AFM contribution to unveil pro- and eukaryotic cell mechanical properties

Penetrating the ultra-tough yeast cell wall with finite element analysis model-aided design of microtools

Microinjecting yeast cells has been challenging for decades with no significant breakthrough due to the ultra-tough cell wall and low stiffness of the traditional injector tip at the micro-scale. Penetrating this protection wall is the key step for artificially bringing foreign substance into the yeast. In this paper, a yeast cell model was built by using finite element analysis (FEA) method to analyze the penetrating process. The key parameters of the yeast cell wall in the model (the Young's modulus, the shear modulus, and the Lame constant) were calibrated according to a general nanoindentation experiment. Then by employing the calibrated model, the injection parameters were optimized to minimize the cell damage (the maximum cell deformation at the critical stress of the cell wall). Key guidelines were suggested for penetrating the cell wall during microinjection.

Holotomography and atomic force microscopy: a powerful combination to enhance cancer, microbiology and nanotoxicology research

Modern imaging strategies are paramount to studying living systems such as cells, bacteria, and fungi and their response to pathogens, toxicants, and nanomaterials (NMs) as modulated by exposure and environmental factors. The need to understand the processes and mechanisms of damage, healing, and cell survivability of living systems continues to motivate the development of alternative imaging strategies. Of particular interest is the use of label-free techniques (microscopy procedures that do not require sample staining) that minimize interference of biological processes by foreign marking substances and reduce intense light exposure and potential photo-toxicity effects. This review focuses on the synergic capabilities of atomic force microscopy (AFM) as a well-developed and robust imaging strategy with demonstrated applications to unravel intimate details in biomedical applications, with the label-free, fast, and enduring Holotomographic Microscopy (HTM) strategy. HTM is a technique that combines holography and tomography using a low intensity continuous illumination laser to investigate (quantitatively and non-invasively) cells, microorganisms, and thin tissue by generating three-dimensional (3D) images and monitoring in real-time inner morphological changes. We first review the operating principles that form the basis for the complementary details provided by these techniques regarding the surface and internal information provided by HTM and AFM, which are essential and complimentary for the development of several biomedical areas studying the interaction mechanisms of NMs with living organisms. First, AFM can provide superb resolution on surface morphology and biomechanical characterization. Second, the quantitative phase capabilities of HTM enable superb modeling and quantification of the volume, surface area, protein content, and mass density of the main components of cells and microorganisms, including the morphology of cells in microbiological systems. These capabilities result from directly quantifying refractive index changes without requiring fluorescent markers or chemicals. As such, HTM is ideal for long-term monitoring of living organisms in conditions close to their natural settings. We present a case-based review of the principal uses of both techniques and their essential contributions to nanomedicine and nanotoxicology (study of the harmful effects of NMs in living organisms), emphasizing cancer and infectious disease control. The synergic impact of the sequential use of these complementary strategies provides a clear drive for adopting these techniques as interdependent fundamental tools.

Nanomotion technology in combination with machine learning: a new approach for a rapid antibiotic susceptibility test for Mycobacterium tuberculosis

Nanomotion technology is a growth-independent approach that can be used to detect and record the vibrations of bacteria attached to cantilevers. We have developed a nanomotion-based antibiotic susceptibility test (AST) protocol for Mycobacterium tuberculosis (MTB). The protocol was used to predict strain phenotype towards isoniazid (INH) and rifampicin (RIF) using a leave-one-out cross-validation (LOOCV) and machine learning techniques. This MTB-nanomotion protocol takes 21 h, including cell suspension preparation, optimized bacterial attachment to functionalized cantilever, and nanomotion recording before and after antibiotic exposure. We applied this protocol to MTB isolates (n = 40) and were able to discriminate between susceptible and resistant strains for INH and RIF with a maximum sensitivity of 97.4% and 100%, respectively, and a maximum specificity of 100% for both antibiotics when considering each nanomotion recording to be a distinct experiment. Grouping recordings as triplicates based on source isolate improved sensitivity and specificity to 100% for both antibiotics. Nanomotion technology can potentially reduce time-to-result significantly compared to the days and weeks currently needed for current phenotypic ASTs for MTB. It can further be extended to other anti-TB drugs to help guide more effective TB treatment.

