Synthesis of crosslinked elastin by an endothelial cell culture

Endothelial Chromatin-Remodeling Enzymes Regulate the Production of Critical ECM Components During Murine Lung Development

BACKGROUND

The chromatin-remodeling enzymes BRG1 (brahma-related gene 1) and CHD4 (chromodomain helicase DNA-binding protein 4) independently regulate the transcription of genes critical for vascular development, but their coordinated impact on vessels in late-stage embryos has not been explored.

METHODS

In this study, we genetically deleted endothelial Brg1 and Chd4 in mixed background mice (Brg1fl/fl;Chd4fl/fl;VE-Cadherin-Cre), and littermates that were negative for Cre recombinase were used as controls. Tissues were analyzed by immunostaining, immunoblot, and flow cytometry. Quantitative reverse transcription polymerase chain reaction was used to determine gene expression, and chromatin immunoprecipitation revealed gene targets of BRG1 and CHD4 in cultured endothelial cells.

RESULTS

We found Brg1/Chd4 double mutants grew normally but died soon after birth with small and compact lungs. Despite having normal cellular composition, distal air sacs of the mutant lungs displayed diminished ECM (extracellular matrix) components and TGFβ (transforming growth factor-β) signaling, which typically promotes ECM synthesis. Transcripts for collagen- and elastin-related genes and the TGFβ ligand Tgfb1 were decreased in mutant lung endothelial cells, but genetic deletion of endothelial Tgfb1 failed to recapitulate the small lungs and ECM defects seen in Brg1/Chd4 mutants. We instead found several ECM genes to be direct targets of BRG1 and CHD4 in cultured endothelial cells.

CONCLUSIONS

Collectively, our data highlight essential roles for endothelial chromatin-remodeling enzymes in promoting ECM deposition in the distal lung tissue during the saccular stage of embryonic lung development.

Elastin, arterial mechanics, and stenosis

Elastin is a long-lived extracellular matrix protein that is organized into elastic fibers that provide elasticity to the arterial wall, allowing stretch and recoil with each cardiac cycle. By forming lamellar units with smooth muscle cells, elastic fibers transduce tissue-level mechanics to cell-level changes through mechanobiological signaling. Altered amounts or assembly of elastic fibers leads to changes in arterial structure and mechanical behavior that compromise cardiovascular function. In particular, genetic mutations in the elastin gene (ELN) that reduce elastin protein levels are associated with focal arterial stenosis, or narrowing of the arterial lumen, such as that seen in supravalvular aortic stenosis and Williams-Beuren syndrome. Global reduction of Eln levels in mice allows investigation of the tissue- and cell-level arterial mechanical changes and associated alterations in smooth muscle cell phenotype that may contribute to stenosis formation. A loxP-floxed Eln allele in mice highlights cell type- and developmental origin-specific mechanobiological effects of reduced elastin amounts. Eln production is required in distinct cell types for elastic layer formation in different parts of the mouse vasculature. Eln deletion in smooth muscle cells from different developmental origins in the ascending aorta leads to characteristic patterns of vascular stenosis and neointima. Dissecting the mechanobiological signaling associated with local Eln depletion and subsequent smooth muscle cell response may help develop new therapeutic interventions for elastin-related diseases.

Polarized Proteins in Endothelium and Their Contribution to Function

Protein localization in endothelial cells is tightly regulated to create distinct signaling domains within their tight spatial restrictions including luminal membranes, abluminal membranes, and interendothelial junctions, as well as caveolae and calcium signaling domains. Protein localization in endothelial cells is also determined in part by the vascular bed, with differences between arteries and veins and between large and small arteries. Specific protein polarity and localization is essential for endothelial cells in responding to various extracellular stimuli. In this review, we examine protein localization in the endothelium of resistance arteries, with occasional references to other vessels for contrast, and how that polarization contributes to endothelial function and ultimately whole organism physiology. We highlight the protein localization on the luminal surface, discussing important physiological receptors and the glycocalyx. The protein polarization to the abluminal membrane is especially unique in small resistance arteries with the presence of the myoendothelial junction, a signaling microdomain that regulates vasodilation, feedback to smooth muscle cells, and ultimately total peripheral resistance. We also discuss the interendothelial junction, where tight junctions, adherens junctions, and gap junctions all convene and regulate endothelial function. Finally, we address planar cell polarity, or axial polarity, and how this is regulated by mechanosensory signals like blood flow.

