Chemical Modifications in Hyaluronic Acid-Based Electrospun Scaffolds

Hyaluronic acid (HA) is a natural, non-sulfated glycosaminoglycan (GAG) present in ECM. It is involved in different biological functions with appealing properties in cosmetics and pharmaceutical preparations as well as in tissue engineering. Generally, HA has been electrospun in blends with natural or synthetic polymers to produce fibers having diameters in the order of nano and micro-scale whose pores can host cells able to regenerate damaged tissues. In the last decade, a rich literature on electrospun HA-based materials arose. Chemical modifications were generally introduced in HA scaffolds to favour crosslinking or conjugation with bioactive molecules. Considering the high solubility of HA in water, HA-based electrospun scaffolds are cross-linked to increase the stability in biological fluids. Crosslinking is necessary also to avoid the release of HA from the hybrid scaffold when implanted in-vivo. Furthermore, to endow the HA based scaffolds with new chemical or biological properties, conjugation of bioactive molecules to HA was widely reported. Herein, we review the existing research classifying chemical modifications on HA and HA-based electrospun fibers into three categories: i) in-situ crosslinking of electrospun HA-based scaffolds ii) off-site crosslinking of electrospun HA-based scaffolds; iii) conjugation of biofunctional molecules to HA with focus on peptides.

Structural and physical basis for the elasticity of elastin

Elastin function is to endow vertebrate tissues with elasticity so that they can adapt to local mechanical constraints. The hydrophobicity and insolubility of the mature elastin polymer have hampered studies of its molecular organisation and structure-elasticity relationships. Nevertheless, a growing number of studies from a broad range of disciplines have provided invaluable insights, and several structural models of elastin have been proposed. However, many questions remain regarding how the primary sequence of elastin (and the soluble precursor tropoelastin) governs the molecular structure, its organisation into a polymeric network, and the mechanical properties of the resulting material. The elasticity of elastin is known to be largely entropic in origin, a property that is understood to arise from both its disordered molecular structure and its hydrophobic character. Despite a high degree of hydrophobicity, elastin does not form compact, water-excluding domains and remains highly disordered. However, elastin contains both stable and labile secondary structure elements. Current models of elastin structure and function are drawn from data collected on tropoelastin and on elastin-like peptides (ELPs) but at the tissue level, elasticity is only achieved after polymerisation of the mature elastin. In tissues, the reticulation of tropoelastin chains in water defines the polymer elastin that bears elasticity. Similarly, ELPs require polymerisation to become elastic. There is considerable interest in elastin especially in the biomaterials and cosmetic fields where ELPs are widely used. This review aims to provide an up-to-date survey of/perspective on current knowledge about the interplay between elastin structure, solvation, and entropic elasticity.

A Study on Thiol-Michael Addition to Semi-Synthetic Elastin-Hyaluronan Material for Electrospun Scaffolds

Thiol-Michael addition is a chemical reaction extensively used for conjugating peptides to polysaccharides with applications as biomaterials. In the present study, for designing a bioactive element in electrospun scaffolds as wound dressing material, a chemical strategy for the semi-synthesis of a hyaluronan-elastin conjugate containing an amide linker (ELAHA) was developed in the presence of tris(2-carboxyethyl)phosphine hydrochloride (TCEP ⋅ HCl). The bioconjugate was electrospun with poly-D,L-lactide (PDLLA), obtaining scaffolds with appealing characteristics in terms of morphology and cell viability of dermal fibroblast cells. For comprehending the factors influencing the efficiency of the bioconjugation reaction, thiolated amino acids were also investigated as nucleophiles toward hyaluronan decorated with Michael acceptors in the presence of TCEP ⋅ HCl through the evaluation of byproducts formation.

Bioconjugation of Carbohydrates to Gelatin Sponges Promoting 3D Cell Cultures

Gelatin sponges are widely employed as hemostatic agents, and are gaining increasing interest as 3D scaffolds for tissue engineering. To broaden their possible application in the field of tissue engineering, a straightforward synthetic protocol able to anchor the disaccharides, maltose and lactose, for specific cell interactions was developed. A high conjugation yield was confirmed by ¹H-NMR and FT-IR spectroscopy, and the morphology of the resulting decorated sponges was characterized by SEM. After the crosslinking reaction, the sponges preserve their porous structure as ascertained by SEM. Finally, HepG2 cells cultured on the decorated gelatin sponges show high viability and significant differences in the cellular morphology as a function of the conjugated disaccharide. More spherical morphologies are observed when cultured on maltose-conjugated gelatin sponges, while a more flattened aspect is discerned when cultured onto lactose-conjugated gelatin sponges. Considering the increasing interest in small-sized carbohydrates as signaling cues on biomaterial surfaces, systematic studies on how small carbohydrates might influence cell adhesion and differentiation processes could take advantage of the described protocol.

