Investigating cell mechanics with atomic force microscopy

Isotope labeled 3D-Raman confocal imaging and atomic force microscopy study on epithelial cells interacting with the fungus Candida albicans

The human pathogenic fungus Candida albicans damages epithelial cells during superficial infections. Here we use three-dimensional-sequential-confocal Raman spectroscopic imaging and atomic force microscopy to investigate the interaction of C. albicans wild type cells, the secreted C. albicans peptide toxin candidalysin and mutant cells lacking candidalysin with epithelial cells. The candidalysin is responsible for epithelial cell damage and exhibits in its deuterated form an identifiable Raman signal in a frequency region distinct from the cellular frequency region. Vibration modes at 2100-2200 cm-1 attributed to carbon‑deuterium bending and at 477 cm-1, attributed to the nitrogen‑deuterium out-of-plane bending, found around the nucleus, can be assigned to deuterated candidalysin. Atomic force microscopy visualized 100 nm deep lesions on the cell and force-distance curves indicate the higher adhesion on pore surrounding after incubation with candidalysin. Candidalysin targets the plasma membrane, but is also found inside of the cytosol of epithelial cells during C. albicans infection.

Using an ER-specific optogenetic mechanostimulator to understand the mechanosensitivity of the endoplasmic reticulum

The ability of cells to perceive and respond to mechanical cues is essential for numerous biological activities. Emerging evidence indicates important contributions of organelles to cellular mechanosensitivity and mechanotransduction. However, whether and how the endoplasmic reticulum (ER) senses and reacts to mechanical forces remains elusive. To fill the knowledge gap, after developing a light-inducible ER-specific mechanostimulator (LIMER), we identify that mechanostimulation of ER elicits a transient, rapid efflux of Ca2+ from ER in monkey kidney COS-7 cells, which is dependent on the cation channels transient receptor potential cation channel, subfamily V, member 1 (TRPV1) and polycystin-2 (PKD2) in an additive manner. This ER Ca2+ release can be repeatedly stimulated and tuned by varying the intensity and duration of force application. Moreover, ER-specific mechanostimulation inhibits ER-to-Golgi trafficking. Sustained mechanostimuli increase the levels of binding-immunoglobulin protein (BiP) expression and phosphorylated eIF2α, two markers for ER stress. Our results provide direct evidence for ER mechanosensitivity and tight mechanoregulation of ER functions, placing ER as an important player on the intricate map of cellular mechanotransduction.

A method for analyzing AFM force mapping data obtained from soft tissue cryosections

Atomic force microscopy (AFM) is a valuable tool for assessing mechanical properties of biological samples, but interpretations of measurements on whole tissues can be difficult due to the tissue's highly heterogeneous nature. To overcome such difficulties and obtain more robust estimates of tissue mechanical properties, we describe an AFM force mapping and data analysis pipeline to characterize the mechanical properties of cryosectioned soft tissues. We assessed this approach on mouse optic nerve head and rat trabecular meshwork, cornea, and sclera. Our data show that the use of repeated measurements, outlier exclusion, and log-normal data transformation increases confidence in AFM mechanical measurements, and we propose that this methodology can be broadly applied to measuring soft tissue properties from cryosections.

Surface-Functionalized Microgels as Artificial Antigen-Presenting Cells to Regulate Expansion of T Cells

Artificial antigen-presenting cells (aAPCs) are currently used to manufacture T cells for adoptive therapy in cancer treatment, but a readily tunable and modular system can enable both rapid T cell expansion and control over T cell phenotype. Here, it is shown that microgels with tailored surface biochemical properties can serve as aAPCs to mediate T cell activation and expansion. Surface functionalization of microgels is achieved via layer-by-layer coating using oppositely charged polymers, forming a thin but dense polymer layer on the surface. This facile and versatile approach is compatible with a variety of coating polymers and allows efficient and flexible surface-specific conjugation of defined peptides or proteins. The authors demonstrate that tethering appropriate stimulatory ligands on the microgel surface efficiently activates T cells for polyclonal and antigen-specific expansion. The expansion, phenotype, and functional outcome of primary mouse and human T cells can be regulated by modulating the concentration, ratio, and distribution of stimulatory ligands presented on microgel surfaces as well as the stiffness and viscoelasticity of the microgels.

Mechanical state transitions in the regulation of tissue form and function

From embryonic development, postnatal growth and adult homeostasis to reparative and disease states, cells and tissues undergo constant changes in genome activity, cell fate, proliferation, movement, metabolism and growth. Importantly, these biological state transitions are coupled to changes in the mechanical and material properties of cells and tissues, termed mechanical state transitions. These mechanical states share features with physical states of matter, liquids and solids. Tissues can switch between mechanical states by changing behavioural dynamics or connectivity between cells. Conversely, these changes in tissue mechanical properties are known to control cell and tissue function, most importantly the ability of cells to move or tissues to deform. Thus, tissue mechanical state transitions are implicated in transmitting information across biological length and time scales, especially during processes of early development, wound healing and diseases such as cancer. This Review will focus on the biological basis of tissue-scale mechanical state transitions, how they emerge from molecular and cellular interactions, and their roles in organismal development, homeostasis, regeneration and disease.

pH-regulated single cell migration

Over the last two decades, extra- and intracellular pH have emerged as fundamental regulators of cell motility. Fundamental physiological and pathological processes relying on appropriate cell migration, such as embryonic development, wound healing, and a proper immune defense on the one hand, and autoimmune diseases, metastatic cancer, and the progression of certain parasitic diseases on the other, depend on surrounding pH. In addition, migrating single cells create their own localized pH nanodomains at their surface and in the cytosol. By this means, the migrating cells locally modulate their adhesion to, and the re-arrangement and digestion of, the extracellular matrix. At the same time, the cytosolic nanodomains tune cytoskeletal dynamics along the direction of movement resulting in concerted lamellipodia protrusion and rear end retraction. Extracellular pH gradients as found in wounds, inflamed tissues, or the periphery of tumors stimulate directed cell migration, and long-term exposure to acidic conditions can engender a more migratory and invasive phenotype persisting for hours up to several generations of cells after they have left the acidic milieu. In the present review, the different variants of pH-dependent single cell migration are described. The underlying pH-dependent molecular mechanisms such as conformational changes of adhesion molecules, matrix protease activity, actin (de-)polymerization, and signaling events are explained, and molecular pH sensors stimulated by H+ signaling are presented.

Dynamically Mapping the Topography and Stiffness of the Leading Edge of Migrating Cells Using AFM in Fast-QI Mode

Cell migration profoundly influences cellular function, often resulting in adverse effects in various pathologies including cancer metastasis. Directly assessing and quantifying the nanoscale dynamics of living cell structure and mechanics has remained a challenge. At the forefront of cell movement, the flat actin modules─the lamellipodium and the lamellum─interact to propel cell migration. The lamellipodium extends from the lamellum and undergoes rapid changes within seconds, making measurement of its stiffness a persistent hurdle. In this study, we introduce the fast-quantitative imaging (fast-QI) mode, demonstrating its capability to simultaneously map both the lamellipodium and the lamellum with enhanced spatiotemporal resolution compared with the classic quantitative imaging (QI) mode. Specifically, our findings reveal nanoscale stiffness gradients in the lamellipodium at the leading edge, where it appears to be slightly thinner and significantly softer than the lamellum. Additionally, we illustrate the fast-QI mode's accuracy in generating maps of height and effective stiffness through a streamlined and efficient processing of force-distance curves. These results underscore the potential of the fast-QI mode for investigating the role of motile cell structures in mechanosensing.

