How cells tiptoe on adhesive surfaces before sticking

LFA-1 nanoclusters integrate TCR stimulation strength to tune T-cell cytotoxic activity

T-cell cytotoxic function relies on the cooperation between the highly specific but poorly adhesive T-cell receptor (TCR) and the integrin LFA-1. How LFA-1-mediated adhesion may scale with TCR stimulation strength is ill-defined. Here, we show that LFA-1 conformation activation scales with TCR stimulation to calibrate human T-cell cytotoxicity. Super-resolution microscopy analysis reveals that >1000 LFA-1 nanoclusters provide a discretized platform at the immunological synapse to translate TCR engagement and density of the LFA-1 ligand ICAM-1 into graded adhesion. Indeed, the number of high-affinity conformation LFA-1 nanoclusters increases as a function of TCR triggering strength. Blockade of LFA-1 conformational activation impairs adhesion to target cells and killing. However, it occurs at a lower TCR stimulation threshold than lytic granule exocytosis implying that it licenses, rather than directly controls, the killing decision. We conclude that the organization of LFA-1 into nanoclusters provides a calibrated system to adjust T-cell killing to the antigen stimulation strength.

Nanoporous electroporation needle for localized intracellular delivery in deep tissues

The exogenous control of intracellular drug delivery has been shown to improve the overall efficacy of therapies by reducing nonspecific off-target toxicity. However, achieving a precise on-demand dosage of a drug in deep tissues with minimal damage is still a challenge. In this study, we report an electric-pulse-driven nanopore-electroporation (nEP) system for the localized intracellular delivery of a model agent in deep tissues. Compared with conventional bulk electroporation, in vitro nEP achieved better transfection efficiency (>60%) with a high cell recovery rate (>95%) under a nontoxic low electroporation condition (40 V). Furthermore, in vivo nEP using a nanopore needle electrode with a side drug-releasing compartment offered better control over the dosage release, time, and location of propidium iodide, which was used as a model agent for intracellular delivery. In a pilot study using experimental animals, the nEP system exhibited two times higher transfection efficiency of propidium iodide in the thigh muscle tissue, while minimizing tissue damage (<20%) compared to that of bulk electroporation. This tissue-penetrating nEP platform can provide localized, safe, and effective intracellular delivery of diverse therapeutics into deep tissues in a controlled manner.

Understanding How Cells Probe the World: A Preliminary Step towards Modeling Cell Behavior?

Cell biologists have long aimed at quantitatively modeling cell function. Recently, the outstanding progress of high-throughput measurement methods and data processing tools has made this a realistic goal. The aim of this paper is twofold: First, to suggest that, while much progress has been done in modeling cell states and transitions, current accounts of environmental cues driving these transitions remain insufficient. There is a need to provide an integrated view of the biochemical, topographical and mechanical information processed by cells to take decisions. It might be rewarding in the near future to try to connect cell environmental cues to physiologically relevant outcomes rather than modeling relationships between these cues and internal signaling networks. The second aim of this paper is to review exogenous signals that are sensed by living cells and significantly influence fate decisions. Indeed, in addition to the composition of the surrounding medium, cells are highly sensitive to the properties of neighboring surfaces, including the spatial organization of anchored molecules and substrate mechanical and topographical properties. These properties should thus be included in models of cell behavior. It is also suggested that attempts at cell modeling could strongly benefit from two research lines: (i) trying to decipher the way cells encode the information they retrieve from environment analysis, and (ii) developing more standardized means of assessing the quality of proposed models, as was done in other research domains such as protein structure prediction.

Observing Membrane and Cell Adhesion via Reflection Interference Contrast Microscopy

Reflection interference contrast microscopy (RICM) is an optical microscopy technique ideally suited for imaging adhesion. While RICM (and the closely related interference reflection microscopy (IRM)) has been extensively used qualitatively or semiquantitatively to image cells, including immune cells, it can also be used quantitatively to measure membrane to surface distance, especially for model membranes. Here, we present a protocol for RICM and IRM imaging and the details of semiquantitative and quantitative analysis.

Gene Electrotransfer into Mammalian Cells Using Commercial Cell Culture Inserts with Porous Substrate

Gene electrotransfer is one of the main non-viral methods for intracellular delivery of plasmid DNA, wherein pulsed electric fields are used to transiently permeabilize the cell membrane, allowing enhanced transmembrane transport. By localizing the electric field over small portions of the cell membrane using nanostructured substrates, it is possible to increase considerably the gene electrotransfer efficiency while preserving cell viability. In this study, we expand the frontier of localized electroporation by designing an electrotransfer approach based on commercially available cell culture inserts with polyethylene-terephthalate (PET) porous substrate. We first use multiscale numerical modeling to determine the pulse parameters, substrate pore size, and other factors that are expected to result in successful gene electrotransfer. Based on the numerical results, we design a simple device combining an insert with substrate containing pores with 0.4 µm or 1.0 µm diameter, a multiwell plate, and a pair of wire electrodes. We test the device in three mammalian cell lines and obtain transfection efficiencies similar to those achieved with conventional bulk electroporation, but at better cell viability and with low-voltage pulses that do not require the use of expensive electroporators. Our combined theoretical and experimental analysis calls for further systematic studies that will investigate the influence of substrate pore size and porosity on gene electrotransfer efficiency and cell viability.

Cells Dynamically Adapt to Surface Geometry by Remodeling Their Focal Adhesions and Actin Cytoskeleton

Cells probe their environment and adapt their shape accordingly via the organization of focal adhesions and the actin cytoskeleton. In an earlier publication, we described the relationship between cell shape and physiology, for example, shape-induced differentiation, metabolism, and proliferation in mesenchymal stem cells and tenocytes. In this study, we investigated how these cells organize their adhesive machinery over time when exposed to microfabricated surfaces of different topographies and adhesive island geometries. We further examined the reciprocal interaction between stress fiber and focal adhesion formation by pharmacological perturbations. Our results confirm the current literature that spatial organization of adhesive sites determines the ability to form focal adhesions and stress fibers. Therefore, cells on roughened surfaces have smaller focal adhesion and fewer stress fibers. Our results further highlight the importance of integrin-mediated adhesion in the adaptive properties of cells and provide clear links to the development of bioactive materials.

