Effect of bone marrow transplantation on lipoprotein metabolism and atherosclerosis in LDL receptor-knockout mice

Top Five Stories of the Cellular Landscape and Therapies of Atherosclerosis: Current Knowledge and Future Perspectives

Atherosclerosis (AS) is characterized by impairment and apoptosis of endothelial cells, continuous systemic and focal inflammation and dysfunction of vascular smooth muscle cells, which is documented as the traditional cellular paradigm. However, the mechanisms appear much more complicated than we thought since a bulk of studies on efferocytosis, transdifferentiation and novel cell death forms such as ferroptosis, pyroptosis, and extracellular trap were reported. Discovery of novel pathological cellular landscapes provides a large number of therapeutic targets. On the other side, the unsatisfactory therapeutic effects of current treatment with lipid-lowering drugs as the cornerstone also restricts the efforts to reduce global AS burden. Stem cell- or nanoparticle-based strategies spurred a lot of attention due to the attractive therapeutic effects and minimized adverse effects. Given the complexity of pathological changes of AS, attempts to develop an almighty medicine based on single mechanisms could be theoretically challenging. In this review, the top stories in the cellular landscapes during the initiation and progression of AS and the therapies were summarized in an integrated perspective to facilitate efforts to develop a multi-targets strategy and fill the gap between mechanism research and clinical translation. The future challenges and improvements were also discussed.

Novel Strategies in the Early Detection and Treatment of Endothelial Cell-Specific Mitochondrial Dysfunction in Coronary Artery Disease

Although elevated cholesterol and other recognised cardiovascular risk factors are important in the development of coronary artery disease (CAD) and heart attack, the susceptibility of humans to this fatal process is distinct from other animals. Mitochondrial dysfunction of cells in the arterial wall, particularly the endothelium, has been strongly implicated in the pathogenesis of CAD. In this manuscript, we review the established evidence and mechanisms in detail and explore the potential opportunities arising from analysing mitochondrial function in patient-derived cells such as endothelial colony-forming cells easily cultured from venous blood. We discuss how emerging technology and knowledge may allow us to measure mitochondrial dysfunction as a potential biomarker for diagnosis and risk management. We also discuss the "pros and cons" of animal models of atherosclerosis, and how patient-derived cell models may provide opportunities to develop novel therapies relevant for humans. Finally, we review several targets that potentially alleviate mitochondrial dysfunction working both via direct and indirect mechanisms and evaluate the effect of several classes of compounds in the cardiovascular context.

The low-density lipoprotein receptor-mTORC1 axis coordinates CD8+ T cell activation

Activation of T cells relies on the availability of intracellular cholesterol for an effective response after stimulation. We investigated the contribution of cholesterol derived from extracellular uptake by the low-density lipoprotein (LDL) receptor in the immunometabolic response of T cells. By combining proteomics, gene expression profiling, and immunophenotyping, we described a unique role for cholesterol provided by the LDLR pathway in CD8+ T cell activation. mRNA and protein expression of LDLR was significantly increased in activated CD8+ compared to CD4+ WT T cells, and this resulted in a significant reduction of proliferation and cytokine production (IFNγ, Granzyme B, and Perforin) of CD8+ but not CD4+ T cells from Ldlr -/- mice after in vitro and in vivo stimulation. This effect was the consequence of altered cholesterol routing to the lysosome resulting in a lower mTORC1 activation. Similarly, CD8+ T cells from humans affected by familial hypercholesterolemia (FH) carrying a mutation on the LDLR gene showed reduced activation after an immune challenge.

Research methods for animal models of atherosclerosis (Review)

Atherosclerosis is a chronic inflammatory disease that threatens human health and lives by causing vascular stenosis and plaque rupture. Various animal models have been employed for elucidating the pathogenesis, drug development and treatment validation studies for atherosclerosis. To the best of our knowledge, the species used for atherosclerosis research include mice, rats, hamsters, rabbits, pigs, dogs, non-human primates and birds, among which the most commonly used ones are mice and rabbits. Notably, apolipoprotein E knockout (KO) or low-density lipoprotein receptor KO mice have been the most widely used animal models for atherosclerosis research since the late 20th century. Although the aforementioned animal models can form atherosclerotic lesions, they cannot completely simulate those in humans with respect to lesion location, lesion composition, lipoprotein composition and physiological structure. Hence, an appropriate animal model needs to be selected according to the research purpose. Additionally, it is necessary for atherosclerosis research to include quantitative analysis results of atherosclerotic lesion size and plaque composition. Laboratory animals can provide not only experimental tissues for in vivo studies but also cells needed for in vitro experiments. The present review first summarizes the common animal models and their practical applications, followed by focus on mouse and rabbit models and elucidating the methods to quantify atherosclerotic lesions. Finally, the methods of culturing endothelial cells, macrophages and smooth muscle cells were elucidated in detail and the experiments involved in atherosclerosis research were discussed.

Macrophages in Atherosclerosis, First or Second Row Players?

