Circulating smooth muscle progenitor cells in atherosclerosis and plaque rupture: Current perspective and methods of analysis

Nonbone Marrow CD34+ Cells Are Crucial for Endothelial Repair of Injured Artery

[Figure: see text].

The anatomical sources of neointimal cells in the arteriovenous fistula

Neointimal cells are an elusive population with ambiguous origins, functions, and states of differentiation. Expansion of the venous intima in arteriovenous fistula (AVF) is one of the most prominent remodeling processes in the wall after access creation. However, most of the current knowledge about neointimal cells in AVFs comes from extrapolations from the arterial neointima in non-AVF systems. Understanding the origin of neointimal cells in fistulas may have important implications for the design and effective delivery of therapies aimed to decrease intimal hyperplasia (IH). In addition, a broader knowledge of cellular dynamics during postoperative remodeling of the AVF may help clarify other transformation processes in the wall that combined with IH determine the successful remodeling or failure of the access. In this review, we discuss the possible anatomical sources of neointimal cells in AVFs and their relative contribution to intimal expansion.

Histological variation of early stage atherosclerotic lesions in baboons after prolonged challenge with high-cholesterol, high-fat diet

INTRODUCTION

The baboon is a well-characterized model of human early stage atherosclerosis. However, histological and morphological changes involved in atherogenesis in baboons are not known. Previously, we challenged baboons with a high-cholesterol, high-fat diet for two years and observed fatty streak and plaque lesions in iliac arteries (RCIA).

METHODS

We evaluated histological and morphological changes of baboon arterial lesions and control arteries. In addition, we evaluated the vascular expression of CD68 and SMαA markers with progression of atherosclerosis.

RESULTS

We observed changes that correlated with extent of atherosclerosis, including increased maximum intimal thickness. We demonstrated at molecular level the infiltration of smooth muscle cells and macrophages into the intimal layer. Further, we observed histological and morphological discordancy between the affected and adjacent areas of the same RCIA.

CONCLUSION

Atherogenesis in baboons is accompanied by histological, morphological, and molecular changes, as in humans, providing insights to evaluate the mechanisms underlying early stage atherosclerosis in target tissues.

High soluble endoglin levels regulate cholesterol homeostasis and bile acids turnover in the liver of transgenic mice

AIMS

Increased plasma soluble endoglin concentrations (sEng) are frequently detected in metabolic disorders accompanied with hypercholesterolemia in serum, but effect of sEng on the cholesterol biochemistry is unknown. Cholesterol and bile acids (BA) are important products of liver metabolism with numerous functions within the organism. Turnover of these substances requires precise regulation due to potential toxicities during their cumulation. In this study, we hypothesized that high sEng levels affect cholesterol homeostasis and BA turnover in mice liver.

MAIN METHODS

Nine-month-old transgenic male mice overexpressing human sEng and wild-type mice underwent plasma, bile, stool, and organ samples analysis by analytical, qRT-PCT and Western blot methods.

KEY FINDINGS

sEng mice demonstrated decreased plasma total and LDL cholesterol concentrations due to upregulation of hepatic Sr-b1 and Ldlr receptors, increased liver cholesterol content, and increased Abcg8-mediated cholesterol efflux into bile. sEng also increased conversion of cholesterol into bile acids (BA) via upregulation of Cyp7a1 and increased Mdr1 expression. Plasma concentrations of BA were increased in sEng mice due to their enhanced reabsorption via ileum. Increased hepatic disposition of BA led to their increased biliary excretion coupled with choleretic activity.

SIGNIFICANCE

For the first time, we have shown that high sEng plasma levels affect cholesterol and BA homeostasis on the basis of complex liver and intestinal effects. The significance of these findings for pathophysiology of diseases associated with increased sEng concentrations remains to be elucidated in prospective studies.

