Platelet Adhesion and Activation in an ECMO Thrombosis-on-a-Chip Model

Use of extracorporeal membrane oxygenation (ECMO) for cardiorespiratory failure remains complicated by blood clot formation (thrombosis), triggered by biomaterial surfaces and flow conditions. Thrombosis may result in ECMO circuit changes, cause red blood cell hemolysis, and thromboembolic events. Medical device thrombosis is potentiated by the interplay between biomaterial properties, hemodynamic flow conditions and patient pathology, however, the contribution and importance of these factors are poorly understood because many in vitro models lack the capability to customize material and flow conditions to investigate thrombosis under clinically relevant medical device conditions. Therefore, an ECMO thrombosis-on-a-chip model is developed that enables highly customizable biomaterial and flow combinations to evaluate ECMO thrombosis in real-time with low blood volume. It is observed that low flow rates, decelerating conditions, and flow stasis significantly increased platelet adhesion, correlating with clinical thrombus formation. For the first time, it is found that tubing material, polyvinyl chloride, caused increased platelet P-selectin activation compared to connector material, polycarbonate. This ECMO thrombosis-on-a-chip model can be used to guide ECMO operation, inform medical device design, investigate embolism, occlusion and platelet activation mechanisms, and develop anti-thrombotic biomaterials to ultimately reduce medical device thrombosis, anti-thrombotic drug use and therefore bleeding complications, leading to safer blood-contacting medical devices.

Endoplasmic Reticulum Protein 72 Regulates Integrin Mac-1 Activity to Influence Neutrophil Recruitment

BACKGROUND

Integrins mediate the adhesion, crawling, and migration of neutrophils during vascular inflammation. Thiol exchange is important in the regulation of integrin functions. ERp72 (endoplasmic reticulum-resident protein 72) is a member of the thiol isomerase family responsible for the catalysis of disulfide rearrangement. However, the role of ERp72 in the regulation of Mac-1 (integrin αMβ2) on neutrophils remains elusive.

METHODS

Intravital microscopy of the cremaster microcirculation was performed to determine in vivo neutrophil movement. Static adhesion, flow chamber, and flow cytometry were used to evaluate in vitro integrin functions. Confocal fluorescent microscopy and coimmunoprecipitation were utilized to characterize the interactions between ERp72 and Mac-1 on neutrophil surface. Cell-impermeable probes and mass spectrometry were used to label reactive thiols and identify target disulfide bonds during redox exchange. Biomembrane force probe was performed to quantitatively measure the binding affinity of Mac-1. A murine model of acute lung injury induced by lipopolysaccharide was utilized to evaluate neutrophil-associated vasculopathy.

RESULTS

ERp72-deficient neutrophils exhibited increased rolling but decreased adhesion/crawling on inflamed venules in vivo and defective static adhesion in vitro. The defect was due to defective activation of integrin Mac-1 but not LFA-1 (lymphocyte function-associated antigen-1) using blocking or epitope-specific antibodies. ERp72 interacted with Mac-1 in lipid rafts on neutrophil surface leading to the reduction of the C654-C711 disulfide bond in the αM subunit that is critical for Mac-1 activation. Recombinant ERp72, via its catalytic motifs, increased the binding affinity of Mac-1 with ICAM-1 (intercellular adhesion molecule-1) and rescued the defective adhesion of ERp72-deficient neutrophils both in vitro and in vivo. Deletion of ERp72 in the bone marrow inhibited neutrophil infiltration, ameliorated tissue damage, and increased survival during murine acute lung injury.

CONCLUSIONS

Extracellular ERp72 regulates integrin Mac-1 activity by catalyzing disulfide rearrangement on the αM subunit and may be a novel target for the treatment of neutrophil-associated vasculopathy.

Fluorescence-coupled Micropipette Aspiration Assay to Investigate Red Blood Cell Mechanosensing

Micropipette aspiration assays have long been a cornerstone for the investigation of live-cell mechanics, offering insights into cellular responses to mechanical stress. This paper details an innovative adaptation of the fluorescence-coupled micropipette aspiration (fMPA) assay. The fMPA assay introduces the capability to administer precise mechanical forces while concurrently monitoring the live-cell mechanotransduction processes mediated by ion channels. The sophisticated setup incorporates a precision-engineered borosilicate glass micropipette connected to a finely regulated water reservoir and pneumatic aspiration system, facilitating controlled pressure application with increments as refined as ± 1 mmHg. A significant enhancement is the integration of epi-fluorescence imaging, allowing for the simultaneous observation and quantification of cell morphological changes and intracellular calcium fluxes during aspiration. The fMPA assay, through its synergistic combination of epi-fluorescence imaging with micropipette aspiration, sets a new standard for the study of cell mechanosensing within mechanically challenging environments. This multifaceted approach is adaptable to various experimental setups, providing critical insights into the single-cell mechanosensing mechanisms.