Macrophages represent a cell type that has been widely described in the context of atherosclerosis since the earliest studies in the 17th century. Their role has long been considered to be preponderant in the onset and aggravation of atherosclerosis, in particular by participating in the establishment of a chronic inflammatory state by the release of pro-inflammatory cytokines and by uncontrolled engorgement of lipids resulting in the formation of foam cells and later of the necrotic core. However, recent evidence from mouse models using an elegant technique of tracing vascular smooth muscle cells (VSMCs) during plaque development revealed that resident VSMCs display impressive plastic properties in response to an arterial injury, allowing them to switch into different cell types within the plaque, including mesenchymal-like cells, macrophage-like cells and osteochondrogenic-like cells. In this review, we oppose the arguments in favor or against the influence of macrophages versus VSMCs in all stages of atherosclerosis including pre-atherosclerosis, formation of lipid-rich foam cells, development of the necrotic core and the fibrous cap as well as calcification and rupture of the plaque. We also analyze the relevance of animal models for the investigation of the pathophysiological mechanisms of atherosclerosis in humans, and discuss potential therapeutic strategies targeting either VSMCs or macrophage to prevent the development of cardiovascular events. Overall, although major findings have been made from animal models, efforts are still needed to better understand and therefore prevent the development of atherosclerotic plaques in humans.

Impact of bone marrow ATP-binding cassette transporter A1 deficiency on atherogenesis is independent of the presence of the low-density lipoprotein receptor

BACKGROUND AND AIMS

There is extensive evidence from bone marrow transplantation studies that hematopoietic ATP binding cassette A1 (Abca1) is atheroprotective in low-density lipoprotein receptor (Ldlr) deficient mice. In contrast, studies using lysosyme M promoter-driven deletion of Abca1 in Ldlr deficient mice failed to show similar effects. It was hypothesized that the discrepancy between these studies might be due to the presence of Ldlr in bone marrow-derived cells in the transplantation model. In this study, we aim to determine the contribution of Ldlr to the atheroprotective effect of hematopoietic Abca1 in the murine bone marrow transplantation model.

METHODS

Wild-type, Ldlr^-/-, Abca1^-/-, and Abca1^-/-Ldlr^-/- bone marrow was transplanted into hypercholesterolemic Ldlr^-/- mice.

RESULTS

Bone marrow Lldr deficiency did not influence the effects of Abca1 on macrophage cholesterol efflux, foam cell formation, monocytosis or plasma cholesterol. Ldlr deficiency did reduce circulating and peritoneal lymphocyte counts, albeit only in animals lacking Abca1 in bone marrow-derived cells. Importantly, the effects of Abca1 deficiency on atherosclerosis susceptibility were unaltered by the presence or absence of Ldlr. Bone marrow Ldlr deficiency did lead to marginally but consistently decreased atherosclerosis, regardless of Abca1 deficiency. Thus, Ldlr expression on bone marrow-derived cells does, to a minimal extent, influence atherosclerotic lesion development, albeit independent of Abca1.

CONCLUSIONS

This study provides novel insight into the relative impact of Ldlr and Abca1 in bone marrow-derived cells on macrophage foam cell formation and atherosclerosis development in vivo. We have shown that Ldlr and Abca1 differentially and independently influence atherosclerosis development in a murine bone marrow transplantation model of atherosclerosis.

Accelerated atherosclerosis development in C57Bl6 mice by overexpressing AAV-mediated PCSK9 and partial carotid ligation

Studying the role of a particular gene in atherosclerosis typically requires a time-consuming and often difficult process of generating double knockouts or transgenics on ApoE^-/- or LDL receptor (LDLR)^-/- background. Recently, it was reported that adeno-associated-virus-8 (AAV8)-mediated overexpression of PCSK9 (AAV8-PCSK9) rapidly induced hyperlipidemia. However, using this method in C57BL6 wild-type (C57) mice, it took ~3 months to develop atherosclerosis. Our partial carotid ligation model is used to rapidly develop atherosclerosis by inducing disturbed flow in the left common carotid artery within 2 weeks in ApoE^-/- or LDLR^-/- mice. Here, we combined these two approaches to develop an accelerated model of atherosclerosis in C57 mice. C57 mice were injected with AAV9-PCSK9 or AAV9-luciferase (control) and high-fat diet was initiated. A week later, partial ligation was performed. Compared to the control, AAV-PCSK9 led to elevated serum PCSK9, hypercholesterolemia, and rapid atherosclerosis development within 3 weeks as determined by gross plaque imaging, and staining with Oil-Red-O, Movat's pentachrome, and CD45 antibody. These plaque lesions were comparable to the atherosclerotic lesions that have been previously observed in ApoE^-/- or LDLR^-/- mice that were subjected to partial carotid ligation and high-fat diet. Next, we tested whether our method can be utilized to rapidly determine the role of a particular gene in atherosclerosis. Using eNOS^-/- and NOX1^-/y mice on C57 background, we found that the eNOS^-/- mice developed more advanced lesions, while the NOX1^-/y mice developed less atherosclerotic lesions as compared to the C57 controls. These results are consistent with the previous findings using double knockouts (eNOS^-/-_ApoE^-/- and NOX1^-/y_ApoE^-/-). AAV9-PCSK9 injection followed by partial carotid ligation is an effective and time-saving approach to rapidly induce atherosclerosis. This accelerated model is well-suited to quickly determine the role of gene(s) interest without generating double or triple knockouts.

