Contribution of bone marrow-derived pericyte precursor cells to corneal vasculogenesis

The Active Role of Pericytes During Neuroinflammation in the Adult Brain

Microvessels in the central nervous system (CNS) have one of the highest populations of pericytes, indicating their crucial role in maintaining homeostasis. Pericytes are heterogeneous cells located around brain microvessels; they present three different morphologies along the CNS vascular tree: ensheathing, mesh, and thin-strand pericytes. At the arteriole-capillary transition ensheathing pericytes are found, while mesh and thin-strand pericytes are located at capillary beds. Brain pericytes are essential for the establishment and maintenance of the blood-brain barrier, which restricts the passage of soluble and potentially toxic molecules from the circulatory system to the brain parenchyma. Pericytes play a key role in regulating local inflammation at the CNS. Pericytes can respond differentially, depending on the degree of inflammation, by secreting a set of neurotrophic factors to promote cell survival and regeneration, or by potentiating inflammation through the release of inflammatory mediators (e.g., cytokines and chemokines), and the overexpression of cell adhesion molecules. Under inflammatory conditions, pericytes may regulate immune cell trafficking to the CNS and play a role in perpetuating local inflammation. In this review, we describe pericyte responses during acute and chronic neuroinflammation.

Lymphatic/blood vessel plasticity: motivation for a future research area based on present and past observations

The lymphatic system plays a significant role in homeostasis and drainage of excess fluid back into venous circulation. Lymphatics are also associated with a number of diseases including lymphedema, tumor metastasis, and various lymphatic malformations. Emerging evidence suggests that lymphatics might have a bigger connection to the blood vascular system than originally presumed. As these two systems are often studied in isolation, several knowledge gaps exist surrounding what constitutes lymphatic vascular plasticity, under what conditions it arises, and where structures characteristic of plasticity can form. The objective of this review is to overview current structural, cell lineage-based, and cell identity-based evidence for lymphatic plasticity. These examples of plasticity will then be considered in the context of potential clinical and surgical implications of this evolving research area. This review details our current understanding of lymphatic plasticity, highlights key unanswered questions in the field, and motivates future research aimed at clarifying the role and therapeutic potential of lymphatic plasticity in disease.

Transcriptional profiling of Hutchinson-Gilford Progeria syndrome fibroblasts reveals deficits in mesenchymal stem cell commitment to differentiation related to early events in endochondral ossification

The expression of a mutant Lamin A, progerin, in Hutchinson-Gilford Progeria Syndrome leads to alterations in genome architecture, nuclear morphology, epigenetic states, and altered phenotypes in all cells of the mesenchymal lineage. Here, we report a comprehensive analysis of the transcriptional status of patient derived HGPS fibroblasts, including nine cell lines not previously reported, in comparison with age-matched controls, adults, and old adults. We find that Progeria fibroblasts carry abnormal transcriptional signatures, centering around several functional hubs: DNA maintenance and epigenetics, bone development and homeostasis, blood vessel maturation and development, fat deposition and lipid management, and processes related to muscle growth. Stratification of patients by age revealed misregulated expression of genes related to endochondral ossification and chondrogenic commitment in children aged 4-7 years old, where this differentiation program starts in earnest. Hi-C measurements on patient fibroblasts show weakening of genome compartmentalization strength but increases in TAD strength. While the majority of gene misregulation occurs in regions which do not change spatial chromosome organization, some expression changes in key mesenchymal lineage genes coincide with lamin associated domain misregulation and shifts in genome compartmentalization.

A Systematic Review of the Evidence of Hematopoietic Stem Cell Differentiation to Fibroblasts

Fibroblasts have an important role in the maintenance of the extracellular matrix of connective tissues by producing and remodelling extracellular matrix proteins. They are indispensable for physiological processes, and as such also associate with many pathological conditions. In recent years, a number of studies have identified donor-derived fibroblasts in various tissues of bone marrow transplant recipients, while others could not replicate these findings. In this systematic review, we provide an overview of the current literature regarding the differentiation of hematopoietic stem cells into fibroblasts in various tissues. PubMed, Embase, and Web of Science (Core Collection) were systematically searched for original articles concerning fibroblast origin after hematopoietic stem cell transplantation in collaboration with a medical information specialist. Our search found 5421 studies, of which 151 were analysed for full-text analysis by two authors independently, resulting in the inclusion of 104 studies. Only studies in animals and humans, in which at least one marker was used for fibroblast identification, were included. The results were described per organ of fibroblast engraftment. We show that nearly all mouse and human organs show evidence of fibroblasts of hematopoietic stem cell transfer origin. Despite significant heterogeneity in the included studies, most demonstrate a significant presence of fibroblasts of hematopoietic lineage in non-hematopoietic tissues. This presence appears to increase after the occurrence of tissue damage.

The Microvascular-Lymphatic Interface and Tissue Homeostasis: Critical Questions That Challenge Current Understanding

Lymphatic and blood microvascular networks play critical roles in the clearance of excess fluid from local tissue spaces. Given the importance of these dynamics in inflammation, tumor metastasis, and lymphedema, understanding the coordinated function and remodeling between lymphatic and blood vessels in adult tissues is necessary. Knowledge gaps exist because the functions of these two systems are typically considered separately. The objective of this review was to highlight the coordinated functional relationships between blood and lymphatic vessels in adult microvascular networks. Structural, functional, temporal, and spatial relationships will be framed in the context of maintaining tissue homeostasis, vessel permeability, and system remodeling. The integration across systems will emphasize the influence of the local environment on cellular and molecular dynamics involved in fluid flow from blood capillaries to initial lymphatic vessels in microvascular networks.

Magic Bullets: The Coming Age of Meaningful Pharmacological Control of the Corneal Responses to Injury and Disease

Corneal injuries from chemical burns, mechanical trauma, infections, immunological rejections, surgical complications, and some diseases are commonly associated with persistent epithelial defects (PED), neurotrophic epitheliopathy, scarring fibrosis, corneal neovascularization (CNV), and/or corneal endothelial damage that lead to vision loss. Several Food and Drug Administration (FDA) approved medications have recently become available, are currently in clinical trials, or are likely to enter clinical trials in the near future. For example, a 2-week course of topical human recombinant nerve growth factor is frequently an effective treatment for corneal neurotrophic epitheliopathy associated with PEDs. Topical losartan, an angiotensin converting enzyme II receptor antagonist that also inhibits TGF beta signaling, has been shown to effectively decrease myofibroblast generation and scarring fibrosis in alkali burn injury and Descemetorhexis rabbit models. Small molecule topical tyrosine kinase inhibitors, such as sunitinib and axitinib, FDA approved as chemotherapeutic agents to treat specific cancers, have also been found to be effective topical inhibitors of CNV in animal and human trials. Rho-kinase inhibitors, such as ripasudil and netarsudil, that are currently approved agents for the treatment of glaucoma in some countries, have been shown to stimulate corneal endothelial proliferation in animal studies and human trials, and may accelerate the regeneration of Descemet's membrane. These agents, as well as other drugs in development, will be used in targeted combinations to treat corneal pathophysiology associated with epithelial healing disorders, stromal scarring fibrosis, CNV, and corneal endothelial injury during the next decade.

