Substrate chemistry influences the morphology and biological function of adsorbed extracellular matrix assemblies

Directed Assembly of Elastic Fibers via Coacervate Droplet Deposition on Electrospun Templates

Elastic fibers provide critical elasticity to the arteries, lungs, and other organs. Elastic fiber assembly is a process where soluble tropoelastin is coacervated into liquid droplets, cross-linked, and deposited onto and into microfibrils. While much progress has been made in understanding the biology of this process, questions remain regarding the timing of interactions during assembly. Furthermore, it is unclear to what extent fibrous templates are needed to guide coacervate droplets into the correct architecture. The organization and shaping of coacervate droplets onto a fiber template have never been previously modeled or employed as a strategy for shaping elastin fiber materials. Using an in vitro system consisting of elastin-like polypeptides (ELPs), genipin cross-linker, electrospun polylactic-co-glycolic acid (PLGA) fibers, and tannic acid surface coatings for fibers, we explored ELP coacervation, cross-linking, and deposition onto fiber templates. We demonstrate that integration of coacervate droplets into a fibrous template is primarily influenced by two factors: (1) the balance of coacervation and cross-linking and (2) the surface energy of the fiber templates. The success of this integration affects the mechanical properties of the final fiber network. Our resulting membrane materials exhibit highly tunable morphologies and a range of elastic moduli (0.8-1.6 MPa) comparable to native elastic fibers.

Investigation of fibrillin microfibrils in the canine cruciate ligament in dogs with different predispositions to ligament rupture

Cranial cruciate ligament disease (CCLD) is the most common cause of pelvic limb lameness in dogs but its precise aetiopathogenesis is uncertain. Fibrillin microfibrils (FM) are complex macro-molecular assemblies found in many tissues including ligaments, where they are thought to play an important mechanical role. We hypothesised that FM ultrastructural variation correlates with the differing predisposition of canine breeds to CCLD. Non-diseased cranial and caudal cruciate ligaments (CCLs and CaCLs) were obtained from Greyhound (GH) and Staffordshire Bull Terrier (SBT) cadavers. Fibrillin microfibrils were extracted from the ligaments by bacterial collagenase digestion, purified by size-exclusion chromatography and subsequently visualized by atomic force microscopy (AFM). With AFM, FMs have a characteristic beads-on-a-string appearance. For each FM, periodicity (bead-bead distance) and length (number of beads/FM) was measured. Fibrillin microfibril length was found to be similar for GH and SBT, with non-significant inter-breed and inter-ligament differences. Fibrillin microfibril periodicity varied when comparing GH and SBT for CCL (GH 60.2 ± 1.4 nm; SBT 56.2 ± 0.8 nm) and CaCL (GH 55.5 ± 1.6 nm; SBT 61.2 ± 1.2 nm). A significant difference was found in the periodicity distribution when comparing CCL for both breeds (P < 0.00001), further, intra-breed differences in CCL vs CaCL were statistically significant within both breeds (P < 0.00001). The breed at low risk of CCLD exhibited a periodicity profile which may be suggestive of a repair and remodelling within the CCL.

Multi-component peptide hydrogels - a systematic study incorporating biomolecules for the exploration of diverse, tuneable biomaterials

Peptide-based supramolecular gels can be designed to be functional "smart" materials that have applications in drug delivery, tissue engineering, and supramolecular chemistry. Although many multi-component gel systems have been designed and reported, many of these applications still rely solely on single-component gel systems which limits the functionalities of the materials. Multi-component self-assembly leads to the formation of highly ordered and complex architectures while offering the possibility to generate hydrogels with interesting properties including functional complexity and diverse morphologies. Being able to incorporate various classes of biomolecules can allow for tailoring the materials' functionalities to specific application needs. Here, a novel peptide amphiphile, myristyl-Phe-Phe (C14-FF), was synthesized and explored for hydrogel formation. The hydrogel possesses a nanofiber matrix morphology, composed of β-sheet aggregates, a record-low gelation concentration for this class of compounds, and a unique solvent-dependent helical switch. The C14-FF hydrogel was then explored with various classes of biomolecules (carbohydrates, vitamins, proteins, building blocks of HA) to generate a multi-component library of gels that have potential to represent the complex natural extracellular matrix. Selected multi-component gels exhibit an excellent compatibility with mesenchymal stem cells showing high cell viability percentages, which holds great promise for applications in regenerative therapy.

