Synthesis of a library of variously modified 4-methylumbelliferyl xylosides and a structure-activity study of human ß4GalT7.

Proteoglycans (PGs) are complex macromolecules that are composed of glycosaminoglycan (GAG) chains covalently attached to a core protein through a tetrasaccharide linker. The biosynthesis of PGs is complex and involves a large number of glycosyltranferases. Here we present a structure-activity study of human ß4GalT7, which transfers the first Gal residue onto a xyloside moiety of the linkage region. An efficient and regiocontrolled synthesis of a library of modified analogs of 4-methylumbelliferyl xyloside (XylMU) is reported herein. Hydroxyl groups at the position C-2, C-3 or C-4 have been epimerized and/or replaced by a hydrogen or a fluorine, while the anomeric oxygen was replaced by either a sulfur or a sulfone. The effect of these compounds on human ß4GalT7 activity in vitro and on GAG biosynthesis in cellulo was then evaluated.

Probing the acceptor active site organization of the human recombinant ß1,4-galactosyltransferase 7 and design of xyloside-based inhibitors.

Among glycosaminoglycan (GAG) biosynthetic enzymes, the human ß1,4-galactosyltransferase 7 (hß4GalT7) is characterized by its unique capacity to take over xyloside derivatives linked to a hydrophobic aglycone as substrates and/or inhibitors. This glycosyltransferase is thus a prime target for the development of regulators of GAG synthesis in therapeutics. Here, we report the structure-guided design of hß4GalT7 inhibitors. By combining molecular modeling, in vitro mutagenesis, and kinetic measurements, and in cellulo analysis of GAG anabolism and decorin glycosylation, we mapped the organization of the acceptor binding pocket, in complex with 4-methylumbelliferone-xylopyranoside as prototype substrate. We show that its organization is governed, on one side, by three tyrosine residues, Tyr(194), Tyr(196), and Tyr(199), which create a hydrophobic environment and provide stacking interactions with both xylopyranoside and aglycone rings. On the opposite side, a hydrogen-bond network is established between the charged amino acids Asp(228), Asp(229), and Arg(226), and the hydroxyl groups of xylose. We identified two key structural features, i.e. the strategic position of Tyr(194) forming stacking interactions with the aglycone, and the hydrogen bond between the His(195) nitrogen backbone and the carbonyl group of the coumarinyl molecule to develop a tight binder of hß4GalT7. This led to the synthesis of 4-deoxy-4-fluoroxylose linked to 4-methylumbelliferone that inhibited hß4GalT7 activity in vitro with a Ki 10 times lower than the Km value and efficiently impaired GAG synthesis in a cell assay. This study provides a valuable probe for the investigation of GAG biology and opens avenues toward the development of bioactive compounds to correct GAG synthesis disorders implicated in different types of malignancies.

Specific implications of the HIV-1 nucleocapsid zinc fingers in the annealing of the primer binding site complementary sequences during the obligatory plus strand transfer.

Synthesis of the HIV-1 viral DNA by reverse transcriptase involves two obligatory strand transfer reactions. The second strand transfer corresponds to the annealing of the (-) and (+) DNA copies of the primer binding site (PBS) sequence which is chaperoned by the nucleocapsid protein (NCp7). NCp7 modifies the (+)/(-)PBS annealing mechanism by activating a loop-loop kissing pathway that is negligible without NCp7. To characterize in depth the dynamics of the loop in the NCp7/PBS nucleoprotein complexes, we investigated the time-resolved fluorescence parameters of a (-)PBS derivative containing the fluorescent nucleoside analogue 2-aminopurine at positions 6, 8 or 10. The NCp7-directed switch of (+)/(-)PBS annealing towards the loop pathway was associated to a drastic restriction of the local DNA dynamics, indicating that NCp7 can 'freeze' PBS conformations competent for annealing via the loops. Moreover, the modifications of the PBS loop structure and dynamics that govern the annealing reaction were found strictly dependent on the integrity of the zinc finger hydrophobic platform. Our data suggest that the two NCp7 zinc fingers are required to ensure the specificity and fidelity of the second strand transfer, further underlining the pivotal role played by NCp7 to control the faithful synthesis of viral HIV-1 DNA.

Chondroitin sulfate N-acetylgalactosaminyltransferase-1 (CSGalNAcT-1) involved in chondroitin sulfate initiation: Impact of sulfation on activity and specificity.

