Cell mechanics: mechanical response, cell adhesion, and molecular deformation

Viscoelasticity of diverse biological samples quantified by Acoustic Force Microrheology (AFMR)

In the context of soft matter and cellular mechanics, microrheology - the use of micron-sized particles to probe the frequency-dependent viscoelastic response of materials - is widely used to shed light onto the mechanics and dynamics of molecular structures. Here we present the implementation of active microrheology in an Acoustic Force Spectroscopy setup (AFMR), which combines multiplexing with the possibility of probing a wide range of forces ( ~ pN to ~nN) and frequencies (0.01-100 Hz). To demonstrate the potential of this approach, we perform active microrheology on biological samples of increasing complexity and stiffness: collagen gels, red blood cells (RBCs), and human fibroblasts, spanning a viscoelastic modulus range of five orders of magnitude. We show that AFMR can successfully quantify viscoelastic properties by probing many beads with high single-particle precision and reproducibility. Finally, we demonstrate that AFMR to map local sample heterogeneities as well as detect cellular responses to drugs.

Enhanced solute transport and steady mechanical stimulation in a novel dynamic perifusion bioreactor increase the efficiency of the in vitro culture of ovarian cortical tissue strips

Introduction: We report the development and preliminary evaluation of a novel dynamic bioreactor to culture ovarian cortical tissue strips that leverages tissue response to enhanced oxygen transport and adequate mechanical stimulation. In vitro multistep ovarian tissue static culture followed by mature oocyte generation, fertilization, and embryo transfer promises to use the reserve of dormant follicles. Unfortunately, static in vitro culture of ovarian tissue does not promote development of primordial to secondary follicles or sustain follicle viability and thereby limits the number of obtainable mature oocytes. Enhancing oxygen transport to and exerting mechanical stimulation on ovarian tissue in a dynamic bioreactor may more closely mimic the physiological microenvironment and thus promote follicle activation, development, and viability. Materials and Methods: The most transport-effective dynamic bioreactor design was modified using 3D models of medium and oxygen transport to maximize strip perifusion and apply tissue fluid dynamic shear stresses and direct compressive strains to elicit tissue response. Prototypes of the final bioreactor design were manufactured with materials of varying cytocompatibility and assessed by testing the effect of leachables on sperm motility. Effectiveness of the bioreactor culture was characterized against static controls by culturing fresh bovine ovarian tissue strips for 7 days at 4.8 × 10-5 m/s medium filtration flux in air at -15% maximal total compressive strain and by assessing follicle development, health, and viability. Results and Conclusions: Culture in dynamic bioreactors promoted effective oxygen transport to tissues and stimulated tissues with strains and fluid dynamic shear stresses that, although non-uniform, significantly influenced tissue metabolism. Tissue strip culture in bioreactors made of cytocompatible polypropylene preserved follicle viability and promoted follicle development better than static culture, less so in bioreactors made of cytotoxic ABS-like resin.

Coleus vettiveroides ethanolic root extract induces cytotoxicity by intrinsic apoptosis in HepG2 cells

Hepatocellular carcinoma (HCC) contributes to more than 80% of all primary cancers globally and ranks fourth in cancer-related deaths, due to the lack of an effective, definite therapeutic drug. Coleus vettiveroides (CV) has been used in Indian traditional medicine to treat diabetes, liver ailments, skin diseases, leukoderma, and leprosy. This study investigates the anticancer effect of CV ethanolic root extract in HepG2 cells. HepG2 cells were treated with CV extract, and its cytotoxicity was analyzed by MTT assay. AO/EB staining, propidium iodide staining, DCFH-DA assay, phalloidine staining, flow cytometry, and qPCR studies were performed for ROS expression, apoptosis and cell cycle analysis. The phytochemical analysis confirmed the presence of quercetin and galangin in CV root extract. The results showed that CV inhibited the proliferation of HepG2 cells, with altered cellular and nuclear morphology. CV was also found to increase intracellular ROS levels and oxidative stress markers in HepG2 cells. CV significantly altered the actin microfilament distribution in HepG2 cells and caused cell cycle arrest at the sub G0 -G1 phase. CV also induced mitochondria-mediated apoptosis, as evidenced by increased expression of p53, Bax, cytochrome C, Apaf-1, PARP, caspase-3 and caspase-9, and downregulated Bcl-2 expression. Therefore, CV exerts its anticancer effect by inducing mitochondrial dysfunction, oxidative stress, cytoskeletal disorganization, cell cycle arrest, and mitochondria-mediated apoptosis, and it could be a potent therapeutic option for HCC.

Computational simulation of applying mechanical vibration to mesenchymal stem cell for mechanical modulation toward bone tissue engineering

Evaluation of cell response to mechanical stimuli at in vitro conditions is known as one of the important issues for modulating cell behavior. Mechanical stimuli, including mechanical vibration and oscillatory fluid flow, act as important biophysical signals for the mechanical modulation of stem cells. In the present study, mesenchymal stem cell (MSC) consists of cytoplasm, nucleus, actin, and microtubule. Also, integrin and primary cilium were considered as mechanoreceptors. In this study, the combined effect of vibration and oscillatory fluid flow on the cell and its components were investigated using numerical modeling. The results of the FEM and FSI model showed that the cell response (stress and strain values) at the frequency of 30 H z mechanical vibration has the highest value. The achieved results on shear stress caused by the fluid flow on the cell showed that the cell experiences shear stress in the range of 0 . 1 - 10 Pa . Mechanoreceptors that bind separately to the cell surface, can be highly stimulated by hydrodynamic pressure and, therefore, can play a role in the mechanical modulation of MSCs at in vitro conditions. The results of this research can be effective in future studies to optimize the conditions of mechanical stimuli applied to the cell culture medium and to determine the mechanisms involved in mechanotransduction.

Biomembrane force probe (BFP): Design, advancements, and recent applications to live-cell mechanobiology

Mechanical forces play a vital role in biological processes at molecular and cellular levels, significantly impacting various diseases such as cancer, cardiovascular disease, and COVID-19. Recent advancements in dynamic force spectroscopy (DFS) techniques have enabled the application and measurement of forces and displacements with high resolutions, providing crucial insights into the mechanical pathways underlying these diseases. Among DFS techniques, the biomembrane force probe (BFP) stands out for its ability to measure bond kinetics and cellular mechanosensing with pico-newton and nano-meter resolutions. Here, a comprehensive overview of the classical BFP-DFS setup is presented and key advancements are emphasized, including the development of dual biomembrane force probe (dBFP) and fluorescence biomembrane force probe (fBFP). BFP-DFS allows us to investigate dynamic bond behaviors on living cells and significantly enhances the understanding of specific ligand-receptor axes mediated cell mechanosensing. The contributions of BFP-DFS to the fields of cancer biology, thrombosis, and inflammation are delved into, exploring its potential to elucidate novel therapeutic discoveries. Furthermore, future BFP upgrades aimed at improving output and feasibility are anticipated, emphasizing its growing importance in the field of cell mechanobiology. Although BFP-DFS remains a niche research modality, its impact on the expanding field of cell mechanobiology is immense.

