A primer to traction force microscopy

Measuring contractile forces in vascular smooth muscle cells

Vascular smooth muscle cell (VSMC) contractility mediates blood vessel tone. Abnormalities in VSMC function and in blood vessel tone can contribute to a variety of cardiovascular diseases. This review examines the role of VSMC contractile force in vascular disease, divided into two primary sections. The first section introducing VSMC mechanical contraction and detailing the molecular mechanisms of VSMC contractility in normal and pathological states. The second section exploring methods of measuring contraction in VSMCs, such as Ca2+ imaging, myography, and traction force microscopy, and highlighting where each method is of best use. Understanding the mechanical properties and contractile profiles of VSMCs offers valuable insights into disease mechanisms. By investigating these aspects, this review describes the potential of VSMC contractile forces as diagnostic markers and therapeutic targets in vascular disease.

Contractile responses of engineered human μmyometrium to prostaglandins and inflammatory cytokines

Preterm labor is a prevalent public health problem and occurs when the myometrium, the smooth muscle layer of the uterus, begins contracting before the fetus reaches full term. Abnormal contractions of the myometrium also underlie painful menstrual cramps, known as dysmenorrhea. Both disorders have been associated with increased production of prostaglandins and cytokines, yet the functional impacts of inflammatory mediators on the contractility of human myometrium have not been fully established, in part due to a lack of effective model systems. To address this, we engineered human myometrial microtissues (μmyometrium) on compliant hydrogels designed for traction force microscopy. We then measured μmyometrium contractility in response to a panel of compounds with known contractile effects and inflammatory mediators. We observed that prostaglandin F2α, interleukin 6, and interleukin 8 induced contraction, while prostaglandin E1 and prostaglandin E2 induced relaxation. Our data suggest that inflammation may be a key factor modulating uterine contractility in conditions including, but not limited to, preterm labor or dysmenorrhea. More broadly, our μmyometrium model can be used to systematically identify the functional impact of many small molecules on human myometrium.

Advances in non-small cell lung cancer mechanomedicine: deciphering the signaling networks that govern tumor-TME interactions

Cells from the tumor microenvironment (TME) interact with tumor cells in non-small cell lung cancer (NSCLC) to form a reciprocal crosstalk which influences tumor growth, proliferation, metastasis and multidrug response. This crosstalk is modulated by TME mechanical inputs, which elicit the processes of mechanosensing and mechanotransduction. Recent advances in unveiling these signaling networks establish the interdisciplinary field of mechanomedicine to exploit emerging diagnostic, predictive and therapeutic tools for more effective NSCLC treatments.

Important functions and molecular mechanisms of aquaporins family on respiratory diseases: potential translational values

Aquaporins (AQPs) are a subgroup of small transmembrane transporters that are distributed in various types of tissues, including the lung, kidney, heart and central nervous system. It is evident that respiratory diseases represent a significant global health concern, with a considerable number of deaths occurring worldwide. Recent researches have demonstrated that AQPs play a pivotal role in respiratory diseases, including chronic obstructive pulmonary disease (COPD), asthma, acute respiratory distress syndrome (ARDS), and particularly non-small cell lung cancer (NSCLC). In the context of NSCLC, the overexpression of AQP1, AQP3, AQP4, and AQP5 has been demonstrated to facilitate tumor angiogenesis, as well as the proliferation, migration, and invasiveness of tumor cells. This review concisely explores the role of AQP family on respiratory diseases, to assess their clinical and translational significance for understanding molecular pathogenesis. However, the potential translation of AQPs biomarkers into clinical applications is promising and the understanding of the precise mechanisms influencing respiratory diseases is still ongoing. Addressing the challenges and outlining the future perspectives in AQPs development is essential for clinical progress in a concise manner.

