Alternative splicing of human elastin mRNA indicated by sequence analysis of cloned genomic and complementary DNA

Human alpha 1-proteinase inhibitor binds to extracellular matrix in vitro

alpha 1-Proteinase inhibitor (alpha 1-PI) is the major endogenous inhibitor of human leukocyte elastase (HLE). We have employed two different methods to quantitate the binding of alpha 1-PI to extracellular matrix (ECM), composed of 51% glycoproteins and proteoglycans, 37% types I and III collagen, and 12% elastin, derived from rat heart smooth muscle cells. alpha 1-PI is tightly bound to ECM via a saturable adsorption process; the bound protein fails to dissociate from the matrix after repeated washing. Binding of alpha 1-PI is unaffected by the prior removal of ECM glycoproteins with trypsin. Binding to ECM is not decreased in the presence of high salt but is decreased at low pH. A 40-fold excess of unlabeled alpha 1-PI displaces only 50% of [125I]alpha 1-PI prebound to ECM. A 30% decrease in the levels of alpha 1-PI bound to ECM is observed after DTT washes of ECM preincubated with alpha 1-PI or when alpha 1-PI is modified with iodoacetamide prior to incubation with ECM, implying that a fraction of bound alpha 1-PI is covalently linked to ECM via disulfide bond formation. Moreover, high molecular weight complexes between [125I]alpha 1-PI and ECM components can be visualized by SDS-PAGE under nonreducing conditions but disappear upon reduction. Approximately 50% of the total alpha 1-PI bound covalently or noncovalently to ECM retains the ability to inhibit HLE-mediated ECM proteolysis. alpha 1-PI-HLE complexes bound to ECM can be visualized by SDS-PAGE following the addition of HLE to ECM that was pretreated with [125I]alpha 1-PI. alpha 1-PI from normal plasma or serum also binds to ECM with retention of immunoreactivity and partial retention of inhibitory activity. However, ECM pretreated with alpha 1-PI-deficient serum retains no HLE-inhibitory activity.

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.

Description of a dinucleotide repeat polymorphism in the human elastin gene and its use to confirm assignment of the gene to chromosome 7

Informative polymorphisms have been very difficult to detect in the elastin gene, and this has hampered the analysis of heritable connective tissue disorders, notably the Marfan syndrome. We have recently detected a dinucleotide repeat polymorphism in intron 17 of the human elastin gene consisting of 8 alleles with sizes between 161 and 175 bp. Analysis of 540 chromosomes from unrelated Caucasian individuals revealed a bimodal frequency distribution typical of (dC-dA)n.(dG-dT)n repeat polymorphisms, with allele frequencies ranging from 0.004 (161 bp) to 0.574 (163 bp). As the elastin gene was originally assigned to chromosome 2q31-ter and because more recent data have suggested an assignment to 7q11.1-21.1, we have genotyped a sub-set of the CEPH pedigrees and carried out pairwise linkage analysis with markers on chromosomes 7 and 2. Lod-scores of between +3.70 and +13.69 were obtained with markers spanning 7p13-q22.1, whilst negative lod-scores were observed with the chromosome 2 markers. Analysis of type II human ovarian teratomas placed the elastin gene within 11 cM of the centromere on chromosome 7. Additionally, we detected the dinucleotide repeat in human-rodent cell hybrids containing chromosome 7, but not those containing chromosome 2. These data confirm the assignment of elastin to chromosome 7 and provide a new, highly informative marker for the analysis of heritable disorders of connective tissue for which elastin is a candidate gene.

A novel tenascin type III repeat is part of a complex of tenascin mRNA alternative splices

Sequence analysis of two human tenascin encoding cDNA clones from a cDNA library of the U251 glioblastoma cell line revealed the presence of a novel 276 bp tenascin type III fibronectin like repeat. This alternatively spliced type III repeat designated AD1 is located between the previously identified repeats 10 and 11 and has sequence homology with human, chicken and mouse tenascin type III repeats. These results show that tenascin has at least 16 consecutive fibronectin like type III repeats. PCR amplification of random primed mRNA with specific type III repeat primers revealed a pattern of multiple alternative splices of AD1 and flanking type III repeats. The alternative splice variants were confirmed by direct sequencing. Differences were observed in the expression of the various alternative splices of tenascin mRNA between tumor and normal cells and may thus indicate differences in tenascin isoform expression and function in normal and tumor cells. PCR and Southern analysis of genomic DNA indicate that AD1 is coded by a single exon present in both human and mouse genome.

