What Are the Key Differences Between Apoe−/− and Ldlr−/− Mice in Atherosclerosis Research?

Apoe−/− and Ldlr−/− mice are commonly used models in atherosclerosis research, but understanding their differences is crucial for accurate interpretation of results; MERCEDES-DIAGNOSTIC-TOOL.EDU.VN provides expertise in navigating these complexities. We offer comprehensive insights into diagnostic tools and repair strategies, ensuring you have the knowledge to address automotive challenges effectively. Explore our resources today for guidance on vehicle diagnostics, maintenance, and repair.

1. What are Apoe−/− and Ldlr−/− Mice Models?

Apoe−/− and Ldlr−/− mice are two extensively utilized murine models in atherosclerosis research, offering valuable insights into the genetic and environmental factors influencing lesion formation and composition. These models, however, exhibit significant differences in their mechanisms of action and lipoprotein profiles, necessitating careful consideration when interpreting research findings.

1.1. Understanding the Apoe−/− Model

Apoe−/− mice lack the apolipoprotein E (apoE) gene, which plays a crucial role in lipid metabolism. ApoE is a key ligand for the uptake of chylomicron and VLDL remnants into hepatocytes via the LDL receptor, LDL receptor-related protein 1, and cell surface heparan sulfate proteoglycans.

1.1.1. How ApoE Deficiency Impacts Lipid Levels

The absence of apoE leads to impaired clearance of these lipoprotein remnants, resulting in hyperlipidemia even on a normal chow diet. This effect is further exacerbated by a high-fat, high-cholesterol diet.

1.1.2. Lipoprotein Profile in Apoe−/− Mice

The predominant lipoproteins that accumulate in Apoe−/− mice are apoB48-containing, cholesteryl ester-rich particles, derived from chylomicron and VLDL remnants. Additionally, these mice typically exhibit low levels of HDL cholesterol compared to wild-type and Ldlr−/− mice.

1.2. Understanding the Ldlr−/− Model

Ldlr−/− mice, on the other hand, lack the LDL receptor, which is responsible for the uptake of LDL particles into cells.

1.2.1. How LDL Receptor Deficiency Impacts Lipid Levels

In Ldlr−/− mice, the primary lipoprotein is apoB100-containing LDL. While these mice do not readily develop atherosclerosis on a standard chow diet, a high-cholesterol diet, with or without high fat, is necessary to induce hyperlipidemia and drive atherogenesis.

1.2.2. Lipoprotein Profile in Ldlr−/− Mice

When fed a high-cholesterol, low-fat diet, Ldlr−/− mice accumulate primarily LDL in their plasma. A high-cholesterol, high-fat diet also elevates VLDL levels, which tend to be more triglyceride-rich compared to those in Apoe−/− mice.

Apoe−/− and Ldlr−/− mice are two murine models that are frequently used to study the development and progression of atherosclerosis.

2. What are the Key Differences in Atherosclerosis Development Between the Models?

Several crucial distinctions exist between Apoe−/− and Ldlr−/− mice concerning atherosclerosis development, including lesion morphology, lipoprotein-driven inflammation, and immune cell involvement. These variations underscore the necessity for cautious interpretation and highlight the importance of considering model-specific characteristics in atherosclerosis research.

2.1. Variance in Lesion Morphology

Morphological differences are evident in atherosclerotic lesions between the two models. A study comparing Apoe−/− and Ldlr−/− mice fed a high-cholesterol diet (1.25% with cholic acid) for 1 to 3 months revealed that Apoe−/− mice exhibited higher plasma cholesterol levels and larger aortic root lesions. These lesions also displayed larger necrotic cores, more chondrocytes and bone formation, and increased smooth muscle cells and matrix after 3 months on the diet, relative to Ldlr−/− mice 4.

