Elastin production by cultured calf pulmonary artery endothelial cells

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.

Pericytes and the Control of Blood Flow in Brain and Heart

Pericytes, attached to the surface of capillaries, play an important role in regulating local blood flow. Using optogenetic tools and genetically encoded reporters in conjunction with confocal and multiphoton imaging techniques, the 3D structure, anatomical organization, and physiology of pericytes have recently been the subject of detailed examination. This work has revealed novel functions of pericytes and morphological features such as tunneling nanotubes in brain and tunneling microtubes in heart. Here, we discuss the state of our current understanding of the roles of pericytes in blood flow control in brain and heart, where functions may differ due to the distinct spatiotemporal metabolic requirements of these tissues. We also outline the novel concept of electro-metabolic signaling, a universal mechanistic framework that links tissue metabolic state with blood flow regulation by pericytes and vascular smooth muscle cells, with capillary K_ATP and Kir2.1 channels as primary sensors. Finally, we present major unresolved questions and outline how they can be addressed.

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.

Into the Tissues: Extracellular Matrix and Its Artificial Substitutes: Cell Signalling Mechanisms

The existence of orderly structures, such as tissues and organs is made possible by cell adhesion, i.e., the process by which cells attach to neighbouring cells and a supporting substance in the form of the extracellular matrix. The extracellular matrix is a three-dimensional structure composed of collagens, elastin, and various proteoglycans and glycoproteins. It is a storehouse for multiple signalling factors. Cells are informed of their correct connection to the matrix via receptors. Tissue disruption often prevents the natural reconstitution of the matrix. The use of appropriate implants is then required. This review is a compilation of crucial information on the structural and functional features of the extracellular matrix and the complex mechanisms of cell-cell connectivity. The possibilities of regenerating damaged tissues using an artificial matrix substitute are described, detailing the host response to the implant. An important issue is the surface properties of such an implant and the possibilities of their modification.

JAGGED1/NOTCH3 activation promotes aortic hypermuscularization and stenosis in elastin deficiency

Obstructive arterial diseases, including supravalvular aortic stenosis (SVAS), atherosclerosis, and restenosis, share 2 important features: an abnormal or disrupted elastic lamellae structure and excessive smooth muscle cells (SMCs). However, the relationship between these pathological features is poorly delineated. SVAS is caused by heterozygous loss-of-function, hypomorphic, or deletion mutations in the elastin gene (ELN), and SVAS patients and elastin-mutant mice display increased arterial wall cellularity and luminal obstructions. Pharmacological treatments for SVAS are lacking, as the underlying pathobiology is inadequately defined. Herein, using human aortic vascular cells, mouse models, and aortic samples and SMCs derived from induced pluripotent stem cells of ELN-deficient patients, we demonstrated that elastin insufficiency induced epigenetic changes, upregulating the NOTCH pathway in SMCs. Specifically, reduced elastin increased levels of γ-secretase, activated NOTCH3 intracellular domain, and downstream genes. Notch3 deletion or pharmacological inhibition of γ-secretase attenuated aortic hypermuscularization and stenosis in Eln-/- mutants. Eln-/- mice expressed higher levels of NOTCH ligand JAGGED1 (JAG1) in aortic SMCs and endothelial cells (ECs). Finally, Jag1 deletion in SMCs, but not ECs, mitigated the hypermuscular and stenotic phenotype in the aorta of Eln-/- mice. Our findings reveal that NOTCH3 pathway upregulation induced pathological aortic SMC accumulation during elastin insufficiency and provide potential therapeutic targets for SVAS.

Structural studies of elastic fibre and microfibrillar proteins

Elastic tissues owe their functional properties to the composition of their extracellular matrices, particularly the range of extracellular, multidomain extensible elastic fibre and microfibrillar proteins. These proteins include elastin, fibrillin, latent TGFβ binding proteins (LTBPs) and collagens, where their biophysical and biochemical properties not only give the matrix structural integrity, but also play a vital role in the mechanisms that underlie tissue homeostasis. Thus far structural information regarding the structure and hierarchical assembly of these molecules has been challenging and the resolution has been limited due to post-translational modification and their multidomain nature leading to flexibility, which together result in conformational and structural heterogeneity. In this review, we describe some of the matrix proteins found in elastic fibres and the new emerging techniques that can shed light on their structure and dynamic properties.

