The effect of the endothelial cell cortex on atomic force microscopy measurements

Viscoelastic properties of doxorubicin-treated HT-29 cancer cells by atomic force microscopy: the fractional Zener model as an optimal viscoelastic model for cells

The malignancy of cancer cells and their response to drug treatments have been traditionally studied using solely their elastic properties. However, the study of the combined viscous and elastic properties provides a richer description of the mechanics of the cell, and achieves a more precise assessment of the effect exerted by anti-cancer treatments. We used an atomic force microscope to obtain the morphological, elastic and viscous properties of HT-29 colorectal cancer cells. Changes in these parameters were observed during exposure of the cells to doxorubicin at different times. The elastic properties were analyzed using the Hertz and Sneddon models. Furthermore, we analyzed the data to study the viscoelasticity of the cells comparing the models known as the standard linear solid, fractional Zener, generalized Maxwell, and power law. A discussion about the optimal model based in the accuracy and physical assumptions for this particular system is included. From the morphological data and viscoelasticity of HT-29 cells exposed to doxorubicin, we found that some parameters were affected differently at shorter or longer exposure times. For instance, the relaxation time suggests a measure of the cell to self-heal and it was observed to increase at shorter exposure times and then to reduce for longer exposure times to the drug. The fractional Zener model better described the mechanical properties of the cell due to the reduced number of parameters and the quality of the fit to experimental data.

Mechanical Point Loading Induces Cortex Stiffening and Actin Reorganization

Global cytoskeleton reorganization is well-recognized when cells are exposed to distinct mechanical stimuli, but the localized responses at a specified region of a cell are still unclear. In this work, we mapped the cell-surface mechanical property of single cells in situ before and after static point loading these cells using atomic force microscopy in PeakForce-Quantitative Nano Mechanics mode. Cell-surface stiffness was elevated at a maximum of 1.35-fold at the vicinity of loading site, indicating an enhanced structural protection of the cortex to the cell. Mechanical modeling also elucidated the structural protection from the stiffened cell cortex, in which 9-15% and 10-19% decrease of maximum stress and strain of the nucleus were obtained. Furthermore, the flat-ended atomic force microscopy probes were used to capture cytoskeleton reorganization after point loading quantitatively, revealing that the larger the applied force and the longer the loading time are, the more pronounced cytoskeleton reorganization is. Also, point loading using a microneedle combined with real-time confocal microscopy uncovered the fast dynamics of actin cytoskeleton reorganization for actin-stained live cells after point loading (<10 s). These results furthered the understandings in the transmission of localized mechanical forces into an adherent cell.

Determination of the Elastic Moduli of a Single Cell Cultured on a Rigid Support by Force Microscopy

The elastic response of a living cell is affected by its physiological state. This property provides mechanical fingerprints of a cell's dysfunctionality. The softness (kilopascal range) and thickness (2-15 μm) of mammalian cells imply that the force exerted by the probe might be affected by the stiffness of the solid support. This observation makes infinite sample thickness models unsuitable to describe quantitatively the forces and deformations on a cell. Here, we report a general theory to determine the true Young's moduli of a single cell from a force-indentation curve. Analytical expressions are deduced for common geometries such as flat punches, paraboloids, cones, needles, and nanowires. For a given cell and indentation, the influence of the solid support on the measurements is reduced by using sharp and high aspect ratio tips. The theory is validated by finite element simulations.

Subsurface Imaging of Cell Organelles by Force Microscopy

The development of high-resolution, label-free, noninvasive, and subsurface microscopy methods of living cells remains a formidable problem. Force-microscopy-based stiffness measurements contribute to our understanding of single-cell nanomechanics. The elastic properties of the cell's outer structures, such as the plasma membrane and actin cytoskeleton, dominate stiffness measurements, which in turns prevents the imaging of intracellular structures. We propose that the above limitation could be overcome by combining 2D sections of the cell's viscoelastic properties. We show the simultaneous imaging of the outer cell's cytoskeleton and the organelles inside the nucleus. The elastic component of interaction force carries information on the cell's outer elements as the cortex and the actin cytoskeleton. The inelastic component is sensitive to the hydrodynamic drag of the inner structures such the nucleoli.

Quantifying the Local Mechanical Properties of Cells in a Fibrous Three-Dimensional Microenvironment

Measurements of the mechanical response of biological cells are critical for understanding injury and disease, for developing diagnostic tools, and for computational models in mechanobiology. Although it is well known that cells are sensitive to the topography of their microenvironment, the current paradigm in mechanical testing of adherent cells is mostly limited to specimens grown on flat two-dimensional substrates. In this study, we introduce a technique in which cellular indentation via optical trapping is performed on cells at a high spatial resolution to obtain their regional mechanical properties while they exist in a more favorable three-dimensional microenvironment. We combine our approach with nonlinear contact mechanics theory to consider the effects of a large deformation. This allows us to probe length scales that are relevant for obtaining overall cell stiffness values. The experimental results herein provide the hyperelastic material properties at both high (∼100 s^-1) and low (∼1-10 s^-1) strain rates of murine central nervous system glial cells. The limitations due to possible misalignment of the indenter in the three-dimensional space are examined using a computational model.

Viscoelastic mapping of cells based on fast force volume and PeakForce Tapping

Development of fast force volume (FFV), PeakForce Tapping (PFT), and related AFM techniques allow fast acquisition and mapping of a sample's mechanical properties. The methods are well-suited for studying soft biological samples like living cells in a liquid environment. However, the question remains how the measured mechanical properties are related to those acquired with the classical force volume (FV) technique conducted at low indentation rates. The difference is coming mostly from the pronounced viscoelastic behavior of cells, making apparent elastic parameters depending on the probing rate. Here, the viscoelastic analysis was applied directly to the force curves acquired with force volume or PeakForce Tapping by their post-processing based on the Ting's model. Maps from classical force volume, FFV and PFT obtained using special PFT cantilevers and cantilevers modified with microspheres were compared here. With the correct viscoelastic model, which was found to be the power-law rheology model, all the techniques have provided self-consistent results. The techniques were further modified for the mapping of the viscoelastic model-independent complex Young's modulus.