Proteomic fingerprints of damage in extracellular matrix assemblies

In contrast to the dynamic intracellular environment, structural extracellular matrix (ECM) proteins with half-lives measured in decades, are susceptible to accumulating damage. Whilst conventional approaches such as histology, immunohistochemistry and mass spectrometry are able to identify age- and disease-related changes in protein abundance or distribution, these techniques are poorly suited to characterising molecular damage. We have previously shown that mass spectrometry can detect tissue-specific differences in the proteolytic susceptibility of protein regions within fibrillin-1 and collagen VI alpha-3. Here, we present a novel proteomic approach to detect damage-induced “peptide fingerprints” within complex multi-component ECM assemblies (fibrillin and collagen VI microfibrils) following their exposure to ultraviolet radiation (UVR) by broadband UVB or solar simulated radiation (SSR). These assemblies were chosen because, in chronically photoaged skin, fibrillin and collagen VI microfibril architectures are differentially susceptible to UVR. In this study, atomic force microscopy revealed that fibrillin microfibril ultrastructure was significantly altered by UVR exposure whereas the ultrastructure of collagen VI microfibrils was resistant. UVR-induced molecular damage was further characterised by proteolytic peptide generation with elastase followed by liquid chromatography tandem mass spectrometry (LC-MS/MS). Peptide mapping revealed that UVR exposure increased regional proteolytic susceptibility within the protein structures of fibrillin-1 and collagen VI alpha-3. This allowed the identification of UVR-induced molecular changes within these two key ECM assemblies. Additionally, similar changes were observed within protein regions of co-purifying, microfibril-associated receptors integrins αv and β1. This study demonstrates that LC-MS/MS mapping of peptides enables the characterisation of molecular post-translational damage (via direct irradiation and radiation-induced oxidative mechanisms) within a complex in vitro model system. This peptide fingerprinting approach reliably allows both the identification of UVR-induced molecular damage in and between proteins and the identification of specific protein domains with increased proteolytic susceptibility as a result of photo-denaturation. This has the potential to serve as a sensitive method of identifying accumulated molecular damage in vivo using conventional mass spectrometry data-sets.

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Mass spectrometry “peptide fingerprinting” can detect post-translational damage within extracellular matrix proteins.

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UVR-induced FBN1 and COL6A3 peptide fingerprints are reproducibly identified from purified microfibrils.

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Peptide mapping reveals increased regional susceptibilities to proteolysis in FBN1 and COL6A3 proteins.

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Regional changes are also observed in protein structures of microfibril-associated receptor integrins αv and β1.

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This “peptide fingerprinting” approach is applicable to conventional LC-MS/MS datasets.

Osteoblast response to zirconia modified-ORMOSILs

In this study, an in vitro evaluation of the human osteoblasts response to Organically Modified Silicate (ORMOSIL) biomaterials was conducted. These materials were synthetized by sol-gel process being modified with zirconia (ZrO₂) and/or Ca²⁺. The materials were immersed into phosphate buffer solution (PBS) in order to test precipitation of mimetic apatite-like on their surfaces. ORMOSILs were characterized by SEM, FT-IR and X-RD analysis. The response of osteoblast to ORMOSILs was analyzed as a measure of cell adhesion, proliferation and differentiation. The results showed that the addition of Ca²⁺ ions modifies the surface morphology of ORMOSILs by forming precipitates of mimetic apatite-like with cauliflower and scales morphologies. On the other hand, biological results suggest that the incorporation of zirconia to ORMOSILs increases their ability to support cell adhesion and proliferation. However, the inclusion of both zirconia and Ca²⁺ in the ORMOSILs decreases their biological compatibility by showing less cell proliferation and lower osteonectin expression, a protein related to osteoblasts. The unfavorable effect of Ca²⁺ on cell proliferation and cell viability could be due to its ability to induce the formation of mimetic apatite-like with incompatible morphology. The analysis of other proteins related to bone formation on ORMOSIL-Zr and ORMOSIL-Zr-Ca surfaces demonstrated clear expression of osteopontin and osteocalcin in cells growth. In the case of ORMOSIL-Zr, the expression of osteonectin occurred at early stages while the expression of osteopontin and osteocalcin begun at later stages, indicating a switch from an early to a mature stage being stimulated by the biomaterial. Together, these results highlight the important role of zirconia and Ca²⁺ ions in the composition of materials regulating their biocompatibility when used as scaffolds in bone regeneration.

Selective proteolysis by matrix metalloproteinases of photo-oxidised dermal extracellular matrix proteins

Photodamage in chronically sun-exposed skin manifests clinically as deep wrinkles and histologically as extensive remodelling of the dermal extracellular matrix (ECM) and in particular, the elastic fibre system. We have shown previously that loss of fibrillin microfibrils, a key elastic fibre component, is a hallmark of early photodamage and that these ECM assemblies are susceptible in vitro to physiologically attainable doses of ultraviolet radiation (UVR). Here, we test the hypotheses that UVR-mediated photo-oxidation is the primary driver of fibrillin microfibril and fibronectin degradation and that prior UVR exposure will enhance the subsequent proteolytic activity of UVR-upregulated matrix metalloproteinases (MMPs). We confirmed that UVB (280-315 nm) irradiation in vitro induced structural changes to both fibrillin microfibrils and fibronectin and these changes were largely reactive oxygen species (ROS)-driven, with increased ROS lifetime (D₂O) enhancing protein damage and depleted O₂ conditions abrogating it. Furthermore, we show that although exposure to UVR alone increased microfibril structural heterogeneity, exposure to purified MMPs (1, -3, -7 and - 9) alone had minimal effect on microfibril bead-to-bead periodicity; however, microfibril suspensions exposed to UVR and then MMPs were more structurally homogenous. In contrast, the susceptibly of fibronectin to proteases was unaffected by prior UVR exposure. These observations suggest that both direct photon absorption and indirect production of ROS are important mediators of ECM remodelling in photodamage. We also show that fibrillin microfibrils are relatively resistant to proteolysis by MMPs -1, -3, -7 and - 9 but that these MMPs may selectively remove damaged microfibril assemblies. These latter observations have implications for predicting the mechanisms of tissue remodelling and targeted repair.