Sourcing of human peripheral blood-derived myeloid angiogenic cells under xeno-free conditions for the treatment of critical limb ischemia

BACKGROUND

Critical limb ischemia (CLI) is the most severe form of peripheral artery disease and exhibits a high risk of lower extremity amputations. As even the most promising experimental approaches based on mesenchymal stem cells (MSCs) demonstrated only moderate therapeutic effects, we hypothesized that other cell types with intrinsic roles in angiogenesis may exhibit a stronger therapeutic potential. We have previously established a protocol to source human peripheral blood-derived angiogenic cells (BDACs). These cells promoted revascularization and took perivascular location at sites of angiogenesis, thus resembling hematopoietic pericytes, which were only described in vivo so far. We thus hypothesized that BDACs might have a superior ability to promote revascularization and rescue the affected limb in CLI.

METHODS

As standard BDAC sourcing techniques involve the use of animal-derived serum, we sought to establish a xeno- and/or serum-free protocol. Next, BDACs or MSCs were injected intramuscularly following the ligation of the iliac artery in a murine model. Their ability to enhance revascularization, impair necrosis and modulate inflammatory processes in the affected limb was investigated. Lastly, the secretomes of both cell types were compared to find potential indications for the observed differences in angiogenic potential.

RESULTS

From the various commercial media tested, one xeno-free medium enabled the derivation of cells that resembled functional BDACs in comparable numbers. When applied to a murine model of CLI, both cell types enhanced limb reperfusion and reduced necrosis, with BDACs being twice as effective as MSCs. This was also reflected in histological evaluation, where BDAC-treated animals exhibited the least muscle tissue degeneration. The BDAC secretome was enriched in a larger number of proteins with pro-angiogenic and anti-inflammatory properties, suggesting that the combination of those factors may be responsible for the superior therapeutic effect.

CONCLUSIONS

Functional BDACs can be sourced under xeno-free conditions paving the way for their safe clinical application. Since BDACs are derived from an easily accessible and renewable tissue, can be sourced in clinically relevant numbers and time frame and exceeded traditional MSCs in their therapeutic potential, they may represent an advantageous cell type for the treatment of CLI and other ischemic diseases.

Central Nervous System Pericytes Contribute to Health and Disease

Successful neuroprotection is only possible with contemporary microvascular protection. The prevention of disease-induced vascular modifications that accelerate brain damage remains largely elusive. An improved understanding of pericyte (PC) signalling could provide important insight into the function of the neurovascular unit (NVU), and into the injury-provoked responses that modify cell-cell interactions and crosstalk. Due to sharing the same basement membrane with endothelial cells, PCs have a crucial role in the control of endothelial, astrocyte, and oligodendrocyte precursor functions and hence blood-brain barrier stability. Both cerebrovascular and neurodegenerative diseases impair oxygen delivery and functionally impair the NVU. In this review, the role of PCs in central nervous system health and disease is discussed, considering their origin, multipotency, functions and also dysfunction, focusing on new possible avenues to modulate neuroprotection. Dysfunctional PC signalling could also be considered as a potential biomarker of NVU pathology, allowing us to individualize therapeutic interventions, monitor responses, or predict outcomes.

On vasa vasorum: A history of advances in understanding the vessels of vessels

The vasa vasorum are a vital microvascular network supporting the outer wall of larger blood vessels. Although these dynamic microvessels have been studied for centuries, the importance and impact of their functions in vascular health and disease are not yet fully realized. There is now rich knowledge regarding what local progenitor cell populations comprise and cohabitate with the vasa vasorum and how they might contribute to physiological and pathological changes in the network or its expansion via angiogenesis or vasculogenesis. Evidence of whether vasa vasorum remodeling incites or governs disease progression or is a consequence of cardiovascular pathologies remains limited. Recent advances in vasa vasorum imaging for understanding cardiovascular disease severity and pathophysiology open the door for theranostic opportunities. Approaches that strive to control angiogenesis and vasculogenesis potentiate mitigation of vasa vasorum-mediated contributions to cardiovascular diseases and emerging diseases involving the microcirculation.

Lipid Keratopathy: A Review of Pathophysiology, Differential Diagnosis, and Management

Lipid keratopathy is a disease in which fat deposits accumulate in the cornea, leading to opacification and decrease of visual acuity. This condition can be idiopathic without signs of previous corneal disease or secondary to ocular or systemic diseases. Lipid keratopathy is usually associated with abnormal vascularization of the cornea, and the lipid classically deposits adjacent to these vessels. Treatment of this condition usually aims to eliminate or prevent abnormal vessel formation, and several modalities have been described. In this review we summarize the etiology, pathophysiology, and clinical presentation of lipid keratopathy and describe current and emerging treatment regimens.

Wound Healing: A Cellular Perspective

Wound healing is one of the most complex processes in the human body. It involves the spatial and temporal synchronization of a variety of cell types with distinct roles in the phases of hemostasis, inflammation, growth, re-epithelialization, and remodeling. With the evolution of single cell technologies, it has been possible to uncover phenotypic and functional heterogeneity within several of these cell types. There have also been discoveries of rare, stem cell subsets within the skin, which are unipotent in the uninjured state, but become multipotent following skin injury. Unraveling the roles of each of these cell types and their interactions with each other is important in understanding the mechanisms of normal wound closure. Changes in the microenvironment including alterations in mechanical forces, oxygen levels, chemokines, extracellular matrix and growth factor synthesis directly impact cellular recruitment and activation, leading to impaired states of wound healing. Single cell technologies can be used to decipher these cellular alterations in diseased states such as in chronic wounds and hypertrophic scarring so that effective therapeutic solutions for healing wounds can be developed.

Lymphatic Vessel Network Structure and Physiology

The lymphatic system is comprised of a network of vessels interrelated with lymphoid tissue, which has the holistic function to maintain the local physiologic environment for every cell in all tissues of the body. The lymphatic system maintains extracellular fluid homeostasis favorable for optimal tissue function, removing substances that arise due to metabolism or cell death, and optimizing immunity against bacteria, viruses, parasites, and other antigens. This article provides a comprehensive review of important findings over the past century along with recent advances in the understanding of the anatomy and physiology of lymphatic vessels, including tissue/organ specificity, development, mechanisms of lymph formation and transport, lymphangiogenesis, and the roles of lymphatics in disease. © 2019 American Physiological Society. Compr Physiol 9:207-299, 2019.

Tunneling nanotubes evoke pericyte/endothelial communication during normal and tumoral angiogenesis

BACKGROUND

Nanotubular structures, denoted tunneling nanotubes (TNTs) have been described in recent times as involved in cell-to-cell communication between distant cells. Nevertheless, TNT-like, long filopodial processes had already been described in the last century as connecting facing, growing microvessels during the process of cerebral cortex vascularization and collateralization. Here we have investigated the possible presence and the cellular origin of TNTs during normal brain vascularization and also in highly vascularized brain tumors.

METHODS

We searched for TNTs by high-resolution immunofluorescence confocal microscopy, applied to the analysis of 20-µm, thick sections from lightly fixed, unembedded samples of both developing cerebral cortex and human glioblastoma (GB), immunolabeled for endothelial, pericyte, and astrocyte markers, and vessel basal lamina molecules.

RESULTS

The results revealed the existence of pericyte-derived TNTs, labeled by proteoglycan NG2/CSPG4 and CD146. In agreement with the described heterogeneity of these nanostructures, ultra-long (> 300 µm) and very thin (< 0.8 µm) TNTs were observed to bridge the gap between the wall of distant vessels, or were detected as short (< 300 µm) bridging cables connecting a vessel sprout with its facing vessel or two apposed vessel sprouts. The pericyte origin of TNTs ex vivo in fetal cortex and GB was confirmed by in vitro analysis of brain pericytes, which were able to form and remained connected by typical TNT structures.

