Astrocytes modulate retinal vasculogenesis: effects on fibronectin expression

Functional diversities within neurons and astrocytes in the adult rat auditory cortex revealed by single-nucleus RNA sequencing

The mammalian cerebral cortex is composed of a rich diversity of cell types. Cortical cells are organized into networks that rely on their functional diversity to ultimately carry out a variety of sophisticated cognitive functions. To investigate the breadth of transcriptional diverse cell types in the sensory cortex, we have used single-nucleus RNA sequencing (snRNA-seq) in the auditory cortex of the adult rat. A variety of unique excitatory and inhibitory neuron types were identified. In addition, we report for the first time a diversity of astrocytes in the auditory cortex that may represent functionally unique subtypes. Together, these results pave the way for building models of how neurons in the sensory cortex work in concert with astrocytes at synapses to fulfill high-cognitive functions like learning and memory.

Loss of Tbx3 in Mouse Eye Causes Retinal Angiogenesis Defects Reminiscent of Human Disease

Purpose

Familial exudative vitreoretinopathy (FEVR) and Norrie disease are examples of genetic disorders in which the retinal vasculature fails to fully form (hypovascular), leading to congenital blindness. While studying the role of a factor expressed during retinal development, T-box factor Tbx3, we discovered that optic cup loss of Tbx3 caused the retina to become hypovascular. The purpose of this study was to characterize how loss of Tbx3 affects retinal vasculature formation.

Methods

Conditional removal of Tbx3 from both retinal progenitors and astrocytes was done using the optic cup-Cre recombinase driver BAC-Dkk3-Cre and was analyzed using standard immunohistochemical techniques.

Results

With Tbx3 loss, the retinas were hypovascular, as seen in patients with retinopathy of prematurity (ROP) and FEVR. Retinal vasculature failed to form the stereotypic tri-layered plexus in the dorsal-temporal region. Astrocyte precursors were reduced in number and failed to form a lattice at the dorsal-temporal edge. We next examined retinal ganglion cells, as they have been shown to play a critical role in retinal angiogenesis. We found that melanopsin expression and Islet1/2-positive retinal ganglion cells were reduced in the dorsal half of the retina. In previous studies, the loss of melanopsin has been linked to hyaloid vessel persistence, which we also observed in the Tbx3 conditional knockout (cKO) retinas, as well as in infants with ROP or FEVR.

Conclusions

To the best of our knowledge, these studies are the first demonstration that Tbx3 is required for normal mammalian eye formation. Together, the results provide a potential genetic model for retinal hypovascular diseases.

The heterogeneity of astrocytes in glaucoma

Glaucoma is a leading cause of blindness with progressive degeneration of retinal ganglion cells. Aging and increased intraocular pressure (IOP) are major risk factors. Lowering IOP does not always stop the disease progression. Alternative ways of protecting the optic nerve are intensively studied in glaucoma. Astrocytes are macroglia residing in the retina, optic nerve head (ONH), and visual brain, which keep neuronal homeostasis, regulate neuronal activities and are part of the immune responses to the retina and brain insults. In this brief review, we discuss the activation and heterogeneity of astrocytes in the retina, optic nerve head, and visual brain of glaucoma patients and animal models. We also discuss some recent transgenic and gene knockout studies using glaucoma mouse models to clarify the role of astrocytes in the pathogenesis of glaucoma. Astrocytes are heterogeneous and play crucial roles in the pathogenesis of glaucoma, especially in the process of neuroinflammation and mitochondrial dysfunction. In astrocytes, overexpression of Stat3 or knockdown of IκKβ/p65, caspase-8, and mitochondrial uncoupling proteins (Ucp2) can reduce ganglion cell loss in glaucoma mouse models. Based on these studies, therapeutic strategies targeting the heterogeneity of reactive astrocytes by enhancing their beneficial reactivity or suppressing their detrimental reactivity are alternative options for glaucoma treatment in the future.

Pericyte dynamics in the mouse germinal matrix angiogenesis

Germinal matrix-intraventricular hemorrhage (GM-IVH) is the most devastating neurological complication in premature infants. GM-IVH usually begins in the GM, a highly vascularized region of the developing brain where glial and neuronal precursors reside underneath the lateral ventricular ependyma. Previous studies using human fetal tissue have suggested increased angiogenesis and paucity of pericytes as key factors contributing to GM-IVH pathogenesis. Yet, despite its relevance, the mechanisms underlying the GM vasculature's susceptibility to hemorrhage remain poorly understood. To gain better understanding on the vascular dynamics of the GM, we performed a comprehensive analysis of the mouse GM vascular endothelium and pericytes during development. We hypothesize that vascular development of the mouse GM will provide a good model for studies of human GM vascularization and provide insights into the role of pericytes in GM-IVH pathogenesis. Our findings show that the mouse GM presents significantly greater vascular area and vascular branching compared to the developing cortex (CTX). Analysis of pericyte coverage showed abundance in PDGFRβ-positive and NG2-positive pericyte coverage in the GM similar to the developing CTX. However, we found a paucity in Desmin-positive pericyte coverage of the GM vasculature. Our results underscore the highly angiogenic nature of the GM and reveal that pericytes in the developing mouse GM exhibit distinct phenotypical and likely functional characteristics compared to other brain regions which might contribute to the high susceptibility of the GM vasculature to hemorrhage.

The Amot/integrin protein complex transmits mechanical forces required for vascular expansion

Vascular development is a complex multistep process involving the coordination of cellular functions such as migration, proliferation, and differentiation. How mechanical forces generated by cells and transmission of these physical forces control vascular development is poorly understood. Using an endothelial-specific genetic model in mice, we show that deletion of the scaffold protein Angiomotin (Amot) inhibits migration and expansion of the physiological and pathological vascular network. We further show that Amot is required for tip cell migration and the extension of cellular filopodia. Exploiting in vivo and in vitro molecular approaches, we show that Amot binds Talin and is essential for relaying forces between fibronectin and the cytoskeleton. Finally, we provide evidence that Amot is an important component of the endothelial integrin adhesome and propose that Amot integrates spatial cues from the extracellular matrix to form a functional vascular network.

Pharmacological depletion of retinal neurons prevents vertical angiogenic sprouting without affecting the superficial vascular plexus

BACKGROUND

In mice, a tri-layered (superficial, intermediate, and deep) vascular structure is formed in the retina during the third postnatal week. Short-term treatment of newborn mice with vascular endothelial growth factor (VEGF) receptor inhibitors delays the formation of superficial vascular plexus and this allows us to investigate the developmental process of superficial and deep vascular plexuses at the same time. Using this model, we examined the effect of pharmacological depletion of retinal neurons on the formation of superficial and deep vascular plexuses.

RESULTS

Neuronal cell loss induced by an intravitreal injection of N-methyl-d-aspartic acid on postnatal day (P) 8 delayed vascular development in the deep layer but not in the superficial layer in mice treated with KRN633, a VEGF receptor inhibitor, on P0 and P1. In KRN633-treated mice, neuronal cell loss decreased the number of vertical sprouts originating from the superficial plexus without affecting the number of angiogenic sprouts growing in front. Neuronal cell loss did not impair networks of fibronectin and astrocytes in the superficial layer.

CONCLUSIONS

Our results suggest that inner retinal neurons play a crucial role in forming the deep vascular plexus by directing the sprouts from the superficial blood vessels to the deep layer.

