Structure of the bovine elastin gene and S1 nuclease analysis of alternative splicing of elastin mRNA in the bovine nuchal ligament

Age-associated proinflammatory elastic fiber remodeling in large arteries

Elastic fibers are the main components of the extracellular matrix of the large arterial wall. Elastic fiber remodeling is an intricate process of synthesis and degradation of the core elastin protein and microfibrils accompanied by the assembly and disassembly of accessory proteins. Age-related morphological, structural, and functional proinflammatory remodeling within the elastic fiber has a profound effect upon the integrity, elasticity, calcification, amyloidosis, and stiffness of the large arterial wall. An age-associated increase in arterial stiffness is a major risk factor for the pathogenesis of diseases of the large arteries such as hypertensive and atherosclerotic vasculopathy. This mini review is an update on the key molecular, cellular, functional, and structural mechanisms of elastic fiber proinflammatory remodeling in large arteries with aging. Targeting structural and functional integrity of the elastic fiber may be an effective approach to impede proinflammatory arterial remodeling with advancing age.

Elastin, arterial mechanics, and cardiovascular disease

Large, elastic arteries are composed of cells and a specialized extracellular matrix that provides reversible elasticity and strength. Elastin is the matrix protein responsible for this reversible elasticity that reduces the workload on the heart and dampens pulsatile flow in distal arteries. Here, we summarize the elastin protein biochemistry, self-association behavior, cross-linking process, and multistep elastic fiber assembly that provide large arteries with their unique mechanical properties. We present measures of passive arterial mechanics that depend on elastic fiber amounts and integrity such as the Windkessel effect, structural and material stiffness, and energy storage. We discuss supravalvular aortic stenosis and autosomal dominant cutis laxa-1, which are genetic disorders caused by mutations in the elastin gene. We present mouse models of supravalvular aortic stenosis, autosomal dominant cutis laxa-1, and graded elastin amounts that have been invaluable for understanding the role of elastin in arterial mechanics and cardiovascular disease. We summarize acquired diseases associated with elastic fiber defects, including hypertension and arterial stiffness, diabetes, obesity, atherosclerosis, calcification, and aneurysms and dissections. We mention animal models that have helped delineate the role of elastic fiber defects in these acquired diseases. We briefly summarize challenges and recent advances in generating functional elastic fibers in tissue-engineered arteries. We conclude with suggestions for future research and opportunities for therapeutic intervention in genetic and acquired elastinopathies.

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.

Elastin in lung development and disease pathogenesis

Elastin is expressed in most tissues that require elastic recoil. The protein first appeared coincident with the closed circulatory system, and was critical for the evolutionary success of the vertebrate lineage. Elastin is expressed by multiple cell types in the lung, including mesothelial cells in the pleura, smooth muscle cells in airways and blood vessels, endothelial cells, and interstitial fibroblasts. This highly crosslinked protein associates with fibrillin-containing microfibrils to form the elastic fiber, which is the physiological structure that functions in the extracellular matrix. Elastic fibers can be woven into many different shapes depending on the mechanical needs of the tissue. In large pulmonary vessels, for example, elastin forms continuous sheets, or lamellae, that separate smooth muscle layers. Outside of the vasculature, elastic fibers form an extensive fiber network that originates in the central bronchi and inserts into the distal airspaces and visceral pleura. The fibrous cables form a looping system that encircle the alveolar ducts and terminal air spaces and ensures that applied force is transmitted equally to all parts of the lung. Normal lung function depends on proper secretion and assembly of elastin, and either inhibition of elastin fiber assembly or degradation of existing elastin results in lung dysfunction and disease.