Glycosaminoglycan (GAG) assembly initiates through the formation of a linkage tetrasaccharide region serving as a primer for both chondroitin sulfate (CS) and heparan sulfate (HS) chain polymerization. A possible role for sulfation of the linkage structure and of the constitutive disaccharide unit of CS chains in the regulation of CS-GAG chain synthesis has been suggested. To investigate this, we determined whether sulfate substitution of galactose (Gal) residues of the linkage region or of N-acetylgalactosamine (GalNAc) of the disaccharide unit influences activity and specificity of chondroitin sulfate N-acetylgalactosaminyltransferase-1 (CSGalNAcT-1), a key glycosyltransferase of CS biosynthesis. We synthesized a series of sulfated and unsulfated analogs of the linkage oligosaccharide and of the constitutive unit of CS and tested these molecules as potential acceptor substrates for the recombinant human CSGalNAcT-1. We show here that sulfation at C4 or C6 of the Gal residues markedly influences CSGalNAcT-1 initiation activity and catalytic efficiency. Kinetic analysis indicates that CSGalNAcT-1 exhibited 3.6-, 1.6-, and 2.2-fold higher enzymatic efficiency due to lower K(m) values toward monosulfated trisaccharides substituted at C4 or C6 position of Gal1, and at C6 of Gal2, respectively, compared with the unsulfated oligosaccharide. This highlights the critical influence of Gal substitution on both CSGalNAcT-1 activity and specifity. No GalNAcT activity was detected toward sulfated and unsulfated analogs of the CS constitutive disaccharide (GlcA-ß1,3-GalNAc), indicating that CSGalNAcT-1 was involved in initiation but not in elongation of CS chains. Our results strongly suggest that sulfation of the linkage region acts as a regulatory signal in CS chain initiation.

A versatile strategy to synthesize N-methyl-anthranilic acid-labelled glycoprobes for fluorescence-based screening assays.

Here we propose a general strategy to label carbohydrates with N-methyl-anthranilic acid at the anomeric position. Through two examples, we demonstrate that the generated glycoprobes are suitable for fluorescence-based binding/competition assays. Our approach is expected to readily generate series of glycoprobes dedicated to screening assays for the discovery of drugs targeting carbohydrate-protein interactions.

8-vinyl-deoxyadenosine, an alternative fluorescent nucleoside analog to 2'-deoxyribosyl-2-aminopurine with improved properties.

We report here the synthesis and the spectroscopic characterization of 8-vinyl-deoxyadenosine (8vdA), a new fluorescent analog of deoxyadenosine. 8vdA was found to absorb and emit in the same wavelength range as 2'-deoxyribosyl-2-aminopurine (2AP), the most frequently used fluorescent nucleoside analog. Though the quantum yield of 8vdA is similar to that of 2AP, its molar absorption coefficient is about twice, enabling a more sensitive detection. Moreover, the fluorescence of 8vdA was found to be sensitive to temperature and solvent but not to pH (around neutrality) or coupling to phosphate groups. Though 8vdA is base sensitive and susceptible to depurination, the corresponding phosphoramidite was successfully prepared and incorporated in oligonucleotides of the type d(CGT TTT XNX TTT TGC) where N = 8vdA and X = A, T or C. The 8vdA-labeled oligonucleotides gave more stable duplexes than the corresponding 2AP-labeled sequences when X = A or T, indicating that 8vdA is less perturbing than 2AP and probably adopts an anti conformation to preserve the Watson-Crick H-bonding. In addition, the quantum yield of 8vdA is significantly higher than 2AP in all tested oligonucleotides in both their single strand and duplex states. The steady-state and time-resolved fluorescence parameters of 8vdA and 2AP were found to depend similarly on the nature of their flanking residues and on base pairing, suggesting that their photophysics are governed by similar mechanisms. Taken together, our data suggest that 8vdA is a non perturbing nucleoside analog that may be used with improved sensitivity for the same applications as 2AP.

'Click'-xylosides as initiators of the biosynthesis of glycosaminoglycans: Comparison of mono-xylosides with xylobiosides.

Different mono-xylosides and their corresponding xylobiosides obtained by a chemo-enzymatic approach featuring various substituents attached to a triazole ring were probed as priming agents for glycosaminoglycan (GAG) biosynthesis in the xylosyltransferase-deficient pgsA-745 Chinese hamster ovary cell line. Xylosides containing a hydrophobic aglycone moiety were the most efficient priming agents. Mono-xylosides induced higher GAG biosynthesis in comparison with their corresponding xylobiosides. The influence of the degree of polymerization of the carbohydrate part on the priming activity was investigated through different experiments. We demonstrated that in case of mono-xylosides, the cellular uptake as well as the affinity and the catalytic efficiency of ß-1,4-galactosyltransferase 7 were higher than for xylobiosides. Altogether, these results indicate that hydrophobicity of the aglycone and degree of polymerization of glycone moiety were critical factors for an optimal priming activity for GAG biosynthesis.

Interaction between dietary bioactive peptides of short length and bile salts in submicellar or micellar state.