Hydrophobicity scale for proteins based on inverse temperature transitions

In general, proteins fold with hydrophobic residues buried, away from water. Reversible protein folding due to hydrophobic interactions results from inverse temperature transitions where folding occurs on raising the temperature. Because homoiothermic animals constitute an infinite heat reservoir, it is the transition temperature, Tt, not the endothermic heat of the transition, that determines the hydrophobically folded state of polypeptides at body temperature. Reported here is a new hydrophobicity scale based on the values of Tt for each amino acid residue as a guest in a natural repeating peptide sequence, the high polymers of which exhibit reversible inverse temperature transitions. Significantly, a number of ways have been demonstrated for changing Tt such that reversibly lowering Tt from above to below physiological temperature becomes a means of isothermally and reversibly driving hydrophobic folding. Accordingly, controlling Tt becomes a mechanism whereby proteins can be induced to carry out isothermal free energy transduction.

Extracellular matrix protein gene expression in atherosclerotic hypertensive pulmonary arteries

Lobar pulmonary arteries from patients with unexplained pulmonary hypertension were obtained at the time of single-lung transplantation to determine the response of large elastic vessels to increased intraluminal pressure. Specifically, human pulmonary arteries were examined to determine if remodeling remained active at the time of surgery and whether remodeling was similar to previously reported remodeling observed in several animal models. Grossly, the hypertensive vessels appeared atherosclerotic. Histochemical stains revealed a thick, diffuse neointima in hypertensive vessels compared with normal vessels. Immunohistochemistry demonstrated elastin protein in the neointima and in situ hybridization studies demonstrated tropoelastin mRNA largely in the neointima. Similarly, immunohistochemistry and in situ hybridization detected cellular fibronectin, thrombospondin and type I collagen protein and mRNA within the thickened intima from hypertensive vessels. These studies provide evidence that hypertensive vessels in patients with severe chronic pulmonary hypertension are actively remodeling but that the pattern of remodeling is different from previously described animal models.

Mapping of binding sites for monoclonal antibodies to chick tropoelastin by recombinant DNA techniques

A fusion molecule consisting of the entire coding sequence of mature chicken tropoelastin preceded by 14 amino acids of the signal peptide and 9 amino acids of vector origin has been expressed in a recombinant bacterial system and purified. The molecule has been used as immunogen for the production of hybridomas. Monoclonal antibodies which bound specifically the immunogen were also reactive with tropoelastin purified from chick aorta and stained elastic fibers in aorta sections by immunofluorescence. The region of tropoelastin containing the antigenic determinant recognized by each antibody has been identified by a recombinant DNA expression strategy based on the use of cDNA clones spanning different portions of the coding sequence. It could be shown that several antibodies were directed against unique epitopes; among these, a group of antibodies bound specifically to the sequence (PGVGV)n. Other antibodies were found to recognize antigenic determinants present more than once in the molecule. The monoclonal antibodies thus characterized will be useful reagents in studying the function of the different domains of tropoelastin.

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)

Collagen metabolism in cutis laxa fibroblasts: increased collagenase gene expression associated with unaltered expression of type I and type III collagen

Collagen metabolism was studied in cutis laxa by analyzing collagen and collagenase gene expression in three dermal fibroblast strains from patients with congenital cutis laxa and comparing them with fibroblasts obtained from age-matched healthy subjects. Normal collagen synthetic activity was observed in the cutis laxa fibroblasts. An increased level of collagenase mRNA and unaltered levels of alpha 1(I) and alpha 1(III) collagen mRNA were found in all cutis laxa cell strains by dot blot hybridization. Reduced levels of elastin mRNA were also detected in these strains. However, no qualitative differences in these mRNA transcripts were detected between the control and cutis laxa fibroblasts by Northern blot analysis. Collagenase activity in fibroblast culture supernatants was then measured using fluorescein isothiocyanate (FITC)-labeled type I collagen. Increased collagenolytic activity in cutis laxa fibroblast culture supernatants was also found. These data suggest that increased collagenase expression of fibroblasts is related to the structural abnormality of dermal connective tissue in cutis laxa.