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.

Layer-By-Layer Fabrication of Large and Thick Human Cardiac Muscle Patch Constructs With Superior Electrophysiological Properties

Engineered cardiac tissues fabricated from human induced pluripotent stem cells (hiPSCs) show promise for ameliorating damage from myocardial infarction, while also restoring function to the damaged left ventricular (LV) myocardium. For these constructs to reach their clinical potential, they need to be of a clinically relevant volume and thickness, and capable of generating synchronous and forceful contraction to assist the pumping action of the recipient heart. Design prerequisites include a structure thickness sufficient to produce a beneficial contractile force, prevascularization to overcome diffusion limitations and sufficient structural development to allow for maximal cell communication. Previous attempts to meet these prerequisites have been hindered by lack of oxygen and nutrient transport due to diffusion limits (100–200 μm) resulting in necrosis. This study employs a layer-by-layer (LbL) fabrication method to produce cardiac tissue constructs that meet these design prerequisites and mimic normal myocardium in form and function. Thick (>2 mm) cardiac tissues created from hiPSC-derived cardiomyocytes, -endothelial cells (ECs) and -fibroblasts (FBs) were assessed, in vitro, over a 4-week period for viability (<6% necrotic cells), cell morphology and functionality. Functional performance assessment showed enhanced t-tubule network development, gap junction communication as well as previously unseen, physiologically relevant conduction velocities (CVs) (>30 cm/s). These results demonstrate that LbL fabrication can be utilized successfully to create prevascularized, functional cardiac tissue constructs from hiPSCs for potential therapeutic applications.

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.

Elastin-derived peptide VGVAPG decreases differentiation of mouse embryo fibroblast (3T3-L1) cells into adipocytes

Elastin is a highly elastic protein present in connective tissue. As a result of protease activity, elastin hydrolysis occurs, and during this process, elastin-derived peptides (EDPs) are released. One of the constitutively repeating elastin and EDP building sequences is the hexapeptide VGVAPG. Therefore, the aim of our research was to define the effect of VGVAPG peptide on adipogenesis in a mouse 3T3-L1 cell line. 3T3-L1 cells were differentiated according to a previously described protocol and exposed to increasing concentrations of VGVAPG or VVGPGA peptide. The obtained results showed that VGVAPG peptide does not stimulate reactive oxygen species (ROS) production, caspase-1 activation, and 3T3-L1 cell proliferation. In the second part of the experiments, it was proved that VGVAPG peptide decreased lipid accumulation as measured by oil red O staining, which was confirmed by the profile of increased expression markers of undifferentiated preadipocytes. In our experiments, 10 nM VGVAPG added for differentiating to adipocytes increased the expression of Pref-1, serpin E1, and adiponectin as compared to rosiglitazone (PPARγ agonist)-treated group and simultaneously decreased the expression of VEGF and resistin as compared to the rosiglitazone-treated group. The obtained results show that VGVAPG peptide sustains 3T3 cells in undifferentiated state.

ABBREVIATIONS

DMSO: dimethyl sulphoxide; EBP: elastin-binding protein; EDPs: elastin-derived peptides; FBS: foetal bovine serum; Glb1: gene for beta-galactosidase; LDL: low-density-lipoprotein; PAI-1 (Serpin E1): plasminogen activator inhibitor-1; PBS: phosphate-buffered saline; PPARγ: peroxisome proliferator-activated receptor gamma; Pref-1: preadipocyte factor 1; ROS: reactive oxygen species; VEGF-A: vascular endothelial growth factor-A; VGVAPG: Val-Gly-Val-Ala-Pro-Gly; β-Gal: beta-galactosidase; ORO: oil red O; IBMX: 3-isobutyl-1-methylxanthine; H2DCFDA: 2',7'-dichlorodihydrofluorescein diacetate; DMEM: Dulbecco's Modified Eagle's Medium; VVGPGA: Val-Val-Gly-Pro-Gly-Ala.

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.

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.