Force-Regulated Spontaneous Conformational Changes of Integrins α5β1 and αVβ3

Integrins are cell surface nanosized receptors crucial for cell motility and mechanosensing of the extracellular environment, which are often targeted for the development of biomaterials and nanomedicines. As a key feature of integrins, their activity, structure and behavior are highly mechanosensitive, which are regulated by mechanical forces down to pico-Newton scale. Using single-molecule biomechanical approaches, we compared the force-modulated ectodomain bending/unbending conformational changes of two integrin species, α5β1 and αVβ3. It was found that the conformation of integrin α5β1 is determined by a threshold head-to-tail tension. By comparison, integrin αVβ3 exhibits bistability even without force and can spontaneously transition between the bent and extended conformations with an apparent transition time under a wide range of forces. Molecular dynamics simulations observed almost concurrent disruption of ∼2 hydrogen bonds during integrin α5β1 unbending, but consecutive disruption of ∼7 hydrogen bonds during integrin αVβ3 unbending. Accordingly, we constructed a canonical energy landscape for integrin α5β1 with a single energy well that traps the integrin in the bent state until sufficient force tilts the energy landscape to allow the conformational transition. In contrast, the energy landscape of integrin αVβ3 conformational changes was constructed with hexa-stable intermediate states and intermediate energy barriers that segregate the conformational change process into multiple small steps. Our study elucidates the different biomechanical inner workings of integrins α5β1 and αVβ3 at the submolecular level, helps understand their mechanosignaling processes and how their respective functions are facilitated by their distinctive mechanosensitivities, and provides useful design principles for the engineering of protein-based biomechanical nanomachines.

Biomembrane force probe (BFP): Design, advancements, and recent applications to live-cell mechanobiology

Mechanical forces play a vital role in biological processes at molecular and cellular levels, significantly impacting various diseases such as cancer, cardiovascular disease, and COVID-19. Recent advancements in dynamic force spectroscopy (DFS) techniques have enabled the application and measurement of forces and displacements with high resolutions, providing crucial insights into the mechanical pathways underlying these diseases. Among DFS techniques, the biomembrane force probe (BFP) stands out for its ability to measure bond kinetics and cellular mechanosensing with pico-newton and nano-meter resolutions. Here, a comprehensive overview of the classical BFP-DFS setup is presented and key advancements are emphasized, including the development of dual biomembrane force probe (dBFP) and fluorescence biomembrane force probe (fBFP). BFP-DFS allows us to investigate dynamic bond behaviors on living cells and significantly enhances the understanding of specific ligand-receptor axes mediated cell mechanosensing. The contributions of BFP-DFS to the fields of cancer biology, thrombosis, and inflammation are delved into, exploring its potential to elucidate novel therapeutic discoveries. Furthermore, future BFP upgrades aimed at improving output and feasibility are anticipated, emphasizing its growing importance in the field of cell mechanobiology. Although BFP-DFS remains a niche research modality, its impact on the expanding field of cell mechanobiology is immense.

A microfluidic approach for early prediction of thrombosis in patients with cancer

Li and colleagues have made a notable advancement in predicting cancer-associated thrombosis with a microfluidic device that monitors circulating platelet activity.1 This tool could improve the management of thrombotic events in patients with cancer, guiding timely treatment and potentially reducing mortality.

3D spheroid-microvasculature-on-a-chip for tumor-endothelium mechanobiology interplay

During the final stage of cancer metastasis, tumor cells embed themselves in distant capillary beds, from where they extravasate and establish secondary tumors. Recent findings underscore the pivotal roles of blood/lymphatic flow and shear stress in this intricate tumor extravasation process. Despite the increasing evidence, there is a dearth of systematic and biomechanical methodologies that accurately mimic intricate 3D microtissue interactions within a controlled hydrodynamic microenvironment. Addressing this gap, we introduce an easy-to-operate 3D Spheroid-Microvasculature-On-A-Chip (SMAC) model. Operating under both static and regulated flow conditions, the SMAC model facilitates the replication of the biomechanical interplay between heterogeneous tumor spheroids and endothelium in a quantitative manner. Serving as an in vitro model for metastasis mechanobiology, our model unveils the phenomena of 3D spheroid-induced endothelial compression and cell-cell junction degradation during tumor migration and expansion. Furthermore, we investigated the influence of shear stress on endothelial orientation, polarization, and tumor spheroid expansion. Collectively, our SMAC model provides a compact, cost-efficient, and adaptable platform for probing the mechanobiology of metastasis.