A to G polymorphism in ELN gene

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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.

Marked longevity of human lung parenchymal elastic fibers deduced from prevalence of D-aspartate and nuclear weapons-related radiocarbon

Normal structure and function of the lung parenchyma depend upon elastic fibers. Amorphous elastin is biochemically stable in vitro, and may provide a metabolically stable structural framework for the lung parenchyma. To test the metabolic stability of elastin in the normal human lung parenchyma, we have (a) estimated the time elapsed since the synthesis of the protein through measurement of aspartic acid racemization and (b) modeled the elastin turnover through measurement of the prevalence of nuclear weapons-related 14C. Elastin purified by a new technique from normal lung parenchyma was hydrolyzed; then the prevalences of D-aspartate and 14C were measured by gas chromatography and accelerator-mass spectrometry, respectively. D-aspartate increased linearly with age; Kasp (1.76 x 10(-3) yr(-1) was similar to that previously found for extraordinarily stable human tissues, indicating that the age of lung parenchymal elastin corresponded with the age of the subject. Radiocarbon prevalence data also were consistent with extraordinary metabolic stability of elastin; the calculated mean carbon residence time in elastin was 74 yr (95% confidence limits, 40-174 yr). These results indicate that airspace enlargement characteristic of "aging lung" is not associated with appreciable new synthesis of lung parenchymal elastin. The present study provides the first tissue-specific evaluation of turnover of an extracellular matrix component in humans and underscores the potential importance of elastin for maintenance of normal lung structure. Most importantly, the present work provides a foundation for strategies to directly evaluate extracellular matrix injury and repair in diseases of lung (especially pulmonary emphysema), vascular tissue, and skin.

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.

Human elastin gene: new evidence for localization to the long arm of chromosome 7

In this study we have utilized human elastin cDNAs in molecular hybridizations to establish the chromosomal location of the human elastin gene. First, in situ hybridizations were performed with metaphase chromosomes from phytohemagglutinin-stimulated human peripheral blood lymphocytes. In three separate experiments using two different regions of human elastin cDNAs, the distribution of grains was found to be concentrated on the long arm of chromosome 7 within the [q11.1-21.1] region, and the peak number of grains coincided with the locus 7q11.2. Second, hybridizations with a panel of human-rodent cell hybrids showed concordance with human chromosome 7. Third, PCR analyses with elastin-specific primers of DNA from a hybrid cell line containing chromosome 7 as the only human chromosome yielded a product of the expected size, while DNA containing human chromosome 2, but not chromosome 7, did not result in a product. The results indicate that the human elastin gene is located in the proximal region of the long arm of chromosome 7. The precise localization of the elastin gene in the human genome is useful in establishing genetic linkage between inheritance of an allele with a mutated elastin gene and a heritable disorder.

Exons--original building blocks of proteins?

In a recent paper, Walter Gilbert's group has estimated the number of original exons from which all extant proteins might have been constructed. The approach used is subjected to a critical analysis here. It is shown that there are flawed assumptions about both the mechanism and generality of exon-shuffling and in the sequence comparison procedures employed, the latter failing to distinguish chance similarity from similarity due to common ancestry. These methodological errors lead to the omission of many known cases of exon-shuffling and the inclusion of others which may not be genuine. In consequence, the analysis from the Gilbert group cannot give a reliable estimate of those modules that actually participated in exon-shuffling and provides no information on the number of protein archetypes that did not participate in these processes.