Localizing factors in atherosclerosis

Atherosclerotic vascular disease is the leading cause of death worldwide. Although the entire vascular bed is constantly exposed to the same risk factors, atheromatous lesions present a distinct intra-individual pattern of localization and progression, being consistently more frequent in specific segments of the arterial vascular bed. This peculiar distribution may be related to selective sensitivity of such locations to the influence of risk factors or to histopathological and flow differences, and has relevant clinical implications, as the prognosis of the disease varies according to localization. We here review the theories that have been formulated to explain such preferential locations, as its understanding can be useful to pursue diagnostic screening strategies and focused preventive measures.

The microfibril-associated glycoproteins (MAGPs) and the microfibrillar niche

The microfibril-associated glycoproteins MAGP-1 and MAGP-2 are extracellular matrix proteins that interact with fibrillin to influence microfibril function. The two proteins are related through a 60 amino acid matrix-binding domain but their sequences differ outside of this region. A distinguishing feature of both proteins is their ability to interact with TGFβ family growth factors, Notch and Notch ligands, and multiple elastic fiber proteins. MAGP-2 can also interact with αvβ3 integrins via a RGD sequence that is not found in MAGP-1. Morpholino knockdown of MAGP-1 expression in zebrafish resulted in abnormal vessel wall architecture and altered vascular network formation. In the mouse, MAGP-1 deficiency had little effect on elastic fibers in blood vessels and lung but resulted in numerous unexpected phenotypes including bone abnormalities, hematopoietic changes, increased fat deposition, diabetes, impaired wound repair, and a bleeding diathesis. Inactivation of the gene for MAGP-2 in mice produced a neutropenia yet had minimal effects on bone or adipose homeostasis. Double knockouts had phenotypes characteristic of each individual knockout as well as several additional traits only seen when both genes are inactivated. A common mechanism underlying all of the traits associated with the knockout phenotypes is altered TGFβ signaling. This review summarizes our current understanding of the function of the MAGPs and discusses ideas related to their role in growth factor regulation.

Composition of intraperitoneally implanted electrospun conduits modulates cellular elastic matrix generation

Improving elastic matrix generation is critical to developing functional tissue engineered vascular grafts. Therefore, this study pursued a strategy to grow autologous tissue in vivo by recruiting potentially more elastogenic cells to conduits implanted within the peritoneal cavity. The goal was to determine the impacts of electrospun conduit composition and hyaluronan oligomer (HA-o) modification on the recruitment of peritoneal cells, and their phenotype and ability to synthesize elastic matrix. These responses were assessed as a function of conduit intra-peritoneal implantation time. This study showed that the blending of collagen with poly(ε-caprolactone) (PCL) promotes a faster wound healing response, as assessed by trends in expression of macrophage and smooth muscle cell (SMC) contractile markers and in matrix deposition, compared to the more chronic response for PCL alone. This result, along with the increase in elastic matrix production, demonstrates the benefits of incorporating as little as 25% w/w collagen into the conduit. In addition, PCR analysis demonstrated the challenges in differentiating between a myofibroblast and an SMC using traditional phenotypic markers. Finally, the impact of the tethered HA-o is limited within the inflammatory environment, unlike the significant response found previously in vitro. In conclusion, these results demonstrate the importance of both careful control of implanted scaffold composition and the development of appropriate delivery methods for HA-o.

Extracellular matrix synthesis in vascular disease: hypertension, and atherosclerosis

Extracellular matrix (ECM) within the vascular network provides both a structural and regulatory role. The ECM is a dynamic composite of multiple proteins that form structures connecting cells within the network. Blood vessels are distended by blood pressure and, therefore, require ECM components with elasticity yet with enough tensile strength to resist rupture. The ECM is involved in conducting mechanical signals to cells. Most importantly, ECM regulates cellular function through chemical signaling by controlling activation and bioavailability of the growth factors. Cells respond to ECM by remodeling their microenvironment which becomes dysregulated in vascular diseases such hypertension, restenosis and atherosclerosis. This review examines the cellular and ECM components of vessels, with specific emphasis on the regulation of collagen type I and implications in vascular disease.

