Distinct kinetic and mechanical properties govern selectin-leukocyte interactions

Nanoscopic Characterization of Cell Migration under Flow Using Optical and Electron Microscopy

Hematopoietic stem/progenitor cell (HSPC) and leukemic cell homing is an important biological phenomenon that takes place through essential interactions with adhesion molecules on an endothelial cell layer. The homing process of HSPCs begins with the tethering and rolling of the cells on the endothelial layer, which is achieved by the interaction between selectins on the endothelium to the ligands on HSPC/leukemic cells under shear stress of the blood flow. Although many studies have been based on in vitro conditions of the cells rolling over recombinant proteins, significant challenges remain when imaging HSPC/leukemic cells on the endothelium, a necessity when considering characterizing cell-to-cell interaction and rolling dynamics during cell migration. Here, we report a new methodology that enables imaging of stem-cell-intrinsic spatiotemporal details during its migration on an endothelium-like cell monolayer. We developed optimized protocols that preserve transiently appearing structures on HSPCs/leukemic cells during its rolling under shear stress for fluorescence and scanning electron microscopy characterization. Our new experimental platform is closer to in vivo conditions and will contribute to indepth understanding of stem-cell behavior during its migration and cell-to-cell interaction during the process of homing.

Selectins-The Two Dr. Jekyll and Mr. Hyde Faces of Adhesion Molecules-A Review

Selectins belong to a group of adhesion molecules that fulfill an essential role in immune and inflammatory responses and tissue healing. Selectins are glycoproteins that decode the information carried by glycan structures, and non-covalent interactions of selectins with these glycan structures mediate biological processes. The sialylated and fucosylated tetrasaccharide sLe^x is an essential glycan recognized by selectins. Several glycosyltransferases are responsible for the biosynthesis of the sLe^x tetrasaccharide. Selectins are involved in a sequence of interactions of circulated leukocytes with endothelial cells in the blood called the adhesion cascade. Recently, it has become evident that cancer cells utilize a similar adhesion cascade to promote metastases. However, like Dr. Jekyll and Mr. Hyde’s two faces, selectins also contribute to tissue destruction during some infections and inflammatory diseases. The most prominent function of selectins is associated with the initial stage of the leukocyte adhesion cascade, in which selectin binding enables tethering and rolling. The first adhesive event occurs through specific non-covalent interactions between selectins and their ligands, with glycans functioning as an interface between leukocytes or cancer cells and the endothelium. Targeting these interactions remains a principal strategy aimed at developing new therapies for the treatment of immune and inflammatory disorders and cancer. In this review, we will survey the significant contributions to and the current status of the understanding of the structure of selectins and the role of selectins in various biological processes. The potential of selectins and their ligands as therapeutic targets in chronic and acute inflammatory diseases and cancer will also be discussed. We will emphasize the structural characteristic of selectins and the catalytic mechanisms of glycosyltransferases involved in the biosynthesis of glycan recognition determinants. Furthermore, recent achievements in the synthesis of selectin inhibitors will be reviewed with a focus on the various strategies used for the development of glycosyltransferase inhibitors, including substrate analog inhibitors and transition state analog inhibitors, which are based on knowledge of the catalytic mechanism.

Nucleolin mediates the binding of cancer cells to L-selectin under conditions of lymphodynamic shear stress

Head and neck squamous cell carcinoma (HNSCC) cells bind to lymphocytes via L-selectin in a shear-dependent manner. This interaction takes place exclusively under low-shear stress conditions, such as those found within the lymph node parenchyma. This represents a novel functional role for L-selectin-selectin ligand interactions. Our previous work has characterized as-of-yet unidentified L-selectin ligands expressed by HNSCC cells that are specifically active under conditions of low shear stress consistent with lymph flow. Using an affinity purification approach, we now show that nucleolin expressed on the surface of HNSCC cells is an active ligand for L-selectin. Parallel plate chamber flow-based experiments and atomic force microscopy (AFM) experiments show that nucleolin is the main functional ligand under these low-force conditions. Furthermore, AFM shows a clear relationship between work of deadhesion and physiological loading rates. Our results reveal nucleolin as the first major ligand reported for L-selectin that operates under low-shear stress conditions.

Dynamic bonds and their roles in mechanosensing

Mechanical forces are ubiquitous in a cell's internal structure and external environment. Mechanosensing is the process that the cell employs to sense its mechanical environment. In receptor-mediated mechanosensing, cell surface receptors interact with immobilized ligands to provide a specific way to receive extracellular force signals to targeted force-transmitting, force-transducing and force-supporting structures inside the cell. Conversely, forces generated endogenously by the cell can be transmitted via cytoplasmic protein-protein interactions and regulate cell surface receptor activities in an 'inside-out' manner. Dynamic force spectroscopy analyzes these interactions on and inside cells to reveal various dynamic bonds. What is more, by integrating analysis of molecular interactions with that of cell signaling events involved in force-sensing and force-responding processes, one can investigate how dynamic bonds regulate the reception, transmission and transduction of mechanical signals.

Adhesion-based sorting of blood cells: an adhesive dynamics simulation study

Blood cells can be sorted in microfluidic devices not only based on their sizes and deformability, but also based on their adhesive properties. In particular, white blood cells have been shown to be sorted out by using adhesive micropatterns made from stripes that are tilted in regard to the direction of shear flow. Here we use adhesive dynamics simulations for round cells to quantitatively investigate this effect and to predict the optimal tilt angle. We then apply our method to predict optimal sorting conditions for malaria-infected red blood cells, which like white blood cells also adhere to and roll on adhesive substrates.

Integrin α4β7 switches its ligand specificity via distinct conformer-specific activation

CCL25, CXCL10, and Mn²⁺ induce three distinct active conformations of integrin α4β7, which have selective high affinity for either MAdCAM-1, VCAM-1, or nonselective high affinity for both ligands. Via this mechanism, integrin α4β7 adopts different active conformations to switch its ligand-binding specificity.

Chemokine (C-C motif) ligand 25 (CCL25) and C-X-C motif chemokine 10 (CXCL10) induce the ligand-specific activation of integrin α4β7 to mediate the selective adhesion of lymphocytes to mucosal vascular addressin cell adhesion molecule-1 (MAdCAM-1) or vascular cell adhesion molecule-1 (VCAM-1). However, the mechanism underlying the selective binding of different ligands by α4β7 remains obscure. In this study, we demonstrate that CCL25 and CXCL10 induce distinct active conformers of α4β7 with a high affinity for either MAdCAM-1 or VCAM-1. Single-cell force measurements show that CCL25 increases the affinity of α4β7 for MAdCAM-1 but decreases its affinity for VCAM-1, whereas CXCL10 has the opposite effect. Structurally, CCL25 induces a more extended active conformation of α4β7 compared with CXCL10-activated integrin. These two distinct intermediate open α4β7 conformers selectively bind to MAdCAM-1 or VCAM-1 by distinguishing their immunoglobulin domain 2. Notably, Mn²⁺ fully opens α4β7 with a high affinity for both ligands. Thus, integrin α4β7 adopts different active conformations to switch its ligand-binding specificity.

