Research progress in quantifying the mechanical properties of single living cells using atomic force microscopy

Advanced optical assessment and modeling of extrusion bioprinting

In the context of tissue engineering, biofabrication techniques are employed to process cells in hydrogel-based matrices, known as bioinks, into complex 3D structures. The aim is the production of functional tissue models or even entire organs. The regenerative production of biological tissues adheres to a multitude of criteria that ultimately determine the maturation of a functional tissue. These criteria are of biological nature, such as the biomimetic spatial positioning of different cell types within a physiologically and mechanically suitable matrix, which enables tissue maturation. Furthermore, the processing, a combination of technical procedures and biological materials, has proven highly challenging since cells are sensitive to stress, for example from shear and tensile forces, which may affect their vitality. On the other hand, high resolutions are pursued to create optimal conditions for subsequent tissue maturation. From an analytical perspective, it is prudent to first investigate the printing behavior of bioinks before undertaking complex biological tests. According to our findings, conventional shear rheological tests are insufficient to fully characterize the printing behavior of a bioink. For this reason, we have developed optical methods that, complementarily to the already developed tests, allow for quantification of printing quality and further viscoelastic modeling of bioinks.

Transcriptomic Analysis Identified ARHGAP Family as a Novel Biomarker Associated With Tumor-Promoting Immune Infiltration and Nanomechanical Characteristics in Bladder Cancer

Bladder cancer (BCa) is a common lethal urinary malignancy worldwide. The role of ARHGAP family genes in BCa and its association with immuno-microenvironment remain largely unknown. ARHGAP family expression and immune infiltration in BCa were analyzed by bioinformatics analysis. Then, we investigated cell proliferation, invasion, and migration in vivo and in vitro of the ARHGAP family. Furthermore, atomic force microscopy (AFM) was employed in measuring cellular mechanical properties of BCa cells. The results demonstrated that ARHGAP family genes correlate with a tumor-promoting microenvironment with a lower Th1/Th2 cell ratio, higher DC cell infiltration, higher Treg cell infiltration, and T-cell exhaustion phenotype. Silencing ARHGAP5, ARHGAP17, and ARHGAP24 suppressed BCa cell proliferation, migration, and metastasis. Knocking down of ARHGAPs in T24 cells caused a relatively higher Young’s modulus and lower adhesive force and cell height. Taken together, ARHGAP family genes promote BCa progressing through establishing a tumor-promoting microenvironment and promoting cancer progression.

Exploration of the Effects of Substrate Stiffness on Biological Responses of Neural Cells and Their Mechanisms

Substrate stiffness, as a critical mechanical factor, has been proven to be an important regulator of biological responses, cellular functions, and disease occurrence. However, the effects of substrate stiffness on the phenotypes and drug responses of neural cells remain largely unknown. In this study, polydimethylsiloxane (PDMS) substrates with different stiffnesses were employed to establish the mechanical microenvironment of tissues of different organs. We studied the influences of stiffness on neural cell phenotypes, including cell viability, cell cycle, cytoskeleton structures, cell stiffness, and drug responses of neural cells for hormesis and therapeutic efficacy in neurodegenerative disorders (NDD). The results showed that the greater the range of maximum stimulatory responses, the bigger the width of the stimulatory dosage and the higher the range of maximum neuroprotective activities of hormetic chemicals in neural cells grown on the soft substrate commensurable to the stiffness of the brain, indicating that neural cells on a rigid substrate are resistant to hormetic and neuroprotective effects of hormetic chemicals against 6-hydroxydopamine (6-OHDA)-induced Parkinson's disease (PD) model. The sensitivity of neural cells on the soft substrate to drug response was attributed to the increased cell viability rate, cell cycle progression, actin stress fibers, focal adhesion formation, and decreased cell stiffness. The promoting effect of the soft substrate and the enhanced hormetic and neuroprotective effect of hormetic chemicals on soft substrates in PC12 cells were confirmed to be mediated by the upregulated EGFR/PI3K/AKT signaling pathway by RNA-Seq and bioinformatics analysis. This study demonstrates that the biomechanical properties of the neural microenvironment play important roles in cell phenotypes and drug responses of neural cells in vitro and suggests that substrate stiffness should be considered in the anti-NDD drug design and treatment.

