Measuring cell adhesion forces with the atomic force microscope at the molecular level

In the past 25 years many techniques have been developed to characterize cell adhesion and to quantify adhesion forces. Atomic force microscopy (AFM) has been used to measure forces in the pico-newton range, an experimental technique known as force spectroscopy. We modified such an AFM to measure adhesion forces between live cells or between cells and surfaces. This strategy required functionalizing the surface of the sensors for immobilizing the cell. We used Dictyostelium discoideum cells which respond to starvation by surface expression of the adhesion molecule csA and consequent aggregation to measure the adhesion force of a single csA-csA bond. Relevant experimental parameters include the duration of contact between the interacting surfaces, the force against which this contact is maintained, the number and specificity of interacting adhesion molecules and the constituents of the medium in which the interaction occurs. This technology also permits the measurement of the viscoelastic properties of single cells or cell layers. Copyright (C) 2002 S, Karger AG, Basel

Imaging Single-Stranded DNA, Antigen-Antibody Reaction and Polymerized Langmuir-Blodgett Films with an Atomic Force Microscope

The combination of an (AFM) atomic force microscope together with microfabricated cantilevers that have integrated tips opens many possibilities for imaging systems of great importance in biology. We have imaged single-stranded 25mer DNA that was adsorbed on treated mica or that was covalently bound with a crosslinker to a polymerized Langmuir-Blodgett (LB) film, the top monolayer of a bilayer system. At low magnification the AFM shows cracks between solid domains, like in an image taken with a fluorescence microscope. At higher magnification, however, the AFM reveals much finer cracks and at still higher magnification it reveals rows of individual molecules in the polymerized LB film with a spacing of 0.45 nm. We have also imaged a LB film consisting of lipids in which 4% of the lipids had hapten molecules chemically bound to the lipid headgroups. Specific antibodies can then bind to these hapten molecules and be imaged with the AFM. This points to the possibility of using the AFM to monitor selective antibody binding

Atomic force microscopy-based mechanobiology

Mechanobiology emerges at the crossroads of medicine, biology, biophysics and engineering and describes how the responses of proteins, cells, tissues and organs to mechanical cues contribute to development, differentiation, physiology and disease. The grand challenge in mechanobiology is to quantify how biological systems sense, transduce, respond and apply mechanical signals. Over the past three decades, atomic force microscopy (AFM) has emerged as a key platform enabling the simultaneous morphological and mechanical characterization of living biological systems. In this Review, we survey the basic principles, advantages and limitations of the most common AFM modalities used to map the dynamic mechanical properties of complex biological samples to their morphology. We discuss how mechanical properties can be directly linked to function, which has remained a poorly addressed issue. We outline the potential of combining AFM with complementary techniques, including optical microscopy and spectroscopy of mechanosensitive fluorescent constructs, super-resolution microscopy, the patch clamp technique and the use of microstructured and fluidic devices to characterize the 3D distribution of mechanical responses within biological systems and to track their morphology and functional state.Peer ReviewedPostprint (published version

Recognition of "mirror-image" DNA by small molecules

May the force be with you: “Mirror-image” hairpin polyamides distinguish the L enantiomer of DNA (L-DNA) in the presence of natural DNA (D-DNA). This specificity is investigated by a molecular force balance at a single-molecule level. The “DNA balance” allows the measurement of rupture forces of match/mismatch diastereomeric complexes in a single experiment

Recognition of "mirror-image" DNA by small molecules

May the force be with you: “Mirror-image” hairpin polyamides distinguish the L enantiomer of DNA (L-DNA) in the presence of natural DNA (D-DNA). This specificity is investigated by a molecular force balance at a single-molecule level. The “DNA balance” allows the measurement of rupture forces of match/mismatch diastereomeric complexes in a single experiment

Mechanisms of Nanonewton Mechanostability in a Protein Complex Revealed by Molecular Dynamics Simulations and Single-Molecule Force Spectroscopy