CONCLUSIONS

None of the multiple roles described for TNTs can be excluded from a possible involvement during the processes of both normal and pathological vessel growth. A possible function, suggested by the pioneering studies made during cerebral cortex vascularization, is in cell searching and cell-to-cell recognition during the processes of vessel collateralization and vascular network formation. According to our results, it is definitely the pericyte-derived TNTs that seem to actively explore the surrounding microenvironment, searching for (site-to-site recognition), and connecting with (pericyte-to-pericyte and/or pericyte-to-endothelial cell communication), the targeted vessels. This idea implies that TNTs may have a primary role in the very early phases of both physiological and tumor angiogenesis in the brain.

Current and emerging therapies for corneal neovascularization

The cornea is unique because of its complete avascularity. Corneal neovascularization (CNV) can result from a variety of etiologies including contact lens wear; corneal infections; and ocular surface diseases due to inflammation, chemical injury, and limbal stem cell deficiency. Management is focused primarily on the etiology and pathophysiology causing the CNV and involves medical and surgical options. Because inflammation is a key factor in the pathophysiology of CNV, corticosteroids and other anti-inflammatory medications remain the mainstay of treatment. Anti-VEGF therapies are gaining popularity to prevent CNV in a number of etiologies. Surgical options including vessel occlusion and ocular surface reconstruction are other options depending on etiology and response to medical therapy. Future therapies should provide more effective treatment options for the management of CNV.

The controversial origin of pericytes during angiogenesis - Implications for cell-based therapeutic angiogenesis and cell-based therapies

Pericytes reside within the basement membrane of small vessels and are often in direct cellular contact with endothelial cells, fulfilling important functions during blood vessel formation and homeostasis. Recently, these pericytes have been also identified as mesenchymal stem cells. Mesenchymal stem cells, and especially their specialized subpopulation of pericytes, represent promising candidates for therapeutic angiogenesis applications, and have already been widely applied in pre-clinical and clinical trials. However, cell-based therapies of ischemic diseases (especially of myocardial infarction) have not resulted in significant long-term improvement. Interestingly, pericytes from a hematopoietic origin were observed in embryonic skin and a pericyte sub-population expressing leukocyte and monocyte markers was described during adult angiogenesis in vivo. Since mesenchymal stem cells do not express hematopoietic markers, the latter cell type might represent an alternative pericyte population relevant to angiogenesis. Therefore, we sourced blood-derived angiogenic cells (BDACs) from monocytes that closely resembled hematopoietic pericytes, which had only been observed in vivo thus far. BDACs displayed many pericytic features and exhibited enhanced revascularization and functional tissue regeneration in a pre-clinical model of critical limb ischemia. Comparison between BDACs and mesenchymal pericytes indicated that BDACs (while resembling hematopoietic pericytes) enhanced early stages of angiogenesis, such as endothelial cell sprouting. In contrast, mesenchymal pericytes were responsible for blood vessel maturation and homeostasis, while reducing endothelial sprouting.Since the formation of new blood vessels is crucial during therapeutic angiogenesis or during integration of implants into the host tissue, hematopoietic pericytes (and therefore BDACs) might offer an advantageous addition or even an alternative for cell-based therapies.

Growth inhibition of formed corneal neovascularization following Fosaprepitant treatment

PURPOSE

The aim of this study was to test the efficacy of Neurokinin-1 Receptor (NK-1R) antagonist -Fosaprepitant- in inducing regression of established corneal neovascularization (CNV).

METHODS

Twenty C57BL/6 mice underwent alkali burn. Seven days later, when corneal neovessels had developed, they received Fosaprepitant 10 mg/ml, administered topically six times a day in the right eye for 10 days. In parallel, a group of 20 causticated mice was treated with normal saline, as control. A second independent experiment was also performed (n = 10/group). Finally, ten healthy mice received the same topical treatment for 10 days to evaluate Fosaprepitant safety. Haemangiogenesis and lymphangiogenesis were measured by means of vesselj plugin (imagej). Secondary endpoints, such as leucocyte infiltration, corneal opacity and corneal fluorescein staining were also evaluated. Inflammatory cell composition was assessed by flow cytometry. Differences between groups were assessed using unpaired t-test, Mann-Whitney U-test or two-way anova, as appropriate.

RESULTS

Topical Fosaprepitant administration induced a significant reduction of (i) CD31⁺ blood corneal neovessels (-27%, p = 0.0132), (ii) LYVE1⁺ lymphatic corneal neovessels (-31%, p = 0.0118) and (iii) CD45⁺ leucocyte infiltration (-36%; p = 0.0237). The second independent experiment confirmed these data. Moreover, Fosaprepitant-treated corneas showed a reduction in opacity, no impairment in corneal fluorescein staining and decreased infiltration of neutrophils (-72%, p < 0.05) and macrophages (-75%, p < 0.01). Finally, topical Fosaprepitant was not toxic to the ocular surface: no signs of conjunctivitis, opacity, perforations or corneal fluorescein staining were detected. Similarly, corneal TUJ1⁺ nerve density was not affected.

CONCLUSIONS

Our data suggest that NK-1R antagonists, such as Fosaprepitant, could be a new, promising therapeutic tool to inhibit CNV after this has been established.

Vascular precursor cells in tissue injury repair

Vascular precursor cells include stem cells and progenitor cells giving rise to all mature cell types in the wall of blood vessels. When tissue injury occurs, local hypoxia and inflammation result in the generation of vasculogenic mediators which orchestrate migration of vascular precursor cells from their niche environment to the site of tissue injury. The intricate crosstalk among signaling pathways coordinates vascular precursor cell proliferation and differentiation during neovascularization. Establishment of normal blood perfusion plays an essential role in the effective repair of the injured tissue. In recent years, studies on molecular mechanisms underlying the regulation of vascular precursor cell function have achieved substantial progress, which promotes exploration of vascular precursor cell-based approaches to treat chronic wounds and ischemic diseases in vital organ systems. Verification of safety and establishment of specific guidelines for the clinical application of vascular precursor cell-based therapy remain major challenges in the field.

Pleiotropic action of CpG-ODN on endothelium and macrophages attenuates angiogenesis through distinct pathways

There is an integral relationship between vascular cells and leukocytes in supporting healthy tissue homeostasis. Furthermore, activation of these two cellular components is key for tissue repair following injury. Toll-like receptors (TLRs) play a role in innate immunity defending the organism against infection, but their contribution to angiogenesis remains unclear. Here we used synthetic TLR9 agonists, cytosine-phosphate-guanosine oligodeoxynucleotides (CpG-ODN), to investigate the role of TLR9 in vascular pathophysiology and identify potential therapeutic translation. We demonstrate that CpG-ODN stimulates inflammation yet inhibits angiogenesis. Regulation of angiogenesis by CpG-ODN is pervasive and tissue non-specific. Further, we noted that synthetic CpG-ODN requires backbone phosphorothioate but not TLR9 activation to render and maintain endothelial stalk cells quiescent. CpG-ODN pre-treated endothelial cells enhance macrophage migration but restrain pericyte mobilisation. CpG-ODN attenuation of angiogenesis, however, remains TLR9-dependent, as inhibition is lost in TLR9 deficient mice. Additionally, CpG-ODNs induce an M1 macrophage phenotype that restricts angiogenesis. The effects mediated by CpG-ODNs can therefore modulate both endothelial cells and macrophages through distinct pathways, providing potential therapeutic application in ocular vascular disease.