Recent advances in AFM-based biological characterization and applications at multiple levels

Atomic force microscopy (AFM) has found a wide range of bio-applications in the past few decades due to its ability to measure biological samples in natural environments at a high spatial resolution. AFM has become a key platform in biomedical, bioengineering and drug research fields, enabling mechanical and morphological characterization of live biological systems. Hence, we provide a comprehensive review on recent advances in the use of AFM for characterizing the biomechanical properties of multi-scale biological samples, ranging from molecule, cell to tissue levels. First, we present the fundamental principles of AFM and two AFM-based models for the characterization of biomechanical properties of biological samples, covering key AFM devices and AFM bioimaging as well as theoretical models for characterizing the elasticity and viscosity of biomaterials. Then, we elaborate on a series of new experimental findings through analysis of biomechanics. Finally, we discuss the future directions and challenges. It is envisioned that the AFM technique will enable many remarkable discoveries, and will have far-reaching impacts on bio-related studies and applications in the future.

An AFM-Based Nanomechanical Study of Ovarian Tissues with Pathological Conditions

BACKGROUND

Different diseases affect both mechanical and chemical features of the involved tissue, enhancing the symptoms.

METHODS

In this study, using atomic force microscopy, we mechanically characterized human ovarian tissues with four distinct pathological conditions: mucinous, serous, and mature teratoma tumors, and non-tumorous endometriosis. Mechanical elasticity profiles were quantified and the resultant data were categorized using K-means clustering method, as well as fuzzy C-means, to evaluate elastic moduli of cellular and non-cellular parts of diseased tissues and compare them among four disease conditions. Samples were stained by hematoxylin-eosin staining to further study the content of different locations of tissues.

RESULTS

Pathological state vastly influenced the mechanical properties of the ovarian tissues. Significant alterations among elastic moduli of both cellular and non-cellular parts were observed. Mature teratoma tumors commonly composed of multiple cell types and heterogeneous ECM structure showed the widest range of elasticity profile and the stiffest average elastic modulus of 14 kPa. Samples of serous tumors were the softest tissues with elastic modulus of only 400 Pa for the cellular part and 5 kPa for the ECM. Tissues of other two diseases were closer in mechanical properties as mucinous tumors were insignificantly stiffer than endometriosis in cellular part, 1300 Pa compared to 1000 Pa, with the ECM average elastic modulus of 8 kPa for both.

CONCLUSION

The higher incidence of carcinoma out of teratoma and serous tumors may be related to the intense alteration of mechanical features of the cellular and the ECM, serving as a potential risk factor which necessitates further investigation.

Atomic force microscopy correlates antimetastatic potentials of HepG2 cell line with its redox/energy status: effects of curcumin and Khaya senegalensis

OBJECTIVE

The fatality of cancer is mostly dependent on the possibility of occurrence of metastasis. Thus, if the development of metastasis can be prevented through novel therapeutic strategies targeted against this process, then the success of cancer treatment will drastically increase. In this study, therefore, we evaluated the antimetastatic potentials of an extract of Khaya senegalensis and curcumin on the metastatic liver cell line HepG2, and also assessed the anticancer property of the extract.

METHODS

Cells were cultured and treated with graded concentrations of test substances for 24, 48, or 72 h with provisions made for negative controls. Treated cells were assessed as follows: nanotechnologically - atomic force microscopy (AFM) was used to determine cell stiffness; biochemically - cell cytotoxicity, glutathione level and adenosine triphosphate status, caspase activation and mitochondrial toxicity were considered; and microbiologically - a carrot disk assay was used to assess the anticancer property of the extract of K. senegalensis.

RESULTS

Curcumin and K. senegalensis increased the cell stiffness by 2.6- and 4.0-fold respectively, indicating their antimetastatic effects. Corresponding changes in redox (glutathione level) and energy (adenosine triphosphate) status of the cells were also demonstrated. Further mechanistic studies indicated that curcumin was not mitotoxic in HepG2 cells unlike the K. senegalensis extract. In addition, the extract potently inhibited the Agrobacterium tumefaciens-induced genetic transformation based on carrot disk assay.

CONCLUSION

Cell elasticity measurement data, using AFM, strongly suggested, for the first time, that both curcumin and the extract of K. senegalensis exhibited antimetastatic properties on HepG2 cells.

Interrogation of drug effects on HeLa cells by exploiting new AFM mechanical biomarkers

New AFM mechanical biomarkers including cell brush length, adhesion work and the factor of viscosity are discovered for drug assays.