Cellulose Nanocrystals as Additives in Electrospun Biocompatible Separators for Aprotic Lithium-Ion Batteries

This work concerns the study of electrospun scaffolds as separators for aprotic lithium-ion batteries (LIBs) composed of the amorphous poly-d,l-lactide (PDLLA), in solution concentrations of 8, 10, and 12 wt % and in different ratios with cellulose nanocrystals (CNCs). PDLLA has been studied for the first time as a separator, taking into account its amorphous character that could facilitate electrolyte incorporation into the polymer matrix and influence ionic conductivity, together with CNCs, for reducing the hydrophobicity of the scaffolds. The embedding of the nanocrystals in the scaffolds was confirmed by X-ray diffraction analysis and attenuated total reflectance Fourier transform infrared spectroscopy. The polymer combination influenced the nanofibrous morphology as evaluated by scanning electron microscopy and modulated the electrochemical behavior of the membranes that was investigated through linear sweep voltammetry, cyclic voltammetry, and electrochemical impedance spectroscopy tests. Among the studied categories, the P12 series displayed a nonhomogeneous electrolyte resistance and electrochemical stability, differently from P10, whose results suggested their application in LIBs with standard formulation, as confirmed by a preliminary performance test of the P10N6 formulation in a full Li-ion cell configuration.

Cucumber mosaic virus Is Unable to Self-Assemble in Tobacco Plants When Transmitted by Seed

Cucumber mosaic virus (CMV), which has great impact on agronomic production worldwide, is both aphid and seed transmitted. Although the mechanisms of aphid transmission have been widely studied, those underlying the ability of CMV to survive and remain infectious during the passage from one generation to the next through the seeds are still to be clarified. Moreover, the viral determinants of seed transmission rate are poorly understood. Three viral genotypes produced from same RNA 1 and 2 components of CMV-Fny but differing in RNA 3 (the wild type CMV-Fny, a pseudorecombinant CMV-Fny/CMV-S and a chimeric CMV previously obtained by our group, named F, FS and CS, respectively) were propagated in Nicotiana tabacum cv Xanthi plants in order to assess differences in tobacco seed transmission rate and persistence through plant generations in the absence of aphid transmission. Seed-growth tests revealed CMV infection in the embryos, but not in the integuments. Seedlings from seed-growth tests showed the presence of all considered viruses but at different rates: from 4% (F, FS) to 16% (CS). Electron microscopy revealed absence (CS) of viral particles or virions without the typical central hole (F and FS). In agreement, structural characteristics of purified CMV particles, assessed by circular dichroism spectroscopy, showed anomalous spectra of nucleic acids rather than the expected nucleoproteins. These alterations resulted in no seed transmission beyond the first plant generation. Altogether, the results show for the first time that correct virion assembly is needed for seed infection from the mother plant but not to seedling invasion from the seed. We propose that incorrect virion formation, self-assembly and architecture stability might be explained if during the first stages of germination and seedling development some tobacco seed factors target viral regions responsible for protein-RNA interactions.

Hydrogels from the Assembly of SAA/Elastin-Inspired Peptides Reveal Non-Canonical Nanotopologies

Peptide-based hydrogels are of great interest in the biomedical field according to their biocompatibility, simple structure and tunable properties via sequence modification. In recent years, multicomponent assembly of peptides have expanded the possibilities to produce more versatile hydrogels, by blending gelating peptides with different type of peptides to add new features. In the present study, the assembly of gelating P5 peptide SFFSF blended with P21 peptide, SFFSFGVPGVGVPGVGSFFSF, an elastin-inspired peptides or, alternatively, with FF dipeptide, was investigated by oscillatory rheology and different microscopy techniques in order to shed light on the nanotopologies formed by the self-assembled peptide mixtures. Our data show that, depending on the added peptides, cooperative or disruptive assembly can be observed giving rise to distinct nanotopologies to which correspond different mechanical properties that could be exploited to fabricate materials with desired properties.

Elastin-Hyaluronan Bioconjugate as Bioactive Component in Electrospun Scaffolds

Hyaluronic acid or hyaluronan (HA) and elastin-inspired peptides (EL) have been widely recognized as bioinspired materials useful in biomedical applications. The aim of the present work is the production of electrospun scaffolds as wound dressing materials which would benefit from synergic action of the bioactivity of elastin peptides and the regenerative properties of hyaluronic acid. Taking advantage of thiol-ene chemistry, a bioactive elastin peptide was successfully conjugated to methacrylated hyaluronic acid (MAHA) and electrospun together with poly-D,L-lactide (PDLLA). To the best of our knowledge, limited reports on peptide-conjugated hyaluronic acid were described in literature, and none of these was employed for the production of electrospun scaffolds. The conformational studies carried out by Circular Dichroism (CD) on the bioconjugated compound confirmed the preservation of secondary structure of the peptide after conjugation while Scanning Electron Microscopy (SEM) revealed the supramolecular structure of the electrospun scaffolds. Overall, the study demonstrates that the bioconjugation of hyaluronic acid with the elastin peptide improved the electrospinning processability with improved characteristics in terms of morphology of the final scaffolds.