A Novel Computational Biomechanics Framework to Model Vascular Mechanopropagation in Deep Bone Marrow

The mechanical stimuli generated by body exercise can be transmitted from cortical bone into the deep bone marrow (mechanopropagation). Excitingly, a mechanosensitive perivascular stem cell niche is recently identified within the bone marrow for osteogenesis and lymphopoiesis. Although it is long known that they are maintained by exercise-induced mechanical stimulation, the mechanopropagation from compact bone to deep bone marrow vasculature remains elusive of this fundamental mechanobiology field. No experimental system is available yet to directly understand such exercise-induced mechanopropagation at the bone-vessel interface. To this end, taking advantage of the revolutionary in vivo 3D deep bone imaging, an integrated computational biomechanics framework to quantitatively evaluate the mechanopropagation capabilities for bone marrow arterioles, arteries, and sinusoids is devised. As a highlight, the 3D geometries of blood vessels are smoothly reconstructed in the presence of vessel wall thickness and intravascular pulse pressure. By implementing the 5-parameter Mooney-Rivlin model that simulates the hyperelastic vessel properties, finite element analysis to thoroughly investigate the mechanical effects of exercise-induced intravascular vibratory stretching on bone marrow vasculature is performed. In addition, the blood pressure and cortical bone bending effects on vascular mechanoproperties are examined. For the first time, movement-induced mechanopropagation from the hard cortical bone to the soft vasculature in the bone marrow is numerically simulated. It is concluded that arterioles and arteries are much more efficient in propagating mechanical force than sinusoids due to their stiffness. In the future, this in-silico approach can be combined with other clinical imaging modalities for subject/patient-specific vascular reconstruction and biomechanical analysis, providing large-scale phenotypic data for personalized mechanobiology discovery.

Force-regulated spontaneous conformational changes of integrins α 5 β 1 and α V β 3

Force can modulate the properties and functions of macromolecules by inducing conformational changes, such as coiling/uncoiling, zipping/unzipping, and folding/unfolding. Here we compared force-modulated bending/unbending of two purified integrin ectodomains, α 5 β 1 and α V β 3 , using single-molecule approaches. Similar to previously characterized mechano-sensitive macromolecules, the conformation of α 5 β 1 is determined by a threshold head-to-tail tension, suggesting a canonical energy landscape with a deep energy well that traps the integrin in the bent state until sufficient force tilts the energy landscape to accelerate transition to the extended state. By comparison, α V β 3 exhibits bi-stability even without force and can spontaneously transition between the bent and extended conformations in a wide range of forces without energy supplies. Molecular dynamics simulations revealed consecutive formation and disruption of 7 hydrogen bonds during α V β 3 bending and unbending, respectively. Accordingly, we constructed an energy landscape with hexa-stable intermediate states to break down the energy barrier separating the bent and extended states into smaller ones, making it possible for the thermal agitation energy to overcome them sequentially and to be accumulated and converted into mechanical work required for α V β 3 to bend against force. Our study elucidates the different inner workings of α 5 β 1 and α V β 3 at the sub-molecular level, sheds lights on how their respectively functions are facilitated by their distinctive mechano-sensitivities, helps understand their signal initiation processes, and provides critical concepts and useful design principles for engineering of protein-based biomechanical nanomachines.

Acoustic Force-Based Cell-Matrix Avidity Measurement in High Throughput

Cancer cells interacting with the extracellular matrix (ECM) in the tumor microenvironment is pivotal for tumorigenesis, invasion, and metastasis. Cell-ECM adhesion has been intensively studied in cancer biology in the past decades to understand the molecular mechanisms underlying the adhesion events and extracellular mechanosensing, as well as develop therapeutic strategies targeting the cell adhesion molecules. Many methods have been established to measure the cell-ECM adhesion strength and correlate it with the metastatic potential of certain cancer types. However, those approaches are either low throughput, not quantitative, or with poor sensitivity and reproducibility. Herein, we developed a novel acoustic force spectroscopy based method to quantify the cell-ECM adhesion strength during adhesion maturation process using the emerging z-Movi^® technology. This can be served as a fast, simple, and high-throughput platform for functional assessment of cell adhesion molecules in a highly predictive and reproducible manner.

