NG2 proteoglycan is expressed exclusively by mural cells during vascular morphogenesis

Defective blood vessel development and pericyte/pvSMC distribution in alpha 4 integrin-deficient mouse embryos

Blood vessel development is in part regulated by pericytes/presumptive vascular smooth muscle cells (PC/pvSMCs). Here, we demonstrate that interactions between PC/pvSMCs and extracellular matrix play a critical role in this event. We show that the cranial vessels in alpha4 integrin-deficient mouse embryos at the stage of vessel remodeling are increased in diameter. This defect is accompanied by a failure of PC/pvSMCs, which normally express alpha4beta1 integrin, to spread uniformly along the vessels. We also find that fibronectin but not VCAM-1 is localized in the cranial vessels at this stage. Furthermore, cultured alpha4 integrin-null PC/pvSMCs plated on fibronectin display a delay in initiating migration, a reduction in migration speed, and a decrease in directional persistence in response to a polarized force of shear flow. These results suggest that specific motile activities of PC/pvSMCs regulated by mechanical signals imposed by the interstitial extracellular matrix may also be required in vivo for the distribution and function of the PC/pvSMCs during blood vessel development.

Targeting of pericytes diminishes neovascularization and lymphangiogenesis in prostate cancer

BACKGROUND

The walls of capillaries in prostate cancer are composed of endothelial cells, and pericytes. NG2 is a transmembrane proteoglycan on nascent pericytes with a functional role in neovascularization.

METHODS

The anti-angiogenic effect of hydron pellets containing NG2 neutralizing antibody was quantified in intracorneal PC-3 and LNCaP xenografts. TRAMP and TRAMP-C1 tumors grafted in NG2 knockout mice represented intrinsic pericyte targeting. TRAMP and TRAMP-C1 grafts were analyzed with confocal microscope for microvascular density (MVD) and lymphatic vascular density (LVD).

RESULTS

NG2 neutralizing antibody decreased corneal neovascularization in PC3 (P<0.0001), and LNCaP (P=0.0079) xenografts. Mean MVD in TRAMP and TRAMP-C1 tumors in NG2 knockout mice were 71% (P=0.0006) and 63% (P=0.0011) lower than wild type controls, respectively. Mean LVD in TRAMP and TRAMP-C1 tumors in NG2 knockout mice were 73% (P=0.0003) and 84% (P<0.0001) lower than wild type controls, respectively.

CONCLUSIONS

Targeting of pericyte-NG2 decreases neovascularization and lymphangiogenesis in prostate cancer significantly.

Angiogenesis with pericyte abnormalities in a transgenic model of prostate carcinoma

BACKGROUND

Previous studies of the TRansgenic Adenocarcinoma of the Mouse Prostate (TRAMP) model vasculature suggest that, as tumors develop, vessels invade the glandular epithelium. However, changes in the vasculature are difficult to study in conventional thin tissue sections. The authors used a new approach to characterize morphologic and architectural changes of blood vessels and pericytes during tumor development in TRAMP mice.

METHODS

Eighty-micron cryostat sections of normal prostate and three histopathologic stages of TRAMP tumor sections, classified by epithelial cell E-cadherin immunoreactivity, were immunostained with vascular endothelial cell and pericyte receptor antibodies and evaluated by confocal microscopy.

RESULTS

In the normal mouse prostate, capillaries were most abundant in the fibromuscular tunica between the epithelium and smooth muscle of the ductules. In the prostatic intraepithelial neoplasia (PIN) stage, vessels accompanied epithelial cell protrusions into the ductule lumen but remained in the connective tissue at the basal side of the epithelium. Well differentiated tissues had extensive angiogenesis with five times the normal mean vascularity outside ductules. Vessels were of variable diameter, were associated with an increased number of pericytes, and some had endothelial sprouts. Angiogenic blood vessels from poorly differentiated adenocarcinomas were tortuous, variable in caliber, and lacked the normal hierarchy. Pericytes on these vessels had an abnormal phenotype manifested by alpha-smooth muscle actin expression and loose association with endothelial cells. Angiogenesis and loss of vascular hierarchy were also found in human prostate carcinoma.

CONCLUSIONS

Vascular abnormalities, which begin at the PIN stage and intensify in well differentiated and poorly differentiated tumors, may be useful readouts for early detection and treatment assessment in prostate carcinoma.

Altered expression of the MCSP/NG2 chondroitin sulfate proteoglycan in collagen VI deficiency

NG2, the rat homologue of the human melanoma chondroitin sulfate proteoglycan (MCSP), is a ligand for collagen VI (COL6). We have examined skeletal muscles of patients affected by Ullrich scleroatonic muscular dystrophy (UCMD), an inherited syndrome caused by COL6 genes mutations. A significant decrease of NG2 immunolabeling was found in UCMD myofibers, as well as in skeletal muscle and cornea of COL6 null-mice. In UCMD muscles, truncated NG2 core protein isoforms were detected. However, real-time RT-PCR analysis revealed marked increase in NG2 mRNA content in UCMD muscle compared to controls. We hypothesize that NG2 immunohistochemical and biochemical behavior may be compromised owing to the absence of its physiological ligand. MCSP/NG2 proteoglycan may be considered an important receptor mediating COL6-sarcolemma interactions, a relationship that is disrupted by the pathogenesis of UCMD muscle.

Targeted disruption of the gene encoding core 1 beta1-3-galactosyltransferase (T-synthase) causes embryonic lethality and defective angiogenesis in mice

The biosynthesis of the core 1 O-glycan (Galbeta1-3GalNAcalpha1-Ser/Thr, T antigen) is controlled by core 1 beta1-3-galactosyltransferase (T-synthase), which catalyzes the addition of Gal to GalNAcalpha1-Ser/Thr (Tn antigen). The T antigen is a precursor for extended and branched O-glycans of largely unknown function. We found that wild-type mice expressed the sialyl-T antigen (NeuAcalpha2-3Galbeta1-3GalNAcalpha1-Ser/Thr) primarily in endothelial, hematopoietic, and epithelial cells during development. Gene-targeted mice lacking T-synthase instead expressed the nonsialylated Tn antigen in these cells and developed brain hemorrhage that was uniformly fatal by embryonic day 14. T-synthase-deficient brains formed a chaotic microvascular network with distorted capillary lumens and defective association of endothelial cells with pericytes and extracellular matrix. These data reveal an unexpected requirement for core 1-derived O-glycans during angiogenesis.

Perivascular Cells Along Venules Upregulate NG2 Expression During Microvascular Remodeling

OBJECTIVE

Recently the authors have shown that neuron-glial antigen 2 (NG2) is expressed by perivascular cells along arterioles and capillaries, but not along venules in quiescent rat mesenteric microvascular networks. To investigate how the spatial distribution of this proteoglycan changes during microvascular remodeling, the objective of this study was to characterize the expression of NG2 in adult rat mesenteric microvascular networks undergoing active remodeling.

METHODS

The distribution of NG2 expression was evaluated in adult rat mesenteric microvascular networks. Tissues were harvested from 250 g, female, Sprague-Dawley rats at 1, 3, and 5 days poststimulation and double immunolabeled for NG2 and CD31 (endothelial cell marker).

RESULTS

After 1 day, NG2 expression was observed along 27 +/- 11% of network draining venules (14-55 microm) and after 3 days, 59 +/- 10% of draining venules (13-59 microm) stained positive for the proteoglycan. By 5 days poststimulation, the percentage of network draining venules (18-59 microm) staining positive for NG2 returned to 18 +/- 7%, indicating a downregulation of the proteoglycan toward quiescent levels along larger-sized venules.

CONCLUSIONS

The results suggest that NG2 proteoglycan expression is transiently upregulated along venules during microvascular remodeling, implicating NG2 as a marker of activated venules.

