Periodic beaded-filament assembly of fibronectin on negatively charged surface

Towards a better understanding of the effect of protein conditioning layers on microbial adhesion: a focused investigation of fibronectin and bovine serum albumin layers on SiO2 surfaces

The interaction of foreign implants with their surrounding environment is significantly influenced by the adsorption of proteins on the biomaterial surfaces, playing a role in microbial adhesion. Therefore, understanding protein adsorption on solid surfaces and its effect on microbial adhesion is essential to assess the associated risk of infection. The aim of this study is to evaluate the effect of conditioning by fibronectin (Fn) or bovine serum albumin (BSA) protein layers of silica (SiO2) surfaces on the adhesion and detachment of two pathogenic microorganisms: Pseudomonas aeruginosa PAO1-Tn7-gfp and Candida albicans CIP 48.72. Experiments are conducted under both static and hydrodynamic conditions using a shear stress flow chamber. Through the use of very low wall shear stresses, the study brings the link between the static and dynamic conditions of microbial adhesion. The results reveal that the microbial adhesion critically depends on: (i) the presence of a protein layer conditioning the SiO2 surface, (ii) the type of protein and (iii) the protein conformation and organization in the conditioning layer. In addition, a very distinct adhesion behaviour of P. aeruginosa is observed towards the two tested proteins, Fn and BSA. This effect is reinforced by the amount of proteins adsorbed on the surface and their organization in the layer. The results are discussed in the light of atomic force microscopy analysis of the organization and conformation of proteins in the layers after adsorption on the SiO2 surface, as well as the specificity in bacterial behaviour when interacting with these protein layers. The study also demonstrates the very distinctive behaviours of the prokaryote P. aeruginosa PAO1-Tn7-gfp compared to the eukaryote C. albicans CIP 48.72. This underscores the importance of considering species-specific interactions between the protein conditioning layer and different pathogenic microorganisms, which appear crucial in designing tailored anti-adhesive surfaces.

Machine learning-based approach for automated classification of cell and extracellular matrix using nanomechanical properties

Fibrosis characterized by excess accumulation of extracellular matrix (ECM) due to complex cell-ECM interactions plays a pivotal role in pathogenesis. Herein, we employ the pancreatic ductal adenocarcinoma (PDAC) model to investigate dynamic alterations in nanomechanical attributes arising from the cell-ECM interactions to study the fibrosis paradigm. Several segregated studies performed on cellular and ECM components fail to recapitulate their complex collaboration. We utilized collagen and fibronectin, the two most abundant PDAC ECM components, and studied their nanomechanical attributes. We demonstrate alteration in morphology and nanomechanical attributes of collagen with varying thicknesses of collagen gel. Furthermore, by mixing collagen and fibronectin in various stoichiometry, their nanomechanical attributes were observed to vary. To demonstrate the dynamicity and complexity of cell-ECM, we utilized Panc-1 and AsPC-1 cells with or without collagen. We observed that Panc-1 and AsPC-1 cells interact differently with collagen and vice versa, evident from their alteration in nanomechanical properties. Further, using nanomechanics data, we demonstrate that ML-based techniques were able to classify between ECM as well as cell, and cell subtypes in the presence/absence of collagen with higher accuracy. This work demonstrates a promising avenue to explore other ECM components facilitating deeper insights into tumor microenvironment and fibrosis paradigm.

Surface-Modified Piezoelectric Copolymer Poly(vinylidene fluoride-trifluoroethylene) Supporting Physiological Extracellular Matrixes to Enhance Mesenchymal Stem Cell Adhesion for Nanoscale Mechanical Stimulation

There is an unmet clinical need to provide viable bone grafts for clinical use. Autologous bone, one of the most commonly transplanted tissues, is often used but is associated with donor site morbidity. Tissue engineering strategies to differentiate an autologous cell source, such as mesenchymal stromal cells (MSCs), into a potential bone-graft material could help to fulfill clinical demand. However, osteogenesis of MSCs can typically require long culture periods that are impractical in a clinical setting and can lead to significant cost. Investigation into strategies that optimize cell production is essential. Here, we use the piezoelectric copolymer poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE), functionalized with a poly(ethyl acrylate) (PEA) coating that drives fibronectin network formation, to enhance MSC adhesion and to present growth factors in the solid phase. Dynamic electrical cues are then incorporated, via a nanovibrational bioreactor, and the MSC response to electromechanical stimulation is investigated.