Alternative splicing and tissue-specific elastin misassembly act as biological modifiers of human elastin gene frameshift mutations associated with dominant cutis laxa

Elastin is the extracellular matrix protein in vertebrates that provides elastic recoil to blood vessels, the lung, and skin. Because the elastin gene has undergone significant changes in the primate lineage, modeling elastin diseases in non-human animals can be problematic. To investigate the pathophysiology underlying a class of elastin gene mutations leading to autosomal dominant cutis laxa, we engineered a cutis laxa mutation (single base deletion) into the human elastin gene contained in a bacterial artificial chromosome. When expressed as a transgene in mice, mutant elastin was incorporated into elastic fibers in the skin and lung with adverse effects on tissue function. In contrast, only low levels of mutant protein incorporated into aortic elastin, which explains why the vasculature is relatively unaffected in this disease. RNA stability studies found that alternative exon splicing acts as a modifier of disease severity by influencing the spectrum of mutant transcripts that survive nonsense-mediated decay. Our results confirm the critical role of the C-terminal region of tropoelastin in elastic fiber assembly and suggest tissue-specific differences in the elastin assembly pathway.

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.

Elastin-elastases and inflamm-aging

Degradation of elastin, the main amorphous component of elastic fibers, by elastases belonging to the serine, metallo, or cysteine families leads to the generation of elastin fragments, designated as elastokines in keeping with their cytokine-like properties. Generation of elastokines from one of the longest lived protein in human might represent a strong tissue repair signal. Indeed, they (1) exhibit potent chemotactic activity for leukocytes, (2) stimulate fibroblast and smooth muscle cell proliferation, and (3) display proangiogenic activity as potent as VEGF. However, continuous exposure of cells to these matrikines, through increased elastase(s) expression with age, can contribute to the formation of a chronic inflammatory state, that is, inflamm-aging. Importantly, binding of elastokines to S-Gal, their cognate receptor, proved to stimulate matrix metalloproteinase expression in normal and cancer cells. Besides, these elastin fragments can polarize lymphocytes toward a Th-1 response or induce an osteogenic response in smooth muscle cells, and arterial wall calcification. In this chapter, emphasis will be made on the contribution of elastokines on the genesis of age-related arterial wall diseases, particularly abdominal aortic aneurysms (AAAs). An elastokine theory of AAAs progression will be proposed. Age is one main risk factor of cancer incidence and development. The myriad of biological effects exerted by elastokines on stromal and inflammatory cells led us to hypothesize that they might be main actors in elaborating a favorable cancerization field in melanoma; for instance these peptides could catalyze the vertical growth phase transition in melanoma through increased expression of gelatinase A and membrane-type 1 matrix metalloproteinase.

Cell instructive polymers

Polymeric materials used in tissue engineering were initially used solely as delivery vehicles for transplanting cells. However, these materials are currently designed to actively regulate the resultant tissue structure and function. This control is achieved through spatial and temporal regulation of various cues (e.g., adhesion ligands, growth factors) provided to interacting cells from the material. These polymeric materials that control cell function and tissue formation are termed cell instructive polymers, and recent trends in their design are outlined in this chapter.

Characterization of peptides resulting from digestion of human skin elastin with elastase

Several pathological disorders are associated with abnormalities in elastic fibers, which are mainly composed of elastin. Understanding the biochemical basis of such disorders requires information about the primary structure of elastin. Since the acquisition of structural information for elastin is hampered by its extreme insolubility in water or any organic solvent, in this study, human skin elastin was digested with elastase to produce water-soluble peptides. Tandem mass spectrometry (MS/MS) experiments were performed using conventional electrospray ionization (ESI) and nano-ESI techniques coupled with ion trap and quadrupole time-of-flight (qTOF) mass analyzers, respectively. The peptides were identified from the fragment spectra using database searching and/or de novo sequencing. The cleavage sites of the enzyme and, for the first time, the extent and location of proline hydroxylation in human skin elastin were determined. A total of 117 peptides were identified with sequence coverage of 58.8%. It has been observed that 25% of proline residues in the sequenced region are hydroxylated. Elastase cleaves predominantly at the C-terminals of the amino acids Gly, Val, Leu, Ala, and Ile, and to a lesser extent at Phe, Pro, Glu, and Arg. Our results confirm a previous report that human skin elastin lacks amino acid sequences expressed by exon 26A.

