Analysis of atherosclerosis susceptibility in mice with genetic defects in platelet function

Platelet Physiology

Platelets are the smallest blood cells, numbering 150 to 350 × 109/L in healthy individuals. The ability of activated platelets to adhere to an injured vessel wall and form aggregates was first described in the 19th century. Besides their long-established roles in thrombosis and hemostasis, platelets are increasingly recognized as pivotal players in numerous other pathophysiological processes including inflammation and atherogenesis, antimicrobial host defense, and tumor growth and metastasis. Consequently, profound knowledge of platelet structure and function is becoming more important in research and in many fields of modern medicine. This review provides an overview of platelet physiology focusing particularly on the structure, granules, surface glycoproteins, and activation pathways of platelets.

Plasma lipidomic profiling in murine mutants of Hermansky-Pudlak syndrome reveals differential changes in pro- and anti-atherosclerotic lipids

Atherosclerosis is characterized by the accumulation of lipid-rich plaques in the arterial wall. Its pathogenesis is very complicated and has not yet been fully elucidated. It is known that dyslipidemia is a major factor in atherosclerosis. Several different Hermansky-Pudlak syndrome (HPS) mutant mice have been shown either anti-atherosclerotic or atherogenic phenotypes, which may be mainly attributed to corresponding lipid perturbation. To explore the effects of different HPS proteins on lipid metabolism and plasma lipid composition, we analyzed the plasma lipid profiles of three HPS mutant mice, pa (Hps9 ^-/-), ru (Hps6 ^-/-), ep (Hps1 ^-/-), and wild-type (WT) mice. In pa and ru mice, some pro-atherosclerotic lipids, e.g. ceramide (Cer) and diacylglycerol (DAG), were down-regulated whereas triacylglycerol (TAG) containing docosahexaenoic acid (DHA) (22:6) fatty acyl was up-regulated when compared with WT mice. Several pro-atherosclerotic lipids including phosphatidic acid (PA), lysophosphatidylserine (LPS), sphingomyelin (SM), and cholesterol (Cho) were up-regulated in ep mice compared with WT mice. The lipid droplets in hepatocytes showed corresponding changes in these mutants. Our data suggest that the pa mutant resembles the ru mutant in its anti-atherosclerotic effects, but the ep mutant has an atherogenic effect. Our findings may provide clues to explain why different HPS mutant mice exhibit distinct anti-atherosclerotic or atherogenic effects after being exposed to high-cholesterol diets.

Genetic Depletion or Hyperresponsiveness of Natural Killer Cells Do Not Affect Atherosclerosis Development

Rationale:

Chronic inflammation is central in the development of atherosclerosis. Both innate and adaptive immunities are involved. Although several studies have evaluated the functions of natural killer (NK) cells in experimental animal models of atherosclerosis, it is not yet clear whether NK cells behave as protective or proatherogenic effectors. One of the main caveats of previous studies was the lack of specificity in targeting loss or gain of function of NK cells.

Objectives:

We used 2 selective genetic approaches to investigate the role of NK cells in atherosclerosis: (1) Ncr1 iCre/+ R26 lsl− DTA/+ mice in which NK cells were depleted and (2) Noé mice in which NK cells are hyperresponsive.

Methods and Results:

No difference in atherosclerotic lesion size was found in Ldlr −/− (low-density lipoprotein receptor null) mice transplanted with bone marrow (BM) cells from Ncr1 iCre R26R lsl− DTA , Noé , or wild-type mice. Also, no difference was observed in plaque composition in terms of collagen content, macrophage infiltration, or the immune profile, although Noé chimera had more IFN (interferon)-γ–producing NK cells, compared with wild-type mice. Then, we investigated the NK-cell selectivity of anti–asialoganglioside M1 antiserum, which was previously used to conclude the proatherogenicity of NK cells. Anti–asialoganglioside M1 treatment decreased atherosclerosis in both Ldlr −/− mice transplanted with Ncr1 iCre R26R lsl− DTA or wild-type bone marrow, indicating that its antiatherogenic effects are unrelated to NK-cell depletion, but to CD8 + T and NKT cells. Finally, to determine whether NK cells could contribute to the disease in conditions of pathological NK-cell overactivation, we treated irradiated Ldlr −/− mice reconstituted with either wild-type or Ncr1 iCre R26R lsl− DTA bone marrow with the viral mimic polyinosinic:polycytidylic acid and found a significant reduction of plaque size in NK-cell–deficient chimeric mice.

