AFM Study of the Elastin-like Biopolymer Poly(ValGlyGlyValGly)

A bio-inspired silkworm 3D cocoon-like hierarchical self-assembled structure from π-conjugated natural aromatic amino acids

The formation of spontaneous 3D self-assembled hierarchical structures from 1D nanofibers is a significant breakthrough in materials science. Overcoming the major challenges associated with developing these 3D structures, such as uncontrolled self-assembly, complex procedures, and machinery, has been a formidable task. However, the current discovery reveals that simple π-system (fluorenyl)-functionalized natural aromatic amino acids, phenylalanine (Fmoc-F) and tyrosine (Fmoc-Y), can form bio-inspired 3D cocoon-like structures. These structures are composed of entangled 1D nanofibers created through supramolecular self-assembly using a straightforward one-step process of solvent casting. The self-assembly process relies on π-π stacking of the fluorenyl (π-system) moieties and intermolecular hydrogen bonding between urethane amide groups. The cocoon-like structures are versatile and independent of concentration, temperature, and humidity, making them suitable for various applications. This discovery has profound implications for materials science and the developed advanced biomaterials, such as Fmoc-F and Fmoc-Y, can serve as flexible foundational components for constructing 3D fiber-based structures.

Atomic Force Microscopy (AFM) on Biopolymers and Hydrogels for Biotechnological Applications-Possibilities and Limits

Atomic force microscopy (AFM) is one of the microscopic techniques with the highest lateral resolution. It can usually be applied in air or even in liquids, enabling the investigation of a broader range of samples than scanning electron microscopy (SEM), which is mostly performed in vacuum. Since it works by following the sample surface based on the force between the scanning tip and the sample, interactions have to be taken into account, making the AFM of irregular samples complicated, but on the other hand it allows measurements of more physical parameters than pure topography. This is especially important for biopolymers and hydrogels used in tissue engineering and other biotechnological applications, where elastic properties, surface charges and other parameters influence mammalian cell adhesion and growth as well as many other effects. This review gives an overview of AFM modes relevant for the investigations of biopolymers and hydrogels and shows several examples of recent applications, focusing on the polysaccharides chitosan, alginate, carrageenan and different hydrogels, but depicting also a broader spectrum of materials on which different AFM measurements are reported in the literature.

Insights into the biophysical forces between proteins involved in elastic fiber assembly

Elastogenesis is a complex process beginning with transcription, translation, and extracellular release of precursor proteins leading to crosslinking, deposition, and assembly of ubiquitous elastic fibers. While the biochemical pathways by which elastic fibers are assembled are known, the biophysical forces mediating the interactions between the constituent proteins are unknown. Using atomic force microscopy, we quantified the adhesive forces among the elastic fiber components, primarily between tropoelastin, elastin binding protein (EBP), fibrillin-1, fibulin-5, and lysyl oxidase-like 2 (LOXL2). The adhesive forces between tropoelastin and other tissue-derived proteins such as insoluble elastin, laminin, and type I collagens were also assessed. The adhesive forces between tropoelastin and laminin were strong (1767 ± 126 pN; p < 10-5vs. all others), followed by forces (≥200 pN) between tropoelastin and human collagen, mature elastin, or tropoelastin. The adhesive forces between tropoelastin and rat collagen, EBP, fibrillin-1, fibulin-5, and LOXL2 coated on fibrillin-1 were in the range of 100-200 pN. The forces between tropoelastin and LOXL2, LOXL2 and fibrillin-1, LOXL2 and fibulin-5, and fibrillin-1 and fibulin-5 were less than 100 pN. Introducing LOXL2 decreased the adhesive forces between the tropoelastin monomers by ∼100 pN. The retraction phase of force-deflection curves was fitted to the worm-like chain model to calculate the rigidity and flexibility of these proteins as they unfolded. The results provided insights into how each constituent's stretching under deformation contributes to structural and mechanical characteristics of these fibers and to elastic fiber assembly.

