Repeating structure of chick tropoelastin revealed by complementary DNA cloning

Exploring LCST- and UCST-like Behavior of Branched Molecules Bearing Repeat Units of Elastin-like Peptides as Side Components

Elastin-like peptides (ELPs) exhibit lower critical solution temperature (LCST)-type behavior, being soluble at low temperatures and insoluble at high temperatures. While the properties of linear, long-chain ELPs are well-studied, short-chain ELPs, especially those with branched architectures, have been less explored. Herein, to obtain further insights into multimeric short ELPs, we investigated the temperature-responsive properties of branched molecules composed of a repeating pentapeptide unit of short ELPs, Phe-Pro-Gly-Val-Gly, as side components and oligo(Glu) as a backbone structure. In turbidimetry experiments, the branched ELPs showed LCST-like behavior similar to conventional ELPs and upper critical solution temperature (UCST)-like behavior, which are rarely observed in ELPs. In addition, the morphological aspects and mechanisms underlying the temperature-responsiveness were investigated. We observed that spherical aggregates formed, and the branched ELPs underwent structural changes through the self-assembly process. This study demonstrates the unique temperature-responsiveness of branched short ELPs, providing new insights into the future development and use of ELPs with tailored properties.

Enhancement of Aggregate Formation Through Aromatic Compound Adsorption in Elastin-like Peptide (FPGVG)5 Analogs

Elastin-like peptides (ELPs) exhibit temperature-dependent reversible self-assembly. Repetitive sequences derived from elastin, such as Val-Pro-Gly-Val-Gly (VPGVG), are essential for the self-assembly of ELPs. Previously, we developed (FPGVG)5 (F5), in which the first valine residue in the VPGVG sequence was replaced with phenylalanine, which showed strong self-aggregation ability. This suggests that interactions through the aromatic amino acid residues of ELPs could play an important role in self-assembly. In this study, we investigated the thermoresponsive behavior of F5 analogs in the presence of aromatic compounds. Turbidimetry, spectroscopy, and fluorescence measurements demonstrated that aromatic compounds interacted with F5 analogs below the transition temperature and enhanced the self-assembly ability of ELPs by stabilizing amyloid-like structures. Furthermore, quantitative high-performance liquid chromatography analyses showed that the F5 analogs could adsorb and remove hydrophobic aromatic compounds from aqueous solutions during aggregate formation. These results suggested that the F5 analogs can be applicable as scavengers of aromatic compounds.

Branched short elastin-like peptides with temperature responsiveness obtained by EDTA-mediated multimerization

Elastin-like peptides (ELPs) exhibit a reversible phase transition, known as coacervation, triggered by temperature changes. This property makes them useful as stimuli-responsive molecular materials for various applications. Among ELPs, short peptide chain lengths have some advantages over long peptide chain lengths because short ELPs can be easily obtained by chemical synthesis, allowing the use of various amino acids, including D-type and unnatural amino acids, at any position in the sequence. Moreover, the incorporated amino acids readily affect the temperature-responsive behavior of ELPs. However, to be utilized in various applications, it is necessary to develop short ELPs and to investigate their temperature-responsive properties. To obtain further insights into the temperature-responsive behavior of the short ELPs, we investigated branched short ELP analogs composed of (FPGVG)_n chains (n = 1 or 2, abbreviated as F1 and F2, respectively). We synthesized multimers composed of four F1 chains or two to four F2 chains using ethylenediaminetetraacetic acid (EDTA) as a central component of multimerization. Our results show that the multimers obtained exhibited coacervation in aqueous solutions whereas linear F1 or F2 did not. Furthermore, the structural features of the obtained multimers were the same as those of linear (FPGVG)₄ . In this study, we demonstrated that molecules capable of coacervation can be obtained by multimerization of F1 or F2. The temperature-responsive molecules obtained using short ELPs make it possible to use them as easy-to-synthesize peptide tags to confer temperature responsiveness to various molecules, which will aid the development of temperature-responsive biomaterials with a wide variety of functions.

Flexible customization of the self-assembling abilities of short elastin-like peptide Fn analogs by substituting N-terminal amino acids

Elastin-like peptides (ELPs) are thermoresponsive biopolymers inspired by the characteristic repetitive sequences of natural elastin. As ELPs exhibit temperature-dependent reversible self-assembly, they are expected to be biocompatible thermoresponsive materials for drug delivery carriers. One of the most widely studied ELPs in this field is the repetitive pentapeptide, (VPGXG)_n . We previously reported that phenylalanine-containing ELP (Fn) analogs, in which the former Val residue of the repetitive sequence (VPGVG)_n is replaced by Phe, show coacervation with a short chain length (n = 5). Owing to their short sequences, Fn analogs are easily modified in amino acid sequences via simple chemical synthesis, and are useful for investigating the relationship between peptide sequences and temperature responsiveness. In this study, we developed Fn analogs by replacing Phe residue(s) with other amino acids or introducing another amino acid at the N-terminus. The temperature responsiveness of the Fn analogs changed drastically with the substitution of a single Phe residue, suggesting that aromatic amino acids play an important role in their self-assembly. In addition, the self-assembling ability of Fn was enhanced by increasing the bulkiness of the N-terminal amino acids. Therefore, the N-terminal residue was considered to be important for hydrophobicity-induced intermolecular interactions between the peptides during coacervation.