Conclusions:

Our findings, using state-of-the-art mouse models, demonstrate that NK cells have no direct effect on the natural development of hypercholesterolemia-induced atherosclerosis, but may play a role when an additional systemic NK-cell overactivation occurs.

BLOC-2 subunit HPS6 deficiency affects the tubulation and secretion of von Willebrand factor from mouse endothelial cells

Hermansky-Pudlak syndrome (HPS) is a recessive disorder with bleeding diathesis, which has been linked to platelet granule defects. Both platelet granules and endothelial Weibel-Palade bodies (WPBs) are members of lysosome-related organelles (LROs) whose formation is regulated by HPS protein associated complexes such as BLOC (biogenesis of lysosome-related organelles complex) -1, -2, -3, AP-3 (adaptor protein complex-3) and HOPS (homotypic fusion and protein sorting complex). Von Willebrand factor (VWF) is critical to hemostasis, which is stored in a highly-multimerized form as tubules in the WPBs. In this study, we found the defective, but varying, release of VWF into plasma after desmopressin (DDAVP) stimulation in HPS1 (BLOC-3 subunit), HPS6 (BLOC-2 subunit), and HPS9 (BLOC-1 subunit) deficient mice. In particular, VWF tubulation, a critical step in VWF maturation, was impaired in HPS6 deficient WPBs. This likely reflects a defective endothelium, contributing to the bleeding tendency in HPS mice or patients. The differentially defective regulated release of VWF in these HPS mouse models suggests the need for precise HPS genotyping before DDAVP administration to HPS patients.

Plant food anthocyanins inhibit platelet granule secretion in hypercholesterolaemia: Involving the signalling pathway of PI3K-Akt

Controlling platelet granule secretion has been considered an effective strategy to dampen thrombosis and prevent atherosclerosis. Anthocyanins are natural plant pigments and possess a wide range of biological activities, including cardiovascular protective activity. In the present study we explored the effects and the potential mechanisms of anthocyanins on platelet granule secretion in hypercholesterolemia. In a randomised, double-blind clinical trial, 150 hypercholesterolaemic individuals were treated with purified anthocyanins (320 mg/day) or placebo for 24 weeks. Anthocyanins consumption significantly reduced plasma levels of β-thromboglobulin (β-TG), soluble P-selectin, and of Regulated on Activation Normal T cell Expressed and Secreted (RANTES) as compared with the placebo. A minor reduction in platelet factor 4 (PF4) and transforming growth factor β1 (TGF-β1) levels were also observed. In in vitro experiments, we observed that puriӿed anthocyanin mixture, as well as its two main anthocyanin components, delphinidin-3-glucoside (Dp-3-g) and cyanidin-3-glucoside (Cy-3g) directly inhibited platelet á-granule, dense granule, and lysosome secretion evaluated by P-selectin, RANTES, β-TG, PF4, TGF-β1, serotonin, ATP, and CD63 release. Further, anthocyanins inhibited platelet PI3K/Akt activation and consequently attenuated eNOS phosphorylation and cGMP production, thus interrupting MAPK activation. LY294002, a PI3K inhibitor, did not cause additional inhibitory efficacy, indicating that anthocyanin-induced effects may be involved in inhibition of the PI3K/Akt signalling pathway. These results provide evidence that by inhibiting platelet granule secretion, anthocyanins may be a potent cardioprotective agent.

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.

