Developmental changes in collagen and elastin biosynthesis in the porcine aorta

Auricular Chondrocytes as a Cell Source for Scaffold-Free Elastic Cartilage Tissue Engineering

Current tissue engineering (TE) methods utilize chondrocytes primarily from costal or articular sources. Despite the robust mechanical properties of neocartilages sourced from these cells, the lack of elasticity and invasiveness of cell collection from these sources negatively impact clinical translation. These limitations invited the exploration of naturally elastic auricular cartilage as an alternative cell source. This study aimed to determine if auricular chondrocytes (AuCs) can be used for TE scaffold-free neocartilage constructs and assess their biomechanical properties. Neocartilages were successfully generated from a small quantity of primary neonatal AuCs of three minipig donors (n = 3). Neocartilage constructs had instantaneous moduli of 200.5 kPa ± 43.34 and 471.9 ± 92.8 kPa at 10% and 20% strain, respectively. TE constructs' relaxation moduli (Er) were 36.99 ± 6.47 kPa Er and 110.3 ± 16.99 kPa at 10% and 20% strain, respectively. The Young's modulus was 2.0 MPa ± 0.63, and the ultimate tensile strength was 0.619 ± 0.177 MPa. AuC-derived neocartilages contained 0.144 ± 0.011 µg collagen, 0.185 µg ± 0.002 glycosaminoglycans per µg dry weight, and 1.7e-3 µg elastin per µg dry weight. In conclusion, this study shows that AuCs can be used as a reliable and easily accessible cell source for TE of biomimetic and mechanically robust elastic neocartilage implants.

Mechanical, structural, and physiologic differences between above and below-knee human arteries

Peripheral Artery Disease (PAD) affects the lower extremities and frequently results in poor clinical outcomes, especially in the vessels below the knee. Understanding the biomechanical and structural characteristics of these arteries is important for improving treatment efficacy, but mechanical and structural data on tibial vessels remain limited. We compared the superficial femoral (SFA) and popliteal (PA) arteries that comprise the above-knee femoropopliteal (FPA) segment to the infrapopliteal (IPA) anterior tibial (AT), posterior tibial (PT), and fibular (FA) arteries from the same 15 human subjects (average age 52, range 42-67 years, 87 % male). Vessels were imaged using μCT, evaluated with biaxial mechanical testing and constitutive modeling, and assessed for elastin, collagen, smooth muscle cells (SMCs), and glycosaminoglycans (GAGs). IPAs were more often diseased or calcified compared to the FPAs. They were also twice smaller, 53 % thinner, and significantly stiffer than the FPA longitudinally, but not circumferentially. IPAs experienced 48 % higher physiologic longitudinal stresses (62 kPa) but 27 % lower circumferential stresses (24 kPa) and similar cardiac cycle stretch of <1.02 compared to the FPA. IPAs had lower longitudinal pre-stretch (1.12) than the FPAs (1.29), but there were no differences in the stored elastic energy during pulsation. The physiologic circumferential stiffness was similar in the above and below-knee arteries (718 kPa vs 754 kPa). Structurally, IPAs had less elastin, collagen, and GAGs than the FPA, but maintained similar SMC content. Our findings contribute to a better understanding of segment-specific human lower extremity artery biomechanics and may inform the development of better medical devices for PAD treatment. STATEMENT OF SIGNIFICANCE: Peripheral Artery Disease (PAD) in the lower extremity arteries exhibits distinct characteristics and results in different clinical outcomes when treating arteries above and below the knee. However, their mechanical, structural, and physiologic differences are poorly understood. Our study compared above- and below-knee arteries from the same middle-aged human subjects and demonstrated distinct differences in size, structure, and mechanical properties, leading to variations in their physiological behavior. These insights could pave the way for creating location-specific medical devices and treatments for PAD, offering a more effective approach to its management. Our findings provide new, important perspectives for clinicians, researchers, and medical device developers interested in treating PAD in both above- and below-knee locations.

Arterial stiffness and cardiac dysfunction in Hutchinson-Gilford Progeria Syndrome corrected by inhibition of lysyl oxidase

The findings show that increased lysyl oxidase abundance is causal for the elevated arterial stiffness present in the arteries of Hutchinson–Gilford Progeria Syndrome mice. Pharmacologic inhibition of lysyl oxidase improves cardiac dysfunction and restores arterial compliance.

Arterial stiffening and cardiac dysfunction are hallmarks of premature aging in Hutchinson–Gilford Progeria Syndrome (HGPS), but the molecular regulators remain unknown. Here, we show that the LaminA^G609G mouse model of HGPS recapitulates the premature arterial stiffening and early diastolic dysfunction seen in human HGPS. Lysyl oxidase (LOX) is up-regulated in the arteries of these mice, and treatment with the LOX inhibitor, β-aminopropionitrile, improves arterial mechanics and cardiac function. Genome-wide and mechanistic analysis revealed reduced expression of the LOX-regulator, miR-145, in HGPS arteries, and forced expression of miR-145 restores normal LOX gene expression in HGPS smooth muscle cells. LOX abundance is also increased in the carotid arteries of aged wild-type mice, but its spatial expression differs from HGPS and its up-regulation is independent of changes in miR-145 abundance. Our results show that miR-145 is selectively misregulated in HGPS and that the consequent up-regulation of LOX is causal for premature arterial stiffening and cardiac dysfunction.

Early impact of moderate preterm birth on the structure, function and gene expression of conduit arteries

NEW FINDINGS

What is the central question of this study? What is the immediate impact of moderate preterm birth on the structure and function of major conduit arteries using a pre-clinical sheep model? What is the main finding and its importance? Postnatal changes in conduit arteries, including a significant decrease in collagen within the thoracic aortic wall (predominately males), narrowed carotid arteries, reduced aortic systolic blood flow, and upregulation of the mRNA expression of cell adhesion and inflammatory markers at 2 days of age in preterm lambs compared to controls, may increase the risk of cardiovascular impairment in later life.

