The pathobiology of aortic aneurysms

Effect of aortic bifurcation geometry on pressure and peak wall stress in abdominal aorta: Fluid-structure interaction study

BACKGROUND AND OBJECTIVE

Geometry of aorto-iliac bifurcation may affect pressure and wall stress in aorta and thus potentially serve as a predictor of abdominal aortic aneurysm (AAA), similarly to hypertension.

METHODS

Effect of aorto-iliac bifurcation geometry was investigated via parametric analysis based on two-way weakly coupled fluid-structure interaction simulations. The arterial wall was modelled as isotropic hyperelastic monolayer, and non-Newtonian behaviour was introduced for the fluid. Realistic boundary conditions of the pulsatile blood flow were used on the basis of experiments in literature and their time shift was tailored to the pulse wave velocity in the model to obtain physiological wave shapes. Eighteen idealized and one patient-specific geometries of human aortic tree with common iliac and renal arteries were considered with different angles between abdominal aorta (AA) and both iliac arteries and different area ratios (AR) of iliac and aortic luminal cross sections.

RESULTS

Peak wall stress (PWS) and systolic blood pressure (SBP) were insensitive to the aorto-iliac angles but sensitive to the AR: when AR decreased by 50%, the PWS and SBP increased by up to 18.4% and 18.8%, respectively.

CONCLUSIONS

Lower AR (as a result of the iliac stenosis or aging), rather than the aorto-iliac angles increases the BP in the AA and may be thus a risk factor for the AAA development.

A quarter of a century biomechanical rupture risk assessment of abdominal aortic aneurysms. Achievements, clinical relevance, and ongoing developments

Abdominal aortic aneurysm (AAA) disease, the local enlargement of the infrarenal aorta, is a serious condition that causes many deaths, especially in men exceeding 65 years of age. Over the past quarter of a century, computational biomechanical models have been developed towards the assessment of AAA risk of rupture, technology that is now on the verge of being integrated within the clinical decision-making process. The modeling of AAA requires a holistic understanding of the clinical problem, in order to set appropriate modeling assumptions and to draw sound conclusions from the simulation results. In this article we summarize and critically discuss the proposed modeling approaches and report the outcome of clinical validation studies for a number of biomechanics-based rupture risk indices. Whilst most of the aspects concerning computational mechanics have already been settled, it is the exploration of the failure properties of the AAA wall and the acquisition of robust input data for simulations that has the greatest potential for the further improvement of this technology.

Homocysteine-Enhanced Proteolytic and Fibrinolytic Processes in Thin Intraluminal Thrombus and Adjacent Wall of Abdominal Aortic Aneurysm: Study In Vitro

Homocysteine (Hcy) may affect the pathogenesis of abdominal aortic aneurysms (AAAs) through enhancement of proteolysis and an impaired coagulation/fibrinolysis system. Intensified haemostatic capacity may promote local proteolytic degradation of the aortic wall. This study aimed to examine the effects of Hcy on haemostatic and proteolytic processes in samples of thick and thin fragments of the ILT and underlying walls. Subjects and Methods. Thirty-six patients who underwent AAA surgery were enrolled. Aneurysm tissue sections were incubated with DL-Hcy (100 and 500 μmol/L) in a series of experiments and analyzed for concentration/activity of proteolytic and haemostatic markers by enzyme-linked immunosorbent assay. Results. Incubation of wall underlying thin ILT segments (B) with DL-Hcy resulted in an increase of active MMP-2 levels compared to control tissue (9.54 ± 5.88 versus 7.44 ± 4.48, p=0.011). DL-Hcy also induced t-PA and plasminogen concentration increases in thin thrombus sections (B1) compared to control tissue (respectively: 1.39 ± 1.65 versus 0.84 ± 0.74, p=0.024; 11.64 ± 5.05 versus 10.34 ± 5.52, p=0.018). In contrast, wall adjacent to thick thrombus segments (A) showed decreases in MMP-2 and TF activities compared to control (respectively, 5.89 ± 3.39 versus 7.26 ± 5.49, p=0.046; 67.13 ± 72.59 versus 114.46 ± 106.29, p=0.007). In thick ILT sections (A1), DL-Hcy decreased MMP-2 activity and t-PA and plasminogen concentrations compared to control tissue (respectively, 2.53 ± 2.02 versus 3.28 ± 2.65, p=0.006; 0.67 ± 0.57 versus 0.96 ± 0.91, p=0.021; 9.25 ± 4.59 versus 12.63 ± 9.56, p=0.017). In addition, analysis revealed positive correlations at all sites between activities/concentrations of MMP-2, TF, and PAI-1 measured in control tissues and after incubation with DL-Hcy. Conclusions. These data indicate the potential for excess Hcy to enhance damage of arterial wall in thinner AAA segments as a result of the increased activity of MMP-2 and fibrinolytic factors.