Monitoring the mass, eigenfrequency, and quality factor of mammalian cells

The regulation of mass is essential for the development and homeostasis of cells and multicellular organisms. However, cell mass is also tightly linked to cell mechanical properties, which depend on the time scales at which they are measured and change drastically at the cellular eigenfrequency. So far, it has not been possible to determine cell mass and eigenfrequency together. Here, we introduce microcantilevers oscillating in the Ångström range to monitor both fundamental physical properties of the cell. If the oscillation frequency is far below the cellular eigenfrequency, all cell compartments follow the cantilever motion, and the cell mass measurements are accurate. Yet, if the oscillating frequency approaches or lies above the cellular eigenfrequency, the mechanical response of the cell changes, and not all cellular components can follow the cantilever motions in phase. This energy loss caused by mechanical damping within the cell is described by the quality factor. We use these observations to examine living cells across externally applied mechanical frequency ranges and to measure their total mass, eigenfrequency, and quality factor. The three parameters open the door to better understand the mechanobiology of the cell and stimulate biotechnological and medical innovations.

Cell Adhesion, Elasticity, and Rupture Forces Guide Microbial Cell Death on Nanostructured Antimicrobial Titanium Surfaces

Naturally occurring and synthetic nanostructured surfaces have been widely reported to resist microbial colonization. The majority of these studies have shown that both bacterial and fungal cells are killed upon contact and subsequent surface adhesion to such surfaces. This occurs because the presence of high-aspect-ratio structures can initiate a self-driven mechanical rupture of microbial cells during the surface adsorption process. While this technology has received a large amount of scientific and medical interest, one important question still remains: what factors drive microbial death on the surface? In this work, the interplay between microbial-surface adhesion, cell elasticity, cell membrane rupture forces, and cell lysis at the microbial-nanostructure biointerface during adsorptive processes was assessed using a combination of live confocal laser scanning microscopy, scanning electron microscopy, in situ amplitude atomic force microscopy, and single-cell force spectroscopy. Specifically, the adsorptive behavior and nanomechanical properties of live Gram-negative (Pseudomonas aeruginosa) and Gram-positive (methicillin-resistant Staphylococcus aureus) bacterial cells, as well as the fungal species Candida albicans and Cryptococcus neoformans, were assessed on unmodified and nanostructured titanium surfaces. Unmodified titanium and titanium surfaces with nanostructures were used as model substrates for investigation. For all microbial species, cell elasticity, rupture force, maximum cell-surface adhesion force, the work of adhesion, and the cell-surface tether behavior were compared to the relative cell death observed for each surface examined. For cells with a lower elastic modulus, lower force to rupture through the cell, and higher work of adhesion, the surfaces had a higher antimicrobial activity, supporting the proposed biocidal mode of action for nanostructured surfaces. This study provides direct quantification of the differences observed in the efficacy of nanostructured antimicrobial surface as a function of microbial species indicating that a universal, antimicrobial surface architecture may be hard to achieve.

A Perspective on Developing Modeling and Image Analysis Tools to Investigate Mechanosensing Proteins

The shift of funding organizations to prioritize interdisciplinary work points to the need for workflow models that better accommodate interdisciplinary studies. Most scientists are trained in a specific field and are often unaware of the kind of insights that other disciplines could contribute to solving various problems. In this paper, we present a perspective on how we developed an experimental pipeline between a microscopy and image analysis/bioengineering lab. Specifically, we connected microscopy observations about a putative mechanosensing protein, obscurin, to image analysis techniques that quantify cell changes. While the individual methods used are well established (fluorescence microscopy; ImageJ WEKA and mTrack2 programs; MATLAB), there are no existing best practices for how to integrate these techniques into a cohesive, interdisciplinary narrative. Here, we describe a broadly applicable workflow of how microscopists can more easily quantify cell properties (e.g., perimeter, velocity) from microscopy videos of eukaryotic (MDCK) adherent cells. Additionally, we give examples of how these foundational measurements can create more complex, customizable cell mechanics tools and models.

Lamellipodia dynamics and microrheology in endothelial cell paracellular gap closure

Vascular endothelial cells (ECs) form a semipermeable barrier separating vascular contents from the interstitium, thereby regulating the movement of water and molecular solutes across small intercellular gaps, which are continuously forming and closing. Under inflammatory conditions, however, larger EC gaps form resulting in increased vascular leakiness to circulating fluid, proteins, and cells, which results in organ edema and dysfunction responsible for key pathophysiologic findings in numerous inflammatory disorders. In this study, we extend our earlier work examining the biophysical properties of EC gap formation and now address the role of lamellipodia, thin sheet-like membrane projections from the leading edge, in modulating EC spatial-specific contractile properties and gap closure. Micropillars, fabricated by soft lithography, were utilized to form reproducible paracellular gaps in human lung ECs. Using time-lapse imaging via optical microscopy, rates of EC gap closure and motility were measured with and without EC stimulation with the barrier-enhancing sphingolipid, sphingosine-1-phosphate. Peripheral ruffle formation was ubiquitous during gap closure. Kymographs were generated to quantitatively compare the lamellipodia dynamics of sphingosine-1-phosphate-stimulated and -unstimulated ECs. Utilizing atomic force microscopy, we characterized the viscoelastic behavior of EC lamellipodia. Our results indicate decreased stiffness and increased liquid-like behavior of expanding lamellipodia compared with regions away from the cellular edge (lamella and cell body) during EC gap closure, results in sync with the rapid kinetics of protrusion/retraction motion. We hypothesize this dissipative EC behavior during gap closure is linked to actomyosin cytoskeletal rearrangement and decreased cross-linking during lamellipodia expansion. In summary, these studies of the kinetic and mechanical properties of EC lamellipodia and ruffles at gap boundaries yield insights into the mechanisms of vascular barrier restoration and potentially a model system for examining the druggability of lamellipodial protein targets to enhance vascular barrier integrity.

Cell nucleus elastography with the adjoint-based inverse solver

BACKGROUND AND OBJECTIVES

The mechanics of the nucleus depends on cellular structures and architecture, and impact a number of diseases. Nuclear mechanics is yet rather complex due to heterogeneous distribution of dense heterochromatin and loose euchromatin domains, giving rise to spatially variable stiffness properties.

METHODS

In this study, we propose to use the adjoint-based inverse solver to identify for the first time the nonhomogeneous elastic property distribution of the nucleus. Inputs of the inverse solver are deformation fields measured with microscopic imaging in contracting cardiomyocytes.

RESULTS

The feasibility of the proposed method is first demonstrated using simulated data. Results indicate accurate identification of the assumed heterochromatin region, with a maximum relative error of less than 5%. We also investigate the influence of unknown Poisson's ratio on the reconstruction and find that variations of the Poisson's ratio in the range [0.3-0.5] result in uncertainties of less than 15% in the identified stiffness. Finally, we apply the inverse solver on actual deformation fields acquired within the nuclei of two cardiomyocytes. The obtained results are in good agreement with the density maps obtained from microscopy images.