Elastomeric Recombinant Protein-based Biomaterials

Elastomeric protein-based biomaterials, produced from elastin derivatives, are widely investigated as promising tissue engineering scaffolds due to their remarkable properties including substantial extensibility, long-term stability, self-assembly, high resilience upon stretching, low energy loss, and excellent biological activity. These elastomers are processed from different sources of soluble elastin such as animal-derived soluble elastin, recombinant human tropoelastin, and elastin-like polypeptides into various forms including three dimensional (3D) porous hydrogels, elastomeric films, and fibrous electrospun scaffolds. Elastin-based biomaterials have shown great potential for the engineering of elastic tissues such as skin, lung and vasculature. In this review, the synthesis and properties of various elastin-based elastomers with their applications in tissue engineering are described.

Tropoelastin--a multifaceted naturally smart material

Tropoelastin dominates the physical performance of human elastic tissue as it is assembled to make elastin. Tropoelastin is increasingly appreciated as a protein monomer with a defined solution shape comprising modular, bridged regions that specialize in elasticity and cell attachment, which collectively participate in macromolecular assembly. This modular, multifaceted molecule is being exploited to enhance the physical performance and biological presentation of engineered constructs to augment and repair human tissues. These tissues include skin and vasculature, and emphasize how growing knowledge of tropoelastin can be powerfully adapted to add value to pre-existing devices like stents and novel, multi-featured biological implants.

Tissue engineering and regenerative strategies to replicate biocomplexity of vascular elastic matrix assembly

Cardiovascular tissues exhibit architecturally complex extracellular matrices, of which the elastic matrix forms a major component. The elastic matrix critically maintains native structural configurations of vascular tissues, determines their ability to recoil after stretch, and regulates cell signaling pathways involved in morphogenesis, injury response, and inflammation via biomechanical transduction. The ability to tissue engineer vascular replacements that incorporate elastic matrix superstructures unique to cardiac and vascular tissues is thus important to maintaining vascular homeostasis. However, the vascular elastic matrix is particularly difficult to tissue engineer due to the inherently poor ability of adult vascular cells to synthesize elastin precursors and organize them into mature structures in a manner that replicates the biocomplexity of elastic matrix assembly during development. This review discusses current tissue engineering materials (e.g., growth factors and scaffolds) and methods (e.g., dynamic stretch and contact guidance) used to promote cellular synthesis and assembly of elastic matrix superstructures, and the limitations of these approaches when applied to smooth muscle cells, the primary elastin-generating cell type in vascular tissues. The potential application of these methods for in situ regeneration of disrupted elastic matrix at sites of proteolytic vascular disease (e.g., abdominal aortic aneurysms) is also discussed. Finally, the review describes the potential utility of alternative cell types to elastic tissue engineering and regenerative matrix repair. Future progress in the field is contingent on developing a thorough understanding of developmental elastogenesis and then mimicking the spatiotemporal changes in the cellular microenvironment that occur during that phase. This will enable us to tissue engineer clinically applicable elastic vascular tissue replacements and to develop elastogenic therapies to restore homeostasis in de-elasticized vessels.

Lung parenchymal mechanics

The lung parenchyma comprises a large number of thin-walled alveoli, forming an enormous surface area, which serves to maintain proper gas exchange. The alveoli are held open by the transpulmonary pressure, or prestress, which is balanced by tissues forces and alveolar surface film forces. Gas exchange efficiency is thus inextricably linked to three fundamental features of the lung: parenchymal architecture, prestress, and the mechanical properties of the parenchyma. The prestress is a key determinant of lung deformability that influences many phenomena including local ventilation, regional blood flow, tissue stiffness, smooth muscle contractility, and alveolar stability. The main pathway for stress transmission is through the extracellular matrix. Thus, the mechanical properties of the matrix play a key role both in lung function and biology. These mechanical properties in turn are determined by the constituents of the tissue, including elastin, collagen, and proteoglycans. In addition, the macroscopic mechanical properties are also influenced by the surface tension and, to some extent, the contractile state of the adherent cells. This chapter focuses on the biomechanical properties of the main constituents of the parenchyma in the presence of prestress and how these properties define normal function or change in disease. An integrated view of lung mechanics is presented and the utility of parenchymal mechanics at the bedside as well as its possible future role in lung physiology and medicine are discussed.