Lineage tracking of origin and fate of smooth muscle cells in atherosclerosis

Advances in lineage-tracking techniques have provided new insights into the origins and fates of smooth muscle cells (SMCs) in atherosclerosis. Yet new tools present new challenges for data interpretation that require careful consideration of the strengths and weaknesses of the methods employed. At the same time, discoveries in other fields have introduced new perspectives on longstanding questions about steps in atherogenesis that remain poorly understood. In this article, we address both the challenges and opportunities for a better understanding of the mechanisms by which cells appearing as or deriving from SMCs accumulate in atherosclerosis.

Vascular stem/progenitor cells: functions and signaling pathways

Vascular stem/progenitor cells (VSCs) are an important source of all types of vascular cells needed to build, maintain, repair, and remodel blood vessels. VSCs, therefore, play critical roles in the development, normal physiology, and pathophysiology of numerous diseases. There are four major types of VSCs, including endothelial progenitor cells (EPCs), smooth muscle progenitor cells (SMPCs), pericytes, and mesenchymal stem cells (MSCs). VSCs can be found in bone marrow, circulating blood, vessel walls, and other extravascular tissues. During the past two decades, considerable progress has been achieved in the understanding of the derivation, surface markers, and differentiation of VSCs. Yet, the mechanisms regulating their functions and maintenance under normal and pathological conditions, such as in eye diseases, remain to be further elucidated. Owing to the essential roles of blood vessels in human tissues and organs, understanding the functional properties and the underlying molecular basis of VSCs is of critical importance for both basic and translational research.

Recent advances and clinical insights into the use of proteomics in the study of atherosclerosis

INTRODUCTION

The application of new proteomics methods may help to identify new diagnostic/predictive molecular markers in an attempt to improve the clinical management of atherosclerosis. Areas covered: Technological advances in proteomics have enhanced its sensitivity and multiplexing capacity, as well as the possibility of studying protein interactions and tissue structure. These advances will help us better understand the molecular mechanisms at play in atherosclerosis as a biological system. Moreover, this should help identify new predictive/diagnostic biomarkers and therapeutic targets that may facilitate effective risk stratification and early diagnosis, with the ensuing rapid implementation of treatment. This review provides a comprehensive overview of the novel methods in proteomics, including state-of-the-art techniques, novel biological samples and applications for the study of atherosclerosis. Expert commentary: Collaboration between clinicians and researchers is crucial to further validate and introduce new molecular markers to manage atherosclerosis that are identified using the most up to date proteomic approaches.

The Therapeutic Potential of Anti-Inflammatory Exerkines in the Treatment of Atherosclerosis

Although many cardiovascular (CVD) medications, such as antithrombotics, statins, and antihypertensives, have been identified to treat atherosclerosis, at most, many of these therapeutic agents only delay its progression. A growing body of evidence suggests physical exercise could be implemented as a non-pharmacologic treatment due to its pro-metabolic, multisystemic, and anti-inflammatory benefits. Specifically, it has been discovered that certain anti-inflammatory peptides, metabolites, and RNA species (collectively termed “exerkines”) are released in response to exercise that could facilitate these benefits and could serve as potential therapeutic targets for atherosclerosis. However, much of the relationship between exercise and these exerkines remains unanswered, and there are several challenges in the discovery and validation of these exerkines. This review primarily highlights major anti-inflammatory exerkines that could serve as potential therapeutic targets for atherosclerosis. To provide some context and comparison for the therapeutic potential of exerkines, the anti-inflammatory, multisystemic benefits of exercise, the basic mechanisms of atherosclerosis, and the limited efficacies of current anti-inflammatory therapeutics for atherosclerosis are briefly summarized. Finally, key challenges and future directions for exploiting these exerkines in the treatment of atherosclerosis are discussed.