CONCLUSIONS

Overall, the proposed approach shows great potential for nuclear elastography, with promising value for emerging fields of mechanobiology and mechanogenetics.

A Method for Analyzing AFM Force Mapping Data Obtained from Soft Tissue Cryosections

Atomic force microscopy (AFM) is a valuable tool for assessing mechanical properties of biological samples, but interpretations of measurements on whole tissues can be difficult due to the tissue's highly heterogeneous nature. To overcome such difficulties and obtain more robust estimates of tissue mechanical properties, we describe an AFM force mapping and data analysis pipeline to characterize the mechanical properties of cryosectioned soft tissues. We assessed this approach on mouse optic nerve head and rat trabecular meshwork, cornea, and sclera. Our data show that the use of repeated measurements, outlier exclusion, and log-normal data transformation increases confidence in AFM mechanical measurements, and we propose that this methodology can be broadly applied to measuring soft tissue properties from cryosections.

New tools reveal PCP-dependent polarized mechanics in the cortex and cytoplasm of single cells during convergent extension

Understanding biomechanics of biological systems is crucial for unraveling complex processes like tissue morphogenesis. However, current methods for studying cellular mechanics in vivo are limited by the need for specialized equipment and often provide limited spatiotemporal resolution. Here we introduce two new techniques, Tension by Transverse Fluctuation (TFlux) and in vivo microrheology, that overcome these limitations. They both offer time-resolved, subcellular biomechanical analysis using only fluorescent reporters and widely available microscopes. Employing these two techniques, we have revealed a planar cell polarity (PCP)-dependent mechanical gradient both in the cell cortex and the cytoplasm of individual cells engaged in convergent extension. Importantly, the non-invasive nature of these methods holds great promise for its application for uncovering subcellular mechanical variations across a wide array of biological contexts.

Summary

Non-invasive imaging-based techniques providing time-resolved biomechanical analysis at subcellular scales in developing vertebrate embryos.

Investigation of TiO2 nanoparticle interactions in the fibroblast NIH-3T3 cells via liquid-mode atomic force microscope

Long before we recognized how significant they were, nanoparticles were already all around in the environment. Since then, an extensive number of synthetic nanoparticles have been engineered to improve our quality of life through rigorous scientific research on their uses in practically every industry, including semiconductor devices, food, medicine, and agriculture. The extensive usage of nanoparticles in commodities that come into proximity with human skin and internal organs through medicine has raised significant concerns over the years. TiO2 nanoparticles (NPs) are widely employed in a wide range of industries, such as cosmetics and food packaging. The interaction and internalization of TiO2 NPs in living cells have been studied by the scientific community for many years. In the present study, we investigated the cell viability, nanomechanical characteristics, and fluorescence response of NIH-3T3 cells treated with sterile DMEM TiO2 nanoparticle solution using a liquid-mode atomic force microscope and a fluorescence microscope. Two different sorts of response systems have been observed in the cells depending on the size of the NPs. TiO2 nanoparticles smaller than 100 nm support its initial stages cell viability, and cells internalize and metabolize NPs. In contrast, bigger TiO2 NPs (> 100 nm) are not completely metabolized and cannot impair cell survival. Furthermore, bigger NPs above 100 nm could not be digested by the cells, therefore hindering cell development, whereas below 100 nm TiO2 stimulated uncontrolled cell growth akin to cancerous type cells. The cytoskeleton softens as a result of particle internalization, as seen by the nanomechanical characteristics of the nanoparticle treated cells. According to our investigations, TiO2 smaller than 100 nm facilitates unintended cancer cell proliferation, whereas larger NPs ultimately suppress cell growth. Before being incorporated into commercial products, similar effects or repercussions that could result from employing different NPs should be carefully examined.

Intracellular remodeling associated with endoplasmic reticulum stress modifies biomechanical compliance of bladder cells

Bladder cells face a challenging biophysical environment: mechanical cues originating from urine flow and regular contraction to enable the filling voiding of the organ. To ensure functional adaption, bladder cells rely on high biomechanical compliance, nevertheless aging or chronic pathological conditions can modify this plasticity. Obviously the cytoskeletal network plays an essential role, however the contribution of other, closely entangled, intracellular organelles is currently underappreciated. The endoplasmic reticulum (ER) lies at a crucial crossroads, connected to both nucleus and cytoskeleton. Yet, its role in the maintenance of cell mechanical stability is less investigated. To start exploring these aspects, T24 bladder cancer cells were treated with the ER stress inducers brefeldin A (10-40nM BFA, 24 h) and thapsigargin (0.1-100nM TG, 24 h). Without impairment of cell motility and viability, BFA and TG triggered a significant subcellular redistribution of the ER; this was associated with a rearrangement of actin cytoskeleton. Additional inhibition of actin polymerization with cytochalasin D (100nM CytD) contributed to the spread of the ER toward cell periphery, and was accompanied by an increase of cellular stiffness (Young´s modulus) in the cytoplasmic compartment. Shrinking of the ER toward the nucleus (100nM TG, 2 h) was related to an increased stiffness in the nuclear and perinuclear areas. A similar short-term response profile was observed also in normal human primary bladder fibroblasts. In sum, the ER and its subcellular rearrangement seem to contribute to the mechanical properties of bladder cells opening new perspectives in the study of the related stress signaling cascades. Video Abstract.

Understanding the Rhodopsin Worldview Through Atomic Force Microscopy (AFM): Structure, Stability, and Activity Studies

Rhodopsin is a G protein-coupled receptor (GPCR) present in the rod outer segment (ROS) of photoreceptor cells that initiates the phototransduction cascade required for scotopic vision. Due to the remarkable advancements in technological tools, the chemistry of rhodopsin has begun to unravel especially over the past few decades, but mostly at the ensemble scale. Atomic force microscopy (AFM) is a tool capable of providing critical information from a single-molecule point of view. In this regard, to bolster our understanding of rhodopsin at the nanoscale level, AFM-based imaging, force spectroscopy, and nano-indentation techniques were employed on ROS disc membranes containing rhodopsin, isolated from vertebrate species both in normal and diseased states. These AFM studies on samples from native retinal tissue have provided fundamental insights into the structure and function of rhodopsin under normal and dysfunctional states. We review here the findings from these AFM studies that provide important insights on the supramolecular organization of rhodopsin within the membrane and factors that contribute to this organization, the molecular interactions stabilizing the structure of the receptor and factors that can modify those interactions, and the mechanism underlying constitutive activity in the receptor that can cause disease.

Recoil Measurements in Drosophila Embryos: from Mounting to Image Analysis

Tension and force propagation play a central role in tissue morphogenesis, as they enable sub- and supra-cellular shape changes required for the generation of new structures. Force is often generated by the cytoskeleton, which forms complex meshworks that reach cell-cell or cell-extracellular matrix junctions to induce cellular rearrangements. These mechanical properties can be measured through laser microdissection, which concentrates energy in the tissue of interest, disrupting its cytoskeleton. If the tissue is undergoing tension, this cut will induce a recoil in the surrounding regions of the cut. This protocol describes how one can perform laser microdissection experiments and subsequently measure the recoil speed of the sample of interest. While we explain how to carry out these experiments in Drosophila embryos, the recoil calibration and downstream analyses can be applied to other types of preparations. Key features Allows measuring tension in live Drosophila embryos with a relatively simple approach. Describes a quick way to mount a high number of embryos. Includes a segmentation-free recoil quantification that reduces bias and speeds up analysis. Graphical overview.

