An in vitro model of aneurysmal disease: effect of leukocyte infiltration and shear stress on MMP production within the arterial wall

Impact of pre-existing elastic matrix on TGFβ1 and HA oligomer-induced regenerative elastin repair by rat aortic smooth muscle cells

Regenerating elastic matrices lost to disease (e.g. in aneurysms) is vital to re-establishing vascular homeostasis but is challenged by the poor elastogenicity of post-neonatal cells. We previously showed that exogenous hyaluronan oligomers (HA-o) and TGFβ1 synergistically enhance tropo- and matrix elastin deposition by healthy adult rat aortic SMCs (RASMCs). Towards treating aortic aneurysms (AAs), which exhibit cause- and site-specific heterogeneity in matrix content/structure and contain proteolytically-injured SMCs, we investigated the impact of pre-existing elastic matrix degeneration on elastogenic induction of injured RASMCs. Elastin-rich RASMC layers at 21 days of culture were treated with 0.15 U/ml (PPE15) and 0.75 U/ml (PPE75) porcine pancreatic elastase to degrade the elastic matrix variably, or left uninjured (control). One set of cultures was harvested at 21 days, before and after injury, to quantify viable cell count, matrix elastin loss. Other injured cell layers were cultured to 42 days with or without factors (0.2 µg/ml HA oligomers, 1 ng/ml TGFβ1). We showed that: (a) the ability of cultures to self-repair and regenerate elastic matrices following proteolysis is limited when elastolysis is severe; (b) HA oligomers and TGFβ1 elastogenically stimulate RASMCs in mildly-injured (i.e. PPE15) cultures to restore both elastic matrix amounts and elastic fibre deposition to levels in healthy cultures; and (c) in severely injured (i.e. PPE75) cultures, the factors stimulate matrix elastin synthesis and crosslinking, although not to control levels. The outcomes underscore the need to enhance elastogenic factor doses based on the severity of elastin loss. This study will help in customizing therapies for elastin regeneration within AAs, based on cause and location.

Experimental models of abdominal aortic aneurysms

Despite being a leading cause of death in the West, the pathophysiology of abdominal aortic aneurysms (AAA) is still incompletely understood. Pharmacotherapy to reduce the growth of small AAAs is limited and techniques for repairing aneurysms continue to evolve. Experimental models play a key role in AAA research, as they allow a detailed evaluation of the pathogenesis of disease progression. This review focuses on in vivo experimental models, which have improved our understanding of the potential mechanisms of AAA development and contributed to the advancement of new treatments.

Loss of elastic fiber integrity and reduction of vascular smooth muscle contraction resulting from the upregulated activities of matrix metalloproteinase-2 and -9 in the thoracic aortic aneurysm in Marfan syndrome

Thoracic aortic aneurysm (TAA) is the life-threatening complication of Marfan syndrome (MFS), a connective tissue disorder caused by mutations in the fibrillin-1 gene. TAA is characterized by degradation of elastic fiber, suggesting the involvement of matrix metalloproteinase (MMP)-2 and -9, the activation of which is regulated by TIMP (tissue inhibitor of MMP) types 1 and 2. We hypothesized that MMP-2 and -9 were upregulated during TAA formation in Marfan syndrome, causing loss of elastic fibers and structural integrity. We studied mice, from 3 to 12 months, heterozygous for a mutant Fbn1 allele encoding a cysteine substitution in fibrillin-1 (Fbn1(C1039G/+), designated as "Marfan" mice) (n=120), the most common class of mutation in Marfan syndrome. The littermates, Fbn1(+/+) served as controls (n=120). In Marfan aneurysmal thoracic aorta, mRNA and protein expression of MMP-2 and -9 were detected at 3 months and peaked at 6 months of age, accompanied by severe elastic fiber fragmentation and degradation. From 3 to 9 months, the MMP-2/TIMP-2 ratio increased by 43% to 63% compared with the controls. Dilated thoracic aorta demonstrated increased elasticity but distention caused a pronounced loss of contraction, suggesting weakening of the aortic wall. Breaking stress of the aneurysmal aorta was 70% of the controls. Contraction in response to depolarization and receptor stimulation decreased in the aneurysmal thoracic aorta by 50% to 80%, but the expression of alpha-smooth muscle actin between the 2 strains was not significantly different. This report demonstrates the upregulation of MMP-2 and -9 during TAA formation in Marfan syndrome. The resulting elastic fiber degeneration with deterioration of the aortic contraction and mechanical properties may explain the pathogenesis of TAA.

A murine model of thoracic aortic aneurysms

BACKGROUND

The mechanisms of thoracic aortic aneurysm (TAA) formation are poorly understood, mainly due to the lack of a useful and reproducible model. Accordingly, the goal of this study was to test the hypothesis that abluminal calcium chloride (CaCl(2)) application could create TAAs in the mouse.

MATERIALS AND METHODS

Adult 129/SvE mice (n = 8) were anesthetized and their thoracic aortas exposed via left thoracotomy. CaCl(2) (0.5M) was applied to the distal descending thoracic aorta for 15 min followed by chest closure. At 4 weeks, the perfusion-fixed aorta was harvested from the root to the renal arteries. Diameter measurements were made using confocal microscopy, and wall thickness was measured from hematoxylin and eosin-stained sections.