Microtubules tune mechanotransduction through NOX2 and TRPV4 to decrease sclerostin abundance in osteocytes

The adaptation of the skeleton to its mechanical environment is orchestrated by mechanosensitive osteocytes, largely by regulating the abundance of sclerostin, a secreted inhibitor of bone formation. We defined a microtubule-dependent mechanotransduction pathway that linked fluid shear stress to reactive oxygen species (ROS) and calcium (Ca²⁺) signals that led to a reduction in sclerostin abundance in cultured osteocytes. We demonstrated that microtubules stabilized by detyrosination, a reversible posttranslational modification of polymerized α-tubulin, determined the stiffness of the cytoskeleton, which set the mechanoresponsive range of cultured osteocytes to fluid shear stress. We showed that fluid shear stress through the microtubule network activated NADPH oxidase 2 (NOX2)-generated ROS that target the Ca²⁺ channel TRPV4 to elicit Ca²⁺ influx. Furthermore, tuning the abundance of detyrosinated tubulin affected cytoskeletal stiffness to define the mechanoresponsive range of cultured osteocytes to fluid shear stress. Finally, we demonstrated that NOX2-ROS elicited Ca²⁺ signals that activated the kinase CaMKII to decrease the abundance of sclerostin protein. Together, these discoveries may identify potentially druggable targets for regulating osteocyte mechanotransduction to affect bone quality.

E-selectin-mediated rolling facilitates pancreatic cancer cell adhesion to hyaluronic acid

Tumor cell extravasation is a multistep process preceded by cell rolling and arrest on the vessel wall via the formation of specific receptor-ligand bonds. The strength, availability, and number of receptor-ligand bonds regulate the rate by which tumor cells tether, roll, and adhere to vascular walls. Although the mechanics of selectin-mediated rolling have been extensively studied, little is known regarding how tumor cell rolling on selectins facilitates adhesion to a distinct substrate-bound protein with different kinetic properties. By using multicomponent protein patterning and a microfluidic system, we evaluated how E-selectin-dependent rolling modulates hyaluronic acid (HA) adhesion as a function of fluid shear, contact time, and the spacing between E-selectin and HA regions patterned on the substrate. We show that tumor cells rolling on E-selectin were ∼40-fold more likely to bind to HA than nonrolling cells in shear flow. Furthermore, E-selectin-dependent rolling promotes adhesion to HA by both physically slowing cells and enabling them to position proximal to the surface, thereby increasing the on rate of adhesion. A better understanding of tumor cell adhesion under physiologic shear would lead to the development of new diagnostic assays and pave the way to clinical approaches aimed ultimately to halt metastasis.-Shea, D. J., Li, Y. W., Stebe, K. J., Konstantopoulos, K. E-selectin-mediated rolling facilitates pancreatic cancer cell adhesion to hyaluronic acid.

P-, but not E- or L-, selectin-mediated rolling adhesion persistence in hemodynamic flow diverges between metastatic and leukocytic cells

The ability of leukocytic cells to engage selectins via rolling adhesion is critical to inflammation, but selectins are also implicated in mediating metastatic dissemination. Using a microfluidic- and flow-based cell adhesion chromatography experimental and analytical technique, we interrogated the cell-subtype differences in engagement and sustainment of rolling adhesion on P-, E-, and L-selectin-functionalized surfaces in physiological flow. Our results indicate that, particularly at low concentrations of P-selectin, metastatic but not leukocytic cells exhibit reduced rolling adhesion persistence, whereas both cell subtypes exhibited reduced persistence on L-selectin and high persistence on E-selectin, differences not revealed by flow cytometry analysis or reflected in the extent or velocity of rolling adhesion. Conditions under which adhesion persistence was found to be significantly reduced corresponded to those exhibiting the greatest sensitivity to a selectin-antagonist. Our results suggest that potentially therapeutically exploitable differences in metastatic and leukocytic cell subtype interactions with selectins in physiological flow are identifiable through implementation of functional assays of adhesion persistence in hemodynamic flow utilizing this integrated, flow-based cell adhesion chromatography analytical technique.

Amino acid polymorphisms in the fibronectin-binding repeats of fibronectin-binding protein A affect bond strength and fibronectin conformation

The Staphylococcus aureus cell surface contains cell wall-anchored proteins such as fibronectin-binding protein A (FnBPA) that bind to host ligands (e.g. fibronectin; Fn) present in the extracellular matrix of tissue or coatings on cardiac implants. Recent clinical studies have found a correlation between cardiovascular infections caused by S. aureus and nonsynonymous SNPs in FnBPA. Atomic force microscopy (AFM), surface plasmon resonance (SPR), and molecular simulations were used to investigate interactions between Fn and each of eight 20-mer peptide variants containing amino acids Ala, Asn, Gln, His, Ile, and Lys at positions equivalent to 782 and/or 786 in Fn-binding repeat-9 of FnBPA. Experimentally measured bond lifetimes (1/k_off) and dissociation constants (K_d = k_off/k_on), determined by mechanically dissociating the Fn·peptide complex at loading rates relevant to the cardiovascular system, varied from the lowest-affinity H782A/K786A peptide (0.011 s, 747 μm) to the highest-affinity H782Q/K786N peptide (0.192 s, 15.7 μm). These atomic force microscopy results tracked remarkably well to metadynamics simulations in which peptide detachment was defined solely by the free-energy landscape. Simulations and SPR experiments suggested that an Fn conformational change may enhance the stability of the binding complex for peptides with K786I or H782Q/K786I (K_d^app = 0.2-0.5 μm, as determined by SPR) compared with the lowest-affinity double-alanine peptide (K_d^app = 3.8 μm). Together, these findings demonstrate that amino acid substitutions in Fn-binding repeat-9 can significantly affect bond strength and influence the conformation of Fn upon binding. They provide a mechanistic explanation for the observation of nonsynonymous SNPs in fnbA among clinical isolates of S. aureus that cause endovascular infections.

Control of interface interactions between natural rubber and solid surfaces through charge effects: an AFM study in force spectroscopic mode

Adhesion of nanoparticles (natural rubber) is monitored by slight changes in the surface charge state of the contacting solid surfaces.

Distinct kinetic and mechanical properties govern mucin 16- and podocalyxin-mediated tumor cell adhesion to E- and L-selectin in shear flow

Selectin-mediated tumor cell tethering to host cells, such as vascular endothelial cells, is a critical step in the process of cancer metastasis. We recently identified sialofucosylated mucin16 (MUC16) and podocalyxin (PODXL) as the major functional E- and L-selectin ligands expressed on the surface of metastatic pancreatic cancer cells. While the biophysics of leukocyte binding to selectins has been well studied, little is known about the mechanics of selectin-mediated adhesion pertinent to cancer metastasis. We thus sought to evaluate the critical parameters of selectin-mediated pancreatic tumor cell tethering and rolling. Using force spectroscopy, we characterized the binding interactions of MUC16 and PODXL to E- and L-selectin at the single-molecule level. To further analyze the response of these molecular interactions under physiologically relevant regimes, we used a microfluidic assay in conjunction with a mathematical model to study the biophysics of selectin-ligand binding as a function of fluid shear stress. We demonstrate that both MUC16 and PODXL-E-selectin-mediated interactions are mechanically stronger than like L-selectin interactions at the single-molecule level, and display a higher binding frequency at all contact times. The single-molecule kinetic and micromechanical properties of selectin-ligand bonds, along with the number of receptor-ligand bonds needed to initiate tethering, regulate the average velocity of ligand-coated microspheres rolling on selectin-coated surfaces in shear flow. Understanding the biophysics of selectin-ligand bonds and their responses to physiologically relevant shear stresses is vital for developing diagnostic assays and/or preventing the metastatic spread of tumor cells by interfering with selectin-mediated adhesion.