Size distribution of human lung elastin-derived peptide antigens generated in vitro and in vivo

The protease-antiprotease hypothesis of emphysema development suggests that degradation of elastin in the lung interstitium may give rise to abnormal quantities of circulating elastin-derived peptides (EDP) during periods of inflammation. Recent studies have shown a relationship between emphysema and high levels of EDP in human plasma. This report characterizes elastin digests on the basis of antigenicity, size, and method of preparation, as well as the size distribution of EDP found in the plasmas of nonsmokers, smokers, and emphysema patients. Gel filtration of elastin digests prepared by hydrolysis of human lung elastin using a low (1:500) ratio of neutrophil elastase to elastin generated a broad protein peak of approximately 70,000 daltons. In contrast, a high (1:25) ratio of neutrophil elastase to human lung elastin gave a broad protein peak, with a size distribution in the 10,000 to 30,000 dalton range. This digest showed distinct immunochemical properties. A polyclonal antibody directed against the low-ratio digest showed a minimum detection of 2 ng/ml for the homologous antigen but required 1,000 ng/ml of the high-ratio digest for detectable inhibition in an indirect ELISA assay. Gel filtration of plasmas from normal nonsmokers and the majority of normal smokers revealed a single immunoreactive EDP fraction of approximately 70,000 daltons. Plasmas from selected normal smokers and emphysema patients with high levels of circulating EDP (greater than 90 ng/ml) fractionated into a complex pattern of peptides in which the 70,000 dalton component represented 50% of the immunoreactive material and several lower molecular weight peptides represented the remaining circulating elastin antigens.(ABSTRACT TRUNCATED AT 250 WORDS)

Three-dimensional structures of uterine elastic fibers: scanning electron microscopic studies

We report findings that demonstrate for the first time that the structure of the elastic fibers of the uterus and cervix are characteristic to these tissues. Elastic fibers of the uterine corpus and cervix were studied by scanning electron microscopy (SEM). Elastic tissues were prepared from non-pregnant human uteri and pregnant rat uteri by three different methods: extraction from the tissue homogenates, in situ digestion by autoclaving of sliced tissue, and in situ formic acid digestion of sliced tissue. In addition, in situ formic acid digestion of the glutaraldehyde-fixed uterine wall of pregnant rats was done. Under SEM, the uterine elastic fibers revealed two distinct structures--fibrils and thin sheets of elastic membranes. Isolated fibers and membranes ranged in thickness from 0.1 to 0.4 micron which is thinner than aortic elastic lamellae (1-2.5 microns in thickness). These thin sheets of elastic membranes and elastic fibrils probably allow the uterus to maintain its elasticity without exerting excess pressure on the growing fetus. Formic acid digestion of fixed uterine walls of pregnant rats preserved the structural organization of elastic tissues near in vivo conditions. In these tissues, the fibers were arranged in a honeycomb structure made up largely of membranes, although sparse fibrils were present. These elastic, membranous sheets were arranged parallel to the plane of the uterine surface and interconnected with the threads of the membranes or fibrils. In the rat uterine wall, at least 12 parallel layers of elastic sheets were present. In contrast, at low magnification, the elastic tissues in the non-pregnant human uterus had no specific architectural arrangement and exhibited a sponge-like structure. Elastic fibers of the cervix were also made up of membranes and fibrils, and these fibers were organized into fishnet-like structures. These cervical membranes had fenestrations and pits with a diameter of 3-5 microns. In these studies, the concentrations of insoluble elastin in human uteri were found to be 1.38 and 1.32-1.41% of dry-defatted tissues for uterine body and cervix, respectively. The concentrations of total collagen were 38.8 and 64.3-72.4% of dry-defatted tissues for uterine body and cervix, respectively.

How big is the universe of exons?

If genes have been assembled from exon subunits, the frequency with which exons are reused leads to an estimate of the size of the underlying exon universe. An exon database was constructed from available protein sequences, and homologous exons were identified on the basis of amino acid identity; statistically significant matches were determined by Monte Carlo methods. It is estimated that only 1000 to 7000 exons were needed to construct all proteins.