A new mechanism of fibronectin fibril assembly revealed by live imaging and super-resolution microscopy

Fibronectin (Fn1) fibrils have long been viewed as continuous fibers composed of extended, periodically aligned Fn1 molecules. However, our live-imaging and single-molecule localization microscopy data are inconsistent with this traditional view and show that Fn1 fibrils are composed of roughly spherical nanodomains containing six to eleven Fn1 dimers. As they move toward the cell center, Fn1 nanodomains become organized into linear arrays, in which nanodomains are spaced with an average periodicity of 105±17 nm. Periodical Fn1 nanodomain arrays can be visualized between cells in culture and within tissues; they are resistant to deoxycholate treatment and retain nanodomain periodicity in the absence of cells. The nanodomain periodicity in fibrils remained constant when probed with antibodies recognizing distinct Fn1 epitopes or combinations of antibodies recognizing epitopes spanning the length of Fn1. Treatment with FUD, a peptide that binds the Fn1 N-terminus and disrupts Fn1 fibrillogenesis, blocked the organization of Fn1 nanodomains into periodical arrays. These studies establish a new paradigm of Fn1 fibrillogenesis. This article has an associated First Person interview with the first author of the paper.

TiO2 Nanotopography-Driven Osteoblast Adhesion through Coulomb's Force Evolution

Nanotopography is an effective method to regulate cells' behaviors to improve Ti orthopaedic implants' in vivo performance. However, the mechanism underlying cellular matrix-nanotopography interactions that allows the modulation of cell adhesion has remained elusive. In this study, we have developed novel nanotopographic features on Ti substrates and studied human osteoblast (HOb) adhesion on nanotopographies to reveal the interactive mechanism regulating cell adhesion and spreading. Through nanoflat, nanoconvex, and nanoconcave TiO₂ nanotopographies, the evolution of Coulomb's force between the extracellular matrix and nanotopographies has been estimated and comparatively analyzed, along with the assessment of cellular responses of HOb. We show that HObs exhibited greater adhesion and spreading on nanoconvex surfaces where they formed super matured focal adhesions and an ordered actin cytoskeleton. It also demonstrated that Coulomb's force on nanoconvex features exhibits a more intense and concentrated evolution than that of nanoconcave features, which may result in a high dense distribution of fibronectin. Thus, this work is meaningful for novel Ti-based orthopaedic implants' surface designs for enhancing their in vivo performance.

A Novel Cell Delivery System Exploiting Synergy between Fresh Titanium and Fibronectin

Delivering and retaining cells in areas of interest is an ongoing challenge in tissue engineering. Here we introduce a novel approach to fabricate osteoblast-loaded titanium suitable for cell delivery for bone integration, regeneration, and engineering. We hypothesized that titanium age influences the efficiency of protein adsorption and cell loading onto titanium surfaces. Fresh (newly machined) and 1-month-old (aged) commercial grade 4 titanium disks were prepared. Fresh titanium surfaces were hydrophilic, whereas aged surfaces were hydrophobic. Twice the amount of type 1 collagen and fibronectin adsorbed to fresh titanium surfaces than aged titanium surfaces after a short incubation period of three hours, and 2.5-times more fibronectin than collagen adsorbed regardless of titanium age. Rat bone marrow-derived osteoblasts were incubated on protein-adsorbed titanium surfaces for three hours, and osteoblast loading was most efficient on fresh titanium adsorbed with fibronectin. The number of osteoblasts loaded using this synergy between fresh titanium and fibronectin was nine times greater than that on aged titanium with no protein adsorption. The loaded cells were confirmed to be firmly attached and functional. The number of loaded cells was strongly correlated with the amount of protein adsorbed regardless of the protein type, with fibronectin simply more efficiently adsorbed on titanium surfaces than collagen. The role of surface hydrophilicity of fresh titanium surfaces in increasing protein adsorption or cell loading was unclear. The hydrophilicity of protein-adsorbed titanium increased with the amount of protein but was not the primary determinant of cell loading. In conclusion, the osteoblast loading efficiency was dependent on the age of the titanium and the amount of protein adsorption. In addition, the efficiency of protein adsorption was specific to the protein, with fibronectin being much more efficient than collagen. This is a novel strategy to effectively deliver osteoblasts ex vivo and in vivo using titanium as a vehicle.