Metal ion scavenging activity of elastin-like peptide analogues containing a cadmium ion binding sequence

The development of simple and safe methods for recovering environmental pollutants, such as heavy metals, is needed for sustainable environmental management. Short elastin-like peptide (ELP) analogues conjugated with metal chelating agents are considered to be useful as metal sequestering agents as they are readily produced, environment friendly, and the metal binding domain can be selected based on any target metal of interest. Due to the temperature dependent self-assembly of ELP, the peptide-based sequestering agents can be transformed from the solution state into the particles that chelate metal ions, which can then be collected as precipitates. In this study, we developed a peptide-based sequestering agent, AADAAC-(FPGVG)₄, by introducing the metal-binding sequence AADAAC on the N-terminus of a short ELP, (FPGVG)₄. In turbidity measurements, AADAAC-(FPGVG)₄ revealed strong self-assembling ability in the presence of metal ions such as Cd²⁺ and Zn²⁺. The results from colorimetric analysis indicated that AADAAC-(FPGVG)₄ could capture Cd²⁺ and Zn²⁺. Furthermore, AADAAC-(FPGVG)₄ that bound to metal ions could be readily recycled by treatment with acidic solution without compromising its metal binding affinity. The present study indicates that the fusion of the metal-binding sequence and ELP is a useful and powerful strategy to develop cost-effective heavy metal scavenging agents with low environmental impacts.

Simple Regulation of the Self-Assembling Ability by Multimerization of Elastin-Derived Peptide (FPGVG) n Using Nitrilotriacetic Acid as a Building Block

Elastin comprises hydrophobic repetitive sequences, such as Val-Pro-Gly-Val-Gly, which are thought to be important for the temperature-dependent reversible self-association (coacervation). Elastin and elastin-like peptides (ELPs), owing to their characteristics, are expected to be applied as base materials for the development of new molecular tools, such as drug-delivery system carrier and metal-scavenging agents. Recently, several studies have been reported on the dendritic or branching ELP analogues. Although the topological difference of the branched ELPs compared to their linear counterparts may lead to useful properties in biomaterials, the available information regarding the effect of branching on molecular architecture and thermoresponsive behavior of ELPs is scarce. To obtain further insight into the thermoresponsive behavior of branched ELPs, novel ELPs, such as nitrilotriacetic acid (NTA)-(FPGVG) _n conjugates, that is, (NTA)-Fn analogues possessing 1-3 (FPGVG) _n (n = 3, 5) molecule(s), were synthesized and investigated for their coacervation ability. Turbidity measurement of the synthesized peptide analogues revealed that (NTA)-Fn analogues showed strong coacervation ability with various strengths. The transition temperature of NTA-Fn analogues exponentially decreased with increasing number of residues. In the circular dichroism measurements, trimerization did not alter the secondary structure of each peptide chain of the NTA-Fn analogue. In addition, it was also revealed that the NTA-Fn analogue possesses one peptide chain that could be utilized as metal-scavenging agents. The study findings indicated that multimerization of short ELPs via NTA is a useful and powerful strategy to obtain thermoresponsive molecules.

Impact of elastin-derived VGVAPG peptide on bidirectional interaction between peroxisome proliferator-activated receptor gamma (Pparγ) and beta-galactosidase (β-Gal) expression in mouse cortical astrocytes in vitro

The process of degradation of the elastin-rich extracellular matrix produces elastin-derived peptides (EDPs). Different types of EDPs are detectable in the cerebrospinal fluid in healthy individuals and in patients after ischemic stroke. To date, it has been demonstrated that EDPs can regulate the development of insulin resistance in mice in a peroxisome proliferator-activated receptor gamma (Pparγ)-dependent manner. Therefore, the aim of this study was to investigate the impact of the elastin-derived valine-glycine-valine-alanine-proline-glycine (VGVAPG) peptide on Pparγ and beta-galactosidase (β-Gal) expression in mouse cortical astrocytes in vitro. Primary astrocytes were maintained in DMEM/F12 without phenol red supplemented with 10% fetal bovine serum. The cells were exposed to 50 nM, 1 and 50 μM of the VGVAPG peptide. After 3 and 6 h (for mRNA) and 24 and 48 h (for the protein) of exposition to the peptide, the expression of Pparγ and β-Gal was measured. Moreover, the siRNA gene knockdown method was applied. Our study showed, for the first time, that the VGVAPG peptide affected β-Gal and Pparγ mRNA and protein expression in mouse astrocytes in vitro. Furthermore, we suggested a bidirectional interaction between Pparγ and β-Gal. Both pioglitazone and rosiglitazone increased β-Gal and Pparγ protein expression in mouse astrocytes in vitro, and this effect was reduced by the VGVAPG peptide. However, due to the lack of sufficient data explaining the molecular mechanism of action of the VGVAPG peptide in the nervous system, more studies are necessary in this field.

