Biomedical subjects

Wendy E Thomas

Publications and source records attributed to Wendy E Thomas.

9 recordsLinked to original sources

PubMed2006

Weak rolling adhesion enhances bacterial surface colonization.

Bacterial adhesion to and subsequent colonization of surfaces are the first steps toward forming biofilms, which are a major concern for implanted medical devices and in many diseases. It has generally been assumed that strong irreversible adhesion is a necessary step for biofilm formation. However, some bacteria, such as Escherichia coli when binding to mannosylated surfaces via the adhesive protein FimH, adhere weakly in a mode that allows them to roll across the surface. Since single-point mutations or even increased shear stress can switch this FimH-mediated adhesion to a strong stationary mode, the FimH system offers a unique opportunity to investigate the role of the strength of adhesion independently from the many other factors that may affect surface colonization. Here we compare levels of surface colonization by E. coli strains that differ in the strength of adhesion as a result of flow conditions or point mutations in FimH. We show that the weak rolling mode of surface adhesion can allow a more rapid spreading during growth on a surface in the presence of fluid flow. Indeed, an attempt to inhibit the adhesion of strongly adherent bacteria by blocking mannose receptors with a soluble inhibitor actually increased the rate of surface colonization by allowing the bacteria to roll. This work suggests that (i) a physiological advantage to the weak adhesion demonstrated by commensal variants of FimH bacteria may be to allow rapid surface colonization and (ii) antiadhesive therapies intended to prevent biofilm formation can have the unintended effect of enhancing the rate of surface colonization.

Adhesins, Escherichia coli↗

PubMed2006

Catch bond-mediated adhesion without a shear threshold: trimannose versus monomannose interactions with the FimH adhesin of Escherichia coli.

The FimH protein is the adhesive subunit of Escherichia coli type 1 fimbriae. It mediates shear-dependent bacterial binding to monomannose (1M)-coated surfaces manifested by the existence of a shear threshold for binding, below which bacteria do not adhere. The 1M-specific shear-dependent binding of FimH is consistent with so-called catch bond interactions, whose lifetime is increased by tensile force. We show here that the oligosaccharide-specific interaction of FimH with another of its ligands, trimannose (3M), lacks a shear threshold for binding, since the number of bacteria binding under static conditions is higher than under any flow. However, similar to 1M, the binding strength of surface-interacting bacteria is enhanced by shear. Bacteria transition from rolling into firm stationary surface adhesion as the shear increases. The shear-enhanced bacterial binding on 3M is mediated by catch bond properties of the 1M-binding subsite within the extended oligosaccharide-binding pocket of FimH, since structural mutations in the putative force-responsive region and in the binding site affect 1M- and 3M-specific binding in an identical manner. A shear-dependent conversion of the adhesion mode is also exhibited by P-fimbriated E. coli adhering to digalactose surfaces.

Adhesins, Bacterial↗

PubMed2006

Elevated shear stress protects Escherichia coli cells adhering to surfaces via catch bonds from detachment by soluble inhibitors.

Soluble inhibitors find widespread applications as therapeutic drugs to reduce the ability of eukaryotic cells, bacteria, or viruses to adhere to surfaces and host tissues. Mechanical forces resulting from fluid flow are often present under in vivo conditions, and it is commonly presumed that fluid flow will further add to the inhibitive effect seen under static conditions. In striking contrast, we discover that when surface adhesion is mediated by catch bonds, whose bond life increases with increased applied force, shear stress may dramatically increase the ability of bacteria to withstand detachment by soluble competitive inhibitors. This shear stress-induced protection against inhibitor-mediated detachment is shown here for the fimbrial FimH-mannose-mediated surface adhesion of Escherichia coli. Shear stress-enhanced reduction of bacterial detachment has major physiological and therapeutic implications and needs to be considered when developing and screening drugs.

Adhesins, Escherichia coli↗

PubMed2006

Uncoiling mechanics of Escherichia coli type I fimbriae are optimized for catch bonds.

We determined whether the molecular structures through which force is applied to receptor-ligand pairs are tuned to optimize cell adhesion under flow. The adhesive tethers of our model system, Escherichia coli, are type I fimbriae, which are anchored to the outer membrane of most E. coli strains. They consist of a fimbrial rod (0.3-1.5 microm in length) built from a helically coiled structural subunit, FimA, and an adhesive subunit, FimH, incorporated at the fimbrial tip. Previously reported data suggest that FimH binds to mannosylated ligands on the surfaces of host cells via catch bonds that are enhanced by the shear-originated tensile force. To understand whether the mechanical properties of the fimbrial rod regulate the stability of the FimH-mannose bond, we pulled the fimbriae via a mannosylated tip of an atomic force microscope. Individual fimbriae rapidly elongate for up to 10 microm at forces above 60 pN and rapidly contract again at forces below 25 pN. At intermediate forces, fimbriae change length more slowly, and discrete 5.0 +/- 0.3-nm changes in length can be observed, consistent with uncoiling and coiling of the helical quaternary structure of one FimA subunit at a time. The force range at which fimbriae are relatively stable in length is the same as the optimal force range at which FimH-mannose bonds are longest lived. Higher or lower forces, which cause shorter bond lifetimes, cause rapid length changes in the fimbria that help maintain force at the optimal range for sustaining the FimH-mannose interaction. The modulation of force and the rate at which it is transmitted from the bacterial cell to the adhesive catch bond present a novel physiological role for the fimbrial rod in bacterial host cell adhesion. This suggests that the mechanical properties of the fimbrial shaft have codeveloped to optimize the stability of the terminal adhesive under flow.

