Synthesis and structural studies of a pentapeptide sequence of elastin. Poly (Val-Gly-Gly-Leu-Gly)

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.

Determining the Sequence Dependency of Self-Assembly of Elastin-Like Peptides Using Short Peptide Analogues with Shuffled Repetitive Sequences

Synthetic elastin-like peptides (ELPs) that possess characteristic tropoelastin-derived hydrophobic repetitive sequences, such as (VPGVG)n, exhibit thermoresponsive reversible self-assembly. Although their thermoresponsive properties have been well-studied, the sequence-dependent and structural requirements for self-assembly remain ambiguous. In particular, it is still unclear whether the amino acid sequences derived from tropoelastin are necessary for self-assembly. In this study, 11 sequence-shuffled ELP analogues based on (FPGVG)5, which is a previously developed short ELP (sELP), were designed to elucidate the sequence-dependent and structural requirements for their self-assembly. Among them, eight shuffled peptides exhibited self-assembling properties, whereas the other three peptides were difficult to dissolve in water. Structural analyses revealed that the structural characteristics of the three insoluble peptides were different from those of their thermoresponsive analogues. Furthermore, the secondary structures of the peptide analogues possessing the self-assembly abilities were different from each other. These results suggest that the potential for self-assembly and water solubility of sELPs depend on the primary structure in each repeated unit. Moreover, several shuffled analogues exhibited more potent self-assembling properties than the original (FPGVG)5, indicating that shorter ELPs can be obtained using their novel motifs as repetitive units. We also observed that the presence of Pro-Gly sequence in the repeating units was advantageous in terms of peptide solubility. Although further analysis will be necessary to elucidate the molecular mechanism underlying the self-assembly of these sELPs, this study provides insights into the relationship between the amino acid sequence and the self-assembling ability of the peptides for developing new sELPs for various applications.

Tropoelastin and Elastin Assembly

Elastic fibers are an important component of the extracellular matrix, providing stretch, resilience, and cell interactivity to a broad range of elastic tissues. Elastin makes up the majority of elastic fibers and is formed by the hierarchical assembly of its monomer, tropoelastin. Our understanding of key aspects of the assembly process have been unclear due to the intrinsic properties of elastin and tropoelastin that render them difficult to study. This review focuses on recent developments that have shaped our current knowledge of elastin assembly through understanding the relationship between tropoelastin’s structure and function.

Elastin-Based Materials: Promising Candidates for Cardiac Tissue Regeneration

Stroke and cardiovascular episodes are still some of the most common diseases worldwide, causing millions of deaths and costing billions of Euros to healthcare systems. The use of new biomaterials with enhanced biological and physical properties has opened the door to new approaches in cardiovascular applications. Elastin-based materials are biomaterials with some of the most promising properties. Indeed, these biomaterials have started to yield good results in cardiovascular and angiogenesis applications. In this review, we explore the latest trends in elastin-derived materials for cardiac regeneration and the different possibilities that are being explored by researchers to regenerate an infarcted muscle and restore its normal function. Elastin-based materials can be processed in different manners to create injectable systems or hydrogel scaffolds that can be applied by simple injection or as patches to cover the damaged area and regenerate it. Such materials have been applied to directly regenerate the damaged cardiac muscle and to create complex structures, such as heart valves or new bio-stents that could help to restore the normal function of the heart or to minimize damage after a stroke. We will discuss the possibilities that elastin-based materials offer in cardiac tissue engineering, either alone or in combination with other biomaterials, in order to illustrate the wide range of options that are being explored. Moreover, although tremendous advances have been achieved with such elastin-based materials, there is still room for new approaches that could trigger advances in cardiac tissue regeneration.

Coarse-grained model of tropoelastin self-assembly into nascent fibrils

Elastin is the dominant building block of elastic fibers that impart structural integrity and elasticity to a range of important tissues, including the lungs, blood vessels, and skin. The elastic fiber assembly process begins with a coacervation stage where tropoelastin monomers reversibly self-assemble into coacervate aggregates that consist of multiple molecules. In this paper, an atomistically based coarse-grained model of tropoelastin assembly is developed. Using the previously determined atomistic structure of tropoelastin, the precursor molecule to elastic fibers, as the basis for coarse-graining, the atomistic model is mapped to a MARTINI-based coarse-grained framework to account for chemical details of protein-protein interactions, coupled to an elastic network model to stabilize the structure. We find that self-assembly of monomers generates up to ∼70 ​nm of dense aggregates that are distinct at different temperatures, displaying high temperature sensitivity. Resulting assembled structures exhibit a combination of fibrillar and globular substructures within the bulk aggregates. The results suggest that the coalescence of tropoelastin assemblies into higher order structures may be reinforced in the initial stages of coacervation by directed assembly, supporting the experimentally observed presence of heterogeneous cross-linking. Self-assembly of tropoelastin is driven by interactions of specific hydrophobic domains and the reordering of water molecules in the system. Domain pair orientation analysis throughout the self-assembly process at different temperatures suggests coacervation is a driving force to orient domains for heterogeneous downstream cross-linking. The model provides a framework to characterize macromolecular self-assembly for elastin, and the formulation could easily be adapted to similar assembly systems.