Soft Hydrogel Inspired by Elastomeric Proteins

Elastin polypeptides based on -VPGVG- repeated motifs are widely used in the production of biomaterials because they are stimuli-responsive systems. On the other hand, glycine-rich sequences, mainly present in tropoelastin terminal domains, are responsible for the elastin self-assembly. In a previous study, we have recombinantly expressed a chimeric polypeptide, named resilin, elastin, and collagen (REC), inspired by glycine-rich motifs of elastin and containing resilin and collagen sequences as well. Herein, a three-block polypeptide, named (REC)₃, was expressed starting from the previous monomer gene by introducing key modifications in the sequence. The choice was mandatory because the uneven distribution of the cross-linking sites in the monomer precluded the hydrogel production. In this work, the cross-linked polypeptide appeared as a soft hydrogel, as assessed by rheology, and the linear un-cross-linked trimer self-aggregated more rapidly than the REC monomer. The absence of cell-adhesive sequences did not affect cell viability, while it was functional to the production of a material presenting antiadhesive properties useful in the integration of synthetic devices in the body and preventing the invasion of cells.

Hyaluronic Acid-Functionalized Hybrid Gelatin-Poly-L-Lactide Scaffolds with Tunable Hydrophilicity

In this study, we describe the production of hybrid gelatin-poly-L-lactide electrospun scaffolds whose hydrophilicity was controlled by binding increasing concentrations of hyaluronic acid (HA). We show that cross-linking has advantages over coating when aiming to functionalize the scaffolds with HA. The here described scaffolds structurely mimicked the complexity of the extracellular matrix, and when excited by second harmonic generation, they produced a signal that is typical of collagen-containing biological fibers. Fluorescence lifetime imaging microscopy (FLIM) was used to marker-independently monitor the growth of human dermal fibroblasts on the electrospun scaffolds using reduced (phosphorylated) nicotinamide adenine dinucleotide as target. Benefitting from the different fluorescence lifetimes of the polymer and the endogenous cellular fluorophore, we were able to distinguish and separate the signals produced by the cells from the signals generated by the electrospun scaffolds. FLIM further allowed the detection of metabolic differences in the cells seeded on the HA-functionalized scaffolds compared with cells that were cultured on nonfunctionalized control scaffolds.

Labeling of Nanofiber-Forming Peptides by Site-Directed Bioconjugation: Effect of Spacer Length on Self-Assembly

BACKGROUND

The conjugation of small organic molecules to self-assembling peptides is a versatile tool to decorate nanostructures with original functionalities. Labeling with chromophores or fluorophores, for example, creates optically active fibers with potential interest in photonic devices.

AIM AND OBJECTIVE

In this work, we present a rapid and effective labeling procedure for a self-assembling peptide able to form nanofibers. Rapid periodate oxidation of the N-terminal serine residue of the peptide and subsequent conjugation with dansyl moiety generated fluorophore-decorated peptides.

RESULTS

Three dansyl-conjugated self-assembling peptides with variable spacer-length were synthesized and characterized and the role of the size of the linker between fluorophore and peptide in self-assembling was investigated. Our results show that a short linker can alter the self-assembly in nanofibers of the peptide.

CONCLUSIONS

Herein we report on an alternative strategy for creating functionalized nanofibrils, able to expand the toolkit of chemoselective bioconjugation strategies to be used in site-specific decoration of self-assembling peptides.

Electrospun poly(d,l-lactide)/gelatin/glass-ceramics tricomponent nanofibrous scaffold for bone tissue engineering

Electrospun scaffolds are emerging as extracellular matrix (ECM)mimicking structures for tissue engineering thanks to their nanofibrous architecture. For the development of suitable electrospun scaffolds for bone tissue engineering, the addition of inorganic components has been implemented with the aim to confer important bioactivity like osteoinduction, osteointegration, and cell adhesion to the scaffolds. In this context, we propose a tricomponent electrospun scaffold composed of poly(d,l-lactide), gelatin and RKKP glass-ceramics. The bioactive RKKP glass-ceramic system has attracted interest, due to the presence of ions such as La³⁺ and Ta⁵⁺ , which turned out to be valuable as growth supporting agents for bones. In this work, RKKP glass-ceramics were embedded inside the microfibers of electrospun scaffolds and the structural and biological properties were investigated. Our results showed that the glass-ceramic microparticles were uniformly distributed in the fibrous structure of the scaffold. Furthermore, the glass-ceramics promoted biomineralization of the scaffolds and improved cell viability and osteogenic differentiation. The mineralized layer formed on RKKP-containing scaffolds after incubation in simulated body fluid medium has been shown to be hydroxyapatite by Raman spectroscopy and X-ray diffraction. The results on differentiation studies of canine adipose-derived mesenchymal stem cells grown on the electrospun scaffolds suggest that on varying the content of RKKP in the scaffold, it is possible to drive the differentiation toward chondrogenic or osteogenic commitment. The presence of ions, like La³⁺ and Ta⁵⁺ , in the RKKP embedded polymeric composite scaffolds could play a role in supporting cell growth and promoting differentiation.