Differentiation of Murine Bone Marrow-Derived Smooth Muscle Progenitor Cells Is Regulated by PDGF-BB and Collagen

Smooth muscle cells (SMCs) are key regulators of vascular disease and circulating smooth muscle progenitor cells may play important roles in vascular repair or remodelling. We developed enhanced protocols to derive smooth muscle progenitors from murine bone marrow and tested whether factors that are increased in atherosclerotic plaques, namely platelet-derived growth factor-BB (PDGF-BB) and monomeric collagen, can influence the smooth muscle specific differentiation, proliferation, and survival of mouse bone marrow-derived progenitor cells. During a 21 day period of culture, bone marrow cells underwent a marked increase in expression of the SMC markers α-SMA (1.93 ± 0.15 vs. 0.0008 ± 0.0003 (ng/ng GAPDH) at 0 d), SM22-α (1.50 ± 0.27 vs. 0.005 ± 0.001 (ng/ng GAPDH) at 0 d) and SM-MHC (0.017 ± 0.004 vs. 0.001 ± 0.001 (ng/ng GAPDH) at 0 d). Bromodeoxyuridine (BrdU) incorporation experiments showed that in early culture, the smooth muscle progenitor subpopulation could be identified by high proliferative rates prior to the expression of smooth muscle specific markers. Culture of fresh bone marrow or smooth muscle progenitor cells with PDGF-BB suppressed the expression of α-SMA and SM22-α, in a rapidly reversible manner requiring PDGF receptor kinase activity. Progenitors cultured on polymerized collagen gels demonstrated expression of SMC markers, rates of proliferation and apoptosis similar to that of cells on tissue culture plastic; in contrast, cells grown on monomeric collagen gels displayed lower SMC marker expression, lower growth rates (319 ± 36 vs. 635 ± 97 cells/mm2), and increased apoptosis (5.3 ± 1.6% vs. 1.0 ± 0.5% (Annexin 5 staining)). Our data shows that the differentiation and survival of smooth muscle progenitors are critically affected by PDGF-BB and as well as the substrate collagen structure.

Origin and differentiation of vascular smooth muscle cells

Vascular smooth muscle cells (SMCs), a major structural component of the vessel wall, not only play a key role in maintaining vascular structure but also perform various functions. During embryogenesis, SMC recruitment from their progenitors is an important step in the formation of the embryonic vascular system. SMCs in the arterial wall are mostly quiescent but can display a contractile phenotype in adults. Under pathophysiological conditions, i.e. vascular remodelling after endothelial dysfunction or damage, contractile SMCs found in the media switch to a secretory type, which will facilitate their ability to migrate to the intima and proliferate to contribute to neointimal lesions. However, recent evidence suggests that the mobilization and recruitment of abundant stem/progenitor cells present in the vessel wall are largely responsible for SMC accumulation in the intima during vascular remodelling such as neointimal hyperplasia and arteriosclerosis. Therefore, understanding the regulatory mechanisms that control SMC differentiation from vascular progenitors is essential for exploring therapeutic targets for potential clinical applications. In this article, we review the origin and differentiation of SMCs from stem/progenitor cells during cardiovascular development and in the adult, highlighting the environmental cues and signalling pathways that control phenotypic modulation within the vasculature.

Coronary Artery Disease Associated Transcription Factor TCF21 Regulates Smooth Muscle Precursor Cells That Contribute to the Fibrous Cap