ABSTRACT

The aim of this work was to compare the structure and function of the conduit arteries of moderately preterm and term-born lambs and to determine whether vascular injury-associated genes were upregulated. Time-mated ewes were induced to deliver either preterm (132 ± 1 days of gestation; n = 11 females and n = 10 males) or at term (147 ± 1 days of gestation; n = 10 females and n = 5 males). Two days after birth, ultrasound imaging of the proximal ascending aorta, main, right and left pulmonary arteries, and right and left common carotid arteries was conducted in anaesthetized lambs. Lambs were then killed and segments of the thoracic aorta and left common carotid artery were either snap frozen for real-time PCR analyses or immersion-fixed for histological quantification of collagen, smooth muscle and elastin within the medial layer. Overall there were few differences in vascular structure between moderately preterm and term lambs. However, there was a significant decrease in the proportion of collagen within the thoracic aortic wall (predominantly in males), narrowing of the common carotid arteries and a reduction in peak aortic systolic blood flow in preterm lambs. In addition, there was increased mRNA expression of the cell adhesion marker P-selectin in the thoracic aortic wall and the pro-inflammatory marker IL-1β in the left common carotid artery in preterm lambs, suggestive of postnatal vascular injury. Early postnatal differences in the function and structure of conduit arteries and evidence of vascular injury in moderately preterm offspring may place them at greater risk of cardiovascular impairment later in life.

Intramural hematoma and penetrating atherosclerotic ulcers of the aorta: uncertainties and controversies

The natural histories of intramural hematoma (IMH) and penetrating atherosclerotic ulcer (PAU) are highly variable as they may progress to aneurysm formation, rupture, or dissection, or even resolve, in the specific case of IMH. Imaging plays an increasingly important role in clinical and surgical management of IMH and PAU. In contrast to ulcer-like projections, images of intramural blood pools have not been widely reported in CT studies of patients with IMH. Understanding the imaging characteristics and the natural course of each of these entities would help clinicians and surgeons to identify patients at greatest risk for bad prognosis and may improve outcomes. This paper discusses the pathophysiology of these entities, the controversies regarding their natural history, and the prognostic factors that should be identified in CT scans.

Aortic stiffness, pressure and flow pulsatility, and target organ damage

Measures of aortic stiffness and pressure and flow pulsatility have emerged as correlates of and potential contributors to cardiovascular disease, dementia, and kidney disease. Higher aortic stiffness and greater pressure and flow pulsatility are associated with excessive pulsatile load on the heart, which increases mass and reduces global longitudinal strain of the left ventricle. Excessive stiffness and pulsatility are also associated with microvascular lesions in high-flow organs, such as the brain and kidney, suggesting that small vessels in these organs are damaged by pulsatility. This brief review will summarize evidence relating aortic stiffness to cardiovascular, brain, and kidney disease.

Unique aspects of the developing lung circulation: structural development and regulation of vasomotor tone

This review summarizes our current knowledge on lung vasculogenesis and angiogenesis during normal lung development and the regulation of fetal and postnatal pulmonary vascular tone. In comparison to that of the adult, the pulmonary circulation of the fetus and newborn displays many unique characteristics. Moreover, altered development of pulmonary vasculature plays a more prominent role in compromised pulmonary vasoreactivity than in the adult. Clinically, a better understanding of the developmental changes in pulmonary vasculature and vasomotor tone and the mechanisms that are disrupted in disease states can lead to the development of new therapies for lung diseases characterized by impaired alveolar structure and pulmonary hypertension.

Transcriptional and posttranscriptional mechanisms contribute to the dysregulation of elastogenesis in Schimke immuno-osseous dysplasia

BACKGROUND

Schimke immuno-osseous dysplasia (SIOD) is an autosomal recessive disorder caused by mutations in SMARCAL1. A frequent complication is arteriosclerosis associated with reduced elastin expression; however, the mechanism underlying the reduced elastin expression remains unknown.

METHODS

Expression of transcriptional regulators of elastin (ELN) and microRNA (miRNA) regulators of ELN messenger RNA (mRNA), ELN promoter methylation, and ELN mRNA poly(A) tail length were assessed by quantitative RT-PCR, bisulfite Sanger sequencing, and the Poly(A) Tail Length Assay Kit, respectively, in unaffected developing human aortae and in an SIOD aorta.

RESULTS

Comparing unaffected fetal and adult aortae, ELN precursor mRNA (pre-mRNA) levels remained nearly constant, whereas mRNA levels declined by ~10(2)-fold. This corresponded with a reduction in poly(A) tail length but not with changes in the other parameters. In contrast, compared to the unaffected fetal aortae, the SIOD aorta had 18-fold less ELN pre-mRNA and 10(4)-fold less mRNA. This corresponded with increased expression of miRNA regulators and shorter ELN mRNA poly(A) tail lengths but not with altered expression of ELN transcriptional regulators or ELN promoter methylation.

CONCLUSION

Posttranscriptional mechanisms account for the reduction in ELN mRNA levels in unaffected aortae, whereas transcriptional and posttranscriptional mechanisms reduce elastin expression in SIOD aorta and predispose to arteriosclerosis.

Bond Strength of Thermally Fused Vascular Tissue Varies With Apposition Force

Surgical tissue fusion devices ligate blood vessels using thermal energy and coaptation pressure, while the molecular mechanisms underlying tissue fusion remain unclear. This study characterizes the influence of apposition force during fusion on bond strength, tissue temperature, and seal morphology. Porcine splenic arteries were thermally fused at varying apposition forces (10-500 N). Maximum bond strengths were attained at 40 N of apposition force. Bonds formed between 10 and 50 N contained laminated medial layers; those formed above 50 N contained only adventitia. These findings suggest that commercial fusion devices operate at greater than optimal apposition forces, and that constituents of the tunica media may alter the adhesive mechanics of the fusion mechanism.

A multi-layered computational model of coupled elastin degradation, vasoactive dysfunction, and collagenous stiffening in aortic aging

Arterial responses to diverse pathologies and insults likely occur via similar mechanisms. For example, many studies suggest that the natural process of aging and isolated systolic hypertension share many characteristics in arteries, including loss of functional elastin, decreased smooth muscle tone, and altered rates of deposition, and/or crosslinking of fibrillar collagen. Our aim is to show computationally how these coupled effects can impact evolving aortic geometry and mechanical behavior. Employing a thick-walled, multi-layered constrained mixture model, we suggest that a coupled loss of elastin and vasoactive function are fundamental mechanisms by which aortic aging occurs. Moreover, it is suggested that collagenous stiffening, although itself generally an undesirable process, can play a key role in attenuating excessive dilatation, perhaps including the enlargement of abdominal aortic aneurysms.