Combined Effect of Matrix Topography and Stiffness on Neutrophil Shape and Motility

The crawling behavior of leukocytes is driven by the cell morphology transition, which is a direct manifestation of molecular motor machinery. The topographical anisotropy and mechanical stiffness of the substrates are the main physical cues that affect leukocytes' shape generation and migratory responses. However, their combined effects on the cell morphology and motility have been poorly understood, particularly for neutrophils, which are the fastest reacting leukocytes against infections and wounds. Here, spatiotemporally correlated physical parameters are shown, which determine the neutrophil shape change during migratory processes, in response to surface topography and elasticity. Guided crawling and shape generation of individual neutrophils, activated by a uniform concentration of a chemoattractant, are analyzed by adopting elasticity-tunable micropatterning and live cell imaging techniques. Whole cell-level image analysis is performed based on a planar geometric quantification of cell shape and motility. The findings show that the pattern anisotropy and elastic modulus of the substrate induce synergic effects on the shape anisotropy, deformability, and polarization/alignment of crawling neutrophils. How the morphology-motility relationship is affected by different surface microstructures and stiffness is demonstrated. These results imply that the neutrophil shape-motility correlations can be utilized for controlling the immune cell functions with predefined physical microenvironments.

Mechanotransduction as a major driver of cell behaviour: mechanisms, and relevance to cell organization and future research

How do cells process environmental cues to make decisions? This simple question is still generating much experimental and theoretical work, at the border of physics, chemistry and biology, with strong implications in medicine. The purpose of mechanobiology is to understand how biochemical and physical cues are turned into signals through mechanotransduction. Here, we review recent evidence showing that (i) mechanotransduction plays a major role in triggering signalling cascades following cell-neighbourhood interaction; (ii) the cell capacity to continually generate forces, and biomolecule properties to undergo conformational changes in response to piconewton forces, provide a molecular basis for understanding mechanotransduction; and (iii) mechanotransduction shapes the guidance cues retrieved by living cells and the information flow they generate. This includes the temporal and spatial properties of intracellular signalling cascades. In conclusion, it is suggested that the described concepts may provide guidelines to define experimentally accessible parameters to describe cell structure and dynamics, as a prerequisite to take advantage of recent progress in high-throughput data gathering, computer simulation and artificial intelligence, in order to build a workable, hopefully predictive, account of cell signalling networks.

The Early Adhesion Effects of Human Gingival Fibroblasts on Bovine Serum Albumin Loaded Hydrogenated Titanium Nanotube Surface

The soft tissue sealing at the transmucal portion of implants is vital for the long-term stability of implants. Hydrogenated titanium nanotubes (H₂-TNTs) as implant surface treatments were proved to promote the adhesion of human gingival fibroblasts (HGFs) and have broad usage as drug delivery systems. Bovine serum albumin (BSA) as the most abundant albumin in body fluid was crucial for cell adhesion and was demonstrated as a normal loading protein. As the first protein arriving on the surface of the implant, albumin plays an important role in initial adhesion of soft tissue cells, it is also a common carrier, transferring and loading different endogenous and exogenous substances, ions, drugs, and other small molecules. The aim of the present work was to investigate whether BSA-loaded H₂-TNTs could promote the early adhesion of HGFs; H₂-TNTs were obtained by hydrogenated anodized titanium dioxide nanotubes (TNTs) in thermal treatment, and BSA was loaded in the nanotubes by vacuum drying; our results showed that the superhydrophilicity of H₂-TNTs is conducive to the loading of BSA. In both hydrogenated titanium nanotubes and non-hydrogenated titanium nanotubes, a high rate of release was observed over the first hour, followed by a period of slow and sustained release; however, BSA-loading inhibits the early adhesion of human gingival fibroblasts, and H₂-TNTs has the best promoting effect on cell adhesion. With the release of BSA after 4 h, the inhibitory effect of BSA on cell adhesion was weakened.

CD8 Co-Receptor Enhances T-Cell Activation without Any Effect on Initial Attachment

The scanning of surrounding tissues by T lymphocytes to detect cognate antigens requires high speed, sensitivity and specificity. T-cell receptor (TCR) co-receptors such as CD8 increase detection performance, but the exact mechanism remains incompletely understood. Here, we used a laminar flow chamber to measure at the single molecule level the kinetics of bond formation and rupture between TCR- transfected CD8+ and CD8− Jurkat cells and surfaces coated with five peptide-exposing major histocompatibility antigens (pMHCs) of varying activating power. We also used interference reflection microscopy to image the spreading of these cells dropped on pMHC-exposing surfaces. CD8 did not influence the TCR–pMHC interaction during the first few seconds following cell surface encounter, but it promoted the subsequent spreading responses, suggesting that CD8 was involved in early activation rather than binding. Further, the rate and extent of spreading, but not the lag between contact and spreading initiation, depended on the pMHC. Elucidating T-lymphocyte detection strategy may help unravel underlying signaling networks.

The use of nanovibration to discover specific and potent bioactive metabolites that stimulate osteogenic differentiation in mesenchymal stem cells

Bioactive metabolites have wide-ranging biological activities and are a potential source of future research and therapeutic tools. Here, we use nanovibrational stimulation to induce osteogenic differentiation of mesenchymal stem cells, in the absence of off-target, nonosteogenic differentiation. We show that this differentiation method, which does not rely on the addition of exogenous growth factors to culture media, provides an artifact-free approach to identifying bioactive metabolites that specifically and potently induce osteogenesis. We first identify a highly specific metabolite, cholesterol sulfate, an endogenous steroid. Next, a screen of other small molecules with a similar steroid scaffold identified fludrocortisone acetate with both specific and highly potent osteogenic-inducing activity. Further, we implicate cytoskeletal contractility as a measure of osteogenic potency and cell stiffness as a measure of specificity. These findings demonstrate that physical principles can be used to identify bioactive metabolites and then enable optimization of metabolite potency can be optimized by examining structure-function relationships.

A Multimodal Platform for Simultaneous T-Cell Imaging, Defined Activation, and Mechanobiological Characterization

T-cell antigen recognition is accompanied by extensive morphological rearrangements of the contact zone between the T-cell and the antigen-presenting cell (APC). This process involves binding of the T-cell receptor (TCR) complex to antigenic peptides presented via MHC on the APC surface, the interaction of costimulatory and adhesion proteins, remodeling of the actin cytoskeleton, and the initiation of downstream signaling processes such as the release of intracellular calcium. However, multiparametric time-resolved analysis of these processes is hampered by the difficulty in recording the different readout modalities at high quality in parallel. In this study, we present a platform for simultaneous quantification of TCR distribution via total internal reflection fluorescence microscopy, of intracellular calcium levels, and of T-cell-exerted forces via atomic force microscopy (AFM). In our method, AFM cantilevers were used to bring single T-cells into contact with the activating surface. We designed the platform specifically to enable the study of T-cell triggering via functionalized fluid-supported lipid bilayers, which represent a widely accepted model system to stimulate T-cells in an antigen-specific manner. In this paper, we showcase the possibilities of this platform using primary transgenic T-cells triggered specifically via their cognate antigen presented by MHCII.

