Microscope-based techniques to study cell adhesion and migration

Magnetically-actuated microcages for cells entrapment, fabricated by laser direct writing via two photon polymerization

The manipulation of biological materials at cellular level constitutes a sine qua non and provocative research area regarding the development of micro/nano-medicine. In this study, we report on 3D superparamagnetic microcage-like structures that, in conjunction with an externally applied static magnetic field, were highly efficient in entrapping cells. The microcage-like structures were fabricated using Laser Direct Writing via Two-Photon Polymerization (LDW via TPP) of IP-L780 biocompatible photopolymer/iron oxide superparamagnetic nanoparticles (MNPs) composite. The unique properties of LDW via TPP technique enabled the reproduction of the complex architecture of the 3D structures, with a very high accuracy i.e., about 90 nm lateral resolution. 3D hyperspectral microscopy was employed to investigate the structural and compositional characteristics of the microcage-like structures. Scanning Electron Microscopy coupled with Energy Dispersive X-Ray Spectroscopy was used to prove the unique features regarding the morphology and the functionality of the 3D structures seeded with MG-63 osteoblast-like cells. Comparative studies were made on microcage-like structures made of IP-L780 photopolymer alone (i.e., without superparamagnetic properties). We found that the cell-seeded structures made by IP-L780/MNPs composite actuated by static magnetic fields of 1.3 T were 13.66 ± 5.11 folds (p < 0.01) more efficient in terms of cells entrapment than the structures made by IP-L780 photopolymer alone (i.e., that could not be actuated magnetically). The unique 3D architecture of the microcage-like superparamagnetic structures and their actuation by external static magnetic fields acted in synergy for entrapping osteoblast-like cells, showing a significant potential for bone tissue engineering applications.

Bio-friendly long-term subcellular dynamic recording by self-supervised image enhancement microscopy

Fluorescence microscopy has become an indispensable tool for revealing the dynamic regulation of cells and organelles. However, stochastic noise inherently restricts optical interrogation quality and exacerbates observation fidelity when balancing the joint demands of high frame rate, long-term recording and low phototoxicity. Here we propose DeepSeMi, a self-supervised-learning-based denoising framework capable of increasing signal-to-noise ratio by over 12 dB across various conditions. With the introduction of newly designed eccentric blind-spot convolution filters, DeepSeMi effectively denoises images with no loss of spatiotemporal resolution. In combination with confocal microscopy, DeepSeMi allows for recording organelle interactions in four colors at high frame rates across tens of thousands of frames, monitoring migrasomes and retractosomes over a half day, and imaging ultra-phototoxicity-sensitive Dictyostelium cells over thousands of frames. Through comprehensive validations across various samples and instruments, we prove DeepSeMi to be a versatile and biocompatible tool for breaking the shot-noise limit.

SAXS Examinations of the Redox-Dependent Formation of a DNA-SOD1 Complex

Cu/Zn superoxide dismutase (SOD1) plays a key role in the maintenance of cellular reactive oxygen species (ROS) homeostasis as an antioxidant enzyme. We recently found that SOD1 is involved in the regulation of gene expression in response to changes in cellular ROS levels by binding to DNA-specific sequences. Moreover, the SOD1 binding to DNA was observed to be redox-dependent in solutions. Thus, we examined the redox-dependent DNA binding of SOD1 by multiple measurements, including small-angle X-ray scattering (SAXS), indicating the redox-dependent formation of a DNA-SOD1 complex in solutions. The redox-dependent formation of the DNA-SOD1 complex could underlie the SOD1 regulation of gene expression.

Molecular Tension Probes to Quantify Cell-Generated Mechanical Forces

Living cells generate, sense, and respond to mechanical forces through their interaction with neighboring cells or extracellular matrix, thereby regulating diverse cellular processes such as growth, motility, differentiation, and immune responses. Dysregulation of mechanosensitive signaling pathways is found associated with the development and progression of various diseases such as cancer. Yet, little is known about the mechanisms behind mechano-regulation, largely due to the limited availability of tools to study it at the molecular level. The recent development of molecular tension probes allows measurement of cellular forces exerted by single ligandreceptor interaction, which has helped in revealing the hitherto unknown mechanistic details of various mechanosensitive processes in living cells. Here, we provide an introductory overview of two methods based on molecular tension probes, tension gauge tether (TGT), and molecular tension fluorescence microscopy (MTFM). TGT utilizes the irreversible rupture of double-stranded DNA tether upon application of force in the piconewton (pN) range, whereas MTFM utilizes the reversible extension of molecular springs such as polymer or single-stranded DNA hairpin under applied pN forces. Specifically, the underlying principle of how molecular tension probes measure cell-generated mechanical forces and their applications to mechanosensitive biological processes are described.

3D-Printed Submicron Patterns Reveal the Interrelation between Cell Adhesion, Cell Mechanics, and Osteogenesis

The surface topography of implantable devices is of crucial importance for guiding the cascade of events that starts from the initial contact of the cells with the surface and continues until the complete integration of the device in its immediate environment. There is, however, limited quantitative information available regarding the relationships between the different stages of such cascade(s) and how the design of surface topography influences them. We, therefore, used direct laser writing to 3D-print submicron pillars with precisely controlled dimensions and spatial arrangements to perform a systematic study of such relationships. Using single-cell force spectroscopy, we measured the adhesion force and the work of adhesion of the preosteoblast cells residing on the different types of surfaces. Not only the adhesion parameters (after 2-60 s) but also the formation of focal adhesions was strongly dependent on the geometry and arrangement of the pillars: sufficiently tall and dense pillars enhanced both adhesion parameters and the formation of focal adhesions. Our morphological study of the cells (after 24 h) showed that those enhancements were associated with a specific way of cell settlement onto the surface (i.e., "top state"). The cells interacting with tall and dense pillars were also characterized by numerous thick actin stress fibers in the perinuclear region and possibly high internal stresses. Furthermore, living cells with highly organized cytoskeletal networks exhibited greater values of the elastic modulus. The early responses of the cells predicted their late response including matrix mineralization: tall and dense submicron pillars significantly upregulated the expression of osteopontin after 21 days of culture under both osteogenic and nonosteogenic conditions. Our findings paint a detailed picture of at least one possible cascade of events that starts from initial cell adhesion and continues to subsequent cellular functions and eventual matrix mineralization. These observations could inform the future developments of instructive surfaces for medical devices based on physical surface cues and early markers.

