Animal models for abdominal aortic aneurysms: Where we are and where we need to go

Abstract

The mortality rate of patients with abdominal aortic aneurysm (AAA) after rupture is extremely high, and this disease has become an important disease endangering the health of the Chinese population. Methods used to model AAA include intraluminal pressurized elastase infusion, chronic infusion of angiotensin II (Ang II) via an osmotic pump, periarterial application of calcium chloride, vascular grafting, and gene modification. AAA models induced by elastase and Ang II are the two most widely used animal models. In the elastase‐induced model, because intraluminal infusion is transient, with the cessation of initial stimulation, the aneurysm lesion tends to be stable and rarely ruptures. The model induced by Ang II infusion often presents with a typical aortic dissection with a false lumen, whereas clinical AAA patients do not necessarily have dissection. Currently, the treatment of AAA in clinical practice remains endovascular, and there is a lack of pharmacological therapy, which is also related to the fact that the pathogenic mechanism has not been fully elucidated. Smoking, old age, male sex, and hypertension are the main risk factors for AAA, but these risk factors have not been fully investigated in the current modeling methods, which may affect the clinical translational application of research results based on animal models. Therefore, this article reviews the most commonly used AAA modeling methods, comments on their applications and limitations, and provides a perspective on the development of novel animal models.

The risk factors for human abdominal aortic aneurysms (AAA) include smoking, old age, male sex, hypertension, and genetics. However, these risk factors are not fully involved in the current modeling methods, which may affect the clinical translational application of research results based on animal models. Porcine pancreatic elastase (PPE), angiotensin II (Ang II), and calcium chloride (CaCl₂) are widely used as reagents to induce experimental animal AAAs. Each animal model of AAA simulates the characteristics of human AAA in different ways, but it also has certain limitations. The new AAA model should better incorporate the risk factors for human AAA for modeling methods, such as smoking and hypertension, to better simulate human AAA and promote the development of new therapies for AAA.

1. INTRODUCTION

Abdominal aortic aneurysm (AAA) refers to irreversible dilation of the abdominal aorta caused by various pathogenic risk factors, and it can be diagnosed when the arterial diameter exceeds 50% of the normal aorta. ¹ , ² , ³ Smoking, male sex, advanced age (>60 years), hypertension, and atherosclerotic status are the main risk factors for the occurrence of AAA. ¹ , ² Clinically, AAAs generally do not cause severe symptoms, but aneurysms can rupture without any symptoms, and the acute rupture mortality rate is as high as 80%, which seriously threatens population health. ¹ , ² , ⁴ Currently, the management of AAA is based mainly on surgery, such as endovascular intervention, which lacks effective clinical drugs because the pathogenic mechanism of this disease has not been fully elucidated. ³ , ⁵ , ⁶ Histopathologically, AAA manifests as the depletion of medial smooth muscle cells (SMCs), disruption of medial elastin, degradation of the extracellular matrix, and local inflammatory responses in the aorta. ⁷ , ⁸ , ⁹ Inflammatory cell infiltration, oxidative stress, and intraluminal thrombosis formation are also considered important characteristics of AAA. ⁹ Because endovascular treatment remains the main therapy, clinical specimens of AAA are becoming increasingly difficult to obtain, and experimental animal models have become the primary tools for studying the pathogenic mechanism and drug therapy of this disease. Currently, animal models of AAA are based mainly on small rodents (mice and rats), with some medium‐ and large laboratory animals (e.g., rabbits, guinea pigs, dogs, and pigs) also being used. ¹⁰ , ¹¹ , ¹² , ¹³ Modeling methods mainly include intraluminal infusion of elastase, chronic infusion of angiotensin II (Ang II), application of calcium chloride (CaCl₂), and vascular tissue grafting. ¹² , ¹⁴