Genetic basis of atherosclerosis: insights from mice and humans

Atherosclerosis is a complex and heritable disease involving multiple cell types and the interactions of many different molecular pathways. The genetic and molecular mechanisms of atherosclerosis have, in part, been elucidated by mouse models; at least 100 different genes have been shown to influence atherosclerosis in mice. Importantly, unbiased genome-wide association studies have recently identified a number of novel loci robustly associated with atherosclerotic coronary artery disease. Here, we review the genetic data elucidated from mouse models of atherosclerosis, as well as significant associations for human coronary artery disease. Furthermore, we discuss in greater detail some of these novel human coronary artery disease loci. The combination of mouse and human genetics has the potential to identify and validate novel genes that influence atherosclerosis, some of which may be candidates for new therapeutic approaches.

Platelet dense-granule secretion plays a critical role in thrombosis and subsequent vascular remodeling in atherosclerotic mice

BACKGROUND

Platelet aggregation plays a critical role in myocardial infarction and stroke; however, the role of platelet secretion in atherosclerotic vascular disease is poorly understood. Therefore, we examined the hypothesis that platelet dense-granule secretion modulates thrombosis, inflammation, and atherosclerotic vascular remodeling after injury.

METHODS AND RESULTS

Functional deletion of the Hermansky-Pudlak syndrome 3 gene (HPS3(-/-)) markedly reduces platelet dense-granule secretion. HPS3(-/-) mice have normal platelet counts, platelet morphology, and alpha-granule number, as well as maximal secretion of the alpha-granule marker P-selectin; however, their capacity to form platelet-leukocyte aggregates is significantly reduced (P<0.05). To examine the role of platelet dense-granule secretion in these processes, atherosclerosis-prone mice with combined genetic deficiency of apolipoprotein E and HPS3 (ApoE(-/-), HPS3(-/-)) were compared with congenic, atherosclerosis-prone mice with normal platelet secretion (ApoE(-/-), HPS3(+/+)). After 16 to 18 weeks on a high-fat diet, both groups of mice had similar fasting cholesterol levels and body weight. Carotid arteries of ApoE(-/-), HPS3(+/+) mice thrombosed rapidly after FeCl(3) injury, but ApoE(-/-), HPS3(-/-) mice were completely resistant to thrombotic arterial occlusion (P<0.01). Three weeks after injury, neointimal hyperplasia (from alpha-smooth muscle actin-positive cells) was significantly less (P<0.001) in arteries from ApoE(-/-), HPS3(-/-) mice. In ApoE(-/-), HPS3(-/-) mice, there were also pronounced reductions in arterial inflammation, as indicated by a 74% decrease in CD45-positive leukocytes (P<0.01) and a 73% decrease in Mac-3-positive macrophages (P<0.05).

CONCLUSIONS

In atherosclerotic mice, reduced platelet dense-granule secretion is associated with marked protection against the development of arterial thrombosis, inflammation, and neointimal hyperplasia after vascular injury.

Mapping genetic loci that regulate lipid levels in a NZB/B1NJxRF/J intercross and a combined intercross involving NZB/B1NJ, RF/J, MRL/MpJ, and SJL/J mouse strains

The NZB/B1NJ (NZB) mouse strain exhibits high cholesterol and HDL levels in blood compared with several other strains of mice. To study the genetic regulation of blood lipid levels, we performed a genome-wide linkage analysis in 542 chow-fed F2 female mice from an NZBxRF/J (RF) intercross and in a combined data set that included NZBxRF and MRL/MpJxSJL/J intercrosses. In the NZBxRF F2 mice, the cholesterol and HDL concentrations were influenced by quantitative trait loci (QTL) on chromosome (Chr) 5 [logarithm of odds (LOD) 17-19; D5Mit10] that was in the region identified earlier in crosses involving NZB mice, but two QTLs on Chr 12 (LOD 4.7; D12Mit182) and Chr 19 (LOD 5.7; D19Mit1) were specific to the NZBxRF intercross. Triglyceride levels were affected by two novel QTLs at D12Mit182 (LOD 8.7) and D15Mit13 (LOD 3.5). The combined-cross linkage analysis (1,054 mice, 231 markers) 1) identified four shared QTLs (Chrs 5, 7, 14, and 17) that were not detected in one of the parental crosses and 2) improved the resolution of two shared QTLs. In summary, we report additional loci regulating lipid levels in NZB mice that had not been identified earlier in crosses involving the NZB strain of mice. The identification of shared loci from multiple crosses increases confidence toward finding the QTL gene.