Dehomogenized Elastic Properties of Heterogeneous Layered Materials in AFM Indentation Experiments

Atomic force microscopy (AFM) is used to study mechanical properties of biological materials at submicron length scales. However, such samples are often structurally heterogeneous even at the local level, with different regions having distinct mechanical properties. Physical or chemical disruption can isolate individual structural elements but may alter the properties being measured. Therefore, to determine the micromechanical properties of intact heterogeneous multilayered samples indented by AFM, we propose the Hybrid Eshelby Decomposition (HED) analysis, which combines a modified homogenization theory and finite element modeling to extract layer-specific elastic moduli of composite structures from single indentations, utilizing knowledge of the component distribution to achieve solution uniqueness. Using finite element model-simulated indentation of layered samples with micron-scale thickness dimensions, biologically relevant elastic properties for incompressible soft tissues, and layer-specific heterogeneity of an order of magnitude or less, HED analysis recovered the prescribed modulus values typically within 10% error. Experimental validation using bilayer spin-coated polydimethylsiloxane samples also yielded self-consistent layer-specific modulus values whether arranged as stiff layer on soft substrate or soft layer on stiff substrate. We further examined a biophysical application by characterizing layer-specific microelastic properties of full-thickness mouse aortic wall tissue, demonstrating that the HED-extracted modulus of the tunica media was more than fivefold stiffer than the intima and not significantly different from direct indentation of exposed media tissue. Our results show that the elastic properties of surface and subsurface layers of microscale synthetic and biological samples can be simultaneously extracted from the composite material response to AFM indentation. HED analysis offers a robust approach to studying regional micromechanics of heterogeneous multilayered samples without destructively separating individual components before testing.

Anisotropy vs isotropy in living cell indentation with AFM

The measurement of local mechanical properties of living cells by nano/micro indentation relies on the foundational assumption of locally isotropic cellular deformation. As a consequence of assumed isotropy, the cell membrane and underlying cytoskeleton are expected to locally deform axisymmetrically when indented by a spherical tip. Here, we directly observe the local geometry of deformation of membrane and cytoskeleton of different living adherent cells during nanoindentation with the integrated Atomic Force (AFM) and spinning disk confocal (SDC) microscope. We show that the presence of the perinuclear actin cap (apical stress fibers), such as those encountered in cells subject to physiological forces, causes a strongly non-axisymmetric membrane deformation during indentation reflecting local mechanical anisotropy. In contrast, axisymmetric membrane deformation reflecting mechanical isotropy was found in cells without actin cap: cancerous cells MDA-MB-231, which naturally lack the actin cap, and NIH 3T3 cells in which the actin cap is disrupted by latrunculin A. Careful studies were undertaken to quantify the effect of the live cell fluorescent stains on the measured mechanical properties. Using finite element computations and the numerical analysis, we explored the capability of one of the simplest anisotropic models – transverse isotropy model with three local mechanical parameters (longitudinal and transverse modulus and planar shear modulus) – to capture the observed non-axisymmetric deformation. These results help identifying which cell types are likely to exhibit non-isotropic properties, how to measure and quantify cellular deformation during AFM indentation using live cell stains and SDC, and suggest modelling guidelines to recover quantitative estimates of the mechanical properties of living cells.

Mapping the creep compliance of living cells with scanning ion conductance microscopy reveals a subcellular correlation between stiffness and fluidity

Living cells exhibit complex material properties, which play a crucial role in many aspects of cell function in health and disease, including migration, proliferation, differentiation, and apoptosis. Various techniques exist to probe the viscoelastic material properties of living cells and a frequent observation is a cell-to-cell correlation between average stiffness and fluidity in populations of cells. However, the origin of this correlation is still under discussion. Here, we introduce an imaging technique based on the scanning ion conductance microscope (SICM) to measure the creep compliance of soft samples, which allowed us to generate images of viscoelastic material properties of living cells with high spatial and temporal resolution. We observe a strong subcellular correlation between the local stiffness and fluidity across the individual living cell: stiff regions exhibit lower fluidity while soft regions exhibit higher fluidity. We find that this subcellular correlation is identical to the previously observed cell-to-cell correlation. The subcellular correlation reversibly vanishes after drug-induced disruption of the cytoskeleton, indicating that the subcellular correlation is a property of the intact cytoskeleton of the living cell.

Mechanical features of endothelium regulate cell adhesive molecule-induced calcium response in neutrophils

Atherosclerosis is caused by chronic inflammation associated with the adhesion of neutrophils and endothelial cells (ECs) that is mediated by their respective cellular adhesive molecules to stiffened blood vessel walls. However, the stiffness dependence of calcium flux on neutrophils remains unclear yet. Here, the effect of substrate stiffness by ECs on neutrophils' calcium spike was quantified when the individual neutrophils that adhered to the human umbilical vascular endothelial cell (HUVEC) monolayer were pre-placed onto a stiffness-varied polyacrylamide substrate (5 or 34.88 kPa) or glass surface. Our data indicated that E-/P-selectins and intercellular adhesion molecule 1 (ICAM-1) on HUVECs and β₂-integrins, P-selectin glycoprotein ligand 1 (PSGL-1), and CD44s on neutrophils were all involved in mediating neutrophil calcium spike in a stiffness-dependent manner, in which the increase in substrate stiffness enhanced the calcium intensity and the oscillation frequency (spike number). Such stiffness-dependent calcium response is associated with the induced selectin related to β₂-integrin activation through the Syk/Src signaling pathway, and F-actin/myosin II are also involved in this. Moreover, tension-activated calcium ion channels displayed critical roles in initiating stiffness-dependent calcium spike. These results provide an insight into understanding how the stiffening of vascular walls could regulate the calcium flux of adhered neutrophils, and thus the immune responses in atherosclerosis.

Quantitative mechanical analysis of indentations on layered, soft elastic materials

Atomic force microscopy (AFM) is becoming an increasingly popular method for studying cell mechanics, however the existing analysis tools for determining the elastic modulus from indentation experiments are unable to quantitatively account for mechanical heterogeneity commonly found in biological samples. In this work, we numerically calculated force-indentation curves onto two-layered elastic materials using an analytic model. We found that the effect of the underlying substrate can be quantitatively predicted by the mismatch in elastic moduli and the homogeneous-case contact radius relative to the layer height for all tested probe geometries. The effect is analogous to one-dimensional Hookean springs in series and was phenomenologically modeled to obtain an approximate closed-form equation for the indentation force onto a two-layered elastic material which is accurate for up to two orders of magnitude mismatch in Young's modulus when the contact radius is less than the layer height. We performed finite element analysis simulations to verify the model and AFM microindentation experiments and macroindentation experiments to demonstrate its ability to deconvolute the Young's modulus of each layer. The model can be broadly used to quantify and serve as a guideline for designing and interpreting indentation experiments into mechanically heterogeneous samples.

