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Estimation of rolling work of adhesion at the nanoscale with soft probing using optical tweezers.

ABSTRACT: Conventionally, the work of adhesion at the nanoscale is estimated using an atomic force microscope with a tip of the size of 10 nm. It is pressed into a surface with nano-Newton forces and then retracted to ascertain when the tip breaks away from the surface. Thus this ensures "hard probing" of a surface. However there can be another configuration where the particle is barely placed into the surface when the work of adhesion attaches the particle to the surface and this can be called "soft probing". In this configuration, if a birefringent particle is confined in linearly polarized optical tweezers, and then the surface is moved in the direction tangential to the plane, a rolling motion can be induced. Study of this rolling motion can also show the work of adhesion. We use this configuration to find the rolling work of adhesion of a 3 μm diameter birefringent particle on a glass surface. We go on to study the effects of changing the surface to a hydrophobic slippery surface like polydimethyl siloxane (PDMS). Then we go on to show that even 500 nm diameter diamonds bearing nitrogen vacancy (NV) centers which are birefringent due to the stresses on the crystal could also be trapped and rolled to generate pitch (out-of-plane rotation) motion with 50 nm contact diameters. We find that this mode of soft probing yields a work of adhesion of about 1 mJ m^-2 while the conventional nanoscale probing with atomic force microscopes (AFM) yields about 50 mJ m^-2.

SUBMITTER: Lokesh M

PROVIDER: S-EPMC8546490 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:IntroductionIn response to external stress, cells alter their morphology, metabolic activity, and functions to mechanically adapt to the dynamic, local environment through cell allostasis. To explore mechanotransduction in cellular allostasis, we applied an integrated micromechanical system that combines an 'ultrasound tweezers'-based mechanical stressor and a Förster resonance energy transfer (FRET)-based molecular force biosensor, termed "actinin-sstFRET," to monitor in situ single-cell allostasis in response to transient stimulation in real time.MethodsThe ultrasound tweezers utilize 1 Hz, 10-s transient ultrasound pulses to acoustically excite a lipid-encapsulated microbubble, which is bound to the cell membrane, and apply a pico- to nano-Newton range of forces to cells through an RGD-integrin linkage. The actinin-sstFRET molecular sensor, which engages the actin stress fibers in live cells, is used to map real-time actomyosin force dynamics over time. Then, the mechanosensitive behaviors were examined by profiling the dynamics in Ca2+ influx, actomyosin cytoskeleton (CSK) activity, and GTPase RhoA signaling to define a single-cell mechanical allostasis.ResultsBy subjecting a 1 Hz, 10-s physical stress, single vascular smooth muscle cells (VSMCs) were observed to remodeled themselves in a biphasic mechanical allostatic manner within 30 min that caused them to adjust their contractility and actomyosin activities. The cellular machinery that underscores the vital role of CSK equilibrium in cellular mechanical allostasis, includes Ca2+ influx, remodeling of actomyosin CSK and contraction, and GTPase RhoA signaling. Mechanical allostasis was observed to be compromised in VSMCs from patients with type II diabetes mellitus (T2DM), which could potentiate an allostatic maladaptation.ConclusionsBy integrating tools that simultaneously permit localized mechanical perturbation and map actomyosin forces, we revealed distinct cellular mechanical allostasis profiles in our micromechanical system. Our findings of cell mechanical allostasis and maladaptation provide the potential for mechanophenotyping cells to reveal their pathogenic contexts and their biophysical mediators that underlie multi-etiological diseases such as diabetes, hypertension, or aging.

Project description:Focal contacts act as mechanosensors allowing cells to respond to their biomechanical environment. Force transmission through newly formed contact sites is a highly dynamic process requiring a stable link between the intracellular cytoskeleton and the extracellular environment. To simultaneously investigate cellular traction forces in several individual maturing adhesion sites within the same cell, we established a custom-built multiple trap optical tweezers setup. Beads functionalized with fibronectin or RGD-peptides were placed onto the apical surface of a cell and trapped with a maximum force of 160 pN. Cells form adhesion contacts around the beads as demonstrated by vinculin accumulation and start to apply traction forces after 30 seconds. Force transmission was found to strongly depend on bead size, surface density of integrin ligands and bead location on the cell surface. Highest traction forces were measured for beads positioned on the leading edge. For mouse embryonic fibroblasts, traction forces acting on single beads are in the range of 80 pN after 5 minutes. If two beads were positioned parallel to the leading edge and with a center-to-center distance less than 10 µm, traction forces acting on single beads were reduced by 40%. This indicates a spatial and temporal coordination of force development in closely related adhesion sites. We also used our setup to compare traction forces, retrograde transport velocities, and migration velocities between two cell lines (mouse melanoma and fibroblasts) and primary chick fibroblasts. We find that maximal force development differs considerably between the three cell types with the primary cells being the strongest. In addition, we observe a linear relation between force and retrograde transport velocity: a high retrograde transport velocity is associated with strong cellular traction forces. In contrast, migration velocity is inversely related to traction forces and retrograde transport velocity.

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Estimation of rolling work of adhesion at the nanoscale with soft probing using optical tweezers.

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