Recent genome wide association studies have identified a number of genes that contribute to the risk for coronary heart disease. One such gene, TCF21, encodes a basic-helix-loop-helix transcription factor believed to serve a critical role in the development of epicardial progenitor cells that give rise to coronary artery smooth muscle cells (SMC) and cardiac fibroblasts. Using reporter gene and immunolocalization studies with mouse and human tissues we have found that vascular TCF21 expression in the adult is restricted primarily to adventitial cells associated with coronary arteries and also medial SMC in the proximal aorta of mouse. Genome wide RNA-Seq studies in human coronary artery SMC (HCASMC) with siRNA knockdown found a number of putative TCF21 downstream pathways identified by enrichment of terms related to CAD, including “vascular disease,” “disorder of artery,” and “occlusion of artery,” as well as disease-related cellular functions including “cellular movement” and “cellular growth and proliferation.” In vitro studies in HCASMC demonstrated that TCF21 expression promotes proliferation and migration and inhibits SMC lineage marker expression. Detailed in situ expression studies with reporter gene and lineage tracing revealed that vascular wall cells expressing Tcf21 before disease initiation migrate into vascular lesions of ApoE^-/- and Ldlr^-/- mice. While Tcf21 lineage traced cells are distributed throughout the early lesions, in mature lesions they contribute to the formation of a subcapsular layer of cells, and others become associated with the fibrous cap. The lineage traced fibrous cap cells activate expression of SMC markers and growth factor receptor genes. Taken together, these data suggest that TCF21 may have a role regulating the differentiation state of SMC precursor cells that migrate into vascular lesions and contribute to the fibrous cap and more broadly, in view of the association of this gene with human CAD, provide evidence that these processes may be a mechanism for CAD risk attributable to the vascular wall.

Coronary artery disease (CAD) is responsible for the majority of deaths in the Western world, and is due in part to environmental and metabolic factors. However, half of the risk for developing heart disease is genetically predetermined. Genome-wide association studies in human populations have identified over 100 sites in the genome that appear to be associated with CAD, however, the mechanisms by which variation in these regions are responsible for predisposition to CAD remain largely unknown. We have begun to study a gene that contributes to CAD risk, the TCF21 gene. Through genomic studies we show that this gene is involved in processes related to alterations in vascular gene expression, and in particular those related to the smooth muscle cell biology. With cell culture models, we show that TCF21 regulates the differentiation state of this cell type, which is believed critical for vascular disease. Using mouse genetic models of atherosclerotic vascular disease we provide evidence that this gene is expressed in precursor cells that migrate into the disease lesions and contribute to the formation of the fibrous cap that is believed to stabilize these lesions and prevent heart attacks.

Mechanisms of Plaque Formation and Rupture

Atherosclerosis causes clinical disease through luminal narrowing or by precipitating thrombi that obstruct blood flow to the heart (coronary heart disease), brain (ischemic stroke), or lower extremities (peripheral vascular disease). The most common of these manifestations is coronary heart disease, including stable angina pectoris and the acute coronary syndromes. Atherosclerosis is a lipoprotein-driven disease that leads to plaque formation at specific sites of the arterial tree through intimal inflammation, necrosis, fibrosis, and calcification. After decades of indolent progression, such plaques may suddenly cause life-threatening coronary thrombosis presenting as an acute coronary syndrome. Most often, the culprit morphology is plaque rupture with exposure of highly thrombogenic, red cell–rich necrotic core material. The permissive structural requirement for this to occur is an extremely thin fibrous cap, and thus, ruptures occur mainly among lesions defined as thin-cap fibroatheromas. Also common are thrombi forming on lesions without rupture (plaque erosion), most often on pathological intimal thickening or fibroatheromas. However, the mechanisms involved in plaque erosion remain largely unknown, although coronary spasm is suspected. The calcified nodule has been suggested as a rare cause of coronary thrombosis in highly calcified and tortious arteries in older individuals. To characterize the severity and prognosis of plaques, several terms are used. Plaque burden denotes the extent of disease, whereas plaque activity is an ambiguous term, which may refer to one of several processes that characterize progression. Plaque vulnerability describes the short-term risk of precipitating symptomatic thrombosis. In this review, we discuss mechanisms of atherosclerotic plaque initiation and progression; how plaques suddenly precipitate life-threatening thrombi; and the concepts of plaque burden, activity, and vulnerability.