AFM study: Cell cycle and probe geometry influences nanomechanical characterization of Panc1 cells

Atomic force microscope (AFM) is emerging as an immensely promising tool to study the cellular morphology with a nanometer scale resolution and to analyze nanomechanical properties (NPs) at various physiological conditions. Advancement of AFM technology enables studying living cells and differentiating cancer cell from normal cells based on topography and NPs. Though the trend overlaps from different literature; numerical values of nanomechanical readouts depict variations over a wide range. These anomalies are associated with the experimental setup under study. In this manuscript, we have identified heterogeneity in cell culture system in addition to the selection of AFM probe with specific tip geometry as the major contributors to the above mentioned anomalies. To test our hypothesis, we have used Panc1 cells, which is a pancreatic ductal adenocarcinoma cell type. Our results suggest that the cellular morphology, membrane roughness and NPs calculated from AFM study are distinctly influenced by cell cycle. Furthermore, we found that the NPs readout is also significantly associated with AFM tip geometries. The cells were found to be softer in their early resting phase when indented with pyramidal probe and became increasingly stiffer as they progressed through the cell cycles. On the contrary, when indented with the spherical probe, cells in G0/G1 phase were observed to be the stiffest. Such an exhaustive study of the role of cell cycle in influencing the NPs in Panc1 cell line along with the impact of tip geometry on NPs is the first of its kind, to the best of our knowledge.

Role of matrix metalloproteinases and histone deacetylase in oxidative stress-induced degradation of the endothelial glycocalyx

The glycocalyx is crucial for normal endothelial function. It also tethers extracellular superoxide dismutase (SOD3), which protects the endothelium against oxidative damage. Proteolytic enzymes [matrix metalloproteinases (MMPs)] are capable of disrupting endothelial cell surface proteins, such as syndecans, resulting in derangements of the endothelial glycocalyx. We sought to test the role of MMPs in oxidative stress-mediated disruption of the endothelial glycocalyx and examine the effect of pharmacological inhibition of MMPs on mitigating this detrimental effect. We also examined the role of histone deacetylase (HDAC) in the oxidative stress-mediated MMP induction and glycocalyx remodeling. Oxidative stress was experimentally induced in human adipose microvascular endothelial cells using H₂O₂ and buthionine sulfoximine in the presence and absence of potent MMP and HDAC inhibitors. H₂O₂ and buthionine sulfoximine resulted in a notable loss of the endothelial glycocalyx; they also increased the expression and proteolytic activity of MMP-2 and MMP-9 and subsequently increased the shedding of syndecan-1 and SOD3 from the endothelial cell surface. MMP upregulation was accompanied by a decline in mRNA and protein levels of their inhibitors, tissue inhibitors of metalloproteinase (TIMPs; TIMP-1 and TIMP-3). Furthermore, oxidative stress induced HDAC activity. Inhibition of MMPs and HDAC reversed syndecan-1 and SOD3 shedding and maintained endothelial glycocalyx integrity. HDAC inhibition increased TIMP expression and reduced MMP expression and activity in endothelial cells. Our findings shed light on MMPs and HDAC as therapeutically targetable mechanisms in oxidative stress-induced glycocalyx remodeling. NEW & NOTEWORTHY Oxidative stress, a hallmark of many diseases, damages the endothelial glycocalyx, resulting in vascular dysfunction. Studying the mechanistic link between oxidative stress and endothelial glycocalyx derangements might help discover new therapeutic targets to preserve vascular function. In this study, we investigated the involvement of matrix metalloproteinases and histone deacetylase in oxidative stress-induced endothelial glycocalyx degradation.

Probe Sensitivity to Cortical versus Intracellular Cytoskeletal Network Stiffness

In development, wound healing, and pathology, cell biomechanical properties are increasingly recognized as being of central importance. To measure these properties, experimental probes of various types have been developed, but how each probe reflects the properties of heterogeneous cell regions has remained obscure. To better understand differences attributable to the probe technology, as well as to define the relative sensitivity of each probe to different cellular structures, here we took a comprehensive approach. We studied two cell types-Schlemm's canal endothelial cells and mouse embryonic fibroblasts (MEFs)-using four different probe technologies: 1) atomic force microscopy (AFM) with sharp tip, 2) AFM with round tip, 3) optical magnetic twisting cytometry (OMTC), and 4) traction microscopy (TM). Perturbation of Schlemm's canal cells with dexamethasone treatment, α-actinin overexpression, or RhoA overexpression caused increases in traction reported by TM and stiffness reported by sharp-tip AFM as compared to corresponding controls. By contrast, under these same experimental conditions, stiffness reported by round-tip AFM and by OMTC indicated little change. Knockout (KO) of vimentin in MEFs caused a diminution of traction reported by TM, as well as stiffness reported by sharp-tip and round-tip AFM. However, stiffness reported by OMTC in vimentin-KO MEFs was greater than in wild type. Finite-element analysis demonstrated that this paradoxical OMTC result in vimentin-KO MEFs could be attributed to reduced cell thickness. Our results also suggest that vimentin contributes not only to intracellular network stiffness but also cortex stiffness. Taken together, this evidence suggests that AFM sharp tip and TM emphasize properties of the actin-rich shell of the cell, whereas round-tip AFM and OMTC emphasize those of the noncortical intracellular network.

Application of atomic force microscopy in cancer research

Atomic force microscopy (AFM) allows for nanometer-scale investigation of cells and molecules. Recent advances have enabled its application in cancer research and diagnosis. The physicochemical properties of live cells undergo changes when their physiological conditions are altered. These physicochemical properties can therefore reflect complex physiological processes occurring in cells. When cells are in the process of carcinogenesis and stimulated by external stimuli, their morphology, elasticity, and adhesion properties may change. AFM can perform surface imaging and ultrastructural observation of live cells with atomic resolution under near-physiological conditions, collecting force spectroscopy information which allows for the study of the mechanical properties of cells. For this reason, AFM has potential to be used as a tool for high resolution research into the ultrastructure and mechanical properties of tumor cells. This review describes the working principle, working mode, and technical points of atomic force microscopy, and reviews the applications and prospects of atomic force microscopy in cancer research.

Determination of the viscoelastic properties of a single cell cultured on a rigid support by force microscopy

Understanding the relationship between the mechanical properties of living cells and physiology is a central issue in mechanobiology. Mechanical properties are used as fingerprints of the pathological state of a single cell. The force exerted on a cell is influenced by the stiffness of the solid support needed to culture it. This effect is a consequence of the cell's boundary conditions. It causes a cell to appear with mechanical properties different from their real values. Here we develop a bottom effect viscoelastic theory to determine the viscoelastic response of a cell. The theory transforms a force-distance curve into the cell's Young's modulus, loss modulus, relaxation time or viscosity coefficient with independence of the stiffness of the rigid support. The theory predicts that, for a given indentation, the force exerted on the cell's periphery will be larger than on a perinuclear region. Results based on the use of semi-infinite contact mechanics models introduce large numerical errors in the determination of the mechanical properties. Finite element simulations confirm the theory and define its range of applicability.