Enhancement of Self-Aggregation Properties of Linear Elastin-Derived Short Peptides by Simple Cyclization: Strong Self-Aggregation Properties of Cyclo[FPGVG] n, Consisting Only of Natural Amino Acids

Elastin-like peptides (ELPs) consist of distinctive repetitive sequences, such as (VPGVG) _n, exhibit temperature-dependent reversible self-assembly (coacervation), and have been considered to be useful for the development of thermoresponsive materials. Further fundamental studies evaluating coacervative properties of novel nonlinear ELPs could present design concepts for new thermoresponsive materials. In this study, we prepared novel ELPs, cyclic (FPGVG) _n (cyclo[FPGVG] _n, n = 1-5), and analyzed their self-assembly properties and structural characteristics. Cyclo[FPGVG] _n ( n = 3-5) demonstrated stronger coacervation capacity than the corresponding linear peptides. The coacervate of cyclo[FPGVG]₅ was able to retain water-soluble dye molecules at 40 °C, which implied that cyclo[FPGVG]₅ could be employed as a base material of DDS (drug delivery system) matrices and other biomaterials. The results of molecular dynamics simulations and circular dichroism measurements suggested that a certain chain length was required for cyclo[FPGVG] _n to demonstrate alterations in molecular structure that were critical to the exhibition of coacervation.

Dimerization effects on coacervation property of an elastin-derived synthetic peptide (FPGVG)5

Elastin, a core protein of the elastic fibers, exhibits the coacervation (temperature-dependent reversible association/dissociation) under physiological conditions. Because of this characteristic, elastin and elastin-derived peptides have been considered to be useful as base materials for developing various biomedical products, skin substitutes, synthetic vascular grafts, and drug delivery systems. Although elastin-derived polypeptide (Val-Pro-Gly-Val-Gly)n also has been known to demonstrate coacervation property, a sufficiently high (VPGVG)n repetition number (n>40) is required for coacervation. In the present study, a series of elastin-derived peptide (Phe-Pro-Gly-Val-Gly)5 dimers possessing high coacervation potential were newly developed. These novel dimeric peptides exhibited coacervation at significantly lower concentrations and temperatures than the commonly used elastin-derived peptide analogs; this result suggests that the coacervation ability of the peptides is enhanced by dimerization. Circular dichroism (CD) measurements indicate that the dimers undergo similar temperature-dependent and reversible conformational changes when coacervation occurs. The molecular dynamics calculation results reveal that the sheet-turn-sheet motif involving a type II β-turn-like structure commonly observed among the dimers and caused formation of globular conformation of them. These synthesized peptide dimers may be useful not only as model peptides for structural analysis of elastin and elastin-derived peptides, but also as base materials for developing various temperature-sensitive biomedical and industrial products.

Development of short and highly potent self-assembling elastin-derived pentapeptide repeats containing aromatic amino acid residues

Tropoelastin is the primary component of elastin, which forms the elastic fibers that make up connective tissues. The hydrophobic domains of tropoelastin are thought to mediate the self-assembly of elastin into fibers, and the temperature-mediated self-assembly (coacervation) of one such repetitive peptide sequence (VPGVG) has been utilized in various bio-applications. To elucidate a mechanism for coacervation activity enhancement and to develop more potent coacervatable elastin-derived peptides, we synthesized two series of peptide analogs containing an aromatic amino acid, Trp or Tyr, in addition to Phe-containing analogs and tested their functional characteristics. Thus, position 1 of the hydrophobic pentapeptide repeat of elastin (X(1)P(2)G(3)V(4)G(5)) was substituted by Trp or Tyr. Eventually, we acquired a novel, short Trp-containing elastin-derived peptide analog (WPGVG)3 with potent coacervation ability. From the results obtained during this process, we determined the importance of aromaticity and hydrophobicity for the coacervation potency of elastin-derived peptide analogs. Generally, however, the production of long-chain synthetic polypeptides in quantities sufficient for commercial use remain cost-prohibitive. Therefore, the identification of (WPGVG)3, which is a 15-mer short peptide consisting simply of five natural amino acids and shows temperature-dependent self-assembly activity, might serve as a foundation for the development of various kinds of biomaterials.

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.

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.

Modification and functional inactivation of the tropoelastin carboxy-terminal domain in cross-linked elastin

The carboxy-terminus of tropoelastin is a highly conserved, atypical region of the molecule with sequences that define both cell and matrix interactions. This domain also plays a critical but unknown role in the assembly and crosslinking of tropoelastin during elastic fiber maturation. Using a competitive ELISA with an antibody to an elastase-resistant epitope in the carboxy-terminus of tropoelastin (domain-36), we quantified levels of the domain-36 sequence in elastase-derived peptides from mature, insoluble elastin. We found that the amount of carboxy-terminal epitope in elastin is approximately 0.2% of the expected value, assuming each tropoelastin monomer that is incorporated into the insoluble polymer has an intact carboxy-terminus. The low levels suggest that the majority of domain-36 sequence is either removed at some stage of elastin assembly or that the antigenic epitope is altered by posttranslational modification. Biochemical evidence is presented for a potential lysine-derived cross-link in this region, which would alter the extractability and antigenicity of the carboxy-terminal epitope. These results show that there is little or no unmodified domain-36 in mature elastin, indicating that the cell and matrix binding activities associated with this region of tropoelastin are lost or modified as elastin matures. A crosslinking function for domain-36 may serve to help register the multiple crosslinking sites in elastin and explains why mutations that alter the domain-36 sequence have detrimental effects on elastic fiber assembly.

