Vascular endothelial growth factor mediates reactive angiogenesis in the postnatal developing brain

Endothelial colony forming cell administration promotes neurovascular unit development in growth restricted and appropriately grown fetal lambs

BACKGROUND

Fetal growth restriction (FGR) is associated with deficits in the developing brain, including neurovascular unit (NVU) dysfunction. Endothelial colony forming cells (ECFC) can mediate improved vascular stability, and have demonstrated potential to enhance vascular development and protection. This investigation examined whether ECFCs from human umbilical cord blood (UCB) enhanced NVU development in FGR and appropriate for gestational age (AGA) fetal sheep.

METHODS

Twin-bearing ewes had surgery performed at 88-90 days' gestation, inducing FGR in one fetus. At 113 days, ECFCs (1 × 10⁷ cells) cultured from human UCB were administered intravenously to fetal sheep in utero. At 127 days, ewes and their fetuses were euthanised, fetal brains collected, and NVU components analysed by immunohistochemistry.

RESULTS

Twenty-four fetal lambs, arranged in four groups: AGA (n = 7), FGR (n = 5), AGA + ECFC (n = 6), and FGR + ECFC (n = 6), were included in analyses. FGR resulted in lower body weight than AGA (P = 0.002) with higher brain/body weight ratio (P = 0.003). ECFC treatment was associated with increased vascular density throughout the brain in both AGA + ECFC and FGR + ECFC groups, as well as increased vascular-astrocyte coverage and VEGF expression in the cortex (P = 0.003, P = 0.0006, respectively) and in the subcortical white matter (P = 0.01, P = 0.0002, respectively) when compared with the untreated groups.

CONCLUSIONS

ECFC administration enhanced development of NVU components in both the AGA and FGR fetal brain. Further investigation is required to assess how to optimise the enhanced angiogenic capabilities of ECFCs to provide a therapeutic strategy to protect the developing NVU against vulnerabilities associated with FGR.

Glial cells in intracerebral transplantation for Parkinson's disease

In the last few decades, intracerebral transplantation has grown from a dubious neuroscientific topic to a plausible modality for treatment of neurological disorders. The possibility for cell replacement opens a new field of perspectives in the therapy of neurodegenerative disorders, ischemia, and neurotrauma, with the most lessons learned from intracerebral transplantation in Parkinson's disease. Multiple animal studies and a few small-scale clinical trials have proven the concept of intracerebral grafting, but still have to provide a uniform and highly efficient approach to the procedure, suitable for clinical application. The success of intracerebral transplantation is highly dependent on the integration of the grafted cells with the host brain. In this process, glial cells are clearly more than passive bystanders. They provide transplanted cells with mechanical support, trophics, mediate synapse formation, and participate in graft vascularization. At the same time, glial cells mediate scarring, graft rejection, and neuroinflammation, which can be detrimental. We can use this information to try to understand the mechanisms behind the glial reaction to intracerebral transplantation. Recognizing and utilizing glial reactivity can move translational research forward and provide an insight not only to post-transplantation events but also to mechanisms of neuronal death and degeneration. Knowledge about glial reactivity to transplanted cells could also be a key for optimization of transplantation protocols, which ultimately should contribute to greater patient benefit.

Fetal Cerebrovascular Maturation: Effects of Hypoxia

The human cerebral vasculature originates in the fourth week of gestation and continues to expand and diversify well into the first few years of postnatal life. A key feature of this growth is smooth muscle differentiation, whereby smooth muscle cells within cerebral arteries transform from migratory to proliferative to synthetic and finally to contractile phenotypes. These phenotypic transformations can be reversed by pathophysiological perturbations such as hypoxia, which causes loss of contractile capacity in immature cerebral arteries. In turn, loss of contractility affects all whole-brain cerebrovascular responses, including those involved in flow-metabolism coupling, vasodilatory responses to acute hypoxia and hypercapnia, cerebral autoregulation, and reactivity to activation of perivascular nerves. Future strategies to minimize cerebral injury following hypoxia-ischemic insults in the immature brain might benefit by targeting treatments to preserve and promote contractile differentiation in the fetal cerebrovasculature. This could potentially be achieved through inhibition of receptor tyrosine kinase-mediated growth factors, such as vascular endothelial growth factor and platelet-derived growth factor, which are mobilized by hypoxic and ischemic injury and which facilitate contractile dedifferentiation. Interruption of the effects of other vascular mitogens, such as endothelin and angiotensin-II, and even some miRNA species, also could be beneficial. Future experimental work that addresses these possibilities offers promise to improve current clinical management of neonates who have suffered and survived hypoxic, ischemic, asphyxic, or inflammatory cerebrovascular insults.

Gestational Hypoxia and Developmental Plasticity

Hypoxia is one of the most common and severe challenges to the maintenance of homeostasis. Oxygen sensing is a property of all tissues, and the response to hypoxia is multidimensional involving complicated intracellular networks concerned with the transduction of hypoxia-induced responses. Of all the stresses to which the fetus and newborn infant are subjected, perhaps the most important and clinically relevant is that of hypoxia. Hypoxia during gestation impacts both the mother and fetal development through interactions with an individual's genetic traits acquired over multiple generations by natural selection and changes in gene expression patterns by altering the epigenetic code. Changes in the epigenome determine "genomic plasticity," i.e., the ability of genes to be differentially expressed according to environmental cues. The genomic plasticity defined by epigenomic mechanisms including DNA methylation, histone modifications, and noncoding RNAs during development is the mechanistic substrate for phenotypic programming that determines physiological response and risk for healthy or deleterious outcomes. This review explores the impact of gestational hypoxia on maternal health and fetal development, and epigenetic mechanisms of developmental plasticity with emphasis on the uteroplacental circulation, heart development, cerebral circulation, pulmonary development, and the hypothalamic-pituitary-adrenal axis and adipose tissue. The complex molecular and epigenetic interactions that may impact an individual's physiology and developmental programming of health and disease later in life are discussed.

Astrogliosis has Different Dynamics after Cell Transplantation and Mechanical Impact in the Rodent Model of Parkinson's Disease

Background:

Transplantation of fetal mesencephalic tissue is a well-established concept for functional reinnervation of the dopamine-depleted rat striatum. However, there is no extensive description of the glial response of the host brain following this procedure.

Aims:

The present study aimed to quantitatively and qualitatively analyse astrogliosis surrounding intrastriatal grafts and compare it to the reaction to mechanical injury with the transplantation instrument only.

Study Design:

Animal experimentation.

Methods:

The standard 6-hydroxydopamine-induced unilateral model of Parkinson’s disease was used. The experimental animals received transplantation of a single-cell suspension of E14 ventral mesencephalic tissue. Control animals (sham-transplanted) were subjected to injury by the transplantation cannula, without injection of a cell suspension. Histological analyses were carried out 7 and 28 days following the procedure by immunohistochemistry assays for tyrosine hydroxylase and glial fibrillary acidic protein. To evaluate astrogliosis, the cell density and immunopositive area were measured in distinct zones within and surrounding the grafts or the cannula tract.

Results:

Statistical analysis revealed that astrogliosis in the grafted striatum increased from day 7 to day 28, as shown by a significant change in both cell density and the immunopositive area. The cell density increased from 816.7±370.6 to 1403±272.1 cells/mm² (p<0.0001) аnd from 523±245.9 to 1164±304.8 cells/mm² (p<0.0001) in the two zones in the graft core, and from 1151±218.6 to 1485±210.6 cells/mm² (p<0.05) for the zone in the striatum immediately adjacent to the graft. The glial fibrillary acidic protein-expressing area increased from 0.3109±0.1843 to 0.7949±0.1910 (p<0.0001) and from 0.1449±0.1240 to 0.702±0.2558 (p<0.0001) for the same zones in the graft core, and from 0.5277±0.1502 to 0.6969±0.1223 (p<0.0001) for the same area adjacent to the graft zone. However, astrogliosis caused by mechanical impact only (control) did not display such dynamics. This finding suggests an influence of the grafted cells on the host’s glia, possibly through cross-talk between astrocytes and transplanted neurons.