Structural and compositional diversity of fibrillin microfibrils in human tissues

Elastic fibers comprising fibrillin microfibrils and elastin are present in many tissues, including the skin, lungs, and arteries, where they confer elasticity and resilience. Although fibrillin microfibrils play distinct and tissue-specific functional roles, it is unclear whether their ultrastructure and composition differ between elastin-rich (skin) and elastin-poor (ciliary body and zonule) organs or after in vitro synthesis by cultured cells. Here, we used atomic force microscopy, which revealed that the bead morphology of fibrillin microfibrils isolated from the human eye differs from those isolated from the skin. Using newly developed pre-MS preparation methods and LC-MS/MS, we detected tissue-specific regions of the fibrillin-1 primary structure that were differentially susceptible to proteolytic extraction. Comparing tissue- and culture-derived microfibrils, we found that dermis- and dermal fibroblast–derived fibrillin microfibrils differ in both bead morphology and periodicity and also exhibit regional differences in fibrillin-1 proteolytic susceptibility. In contrast, collagen VI microfibrils from the same dermal or fibroblast samples were invariant in ultrastructure (periodicity) and protease susceptibility. Finally, we observed that skin- and eye-derived microfibril suspensions were enriched in elastic fiber– and basement membrane–associated proteins, respectively. LC-MS/MS also identified proteins (such as calreticulin and protein-disulfide isomerase) that are potentially fundamental to fibrillin microfibril biology, regardless of their tissue source. Fibrillin microfibrils synthesized in cell culture lacked some of these key proteins (MFAP2 and -4 and fibrillin-2). These results showcase the structural diversity of these key extracellular matrix assemblies, which may relate to their distinct roles in the tissues where they reside.

Human-derived extracellular matrix from Wharton's jelly: An untapped substrate to build up a standardized and homogeneous coating for vascular engineering

One of the outstanding goals in tissue engineering is to develop a natural coating surface which is easy to manipulate, effective for cell adhesion and fully biocompatible. The ideal surface would be derived from human tissue, perfectly controllable, and pathogen-free, thereby satisfying all of the standards of the health authorities. This paper reports an innovative approach to coating surfaces using a natural extracellular matrix (ECM) extracted from the Wharton's jelly (WJ) of the umbilical cord (referred to as WJ-ECM). We have shown by atomic force microscopy (AFM), that the deposition of WJ-ECM on surfaces is homogenous with a controllable thickness, and that this easily-prepared coating is appropriate for both the adhesion and proliferation of human mesenchymal stem cells and mature endothelial cells. Furthermore, under physiological shear stress conditions, a larger number of cells remained adhered to WJ-ECM than to a conventional coating such as collagen - a result supported by the higher expression of both integrins α2 and β1 in cells cultured on WJ-ECM. Our data clearly show that Wharton's jelly is a highly promising coating for the design of human biocompatible surfaces in tissue engineering as well as in regenerative medicine.

STATEMENT OF SIGNIFICANCE

Discovery and design of biomaterial surface are a hot spot in the tissue engineering field. Natural matrix is preferred to mimic native cell microenvironment but its use is limited due to poor resource availability. Moreover, current studies often use single or several components of natural polymers, which is not the case in human body. This paper reports a natural extracellular matrix with full components derived from healthy human tissue: Wharton's jelly of umbilical cord. Reconstituting this matrix as a culture surface, our easily-prepared coating provides superior biocompatibility for stem and mature cells. Furthermore, we observed improved cell performance on this coating under both static and dynamic condition. This novel human derived ECM would be a promising choice for regenerative medicine.

A potential role for endogenous proteins as sacrificial sunscreens and antioxidants in human tissues

Excessive ultraviolet radiation (UVR) exposure of the skin is associated with adverse clinical outcomes. Although both exogenous sunscreens and endogenous tissue components (including melanins and tryptophan-derived compounds) reduce UVR penetration, the role of endogenous proteins in absorbing environmental UV wavelengths is poorly defined. Having previously demonstrated that proteins which are rich in UVR-absorbing amino acid residues are readily degraded by broadband UVB-radiation (containing UVA, UVB and UVC wavelengths) here we hypothesised that UV chromophore (Cys, Trp and Tyr) content can predict the susceptibility of structural proteins in skin and the eye to damage by physiologically relevant doses (up to 15.4 J/cm²) of solar UVR (95% UVA, 5% UVB). We show that: i) purified suspensions of UV-chromophore-rich fibronectin dimers, fibrillin microfibrils and β- and γ-lens crystallins undergo solar simulated radiation (SSR)-induced aggregation and/or decomposition and ii) exposure to identical doses of SSR has minimal effect on the size or ultrastructure of UV chromophore-poor tropoelastin, collagen I, collagen VI microfibrils and α-crystallin. If UV chromophore content is a factor in determining protein stability in vivo, we would expect that the tissue distribution of Cys, Trp and Tyr-rich proteins would correlate with regional UVR exposure. From bioinformatic analysis of 244 key structural proteins we identified several biochemically distinct, yet UV chromophore-rich, protein families. The majority of these putative UV-absorbing proteins (including the late cornified envelope proteins, keratin associated proteins, elastic fibre-associated components and β- and γ-crystallins) are localised and/or particularly abundant in tissues that are exposed to the highest doses of environmental UVR, specifically the stratum corneum, hair, papillary dermis and lens. We therefore propose that UV chromophore-rich proteins are localised in regions of high UVR exposure as a consequence of an evolutionary pressure to express sacrificial protein sunscreens which reduce UVR penetration and hence mitigate tissue damage.

