Low or No Inhibitory Potency of the Canonical Galectin Carbohydrate-binding Site by Pectins and Galactomannans.

Some complex plant-derived polysaccharides, such as modified citrus pectins and galactomannans, have been shown to have promising anti-inflammatory and anti-cancer effects. Most reports propose or claim that these effects are due to interaction of the polysaccharides with galectins because the polysaccharides contain galactose-containing side chains that might bind this class of lectin. However, their direct binding to and/or inhibition of the evolutionarily conserved galactoside-binding site of galectins has not been demonstrated. Using a well established fluorescence anisotropy assay, we tested the direct interaction of several such polysaccharides with physiological concentrations of a panel of galectins. The bioactive pectic samples tested were very poor inhibitors of the canonical galactoside-binding site for the tested galectins, with IC50 values >10 mg/ml for a few or in most cases no inhibitory activity at all. The galactomannan Davanat® was more active, albeit not a strong inhibitor (IC50 values ranging from 3 to 20 mg/ml depending on the galectin). Pure synthetic oligosaccharide fragments found in the side chains and backbone of pectins and galactomannans were additionally tested. The most commonly found galactan configuration in pectins had no inhibition of the galectins tested. Galactosylated tri- and pentamannosides, representing the structure of Davanat®, had an inhibitory effect of galectins comparable with that of free galactose. Further evaluation using cell-based assays, indirectly linked to galectin-3 inhibition, showed no inhibition of galectin-3 by the polysaccharides. These data suggest that the physiological effects of these plant polysaccharides are not due to inhibition of the canonical galectin carbohydrate-binding site.

Structural basis of pharmacological chaperoning for human ß-galactosidase.

GM1 gangliosidosis and Morquio B disease are autosomal recessive diseases caused by the defect in the lysosomal ß-galactosidase (ß-Gal), frequently related to misfolding and subsequent endoplasmic reticulum-associated degradation. Pharmacological chaperone (PC) therapy is a newly developed molecular therapeutic approach by using small molecule ligands of the mutant enzyme that are able to promote the correct folding and prevent endoplasmic reticulum-associated degradation and promote trafficking to the lysosome. In this report, we describe the enzymological properties of purified recombinant human ß-Gal(WT) and two representative mutations in GM1 gangliosidosis Japanese patients, ß-Gal(R201C) and ß-Gal(I51T). We have also evaluated the PC effect of two competitive inhibitors of ß-Gal. Moreover, we provide a detailed atomic view of the recognition mechanism of these compounds in comparison with two structurally related analogues. All compounds bind to the active site of ß-Gal with the sugar-mimicking moiety making hydrogen bonds to active site residues. Moreover, the binding affinity, the enzyme selectivity, and the PC potential are strongly affected by the mono- or bicyclic structure of the core as well as the orientation, nature, and length of the exocyclic substituent. These results provide understanding on the mechanism of action of ß-Gal selective chaperoning by newly developed PC compounds.

A bicyclic 1-deoxygalactonojirimycin derivative as a novel pharmacological chaperone for GM1 gangliosidosis.

Lysosomal ß-galactosidase (ß-Gal) deficiency causes a group of disorders that include neuronopathic GM1 gangliosidosis and non-neuronopathic Morquio B disease. We have previously proposed the use of small molecule ligands of ß-Gal as pharmacological chaperones (PCs) for the treatment of GM1 gangliosidosis brain pathology. Although it is still under development, PC therapy has yielded promising preclinical results in several lysosomal diseases. In this study, we evaluated the effect of bicyclic 1-deoxygalactonojirimycin (DGJ) derivative of the sp(2)-iminosugar type, namely 5N,6S-(N'-butyliminomethylidene)-6-thio-1- deoxygalactonojirimycin (6S-NBI-DGJ), as a novel PC for human mutant ß-Gal. In vitro, 6S-NBI-DGJ had the ability to inhibit the activity of human ß-Gal in a competitive manner and was able to protect this enzyme from heat-induced degradation. Computational analysis supported that the rigid glycone bicyclic core of 6S-NBI-DGJ binds to the active site of the enzyme, with the aglycone N'-butyl substituent, in a precise E-orientation, located at a hydrophobic region nearby. Chaperone potential profiling indicated significant increases of enzyme activity in 24 of 88 ß-Gal mutants, including four common mutations. Finally, oral administration of 6S-NBI-DGJ ameliorated the brain pathology of GM1 gangliosidosis model mice. These results suggest that 6S-NBI-DGJ is a novel PC that may be effective on a broad range of ß-Gal mutants.