2. ELASTASE‐INDUCED AAA MODEL

The porcine pancreatic elastase (PPE) infusion‐induced AAA model was initially established in rats in 1990 and has since been extensively utilized for AAA modeling in mice as well. ¹⁵ , ¹⁶ In this model, the infrarenal abdominal aortic segment of a mouse (or rat) is surgically isolated, the infrarenal abdominal aorta is temporarily ligated to the iliac artery segment with a slipknot, and then PPE solution is infused into the infrarenal abdominal aorta at a given pressure through an infusion pump. ¹⁶ After ~5 min of pressurized infusion, surgery can be completed. ⁹ Pressurized infusion causes local aortic segment dilation and injury to the arterial wall. Moreover, elastase may penetrate the media of the aorta, thereby causing inflammation of the vascular wall and degenerative changes in elastin, and aneurysms gradually form. It manifests as typical inflammatory cell infiltration, medial SMC depletion, elastin breakage, and abnormal mural angiogenesis. In rats, intraluminal thrombosis can be observed, whereas in mice, thrombosis usually does not occur. ¹⁷ Because the PPE‐induced surgical model requires a relatively high level of microsurgical operation skills, researchers have established a model via PPE periarterial incubation. Although the operation method is relatively simple, the incidence of AAA is relatively low, and there is less aortic dilation than in infusion models. ⁸ , ¹⁸ Researchers have also made some modifications by combining the peri‐adventitial application of PPE with β‐aminopropionitrile exposure. ¹⁹ , ²⁰ , ²¹ This combination has been demonstrated to be a more efficacious method for inducing infrarenal expansion. ¹⁹ , ²⁰ , ²¹ , ²² This model is easier to perform surgically, leads to large intraluminal thrombus that forms typically after 6 weeks, can be developed in both male and female mice, and even allows for the study of AAA‐related intraluminal thrombus. The PPE intraluminal infusion model is widely used as a classical AAA model. In addition, several other models have been derived, such as replacing PPE with papain and incubating the abdominal aorta in vivo with papain to induce AAA formation. ²³

The PPE model is an acute inflammation model with obvious inflammatory characteristics. This model effectively simulates the role of inflammation in the occurrence and development of AAA but requires a high level of microsurgical operation skills. Notably, different mouse strains have different sensitivities to elastase, indicating that there may be genetic susceptibility in the formation of AAAs. ⁹ , ²⁴ This method has been successfully used to establish AAA models in animals such as mice, rats, guinea pigs, and rabbits.

3. AAA MODEL INDUCED BY PERIARTERIAL APPLICATION OF CaCl₂

This model is relatively simple to develop and does not require complex surgical operations. Typically, cotton gauze soaked with CaCl₂ is applied periarterially to the previously isolated infrarenal abdominal aorta of a rat or mouse and incubated for ~15 min. ²⁵ The formation of AAAs in this model starts with injury to the adventitia caused by CaCl₂. After a few days, aneurysm progression tends to be stable, and aneurysms rarely rupture. After high‐concentration calcium ions in the local aorta enter the cells of the aortic wall, they can be further converted into calcium phosphate (CaPO₄) by alkaline phosphatase. CaPO₄ precipitates on medial elastin, resulting in the degeneration or breakage of elastic fibers. The deposition of calcium ions destroys the media of the aortic wall and damages endothelial cells, thereby triggering thickening of the vascular wall, activating the inflammatory response, and gradually causing the formation of AAAs. ²⁶ The model induced by CaCl₂ is caused by adventitia damage, so it is more suitable for studying the role of adventitia stress factors in the pathogenic mechanism of AAA. This model simulates the role of vascular stiffness (vascular calcification) in the development of AAA, which is promoted by localized excess calcium salt deposition in the aorta. The CaCl₂‐induced AAA model is relatively easy to develop, and AAAs can be located in infrarenal abdominal aorta segments without microsurgical operations. The model has several characteristics of human AAA, such as aortic tissue calcification, elastic fiber damage, and vascular SMC (vSMC) apoptosis. The AAA model induced by CaCl₂ is important for studying the pathogenesis of aneurysms and provides an important reference for exploring effective drug treatment targets for AAA. This model is mostly used for AAA modeling in mice, rats, and rabbits.