Major structural proteins such as collagen I and tropoelastin are UVA-resistant.

In contrast, proteins which are rich in Cys, Trp and Tyr residues are UV-susceptible.

These proteins are concentrated in UV exposed tissues.

UV-chromophore (Cys, Trp, Tyr)-rich proteins may act as endogenous sunscreens.

Localized micro- and nano-scale remodelling in the diabetic aorta

Diabetes is strongly associated with cardiovascular disease, but the mechanisms, structural and biomechanical consequences of aberrant blood vessel remodelling remain poorly defined. Using an experimental (streptozotocin, STZ) rat model of diabetes, we hypothesized that diabetes enhances extracellular protease activity in the aorta and induces morphological, compositional and localized micromechanical tissue remodelling. We found that the medial aortic layer underwent significant thickening in diabetic animals but without significant changes in collagen or elastin (abundance). Scanning acoustic microscopy demonstrated that such tissue remodelling was associated with a significant decrease in acoustic wave speed (an indicator of reduced material stiffness) in the inter-lamellar spaces of the vessel wall. This index of decreased stiffness was also linked to increased extracellular protease activity (assessed by semi-quantitative in situ gelatin zymography). Such a proteolytically active environment may affect the macromolecular structure of long-lived extracellular matrix molecules. To test this hypothesis, we also characterized the effects of diabetes on the ultrastructure of an important elastic fibre component: the fibrillin microfibril. Using size exclusion chromatography and atomic force microscopy, we isolated and imaged microfibrils from both healthy and diabetic aortas. Microfibrils derived from diabetic tissues were fragmented, morphologically disrupted and weakened (as assessed following molecular combing). These structural and functional abnormalities were not replicated by in vitro glycation. Our data suggest that proteolysis may be a key driver of localized mechanical change in the inter-lamellar space of diabetic rat aortas and that structural proteins (such as fibrillin microfbrils) may be biomarkers of diabetes induced damage.

Damage to skin extracellular matrix induced by UV exposure

SIGNIFICANCE

Chronic exposure to environmental ultraviolet radiation (UVR) plays a key role in both photocarcinogenesis and induction of accelerated skin aging. Although the spatiotemporal consequences of UVR exposure for the composition and architecture of the dermal extracellular matrix (ECM) are well characterized, the pathogenesis of photoaging remains poorly defined. Given the compelling evidence for the role of reactive oxygen species (ROS) as mediators of photoaging, UVR-exposed human skin may be an accessible model system in which to characterize the role of oxidative damage in both internal and external tissues.

RECENT ADVANCES

Although the cell-mediated degradation of dermal components via UVR-induced expression of ECM proteases has long been identified as an integral part of the photoaging pathway, the relative importance and identity of cellular and extracellular photosensitizers (direct hit and bystanders models, respectively) in initiating this enzymatic activity is unclear. Recently, both age-related protein glycation and relative amino-acid composition have been identified as potential risk factors for photo-ionization and/or photo-sensitization. Here, we propose a selective multi-hit model of photoaging.

CRITICAL ISSUES

Bioinformatic analyses can be employed to identify candidate UVR targets/photosensitizers, but the action of UVR on protein structure and/or ROS production should be verified experimentally. Crucially, in the case of biochemically active ECM components such as fibronectin and fibrillin, the downstream effects of photo-degradation on tissue homeostasis remain to be confirmed.

FUTURE DIRECTIONS

Both topical antioxidants and inhibitors of detrimental cell signaling may be effective in abrogating the effects of specific UVR-mediated protein degradation in the dermis.

Extracellular matrix protein adsorption to phosphate-functionalized gels from serum promotes osteogenic differentiation of human mesenchymal stem cells

One of the primary goals for tissue engineering is to induce new tissue formation by stimulating specific cell function. Human mesenchymal stem cells (hMSCs) are a particularly important cell type that has been widely studied for differentiation down the osteogenic (bone) lineage, and we recently found that simple phosphate functional groups incorporated into poly(ethylene glycol) (PEG) hydrogels could induce osteogenesis without using differentiation medium by unknown mechanisms. Here, we aimed to determine whether direct or indirect cell/materials interactions were responsible for directing hMSCs down the osteogenic lineage on phosphate (PO(4))-functionalized PEG hydrogels. Our results indicated that serum components adsorbed onto PO(4)-PEG hydrogels from medium in a presoaking step were sufficient for attachment and spreading of hMSCs, even when seeded in serum-free conditions. Blocking antibodies for collagen and fibronectin (targeted to the hydrogel), as well as β1 and β3 integrin blocking antibodies (targeted to the cells), each reduced attachment of hMSCs to PO(4)-PEG hydrogels, suggesting that integrin-mediated interactions between cells and adsorbed matrix components facilitate attachment and spreading. Outside-in signaling, and not merely shape change, was found to be required for osteogenesis, as alkaline phosphatase activity and expression of CBFA1, osteopontin and collagen-1 were each significantly down regulated upon inhibition of focal adhesion kinase phosphorylation even though the focal adhesion structure or cell shape was unchanged. Our results demonstrate that complex function (i.e. osteogenic differentiation) can be controlled using simple functionalization strategies, such as incorporation of PO(4), but that the role of these materials may be due to more complex influences than has previously been appreciated.