Cellular interactions with elastin

Elastin is a key structural component of the extracellular matrix. Tropoelastin is the soluble precursor of elastin. In addition to providing elastic recoil to various tissues such as the aorta and lung, elastin, tropoelastin and elastin degradation products are able to influence cell function and promote cellular responses. These responses include chemotaxis, proliferation and cell adhesion. The interaction of elastin products with cells has been attributed to the elastin receptor. However, additional cell-surface receptors have also been identified. These include G protein-coupled receptors and integrins. The potential roles of these receptors in cell-elastin interactions, with particular focus on elastin formation are discussed.

The structure and organization of lamprin genes: multiple-copy genes with alternative splicing and convergent evolution with insect structural proteins

Lamprin is a unique structural protein which forms the extracellular matrix of several cartilaginous structures found in the lamprey. Lamprin is noncollagenous in nature but shows sequence similarities to elastins and to insect structural proteins. Here, we characterize the structure and organization of lamprin genes, demonstrating the presence of multiple similar but not identical copies of the lamprin gene in the genome of the lamprey. In at least one species of lamprey, Lampetra richardsoni, the multiple gene copies are arranged in tandem in the genome in a head-to-tail orientation. Lamprin genes from Petromyzon marinus contain either seven or eight exons, with exon 4 being alternatively spliced in all genes, resulting in a total of six different lamprin transcripts. All exon junctions are of class 1,1. An unusual feature of the lamprin gene structure is the distribution of the 3' untranslated region sequence among multiple exons. A TATA box and cap sequence have been identified in upstream sequences in close proximity to the transcription start site, but no CAAT box could be identified. Sequence and gene structure comparisons between lamprins, elastins, and insect structural proteins suggest that the regions of sequence similarity are the result of a process of convergent evolution.

The EMILIN protein family

The EMILINs are a new family of glycoproteins of the extracellular matrix. The prototype of this family is the chicken EMILIN that was originally identified in extracts of aortas; it was then found to be widely distributed in several tissues associated with elastin and localized at the interface between amorphous elastin and microfibrils. Based on peptide sequences, chicken and human cDNAs coding for EMILIN were isolated by RT/PCR by screening kidney and heart cDNA libraries. By using a C-terminal fragment of human EMILIN-1 as a bait in the yeast two-hybrid system, a second family member, EMILIN-2, has also been isolated. EMILINs are characterized by a C-terminal gC1q globular domain, a short collagenous sequence, a long coiled-coil region and a new cysteine-rich N-terminal domain that can be considered a hallmark of the family being present also in multimerin. The gene for EMILIN-1 was mapped on chromosome 2p23 overlapping with the promoter region of the ketohexokinase gene. The gC1q domain of EMILIN-1 can form relatively stable and compact homotrimers and this association is then followed by a multimeric assembly of disulfide-bonded protomers. Recombinant EMILIN-1 purified from the supernatant of 293 cells represents a very efficient ligand for cell adhesion of several cell types.

Expression of the elastin promoter in novel tissue sites in transgenic mouse embryos

We have previously shown in a transgenic mouse line, in which 5.2 kb of the elastin promoter was linked to the reporter enzyme chloramphenicol acetyltransferase (CAT), that the highest levels of expression were found in embryonic lungs and aorta, while lower levels were detected in other elastin-containing tissues. Furthermore, in general, expression of the transgene showed developmental regulation similar to that of the endogenous gene. However, the precise location of cellular expression could not be determined in this model. To overcome this limitation, we have developed a similar model, but replaced CAT with the reporter enzyme beta-galactosidase. Enzyme activity was readily detected in the transgenic mouse embryos in expected regions of tissue forming elastic fibers, including the dermis and elastic cartilage. Of considerable interest, however, was the novel finding of expression in specific areas of neuroepithelium of the brain and in the perichondrium surrounding areas destined to form hyaline cartilage in endochondral bone formation. These latter areas included all the bones of the limbs, the spine and rib cage. It appeared that these segments of elastin expression demarcated the border between the developing cartilage and the surrounding mesenchymal tissue. Elastin promoter expression was also found in developing somites, in the mesenchymal layer of the forming cornea of the eye, in the genital tubercle and in the epithelium destined to form the olfactory epithelium. These findings indicate that the elastin promoter is activated during embryonic development in a variety of tissues, suggesting that elastin gene expression may play a role in organizing cutaneous, skeletal and neural structures.