2.2. Lipoproteins Drive the Inflammatory Process

The specific lipoproteins driving the inflammatory process also differ between the models. In Apoe−/− mice, the absence of apoE results in the accumulation of chylomicron and VLDL remnants, which are rich in cholesteryl esters. These remnants contribute to the initiation and progression of atherosclerotic lesions.

2.2.1. Role of VLDL Cholesterol

In Ldlr−/− mice, a strong correlation exists between aortic root lesion size and VLDL cholesterol concentration when fed a Western-type diet for 12 weeks 9. Conversely, in Apoe−/− mice maintained on a standard chow diet for 27 weeks, the inverse of HDL cholesterol was found to be the best predictor of aortic root lesion size. This suggests that lipoproteins may not drive atherogenesis identically at different vascular sites in the two models.

2.3. Involvement of Macrophages

Macrophages, critical cells in developing lesions, exhibit differences in their behavior between the two models. Macrophage-derived apoE can rescue the phenotype of Apoe−/− mice by reducing plasma lipoprotein levels and atherosclerosis, as demonstrated by bone marrow transplantation studies 20, 21.

2.3.1. Impact of LDL Receptor Expression

The LDL receptor is down-regulated in the presence of hyperlipidemia, and transplantation of Ldlr-expressing bone marrow has minimal impact on atherosclerosis in Ldlr−/− mice 22. Furthermore, macrophage-specific expression of a human apoE transgene in Apoe−/− mice reduces atherosclerosis even in animals with matched plasma cholesterol levels 24.

2.4. The Role of Lymphocytes

While monocyte/macrophages are required for the development of atherosclerosis, this is not the case for T and B cells. Animals lacking these latter cells develop fairly robust atherosclerotic lesions [35](#R35], [36](#R36]. However, this conclusion is not as straightforward as it suggests, since there are several types of T and B cells that could exert either pro-inflammatory or anti-inflammatory effects [37](#R37].

Macrophages, which are derived from blood monocytes, are the first and critical cells to accumulate in the developing lesions, forming foam cells and promoting lesion progression.

3. How Do Lipoproteins Influence the Initiation of Atherosclerosis?

Lipoproteins play a pivotal role in initiating atherosclerosis, with the influx and retention of apoB-containing lipoproteins into the subendothelial space at atherosusceptible vascular sites being an early hallmark of the disease. Understanding the specific mechanisms by which lipoproteins contribute to atherogenesis in Apoe−/− and Ldlr−/− mice can offer insights into the broader processes of atherosclerosis.

3.1. Lipoprotein Retention and Aggregation

One of the earliest events in atherogenesis is the influx of apoB-containing lipoproteins into the subendothelial space at atherosusceptible vascular sites, where they are retained and aggregate 10. This process occurs before macrophage accumulation [11](#R11].

3.1.1. Ultrastructural Analysis

Ultrastructural analysis of the subendothelial space in Apoe−/− mice reveals clusters of lipoprotein particles ranging from 33 to 66 nm in diameter, corresponding to the sizes of chylomicron remnants and intermediate-density lipoproteins.

3.2. Influence of Lipoprotein Size and Composition

Lipoprotein size, permeability, composition, and association with matrix components determine their retention in the subendothelial space.

3.2.1. Resolving the Difficulty

Young and colleagues 12 engineered Apoe−/− and Ldlr−/− mice expressing only apoB100 (apoB100/100) to address this complexity. These models allowed for a comparison of lipoprotein effects while controlling for plasma cholesterol levels. Female Ldlr−/− apoB100/100 and Apoe−/− apoB100/100 mice fed standard chow exhibited almost identical plasma cholesterol levels, but the size of their lipoproteins differed significantly.

3.2.2. Lipoprotein Size Matters

The Ldlr−/− apoB100/100 mice had LDL with a mean diameter of 24 nm as the predominant plasma lipoprotein, while the Apoe−/− apoB100/100 mice primarily had larger VLDL-sized particles with a mean diameter of 63 nm 13. Despite similar cholesterol levels, the extent of atherosclerosis in the whole aorta was higher in the Ldlr−/− apoB100/100 mice. This suggests that the higher number of LDL-sized particles in Ldlr−/− apoB100/100 mice, increased vascular wall permeability to LDL-sized particles, or their retention through interaction with matrix components is more atherogenic.