Advances in biomimetic regeneration of elastic matrix structures

Elastin is a vital component of the extracellular matrix, providing soft connective tissues with the property of elastic recoil following deformation and regulating the cellular response via biomechanical transduction to maintain tissue homeostasis. The limited ability of most adult cells to synthesize elastin precursors and assemble them into mature crosslinked structures has hindered the development of functional tissue-engineered constructs that exhibit the structure and biomechanics of normal native elastic tissues in the body. In diseased tissues, the chronic overexpression of proteolytic enzymes can cause significant matrix degradation, to further limit the accumulation and quality (e.g., fiber formation) of newly deposited elastic matrix. This review provides an overview of the role and importance of elastin and elastic matrix in soft tissues, the challenges to elastic matrix generation in vitro and to regenerative elastic matrix repair in vivo, current biomolecular strategies to enhance elastin deposition and matrix assembly, and the need to concurrently inhibit proteolytic matrix disruption for improving the quantity and quality of elastogenesis. The review further presents biomaterial-based options using scaffolds and nanocarriers for spatio-temporal control over the presentation and release of these biomolecules, to enable biomimetic assembly of clinically relevant native elastic matrix-like superstructures. Finally, this review provides an overview of recent advances and prospects for the application of these strategies to regenerating tissue-type specific elastic matrix structures and superstructures.

Low-dose ultraviolet radiation selectively degrades chromophore-rich extracellular matrix components

Photoageing of human skin due to chronic exposure to ultraviolet radiation (UVR) is characterized histologically by extensive remodelling of the dermal elastic fibre system. Whilst enzymatic pathways are thought to play a major role in mediating extracellular matrix (ECM) degeneration in UV-exposed skin, the substrate specificity of UVR-up-regulated and activated matrix metalloproteinases (MMPs) is low. It is unclear, therefore, how such cell-mediated mechanisms alone could be responsible for the reported selective degradation of elastic fibre components such as fibrillin-1 and fibulin-5 during the early stages of photoageing. Here we use atomic force microscopy (AFM) and scanning transmission electron microscopy (STEM) to demonstrate that physiologically attainable doses (20-100 mJ/cm(2)) of direct UV-B radiation can induce profound, dose-dependent, changes in the structure of, and mass distribution within, isolated fibrillin microfibrils. Furthermore, using reducing and native PAGE in combination with AFM, we show that, whilst exposure to low-dose UV-B radiation significantly alters the macromolecular and quaternary structures of both UV chromophore (Cys, His, Phe, Trp and Tyr)-rich fibrillin microfibrils (fibrillin-1, 21.0%) and fibronectin dimers (fibronectin, 12.9%), similar doses have no detectable effect on UV chromophore-poor type I collagen monomers (2.2%). Analysis of the published primary amino acid sequences of 49 dermal ECM components demonstrates that most elastic fibre-associated proteins, but crucially neither elastin nor members of the collagen family, are rich in UV chromophores. We suggest, therefore, that the amino acid composition of elastic fibre-associated proteins [including the fibrillins, fibulins, latent TGFbeta binding proteins (LTBPs) and the lysyl oxidase family of enzymes (LOK/LOXLs)] may predispose them to direct degradation by UVR. As a consequence, this selective acellular photochemical pathway may play an important role in initiating and/or exacerbating cell-mediated ECM remodelling in UVR-exposed skin.

Statistical approach of chemistry and topography effect on human osteoblast adhesion

Our objective in this work was to determine statistically the relative influence of surface topography and surface chemistry of metallic substrates on long-term adhesion of human bone cell quantified by the adhesion power (AP). Pure titanium, titanium alloy, and stainless steel substrates were processed with electro-erosion, sandblasting, or polishing giving various morphologies and amplitudes. The surface chemistry was characterized by X-ray photoelectron spectroscopy (XPS) associated with an extensive analysis of surface topography. The statistical analysis demonstrated that the effect on AP of the material composition was not significant. More, no correlation was found between AP and the surface element concentrations determined by XPS demonstrating that the surface chemistry was not an influencing parameter for long-term adhesion. In the same way, the roughness amplitude, independently of the process, had no influence on AP, meaning that roughness amplitude is not an intrinsic parameter of long-term adhesion. On the contrary, the elaboration process alone had a significant effect on AP. For a same surface elaboration process, the number of inflexion points, or G parameter, was the most pertinent roughness parameter for describing the topography influence on long-term adhesion. Thus, more the inflexion points, more the discontinuities, higher the long-term adhesion.

Modulation of hepatocarcinoma cell morphology and activity by parylene-C coating on PDMS

Background

The ability to understand and locally control the morphogenesis of mammalian cells is a fundamental objective of cell and developmental biology as well as tissue engineering research. We present parylene-C (ParC) deposited on polydimethylsiloxane (PDMS) as a new substratum for in vitro advanced cell culture in the case of Human Hepatocarcinoma (HepG2) cells.

Principal Findings

Our findings establish that the intrinsic properties of ParC-coated PDMS (ParC/PDMS) influence and modulate initial extracellular matrix (ECM; here, type-I collagen) surface architecture, as compared to non-coated PDMS substratum. Morphological changes induced by the presence of ParC on PDMS were shown to directly affect liver cell metabolic activity and the expression of transmembrane receptors implicated in cell adhesion and cell-cell interaction. These changes were characterized by atomic force microscopy (AFM), which elucidated differences in HepG2 cell adhesion, spreading, and reorganization into two- or three-dimensional structures by neosynthesis of ECM components. Local modulation of cell aggregation was successfully performed using ParC/PDMS micropatterns constructed by simple microfabrication.