Conclusion:

This bidirectional relationship is affected by multiple factors beyond the mechanical trauma. Elucidation of these factors might help achieve better functional outcomes after intracerebral transplantation.

The role of endothelial HIF-1 αin the response to sublethal hypoxia in C57BL/6 mouse pups

Chronic sublethal hypoxia, a complication of premature birth, is associated with cognitive and motor handicaps. Responsiveness to and recovery from this hypoxic environment is dependent on induction of HIF-1 α in the cells affected. Microvascular endothelial-glial and microvascular endothelial-neuronal precursor interactions have been found to be dynamic and reciprocal, involving autocrine and paracrine signaling, with response and recovery correlated with baseline levels and levels of induction of HIF-1 α.To ascertain the roles of endothelial HIF-1 α in the responses of brain microvascular endothelial cells (EC) and neuronal precursors to hypoxia, we examined the effects of the presence and absence of endothelial HIF-1 α expression in culture and in cells comprising the subventricular zone (SVZ) and dentate gyrus under normoxic and hypoxic conditions. We used C57BL/6 WT and EC HIF-1 α -deficient mice and brain microvascular ECs isolated from these mice in western blots, immunofluorescence, and behavioral studies to examine the roles of EC HIF-1 α behaviors of endothelial and neuronal precursor cells (NPCs) in SVZ and hippocampal tissues under normoxic and hypoxic conditions and behaviors of these mice in open field activity tests. Analyses of ECs and SVZ and dentate gyrus tissues revealed effects of the absence of endothelial HIF-1 α on proliferation and apoptosis as well as open field activity, with both ECs and neuronal cells exhibiting decreased proliferation, increased apoptosis, and pups exhibiting gender-specific differences in open field activities. Our studies demonstrate the autocrine and paracrine effects of EC HIF-1 α-modulating proliferative and apoptotic behaviors of EC and NPC in neurogenic regions of the brain and gender-specific behaviors in normoxic and hypoxic settings.

Krabbe's leukodystrophy: Approaches and models in vitro

This Review describes some in vitro approaches used to investigate the mechanisms involved in Krabbe's disease, with particular regard to the cellular systems employed to study processes of inflammation, apoptosis, and angiogenesis. The aim was to update the knowledge on the results obtained from in vitro models of this neurodegenerative disorder and provide stimuli for future research. For a long time, the nonavailability of established neural cells has limited the understanding of neuropathogenic mechanisms in Krabbe's leukodystrophy. More recently, the development of new Krabbe's disease cell models has allowed the identification of neurologically relevant pathogenic cascades, including the major role of elevated psychosine levels. Thus, direct and/or indirect roles of psychosine in the release of cytokines, reactive oxygen species, and nitric oxide and in the activation of kinases, caspases, and angiogenic factors results should be clearer. In parallel, it is now understood that the presence of globoid cells precedes oligodendrocyte apoptosis and demyelination. The information described here will help to continue the research on Krabbe's leukodystrophy and on potential new therapeutic approaches for this disease that even today, despite numerous attempts, is without cure. © 2016 Wiley Periodicals, Inc.

Neonatal Platelet Transfusions and Future Areas of Research

Thrombocytopenia affects approximately one fourth of neonates admitted to neonatal intensive care units, and prophylactic platelet transfusions are commonly administered to reduce bleeding risk. However, there are few evidence-based guidelines to inform clinicians' decision-making process. Developmental differences in hemostasis and differences in underlying disease processes make it difficult to apply platelet transfusion practices from other patient populations to neonates. Thrombocytopenia is a risk factor for common preterm complications such as intraventricular hemorrhage; however, a causal link has not been established, and platelet transfusions have not been shown to reduce risk of developing intraventricular hemorrhage. Platelet count frequently drives the decision of whether to transfuse platelets, although there is little evidence to demonstrate what a safe platelet nadir is in preterm neonates. Current clinical assays of platelet function often require large sample volumes and are not valid in the setting of thrombocytopenia; however, evaluation of platelet function and/or global hemostasis may aid in the identification of neonates who are at the highest risk of bleeding. Although platelets' primary role is in establishing hemostasis, platelets also carry pro- and antiangiogenic factors in their granules. Aberrant angiogenesis underpins common complications of prematurity including intraventricular hemorrhage and retinopathy of prematurity. In addition, platelets play an important role in host immune defenses. Infectious and inflammatory conditions such as sepsis and necrotizing enterocolitis are commonly associated with late-onset thrombocytopenia in neonates. Severity of thrombocytopenia is correlated with mortality risk. The nature of this association is unclear, but preclinical data suggest that thrombocytopenia contributes to mortality rather than simply being a proxy for disease severity. Neonates are a distinct patient population in whom thrombocytopenia is common. Their unique physiology and associated complications make the risks and benefits of platelet transfusions difficult to understand. The goal of this review was to highlight research areas that need to be addressed to better understand the risks and benefits of platelet transfusions in neonates. Specifically, it will be important to identify neonates at risk of bleeding who would benefit from a platelet transfusion and to determine whether platelet transfusions either abrogate or exacerbate common neonatal complications such as sepsis, chronic lung disease, necrotizing enterocolitis, and retinopathy of prematurity.

Endothelial cell dysfunction in globoid cell leukodystrophy

Angiogenesis plays a pivotal role in the physiology and pathology of the brain. Microvascular alterations have been observed in various neurodegenerative disorders, including genetic leukodystrophies. Globoid cell leukodystrophy (GLD) is a lysosomal storage disease caused by β-galactosylceramidase (GALC) deficiency and characterized by the accumulation of the neurotoxic metabolite psychosine in the central nervous system and peripheral tissues. Structural and functional alterations occur in the microvascular endothelium of the brain of GLD patients and twitcher mice, a murine model of the disease. In addition, increased vessel permeability and a reduced capacity to respond to proangiogenic stimuli characterize the endothelium of twitcher animals. On the one hand, these alterations may depend, at least in part, on the local and systemic angiostatic activity exerted by psychosine on endothelial cells. On the other hand, studies performed in vivo on zebrafish embryos and in vitro on human endothelial cells suggest that GALC downregulation may also lead to psychosine-independent neuronal and vascular defects. Together, experimental observations indicate that endothelial cell dysfunctions may represent a novel pathogenic mechanism in human leukodystrophies, including GLD. A better understanding of the molecular mechanisms responsible for these microvascular alterations may provide new insights for the therapy of GLD. © 2016 Wiley Periodicals, Inc.

Cerebrovascular adaptations to chronic hypoxia in the growth restricted lamb

Chronic moderate hypoxia induces angiogenic adaptation in the brain, reflecting a modulatory role for oxygen in determining cerebrovascular development. Chronic intrauterine fetal hypoxia, such as occurs in intrauterine growth restriction (IUGR) is likely to lead to a reduction in oxygen delivery to the brain and long-term neurological abnormalities. Thus we investigated whether vascular remodeling and vascular abnormalities were evident in the brain of IUGR newborn lambs that were chronically hypoxic in utero. Single uterine artery ligation (SUAL) surgery was performed in fetuses at ∼ 105 days gestation (term ∼ 145 days) to induce placental insufficiency and IUGR. Ewes delivered naturally at term and lambs were euthanased 24h later. IUGR brains (n = 9) demonstrated a significant reduction in positive staining for the number of blood vessels (laminin immunohistochemistry) compared with control (n = 8): from 1650 ± 284 to 416 ± 47 cells/mm(2) in subcortical white matter (SCWM) 1793 ± 298 to 385 ± 20 cells/mm(2) in periventricular white matter (PVWM), and 1717 ± 161 to 405 ± 84 cells/mm(2) in the subventricular zone (SVZ). The decrease in vascular density was associated with a significant decrease in VEGF immunoreactivity. The percentage of blood vessels exhibiting endothelial cell proliferation (Ki67 positive) varied regionally between 14 to 22% in white matter of control lambs, while only 1-3% of blood vessels in IUGR brains showed proliferation. A 66% reduction in pericyte coverage (α-SMA and desmin) of blood vessels was observed in SCWM, 71% in PVWM, and 73% in SVZ of IUGR lambs, compared to controls. A reduction in peri-vascular astrocytes (GFAP and laminin) was also observed throughout the white matter of IUGR lambs, and extravasation of albumin into the brain parenchyma was present, indicative of increased permeability of the blood brain barrier. Chronic hypoxia associated with IUGR results in a reduction in vascular density in the white matter of IUGR newborn brains. Vascular pericyte coverage and peri-vascular astrocytes, both of which are essential for stabilisation of blood vessels and the maintenance of vascular permeability, were also decreased in the white matter of IUGR lambs. In turn, these vascular changes could lead to inadequate oxygen supply and contribute to under-perfusion and increased vulnerability of white matter in IUGR infants.

