Synthetic multivalent ligands as probes of signal transduction

Covalently immobilized biosignal molecule materials for tissue engineering

Immobilization of biosignal molecules including growth factors and cytokines is important for developing biologically active materials which can contribute to tissue engineering as a component. The immobilization has more meanings than only immobilization of the enzyme in a bioreactor or ligand-receptor interactions, because the immobilized biosignal molecules work on cells which have very complex structures and functions. This review discusses recent progress in immobilization of biosignal molecules, including the mechanisms and design concepts.

Trivalent ligands with rigid DNA spacers reveal structural requirements for IgE receptor signaling in RBL mast cells

Antigen-mediated cross-linking of IgE bound to its receptor, FcRI, stimulates degranulation, phospholipid metabolism, and cytokine production in mast cells and basophils to initiate inflammatory and allergic responses. Previous studies suggested that spatial organization of the clustered receptors affects the assembly of the transmembrane signaling complexes. To investigate systematically the structural constraints in signal initiation, we utilized rigid double-stranded DNA scaffolds to synthesize ligands with tunable lengths. We characterized a series of symmetric trivalent DNA ligands with rigid spacing between 2,4-dinitrophenyl (DNP) haptenic groups in the range of 5-15 nm. These ligands all bind to anti-DNP IgE on RBL mast cells with similar avidity, and they all cross-link IgE-FcRI complexes effectively. We observe length-dependent stimulation of tyrosine phosphorylation of FcRI beta and gamma subunits and the adaptor protein LAT: the shortest ligand is approximately 5-10-fold more potent than the longest. Stimulated Ca2+ mobilization and degranulation also exhibits kinetics and magnitudes that differ as a function of ligand length. In contrast, tyrosine phosphorylation of phospholipase Cgamma1 and consequent Ca2+ release from intracellular stores do not show this dependence on ligand length. Our results with these rigid, DNA-based ligands provide direct support for receptor transphosphorylation as a key step in amplified signaling leading to degranulation, and they further reveal branching of pathways in signaling events.

Signaling by committee: receptor clusters determine pathways of cellular activation

Receptor clustering is a common signaling mechanism for cell surface receptors. Exogenous ligands such as antibodies or synthetic analogues can be used to artificially induce clustering. New studies using defined synthetic ligands suggest that the spatial organization of these clusters attenuates signaling in one pathway but has no effect in another.

Effects of Saccharide Spacing and Chain Extension on Toxin Inhibition by Glycopolypeptides of Well-Defined Architecture

Many recognition events important in biology are mediated via multivalent interactions between relevant oligosaccharides and multiple saccharide receptors present on lectins, viruses, toxins, and cell surfaces. Because of the important role played by protein-carbohydrate interactions in these pathogenic recognition events and in other human diseases, considerable effort has been devoted toward the development of multivalent polymeric ligands for carbohydrate-binding proteins. In this work, we report the synthesis of new polypeptide-based glycopolymers produced via a combination of protein engineering and chemical methods. These methodologies permit control over the number and the spacing of saccharides on the scaffold, as well as the conformation of the polymer backbone, and allow a more purposeful design of polymers for manipulation of multivalent binding events. Two families of galactose-bearing glycopolypeptides with random coil conformations, [(AG)(3)PEG](y) (y = 10 and 16) and {[(AG)(2)PSG](2)[(AG)(2)PEG][(AG)(2)PSG](2)}(y) (y = 6), have been synthesized. The carboxylic acid functionality of the glutamic acid residues allowed subsequent modification with amino-saccharides to yield the desired glycopolypeptides; selective placement of the glutamic acid group permitted investigation of the effects of multivalency and saccharide spacing on toxin inhibition. In addition, a family of galactose-functionalized PGA-based glycopolymers of varying molecular weights was also synthesized to compare the effects of backbone flexibility and hydrodynamic volume, relative to the recombinant glycopolypeptides, on toxin inhibition. Glycopolypeptides were characterized via (1)H NMR, MALDI-TOF mass spectrometry, SDS-PAGE analysis, and spectrophotometric assays. They were tested as inhibitors of the binding of the cholera toxin B subunit via direct enzyme-linked assays. The data from these experiments confirm the relevance of appropriate saccharide spacing on controlling the binding event and also indicate the influence of chain extension in improving inhibition.