Conclusion/Significance

We demonstrated for the first time the modulation of HepG2 cells' behavior in relation to the intrinsic physical properties of PDMS and ParC, enabling the local modulation of cell spreading in a 2D or 3D manner by simple microfabrication techniques. This work will provide promising insights into the development of cell-based platforms that have many applications in the field of in vitro liver tissue engineering, pharmacology and therapeutics.

Nanofibrous composites for tissue engineering applications

Development of artificial matrices for tissue engineering is a crucial area of research in the field of regenerative medicine. Successful tissue scaffolds, in analogy with the natural mammalian extracellular matrix (ECM), are multi-component, fibrous, and on the nanoscale. In addition, to this key morphology, artificial scaffolds must have mechanical, chemical, surface, and electrical properties that match the ECM or basement membrane of the specific tissue desired. In particular, these material properties may vary significantly for the four primary tissues in the body: nerve, muscle, epithelial, and connective. In order to address this complex array of attributes with a polymeric material, a nanocomposite approach, employing a blend of materials, addition of a particle to enhance particular properties, or a surface treatment, is likely to be required. In this review, we examine nanocomposite approaches to address these diverse needs as a function of tissue type. The review is intended as a bridge between material scientists and biomedical researchers to give basic background information on tissue biology to the former, and on material processing approaches to the latter, in a general manner, and specifically review fibrous nanocomposite materials that have previously been used for cell studies, either in vivo or in vitro.

Specific DNA-protein interactions on mica investigated by atomic force microscopy

DNA processing by site-specific proteins on surface remains a challenging issue for nanobioscience applications and, in particular, for high-resolution imaging by atomic force microscopy (AFM). To obtain high-resolution conditions, mica, an atomically flat and negatively charged surface, is generally used. However, even though many specific DNA/protein interactions have already been observed by AFM, little is known about DNA accessibility to specific enzymes on mica. Here we measured the accessibility of adsorbed DNA to restriction endonucleases (EcoRI and EcoRV) using AFM. By increasing the concentration of divalent or multivalent salts, DNA adsorption on mica switches from weak to strong binding. Interestingly, while the accessibility of strongly bound DNA was inhibited, loosely adsorbed DNA was efficiently cleaved on mica. This result opens new perspective to study DNA/protein interaction by AFM or to modify specifically DNA on surface.

Cell behaviors on polysaccharide-wrapped single-wall carbon nanotubes: a quantitative study of the surface properties of biomimetic nanofibrous scaffolds

Natural polysaccharides such as amylose (AMY), alginate sodium (ALG), and chitosan (CHI) have been noncovalently wrapped onto single-wall carbon nanotubes (SWCNTs) to give a series of SWCNT scaffolds, termed as AMY-SWCNT, ALG-SWCNT, CHI-SWCNT, and CHI/ALG-SWCNT scaffolds. Compared to purified SWCNTs and oxidized SWCNTs, the polysaccharide-wrapped SWCNTs can well mimic nanofibrous extracellular matrix and significantly enhance cell adhesion and proliferation. The surface properties of the SWCNT scaffolds, such as functional groups, surface charge, and hydrophilicity, can all directly influence the protein adsorption and lead to changes in cellular FAK expression, thus affect the mammalian cell morphology and proliferation. By quantitatively studying the surface properties of these SWCNT scaffolds, it can be concluded that relatively positively charged hydrophilic scaffolds that bear -OH groups can remarkably promote cell growth. Considering all properties, the relatively electrical neutral and hydrophilic AMY-SWCNT scaffolds bearing only -OH groups are able to sustain the highest cell viability after 72 h culturing.

Effects of poly(L-lysine), poly(acrylic acid) and poly(ethylene glycol) on the adhesion, proliferation and chondrogenic differentiation of human mesenchymal stem cells

Microenvironments, composed of many kinds of cytokines and growth factors plus extracellular matrices with diverse electrostatic properties, play key roles in controlling cell functions in vivo. In this study, three kinds of water-soluble polymers, positively charged poly(L-lysine) (PLL), negatively charged poly(acrylic acid) (PAAc) and neutral poly(ethylene glycol) (PEG), were compared based on their effects on the adhesion, spread, proliferation and chondrogenic differentiation of human mesenchymal stem cells (MSCs). The MSCs were seeded and cultured in the presence of polymers of different concentrations applied by methods using coating, mixing or covering. The effects of the water-soluble polymers depended on their electrostatic properties and method of application. The methods were in the order of coating, mixing and covering in terms of high to low influence. A low concentration of PLL promoted MSC adhesion, spread, proliferation and chondrogenic differentiation, while a high concentration of PLL was toxic. The PEG-coated surface facilitated cell aggregation and spheroid formation by inhibiting cell adhesion. A high concentration of mixed PEG (10 microg/ml) promoted cell proliferation in serum-free medium. PAAc showed no obvious effects on MSC adhesion, spread, proliferation, or chondrogenic differentiation.

