Elsevier

Atherosclerosis

Volume 226, Issue 1, January 2013, Pages 29-39
Atherosclerosis

Review
The calcium chloride-induced rodent model of abdominal aortic aneurysm

https://doi.org/10.1016/j.atherosclerosis.2012.09.010Get rights and content

Abstract

Abdominal aortic aneurysm (AAA) affects ∼5% men aged over 65 years and is an important cause of death in this population. Research into AAA pathogenesis has been fuelled by the need to identify new diagnostic biomarkers and therapeutic targets for this disease. One animal model of AAA involves peri-vascular application of calcium chloride (CaCl2) onto the infra-renal aorta of mice and rats to induce extracellular matrix remodelling. Twenty-three studies assessing CaCl2-induced AAA and six studies assessing AAA induced by a modified CaCl2 method were identified. In the current report the preparation and pathological features of this AAA model are discussed. We also compared this animal model to human AAA. CaCl2-induced AAA shows the following pathological characteristics typically found in human AAA: calcification, inflammatory cell infiltration, oxidative stress, neovascularisation, elastin degradation and vascular smooth muscle cell apoptosis. A number of mechanisms involved in CaCl2-induced AAA have been identified which may be relevant to the pathogenesis of human AAA. Key molecules include c-Jun N-terminal kinase, peroxisome proliferator-activated receptor-γ, chemokine (C–C motif) receptor 2, group x secretory phospholipase A2 and plasminogen. CaCl2-induced AAA does not display aortic thrombus, atherosclerosis and rupture which are classical features of human AAA. Advantages of the CaCl2-induced AAA technique include (1) it can be applied to wild type mice making assessment of transgenic rodent models more straight forward and rapid; and (2) CaCl2-induced AAAs are usually developed in the infra-renal abdominal aorta, which is the most common location of human AAA. Currently findings obtained from the CaCl2-induced AAA model or other animal models of AAA have not been translated into the human situation. It is hoped that this deficiency will be corrected over the next decade with a number of clinical trials currently examining novel treatment options for AAA patients.

Introduction

Aortic aneurysm is the general term for any dilation (aneurysm) of the aorta to greater than 1.5 times normal size [1]. Abdominal aortic aneurysm (AAA) represents a weakened and dilated region of the abdominal aorta usually affecting the infra-renal segment [2]. AAA has previously been reported to affect ∼5% men aged over 65 years [3], [4]. Recent studies suggest that AAA prevalence is decreasing. A recent large screening study in Sweden reported an AAA prevalence of 1.7% in 65-year-old men [5]. A recent epidemiological study in England and Wales suggests that AAA mortality is decreasing and AAA presentation is shifting to an older population of ≥75 years [6]. The pathological features of AAA include increased elastin degradation, vascular smooth muscle cell (VSMC) apoptosis, oxidative stress and inflammation [7], [8]. The risk of AAA rupture increases with increasing aortic diameter [9]. Mortality after AAA rupture is about 80% for those who reach hospital and ∼50% for those who undergo surgery [10], [11]. There is no established therapy for small AAAs [12]. Larger (greater than 5.5 cm in diameter) or symptomatic AAAs usually undergo open surgical or endovascular repair [13]. Both surgical repair methods have some limitations. Open surgery is associated with a significant peri-operative mortality and morbidity; while up to 20% patients who undergo endovascular repair require reintervention within 5 years [14].

AAA diagnosis is problematic because most aneurysms are asymptomatic until rupture. While ultrasound screening has been introduced in older males in some countries, the feasibility of continuing such programs is in question with the evidence suggesting that AAA incidence is decreasing [6]. There is therefore great current interest in identifying novel diagnostic markers and appropriate molecular targets for drug development [15]. The mechanisms underlying AAA initiation and progression remain incompletely understood, contributing to significant shortfalls in the diagnosis and management of AAA. Experiments using human biopsies and cells derived from these biopsies are helpful in providing information on end-stage AAA pathogenesis. Such studies are limited by the decreasing availability of samples from open AAA repair and the inherent difficulties in modelling in vivo pathology. Animal models enable the study of AAA pathogenesis in vivo.

One of the animal models used to investigate AAA is that induced by calcium chloride (CaCl2) [16], [17], and research employing this model has provided insight into the pathogenesis of AAA. The current systematic review describes the technical aspects, cellular and molecular features of this animal model and comments on the relevance of this model to human AAA.

Section snippets

The development of the CaCl2 model

Calcification of the human aorta is common in older adults [18], [19]. A study in which 582 aortic specimens from the proximal portion of the aortic arch were examined demonstrated that calcification of the aortic media starts in people younger than 19 years and that the presence and severity of calcification increases with age [19]. Intimal calcification is present in most patients who have atherosclerosis and more severe aortic calcification has been shown to predict worse long term outcome

Technical aspects of the CaCl2-induced AAA model

By searching the PUBMED database and hand searching of the reference lists of relevant articles we identified 23 articles which reported employing the CaCl2-induced AAA model in rodents and 6 articles which reported employing modifications of the CaCl2 method in rodents. The technical aspects of the CaCl2-induced AAA model employed in these 23 studies were similar. Briefly, after the animals were anesthetized, the fur on the abdomen was removed and a midline abdominal incision was performed.

Cellular and molecular features of the CaCl2-induced AAA model

Peri-aortic application of CaCl2 has a large range of effects on aortic endothelial cells (ECs) and VSMCs. Peri-aortic application of CaCl2 to the rat aorta increases endothelial permeability and induces VSMC apoptosis [44]. VSMCs isolated from AAAs induced by CaCl2 produce large amounts of MMP-2 and MMP-9 [40] and less collagen, tropoelastin, and matrix elastin, compared to healthy VSMCs [40]. These characteristics of VSMCs are typically found in VSMCs isolated from human AAA tissue [50], [51]

The relevant of the CaCl2-induced rodent AAA model to human AAA

This animal model share many pathological features with human AAA, including aortic calcification, inflammatory cell infiltration, oxidative stress, neovascularisation, elastin degradation and VSMC apoptosis [13], [17], [21], [24], [25], [28], [29], [32], [34], [35], [38], [39], [40], [41], [42], [43], [44], [74], [75], [76], [77], [78], [79], [80], [81] (Table 6). On the other hand, CaCl2-induced AAA does not display thrombus, atherosclerosis and rupture, which are important features of human

Conclusion

The CaCl2-induced AAA model resembles many pathological features of human AAA, such as marked aortic calcification, inflammation, oxidative stress, MMP activity, neovascularisation, elastin degradation and VSMC apoptosis. In addition, studies using this model have identified some molecules important for AAA development in rodents which may be relevant to human AAA. These molecules include JNK, PPAR-γ, CCR2, sPLA2-X and plasminogen. On the other hand, this model does not display intraluminal

Conflict of interest

The authors declare that they have no conflict of interest.

Acknowledgement

This work is funded by grants from the National Health and Medical Research Council (540404, 1021416) and the BUPA Foundation. JG holds a Practitioner Fellowship from the National Health and Medical Research Council, Australia (1019921) and a Senior Clinical Research Fellowship from the Queensland Government.

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