Isolation of tropoelastin a from lathyritic chick aortae

Methods in elastic tissue biology: elastin isolation and purification

Elastin provides recoil to tissues subjected to repeated stretch, such as blood vessels and the lung. It is encoded by a single gene in mammals and is secreted as a 60-70 kDa monomer called tropoelastin. The functional form of the protein is that of a large, highly crosslinked polymer that organizes as sheets or fibers in the extracellular matrix. Purification of mature, crosslinked elastin is problematic because its insolubility precludes its isolation using standard wet-chemistry techniques. Instead, relatively harsh experimental approaches designed to remove non-elastin 'contaminates' are employed to generate an insoluble product that has the amino acid composition expected of elastin. Although soluble, tropoelastin also presents problems for isolation and purification. The protein's extreme stickiness and susceptibility to proteolysis requires careful attention during purification and in tropoelastin-based assays. This article describes the most common approaches for purification of insoluble elastin and tropoelastin. It also addresses key aspects of studying tropoelastin production in cultured cells, where elastin expression is highly dependent upon cell type, culture conditions, and passage number.

Structure of the elastin gene

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

Specificity in the coacervation of tropoelastin: solvent exposed lysines

Tropoelastin protein monomers associate by coacervation and are cross-linked in vivo to form elastin macro-assemblies. We provide evidence for specific protein domain contact points between tropoelastin monomers during association by coacervation. The homobifunctional cross-linker bis(sulfosuccinimidyl) suberate served as a rapid reporter of adjacent lysines and preferentially exposed domains. Intact cross-linked peptide pairs were identified after protease digestion and high-resolution electrospray mass spectrometry followed by MS/MS sequencing. Mapping of the assigned sequences indicated that the region in the monomer spanning domains 19-25 was readily accessible to solvent and enriched in cross-linking. Domains 12 and 36 were also prevalent, where these two regions were not previously thought to play a major role in the formation of mature elastin. A specificity for particular lysines allowed for the construction of a model for the first close contacts between domains and the first detailed study of the cross-linking of tropoelastin.

Codistribution analysis of elastin and related fibrillar proteins in early vertebrate development

Elastin is an extracellular matrix protein found in adult and neonatal vasculature, lung, skin and connective tissue. It is secreted as tropoelastin, a soluble protein that is cross-linked in the tissue space to form an insoluble elastin matrix. Cross-linked elastin can be found in association with several microfibril-associated proteins including fibrillin-1, fibrillin-2 and fibulin-1 suggesting that these proteins contribute to elastic fiber assembly, structure or function. To date, the earliest reported elastin expression was in the conotruncal region of the developing avian heart at 3.5 days of gestation. Here we report that elastin expression begins at significantly earlier developmental stages. Using a novel immunolabeling method, the deposition of elastin, fibrillin-1 and -2 and fibulin-1 was analyzed in avian embryos at several time points during the first 2 days of development. Elastin was found at the midline associated with axial structures such as the notochord and somites at 23 h of development. Fibrillin-1 and -2 and fibulin-1 were also expressed at the embryonic midline at this stage with fibrillin-1 and fibulin-1 showing a high degree of colocalization with elastin in fibers surrounding midline structures. The expression of these genes was confirmed by conventional immunoblotting and mRNA detection methods. Our results demonstrate that elastin polypeptide deposition occurs much earlier than was previously appreciated. Furthermore, the results suggest that elastin deposition at the early embryonic midline is accompanied by the deposition and organization of a number of extracellular matrix polypeptides. These filamentous extracellular matrix structures may act to transduce or otherwise stabilize dynamic forces generated during embryogenesis.

Regulation of elastin gene expression

Recent isolation and characterization of cDNAs encompassing the full length of chicken, cow, and human elastin mRNA have led to the elucidation of the primary structure of the respective tropoelastins. Comparison of the tropoelastin from the different species has revealed that large segments of the sequence are conserved, but considerable variation also exists, ranging in extent from relatively small alterations, such as conservative amino acid substitutions, to large-scale deletions and insertions. Several distinct approaches have yielded compelling evidence of a single elastin gene per haploid genome. Analysis of the bovine and human elastin genes revealed that functionally distinct hydrophobic and cross-link domains of the protein are encoded in separate exons which alternate in the genes. The human gene contains 34 exons, the intron/exon ratio is unusually large (20:1), and the introns contain large amounts of repetitive sequences that may predispose to genetic instability. Comparison of the cDNA and genomic sequences has demonstrated that the primary transcript of both species is subject to considerable alternative splicing, which can account for the presence of multiple tropoelastin isoforms. It is likely that the conformation of elastin is, at least in part, that of a random coil, and therefore it might be expected that the stringency for conservation of the amino acid sequence would be less than that for other proteins with unique conformations. This suggests that functional elastin molecules that vary in their sequence and fitness may exist in the human population and be compatible with a normal life. Potentially though, these variations could have profound consequences on the properties of vital tissues found in the cardiovascular and pulmonary systems over the lifetime of the individual. Consequently, analysis of the structure of the elastin gene and its variation in what is regarded as the normal human population, rather than in those individuals with clearly heritable diseases, assumes greater importance. The 5'-flanking region of the gene is G + C rich and contains several SP-1 and AP2 binding sites, as well as putative glucocorticoid, cAMP, and TPA responsive elements, but no consensus TATA box or functional CAAT box. Primer extension and S1 mapping of the elastin mRNA indicated that transcription was initiated at multiple sites. Transfection experiments using promoter elements/reporter gene constructs demonstrated that the basic promoter element was found within region -128 to -1. In addition, three distinct up-regulatory and two down-regulatory regions were delineated. Taken together, these findings suggest that the regulation of elastin gene expression is complex and takes place at several levels.

