Structure and composition of the elastin fibre in normal and pathological conditions

Natural Piezoelectric Biomaterials: A Biocompatible and Sustainable Building Block for Biomedical Devices

The piezoelectric effect has been widely observed in biological systems, and its applications in biomedical field are emerging. Recent advances of wearable and implantable biomedical devices bring promise as well as requirements for the piezoelectric materials building blocks. Owing to their biocompatibility, biosafety, and environmental sustainability, natural piezoelectric biomaterials are known as a promising candidate in this emerging field, with a potential to replace conventional piezoelectric ceramics and synthetic polymers. Herein, we provide a thorough review of recent progresses of research on five major types of piezoelectric biomaterials including amino acids, peptides, proteins, viruses, and polysaccharides. Our discussion focuses on their structure- and phase-related piezoelectric properties and fabrication strategies to achieve desired piezoelectric phases. We compare and analyze their piezoelectric performance and further introduce and comment on the approaches to improve their piezoelectric property. Representative biomedical applications of this group of functional biomaterials including energy harvesting, sensing, and tissue engineering are also discussed. We envision that molecular-level understanding of the piezoelectric effect, piezoelectric response improvement, and large-scale manufacturing are three main challenges as well as research and development opportunities in this promising interdisciplinary field.

Allysine modifications perturb tropoelastin structure and mobility on a local and global scale

Elastin provides elastic tissues with resilience through stretch and recoil cycles, and is primarily made of its extensively cross-linked monomer, tropoelastin. Here, we leverage the recently published full atomistic model of tropoelastin to assess how allysine modifications, which are essential to cross-linking, contribute to the dynamics and structural changes that occur in tropoelastin in the context of elastin assembly. We used replica exchange molecular dynamics to generate structural ensembles of allysine containing tropoelastin. We conducted principal component analysis on these ensembles and found that the molecule departs from the canonical structural ensemble. Furthermore, we showed that, while the canonical scissors-twist movement was retained, new movements emerged that deviated from those of the wild type protein, providing evidence for the involvement of a variety of molecular motions in elastin assembly. Additionally, we highlighted secondary structural changes and linked these perturbations to the longevity of specific salt bridges. We propose a model where allysines in tropoelastin contribute to hierarchical elastin assembly through global and local perturbations to molecular structure and dynamics.

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converting lysine to allysine by lysyl oxidases is needed to generate crosslinks between tropoelastin molecules in order to make elastin

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structural changes in the intact tropoelastin molecule ensue where modified tropoelastin molecules structurally depart from the canonical ensemble

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new molecular motions deviate from those of unmodified tropoelastin

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persistence times of specific salt bridges contribute to these perturbations

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allysines in tropoelastin contribute to hierarchical elastin assembly through global and local perturbations to molecular structure and dynamics

Exposure of tropoelastin to peroxynitrous acid gives high yields of nitrated tyrosine residues, di-tyrosine cross-links and altered protein structure and function

Elastin is an abundant extracellular matrix protein in elastic tissues, including the lungs, skin and arteries, and comprises 30-57% of the aorta by dry mass. The monomeric precursor, tropoelastin (TE), undergoes complex processing during elastogenesis to form mature elastic fibres. Peroxynitrous acid (ONOOH), a potent oxidising and nitrating agent, is formed in vivo from superoxide and nitric oxide radicals. Considerable evidence supports ONOOH formation in the inflamed artery wall, and a role for this species in the development of human atherosclerotic lesions, with ONOOH-damaged extracellular matrix implicated in lesion rupture. We demonstrate that TE is highly sensitive to ONOOH, with this resulting in extensive dimerization, fragmentation and nitration of Tyr residues to give 3-nitrotyrosine (3-nitroTyr). This occurs with equimolar or greater levels of oxidant and increases in a dose-dependent manner. Quantification of Tyr loss and 3-nitroTyr formation indicates extensive Tyr modification with up to two modified Tyr per protein molecule, and up to 8% conversion of initial ONOOH to 3-nitroTyr. These effects were modulated by bicarbonate, an alternative target for ONOOH. Inter- and intra-protein di-tyrosine cross-links have been characterized by mass spectrometry. Examination of human atherosclerotic lesions shows colocalization of 3-nitroTyr with elastin epitopes, consistent with TE or elastin modification in vivo, and also an association of 3-nitroTyr containing proteins and elastin with lipid deposits. These data suggest that exposure of TE to ONOOH gives marked chemical and structural changes to TE and altered matrix assembly, and that such damage accumulates in human arterial tissue during the development of atherosclerosis.