Fibrillin microfibrils: a key role for the interbead region in elasticity

Fibrillin microfibrils have essential roles in elastic fiber formation and elastic tissue homeostasis, as well as transforming growth factor-beta sequestration. A role for fibrillin microfibrils in tissue elasticity has been implied by their ability to increase periodicity from 56 to 150 nm. In this study, we found that microfibril periodicity and structure are dependent on the ionic strength of the buffer and Ca(2+) concentration; we then used these properties of the microfibril to trap conformation intermediates. Transmission electron microscopy imaging of microfibrils with a range of periodicities between 56 and 154 nm revealed a gross conformational change in the interbead region that accommodates the length change. At periodicities below 85 nm, four thin filaments are visualized in the interbead region, but at periodicities greater than 85 nm, two thick filaments are seen. The diameter of the bead remains almost constant at all periodicities, but there is a decrease in stain-exclusion above 85 nm periodicity, which is likely to correspond to a decrease in bead mass. Additionally, we identified eight molecules in cross-section through a microfibril, allowing us to understand microfibril organization in three dimensions. In conclusion, when microfibrils extend, there is a large molecular rearrangement within the interbead region, and this highlights a possible role for Ca(2+) in stabilizing the microfibril architecture and moderating extension in vivo.

Blending polysaccharides with biodegradable polymers. II. Structure and biological response of chitosan/polycaprolactone blends

Blends of polycaprolactone (PCL) and chitosan (CHT) were prepared by casting from the mixture of solutions of both components in suitable solvents. PCL, and CHT, form phase separated blends with improved mechanical properties and increased water sorption ability with respect to pure PCL. The morphology of the system was investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM) and confocal microscopy. Dispersed domains of CHT in the semicrystalline PCL matrix were found in samples with less than 20% CHT but cocontinuous phase morphologies are found in blends with 20% or more CHT. This feature was corroborated by the temperature dependence of the elastic modulus measured by dynamic mechanical properties as a function of temperature. It was observed that for those blends above 20 wt% CHT, the mechanical stability of the system was kept even after melting of the PCL phase. Primary human chondrocytes were cultured on the different substrates. Cell morphology was studied by SEM and the viability and proliferation was investigated by the colorimetric MTT assay. Different protein conformations were found by AFM on CHT and PCL samples which were related to the biological performance of the substrates. Hydrophilicty of the material is not directly related to the biological response and the sample with 20 wt% CHT shows better results than the other blends with respect to chondrocyte viability and proliferation. However, the results obtained in the blends are worse than in pure PCL. It seems to be correlated with the surface energy of the different blends rather than hydrophilicity.

ECM macromolecules: height-mapping and nano-mechanics using atomic force microscopy

The atomic force microscope (AFM) may be used to collect quantitative height data from extracellular matrix molecules and macro-molecular assemblies adsorbed to a wide range of solid substrates. The advantages of atomic force microscopy include rapid specimen preparation, which does not rely on chemical fixation, dehydration or heavy-metal staining, and sub-nanometre resolution imaging with a high signal-noise ratio. In combination with complimentary techniques such as molecular combing and by exploiting the ability to act as a force spectrometer, the AFM can provide valuable information on the nano-mechanical properties of extracellular matrix components.

The role of endothelial cell attachment to elastic fibre molecules in the enhancement of monolayer formation and retention, and the inhibition of smooth muscle cell recruitment

The endothelium is an essential modulator of vascular tone and thrombogenicity and a critical barrier between the vessel wall and blood components. In tissue-engineered small-diameter vascular constructs, endothelial cell detachment in flow can lead to thrombosis and graft failure. The subendothelial extracellular matrix provides stable endothelial cell anchorage through interactions with cell surface receptors, and influences the proliferation, migration, and survival of both endothelial cells and smooth muscle cells. We have tested the hypothesis that these desired physiological characteristics can be conferred by surface coatings of natural vascular matrix components, focusing on the elastic fiber molecules, fibrillin-1, fibulin-5 and tropoelastin. On fibrillin-1 or fibulin-5-coated surfaces, endothelial cells exhibited strong integrin-mediated attachment in static conditions (82% and 76% attachment, respectively) and flow conditions (67% and 78% cell retention on fibrillin-1 or fibulin-5, respectively, at 25 dynes/cm2), confluent monolayer formation, and stable functional characteristics. Adhesion to these two molecules also strongly inhibited smooth muscle cell migration to the endothelial monolayer. In contrast, on elastin, endothelial cells attached poorly, did not spread, and had markedly impaired functional properties. Thus, fibrillin-1 and fibulin-5, but not elastin, can be exploited to enhance endothelial stability, and to inhibit SMC migration within vascular graft scaffolds. These findings have important implications for the design of vascular graft scaffolds, the clinical performance of which may be enhanced by exploiting natural cell-matrix biology to regulate cell attachment and function.

Extracellular matrix protein coatings for facilitation of urothelial cell attachment

Synthetic urothelium is an important goal for the tissue-engineering field that would have great utility for treating diseases and congenital defects affecting the urinary tract. A key step in the development of synthetic tissue is optimizing the conditions for coating biomaterials with cells of interest. Initial cell attachment is an important consideration when designing tissue-engineering scaffolds. The scaffold environment must also be conducive to cell proliferation and differentiation. The most popular materials for tissue-engineering scaffold often have suboptimal properties when analyzed for cell attachment and growth. It would then be of interest to know, for urinary tract tissue-engineering applications, which extracellular matrix protein coatings can facilitate urothelial cell attachment and encourage growth. Cells grown on 96-well cycloolefin plates coated with type IV or type I collagen exhibited improved initial attachment over plates coated with fibronectin or laminin. After 20 h, deoxyribonucleic acid synthesis was found to increase in cultures grown on type IV collagen, fibronectin, and laminin. Total metabolic activity of urothelial cell cultures was also monitored, and no difference was seen between any protein-coating conditions. The development of such reliable assays will be beneficial in monitoring the fate of scaffolds seeded with human urothelial cells.