Mechano-Redox Control of Integrins in Thromboinflammation

Significance: How mechanical forces and biochemical cues are coupled remains a miracle for many biological processes. Integrins, well-known adhesion receptors, sense changes in mechanical forces and reduction-oxidation reactions (redox) in their environment to mediate their adhesive function. The coupling of mechanical and redox function is a new area of investigation. Disturbance of normal mechanical forces and the redox balance occurs in thromboinflammatory conditions; atherosclerotic plaques create changes to the mechanical forces in the circulation. Diabetes induces redox changes in the circulation by the production of reactive oxygen species and vascular inflammation. Recent Advances: Integrins sense changes in the blood flow shear stress at the level of focal adhesions and respond to flow and traction forces by increased signaling. Talin, the integrin-actin linker, is a traction force sensor and adaptor. Oxidation and reduction of integrin disulfide bonds regulate their adhesion. A conserved disulfide bond in integrin αlpha IIb beta 3 (αIIbβ3) is directly reduced by the thiol oxidoreductase endoplasmic reticulum protein 5 (ERp5) under shear stress. Critical Issues: The coordination of mechano-redox events between the extracellular and intracellular compartments is an active area of investigation. Another fundamental issue is to determine the spatiotemporal arrangement of key regulators of integrins' mechanical and redox interactions. How thromboinflammatory conditions lead to mechanoredox uncoupling is relatively unexplored. Future Directions: Integrated approaches, involving disulfide bond biochemistry, microfluidic assays, and dynamic force spectroscopy, will aid in showing that cell adhesion constitutes a crossroad of mechano- and redox biology, within the same molecule, the integrin. Antioxid. Redox Signal. 37, 1072-1093.

The soluble N-terminal autoinhibitory module of the A1 domain in von Willebrand factor partially suppresses its catch bond with glycoprotein Ibα in a sandwich complex

von Willebrand factor (VWF) senses and responds to the hemodynamic forces to interact with the circulatory system and platelets in hemostasis and thrombosis. The dark side of this mechanobiology is implicated in atherothrombosis, stroke, and, more recently, the COVID-19 thrombotic symptoms. The force-responsive element controlling VWF activation predominantly resides in the N terminal auto-inhibitory module (N-AIM) flanking its A1 domain. Nevertheless, the detailed mechano-chemistry of soluble VWF N-AIM is poorly understood at the sub-molecular level as it is assumed to be unstructured loops. Using the free molecular dynamics (MD) simulations, we first predicted a hairpin-like structure of the soluble A1 N-AIM derived polypeptide (Lp; sequences Q1238-E1260). Then we combined molecular docking and steered molecular dynamics (SMD) simulations to examine how Lp regulates the A1-GPIbα interaction under tensile forces. Our simulation results indicate that Lp suppresses the catch bond in a sandwich complex of A1-Lp-GPIbα yet contributes an additional catch-bond residue D1249. To experimentally benchmark the binding kinetics for A1-GPIbα in the absence or presence of Lp, we conducted the force spectroscopy-biomembrane force probe (BFP) assays. We found similar suppression on the A1-GPIbα catch bond with soluble Lp in presence. Clinically, as more and more therapeutic candidates targeting the A1-GPIbα axis have entered clinical trials to treat patients with TTP and acute coronary syndrome, our work represents an endeavor further towards an effective anti-thrombotic approach without severe bleeding side effects as most existing drugs suffer.