Tissue elasticity and the ageing elastic fibre

The ability of elastic tissues to deform under physiological forces and to subsequently release stored energy to drive passive recoil is vital to the function of many dynamic tissues. Within vertebrates, elastic fibres allow arteries and lungs to expand and contract, thus controlling variations in blood pressure and returning the pulmonary system to a resting state. Elastic fibres are composite structures composed of a cross-linked elastin core and an outer layer of fibrillin microfibrils. These two components perform distinct roles; elastin stores energy and drives passive recoil, whilst fibrillin microfibrils direct elastogenesis, mediate cell signalling, maintain tissue homeostasis via TGFβ sequestration and potentially act to reinforce the elastic fibre. In many tissues reduced elasticity, as a result of compromised elastic fibre function, becomes increasingly prevalent with age and contributes significantly to the burden of human morbidity and mortality. This review considers how the unique molecular structure, tissue distribution and longevity of elastic fibres pre-disposes these abundant extracellular matrix structures to the accumulation of damage in ageing dermal, pulmonary and vascular tissues. As compromised elasticity is a common feature of ageing dynamic tissues, the development of strategies to prevent, limit or reverse this loss of function will play a key role in reducing age-related morbidity and mortality.

Computational study of growth and remodelling in the aortic arch

Opening angles (OAs) are associated with growth and remodelling in arteries. One curiosity has been the relatively large OAs found in the aortic arch of some animals. Here, we use computational models to explore the reasons behind this phenomenon. The artery is assumed to contain a smooth muscle/collagen phase and an elastin phase. In the models, growth and remodelling of smooth muscle/collagen depends on wall stress and fluid shear stress. Remodelling of elastin, which normally turns over very slowly, is neglected. The results indicate that OAs generally increase with longitudinal curvature (torus model), earlier elastin production during development, and decreased wall stiffness. Correlating these results with available experimental data suggests that all of these effects may contribute to the large OAs in the aortic arch. The models also suggest that the slow turnover rate of elastin limits longitudinal growth. These results should promote increased understanding of the causes of residual stress in arteries.

Growth and remodeling in a thick-walled artery model: effects of spatial variations in wall constituents

A mathematical model is presented for growth and remodeling of arteries. The model is a thick-walled tube composed of a constrained mixture of smooth muscle cells, elastin and collagen. Material properties and radial and axial distributions of each constituent are prescribed according to previously published data. The analysis includes stress-dependent growth and contractility of the muscle and turnover of collagen fibers. Simulations were conducted for homeostatic conditions and for the temporal response following sudden hypertension. Numerical pressure-radius relations and opening angles (residual stress) show reasonable agreement with published experimental results. In particular, for realistic material and structural properties, the model predicts measured variations in opening angles along the length of the aorta with reasonable accuracy. These results provide a better understanding of the determinants of residual stress in arteries and could lend insight into the importance of constituent distributions in both natural and tissue-engineered blood vessels.

Cellular interactions with elastin

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

Effects of angiotensin-converting enzyme polymorphism on aortic elastic parameters in athletes

BACKGROUND

Physiologic adaptations in an athlete's heart include increased left and right ventricular chamber size, left ventricular wall thickness and mass. Angiotensin-converting enzyme (ACE) is a key enzyme in angiotensin II production causing cardiac hypertrophy. The cloning of the ACE gene has made it possible to identify a deletion (D)-insertion (I) polymorphism that appears to affect the level of serum ACE activity. Therefore, the ACE genes, which have been shown to be polymorphic, could be candidate genes for large-artery stiffness.

METHODS

56 endurance athletes and 46 sedentary subjects were included in this study, and they underwent both complete echocardiographic examination, and analysis of ACE insertion (I) and deletion (D) allele frequencies in peripheral blood. The aortic diameter was recorded by M-mode echocardiography at a level 3 cm above the aortic valve. Aortic systolic diameter was measured at the time of full opening of the aortic valve, and diastolic diameter was measured at the peak of QRS. Aortic strain, stiffness index and distensibility were calculated.

RESULTS

Left ventricular mass index and left ventricular ejection fraction were significantly higher in athletes than controls (p < 0.001). The aortic distensibility index and strain were significantly greater in athletes compared with controls (respectively: 5.8 +/- 2.7 vs. 4.7 +/- 1.8 cm(-2) dyn(-1) 10(-6), p = 0.017; 12.3 +/- 2.4 vs. 9.3 +/- 3.1, p < 0.001). The aortic stiffness index was significantly lower in athletes than in controls (4.8 +/- 1.9 vs. 6.1 +/- 2.1, p < 0.001). The aortic distensibility index and strain were statistically different in ACE DD vs. DI groups and DD vs. II groups of athletes. The aortic stiffness index was statistically different in ACE DD vs. II groups of athletes. Aortic parameters were similar according to ACE genotypes in controls.

CONCLUSION

The results of this study indicate that aortic distensibility was increased by prolonged training in endurance athletes, particularly in those with the ACE II genotype. This effect represents an extracardiac adaptation to chronic prolonged training in athletes.

Vascular development, pulse pressure, and the mechanisms of hypertension

For a given cardiac function, the cyclic blood pressure (BP) curve results from 2 different phenotypes: the mean arterial pressure (MAP), a steady component reflecting the resistance of the microvascular network, and pulse pressure (PP), another component corresponding to large artery stiffness and wave reflections. Around birth, cardiovascular (CV) survival is critically influenced by the coupling between the heart and thoracic aorta, and hence, the adequacy of the Windkessel function, the magnitude of aortic elastin accumulation and the PP level. The maturation of the aortic trunk and its branches results from adaptative mechanisms involving shear and tensile stress, with major potential consequences on heart rate control, transit of wave reflections, and coronary perfusion. An adequate optimization of the Windkessel function, and hence PP, diastolic coronary perfusion and CV survival needs a critical MAP level to be reached in each individual during the postnatal period. The achievement of this MAP level requires the development of multiple resistance segments of the microvascular network, particularly within the kidney. Translated in adult populations, this pathophysiological process gives rise to a Gaussian BP distribution, with individuals remaining in the same BP percentile from birth onward (BP tracking). We suggest that hypertension results from early developmental vascular mechanisms that direct BP toward the higher percentiles of the Gaussian distribution curve.

