Harnessing atomic force microscopy-based single-cell analysis to advance physical oncology

Single-cell analysis is an emerging and promising frontier in the field of life sciences, which is expected to facilitate the exploration of fundamental laws of physiological and pathological processes. Single-cell analysis allows experimental access to cell-to-cell heterogeneity to reveal the distinctive behaviors of individual cells, offering novel opportunities to dissect the complexity of severe human diseases such as cancers. Among the single-cell analysis tools, atomic force microscopy (AFM) is a powerful and versatile one which is able to nondestructively image the fine topographies and quantitatively measure multiple mechanical properties of single living cancer cells in their native states under aqueous conditions with unprecedented spatiotemporal resolution. Over the past few decades, AFM has been widely utilized to detect the structural and mechanical behaviors of individual cancer cells during the process of tumor formation, invasion, and metastasis, yielding numerous unique insights into tumor pathogenesis from the biomechanical perspective and contributing much to the field of cancer mechanobiology. Here, the achievements of AFM-based analysis of single cancer cells to advance physical oncology are comprehensively summarized, and challenges and future perspectives are also discussed. RESEARCH HIGHLIGHTS: Achievements of AFM in characterizing the structural and mechanical behaviors of single cancer cells are summarized, and future directions are discussed. AFM is not only capable of visualizing cellular fine structures, but can also measure multiple cellular mechanical properties as well as cell-generated mechanical forces. There is still plenty of room for harnessing AFM-based single-cell analysis to advance physical oncology.

The influence of different liquid environments on the atomic force microscopy detection of living bEnd.3 cells

Atomic force microscopy (AFM) is one of the most important tools in the field of biomedical science, and it can be used to perform the high-resolution three-dimensional imaging of samples in liquid environments to obtain their physical properties (such as surface potentials and mechanical properties). The influence of the liquid environment on the image quality of the sample and the detection results cannot be ignored. In this work, quantitative imaging (QI) mode AFM imaging and mechanical detection were performed on mouse brain microvascular endothelial (bEnd.3) cells in different liquid environments. The gray-level variance product (SMD2) function was used to evaluate the imaging quality of the cells in liquids with different physical properties, and the variations in cell mechanical properties were quantitatively analyzed. An AFM detection liquid containing less ions and organics compared with the traditional culture medium, which is beneficial for improving the imaging quality, is introduced, and it shows similar mechanical detection results within 3 h. This can greatly reduce the detection costs and could have positive significance in the field of AFM living-cell detection.

Progress in Nanorobotics for Advancing Biomedicine

Nanorobotics, which has long been a fantasy in the realm of science fiction, is now a reality due to the considerable developments in diverse fields including chemistry, materials, physics, information and nanotechnology in the past decades. Not only different prototypes of nanorobots whose sizes are nanoscale are invented for various biomedical applications, but also robotic nanomanipulators which are able to handle nano-objects obtain substantial achievements for applications in biomedicine. The outstanding achievements in nanorobotics have significantly expanded the field of medical robotics and yielded novel insights into the underlying mechanisms guiding life activities, remarkably showing an emerging and promising way for advancing the diagnosis & treatment level in the coming era of personalized precision medicine. In this review, the recent advances in nanorobotics (nanorobots, nanorobotic manipulations) for biomedical applications are summarized from several facets (including molecular machines, nanomotors, DNA nanorobotics, and robotic nanomanipulators), and the future perspectives are also presented.

A multi-scale approach to study biochemical and biophysical aspects of resveratrol on diesel exhaust particle-human primary lung cell interaction

Diesel exhaust particles (DEPs) are major air pollutants that lead to numerous human disorders, especially pulmonary diseases, partly through the induction of oxidative stress. Resveratrol is a polyphenol that ameliorates the production of reactive oxygen species (ROS) and delays aging-related processes. Herein we studied the cytoprotective effect of resveratrol on DEP-exposed human lung cells in a factorial experimental design. This work investigates biophysical features including cellular compositions and biomechanical properties, which were measured at the single-cell level using confocal Raman microspectroscopy (RM) and atomic force microscopy (AFM), respectively. Principal component analysis (PCA), hierarchical cluster analysis (HCA) and partial least square regression (PLS) analysis were applied to analyze Raman spectra with and without resveratrol protection. The health status of individual cells could be effectively predicted using an index derived from characteristic Raman spectral peak (e.g., 1006 cm⁻¹) based on PLS model. AFM measurements indicated that cellular adhesion force was greatly reduced, while Young’s modulus was highly elevated in resveratrol treated DEP-exposed cells. Anti-oxidant resveratrol reduced DEP-induced ROS production and suppressed releases of several cytokines and chemokines. These findings suggest resveratrol may enhance resistance of human lung cells (e.g., SAEC) to air pollutants (e.g. DEPs).

