Tropoelastin heterogeneity: implications for protein function and disease

Emerging mechanisms of elastin transcriptional regulation

Elastin provides recoil to tissues that stretch such as the lung, blood vessels, and skin. It is deposited in a brief window starting in the prenatal period and extending to adolescence in vertebrates, and then slowly turns over. Elastin insufficiency is seen in conditions such as Williams-Beuren syndrome and elastin-related supravalvar aortic stenosis, which are associated with a range of vascular and connective tissue manifestations. Regulation of the elastin (ELN) gene occurs at multiple levels including promoter activation/inhibition, mRNA stability, interaction with microRNAs, and alternative splicing. However, these mechanisms are incompletely understood. Better understanding of the processes controlling ELN gene expression may improve medicine's ability to intervene in these rare conditions, as well as to replace age-associated losses by re-initiating elastin production. This review describes what is known about the ELN gene promoter structure, transcriptional regulation by cytokines and transcription factors, and posttranscriptional regulation via mRNA stability and micro-RNA and highlights new approaches that may influence regenerative medicine.

Elastic fibers: formation, function, and fate during aging and disease

Elastic fibers are extracellular components of higher vertebrates and confer elasticity and resilience to numerous tissues and organs such as large blood vessels, lungs, and skin. Their formation and maturation take place in a complex multistage process called elastogenesis. It requires interactions between very different proteins but also other molecules and leads to the deposition and crosslinking of elastin's precursor on a scaffold of fibrillin-rich microfibrils. Mature fibers are exceptionally resistant to most influences and, under healthy conditions, retain their biomechanical function over the life of the organism. However, due to their longevity, they accumulate damages during aging. These are caused by proteolytic degradation, formation of advanced glycation end products, calcification, oxidative damage, aspartic acid racemization, lipid accumulation, carbamylation, and mechanical fatigue. The resulting changes can lead to diminution or complete loss of elastic fiber function and ultimately affect morbidity and mortality. Particularly, the production of elastokines has been clearly shown to influence several life-threatening diseases. Moreover, the structure, distribution, and abundance of elastic fibers are directly or indirectly influenced by a variety of inherited pathological conditions, which mainly affect organs and tissues such as skin, lungs, or the cardiovascular system. A distinction can be made between microfibril-related inherited diseases that are the result of mutations in diverse microfibril genes and indirectly affect elastogenesis, and elastinopathies that are linked to changes in the elastin gene. This review gives an overview on the formation, structure, and function of elastic fibers and their fate over the human lifespan in health and disease.

Unique molecular networks: Formation and role of elastin cross-links

Elastic fibers are essential assemblies of vertebrates and confer elasticity and resilience to various organs including blood vessels, lungs, skin, and ligaments. Mature fibers, which comprise a dense and insoluble elastin core and a microfibrillar mantle, are extremely resistant toward intrinsic and extrinsic influences and maintain elastic function over the human lifespan in healthy conditions. The oxidative deamination of peptidyl lysine to peptidyl allysine in elastin's precursor tropoelastin is a crucial posttranslational step in their formation. The modification is catalyzed by members of the family of lysyl oxidases and the starting point for subsequent manifold condensation reactions that eventually lead to the highly cross-linked elastomer. This review summarizes the current understanding of the formation of cross-links within and between the monomer molecules, the molecular sites, and cross-link types involved and the pathological consequences of abnormalities in the cross-linking process.

Collagen and Elastin Biomaterials for the Fabrication of Engineered Living Tissues

Collagen and elastin represent the two most predominant proteins in the body and are responsible for modulating important biological and mechanical properties. Thus, the focus of this review is the use of collagen and elastin as biomaterials for the fabrication of living tissues. Considering the importance of both biomaterials, we first propose the notion that many tissues in the human body represent a reinforced composite of collagen and elastin. In the rest of the review, collagen and elastin biosynthesis and biophysics, as well as molecular sources and biomaterial fabrication methodologies, including casting, fiber spinning, and bioprinting, are discussed. Finally, we summarize the current attempts to fabricate a subset of living tissues and, based on biochemical and biomechanical considerations, suggest that future tissue-engineering efforts consider direct incorporation of collagen and elastin biomaterials.

Is tissue engineering and biomaterials the future for lower urinary tract dysfunction (LUTD)/pelvic organ prolapse (POP)?