Scalable syntheses of both enantiomers of DNJNAc and DGJNAc from glucuronolactone: the effect of N-alkylation on hexosaminidase inhibition.

The efficient scalable syntheses of 2-acetamido-1,2-dideoxy-D-galacto-nojirimycin (DGJNAc) and 2-acetamido-1,2-dideoxy-D-gluco-nojirimycin (DNJNAc) from D-glucuronolactone, as well as of their enantiomers from L-glucuronolactone, are reported. The evaluation of both enantiomers of DNJNAc and DGJNAc, along with their N-alkyl derivatives, as glycosidase inhibitors showed that DGJNAc and its N-alkyl derivatives were all inhibitors of α-GalNAcase but that none of the epimeric DNJNAc derivatives inhibited this enzyme. In contrast, both DGJNAc and DNJNAc, as well as their alkyl derivatives, were potent inhibitors of ß-GlcNAcases and ß-GalNAcases. Neither of the L-enantiomers showed any significant inhibition of any of the enzymes tested. Correlation of the in vitro inhibition with the cellular data, by using a free oligosaccharide analysis of the lysosomal enzyme inhibition, revealed the following structure-property relationship: hydrophobic side-chains preferentially promoted the intracellular access of iminosugars to those inhibitors with more-hydrophilic side-chain characteristics.

Bicyclic (galacto)nojirimycin analogues as glycosidase inhibitors: effect of structural modifications in their pharmacological chaperone potential towards ß-glucocerebrosidase.

A molecular-diversity-oriented approach for the preparation of bicyclic sp(2)-iminosugar glycomimetics related to nojirimycin and galactonojirimycin is reported. The synthetic strategy takes advantage of the ability of endocyclic pseudoamide-type atoms in five-membered cyclic iso(thio)ureas and guanidines to undergo intramolecular nucleophilic addition to the masked carbonyl group of monosaccharides. The stereochemistry of the resulting hemiaminal stereocenter is governed by the anomeric effect, with a large preference for the axial (pseudo-α) orientation. A library of compounds differing in the stereochemistry at the position equivalent to C-4 in monosaccharides (D-gluco and D-galacto), the heterocyclic core (cyclic isourea, isothiourea or guanidine) and the nature of the exocyclic nitrogen substituent (apolar, polar, linear or branched) has been thus prepared and the glycosidase inhibitory activity evaluated against commercial glycosidases. Compounds bearing lipophilic substituents behaved as potent and very selective inhibitors of ß-glucosidases. They further proved to be good competitive inhibitors of the recombinant human ß-glucocerebrosidase (imiglucerase) used in enzyme replacement therapy (ERT) for Gaucher disease. The potential of these compounds as pharmacological chaperones was assessed by measuring their ability to inhibit thermal-induced denaturation of the enzyme in comparison with N-nonyl-1-deoxynojirimycin (NNDNJ). The results indicated that amphiphilic sp(2)-iminosugars within this series are more efficient than NNDNJ at stabilizing ß-glucocerebrosidase and have a strong potential in pharmacological chaperone (PC) and ERT-PC combined therapies.

Cyclodextrin-mediated crystallization of acid ß-glucosidase in complex with amphiphilic bicyclic nojirimycin analogues.