Pericytes and endothelial precursor cells: cellular interactions and contributions to malignancy

Tumor vasculature is irregular, abnormal, and essential for tumor growth. Pericytes and endothelial precursor cells (EPC) contribute to the formation of blood vessels under angiogenic conditions. As primary cells in culture, pericytes and EPC share many properties such as tube/network formation and response to kinase inhibitors selective for angiogenic pathways. Expression of cell surface proteins including platelet-derived growth factor receptor, vascular cell adhesion molecule, intercellular adhesion molecule, CD105, desmin, and neural growth proteoglycan 2 was similar between pericytes and EPC, whereas expression of P1H12 and lymphocyte function-associated antigen-1 clearly differentiates the cell types. Further distinction was observed in the molecular profiles for expression of angiogenic genes. Pericytes or EPC enhanced the invasion of MDA-MB-231 breast cancer cells in a coculture assay system. The s.c. coinjection of live pericytes or EPC along with MDA-MB-231 cells resulted in an increased rate of tumor growth compared with coinjection of irradiated pericytes or EPC. Microvessel density analysis indicated there was no difference in MDA-MB-231 tumors with or without EPC or pericytes. However, immunohistochemical staining of vasculature suggested that EPC and pericytes may stabilize or normalize vasculature rather than initiate vasculogenesis. In addition, tumors arising from the coinjection of EPC and cancer cells were more likely to develop lymphatic vessels. These results support the notion that pericytes and EPC contribute to malignancy and that these cell types can be useful as cell-based models for tumor vascular development and selection of agents that may provide therapeutic benefit.

Tumor vascular targeting therapy with viral vectors

Tumor angiogenesis is crucial for the progression and metastasis of cancer. The vasculature of tumor tissue is different from normal vasculature. Therefore, tumor vascular targeting therapy could represent an effective therapeutic strategy with which to suppress both primary tumor growth and tumor metastasis. The use of viral vectors for tumor vascular targeting therapy is a promising strategy based on the unique properties of viral vectors. In order to circumvent the potential problems of antiviral neutralizing antibodies, poor access to extravascular tumor tissue, and toxicities to normal tissue, viral vectors need to be modified to target the tumor endothelial cells. Viral vectors that could be used for tumor vascular targeting therapy include adenoviral vectors, adeno-associated viral vectors, retroviral vectors, lentiviral vectors, measles virus, and herpes simplex viral vectors. In this review, we will summarize the strategies available for targeting viral vectors for tumor vascular targeting therapy.

The role of pericytes in blood-vessel formation and maintenance

Blood vessels are composed of two interacting cell types. Endothelial cells form the inner lining of the vessel wall, and perivascular cells--referred to as pericytes, vascular smooth muscle cells or mural cells--envelop the surface of the vascular tube. Over the last decades, studies of blood vessels have concentrated mainly on the endothelial cell component, especially when the first angiogenic factors were discovered, while the interest in pericytes has lagged behind. Pericytes are, however, functionally significant; when vessels lose pericytes, they become hemorrhagic and hyperdilated, which leads to conditions such as edema, diabetic retinopathy, and even embryonic lethality. Recently, pericytes have gained new attention as functional and critical contributors to tumor angiogenesis and therefore as potential new targets for antiangiogenic therapies. Pericytes are complex. Their ontogeny is not completely understood, and they perform various functions throughout the body. This review article describes the current knowledge about the nature of pericytes and their functions during vessel growth, vessel maintenance, and pathological angiogenesis.

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

PURPOSE

Bone-marrow (BM)-derived hematopoietic precursor cells are thought to participate in the growth of blood vessels during postnatal vasculogenesis. In this investigation, multichannel laser scanning confocal microscopy and quantitative image analysis were used to study the fate of BM-derived hematopoietic precursor cells in corneal neovascularization.

METHODS

A BM-reconstituted mouse model was used in which the BM from enhanced green fluorescent protein (GFP)-positive mice was transplanted into C57BL/6 mice. Basic fibroblast growth factor (bFGF) was used to induce corneal neovascularization in mice. The vasculogenic potential of adult BM-derived cells and their progeny were tested in this in vivo model. Seventy-two histologic sections selected by systematic random sampling from four mice were immunostained and imaged with a confocal microscope and analyzed with image-analysis software.

RESULTS

BM-derived endothelial cells did not contribute to bFGF-induced neovascularization in the cornea. BM-derived periendothelial vascular mural cells (pericytes) were detected at sites of neovascularization, whereas endothelial cells of blood vessels originated from preexisting blood vessels in limbal capillaries. Fifty three percent of all neovascular pericytes originated from BM, and 47% of them originated from preexisting corneoscleral limbus capillaries. Ninety-six percent and 92% of BM-derived pericytes also expressed CD45 and CD11b, respectively, suggesting their hematopoietic origin from the BM.

CONCLUSIONS

Pericytes of new corneal vessels have a dual source: BM and preexisting limbal capillaries. These findings establish BM as a significant effector organ in corneal disorders associated with neovascularization.

Expression of the homeobox gene Pitx2 in neural crest is required for optic stalk and ocular anterior segment development

Heterozygous mutations in the homeobox gene, PITX2, result in ocular anterior segment defects and a high incidence of early-onset glaucoma. Pitx2 is expressed in both the neural crest and the mesoderm-derived precursors of the periocular mesenchyme. Complete loss of function in mice results in agenesis or severe disruption of periocular mesenchyme structures and extrinsic defects in early optic nerve development. However, the specific requirements for Pitx2 in neural crest versus mesoderm could not be determined using these mice, and only roles in the initial stages of eye development could be assessed due to early embryonic lethality. To determine the specific roles of Pitx2 in the neural crest precursor pool, we generated neural crest-specific Pitx2 knockout mice (Pitx2-ncko). Because Pitx2-nkco mice are viable, we also analyzed gene function in later eye development. Pitx2 is intrinsically required in neural crest for specification of corneal endothelium, corneal stroma and the sclera. Pitx2 function in neural crest is also required for normal development of ocular blood vessels. Pitx2-ncko mice exhibit a unique optic nerve phenotype in which the eyes are progressively displaced towards the midline until they are directly attached to the ventral hypothalamus. As Pitx2 is not expressed in the optic stalk, an essential function of PITX2 protein in neural crest is to regulate an extrinsic factor(s) required for development of the optic nerve. We propose a revised model of optic nerve development and new mechanisms that may underlie the etiology of glaucoma in Axenfeld-Rieger patients.

NG2 expression regulates vascular morphology and function in human brain tumours

Tumour angiogenesis is a tightly regulated process involving cross-talk between tumour cells and the host tissue. The underlying mechanisms that regulate such interactions remain largely unknown. NG2 is a transmembrane proteoglycan whose presence on transformed cells has been demonstrated to increase proliferation in vitro and angiogenesis in vivo. To study the effects of NG2 during tumour growth and progression, we engineered an NG2 positive human glioma cell line (U251-NG2) from parental NG2 negative cells (U251-WT) and implanted both cell types stereotactically into immunodeficient nude rat brains. The tumours were longitudinally monitored in vivo using multispectral MRI employing two differently sized contrast agents (Gd-DTPA-BMA and Gadomer) to assess vascular leakiness, vasogenic oedema, tumour volumes and necrosis. Comparisons of Gd-DTPA-BMA and Gadomer revealed differences in their spatial distribution in the U251-NG2 and U251-WT tumours. The U251-NG2 tumours exhibited a higher leakiness of the larger molecular weight Gadomer and displayed a stronger vasogenic oedema (69.9 +/- 15.2, P = 0.018, compared to the controls (10.7 +/- 7.7). Moreover, immunohistochemistry and electron microscopy revealed that the U251-NG2 tumours had a higher microvascular density (11.81 +/- 0.54; P = 0.0010) compared to controls (5.76 +/- 0.87), with vessels that displayed larger gaps between the endothelial cells. Thus, tumour cells can regulate both the function and structure of the host-derived tumour vasculature through NG2 expression, suggesting a role for NG2 in the cross-talk between tumour-host compartments.