Fibrillar Self-Assembly of a Chimeric Elastin-Resilin Inspired Engineered Polypeptide

In the field of tissue engineering, recombinant protein-based biomaterials made up of block polypeptides with tunable properties arising from the functionalities of the individual domains are appealing candidates for the construction of medical devices. In this work, we focused our attention on the preparation and structural characterization of nanofibers from a chimeric-polypeptide-containing resilin and elastin domain, designed on purpose to enhance its cell-binding ability by introducing a specific fibronectin-derived Arg-Gly-Asp (RGD) sequence. The polypeptide ability to self-assemble was investigated. The molecular and supramolecular structure was characterized by Scanning Electronic Microscopy (SEM) and Atomic Force Microscopy (AFM), circular dichroism, state-of-the-art synchrotron radiation-induced techniques X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The attained complementary results allow us to assess as H-bonds influence the morphology of the aggregates obtained after the self-assembling of the chimeric polypeptide. Finally, a preliminary investigation of the potential cytotoxicity of the polypeptide was performed by culturing human fetal foreskin fibroblast (HFFF2) for its use as biomedical device.

Controlled and tuneable drug release from electrospun fibers and a non-invasive approach for cytotoxicity testing

Electrospinning is an attractive method to generate drug releasing systems. In this work, we encapsulated the cell death-inducing drug Diclofenac (DCF) in an electrospun poly-L-lactide (PLA) scaffold. The scaffold offers a system for a sustained and controlled delivery of the cytotoxic DCF over time making it clinically favourable by achieving a prolonged therapeutic effect. We exposed human dermal fibroblasts (HDFs) to the drug-eluting scaffold and employed multiphoton microscopy and fluorescence lifetime imaging microscopy. These methods were suitable for non-invasive and marker-independent assessment of the cytotoxic effects. Released DCF induced changes in cell morphology and glycolytic activity. Furthermore, we showed that drug release can be influenced by adding dimethyl sulfoxide as a co-solvent for electrospinning. Interestingly, without affecting the drug diffusion mechanism, the resulting PLA scaffolds showed altered fibre morphology and enhanced initial DCF burst release. The here described model could represent an interesting way to control the diffusion of encapsulated bio-active molecules and test them using a marker-independent, non-invasive approach.

Tuning of hydrogel stiffness using a two-component peptide system for mammalian cell culture

Self-assembling peptide hydrogels (SAPHs) represent emerging cell cultures systems in several biomedical applications. The advantages of SAPHs are mainly ascribed to the absence of toxic chemical cross-linkers, the presence of ECM-like fibrillar structures and the possibility to produce hydrogels with a large range of different mechanical properties. We will present a two-component peptide system with tuneable mechanical properties, consisting of a small pentapeptide (SFFSF-NH2 , SA5N) that acts as a gelator and a larger 21-mer peptide (SFFSF-GVPGVGVPGVG-SFFSF, SA21) designed as a physical cross-linker. The hydrogels formed by different mixtures of the two peptides are made up mainly of antiparallel β-sheet nanofibers entangling in an intricate network. The effect of the addition of SA21 on the morphology of the hydrogels was investigated by atomic force microscopy and transmission electron microscopy and correlated to the mechanical properties of the hydrogel. Finally, the biocompatibility of the hydrogels using 2D cell cultures was tested. © 2018 The Authors. journal Of Biomedical Materials Research Part A Published By Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 535-544, 2019.

Non-invasive characterization of hybrid gelatin:poly-l-lactide electrospun scaffolds using second harmonic generation and multiphoton imaging

Hybrid scaffolds composed of synthetic polymers and naturally occurring components have become more relevant in the field of tissue engineering and regenerative medicine. Synthetic polymers are responsible for scaffold durability, strength and structural integrity; however, often do not provide biological signals. Introducing a biological component leads to more advanced and biocompatible scaffolds. In order to use these scaffolds as implants, a deeper knowledge of material characteristics and the impact of the biological component on the scaffold mechanical properties are required. Furthermore, it is necessary to implement fast, easy and non-invasive methods to determine material characteristics. In this work, we aimed to generate gelatin-poly-l-lactide (PLA) hybrids via electrospinning with defined, controllable and tunable scaffold characteristics. Using Raman microspectroscopy, we demonstrated the effectiveness of the cross-linking reaction and evaluated the increasing PLA content in the hybrid scaffolds with a non-invasive approach. Using multiphoton microscopy, we showed that gelatin fibers electrospun from a fluorinated solvent exhibit a second harmonic generation (SHG) signal typical for collagen-like structures. Compared to pure gelatin, where the SHG signal vanishes after cross-linking, the signal could be preserved in the hybrid scaffolds even after cross-linking. Furthermore, we non-invasively imaged cellular growth of human dermal fibroblasts on the hybrid electrospun scaffolds and performed fluorescence lifetime imaging microscopy on the cell-seeded hybrids, where we were able to discriminate between cells and scaffolds. Here, we successfully employed non-invasive methods to evaluate scaffold characteristics and investigate cell-material interactions.