Dilating Vascular Diseases: Pathophysiology and Clinical Aspects

Atherosclerotic disease of the vessels is a significant problem affecting mortality and morbidity all over the world. However, dilatation of the vessels either in the arterial system or in the venous territory is another vessel disease. Varicocele, pelvic, and peripheral varicose veins and hemorrhoids are aneurysms of the venous vascular regions and have been defined as dilating venous disease, recently. Coronary artery ectasia, intracranial aneurysm, and abdominal aortic aneurysm are examples of arterial dilating vascular diseases. Mostly, they have been defined as variants of atherosclerosis. Although there are some similarities in terms of pathogenesis, they are distinct from atherosclerotic disease of the vessels. In addition, pathophysiological and histological similarities and clinical coexistence of these diseases have been demonstrated both in the arterial and in the venous system. This situation underlies the thought that dilatation of the vessels in any vascular territory should be considered as a systemic vessel wall disease rather than being a local disease of any vessel. These patients should be evaluated for other dilating vascular diseases in a systematic manner.

Pathobiology of Idiopathic Ascending Aortic Aneurysms

The majority of ascending aortic aneurysms cannot be related to any specific etiology and should be qualified as idiopathic. The pathobiology of ascending aortic aneurysms remains incompletely understood. Data from direct study are still scarce and often limited because of patient heterogenicity. Currently available information suggests that destructive remodeling of the aortic wall, inflammation and angiogenesis, biomechanical wall stress, and molecular genetics are relevant mechanisms of idiopathic ascending aortic aneurysm formation and progression. Further understanding of these mechanisms will likely provide novel diagnostic, prognostic, and therapeutical tools for the clinician.

Ascending Aortic Aneurysm in Angiotensin II-Infused Mice: Formation, Progression, and the Role of Focal Dissections

OBJECTIVE

To understand the anatomy and physiology of ascending aortic aneurysms in angiotensin II-infused ApoE(-/-) mice.

APPROACH AND RESULTS

We combined an extensive in vivo imaging protocol (high-frequency ultrasound and contrast-enhanced microcomputed tomography at baseline and after 3, 10, 18, and 28 days of angiotensin II infusion) with synchrotron-based ultrahigh resolution ex vivo imaging (phase contrast X-ray tomographic microscopy) in n=47 angiotensin II-infused mice and 6 controls. Aortic regurgitation increased significantly over time, as did the luminal volume of the ascending aorta. In the samples that were scanned ex vivo, we observed one or several focal dissections, with the largest located in the outer convex aspect of the ascending aorta. The volume of the dissections moderately correlated to the volume of the aneurysm as measured in vivo (r(2)=0.46). After 3 days of angiotensin II infusion, we found an interlaminar hematoma in 7/12 animals, which could be linked to an intimal tear. There was also a significant increase in single laminar ruptures, which may have facilitated a progressive enlargement of the focal dissections over time. At later time points, the hematoma was resorbed and the medial and adventitial thickness increased. Fatal transmural dissection occurred in 8/47 mice at an early stage of the disease, before adventita remodeling.

CONCLUSIONS

We visualized and quantified the dissections that lead to ascending aortic aneurysms in angiotensin II-infused mice and provided unique insight into the temporal evolution of these lesions.

Comparison of Aortic Collagen Fiber Angle Distribution in Mouse Models of Atherosclerosis Using Second-Harmonic Generation (SHG) Microscopy

Characterization of collagen fiber angle distribution throughout the blood vessel wall provides insight into the mechanical behavior of healthy and diseased arteries and their capacity to remodel. Atherosclerotic plaque contributes to the overall mechanical behavior, yet little is known experimentally about how collagen fiber orientation is influenced by atherogenesis. We hypothesized that atherosclerotic lesion development, and the factors contributing to lesion development, leads to a shift in collagen fiber angles within the aorta. Second-harmonic generation microscopy was used to visualize the three-dimensional organization of collagen throughout the aortic wall and to examine structural differences in mice maintained on high-fat Western diet versus age-matched chow diet mice in a model of atherosclerosis. Image analysis was performed on thoracic and abdominal sections of the aorta from each mouse to determine fiber orientation, with the circumferential (0°) and blood flow directions (axial ±90°) as the two reference points. All measurements were used in a multiple regression analysis to determine the factors having a significant influence on mean collagen fiber angle. We found that mean absolute angle of collagen fibers is 43° lower in Western diet mice compared with chow diet mice. Mice on a chow diet have a mean collagen fiber angle of ±63°, whereas mice on a Western diet have a more circumferential fiber orientation (~20°). This apparent shift in absolute angle coincides with the development of extensive aortic atherosclerosis, suggesting that atherosclerotic factors contribute to collagen fiber angle orientation.

Incidence, severity, mortality, and confounding factors for dissecting AAA detection in angiotensin II-infused mice: a meta-analysis

AIMS

While angiotensin II-infused mice are the most popular model for preclinical aneurysm research, representative data on incidence, severity, and mortality of dissecting abdominal aortic aneurysms (AAAs) have never been established, and the influence of confounding factors is unknown.

