Technical Advance: The rat aorta contains resident mononuclear phagocytes with proliferative capacity and proangiogenic properties

Paracrine Signals in Calcified Conditioned Media Elicited Differential Responses in Primary Aortic Vascular Smooth Muscle Cells and in Adventitial Fibroblasts

Our goal was to determine if paracrine signals from different aortic layers can impact other cell types in the diabetic microenvironment, specifically medial vascular smooth muscle cells (VSMCs) and adventitial fibroblasts (AFBs). The diabetic hyperglycemic aorta undergoes mineral dysregulation, causing cells to be more responsive to chemical messengers eliciting vascular calcification. Advanced glycation end-products (AGEs)/AGE receptors (RAGEs) signaling has been implicated in diabetes-mediated vascular calcification. To elucidate responses shared between cell types, pre-conditioned calcified media from diabetic and non-diabetic VSMCs and AFBs were collected to treat cultured murine diabetic, non-diabetic, diabetic RAGE knockout (RKO), and non-diabetic RKO VSMCs and AFBs. Calcium assays, western blots, and semi-quantitative cytokine/chemokine profile kits were used to determine signaling responses. VSMCs responded to non-diabetic more than diabetic AFB calcified pre-conditioned media. AFB calcification was not significantly altered when VSMC pre-conditioned media was used. No significant changes in VSMCs signaling markers due to treatments were reported; however, genotypic differences existed. Losses in AFB α-smooth muscle actin were observed with diabetic pre-conditioned VSMC media treatment. Superoxide dismutase-2 (SOD-2) increased with non-diabetic calcified + AGE pre-conditioned VSMC media, while same treatment decreased diabetic AFBs levels. Overall, non-diabetic and diabetic pre-conditioned media elicited different responses from VSMCs and AFBs.

Vascular Stem/Progenitor Cells in Vessel Injury and Repair

Vascular repair upon vessel injury is essential for the maintenance of arterial homeostasis and function. Stem/progenitor cells were demonstrated to play a crucial role in regeneration and replenishment of damaged vascular cells during vascular repair. Previous studies revealed that myeloid stem/progenitor cells were the main sources of tissue regeneration after vascular injury. However, accumulating evidences from developing lineage tracing studies indicate that various populations of vessel-resident stem/progenitor cells play specific roles in different process of vessel injury and repair. In response to shear stress, inflammation, or other risk factors-induced vascular injury, these vascular stem/progenitor cells can be activated and consequently differentiate into different types of vascular wall cells to participate in vascular repair. In this review, mechanisms that contribute to stem/progenitor cell differentiation and vascular repair are described. Targeting these mechanisms has potential to improve outcome of diseases that are characterized by vascular injury, such as atherosclerosis, hypertension, restenosis, and aortic aneurysm/dissection. Future studies on potential stem cell-based therapy are also highlighted.

RAGE Differentially Altered in vitro Responses in Vascular Smooth Muscle Cells and Adventitial Fibroblasts in Diabetes-Induced Vascular Calcification

The Advanced Glycation End-Products (AGE)/Receptor for AGEs (RAGE) signaling pathway exacerbates diabetes-mediated vascular calcification (VC) in vascular smooth muscle cells (VSMCs). Other cell types are involved in VC, such as adventitial fibroblasts (AFBs). We hope to elucidate some of the mechanisms responsible for differential signaling in diabetes-mediated VC with this work. This work utilizes RAGE knockout animals and in vitro calcification to measure calcification and protein responses. Our calcification data revealed that VSMCs calcification was AGE/RAGE dependent, yet AFBs calcification was not an AGE-mediated RAGE response. Protein expression data showed VSMCs lost their phenotype marker, α-smooth muscle actin, and had a higher RAGE expression over non-diabetics. RAGE knockout (RKO) VSMCs did not show changes in phenotype markers. P38 MAPK, a downstream RAGE-associated signaling molecule, had significantly increased activation with calcification in both diabetic and diabetic RKO VSMCs. AFBs showed a loss in myofibroblast marker, α-SMA, due to calcification treatment. RAGE expression decreased in calcified diabetic AFBs, and P38 MAPK activation significantly increased in diabetic and diabetic RKO AFBs. These findings point to potentially an alternate receptor mediating the calcification response in the absence of RAGE. Overall, VSMCs and AFBs respond differently to calcification and the application of AGEs.