Auxeticity as a Mechanobiological Tool to Create Meta-Biomaterials

Mechanical and morphological design parameters, such as stiffness or porosity, play important roles in creating orthopedic implants and bone substitutes. However, we have only a limited understanding of how the microarchitecture of porous scaffolds contributes to bone regeneration. Meta-biomaterials are increasingly used to precisely engineer the internal geometry of porous scaffolds and independently tailor their mechanical properties (e.g., stiffness and Poisson's ratio). This is motivated by the rare or unprecedented properties of meta-biomaterials, such as negative Poisson's ratios (i.e., auxeticity). It is, however, not clear how these unusual properties can modulate the interactions of meta-biomaterials with living cells and whether they can facilitate bone tissue engineering under static and dynamic cell culture and mechanical loading conditions. Here, we review the recent studies investigating the effects of the Poisson's ratio on the performance of meta-biomaterials with an emphasis on the relevant mechanobiological aspects. We also highlight the state-of-the-art additive manufacturing techniques employed to create meta-biomaterials, particularly at the micrometer scale. Finally, we provide future perspectives, particularly for the design of the next generation of meta-biomaterials featuring dynamic properties (e.g., those made through 4D printing).

Molecular and cellular level characterization of cytoskeletal mechanics using a quartz crystal microbalance

A quartz crystal microbalance (QCM) is an instrument that has the ability to measure nanogram-level changes in mass on a quartz sensor and is traditionally used to probe surface interactions and assembly kinetics of synthetic systems. The addition of dissipation monitoring (QCM-D) facilitates the study of viscoelastic systems, such as those relevant to molecular and cellular mechanics. Due to real-time recording of frequency and dissipation changes and single protein-level precision, the QCM-D is effective in interrogating the viscoelastic properties of cell surfaces and in vitro cellular components. However, few studies focus on the application of this instrument to cytoskeletal systems, whose dynamic parts create interesting emergent mechanics as ensembles that drive essential tasks, such as division and motility. Here, we review the ability of the QCM-D to characterize key kinetic and mechanical features of the cytoskeleton through in vitro reconstitution and cellular assays and outline how QCM-D studies can yield insightful mechanical data alone and in tandem with other biophysical characterization techniques.

Optical micro-elastography with magnetic excitation for high frequency rheological characterization of soft media

The propagation of shear waves in elastography at high frequency (>3 kHz) in viscoelastic media has not been extensively studied due to the high attenuation and technical limitations of current techniques. An optical micro-elastography (OME) technique using magnetic excitation for generating and tracking high frequency shear waves with enough spatial and temporal resolution was proposed. Ultrasonics shear waves (above 20 kHz) were generated and observed in polyacrylamide samples. A cutoff frequency, from where the waves no longer propagate, was observed to vary depending on the mechanical properties of the samples. The ability of the Kelvin-Voigt (KV) model to explain the high cutoff frequency was investigated. Two alternative measurement techniques, Dynamic Mechanical Analysis (DMA) and Shear Wave Elastography (SWE), were used to complete the whole frequency range of the velocity dispersion curve while avoid capturing guided waves in the low frequency range (<3 kHz). The combination of the three measurement techniques provided rheology information from quasi-static to ultrasonic frequency range. A key observation was that the full frequency range of the dispersion curve was necessary if one wanted to infer accurate physical parameters from the rheological model. By comparing the low frequency range with the high frequency range, the relative errors for the viscosity parameter could reach 60 % and they could be higher with higher dispersive behavior. The high cutoff frequency may be predicted in materials that follow a KV model over their entire measurable frequency range. The mechanical characterization of cell culture media could benefit from the proposed OME technique.

Measuring pico-Newton Forces with Lipid Anchors as Force Sensors in Molecular Dynamics Simulations

Binding forces between biomolecules are ubiquitous in nature but sometimes as weak as a few pico-Newtons (pN). In many cases, the binding partners are attached to biomembranes with the help of a lipid anchor. One important example are glycolipids that promote membrane adhesion through weak carbohydrate-carbohydrate binding between adjacent membranes. Here, we use molecular dynamics (MD) simulations to quantify the forces generated by bonds involving membrane-anchored molecules. We introduce a method in which the protrusion of the lipid anchors from the membrane acts as the force sensor. Our results with two different glycolipids reveal binding forces of up to 20 pN and corroborate the recent notion that carbohydrate-carbohydrate interactions are generic rather than specific.

Insights in Cell Biomechanics through Atomic Force Microscopy

We review the advances obtained by using Atomic Force Microscopy (AFM)-based approaches in the field of cell/tissue mechanics and adhesion, comparing the solutions proposed and critically discussing them. AFM offers a wide range of detectable forces with a high force sensitivity, thus allowing a broad class of biological issues to be addressed. Furthermore, it allows for the accurate control of the probe position during the experiments, providing spatially resolved mechanical maps of the biological samples with subcellular resolution. Nowadays, mechanobiology is recognized as a subject of great relevance in biotechnological and biomedical fields. Focusing on the past decade, we discuss the intriguing issues of cellular mechanosensing, i.e., how cells sense and adapt to their mechanical environment. Next, we examine the relationship between cell mechanical properties and pathological states, focusing on cancer and neurodegenerative diseases. We show how AFM has contributed to the characterization of pathological mechanisms and discuss its role in the development of a new class of diagnostic tools that consider cell mechanics as new tumor biomarkers. Finally, we describe the unique ability of AFM to study cell adhesion, working quantitatively and at the single-cell level. Again, we relate cell adhesion experiments to the study of mechanisms directly or secondarily involved in pathologies.

Uncovering the cytotoxic effects of air pollution with multi-modal imaging of in vitro respiratory models

Annually, an estimated seven million deaths are linked to exposure to airborne pollutants. Despite extensive epidemiological evidence supporting clear associations between poor air quality and a range of short- and long-term health effects, there are considerable gaps in our understanding of the specific mechanisms by which pollutant exposure induces adverse biological responses at the cellular and tissue levels. The development of more complex, predictive, in vitro respiratory models, including two- and three-dimensional cell cultures, spheroids, organoids and tissue cultures, along with more realistic aerosol exposure systems, offers new opportunities to investigate the cytotoxic effects of airborne particulates under controlled laboratory conditions. Parallel advances in high-resolution microscopy have resulted in a range of in vitro imaging tools capable of visualizing and analysing biological systems across unprecedented scales of length, time and complexity. This article considers state-of-the-art in vitro respiratory models and aerosol exposure systems and how they can be interrogated using high-resolution microscopy techniques to investigate cell-pollutant interactions, from the uptake and trafficking of particles to structural and functional modification of subcellular organelles and cells. These data can provide a mechanistic basis from which to advance our understanding of the health effects of airborne particulate pollution and develop improved mitigation measures.