Effects of substrate stiffness and actin velocity on in silico fibronectin fibril morphometry and mechanics

Assembly of the extracellular matrix protein fibronectin (FN) into insoluble, viscoelastic fibrils is a critical step during embryonic development and wound healing; misregulation of FN fibril assembly has been implicated in many diseases, including fibrotic diseases and cancer. We have previously developed a computational model of FN fibril assembly that recapitulates the morphometry and mechanics of cell-derived FN fibrils. Here we use this model to probe two important questions: how is FN fibril formation affected by the contractile phenotype of the cell, and how is FN fibril formation affected by the stiffness of the surrounding tissue? We show that FN fibril formation depends strongly on the contractile phenotype of the cell, but only weakly on in vitro substrate stiffness, which is an analog for in vivo tissue stiffness. These results are consistent with previous experimental data and provide a better insight into conditions that promote FN fibril assembly. We have also investigated two distinct phenotypes of FN fibrils that we have previously identified; we show that the ratio of the two phenotypes depends on both substrate stiffness and contractile phenotype, with intermediate contractility and high substrate stiffness creating an optimal condition for stably stretched fibrils. Finally, we have investigated how re-stretch of a fibril affects cellular response. We probed how the contractile phenotype of the re-stretching cell affects the mechanics of the fibril; results indicate that the number of myosin motors only weakly affects the cellular response, but increasing actin velocity results in a decrease in the apparent stiffness of the fibril and a decrease in the stably-applied force to the fibril. Taken together, these results give novel insights into the combinatorial effects of substrate stiffness and cell contractility on FN fibril assembly.

Biophysical Techniques to Analyze Elastic Tissue Extracellular Matrix Proteins Interacting with ADAMTS Proteins

Multidomain matrix-associated zinc extracellular proteases ADAMTS and ADAMTS-like proteins have important biological activities in cells and tissues. Beyond their traditional role in procollagen and von Willebrand factor processing and proteoglycan cleavage, ADAMTS/ADAMTSL likely participate in or at least have some role in ECM assembly as some of these proteins bind ECM proteins including fibrillins, fibronectin, and LTBPs. In this chapter, we present four biophysical techniques largely used for the characterization, multimerization, and interaction of proteins: surface plasmon resonance spectroscopy, dynamic light scattering, atomic force microscopy, and circular dichroism spectroscopy.

Formation of Multi-Component Extracellular Matrix Protein Fibers

The extracellular matrix (ECM) consists of polymerized protein monomers that form a unique fibrous network providing stability and structural support to surrounding cells. We harnessed the fibrillogenesis mechanisms of naturally occurring ECM proteins to produce artificial fibers with a heterogeneous protein makeup. Using ECM proteins as fibril building blocks, we created uniquely structured multi-component ECM fibers. Sequential incubation of fibronectin (FN) and laminin (LAM) resulted in self-assembly into locally stacked fibers. In contrast, simultaneous incubation of FN with LAM or collagen (COL) produced molecularly stacked multi-component fibers because both proteins share a similar assembly mechanism or possess binding domains specific to each other. Sequential incubation of COL on FN fibers resulted in fibers with sandwiched layers because COL molecules bind to the external surface of FN fibers. By choosing proteins for incubation according to the interplay of their fibrillogenesis mechanisms and their binding domains (exposed when they unfold), we were able to create ECM protein fibers that have never before been observed.

Measuring the Poisson's Ratio of Fibronectin Using Engineered Nanofibers

The extracellular matrix (ECM) is a fibrillar protein-based network, the physical and chemical properties of which can influence a multitude of cellular processes. Despite having an important role in cell and tissue signaling, a complete chemo-mechanical characterization of ECM proteins such as fibronectin (FN) is lacking. In this study, we engineered monodisperse FN nanofibers using a surface-initiated assembly technique in order to provide new insight into the elastic behavior of this material over large deformations. FN nanofibers were patterned on surfaces in a pre-stressed state and when released from the surface underwent rapid contraction. We found that the FN nanofibers underwent 3.3-fold and 9-fold changes in length and width, respectively, and that the nanofiber volume was conserved. Volume was also conserved following uniaxial extension of the FN nanofibers of ~2-fold relative to the patterned state. This data suggests that the FN networks we engineered formed an incompressible material with a Poisson’s ratio of ~0.5. While the Poisson’s ratio of cells and other biological materials are widely estimated as 0.5, our experimental results demonstrate that for FN networks this is a reasonable approximation.