Applying elastic fibre biology in vascular tissue engineering

For the treatment of vascular disease, the major cause of death in Western society, there is an urgent need for tissue-engineered, biocompatible, small calibre artery substitutes that restore biological function. Vascular tissue engineering of such grafts involves the development of compliant synthetic or biomaterial scaffolds that incorporate vascular cells and extracellular matrix. Elastic fibres are major structural elements of arterial walls that can enhance vascular graft design and patency. In blood vessels, they endow vessels with the critical property of elastic recoil. They also influence vascular cell behaviour through direct interactions and by regulating growth factor activation. This review addresses physiological elastic fibre assembly and contributions to vessel structure and function, and how elastic fibre biology is now being exploited in small diameter vascular graft design.

Substrate chemistry-dependent conformations of single laminin molecules on polymer surfaces are revealed by the phase signal of atomic force microscopy

The conformation of single laminin molecules adsorbed on synthetic substrates is directly observed making use of the phase magnitude in tapping mode atomic force microscopy (AFM). With AFM, it is not possible to differentiate the proteins on the substrate if use is made of the height signal, since the roughness of the material becomes of the same order of magnitude as the adsorbed protein, typically 10 nm height. This work shows how AFM can be exploited to reveal protein conformation on polymer materials. Different laminin morphologies are observed on a series of different copolymers based on ethyl acrylate and hydroxyethyl acrylate as a function of the surface density of -OH groups: from globular to completely extended morphologies of the protein molecules are obtained, and the onset of laminin network formation on some substrates can be clearly identified. The results stress the importance of the underlying synthetic substrate's surface chemistry for the biofunctional conformation of adsorbed proteins.

Micropatterned surfaces of PDMS as growth templates for HEK 293 cells

In this paper the easy and reliable preparation of precise micropatterns on PDMS surfaces is described and the growth of HEK 293 cells on those patterns during culture over several days is examined. The first patterning approach described is based on soft-lithography and polyelectrolyte multilayer deposition. Two different soft-lithographic techniques are employed for creating surface patterns of PAH, PSS, untreated and oxidized PDMS. The growth behavior of HEK 293 cells is investigated on all the dual combinations of the four surfaces, and decreasing preference of the cells for the surfaces in the order PAH (-NH2)>ox-PDMS (-OH)>>PSS (-SO3-)>PDMS (-CH3) is revealed. As the second patterning approach a method is introduced, which allows the deposition of gel droplets in a microarray format utilizing differences in the surface wettability. This concept is new and expected to be very useful for various applications. Finally, a speculative explanation for the different cell spreading behavior is provided considering the interplay between individual cell-surface interactions and a permanent cell tractional force.

Influence of the substrate's hydrophilicity on thein vitro Schwann cells viability

A series of polymeric biomaterials including poly (methyl acrylate) (PMA), chitosan (CHT), poly(ethyl acrylate) (PEA), poly(hydroxyethyl acrylate) (PHEA), and a series of random copolymers containing ethyl acrylate and hydroxyethyl acrylate monomeric units were tested in vitro as culture substrates and compared for their impact on the proliferation and expansion of Schwann cells (SCs). Immunocytochemical staining assay and scanning electron microscopy techniques were applied to perform a quantitative analysis to determine the correct maintenance of the cultured glial cells on the different biomaterials. The results strongly suggest that cell attachment and proliferation is influenced by the substrate's surface chemistry, and that hydrophobic biomaterials based on PMA, PEA, and the copolymers PEA and PHEA in a narrow composition window are suitable substrates to promote cell attachment and proliferation of SCs in vitro.

Characterization of collagen fibers in Bruch’s membrane using chemical force microscopy

Bruch's membrane is a layer composed of collagen fibers located just beneath the retina. This study validates a strategy used to map the morphological and adhesion characteristics of collagen fibers in Bruch's membrane. Atomic force microscopy tips were functionalized with different chemical groups and used to map the hydrophilic and hydrophobic regions on the surface of the eye tissue. The largest adhesion forces were observed when tips functionalized with NH(2) groups were used. The trend in the adhesion forces was rationalized based on the distribution of different functional groups in the triple-helical structure of the collagen fibers. The results of this study can be used to design more effective strategies to treat eye diseases such as age-related macular degeneration.

Survival and differentiation of embryonic neural explants on different biomaterials

Biomaterials prepared from polyacrylamide, ethyl acrylate (EA), and hydroxyethyl acrylate (HEA) in various blend ratios, methyl acrylate and chitosan, were tested in vitro as culture substrates and compared for their ability to be colonized by the cells migrating from embryonic brain explants. Neural explants were isolated from proliferative areas of the medial ganglionic eminence and the cortical ventricular zone of embryonic rat brains and cultured in vitro on the different biomaterials. Chitosan, poly(methyl acrylate), and the 50% wt copolymer of EA and HEA were the most suitable substrates to promote cell attachment and differentiation of the neural cells among those tested. Immunofluorescence microscopy analysis showed that progenitor cells had undergone differentiation and that the resulting glial and neuronal cells expressed their intrinsic morphological characteristics in culture.