Elastic modulus and migration capability of drug treated leukemia cells K562.

Leukemia is a commonly seen disease caused by abnormal differentiation of hematopoietic stem cells and blasting in bone marrow. Despite drugs are used to treat the disease clinically, the influence of these drugs on leukemia cells' biomechanical properties, which are closely related to complications like leukostasis or infiltration, is still unclear. Due to non-adherent and viscoelastic nature of leukemia cells, accurate measurement of their elastic modulus is still a challenging issue. In this study, we adopted rate-jump method together with optical tweezers indentation to accurately measure elastic modulus of leukemia cells K562 after phorbol 12-myristate 13-acetate (PMA), all-trans retinoic acid (ATRA), Cytoxan (CTX), and Dexamethasone (DEX) treatment, respectively. We found that compared to control sample, K562 cells treated by PMA showed nearly a threefold increase in elastic modulus. Transwell experiment results suggested that the K562 cells treated with PMA have the lowest migration capability. Besides, it was shown that the cytoskeleton protein gene α-tubulin and vimentin have a significant increase in expression after PMA treatment by qPCR. The results indicate that PMA has a significant influence on protein expression, stiffness, and migration ability of the leukemia cell K562, and may also play an important role in the leukostasis in leukemia.

Actin cytoskeleton stiffness grades metastatic potential of ovarian carcinoma Hey A8 cells via nanoindentation mapping.

Recent studies have indicated that the nanoindentation measured stiffness of carcinoma adherent cells is in general lower than normal cells, thus suggesting that cell stiffness may serve as a bio-marker for carcinoma. However, the proper establishment of such a conclusion would require biophysical understanding of the underlying mechanism of the cell stiffness. In this work, we compared the elastic moduli of the actin cytoskeletons of Hey A8 ovarian carcinoma cells with and without metastasis (HM and NM), as measured by 2D atomic force microscopy (AFM) with low-depth nanoindentation via a rate-jump method. The results indicate clearly that HM cells showed lower actin cytoskeleton stiffness atop of their nucleus position and higher actin cytoskeleton stiffness at their rims, compared to NM cells, suggesting that the local stiffness on the cytoskeleton can reflect actin filament distribution. Immunofluorescence staining and scanning electron microscopy (SEM) also indicated that the difference in stiffness in Hey A8 cells with different metastasis is associated with their F-actin rearrangement. Finite-element modelling (FEM) shows that a migrating cell would have its actin filaments bundled together to form stress fibers, which would exhibit lower indentation stiffness than the less aligned arrangement of filaments in a non-migrating cell. The results here indicate that the actin cytoskeleton stiffness can serve as a reliable marker for grading the metastasis of adherent carcinoma cells due to their cytoskeleton change and potentially predicting the migration direction of the cells.

Cell-structure specific necrosis by optical-trap induced intracellular nuclear oscillation.

A drug-free procedure for killing malignant cells in a cell-type specific manner would represent a significant breakthrough for leukemia treatment. Here, we show that mechanically vibrating a cell in a specific oscillation condition can significantly promote necrosis. Specifically, oscillating the cell by a low-power laser trap at specific frequencies of a few Hz was found to result in increased death rate of 50% or above in different types of myelogenous leukemia cells, while normal leukocytes showed very little response to similar laser manipulations. The alteration of cell membrane permeability and cell volume, detected from ethidium bromide staining and measurement of intracellular sodium ion concentration, together with the observed membrane blebbing within 10min, suggest cell necrosis. Mechanics modelling reveals severe distortion of the cytoskeleton cortex at frequencies in the same range for peaked cell death. The disruption of cell membrane leading to cell death is therefore due to the cortex distortion, and the frequency at which this becomes significant is cell-type specific. Our findings lay down a new concept for treating leukemia based on vibration induced disruption of membrane in targeted malignant cells.

Characterizing the malignancy and drug resistance of cancer cells from their membrane resealing response.

In this report, we showed that two tumor cell characteristics, namely the malignancy and drug-resistance status can be evaluated by their membrane resealing response. Specifically, membrane pores in a number of pairs of cancer and normal cell lines originated from nasopharynx, lung and intestine were introduced by nano-mechanical puncturing. Interestingly, such nanometer-sized holes in tumor cells can reseal ~2-3 times faster than those in the corresponding normal cells. Furthermore, the membrane resealing time in cancer cell lines exhibiting resistance to several leading chemotherapeutic drugs was also found to be substantially shorter than that in their drug-sensitive counterparts, demonstrating the potential of using this quantity as a novel marker for future cancer diagnosis and drug resistance detection. Finally, a simple model was proposed to explain the observed resealing dynamics of cells which suggested that the distinct response exhibited by normal, tumor and drug resistant cells is likely due to the different tension levels in their lipid membranes, a conclusion that is also supported by direct cortical tension measurement.

Mechanical oscillations enhance gene delivery into suspended cells.