Mouse model for hereditary hemorrhagic telangiectasia has a generalized vascular abnormality

BACKGROUND

Mutations in endoglin or activin like kinase-1, both involved in the endothelial transforming growth factor-beta signaling pathway, cause the autosomal dominant bleeding disorder hereditary hemorrhagic telangiectasia. We and others have reported mouse models for this disease that share the characteristic phenotype of dilated vessels and sporadic hemorrhage. The reasons for the variable phenotype in hereditary hemorrhagic telangiectasia are not understood.

METHODS AND RESULTS

After a detailed immunohistochemical analysis of 129/Ola mice, which are heterozygous for a targeted deletion in the endoglin gene, we observed intrinsic abnormalities in the vascular walls throughout the cutaneous vasculature. Postcapillary venules were dilated, and up to 70% of the vascular wall had no smooth muscle cells. The supporting layers of collagens and elastin were irregular, with thin areas, adding to the fragility of these vessels. A variable hemorrhagic phenotype was observed in which local bleeding is associated not only with fragile vessels but also with regions of inflammation.

CONCLUSIONS

These findings have relevance to our understanding of the molecular basis of vascular integrity in a wide range of diseases.

Pulmonary and systemic vascular tissue collagen, growth factor, and cytokine gene expression in the rabbit

During development, the vascular wall composition of the pulmonary and systemic capacitance vessels and their intravascular pressure changes. Little is known, however, about the factors controlling vascular collagen gene expression in both circulations during growth and development. The purpose of this study was to compare the developmental changes in collagen, major growth factors, and cytokines gene expression, in order to ascertain whether a circulation specific pattern is present in the rabbit. Fetal, neonatal, and adult rabbit extrapulmonary and aortic tissues were obtained and the mRNA levels for collagen I and III, as well as major growth factors and cytokines, were measured by a semi-quantitative RT-PCR technique. Collagen I, but not collagen III, expression was developmentally regulated in pulmonary vascular and aorta tissues. Collagen I expression was greatest during the fetal and neonatal period (P < 0.01) and higher in the aorta as compared with the pulmonary artery at these ages (P < 0.05). Significant developmental changes in growth factor mRNA levels were observed for TGF-beta, IGF-2, and bFGF (P < 0.01). IGF-2 mRNA levels significantly declined in both arteries from neonatal to adult, but bFGF increased only in the pulmonary artery during this transition. With regards to inducible enzymes, COX-2 mRNA levels changed developmentally, whereas iNOS mRNA levels were similar for both vessels at all ages. When comparing the two vessels, COX-2 transcripts were relatively more abundant in the adult pulmonary artery tissue and fetal aorta, with similar levels in the newborn. We conclude that circulation specific developmental regulation of collagen gene expression is present in the rabbit in a pattern that is unrelated to the intravascular pressure.

Key words: developmental changes, vascular, collagen, mRNA expression, growth factors.

Endothelial and smooth muscle changes of the thoracic and abdominal aorta with various types of cryopreservation

BACKGROUND

There is a need for cryopreserved arterial allografts in vascular surgery. Vitrification was examined as an alternative to slow cooling. Function of endothelial and adjacent cells was evaluated in the thoracic and abdominal regions of the porcine aorta.

MATERIAL AND METHODS

Aortas from young pigs were harvested and cryopreserved with several treatments: slow cooling at 1 degrees C/min with 10% dimethyl sulfoxide (DMSO) and vitrification with a vitrification solution (VS), performed either in the presence (medium) or in the absence (air) of the cryoprotective medium surrounding the sample. Tetrazolium salt reduction and oxygen consumption were used to assess the mitochondrial activity of the endothelial cells and smooth muscle cells from the aorta.

RESULTS

Fresh aorta showed an increased function from below the aortic arch to the iliac branch. Exposure to cryoprotective solutions reduced significantly the oxygen consumption of the abdominal sections. Samples from the thoracic region had a better response to cryopreservation than those from the abdominal region. There was a significant reduction in tetrazolium salt reduction and oxygen consumption when the samples were cryopreserved immersed in the medium. Significant flaking of the endothelium was observed after vitrification and contributes generally to lower the function.

CONCLUSION

The thoracic region of the aortic wall was more tolerant to the preservation injury, and freezing with DMSO in air offered a better choice of preservation.

Influence of elastin gene polymorphism on the elastin content of the aorta: A study in 2 strains of rat

The elastin content in the thoracic aorta of male Brown-Norway (BN) rats is 31.4+/-1.2% (dry weight), whereas that of male LOU rats is 37.2+/-1.0%. A similar difference in the elastin content of the thoracic aorta is also observed in female animals. Furthermore, in the thoracic aorta of young, growing rats as well as in cultured aortic smooth muscle cells, the steady-state level of elastin mRNA is significantly lower in the BN than in the LOU strain. These results suggested that 1 or more genes control the elastin mRNA level and the elastin content in the aortas of BN and LOU rats. A possible relationship between a polymorphism in the elastin gene and the elastin content of the aorta was tested. For this purpose, the aortic elastin content was measured in F(1) and F(2) generations bred from LOU and BN rats and was compared with that of the F(0) (parental) generation. A polymorphic marker located in intron 25 of the elastin gene has been used to genotype the F(2) rats. The degree of genetic determination of aortic elastin content was estimated to be 73% in the F(2) cohort, but the elastin locus accounts for only 3. 9% of the total variance in aortic elastin content. Other genes are thus responsible for the major part of the observed interstrain difference by regulating the transcription of the gene, the stability of elastin mRNA, and/or posttranslational events.