Fluorescence-coupled micropipette aspiration assay to examine calcium mobilization caused by red blood cell mechanosensing

Mechanical stimuli such as tension, compression, and shear stress play critical roles in the physiological functions of red blood cells (RBCs) and their homeostasis, ATP release, and rheological properties. Intracellular calcium (Ca²⁺) mobilization reflects RBC mechanosensing as they transverse the complex vasculature. Emerging studies have demonstrated the presence of mechanosensitive Ca²⁺ permeable ion channels and their function has been implicated in the regulation of RBC volume and deformability. However, how these mechanoreceptors trigger Ca²⁺ influx and subsequent cellular responses are still unclear. Here, we introduce a fluorescence-coupled micropipette aspiration assay to examine RBC mechanosensing at the single-cell level. To achieve a wide range of cell aspirations, we implemented and compared two negative pressure adjusting apparatuses: a homemade water manometer (- 2.94 to 0 mmH₂O) and a pneumatic high-speed pressure clamp (- 25 to 0 mmHg). To visualize Ca²⁺ influx, RBCs were pre-loaded with an intensiometric probe Cal-520 AM, then imaged under a confocal microscope with concurrent bright-field and fluorescent imaging at acquisition rates of 10 frames per second. Remarkably, we observed the related changes in intracellular Ca²⁺ levels immediately after aspirating individual RBCs in a pressure-dependent manner. The RBC aspirated by the water manometer only displayed 1.1-fold increase in fluorescence intensity, whereas the RBC aspirated by the pneumatic clamp showed up to threefold increase. These results demonstrated the water manometer as a gentle tool for cell manipulation with minimal pre-activation, while the high-speed pneumatic clamp as a much stronger pressure actuator to examine cell mechanosensing directly. Together, this multimodal platform enables us to precisely control aspiration and membrane tension, and subsequently correlate this with intracellular calcium concentration dynamics in a robust and reproducible manner.

Micropipette-based biomechanical nanotools on living cells

Mechanobiology is an emerging field at the interface of biology and mechanics, investigating the roles of mechanical forces within biomolecules, organelles, cells, and tissues. As a highlight, the recent advances of micropipette-based aspiration assays and dynamic force spectroscopies such as biomembrane force probe (BFP) provide unprecedented mechanobiological insights with excellent live-cell compatibility. In their classic applications, these assays measure force-dependent ligand-receptor-binding kinetics, protein conformational changes, and cellular mechanical properties such as cortical tension and stiffness. In recent years, when combined with advanced microscopies in high spatial and temporal resolutions, these biomechanical nanotools enable characterization of receptor-mediated cell mechanosensing and subsequent organelle behaviors at single-cellular and molecular level. In this review, we summarize the latest developments of these assays for live-cell mechanobiology studies. We also provide perspectives on their future upgrades with multimodal integration and high-throughput capability.

Platelet Mechanobiology Inspired Microdevices: From Hematological Function Tests to Disease and Drug Screening

Platelet function tests are essential to profile platelet dysfunction and dysregulation in hemostasis and thrombosis. Clinically they provide critical guidance to the patient management and therapeutic evaluation. Recently, the biomechanical effects induced by hemodynamic and contractile forces on platelet functions attracted increasing attention. Unfortunately, the existing platelet function tests on the market do not sufficiently incorporate the topical platelet mechanobiology at play. Besides, they are often expensive and bulky systems that require large sample volumes and long processing time. To this end, numerous novel microfluidic technologies emerge to mimic vascular anatomies, incorporate hemodynamic parameters and recapitulate platelet mechanobiology. These miniaturized and cost-efficient microfluidic devices shed light on high-throughput, rapid and scalable platelet function testing, hematological disorder profiling and antiplatelet drug screening. Moreover, the existing antiplatelet drugs often have suboptimal efficacy while incurring several adverse bleeding side effects on certain individuals. Encouraged by a few microfluidic systems that are successfully commercialized and applied to clinical practices, the microfluidics that incorporate platelet mechanobiology hold great potential as handy, efficient, and inexpensive point-of-care tools for patient monitoring and therapeutic evaluation. Hereby, we first summarize the conventional and commercially available platelet function tests. Then we highlight the recent advances of platelet mechanobiology inspired microfluidic technologies. Last but not least, we discuss their future potential of microfluidics as point-of-care tools for platelet function test and antiplatelet drug screening.

Microfluidic post method for 3-dimensional modeling of platelet–leukocyte interactions

These studies demonstrate the versatility and relevance of a novel ‘platelet post’ model to examine the adhesive interactions between platelets and neutrophils under 3D disturbed flow conditions relevant to thromboinflammation.

The N-terminal autoinhibitory module of the A1 domain in von Willebrand factor stabilizes the mechanosensor catch bond

The N-AIM of VWF-A1 forms a Rotini-like structure, therefore partially autoinhibit VWF-A1–GPIbα interaction. The N-AIM acts as a defending sword to protect and stabilize the VWF-A1 structure under harsh environments.