Vascular elastic fiber heterogeneity in health and disease

PURPOSE OF REVIEW

Elastin has historically been described as an amorphous protein that functions to provide recoil to tissues that stretch. However, evidence is growing that elastin's role may not be limited to biomechanics. In this minireview, we will summarize current knowledge regarding vascular elastic fibers, focusing on structural differences along the arterial tree and how those differences may influence the behavior of affiliated cells.

RECENT FINDINGS

Regional heterogeneity, including differences in elastic lamellar number, density and cell developmental origin, plays an important role in vessel health and function. These differences impact cell-cell communication, proliferation and movement. Perturbations of normal cell-matrix interactions are correlated with human diseases including aneurysm, atherosclerosis and hypertension.

SUMMARY

Although classically described as a structural protein, recent data suggest that differences in elastin deposition along the arterial tree have important effects on heterotypic cell interactions and human disease.

Heterogeneous Cellular Contributions to Elastic Laminae Formation in Arterial Wall Development

RATIONALE

Elastin is an important ECM (extracellular matrix) protein in large and small arteries. Vascular smooth muscle cells (SMCs) produce the layered elastic laminae found in elastic arteries but synthesize little elastin in muscular arteries. However, muscular arteries have a well-defined internal elastic lamina (IEL) that separates endothelial cells (ECs) from SMCs. The extent to which ECs contribute elastin to the IEL is unknown.

OBJECTIVE

To use targeted elastin (Eln) deletion in mice to explore the relative contributions of SMCs and ECs to elastic laminae formation in different arteries.

METHODS AND RESULTS

We used SMC- and EC-specific Cre recombinase transgenes with a novel floxed Eln allele to focus gene inactivation in mice. Inactivation of Eln in SMCs using Sm22aCre resulted in depletion of elastic laminae in the arterial wall with the exception of the IEL and SMC clusters in the outer media near the adventitia. Inactivation of elastin in ECs using Tie2Cre or Cdh5Cre resulted in normal medial elastin and a typical IEL in elastic arteries. In contrast, the IEL was absent or severely disrupted in muscular arteries. Interruptions in the IEL resulted in neointimal formation in the ascending aorta but not in muscular arteries.

CONCLUSIONS

Combined with lineage-specific fate mapping systems, our knockout results document an unexpected heterogeneity in vascular cells that produce the elastic laminae. SMCs and ECs can independently form an IEL in most elastic arteries, whereas ECs are the major source of elastin for the IEL in muscular and resistance arteries. Neointimal formation at IEL disruptions in the ascending aorta confirms that the IEL is a critical physical barrier between SMCs and ECs in the large elastic arteries. Our studies provide new information about how SMCs and ECs contribute elastin to the arterial wall and how local elastic laminae defects may contribute to cardiovascular disease.

Cell type specific expression of Follistatin-like 1 (Fstl1) in mouse embryonic lung development

Follistatin like-1 (Fstl1) is a secreted glycoprotein and can be up-regulated by TGF-β1. To better study the function of Fstl1 in lung development, we examined Fstl1 expression in the developing lung, in a cell type specific manner, using a tamoxifen inducible Fstl1-reporter mouse strain. Our results show that Fstl1 is ubiquitously expressed at saccular stage in the developing lung. At E18.5, Fstl1 expression is robust in most type of mesenchymal cells, including airway smooth muscle cells surrounding airways, vascular smooth muscle cells, endothelial cells, and vascular pericytes from blood vessel, but not PDGFRα⁺ fibroblasts in the distal alveolar sacs. Meanwhile, relative weak and sporadic signals of Fstl1 expression are observed in epithelium, including a subgroup of club cells in proximal airways and a few type II alveolar epithelial cells in distal airways. Our data help to understand the critical role of Fstl1 in lung development and lung disease pathogenesis.

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 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.

Vascular extracellular matrix and arterial mechanics

An important factor in the transition from an open to a closed circulatory system was a change in vessel wall structure and composition that enabled the large arteries to store and release energy during the cardiac cycle. The component of the arterial wall in vertebrates that accounts for these properties is the elastic fiber network organized by medial smooth muscle. Beginning with the onset of pulsatile blood flow in the developing aorta, smooth muscle cells in the vessel wall produce a complex extracellular matrix (ECM) that will ultimately define the mechanical properties that are critical for proper function of the adult vascular system. This review discusses the structural ECM proteins in the vertebrate aortic wall and will explore how the choice of ECM components has changed through evolution as the cardiovascular system became more advanced and pulse pressure increased. By correlating vessel mechanics with physiological blood pressure across animal species and in mice with altered vessel compliance, we show that cardiac and vascular development are physiologically coupled, and we provide evidence for a universal elastic modulus that controls the parameters of ECM deposition in vessel wall development. We also discuss mechanical models that can be used to design better tissue-engineered vessels and to test the efficacy of clinical treatments.