Cell-substrate distance fluctuations of confluent cells enable fast and coherent collective migration

Collective cell migration is an emergent phenomenon, with long-range cell-cell communication influenced by various factors, including transmission of forces, viscoelasticity of individual cells, substrate interactions, and mechanotransduction. We investigate how alterations in cell-substrate distance fluctuations, cell-substrate adhesion, and traction forces impact the average velocity and temporal-spatial correlation of confluent monolayers formed by either wild-type (WT) MDCKII cells or zonula occludens (ZO)-1/2-depleted MDCKII cells (double knockdown [dKD]) representing highly contractile cells. The data indicate that confluent dKD monolayers exhibit decreased average velocity compared to less contractile WT cells concomitant with increased substrate adhesion, reduced traction forces, a more compact shape, diminished cell-cell interactions, and reduced cell-substrate distance fluctuations. Depletion of basal actin and myosin further supports the notion that short-range cell-substrate interactions, particularly fluctuations driven by basal actomyosin, significantly influence the migration speed of the monolayer on a larger length scale.

Massive field-of-view sub-cellular traction force videography enabled by Single-Pixel Optical Tracers (SPOT)

We report a massive field-of-view and high-speed videography platform for measuring the sub-cellular traction forces of more than 10,000 biological cells over 13 mm2 at 83 frames per second. Our Single-Pixel Optical Tracers (SPOT) tool uses 2-dimensional diffraction gratings embedded into a soft substrate to convert cells' mechanical traction force into optical colors detectable by a video camera. The platform measures the sub-cellular traction forces of diverse cell types, including tightly connected tissue sheets and near isolated cells. We used this platform to explore the mechanical wave propagation in a tightly connected sheet of Neonatal Rat Ventricular Myocytes (NRVMs) and discovered that the activation time of some tissue regions are heterogeneous from the overall spiral wave behavior of the cardiac wave.

Exploiting Matrix Stiffness to Overcome Drug Resistance

Drug resistance is arguably one of the biggest challenges facing cancer research today. Understanding the underlying mechanisms of drug resistance in tumor progression and metastasis are essential in developing better treatment modalities. Given the matrix stiffness affecting the mechanotransduction capabilities of cancer cells, characterization of the related signal transduction pathways can provide a better understanding for developing novel therapeutic strategies. In this review, we aimed to summarize the recent advancements in tumor matrix biology in parallel to therapeutic approaches targeting matrix stiffness and its consequences in cellular processes in tumor progression and metastasis. The cellular processes governed by signal transduction pathways and their aberrant activation may result in activating the epithelial-to-mesenchymal transition, cancer stemness, and autophagy, which can be attributed to drug resistance. Developing therapeutic strategies to target these cellular processes in cancer biology will offer novel therapeutic approaches to tailor better personalized treatment modalities for clinical studies.

Optical mapping of ground reaction force dynamics in freely behaving Drosophila melanogaster larvae

During locomotion, soft-bodied terrestrial animals solve complex control problems at substrate interfaces, but our understanding of how they achieve this without rigid components remains incomplete. Here, we develop new all-optical methods based on optical interference in a deformable substrate to measure ground reaction forces (GRFs) with micrometre and nanonewton precision in behaving Drosophila larvae. Combining this with a kinematic analysis of substrate-interfacing features, we shed new light onto the biomechanical control of larval locomotion. Crawling in larvae measuring ~1 mm in length involves an intricate pattern of cuticle sequestration and planting, producing GRFs of 1-7 µN. We show that larvae insert and expand denticulated, feet-like structures into substrates as they move, a process not previously observed in soft-bodied animals. These 'protopodia' form dynamic anchors to compensate counteracting forces. Our work provides a framework for future biomechanics research in soft-bodied animals and promises to inspire improved soft-robot design.