The process of revascularization in the neonatal mouse retina following short-term blockade of vascular endothelial growth factor receptors

Misdirected vascular growth frequently occurs in the neovascular diseases in the retina. However, the mechanisms are still not fully understood. In the present study, we created capillary-free zones in the central and peripheral retinas in neonatal mice by pharmacological blockade of vascular endothelial growth factor (VEGF) signaling. Using this model, we investigated the process and mechanisms of revascularization in the central and peripheral avascular areas. After the completion of a 2-day treatment with the VEGF receptor tyrosine kinase inhibitor KRN633 on postnatal day (P) 4 and P5, revascularization started on P8 in the central avascular area where capillaries had been dropped out. The expression levels of VEGF were higher in the peripheral than in the central avascular area. However, the expansion of the vasculature in the peripheral avascular retina remained suppressed until revascularization had been completed in the central avascular area. Additionally, we found disorganized endothelial cell division, misdirected blood vessels with irregular diameters, and abnormal fibronectin networks at the border of the vascular front and the avascular retina. In the central avascular area, a slight amount of fibronectin as non-vascular component re-formed to provide a scaffold for revascularization. Mechanistic analysis revealed that higher levels of VEGF attenuated the migratory response of endothelial cells without decreasing the proliferative activity. These results suggest that the presence of concentration range of VEGF, which enhances both migration and proliferation of the endothelial cells, and the structurally normal fibronectin network contribute to determine the proper direction of angiogenesis.

Cellular Mechanisms of Angiogenesis in Neonatal Rat Models of Retinal Neurodegeneration

Νeuronal and glial cells play an important role in the development of vasculature in the retina. In this study, we investigated whether re-vascularization occurs in retinal neurodegenerative injury models. To induce retinal injury, N-methyl-D-aspartic acid (NMDA, 200 nmol) or kainic acid (KA, 20 nmol) was injected into the vitreous chamber of the eye on postnatal day (P)7. Morphological changes in retinal neurons and vasculature were assessed on P14, P21, and P35. Prevention of vascular growth and regression of some capillaries were observed on P14 in retinas of NMDA- and KA-treated eyes. However, vascular growth and re-vascularization started on P21, and the retinal vascular network was established by P35 in retinas with neurodegenerative injuries. The re-vascularization was suppressed by a two-day treatment with KRN633, an inhibitor of VEGF receptor tyrosine kinase, on P21 and P22. Astrocytes and Müller cells expressed vascular endothelial growth factor (VEGF), and the distribution pattern of VEGF was almost the same between the control and the NMDA-induced retinal neurodegenerative injury model, except for the difference in the thickness of the inner retinal layer. During re-vascularization, angiogenic sprouts from pre-existing blood vessels were present along the network of fibronectins formed by astrocytes. These results suggest that glial cells contribute to angiogenesis in neonatal rat models of retinal neurodegeneration.

Cyp1b1-deficient retinal astrocytes are more proliferative and migratory and are protected from oxidative stress and inflammation

Astrocytes (ACs) are the most abundant cells in the central nervous system. Retinal ACs play an important role in maintaining the integrity of retinal neurovascular function, and their dysfunction contributes to the pathogenesis of various eye diseases including diabetic retinopathy. Cytochrome P450 1B1 (CYP1B1) expression in the neurovascular structures of the central nervous system including ACs has been reported. We previously showed that CYP1B1 expression is a key regulator of redox homeostasis in retinal vascular cells. Its deficiency in mice resulted in increased oxidative stress and attenuation of angiogenesis in vivo and proangiogenic activity of retinal vascular cells in vitro. Here, using retinal ACs prepared from wild-type (Cyp1b1+/+) and Cyp1b1-deficient (Cyp1b1-/-) mice, we determined the impact of Cyp1b1 expression on retinal AC function. We showed that Cyp1b1-/- retinal ACs were more proliferative and migratory. These cells also produced increased amounts of fibronectin and its receptors, αvβ3- and α5β1-integrin. These results were consistent with the increased adhesive properties of Cyp1b1-/- ACs and their lack of ability to form a network in Matrigel. This was reversed by reexpression of Cyp1b1 in Cyp1b1-/- ACs. Although no significant changes were observed in Akt/SRC/MAPK signaling pathways, production of inflammatory mediators bone morphogenetic protein-7 (BMP-7) and monocyte chemoattractant protein-1 (MCP-1) was decreased in Cyp1b1-/- ACs. Cyp1b1-/- ACs also showed increased levels of connexin 43 phosphorylation and cluster of differentiation 38 expression when challenged with H2O2. These results are consistent with increased proliferation and diminished oxidative stress in Cyp1b1-/- cells. Thus, Cyp1b1 expression in ACs plays an important role in retinal neurovascular homeostasis.

Increased Extracellular Matrix Protein Production in Chronic Diabetic Complications: Implications of Non-Coding RNAs

Management of chronic diabetic complications remains a major medical challenge worldwide. One of the characteristic features of all chronic diabetic complications is augmented production of extracellular matrix (ECM) proteins. Such ECM proteins are deposited in all tissues affected by chronic complications, ultimately causing organ damage and dysfunction. A contributing factor to this pathogenetic process is glucose-induced endothelial damage, which involves phenotypic transformation of endothelial cells (ECs). This phenotypic transition of ECs, from a quiescent state to an activated dysfunctional state, can be mediated through alterations in the synthesis of cellular proteins. In this review, we discussed the roles of non-coding RNAs, specifically microRNAs (miRNAs) and long non-coding RNAs (lncRNAs), in such processes. We further outlined other epigenetic mechanisms regulating the biogenesis and/or function of non-coding RNAs. Overall, we believe that better understanding of such molecular processes may lead to the development of novel biomarkers and therapeutic strategies in the future.

Bioactive lipids and pathological retinal angiogenesis

Angiogenesis, disruption of the retinal barrier, leukocyte-adhesion and oedema are cardinal signs of proliferative retinopathies that are associated with vision loss. Therefore, identifying factors that regulate these vascular dysfunctions is critical to target pathological angiogenesis. Given the conflicting role of bioactive lipids reported in the current literature, the goal of this review is to provide the reader a clear road map of what has been accomplished so far in the field with specific focus on the role of polyunsaturated fatty acids (PUFAs)-derived metabolites in proliferative retinopathies. This necessarily entails a description of the different retina cells, blood retina barriers and the role of (PUFAs)-derived metabolites in diabetic retinopathy, retinopathy of prematurity and age-related macular degeneration as the most common types of proliferative retinopathies.

Detection of an Integrin-Binding Mechanoswitch within Fibronectin during Tissue Formation and Fibrosis

Fibronectin (Fn) is an extracellular matrix protein that orchestrates complex cell adhesion and signaling through cell surface integrin receptors during tissue development, remodeling, and disease, such as fibrosis. Fn is sensitive to mechanical forces in its tandem type III repeats, resulting in extensive molecular enlongation. As such, it has long been hypothesized that cell- and tissue-derived forces may activate an "integrin switch" within the critical integrin-binding ninth and 10th type III repeats-conferring differential integrin-binding specificity, leading to differential cell responses. Yet, no direct evidence exists to prove the hypothesis nor demonstrate the physiological existence of the switch. We report direct experimental evidence for the Fn integrin switch both in vitro and ex vivo using a scFv engineered to detect the transient, force-induced conformational change, representing an opportunity for detection and targeting of early molecular signatures of cell contractile forces in tissue repair and disease.