The Role of Membrane-Tethered Mucins in Axial Epithelial Adhesion in Controlled Normal Stress Environments

The collective adhesive behavior of epithelial cell layers mediated by complex macromolecular fluid environments plays a vital role in many biological processes. Mucins, a family of highly glycosylated proteins, are known to lubricate cell-on-cell contacts in the shear direction. However, the role of mucins mediating axial epithelial adhesion in the direction perpendicular to the plane of the cell sheet has received less attention. This article subjects cell-on-cell layers of live ocular epithelia that express mucins on their apical surfaces to compression/decompression cycles and tensile loading using a customized instrument. In addition to providing compressive moduli of native cell-on-cell layers, it is found that the mucin layer between the epithelia acts as a soft cushion between the epithelial cell layers. Decompression experiments reveal mucin layers act as soft, nonlinear springs in the axial direction. The cell-on-cell layers withstand decompression before fracturing by a cohesive failure within the mucin layer. When mucin deficiency is induced via a protease treatment, it is found that the axial adhesion between the cell layers is increased. The findings which correlate changes in biological factors with changes in mechanical properties might be of interest to challenges in ophthalmology, vision care, and mucus research.

Nano- and Microemulsions in Biomedicine: From Theory to Practice

Nano- and microemulsions are colloidal systems that are widely used in various fields of biomedicine, including wound and burn healing, cosmetology, the development of antibacterial and antiviral drugs, oncology, etc. The stability of these systems is governed by the balance of molecular interactions between nanodomains. Microemulsions as a colloidal form play a special important role in stability. The microemulsion is the thermodynamically stable phase from oil, water, surfactant and co-surfactant which forms the surface of drops with very small surface energy. The last phenomena determines the shortage time of all fluid dispersions including nanoemulsions and emulgels. This review examines the theory and main methods of obtaining nano- and microemulsions, particularly focusing on the structure of microemulsions and methods for emulsion analysis. Additionally, we have analyzed the main preclinical and clinical studies in the field of wound healing and the use of emulsions in cancer therapy, emphasizing the prospects for further developments in this area.

Principles and advances in ultrafast photoacoustics; applications to imaging cell mechanics and to probing cell nanostructure

In this article we first present the foundations of ultrafast photoacoustics, a technique where the acoustic wavelength in play can be considerably shorter than the optical wavelength. The physics primarily involved in the conversion of short light pulses into high frequency sound is described. The mechanical disturbances following the relaxation of hot electrons in metals and other processes leading to the breaking of the mechanical balance are presented, and the generation of bulk shear-waves, of surface and interface waves and of guided waves is discussed. Then, efforts to overcome the limitations imposed by optical diffraction are described. Next, the principles behind the detection of the so generated coherent acoustic phonons with short light pulses are introduced for both opaque and transparent materials. The striking instrumental advances, in the detection of acoustic displacements, ultrafast acquisition, frequency and space resolution are discussed. Then secondly, we introduce picosecond opto-acoustics as a remote and label-free novel modality with an excellent capacity for quantitative evaluation and imaging of the cell's mechanical properties, currently with micron in-plane and sub-optical in depth resolution. We present the methods for time domain Brillouin spectroscopy in cells and for cell ultrasonography. The current applications of this unconventional means of addressing biological questions are presented. This microscopy of the nanoscale intra-cell mechanics, based on the optical monitoring of coherent phonons, is currently emerging as a breakthrough method offering new insights into the supra-molecular structural changes that accompany cell response to a myriad of biological events.

An isoform of the giant protein titin is a master regulator of human T lymphocyte trafficking

Response to multiple microenvironmental cues and resilience to mechanical stress are essential features of trafficking leukocytes. Here, we describe unexpected role of titin (TTN), the largest protein encoded by the human genome, in the regulation of mechanisms of lymphocyte trafficking. Human T and B lymphocytes express five TTN isoforms, exhibiting cell-specific expression, distinct localization to plasma membrane microdomains, and different distribution to cytosolic versus nuclear compartments. In T lymphocytes, the LTTN1 isoform governs the morphogenesis of plasma membrane microvilli independently of ERM protein phosphorylation status, thus allowing selectin-mediated capturing and rolling adhesions. Likewise, LTTN1 controls chemokine-triggered integrin activation. Accordingly, LTTN1 mediates rho and rap small GTPases activation, but not actin polymerization. In contrast, chemotaxis is facilitated by LTTN1 degradation. Finally, LTTN1 controls resilience to passive cell deformation and ensures T lymphocyte survival in the blood stream. LTTN1 is, thus, a critical and versatile housekeeping regulator of T lymphocyte trafficking.

Effects of cell deformability and adhesion strength on dynamic cell seeding: Cell-scale investigation via mesoscopic modeling

The flow of cell suspension through a porous scaffold is a common process in dynamic cell seeding, which determines the initial distribution of cells for constructing tissue-engineered grafts. Physical insights into the transport and adhesion behaviors of cells in this process are of great significance to the precise control of cell density and its distribution in the scaffold. Revealing of dynamic mechanisms underlying these cell behaviors through experiments is still difficult. The numerical approach therefore plays an important role in such studies. However, existing studies have mostly focused on external factors (e.g., flow conditions and scaffold architecture) but ignored the intrinsic biomechanical properties of cells as well as their associated effects. The present work utilized a well-established mesoscopic model to simulate the dynamic cell seeding within a porous scaffold, based on which a thorough investigation of the effects of cell deformability and cell-scaffold adhesion strength on the seeding process was carried out. The results show that the increase in either the stiffness or the bond strength of cells would augment the firm-adhesion rate and thus enhance seeding efficiency. In comparison to cell deformability, bond strength seems to play a more dominant role. Especially in the cases with weak bond strength, remarkable losses of seeding efficiency and distribution uniformity are observed. Noteworthily, it is found that both the firm-adhesion rate and the seeding efficiency are quantiatively related to the adhesion strength which is measured as the detachment force, suggesting a straightforward way to estimate the seeding outcome.