Vitronectin as a Micromanager of Cell Response in Material-Driven Fibronectin Nanonetworks

Surface functionalization strategies of synthetic materials for regenerative medicine applications comprise the development of microenvironments that recapitulate the physical and biochemical cues of physiological extracellular matrices. In this context, material-driven fibronectin (FN) nanonetworks obtained from the adsorption of the protein on poly(ethyl acrylate) provide a robust system to control cell behavior, particularly to enhance differentiation. This study aims at augmenting the complexity of these fibrillar matrices by introducing vitronectin, a lower-molecular-weight multifunctional glycoprotein and main adhesive component of serum. A cooperative effect during co-adsorption of the proteins is observed, as the addition of vitronectin leads to increased fibronectin adsorption, improved fibril formation, and enhanced vitronectin exposure. The mobility of the protein at the material interface increases, and this, in turn, facilitates the reorganization of the adsorbed FN by cells. Furthermore, the interplay between interface mobility and engagement of vitronectin receptors controls the level of cell fusion and the degree of cell differentiation. Ultimately, this work reveals that substrate-induced protein interfaces resulting from the cooperative adsorption of fibronectin and vitronectin fine-tune cell behavior, as vitronectin micromanages the local properties of the microenvironment and consequently short-term cell response to the protein interface and higher order cellular functions such as differentiation.

Stretch-dependent changes in molecular conformation in fibronectin nanofibers

Fibronectin (FN) is an extracellular matrix (ECM) glycoprotein that plays an important role in a wide range of biological processes including embryonic development, wound healing, and fibrosis. Recent evidence has demonstrated that FN is mechanosensitive, where the application of force induces conformational changes within the FN molecule to expose otherwise cryptic binding domains. However, it has proven technically challenging to dynamically monitor how the nanostructure of FN fibers changes as a result of force-induced extension, due in part to the inherent complexity of FN networks within tissue and cell-generated extracellular matrix (ECM). This has limited our understanding of FN matrix mechanobiology and the complex bi-directional signaling between cells and the ECM, and de novo FN fiber fabrication strategies have only partially addressed this. Towards addressing this need, we have developed a modified surface-initiated assembly (SIA) technique to engineer FN nanofibers that we can uniaxially stretch to >7-fold extensions and subsequently immobilize them in the stretched state for high resolution atomic force microscopy (AFM) imaging. Using this approach, we analyzed how the nanostructure of FN molecules within the nanofibers changed with stretch. In fully contracted FN nanofibers, we observed large, densely packed, isotropically-oriented nodules. With intermediate extension, uniaxially-aligned fibrillar regions developed and nodules became progressively smaller. At high extension, the nanostructure consisted of highly aligned fibrils with small nodules in a beads-on-a-string arrangement. In summary, we have established a methodology to uniaxially stretch FN fibers and monitor changes in nanostructure using AFM. Our results provide new insight into how FN fiber extension can affect the morphology of the constituent FN molecules.

Studying early stages of fibronectin fibrillogenesis in living cells by atomic force microscopy

Time-lapse atomic force microscopy imaging is used to visualize the initial stages of fibronectin fibrillogenesis directly in living cells with high resolution. This approach provides new structural and mechanistic details, such as a stepwise extension mechanism and an accelerating effect of extracellular Mn²⁺ on early FN fibrillogenesis.