Bone marrow-derived cells in ocular neovascularization: contribution and mechanisms

Ocular neovascularization often leads to severe vision loss. The role of bone marrow-derived cells (BMCs) in the development of ocular neovascularization, and its significance, is increasingly being recognized. In this review, we discuss their contribution and the potential mechanisms that mediate the effect of BMCs on the progression of ocular neovascularization. The sequence of events by which BMCs participate in ocular neovascularization can be roughly divided into four phases, i.e., mobilization, migration, adhesion and differentiation. This process is delicately regulated and liable to be affected by multiple factors. Cytokines such as vascular endothelial growth factor, granulocyte colony-stimulating factor and erythropoietin are involved in the mobilization of BMCs. Studies have also demonstrated a key role of cytokines such as stromal cell-derived factor-1, tumor necrosis factor-α, as well as vascular endothelial growth factor, in regulating the migration of BMCs. The adhesion of BMCs is mainly regulated by vascular cell adhesion molecule-1, intercellular adhesion molecule-1 and vascular endothelial cadherin. However, the mechanisms regulating the differentiation of BMCs are largely unknown at present. In addition, BMCs secrete cytokines that interact with the microenvironment of ocular neovascularization; their contribution to ocular neovascularization, especially choroidal neovascularization, can be aggravated by several risk factors. An extensive regulatory network is thought to modulate the role of BMCs in the development of ocular neovascularization. A comprehensive understanding of the involved mechanisms will help in the development of novel therapeutic strategies related to BMCs. In this review, we have limited the discussion to the recent progress in this field, especially the research conducted at our laboratory.

Brain and Retinal Pericytes: Origin, Function and Role

Pericytes are specialized mural cells located at the abluminal surface of capillary blood vessels, embedded within the basement membrane. In the vascular network these multifunctional cells fulfil diverse functions, which are indispensable for proper homoeostasis. They serve as microvascular stabilizers, are potential regulators of microvascular blood flow and have a central role in angiogenesis, as they for example regulate endothelial cell proliferation. Furthermore, pericytes, as part of the neurovascular unit, are a major component of the blood-retina/brain barrier. CNS pericytes are a heterogenic cell population derived from mesodermal and neuro-ectodermal germ layers acting as modulators of stromal and niche environmental properties. In addition, they display multipotent differentiation potential making them an intriguing target for regenerative therapies. Pericyte-deficiencies can be cause or consequence of many kinds of diseases. In diabetes, for instance, pericyte-loss is a severe pathological process in diabetic retinopathy (DR) with detrimental consequences for eye sight in millions of patients. In this review, we provide an overview of our current understanding of CNS pericyte origin and function, with a special focus on the retina in the healthy and diseased. Finally, we highlight the role of pericytes in de- and regenerative processes.

Macrophages: An Inflammatory Link Between Angiogenesis and Lymphangiogenesis

Angiogenesis and lymphangiogenesis often occur in response to tissue injury or in the presence of pathology (e.g., cancer), and it is these types of environments in which macrophages are activated and increased in number. Moreover, the blood vascular microcirculation and the lymphatic circulation serve as the conduits for entry and exit for monocyte-derived macrophages in nearly every tissue and organ. Macrophages both affect and are affected by the vessels through which they travel. Therefore, it is not surprising that examination of macrophage behaviors in both angiogenesis and lymphangiogenesis has yielded interesting observations that suggest macrophages may be key regulators of these complex growth and remodeling processes. In this review, we will take a closer look at macrophages through the lens of angiogenesis and lymphangiogenesis, examining how their dynamic behaviors may regulate vessel sprouting and function. We present macrophages as a cellular link that spatially and temporally connects angiogenesis with lymphangiogenesis, in both physiological growth and in pathological adaptations, such as tumorigenesis. As such, attempts to therapeutically target macrophages in order to affect these processes may be particularly effective, and studying macrophages in both settings will accelerate the field's understanding of this important cell type in health and disease.

Retinal Angiogenesis Effects of TGF-β1 and Paracrine Factors Secreted From Human Placental Stem Cells in Response to a Pathological Environment

Abnormal angiogenesis is a primary cause of many eye diseases, including diabetic retinopathy, age-related macular degeneration, and retinopathy of prematurity. Mesenchymal stem cells (MSCs) are currently being investigated as a treatment for several such retinal diseases based on their neuroprotective and angiogenic potentials. In this study, we evaluated the role of systemically injected human placental amniotic membrane-derived MSCs (AMSCs) on pathological neovascularization of proliferative retinopathy. We determined that AMSCs secrete higher levels of transforming growth factor-β (TGF-β1) than other MSCs, and the secreted TGF-β1 directly suppresses the proliferation of endothelial cells under pathological conditions in vitro. Moreover, in a mouse model of oxygen-induced retinopathy, intraperitoneally injected AMSCs migrated into the retina and suppressed excessive neovascularization of the vasculature via expression of TGF-β1, and the antineovascular effect of AMSCs was blocked by treatment with TGF-β1 siRNA. These findings are the first to demonstrate that TGF-β1 secreted from AMSCs is one of the key factors to suppress retinal neovascularization in proliferative retinopathy and further elucidate the therapeutic function of AMSCs for the treatment of retinal neovascular diseases.

Sourcing of an alternative pericyte-like cell type from peripheral blood in clinically relevant numbers for therapeutic angiogenic applications

Autologous cells hold great potential for personalized cell therapy, reducing immunological and risk of infections. However, low cell counts at harvest with subsequently long expansion times with associated cell function loss currently impede the advancement of autologous cell therapy approaches. Here, we aimed to source clinically relevant numbers of proangiogenic cells from an easy accessible cell source, namely peripheral blood. Using macromolecular crowding (MMC) as a biotechnological platform, we derived a novel cell type from peripheral blood that is generated within 5 days in large numbers (10-40 million cells per 100 ml of blood). This blood-derived angiogenic cell (BDAC) type is of monocytic origin, but exhibits pericyte markers PDGFR-β and NG2 and demonstrates strong angiogenic activity, hitherto ascribed only to MSC-like pericytes. Our findings suggest that BDACs represent an alternative pericyte-like cell population of hematopoietic origin that is involved in promoting early stages of microvasculature formation. As a proof of principle of BDAC efficacy in an ischemic disease model, BDAC injection rescued affected tissues in a murine hind limb ischemia model by accelerating and enhancing revascularization. Derived from a renewable tissue that is easy to collect, BDACs overcome current short-comings of autologous cell therapy, in particular for tissue repair strategies.