Are elastic moduli of biological cells depth dependent or not? Another explanation using a contact mechanics model with surface tension

Atomic force microscopy (AFM) has become the most commonly used tool to measure the mechanical properties of biological cells. In AFM indentation experiments, the Hertz and Sneddon models of contact mechanics are usually adopted to extract the elastic modulus by analyzing the load-indent depth curves for spherical and conical tips, respectively. However, the effects of surface tension, neglected in existing contact models, become more significant in indentation responses due to the lower elastic moduli of living cells. Here, we present two simple yet robust relations between load and indent depth considering surface tension effects for spherical and conical indentations, through dimensional analysis and finite element simulations. When the indent depth is smaller than the intrinsic length defined as the ratio of surface tension to elastic modulus, the elastic modulus obtained by classical contact mechanics theories would be overestimated. Contrary to the majority of reported results, we find that the elastic modulus of a cell could be independent of indent depths if surface tension is taken into account. Our model seems to be in agreement with experimental data available. A comprehensive comparison will be done in the future.

Glaucoma Drugs in the Pipeline

Glaucoma is a chronic disease that can be challenging to treat for both patients and physicians. Most patients will require more than 1 medication over time to maintain their intraocular pressure (IOP) at a physiologically benign level. Patients may become refractory to existing compounds and many struggle with adherence to multiple topical drop regimens. The field of glaucoma therapeutics has been advancing rapidly with an emphasis on compounds comprising multiple molecules/mechanisms of action that offer additivity and are complementary to current therapeutics. Several new topical drop compounds directly targeting the trabecular meshwork (TM)/Schlemm canal/conventional outflow pathway to reduce outflow resistance have obtained US Food and Drug Administration approval in the past year. These include rho kinase inhibitors and nitric oxide donating compounds. Alternative therapies that offer long-term IOP lowering while removing the patient from the drug delivery system are moving forward in development. These include gene therapy and stem cell strategies, which could ease or eliminate the burden of topical drop self-administration for several years. Additionally, a variety of novel formulations and devices are in development that aim for controlled, steady state delivery of therapeutics over periods of months. The future of glaucoma therapy is focusing on an increase in specificity for the individual patient: their type of glaucoma; underlying mechanisms; genetic make-up; comorbid conditions; and rate of progression. Maintaining functional vision and improving patient outcomes remains the goal in glaucoma therapeutics. The current collection of novel therapeutics offers an expanded set of tools to achieve that goal.

Biomechanical Rigidity and Quantitative Proteomics Analysis of Segmental Regions of the Trabecular Meshwork at Physiologic and Elevated Pressures

Purpose

The extracellular matrix (ECM) of the trabecular meshwork (TM) modulates resistance to aqueous humor outflow, thereby regulating IOP. Glaucoma, a leading cause of irreversible blindness worldwide, is associated with changes in the ECM of the TM. The elastic modulus of glaucomatous TM is larger than age-matched normal TM; however, the biomechanical properties of segmental low (LF) and high flow (HF) TM regions and their response to elevated pressure, are unknown.

Methods

We perfused human anterior segments at two pressures using an ex vivo organ culture system. After extraction, we measured the elastic modulus of HF and LF TM regions by atomic force microscopy and quantitated protein differences by proteomics analyses.

Results

The elastic modulus of LF regions was 2.3-fold larger than HF regions at physiological (1×) pressure, and 7.4-fold or 3.5-fold larger than HF regions at elevated (2×) pressure after 24 or 72 hours, respectively. Using quantitative proteomics, comparisons were made between HF and LF regions at 1× or 2× pressure. Significant ECM protein differences were observed between LF and HF regions perfused at 2×, and between HF regions at 1× compared to 2× pressures. Decorin, TGF-β–induced protein, keratocan, lumican, dermatopontin, and thrombospondin 4 were common differential candidates in both comparisons.

Conclusions

These data show changes in biomechanical properties of segmental regions within the TM in response to elevated pressure, and levels of specific ECM proteins. Further studies are needed to determine whether these ECM proteins are specifically involved in outflow resistance and IOP homeostasis.

Modulation of Schlemm's canal endothelial cell stiffness via latrunculin loaded block copolymer micelles

Increased stiffness of Schlemm's canal endothelial cells (SC cells) is a major contributing factor to the increased pressure characteristic of primary open-angle glaucoma. New treatments for glaucoma are being developed using actin depolymerizers and rho kinase inhibitors to address this increased stiffness. However, these agents have off-target effects and are not as potent as had been hoped. We have developed a micellar nanocarrier assembled from poly(ethylene glycol)-bl-poly(propylene sulfide) copolymers capable of encapsulating latrunculin A (Lat A) with the goal of modulating SC cell stiffness. Lat A-loaded nanocarriers were similar in size and morphology to unloaded poly (ethylene glycol)-bl-poly(propylene sulfide) (PEG-bl-PPS) micelles, loaded Lat A at 62% encapsulation efficiency, and retained loaded Lat A for at least 22 days. The continued functional activity of Lat A following encapsulation within micelles was verified in murine macrophages, which are known to display decreased endocytosis in response to Lat A-dependent cytoskeletal disruption. Endocytic inhibition remained unchanged when comparing equal concentrations of micelle-loaded versus free form Lat A. Uptake of Lat A-loaded micelles by human SC cells was verified in vitro with no sign of cytotoxicity, and modulation of SC cell stiffness was measured by atomic force microscopy. Lat A-loaded micelles significantly decreased SC cell stiffness, which resulted in visible changes in cell morphology as observed by confocal microscopy. Our results demonstrate that PEG-bl-PPS micelles represent a tunable platform for the controlled intracellular delivery of latrunculin. These self-assembled polymeric nanobiomaterials may support the rational design and engineering of delivery systems for the treatment of glaucoma. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 1771-1779, 2018.

Nanoscale imaging and force probing of biomolecular systems using atomic force microscopy: from single molecules to living cells

Due to the lack of adequate tools for observation, native molecular behaviors at the nanoscale have been poorly understood. The advent of atomic force microscopy (AFM) provides an exciting instrument for investigating physiological processes on individual living cells with molecular resolution, which attracts the attention of worldwide researchers. In the past few decades, AFM has been widely utilized to investigate molecular activities on diverse biological interfaces, and the performances and functions of AFM have also been continuously improved, greatly improving our understanding of the behaviors of single molecules in action and demonstrating the important role of AFM in addressing biological issues with unprecedented spatiotemporal resolution. In this article, we review the related techniques and recent progress about applying AFM to characterize biomolecular systems in situ from single molecules to living cells. The challenges and future directions are also discussed.