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.

Bioactive scaffolds based on elastin-like materials for wound healing

Wound healing is a complex process that, in healthy tissues, starts immediately after the injury. Even though it is a natural well-orchestrated process, large trauma wounds, or injuries caused by acids or other chemicals, usually produce a non-elastic deformed tissue that not only have biological reduced properties but a clear aesthetic effect. One of the main drawbacks of the scaffolds used for wound dressing is the lack of elasticity, driving to non-elastic and contracted tissues. In the last decades, elastin based materials have gained in importance as biomaterials for tissue engineering applications due to their good cyto- and bio-compatibility, their ease handling and design, production and modification. Synthetic elastin or elastin like-peptides (ELPs) are the two main families of biomaterials that try to mimic the outstanding properties of natural elastin, elasticity amongst others; although there are no in vivo studies that clearly support that these two families of elastin based materials improve the elasticity of the artificial scaffolds and of the regenerated skin. Within the next pages a review of the different forms (coacervates, fibres, hydrogels and biofunctionalized surfaces) in which these two families of biomaterials can be processed to be applied in the wound healing field have been done. Here, we explore the mechanical and biological properties of these scaffolds as well as the different in vivo approaches in which these scaffolds have been used.

Stepwise Mechanism of Temperature-Dependent Coacervation of the Elastin-like Peptide Analogue Dimer, (C(WPGVG)3)2

Elastin-like peptides (ELPs) are distinct, repetitive, hydrophobic sequences, such as (VPGVG) _n, that exhibit coacervation, the property of reversible, temperature-dependent self-association and dissociation. ELPs can be found in elastin and have been developed as new scaffold biomaterials. However, the detailed relationship between their amino acid sequences and coacervation properties remains obscure because of the structural flexibility of ELPs. In this study, we synthesized a novel, dimeric ELP analogue (H-C(WPGVG)₃-NH₂)₂, henceforth abbreviated (CW3)2, and analyzed its self-assembly properties and structural factors as indicators of coacervation. Turbidity measurements showed that (CW3)2 demonstrated coacervation at a concentration much lower than that of its monomeric form and another ELP. In addition, the coacervate held water-soluble dye molecules. Thus, potent and distinct coacervation was obtained with a remarkably short sequence of (CW3)2. Furthermore, fluorescence microscopy, dynamic light scattering, and optical microscopy revealed that the coacervation of (CW3)2 was a stepwise process. The structural factors of (CW3)2 were analyzed by molecular dynamics simulations and circular dichroism spectroscopy. These measurements indicated that helical structures primarily consisting of proline and glycine became more disordered at high temperatures with concurrent, significant exposure of their hydrophobic surfaces. This extreme change in the hydrophobic surface contributes to the potent coacervation observed for (CW3)2. These results provide important insights into more efficient applications of ELPs and their analogues, as well as the coacervation mechanisms of ELP and elastin.

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.

Tropoelastin: a versatile, bioactive assembly module

Elastin provides structural integrity, biological cues and persistent elasticity to a range of important tissues, including the vasculature and lungs. Its critical importance to normal physiology makes it a desirable component of biomaterials that seek to repair or replace these tissues. The recent availability of large quantities of the highly purified elastin monomer, tropoelastin, has allowed for a thorough characterization of the mechanical and biological mechanisms underpinning the benefits of mature elastin. While tropoelastin is a flexible molecule, a combination of optical and structural analyses has defined key regions of the molecule that directly contribute to the elastomeric properties and control the cell interactions of the protein. Insights into the structure and behavior of tropoelastin have translated into increasingly sophisticated elastin-like biomaterials, evolving from classically manufactured hydrogels and fibers to new forms, stabilized in the absence of incorporated cross-linkers. Tropoelastin is also compatible with synthetic and natural co-polymers, expanding the applications of its potential use beyond traditional elastin-rich tissues and facilitating finer control of biomaterial properties and the design of next-generation tailored bioactive materials.