Cyclodextrin-based host-guest chemistry has been exploited to facilitate co-crystallization of recombinant human acid ß-glucosidase (ß-glucocerebrosidase, GlcCerase) with amphiphilic bicyclic nojirimycin analogues of the sp(2)-iminosugar type. Attempts to co-crystallize GlcCerase with 5-N,6-O-[N'-(n-octyl)iminomethylidene]nojirimycin (NOI-NJ) or with 5-N,6-S-[N'-(n-octyl)iminomethylidene]-6-thionojirimycin (6S-NOI-NJ), two potent inhibitors of the enzyme with promising pharmacological chaperone activity for several Gaucher disease-associated mutations, were unsuccessful probably due to the formation of aggregates that increase the heterogeneity of the sample and affect nucleation and growth of crystals. Cyclomaltoheptaose (ß-cyclodextrin, ßCD) efficiently captures NOI-NJ and 6S-NOI-NJ in aqueous media to form inclusion complexes in which the lipophilic tail is accommodated in the hydrophobic cavity of the cyclooligosaccharide. The dissociation constant of the complex of the amphiphilic sp(2)-iminosugars with ßCD is two orders of magnitude higher than that of the corresponding complex with GlcCerase, allowing the efficient transfer of the inhibitor from the ßCD cavity to the GlcCerase active site. Enzyme-inhibitor complexes suitable for X-ray analysis were thus grown in the presence of ßCD. In contrast to what was previously observed for the complex of GlcCerase with the more basic derivative, 6-amino-6-deoxy-5-N,6-N-[N'-(n-octyl)iminomethylidene]nojirimycin (6N-NOI-NJ), the ß-anomers of both NOI-NJ and 6S-NOI-NJ were seen in the active site, even though the α-anomer was exclusively detected both in aqueous solution and in the corresponding ßCD:sp(2)-iminosugar complexes. Our results further suggest that cyclodextrin derivatives might serve as suitable delivery systems of amphiphilic glycosidase inhibitors in a biomedical context.

A Fluorescent sp2-iminosugar with pharmacological chaperone activity for gaucher disease: synthesis and intracellular distribution studies.

Gaucher disease (GD) is the most prevalent lysosomal-storage disorder, it is caused by mutations of acid ß-glucosidase (ß-glucocerebrosidase; ß-Glu). Recently, we found that bicyclic nojirimycin (NJ) derivatives of the sp(2)-iminosugar type, including the 6-thio-N'-octyl-(5N,6S)-octyliminomethylidene derivative (6S-NOI-NJ), behaved as very selective competitive inhibitors of the lysosomal ß-Glu and exhibited remarkable chaperone activities for several GD mutations. To obtain information about the cellular uptake pathway and intracellular distribution of this family of chaperones, we have synthesized a fluorescent analogue that maintains the fused piperidine-thiazolidine bicyclic skeleton and incorporates a dansyl group in the N'-substituent, namely 6-thio-(5N,6S)-[4-(N'-dansylamino)butyliminomethylidene]nojirimycin (6S-NDI-NJ). This structural modification does not significantly modify the biological activity of the glycomimetic as a chemical chaperone. Our study showed that 6S-NDI-NJ is mainly located in lysosome-related organelles in both normal and GD fibroblasts, and the fluorescent intensity of 6S-NDI-NJ in the lysosome is related to the ß-Glu concentration level. 6S-NDI-NJ also can enter cultured neuronal cells and act as a chaperone. Competitive inhibition studies of 6S-NDI-NJ uptake in fibroblasts showed that high concentrations of D-glucose have no effect on chaperone internalization, suggesting that it enters the cells through glucose-transporter-independent mechanisms.

Fluorescent-tagged sp2-iminosugars with potent ß-glucosidase inhibitory activity.

New fluorescently-labelled sp(2)-iminosugars based on the 5N,6S-[N'-(4-aminobutyl)iminomethylidene]-6-thionojirimycin skeleton have been synthesized as photoprobes to monitor glycosidase binding. Dansyl, dapoxyl and coumarin fluorophores were appended to the terminal amino group at the N'-substituent by either sulfonamide or amide bridging reaction. All the conjugates behaved as strong (low micromolar to nanomolar) and selective inhibitors of ß-glucosidases (almonds and bovine liver) and naringinase, in agreement with the inhibition pattern previously encountered for related iso(thio)urea-type bicyclic sp(2)-iminosugars. The presence of the fluorescent probe allows real-time and continuous monitoring of ß-glucosidase inhibition by fluorescence resonance energy transfer (FRET), taking advantage of the intrinsic tryptophan-associated fluorescence of the protein.