Robust Antibacterial Activity of Xanthan-Gum-Stabilized and Patterned CeO2-x-TiO2 Antifog Films

Increased occurrence of antimicrobial resistance leads to a huge burden on patients, the healthcare system, and society worldwide. Developing antimicrobial materials through doping rare-earth elements is a new strategy to overcome this challenge. To this end, we design antibacterial films containing CeO_2-x-TiO₂, xanthan gum, poly(acrylic acid), and hyaluronic acid. CeO_2-x-TiO₂ inks are additionally integrated into a hexagonal grid for prominent transparency. Such design yields not only an antibacterial efficacy of ∼100% toward Staphylococcus aureus and Escherichia coli but also excellent antifog performance for 72 h in a 100% humidity atmosphere. Moreover, FluidFM is employed to understand the interaction in-depth between bacteria and materials. We further reveal that reactive oxygen species (ROS) are crucial for the bactericidal activity of E. coli through fluorescent spectroscopic analysis and SEM imaging. We meanwhile confirm that Ce³⁺ ions are involved in the stripping phosphate groups, damaging the cell membrane of S. aureus. Therefore, the hexagonal mesh and xanthan-gum cross-linking chains act as a reservoir for ROS and Ce³⁺ ions, realizing a long-lasting antibacterial function. We hence develop an antibacterial and antifog dual-functional material that has the potential for a broad application in display devices, medical devices, food packaging, and wearable electronics.

Building a tissue: mesenchymal and epithelial cell spheroids' mechanical properties at micro- and nanoscale

Cell transitions between the epithelial and mesenchymal phenotypes provide the regulated morphogenesis and regeneration throughout the ontogenesis. The tissue mechanics and mechanotransduction play an essential role in these processes. Cell spheroids reproduce the cell density of native tissues and represent simple building blocks for the tissue engineering purposes. The mechanical properties of mesenchymal and epithelial cells have been extensively studied in 2D monolayer cultures, but have not been sufficiently compared in spheroids. Here, we have simultaneously applied several techniques to assess the mechanical parameters of such spheroids. The local surface mechanical properties were measured by AFM, and the bulk properties were analyzed with parallel-plate compression, as well as by observing cut opening after microdissection. The comparison of the collected data allowed us to apply the model of a solid body with surface tension, and estimate the parameters of this model. We found an expectedly higher surface tension in mesenchymal spheroids, as well as a higher bulk modulus and relaxation time. The two latter parameters agree with the bulk poroelastic behavior of spheroids, and with the higher cell density and extracellular matrix content in mesenchymal spheroids. The higher tension of the surface layer cells in mesenchymal cell spheroids was also confirmed by the viscoelastic AFM characterization. The cell phenotype affected the self-organization during the spheroid formation, as well as the structure, biomechanical properties, and spreading of spheroids. The obtained results will contribute to a more detailed description of spheroid and tissue biomechanics, and will help in controlling the tissue regeneration and morphogenesis. STATEMENT OF SIGNIFICANCE: Spheroids are widely used as building blocks for scaffold-based and scaffold-free strategies in tissue engineering. In the majority of the past studies, either the concept of a solid body or a liquid with surface tension was used to describe the biomechanical behavior of spheroids. Here, we have used a model which combines both aspects, a solid body with surface tension. The "solid" aspect was described as a visco-poroelastic material, affected by the liquid redistribution through the cells and ECM at the scale of the whole spheroid. A higher surface tension was found for mesenchymal spheroids than that for epithelial spheroids, observed as a higher stiffness of the spheroid surface, as well as a larger spontaneous opening of the cut edges after microdissection.

