Tropoelastin as a thermodynamically unfolded premolten globule protein: The effect of trimethylamine N-oxide on structure and coacervation

Emerging mechanisms of elastin transcriptional regulation

Elastin provides recoil to tissues that stretch such as the lung, blood vessels, and skin. It is deposited in a brief window starting in the prenatal period and extending to adolescence in vertebrates, and then slowly turns over. Elastin insufficiency is seen in conditions such as Williams-Beuren syndrome and elastin-related supravalvar aortic stenosis, which are associated with a range of vascular and connective tissue manifestations. Regulation of the elastin (ELN) gene occurs at multiple levels including promoter activation/inhibition, mRNA stability, interaction with microRNAs, and alternative splicing. However, these mechanisms are incompletely understood. Better understanding of the processes controlling ELN gene expression may improve medicine's ability to intervene in these rare conditions, as well as to replace age-associated losses by re-initiating elastin production. This review describes what is known about the ELN gene promoter structure, transcriptional regulation by cytokines and transcription factors, and posttranscriptional regulation via mRNA stability and micro-RNA and highlights new approaches that may influence regenerative medicine.

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

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

Alterations of elastin in female reproductive tissues arising from advancing parity

Female reproductive tissues undergo significant alterations during pregnancy, which may compromise the structural integrity of extracellular matrix proteins. Here, we report on modifications of elastic fibers, which are primarily composed of elastin and believed to provide a scaffold to the reproductive tissues, due to parity and parturition. Elastic fibers from the upper vaginal wall of virgin Sprague Dawley rats were investigated and compared to rats having undergone one, three, or more than five pregnancies. Optical microscopy was used to study fiber level changes. Mass spectrometry, ¹³C and ²H NMR, was applied to study alterations of elastin from the uterine horns. Spectrophotometry was used to measure matrix metalloproteinases-2,9 and tissue inhibitor of metalloproteinase-1 concentration changes in the uterine horns. Elastic fibers were found to exhibit increase in tortuosity and fragmentation with increased pregnancies. Surprisingly, secondary structure, dynamics, and crosslinking of elastin from multiparous cohorts appear similar to healthy mammalian tissues, despite fragmentation observed at the fiber level. In contrast, elastic fibers from virgin and single pregnancy cohorts are less fragmented and comprised of elastin exhibiting structure and dynamics distinguishable from multiparous groups, with reduced crosslinking. These alterations were correlated to matrix metalloproteinases-2,9 and tissue inhibitor of metalloproteinase-1 concentrations. This work indicates that fiber level alterations resulting from pregnancy and/or parturition, such as fragmentation, rather than secondary structure (e.g. elastin crosslinking density), appear to govern scaffolding characteristics in the female reproductive tissues.

Elastin-driven genetic diseases

Elastic fibers provide recoil to tissues that undergo repeated deformation, such as blood vessels, lungs and skin. Composed of elastin and its accessory proteins, the fibers are produced within a restricted developmental window and are stable for decades. Their eventual breakdown is associated with a loss of tissue resiliency and aging. Rare alteration of the elastin (ELN) gene produces disease by impacting protein dosage (supravalvar aortic stenosis, Williams Beuren syndrome and Williams Beuren region duplication syndrome) and protein function (autosomal dominant cutis laxa). This review highlights aspects of the elastin molecule and its assembly process that contribute to human disease and also discusses potential therapies aimed at treating diseases of elastin insufficiency.

Design of an elastin-layered dermal regeneration template

We demonstrate a novel approach for the production of tunable quantities of elastic fibers. We also show that exogenous tropoelastin is rate-limiting for elastin synthesis regardless of the age of the dermal fibroblast donor. Additionally, we provide a strategy to further enhance synthesis by older cells through the application of conditioned media. We show that this approach delivers an elastin layer on one side of the leading dermal repair template for contact with the deep dermis in order to deliver prefabricated elastic fibers to a physiologically appropriate site during subsequent surgery. This system is attractive because it provides for the first time a viable path for sufficient, histologically detectable levels of patient elastin into full-thickness wound sites that have until now lacked this elastic underlayer.