Endothelial survival factors and spatial completion, but not pericyte coverage of retinal capillaries determine vessel plasticity

Pericyte loss and capillary regression are characteristic for incipient diabetic retinopathy. Pericyte recruitment is involved in vessel maturation, and ligand-receptor systems contributing to pericyte recruitment are survival factors for endothelial cells in pericyte-free in vitro systems. We studied pericyte recruitment in relation to the susceptibility toward hyperoxia-induced vascular remodeling using the pericyte reporter X-LacZ mouse and the mouse model of retinopathy of prematurity (ROP). Pericytes were found in close proximity to vessels, both during formation of the superficial and the deep capillary layers. When exposure of mice to the ROP was delayed by 24 h, i.e., after the deep retinal layer had formed [at postnatal (p) day 8], preretinal neovascularizations were substantially diminished at p18. Mice with a delayed ROP exposure had 50% reduced avascular zones. Formation of the deep capillary layers at p8 was associated with a combined up-regulation of angiopoietin-1 and PDGF-B, while VEGF was almost unchanged during the transition from a susceptible to a resistant capillary network. Inhibition of Tie-2 function either by soluble Tie-2 or by a sulindac analog, an inhibitor of Tie-2 phosphorylation, resensitized retinal vessels to neovascularizations due to a reduction of the deep capillary network. Inhibition of Tie-2 function had no effect on pericyte recruitment. Our data indicate that the final maturation of the retinal vasculature and its resistance to regressive signals such as hyperoxia depend on the completion of the multilayer structure, in particular the deep capillary layers, and are independent of the coverage by pericytes.

N-cadherin deficiency impairs pericyte recruitment, and not endothelial differentiation or sprouting, in embryonic stem cell-derived angiogenesis

Endothelial cells express two classical cadherins, VE-cadherin and N-cadherin. VE-cadherin is absolutely required for vascular morphogenesis, but N-cadherin is thought to participate in vessel stabilization by interacting with periendothelial cells during vessel formation. However, recent data suggest a more critical role for N-cadherin in endothelium that would regulate angiogenesis, in part by controlling VE-cadherin expression. In this study, we have assessed N-cadherin function in vascular development using an in vitro model derived from embryonic stem (ES) cell differentiation. We show that pluripotent ES cells genetically null for N-cadherin can differentiate normally into endothelial cells. In addition, sprouting angiogenesis was unaltered, suggesting that N-cadherin is not essential for the early events of angiogenesis. However, the lack of N-cadherin led to an impairment in pericyte covering of endothelial outgrowths. We conclude that N-cadherin is necessary neither for vasculogenesis nor proliferation and migration of endothelial cells but is required for the subsequent maturation of endothelial sprouts by interacting with pericytes.

PDGFRbeta+ perivascular progenitor cells in tumours regulate pericyte differentiation and vascular survival

The microvasculature consists of endothelial cells and their surrounding pericytes. Few studies on the regulatory mechanisms of tumour angiogenesis have focused on pericytes. Here we report the identification of tumour-derived PDGFRbeta (+) (platelet-derived growth factor receptor beta) progenitor perivascular cells (PPCs) that have the ability to differentiate into pericytes and regulate vessel stability and vascular survival in tumours. A subset of PDGFRbeta (+) PPCs is recruited from bone marrow to perivascular sites in tumours. Specific inhibition of PDGFRbeta signalling eliminates PDGFRbeta (+) PPCs and mature pericytes around tumour vessels, leading to vascular hyperdilation and endothelial cell apoptosis in pancreatic islet tumours of transgenic Rip1Tag2 mice.

Antitumor activity of rapamycin in a transgenic mouse model of ErbB2-dependent human breast cancer

The ErbB2 (Neu) receptor tyrosine kinase is frequently overexpressed in human breast cancers, and this phenotype correlates with a poor clinical prognosis. We examined the effects of the mammalian target of rapamycin inhibitor, rapamycin, on mammary tumorigenesis in transgenic mice bearing an activated ErbB2 (NeuYD) transgene in the absence or presence of a second transgene encoding vascular endothelial growth factor (VEGF). Treatment of NeuYD or NeuYD x VEGF mice with rapamycin dramatically inhibited tumor growth accompanied by a marked decrease in tumor vascularization. Two key events that may underlie the antitumor activity of rapamycin were decreased expression of ErbB3 and inhibition of hypoxia-inducible factor-1-dependent responses to hypoxic stress. Rapamycin exposure caused only a modest inhibition of the proliferation of tumor-derived cell lines in standard monolayer cultures, but dramatically inhibited the growth of the same cells in three-dimensional cultures, due in part to the induction of apoptotic cell death. These studies underscore the therapeutic potential of mammalian target of rapamycin inhibitors in ErbB2-positive breast cancers and indicate that, relative to monolayer cultures, three-dimensional cell cultures are more predictive in vitro models for studies of the antitumor mechanisms of rapamycin and related compounds.

The postnatal rat aorta contains pericyte progenitor cells that form spheroidal colonies in suspension culture

Pericytes play an important role in modulating angiogenesis, but the origin of these cells is poorly understood. To evaluate whether the mature vessel wall contains pericyte progenitor cells, nonendothelial mesenchymal cells isolated from the rat aorta were cultured in a serum-free medium optimized for stem cells. This method led to the isolation of anchorage-independent cells that proliferated slowly in suspension, forming spheroidal colonies. This process required basic fibroblast growth factor (bFGF) in the culture medium, because bFGF withdrawal caused the cells to attach to the culture dish and irreversibly lose their capacity to grow in suspension. Immunocytochemistry and RT-PCR analysis revealed the expression of the precursor cell markers CD34 and Tie-2 and the absence of endothelial cell markers (CD31 and endothelial nitric oxide synthase, eNOS) and smooth muscle cell markers (alpha-smooth muscle actin, alpha-SMA). In addition, spheroid-forming cells were positive for NG2, nestin, PDGF receptor (PDGFR)-alpha, and PDGFR-beta. Upon exposure to serum, these cells lost CD34 expression, acquired alpha-SMA, and attached to the culture dish. Returning these cells to serum-free medium failed to restore their original spheroid phenotype, suggesting terminal differentiation. When embedded in collagen gels, spheroid-forming cells rapidly migrated in response to PDGF-BB and became dendritic. Spheroid-forming cells cocultured in collagen with angiogenic outgrowths of rat aorta or isolated endothelial cells transformed into pericytes. These results demonstrate that the rat aorta contains primitive mesenchymal cells capable of pericyte differentiation. These immature cells may represent an important source of pericytes during angiogenesis in physiological and pathological processes. They may also provide a convenient supply of mural cells for vascular bioengineering applications.

Differential arterial/venous expression of NG2 proteoglycan in perivascular cells along microvessels: identifying a venule-specific phenotype

OBJECTIVE

Similar to other vascular pericyte markers, including smooth muscle (SM) alpha-actin, desmin, and PDGF-beta-receptor, NG2 proteoglycan is not pericyte specific. Therefore, the use of NG2 as a pericyte marker, especially in cell lineage studies, in comparison to other nonspecific pericyte markers requires an understanding of how its expression varies spatially within a microvascular network. The objective of this study was to characterize NG2 expression along vessels within rat microvascular networks and compare this to SM alpha-actin expression.

METHODS

Mesenteric tissue, subcutaneous tissue, spinotrapezius muscle, and gracilis muscle were harvested from 250-g, female, Sprague-Dawley rats and stained for NG2 and SM alpha-actin. The distribution of NG2 expression was evaluated in mesenteric networks (n = 28) with complementary observations in subcutaneous tissue and skeletal muscle.

RESULTS

Perivascular cells, including mature smooth muscle cells (SMCs), immature SMCs, and pericytes, expressed NG2. Most importantly, NG2 expression was primarily confined to perivascular cells along arterioles and capillaries, and continuous expression was not observed along venules beyond the immediate postcapillary vessels. The differential expression of NG2 along the arteriolar side of microvascular networks was also observed in rat subcutaneous and skeletal muscle.

CONCLUSIONS

The results indicate that NG2 is expressed by all perivascular cells along arterioles, and its absence denotes a venule-specific phenotype. These results identify for the first time a marker that differentiates venous smooth muscle and pericytes from other capillary- and arteriole-associated perivascular cells.

Arf-dependent regulation of Pdgf signaling in perivascular cells in the developing mouse eye

We have established that the Arf tumor suppressor gene regulates mural cell biology in the hyaloid vascular system (HVS) of the developing eye. In the absence of Arf, perivascular cells accumulate within the HVS and prevent its involution. We now demonstrate that mural cell accumulation evident at embryonic day (E) 13.5 in Arf(-/-) mice was driven by excess proliferation at E12.5, when Arf expression was detectable in vitreous pericyte-like cells. Their expression of Arf overlapped with Pdgf receptor beta (Pdgfrbeta), which is essential for pericyte accumulation in the mouse. In cultured cells, p19Arf decreased Pdgfrbeta and blocked Pdgf-B-driven proliferation independently of Mdm2 and p53. The presence of a normal Arf allele correlated with decreased Pdgfrbeta in the embryonic vitreous. Pdgfrbeta was required for vitreous cell accumulation in the absence of Arf. Our findings demonstrate a novel, p53- and Mdm2-independent function for p19Arf. Instead of solely sensing excessive mitogenic stimuli, developmental cues induce Arf to block Pdgfrbeta-dependent signals and prevent the accumulation of perivascular cells selectively in a vascular bed destined to regress.