A new 3D finite element-based approach for computing cell surface tractions assuming nonlinear conditions

Advances in methods for determining the forces exerted by cells while they migrate are essential for attempting to understand important pathological processes, such as cancer or angiogenesis, among others. Precise data from three-dimensional conditions are both difficult to obtain and manipulate. For this purpose, it is critical to develop workflows in which the experiments are closely linked to the subsequent computational postprocessing. The work presented here starts from a traction force microscopy (TFM) experiment carried out on microfluidic chips, and this experiment is automatically joined to an inverse problem solver that allows us to extract the traction forces exerted by the cell from the displacements of fluorescent beads embedded in the extracellular matrix (ECM). Therefore, both the reconstruction of the cell geometry and the recovery of the ECM displacements are used to generate the inputs for the resolution of the inverse problem. The inverse problem is solved iteratively by using the finite element method under the hypothesis of finite deformations and nonlinear material formulation. Finally, after mathematical postprocessing is performed, the traction forces on the surface of the cell in the undeformed configuration are obtained. Therefore, in this work, we demonstrate the robustness of our computational-based methodology by testing it under different conditions in an extreme theoretical load problem and then by applying it to a real case based on experimental results. In summary, we have developed a new procedure that adds value to existing methodologies for solving inverse problems in 3D, mainly by allowing for large deformations and not being restricted to any particular material formulation. In addition, it automatically bridges the gap between experimental images and mechanical computations.

A Review of Single-Cell Adhesion Force Kinetics and Applications

Cells exert, sense, and respond to the different physical forces through diverse mechanisms and translating them into biochemical signals. The adhesion of cells is crucial in various developmental functions, such as to maintain tissue morphogenesis and homeostasis and activate critical signaling pathways regulating survival, migration, gene expression, and differentiation. More importantly, any mutations of adhesion receptors can lead to developmental disorders and diseases. Thus, it is essential to understand the regulation of cell adhesion during development and its contribution to various conditions with the help of quantitative methods. The techniques involved in offering different functionalities such as surface imaging to detect forces present at the cell-matrix and deliver quantitative parameters will help characterize the changes for various diseases. Here, we have briefly reviewed single-cell mechanical properties for mechanotransduction studies using standard and recently developed techniques. This is used to functionalize from the measurement of cellular deformability to the quantification of the interaction forces generated by a cell and exerted on its surroundings at single-cell with attachment and detachment events. The adhesive force measurement for single-cell microorganisms and single-molecules is emphasized as well. This focused review should be useful in laying out experiments which would bring the method to a broader range of research in the future.

Calcium Signaling in Glioma Cells: The Role of Nucleotide Receptors

Calcium signaling is probably one of the evolutionary oldest and the most common way by which the signal can be transmitted from the cell environment to the cytoplasmic calcium binding effectors. Calcium signal is fast and due to diversity of calcium binding proteins it may have a very broad effect on cell behavior. Being a crucial player in neuronal transmission it is also very important for glia physiology. It is responsible for the cross-talk between neurons and astrocytes, for microglia activation and motility. Changes in calcium signaling are also crucial for the behavior of transformed glioma cells. The present chapter summarizes molecular mechanisms of calcium signal formation present in glial cells with a strong emphasis on extracellular nucleotide-evoked signaling pathways. Some aspects of glioma C6 signaling such as the cross-talk between P2Y₁ and P2Y₁₂ nucleotide receptors in calcium signal generation will be discussed in-depth, to show complexity of machinery engaged in formation of this signal. Moreover, possible mechanisms of modulation of the calcium signal in diverse environments there will be presented herein. Finally, the possible role of calcium signal in glioma motility is also discussed. This is a very important issue, since glioma cells, contrary to the vast majority of neoplastic cells, cannot spread in the body with the bloodstream and, at least in early stages of tumor development, may expand only by means of sheer motility.

Photo-triggered Drug Delivery Systems for Neuron-related Applications

The development of materials, chemistry and genetics has created a great number of systems for delivering antibiotics, neuropeptides or other drugs to neurons in neuroscience research, and has also provided important and powerful tools in neuron-related applications. Although these drug delivery systems can facilitate the advancement of neuroscience studies, they still have limited applications due to various drawbacks, such as difficulty in controlling delivery molecules or drugs to the target region, and trouble of releasing them in predictable manners. The combination of optics and drug delivery systems has great potentials to address these issues and deliver molecules or drugs to the nervous system with extraordinary spatiotemporal selectivity triggered by light. In this review, we will introduce the development of photo-triggered drug delivery systems in neuroscience research and their neuron-related applications including regulating neural activities, treating neural diseases and inducing nerve regenerations.

