Constructing modular and universal single molecule tension sensor using protein G to study mechano-sensitive receptors

Analytical methods in studying cell force sensing: principles, current technologies and perspectives

Mechanical stimulation plays a crucial role in numerous biological activities, including tissue development, regeneration and remodeling. Understanding how cells respond to their mechanical microenvironment is vital for investigating mechanotransduction with adequate spatial and temporal resolution. Cell force sensing-also known as mechanosensation or mechanotransduction-involves force transmission through the cytoskeleton and mechanochemical signaling. Insights into cell-extracellular matrix interactions and mechanotransduction are particularly relevant for guiding biomaterial design in tissue engineering. To establish a foundation for mechanical biomedicine, this review will provide a comprehensive overview of cell mechanotransduction mechanisms, including the structural components essential for effective mechanical responses, such as cytoskeletal elements, force-sensitive ion channels, membrane receptors and key signaling pathways. It will also discuss the clutch model in force transmission, the role of mechanotransduction in both physiology and pathological contexts, and biomechanics and biomaterial design. Additionally, we outline analytical approaches for characterizing forces at cellular and subcellular levels, discussing the advantages and limitations of each method to aid researchers in selecting appropriate techniques. Finally, we summarize recent advancements in cell force sensing and identify key challenges for future research. Overall, this review should contribute to biomedical engineering by supporting the design of biomaterials that integrate precise mechanical information.

Unravelling molecular mechanobiology using DNA-based fluorogenic tension sensors

Investigations of the biological system have revealed many principles that govern regular life processes. Recently, the analysis of tiny mechanical forces associated with many biological processes revealed their significance in understanding biological functions. Consequently, this piqued the interest of researchers, and a series of technologies have been developed to understand biomechanical cues at the molecular level. Notable techniques include single-molecule force spectroscopy, traction force microscopy, and molecular tension sensors. Well-defined double-stranded DNA structures could possess programmable mechanical characteristics, and hence, they have become one of the central molecules in molecular tension sensor technology. With the advancement of DNA technology, DNA or nucleic acid-based robust tension sensors offer the possibility of understanding mechanobiology in the bulk to single-molecule level range with desired spatiotemporal resolution. This review presents a comprehensive account of molecular tension sensors with a special emphasis on DNA-based fluorogenic tension sensors. Along with a detailed discussion on irreversible and reversible DNA-based tension sensors and their application in super-resolution microscopy, a discussion on biomolecules associated with cellular mechanotransduction and key findings in the field are included. This review ends with an elaborate discussion on the current challenges and future prospects of molecular tension sensors.

Engineering tunable catch bonds with DNA

Unlike most adhesive bonds, biological catch bonds strengthen with increased tension. This characteristic is essential to specific receptor-ligand interactions, underpinning biological adhesion dynamics, cell communication, and mechanosensing. While artificial catch bonds have been conceived, the tunability of their catch behaviour is limited. Here, we present the fish-hook, a rationally designed DNA catch bond that can be finely adjusted to a wide range of catch behaviours. We develop models to design these DNA structures and experimentally validate different catch behaviours by single-molecule force spectroscopy. The fish-hook architecture supports a vast sequence-dependent behaviour space, making it a valuable tool for reprogramming biological interactions and engineering force-strengthening materials.

In-Silico Analyses of Molecular Force Sensors for Mechanical Characterization of Biological Systems

Mechanical forces play key roles in biological processes such as cell migration and sensory perception. In recent years molecular force sensors have been developed as tools for in situ force measurements. Here we use all-atom steered molecular dynamics simulations to predict and study the relationship between design parameters and mechanical properties for three types of molecular force sensors commonly used in cellular biological research: two peptide- and one DNA-based. The peptide-based sensors consist of a pair of fluorescent proteins, which can undergo Förster resonance energy transfer (FRET), linked by spider silk (GPGGA)n or synthetic (GGSGGS)n disordered regions. The DNA-based sensor consists of two fluorophore-labeled strands of DNA that can be unzipped or sheared upon force application with a FRET signal as readout of dissociation. We simulated nine sensors, three of each kind. After equilibration, flexible peptide linkers of three different lengths were stretched by applying forces to their N- and C-terminal Cα atoms in opposite directions. Similarly, we equilibrated a DNA-based sensor and pulled on the phosphate atom of the terminal guanine of one strand and a selected phosphate atom on the other strand in the opposite direction. These simulations were performed at constant velocity (0.01 nm/ns - 10 nm/ns) and constant force (10 pN - 500 pN) for all versions of the sensors. Our results show how the force response of these sensors depends on their length, sequence, configuration and loading rate. Mechanistic insights gained from simulations analyses indicate that interpretation of experimental results should consider the influence of transient formation of secondary structure in peptide-based sensors and of overstretching in DNA-based sensors. These predictions can guide optimal fluorophore choice and facilitate the rational design of new sensors for use in protein, DNA, hybrid systems, and molecular devices.

Altering Cell Junctional Tension in Spheroids through E-Cadherin Engagement Modulation

Cadherin-mediated tension at adherens junctions (AJs) is fundamental for cell-cell adhesion and maintaining epithelial integrity. Despite the importance of manipulating AJs to dissect cell-cell interactions, existing three-dimensional (3D) multicellular models have not adequately addressed the precise manipulation of these junctions. To fill this gap, we introduce E-cadherin-modified tension gauge tethers (TGTs) at the junctions within spheroids. The system enables both quantification and modulation of junctional tension with specific DNA triggers. Using rupture-induced fluorescence, we successfully measure mechanical forces in 3D spheroids. Furthermore, mechanically strong TGTs can maintain normal E-cadherin-mediated adhesion. Employing toehold-mediated strand displacement allowed us to disrupt E-cadherin-specific cell-cell adhesion, consequently altering intracellular tension within the spheroids. Our methodology offers a robust and precise way to manipulate cell-cell adhesion and intracellular mechanics in spheroid models.