Uncertainty-Aware Traction Force Microscopy

Traction Force Microscopy (TFM) is a versatile tool to quantify cell-exerted forces by imaging and tracking fiduciary markers embedded in elastic substrates. The computations involved in TFM are ill-conditioned, and data smoothing or regularization is required to avoid overfitting the noise in the tracked substrate displacements. Most TFM calculations depend critically on the heuristic selection of regularization (hyper)parameters affecting the balance between overfitting and smoothing. However, TFM methods rarely estimate or account for measurement errors in substrate deformation to adjust the regularization level accordingly. Moreover, there is a lack of tools to quantify how these errors propagate to the recovered traction stresses. These limitations make it difficult to interpret TFM readouts and hinder comparing different experiments. This manuscript presents an uncertainty-aware TFM technique that estimates the variability in the magnitude and direction of the traction stress vector recovered at each point in space and time of each experiment. In this technique, substrate deformation and its uncertainty are quantified using a non-parametric bootstrap PIV method by resampling the microscopy image pixels (PIV-UQ). This information is passed to a hierarchical Bayesian framework that automatically selects its hyperparameters to perform spatially adaptive regularization conditioned on image quality and propagates the uncertainty to the traction stress readouts (TFM-UQ). We validate the performance of PIV-UQ and TFM-UQ using synthetic datasets with prescribed image quality variations and demonstrate the application of PIV-UQ and TFM-UQ to experimental datasets. These studies show that TFM-UQ locally adapts the level of smoothing, outperforming traditional regularization methods. They also illustrate how uncertainty-aware TFM tools can be used to objectively choose key image analysis parameters like PIV-UQ interrogation window size. We anticipate that these tools will allow for decoupling biological heterogeneity from measurement variability and facilitate automating the analysis of large datasets by parameter-free, input data-based regularization.

Multimodal analysis of traction forces and the temperature dynamics of living cells with a diamond-embedded substrate

Cells and tissues are constantly exposed to chemical and physical signals that regulate physiological and pathological processes. This study explores the integration of two biophysical methods: traction force microscopy (TFM) and optically detected magnetic resonance (ODMR) to concurrently assess cellular traction forces and the local relative temperature. We present a novel elastic substrate with embedded nitrogen-vacancy microdiamonds that facilitate ODMR-TFM measurements. Optimization efforts focused on minimizing sample illumination and experiment duration to mitigate biological perturbations. Our hybrid ODMR-TFM technique yields TFM maps and achieves approximately 1 K precision in relative temperature measurements. Our setup employs a simple wide-field fluorescence microscope with standard components, demonstrating the feasibility of the proposed technique in life science laboratories. By elucidating the physical aspects of cellular behavior beyond the existing methods, this approach opens avenues for a deeper understanding of cellular processes and may inspire the development of diverse biomedical applications.

3D Traction Force Microscopy in Biological Gels: From Single Cells to Multicellular Spheroids

Cell traction force plays a critical role in directing cellular functions, such as proliferation, migration, and differentiation. Current understanding of cell traction force is largely derived from 2D measurements where cells are plated on 2D substrates. However, 2D measurements do not recapitulate a vital aspect of living systems; that is, cells actively remodel their surrounding extracellular matrix (ECM), and the remodeled ECM, in return, can have a profound impact on cell phenotype and traction force generation. This reciprocal adaptivity of living systems is encoded in the material properties of biological gels. In this review, we summarize recent progress in measuring cell traction force for cells embedded within 3D biological gels, with an emphasis on cell-ECM cross talk. We also provide perspectives on tools and techniques that could be adapted to measure cell traction force in complex biochemical and biophysical environments.

Enhancing PKA-dependent mesothelial barrier integrity reduces ovarian cancer transmesothelial migration via inhibition of contractility

Cancer-mesothelial cell interactions are critical for multiple solid tumors to colonize the surface of peritoneal organs. Understanding mechanisms of mesothelial barrier dysfunction that impair its protective function is critical for discovering mesothelial-targeted therapies to combat metastatic spread. Here, we utilized a live cell imaging-based assay to elucidate the dynamics of ovarian cancer spheroid transmesothelial migration and mesothelial-generated mechanical forces. Treatment of mesothelial cells with the adenylyl cyclase agonist forskolin strengthens cell-cell junctions, reduces actomyosin fibers, contractility-driven matrix displacements, and cancer spheroid transmigration in a protein kinase A (PKA)-dependent mechanism. We also show that inhibition of the cytoskeletal regulator Rho-associated kinase in mesothelial cells phenocopies the anti-metastatic effects of forskolin. Conversely, upregulation of contractility in mesothelial cells disrupts cell-cell junctions and increases the clearance rates of ovarian cancer spheroids. Our findings demonstrate the critical role of mesothelial cell contractility and mesothelial barrier integrity in regulating metastatic dissemination within the peritoneal microenvironment.