Effect of coacervated alpha-elastin on proliferation of vascular smooth muscle and endothelial cells

The arterial wall injury associated with arterial graft implantation causes smooth muscle cells (SMCs) in the media to migrate and proliferate in the intima at the graft-artery junction resulting in anastomotic intimal hyperplasia (AIH). An important step in developing a small-diameter prosthesis may be to stimulate endothelialization and thereby inhibit AIH. In this study, we investigated the effect of coacervated and crosslinked alpha-elastin on proliferation of SMCs and endothelial cells (ECs) in vitro. Coacervation is an important step in the conversion of proelastin to make an elastin fiber in vivo. SMCs and ECs were prepared from porcine aortic media and endothelium, respectively. SMCs and ECs (three to five passages, 4 x 10[4] cells/well) were seeded onto 12 well plates, coated and crosslinked with 0 or 10 mg/mL of coacervated alpha-elastin. After the 1st, 2nd, or 3rd day of cultivation, proliferation was assayed by scintillation counting of [3H]-thymidine incorporation. For the 4th day only, 0, 0.1, 1, 10 mg/mL concentration of coacervated alpha-elastin was coated and crosslinked. SMC proliferation (1st, 2nd day: p<0.005; 3rd, 4th day: p<0.0001) was significantly inhibited over time and dose dependently, eg, 0.1 mg/mL (45.7+/-2.3%: % of control p<0.005), 1 mg/mL (5.9+/-0.7%, p<0.0005), 10 mg/mL (2.8+/-0.4%, p<0.0005). EC proliferation was inhibited over time by 10 mg/mL of coacervated alpha-elastin (2nd, 3rd day: p<0.005; 4th day: p<0.0001), but proliferation (132.8+/-9.9%: % of control p=NS) was stimulated by 0.1 mg/mL of coacervated alpha-elastin. These results suggest that coating and crosslinking a coacervated alpha-elastin into the structure of arterial prosthesis may inhibit AIH and stimulate endothelialization.

Inhibitory effect of type 1 collagen gel containing alpha-elastin on proliferation and migration of vascular smooth muscle and endothelial cells

The purpose of this study was to investigate in vitro the potential effect of type 1 collagen gel containing alpha-elastin on the proliferation of vascular smooth muscle cells and vascular endothelial cells, and on smooth muscle cell migration. Vascular smooth muscle cell and endothelial cell were cultured in 12-well plates precoated with collagen gels and alpha-elastin. Cell proliferation rates were measured by monitoring [3H]-thymidine incorporation. After 2, 3 or 4 days of culture, the proliferation rate of both smooth muscle cells and endothelial cells was significantly decreased on collagen gel containing 10 mg/ml alpha-elastin compared with collagen gel only as control. Smooth muscle cell proliferation on collagen gel containing alpha-elastin on the 4th day of culture was decreased dose-dependently, e.g. 1 mg/ml of alpha-elastin (74.8(2.3)% of control, P=n.s.); 5 mg/ml (56.7(2.1)%; P<0.05); 10 mg/ml (30.3(3.1)%; P<0.005). In the case of cultured endothelial cells, however, [3H]-thymidine incorporation was not decreased significantly in the presence of 5 mg/ml alpha-elastin (83.1(7.9)%, P=n.s.). After stimulation by platelet-derived growth factor, the smooth muscle cell migration rate on collagen gel containing alpha-elastin (5 mg/ml) was decreased over time. The area of migration on the 6th day of culture was also significantly decreased dose-dependently in the presence of alpha-elastin, e.g. 1 mg/ml (72.6(3.4)% of control, P<0.05), 5 mg/ml (56.9%(1.5)%; P<0.05); 10 mg/ml (37.3(2.7)%; P<0.0005). In conclusion, alpha-elastin inhibited the proliferation and migration of smooth muscle cell in a dose-dependent manner on collagen gel culture, however, at high concentrations of alpha-elastin (10 mg/ml), the endothelial cell proliferation rate was also inhibited. At 5 mg/ml, alpha-elastin significantly inhibited smooth muscle cell proliferation and migration but did not significantly inhibit endothelial cell proliferation. Incorporation of collagen gel containing alpha-elastin into the structure of arterial prosthesis offers the possibility of inhibiting smooth muscle cell hyperplasia without significant effect on endothelial cell formation.