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.

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.

Structural disorder and dynamics of elastin

Elastin is a self-assembling, extracellular-matrix protein that is the major provider of tissue elasticity. Here we review structural studies of elastin from over four decades, and draw together evidence for solution flexibility and conformational disorder that is inherent in all levels of structural organization. The characterization of disorder is consistent with an entropy-driven mechanism of elastic recoil. We conclude that conformational disorder is a constitutive feature of elastin structure and function.

Chemical conjugation of linear and cyclic RGD moieties to a recombinant elastin-mimetic polypeptide--a versatile approach towards bioactive protein hydrogels

An elastin-mimetic polypeptide, (EMM)(7), with the amino-acid sequence GRDPSS [VPGVG VPGKG VPGVG VPGVG VPGEG VPGIG](7) was used for chemical conjugation of various integrin ligands (RGD peptides) to prepare bioactive hydrogels. The chemical approach involved (1) chemical protection of lysine residues with Fmoc or Boc groups, (2) chemical ligation of a protected linear or cyclic RGD ligand, with or without a hexanoic-acid spacer to the glutamic acid residue, (3) deprotection of the lysine functionalities and the RGD moieties and (4) cross-linking to form a bioactive hydrogel. (1)H NMR spectroscopy was used to quantify the multiple steps in the reaction. The chemical protection was found to be between 65 and 93% for Fmoc and Boc, respectively. The ligands studied included linear RGD cell-binding [H-FGRGDS-OH (1-l-RGD), H-Ahx--FGRGDS-OH (2-Ahx-FGRGDS) and a cyclic -H(2)N-(CH(2))(6)COHN-cyclo(-RGDfK-) (H-Ahx-c(-RGDfK-)) peptide also with a hexanoic-acid spacer. Cell adhesion with mouse osteoblast cells was dependent on the ligand type, ligand density and the use of a spacer.

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.

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.

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.

Flexibility in the solution structure of human tropoelastin

We investigated the flexibility of full-length tropoelastin in solution by using far- and near-ultraviolet circular dichroism (UV CD) and fluorescence spectroscopy to probe for structural flexibility and residue mobility within secondary and tertiary features of the monomer. Fluorescence spectroscopy revealed the presence of exposed hydrophobicity through the binding of the hydrophobic probe 4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonate (bis-ANS), which demonstrates that hydrophobic regions form clusters and are not confined to a molecular core. Near-UV CD indicated substantial mobility of aromatic residues. Structural prediction programs (PONDR, DisEMBL, and Globplot version 2.0) estimated 75 +/- 2% disorder in the tertiary structure of tropoelastin on the basis of primary sequence information. A single-site substitution of Trp for Gln (Q513W) at the tropoelastin domain 25-26 interface facilitated fluorescence spectroscopy for revealing that this region is exposed to solvent. Polarization anisotropy demonstrated substantial flexibility of W513 and little change upon denaturation of the monomer with guanidine hydrochloride. Comparable movement was found for native sequence aromatic residues in the presence of glycosaminoglycans and trifluoroethanol. These data prove the intrinsic flexibility of specific residues and adjacent sequences in any native conformation(s) they may take. This study is the first characterization of the level of mobility in defined regions of the full-length tropoelastin monomer and provides direct evidence for regions of flexible structure in tropoelastin.

Tropoelastin massively associates during coacervation to form quantized protein spheres

Tropoelastin, the precursor of elastin, undergoes a rapid monomer to multimer association in an inverse temperature transition. This association culminates in the rapid formation of stable, optically distinct droplets of tropoelastin. Light scattering and microscope measurements reveal that these droplets are 2-6 microm in diameter. Scanning electron microscopy confirms that the droplets are spherical. Three-dimensional confocal image stacks based on the autofluorescence of tropoelastin reveal that droplets are loaded with hydrated tropoelastin. Droplets are viable intermediates in synthetic elastin macroassembly. Dense clusters of aggregated droplets and partially formed fibers develop when droplets are incubated in the presence of a lysyl oxidase. Lysine-reacting chemical and enzyme-assisted cross-linking conditions generate cross-linked beads due to interactions between multiple, surface-exposed lysine epsilon-amino groups. Droplets represent an efficient mechanism for the bolus delivery during elastogenesis of quantized packages of preaccreted tropoelastin.