Relationship between perlecan and tropoelastin gene expression and cell replication in the developing rat pulmonary vasculature

Smooth-muscle-cell (SMC) replication and extracellular matrix protein expression are two vital and interrelated processes necessary for normal development of the vasculature. To understand better the nature of this relationship in the developing rat lung, we investigated the relationship between SMC proliferation and the expression of perlecan, a basement membrane (BM) heparan sulfate proteoglycan implicated in the control of SMC growth and differentiation, and tropoelastin (TE), a structural matrix protein not known to influence directly the replicative state of SMCs. Using bromodeoxyuridine (BrdU) incorporation to assess DNA synthesis, we first established the time course of SMC proliferation in the hilar pulmonary artery (PA) from embryonic to adult life. We found a labeling index of > 80% during the embryonic period (embryonic Day 13 [e13] to fetal Day 18 [f18]), a dramatic decline to approximately 40% during the fetal period of development, and a steady decrease in proliferation rates following birth such that, by 30 d of age, a labeling index of < 2% was noted. Using in situ hybridization, we found that although peak expression of both perlecan and TE messenger RNA (mRNA) occurred in the fetal and early postnatal periods following the major decrease in cell replication, TE mRNA expression was clearly observed in the PA as early as embryonic Day 14, whereas perlecan transcripts were virtually undetectable until fetal Day 19. Therefore, to evaluate further the relationship between cell replication and perlecan and/or TE gene expression, we used a combined in situ hybridization/BrdU immunohistochemistry technique and demonstrated that, on an individual cell basis, perlecan message was predominantly expressed by nonreplicating (BrdU-negative) PA, whereas TE mRNA was equally expressed in replicating and nonreplicating PA SMCs. Interestingly, a very similar pattern of replication and relationship to perlecan and TE mRNA expression was noted in airway SMCs and epithelial cells. Thus, in the lung as a whole, maximal expression of both the BM protein perlecan and the interstitial matrix protein TE occurs coordinately and follows the period of maximal SMC proliferation. However, in individual SMCs, perlecan mRNA expression varies inversely with DNA synthesis, whereas TE mRNA expression appears independent of the proliferative state of the cell.

Tetraspanin CD9 is associated with very late-acting integrins in human vascular smooth muscle cells and modulates collagen matrix reorganization

CD9, a member of the tetraspanin family, and very late-acting (VLA) integrins are known to associate and form functional units on the surface of several cell types. We studied the changes in expression of CD9 and beta1-integrins (CD29, VLA) in human vascular smooth muscle cells (VSMCs) under in vitro culture conditions mimicking proliferative vascular diseases. We also investigated possible interactions between CD9 and VLA integrins in VSMCs. We found that CD9 is highly expressed in VSMCs and is subject to modulation, depending on the proliferative/contractile state of the cells. In the contractile phenotype, the levels of CD9, CD81, another tetraspanin, and CD29 are approximately 50% of those found in the proliferative phenotype. Coimmunoprecipitation experiments showed physical association between CD9 and CD29. CD9 was mainly associated with alpha2 and alpha3-integrins (CD49b and c) and also with alpha5-integrin to a weaker extent. Functionally, the addition of anti-CD9 monoclonal antibodies (MoAbs) doubled the extent of collagen gel contraction mediated by VSMCs, a model for the reorganization of the extracellular collagen matrix occurring in the vessel wall. Anti-CD29 MoAbs inhibited gel contraction, but anti-CD9 MoAbs counteracted this inhibitory effect of anti-CD29 MoAbs. Transfection of human CD9 into Chinese hamster ovary cells more than doubled the extent of Chinese hamster ovary cell-mediated collagen gel contraction (130% stimulation), confirming a role for CD9 in extracellular matrix reorganization. Thus, CD9 seems to be involved in the modulation of VLA integrin-mediated collagen matrix reorganization by VSMCs. These findings suggest that high CD9 expression is associated with a proliferative state of VSMCs. The role of CD9 could be to modulate the function of VLA integrins on the surface of VSMCs.

Onset of elastogenesis and downregulation of smooth muscle actin as distinguishing phenomena in artery differentiation in the chick embryo

During development, the arterial system is grossly divided into elastic and muscular vessel types. Apart from local environmental factors, it has been suggested that vascular smooth muscle cell origin (mesoderm or neural crest) is involved in this, as yet poorly understood, arterial differentiation. We describe differentiation of the thoracic arterial system in the chick embryo, using immunohistochemical techniques staining for muscle-specific actin, vinculin and desmin and histological staining to visualise elastin. The initial developmental stages of the vessel wall in all arteries appeared to be highly similar, with all arteries showing peri-endothelial actin and vinculin staining. Major alterations did not occur until the start of elastogenesis, which coincided with complete loss of actin staining from the proximal part of the great arteries. Later in development, however, actin was re-expressed in a subpopulation of medial cells, which also expressed vinculin and desmin. Concomitantly another, nonmuscular, cell type became evident in the great arteries. Transient loss of actin expression and segregation of very distinct cell populations occurred only in vessels prone to elastic development and known to receive a neural crest contribution. In contrast, arteries that developed a muscular phenotype never lost the initially acquired peri-endothelial actin expression. We also show a significant difference in the organisation of elastic fibres between elastic vessels that contain neural crest derivatives and those that do not. The ductus arteriosus still presents as an enigma in the sense that it is the only part of the pharyngeal arch complex that develops a muscular phenotype.

Tropoelastin gene expression in individual vascular smooth muscle cells. Relationship to DNA synthesis during vascular development and after arterial injury

After vascular injury, quiescent adult smooth muscle cells (SMCs) revert to a more immature synthetic-state phenotype concomitant with the onset of cell replication. The relationship between SMC proliferation and the reexpression of genes characteristic of immature SMCs (eg,tropoelastin [TE]), on an individual cell basis, has not been determined. Using a combined bromodeoxyuridine (BrdU) immunocytochemistry-TE in situ hybridization technique, we determined the relationship between DNA synthesis and TE gene expression in the rat vascular wall during development of the aortic media (embryonic days 13 to 18), low but detectable levels of TE expression occurred equally in both replicating and nonreplicating SMCs. TE message levels dramatically increased in the late fetal and early postnatal periods (fetal day 19 to 1 month postpartum), after a precipitous drop in SMC replication, and then decreased to undetectable levels by postpartum day 60. After a balloon catheter injury in the adult, a developmental sequence of SMC replication followed by TE gene expression was reiterated in both the media and in the developing neointima. On an individual cell basis, adult SMCs replicating after injury expressed little or no TE message; detectable TE gene expression occurred only in nonreplicating SMCs. The most important implications of these data are that (1) adult SMCs replicating after injury appear to revert to a pre-elastogenic embryonic phenotype; (2) maximal TE expression occurs in SMCs only after the cessation of cell replication; and (3) in both the media and the neointima, adult SMCs responding to injury undergo temporarily sequential changes in phenotype reflective of SMC development.