Glycosaminoglycans restrained in a fibrin matrix improve ECM remodelling by endothelial cells grown for vascular tissue engineering

The success of a biocompatible vascular graft depends upon its mechanical attributes and post-implantation healing responses. Mechanical strength is a paramount issue because grafts placed in the arterial circulation must be capable of withstanding long-term haemodynamic stress without graft failure. Extracellular matrix (ECM) proteins that are deposited by the cells to remodel the environment play a major role in determining the construct stability and strength. A suitable scaffold that stimulates ECM deposition and remodelling by cells grown in vitro may generate tissues with normal function. The objective of this study was to prove that fibrin matrix composition can be modified with growth factors (GFs) and glycosaminoglycans (GAGs) to promote ECM remodelling coupled with endothelial cell (EC) growth. Effect of GFs and GAGs on ECM production and remodelling was studied separately and in combination. Matrices recovered after EC cultures were analysed after immunochemical staining and it was observed that GFs and GAGs influence collagen IV and elastin deposition. Quantitative PCR analysis of mRNA after specific periods of culture demonstrated significant upregulation of elastin and collagen expression in EC by combination of GFs and GAGS when compared to their individual effects. The results of experiments conducted with various combinations of GFs and GAGs show that a biomimetic approach of immobilization of signalling molecules in fibrin can upregulate ECM remodelling with simultaneous degradation of the fibrin matrix and deposition of collagen IV and elastin. Hence, this combination may be suitable for cardiovascular tissue generation in vitro.

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.

Mice lacking the extracellular matrix protein MAGP1 display delayed thrombotic occlusion following vessel injury

Mice lacking the extracellular matrix protein microfibril-associated glycoprotein-1 (MAGP1) display delayed thrombotic occlusion of the carotid artery following injury as well as prolonged bleeding from a tail vein incision. Normal occlusion times were restored when recombinant MAGP1 was infused into deficient animals prior to vessel wounding. Blood coagulation was normal in these animals as assessed by activated partial thromboplastin time and prothrombin time. Platelet number was lower in MAGP1-deficient mice, but the platelets showed normal aggregation properties in response to various agonists. MAGP1 was not found in normal platelets or in the plasma of wild-type mice. In ligand blot assays, MAGP1 bound to fibronectin, fibrinogen, and von Willebrand factor, but von Willebrand factor was the only protein of the 3 that bound to MAGP1 in surface plasmon resonance studies. These findings show that MAGP1, a component of microfibrils and vascular elastic fibers, plays a role in hemostasis and thrombosis.

Growth factors upregulate deposition and remodeling of ECM by endothelial cells cultured for tissue-engineering applications

Appropriate matrix formation, turnover and remodeling in tissue-engineered small diameter vascular conduits are crucial for their long-term function. The interaction between cells and extra-cellular components is indispensable in determining cellular behavior in tissues and on biomaterials. The fibrin that contains fibronectin shows promise in most aspects as a tissue engineering scaffold, whereas, deposition of elastin and collagen by endothelial cells grown in the lumen of the construct is desirable to improve post implant retention, mechanical stability and vaso-responsiveness. So far there is no report on production of extra-cellular matrix (ECM) proteins, elastin and collagen by endothelial cells (EC) in in vitro culture conditions. In this study, we have used a biomimetic approach of providing multiple growth factors (GF) in the fibronectin (FN)-containing fibrin matrix to induce production of elastin and collagen by the endothelial cells for application in vascular tissue engineering. Deposition of elastin and collagens with matrix remodeling is demonstrated through qualitative analysis of the matrices that were recovered after growing cells on the initial fibrin-FN-GF matrix. Expressions of mRNA for both proteins were assessed by real time polymerase chain reaction (RT-PCR) to estimate the effects of multiple growth factor compositions. Marked deposition of elastin and collagen was evidenced by staining the recovered matrix after different culture intervals. Obviously, the biomimetic environment created by adding angiogenic and platelet growth factors in the fibrin-fibronectin-gelatin matrix can induce deposition of collagens and elastin by EC.

Comparison of the endothelial surface and subjacent elastic lamina of anterior descending coronary arteries at the location of atheromatous lesions with internal thoracic arteries of the same subjects: a scanning electron microscopic study

AIM

Current theories fail to explain the localisation of atheromatous lesions or their variable incidence in different arteries of the same subject. The objective of this study was to compare by scanning electron microscopy (SEM) the endothelial surface and the subjacent elastic lamina of human coronary arteries at the location of areas showing infiltration by lipid and cells, with the same components of internal thoracic arteries of the same subjects.