Micro/nano topography with altered nanotube diameter differentially trigger endoplasmic reticulum stress to mediate bone mesenchymal stem cell osteogenic differentiation

Micro/nano-topography (MNT) can promote osteogenic differentiation of stem cells, but the mechanism of topographical signaling transduction remains unclear. We have confirmed MNT, as a stressor, triggers endoplasmic reticulum (ER) stress and activates unfolded protein response in rat bone marrow mesenchymal stem cells, and such topography-induced ER stress promotes osteogenic differentiation. In order to reveal the influence of nanotube dimensions on ER stress, MNTs containing vertically oriented TiO2 nanotubes of diameters ranging from 30 nm to 100 nm were fabricated on pure titanium (Ti) foils, and ER stress and osteogenic differentiation of cells were systematically studied. After 12 h of cultivation, the transmission electron microscopy showed that cells on MNTs presented gross distortions of rough ER morphology containing the electron-dense material, and the expansion of the ER lumen became more pronounced as the dimension of nanotubes increased. Additionally, PCR and western blotting showed that the ER stress-related gene, the ER chaperone 78 kDa glucose-regulated protein, also known as binding-immunoglobulin protein (GRP78/BiP), was up-regulated, which was consistent with the osteogenesis-inducing ability of MNTs. Based on our previous studies, the findings in this article further revealed the mechanism for topographical cues modulating osteogenic differentiation of cells, which may provide an innovative approach for the optimal design of implant surface topography.

Recent Advances and Prospects in the Research of Nascent Adhesions

Nascent adhesions are submicron transient structures promoting the early adhesion of cells to the extracellular matrix. Nascent adhesions typically consist of several tens of integrins, and serve as platforms for the recruitment and activation of proteins to build mature focal adhesions. They are also associated with early stage signaling and the mechanoresponse. Despite their crucial role in sampling the local extracellular matrix, very little is known about the mechanism of their formation. Consequently, there is a strong scientific activity focused on elucidating the physical and biochemical foundation of their development and function. Precisely the results of this effort will be summarized in this article.

In vitro analysis of the trajectories of adhesive microbubbles approaching endothelial cells

Adhesion is a key process when ultrasound contrast agents, i.e. microbubbles, approach pathological tissues. A way to accomplish tumour targeting is to tether surface engineered microbubbles to endothelial cells of the up-regulated vascularization of cancer tissues. This can be achieved by coupling the microbubbles surface with the Arginine-Glycine-Aspartate, RGD, sequence. Such molecule interacts with the integrin receptors placed on the endothelial cells. Stability and trajectories of RGD modified lipid shelled MBs have been analysed in vitro using microchannels coated with human umbilical vein endothelial cells, HUVEC. In the microchannels realistic conditions, close to the physiological ones, were reproduced replicating shear rate, roughness comparable to the endothelium and channel size mimicking the postcapillary venules. In these conditions, the analysis of the trajectories close to the walls highlights a substantial difference between the modified MBs and the plain ones. Moreover, MBs adhesion has dynamic features recalling the motion of neutrophils engaged near the substrate such as rolling, translations and transient detachments. These findings are useful for the optimization of in vivo imaging and targeting functions.

Statistical Mechanics of an Elastically Pinned Membrane: Equilibrium Dynamics and Power Spectrum

In biological settings, membranes typically interact locally with other membranes: the extracellular matrix in the exterior or internal cellular structures such as the cytoskeleton, locally pinning the membrane. Characterizing the dynamical properties of such interactions presents a difficult task. Significant progress has been achieved through simulations and experiments, yet analytical progress in modeling pinned membranes has been impeded by the complexity of governing equations. Here, we circumvent these difficulties by calculating analytically the time-dependent Green's function of the operator governing the dynamics of an elastically pinned membrane in a hydrodynamic surrounding and subject to external forces. This enables us to calculate the equilibrium power spectral density for an overdamped membrane pinned by an elastic, permanently attached spring subject to thermal excitations. By considering the effects of the finite experimental resolution on the measured spectra, we show that the elasticity of the pinning can be extracted from the experimentally measured spectrum. Membrane fluctuations can thus be used as a tool to probe mechanical properties of the underlying structures. Such a tool may be particularly relevant in the context of cell mechanics, in which the elasticity of the membrane's attachment to the cytoskeleton could be measured.

Dynamic adaptation of mesenchymal stem cell physiology upon exposure to surface micropatterns

Human mesenchymal stem (hMSCs) are defined as multi-potent colony-forming cells expressing a specific subset of plasma membrane markers when grown on flat tissue culture polystyrene. However, as soon as hMSCs are used for transplantation, they are exposed to a 3D environment, which can strongly impact cell physiology and influence proliferation, differentiation and metabolism. Strategies to control in vivo hMSC behavior, for instance in stem cell transplantation or cancer treatment, are skewed by the un-physiological flatness of the standard well plates. Even though it is common knowledge that cells behave differently in vitro compared to in vivo, only little is known about the underlying adaptation processes. Here, we used micrometer-scale defined surface topographies as a model to describe the phenotype of hMSCs during this adaptation to their new environment. We used well established techniques to compare hMSCs cultured on flat and topographically enhanced polystyreneand observed dramatically changed cell morphologies accompanied by shrinkage of cytoplasm and nucleus, a decreased overall cellular metabolism, and slower cell cycle progression resulting in a lower proliferation rate in cells exposed to surface topographies. We hypothesized that this reduction in proliferation rate effects their sensitivity to certain cancer drugs, which was confirmed by higher survival rate of hMSCs cultured on topographies exposed to paclitaxel. Thus, micro-topographies can be used as a model system to mimic the natural cell micro-environment, and be a powerful tool to optimize cell treatment in vitro.

Relevance of Topographic Parameters on the Adhesion and Proliferation of Human Gingival Fibroblasts and Oral Bacterial Strains

Dental implantology allows replacement of failing teeth providing the patient with a general improvement of health. Unfortunately not all reconstructions succeed, as a consequence of the development of infections of bacterial origin on the implant surface. Surface topography is known to modulate a differential response to bacterial and mammalian cells but topographical measurements are often limited to vertical parameters. In this work we have extended the topographical measurements also to lateral and hybrid parameters of the five most representative implant and prosthetic component surfaces and correlated the results with bacterial and mammalian cell adhesion and proliferation outcomes. Primary human oral gingival fibroblast (gum cells) and the bacterial strains: Streptococcus mutans, Streptococcus sanguinis and Aggregatibacter actinomycetemcomitans, implicated in infectious processes in the oral/implant environment were employed in the presence or absence of human saliva. The results confirm that even though not all the measured surface is available for bacteria to adhere, the overall race for the surface between cells and bacteria is more favourable to the smoother surfaces (nitrided, as machined or lightly acid etched) than to the rougher ones (strong acid etched or sandblasted/acid etched).