Multi-germ layer lineage central nervous system repair: nerve and vascular cell generation by embryonic stem cells transplanted in the injured brain

OBJECT

To restore proper function to a damaged central nervous system (CNS) through transplantation, it is necessary to replace both neural and nonneural elements that arise from different germ layers in the embryo. Mounting evidence indicates the importance of signals related to vasculogenesis in governing neural proliferation and differentiation in early CNS development. Here, the authors examined whether embryonic stem cell (ESC)-derived progenitors can selectively generate both neural and endothelial cells after transplantation in the damaged CNS.

METHODS

Injections of 20 nmol N-methyl-D-aspartate created a unilateral striatal injury in 7-day-old rats. One week postinjury, murine ESCs, neural-induced with retinoic acid, were transplanted into the injured striatum. Histological staining, laser confocal microscopy, and transmission electron microscopy of grafted ESCs were performed 1 week posttransplantation.

CONCLUSIONS

Transplanted ESCs differentiated into neural cells, which segregated into multiple pools and formed neurons that conformed to host cytoarchitecture. The ESCs also generated endothelial cells, which integrated with host cells to form chimeric vasculature. The combination of ESC pluripotentiality and multiple germ layer differentiation provides a new conceptual framework for CNS repair.

Differential responses of spinal axons to transection: influence of the NG2 proteoglycan

Spinal cord transections were performed in wild type and NG2 proteoglycan null mice in order to study penetration of regenerating axons into the scar that forms in response to this type of injury. Aside from the presence or absence of NG2, the features of the transection scar did not differ between the two genotypes. In both cases, the rostral and caudal spinal cord stumps were separated by collagenous connective tissue that was continuous with the spinal cord meninges. In wild type mice, oligodendrocyte progenitors, macrophages, and microvascular pericytes contributed to up-regulation of NG2 expression in and around the scar. Substantial amounts of non-cell associated NG2 were also observed in the scar. The abilities of two classes of spinal axons to penetrate the transection scar were examined. Serotonergic efferents and calcitonin gene-related peptide-positive sensory afferents both were observed within the lesion, with calcitonin gene-related peptide-positive axons exhibiting a greater capability to penetrate deeply into the scar tissue. These observations demonstrate inherent differences in the abilities of distinct types of neurons to penetrate the scar. Significantly, growth of serotonergic axons into the transection scar was observed twice as frequently in wild type mice as in NG2 knockout mice, suggesting a stimulatory role for the proteoglycan in regeneration of these fibers. These findings run counter to in vitro evidence implicating NG2 as an inhibitor of nerve regeneration. This work therefore emphasizes the importance of including in vivo models in evaluating the responses of specific types of neurons to spinal cord injury.

Targeting neovascular pericytes in neurofibromatosis type 1

Apart from tumor-driven neovascularization, a less-appreciated consequence of neurofibromatosis type 1 (NF1) is the hyperproliferation of vascular mural cells (pericytes). This study aims at establishing a role for pericytes in NF1, and determining whether interference with the function of a key pericyte component (NG2 proteoglycan) inhibits NF1 tumor neovascularization. Neovascularization in NF1 was studied in Nf+/+(control), Nf1+/-, and Nf1-/-embryos at E-10, ischemia-induced retinal angiogenesis model in 24 eyes of Nf1+/-, Nf1+/+mice, and in malignant peripheral nerve sheath tumors (MPNSTs) derived from NF1 patients (ST88-14, NMS-2PC) orthotopically grown in nude mice (Crl: nu/nu). The anti-angiogenic effect of intracorneal polymer pellets containing anti-NG2 neutralizing antibody was quantified in the nude-mouse corneal angiogenesis model in which angiogenesis was induced by xenografting NMS-2PC tumor into the corneal stroma of 22 eyes. By using confocal microscopy, immunohistochemistry, and BrdU proliferation assay, the pericyte/endothelium ratios and proliferation rates were measured. Activated pericytes were present at the leading tip of the angiogenic sprouts. Pericytes showed continuous investment of endothelium in both NMS-2PC and ST88-14 MPNST tumor xenografts. Mean corneal angiogenesis induced by NMS-2PC tumor grafts in NG2-antibody treated eyes was 1.491 and 3.186 mm2 in isotype-matched non-immunoglobulin treated eyes (control) (P=0.0002). A total of 193.8 vascular nuclei (a measure of ischemia-induced retinal angiogenesis) was present in angiogenic retinal tufts in Nf1+/- mice compared to 89.23 in Nf1+/+ mice (control) (P<0.0001). Mean pericyte/endothelium investment ratios were 1.015, 1.380, and 2.084 in control, Nf1+/-, and Nf1-/-embryos, respectively. Pericytes were 23% (control), 49% (Nf1+/-), and 69% (Nf1-/-) BrdU-positive. Endothelial cells from the same embryos were 29% (control), 47% (Nf1+/-), and 62% (Nf1-/-) BrdU-positive. Angiogenesis is accelerated in NF1 due to hyperproliferation of pericytes and endothelial cells. Mitotically activated NG2-positive pericytes, and endothelial cells may serve as potential therapeutic targets in NF1.

Perivascular cells expressing annexin A5 define a novel mesenchymal stem cell-like population with the capacity to differentiate into multiple mesenchymal lineages

The annexin A5 gene (Anxa5) was recently found to be expressed in the developing and adult vascular system as well as the skeletal system. In this paper, the expression of an Anxa5-lacZ fusion gene was used to define the onset of expression in the vasculature and to characterize these Anxa5-lacZ-expressing vasculature-associated cells. After blastocyst implantation, Anxa5-lacZ-positive cells were first detected in extra-embryonic tissues and in angioblast progenitors forming the primary vascular plexus. Later, expression is highly restricted to perivascular cells in most blood vessels resembling pericytes or vascular smooth muscle cells. Viable Anxa5-lacZ+ perivascular cells were isolated from embryos as well as adult brain meninges by specific staining with fluorescent X-gal substrates and cell-sorting. These purified lacZ+ cells specifically express known markers of pericytes, but also markers characteristic for stem cell populations. In vitro and in vivo differentiation experiments show that this cell pool expresses early markers of chondrogenesis, is capable of forming a calcified matrix and differentiates into adipocytes. Hence, Anxa5 expression in perivascular cells from mouse defines a novel population of cells with a distinct developmental potential.

Pathological angiogenesis is reduced by targeting pericytes via the NG2 proteoglycan

The NG2 proteoglycan is expressed by nascent pericytes during the early stages of angiogenesis. To investigate the functional role of NG2 in neovascularization, we have compared pathological retinal and corneal angiogenesis in wild type and NG2 null mice. During ischemic retinal neovascularization, ectopic vessels protruding into the vitreous occur twice as frequently in wild type retinas as in NG2 null retinas. In the NG2 knock-out retina, proliferation of both pericytes and endothelial cells is significantly reduced, and the pericyte:endothelial cell ratio falls to 0.24 from the wild type value of 0.86. Similarly, bFGF-induced angiogenesis is reduced more than four-fold in the NG2 null cornea compared to that seen in the wild type retina. Significantly, NG2 antibody is effective in reducing angiogenesis in the wild type cornea, suggesting that the proteoglycan can be an effective target for anti-angiogenic therapy. These experiments therefore demonstrate both the functional importance of NG2 in pericyte development and the feasibility of using pericytes as anti-angiogenic targets.