Emerging Microfluidic Approaches for Platelet Mechanobiology and Interplay With Circulatory Systems

Understanding how platelets can sense and respond to hemodynamic forces in disturbed blood flow and complexed vasculature is crucial to the development of more effective and safer antithrombotic therapeutics. By incorporating diverse structural and functional designs, microfluidic technologies have emerged to mimic microvascular anatomies and hemodynamic microenvironments, which open the floodgates for fascinating platelet mechanobiology investigations. The latest endothelialized microfluidics can even recapitulate the crosstalk between platelets and the circulatory system, including the vessel walls and plasma proteins such as von Willebrand factor. Hereby, we highlight these exciting microfluidic applications to platelet mechanobiology and platelet–circulatory system interplay as implicated in thrombosis. Last but not least, we discuss the need for microfluidic standardization and summarize the commercially available microfluidic platforms for researchers to obtain reproducible and consistent results in the field.

Molecular Spring Constant Analysis by Biomembrane Force Probe Spectroscopy

A biomembrane force probe (BFP) has recently emerged as a native-cell-surface or in situ dynamic force spectroscopy (DFS) nanotool that can measure single-molecular binding kinetics, assess mechanical properties of ligand-receptor interactions, visualize protein dynamic conformational changes and more excitingly elucidate receptor mediated cell mechanosensing mechanisms. More recently, BFP has been used to measure the spring constant of molecular bonds. This protocol describes the step-by-step procedure to perform molecular spring constant DFS analysis. Specifically, two BFP operation modes are discussed, namely the Bead-Cell and Bead-Bead modes. This protocol focuses on deriving spring constants of the molecular bond and cell from DFS raw data.

Enabling peristalsis of human colon tumor organoids on microfluidic chips

Peristalsis in the digestive tract is crucial to maintain physiological functions. It remains challenging to mimic the peristaltic microenvironment in gastrointestinal organoid culture. Here, we present a method to model the peristalsis for human colon tumor organoids on a microfluidic chip. The chip contains hundreds of lateral microwells and a surrounding pressure channel. Human colon tumor organoids growing in the microwell were cyclically contracted by pressure channel, mimicking thein vivomechano-stimulus by intestinal muscles. The chip allows the control of peristalsis amplitude and rhythm and the high throughput culture of organoids simultaneously. By applying 8% amplitude with 8 ∼ 10 times min^-1, we observed the enhanced expression of Lgr5 and Ki67. Moreover, ellipticine-loaded polymeric micelles showed reduced uptake in the organoids under peristalsis and resulted in compromised anti-tumor efficacy. The results indicate the importance of mechanical stimuli mimicking the physiological environment when usingin vitromodels to evaluate nanoparticles. This work provides a method for attaining more reliable and representative organoids models in nanomedicine.

Modified N-linked glycosylation status predicts trafficking defective human Piezo1 channel mutations

Mechanosensitive channels are integral membrane proteins that sense mechanical stimuli. Like most plasma membrane ion channel proteins they must pass through biosynthetic quality control in the endoplasmic reticulum that results in them reaching their destination at the plasma membrane. Here we show that N-linked glycosylation of two highly conserved asparagine residues in the 'cap' region of mechanosensitive Piezo1 channels are necessary for the mature protein to reach the plasma membrane. Both mutation of these asparagines (N2294Q/N2331Q) and treatment with an enzyme that hydrolyses N-linked oligosaccharides (PNGaseF) eliminates the fully glycosylated mature Piezo1 protein. The N-glycans in the cap are a pre-requisite for N-glycosylation in the 'propeller' regions, which are present in loops that are essential for mechanotransduction. Importantly, trafficking-defective Piezo1 variants linked to generalized lymphatic dysplasia and bicuspid aortic valve display reduced fully N-glycosylated Piezo1 protein. Thus the N-linked glycosylation status in vitro correlates with efficient membrane trafficking and will aid in determining the functional impact of Piezo1 variants of unknown significance.

Upconversion Nonlinear Structured Illumination Microscopy

Video-rate super-resolution imaging through biological tissue can visualize and track biomolecule interplays and transportations inside cellular organisms. Structured illumination microscopy allows for wide-field super resolution observation of biological samples but is limited by the strong extinction of light by biological tissues, which restricts the imaging depth and degrades its imaging resolution. Here we report a photon upconversion scheme using lanthanide-doped nanoparticles for wide-field super-resolution imaging through the biological transparent window, featured by near-infrared and low-irradiance nonlinear structured illumination. We demonstrate that the 976 nm excitation and 800 nm upconverted emission can mitigate the aberration. We found that the nonlinear response of upconversion emissions from single nanoparticles can effectively generate the required high spatial frequency components in the Fourier domain. These strategies lead to a new modality in microscopy with a resolution below 131 nm, 1/7th of the excitation wavelength, and an imaging rate of 1 Hz.

