Characterizing Biophysical Parameters of Single TCR-pMHC Interactions Using Optical Tweezers

αβ T cells are mechanosensors that leverage bioforces during immune surveillance for highly sensitive and specific antigen discrimination. Single-molecule studies are used to profile the initial TCRαβ-pMHC binding event, and various biophysical parameters can be identified. Isolating purified TCRαβ and pMHC molecules on a coverslip allows for direct measurements of the kinetics and conformational changes in the system and removes cellular components along the load pathway that may interfere with or mask subtle changes. Optical tweezers provide high resolution position and force information that map the bonding profile, including catch bond, and the ability to measure distinct conformational changes driven by forces. The present method describes the single-molecule optical tweezers assay setup, considerations, and execution. This model can be used for various TCR-pMHC pairs or expanded to measure a wide variety of receptor-ligand interactions operative in multiple biological systems.

Measuring αβ T-Cell Receptor-Mediated Mechanosensing Using Optical Tweezers Combined with Fluorescence Imaging

T-cell antigen receptors (TCRs) are mechanosensors, which initiate a signaling cascade upon ligand recognition resulting in T-cell differentiation, homeostasis, effector and regulatory functions. An optical trap combined with fluorescence permits direct monitoring of T-cell triggering in response to force application at various concentrations of peptide-bound major histocompatibility complex molecules (pMHC). The technique mimics physiological shear forces applied as cells crawl across antigen-presenting surfaces during immune surveillance. True single molecule studies performed on single cells profile force-bond lifetime, typically seen as a catch bond, and conformational change at the TCR-pMHC bond on the surface of the cell upon force loading. Together, activation and single molecule single cell studies provide chemical and physical triggering thresholds as well as insight into catch bond formation and quaternary structural changes of single TCRs. The present methods detail assay design, preparation, and execution, as well as data analysis. These methods may be applied to a wide range of pMHC-TCR interactions and have potential for adaptation to other receptor-ligand systems.

Force dependence of unbinding rate of kinesin motor during its processive movement on microtubule

Kinesin is a biological molecular motor that can move continuously on microtubule until it unbinds. Here, we studied computationally the force dependence of the unbinding rate of the motor. Our results showed that while the unbinding rate under the forward load has the expected characteristic of "slip bond", with the unbinding rate increasing monotonically with the increase of the forward load, the unbinding rate under the backward load shows counterintuitive characteristic of "slip-catch-slip bond": as the backward load increases, the unbinding rate increases exponentially firstly, then drops rapidly and then increases again. Our calculated data are in agreement with the available single-molecule data from different research groups. The mechanism of the slip-catch-slip bond was revealed.

Biophysical basis of cadherin mediated cell-cell adhesion

Classical cadherin transmembrane cell-cell adhesion proteins play essential roles in tissue morphogenesis and in mediating tissue integrity. Cadherin ectodomains from opposing cells interact to form load-bearing trans dimers that mechanically couple cells. Cell-cell adhesion is believed to be strengthened by cis clustering of cadherins on the same cell surface. This review summarizes biophysical studies of the structure, interaction kinetics and biomechanics of classical cadherin ectodomains. We first discuss the structure and equilibrium binding kinetics of classical cadherin trans and cis dimers. We then discuss how mechanical stimuli alters the kinetics of cadherin interaction and tunes adhesion. Finally, we highlight open questions on the role of mechanical forces in influencing cadherin structure, function and organization on the cell surface.

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.