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.

Cloning of full-length elastin cDNAs from a human skin fibroblast recombinant cDNA library: further elucidation of alternative splicing utilizing exon-specific oligonucleotides

A human cDNA library was constructed utilizing RNA isolated from cultured skin fibroblasts. Recombinant clones containing elastin sequences were identified by plaque hybridizations with previously characterized human placental elastin cDNAs. Seven positive recombinant clones with inserts of approximately 3.2-2.2 kb were isolated. Characterization of the clones by restriction endonuclease analysis and dot-blot hybridizations with exon-specific synthetic oligonucleotides demonstrated considerable variability in the primary nucleotide sequence. Dideoxy nucleotide sequencing confirmed this finding. The variability is most likely a result of alternative splicing of exons from the primary elastin transcripts. The two largest clones contained approximately 1 kb of 3' untranslated sequence and approximately 2.2 kb of translated sequence encoding 730 amino acids. Six amino acids, encoded by exon 12A, have not been previously noted in human elastin cDNAs. In addition, these human skin fibroblast clones contained a 49 bp 5' untranslated sequence. These results demonstrate that there is considerable variability in the processed nucleotide sequence of the elastin mRNAs. These transcripts may code for isoforms of tropoelastin with different biologic properties.

Origin and propagation of elastogenesis in the developing cardiovascular system

Ectomesenchyme derived from cardiac neural crest is critical to aorticopulmonary septation in the heart. However, any unique contribution of the cardiac ectomesenchyme to the extracellular matrix of the conotruncus has not been demonstrated previously. In this study the chronology and topography of soluble tropoelastin (STE) and the aldehyde-rich protein (ARP) of the elastic connective tissues have been examined in the chick embryo, stages 21-38, and in the quail-chick chimera, stages 24-35 (quail neural fold grafted onto a chick embryo). STE was located with immunofluorescence histochemistry, and ARP with Schiff's reagent. With these procedures prevenient sites of elastin synthesis are observed readily. The results show that the myocardium proper appears to have a role in the instigation of elastogenesis and in elastic fiber orientation; that the mesenchymal cells whose matrix contains elastic fibers are ectomesenchymal, of neural crest origin; and that elastin is deployed in an orderly proximal-distal sequence. It is hypothesized that elastogenesis is a critical event in aorticopulmonary septation.

Sequence variation of bovine elastin mRNA due to alternative splicing

Poly A+ RNA, isolated from a single 210 day fetal bovine nuchal ligament, was used to synthesize cDNA by the RNase H method, using AMV reverse transcriptase for first strand synthesis and DNA polymerase I for the second strand. The cDNA was inserted into lambda gt10 using EcoRI linkers, and recombinant phage containing elastin sequences were identified by hybridization with a 1.3 kb sheep elastin cDNA clone, pcSELI (Yoon, K. et al., Biochem. Biophys. Res. Comm. 118: 261-265, 1984). Three clones containing the largest inserts of 2.9, 2.8, and 2.6 kb were selected for further study. The complete sequence analysis of the 3 clones was correlated with the sequence of 10.2 kb of the bovine elastin gene. The analyses: (i) showed that the cDNA encompassed the great majority of the translated sequence, (ii) ordered the tryptic peptides of porcine tropoelastin, (iii) determined new amino acid sequences not previously found in the porcine peptides and (iv) demonstrated that alternative splicing of the primary transcript leads to significant variation in the sequence of the translated portion of the mRNA.

The gene coding for tropoelastin is represented as a single copy sequence in the haploid sheep genome

The identity of the primary in vitro translation products of fetal sheep nuchal ligament elastin mRNA was confirmed as two distinct polypeptides of 63 Kdal and 65 Kdal in both rabbit reticulocyte and wheat germ extract cell-free translation systems. Both polypeptides were co-translationally processed by a microsomal membrane signal peptidase, with the removal of 20-25 amino acid residues. A single (3,5 kb) RNA species encodes both tropoelastin polypeptides. Restriction endonuclease mapping of sheep genomic DNA by hydridization with two radiolabelled genomic DNA fragments containing sequences coding for sheep tropoelastin (pSE1-1,3 and pSE1-0.7,) indicated the presence of a single elastin gene. The elastin gene copy number was further quantitated by comparison of hybridisation of pSE1-1.3 and pSE1-0.7 to slot-blots and Southern transfers of sheep genomic DNA and to standard curves constructed with each clone. These results clearly demonstrate that each of these sequences is represented only once per haploid genome, suggesting that the two tropoelastin polypeptides are products of a single elastin gene.

Bovine elastin cDNA clones: evidence for the occurrence of a new elastin-related protein in fetal calf ligamentum nuchae

Poly (A+) mRNA was isolated from fetal calf ligamentum nuchae and used for the construction of cDNA libraries. A fraction highly enriched in elastin mRNA was used to prepare the cDNA probes for screening the libraries. A 2 kb clone, pRE1, gave the most positive signal in colony hybridization. It hybridized to a mRNA of the same size as reported for elastin mRNAs from chick and sheep. Hybrid-arrested translation showed that translation of mRNAs for proteins other than elastin doublet was not inhibited by pRE1. Southern blot analysis showed that pRE1 has sequence homology with pVE6 and pVE10, which were tentatively identified as elastin-related cDNA clones representing two distinct mRNAs. DNA sequence data from the 5' end of pRE1 show that the translated amino acid sequence is not typical of known elastin sequences but contains some elastin-like sequences. All of this evidence strongly suggests the occurrence in fetal calf nuchal ligament of a mRNA which codes for a previously unknown elastin-related protein.