Cooperativity between the Hydrophobic and Cross-linking Domains of Elastin

The principal protein component of the elastic fiber found in elastic tissues is elastin, an amorphous, cross-linked biopolymer that is assembled from a high molecular weight monomer. The hydrophobic and cross-linking domains of elastin have been considered separate and independent, such that changes to one region are not thought to affect the other. However, results from these solid-state 13C NMR experiments demonstrate that cooperativity in protein folding exists between the two domain types. The sequence of the EP20-24-24 polypeptide has three hydrophobic sequences from exons 20 and 24 of the soluble monomer tropoelastin, interspersed with cross-linking domains constructed from exons 21 and 23. In the middle of each cross-linking domain is a "hinge" sequence. When this pentapeptide is replaced with alanines, as in EP20-24-24[23U], its properties are changed. In addition to the expected increase in alpha-helical content and the resulting increase in rigidity of the cross-linking domains, changes to the organization of the hydrophobic regions are also observed. Using one-dimensional CPMAS (cross-polarization with magic angle spinning) techniques, including spectral editing and relaxation measurements, evidence for a change in dynamics to both domain types is observed. Furthermore, it is likely that the methyl groups of the leucines of the hydrophobic domains are also affected by the substitution to the hinge region of the cross-linking sequences. This cooperativity between the two domain types brings new questions to the phenomenon of coacervation in elastin polypeptides and strongly suggests that functional models for the protein must include a role for the cross-linking regions.

Specificity in the coacervation of tropoelastin: solvent exposed lysines

Tropoelastin protein monomers associate by coacervation and are cross-linked in vivo to form elastin macro-assemblies. We provide evidence for specific protein domain contact points between tropoelastin monomers during association by coacervation. The homobifunctional cross-linker bis(sulfosuccinimidyl) suberate served as a rapid reporter of adjacent lysines and preferentially exposed domains. Intact cross-linked peptide pairs were identified after protease digestion and high-resolution electrospray mass spectrometry followed by MS/MS sequencing. Mapping of the assigned sequences indicated that the region in the monomer spanning domains 19-25 was readily accessible to solvent and enriched in cross-linking. Domains 12 and 36 were also prevalent, where these two regions were not previously thought to play a major role in the formation of mature elastin. A specificity for particular lysines allowed for the construction of a model for the first close contacts between domains and the first detailed study of the cross-linking of tropoelastin.

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.

Dissection of human tropoelastin: supramolecular organization of polypeptide sequences coded by particular exons

Polypeptide sequences encoded by some exons of the human tropoelastin gene (EDP, elastin-derived peptide) have been analysed for their ability to coacervate and to self-assembly. The great majority of them were shown to form organized structures, but only a few were indeed able to coacervate. Negative staining and rotary shadowing transmission electron microscopy showed the polypeptides to adopt a variety of supramolecular organization, from filaments, as those typical of tropoelastin, to amyloid-like fibers. The results obtained gave significant insight to the possible roles played by specific polypeptide sequences of tropoelastin.

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

In this paper, we report an AFM study on the supramolecular structures adopted by the synthetic polypentapeptide poly(ValGlyGlyValGly), whose monomeric sequence is an abundant, simple building block of elastin. The polypeptide was analyzed by deposition from both methanolic and aqueous suspensions, showing different behaviors. In methanol, the polypeptide is able to evolve, in a time-dependent way, from layers to ribbons to beaded filaments. When the equilibrium is reached, the formation of well-defined dendritic structures is also observed. This restructuring of the polypentapeptide seems to be reminiscent of a sort of Rayleigh instability. When deposited from aqueous suspensions, the polypeptide self-assembles either in fibrillar networks or in amyloid-like patterns, both of them being found in elastin or elastin-related polypeptides. As a general finding, poly(ValGlyGlyValGly) seems to constitute an excellent mimetic of the supramolecular properties of native elastin.

Dissection of human tropoelastin: exon-by-exon chemical synthesis and related conformational studies

Polypeptide sequences encoding the single exons of human tropoelastin were synthesized and their conformations were studied in different solvents and at different temperatures by CD and (1)H NMR. The results 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 [Debelle, L., and Tamburro, A. M. (1999) Int. J. Biochem. Cell. Biol. 31, 261-272].