Heterogeneous subpopulations of adventitial progenitor cells regulate vascular homeostasis and pathological vascular remodeling

Cardiovascular diseases are characterized by chronic vascular dysfunction and provoke pathological remodeling events such as neointima formation, atherosclerotic lesion development, and adventitial fibrosis. While lineage-tracing studies have shown that phenotypically modulated smooth muscle cells (SMCs) are the major cellular component of neointimal lesions, the cellular origins and microenvironmental signaling mechanisms that underlie remodeling along the adventitial vascular layer are not fully understood. However, a growing body of evidence supports a unique population of adventitial lineage-restricted progenitor cells expressing the stem cell marker, stem cell antigen-1 (Sca1; AdvSca1 cells) as important effectors of adventitial remodeling and suggests that they are at least partially responsible for subsequent pathological changes that occur in the media and intima. AdvSca1 cells are being studied in murine models of atherosclerosis, perivascular fibrosis, and neointima formation in response to acute vascular injury. Depending on the experimental conditions, AdvSca1 cells exhibit the capacity to differentiate into SMCs, endothelial cells, chondrocytes, adipocytes, and pro-remodeling cells such as myofibroblasts and macrophages. These data indicate that AdvSca1 cells may be a targetable cell population to influence the outcomes of pathologic vascular remodeling. Important questions remain regarding the origins of AdvSca1 cells and the essential signaling mechanisms and microenvironmental factors that regulate both maintenance of their stem-like, progenitor phenotype and their differentiation into lineage-specified cell types. Adding complexity to the story, recent data indicate that the collective population of adventitial progenitor cells is likely composed of several smaller, lineage-restricted subpopulations which are not fully defined by their transcriptomic profile and differentiation capabilities. The aim of this review is to outline the heterogeneity of Sca1+ adventitial progenitor cells, summarize their role in vascular homeostasis and remodeling, and comment on their translational relevance in humans.

Modifications to the classical rat aortic ring model to allow vascular degeneration studies

The classical aortic ring model is well suited for deciphering pro-angiogenic processes. Here, we propose simple modifications of the standard protocol to study various anti-angiogenic processes from growth arrest to capillary degeneration. Aortic rings are cultured under basal conditions for 6 days to allow physiological vessel sprouting and then split into treatment groups to follow capillary growth or degeneration for an additional 2 days.

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Aortic rings sprout under basal conditions until day 5 and are treated on day 6 only

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Growth rate analysis allows evaluation of vascular degeneration

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The protocol can distinguish angiostatic from vascular degenerative processes

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Paired statistical analysis confers additional power to the analysis

Engineered human myocardium with local release of angiogenic proteins improves vascularization and cardiac function in injured rat hearts

Heart regeneration after myocardial infarction requires new cardiomyocytes and a supportive vascular network. Here, we evaluate the efficacy of localized delivery of angiogenic factors from biomaterials within the implanted muscle tissue to guide growth of a more dense, organized, and perfused vascular supply into implanted engineered human cardiac tissue on an ischemia/reperfusion injured rat heart. We use large, aligned 3-dimensional engineered tissue with cardiomyocytes derived from human induced pluripotent stem cells in a collagen matrix that contains dispersed alginate microspheres as local protein depots. Release of angiogenic growth factors VEGF and bFGF in combination with morphogen sonic hedgehog from the microspheres into the local microenvironment occurs from the epicardial implant site. Analysis of the 3D vascular network in the engineered tissue via Microfil® perfusion and microCT imaging at 30 days shows increased volumetric network density with a wider distribution of vessel diameters, proportionally increased branching and length, and reduced tortuosity. Global heart function is increased in the angiogenic factor-loaded cardiac implants versus sham. These findings demonstrate for the first time the efficacy of a combined remuscularization and revascularization therapy for heart regeneration after myocardial infarction.