Structure, chromosomal localization, and promoter analysis of the human elastin microfibril interfase located proteIN (EMILIN) gene

Elastin microfibril interfase-located protein (EMILIN) is an extracellular matrix glycoprotein abundantly expressed in elastin-rich tissues such as the blood vessels, skin, heart, and lung. It occurs with elastic fibers at the interface between amorphous elastin and microfibrils. In vitro experiments suggested a role for EMILIN in the process of elastin deposition. This multimodular protein consists of 995 amino acids; the domain organization includes a C1q-like globular domain at the C terminus, a short collagenous stalk, a region containing two leucine zippers, and at least four heptad repeats with a high potential for forming coiled-coil alpha-helices and, at the N terminus, a cysteine-rich sequence characterized by a partial epidermal growth factor-like motif and homologous to a region of multimerin. Here we report the complete characterization of the human and murine EMILIN gene, their chromosomal assignment, and preliminary functional data of the human promoter. A cDNA probe corresponding to the C terminus of EMILIN was used to isolate two genomic clones from a human BAC library. Sequencing of several derived subclones allowed the characterization of the whole gene that was found to be about 8 kilobases in size and to contain 8 exons and 7 introns. The internal exons range in size from 17 base pairs to 1929 base pairs. All internal intron/exon junctions are defined by canonical splice donor and acceptor sites, and the different domains potentially involved in the formation of a coiled-coil structure are clustered in the largest exon. The 3'-end of the EMILIN gene overlaps with the 5'-end of the promoter region of the ketohexokinase gene, whose chromosomal position is between markers D2S305 and D2S165 on chromosome 2. A 1600-base pair-long sequence upstream of the translation starting point was evaluated for its promoter activity; five deletion constructs were assayed after transfection in primary chicken fibroblasts and in a human rhabdomyosarcoma cell line. This analysis indicates the existence of two contiguous regions able to modulate luciferase expression in both cell types used, one with a strong activatory function, ranging from positions -204 to -503, and the other, ranging from positions -504 to -683, with a strong inhibitory function.

Temporal expression of elastic fiber components in bladder development

Fetal and postnatal bovine bladders were examined for expression of elastic fiber components by immunohistochemistry as well as by measurement of steady state mRNA levels. Expression of fibrillin-1, microfibril-associated glycoprotein (MAGP) and elastin during the fetal period were compared with that of postnatal two year old animals (heifers) and adults. Each bladder was separated into two distinct tissue samples: 1) the outer smooth muscle layer (detrusor) and 2) the inner epithelium (urothelium) lined lamina propria (urotherial-lamina propria). Each of these samples was analyzed separately. Distribution of the elastic fiber components, determined by immunohistochemistry with matrix-specific antibodies, was different depending upon the region of the bladder wall examined and its developmental stage. In particular, MAGP and fibrillin-1 were conspicuously present in the urothelium during the later fetal stages. RNA products of elastic fiber genes were detectable both in the detrusor smooth muscle and urothelial-lamina propria fractions. The highest level of expression occurred in the urothelial-lamina propria fraction during the late second-early third trimester. Elastin expression was different from that of MAGP and fibrillin-1. The highest levels of steady-state elastin mRNA occurred at the earliest developmental stages examined and then progressively decreased through term. A high level of elastin expression occurred within the inner or lamina propria layer of the bladder. Since this layer is the functional capacitance layer within the bladder, its flexibility is likely related to the structural integration of elastin and associated microfibrillar components.