3D nanomechanical mapping of subcellular and sub-nuclear structures of living cells by multi-harmonic AFM with long-tip microcantilevers

Recent developments such as multi-harmonic Atomic Force Microscopy (AFM) techniques have enabled fast, quantitative mapping of nanomechanical properties of living cells. Due to their high spatiotemporal resolution, these methods provide new insights into changes of mechanical properties of subcellular structures due to disease or drug response. Here, we propose three new improvements to significantly improve the resolution, identification, and mechanical property quantification of sub-cellular and sub-nuclear structures using multi-harmonic AFM on living cells. First, microcantilever tips are streamlined using long-carbon tips to minimize long-range hydrodynamic interactions with the cell surface, to enhance the spatial resolution of nanomechanical maps and minimize hydrodynamic artifacts. Second, simultaneous Spinning Disk Confocal Microscopy (SDC) with live-cell fluorescent markers enables the unambiguous correlation between observed heterogeneities in nanomechanical maps with subcellular structures. Third, computational approaches are then used to estimate the mechanical properties of sub-nuclear structures. Results are demonstrated on living NIH 3T3 fibroblasts and breast cancer MDA-MB-231 cells, where properties of nucleoli, a deep intracellular structure, were assessed. The integrated approach opens the door to study the mechanobiology of sub-cellular structures during disease or drug response.

Mechanical properties of cell sheets and spheroids: the link between single cells and complex tissues

Cell aggregates, including sheets and spheroids, represent a simple yet powerful model system to study both biochemical and biophysical intercellular interactions. However, it is becoming evident that, although the mechanical properties and behavior of multicellular structures share some similarities with individual cells, yet distinct differences are observed in some principal aspects. The description of mechanical phenomena at the level of multicellular model systems is a necessary step for understanding tissue mechanics and its fundamental principles in health and disease. Both cell sheets and spheroids are used in tissue engineering, and the modulation of mechanical properties of cell constructs is a promising tool for regenerative medicine. Here, we review the data on mechanical characterization of cell sheets and spheroids, focusing both on advances in the measurement techniques and current understanding of the subject. The reviewed material suggest that interplay between the ECM, intercellular junctions, and cellular contractility determines the behavior and mechanical properties of the cell aggregates.

Hypergravity affects cell traction forces of fibroblasts

Cells sense and react on changes of the mechanical properties of their environment and, likewise, respond to external mechanical stress applied to them. However, whether the gravitational field as overall body force modulates cellular behavior is unclear. Different studies demonstrated that micro- and hypergravity influences the shape and elasticity of cells, initiate cytoskeleton reorganization, and influence cell motility. All these cellular properties are interconnected and contribute to forces that cells apply on their surrounding microenvironment. Yet, studies that investigated changes of cell traction forces under hypergravity conditions are scarce. Here, we performed hypergravity experiments on 3T3 fibroblast cells using the large-diameter centrifuge at the European Space Agency - European Space Research and Technology Centre. Cells were exposed to hypergravity of up to 19.5 g for 16 h in both the upright and the inverted orientation with respect to the g-force vector. We observed a decrease in cellular traction forces when the gravitational field was increased up to 5.4 g, followed by an increase of traction forces for higher gravity fields up to 19.5 g independent of the orientation of the gravity vector. We attribute the switch in cellular response to shear thinning at low g-forces, followed by significant rearrangement and enforcement of the cytoskeleton at high g-forces.

Heterogeneity of nanomechanical properties of the human umbilical vein endothelial cell surface

Endothelial cells, due to heterogeneity in the cell structure, can potentially form an inhomogeneous on structural and mechanical properties of the inner layer of the capillaries. Using quantitative nanomechanical mapping mode of atomic force microscopy, the parameters of the structural, elastic, and adhesive properties of the cell surface for living and glutaraldehyde-fixed human umbilical vein endothelial cells were studied. A significant difference in the studied parameters for three cell surface zones (peripheral, perinuclear, and nuclear zones) was established. The perinuclear zone appeared to be the softest zone of the endothelial cell surface. The heterogeneity of the endothelial cell mechanical properties at the nanoscale level can be an important mechanism in regulating the endothelium functions in blood vessels.