Vasculogenic properties of adventitial Sca-1+CD45+ progenitor cells in mice: a potential source of vasa vasorum in atherosclerosis

The cellular origins of vasa vasorum are ill-defined and may involve circulating or local progenitor cells. We previously discovered that murine aortic adventitia contains Sca-1⁺CD45⁺ progenitors that produce macrophages. Here we investigated whether they are also vasculogenic. In aortas of C57BL/6 mice, Sca-1⁺CD45⁺ cells were localised to adventitia and lacked surface expression of endothelial markers (<1% for CD31, CD144, TIE-2). In contrast, they did show expression of CD31, CD144, TIE-2 and VEGFR2 in atherosclerotic ApoE^-/- aortas. Although Sca-1⁺CD45⁺ cells from C57BL/6 aorta did not express CD31, they formed CD31⁺ colonies in endothelial differentiation media and produced interconnecting vascular-like cords in Matrigel that contained both endothelial cells and a small population of macrophages, which were located at branch points. Transfer of aortic Sca-1⁺CD45⁺ cells generated endothelial cells and neovessels de novo in a hindlimb model of ischaemia and resulted in a 50% increase in perfusion compared to cell-free control. Similarly, their injection into the carotid adventitia of ApoE^-/- mice produced donor-derived adventitial and peri-adventitial microvessels after atherogenic diet, suggestive of newly formed vasa vasorum. These findings show that beyond its content of macrophage progenitors, adventitial Sca-1⁺CD45⁺ cells are also vasculogenic and may be a source of vasa vasorum during atherogenesis.

The Impact of Natural Antioxidants on the Regenerative Potential of Vascular Cells

With advances in technology, the impact of natural antioxidants on vascular cell regeneration is attracting enormous attention as many current studies are now exploring the clinical potential of antioxidants in regenerative medicine. Natural antioxidants are an important step for improving future treatment and prevention of various diseases such as cardiovascular, cancer, neurodegenerative, and diabetes. The use of natural antioxidants which have effects on several types of stem cells with the potential to differentiate into functional endothelium and smooth muscle cells (known as vascular progenitors) for vascular regeneration might override pharmaceutical and surgical treatments. The natural antioxidant systems comprise of several components present in fruits, vegetables, legumes, medicinal plants, and other animal-derived products that interact with reactive free radicals such as oxygen and nitrogen species to neutralize their oxidative damaging effects on vascular cells. Neutralization by antioxidants involves the breaking down of the oxidative cascade chain reactions in the cell membranes in order to fine-tune the free radical levels. The effect of natural antioxidants on vascular regeneration includes restoration or establishment of new vascular structures and functions. In this review, we highlight the significant effects of natural antioxidants on modulating vascular cells to regenerate vessels, as well as possible mechanisms of action and the potential therapeutic benefits on health. The role of antioxidants in regenerating vessels may be critical for the future of regenerative medicine in terms of the maintenance of the normal functioning of vessels and the prevention of multiple vascular diseases.

Consensus guidelines for the use and interpretation of angiogenesis assays

The formation of new blood vessels, or angiogenesis, is a complex process that plays important roles in growth and development, tissue and organ regeneration, as well as numerous pathological conditions. Angiogenesis undergoes multiple discrete steps that can be individually evaluated and quantified by a large number of bioassays. These independent assessments hold advantages but also have limitations. This article describes in vivo, ex vivo, and in vitro bioassays that are available for the evaluation of angiogenesis and highlights critical aspects that are relevant for their execution and proper interpretation. As such, this collaborative work is the first edition of consensus guidelines on angiogenesis bioassays to serve for current and future reference.

Vascular Stem/Progenitor Cell Migration and Differentiation in Atherosclerosis

SIGNIFICANCE

Atherosclerosis is a major cause for the death of human beings, and it takes place in large- and middle-sized arteries. The pathogenesis of the disease has been widely investigated, and new findings on vascular stem/progenitor cells could have an impact on vascular regeneration. Recent Advances: Recent studies have shown that abundant stem/progenitor cells present in the vessel wall are mainly responsible for cell accumulation in the intima during vascular remodeling. It has been demonstrated that the mobilization and recruitment of tissue-resident stem/progenitor cells give rise to endothelial and smooth muscle cells (SMCs) that participate in vascular repair and remodeling such as neointimal hyperplasia and arteriosclerosis. Interestingly, cell lineage tracing studies indicate that a large proportion of SMCs in neointimal lesions is derived from adventitial stem/progenitor cells.