Nanomechanical Profiling of Aβ42 Oligomer-Induced Biological Changes in Single Hippocampus Neurons

Understanding how Aβ42 oligomers induce changes in neurons from a mechanobiological perspective has important implications in neuronal dysfunction relevant to neurodegenerative diseases. However, it remains challenging to profile the mechanical responses of neurons and correlate the mechanical signatures to the biological properties of neurons given the structural complexity of cells. Here, we quantitatively investigate the nanomechanical properties of primary hippocampus neurons upon exposure to Aβ42 oligomers at the single neuron level by using atomic force microscopy (AFM). We develop a method termed heterogeneity-load-unload nanomechanics (HLUN), which exploits the AFM force spectra in the whole loading-unloading cycle, allowing comprehensive profiling of the mechanical properties of living neurons. We extract four key nanomechanical parameters, including the apparent Young's modulus, cell spring constant, normalized hysteresis, and adhesion work, that serve as the nanomechanical signatures of neurons treated with Aβ42 oligomers. These parameters are well-correlated with neuronal height increase, cortical actin filament strengthening, and calcium concentration elevation. Thus, we establish an HLUN method-based AFM nanomechanical analysis tool for single neuron study and build an effective correlation between the nanomechanical profile of the single neurons and the biological effects triggered by Aβ42 oligomers. Our finding provides useful information on the dysfunction of neurons from the mechanobiological perspective.

Strain-dependent elastography of cancer cells reveals heterogeneity and stiffening due to attachment

Because cells vary in thickness and in biomechanical properties, the use of a constant force trigger during atomic force microscopy (AFM) stiffness mapping produces a varied nominal strain that can obfuscate the comparison of local material properties. In this study, we measured the biomechanical spatial heterogeneity of ovarian and breast cancer cells by using an indentation-dependent pointwise Hertzian method. Force curves and surface topography were used together to determine cell stiffness as a function of nominal strain. By recording stiffness at a particular strain, it may be possible to improve comparison of the material properties of cells and produce higher contrast representations of cell mechanical properties. Defining a linear region of elasticity that corresponds to a modest nominal strain, we were able to clearly distinguish the mechanics of the perinuclear region of cells. We observed that, relative to the lamelopodial stiffness, the perinuclear region was softer for metastatic cancer cells than their nonmetastatic counterparts. Moreover, contrast in the strain-dependent elastography in comparison to conventional force mapping with Hertzian model analysis revealed a significant stiffening phenomenon in the thin lamellipodial region in which the modulus scales inversely and exponentially with cell thickness. The observed exponential stiffening is not affected by relaxation of cytoskeletal tension, but finite element modeling indicates it is affected by substrate adhesion. The novel cell mapping technique explores cancer cell mechanical nonlinearity that results from regional heterogeneity, which could help explain how metastatic cancer cells can show soft phenotypes while simultaneously increasing force generation and invasiveness.

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

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

Application of self-organizing maps to AFM-based viscoelastic characterization of breast cancer cell mechanics

Cell mechanical properties have been proposed as label free markers for diagnostic purposes in diseases such as cancer. Cancer cells show altered mechanical phenotypes compared to their healthy counterparts. Atomic Force Microscopy (AFM) is a widely utilized tool to study cell mechanics. These measurements often need skilful users, physical modelling of mechanical properties and expertise in data interpretation. Together with the need to perform many measurements for statistical significance and to probe wide enough areas in tissue structures, the application of machine learning and artificial neural network techniques to automatically classify AFM datasets has received interest recently. We propose the use of self-organizing maps (SOMs) as unsupervised artificial neural network applied to mechanical measurements performed via AFM on epithelial breast cancer cells treated with different substances that affect estrogen receptor signalling. We show changes in mechanical properties due to treatments, as estrogen softened the cells, while resveratrol led to an increase in cell stiffness and viscosity. These data were then used as input for SOMs. Our approach was able to distinguish between estrogen treated, control and resveratrol treated cells in an unsupervised manner. In addition, the maps enabled investigation of the relationship of the input variables.

Atomic force microscopy-based indentation of cells: modelling the effect of a pericellular coat

A simple analytical model is built up to account for the interface deformation effect in a spherical atomic force microscopy (AFM)-based quasi-static indentation of a living cell covered with a pericellular brush. The compression behaviour of the pericellular coat is described using the Alexander–de Gennes model that allows for nonlinear deformation. An approximate second-order relation between contact force and indenter displacement is obtained in implicit form, using the Hertzian solution as a first-order approximation. A method of fitting the indentation brush/cell model to experimental data is suggested based on the non-dimensionalized version of the displacement–force relation in the parametric form and illustrated with a specific example of AFM raw data taken from the literature.

Bladder Wall Stiffness after Cystectomy in Bladder Cancer Patients: A Preliminary Study

Bladder cancer (BlCa), specifically urothelial carcinomas, is a heterogeneous disease that derives from the urothelial lining. Two main classes of BlCa are acknowledged: the non-muscle invasive BlCa and the muscle-invasive BlCa; the latter constituting an aggressive disease which invades locally and metastasizes systemically. Distinguishing the specific microenvironment that cancer cells experience between mucosa and muscularis propria layers can help elucidate how these cells acquire invasive capacities. In this work, we propose to measure the micromechanical properties of both mucosa and muscularis propria layers of the bladder wall of BlCa patients, using atomic force microscopy (AFM). To do that, two cross-sections of both the macroscopically normal urinary bladder wall and the bladder wall adjacent to the tumor were collected and immediately frozen, prior to AFM samples analysis. The respective "twin" formalin-fixed paraffin-embedded tissue fragments were processed and later evaluated for histopathological examination. H&E staining suggested that tumors promoted the development of muscle-like structures in the mucosa surrounding the neoplastic region. The average Young's modulus (cell stiffness) in tumor-adjacent specimens was significantly higher in the muscularis propria than in the mucosa. Similarly, the tumor-free specimens had significantly higher Young's moduli in the muscularis propria than in the urothelium. Young's moduli were higher in all layers of tumor-adjacent tissues when compared with tumor-free samples. Here we provide insights into the stiffness of the bladder wall layers, and we show that the presence of tumor in the surrounding mucosa leads to an alteration of its smooth muscle content. The quantitative assessment of stiffness range here presented provides essential data for future research on BlCa and for understanding how the biomechanical stimuli can modulate cancer cells' capacity to invade through the different bladder layers.

Fluidic Force Microscopy and Atomic Force Microscopy Unveil New Insights into the Interactions of Preosteoblasts with 3D-Printed Submicron Patterns

Physical patterns represent potential surface cues for promoting osteogenic differentiation of stem cells and improving osseointegration of orthopedic implants. Understanding the early cell-surface interactions and their effects on late cellular functions is essential for a rational design of such topographies, yet still elusive. In this work, fluidic force microscopy (FluidFM) and atomic force microscopy (AFM) combined with optical and electron microscopy are used to quantitatively investigate the interaction of preosteoblasts with 3D-printed patterns after 4 and 24 h of culture. The patterns consist of pillars with the same diameter (200 nm) and interspace (700 nm) but distinct heights (500 and 1000 nm) and osteogenic properties. FluidFM reveals a higher cell adhesion strength after 24 h of culture on the taller pillars (32 ± 7 kPa versus 21.5 ± 12.5 kPa). This is associated with attachment of cells partly on the sidewalls of these pillars, thus requiring larger normal forces for detachment. Furthermore, the higher resistance to shear forces observed for these cells indicates an enhanced anchorage and can be related to the persistence and stability of lamellipodia. The study explains the differential cell adhesion behavior induced by different pillar heights, enabling advancements in the rational design of osteogenic patterns.