Detection of CD28/CD86 co-stimulatory molecules and surface properties of T and dendritic cells: An AFM study

Although the importance of B7/CD28 co-stimulation has been widely studied, little is known about their nano-spatial localization and their corresponding cells' biophysical and biomechanical properties. Here, we investigated the morphological, biophysical, and biomechanical properties of T cells and dendritic cells (DCs) by atomic force microscopy (AFM) and force curves. The nano-spatial distribution of CD28 and CD86 antigen on T cells and DCs was detected by CD86 or CD28 antibody-functionalized AFM tip. Single-molecule force spectroscopy (SMFS)-based force volumes and quantum dots (QDs)-based fluorescence imaging demonstrated that the co-stimulatory molecules were not randomly distributed over the cells' surface, but more than 80% of CD28 and CD86 molecules appeared to be expressed as 100-200 nm nanoclusters and polarize dominantly in the peak of the cell membrane fluctuations. AFM imaging and quantitative analysis showed that the roughness of mature DCs (mDCs) was higher than that of immature DCs (iDCs). The adhesion force distribution of iDCs and mDCs was heterogeneous while the elasticity distribution was homogeneous locally. In addition, mDCs had a fourfold increase of Young's modulus of iDCs, indicating the contribution of the actin cytoskeleton to the elastic properties of the cells. Taken together, the nano-cluster distribution of CD28 and CD86, the rough mDCs surface, the higher adhesion force and elasticity of mDCs may facilitate to the occurrence of B7/CD28 co-stimulation signals and the formation of immune synapse. These nanoscale findings provide new insights into the antigen-presenting function of DCs, the T cell activation and ultimate immune response. SCANNING 38:365-375, 2016. © 2015 Wiley Periodicals, Inc.

Effects of temperature and cellular interactions on the mechanics and morphology of human cancer cells investigated by atomic force microscopy

Cell mechanics plays an important role in cellular physiological activities. Recent studies have shown that cellular mechanical properties are novel biomarkers for indicating the cell states. In this article, temperature-controllable atomic force microscopy (AFM) was applied to quantitatively investigate the effects of temperature and cellular interactions on the mechanics and morphology of human cancer cells. First, AFM indenting experiments were performed on six types of human cells to investigate the changes of cellular Young's modulus at different temperatures and the results showed that the mechanical responses to the changes of temperature were variable for different types of cancer cells. Second, AFM imaging experiments were performed to observe the morphological changes in living cells at different temperatures and the results showed the significant changes of cell morphology caused by the alterations of temperature. Finally, by co-culturing human cancer cells with human immune cells, the mechanical and morphological changes in cancer cells were investigated. The results showed that the co-culture of cancer cells and immune cells could cause the distinct mechanical changes in cancer cells, but no significant morphological differences were observed. The experimental results improved our understanding of the effects of temperature and cellular interactions on the mechanics and morphology of cancer cells.

Nanoscale monitoring of drug actions on cell membrane using atomic force microscopy

Knowledge of the nanoscale changes that take place in individual cells in response to a drug is useful for understanding the drug action. However, due to the lack of adequate techniques, such knowledge was scarce until the advent of atomic force microscopy (AFM), which is a multifunctional tool for investigating cellular behavior with nanometer resolution under near-physiological conditions. In the past decade, researchers have applied AFM to monitor the morphological and mechanical dynamics of individual cells following drug stimulation, yielding considerable novel insight into how the drug molecules affect an individual cell at the nanoscale. In this article we summarize the representative applications of AFM in characterization of drug actions on cell membrane, including topographic imaging, elasticity measurements, molecular interaction quantification, native membrane protein imaging and manipulation, etc. The challenges that are hampering the further development of AFM for studies of cellular activities are aslo discussed.

Nanomechanical properties of the sea-water bacterium Paracoccus seriniphilus--a scanning force microscopy approach

The measurement of force-distance curves on a single bacterium provides a unique opportunity to detect properties such as the turgor pressure under various environmental conditions. Marine bacteria are very interesting candidates for the production of pharmaceuticals, but are only little studied so far. Therefore, the elastic behavior of Paracoccus seriniphilus, an enzyme producing marine organism, is presented in this study. After a careful evaluation of the optimal measurement conditions, the spring constant and the turgor pressure are determined as a function of ionic strength and pH. Whereas the ionic strength changes the turgor pressure passively, the results give a hint that the change to acidic pH increases the turgor pressure by an active mechanism. Furthermore, it could be shown, that P. seriniphilus has adhesive protrusions outside its cell wall.