The fields of tissue engineering and regenerative medicine have seen major advances over the span of the past two decades, with biomaterials playing a central role. Although the term "regenerative medicine" has been applied to encompass most fields of medicine, in fact urology has been one of the most progressive. Many urological applications have been investigated over the past decades, with the culmination of these technologies in the introduction of the first laboratory-produced organ to be placed in a human body.1 With the quality of life issues associated with urinary incontinence, there is a strong driver to identify and introduce new technologies and the potential exists for further major advancements from regenerative medicine approaches using biomaterials, cells or a combination of both. A central question is why use biomaterials? The answer rests on the need to make up for inadequate or lack of autologous tissue, to decrease morbidity and to improve long-term efficacy. Thus, the ideal biomaterial needs to meet the following criteria: (1) Provide mechanical and structural support, (2) Maintain compliance and be biocompatible with surrounding tissues, and (3) Be "fit for purpose" by meeting specific application needs ranging from static support to bioactive cell signaling. In essence, this represents a wide range of biomaterials with a spectrum of potential applications, from use as a supportive or bulking implant alone, to implanted biomaterials that promote integration and eventual replacement by infiltrating host cells, or scaffolds pre-seeded with cells prior to implant. In this review we shall discuss the structural versus the integrative uses of biomaterials by referring to two key areas in urology of (1) pelvic organ support for prolapse and stress urinary incontinence, and (2) bladder replacement/augmentation.

Solubilized elastin substrate for continuous fluorimetric assay of kinetics of elastases

Elastolysis is central to progression of emphysema and aortic aneurysms. Characterization of steady-state enzyme kinetics of elastolysis is fettered by the insolubility of mature elastin and the polydispersity of solubilized elastin. We prepared a fluor-tagged, 100-kDa fraction (fEln-100) from commercial α-elastin. It is soluble, less heterogeneous in mass, cross-linked like mature elastin, and likely to retain the capacity of α-elastin to self-assemble. fEln-100 has introduced the ability to compare quantitatively the apparent k(cat) and K(m) of elastases. For example, metalloelastase (MMP-12) displays higher apparent affinity for fEln-100, while MMP-2 displays faster catalytic turnover.

NMR and bioinformatics discovery of exosites that tune metalloelastase specificity for solubilized elastin and collagen triple helices

The catalytic domain of metalloelastase (matrix metalloproteinase-12 or MMP-12) is unique among MMPs in exerting high proteolytic activity upon fibrils that resist hydrolysis, especially elastin from lungs afflicted with chronic obstructive pulmonary disease or arteries with aneurysms. How does the MMP-12 catalytic domain achieve this specificity? NMR interface mapping suggests that α-elastin species cover the primed subsites, a strip across the β-sheet from β-strand IV to the II-III loop, and a broad bowl from helix A to helix C. The many contacts may account for the comparatively high affinity, as well as embedding of MMP-12 in damaged elastin fibrils in vivo. We developed a strategy called BINDSIght, for bioinformatics and NMR discovery of specificity of interactions, to evaluate MMP-12 specificity without a structure of a complex. BINDSIght integration of the interface mapping with other ambiguous information from sequences guided choice mutations in binding regions nearer the active site. Single substitutions at each of ten locations impair specific activity toward solubilized elastin. Five of them impair release of peptides from intact elastin fibrils. Eight lesions also impair specific activity toward triple helices from collagen IV or V. Eight sites map to the "primed" side in the III-IV, V-B, and S1' specificity loops. Two map to the "unprimed" side in the IV-V and B-C loops. The ten key residues circumscribe the catalytic cleft, form an exosite, and are distinctive features available for targeting by new diagnostics or therapeutics.

Elastic fibre assembly: macromolecular interactions

To investigate the mechanisms behind elastic fibre assembly, we studied the molecular interactions between elastin and microfibrillar components using solid-phase binding assays. Fibrillin 1, purified from tissue using reductive-saline extraction, showed no binding to microfibril-associated glycoprotein (MAGP) or tropoelastin. MAGP, however, was found to bind specifically to tropoelastin in a divalent-cation independent manner. Antibody inhibition studies indicated that the C-terminus of tropoelastin defined the interactive site with MAGP. MAGP and fibrillin were also substrates for transglutaminase, which may provide an important mechanism for stabilizing microfibrillar structure. In other studies we found that a major cross-linking region in elastin is formed through the association of domains encoded by exons 10, 19 and 25 of tropoelastin and that the three chains are joined together by one desmosine and two lysinonorleucine cross-links.