Multiple forms of chicken alpha 3(VI) collagen chain generated by alternative splicing in type A repeated domains

Type VI collagen is a structurally unique component widely distributed in connective tissues. Its molecular structure consists of monomers that have the potential to assemble intracellularly into dimers and tetramers which, once secreted, can form microfilaments by end-to-end association. Individual monomers are composed of chains of Mr = approximately 140,000 (alpha 1 and alpha 2) and greater than 300,000 (alpha 3). Type VI collagen molecules contain a short triple helix with large globular domains at both ends. These domains are made for their greatest part of repetitive units similar to type A repeats of von Willebrand Factor. The alpha 3(VI) chain, contributing most of the mass of the NH2-terminal globule, appeared heterogenous both at the mRNA and protein level. Several alpha 3(VI)-specific clones that lack the sequences corresponding to repeats A8 and A6 were isolated from a chicken aorta cDNA library. Northern blot hybridization of poly (A+)-enriched RNA from chicken gizzard with cDNA fragments corresponding to several individual type A repeats showed that A8- and A6-specific probes did not hybridize to the lower Mr transcripts. Clones spanning approximately 20 kb of the 5'-end of the alpha 3(VI) gene were isolated from a chicken genomic library and subjected to analysis by restriction mapping, Southern blotting, and selective sequencing of the intron-exon boundaries. At the most 5'-end of the gene an additional type A repeat (A9), previously undetected in cDNA clones, was identified. Furthermore, it was determined that the presumed signal peptide and repeats A9 through A6 are encoded within individual exons. Reverse transcription and polymerase chain reaction of aorta RNA suggested that a mechanism of alternative mRNA splicing by a phenomenon of exon skipping generates alpha 3(VI) isoform variants that contain different numbers of type A repeats. Immunohistochemistry of frozen sections of chicken embryo tissues with repeat-specific mAbs showed that an antibody directed against a conditional exon has a more restricted tissue distribution compared to an antibody against a constitutive exon.

Transforming growth factor-beta increases elastin production by neonatal rat lung fibroblasts

While elastin degradation is a hallmark of pulmonary emphysema, it is likely that elastin synthesis also occurs. However, the supramolecular structure and function of the newly synthesized elastin are abnormal. Very little is known about the regulation of elastin synthesis during the development of emphysema when prominent collections of mononuclear phagocytes are found in and near the alveolar interstitium. Transforming growth factor-beta (TGF-beta) is an important regulator of collagen and fibronectin production in wound healing, which is also accompanied by an influx of mononuclear phagocytes. We hypothesized that TGF-beta may influence elastin production by fibroblasts in the pulmonary interstitium. Therefore, we examined the influence of TGF-beta on the production of elastin by postconfluent cultures of neonatal rat lung fibroblasts. Elastin production was quantitated by analyzing the incorporation of [3H]valine into the soluble elastin precursor tropoelastin (TE). The incorporation of [3H]valine into TE was approximately 2-fold greater in the presence of 40 or 100 pM TGF-beta than in its absence. The intracellular, free [3H]valine pool was increased by 18% in the presence of TGF-beta. Therefore, TGF-beta-related differences in the precursor pool size were not solely responsible for the observed increase in [3H]valine incorporation. Northern analysis demonstrated that the increase in TE was accompanied by a smaller but significant increase in the steady-state level of elastin mRNA. Thus, the observed increase in TE production can be at least partially attributed to a pretranslational effect of TGF-beta.

Degradation of immobilized soluble elastin by tumor cells in culture: quantitation by ELISA

A new, sensitive assay based on the enzyme-linked immunosorbent assay has been developed for measuring elastolytic activity produced by invasive and/or metastatic tumor cells in culture. Elastin peptides, obtained by treating the insoluble protein with either oxalic acid, KOH, or chymotrypsin, are adsorbed onto the surface of cell culture microtiter plastic wells, and incubated with dilution of standard proteinases or viable normal or tumor cells. The total amount of immobilized elastin peptides is revealed by the mean of specific antibodies, and detected by a microplate reader, while dose- and time-dependent reduction of bound antibodies after incubation with proteases or cells is taken as a measure of elastin degradation. Adsorbed elastin has been found to be available as a substrate for purified enzymes, as well as for living melanoma cells (A2058 and B16-BL6), c-Ha-ras transformed rat embryo fibroblasts, and human pulmonary macrophages, as demonstrated by the release into the culture medium of lower molecular weight digestion products. No degradation was achieved by BALB/3T3 and rat embryo control fibroblasts, and no inhibition was produced by the presence of fetal calf serum which, on the contrary, potentiated the degradation by active cells. This new method, revealing degradation of only a few nanograms of soluble elastin peptides, can be used for studying the importance in tissue invasion and metastasis of elastolytic proteinases produced by cells in culture.