Decreased elastin synthesis in normal development and in long-term aortic organ and cell cultures is related to rapid and selective destabilization of mRNA for elastin

We have previously shown that aortic organ cultures from 1- to 3-day-old chickens initially mimic the high levels of elastin production seen in vivo. However, more prolonged incubation of these tissues results in decreased synthesis of elastin. In the present study, we demonstrate that decreased production of elastin in these aortic organ cultures is selective for elastin compared with collagen and is correlated with decreased steady state levels of mRNA for elastin. These decreases in steady state levels of elastin mRNA are due at least in part to a rapid and selective destabilization of mRNA for elastin, the half-life of which falls from approximately 25 hours in fresh aortic tissues to approximately 15 hours after incubation for only 8 hours. Destabilization of elastin mRNA can be prevented by incubation in the presence of blockers of DNA transcription (5,6-dichlorobenzimidazole riboside and actinomycin D) and mRNA translation (cycloheximide). Furthermore, the half-life of aortic elastin mRNA decreases from approximately 25 hours in the 1-day-old chicken to approximately 7 hours in the 8-week-old chicken, demonstrating that destabilization of mRNA is an important contributing factor in the decline in production of aortic elastin taking place during normal postnatal growth.

Multiple phenotypically distinct smooth muscle cell populations exist in the adult and developing bovine pulmonary arterial media in vivo

Different smooth muscle cell (SMC) functions may require different cell phenotypes. Because the main pulmonary artery performs diverse functions, we hypothesized that it would contain heterogeneous SMC populations. If the hypothesis were confirmed, we wished to determine the developmental origin of the different populations. Using specific antibodies, we analyzed the expression of smooth muscle (SM) contractile and cytoskeletal proteins (alpha-SM-actin, SM myosin, calponin, desmin, and meta-vinculin) in the main pulmonary artery of fetal (60 to 270 days of gestation), neonatal, and adult animals. We demonstrated the existence of a complex, site-specific heterogeneity in the structure and cellular composition of the pulmonary arterial wall. We found that at least four cell/SMC phenotypes, based on immunobiochemical characteristics, cell morphology, and elastic lamellae arrangement pattern, were simultaneously expressed within the mature arterial media. Further, we were able to assess phenotypic alterations in each of the four identified cell populations during development. We found that each cell population within the arterial media expressed alpha-SM-actin at least at certain stages of development, thus demonstrating its smooth muscle identity. However, each cell population progressed along different developmental pathways, suggesting the existence of multiple and distinct cell lineages. A novel anti-metavinculin antibody described in this study reliably distinguished one SMC population from the others during all the developmental stages analyzed. We conclude that the pulmonary arterial media is indeed composed of multiple phenotypically distinct cell/SMC populations with unique lineages. We speculate that these distinct cell populations may serve different functions within the arterial media and may also respond in unique ways to pathophysiological stimuli.

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

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

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.

Phenotypic stability and variation in cells of the porcine aorta: collagen and elastin production

The extracellular matrix of the developing vasculature varies in composition as a function of time and position. Cellular models of vascular biology and pathology depend on the assumption that stable phenotypic characteristics of vascular cells can be propagated through several generations of in vitro cultivation. We show that the positional and developmental heterogeneity of matrix phenotypes in the porcine aorta are expressed by explanted vascular smooth muscle cell (SMC) and adventitial cell populations for a limited number of passages. Elastin was expressed most highly by thoracic SMC while interstitial collagen production was usually maximal in abdominal segments. Parallel gradients of collagen types I, III and V, detected by specific ELISA assays, were expressed in early-passage SMC. Adventitial cell populations from the abdominal aorta of the neonatal pig accumulated significant levels of collagen, while these fibroblasts produced less than 10% of the elastin made by SMC. All cell populations expressed alpha-smooth muscle actin in vitro. Gradients of collagen and elastin expression were evident for no more than three passages, and direct outgrowth of cells without limited digestion of the matrix further reduced phenotypic stability. Variation and decline of the elastin phenotype could be due to hypermethylation of regulatory sequences in the elastin gene or trans-acting factors, but elastin production was dose-dependently stimulated to a similar extent (100%; 10 microM 5-azacytidine) in all segmental SMC populations at early (p1) and late (p3) passage. These data indicated that faithful reflection of in vivo SMC behavior was limited to a few population doublings, at least under standard culture conditions. Modification of the cellular environment by reducing serum factors, changing matrix, or adding mechanical stimulation may increase phenotypic stability.

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.

Stability of elastin in the developing mouse aorta: a quantitative radioautographic study

Elastic lamina growth during development and the ultimate stability of elastin in the mouse aortic media was investigated by light and electron microscopic radioautography. Following a single subcutaneous injection of L-[3,4-3H]valine at 3 days of age, animals were killed at 9 subsequent time intervals up to 4 months of age. One day after injection, radioautographic silver grains were primarily observed over the elastic laminae; however, silver grains were also seen over the smooth muscle cells and extracellular matrix. By 21 to 28 days of age, the silver grains were almost exclusively located over the elastic laminae. From 28 days to 4 months of age, the distribution of silver grains appeared relatively unchanged. Quantitation of silver grain number/micron2 of elastin showed a steady decrease in the concentration of silver grains associated with the elastic laminae from 4 to 21 days of age. After this time, no significant difference in silver grain concentration was observed. Since the initial decrease in grains/micron2 of elastin corresponds to a period of rapid post-natal growth, the decrease is likely to be a result of dilution of the radiolabel due to new elastin synthesis. With the assumption that little or no significant turnover occurs during this time, a constant growth rate of 4.3% per day was predicted by linear regression analysis. Since no significant difference in the concentration of silver grains was observed from 28 to 118 days of age, no new growth or turnover of elastin can be said to occur during this time period. This is supported by the observation that animals injected with radiolabeled valine at 28 days and 8 months of age showed no significant incorporation of radiolabel into the elastic laminae. The results from this study present the first long-term radioautographic evidence of the stability of aortic elastin and emphasize that initial deposition of elastin and proper assembly of elastic laminae is a critical event in vessel development.