Mapping Nanomechanical Properties of Basal Surfaces in Metastatic Intestinal 3D Living Organoids with High-Speed Scanning Ion Conductance Microscopy

Studying mechanobiology is increasing of scientific interests in life science and nanotechnology since its impact on cell activities (e.g., adhesion, migration), physiology, and pathology. The role of apical surface (AS) and basal surface (BS) of cells played in mechanobiology is significant. The mechanical mapping and analysis of cells mainly focus on AS while little is known about BS. Here, high-speed scanning ion conductance microscope as a powerful tool is utilized to simultaneously reveal morphologies and local elastic modulus (E) of BS of genotype-defined metastatic intestinal organoids. A simple method is developed to prepare organoid samples allowing for long-term BS imaging. The multiple nano/microstructures, i.e., ridge-like, stress-fiber, and E distributions on BS are dynamically revealed. The statistic E analysis shows softness of BS derived from eight types of organoids following a ranking: malignant tumor cells > benign tumor cells > normal cells. Moreover, the correlation factor between morphology and E is demonstrated depending on cell types. This work as first example reveals the subcellular morphologies and E distributions of BS of cells. The results would provide a clue for correlating genotype of 3D cells to malignant phenotype reflected by E and offering a promising strategy for early-stage diagnosis of cancer.

Optothermal rotation of micro-/nano-objects

Due to its contactless and fuel-free operation, optical rotation of micro-/nano-objects provides tremendous opportunities for cellular biology, three-dimensional (3D) imaging, and micro/nanorobotics. However, complex optics, extremely high operational power, and the applicability to limited objects restrict the broader use of optical rotation techniques. This Feature Article focuses on a rapidly emerging class of optical rotation techniques, termed optothermal rotation. Based on light-mediated thermal phenomena, optothermal rotation techniques overcome the bottlenecks of conventional optical rotation by enabling versatile rotary control of arbitrary objects with simpler optics using lower powers. We start with the fundamental thermal phenomena and concepts: thermophoresis, thermoelectricity, thermo-electrokinetics, thermo-osmosis, thermal convection, thermo-capillarity, and photophoresis. Then, we highlight various optothermal rotation techniques, categorizing them based on their rotation modes (i.e., in-plane and out-of-plane rotation) and the thermal phenomena involved. Next, we explore the potential applications of these optothermal manipulation techniques in areas such as single-cell mechanics, 3D bio-imaging, and micro/nanomotors. We conclude the Feature Article with our insights on the operating guidelines, existing challenges, and future directions of optothermal rotation.

Phase angle, muscle tissue, and resistance training

The biophysical response of the human body to electric current is widely appreciated as a barometer of fluid distribution and cell function. From distinct raw bioelectrical impedance (BIA) variables assessed in the field of body composition, phase angle (PhA) has been repeatedly indicated as a functional marker of the cell's health and mass. Although resistance training (RT) programs have demonstrated to be effective to improve PhA, with varying degrees of change depending on other raw BIA variables, there is still limited research explaining the biological mechanisms behind these changes. Here, we aim to provide the rationale for the responsiveness of PhA determinants to RT, as well as to summarize all available evidence addressing the effect of varied RT programs on PhA of different age groups. Available data led us to conclude that RT modulates the cell volume by increasing the levels of intracellular glycogen and water, thus triggering structural and functional changes in different cell organelles. These alterations lead, respectively, to shifts in the resistive path of the electric current (resistance, R) and capacitive properties of the human body (reactance, Xc), which ultimately impact PhA, considering that it is the angular transformation of the ratio between Xc and R. Evidence drawn from experimental research suggests that RT is highly effective for enhancing PhA, especially when adopting high-intensity, volume, and duration RT programs combining other types of exercise. Still, additional research exploring the effects of RT on whole-body and regional BIA variables of alternative population groups is recommended for further knowledge development.

Simulation of a tumor cell flowing through a symmetric bifurcated microvessel

Microvessel bifurcations serve as the major sites of tumor cell adhesion and further extravasation. In this study, the movement, deformation, and adhesion of a circulating tumor cell flowing in a symmetric microvessel with diverging and converging bifurcations were simulated by dissipative particle dynamics combined with a spring-based network model. Effects of the initial position of the CTC, externally-applied acceleration and the presence of RBCs on the motion of the CTC were investigated. The results demonstrated that the CTC released at the centerline of the parent vessel would attach to the vessel wall when arriving at the apex of diverging bifurcation and slide into the daughter branch determined by its centroid deflection and finally form firm adhesion at relatively lower flow rates. As the external acceleration increases, the increasing shear force enlarges the contact area for the adherent CTC on the one hand and reduces the residence time on the other hand. With the presence of RBCs in the bloodstream, the collision between the adherent tumor cell at the diverging bifurcation and flowing RBCs promotes the firm adhesion of CTC at lower flow rates.

Effect of the Rho-Kinase/ROCK Signaling Pathway on Cytoskeleton Components

The mechanical properties of cells are important in tissue homeostasis and enable cell growth, division, migration and the epithelial-mesenchymal transition. Mechanical properties are determined to a large extent by the cytoskeleton. The cytoskeleton is a complex and dynamic network composed of microfilaments, intermediate filaments and microtubules. These cellular structures confer both cell shape and mechanical properties. The architecture of the networks formed by the cytoskeleton is regulated by several pathways, a key one being the Rho-kinase/ROCK signaling pathway. This review describes the role of ROCK (Rho-associated coiled-coil forming kinase) and how it mediates effects on the key components of the cytoskeleton that are critical for cell behaviour.

Modulating cell signalling in vivo with magnetic nanotransducers

Weak magnetic fields offer nearly lossless transmission of signals within biological tissue. Magnetic nanomaterials are capable of transducing magnetic fields into a range of biologically relevant signals in vitro and in vivo. These nanotransducers have recently enabled magnetic control of cellular processes, from neuronal firing and gene expression to programmed apoptosis. Effective implementation of magnetically controlled cellular signalling relies on careful tailoring of magnetic nanotransducers and magnetic fields to the responses of the intended molecular targets. This primer discusses the versatility of magnetic modulation modalities and offers practical guidelines for selection of appropriate materials and field parameters, with a particular focus on applications in neuroscience. With recent developments in magnetic instrumentation and nanoparticle chemistries, including those that are commercially available, magnetic approaches promise to empower research aimed at connecting molecular and cellular signalling to physiology and behaviour in untethered moving subjects.

Reciprocity of Cell Mechanics with Extracellular Stimuli: Emerging Opportunities for Translational Medicine

Human cells encounter dynamic mechanical cues in healthy and diseased tissues, which regulate their molecular and biophysical phenotype, including intracellular mechanics as well as force generation. Recent developments in bio/nanomaterials and microfluidics permit exquisitely sensitive measurements of cell mechanics, as well as spatiotemporal control over external mechanical stimuli to regulate cell behavior. In this review, the mechanobiology of cells interacting bidirectionally with their surrounding microenvironment, and the potential relevance for translational medicine are considered. Key fundamental concepts underlying the mechanics of living cells as well as the extracelluar matrix are first introduced. Then the authors consider case studies based on 1) microfluidic measurements of nonadherent cell deformability, 2) cell migration on micro/nano-topographies, 3) traction measurements of cells in three-dimensional (3D) matrix, 4) mechanical programming of organoid morphogenesis, as well as 5) active mechanical stimuli for potential therapeutics. These examples highlight the promise of disease diagnosis using mechanical measurements, a systems-level understanding linking molecular with biophysical phenotype, as well as therapies based on mechanical perturbations. This review concludes with a critical discussion of these emerging technologies and future directions at the interface of engineering, biology, and medicine.