Specific integrin labeling in living cells using functionalized nanocrystals

We present an integrin labeling method using functionalized quantum dots (QDs). Cyclic Arg-Gly-Asp (RGD) peptides and a biotin-streptavidin linkage are used to specifically couple individual QDs to integrins of living cells. The spacer distance between the RGD sequence and the QD surface is a crucial parameter to ensure specific binding to individual alpha(v)beta(3) integrins of osteoblast cells. Despite blinking, the position of single QDs is tracked with nanometer precision and localized diffusive behavior is observed. We show that blinking events do not prevent the acquisition of quantitative parameters from the QD trajectories.

Mannosylated G(0) Dendrimers with Nanomolar Affinities toEscherichia coli FimH

Pentaerythritol and bis-pentaerythritol scaffolds were used for the preparation of first generation glycodendrimers bearing aryl alpha-D-mannopyranoside residues assembled using single-step Sonogashira and click chemistry. The carbohydrate precursors were built with either para-iodophenyl, propargyl, or 2-azidoethyl aglycones whereas the pentaerythritol moieties were built with terminal azide or propargyl groups, respectively. Cross-linking abilities of this series of glycodendrimers were first evaluated with the lectin from Canavalia ensiformis (Concanavalin A). Surface plasmon resonance measurements showed these two families of mannosylated clusters as the best ligands known to date toward Escherichia coli K12 FimH with subnanomolar affinities. Tetramer 4 had a K(d) of 0.45 nM. These clusters were 1000 times more potent than mannose for their capacity to inhibit the binding of E. coli to erythrocytes in vitro.

Protein-directed assembly of binary monolayers at the interface and surface patterns of protein on the monolayers

Ferritin-directed assembly of binary monolayers of zwitterionic dipalmitoylphosphatidylcholine and cationic dioctadecyldimethylammonium bromide (DOMA) at the interface and surface patterns of ferritin on the monolayers have been investigated using a combination of infrared reflection absorption spectroscopy, surface plasmon resonance, and atomic force microscopy. Ferritin binding to the binary monolayers at the air-water interface at the surface pressure 30 mN/m, primarily driven by the electrostatic interaction, gives rise to a change in tilt angle of hydrocarbon chains from 15 degrees +/- 1 degrees to 10 degrees +/- 1 degrees with respect to the normal of the monolayer at the mole fraction of DOMA (XDOMA) of 0.1. The chains at XDOMA = 0.3 are oriented vertical to the water surface before and after protein binding. A new mechanism for protein binding to the binary monolayers is proposed. The secondary structures of the adsorbed ferritin are prevented from changing to some extent due to the existence of the monolayers. The amounts of the bound protein on the monolayers at the air-water interface are increased in comparison with those on the pre-immobilized monolayers at low XDOMA. The increased amounts and different patterns of the adsorbed protein at the monolayers are mostly attributed to the formation of multiple binding sites available for ferritin, which is due to the lateral reorganization of the lipid components in the monolayers induced by the protein in the subphase. The created multiple binding sites on the monolayer surfaces through the protein-directed assembly can be preserved for subsequent protein binding.

Defined substrates for human embryonic stem cell growth identified from surface arrays

Methods for the rapid identification of defined cell growth conditions are lacking. This deficiency is a major barrier to the investigation and application of human embryonic stem (ES) cells. To address this problem, we developed a method for generating arrays of self-assembled monolayers (SAMs) in which each element constitutes a defined surface. By screening surface arrays, we identified peptidic surfaces that support ES cell growth and self-renewal. The ability of the active surface array elements to support ES cell growth depends on their composition: both the density of the peptide presented and its sequence are critical. These findings support a role for specific surface-cell interactions. Moreover, the data from the surface arrays are portable. They can be used to design an effective 3D synthetic scaffold that supports the growth of undifferentiated human ES cells. Our results demonstrate that synthetic substrates for promoting and probing human ES cell self-renewal can be discovered through SAM surface arrays.