Mouse models of atherosclerosis: a historical perspective and recent advances

Atherosclerosis represents a significant cause of morbidity and mortality in both the developed and developing countries. Animal models of atherosclerosis have served as valuable tools for providing insights on its aetiology, pathophysiology and complications. They can be used for invasive interrogation of physiological function and provide a platform for testing the efficacy and safety of different pharmacological therapies. Compared to studies using human subjects, animal models have the advantages of being easier to manage, with controllable diet and environmental risk factors. Moreover, pathophysiological changes can be induced either genetically or pharmacologically to study the harmful effects of these interventions. There is no single ideal animal model, as different systems are suitable for different research objectives. A good understanding of the similarities and differences to humans enables effective extrapolation of data for translational application. In this article, we will examine the different mouse models for the study and elucidation of the pathophysiological mechanisms underlying atherosclerosis. We also review recent advances in the field, such as the role of oxidative stress in promoting endoplasmic reticulum stress, mitochondrial dysfunction and mitochondrial DNA damage, which can result in vascular inflammation and atherosclerosis. Finally, novel therapeutic approaches to reduce vascular damage caused by chronic inflammation using microRNA and nano-medicine technology, are discussed.

Do the Apoe-/- and Ldlr-/- Mice Yield the Same Insight on Atherogenesis?

Murine models of atherosclerosis are useful for investigating the environmental and genetic influences on lesion formation and composition. Apoe(-/-) and Ldlr(-/-) mice are the 2 most extensively used models. The models differ in important ways with respect to the precise mechanism by which their absence enhances atherosclerosis, including differences in plasma lipoproteins. The majority of the gene function studies have utilized only 1 model, with the results being generalized to atherogenic mechanisms. In only a relatively few cases have studies been conducted in both atherogenic murine models. This review will discuss important differences between the 2 atherogenic models and will point out studies that have been performed in the 2 models where results are comparable and those where different results were obtained.

Bone Marrow Transplantation in Mice to Study the Role of Hematopoietic Cells in Atherosclerosis

Hematopoietic stem cell transplantation or bone marrow transplantation is a common approach to reconstitute the immune system of mice that have been subjected to marrow-ablative doses of radiation. This method can be used in the field of atherosclerosis to assess the contribution of hematopoietic cells of a desired genotype to disease pathogenesis. The engraftment of atherosclerosis-prone mice with donor cells that contain genetic alterations in cells of the innate or adaptive immune system has been invaluable to define the role of multiple gene products in atherosclerosis. Here, we describe the different steps involved in the bone marrow transplantation protocol along with specific guidelines regarding the theoretical and technical details of the procedure.

Splenocytes seed bone marrow of myeloablated mice: implication for atherosclerosis

Extramedullary hematopoiesis has been shown to contribute to the pathogenesis of a variety of diseases including cardiovascular diseases. In this process, the spleen is seeded with mobilized bone marrow cells that augment its hematopoietic ability. It is unclear whether these immigrant cells that are produced/reprogrammed in spleen are similar or different from those found in the bone marrow. To begin to understand this, we investigated the relative potency of adult splenocytes per se to repopulate bone marrow of lethally-irradiated mice and its functional consequences in atherosclerosis. The splenocytes were harvested from GFP donor mice and transplanted into myeloablated wild type recipient mice without the inclusion of any bone marrow helper cells. We found that adult splenocytes repopulated bone marrow of myeloablated mice and the transplanted cells differentiated into a full repertoire of myeloid cell lineages. The level of monocytes/macrophages in the bone marrow of recipient mice was dependent on the cell origin, i.e., the donor splenocytes gave rise to significantly more monocytes/macrophages than the donor bone marrow cells. This occurred despite a significantly lower number of hematopoietic stem cells being present in the donor splenocytes when compared with donor bone marrow cells. Atherosclerosis studies revealed that donor splenocytes displayed a similar level of atherogenic and atheroprotective activities to those of donor bone marrow cells. Cell culture studies showed that the phenotype of macrophages derived from spleen is different from those of bone marrow. Together, these results demonstrate that splenocytes can seed bone marrow of myeloablated mice and modulate atherosclerosis. In addition, our study shows the potential of splenocytes for therapeutic interventions in inflammatory disease.