Nuclear reprogramming and its role in vascular smooth muscle cells

In general terms, "nuclear reprogramming" refers to a change in gene expression profile that results in a significant switch in cellular phenotype. Nuclear reprogramming was first addressed by pioneering studies of cell differentiation during embryonic development. In recent years, nuclear reprogramming has been studied in great detail in the context of experimentally controlled dedifferentiation and transdifferentiation of mammalian cells for therapeutic purposes. In this review, we present a perspective on nuclear reprogramming in the context of spontaneous, pathophysiological phenotypic switch of vascular cells occurring in the atherosclerotic lesion. In particular, we focus on the current knowledge of epigenetic mechanisms participating in the extraordinary flexibility of the gene expression profile of vascular smooth muscle cells and other cell types participating in atherogenesis. Understanding how epigenetic changes participate in vascular cell plasticity may lead to effective therapies based on the remodelling of the vascular architecture.

Myocardin regulates vascular response to injury through miR-24/-29a and platelet-derived growth factor receptor-β

OBJECTIVE

Myocardin, a potent transcriptional coactivator of serum response factor, is involved in vascular development and promotes a contractile smooth muscle phenotype. Myocardin levels are reduced during vascular injury, in association with phenotypic switching of smooth muscle cells (SMCs). However, the direct role of myocardin in vascular disease is unclear.

APPROACH AND RESULTS

We show that re-expression of myocardin prevents the vascular injury response in murine carotid arteries, with reduced neointima formation due to decreased SMC migration and proliferation. Myocardin reduced SMC migration by downregulating platelet-derived growth factor receptor-β (PDGFRB) expression. Pdgfrb was regulated by myocardin-induced miR-24 and miR-29a expression, and antagonizing these microRNAs restored SMC migration. Furthermore, using miR-24 and miR-29a mimics, we demonstrated that miR-29a directly regulates Pdgfrb expression at the 3' untranslated region while miR-24 has an indirect effect on Pdgfrb levels. Myocardin heterozygous-null mice showed an augmented neointima formation with increased SMC migration and proliferation, demonstrating that endogenous levels of myocardin are a critical regulator of vessel injury responses.

CONCLUSIONS

Our results extend the function of myocardin from a developmental role to a pivotal regulator of SMC phenotype in response to injury, and this transcriptional coactivator may be an attractive target for novel therapeutic strategies in vascular disease.

Increased amount of bone marrow-derived smooth muscle-like cells and accelerated atherosclerosis in diabetic apoE-deficient mice

AIMS

Atherosclerotic plaque development is accelerated in patients with diabetes. Bone marrow-derived smooth muscle-like cells have been detected in neointima and diabetes has a numerical and functional effect on circulating vascular progenitor cells. We hypothesized that an increased number of bone marrow-derived smooth muscle-like cells correlates with accelerated atherosclerosis in diabetic apoE-deficient mice.

METHODS

ApoE(-/-) mice were subjected to total body irradiation and transplanted with bone marrow cells from GFP-transgenic mice. Mice were rendered diabetic by streptozotocin injection and examined after 4, 8, 11 and 15 weeks of diabetes.

RESULTS

Diabetic mice showed a larger plaque area and a higher number of smooth muscle-like cells compared to non-diabetic mice at 11 and 15 weeks after diabetes induction. Bone marrow-derived smooth muscle-like cells were detected in atherosclerotic plaques of both diabetic and control mice, but numbers were higher in plaques of diabetic mice 11 weeks after induction of diabetes. The higher number of bone marrow-derived smooth muscle-like cells in plaque was associated with an increase in in vitro differentiation of smooth muscle-like cells from spleen mononuclear cells in diabetic mice.

CONCLUSIONS

Diabetes increases the number of bone marrow-derived smooth muscle-like cells in atherosclerotic plaques and the differentiation of mononuclear cells towards smooth muscle-like cells, which may contribute to accelerated atherosclerotic plaque development in diabetic apoE(-/-) mice.