GM1 clustering inhibits cholera toxin binding in supported phospholipid membranes

The present studies explore multivalent ligand-receptor interactions between pentameric cholera toxin B subunits (CTB) and the corresponding membrane ligand, ganglioside GM1. CTB binding was monitored on supported phospholipid bilayers coated on the walls and floors of microfluidic channels. Measurements were made by total internal reflection fluorescence microscopy (TIRFM). Apparent dissociation constants were extracted by fitting the binding data to both the Hill-Waud and Langmuir adsorption isotherm equations. Studies of the effect of ligand density on multivalent CTB-GM1 interactions revealed that binding weakened with increasing GM1 density from 0.02 mol % to 10.0 mol %. Such a result could be explained by the clustering of GM1 on the supported phospholipid membranes, which in turn inhibited the binding of CTB. Atomic force microscopy (AFM) experiments directly verified GM1 clustering within the supported POPC bilayers.

Synthesis and Characterization of Covalent Mimics of Phosphatidylinositol-4,5-bisphosphate Micelles

There is increasing evidence that multivalency plays an important role in protein-lipid recognition and membrane targeting in biological systems. We describe here the preparation and characterization of multivalent analogues of the signaling lipid phosphatidylinositol-4,5-bisphosphate (PIP2). Tetherable analogues of the PIP2 headgroup were appended to polyamidoamine dendrimers via a squarate linker to afford polymers displaying four or eight headgroup moieties. This class of molecules should provide a powerful tool for the study of protein-lipid interactions.

Squalene-derived flexible linkers for bioactive peptides

A regiochemical and stereochemical mixture of flexible linkers bearing terminal azide functionality was synthesized in two steps from squalene and was used to connect two high affinity NDP-alpha-MSH ligands or two low affinity MSH(4) ligands. The ligands were N-terminally acylated using N-hydroxysuccinimidoyl 5-hexynoate and were subsequently attached to the linker via copper-catalyzed 'click' 3+2 cyclization of the azide and alkyne moieties. In vitro biological evaluations showed that the binding affinity to the human melanocortin 4 receptor was not diminished for most linker-ligand combinations relative to the corresponding parental ligand. Statistical and cooperative binding effects were observed for dimeric constructs containing the low affinity ligand MSH(4), but not for dimeric NDP-alpha-MSH constructs, presumably due to slow off rates for this high affinity ligand.

Functionalization of oligosaccharide mimetics and multimerization using squaric diester-mediated coupling

Functionalized carbohydrate-centered glycoclusters formed the starting material for the synthesis of tagged oligosaccharide and glycoconjugate mimetics, which were obtained by thiourea-bridging, peptide coupling and in particular squaric diester-mediated coupling. The latter method could also be utilized to provide new multivalent glycoconjugates, which were tested for their anti-adhesive properties in an ELISA with Escherichia coli bacteria.

Neoglycopeptide dendrimer libraries as a source of lectin binding ligands

[structure: see text] A 15 625-membered peptide dendrimer combinatorial library was acylated with an alpha-C-fucosyl residue at its four N-termini and screened for binding to fucose-specific lectins. Dendrimer FD2 (Fuc-alpha-CH2CO-Lys-Pro-Leu)4(Lys-Phe-Lys-Ile)2Lys-His-Ile-NH2 was identified as a potent ligand against Ulex europaeus lectin UEA-I (IC50 = 11 microM) and Pseudomonas aeruginosa lectin PA-IIL (IC50 = 0.14 microM).

Affinity of galectin-8 and its carbohydrate recognition domains for ligands in solution and at the cell surface

Galectin-8 has two different carbohydrate recognition domains (CRDs), the N-terminal Gal-8N and the C-terminal Gal-8C linked by a peptide, and has various effects on cell adhesion and signaling. To understand the mechanism for these effects further, we compared the binding activities of galectin-8 in solution with its binding and activation of cells. We used glycan array analysis to broaden the specificity profile of the two galectin-8 CRDs, as well as intact galectin-8s (short and long linker), confirming the unique preference for sulfated and sialylated glycans of Gal-8N. Using a fluorescence anisotropy assay, we examined the solution affinities for a subset of these glycans, the highest being 50 nM for NeuAcalpha2,3Lac by Gal-8N. Thus, carbohydrate-protein interactions can be of high affinity without requiring multivalency. More importantly, using fluorescence polarization, we also gained information on how the affinity is built by multiple weak interactions between different fragments of the glycan and its carrier molecule and the galectin CRD subsites (A-E). In intact galectin-8 proteins, the two domains act independently of each other in solution, whereas at a surface they act together. Ligands with moderate or weak affinity for the isolated CRDs on the array are bound strongly by intact galectin-8s. Also galectin-8 binding and signaling at cell surfaces can be explained by combined binding of the two CRDs to low or medium affinity ligands, and their highest affinity ligands, such as sialylated galactosides, are not required.

