Structure-function relationship in the evolution of elastin

Zebrafish as a Model to Study Vascular Elastic Fibers and Associated Pathologies

Many extensible tissues such as skin, lungs, and blood vessels require elasticity to function properly. The recoil of elastic energy stored during a stretching phase is provided by elastic fibers, which are mostly composed of elastin and fibrillin-rich microfibrils. In arteries, the lack of elastic fibers leads to a weakening of the vessel wall with an increased risk to develop cardiovascular defects such as stenosis, aneurysms, and dissections. The development of new therapeutic molecules involves preliminary tests in animal models that recapitulate the disease and whose response to drugs should be as close as possible to that of humans. Due to its superior in vivo imaging possibilities and the broad tool kit for forward and reverse genetics, the zebrafish has become an important model organism to study human pathologies. Moreover, it is particularly adapted to large scale studies, making it an attractive model in particular for the first steps of investigations. In this review, we discuss the relevance of the zebrafish model for the study of elastic fiber-related vascular pathologies. We evidence zebrafish as a compelling alternative to conventional mouse models.

Extracellular Matrix in Vascular Disease, Part 2/4: JACC Focus Seminar

Medium-sized and large arteries consist of 3 layers: the tunica intima, tunica media, and tunica adventitia. The tunica media accounts for the bulk of the vessel wall and is the chief determinant of mechanical compliance. It is primarily composed of circumferentially arranged layers of vascular smooth muscle cells that are separated by concentrically arranged elastic lamellae; a form of extracellular matrix (ECM). The tunica media is separated from the tunica intima and tunica adventitia, the innermost and outermost layers, respectively, by the internal and external elastic laminae. This second part of a 4-part JACC Focus Seminar discusses the contributions of the ECM to vascular homeostasis and pathology. Advances in genetics and proteomics approaches have fostered significant progress in our understanding of vascular ECM. This review highlights the important role of the ECM in vascular disease and the prospect of translating these discoveries into clinical disease biomarkers and potential future therapies.

Mechanisms of soft tissue and protein preservation in Tyrannosaurus rex

The idea that original soft tissue structures and the native structural proteins comprising them can persist across geological time is controversial, in part because rigorous and testable mechanisms that can occur under natural conditions, resulting in such preservation, have not been well defined. Here, we evaluate two non-enzymatic structural protein crosslinking mechanisms, Fenton chemistry and glycation, for their possible contribution to the preservation of blood vessel structures recovered from the cortical bone of a Tyrannosaurus rex (USNM 555000 [formerly, MOR 555]). We demonstrate the endogeneity of the fossil vessel tissues, as well as the presence of type I collagen in the outermost vessel layers, using imaging, diffraction, spectroscopy, and immunohistochemistry. Then, we use data derived from synchrotron FTIR studies of the T. rex vessels to analyse their crosslink character, with comparison against two non-enzymatic Fenton chemistry- and glycation-treated extant chicken samples. We also provide supporting X-ray microprobe analyses of the chemical state of these fossil tissues to support our conclusion that non-enzymatic crosslinking pathways likely contributed to stabilizing, and thus preserving, these T. rex vessels. Finally, we propose that these stabilizing crosslinks could play a crucial role in the preservation of other microvascular tissues in skeletal elements from the Mesozoic.

A comprehensive map of human elastin cross-linking during elastogenesis

Elastin is an essential structural protein in the extracellular matrix of vertebrates. It is the core component of elastic fibers, which enable connective tissues such as those of the skin, lungs or blood vessels to stretch and recoil. This function is provided by elastin's exceptional properties, which mainly derive from a unique covalent cross-linking between hydrophilic lysine-rich motifs of units of the monomeric precursor tropoelastin. To date, elastin's cross-linking is poorly investigated. Here, we purified elastin from human tissue and cleaved it into soluble peptides using proteases with different specificities. We then analyzed elastin's molecular structure by identifying unmodified residues, post-translational modifications and cross-linked peptides by high-resolution mass spectrometry and amino acid analysis. The data revealed the presence of multiple isoforms in parallel and a complex and heterogeneous molecular interconnection. We discovered that the same lysine residues in different monomers were simultaneously involved in various cross-link types or remained unmodified. Furthermore, both types of cross-linking domains, Lys-Pro and Lys-Ala domains, participate not only in bifunctional inter- but also in intra-domain cross-links. We elucidated the sequences of several desmosine-containing peptides and the contribution of distinct domains such as 6, 14 and 25. In contrast to earlier assumptions proposing that desmosine cross-links are formed solely between two domains, we elucidated the structure of a peptide that proves a desmosine formation with participation of three Lys-Ala domains. In summary, these results provide new and detailed insights into the cross-linking process, which takes place within and between human tropoelastin units in a stochastic manner.