Comparison of human and mouse E-selectin binding to Sialyl-Lewis(x)

Background

During inflammation, leukocytes are captured by the selectin family of adhesion receptors lining blood vessels to facilitate exit from the bloodstream. E-selectin is upregulated on stimulated endothelial cells and binds to several ligands on the surface of leukocytes. Selectin:ligand interactions are mediated in part by the interaction between the lectin domain and Sialyl-Lewis x (sLe^x), a tetrasaccharide common to selectin ligands. There is a high degree of homology between selectins of various species: about 72 and 60 % in the lectin and EGF domains, respectively. In this study, molecular dynamics, docking, and steered molecular dynamics simulations were used to compare the binding and dissociation mechanisms of sLe^x with mouse and human E-selectin. First, a mouse E-selectin homology model was generated using the human E-selectin crystal structure as a template.

Results

Mouse E-selectin was found to have a greater interdomain angle, which has been previously shown to correlate with stronger binding among selectins. sLe^x was docked onto human and mouse E-selectin, and the mouse complex was found to have a higher free energy of binding and a lower dissociation constant, suggesting stronger binding. The mouse complex had higher flexibility in a few key residues. Finally, steered molecular dynamics was used to dissociate the complexes at force loading rates of 2000–5000 pm/ps². The mouse complex took longer to dissociate at every force loading rate and the difference was statistically significant at 3000 pm/ps². When sLe^x-coated microspheres were perfused through microtubes coated with human or mouse E-selectin, the particles rolled more slowly on mouse E-selectin.

Conclusions

Both molecular dynamics simulations and microsphere adhesion experiments show that mouse E-selectin protein binds more strongly to sialyl Lewis x ligand than human E-selectin. This difference was explained by a greater interdomain angle for mouse E-selectin, and greater flexibility in key residues. Future work could introduce similar amino acid substitutions into the human E-selectin sequence to further modulate adhesion behavior.

Confinement Sensing and Signal Optimization via Piezo1/PKA and Myosin II Pathways

Cells adopt distinct signaling pathways to optimize cell locomotion in different physical microenvironments. However, the underlying mechanism that enables cells to sense and respond to physical confinement is unknown. Using microfabricated devices and substrate-printing methods along with FRET-based biosensors, we report that, as cells transition from unconfined to confined spaces, intracellular Ca²⁺ level is increased, leading to phosphodiesterase 1 (PDE1)-dependent suppression of PKA activity. This Ca²⁺ elevation requires Piezo1, a stretch-activated cation channel. Moreover, differential regulation of PKA and cell stiffness in unconfined versus confined cells is abrogated by dual, but not individual, inhibition of Piezo1 and myosin II, indicating that these proteins can independently mediate confinement sensing. Signals activated by Piezo1 and myosin II in response to confinement both feed into a signaling circuit that optimizes cell motility. This study provides a mechanism by which confinement-induced signaling enables cells to sense and adapt to different physical microenvironments.

Analytical cell adhesion chromatography reveals impaired persistence of metastatic cell rolling adhesion to P-selectin

Selectins facilitate the recruitment of circulating cells from the bloodstream by mediating rolling adhesion, which initiates the cell-cell signaling that directs extravasation into surrounding tissues. To measure the relative efficiency of cell adhesion in shear flow for in vitro drug screening, we designed and implemented a microfluidic-based analytical cell adhesion chromatography system. The juxtaposition of instantaneous rolling velocities with elution times revealed that human metastatic cancer cells, but not human leukocytes, had a reduced capacity to sustain rolling adhesion with P-selectin. We define a new parameter, termed adhesion persistence, which is conceptually similar to migration persistence in the context of chemotaxis, but instead describes the capacity of cells to resist the influence of shear flow and sustain rolling interactions with an adhesive substrate that might modulate the probability of extravasation. Among cell types assayed, adhesion persistence to P-selectin was specifically reduced in metastatic but not leukocyte-like cells in response to a low dose of heparin. In conclusion, we demonstrate this as an effective methodology to identify selectin adhesion antagonist doses that modulate homing cell adhesion and engraftment in a cell-subtype-selective manner.

Characterization of monobody scaffold interactions with ligand via force spectroscopy and steered molecular dynamics

Monobodies are antibody alternatives derived from fibronectin that are thermodynamically stable, small in size, and can be produced in bacterial systems. Monobodies have been engineered to bind a wide variety of target proteins with high affinity and specificity. Using alanine-scanning mutagenesis simulations, we identified two scaffold residues that are critical to the binding interaction between the monobody YS1 and its ligand, maltose-binding protein (MBP). Steered molecular dynamics (SMD) simulations predicted that the E47A and R33A mutations in the YS1 scaffold substantially destabilize the YS1-MBP interface by reducing the bond rupture force and the lifetime of single hydrogen bonds. SMD simulations further indicated that the R33A mutation weakens the hydrogen binding between all scaffold residues and MBP and not just between R33 and MBP. We validated the simulation data and characterized the effects of mutations on YS1-MBP binding by using single-molecule force spectroscopy and surface plasmon resonance. We propose that interfacial stability resulting from R33 of YS1 stacking with R344 of MBP synergistically stabilizes both its own bond and the interacting scaffold residues of YS1. Our integrated approach improves our understanding of the monobody scaffold interactions with a target, thus providing guidance for the improved engineering of monobodies.

Probing time-dependent mechanical behaviors of catch bonds based on two-state models

With lifetime counter-intuitively being prolonged under forces, catch bonds can play critical roles in various sub-cellular processes. By adopting different “catching” strategies within the framework of two-state models, we construct two types of catch bonds that have a similar force-lifetime profile upon a constant force-clamp load. However, when a single catch bond of either type is subjected to varied forces, we find that they can behave very differently in both force history dependence and bond strength. We further find that a cluster of catch bonds of either type generally becomes unstable when subjected to a periodically oscillating force, which is consistent with experimental results. These results provide important insights into versatile time-dependent mechanical behaviors of catch bonds. We suggest that it is necessary to further differentiate those bonds that are all phenomenologically referred to as “Catch bonds”.

β-Catenin serves as a clutch between low and high intercellular E-cadherin bond strengths

A wide range of invasive pathological outcomes originate from the loss of epithelial phenotype and involve either loss of function or downregulation of transmembrane adhesive receptor complexes, including Ecadherin (Ecad) and binding partners β-catenin and α-catenin at adherens junctions. Cellular pathways regulating wild-type β-catenin level, or direct mutations in β-catenin that affect the turnover of the protein have been shown to contribute to cancer development, through induction of uncontrolled proliferation of transformed tumor cells, particularly in colon cancer. Using single-molecule force spectroscopy, we show that depletion of β-catenin or the prominent cancer-related S45 deletion mutation in β-catenin present in human colon cancers both weaken tumor intercellular Ecad/Ecad bond strength and diminishes the capacity of specific extracellular matrix proteins-including collagen I, collagen IV, and laminin V-to modulate intercellular Ecad/Ecad bond strength through α-catenin and the kinase activity of glycogen synthase kinase 3 (GSK-3β). Thus, in addition to regulating tumor cell proliferation, cancer-related mutations in β-catenin can influence tumor progression by weakening the adhesion of tumor cells to one another through reduced individual Ecad/Ecad bond strength and cellular adhesion to specific components of the extracellular matrix and the basement membrane.