Unraveling the Dual-Stretch-Mode Impact on Tension Gauge Tethers' Mechanical Stability

Tension gauge tethers (TGTs), short DNA segments serving as extracellular tension sensors, are instrumental in assessing the tension dynamics in mechanotransduction. These TGTs feature an initial shear-stretch region and an unzip-stretch region. Despite their utility, no theoretical model has been available to estimate their tension-dependent lifetimes [τ(f)], restricting insights from cellular mechanotransduction experiments. We have now formulated a concise expression for τ(f) of TGTs, accommodating contributions from both stretch regions. Our model uncovers a tension-dependent energy barrier shift occurring when tension surpasses a switching force of approximately 13 pN for the recently developed TGTs, greatly influencing τ(f) profiles. Experimental data from several TGTs validated our model. The calibrated expression can predict τ(f) of TGTs at 37 °C based on their sequences with minor fold changes, supporting future applications of TGTs.

DNA-functionalized artificial mechanoreceptor for de novo force-responsive signaling

Synthetic signaling receptors enable programmable cellular responses coupling with customized inputs. However, engineering a designer force-sensing receptor to rewire mechanotransduction remains largely unexplored. Herein, we introduce nongenetically engineered artificial mechanoreceptors (AMRs) capable of reprogramming non-mechanoresponsive receptor tyrosine kinases (RTKs) to sense user-defined force cues, enabling de novo-designed mechanotransduction. AMR is a modular DNA-protein chimera comprising a mechanosensing-and-transmitting DNA nanodevice grafted on natural RTKs via aptameric anchors. AMR senses intercellular tensile force via an allosteric DNA mechano-switch with tunable piconewton-sensitive force tolerance, actuating a force-triggered dynamic DNA assembly to manipulate RTK dimerization and activate intracellular signaling. By swapping the force-reception ligands, we demonstrate the AMR-mediated activation of c-Met, a representative RTK, in response to the cellular tensile forces mediated by cell-adhesion proteins (integrin, E-cadherin) or membrane protein endocytosis (CI-M6PR). Moreover, AMR also allows the reprogramming of FGFR1, another RTK, to customize mechanobiological function, for example, adhesion-mediated neural stem cell maintenance.

Imaging and detecting intercellular tensile forces in spheroids and embryoid bodies using lipid-modified DNA probes

Cells continuously experience and respond to different physical forces that are used to regulate their physiology and functions. Our ability to measure these mechanical cues is essential for understanding the bases of various mechanosensing and mechanotransduction processes. While multiple strategies have been developed to study mechanical forces within two-dimensional (2D) cell culture monolayers, the force measurement at cell-cell junctions in real three-dimensional (3D) cell models is still pretty rare. Considering that in real biological systems, cells are exposed to forces from 3D directions, measuring these molecular forces in their native environment is thus highly critical for the better understanding of different development and disease processes. We have recently developed a type of DNA-based molecular probe for measuring intercellular tensile forces in 2D cell models. Herein, we will report the further development and first-time usage of these molecular tension probes to visualize and detect mechanical forces within 3D spheroids and embryoid bodies (EBs). These probes can spontaneously anchor onto live cell membranes via the attached lipid moieties. By varying the concentrations of these DNA probes and their incubation time, we have first characterized the kinetics and efficiency of probe penetration and loading onto tumor spheroids and stem cell EBs of different sizes. After optimization, we have further imaged and measured E-cadherin-mediated forces in these 3D spheroids and EBs for the first time. Our results indicated that these DNA-based molecular tension probes can be used to study the spatiotemporal distributions of target mechanotransduction processes. These powerful imaging tools may be potentially applied to fill the gap between ongoing research of biomechanics in 2D systems and that in real 3D cell complexes.

Tension-tuned receptors for synthetic mechanotransduction and intercellular force detection

Cells interpret mechanical stimuli from their environments and neighbors, but the ability to engineer customized mechanosensing capabilities has remained a synthetic and mechanobiology challenge. Here we introduce tension-tuned synthetic Notch (SynNotch) receptors to convert extracellular and intercellular forces into specifiable gene expression changes. By elevating the tension requirements of SynNotch activation, in combination with structure-guided mutagenesis, we designed a set of receptors with mechanical sensitivities spanning the physiologically relevant picoNewton range. Cells expressing these receptors can distinguish between varying tensile forces and respond by enacting customizable transcriptional programs. We applied these tools to design a decision-making circuit, through which fibroblasts differentiate into myoblasts upon stimulation with distinct tension magnitudes. We also characterize cell-generated forces transmitted between cells during Notch signaling. Overall, this work provides insight into how mechanically induced changes in protein structure can be used to transduce physical forces into biochemical signals. The system should facilitate the further programming and dissection of force-related phenomena in biological systems.

Advanced Mechanical Testing Technologies at the Cellular Level: The Mechanisms and Application in Tissue Engineering

Mechanics, as a key physical factor which affects cell function and tissue regeneration, is attracting the attention of researchers in the fields of biomaterials, biomechanics, and tissue engineering. The macroscopic mechanical properties of tissue engineering scaffolds have been studied and optimized based on different applications. However, the mechanical properties of the overall scaffold materials are not enough to reveal the mechanical mechanism of the cell-matrix interaction. Hence, the mechanical detection of cell mechanics and cellular-scale microenvironments has become crucial for unraveling the mechanisms which underly cell activities and which are affected by physical factors. This review mainly focuses on the advanced technologies and applications of cell-scale mechanical detection. It summarizes the techniques used in micromechanical performance analysis, including atomic force microscope (AFM), optical tweezer (OT), magnetic tweezer (MT), and traction force microscope (TFM), and analyzes their testing mechanisms. In addition, the application of mechanical testing techniques to cell mechanics and tissue engineering scaffolds, such as hydrogels and porous scaffolds, is summarized and discussed. Finally, it highlights the challenges and prospects of this field. This review is believed to provide valuable insights into micromechanics in tissue engineering.