Effects of oxidized phospholipids on gene expression in RAW 264.7 macrophages: a microarray study

Oxidized phospholipids (oxPLs) are components of oxidized LDL (oxLDL). It is known that oxLDL activates expression of a series of atherogenic genes and their oxPLs contribute to their biological activities. In this study we present the effects of 1-palmitoyl-2-glutaroyl-sn-glycero-3-phosphocholine (PGPC) and 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphocholine (POVPC) on gene expression in RAW 264.7 macrophages using cDNA microarrays. PGPC affected the regulation of 146 genes, whereas POVPC showed only very minor effects. PGPC preferentially influenced expression of genes related to cell death, angiogenesis, cholesterol efflux, procoagulant mechanisms, atherogenesis, inflammation, and cell cycle. Many of these effects are known from studies with oxLDL or oxidized 1-hexadecanoyl-2-eicosatetra-5′,8′,11′,14′-enoyl-sn-glycero-3-phosphocholine (oxPAPC), containing PGPC in addition to other oxPL species. It is known that POVPC efficiently reacts with proteins by Schiff base formation, whereas PGPC only physically interacts with its biological targets. POVPC seems to affect cell physiology to a great extent on the protein level, whereas PGPC gives rise to both the modulation of protein function and regulation on the transcriptional level.

Mechanisms that regulate macrophage burden in atherosclerosis

Mononuclear phagocytes (MPs) relevant to atherosclerosis include monocytes, macrophages, and dendritic cells. A decade ago, studies on macrophage behavior in atherosclerotic lesions were often limited to quantification of total macrophage area in cross-sections of plaques. Although technological advances are still needed to examine plaque MP populations in an increasingly dynamic and informative manner, innovative methods to interrogate the biology of MPs in atherosclerotic plaques developed in the past few years point to several mechanisms that regulate the accumulation and function of MPs within plaques. Here, I review the evolution of atherosclerotic plaques with respect to changes in the MP compartment from the initiation of plaque to its progression and regression, discussing the roles that recruitment, proliferation, and retention of MPs play at these different disease stages. Additional work in the future will be needed to better distinguish macrophages and dendritic cells in plaque and to address some basic unknowns in the field, including just how cholesterol drives accumulation of macrophages in lesions to build plaques in the first place and how macrophages as major effectors of innate immunity work together with components of the adaptive immune response to drive atherosclerosis. Answers to these questions are sought with the goal in mind of reversing disease where it exists and preventing its development where it does not.

Stem cell-based therapies for atherosclerosis: perspectives and ongoing controversies

Atherosclerosis is a major contributor to life-threatening cardiovascular events, the leading cause of death worldwide. Since the mechanisms of atherosclerosis have not been fully understood, currently, there are no effective approaches to regressing atherosclerosis. Therefore, there is a dire need to explore the mechanisms and potential therapeutic strategies to prevent or reverse the progression of atherosclerosis. In recent years, stem cell-based therapies have held promises to various diseases, including atherosclerosis. Unfortunately, the efficacy of stem cell-based therapies for atherosclerosis as reported in the literature has been inconsistent or even conflicting. In this review, we summarize the current literature of stem cell-based therapies for atherosclerosis and discuss possible mechanisms and future directions of these potential therapies.

Activation of liver X receptor decreases atherosclerosis in Ldlr⁻/⁻ mice in the absence of ATP-binding cassette transporters A1 and G1 in myeloid cells

OBJECTIVE

Liver X receptor (LXR) activators decrease atherosclerosis in mice. LXR activators (1) directly upregulate genes involved in reverse cholesterol transport and (2) exert anti-inflammatory effects mediated by transrepression of nuclear factor-κB target genes. We investigated whether myeloid cell deficiency of ATP-binding cassette transporters A1 and G1 (ABCA1/G1), principal targets of LXR that promote macrophage cholesterol efflux and initiate reverse cholesterol transport, would abolish the beneficial effects of LXR activation on atherosclerosis.

APPROACH AND RESULTS

LXR activator T0901317 substantially reduced inflammatory gene expression in macrophages lacking ABCA1/G1. Ldlr(-/-) mice were transplanted with Abca1(-/-)Abcg1(-/-) or wild-type bone marrow (BM) and fed a Western-type diet for 6 weeks with or without T0901317 supplementation. Abca1/g1 BM deficiency increased atherosclerotic lesion complexity and inflammatory cell infiltration into the adventitia and myocardium. T0901317 markedly decreased lesion area, complexity, and inflammatory cell infiltration in the Abca1(-/-)Abcg1(-/-) BM-transplanted mice. To investigate whether this was because of macrophage Abca1/g1 deficiency, Ldlr(-/-) mice were transplanted with LysmCreAbca1(fl/fl)Abcg1(fl/fl) or Abca1(fl/fl)Abcg1(fl/fl) BM and fed Western-type diet with or without the more specific LXR agonist GW3965 for 12 weeks. GW3965 decreased lesion size in both groups, and the decrease was more prominent in the LysmCreAbca1(fl/fl)Abcg1(fl/fl) group.

CONCLUSIONS

The results suggest that anti-inflammatory effects of LXR activators are of key importance to their antiatherosclerotic effects in vivo independent of cholesterol efflux pathways mediated by macrophage ABCA1/G1. This has implications for the development of LXR activators that lack adverse effects on lipogenic genes while maintaining the ability to transrepress inflammatory genes.