Vitamin E attenuates induction of elastase-like activity by tumor necrosis factor-alpha, cholestan-3 beta,5 alpha,6 beta-triol and linoleic acid in cultured endothelial cells

Disturbances in arterial wall elastin metabolism appear to be important factors in atherosclerosis development. To evaluate this hypothesis, elastase-like activity was determined in cultured endothelial cells and their surrounding media after exposure to tumor necrosis factor-alpha (TNF), cholestan-3 beta,5 alpha,6 beta-triol (Triol) and linoleic acid (18:2). Significant increases in elastase-like activity both in the cells and in the media were observed when subconfluent endothelial cells were treated with 12 microM Triol, 500 U TNF/ml, or 90 microM 18:2, for 72 h in the presence of 5% calf serum. Even higher activities were measured when endothelial cells were seeded directly into media enriched with 18:2, TNF or Triol and treated for 72 h. Vitamin E supplementation (25 microM) attenuated elastase-like activity in cells and media, independent of treatment. These results suggest that elastase-like enzyme induction in endothelial cells may be involved in cellular perturbations induced by certain lipids and cytokines. Vitamin E may provide a protective function by preventing the induction of elastolytic enzymes. This may have implications in elastin metabolism and atherosclerosis.

Biochemical characterization of elastin in neointimal hyperplasia of rabbit aorta

Elastin synthesized in response to vascular injury was characterized in terms of its amino acid composition, the biosynthetic labeling of the desmosines and of the heat coacervable polypeptides present in the 2 M urea extract. Neointimal hyperplasia of the chronic variety was induced in rabbit aorta by superficial mechanical lesions. At 4 months following injury the reendothelialized neointimal thickening and the media were excised. Aliquot samples were incubated with [3H] lysine, extracted with 2 M urea, 0.1 M Tris, pH 7.4 and hydrolysed with collagenase. In the residue of the digests the [3H] desmosines were quantified after electrophoretic separation. Elastin was purified from the nonlabeled aliquots of the media and neointimal hyperplasia. It accounted for 60% and 25% of the dry weight of the media and the neointima respectively. Elastin isolated from the media and the neointima had essentially the same amino acid composition. The incorporation of [3H] lysine into desmosines and into coacervable polypeptides indicated that the synthesis of crosslinked elastin is still active in the hyperplasia at 4 months following injury.

Cell biology of endothelial cells

Endothelial cells are a source of physiologically important molecules synthesized therein and secreted to the blood and/or to the subendothelial extracellular matrix. These molecules participate in formation of platelet and fibrin thrombi (e.g., von Willebrand factor and tissue factor) and contribute to antithrombotic properties of the endothelium (e.g., prostacyclin, thrombomodulin, and heparan sulfate). Endothelial cells synthesize and secrete plasminogen activator and inhibitors. They are the source of molecules regulating the growth of other cells; they synthesize angiotensin-converting enzyme, and bind lipoproteins and hormones. Finally, they are the target for, and participant in, immune reactions. Thus, endothelial cells constitute not only the first barrier between the blood and the extravascular space but also serve as a source of molecules influencing the structural and functional integrity of the circulation.

Histologic studies on normal and persistent ductus arteriosus in the dog

The process of anatomic closure of the ductus arteriosus was studied at the ultrastructural level in 15 normal beagles (age 0 hour to 13 days) and in 18 specimens from a strain of dogs with hereditary persistent ductus arteriosus (age 4 hours to 27 days). Normal ductal closure takes place from the pulmonary artery to the aortic end. It is accompanied by a series of histologic changes: 1) separation of the endothelial cells from the internal elastic lamina resulting in a wide region of subendothelial edema; 2) ingrowth and infolding of endothelial cells and migration of undifferentiated smooth muscle cells from the inner media into the subendothelial region; 3) apposition of endothelial cells bordering the lumen; and 4) degenerative changes. In persistent ductus arteriosus, these changes do not occur. The endothelial cells remain closely adhered to the internal elastic lamina and the underlying media is abnormal in structure. In the case of partial persistent ductus arteriosus (ductus diverticulum), both the normal and the abnormal type of wall are found in a single ductus arteriosus. The histologic features of the normal and the persistent ductus arteriosus in the dog resemble those of the normal and the persistent ductus arteriosus in humans, suggesting a similar pathogenesis.

The pattern of elastin in the aorta and large arteries of mammals

The arteries of mammals contain large amounts of elastin arranged in concentric lamellae known as medial lamellar units (MLU). In adult mammals of a variety of species the number of lamellar units is roughly proportional to the radius of the artery and the tension/MLU ratio is roughly constant in all species, but greater in the abdominal than in the thoracic aorta. Re-analysis of these data shows that the number of MLU of the abdominal aorta is linearly related to the pulse pressure, while the number of MLU in both the thoracic and abdominal aorta increases exponentially with stroke volume. Preliminary data are presented showing the decrease in number of MLU along the thoracic aorta of both fetal lambs and sheep, and evidence is provided that some of this elastin may be involved in the formation of the small arteries, such as the intercostals, which arise from the aorta. Scanning electron microscopy showed that the elastin on the intimal side of the media was in the form of fenestrated sheets while that on the adventitial side was a fibrous network. The size and density of the fenestrations was greater in fetal lambs and may play a role in allowing growth of the artery.