Effects of physical exercise on the elasticity and elastic components of the rat aorta

To evaluate the effects of exercise on aortic wall elasticity and elastic components, young male rats underwent various exercise regimes for 16 weeks. In the exercised rats, the aortic incremental elastic modulus decreased significantly when under physiological strain. The aortic content of elastin increased significantly and the calcium content of elastin decreased significantly in the exercised group. The accumulated data from the exercised and sedentary groups revealed that the elastin calcium content was related positively to the incremental elastic modulus. We concluded that physical exercise from an early age decreases the calcium deposit in aortic wall elastin and that this effect probably produced in the exercised rats a distensible aorta.

Regulation of the human elastin promoter in chick embryo cells. Tissue-specific effect of TGF-beta

The function of the 5' flanking region of the human elastin gene on transcription regulation has been investigated in chick embryo aorta cells by transient DNA transfer experiments with elastin-chloramphenicol acetyltransferase (CAT) fusions. The results have shown that the region comprised within -129 and -12 bp from the translation start site is essential for transcription and probably contains different control sequences. Expression of the reporter gene was increased 2-4-fold by addition of TGF-beta to the cell cultures. Analysis of CAT expression from different deletion constructs suggests that sequences in the region -196 to -12 play a major role in TGF-beta induction. The stimulating effect of the growth factor could not be observed when transfections were performed with chick embryo tendon fibroblasts. This suggests that transcriptional regulation of elastin by TGF-beta is tissue specific.

Relative efficiency of incorporation of newly synthesized elastin and collagen into aorta, pulmonary artery and pulmonary vein of growing pigs

It is generally accepted that, once laid down in the extracellular matrix, elastin turns over only very slowly if at all. However, much of the evidence for minimal turnover of elastin comes from aortic tissue. In this study we have compared the relative synthesis rates of elastin and collagen with their relative accumulation rates in segments of aorta (AO), pulmonary artery (PA), and pulmonary vein (PV) of young, growing pigs. While rates of elastin synthesis are comparable in the AO and PA, the PA accumulates only 33% of the elastin deposited in the AO. Similarly, while the rate of elastin synthesis in the PV is 60% of that in the AO, accumulation of elastin is only 20% of that in the AO. Similar discrepancies between collagen synthesis and accumulation were seen among these three vessels. These data suggest that, at least in growing animals, the efficiency of permanent incorporation of elastin and collagen into the extracellular matrix may be an important factor in determining the final contents of these connective tissue proteins in vascular tissues.

Role of the extracellular matrix in age-related modifications of the rat aorta. Ultrastructural, morphometric, and enzymatic evaluations

Connective tissues such as blood vessels are known to be greatly affected by age because of impaired functional properties and increased susceptibility to diseases. With the aim of providing further information on the role of the extracellular matrix in age-related modifications, we investigated the aorta in the rat model from birth to senescence by means of morphological and morphometric observations and by evaluation of lysyl oxidase activity. Results focused on the dramatic vascular rearrangements due to progressive fibrosis of the extracellular matrix and on prominent elastin modifications. The presence of lysyl oxidase activity, even in the oldest animals, might be at least partly responsible for the increased stiffness of the aging extracellular matrix. The striking age-related remodeling of the aortic architecture and the alterations of the interactions between cellular and extracellular compartments might greatly influence the functional properties of the arterial wall in senescence, at least contributing to the consequences of some apparently age-related vascular disorders.

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.

Changes of cellular expression of mRNA for tropoelastin in the intraembryonic arterial vessels of developing chick revealed by in situ hybridization

The pattern of expression of tropoelastin mRNA in the arterial tree of developing chick has been studied by in situ hybridization. Significant hybridization was noted in 5.5-day embryos in the region of the truncus arteriosus where aorta and pulmonary artery had newly separated. The activation of expression then propagated centrifugally and longitudinal gradients of mRNA decreasing from the heart to the periphery were established. For almost two-thirds of the embryonic period, the hybridization signal was rather uniform over the entire wall of the arterial vessels. Later, however, its distribution varied depending on the type of artery (elastic or muscular) and on the developmental stage. A radial gradient of tropoelastin mRNA expression decreasing in the in-out direction was formed in elastic arteries. This was first seen in the pulmonary artery (15-day chick embryos) and became detectable in the vessels of the general circulation only much later (2 weeks after hatching). The appearance of the radial gradient was followed by a general reduction of mRNA synthesis. In muscular arteries radial gradients were also established, but had, however, an opposite polarity; in small arteries a ring of hybridization was evident at the media-adventitia border. The results indicate that the expression of the tropoelastin gene in cells of the arterial wall is finely regulated, depending on the coordinates in the arterial tree, the type of artery and the organ supplied.

Rapid accumulation of elastin and collagen in the aortas of sheep in the immediate perinatal period

While characterizing developmental changes in aortic wall composition in sheep, we observed very rapid accumulation of elastin and collagen in the immediate perinatal period. Thoracic aortic elastin content increased by 41%, and collagen content increased by 49% in approximately 1 week, between 140 days gestation and 3 days postpartum (term = 145 days). Even larger changes were observed in the abdominal aorta. Elastin content increased by 66%, and collagen increased by 57%. The pronounced increase in wall tissue accumulation near birth preceded a marked postnatal increase in arterial pressure. We propose that this elastin and collagen accumulation is a preadaptive response in preparation for the later increase in pressure. The prenatal and postnatal events that initiate this synthesis and accumulation are not known. We also found that, in the 3 weeks after this initial rapid increase, accumulation of elastin and collagen was markedly reduced in the abdominal, but not the thoracic, aorta. This latter finding may be linked to the dramatic decrease in flow through this vessel that results from the loss of the placental circulation. Finally, we observed that relatively high smooth muscle cell replication rates in the abdominal aorta postpartum resulted in no net DNA accumulation. This finding indicates that cell turnover plays an important role in postnatal arterial growth and development.

Pattern of accumulation of elastin and the level of mRNA for elastin in aortic tissue of growing chickens

Synthesis and accumulation of elastin in many elastic tissues begins in the last third of fetal development, reaches a maximum shortly after birth, and then declines rapidly. For the aorta of the chick and the pig and the ligamentum nuchae and lung of the sheep, it has been shown that increased levels of elastin production with fetal development are correlated with increased levels of elastin mRNA in the tissue, measured both by cell-free translation and by hybridization to cDNA probes. In this study we examine the relationship between insoluble elastin accumulation and message levels for tropoelastin in aortic tissue of chickens during posthatching development and growth. Whether evaluated by cell-free translation or by dot blot hybridization, steady state levels of tropoelastin message increase to a maximum at 2 weeks after hatching, and then fall rapidly with further development and growth. This pattern correlates well with production of insoluble elastin by the aorta, determined either by direct measurements of synthesis or by rate of accumulation of insoluble elastin. The data indicate that the major site of regulation of elastin production is pretranslational throughout the entire period of development and growth of the chicken aorta.