Time-resolved nanomechanics of a single cell under the depolymerization of the cytoskeleton

Single cell stiffness measurements consider cells as passive and elastic materials which react instantaneously to an external force. This approximation is at odds with the complex structure of the cell which includes solid and liquid components. Here we develop a force microscopy method to measure the time and frequency dependencies of the elastic modulus, the viscosity coefficient, the loss modulus and the relaxation time of a single live cell. These parameters have different time and frequency dependencies. At low modulation frequencies (0.2-4 Hz), the elastic modulus remains unchanged; the loss modulus increases while the viscosity and the relaxation time decrease. We have followed the evolution of a fibroblast cell subjected to the depolymerization of its F-actin cytoskeleton. The elastic modulus, the loss modulus and the viscous coefficient decrease with the exposure time to the depolymerization drug while the relaxation time increases. The latter effect reflects that the changes in the elastic response happen at a higher rate than those affecting the viscous flow. The observed behavior is compatible with a cell mechanical response described by the poroelastic model.

Characterization of Cell Scaffolds by Atomic Force Microscopy

This review reports on the use of the atomic force microscopy (AFM) in the investigation of cell scaffolds in recent years. It is shown how the technique is able to deliver information about the scaffold surface properties (e.g., topography), as well as about its mechanical behavior (Young's modulus, viscosity, and adhesion). In addition, this short review also points out the utilization of the atomic force microscope technique beyond its usual employment in order to investigate another type of basic questions related to materials physics, chemistry, and biology. The final section discusses in detail the novel uses that those alternative measuring modes can bring to this field in the future.

Mechanical Criterion for the Rupture of a Cell Membrane under Compression

We investigate the mechanical conditions leading to the rupture of the plasma membrane of an endothelial cell subjected to a local, compressive force. Membrane rupture is induced by tilted microindentation, a technique used to perform mechanical measurements on adherent cells. In this technique, the applied force can be deduced from the measured horizontal displacement of a microindenter's tip, as imaged with an inverted microscope and without the need for optical sensors to measure the microindenter's deflection. We show that plasma membrane rupture of endothelial cells occurs at a well-defined value of the applied compressive stress. As a point of reference, we use numerical simulations to estimate the magnitude of the compressive stresses exerted on endothelial cells during the deployment of a stent.

Atomic Force Microscopy in Characterizing Cell Mechanics for Biomedical Applications: A Review

Cell mechanics is a novel label-free biomarker for indicating cell states and pathological changes. The advent of atomic force microscopy (AFM) provides a powerful tool for quantifying the mechanical properties of single living cells in aqueous conditions. The wide use of AFM in characterizing cell mechanics in the past two decades has yielded remarkable novel insights in understanding the development and progression of certain diseases, such as cancer, showing the huge potential of cell mechanics for practical applications in the field of biomedicine. In this paper, we reviewed the utilization of AFM to characterize cell mechanics. First, the principle and method of AFM single-cell mechanical analysis was presented, along with the mechanical responses of cells to representative external stimuli measured by AFM. Next, the unique changes of cell mechanics in two types of physiological processes (stem cell differentiation, cancer metastasis) revealed by AFM were summarized. After that, the molecular mechanisms guiding cell mechanics were analyzed. Finally the challenges and future directions were discussed.

Fluorescence Microscopy Imaging Calibration for Quantifying Nanocarrier Binding to Cells During Shear Flow Exposure

Targeted drug delivery is a fast growing industry in healthcare and technologies are being developed for applications utilizing nanocarriers as vehicles for drug transport. As the size scale of these particles becomes further reduced, advanced fluorescence microscopy and image analysis techniques become increasingly important for facilitating our understanding of nanocarrier binding and avidity, thereby establishing the basis for nanocarrier design optimization. While there is a significant body of published work using nanocarriers in vitro and in vivo, the advent of smaller particles that have typically been studied (~500 nm) limits the ability to attain quantitative measurements of nanocarrier binding dynamics since image acquisition and analysis methods are restricted by microscopy pixel size. This work demonstrates the use of a novel calibration technique based on radioisotope counting and fluorescence imaging for enabling quantitative determination of nanocarrier binding dynamics. The technique is then applied to assess the temporal profile of endothelial cell binding of two antibody targeted nanocarrier types in the presence of fluid shear stress. Results are provided for binding of nanoparticles smaller than a microscopy image pixel.

How Nucleus Mechanics and ECM Microstructure Influence the Invasion of Single Cells and Multicellular Aggregates

In order to move in a three-dimensional extracellular matrix, the nucleus of a cell must squeeze through the narrow spacing among the fibers and, by adhering to them, the cell needs to exert sufficiently strong traction forces. If the nucleus is too stiff, the spacing too narrow, or traction forces too weak, the cell is not able to penetrate the network. In this article, we formulate a mathematical model based on an energetic approach, for cells entering cylindrical channels composed of extracellular matrix fibers. Treating the nucleus as an elastic body covered by an elastic membrane, the energetic balance leads to the definition of a necessary criterion for cells to pass through the regular network of fibers, depending on the traction forces exerted by the cells (or possibly passive stresses), the stretchability of the nuclear membrane, the stiffness of the nucleus, and the ratio of the pore size within the extracellular matrix with respect to the nucleus diameter. The results obtained highlight the importance of the interplay between mechanical properties of the cell and microscopic geometric characteristics of the extracellular matrix and give an estimate for a critical value of the pore size that represents the physical limit of migration and can be used in tumor growth models to predict their invasive potential in thick regions of ECM.

Microvasculature on a chip: study of the Endothelial Surface Layer and the flow structure of Red Blood Cells

Microvasculatures-on-a-chip, i.e. in vitro models that mimic important features of microvessel networks, have gained increasing interest in recent years. Such devices have allowed investigating pathophysiological situations involving abnormal biophysical interactions between blood cells and vessel walls. Still, a central question remains regarding the presence, in such biomimetic systems, of the endothelial glycocalyx. The latter is a glycosaminoglycans-rich surface layer exposed to blood flow, which plays a crucial role in regulating the interactions between circulating cells and the endothelium. Here, we use confocal microscopy to characterize the layer expressed by endothelial cells cultured in microfluidic channels. We show that, under our culture conditions, endothelial cells form a confluent layer on all the walls of the circuit and display a glycocalyx that fully lines the lumen of the microchannels. Moreover, the thickness of this surface layer is found to be on the order of 600 nm, which compares well with measurements performed ex or in vivo on microcapillaries. Furthermore, we investigate how the presence of endothelial cells in the microchannels affects their hydrodynamic resistance and the near-wall motion of red blood cells. Our study thus provides an important insight into the physiological relevance of in vitro microvasculatures.