Hyperosmotic stimulus induces reversible angiogenesis within the hypothalamic magnocellular nuclei of the adult rat: a potential role for neuronal vascular endothelial growth factor

Background

In mammals, the CNS vasculature is established during the postnatal period via active angiogenesis, providing different brain regions with capillary networks of various densities that locally supply adapted metabolic support to neurons. Thereafter this vasculature remains essentially quiescent excepted for specific pathologies. In the adult rat hypothalamus, a particularly dense network of capillary vessels is associated with the supraoptic (SON) and paraventricular (PVN) nuclei containing the magnocellular neurons secreting vasopressin and oxytocin, two neurohormones involved in the control of the body fluid homoeostasis. In the seventies, it was reported that proliferation of astrocytes and endothelial cells occurs within these hypothalamic nuclei when strong metabolic activation of the vasopressinergic and oxytocinergic neurons was induced by prolonged hyperosmotic stimulation. The aim of the present study was to determine whether such proliferative response to osmotic stimulus is related to local angiogenesis and to elucidate the cellular and molecular mechanisms involved.

Results

Our results provide evidence that cell proliferation occurring within the SON of osmotically stimulated adult rats corresponds to local angiogenesis. We show that 1) a large majority of the SON proliferative cells is associated with capillary vessels, 2) this proliferative response correlates with a progressive increase in density of the capillary network within the nucleus, and 3) SON capillary vessels exhibit an increased expression of nestin and vimentin, two markers of newly formed vessels. Contrasting with most adult CNS neurons, hypothalamic magnocellular neurons were found to express vascular endothelial growth factor (VEGF), a potent angiogenic factor whose production was increased by osmotic stimulus. When VEGF was inhibited by dexamethasone treatment or by the local application of a blocking antibody, the angiogenic response was strongly inhibited within the hypothalamic magnocellular nuclei of hyperosmotically stimulated rats.

Conclusion

This study shows that the functional stimulation of hypothalamic magnocellular neurons of adult rats induces reversible angiogenesis via the local secretion of neuronal VEGF. Since many diseases are driven by unregulated angiogenesis, the hypothalamic magnocellular nuclei should provide an interesting model to study the cellular and molecular mechanisms involved in the regulation of angiogenesis processes within the adult CNS.

Role of pericytes in vascular morphogenesis

Pericytes are solitary, smooth muscle-like mural cells that invest the wall of microvessels. For a long time, the functional significance of the presence and distribution of pericytes in the microvasculature was unclear. However, in recent years, the application of experimental genetics to the PDGF-B/PDGFRbeta signaling pathway in mice has provided a range of mutants with primary defects in pericytes, allowing for studies of the physiological consequences of pericyte deficiency in developmental angiogenesis and adult physiology. Interestingly, some of the phenotypic consequences of these mutations resemble human diseases, such as diabetic retinopathy. The studies have also led to the discovery of critical mechanisms involved in pericyte recruitment and differentiation. The present review focuses on genetic data suggesting that pericytes take active part in developmental angiogenic processes.

Protocadherin 12 (VE-cadherin 2) is expressed in endothelial, trophoblast, and mesangial cells

Protocadherin 12 protein (PCDH12, VE-cadherin 2) is a cell adhesion molecule that has been isolated from endothelial cells. Here, we have used Northern and Western blots, immunohistology, and flow cytometry to examine the distribution of PCDH12 in mouse tissues. It is an N-glycosylated protein of 150-kDa mass. In the endothelium, PCDH12 immunoreactivity was variable and dependent upon the vascular bed. In both the embryo and embryonic stem cell differentiation system, signals were localized in vasculogenic rather than angiogenic endothelium. In addition, the protein was strongly expressed in a subset of invasive cells of the placenta, which were identified as glycogen-rich trophoblasts. In adult mice, strong PCDH12 signals were observed in mesangial cells of kidney glomeruli whereas expression was not detected in other types of perivascular cells. As opposed to most protocadherins, PCDH12 is not expressed in early embryonic (day 12.5) and adult brains. As a first approach to obtain insight into PCDH12 function, we produced transgenic mice deficient in PCDH12, which were viable and fertile. They did not display any obvious histomorphological defects. We conclude that PCDH12 has a unique expression pattern and that its deficiency does not lead to conspicuous abnormalities. Moreover, PCDH12 is the first specific marker for both glycogen-rich trophoblasts and mesangial cells.

Dysmorphogenesis of kidney cortical peritubular capillaries in angiopoietin-2-deficient mice

Angiopoietin-2 (Ang-2) modulates Tie-2 receptor activation. In mouse kidney maturation, Ang-2 is expressed in arteries, with lower levels in tubules, whereas Tie-2 is expressed by endothelia. We hypothesized that Ang-2 deficiency disrupts kidney vessel patterning. The normal renal cortical peritubular space contains fenestrated capillaries, which have few pericytes; they receive water and solutes which proximal tubules reclaim from the glomerular filtrate. In wild-type neonates, alpha smooth muscle actin (alpha SMA), platelet-derived growth factor receptor beta (PDGFR beta), and desmin-expressing cells were not prominent in this compartment. In Ang-2 null mutants, alpha SMA, desmin, and PDGFR beta prominently immunolocalized in cortical peritubular locations. Some alpha SMA-positive cells were closely associated with CD31- and Tie-2-positive peritubular capillary endothelia, and some of the alpha SMA-positive cells expressed PDGFR beta, desmin, and neural/glial cell 2 (NG2), consistent with a pericyte-like identity. Immunoblotting suggested an increase of total and tyrosine-phosphorylated Tie-2 proteins in null mutant versus wild-type kidneys, and electron microscopy confirmed disorganized capillaries and adjacent cells in cortical peritubular spaces in mutant neonate kidneys. Hence, Ang-2 deficiency causes dysmorphogenesis of cortical peritubular capillaries, with adjacent cells expressing pericyte-like markers; we speculate the latter effect is caused by disturbed paracrine signaling between endothelial and surrounding mesenchymal precursor cells.

Early contribution of pericytes to angiogenic sprouting and tube formation

Immunostaining with endothelial and pericyte markers was used to evaluate the cellular composition of angiogenic sprouts in several types of tumors and in the developing retina. Confocal microscopy revealed that, in addition to conventional endothelial tubes heavily invested by pericytes, all tissues contained small populations of endothelium-free pericyte tubes in which nerve/glial antigen 2 (NG2) positive, platelet-derived growth factor beta (PDGF beta ) receptor-positive perivascular cells formed the lumen of the microvessel. Perfusion of tumor-bearing mice with FITC-dextran, followed by immunohistochemical staining of tumor vasculature, demonstrated direct apposition of pericytes to FITC-dextran in the lumen, confirming functional connection of the pericyte tube to the circulation. Transplantation of prostate and mammary tumor fragments into NG2-null mice led to the formation of tumor microvasculature that was invariably NG2-negative, demonstrating that pericytes associated with tumor microvessels are derived from the host rather than from the conversion of tumor cells to a pericyte phenotype. The existence of pericyte tubes reflects the early participation of pericytes in the process of angiogenic sprouting. The ability to study these precocious contributions of pericytes to neovascularization depends heavily on the use of NG2 and PDGF beta -receptor as reliable early markers for activated pericytes.

Progenitor cells of the testosterone-producing Leydig cells revealed

The cells responsible for production of the male sex hormone testosterone, the Leydig cells of the testis, are post-mitotic cells with neuroendocrine characteristics. Their origin during ontogeny and regeneration processes is still a matter of debate. Here, we show that cells of testicular blood vessels, namely vascular smooth muscle cells and pericytes, are the progenitors of Leydig cells. Resembling stem cells of the nervous system, the Leydig cell progenitors are characterized by the expression of nestin. Using an in vivo model to induce and monitor the synchronized generation of a completely new Leydig cell population in adult rats, we demonstrate specific proliferation of vascular progenitors and their subsequent transdifferentiation into steroidogenic Leydig cells which, in addition, rapidly acquire neuronal and glial properties. These findings, shown to be representative also for ontogenetic Leydig cell formation and for the human testis, provide further evidence that cellular components of blood vessels can act as progenitor cells for organogenesis and repair.

Pathogenesis of persistent hyperplastic primary vitreous in mice lacking the arf tumor suppressor gene

PURPOSE

Persistent hyperplastic primary vitreous (PHPV) is an idiopathic developmental eye disease associated with failed involution of the hyaloid vasculature. The present work addressed the pathogenesis of PHPV in a mouse model that replicates many aspects of the human disease.