Glia-neuron interactions in the mammalian retina

The mammalian retina provides an excellent opportunity to study glia-neuron interactions and the interactions of glia with blood vessels. Three main types of glial cells are found in the mammalian retina that serve to maintain retinal homeostasis: astrocytes, Müller cells and resident microglia. Müller cells, astrocytes and microglia not only provide structural support but they are also involved in metabolism, the phagocytosis of neuronal debris, the release of certain transmitters and trophic factors and K(+) uptake. Astrocytes are mostly located in the nerve fibre layer and they accompany the blood vessels in the inner nuclear layer. Indeed, like Müller cells, astrocytic processes cover the blood vessels forming the retinal blood barrier and they fulfil a significant role in ion homeostasis. Among other activities, microglia can be stimulated to fulfil a macrophage function, as well as to interact with other glial cells and neurons by secreting growth factors. This review summarizes the main functional relationships between retinal glial cells and neurons, presenting a general picture of the retina recently modified based on experimental observations. The preferential involvement of the distinct glia cells in terms of the activity in the retina is discussed, for example, while Müller cells may serve as progenitors of retinal neurons, astrocytes and microglia are responsible for synaptic pruning. Since different types of glia participate together in certain activities in the retina, it is imperative to explore the order of redundancy and to explore the heterogeneity among these cells. Recent studies revealed the association of glia cell heterogeneity with specific functions. Finally, the neuroprotective effects of glia on photoreceptors and ganglion cells under normal and adverse conditions will also be explored.

Scaffolding the retina: the interstitial extracellular matrix during rat retinal development

PURPOSE

To examine the expression of interstitial extracellular matrix components and their role during retinal development.

MATERIAL AND METHODS

Fibronectin (FN), collagen IV (Coll IV) and laminin 5 (Lam 5) expression in rat retinas from developmental stages E17 to adult were studied. In addition, PN5 full-thickness retinas were cultured for 7 days with dispase, which selectively cleaves FN and Coll IV, at either 0.5 U/ml or 5.0 U/ml for 3 or 24h. Eyecups and retinal cultures were examined morphologically using hematoxylin and eosin staining and immunohistochemistry.

RESULTS

Coll IV, Lam 5 and FN were all transiently expressed in the interstitial matrix of the retinal layers during development. The retinal layers in dispase treated explants was severely disturbed in a dose and time dependent manner.

CONCLUSIONS

FN, Lam 5 and Coll IV, are present in the interstitial extracellular matrix during rat retinal development. Enzymatic cleavage of FN and Coll IV early in the lamination process disrupts the retinal layers implicating their pivotal role in this process.

Involvement of plasmalogens in post-natal retinal vascular development

Objective

Proper development of retinal blood vessels is essential to ensure sufficient oxygen and nutrient supplies to the retina. It was shown that polyunsaturated fatty acids (PUFAs) could modulate factors involved in tissue vascularization. A congenital deficiency in ether-phospholipids, also termed “plasmalogens”, was shown to lead to abnormal ocular vascularization. Because plasmalogens are considered to be reservoirs of PUFAs, we wished to improve our understanding of the mechanisms by which plasmalogens regulate retinal vascular development and whether the release of PUFAs by calcium-independent phospholipase A2 (iPLA2) could be involved.

Methods and Results

By characterizing the cellular and molecular steps of retinal vascular development in a mouse model of plasmalogen deficiency, we demonstrated that plasmalogens modulate angiogenic processes during the early phases of retinal vascularization. They influence glial activity and primary astrocyte template formation, endothelial cell proliferation and retinal vessel outgrowth, and impact the expression of the genes involved in angiogenesis in the retina. These early defects led to a disorganized and dysfunctional retinal vascular network at adult age. By comparing these data to those obtained on a mouse model of retinal iPLA2 inhibition, we suggest that these processes may be mediated by PUFAs released from plasmalogens and further signalling through the angiopoietin/tie pathways.

Conclusions

These data suggest that plasmalogens play a crucial role in retinal vascularization processes.

Angiopoietin-1 guides directional angiogenesis through integrin αvβ5 signaling for recovery of ischemic retinopathy

Retinopathy of prematurity (ROP) and proliferative diabetic retinopathy (PDR) are ischemic retinal diseases caused by insufficient vascular network formation and vascular regression in addition to aberrant angiogenesis. We examined the role of angiopoietin-1 (Ang1) in retinal vascular network formation during postnatal development using Ang1 gain- and loss-of-function mouse models, and tested the effects of intraocular administration of Ang1 in an oxygen-induced retinopathy (OIR) mouse model that mimics cardinal features of ROP and PDR. We observed that Ang1 plays a substantial role in the formation of the retinal vascular network during postnatal development and that Ang1 supplementation can rescue vascular retinopathies by simultaneously promoting healthy vascular network formation and inhibiting subsequent abnormal angiogenesis, vascular leakage, and neuronal dysfunction in the retinas of the OIR model. We attribute these Ang1-induced effects to a dual signaling pathway-Tie2 signaling in the vascular region and integrin αvβ5 signaling in the astrocytes. The activation of integrin αvβ5 signaling promoted fibronectin accumulation and radial distribution along the sprouting endothelial cells, which consequently stimulated guided angiogenesis in the retina. These findings shed light on the role of Ang1 in the recovery of ischemic retinopathies such as ROP, PDR, and retinal vascular occlusive disease.

Aging is not a disease: distinguishing age-related macular degeneration from aging

Age-related macular degeneration (AMD) is a disease of the outer retina, characterized most significantly by atrophy of photoreceptors and retinal pigment epithelium accompanied with or without choroidal neovascularization. Development of AMD has been recognized as contingent on environmental and genetic risk factors, the strongest being advanced age. In this review, we highlight pathogenic changes that destabilize ocular homeostasis and promote AMD development. With normal aging, photoreceptors are steadily lost, Bruch's membrane thickens, the choroid thins, and hard drusen may form in the periphery. In AMD, many of these changes are exacerbated in addition to the development of disease-specific factors such as soft macular drusen. Para-inflammation, which can be thought of as an intermediate between basal and robust levels of inflammation, develops within the retina in an attempt to maintain ocular homeostasis, reflected by increased expression of the anti-inflammatory cytokine IL-10 coupled with shifts in macrophage plasticity from the pro-inflammatory M1 to the anti-inflammatory M2 polarization. In AMD, imbalances in the M1 and M2 populations together with activation of retinal microglia are observed and potentially contribute to tissue degeneration. Nonetheless, the retina persists in a state of chronic inflammation and increased expression of certain cytokines and inflammasomes is observed. Since not everyone develops AMD, the vital question to ask is how the body establishes a balance between normal age-related changes and the pathological phenotypes in AMD.

Identification of novel drug targets for the treatment of diabetic retinopathy

Vision loss in diabetic retinopathy (DR) is attributable to retinal vascular disorders that result in macular edema and neoangiogenesis. In addition to laser photocoagulation therapy, intraocular injections of antivascular endothelial growth factor drugs have contributed to the treatment of these disease conditions. Nonetheless, the clinical feasibility of intraocular drug administration has raised an increasing demand to develop alternative drugs that can fundamentally ameliorate the retinal vascular dysfunctions in DR. For this purpose, experimental animal models that reproduce human DR would be of clinical benefit. Despite the unavailability of DR models in rats or mice, pharmacological and genetic manipulations without hyperglycemia have successfully recapitulated retinal edema and neoangiogenesis in postnatal mouse retinas, thereby enabling the understanding of the pathophysiology underlying DR. This article highlights the utility of experimental mouse models of retinal vascular abnormalities and discusses cellular and molecular mechanisms responsible for the onset and progression of DR. These approaches will lead to the identification of novel drug targets for the restoration of vascular integrity and regeneration of functional capillaries in DR.