Isolation of microvascular endothelial cells from cadaveric corneal limbus

Limbal microvascular endothelial cells (L-MVEC) contribute to formation of the corneal-limbal stem cell niche and to neovascularization of diseased and injuries corneas. Nevertheless, despite these important roles in corneal health and disease, few attempts have been made to isolate L-MVEC with the view to studying their biology in vitro. We therefore explored the feasibility of generating primary cultures of L-MVEC from cadaveric human tissue. We commenced our study by evaluating growth conditions (MesenCult-XF system) that have been previously found to be associated with expression of the endothelial cell surface marker thrombomodulin/CD141, in crude cultures established from collagenase-digests of limbal stroma. The potential presence of L-MVEC in these cultures was examined by flow cytometry using a more specific marker for vascular endothelial cells, CD31/PECAM-1. These studies demonstrated that the presence of CD141 in crude cultures established using the MesenCult-XF system is unrelated to L-MVEC. Thus we subsequently explored the use of magnetic assisted cell sorting (MACS) for CD31 as a tool for generating cultures of L-MVEC, in conjunction with more traditional endothelial cell growth conditions. These conditions consisted of gelatin-coated tissue culture plastic and MCDB-131 medium supplemented with foetal bovine serum (10% v/v), D-glucose (10 mg/mL), epidermal growth factor (10 ng/mL), heparin (50 μg/mL), hydrocortisone (1 μg/mL) and basic fibroblast growth factor (10 ng/mL). Our studies revealed that use of endothelial growth conditions are insufficient to generate significant numbers of L-MVEC in primary cultures established from cadaveric corneal stroma. Nevertheless, through use of positive-MACS selection for CD31 we were able to routinely observe L-MVEC in cultures derived from collagenase-digests of limbal stroma. The presence of L-MVEC in these cultures was confirmed by immunostaining for von Willebrand factor (vWF) and by ingestion of acetylated low-density lipoprotein. Moreover, the vWF(+) cells formed aligned cell-to-cell 'trains' when grown on Geltrex™. The purity of L-MVEC cultures was found to be unrelated to tissue donor age (32-80 years) or duration in eye bank corneal preservation medium prior to use (3-10 days in Optisol) (using multiple regression test). Optimal purity of L-MVEC cultures was achieved through use of two rounds of positive-MACS selection for CD31 (mean ± s.e.m, 65.0 ± 20.8%; p < 0.05). We propose that human L-MVEC cultures generated through these techniques, in conjunction with other cell types, will provide a useful tool for exploring the mechanisms of blood vessel cell growth in vitro.

Pericyte dynamics during angiogenesis: new insights from new identities

Therapies aimed at manipulating the microcirculation require the ability to control angiogenesis, defined as the sprouting of new capillaries from existing vessels. Blocking angiogenesis would be beneficial in many pathologies (e.g. cancer, retinopathies and rheumatoid arthritis). In others (e.g. myocardial infarction, stroke and hypertension), promoting angiogenesis would be desirable. We know that vascular pericytes elongate around endothelial cells (ECs) and are functionally associated with regulating vessel stabilization, vessel diameter and EC proliferation. During angiogenesis, bidirectional pericyte-EC signaling is critical for capillary sprout formation. Observations of pericytes leading capillary sprouts also implicate their role in EC guidance. As such, pericytes have recently emerged as a therapeutic target to promote or inhibit angiogenesis. Advancing our basic understanding of pericytes and developing pericyte-related therapies are challenged, like in many other fields, by questions regarding cell identity. This review article discusses what we know about pericyte phenotypes and the opportunity to advance our understanding by defining the specific pericyte cell populations involved in capillary sprouting.

Direct cell-cell contact between mesenchymal stem cells and endothelial progenitor cells induces a pericyte-like phenotype in vitro

Tissue engineering techniques for the regeneration of large bone defects require sufficient vascularisation of the applied constructs to ensure a sufficient supply of oxygen and nutrients. In our previous work, prevascularised 3D scaffolds have been successfully established by coculture of bone marrow derived stem cells (MSCs) and endothelial progenitor cells (EPCs). We identified stabilising pericytes (PCs) as part of newly formed capillary-like structures. In the present study, we report preliminary data on the interactions between MSCs and EPCs, leading to the differentiation of pericyte-like cells. MSCs and EPCs were seeded in transwell cultures, direct cocultures, and single cultures. Cells were cultured for 10 days in IMDM 10% FCS or IMDM 5% FCS 5% platelet lysate medium. Gene expression of PC markers, CD146, NG2, αSMA, and PDGFR-β, was analysed using RT-PCR at days 0, 3, 7, and 10. The upregulation of CD146, NG2, and αSMA in MSCs in direct coculture with EPCs advocates the MSCs' differentiation towards a pericyte-like phenotype in vitro. These results suggest that pericyte-like cells derive from MSCs and that cell-cell contact with EPCs is an important factor for this differentiation process. These findings emphasise the concept of coculture strategies to promote angiogenesis for cell-based tissue engineered bone grafts.

Bone marrow chimera experiments to determine the contribution of hematopoietic stem cells to cerebral angiogenesis

The generation of bone marrow chimera in mice is a valuable tool to study a variety of cellular processes. Donor bone marrow cells expressing reporter genes have been used to study the process of cell differentiation and the mechanisms involved in bone marrow cell recruitment. Bone marrow cells bearing genetic manipulation have been used in bone marrow chimeras to elucidate the role of molecules in different physiological and pathological settings. Since in the normal adult brain angiogenesis does not occur, models of brain injury like ischemia and tumor growth have been used to study the contribution of bone marrow-derived cells to the cerebral vasculature. This chapter describes the procedures to perform bone marrow transplantation in order to study the contribution of bone marrow-derived cells to vascularization in an orthotopic glioma model.

Bone marrow transplantation transfers age-related susceptibility to neovascular remodeling in murine laser-induced choroidal neovascularization

PURPOSE

Neovascular remodeling (NVR), the progression of small capillaries into large-caliber arterioles with perivascular fibrosis, represents a major therapeutic challenge in neovascular age-related macular degeneration (AMD). Neovascular remodeling occurs after laser-induced choroidal neovascularization (CNV) in aged but not young mice. Additionally, bone marrow-derived cells, including macrophages, endothelial precursor cells, and mesenchymal precursor cells, contribute to CNV severity. In this study, we investigated the impact of aged bone marrow transplantation (BMT) on the degree of fibrosis, size, and vascular morphology of CNV lesions in a mouse model of laser-induced CNV.

METHODS

Young (2 months) and old (16 months) mice were transplanted with green fluorescent protein (GFP)-labeled bone marrow isolated from either young or old donors. Laser CNV was induced 1 month following transplant, and eyes were analyzed via choroidal flat mounts and immunohistochemistry 1 month postlaser. The identity of cells infiltrating CNV lesions was determined using specific markers for the labeled transplanted cells (GFP+), macrophages (F4/80+), perivascular mesenchymal-derived cells (smooth muscle actin, SMA+), and endothelial cells (CD31+).

RESULTS

Bone marrow transplantation from aged mice transferred susceptibility to NVR into young recipients. Inversely, transplantation of young marrow into old mice prevented NVR, preserving small size and minimal fibrosis. Mice with NVR demonstrated a greater relative contribution of marrow-derived SMA+ perivascular mesenchymal cells as compared to other cells.

CONCLUSIONS

Our findings indicate that the status of bone marrow is an important determining factor of neovascular severity. Furthermore, we find that perivascular mesenchymal cells, rather than endothelial cells, derived from aged bone marrow may contribute to increased CNV severity in this murine model of experimental neovascularization.

Whole brain radiation-induced vascular cognitive impairment: mechanisms and implications

Mild cognitive impairment is a well-documented consequence of whole brain radiation therapy (WBRT) that affects 40-50% of long-term brain tumor survivors. The exact mechanisms for the decline in cognitive function after WBRT remain elusive and no treatment or preventative measures are available for use in the clinic. Here, we review recent findings indicating how changes in the neurovascular unit may contribute to the impairments in learning and memory. In addition to affecting neuronal development, WBRT induces profound capillary rarefaction within the hippocampus - a region of the brain important for learning and memory. Therapeutic strategies such as hypoxia, which restore the capillary density, result in the rescue of cognitive function. In addition to decreasing vascular density, WBRT impairs vasculogenesis and/or angiogenesis, which may also contribute to radiation-induced cognitive decline. Further studies aimed at uncovering the specific mechanisms underlying these WBRT-induced changes in the cerebrovasculature are essential for developing therapies to mitigate the deleterious effects of WBRT on cognitive function.