Fibronectin (FN) is an extracellular matrix protein that can be assembled by cells into large fibrillar networks, but the dynamics of FN remodeling and the transition through intermediate fibrillar stages are incompletely understood. Here we used a combination of fluorescence microscopy and time-lapse atomic force microscopy (AFM) to visualize initial stages of FN fibrillogenesis in living fibroblasts at high resolution. Initial FN nanofibrils form within <5 min of cell–matrix contact and subsequently extend at a rate of 0.25 μm/min at sites of cell membrane retraction. FN nanofibrils display a complex linear array of globular features spaced at varying distances, indicating the coexistence of different conformational states within the fibril. In some cases, initial fibrils extended in discrete increments of ∼800 nm during a series of cyclical membrane retractions, indicating a stepwise fibrillar extension mechanism. In presence of Mn²⁺, a known activator of integrin adhesion to FN, fibrillogenesis was accelerated almost threefold to 0.68 μm/min and fibrillar dimensions were increased, underlining the importance of integrin activation for early FN fibrillogenesis. FN fibrillogenesis visualized by time-lapse AFM thus provides new structural and mechanistic insight into initial steps of cell-driven FN fibrillogenesis.

Extracellular Matrix Revisited: Roles in Tissue Engineering

The extracellular matrix (ECM) is a heterogeneous, connective network composed of fibrous glycoproteins that coordinate in vivo to provide the physical scaffolding, mechanical stability, and biochemical cues necessary for tissue morphogenesis and homeostasis. This review highlights some of the recently raised aspects of the roles of the ECM as related to the fields of biophysics and biomedical engineering. Fundamental aspects of focus include the role of the ECM as a basic cellular structure, for novel spontaneous network formation, as an ideal scaffold in tissue engineering, and its essential contribution to cell sheet technology. As these technologies move from the laboratory to clinical practice, they are bound to shape the vast field of tissue engineering for medical transplantations.

Protein selective adsorption properties of a polyethylene terephtalate artificial ligament grafted with poly(sodium styrene sulfonate) (polyNaSS): correlation with physicochemical parameters of proteins

Immediately after surgical placement of biomaterials, a first step consists in the adsorption of proteins from the biological environment on the artificial surfaces. Because the composition of the adsorbed protein layer modulates the cell response to the implanted material, researchers in the biomaterials field have focused on coating proteins or peptides onto surfaces to improve cell response and therefore the long-term compatibility of the implant. However, some materials used in tissue engineering, mainly synthetic polymers, are too hydrophobic to allow the optimal adsorption of proteins and have to be first submitted to physical or chemical treatments. In our laboratory, we have demonstrated that grafting of poly(sodium styrene sulfonate) (polyNaSS) onto biomaterials can strongly modulate the protein adsorption and the cellular response compared to unmodified surfaces. In this study, we used a liquid chromatography strategy coupled to proteomics to evaluate the adsorptive properties of a polyethylene terephtalate (PET) artificial ligament grafted with polyNaSS, and to identify and analyse proteins adsorbed on PET fibers. Results obtained with platelet rich plasma (PRP) proteins demonstrated that grafting significantly increases the protein adsorption of the PET and also selectively modulates the adsorption of proteins on PET fibers. Finally, regarding physicochemical parameters calculated from the amino acid sequence of identified proteins, we found that the aliphatic index is highly correlated with the selective adsorption of proteins onto the polyNaSS/PET surface. Therefore, the proteomic approach complemented with physicochemical property evaluation could provide a powerful tool for the elaboration of new biomaterials based on protein layer deposition.

Nanopore Diameters Tune Strain in Extruded Fibronectin Fibers

Fibronectin is present in the extracellular matrix and can be assembled into nanofibers in vivo by undergoing conformational changes. Here, we present a novel approach to prepare fibronectin nanofibers under physiological conditions using an extrusion approach through nanoporous aluminum oxide membranes. This one-step process can prepare nanofiber bundles up to a millimeter in length and with uniform fiber diameters in the nanometer range. Most importantly, by using different pore diameters and protein concentrations in the extrusion process, we could induce varying lasting structural changes in the fibers, which were monitored by Förster resonance energy transfer and should impose different physiological functions.

Studying early stages of fibronectin fibrillogenesis in living cells by atomic force microscopy

Fibronectin (FN) is an extracellular matrix protein that can be assembled by cells into large fibrillar networks, but the dynamics of FN remodeling and the transition through intermediate fibrillar stages are incompletely understood. Here we used a combination of fluorescence microscopy and time-lapse atomic force microscopy (AFM) to visualize initial stages of FN fibrillogenesis in living fibroblasts at high resolution. Initial FN nanofibrils form within <5 min of cell–matrix contact and subsequently extend at a rate of 0.25 μm/min at sites of cell membrane retraction. FN nanofibrils display a complex linear array of globular features spaced at varying distances, indicating the coexistence of different conformational states within the fibril. In some cases, initial fibrils extended in discrete increments of ∼800 nm during a series of cyclical membrane retractions, indicating a stepwise fibrillar extension mechanism. In presence of Mn2+, a known activator of integrin adhesion to FN, fibrillogenesis was accelerated almost threefold to 0.68 μm/min and fibrillar dimensions were increased, underlining the importance of integrin activation for early FN fibrillogenesis. FN fibrillogenesis visualized by time-lapse AFM thus provides new structural and mechanistic insight into initial steps of cell-driven FN fibrillogenesis.