METHODS

The endothelial surface and the subjacent elastic lamina of localised atheromatous areas of 146 anterior descending coronary arteries were compared with the same structural components of the internal thoracic arteries of the same subjects, using SEM, transverse paraffin sections and freeze-fracture. Some arteries were digested with formic acid to destroy the endothelium and interstitial tissue, and reveal the elastin fibre structure of the elastic laminae.

RESULTS

Coronary arteries showed localised defects of the endothelial surface and of the elastin fibre structure of the subjacent elastic membrane, with the presence of lipid and cells in transverse sections of the intima. Internal thoracic arteries showed such changes only rarely, more particularly in older age groups.

CONCLUSIONS

In localised areas of the coronary arteries showing infiltration of the wall by lipid and cells, there were imperfections of the endothelial surface and of the elastin fibre structure of the subjacent elastic lamina. These imperfections were not in general present in the endothelial surface, or subendothelial elastic lamina of the internal thoracic arteries in age groups below 50, and only rarely in older subjects.

The importance of a substantial elastic lamina subjacent to the endothelium in limiting the progression of atherosclerotic changes

This study examines the hypothesis that progressive intimal thickening and atherosclerosis in the larger pulsatile arteries arise from failure to maintain, subjacent to the endothelial cells, a substantial elastin membrane, a component which has been shown to be of special structural significance. The internal thoracic arteries of 293 subjects of all ages up to 60 years were compared histologically with the anterior descending coronary arteries of the same individuals by light- and electronmicroscopy and immunoperoxidase staining for macromolecules. The internal thoracic arteries usually developed a new robust reduplicated internal elastic lamina at an early age, no further intimal thickening, and no significant entry of lipid or cells to the intima. The coronary arteries showed areas of rapid intimal thickening with poor and incomplete reduplicated internal elastic laminae, entry of lipid, macrophages, and other cells to the intima. The reduplicated internal elastic laminae appeared to be formed primarily by the endothelial cells themselves. An elastin membrane subjacent to the endothelial cells appears to be essential. It provides a secure attachment for the cells and a barrier to the entry of macromolecules and cells to the intima. Its absence is associated with progressive intimal thickening and atherosclerosis.

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.

Ultrastructure appearance of atherosclerosis in human and experimentally-induced animal models

We describe here the basic structure of the aorta, the changes with aging and ultrastructural appearance of atherosclerosis of human and animal models. The architecture of the aortic wall is highly organized, for adaptation to changes of blood pressure. The main cells composing the vessel are endothelial cells and smooth muscle cells. They maintain the integrity and homeostasis of the aorta along with the extracellular matrix of collagen fibrils, elastic fibers and glycosaminoglycans. The structural changes with aging and atherogenesis are a compensative or degenerative phenomenon caused by many factors. Three major cells are the endothelial cell, smooth muscle cell and monocyte-derived macrophages (as well as platelets) all of which are involved in atherogenesis. Foam cells in atheromatous lesions are derived from macrophages and smooth muscle cells. Recently, the molecular biological nature and function of these cells and their derived-factors have been thoroughly investigated in cell culture and in experimental animal models caused by a mechanical injury of the endothelium or by a dietary induced hypercholesterolemia. However, the mechanism of the endothelial injury in vivo as well as formation of atheromatous cores of human atherosclerosis is not exactly understood. Some structural and functional changes inherent to the arterial wall during aging may play an important role in initiation or progression of human atherosclerosis.

Developmental changes in tropoelastin gene expression in the rat lung studied by in situ hybridization

Gene expression for tropoelastin, the proprotein for elastin, was examined in the rat lung from 17 days of gestation (pseudoglandular stage) to adulthood by in situ hybridization using a rat-specific 35S-radiolabeled riboprobe. The tropoelastin message was present in vascular and airway smooth muscle, endothelial, septal interstitial, alveolar wall, and mesothelial cells but not in epithelial cells. With alveolar septal formation, the message in the interstitium increased progressively from 17 days of gestation, reaching a peak at 7 to 11 days postnatal. The signal in the arterial walls, in contrast, peaked between 19 days of gestation to 1 day postnatal and thereafter declined first from the outer media. The signal in general declined significantly by 21 days postnatal, and elastogenesis was virtually absent in the adult. These results support the idea that tropoelastin gene expression in the interstitium is closely associated with the centripetal progression of alveolarization, and the early postnatal decrease of tropoelastin expression in blood vessels corresponds with the sudden postnatal changes in the pulmonary hemodynamics. Furthermore, in the rat fetus and neonate, endothelial cells expressed the gene for tropoelastin and hence probably play a significant role in the formation of internal elastic lamina in vivo.

