A DNA Aptameric Ligand of Human Transferrin Receptor Generated by Cell-SELEX

General cancer-targeted ligands that can deliver drugs to cells have been given considerable attention. In this paper, a high-affinity DNA aptamer (HG1) generally binding to human tumor cells was evolved by cell-SELEX, and was further optimized to have 35 deoxynucleotides (HG1-9). Aptamer HG1-9 could be taken up by live cells, and its target protein on a cell was identified to be human transferrin receptor (TfR). As a man-made ligand of TfR, aptamer HG1-9 was demonstrated to bind at the same site of human TfR as transferrin with comparable binding affinity, and was proved to cross the epithelium barrier through transferrin receptor-mediated transcytosis. These results suggest that aptamer HG1-9 holds potential as a promising ligand to develop general cancer-targeted diagnostics and therapeutics.     

Structural Model for the Binding Sites of Allosterically Potentiating Ligands on Nicotinic Acetylcholine Receptors

Current treatments of Alzheimer's disease include the allosteric potentiation of nicotinic acetylcholine receptor (nAChR) response. The location of the binding site for allosteric potentiating ligands (APLs) within the receptor is not yet fully understood. Based on homology models for the ligand binding domain of human α7, human α4β2, and chicken α7 receptors, as well as blind docking experiments with galanthamine, physostigmine, codeine, and 5HT, we identified T197 as an essential element of the APL binding site at the outer surface of the ligand binding domain (LBD) of nAChR. We also found the previously known galanthamine binding site in the region of K123 at the inside of the receptor funnel, which, however, was shown to not be part of the APL site. Our results are verified by site‐directed mutagenesis and electrophysiological experiments, and suggest that APL and ACh bind to different sites on nicotinic receptors and that allosteric potentiation may arise from a direct interplay between both these sites.     

Hetero‐bivalent GLP‐1/Glibenclamide for Targeting Pancreatic β‐Cells

G protein‐coupled receptor (GPCR) cell signalling cascades are initiated upon binding of a specific agonist ligand to its cell surface receptor. Linking multiple heterologous ligands that simultaneously bind and potentially link different receptors on the cell surface is a unique approach to modulate cell responses. Moreover, if the target receptors are selected based on analysis of cell‐specific expression of a receptor combination, then the linked binding elements might provide enhanced specificity of targeting the cell type of interest, that is, only to cells that express the complementary receptors. Two receptors whose expression is relatively specific (in combination) to insulin‐secreting pancreatic β‐cells are the sulfonylurea‐1 (SUR1) and the glucagon‐like peptide‐1 (GLP‐1) receptors. A heterobivalent ligand was assembled from the active fragment of GLP‐1 (7–36 GLP‐1) and glibenclamide, a small organic ligand for SUR1. The synthetic construct was labelled with Cy5 or europium chelated in DTPA to evaluate binding to β‐cells, by using fluorescence microscopy or time‐resolved saturation and competition binding assays, respectively. Once the ligand binds to β‐cells, it is rapidly capped and presumably removed from the cell surface by endocytosis. The bivalent ligand had an affinity approximately fivefold higher than monomeric europium‐labelled GLP‐1, likely a result of cooperative binding to the complementary receptors on the βTC3 cells. The high‐affinity binding was lost in the presence of either unlabelled monomer, thus demonstrating that interaction with both receptors is required for the enhanced binding at low concentrations. Importantly, bivalent enhancement was accomplished in a cell system with physiological levels of expression of the complementary receptors, thus indicating that this approach might be applicable for β‐cell targeting in vivo.     

Molecular dynamics of the 5-HT1a receptor and ligands

A 3-D model of the human 5-HT_1a receptor was constructed fromits amino acid sequence by computer graphics techniques, molecularmechanics calculations and molecular dynamics simulations. Themodel has seven -helical membrane spanning segments, which forma central core containing a putative ligand binding site. Electrostaticpotentials 1.4 Å outside the water accessible surfacewere mainly negative on the synaptic side of the receptor modeland at the postulated ligand binding site, and positive in thecytoplasmic domains. The negative electrostatic potentials aroundthe synaptic domains indicate that positively charged ligandsare attracted to the receptor by electrostatic forces. Moleculardynamics simulations of the receptor model with serotonin, ipsapirone,R(–)-methiothepin or S(+)- methiothepin in the centralcore suggested that up to 22 different amino acid residues mayform a ligand binding pocket, and contribute to the specificityof ligand recognition and binding.     