Targeted Modulation of Tropoelastin Structure and Assembly

Tropoelastin, as the monomer unit of elastin, assembles into elastic fibers that impart strength and resilience to elastic tissues. Tropoelastin is also widely used to manufacture versatile materials with specific mechanical and biological properties. The assembly of tropoelastin into elastic fibers or biomaterials is crucially influenced by key submolecular regions and specific residues within these domains. In this work, we identify the functional contributions of two rarely occurring negatively charged residues, glutamate 345 in domain 19 and glutamate 414 in domain 21, in jointly maintaining the native conformation of the tropoelastin hinge, bridge and foot regions. Alanine substitution of E345 and/or E414 variably alters the positioning and interactive accessibility of these regions, as illustrated by nanostructural studies and detected by antibody and cell probes. These structural changes are associated with a lower propensity for monomer coacervation, cross-linking into morphologically and functionally atypical hydrogels, and markedly impaired and abnormal elastic fiber formation. Our work indicates the crucial significance of both E345 and E414 residues in modulating specific local structure and higher-order assembly of human tropoelastin.

Ferroelectric switching of elastin

Ferroelectricity has long been speculated to have important biological functions, although its very existence in biology has never been firmly established. Here, we present compelling evidence that elastin, the key ECM protein found in connective tissues, is ferroelectric, and we elucidate the molecular mechanism of its switching. Nanoscale piezoresponse force microscopy and macroscopic pyroelectric measurements both show that elastin retains ferroelectricity at 473 K, with polarization on the order of 1 μC/cm(2), whereas coarse-grained molecular dynamics simulations predict similar polarization with a Curie temperature of 580 K, which is higher than most synthetic molecular ferroelectrics. The polarization of elastin is found to be intrinsic in tropoelastin at the monomer level, analogous to the unit cell level polarization in classical perovskite ferroelectrics, and it switches via thermally activated cooperative rotation of dipoles. Our study sheds light onto a long-standing question on ferroelectric switching in biology and establishes ferroelectricity as an important biophysical property of proteins. This is a critical first step toward resolving its physiological significance and pathological implications.

Glucose suppresses biological ferroelectricity in aortic elastin

Elastin is an intriguing extracellular matrix protein present in all connective tissues of vertebrates, rendering essential elasticity to connective tissues subjected to repeated physiological stresses. Using piezoresponse force microscopy, we show that the polarity of aortic elastin is switchable by an electrical field, which may be associated with the recently discovered biological ferroelectricity in the aorta. More interestingly, it is discovered that the switching in aortic elastin is largely suppressed by glucose treatment, which appears to freeze the internal asymmetric polar structures of elastin, making it much harder to switch, or suppressing the switching completely. Such loss of ferroelectricity could have important physiological and pathological implications from aging to arteriosclerosis that are closely related to glycation of elastin.

Tropoelastin bridge region positions the cell-interactive C terminus and contributes to elastic fiber assembly

The tropoelastin monomer undergoes stages of association by coacervation, deposition onto microfibrils, and cross-linking to form elastic fibers. Tropoelastin consists of an elastic N-terminal coil region and a cell-interactive C-terminal foot region linked together by a highly exposed bridge region. The bridge region is conveniently positioned to modulate elastic fiber assembly through association by coacervation and its proximity to dominant cross-linking domains. Tropoelastin constructs that either modify or remove the entire bridge and downstream regions were assessed for elastogenesis. These constructs focused on a single alanine substitution (R515A) and a truncation (M155n) at the highly conserved arginine 515 site that borders the bridge. Each form displayed less efficient coacervation, impaired hydrogel formation, and decreased dermal fibroblast attachment compared to wild-type tropoelastin. The R515A mutant protein additionally showed reduced elastic fiber formation upon addition to human retinal pigmented epithelium cells and dermal fibroblasts. The small-angle X-ray scattering nanostructure of the R515A mutant protein revealed greater conformational flexibility around the bridge and C-terminal regions. This increased flexibility of the R515A mutant suggests that the tropoelastin R515 residue stabilizes the structure of the bridge region, which is critical for elastic fiber assembly.