Selective tumor cell targeting using low-affinity, multivalent interactions

This report highlights the advantages of low-affinity, multivalent interactions to recognize one cell type over another. Our goal was to devise a strategy to mediate selective killing of tumor cells, which are often distinguished from normal cells by their higher levels of particular cell surface receptors. To test whether multivalent interactions could lead to highly specific cell targeting, we used a chemically synthesized small-molecule ligand composed of two distinct motifs: (1) an Arg-Gly-Asp (RGD) peptidomimetic that binds tightly (Kd approximately 10(-9)M) to alphavbeta3 integrins and (2) the galactosyl-alpha(1-3)galactose (alpha-Gal epitope), which is recognized by human anti-alpha-galactosyl antibodies (anti-Gal). Importantly, anti-Gal binding requires a multivalent presentation of carbohydrate residues; anti-Gal antibodies interact weakly with the monovalent oligosaccharide (Kd approximately 10(-5)M) but bind tightly (Kd approximately 10(-11) M) to multivalent displays of alpha-Gal epitopes. Such a display is generated when the bifunctional conjugate decorates a cell possessing a high level of alphavbeta3 integrin; the resulting cell surface, which presents many alpha-Gal epitopes, can recruit anti-Gal, thereby triggering complement-mediated lysis. Only those cells with high levels of the integrin receptor are killed. In contrast, doxorubicin tethered to the RGD-based ligand affords indiscriminate cell death. These results highlight the advantages of exploiting the type of the multivalent recognition processes used by physiological systems to discriminate between cells. The selectivity of this strategy is superior to traditional, abiotic, high-affinity targeting methods. Our results have implications for the treatment of cancer and other diseases characterized by the presence of deleterious cells.

Directed Assembly of Binary Monolayers with a High Protein Affinity: Infrared Reflection Absorption Spectroscopy (IRRAS) and Surface Plasmon Resonance (SPR)

Infrared reflection absorption spectroscopy (IRRAS) and surface plasmon resonance (SPR) techniques have been employed to investigate human serum albumin (HSA) binding to binary monolayers of zwitterionic dipalmitoylphosphatidylcholine (DPPC) and cationic dioctadecyldimethylammonium bromide (DOMA). At the air-water interface, the favorable electrostatic interaction between DPPC and DOMA leads to a dense chain packing. The tilt angle of the hydrocarbon chains decreases with increasing mole fraction of DOMA (X(DOMA)) in the monolayers at the surface pressure 30 mN/m: DPPC ( approximately 30 degrees ), X(DOMA) = 0.1 ( approximately 15 degrees ), and X(DOMA) = 0.3 ( approximately 0 degrees ). Negligible protein binding to the DPPC monolayer is observed in contrast to a significant binding to the binary monolayers. After HSA binding, the hydrocarbon chains at X(DOMA) = 0.1 undergo an increase in tilt angle from 15 degrees to 25 approximately 30 degrees , and the chains at X(DOMA) = 0.3 remain almost unchanged. The two components in the monolayers deliver through lateral reorganization, induced by the protein in the subphase, to form multiple interaction sites favorable for protein binding. The surfaces with a high protein affinity are created through the directed assembly of binary monolayers for use in biosensing.

Salicylaldimine Ruthenium Alkylidene Complexes: Metathesis Catalysts Tuned for Protic Solvents

Tuning the electronic and steric environment of olefin metathesis catalysts with specialized ligands can adapt them to broader applications, including metathesis in aqueous solvents. Bidentate salicylaldimine ligands are known to stabilize ruthenium alkylidene complexes, as well as allow ring-closing metathesis in protic media. Here, we report the synthesis and characterization of exceptionally robust olefin metathesis catalysts bearing both bidentate salicylaldimine and N-heterocyclic carbene ligands, including a trimethylammonium-functionalized complex adapted for polar solvents. NMR spectroscopy and X-ray crystallographic analysis confirm the structures of the complexes. Although the N-heterocyclic carbene-salicylaldimine ligand combination limits the activity of these catalysts in nonpolar solvents, this pairing enables efficient ring-closing metathesis of both dienes and enynes in methanol and methanol-water mixtures under air.