抗Ⅰ型脊髓灰质炎病毒单克隆抗体的制备

目的制备抗脊髓灰质炎病毒单克隆抗体,为ELISA检测方法的建立奠定基础。方法将Ⅰ型脊髓灰质炎减毒疫苗原液和灭活疫苗原液经超滤、超离纯化,以纯化的减毒脊髓灰质炎病毒D抗原免疫BALB/c小鼠,得到的脾细胞与SP2/0-A14小鼠骨髓瘤细胞融合,以纯化的灭活脊髓灰质炎病毒D抗原建立间接ELISA法,筛选分泌特异性单抗的阳性细胞株,诱生小鼠腹水,通过正辛酸-硫酸铵法纯化抗体,并进行各项鉴定。结果获得两株稳定分泌抗脊髓灰质炎病毒单抗的细胞株7G5及5E4。纯化的7G5和5E4株单抗分别可与脊髓灰质炎病毒D抗原的VP2、VP3衣壳蛋白结合;在不同温度下放置均比较稳定;pH4不利于7G5株单抗的保存;5E4株单抗的相对亲和力大于7G5株;7G5株单抗的亚类为IgM,5E4株单抗的亚类为IgG2a;两株单抗可识别相似表位;7G5株和5E4株单抗的中和效价分别为1:64和1:128。结论已成功制备了两株分泌抗脊髓灰质炎病毒的单抗。     

Lactose‐Functionalized Dendrimers Arbitrate the Interaction of Galectin‐3/MUC1 Mediated Cancer Cellular Aggregation

By using lactose‐functionalized poly(amidoamine) dendrimers as a tunable multivalent platform, we studied cancer cell aggregation in three different cell lines (A549, DU‐145, and HT‐1080) with galectin‐3. We found that small lactose‐functionalized G(2)‐dendrimer 1 inhibited galectin‐3‐induced aggregation of the cancer cells. In contrast, dendrimer 4 (a larger, generation 6 dendrimer with 100 carbohydrate end groups) caused cancer cells to aggregate through a galectin‐3 pathway. This study indicates that inhibition of cellular aggregation occurred because 1 provided competitive binding sites for galectin‐3 (compared to its putative cancer cell ligand, TF‐antigen on MUC1). Dendrimer 4 , in contrast, provided an excess of ligands for galectin‐3 binding; this caused crosslinking and aggregation of cells to be increased.     

Structure-activity relationships and mechanism of action of Eph-ephrin antagonists: interaction of cholanic acid with the EphA2 receptor

The Eph-ephrin system, including the EphA2 receptor and the ephrinA1 ligand, plays a critical role in tumor and vascular functions during carcinogenesis. We previously identified (3α,5β)-3-hydroxycholan-24-oic acid (lithocholic acid) as an Eph-ephrin antagonist that is able to inhibit EphA2 receptor activation; it is therefore potentially useful as a novel EphA2 receptor-targeting agent. Herein we explore the structure-activity relationships of a focused set of lithocholic acid derivatives based on molecular modeling investigations and displacement binding assays. Our exploration shows that while the 3-α-hydroxy group of lithocholic acid has a negligible role in recognition of the EphA2 receptor, its carboxylate group is critical for disrupting the binding of ephrinA1 to EphA2. As a result of our investigation, we identified (5β)-cholan-24-oic acid (cholanic acid) as a novel compound that competitively inhibits the EphA2-ephrinA1 interaction with higher potency than lithocholic acid. Surface plasmon resonance analysis indicates that cholanic acid binds specifically and reversibly to the ligand binding domain of EphA2, with a steady-state dissociation constant (K(D) ) in the low micromolar range. Furthermore, cholanic acid blocks the phosphorylation of EphA2 as well as cell retraction and rounding in PC3 prostate cancer cells, two effects that depend on EphA2 activation by the ephrinA1 ligand. These findings suggest that cholanic acid can be used as a template structure for the design of effective EphA2 antagonists, and may have potential impact in the elucidation of the role played by this receptor in pathological conditions.     