Heparan sulfates facilitate harmless amyloidogenic fibril formation interacting with elastin-like peptides

Heparan sulfates (HSs) modulate tissue elasticity in physiopathological conditions by interacting with various matrix constituents as tropoelastin and elastin-derived peptides. HSs bind also to protein moieties accelerating amyloid formation and influencing cytotoxic properties of insoluble fibrils. Interestingly, amyloidogenic polypeptides, despite their supposed pathogenic role, have been recently explored as promising bio-nanomaterials due to their unique and interesting properties. Therefore, we investigated the interactions of HSs, obtained from different sources and exhibiting various degree of sulfation, with synthetic amyloidogenic elastin-like peptides (ELPs), also looking at the effects of these interactions on cell viability and cell behavior using in vitro cultured fibroblasts, as a prototype of mesenchymal cells known to modulate the soft connective tissue environment. Results demonstrate, for the first time, that HSs, with differences depending on their sulfation pattern and chain length, interact with ELPs accelerating aggregation kinetics and amyloid-like fibril formation as well as self-association. Furthermore, these fibrils do not negatively affect fibroblasts’ cell growth and parameters of redox balance, and influence cellular adhesion properties. Data provide information for a better understanding of the interactions altering the elastic component in aging and in pathologic conditions and may pave the way for the development of composite matrix-based biomaterials.

Characterization of a Crosslinked Elastomeric-Protein Inspired Polypeptide

Materials inspired by natural proteins have a great appeal in tissue engineering for their biocompatibility and similarity to extracellular matrix (ECM). Chimeric polypeptides inspired by elastomeric proteins such as silk, elastin, and collagen are of outstanding interest in the field. A recombinant polypeptide constituted of three different blocks, each of them having sequences derived from elastin, resilin, and collagen proteins, was demonstrated to be a good candidate as biomaterial for its self-assembling characteristics and biocompatibility. Herein, taking advantage of the primary amine functionalities present in the linear polypeptide, we crosslinked it with 1,6-hexamethylene-diisocyanate (HMDI). The characterization of the obtained polypeptide was realized by CD spectroscopy, AFM, and SEM microscopies. The obtained results, although not conclusive, demonstrate that the crosslinked polypeptide gave rise to porous networks, thin nanowires, and films not observable for the linear polypeptide. Chirality 28:606-611, 2016. © 2016 Wiley Periodicals, Inc.

Combined effects of solvation and aggregation propensity on the final supramolecular structures adopted by hydrophobic, glycine-rich, elastin-like polypeptides

Previous work on elastin-like polypeptides (ELPs) made of hydrophobic amino acids of the type XxxGlyGlyZzzGly (Xxx, Zzz = Val, Leu) has consistently shown that differing dominant supramolecular structures were formed when the suspending media were varied: helical, amyloid-like fibers when suspended in water and globules evolving into "string of bead" structures, poly(ValGlyGlyValGly), or cigar-like bundles, poly(ValGlyGlyLeuGly), when suspended in methyl alcohol. Comparative experiments with poly(LeuGlyGlyValGly) have further indicated that the interface energy plays a significant role and that solvation effects act in concomitance with the intrinsic aggregation propensity of the repeat sequence. Continuing our investigation on ELPs using surface (X-ray photoelectron spectroscopy, atomic force microscopy) and bulk (circular dichroism, Fourier transform infrared spectroscopy) techniques for their characterization, here we have compared the effect of suspending solvents (H(2)O, dimethylsulfoxide, ethylene glycol, and MeOH) on poly(ValGlyGlyValGly), the polypeptide most inclined to form long and well-refined helical fibers in water, searching for the signature of intermolecular interactions occurring between the polypeptide chains in the given suspension. The influence of sequence specificities has been studied by comparing poly(ValGlyGlyValGly) and poly(LeuGlyGlyValGly) with a similar degree of polymerization. Deposits on substrates of the polypeptides were characterized taking into account the differing evaporation rate of solvents, and tests on their stability in ultra high vacuum were performed. Finally, combining experimental and computational studies, we have revaluated the three-dimensional modeling previously proposed for the supramolecular assembly in water of poly(ValGlyGlyValGly). The results were discussed and rationalized also in the light of published data.

Elastin peptides in aging and pathological conditions

Elastin is the protein responsible for the resilience of vertebrate tissue. It is an extremely stable protein deposited during the early stages of life and experiencing almost no renewal. As a consequence, it can be considered that each individual has an elastin capital for life. Despite its extreme stability, elastin can be degraded by several enzymes termed elastases. Elastases are among the most aggressive proteases, and their presence is increased with age. As a consequence, elastin fragmentation resulting in the generation of elastin peptides is one of the hallmarks of aging. This review will examine their nature and further expose our current understanding of the role played by these peptides in aging and their contribution to tissue homeostasis and several pathologies.