3.3. Lipoprotein Digestion and Matrix Interaction

The digestion of lipoproteins by sphingomyelinase secreted from endothelial cells or macrophages leads to protein aggregation and interaction with matrix proteins and proteoglycans 14. Although Apoe−/− mice have lipoproteins enriched in sphingomyelin compared to Ldlr−/− mice 15, the deletion of sphingomyelinase results in similarly reduced lesion formation in both models 16.

3.4. ApoB and Proteoglycan Binding

The regions of apoB that mediate the association of accumulating lipoproteins with proteoglycans in the intima differ between the two models. In Ldlr−/− mice, LDL primarily associates with heparin sulfate proteoglycans via interaction with apoB100 residues 3359-3369 17, 18. This sequence is absent in apoB48, the predominant protein of lipoproteins in Apoe−/− mice. In apoB48-containing lipoproteins, residues 84-94 are responsible for binding to proteoglycans, a site potentially masked in apoB100-containing lipoproteins 19.

Lipoproteins play a role in the initation of atherosclerosis.

4. How Do Macrophages Contribute to Atherogenesis in Apoe−/− and Ldlr−/− Mice?

Macrophages play a central role in atherogenesis, acting as key mediators in the inflammatory processes that drive lesion formation. These cells express both apoE and the LDL receptor, but only apoE is a secreted protein, enabling it to function extracellularly. Examining the specific roles and functions of macrophages in Apoe−/− and Ldlr−/− mice can reveal critical insights into their respective contributions to atherosclerosis.

4.1. Macrophage-Derived ApoE and Atheroprotection

Macrophage-derived apoE has the capacity to rescue the phenotype of Apoe−/− mice, reducing plasma lipoprotein levels and atherosclerosis. This was initially demonstrated with the transplantation of wild-type bone marrow expressing apoE into Apoe−/− recipients 20, [21](#R21].

4.1.1. Bone Marrow Transplantation Studies

Bone marrow transplantations are frequently performed to determine whether a gene of interest operates mainly in bone marrow-derived cells (e.g., macrophages) or non-hematopoietic vascular cells. Given that macrophage-derived apoE has a profound impact on atherosclerosis, bone marrow transplantation studies are generally performed in Ldlr−/− recipients to avoid confounding effects.

4.2. LDL Receptor and Atherosclerosis

The LDL receptor is down-regulated in the presence of hyperlipidemia, and the transplantation of Ldlr-expressing bone marrow has very little if any effect per se on atherosclerosis in Ldlr−/− mice 22.

4.2.1. Macrophage-Specific ApoE Expression

In addition to bone marrow transplantation, the macrophage-selective repair of the hypomorphic apoE gene 23 also is atheroprotective. In these experiments, the reduction in plasma lipoproteins and atherosclerosis go hand in hand. Macrophage-specific expression of a human apoE transgene in Apoe−/− mice reduces atherosclerosis [24](#R24] even in animals matched for plasma cholesterol levels. Furthermore, mice transgenic for adrenal apoE expression at different levels show that at low apoprotein levels, atherosclerosis may be reduced with little or no effect on plasma lipids [25](#R25].

4.3. Additional Atheroprotective Functions of ApoE

These studies highlight two features of the extrahepatic production of apoE in securing atheroprotection. First, macrophages are not unique in their capacity to rescue the apoE-deficient phenotype. Second, apoE has additional atheroprotective functions beyond its capacity to lower plasma lipids.

4.3.1. Anti-Inflammatory Properties

ApoE has been shown to exhibit anti-inflammatory properties, polarizing macrophages from the pro-inflammatory M1 subset to the anti-inflammatory M2 subset [26](#R26].