Atomic force microscopy reveals a role for endothelial cell ICAM-1 expression in bladder cancer cell adherence

Cancer metastasis is a complex process involving cell-cell interactions mediated by cell adhesive molecules. In this study we determine the adhesion strength between an endothelial cell monolayer and tumor cells of different metastatic potentials using Atomic Force Microscopy. We show that the rupture forces of receptor-ligand bonds increase with retraction speed and range between 20 and 70 pN. It is shown that the most invasive cell lines (T24, J82) form the strongest bonds with endothelial cells. Using ICAM-1 coated substrates and a monoclonal antibody specific for ICAM-1, we demonstrate that ICAM-1 serves as a key receptor on endothelial cells and that its interactions with ligands expressed by tumor cells are correlated with the rupture forces obtained with the most invasive cancer cells (T24, J82). For the less invasive cancer cells (RT112), endothelial ICAM-1 does not seem to play any role in the adhesion process. Moreover, a detailed analysis of the distribution of rupture forces suggests that ICAM-1 interacts preferentially with one ligand on T24 cancer cells and with two ligands on J82 cancer cells. Possible counter receptors for these interactions are CD43 and MUC1, two known ligands for ICAM-1 which are expressed by these cancer cells.

Adhesion and fusion efficiencies of human immunodeficiency virus type 1 (HIV-1) surface proteins

In about half of patients infected with HIV-1 subtype B, viral populations shift from utilizing the transmembrane protein CCR5 to CXCR4, as well as or instead of CCR5, during late stage progression of the disease. How the relative adhesion efficiency and fusion competency of the viral Env proteins relate to infection during this transition is not well understood. Using a virus-cell fusion assay and live-cell single-molecule force spectroscopy, we compare the entry competency of viral clones to tensile strengths of the individual Env-receptor bonds of Env proteins obtained from a HIV-1 infected patient prior to and during coreceptor switching. The results suggest that the genetic determinants of viral entry were predominantly enriched in the C3, HR1 and CD regions rather than V3. Env proteins can better mediate entry into cells after coreceptor switch; this effective entry capacity does not correlate with the bond strengths between viral Env and cellular receptors.

Mechanochemitry: a molecular biomechanics view of mechanosensing

Molecular biomechanics includes two themes: the study of mechanical aspects of biomolecules and the study of molecular biology of the cell using mechanical tools. The two themes are interconnected for obvious reasons. The present review focuses on one of the interconnected areas-the mechanical regulation of molecular interaction and conformational change. Recent conceptual developments are summarized, including catch bonds, regulation of molecular interaction by the history of force application, and cyclic mechanical reinforcement. These studies elucidate the mechanochemistry of some of the candidate mechanosensing molecules, thereby providing a natural connection to mechanobiology.

Single-cell force spectroscopy, an emerging tool to quantify cell adhesion to biomaterials

Cell adhesion receptors play a central role in sensing and integrating signals provided by the cellular environment. Thus, understanding adhesive interactions at the cell-biomaterial interface is essential to improve the design of implants that should emulate certain characteristics of the cell's natural environment. Numerous cell adhesion assays have been developed; among these, atomic force microscopy-based single-cell force spectroscopy (AFM-SCFS) provides a versatile tool to quantify cell adhesion at physiological conditions. Here we discuss how AFM-SCFS can be used to quantify the adhesion of living cells to biomaterials and give examples of using AFM-SCFS in tissue engineering and regenerative medicine. We anticipate that in the near future, AFM-SCFS will be established in the biomaterial field as an important technique to quantify cell-biomaterial interactions and thereby will contribute to the optimization of implants, scaffolds, and medical devices.

Distinct kinetic and molecular requirements govern CD44 binding to hyaluronan versus fibrin(ogen)

CD44 is a multifunctional glycoprotein that binds to hyaluronan and fibrin(ogen). Alternative splicing is responsible for the generation of numerous different isoforms, the smallest of which is CD44s. Insertion of variant exons into the extracellular membrane proximal region generates the variant isoforms (CD44v). Here, we used force spectroscopy to delineate the biophysical and molecular requirements of CD44-HA and CD44-fibrin(ogen) interactions at the single-molecule level. CD44v-HA and CD44s-HA single bonds exhibit similar kinetic and micromechanical properties because the HA-binding motif on CD44 is common to all of the isoforms. Although this is the primary binding site, O- and N-linked glycans and sulfation also contribute to the tensile strength of the CD44-HA bond. The CD44s-fibrin pair has a lower unstressed dissociation rate and a higher tensile strength than CD44s-fibrinogen but is weaker than the CD44-HA bond. In contrast to CD44-HA binding, the molecular interaction between CD44 and fibrin(ogen) is predominantly mediated by the chondroitin sulfate and dermatan sulfate on CD44. Blocking sulfation on CD44s modestly decreases the tensile strength of CD44s-fibrin(ogen) binding, which is in stark contrast to CD44v-fibrin interaction. Collectively, the results obtained by force spectroscopy in conjunction with biochemical interventions enable us to delineate the biophysical parameters and molecular constituents of CD44 binding to hyaluronan and fibrin(ogen).

A High-Throughput Technique Reveals the Load- and Site Density-Dependent Kinetics of E-Selectin

The kinetics of bond rupture between receptors and ligand are critically dependent on applied mechanical force. Force spectroscopy of single receptor-ligand pairs to measure kinetics is a laborious and time-consuming process that is generally performed using individual force probes and making one measurement at a time when typically hundreds of measurements are needed. A high-throughput approach is thus desirable. We report here a magnetic bond puller that provides high-throughput measurements of single receptor-ligand bond kinetics. Electromagnets are used to apply pN tensile and compressive forces to receptor-coated magnetic microspheres while monitoring their contact with a ligand-coated surface. Bond lifetimes and the probability of forming a bond are measured via videomicroscopy, and the data are used to determine the load dependent rates of bond rupture and bond formation. The approach is simple, customizable, relatively inexpensive, and can make dozens of kinetic measurements simultaneously. We used the device to investigate how compressive and tensile forces affect the rates of formation and rupture, respectively, of bonds between E-selectin and sialyl Lewisa (sLea), a sugar on P-selectin glycoprotein ligand-1 to which selectins bind. We confirmed earlier findings of a load-dependent rate of bond formation between these two molecules, and that they form a catch-slip bond like other selectin family members. We also make the novel observation of an "ideal" bond in a highly multivalent system of this receptor-ligand pair.