Qualitative and quantitative differences in protein synthesis comparing fetal lamb ductus arteriosus endothelium and smooth muscle with cells from adjacent vascular sites

During late gestation, intimal cushions form in the ductus arteriosus (DA) and these cause the vessel to close when it constricts in the postnatal period. The formation of intimal cushions suggests highly specialized functions of DA endothelial and smooth muscle cells. To investigate these properties, we established, from fetal lambs on Day 138 of a 148-day term gestation, primary cell cultures of DA endothelium and smooth muscle and compared them to cells derived from the adjacent pulmonary artery and aorta. Purity of the endothelial cell cultures from each vascular site was assessed by the contact inhibited "cobblestone" monolayer phenotype, by positive immunofluorescence for factor VIII and by angiotensin converting enzyme activity. Purity of smooth muscle cell cultures at each vascular site was assessed by the "hills and valleys" phenotype and by positive immunofluorescence with a smooth muscle actin specific monoclonal antibody. Endothelial and smooth muscle cells had different growth curves, ultrastructural features, and protein profiles on single and two-dimensional SDS-polyacrylamide gel electrophoresis (PAGE), but vascular sites were similar. To further determine whether differences related to DA origin were indeed present, endothelial and smooth muscle cells from all three vascular sites were incubated with the radiolabeled amino acids [14C]leucine, [14C]proline, and [14C]valine and the proteins in both the cells and the conditioned medium were analyzed by autoradiography after SDS-PAGE. A dense band corresponding to a 42-kDa protein was observed in valine-labeled DA endothelial cells and conditioned medium and a 52-kDa protein was observed in the conditioned medium of leucine-labeled DA smooth muscle cells only. Further isolation and characterization of these endothelial and smooth muscle proteins will be necessary to determine whether they are related to the mechanism of intimal cushion formation in the late gestation DA or are present abnormally in association with the intimal proliferation observed in pulmonary and systemic vascular disease.

The elastin receptor: a galactoside-binding protein

The elastin receptor complex contains a component of 67 kilodaltons that binds to a glycoconjugate affinity column containing beta-galactoside residues and is eluted from this column with lactose. This protein component is also released from the surface of cultured chondroblasts by incubation with lactose, and its association with immobilized elastin is inhibited by lactose. Since lactose also blocks elastic fiber formation by cultured chondroblasts, the galactoside-binding property of the elastin receptor is implicated in this process.

Widespread codistribution of glycoprotein gp 115 and elastin in chick eye and other tissues

Frozen sections of chick tissues were exposed to affinity-purified monoclonal antibodies raised against chick gp 115 and to affinity-purified antibodies raised against chick tropoelastin to study the distribution pattern of the corresponding antigens by the avidin-biotin immunoperoxidase technique. Laminin and fibronectin antibodies were used for comparison. Gp 115 and tropoelastin antibodies localized to the same structure in several of the tissues examined. The endothelial membrane of the cornea and Bruch's membrane in the choroid were positive, while the corneal epithelial membrane was negative. Both antibodies displayed a peculiar punctate reactivity in the corneal stroma and a very fine fibrillar pattern in the conjunctiva and at the corneal-scleral junction. Liver, heart and large vessels, striated muscle and skin showed a similar pattern both for tropoelastin and gp 115 antibodies. Few differences were seen in the distribution of the reactivity: the pericellular matrix of intestinal smooth muscle cells was stained by gp 115 but not by tropoelastin antibodies. However, the reactivity of gp 115 and tropoelastin antibodies was similarly distributed in the lung smooth muscle cell clusters. The peritubular matrix in the kidney did also not react with tropoelastin antibodies as did the brain intraparenchymal vessels; whereas gp 115 antibody reactivity was present in both sites. We interpret these lack of apparent codistribution in some tissues as a variation in the relative availability of the target antigen for the reaction with the antibody and not as a consequence of a qualitative difference in the distribution of gp 115 and tropoelastin. By the use of anti gp 115 monoclonal antibodies that do not cross-react, and presumably recognize different epitopes, it was shown that some but not all antibodies, react with brain intraparenchymal blood vessels; whereas the pattern of distribution in other tissues was the same. This suggests that in vessels with an undetectable level of elastin, certain epitopes of gp 115 molecule might not be recognized as a result of being masked by other components or by a different conformation of the molecule.