VEGF receptors mediate hypoxic remodeling of adult ovine carotid arteries

Recent studies suggest that VEGF contributes to hypoxic remodeling of arterial smooth muscle, although hypoxia produces only transient increases in VEGF that return to normoxic levels despite sustained changes in arterial structure and function. To explore how VEGF might contribute to long-term hypoxic vascular remodeling, this study explores the hypothesis that chronic hypoxia produces sustained increases in smooth muscle VEGF receptor density that mediate long-term vascular effects of hypoxia. Carotid arteries from adult sheep maintained at sea level or altitude (3,820 m) for 110 days were harvested and denuded of endothelium. VEGF levels were similar in chronically hypoxic and normoxic arteries, as determined by immunoblotting. In contrast, VEGF receptor levels were significantly increased by 107% (VEGF-R1) and 156% (VEGF-R2) in hypoxic compared with normoxic arteries. In arteries that were organ cultured 24 h with 3 nM VEGF, VEGF replicated effects of hypoxia on abundances of smooth muscle α actin (SMαA), myosin light chain kinase (MLCK), and MLC20 and the effects of hypoxia on colocalization of MLC20 with SMαA, as measured via confocal microscopy. VEGF did not replicate the effects of chronic hypoxia on colocalization of MLCK with SMαA or MLCK with MLC20, suggesting that VEGF's role in hypoxic remodeling is highly protein specific, particularly for contractile protein organization. VEGF effects in organ culture were inhibited by VEGF receptor blockers vatalinib (240 nM) and dasatinib (6.3 nM). These findings support the hypothesis that long-term upregulation of VEGF receptors help mediate sustained effects of hypoxia on the abundance and colocalization of contractile proteins in arterial smooth muscle.

Vasotrophic regulation of age-dependent hypoxic cerebrovascular remodeling

Hypoxia can induce functional and structural vascular remodeling by changing the expression of trophic factors to promote homeostasis. While most experimental approaches have been focused on functional remodeling, structural remodeling can reflect changes in the abundance and organization of vascular proteins that determine functional remodeling. Better understanding of age-dependent hypoxic macrovascular remodeling processes of the cerebral vasculature and its clinical implications require knowledge of the vasotrophic factors that influence arterial structure and function. Hypoxia can affect the expression of transcription factors, classical receptor tyrosine kinase factors, non-classical G-protein coupled factors, catecholamines, and purines. Hypoxia's remodeling effects can be mediated by Hypoxia Inducible Factor (HIF) upregulation in most vascular beds, but alterations in the expression of growth factors can also be independent of HIF. PPARγ is another transcription factor involved in hypoxic remodeling. Expression of classical receptor tyrosine kinase ligands, including vascular endothelial growth factor, platelet derived growth factor, fibroblast growth factor and angiopoietins, can be altered by hypoxia which can act simultaneously to affect remodeling. Tyrosine kinase-independent factors, such as transforming growth factor, nitric oxide, endothelin, angiotensin II, catecholamines, and purines also participate in the remodeling process. This adaptation to hypoxic stress can fundamentally change with age, resulting in different responses between fetuses and adults. Overall, these mechanisms integrate to assure that blood flow and metabolic demand are closely matched in all vascular beds and emphasize the view that the vascular wall is a highly dynamic and heterogeneous tissue with multiple cell types undergoing regular phenotypic transformation.

Inhibition of angiogenesis by β-galactosylceramidase deficiency in globoid cell leukodystrophy

Globoid cell leukodystrophy (Krabbe disease) is a neurological disorder of infants caused by genetic deficiency of the lysosomal enzyme β-galactosylceramidase leading to accumulation of the neurotoxic metabolite 1-β-d-galactosylsphingosine (psychosine) in the central nervous system. Angiogenesis plays a pivotal role in the physiology and pathology of the brain. Here, we demonstrate that psychosine has anti-angiogenic properties by causing the disassembling of endothelial cell actin structures at micromolar concentrations as found in the brain of patients with globoid cell leukodystrophy. Accordingly, significant alterations of microvascular endothelium were observed in the post-natal brain of twitcher mice, an authentic model of globoid cell leukodystrophy. Also, twitcher endothelium showed a progressively reduced capacity to respond to pro-angiogenic factors, defect that was corrected after transduction with a lentiviral vector harbouring the murine β-galactosylceramidase complementary DNA. Finally, RNA interference-mediated β-galactosylceramidase gene silencing causes psychosine accumulation in human endothelial cells and hampers their mitogenic and motogenic response to vascular endothelial growth factor. Accordingly, significant alterations were observed in human microvasculature from brain biopsy of a globoid cell leukodystrophy case. Together these data demonstrate that β-galactosylceramidase deficiency induces significant alterations in endothelial neovascular responses that may contribute to central nervous system and systemic damages that occur in globoid cell leukodystrophy.

Multifocal haemorrhagic brain damage following hypoxia and blood pressure lability: case report and rat model

AIMS

Haemorrhagic brain damage is frequently encountered as a complication of premature birth. Much less frequently, multifocal petechial haemorrhage is identified in asphyxiated term newborns. Our goal was to develop an experimental rat model to reproduce this pattern of brain damage.

METHODS

Neonatal rat pups were exposed to a 24-h period of 10% or 8% hypoxia followed by a single dose of phenylephrine. Acute and subacute changes, as well as long-term outcomes, were investigated by histology, brain magnetic resonance imaging and behavioural assessment. Immunostaining for vascular endothelial growth factor and caveolin-1 was performed in the rat brains as well as in a 17-day human case.

RESULTS

Small foci of haemorrhage were identified in almost all regions of the rat brain subjected to hypoxia plus phenylephrine, but not hypoxia alone. Exposure to 8% hypoxia was associated with more haemorrhagic foci than 10% hypoxia. With rare exceptions, the blood deposits were too small to be detected by magnetic resonance imaging. Altered immunohistochemical detection of vascular endothelial growth factor and caveolin-1 in the child and the rat model suggests a role for blood-brain barrier compromise. There were no clear behavioural changes and no residual morphological abnormalities in the 78-day follow-up of the rats.

CONCLUSIONS

We conclude that transient hypoxia, in a dose-dependent manner, can weaken the vasculature and predispose to brain haemorrhage in the situation of labile blood pressure. Persistent hypoxia is likely to be important in the genesis of permanent severe brain damage.

VEGF expression and microvascular responses to severe transient hypoxia in the fetal sheep brain

BACKGROUND

Fetal hypoxia contributes significantly to the pathogenesis of permanent perinatal brain injury. We hypothesized that hypoxia-induced cerebral angiogenesis and microvascular changes would occur in fetal sheep subjected to a severe hypoxic insult produced by umbilical cord occlusion (UCO) for 10 min.

METHODS

At 124-126 d of gestation, singleton fetal sheep underwent surgery for implantation of catheters and placement of an inflatable cuff around the umbilical cord. A 10-min UCO or sham UCO (n = 5) was induced at 130 d gestation. The fetal brain was collected at 24 h (n = 5) or 48 h (n = 4) after UCO for immunohistochemical analysis of vascular endothelial growth factor (VEGF), Ki67, and serum albumin.

RESULTS

By 48 h after UCO, the percentage of blood vessels expressing VEGF had increased in the subventricular zone, periventricular and subcortical white matter, corpus callosum, and cortex. Alterations in vascular permeability (albumin extravasation) were observed only in the periventricular and subcortical white matter and the subventricular zone following UCO.

CONCLUSION

The upregulation of VEGF expression and increased leakage of plasma protein in the fetal sheep brain show that the microvasculature in white matter is sensitive to hypoxia in the near-term brain.