4. ANG II INFUSION‐INDUCED AAA MODEL

The Ang II‐induced model was initially established by Daugherty et al. ²⁷ , ²⁸ This model usually requires mice with an ApoE ^−/− or Ldlr ⁻⁻ genetic background and combines the continuous infusion of Ang II in vivo via an osmotic pump to induce AAA. ²⁷ The hyperlipidemic state can increase the success rate of AAA modeling. The inflammatory state caused by hyperlipidemia, combined with hypertension caused by Ang II, can cause tearing of the aortic intima, entry of blood into the aortic media to form an intramural hematoma, and finally local dilation of the suprarenal aorta in mice. ²⁷ , ²⁸ The typical manifestations of this model are the aggregation of macrophages in the early intima and the appearance of aortic dissection, resulting in the degradation of medial elastin and the infiltration of T and B lymphocytes. Because the model is established with a background of hyperlipidemia, it usually involves processes such as atherosclerosis, intimal thickening, aggregation of macrophages in the outer elastic layer, and thrombosis formation, which is similar to the formation of clinical AAAs. During the 4‐week infusion of Ang II in mice, the incidence of aneurysms was approximately 60%–70%, and the mortality rate reached more than 20%. In addition, the model requires the use of osmotic pumps and gene‐knockout mice, so the experimental modeling cost is relatively high. ²⁷ This model can also be considered a commonly used AAA mouse model that can be combined with atherosclerosis. When exploring pathways such as blood pressure, blood lipids, and oxidative stress and studying dissected AAAs, the Ang II model is a good choice. With the development of biotechnology, injecting rAAV‐D377Y‐mPCSK9 into wild‐type mice can also reduce the expression of low density lipoprotein receptor (LDLR), simulating Ldlr deficiency, which expands the application of this model. ²⁹ , ³⁰

5. OTHER AAA ANIMAL MODELS

5.1. Aortic transplantation AAA model

A decellularized xenogeneic aortic transplantation AAA model was developed by Allaire et al. ³¹ The infrarenal aorta of guinea pigs was obtained and decellularized using sodium dodecyl sulfate. After decellularization, the SMCs of the transplanted aortas were absent, whereas the elastin and collagen networks were intact. After the decellularized aorta was washed several times, it was transplanted into the rats via microsurgery. The decellularized xenogeneic aortic graft becomes the target organ of the immune rejection reaction, ultimately resulting in degradation of the extracellular matrix and persistent aortic dilation. ³² After 14 days of surgery, the diameter of the xenogeneic transplanted aorta increased by more than 50%, resulting in the formation of an aneurysm. This model is based mainly on the removal of medial SMCs and xenogeneic immune rejection reactions to form a model.

5.2. Erythropoietin‐induced AAA model

After continuous intraperitoneal injection of erythropoietin (EPO) into wild‐type mice for 15 days, AAA can be induced in some mice, and continuing EPO injection for 4 weeks can cause a dose‐dependent increase in the mortality rate of these mice. ³³ , ³⁴ EPO may promote the proliferation and migration of endothelial cells and the production of matrix metallopeptidase 2 (MMP2) to stimulate angiogenesis and induce the formation of AAA. ³⁴ The pathophysiological manifestations of AAA induced by EPO are abnormal angiogenesis in the adventitia, macrophage infiltration, increased MMP secretion, and decreased collagen and vSMC. The advantage of this model is that the operation is relatively simple, and no surgical operation is needed. Intraperitoneal injection can be used for drug administration. This model can be used to study the characteristics of human aneurysms, such as hemoglobin concentration, angiogenesis, inflammation, and the relationship between matrix metalloproteinases and AAA.

5.3. Gene‐modified AAA animal models

The binding of transforming growth factor‐β (TGF‐β) to its receptor can activate the classic (Smad) and nonclassic (Erk) signaling pathways, which play important roles in maintaining vascular homeostasis. ³⁵ Mutations/deletions of TGF‐β or its receptors, or Smad, can also cause the formation of aortic dissection and AAA. ³⁵ , ³⁶ , ³⁷ These models are convenient for studying the relationships between specific molecular signaling pathways and AAA.

6. PROSPECTIVE

Animal models have been widely used to study the pathogenic mechanisms of AAA and to develop new prevention and treatment strategies. Different models are not always the same in terms of lesion location, lesion characteristics, intraluminal thrombus formation, aneurysm dilation characteristics, and acute rupture. A comparison of the major AAA models and the clinical presentations of human AAA patients is presented in Table 1. The PPE model well simulates the role of inflammation in the development of AAA and can control the site of aneurysm development, making it a good model for studying the relationship between inflammation and AAA. The Ang II‐induced AAA model may better represent aortic dissection and not fusiform AAAs. Currently, the most important method for the treatment of AAAs in clinical practice is still endovascular surgery. ² The development of new therapies depends on the successful identification of pathogenic mechanisms and effective targets. Many experimental studies on the pathogenesis of AAA and the development of new research strategies have been conducted in existing AAA animal models, but challenges remain. First, the modeling factors of these models often differ from the risk factors for the occurrence of human AAA; thus, the clinical translation of some research results based on the current models faces challenges. In addition, the existing models cannot simulate the entire process from onset to rupture as human AAAs do. This is particularly evident in the PPE model, the CaCl₂ model, and the Ang II infusion model because of the limited infusion/modeling time. Once external stimulation is removed, the AAA progresses through stages such as disease progression, repair, and partial regression and even maintains a stable state.