Tuning self-assembly in elastin-derived peptides

Elastin-derived peptides are gaining increasing interest as potential biomaterials. Previous studies have demonstrated that short elastin-derived peptides are able to self-assemble into fibrils as the entire elastin protein. The motif responsible for that is the XGGZG motif at least three-fold repeated. In this work we have synthesized and studied, at molecular and supramolecular levels, four pentadecapeptides obtained by switching the X and Z residue with leucine and/or valine. We found that the four peptides formed different supramolecular structures corresponding to specific molecular conformations. Our results show that not only the residue type but also the exact position occupied by the residue in the motif is crucial in driving the self-aggregation. The aim of this work is to provide the basis for designing elastin-derived peptides with tunable supramolecular architecture.

Nanospheres from the self-assembly of an elastin-inspired triblock peptide

The self-assembly of an elastin-inspired triblock peptide into nanospheres highlights the important role of conformational flexibility and π–π stacking.

Biological and structural characterization of a naturally inspired material engineered from elastin as a candidate for tissue engineering applications

The adoption of a biomimetic approach in the design and fabrication of innovative materials for biomedical applications is encountering a growing interest. In particular, new molecules are being engineered on the basis of proteins present in the extracellular matrix, such as fibronectin, collagen, or elastin. Following this approach scientists expect to be able not only to obtain materials with tailored mechanical properties but also to elicit specific biological responses inherited by the mimicked tissue. In the present work, a novel peptide, engineered starting from the sequence encoded by exon 28 of human tropoelastin, was characterized from a chemical, physical, and biological point of view. The obtained molecule was observed to aggregate at high temperatures, forming a material able to induce a biological effect similar to what elastin does in the physiological context. This material seems to be a good candidate to play a relevant role in future biomedical applications with special reference to vascular surgery.

Investigating the role of (2S,4R)-4-hydroxyproline in elastin model peptides

Post-translational modifications play a key role in defining the biological functions of proteins. Among them, the hydroxylation of proline producing the (2S,4R)-4-hydroxyproline (Hyp) is one of the most frequent modifications observed in vertebrates, being particularly abundant in the proteins of the extracellular matrix. In collagen, hydroxylation of proline plays a critical role, conferring the correct structure and mechanical strength to collagen fibers. In elastin, the exact role of this modification is not yet understood. Here we show that Hyp-containing elastin polypeptides have flexible molecular structures, analogously to proline-containing polypeptides. In turn, the self-assembly of the elastin peptides is significantly altered by the presence of Hyp, evidencing different supramolecular structures. Also the in vitro susceptibility to protease digestion is changed. These findings give a better insight into the elastic fiber formation and degradation processes in the extracellular matrix. Furthermore, our results could contribute in defining the subtle role of proline structural variants in the folding and self-assembly of elastin-inspired peptides, helping the rational design of elastin biomaterials.

Multiscale characterization of a chimeric biomimetic polypeptide for stem cell culture

Mesenchymal stem cells have attracted great interest in the field of tissue engineering and regenerative medicine because of their multipotentiality and relative ease of isolation from adult tissues. The medical application of this cellular system requires the inclusion in a growth and delivery scaffold that is crucial for the clinical effectiveness of the therapy. In particular, the ideal scaffolding material should have the needed porosity and mechanical strength to allow a good integration with the surrounding tissues, but it should also assure high biocompatibility and full resorbability. For such a purpose, protein-inspired biomaterials and, in particular, elastomeric-derived polypeptides are playing a major role, in which they are expected to fulfil many of the biological and mechanical requirements. A specific chimeric protein, designed starting from elastin, resilin and collagen sequences, was characterized over different length scales. Single-molecule mechanics, aggregation properties and compatibility with human mesenchymal stem cells were tested, showing that the engineered compound is a good candidate as a stem cell scaffold to be used in tissue engineering applications.

Role of polyproline II conformation in human tropoelastin structure

In this review, we present a comprehensive overview of the molecular studies on human tropoelastin domains accomplished by Tamburro and co-workers in the last decade. The used approach is the reductionist approach applied to human tropoelastin and is based on the observation that the tropoelastin gene exhibits a cassette-like organization, with a regular alternation of cross-linking and hydrophobic domains putatively responsible for the elasticity of the protein. The peculiar structure of human tropoelastin gene prompted us to study the isolated domains encoded by the exons of tropoelastin, with the perspective to get deep insights into the structural properties of the whole protein. At the molecular level, the results clearly evidence large flexibility of the polypeptide chains in the hydrophobic domains, which oscillate between rather extended and folded conformations. An important role was assigned to poly-proline II conformation considered as the hinge structure in the dynamic conformational equilibrium suggested for the hydrophobic domains. For the lysine-rich cross-linking domains, the structural studies exactly localized α-helix along the polypeptide sequence. Furthermore, at supramolecular level, these studies showed that several domains are able to self-assemble in two different aggregation patterns, the fibrous elastin-like structure for some proline-rich hydrophobic domains and the amyloid-like for some glycine-rich hydrophobic domains. Accordingly, the studies suggest that the reductionist approach was a valid tool for studying a complex protein, such as elastin, elucidating not only the structure but also the specific role played by its constituent domains.