Binding Characterization of Recombinant Odorant-binding Proteins from the Parasitic Wasp, <Emphasis Type="Italic">Microplitis mediator</Emphasis> (Hymenoptera: Braconidae)

Chemoreception in insects is mediated by small odorant-binding proteins (OBPs) that are believed to carry lipophilic stimuli to the olfactory receptor cells through the aqueous sensillar lymph. Binding experiments and recent structural studies of OBPs have illustrated their versatility and ability to accommodate ligands of different shapes and chemical structures. We expressed and purified seven recombinant OBPs (MmedOBP1-MmedOBP7) from the parasitic wasp, Microplitis mediator (Hymenoptera: Braconidae) in a prokaryotic expression system. With 4,4′-dianilino-1,1′-binaphthyl-5,5′-sulfonic acid (bis-ANS) as a fluorescent probe, the ligand-binding specificities of these seven MmedOBPs with 50 small organic compounds were investigated in vitro. The results revealed that all of the M. mediator OBPs can bind a wide variety of odorant molecules with different binding affinities. The best ligand for all seven MmedOBPs was β-ionone. MmedOBP2 showed affinity for some aromatic compounds, whereas MmedOBP4 and MmedOBP6 bound several terpenoids. MmedOBP5 bound β-ionone, but did not bind any of the other potential ligands that we tested.     

The predicted 3D structures of the human M1 muscarinic acetylcholine receptor with agonist or antagonist bound

The muscarinic acetylcholine G-protein-coupled receptors are implicated in diseases ranging from cognitive dysfunctions to smooth-muscle disorders. To provide a structural basis for drug design, we used the MembStruk computational method to predict the 3D structure of the human M1 muscarinic receptor. We validated this structure by using the HierDock method to predict the binding sites for three agonists and four antagonists. The intermolecular ligand-receptor contacts at the predicted binding sites agree well with deductions from available mutagenesis experiments, and the calculated relative binding energies correlate with measured binding affinities. The predicted binding site of all four antagonists is located between transmembrane (TM) helices 3, 4, 5, 6, and 7, whereas the three agonists prefer a site involving residues from TM3, TM6, and TM7. We find that Trp 157(4) contributes directly to antagonist binding, whereas Pro 159(4) provides an indirect conformational switch to position Trp 157(4) in the binding site (the number in parentheses indicates the TM helix). This explains the large decrease in ligand binding affinity and signaling efficacy by mutations of Trp 157(4) and Pro 159(4) not previously explained by homology models. We also found that Asp 105(3) and aromatic residues Tyr 381(6), Tyr 404(7), and Tyr 408(7) are critical for binding the quaternary ammonium head group of the ligand through cation-pi interactions. For ligands with a charged tertiary amine head group, we suggest that proton transfer from the ligand to Asp 105(3) occurs upon binding. Furthermore, we found that an extensive aromatic network involving Tyr 106(3), Trp 157(4), Phe 197(5), Trp 378(6), and Tyr 381(6) is important in stabilizing antagonist binding. For antagonists with two terminal phenyl rings, this aromatic network extends to Trp 164(4), Tyr 179(extracellular loop 2), and Phe 390(6) located at the extracellular end of the TMs. We find that Asn 382(6) forms hydrogen bonds with selected antagonists. Tyr381(6) and Ser 109(3) form hydrogen bonds with the ester moiety of acetylcholine, which binds in the gauche conformation.     

Evaluation of a fluorescent derivative of AMD3100 and its interaction with the CXCR4 chemokine receptor