All-Covalent Nuclease-Resistant and Hydrogel-Tethered DNA Hairpin Probes Map pN Cell Traction Forces

Cells sense and respond to the physical properties of their environment through receptor-mediated signaling, a process known as mechanotransduction, which can modulate critical cellular functions such as proliferation, differentiation, and survival. At the molecular level, cell adhesion receptors, such as integrins, transmit piconewton (pN)-scale forces to the extracellular matrix, and the magnitude of the force plays a critical role in cell signaling. The most sensitive approach to measuring integrin forces involves DNA hairpin-based sensors, which are used to quantify and map forces in living cells. Despite the broad use of DNA hairpin sensors to study a variety of mechanotransduction processes, these sensors are typically anchored to rigid glass slides, which are orders of magnitude stiffer than the extracellular matrix and hence modulate native biological responses. Here, we have developed nuclease-resistant DNA hairpin probes that are all covalently tethered to PEG hydrogels to image cell traction forces on physiologically relevant substrate stiffness. Using HeLa cells as a model cell line, we show that the molecular forces transmitted by integrins are highly sensitive to the bulk modulus of the substrate, and cells cultured on the 6 and 13 kPa gels produced a greater number of hairpin unfolding events compared to the 2 kPa substrates. Tension signals are spatially colocalized with pY118-paxillin, confirming focal adhesion-mediated probe opening. Additionally, we found that integrin forces are greater than 5.8 pN but less than 19 pN on 13 kPa gels. This work provides a general strategy to integrate molecular tension probes into hydrogels, which can better mimic in vivo mechanotransduction.

RAD-TGTs: high-throughput measurement of cellular mechanotype via rupture and delivery of DNA tension probes

Mechanical forces drive critical cellular processes that are reflected in mechanical phenotypes, or mechanotypes, of cells and their microenvironment. We present here "Rupture And Deliver" Tension Gauge Tethers (RAD-TGTs) in which flow cytometry is used to record the mechanical history of thousands of cells exerting forces on their surroundings via their propensity to rupture immobilized DNA duplex tension probes. We demonstrate that RAD-TGTs recapitulate prior DNA tension probe studies while also yielding a gain of fluorescence in the force-generating cell that is detectable by flow cytometry. Furthermore, the rupture propensity is altered following disruption of the cytoskeleton using drugs or CRISPR-knockout of mechanosensing proteins. Importantly, RAD-TGTs can differentiate distinct mechanotypes among mixed populations of cells. We also establish oligo rupture and delivery can be measured via DNA sequencing. RAD-TGTs provide a facile and powerful assay to enable high-throughput mechanotype profiling, which could find various applications, for example, in combination with CRISPR screens and -omics analysis.

Tension Gauge Tethers as Tension Threshold and Duration Sensors

Mechanotransduction, the process by which cells respond to tension transmitted through various supramolecular linkages, is important for understanding cellular behavior. Tension gauge tethers (TGTs), short fragments of double-stranded DNA that irreversibly break under shear-stretch conditions, have been used in live cell experiments to study mechanotransduction. However, our current understanding of TGTs' mechanical responses is limited, which limits the information that can be gleaned from experimental observations. In this study, we quantified the tension-dependent lifetime of TGTs to better understand their mechanical stability under various physiologically relevant stretching conditions. This work has broad applications for using TGTs as tension threshold and duration sensors and also suggests the need to revisit previous interpretations of experimental observations.

Recent advances in label-free imaging of cell-matrix adhesions

Cell-matrix adhesions play an essential role in mediating and regulating many biological processes. The adhesion receptors, typically transmembrane integrins, provide dynamic correlations between intracellular environments and extracellular matrixes (ECMs) by bi-directional signaling. In-depth investigations of cell-matrix adhesion and integrin-mediated cell adhesive force are of great significance in biology and medicine. The emergence of advanced imaging techniques and principles has facilitated the understanding of the molecular composition and structure dynamics of cell-matrix adhesions, especially the label-free imaging methods that can be used to study living cell dynamics without immunofluorescence staining. This highlight article aims to give an overview of recent developments in imaging cell-matrix adhesions in a label-free manner. Electrochemiluminescence microscopy (ECLM) and surface plasmon resonance microscopy (SPRM) are briefly introduced and their applications in imaging analysis of cell-matrix adhesions are summarized. Then we highlight the advances in mapping cell-matrix adhesion force based on molecular tension probes and fluorescence microscopy (collectively termed as MTFM). The biomaterials including polyethylene glycol (PEG), peptides and DNA for constructing tension probes in MTFM are summarized. Finally, the outlook and perspectives on the further developments of cell-matrix adhesion imaging are presented.

Facile detection of mechanical forces across proteins in cells with STReTCh

Numerous proteins experience and respond to mechanical forces as an integral part of their cellular functions, but measuring these forces remains a practical challenge. Here, we present a compact, 11-kDa molecular tension sensor termed STReTCh (sensing tension by reactive tag characterization). Unlike existing genetically encoded tension sensors, STReTCh does not rely on experimentally demanding measurements based on Förster resonance energy transfer and is compatible with typical fix-and-stain protocols. Using a magnetic tweezers assay, we calibrate the STReTCh module and show that it responds to physiologically relevant, piconewton forces. As proof of concept, we use an extracellular STReTCh-based sensor to visualize cell-generated forces at integrin-based adhesion complexes. In addition, we incorporate STReTCh into vinculin, a cytoskeletal adaptor protein, and show that STReTCh reports on forces transmitted between the cytoskeleton and cellular adhesion complexes. These data illustrate the utility of STReTCh as a tool for visualizing molecular-scale forces in biological systems.