Advanced glycation end-products disrupt the blood-brain barrier by stimulating the release of transforming growth factor-β by pericytes and vascular endothelial growth factor and matrix metalloproteinase-2 by endothelial cells in vitro

Diabetic encephalopathy is now accepted as an important complication of diabetes. The breakdown of the blood-brain barrier (BBB) is associated with dementia in patients with type 2 diabetes mellitus (T2DM). The purpose of this study was to identify the possible mechanisms responsible for the disruption of the BBB after exposure to advanced glycation end-products (AGEs). We investigated the effect of AGEs on the basement membrane and the barrier property of the BBB by Western blot analysis, using our newly established lines of human brain microvascular endothelial cell (BMEC), pericytes, and astrocytes. AGEs reduced the expression of claudin-5 in BMECs by increasing the autocrine signaling through vascular endothelial growth factor (VEGF) and matrix metalloproteinase-2 (MMP-2) secreted by the BMECs themselves. Furthermore, AGEs increased the amount of fibronectin in the pericytes through a similar up-regulation of the autocrine transforming growth factor (TGF)-β released by pericytes. These results indicated that AGEs induce basement membrane hypertrophy of the BBB by increasing the degree of autocrine TGF-β signaling by pericytes, and thereby disrupt the BBB through the up-regulation of VEGF and MMP-2 in BMECs under diabetic conditions.

Integrin-dependent and -independent functions of astrocytic fibronectin in retinal angiogenesis

Fibronectin (FN) is a major component of the extracellular matrix and functions in cell adhesion, cell spreading and cell migration. In the retina, FN is transiently expressed and assembled on astrocytes (ACs), which guide sprouting tip cells and deposit a provisional matrix for sprouting angiogenesis. The precise function of FN in retinal angiogenesis is largely unknown. Using genetic tools, we show that astrocytes are the major source of cellular FN during angiogenesis in the mouse retina. Deletion of astrocytic FN reduces radial endothelial migration during vascular plexus formation in a gene dose-dependent manner. This effect correlates with reduced VEGF receptor 2 and PI3K/AKT signalling, and can be mimicked by selectively inhibiting VEGF-A binding to FN through intraocular injection of blocking peptides. By contrast, AC-specific replacement of the integrin-binding RGD sequence with FN-RGE or endothelial deletion of itga5 shows little effect on migration and PI3K/AKT signalling, but impairs filopodial alignment along AC processes, suggesting that FN-integrin α5β1 interaction is involved in filopodial adhesion to the astrocytic matrix. AC FN shares its VEGF-binding function and cell-surface distribution with heparan-sulfate (HS), and genetic deletion of both FN and HS together greatly enhances the migration defect, indicating a synergistic function of FN and HS in VEGF binding. We propose that in vivo the VEGF-binding properties of FN and HS promote directional tip cell migration, whereas FN integrin-binding functions to support filopodia adhesion to the astrocytic migration template.

Advanced glycation end-products induce basement membrane hypertrophy in endoneurial microvessels and disrupt the blood-nerve barrier by stimulating the release of TGF-β and vascular endothelial growth factor (VEGF) by pericytes

AIMS/HYPOTHESIS

The breakdown of the blood-nerve barrier (BNB) is considered to be a key step in diabetic neuropathy. Although basement membrane hypertrophy and breakdown of the BNB are characteristic features of diabetic neuropathy, the underlying pathogenesis remains unclear. The purpose of the present study was to identify the possible mechanisms responsible for inducing the hypertrophy of basement membrane and the disruption of the BNB after exposure to AGEs.

METHODS

The newly established human peripheral nerve microvascular endothelial cell (PnMEC) and pericyte cell lines were used to elucidate which cell types constituting the BNB regulate the basement membrane and to investigate the effect of AGEs on the basement membrane of the BNB using western blot analysis.

RESULTS

Fibronectin, collagen type IV and tissue inhibitor of metalloproteinase (TIMP-1) protein were produced mainly by peripheral nerve pericytes, indicating that the basement membrane of the BNB is regulated mainly by these cells. AGEs reduced the production of claudin-5 in PnMECs by increasing autocrine signalling through vascular endothelial growth factor (VEGF) secreted by the PnMECs themselves. Furthermore, AGEs increased the amount of fibronectin, collagen type IV and TIMP-1 in pericytes through a similar upregulation of autocrine VEGF and transforming growth factor (TGF)-β released by pericytes.

CONCLUSIONS/INTERPRETATION

These results indicate that pericytes may be the main regulators of the basement membrane at the BNB. AGEs induce basement membrane hypertrophy and disrupt the BNB by increasing autocrine VEGF and TGF-β signalling by pericytes under diabetic conditions.

The CNS microvascular pericyte: pericyte-astrocyte crosstalk in the regulation of tissue survival

The French scientist Charles Benjamin Rouget identified the pericyte nearly 140 years ago. Since that time the role of the pericyte in vascular function has been difficult to elucidate. It was not until the development of techniques to isolate and culture pericytes that scientists have begun to understand the true impact of this unique cell in the maintenance of tissue homeostasis. In the brain the pericyte is an integral cellular component of the blood-brain barrier and, together with other cells of the neurovascular unit (endothelial cells, astrocytes and neurons) the pericyte makes fine-tuned regulatory adjustments and adaptations to promote tissue survival. These regulatory changes involve trans-cellular communication networks between cells. In this review we consider evidence for cell-to-cell crosstalk between pericytes and astrocytes during development and in adult brain.

Conditional knockout of fibronectin abrogates mouse mammary gland lobuloalveolar differentiation

Fibronectin (Fn) plays an important part in the branching morphogenesis of salivary gland, lung, and kidney. Here, we examine the effect of the conditional knockout of Fn in the mammary epithelium [Fn(MEp-/-)] on postnatal mammary gland development, using Cre-loxP-mediated gene knockout technology. Our data show that Fn deletion causes a moderate retardation in outgrowth and branching of the ductal tree in 5-week-old mice. These defects are partially compensated in virgin 16-week-old mice. However, mammary glands consisting of Fn-deficient epithelial cells fail to undergo normal lobuloalveolar differentiation during pregnancy. The severity of lobuloalveolar impairment ranged from lobular hypoplasia to aplasia in some cases and was associated with the amount of Fn protein recovered from these glands. Decreased rates of mammary epithelial cell proliferation accounted for delayed ductal outgrowth in virgin and lack of alveologenesis in pregnant Fn(MEp-/-) mice. Concomitant decreased expression of integrin beta(1) (Itgb1) and lack of autophosphorylation of focal adhesion kinase (Fak) suggest that this pathology might, at least in part, be mediated by disruption of the Fn/Itgb1/Fak signaling pathway.