Suspended cells are difficult to be transfected by common biochemical methods which require cell attachment to a substrate. Mechanical oscillations of suspended cells at certain frequencies are found to result in significant increase in membrane permeability and potency for delivery of nano-particles and genetic materials into the cells. Nanomaterials including siRNAs are found to penetrate into suspended cells after subjecting to short-time mechanical oscillations, which would otherwise not affect the viability of the cells. Theoretical analysis indicates significant deformation of the actin-filament network in the cytoskeleton cortex during mechanical oscillations at the experimental frequency, which is likely to rupture the soft phospholipid bilayer leading to increased membrane permeability. The results here indicate a new method for enhancing cell transfection.

Volumetric deformation of live cells induced by pressure-activated cross-membrane ion transport.

In this work, we developed a method that allows precise control over changes in the size of a cell via hydrostatic pressure changes in the medium. Specifically, we show that a sudden increase, or reduction, in the surrounding pressure, in the physiologically relevant range, triggers cross-membrane fluxes of sodium and potassium ions in leukemia cell lines K562 and HL60, resulting in reversible volumetric deformation with a characteristic time of around 30 min. Interestingly, healthy leukocytes do not respond to pressure shocks, suggesting that the cancer cells may have evolved the ability to adapt to pressure changes in their microenvironment. A model is also proposed to explain the observed cell deformation, which highlights how the apparent viscoelastic response of cells is governed by the microscopic cross-membrane transport.

Frequency-dependent cell death by optical tweezers manipulation.

Optical tweezers were used to scan individual Chronic Myelogenous Leukemia cells to determine if the cell death depends on the scanning conditions. Although increasing the scanning frequency or amplitude means greater force applied to the cells, their effects on cell death are not a simple increasing trend, as observed in the optical microscopy. Indeed, cell death sharply increased at particular screening frequencies and amplitudes, whereas other frequencies or amplitudes were less detrimental. These results suggest that cell damage was more sensitive to certain scanning conditions, rather than simply high-applied forces.

Compression-induced alignment and elongation of human mesenchymal stem cell (hMSC) in 3D collagen constructs is collagen concentration dependent.

Controlling cell organization is important in tissue engineering. Guidance by aligned features on scaffolds or stimulation by physical signals can be used to induce cell alignment. We have previously demonstrated a preferred alignment of human MSCs (hMSCs) along the compression loading axis in 3D collagen construct. In this study, we aim to investigate the collagen concentration dependence of the compression-induced hMSC organization. Results demonstrated that the compression-induced alignment and elongation of hMSCs exhibited a biphasic dose-dependent relationship with collagen concentration, and associated well with both collagen ligand density and elastic modulus of the constructs. Moreover, collagen concentration and compression loading significantly affected the expression level of integrin beta 1 and antibody neutralization against this molecule aborted the compression-induced alignment and elongation responses.

Reliable measurement of elastic modulus of cells by nanoindentation in an atomic force microscope.

The elastic modulus of an oral cancer cell line UM1 is investigated by nanoindentation in an atomic force microscope with a flat-ended tip. The commonly used Hertzian method gives apparent elastic modulus which increases with the loading rate, indicating strong effects of viscoelasticity. On the contrary, a rate-jump method developed for viscoelastic materials gives elastic modulus values which are independent of the rate-jump magnitude. The results show that the rate-jump method can be used as a standard protocol for measuring elastic stiffness of living cells, since the measured values are intrinsic properties of the cells.

A method to quantitatively measure the elastic modulus of materials in nanometer scale using atomic force microscopy.

A method is proposed for quantitatively measuring the elastic modulus of materials using atomic force microscopy (AFM) nanoindentation. In this method, the cantilever deformation and the tip-sample interaction during the early loading portion are treated as two springs in series, and based on Sneddon's elastic contact solution, a new cantilever-tip property α is proposed which, together with the cantilever sensitivity A, can be measured from AFM tests on two reference materials with known elastic moduli. The measured α and A values specific to the tip and machine used can then be employed to accurately measure the elastic modulus of a third sample, assuming that the tip does not get significantly plastically deformed during the calibration procedure. AFM nanoindentation tests were performed on polypropylene (PP), fused quartz and acrylic samples to verify the validity of the proposed method. The cantilever-tip property and the cantilever sensitivity measured on PP and fused quartz were 0.514 GPa and 51.99 nm nA(-1), respectively. Using these measured quantities, the elastic modulus of acrylic was measured to be 3.24 GPa, which agrees well with the value measured using conventional depth-sensing indentation in a commercial nanoindenter.

A novel artificial prosthetic replacement for the proximal interphalangeal joint of the hand--from concept to prototype.

This article describes the development of a proximal interphalangeal (PIP) joint prosthesis based on the principles of replicating anatomical surface components, the use of macrolocking intramedullary stem and the use of a cobalt-chrome alloy material. The design features are intended to obtain an optimal range of motion while retaining stability and longevity. The final prototype, for which a patent has been filed, is described.