Traction force microscopy with optimized regularization and automated Bayesian parameter selection for comparing cells

Adherent cells exert traction forces on to their environment which allows them to migrate, to maintain tissue integrity, and to form complex multicellular structures during developmental morphogenesis. Traction force microscopy (TFM) enables the measurement of traction forces on an elastic substrate and thereby provides quantitative information on cellular mechanics in a perturbation-free fashion. In TFM, traction is usually calculated via the solution of a linear system, which is complicated by undersampled input data, acquisition noise, and large condition numbers for some methods. Therefore, standard TFM algorithms either employ data filtering or regularization. However, these approaches require a manual selection of filter- or regularization parameters and consequently exhibit a substantial degree of subjectiveness. This shortcoming is particularly serious when cells in different conditions are to be compared because optimal noise suppression needs to be adapted for every situation, which invariably results in systematic errors. Here, we systematically test the performance of new methods from computer vision and Bayesian inference for solving the inverse problem in TFM. We compare two classical schemes, L1- and L2-regularization, with three previously untested schemes, namely Elastic Net regularization, Proximal Gradient Lasso, and Proximal Gradient Elastic Net. Overall, we find that Elastic Net regularization, which combines L1 and L2 regularization, outperforms all other methods with regard to accuracy of traction reconstruction. Next, we develop two methods, Bayesian L2 regularization and Advanced Bayesian L2 regularization, for automatic, optimal L2 regularization. Using artificial data and experimental data, we show that these methods enable robust reconstruction of traction without requiring a difficult selection of regularization parameters specifically for each data set. Thus, Bayesian methods can mitigate the considerable uncertainty inherent in comparing cellular tractions in different conditions.

Automated quantification of three-dimensional organization of fiber-like structures in biological tissues

Fiber-like structures are prevalent in biological tissues, yet quantitative approaches to assess their three-dimensional (3D) organization are lacking. We develop 3D directional variance, as a quantitative biomarker of truly 3D fibrillar organization by extending the directional statistics formalism developed for describing circular data distributions (i.e. when 0° and 360° are equivalent) to axial ones (i.e. when 0° and 180° are equivalent). Significant advantages of this analysis include its time efficiency, sensitivity and ability to provide quantitative readouts of organization over different size scales of a given data set. We establish a broad range of applications for this method by characterizing collagen fibers, neuronal axons and fibroblasts in the context of cancer diagnostics, traumatic brain injury and cell-matrix interactions in developing engineered tissues. This method opens possibilities for unraveling in a sensitive, and quantitative manner the organization of essential fiber-like structures in tissues and ultimately its impact on tissue function.

Myofibril growth during cardiac hypertrophy is regulated through dual phosphorylation and acetylation of the actin capping protein CapZ

The mechanotransduction signaling pathways initiated in heart muscle by increased mechanical loading are known to lead to long-term transcriptional changes and hypertrophy, but the rapid events for adaptation at the sarcomeric level are not fully understood. The goal of this study was to test the hypothesis that actin filament assembly during cardiomyocyte growth is regulated by post-translational modifications (PTMs) of CapZβ1. In rapidly hypertrophying neonatal rat ventricular myocytes (NRVMs) stimulated by phenylephrine (PE), two-dimensional gel electrophoresis (2DGE) of CapZβ1 revealed a shift toward more negative charge. Consistent with this, mass spectrometry identified CapZβ1 phosphorylation on serine-204 and acetylation on lysine-199, two residues which are near the actin binding surface of CapZβ1. Ectopic expression of dominant negative PKCɛ (dnPKCɛ) in NRVMs blunted the PE-induced increase in CapZ dynamics, as evidenced by the kinetic constant (Kfrap) of fluorescence recovery after photobleaching (FRAP), and concomitantly reduced phosphorylation and acetylation of CapZβ1. Furthermore, inhibition of class I histone deacetylases (HDACs) increased lysine-199 acetylation on CapZβ1, which increased Kfrap of CapZ and stimulated actin dynamics. Finally, we show that PE treatment of NRVMs results in decreased binding of HDAC3 to myofibrils, suggesting a signal-dependent mechanism for the regulation of sarcomere-associated CapZβ1 acetylation. Taken together, this dual regulation through phosphorylation and acetylation of CapZβ1 provides a novel model for the regulation of myofibril growth during cardiac hypertrophy.

Active limited-angle tomographic phase microscope

We demonstrate an active, holographic tomography system, working with limited angle of projections, realized by optical-only, diffraction-based beam steering. The system created for this purpose is a Mach–Zehnder interferometer modified to serve as a digital holographic microscope with a high numerical aperture illumination module and a spatial light modulator (SLM). Such a solution is fast and robust. Apart from providing an elegant solution to viewing angle shifting, it also adds new capabilities of the holographic microscope system. SLM, being an active optical element, allows wavefront correction in order to improve measurement accuracy. Integrated phase data captured with different illumination scenarios within a highly limited angular range are processed by a new tomographic reconstruction algorithm based on the compressed sensing technique: total variation minimization, which is applied here to reconstruct nonpiecewise constant samples. Finally, the accuracy of full measurement and the proposed processing path is tested for a calibrated three-dimensional micro-object as well as a biological object--C2C12 myoblast cell.

Actin dynamics is rapidly regulated by the PTEN and PIP2 signaling pathways leading to myocyte hypertrophy

Mature cardiac myocytes are terminally differentiated, and the heart has limited capacity to replace lost myocytes. Thus adaptation of myocyte size plays an important role in the determination of cardiac function. The hypothesis tested is that regulation of the dynamic exchange of actin leads to cardiac hypertrophy. ANG II was used as a hypertrophic stimulant in mouse heart and neonatal rat ventricular myocytes (NRVMs) in culture for assessment of a mechanism for regulation of actin dynamics by phosphatidylinositol 4,5-bisphosphate (PIP2). Actin dynamics in NRVMs rapidly increased in a PIP2-dependent manner, measured by imaging and fluorescence recovery after photobleaching (FRAP). A significant increase in PIP2 levels was found by immunoblotting in both adult mouse heart tissue and cultured NRVMs. Inhibition of phosphatase and tensin homolog (PTEN) in NRVMs markedly blunted ANG II-induced increases in actin dynamics, the PIP2 level, and cell size. Furthermore, PTEN activity was dramatically upregulated in ANG II-treated NRVMs but downregulated when PTEN inhibitors were used. The time course of the rise in the PIP2 level was inversely related to the fall in the PIP3 level, which was significant by 30 min in ANG II-treated NRVMs. However, significant translocation of PTEN to the plasma membrane occurred by 10 min, suggesting a crucial initial step for PTEN for the cellular responses to ANG II. In conclusion, PTEN and PIP2 signaling may play an important role in myocyte hypertrophy by the regulation of actin filament dynamics, which is induced by ANG II stimulation.