4.3.2. Anti-Oxidative Activity

ApoE also has anti-oxidative activity [27](#R27], 28, which could contribute to its capacity to reduce atherosclerosis. It also promotes cellular cholesterol efflux from macrophages. None of these properties is shared by the bone marrow or extrahepatic expression of the LDL receptor.

4.4. ApoE Secretion in Ldlr−/− Mice

Peritoneal macrophages isolated from Ldlr−/− mice fed a Western-type diet show reduced secretion of apoE 29. If peritoneal macrophages reflect the phenotype of lesional macrophages, then reduced apoE production in aortic macrophages may also contribute to the development of atherosclerosis in the Ldlr−/− mice.

4.5. Bone Marrow-Derived Cells in Atherosclerotic Lesions

Macrophages are but one of the cells derived from the bone marrow that populate the atherosclerotic lesion and modulate its evolution.

4.5.1. Myeloid Specific Knockout of ABCA1

Myeloid-specific knockout of ABCA1 in the Ldlr−/− background had little impact on atherosclerosis, though prior studies using bone marrow transplantation from global Abca1-deficient donors had suggested that the transporter was atheroprotective [30](#R30].

4.6. Monocyte Accumulation and Cholesterol Homeostasis

Macrophages in lesions are derived from the influx of blood monocytes, although recent work has highlighted the importance of local proliferation, especially in advanced lesions, to the macrophage content of the lesion [31](#R31], [32](#R32].

4.6.1. Leukocyte Levels and ApoE Function

The level of blood monocytes and neutrophils is a risk factor for atherosclerosis and their level can be regulated by cholesterol homeostasis in the progenitor cells of the bone marrow. In hyperlipidemic or ABCA1/ABCG1-deficient animals, the plasma membranes of these progenitor cells are enriched in lipid rafts, where signaling receptors, including IL-3 and GM-CSF receptors, are concentrated.

4.6.2. Cell Surface ApoE and Cholesterol Removal

Cell surface apoE bound to heparan sulfate proteoglycans functions in a cell-autonomous fashion in the progenitor cells to facilitate the removal of cellular cholesterol. This property of apoE probably accounts for the higher leukocytosis in Western-type diet-fed Apoe−/− animals than in Ldlr−/− animals [33](#R33]. The cell-autonomous apoE may also function in peripheral macrophages, such as those in the lesions, to enhance cholesterol efflux from macrophage foam cells [34](#R34].

Bone marrow transplantation is a common method used to investigate whether a gene product or cell type influences atherosclerosis development.

5. What Role Do Lymphocytes Play in Atherosclerosis in These Models?

The role of lymphocytes, including T and B cells, in atherosclerosis is complex and not as straightforward as that of macrophages. While macrophages are essential for lesion development, the necessity of T and B cells is less clear. The impact of lymphocyte subsets can differ by arterial site and the specific model used, underscoring the intricate interplay of immune cells in atherogenesis.

5.1. T and B Cell Deficiency and Atherosclerosis

Animals lacking T and B cells can still develop robust atherosclerotic lesions, indicating that these cells are not strictly required for atherogenesis [35](#R35], [36](#R36].

5.1.1. Complexity of T and B Cell Function

However, this conclusion is not as straightforward as it suggests, since there are several types of T and B cells that could exert either pro-inflammatory or anti-inflammatory effects [37](#R37]. Th1 and invariant natural killer T (iNKT) cells are pro-inflammatory, while regulatory T cells (Tregs) and possibly Th2 cells are anti-inflammatory. Conflicting results have been reported for the role Th17 in atherosclerosis [38](#R38].

5.2. Arterial Site Specificity

The relative contribution of these cells to the atherogenic process may differ by arterial site at which lesions develop.

5.2.1. Adaptive Immune System Deficiency

Deficiency of the adaptive immune system in Apoe−/− mice and Ldlr−/− mice reduces lesion formation at the aortic root but not at the innominate artery [35](#R35], [39](#R39]. Several studies indicate the complexity of these counterbalances.