Elastin gene mutations in transgenic mice

We have constructed several rat tropoelastin minigene recombinants encoding the complete sequence of rat tropoelastin, two isoforms of rat tropoelastin and a truncated tropoelastin lacking the domains encoded by exons 19-31 of the rat gene. Coding and non-coding domains in all these recombinants were placed under the transcriptional control of 3 kb of the promoter domain of the rat tropoelastin gene. These minigenes were used to prepare a total of 28 separate founder lines of transgenic mice. A species-specific reverse-transcriptase polymerase chain reaction (RT-PCR) assay was established to demonstrate the synthesis of rat and mouse tropoelastin mRNA in several tissues obtained from both neonatal and adult transgenic mice. Thermolytic digestion of insoluble elastin isolated from several neonatal mouse tissues revealed the presence of rat tropoelastin peptides in progeny from all those founder mice in which detectable levels of rat tropoelastin mRNA were noted. Phenotypic and histopathological assessment of transgenic and non-transgenic animals revealed the development of two diverse elastic tissue disorders. The progeny of two separate founder lines overexpressing the rat tropoelastin isoform lacking exon 33, developed an emphysematous phenotype in early adulthood. In contrast, transgenic mice, in which expression of the truncated rat tropoelastin minigene lacking exons 19-31 had been observed, died of a ruptured ascending aortic aneurysm. Tropoelastin gene mutations, therefore, will result in heritable disorders of elastic tissue. Moreover, different mutations in the tropoelastin gene will be responsible for very different abnormalities in elastic tissue function.

Elastic fibres and vascular structure in hypertension

Blood vessels are dynamic structures composed of cells and extracellular matrix (ECM), which are in continuous cross-talk with each other. Thus, cellular changes in phenotype or in proliferation/death rate affect ECM synthesis. In turn, ECM elements not only provide the structural framework for vascular cells, but they also modulate cellular function through specific receptors. These ECM-cell interactions, together with neurotransmitters, hormones and the mechanical forces imposed by the heart, modulate the structural organization of the vascular wall. It is not surprising that pathological states related to alterations in the nervous, humoral or haemodynamic environment-such as hypertension-are associated with vascular wall remodeling, which, in the end, is deleterious for cardiovascular function. However, the question remains whether these structural alterations are simply a consequence of the disease or if there are early cellular or ECM alterations-determined either genetically or by environmental factors-that can predispose to vascular remodeling independent of hypertension. Elastic fibres might be key elements in the pathophysiology of hypertensive vascular remodeling. In addition to the well known effects of hypertension on elastic fibre fatigue and accelerated degradation, leading to loss of arterial wall resilience, recent investigations have highlighted new roles for individual components of elastic fibres and their degradation products. These elements can act as signal transducers and regulate cellular proliferation, migration, phenotype, and ECM degradation. In this paper, we review current knowledge regarding components of elastic fibres and discuss their possible pathomechanistic associations with vascular structural abnormalities and with hypertension development or progression.

Autosomal dominant cutis laxa with severe lung disease: synthesis and matrix deposition of mutant tropoelastin

Cutis laxa (CL) is a heterogeneous group of genetic and acquired disorders with at least two autosomal dominant forms caused by mutations in the elastin and fibulin-5 genes, respectively. To define the molecular basis of CL in patients negative for point mutations in the elastin gene, metabolic labeling and immunoprecipitation experiments were used to study the synthesis of elastin in dermal fibroblasts. In addition to the normal 68 kDa tropoelastin (TE) protein, an abnormal, 120 kDa polypeptide was detected in the proband and her affected daughter in a CL family characterized by hernias and unusually severe and early-onset pulmonary disease including bronchiectasis and pulmonary emphysema. Mutational and gene expression studies established that affected individuals in this family carried a partial tandem duplication in the elastin locus. Immunoprecipitation experiments showed that the mutant TE was partially secreted and partially retained intracellularly. A polyclonal antibody raised against a unique peptide in the mutant TE molecule showed both intracellular and matrix staining. We conclude that elastin mutations can cause CL associated with a severe pulmonary phenotype. Synthesis of abnormal TE may interfere with elastic fiber function through a dominant-negative or a gain of function mechanism.