Structural changes and facilitated association of tropoelastin

Circular dichroism studies of tropoelastin secondary structure show 4+/-1% alpha-helix in aqueous solutions. This is in contrast to the substantially higher amounts (up to 23+/-7%) of alpha-helix predicted by computer algorithms, which propose that regions of alpha-helix are limited to the alanine-rich cross-linking domains. Through the addition of trifluoroethanol, the amount of alpha-helix increased to 17+/-1%, equivalent to that expected on the basis of primary structure. The physiological ability of the protein to coacervate and the critical concentration of monomer required for coacervation were unaffected by levels of alpha-helix. However, the temperature required for coacervation decreased linearly with increasing alpha-helical structure, which correlates with the participation of alpha-helices in association. We propose that the alanine-rich cross-linking domains exist as nascent helices in tropoelastin in aqueous solution. We further suggest a novel mechanism for coacervation whereby formation of alpha-helices and subsequent helical side chain interactions limit the conformational flexibility of the polypeptide, to facilitate associations between hydrophobic domains during elastogenesis.

Molecular mobility of elastin: effect of molecular architecture

The thermal and dielectric properties of elastin and two soluble derivatives (kappa-elastin and derived elastin peptides from enzymatic elastolysis) were investigated in the freeze-dried state in a wide temperature range (from -180 to +220 degrees C). The glass transition of these amorphous proteins was studied by differential scanning calorimetry (DSC). The dielectric relaxations of both proteins were followed by thermally stimulated currents (TSC), an isochronal dielectric spectrometry running at variable temperature, analogous to a low-frequency spectroscopy (10(-3)-10(-2) Hz) and by dynamic dielectric spectroscopy (DDS), performed isothermally with the frequency varying from 10(-2) to 3 x 10(6) Hz. The combination of TSC and DDS experiments and the determination of the activation parameters of the relaxation times inform about the molecular mobility of the proteins, both in the glassy state and in the liquid state. Major differences between the relaxation behavior of elastin and its soluble derivatives have been discussed and correlated with the molecular architecture of the proteins.

Elastin-based biopolymers: chemical synthesis and structural characterization of linear and cross-linked poly(OrnGlyGlyOrnGly)

Poly(OrnGlyGlyOrnGly) was synthesized by classical procedures in solution. The monomeric sequence -OrnGlyGlyOrnGly- was chosen as a modification of -ValGlyGlyValGly-, typical of elastin, to impart primary amine functionality, susceptible to cross-linking with appropriate bifunctional reagents. Herein we focus on the cross-linking of poly(OrnGlyGlyOrnGly) with glutaraldehyde. The polymers, both linear and cross-linked, were characterized and investigated for their molecular and supramolecular properties. Circular dichroism studies performed on linear poly(OrnGlyGlyOrnGly) revealed a variety of conformations similar to elastin. At a supramolecular level, different kinds of aggregates were found such as the elastin-like twisted-rope pattern of filaments and fibrils, together with other specific morphologies, similar to those recently identified in some elastin-mimetic polypeptides.

Elastin as a self-organizing biomaterial: use of recombinantly expressed human elastin polypeptides as a model for investigations of structure and self-assembly of elastin

Elastin is the major extracellular matrix protein of large arteries such as the aorta, imparting characteristics of extensibility and elastic recoil. Once laid down in tissues, polymeric elastin is not subject to turnover, but is able to sustain its mechanical resilience through thousands of millions of cycles of extension and recoil. Elastin consists of ca. 36 domains with alternating hydrophobic and cross-linking characteristics. It has been suggested that these hydrophobic domains, predominantly containing glycine, proline, leucine and valine, often occurring in tandemly repeated sequences, are responsible for the ability of elastin to align monomeric chains for covalent cross-linking. We have shown that small, recombinantly expressed polypeptides based on sequences of human elastin contain sufficient information to self-organize into fibrillar structures and promote the formation of lysine-derived cross-links. These cross-linked polypeptides can also be fabricated into membrane structures that have solubility and mechanical properties reminiscent of native insoluble elastin. Understanding the basis of the self-organizational ability of elastin-based polypeptides may provide important clues for the general design of self-assembling biomaterials.