Recent developments in DNA-based mechanical nanodevices

Cellular processes and functions can be regulated by mechanical forces. Nanodevices that can measure and manipulate these forces are critical tools in chemical and cellular biology. Synthetic DNA oligonucleotides have been used to develop a wide range of powerful nanodevices due to their programmable nature and precise and predictable self-assembly. In recent years, various types of DNA-based mechanical nanodevices have been engineered for studying molecular-level forces. With the help of these nanodevices, our understanding of cellular responses to physical forces has been significantly advanced. In this article, we have reviewed some recent developments in DNA-based mechanical sensors and regulators for application in the characterization of cellular biomechanics and the manipulation of cellular morphology, motion and other functions. The design principles discussed in this article can be further used to inspire other types of powerful DNA-based mechanical nanodevices.

Immobilization of Jagged1 Enhances Vascular Smooth Muscle Cells Maturation by Activating the Notch Pathway

In Notch signaling, the Jagged1-Notch3 ligand-receptor pairing is implicated for regulating the phenotype maturity of vascular smooth muscle cells. However, less is known about the role of Jagged1 presentation strategy in this regulation. In this study, we used bead-immobilized Jagged1 to direct phenotype control of primary human coronary artery smooth muscle cells (HCASMC), and to differentiate embryonic multipotent mesenchymal progenitor (10T1/2) cell towards a vascular lineage. This Jagged1 presentation strategy was sufficient to activate the Notch transcription factor HES1 and induce early-stage contractile markers, including smooth muscle α-actin and calponin in HCASMCs. Bead-bound Jagged1 was unable to regulate the late-stage markers myosin heavy chain and smoothelin; however, serum starvation and TGFβ1 were used to achieve a fully contractile smooth muscle cell. When progenitor 10T1/2 cells were used for Notch3 signaling, pre-differentiation with TGFβ1 was required for a robust Jagged1 specific response, suggesting a SMC lineage commitment was necessary to direct SMC differentiation and maturity. The presence of a magnetic tension force to the ligand-receptor complex was evaluated for signaling efficacy. Magnetic pulling forces downregulated HES1 and smooth muscle α-actin in both HCASMCs and progenitor 10T1/2 cells. Taken together, this study demonstrated that (i) bead-bound Jagged1 was sufficient to activate Notch3 and promote SMC differentiation/maturation and (ii) magnetic pulling forces did not activate Notch3, suggesting the bead alone was able to provide necessary clustering or traction forces for Notch activation. Notch is highly context-dependent; therefore, these findings provide insights to improve biomaterial-driven Jagged1 control of SMC behavior.

Quantification of fast molecular adhesion by fluorescence footprinting

Rolling adhesion is a unique process in which the adhesion events are short-lived and operate under highly nonequilibrium conditions. These characteristics pose a challenge in molecular force quantification, where in situ measurement of these forces cannot be achieved with molecular force sensors that probe near equilibrium. Here, we demonstrated a quantitative adhesion footprint assay combining DNA-based nonequilibrium force probes and modeling to measure the molecular force involved in fast rolling adhesion. We were able to directly profile the ensemble molecular force distribution in our system during rolling adhesion with a dynamic range between 0 and 18 pN. Our results showed that the shear stress driving bead rolling motility directly controls the molecular tension on the probe-conjugated adhesion complex. Furthermore, the shear stress can steer the dissociation bias of components within the molecular force probe complex, favoring either DNA probe dissociation or receptor-ligand dissociation.

Molecular Nanomechanical Mapping of Histamine-Induced Smooth Muscle Cell Contraction and Shortening

Mechanical response to external stimuli is a conserved feature of many cell types. For example, neurotransmitters (e.g., histamine) trigger calcium signals that induce actomyosin-regulated contraction of airway smooth muscle (ASM); the resulting cell shortening causes airway narrowing, the excess of which can cause asthma. Despite intensive studies, however, it remains unclear how physical forces are propagated through focal adhesion (FA)-the major force-transmission machinery of the cell-during ASM shortening. We provide a nanomechanical platform to directly image single molecule forces during ASM cell shortening by repurposing DNA tension sensors. Surprisingly, cell shortening and FA disassembly that immediately precedes it occurred long after histamine-evoked increases in intracellular calcium levels ([Ca2+]i). Our mathematical model that fully integrates cell edge protrusion and retraction with contractile forces acting on FA predicted that (1) stabilization of FA impedes cell shortening and (2) the disruption of FAs is preceded by their strengthening through actomyosin-activated molecular tension. We confirmed these predictions via real-time imaging and molecular force measurements. Together, our work highlights a key role of FA dynamics in regulating ASM contraction induced by an allergen with potential therapeutic implications.

Cytoskeletal prestress: The cellular hallmark in mechanobiology and mechanomedicine

Increasing evidence demonstrates that mechanical forces, in addition to soluble molecules, impact cell and tissue functions in physiology and diseases. How living cells integrate mechanical signals to perform appropriate biological functions is an area of intense investigation. Here, we review the evidence of the central role of cytoskeletal prestress in mechanotransduction and mechanobiology. Elevating cytoskeletal prestress increases cell stiffness and reinforces cell stiffening, facilitates long-range cytoplasmic mechanotransduction via integrins, enables direct chromatin stretching and rapid gene expression, spurs embryonic development and stem cell differentiation, and boosts immune cell activation and killing of tumor cells whereas lowering cytoskeletal prestress maintains embryonic stem cell pluripotency, promotes tumorigenesis and metastasis of stem cell-like malignant tumor-repopulating cells, and elevates drug delivery efficiency of soft-tumor-cell-derived microparticles. The overwhelming evidence suggests that the cytoskeletal prestress is the governing principle and the cellular hallmark in mechanobiology. The application of mechanobiology to medicine (mechanomedicine) is rapidly emerging and may help advance human health and improve diagnostics, treatment, and therapeutics of diseases.