AMD3100 is a potent and selective antagonist of the CXCR4 receptor; it has been shown to block the route of entry of HIV into host T-cells. This compound and its analogues have since been found to act as haematopoietic stem cell mobilisation agents and, more recently, as anti-cancer agents. Here, we have examined a fluorescent derivative of AMD3100, L(1), which offered the potential to assess the behaviour of AMD3100 at the cell surface by using optical imaging modalities. The binuclear Zn(II) , Cu(II) and Ni(II) complexes of L(1) have also been investigated as these metals have been previously shown to enhance the binding properties of AMD3100. Furthermore, Zn(II) and Cu(II) are known to enhance and quench, respectively, the fluorescence of similar anthracenyl-based ligands. Whilst L(1) demonstrates an ability to inhibit the binding of the anti-CXCR4 monoclonal antibody 12G5 (IC(50) =0.25-0.9 μM), the incorporation of an anthracenyl moiety resulted in a significantly reduced affinity for CXCR4 compared to AMD3100 (IC(50) =10 nM). We observed no significant increase in fluorescence intensity following incubation with murine pre-B cells overexpressing CXCR4 compared to a control cell line. This limits the usefulness of L(1) as a fluorescent imaging probe. Interestingly, the Zn(II) complex, which carries an overall +4 charge, revealed marginally higher specificity and reduced toxicity in vitro compared to the free ligand, albeit with reduced affinity for CXCR4 (IC(50) =1.8-5 μM). We suggest that the incorporation of an anthracenyl group contributes to the lipophilic character of the free ligand, thereby resulting in transport across the plasma membrane. This effect is seemingly diminished when the ligand is complexed to charged metal ions.     

Binding of the Phage Display Derived Peptide CaIX-P1 on Human Colorectal Carcinoma Cells Correlates with the Expression of Carbonic Anhydrase IX

Phage display represents an attractive screening strategy for the identification of novel, specific binding ligands that could be used for tumor targeting. Recently, a new peptide (CaIX-P1) with affinity for human carbonic anhydrase IX (CAIX) was identified and evaluated. The aim of the present study is to characterize the properties of CaIX-P1 for targeting human colorectal carcinoma and investigate the correlation of peptide binding with the expression of carbonic anhydrase IX. Human colorectal carcinoma HCT116 and HT29 cells were investigated for CAIX expression using Western Blot analysis. Binding and competition studies of ¹²⁵I-radiolabeled CaIX-P1 were performed on HCT116 cells in vitro. FACS analysis and fluorescence microscopy studies were carried out after cell incubation with fluorescein-labeled CaIX-P1 and rhodamine-labeled anti-human CAIX-mAb. Our studies revealed an enhanced in vitro expression of carbonic anhydrase IX in HCT116 and HT29 cells with increasing cell density. Binding of ¹²⁵I-labeled-CaIX-P1 on HCT116 cells increased with increasing cell density and correlated to the CAIX expression. FACS analysis demonstrated a correlation of cell labeling between FITC-CaIX-P1 and rhodamine-labeled anti-CAIX-mAb in both HCT116 and HT29 cells. The results of our study indicate that the phage display identified peptide CaIX-P1 might be an attractive candidate for the development of a ligand targeting CAIX in colorectal cancer.     

Synthesis and evaluation of an intercalator-polyamide hairpin designed to target the inverted CCAAT box 2 in the topoisomerase IIalpha promoter

The synthesis and DNA-binding properties of a novel naphthalimide-polyamide hairpin (3) designed to target the inverted CCAAT box 2 (ICB2) site on the topoisomerase IIalpha (topoIIalpha) promoter are described. The polyamide component of 3 was derived from the minor-groove binder, 2, and tailored to bind to the 5'-TTGGT sequence found in and flanking ICB2. The propensity of mitonafide 4 to intercalate between G-C base pairs was exploited by the incorporation of a naphthalimide moiety at the N terminus of 2. Hybrid 3 targeted 5'-CGATTGGT and covered eight contiguous base pairs, which included the underlined ICB2 site. DNase I footprinting analysis with the topoIIalpha promoter sequence demonstrated that 3 bound selectively to the ICB2 and ICB3 sites. Thermal-denaturation studies confirmed these results, and the highest degree of stabilization was found for ICB2 and -3 in preference to ICB1 (4.1, 4.6, and 0.6 degrees C, respectively). CD studies confirmed minor-groove binding and suggested a 1:1 binding stoichiometry. Emission-titration experiments established intercalative binding. Surface plasmon resonance results showed strong binding to ICB2 (2.5x10(7) M(-1)) with no observable binding to ICB1. Furthermore, the binding constant of 3 to ICB2 was larger than that of the parent polyamide 2. The increased binding affinity was primarily due to a reduction in the dissociation-rate constant of the polyamide-DNA complex, which can be attributed to the N-terminal naphthalimide moiety. In addition, the binding site of 3 was larger than that of 2, which innately improved sequence selectivity. We conclude that the polyamide-naphthalimide 3 selectively binds to the ICB2 site by simultaneous intercalation and minor-groove binding, and warrants further investigation as a model compound for the regulation of topoIIalpha gene expression.     