Erythropoietin as a neuroprotectant for neonatal brain injury: animal models

Prematurity and perinatal hypoxia-ischemia are common problems that result in significant neurodevelopmental morbidity and high mortality worldwide. The Vannucci model of unilateral brain injury was developed to model perinatal brain injury due to hypoxia-ischemia. Because the rodent brain is altricial, i.e., it develops postnatally, investigators can model either preterm or term brain injury by varying the age at which injury is induced. This model has allowed investigators to better understand developmental changes that occur in susceptibility of the brain to injury, evolution of brain injury over time, and response to potential neuroprotective treatments. The Vannucci model combines unilateral common carotid artery ligation with a hypoxic insult. This produces injury of the cerebral cortex, basal ganglia, hippocampus, and periventricular white matter ipsilateral to the ligated artery. Varying degrees of injury can be obtained by varying the depth and duration of the hypoxic insult. This chapter details one approach to the Vannucci model and also reviews the neuroprotective effects of erythropoietin (Epo), a neuroprotective treatment that has been extensively investigated using this model and others.

Vulnerability of the developing brain to hypoxic-ischemic damage: contribution of the cerebral vasculature to injury and repair?

As clinicians attempt to understand the underlying reasons for the vulnerability of different regions of the developing brain to injury, it is apparent that little is known as to how hypoxia-ischemia may affect the cerebrovasculature in the developing infant. Most of the research investigating the pathogenesis of perinatal brain injury following hypoxia-ischemia has focused on excitotoxicity, oxidative stress and an inflammatory response, with the response of the developing cerebrovasculature receiving less attention. This is surprising as the presentation of devastating and permanent injury such as germinal matrix-intraventricular haemorrhage (GM-IVH) and perinatal stroke are of vascular origin, and the origin of periventricular leukomalacia (PVL) may also arise from poor perfusion of the white matter. This highlights that cerebrovasculature injury following hypoxia could primarily be responsible for the injury seen in the brain of many infants diagnosed with hypoxic-ischemic encephalopathy (HIE). Interestingly the highly dynamic nature of the cerebral blood vessels in the fetus, and the fluctuations of cerebral blood flow and metabolic demand that occur following hypoxia suggest that the response of blood vessels could explain both regional protection and vulnerability in the developing brain. However, research into how blood vessels respond following hypoxia-ischemia have mostly been conducted in adult models of ischemia or stroke, further highlighting the need to investigate how the developing cerebrovasculature responds and the possible contribution to perinatal brain injury following hypoxia. This review discusses the current concepts on the pathogenesis of perinatal brain injury, the development of the fetal cerebrovasculature and the blood brain barrier (BBB), and key mediators involved with the response of cerebral blood vessels to hypoxia.

Angiogenic signalling pathways altered in gliomas: selection mechanisms for more aggressive neoplastic subpopulations with invasive phenotype

The angiogenesis process is a key event for glioma survival, malignancy and growth. The start of angiogenesis is mediated by a cascade of intratumoural events: alteration of the microvasculature network; a hypoxic microenvironment; adaptation of neoplastic cells and synthesis of pro-angiogenic factors. Due to a chaotic blood flow, a consequence of an aberrant microvasculature, tissue hypoxia phenomena are induced. Hypoxia inducible factor 1 is a major regulator in glioma invasiveness and angiogenesis. Clones of neoplastic cells with stem cell characteristics are selected by HIF-1. These cells, called "glioma stem cells" induce the synthesis of vascular endothelial growth factor. This factor is a pivotal mediator of angiogenesis. To elucidate the role of these angiogenic mediators during glioma growth, we have used a rat endogenous glioma model. Gliomas induced by prenatal ENU administration allowed us to study angiogenic events from early to advanced tumour stages. Events such as microvascular aberrations, hypoxia, GSC selection and VEGF synthesis may be studied in depth. Our data showed that for the treatment of gliomas, developing anti-angiogenic therapies could be aimed at GSCs, HIF-1 or VEGF. The ENU-glioma model can be considered to be a useful option to check novel designs of these treatment strategies.

Serum levels of platelet-derived growth factor BB homodimers are increased in male children with autism

BACKGROUND

The neurobiological basis of autism remains poorly understood. To examine the role played by serum cytokines in brain development, we hypothesized that Platelet-Derived Growth Factor (PDGF) and Vascular Endothelial Growth Factor (VEGF) may be associated with pathophysiology of autism. In this study, we screened serum levels of these growth factors in young male subjects with autism.

METHODS

We measured serum levels of PDGF subtypes and VEGF in the 31 male children with autism (6-19 years old) and 31 healthy age- and gender-matched subjects.

RESULTS

The serum levels of PDGF-BB in male children with autism (N=31, 5624.5+/-1651.8 pg/mL [mean+/-SD]) were significantly higher (two-tailed Student's t-test: p=0.0188) than those of normal control subjects (N=31, 4758.2+/-1521.5 pg/mL [mean+/-SD]). There was a significant and positive correlation (Pearson's r=0.5320, p=0.0010) between the serum levels of PDGF-BB and the Autism Diagnostic Interview-Revised (ADI-R) domain C scores, which represent stereotyped patterns of behavior in the children with autism. However, there were no marked or significant correlations between serum PDGF-BB levels and clinical variables, including the other ADI-R scores and Intellectual Quotient (IQ) scores by WAIS-R. There were no significant change and correlations with clinical variables in serum PDGF-AA, PDGF-AB, and VEGF levels in the children with autism.

CONCLUSIONS

Increased levels of serum PDGF-BB homodimers might be implicated in the pathophysiology of autism.

Vascular endothelial growth factor increases human cardiac microvascular endothelial cell permeability to low-density lipoproteins

BACKGROUND

Endothelial cell hyperpermeability is a proposed mechanism of increased lipid insudation into the vessel walls of allografts. Vascular endothelial growth factor (VEGF) is a potent inducer of vascular permeability and its expression is upregulated in human heart allografts. The goal of these experiments was to investigate the effects of VEGF on low-density lipoprotein (LDL) permeability through confluent monolayers of human cardiac microvascular endothelial cells (HCMEC) in vitro.

METHODS

VEGF mRNA and protein expression was characterized in coronary arteries from cardiac allograft vasculopathy patients as compared with healthy controls using in situ hybridization and immunohistochemical staining of sub-adjacent sections. HCMEC were grown to confluence and treated with VEGF-A(121) or VEGF-A(165). Permeability of LDL in confluent endothelial monolayers was measured using fluorometry. Transendothelial electrical resistance (TER) measurements were used to indirectly measure the tight junctional status. Immunocytochemical staining was performed to visualize changes in CD31 and zonula occludens-1.

RESULTS

We observed significant increases in VEGF expression within the superficial and deep intima and media of coronaries from allografts, as compared with controls. In vitro treatment with VEGF-A(121) and VEGF-A(165) significantly increased LDL passage through endothelial monolayers. We further showed that VEGF-A(121) and VEGF-A(165) caused significant decreases in TER at 2 to 4 hours post-treatment. Also, VEGF induced disruption of tight junctions, resulting in an increase in the intercellular gap formation.

CONCLUSIONS

These results demonstrate that VEGF increases low-density lipoprotein permeability through endothelial monolayers, and this effect is correlated with VEGF-induced disruption of endothelial tight junctions resulting in the formation of intercellular gaps.

Stroke in the immature brain: review of pathophysiology and animal models of pediatric stroke

Pediatric stroke research presents many challenges. Relatively low incidence, need for age stratification, diverse etiologies, delays in diagnosis, lack of an established age-based stroke severity scale and outcome measures are only some of the issues that have prevented the implementation of clinical trials in infants and children with stroke. Experimental animal models of pediatric stroke, therefore, are critical to understanding the pathophysiology and management of ischemic brain damage in the immature brain, and provide the necessary platform for future clinical trials. In this review we discuss the pertinent clinical aspects of pediatric stroke, the pathophysiology of stroke in the developing brain and the animal models established to study basic mechanisms as well as translational issues in pediatric stroke.