Arterial endothelial cells: still the craftsmen of regenerated endothelium

For more than a decade, a prevailing hypothesis in research related to arterial disease has been that circulating endothelial progenitor cells (EPCs) provide protection by their innate ability to replace dysfunctional or damaged endothelium. This paradigm has led to extensive investigation of EPCs in the hope of finding therapeutic targets to control their homing and differentiation. However, from the very beginning, the nomenclature and the phenotype of EPCs have been subject to controversy and there are currently no specific markers that can unambiguously identify these cells. Moreover, many of the initial observations that EPCs differentiate to endothelial cells in the course of arterial disease have been criticized for methodological problems. The present review discusses the contrasting experimental evidence as to the role of EPCs in contributing to relining of the endothelium and highlights some of the methodological pitfalls and terminological ambiguities that confuse the field.

Towards the therapeutic use of vascular smooth muscle progenitor cells

Recent advances in the development of alternative proangiogenic and revascularization processes, including recombinant protein delivery, gene therapy, and cell therapy, hold the promise of greater efficacy in the management of cardiovascular disease in the coming years. In particular, vascular progenitor cell-based strategies have emerged as an efficient treatment approach to promote vessel formation and repair and to improve tissue perfusion. During the past decade, considerable progress has been achieved in understanding therapeutic properties of endothelial progenitor cells, while the therapeutic potential of vascular smooth muscle progenitor cells (SMPC) has only recently been explored; the number of the circulating SMPC being correlated with cardiovascular health. Several endogenous SMPC populations with varying phenotypes have been identified and characterized in the peripheral blood, bone marrow, and vascular wall. While the phenotypic entity of vascular SMPC is not fully defined and remains an evolving area of research, SMPC are increasingly recognized to play a special role in cardiovascular biology. In this review, we describe the current approaches used to define vascular SMPC. We further summarize the data on phenotype and functional properties of SMPC from various sources in adults. Finally, we discuss the role of SMPC in cardiovascular disease, including the contribution of SMPC to intimal proliferation, angiogenesis, and atherosclerotic plaque instability as well as the benefits resulting from the therapeutic use of SMPC.

Circulating endothelial progenitor cells do not contribute to regeneration of endothelium after murine arterial injury

AIMS

Endothelial regeneration after vascular injury, including percutaneous coronary intervention, is essential for vascular homeostasis and inhibition of neointima formation. Circulating endothelial progenitor cells (EPCs) have been implicated to contribute by homing and differentiating into endothelial cells (ECs). We tested this theory in a murine arterial injury model using carotid artery transplants and fluorescent reporter mice.

METHODS AND RESULTS

Wire-injured carotid artery segments from wild-type mice were transplanted into TIE2-GFP transgenic mice expressing green fluorescent protein (GFP) in ECs. We found that the endothelium regenerated with GFP(+) ECs as a function of time, evolving from the anastomosis sites towards the centre of the transplant. A migration front of ECs at Day 7 was verified by scanning electron microscopy and by bright-field microscopy using recipient TIE2-lacZ mice with endothelial β-galactosidase expression. These experiments indicated migration of flanking ECs rather than homing of circulating cells as the underlying mechanism. To confirm this, we interposed non-injured wild-type carotid artery segments between the denuded transplant and the TIE2-GFP recipient mouse. Among 1186 ECs identified in re-endothelialized transplants (n= 5) by staining for von Willebrand Factor or vascular endothelial-cadherin, we did not find any blood-derived (GFP(+)) cells.

CONCLUSION

Endothelial regeneration after vascular injury did not involve circulating EPCs but was mediated solely by migration of ECs from the adjacent healthy endothelium.