Vascular endothelial growth factor in eye disease

Collectively, angiogenic ocular conditions represent the leading cause of irreversible vision loss in developed countries. In the US, for example, retinopathy of prematurity, diabetic retinopathy and age-related macular degeneration are the principal causes of blindness in the infant, working age and elderly populations, respectively. Evidence suggests that vascular endothelial growth factor (VEGF), a 40kDa dimeric glycoprotein, promotes angiogenesis in each of these conditions, making it a highly significant therapeutic target. However, VEGF is pleiotropic, affecting a broad spectrum of endothelial, neuronal and glial behaviors, and confounding the validity of anti-VEGF strategies, particularly under chronic disease conditions. In fact, among other functions VEGF can influence cell proliferation, cell migration, proteolysis, cell survival and vessel permeability in a wide variety of biological contexts. This article will describe the roles played by VEGF in the pathogenesis of retinopathy of prematurity, diabetic retinopathy and age-related macular degeneration. The potential disadvantages of inhibiting VEGF will be discussed, as will the rationales for targeting other VEGF-related modulators of angiogenesis.

How to create the vascular tree? (Latest) help from the zebrafish

The cardiovascular system provides oxygen, nutrients and hormones to organs, it directs traffic of metabolites and it maintains tissue homeostasis. It is one of the first organs assembled during vertebrate development and it is essential to life from early stages to adult. For these reasons, the process of vessel formation has being studied for more than a century, but it is only in the late eighties that there has been an explosion of research in the field with the employment of various in vitro and in vivo model systems. The zebrafish (Danio rerio) offers several advantages for in vivo studies; it played a fundamental role in new discoveries and helped to refine our knowledge of the vascular system. This review recapitulates the zebrafish data on vasculogenesis and angiogenesis, including the specification of the haemangioblasts from the mesoderm, their migration to form the vascular cord followed by axial vessels specification, the primary and secondary sprouting of intersomitic vessels, the formation of the lumen, the arterial versus venous specification and patterning. To emphasize the strengths of the zebrafish system in the vascular field, we summarize main tools, such as gene expression and mutagenesis screens, knock down technologies, transgenic lines and imaging, which played a major role in the development of the field and allowed significant discoveries, for instance the recent visualization of the lymphatic system in zebrafish. This information contributes to the prospective of drug discovery to cure human diseases linked to angiogenesis, not last tumours.

Increased expression of fibronectin and the alpha 5 beta 1 integrin in angiogenic cerebral blood vessels of mice subject to hypobaric hypoxia

The extracellular matrix (ECM) is an important regulator of angiogenesis and vascular remodeling. We showed previously that angiogenic capillaries in the developing CNS express high levels of fibronectin and its receptor alpha5beta1 integrin, and that this expression is developmentally downregulated. As cerebral hypoxia leads to an angiogenic response, we sought to determine whether angiogenic vessels in the adult CNS re-express fibronectin and the alpha5beta1 integrin. Ten-week old mice were subject to hypobaric hypoxia for 0, 4, 7 and 14 days, and fibronectin/integrin expression examined. Fibronectin and the alpha5 integrin subunit were strongly upregulated on capillaries in the hypoxic CNS, with the effect maximal at the earliest time point examined (4 days). Immunofluorescent studies demonstrated that the alpha5 integrin was expressed by angiogenic endothelial cells. In light of the defined angiogenic role for fibronectin in other systems, this work suggests that induction of fibronectin-alpha5beta1 integrin expression may be an important molecular switch driving angiogenesis in the hypoxic CNS.

Altered expression of basement membrane-related molecules in rat brain pericyte, endothelial, and astrocyte cell lines after transforming growth factor-beta1 treatment

The basement membrane at the blood-brain barrier (BBB) plays important roles in maintaining the structure and function of capillary vessels. The BBB is constructed from endothelial cells, astrocytes and pericytes, but their interactions in the formation or maintenance of basement membrane have not been established. Transforming growth factor-beta1 (TGF-beta1) is known to increase fibronectin in brain capillary basement membrane with deposition of beta-amyloid. We previously reported that the mRNA level of alpha-smooth muscle actin in a brain capillary pericyte cell line TR-PCT1 was increased by treatment with TGF-beta1. In this study, expression of mRNAs encoding basement membrane-related molecules in TR-PCT1, a rat endothelial cell line TR-BBB13, and a type 2 astrocyte cell line TR-AST4 was evaluated by RT-PCR. The effects of TGF-beta1 on expression of basement membrane-related genes in these cell lines were also examined. Fibronectin, MMP-9, tPA, TIMP-1, and PAI-l in TR-PCT1 were higher than in TR-BBB13 and TR-AST4. In TR-PCT1 treated with TGF-beta1, collagen type IV, PAI-1, and MMP-9 were increased, and TIMP-2 was reduced. The change in PAI-1 mRNA was faster than those in MMP-9, TIMP-2, collagen type IV mRNAs. These results suggest that pericytes may be key cells in the maintenance of the basement membrane at the BBB.

A transgene-assisted genetic screen identifies essential regulators of vascular development in vertebrate embryos

Formation of a functional vasculature during mammalian development is essential for embryonic survival. In addition, imbalance in blood vessel growth contributes to the pathogenesis of numerous disorders. Most of our understanding of vascular development and blood vessel growth comes from investigating the Vegf signaling pathway as well as the recent observation that molecules involved in axon guidance also regulate vascular patterning. In order to take an unbiased, yet focused, approach to identify novel genes regulating vascular development, we performed a three-step ENU mutagenesis screen in zebrafish. We first screened live embryos visually, evaluating blood flow in the main trunk vessels, which form by vasculogenesis, and the intersomitic vessels, which form by angiogenesis. Embryos that displayed reduced or absent circulation were fixed and stained for endogenous alkaline phosphatase activity to reveal blood vessel morphology. All putative mutants were then crossed into the Tg(flk1:EGFP)(s843) transgenic background to facilitate detailed examination of endothelial cells in live and fixed embryos. We screened 4015 genomes and identified 30 mutations affecting various aspects of vascular development. Specifically, we identified 3 genes (or loci) that regulate the specification and/or differentiation of endothelial cells, 8 genes that regulate vascular tube and lumen formation, 8 genes that regulate vascular patterning, and 11 genes that regulate vascular remodeling, integrity and maintenance. Only 4 of these genes had previously been associated with vascular development in zebrafish illustrating the value of this focused screen. The analysis of the newly defined loci should lead to a greater understanding of vascular development and possibly provide new drug targets to treat the numerous pathologies associated with dysregulated blood vessel growth.

Induction of directional sprouting angiogenesis by matrix gradients

The fate of any tissue engineering implant relies upon an adequate oxygen and nutrients supply throughout the cellular construct and, hence, by the ability of the scaffold to induce and guide vascular ingrowth. However, implant vascularization is usually an uncontrolled process that takes several weeks. In this work, we assessed the feasibility of controlling vascular sprout rate and direction within three-dimensional collagen-hyaluronic acid semi-interpenetrated networks by modulating the spatial distribution of the matricellular cues. Results indicated that increasing amount of hyaluronic acid (HA) within the matrix led to a progressive inhibition of sprouting. In HA-rich matrices, the sprout number and the propagation rate showed a 2.7- and 4-fold reduction, respectively, compared to collagen matrices. Furthermore, by creating HA gradients within the collagen network, we were able to direct and enhance the sprouting rate. This study provides an experimental platform for controlling vascularization of engineered tissues.