METHODS AND RESULTS

We performed a meta-analysis including 194 manuscripts representing 1679 saline-infused, 4729 non-treated angiotensin II-infused, and 4057 treated angiotensin II-infused mice. Incidence (60%) and mortality (20%) rates are reported overall as well as for grade I (22%), grade II (26%), grade III (29%), and grade IV (24%) aneurysms. Dissecting AAA incidence was significantly (P < 0.05) influenced by sex, age, genetic background, infusion time, and dose of angiotensin II. Mortality was influenced by sex, genetic background, and dose, but not by age or infusion time. Surprisingly, both incidence and mortality were significantly different (P < 0.05) when comparing angiotensin II-infused mice in descriptive studies (56% incidence and 19% mortality) with angiotensin II-infused mice that served as control animals in treatment studies designed to either enhance (35% incidence and 13% mortality) or reduce (73% incidence and 25% mortality) dissecting AAA formation. After stratification to account for confounding factors (selection bias), the observed effect was still present for incidence, but not for mortality. Possible underlying causes are detection bias (non-uniform definition for detection and quantification of dissecting AAA in mice) or publication bias (studies with negative results, related to incidence in the control group, not being published).

CONCLUSIONS

Our data provide a new meta-analysis-based reference for incidence and mortality of dissecting AAA in angiotensin II-infused mice, and indicate that treatment studies using this mouse model should be interpreted with caution.

Automatic Evaluation of Collagen Fiber Directions from Polarized Light Microscopy Images

Mechanical properties of the arterial wall depend largely on orientation and density of collagen fiber bundles. Several methods have been developed for observation of collagen orientation and density; the most frequently applied collagen-specific manual approach is based on polarized light (PL). However, it is very time consuming and the results are operator dependent. We have proposed a new automated method for evaluation of collagen fiber direction from two-dimensional polarized light microscopy images (2D PLM). The algorithm has been verified against artificial images and validated against manual measurements. Finally the collagen content has been estimated. The proposed algorithm was capable of estimating orientation of some 35 k points in 15 min when applied to aortic tissue and over 500 k points in 35 min for Achilles tendon. The average angular disagreement between each operator and the algorithm was -9.3±8.6° and -3.8±8.6° in the case of aortic tissue and -1.6±6.4° and 2.6±7.8° for Achilles tendon. Estimated mean collagen content was 30.3±5.8% and 94.3±2.7% for aortic media and Achilles tendon, respectively. The proposed automated approach is operator independent and several orders faster than manual measurements and therefore has the potential to replace manual measurements of collagen orientation via PLM.

Reasons to Investigate the Soluble Receptor for Advanced Glycation End-Product (sRAGE) Pathway in Aortic Disease

Aortic disease has a high rate of morbidity and mortality, and there are no documented screening methods to date. Yet biochemical research does show a significant link between soluble receptor for advanced glycation end-products (sRAGE) protein and cardiovascular disease. Therefore, it can be hypothesized that sRAGE plasma levels may help differentiate patients with aortic disease from the general population, which this paper will review and present.

Hemodynamic variations due to spiral blood flow through four patient-specific bifurcated stent graft configurations for the treatment of abdominal aortic aneurysms

Endovascular repair is now a recognised procedure for treating abdominal aortic aneurysms. However, post-operative complications such as stent graft migration and thrombus may still occur. To assess these complications numerically, the correct input boundary conditions, which include the full human aorta with associated branching, should be included. Four patient-specific computed tomography scanned bifurcated stent grafts (SGs) were modelled and attached onto a full human aorta, which included the ascending, aortic arch and descending aortas. Two of the SG geometries had a twisted leg configuration, while the other two had conventional nontwisted leg configurations. Computational fluid dynamics was completed for both geometries and the hemodynamics assessed. The complexity of the flow patterns and secondary flows were influenced by the inclusion of the full human aorta at the SG proximal section. During the decelerating phase significant recirculations occurred along the main body of all SG configurations. The inclusion of the full human aorta did not impact the velocity contours within the distal legs and there was no difference in drag forces with the SG containing the full human aorta and those without. A twisted leg configuration further promoted a spiral flow formation along its distal legs.

Do Adventitial Mast Cells Contribute to the Pathogenesis of Ascending Thoracic Aorta Aneurysm?

The precise pathogenesis of the ascending aortic aneurysm (AscAA) remains to be determined. Mast cells in the adventitia of human AscAA lesions may play a role in this pathogenesis. Adventitial mast cell density per 10 high-power fields (0.25 mm2) was assessed in multiple biopsy samples, from aneurysmal aortic sections (n = 41) and control (nondilated) aortic specimens (n = 50), stained by orcein-Giemsa method, an inexpensive (<$1) method. In a multivariable adjusted logistic regression model, using AscAA as the dependent variable, mast cell density was found to be an independent predictor of AscAA occurrence (odds ratio = 2.21; 95% confidence interval = 1.58-3.08; P < .001). Receiver operating characteristic curve analysis showed that the proposed cutoff value of ≥3 mast cells per 10 high-power fields was very sensitive to detect AscAA occurrence, yielding a sensitivity of 90% with a specificity of 80%. In conclusion, a significant increase in the number of mast cells in the adventitia of human ascending aortic lesions proposes a role for these cells in the pathogenesis of AscAA.