Structure of the elastin gene

The isolation and characterization of cDNAs encompassing the full length of chicken, cow, rat and human elastin mRNA have led to the elucidation of the primary structure of the respective tropoelastins. Large segments of the sequence are conserved but there are also considerable variations which range in extent from relatively small alterations, such as conservative amino acid substitutions, to variation in the length of hydrophobic segments and largescale deletions and insertions. In general, smaller differences are found among mammalian tropoelastins and greater ones between chicken and mammalian tropoelastins. Although only a single elastin gene is found per haploid genome, the primary transcript is subject to considerable alternative splicing, resulting in multiple tropoelastin isoforms. Functionally distinct hydrophobic and cross-link domains of the protein are encoded in separate exons which alternate in the gene. The introns of the human gene are rich in Alu repetitive sequences, which may be the site of recombinational events, and there are also several dinucleotide repeats, which may exhibit polymorphism and, therefore, be effective genetic markers. The 5' flanking region is G+C rich and contains potential binding sites for numerous modulating factors, but no TATA box or functional CAAT box. The basic promoter is contained within a 136 bp segment and transcription is initiated at multiple sites. These findings suggest that the regulation of elastin gene expression is complex and takes place at several levels.

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.

Sequences and domain structures of mammalian, avian, amphibian and teleost tropoelastins: Clues to the evolutionary history of elastins

Tropoelastin is the monomeric form of elastin, a polymeric extracellular matrix protein responsible for properties of extensibility and elastic recoil in connective tissues of most vertebrates. As an approach to investigate how sequence and structural characteristics of tropoelastin assist in polymeric assembly and account for the elastomeric properties of this polymer, and to better understand the evolutionary history of elastin, we have identified and characterized tropoelastins from frog (Xenopus tropicalis) and zebrafish (Danio rerio), comparing these to their mammalian and avian counterparts. Unlike other species, two tropoelastin genes were expressed in zebrafish. All tropoelastins shared a predominant and characteristic alternating domain arrangement, as well as the fundamental crosslinking sequence motifs. However, zebrafish and frog tropoelastins had several unusual characteristics, including increased exon numbers and protein molecular weights, and decreased hydropathies. For all tropoelastins there was evidence of evolutionary expansion of the proteins by extensive replication of a hydrophobic-crosslinking exon pair. This was particularly apparent for zebrafish and frog tropoelastin genes, where remnants of sequence similarity were also seen in introns flanking the replicated exon pair. While overall alignment of mammalian, avian, frog and zebrafish tropoelastin sequences was not possible because of sequence variability, the C-terminal exon was well-conserved in all species. In addition, good sequence alignment was possible for several exons just upstream of the putative region of replication, suggesting that these conserved domains may represent 'primordial' core sequences present in the ancestral sequence common to all tropoelastins and in some way essential to the structure/function of elastin.

Characterization of peptides resulting from digestion of human skin elastin with elastase

Several pathological disorders are associated with abnormalities in elastic fibers, which are mainly composed of elastin. Understanding the biochemical basis of such disorders requires information about the primary structure of elastin. Since the acquisition of structural information for elastin is hampered by its extreme insolubility in water or any organic solvent, in this study, human skin elastin was digested with elastase to produce water-soluble peptides. Tandem mass spectrometry (MS/MS) experiments were performed using conventional electrospray ionization (ESI) and nano-ESI techniques coupled with ion trap and quadrupole time-of-flight (qTOF) mass analyzers, respectively. The peptides were identified from the fragment spectra using database searching and/or de novo sequencing. The cleavage sites of the enzyme and, for the first time, the extent and location of proline hydroxylation in human skin elastin were determined. A total of 117 peptides were identified with sequence coverage of 58.8%. It has been observed that 25% of proline residues in the sequenced region are hydroxylated. Elastase cleaves predominantly at the C-terminals of the amino acids Gly, Val, Leu, Ala, and Ile, and to a lesser extent at Phe, Pro, Glu, and Arg. Our results confirm a previous report that human skin elastin lacks amino acid sequences expressed by exon 26A.

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.

Nitric oxide stimulates elastin expression in chick aortic smooth muscle cells

Nitric oxide (NO), an endothelium-dependent relaxing factor, regulates relaxation, proliferation, and migration of smooth muscle cells (SMCs) and most likely attenuates developing vascular disease such as atherosclerosis. We investigated whether or not NO is associated with regulation of aortic elasticity. S-Nitrosoglutathione (GSNO), a NO donor, stimulated tropoelastin synthesis in cultured SMCs during both the quiescent and proliferating phases. The stimulation of tropoelastin synthesis was dose-dependent within 1-100 nM. Maximum stimulation was detected by treatment with 100 nM GSNO for 24 h. 8-Bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP), an exogenous cyclic GMP analog, also upregulated tropoelastin synthesis. Tropoelastin and lysyl oxidase mRNA expression, as assessed by Northern blot analysis, was also stimulated by GSNO. Administration of KT5823, a cyclic GMP-dependent protein kinase inhibitor, inhibited the GSNO-induced tropoelastin synthesis. These results indicate that the stimulatory effects of GSNO are due to cyclic GMP dependent protein kinase (PKG) activation by NO. In conclusion, NO seems to enhance aortic elasticity via tropoelastin and lysyl oxidase upregulation.