Human T cells monitored by impedance spectrometry using field-effect transistor arrays: a novel tool for single-cell adhesion and migration studies

Cytotoxic T lymphocytes (CTLs) play an important role in the immune system by recognizing and eliminating pathogen-infected and tumorigenic cells. In order to achieve their function, T cells have to migrate throughout the whole body and identify the respective targets. In conventional immunology studies, interactions between CTLs and targets are usually investigated using tedious and time-consuming immunofluorescence imaging. However, there is currently no straightforward measurement tool available to examine the interaction strengths. In the present study, adhesion strengths and migration of single human CD8(+) T cells on pre-coated field-effect transistor (FET) devices (i.e. fibronectin, anti-CD3 antibody, and anti-LFA-1 antibody) were measured using impedance spectroscopy. Adhesion strengths to different protein and antibody coatings were compared. By fitting the data to an electronically equivalent circuit model, cell-related parameters (cell membrane capacitance referring to cell morphology and seal resistance referring to adhesion strength) were obtained. This electronically-assessed adhesion strength provides a novel, fast, and important index describing the interaction efficiency. Furthermore, the size of our detection transistor gates as well as their sensitivity reaches down to single cell resolution. Real-time motions of individually migrating T cells can be traced using our FET devices. The in-house fabricated FETs used in the present study are providing a novel and very efficient insight to individual cell interactions.

S137 phosphorylation of profilin 1 is an important signaling event in breast cancer progression

BACKGROUND

Profilins are actin-modulating proteins regulating many intracellular functions based on their multiple and diverse ligand interactions. They have been implicated to play a role in many pathological conditions such as allergies, cardiovascular diseases, muscular atrophy, diabetes, dementia and cancer. Post-translational modifications of profilin 1 can alter its properties and subsequently its function in a cell. In the present study, we identify the importance of phosphorylation of profilin 1 at serine 137 (S137) residue in breast cancer progression.

METHODS/PRINCIPAL FINDINGS

We found elevated profilin 1 (PFN) in human breast cancer tissues when compared to adjacent normal tissues. Overexpression of wild-type profilin 1 (PFN-WT) in breast cancer MCF7 cells made them more migratory, invasive and adherent independent in comparison to empty vector transfected cells. Mutation in serine phosphorylation site (S137) of profilin 1 (PFN-S137A) significantly abrogated these properties. Mutation affecting actin-binding ability (PFN-R74E) of profilin 1 enhanced its tumorigenic function whereas mutation affecting its poly-L-proline binding function (PFN-H133S) alleviated these mechanisms in breast cancer cells. PFN-WT was found to activate matrix metalloproteinases by zymography, MMP2 and MMP9 in presence of PDBu (phorbol 12, 13 dibutyrate, PI3K agonist) to enhance migration and invasion in MCF7 cells while PFN-S137A did not. Phosphorylation increased migration and invasion in other mutants of profilin 1. Nuclear profilin levels also increased in the presence of PDBu.

CONCLUSIONS

Previous studies show that profilin could be executing a dual role in cancer by either suppressing or promoting tumorigenesis in a context dependent manner. In this study we demonstrate for the first time that phosphorylation of profilin 1 at serine 137 enhances oncogenic properties in breast cancer cells. Inhibitors targeting profilin 1 phosphorylation directly or indirectly through inhibition of kinases that phosphorylate profilin could be valuable therapeutic agents that can alter its activity and thereby control the progression of cancer.

Combining surface topography with polymer chemistry: exploring new interfacial biological phenomena

This review focuses on combining surface topography and surface chemical modification by the grafting of polymers to develop optimal material interfaces with synergistic properties and functions for biological and biomedical applications.

Phosphatidylinositol 4,5-bisphosphate regulates CapZβ1 and actin dynamics in response to mechanical strain

Mechanical stress causes filament remodeling leading to myocyte hypertrophy and heart failure. The actin capping protein Z (CapZ) tightly binds to the barbed end of actin filaments, thus regulating actin assembly. The hypothesis is that the binding between CapZ and the actin filament is modulated through phosphatidylinositol 4,5-bisphosphate (PIP2) and how the COOH-terminus of CapZβ1 regulates this binding. Primary neonatal rat ventricular myocytes (NRVMs) were strained at 10% amplitude and 1-Hz frequency. Dot blotting measured the PIP2 amount, and affinity precipitation assay assessed the direct interaction between PIP2 and CapZβ1. Fluorescence recovery after photobleaching of green fluorescent protein-CapZβ1 and actin-green fluorescent protein after 1 h of strain shows the dynamics significantly increased above the unstrained group. The increases in CapZ and actin dynamics were blunted by neomycin, suggesting PIP2 signaling is involved. The amount of PIP2 dramatically increased in NRVMs strained for 1 h. With a ROCK or RhoA inhibitor, changes were markedly reduced. Subcellular fractionation and antibody localization showed PIP2 distributed to the sarcomeres. More PIP2-bound CapZβ1 was found in strained NRVMs. Less PIP2 bound to the CapZβ1 with its COOH-terminus intact than in the COOH-terminal mutant of CapZβ1, suggesting some inhibitory role for the COOH-terminus. Myocyte hypertrophy normally induced by 48 h of cyclic strain was blunted by dominant negative RhoA or neomycin. This suggests that after many hours of cyclic strain, a possible mechanism for cell hypertrophy is the accumulation of thin filament assembly triggered partially by the increased PIP2 level and its binding to CapZ.