METHODS

Ophthalmoscopic and histologic analyses documented pathologic processes in eyes of mice lacking the Arf gene compared with Ink4a-deficient and wild-type control animals. Immunohistochemical staining, in situ hybridization, and RT-PCR demonstrated the expression of relevant gene products. Arf gene expression was determined by in situ hybridization using wholemounts of wild-type mouse eyes and by immunofluorescence staining for green fluorescent protein (GFP) in Arf(+/GFP) heterozygous knock-in mouse eyes.

RESULTS

Abnormalities in Arf(-/-) mice mimicked those found in patients with severe PHPV. The mice had microphthalmia; fibrovascular, retrolental tissue containing retinal pigment epithelial cells and remnants of the hyaloid vascular system; posterior lens capsule destruction with lens degeneration and opacity; and severe retinal dysplasia and detachment. Eyes of mice lacking the overlapping Ink4a gene were normal. Arf was selectively expressed in perivascular cells within the vitreous of the postnatal eye. Cells composing the retrolental mass in Arf(-/-) mice expressed the Arf promoter. The remnant hyaloid vessels expressed Flk-1. Its ligand, vascular endothelial growth factor (Vegf), was expressed in the retrolental tissue and the adjacent dysplastic neuroretina.

CONCLUSIONS

Arf(-/-) mice have features that accurately mimic severe PHPV. In the HVS, Arf expression in perivascular cells may block their accumulation or repress Vegf expression to promote HVS involution and prevent PHPV.

Desmin ensheathment ratio as an indicator of vessel stability: evidence in normal development and in retinopathy of prematurity

We developed a measure of pericyte/endothelial interaction, the desmin ensheathment ratio (DER), using the intermediate filament desmin as an indicator of pericyte ensheathment and have examined the DER in normal retinal vascular development and in the kitten retinopathy of prematurity (ROP) model. We also examined the role of mural cells in the pathogenesis of ROP. Postnatal day 1 to 45 kitten retinae were labeled for desmin, alpha-smooth muscle actin (SMA), and isolectin-B4. Newborn kittens exposed to hyperoxia and then returned to room air for 0 to 40 days (dRA) were similarly labeled. The ratio of desmin to lectin labeling on confocal images yielded the DER. Ultrastructural studies showed that mural cells were present on even the most primitive vessels. During normal development, immature vascular beds had DERs of 0.3 to 0.6 whereas mature beds, which predominated by postnatal day 28, had DERs greater than 0.9. Immature pericytes and smooth muscle cells did not prevent hyperoxia-induced vessel regression. During the vasoproliferative stage of ROP, the DERs of intra- and preretinal vessels ranged between 0.2 and 0.5. In the recovery stage, the DER increased in parallel with regression of pathology, reaching 0.9 at 34 dRA. Stabilization of the DER by the fifth postnatal week was temporally coincident with the development of resistance to hyperoxia-induced vessel regression previously reported in the kitten. These observations lead us to suggest that a DER of 0.9 represents a vascular stability threshold and that a low DER observed during ROP raises the possibility that mural cell abnormalities play a key role in the pathogenesis of ROP.

Regulated angiogenesis and vascular regression in mice overexpressing vascular endothelial growth factor in airways

Angiogenesis and vascular remodeling occurs in many inflammatory diseases, including asthma. In this study, we determined the time course and reversibility of the angiogenesis and vascular remodeling produced by vascular endothelial growth factor (VEGF) in a tet-on inducible transgenic system driven by the CC10 promoter in airway epithelium. One day after switching on VEGF expression, endothelial sprouts arose from venules, grew toward the epithelium, and were abundant by 3 to 5 days. Vessel density reached twice baseline by 7 days. Many new vessels were significantly larger than normal, were fenestrated, and penetrated the epithelium. Despite their mature appearance at 7 days suggested by their pericyte coat and basement membrane, the new vessels started to regress within 3 days when VEGF was switched off, showing stasis and luminal occlusion, influx of inflammatory cells, and retraction and apoptosis of endothelial cells and pericytes. Vessel density returned to normal within 28 days after VEGF withdrawal. Our study showed the dynamic nature of airway angiogenesis and regression. Blood vessels can respond to VEGF by sprouting angiogenesis within a few days, but regress more slowly after VEGF withdrawal, and leave a historical record of their previous extent in the form of empty basement membrane sleeves.

Phosphorylation of NG2 proteoglycan by protein kinase C-alpha regulates polarized membrane distribution and cell motility

Protein kinase C (PKC)-alpha phosphorylation of recombinant NG2 cytoplasmic domain and phorbol ester-induced PKC-dependent phosphorylation of full-length NG2 expressed in U251 cells are both blocked by mutation of Thr(2256), identifying this residue as a primary phosphorylation site. In untreated U251/NG2 cells, NG2 is present along with ezrin and alpha(3)beta(1) integrin in apical cell surface protrusions. Phorbol ester treatment causes redistribution of all three components to lamellipodia, accompanied by increased cell motility. U251 cells expressing NG2 with a valine substitution at position 2256 are resistant to phorbol ester treatment: NG2 remains in membrane protrusions and cell motility is unchanged. In contrast, NG2 with a glutamic acid substitution at position 2256 redistributes to lamellipodia even without phorbol ester treatment, rendering transfected U251 cells spontaneously motile. PKC-alpha-mediated NG2 phosphorylation at Thr(2256) is therefore a key step for initiating cell polarization and motility.

Defective valves and abnormal mural cell recruitment underlie lymphatic vascular failure in lymphedema distichiasis

Lymphatic vessels are essential for the removal of interstitial fluid and prevention of tissue edema. Lymphatic capillaries lack associated mural cells, and collecting lymphatic vessels have valves, which prevent lymph backflow. In lymphedema-distichiasis (LD), lymphatic vessel function fails because of mutations affecting the forkhead transcription factor FOXC2. We report that Foxc2(-/-) mice show abnormal lymphatic vascular patterning, increased pericyte investment of lymphatic vessels, agenesis of valves and lymphatic dysfunction. In addition, an abnormally large proportion of skin lymphatic vessels was covered with smooth muscle cells in individuals with LD and in mice heterozygous for Foxc2 and for the gene encoding lymphatic endothelial receptor, Vegfr3 (also known as Flt4). Our data show that Foxc2 is essential for the morphogenesis of lymphatic valves and the establishment of a pericyte-free lymphatic capillary network and that it cooperates with Vegfr3 in the latter process. Our results indicate that an abnormal interaction between the lymphatic endothelial cells and pericytes, as well as valve defects, underlie the pathogenesis of LD.

NG2 proteoglycan expression in the peripheral nervous system: upregulation following injury and comparison with CNS lesions

The chondroitin sulphate proteoglycan NG2 blocks neurite outgrowth in vitro and thus may be able to inhibit axonal regeneration in the CNS. We have used immunohistochemistry to compare the expression of NG2 in the PNS, where axons regenerate, and the spinal cord, where regeneration fails. NG2 is expressed by satellite cells in dorsal root ganglia (DRG) and in the perineurium and endoneurium of intact sciatic nerves of adult rats. Endoneurial NG2-positive cells were S100-negative. Injury to dorsal roots, ventral rami or sciatic nerves had no effect on NG2 expression in DRG but sciatic nerve section or crush caused an upregulation of NG2 in the damaged nerve. Strongly NG2-positive cells in damaged nerves were S100-negative. The proximal stump of severed nerves was capped by dense NG2, which surrounded bundles of regenerating axons. The distal stump, into which axons regenerated, also contained many NG2-positive/S100-negative cells. Immunoelectron microscopy revealed that most NG2-positive cells in distal stumps had perineurial or fibroblast-like morphologies, with NG2 being concentrated at the poles of the cells in regions exhibiting microvillus-like protrusions or caveolae. Compression and partial transection injuries to the spinal cord also caused an upregulation of NG2, and NG2-positive cells and processes invaded the lesion sites. Transganglionically labelled ascending dorsal column fibres, stimulated to sprout by a conditioning sciatic nerve injury, ended in the borders of lesions among many NG2-positive processes. Thus, NG2 upregulation is a feature of the response to injury in peripheral nerves and in the spinal cord, but it does not appear to limit regeneration in the sciatic nerve.

