Properties of chick tropoelastin

The biology of vascular calcification

Vascular calcification (VC), characterized by different mineral deposits (i.e., carbonate apatite, whitlockite and hydroxyapatite) accumulating in blood vessels and valves, represents a relevant pathological process for the aging population and a life-threatening complication in acquired and in genetic diseases. Similarly to bone remodeling, VC is an actively regulated process in which many cells and molecules play a pivotal role. This review aims at: (i) describing the role of resident and circulating cells, of the extracellular environment and of positive and negative factors in driving the mineralization process; (ii) detailing the types of VC (i.e., intimal, medial and cardiac valve calcification); (iii) analyzing rare genetic diseases underlining the importance of altered pyrophosphate-dependent regulatory mechanisms; (iv) providing therapeutic options and perspectives.

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.

Structure of the elastin gene and alternative splicing of elastin mRNA: implications for human disease

The protein elastin is largely responsible for the elastic properties of vertebrate lungs, large blood vessels, and skin. The structure of the human, bovine, and chick elastin gene and protein monomer, tropoelastin, has recently been elucidated by using techniques of molecular biology. Extensive homology of amino acid sequence exists among the mammalian species and there is in addition strong conservation of nucleotide sequences in the 3' untranslated region of the gene. The translated exons are small and embedded in large expanses of introns. Sequences coding for the hydrophobic regions, responsible for the elastic properties of the molecule, and the alanine-lysine rich regions, responsible for crosslink formation between molecules, reside in separate exons and alternate for the most part in the elastin gene. S1 analyses and sequence analysis of cDNA and genomic clones have indicated that there is substantial alternative splicing of the primary elastin transcript. Variations in the structure of mRNAs resulting from alternative splicing could explain the existence of the multiple forms of tropoelastin observed electrophoretically in several species. Different kinds of splicing patterns could occur in human populations and may contribute to aging and pathological situations in the cardiovascular and pulmonary systems.

Lysyl oxidase activity and elastin/glycosaminoglycan interactions in growing chick and rat aortas

Hydrophobic tropoelastin molecules aggregate in vitro in physiological conditions and form fibers very similar to natural ones (Bressan, G. M., I. Pasquali Ronchetti, C. Fornieri, F. Mattioli, I. Castellani, and D. Volpin, 1986, J. Ultrastruct. Molec. Struct. Res., 94:209-216). Similar hydrophobic interactions might be operative in in vivo fibrogenesis. Data are presented suggesting that matrix glycosaminoglycans (GAGs) prevent spontaneous tropoelastin aggregation in vivo, at least up to the deamination of lysine residues on tropoelastin by matrix lysyl oxidase. Lysyl oxidase inhibitors beta-aminopropionitrile, aminoacetonitrile, semicarbazide, and isonicotinic acid hydrazide were given to newborn chicks, to chick embryos, and to newborn rats, and the ultrastructural alterations of the aortic elastic fibers were analyzed and compared with the extent of the enzyme inhibition. When inhibition was greater than 65% all chemicals induced alterations of elastic fibers in the form of lateral aggregates of elastin, which were always permeated by cytochemically and immunologically recognizable GAGs. The number and size of the abnormal elastin/GAGs aggregates were proportional to the extent of lysyl oxidase inhibition. The phenomenon was independent of the animal species. All data suggest that, upon inhibition of lysyl oxidase, matrix GAGs remain among elastin molecules during fibrogenesis by binding to positively charged amino groups on elastin. Newly synthesized and secreted tropoelastin has the highest number of free epsilon amino groups, and, therefore, the highest capability of binding to GAGs. These polyanions, by virtue of their great hydration and dispersing power, could prevent random spontaneous aggregation of hydrophobic tropoelastin in the extracellular space.

Newly determined carboxy terminal sequences in tropoelastin: immunologic identification in insoluble elastin

The carboxy terminal sequence of sheep, bovine and human tropoelastin (GFPGGACLGKA/SCGRKRK) has been inferred in earlier studies from sequencing of cloned complementary and genomic DNA. However, this putative carboxy terminal sequence was not found previously in peptides recovered from tryptic digests of tropoelastin. In order to determine whether the amino acid sequence described above is found in insoluble elastin, antibodies were raised against the chemically synthesized peptides with the appropriate sequences and the antibodies were shown to react with peptides derived from human, bovine, porcine, dog and hamster insoluble elastins. These results strongly suggest that the sequence (GFPGGACLGKA/SCGRKRK) at the carboxy terminus of tropoelastin is found in the elastins of many species.