Molecular variation of the human elastin (ELN) gene in a normal human population

DNA sequence diversity in the human elastin genomic region has been estimated by RFLP analysis in a normal human population. The proportion of polymorphic nucleotides and the degree of nucleotide diversity were 0.0034 and 0.0018 respectively. It is argued that the estimate of nucleotide diversity does not indicate strong purifying selection in the region. A total of 144 restriction sites were sampled in each of 80 independent chromosomes representing the screening of 58080 bp overall. Six main haplotypes were constructed; they represent at least 84% of the 80 chromosomes sampled. Analysis for linkage disequilibrium revealed two statistically significant comparisons out of 54 tests, approximately the proportion that would be statistically significant at the 5% level by chance. A higher order quadrigenic disequilibrium was detected. The relationship between the physical distance separating polymorphic restriction sites and linkage disequilibrium is discussed. The development of elastin haplotypes and knowledge of the pattern of linkage disequilibrium should aid the study of elastin related disease and human evolution.

Transcriptional regulation of the elastin gene by insulin-like growth factor-I involves disruption of Sp1 binding. Evidence for the role of Rb in mediating Sp1 binding in aortic smooth muscle cells

We have recently identified a novel element (EFE 5/6) in the human elastin gene promoter that modulates the ability of insulin-like growth factor I (IGF-I) to up-regulate elastin gene transcription in aortic smooth muscle cells. In the present study, we have pursued the identification of those nuclear proteins binding to the EFE 5/6 element and affected by IGF-I treatment. Chelation inactivation and metal reactivation experiments together with supershift gel analyses demonstrated that Sp1 was one of the proteins affected by IGF-I. Southwestern and Western analyses showed that Sp1 was present in IGF-I nuclear extracts and capable of binding DNA after fractionation. Addition of retinoblastoma gene product (Rb) antibody mimicked the effect of IGF-I in gel shift analysis, suggesting that Sp1 binding may be regulated by an inhibitor normally associated with Rb. The fact that the phosphorylation state of Rb was affected by IGF-I was shown by Western blot analysis. The control smooth muscle cells transcribed the elastin gene at a high level without addition of IGF-I, so it is likely that disruption of Sp1 binding is the first step in allowing the binding of a more potent activating factor.

Evidence for the presence of a functional TATA box (ATAAAA) sequence in the gene for bovine elastin

The wild type sequence (wt; ATAAAA) was mutated and the effects of the mutants were determined by assaying the expression of the chloramphenicol acetyltransferase (CAT)-encoding gene (cat) cloned downstream. The negative mutant (neg; ACGAAA) practically abolished the activities of the 129 bp and the 416 bp elastin promoters in NIH 3T3 and neonatal rat aortic smooth muscle cells, respectively. However, when the positive mutant (pos; TATAAAA) was assayed in parallel experiments, there was enhancement of activity. The TATA box-binding protein (TBP) was shown to bind to the ATAAAA sequence and the retardation of the band was abolished by competition assay using unlabeled wt and pos sequences, but not by the neg mutant. These results provide evidence for the presence of a functional TATA box in the gene for elastin.

Evidence for a cell-specific negative regulatory element in the first intron of the gene for bovine elastin

A cell-specific negative regulatory element has been identified in the first intron of the gene for elastin in a region between 442 and 464 bp from the translational start site. This regulatory element functions both when it is located 5' of the promoter and 3' of the chloramphenicol acetyltransferase (CAT) gene. The inhibition is observed both with the homologous elastin promoter and the heterologous SV1 promoter in transient expression experiments using rat aortic smooth-muscle cells. No inhibition was observed with NIH 3T3, Hep G2 and little, if any, with HeLa cells. Cell specificity was further confirmed by DNA mobility shift assays and the position of the negative regulatory element was localized with the use of synthetic duplex oligomers. It is proposed that this negative element plays a significant role in the modulation of the expression of the gene for elastin in the smooth-muscle cells of the aorta during development.

Alternate exon usage is a commonly used mechanism for increasing coding diversity within genes coding for extracellular matrix proteins

Extracellular matrix proteins are a diverse family of secreted proteins and glycoproteins that are responsible for a variety of critical functions in different tissues. A large number of multiexon genes encode these proteins of the extracellular matrix. Over the last few years, it has become evident that the processing of the pre-mRNA from several of these genes involves alternative splicing. This review summarizes the known examples of alternative splicing in genes coding for the extracellular matrix and attempts to relate the increase in coding diversity generated by alternate exon usage to the function(s) of individual extracellular matrix proteins.