Design and production of a chimeric resilin-, elastin-, and collagen-like engineered polypeptide

Protein-inspired biomaterials have gained great interest as an alternative to synthetic polymers, in particular, for their potential use as biomedical devices. The potential inspiring models are mainly proteins able to confer mechanical properties to tissues and organs, such as elasticity (elastin, resilin, spider silk) and strength (collagen, silk). The proper combination of repetitive sequences, each of them derived from different proteins, represents a useful tool for obtaining biomaterials with tailored mechanical properties and biological functions. In this report we describe the design, the production, and the preliminary characterization of a chimeric polypeptide, based on sequences derived from the highly resilient proteins resilin and elastin and from collagen-like sequences. The results show that the obtained chimeric recombinant material exhibits promising self-assembling properties. Young's modulus of the fibers was determined by AFM image analysis and lies in the range of 0.1-3 MPa in agreement with the expectations for elastin-like and resilin-like materials.

Effect of chemical cross-linking on the mechanical properties of elastomeric peptides studied by single molecule force spectroscopy

Mechanical properties of animal tissues are mainly provided by the assembly of single elastomeric proteins into a complex network of filaments. Even if the overall elastic properties of such a reticulated structure depend on the mechanical characteristics of the constituents, it is not the only aspect to be considered. In addition, the aggregation mechanism has to be clarified to attain a full knowledge of the molecular basis of the elastic properties of natural nanostructured materials. This aim is even more crucial in the process of rational design of biomaterials with selected mechanical properties, in which not only the mechanics of single molecules but also of their assemblies has to be cared of. In this study, this aspect was approached by means of single molecule stretching experiments. In particular, the effect of chemical cross-linking on the mechanical properties of a naturally inspired elastomeric peptide was investigated. Accordingly, we observed that, in order to preserve the elastic properties of the single filament, the two strands of the dimer have to interact with each other. The results thus confirm that the influence of the aggregation process on the mechanical properties of a molecular assembly cannot be neglected.

Influence of amino acid specificities on the molecular and supramolecular organization of glycine-rich elastin-like polypeptides in water

Elastin-like polypeptides adopt complex supramolecular structures, showing either a hydrophobic or a hydrophilic surface, depending on their surrounding environment and the supporting substrate. The preferred organization is important in many situations ranging from biocompatibility to bio-function. Here we compare the n-repeat pentamer LeuGlyGlyValGly (n = 7) with the analogue ValGlyGlyValGly (n = 5), as water suspensions and as deposits on silicon substrates. These sequences contain the repeat XxxGlyGlyZzzGly (Xxx, Zzz = Val, Leu) motif belonging to the hydrophobic glycine-rich domain of elastin and represent a simplified model from which to obtain information on molecular interactions functional to elastin itself. The compounds studied differ only by the presence of the -CH(2)- spacer in the Leu moiety and thus the work was aimed at revealing the influence of this spacer element on self assembly. Both polypeptides were studied under identical conditions, using combined techniques, to identify differences in their conformational states both at molecular (CD, FTIR) and supramolecular (XPS, AFM) levels. By these means, together with a Congo Red spectroscopic assay of β-sheet formation in water, a clear correlation between amino acid sequences (sequence specificity) and their kinetics and ordering of aggregation has emerged. The novel outcomes of this work are from the supplementary measurements, made to augment the AFM and XPS studies, showing that the significant step in the self assembly of both polypeptides takes place in the liquid phase and from the finding that the substitution of Val by Leu in the first position of the pentapeptide effectively inhibits the formation of amyloidal fibers.

Human tropoelastin sequence: dynamics of polypeptide coded by exon 6 in solution

Calorimetric studies were performed on exon 6 in powdered form and in solution [water and 2,2,2-trifluoroethanol (TFE), a structure-inducing solvent or cosolvent]. Dynamic dielectric spectroscopy (DDS) analyses were realized in water and 20% TFE. The major role of solvent-peptide organization is evidenced with these techniques. Calorimetric measurements reveal the structural water organization around the polypeptide as well as the presence of hydrophobic interactions in TFE solution. Dielectric measurements showed for exon 6/water a decrease of relaxations times of bulk solvent implying a faster dynamics with a slight increase of the activation entropy, suggesting that exon 6 probably creates disorder within the solvent. For TFE/water mixtures, an influence of exon 6 on its environment was seen with a relaxation associated with the exon 6/solvent interactions reinforced by storage of 72 h. Finally, exon 6/solvent interactions were clearly observed with addition of TFE.

On enhancers and inhibitors of elastin-derived amyloidogenesis

Aims: The main aim of this study is to better understand the self-aggregation mechanism of amyloid-like elastin-derived fibers in order to design and produce new powerful drugs that will inhibit the onset of ‘amyloidosis’. Materials & methods: Atomic force microscopy (AFM), Congo Red birefringence assay and Thioflavin T fluorescence measurements were used to demonstrate the amyloid-like behavior of some fragments of elastin protein (exon 30 [EX30] and exon 28 [EX28]). Turbidimetry on apparent absorbance technique was used to investigate the effect either of enhancers or of inhibitors on the amyloidogenic elastin-like peptides. Circular-dichroism spectroscopy was used to study the secondary structures of the peptides. Results & discussion: We used Congo Red birefringence assay, Thioflavin T fluorescence measurements and AFM measurements that are used commonly to demonstrate the formation of amyloids. The elastin fibrillogenesis is amyloid-like. Then, the elastin amyloidogenesis is inhibited by particular pentapeptides. Conclusions: We have reported herein that the fibrillogenesis of elastin-derived EX28 and EX30 polypeptides is facilitated significantly by the effect of sodium taurocholate bile salt and is inhibited by a classical inhibitor of Aβ-amyloid peptide, such as KLVFF, as well as by novel inhibitors, designed by us on the basis of some elastin sequences.