Mechanics, mechanobiology, and modeling of human abdominal aorta and aneurysms

Biomechanical factors play fundamental roles in the natural history of abdominal aortic aneurysms (AAAs) and their responses to treatment. Advances during the past two decades have increased our understanding of the mechanics and biology of the human abdominal aorta and AAAs, yet there remains a pressing need for considerable new data and resulting patient-specific computational models that can better describe the current status of a lesion and better predict the evolution of lesion geometry, composition, and material properties and thereby improve interventional planning. In this paper, we briefly review data on the structure and function of the human abdominal aorta and aneurysmal wall, past models of the mechanics, and recent growth and remodeling models. We conclude by identifying open problems that we hope will motivate studies to improve our computational modeling and thus general understanding of AAAs.

The activity of class I, II, III and IV of alcohol dehydrogenase (ADH) isoenzymes and aldehyde dehydrogenase (ALDH) in the wall of abdominal aortic aneurysms

OBJECTIVE

Human blood vessels contain a huge amount of alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH), which play a significant role in the metabolism of many biological substances and participate in various metabolic pathways. The aim of this study was the investigation of the differences between the activities of ADH and ALDH in the wall of aortic aneurysm and wall of healthy aorta, that can explain the pathological background of aneurysm development.

METHODS

For the measurement of the activity of class I and II ADH isoenzymes and ALDH activity the fluorometric methods was employed. The total ADH activity and activity of class III and IV isoenzymes was measured by the photometric method. The study material consisted of vessels wall samples obtained from 45 abdominal aortic aneurysm.

RESULTS

The activity of the class I ADH isoenzyme was significantly lower in the wall of aortic aneurysm than in healthy aorta. The other tested classes of ADH showed the tendency to lower level of the activity in aneurysm tissue than that in wall of unchanged aorta. The activities of total ADH and ALDH were also not significantly lower in the aneurysms.

CONCLUSION

The decrease of the activity of class I ADH isoenzymes in the wall of aortic aneurysm may be a factor of some disorders in metabolic pathways with participation of these isoenzymes.

Mediators of neutrophil recruitment in human abdominal aortic aneurysms

Aims

Neutrophils/platelet interactions are involved in abdominal aortic aneurysm (AAA). The intraluminal thrombus (ILT) is a human model of platelet/neutrophil interactions. The present study focused on mediators involved in neutrophil recruitment in AAA.

Methods and results

Conditioned media from luminal, intermediate, and abluminal layers of 29 human ILTs were analysed for neutrophil markers [elastase/α₁-antitrypsin and MMP9/NGAL complexes, myeloperoxidase (MPO), and α-defensin peptides], RANTES, platelet factor 4 (PF4), and interleukin-8 (IL-8). Their time-dependent release into serum from clots generated in vitro and their plasma concentrations in AAA patients and controls were determined. Immunohistochemistry for neutrophils, platelets, IL-8, PF4, and RANTES on AAA sections was performed; and molecules involved in ILT neutrophil chemotactic function were analysed in vitro. Neutrophils and platelets colocalized in the luminal layer of the thrombus. Consistently, neutrophil markers and platelet-derived RANTES and PF4 were released predominantly by the luminal thrombus pole, where their concentrations were significantly correlated. The luminal ILT layer was also the main source of IL-8, whose immunostaining colocalized with neutrophils. All were also released time dependently from clots and were increased in plasma of AAA patients. Luminal ILT layers displayed potent neutrophil chemotactic activity in vitro, which was inhibited by RANTES- and IL-8-blocking antibodies as well as by reparixin, an antagonist of the IL-8 receptors CXCR1 and CXCR2.

Conclusion

Taken together, these results suggest that platelet-derived RANTES and neutrophil-derived IL-8 are involved in attracting neutrophils to the luminal layer of AAA ILT.

Relation of common carotid artery lumen diameter to general arterial dilating diathesis and abdominal aortic aneurysms: the Tromsø Study

In a cross-sectional, population-based study in Tromsø, Norway, the authors investigated correlations between lumen diameter in the right common carotid artery (CCA) and the diameters of the femoral artery and abdominal aorta and whether CCA lumen diameter was a risk factor for abdominal aortic aneurysm (AAA). Ultrasonography was performed in 6,400 men and women aged 25-84 years during 1994-1995. An AAA was considered present if the aortic diameter at the level of renal arteries was greater than or equal to 35 mm, the infrarenal aortic diameter was greater than or equal to 5 mm larger than the diameter of the level of renal arteries, or a localized dilation of the aorta was present. CCA lumen diameter was positively correlated with abdominal aortic diameter (r = 0.3, P < 0.01) and femoral artery diameter (r = 0.2, P < 0.01). In a multivariable adjusted model, CCA lumen diameter was a significant predictor of AAA in both men and women (for the fifth quintile vs. the third, odds ratios were 1.9 (95% confidence interval: 1.2, 2.9) and 4.1 (95% confidence interval: 1.5, 10.8), respectively). Thus, CCA lumen diameter was positively correlated with femoral and abdominal aortic artery diameter and was an independent risk factor for AAA.