Coacervation of tropoelastin

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

Modular design in natural and biomimetic soft materials

Under eons of evolutionary and environmental pressure, biological systems have developed strong and lightweight peptide-based polymeric materials by using the 20 naturally occurring amino acids as principal monomeric units. These materials outperform their man-made counterparts in the following ways: 1) multifunctionality/tunability, 2) adaptability/stimuli-responsiveness, 3) synthesis and processing under ambient and aqueous conditions, and 4) recyclability and biodegradability. The universal design strategy that affords these advanced properties involves "bottom-up" synthesis and modular, hierarchical organization both within and across multiple length-scales. The field of "biomimicry"-elucidating and co-opting nature's basic material design principles and molecular building blocks-is rapidly evolving. This Review describes what has been discovered about the structure and molecular mechanisms of natural polymeric materials, as well as the progress towards synthetic "mimics" of these remarkable systems.

Progress in the methodological strategies for the detection in real samples of desmosine and isodesmosine, two biological markers of elastin degradation

Desmosines are crosslinking amino acids unique to mature elastin in humans. Owing to this unicity, they have been discussed as potentially attractive indicators of connective tissue disorders whose clinical manifestations are mostly the result of elastin degradation. This review covers advances in immunochemical, chromatographic, and electrophoretic procedures applied in the last 25 years to detect and quantitate these crosslinksin a variety of biological samples. Recent applications of CE with LIF detection (CE-LIF) for investigating the content of desmosines in different fluids will also be discussed.

Cell-matrix interactions of in vitro human skin fibroblasts upon addition of hyaluronan

Normal human skin fibroblasts were grown in a three-dimensional collagen gel or in monolayer in the presence or absence of high molecular weight hyaluronan (HA) to assess the influence of extracellular HA on cell-matrix interactions. HA incorporated into the collagen gel or added to the culture medium did not modify lattice retraction with time. The effect was independent from HA molecular weight (from 7.5 x 10(5) to 2.7 x 10(6) Da) and concentration (from 0.1 up to 1 mg/ml). HA did not affect shape and distribution of fibroblasts within the gel, whereas it induced the actin filaments to organise into thicker cables running underneath the plasma membrane. The same phenomenon was observed in fibroblasts grown in monolayer. By contrast, vimentin cytoskeleton and cell-substrate focal adhesions were not modified by exogenous HA. The number of fibroblasts attached to HA-coated dishes was always significantly lower compared to plastic and to collagen type I-coated plates. By contrast, adhesion was not affected by soluble HA added to the medium nor by anti-CD44 and anti-RHAMM-IHABP polyclonals. After 24-h seeding on collagen type I or on plastic, cells were large and spread. Conversely, cells adherent to HA-coated surfaces were long, thin and aligned into rows; alcian blue showed that cells were attached to the plastic in between HA bundles. Therefore, normal human skin fibroblasts exhibit very scarce, if any, adhesion to matrix HA, either soluble or immobilised. Moreover, even at high concentration, HA molecules do not exert any visco-mechanical effect on lattice retraction and do not interfere with fibroblast-collagen interactions nor with focal adhesion contacts of fibroblasts with the substrate. This is probably relevant in organogenesis and wound repair. By contrast, HA greatly modifies the organisation of the actin cytoskeleton, suggesting that CD44-mediated signal transduction by HA may affect cell locomotion and orientation, as indicated by the fusiform shape of fibroblasts grown in the presence of immobilised HA. A role of HA in cell orientation could be relevant for the deposition of collagen fibrils in regeneration and tissue remodelling.

