Mechanosensing depicts the ability of a cell to sense mechanical cues, which under some circumstances is mediated by the surface receptors. In this review, a four-step model is described for receptor-mediated mechanosensing. Platelet GPIb, T-cell receptor, and integrins are used as examples to illustrate the key concepts and players in this process.

Mechanical forces are central to most, if not all, biological processes, including cell development, immune recognition, and metastasis. Because the cellular machinery mediating mechano-sensing and force generation is dependent on the nanoscale organization and geometry of protein assemblies, a current need in the field is the development of force-sensing probes that can be customized at the nanometer-length scale. In this work, we describe a DNA origami tension sensor that maps the piconewton (pN) forces generated by living cells. As a proof-of-concept, we engineered a novel library of six-helix-bundle DNA-origami tension probes (DOTPs) with a tailorable number of tension-reporting hairpins (each with their own tunable tension response threshold) and a tunable number of cell-receptor ligands. We used single-molecule force spectroscopy to determine the probes' tension response thresholds and used computational modeling to show that hairpin unfolding is semi-cooperative and orientation-dependent. Finally, we use our DOTP library to map the forces applied by human blood platelets during initial adhesion and activation. We find that the total tension signal exhibited by platelets on DOTP-functionalized surfaces increases with the number of ligands per DOTP, likely due to increased total ligand density, and decreases exponentially with the DOTP's force-response threshold. This work opens the door to applications for understanding and regulating biophysical processes involving cooperativity and multivalency.

SignificanceSingle-molecule localization microscopy (SMLM) is useful for deciphering dynamic organizations of structures densely labeled by specific proteins in the cellular context with nanoscopic resolution not attainable by conventional imaging tools. Here we employed SMLM to investigate the mechanism by which the HIV-1 viral RNA (vRNA) mediates the assembly of thousands of Gag proteins into a virus particle at the plasma membrane. In contrast to the general notion that vRNA only triggers Gag assembly and is dispensable for subsequent assembly, we found that vRNA is indispensable throughout assembly, scaffolding the formation of assembly intermediates and maintaining their architectures via balancing of external forces acting on the assembly environment. These previously unidentified features may facilitate understanding of HIV-1 and, potentially, other retroviruses.