Quantitation of tropoelastin mRNA and assessment of alternative splicing in human skin fibroblasts by reverse transcriptase-polymerase chain reaction

We have developed a reverse transcriptase-polymerase chain reaction (RT-PCR) assay for the quantitative measurement of levels of tropoelastin mRNA in total RNA preparations from skin fibroblasts. This method facilitates the reproducible detection of low abundance tropoelastin mRNA in the range of 10-1000 copies per cell. The procedure is based on a competitive RT-PCR assay where a tropoelastin cDNA-derived internal RNA standard is cotranscribed and coamplified together with the sample derived-endogenous target mRNA. In addition, RT-PCR of several domains of tropoelastin mRNA, followed by DNA sequence analysis of asymmetric PCR products, revealed a previously unknown pattern of alternate exon usage at the 3' end of the tropoelastin gene in human skin fibroblasts.

PGAIPG, a repeated hexapeptide of bovine tropoelastin, is a ligand for the 67-kDa bovine elastin receptor

Tropoelastin is composed of alternating hydrophobic and hydrophilic domains. A hydrophobic peptide, VGVAPG, has been shown to be a ligand for a 67-kDa elastin cell surface receptor expressed on fetal bovine auricular chondrocytes and ligamentum nuchae fibroblasts. To explore the possibility that tropoelastin contains additional peptide ligands for this elastin receptor, we have constructed two deletion proteins that are expressed in E. coli and lack the repeated VGVAPG sequence. These proteins supported bovine fibroblast attachment implying the presence of a receptor binding site. Experiments using synthetic peptides contained within these proteins identify a chemotactic peptide, PGAIPG, and a chemokinetic peptide, GAIPG, PGAIPG was identified as a ligand for the bovine elastin receptor.

Phorbol ester-mediated downregulation of tropoelastin expression is controlled by a posttranscriptional mechanism

Expression of tropoelastin, the principal precursor of elastic fibers, is tissue-specific and is limited to a brief developmental period. Little is known, however, about the mechanisms that regulate the tissue- and temporal-specific expression of elastogenesis. The tropoelastin promoter contains putative phorbol ester responsive elements, or AP-1 binding sites, but the functional significance of these sequences is unknown. To test if tropoelastin expression is influenced by phorbol esters, we exposed elastogenic fetal bovine chondrocytes to 10(-7) M 12-O-tetradecanoylphorbol 13-acetate (TPA). Tropoelastin mRNA levels decreased greater than 10-fold in response to TPA, and this downregulation was paralleled by a decline in the secretion of tropoelastin protein into the culture medium. As determined by nuclear-runoff assay and transient transfection with a human gene promoter-CAT construct, tropoelastin transcription was unaffected after exposure to TPA. As indicated by actinomycin D experiments, the half-life of tropoelastin mRNA in control cells was about 20 h, but exposure to TPA resulted in an accelerated decay of the tropoelastin transcript (t1/2 = 2.2 h). These data indicate that downregulation of tropoelastin expression was controlled by a posttranscriptional mechanism and that the AP-1 elements in the bovine tropoelastin promoter may not be involved in regulation of production.

Elements of the rat tropoelastin gene associated with alternative splicing

Multiple isoforms of tropoelastin, the soluble precursor of elastin, are the products of translation of splice-variant mRNAs derived from the single-copy tropoelastin gene. Previous data had demonstrated DNA sequence heterogeneity in three domains of rat tropoelastin mRNA, indicating alternative splicing of several exons of the rat tropoelastin gene. Rat tropoelastin genomic clones encompassing the sites of alternative splicing were isolated and sequenced. Two sites of alternative splicing identified in rat tropoelastin mRNA sequences corresponded to exons 13-15 and exon 33 of the rat tropoelastin gene. Furthermore, the variable inclusion of an alanine codon in exon 16 resulted from two functional acceptor sites separated by three nucleotides. DNA sequences flanking exons subject to alternative splicing were analyzed. These exons contained splicing signals that differed from consensus sequences and from splicing signals of constitutively spliced exons. Introns immediately 5' of exons 14 and 33, for example, lacked typical polypyrimidine tracts and had weak, overlapping branch point sequences. Further, a region of secondary structure encompassing the acceptor site of exon 13 may influence alternative splicing of this exon. These results demonstrate that multiple cis-acting sequence elements may contribute to alternative splicing of rat tropoelastin pre-mRNA.