Gold Nano-Bio-Interaction to Modulate Mechanobiological Responses for Cancer Therapy Applications

In the present study, we investigate the mechanobiological responses of human lung cancer that may occur through their interactions with two different types of gold nanoparticles: nanostars and nanospheres. Hyperspectral images of nanoparticle-treated cells revealed different spatial distributions of nanoparticles in cells depending on their morphology, with nanospheres being more uniformly distributed in cells than nanostars. Gold nanospheres were also found to be more effective in mechanobiological modulations. They significantly suppressed the migratory ability of cells under different incubation times while lowering the bulk stiffness and adhesion of cells. This in vitro study suggests the potential applications of gold nanoparticles to manage cell migration. Nano-bio-interactions appeared to impact the cytoskeletal organization of cells and consequently alter the mechanical properties of cells, which could influence the cellular functions of cells. According to the results and migratory index model, it is thought that nanoparticle-treated cells experience mechanical changes in their body, which largely reduces their migratory potentials. These findings provide a better understanding of nano-bio-interaction in terms of cell mechanics and highlight the importance of mechanobiological responses in designing gold nanoparticles for cancer therapy.

Biophysics involved in the process of tumor immune escape

Much of the current research into immune escape from cancer is focused on molecular and cellular biology, an area of biophysics that is easily overlooked. A large number of immune drugs entering the clinic are not effective for all patients. Apart from the molecular heterogeneity of tumors, the biggest reason for this may be that knowledge of biophysics has not been considered, and therefore an exploration of biophysics may help to address this challenge. To help researchers better investigate the relationship between tumor immune escape and biophysics, this paper provides a brief overview on recent advances and challenges of the biophysical factors and strategies by which tumors acquire immune escape and a comprehensive analysis of the relevant forces acting on tumor cells during immune escape. These include tumor and stromal stiffness, fluid interstitial pressure, shear stress, and viscoelasticity. In addition, advances in biophysics cannot be made without the development of detection tools, and this paper also provides a comprehensive summary of the important detection tools available at this stage in the field of biophysics.

Force-dependent remodeling of cytoplasmic ZO-1 condensates contributes to cell-cell adhesion through enhancing tight junctions

The physiological importance of biomolecular condensates is widely recognized, but how it is controlled in time and space during development is largely unknown. Here, we show that a tight junction protein ZO-1 forms cytoplasmic condensates in the trophectoderm (TE) of the mouse embryo before E4.0. These disappear via dissolution, and ZO-1 accumulates at the cell junction as the blastocyst cavity grows and internal pressure on TE cells increases. In contrast, this dissolution was less evident in TE cells attached to the inner cell mass because they receive weaker tensile forces. Furthermore, analyses using MDCK cells demonstrated that the ZO-1 condensates are generated and maintained by liquid-liquid phase separation. Our study also highlights that the dynamics of these condensates depends on the physical environment via an interaction between ZO-1 and F-actin. We propose that the force-dependent regulation of ZO-1 condensation contributes to the establishment of robust cell-cell adhesion during early development.

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ZO-1 forms cytoplasmic droplets via liquid-liquid phase separation

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In hatching mouse embryos, ZO-1 droplets dissolve and it localizes to cell junctions

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In MDCK cells, ZO-1 forms droplets in response to mechanical environments

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Interaction with F-actin negatively regulates ZO-1 phase separation

A Continuum-Tensegrity Computational Model for Chondrocyte Biomechanics in AFM Indentation and Micropipette Aspiration

Mechanical stimuli are fundamental in the development of organs and tissues, their growth, regeneration or disease. They influence the biochemical signals produced by the cells, and, consequently, the development and spreading of a disease. Moreover, tumour cells are usually characterized by a decrease in the cell mechanical properties that may be directly linked to their metastatic potential. Thus, recently, the experimental and computational study of cell biomechanics is facing a growing interest. Various experimental approaches have been implemented to describe the passive response of cells; however, cell variability and complex experimental procedures may affect the obtained mechanical properties. For this reason, in-silico computational models have been developed through the years, to overcome such limitations, while proposing valuable tools to understand cell mechanical behaviour. This being the case, we propose a combined continuous-tensegrity finite element (FE) model to analyse the mechanical response of a cell and its subcomponents, observing how every part contributes to the overall mechanical behaviour. We modelled both Atomic Force Microscopy (AFM) indentation and micropipette aspiration techniques, as common mechanical tests for cells and elucidated also the role of cell cytoplasm and cytoskeleton in the global cell mechanical response.

Cytoskeletal Tensegrity in Microgravity

In order for Man to venture further into Space he will have to adapt to its conditions, including microgravity. Life as we know it has evolved on Earth with a substantial gravitational field. If they spend considerable time away from Earth, astronauts experience physiological, mental, and anatomical changes. It is not clear if these are pathological or adaptations. However, it is true that they experience difficulties on their return to stronger gravity. The cytoskeleton is a key site for the detection of gravitational force within the body, due to its tensegrity architecture. In order to understand what happens to living beings in space, we will need to unravel the role cytoskeletal tensegrity architecture plays in the building and function of cells, organs, the body, and mind.

Energy Landscape of Ubiquitin Is Weakly Multidimensional

Single molecule pulling experiments report time-dependent changes in the extension (X) of a biomolecule as a function of the applied force (f). By fitting the data to one-dimensional analytical models of the energy landscape, we can extract the hopping rates between the folded and unfolded states in two-state folders as well as the height and the location of the transition state (TS). Although this approach is remarkably insightful, there are cases for which the energy landscape is multidimensional (catch bonds being the most prominent). To assess if the unfolding energy landscape in small single domain proteins could be one-dimensional, we simulated force-induced unfolding of ubiquitin (Ub) using the coarse-grained self-organized polymer-side chain (SOP-SC) model. Brownian dynamics simulations using the SOP-SC model reveal that the Ub energy landscape is weakly multidimensional (WMD), governed predominantly by a single barrier. The unfolding pathway is confined to a narrow reaction pathway that could be described as diffusion in a quasi-1D X-dependent free energy profile. However, a granular analysis using the P_fold analysis, which does not assume any form for the reaction coordinate, shows that X alone does not account for the height and, more importantly, the location of the TS. The f-dependent TS location moves toward the folded state as f increases, in accord with the Hammond postulate. Our study shows that, in addition to analyzing the f-dependent hopping rates, the transition state ensemble must also be determined without resorting to X as a reaction coordinate to describe the unfolding energy landscapes of single domain proteins, especially if they are only WMD.