Properties of endothelial cell and smooth muscle cell cultured in ambient pressure

Cultures of umbilical vein endothelial cells and smooth muscle cells were studied in a constant pressure chamber. The following results were obtained: (a) Endothelial cell growth was maximal at 80 mmHg and minimal at 0 mmHg (atmospheric pressure) for the first 2 d of incubation. However, these growth rates were reversed during the following 6 d because of steady increase in growth at 0 mmHg, and a decrease in growth at higher pressures. A degeneration of endothelial cells began at 120 mmHg and marked degeneration was noted at 160 mmHg. Growth of smooth muscle cells was not influenced by ambient pressure and a steady increase in labeled nuclei continued throughout the period of culture. (b) Elastin, stainable with tannic acid, was noted electronmicroscopically in both endothelial and smooth muscle cells. (c) Production of prostacyclin by endothelial cells was maximal at 0 mmHg and minimal at 80 mmHg, in contrast to the growth pattern of these cells. Production of thromboxane B2 by endothelial cells and prostacyclin and thromboxane B2 by smooth muscle cells was very slight and not significantly different. Although it is not known at present what mechanism acts on the vascular cells when cultured in ambient pressure, these results may indicate a new concept of the behavioral relationship between endothelial cell, smooth muscle cell, and blood pressure in vivo.

Smooth muscle-mediated connective tissue remodeling in pulmonary hypertension

Abnormal accumulation of connective tissue in blood vessels contributes to alterations in vascular physiology associated with disease states such as hypertension and atherosclerosis. Elastin synthesis was studied in blood vessels from newborn calves with severe pulmonary hypertension induced by alveolar hypoxia in order to investigate the cellular stimuli that elicit changes in pulmonary arterial connective tissue production. A two- to fourfold increase in elastin production was observed in pulmonary artery tissue and medial smooth muscle cells from hypertensive calves. This stimulation of elastin production was accompanied by a corresponding increase in elastin messenger RNA consistent with regulation at the transcriptional level. Conditioned serum harvested from cultures of pulmonary artery smooth muscle cells isolated from hypertensive animals contained one or more low molecular weight elastogenic factors that stimulated the production of elastin in both fibroblasts and smooth muscle cells and altered the chemotactic responsiveness of fibroblasts to elastin peptides. These results suggest that connective tissue changes in the pulmonary vasculature in response to pulmonary hypertension are orchestrated by the medial smooth muscle cell through the generation of specific differentiation factors that alter both the secretory phenotype and responsive properties of surrounding cells.

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.

Cellular changes during hypertension: a quantitative study of the rat aorta

Using rats made hypertensive by aortic ligation or by the one kidney--one clip method, we searched the aorta for morphologic clues that could explain why hypertension aggravates atherosclerosis. Both atherosclerosis and hypertension are characterized by an increased migration of mononuclear cells into the aortic intima; we therefore quantitated this phenomenon and studied its time course. In the thoracic aorta of hypertensive rats intimal cells (emigrated mononuclear cells) increased up to 15 times 2 weeks after surgery and remained stationary thereafter. In both control and experimental rats, leukocyte emigration was heavier in the thoracic aorta than in the abdominal region. A two- to threefold increase in medial smooth muscle herniae into the intima (myointimal herniae) was also found at 8 weeks, indicating a smooth muscle cell dysfunction. Electron microscopic study of the intima showed that its thickening was due to blood-borne material and also to extracellular matrix synthesized by the endothelium. Heightened secretion reflects cell activation, a condition that (in the endothelium) leads also to leukocyte adhesion. These data suggest that, in renovascular hypertension, the aortic endothelium is in an activated state, possibly through a hormonal stimulus.