Chronic hydrocephalus-induced hypoxia: increased expression of VEGFR-2+ and blood vessel density in hippocampus

Chronic hydrocephalus (CH) is a neurological disease characterized by increased cerebrospinal fluid volume and pressure that is often associated with impaired cognitive function. By and large, CH is a complex and heterogeneous cerebrospinal fluid (CSF) disorder where the exact site of brain insult is uncertain. Several mechanisms including neural compression, fiber stretch, and local or global hypoxia have been implicated in the underlying pathophysiology of CH. Specifically, the hippocampus, which plays a significant role in memory processing and is in direct contact with expanding CSF ventricles, may be involved. Using our model of chronic hydrocephalus, we quantified the density of vascular endothelial growth factor receptor 2 (VEGFR-2(+)) neurons, glial, endothelial cells, and blood vessels in hippocampal regions CA1, CA2-3, dentate gyrus and hilus using immunohistochemical and stereological methods. Density and %VEGFR-2(+) cell populations were estimated for CH animals (2-3 weeks vs. 12-16 weeks) and surgical controls (SC). Overall, we found approximately six- to eightfold increase in the cellular density of VEGFR-2(+) and more than double blood vessel density (BVd) in the hippocampus of CH compared with SC. There were no significant regional differences in VEGFR-2(+) cellular and BVd expression in the CH group. VEGFR-2(+) and BVds were significantly related to changes in CSF volume (P<or=0.05), and not intracranial pressure (ICP). The %VEGFR-2(+) was significantly greater in CH than SC (P<or=0.05), and was significantly correlated with BVd (P<or=0.05). These results showed that CH elicited a profound increase in VEGFR-2(+) in hippocampus that corresponded to increased BVd. It was unclear whether increased VEGFR-2(+) and blood vessel expression was related to focal compression alone or in combination with global ischemia/hypoxia conditions as previously described. These findings suggest that VEGFR-2 may play an adaptive role in angiogenesis after CH-induced hypoxia. Modulation of vascular endothelial growth factor/VEGFR-2(+) may be important in developing treatments for hypoxic conditions including hydrocephalus and other forms of cerebral ischemia.

Novel three-dimensional analysis tool for vascular trees indicates complete micro-networks, not single capillaries, as the angiogenic endpoint in mice overexpressing human VEGF(165) in the brain

To adequately supply tissues with oxygen and nutrients, the formation of functional vascular networks requires generation of normal, healthy vessels and their arrangement into an effective network architecture. While our knowledge about the development of single vessels significantly increased during the last years, mechanisms responsible for network formation are still poorly understood. This is probably due to the lack of suitable methods for quantification of structural properties of microvascular networks. Previously we showed that cerebral blood flow is not increased in mice exhibiting a 2- to 3-fold higher density of normal and perfused capillaries as a result of transgenic overexpression of the human vascular endothelial growth factor (VEGF(165)). Here we used vascular corrosion casting and hierarchical micro-computed tomography combined with a new network analysis tool to characterize the vascular architecture in gray and white matter of these mice. Our results indicate that VEGF overexpression leads to formation of additional micro-networks connected to higher order vessels rather than insertion of individual capillaries into the existing vessel structure. This implies that the smallest "angiogenic quantum", i.e. the final, stable result of angiogenesis and subsequent remodeling, is not a single microvessel, but a complete micro-network. In conclusion, high-resolution 3D imaging combined with network analysis can substantially improve our understanding of vascular architecture, beneficial for the development of therapeutic angiogenesis as a clinical tool for applications such as wound healing or treatment of ischemic diseases.

Prenatal Cord Clamping in Newborn Macaca nemestrina: A Model of Perinatal Asphyxia

Our objective was to establish a nonhuman primate model of perinatal asphyxia appropriate for preclinical evaluation of neuroprotective treatment strategies under conditions that closely resemble human neonatal emergencies, and to begin testing the safety and efficacy of erythropoietin neuroprotective treatment. Prior to delivery by hysterotomy, the umbilical cords of near term Macaca nemestrina (n = 8) were clamped for times ranging between 12 and 15 min. Animals received erythropoietin (5,000 U/kg/dose x 2 i.v., n = 3), or vehicle (n = 5) after resuscitation. We assessed physiologic parameters, continuous electroencephalogram, magnetic resonance imaging/spectroscopy, safety parameters and behavior. Animals were euthanized at 4 months of age. Mean birth weight was 507 +/- 62 g. Initial arterial pH ranged from 6.75 to 7.12, with base deficits of 17-25 mEq. Animals were flaccid at birth, with attenuated electroencephalograms, and seizures occurred in 3 of 8 animals. We demonstrated magnetic resonance imaging/spectroscopy changes consistent with hypoxia (elevated lactate levels were present in some animals), significant motor and behavioral abnormalities (particularly with 15 min of cord clamping), and evidence of gliosis at the time of death. We have established a reproducible model of moderate to severe perinatal hypoxic-ischemic injury in M. nemestrina newborns. This model, which combines structural, biochemical, and behavioral assessments over time can be used to assess the safety and efficacy of neuroprotective strategies.

From angiogenesis to neuropathology

Angiogenesis--the growth of new blood vessels--is a crucial force for shaping the nervous system and protecting it from disease. Recent advances have improved our understanding of how the brain and other tissues grow new blood vessels under normal and pathological conditions. Angiogenesis factors, especially vascular endothelial growth factor, are now known to have roles in the birth of new neurons (neurogenesis), the prevention or mitigation of neuronal injury (neuroprotection), and the pathogenesis of stroke, Alzheimer's disease and motor neuron disease. As our understanding of pathophysiology grows, these developments may point the way towards new molecular and cell-based therapies.

Reactive astrogliosis induces astrocytic differentiation of adult neural stem/progenitor cells in vitro

Neural stem cells reside in defined areas of the adult mammalian brain, including the dentate gyrus of the hippocampus. Rat neural stem/progenitor cells (NSPCs) isolated from this region retain their multipotency in vitro and in vivo after grafting into the adult brain. Recent studies have shown that endogenous or grafted NSPCs are activated after an injury and migrate toward lesioned areas. In these areas, reactive astrocytes are present and secrete numerous molecules and growth factors; however, it is not currently known whether reactive astrocytes can influence the lineage selection of NSPCs. We investigated whether reactive astrocytes could affect the differentiation, proliferation, and survival of adult NSPCs by modelling astrogliosis in vitro, using mechanical lesion of primary astrocytes. Initially, it was found that conditioned medium from lesioned astrocytes induced astrocytic differentiation of NSPCs without affecting neuronal or oligodendrocytic differentiation. In addition, NSPCs in coculture with lesioned astrocytes also displayed increased astrocytic differentiation and some of these NSPC-derived astrocytes participated in glial scar formation in vitro. When proliferation and survival of NSPCs were analyzed, no differential effects were observed between lesioned and nonlesioned astrocytes. To investigate the molecular mechanisms of the astrocyte-inducing activity, the expression of two potent inducers of astroglial differentiation, ciliary neurotrophic factor and leukemia inhibitory factor, was analyzed by Western blot and shown to be up-regulated in conditioned medium from lesioned astrocytes. These results demonstrate that lesioned astrocytes can induce astroglial differentiation of NSPCs and provide a mechanism for astroglial differentiation of these cells following brain injury.

Effect of chronic continuous or intermittent hypoxia and reoxygenation on cerebral capillary density and myelination

Chronic hypoxia, whether continuous (CCH) or intermittent (CIH), occurs in many neonatal pathological conditions, such as bronchopulmonary dysplasia and obstructive sleep apnea. In this study, we explored the effect of CCH and CIH on cerebral capillary density and myelination. We subjected CD-1 mice starting at postnatal day 2 to either CCH 11% oxygen (O(2)), or CIH 11% O(2) (4-min cycles), for periods of 2 and 4 wk followed by reoxygenation for 4 wk. Mice were deeply anesthetized and perfused. Brains were removed to fixative for 24 h, then paraffin-embedded. Coronal brain sections were taken for analysis. Immunocytochemistry for glucose transporter 1 was used to assess angiogenesis, and Luxol fast blue and fluoromyelin stains were used to assess myelination. Capillary density increased after 2-wk exposure to CIH and CCH. By 4 wk, capillary density increased in both CIH and CCH by 25% and 47%, respectively, in cortex and by 29% and 44%, respectively, in hippocampus (P < 0.05). There was a decrease in myelination in the corpus callosum of mice exposed to CIH (75% of control) and CCH (50% of control) (P < 0.05). Reoxygenation reversed the increased capillary density seen in CCH to normoxic values. However, dysmyelination that occurred in CCH-exposed mice did not show any improvement upon reoxygenation. We conclude that neonatal chronic hypoxia 1) induces brain angiogenesis, which is reversible with reoxygenation, and 2) irreversibly reduces the extent of myelination in the corpus callosum. This potential irreversible effect on myelination in early life can, therefore, have long-term and devastating effects.