In Vitro Innovation of Tendon Tissue Engineering Strategies

Tendinopathy is the term used to refer to tendon disorders. Spontaneous adult tendon healing results in scar tissue formation and fibrosis with suboptimal biomechanical properties, often resulting in poor and painful mobility. The biomechanical properties of the tissue are negatively affected. Adult tendons have a limited natural healing capacity, and often respond poorly to current treatments that frequently are focused on exercise, drug delivery, and surgical procedures. Therefore, it is of great importance to identify key molecular and cellular processes involved in the progression of tendinopathies to develop effective therapeutic strategies and drive the tissue toward regeneration. To treat tendon diseases and support tendon regeneration, cell-based therapy as well as tissue engineering approaches are considered options, though none can yet be considered conclusive in their reproduction of a safe and successful long-term solution for full microarchitecture and biomechanical tissue recovery. In vitro differentiation techniques are not yet fully validated. This review aims to compare different available tendon in vitro differentiation strategies to clarify the state of art regarding the differentiation process.

Dependence of Membrane Tether Strength on Substrate Rigidity Probed by Single-Cell Force Spectroscopy

Substrate rigidity modulates cell mechanics, which affect cell migration and proliferation. Quantifying the effects of substrate rigidity on cancer cell mechanics requires a quantifiable parameter that can be measured for individual cells, as well as a substrate platform with rigidity being the only variable. Here we used single-cell force spectroscopy to pull cancer cells on substrates varying only in rigidity, and extracted a parameter from the force-distance curves to be used to quantify the properties of membrane tethers. Our results showed that tether force increases with substrate rigidity until it reaches its asymptotic limit. The variations are similar for all three cancer cell lines studied, and the largest change occurs in the rigidity regions of softer tissues, indicating a universal response of cancer cell elasticity to substrate rigidity.

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.

Self-assembly in elastin-like recombinamers: a mechanism to mimic natural complexity

The topic of self-assembled structures based on elastin-like recombinamers (ELRs, i.e., elastin-like polymers recombinantly bio-produced) has released a noticeable amount of references in the last few years. Most of them are intended for biomedical applications. In this review, a complete revision of the bibliography is carried out. Initially, the self-assembly (SA) concept is considered from a general point of view, and then ELRs are described and characterized based on their intrinsic disorder. A classification of the different self-assembled ELR-based structures is proposed based on their morphologies, paying special attention to their tentative modeling. The impact of the mechanism of SA on these biomaterials is analyzed. Finally, the implications of ELR SA in biological systems are considered.

Visualization of the temperature dependent rearrangement of SynB1 elastin-like polypeptide on silica using scanning electron microscopy

In this study, scanning electron microscopy (SEM) was used to observe the interaction between de-solvated SynB1-elastin-like polypeptide (SynB1-ELP) and silica at a temperature above ELP's lower critical solution temperature (LCST). ELP was seen to initially wet the surface of the silica before rearranging to form narrowly distributed spherical particles. After formation, the ELP particles dynamically rearranged to increase and subsequently decrease in size until 24 h at which time they collapsed. SEM and Energy Dispersive X-ray Spectroscopy revealed that the formation of a thin layer of salt from the PBS solution preceded the initial wetting of ELP on silica, which was shown to play a role in the continuous rearrangement of ELP. FT-IR revealed that the salt, in combination with the hydrophilic silica, trapped water that provided a repulsive surface to the hydrophobic ELP and forced the ELP to continuously minimize its surface area until the water evaporated. This behavior shows that ELP's thermo-responsive nature coupled with its hydrophobicity can be used to create ELP particles and surfaces that can reorganize with minimal water present.

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.

Beaded nanofibers assembled from double-hydrophobic elastin-like block polypeptides: Effects of trifluoroethanol

A "double-hydrophobic" elastin-like triblock polypeptide GPG has been constructed by mimicking the localization of proline- and glycine-rich hydrophobic domains of native elastin, a protein that provides elasticity and resilience to connective tissues. In this study, the effects of trifluoroethanol (TFE), an organic solvent that strongly affects secondary structures of polypeptides on self-assembly of GPG in aqueous solutions were systematically studied. Beaded nanofiber formation of GPG, where nanoparticles are initially formed by coacervation of the polypeptides followed by their connection into one-dimensional nanostructures, is accelerated by the addition of TFE at the concentrations up to 30% (v/v), whereas aggregates of nanoparticles are formed at 60% TFE. The concentration-dependent assembly pattern discussed is based on the influence of TFE on the secondary structures of GPG. Well-defined nanofibers whose diameter and secondary structures are controlled by TFE concentration may be ideal building blocks for constructing bioelastic materials in tissue engineering.

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.