Statistical Mechanics of an Elastically Pinned Membrane: Static Profile and Correlations

The relation between thermal fluctuations and the mechanical response of a free membrane has been explored in great detail, both theoretically and experimentally. However, understanding this relationship for membranes locally pinned by proteins is significantly more challenging. Given that the coupling of the membrane to the cell cytoskeleton, to the extracellular matrix, and to other internal structures is crucial for the regulation of a number of cellular processes, understanding the role of the pinning is of great interest. In this manuscript, we consider a single protein (elastic spring of a finite rest length) pinning a membrane modeled in the Monge gauge. First, we determine the Green's function for the system and complement this approach by the calculation of the mode-coupling coefficients for the plane wave expansion and the orthonormal fluctuation modes, in turn building a set of tools for numerical and analytic studies of a pinned membrane. Furthermore, we explore static correlations of the free and the pinned membrane, as well as the membrane shape, showing that all three are mutually interdependent and have an identical long-range behavior characterized by the correlation length. Interestingly, the latter displays a nonmonotonic behavior as a function of membrane tension. Importantly, exploiting these relations allows for the experimental determination of the elastic parameters of the pinning. Last but not least, we calculate the interaction potential between two pinning sites and show that even in the absence of the membrane deformation, the pinnings will be subject to an attractive force because of changes in membrane fluctuations.

Lamellipod Reconstruction by Three-Dimensional Reflection Interference Contrast Nanoscopy (3D-RICN)

There are very few techniques to reconstruct the shape of a cell at nanometric resolution, and those that exist are almost exclusively based on fluorescence, implying limitations due to staining constraints and artifacts. Reflection interference contrast microscopy (RICM), a label-free technique, permits the measurement of nanometric distances between refractive objects. However, its quantitative application to cells has been largely limited due to the complex interferometric pattern caused by multiple reflections on internal or thin structures like lamellipodia. Here we introduce 3D reflection interference contrast nanoscopy, 3D-RICN, which combines information from multiple illumination wavelengths and aperture angles to characterize the lamellipodial region of an adherent cell in terms of its distance from the surface and its thickness. We validate this new method by comparing data obtained on fixed cells imaged with atomic force microscopy and quantitative phase imaging. We show that as expected, cells adhering to micropatterns exhibit a radial symmetry for the lamellipodial thickness. We demonstrate that the substrate-lamellipod distance may be as high as 100 nm. We also show how the method applies to living cells, opening the way for label-free dynamical study of cell structures with nanometric resolution.

Control of cell behaviour through nanovibrational stimulation: nanokicking

Mechanical signals are ubiquitous in our everyday life and the process of converting these mechanical signals into a biological signalling response is known as mechanotransduction. Our understanding of mechanotransduction, and its contribution to vital cellular responses, is a rapidly expanding field of research involving complex processes that are still not clearly understood. The use of mechanical vibration as a stimulus of mechanotransduction, including variation of frequency and amplitude, allows an alternative method to control specific cell behaviour without chemical stimulation (e.g. growth factors). Chemical-independent control of cell behaviour could be highly advantageous for fields including drug discovery and clinical tissue engineering. In this review, a novel technique is described based on nanoscale sinusoidal vibration. Using finite-element analysis in conjunction with laser interferometry, techniques that are used within the field of gravitational wave detection, optimization of apparatus design and calibration of vibration application have been performed. We further discuss the application of nanovibrational stimulation, or 'nanokicking', to eukaryotic and prokaryotic cells including the differentiation of mesenchymal stem cells towards an osteoblast cell lineage. Mechanotransductive mechanisms are discussed including mediation through the Rho-A kinase signalling pathway. Optimization of this technique was first performed in two-dimensional culture using a simple vibration platform with an optimal frequency and amplitude of 1 kHz and 22 nm. A novel bioreactor was developed to scale up cell production, with recent research demonstrating that mesenchymal stem cell differentiation can be efficiently triggered in soft gel constructs. This important step provides first evidence that clinically relevant (three-dimensional) volumes of osteoblasts can be produced for the purpose of bone grafting, without complex scaffolds and/or chemical induction. Initial findings have shown that nanovibrational stimulation can also reduce biofilm formation in a number of clinically relevant bacteria. This demonstrates additional utility of the bioreactor to investigate mechanotransduction in other fields of research.This article is part of a discussion meeting issue 'The promises of gravitational-wave astronomy'.

Membrane-Mediated Cooperativity of Proteins

Besides direct protein-protein interactions, indirect interactions mediated by membranes play an important role for the assembly and cooperative function of proteins in membrane shaping and adhesion. The intricate shapes of biological membranes are generated by proteins that locally induce membrane curvature. Indirect curvature-mediated interactions between these proteins arise because the proteins jointly affect the bending energy of the membranes. These curvature-mediated interactions are attractive for crescent-shaped proteins and are a driving force in the assembly of the proteins during membrane tubulation. Membrane adhesion results from the binding of receptor and ligand proteins that are anchored in the apposing membranes. The binding of these proteins strongly depends on nanoscale shape fluctuations of the membranes, leading to a fluctuation-mediated binding cooperativity. A length mismatch between receptor-ligand complexes in membrane adhesion zones causes repulsive curvature-mediated interactions that are a driving force for the length-based segregation of proteins during membrane adhesion.

Mapping Cell Membrane Fluctuations Reveals Their Active Regulation and Transient Heterogeneities

Shape fluctuations of the plasma membrane occur in all cells, are incessant, and are proposed to affect membrane functioning. Although studies show how membrane fluctuations are affected by cellular activity in adherent cells, their spatial regulation and the corresponding change in membrane mechanics remain unclear. In this article, we study how ATP-driven activities and actomyosin cytoskeleton impact basal membrane fluctuations in adherent cells. Using interference imaging, we map height fluctuations within single cells and compare the temporal spectra with existing theoretical models to gain insights about the underlying membrane mechanics. We find that ATP-dependent activities enhance the nanoscale z fluctuations but stretch out the membrane laterally. Although actin polymerization or myosin-II activity individually enhances fluctuations, the cortex in unperturbed cells stretches out the membrane and dampens fluctuations. Fitting with models suggest this dampening to be due to confinement by the cortex. However, reduced fluctuations on mitosis or on ATP-depletion/stabilization of cortex correlate with increased tension. Both maps of fluctuations and local temporal autocorrelation functions reveal ATP-dependent transient short-range (<2 μm) heterogeneities. Together, our results show how various ATP-driven processes differently affect membrane mechanics and hence fluctuations, while creating distinct local environments whose functional role needs future investigation.