CRITICAL ISSUES

The influence of stem/progenitor cell behavior on the development of atherosclerosis is crucial. An understanding of the regulatory mechanisms that control stem/progenitor cell migration and differentiation is essential for stem/progenitor cell therapy for vascular diseases and regenerative medicine.

FUTURE DIRECTIONS

Identification of the detailed process driving the migration and differentiation of vascular stem/progenitor cells during the development of atherosclerosis, discovery of the environmental cues, and signaling pathways that control cell fate within the vasculature will facilitate the development of new preventive and therapeutic strategies to combat atherosclerosis. Antioxid. Redox Signal. 00, 000-000.

Role of Resident Stem Cells in Vessel Formation and Arteriosclerosis

Vascular, resident stem cells are present in all 3 layers of the vessel wall; they play a role in vascular formation under physiological conditions and in remodeling in pathological situations. Throughout development and adult early life, resident stem cells participate in vessel formation through vasculogenesis and angiogenesis. In adults, the vascular stem cells are mostly quiescent in their niches but can be activated in response to injury and participate in endothelial repair and smooth muscle cell accumulation to form neointima. However, delineation of the characteristics and of the migration and differentiation behaviors of these stem cells is an area of ongoing investigation. A set of genetic mouse models for cell lineage tracing has been developed to specifically address the nature of these cells and both migration and differentiation processes during physiological angiogenesis and in vascular diseases. This review summarizes the current knowledge on resident stem cells, which has become more defined and refined in vascular biology research, thus contributing to the development of new potential therapeutic strategies to promote endothelial regeneration and ameliorate vascular disease development.

Targeted gold-coated iron oxide nanoparticles for CD163 detection in atherosclerosis by MRI

CD163 is a membrane receptor expressed by macrophage lineage. Studies performed in atherosclerosis have shown that CD163 expression is increased at inflammatory sites, pointing at the presence of intraplaque hemorrhagic sites or asymptomatic plaques. Hence, imaging of CD163 expressing macrophages is an interesting strategy in order to detect atherosclerotic plaques. We have prepared a targeted probe based on gold-coated iron oxide nanoparticles vectorized with an anti-CD163 antibody for the specific detection of CD163 by MRI. Firstly, the specificity of the targeted probe was validated in vitro by incubation of the probe with CD163(+) or (-) macrophages. The probe was able to selectively detect CD163(+) macrophages both in human and murine cells. Subsequently, the targeted probe was injected in 16 weeks old apoE deficient mice developing atherosclerotic lesions and the pararenal abdominal aorta was imaged by MRI. The accumulation of probe in the site of interest increased over time and the signal intensity decreased significantly 48 hours after the injection. Hence, we have developed a highly sensitive targeted probe capable of detecting CD163-expressing macrophages that could provide useful information about the state of the atheromatous lesions.

Vascular wall progenitor cells in health and disease

The vasculature plays an indispensible role in organ development and maintenance of tissue homeostasis, such that disturbances to it impact greatly on developmental and postnatal health. Although cell turnover in healthy blood vessels is low, it increases considerably under pathological conditions. The principle sources for this phenomenon have long been considered to be the recruitment of cells from the peripheral circulation and the re-entry of mature cells in the vessel wall back into cell cycle. However, recent discoveries have also uncovered the presence of a range of multipotent and lineage-restricted progenitor cells in the mural layers of postnatal blood vessels, possessing high proliferative capacity and potential to generate endothelial, smooth muscle, hematopoietic or mesenchymal cell progeny. In particular, the tunica adventitia has emerged as a progenitor-rich compartment with niche-like characteristics that support and regulate vascular wall progenitor cells. Preliminary data indicate the involvement of some of these vascular wall progenitor cells in vascular disease states, adding weight to the notion that the adventitia is integral to vascular wall pathogenesis, and raising potential implications for clinical therapies. This review discusses the current body of evidence for the existence of vascular wall progenitor cell subpopulations from development to adulthood and addresses the gains made and significant challenges that lie ahead in trying to accurately delineate their identities, origins, regulatory pathways, and relevance to normal vascular structure and function, as well as disease.