Cell-surface contacts determine volume and mechanical properties of human embryonic kidney 293 T cells

Alterations in the organization of the cytoskeleton precede the escape of adherent cells from the framework of cell-cell and cell-matrix interactions into suspension. With cytoskeletal dynamics being linked to cell mechanical properties, many studies elucidated this relationship under either native adherent or suspended conditions. In contrast, tethered cells that mimic the transition between both states have not been the focus of recent research. Using human embryonic kidney 293 T cells we investigated all three conditions in the light of alterations in cellular shape, volume, as well as mechanical properties and relate these findings to the level, structure, and intracellular localization of filamentous actin (F-actin). For cells adhered to a substrate, our data shows that seeding density affects cell size but does not alter their elastic properties. Removing surface contacts leads to cell stiffening that is accompanied by changes in cell shape, and a reduction in cellular volume but no alterations in F-actin density. Instead, we observe changes in the organization of F-actin indicated by the appearance of blebs in the semi-adherent state. In summary, our work reveals an interplay between molecular and mechanical alterations when cells detach from a surface that is mainly dominated by cell morphology.

Simultaneous assessment of radial and axial myocyte mechanics by combining atomic force microscopy and carbon fibre techniques

Cardiomyocytes sense and shape their mechanical environment, contributing to its dynamics by their passive and active mechanical properties. While axial forces generated by contracting cardiomyocytes have been amply investigated, the corresponding radial mechanics remain poorly characterized. Our aim is to simultaneously monitor passive and active forces, both axially and radially, in cardiomyocytes freshly isolated from adult mouse ventricles. To do so, we combine a carbon fibre (CF) set-up with a custom-made atomic force microscope (AFM). CF allows us to apply stretch and to record passive and active forces in the axial direction. The AFM, modified for frontal access to fit in CF, is used to characterize radial cell mechanics. We show that stretch increases the radial elastic modulus of cardiomyocytes. We further find that during contraction, cardiomyocytes generate radial forces that are reduced, but not abolished, when cells are forced to contract near isometrically. Radial forces may contribute to ventricular wall thickening during contraction, together with the dynamic re-orientation of cells and sheetlets in the myocardium. This new approach for characterizing cell mechanics allows one to obtain a more detailed picture of the balance of axial and radial mechanics in cardiomyocytes at rest, during stretch, and during contraction. This article is part of the theme issue 'The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease'.

Possibilities of using T-cell biophysical biomarkers of ageing

Ageing is interrelated with the development of immunosenescence. This article focuses on one of the cell sets of the adaptive immune system, T cells, and provides a review of the known changes in T cells associated with ageing. Such fundamental changes affect both cell molecular content and internal ordering. However, acquiring a complete description of the changes at these levels would require extensive measurements of parameters and, furthermore, important fine details of the internal ordering that may be difficult to detect. Therefore, an alternative approach for the characterisation of cells consists of the performance of physical measurements of the whole cell, such as deformability measurements or migration measurements: the physical parameters, complementing the commonly used chemical biomarkers, may contribute to a better understanding of the evolution of T-cell states during ageing. Mechanical measurements, among other biophysical measurements, have the advantage of their relative simplicity: one single parameter agglutinates the complex effects of the variety of changes that gradually appear in cells during ageing.

Large extracellular vesicles do not mitigate the harmful effect of hyperglycemia on endothelial cell mobility

Extracellular vesicles, especially the larger fraction (LEVs - large extracellular vesicles), are believed to be an important means of intercellular communication. Earlier studies on LEVs have shown their healing properties, especially in the vascular cells of diabetic patients. Uptake of LEVs by endothelial cells and internalization of their cargo have also been demonstrated. Endothelial cells change their properties under hyperglycemic conditions (HGC), which reduces their activity and is the cause of endothelial dysfunction. The aim of our study was to investigate how human umbilical vein endothelial cells (HUVECs) change their biological properties: shape, mobility, cell surface stiffness, as well as describe the activation of metabolic pathways after exposure to the harmful effects of HGC and the administration of LEVs released by endothelial cells. We obtained LEVs from HUVEC cultures in HGC and normoglycemia (NGC) using the filtration and ultracentrifugation methods. We assessed the size of LEVs and the presence of biomarkers such as phosphatidylserine, CD63, beta-actin and HSP70. We analyzed the LEVs uptake efficiency by HUVECs, HUVEC shape, actin cytoskeleton remodeling, surface stiffness and finally gene expression by mRNA analysis. Under HGC conditions, HUVECs were larger and had a stiffened surface and a strengthened actin cortex compared to cells under NGC condition. HGC also altered the activation of metabolic pathways, especially those related to intracellular transport, metabolism, and organization of cellular components. The most interesting observation in our study is that LEVs did not restore cell motility disturbed by HGC. Although, LEVs were not able to reverse this deleterious effect of HGC, they activated transcription of genes involved in protein synthesis and vesicle trafficking in HUVECs.

Quantitative Phase Imaging: Recent Advances and Expanding Potential in Biomedicine

Quantitative phase imaging (QPI) is a label-free, wide-field microscopy approach with significant opportunities for biomedical applications. QPI uses the natural phase shift of light as it passes through a transparent object, such as a mammalian cell, to quantify biomass distribution and spatial and temporal changes in biomass. Reported in cell studies more than 60 years ago, ongoing advances in QPI hardware and software are leading to numerous applications in biology, with a dramatic expansion in utility over the past two decades. Today, investigations of cell size, morphology, behavior, cellular viscoelasticity, drug efficacy, biomass accumulation and turnover, and transport mechanics are supporting studies of development, physiology, neural activity, cancer, and additional physiological processes and diseases. Here, we review the field of QPI in biology starting with underlying principles, followed by a discussion of technical approaches currently available or being developed, and end with an examination of the breadth of applications in use or under development. We comment on strengths and shortcomings for the deployment of QPI in key biomedical contexts and conclude with emerging challenges and opportunities based on combining QPI with other methodologies that expand the scope and utility of QPI even further.

Compression-induced buckling of a semiflexible filament in two and three dimensions

The ability of biomolecules to exert forces on their surroundings or resist compression from the environment is essential in a variety of biologically relevant contexts. For filaments in the low-temperature limit and under a constant compressive force, Euler buckling theory predicts a sudden transition from a compressed to a bent state in these slender rods. In this paper, we use a mean-field theory to show that if a semiflexible chain is compressed at a finite temperature with a fixed end-to-end distance (permitting fluctuations in the compressive forces), it exhibits a continuous phase transition to a buckled state at a critical level of compression. We determine a quantitatively accurate prediction of the transverse position distribution function of the midpoint of the chain that indicates this transition. We find the mean compressive forces are non-monotonic as the extension of the filament varies, consistent with the observation that strongly buckled filaments are less able to bear an external load. We also find that for the fixed extension (isometric) ensemble, the buckling transition does not coincide with the local minimum of the mean force (in contrast to Euler buckling). We also show the theory is highly sensitive to fluctuations in length in two dimensions, and that the buckling transition can still be accurately recovered by accounting for those fluctuations. These predictions may be useful in understanding the behavior of filamentous biomolecules compressed by fluctuating forces, relevant in a variety of biological contexts.