Binding equilibrium and kinetics of membrane-anchored receptors and ligands in cell adhesion: Insights from computational model systems and theory

The adhesion of cell membranes is mediated by the binding of membrane-anchored receptor and ligand proteins. In this article, we review recent results from simulations and theory that lead to novel insights on how the binding equilibrium and kinetics of these proteins is affected by the membranes and by the membrane anchoring and molecular properties of the proteins. Simulations and theory both indicate that the binding equilibrium constant [Formula: see text] and the on- and off-rate constants of anchored receptors and ligands in their 2-dimensional (2D) membrane environment strongly depend on the membrane roughness from thermally excited shape fluctuations on nanoscales. Recent theory corroborated by simulations provides a general relation between [Formula: see text] and the binding constant [Formula: see text] of soluble variants of the receptors and ligands that lack the membrane anchors and are free to diffuse in 3 dimensions (3D).

Use of nanoscale mechanical stimulation for control and manipulation of cell behaviour

The ability to control cell behaviour, cell fate and simulate reliable tissue models in vitro remains a significant challenge yet is crucial for various applications of high throughput screening e.g. drug discovery. Mechanotransduction (the ability of cells to convert mechanical forces in their environment to biochemical signalling) represents an alternative mechanism to attain this control with such studies developing techniques to reproducibly control the mechanical environment in techniques which have potential to be scaled. In this review, the use of techniques such as finite element modelling and precision interferometric measurement are examined to provide context for a novel technique based on nanoscale vibration, also known as "nanokicking". Studies have shown this stimulus to alter cellular responses in both endothelial and mesenchymal stem cells (MSCs), particularly in increased proliferation rate and induced osteogenesis respectively. Endothelial cell lines were exposed to nanoscale vibration amplitudes across a frequency range of 1-100 Hz, and MSCs primarily at 1 kHz. This technique provides significant potential benefits over existing technologies, as cellular responses can be initiated without the use of expensive engineering techniques and/or chemical induction factors. Due to the reproducible and scalable nature of the apparatus it is conceivable that nanokicking could be used for controlling cell behaviour within a wide array of high throughput procedures in the research environment, within drug discovery, and for clinical/therapeutic applications.

STATEMENT OF SIGNIFICANCE

The results discussed within this article summarise the potential benefits of using nanoscale vibration protocols for controlling cell behaviour. There is a significant need for reliable tissue models within the clinical and pharma industries, and the control of cell behaviour and stem cell differentiation would be highly beneficial. The full potential of this method of controlling cell behaviour has not yet been realised.

Quantitative Evaluation of Cancer Cell Adhesion to Self-Assembled Monolayer-Patterned Substrates by Reflection Interference Contrast Microscopy

Adhesion of cancer cells with different metastatic potential and anticancer drug resistance has been quantitatively evaluated by using self-assembled monolayer (SAM)-patterned substrates and reflection interference contrast microscopy (RICM). Cell-adhesive SAM spots with optimized diameter could prevent cell-cell adhesion and thus allowed the systematic evaluation of statistically reliable numbers of contact area between single cancer cells and substrates by RICM. The statistical image analysis revealed that highly metastatic mouse melanoma cells showed larger contact area than lowly metastatic cells. We also found that both cancer cell types exhibited distinct transition from the "strong" to "weak" adhesion states with increase in the concentration of (-)-epigallocatechin gallate (EGCG), which is known to exhibit cancer preventive activity. Mathematical analysis of the adhesion transition revealed that adhesion of the highly metastatic mouse melanoma cells showed more EGCG tolerance than that of lowly metastatic cells. Moreover, time-lapse RICM observation revealed that EGCG weakened cancer cell adhesion in a stepwise manner, probably via focal adhesion complex. These results clearly indicate that contact area can be used as a quantitative measure for the determination of cancer phenotypes and their drug resistance, which will provide physical insights into the mechanism of cancer metastasis and cancer prevention.

Early T cell activation: integrating biochemical, structural, and biophysical cues

T cells carry out the formidable task of identifying small numbers of foreign antigenic peptides rapidly and specifically against a very noisy environmental background of endogenous self-peptides. Early steps in T cell activation have thus fascinated biologists and are among the best-studied models of cell stimulation. This remarkable process, critical in adaptive immune responses, approaches and even seems to exceed the limitations set by the physical laws ruling molecular behavior. Despite the enormous amount of information concerning the nature of molecules involved in the T cell antigen receptor (TCR) signal transduction network, and the description of the nanoscale organization and real-time analysis of T cell responses, the general principles of information gathering and processing remain incompletely understood. Here we review currently accepted key data on TCR function, discuss the limitations of current research strategies, and suggest a novel model of TCR triggering and a few promising ways of going further into the integration of available data.

Measuring fast stochastic displacements of bio-membranes with dynamic optical displacement spectroscopy

Stochastic displacements or fluctuations of biological membranes are increasingly recognized as an important aspect of many physiological processes, but hitherto their precise quantification in living cells was limited due to a lack of tools to accurately record them. Here we introduce a novel technique—dynamic optical displacement spectroscopy (DODS), to measure stochastic displacements of membranes with unprecedented combined spatiotemporal resolution of 20 nm and 10 μs. The technique was validated by measuring bending fluctuations of model membranes. DODS was then used to explore the fluctuations in human red blood cells, which showed an ATP-induced enhancement of non-Gaussian behaviour. Plasma membrane fluctuations of human macrophages were quantified to this accuracy for the first time. Stimulation with a cytokine enhanced non-Gaussian contributions to these fluctuations. Simplicity of implementation, and high accuracy make DODS a promising tool for comprehensive understanding of stochastic membrane processes.

Precise quantification of stochastic motions of biological membranes is limited by a lack of suitable detection methods. Here Monzel et al. develop dynamic optical displacement spectroscopy to measure stochastic membrane displacements at 20 nm/10 μs spatiotemporal resolution.

How actin/myosin crosstalks guide the adhesion, locomotion and polarization of cells

Cell-tissue-tissue interaction is determined by specific short range forces between cell adhesion molecules (CAMs) and ligands of the tissue, long range repulsion forces mediated by cell surface grafted macromolecules and adhesion-induced elastic stresses in the cell envelope. This interplay of forces triggers the rapid random clustering of tightly coupled linkers. By coupling of actin gel patches to the intracellular domains of the CAMs, these clusters can grow in a secondary process resulting in the formation of functional adhesion microdomains (ADs). The ADs can act as biochemical steering centers by recruiting and activating functional proteins, such as GTPases and associated regulating proteins, through electrostatic-hydrophobic forces with cationic lipid domains that act as attractive centers. First, I summarize physical concepts of cell adhesion revealed by studies of biomimetic systems. Then I describe the role of the adhesion domains as biochemical signaling platforms and force transmission centers promoting cellular protrusions, in terms of a shell string model of cells. Protrusion forces are generated by actin gelation triggered by molecular machines (focal adhesion kinase (FAK), Src-kinases and associated adaptors) which assemble around newly formed integrin clusters. They recruit and activate the GTPases Rac-1 and actin gelation promoters to charged membrane domains via electrostatic-hydrophobic forces. The cell front is pushed forward in a cyclic and stepwise manner and the step-width is determined by the dynamics antagonistic interplay between Rac-1 and RhoA. The global cell polarization in the direction of motion is mediated by the actin-microtubule (MT) crosstalk at adhesion domains. Supramolecular actin-MT assemblies at the front help to promote actin polymerization. At the rear they regulate the dismantling of the ADs through the Ca(++)-mediated activation of the protease calpain and trigger their disruption by RhoA mediated contraction via stress fibers. This article is part of a Special Issue entitled: Mechanobiology.