Calcium signaling in glioma cells--the role of nucleotide receptors

Calcium signaling is probably one of the evolutionary oldest and the most common way by which the signal can be transmitted from the cell environment to the cytoplasmic calcium binding effectors. Calcium signal is fast and due to diversity of calcium binding proteins it may have a very broad effect on cell behavior. Being a crucial player in neuronal transmission it is also very important for glia physiology. It is responsible for the cross-talk between neurons and astrocytes, for microglia activation and motility. Changes in calcium signaling are also crucial for the behavior of transformed glioma cells. The present Chapter summarizes molecular mechanisms of calcium signal formation present in glial cells with a strong emphasis on extracellular nucleotide-evoked signaling pathways. Some aspects of glioma C6 signaling such as the cross-talk between P2Y(1) and P2Y(12) nucleotide receptors in calcium signal generation will be discussed in-depth, to show complexity of machinery engaged in formation of this signal. Moreover, possible mechanisms of modulation of the calcium signal in diverse environments there will be presented herein. Finally, the possible role of calcium signal in glioma motility is also discussed. This is a very important issue, since glioma cells, contrary to the vast majority of neoplastic cells, cannot spread in the body with the bloodstream and, at least in early stages of tumor development, may expand only by means of sheer motility.

Modeling capping protein FRAP and CALI experiments reveals in vivo regulation of actin dynamics

To gain insights on cellular mechanisms regulating actin polymerization, we used the Virtual Cell to model fluorescence recovery after photobleaching (FRAP) and chromophore-assisted laser inactivation (CALI) experiments on EGFP-capping protein (EGFP-CP). Modeling the FRAP kinetics demonstrated that the in vivo rate for the dissociation of CP from actin filaments is much faster (approximately 0.1 s(-1)) than that measured in vitro (0.01-0.0004 s(-1)). The CALI simulation revealed that in order to induce sustainable changes in cell morphology after CP inactivation, the cells should exhibit anticapping ability. We included the VASP protein as the anticapping agent in the modeling scheme. The model predicts that VASP affinity for barbed ends has a cooperative dependence on the concentration of VASP-barbed end complexes. This dependence produces a positive feedback that stabilizes the complexes and allows sustained growth at clustered filament tips. We analyzed the range of laser intensities that are sufficient to induce changes in cell morphology. This analysis demonstrates that FRAP experiments with EGFP-CP can be performed safely without changes in cell morphology, because, the intensity of the photobleaching beam is not high enough to produce the critical concentration of free barbed ends that will induce filament growth before diffusional replacement of EGFP-CP occurs.

Systems microscopy approaches to understand cancer cell migration and metastasis

Cell migration is essential in a number of processes, including wound healing, angiogenesis and cancer metastasis. Especially, invasion of cancer cells in the surrounding tissue is a crucial step that requires increased cell motility. Cell migration is a well-orchestrated process that involves the continuous formation and disassembly of matrix adhesions. Those structural anchor points interact with the extra-cellular matrix and also participate in adhesion-dependent signalling. Although these processes are essential for cancer metastasis, little is known about the molecular mechanisms that regulate adhesion dynamics during tumour cell migration. In this review, we provide an overview of recent advanced imaging strategies together with quantitative image analysis that can be implemented to understand the dynamics of matrix adhesions and its molecular components in relation to tumour cell migration. This dynamic cell imaging together with multiparametric image analysis will help in understanding the molecular mechanisms that define cancer cell migration.

Fluorescence Resonance Energy Transfer (FRET)-based Detection of Profilin-VASP Interaction

Profilins belong to a family of small G-actin binding proteins which are thought to assist in F-actin elongation at the leading edge of migrating cells through their interactions with a host of actin-binding proteins including Ena (enabled)/VASP (vasodilator stimulated phosphoprotein). Profilin's interactions with the major actin regulators have been studied almost exclusively using biochemical methods. Therefore spatiotemporal features of these protein-protein interactions have not been resolved so far. In this paper, we for the first time demonstrate the feasibility of GFP-based fluorescence resonance energy transfer (FRET) technique to detect VASP's interaction with profilin-1, a ubiquitously expressed member of profilin family of genes. Specifically, we performed acceptor photobleaching FRET in MDA-MB-231 breast cancer cells to show prominent VASP-Pfn1 interaction at the membrane ruffles near the leading edge.

MEASUREMENT of Protein 53 Diffusion Coefficient in Live HeLa Cells Using Raster Image Correlation Spectroscopy (RICS)

We have applied Raster Image Correlation Spectroscopy (RICS) technique to characterize the dynamics of protein 53 (p53) in living cells before and after the treatment with DNA damaging agents. HeLa cells expressing Green Fluorescent Protein (GFP) tagged p53 were incubated with and without DNA damaging agents, cisplatin or eptoposide, which are widely used as chemotherapeutic drugs. Then, the diffusion coefficient of GFP-p53 was determined by RICS and it was significantly reduced after the drug treatment while that of the one without drug treatment was not. It is suggested that the drugs induced the interaction of p53 with either other proteins or DNA. Together, our results demonstrated that RICS is able to detect the protein dynamics which may be associated with protein-protein or protein-DNA interactions in living cells and it may be useful for the drug screening.

Quantifying cellular traction forces in three dimensions

Cells engage in mechanical force exchange with their extracellular environment through tension generated by the cytoskeleton. A method combining laser scanning confocal microscopy (LSCM) and digital volume correlation (DVC) enables tracking and quantification of cell-mediated deformation of the extracellular matrix in all three spatial dimensions. Time-lapse confocal imaging of migrating 3T3 fibroblasts on fibronectin (FN)-modified polyacrylamide gels of varying thickness reveals significant in-plane (x, y) and normal (z) displacements, and illustrates the extent to which cells, even in nominally two-dimensional (2-D) environments, explore their surroundings in all three dimensions. The magnitudes of the measured displacements are independent of the elastic moduli of the gels. Analysis of the normal displacement profiles suggests that normal forces play important roles even in 2-D cell migration.