Participatory role of natural killer and natural killer T cells in atherosclerosis: lessons learned from in vivo mouse studies

Atherosclerosis is a multifactor, highly complex disease with numerous aetiologies that work synergistically to promote lesion development. One of the emerging components that drive the development of both early- and late-stage atherosclerotic lesions is the participation of both the innate and acquired immune systems. In both humans and animal models of atherosclerosis, the most prominent cells that infiltrate evolving lesions are macrophages and T lymphocytes. The functional loss of either of these cell types reduces the extent of atherosclerosis in mice that were rendered susceptible to the disease by deficiency of either apolipoprotein E or the LDL (low density lipoprotein) receptor. In addition to these major immune cell participants, a number of less prominent leukocyte populations that can modulate the atherogenic process are also involved. This review will focus on the participatory role of two "less prominent" immune components, namely natural killer (NK) cells and natural killer T (NKT) cells. Although this review will highlight the fact that both NK and NKT cells are not sufficient for causing the disease, the roles played by both these cells types are becoming increasingly important in understanding the complexity of this disease process.

Platelets in atherothrombosis: lessons from mouse models

Platelets play a central role in hemostasis and thrombosis but also in the initiation of atherosclerosis, making platelet receptors and their intracellular signaling pathways important molecular targets for antithrombotic and anti-inflammatory therapy. Historically, much of the knowledge about hemostasis and thrombosis has been derived from patients suffering from bleeding and thrombotic disorders and the identification of the underlying molecular defects. In recent years, the availability of genetically modified mouse strains with defined defects in platelet function and the development of in vivo models to assess platelet-related physiologic and pathophysiologic processes have opened new ways to identify the individual roles and the interplay of platelet proteins in adhesion, activation, aggregation, secretion, and procoagulant activity in vitro and in vivo. This review will summarize key findings made by these approaches and discuss them in the context of human disease.

Relevance of Toll-like receptor-4 polymorphisms in renal transplantation

BACKGROUND

Polymorphisms in Toll-like receptor-4 (TLR4) have been reported to be associated with a blunted immune response to microbial pathogens, as well as a decreased risk of atherosclerosis in the general population. We assessed the impact of the two TLR4 variants on the risk of severe infection, the incidence of acute rejection, and the occurrence of atherosclerotic complications in renal transplant recipients (RTR).

METHODS

TLR-4 polymorphisms were assessed in a cohort of 238 RTR. Post-transplant atherosclerotic events, acute rejection, severe bacterial infection, cytomegalovirus (CMV) disease, and opportunistic infections were evaluated as outcomes.

RESULTS

The patients were followed for a mean duration of 95 +/- 29 months after transplant. TLR4 polymorphism was observed in 27 (11.3%) RTR. Subjects with TLR4 polymorphisms were less likely to experience post-transplant atherosclerotic events (RR 0.44; 95% CI 0.21 to 0.93; P= 0.02) and acute rejection (RR 0.41; 95% CI 0.30 to 0.83; P= 0.01), but presented severe bacterial infections (RR 1.33; 95% CI 1.12 to 1.67; P= 0.01) and opportunistic infections (RR 3.03; 95% CI 1.72 to 8.29; P= 0.008) more frequently. TLR4 polymorphism was marginally associated with CMV disease (RR 1.47; 95% CI 0.95 to 2.64; P= 0.08).