Review of Current Theories for Abdominal Aortic Aneurysm Pathogenesis

Atherosclerotic plaques are a feature of abdominal aortic aneurysms (AAAs). Atherosclerosis and AAA appear to share similar risk factors. These observations have led to the conclusion that AAAs are a consequence of advanced atherosclerosis.

This review explores current theories regarding the pathogenesis of AAA and their implications for treatment.

A systematic literature search was conducted using the search terms abdominal aortic aneurysm, atherosclerosis, pathogenesis, and systemic disease. Articles were categorized according to the association of AAAs with atherosclerosis, arteriomegaly, peripheral aneurysm, systemic expression, genetics, autoimmunity, oxidative stress, and systemic disease. Twenty-nine articles reporting changes in the systemic vasculature associated with AAA and 12 articles examining the shared risk factor hypothesis were identified.

There is insufficient evidence to confirm that AAAs are the result of advanced atherosclerosis. The bulk of evidence points to AAA disease being a systemic disease of the vasculature, with a predetermined genetic susceptibility leading to a phenotype governed by environmental factors.

[Pathobiology of idiopathic ascending aortic aneurysms]

BACKGROUND

The majority of ascending aortic aneurysms cannot be related to any specific etiology and should be qualified as idiopathic. The incidence of this disease is increasing in the population of the developed countries but its pathobiology is poorly understood.

AIM

This article is reviewing the publications concerning the pathobiology of idiopathic ascending aortic aneurysms.

SOURCES

A PubMed search on articles published in English or French, between January 1965 and December 2007, on key-words << aortic root >>, << ascending aorta >>, << aortic arch >>, << thoracic aorta >>, << aneurysm >>, << dilatation >> and << dissection >> was undertaken. Articles on aneurysms related to inflammatory and infectious diseases, congenital or genetic syndromes were excluded.

RESULTS

The presented data suggests that destructive remodeling of the aortic wall, inflammation and angiogenesis, biomechanical wall stress, and molecular genetics are relevant mechanisms of idiopathic ascending aortic aneurysm formation and progression.

LIMITS

Sparse data available from few direct studies offer limited knowledge on pathobiology of idiopathic ascending aortic aneurysms.

CONCLUSION

A more intimate knowledge of the triggers and perpetrating factors of this disease might offer new diagnostic and treatment options.

Transforming growth factor-beta signaling in thoracic aortic aneurysm development: a paradox in pathogenesis

Thoracic aortic aneurysms (TAAs) are potentially devastating, and due to their asymptomatic behavior, pose a serious health risk characterized by the lack of medical treatment options and high rates of surgical morbidity and mortality. Independent of the inciting stimuli (biochemical/mechanical), TAA development proceeds by a multifactorial process influenced by both cellular and extracellular mechanisms, resulting in alterations of the structure and composition of the vascular extracellular matrix (ECM). While the role of enhanced ECM proteolysis in TAA formation remains undisputed, little attention has been focused on the upstream signaling events that drive the remodeling process. Recent evidence highlighting the dysregulation of transforming growth factor-beta (TGF-beta) signaling in ascending TAAs from Marfan syndrome patients has stimulated an interest in this intracellular signaling pathway. However, paradoxical discoveries have implicated both enhanced TGF-beta signaling and loss of function TGF-beta receptor mutations, in aneurysm formation; obfuscating a clear functional role for TGF-beta in aneurysm development. In an effort to elucidate this subject, TGF-beta signaling and its role in vascular remodeling and pathology will be reviewed, with the aim of identifying potential mechanisms of how TGF-beta signaling may contribute to the formation and progression of TAA.

Xylosyltransferase I variants and their impact on abdominal aortic aneurysms

BACKGROUND

The formation of abdominal aortic aneurysm (AAA) is caused by a destructive remodeling of the extracellular matrix in the vascular wall. Proteoglycan content and biosynthesis have been shown to be altered in AAA. Xylosyltransferase I (XT-I) is the initial and rate-limiting enzyme in the biosynthesis of the proteoglycan-linked glycosaminoglycan chains. A familial predisposition to AAA is well recognized. Thus, variations in the XT-I coding gene XYLT1 might be risk factors for AAA formation.

METHODS

We performed genotyping of two genetic variations in the XYLT1 gene which, have been already linked to proteoglycan-associated diseases, in 129 AAA patients and 129 age- and sex-matched healthy controls.