Adult bone marrow-derived cells recruited during angiogenesis comprise precursors for periendothelial vascular mural cells

Bone marrow (BM)-derived cells are thought to participate in the growth of blood vessels during postnatal vascular regeneration and tumor growth, a process previously attributed to stem and precursor cells differentiating to endothelial cells. We used multichannel laser scanning confocal microscopy of whole-mounted tissues to study angiogenesis in chimeric mice created by reconstituting C57BL mice with genetically marked syngeneic BM. We show that BM-derived endothelial cells do not significantly contribute to tumor- or cytokine-induced neoangiogenesis. Instead, BM-derived periendothelial vascular mural cells were persistently detected at sites of tumor- or vascular endothelial growth factor-induced angiogenesis. Subpopulations of these cells expressed the pericyte-specific NG2 proteoglycan, or the hematopoietic markers CD11b and CD45, but did not detectably express the smooth muscle markers smooth muscle alpha-actin or desmin. Thus, the major contribution of the BM to angiogenic processes is not endothelial, but may come from progenitors for periendothelial vascular mural and hematopoietic effector cells.

NG2 proteoglycan promotes endothelial cell motility and angiogenesis via engagement of galectin-3 and alpha3beta1 integrin

The NG2 proteoglycan is expressed by microvascular pericytes in newly formed blood vessels. We have used in vitro and in vivo models to investigate the role of NG2 in cross-talk between pericytes and endothelial cells (EC). Binding of soluble NG2 to the EC surface induces cell motility and multicellular network formation in vitro and stimulates corneal angiogenesis in vivo. Biochemical data demonstrate the involvement of both galectin-3 and alpha3beta1 integrin in the EC response to NG2 and show that NG2, galectin-3, and alpha3beta1 form a complex on the cell surface. Transmembrane signaling via alpha3beta1 is responsible for EC motility and morphogenesis in this system. Galectin-3-dependent oligomerization may potentiate NG2-mediated activation of alpha3beta1. In conjunction with recent studies demonstrating the early involvement of pericytes in angiogenesis, these data suggest that pericyte-derived NG2 is an important factor in promoting EC migration and morphogenesis during the early stages of neovascularization.

Defective angiogenesis and fatal embryonic hemorrhage in mice lacking core 1-derived O-glycans

The core 1 β1-3-galactosyltransferase (T-synthase) transfers Gal from UDP-Gal to GalNAcα1-Ser/Thr (Tn antigen) to form the core 1 O-glycan Galβ1-3GalNAcα1-Ser/Thr (T antigen). The T antigen is a precursor for extended and branched O-glycans of largely unknown function. We found that wild-type mice expressed the NeuAcα2-3Galβ1-3GalNAcα1-Ser/Thr primarily in endothelial, hematopoietic, and epithelial cells during development. Gene-targeted mice lacking T-synthase instead expressed the nonsialylated Tn antigen in these cells and developed brain hemorrhage that was uniformly fatal by embryonic day 14. T-synthase–deficient brains formed a chaotic microvascular network with distorted capillary lumens and defective association of endothelial cells with pericytes and extracellular matrix. These data reveal an unexpected requirement for core 1–derived O-glycans during angiogenesis.

Synaptic signaling between neurons and glia

Rapid signaling between vertebrate neurons occurs primarily at synapses, intercellular junctions where quantal release of neurotransmitter triggers rapid changes in membrane conductance through activation of ionotropic receptors. Glial cells express many of these same ionotropic receptors, yet little is known about how receptors in glial cells become activated in situ. Because synapses were thought to be the sole provenance of neurons, it has been assumed that these receptors must be activated following diffusion of transmitter out of the synaptic cleft, or through nonsynaptic mechanisms such as transporter reversal. Two recent reports show that a ubiquitous class of progenitors that express the proteoglycan NG2 (NG2 cells) engage in rapid signaling with glutamatergic and gamma-aminobutyric acid (GABA)ergic neurons through direct neuron-glia synapses. Quantal release of transmitter from neurons at these sites triggers rapid activation of aminomethylisoxazole propionic acid (AMPA) or GABA(A) receptors in NG2 cells. These currents exhibit properties consistent with direct rather than spillover-mediated transmission, and electron micrographic analyses indicate that nerve terminals containing clusters of synaptic vesicles form discrete junctions with NG2 cell processes. Although activation of AMPA or GABA(A) receptors depolarize NG2 cells, these receptors are more likely to serve as routes for ion flux rather than as current sources for depolarization, because the amplitudes of the synaptic transients are small and the resting membrane potential of NG2 cells is highly negative. The ability of both glutamate and GABA to influence the morphology, physiology, and development of NG2 cells in vitro suggests that this rapid form of signaling may play important roles in adapting the behavior of these cells to the needs of surrounding neurons in vivo.

Matrix metalloproteinase-9 facilitates remyelination in part by processing the inhibitory NG2 proteoglycan

Remyelination is a critical repair process that is initiated after a demyelinating insult. The failure to remyelinate contributes to neurological diseases such as multiple sclerosis. Here, we test the hypothesis that proteinase activity is required for the extensive remodeling of the extracellular matrix that occurs during remyelination. We show that mice lacking matrix metalloproteinase (MMP)-9 are impaired in myelin reformation after lysolecithin-induced demyelination. This deficiency may be explained at least in part by the failure to clear the accumulation of NG2, an inhibitory proteoglycan that retards the maturation and differentiation of oligodendrocytes that are needed for remyelination. These results emphasize for the first time that upregulation of MMP activity can be important for facilitating regeneration from some types of CNS injury.

Transforming growth factor beta1 induction of vascular endothelial growth factor receptor 1: mechanism of pericyte-induced vascular survival in vivo

Degeneration of vessels precedes and precipitates the devastating ischemia of many diseases, including retinopathy of prematurity and diabetic retinopathy. Ischemia then leads to proliferative retinopathy and blindness. Understanding the mechanisms of blood vessel degeneration is critical to prevention of these diseases. Vessel loss is associated with oxygen-induced suppression of vascular endothelial growth factor (VEGF) and with pericyte (vascular smooth muscle cell) dropout. The molecular mechanism of pericyte protection of the vasculature is unknown. We show that transforming growth factor beta1 (TGF-beta1)-expressing pericytes are specifically found on vessels resistant to oxygen-induced loss. TGF-beta1 potently induces VEGF receptor 1 (VEGFR-1) expression in endothelial cells and thereby prevents oxygen-induced vessel loss in vivo. Vessel survival is further stimulated with a VEGFR-1-specific ligand, placental growth factor 1. TGF-beta1 induction of VEGFR-1 in endothelial cells explains pericyte protection of vessels and the selective vulnerability of neonatal vessels to oxygen. These results implicate induction and activation of VEGFR-1 as critical targets to prevent vessel loss.

Pericyte production of cell-associated VEGF is differentiation-dependent and is associated with endothelial survival

Pericytes have been suggested to play a role in regulation of vessel stability; one mechanism for this stabilization may be via pericyte-derived vascular endothelial growth factor (VEGF). To test the hypothesis that differentiation of mesenchymal cells to pericytes/smooth muscle cells (SMC) is accompanied by VEGF expression, we used endothelial cell (EC) and mesenchymal cell cocultures to model cell-cell interactions that occur during vessel development. Coculture of EC and 10T1/2 cells, multipotent mesenchymal cells, led to induction of VEGF expression by 10T1/2 cells. Increased VEGF expression was dependent on contact between EC-10T1/2 and was mediated by transforming growth factorbeta (TGFbeta). A majority of VEGF produced in coculture was cell- and/or matrix-associated. Treatment of cells with high salt, protamine, heparin, or suramin released significant VEGF, suggesting that heparan sulfate proteoglycan might be sequestering some of the VEGF. Inhibition of VEGF in cocultures led to a 75% increase in EC apoptosis, indicating that EC survival in cocultures is dependent on 10T1/2-derived VEGF. VEGF gene expression in developing retinal vasculature was observed in pericytes contacting newly formed microvessels. Our observations indicate that differentiated pericytes produce VEGF that may act in a juxtacrine/paracrine manner as a survival and/or stabilizing factor for EC in microvessels.