Selectin-mediated adhesion in shear flow using micropatterned substrates: multiple-bond interactions govern the critical length for cell binding

Receptor-ligand adhesive interactions play a pivotal role in diverse biological processes including inflammation and cancer metastasis. Cell adhesion is mediated by the molecular recognition of membrane-bound receptors by their cognate ligands on apposing cells. Cell-cell binding is regulated by distinct parameters such as the receptor-ligand binding kinetics, the tensile strength of individual bonds, the involvement of multiple bonds and their modulation by hydrodynamic shear. This work aims to investigate the interplay of these parameters on selectin-mediated cell adhesion in shear flow. We designed a microfluidic device that delivers cells in a single file over a receptor-functionalized substrate, thereby permitting accurate determination of the cell flux. The selectin(s) was presented on striped patches of fixed width and varying length. We identified the critical patch lengths of P- and L-selectin for the initiation of HL-60 cell binding in shear flow. This characteristic length is governed by the time required to form multiple-bond interactions, as revealed by a multiple-bond mathematical model. The number of bonds required to support cell binding increases with the applied shear stress (0.5-2 dyn cm(-2)) for L- but not P-selectin. This finding is explained by differences in the tensile strength of P- and L-selectin for PSGL-1. Our integrated experimental and mathematical approach advances our understanding of receptor-mediated cell adhesion in the vasculature. Detailed knowledge of how molecular interactions modulate macroscopic cell binding behavior pertinent to inflammation and metastasis would facilitate the development of promising diagnostic tools to combat these diseases.

A hot-spot motif characterizes the interface between a designed ankyrin-repeat protein and its target ligand

Nonantibody scaffolds such as designed ankyrin repeat proteins (DARPins) can be rapidly engineered to detect diverse target proteins with high specificity and offer an attractive alternative to antibodies. Using molecular simulations, we predicted that the binding interface between DARPin off7 and its ligand (maltose binding protein; MBP) is characterized by a hot-spot motif in which binding energy is largely concentrated on a few amino acids. To experimentally test this prediction, we fused MBP to a transmembrane domain to properly orient the protein into a polymer-cushioned lipid bilayer, and characterized its interaction with off7 using force spectroscopy. Using this, to our knowledge, novel technique along with surface plasmon resonance, we validated the simulation predictions and characterized the effects of select mutations on the kinetics of the off7-MBP interaction. Our integrated approach offers scientific insights on how the engineered protein interacts with the target molecule.

Dissociation rate constants of human fibronectin binding to fibronectin-binding proteins on living Staphylococcus aureus isolated from clinical patients

Staphylococcus aureus is part of the indigenous microbiota of humans. Sometimes, S. aureus bacteria enter the bloodstream, where they form infections on implanted cardiovascular devices. A critical, first step in such infections is a bond that forms between fibronectin-binding protein (FnBP) on S. aureus and host proteins, such as fibronectin (Fn), that coat the surface of implants in vivo. In this study, native FnBPs on living S. aureus were shown to form a mechanically strong conformational structure with Fn by atomic force microscopy. The tensile acuity of this bond was probed for 46 bloodstream isolates, each from a patient with a cardiovascular implant. By analyzing the force spectra with the worm-like chain model, we determined that the binding events were consistent with a multivalent, cluster bond consisting of ~10 or ~80 proteins in parallel. The dissociation rate constant (k(off), s(-1)) of each multibond complex was determined by measuring strength as a function of the loading rate, normalized by the number of bonds. The bond lifetime (1/k(off)) was two times longer for bloodstream isolates from patients with an infected device (1.79 or 69.47 s for the 10- or 80-bond clusters, respectively; n = 26 isolates) relative to those from patients with an uninfected device (0.96 or 34.02 s; n = 20 isolates). This distinction could not be explained by different amounts of FnBP, as confirmed by Western blots. Rather, amino acid polymorphisms within the Fn-binding repeats of FnBPA explain, at least partially, the statistically (p < 0.05) longer bond lifetime for isolates associated with an infected cardiovascular device.

Mucin 16 is a functional selectin ligand on pancreatic cancer cells

Selectins promote metastasis by mediating specific interactions between selectin ligands on tumor cells and selectin-expressing host cells in the microvasculature. Using affinity chromatography in conjunction with tandem mass spectrometry and bioinformatics tools, we identified mucin 16 (MUC16) as a novel selectin ligand expressed by metastatic pancreatic cancer cells. While up-regulated in many pancreatic cancers, the biological function of sialofucosylated MUC16 has yet to be fully elucidated. To address this, we employed blot rolling and cell-free flow-based adhesion assays using MUC16 immunopurified from pancreatic cancer cells and found that it efficiently binds E- and L- but not P-selectin. The selectin-binding determinants are sialofucosylated structures displayed on O- and N-linked glycans. Silencing MUC16 expression by RNAi markedly reduces pancreatic cancer cell binding to E- and L-selectin under flow. These findings provide a novel integrated perspective on the enhanced metastatic potential associated with MUC16 overexpression and the role of selectins in metastasis.

Biomechanics of leukocyte rolling

Leukocyte rolling on endothelial cells and other P-selectin substrates is mediated by P-selectin binding to P-selectin glycoprotein ligand-1 expressed on the tips of leukocyte microvilli. Leukocyte rolling is a result of rapid, yet balanced formation and dissociation of selectin-ligand bonds in the presence of hydrodynamic shear forces. The hydrodynamic forces acting on the bonds may either increase (catch bonds) or decrease (slip bonds) their lifetimes. The force-dependent 'catch-slip' bond kinetics are explained using the 'two pathway model' for bond dissociation. Both the 'sliding-rebinding' and the 'allosteric' mechanisms attribute 'catch-slip' bond behavior to the force-induced conformational changes in the lectin-EGF domain hinge of selectins. Below a threshold shear stress, selectins cannot mediate rolling. This 'shear-threshold' phenomenon is a consequence of shear-enhanced tethering and catch bond-enhanced rolling. Quantitative dynamic footprinting microscopy has revealed that leukocytes rolling at venular shear stresses (>0.6 Pa) undergo cellular deformation (large footprint) and form long tethers. The hydrodynamic shear force and torque acting on the rolling cell are thought to be synergistically balanced by the forces acting on tethers and stressed microvilli, however, their relative contribution remains to be determined. Thus, improvement beyond the current understanding requires in silico models that can predict both cellular and microvillus deformation and experiments that allow measurement of forces acting on individual microvilli and tethers.

Regulation of catch bonds by rate of force application

The current paradigm for receptor-ligand dissociation kinetics assumes off-rates as functions of instantaneous force without impact from its prior history. This a priori assumption is the foundation for predicting dissociation from a given initial state using kinetic equations. Here we have invalidated this assumption by demonstrating the impact of force history with single-bond kinetic experiments involving selectins and their ligands that mediate leukocyte tethering and rolling on vascular surfaces during inflammation. Dissociation of bonds between L-selectin and P-selectin glycoprotein ligand-1 (PSGL-1) loaded at a constant ramp rate to a constant hold force behaved as catch-slip bonds at low ramp rates that transformed to slip-only bonds at high ramp rates. Strikingly, bonds between L-selectin and 6-sulfo-sialyl Lewis X were impervious to ramp rate changes. This ligand-specific force history effect resembled the effect of a point mutation at the L-selectin surface (L-selectinA108H) predicted to contact the former but not the latter ligand, suggesting that the high ramp rate induced similar structural changes as the mutation. Although the A108H substitution in L-selectin eliminated the ramp rate responsiveness of its dissociation from PSGL-1, the inverse mutation H108A in P-selectin acquired the ramp rate responsiveness. Our data are well explained by the sliding-rebinding model for catch-slip bonds extended to incorporate the additional force history dependence, with Ala-108 playing a pivotal role in this structural mechanism. These results call for a paradigm shift in modeling the mechanical regulation of receptor-ligand bond dissociation, which includes conformational coupling between binding pocket and remote regions of the interacting molecules.

