Reducing Myosin II and ATP-Dependent Mechanical Activity Increases Order and Stability of Intracellular Organelles.

Organization of intracellular content is affected by multiple simultaneous processes, including diffusion in a viscoelastic and structured environment, intracellular mechanical work and vibrations. The combined effects of these processes on intracellular organization are complex and remain poorly understood. Here, we studied the organization and dynamics of a free Ca++ probe as a small and mobile tracer in live T cells. Ca++, highlighted by Fluo-4, is localized in intracellular organelles. Inhibiting intracellular mechanical work by myosin II through blebbistatin treatment increased cellular dis-homogeneity of Ca++-rich features in length scale < 1.1 µm. We detected a similar effect in cells imaged by label-free bright-field (BF) microscopy, in mitochondria-highlighted cells and in ATP-depleted cells. Blebbistatin treatment also reduced the dynamics of the Ca++-rich features and generated prominent negative temporal correlations in their signals. Following Guggenberger et al. and numerical simulations, we suggest that diffusion in the viscoelastic and confined medium of intracellular organelles may promote spatial dis-homogeneity and stability of their content. This may be revealed only after inhibiting intracellular mechanical work and related cell vibrations. Our described mechanisms may allow the cell to control its organization via balancing its viscoelasticity and mechanical activity, with implications to cell physiology in health and disease.

Fast and synchronized fluctuations of cortical actin negatively correlate with nucleoli liquid-liquid phase separation in T cells.

Liquid-liquid phase separation is an important mechanism by which eukaryotic cells functionally organize their intracellular content and has been related to cell malignancy and neurodegenerative diseases. These cells also undergo ATP-driven mechanical fluctuations, yet the effect of these fluctuations on the liquid-liquid phase separation remains poorly understood. Here, we employ high-resolution microscopy and atomic force microscopy of live Jurkat T cells to characterize the spectrum of their mechanical fluctuations, and to relate these fluctuations to the extent of nucleoli liquid-liquid phase separation (LLPS). We find distinct fluctuation of the cytoskeleton and of the cell diameter around 110 Hz, which depend on ATP and on myosin activity. Importantly, these fluctuations negatively correlate to nucleoli LLPS. According to a model of cell viscoelasticity, we propose that these fluctuations generate mechanical work that increases intracellular homogeneity by inhibiting LLPS. Thus, active mechanical fluctuations serve as an intracellular regulatory mechanism that could affect multiple pathophysiological conditions.

Discrimination of leukemic Jurkat cells from normal lymphocytes via novo label-free cytometry based on fluctuation of image gray values.

We introduce a simple, label-free cytometry technique, based on the spatio-temporal fluctuation analysis of pixel gray levels of a cell image utilizing the Gray Level Information Entropy (GLIE) function. In this study, the difference in GLIE random fluctuations and its biophysical etiology in a comparison cell model of leukemic Jurkat cells and human healthy donor lymphocytes was explored. A combination of common bright field microscopy and a unique imaging dish wherein cells are individually held untethered in a picoliter volume matrix of optical chambers was used. Random GLIE fluctuations were found to be greater in malignant Jurkat cells than in benign lymphocytes, while these fluctuations correlate with intracellular vesicle Mean Square Displacement (MSD) values and are inhibited by myosin-2 and adenosine triphosphate (ATP) inhibitors. These results suggest that the incoherent active forces acting on the cytoskeleton which cause mechanical dissipative fluctuation of the cytoskeletal and related intracellular content are the biophysical cellular mechanism behind the GLIE random fluctuation results. Analysis of the results in Jurkat cells and normal lymphocytes suggests the possible potential of this simple and automated label-free cytometry to identify malignancy, particularly in a diagnostic setup of multiple cell examination.

Cell Surface Vibrations Distinguish Malignant from Benign Cells.

The mechanical properties of living cells, including their shape, rigidity, and internal dynamics play a crucial role in their physiology and pathology. Still, the relations between the physiological cell state and its rigidity and surface vibrations remain poorly understood. Here, we have employed AFM measurements on T cells and found a negative relation between cell surface stiffness and its vibrations. Blocking T-type Ca++-channels using Mibefradil reduced cortical actin tension in these cells and enhanced their membrane vibrations and dissipation of intracellular mechanical work to the cell surroundings. We also found increased vibrations of cell membranes in five different malignant cells lines derived from T cell leukemia, lung, prostate, bladder, and melanoma cancers, as compared to their corresponding benign cells. This was demonstrated by utilizing TIRF microscopy in single cells and dynamic laser speckles measurements in an in vitro model of multiple cells in a tissue. Our results show that cell membrane vibrations and dissipation of mechanical work are higher in malignant cells relative to benign cells. Accordingly, these properties may be used to detect and monitor cellular and tissue malignancies.

Spectral Analysis of ATP-Dependent Mechanical Vibrations in T Cells.

Mechanical vibrations affect multiple cell properties, including its diffusivity, entropy, internal content organization, and thus-function. Here, we used Differential Interference Contrast (DIC), confocal, and Total Internal Reflection Fluorescence (TIRF) microscopies to study mechanical vibrations in live (Jurkat) T cells. Vibrations were measured via the motion of intracellular particles and plasma membrane. These vibrations depend on adenosine triphosphate (ATP) consumption and on Myosin II activity. We then used spectral analysis of these vibrations to distinguish the effects of thermal agitation, ATP-dependent mechanical work and cytoskeletal visco-elasticity. Parameters of spectral analyses could be related to mean square displacement (MSD) analyses with specific advantages in characterizing intracellular mechanical work. We identified two spectral ranges where mechanical work dominated vibrations of intracellular components: 0-3 Hz for intracellular particles and the plasma-membrane, and 100-150 Hz for the plasma-membrane. The 0-3 Hz vibrations of the cell membrane that we measured in an experimental model of immune synapse (IS) are expected to affect the IS formation and function in effector cells. It may also facilitate immunological escape of extensively vibrating malignant cells.

In situ label-free static cytometry by monitoring spatiotemporal fluctuations of image gray values.

Spatiotemporal fluctuation of homogeneity and randomness of gray values within an image was explored and utilized as a label-free means for cell examination. This was done by utilizing a user-friendly combination of simple bright field microscope and Cytocapture dish, wherein cells are individually held, each within a picoliter optical chamber, forming an array of cells to be repeatedly measured over time and biomanipulated in situ at single-cell resolution. First, the measured gray level information entropy (GLIE) was used and, based on the fact that living cells are not in a state of thermodynamic equilibrium but rather in a metastable state, two fluctuation-sensitive measures were proposed and examined: ASDE­the spatial average of temporal standard deviation (SD) of GLIE, and AA­the average time autocorrelation of GLIE. System performance was validated on cell-free solutions. This was followed by examining the performance of the measures AGLIE, ASDE, and AA to distinguish among individual live-still, dead and live cells from various cell lines, as well as between cells which were and were not induced to differentiate. Results, which were obtained on four types of cells, indicate advantages of the proposed measures which are believed to be significant additions to the microscope-based probe-free toolbox.