Quantitative interpretation of cell rolling velocity distribution

Leukocyte rolling adhesion, facilitated by selectin-mediated interactions, is a highly dynamic process in which cells roll along the endothelial surface of blood vessel walls to reach the site of infection. The most common approach to investigate cell-substrate adhesion is to analyze the cell rolling velocity in response to shear stress changes. It is assumed that changes in rolling velocity indicate changes in adhesion strength. In general, cell rolling velocity is studied at the population level as an average velocity corresponding to given shear stress. However, no statistical investigation has been performed on the instantaneous velocity distribution. In this study, we first developed a method to remove systematic noise and revealed the true velocity distribution to exhibit a log-normal profile. We then demonstrated that the log-normal distribution describes the instantaneous velocity at both the population and single-cell levels across the physiological flow rates. The log-normal parameters capture the cell motion more accurately than the mean and median velocities, which are prone to systematic error. Lastly, we connected the velocity distribution to the molecular adhesion force distribution and showed that the slip-bond regime of the catch-slip behavior of the P-selectin/PSGL-1 interaction is responsible for the variation of cell velocity.

Multivalent molecular tension probes as anisotropic mechanosensors: concept and simulation

Cells use protein-based mechanosensors to measure the physical properties of their surroundings. Synthetic tension sensors made of proteins, DNA, and other molecular building blocks have recently emerged as tools to visualize and perturb the mechanics of these mechanosensors. While almost all synthetic tension sensors are designed to exhibit orientation-independent force responses, recent work has shown that biological mechanosensors often function in a manner that is highly dependent on force orientation. Accordingly, the design of synthetic mechanosensors with orientation-dependent force responses can provide a means to study the role of orientation in mechanosensation. Furthermore, the process of designing anisotropic force responses may yield insight into the physical basis for orientation-dependence in biological mechanosensors. Here, we propose a DNA-based molecular tension sensor design wherein multivalency is used to create an orientation-dependent force response. We apply chemomechanical modeling to show that multivalency can be used to create synthetic mechanosensors with force response thresholds that vary by tens of pN with respect to force orientation.

Recent Advances in Cell Adhesive Force Microscopy

Cell adhesive force, exerting on the local matrix or neighboring cells, plays a critical role in regulating many cell functions and physiological processes. In the past four decades, significant efforts have been dedicated to cell adhesive force detection, visualization and quantification. A recent important methodological advancement in cell adhesive force visualization is to adopt force-to-fluorescence conversion instead of force-to-substrate strain conversion, thus greatly improving the sensitivity and resolution of force imaging. This review summarizes the recent development of force imaging techniques (collectively termed as cell adhesive force microscopy or CAFM here), with a particular focus on the improvement of CAFM's spatial resolution and the biomaterial choices for constructing the tension sensors used in force visualization. This review also highlights the importance of DNA-based tension sensors in cell adhesive force imaging and the recent breakthrough in the development of super-resolution CAFM.

Mechanical Flexibility of DNA: A Quintessential Tool for DNA Nanotechnology

The mechanical properties of DNA have enabled it to be a structural and sensory element in many nanotechnology applications. While specific base-pairing interactions and secondary structure formation have been the most widely utilized mechanism in designing DNA nanodevices and biosensors, the intrinsic mechanical rigidity and flexibility are often overlooked. In this article, we will discuss the biochemical and biophysical origin of double-stranded DNA rigidity and how environmental and intrinsic factors such as salt, temperature, sequence, and small molecules influence it. We will then take a critical look at three areas of applications of DNA bending rigidity. First, we will discuss how DNA's bending rigidity has been utilized to create molecular springs that regulate the activities of biomolecules and cellular processes. Second, we will discuss how the nanomechanical response induced by DNA rigidity has been used to create conformational changes as sensors for molecular force, pH, metal ions, small molecules, and protein interactions. Lastly, we will discuss how DNA's rigidity enabled its application in creating DNA-based nanostructures from DNA origami to nanomachines.

Force-dependent trans-endocytosis by breast cancer cells depletes costimulatory receptor CD80 and attenuates T cell activation

In this study, we investigated the biophysical interaction between cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) and CD80. CTLA-4 is a key molecule in immunosuppression, and CD80 is a costimulatory receptor promoting T cell activation. We observed that after cell-cell contact was established between breast cancer cells and antigen presenting cells (APCs), CTLA-4 expressed on the breast cancer cells bind to CD80 expressed on the APCs, and underwent trans-endocytosis to deplete CD80. Force measurement and live cell imaging revealed that upon binding to CD80, forces generated by breast cancer cells and transmitted via CTLA-4 were sufficiently strong to displace CD80 from the surface of APCs to be internalized by breast cancer cells. We further demonstrated that because of the force-dependent trans-endocytosis of CD80, the capacity of APCs to activate T cells was significantly attenuated. Furthermore, inhibiting force generation in cancer cells would increase the T cell activating capacity of APCs. Our results provide a possible mechanism behind the immunosuppression commonly seen in breast cancer patients, and may lead to a new strategy to restore anti-tumor immunity by inhibiting pathways of force-generation.

Quantifying tensile forces at cell-cell junctions with a DNA-based fluorescent probe

Cells are physically contacting with each other. Direct and precise quantification of forces at cell–cell junctions is still challenging. Herein, we have developed a DNA-based ratiometric fluorescent probe, termed DNAMeter, to quantify intercellular tensile forces. These lipid-modified DNAMeters can spontaneously anchor onto live cell membranes. The DNAMeter consists of two self-assembled DNA hairpins of different force tolerance. Once the intercellular tension exceeds the force tolerance to unfold a DNA hairpin, a specific fluorescence signal will be activated, which enables the real-time imaging and quantification of tensile forces. Using E-cadherin-modified DNAMeter as an example, we have demonstrated an approach to quantify, at the molecular level, the magnitude and distribution of E-cadherin tension among epithelial cells. Compatible with readily accessible fluorescence microscopes, these easy-to-use DNA tension probes can be broadly used to quantify mechanotransduction in collective cell behaviors.