Elastin

Elastin is a key extracellular matrix protein that is critical to the elasticity and resilience of many vertebrate tissues including large arteries, lung, ligament, tendon, skin, and elastic cartilage. Tropoelastin associates with multiple tropoelastin molecules during the major phase of elastogenesis through coacervation, where this process is directed by the precise patterning of mostly alternating hydrophobic and hydrophilic sequences that dictate intermolecular alignment. Massively crosslinked arrays of tropoelastin (typically in association with microfibrils) contribute to tissue structural integrity and biomechanics through persistent flexibility, allowing for repeated stretch and relaxation cycles that critically depend on hydrated environments. Elastin sequences interact with multiple proteins found in or colocalized with microfibrils, and bind to elastogenic cell surface receptors. Knowledge of the major stages in elastin assembly has facilitated the construction of in vitro models of elastogenesis, leading to the identification of precise molecular regions that are critical to elastin-based protein interactions.

Multi-species sequence comparison reveals dynamic evolution of the elastin gene that has involved purifying selection and lineage-specific insertions/deletions

Background

The elastin gene (ELN) is implicated as a factor in both supravalvular aortic stenosis (SVAS) and Williams Beuren Syndrome (WBS), two diseases involving pronounced complications in mental or physical development. Although the complete spectrum of functional roles of the processed gene product remains to be established, these roles are inferred to be analogous in human and mouse. This view is supported by genomic sequence comparison, in which there are no large-scale differences in the ~1.8 Mb sequence block encompassing the common region deleted in WBS, with the exception of an overall reversed physical orientation between human and mouse.

Results

Conserved synteny around ELN does not translate to a high level of conservation in the gene itself. In fact, ELN orthologs in mammals show more sequence divergence than expected for a gene with a critical role in development. The pattern of divergence is non-conventional due to an unusually high ratio of gaps to substitutions. Specifically, multi-sequence alignments of eight mammalian sequences reveal numerous non-aligning regions caused by species-specific insertions and deletions, in spite of the fact that the vast majority of aligning sites appear to be conserved and undergoing purifying selection.

Conclusions

The pattern of lineage-specific, in-frame insertions/deletions in the coding exons of ELN orthologous genes is unusual and has led to unique features of the gene in each lineage. These differences may indicate that the gene has a slightly different functional mechanism in mammalian lineages, or that the corresponding regions are functionally inert. Identified regions that undergo purifying selection reflect a functional importance associated with evolutionary pressure to retain those features.

Altered bladder function in transgenic mice expressing rat elastin

The elasticity of tissues subjected to repeated deformation is provided by the presence of elastic fibers in the extracellular matrix (ECM). The most abundant component of elastic fibers is elastin, whose soluble precursor is tropoelastin. To establish the role elastin plays in the bladder, this study describes the biosynthetic, histologic, and physiologic consequences of expression of an isoform of rat tropoelastin in transgenic mouse bladder. The polymerase chain reaction (PCR) was used to determine expression of a rat tropoelastin minigene in transgenic mice. Histochemical methods were used to demonstrate changes in elastic fibers in frozen sections of bladder. Cystometric analysis was carried out in transgenic and non-transgenic mice, prior to and after 3 weeks of partial outlet obstruction. The PCR assay demonstrated that bladder tissue of transgenic mice expressed rat tropoelastin mRNA, whereas non-transgenes did not. Increased deposition of elastic fibers was demonstrated with the Verhoeff-van Gieson stain. Bladders of transgenic animals were more compliant than bladders of their non-transgenic littermates. Partial outlet obstruction resulted in increased bladder volume and more compliant bladders in non-transgenic mice. In contrast, the bladder volume and compliance in transgenes was almost unchanged by obstruction. This study demonstrates that normal elastic fiber assembly is prerequisite for the compliant properties of the bladder wall. Moreover, the response of the bladder to obstruction is critically influenced by elastin synthesis.