Immortalized multipotent pericytes derived from the vasa vasorum in the injured vasculature. A cellular tool for studies of vascular remodeling and regeneration

Adventitial microvessels, vasa vasorum in the vessel walls, have an active role in the vascular remodeling, although its mechanisms are still unclear. It has been reported that microvascular pericytes (PCs) possess mesenchymal plasticity. Therefore, microvessels would serve as a systemic reservoir of stem cells and contribute to the tissues remodeling. However, most aspects of the biology of multipotent PCs (mPCs), in particular of pathological microvessels are still obscure because of the lack of appropriate methods to detect and isolate these cells. In order to examine the characteristics of mPCs, we established immortalized cells residing in adventitial capillary growing at the injured vascular walls. We recently developed in vivo angiogenesis to observe adventitial microvessels using collagen-coated tube (CCT), which also can be used as an adventitial microvessel-rich tissue. By using the CCT, CD146- or NG2-positive cells were isolated from the adventitial microvessels in the injured arteries of mice harboring a temperature-sensitive SV40 T-antigen gene. Several capillary-derived endothelial cells (cECs) and PCs (cPCs) cell lines were established. cECs and cPCs maintain a number of key endothelial and PC features. Co-incubation of cPCs with cECs formed capillary-like structure in Matrigel. Three out of six cPC lines, termed capillary mPCs demonstrated both mesenchymal stem cell- and neuronal stem cell-like phenotypes, differentiating effectively into adipocytes, osteoblasts, as well as schwann cells. mPCs differentiated to ECs and PCs, and formed capillary-like structure on their own. Transplanted DsRed-expressing mPCs were resident in the capillary and muscle fibers and promoted angiogenesis and myogenesis in damaged skeletal muscle. Adventitial mPCs possess transdifferentiation potential with unique phenotypes, including the reconstitution of capillary-like structures. Their phenotype would contribute to the pathological angiogenesis associated with vascular remodeling. These cell lines also provide a reproducible cellular tool for high-throughput studies on angiogenesis, vascular remodeling, and regeneration as well.

Vitamin D promotes vascular regeneration

BACKGROUND

Vitamin D deficiency in humans is frequent and has been associated with inflammation. The role of the active hormone 1,25-dihydroxycholecalciferol (1,25-dihydroxy-vitamin D3; 1,25-VitD3) in the cardiovascular system is controversial. High doses induce vascular calcification; vitamin D3 deficiency, however, has been linked to cardiovascular disease because the hormone has anti-inflammatory properties. We therefore hypothesized that 1,25-VitD3 promotes regeneration after vascular injury.

METHODS AND RESULTS

In healthy volunteers, supplementation of vitamin D3 (4000 IU cholecalciferol per day) increased the number of circulating CD45-CD117+Sca1+Flk1+ angiogenic myeloid cells, which are thought to promote vascular regeneration. Similarly, in mice, 1,25-VitD3 (100 ng/kg per day) increased the number of angiogenic myeloid cells and promoted reendothelialization in the carotid artery injury model. In streptozotocin-induced diabetic mice, 1,25-VitD3 also promoted reendothelialization and restored the impaired angiogenesis in the femoral artery ligation model. Angiogenic myeloid cells home through the stromal cell-derived factor 1 (SDF1) receptor CXCR4. Inhibition of CXCR4 blocked 1,25-VitD3-stimulated healing, pointing to a role of SDF1. The combination of injury and 1,25-VitD3 increased SDF1 in vessels. Conditioned medium from injured, 1,25-VitD3-treated arteries elicited a chemotactic effect on angiogenic myeloid cells, which was blocked by SDF1-neutralizing antibodies. Conditional knockout of the vitamin D receptor in myeloid cells but not the endothelium or smooth muscle cells blocked the effects of 1,25-VitD3 on healing and prevented SDF1 formation. Mechanistically, 1,25-VitD3 increased hypoxia-inducible factor 1-α through binding to its promoter. Increased hypoxia-inducible factor signaling subsequently promoted SDF1 expression, as revealed by reporter assays and knockout and inhibitory strategies of hypoxia-inducible factor 1-α.