Adhesins, Bacterial↗

PubMed2005

Distinctive features of the biological catch bond in the jump-ramp force regime predicted by the two-pathway model.

The receptor-ligand unbinding in the biological catch bond is analyzed within a simple model that comprises a single bound state and two unbinding pathways. This model is investigated in detail for the jump-ramp force regime, where the pulling force quickly jumps to a finite value and then is ramped linearly with time. Two qualitative criteria are identified that distinguish the catch bond from the slip bond. First, the rupture force probability density of the catch-bond exhibits a maximum-minimum pair, which develops at finite forces. In contrast, the slip bond produces a maximum that first appears at zero force. Second, the catch bond can be identified over a wide range of ramp rates by high rupture probabilities at low forces relative to the probability at the maximum, in contrast to the slip bond, where the probability at the maximum always corresponds to the most likely rupture force. Both distinctive features of the catch bond are masked by large jump forces, indicating that the catch bond is best identified in experiments with moderate loading rates and small jump forces. The catch-bond lifetime in the constant force regime is related to the probability density in the jump-ramp regime, allowing one to determine the bond lifetime for a constant force by measuring the initial probability density in the jump-ramp experiments with different jump forces and a fixed ramp rate. The key analytic results are illustrated with the P -selectin/P-selectin glucoprotein ligand-1 bond.

Biophysics↗

PubMed2005

The two-pathway model for the catch-slip transition in biological adhesion.

Some recently studied biological noncovalent bonds have shown increased lifetime when stretched by mechanical force. In each case these counterintuitive "catch-bonds" have transitioned into ordinary "slip-bonds" that become increasingly shorter lived as the tensile force on the bond is further increased. We describe analytically how these results are supported by a physical model whereby the ligand escapes the receptor binding site via two alternative routes, a catch-pathway that is opposed by the applied force and a slip-pathway that is promoted by force. The model predicts under what conditions and at what critical force the catch-to-slip transition would be observed, as well as the degree to which the bond lifetime is enhanced at the critical force. The model is applied to four experimentally studied systems taken from the literature, involving the binding of P- and L-selectins to sialyl Lewis(X) oligosaccharide-containing ligands. Good quantitative fit to the experimental data is obtained, both for experiments with a constant force and for experiments where the force increases linearly with time.

Adhesiveness↗

PubMed2004

Shear-dependent 'stick-and-roll' adhesion of type 1 fimbriated Escherichia coli.

It is generally assumed that bacteria are washed off surfaces as fluid flow increases because they adhere through 'slip-bonds' that weaken under mechanical force. However, we show here that the opposite is true for Escherichia coli attachment to monomannose-coated surfaces via the type 1 fimbrial adhesive subunit, FimH. Raising the shear stress (within the physiologically relevant range) increased accumulation of type 1 fimbriated bacteria on monomannose surfaces by up to two orders of magnitude, and reducing the shear stress caused them to detach. In contrast, bacterial binding to anti-FimH antibody-coated surfaces showed essentially the opposite behaviour, detaching when the shear stress was increased. These results can be explained if FimH is force-activated; that is, that FimH mediates 'catch-bonds' with mannose that are strengthened by tensile mechanical force. As a result, on monomannose-coated surfaces, bacteria displayed a complex 'stick-and-roll' adhesion in which they tended to roll over the surface at low shear but increasingly halted to stick firmly as the shear was increased. Mutations in FimH that were predicted earlier to increase or decrease force-induced conformational changes in FimH were furthermore shown here to increase or decrease the probability that bacteria exhibited the stationary versus the rolling mode of adhesion. This 'stick-and-roll' adhesion could allow type 1 fimbriated bacteria to move along mannosylated surfaces under relatively low flow conditions and to accumulate preferentially in high shear regions.

Adhesins, Escherichia coli↗

PubMed2002

Bacterial adhesion to target cells enhanced by shear force.

Surface adhesion of bacteria generally occurs in the presence of shear stress, and the lifetime of receptor bonds is expected to be shortened in the presence of external force. However, by using Escherichia coli expressing the lectin-like adhesin FimH and guinea pig erythrocytes in flow chamber experiments, we show that bacterial attachment to target cells switches from loose to firm upon a 10-fold increase in shear stress applied. Steered molecular dynamics simulations of tertiary structure of the FimH receptor binding domain and subsequent site-directed mutagenesis studies indicate that shear-enhancement of the FimH-receptor interactions involves extension of the interdomain linker chain under mechanical force. The ability of FimH to function as a force sensor provides a molecular mechanism for discrimination between surface-exposed and soluble receptor molecules.

Adhesins, Bacterial↗

PubMed2002

A structural model for force regulated integrin binding to fibronectin's RGD-synergy site.

The synergy site on fibronectin's FN-III(9) module, located approximately 32 A away from the RGD-loop on FN-III(10), greatly enhances integrin alpha(5)beta(1) mediated cell binding. Since fibronectin is exposed to mechanical forces acting on the extracellular matrix in vivo, we used steered molecular dynamics to study how mechanical stretching of FN-III(9-10) affects the relative distance between these two synergistic sites. Our simulations predict the existence of an intermediate state prior to unfolding. In this state, the synergy-RGD distance is increased from 32 A to approximately 55 A, while the conformations of both sites remain unperturbed. This distance is too large for both sites to co-bind the same receptor, as indicated by experiments that confirm that increasing the length of the linker chain between FN-III(9) and FN-III(10) reduces alpha(5)beta(1) binding. Our simulations thus suggest that increased alpha(5)beta(1)-binding attributed to the synergy site, along with the associated downstream cell-signaling events, can be turned off mechanically by stretching FN-III(9-10) into this intermediate state. The potential physiological implications are discussed.

Computer Simulation↗