The role of the carboxy terminus of tropoelastin in its assembly into the elastic fiber

Tropoelastin, the soluble precursor protein of insoluble amorphous elastin, contains repeating segments that are important for the characteristic elasticity and crosslinking sites of mature elastin. In addition, there is a unique carboxy terminal domain that is encoded by exon 36 of the elastin gene, and it has been suggested that this region may play a role in the process of insolubilization. The contribution of exon 36 to the maturation of tropoelastin into insoluble elastin was probed in these studies. Neonatal rat aortic smooth muscle cells were cultured and the fate of [3H] Lys labeled human recombinant tropoelastin (hrTE) molecules added to the cultures was monitored. In comparison to the hrTE containing the region encoded by exon 36, hrTE molecules lacking this domain were less efficiently incorporated into elastin, as evidenced by a decrease in NaOH insoluble radioactivity. Specific residues within the domain encoded by exon 36 were targeted for further study in experiments in which the two Cys residues were reduced and alkylated, and/or the four basic Arg-Lys-Arg-Lys residues at the carboxy terminus were removed. Both of these modifications resulted in decreased incorporation into elastin equivalent to the complete removal of the carboxy terminus. Prior treatment of the cell layer with elastase reduced the efficiency of insolubilization of hrTE containing the domain encoded by exon 36, but had no effect on the processing of molecules lacking this region. These data suggest that exon 36 of the elastin gene contributes to normal efficient incorporation of tropoelastin into the elastin fiber.

Deficient coacervation of two forms of human tropoelastin associated with supravalvular aortic stenosis

Human tropoelastin associates by coacervation and is subsequently cross-linked to make elastin. In Williams syndrome, defective elastin deposition is associated with hemizygous deletion of the tropoelastin gene in supravalvular aortic stenosis (SVAS). Remarkably, point-mutation forms of SVAS correspond to incomplete forms of tropoelastin which include in-frame termination by nonsense mutations, yet the resulting phenotype of these disorders is not explained because expression variably occurs from both normal and mutant alleles. Proteins corresponding to two truncated tropoelastin mutants were expressed and purified to homogeneity. Coacervation of these proteins occurred as expected with increasing temperature, but substantially contrasted with that of the performance of a normal tropoelastin. Significantly, association by coacervation of the truncated SVAS tropoelastin molecules was negligible at 37 degrees C, which contrasted with the substantial coacervation seen for normal tropoelastin. Furthermore their midpoints of coacervation increased and correlated with the extent of deletion, in accord with the loss of hydrophobic regions required for tropoelastin association. Their secondary structures are similar, as evidenced by CD studies. We propose a model for point-mutation SVAS in which aberrant tropoelastin molecules are incompetent and are mainly excluded from participation in coacervation and consequently in elastogenesis. These forms of SVAS may consequently be considered functionally similar to a hemizygous deletion, and mark point-mutation SVAS as a disorder of defective coacervation.

Identification of a large region of secondary structure in the 3'-untranslated region of chicken elastin mRNA with implications for the regulation of mRNA stability

Synthesis of aortic elastin peaks in the perinatal period and then is strongly down-regulated with postnatal vascular development. Our laboratory has previously shown that changes in elastin mRNA stability contribute to this developmental decrease in elastin production. Here we identify a large region of stable secondary structure in the 3'-untranslated region (3'-UTR) of chicken elastin mRNA. Reverse transcriptase polymerase chain reaction or polymerase chain reaction amplification of the 3'-UTR consistently resulted in products with an approximately 328-bp deletion from the central region of the 3'-UTR, suggesting the presence of secondary structure. The presence of this structure was confirmed by probing the 3'-UTR with RNases with selectivity for single- or double-stranded RNA. Gel migration shift assays using cytosolic extracts from 2-day old chicken aorta demonstrate specific binding of a cytosolic protein to riboprobes containing the 3'-UTR of elastin but not to riboprobes either corresponding to other areas of the message or containing the 3'-UTR but lacking the region of secondary structure. Binding of cytosolic protein was particularly prominent in aortic extracts from 2-day old chickens, a time when elastin message is stable, as compared with 8- and 15-week old chickens, when the elastin message is relatively unstable, suggesting that this region of secondary structure may play a role in developmental regulation of stability of elastin mRNA.

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.

Quantum molecular modeling of the elastinic tetrapeptide Val-Pro-Gly-Gly

The free Val-Pro-Gly-Gly tetrapeptide belonging to the Proline-rich sequences of elastin has been studied both theoretically and experimentally. The molecular modelisation was carried out using AM1 and ab initio quantum computations while the conformation in solution was ascertained by circular dichroism spectroscopy performed on the synthesized tetrapeptide. Experimental and theoretical investigations lead to the conclusion that the most probable structure is constituted by a type II beta-turn.

Spinalin, a new glycine- and histidine-rich protein in spines of Hydra nematocysts

Here we present the cloning, expression and immunocytochemical localization of a novel 24 kDa protein, designated spinalin, which is present in the spines and operculum of Hydra nematocysts. Spinalin cDNA clones were identified by in situ hybridization to differentiating nematocytes. Sequencing of a full-length clone revealed the presence of an N-terminal signal peptide, suggesting that the mature protein is sorted via the endoplasmic reticulum to the post-Golgi vacuole in which the nematocyst is formed. The N-terminal region of spinalin (154 residues) is very rich in glycines (48 residues) and histidines (33 residues). A central region of 35 residues contains 19 glycines, occurring mainly as pairs. For both regions a polyglycine-like structure is likely and this may be stabilized by hydrogen bond-mediated chain association. Similar sequences found in loricrins, cytokeratins and avian keratins are postulated to participate in formation of supramolecular structures. Spinalin is terminated by a basic region (6 lysines out of 15 residues) and an acidic region (9 glutamates and 9 aspartates out of 32 residues). Western blot analysis with a polyclonal antibody generated against a recombinant 19 kDa fragment of spinalin showed that spinalin is localized in nematocysts. Following dissociation of the nematocyst's capsule wall with DTT, spinalin was found in the insoluble fraction containing spines and the operculum. Immunocytochemical analysis of developing nematocysts revealed that spinalin first appears in the matrix but then is transferred through the capsule wall at the end of morphogenesis to form spines on the external surface of the inverted tubule and the operculum.