Cellular expression of tropoelastin mRNA splice variants

The primary transcript of tropoelastin is alternatively spliced into multiple mRNAs. The pattern and frequency of exon splicing is developmentally regulated, but the cellular profile of isoform expression within and among elastic tissues is not known. We used splice-variant specific antisense oligomeric deoxyribonucleotide probes in an in situ hybridization assay to assess the distribution of cells undergoing specific alternative splicing of tropoelastin pre-mRNA in developing bovine elastic tissues. Antisense oligomers were synthesized to exon sequences that are not alternatively spliced (exon 36) and to sequences that become abutted after high frequency (exon 33) and low frequency (exons 13 and 14) alternative splicing. The specificity of these probes for tropoelastin splice variants was verified by Southern hybridization to tropoelastin cDNAs with known exon deletions, and their specificity for tropoelastin mRNA was demonstrated by Northern hybridization. In situ hybridization with [35S]-labeled oligomers on sections of bovine lobar pulmonary artery and other elastic tissues showed that all elastogenic cells produce multiple forms of tropoelastin mRNA. These observations suggest that the production of tropoelastin isoforms is common to all cells within an elastin tissue and that this multiplicity may not be involved in regional differences in elastic tissue architecture.

Alternative splicing of rat tropoelastin mRNA is tissue-specific and developmentally regulated

Sequence analysis of cDNA clones coding for rat tropoelastin previously has identified two variants that potentially corresponded to alternatively spliced tropoelastin mRNAs (Pierce et al., 1990). We have now used S1 nuclease protection analysis of total RNA from aorta, skin and lungs of 10-day and 6-week old rats to localize all sites of alternative splicing in the tropoelastin mRNA and to examine tissue-specific and developmental regulation of the use of these sites. This analysis revealed multiple sites of alternative splicing involving rat tropoelastin coding sequences corresponding to exons 12 through 15 of the bovine tropoelastin gene and a single site of alternative splicing at sequences corresponding to exon 33. Messenger RNAs from all three tissues at both developmental stages were alternatively spliced at the same sites; there was no evidence for the use of an alternative splice site unique to a particular tissue or developmental stage. However, both tissue-specific and developmentally regulated differences were apparent in the proportion of rat tropoelastin mRNA alternatively spliced at exon 33. Tropoelastin mRNA from the aorta and lungs of neonatal rats was alternatively spliced at exon 33 ten time more frequently than tropoelastin mRNA from skin. Between 10 days and 6 weeks of development, the use of this site of alternative splicing decreased by twenty-fold in RNA from skin, ten-fold in RNA from lungs and two-fold in RNA from aorta. In contrast, alternative splicing at exons 12 through 15 occurred in a small percentage of the mRNA and use of these sites exhibited minimal tissue-specific differences or developmental regulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Mammalian tropoelastin: multiple domains of the protein define an evolutionarily divergent amino acid sequence

We have recently derived the complete amino acid sequence of rat tropoelastin from a series of overlapping cDNA clones. Comparison of this protein sequence to bovine and human tropoelastin has revealed significant differences in the rates of evolutionary divergence of the various domains of tropoelastin. The overall rate of divergence of the hydrophobic domains of tropoelastin was twice as fast as the cross-link domains of the protein. Certain hydrophobic domains, however, are as conserved as cross-link regions, particularly the hydrophobic sequence coded for by exon 33, the only exon subject to alternate usage in all three mammalian species and the most conserved domain in rat, bovine and human tropoelastin. This conservation of sequence strongly suggests a more complex function of the hydrophobic region encoded by exon 33, beyond the elastic recoil characteristic of all hydrophobic domains of tropoelastin. A comparison of average rates of divergence of hydrophobic and cross-link domains of tropoelastin to functionally-defined domains of other structural proteins, such as collagen, has also revealed that overall, tropoelastin is a highly divergent amino acid sequence, comparable to proteins such as globin and the fibrino-peptides.