A DNA-based fluorescent probe to quantify the magnitude and distribution of tensile forces at cell–cell junctions.

EGFR activation attenuates the mechanical threshold for integrin tension and focal adhesion formation

Mechanical forces, growth factors and the extracellular matrix all play crucial roles in cell adhesion. To understand how epidermal growth factor receptor (EGFR) impacts the mechanics of adhesion, we employed tension gauge tether (TGT) probes displaying the integrin ligand cRGDfK and quantified integrin tension. EGF exposure significantly increased spread area, cell circularity, integrated integrin tension, mechanical rupture density, radial organization and size of focal adhesions in Cos-7 cells on TGT surfaces. These findings suggest that EGFR regulates integrin tension and the spatial organization of focal adhesions. Additionally, we found that the mechanical tension threshold for outside-in integrin activation is tunable by EGFR. Parallel genetic and pharmacologic strategies demonstrated that these phenotypes are driven by ligand-dependent EGFR signaling. Our results establish a novel mechanism whereby EGFR regulates integrin activation and cell adhesion, providing control over cellular responses to the environment.This article has an associated First Person interview with the first author of the paper.

Quantifying molecular tension-classifications, interpretations and limitations of force sensors

Molecular force sensors (MFSs) have grown to become an important tool to study the mechanobiology of cells and tissues. They provide a minimally invasive means to optically report mechanical interactions at the molecular level. One of the challenges in molecular force sensor studies is the interpretation of the fluorescence readout. In this review, we divide existing MFSs into three classes based on the force-sensing mechanism (reversibility) and the signal output (analog/digital). From single-molecule force spectroscopy (SMFS) perspectives, we provided a critical discussion on how the sensors respond to force and how the different sensor designs affect the interpretation of their fluorescence readout. Lastly, the review focuses on the limitations and attention one must pay in designing MFSs and biological experiments using them; in terms of their tunability, signal-to-noise ratio (SNR), and perturbation of the biological system under investigation.

Molecular Tension Sensors: Moving Beyond Force

Nearly all cellular processes are sensitive to mechanical inputs, and this plays a major role in diverse physiological processes. Mechanical stimuli are thought to be primarily detected through force-induced changes in protein structure. Approximately a decade ago, molecular tension sensors were created to measure forces across proteins within cells. Since then, an impressive assortment of sensors has been created and provided key insights into mechanotransduction, but comparisons of measurements between various sensors are challenging. In this review, we discuss the different types of molecular tension sensors, provide a system of classification based on their molecular-scale mechanical properties, and highlight how new applications of these sensors are enabling measurements beyond the magnitude of tensile load. We suggest that an expanded understanding of the functionality of these sensors, as well as integration with other techniques, will lead to consensus amongst measurements as well as critical insights into the underlying mechanisms of mechanotransduction.

Highly Polyvalent DNA Motors Generate 100+ pN of Force via Autochemophoresis

Motor proteins such as myosin, kinesin, and dynein are essential to eukaryotic life and power countless processes including muscle contraction, wound closure, cargo transport, and cell division. The design of synthetic nanomachines that can reproduce the functions of these motors is a longstanding goal in the field of nanotechnology. DNA walkers, which are programmed to "walk" along defined tracks via the burnt bridge Brownian ratchet mechanism, are among the most promising synthetic mimics of these motor proteins. While these DNA-based motors can perform useful tasks such as cargo transport, they have not been shown to be capable of cooperating to generate large collective forces for tasks akin to muscle contraction. In this work, we demonstrate that highly polyvalent DNA motors (HPDMs), which can be viewed as cooperative teams of thousands of DNA walkers attached to a microsphere, can generate and sustain substantial forces in the 100+ pN regime. Specifically, we show that HPDMs can generate forces that can unzip and shear DNA duplexes (∼12 and ∼50 pN, respectively) and rupture biotin-streptavidin bonds (∼100-150 pN). To help explain these results, we present a variant of the burnt-bridge Brownian ratchet mechanism that we term autochemophoresis, wherein many individual force generating units generate a self-propagating chemomechanical gradient that produces large collective forces. In addition, we demonstrate the potential of this work to impact future engineering applications by harnessing HPDM autochemophoresis to deposit "molecular ink" via mechanical bond rupture. This work expands the capabilities of synthetic DNA motors to mimic the force-generating functions of biological motors. Our work also builds upon previous observations of autochemophoresis in bacterial transport processes, indicating that autochemophoresis may be a fundamental mechanism of pN-scale force generation in living systems.

A Supramolecular Platform for the Introduction of Fc-Fusion Bioactive Proteins on Biomaterial Surfaces

Bioorthogonal chemistry is an excellent method for functionalization of biomaterials with bioactive molecules, as it allows for decoupling of material processing and bioactivation. Here, we report on a modular system created by means of tetrazine/trans-cyclooctene (Tz/TCO) click chemistry undergoing an inverse electron demand Diels-Alder cycloaddition. A reactive supramolecular surface based on ureido-pyrimidinones (UPy) is generated via a UPy-Tz additive, in order to introduce a versatile TCO-protein G conjugate for immobilization of Fc-fusion proteins. As a model bioactive protein, we introduced Fc-Jagged1, a Notch ligand, to induce Notch signaling activity on the material. Interestingly, HEK293 FLN1 cells expressing the Notch1 receptor were repelled by films modified with TCO-protein G but adhered and spread on functionalized electrospun meshes. This indicates that the material processing method influences the biocompatibility of the postmodification. Notch signaling activity was upregulated 5.6-fold with respect to inactive controls on electrospun materials modified with TCO-protein G/Fc-Jagged1. Furthermore, downstream effects of Notch signaling were detected on the gene level in vascular smooth muscle cells expressing the Notch3 receptor. Taken together, our results demonstrate the successful use of a modular supramolecular system for the postprocessing modification of solid materials with functional proteins.