Synthesis of DOTA-conjugated multivalent cyclic-RGD peptide dendrimers via 1,3-dipolar cycloaddition and their biological evaluation: implications for tumor targeting and tumor imaging purposes

This report describes the design and synthesis of a series of alpha(V)beta(3) integrin-directed monomeric, dimeric and tetrameric cyclo[Arg-Gly-Asp-d-Phe-Lys] dendrimers using "click chemistry". It was found that the unprotected N-epsilon-azido derivative of cyclo[Arg-Gly-Asp-d-Phe-Lys] underwent a highly chemoselective conjugation to amino acid-based dendrimers bearing terminal alkynes using a microwave-assisted Cu(I)-catalyzed 1,3-dipolar cycloaddition. The alpha(V)beta(3) binding characteristics of the dendrimers were determined in vitro and their in vivoalpha(V)beta(3) targeting properties were assessed in nude mice with subcutaneously growing human SK-RC-52 tumors. The multivalent RGD-dendrimers were found to have enhanced affinity toward the alpha(V)beta(3) integrin receptor as compared to the monomeric derivative as determined in an in vitro binding assay. In case of the DOTA-conjugated (111)In-labeled RGD-dendrimers, it was found that the radiolabeled multimeric dendrimers showed specifically enhanced uptake in alpha(V)beta(3) integrin expressing tumors in vivo. These studies showed that the tetrameric RGD-dendrimer had better tumor targeting properties than its dimeric and monomeric congeners.

The Binding Avidity of a Nanoparticle-Based Multivalent Targeted Drug Delivery Platform

Dendrimer-based anticancer nanotherapeutics containing approximately 5 folate molecules have shown in vitro and in vivo efficacy in cancer cell targeting. Multivalent interactions have been inferred from observed targeting efficacy, but have not been experimentally proven. This study provides quantitative and systematic evidence for multivalent interactions between these nanodevices and folate-binding protein (FBP). A series of the nanodevices were synthesized by conjugation with different amounts of folate. Dissociation constants (K(D)) between the nanodevices and FBP measured by SPR are dramatically enhanced through multivalency ( approximately 2,500- to 170,000-fold). Qualitative evidence is also provided for a multivalent targeting effect to KB cells using flow cytometry. These data support the hypothesis that multivalent enhancement of K(D), not an enhanced rate of endocytosis, is the key factor resulting in the improved biological targeting by these drug delivery platforms.

Vesicle aggregation by multivalent ligands: relating crosslinking ability to surface affinity

In an effort to improve the stability of our tissue-mimetic vesicle aggregates, we have investigated how increasing the valency of our multivalent crosslinking ligand, poly-l-histidine, affected both the extent of vesicle aggregation and the affinity of the multivalent ligand for the synthetic receptor Cu(1) embedded in the vesicle membranes. Although increasing ligand valency gave the anticipated increase in the size of the vesicle aggregates, isothermal calorimetric studies did not show the expected increase in the valence-corrected binding constant for the embedded receptors. To explain both observations, we have developed a simple new binding model that encompasses both multivalent binding to receptors on a single vesicle surface (intramembrane binding) and vesicle crosslinking (intermembrane binding).

Self-assembled multivalent carbohydrate ligands

Materials that display multiple carbohydrate residues have gained much attention due to their potential to inhibit or modulate biological multivalent interactions. These materials can be grouped accordingly to the way they are prepared, as unimolecular or as self-assembled systems. Both systems take advantage of the fact that multivalent interactions have significantly higher binding affinity than the corresponding monovalent interactions. The self-assembled system is a more recent field of research compared to the unimolecular system. In this review, we describe current efforts to realize multivalent carbohydrate interactions from the perspective of synthetic self-assembled systems. We limit the scope to self-assembled systems that are stable, soluble in aqueous solution and morphologically discrete. We grouped them into two separate categories. In the first category carbohydrate ligands self-assemble onto a pre-organized nanostructure, and in the second carbohydrate-conjugated block molecules spontaneously assemble to construct morphologically distinct nanostructures.