Tropoelastin expression is up-regulated during activation of hepatic stellate cells and in the livers of CCl(4)-cirrhotic rats

BACKGROUND/AIMS

Activated hepatic stellate cells (HSC) are regarded as the principal cells synthesizing extracellular matrix components in fibrotic liver. Elastin content is increased in cirrhotic livers, but the cellular source is not known. Contribution of HSC to the production of elastin was investigated.

METHODS

Expression of elastin in CCl(4)-cirrhotic rat liver was studied by immunohistochemistry and in situ hybridization, liver myofibroblasts were identified in histological sections by alpha-smooth muscle actin (alpha-SMA) staining. LightCycler PCR and Northern blotting were used to detect tropoelastin mRNA in isolated HSC; tropoelastin protein was detected in the cells and in cell-conditioned medium by Western blotting.

RESULTS

HSC, isolated from normal rat liver, displayed increasing tropoelastin mRNA expression during transdifferentiation in culture. Expression of tropoelastin mRNA was accompanied by the production of tropoelastin protein in vitro. Increased levels of tropoelastin transcripts were found in the connective tissue septa of CCl(4)-cirrhotic rats and co-localized with alpha-SMA positive cells. Immunohistochemistry demonstrated elastin presence in the septa.

CONCLUSION

HSC express tropoelastin and its expression increases during transdifferentiation to myofibroblast-like cells.

Study of elastic fiber organization by scanning force microscopy

Elastic fibers of beef ligamentum nuchae were observed by atomic force microscopy and data compared with those obtained by conventional and freeze-fracture electron microscopy. Fresh isolated elastin fibers as well as thin sections of ligament fragments, which were fixed and embedded either in relaxed or in stretched conditions, were analysed. The results confirm that, at least in beef ligamentum nuchae, elastic fibers consist of beaded filaments which can be oriented by stretching in the direction of the force applied. Moreover, atomic force microscopy revealed that these beaded filaments are laterally connected by periodical bridges which become more pronounced upon stretching. The data clearly show that elastin molecules are organized in a rather ordered array, at least at the super-molecular level, and a depiction of the elastin organization in beef ligamentum nuchae is attempted.

Microfibrillar elements of the dermal matrix

Connective tissue microfibrils are key structural elements of the dermal matrix which play major roles in establishing and maintaining the structural and mechanical integrity of this complex tissue. Type VI collagen microfibrils form extensive microfibrillar networks which intercalate between the major collagen fibrils and are juxtaposed to cellular basement membranes, blood vessels and other interstitial structures. Fibrillin microfibrils define the continuous elastic network of skin, and are present in dermis as microfibril bundles devoid of measureable elastin extending from the dermal-epithelial junction and as components of the thick elastic fibres present in the deep reticular dermis. Electron microscopic analyses have revealed both classes of microfibrils to have complex ultrastructures. The ability to isolate intact native microfibrils from skin has enabled a combination of high resolution and biochemical techniques to be applied to elucidate their structure:function relationships. These approaches have generated new information about their molecular organisation and physiological interactions in health and disease.

Glycosaminoglycans in porcine lung: an ultrastructural study using cupromeronic blue

Glycosaminoglycans (GAGs) are essential components of the extracellular matrix contributing to the mechanical properties of connective tissues as well as to cell recognition and growth regulation. The ultrastructural localization of GAGs in porcine lung was studied by means of the dye Cupromeronic Blue in the presence of 0.3 M MgCl2 according to Scott's critical electrolyte concentration technique. GAGs were observed in locations described as follows. Pleura: Dermatan sulphate (DS) and chondroitin sulphate (CS) attached in the region of the d-band of collagen fibrils, interconnecting the fibrils; heparan sulphate (HS) at the surface of elastic fibers and in the basement membrane of the mesothelium and blood vessels. Bronchial cartilage: Abundant amounts of GAGs were observed in three zones: pericellular, in the intercellular matrix and at the perichondrial collagen. By enzyme digestion a superficial cartilage layer with predominantly CS could be distinguished from a deep zone with CS and keratan sulphate. The structure of the large aggregating cartilage proteoglycan was confirmed in situ. Airway epithelium: HS at the whole surface of cilia and microvilli and in the basement membrane of the epithelial cells. Alveolar wall: CS/DS at collagen fibrils, HS at the surface of elastic fibers and in the basement membranes of epithelium and endothelium.