Multidomain Assembler (MDA) Generates Models of Large Multidomain Proteins

Homology modeling predicts protein structures using known structures of related proteins as templates. We developed MULTIDOMAIN ASSEMBLER (MDA) to address the special problems that arise when modeling proteins with large numbers of domains, such as fibronectin with 30 domains, as well as cases with hundreds of templates. These problems include how to spatially arrange nonoverlapping template structures, and how to get the best template coverage when some sequence regions have hundreds of available structures while other regions have a few distant homologs. MDA automates the tasks of template searching, visualization, and selection followed by multidomain model generation, and is part of the widely used molecular graphics package UCSF CHIMERA (University of California, San Francisco). We demonstrate applications and discuss MDA's benefits and limitations.

Multiscale relationships between fibronectin structure and functional properties

Cell behavior is tightly coupled to the properties of the extracellular matrix (ECM) to which they attach. Fibronectin (Fn) forms a supermolecular, fibrillar component of the ECM that is prominent during development, wound healing and the progression of numerous diseases. This indicates that Fn has an important function in controlling cell behavior during dynamic events in vivo. The multiscale architecture of Fn molecules assembled into these fibers determines the ligand density of cell adhesion sites on the surface of the Fn fiber, Fn fiber porosity for cell signaling molecules such as growth factors, the mechanical stiffness of the Fn matrix and the adhesivity of Fn for its numerous soluble ligands. These parameters are altered by mechanical strain applied to the ECM. Recent efforts have attempted to link the molecular properties of Fn with bulk properties of Fn matrix fibers. Studies of isolated Fn fibers have helped to characterize the fiber's material properties and, in combination with models of Fn molecular behavior in the fibers, have begun to provide insights into the Fn molecular arrangement and intermolecular adhesions within the fibers. A review of these studies allows the development of an understanding of the mechanobiological functions of Fn.

Electrochemical modulation of plasma fibronectin surface conformation enables filament formation and control of endothelial cell–surface interactions

Electrochemical modulation of a gold surface charge induces conformational changes in fibronectin when immobilized on the surface. A negatively-charged surface yields an open and filamentous fibronectin which significantly improves endothelial cell adhesion.

Using molecular mechanics to predict bulk material properties of fibronectin fibers

The structural proteins of the extracellular matrix (ECM) form fibers with finely tuned mechanical properties matched to the time scales of cell traction forces. Several proteins such as fibronectin (Fn) and fibrin undergo molecular conformational changes that extend the proteins and are believed to be a major contributor to the extensibility of bulk fibers. The dynamics of these conformational changes have been thoroughly explored since the advent of single molecule force spectroscopy and molecular dynamics simulations but remarkably, these data have not been rigorously applied to the understanding of the time dependent mechanics of bulk ECM fibers. Using measurements of protein density within fibers, we have examined the influence of dynamic molecular conformational changes and the intermolecular arrangement of Fn within fibers on the bulk mechanical properties of Fn fibers. Fibers were simulated as molecular strands with architectures that promote either equal or disparate molecular loading under conditions of constant extension rate. Measurements of protein concentration within micron scale fibers using deep ultraviolet transmission microscopy allowed the simulations to be scaled appropriately for comparison to in vitro measurements of fiber mechanics as well as providing estimates of fiber porosity and water content, suggesting Fn fibers are approximately 75% solute. Comparing the properties predicted by single molecule measurements to in vitro measurements of Fn fibers showed that domain unfolding is sufficient to predict the high extensibility and nonlinear stiffness of Fn fibers with surprising accuracy, with disparately loaded fibers providing the best fit to experiment. This work shows the promise of this microstructural modeling approach for understanding Fn fiber properties, which is generally applicable to other ECM fibers, and could be further expanded to tissue scale by incorporating these simulated fibers into three dimensional network models.