Elastin is heterogeneously cross-linked

Elastin is an essential vertebrate protein responsible for the elasticity of force-bearing tissues such as those of the lungs, blood vessels, and skin. One of the key features required for the exceptional properties of this durable biopolymer is the extensive covalent cross-linking between domains of its monomer molecule tropoelastin. To date, elastin's exact molecular assembly and mechanical properties are poorly understood. Here, using bovine elastin, we investigated the different types of cross-links in mature elastin to gain insight into its structure. We purified and proteolytically cleaved elastin from a single tissue sample into soluble cross-linked and noncross-linked peptides that we studied by high-resolution MS. This analysis enabled the elucidation of cross-links and other elastin modifications. We found that the lysine residues within the tropoelastin sequence were simultaneously unmodified and involved in various types of cross-links with different other domains. The Lys-Pro domains were almost exclusively linked via lysinonorleucine, whereas Lys-Ala domains were found to be cross-linked via lysinonorleucine, allysine aldol, and desmosine. Unexpectedly, we identified a high number of intramolecular cross-links between lysine residues in close proximity. In summary, we show on the molecular level that elastin formation involves random cross-linking of tropoelastin monomers resulting in an unordered network, an unexpected finding compared with previous assumptions of an overall beaded structure.

Molecular assembly and mechanical properties of the extracellular matrix: A fibrous protein perspective

The extracellular matrix is an integral and dynamic component of all tissues. Macromolecular compositions and structural architectures of the matrix are tissue-specific and typically are strongly influenced by the magnitude and direction of biomechanical forces experienced as part of normal tissue function. Fibrous extracellular networks of collagen and elastin provide the dominant response to tissue mechanical forces. These matrix proteins enable tissues to withstand high tensile and repetitive stresses without plastic deformation or rupture. Here we provide an overview of the hierarchical molecular and supramolecular assembly of collagens and elastic fibers, and review their capacity for mechanical behavior in response to force. This article is part of a Special Issue entitled: Fibrosis: Translation of basic research to human disease.

Modelling the self-assembly of elastomeric proteins provides insights into the evolution of their domain architectures

Elastomeric proteins have evolved independently multiple times through evolution. Produced as monomers, they self-assemble into polymeric structures that impart properties of stretch and recoil. They are composed of an alternating domain architecture of elastomeric domains interspersed with cross-linking elements. While the former provide the elasticity as well as help drive the assembly process, the latter serve to stabilise the polymer. Changes in the number and arrangement of the elastomeric and cross-linking regions have been shown to significantly impact their assembly and mechanical properties. However, to date, such studies are relatively limited. Here we present a theoretical study that examines the impact of domain architecture on polymer assembly and integrity. At the core of this study is a novel simulation environment that uses a model of diffusion limited aggregation to simulate the self-assembly of rod-like particles with alternating domain architectures. Applying the model to different domain architectures, we generate a variety of aggregates which are subsequently analysed by graph-theoretic metrics to predict their structural integrity. Our results show that the relative length and number of elastomeric and cross-linking domains can significantly impact the morphology and structural integrity of the resultant polymeric structure. For example, the most highly connected polymers were those constructed from asymmetric rods consisting of relatively large cross-linking elements interspersed with smaller elastomeric domains. In addition to providing insights into the evolution of elastomeric proteins, simulations such as those presented here may prove valuable for the tuneable design of new molecules that may be exploited as useful biomaterials.