Evidence for epitope-specific thymus-independent response against a repeat sequence in a protein antigen

We have studied the immunogenicity of a recombinant heat-shock protein-related stress protein of 70,000 MW (Pfhsp) of the human malaria parasite Plasmodium falciparum in H-2 congenic and non-congenic strains of inbred and outbred mice. Most mice of different genetic backgrounds produced antibodies to Pfhsp, indicating a lack of any apparent genetic restriction of immune response. A peptide corresponding to the GGMP repeat sequence in the C-terminal region of Pfhsp was recognized by more than 75% of sera from immunized mice. The GGMP repeat epitope-specific antibodies were largely of the IgM isotype, especially in all seven inbred strains of mice tested. The lack of significant boosting of the immune response, a predominantly IgM isotype of antibodies and generation of antibody responses in athymic nude mice suggest a thymus-independent response against the GGMP repeat epitope in the Pfhsp molecule.

Alternative splicing of VWFA modules generates variants of type VI collagen alpha 3 chain with a distinctive expression pattern in embryonic chicken tissues and potentially different adhesive function

Type VI collagen, a ubiquitous extracellular cell adhesion molecule, is formed by heterotrimeric monomers which associate into dimers and tetramers and assemble into larger oligomers constituting the 100 nm-long periodic microfilaments of connective tissues. One distinctive structural characteristic of type VI collagen is represented by an alpha 3 chain with a much larger molecular mass compared to the other two chains and with an extensive size heterogeneity, exemplified by the separation into up to five polypeptides in SDS-PAGE. There is evidence that the alpha 3(VI) mRNA can undergo alternative splicing of three VWFA modules at the 5'-end, potentially resulting in the expression of protein variants. Here we report that alternative splicing of alpha 3(VI) mRNA in chicken embryo did not result in the absolute predominance of a particular alpha 3(VI) form in any tissue; instead, the expression of variants including exons A9, A8 and A6 increased with age. In addition, these variants had a more restricted tissue distribution pattern compared to variants including only constitutive exons: A9+ were the rarest and were present almost exclusively in skin and skeletal muscle; A6+ were expressed in several of the examined tissues with local variations; A8+ had intermediate levels and were less widely distributed than A6+ variants. Quantitative densitometric scanning of immunoblots of type VI collagen purified from gizzard and stained with VWFA module-specific antibodies indicated that the polymorphic migration pattern of alpha 3(VI) polypeptides is contributed by concurrent or independent splicing of two exons (A8 and A6) and probably by processing and/or proteolysis at the N- and C-terminus. Three exon-specific recombinant polypeptides were examined in cell adhesion assays, and A6 appeared to be the most active, particularly at low substrate concentrations. The adhesion to the recombinant modules was not abrogated by EDTA nor by mAbs against the integrin beta 1 or alpha 2 subunits. Over all, these results suggest that the splicing of the alpha 3(VI) mRNA and the tissue distribution pattern of type VI collagen variants, apart from promoting cell adhesion to different extents, might also affect additional structural as well as functional properties of this molecule, including microfilament formation and interaction with other extracellular matrix molecules.

Characterization of exoskeletal proteins from the American lobster, Homarus americanus

Proteins from the calcified exoskeleton of the lobster, Homarus americanus, were extracted and separated by two-dimensional gel-electrophoresis. Electroblotting the proteins onto polyvinylidene difluoride (PVDF) membranes followed by sequence determination gave 16 N-terminal amino-acid sequences and revealed that further eight proteins were N-terminally blocked. The relative molecular mass, M(r), was obtained for most of the electrophoretically separated proteins by means of matrix-assisted laser desorption mass spectrometry (MALDIMS) after electroelution from Coomassie-stained two-dimensional polyacrylamide gels. Eleven proteins were purified from extracts of the exoskeleton by low pressure ion exchange chromatography and reversed-phase high performance chromatography, and their sequences were determined by combined use of Edman degradation and mass spectrometry. Good agreement was obtained between the M(r)-values measured by mass spectrometry and those calculated from the sequences. Five of the sequenced proteins contain two copies of a previously observed 18-residue sequence motif, while a couple of the remaining sequences show similarity to sequences of exoskeletal proteins from shrimps and spiders. Only limited similarity to insect cuticular proteins was observed.

Stimulation of cell proliferation and autoregulation of elastin expression by elastin peptide VPGVG in cultured chick vascular smooth muscle cells

Synthetic elastin peptides, VPGVG or its polymer (VPGVG)n, enhanced the proliferation of smooth muscle cells 1.5-fold during 48 h treatment at the concentrations over 10(-6) M or 1.0 microgram/ml, respectively. Monomeric and polymeric VPGVG sequences reduced elastin synthesis and its mRNA level to one-third and one-half of control respectively under the conditions in which the proliferation of cells were enhanced, but did not change collagen synthesis as measured by bacterial collagenase digestion. The elastin-specific autoregulation by elastin fragments may reflect the feedback regulation of elastin expression which may play an essential role in elastin metabolism under the normal and diseased conditions.