T lymphocytes need less than 3 min to discriminate between peptide MHCs with similar TCR-binding parameters

T lymphocytes need to detect rare cognate foreign peptides among numerous foreign and self-peptides. This discrimination seems to be based on the kinetics of TCRs binding to their peptide-MHC (pMHC) ligands, but there is little direct information on the minimum time required for processing elementary signaling events and deciding to initiate activation. Here, we used interference reflection microscopy to study the early interaction between transfected human Jurkat T cells expressing the 1G4 TCR and surfaces coated with five different pMHC ligands of 1G4. The pMHC concentration required for inducing 50% maximal IFN-γ production by T cells, and 1G4-pMHC dissociation rates measured in soluble phase or on surface-bound molecules, displayed six- to sevenfold variation among pMHCs. When T cells were dropped onto pMHC-coated surfaces, rapid spreading occurred after a 2-min lag. The initial spreading rate measured during the first 45 s, and the contact area, were strongly dependent on the encountered TCR ligand. However, the lag duration did not significantly depend on encountered ligand. In addition, spreading appeared to be an all-or-none process, and the fraction of spreading cells was tightly correlated to the spreading rate and spreading area. Thus, T cells can discriminate between fairly similar TCR ligands within 2 min.

Nanoscale stimulation of osteoblastogenesis from mesenchymal stem cells: nanotopography and nanokicking

AIM

Mesenchymal stem cells (MSCs) have large regenerative potential to replace damaged cells from several tissues along the mesodermal lineage. The potency of these cells promises to change the longer term prognosis for many degenerative conditions currently suffered by our aging population. We have endeavored to demonstrate our ability to induce osteoblatogenesis in MSCs using high-frequency (1000-5000 Hz) piezo-driven nanodisplacements (16-30 nm displacements) in a vertical direction.

MATERIALS & METHODS

Osteoblastogenesis has been determined by the upregulation of osteoblasic genes such as osteonectin (ONN), RUNX2 and Osterix, assessed via quantitative real-time PCR; the increase of osteocalcin (OCN) and osteopontin (OPN) at the protein level and the deposition of calcium phosphate determined by histological staining.

RESULTS

Intriguingly, we have observed a relationship between nanotopography and piezo-stimulated mechanotransduction and possibly see evidence of two differing osteogenic mechanisms at work. These data provide confidence in nanomechanotransduction for stem cell differentiation without dependence on soluble factors and complex chemistries.

CONCLUSION

In the future it is envisaged that this technology may have beneficial therapeutic applications in the healthcare industry, for conditions whose overall phenotype maybe characterized by weak or damaged bones (e.g., osteoporosis and bone fractures), and which can benefit from having an increased number of osteoblastic cells in vivo.

Topography-dependent antibacterial, osteogenic and anti-aging properties of pure titanium

Topography-dependent antibacterial, osteogenic properties of pure titanium and its biological aging mechanism.

Use of TIRF to Monitor T-Lymphocyte Membrane Dynamics with Submicrometer and Subsecond Resolution

A key step of adaptive immune responses is the T lymphocyte capacity to detect the presence of foreign antigens on specialized cells with high speed and specificity during contacts lasting a few minutes. Much evidence suggests that there is a deep link between the lifetime of molecular interactions between T cell receptors and ligands and T cell activation, but the precise mechanisms of bond formation and dissociation remain incompletely understood. Previous experiments done with interference reflection microscopy/reflection interference contrast microscopy disclosed transverse motions with several nanometer average amplitude of micrometer size membrane zones. More recently, total internal reflection fluorescence microscopy was used to show that the initial interaction between primary T lymphocytes and model surfaces involved the tip of microvilli (typically 0.2 µm² area) generating apparent contacts of a few seconds that allowed cells to detect ligands of their membrane receptors. Here we show that these microvilli displayed minimal lateral displacements but quantitative fluorescence measurement suggested the occurrence of spontaneous transverse fluctuations of order of 67 nm amplitude during 1-s observation periods. This may play a major role in membrane receptor engagement and ensuing signal generation.

Fibroblast extracellular matrix and adhesion on microtextured polydimethylsiloxane scaffolds

The immediate physical and chemical surroundings of cells provide important biochemical cues for their behavior. Designing and tailoring biomaterials for controlled cell signaling and extracellular matrix (ECM) can be difficult due to the complexity of the cell-surface relationship. To address this issue, our research has led to the development of a polydimethylsiloxane (PDMS) scaffold with defined microtopography and chemistry for surface driven ECM assembly. When human fibroblasts were cultured on this microtextured PDMS with 2-6 µm wide vertical features, significant changes in morphology, adhesion, actin cytoskeleton, and fibronectin generation were noted when compared with cells cultured on unmodified PDMS. Investigation of cellular response and behavior was performed with atomic force microscopy in conjunction with fluorescent labeling of focal adhesion cites and fibronectin in the ECM. Changes in the surface topography induced lower adhesion, an altered actin cytoskeleton, and compacted units of fibronectin similar to that observed in vivo. Overall, these findings provide critical information of cell-surface interactions with a microtextured, polymer substrate that can be used in the field of tissue engineering for controlling cellular ECM interactions.

Measuring physical properties of neuronal and glial cells with resonant microsensors

Microelectromechanical systems (MEMS) resonant sensors provide a high degree of accuracy for measuring the physical properties of chemical and biological samples. These sensors enable the investigation of cellular mass and growth, though previous sensor designs have been limited to the study of homogeneous cell populations. Population heterogeneity, as is generally encountered in primary cultures, reduces measurement yield and limits the efficacy of sensor mass measurements. This paper presents a MEMS resonant pedestal sensor array fabricated over through-wafer pores compatible with vertical flow fields to increase measurement versatility (e.g., fluidic manipulation and throughput) and allow for the measurement of heterogeneous cell populations. Overall, the improved sensor increases capture by 100% at a flow rate of 2 μL/min, as characterized through microbead experiments, while maintaining measurement accuracy. Cell mass measurements of primary mouse hippocampal neurons in vitro, in the range of 0.1-0.9 ng, demonstrate the ability to investigate neuronal mass and changes in mass over time. Using an independent measurement of cell volume, we find cell density to be approximately 1.15 g/mL.