A novel three-dimensional system to study interactions between endothelial cells and neural cells of the developing central nervous system

Background

During angiogenesis in the developing central nervous system (CNS), endothelial cells (EC) detach from blood vessels growing on the brain surface, and migrate into the expanding brain parenchyma. Brain angiogenesis is regulated by growth factors and extracellular matrix (ECM) proteins secreted by cells of the developing CNS. In addition, recent evidence suggests that EC play an important role in establishing the neural stem cell (NSC) niche. Therefore, two-way communication between EC and neural cells is of fundamental importance in the developing CNS. To study the interactions between brain EC and neural cells of the developing CNS, a novel three-dimensional (3-D) murine co-culture system was developed. Fluorescent-labelled brain EC were seeded onto neurospheres; floating cellular aggregates that contain NSC/neural precursor cells (NPC) and smaller numbers of differentiated cells. Using this system, brain EC attachment, survival and migration into neurospheres was evaluated and the role of integrins in mediating the early adhesive events addressed.

Results

Brain EC attached, survived and migrated deep into neurospheres over a 5-day period. Neurospheres express the ECM proteins fibronectin and laminin, and brain EC adhesion to neurospheres was inhibited by RGD peptides and antibodies specific for the β1, but not the α6 integrin subunit.

Conclusion

A novel 3-D co-culture system for analysing the interactions between EC and neural cells of the developing CNS is presented. This system could be used to investigate the reciprocal influence of EC and NSC/NPC; to examine how NSC/NPC influence cerebral angiogenesis, and conversely, to examine how EC regulate the maintenance and differentiation of NSC/NPC. Using this system it is demonstrated that EC attachment to neurospheres is mediated by the fibronectin receptor, α5β1 integrin.

Possible involvement of increase in spinal fibronectin following peripheral nerve injury in upregulation of microglial P2X4, a key molecule for mechanical allodynia

We have recently demonstrated that the P2X4 receptor, an ATP-gated cation channel, in spinal microglia is a key molecule that mediates the mechanical allodynia induced by peripheral nerve injury. Although microglial P2X4 receptor expression is increased after peripheral nerve injury, the molecular mechanism(s) underlying its upregulation remains largely unknown. Fibronectin is a member of the extracellular matrix molecules and is actively produced in response to injury and diseases in the CNS. Here, we describe the influence of fibronectin on P2X4 receptor expression in microglia and the upregulation of fibronectin after peripheral nerve injury. Microglia that were cultured on fibronectin-coated dishes showed a marked increase in P2X4 receptor expression, both at the mRNA and protein levels, as compared to those cultured on control dishes. Fibronectin also enhanced the microglial Ca2+ responses mediated by P2X4 receptors. Moreover, Western blot examination of the spinal cord from a rat with spinal nerve injury indicated that fibronectin was upregulated on the ipsilateral side. Interestingly, intrathecal injection of ATP-stimulated microglia to the rat lumber spinal cord revealed that microglia cultured on fibronectin-coated dishes was more effective in the induction of allodynia than microglia cultured on control dishes. Taken together, our results suggest that spinal fibronectin is elevated after the peripheral nerve injury and it may be involved in the upregulation of the P2X4 receptor in microglia, which leads to the induction of neuropathic pain.

Mechanisms of endothelial cell guidance and vascular patterning in the developing mouse retina

The appropriate guidance and patterning of vessels during vascular development is critical for proper tissue function. The loss of these guidance mechanisms can lead to abnormal vascularization and a number of pathological conditions. The molecular basis of endothelial cell guidance and subsequent tissue specific vascular patterning remains largely unknown in spite of its clinical relevance and biological importance. In this regard, retinal vascular development offers many advantages for studying endothelial cell guidance and the mechanisms by which characteristic vascular patterns are formed. In this review, we will provide an overview of the known mechanisms that mediate vascular patterning during mouse retinal development, synthesizing these data to formulate a model of how growth factors, cellular adhesion molecules, and vascular-associated cells mediate directed endothelial cell migration and appropriate vascular remodeling. Finally, we will discuss the many aspects of retinal vascular development that remain unknown and cite evidence that many of these gaps may be addressed by further studying the guidance cues shared by vascular and neuronal elements in the retina and other parts of the central nervous system.

The effect of matrix composition of 3D constructs on embryonic stem cell differentiation

The use of embryonic stem (ES) cells as unlimited cell source in tissue engineering has ignited the hope of regenerating any kind of tissue in vitro. However, the role of the material in control and guidance of their development and commitment into complex and viable three-dimensional (3D) tissues is still poorly understood. In this work, we investigate the role of material composition and structure on promoting ES cells growth and differentiation, by culturing mouse ES cell-derived embryoid bodies (EBs) in various semi-interpenetrating polymer networks (SIPNs), made of collagen, fibronectin (FN) and laminin (LM). We show that both composition and strength of the supportive matrix play an important role in EBs development. High collagen concentrations inhibit EBs cavitation and hence the following EBs differentiation, by inhibiting apoptosis. The presence of FN in 3D collagen constructs strongly stimulates endothelial cell differentiation and vascularization. Conversely, LM increases the ability of ES cells to differentiate into beating cardiomyocytes. Our data suggest that matrix composition has an important role in EBs development and that it is possible to influence stem cell differentiation toward preferential pattern, by modulating the physical and biochemical properties of the scaffold.

Developmental abnormalities in the Nuc1 rat retina: a spontaneous mutation that affects neuronal and vascular remodeling and retinal function

The retina serves as an excellent model in which to study vertebrate CNS development. We have discovered a spontaneous mutation in the Sprague-Dawley rat that results in a novel and unusual ocular phenotype, including retinal abnormalities, that we have named Nuc1. We have previously shown that the Nuc1 mutation appears to suppress programmed cell death in the developing retina. Here we report that maturation of both the retinal neurons and the retinal vessels is abnormal in Nuc1 homozygous rats. The developmental changes in the retinal neurons and vasculature are correlated with regard to degree of abnormality. As Nuc1 homozygotes mature, focal retinal detachment begins at approximately 3 months after birth, and near total traction retinal detachment, associated with pre-retinal fibrosis and neovascularization, is evident by 18 months. Electroretinographic studies at 2.5 months of age indicate that functional retinal degeneration precedes retinal detachment. The functional abnormality is most evident in rods and the inner retina, and is present in homozygous but not heterozygous mutants. Immunocytochemical studies of rod and cone photoreceptors indicate abnormalities in rod, but not cone, photoreceptors in Nuc1 homozygotes, consistent with the electroretinographic findings. In Nuc1 animals, the Muller cells are activated. Although such activation may result from inflammation, Muller cells in Nuc1 may be reacting to a neuronal influence. It appears that the Nuc1 mutation plays a regulatory role in both developing and maturing ocular tissues. The Nuc1 mutation may also serve as an important genetic tool to explore the relationships that may exist among gliosis, normal neuronal development, and normal vascular development and how abnormalities in these associations lead to common retinal diseases.