Screen Key Genes Associated with Distraction-Induced Osteogenesis of Stem Cells Using Bioinformatics Methods

Background: Applying mesenchymal stem cells (MSCs), together with the distraction osteogenesis (DO) process, displayed enhanced bone quality and shorter treatment periods. The DO guides the differentiation of MSCs by providing mechanical clues. However, the underlying key genes and pathways are largely unknown. The aim of this study was to screen and identify hub genes involved in distraction-induced osteogenesis of MSCs and potential molecular mechanisms. Material and Methods: The datasets were downloaded from the ArrayExpress database. Three samples of negative control and two samples subjected to 5% cyclic sinusoidal distraction at 0.25 Hz for 6 h were selected for screening differentially expressed genes (DEGs) and then analysed via bioinformatics methods. The Gene Ontology (GO) terms and Kyoto Encyclopaedia of Genes and Genomes (KEGG) pathway enrichment were investigated. The protein–protein interaction (PPI) network was visualised through the Cytoscape software. Gene set enrichment analysis (GSEA) was conducted to verify the enrichment of a self-defined osteogenic gene sets collection and identify osteogenic hub genes. Results: Three hub genes (IL6, MMP2, and EP300) that were highly associated with distraction-induced osteogenesis of MSCs were identified via the Venn diagram. These hub genes could provide a new understanding of distraction-induced osteogenic differentiation of MSCs and serve as potential gene targets for optimising DO via targeted therapies.

The Cell as Matter: Connecting Molecular Biology to Cellular Functions

Viewing cell as matter to understand the intracellular biomolecular processes and multicellular tissue behavior represents an emerging research area at the interface of physics and biology. Cellular material displays various physical and mechanical properties, which can strongly affect both intracellular and multicellular biological events. This review provides a summary of how cells, as matter, connect molecular biology to cellular and multicellular scale functions. As an impact in molecular biology, we review recent progresses in utilizing cellular material properties to direct cell fate decisions in the communities of immune cells, neurons, stem cells, and cancer cells. Finally, we provide an outlook on how to integrate cellular material properties in developing biophysical methods for engineered living systems, regenerative medicine, and disease treatments.

Transient cell stiffening triggered by magnetic nanoparticle exposure

Background

The interactions between nanoparticles and the biological environment have long been studied, with toxicological assays being the most common experimental route. In parallel, recent growing evidence has brought into light the important role that cell mechanics play in numerous cell biological processes. However, despite the prevalence of nanotechnology applications in biology, and in particular the increased use of magnetic nanoparticles for cell therapy and imaging, the impact of nanoparticles on the cells’ mechanical properties remains poorly understood.

Results

Here, we used a parallel plate rheometer to measure the impact of magnetic nanoparticles on the viscoelastic modulus G*(f) of individual cells. We show how the active uptake of nanoparticles translates into cell stiffening in a short time scale (< 30 min), at the single cell level. The cell stiffening effect is however less marked at the cell population level, when the cells are pre-labeled under a longer incubation time (2 h) with nanoparticles. 24 h later, the stiffening effect is no more present. Imaging of the nanoparticle uptake reveals almost immediate (within minutes) nanoparticle aggregation at the cell membrane, triggering early endocytosis, whereas nanoparticles are almost all confined in late or lysosomal endosomes after 2 h of uptake. Remarkably, this correlates well with the imaging of the actin cytoskeleton, with actin bundling being highly prevalent at early time points into the exposure to the nanoparticles, an effect that renormalizes after longer periods.

Conclusions

Overall, this work evidences that magnetic nanoparticle internalization, coupled to cytoskeleton remodeling, contributes to a change in the cell mechanical properties within minutes of their initial contact, leading to an increase in cell rigidity. This effect appears to be transient, reduced after hours and disappearing 24 h after the internalization has taken place.

Hyperglycemia and hyperlipidemia can induce morphophysiological changes in rat cardiac cell line

H9c2 cardiac cells were incubated under the control condition and at different hyperglycemic and hyperlipidemic media, and the following parameters were determined and quantified: a) cell death, b) type of cell death, and c) changes in cell length, width and height. Of all the proven media, the one that showed the greatest differences compared to the control was the medium glucose (G) 33 mM + 500 μM palmitic acid. This condition was called the hyperglycemic and hyperlipidemic condition (HHC). Incubation of H9c2 cells in HHC promoted 5.2 times greater total cell death when compared to the control. Of the total death ofthe HHC cells, 38.6% was late apoptotic and 8.3% early apoptotic. HHC also changes cell morphology. The reordering of the actin cytoskeleton and cell stiffness was also studied in control and HHC cells. The actin cytoskeleton was quantified and the number and distance of actin bundles were not the same in the control as under HHC. Young's modulus images show a map of cell stiffness. Cells incubated in HHC with the reordered actin cytoskeleton were stiffer than those incubated in control. The region of greatest stiffness was the peripheral zone of HHC cells (where the number of actin bundles was higher and the distance between them smaller). Our results suggest a correlation between the reordering of the actin cytoskeleton and cell stiffness. Thus, our study showed that HHC can promote morphophysiological changes in rat cardiac cells confirming that gluco-and lipotoxicity may play a central role in the development of diabetic cardiomyopathy.

Quantitative Methodologies to Dissect Immune Cell Mechanobiology

Mechanobiology seeks to understand how cells integrate their biomechanics into their function and behavior. Unravelling the mechanisms underlying these mechanobiological processes is particularly important for immune cells in the context of the dynamic and complex tissue microenvironment. However, it remains largely unknown how cellular mechanical force generation and mechanical properties are regulated and integrated by immune cells, primarily due to a profound lack of technologies with sufficient sensitivity to quantify immune cell mechanics. In this review, we discuss the biological significance of mechanics for immune cells across length and time scales, and highlight several experimental methodologies for quantifying the mechanics of immune cells. Finally, we discuss the importance of quantifying the appropriate mechanical readout to accelerate insights into the mechanobiology of the immune response.

In vitro evaluation of nanoparticle drug-coated balloons: a pectin-RGDS-OC8H17-paclitaxel solution