STATEMENT OF SIGNIFICANCE

The scars of full thickness wounds typically lack elasticity. Elastin is essential for skin elasticity and is enriched in the deep dermis. This paper is significant because it shows that: (1) we can generate elastic fibers in tunable quantities, (2) tropoelastin is the rate-limiting component in elastin synthesis in vitro, (3) we can generate elastin fibers regardless of donor age, (4) we describe a novel approach to further increase the numbers and thickness of elastic fibers for older donors, (5) we improve on Integra Dermal Regeneration Template and generate a new hybrid biomaterial intended to subsequently surgically deliver these elastic fibers, (6) the elastic fiber layer is presented on the side of Integra that is intended for delivery into its physiologically appropriate site i.e. the deep dermis.

Mechanically Robust Plasma-Activated Interfaces Optimized for Vascular Stent Applications

The long-term performance of many medical implants is limited by the use of inherently incompatible and bioinert materials. Metallic alloys, ceramics, and polymers commonly used in cardiovascular devices encourage clot formation and fail to promote the appropriate molecular signaling required for complete implant integration. Surface coating strategies have been proposed for these materials, but coronary stents are particularly problematic as the large surface deformations they experience in deployment require a mechanically robust coating interface. Here, we demonstrate a single-step ion-assisted plasma deposition process to tailor plasma-activated interfaces to meet current clinical demands for vascular implants. Using a process control-feedback strategy which predicts crucial coating growth mechanisms by adopting a suitable macroscopic plasma description in combination with noninvasive plasma diagnostics, we describe the optimal conditions to generate highly reproducible, industry-scalable stent coatings. These interfaces are mechanically robust, resisting delamination even upon plastic deformation of the underlying material, and were developed in consideration of the need for hemocompatibility and the capacity for biomolecule immobilization. Our optimized coating conditions combine the best mechanical properties with strong covalent attachment capacity and excellent blood compatibility in initial testing with plasma and whole blood, demonstrating the potential for improved vascular stent coatings.

Plasma-Activated Tropoelastin Functionalization of Zirconium for Improved Bone Cell Response

The mechanical strength, durability, corrosion resistance, and biocompatibility of metal alloys based on zirconium (Zr) and titanium (Ti) make them desirable materials for orthopedic implants. However, as bioinert metals, they do not actively promote bone formation and integration. Here we report a plasma coating process for improving integration of such metal implants with local bone tissue. The coating is a stable carbon-based plasma polymer layer that increased surface wettability by 28%, improved surface elasticity to the range exhibited by natural bone, and additionally covalently bound the extracellular matrix protein, tropoelastin, in an active conformation. The thus biofunctionalized material was significantly more resistant to medical-grade sterilization by steam, autoclaving or gamma-ray irradiation, retaining >60% of the adhered tropoelastin molecules and preserving full bioactivity. The interface of the coating and metal was robust so as to resist delamination during surgical insertion and in vivo deployment, and the plasma process employed was utilized to also coat the complex 3D geometries typical of orthopedic implants. Osteoblast-like osteosarcoma cells cultured on the biofunctionalized Zr surface exhibited a significant 30% increase in adhesion and up to 70% improvement in proliferation. Cells on these materials also showed significant early stage up-regulation of bone marker expression (alkaline phosphatase, 1.8 fold; osteocalcin, 1.4 fold), and sustained up-regulation of these genes (alkaline phosphatase, 1.3 fold; osteocalcin, 1.2 fold) in osteogenic conditions. In addition, alkaline phosphatase production significantly increased (2-fold) on the functionalized surfaces, whereas bone mineral deposition increased by 30% above background levels compared to bare Zr. These findings have the potential to be readily translated to the development of improved Zr and Ti-based implants for accelerated bone repair.

Subtle balance of tropoelastin molecular shape and flexibility regulates dynamics and hierarchical assembly

The assembly of the tropoelastin monomer into elastin is vital for conferring elasticity on blood vessels, skin, and lungs. Tropoelastin has dual needs for flexibility and structure in self-assembly. We explore the structure-dynamics-function interplay, consider the duality of molecular order and disorder, and identify equally significant functional contributions by local and global structures. To study these organizational stratifications, we perturb a key hinge region by expressing an exon that is universally spliced out in human tropoelastins. We find a herniated nanostructure with a displaced C terminus and explain by molecular modeling that flexible helices are replaced with substantial β sheets. We see atypical higher-order cross-linking and inefficient assembly into discontinuous, thick elastic fibers. We explain this dysfunction by correlating local and global structural effects with changes in the molecule's assembly dynamics. This work has general implications for our understanding of elastomeric proteins, which balance disordered regions with defined structural modules at multiple scales for functional assembly.