CONCLUSIONS

By inducing SDF1, vitamin D3 is a novel approach to promote vascular repair.

Regulation of angiogenesis, mural cell recruitment and adventitial macrophage behavior by Toll-like receptors

The angiogenic response to injury can be studied by culturing rat or mouse aortic explants in collagen gels. Gene expression studies show that aortic angiogenesis is preceded by an immune reaction with overexpression of Toll-like receptors (TLRs) and TLR-inducible genes. TLR1, 3, and 6 are transiently upregulated at 24 h whereas TLR2, 4, and 8 expression peaks at 24 h but remains elevated during angiogenesis and vascular regression. Expression of TLR5, 7 and 9 steadily increases over time and is highest during vascular regression. Studies with isolated cells show that TLRs are expressed at higher levels in aortic macrophages compared to endothelial or mural cells with the exception of TLR2 and TLR9 which are more abundant in the aortic endothelium. LPS and other TLR ligands dose dependently stimulate angiogenesis and vascular endothelial growth factor production. TLR9 ligands also influence the behavior of nonendothelial cell types by blocking mural cell recruitment and inducing formation of multinucleated giant cells by macrophages. TLR9-induced mural cell depletion is associated with reduced expression of the mural cell recruiting factor PDGFB. The spontaneous angiogenic response of the aortic rings to injury is reduced in cultures from mice deficient in myeloid differentiation primary response 88 (MyD88), a key adapter molecule of TLRs, and following treatment with an inhibitor of the NFκB pathway. These results suggest that the TLR system participates in the angiogenic response of the vessel wall to injury and may play an important role in the regulation of inflammatory angiogenesis in reactive and pathologic processes.

The adventitia: essential regulator of vascular wall structure and function

The vascular adventitia acts as a biological processing center for the retrieval, integration, storage, and release of key regulators of vessel wall function. It is the most complex compartment of the vessel wall and is composed of a variety of cells, including fibroblasts, immunomodulatory cells (dendritic cells and macrophages), progenitor cells, vasa vasorum endothelial cells and pericytes, and adrenergic nerves. In response to vascular stress or injury, resident adventitial cells are often the first to be activated and reprogrammed to influence the tone and structure of the vessel wall; to initiate and perpetuate chronic vascular inflammation; and to stimulate expansion of the vasa vasorum, which can act as a conduit for continued inflammatory and progenitor cell delivery to the vessel wall. This review presents the current evidence demonstrating that the adventitia acts as a key regulator of vascular wall function and structure from the outside in.

The acute phase reactant orosomucoid-1 is a bimodal regulator of angiogenesis with time- and context-dependent inhibitory and stimulatory properties

Background

Tissues respond to injury by releasing acute phase reaction (APR) proteins which regulate inflammation and angiogenesis. Among the genes upregulated in wounded tissues are tumor necrosis factor-alpha (TNFα) and the acute phase reactant orosomucoid-1 (ORM1). ORM1 has been shown to modulate the response of immune cells to TNFα, but its role on injury- and TNFα-induced angiogenesis has not been investigated. This study was designed to characterize the role of ORM1 in the angiogenic response to injury and TNFα.

Methods and Results

Angiogenesis was studied with in vitro, ex vivo, and in vivo angiogenesis assays. Injured rat aortic rings cultured in collagen gels produced an angiogenic response driven by macrophage-derived TNFα. Microarray analysis and qRT-PCR showed that TNFα and ORM1 were upregulated prior to angiogenic sprouting. Exogenous ORM1 delayed the angiogenic response to injury and inhibited the proangiogenic effect of TNFα in cultures of aortic rings or isolated endothelial cells, but stimulated aortic angiogenesis over time while promoting VEGF production and activity. ORM1 inhibited injury- and TNFα-induced phosphorylation of MEK1/2 and p38 MAPK in aortic rings, but not of NFκB. This effect was injury/TNFα-specific since ORM1 did not inhibit VEGF-induced signaling, and cell-specific since ORM1 inhibited TNFα-induced phosphorylation of MEK1/2 and p38 MAPK in macrophages and endothelial cells, but not mural cells. Experiments with specific inhibitors demonstrated that the MEK/ERK pathway was required for angiogenesis. ORM1 inhibited angiogenesis in a subcutaneous in vivo assay of aortic ring-induced angiogenesis, but stimulated developmental angiogenesis in the chorioallantoic membrane (CAM) assay.