Structure–Activity Relationships of Quinoxaline‐Based 5‐HT3A and 5‐HT3AB Receptor‐Selective Ligands

Until recently, discriminating between homomeric 5‐HT₃A and heteromeric 5‐HT₃AB receptors was only possible with ligands that bind in the receptor pore. This study describes the first series of ligands that can discriminate between these receptor types at the level of the orthosteric binding site. During a recent fragment screen, 2‐chloro‐3‐(4‐methylpiperazin‐1‐yl)quinoxaline (VUF10166) was identified as a ligand that displays an 83‐fold difference in [³H]granisetron binding affinity between 5‐HT₃A and 5‐HT₃AB receptors. Fragment hit exploration, initiated from VUF10166 and 3‐(4‐methylpiperazin‐1‐yl)quinoxalin‐2‐ol, resulted in a series of compounds with higher affinity at either 5‐HT₃A or 5‐HT₃AB receptors. These ligands reveal that a single atom is sufficient to change the selectivity profile of a compound. At the extremes of the new compounds were 2‐amino‐3‐(4‐methylpiperazin‐1‐yl)quinoxaline, which showed 11‐fold selectivity for the 5‐HT₃A receptor, and 2‐(4‐methylpiperazin‐1‐yl)quinoxaline, which showed an 8.3‐fold selectivity for the 5‐HT₃AB receptor. These compounds represent novel molecular tools for studying 5‐HT₃ receptor subtypes and could help elucidate their physiological roles.     

Deriving Ligand Orientation in Weak Protein–Ligand Complexes by DEEP-STD NMR Spectroscopy in the Absence of Protein Chemical-Shift Assignment

Differential epitope mapping saturation transfer difference (DEEP-STD) NMR spectroscopy is a recently developed powerful approach for elucidating the structure and pharmacophore of weak protein–ligand interactions, as it reports key information on the orientation of the ligand and the architecture of the binding pocket. 1 The method relies on selective saturation of protein residues in the binding site and the generation of a differential epitope map by observing the ligand, which depicts the nature of the protein residues making contact with the ligand in the bound state. Selective saturation requires knowledge of the chemical-shift assignment of the protein residues, which can be obtained either experimentally by NMR spectroscopy or predicted from 3D structures. Herein, we propose a simple experimental procedure to expand the DEEP-STD NMR methodology to protein–ligand cases in which the spectral assignment of the protein is not available. This is achieved by experimentally identifying the chemical shifts of the residues present in binding hot-spots on the surface of the receptor protein by using 2D NMR experiments combined with a paramagnetic probe.     

Stability of fatty acyl-coenzyme a thioester ligands of hepatocyte nuclear factor-4α and peroxisome proliferator-activated receptor-α

Although long-chain fatty acyl-coenzyme A (LCFA-CoA) thioesters are specific high-affinity ligands for hepatocyte nuclear factor-4α (HNF-4α) and peroxisome proliferator-activated receptor-α (PPARα), X-ray crystals of the respective purified recombinant ligand-binding domains (LBD) do not contain LCFA-CoA, but instead exhibit bound LCFA or have lost all ligands during the purification process, respectively. As shown herein: (i) The acyl chain composition of LCFA bound to recombinant HNF-4α reflected that of the bacterial LCFA-CoA pool, rather than the bacterial LCFA pool. (ii) Bacteria used to produce the respective HNF-4α and PPARα contained nearly 100-fold less LCFA-CoA than LCFA. (iii) Under conditions used to crystallize LBD (at least 3 wk at room temperature in aqueous buffer), 16∶1-CoA was very unstable in buffer alone. (iv) In the presence of the respective nuclear receptor (i.e., HNF-4α and PPARα), LBD 70–75% of 16∶1-CoA was degraded after 1 d at room temperature in the crystallization buffer, whereas as much as 94–97% of 16∶1-CoA was degraded by 3 wk. (v) Cytoplasmic LCFA-CoA binding proteins such as acyl-CoA binding protein, sterol carrier protein-2, and liver-FA binding protein slowed the process of 16∶1-CoA degradation proportional to their respective affinities for this ligand. Taken together, these data for the first time indicated that the absence of LCFA-CoA in the crystallized HNF-4α and PPARα was due to the paucity of LCFA-CoA in bacteria as well as to the instability of LCFA-CoA in aqueous buffers and the conditions used for LBD crystallization. Furthermore, instead of protecting bound LCFA-CoA from autohydrolysis like several cytoplasmic LCFA-CoA binding proteins, these nuclear receptors facilitated LCFA-CoA degradation.     