The role of the carboxy terminus of tropoelastin in its assembly into the elastic fiber

Tropoelastin, the soluble precursor protein of insoluble amorphous elastin, contains repeating segments that are important for the characteristic elasticity and crosslinking sites of mature elastin. In addition, there is a unique carboxy terminal domain that is encoded by exon 36 of the elastin gene, and it has been suggested that this region may play a role in the process of insolubilization. The contribution of exon 36 to the maturation of tropoelastin into insoluble elastin was probed in these studies. Neonatal rat aortic smooth muscle cells were cultured and the fate of [3H] Lys labeled human recombinant tropoelastin (hrTE) molecules added to the cultures was monitored. In comparison to the hrTE containing the region encoded by exon 36, hrTE molecules lacking this domain were less efficiently incorporated into elastin, as evidenced by a decrease in NaOH insoluble radioactivity. Specific residues within the domain encoded by exon 36 were targeted for further study in experiments in which the two Cys residues were reduced and alkylated, and/or the four basic Arg-Lys-Arg-Lys residues at the carboxy terminus were removed. Both of these modifications resulted in decreased incorporation into elastin equivalent to the complete removal of the carboxy terminus. Prior treatment of the cell layer with elastase reduced the efficiency of insolubilization of hrTE containing the domain encoded by exon 36, but had no effect on the processing of molecules lacking this region. These data suggest that exon 36 of the elastin gene contributes to normal efficient incorporation of tropoelastin into the elastin fiber.

An open reading frame element mediates posttranscriptional regulation of tropoelastin and responsiveness to transforming growth factor beta1

Elastin, an extracellular component of arteries, lung, and skin, is produced during fetal and neonatal growth. We reported previously that the cessation of elastin production is controlled by a posttranscriptional mechanism. Although tropoelastin pre-mRNA is transcribed at the same rate in neonates and adults, marked instability of the fully processed transcript bars protein production in mature tissue. Using RNase protection, we identified a 10-nucleotide sequence in tropoelastin mRNA near the 5' end of the sequences coded by exon 30 that interacts specifically with a developmentally regulated cytosolic 50-kDa protein. Binding activity increased as tropoelastin expression dropped, being low in neonatal fibroblasts and high in adult cells, and treatment with transforming growth factor beta1 (TGF-beta1), which stimulates tropoelastin expression by stabilizing its mRNA, reduced mRNA-binding activity. No other region of tropoelastin mRNA interacted with cellular proteins, and no binding activity was detected in nuclear extracts. The ability of the exon-30 element to control mRNA decay and responsiveness to TGF-beta1 was assessed by three distinct functional assays: (i) insertion of exon 30 into a heterologous gene conferred increased reporter activity after exposure to TGF-beta1; (ii) addition of excess exon 30 RNA slowed tropoelastin mRNA decay in an in vitro polysome degradation assay; and (iii) a mutant tropoelastin cDNA lacking exon 30, compared to wild-type cDNA, produced a stable transcript whose levels were not affected by TGF-beta1. These findings demonstrate that posttranscriptional regulation of elastin production in mature tissue is conferred by a specific element within the open reading frame of tropoelastin mRNA.

Extensive alternate exon usage at the 5' end of the sheep tropoelastin gene

Several overlapping cDNA clones were isolated from a lambda gt10 cDNA library constructed using poly A+ RNA from neonatal sheep lung. DNA sequence analysis of these cDNA recombinants revealed the complete derived amino acid sequence of sheep tropoelastin. A comparison of DNA sequences from individual sheep tropoelastin cDNA also confirmed the presence of several tropoelastin mRNA isoforms in neonatal lung tissue. Coding domains corresponding to exons 13, 14 and 33 were present in several of the sheep tropoelastin cDNA fragments but absent in others. The relative amount of alternate usage of these exons was quantitated by polymerase chain amplification. In confirmation of previous studies in other mammalian species, extensive alternate usage of exon 33 was observed in total RNA isolated from aorta, nuchal ligament and pulmonary artery from neonatal sheep. In striking contrast to all previous studies, however, exons 13 and 14 were shown to be subject to almost the same level of alternate usage as exon 33 in all three neonatal sheep tissues examined.

A 5' splice site mutation affecting the pre-mRNA splicing of two upstream exons in the collagen COL1A1 gene. Exon 8 skipping and altered definition of exon 7 generates truncated pro alpha 1(I) chains with a non-collagenous insertion destabilizing the triple helix