Chemotherapeutic resistance: a nano-mechanical point of view

Chemotherapeutic resistance is one of the main obstacles for cancer remission. To understand how cancer cells acquire chemotherapeutic resistance, biochemical studies focusing on drug target alteration, altered cell proliferation, and reduced susceptibility to apoptosis were performed. Advances in nano-mechanobiology showed that the enhanced mechanical deformability of cancer cells accompanied by cytoskeletal alteration is a decisive factor for cancer development. Furthermore, atomic force microscopy (AFM)–based nano-mechanical studies showed that chemotherapeutic treatments reinforced the mechanical stiffness of drug-sensitive cancer cells. However, drug-resistant cancer cells did not show such mechanical responses following chemotherapeutic treatments. Interestingly, drug-resistant cancer cells are mechanically heterogeneous, with a subpopulation of resistant cells showing higher stiffness than their drug-sensitive counterparts. The signaling pathways involving Rho, vinculin, and myosin II were found to be responsible for these mechanical alterations in drug-resistant cancer cells. In the present review, we highlight the mechanical aspects of chemotherapeutic resistance, and suggest how mechanical studies can contribute to unravelling the multifaceted nature of chemotherapeutic resistance.

Cryopreserved Cells Regeneration Monitored by Atomic Force Microscopy and Correlated With State of Cytoskeleton and Nuclear Membrane

Atomic force microscopy (AFM) helps to describe and explain the mechanobiological properties of living cells on the nanoscale level under physiological conditions. The stiffness of cells is an important parameter reflecting cell physiology. Here, we have provided the first study of the stiffness of cryopreserved cells during post-thawing regeneration using AFM combined with confocal fluorescence microscopy. We demonstrated that the nonfrozen cell stiffness decreased proportionally to the cryoprotectant concentration in the medium. AFM allowed us to map cell surface reconstitution in real time after a freeze/thaw cycle and to monitor the regeneration processes at different depths of the cell and even different parts of the cell surface (nucleus and edge). Fluorescence microscopy showed that the cytoskeleton in fibroblasts, though damaged by the freeze/thaw cycle, is reconstructed after long-term plating. Confocal microscopy confirmed that structural changes affect the nuclear envelopes in cryopreserved cells. AFM nanoindentation analysis could be used as a noninvasive method to identify cells that have regenerated their surface mechanical properties with the proper dynamics and to a sufficient degree. This identification can be important particularly in the field of in vitro fertilization and in future cell-based regeneration strategies.

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

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

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

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

Effects of methotrexate on the viscoelastic properties of single cells probed by atomic force microscopy

Methotrexate is a commonly used anti-cancer chemotherapy drug. Cellular mechanical properties are fundamental parameters that reflect the physiological state of a cell. However, so far the role of cellular mechanical properties in the actions of methotrexate is still unclear. In recent years, probing the behaviors of single cells with the use of atomic force microscopy (AFM) has contributed much to the field of cell biomechanics. In this work, with the use of AFM, the effects of methotrexate on the viscoelastic properties of four types of cells were quantitatively investigated. The inhibitory and cytotoxic effects of methotrexate on the proliferation of cells were observed by optical and fluorescence microscopy. AFM indenting was used to measure the changes of cellular viscoelastic properties (Young's modulus and relaxation time) by using both conical tip and spherical tip, quantitatively showing that the stimulation of methotrexate resulted in a significant decrease of both cellular Young's modulus and relaxation times. The morphological changes of cells induced by methotrexate were visualized by AFM imaging. The study improves our understanding of methotrexate action and offers a novel way to quantify drug actions at the single-cell level by measuring cellular viscoelastic properties, which may have potential impacts on developing label-free methods for drug evaluation.

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.