Ultra-stable Biomembrane Force Probe for Accurately Determining Slow Dissociation Kinetics of PD-1 Blockade Antibodies on Single Living Cells

Immune checkpoint blockade with monoclonal antibodies (mAbs) that target programmed cell death protein-1 (PD-1) has remarkably revolutionized cancer therapy. Their binding kinetics measured by surface plasmon resonance does not always correlate well with their immunotherapeutic efficacies, mainly due to the lack of two-dimensional cell plasma membrane and the capability of force sensing and manipulation. In this regard, based on a more suitable and ultra-sensitive biomechanical nanotool, biomembrane force probe (BFP), we developed a Double-edge Smart Feedback control system as an ultra-stable platform to characterize ultra-long bond lifetimes of receptor-ligand binding on living cells. We further benchmarked the dissociation kinetics for three clinically approved PD-1 blockade mAbs (Nivolumab, Pembrolizumab, and Camrelizumab), intriguingly correlating well with the objective response rates in the hepatocellular carcinoma second-line treatment. This ultra-stable BFP potentially provides a compelling kinetic platform to direct the screening, optimization, and clinical selection of therapeutic antibodies in the future.

Biomechanical thrombosis: the dark side of force and dawn of mechano-medicine

Arterial thrombosis is in part contributed by excessive platelet aggregation, which can lead to blood clotting and subsequent heart attack and stroke. Platelets are sensitive to the haemodynamic environment. Rapid haemodynamcis and disturbed blood flow, which occur in vessels with growing thrombi and atherosclerotic plaques or is caused by medical device implantation and intervention, promotes platelet aggregation and thrombus formation. In such situations, conventional antiplatelet drugs often have suboptimal efficacy and a serious side effect of excessive bleeding. Investigating the mechanisms of platelet biomechanical activation provides insights distinct from the classic views of agonist-stimulated platelet thrombus formation. In this work, we review the recent discoveries underlying haemodynamic force-reinforced platelet binding and mechanosensing primarily mediated by three platelet receptors: glycoprotein Ib (GPIb), glycoprotein IIb/IIIa (GPIIb/IIIa) and glycoprotein VI (GPVI), and their implications for development of antithrombotic 'mechano-medicine' .

Dynamic bonds and their roles in mechanosensing

Mechanical forces are ubiquitous in a cell's internal structure and external environment. Mechanosensing is the process that the cell employs to sense its mechanical environment. In receptor-mediated mechanosensing, cell surface receptors interact with immobilized ligands to provide a specific way to receive extracellular force signals to targeted force-transmitting, force-transducing and force-supporting structures inside the cell. Conversely, forces generated endogenously by the cell can be transmitted via cytoplasmic protein-protein interactions and regulate cell surface receptor activities in an 'inside-out' manner. Dynamic force spectroscopy analyzes these interactions on and inside cells to reveal various dynamic bonds. What is more, by integrating analysis of molecular interactions with that of cell signaling events involved in force-sensing and force-responding processes, one can investigate how dynamic bonds regulate the reception, transmission and transduction of mechanical signals.

An integrin αIIbβ3 intermediate affinity state mediates biomechanical platelet aggregation

Integrins are membrane receptors that mediate cell adhesion and mechanosensing. The structure-function relationship of integrins remains incompletely understood, despite the extensive studies carried out because of its importance to basic cell biology and translational medicine. Using a fluorescence dual biomembrane force probe, microfluidics and cone-and-plate rheometry, we applied precisely controlled mechanical stimulations to platelets and identified an intermediate state of integrin α_IIbβ₃ that is characterized by an ectodomain conformation, ligand affinity and bond lifetimes that are all intermediate between the well-known inactive and active states. This intermediate state is induced by ligand engagement of glycoprotein (GP) Ibα via a mechanosignalling pathway and potentiates the outside-in mechanosignalling of α_IIbβ₃ for further transition to the active state during integrin mechanical affinity maturation. Our work reveals distinct α_IIbβ₃ state transitions in response to biomechanical and biochemical stimuli, and identifies a role for the α_IIbβ₃ intermediate state in promoting biomechanical platelet aggregation.