Elastin exhibits a distinctive temporal and spatial pattern of distribution in the developing chick limb in association with the establishment of the cartilaginous skeleton

In this work we have analyzed the presence of elastic components in the extracellular matrices of the developing chick leg bud. The distributions of elastin and fibrillin were studied immunohistochemically in whole-mount preparations using confocal laser microscopy. The association of these constituents of the elastic matrix with other components of the extracellular matrix was also studied, using several additional antibodies. Our results reveal the transient presence of an elastin-rich scaffold of extracellular matrix fibrillar material in association with the establishment of the cartilaginous skeleton of the leg bud. The scaffold consisted of elastin-positive fibers extending from the ectodermal surface of the limb to the central cartilage-forming regions and between adjacent cartilages. Fibrillin immunolabeling was negative in this fibrillar scaffold while other components of the extracellular matrix including: tenascin, laminin and collagens type I, type III and type VI; appeared codistributed with elastin in some regions of the scaffold. Progressive changes in the spatial pattern of distribution of the elastin-positive scaffold were detected in explant cultures in which one expects a modification in the mechanical stresses of the tissues related to growth. A scaffold of elastin comparable to that found in vivo was also observed in high-density micromass cultures of isolated limb mesodermal cells. In this case the elastic fibers are observed filling the spaces located between the cartilaginous nodules. The fibers become reoriented and attach to the ectodermal basal surface when an ectodermal fragment is located at the top of the growing micromass. Our results suggest that the formation of the cartilaginous skeleton of the limb involves the segregation of the undifferentiated limb mesenchyme into chondrogenic and elastogenic cell lineages. Further, a role for the elastic fiber scaffold in coordinating the size and the spatial location of the cartilaginous skeletal elements within the limb bud is also suggested from our observations.

Elastic extracellular matrix of the embryonic chick heart: an immunohistological study using laser confocal microscopy

The "elastic matrix" constitutes a specialized component of the extracellular matrix which confers resiliency to tissues and organs subjected to repeated deformations. The role of the elastic matrix in living organisms appears to be of key importance since diseases characterized by expression of defective inherited genes which encode components of the elastic matrix lead to premature death. While the elastic matrix of adult organs has received a great deal of attention, little is known about when it first appears in embryonic tissues or its possible role in developing organs. In the present study we have performed an immunohistochemical study of the distribution of elastin and three additional components often associated with elastic matrices in adult tissues (i.e., fibrillin, emilin, and type VI collagen) during the development of the chicken embryonic heart. The three-dimensional arrangement of these components was established through the observation of whole-amount specimens with scanning laser confocal microscopy. Our results revealed three different periods of heart development regarding the composition of the elastic matrix. Prior to stage 21 the embryonic heart lacks elastin but exhibits a matrix scaffold of fibrillin and emilin associated with the endocardium and the developing cardiac jelly. Between stages 22 and 29 the heart shows a transient elastic scaffold in the outflow tract which contains elastin, fibrillin, and emilin. Elastin-positive fibrillar material is also observed during these stages in the base of the atrioventricular cushion adjacent to the myocardial wall. In addition, emilin-positive material appears to be associated with the zones of formation of ventricular trabeculae. Collagen type VI was not detected during these early stages. From stage 30 to stage 40 a progressive modification of the pattern of distribution of elastin, fibrillin, emilin, and collagen type VI is observed in association with the formation of the definitive four-chambered heart. The distribution of the elastic scaffold in the outflow tract appears to be rearranged and becomes restricted to the roots of the main arteries. Each of the components studied here is also deposited at increasing levels in the developing valvular apparatus including the valve leaflets and the chordae tendinea. The components are also present in the subendocardial space where they form aligned fibrillar tracts, an arrangement suggestive of a role in ventricular contractile function. The epicardium constitutes an additional region of elastic matrix deposition during these later stages and contains elastic, fibrillin, and collagen type VI.(ABSTRACT TRUNCATED AT 400 WORDS)