RESULTS

The control (n = 15) distal descending thoracic aortic diameter was 0.60 +/- 0.04 mm and increased by 25% (0.76 +/- 0.06 mm, P < 0.05) following CaCl(2) treatment. Control aortic wall thickness was 48 +/- 9 mum and decreased by 47% in corresponding CaCl(2)-exposed segments (25 +/- 8 mum, P < 0.05). The diameter and wall thickness of the ascending aorta (used as an internal control) were not significantly different between groups. Picrosirius red staining of the TAA showed adventitial collagen breakdown and disruption of lamellar organization.

CONCLUSIONS

We conclude that abluminal application of CaCl(2) to the thoracic aorta reliably produces dilation, wall-thinning, and disruption of mural architecture, the hallmark signs of aneurysm formation. To our knowledge, these findings describe for the first time the generation of a reproducible model of isolated TAA formation in a murine system.

Review: Clinical aspects of vascular remodeling

Vascular remodeling represents a spectrum of structural changes whereby the vascular wall responds to changes in its hemodynamic environment. Such changes may be classified as vessel enlargement (outward remodeling), diminution (inward remodeling), alternatively as adaptive (compensatory, appropriate to the hemodynamic stimulus), or maladaptive (dysfunctional, inappropriate). The direction and scale of remodeling are coordinated by endothelial production of growth factors, proteases, and cellular adhesion molecules in response to sensed changes in blood flow. In early atherosclerosis, outward remodeling preserves lumen size. Although protective in the long-term, the matrix degradation involved in this process may predispose atherosclerotic plaques to rupture, hence increasing the risks of acute coronary syndromes. Inward remodeling also occurs in advanced atherosclerotic lesions, whereby the vessel shrinks rather than enlarging, exacerbating rather than ameliorating stenosis. In transplant coronary artery disease, early inward remodeling may be a more important component of vessel stenosis than intimal thickening, while inappropriate inward remodeling appears to be as least as important as excessive intimal growth in the development of restenosis after angioplasty. Increased awareness of vascular remodeling, and in particular its malaptive forms, may provide new therapeutic insights for the future.

Production and inhibition of the gelatinolytic matrix metalloproteinases in a human model of vein graft stenosis

OBJECTIVES

human vein graft stenoses are caused by intimal hyperplasia, a process which is characterised by extensive degradation and accumulation of extracellular matrix. This study investigated the role of the matrix metalloproteinases (MMPs) - the principal physiological mediators of extracellular matrix degradation - in the development of intimal hyperplasia in cultured human long saphenous vein.

DESIGN

experimental study.

MATERIALS AND METHODS

paired venous segments with the endothelium intact or denuded were cultured in standard conditions for 14 days. At the termination of culture, MMPs were extracted from one half of the tissue, whilst the remainder of the vein was prepared for histological examination.

RESULTS

stereologic analysis revealed that the endothelium intact veins developed a significantly thicker neointima when compared to the denuded venous segments (20 micron v. 0 micron, p=0.006). Quantification of MMPs by substrate gel enzymography demonstrated that the development of a neointima was associated with increased production of the gelatinolytic MMP-9 (p=0. 03) in intact veins. Immunocytochemistry showed that the MMP-9 localised to the internal elastic lumina, which suggested a role in facilitating smooth-muscle-cell migration into the intima. The role of MMPs-2 and -9 in intimal hyperplasia was further investigated by culturing intact venous segments with a therapeutic concentration of doxycycline--a potent MMP inhibitor. These experiments demonstrated that a therapeutic dose of doxycycline significantly reduced neointimal thickness (control 21 micron, doxycycline 10 mg/l-5.5 micron), in conjunction with a significant reduction in the production of MMP-9.

CONCLUSIONS

these data suggest that elevated levels of MMPs may play a significant role in the development of human intimal hyperplasia and that inhibition of these enzymes may offer a potential therapeutic strategy for the prevention of hyperplastic lesions.

Doxycycline inhibits elastin degradation and reduces metalloproteinase activity in a model of aneurysmal disease

PURPOSE

Abdominal aortic aneurysms are characterized by degradation of the extracellular matrix, with a reduction in the elastin concentration of the arterial media. These changes are mediated by increased levels of endogenous metalloproteinases (MMPs) within the aorta, which provide a potential therapeutic target for pharmacologic agents aimed at reducing the growth rate of small aneurysms. In this study, the ability of doxycycline--an MMP inhibitor--to reduce matrix degradation was assessed in a previously described model of aneurysmal disease that used a brief pulse of elastase to induce MMP production and elastin degradation in arterial organ cultures.

METHODS

Porcine aortic segments (n = 8) were preincubated in exogenous pancreatic elastase for 24 hours before culture in standard conditions for 13 days with both 1 and 10 mg/L doxycycline. Control segments were cultured both without doxycycline and without elastase. At the termination of culture, MMPs were extracted from the tissue and quantified by a combination of substrate gel enzymography and immunoblotting. The volume fractions of elastin and collagen were determined by stereologic analysis of sections stained with Miller's elastin and van Gieson's stain.

RESULTS

Stereologic analysis demonstrated a significant preservation of elastin in aorta treated with doxycycline 10 mg/L (p < 0.001) and demonstrated that this preservation was accompanied by a significant reduction in MMP-9 activity (p < 0.02). Immunoblotting for tissue inhibitors of metalloproteinases (TIMP-1 and TIMP-2) showed no decreased production in the doxycycline-treated groups.

CONCLUSIONS

Therapeutic ranges of doxycycline significantly inhibited elastin degradation and MMP-9 production within aortic organ cultures. These data suggest that doxycycline may have a potential application in reducing the growth rates of small abdominal aortic aneurysms.