Molecular biology of atherosclerosis

At least 468 individual genes have been manipulated by molecular methods to study their effects on the initiation, promotion, and progression of atherosclerosis. Most clinicians and many investigators, even in related disciplines, find many of these genes and the related pathways entirely foreign. Medical schools generally do not attempt to incorporate the relevant molecular biology into their curriculum. A number of key signaling pathways are highly relevant to atherogenesis and are presented to provide a context for the gene manipulations summarized herein. The pathways include the following: the insulin receptor (and other receptor tyrosine kinases); Ras and MAPK activation; TNF-α and related family members leading to activation of NF-κB; effects of reactive oxygen species (ROS) on signaling; endothelial adaptations to flow including G protein-coupled receptor (GPCR) and integrin-related signaling; activation of endothelial and other cells by modified lipoproteins; purinergic signaling; control of leukocyte adhesion to endothelium, migration, and further activation; foam cell formation; and macrophage and vascular smooth muscle cell signaling related to proliferation, efferocytosis, and apoptosis. This review is intended primarily as an introduction to these key signaling pathways. They have become the focus of modern atherosclerosis research and will undoubtedly provide a rich resource for future innovation toward intervention and prevention of the number one cause of death in the modern world.

Hematopoietic G-protein-coupled receptor kinase 2 deficiency decreases atherosclerotic lesion formation in LDL receptor-knockout mice

Leukocyte chemotaxis is deemed instrumental in initiation and progression of atherosclerosis. It is mediated by G-protein-coupled receptors (e.g., CCR2 and CCR5), the activity of which is controlled by G-protein-coupled receptor kinases (GRKs). In this study, we analyzed the effect of hematopoietic deficiency of a potent regulator kinase of chemotaxis (GRK2) on atherogenesis. LDL receptor-deficient (LDLr(-/-)) mice with heterozygous hematopoietic GRK2 deficiency, generated by bone marrow transplantation (n=15), displayed a dramatic attenuation of plaque development, with 79% reduction in necrotic core and increased macrophage content. Circulating monocytes decreased and granulocytes increased in GRK2(+/-) chimeras, which could be attributed to diminished granulocyte colony-forming units in bone marrow. Collectively, these data pointed to myeloid cells as major mediators of the impaired atherogenic response in GRK2(+/-) chimeras. LDLr(-/-) mice with macrophage/granulocyte-specific GRK2 deficiency (LysM-Cre GRK2(flox/flox); n=8) failed to mimic the aforementioned phenotype, acquitting these cells as major responsible subsets for GRK2 deficiency-associated atheroprotection. To conclude, even partial hematopoietic GRK2 deficiency prevents atherosclerotic lesion progression beyond the fatty streak stage, identifying hematopoietic GRK2 as a potential target for intervention in atherosclerosis.

Macrophage adipose triglyceride lipase deficiency attenuates atherosclerotic lesion development in low-density lipoprotein receptor knockout mice

OBJECTIVE

The consequences of macrophage triglyceride (TG) accumulation on atherosclerosis have not been studied in detail so far. Adipose triglyceride lipase (ATGL) is the rate-limiting enzyme for the initial step in TG hydrolysis. Because ATGL knockout (KO) mice exhibit massive TG accumulation in macrophages, we used ATGL KO mice to study the effects of macrophage TG accumulation on atherogenesis.

METHODS AND RESULTS

Low-density lipoprotein receptor (LDLr) KO mice were transplanted with bone marrow from ATGL KO (ATGL KO→LDLr KO) or wild-type (WT→LDLr KO) mice and challenged with a Western-type diet for 9 weeks. Despite TG accumulation in ATGL KO macrophages, atherosclerosis in ATGL KO→LDLr KO mice was 43% reduced associated with decreased plasma monocyte chemoattractant protein-1 (MCP-1) and macrophage interleukin-6 concentrations. This coincided with a reduced amount of macrophages, possibly because of a 39% increase in intraplaque apoptosis and a decreased migratory capacity of ATGL KO macrophages. The reduced number of white blood cells might be due to a 36% decreased Lin(-)Sca-1(+)cKit(+) hematopoietic stem cell population.

CONCLUSIONS

We conclude that the attenuation of atherogenesis in ATGL KO→LDLr KO mice is due to decreased infiltration of less inflammatory macrophages into the arterial wall and increased macrophage apoptosis.