Supramolecular organization of elastin and elastin-related nanostructured biopolymers

The ultrastructure of elastin has been extensively analyzed by different methodologies. Starting from the first descriptions, where elastin was depicted as an amorphous structure, more complex and, in some cases, varied morphologies were revealed. The supramolecular structures found for elastin have been compared with those found for other elastin-related polypeptides, such as alpha-elastin and tropoelastin, and very similar features emerged. This review will deal with the supramolecular organization exhibited by many elastin-related compounds, starting from elastin, going through polypeptides constituted by different domains of tropoelastin, up to polymers containing repetitive sequences of elastin. In particular, recent developments on biopolymers of general type poly(Val-Pro-Gly-Xaa-Gly) and poly(Xaa-Gly-Gly-Zaa-Gly) (Xaa, Zaa = Val, Leu, Lys, Glu, Orn) obtained either by chemical synthesis or recombinant DNA techniques will be discussed in detail. The general aim is to describe the supramolecular features useful for the identification of elastin-inspired nanostructured biopolymers for developing highly functional and biocompatible vascular grafts as well as scaffolds for tissue regeneration.

Elastic fibers and amyloid deposition in vascular tissue

Amyloid fibrils are associated with a large number of diseases, such as Alzheimer’s dementia and others. Evidence links Alzheimer’s dementia with vascular diseases and only few data connect amyloids and atherosclerosis and aging via deposits in the aortic intima. Recent results demonstrate that some elastin polypeptide sequences are also able to produce amyloid fibers. This finding could have useful implications in the study of amyloids in cardiovascular tissue whose main constituent is elastin. In this review, we have also outlined the main characterizing features regarding the structure of amyloid fibrils. Finally, we describe, as a future perspective, the design of proper inhibitors of amyloid deposition in vascular walls as potential therapeutic drugs.

Molecular and supramolecular structural studies on human tropoelastin sequences

One of the unusual properties of elastin is its ability to coacervate, which has been proposed to play an important role in the alignment of monomeric elastin for cross-linking into the polymeric elastin matrix. The temperature at which this transition takes place depends on several factors including protein concentration, ionic strength, and pH. Previously, polypeptide sequences encoded by different exons of the human tropoelastin gene have been analyzed for their ability to coacervate and to self-assemble. Few of them were indeed able to coacervate and only one, that encoded by exon 30 (EX30), gave amyloid fibers. In this article, we report on two chemically synthesized peptides-a decapeptide and an octadecapeptide-whose sequences are contained in the longer EX30 peptide and on a polypeptide (EX1-7) of 125 amino-acid residues corresponding to the sequence coded by the exons 1-7 and on a polypeptide (EX2-7) of 99 amino-acid residues encoded by exons 2-7 of human tropoelastin obtained by recombinant DNA techniques. Molecular and supramolecular structural characterization of these peptides showed that a minimum sequence of approximately 20 amino acids is needed to form amyloid fibers in the exon 30-derived peptides. The N-terminal region of mature tropoelastin (EX2-7) gives rise to a coacervate and forms elastinlike fibers, whereas the polypeptide sequence containing the signal peptide (EX1-7) forms mainly amyloid fibers. Circular dichroism spectra show that beta-structure is ubiquitous in all the sequences studied, suggesting that the presence of a beta-structure is a necessary, although not sufficient, requirement for the appearance of amyloid fibers.

Localizing alpha-helices in human tropoelastin: assembly of the elastin "puzzle"

Polyalanine cross-linking domains encoded by exons 6, 15, 17, 19, 21, 23, 25, 27, 29, 31 of human tropoelastin were synthesized, and their conformations were studied in different solutions and at different temperatures by CD and (1)H NMR. The results demonstrated the presence of poly-proline II helix (PPII) in aqueous solvent and of alpha-helical conformation in TFE. The (1)H NMR results allowed the precise localization of the helices along the peptide sequence. These data were further refined by prediction algorithms in order to take into account the reduced helix stability at the end of the peptides. Furthermore, the influence of flanking residues was checked by synthesizing and by determining the structure of a peptide spanning exon 31 coded domain and the first five residues of the following exon 32 coded domain. These studies, together with those previously published [Tamburro, A. M., Bochicchio, B., and Pepe, A. (2003) Biochemistry 42, 13147-62], are used to propose a coherent recomposition of the elastin pieces (domains) in order to give an acceptable solution to the elastin structure-function problem.