5.3. T Cell Density and Lesion Maturation

T cell density in maturing lesions in both models declines between 1-3 months on a cholate-containing atherogenic diet [4](#R4]. However, this affects the Apoe−/− animals more profoundly than the Ldlr−/− animals, which have a higher T cell density at all times.

5.4. Effector T Cells and Treg Cells

When effector T cells and Treg cells were separately examined in Ldlr−/− mice fed a cholate-free atherogenic diet, effector T cells increased in lesions with time on diet, while circulating and lesion Tregs peaked at 4 weeks of diet and dropped dramatically thereafter [40](#R40].

5.4.1. Ratio of Lesion Effector T/Treg Cells

Thus, the ratio of lesion effector T/Treg cells was profoundly reduced by the end of 20 weeks of diet, illustrating how sensitive the lesion composition is to the time at which lesions are sampled.

5.5. Oxidation of Lipoproteins and Autoantibodies

Oxidation of lipoproteins is thought to be a major mediator of atherogenesis. Inasmuch as apoE has anti-oxidative properties, the oxidation of lipoproteins may be more prominent in the Apoe−/− model [41](#R41], [42](#R42]. These oxidatively modified lipoproteins serve as autoantigens eliciting the production of autoantibodies.

5.5.1. Autoantibody Production

The B1 cell-produced antibody EO6/T15, which recognizes the phosphocholine head group of oxidized phospholipids, was cloned from the Apoe−/− mouse [43](#R43], given that antibodies to oxidized LDL epitopes are especially high in this model. Accumulating evidence suggests that these autoantibodies are atheroprotective [37](#R37].

5.6. Response to ApoA-I Deficiency

An unusual differential effect of T cells in these two animal models is seen in their response to apoA-I deficiency. The Ldlr−/− Apoa1−/− mice fed palm oil and quite modest levels of dietary cholesterol exhibit obvious dermatitis, lymphadenopathy with an increase in the number and sterol loading of T cells [44](#R44].

5.6.1. Cholesterol Load in Macrophages

Peritoneal macrophages from the double knockout mice did not have an increased load of cholesterol compared to Ldlr−/− mice [45](#R45], perhaps because the plasma cholesterol was substantially lower in the double knockouts. With Apoe−/−Apoa1−/− mice fed a Western-type diet, no obvious skin lesions were noted, and the peritoneal macrophages isolated at 6 weeks of diet had twice the cholesterol load compared to their single Apoe−/− counterparts [46](#R46].

5.6.2. ApoE’s Role in Cholesterol Equilibrium

Whether apoE, either exogenous or cell autonomous, contributes to the equilibrium of cholesterol in skin and peritoneal macrophages in these models needs further study.

T cells are involved in the adaptive immune response and have been implicated in atherosclerosis development.

6. Are Atherosclerosis-Relevant Gene Expressions Similar in Both Models?

Both the Apoe−/− and Ldlr−/− models have been employed as platforms to assess the role of a large variety of genes in atherogenesis [47](#R47], [48](#R48]. The majority of the gene functions examined have utilized only one model, with most studies using the Apoe−/− mice. In most studies, this involved generating double knockouts of the gene of interest, although in some cases, especially with the Ldlr−/− model, bone marrow transplantation was used.

6.1. Challenges in Assessing Gene Influence

To obtain a true direct sense of how these genes influence atherogenesis, atherosclerosis should be examined with both models subject to the same gene knockout in the same vivarium (i.e., microbiome effects), subject to the same diet or the same plasma cholesterol load, and sampled at multiple arterial sites of both males and females and at several times during the evolution of lesions, given the highly dynamic inflammatory process associated with atherogenesis.

6.2. Factors to Consider in Evaluating Model Similarity

Satisfying these requirements is certainly very difficult and may be impossible in view of the differences in plasma lipoproteins in the two models. It is perhaps not surprising, therefore, that few studies approach these ideal comparisons. In any event, these considerations must be borne in mind in evaluating questions of the similarity of insight into the atherogenic process provided by the two models.