Phenotype and differentiation of bone marrow-derived smooth muscle progenitor cells

1. Smooth muscle progenitor cells (SPC) are undifferentiated vascular smooth muscle cells implicated in many hyperplastic diseases of the blood vessels. However, few in vitro studies have investigated the characteristics of SPC. 2. In the present study, we constructed a recombinant plasmid with the enhanced green fluorescent protein (GFP) gene and a rat SM22α promoter, which was exclusively promoted in a smooth muscle cell lineage. Constructs were then transferred into adherent mononuclear cells derived from rat bone marrow. After 3 days, GFP-positive cells, which should be SPC, were isolated by flow cytometry. 3. Flow cytometric analysis and dual immunofluorescent staining showed that the GFP-positive cells expressed both α-smooth muscle actin (a specific marker for smooth muscle) and the chemokine receptor CXCR4 (abundant on precursor cells), but not calmodulin or CD31. After stimulation of SPC with 50 ng/mL platelet-derived growth factor-BB, CXCR4 levels decreased and calmodulin protein content increased, as determined by western blot analysis. 4. On the basis of these results, we conclude that SPC have dual characteristics of both precursor and smooth muscle cells, and might well differentiate into smooth muscle-like cells under certain conditions.

Musashi-1 expression in atherosclerotic arteries and its relevance to the origin of arterial smooth muscle cells: histopathological findings and speculations

The origin of smooth muscle cells in developing atherosclerotic lesions is a controversial topic with accumulating evidence indicating that at least some arterial smooth muscle cells might originate from bone marrow-derived smooth muscle cell precursors circulating in the blood. The stem cell markers currently used for the identification of stem cells in the arterial intima can be expressed by a number of different cell types residing in the arterial wall, such as mast cells, endothelial cells and dendritic cells, which can make interpretation of the data obtained somewhat ambiguous. In the present study we examined whether the putative intestinal stem cell marker Musashi-1 is expressed in the arterial wall. Using a multiplexed tandem polymerase chain reaction method (MT-PCR) and immunohistochemistry, Musashi-1 expression was revealed in human coronary arterial wall tissue segments, and this finding was followed by the demonstration of significantly higher expression levels of Musashi-1 in atherosclerotic plaques compared with those in undiseased intimal sites. Double immunohistochemistry demonstrated that in the arterial wall Musashi-1 positive cells either did not display any specific markers of cells that are known to reside in the arterial intima or Musashi-1 was co-expressed by smooth muscle α-actin positive cells. Some Musashi-1 positive cells were found along the luminal surface of arteries as well as within microvessels formed in atherosclerotic plaques by neovascularization, which supports the possibility that Musashi-1 positive cells might intrude into the arterial wall from the blood and might even represent circulating smooth muscle cell precursors.

Flanking recipient vasculature, not circulating progenitor cells, contributes to endothelium and smooth muscle in murine allograft vasculopathy

OBJECTIVE

The prevailing view assumes that circulating endothelial and smooth muscle progenitor cells participate in allograft vasculopathy (AV), although the seminal studies in the field were not designed to distinguish between circulating and migrating cells of recipient origin. We developed a double-transplantation technique to overcome this problem and reinvestigated the origin of endothelial cells (ECs) and smooth muscle cells (SMCs) in murine AV.

METHODS AND RESULTS

Carotid artery segments from BALB/c mice were allografted to apolipoprotein E(-/-) B6 mice with or without a "flanking" isograft interpositioned between the allograft and the recipient artery. Either recipient mice or interpositioned isografts expressed enhanced green fluorescent protein, and consequently, cells migrating into the allograft from the flanking vasculature could easily be tracked and distinguished from recruited circulating cells. Without immunosuppression, allograft donor cells vanished as expected, and AV developed by replacement and accumulation of ECs and SMCs of recipient origin. The double transplantation models revealed that all ECs and SMCs in AV had migrated into the allograft from the flanking vasculature without any contribution from putative progenitor cells in the blood.

CONCLUSIONS

Migrating cells from the flanking vasculature, not circulating progenitor cells, are the source of recipient-derived ECs and SMCs in murine AV.