Presence of elastin-related 45-kDa fragment in culture medium: specific cleavage product of tropoelastin in vascular smooth muscle cell culture

Significant amount of 45-kDa polypeptide was found to be present in the cultured medium of chick aortic smooth muscle cells. The polypeptide as well as tropoelastin (65-kDa) reacted with monoclonal antibody for tropoelastin. Pulse-chase experiments revealed that the relative density of the 45-kDa polypeptide to tropoelastin increased with chase periods. Partially purified radioactive tropoelastin (65-kDa) was converted to a 45-kDa polypeptide fragment in the test tube. The processing from the 65- to the 45-kDa polypeptide in the test tube was inhibited by ethylenediaminetetraacetic acid but not by N-ethylmaleimide and aminophenylmethylsulfonyl fluoride. These results indicate that the 45-kDa fragment is a degradation product of tropoelastin and that processing is mediated by enzymatic cleavages with metal proteinase. Fully hydroxylated tropoelastin treated with ascorbic acid was more resistant to the enzymes than underhydroxylated tropoelastin with scorbutic condition, suggesting that the structural stability of tropoelastin is also involved in the processing rate.

Conformational study of the Thr-Gly repeat in the Drosophila clock protein, PERIOD

Recent results with the Drosophila melanogaster period gene suggest that the apparently conserved repetitive motif (Thr-Gly)n encoded by this gene may play an important role in the temperature compensation of the circadian clock. We have therefore initiated both a theoretical and experimental conformational analysis of (Thr-Gly)n peptides. By using a build-up method, it is clear that the hexapeptide (Thr-Gly)3 represents a 'conformational monomer' and generates a stable type II or type III beta-turn. Circular dichroism and nuclear magnetic resonance spectra of synthetic (Thr-Gly)3 and poly(Thr-Gly) peptides revealed that these peptides exhibit flexible conformations, especially in more polar environments and at higher temperatures. We speculate that this flexibility may illuminate our understanding of both the molecular mechanism of temperature compensation and the systematic geographical distribution within Europe of the Thr-Gly length polymorphism in D. melanogaster.

Stimulation of elastin expression by minoxidil in chick skin fibroblasts

Minoxidil inhibited the proliferation of embryonic skin fibroblasts during the growth phase but not during the stationary phase. Minoxidil stimulated elastin synthesis two-fold in a dose-dependent manner at a concentration of 1 mM during the stationary phase. The stimulation of elastin synthesis paralleled a comparable increase in elastin mRNA level. These results suggest that the stimulation of elastin expression by minoxidil in skin fibroblasts was controlled at the elastin mRNA level and also suggest that its elastin-stimulating effect is not related to the suppressive effect on cell proliferation. Minoxidil appears to be a potent stimulator for elastin expression in skin fibroblasts.

Total synthesis and expression in Escherichia coli of a gene encoding human tropoelastin

To elucidate the structural features and interactions of tropoelastin (TEL) molecules which assist in giving the elastic fibre its physical properties, a 2210-bp synthetic human TEL-encoding gene (SHEL) was constructed for expression in Escherichia coli. To this end, a model of codon adjustment was tested which better suits the polypeptide biosynthetic needs of E. coli than the human sequence, where over one-third of this natural sequence contains expression-limiting rare codons and 4 amino acids alone account for 75% of the resulting polypeptide. This large synthetic TEL gene was expressed at a high level as the recombinant counterpart of human TEL and as a C-terminal fusion with glutathione S-transferase. This demonstrates that a synthetic approach based upon matching codon usage to that of the host organism can support significant expression of recombinant sequences. The synthetic gene incorporates the facility for simple cassette replacement in future insertion, deletion and mutagenesis experiments, including the introduction and removal of exon homologues. The resulting soluble polypeptide is easily purified and displays properties expected for this protein.

The evolution and recognition of protein sequence repeats

Many proteins sequences contain motifs which display similarity. The similarities between the repeats are a result of gene duplication and/or gene fusion. The evolutionary role of repeats within protein sequences is considered and some repeat examples are given ranging from tandem repeats to multiple types of repeats which are sequentially interspersed. Existing computer methods to delineate repeats in individual protein sequences are discussed and a novel sensitive repeat recognition method is introduced.

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)

A method to recognize distant repeats in protein sequences

An automated algorithm is presented that delineates protein sequence fragments which display similarity. The method incorporates a selection of a number of local nonoverlapping sequence alignments with the highest similarity scores and a graph-theoretical approach to elucidate the consistent start and end points of the fragments comprising one or more ensembles of related subsequences. The procedure allows the simultaneous identification of different types of repeats within one sequence. A multiple alignment of the resulting fragments is performed and a consensus sequence derived from the ensemble(s). Finally, a profile is constructed from the multiple alignment to detect possible and more distant members within the sequence. The method tolerates mutations in the repeats as well as insertions and deletions. The sequence spans between the various repeats or repeat clusters may be of different lengths. The technique has been applied to a number of proteins where the repeating fragments have been derived from information additional to the protein sequences.

