Suitable Preclinical Atherosclerosis Research Model: B6J-Apoe Knockout Mice

In this issue, we introduce the B6J-Apoe knockout (KO) mouse model for atherosclerosis research. Atherosclerosis, a specific type of arteriosclerosis, is a chronic immune-metabolic inflammatory disease that causes the buildup of fats, cholesterol and other substances on the artery walls. Since the 1990s, mice have been the primary animal model for studying atherosclerosis. Currently, the most commonly used mouse models of atherosclerosis are Apoe knockout mice and Ldlr knockout mice, respectively accounting for about 70% and 27% of research on the disease.

The Key Target Of Disease Pathology: APOE

APOE is a polymorphic gene with three (3) major alleles (APOE-ε2, APOE-ε3, and APOE-ε4) which encode three protein isoforms (apoE2, apoE3, and apoE4). The  APOE gene encodes Apolipoprotein E (Apo-E, ApoE), which is a polymorphic carrier protein associated with lipid particles that is a core component of plasma lipoproteins and is involved in the production, transport, and clearance of lipoproteins. ApoE protein is associated with chylomicrons, chylomicron remnants, high-density lipoproteins (HDL), very low-density lipoproteins (VLDL), and intermediate-density lipoproteins (IDL), showing a particular affinity for binding with high-density lipoproteins (HDL). ^[1]

ApoE is responsible for the transport of chylomicrons, serving as the most important lipid transport protein in the body due to its significant impact on lipid metabolism. Its interaction with low-density lipoprotein receptors (LDLR) is crucial for the normal processing (breakdown and metabolism) of triglyceride-rich lipoproteins. ^[2] In peripheral tissues, ApoE protein is mainly produced by the liver and macrophages, mediating cholesterol metabolism. Whereas in the central nervous system, it is primarily produced by astrocytes, acting as the major cholesterol carrier in the brain and is essential for the transport of cholesterol from astrocytes to neurons. ^[1-4] Additionally, ApoE protein, by forming complexes with activated C1q, becomes a target for the checkpoint inhibitor of the classical complement pathway. ^[5]

APOE Gene Variants and Disease Risk

APOE is polymorphic, with three major alleles among the human population: epsilon 2 (ε2), epsilon 3 (ε3), and epsilon 4 (ε4). These allelic forms differ by 1-2 amino acids at positions 112 and 158, which result in altered APOE structure and function. Notable features of each allele polymorphism are noted below:

• APOE3 has the highest worldwide allele frequency and is considered the neutral (wild type, WT) APOE genotype.
• Both APOE2 and APOE4 have unique biological implications and relevance for a range of neurological and cardiovascular diseases.
• APOE ε4 is a major genetic risk factor for Alzheimer disease: discussed in our Gene of the Week article.

Polymorphisms in the APOE gene are associated with Alzheimer's disease and various cardiovascular diseases and risks, including lipid accumulation, hyperlipidemia, atherosclerosis, hypercholesterolemia, and some mutations can cause familial lipoproteinemia or type III hyperlipoproteinemia (HLP III). In addition to the roles APOE plays in developmental diseases such as Alzheimer’s, studies have shown APOE to be implicated in the host response to a range of infectious pathogens, including herpes simplex virus type I (HSV1) and hepatitis C virus (HCV). Growing evidence supports the theory that host genetics are a factor in determining both susceptibility to and outcome of infections, which is exemplified by the APOE isoforms.

More Resources on How APOE Influences Risk for Multiple Pathologies

The significance of the APOE4 variant as the single largest genetic risk factor for late-onset AD among the human population was discussed in our Gene of the Week article:

“[Gene of the Week] Alzheimer's Disease and Genes - APOE (Apolipoprotein E)” >>

In 1997, the lab of Dr. Ruth Itzhaki was among the first to study the role of specific APOE alleles for their involvement in a range of diseases caused by infectious agents. Since this time, related evidence has suggested that certain APOE isoforms, such as carriers of an APOE-e4 allele, may confer various influences on pathogenic infectivity, disease severity, and even overall damage caused by infection. Several studies performed by the lab of Dr. Itzhaki have revealed the APOE genotype to have a modulatory effect on either susceptibility to or severity of damage by the pathogen in several infectious diseases, such as HSV1. We were able to discuss the connections between APOE polymorphisms, Alzheimer’s Disease, and Neurovirology with Dr. Ruth Itzhaki in the following article:

“Beyond Alzheimer’s Disease – Implications of APOE in Viral Pathology” >>

Development of Apoe Gene Knockout Mouse Model

The B6J-Apoe KO strain (Product Code: C001507) is an ApoE protein-deficient mouse model, created by using gene editing technology to knockout (KO) the human APOE gene homolog, Apoe, in mice. This results in the disruption of ApoE protein synthesis in the mouse, leading to elevated cholesterol levels and the spontaneous development of atherosclerosis. The elevation in cholesterol levels is further exacerbated when the mice are fed a high-fat diet (HFD).

Homozygous B6J-Apoe KO mice can survive and spontaneously develop atherosclerotic plaques with a normal diet, presenting mild symptoms. However, aortic atherosclerosis induced by a HFD is exacerbated and symptoms are more severe. The B6J-Apoe KO mouse model can be used for research in the fields of hypercholesterolemia, atherosclerosis, and Alzheimer's disease.

B6J-Apoe Knockout Mouse Model Validation Data

(1) Growth Curves

Figure 1. Body weight trajectories of B6J-Apoe KO and wild-type (WT) mice on normal diet (ND) and high-fat diet (HFD) conditions.