Relevance of aggregation properties of tropoelastin to the assembly and structure of elastic fibers

Solutions of tropoelastin incubated under different experimental conditions were examined by electron microscopy after negative staining and after fixation and embedding. Below 37 degrees C only polymorphous structureless elements of variable size could be found. In samples kept for a few minutes at 40 degrees C, flexible, isolated filaments of 5 nm diameter and variable length, together with a few small aggregates of filaments, were seen. No single filaments, but only bundles of filaments were detectable after incubation at 40 degrees C for longer than 5-10 min. Tropoelastin kept at 40 degrees C for longer than 10 hr formed a white precipitate, which, when fixed and embedded as in conventional electron microscopy, consisted of 0.5-2 microns thick, amorphous and branching fibers, identical to those seen in identically processed normal tissues. From these observations a model for the assembly and structure of elastic fibers is proposed.

Analysis of the 3' region of the sheep elastin gene

The nucleotide sequences of a 1279-bp sheep elastin cDNA clone, pcSEL1 [Yoon et al. (1984) Biochem. Biophys. Res. Commun. 118, 261-269], and a 1230-bp sheep elastin genomic subclone, pSS1 [Davidson et al. (1984) Biochem. J. 220, 643-652], corresponding to a portion of the cDNA clone, were determined. These analyses permitted determination of the 100 amino acids at the carboxy terminus of sheep tropoelastin. A portion of this sequence showed strong homology to known sequences of pig tropoelastin, but most of the sequence had not been previously determined through protein sequencing. Novel aspects of the tropoelastin molecule which have been revealed by the present analyses are (i) the presence of an unusual sequence, KPPKP, which may contribute to crosslink formation; and (ii) the finding of cysteine within a sequence, CLGKSCGRKRK, at the putative carboxy terminus of tropoelastin. Because of the presence of these sequences, it is speculated that the carboxy-terminal region may be of importance in crosslinking tropoelastin molecules to themselves or to other matrix macromolecules. The nucleotide analyses revealed that sheep elastin mRNA contains a 974-bp untranslated sequence at the 3' end, which appears to be strongly conserved among species.

Structure of the 3' portion of the bovine elastin gene

A bovine genomic library constructed by partial Sau3A digestion and contained in lambda Charon 30 was screened by in situ hybridization with a 1.3-kilobase (kb) sheep elastin cDNA clone [Yoon, K., May, M., Goldstein, N., Indik, Z., Oliver, L., Boyd, C., & Rosenbloom, J. (1984) Biochem. Biophys. Res. Commun. 118, 261-269]. Three clones encompassing 10 kb of the bovine elastin gene were identified and characterized by restriction mapping and DNA sequencing of the 6.2 kb of the most 3' region of the gene. These analyses have permitted localization of eight exons in the 6.2 kb in which the translated exons vary in size from 27 to 69 base pairs, and there is an approximately 1-kb untranslated region at the 3' end. In addition to identification of sequences homologous to those found in porcine tropoelastin, the analyses defined a 58 amino acid sequence that forms the carboxy-terminal region of tropoelastin, and this sequence, which contains two cysteine residues, was previously not observed in the protein sequence data. The analyses also suggest that functionally distinct cross-link and hydrophobic domains of the protein are encoded in separate exons.