Molecularly engineered surfaces for cell biology: from static to dynamic surfaces

Surfaces with a well-defined presentation of ligands for receptors on the cell membrane can serve as models of the extracellular matrix for studying cell adhesion or as model cell surfaces for exploring cell-cell contacts. Because such surfaces can provide exquisite control over, for example, the density of these ligands or when the ligands are presented to the cell, they provide a very precise strategy for understanding the mechanisms by which cells respond to external adhesive cues. In the present feature article, we present an overview of the basic biology of cell adhesion before discussing surfaces that have a static presentation of immobile ligands. We outline the biological information that such surfaces have given us, before progressing to recently developed switchable surfaces and surfaces that mimic the lipid bilayer, having adhesive ligands that can move around the membrane and be remodeled by the cell. Finally, the feature article closes with some of the biological information that these new types of surfaces could provide.

Physics of cell adhesion: some lessons from cell-mimetic systems

Cell adhesion is a paradigm of the ubiquitous interplay of cell signalling, modulation of material properties and biological functions of cells. It is controlled by competition of short range attractive forces, medium range repellant forces and the elastic stresses associated with local and global deformation of the composite cell envelopes. We review the basic physical rules governing the physics of cell adhesion learned by studying cell-mimetic systems and demonstrate the importance of these rules in the context of cellular systems. We review how adhesion induced micro-domains couple to the intracellular actin and microtubule networks allowing cells to generate strong forces with a minimum of attractive cell adhesion molecules (CAMs) and to manipulate other cells through filopodia over micrometer distances. The adhesion strength can be adapted to external force fluctuations within seconds by varying the density of attractive and repellant CAMs through exocytosis and endocytosis or protease-mediated dismantling of the CAM-cytoskeleton link. Adhesion domains form local end global biochemical reaction centres enabling the control of enzymes. Actin-microtubule crosstalk at adhesion foci facilitates the mechanical stabilization of polarized cell shapes. Axon growth in tissue is guided by attractive and repulsive clues controlled by antagonistic signalling pathways.

Improved ligand discrimination by force-induced unbinding of the T cell receptor from peptide-MHC

T cell activation is mediated via the recognition of peptides by the T cell receptor (TCR). This receptor ligand interaction is highly specific, and the TCR has to discriminate between a huge number of peptides presented by the products of the major histocompatibility complexes (MHCs). Recent studies indicate that cells probe the TCR-pMHC interaction by imposing force on the interaction. Here we investigated in a theoretical analysis the consequences of such force-induced unbinding for T cell recognition. Our findings are as follows. First, the bond rupture under force is much faster, improving the time resolution of the discrimination process. Second, cells can access additional parameters characterizing the shape of the binding energy surface. Third, load-induced unbinding yields a reduced coefficient of variation of the bond lifetimes, which improves the discriminative power even between peptide/MHCs (pMHCs) with similar off-rates.

Cyclic tensile strain controls cell shape and directs actin stress fiber formation and focal adhesion alignment in spreading cells

The actin cytoskeleton plays a crucial role for the spreading of cells, but is also a key element for the structural integrity and internal tension in cells. In fact, adhesive cells and their actin stress fiber–adhesion system show a remarkable reorganization and adaptation when subjected to external mechanical forces. Less is known about how mechanical forces alter the spreading of cells and the development of the actin–cell-matrix adhesion apparatus. We investigated these processes in fibroblasts, exposed to uniaxial cyclic tensile strain (CTS) and demonstrate that initial cell spreading is stretch-independent while it is directed by the mechanical signals in a later phase. The total temporal spreading characteristic was not changed and cell protrusions are initially formed uniformly around the cells. Analyzing the actin network, we observed that during the first phase the cells developed a circumferential arc-like actin network, not affected by the CTS. In the following orientation phase the cells elongated perpendicular to the stretch direction. This occurred simultaneously with the de novo formation of perpendicular mainly ventral actin stress fibers and concurrent realignment of cell-matrix adhesions during their maturation. The stretch-induced perpendicular cell elongation is microtubule-independent but myosin II-dependent. In summary, a CTS-induced cell orientation of spreading cells correlates temporary with the development of the acto-myosin system as well as contact to the underlying substrate by cell-matrix adhesions.

T lymphocytes sense antigens within seconds and make a decision within one minute

Adaptive immune responses are triggered by the rapid and sensitive detection of MHC-bound peptides by TCRs. The kinetics of early TCR/APC contacts are incompletely known. In this study, we used total internal reflection fluorescence microscopy to image human T cell membranes near model surfaces: contact was mediated by mobile protrusions of <0.4 μm diameter. The mean lifetime of contacts with a neutral surface was 8.6 s. Adhesive interactions increased mean contact time to 27.6 s. Additional presence of TCR ligands dramatically decreased contact to 13.7 s, thus evidencing TCR-mediated triggering of a pulling motion within seconds after ligand encounter. After an interaction typically involving 30-40 contacts formed during a 1-min observation period, TCR stimulation triggered a rapid and active cell spreading. Pulling events and cell spreading were mimicked by pharmacological phospholipase Cγ1 activation, and they were prevented by phospholipase Cγ1 inhibition. These results provide a quantitative basis for elucidating the earliest cell response to the detection of foreign Ags.

Evaluating the diffusion coefficient of dopamine at the cell surface during amperometric detection: disk vs ring microelectrodes

During exocytosis, small quantities of neurotransmitters are released by the cell. These neurotransmitters can be detected quantitatively using electrochemical methods, principally with disk carbon fiber microelectrode amperometry. An exocytotic event then results in the recording of a current peak whose characteristic features are directly related to the mechanisms of exocytosis. We have compared two exocytotic peak populations obtained from PC12 cells with a disk carbon fiber microelectrode and with a pyrolyzed carbon ring microelectrode array, with a 500 nm ring thickness. The specific shape of the ring electrode allows for precise analysis of diffusion processes at the vicinity of the cell membrane. Peaks obtained with a ring microelectrode array show a distorted average shape, owing to increased diffusion pathways. This result has been used to evaluate the diffusion coefficient of dopamine at the surface of a cell, which is up to an order of magnitude smaller than that measured in free buffer. The lower rate of diffusion is discussed as resulting from interactions with the glycocalyx.

Cell interactions at the nanoscale: piezoelectric stimulation

Nanometric movements of the substrate on which endothelial cells are growing, driven by periodic sinusoidal vibration from 1 Hz to 50 Hz applied by piezo actuators, upregulate endothelin-1 and Kruppel-like factor 2 expression, and increase cell adhesion. These movements are in the z (vertical) axis and ranges from 5 to 50 nm and are similar in vertical extent to protrusions from the cells themselves already reported in the literature. White noise vibrations do not to produce these effects. Vibrational sweeps, if suitably confined within a narrow frequency range, produce similar stimulatory effects but not at wider sweeps. These effects suggest that coherent vibration is crucial for driving these cellular responses. In addition to this, the applied stimulations are observed to be close to or below the random seismic noise of the surroundings, which may suggest stochastic resonance is being employed. The stimulations also interact with the effects of nanometric patterning of the substrates on cell adhesion and Kruppel-like factor 2 and endothelin-1 expression thus linking cell reactions to nanotopographically patterned surfaces with those to mechanical stimulation.