Tensile and compressive force regulation on cell mechanosensing

Receptor-mediated cell mechanosensing plays critical roles in cell spreading, migration, growth, and survival. Dynamic force spectroscopy (DFS) techniques have recently been advanced to visualize such processes, which allow the concurrent examination of molecular binding dynamics and cellular response to mechanical stimuli on single living cells. Notably, the live-cell DFS is able to manipulate the force "waveforms" such as tensile versus compressive, ramped versus clamped, static versus dynamic, and short versus long lasting forces, thereby deriving correlations of cellular responses with ligand binding kinetics and mechanical stimulation profiles. Here, by differentiating extracellular mechanical stimulations into two major categories, tensile force and compressive force, we review the latest findings on receptor-mediated mechanosensing mechanisms that are discovered by the state-of-the-art live-cell DFS technologies.

Straight Channel Microfluidic Chips for the Study of Platelet Adhesion under Flow

Microfluidic devices have become an integral method of cardiovascular research as they enable the study of shear force in biological processes, such as platelet function and thrombus formation. Furthermore, microfluidic chips offer the benefits of ex vivo testing of platelet adhesion using small amounts of blood or purified platelets. Microfluidic chips comprise flow channels of varying dimensions and geometries which are connected to a syringe pump. The pump draws blood or platelet suspensions through the channel(s) allowing for imaging of platelet adhesion and thrombus formation by fluorescence microscopy. The chips can be fabricated from various blood-compatible materials. The current protocol uses commercial plastic or in-house polydimethylsiloxane (PDMS) chips. Commercial biochips offer the advantage of standardization whereas in-house chips offer the advantage of decreased cost and flexibility in design. Microfluidic devices are a powerful tool to study the biorheology of platelets and other cell types with the potential of a diagnostic and monitoring tool for cardiovascular diseases.

Fast Force Loading Disrupts Molecular Binding Stability in Human and Mouse Cell Adhesions

Force plays critical roles in cell adhesion and mechano-signaling, partially by regulating the dissociation rate, i.e., off-rate, of receptor-ligand bonds. However, the mechanism of such regulation still remains elusive. As a controversial topic of the field, when measuring the "off-rate vs. force" relation of the same molecular system, different dynamic force spectroscopy (DFS) assays (namely, force-clamp and force-ramp assays) often yield contradictive results. Such discrepancies hurdled our further understanding of molecular binding, and casted doubt on the existing theoretical models. In this work, we used a live-cell DFS technique, biomembrane force probe, to measure the single-bond dissociation in three receptor-ligand systems which respectively have important functions in vascular and immune systems: human platelet GPIbα-VWF, mouse T cell receptor-OVA peptide:MHC, and mouse platelet integrin α_IIbβ₃-fibrinogen. Using force-clamp and force-ramp assays in parallel, we identified that the force loading disrupted the stability of molecular bonds in a rate-dependent manner. This disruptive effect was achieved by the transitioning of bonds between two dissociation states: faster force loading induces more bonds to adopt the fast-dissociating state (and less to adopt the slow-dissociating state). Based on this mechanism, a new biophysical model of bond dissociation was established which took into account the effects of both force magnitude and loading rate. Remarkably, this model reconciled the results from the two assays in all three molecular systems under study. Our discoveries provided a new paradigm for understanding how force regulates receptor-ligand interactions and a guideline for the proper use of DFS technologies. Furthermore, our work highlighted the opportunity of using different DFS assays to answer specific biological questions in the field of cell adhesion and mechano-signaling.

Biophysical nanotools for single-molecule dynamics

The focus of the cell biology field is now shifting from characterizing cellular activities to organelle and molecular behaviors. This process accompanies the development of new biophysical visualization techniques that offer high spatial and temporal resolutions with ultra-sensitivity and low cell toxicity. They allow the biology research community to observe dynamic behaviors from scales of single molecules, organelles, cells to organoids, and even live animal tissues. In this review, we summarize these biophysical techniques into two major classes: the mechanical nanotools like dynamic force spectroscopy (DFS) and the optical nanotools like single-molecule and super-resolution microscopy. We also discuss their applications in elucidating molecular dynamics and functionally mapping of interactions between inter-cellular networks and intra-cellular components, which is key to understanding cellular processes such as adhesion, trafficking, inheritance, and division.