Bile salts act as steroidal detergents in the gut, and could also interact with peptides and improve their bioavailability, although the mechanism is unclear. The occurrence of direct interaction between milk bioactive peptides, Ile-Asn-Tyr-Trp, Leu-Asp-Gln-Trp, and Leu-Gln-Lys-Trp, and different bile salts in the submicellar or micellar state was investigated by intrinsic fluorescence measurement and dynamic light scattering, above the critical micellar concentration, the latter being determined by isothermal titration calorimetry. The peptides form aggregates, spontaneously. In the presence of bile salts, some released peptide monomers were bound to the micellar surface. The lack of hydrogen bonding involving the C12OH group of the steroid skeleton, and the acidic function of some bile salts, might promote the interaction with the peptides, as could the lack of the C12OH group, rather than that of the C7OH group. At submicellar concentrations, sodium taurochenodeoxycholate and taurodeoxycholate readily interacted with the most hydrophobic peptide Ile-Asn-Tyr-Trp.

HIV-1 nucleocapsid protein binds to the viral DNA initiation sequences and chaperones their kissing interactions.

The chaperone properties of the human immunodeficiency virus type 1 (HIV-1) nucleocapsid protein (NC) are required for the two obligatory strand transfer reactions occurring during viral DNA synthesis. The second strand transfer relies on the destabilization and the subsequent annealing of the primer binding site sequences (PBS) at the 3' end of the (-) and (+) DNA strands. To characterize the binding and chaperone properties of NC on the (-)PBS and (+)PBS sequences, we monitored by steady-state and time-resolved fluorescence spectroscopy as well as by fluorescence correlation spectroscopy the interaction of NC with wild type and mutant oligonucleotides corresponding to the (-)PBS and (+)PBS hairpins. NC was found to bind with high affinity to the loop, the stem and the single-stranded protruding sequence of both PBS sequences. NC induces only a limited destabilization of the secondary structure of both sequences, activating the transient melting of the stem only during its "breathing" period. This probably results from the high stability of the PBS due to the four G-C pairs in the stem. In contrast, NC directs the formation of "kissing" homodimers efficiently for both (-)PBS and (+)PBS sequences. Salt-induced dimerization and mutations in the (-)PBS sequence suggest that these homodimers may be stabilized by two intermolecular G-C Watson-Crick base-pairs between the partly self-complementary loops. The propensity of NC to promote the dimerization of partly complementary sequences may favor secondary contacts between viral sequences and thus, recombination and viral diversity.

The UDP-glucuronosyltransferases of the blood-brain barrier: their role in drug metabolism and detoxication.

UDP-glucuronosyltransferases (UGTs) form a multigenic family of membrane-bound enzymes expressed in various tissues, including brain. They catalyze the formation of ß-D-glucuronides from structurally unrelated substances (drugs, other xenobiotics, as well as endogenous compounds) by the linkage of glucuronic acid from the high energy donor, UDP-α-D-glucuronic acid. In brain, UGTs actively participate to the overall protection of the tissue against the intrusion of potentially harmful lipophilic substances that are metabolized as hydrophilic glucuronides. These metabolites are generally inactive, except for important pharmacologically glucuronides such as morphine-6-glucuronide. UGTs are mainly expressed in endothelial cells and astrocytes of the blood brain barrier (BBB). They are also associated to brain interfaces devoid of BBB, such as circumventricular organ, pineal gland, pituitary gland and neuro-olfactory tissues. Beside their key-role as a detoxication barrier, UGTs play a role in the steady-state of endogenous compounds, like steroids or dopamine (DA) that participate to the function of the brain. UGT isoforms of family 1A, 2A, 2B and 3A are expressed in brain tissues to various levels and are known to present distinct but overlapping substrate specificity. The importance of these enzyme species with regard to the formation of toxic, pharmacologically or physiologically relevant glucuronides in the brain will be discussed.

Structure-function relationships and crystal structures of the vitamin D receptor bound 2 alpha-methyl-(20S,23S)- and 2 alpha-methyl-(20S,23R)-epoxymethano-1 alpha,25-dihydroxyvitamin D3.

The vitamin D nuclear receptor is a ligand-dependent transcription factor that controls multiple biological responses such as cell proliferation, immune responses, and bone mineralization. Numerous 1 alpha,25(OH)(2)D(3) analogues, which exhibit low calcemic side effects and/or antitumoral properties, have been synthesized. We recently showed that the synthetic analogue (20S,23S)-epoxymethano-1 alpha,25-dihydroxyvitamin D(3) (2a) acts as a 1 alpha,25(OH)(2)D(3) superagonist and exhibits both antiproliferative and prodifferentiating properties in vitro. Using this information and on the basis of the crystal structures of human VDR ligand binding domain (hVDR LBD) bound to 1 alpha,25(OH)(2)D(3), 2 alpha-methyl-1 alpha,25(OH)(2)D(3), or 2a, we designed a novel analogue, 2 alpha-methyl-(20S,23S)-epoxymethano-1 alpha,25-dihydroxyvitamin D(3) (4a), in order to increase its transactivation potency. Here, we solved the crystal structures of the hVDR LBD in complex with the 4a (C23S) and its epimer 4b (C23R) and determined their correlation with specific biological outcomes.