Tropoelastin gene expression in the developing vascular system of the chicken: an in situ hybridization study

Temporal and spatial patterns in the accumulation of Tropoelastin (TE) mRNA during development of the chick embryo were established by in situ hybridization. Radiolabeled oligonucleotide probes of high specific activity were hybridized to serial sections of the cardiovascular system from embryonic day 3.5 (ED 3.5) to ED 19. Tropoelastin mRNA was observed as early as ED 3.5 in the dorsal part of the arterial trunk. During septation varying levels of TE mRNA were seen in the pulmonary trunk, the aorta and the aorticopulmonary septum. Thereafter TE mRNA levels increased up to ED 12, and the appearance of message was distributed distally in the walls of developing arteries. From ED 4.5 on, we found a decreasing proximo-distal gradient of the hybridization signal along the trunks and later along the main arteries (longitudinal gradient), and a radial gradient through the arterial vessel wall with the highest levels of TE mRNA in the outer layers of the media. Both gradients persisted in all major arterial vessels except in the proximal systemic and pulmonary trunks, where the original radial gradient was inverted or locally bimodal during the second half of development. The valvular region of aortic and pulmonary trunks showed particularly striking patterns of TE mRNA distribution, notably a prominent label on the endothelial cell layer on aortic and pulmonary valves. Outside the cardiovascular system, TE mRNA was mainly present in prochondral or perichondral cells in trachea and growing skeleton, and in the gap of growing joints. In kidney or nephric primordia, TE mRNA was only detectable in the wall of renal arteries. A hybridization signal was observed on mesenchyme of pulmonary septae at ED 16. Our results suggest a complex regulation of elastin gene expression during development, particularly within the proximal regions of the large arterial vessels.

Quantitation of tropoelastin mRNA and assessment of alternative splicing in human skin fibroblasts by reverse transcriptase-polymerase chain reaction

We have developed a reverse transcriptase-polymerase chain reaction (RT-PCR) assay for the quantitative measurement of levels of tropoelastin mRNA in total RNA preparations from skin fibroblasts. This method facilitates the reproducible detection of low abundance tropoelastin mRNA in the range of 10-1000 copies per cell. The procedure is based on a competitive RT-PCR assay where a tropoelastin cDNA-derived internal RNA standard is cotranscribed and coamplified together with the sample derived-endogenous target mRNA. In addition, RT-PCR of several domains of tropoelastin mRNA, followed by DNA sequence analysis of asymmetric PCR products, revealed a previously unknown pattern of alternate exon usage at the 3' end of the tropoelastin gene in human skin fibroblasts.

Solid-state studies on synthetic fragments and analogues of elastin

A series of synthetic fragments and analogues of elastin have been investigated, in the solid state, by means of differential scanning calorimetry and thermally stimulated current. Most of the polypeptides were shown to possess both amorphous regions and segments of long-range order. Water, which interacts preferentially with the amorphous zones, behaves as plasticizer, i.e. facilitates the localized motions of polypeptide chains. The results obtained have been correlated with elastin elasticity, in particular as far as the fundamental destructuring role of water is concerned.

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.

Ascidian entactin/nidogen. Implication of evolution by shuffling two kinds of cysteine-rich motifs

Entactin/nidogen, a major component of the basement membrane, has a domain structure comprising three globular domains, and thread-like and rod-like domains connecting them. It contains six epidermal-growth-factor-(EGF)-like motifs and one thyroglobulin-like motif. In the present study, ascidian entactin/nidogen has been identified by a monoclonal antibody technique. We prepared anti-(ascidian entactin/nidogen)IgG, named anti-AsEnt1, then cloned the cDNA of ascidian entactin/nidogen using anti-AsEnt1 as a probe, and determined its entire sequence. Mainly because the deduced amino acid sequence exhibited high similarity to mouse entactin and human nidogen, and because the antigen localized in basement membrane of ascidian body-wall muscle, we have concluded that the antigen anti-AsEnt1 corresponds to the ascidian entactin/nidogen homologue. The deduced amino acid sequence of ascidian entactin/nidogen clearly showed that the ascidian homologue also has a domain structure. However, the ascidian homologue lacked the thread-like domain, and the rod-like domain differed from that of mouse entactin in composition, consisting of two kinds of cysteine-rich motifs, that is, the EGF-like motif and the thyroglobulin-like motif. These results suggest that entactin/nidogen have evolved by modifying the domains, especially by shuffling the two kinds of cysteine-rich motifs.

PGAIPG, a repeated hexapeptide of bovine tropoelastin, is a ligand for the 67-kDa bovine elastin receptor

Tropoelastin is composed of alternating hydrophobic and hydrophilic domains. A hydrophobic peptide, VGVAPG, has been shown to be a ligand for a 67-kDa elastin cell surface receptor expressed on fetal bovine auricular chondrocytes and ligamentum nuchae fibroblasts. To explore the possibility that tropoelastin contains additional peptide ligands for this elastin receptor, we have constructed two deletion proteins that are expressed in E. coli and lack the repeated VGVAPG sequence. These proteins supported bovine fibroblast attachment implying the presence of a receptor binding site. Experiments using synthetic peptides contained within these proteins identify a chemotactic peptide, PGAIPG, and a chemokinetic peptide, GAIPG, PGAIPG was identified as a ligand for the bovine elastin receptor.