Elastin production by cultured calf pulmonary artery endothelial cells

Calf pulmonary artery (CPA) endothelial cells synthesize and secrete soluble elastin when incubated in medium conditioned by arterial smooth muscle cells. Endothelial cell tropoelastin cross-reacts with antiserum to bovine ligamentum nuchae elastin and comigrates on SDS-PAGE with tropoelastins from fetal bovine ligamentum nuchae fibroblasts, aortic smooth muscle cells, and ear chondroblasts at an apparent molecular weight of 70,000. Endothelial cells synthesize only one-third as much elastin as these other cell types, however. Approximately 80% of the elastin synthesized by endothelial cells in confluent culture is released into the culture medium. The remaining 20% remains associated with the cell layer and is readily extractable with dilute acetic acid as un-cross-linked, 70,000-dalton tropoelastin. The addition of beta-aminopropionitrile to culture medium did not alter the ratio of tropoelastin in the medium and cell layer, suggesting that cross-linking of tropoelastin does not occur in culture. Immunofluorescent staining of confluent endothelial cell cultures with antielastin serum demonstrated elastin occurring as a web-like network of fine filaments extending throughout the extracellular space. The fibrous elastin was different in organization and distribution from fibers stained with antifibronectin serum, which were localized primarily beneath the cell layer and in regions of cell-cell contact. Extracellular matrix remaining after solubilization of cellular material with Triton X-100 stained positive for fibronectin, but not for elastin.

Healing of biodegradable vascular prosthesis. Incorporation of 3H-valine into proteins in the subendothelial scar and host intima-media of rat aorta

Heparin treated and aldehyde crosslinked rat aorta segments were implanted in infrarenal aorta of homologous rats. One year following aortic replacement, the subendothelial scar and the prosthetic remnants were excised. The scar and the host intima-media were incubated with 3H-valine for 4 h and extracted with 5 M guanidinium chloride--0.05 M dithiothreitol--0.1 M Tris--0.1% EDTANa2 at pH 7.5 prior (Extract 1) and following (Extract 2) hydrolysis of collagen. The radioactivity of extract 1 accounted for approximately 80% of the total label incorporated in the scar and host intima-media. The 3H-label of extract 1 adjusted for the tissue collagen content was about twenty times higher in the scar than in the host aorta. The major 3H protein peaks from Extract 1 of scar and host aorta were of 130 K, 100 K and 70 K apparent molecular weight, based on polyacrylamide gel electrophoresis in SDS. Hydrolysis with 2N KOH of the extraction residue from the host aorta and scar yielded 3H-val-pro dipeptides and hydrolysis with 6N HCl desmosines. The incorporation pattern of 3H-valine into proteins and the presence of elastin synthesized de novo in the scar replacing the prosthesis indicate macromolecular repair of the host aortic wall.

Isolation and partial characterization of an elastase-type protease in human vulva fibroblasts: its possible involvement in vulvar elastic tissue destruction of patients with lichen sclerosus et atrophicus

A complete disappearance of orcein positive material was observed in the superficial dermis of patients suffering from Lichen sclerosus et atrophicus. An elastase-type protease was isolated and partially purified from Triton X-100 extracts of human vulvar fibroblasts by gel permeation chromatography. It presents the characteristics of a metalloenzyme hydrolyzing Succinoyl-tri-alanine paranitroanilide maximally at pH 8.0 and is also active towards insoluble elastin. When partially purified enzyme is directly applied on to rabbit skin sections or when injected intradermally to young rabbits, it produces appreciable degradation of elastic fibers. The involvement of this protease in the disappearance of elastic fibers in Lichen sclerosus et atrophicus is postulated.

Separation of elastin components by thin layer chromatography and electrophoresis

Several methods are described which employ thin layer chromatography and electrophoresis for investigations into the structure and synthesis of crosslinked elastin. These procedures are more sensitive and involve less elaborate equipment than separation on paper. The methods described include an electrophoretic procedure for detecting the cross-linking amino acids in elastin hydrolysates, a method for completely separating desmosine, isodesmosine and merodesmosine in these hydrolysates, and a method for producing two-dimensional peptide maps of elastase digests of elastin.