Nanomotion Detection-Based Rapid Antibiotic Susceptibility Testing

Rapid antibiotic susceptibility testing (AST) could play a major role in fighting multidrug-resistant bacteria. Recently, it was discovered that all living organisms oscillate in the range of nanometers and that these oscillations, referred to as nanomotion, stop as soon the organism dies. This finding led to the development of rapid AST techniques based on the monitoring of these oscillations upon exposure to antibiotics. In this review, we explain the working principle of this novel technique, compare the method with current ASTs, explore its application and give some advice about its implementation. As an illustrative example, we present the application of the technique to the slowly growing and pathogenic Bordetella pertussis bacteria.

Enhanced Osteogenic Differentiation of Human Bone Marrow-Derived Mesenchymal Stem Cells by a Hybrid Hydroxylapatite/Collagen Scaffold

Human bone marrow-derived mesenchymal stem cells (hBMSCs) and their derivative enhanced green fluorescent protein (eGFP)-hBMSCs were employed to evaluate an innovative hybrid scaffold composed of granular hydroxylapatite and collagen hemostat (Coll/HA). The cellular morphology/cytoskeleton organization and cell viability were investigated by immunohistochemistry (IHC) and AlamarBlue metabolic assay, respectively. The expression of osteopontin and osteocalcin proteins was analyzed by IHC and ELISA, whereas osteogenic genes were investigated by quantitative PCR (Q-PCR). Cell morphology of eGFP-hBMSCs was indistinguishable from that of parental hBMSCs. The cytoskeleton architecture of hBMSCs grown on the scaffold appeared to be well organized, whereas its integrity remained uninfluenced by the scaffold during the time course. Metabolic activity measured in hBMSCs grown on a biomaterial was increased during the experiments, up to day 21 (p < 0.05). The biomaterial induced the matrix mineralization in hBMSCs. The scaffold favored the expression of osteogenic proteins, such as osteocalcin and osteopontin. In hBMSC cultures, the scaffold induced up-regulation in specific genes that are involved in ossification process (BMP2/3, SPP1, SMAD3, and SP7), whereas they showed an up-regulation of MMP9 and MMP10, which play a central role during the skeletal development. hBMSCs were induced to chondrogenic differentiation through up-regulation of COL2A1 gene. Our experiments suggest that the innovative scaffold tested herein provides a good microenvironment for hBMSC adhesion, viability, and osteoinduction. hBMSCs are an excellent in vitro cellular model to assay scaffolds, which can be employed for bone repair and bone tissue engineering.

Dynamic AFM detection of the oxidation-induced changes in size, stiffness, and stickiness of low-density lipoprotein

Background

Low-density lipoprotein (LDL) is an important plasma lipoprotein transporting lipids to peripheral tissues/cells. The oxidation of LDL plays critical roles in atherogenesis and its oxidized form (oxLDL) is an important risk factor of atherosclerosis. The biomechanical properties of LDL/oxLDL are closely correlated with the disease. To date, however, the oxidation-induced changes in size and biomechanical properties (stiffness and stickiness) of LDL particles are less investigated.

Methods

In this study, copper-induced LDL oxidation was confirmed by detecting electrophoretic mobility, malondialdehyde production, and conjugated diene formation. Then, the topographical and biomechanical mappings of LDL particles before/after and during oxidation were performed by using atomic force microscopy (AFM) and the size and biomechanical forces of particles were measured and quantitatively analyzed.

Results

Oxidation induced a significant decrease in size and stiffness (Young’s modulus) but a significant increase in stickiness (adhesion force) of LDL particles. The smaller, softer, and stickier characteristics of oxidized LDL (oxLDL) partially explains its pro-atherosclerotic role.