Piezo1 Channel as a Potential Target for Hindering Cardiac Fibrotic Remodeling

Fibrotic tissues share many common features with neoplasms where there is an increased stiffness of the extracellular matrix (ECM). In this review, we present recent discoveries related to the role of the mechanosensitive ion channel Piezo1 in several diseases, especially in regulating tumor progression, and how this can be compared with cardiac mechanobiology. Based on recent findings, Piezo1 could be upregulated in cardiac fibroblasts as a consequence of the mechanical stress and pro-inflammatory stimuli that occurs after myocardial injury, and its increased activity could be responsible for a positive feedback loop that leads to fibrosis progression. The increased Piezo1-mediated calcium flow may play an important role in cytoskeleton reorganization since it induces actin stress fibers formation, a well-known characteristic of fibroblast transdifferentiation into the activated myofibroblast. Moreover, Piezo1 activity stimulates ECM and cytokines production, which in turn promotes the phenoconversion of adjacent fibroblasts into new myofibroblasts, enhancing the invasive character. Thus, by assuming the Piezo1 involvement in the activation of intrinsic fibroblasts, recruitment of new myofibroblasts, and uncontrolled excessive ECM production, a new approach to blocking the fibrotic progression can be predicted. Therefore, targeted therapies against Piezo1 could also be beneficial for cardiac fibrosis.

Mechanical Forces Govern Interactions of Host Cells with Intracellular Bacterial Pathogens

To combat infectious diseases, it is important to understand how host cells interact with bacterial pathogens. Signals conveyed from pathogen to host, and vice versa, may be either chemical or mechanical. While the molecular and biochemical basis of host-pathogen interactions has been extensively explored, relatively less is known about mechanical signals and responses in the context of those interactions. Nevertheless, a wide variety of bacterial pathogens appear to have developed mechanisms to alter the cellular biomechanics of their hosts in order to promote their survival and dissemination, and in turn many host responses to infection rely on mechanical alterations in host cells and tissues to limit the spread of infection. In this review, we present recent findings on how mechanical forces generated by host cells can promote or obstruct the dissemination of intracellular bacterial pathogens. In addition, we discuss how in vivo extracellular mechanical signals influence interactions between host cells and intracellular bacterial pathogens. Examples of such signals include shear stresses caused by fluid flow over the surface of cells and variable stiffness of the extracellular matrix on which cells are anchored. We highlight bioengineering-inspired tools and techniques that can be used to measure host cell mechanics during infection. These allow for the interrogation of how mechanical signals can modulate infection alongside biochemical signals. We hope that this review will inspire the microbiology community to embrace those tools in future studies so that host cell biomechanics can be more readily explored in the context of infection studies.

Noble classical and quantum approach to model the optical properties of metallic nanoparticles to enhance the sensitivity of optoplasmonic sensors

The bright light obtained from the quantum principle has a key role in the construction of optical sensors. Yet, theoretical and experimental work highlights the challenges of overcoming the high cost and low efficiency of such sensors. Therefore, we report a metallic nanoparticle-based metasurface plasmons polariton using quantum and classical models. We have investigated the material properties, absorption cross-section, scattering cross-section, and efficiency of the classical model. By quantizing light-matter interaction, the quantum features of light - degree of squeezing, correlation, and entanglement are quantified numerically and computationally. In addition, we note the penetration depth and propagation length from a hybrid model in order to enhance the optoplasmonic sensor performance for imaging, diagnosing, and early perception of cancer cells with label-free, direct, and real-time detection. Our study findings conclude that the frequency of incident light, size, shape, and type of nanoparticles has a significant impact on the optical properties of metallic nanoparticles and the nonlinear optical properties of metallic nanoparticles are dynamic, enhancing the sensitivity of the optoplasmonic sensor. Moreover, the resulting bright light shows the systematic potential for further medical image processing.

Scaling microfluidic throughput with flow-balanced manifolds to simply control devices with multiple inlets and outlets

Microfluidics can bring unique functionalities to cell processing, but the small channel dimensions often limit the throughput for cell processing that prevents scaling necessary for key applications. While processing throughput can be improved by increasing cell concentration or flow rate, an excessive number or velocity of cells can result in device failure. Designing parallel channels can linearly increase the throughput by channel number, but for microfluidic devices with multiple inlets and outlets, the design of the channel architecture with parallel channels can result in intractable numbers of inlets and outlets. We demonstrate an approach to use multiple parallel channels for complex microfluidic designs that uses a second manifold layer to connect three inlets and five outlets per channel in a manner that balances flow properties through each channel. The flow balancing in the individual microfluidic channels was accomplished through a combination of analytical and finite element analysis modeling. Volumetric flow and cell flow velocity were measured in each multiplexed channel to validate these models. We demonstrate eight-channel operation of a label-free mechanical separation device that retains the accuracy of a single channel separation. Using the parallelized device and a model biomechanical cell system for sorting of cells based on their viability, we processed over 16 × 10⁶ cells total over three replicates at a rate of 5.3 × 10⁶ cells per hour. Thus, parallelization of complex microfluidics with a flow-balanced manifold system can enable higher throughput processing with the same number of inlet and outlet channels to control.

Shear-Driven Solidification and Nonlinear Elasticity in Epithelial Tissues

Biological processes, from morphogenesis to tumor invasion, spontaneously generate shear stresses inside living tissue. The mechanisms that govern the transmission of mechanical forces in epithelia and the collective response of the tissue to bulk shear deformations remain, however, poorly understood. Using a minimal cell-based computational model, we investigate the constitutive relation of confluent tissues under simple shear deformation. We show that an initially undeformed fluidlike tissue acquires finite rigidity above a critical applied strain. This is akin to the shear-driven rigidity observed in other soft matter systems. Interestingly, shear-driven rigidity can be understood by a critical scaling analysis in the vicinity of the second order critical point that governs the liquid-solid transition of the undeformed system. We further show that a solidlike tissue responds linearly only to small strains and but then switches to a nonlinear response at larger stains, with substantial stiffening. Finally, we propose a mean-field formulation for cells under shear that offers a simple physical explanation of shear-driven rigidity and nonlinear response in a tissue.

Seasonal changes in membrane structure and excitability in retinal neurons of goldfish (Carassius auratus) under constant environmental conditions

Seasonal modifications in the structure of cellular membranes occur as an adaptive measure to withstand exposure to prolonged environmental change. Little is known about whether such changes may occur independently of external cues, such as photoperiod or temperature, or how they may impact the central nervous system. We compared membrane properties of neurons isolated from the retina of goldfish (Carassius auratus), an organism well-adapted to extreme environmental change, during the summer and winter months. Goldfish were maintained in a facility under constant environmental conditions throughout the year. Analysis of whole-retina phospholipid composition using mass spectrometry-based lipidomics revealed a two-fold increase in phosphatidylethanolamine species during the winter, suggesting an increase in cell membrane fluidity. Atomic force microscopy was used to produce localized, nanoscale-force deformation of neuronal membranes. Measurement of Young's modulus indicated increased membrane-cortical stiffness (or decreased elasticity) in neurons isolated during the winter. Voltage-clamp electrophysiology was used to assess physiological changes in neurons between seasons. Winter neurons displayed a hyperpolarized reversal potential (Vrev) and a significantly lower input resistance (Rin) compared to summer neurons. This was indicative of a decrease in membrane excitability during the winter. Subsequent measurement of intracellular Ca2+ activity using Fura-2 microspectrofluorometry confirmed a reduction in action potential activity, including duration and action potential profile, in neurons isolated during the winter. These studies demonstrate chemical and biophysical changes that occur in retinal neurons of goldfish throughout the year without exposure to seasonal cues, and suggest a novel mechanism of seasonal regulation of retinal activity.