Hydrophobic domains of human tropoelastin interact in a context-dependent manner

Tropoelastin is the soluble precursor of elastin, the major component of the extracellular elastic fiber. Tropoelastin undergoes self-association via an inverse temperature transition termed coacervation, which is a crucial step in elastogenesis. Coacervation of tropoelastin takes place through multiple intermolecular interactions of its hydrophobic domains. Previous work has implicated those hydrophobic domains located near the center of the polypeptide as playing a dominant role in coacervation. Short constructs of domains 18, 20, 24, and a mutated form of domain 26 were largely disordered at 20 degrees C but displayed increased order on heating that was consistent with the formation of beta-structures. However, their conformational transitions were not sensitive to physiological temperature in contrast to the observed behavior of the native domain 26. A polypeptide consisting of domains 17-27 of tropoelastin coacervated at temperatures above 60 degrees C, whereas individually expressed hydrophobic regions were not capable of coacervation. We conclude that coacervation depends on the hydrophobicity of the molecule and, by inference, the number of hydrophobic domains. Tropoelastin mutants were constructed to contain a Pro --> Ala mutation in domain 26, separate deletions of domains 18 and 26, and a displacement of domain 26. These constructs displayed unequal capacities for coacervation, even when they contained the same number of hydrophobic regions and comparable levels of secondary structure. Thus, the capability for coacervation is determined by contributions from individual hydrophobic domains for which function should be considered in the context of their positions in the intact tropoelastin molecule.

Self-aggregation characteristics of recombinantly expressed human elastin polypeptides

Elastin is an extracellular matrix protein found in tissues requiring extensibility and elastic recoil. Monomeric elastin has the ability to aggregate into fibrillar structures in vitro, and has been suggested to participate in the organization of its own assembly into a polymeric matrix in vivo. Although hydrophobic sequences in elastin have been suggested to be involved in this process of self-organization, the contributions of specific hydrophobic and crosslinking domains to the propensity of elastin to self-assemble have received less attention. We have used a series of defined, recombinant human elastin polypeptides to investigate the factors contributing to elastin self-assembly. In general, coacervation temperature of these polypeptides, used as a measure of their propensity to self-assemble, was influenced both by salt concentration and polypeptide concentration. In addition, hydrophobic domains appeared to be essential for the ability of these polypeptides to self-assemble. However, neither overall molecular mass, number of hydrophobic domains nor general hydropathy of the polypeptides provided a complete explanation for differences in coacervation temperature, suggesting that the specific nature of the sequences of these hydrophobic domains are an important determinant of the ability of elastin polypeptides to self-assemble.

Chemical synthesis of cross-linked poly(KGGVG), an elastin-like biopolymer

Previous studies afforded on peptides and polypeptides containing repetitive sequences of elastin have largely demonstrated that their molecular and supramolecular properties are fully representative of those of tropoelastin, the soluble, linear precursor of elastin itself. In the attempt to synthesize cross-linked elastin-mimetic polypeptides, the repeating sequence VGGVG (V: valine; G: glycine), typical of elastin, was modified to incorporate lysine residues, yielding the polymer poly(KGGVG) (K: lysine). This imparts primary amine functionality susceptible to cross-linking reaction with appropriate bifunctional cross-linking reagents. We report herein the chemical synthesis and cross-linking of poly(KGGVG) with glutaraldehyde (GTA) and with disuccinimidyl glutarate (DSG). In both cases, the characterization of the polymers, both linear and cross-linked, has been carried out by CD spectroscopy and transmission electron microscopy measurements. The obtained results, although not conclusive, demonstrate that poly(KGGVG), both linear and cross-linked, may be considered very similar to tropoelastin and mature elastin, as concerns its molecular and supramolecular properties.

Alterations in the chain dynamics of insoluble elastin upon proteolysis by serine elastases

The high temperature dielectric relaxations of purified and elastolized ligamentum nuchae elastin in the dry state have been investigated by thermally stimulated depolarization current spectrometry, with an equivalent frequency comprised between 10(-2) and 10(-3) Hz. A main relaxation mode, located close to 150 degrees C and attributed to the dielectric manifestation of a glass transition, is found for all samples. After decomposition by the fractional polarization method, the analysis of the high temperature mode shows the existence of two relaxation mechanisms: a cooperative one, associated with flexible zones of the protein, and an isoenthalpic one, corresponding to more ordered and constrained zones. The activation parameters of the two mechanisms are dependent on the extent of elastolysis and on the nature of enzyme (pancreatic elastase vs leukocyte elastase). Both enzymes influence the dielectric behavior of elastin in a similar way: the activation enthalpy maximum of the relaxing units located in the flexible zones, characteristic of the cooperative length, decreases with increasing hydrolysis. Moreover, the isoenthalpic mechanism becomes cooperative at the highest extent of elastolysis, which highlights release of constraints in ordered zones. Nevertheless, the differences found between the two enzymatic hydrolyses are characteristic of distinct sites of cleavage in the elastin network.