Connection of pericyte-angiopoietin-Tie-2 system in diabetic retinopathy: friend or foe?

Pericytes are distinctive regulators of vascular morphologenesis and function during vascular development and homeostasis. Pericytes have recently come into focus as implications of aberrant interactions between pericytes and endothelial cells in number of pathological angiogenesis conditions, including diabetic retinopathy and tumor angiogenesis. Pericyte dropout is a hallmark of early diabetic retinopathy. Abnormal angiopoietin (Ang)-Tie-2 signaling is one principal system participating in pericyte/endothelial cell dissociation during early stages of diabetic retinopathy. Angiopoietin 2 (Ang-2) is among the relevant growth factors induced by hypoxia and plays an important role in the initiation of retinal neovascularization and cause pericyte loss. Furthermore, high levels of VEGF synergize Ang-Tie-2 signaling during the development of diabetic retinopathy. An accelerated rate of clinical development Ang-Tie-2-manipulating drugs requests a better mechanistic understanding the connection between pericytes and Ang-Tie-2 systems both under normal and disease conditions. We summarize recent advances in pericyte study in conjunction with Ang-Tie-2 signaling and also discuss possible therapeutic strategies for diabetic retinopathy by targeting pericytes through manipulating Ang-Tie-2 signaling.

Adventitial pericyte progenitor/mesenchymal stem cells participate in the restenotic response to arterial injury

Restenosis is a major complication of coronary angioplasty, at least partly due to the fact that the origin and identity of contributing cell types are not well understood. In this study, we have investigated whether pericyte-like cells or mesenchymal stem cells (MSCs) from the adventitia contribute to restenosis. We demonstrate that while cells expressing the pericyte markers NG2, platelet-derived growth factor receptor β, and CD146 are rare in the adventitia of uninjured mouse femoral arteries, following injury their numbers strongly increase. Some of these adventitial pericyte-like cells acquire a more MSC-like phenotype (CD90+ and CD29+ are up-regulated) and also appear in the restenotic neointima. Via bone marrow transplantation and ex vivo artery culture approaches, we demonstrate that the pericyte-like MSCs of the injured femoral artery are not derived from the bone marrow, but originate in the adventitia itself mainly via the proliferation of resident pericyte-like cells. In summary, we have identified a population of resident adventitial pericyte-like cells or MSCs that contribute to restenosis following arterial injury. These cells are different from myofibroblasts, smooth muscle cells, and other progenitor populations that have been shown to participate in the restenotic process.

Pericytes on the tumor vasculature: jekyll or hyde?

The induction of tumor vasculature, known as the 'angiogenic switch', is a rate-limiting step in tumor progression. Normal blood vessels are composed of two distinct cell types: endothelial cells which form the channel through which blood flows, and mural cells, the pericytes and smooth muscle cells which serve to support and stabilize the endothelium. Most functional studies have focused on the responses of endothelial cells to pro-angiogenic stimuli; however, there is mounting evidence that the supporting mural cells, particularly pericytes, may play key regulatory roles in both promoting vessel growth as well as terminating vessel growth to generate a mature, quiescent vasculature. Tumor vessels are characterized by numerous structural and functional abnormalities, including altered association between endothelial cells and pericytes. These dysfunctional, unstable vessels contribute to hypoxia, interstitial fluid pressure, and enhanced susceptibility to metastatic invasion. Increasing evidence points to the pericyte as a critical regulator of endothelial activation and subsequent vessel development, stability, and function. Here we discuss both the stimulatory and inhibitory effects of pericytes on the vasculature and the possible utilization of vessel normalization as a therapeutic strategy to combat cancer.

Corneal neovascularization and the utility of topical VEGF inhibition: ranibizumab (Lucentis) vs bevacizumab (Avastin)

Corneal avascularity is necessary for the preservation of optimal vision. The cornea maintains a dynamic balance between pro- and antiangiogenic factors that allows it to remain avascular under normal homeostatic conditions; however, corneal avascularity can be compromised by pathologic conditions that negate the cornea's "angiogenic privilege." The clinical relevance of corneal neovascularization has long been recognized, but management of this condition has been hindered by a lack of safe and effective therapeutic modalities. Herein, the etiology, epidemiology, pathogenesis, and treatment of corneal neovascularization are reviewed. Additionally, the authors' recent findings regarding the clinical utility of topical ranibizumab (Lucentis®) and bevacizumab (Avastin®) in the treatment of corneal neovascularization are summarized. These findings clearly indicate that ranibizumab and bevacizumab are safe and effective treatments for corneal neovascularization when appropriate precautions are observed. Although direct comparisons are not conclusive, the results suggest that ranibizumab may be modestly superior to bevacizumab in terms of both onset of action and degree of efficacy. In order to justify the increased cost of ranibizumab, it will be necessary to demonstrate meaningful treatment superiority in a prospective, randomized, head-to-head comparison study.

Pericyte requirement for anti-leak action of angiopoietin-1 and vascular remodeling in sustained inflammation

Blood vessel leakiness is an early, transient event in acute inflammation but can also persist as vessels undergo remodeling in sustained inflammation. Angiopoietin/Tie2 signaling can reduce the leakiness through changes in endothelial cells. The role of pericytes in this action has been unknown. We used the selective PDGF-B-blocking oligonucleotide aptamer AX102 to determine whether disruption of pericyte-endothelial crosstalk alters vascular leakiness or remodeling in the airways of mice under four different conditions: i) baseline, ii) acute inflammation induced by bradykinin, iii) sustained inflammation after 7-day infection by the respiratory pathogen Mycoplasma pulmonis, or iv) leakage after bradykinin challenge in the presence of vascular stabilization by the angiopoietin-1 (Ang1) mimic COMP-Ang1 for 7 days. AX102 reduced pericyte coverage but did not alter the leakage of microspheres from tracheal blood vessels at baseline or after bradykinin; however, AX102 exaggerated leakage at 7 days after M. pulmonis infection and increased vascular remodeling and disease severity at 14 days. AX102 also abolished the antileakage effect of COMP-Ang1 at 7 days. Together, these findings show that pericyte contributions to endothelial stability have greater dependence on PDGF-B during the development of sustained inflammation, when pericyte dynamics accompany vascular remodeling, than under baseline conditions or in acute inflammation. The findings also show that the antileakage action of Ang1 requires PDGF-dependent actions of pericytes in maintaining endothelial stability.

Pericytopathy: oxidative stress and impaired cellular longevity in the pancreas and skeletal muscle in metabolic syndrome and type 2 diabetes

The pericyte's role has been extensively studied in retinal tissues of diabetic retinopathy; however, little is known regarding its role in such tissues as the pancreas and skeletal muscle. This supportive microvascular mural cell plays an important and novel role in cellular and extracellular matrix remodeling in the pancreas and skeletal muscle of young rodent models representing the metabolic syndrome and type 2 diabetes mellitus (T2DM). Transmission electron microscopy can be used to evaluate these tissues from young rodent models of insulin resistance and T2DM, including the transgenic Ren2 rat, db/db obese insulin resistantߞT2DM mouse, and human islet amyloid polypeptide (HIP) rat model of T2DM. With this method, the earliest pancreatic remodeling change was widening of the islet exocrine interface and pericyte hypercellularity, followed by pericyte differentiation into islet and pancreatic stellate cells with early fibrosis involving the islet exocrine interface and interlobular interstitium. In skeletal muscle there was a unique endothelial capillary connectivity via elongated longitudinal pericyte processes in addition to pericyte to pericyte and pericyte to myocyte cellcell connections allowing for paracrine communication. Initial pericyte activation due to moderate oxidative stress signaling may be followed by hyperplasia, migration and differentiation into adult mesenchymal cells. Continued robust oxidative stress may induce pericyte apoptosis and impaired cellular longevity. Circulating antipericyte autoantibodies have recently been characterized, and may provide a screening method to detect those patients who are developing pericyte loss and are at greater risk for the development of complications of T2DM due to pericytopathy and rarefaction. Once detected, these patients may be offered more aggressive treatment strategies such as early pharmacotherapy in addition to lifestyle changes targeted to maintaining pericyte integrity. In conclusion, we have provided a review of current knowledge regarding the pericyte and novel ultrastructural findings regarding its role in metabolic syndrome and T2DM.