Chaperone activity of bicyclic nojirimycin analogues for Gaucher mutations in comparison with N-(n-nonyl)deoxynojirimycin.

Gaucher disease (GD), the most prevalent lysosomal storage disorder, is caused by mutations of lysosomal beta-glucosidase (acid beta-Glu, beta-glucocerebrosidase); these mutations result in protein misfolding. Some inhibitors of this enzyme, such as the iminosugar glucomimetic N-(n-nonyl)-1-deoxynojirimycin (NN-DNJ), are known to bind to the active site and stabilize the proper folding for the catalytic form, acting as "chemical chaperones" that facilitate transport and maturation of acid beta-Glu. Recently, bicyclic nojirimycin (NJ) analogues with structure of sp2 iminosugars were found to behave as very selective, competitive inhibitors of the lysosomal beta-Glu. We have now evaluated the glycosidase inhibitory profile of a series of six compounds within this family, namely 5-N,6-O-(N'-octyliminomethylidene-NJ (NOI-NJ), the 6-thio and 6-amino-6-deoxy derivatives (6S-NOI-NJ and 6N-NOI-NJ) and the corresponding galactonojirimycin (GNJ) counterparts (NOI-GNJ, 6S-NOI-GNJ and 6N-NOI-GNJ), against commercial as well as lysosomal glycosidases. The chaperone effects of four selected candidates (NOI-NJ, 6S-NOI-NJ, 6N-NOI-NJ, and 6S-NOI-GNJ) were further evaluated in GD fibroblasts with various acid beta-Glu mutations. The compounds showed enzyme enhancement on human fibroblasts with N188S, G202R, F213I or N370S mutations. The chaperone effects of the sp2 iminosugar were generally stronger than those observed for NN-DNJ; this suggests that these compounds are promising candidates for clinical treatment of GD patients with a broad range of beta-Glu mutations, especially for neuronopathic forms of Gaucher disease.

Synthesis of thiohydantoin-castanospermine glycomimetics as glycosidase inhibitors.

The preparation of bicyclic carbohydrate mimics related to (+)-castanospermine incorporating a thiohydantoin moiety is reported. The synthetic approach is compatible with molecular diversity-oriented strategies and involves alpha-azidoesters, built at the C-5/C-6 segment in gluco- or galactofuranose scaffolds, as the key precursors. Reduction to the corresponding alpha-amino ester and in situ coupling with isothiocyanates afford thioureidoester intermediates that undergo spontaneous cyclization to the corresponding hydantoins, beta-elimination, and furanose --> indolizidine rearrangement in a tandem manner. Biological evaluation of the new sp(2)-iminosugar-type glycomimetics evidenced a strong influence of the nature of the substituents at the nitrogen or oxygen atoms on the glycosidase inhibitory properties.

Glycosidase inhibition by ring-modified castanospermine analogues: tackling enzyme selectivity by inhibitor tailoring.

Synthesis of a panel of iso(thio)urea-type ring-modified castanospermine analogues bearing a freely mutarotating pseudoanomeric hydroxyl group results in tight-binding beta-glucosidase inhibitors with unusual binding signatures; the presence of an N-octyl substituent imparts a remarkable anomeric selectivity, promoting strong binding of the appropriate beta-anomer by the beta-glucosidase.

Generalized anomeric effect in gem-diamines: stereoselective synthesis of alpha-N-linked disaccharide mimics.

The orbital (negative hyperconjugation) contribution to the generalized anomeric effect is highly increased in bicyclic gem-diamines with a pseudoamide-type endocyclic nitrogen atom, which has been exploited for the stereoselective synthesis of configurationally stable alpha-N-linked azadisaccharide heteroanalogues of the natural disaccharides maltose and isomaltose as aglycon-sensitive inhibitors of isomaltase.