A Uniform Shear Assay for Human Platelet and Cell Surface Receptors via Cone-plate Viscometry

Many biological cells/tissues sense the mechanical properties of their local environments via mechanoreceptors, proteins that can respond to forces like pressure or mechanical perturbations. Mechanoreceptors detect their stimuli and transmit signals via a great diversity of mechanisms. Some of the most common roles for mechanoreceptors are in neuronal responses, like touch and pain, or hair cells which function in balance and hearing. Mechanosensation is also important for cell types which are regularly exposed to shear stress such as endothelial cells, which line blood vessels, or blood cells which experience shear in normal circulation. Viscometers are devices that detect the viscosity of fluids. Rotational viscometers may also be used to apply a known shear force to fluids. The ability of these instruments to introduce uniform shear to fluids has been exploited to study many biological fluids including blood and plasma. Viscometry may also be used to apply shear to the cells in a solution, and to test the effects of shear on specific ligand-receptor pairs. Here, we utilize cone-plate viscometry to test the effects of endogenous levels of shear stress on platelets treated with antibodies against the platelet mechanosensory receptor complex GPIb-IX.

DNA Nanotechnology as an Emerging Tool to Study Mechanotransduction in Living Systems

The ease of tailoring DNA nanostructures with sub-nanometer precision has enabled new and exciting in vivo applications in the areas of chemical sensing, imaging, and gene regulation. A new emerging paradigm in the field is that DNA nanostructures can be engineered to study molecular mechanics. This new development has transformed the repertoire of capabilities enabled by DNA to include detection of molecular forces in living cells and elucidating the fundamental mechanisms of mechanotransduction. This Review first describes fundamental aspects of force-induced melting of DNA hairpins and duplexes. This is then followed by a survey of the currently available force sensing DNA probes and different fluorescence-based force readout modes. Throughout the Review, applications of these probes in studying immune receptor signaling, including the T cell receptor and B cell receptor, as well as Notch and integrin signaling, are discussed.

Quantifying Molecular Forces with Serially Connected Force Sensors

To understand the mechanical forces involved in cell adhesion, molecular force sensors have been developed to study tension through adhesion proteins. Recently, a class of molecular force sensors called tension gauge tethers (TGTs) have been developed that rely on irreversible force-dependent dissociation of a DNA duplex to study cell adhesion forces. Although the TGT offers a high signal-to-noise ratio and is ideal for studying fast/single-molecular adhesion processes, quantitative interpretation of experimental results has been challenging. Here, we use a computational approach to investigate how TGT fluorescence readout can be quantitatively interpreted. In particular, we studied force sensors made of a single TGT, multiplexed single TGTs, and two TGTs connected in series. Our results showed that fluorescence readout using a single TGT can result from drastically different combinations of force history and adhesion event density that span orders of magnitude. In addition, the apparent behavior of the TGT is influenced by the tethered receptor-ligand, making it necessary to calibrate the TGT with every new receptor-ligand. To solve this problem, we proposed a system of two serially connected TGTs. Our result shows that not only is the ratiometric readout of serial TGT independent of the choice of receptor-ligand, it is able to reconstruct force history with sub-pN force resolution. This is also not possible by simply multiplexing different types of TGTs together. Last, we systematically investigated how the sequence composition of the two serially connected TGTs can be tuned to achieve different dynamic range. This computational study demonstrated how serially connected irreversible molecular dissociation processes can accurately quantify molecular force and laid the foundation for subsequent experimental studies.

Cdc42-dependent modulation of rigidity sensing and cell spreading in tumor repopulating cells

Recently, a robust mechanical method has been established to isolate a small subpopulation of highly tumorigenic tumor repopulating cells (TRCs) from parental melanoma cells. In order to characterize the molecular and mechanical properties of TRCs, we utilized the tension gauge tether (TGT) single-molecule platform and investigated force requirements during early cell spreading events. TRCs required the peak single molecular tension of around 40 pN through integrins for initial adhesion like the parental control cells, but unlike the control cells, they did not spread and formed very few mature focal adhesions (FAs). Single molecule resolution RNA quantification of three Rho GTPases showed that downregulation of Cdc42, but not Rac1, is responsible for the unusual biophysical features of TRCs and that a threshold level of Cdc42 transcripts per unit cell area is required to initiate cell spreading. Cdc42 overexpression rescued TRC spreading through FA formation and restored the sensitivity to tension cues such that TRCs, like parental control cells, increase cell spreading with increasing single-molecular tension cues. Our single molecule studies identified an unusual biophysical feature of suppressed spreading of TRCs that may enable us to distinguish TRC population from a pool of heterogeneous tumor cell population.

Ultrashort Nucleic Acid Duplexes Exhibit Long Wormlike Chain Behavior with Force-Dependent Edge Effects

Despite their importance in biology and use in nanotechnology, the elastic behavior of nucleic acids on "ultrashort" (<15  nt) length scales remains poorly understood. Here, we use optical tweezers combined with fluorescence imaging to observe directly the hybridization of oligonucleotides (7-12 nt) to a complementary strand under tension and to measure the difference in end-to-end extension between the single-stranded and duplex states. Data are consistent with long-polymer models at low forces (<8  pN) but smaller than predicted at higher forces (>8  pN), the result of the sequence-dependent duplex edge effects.