Perspectives on the Molecular and Biological Implications of Tropoelastin in Human Tissue Elasticity

The elasticity of a range of vertebrate and particularly human tissues depends on the dynamic and persistent protein elastin. This elasticity is diverse, and comprises skin, blood vessels, and lung, and is essential for tissue viability. Elastin is predominantly made by assembling tropoelastin, which is an asymmetric 20-nm-long protein molecule. This overview considers tropoelastin’s molecular features and biological interactions in the context of its value in tissue repair.

A negatively charged residue stabilizes the tropoelastin N-terminal region for elastic fiber assembly

Tropoelastin is an extracellular matrix protein that assembles into elastic fibers that provide elasticity and strength to vertebrate tissues. Although the contributions of specific tropoelastin regions during each stage of elastogenesis are still not fully understood, studies predominantly recognize the central hinge/bridge and C-terminal foot as the major participants in tropoelastin assembly, with a number of interactions mediated by the abundant positively charged residues within these regions. However, much less is known about the importance of the rarely occurring negatively charged residues and the N-terminal coil region in tropoelastin assembly. The sole negatively charged residue in the first half of human tropoelastin is aspartate 72. In contrast, the same region comprises 17 positively charged residues. We mutated this aspartate residue to alanine and assessed the elastogenic capacity of this novel construct. We found that D72A tropoelastin has a decreased propensity for initial self-association, and it cross-links aberrantly into denser, less porous hydrogels with reduced swelling properties. Although the mutant can bind cells normally, it does not form elastic fibers with human dermal fibroblasts and forms fewer atypical fibers with human retinal pigmented epithelial cells. This impaired functionality is associated with conformational changes in the N-terminal region. Our results strongly point to the role of the Asp-72 site in stabilizing the N-terminal segment of human tropoelastin and the importance of this region in facilitating elastic fiber assembly.

Molecular-level characterization of elastin-like constructs and human aortic elastin

This study aimed to characterize the structures of two elastin-like constructs, one composed of a cross-linked elastin-like polypeptide and the other one of cross-linked tropoelastin, and native aortic elastin. The structures of the insoluble materials and human aortic elastin were investigated using scanning electron microscopy. Additionally, all samples were digested with enzymes of different specificities, and the resultant peptide mixtures were characterized by ESI mass spectrometry and MALDI mass spectrometry. The MS(2) data was used to sequence linear peptides, and cross-linked species were analyzed with the recently developed software PolyLinX. This enabled the identification of two intramolecularly cross-linked peptides containing allysine aldols in the two constructs. The presence of the tetrafunctional cross-link desmosine was shown for all analyzed materials and its quantification revealed that the cross-linking degree of the two in vitro cross-linked materials was significantly lower than that of native elastin. Molecular dynamics simulations were performed, based on molecular species identified in the samples, to follow the formation of elastin cross-links. The results provide evidence for the significance of the GVGTP hinge region of domain 23 for the formation of elastin cross-links. Overall, this work provides important insight into structural similarities and differences between elastin-like constructs and native elastin. Furthermore, it represents a step toward the elucidation of the complex cross-linking pattern of mature elastin.

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.

Engineered cell-laden human protein-based elastomer

Elastic tissue equivalence is a vital requirement of synthetic materials proposed for many resilient, soft tissue engineering applications. Here we present a bioelastomer made from tropoelastin, the human protein that naturally facilitates elasticity and cell interactions in all elastic tissues. We combined this protein's innate versatility with fast non-toxic fabrication techniques to make highly extensible, cell compatible hydrogels. These hydrogels can be produced in less than a minute through photocrosslinking of methacrylated tropoelastin (MeTro) in an aqueous solution. The fabricated MeTro gels exhibited high extensibility (up to 400%) and superior mechanical properties that outperformed other photocrosslinkable hydrogels. MeTro gels were used to encapsulate cells within a flexible 3D environment and to manufacture highly elastic 2D films for cell attachment, growth, and proliferation. In addition, the physical properties of this fabricated bioelastomer such as elasticity, stiffness, and pore characteristics were tuned through manipulation of the methacrylation degree and protein concentration. This photocrosslinkable, functional tissue mimetic gel benefits from the innate biological properties of a human elastic protein and opens new opportunities in tissue engineering.