Immunohistochemical study of angiogenesis and proliferative activity in epiretinal membranes

Formation of epiretinal membranes (ERMs) is a serious complication of retinal diseases, the most important being proliferative diabetic retinopathy (PDR) and proliferative vitreoretinopathy (PVR). In this study, our goal was to (i) calculate the microvessel density (MVD), (ii) evaluate vascular endothelial growth factor (VEGF) expression and (iii) correlate angiogenesis with the proliferative activity as expressed by the expression of Ki67 marker, in both membrane types. We performed immunohistochemistry in 14 PVR and eight PDR membranes, using antibodies against CD34, VEGF, Ki67 and glial fibrillary acidic protein. PDR membranes presented higher average count of microvessels compared with PVR membranes (p = 0.0015). No differences were observed concerning VEGF expression (p = 0.1). The expression of Ki67 was not correlated with microvessel number or VEGF expression. Our study confirms the presence of vascularisation in PDR membranes, as well as the presence of VEGF even in avascular PVR membranes, suggesting that immunoreactivity for VEGF may not be accompanied by angiogenesis.

Immunohistochemical study of extracellular matrix components in epiretinal membranes of vitreoproliferative retinopathy and proliferative diabetic retinopathy

PURPOSE

The migration, proliferation, differentiation, and adhesion of cells and other cellular functions are influenced by the surrounding extracellular matrix in normal and wound healing conditions. The formation of epiretinal membranes, a wound healing process, is a serious complication of retinal diseases, the most important being proliferative diabetic retinopathy (PDR) and proliferative vitreoretinopathy (PVR). In the present study, the authors investigated the expression of various extracellular matrix components and in particular tenascin, fibronectin, laminin, collagen IV, and MMP-3 glycoprotein as well as the expression of glial fibrillary acidic protein in each type of epithelial membrane in order to elucidate the role of these molecules in the formation of these two types of membranes.

METHODS

The authors performed immunohistochemistry in 14 PVR and 14 PDR membranes, using antibodies against the above mentioned extracellular matrix components. Tenascin and fibronectin were observed as major components in the extracellular matrix, while laminin and collagen type IV were detected as minor components in both types of membranes. A higher fibronectin expression in PVR compared with PDR membranes was found (p=0.0035). A positive relationship of its expression with the proliferative activity (p=0.15) and collagen type IV expression (p<0.0001) was also observed.

RESULTS

Tenascin expression was positively correlated with glial fibrillary acidic protein positive cells in PDR membranes (p=0.04). Collagen type IV localized around vessels was observed with high levels in PDR membranes (p=0.0031).

CONCLUSIONS

The results indicated that the extracellular matrix components seem to be involved in PVR and PDR, contributing to tissue remodeling and perhaps by different pathogenetic pathways, which could reflect different stages of development in these two types of membranes.

Cell adhesion and angiogenesis

Cell-adhesion mechanisms play a fundamental role during angiogenesis. This article summarizes the role of various cell-adhesive events in blood vessel formation, including general aspects of cell-matrix and cell-cell interactions. In particular, the authors discuss the role of integrin alphavbeta3 in vascular cell survival, proliferation and invasion during the complex process of angiogenesis.

Under stress, the absence of intermediate filaments from Müller cells in the retina has structural and functional consequences

In epithelial and muscle cells, intermediate filaments (IFs) are important for resistance to mechanical stress. The aim of this study was to elucidate whether IFs are also important for providing resistance to mechanical stress in the Müller cells of the retina and whether this has any pathophysiological consequences. We used mice deficient in IF proteins glial fibrillary acidic protein and/or vimentin (GFAP(-/-), Vim(-/-) and GFAP(-/-) Vim(-/-)), and stress on the retina was applied by excision of the eyes immediately post mortem (compared with in situ fixation) or by inducing a neovascular response to oxygen-induced retinopathy (OIR). The structure of unchallenged retinas was normal, but mechanical stress caused local separation of the inner limiting membrane (ILM) and adjacent tissue from the rest of the retina in GFAP(-/-) Vim(-/-) mice and, to a lesser extent, in Vim(-/-) mice. This detachment occurred within the endfeet of Müller cells, structures normally rich in IFs but IF-free in GFAP(-/-) Vim(-/-) mice. Hypoxia-induced neovascularization was comparable in all groups of mice with respect to the retinal surface area occupied by new vessels. However, the vessels traversed the ILM and penetrated the vitreous body less frequently than in wild-type retinas (31-55% in Vim(-/-), 66-79% in GFAP(-/-) Vim(-/-)). We conclude that IFs are important for maintaining the mechanical integrity of Müller-cell endfeet and the inner retinal layers under a mechanical challenge. Furthermore, the absence of IFs in Müller cells leads to an abnormal response of the vascular system to ischemia, specifically decreased ability of newly formed blood vessels to traverse the ILM.

Disappearance of the post-lesional laminin immunopositivity of brain vessels is parallel with the formation of gliovascular junctions and common basal lamina. A double-labelling immunohistochemical study

Previous studies revealed that during development the laminin immunopositivity gradually disappeared from the brain vessels, but temporarily re-appeared in them around lesions. The question of the present study was the correlation between the post-lesional vascular immunopositivity to laminin and the glial reaction. Following stab wounds, double fluorescent immunohistochemical labelling was performed against laminin (using a polyclonal antiserum against laminin 1) and glial fibrillary acidic protein. A number of vessels exhibited intense immunopositivity to laminin within the lesioned tissue. Where these laminin immunopositive vessels entered the perilesional brain substance, the astroglia formed contacts on them, and the separate vascular and glial basal laminae fused. The disappearance of the post-lesional laminin immunopositivity seemed to coincide with these phenomena. When monoclonal antibodies were applied against the beta1 and gamma1 laminin chains, vessels proved to be immunopositive at the lesion, but none in the intact brain tissue. No immunoreactivity was detected in the cases of alpha2 and beta2 chains. The results suggest that the disappearance of laminin immunopositivity may be attributed to that the epitopes become inaccessible for antibodies owing to the formation of gliovascular junctions and common basal lamina between astroglia and vessel. The possible role of an alteration in the laminin composition and the effect of fixation are discussed.

Increased fibronectin expression in sturge-weber syndrome fibroblasts and brain tissue

Sturge-Weber syndrome (SWS) is a neurocutaneous disorder that presents with a facial port-wine stain and a leptomeningeal angioma. Fibronectin expression regulates angiogenesis and vasculogenesis and participates in brain tissue responses to ischemia and seizures. We therefore hypothesized that abnormal gene expression of fibronectin and other extracellular matrix genes would be found in SWS brain tissue and SWS port-wine skin fibroblasts. Fibronectin gene and protein expression from port-wine-derived fibroblasts were compared with that from normal skin-derived fibroblasts of four individuals with SWS using microarrays, reverse transcriptase-PCR, Western analysis, and immunocytochemistry. Fibronectin gene and/or protein expression from eight SWS surgical brain samples was compared with that in two surgical epilepsy brain samples and six postmortem brain samples using microarrays, reverse transcriptase-PCR, and Western analysis. The gene expression of fibronectin was significantly increased (p < 0.05) in the SWS port-wine-derived fibroblasts compared with that of fibroblasts from SWS normal skin. A trend for increased protein levels of fibronectin in port-wine fibroblasts was found by Western analysis. No difference in the pattern of fibronectin staining was detected. The gene expression of fibronectin was significantly increased (p < 0.05), and a trend for increased fibronectin protein expression was found in the SWS surgical brain samples compared with the postmortem controls. These results suggest a potential role for fibronectin in the pathogenesis of SWS and in the brain's response to chronic ischemic injury in SWS. The reproducible differences in fibronectin gene expression between the SWS port-wine-derived fibroblasts and the SWS normal skin-derived fibroblasts are consistent with the presence of a hypothesized somatic mutation underlying SWS.

