in part, at least utilizes the phosphatidyl inositol pathway. Finally, it is noteworthy that m phi activity is influenced by the sequence in which GSIB4 is presented to the m phi relative to the IFN/LPS treatment.