The physics of cancer: the role of physical interactions and mechanical forces in metastasis

Metastasis is a complex, multistep process responsible for >90% of cancer-related deaths. In addition to genetic and external environmental factors, the physical interactions of cancer cells with their microenvironment, as well as their modulation by mechanical forces, are key determinants of the metastatic process. We reconstruct the metastatic process and describe the importance of key physical and mechanical processes at each step of the cascade. The emerging insight into these physical interactions may help to solve some long-standing questions in disease progression and may lead to new approaches to developing cancer diagnostics and therapies.

Single-molecule binding of CD44 to fibrin versus P-selectin predicts their distinct shear-dependent interactions in cancer

P-selectin and fibrin(ogen) have pivotal roles in the hematogenous dissemination of tumor cells. CD44 variant isoforms, CD44v, have been identified as the major functional P-selectin ligands and fibrin receptors on metastatic colon carcinoma cells. The molecular recognition of CD44v by fibrin mediates firm adhesion at low shear, whereas CD44v-P-selectin binding supports transient rolling interactions at elevated shear stresses and low site densities of P-selectin. We used single-molecule force spectroscopy to provide a molecular interpretation for these two distinct adhesion events. The CD44v-P-selectin bond has a longer unstressed equilibrium lifetime, a lower reactive compliance and a higher tensile strength relative to the CD44v-fibrin bond. These intrinsic differences confer the ability to the CD44v-P-selectin pair to mediate binding at higher shear stresses. Increasing the duration of receptor-ligand contact (2-200 milliseconds) did not affect the micromechanical properties of the CD44v-P-selectin bond, but it increased the tensile strength and the depth of the free energy barrier of the CD44v-fibrin bond and decreased its reactive compliance. This bond strengthening at longer interaction times might explain why CD44v binding to immobilized fibrin occurs at low shear. Single-molecule characterization of receptor-ligand binding can predict the shear-dependent adhesive interactions between cells and substrates observed both in vitro and in vivo.

Sialyl Lewisx-dependent binding of human monocyte-derived dendritic cells to selectins

The limited efficacy of monocyte-derived dendritic cell (mo-DC)-based vaccines is primarily attributed to the reduced mo-DC migratory capacity. One undefined aspect is the initial binding of mo-DCs to endothelial cells and vascular selectins. In this study, we investigated the role and modulation of the selectin binding determinant sialyl Lewis(x) (sLe(x)) in selectin-dependent mo-DC binding. Our data reveal that sLe(x) is required for maximal binding of mo-DCs to tumor necrosis factor (TNF)-α-activated endothelial cells under static conditions, as evidenced by the use of sialidase. Sialidase treatment also abrogated mo-DC cell tethering to immobilized, purified P-, L-, or E-selectin under flow. The requirement of sLe(x)-dependent binding of mo-DC to selectins was further substantiated by using sLe(x) free sugar and anti-sLe(x) antibody, which significantly suppressed mo-DC-selectin binding. P-selectin glycoprotein ligand-1 is required for mo-DC binding to both P- and L-selectin, but it is dispensable for E-selectin recognition. Interestingly, the extent of mo-DC tethering was maximal on P-selectin, followed by E- and L- selectin. Accordingly, L-selectin mediated faster mo-DC rolling than E- or P-selectin. Interferon (IFN)-γ induces a significant increase in mo-DC surface sLe(x) expression, which is probably due to the enhanced synthesis of C2GnT-I. These findings may contribute to improving mo-DC-based vaccination protocols.

Molecular mechanistic insights into the endothelial receptor mediated cytoadherence of Plasmodium falciparum-infected erythrocytes

Cytoadherence or sequestration is essential for the pathogenesis of the most virulent human malaria species, Plasmodium falciparum (P. falciparum). Similar to leukocyte-endothelium interaction in response to inflammation, cytoadherence of P. falciparum infected red blood cells (IRBCs) to endothelium occurs under physiological shear stresses in blood vessels and involves an array of molecule complexes which cooperate to form stable binding. Here, we applied single-molecule force spectroscopy technique to quantify the dynamic force spectra and characterize the intrinsic kinetic parameters for specific ligand-receptor interactions involving two endothelial receptor proteins: thrombospondin (TSP) and CD36. It was shown that CD36 mediated interaction was much more stable than that mediated by TSP at single molecule level, although TSP-IRBC interaction appeared stronger than CD36-IRBC interaction in the high pulling rate regime. This suggests that TSP-mediated interaction may initiate cell adhesion by capturing the fast flowing IRBCs whereas CD36 functions as the ‘holder’ for providing stable binding.

Biophysics of selectin-ligand interactions in inflammation and cancer

Selectins (L-, E- and P-selectin) are calcium-dependent transmembrane glycoproteins that are expressed on the surface of circulating leukocytes, activated platelets, and inflamed endothelial cells. Selectins bind predominantly to sialofucosylated glycoproteins and glycolipids (E-selectin only) present on the surface of apposing cells, and mediate transient adhesive interactions pertinent to inflammation and cancer metastasis. The rapid turnover of selectin-ligand bonds, due to their fast on- and off-rates along with their remarkably high tensile strengths, enables them to mediate cell tethering and rolling in shear flow. This paper presents the current body of knowledge regarding the role of selectins in inflammation and cancer metastasis, and discusses experimental methodologies and mathematical models used to resolve the biophysics of selectin-mediated cell adhesion. Understanding the biochemistry and biomechanics of selectin-ligand interactions pertinent to inflammatory disorders and cancer metastasis may provide insights for developing promising therapies and/or diagnostic tools to combat these disorders.

Biophysical methods to probe claudin-mediated adhesion at the cellular and molecular level

Claudins are a family of tetraspan membrane proteins that localize at tight junctions in an epithelial monolayer forming a selective barrier to diffusion of solutes via the intercellular spaces. It is widely accepted that the interaction between the extracellular loops of claudin molecules from adjacent cells is critical for this function. Though previous experiments utilizing traditional biological, biochemical, morphological, and electrophysiological approaches have provided significant insights into the role of claudins in regulating ion permeability, the interaction kinetics between these molecules has not been characterized. In this chapter, we describe two experimental procedures to study the adhesion forces imparted by claudins: (a) dual micropipette assay to quantify the adhesion forces at the cellular level and (b) single molecule force spectroscopy using atomic force microscopy to characterize the interaction kinetics at the molecular level. Though the experimental procedures are described for claudins, they can be easily modified for studying the interaction properties of a wide variety of other proteins.