2-[4-(4-氟苄基)哌嗪-1-基甲基]吡唑并[1,5-a]吡啶的合成及其体外受体结合分析

以吡唑并[1,5-a]吡啶-2-甲醛和N-甲酰基哌嗪为原料,经过还原胺化反应、水解反应、N-烷基化反应,合成了2-[4-(4-氟苄基)哌嗪-1-基甲基]吡唑并[1,5-a]吡啶,总收率29.5%,用1HNMR、19FNMR、ESI-MS对中间体及目标化合物进行了结构表征,并通过体外受体结合实验,测定目标化合物对多巴胺D4受体的亲和常数为1.2nmol/L,对D2、D3受体的亲和常数分别为3 900、1 890 nmol/L,显示对多巴胺D4受体具有较高的亲和性与选择性,是一种潜在的多巴胺D4受体配基。     

TRFIA螯合剂中间体2,6-二(3′-溴甲基-1′-吡唑基)-4-溴吡啶的合成、分离与表征

以2,6-二（3-甲基-1-H-吡唑基）-4-溴吡啶为原料,经重氮化、溴化合成了新型时间分辨荧光免疫分析（TR-FIA）双功能螯合剂中间体2,6-二（3＇-溴甲基-1＇-吡唑基）-4-溴吡啶,通过IR、GC-MS1、HNMR和元素分析等对其结构进行了确认,探讨了合成条件及反应机理。同时,通过GC-MS1、HNMR和元素分析等对第一副产物4-溴-2-（3＇-溴甲基-1＇-吡唑基）-6-（3＇-甲基-1＇-吡唑基）吡啶的结构也进行了确认,以其为原料可继续合成目标化合物,大幅提高总产率。     

Charge-carrier relaxation dynamics of poly(3-hexylthiophene)-coated gold hybrid nanoparticles

Poly(3-hexylthiophene)-coated gold (Au@P3HT) hybrid nanoparticles have been prepared via a “grafting-to” approach, and compared with pristine P3HT to investigate the dynamics of exciton relaxation using time-resolved transient-absorption and fluorescence spectroscopy. In efforts to facilitate the efficient dissociation of photo-generated excitons, we have incorporated gold nanoparticles having surface-plasmon resonances into thiol-terminated P3HT to fabricate Au@P3HT nanocomposites. The first-excited singlet state of Au@P3HT decays faster than that of pristine P3HT due to interactions with surface-plasmon resonances; excitons undergo dissociation via energy transfer from P3HT to the surface-plasmon state of a gold nanoparticle in Au@P3HT nanocomposites. The lowest triplet state of P3HT is also less populated due to the energy transfer while its lifetime is slightly reduced by the presence of gold nanoparticles. Thus, it is suggested that our incorporation of gold nanoparticles into P3HT reduces the recombination of geminate excitons and, thereby, increases the probability of exciton dissociation.     

Engineering the Ligand Specificity of the Human Galectin-1 by Incorporation of Tryptophan Analogues

Galectin-1 is a β-galactoside-binding lectin with manifold biological functions. A single tryptophan residue (W68) in its carbohydrate binding site plays a major role in ligand binding and is highly conserved among galectins. To fine tune galectin-1 specificity, we introduced several non-canonical tryptophan analogues at this position of human galectin-1 and analyzed the resulting variants using glycan microarrays. Two variants containing 7-azatryptophan and 7-fluorotryptophan showed a reduced affinity for 3’-sulfated oligosaccharides. Their interaction with different ligands was further analyzed by fluorescence polarization competition assay. Using molecular modeling we provide structural clues that the change in affinities comes from modulated interactions and solvation patterns. Thus, we show that the introduction of subtle atomic mutations in the ligand binding site of galectin-1 is an attractive approach for fine-tuning its interactions with different ligands.