Effect of static stretching on elastin production by porcine aortic smooth muscle cells

An in vitro model was developed to assay the effects of static stretching on soluble elastin (tropoelastin, TE) synthesis by porcine aorta smooth muscle cells (SMC). Culture dishes containing SMC adherent to the deformable bottoms were placed over hard convex templates. Stress was applied by placing a weight on the dish covers. Measurement of TE was accomplished by a specific ELISA assay. With this model we demonstrated reproducible stimulation of TE synthesis by stretched SMC. Significant results (161.4% of control; p = less than 0.003 by Student's t-test) were obtained by stretching passage 2 SMC for 3 days with a medium change after the first 18-24 hours, use of 5% newborn calf serum in cultures during stretching, and 50-g weight. DNA content in stretched cultures did not increase over control values. Thus, stretching alone did not cause hyperplasia or hyperploidy in these SMC and, in the absence of other vascular cell elements, was sufficient to increase production of this extracellular matrix protein. Transduction of mechanical force into elastin gene expression by SMC may contribute to the development of thickened arterial tunica media characteristic of hypertensive vessels.

Quantitation of elastin in tissues and culture: problems related to the accurate measurement of small amounts of elastin with special emphasis on the rat

Both rat and sheep elastin can be quantified by measurement of discrete peptides released from the insoluble protein by thermolysin digestion. These peptides are easily visualized and measured by HPLC. With the sheep the tallest peak on the chromatogram represents the VGVPG pentapeptide derived from a repeating sequence seen in elastin from many species. This repeating sequence allows for amplification of the signal significantly above background so that accurate quantitation can be carried out. The measurement is reproducible over a wide range of protein concentrations. With the rat however the pentapeptide is not present but appears to be replaced by other repeating sequences. We quantitated and determined amino acid sequence on 8 peaks present in the early portion of the chromatogram for purposes of quantifying rat elastin. That signal most reliably present over a range of concentrations was tyrosyl-glycine (YG) which eluted at 8.5 minutes. We have used YG as a basis for quantitation of rat elastin both from tissues and tissue culture. We have also shown that the desmosine crosslinks are not constant in elastin produced in a neonatal rat smooth muscle culture system but vary with the age of the culture. We thus propose that an index of maturation be considered for a given elastin in the form of mumoles of crosslink per gram of elastin so as to better define its quality.

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

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

Development of the aorta in the chick embryo: structural and ultrastructural study

A structural and ultrastructural study was designed to analyze systematically the cellular events which take place in the aortic wall between days 7 and 21 of chick embryo development. Between days 7 and 18, increase in total diameter, number of cell layers, and aortic wall thickness are highly correlated, whereas between days 18 and 21 the total diameter increase is correlated mainly with an increase in vessel lumen diameter. Cell layers of smooth muscle cells showing an immature or synthetic phenotype arise from progressive association and organization of mesenchymal cells originated from an endothelial activation process in which a hyaluronic acid-rich extracellular matrix seems to be involved. It is suggested that the process of endothelial activation takes place between days 7 and 18 of embryonic development provided that within that period the typical cellular events which are involved in such a process take place (hypertrophy, reorientation, invagination, mitotic activity, acquisition of migratory appendages, endothelial detachment and incorporation into adjacent spaces). This endothelial activation has been recognized as a selective multiphasic process required for the transition of endothelial cells into mesenchyma.

Terminal differentiation of nuchal ligament fibroblasts: characterization of synthetic properties and responsiveness to external stimuli

The temporal expression of elastogenesis is unique among connective tissues in that elastin production occurs primarily during late fetal and early neonatal periods and is essentially fully repressed once fiber assembly is completed. To test whether elastin synthesis in adult nuchal ligament fibroblasts is permanently repressed or whether the cells retain the ability to reinitiate production upon proper stimulation, we examined in adult ligament cells various parameters known to be involved in the regulation of elastin production. Elastin synthetic capacity, as determined by the levels of steady-state tropoelastin mRNA, of adult tissue was significantly decreased relative to fetal tissue. Likewise, fibroblasts grown from explants of adult ligament had about a fourfold decrease in elastin production and elastin-specific mRNA levels. On the other hand, adult cells were similar to fetal ligament cells in that they were sensitive to glucocorticoid stimulation and demonstrated chemotactic responsiveness to elastin peptides. Since our previous studies have shown that the extracellular matrix (ECM) plays an important role in influencing elastin phenotypic expression, fetal and adult fibroblasts were grown on slices of nonviable adult ligament to test if repression of elastin production was directed by factors in ECM of adult tissues. No change in elastin synthesis was detected with either cell type grown on adult ligament, whereas both fetal and adult cells demonstrated increased elastin production in response to contact with fetal ligament. These results suggest that adult ligament ECM does not provide a metabolic signal to shut off the elastin gene and that adult cells remain responsive to external stimuli that may reinitiate high levels of elastin synthesis.

The elastogenic effect of recombinant transforming growth factor-beta on porcine aortic smooth muscle cells

Transforming growth factor-beta, a peptide growth factor, is known to be a multifunctional regulator of cellular activity. The effect of this growth factor on extracellular matrix formation is well established, but its effects on elastin, a critical component of lung, skin, and blood vessels are unknown. In the present study, by use of an Enzyme-Linked Immunoassay method, we found that transforming growth factor-beta strongly increased elastin production in cultured porcine aortic smooth muscle cells. In a dosage-dependent study, 1.0-10.0 ng/ml transforming growth factor-beta promoted elastin production 2-3 fold. In a time-dependent study, at least an 8 h pre-treatment with 10.0 ng/ml transforming growth factor-beta was required for sustained increases in elastin production. The effects of transforming growth factor-beta on cultured aortic smooth muscle cells suggest that this cytokine may be an important mediator of elastin formation during atherosclerosis and hypertension.