RESULTS

The T-allele of the polymorphism c.343G>T (p.A115S) was found to be significantly more frequent in AAA patients compared to the healthy control group, demonstrating that carriers of the T-allele have a 5-fold increased risk of developing AAA (odds ratio 4.87, 95%-CI 1.38-17.19; p=0.011).

CONCLUSIONS

Our results show that XT-I polymorphisms potentially confer to the genetic susceptibility of AAA.

Intracranial and abdominal aortic aneurysms: similarities, differences, and need for a new class of computational models

Intracranial saccular and abdominal aortic aneurysms (ISAs and AAAs, respectively) result from different underlying disease processes and exhibit different rupture potentials, yet they share many histopathological and biomechanical characteristics. Moreover, as in other vascular diseases, hemodynamics and wall mechanics play important roles in the natural history and possible treatment of these two types of lesions. The goals of this review are twofold: first, to contrast the biology and mechanics of intracranial and abdominal aortic aneurysms to emphasize that separate advances in our understanding of each disease can aid in our understanding of the other disease, and second, to suggest that research on the biomechanics of aneurysms must embrace a new paradigm for analysis. That is, past biomechanical studies have provided tremendous insight but have progressed along separate lines, focusing on either the hemodynamics or the wall mechanics. We submit that there is a pressing need to couple in a new way the separate advances in vascular biology, medical imaging, and computational biofluid and biosolid mechanics to understand better the mechanobiology, pathophysiology, and treatment of these lesions, which continue to be responsible for significant morbidity and mortality. We refer to this needed new class of computational tools as fluid-solid-growth (FSG) models.

Risks of subarachnoid hemorrhage in siblings: a nationwide epidemiological study from Sweden

This nationwide study aimed to enhance available data by determining sibling risks of subarachnoid hemorrhage in a total population. The MigMed database at the Karolinska Institute, Stockholm, was used to identify all cases of subarachnoid hemorrhage diagnosed in Sweden between 1987 and 2001. Incidence ratios standardized for age, region, and socioeconomic status (SIRs) were calculated for persons with at least 1 sibling with subarachnoid hemorrhage. The reference group consisted of persons whose siblings had no subarachnoid hemorrhage. A total of 90 affected siblings were identified; their SIR of subarachnoid hemorrhage was 2.75. The risk decreased with increasing age in both men and women. Within the limits of the sample size, no sex differences could be observed. The relatively high sibling risks are likely to be due to heritable causes and shared environmental factors. Genetic causes possibly weigh more in early- than late-onset cases. This study shows the feasibility of carrying out nationwide family studies on subarachnoid hemorrhage.

Systemic Inflammation and Repair of Abdominal Aortic Aneurysm

BACKGROUND

Inflammation is integral to the pathogenesis of abdominal aortic aneurysm (AAA). This study examines preoperative biomarkers of systemic inflammation in patients undergoing open repair of intact and ruptured AAA.

METHODS

One-hundred twelve patients were entered into a prospective observational study. Preoperative POSSUM physiology score, C-reactive protein (CRP), white blood count (WBC), platelet count, fibrinogen, and albumin were recorded and related to clinical variables using univariate analysis.

RESULTS

Sixty-one patients with a ruptured AAA, 39 with an asymptomatic intact AAA, and 12 with an acutely symptomatic intact AAA underwent attempted repair. There were two inflammatory asymptomatic aneurysms and one inflammatory ruptured aneurysm. No patient had clinical evidence of coexistent inflammatory disease. Patients with a symptomatic intact AAA had a significantly greater level of CRP and fibrinogen, higher WBC, and lower serum albumin, than those with an asymptomatic intact AAA. Patients with a ruptured aneurysm had a significantly greater level of CRP, higher WBC, and lower serum albumin than those with an asymptomatic intact aneurysm. Patients with a symptomatic intact AAA had a significantly higher CRP level, but lower WBC, than those with a ruptured AAA. There was no difference in CRP level, WBC, or serum albumin between survivors and non-survivors of attempted repair of asymptomatic, symptomatic and ruptured AAA.

CONCLUSIONS

Acutely symptomatic and ruptured AAAs are associated with an early elevation in systemic inflammatory biomarkers. This early activation of the inflammatory response might influence perioperative outcome.

Spatiotemporal expression and localization of matrix metalloproteinas-9 in a murine model of thoracic aortic aneurysm

OBJECTIVE

Matrix metalloproteinase-9 (MMP-9) has been widely described to play a critical role in aneurysm development. The goal of this study was to determine the spatiotemporal changes in MMP-9 expression and abundance in the early stages of aortic dilatation during the course of thoracic aortic aneurysm (TAA) formation in a mouse model.