Antibody-immobilized column for quick cell separation based on cell rolling

Cell separation using methodological standards that ensure high purity is a very important step in cell transplantation for regenerative medicine and for stem cell research. A separation protocol using magnetic beads has been widely used for cell separation to isolate negative and positive cells. However, not only the surface marker pattern, e.g., negative or positive, but also the density of a cell depends on its developmental stage and differentiation ability. Rapid and label-free separation procedures based on surface marker density are the focus of our interest. In this study, we have successfully developed an antiCD34 antibody-immobilized cell-rolling column, that can separate cells depending on the CD34 density of the cell surfaces. Various conditions for the cell-rolling column were optimized including graft copolymerization, and adjustment of the column tilt angle, and medium flow rate. Using CD34-positive and -negative cell lines, the cell separation potential of the column was established. We observed a difference in the rolling velocities between CD34-positive and CD34-negative cells on antibody-immobilized microfluidic device. Cell separation was achieved by tilting the surface 20 degrees and the increasing medium flow. Surface marker characteristics of the isolated cells in each fraction were analyzed using a cell-sorting system, and it was found that populations containing high density of CD34 were eluted in the delayed fractions. These results demonstrate that cells with a given surface marker density can be continuously separated using the cell rolling column.

The viscoelastic properties of microvilli are dependent upon the cell-surface molecule

We studied at nanometer resolution the viscoelastic properties of microvilli and tethers pulled from myelogenous cells via P-selectin glycoprotein ligand 1 (PSGL-1) and found that in contrast to pure membrane tethers, the viscoelastic properties of microvillus deformations are dependent upon the cell-surface molecule through which load is applied. A laser trap and polymer bead coated with anti-PSGL-1 (KPL-1) were used to apply step loads to microvilli. The lengthening of the microvillus in response to the induced step loads was fitted with a viscoelastic model. The quasi-steady state force on the microvillus at any given length was approximately fourfold lower in cells treated with cytochalasin D or when pulled with concanavalin A-coated rather than KPL-1-coated beads. These data suggest that associations between PSGL-1 and the underlying actin cytoskeleton significantly affect the early stages of leukocyte deformation under flow.

The Effects of Load on E-Selectin Bond Rupture and Bond Formation

Molecular dissociation rates have long been known to be sensitive to applied force. We use a laser trap to provide evidence that rates of association may also be force-dependent. We use the thermal fluctuation assay to study single bonds between E-selectin and sialyl Lewis(a) (sLe(a)), the sugar on PSGL-1 to which the three selectins bind. Briefly, an E-selectin-coated bead is held in a laser trap and pressed with various compressive loads against the vertical surface of a bead coated with sLe(a). The time it takes for a bond to form is used to calculate a specific two-dimensional on-rate, kono. We observe an increase in kono with increasing compressive force, providing single molecule evidence that on-rate, in addition to off-rate, is influenced by load. By measuring bond lifetimes at known tensile loads, we show that E-selectin, like its family members L- and P-selectin, is capable of forming catch bonds. Our data support a reverse Bell model, in which compressive forces lower the activation energy for binding. Load-dependent on-rates may be a general feature of all intermolecular bonds.

LFA-1 binding destabilizes the JAM-A homophilic interaction during leukocyte transmigration

Leukocyte transendothelial migration into inflamed areas is regulated by the integrity of endothelial cell junctions and is stabilized by adhesion molecules including junctional adhesion molecule-A (JAM-A). JAM-A has been shown to participate in homophilic interactions with itself and in heterophilic interactions with leukocyte function-associated antigen-1 (LFA-1) via its first and second immunoglobulin domains, respectively. Using competitive binding assays in conjunction with atomic force microscopy adhesion measurements, we provide compelling evidence that the second domain of JAM-A stabilizes the homophilic interaction because its deletion suppresses the dynamic strength of the JAM-A homophilic interaction. Moreover, binding of the LFA-1 inserted domain to the second domain of JAM-A reduces the dynamic strength of the JAM-A homophilic interaction to the level measured with the JAM-A domain 2 deletion mutant. This finding suggests that LFA-1 binding cancels the stabilizing effects of the second immunoglobulin domain of JAM-A. Finally, our atomic force microscopy measurements reveal that the interaction of JAM-A with LFA-1 is stronger than the JAM-A homophilic interaction. Taken together, these results suggest that LFA-1 binding to JAM-A destabilizes the JAM-A homophilic interaction. In turn, the greater strength of the LFA-1/JAM-A complex permits it to support the tension needed to disrupt the JAM-A homophilic interaction, thus allowing transendothelial migration to proceed.

Cancer cells in transit: the vascular interactions of tumor cells

Metastasis is a highly regulated, multistep process in which cancerous cells shed from the primary tumor and enter the circulatory system, where they interact extensively with host cells before they lodge and colonize the target organ. The adhesive interactions of circulating tumor cells with platelets, leukocytes, and endothelial cells facilitate their survival and extravasation from the vasculature, thus representing critical kick-off events for the colonization of distant organs. This review presents our current mechanistic knowledge on vascular interactions of tumor cells, and it discusses biochemical and cell and molecular biology techniques used for the identification of novel receptor-ligand pairs mediating these interactions. This review brings together diverse observations about the contributions of key molecular constituents, including selectins, fibrin(ogen), and CD44, in one mechanistic interpretation. Understanding the molecular underpinnings of adhesive interactions between tumor cells and host cells may provide guidelines for developing promising antimetastatic therapies when initiated early in the course of disease progression.

Action at a distance: lengthening adhesion bonds with poly(ethylene glycol) spacers enhances mechanically stressed affinity for improved vascular targeting of microparticles

Poly(ethylene glycol) (PEG) chains were used to decorate microparticles with long adhesion ligands to emulate the efficacy of selectin-mediated leukocyte homing mechanisms. Ligands for P-selectin, an endothelial cell inflammatory marker, were coupled to PEG spacers of two sizes (MW 3400 and 10,000 Da) to investigate the effects on adhesion kinetics to P-selectin substrates. Under shear flow 80 nm PEG spacers improved P-selectin-antibody adhesion frequency by up to 4.5-fold and bond lifetimes by 7-fold compared to microparticles bearing chemisorbed antibody. Presentation of the glycosulfopeptide P-selectin ligands (2-GSP-6) and its nonsulfated low affinity form (2-GP-6) by long PEG spacers led to improved lifetimes of stressed bonds formed with P-selectin in shear flow and the rolling fluxes. Thus, structural features far removed from the binding pocket of a receptor that increase molecular contour length may enhance affinity in mechanically stressed environments such as those existing within the confines of the blood vessel. Such features may be useful for improving the performance of vascular-targeted micro- and nanoparticles used for drug, gene, and image contrast delivery. Ligand presentation on molecularly extended stalks may also serve to enhance any particle-surface interaction that takes place in laminar shear flow.