Drug-coated balloons have proved to be an effective technology in percutaneous transluminal angioplasty in treating peripheral artery disease. Paclitaxel-based coating is mainly used. Solutions to such problems as drug loss and inefficient drug release during operations, however, have not been found yet. This study aims to explore the activity of a newly designed paclitaxel-coated balloon in vitro using pectin as the excipient (pectin-paclitaxel) compared with the commercially available shellac excipient balloon, and to characterize the novel nanoparticle paclitaxel-coated balloon with peptide (Arg-Gly-Asp-Ser, RGDS) derivative RGDS-OC8H17 (pectin-RGDS-OC8H17-paclitaxel). Two coating solutions, pectin-paclitaxel and pectin-RGDS-OC8H17-paclitaxel, were successively designed and prepared. The morphology of both coating solutions was first characterized compared with the control group, the commercially available paclitaxel-coated balloon. Then the in vitro experiments were conducted to determine the drug-releasing profiles of both pectin-paclitaxel and pectin-RGDS-OC8H17-paclitaxel coatings. The pectin-RGDS-OC8H17-paclitaxel-coated balloon was smoother and more homogeneous compared with the commercially available paclitaxel-coated balloon and the pectin-paclitaxel-coated balloon. This difference was more obvious when paclitaxel was at low concentration. During the in vitro trial, the drug-releasing curve of the pectin-RGDS-OC8H17-paclitaxel model showed an adjustable paclitaxel-releasing: more than 90% of the paclitaxel released in 2 h at 300 rpm and more than 99% released in 10 min at 1200 rpm. Compared to the performance of the current commercially available shellac excipient products and the pectin-paclitaxel coating, pectin-RGDS-OC8H17-paclitaxel coating provided higher drug-releasing speed. However, the clinical outcomes of this finding need to be further demonstrated. Paclitaxel-coated balloons as an effective therapeutic strategy currently in treating peripheral arterial disease need to be further improved in terms of its efficiency in anti-proliferative drug delivery and release. The pectin-RGDS-OC8H17-paclitaxel coating solution developed in this study exhibited excellent drug-releasing properties. Further experiments are still needed to demonstrate the performance of this novel drug-coated balloon in vivo and its clinical importance.

Human mammary epithelial cells in a mature, stratified epithelial layer flatten and stiffen compared to single and confluent cells

BACKGROUND

The epithelium forms a protective barrier against external biological, chemical and physical insults. So far, AFM-based, micro-mechanical measurements have only been performed on single cells and confluent cells, but not yet on cells in mature layers.

METHODS

Using a combination of atomic force, fluorescence and confocal microscopy, we determined the changes in stiffness, morphology and actin distribution of human mammary epithelial cells (HMECs) as they transition from single cells to confluency to a mature layer.

RESULTS

Single HMECs have a tall, round (planoconvex) morphology, have actin stress fibers at the base, have diffuse cortical actin, and have a stiffness of 1 kPa. Confluent HMECs start to become flatter, basal actin stress fibers start to disappear, and actin accumulates laterally where cells abut. Overall stiffness is still 1 kPa with two-fold higher stiffness in the abutting regions. As HMECs mature and form multilayered structures, cells on apical surfaces become flatter (apically more level), wider, and seven times stiffer (mean, 7 kPa) than single and confluent cells. The main drivers of these changes are actin filaments, as cells show strong actin accumulation in the regions where cells adjoin, and in the apical regions.

CONCLUSIONS

HMECs stiffen, flatten and redistribute actin upon transiting from single cells to mature, confluent layers.

GENERAL SIGNIFICANCE

Our findings advance the understanding of breast ductal morphogenesis and mechanical homeostasis.

Understanding and Regulating Cell-Matrix Interactions Using Hydrogels of Designable Mechanical Properties

Similar to natural tissues, hydrogels contain abundant water, so they are considered as promising biomaterials for studying the influence of the mechanical properties of extracellular matrices (ECM) on various cell functions. In recent years, the growing research on cellular mechanical response has revealed that many cell functions, including cell spreading, migration, tumorigenesis and differentiation, are related to the mechanical properties of ECM. Therefore, how cells sense and respond to the extracellular mechanical environment has gained considerable attention. In these studies, hydrogels are widely used as the in vitro model system. Hydrogels of tunable stiffness, viscoelasticity, degradability, plasticity, and dynamical properties have been engineered to reveal how cells respond to specific mechanical features. In this review, we summarize recent process in this research direction and specifically focus on the influence of the mechanical properties of the ECM on cell functions, how cells sense and respond to the extracellular mechanical environment, and approaches to adjusting the stiffness of hydrogels.

Probing coordinated co-culture cancer related motility through differential micro-compartmentalized elastic substrates

Cell development and behavior are driven by internal genetic programming, but the external microenvironment is increasingly recognized as a significant factor in cell differentiation, migration, and in the case of cancer, metastatic progression. Yet it remains unclear how the microenvironment influences cell processes, especially when examining cell motility. One factor that affects cell motility is cell mechanics, which is known to be related to substrate stiffness. Examining how cells interact with each other in response to mechanically differential substrates would allow an increased understanding of their coordinated cell motility. In order to probe the effect of substrate stiffness on tumor related cells in greater detail, we created hard–soft–hard (HSH) polydimethylsiloxane (PDMS) substrates with alternating regions of different stiffness (200 and 800 kPa). We then cultured WI-38 fibroblasts and A549 epithelial cells to probe their motile response to the substrates. We found that when the 2 cell types were exposed simultaneously to the same substrate, fibroblasts moved at an increased speed over epithelial cells. Furthermore, the HSH substrate allowed us to physically guide and separate the different cell types based on their relative motile speed. We believe that this method and results will be important in a diversity of areas including mechanical microenvironment, cell motility, and cancer biology.

Recombinant human lactoferrin carrying humanized glycosylation exhibits antileukemia selective cytotoxicity, microfilament disruption, cell cycle arrest, and apoptosis activities

Lactoferrin has gained extensive attention due to its ample biological properties. In this study, recombinant human lactoferrin carrying humanized glycosylation (rhLf-h-glycan) expressed in the yeast Pichia pastoris SuperMan5, which is genetically glycoengineered to efficiently produce functional humanized glycoproteins inclosing (Man)₅(GlcNAc)₂ Asn-linked glycans, was analyzed, inspecting its potential toxicity against cancer cells. The live-cell differential nuclear staining assay was used to quantify the rhLf-h-glycan cytotoxicity, which was examined in four human cell lines: acute lymphoblastic leukemia (ALL) CCRF-CEM, T-cell lymphoblastic lymphoma SUP-T1, cervical adenocarcinoma HeLa, and as control, non-cancerous Hs27 cells. The defined CC₅₀ values of rhLf-h-glycan in CCRF-CEM, SUP-T1, HeLa, and Hs27 cells were 144.45 ± 4.44, 548.47 ± 64.41, 350 ± 14.82, and 3359.07 ± 164 µg/mL, respectively. The rhLf-h-glycan exhibited a favorable selective cytotoxicity index (SCI), preferentially killing cancer cells: 23.25 for CCRF-CEM, 9.59 for HeLa, and 6.12 for SUP-T1, as compared with Hs27 cells. Also, rhLf-h-glycan showed significant antiproliferative activity (P < 0.0001) at 24, 48, and 72 h of incubation on CCRF-CEM cells. Additionally, it was observed via fluorescent staining and confocal microscopy that rhLf-h-glycan elicited apoptosis-associated morphological changes, such as blebbing, nuclear fragmentation, chromatin condensation, and apoptotic bodies in ALL cells. Furthermore, rhLf-h-glycan-treated HeLa cells revealed shrinkage of the microfilament structures, generating a speckled/punctuated pattern and also caused PARP-1 cleavage, a hallmark of apoptosis. Moreover, in ALL cells, rhLf-h-glycan altered cell cycle progression inducing the G2/M phase arrest, and caused apoptotic DNA fragmentation. Overall, our findings revealed that rhLf-h-glycan has potential as an anticancer agent and therefore deserves further in vivo evaluation.