Heat-induced fibrillation of BclXL apoptotic repressor

The BclXL apoptotic repressor bears the propensity to associate into megadalton oligomers in solution, particularly under acidic pH. Herein, using various biophysical methods, we analyze the effect of temperature on the oligomerization of BclXL. Our data show that BclXL undergoes irreversible aggregation and assembles into highly-ordered rope-like homogeneous fibrils with length in the order of mm and a diameter in the μm-range under elevated temperatures. Remarkably, the formation of such fibrils correlates with the decay of a largely α-helical fold into a predominantly β-sheet architecture of BclXL in a manner akin to the formation of amyloid fibrils. Further interrogation reveals that while BclXL fibrils formed under elevated temperatures show no observable affinity toward BH3 ligands, they appear to be optimally primed for insertion into cardiolipin bicelles. This salient observation strongly argues that BclXL fibrils likely represent an on-pathway intermediate for insertion into mitochondrial outer membrane during the onset of apoptosis. Collectively, our study sheds light on the propensity of BclXL to form amyloid-like fibrils with important consequences on its mechanism of action in gauging the apoptotic fate of cells in health and disease.

Lipid loading of human vascular smooth muscle cells induces changes in tropoelastin protein levels and physical structure

Aggregated low-density lipoprotein (agLDL), one of the main LDL modifications in the arterial intima, contributes to massive intracellular cholesteryl ester (CE) accumulation in human vascular smooth muscle cells (VSMC), which are major producers of elastin in the vascular wall. Our aim was to analyze the levels, physical structure, and molecular mobility of tropoelastin produced by agLDL-loaded human VSMC (agLDL-VSMC) versus that produced by control VSMC. Western blot analysis demonstrated that agLDL reduced VSMC-tropoelastin protein levels by increasing its degradation rate. Moreover, our results demonstrated increased levels of precursor and mature forms of cathepsin S in agLDL-VSMC. Fourier transform infrared analysis revealed modifications in the secondary structures of tropoelastin produced by lipid-loaded VSMCs. Thermal and dielectric analyses showed that agLDL-VSMC tropoelastin has decreased glass transition temperatures and distinct chain dynamics that, in addition to a loss of thermal stability, lead to strong changes in its mechanical properties. In conclusion, agLDL lipid loading of human vascular cells leads to an increase in cathepsin S production concomitantly with a decrease in cellular tropoelastin protein levels and dramatic changes in secreted tropoelastin physical structure. Therefore, VSMC-lipid loading likely determines alterations in the mechanical properties of the vascular wall and plays a crucial role in elastin loss during atherosclerosis.

Polymorphisms in the human tropoelastin gene modify in vitro self-assembly and mechanical properties of elastin-like polypeptides

Elastin is a major structural component of elastic fibres that provide properties of stretch and recoil to tissues such as arteries, lung and skin. Remarkably, after initial deposition of elastin there is normally no subsequent turnover of this protein over the course of a lifetime. Consequently, elastic fibres must be extremely durable, able to withstand, for example in the human thoracic aorta, billions of cycles of stretch and recoil without mechanical failure. Major defects in the elastin gene (ELN) are associated with a number of disorders including Supravalvular aortic stenosis (SVAS), Williams-Beuren syndrome (WBS) and autosomal dominant cutis laxa (ADCL). Given the low turnover of elastin and the requirement for the long term durability of elastic fibres, we examined the possibility for more subtle polymorphisms in the human elastin gene to impact the assembly and long-term durability of the elastic matrix. Surveys of genetic variation resources identified 118 mutations in human ELN, 17 being non-synonymous. Introduction of two of these variants, G422S and K463R, in elastin-like polypeptides as well as full-length tropoelastin, resulted in changes in both their assembly and mechanical properties. Most notably G422S, which occurs in up to 40% of European populations, was found to enhance some elastomeric properties. These studies reveal that even apparently minor polymorphisms in human ELN can impact the assembly and mechanical properties of the elastic matrix, effects that over the course of a lifetime could result in altered susceptibility to cardiovascular disease.