One-Pot Procedure for Diazo Transfer and Azide−Alkyne Cycloaddition: Triazole Linkages from Amines

[reaction: see text] A one-pot reaction for Cu(II)-catalyzed diazo transfer and Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition (sometimes called click reaction) is reported. 1,4-Disubstituted 1,2,3-triazoles are obtained in excellent yields from a variety of readily available amines without the need for isolation of the azide intermediates. The reaction has a broad scope and is especially practical for the synthesis of multivalent structures because compounds substituted with multiple azides are potentially unstable.

Supramolecular Polypseudorotaxanes Composed of Star-Shaped Porphyrin-Cored Poly(ε-caprolactone) and α-Cyclodextrin

Star-shaped porphyrin-cored poly(epsilon-caprolactone) (SPPCL) was synthesized using a tetrahydroxyethyl-terminated porphyrin as a core initiator and stannous octoate as a catalyst in bulk at 120 degrees C. The molecular weight of as-synthesized polymer could be adjusted linearly by controlling the molar ratio of epsilon-caprolactone to porphyrin core initiator, and the molecular weight distribution was reasonably narrow. Supramolecular polypseudorotaxanes were prepared by inclusion complexation of SPPCL with alpha-cyclodextrin (alpha-CD) and thoroughly characterized by means of FT-IR, 1H NMR, 13C CP/MAS NMR, DSC, TGA, and WAXD. The results demonstrated that the porphyrin-cored polypseudorotaxanes formed through alpha-CD molecules threading onto the branch chains of star-shaped SPPCL polymers, and they had a channel-type crystalline structure. Meanwhile, the original crystallization of SPPCL polymers within the polypseudorotaxanes was completely suppressed in the alpha-CD cavities. Moreover, inclusion complexation between SPPCL and alpha-CD enhanced the thermal stability of both the guest SPPCL polymers and the host alpha-CD. Furthermore, both the SPPCL polymers and the polypseudorotaxanes showed similar fluorescent and UV-vis spectra compared with porphyrin core initiator. Consequently, this will not only provide potentially porphyrin-cored poly(epsilon-caprolactone) and its polypseudorotaxanes for photodynamic therapy but also improve the compatibility between poly(epsilon-caprolactone) and peptide drugs for drug delivery.

Oligomannoside mimetics by glycosylation of 'octopus glycosides' and their investigation as inhibitors of type 1 fimbriae-mediated adhesion of Escherichia coli

The glycocalyx of eukaryotic cells is composed of glycoconjugates, which carry highly complex oligosaccharide portions. To elucidate the biological role and function of the glycocalyx in cell-cell communication and cellular adhesion processes, glycomimetics have become targets of glycosciences, which resemble the composition and structural complexity of the glycocalyx constituents. Here, we report about the synthesis of a class of oligosaccharide mimetics of a high-mannose type, which were obtained by mannosylation of spacered mono- and oligosaccharide cores. These carbohydrate-centered cluster mannosides have been targeted as inhibitors of mannose-specific bacterial adhesion, which is mediated by so-called type 1 fimbriae. Their inhibitory potencies were measured by ELISA and compared to methyl mannoside as well as to a series of mannobiosides, and finally to the polysaccharide mannan. The obtained results suggest a new interpretation of the mechanisms of bacterial adhesion according to a macromolecular rather than a multivalency effect.

Sequence-Specific Recognition and Cooperative Dimerization of N-Terminal Aromatic Peptides in Aqueous Solution by a Synthetic Host

This article describes the selective recognition and noncovalent dimerization of N-terminal aromatic peptides in aqueous solution by the synthetic host compound, cucurbit[8]uril (Q8). Q8 is known to bind two aromatic guests simultaneously and, in the presence of methyl viologen, to recognize N-terminal tryptophan over internal and C-terminal sequence isomers. Here, the binding of Q8 to aromatic peptides in the absence of methyl viologen was studied by isothermal titration calorimetry (ITC), (1)H NMR spectroscopy, and X-ray crystallography. The peptides studied were of sequence X-Gly-Gly, Gly-X-Gly, and Gly-Gly-X (X = Trp, Phe, Tyr, and His). Q8 selectively binds and dimerizes Trp-Gly-Gly (1) and Phe-Gly-Gly (4) with high affinity (ternary K = 10(9)-10(11) M(-)(2)); binding constants for the other 10 peptides were too small to be measured by ITC. Both peptides bound in a stepwise manner, and peptide 4 bound with positive cooperativity. Crystal structures of Q8.1 and Q8.4(2) reveal the basis for selective recognition as simultaneous inclusion of the hydrophobic aromatic side chain into the cavity of Q8 and chelation of the proximal N-terminal ammonium group by carbonyl groups of Q8. The peptide sequence selectivity and positively cooperative dimerization reported here are, to the best of our knowledge, unprecedented for synthetic hosts in aqueous solution. Specific peptide recognition and dimerization by synthetic hosts such as Q8 should be important in the study of dimer-mediated biochemical processes and for the separation of peptides and proteins.