6.3. Potential for False Results

Additionally, it is necessary to take account of the possibility of false positive or false negative results, which, if present, would compound the interpretation of the differences discussed below [49](#R49]. Unfortunately, the competitive nature of publication decisions does not make for ready publication of replicate studies, which would be important to demonstrate the reproducibility of results, especially in light of the many experimental differences that may influence study outcomes.

6.4. Genes with Similar Effects in Both Models

As tabulated by Hopkins [48](#R48], several gene functions have been examined in the two models, with atherosclerosis responding in a similar direction (either increase or decrease of lesions or no effect). In most cases, changes in lesion area or coverage are evaluated. Whether this is attributable to a change in the number of cells in the lesion or some cell-intrinsic changes is quite uncertain, partly because few investigators set out to make this distinction.

6.5. Apoptosis Study

One study examining apoptosis in the aortic root in Apoe−/−Chop−/− and Ldlr−/−Chop−/− male mice, performed in the same laboratory, comes close to meeting the above-specified requirements [50](#R50]. A similar reduction in lesion area and necrotic plaque area was observed in both models, suggesting that the differences in the two models do not impact responsiveness to the removal of CHOP function.

6.6. IFNγ Mice

The findings are more complex in Ifng−/− mice. Male and female Ldlr−/− and Ldlr−/− Ifng−/− mice were fed a cholesterol-enriched diet for either 8 or 20 weeks [51](#R51]. Lesions in the aortic arch and descending aorta were reduced in the absence of IFNγ. Macrophages and smooth muscle cells were reduced in the lesions after 8 weeks of diet but not at 20 weeks, and there was a marked decline in lesional T cells between 8 and 20 weeks regardless of the presence of the cytokine.

6.7. Differential Cytokine Interaction

In the Apoe−/−Ifng−/− mice, though both genders were examined, only male mice exhibited a reduction in lesions (in the aortic arch and ascending aorta) when fed a chow diet or Western-type diet [52](#R52]. Only in the male mice on the Western-type diet was a reduction in T cells in the lesions observed. These results with the different mouse models, though not performed in the same laboratory, suggest that there was a complex interaction between the cytokine and gender between the two models.

6.8. Genes Exhibiting Different Phenotypes

There are also gene knockouts that exhibit different phenotypes in the two models. FXR is a bile acid-activated nuclear receptor. Female Apoe−/− Fxr−/− mice fed a high-fat diet developed less aortic root atherosclerosis than did Apoe−/− control mice [53](#R53]. On the other hand, double knockout mice in the Ldlr−/− background fed a Western-type diet also had a lower lesion area (as measured by en face analysis) but only in male mice [54](#R54].

6.9. Hepatic Lipase

Hepatic lipase is a plasma lipase that not only hydrolyzes plasma lipoprotein phospholipids but can also function as a ligand for the uptake of lipoproteins in the liver. Knocking out this function is associated with increased lesion formation in Ldlr−/− mice [55](#R55] and decreased lesion formation in Apoe−/− mice [56](#R56].

6.10. IL-6 Function

In the Ldlr−/− model, the removal of IL-6 function had no impact on plasma lipids or atherosclerotic lesions in the aortic root [57](#R57], while in the Apoe−/− model, loss of this function was found to be associated with an increment in lesions in the whole aorta and the aortic arch, associated with a decline in IL-10 levels [58](#R58]. It is possible that changes in VLDL and LDL levels in the Apoe−/− Il6−/− model contributed to the lesion enhancement.