Migration of isogenic cell lines quantified by dynamic multivariate analysis of single-cell motility

Cell migration is essential in many physiological and pathological processes. To understand this complex behavior, researchers have turned to quantitative, in vitro, image-based measurements to dissect the steps of cellular motility. With the rise of automated microscopy, the bottleneck in these approaches is no longer data acquisition, but data analysis. Using time-lapse microscopy and computer-assisted image analysis, we have developed a novel, quantitative assay that extracts a multivariate profile for cellular motility. This technique measures three dynamic parameters per single cell: speed, surface area, and an in-dex of cell expansion/contraction activity (DECCA). Our assay can be used in combination with a variety of extracellular matrix components, or other soluble agents, to analyze the effects of the microenvironment on cellular migration dynamics in vitro. Our application was developed and tested using A431 and HT-1080 cell lines plated on laminin-332 or fibronectin substrates. Our results indicate that HT-1080 cells migrate faster, have a greater surface area, and have a higher DECCA index than A431 cells on both matrices (for all parameters, p < 0.05). Spearman's correlation coefficients suggest that for these cell lines and matrices, various combinations of the three measurements display low to medium-high levels of correlation. These findings compare well with previous literature. Our approach provides new tools to measure cellular migration dynamics and address questions on the relationship between cell motility and the microenvironment, using only common microscopy techniques, accessible image analysis applications, and a basic desktop computer for image processing.

A novel assay based on fluorescence microscopy and image processing for determining phenotypic distributions of rod-shaped bacteria

Cell population balance (CPB) models can account for the phenotypic heterogeneity that characterizes isogenic cell populations. To utilize the predictive power of these models, however, we must determine the single-cell reaction and division rates as well as the partition probability density function of the cell population. These functions can be obtained through the Collins-Richmond inverse CPB modeling methodology, if we know the phenotypic distributions of (a) the overall cell population, (b) the dividing cell subpopulation, and (c) the newborn cell subpopulation. This study presents the development of a novel assay that combines fluorescence microscopy and image processing to determine these distributions. The method is generally applicable to rod-shaped cells dividing through the formation of a characteristic constriction. Morphological criteria were developed for the automatic identification of dividing cells and validated through direct comparison with manually obtained measurements. The newborn cell subpopulation was obtained from the corresponding dividing cell subpopulation by collecting information from the two compartments separated by the constriction. The method was applied to E. coli cells carrying the genetic toggle network with a green fluorescent marker. Our measurements for the overall cell population were in excellent agreement with the distributions obtained via flow cytometry. The new assay constitutes a powerful tool that can be used in conjunction with inverse CPB modeling to rigorously quantify single-cell behavior from data collected from highly heterogeneous cell populations.

Decomposition cross-correlation for analysis of collagen matrix deformation by single smooth muscle cells

Microvascular remodeling is known to depend on cellular interactions with matrix tissue. However, it is difficult to study the role of specific cells or matrix elements in an in vivo setting. The aim of this study is to develop an automated technique that can be employed to obtain and analyze local collagen matrix remodeling by single smooth muscle cells. We combined a motorized microscopic setup and time-lapse video microscopy with a new cross-correlation based image analysis algorithm to enable automated recording of cell-induced matrix reorganization. This method rendered 60–90 single cell studies per experiment, for which collagen deformation over time could be automatically derived. Thus, the current setup offers a tool to systematically study different components active in matrix remodeling.

Cellular mechanobiology and cancer metastasis

The primary cause of cancer treatment failure is invasion and metastasis, and invading tumor cells utilize many of the motility patterns that have been documented for normal morphogenesis. Recently, the role of mechanical forces in guiding various tissue and cell movements in embryonic development has been systematically analyzed with new experimental and computational methods. The tissue and cellular mechanobiology approach also holds promise for increasing the understanding of tumor invasion. In fact, the mechanical stiffness of tumors has correlated with invasiveness, and manipulation of extracellular matrix (ECM) stiffness in vitro has suppressed the cancer phenotype. Several important signaling molecules reside on the cytoskeleton, which is affected by external stress imparted by the ECM, and deformation of the nucleus can trigger the activation of certain genes. All these observations suggest that a synthesis of the biology of cancer cell invasion and cellular mechanobiology may offer new targets for the treatment of malignant disease. Accordingly, sensitive and relevant in vivo models and methods to study cancer mechanobiology are needed.

High resolution traction force microscopy based on experimental and computational advances

Cell adhesion and migration crucially depend on the transmission of actomyosin-generated forces through sites of focal adhesion to the extracellular matrix. Here we report experimental and computational advances in improving the resolution and reliability of traction force microscopy. First, we introduce the use of two differently colored nanobeads as fiducial markers in polyacrylamide gels and explain how the displacement field can be computationally extracted from the fluorescence data. Second, we present different improvements regarding standard methods for force reconstruction from the displacement field, which are the boundary element method, Fourier-transform traction cytometry, and traction reconstruction with point forces. Using extensive data simulation, we show that the spatial resolution of the boundary element method can be improved considerably by splitting the elastic field into near, intermediate, and far field. Fourier-transform traction cytometry requires considerably less computer time, but can achieve a comparable resolution only when combined with Wiener filtering or appropriate regularization schemes. Both methods tend to underestimate forces, especially at small adhesion sites. Traction reconstruction with point forces does not suffer from this limitation, but is only applicable with stationary and well-developed adhesion sites. Third, we combine these advances and for the first time reconstruct fibroblast traction with a spatial resolution of approximately 1 microm.