Hydrolysis of glycine-containing elastin pentapeptides by LasA, a metalloelastase from Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen that causes severe infections in vulnerable hosts. It may produce various virulence factors including proteases. Among them, LasA possesses both elastolytic and staphylolytic (hydrolysis of pentaglycine cross-links in the cell wall peptidoglycan) activities. To understand if its elastolytic activity results from a preference for glycine-rich substrates, we studied its ability to hydrolyse the 65 pentapeptides of human tropoelastin containing at least three glycines. As demonstrated by capillary electrophoresis (CE), 22 of these peptides were hydrolysed by LasA, generally at a single peptide bond and the catalytic ratio kcat/KM was determined for most of them. The highest value was obtained for LGGGA, 59 +/- 9 min(-1) x mmol(-1) x L. The specificity of hydrolysis was elucidated by CE, liquid secondary ion mass spectrometry and, in some cases, collision activated dissociation-mass analysis of ion kinetic energy. The preferred cleavage sites are GG and GA peptide bonds, the sequence GG(cleavage site)A being especially sensitive to hydrolysis. Both positions P2 and P'2 must be occupied for hydrolysis and the presence of an amino acid in P3 (but not in P'3) significantly increases the catalytic ratio. Considering these results, about 30 GGX sequences (X: G, A or Y) of human tropoelastin could be susceptible to LasA elastolysis.

Domain 26 of tropoelastin plays a dominant role in association by coacervation

The temperature-dependent association of tropoelastin molecules through coacervation is an essential step in their assembly leading to elastogenesis. The relative contributions of C-terminal hydrophobic domains in coacervation were assessed. Truncated tropoelastins were constructed with N termini positioned variably downstream of domain 25. The purified proteins were assessed for their ability to coacervate. Disruption to domain 26 had a substantial effect and abolished coacervation. Circular dichroism spectroscopy of an isolated peptide comprising domain 26 showed that it undergoes a structural transition to a state of increased order with increasing temperature. Protease mapping demonstrated that domain 26 is flanked by surface sites and is likely to be in an exposed position on the surface of the tropoelastin molecule. These results suggest that the hydrophobic domain 26 is positioned to play a dominant role in the intermolecular interactions that occur during coacervation.

Elastin: molecular description and function

Elastin, the protein responsible for the elastic properties of vertebrate tissues, has been thought to be solely restricted to that role. As a consequence, elastin was conventionally described as an amorphous polymer. Recent results in the biomedical, biochemical and biophysical fields have lead to the conclusion that the presence of elastin in the extracellular space has very complex implications involving many other molecules. The present review describes the current state of knowledge concerning elastin as an elastic macromolecule. First, the genetic, biological, biochemical and biophysical processes leading to a functional polymer are described. Second, the elastic function of elastin is discussed. The controversy on elastin structure and elasticity is discussed and a novel dynamic mechanism of elasticity proposed. Finally, pathologies where the elastin molecule is involved are considered. This updated description of functional elastin provides the required background for the understanding of its pathologies and defines clearly the properties a substance should possess to be qualified as a good elastic biomaterial.

Inhibition of aortic wall calcification in bioprosthetic heart valves by ethanol pretreatment: biochemical and biophysical mechanisms

The effectiveness of ethanol pretreatment on preventing calcification of glutaraldehyde-fixed porcine aortic bioprosthetic heart valve (BPHV) cusps was previously demonstrated, and the mechanism of action of ethanol was attributed in part to both lipid removal and a specific collagen conformational change. In the present work, the effect of ethanol pretreatment on BPHV aortic wall calcification was investigated using both rat subdermal and sheep circulatory implants. Ethanol pretreatment significantly inhibited calcification of BPHV aortic wall, but with less than complete inhibition. The maximum inhibition of calcification of BPHV aortic wall was achieved using an 80% ethanol pretreatment; calcium levels were 71.80+/-8.45 microg/mg with 80% ethanol pretreatment compared to the control calcium level of 129.90+/-7.24 microg/mg (p = 0.001). Increasing the duration of ethanol exposure did not significantly improve the inhibitory effect of ethanol on aortic wall calcification. In the sheep circulatory implants, ethanol pretreatment partly prevented BPHV aortic wall calcification with a calcium level of 28.02+/-4.42 microg/mg compared to the control calcium level of 56.35+/-6.14 microg/mg (p = 0.004). Infrared spectroscopy (ATR-FTIR) studies of ethanol-pretreated BPHV aortic wall (vs. control) demonstrated a significant change in protein structure due to ethanol pretreatment. The water content of the aortic wall tissue and the spin-lattice relaxation times (T1) as assessed by proton nuclear magnetic resonance spectroscopy did not change significantly owing to ethanol pretreatment. The optimum condition of 80% ethanol pretreatment almost completely extracted both phospholipids and cholesterol from the aortic wall; despite this, significant calcification occurred. In conclusion, these results clearly demonstrate that ethanol pretreatment is significantly but only partially effective for inhibition of calcification of BPHV aortic wall and this effect may be due in part to lipid extraction and protein structure changes caused by ethanol. It is hypothesized that ethanol pretreatment may be of benefit for preventing bioprosthetic aortic wall calcification only in synergistic combination with another agent.