Fabricated Elastin

The mechanical stability, elasticity, inherent bioactivity, and self-assembly properties of elastin make it a highly attractive candidate for the fabrication of versatile biomaterials. The ability to engineer specific peptide sequences derived from elastin allows the precise control of these physicochemical and organizational characteristics, and further broadens the diversity of elastin-based applications. Elastin and elastin-like peptides can also be modified or blended with other natural or synthetic moieties, including peptides, proteins, polysaccharides, and polymers, to augment existing capabilities or confer additional architectural and biofunctional features to compositionally pure materials. Elastin and elastin-based composites have been subjected to diverse fabrication processes, including heating, electrospinning, wet spinning, solvent casting, freeze-drying, and cross-linking, for the manufacture of particles, fibers, gels, tubes, sheets and films. The resulting materials can be tailored to possess specific strength, elasticity, morphology, topography, porosity, wettability, surface charge, and bioactivity. This extraordinary tunability of elastin-based constructs enables their use in a range of biomedical and tissue engineering applications such as targeted drug delivery, cell encapsulation, vascular repair, nerve regeneration, wound healing, and dermal, cartilage, bone, and dental replacement.

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.

Elastin-like polypeptides as a promising family of genetically-engineered protein based polymers

Elastin-like polypeptides (ELP) are artificial, genetically encodable biopolymers, belonging to elastomeric proteins, which are widespread in a wide range of living organisms. They are composed of a repeating pentapeptide sequence Val-Pro-Gly-Xaa-Gly, where the guest residue (Xaa) can be any naturally occurring amino acid except proline. These polymers undergo reversible phase transition that can be triggered by various environmental stimuli, such as temperature, pH or ionic strength. This behavior depends greatly on the molecular weight, concentration of ELP in the solution and composition of the amino acids constituting ELPs. At a temperature below the inverse transition temperature (Tt), ELPs are soluble, but insoluble when the temperature exceeds Tt. Furthermore, this feature is retained even when ELP is fused to the protein of interest. These unique properties make ELP very useful for a wide variety of biomedical applications (e.g. protein purification, drug delivery etc.) and it can be expected that smart biopolymers will play a significant role in the development of most new materials and technologies. Here we present the structure and properties of thermally responsive elastin-like polypeptides with a particular emphasis on biomedical and biotechnological application.

NANO AND MICRO-STRUCTURES OF ELASTIN-LIKE POLYPEPTIDE-BASED MATERIALS AND THEIR APPLICATIONS: RECENT DEVELOPMENTS

Elastin-like polypeptide (ELP) containing materials have spurred significant research interest for biomedical applications exploiting their biocompatible, biodegradable and nonimmunogenic nature while maintaining precise control over their chemical structure and functionality through genetic engineering. Physical, mechanical and biological properties of ELPs could be further manipulated using genetic engineering or through conjugation with a variety of chemical moieties. These chemical and physical modifications also achieve interesting micro- and nanostructured ELP-based materials. Here, we review the recent developments during the past decade in the methods to engineer elastin-like materials, available genetic and chemical modification methods and applications of ELP micro and nanostructures in tissue engineering and drug delivery.

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.

Coacervation of tropoelastin

The coacervation of tropoelastin represents the first major stage of elastic fiber assembly. The process has been modeled in vitro by numerous studies, initially with mixtures of solubilized elastin, and subsequently with synthetic elastin peptides that represent hydrophobic repeat units, isolated hydrophobic domains, segments of alternating hydrophobic and cross-linking domains, or the full-length monomer. Tropoelastin coacervation in vitro is characterized by two stages: an initial phase separation, which involves a reversible inverse temperature transition of monomer to n-mer; and maturation, which is defined by the irreversible coalescence of coacervates into large species with fibrillar structures. Coacervation is an intrinsic ability of tropoelastin. It is primarily influenced by the number, sequence, and contextual arrangement of hydrophobic domains, although hydrophilic sequences can also affect the behavior of the hydrophobic domains and thus affect coacervation. External conditions including ionic strength, pH, and temperature also directly influence the propensity of tropoelastin to self-associate. Coacervation is an endothermic, entropically-driven process driven by the cooperative interactions of hydrophobic domains following destabilization of the clathrate-like water shielding these regions. The formation of such assemblies is believed to follow a helical nucleation model of polymerization. Coacervation is closely associated with conformational transitions of the monomer, such as increased β-structures in hydrophobic domains and α-helices in cross-linking domains. Tropoelastin coacervation in vivo is thought to mainly involve the central hydrophobic domains. In addition, cell-surface glycosaminoglycans and microfibrillar proteins may regulate the process. Coacervation is essential for progression to downstream elastogenic stages, and impairment of the process can result in elastin haploinsufficiency disorders such as supravalvular aortic stenosis.