Interaction between elastin and tumor cell lines with different metastatic potential; in vitro and in vivo studies

Interactions between the extracellular matrix macromolecules and tumor cells are critical in the process of metastasis formation. We show here that elastins (both mature insoluble elastin and a 75-kDa soluble peptide: K-elastin) adhere rapidly to two cell lines with high metastatic capacities: a metastatic lung carcinoma cell line (3LL-HM) and a human amelanotic melanoma cell line (A-2058); by contrast the low-metastatic Lewis lung carcinoma cell line variant as well as a rhabdomyosarcoma cell line with a low metastatic potential bind to elastins to a much lower extent. 3H-labelled K-elastin was used in order to study elastin--3LL-HM interaction. It was found to be saturable (2 ng 3H-labelled K-elastin/10(6) cells), with one class of high-affinity binding sites having Kd equal to 1.3 nM and 16,000 sites/cell. The binding of K-elastin to 3LL-HM cells at its receptor triggered several cell responses; (a) increase of intracellular Ca2+ concentration; (b) induction of 3LL-HM chemotaxis toward the K-elastin gradient; (c) stimulation of the adherence of mature insoluble elastin. In contrast to non-transformed cells such as fibroblasts and smooth muscle cells, the adhesion kinetics of insoluble elastin to 3LL-HM did not exhibit a lag period; the rapid binding of insoluble elastin to the tumor cells was followed by its slow detachment from the cells, which lasted for 6 h. 3LL-HM cells but not human skin fibroblasts were shown to secrete elastinolytic activity inhibitable by metal-chelating agents. In vivo studies were performed in order to evaluate the influence of K-elastin binding to 3LL-HM cells on their ability to form lung colonies in mice. It was shown that pretreatment of 10(4) 3LL-HM cells with 10 microM K-elastin and the simultaneous i.v. injection into mice of 750 micrograms K-elastin together with the highly metastatic cells was able to reduce the number of lung colonies by more than 70% after 12 days.

Fibroblast adhesion to recombinant tropoelastin expressed as a protein A-fusion protein

A bovine tropoelastin cDNA encoding exons 15-36 that includes the elastin-receptor binding site was expressed in Escherichia coli as a fusion protein with Protein A from Staphylococcus aureus. After isolation of the fusion protein by affinity chromatography on Ig-Sepharose, the tropoelastin domain was separated from plasmid-pR1T2T-encoded Protein A (Protein A') by CNBr cleavage. Cell-adhesion assays demonstrated specific adhesion to the recombinant tropoelastin. Furthermore, the data indicate that interactions involving the bovine elastin receptor mediate nuchalligament fibroblast adhesion to the recombinant protein. In agreement with earlier studies of fibroblast chemotaxis to bovine tropoelastin, nuchal-ligament fibroblast adhesion demonstrated developmental regulation of the elastin receptor.

Tropoelastin heterogeneity: implications for protein function and disease

The organization of the tropoelastin gene is similar to that of other genes coding for matrix proteins in that the exons code for distinct domains of the protein. An unusual feature of tropoelastin expression is that the primary transcript of the gene coding for tropoelastin undergoes extensive, developmentally regulated alternative splicing, resulting in numerous protein isoforms. Although the significance of this heterogeneity is unknown, the multiple sequence variations may affect the function of tropoelastin. Without an understanding of the importance of the domains of tropoelastin and the process of fibrillogenesis, characterization of defects resulting in aberrant elastin production will be hindered. In this update, we review recent findings on tropoelastin and speculate as to the structural and regulatory role of various regions of this matrix protein.

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.