Discrete network models of endothelial cells and their interactions with the substrate

Endothelial cell monolayers line the inner surfaces of blood and lymphatic vessels. They are continuously exposed to different mechanical loads, which may trigger mechanobiological signals and hence play a role in both physiological and pathological processes. Computer-based mechanical models of cells contribute to a better understanding of the relation between cell-scale loads and cues and the mechanical state of the hosting tissue. However, the confluency of the endothelial monolayer complicates these approaches since the intercellular cross-talk needs to be accounted for in addition to the cytoskeletal mechanics of the individual cells themselves. As a consequence, the computational approach must be able to efficiently model a large number of cells and their interaction. Here, we simulate cytoskeletal mechanics by means of molecular dynamics software, generally suitable to deal with large, locally interacting systems. Methods were developed to generate models of single cells and large monolayers with hundreds of cells. The single-cell model was considered for a comparison with experimental data. To this end, we simulated cell interactions with a continuous, deformable substrate, and computationally replicated multistep traction force microscopy experiments on endothelial cells. The results indicate that cell discrete network models are able to capture relevant features of the mechanical behaviour and are thus well-suited to investigate the mechanics of the large cytoskeletal network of individual cells and cell monolayers.

Harnessing atomic force microscopy-based single-cell analysis to advance physical oncology

Single-cell analysis is an emerging and promising frontier in the field of life sciences, which is expected to facilitate the exploration of fundamental laws of physiological and pathological processes. Single-cell analysis allows experimental access to cell-to-cell heterogeneity to reveal the distinctive behaviors of individual cells, offering novel opportunities to dissect the complexity of severe human diseases such as cancers. Among the single-cell analysis tools, atomic force microscopy (AFM) is a powerful and versatile one which is able to nondestructively image the fine topographies and quantitatively measure multiple mechanical properties of single living cancer cells in their native states under aqueous conditions with unprecedented spatiotemporal resolution. Over the past few decades, AFM has been widely utilized to detect the structural and mechanical behaviors of individual cancer cells during the process of tumor formation, invasion, and metastasis, yielding numerous unique insights into tumor pathogenesis from the biomechanical perspective and contributing much to the field of cancer mechanobiology. Here, the achievements of AFM-based analysis of single cancer cells to advance physical oncology are comprehensively summarized, and challenges and future perspectives are also discussed. RESEARCH HIGHLIGHTS: Achievements of AFM in characterizing the structural and mechanical behaviors of single cancer cells are summarized, and future directions are discussed. AFM is not only capable of visualizing cellular fine structures, but can also measure multiple cellular mechanical properties as well as cell-generated mechanical forces. There is still plenty of room for harnessing AFM-based single-cell analysis to advance physical oncology.

Arterial Stiffness: From Basic Primers to Integrative Physiology

The elastic properties of conductance arteries are one of the most important hemodynamic functions in the body, and data continue to emerge regarding the importance of their dysfunction in vascular aging and a range of cardiovascular diseases. Here, we provide new insight into the integrative physiology of arterial stiffening and its clinical consequence. We also comprehensively review progress made on pathways/molecules that appear today as important basic determinants of arterial stiffness, particularly those mediating the vascular smooth muscle cell (VSMC) contractility, plasticity and stiffness. We focus on membrane and nuclear mechanotransduction, clearance function of the vascular wall, phenotypic switching of VSMCs, immunoinflammatory stimuli and epigenetic mechanisms. Finally, we discuss the most important advances of the latest clinical studies that revisit the classical therapeutic concepts of arterial stiffness and lead to a patient-by-patient strategy according to cardiovascular risk exposure and underlying disease.