Conclusions

The data implies that LDL oxidation probably aggravates atherogenesis by changing the size and biomechanical properties of LDL particles. The data may provide important information for a better understanding of LDL/oxLDL and atherosclerosis.

Viscoelasticity and Volume of Cortical Neurons under Glutamate Excitotoxicity and Osmotic Challenges

Neural activity depends on the maintenance of ionic and osmotic homeostasis. Under these conditions, the cell volume must be regulated to maintain optimal neural function. A disturbance in the neuronal volume regulation often occurs in pathological conditions such as glutamate excitotoxicity. The cell volume, mechanical properties, and actin cytoskeleton structure are tightly connected to achieve the cell homeostasis. Here, we studied the effects of glutamate-induced excitotoxicity, external osmotic pressure, and inhibition of actin polymerization on the viscoelastic properties and volume of neurons. Atomic force microscopy was used to map the viscoelastic properties of neurons in time-series experiments to observe the dynamical changes and a possible recovery. The data obtained on cultured rat cortical neurons were compared with the data obtained on rat fibroblasts. The neurons were found to be more responsive to the osmotic challenges but less sensitive to the inhibition of actin polymerization than fibroblasts. The alterations of the viscoelastic properties caused by glutamate excitotoxicity were similar to those induced by the hypoosmotic stress, but, in contrast to the latter, they did not recover after the glutamate removal. These data were consistent with the dynamic volume changes estimated using ratiometric fluorescent dyes. The recovery after the glutamate-induced excitotoxicity was slow or absent because of a steady increase in intracellular calcium and sodium concentrations. The viscoelastic parameters and their changes were related to such parameters as the actin cortex stiffness, tension, and cytoplasmic viscosity.

Mechanistic Investigation of a Self-Assembling Peptide against Escherichia coli

Because of their distinctive mode of action in targeting bacterial cell membranes, antimicrobial peptides (AMPs) are increasingly regarded as a potential candidate for the development of novel antibiotics to combat the wide spread of bacterial resistance. To date, understanding of the exact molecular process by which AMPs act on the real bacterial envelope remains challenging. Simultaneously, the aggregated state of AMPs upon interaction with bacterial envelopes is still elusive. Previously, we have demonstrated that the potent antibacterial activity of a designed surfactant-like peptide Ac-A₉K-NH₂ benefited greatly from its high self-assembling ability and appropriate self-assembled morphologies and sizes. By using high-resolution atomic force microscopy, we here not only follow the variations of the Escherichia coli cell envelope in the presence of Ac-A₉K-NH₂ but also characterize the peptide aggregates on the bacterial surface as well as on the substrate surface. The results, together with those from fluorescence, zeta potential, circular dichroism, and scanning electron microscopy measurements, indicate that both the positively charged peptide monomers and self-assembled nanostructures can directly act on the negatively charged bacterial surface, followed by their insertion into the bacterial membrane, the formation of surface nanopores, and membrane lysis. The mechanism of Ac-A₉K-NH₂ against E. coli is thus consistent with the detergent-like mode of action. This work enhances our mechanistic understanding of the antibacterial behaviors of self-assembling peptides that will be valuable in exploring their biomedical applications.

Measurement of mechanical properties of naked cell membranes using atomic force microscope puncture test

Cell membrane defines the physical boundary of the cell, maintains their shape and volume, and meanwhile dominates the cells response to mechanical force. There are many limitations in the mechanical analysis of naked cell membrane, such as cell condition, the supporting of cytoskeleton and cytoplasm. To reduce these influences, non-supported membrane was prepared on high-ordered pore array silicon substrate. Rupture forces and Young's modulus of three non-supported membranes, red blood cell membrane, somatic cell (MDCK) membrane and phospholipids membrane, are obtained quantitatively using AFM puncture tests. Results indicate that the sequence of both rupture forces and Young's modulus of the three membranes is MDCK cell membrane > red blood cell membrane > phospholipids membrane. The determinant of naked cell membrane's mechanical properties is the constituent of membrane itself, including the quantity and distribution of membrane proteins. Focused on the distribution of membrane proteins, two proposed models of cell membrane-the semi-mosaic model of red blood cell membrane and the Protein Layer-Lipid-Protein Island (PLLPI) model of nucleated mammalian cell membrane can be used to explain the different mechanical properties of naked cell membranes.