CONCLUSION

RTR with TLR4 polymorphism present a lower risk of post-transplant atherosclerotic events and acute allograft rejection, but experience severe infectious episodes more frequently. This subset of RTR may benefit from a less potent immunosuppression regimen, along with increased preventive measures against infectious agents.

P-selectin in haemostasis

During the past decade, interrelationships between inflammation and thrombosis have been the subject of extensive works, and it is now commonly recognized that inflammation (notably leucocyte recruitment) directly affects thrombosis, and that thrombosis also constitutes a pro-inflammatory event. This tight link is partly attributable to P-selectin, which is functional not only when expressed on the surface of activated platelets and endothelial cells, but also when shed, generating its soluble form, termed sP-selectin. In this review, we will provide an overview of the relative roles of the different compartments of P-selectin (platelet, endothelial cell, plasma) in haemostasis and vascular pathologies, and the potential therapeutic benefits achievable in targeting this molecule.

Depletion of natural killer cell function decreases atherosclerosis in low-density lipoprotein receptor null mice

OBJECTIVE

Natural killer (NK) cells are a key component of innate immunity. Despite being identified in human and mouse atherosclerotic lesions, the role of NK cells in the disease process in unknown. To determine this role, we created chimeric atherosclerosis-susceptible low-density lipoprotein (LDL) receptor null (ldl-r-/-) mice that were deficient in functional NK cells through expression of a transgene encoding for Ly49A.

METHODS AND RESULTS

Bone marrow cells from Ly49A transgenic and nontransgenic littermates were used to repopulate the hematopoietic system of lethally-irradiated female ldl-r-/- mice. After a recovery period to permit sufficient engraftment, mice were placed on a diet enriched in saturated fat and cholesterol. After 8 weeks, there was no difference in either serum total cholesterol concentrations or lipoprotein cholesterol distribution in mice repopulated with nontransgenic versus Ly49A transgenic marrow cells. Using immunohistochemistry, we detected NK cells in atherosclerotic lesions of both groups of mice. However, deficiency of functional NK cells significantly reduced the size of atherosclerosis by 70% (P=0.0002) in cross-sectional analysis of the aortic root and by 38% (P=0.004) in en face analysis of the intimal surface of the aortic arch.

CONCLUSIONS

These studies demonstrate that NK cells infiltrate the vessel wall and promote atherosclerotic lesion development.

Participation of innate and acquired immunity in atherosclerosis

Coronary artery disease, the major manifestation of atherosclerosis, is the leading cause of death in the Western world. However, the pathogenesis of atherosclerosis is still poorly understood. Controversy exists regarding the participation of innate immunity involving macrophages and natural killer (NK) cells vs antigen-specific acquired immunity involving lymphocytes. Macrophages predominate in atherosclerotic lesions. NK cells, although smaller in number, are present as well. Furthermore, T lymphocytes that participate in acquired immunity are frequently observed in lesions and can modulate lesion progression. By using mouse models of hyperlipidemia, our laboratory is addressing in vivo the participation of both innate inflammatory responses and acquired immune responses in atherosclerosis.

Mouse models of Hermansky Pudlak syndrome: a review

Hermansky Pudlak Syndrome (HPS) is a recessively inherited disease affecting the contents and/or the secretion of several related subcellular organelles including melanosomes, lysosomes, and platelet dense granules. It presents with disorders of pigmentation, prolonged bleeding, and ceroid deposition, often accompanied by severe fibrotic lung disease and colitis. In the mouse, the disorder is clearly multigenic, caused by at least 14 distinct mutations. Studies on the mouse mutants have defined the granule abnormalities of HPS and have shown that the disease is associated with a surprising variety of phenotypes affecting many tissues. This is an exciting time in HPS research because of the recent molecular identification of the gene causing a major form of human HPS and the expected identifications of several mouse HPS genes. Identifications of mouse HPS genes are expected to increase our understanding of intracellular vesicle trafficking, lead to discovery of new human HPS genes, and suggest diagnostic and therapeutic approaches toward the more severe clinical consequences of the disease.