Cytoskeletal regulation couples LFA-1 conformational changes to receptor lateral mobility and clustering

The alpha(L)beta(2) integrin (leukocyte function-associated antigen-1 [LFA-1]) is regulated to engage and maintain T cell adhesion. Conformational changes in the receptor are associated with changes in receptor-ligand affinity and are necessary for firm adhesion. Less well understood is the relationship between receptor conformation and the regulation of its lateral mobility. We have used fluorescence photobleaching recovery and single-particle tracking to measure the lateral mobility of specific conformations of LFA-1. These measurements show that different receptor conformations have distinct diffusion profiles and that these profiles vary according to the activation state of the cell. Notably, a high-affinity conformation of LFA-1 is mobile on resting cells but immobile on phorbol-12-myristate-13-acetate-activated cells. This activation-induced immobilization is prevented by a calpain inhibitor and by an allosteric LFA-1 inhibitor. Our results suggest that current models of LFA-1 regulation are incomplete and that LFA-1 confinement by cytoskeletal attachment regulates cell adhesion both negatively and positively.

Romping the cellular landscape: linear scaffolds for molecular recognition

Multivalent molecules with a precise array of recognition elements that interact with specific cell types are important for characterizing the topology of molecules on a cell surface. Applications ranging from the control of cellular signaling to drug delivery and tissue imaging rely on these surface-mapping molecules. Linear polymers provide a molecular scaffold that is advantageous for these types of applications and their synthesis can be amenable to the introduction of different recognition elements. Recently, advances have been made in the development of synthetic approaches for preparing linear polymeric substrates with highly controlled lengths and recognition element spacing.

Evaluation of the carbohydrate recognition domain of the bacterial adhesin FimH: design, synthesis and binding properties of mannoside ligands

Fimbriae are proteinogeneous appendages on the surface of bacteria, which mediate bacterial adhesion to the host cell glycocalyx. The so-called type 1 fimbriae exhibit specificity for alpha-d-mannosides and, therefore, they are assumed to mediate bacterial adhesion via the interaction of a fimbrial lectin and alpha-d-mannosyl residues exposed on the host cell surface. This carbohydrate-specific adhesive protein subunit of type 1 fimbriae has been identified as a protein called FimH. The crystal structure of this lectin is known and, based on this information, the molecular details of the interaction of mannoside ligands and FimH are addressed in this paper. Computer-based docking methods were used to evaluate known ligands as well as to design new ones. Then, a series of new mannosides with extended aglycon was synthesized and tested as inhibitors of type 1 fimbriae-mediated bacterial adhesion in an ELISA. The results obtained were compared to the predictions and findings as delivered by molecular modeling. This study led to an improved understanding of the ligand-receptor interactions under investigation.

A flexible method for preparation of peptide homo- and heterodimers functionalized with affinity probes, chelating ligands, and latent conjugating groups

Dimerization of peptides can provide high binding entities to serve as targeted diagnostics or therapeutics. We developed methods for the preparation of homo- and heterodimer peptides bearing functional molecules (affinity probes, chelating ligands, or latent conjugating moieties). Monomer peptides, optionally bearing spacer groups, are tethered using a bifunctional linker, (di-succinimidyl glutarate, DSG) to provide the dimers. Protected or unprotected peptides can be employed for dimer assembly. Multiple lysine N(epsilon)-amino groups are controlled using the (4,4-dimethyl-2,6-dioxocyclohex-1-ylidene)-3-methylbutyl (ivDde) protecting group. Functional molecules are optionally incorporated into the component peptides or into the assembled dimer. The methods are efficient and scaleable.