Yeast Nanometric Scale Oscillations Highlights Fibronectin Induced Changes in C. albicans

Yeast resistance to antifungal drugs is a major public health issue. Fungal adhesion onto the host mucosal surface is still a partially unknown phenomenon that is modulated by several actors among which fibronectin plays an important role. Targeting the yeast adhesion onto the mucosal surface could lead to potentially highly efficient treatments. In this work, we explored the effect of fibronectin on the nanomotion pattern of different Candida albicans strains by atomic force microscopy (AFM)-based nanomotion detection and correlated the cellular oscillations to the yeast adhesion onto epithelial cells. Preliminary results demonstrate that strongly adhering strains reduce their nanomotion activity upon fibronectin exposure whereas low adhering Candida remain unaffected. These results open novel avenues to explore cellular reactions upon exposure to stimulating agents and possibly to monitor in a rapid and simple manner adhesive properties of C. albicans.

Evaluation of Nanomechanical Properties of Tomato Root by Atomic Force Microscopy

Here, different tissue surfaces of tomato root were characterized employing atomic force microscopy on day 7 and day 21 of growth through Young's modulus and plasticity index. These parameters provide quantitative information regarding the mechanical behavior of the tomato root under fresh conditions in different locations of the cross-section of root [cell surface of the epidermis, parenchyma (Pa), and vascular bundles (Vb)]. The results show that the mechanical parameters depend on the indented region, tissue type, and growth time. Thereby, the stiffness increases in the cell surface of epidermal tissue with increasing growth time (from 9.19 ± 0.68 to 13.90 ± 1.68 MPa) and the cell surface of Pa tissue displays the opposite behavior (from 1.74 ± 0.49 to 0.48 ± 0.55); the stiffness of cell surfaces of Vb tissue changes from 10.60 ± 0.58 to 6.37 ± 0.53 MPa, all cases showed a statistical difference (p &lt; 0.05). Viscoelastic behavior dominates the mechanical forces in the tomato root. The current study is a contribution to a better understanding of the cell mechanics behavior of different tomato root tissues during growth.

Evaluating the effect of aminoglycosides on the interaction between bovine serum albumins by atomic force microscopy

Characterization and determination of protein-protein interactions (PPIs) plays an important role in molecular biological science. In this study, the effect of aminoglycosides (AGs: streptomycin, gentamycin, lincomycin and clindamycin) on interactions between bovine serum albumin (BSA) was evaluated employing imaging and probing adhesion event by AFM. Multi-spectroscopy and molecular docking were supplementary to investigate the acting forces of the effect. AFM measurements revealed the aggregation of BSA grains and changes of adhesion forces at single molecule level. With adhesion forces between BSA pairs decomposed by Poisson method, specific forces in streptomycin, gentamycin, lincomycin and climdamycin were obviously decreased with the rate of 33.1%, 26.4%, 32.3% and 31.3% while non-specific forces slightly decreased with 5.5%, 3.3%, 4.0% and 7.7%. Combined with results of multi-spectroscopy as well as molecular docking, the whole determination showed AGs affected PPIs by multiple forces, where the hydrogen bonding and hydration effect were the main reasons. The binding of drugs and proteins acted by hydrogen bonding affected the interaction forces between BSA. Consequently, AFM was proposed to be an effective and precise tool in application including evaluating the effects of exogenous compounds on biomacromolecular interactions and rapid screening of drug candidates to avoid potential damages in disease treatment.