Progenitor cells in pulmonary vascular remodeling

Pulmonary hypertension is characterized by cellular and structural changes in the walls of pulmonary arteries. Intimal thickening and fibrosis, medial hypertrophy and fibroproliferative changes in the adventitia are commonly observed, as is the extension of smooth muscle into the previously non-muscularized vessels. A majority of these changes are associated with the enhanced presence of α-SM-actin+ cells and inflammatory cells. Atypical abundances of functionally distinct endothelial cells, particularly in the intima (plexiform lesions), and also in the perivascular regions, are also described. At present, neither the origin(s) of these cells nor the molecular mechanisms responsible for their accumulation, in any of the three compartments of the vessel wall, have been fully elucidated. The possibility that they arise from either resident vascular progenitors or bone marrow-derived progenitor cells is now well established. Resident vascular progenitor cells have been demonstrated to exist within the vessel wall, and in response to certain stimuli, to expand and express myofibroblastic, endothelial or even hematopoietic markers. Bone marrow-derived or circulating progenitor cells have also been shown to be recruited to sites of vascular injury and to assume both endothelial and SM-like phenotypes. Here, we review the data supporting the contributory role of vascular progenitors (including endothelial progenitor cells, smooth muscle progenitor cells, pericytes, and fibrocytes) in vascular remodeling. A more complete understanding of the processes by which progenitor cells modulate pulmonary vascular remodeling will undoubtedly herald a renaissance of therapies extending beyond the control of vascular tonicity and reduction of pulmonary artery pressure.

Circulating smooth muscle progenitor cells in arterial remodeling

The proliferation and migration of vascular smooth muscle cells (SMCs) from the media toward the intimal layer are key components in vascular proliferative diseases. In addition, the differentiation of circulating bone marrow-derived mononuclear cells (BMMCs) into SMCs has been described to contribute to lesion progression in experimental models of atherosclerosis, transplant arteriosclerosis, and neointima formation. In vitro, CD14(+) BMMCs from peripheral blood acquire a spindle-shaped phenotype and express specific SMC markers in response to platelet-derived growth factor-BB. However, the 'trans-differentiation' capacity of BMMCs into definitive SMCs in vivo remains a highly controversial issue. Whereas SMCs within atherosclerotic plaques have been demonstrated to be exclusively of local origin, more severe injury models have shown a wide diversity of SMCs or smooth muscle-like cells derived from BMMCs. In hindsight, these discrepancies may be attributed to methodological differences, e.g., the use of high-resolution microscopy or the specificity of the SMC marker proteins. In fact, the analysis of mouse strains that express marker genes under the control of a highly specific smooth muscle-myosin heavy chain (SM-MHC) promoter and a time-course analysis on the dynamic process of neointima formation have recently shown that BMMCs temporarily express α-smooth muscle actin, not SM-MHC. Additionally, BM-derived cells disappear from the neointimal lesion after the inflammatory response to the injury has subsided. Although CD14(+)/CD68(+) have important paracrine effects on arterial lesion progression, BMMCs account for more of the 'SMC-like macrophages' than the highly 'trans-differentiated' and definitive SMCs in vivo. This article is part of a special issue entitled, "Cardiovascular Stem Cells Revisited".

Atherosclerotic lesions in mouse and man: is it the same disease?

PURPOSE OF REVIEW

Genetically-engineered mice with hyperlipidemia are the most widely used atherosclerosis models today, but recent advances in transgenesis open the possibility to create new models in alternative species, such as the rat and pig. It seems relevant at this point in time to review some of the strengths and weaknesses of the mouse.

RECENT FINDINGS

The histology of lesion development in mouse and man has more similarities than differences, and comparative genetics show that many mechanisms of murine and human atherogenesis are shared. Unfortunately, the most feared complication of human atherosclerosis, that is, plaque rupture and thrombosis, occur extremely rarely in mice. This is a major problem. Most patients today are not treated before symptoms ensue, and at this late stage of the disease, mechanisms identified during plaque development in the mouse may not be very important.

SUMMARY

Murine atherosclerosis models are highly valuable for identifying atherogenic mechanisms that can be targeted by preventive medicine. However, models with thrombotic complications and large animal models suitable for interventional procedures and imaging would be more supportive for current clinical practice and are highly wanted.