METHODS

In this study, TAAs were surgically induced in a transgenic reporter mouse strain expressing the beta-galactosidase (beta-gal) gene under control of the MMP-9 promoter. Terminal studies were performed during the early stages of TAA development at 1 week (n = 6), 2 weeks (n = 6), and 4 weeks (n = 6) post-TAA induction surgery. Changes in aortic outer diameter were determined in vivo by video micrometry. MMP-9 transcriptional activity (beta-gal staining) and protein content (immunohistochemistry) were quantified at each time point and expressed as a percentage of unoperated reference control mice (n = 6).

RESULTS

Aortic dilatation was evident at 1 week and reached maximal size at 2 weeks (21% +/- 6% increase from baseline, P < .05). MMP-9 transcriptional activity was detected at 1 week post-TAA induction (722% +/- 323%, P = .19), reached a maximum within the adventitia at 2 weeks (1770% +/- 505%, P < .05), and returned to baseline by 4 weeks (167% +/- 47%, P = .21). MMP-9 transcription at 2 weeks colocalized with fibroblasts and smooth muscle cells. MMP-9 protein content within the aortic adventitia was increased at 2 weeks post-TAA induction (413% +/- 124%, P < .05) and remained elevated at 4 weeks (222% +/- 41%, P < .05). MMP-9 staining was most intense at the adventitial-medial border and could be detected throughout the elastic media.

CONCLUSIONS

These findings demonstrate a unique spatiotemporal pattern of MMP-9 transcriptional activation and protein content in the developing TAA. Colocalization studies suggest that early dilatation may be driven in part by MMP-9 produced by endogenous cells residing within the aortic vascular wall.

The genetics of intracranial aneurysms

The rupture of an intracranial aneurysm (IA) leads to a subarachnoid hemorrhage, a sudden onset disease that can lead to severe disability and death. Several risk factors such as smoking, hypertension and excessive alcohol intake are associated with subarachnoid hemorrhage. IAs, ruptured or unruptured, can be treated either surgically via a craniotomy (through an opening in the skull) or endovascularly by placing coils through a catheter in the femoral artery. Even though the etiology of IA formation is mostly unknown, several studies support a certain role of genetic factors. In reports so far, genome-wide linkage studies suggest several susceptibility loci that may contain one or more predisposing genes. Studies of several candidate genes report association with IAs. To date, no single gene has been identified as responsible for IA formation or rupture. The identification of susceptible genes may lead to the understanding of the mechanism of formation and rupture and possibly lead to the development of a pharmacological therapy.

Deficiencies in estrogen-mediated regulation of cerebrovascular homeostasis may contribute to an increased risk of cerebral aneurysm pathogenesis and rupture in menopausal and postmenopausal women

Despite the catastrophic consequence of ruptured intracranial aneurysms, very little is understood regarding their pathogenesis, and there are no reliable predictive markers for identifying at-risk individuals. Few studies have addressed the molecular pathological basis and mechanisms of intracranial aneurysm formation, growth, and rupture. The pathogenesis and rupture of cerebral aneurysms have been associated with inflammatory processes, and these have been implicated in the digestion and breakdown of vascular wall matrix. Epidemiological data indicate that the risk of cerebral aneurysm pathogenesis and rupture in women rises during and after menopause as compared to premenopausal women, and has been attributed to hormonal factors. Moreover, experimental evidence supports a role for estrogen in the modulation of each phase of the inflammatory response implicated in cerebral aneurysm pathogenesis and rupture. While the risk of aneurysm rupture in men also increases with age, this increased risk has been attributed to other recognized risk factors including cigarette smoking, use of alcohol, and history of hypertension, all of which are more common in men than women. We hypothesize, therefore, that decreases in both circulating estrogen levels and cerebrovascular estrogen receptor density may contribute to an increased risk of cerebral aneurysm pathogenesis and rupture in women during and after menopause. To test our hypothesis, experiments are needed to identify genes regulated by estrogen and to evaluate gene expression and intracellular mechanisms in cells/tissues exposed to varying concentrations and duration of treatment with estrogen, metabolites of estrogen, and selective estrogen receptor modulators (SERMs). Furthermore, it is not likely that the regulation of cerebrovascular homeostasis is due to the actions of estrogen alone, but rather the interplay of estrogen and other hormones and their associated receptor expression. The potential interactions of these hormones in the maintenance of normal cerebrovascular tone need to be elucidated. Additional studies are needed to define the role that estrogen and other sex hormones may play in the cerebrovascular circulation and the pathogenesis and rupture of cerebral aneurysms. Efforts directed at understanding the basic pathophysiological mechanisms of aneurysm pathogenesis and rupture promise to yield dividends that may have important therapeutic and clinical implications. The development of non-invasive tools such as molecular MRI for the detection of specific cells, molecular markers, and tissues may facilitate early diagnosis of initial pathophysiological changes that are undetectable by clinical examination or other diagnostic tools, and can also be used to evaluate the state of activity of cerebral aneurysm pathogenesis before, during, and after treatment.