Bone marrow transplantation in mice as a tool for studying the role of hematopoietic cells in metabolic and cardiovascular diseases

Hematopoietic cells have been established as major players in cardiovascular disease, with an important role in the etiology of atherosclerotic plaque. In addition, hematopoietic cells, and in particular the cells of monocyte and macrophage lineages, have recently been unmasked as one of the main causes of metabolic abnormalities leading to insulin resistance and type 2 diabetes. With the availability of transgenic mouse models that reproduce many aspects of these diseases, research in these areas has been able to make exceptional progress. Much of the work exploring the role of hematopoietic cells has been carried out on chimeric mice made by the recipient disease model mice being given donor bone marrow cells from transgenic mice harboring a genetic alteration in a relevant pathway. Here, we will describe the potential of the bone marrow transplantation approach and discuss recent developments, including the use of virally transduced cells. We will explain some of the caveats, their effect on the experimental outcomes, and some possible solutions. Taken as a whole, this technology offers great advantages in efficiency and cost-effectiveness, and it is expected to continue to be a crucial technique in cardiovascular research work.

The relative atherogenicity of VLDL and LDL is dependent on the topographic site

To evaluate whether the relative atherogenicity of VLDL and LDL is dependent on the topographic site, atherosclerosis was compared at four topographic sites in homozygous LDL receptor (LDLr)-deficient rabbits fed normal chow and in heterozygous LDLr-deficient rabbits with the same genetic background fed a 0.15% cholesterol diet to match cholesterol levels. VLDL cholesterol was significantly higher and LDL cholesterol significantly lower in LDLr(+/-) diet rabbits compared with LDLr(-/-) rabbits. Intimal area in the ascending thoracic aorta and in the abdominal aorta at the level of the renal arteries was 1.4-fold (P < 0.05) and 1.5-fold (P < 0.05) higher, respectively, in LDLr(-/-) rabbits than in LDLr(+/-) diet rabbits, whereas no significant difference occurred in the descending thoracic aorta and in the abdominal aorta just above the bifurcation. Differences remained statistically significant after adjustment for plasma cholesterol, triglycerides, and sex. Compared with LDLr(+/-) diet rabbits, higher intimal lipoprotein lipase (LPL) and apolipoprotein (apo) B levels were observed in LDLr(-/-) rabbits only at the level of the descending thoracic aorta. Intimal apo E levels in LDLr(-/-) rabbits were significantly lower in sites with a larger intima than in LDLr(+/-) diet rabbits. In conclusion, the relative atherogenicity of VLDL and LDL is dependent on the topographic site.

Apolipoprotein E4 in macrophages enhances atherogenesis in a low density lipoprotein receptor-dependent manner

Apolipoprotein E (apoE) and the low density lipoprotein receptor (LDLr) are well recognized determinants of atherosclerosis. In addition to hepatocytes, where both are highly expressed and contribute to plasma lipoprotein clearance, they are expressed in vascular cells and macrophages. In this study, we examined the effects of human apoE isoforms and LDLr levels in atherogenic pathways in primary macrophages ex vivo and atherosclerosis development after bone marrow transfer in vivo using mice expressing human apoE isoforms and different levels of LDLr expression. Increases in LDLr expression significantly increased cholesterol delivery into macrophages in culture, and the effects were more prominent with lipoproteins containing apoE4 than those containing apoE3. Conversely, increased LDLr expression reduced cholesterol efflux in macrophages expressing apoE4 but not in macrophages expressing apoE3. Furthermore, apoE3 protected VLDL from oxidation in vitro more than did apoE4. In LDLr-deficient mice expressing the human apoE4 isoform, Apoe4/4 Ldlr-/-, the replacement of bone marrow cells with those expressing LDLr increased atherosclerotic lesions in a dose-dependent manner compared with mice transplanted with cells having no LDLr. In contrast, atherosclerosis in Apoe3/3 Ldlr-/- mice, expressing the human apoE3 isoform, did not differ by the levels of macrophage LDLr expression. Our results demonstrate that apoE4, but not apoE3, in macrophages enhances atherosclerotic plaque development in mice in an LDLr-dependent manner and suggests that this interaction may contribute to the association of apoE4 with an increased cardiovascular risk in humans.

Lentiviral shRNA silencing of murine bone marrow cell CCR2 leads to persistent knockdown of CCR2 function in vivo

A major barrier in hematopoietic gene function studies is posed by the laborious and time-consuming generation of knockout mice with an appropriate genetic background. Here we present a novel lentivirus-based strategy for the in situ generation of hematopoietic knockdowns. A short hairpin RNA (shRNA) was designed targeting murine CC-chemokine receptor 2 (CCR2), which was able to specifically blunt CCR2 expression at the mRNA, protein, and functional levels in vitro. Reconstitution of irradiated recipient mice with autologous bone marrow that had been ex vivo transduced with shRNA lentivirus led to persistent down-regulation of CCR2 expression, which translated into a 70% reduction in CCR2-dependent recruitment of macrophages to an inflamed peritoneal cavity without noticeable side effects on related chemokine receptors or general inflammation status. These findings clearly demonstrate the potential of shRNA lentivirus-infected bone marrow transplantation as a rapid and effective method to generate hematopoietic knockdowns for leukocyte gene function studies.

Interferon-gamma produced by bone marrow-derived cells attenuates atherosclerotic lesion formation in LDLR-deficient mice

BACKGROUND

We evaluated the role of IFN-gamma produced by bone marrow-derived cells in atherogenesis in LDLR(-/-) mice using bone marrow transplantation (BMT).