6.11. CD40 Results

Results with abrogation of CD40, a costimulatory molecule, are complex. The lesions in Ldlr−/−Cd40−/− mice are similar in size to the lesions in Ldlr−/− mice [59](#R59], though the transplantation of bone marrow from CD40-deficient mice into Ldlr−/− recipients fed the Western diet had reduced lesions [60](#R60]. On the other hand, Apoe−/−Cd40−/− mice fed chow have notably reduced lesions, apparently as a result of impaired signaling via TRAF6 (TNF receptor-associated factor 6) [60](#R60]. The basis for the difference between these two studies is not clear.

6.12. PECAM-1 Knockout Results

Although platelet-endothelial cell adhesion molecule 1 (PECAM-1) knockout results in a reduction in aortic arch atherosclerosis, particularly the inner curvature, in both murine models 61-[64](#R64], there was a difference in the atherosclerotic response in other vascular sites. No difference was noted in the extent of atherosclerosis in the thoracic descending and abdominal aortas of Apoe−/−Pecam1−/− and Apoe−/− mice fed chow, but the atherosclerotic lesions at these arterial sites were increased in the double knockout mice compared to Apoe−/− mice when the animals were fed the Western-type diet for 13 weeks in one study [64](#R64], but not in another [62](#R62]. Similarly, these lesions were increased in Western-type diet-fed Ldlr−/− Pecam1−/− mice [61](#R61].

6.12.1. Bone Marrow Transplantation Experiments

Bone marrow transplantation experiments indicated that PECAM1 expressed on endothelial cells and hematopoietic cells (leukocytes and platelets) inhibits lesion formation in the thoracic and abdominal aortas in Apoe−/− [64](#R64] and Ldlr−/− mice [61](#R61]. Thus, PECAM-1 regulation of lesion formation was subject to complex influences by model and site of aorta examined. Much further work is required to understand the mechanistic basis of these differences.

6.13. GM-CSF Knockout Results

Complex results were also seen with the knockout of Gmcsf in the two models, although in both models, the plaque phenotype is apparently independent of the growth-promoting action of GM-CSF. In the Ldlr−/− model, mice fed the Western diet for 12 weeks revealed no change in lesion size in the aortic root, though there was a reduction in macrophage apoptosis and plaque necrosis in the absence of GM-CSF [65](#R65].

6.13.1. Reduced GM-CSF Induction

This plaque phenotype is apparently attributable to reduced GM-CSF induction of the expression of IL-23, leading to increased levels of Bcl-2 in the lesions that likely account for the attenuated apoptosis. In contrast, Apoe−/− mice lacking GM-CSF exhibited increased size of the aortic root lesions with increased macrophage content and reduced PPARγ and ABCA1 expression [66](#R66]. PPARγ and ABCA1 expression in the aortic root were not affected by Gmcsf deficiency in Ldlr−/− mice [65](#R65].

6.13.2. Signaling Results

Signaling via the GM-CSF receptor and the IL-3 receptor in Apoe−/− bone marrow transplanted into Ldlr−/− mice also did not affect lesion size, though the lesions had fewer macrophages and increased necrosis that was accompanied by reduced blood monocytes and neutrophils and their precursors in the bone marrow [67](#R67].

The effect of gene manipulation on atherosclerotic lesion area in the two murine models.

7. How Does Coronary Artery Atherosclerosis Develop in These Models?

Neither Apoe−/− nor Ldlr−/− mice are characterized by significant coronary artery atherosclerosis under normal conditions. However, modified models based on these backgrounds have been developed in which coronary artery atherosclerosis is a prominent feature.

7.1. Models with Coronary Artery Involvement

Mice doubly deficient in Apoe and the Ldlr develop obstructive coronary artery atherosclerosis and myocardial infarction, associated with hyperlipidemia, when fed a Western-type diet [68](#R68]. Macrophage-specific overexpression of urokinase on an Apoe−/− background also results in obstructive, lipid-rich coronary lesions associated with myocardial infarction [69](#R69].

7.2. Models with Premature Mortality

Perhaps the most dramatic model involving premature mortality resulting from occlusive coronary artery atherosclerosis leading to myocardial infarction and cardiomegaly is seen in chow-fed mice lacking both Apoe and scavenger receptor B1 (Srb1) [70](#R70].