Rac1 promotes kidney collecting duct repair by mechanically coupling cell morphology to mitotic entry

Prolonged obstruction of the ureter, which leads to injury of the kidney collecting ducts, results in permanent structural damage, while early reversal allows for repair. Cell structure is defined by the actin cytoskeleton, which is dynamically organized by small Rho guanosine triphosphatases (GTPases). In this study, we identified the Rho GTPase, Rac1, as a driver of postobstructive kidney collecting duct repair. After the relief of ureteric obstruction, Rac1 promoted actin cytoskeletal reconstitution, which was required to maintain normal mitotic morphology allowing for successful cell division. Mechanistically, Rac1 restricted excessive actomyosin activity that stabilized the negative mitotic entry kinase Wee1. This mechanism ensured mechanical G2-M checkpoint stability and prevented premature mitotic entry. The repair defects following injury could be rescued by direct myosin inhibition. Thus, Rac1-dependent control of the actin cytoskeleton integrates with the cell cycle to mediate kidney tubular repair by preventing dysmorphic cells from entering cell division.

Micro-engineering and nano-engineering approaches to investigate tumour ecosystems

The interactions among tumour cells, the tumour microenvironment (TME) and non-tumour tissues are of interest to many cancer researchers. Micro-engineering approaches and nanotechnologies are under extensive exploration for modelling these interactions and measuring them in situ and in vivo to investigate therapeutic vulnerabilities in cancer and extend a systemic view of tumour ecosystems. Here we highlight the greatest opportunities for improving the understanding of tumour ecosystems using microfluidic devices, bioprinting or organ-on-a-chip approaches. We also discuss the potential of nanosensors that can transmit information from within the TME or elsewhere in the body to address scientific and clinical questions about changes in chemical gradients, enzymatic activities, metabolic and immune profiles of the TME and circulating analytes. This Review aims to connect the cancer biology and engineering communities, presenting biomedical technologies that may expand the methodologies of the former, while inspiring the latter to develop approaches for interrogating cancer ecosystems.

2.5D Traction Force Microscopy: Imaging three-dimensional cell forces at interfaces and biological applications

The forces that cells, tissues, and organisms exert on the surface of a soft substrate can be measured using Traction Force Microscopy (TFM), an important and well-established technique in Mechanobiology. The usual TFM technique (two-dimensional, 2D TFM) treats only the in-plane component of the traction forces and omits the out-of-plane forces at the substrate interfaces (2.5D) that turn out to be important in many biological processes such as tissue migration and tumour invasion. Here, we review the imaging, material, and analytical tools to perform "2.5D TFM" and explain how they are different from 2D TFM. Challenges in 2.5D TFM arise primarily from the need to work with a lower imaging resolution in the z-direction, track fiducial markers in three-dimensions, and reliably and efficiently reconstruct mechanical stress from substrate deformation fields. We also discuss how 2.5D TFM can be used to image, map, and understand the complete force vectors in various important biological events of various length-scales happening at two-dimensional interfaces, including focal adhesions forces, cell diapedesis across tissue monolayers, the formation of three-dimensional tissue structures, and the locomotion of large multicellular organisms. We close with future perspectives including the use of new materials, imaging and machine learning techniques to continuously improve the 2.5D TFM in terms of imaging resolution, speed, and faithfulness of the force reconstruction procedure.

Extracellular vimentin is sufficient to promote cell attachment, spreading, and motility by a mechanism involving N-acetyl glucosamine-containing structures

Vimentin intermediate filaments form part of the cytoskeleton of mesenchymal cells, but under pathological conditions often associated with inflammation, vimentin filaments depolymerize as the result of phosphorylation or citrullination, and vimentin oligomers are secreted or released into the extracellular environment. In the extracellular space, vimentin can bind surfaces of cells and the extracellular matrix, and the interaction between extracellular vimentin and cells can trigger changes in cellular functions, such as activation of fibroblasts to a fibrotic phenotype. The mechanism by which extracellular vimentin binds external cell membranes and whether vimentin alone can act as an adhesive anchor for cells is largely uncharacterized. Here, we show that various cell types (normal and vimentin null fibroblasts, mesenchymal stem cells, and A549 lung carcinoma cells) attach to and spread on polyacrylamide hydrogel substrates covalently linked to vimentin. Using traction force microscopy and spheroid expansion assays, we characterize how different cell types respond to extracellular vimentin. Cell attachment to and spreading on vimentin-coated surfaces is inhibited by hyaluronic acid degrading enzymes, hyaluronic acid synthase inhibitors, soluble heparin or N-acetyl glucosamine, all of which are treatments that have little or no effect on the same cell types binding to collagen-coated hydrogels. These studies highlight the effectiveness of substrate-bound vimentin as a ligand for cells and suggest that carbohydrate structures, including the glycocalyx and glycosylated cell surface proteins that contain N-acetyl glucosamine, form a novel class of adhesion receptors for extracellular vimentin that can either directly support cell adhesion to a substrate or fine-tune the glycocalyx adhesive properties.