Covalently immobilized VEGF-mimicking peptide with gelatin methacrylate enhances microvascularization of endothelial cells

Clinically usable tissue-engineered constructs are currently limited due to their inability of forming microvascular networks necessary for adequate cellular oxygen and nutrient supply upon implantation. The aim of this study is to investigate the conditions necessary for microvascularization in a tissue-engineered construct using vascular endothelial growth factor (VEGF). The construct was made of gelatin methacrylate (GelMA) based cell-laden hydrogel system, which was then covalently linked with VEGF-mimicking peptide (AcQK), using human umbilical vein endothelial cells (HUVECs) as the model cell. The results of the mechanics and gene expression analysis indicated significant changes in mechanical properties and upregulation of vascular-specific genes. The major finding of this study is that the increased expression of vascular-specific genes could be achieved by employing AcQK in the GelMA based hydrogel system, leading to accelerated microvascularization. We conclude that GelMA with covalently-linked angiogenic peptide is a useful tissue engineered construct suitable for microvascularization. STATEMENT OF SIGNIFICANCE: (1) This study reports the conditions necessary for microvascularization in a tissue-engineered construct using vascular endothelial growth factor (VEGF). (2) The construct was made of gelatin methacrylate based cell-laden hydrogel system. (3) There is a significant change observed in mechanical properties and upregulation of vascular-specific genes, in particular CD34, when AcQK is used. (4) The major finding of this study is that the increased expression of vascular-specific genes, i.e., CD34 could be achieved by employing AcQK in the GelMA based hydrogel system, leading to accelerated microvascularization.

Self-organizing actin patterns shape membrane architecture but not cell mechanics

Cell-free studies have demonstrated how collective action of actin-associated proteins can organize actin filaments into dynamic patterns, such as vortices, asters and stars. Using complementary microscopic techniques, we here show evidence of such self-organization of the actin cortex in living HeLa cells. During cell adhesion, an active multistage process naturally leads to pattern transitions from actin vortices over stars into asters. This process is primarily driven by Arp2/3 complex nucleation, but not by myosin motors, which is in contrast to what has been theoretically predicted and observed in vitro. Concomitant measurements of mechanics and plasma membrane fluidity demonstrate that changes in actin patterning alter membrane architecture but occur functionally independent of macroscopic cortex elasticity. Consequently, tuning the activity of the Arp2/3 complex to alter filament assembly may thus be a mechanism allowing cells to adjust their membrane architecture without affecting their macroscopic mechanical properties.

Diffusion and Binding of Mismatch Repair Protein, MSH2, in Breast Cancer Cells at Different Stages of Neoplastic Transformation

The interior of cells is a highly complex medium, containing numerous organelles, a matrix of different fibers and a viscous, aqueous fluid of proteins and small molecules. The interior of cells is also a highly dynamic medium, in which many components move, either by active transport or passive diffusion. The mobility and localization of proteins inside cells can provide important insights into protein function and also general cellular properties, such as viscosity. Neoplastic transformation affects numerous cellular properties, and our goal was to investigate the diffusional and binding behavior of the important mismatch repair (MMR) protein MSH2 in live human cells at various stages of neoplastic transformation. Toward this end, noncancerous, immortal, tumorigenic, and metastatic mammary epithelial cells were transfected with EGFP and EGFP-tagged MSH2. MSH2 forms two MMR proteins (MutSα and MutSβ) and we assume MSH2 is in the complex MutSα, though our results are similar in either case. Unlike the MutS complexes that bind to nuclear DNA, EGFP diffuses freely. EGFP and MutSα-EGFP diffusion coefficients were determined in the cytoplasm and nucleus of each cell type using fluorescence recovery after photobleaching. Diffusion coefficients were 14-24 μm2/s for EGFP and 3-7 μm2/s for MutSα-EGFP. EGFP diffusion increased in going from noncancerous to immortal cells, indicating a decrease in viscosity, with smaller changes in subsequent stages. MutSα produces an effective diffusion coefficient that, coupled with the free EGFP diffusion measurements, can be used to extract a pure diffusion coefficient and a pseudo-equilibrium constant K*. The MutSα nuclear K* increased sixfold in the first stage of cancer and then decreased in the more advanced stages. The ratio of nuclear to cytoplasmic K*for MutSα increased almost two orders of magnitude in going from noncancerous to immortal cells, suggesting that this quantity may be a sensitive metric for recognizing the onset of cancer.

Nanoindentation characterisation of human colorectal cancer cells considering cell geometry, surface roughness and hyperelastic constitutive behaviour

Characterisation of the mechanical behaviour of cancer cells is an issue of crucial importance as specific cell mechanical properties have been measured and utilized as possible biomarkers of cancer progression. Atomic force microscopy certainly occupies a prominent place in the field of the mechanical characterisation devices. We developed a hybrid approach to characterise different cell lines (SW620 and SW480) of the human colon carcinoma submitted to nanoindentation measurements. An ad hoc algorithm was written that compares the force-indentation curves experimentally retrieved with those predicted by a finite element model that simulates the nanoindentation process and reproduces the cell geometry and the surface roughness. The algorithm perturbs iteratively the values of the cell mechanical properties implemented in the finite element model until the difference between the experimental and numerical force-indentation curves reaches the minimum value. The occurrence of this indicates that the implemented material properties are very close to the real ones. Different hyperelastic constitutive models, such as Arruda-Boyce, Mooney-Rivlin and Neo-Hookean were utilized to describe the structural behaviour of indented cells. The algorithm was capable of separating, for all the cell lines investigated, the mechanical properties of cell cortex and cytoskeleton. Material properties determined via the algorithm were different with respect to those obtained with the Hertzian contact theory. This demonstrates that factors such as: the cell geometry/anatomy and the hyperelastic constitutive behaviour, which are not contemplated in the Hertz's theory hypotheses, do affect the nanoindentation measurements. The proposed approach represents a powerful tool that, only on the basis of nanoindentation measurements, is capable of characterising material at the subcellular level.

Structural Modeling of Mechanosensitivity in Non-Muscle Cells: Multiscale Approach to Understand Cell Sensing

The contraction and spreading of nonmuscle cells are important phenomena in a number of cellular processes such as differentiation, morphogenesis, and tissue growth. Recent experimental work has shown that the topology and the mechanical properties of the underlying substrate play a significant role in directing the cell's response. In this work, we introduce a multiscale model to understand the sensing, activation, and contraction of the actin cytoskeleton of nonmuscle cells based on the idea that acto-myosin cross-bridges display a catch-bond response. After investigating the respective roles of bond catchiness and acto-myosin assembly on the mechano-sensitivity of stress fibers, we present full simulations of cells laying on arrays of micropillars. Model predictions show good qualitative agreements with experimental observation, suggesting that acto-myosin catch bonds are a major mechano-sensing element in nonmuscle cells.