7.2.1. Hypomorphic ApoE Mice

The hypomorphic apoE (Apoeh/h) Srb1−/− fed an atherogenic diet containing cholate produces a similar coronary artery and cardiac phenotype [71](#R71]. The binding of HDL to SR-B1 triggers activation of the adapter protein PDZK1, Akt1, and eNOS in endothelial cells. Indeed, mice with Pdzk1, Akt1, or Enos deficiency in the Apoe−/− background develop coronary atherosclerosis [72](#R72], but an atherogenic diet is required to observe the phenotype.

7.3. Akt1 Deficiency and Bone Marrow Transplantation

In the case of Akt1, bone marrow transplantation studies indicate that the expression of this protein in the vasculature, probably endothelial cells, is more important for coronary artery atheroprotection than its expression on hematopoietic cells [73](#R73]. Akt has at least two isoforms, Akt1 and Akt3.

7.4. Akt Isoforms and Atherosclerosis

Unlike the atherosclerosis phenotype of Akt1 deficiency in the Apoe−/− background, Akt3 deficiency in this background is associated with an increase in atherosclerosis. This appears to be attributable to deficient expression of this isoform in macrophages and increased macrophage lipoprotein uptake, likely mediated by pinocytosis [74](#R74].

7.4.1. SRB1 Expression

This is in contrast to the attenuation of this phenotype upon transplantation of Srb1-expressing bone marrow into the hypomorphic apoE mice [75](#R75], suggesting that SBR1 expression on bone marrow-derived cells protects against coronary artery atherosclerosis. This suggests that SRB1 expressed in either macrophages or endothelial cells may contribute in a complex way to the coronary artery lesions.

7.5. Ldlr−/− Srb1−/− Mice

Coronary artery lesions are also seen in Ldlr−/− Srb1−/− mice, but again only if fed an atherogenic diet [76](#R76].

Although neither Apoe−/− nor Ldlr−/− mice spontaneously develop coronary artery atherosclerosis, genetic manipulation or diet can promote its development.

8. Future Perspective

The question of how Apoe−/− and Ldlr−/− mice compare in atherosclerosis research is not easily answered. This is largely due to the difficulty of normalizing the influences on atherosclerosis, as reflected by the level and nature of the apoB-containing lipoproteins in the two models, and differences in experimental design, including the duration of the experiment, the dietary composition, the gender of the mice, the protocol of arterial sampling for the assessment of lesions at one or more arterial sites, and the possible influence of the intestinal microbiome that may vary between vivariums. There are very few reports in which these influences are the same or very similar.

8.1. Hypercholesterolemia Induction

What is common to both models is the need to induce hypercholesterolemia. For the Ldlr−/− model, it is largely the defect in the hepatic receptor that accounts for the hypercholesterolemia, while deficiencies in apoE in hepatocytes or other cells, notably macrophages, may contribute to the hyperlipidemia.

8.2. Hepatic LDL Receptor Deficiency

Even though the deficiency of the hepatic LDL receptor is critical for the development of hypercholesterolemia, the possible influence of the receptor on other relevant vascular and immune cells has not been intensively studied.

8.3. Emerging Risk Factors

Recent work has drawn important attention to risk factors other than hypercholesterolemia per se. Most notable of these is the blood leukocytes, especially monocytes, which appear to be differentially affected by cell-autonomous apoE.

8.4. Cholesterol Efflux

Cholesterol efflux, which may play an important atheroprotective role, may also respond to cell-autonomous apoE in the macrophages in the atherosclerotic lesions. Thus, apoE may contribute to atheroprotection separately from its role in controlling hypercholesterolemia.

8.5. Environmental and Genetic Influences

Many investigators have employed one or the other model in which to explore environmental or genetic influences on lesion formation, composition, and stability. These experiments are often performed without regard to whether each atherosusceptible model would yield comparable results.