METHODS AND RESULTS

We generated IFN-gamma-deficient bone marrow transplanted LDLR(-/- )mice (IFN-gamma(-/-) BMT mice), and compared them with controls (IFN-gamma(+/+) BMT mice). These mice were fed a high-fat diet (HFD). Plasma total cholesterol and triglyceride levels did not differ between these two groups. After 6 weeks of HFD feeding, the atherosclerotic lesions of IFN-gamma(-/-) BMT mice were larger than those of IFN-gamma(+/+) BMT mice at the aortic sinus, aortic arch and abdominal aorta. After 12 weeks of HFD feeding, the significant differences between the two groups disappeared except for the atherosclerotic lesion in the aortic sinus. MOMA2, CD4, CD8 or alpha-smooth muscle actin-positive cells were detected in the atherosclerotic lesions. The cellular composition of the lesions was identical between the two groups, but the cellular density showed decreased concomitant with the increased extracellular matrix deposition in IFN-gamma(-/- )BMT mice.

CONCLUSIONS

These findings demonstrate that IFN-gamma produced by bone marrow-derived cells delays the progression of atherosclerosis without any effect on plasma lipids, and this suppression may be due to decreased extracellular matrix deposition.

The presence of leukocyte CC-chemokine receptor 2 in CCR2 knockout mice promotes atherogenesis

OBJECTIVE

To selectively determine the role of leukocyte CC-chemokine receptor 2 (CCR2) in atherogenesis.

METHODS AND RESULTS

Bone marrow progenitor cells harvested from CCR2(+/+) mice were transplanted into irradiated CCR2(-/-) mice, representing the whole-body absence of CCR2 except in leukocytes. Transplantation of CCR2(-/-) bone marrow into CCR2(-/-) mice served as control. Eight weeks after bone marrow transplantation, the diet of regular chow was switched to a high-cholesterol diet for another 10 weeks in order to induce atherosclerosis. No significant differences in serum cholesterol and triglyceride levels were observed between the two groups. However, the mean cross-sectional aortic root lesion area of CCR2(+/+)-->CCR2(-/-) mice amounted up to 12.28+/-3.28x10(4) microm(2), compared with only 3.08+/-0.74 x 10(4) microm(2) observed in the CCR2(-/-)-->CCR2(-/-) group. Thus, the presence of CCR2 exclusively on leukocytes induces a fourfold increase in aortic lesion area. This extent of lesion development was comparable to C57Bl/6 mice receiving CCR2(+/+) bone marrow (10.08+/-3.30x10(4) microm(2)).

CONCLUSION

These results point at a dominant role of leukocyte CCR2 in atherogenesis, implying that CCR2 from nonleukocyte sources, like endothelial cells or smooth muscle cells, is less critical in the initiation of atherosclerosis. Pharmacological inhibition of leukocyte CCR2 function might be a promising strategy to prevent atherosclerosis.

Reduced macrophage apoptosis is associated with accelerated atherosclerosis in low-density lipoprotein receptor-null mice

OBJECTIVE

The majority of apoptotic cells in atherosclerotic lesions are macrophages. However, the pathogenic role of macrophage apoptosis in the development of atherosclerosis remains unclear. Elevated expression of Bax, one of the pivotal proapoptotic proteins of the Bcl-2 family, has been found in human atherosclerotic plaques. Activation of Bax also occurs in free cholesterol-loaded and oxysterol-treated mouse macrophages. In this study, we examined the effect of Bax deficiency in bone marrow-derived leukocytes on the development of atherosclerosis in low-density lipoprotein receptor-null (LDLR-/-) mice.

METHODS AND RESULTS

Fourteen 8-week-old male LDLR-/- mice were lethally irradiated and reconstituted with either wild-type (WT) C57BL6 or Bax-null (Bax-/-) bone marrow. Three weeks later, the mice were challenged with a Western diet for 10 weeks. No differences were found in the plasma cholesterol level between the WT and Bax-/- group. However, quantitation of cross sections from proximal aorta revealed a 49.2% increase (P=0.0259) in the mean lesion area of the Bax-/- group compared with the WT group. A 53% decrease in apoptotic macrophages in the Bax-/- group was found by TUNEL staining (P<0.05).

CONCLUSIONS

The reduction of apoptotic activity in macrophages stimulates atherosclerosis in LDLR-/- mice, which is consistent with the hypothesis that macrophage apoptosis suppresses the development of atherosclerosis.