Ex vivo cultured limbal epithelial transplantation. A clinical perspective

The term ex vivo cultured limbal epithelial transplantation (CLET) refers to the process of culturing a sheet of human limbal epithelium in the laboratory and transplanting this sheet back onto the limbal stem cell-deficient cornea of the same patient or another recipient. This emerging technology represents one of the earliest successes in regenerative medicine. CLET is, at present, best suited to patients who have unilateral total limbal stem cell deficiency arising from chemical injury and who are suitable for autologous cell culture and transplantation. Although the results of allogeneic cell transplantation are encouraging and superior to conventional stem cell transplantation techniques, insufficient follow-up precludes conclusions regarding the long-term outcomes. Other tissues, such as oral mucosal epithelium, are emerging as viable alternative sources of cells, especially for patients with bilateral disease.

More insight into mesenchymal stem cells and their effects inside the body

IMPORTANCE OF THE FIELD

The pan-tissue existence and multipotency of differentiation make mesenchymal stem cells (MSCs) an attractive source of cells as tissue repair cells, seeds of engineered tissue, vehicles for gene therapy or in combination to promote tissue regeneration in wound healing and disease recovery.

AREAS COVERED IN THIS REVIEW

This review focuses on recent understanding on MSC's basic biological characteristics and the mechanisms underlying the therapeutic effects of MSCs in vivo.

WHAT THE READER WILL GAIN

The gene expression profiles for mRNA, protein, microRNA and cell surface marker of MSCs are summarized. Special attention is given to miRNA expression and its relationship with the characteristics of MSCs. The mechanisms of therapeutic effects of MSCs are attributed to their ability to migrate along chemokine gradients, differentiate into tissue-specific cells, enhance angiogenesis of wound tissue and regulate immune response. As examples, a detailed description is given on the regeneration of functional sweat glands on burned skin as well as neural cells in middle cerebral artery occlusion (MCAO) animals upon MSC transplantation.

TAKE HOME MESSAGE

Based on current data, although limited, the mesenchymal-epithelial transition is proposed to be one of the important ways for MSCs to participate tissue repair.

CNS pericytes: concepts, misconceptions, and a way out

Rouget, in 1873, was the first to describe a population of cells surrounding capillaries, which he regarded as contractile elements. Fifty years later, Zimmermann termed these cells "pericytes" and distinguished three subtypes along the vascular tree. Since then, the discussion concerning the contractile ability of pericytes has never ceased. Current concepts of pericyte biology rather suggest critical roles in the maintenance of homeostasis, blood-brain barrier (BBB) integrity, angiogenesis, and neovascularization. In addition, data from models of brain pathology suggest that novel pericytes are recruited from the bone marrow, but their respective precursor remains enigmatic. Recent data also suggest an important role in the regulation of cerebral blood flow, thus confirming Rouget's original idea. However, comparison of data from different studies is often constrained by the fact that pericytes were questionably identified. Although a clear-cut definition exists, defining pericytes as part of the vascular wall being enclosed in its basement membrane, pericytes are often mixed up with adjacent cell types of the vascular wall, the perivascular space, and the juxtavascular parenchyma. In fact, their identification is difficult-if not impossible-in standard histological sections. An unambiguous distinction, however, is possible at the ultrastructural level and in semi-thin sections, where their location within the vascular basement membrane can be displayed. Using these techniques in combination with immunological staining methods allows demarking their unique morphology and location. Here, we review original papers describing pericytes, briefly outline their topography within the vascular compartments, describe methods for their identification, and summarize current concepts of their function.

Bone marrow-derived cells are differentially involved in pathological and physiological retinal angiogenesis in mice

PURPOSE

Bone marrow-derived cells have been shown to play roles in angiogenesis. Although these cells have been shown to promote angiogenesis, it is not yet clear whether these cells affect all types of angiogenesis. This study investigated the involvement of bone marrow-derived cells in pathological and physiological angiogenesis in the murine retina.

MATERIALS AND METHODS

The oxygen-induced retinopathy (OIR) model was used as a retinal angiogenesis model in newborn mice. To block the influence of bone marrow-derived cells, the mice were irradiated with a 4-Gy dose of radiation from a (137)Cs source. Irradiation was performed in four different conditions with radio dense 2-cm thick lead disks; (1) H group, the head were covered with these discs to protect the eyes from radiation; (2) A group, all of the body was covered with these discs; (3) N group, mice were completely unshielded; (4) C group, mice were put in the irradiator but were not irradiated. On P17, the retinal areas showing pathological and physiological retinal angiogenesis were measured and compared to the retinas of nonirradiated mice.

RESULTS

Although irradiation induced leukocyte depletion, it did not affect the number of other cell types or body weight. Retinal nonperfusion areas were significantly larger in irradiated mice than in control mice (P<0.05), indicating that physiological angiogenesis was impaired. However, the formation of tuft-like angiogenesis processes was more prominent in the irradiated mice (P<0.05), indicating that pathological angiogenesis was intact.

CONCLUSIONS

Bone marrow-derived cells seem to be differentially involved in the formation of physiological and pathological retinal vessels. Pathological angiogenesis in the murine retina does not require functional bone marrow-derived cells, but these cells are important for the formation of physiological vessels. Our results add a new insight into the pathology of retinal angiogenesis and bolster the hypothesis that bone marrow cells are involved in the pathology or severity of retinal angiogenic diseases.

Discovery of microvascular miRNAs using public gene expression data: miR-145 is expressed in pericytes and is a regulator of Fli1

Background

A function for the microRNA (miRNA) pathway in vascular development and angiogenesis has been firmly established. miRNAs with selective expression in the vasculature are attractive as possible targets in miRNA-based therapies. However, little is known about the expression of miRNAs in microvessels in vivo. Here, we identified candidate microvascular-selective miRNAs by screening public miRNA expression datasets.

Methods

Bioinformatics predictions of microvascular-selective expression were validated with real-time quantitative reverse transcription PCR on purified microvascular fragments from mouse. Pericyte expression was shown with in situ hybridization on tissue sections. Target sites were identified with 3' UTR luciferase assays, and migration was tested in a microfluid chemotaxis chamber.

Results

miR-145, miR-126, miR-24, and miR-23a were selectively expressed in microvascular fragments isolated from a range of tissues. In situ hybridization and analysis of Pdgfb retention motif mutant mice demonstrated predominant expression of miR-145 in pericytes. We identified the Ets transcription factor Friend leukemia virus integration 1 (Fli1) as a miR-145 target, and showed that elevated levels of miR-145 reduced migration of microvascular cells in response to growth factor gradients in vitro.