Extracellular matrix

A succinct overview of recent results on the biochemistry of extracellular matrix (ECM) is presented. The rapid expansion of this discipline over the best decades renders impossible to give an even approximately complete coverage of matrix biology. Some selected results concerning the four major families of macromolecules composing the ECM, that is, collagens (14 types described), elastin(s), proteoglycans and structural glycoproteins (especially fibronectin) are described. Special attention is directed to a crucial aspect of matrix biology: cell-matrix interactions. A number of cell membrane receptors were recently described mediating the two way information flow from the cells to the matrix via the 'programme' of ECM synthesis coded in the genome and unfolding during differentiation and from the ECM to the cells through the membrane receptors which contact the cytoskeleton. One of them at least, the elastin receptor was shown to be linked through a G-protein-phospholipase C-IP3 mediated relay to the regulation of intracellular calcium. Modifications of the ECM will therefore influence cell behaviour. Derangements of this informational feed back mechanisms appear to be involved in most age-related connective tissue diseases.

Two polymorphisms for the human elastin gene

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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.

Susceptibility of baboon aorta elastin to proteolysis

Elastin was purified from baboon aorta using Achromobacter collagenase and its susceptibility to proteolysis by various enzymes was studied. Human leukocyte elastase (HLE) hydrolysed baboon aortic elastin 8 times faster than human cathepsin G. Bovine chymotrypsin had virtually no activity against this substrate. The kinetic constants V and [S50] of aortic elastin hydrolysis by HLE (0.15 microM) were 0.00286 mg x ml-1 x min-1 and 0.158 mg x ml-1, respectively. One mg of this elastin could be saturated with 5.6 micrograms of HLE. As with elastins isolated from other sources, the hydrolysis of baboon aortic elastin by HLE was highly sensitive to ionic strength, and a biphasic effect was obtained with increasing NaCl concentrations. A nearly 2-fold stimulation of elastolysis was observed at a 0.15M NaCl concentration. Further increase in ionic strength led to a continuous decrease of the rate of elastolysis which paralleled the decrease of adsorption of elastase to baboon aortic elastin. Cathepsin G, but not bovine alpha-chymotrypsin, was able to stimulate the rate of hydrolysis of baboon aortic elastin by HLE. A 1.7 fold stimulation was observed for a 1:1 molar ratio of the two proteinases and rose to 2.1 for a HLE/Cat. G ratio equal to 8.

Regulation of elastin gene expression: evidence for functional promoter activity in the 5'-flanking region of the human gene

Analysis of nucleotide sequences in the 5'-flanking region of the human elastin gene has revealed several unusual features, suggesting that regulation of elastin gene expression is complex. To identify any cis-acting regulatory promoter elements, a 35-kb fragment of DNA (CosE) was isolated from a human genomic cosmid library by hybridizations with a human elastin cDNA. Southern blots of EcoRI digests of CosE DNA, utilizing a 5'-end labeled 21-mer oligonucleotide corresponding to the signal sequence of elastin, revealed the presence of a single 7.8-kb genomic fragment. Partial dideoxynucleotide sequencing of this EcoRI genomic subclone revealed that it extended approximately 2.5 kb 3' of the translation initiation site (ATG), encompassing exon 1 and a portion of the first intron, while the remaining DNA encompassed the 5'-flanking region. Exonuclease III digestion (3'----5') was performed to remove sequences of the first intron and first exon, including the ATG site. One clone, approximately 5 kb in size, had the 3' end located 14 bp upstream of the ATG site. A 462-bp 3' portion of this 5-kb fragment was subcloned into a Bluescript/CAT chimeric plasmid (pBS0CAT) to generate an elastin gene promoter/CAT reporter gene construct (pEP6CAT). Transient transfection experiments with pEP6CAT using human skin fibroblasts, human HT-1080, mouse NIH-3T3, or freshly isolated neonatal rat aortic smooth muscle cells revealed significant CAT activity in each cell line. These results suggest that the 5'-flanking region of the elastin gene contains the cis-acting regulatory elements necessary for transcription. The chimeric plasmid pEP6CAT provides a means to study the transcriptional control of elastin gene expression by exogenous affector molecules, as well as in human dermatologic diseases.