Essential role for the (hepatic) LDL receptor in macrophage apolipoprotein E-induced reduction in serum cholesterol levels and atherosclerosis

Apolipoprotein E (apoE) is a high affinity ligand for several receptor systems in the liver, including the low-density lipoprotein (LDL) receptor, and non-LDL receptor sites, like the LDL receptor-related protein (LRP), the putative remnant receptor and/or proteoglycans. Although the liver is the major source of apoE synthesis, apoE is also produced by a wide variety of other cell types, including macrophages. In the present study, the role of the LDL receptor in the removal of lipoprotein remnants, enriched with macrophage-derived apoE from the circulation, was determined using the technique of bone marrow transplantation (BMT). Reconstitution of macrophage apoE production in apoE-deficient mice resulted in a serum apoE concentration of only 2% of the concentration in wild-type C57Bl/6 mice. This low level of apoE nevertheless reduced VLDL and LDL cholesterol 12-fold (P<0.001) and fourfold (P<0.001), respectively, thereby reducing serum cholesterol levels and the susceptibility to atherosclerosis. In contrast, reconstitution of macrophage apoE synthesis in mice lacking both apoE and the LDL receptor induced only a twofold (P<0.001) reduction in VLDL cholesterol and had no significant effect on atherosclerotic lesion development, although serum apoE levels were 93% of the concentration in normal C57Bl/6 mice. In conclusion, a functional (hepatic) LDL receptor is essential for the efficient removal of macrophage apoE-enriched lipoprotein remnants from the circulation and thus for normalization of serum cholesterol levels and protection against atherosclerotic lesion development in apoE-deficient mice.

Comparison of astrocytic and myocytic metabolic dysregulation in apolipoprotein E deficient and human apolipoprotein E transgenic mice

The accumulation of tubular aggregates in type II skeletal muscle fibres and fibrillo-granular inclusions in hippocampal protoplasmic astrocytes are characteristic lesions of apolipoprotein E deficient mice. Moreover these inclusions reacted immunocytochemically with an antibody specific to fragment 17-24 of the published sequence of Alzheimer's amyloid peptide. In an effort to evaluate the role of apolipoprotein E in the formation of these abnormal structures, we examined the tibialis anterior muscle and the hippocampus of several groups of animals including: (i) apolipoprotein E "knockout" mice which had been whole body irradiated with 1200 rads and bone marrow replenished with apolipoprotein E sufficient marrow; and (ii) three transgenic murine strains that had been genetically engineered to express either human apolipoprotein E2, E3 or E4 protein on an apoE deficient background. The results of this study showed that the presence of murine apolipoprotein E (even in subnormal levels in the serum) in irradiated bone marrow replenished mice and in all three (E2, E3 or E4) human apoE transgenic strains was sufficient to prevent the aggregation of sarcoplasmic tubules in the tibialis anterior type II muscle fibres. Similarly apolipoprotein E "knockout" bone marrow replenished mice and all three transgenic strains expressing the different human apolipoprotein E alleles reduced the number of the astrocytic inclusions in the hippocampus to levels not significantly different to those observed in control C57Bl6J animals. The data obtained in this study indicate that neurological and neuromuscular abnormalities found in apoE deficient mice are reversed when apoE protein is replaced in the circulation, either by bone marrow transplantation of normal apoE sufficient marrow, or by gene therapy with the apoE gene, albeit of human origin and irrespective of the allele used.

A direct role for the macrophage low density lipoprotein receptor in atherosclerotic lesion formation

To evaluate the contribution of the macrophage low density lipoprotein receptor (LDLR) to atherosclerotic lesion formation, we performed bone marrow transplantation studies in different mouse strains. First, LDLR(-/-) mice were transplanted with either LDLR(+/+) marrow or LDLR(-/-) marrow and were challenged with an atherogenic Western type diet. The diet caused severe hypercholesterolemia of a similar degree in the two groups, and no differences in the aortic lesion area were detected. Thus, macrophage LDLR expression does not influence foam cell lesion formation in the setting of extreme LDL accumulation. To determine whether macrophage LDLR expression affects foam cell formation under conditions of moderate, non-LDL hyperlipidemia, we transplanted C57BL/6 mice with either LDLR(-/-) marrow (experimental group) or LDLR(+/+) marrow (controls). Cholesterol levels were not significantly different between the two groups at baseline or after 6 weeks on a butterfat diet, but were 40% higher in the experimental mice after 13 weeks, mostly due to accumulation of beta-very low density lipoprotein (beta-VLDL). Despite the increase in cholesterol levels, mice receiving LDLR(-/-) marrow developed 63% smaller lesions than controls, demonstrating that macrophage LDLR affects the rate of foam cell formation when the atherogenic stimulus is beta-VLDL. We conclude that the macrophage LDLR is responsible for a significant portion of lipid accumulation in foam cells under conditions of dietary stress.

Macrophages, lipoprotein metabolism, and atherosclerosis: insights from murine bone marrow transplantation studies

The macrophage expresses a variety of genes that may contribute to lipoprotein metabolism and atherosclerotic lesion formation. Bone marrow transplantation experiments using gene-targeted mice, both as donors and recipients, provide a useful approach to examine the contribution of macrophage gene expression to lipoprotein metabolism and atherogenesis in vivo. This article describes recent insights into the role of macrophage expression of apolipoprotein E and the LDL receptor gained from bone marrow transplantation studies in the mouse.