Results showed that B6J-Apoe KO mice grew at a similar rate under both normal dietary and HFD conditions. WT mice fed HFD gained weight, and internal inspection data showed that at 20 weeks, the aortic arch and aortic vessels in WT mice were normal and no atherosclerotic plaque was formed.*

*High-fat diet (HFD) was purchased from Medicine Ltd. (catalog number: MD12015). High-fat feeding protocol: Mice were started on a gradual transition to the high-fat diet at week 5 and began formal feeding on the high-fat diet at week 6.

(2) Biochemical Analysis of Blood Lipid Indicators: Male Mice

Figure 2. Serum lipid biochemical indicators of male B6J-Apoe KO and wild-type (WT) mice on normal diet and high-fat diet (HFD) conditions.

Data showed that under normal dietary conditions, male B6J-Apoe KO mice had different degrees of increases across the levels of triglyceride (TRIG), cholesterol (CHOL), and low-density lipoprotein (LDL-C) in their bodies compared with wild-type (WT) mice. HFD administration further increased the levels of these indicators in male B6J-Apoe KO mice. There was no significant difference in high-density lipoprotein (HDL-C) levels between male B6J-Apoe KO mice and WT mice.

(3) Biochemical Analysis of Blood Lipid Indicators: Female Mice

Figure 3. Serum lipid biochemical indicators of female B6J-Apoe KO and wild-type (WT) mice on normal diet and high-fat diet (HFD) conditions.

Under normal dietary conditions, female B6J-Apoe KO mice had significantly elevated levels of cholesterol (CHOL) and low-density lipoprotein (LDL-C) compared with wild-type (WT) mice. HFD administration further increased the levels of cholesterol (CHOL) and low-density lipoprotein (LDL-C) in female B6J-Apoe KO mice. There were no significant differences in high-density lipoprotein (HDL-C) and triglyceride (TRIG) levels between female B6J-Apoe KO mice and WT mice.

(4) Aortic pathology in male B6J-Apoe KO mice

Figure 4. Detection of atherosclerotic plaques in the aorta of male B6J-Apoe KO mice on normal diet (ND) and high-fat diet (HFD) conditions.

Male B6J-Apoe KO mice can spontaneously form atherosclerotic plaques in the aorta after 10 weeks on a normal diet, with mild symptoms. After induction by HFD feeding, the atherosclerotic plaques in the aorta were exacerbated, and the symptoms became more obvious after week 12 of HFD; the symptoms continued to worsen at week 14 and reached a mid-to-late stage pathological phenotype at week 16.

(5)  Aortic pathology in female B6J-Apoe KO mice

Figure 5. Detection of atherosclerotic plaques in the aorta of female B6J-Apoe KO mice on normal diet (ND) and high-fat diet (HFD) conditions.

Female B6J-Apoe KO mice can spontaneously form atherosclerotic plaques in the aorta after 12 weeks on a normal diet, with mild symptoms. After induction by high-fat diet (HFD) feeding, the aortic atherosclerotic plaques were exacerbated, with the symptoms becoming more obvious after week 14 and continuing to worsen during week 16.

Recommended Mouse Models for Metabolic and Cardiovascular Disease Research

As mentioned earlier, the current most commonly-used models for atherosclerosis research primarily involve Apoe and Ldlr gene knockouts (KO). In addition to the B6J-Apoe KO mouse model detailed in this article, Cyagen Biosciences has also developed the Ldlr KO (em) mouse model for research into atherosclerosis and hypercholesterolemia, among other diseases (such as Nonalcoholic Steatohepatitis). Furthermore, we can support preclinical drug development by providing multiple types of metabolic-related genetically engineered models to such as B6-ob/ob mice (Lep KO mice), Uox-KO Mice, Atp7b KO mice, and Foxj1 KO mice. If you are interested in our metabolic and cardiovascular disease models or wish to collaborate with us to develop models that meet your research needs, please contact us to receive a complimentary consultation and quote!

References:

[1]Huang Y, Mahley RW. Apolipoprotein E: structure and function in lipid metabolism, neurobiology, and Alzheimer's diseases. Neurobiol Dis. 2014 Dec;72 Pt A:3-12.

[2]Mahley RW, Weisgraber KH, Huang Y. Apolipoprotein E: structure determines function, from atherosclerosis to Alzheimer's disease to AIDS. J Lipid Res. 2009 Apr;50 Suppl(Suppl):S183-8.

[3]Wang H, Kulas JA, Wang C, Holtzman DM, Ferris HA, Hansen SB. Regulation of beta-amyloid production in neurons by astrocyte-derived cholesterol. Proc Natl Acad Sci U S A. 2021 Aug 17;118(33):e2102191118.

[4]Serrano-Pozo A, Das S, Hyman BT. APOE and Alzheimer's disease: advances in genetics, pathophysiology, and therapeutic approaches. Lancet Neurol. 2021 Jan;20(1):68-80. 

[5]Yin C, Ackermann S, Ma Z, Mohanta SK, Zhang C, Li Y, Nietzsche S, Westermann M, Peng L, Hu D, Bontha SV, Srikakulapu P, Beer M, Megens RTA, Steffens S, Hildner M, Halder LD, Eckstein HH, Pelisek J, Herms J, Roeber S, Arzberger T, Borodovsky A, Habenicht L, Binder CJ, Weber C, Zipfel PF, Skerka C, Habenicht AJR. ApoE attenuates unresolvable inflammation by complex formation with activated C1q. Nat Med. 2019 Mar;25(3):496-506.