Hypoxia inducible factor-1α and expression of vascular endothelial growth factor and its receptors in cerebral arteriovenous malformations

BACKGROUND

Vascular endothelial growth factor (VEGF) and its tyrosine kinase family of receptors (VEGFR) (Flt-1, Flk-1, Flt-4) have been implicated in vascular angiogenesis and remodelling in cerebral arteriovenous malformations (CAVM). In this study, we investigate the role of hypoxia inducible factor-1 (HIF-1alpha) in CAVM and its relationship to VEGF and VEGFR.

METHODS

Surgical specimens from 26 patients undergoing CAVM resection were studied for HIF-1alpha , VEGF, Flt-1, Flk-1 and Flt-4. The mean age was 34.08 +/- 14.18 years. Twenty-one patients presented with intracerebral haemorrhage.

RESULTS

VEGF, Flt-1 and Flt-4 were expressed in all specimens. Flk-1 was expressed in 15 of 26 patients. HIF-1alpha was expressed in 15 of 26 patients. HIF-1alpha expression was significantly associated with VEGF, Flt-1 and Flk-1 expression (p < 0.05)

CONCLUSIONS

HIF-1alpha is expressed in human CAVM. The expression of HIF-1alpha is significantly related to VEGF and VEGFR expression, suggesting a possible role for its induction and role in maintaining angiogenesis and vascular remodelling.

Paracrine and autocrine functions of brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) in brain-derived endothelial cells

Brain-derived neurotrophic factor (BDNF) is expressed by endothelial cells. We investigated the characteristics of BDNF expression by brain-derived endothelial cells and tested the hypothesis that BDNF serves paracrine and autocrine functions affecting the vasculature of the central nervous system. In addition to expressing TrkB and p75NTR and BDNF under normoxic conditions, these cells increased their expression of BDNF under hypoxia. While the expression of TrkB is unaffected by hypoxia, TrkB exhibits a base-line phosphorylation under normoxic conditions and an increased phosphorylation when BDNF is added. TrkB phosphorylation is decreased when endogenous BDNF is sequestered by soluble TrkB. Exogenous BDNF elicits robust angiogenesis and survival in three-dimensional cultures of these endothelial cells, while sequestration of endogenous BDNF caused significant apoptosis. The effects of BDNF engagement of TrkB appears to be mediated via the phosphatidylinositol (PI) 3-kinase-Akt pathway. Modulation of BDNF levels directly correlate with Akt phosphorylation and inhibitors of PI 3-kinase abrogate the BDNF responses. BDNF-mediated effects on endothelial cell survival/apoptosis correlated directly with activation of caspase 3. These endothelial cells also express p75NTR and respond to its preferred ligand, pro-nerve growth factor (pro-NGF), by undergoing apoptosis. These data support a role for neurotrophins signaling in the dynamic maintenance/differentiation of central nervous system endothelia.

Neurotrophic effects of vascular endothelial growth factor on organotypic cortical explants and primary cortical neurons

Vascular endothelial growth factor (VEGF) is well known to play an important regulatory role in vascular growth and development. Because gene knock-outs of VEGF and its receptors flk-1 and flt-1 result in early embryonic lethality, determining roles for VEGF in CNS development has been particularly difficult. Recent studies have shown that VEGF is upregulated after various injuries to the adult brain and that the cytokine affords protection to cultured neurons affected by oxidative or excitotoxic stress. The present study demonstrates, for the first time, that VEGF is directly neurotrophic to CNS neurons in culture. We applied VEGF to normoxic fetal organotypic cortical explants as a model of CNS neuropil, in addition to primary cortical neurons, to assess direct growth effects absent vascular or astroglial activity. We found that VEGF provided a significant dose-responsive increase in the neuronal microtubule markers TUJ1 and MAP-2, as well as mRNA for MAP-2 and flk-1. Antisense oligodeoxynucleotides to flk-1, but not flt-1, inhibited neuritic outgrowth, whereas inhibitors of the signaling pathways MEK1 and P13-AKT both abrogated VEGF-induced growth. VEGF applied to primary cortical neurons produced significant increases in neuronal cell body diameter and the number of emerging neurites mediated by flk-1. Possibly, VEGF achieves its effects by acting on the neuronal microtubular content, which is involved with growth, stability and maturation. Several studies have now shown that VEGF is neurotrophic and neuroprotective independent of a vascular component; we suggest that VEGF plays seminal pleiotrophic roles in CNS development and repair.

Brain capillaries and cholinergic neurons persist in organotypic brain slices in the absence of blood flow

Angiogenesis plays an important role during development of the brain and under pathological conditions. The aim of the present study was to observe interaction of brain capillaries and cholinergic neurons in organotypic brain slices. Immunohistochemistry was used to visualize brain capillary-like structures (RECA-1 antigen) and cholinergic neurons (choline acetyltransferase). Under normal culture conditions, a very low number of brain capillaries was found in 2- and 4-week-old cortex brain slices. Treatment of slices with acidic medium (pH 6) or hyperthermia (42 degrees C) markedly enhanced the number of brain capillaries. Incubation with 10 ng/mL vascular endothelial growth factor only enhanced angiogenesis in more developed slices. Cholinergic neurons survived in slices of the basal nucleus of Meynert; however, hyperthermia but not acidosis markedly decreased their number. In coslices of the basal nucleus of Meynert and cortex (pretreated with acidic medium), a high number of RECA-1-positive capillaries and cholinergic neurons persisted and displayed strong nerve fibre growth of cholinergic fibres into the cortex. In conclusion, we have demonstrated that RECA-1-positive capillaries and cholinergic neurons can be studied in slice cultures in the absence of blood perfusion, and that this model could provide a system to study mechanisms involved in vascular dementia.

VEGF-induced activation of the PI3-K/Akt pathway reduces mutant SOD1-mediated motor neuron cell death

The increased oxidative stress induced by mutant SOD1 is associated with motor neuron degeneration in both human ALS and transgenic mice expressing mutant SOD1. Vascular endothelial growth factor (VEGF) is neurotrophic and also protects from hypoxia-induced neuronal injury. The potential role of VEGF in preventing mutant SOD1-mediated motor neuron cell death was examined using a mouse NSC34 motor neuron-like cell culture system. Infection with adenovirus containing mutant G93A-SOD1, but not vector control or wild-type SOD1, increased cellular oxidative stress and motor neuron-like cell death. However, NSC34 cells pretreated with VEGF displayed a dose-dependent resistance to oxidative damage from hydrogen peroxide, TNF-alpha, and mutant G93A-SOD1. VEGF activated both PI3-K and MAPK activities in mouse NSC34 motor neuron-like cells. Pharmacological inhibitors and constitutively active as well as dominant negative mutants of MAPK and PI3-K revealed that the protective effects of VEGF were mediated via the PI3-K activity, and that MAPK activation was not associated with NSC34 cell survival. Furthermore, VEGF-induced downstream Akt activation promoted motor neuron-like NSC34 cell survival in the presence of mutant G93A-SOD1. Thus, VEGF protected mouse NSC34 motor neuron-like cell death from mutant G93A-SOD1 effects via PI3-K/Akt activation.

Cerebral capillary endothelial cell mitogenesis and morphogenesis induced by astrocytic epoxyeicosatrienoic Acid

Background and Purpose- Epoxyeicosatrienoic acids (EETs) are products of cytochrome P450 epoxygenation of arachidonic acid. We have previously demonstrated that astrocyte-conditioned medium induced mitogenesis in brain capillary endothelial cells. The goals of the present studies are to further define the mechanism through which this can occur and to confirm that EETs are derived from astrocytes, through which astrocytic activity can regulate cerebral angiogenesis in response to neuronal activation.