Elastomeric proteins such as elastin, resilin, abductin and wheat gluten represent a remarkable class of self-assembling proteins that provide properties of extensibility and elastic recoil. Although unrelated from an evolutionary viewpoint, these proteins nonetheless share a common sequence design involving highly repetitive elastomeric regions interspersed with elements capable of forming cross-links that help stabilize the formation of polymers. Attempts to explore the influence of domain architecture on the self-assembly and mechanical properties of elastomeric proteins at the molecular level have largely been hindered by a general lack of detailed structural information. Here we introduce a novel theoretical study based on random walks to simulate the self-assembly of elastomeric proteins. Applying this model, we explored the impact of different configurations of elastomeric and cross-linking elements on the stability of the resultant polymer. Through exploring the complex relationships between elastomeric domains, required to drive self-assembly, and cross-linking domains, required for structural integrity, results from these simulations provide insights into the molecular basis for the evolution of elastomeric proteins as well as help guide the rational design of novel elastomeric-peptides.

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.

Function-structure relationship of elastic arteries in evolution: from microfibrils to elastin and elastic fibres

Evolution of species has led to the appearance of circulatory systems including blood vessels and one or more pulsatile pumps, typically resulting in a low-pressurised open circulation in most invertebrates and a high-pressurised closed circulation in vertebrates. In both open and closed circulations, the large elastic arteries proximal to the heart damp out the pulsatile flow and blood pressure delivered by the heart, in order to limit distal shear stress and to allow regular irrigation of downstream organs. To achieve this goal, networks of resilient and stiff proteins adapted to each situation--i.e. low or high blood pressure--have been developed in the arterial wall to provide it with non-linear elasticity. In the low-pressurised circulation of some invertebrates, the mechanical properties of arteries can almost be entirely microfibril-based, whereas, in high-pressurised circulations, they are due to an interplay between a highly resilient protein, an elastomer in the octopus and elastin in most vertebrates, and the rather stiff protein collagen. In vertebrate development, elastin is incorporated in elastic fibres, on a earlier deposited scaffold of microfibrils. The elastic fibres are then arranged in functional concentric elastic lamellae and, with the smooth muscle cells, lamellar units. The microfibrils may also play a direct functional role in all mature arteries of high- and low-pressurised circulations. Finally, since blood pressure regularly increases with developmental stages, it appears possible that the early deposition of microfibrils, which are highly-conserved in evolution, corresponds, at least in part, to an early microfibril-driven elasticity in low-pressurised arteries, present across species. In vertebrates, when pressure developmentally rises above a threshold value, the vascular wall stress may turn on the expression of other resilient protein genes, including the elastin gene. Elastin would then be deposited on microfibrils and resulting in the elastic fibre network and elastic lamellae whose mechanical properties are adapted to allow for proper arterial work at higher pressures.

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

Poly (Val-Gly-Gly-Leu-Gly), a polypeptide mimicking the physico-chemical properties of the glycine-rich regions of elastin, has been synthesized and studied both in solution and in the aggregated state. By comparison, also the conformation of different "monomeric" units has been investigated. The polymer showed increased disorder with respect to the "monomers", the molecular conformation being accounted for by a more or less random collection of isolated beta-turns. Nevertheless, in the solid state the polymer is able to adopt supramolecular structures reminiscent of those found for elastin.

A re-evaluation of the distribution of the elastic meshwork within the periodontal ligament of the mouse