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.

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.

Recombinamers: combining molecular complexity with diverse bioactivities for advanced biomedical and biotechnological applications

The rapid development of polymer science has led to literally thousands of different monomers and an almost endless number of possibilities arising from their combination. The most promising strategy to date has been to consider natural products as macromolecules that provide the best option for obtaining functional materials. Proteins, with their high levels of complexity and functionality, are one of the best examples of this approach. In addition, the development of genetic engineering has permitted the design and highly controlled synthesis of proteinaceous materials with complex and advanced functionalities. Elastin-like recombinamers (ELRs) are presented herein as an example of an extraordinary convergence of different properties that is not found in any other synthetic polymer system. These materials are highly biocompatible, stimuli-responsive, show unusual self-assembly properties, and can incorporate bioactive domains and other functionalities along the polypeptide chain. These attributes are an important factor in the development of biomedical and biotechnological applications such as tissue engineering, drug delivery, purification of recombinant proteins, biosensors or stimuli-responsive surfaces.

The effect of PEO length on the self-assembly of poly(ethylene oxide)-tetrapeptide conjugates prepared by "Click" chemistry

Two series of poly(ethylene oxide)-tetrapeptide conjugates have been prepared using a "Click" reaction between an alkyne-modified tetra(phenylalanine) or tetra(valine) and various azide-terminated poly(ethylene oxide) (PEO) oligomers. Three different PEO precursors were used to prepare these conjugates, with number-average molecular weights of 350, 1200, and 1800 Da. Assembly of mPEO-F4-OEt and mPEO-V4-OEt conjugates was achieved by dialysis of a THF solution of the conjugate against water or by direct aqueous rehydration of a thin film. The PEO length has a profound effect on the outcome of the self-assembly, with the F4 conjugates giving rise to nanotubes, fibers, and wormlike micelles, respectively, as the length of the PEO block is increased. For the V4 series, the propensity to form beta-sheets dominates, and hence, the self-assembled structures are reminiscent of those formed by peptides alone, even at the longer PEO lengths. Thus, this systematic study demonstrates that the self-assembly of PEO-peptides depends on both the nature of the peptides and the relative PEO block length.

A never-ending love story with elastin: a scientific autobiography

The author describes, in a quite unconventional way, the most important results achieved in the last 50 years in the field of elastin structure–elasticity relationships, beginning with the first invaluable findings of Partridge on desmosines and isodesmosines until the most recent theories on elastomeric proteins. The author also relates a scientific autobiography characterized by his greatest passion, elastin.

Soft hydrogels from nanotubes of poly(ethylene oxide)-tetraphenylalanine conjugates prepared by click chemistry

A new poly(ethylene oxide)-tetraphenylalanine polymer-peptide conjugate has been prepared via a "click" reaction between an alkyne-modified peptide and an azide-terminated PEO oligomer. Self-assembled nanotubes are formed after dialysis of a THF solution of this polymer-peptide conjugate against water. The structure of these nanotubes has been probed by circular dichroism, IR, TEM, and SAXS. From these data, it is apparent that self-assembly involves the formation of antiparallel beta-sheets and pi-pi-stacking. Nanotubes are formed at concentrations between 2 and 10 mg mL(-1). Entanglement between adjacent nanotubes occurs at higher concentrations, resulting in the formation of soft hydrogels. Gel strength increases at higher polymer-peptide conjugate concentration, as expected.

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.

Effects of Hofmeister anions on the phase transition temperature of elastin-like polypeptides