METHODS

Astrocytes and cerebral capillary endothelial cells in primary cultures were cocultured to examine the interaction of the 2 cell types. We used multiple immunohistochemical techniques to characterize the multicellular nature of the capillaries, which is not simply an artifact related to the culture conditions. The mitogenic effect of EETs was determined by (3)H-thymidine incorporation and cell proliferation assay. Endothelial tube formation was examined in vitro and in vivo with the use of a reconstituted basement membrane (Matrigel) assay.

RESULTS

In cocultures of astrocytes and capillary endothelium, we observed morphological changes in both cell types such that each assumed certain physiological characteristics, ie, endothelial networks and astrocytes with "footlike" projections as well as intermittent gap junctions forming within the endothelial cells. EETs from astrocytes as well as synthetic EETs promoted mitogenesis of endothelial cells, a process sensitive to inhibition of tyrosine kinase with genistein. Treatments with exogenous EETs were sufficient for endothelial cells to differentiate into capillary-like structures in culture as well as in vivo in a Matrigel matrix.

CONCLUSIONS

The 2 major conclusions from these data are that astrocytes may play an important role in regulating angiogenesis in the brain and that cytochrome P450-derived EETs from astrocytes are mitogenic and angiogenic.

Molecular control of capillary growth in skeletal muscle

Angiogenesis, the growth of new capillaries, enhances the oxygen delivery capacity of an existing vascular network. This adaptation is a well-documented occurrence in exercising skeletal muscle. The purpose of this review is to summarize our current understanding of the various stimuli that are involved in the initiation of capillary growth in skeletal muscle. The roles of humoral and mechanical signals in the cellular regulation of several key angiogenic players, vascular endothelial cell growth factor and matrix metalloproteinases, will be discussed. Evidence will be presented supporting the existence of angiogenesis processes that are distinct from the "classically" defined process. Determining how specific angiogenic stimuli can initiate unique patterns of capillary growth will provide insight into the complex task of developing effective pro-angiogenic therapies.

Chronic hypoxia up-regulates fibroblast growth factor ligands in the perinatal brain and induces fibroblast growth factor-responsive radial glial cells in the sub-ependymal zone

A number of signaling molecules have been implicated in the acute response to hypoxia/ischemia in the adult brain. In contrast, the reaction to chronic hypoxemia is largely unexplored. We used a protocol of chronic hypoxia in rat pups during the first three postnatal weeks, encompassing the period of cellular plasticity in the cerebral cortex. We find that the levels of fibroblast growth factor 1 (FGF1) and FGF2, two members of the FGF family, increase after 2 weeks of chronic hypoxia. In contrast, members of the neurotrophin family are unaffected. FGF2 is normally expressed in the nucleus of mature, glial fibrillary acidic protein (GFAP)-containing astrocytes. Under hypoxia, most FGF2-containing cells do not express detectable levels of GFAP, suggesting that chronic low O(2) induces their transformation into more immature glial phenotypes. Remarkably, hypoxia promotes the appearance of radial glia throughout the sub-ventricular and ependymal zones. Most of these cells express vimentin and brain lipid binding protein. A subset of these radial glial cells expresses FGF receptor 1, and are in close contact with FGF2-positive cells in the sub-ventricular zone. Thus, FGF receptor signaling in radial glia may foster cell genesis after chronic hypoxic damage. From the results of this study we suggest that after the chronic exposure to low levels of oxygen during development, the expression of radial glia increases in the forebrain periventricular region. We envision that astroglia, which are the direct descendants of radial glia, are reverting back to immature glial cells. Alternatively, hypoxia hinders the normal maturation of radial glia into GFAP-expressing astrocytes. Interestingly, hypoxia increases the levels of expression of FGF2, a factor that is essential for neuronal development. Furthermore, chronic hypoxia up-regulated FGF2's major receptor in the periventricular region. Because radial glia have been suggested to play a key role in neurogenesis and cell migration, our data suggests that hypoxia-induced FGF signaling in radial glia may represent part of a conserved program capable of regenerating neurons in the brain after injury.

Angiogenic and astroglial responses to vascular endothelial growth factor administration in adult rat brain

The effects of exogenous vascular endothelial growth factor (VEGF) on angiogenesis, blood-brain barrier permeability and astroglial proliferation in the adult rat CNS in situ were investigated. Recombinant human VEGF(165) (25 or 50 ng/ml) was delivered for up to 1 week using either intracerebral osmotic minipumps or less traumatic subdural gelatin sponge placement. By 3 days, VEGF delivery caused significantly increased cerebral angiogenesis (25 ng/ml was most effective) in both experimental models when compared to saline controls; VEGF infusion resulted in a 100% increase in an index of vascular proliferation, and gelatin sponge delivery produced a 65% increase. The blood-brain barrier hallmark endothelial glucose transporter-1 was not present in nascent vascular sprouts. Infusion of VEGF produced extensive protein leakage that persisted after saline-induced permeability was mostly resolved, while gelatin sponge administration caused milder barrier dysfunction. Administration of the angiogenic factor had unexpected proliferative effects on astroglia in both models, resulting in an 80-85% increase in mitotically active astroglia when compared to controls. Immunohistochemical results and semi-quantitative reverse transcriptase-polymerase chain reaction indicated that the VEGF receptors flk-1 and flt-1 were up-regulated in response to the infusion trauma; flt-1 was localized to reactive astroglia, while flk-1 was expressed in vascular endothelium but predominantly in neuronal somata and processes adjacent to the delivery site. mRNA for the VEGF(121), VEGF(165) and VEGF(188) isoforms was also increased after delivery of the recombinant protein. These data show that VEGF application has substantial proliferative effects on CNS endothelium and astroglia and causes up-regulation of its own message. Flt-1 and flk-1 receptor mRNAs and proteins are up-regulated in both vascular and non-vascular cell types following infusion trauma. From these results we suggest that administered VEGF has heretofore unanticipated pleiotrophic effects in the adult CNS.

Astrocyte-derived VEGF mediates survival and tube stabilization of hypoxic brain microvascular endothelial cells in vitro

Chronic sublethal hypoxia has been associated with changes in neurovascular behavior, mediated, in part, by induction of vascular endothelial growth factor-A (VEGF-A(165)). In this report we demonstrate that RBE4 cells (derived from rodent cerebral microvasculature), when cultured in three-dimensional collagen gels: (1) Are induced to undergo increased tube formation in response to VEGF-A(165) in a dose-dependent manner; (2) undergo apoptosis under mild hypoxic conditions; (3) are rescued from the effects of hypoxia by the addition of exogenous VEGF-A(165) in a dose-dependent and inhibitable manner or by co-culture with primary newborn rat astrocytes, which are induced to express increased amounts of VEGF-A in hypoxic conditions. Further, we demonstrate that: (4) The observed astrocyte-produced, VEGF-mediated protection from apoptosis (survival) is inhibitable with soluble recombinant VEGF receptor-1 (sFlt), and is associated with a robust induction of MAPK tyrosine phosphorylation. These findings illustrate the importance of VEGF in the process of neurovascular survival in response to injury in developing brain and provide insight into the signaling pathways involved.

Multiple molecular penumbras after focal cerebral ischemia

Though the ischemic penumbra has been classically described on the basis of blood flow and physiologic parameters, a variety of ischemic penumbras can be described in molecular terms. Apoptosis-related genes induced after focal ischemia may contribute to cell death in the core and the selective cell death adjacent to an infarct. The HSP70 heat shock protein is induced in glia at the edges of an infarct and in neurons often at some distance from the infarct. HSP70 proteins are induced in cells in response to denatured proteins that occur as a result of temporary energy failure. Hypoxia-inducible factor (HIF) is also induced after focal ischemia in regions that can extend beyond the HSP70 induction. The region of HIF induction is proposed to represent the areas of decreased cerebral blood flow and decreased oxygen delivery. Immediate early genes are induced in cortex, hippocampus, thalamus, and other brain regions. These distant changes in gene expression occur because of ischemia-induced spreading depression or depolarization and could contribute to plastic changes in brain after stroke.