Conclusions

miR-126, miR-24 and miR-23a are selectively expressed in microvascular endothelial cells in vivo, whereas miR-145 is expressed in pericytes. miR-145 targets the hematopoietic transcription factor Fli1 and blocks migration in response to growth factor gradients. Our findings have implications for vascular disease and provide necessary information for future drug design against miRNAs with selective expression in the microvasculature.

The EGFR inhibitor gefitinib suppresses recruitment of pericytes and bone marrow-derived perivascular cells into tumor vessels

Drugs that target EGFR have established anti-tumor effect and are used in the clinic. Here we addressed whether inhibition of EGFR tyrosine kinase activity by gefitinib in tumor microenvironment affected tumor angiogenesis or vasculogenesis. A syngeneic tumor model of mice with grafted GFP-labeled bone marrow cells was used to analyze the effects of gefitinib on different cellular components of tumor vasculature. To characterize tumor cell-independent stromal effects of EGFR targeting, the mice were injected with B16 melanoma cells not expressing significant quantities of EGFR, and treated with gefitinib for seven days, a period not sufficient for significant reduction in total tumor volume. Numbers of vessels as well as cell surface areas covered by markers of endothelial, pericyte and bone marrow-derived progenitor cells were quantified by image analysis of tumor sections. Quantitative analysis of immunohistochemical data demonstrated that gefitinib decreased the coverage of small CD31-positive vessels with NG2-positive pericytes, as well as reduced the recruitment of perivascular GFP-positive bone marrow-derived progenitor cells within the tumor tissue. These results suggest that inhibition of EGFR activity in tumors has vascular effects in the absence of direct effect on tumor cells. EGFR targeting may lead to suppressed mobilization of pericytes needed for vessel stabilization, as well as of bone marrow-derived perivascular progenitor cells. These findings introduce novel cellular mechanisms by which EGFR targeted drugs may suppress tumor growth.

Influence of adult mesenchymal stem cells on in vitro vascular formation

The effective delivery of bioactive molecules to wound sites hasten repair. Cellular therapies provide a means for the targeted delivery of a complex, multiple arrays of bioactive factors to wound sites. Thus, the identification of ideal therapeutic populations is an essential aspect of this approach. In vitro assays can provide an important first step toward this goal by selecting populations that are likely suitable for more expensive and time-consuming in vivo assays. In this study, bone marrow-derived mesenchymal stem cells (BM-MSCs) were integrated into a three-dimensional coculture system that supports the development and stabilization of vascular tube-like structures. The presence of a limited number of BM-MSCs resulted in their coalignment with vascular structures, and it further resulted in increased tubule numbers and complexity. Thus, these studies suggest that BM-MSCs functionally interacted with and were attracted to in vitro formed vascular structures. Further, these cells also provided sufficient bioactive factors and matrix molecules to support the formation of tubular arrays and the stabilization of these arrays. This in vitro system provides a means for assessing the function of BM-MSCs in aspects of the angiogenic component of wound repair.

CEACAM1+ myeloid cells control angiogenesis in inflammation

Local inflammation during cutaneous leishmaniasis is accompanied by accumulation of CD11b+ cells at the site of the infection. A functional role for these monocytic cells in local angiogenesis in leishmaniasis has not been described so far. Here, we show that CD11b+ cells express high levels of the myeloid differentiation antigen carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1). In experimental cutaneous leishmaniasis in C57BL/6 wild-type (B6.WT) and B6.Ceacam1−/− mice, we found that only B6.Ceacam1−/− mice develop edemas and exhibit impairment of both hemangiogenesis and lymphangiogenesis. Because CEACAM1 expression correlates with functional angiogenesis, we further analyzed the role of the CD11b+ population. In B6.Ceacam1−/− mice, we found systemic reduction of Ly-6Chigh/CD11bhigh monocyte precursors. To investigate whether CEACAM1+ myeloid cells are causally related to efficient angiogenesis, we used reverse bone marrow transplants (BMTs) to restore CEACAM1+ or CEACAM1− bone marrow in B6.Ceacam1−/− or B6.WT recipients, respectively. We found that angiogenesis was restored by CEACAM1+ BMT only. In addition, we observed reduced morphogenic potential of inflammatory cells in Matrigel implants in CEACAM1− backgrounds or after systemic depletion of CD11bhigh macrophages. Taken together, we show for the first time that CEACAM1+ myeloid cells are crucial for angiogenesis in inflammation.

Stem cells of the adult cornea: from cytometric markers to therapeutic applications

The cornea is a major protective shield of the interior of the eye and represents two thirds of its refractive power. It is made up of three tissue layers that have different developmental origins: the outer, epithelial layer develops from the ectoderm overlying the lens vesicle, whereas the stroma and the endothelium have mesenchymal origin. In the adult organism, the outermost corneal epithelium is the most exposed to environmental damage, and its constant renewal is assured by the epithelial stem cells that reside in the limbus, the circular border of the cornea. Cell turnover in the stromal layer is very slow and the endothelial cells probably do not reproduce in the adult organism. However, recent experimental evidence indicates that stem cells may be found in these layers. Damage to any of the corneal layers leads to loss of transparency and low vision. Corneal limbal stem cell deficiency results in severe ocular surface disease and its treatment by transplantating ex vivo expanded limbal epithelial cells is becoming widely accepted today. Stromal and endothelial stem cells are potential tools of tissue engineering and regenerative therapies of corneal ulcers and endothelial cell loss. In the past few years, intensive research has focused on corneal stem cells aiming to improve the outcomes of the current corneal stem cell transplantation techniques. This review summarizes the current state of knowledge on corneal epithelial, stromal and endothelial stem cells. Special emphasis is placed on the molecular markers that may help to identify these cells, and the recently revealed mechanisms that could maintain their "stemness" or drive their differentiation. The techniques for isolating and culturing/expanding these cells are also described.

Ultrastructure of islet microcirculation, pericytes and the islet exocrine interface in the HIP rat model of diabetes

CONTEXT

The transgenic human islet amyloid polypeptide (HIP) rat model of type 2 diabetes mellitus (T2DM) parallels the functional and structural changes in human islets with T2DM.

OBJECTIVE

The transmission electron microscope (TEM) was utilized to observe the ultrastructural changes in islet microcirculation.

METHODS

Pancreatic tissue from male Sprague Dawley rats (2, 4, 8, 14 months) were used as controls (SDC) and compared to the 2-, 4-, 8- and 14-month-old HIP rat models.

RESULTS

The 2-month-old HIP model demonstrated no islet or microcirculation remodeling changes when compared to the SDC models. The 4-month-old HIP model demonstrated significant pericapillary amyloid deposition and diminution of pericyte foot processes as compared to the SDC models. The 8-month-old model demonstrated extensive islet amyloid deposition associated with pericyte and beta-cell apoptosis when compared with SDC. The 14-month-old HIP model demonstrated a marked reduction of beta-cells and intra-islet capillaries with near complete replacement of islets by amyloidoses. Increased cellularity in the region of the islet exocrine interface was noted in the 4- to 14-month-old HIP models as compared to SDC. In contrast to intra-islet capillary rarefaction there was noticeable angiogenesis in the islet exocrine interface. Pericytes seemed to be closely associated with collagenosis, intra-islet adipogenesis and angiogenesis in the islet exocrine interface.

CONCLUSION

The above novel findings regarding the microcirculation and pericytes could assist researchers and clinicians in a better morphological understanding of T2DM and lead to new strategies for prevention and treatment of T2DM.