Familial risks of aortic aneurysms among siblings in a nationwide Swedish study

PURPOSE

Aortic aneurysms have a high fatality rate that could be reduced with control of risk factors and use of available screening methods for detection of early changes in aortic walls. The available data on familial risks, a potential indication for screening, are mainly limited to abdominal aortic aneurysms.

METHODS

A nationwide Swedish cohort was constructed by linking the Multigeneration Register on 0- to 69-year-old siblings to the Hospital Discharge Register and the Cause of Death Register for data on aortic aneurysms from years 1987 to 2001. Standardized incidence ratios (SIRs) were calculated for affected siblings by comparing with those whose siblings had no aneurysm.

RESULTS

A total of 71 affected siblings were identified with a familial SIR of 8.71; when one sibling was diagnosed before age 50 years, the SIR was 19.69. For concordant thoracic or concordant abdominal aneurysms, the SIRs were 21.68 and 13.06, respectively. For brothers, the risk of abdominal aneurysms was 14.63, and 49.50 for diagnosis before age 50 years. Familial risks and the effects of early diagnostic age were shared by the anatomic subtypes of aneurysms. Within limits of the sample size, no gender differences could be observed. Affected siblings constituted 2.2% of all diagnosed patients.

CONCLUSIONS

A family history of any aortic aneurysms and age groups younger than 50 years should be considered in recommendations for screening. The high familial risks are likely to be the result of heritable genes, the identification of which would allow gene testing and preventive counseling.

ATP stimulates MMP-2 release from human aortic smooth muscle cells via JNK signaling pathway

Aortic smooth muscle cell release of matrix metalloproteinase-2 (MMP-2) and tissue inhibitor of metalloproteinase-2 (TIMP-2) has been implicated in aortic aneurysm pathogenesis, but proximal modulation of release is poorly understood. Extracellular nucleotides regulate vascular smooth muscle cell metabolism in response to physiochemical stresses, but nucleotide modulation of MMP and/or TIMP release has not been reported. We hypothesized that nucleotides modulate MMP-2 and TIMP-2 release from human aortic smooth muscle cells (HASMCs) via distinct purinergic receptors and signaling pathways. We exposed HASMCs to exogenous ATP and other nucleotides with and without interleukin-1beta (IL-1beta). HASMCs were pretreated in some experiments with apyrase, which degrades ATP, and inhibitors of ERK1/2, JNK, and p38 MAPK. MMP-2 and TIMP-2 released into supernatant were assessed using ELISA and Western blotting. ATP, adenosine, and UTP significantly stimulated MMP-2 release in the presence of IL-1beta (300 nM ATP: 181 +/- 22%, P = 0.003; 30 microm adenosine: 244 +/- 150%, P = 0.001; and 200 microm UTP: 153 +/- 40%, P = 0.015; vs. 100% constitutive). ATP also stimulated MMP-2 release in the absence of IL-1beta (100 microm ATP: 148 +/- 38% vs. 100% constitutive). Apyrase significantly reduced ATP-stimulated MMP-2 release (apyrase + 500 nM ATP: 59 +/- 3% vs. 124 +/- 7% with 500 nM ATP). Rank-order agonist potency for MMP-2 release was consistent with ATP activation of PAY and PAY receptors. ATP induced phosphorylation of intracellular JNK, and inhibition of the JNK pathway blocked ATP-stimulated MMP-2 release, indicating signaling via this pathway. Nucleotides are thus novel stimulants of MMP-2 release from HASMCs and may provide a mechanistic link between physiochemical stress in the aorta and aneurysms, especially in the context of inflammation.

Evaluation of enzymes involved in proteoglycan degradation in the wall of abdominal aortic aneurysms

The abdominal aortic aneurysm (AAA) wall represents an extreme example of arterial remodeling with disturbed elastin, collagen and proteoglycan metabolism. The aim of this study was to evaluate enzymes involved in the degradation of glycosaminoglycan chains and core proteins of proteoglycans in the AAA wall. The study material consisted of wall samples from 10 AAA. Fragments of 5 normal abdominal aortas from organ donors were used as a control. The activity of endoglycosidases, exoglycosidases and sulfatases was measured using colorimetric methods. To assess matrix metalloproteinases (MMPs), Western blot and zymography were performed. The activity of endoglycosidase degrading chondroitin-4-sulfate was lower in the AAA wall. Endoglycosidase degrading heparan sulfate and dermatan sulfate, arylosulfatase B, as well as all the exoglycosidases assessed demonstrated higher activities in the AAA wall. Furthermore, increased expression of MMP1, MMP2 and MMP9 was also shown in the AAA wall. Zymography revealed decreased activity of pro-MMP2 and presence of pro-MMP9 in the AAA wall compared to the wall of normal aorta. Extensive changes in the activity of glycosaminoglycan-degrading enzymes and MMPs may influence the organization of the extracellular matrix network and lead to previously demonstrated changes in the proteoglycan and glycosaminoglycan content in the AAA wall.