TABLE 1.

Comparison of AAA clinical characteristics between human and animal models.

Lesion characteristics	Human AAA	PPE‐induced model	CaCl2‐induced model	Ang II‐induced model
Lesion site	Infrarenal	Infrarenal	Infrarenal	Suprarenal
Aortic expanding curve	Continuous increase	With platform	With platform	Continuous increase
Aortic dissection	Partly	No	No	Yes
SMC depletion	Yes	Yes/severe	Yes	Yes/mild
Leucocyte infiltration	Yes	Yes/severe	Yes	Yes/mild
Rupture	Partly	Rare	Rare	Partly
Thrombus	Common	Rare	No	Yes
Atherosclerotic lesion	Common	No	No	Partly
Surgical procedure	NA	Difficult	Easy	Easy
Postoperative mortality	NA	Low	Lower	Mild
Modeling time	NA	2 weeks	4–6 weeks	4 weeks
Success rate	NA	Very high	Higher	Higher
Heterogeneity	NA	Low	Higher	Higher
Cost	NA	Low	Low	High

Abbreviations: AAA, abdominal aortic aneurysm; Ang II, angiotensin II; NA, not applicable; PPE, porcine pancreatic elastase; SMC, smooth muscle cell.

Because the current AAA models have limitations, the development of new AAA models should consider how to incorporate the disease risk factors observed in clinical AAAs, such as smoking, into the modeling method. Although many studies have analyzed the role of nicotine in the initiation and progression of AAA, the success rate of inducing AAA by infusing nicotine through an osmotic pump is too low, at only approximately 20%, and nicotine cannot be widely used in the laboratory. ³⁸ Considering that the mechanism of human AAA formation is complex, it is a long process involving repeated vascular stress and remodeling. Researchers may consider introducing the “two‐hit” theory, such as the intraluminal infusion of nicotine solution under pressure, which leads to the formation of AAAs under the dual action of mechanical tension stimulation and nicotine penetration‐induced local aortic inflammation, into AAA models. Moderate‐intensity hypertension (~140 mmHg), another risk factor for AAA, can lead to AAA formation without causing dissection. The development of these models is conducive to truly simulating the formation process of human AAAs and is conducive to translational research on AAAs. We can draw inspiration from the research and development history of statins, which are therapeutic drugs for another peripheral vascular disease, atherosclerosis. ³⁹ Although there are multiple theories regarding the pathogenesis of atherosclerosis, to date, statins have been successfully developed based on the theory of dyslipidemia and are thus used for the prevention and treatment of atherosclerotic diseases. Atherosclerosis in animal models is primarily induced by high‐fat and high‐cholesterol diets, similar to atherosclerotic risk factors in humans, which may be among the most important factors contributing to the successful clinical translation of statins. ³⁹ , ⁴⁰ , ⁴¹ Therefore, the development of new AAA models that incorporate real risk factors for human AAAs, which can better resemble human aneurysms, is one of the major directions for future modeling.

AUTHOR CONTRIBUTIONS

Kangli Tian: Conceptualization; funding acquisition; investigation; writing – original draft. Fizza Malik: Conceptualization; investigation; writing – review and editing. Sihai Zhao: Conceptualization; funding acquisition; writing – review and editing.

FUNDING INFORMATION

This study was supported by the Natural Science of Shaanxi Province (2023‐CX‐PT‐17) and the General Project of Natural Science Research in Luoyang Polytechnic College (2024B01).

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.

ETHICS STATEMENT

None.

Tian K, Malik F, Zhao S. Animal models for abdominal aortic aneurysms: Where we are and where we need to go. Anim Models Exp Med. 2025;8:573‐577. doi: 10.1002/ame2.12572