Loss of alpha-catenin decreases the strength of single E-cadherin bonds between human cancer cells

The progression of several human cancers correlates with the loss of cytoplasmic protein alpha-catenin from E-cadherin-rich intercellular junctions and loss of adhesion. However, the potential role of alpha-catenin in directly modulating the adhesive function of individual E-cadherin molecules in human cancer is unknown. Here we use single-molecule force spectroscopy to probe the tensile strength, unstressed bond lifetime, and interaction energy between E-cadherins expressed on the surface of live human parental breast cancer cells lacking alpha-catenin and these cells where alpha-catenin is re-expressed. We find that the tensile strength and the lifetime of single E-cadherin/E-cadherin bonds between parental cells are significantly lower over a wide range of loading rates. Statistical analysis of the force displacement spectra reveals that single cadherin bonds between cancer cells feature an exceedingly low energy barrier against tensile forces and low molecular stiffness. Disassembly of filamentous actin using latrunculin B has no significant effect on the strength of single intercellular E-cadherin bonds. The absence of alpha-catenin causes a dominant negative effect on both global cell-cell adhesion and single E-cadherin bond strength. These results suggest that the loss of alpha-catenin alone drastically reduces the adhesive force between individual cadherin pairs on adjoining cells, explain the global loss of cell adhesion in human breast cancer cells, and show that the forced expression of alpha-catenin in cancer cells can restore both higher intercellular avidity and intercellular E-cadherin bond strength.

Atomistic simulation combined with analytic theory to study the response of the P-selectin/PSGL-1 complex to an external force

Steered molecular dynamics simulations are combined with analytic theory in order to gain insights into the properties of the P-selectin/PSGL-1 catch-slip bond at the atomistic level of detail. The simulations allow us to monitor the conformational changes in the P-selectin/PSGL-1 complex in response to an external force, while the theory provides a unified framework bridging the simulation data with experiment over 9 orders of magnitude. The theory predicts that the probability of bond dissociation by the catch mechanism is extremely low in the simulations; however, a few or even a single trajectory can be sufficient for characterization of the slip mechanism. Theoretical analysis of the simulation data shows that the bond responds to the force in a highly nonlinear way, with the bond stiffness changing considerably as a function of the force ramp rate. The Langevin description of the simulation provides spring constants of the proteins and the binding interaction and gives the friction coefficient associated with the receptor-ligand motion in water. The estimated relaxation time shows that the simple probabilistic description is accurate for the experimental regime and remains approximately valid for the high ramp rates used in simulations. The simulations establish that bond deformation occurs primarily within the P-selectin receptor region. The two interaction sites within the binding pocket dissociate sequentially, raising the possibility of observing these independent rupture events in experiment. The stronger interaction that determines the overall properties of the bond dissociates first, indicating that the experimental data indeed capture the main rupture event and not the secondary weaker site rupture. The main rupture event involves the interaction between the calcium ion of the receptor and the ligand residue FUC-623. It is followed by new interactions, supporting the sliding-rebinding behavior observed in the earlier simulation [ Lou, J. Zhu, C. Biophys. J. 2007 , 92 , 1471 - 1485 ]. The weaker binding site shows fewer interaction features, suggesting that the sliding-rebinding behavior may be determined by the unique properties of the calcium site. The agreement between simulation and experiment provided by the two-pathway and deformation models, each containing only four parameters, indicates that the essential physics of the catch-slip bond should be relatively simple and robust over a wide range of pulling regimes.

Podocalyxin-like protein is an E-/L-selectin ligand on colon carcinoma cells: comparative biochemical properties of selectin ligands in host and tumor cells

Selectins facilitate metastasis and tumor cell arrest in the microvasculature by mediating binding of selectin-expressing host cells to ligands on tumor cells. We recently identified CD44 variant isoforms as functional P-, but not E-/L-, selectin ligands on colon carcinoma cells. Furthermore, a approximately 180-kDa sialofucosylated glycoprotein(s) mediated selectin binding in CD44-knockdown cells. Using immunoaffinity chromatography and tandem mass spectrometry, we identify podocalyxin-like protein (PCLP) as an alternative selectin ligand. Blot rolling and cell-free flow-based adhesion assays disclose that PCLP on LS174T colon carcinoma cells possesses E-/L-, but not P-, selectin binding activity. The selectin-binding determinants on LS174T PCLP are non-MECA-79-reactive sialofucosylated structures displayed on O-linked glycans, distinct from the MECA-79-reactive O-glycans on PCLP expressed by high endothelial venules, which is an L-selectin ligand. PCLP on CD44-knockdown LS174T cells exhibits higher HECA-452 immunoreactivity than PCLP on wild-type cells, suggesting that PCLP functions as an alternative acceptor for selectin-binding glycans. The enhanced expression of HECA-452 reactivity on PCLP from CD44-knockdown cells correlates with the increased avidity of PCLP for E- but not L-selectin. The novel finding that PCLP is an E-/L-selectin ligand on carcinoma cells offers a unifying perspective on the apparent enhanced metastatic potential associated with tumor cell PCLP overexpression and the role of selectins in metastasis.

{alpha}-Catenin mediates initial E-cadherin-dependent cell-cell recognition and subsequent bond strengthening

alpha-Catenin is essential in cadherin-mediated epithelium development and maintenance of tissues and in cancer progression and metastasis. However, recent studies question the conventional wisdom that alpha-catenin directly bridges the cadherin adhesion complex to the actin cytoskeleton. Therefore, whether alpha-catenin plays a direct role in cadherin-dependent cell adhesion is unknown. Here, single-molecule force spectroscopy measurements in cells depleted of alpha-catenin or expressing the hereditary diffuse gastric cancer associated V832M E-cadherin germ-line missense mutation show that alpha-catenin plays a critical role in cadherin-mediated intercellular recognition and subsequent multibond formation within the first 300 ms of cell contact. At short contact times, alpha-catenin mediates a 30% stronger interaction between apposing E-cadherin molecules than when it cannot bind the E-cadherin-beta-catenin complex. As contact time between cells increases, alpha-catenin is essential for the strengthening of the first intercellular cadherin bond and for the ensuing formation of additional bonds between the cells, all without the intervention of actin. These results suggest that a critical decision to form an adhesion complex between 2 cells occurs within an extremely short time span and at a single-molecule level and identify a previously unappreciated role for alpha-catenin in these processes.

A comparative molecular force spectroscopy study of homophilic JAM-A interactions and JAM-A interactions with reovirus attachment protein sigma1

JAM-A belongs to a family of immunoglobulin-like proteins called junctional adhesion molecules (JAMs) that localize at epithelial and endothelial intercellular tight junctions. JAM-A is also expressed on dendritic cells, neutrophils, and platelets. Homophilic JAM-A interactions play an important role in regulating paracellular permeability and leukocyte transmigration across epithelial monolayers and endothelial cell junctions, respectively. In addition, JAM-A is a receptor for the reovirus attachment protein, sigma1. In this study, we used single molecular force spectroscopy to compare the kinetics of JAM-A interactions with itself and sigma1. A chimeric murine JAM-A/Fc fusion protein and the purified sigma1 head domain were used to probe murine L929 cells, which express JAM-A and are susceptible to reovirus infection. The bond half-life (t(1/2)) of homophilic JAM-A interactions was found to be shorter (k(off)(o) = 0.688 +/- 0.349 s(-1)) than that of sigma1/JAM-A interactions (k(off)(o) = 0.067 +/- 0.041 s(-1)). These results are in accordance with the physiological functions of JAM-A and sigma1. A short bond lifetime imparts a highly dynamic nature to homophilic JAM-A interactions for regulating tight junction permeability while stable interactions between sigma1 and JAM-A likely anchor the virus to the cell surface and facilitate viral entry.