Can molecular dynamics simulations predict the mechanical behavior of protein complexes? Can simulations decipher the role of protein domains of unknown function in large macromolecular complexes? Here, we employ a wide-sampling computational approach to demonstrate that molecular dynamics simulations, when carefully performed and combined with single-molecule atomic force spectroscopy experiments, can predict and explain the behavior of highly mechanostable protein complexes. As a test case, we studied a previously unreported homologue from; Ruminococcus flavefaciens; called X-module-Dockerin (XDoc) bound to its partner Cohesin (Coh). By performing dozens of short simulation replicas near the rupture event, and analyzing dynamic network fluctuations, we were able to generate large simulation statistics and directly compare them with experiments to uncover the mechanisms involved in mechanical stabilization. Our single-molecule force spectroscopy experiments show that the XDoc-Coh homologue complex withstands forces up to 1 nN at loading rates of 10; 5; pN/s. Our simulation results reveal that this remarkable mechanical stability is achieved by a protein architecture that directs molecular deformation along paths that run perpendicular to the pulling axis. The X-module was found to play a crucial role in shielding the adjacent protein complex from mechanical rupture. These mechanisms of protein mechanical stabilization have potential applications in biotechnology for the development of systems exhibiting shear enhanced adhesion or tunable mechanics

Restoration of Visual Function by Expression of a Light-Gated Mammalian Ion Channel in Retinal Ganglion Cells or ON-Bipolar Cells

Most inherited forms of blindness are caused by mutations that lead to photoreceptor cell death but spare second- and third-order retinal neurons. Expression of the light-gated excitatory mammalian ion channel light-gated ionotropic glutamate receptor (LiGluR) in retinal ganglion cells (RGCs) of the retina degeneration (rd1) mouse model of blindness was previously shown to restore some visual functions when stimulated by UV light. Here, we report restored retinal function in visible light in rodent and canine models of blindness through the use of a second-generation photoswitch for LiGluR, maleimide-azobenzene-glutamate 0 with peak efficiency at 460 nm (MAG0460). In the blind rd1 mouse, multielectrode array recordings of retinal explants revealed robust and uniform light-evoked firing when LiGluR-MAG0460 was targeted to RGCs and robust but diverse activity patterns in RGCs when LiGluR-MAG0460 was targeted to ON-bipolar cells (ON-BCs). LiGluR-MAG0460 in either RGCs or ON-BCs of the rd1 mouse reinstated innate light-avoidance behavior and enabled mice to distinguish between different temporal patterns of light in an associative learning task. In the rod-cone dystrophy dog model of blindness, LiGluR-MAG0460 in RGCs restored robust light responses to retinal explants and intravitreal delivery of LiGluR and MAG0460 was well tolerated in vivo. The results in both large and small animal models of photoreceptor degeneration provide a path to clinical translation

Ultrastable cellulosome-adhesion complex tightens under load

Challenging environments have guided nature in the development of ultrastable protein complexes. Specialized bacteria produce discrete multi-component protein networks called cellulosomes to effectively digest lignocellulosic biomass. While network assembly is enabled by protein interactions with commonplace affinities, we show that certain cellulosomal ligand-receptor interactions exhibit extreme resistance to applied force. Here, we characterize the ligand-receptor complex responsible for substrate anchoring in the Ruminococcus flavefaciens cellulosome using single-molecule force spectroscopy and steered molecular dynamics simulations. The complex withstands forces of 600-750 pN, making it one of the strongest bimolecular interactions reported, equivalent to half the mechanical strength of a covalent bond. Our findings demonstrate force activation and inter-domain stabilization of the complex, and suggest that certain network components serve as mechanical effectors for maintaining network integrity. This detailed understanding of cellulosomal network components may help in the development of biocatalysts for production of fuels and chemicals from renewable plant-derived biomass

Understanding Body-Worn Camera Diffusion in U.S. Policing

By 2016, approximately one half of American police agencies had adopted body-worn cameras (BWCs). Although a growing body of research has examined the impact of BWCs on outcomes such as use of force, complaints, and perceptions of police, few have considered how and why some agencies adopted BWCs, while others have not. With guidance from the diffusion of innovations paradigm, this study explores variation in BWC adoption by police agencies. Drawing on a survey administered to a national probability sample of 665 municipal police executives in the spring of 2018, we found agency size, region, and the demographic composition of municipalities were associated with BWC usage. We then examined executives’ support for (or opposition to) legislation that would require BWC footage to be released publicly. Results suggest (a) a variety of environmental factors were associated with support and (b) the correlates of support varied across agencies of different sizes