Marfan syndrome: exclusion of genetic linkage to five genes coding for connective tissue components in the long arm of chromosome 2

Marfan syndrome represents a heterogeneous connective tissue disease, the symptoms arising in several tissues and organs. The defective gene(s) behind this autosomal dominant condition has not been found despite considerable research. The main targets of the research have been the genes coding for connective tissue components. Several of the candidate genes suspected to be defective in Marfan syndrome are located on the long arm of chromosome 2. These genes include a cluster of two genes coding for fibrillar collagens COL3A1 and COL5A2, and a third member of the collagen gene family: COL6A3. Furthermore, genes for elastin (ELN) and fibronectin (FN) are also located in this area of chromosome 2. We studied this chromosomal area using restriction fragment length polymorphism (RFLP) linkage analysis in five Finnish Marfan families with affected members in three generations. In two point linkage analyses, Lod scores of -3.192 (theta = 0.1) to COL3A1, -1.683 (theta = 0) to COL6A3 and -2.664 (theta = 0.01) to FN were obtained, whereas the linkage analysis between elastin and the disease was non-informative (Lod score 0.444, theta = 0). With the multipoint linkage analysis that permits simultaneous examination of several loci and more efficient use of family data, we obtained an exclusion of all these loci as the site of the mutation leading to Marfan syndrome in these families.

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.

Developmental regulation of the mRNAs for elastins a, b and c in foetal-calf nuchal ligament and aorta

The data presented clearly suggest that relative amounts of mRNAs for elastins a, b and c are developmentally regulated in foetal-calf nuchal ligament and aorta and that this regulation is tissue-specific. In nuchal ligament, at earlier stages of foetal development, the relative amounts of mRNAs for elastins a and b are very low. After the foetal age of about 6 months the relative amount of mRNA for elastin b begins to increase. This is followed by an increase in the relative amount of mRNA for elastin a. In aorta, with increasing foetal age, the relative amounts of mRNAs for elastins b and c increase and decrease alternately. The relative amounts of mRNA for elastin a remain low, with only marginal increases with foetal age. A possible self-aggregation role of elastin a in elastogenesis is proposed.

Heterogeneity of elastin expression in cutis laxa fibroblast strains

Cutis laxa is a genetically heterogeneous connective tissue disease that occurs in both inherited and acquired forms. The most apparent defect is loose, redundant, nonresilient skin, but systemic connective tissue abnormalities exist, especially in conjunction with the early onset or autosomal recessive variety. The elastic fiber shows morphologic alterations. We studied dermal skin biopsies and cultured skin fibroblasts from 6 patients with congenital forms of cutis laxa in an effort to correlate alterations in elastin morphology and metabolism. In general, ultrastructural analysis revealed occasional variance in collagen fiber diameter, whereas elastic tissue varied in content, appearance, and the proportion and manner by which elastin and microfibrillar component associated. Fibroblast cell lines comprised of normal donors from a similar age group produced an average of 35 +/- 10 X 10(3) tropoelastin molecular equivalents per cell per hour, as measured by an ELISA. Three of six cutis laxa cell strains were markedly (5-20-fold) reduced in tropoelastin production. Two of these cell strains had specifically reduced levels of tropoelastin production relative to total protein synthesis. Analysis of elastin specific messenger RNA levels indicated this reduced expression of tropoelastin was regulated at a pretranslational level. In other strains, diminished production of elastin did not appear to be the primary defect, underscoring the heterogeneous nature of cutis laxa at both the biochemical and ultrastructural levels.