A heterozygous de novo G to A point mutation in intron 8 at the +5 position of the splice donor site of the gene for the pro alpha 1(I) chain of type I procollagen, COL1A1, was defined in a patient with type IV osteogenesis imperfecta. The splice donor site mutation resulted not only in the skipping of the upstream exon 8 but also unexpectedly had the secondary effect of activating a cryptic splice site in the next upstream intron, intron 7, leading to re-definition of the 3' limit of exon 7. These pre-mRNA splicing aberrations cause the deletion of exon 8 sequences from the mature mRNA and the inclusion of 96 bp of intron 7 sequence. Since the mis-splicing of the mutant allele product resulted in the maintenance of the correct codon reading frame, the resultant pro alpha 1(I) chain contained a short non-collagenous 32-amino-acid sequence insertion within the repetitive Gly-Xaa-Yaa collagen sequence motif. At the protein level, the mutant alpha 1(I) chain was revealed by digestion with pepsin, which cleaved the mutant procollagen within the protease-sensitive non-collagenous insertion, producing a truncated alpha 1(I). This protease sensitivity demonstrated the structural distortion to the helical structure caused by the insertion. In long-term culture with ascorbic acid, which stimulates the formation of a mature crosslinked collagen matrix, and in tissues, there was no evidence of the mutant chain, suggesting that during matrix formation the mutant chain was unable to stably incorporated into the matrix and was degraded proteolytically.

Persistence of the fetal pattern of tropoelastin gene expression in severe neonatal bovine pulmonary hypertension

Neonatal hypoxic pulmonary hypertension causes increases and spatial changes in tropoelastin expression in pulmonary arteries. However, it is not clear if this is due to recruitment of quiescent smooth muscle cells (SMC) into an elastin-producing phenotype or persistence of the fetal pattern of tropoelastin gene expression. We evaluated the distribution and relative concentration of tropoelastin mRNA in intralobar pulmonary arteries from late gestation fetuses and in animals exposed to hypobaric hypoxia (430 mmHg) from birth for 1, 3, 7, or 14 d, as well as in age-matched and adult room air-breathing controls. In situ hybridization demonstrated that tropoelastin mRNA was distributed throughout the entire radius of the pulmonary vessel wall in the fetus and newborn calf. By 15 d of age, only cells in the inner third of the media expressed tropoelastin mRNA, and by adulthood no tropoelastin mRNA was detected in the vessel wall. These findings demonstrated that tropoelastin expression shuts off in a spatially specific pattern, moving from the abluminal to the luminal side of the medial in the neonatal pulmonary artery when pressures and resistance are falling. In the aorta of 15-d-old calves, tropoelastin mRNA expression was seen equally throughout the media, indicating tissue-specific regulation of elastin in the neonatal period. In contrast, intralobar pulmonary arteries from calves exposed to hypoxia, which prevented the normal postnatal decline in pulmonary artery pressure, maintained the fetal pattern and levels of tropoelastin mRNA expression at all time points. Thus, rather than causing a recruitment of SMC into an elastin-producing phenotype, neonatal pulmonary hypertension caused a persistence of the fetal pattern of tropoelastin expression in medial SMC. Cell-free translation showed that the same tropoelastin isoforms were made by mRNA from control and hypertensive calves and, unlike the ligamentum nuchae, did not change during the transition from fetal to neonatal life. We conclude that pulmonary hypertension in the neonate perturbs the normal postpartum repression of tropoelastin expression resulting in a persistence of the fetal spacial and isoform patterns of tropoelastin gene expression.

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.

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.

Changes in aortic levels of tropoelastin mRNA following treatment of rats with the antihypertensive drugs captopril and hydralazine

This manuscript describes changes in the steady state levels of aortic tropoelastin mRNA in spontaneously hypertensive rats (SHR) and normotensive controls (WKY) following treatment with two antihypertensive drugs. Three-week-old WKY and SHR rats were treated with hydralazine (15 mg/kg/day) or captopril (25 mg/kg/day). Tail artery blood pressure was monitored twice weekly. Both drugs prevented the development of hypertension in the SHR rat. At 6 weeks of age, total aortic RNA was extracted and the steady state levels of mRNAs coding for tropoelastin and pro alpha 1 (III) collagen were determined by slot blot hybridization analysis using radiolabeled tropoelastin and pro alpha 1 (III) collagen cDNA clones. Hydralazine treatment resulted in a threefold increase in tropoelastin mRNA levels in both the SHR and the WKY animals (P less than 0.01). Captopril-treated SHR animals demonstrated a similar significant increase. In contrast, no differences in pro alpha 1 (III) collagen mRNA levels were observed in the aorta of SHR or WKY rats following treatment with either captopril or hydralazine. These data suggest that antihypertensive agents can act specifically to directly induce tropoelastin mRNA levels in large arteries and thus may induce vascular remodeling independent of an increase in blood pressure.

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.