A fusion pore phenotype in mast cells of the ruby-eye mouse

Using patch-clamp capacitance and amperometric techniques, we have identified an exocytotic phenotype that affects the function of the fusion pore, the molecular structure that connects the lumen of a secretory vesicle with the extracellular environment during exocytosis. Direct observation of individual exocytotic events in mast cells from the ruby-eye mouse (ru/ru) showed a 3-fold increase in the fraction and duration of transient fusion events with respect to wild-type mice. The fraction of the total fusion events that were transient increased from 0.22 +/- 0.02 (wild type) to 0.65 +/- 0.02 (ru/ru), and the average duration of these events increased from 418 +/- 32 ms (wild type) to 1207 +/- 89 ms (ru/ru). We also show that this phenotype can reduce and delay an evoked secretory response by causing the fusion of vesicles that have been previously emptied by repeated cycles of transient fusion. The exocytotic phenotype that we describe here may be a cause of diseases like platelet storage pool deficiency and prolonged bleeding times for which the ruby-eye mouse serves as an animal model. Furthermore, the identification of the gene causing the fusion pore phenotype reported here will illuminate the molecular mechanisms regulating exocytotic fusion.

Effect of human apoE4 on the clearance of chylomicron-like lipid emulsions and atherogenesis in transgenic mice

Apolipoprotein (apo) E is a ligand for lipoprotein receptors and mediates the cellular uptake of several different lipoproteins. Human apoE occurs in three allelic forms designated E2, E3, and E4. The E2 isoform is associated with changes in lipoprotein metabolism, and the E4 isoform is associated with Alzheimer's disease and an increased risk of coronary heart disease. In this study transgenic mice were generated to assess the effect of a sustained increase in plasma apoE4 concentration. The transgenic animals had three- to sixfold increases in total plasma apoE, associated primarily with the non-high-density lipoprotein (HDL) fractions of plasma lipoproteins. In response to an atherogenic diet the transgenic mice developed hypercholesterolemia similar to that in nontransgenic mice but did not experience the decrease in HDL cholesterol normally observed in this strain of C57BL/6 mice. The rate of plasma clearance of a lipid emulsion mimicking lymph chylomicrons was measured in transgenic mice expressing the human apoE4 gene and compared with the clearance rate in nontransgenic control animals. In animals fed a low-fat diet the emulsion lipids were cleared significantly more rapidly from the plasma of transgenic than control mice. In animals adapted to a high-fat diet, the clearance of chylomicron remnants was slowed markedly in both transgenic and control mice and was not significantly accelerated in transgenic compared with control animals. We also investigated the effect of increasing the plasma concentration of apoE4 on the progression of atherosclerotic heart disease.(ABSTRACT TRUNCATED AT 250 WORDS)

Coagulation factors X, Xa, and protein S as potent mitogens of cultured aortic smooth muscle cells

Smooth muscle cells (SMCs) in the rat carotid artery leave the quiescent state and proliferate after balloon catheter injury. The precise signals responsible for this SMC mitogenesis need to be elucidated. Although platelet-derived growth factor (PDGF), a potent SMC mitogen, is released from activated platelets, damaged endothelium, and macrophages, it cannot be solely responsible for this proliferation. In search of other SMC growth factors, we have examined several proteins of the coagulation cascade. At nanomolar concentrations, factors X, Xa, and protein S promote cultured rat aortic SMC mitosis. In contrast, factor IX is only weakly mitogenic, whereas factor VII and protein C fail to stimulate SMC division. Protein S, the most mitogenic of these coagulation cascade factors, stimulates DNA synthesis in cultured SMCs with a time course similar to that of PDGF-AA and without the delay observed for transforming growth factor beta. Antistasin and tick anticoagulant peptide, two specific factor Xa inhibitors, inhibit SMC mitogenesis due to Xa and protein S. Coagulation factors that possess mitogenic activity may contribute to intimal SMC proliferation after vascular injury as a result of angioplasty or vascular compromise during atherogenesis.