Cell monolayer deformation microscopy reveals mechanical fragility of cell monolayers following EMT

Tissue and cell mechanics are crucial factors in maintaining homeostasis and in development, with aberrant mechanics contributing to many diseases. During the epithelial-to-mesenchymal transition (EMT), a highly conserved cellular program in organismal development and cancer metastasis, cells gain the ability to detach from their original location and autonomously migrate. While a great deal of biochemical and biophysical changes at the single-cell level have been revealed, how the physical properties of multicellular assemblies change during EMT, and how this may affect disease progression, is unknown. Here we introduce cell monolayer deformation microscopy (CMDM), a new methodology to measure the planar mechanical properties of cell monolayers by locally applying strain and measuring their resistance to deformation. We employ this new method to characterize epithelial multicellular mechanics at early and late stages of EMT, finding the epithelial monolayers to be relatively compliant, ductile, and mechanically homogeneous. By comparison, the transformed mesenchymal monolayers, while much stiffer, were also more brittle, mechanically heterogeneous, displayed more viscoelastic creep, and showed sharp yield points at significantly lower strains. Here, CMDM measurements identify specific biophysical functional states of EMT and offer insight into how cell aggregates fragment under mechanical stress. This mechanical fingerprinting of multicellular assemblies using new quantitative metrics may also offer new diagnostic applications in healthcare to characterize multicellular mechanical changes in disease.

It Takes More than Two to Tango: Complex, Hierarchal, and Membrane-Modulated Interactions in the Regulation of Receptor Tyrosine Kinases

The search for an understanding of how cell fate and motility are regulated is not a purely scientific undertaking, but it can also lead to rationally designed therapies against cancer. The discovery of tyrosine kinases about half a century ago, the subsequent characterization of certain transmembrane receptors harboring tyrosine kinase activity, and their connection to the development of human cancer ushered in a new age with the hope of finding a treatment for malignant diseases in the foreseeable future. However, painstaking efforts were required to uncover the principles of how these receptors with intrinsic tyrosine kinase activity are regulated. Developments in molecular and structural biology and biophysical approaches paved the way towards better understanding of these pathways. Discoveries in the past twenty years first resulted in the formulation of textbook dogmas, such as dimerization-driven receptor association, which were followed by fine-tuning the model. In this review, the role of molecular interactions taking place during the activation of receptor tyrosine kinases, with special attention to the epidermal growth factor receptor family, will be discussed. The fact that these receptors are anchored in the membrane provides ample opportunities for modulatory lipid-protein interactions that will be considered in detail in the second part of the manuscript. Although qualitative and quantitative alterations in lipids in cancer are not sufficient in their own right to drive the malignant transformation, they both contribute to tumor formation and also provide ways to treat cancer. The review will be concluded with a summary of these medical aspects of lipid-protein interactions.

Viscoelasticity, Like Forces, Plays a Role in Mechanotransduction

Viscoelasticity and its alteration in time and space has turned out to act as a key element in fundamental biological processes in living systems, such as morphogenesis and motility. Based on experimental and theoretical findings it can be proposed that viscoelasticity of cells, spheroids and tissues seems to be a collective characteristic that demands macromolecular, intracellular component and intercellular interactions. A major challenge is to couple the alterations in the macroscopic structural or material characteristics of cells, spheroids and tissues, such as cell and tissue phase transitions, to the microscopic interferences of their elements. Therefore, the biophysical technologies need to be improved, advanced and connected to classical biological assays. In this review, the viscoelastic nature of cytoskeletal, extracellular and cellular networks is presented and discussed. Viscoelasticity is conceptualized as a major contributor to cell migration and invasion and it is discussed whether it can serve as a biomarker for the cells’ migratory capacity in several biological contexts. It can be hypothesized that the statistical mechanics of intra- and extracellular networks may be applied in the future as a powerful tool to explore quantitatively the biomechanical foundation of viscoelasticity over a broad range of time and length scales. Finally, the importance of the cellular viscoelasticity is illustrated in identifying and characterizing multiple disorders, such as cancer, tissue injuries, acute or chronic inflammations or fibrotic diseases.

Using Polyacrylamide Hydrogels to Model Physiological Aortic Stiffness Reveals that Microtubules Are Critical Regulators of Isolated Smooth Muscle Cell Morphology and Contractility

Vascular smooth muscle cells (VSMCs) are the predominant cell type in the medial layer of the aortic wall and normally exist in a quiescent, contractile phenotype where actomyosin-derived contractile forces maintain vascular tone. However, VSMCs are not terminally differentiated and can dedifferentiate into a proliferative, synthetic phenotype. Actomyosin force generation is essential for the function of both phenotypes. Whilst much is already known about the mechanisms of VSMC actomyosin force generation, existing assays are either low throughput and time consuming, or qualitative and inconsistent. In this study, we use polyacrylamide hydrogels, tuned to mimic the physiological stiffness of the aortic wall, in a VSMC contractility assay. Isolated VSMC area decreases following stimulation with the contractile agonists angiotensin II or carbachol. Importantly, the angiotensin II induced reduction in cell area correlated with increased traction stress generation. Inhibition of actomyosin activity using blebbistatin or Y-27632 prevented angiotensin II mediated changes in VSMC morphology, suggesting that changes in VSMC morphology and actomyosin activity are core components of the contractile response. Furthermore, we show that microtubule stability is an essential regulator of isolated VSMC contractility. Treatment with either colchicine or paclitaxel uncoupled the morphological and/or traction stress responses of angiotensin II stimulated VSMCs. Our findings support the tensegrity model of cellular mechanics and we demonstrate that microtubules act to balance actomyosin-derived traction stress generation and regulate the morphological responses of VSMCs.

Nonlinear large deformation of a spherical red blood cell induced by ultrasonic standing wave

A computational model is developed to investigate the nonlinear static deformation of a spherical (osmotically swollen) red blood cell (RBC) induced by ultrasonic standing wave. The ultrasonic standing wave can generate steady acoustic radiation stress to deform the cell, and in turn, the deformed cell reshapes the acoustic field. This is a real-time coupling problem between the acoustic field and the mechanical field. In the computational model, the acoustic radiation stress acting on the RBC membrane is modeled by adopting the nonviscous momentum flux theory. The RBC membrane is modeled as a hyperelastic shell considering the in-plane elasticity, bending elasticity, and surface tension of the membrane. The volume conservation constraint of the membrane sealing fluid is applied to ensure the osmotic balance of the membrane. To address this real-time coupling problem, the computational model is implemented by a finite element method algorithm. The numerical results are compared with the existing theoretical model and experimental data, and the strain hardening trend of the experimental data is successfully predicted, which verifies the accuracy and effectiveness of the computational model. The computational model can accurately extract the mechanical properties of cells from acoustic deformation experiments, which is helpful for the diagnosis of some human diseases.