Structural disorder and protein elasticity

An emerging class of disordered proteins underlies the elasticity of many biological tissues. Elastomeric proteins are essential to the function of biological machinery as diverse as the human arterial wall, the capture spiral of spider webs and the jumping mechanism of fleas. In this chapter, we review what is known about the molecular basis and the functional role of structural disorder in protein elasticity. In general, the elastic recoil of proteins is due to a combination of internal energy and entropy. In rubber-like elastomeric proteins, the dominant driving force is the increased entropy of the relaxed state relative to the stretched state. Aggregates of these proteins are intrinsically disordered or fuzzy, with high polypeptide chain entropy. We focus our discussion on the sequence, structure and function of five rubber-like elastomeric proteins, elastin, resilin, spider silk, abductin and ColP. Although we group these disordered elastomers together into one class of proteins, they exhibit a broad range of sequence motifs, mechanical properties and biological functions. Understanding how sequence modulates both disorder and elasticity will help advance the rational design of elastic biomaterials such as artificial skin and vascular grafts.

Shape of tropoelastin, the highly extensible protein that controls human tissue elasticity

Elastin enables the reversible deformation of elastic tissues and can withstand decades of repetitive forces. Tropoelastin is the soluble precursor to elastin, the main elastic protein found in mammals. Little is known of the shape and mechanism of assembly of tropoelastin as its unique composition and propensity to self-associate has hampered structural studies. In this study, we solve the nanostructure of full-length and corresponding overlapping fragments of tropoelastin using small angle X-ray and neutron scattering, allowing us to identify discrete regions of the molecule. Tropoelastin is an asymmetric coil, with a protruding foot that encompasses the C-terminal cell interaction motif. We show that individual tropoelastin molecules are highly extensible yet elastic without hysteresis to perform as highly efficient molecular nanosprings. Our findings shed light on how biology uses this single protein to build durable elastic structures that allow for cell attachment to an appended foot. We present a unique model for head-to-tail assembly which allows for the propagation of the molecule's asymmetric coil through a stacked spring design.

Proline periodicity modulates the self-assembly properties of elastin-like polypeptides

Elastin is a self-assembling protein of the extracellular matrix that provides tissues with elastic extensibility and recoil. The monomeric precursor, tropoelastin, is highly hydrophobic yet remains substantially disordered and flexible in solution, due in large part to a high combined threshold of proline and glycine residues within hydrophobic sequences. In fact, proline-poor elastin-like sequences are known to form amyloid-like fibrils, rich in β-structure, from solution. On this basis, it is clear that hydrophobic elastin sequences are in general optimized to avoid an amyloid fate. However, a small number of hydrophobic domains near the C terminus of tropoelastin are substantially depleted of proline residues. Here we investigated the specific contribution of proline number and spacing to the structure and self-assembly propensities of elastin-like polypeptides. Increasing the spacing between proline residues significantly decreased the ability of polypeptides to reversibly self-associate. Real-time imaging of the assembly process revealed the presence of smaller colloidal droplets that displayed enhanced propensity to cluster into dense networks. Structural characterization showed that these aggregates were enriched in β-structure but unable to bind thioflavin-T. These data strongly support a model where proline-poor regions of the elastin monomer provide a unique contribution to assembly and suggest a role for localized β-sheet in mediating self-assembly interactions.

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.

UV and X-ray structural studies of a 101-residue long Tat protein from a HIV-1 primary isolate and of its mutated, detoxified, vaccine candidate

The 101-residue long Tat protein of primary isolate 133 of the human immunodeficiency virus type 1 (HIV-1), wt-Tat(133) displays a high transactivation activity in vitro, whereas the mutant thereof, STLA-Tat(133), a vaccine candidate for HIV-1, has none. These two proteins were chemically synthesized and their biological activity was validated. Their structural properties were characterized using circular dichroism (CD), fluorescence emission, gel filtration, dynamic light scattering, and small angle X-ray scattering (SAXS) techniques. SAXS studies revealed that both proteins were extended and belong to the family of intrinsically unstructured proteins. CD measurements showed that wt-Tat(133) or STLA-Tat(133) underwent limited structural rearrangements when complexed with specific fragments of antibodies. Crystallization trials have been performed on the two forms, assuming that the Tat(133) proteins might have a better propensity to fold in supersaturated conditions, and small crystals have been obtained. These results suggest that biologically active Tat protein is natively unfolded and requires only a limited gain of structure for its function.