There is growing awareness of the role of mechanical properties within biological tissues. Cells both generate force and are sensitive to applied forces, however nuanced sensitivity to externally applied forces also extends outside the cell to the fibrous structural proteins of the extracellular matrix. It has been shown that stretching these proteins under force can change their biochemical properties in a way that impacts tissue function. In this work we were able, for the first time, to measure the concentration of protein within fibronectin extracellular matrix fibers. This key measurement then enabled us to evaluate a model that links mechanical properties of fibers directly to molecular structural changes that form the physical basis for force sensitivity. The model was found to be predictive of fiber mechanical properties without fitting. This combination of modeling and experiment also offers insights into molecular forces, as well as estimates of fiber hydration and porosity.

Coassembly of aromatic dipeptides into biomolecular necklaces

This paper describes the formation of complex peptide-based structures by the coassembly of two simple peptides, the diphenylalanine peptide and its tert-butyl dicarbonate (Boc) protected analogue. Each of these peptides can self-assemble into a distinct architecture: the diphenylalanine peptide into tubular structures and its analogue into spheres. Integrated together, these peptides coassemble into a construction of beaded strings, where spherical assemblies are connected by elongated elements. Electron and scanning force microscopy demonstrated the morphology of these structures, which we termed "biomolecular necklaces". Additional experiments indicated the reversibility of the coassembly process and the stability of the structures. Furthermore, we suggest a possible mechanism of formation for the biomolecular necklaces. Our suggestion is based on the necklace model for polyelectrolyte chains, which proposes that a necklace structure appears as a result of counterion condensation on the backbone of a polyelectrolyte. Overall, the approach of coassembly, demonstrated using aromatic peptides, can be adapted to any peptides and may lead to the development and discovery of new self-assembled architectures formed by peptides and other biomolecules.

Adhesion of osteoblasts to a nanorough titanium implant surface

This work considers the adhesion of cells to a nanorough titanium implant surface with sharp edges. The basic assumption was that the attraction between the negatively charged titanium surface and a negatively charged osteoblast is mediated by charged proteins with a distinctive quadrupolar internal charge distribution. Similarly, cation-mediated attraction between fibronectin molecules and the titanium surface is expected to be more efficient for a high surface charge density, resulting in facilitated integrin mediated osteoblast adhesion. We suggest that osteoblasts are most strongly bound along the sharp convex edges or spikes of nanorough titanium surfaces where the magnitude of the negative surface charge density is the highest. It is therefore plausible that nanorough regions of titanium surfaces with sharp edges and spikes promote the adhesion of osteoblasts.

Pre-osteoblasts on poly(L-lactic acid) and silicon oxide: Influence of fibronectin and albumin adsorption

Cell adhesion and subsequent viability are critical initial steps in biomaterial-tissue integration and are strongly dependent on the material properties and the presence of matrix proteins. In the present study MC3T3-E1 osteoblast-like cell behavior on silicon oxide (SO) and poly(L-lactic acid) (PLLA) substrates has been examined, with a focus on the influence of the adhesive protein fibronectin and the non-adhesive protein albumin adsorbed on the substrates. Quartz crystal microgravimetry showed adsorption of fibronectin and albumin to be nearly identical on SO and PLLA. Subsequent exposure a previously adsorbed fibronectin layer to albumin decreased the rigidity of the adsorbed layer without any measurable increase in adsorbed mass. Cell adhesion and spreading were significantly enhanced on both SO and PLLA substrates coated with fibronectin or with fibronectin and albumin, compared with uncoated or albumin-coated substrates. The only statistically significant difference between the two substrates in these assays was increased spreading on PLLA compared with SO in the presence of fibronectin and albumin. Cell proliferation was significantly higher on SO compared with PLLA after 7 days culture, but depended on the presence of fibronectin only in the PLLA system. In contrast, mitochondrial activity was higher on PLLA than on SO, and was enhanced by fibronectin on both substrates. PLLA substrates coated with fibronectin and subsequently exposed to albumin exhibited the highest level of cell differentiation, as assayed via alkaline phosphatase activity. These results demonstrate the importance of adsorbed proteins on osteoblast-like cell-surface interactions.