Bovine elastin and kappa-elastin secondary structure determination by optical spectroscopies

Elastin is the macromolecular polymer of tropoelastin molecules responsible for the elastic properties of tissues. The understanding of its specific elasticity is uncertain because its structure is still unknown. Here, we report the first experimental quantitative determination of bovine elastin secondary structures as well as those of its corresponding soluble kappa-elastin. Using circular dichroism and Fourier transform infrared and near infrared Fourier transform Raman spectroscopic data, we estimated the secondary structure contents of elastin to be approximately 10% alpha-helices, approximately 45% beta-sheets, and approximately 45% undefined conformations. These values were very close to those we had previously determined for the free monomeric tropoelastin molecule, suggesting thus that elastin would be constituted of a closely packed assembly of globular beta structural class tropoelastin molecules cross-linked to form the elastic network (liquid drop model of elastin architecture). The presence of a strong hydration shell is demonstrated for elastin, and its possible contribution to elasticity is discussed.

Pepstatin A: polymerization of an oligopeptide

Pepstatin A, a pentapeptide with the molecular weight of 686, is a naturally occurring inhibitor of aspartyl proteases secreted by Streptomyces species. Above a critical concentration of 0.1 mM at low ionic strength and neutral pH, it can polymerize into filaments which may extend over several micrometers. After negative staining, these filaments show a helical substructure with characteristic diameters ranging from 6 to 12 nm. Selected images at higher magnification suggest the filaments are composed of two intertwined 6 nm strands. This is in agreement with the optical diffraction analysis which additionally established a periodic pitch of 25 nm for the helical intertwining. Rotary shadowing of the pepstatin A filaments clearly demonstrated the right-handedness of the helical twist. In physiological salt solution or at higher concentrations of pepstatin A, a variety of higher order structures were observed, including ribbons, sheets and cylinders with both regular and twisted or irregular geometries. Pepstatin A can interact with intermediate filament subunit proteins. These proteins possess a long, alpha-helical rod domain that forms coiled-coil dimers, which through both hydrophobic and ionic interactions form tetramers which, in turn, in the presence of physiological salt concentrations, polymerize into the 10 nm intermediate filaments. In the absence of salt, pepstatin A and intermediate filament proteins polymerize into long filaments with a rough surface and a diameter of 15-17 nm. This polymerization appears to be primarily driven by nonionic interactions between pepstatin A and polymerization-competent forms of intermediate filament proteins, resulting in a composite filament. Polymerization-incompetent proteolytic fragments of vimentin, lacking portions of the head and/or tail domain, failed to copolymerize with pepstatin A into long filaments under these conditions. These peptides, as well as bovine serum albumin, were found to stick to the surface of pepstatin A filaments, ribbons and sheets. Independent evidence for direct association of pepstatin A with intermediate filament subunit proteins was provided not only by electron microscopy but also by UV difference spectra. Pepstatin A loses its ability to inhibit the aspartyl protease of the human immunodeficiency virus type 1 following polymerization into the higher order structures described here. The amazing fact that pepstatin A can spontaneously self-associate to form very large polymers seems to be a more rare event for such small peptides. The other examples of synthetic or naturally occurring oligopeptides discussed in this review which are able to polymerize into higher order structures possess a common property, their hydrophobicity, often manifested by clusters of valine or isoleucine residues.(ABSTRACT TRUNCATED AT 400 WORDS)

An aggregating elastin-like pentapeptide

Synthetic VGGVG, a "monomeric" unit of the glycine-rich regions of elastin, has been investigated for its molecular and supramolecular properties. In aqueous solution the pentapeptide showed conformational features strongly concentration-dependent. CD and NMR studies suggested a partial unfolding on increasing the concentration. Electron microscopy, on the other hand, evidenced extensive aggregation of the pentapeptide yielding elastin-like supramolecular structures constituted either by twisted ropes or by banded fibrils.