Conclusion

ORM1 regulates injury-induced angiogenesis in a time- and context-dependent manner by sequentially dampening the initial TNFα-induced angiogenic response and promoting the downstream stimulation of the angiogenic process by VEGF. The context-dependent nature of ORM1 angioregulatory function is further demonstrated in the CAM assay where ORM1 stimulates developmental angiogenesis without exerting any inhibitory activity.

Identification of a monocyte-predisposed hierarchy of hematopoietic progenitor cells in the adventitia of postnatal murine aorta

BACKGROUND

Hematopoiesis originates from the dorsal aorta during embryogenesis. Although adult blood vessels harbor progenitor populations for endothelial and smooth muscle cells, it is not known if they contain hematopoietic progenitor or stem cells. Here, we hypothesized that the arterial wall is a source of hematopoietic progenitor and stem cells in postnatal life.

METHODS AND RESULTS

Single-cell aortic disaggregates were prepared from adult chow-fed C57BL/6 and apolipoprotein E-null (ApoE(-/-)) mice. In short- and long-term methylcellulose-based culture, aortic cells generated a broad spectrum of multipotent and lineage-specific hematopoietic colony-forming units, with a preponderance of macrophage colony-forming units. This clonogenicity was higher in lesion-free ApoE(-/-) mice and localized primarily to stem cell antigen-1-positive cells in the adventitia. Expression of stem cell antigen-1 in the aorta colocalized with canonical hematopoietic stem cell markers, as well as CD45 and mature leukocyte antigens. Adoptive transfer of labeled aortic cells from green fluorescent protein transgenic donors to irradiated C57BL/6 recipients confirmed the content of rare hematopoietic stem cells (1 per 4 000 000 cells) capable of self-renewal and durable, low-level reconstitution of leukocytes. Moreover, the predominance of long-term macrophage precursors was evident by late recovery of green fluorescent protein-positive colonies from recipient bone marrow and spleen that were exclusively macrophage colony-forming units. Although trafficking from bone marrow was shown to replenish some of the hematopoietic potential of the aorta after irradiation, the majority of macrophage precursors appeared to arise locally, suggesting long-term residence in the vessel wall.

CONCLUSIONS

The postnatal murine aorta contains rare multipotent hematopoietic progenitor/stem cells and is selectively enriched with stem cell antigen-1-positive monocyte/macrophage precursors. These populations may represent novel, local vascular sources of inflammatory cells.

The adventitia: a progenitor cell niche for the vessel wall

Recent observations suggest that the adventitial layer of blood vessels exhibits properties resembling a stem/progenitor cell niche. Progenitor cells have been isolated from the adventitia of both murine and human blood vessels with the potential to form endothelial cells, mural cells, osteogenic cells, and adipocytes. These progenitors appear to cluster at or near the border zone between the outer media and inner adventitia. In the mouse, this border zone region corresponds to a localized site of sonic hedgehog signaling in the artery wall. This brief review will discuss the emerging evidence that the tunica adventitia may provide a niche-like signaling environment for resident progenitor cells and will address the role of the adventitia in growth, remodeling, and repair of the artery wall.