The nanomechanical role of melanin granules in the retinal pigment epithelium

Nanomechanical properties of cells and tissues, in particular their elasticity, play an important role in different physiological and pathological processes. Recently, we have demonstrated that melanin granules dramatically modify nanomechanical properties of melanoma cells making them very stiff and, as a result, less aggressive. Although the mechanical effect of melanin granules was demonstrated in pathological cells, it was never studied in the case of normal cells. In this work, we analyzed the impact of melanin granules on nanomechanical properties of primary retinal pigment epithelium tissue fragments isolated from porcine eyes. The obtained results clearly show that melanin granules are responsible for the exceptional nanomechanical properties of the tissue. Our findings suggest that melanin granules in the retinal pigment epithelium may play an important role in sustaining the stiffness of this single cell layer, which functions as a natural mechanical barrier separating the retina from the choroid.

Biomechanics of subcellular structures by non-invasive Brillouin microscopy

Cellular biomechanics play a pivotal role in the pathophysiology of several diseases. Unfortunately, current methods to measure biomechanical properties are invasive and mostly limited to the surface of a cell. As a result, the mechanical behaviour of subcellular structures and organelles remains poorly characterised. Here, we show three-dimensional biomechanical images of single cells obtained with non-invasive, non-destructive Brillouin microscopy with an unprecedented spatial resolution. Our results quantify the longitudinal elastic modulus of subcellular structures. In particular, we found the nucleoli to be stiffer than both the nuclear envelope (p < 0.0001) and the surrounding cytoplasm (p < 0.0001). Moreover, we demonstrate the mechanical response of cells to Latrunculin-A, a drug that reduces cell stiffness by preventing cytoskeletal assembly. Our technique can therefore generate valuable insights into cellular biomechanics and its role in pathophysiology.

Morphological and nanomechanical changes in mechanosensitive endothelial cells induced by colloidal AFM probes

Mechanotransduction is one of the main properties of endothelial cells (ECs) phenotype. Hemodynamic forces like flow-generated endothelial shear stress play a fundamental role in ECs cytoskeletal remodeling and activate signaling cascades in ECs. AFM methods are widely used to characterize morphology as well as mechanical properties of cells. In both cases AFM probes directly interact with cell surface exerting mechanical forces on the cellular membrane, which in turn may stimulate mechanosensitive receptors present in EC. This article presents examples of how the colloidal AFM probes influence ECs during multiple scans. The results revealed that multiple scans of the ECs significantly influenced the morphology and elasticity of cells. Moreover, changes in the cell shape and mechanical properties were dependent on the scan direction (across or along the main axis of the cell). Multiple scans with a colloidal probe leaded to reorientation of the cell main axis and this effect was similar to the action of the shear stress induced by flow conditions. Furthermore, the correlation between the tip-induced modification of the cell properties and the remodeling of the cell's glycocalyx was observed. SCANNING 38:654-664, 2016. © 2016 Wiley Periodicals, Inc.

A beginner's guide to atomic force microscopy probing for cell mechanics

Atomic Force microscopy (AFM) is becoming a prevalent tool in cell biology and biomedical studies, especially those focusing on the mechanical properties of cells and tissues. The newest generation of bio-AFMs combine ease of use and seamless integration with live-cell epifluorescence or more advanced optical microscopies. As a unique feature with respect to other bionanotools, AFM provides nanometer-resolution maps for cell topography, stiffness, viscoelasticity, and adhesion, often overlaid with matching optical images of the probed cells. This review is intended for those about to embark in the use of bio-AFMs, and aims to assist them in designing an experiment to measure the mechanical properties of adherent cells. In addition to describing the main steps in a typical cell mechanics protocol and explaining how data is analysed, this review will also discuss some of the relevant contact mechanics models available and how they have been used to characterize specific features of cellular and biological samples. Microsc. Res. Tech. 80:75-84, 2017. © 2016 Wiley Periodicals, Inc.

Bio- chemical and physical characterizations of mesenchymal stromal cells along the time course of directed differentiation

Cellular biophysical properties are novel biomarkers of cell phenotypes which may reflect the status of differentiating stem cells. Accurate characterizations of cellular biophysical properties, in conjunction with the corresponding biochemical properties could help to distinguish stem cells from primary cells, cancer cells, and differentiated cells. However, the correlated evolution of these properties in the course of directed stem cells differentiation has not been well characterized. In this study, we applied video particle tracking microrheology (VPTM) to measure intracellular viscoelasticity of differentiating human mesenchymal stromal/stem cells (hMSCs). Our results showed that osteogenesis not only increased both elastic and viscous moduli, but also converted the intracellular viscoelasticity of differentiating hMSCs from viscous-like to elastic-like. In contrast, adipogenesis decreased both elastic and viscous moduli while hMSCs remained viscous-like during the differentiation. In conjunction with bio- chemical and physical parameters, such as gene expression profiles, cell morphology, and cytoskeleton arrangement, we demonstrated that VPTM is a unique approach to quantify, with high data throughput, the maturation level of differentiating hMSCs and to anticipate their fate decisions. This approach is well suited for time-lapsed study of the mechanobiology of differentiating stem cells especially in three dimensional physico-chemical biomimetic environments including porous scaffolds.

Investigating the mechanical properties of zona pellucida of whole human oocytes by atomic force spectroscopy

The role of mechanics in numerous biological processes is nowadays recognized, while in others, such as the fertilization process, it is still neglected. In the case of oocytes the description of their mechanical properties could improve the comprehension of the oocyte-spermatozoon interaction and be helpful for application in in vitro fertilization (IVF) clinics. Herein the mechanical properties of whole human oocytes (HOs) immediately after retrieval are investigated by indentation measurements with atomic force spectroscopy under physiological conditions. Measurements are performed on immature (metaphase I - MI) and mature (metaphase II - MII) HOs. According to their morphological characteristics MII-HOs are classified as "suitable" and "rejected"; these latter would be usually rejected for intracytoplasmic sperm injection (ICSI). For all maturation stages we observe that the elastic response of the zona pellucida (ZP) outer layer was different and distinguishable from the rest of the ZP-HO. The elasticity of this ZP outer layer varies with maturation and quality: stiffness decreases from immature MI to good quality MII, up to poor-quality rejected MII. An indirect analysis with IVF outcome indicates that the ZP outer layer of analysed HOs donated by women who achieved pregnancy is stiffer than that of HOs from women with negative outcome. Our findings suggest that mechanical properties can represent important oocyte quality indicators that may be exploited for the design of innovative ICSI dedicated cell sorters.