Probing cellular microenvironments and tissue remodeling by atomic force microscopy

The function of cells is strongly determined by the properties of their extracellular microenvironment. Biophysical parameters like environmental stiffness and fiber orientation in the surrounding matrix are important determinants of cell adhesion and migration. Processes like tissue maintenance, wound repair, cancer cell invasion, and morphogenesis depend critically on the ability of cells to actively sense and remodel their surroundings. Pericellular proteolytic activity and adaptation of migration tactics to the environment are strategies to achieve this aim. Little is known about the distinct regulatory mechanisms that are involved in these processes. The system's critical biophysical and biochemical determinants are well accessible by atomic force microscopy (AFM), a unique tool for functional, nanoscale probing and morphometric, high-resolution imaging of processes in live cells. This review highlights common principles of tissue remodeling and focuses on application examples of different AFM techniques, for example elasticity mapping, the combination of AFM and fluorescence microscopy, the morphometric imaging of proteolytic activity, and force spectroscopy applications of single molecules or individual cells. To achieve a more complete understanding of the processes underlying the interaction of cells with their environments, the combination of AFM force spectroscopy experiments will be essential.

Sensitivity of CFP/YFP and GFP/mCherry pairs to donor photobleaching on FRET determination by fluorescence lifetime imaging microscopy in living cells

Fluorescent protein-based FRET is a powerful method for visualizing protein-protein interactions and biochemical reactions in living cells. It can be difficult, however, to avoid photobleaching when observing fluorescent cells under the microscope, especially those expressing CFP. We compared the sensitivity of two protein-based FRET pairs to light-induced fluorescence changes in the donor, on FRET determination by fluorescence lifetime imaging microscopy (FLIM). Thanks to the very low excitation light levels of the time- and space-correlated single photon counting (TSCSPC) method, FLIM acquisitions were achieved without donor photobleaching. Here, we show that photobleaching of CFP by a mercury lamp under the microscope induced a decrease in the mean fluorescence lifetime, which interfered with FRET determination between CFP and YFP. Importantly, the range of light-induced variation of the mean fluorescence lifetime of CFP was not proportional to the decrease in the steady state fluorescence intensity and varied from cell to cell. The choice of the CFP/YFP pair therefore requires that the cells be observed and analyzed at very low light levels during the whole FRET experiment. In contrast, the GFP/mCherry pair provided an accurate FRET measurement by FLIM, even if some GFP photobleaching took place. We thus demonstrate that CFP can be an unreliable donor for FRET determination in living cells, due to its photosensitivity properties. We demonstrate that the GFP/mCherry pair is better suited for FRET measurement by FLIM in living cells than the CFP/YFP pair.

Clustering of raft-associated proteins in the external membrane leaflet modulates internal leaflet H-ras diffusion and signaling

One of the least-explored aspects of cholesterol-enriched domains (rafts) in cells is the coupling between such domains in the external and internal monolayers and its potential to modulate transbilayer signal transduction. Here, we employed fluorescence recovery after photobleaching to study the effects of antibody-mediated patching of influenza hemagglutinin (HA) proteins [raft-resident wild-type HA and glycosylphosphatidylinositol-anchored HA, or the nonraft mutant HA(2A520)] on the lateral diffusion of internal-leaflet raft and nonraft Ras isoforms (H-Ras and K-Ras, respectively). Our studies demonstrate that the clustering of outer-leaflet or transmembrane raft-associated HA proteins (but not their nonraft mutants) retards the lateral diffusion of H-Ras (but not K-Ras), suggesting stabilized interactions of H-Ras with the clusters of raft-associated HA proteins. These modulations were paralleled by specific effects on the activity of H-Ras but not of the nonraft K-Ras. Thus, clustering raft-associated HA proteins facilitated the early step whereby H-Ras is converted to an activated, GTP-loaded state but inhibited the ensuing step of downstream signaling via the Mek/Erk pathway. We propose a model for the modulation of transbilayer signaling by clustering of raft proteins, where external clustering (antibody or ligand mediated) enhances the association of internal-leaflet proteins with the stabilized clusters, promoting either enhancement or inhibition of signaling.

Physical determinants of cell organization in soft media

Cell adhesion is an integral part of many physiological processes in tissues, including development, tissue maintenance, angiogenesis, and wound healing. Recent advances in materials science (including microcontact printing, soft lithography, microfluidics, and nanotechnology) have led to strongly improved control of extracellular ligand distribution and of the properties of the micromechanical environment. As a result, the investigation of cellular response to the physical properties of adhesive surfaces has become a very active area of research. Sophisticated use of elastic substrates has revealed that cell organization in soft media is determined by active mechanosensing at cell-matrix adhesions. In order to determine the underlying mechanisms, quantification and biophysical modelling are essential. In tissue engineering, theory might help to design new environments for cells.

MAP kinases and cell migration

Recent studies have demonstrated that mitogen-activated protein kinases (MAPKs), including Jun N-terminus kinase (JNK), p38 and Erk, play crucial roles in cell migration. JNK, for example, regulates cell migration by phosphorylating paxillin, DCX, Jun and microtubule-associated proteins. Studies of p38 show that this MAPK modulates migration by phosphorylating MAPK-activated protein kinase 2/3 (MAPKAP 2/3), which appears to be important for directionality of migration. Erk governs cell movement by phosphorylating myosin light chain kinase (MLCK), calpain or FAK. Thus, the different kinases in the MAPK family all seem able to regulate cell migration but by distinct mechanisms.

The mechanism of cortical ingression during early cytokinesis: thinking beyond the contractile ring hypothesis

Owing to the rapid advances in genomic, proteomic and imaging technologies, the field of cytokinesis has seen rapid advances during the past decade. However, the basic model for the early stage of ingression, known as the contractile ring hypothesis, remains largely unchanged. From recent observations, it is becoming clear that early cytokinesis of animal cells involves a more extensive set of events, both temporally and spatially, than what is encompassed by the original contractile ring hypothesis. Activities relevant to cytokinesis, such as cortical contraction, can initiate well before onset of anaphase. Furthermore, equatorial ingression can involve multiple events in different regions of the cortex, including the establishment of anterior-posterior polarity, the modulation of cortical deformability, the expansion and compression of the cell cortex, and forces directed towards the interior of the cell or away from the equator. In this article (which is part of the Cytokinesis series), I evaluate critically key observations on when, where and how early ingression of animal cells takes place.