Nonlinear Elastic and Inelastic Properties of Cells

Mechanical forces play an important role in various physiological processes, such as morphogenesis, cytokinesis, and migration. Thus, in order to illuminate mechanisms underlying these physiological processes, it is crucial to understand how cells deform and respond to external mechanical stimuli. During recent decades, the mechanical properties of cells have been studied extensively using diverse measurement techniques. A number of experimental studies have shown that cells are far from linear elastic materials. Cells exhibit a wide variety of nonlinear elastic and inelastic properties. Such complicated properties of cells are known to emerge from unique mechanical characteristics of cellular components. In this review, we introduce major cellular components that largely govern cell mechanical properties and provide brief explanations of several experimental techniques used for rheological measurements of cell mechanics. Then, we discuss the representative nonlinear elastic and inelastic properties of cells. Finally, continuum and discrete computational models of cell mechanics, which model both nonlinear elastic and inelastic properties of cells, will be described.

Numerical study on the adhesion of a circulating tumor cell in a curved microvessel

The adhesion of a circulating tumor cell (CTC) in a three-dimensional curved microvessel was numerically investigated. Simulations were first performed to characterize the differences in the dynamics and adhesion of a CTC in the straight and curved vessels. After that, a parametric study was performed to investigate the effects of the applied driven force density f (or the flow Reynolds number Re) and the CTC membrane bending modulus K_b on the CTC adhesion. Our simulation results show that the CTC prefers to adhere to the curved vessel as more bonds are formed around the transition region of the curved part due to the increased cell-wall contact by the centrifugal force. The parametric study also indicates that when the flow driven force f (or Re) increases or when the CTC becomes softer (K_b decreases), the bond formation probability increases and the bonds will be formed at more sites of a curved vessel. The increased f (or Re) brings a larger centrifugal force, while the decreased K_b generates more contact areas at the cell-wall interface, both of which are beneficial to the bond formation. In the curved vessel, it is found that the site where bonds are formed the most (hotspot) varies with the applied f and the K_b. For our vessel geometry, when f is small, the hotspot tends to be within the first bend of the vessel, while as f increases or K_b decreases, the hotspot may shift to the second bend of the vessel.

A quantitative model for the caveolae under cell membrane stretch

In this paper, we proposed a two-state model to quantitatively illustrate the working mechanism of caveolae structure in the regulation of cell membrane stretch. First, we derived the free energy compositions of one caveola in an arbitrary state. This model predicts two local minima for the free energy of one caveola at the completely open and completely closed states at the bottom of a relatively deep curve of free energy at different shape coefficients. The behavior of one caveola is generalized to large number of caveolae in cell membranes using the central limit theory. The dynamic stretching behavior of membrane during a constant-speed tether-pulling experiment is also investigated by making a hypothesis of transition rate between the closed and open states.

Finite element analysis reveals an important role for cell morphology in response to mechanical compression

Mechanical loading naturally controls cell phenotype, development, motility and various other biological functions; however, prolonged or substantial loading can cause cell damage and eventual death. Loading-induced mechanobiological and mechanostructural responses of different cell types affect their morphology and the internal architecture and the mechanics of the cellular components. Using single, mesenchymal stem cells, we have developed a cell-specific three-dimensional finite-element model; cell models were developed from phase-contrast microscopy images. This allowed us to evaluate the mechanostructural response of the naturally occurring variety of cell morphologies to increase sustained compressive loading. We focus on the morphology of the cytoplasm and the nucleus, as the main mechanically responsive elements, and evaluate formation of tensional strains and area changes in cells undergoing increasing uniaxial compressions. Here, we study mesenchymal stem cells as a model, due to their important role in tissue engineering and regenerative medicine; the method and findings are, however, applicable to any cell type. We observe variability in the cell responses to compression, which correlate directly with the morphology of the cells. Specifically, in cells with or without elongated protrusions (i.e., lamellipodia) tensional strains were, respectively, distributed mostly in the thin extensions or concentrated around the stiff nucleus. Thus, through cell-specific computational modeling of mechanical loading we have identified an underlying cause for stiffening (by actin recruitment) along the length of lamellipodia as well as a role for cell morphology in inducing cell-to-cell variability in mechanostructural response to loading.

Evaluating viscoelastic properties and membrane electrical charges of red blood cells with optical tweezers and cationic quantum dots - applications to β-thalassemia intermedia hemoglobinopathy

Biomechanical and electrical properties are important to the performance and survival of red blood cells (RBCs) in the microcirculation. This study proposed and explored methodologies based on optical tweezers and cationic quantum dots (QDs) as biophotonic tools to characterize, in a complementary way, viscoelastic properties and membrane electrical charges of RBCs. The methodologies were applied to normal (HbA) and β-thalassemia intermedia (Hbβ) RBCs. The β-thalassemia intermedia disease is a hereditary hemoglobinopathy characterized by a reduction (or absence) of β-globin chains, which leads to α-globin chains precipitation. The apparent elasticity (μ) and membrane viscosity (η_m) of RBCs captured by optical tweezers were obtained in just a single experiment. Besides, the membrane electrical charges were evaluated by flow cytometry, exploring electrostatic interactions between cationic QDs, stabilized with cysteamine, with the negatively charged RBC surfaces. Results showed that Hbβ RBCs are less elastic, have a higher η_m, and presented a reduction in membrane electrical charges, when compared to HbA RBCs. Moreover, the methodologies based on optical tweezers and QDs, here proposed, showed to be capable of providing a deeper and integrated comprehension on RBC rheological and electrical changes, resulting from diverse biological conditions, such as the β-thalassemia intermedia hemoglobinopathy.

Cellular nanoscale stiffness patterns governed by intracellular forces

Cell stiffness measurements have led to insights into various physiological and pathological processes^1,2. Although many cellular behaviours are influenced by intracellular mechanical forces^3-6 that also alter the material properties of the cell, the precise mechanistic relationship between intracellular forces and cell stiffness remains unclear. Here we develop a cell mechanical imaging platform with high spatial resolution that reveals the existence of nanoscale stiffness patterns governed by intracellular forces. On the basis of these findings, we develop and validate a cellular mechanical model that quantitatively relates cell stiffness to intracellular forces. This allows us to determine the magnitude of tension within actin bundles, cell cortex and plasma membrane from the cell stiffness patterns across individual cells. These results expand our knowledge on the mechanical interaction between cells and their environments, and offer an alternative approach to determine physiologically relevant intracellular forces from high-resolution cell stiffness images.