Trabecular meshwork stiffness in glaucoma

Alterations in stiffness of the trabecular meshwork (TM) may play an important role in primary open-angle glaucoma (POAG), the second leading cause of blindness. Specifically, certain data suggest an association between elevated intraocular pressure (IOP) and increased TM stiffness; however, the underlying link between TM stiffness and IOP remains unclear and requires further study. We here first review the literature on TM stiffness measurements, encompassing various species and based on a number of measurement techniques, including direct approaches such as atomic force microscopy (AFM) and uniaxial tension tests, and indirect methods based on a beam deflection model. We also briefly review the effects of several factors that affect TM stiffness, including lysophospholipids, rho-kinase inhibitors, cytoskeletal disrupting agents, dexamethasone (DEX), transforming growth factor-β₂ (TGF-β₂), nitric oxide (NO) and cellular senescence. We then describe a method we have developed for determining TM stiffness measurement in mice using a cryosection/AFM-based approach, and present preliminary data on TM stiffness in C57BL/6J and CBA/J mouse strains. Finally, we investigate the relationship between TM stiffness and outflow facility between these two strains. The method we have developed shows promise for further direct measurements of mouse TM stiffness, which may be of value in understanding mechanistic relations between outflow facility and TM biomechanical properties.

Role of catch bonds in actomyosin mechanics and cell mechanosensitivity

We propose a mechanism of adherent cell mechanosensing, based on the idea that the contractile actomyosin machinery behaves as a catch bond. For this, we construct a simplified model of the actomyosin structure that constitutes the building block of stress fibers and express the stability of cross bridges in terms of the force-dependent bonding energy of the actomyosin bond. Consistent with experimental measurements, we then consider that the energy barrier of the actomyosin bond increases for tension and show that this response is enough to explain the force-induced stabilization of a stress fiber. Further numerical simulations at the cellular level show that the catch-bond hypothesis can help in understanding and predict the sensitivity of adherent cells to substrate stiffness.

An Automated High-throughput Array Microscope for Cancer Cell Mechanics

Changes in cellular mechanical properties correlate with the progression of metastatic cancer along the epithelial-to-mesenchymal transition (EMT). Few high-throughput methodologies exist that measure cell compliance, which can be used to understand the impact of genetic alterations or to screen the efficacy of chemotherapeutic agents. We have developed a novel array high-throughput microscope (AHTM) system that combines the convenience of the standard 96-well plate with the ability to image cultured cells and membrane-bound microbeads in twelve independently-focusing channels simultaneously, visiting all wells in eight steps. We use the AHTM and passive bead rheology techniques to determine the relative compliance of human pancreatic ductal epithelial (HPDE) cells, h-TERT transformed HPDE cells (HPNE), and four gain-of-function constructs related to EMT. The AHTM found HPNE, H-ras, Myr-AKT, and Bcl2 transfected cells more compliant relative to controls, consistent with parallel tests using atomic force microscopy and invasion assays, proving the AHTM capable of screening for changes in mechanical phenotype.

Keratin network modifications lead to the mechanical stiffening of the hair follicle fiber

The complex mechanical properties of biomaterials such as hair, horn, skin, or bone are determined by the architecture of the underlying fibrous bionetworks. Although much is known about the influence of the cytoskeleton on the mechanics of isolated cells, this has been less studied in tridimensional tissues. We used the hair follicle as a model to link changes in the keratin network composition and architecture to the mechanical properties of the nascent hair. We show using atomic force microscopy that the soft keratinocyte matrix at the base of the follicle stiffens by a factor of ∼360, from 30 kPa to 11 MPa along the first millimeter of the follicle. The early mechanical stiffening is concomitant to an increase in diameter of the keratin macrofibrils, their continuous compaction, and increasingly parallel orientation. The related stiffening of the material follows a power law, typical of the mechanics of nonthermal bending-dominated fiber networks. In addition, we used X-ray diffraction to monitor changes in the (supra)molecular organization within the keratin fibers. At later keratinization stages, the inner mechanical properties of the macrofibrils dominate the stiffening due to the progressive setting up of the cystine network. Our findings corroborate existing models on the sequence of biological and structural events during hair keratinization.

Actin kinetics shapes cortical network structure and mechanics

The actin cortex of animal cells is the main determinant of cellular mechanics. The continuous turnover of cortical actin filaments enables cells to quickly respond to stimuli. Recent work has shown that most of the cortical actin is generated by only two actin nucleators, the Arp2/3 complex and the formin Diaph1. However, our understanding of their interplay, their kinetics, and the length distribution of the filaments that they nucleate within living cells is poor. Such knowledge is necessary for a thorough comprehension of cellular processes and cell mechanics from basic polymer physics principles. We determined cortical assembly rates in living cells by using single-molecule fluorescence imaging in combination with stochastic simulations. We find that formin-nucleated filaments are, on average, 10 times longer than Arp2/3-nucleated filaments. Although formin-generated filaments represent less than 10% of all actin filaments, mechanical measurements indicate that they are important determinants of cortical elasticity. Tuning the activity of actin nucleators to alter filament length distribution may thus be a mechanism allowing cells to adjust their macroscopic mechanical properties to their physiological needs.

A Parallel-Plate Flow Chamber for Mechanical Characterization of Endothelial Cells Exposed to Laminar Shear Stress

Shear stresses induced by laminar fluid flow are essential to properly recapitulate the physiological microenvironment experienced by endothelial cells (ECs). ECs respond to these stresses via mechanotransduction by modulating their phenotype and biomechanical characteristics, which can be characterized by Atomic Force Microscopy (AFM). Parallel Plate Flow Chambers (PPFCs) apply unidirectional laminar fluid flow to EC monolayers in vitro. Since ECs in sealed PPFCs are inaccessible to AFM probes, cone-and-plate viscometers (CPs) are commonly used to apply shear stress. This paper presents a comparison of the efficacies of both methods. Computational Fluid Dynamic simulation and validation testing using EC responses as a metric have indicated limitations in the use of CPs to apply laminar shear stress. Monolayers subjected to laminar fluid flow in a PPFC respond by increasing cortical stiffness, elongating, and aligning filamentous actin in the direction of fluid flow to a greater extent than CP devices. Limitations using CP devices to provide laminar flow across an EC monolayer suggest they are better suited when studying EC response for disturbed flow conditions. PPFC platforms allow for exposure of ECs to laminar fluid flow conditions, recapitulating cellular biomechanical behaviors, whereas CP platforms allow for mechanical characterization of ECs under secondary flow.