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.

Tunable Hybrid Biopolymeric Hydrogel Scaffolds Based on Atomic Force Microscopy Characterizations for Tissue Engineering

Developing adequate biomaterials to engineer cell-scaffold interactions has become a promising way for physically regulating the biological behaviors of cells in the field of tissue engineering. Biopolymeric hydrogels have shown great merits as cellular scaffolds due to their biocompatible and biodegradable characteristics. In particular, the advent of atomic force microscopy (AFM) provides a powerful tool for characterizing native specimens at the micro/nanoscale, but utilizing AFM to investigate the detailed structures and properties of hydrogel scaffolds has been still scarce. In this paper, hybrid natural biopolymers are used to form hydrogel scaffolds which exhibit tunable structural and mechanical properties characterized by AFM peak force tapping imaging, and the applications of the formed hydrogel scaffolds in tissue engineering are studied. AFM morphological images showed that the cross-linking reactions of sodium alginate and gum arabic via calcium cations yielded the porous hydrogel scaffolds. By altering the component ratios, AFM mechanical images showed that the porous and mechanical properties (Young's modulus and adhesion force) of the hydrogel scaffolds were tunable. Next, the nanoscale structural and mechanical dynamics of the fabricated hydrogel scaffolds during the degradation process were revealed by AFM peak force tapping imaging. The experimental results on three different types of cells showed that the fabricated hydrogel scaffolds facilitate the formation of cellular spheroids. The research provides a novel idea to design tunable hydrogel scaffolds based on AFM characterizations for investigating cell-scaffold interactions, which will have potential impacts on tissue engineering.

Biomechanical Characterization at the Cell Scale: Present and Prospects

The rapidly growing field of mechanobiology demands for robust and reproducible characterization of cell mechanical properties. Recent achievements in understanding the mechanical regulation of cell fate largely rely on technological platforms capable of probing the mechanical response of living cells and their physico–chemical interaction with the microenvironment. Besides the established family of atomic force microscopy (AFM) based methods, other approaches include optical, magnetic, and acoustic tweezers, as well as sensing substrates that take advantage of biomaterials chemistry and microfabrication techniques. In this review, we introduce the available methods with an emphasis on the most recent advances, and we discuss the challenges associated with their implementation.

Receptor-mediated endocytosis generates nanomechanical force reflective of ligand identity and cellular property

Whether environmental (thermal, chemical, and nutrient) signals generate quantifiable, nanoscale, mechanophysical changes in the cellular plasma membrane has not been well elucidated. Assessment of such mechanophysical properties of plasma membrane may shed lights on fundamental cellular process. Atomic force microscopic (AFM) measurement of the mechanical properties of live cells was hampered by the difficulty in accounting for the effects of the cantilever motion and the associated hydrodynamic force on the mechanical measurement. These challenges have been addressed in our recently developed control-based AFM nanomechanical measurement protocol, which enables a fast, noninvasive, broadband measurement of the real-time changes in plasma membrane elasticity in live cells. Here we show using this newly developed AFM platform that the plasma membrane of live mammalian cells exhibits a constant and quantifiable nanomechanical property, the membrane elasticity. This mechanical property sensitively changes in response to environmental factors, such as the thermal, chemical, and growth factor stimuli. We demonstrate that different chemical inhibitors of endocytosis elicit distinct changes in plasma membrane elastic modulus reflecting their specific molecular actions on the lipid configuration or the endocytic machinery. Interestingly, two different growth factors, EGF and Wnt3a, elicited distinct elastic force profiles revealed by AFM at the plasma membrane during receptor-mediated endocytosis. By applying this platform to genetically modified cells, we uncovered a previously unknown contribution of Cdc42, a key component of the cellular trafficking network, to EGF-stimulated endocytosis at plasma membrane. Together, this nanomechanical AFM study establishes an important foundation that is expandable and adaptable for investigation of cellular membrane evolution in response to various key extracellular signals.