Label-free multi-parametric imaging of single cells: dual picosecond optoacoustic microscopy

Advances in microscopy with new visualization possibilities often bring dramatic progress to our understanding of the intriguing cellular machinery. Picosecond optoacoustic micro-spectroscopy is an optical technique based on ultrafast pump-probe generation and detection of hypersound on time durations of picoseconds and length scales of nanometers. It is experiencing a renaissance as a versatile imaging tool for cell biology research after a plethora of applications in solid-state physics. In this emerging context, this work reports on a dual-probe architecture to carry out real-time parallel detection of the hypersound propagation inside a cell that is cultured on a metallic substrate, and of the hypersound reflection at the metal/cell adhesion interface. Using this optoacoustic modality, several biophysical properties of the cell can be measured in a noncontact and label-free manner. Its abilities are demonstrated with the multiple imaging of a mitotic macrophage-like cell in a single run experiment.

Characterization of Lipid-Protein Interactions and Lipid-Mediated Modulation of Membrane Protein Function through Molecular Simulation

The cellular membrane constitutes one of the most fundamental compartments of a living cell, where key processes such as selective transport of material and exchange of information between the cell and its environment are mediated by proteins that are closely associated with the membrane. The heterogeneity of lipid composition of biological membranes and the effect of lipid molecules on the structure, dynamics, and function of membrane proteins are now widely recognized. Characterization of these functionally important lipid-protein interactions with experimental techniques is however still prohibitively challenging. Molecular dynamics (MD) simulations offer a powerful complementary approach with sufficient temporal and spatial resolutions to gain atomic-level structural information and energetics on lipid-protein interactions. In this review, we aim to provide a broad survey of MD simulations focusing on exploring lipid-protein interactions and characterizing lipid-modulated protein structure and dynamics that have been successful in providing novel insight into the mechanism of membrane protein function.

The effects of mechanical stretch on the biological characteristics of human adipose-derived stem cells

Adipose‐derived stem cells (ADSCs) are a subset of mesenchymal stem cells (MSCs), which have promised a vast therapeutic potential in tissue regeneration. Recent studies have demonstrated that combining stem cells with mechanical stretch may strengthen the efficacy of regenerative therapies. However, the exact influences of mechanical stretch on MSCs still remain inconclusive. In this study, human ADSCs (hADSCs) were applied cyclic stretch stimulation under an in vitro stretching model for designated duration. We found that mechanical stretch significantly promoted the proliferation, adhesion and migration of hADSCs, suppressing cellular apoptosis and increasing the production of pro‐healing cytokines. For differentiation of hADSCs, mechanical stretch inhibited adipogenesis, but enhanced osteogenesis. Long‐term stretch could promote ageing of hADSCs, but did not alter the cell size and typical immunophenotypic characteristics. Furthermore, we revealed that PI3K/AKT and MAPK pathways might participate in the effects of mechanical stretch on the biological characteristics of hADSCs. Taken together, mechanical stretch is an effective strategy for enhancing stem cell behaviour and regulating stem cell fate. The synergy between hADSCs and mechanical stretch would most likely facilitate tissue regeneration and promote the development of stem cell therapy.

Emerging Biomimetic Applications of DNA Nanotechnology

Re-engineering cellular components and biological processes has received great interest and promised compelling advantages in applications ranging from basic cell biology to biomedicine. With the advent of DNA nanotechnology, the programmable self-assembly ability makes DNA an appealing candidate for rational design of artificial components with different structures and functions. This Forum Article summarizes recent developments of DNA nanotechnology in mimicking the structures and functions of existing cellular components. We highlight key successes in the achievements of DNA-based biomimetic membrane proteins and discuss the assembly behavior of these artificial proteins. Then, we focus on the construction of higher-order structures by DNA nanotechnology to recreate cell-like structures. Finally, we explore the current challenges and speculate on future directions of DNA nanotechnology in biomimetics.

Comparison of cell mechanical measurements provided by Atomic Force Microscopy (AFM) and Micropipette Aspiration (MPA)

A comparative analysis of T-lymphocyte mechanical data obtained from Micropipette Aspiration (MPA) and Atomic Force Microscopy (AFM) is presented. Results obtained by fitting the experimental data to simple Hertz and Theret models led to non-Gaussian distributions and significantly different values of the elastic moduli obtained by both techniques. The use of more refined models, taking into account the finite size of cells (simplified double contact and Zhou models) reduces the differences in the values calculated for the elastic moduli. Several possible sources for the discrepancy between the techniques are considered. The analysis suggests that the local nature of AFM measurements compared with the more general character of MPA measurements probably contributed to the differences observed.

Modeling Cell Adhesion and Extravasation in Microvascular System

The blood flow behaviors in the microvessels determine the transport modes and further affect the metastasis of circulating tumor cells (CTCs). Much biochemical and biological efforts have been made on CTC metastasis; however, precise experimental measurement and accurate theoretical prediction on its mechanical mechanism are limited. To complement these, numerical modeling of a CTC extravasation from the blood circulation, including the steps of adhesion and transmigration, is discussed in this chapter. The results demonstrate that CTCs prefer to adhere at the positive curvature of curved microvessels, which is attributed to the positive wall shear stress/gradient. Then, the effects of particulate nature of blood on CTC adhesion are investigated and are found to be significant in the microvessels. Furthermore, the presence of red blood cell (RBC) aggregates is also found to promote the CTC adhesion by providing an additional wall-directed force. Finally, a single cell passing through a narrow slit, mimicking CTC transmigration, was examined under the effects of cell deformability. It showed that the cell shape and surface area increase play a more important role than the cell elasticity in cell transit across the narrow slit.

A one-step procedure to probe the viscoelastic properties of cells by Atomic Force Microscopy

The increasingly recognised importance of viscoelastic properties of cells in pathological conditions requires rapid development of advanced cell microrheology technologies. Here, we present a novel Atomic Force Microscopy (AFM)-microrheology (AFM²) method for measuring the viscoelastic properties in living cells, over a wide range of continuous frequencies (0.005 Hz ~ 200 Hz), from a simple stress-relaxation nanoindentation. Experimental data were directly analysed without the need for pre-conceived viscoelastic models. We show the method had an excellent agreement with conventional oscillatory bulk-rheology measurements in gels, opening a new avenue for viscoelastic characterisation of soft matter using minute quantity of materials (or cells). Using this capability, we investigate the viscoelastic responses of cells in association with cancer cell invasive activity modulated by two important molecular regulators (i.e. mutation of the p53 gene and Rho kinase activity). The analysis of elastic (G′(ω)) and viscous (G″(ω)) moduli of living cells has led to the discovery of a characteristic transitions of the loss tangent (G″(ω)/G′(ω)) in the low frequency range (0.005 Hz ~ 0.1 Hz) that is indicative of the capability for cell restructuring of F-actin network. Our method is ready to be implemented in conventional AFMs, providing a simple yet powerful tool for measuring the viscoelastic properties of living cells.