The modulation of the lower critical solution temperature (LCST) of two elastin-like polypeptides (ELPs) was investigated in the presence of 11 sodium salts that span the Hofmeister series for anions. It was found that the hydrophobic collapse/aggregation of these ELPs generally followed the series. Specifically, kosmotropic anions decreased the LCST by polarizing interfacial water molecules involved in hydrating amide groups on the ELPs. On the other hand, chaotropic anions lowered the LCST through a surface tension effect. Additionally, chaotropic anions showed salting-in properties at low salt concentrations that were related to the saturation binding of anions with the biopolymers. These overall mechanistic effects were similar to those previously found for the hydrophobic collapse and aggregation of poly(N-isopropylacrylamide), PNIPAM. There is, however, a crucial difference between PNIPAM and ELPs. Namely, PNIPAM undergoes a two-step collapse process as a function of temperature in the presence of sufficient concentrations of kosmotropic salts. By contrast, ELPs undergo collapse in a single step in all cases studied herein. This suggests that the removal of water molecules from around the amide moieties triggers the removal of hydrophobic hydration waters in a highly coupled process. There are also some key differences between the LCST behavior of the two ELPs. Specifically, the more hydrophilic ELP V5A2G(3)-120 construct displays collapse/aggregation behavior that is consistent with a higher concentration of anions partitioning to polymer/aqueous interface as compared to the more hydrophobic ELP V(5)-120. It was also found that larger anions could bind with ELP V5A2G(3)-120 more readily in comparison with ELP V(5)-120. These latter results were interpreted in terms of relative binding site accessibility of the anion for the ELP.

Dynamic light scattering and atomic force microscopy imaging on fragments of beta-connectin from human cardiac muscle

In order to investigate the protein folding-unfolding process, dynamic light scattering (DLS) and atomic force microscopy (AFM) imaging were used to study two fragments of the muscle cardiac protein beta-connectin, also known as titin. Both fragments belong to the I band of the sarcomer, and they are composed of four domains from I(27) to I(30) (tetramer) and eight domains from I(27) to I(34) (octamer). DLS measurements provide the size of both fragments as a function of temperature from 20 up to 86 degrees C, and show a thermal denaturation due to temperature increase. AFM imaging of both fragments in the native state reveals a homogeneous and uniform distribution of comparable structures. The DLS and AFM techniques turn out to be complementary for size measurements of the fragments and fragment aggregates. An unexpected result is that the octamer folds into a smaller structure than the tetramer and the unfolded octamer is also smaller than the unfolded tetramer. This feature seems related to the significance of the hydrophobic interactions between domains of the fragment. The longer the fragment, the more easily the hydrophobic parts of the domains interact with each other. The fragment aggregation behavior, in particular conditions, is also revealed by both DLS and AFM as a process that is parallel to the folding-unfolding transition.

Charge transport and intrinsic fluorescence in amyloid-like fibrils

The self-assembly of polypeptides into stable, conductive, and intrinsically fluorescent biomolecular nanowires is reported. We have studied the morphology and electrical conduction of fibrils made of an elastin-related polypeptide, poly(ValGlyGlyLeuGly). These amyloid-like nanofibrils, with a diameter ranging from 20 to 250 nm, result from self-assembly in aqueous solution at neutral pH. Their morphological properties and conductivity have been investigated by atomic force microscopy, scanning tunneling microscopy, and two-terminal transport experiments at the micro- and nanoscales. We demonstrate that the nanofibrils can sustain significant electrical conduction in the solid state at ambient conditions and have remarkable stability. We also show intrinsic blue-green fluorescence of the nanofibrils by confocal microscopy analyses. These results indicate that direct (label-free) excitation can be used to investigate the aggregation state or the polymorphism of amyloid-like fibrils (and possibly of other proteinaceous material) and open up interesting perspectives for the use of peptide-based nanowire structures, with tunable physical and chemical properties, for a wide range of nanobiotechnological and bioelectronic applications.

Nanobiotechnological approach to engineered biomaterial design: the example of elastin-like polymers

Today, the development of advanced biomaterials is still lacking an appropriate tailored engineering approach. Most of the biomaterials currently used have their origin in materials developed for other technological applications. This lack of adequate biomaterial design is probably due to the peculiar environment where those materials must operate. On the one hand, this environment is dominated by the immune rejection system. On the other hand, the functionality of natural biomolecules is based on complex topological physical-chemical function distributions at the nanometer level. This review presents arguments concerning the role of biotechnology and nanotechnology in the future development of new advanced biomaterials and the potential of these biomaterials as a way to achieve highly biofunctional and truly biocompatible biomaterials for hot areas, such as regenerative medicine and controlled release. Recombinant protein-polymers will be presented as an example of candidates for this new paradigm in biomaterial design and production.

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.