Secretion of elastin in the embryonic chick aorta as visualized by immunoelectron microscopy

Recently, significant advances have been made in characterizing the pathway of elastin biosynthesis from the biochemical point of view and a 70,000 dalton protein, designated tropoelastin, appears to be the primary translation product and soluble intermediate of the insoluble elastin. However, relatively little is known concerning the intracellular secretory pathway of tropoelastin. We previously developed an electron microscopic technique using elastin-specific antibody and ferritin-conjugated secondary antibody to identify intracellular elastin and to identify, provisionally, intracellular vesicles containing elastin ( Damiano et al., Conn. Tiss . Res. 8: 185-188, 1981). However, the method did not permit localization of elastin in other intracellular organelles. We now describe an improved post-embedding technique using the peroxidase-antiperoxidase method to detect the primary elastin antibody and have localized elastin in both the endothelial and medial cells of the embryonic chick aorta. Specific staining was visualized in the cisternae of the endoplasmic reticulum, in the Golgi apparatus, and in vesicles forming on the trans side of the Golgi. Some of these smaller vesicles appeared to fuse, forming larger vesicles which may have a storage function. Both types of vesicles were seen fusing with the cell plasma membrane, suggesting that elastin is secreted by an exocytotic process. These results suggest that tropoelastin follows the classical pathway for protein secretion.

Studies on the binding of plasma low density lipoproteins to arterial elastin

The mechanism of in vitro complex formation between plasma low density lipoproteins (LDL) and arterial elastin was studied. Rosette formation and decreased binding of the chemically modified LDL suggested that the intact protein moiety of lipoproteins was essential for the transfer of lipids from LDL to elastin. However, subsequent treatment of the elastin-LDL complex with trypsin removed the greater part of the lipoprotein protein but not the transferred cholesterol, indicating that the protein moiety of the lipoprotein did not take part in the retention of lipids on the elastin. In view of the observed effects of pH, ionic strength, various types of detergents and polarity of elastin preparations, it appears that the charged groups of the protein moiety of lipoproteins and the hydrophobicity of elastin proteins may play important parts in the binding of lipoproteins to arterial elastin.

Control of elastin synthesis

mRNA was isolated from the thoracic aortas of 16-day chick embryos and used to synthesize blunt-ended heteroduplex molecules consisting of one strand of mRNA and one of cDNA using AMV reverse transcriptase and S1 nuclease. The duplexes were tailed with dCTP and hybridized to the plasmid pBR322 which had been restricted with Pst I and tailed with dGTP. Recombinant plasmids were used to transform E. coli C600 and colonies containing elastin cDNA were selected by in situ hybridization with 32P labeled elastin mRNA and by hybrid selected translation using the nuclease-treated reticulocyte lysate system. mRNA recovered from hybridization to DNA of one clone, pWB1, markedly stimulated incorporation of [3H]valine into a protein which was immunoprecipitable with elastin-specific antibody and had a molecular weight of 72,000, characteristic of tropoelastin. The 230 bp insert of pWB1 was sequenced by the technique of Maxam and Gilbert and found to be derived from a nontranslated region of the 3' end of the mRNA. Nick-translated pWB1 was used to identify and to estimate the relative amounts of elastin mRNA in the developing chick embryo aorta by blot hybridization. A single mRNA species of 3.5 kb hybridized to the pWB1 probe and this species increased greatly in amount between day 7 and day 14. This increase was paralleled by an increase in translatable elastin mRNA and by the rate of elastin synthesis of aortas from various age embryos incubated in vivo. The injection of 150 microgram of hydrocortisone 21-phosphate into 8-day eggs produced a significant increase in both the relative rate of tropoelastin synthesized by the isolated aortas and the relative amount of elastin mRNA. These results suggest that the observed changes in elastin synthesis during development and after hydrocortisone administration are governed by the elastin mRNA content of the aortas.

Fibrogenesis and biosynthesis of elastin in cartilage

This study presents direct evidence that dissociated chondroblasts from rabbit ear cartilage grown in vitro are capable of synthesizing insoluble elastin. Ultrastructural examination indicated that at an early stage of tissue development, elastogenesis is initiated producing a form of primary fibrils which later condense into an electron dense amorphous material which, unlike other elastin-containing tissues, is heavily stained by metal cations and lacks peripheral microfibrils. Native elastic fibrils and the mature elastic fiber bundles are both susceptible to elastase digestion. Transmission electronmicroscopy demonstrated the presence of many intracellular filaments all showing a substructural organization and localized in close proximity to the nucleus. Their possible contractile nature is discussed. Amino acid analysis of cartilage elastin and of the elastin synthesized in vitro revealed a close chemical similarity between the two molecules. Ultrastructural analysis of the in vitro elastin demonstrated a substructural organization quite similar to that of the elastin observed in an in vivo system.