The "Phantom Effect" of the Rexinoid LG100754: structural and functional insights.

Retinoic acid receptors (RARs) and Retinoid X nuclear receptors (RXRs) are ligand-dependent transcriptional modulators that execute their biological action through the generation of functional heterodimers. RXR acts as an obligate dimer partner in many signalling pathways, gene regulation by rexinoids depending on the liganded state of the specific heterodimeric partner. To address the question of the effect of rexinoid antagonists on RAR/RXR function, we solved the crystal structure of the heterodimer formed by the ligand binding domain (LBD) of the RARα bound to its natural agonist ligand (all-trans retinoic acid, atRA) and RXRα bound to a rexinoid antagonist (LG100754). We observed that RARα exhibits the canonical agonist conformation and RXRα an antagonist one with the C-terminal H12 flipping out to the solvent. Examination of the protein-LG100754 interactions reveals that its propoxy group sterically prevents the H12 associating with the LBD, without affecting the dimerization or the active conformation of RAR. Although LG100754 has been reported to act as a 'phantom ligand' activating RAR in a cellular context, our structural data and biochemical assays demonstrate that LG100754 mediates its effect as a full RXR antagonist. Finally we show that the 'phantom ligand effect' of the LG100754 is due to a direct binding of the ligand to RAR that stabilizes coactivator interactions thus accounting for the observed transcriptional activation of RAR/RXR.

How the HIV-1 nucleocapsid protein binds and destabilises the (-)primer binding site during reverse transcription.

The human immunodeficiency virus type 1 nucleocapsid protein (NCp7) plays an important role in the second strand transfer during reverse transcription. It promotes annealing of the 18-nucleotide complementary DNA primer-binding site (PBS) sequences at the 3' ends of (-)DNA and (+)DNA. NMR studies show that NCp7(12-55) and NCp7(1-55) interact at the 5' end of the loop of DeltaP(-)PBS, a (-)PBS derivative without the 3' protruding sequence, in a slow-exchange equilibrium. This interaction is mediated through the binding of the hydrophobic plateau (Val13, Phe16, Thr24, Ala25, Trp37, and Met46) on the zinc finger domain of both peptides to the 5-CTG-7 sequence of DeltaP(-)PBS. The stacking of the Trp37 aromatic ring with the G7 residue likely constitutes the determinant factor of the interaction. Although NCp7(12-55) does not melt the DeltaP(-)PBS stem-loop structure, it opens the loop and weakens the C5.G11 base pair next to the loop. Moreover, NCp7(12-55) was also found to bind but with lower affinity to the 10-CGG-12 sequence in an intermediate-exchange equilibrium on the NMR time scale. The loop modifications may favour a kissing interaction with the complementary (+)PBS loop. Moreover, the weakening of the upper base pair of the stem likely promotes the melting of the stem that is required to convert the kissing complex into the final (+/-)PBS extended duplex.

Investigating the mechanism of the nucleocapsid protein chaperoning of the second strand transfer during HIV-1 DNA synthesis.

Conversion of the human immunodeficiency virus type 1 (HIV-1) genomic RNA into the proviral DNA by reverse transcriptase involves two obligatory strand transfers that are chaperoned by the nucleocapsid protein (NC). The second strand transfer relies on the annealing of the (-) and (+) copies of the primer binding site, (-)PBS and (+) PBS, which fold into complementary stem-loops (SLs) with terminal single-stranded overhangs. To understand how NC chaperones their hybridization, we investigated the annealing kinetics of fluorescently labelled (+)PBS with various (-)PBS derivatives. In the absence of NC, the (+)/(-)PBS annealing was governed by a second-order pathway nucleated mainly by the single-stranded overhangs of the two PBS SLs. The annealing reaction appeared to be rate-limited by the melting of the stable G.C-rich stem subsequent to the formation of the partially annealed intermediate. A second pathway nucleated through the loops could be detected, but was very minor. NC(11-55), which consists primarily of the zinc finger domain, increased the (-)/(+) PBS annealing kinetics by about sixfold, by strongly activating the interaction between the PBS loops. NC(11-55) also activated (-)/(+) PBS annealing through the single-strand overhangs, but by a factor of only 2. Full-length NC(1-55) further increased the (-)/(+)PBS annealing kinetics by tenfold. The NC-promoted (-)/(+)PBS mechanism proved to be similar with extended (-)DNA molecules, suggesting that it is relevant in the context of proviral DNA synthesis. These findings favour the notion that the ubiquitous role of NC in the viral life-cycle probably relies on the ability of NC to chaperone nucleic acid hybridization via different mechanisms.