Transformation of Amyloid-like Fibers, Formed from an Elastin-Based Biopolymer, into a Hydrogel: An X-ray Photoelectron Spectroscopy and Atomic Force Microscopy Study

Previous studies have revealed the propensity of elastin-based biopolymers to form amyloid-like fibers when dissolved in water. These are of interest when considered as "ancestral units" of elastin in which they represent the simplest sequences in the hydrophobic regions of the general type XxxGlyGlyZzzGly (Xxx, Zzz = Val, Leu). We normally refer to these biopolymers based on elastin or related to elastin units as "elastin-like polypeptides". The requirement of water for the formation of amyloids seems quite interesting and deserves investigation, the water representing the natural transport medium in human cells. As a matter of fact, the "natural" supramolecular organization of elastin is in the form of beaded-string-like filaments and not in the form of amyloids whose "in vivo" deposition is associated with some important human diseases. Our work is directed, therefore, to understanding the mechanism by which such hydrophobic sequences form amyloids and any conditions by which they might regress to a non-amyloid filament. The elastin-like sequence here under investigation is the ValGlyGlyValGly pentapeptide that has been previously analyzed both in its monomer and polymer form. In particular, we have focused our investigation on the apparent stability of amyloids formed from poly(ValGlyGlyValGly), and we have observed these fibers evolving to a hydrogel after prolonged aging in water. We will show how atomic force microscopy can be combined with X-ray photoelectron spectroscopy to gain an insight into the spontaneous organization of an elastin-like polypeptide driven by interfacial interactions. The results are discussed also in light of fractal-like assembly and their implications from a biomedical point of view.

Proline and Glycine Control Protein Self-Organization into Elastomeric or Amyloid Fibrils

Elastin provides extensible tissues, including arteries and skin, with the propensity for elastic recoil, whereas amyloid fibrils are associated with tissue-degenerative diseases, such as Alzheimer's. Although both elastin-like and amyloid-like materials result from the self-organization of proteins into fibrils, the molecular basis of their differing physical properties is poorly understood. Using molecular simulations of monomeric and aggregated states, we demonstrate that elastin-like and amyloid-like peptides are separable on the basis of backbone hydration and peptide-peptide hydrogen bonding. The analysis of diverse sequences, including those of elastin, amyloids, spider silks, wheat gluten, and insect resilin, reveals a threshold in proline and glycine composition above which amyloid formation is impeded and elastomeric properties become apparent. The predictive capacity of this threshold is confirmed by the self-assembly of recombinant peptides into either amyloid or elastin-like fibrils. Our findings support a unified model of protein aggregation in which hydration and conformational disorder are fundamental requirements for elastomeric function.

Tailored recombinant elastin-like polymers for advanced biomedical and nano(bio)technological applications

The genetic engineering of protein-based polymers is a method that enables, in an easy way, the design of complex and highly functional macromolecules. As examples of this approach, different molecular designs are presented, with increasing degree of complexity, showing how the controlled increase in their complexity yields (multi)functional materials with more selected and sophisticated properties. The simplest designs show interesting properties already, but the adequate introduction of given chemical functions along the polymer chain provides an opportunity to expand the range of properties to enhanced smart behavior and self-assembly. Finally, examples are given where those molecular designs further incorporate selected bioactivities in order to develop materials for the most cutting edge applications in biomedicine and nano(bio)technology.

The dissection of human tropoelastin: from the molecular structure to the self-assembly to the elasticity mechanism

After a historical introduction the authors describe their most recent results on the structure, assembly and elasticity of elastin. Recent results obtained by analyzing the conformation of polypeptide sequences encoded by the single exons of human tropoelastin demonstrated the presence of labile conformations such as poly-proline II helix (PPII) and beta-turns whose stability is strongly dependent on the microenvironment. Stable, periodic structures, such as alpha-helices, are only present in the poly-alanine cross-linking domains. These findings give a strong experimental basis to the understanding of the molecular mechanism of elasticity of elastin. In particular, they strongly support the description of the native relaxed state of the protein in terms of trans-conformational equilibria between extended and folded structures as previously proposed [Int. J. Biochem. Cell. Biol. 31 (1999) 261]. The same polypeptide sequences have been analyzed for their ability to coacervate and to self-assembly. Although the great majority of them were shown to be able to adopt more or less organized structures, only a few were indeed able to coacervate. Studies carried out by transmission electron microscopy showed the polypeptides to adopt a variety of supramolecular structures going from a filamentous organization (typical of elastin) to amyloid-like fibers. On the whole, the results obtained gave significant insight to the roles played by specific polypeptide sequences in self-assembly and possibly in elasticity.