A simple method to study cellular migration

We describe here a simple and fast method for the characterisation of cell motion. By projecting on a single plane different positions of the cell a ribbon is generated, whose characteristics can be related to the type of motion. The proposed method allows both to determine, very quickly, the motility of a population of cells and to investigate and characterise properties of a single cell's motion. The methodology presented here can be applied to a large range of cell movement and also adapted and extended to other problems involving biological motion.

Local, Three-Dimensional Strain Measurements Within Largely Deformed Extracellular Matrix Constructs

The ability to create extracellular matrix (ECM) constructs that are mechanically and biochemically similar to those found in vivo and to understand how their properties affect cellular responses will drive the next generation of tissue engineering strategies. To date, many mechanisms by which cells biochemically communicate with the ECM are known. However, the mechanisms by which mechanical information is transmitted between cells and their ECM remain to be elucidated. “Self-assembled” collagen matrices provide an in vitro-model system to study the mechanical behavior of ECM. To begin to understand how the ECM and the cells interact mechanically, the three-dimensional (3D) mechanical properties of the ECM must be quantified at the micro-(local) level in addition to information measured at the macro-(global) level. Here we describe an incremental digital volume correlation (IDVC) algorithm to quantify large (>0.05) 3D mechanical strains in the microstructure of 3D collagen matrices in response to applied mechanical loads. Strain measurements from the IDVC algorithm rely on 3D confocal images acquired from collagen matrices under applied mechanical loads. The accuracy and the precision of the IDVC algorithm was verified by comparing both image volumes collected in succession when no deformation was applied to the ECM (zero strain) and image volumes to which simulated deformations were applied in both 1D and 3D (simulated strains). Results indicate that the IDVC algorithm can accurately and precisely determine the 3D strain state inside largely deformed collagen ECMs. Finally, the usefulness of the algorithm was demonstrated by measuring the microlevel 3D strain response of a collagen ECM loaded in tension.

Cell migration/invasion assays and their application in cancer drug discovery

Invasive capacity is the single most important trait that distinguishes benign from malignant lesions. Tumour cells, during intravasation and extravasation of blood and lymphatic channels and when establishing colonies at secondary sites, must move through tissue boundaries that normal adult cells (other than, for example activated leukocytes) do not cross. Similar mechanisms are also utilised by activated endothelial cells during the generation of new blood vessels that enable the sustained growth and dissemination of tumours. It is now increasingly recognised that these processes--cell motility and invasion--might provide a rich source of novel targets for cancer therapy and that appropriate inhibitors may restrain both metastasis and neoangiogenesis. This new paradigm demands screening assays that can rapidly and quantitatively measure cell movement and the ability to traverse physiological barriers. We also need to consider whether simple reductionist in vitro approaches can reliably model the complexity of in vivo tumour invasion/neoangiogenesis. There are both opportunities and challenges ahead in developing a balanced portfolio of assays that will be able to evaluate accurately and finally deliver novel anti-invasive agents with therapeutic potential for clinical use.

Internet-based image analysis quantifies contractile behavior of individual fibroblasts inside model tissue

In a cell-populated collagen gel, intrinsic fiber structure visible in differential interference contrast images can provide markers for an in situ strain gauge to quantify cell-gel mechanics, while optical sections of fluorescent protein distribution capture cytoskeletal kinematics. Mechanics quantification can be derived automatically from timelapse differential interference contrast images using a Deformation Quantification and Analysis software package accessible online at http://dqa.web.cmu.edu. In our studies, fibroblast contractile machinery was observed to function entirely within pseudopods, while GFP-alpha-actinin concentrated in pseudopod tips and cortex. Complex strain patterns around individual cells showed instances of both elastic and inelastic strain transmission, suggesting a role in observed long-range alignment of cells.

Unresolved Issues in Mammalian Fertilization

This review considers the role of the sperm in fertilization, addressing areas of misunderstanding and unfounded assumptions and taking particular advantage of the large body of data resulting from work with rodent species in vitro. Considerable attention is given to the appropriate use and interpretation of assays for capacitation, acrosomal exocytosis, hyperactivation, and sperm protein phosphorylation, as well as tests for sperm-zona and sperm-oocyte membrane interactions. The lack of general agreement on the means of sperm adhesion to and penetration of the zona pellucida is addressed, and the need for new approaches to this problem is pointed out. Some molecular advances in our understanding of specific steps in the process of fertilization are discussed in the context of intact cell-matrix and cell-cell interaction. This review should provide practical information for researchers just beginning the study of fertilization and interesting but not widely known observations to stimulate new ideas in experienced scientists.

Adhesion-dependent cell mechanosensitivity

The conversion of physical signals, such as contractile forces or external mechanical perturbations, into chemical signaling events is a fundamental cellular process that occurs at cell-extracellular matrix contacts, known as focal adhesions. At these sites, transmembrane integrin receptors are associated via their cytoplasmic domains with the actin cytoskeleton. This interaction with actin is mediated by a submembrane plaque, consisting of numerous cytoskeletal and signaling molecules. Application of intrinsic or external forces to these structures dramatically affects their assembly and triggers adhesion-mediated signaling. In this review, we discuss the structure-function relationships of focal adhesions and the possible mode of action of the putative mechanosensor associated with them. We also discuss the general phenomenon of mechanosensitivity, and the approaches used to measure local forces at adhesion sites, the cytoskeleton-mediated regulation of local contractility, and the nature of the signaling networks that both affect contractility and are affected by it.