Photoreceptor-specific overexpression of platelet-derived growth factor induces proliferation of endothelial cells, pericytes, and glial cells and aberrant vascular development: an ultrastructural and immunocytochemical study

Platelet-derived growth factor (PDGF) is necessary for the normal development of the retinal vasculature and its overexpression is likely to contribute to proliferative retinal disorders, such as proliferative vitreoretinopathy. Transgenic mice that overexpress PDGF-B in the photoreceptors (rho/PDGF-B mice) develop traction retinal detachment. In the present study, a detailed histopathological analysis was performed in rho/PDGF-B mice. In these transgenic mice, endothelial cells, pericytes, and glial cells begin to proliferate at postnatal day 7 (P7). All three cell types increase in numbers, forming a highly vascularized cell mass, which reaches a maximum thickness at P14. Cords of endothelial cells and glia invade the retina and exert traction, generating retinal folds; however, the deep capillary bed never forms. Griffonia simplicifolia isolectin B4 (GSA)-positive endothelial cells form tubes and penetrate the retina to the level of the outer plexiform layer, but they never interconnect to form the deep capillary bed. The vessels within the cell mass are patent, but have a very immature morphology. They often are thin-walled with fenestrations. Pericytes and glial cells are usually found in clusters and are not associated with the abnormal vessels. The lack of this association may account for the failure to form a mature vasculature.

Cell-cell interactions in vascular development

Research into areas as divergent as hemangiopoiesis and cardiogenesis as well as investigations of diseases such as cancer and diabetic retinopathy have converged to form the face of research in vascular development today. This convergence of disparate topics has resulted in rapid advances in many areas of vascular research. The focus of this review has been the role of cell-cell interactions in the development of the vascular system, but we have included discussions of pathology where the mechanism of disease progression may have parallels with developmental processes. A number of intriguing questions remain unanswered. For example, what triggers abnormal angiogenesis in the disease state? Are the mechanisms similar to those that control developmental neovascularization? Perhaps the difference in development in angiogenesis versus in disease is context driven, that is, an adult versus an embryonic organism. If this is the case, can the controls that curtail developmental vessel formation be applied in pathologies? Can cell-cell interactions be targeted as a control point for new vessel formation? For instance, can perivascular cells be stimulated or eliminated to result in increased vessel stability or instability, respectively? If the hypothesis that mural cell association is required for vessel stabilization is accurate, are there mechanisms to promote or inhibit mural cell recruitment and differentiation as needed? These and other questions lie in wait for the next generation of approaches to discern the mechanisms and the nature of the cell-cell interactions and the influence of the microenvironment on vascular development.

The 67-kDa laminin-binding protein is involved in shear stress-dependent endothelial nitric-oxide synthase expression

It has been suggested that the mechanical forces acting on endothelial cells may be sensed in part by cell-matrix connections. We therefore studied the role of different matrix proteins, in particular laminin I, on a shear stress-dependent endothelial response, namely nitric-oxide synthase (eNOS) expression. Primary porcine aortic endothelial cells were seeded onto glass plates either noncoated (NC cells) or precoated with fibronectin (FN cells), laminin (LN cells), or collagen I (CN cells). Western blots were used to detect differences in the final matrix composition of these cells. A shear stress of 16 dyn/cm2 was applied for 6 h. Only LN cells showed detectable amounts of laminin I in their underlying matrix when they reached confluence. They reacted with a 2-fold increase of eNOS expression (n = 16, p < 0.001) to the exposure of shear stress, which went along with enhanced eNOS protein and NO release. In contrast, neither FN cells (n = 9) nor NC cells (n = 13) showed a significant increase of eNOS expression under shear stress. The increase in CN cells was borderline (1.4-fold; n = 9, p < 0.05) and was not associated with an increase of eNOS protein. The shear-induced increase in eNOS expression of LN cells was abolished by the peptide YIGSR, which blocks the cellular binding to laminin I via a 67-kDa laminin-binding protein, whereas a control peptide (YIGSK) had no effect. The induction of eNOS expression by shear stress is stimulated by an interaction of endothelial cells with laminin which is, at least in part, mediated by a 67-kDa laminin-binding protein.

The pecten oculi of the chicken: a model system for vascular differentiation and barrier maturation

The pecten oculi is a convolute of blood vessels in the vitreous body of the avian eye. This structure is well known for more than a century, but its functions are still a matter of controversies. One of these functions must be the formation of a blood-retina barrier because there is no diffusion barrier for blood-borne compounds available between the pecten and the retina. Surprisingly, the blood-retina barrier characteristics of this organ have not been studied so far, although the pecten oculi may constitute a fascinating model of vascular differentiation and barrier maturation: Pectinate endothelial cells grow by angiogenesis from the ophthalmotemporal artery into the pecten primordium and consecutively gain barrier properties. The pectinate pigmented cells arise during development from retinal pigment epithelial cells and subsequently lose barrier properties. These inverse transdifferentiation processes may be triggered by the peculiar microenvironment in the vitreous body. In addition, the question is discussed whether the avascularity of the avian retina may be due to the specific metabolic activity of the pecten.

Normal and pathological mechanisms in retinal vascular development

Angiogenesis is a complex biologic process that occurs normally in development and in turnover and remodeling of mature vascular networks. Pathological angiogenesis and neovascularization occur in association with retinal and ocular ischemic diseases, in retinopathy of prematurity and other developmental disorders, and in tumor growth and metastasis. We describe current understanding of cellular and molecular mechanisms of retinal vascular development, highlighting aspects that relate to eye diseases, that provide sites of therapeutic intervention in ophthalmology and that are potential avenues for research.

Angiostatic role of astrocytes: suppression of vascular endothelial cell growth by TGF-beta and other inhibitory factor(s)

Our previous in vivo analyses have suggested that astrocytes play a key role in retinal vascularization by inducing endothelial cell differentiation. Here we demonstrate that medium conditioned by cultured rat brain astrocytes (ACM) contains factors, including transforming growth factor-beta (TGF-beta), that inhibit endothelial cell growth. Serum-free medium conditioned for 1-3 days was tested on exponentially growing bovine retinal microvascular endothelial, aortic endothelial, mink lung epithelial CCL-64, and Swiss mouse 3T3 fibroblast cells. The growth of all four cell types was inhibited in a dose- and time-dependent manner. CCL cells, which are used as a model for assaying TGF-beta activity, were more sensitive than the endothelial cells, suggesting that ACM contains TGF-beta. Moreover, acid treatment significantly increased the inhibitory activity of ACM, indicating that TGF-beta in ACM is predominantly in the latent form. Mouse fibroblasts, which are not affected by TGF-beta treatment under the same conditions, were also inhibited by ACM. This suggests that other inhibitory factors in addition to TGF-beta may be involved. Adsorption by an anti-TGF-beta polyclonal antibody column substantially reduced but did not eliminate the inhibitory activity of ACM for CCL and endothelial cells. Western blot analysis of ACM and proteins eluted from the affinity column revealed a 25 kDa band that co-migrates with TGF-beta. Comparative densitometry of the 25 kDa bands on Western blot indicated that the amount of TGF-beta in ACM is not sufficient to account for the total growth-inhibitory activity. These experiments demonstrate directly that rat brain astrocytes express TGF-beta. They also indicate that astrocytes may produce other growth-inhibitory factor(s) yet to be identified.