Massively Concurrent Sub-Cellular Traction Force Videography enabled by Single-Pixel Optical Tracers (SPOTs)

We report a large field-of-view and high-speed videography platform for measuring the sub-cellular traction forces of more than 10,000 biological cells over 13mm 2 at 83 frames per second. Our Single-Pixel Optical Tracers (SPOT) tool uses 2-dimensional diffraction gratings embedded into a soft substrate to convert cells' mechanical traction stress into optical colors detectable by a video camera. The platform measures the sub-cellular traction forces of diverse cell types, including tightly connected tissue sheets and near isolated cells. We used this platform to explore the mechanical wave propagation in a tightly connected sheet of Neonatal Rat Ventricular Myocytes (NRVMs) and discovered that the activation time of some tissue regions are heterogeneous from the overall spiral wave behavior of the cardiac wave.

One-Sentence Summary

An optical platform for fast, concurrent measurements of cell mechanics at 83 frames per second, over a large area of 13mm 2 .

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.

Reflections on the Biology of Cell Culture Models: Living on the Edge of Oxidative Metabolism in Cancer Cells

Nowadays, the study of cell metabolism is a hot topic in cancer research. Many studies have used 2D conventional cell cultures for their simplicity and the facility to infer mechanisms. However, the limitations of bidimensional cell cultures to recreate architecture, mechanics, and cell communication between tumor cells and their environment, have forced the development of other more realistic in vitro methodologies. Therefore, the explosion of 3D culture techniques and the necessity to reduce animal experimentation to a minimum has attracted the attention of researchers in the field of cancer metabolism. Here, we revise the limitations of actual culture models and discuss the utility of several 3D culture techniques to resolve those limitations.

Quantifying Immune Cell Force Generation Using Traction Force Microscopy

Immune cells rely on the generation of mechanical force to carry out their function. Consequently, there is a pressing need for quantitative methodologies that permit the probing of the spatio-temporal distribution of mechanical forces generated by immune cells. In this chapter, we provide a guide to quantify immune cell force generation using traction force microscopy (TFM), with a specific focus on its application to the study of the T-cell immunological synapse.

The role of cellular traction forces in deciphering nuclear mechanics

Cellular forces exerted on the extracellular matrix (ECM) during adhesion and migration under physiological and pathological conditions regulate not only the overall cell morphology but also nuclear deformation. Nuclear deformation can alter gene expression, integrity of the nuclear envelope, nucleus-cytoskeletal connection, chromatin architecture, and, in some cases, DNA damage responses. Although nuclear deformation is caused by the transfer of forces from the ECM to the nucleus, the role of intracellular organelles in force transfer remains unclear and a challenging area of study. To elucidate nuclear mechanics, various factors such as appropriate biomaterial properties, processing route, cellular force measurement technique, and micromanipulation of nuclear forces must be understood. In the initial phase of this review, we focused on various engineered biomaterials (natural and synthetic extracellular matrices) and their manufacturing routes along with the properties required to mimic the tumor microenvironment. Furthermore, we discussed the principle of tools used to measure the cellular traction force generated during cell adhesion and migration, followed by recently developed techniques to gauge nuclear mechanics. In the last phase of this review, we outlined the principle of traction force microscopy (TFM), challenges in the remodeling of traction forces, microbead displacement tracking algorithm, data transformation from bead movement, and extension of 2-dimensional TFM to multiscale TFM.