Region-specific and stage-dependent regulation of Olig gene expression and oligodendrogenesis by Nkx6.1 homeodomain transcription factor

During early neural development, the Nkx6.1 homeodomain neural progenitor gene is specifically expressed in the ventral neural tube, and its activity is required for motoneuron generation in the spinal cord. We report that Nkx6.1 also controls oligodendrocyte development in the developing spinal cord, possibly by regulating Olig gene expression in the ventral neuroepithelium. In Nkx6.1 mutant spinal cords, expression of Olig2 in the motoneuron progenitor domain is diminished, and the generation and differentiation of oligodendrocytes are significantly delayed and reduced. The regulation of Olig gene expression by Nkx6.1 is stage dependent, as ectopic expression of Nkx6.1 in embryonic chicken spinal cord results in an induction of Olig2 expression at early stages, but an inhibition at later stages. Moreover, the regulation of Olig gene expression and oligodendrogenesis by Nkx6.1 also appears to be region specific. In the hindbrain, unlike in the spinal cord, Olig1 and Olig2 can be expressed both inside and outside the Nkx6.1-expressing domains and oligodendrogenesis in this region is not dependent on Nkx6.1 activity.

Losses of NG2 and NeuN immunoreactivity but not astrocytic markers during early reperfusion following severe focal cerebral ischemia

The ability of glia to recover essential functions following a period of focal cerebral ischemia is likely to be one important factor influencing the severity of tissue damage that subsequently develops. In this study, we have compared changes in immunoreactivity of markers specific for astrocytes, NG2-positive glia and neurons in tissue subregions during early reperfusion following 3 h of middle cerebral artery occlusion to provide insights into possible differential susceptibility of these cell populations. Under the conditions used, infarction ultimately encompasses most of the perfusion territory of the occluded artery. Nonetheless, alterations in immunoreactivity during the first 3 h of recirculation were restricted to brain regions that had been subjected to severe ischemia. In the striatum, cellular immunoreactivity for NG2 and neuronal markers, NeuN and microtubule-associated protein 2, was greatly reduced by 1 h of reperfusion and declined further at 3 h. NG2 labeling of blood vessels in the striatum appeared post-ischemically, mimicking expression of this protein during development. Less severe changes were seen in the neuronal markers in overlying cerebral cortex. In contrast to the losses of other cellular proteins, immunoreactivity for the astrocytic marker, glial fibrillary acidic protein, was preserved in all tissue that had been subjected to severe ischemia and labeling of another astrocytic protein, glutamine synthetase, was increased by 3 h of reperfusion. These findings provide the first evidence of marked sensitivity of NG2-immunoreactivity to severe ischemia and suggest a greater initial resistance of astrocytes compared with neurons and NG2-positive glia to ischemia-reperfusion damage.

Expression of NG2 proteoglycan during endochondral and intramembranous ossification

We have used immunohistochemistry to study the distribution of the NG2 proteoglycan during bone development in the mouse. At embryonic day 15.5, NG2 was strongly detected in the immature cartilage of developing limbs. After transient down-regulation in mature chondrocytes, NG2 was up-regulated during primary ossification, colocalizing with alkaline phosphatase and tenascin C. In the epiphyseal growth plates of newborn mouse tibia, NG2 and alkaline phosphatase exhibited overlapping patterns of expression by hypertrophic chondrocytes and by osteoblasts surrounding newly formed bone trabeculae. NG2 was down-regulated after puberty, being only faintly detectable in the tibial growth plates of 3-month-old mice. In cranial sutures, NG2 was strongly labeled in osteogenic bone fronts and in the suture matrix. Our results indicate that NG2 expression is up-regulated during both endochondral and intramembranous ossification, but is down-regulated as ossification is completed.

Imaging of angiogenesis: from microscope to clinic

Advances in imaging are transforming our understanding of angiogenesis and the evaluation of drugs that stimulate or inhibit angiogenesis in preclinical models and human disease. Vascular imaging makes it possible to quantify the number and spacing of blood vessels, measure blood flow and vascular permeability, and analyze cellular and molecular abnormalities in blood vessel walls. Microscopic methods ranging from fluorescence, confocal and multiphoton microscopy to electron microscopic imaging are particularly useful for elucidating structural and functional abnormalities of angiogenic blood vessels. Magnetic resonance imaging (MRI), computed tomography (CT), positron emission tomography (PET), ultrasonography and optical imaging provide noninvasive, functionally relevant images of angiogenesis in animals and humans. An ongoing dilemma is, however, that microscopic methods provide their highest resolution on preserved tissue specimens, whereas clinical methods give images of living tissues deep within the body but at much lower resolution and specificity and generally cannot resolve vessels of the microcirculation. Future challenges include developing new imaging methods that can bridge this resolution gap and specifically identify angiogenic vessels. Another goal is to determine which microscopic techniques are the best benchmarks for interpreting clinical images. The importance of angiogenesis in cancer, chronic inflammatory diseases, age-related macular degeneration and reversal of ischemic heart and limb disease provides incentive for meeting these challenges.

Human uveal melanoma expresses NG2 immunoreactivity

BACKGROUND/AIMS

NG2 is the rat homologue of the human melanoma proteoglycan (HMP), also known as the high molecular weight melanoma associated antigen. Most cutaneous melanomas, as well as glioblastomas, chondrosarcomas, and some leukaemias express NG2 immunoreactivity, recognised using monoclonal antibody (mAb) 9.2.27. This antibody has also been used for molecular targeting in targeted alpha therapy for melanoma. The purpose of this study was to evaluate the expression of NG2 immunoreactivity in human uveal melanoma and normal ocular tissue using mAb 9.2.27.

METHODS

Enucleated eyes from 26 patients with choroidal or ciliary body melanoma (n=26) were available as paraffin sections, and stained with haematoxylin and eosin to assess for tumour cell type and histopathology. Additional slides were investigated for NG2 immunoreactivity using mAb 9.2.27 and alkaline phosphatase anti-alkaline phosphatase (APAAP) immunostaining. Two independent observers graded immunostaining using a semiquantitative scale from 0 (negative) to 3 (strong).

RESULTS

Immunostaining for mAb 9.2.27 could not be graded in 7/26 cases with dense pigmentation of the tumour. For the remaining cases, grade 2 (moderate) or more immunostaining was seen in 18/19 tumours (95%). The retina, retinal pigment epithelium (RPE), and choroid displayed weak immunostaining (grade 0.5-1.5) in the majority of melanoma affected eyes. Normal retina and choroid (n=5) appeared negative for mAb 9.2.27. Optic nerve axon bundles in both control and melanoma affected eyes displayed moderate immunostaining.

CONCLUSION

In the present study, the majority of human uveal melanomas expressed NG2 immunoreactivity, as detected using mAb 9.2.27. This antibody may be a suitable candidate for radioimmunotherapy to target ocular melanoma.

Transcription profiling of platelet-derived growth factor-B-deficient mouse embryos identifies RGS5 as a novel marker for pericytes and vascular smooth muscle cells

All blood capillaries consist of endothelial tubes surrounded by mural cells referred to as pericytes. The origin, recruitment, and function of the pericytes is poorly understood, but the importance of these cells is underscored by the severe cardiovascular defects in mice genetically devoid of factors regulating pericyte recruitment to embryonic vessels, and by the association between pericyte loss and microangiopathy in diabetes mellitus. A general problem in the study of pericytes is the shortage of markers for these cells. To identify new markers for pericytes, we have taken advantage of the platelet-derived growth factor (PDGF)-B knockout mouse model, in which developing blood vessels in the central nervous system are almost completely devoid of pericytes. Using cDNA microarrays, we analyzed the gene expression in PDGF-B null embryos in comparison with corresponding wild-type embryos and searched for down-regulated genes. The most down-regulated gene present on our microarray was RGS5, a member of the RGS family of GTPase-activating proteins for G proteins. In situ hybridization identified RGS5 expression in brain pericytes, and in pericytes and vascular smooth muscle cells in certain other, but not all, locations. Absence of RGS5 expression in PDGF-B and PDGFR beta-null embryos correlated with pericyte loss in these mice. Residual RGS5 expression in rare pericytes suggested that RGS5 is a pericyte marker expressed independently of PDGF-B/R beta signaling. With RGS5 as a proof-of-principle, our data demonstrate the usefulness of microarray analysis of mouse models for abnormal pericyte development in the identification of new pericyte-specific markers.