The elastic properties of the periodontal ligament have been attributed, in part, to oxytalan fibers, as no other types of elastic fibers are described there. It has been difficult to study the periodontal elastic meshwork by standard microscopic techniques because it is partially obscured by the adjacent periodontal ligament collagen fibers. Our study employed methods which either completely or partially removed mandibular molar periodontal ligament collagen fibers, exposing a previously undescribed periodontal elastic meshwork. The periodontal elastic meshwork was composed of many elastin lamellae containing both peripheral microfibrils of regular arrangement and central microfibrils of irregular arrangement, which could only be demonstrated in oxidized tissues. Peripheral, regularly arranged bundles of microfibrils resembled oxytalan fibers, which were often adherent to the border of the elastin lamella. Elastin lamellae containing irregular microfibrils resembled elaunin fibers. These fibers probably enclosed either blood vessels, nerves or collagen fiber bundles. Peripheral microfibrils attached elaunin to cementum, alveolar bone, blood vessels, and principal periodontal collagen fibers. Thus, the periodontal elastic meshwork is composed of both oxytalan and elaunin fibers. Microfibrils attach elaunin fibers to the adjacent non-elastic tissue and also form bundles which traverse the periodontal ligament space and are probably the oxytalan fibers demonstrable by light microscopic techniques. This meshwork of oxytalan and elaunin fibers probably contributes to tooth support and maintenance of periodontal homeostasis by dissipating chewing forces and maintaining patency of periodontal blood vessels.

Characterization of elastin protein and mRNA from salmonid fish (Oncorhynchus kisutch)

1. Elastin was isolated from the bulbus arteriosus of a salmonid fish. Monoclonal and polyclonal antibodies, elicited against a CNBr digest of this protein, immunoprecipitated a polypeptide of Mr 43,000 from fish cell culture medium. 2. Cell-free translation of salmon poly A+ RNA produced a protein of approximately 43 kD that was immunoprecipitated with anti-elastin antibodies. The corresponding mRNA had an approximate Mr of 2 kb. 3. Despite similarities in amino acid composition, the differences in Mr between mammalian and salmon mRNA and protein suggest a divergence of fish and higher vertebrate elastins from an earlier ancestral gene.

Sequence analysis of elastin cDNA from chick aorta and tissue-specific transcription of the elastin gene in developing chick embryo

A portion of elastin cDNA with a size of 1.5 kilobase pairs (kb) was cloned from chick aorta. Sequence analysis revealed that the cDNA consists of 0.9 kb of coding region and 0.6 kb of 3'-untranslatable region. The primary structure of the peptide deduced from the coding sequence exhibited a strong homology with the published data from sheep and bovine elastin cDNA. The abundance of elastin mRNA in the aorta and skin was studied in developing chick embryo by Northern analysis using the cDNA as a probe. The elastin mRNA level in the aorta gradually decreased in the late half of development, while the elastin mRNA level in the skin was dramatically elevated between the 18th and 21st days. These results strongly suggest that the transcription of the elastin gene was controlled specifically in the respective organ during development.

Schistosoma mansoni: degradation of host extracellular matrix by eggs and miracidia

A radioactively labeled in vitro model of the extracellular matrix of the mammalian intestinal wall and of snail tissue was used to determine whether proteolytic enzymes released by eggs and miracidia of Schistosoma mansoni could degrade connective tissue macromolecules in the type of interactive framework found in vivo. Eggs were collected and miracidia hatched in the presence of antibiotics to eliminate bacterial contamination. Uninfected livers were used as controls to ensure that the tissue dissociation and egg collection procedures did not produce proteolytic activity. One thousand live eggs incubated with the extracellular matrix for 72 hr at 37 C degraded 31% of the glycoprotein in the matrix; there was no degradation of elastin or collagen. Medium conditioned by incubation with eggs degraded 60% as much of the matrix as the live eggs themselves. The proteolytic activity of the egg-conditioned medium was greater in the presence of dithiothreitol. Miracidia incubated with the extracellular matrix in tissue culture medium at 27 or 37 C rapidly transformed to living sporocysts. This transformation was accompanied by a release of proteolytic activity, resulting in the degradation of 49 to 58% of the glycoprotein in the extracellular matrix by 1000 miracidia. Again, no elastin or collagen was degraded. The time course of degradation by miracidia was rapid over 24 hr and thus similar to that previously reported for cercariae. Degradation by eggs occurred more slowly over 72 hr. These data confirm that both eggs and miracidia secrete proteinases which are capable of degrading at least the glycoprotein components of extracellular matrix to facilitate their migration through intestinal wall or penetration of snail tissue.