The adventitia: a dynamic interface containing resident progenitor cells

Conventional views of the tunica adventitia as a poorly organized layer of vessel wall composed of fibroblasts, connective tissue, and perivascular nerves are undergoing revision. Recent studies suggest that the adventitia has properties of a stem/progenitor cell niche in the artery wall that may be poised to respond to arterial injury. It is also a major site of immune surveillance and inflammatory cell trafficking and harbors a dynamic microvasculature, the vasa vasorum, that maintains the medial layer and provides an important gateway for macrophage and leukocyte migration into the intima. In addition, the adventitia is in contact with tissue that surrounds the vessel and may actively participate in exchange of signals and cells between the vessel wall and the tissue in which it resides. This brief review highlights recent advances in our understanding of the adventitia and its resident progenitor cells and discusses progress toward an integrated view of adventitial function in vascular development, repair, and disease.

Macrophage-derived tumor necrosis factor-alpha is an early component of the molecular cascade leading to angiogenesis in response to aortic injury

OBJECTIVE

The goal of this study was to define the role of tumor necrosis factor-α (TNFα) in the cascade of gene activation that regulates aortic angiogenesis in response to injury.

METHODS AND RESULTS

Angiogenesis was studied by culturing rat or mouse aortic rings in collagen gels. Gene expression was evaluated by quantitative reverse transcription-polymerase chain reaction, microarray analysis, immunocytochemistry, and ELISA. TNFα gene disruption and recombinant TNFα or blocking antibodies against vascular endothelial growth factor (VEGF) or TNF receptors were used to investigate TNFα-mediated angiogenic mechanisms. Resident aortic macrophages were depleted with liposomal clodronate. Angiogenesis was preceded by overexpression of TNFα and TNFα-inducible genes. Studies with isolated cells showed that macrophages were the main source of TNFα. Angiogenesis, VEGF production, and macrophage outgrowth were impaired by TNFα gene disruption and promoted by exogenous TNFα. Antibody-mediated inhibition of TNF receptor 1 significantly inhibited angiogenesis. The proangiogenic effect of TNFα was suppressed by blocking VEGF or by ablating aortic macrophages. Exogenous TNFα, however, maintained a limited proangiogenic capacity in the absence of macrophages and macrophage-mediated VEGF production.

CONCLUSIONS

Overexpression of TNFα is required for optimal VEGF production and angiogenesis in response to injury. This TNFα/VEGF-mediated angiogenic pathway requires macrophages. The residual capacity of TNFα to stimulate angiogenesis in macrophage-depleted aortic cultures implies the existence of a VEGF-independent alternate pathway of TNFα-induced angiogenesis.

A two-way communication between microglial cells and angiogenic sprouts regulates angiogenesis in aortic ring cultures

BACKGROUND

Myeloid cells have been associated with physiological and pathological angiogenesis, but their exact functions in these processes remain poorly defined. Monocyte-derived tissue macrophages of the CNS, or microglial cells, invade the mammalian retina before it becomes vascularized. Recent studies correlate the presence of microglia in the developing CNS with vascular network formation, but it is not clear whether the effect is directly caused by microglia and their contact with the endothelium.

METHODOLOGY/PRINCIPAL FINDINGS

We combined in vivo studies of the developing mouse retina with in vitro studies using the aortic ring model to address the role of microglia in developmental angiogenesis. Our in vivo analyses are consistent with previous findings that microglia are present at sites of endothelial tip-cell anastomosis, and genetic ablation of microglia caused a sparser vascular network associated with reduced number of filopodia-bearing sprouts. Addition of microglia in the aortic ring model was sufficient to stimulate vessel sprouting. The effect was independent of physical contact between microglia and endothelial cells, and could be partly mimicked using microglial cell-conditioned medium. Addition of VEGF-A promoted angiogenic sprouts of different morphology in comparison with the microglial cells, and inhibition of VEGF-A did not affect the microglia-induced angiogenic response, arguing that the proangiogenic factor(s) released by microglia is distinct from VEGF-A. Finally, microglia exhibited oriented migration towards the vessels in the aortic ring cultures.

CONCLUSIONS/SIGNIFICANCE

Microglia stimulate vessel sprouting in the aortic ring cultures via a soluble microglial-derived product(s), rather than direct contact with endothelial cells. The observed migration of microglia towards the growing sprouts suggests that their position near endothelial tip-cells could result from attractive cues secreted by the vessels. Our data reveals a two-way communication between microglia and vessels that depends on soluble factors and should extend the understanding of how microglia promote vascular network formation.