Biophysical description of multiple events contributing blood leukocyte arrest on endothelium

Blood leukocytes have a remarkable capacity to bind to and stop on specific blood vessel areas. Many studies have disclosed a key role of integrin structural changes following the interaction of rolling leukocytes with surface-bound chemoattractants. However, the functional significance of structural data and mechanisms of cell arrest are incompletely understood. Recent experiments revealed the unexpected complexity of several key steps of cell-surface interaction: (i) ligand-receptor binding requires a minimum amount of time to proceed and this is influenced by forces. (ii) Also, molecular interactions at interfaces are not fully accounted for by the interaction properties of soluble molecules. (iii) Cell arrest depends on nanoscale topography and mechanical properties of the cell membrane, and these properties are highly dynamic. Here, we summarize these results and we discuss their relevance to recent functional studies of integrin-receptor association in cells from a patient with type III leukocyte adhesion deficiency. It is concluded that an accurate understanding of all physical events listed in this review is needed to unravel the precise role of the multiple molecules and biochemical pathway involved in arrest triggering.

Single cell amperometry reveals glycocalyx hinders the release of neurotransmitters during exocytosis

The diffusional hindrance of the glycocalyx along the cell surface on exocytotic peaks, observed with single cell amperometry, was investigated. Partial digestion of the glycocalyx with neuraminidase led to the observation of faster peaks, as shown by varied peak parameters. This result indicates that diffusion of small molecules in the partially digested glycocalyx is 2.2 faster than in the intact glycocalyx. Similarly, neutralization of the negative charges present in the cell microenvironment led to faster peak kinetics. The analysis of the vesicular efflux indicates that the diffusion coefficient of dopamine at the cell surface is at most 45% of the diffusion coefficient in free solution. This study shows that the glycocalyx plays an important role in the diffusion kinetics of processes along the cell surface, including exocytotic events.

Osteogenesis of mesenchymal stem cells by nanoscale mechanotransduction

It is likely that mesenchymal stem cells will find use in many autologous regenerative therapies. However, our ability to control cell stem growth and differentiation is presently limited, and this is a major hurdle to the clinical use of these multipotent cells especially when considering the desire not to use soluble factors or complex media formulations in culture. Also, the large number of cells required to be clinically useful is currently a hurdle to using materials-based (stiffness, chemistry, nanotopography, etc.) culture substrates. Here we give a first demonstration of using nanoscale sinusoidal mechanotransductive protocols (10-14 nm displacements at 1 kHz frequency), "nanokicking", to promote osteoblastogenesis in human mesenchymal stem cell cultures. On the basis of application of the reverse piezo effect, we use interferometry to develop the optimal stem cell stimulation conditions, allowing delivery of nanoscale cues across the entire surface of the Petri dishes used. A combination of immunofluorescence, PCR, and microarray has then been used to demonstrate osteoblastogenesis, and the arrays implicate RhoA as central to osteoblastic differentiation in agreement with materials-based strategies. We validate this with pharmacological inhibition of RhoA kinase. It is easy to envisage such stimulation protocols being up-scaled to form large-scale osteoblast bioreactors as standard cell culture plates and incubators are used in the protocol.

Bony defect repair in rabbit using hybrid rapid prototyping polylactic-co-glycolic acid/β-tricalciumphosphate collagen I/apatite scaffold and bone marrow mesenchymal stem cells

BACKGROUND

In bone tissue engineering, extracellular matrix exerts critical influence on cellular interaction with porous biomaterial and the apatite playing an important role in the bonding process of biomaterial to bone tissue. The aim of this study was to observe the therapeutic effects of hybrid rapid prototyping (RP) scaffolds comprising polylactic-co-glycolic acid (PLGA), β-tricalciumphosphate (β-TCP), collagen I and apatite (PLGA/β-TCP-collagen I/apatite) on segmental bone defects in conjunction with combination with bone marrow mesenchymal stem cells (BMSCs).

MATERIALS AND METHODS

BMSCs were seeded into the hybrid RP scaffolds to repair 15 mm defect in the radius of rabbits. Radiograph, microcomputed tomography and histology were used to evaluate new bone formation.

RESULTS

Radiographic analysis done from 12 to 36 weeks postoperative period demonstrated that new bone formed at the radial defect site and continues to increase until the medullary cavity is recanalized and remodelling is complete. The bone defect remained unconnected in the original RP scaffolds (PLGA/β-TCP) during the whole study. Histological observations conformed to the radiographic images. In hybrid RP scaffold group, woven bone united the radial defect at 12 weeks and consecutively remodeled into lamellar bone 24 weeks postoperation and finally matured into cortical bone with normal marrow cavity after another 12 weeks. No bone formation but connective tissue has been detected in RP scaffold at the same time.

CONCLUSION

Collagen I/apatite sponge composite coating could improve new bone formation in vivo. The hybrid RP scaffold of PLGA/β-TCP skeleton with collagen I/apatite sponge composite coating is a promising candidate for bone tissue engineering.

Bacterial attachment on sub-nanometrically smooth titanium substrata

Despite the volume of work that has been conducted on the topic, the role of surface topography in mediating bacterial cell adhesion is not well understood. The primary reason for this lack of understanding is the relatively limited extent of topographical characterisation employed in many studies. In the present study, the topographies of three sub-nanometrically smooth titanium (Ti) surfaces were comprehensively characterised, using nine individual parameters that together describe the height, shape and distribution of their surface features. This topographical analysis was then correlated with the adhesion behaviour of the pathogenic bacteria Staphylococcus aureus and Pseudomonas aeruginosa, in an effort to understand the role played by each aspect of surface architecture in influencing bacterial attachment. While P. aeruginosa was largely unable to adhere to any of the three sub-nanometrically smooth Ti surfaces, the extent of S. aureus cell attachment was found to be greater on surfaces with higher average, RMS and maximum roughness and higher surface areas. The cells also attached in greater numbers to surfaces that had shorter autocorrelation lengths and skewness values that approached zero, indicating a preference for less ordered surfaces with peak heights and valley depths evenly distributed around the mean plane. Across the sub-nanometrically smooth range of surfaces tested, it was shown that S. aureus more easily attached to surfaces with larger features that were evenly distributed between peaks and valleys, with higher levels of randomness. This study demonstrated that the traditionally employed amplitudinal roughness parameters are not the only determinants of bacterial adhesion, and that spatial parameters can also be used to predict the extent of attachment.