Molecular Tension Probes for Imaging Forces at the Cell Surface

Mechanical forces are essential for a variety of biological processes ranging from transcription and translation to cell adhesion, migration, and differentiation. Through the activation of mechanosensitive signaling pathways, cells sense and respond to physical stimuli from the surrounding environment, a process widely known as mechanotransduction. At the cell membrane, many signaling receptors, such as integrins, cadherins and T- or B-cell receptors, bind to their ligands on the surface of adjacent cells or the extracellular matrix (ECM) to mediate mechanotransduction. Upon ligation, these receptor-ligand bonds transmit piconewton (pN) mechanical forces that are generated, in part, by the cytoskeleton. Importantly, these forces expose cryptic sites within mechanosensitive proteins and modulate the binding kinetics (on/off rate) of receptor-ligand complexes to further fine-tune mechanotransduction and the corresponding cell behavior. Over the past three decades, two categories of methods have been developed to measure cell receptor forces. The first class is traction force microscopy (TFM) and micropost array detectors (mPADs). In these methods, cells are cultured on elastic polymers or microstructures that deform under mechanical forces. The second category of techniques is single molecule force spectroscopy (SMFS) including atomic force microscopy (AFM), optical or magnetic tweezers, and biomembrane force probe (BFP). In SMFS, the experimenter applies external forces to probe the mechanics of individual cells or single receptor-ligand complexes, serially, one bond at a time. Although these techniques are powerful, the limited throughput of SMFS and the nN force sensitivity of TFM have hindered further elucidation of the molecular mechanisms of mechanotransduction. In this Account, we introduce the recent advent of molecular tension fluorescence microscopy (MTFM) as an emerging tool for molecular imaging of receptor mechanics in living cells. MTFM probes are composed of an extendable linker, such as polymer, oligonucleotide, or protein, and flanked by a fluorophore and quencher. By measuring the fluorescence emission of immobilized MTFM probes, one can infer the extension of the linker and the externally applied force. Thus, MTFM combines aspects of TFM and SMFS to optically report receptor forces across the entire cell surface with pN sensitivity. Specifically, we provide an in-depth review of MTFM probe design, which includes the extendable "spring", spectroscopic ruler, surface immobilization chemistry, and ligand design strategies. We also demonstrate the strengths and weaknesses of different versions of MTFM probes by discussing case studies involving the pN forces involved in epidermal growth factor receptor, integrin, and T-cell receptor signaling pathways. Lastly, we present a brief future outlook, primarily from a chemists' perspective, on the challenges and opportunities for the design of next generation MTFM probes.

Mapping cell surface adhesion by rotation tracking and adhesion footprinting

Rolling adhesion, in which cells passively roll along surfaces under shear flow, is a critical process involved in inflammatory responses and cancer metastasis. Surface adhesion properties regulated by adhesion receptors and membrane tethers are critical in understanding cell rolling behavior. Locally, adhesion molecules are distributed at the tips of membrane tethers. However, how functional adhesion properties are globally distributed on the individual cell’s surface is unknown. Here, we developed a label-free technique to determine the spatial distribution of adhesive properties on rolling cell surfaces. Using dark-field imaging and particle tracking, we extract the rotational motion of individual rolling cells. The rotational information allows us to construct an adhesion map along the contact circumference of a single cell. To complement this approach, we also developed a fluorescent adhesion footprint assay to record the molecular adhesion events from cell rolling. Applying the combination of the two methods on human promyelocytic leukemia cells, our results surprisingly reveal that adhesion is non-uniformly distributed in patches on the cell surfaces. Our label-free adhesion mapping methods are applicable to the variety of cell types that undergo rolling adhesion and provide a quantitative picture of cell surface adhesion at the functional and molecular level.

Notch-Jagged complex structure implicates a catch bond in tuning ligand sensitivity

Notch receptor activation initiates cell fate decisions and is distinctive in its reliance on mechanical force and protein glycosylation. The 2.5-angstrom-resolution crystal structure of the extracellular interacting region of Notch1 complexed with an engineered, high-affinity variant of Jagged1 (Jag1) reveals a binding interface that extends ~120 angstroms along five consecutive domains of each protein. O-Linked fucose modifications on Notch1 epidermal growth factor-like (EGF) domains 8 and 12 engage the EGF3 and C2 domains of Jag1, respectively, and different Notch1 domains are favored in binding to Jag1 than those that bind to the Delta-like 4 ligand. Jag1 undergoes conformational changes upon Notch binding, exhibiting catch bond behavior that prolongs interactions in the range of forces required for Notch activation. This mechanism enables cellular forces to regulate binding, discriminate among Notch ligands, and potentiate Notch signaling.

Defining Single Molecular Forces Required for Notch Activation Using Nano Yoyo

Notch signaling, involved in development and tissue homeostasis, is activated at the cell-cell interface through ligand-receptor interactions. Previous studies have implicated mechanical forces in the activation of Notch receptor upon binding to its ligand. Here we aimed to determine the single molecular force required for Notch activation by developing a novel low tension gauge tether (LTGT). LTGT utilizes the low unbinding force between single-stranded DNA (ssDNA) and Escherichia coli ssDNA binding protein (SSB) (∼4 pN dissociation force at 500 nm/s pulling rate). The ssDNA wraps around SSB and, upon application of force, unspools from SSB, much like the unspooling of a yoyo. One end of this nano yoyo is attached to the surface though SSB, while the other end presents a ligand. A Notch receptor, upon binding to its ligand, is believed to undergo force-induced conformational changes required for activating downstream signaling. If the required force for such activation is larger than 4 pN, ssDNA will unspool from SSB, and downstream signaling will not be activated. Using these LTGTs, in combination with the previously reported TGTs that rupture double-stranded DNA at defined forces, we demonstrate that Notch activation requires forces between 4 and 12 pN, assuming an in vivo loading rate of 60 pN/s. Taken together, our study provides a direct link between single-molecular forces and Notch activation.