Changes in the state of ionization of carboxyl groups in elastin in response to the binding of sodium dodecyl sulfate

The interaction of sodium dodecyl sulfate with elastin has been studied by complexometric titration. Approximately 1.2 mumoles of protons with a pKapp of 5.45 are taken up by 10 milligrams of insoluble elastin upon the binding of detergent, apparently due to the protonation of normally ionized carboxylate functions in this protein. Since ionized carboxylate functions of elastin are essential for its interaction with elastase and, possibly, metallic cations, these results may have physiological significance in view of the affinity of elastin for lipid-like ligands.

Intrinsic enzyme activity associated with tropoelastin

The presence of an enzyme(s) associated with purified tropoelastin has been established. Results indicate that the enzyme(s) remains closely associated with the soluble elastin throughout the entire purification procedure suggesting that it is very tightly bound. Enzymatic activity is optimum through the pH range 7-9 (37 degrees C) and can be inhibited by disodium ethylenediaminetetraacetate, N-ethylmaleimide, sulfite, soybean trypsin inhibitor and human alpha-1-antitrypsin. The fragmentation pattern appears to be specific and reproducible.

Arterial elastin synthesis in the young chick

In the growing chick a marked stimulation in soluble and mature arterial elastin synthesis occurs 2 and 5 weeks after hatching. Measurement of [3H]valine and [3H]proline incorporation into arterial soluble protein during this period indicated that most of the label is found in a 70 000 dalton protein subunit. The labeled soluble subunit had the characteristics of native soluble elastin or tropoelastin. During the period in which the greatest percentage increase in mature elastin occurs, the highest specific activities of soluble [3H]valine-labeled protein were observed. These changes were striking and suggest a developmental period for the growing chick in which factors related to elastin metabolism may be more easily studied.

Radioimmunological identification of tropoelastin

Antiserum was prepared in sheep against insoluble elastin isolated from embryonic-chick aortae. In an indirect immunoprecipitation test, the antiserum reacted quantitatively with small amounts of radioactively labelled purified tropoelastin prepared from embryonic-chick aortae. The antiserum did not cross-react with chick procollagen, and the antiserum uas used to identify radioactively labelled tropoelastin secreted by chick aorta cells in suspension culture.

A convenient method for the identification and estimation of soluble elastin synthesis in vitro

A method is described by which newly synthesized soluble elastin can be routinely and conveniently identified and estimated in vitro without the need for unlabelled carrier tropoelastin. Aortic tissue from 11 day old chick embryos is incubated in the presence of [14C] L-proline and [3H] L-valine and the distributions of [14C] proline-, [14C] hydroxyproline-and [3H] valine-labelled proteins on SDS-polyacrylamide gels are determined. Soluble elastin is identified as a [14C] hydroxyproline-labelled peak of molecular weight approximately 70,000 daltons which also incorporates large quantities of [3H] valine and has a [14C] hydroxyproline/[14C] proline ratio of about 0.2. Whereas the hydroxyproline label can be used to estimate newly synthesized soluble elastin even after several hours of incubation, similar use of the [3H] valine label is limited to short incubation times. Paradoxically, the quantity of [14C] hydroxyproline-labelled soluble elastin detected by the assay decreases with increased incubation time. This fall-off in labelled soluble elastin is not due to an increase in the rate of crosslinking of the protein over the course of the incubation. Soluble elastin is detectable both in tissue extract and medium fractions. The presence of hydroxyproline-labelled protein fragments in the medium fraction is evidence of proteolytic breakdown of collagen or elastin or both proteins. This proteolytic activity is augmented by the inclusion of serum in the incubation medium and is not inhibited by phenylmethylsulfonylfluoride. The method provides a convenient assay by which factors affecting the synthesis and secretion of soluble elastin may be studied.

Scanning electron microscopy and electron probe microanalysis of calcified alpha-elastin coacervates

The uptake of calcium and phosphorus from a serum calcification medium by coacervated alpha-elastin was studied by electron probe microanalysis and scanning electron microscopy. Calcium and phosphorus were bound by the coacervate in ratios similar to that of hydroxyapatite. A significant difference was observed in the secondary electron image of the coacercules from the serum during calcification of the coacervate.