Effect of prolonged O2 deprivation on Na+ channels: differential regulation in adult versus fetal rat brain

Neuronal Na+ channels are functionally inhibited in the adult in response to acute O2 deprivation. Since prolonged hypoxia may not only affect channel function, but also its expression, we hypothesized that long-term hypoxia alters Na+ channel density. This alteration may depend on age, because we have found major differences in neuronal responses to hypoxia between the immature and adult. In the present work, we used northern blots, slot blots, saxitoxin binding and autoradiography to ask whether: (i) prolonged hypoxia alters Na+ channel messenger RNA and protein levels in the brain; (ii) there is a difference between the adult and prenatal brains regarding Na+ channel expression with hypoxic exposure; and (iii) regional differences in Na+ channel expression occur in hypoxia-exposed brains. Our results show the following. (1) Na+ channel messenger RNA and saxitoxin binding density decreased after prolonged hypoxia in adult brain homogenates; this is in sharp contrast to the changes observed in fetal brains, which tended to increase Na+ channel messenger RNA and protein after hypoxia. (2) Changes in saxitoxin binding density are related to alterations in the number of saxitoxin binding sites and not to binding affinity, since there was no major change in Kd values between the hypoxia and naive groups. (3) The hypoxia-induced Na+ channel expression was heterogeneous, with major differences between rostral regions (e.g., the cortex) and caudal regions (e.g., the medulla and pons). We speculate that down-regulation of Na+ channels during long-term hypoxia in mature brains is an adaptive cellular response, aimed at minimizing the mismatch between energy supply and demand, since maintenance of Na+ gradients is a major energy-requiring process. However, the prenatal brain does not depend on this adaptive mechanism in response to hypoxic stress.

Neuronal VEGF expression correlates with angiogenesis in postnatal developing rat brain

When exposed to chronic sublethal hypoxia the developing brain responds with increases in permeability and angiogenesis. Vascular endothelial growth factor (VEGF) may mediate this response. Here, we present data on the localization of VEGF in the rat brain cortex during postnatal development and its correlation to vascularization. We reared newborn rats under normoxic conditions and in hypoxic chambers (FiO(2) 9.5%), removed them at postnatal days (P) 3, 8, 13, 24, and 33 and prepared the cortical brain tissue for immunohistochemistry, in situ hybridization (ISH), Western blot analyses and vessel density counting. When compared to age-matched controls, hypoxic-reared animals displayed a significant increase in platelet endothelial cell adhesion molecule 1 (PECAM-1) protein levels, cerebral microvascular lumen diameter and number and density of vessels (number of capillaries per area). In control animals, ISH and immunohistochemistry revealed that localization of VEGF is restricted almost exclusively to cortical neurons at early stages of development. As the vascular bed begins to stabilize, predominant VEGF expression switches to maturing glial cells which invest vessels while neuronal expression is reduced to a basal level. In hypoxic animals, early localization of VEGF is also restricted to cortical neurons, however, during later developmental stages, glial cells express elevated levels of VEGF protein and high neuronal expression also persists. Thus chronic sublethal hypoxia disrupts the temporal-spatial expression of VEGF, which correlates with continuing hypoxia-driven angiogenesis.

THYMUS AND LYMPH NODE CELL CD4(+)AND CD8(+)MARKER EXPRESSION AND THEIR ANGIOGENIC ACTIVITY OF OFFSPRING OF SALBUTAMOL-TREATED PREGNANT C3H MICE

Salbutamol, beta(2)-adrenoceptor agonist is a first choice drug in preterm delivery treatment. The aim of the present study was to determine whether salbutamol can cause any alterations in neonatal immune systems and therefore should be considered in newborn care. The experiments were performed on 4-5- and 6-7-week-old female and male offspring of salbutamol-treated C3H inbred mice. Thymus and lymph node weight, cellularity and lymphatic organs, lymphocytes phenotypes and their angiogenic activity were evaluated. We observed lower thymus weight in 6-7-week-old progeny of salbutamol-treated mothers and in the same time lower thymus cell number in both age groups. Lower lymph node weight was developed in older progeny while cellularity was diminished both in 4-5- and 6-7-week-old offspring of salbutamol-treated mothers. We have not detected any changes in percentage of CD4, CD8, CD3 and CD4CD8 positive lymphocytes in progeny of salbutamol-treated mothers. As far as lymph node lymphocytes phenotype is considered we showed in both age groups lowering of CD4 and CD3 positive cells in experimental groups. In the LIA test (lymphocyte-induced angiogenesis) we showed lower lymph node cell angiogenic activity of salbutamol-treated mothers' progeny in both age groups. In the case of thymus lymphocytes we have not observed any alterations in their angiogenic activity. The differences in histological examination of thymus and lymph nodes were not detected in experimental and control groups. 2000 Academic Press@p$hr Copyright 2000 Academic Press.

Transient coexpression of nestin, GFAP, and vascular endothelial growth factor in mature reactive astroglia following neural grafting or brain wounds

The spatial and temporal immunoexpression of the intermediate filament (IF) protein nestin and its relationship to glial fibrillary acidic protein (GFAP), vascular endothelial growth factor (VEGF), and its receptor flt-1 (VEGF-R1) in reactive astroglia was examined following stab wounds or transplants of fetal CNS tissue into the adult brain. Since developmentally regulated proteins and gene transcripts can be reexpressed in reactive astroglia following certain brain injuries, we analyzed the nestin profile in these experimental paradigms in order to more fully understand the nature of the gliotic "scar." Nestin expression was transiently up-regulated in some but not all astrocytes which often had a different morphology than the typical stout, stellate GFAP (+) cells; the processes of the nestin (+) cells tended to be slender and elongated. In reactive astroglia from the mature brain, nestin expression was robust but generally localized to the wound or graft site, peaked at 7-10 days postoperative, and was absent by 28 days, whereas GFAP (+) astrocytes were far more widespread and persisted for many months. Only nestin was strongly expressed immediately adjacent to early stab wounds, whereas GFAP (+) cells were located further from the wound sites. In contrast, there was marked nestin/GFAP colocalization at the graft/host interface. Semiquantitative analysis combined with confocal microscopy revealed a unique compartmentalization of protein expression; processes from single astrocytes could be entirely nestin (+), GFAP (+), or could show coexpression. At 4, 7, and 14 days postoperative, 41, 58, and 32% of the immunoexpression, respectively, was accounted for by nestin at the graft/host interface, and it was essentially undetectable at 28 days postoperative. In situ hybridization studies showed nestin transcripts within GFAP (+) cells primarily between 4 and 10 days postoperative and absent by 28 days. Many nestin (+) astrocytes, as shown by electron microscopy, were closely related to the vasculature. Therefore we further examined the expression of vascular endothelial growth factor (VEGF), an endothelial cell mitogen associated with angiogenesis. Nestin colocalized with VEGF in some astrocytes (7%) but far more prominently with the VEGF flt-1 receptor (25%). Early astroglial activation may involve several different IF components and possibly a distinct astrocytic population that shows a rapid, transient nestin expression adjacent to injury sites. Expression of the nestin IF phenotype within affected astrocytes in the surgical vicinity may be indicative of a reversion to an immature phenotype that might be less susceptible to attendant hypoxia after injury. Since injured astrocytes are well known to express many bioactive compounds, such transient reexpression of early, developmentally regulated proteins may be a hallmark for the elaboration of growth factors such as VEGF.

Vascular, glial and neuronal effects of vascular endothelial growth factor in mesencephalic explant cultures

Vascular endothelial growth factor is a highly conserved, heparin-binding protein which mediates a number of critical developmental processes in both vertebrates and invertebrates, including angiogenesis, vasculogenesis and hematopoiesis. We employed an organotypic rat explant model (produced from embryonic day 17 fetuses) to assess the effects of vascular endothelial growth factor on brain microvasculature in general and the ventral midbrain specifically. Immunohistochemistry using antisera to rat endothelial cell antigen and laminin demonstrated a robust, dose-dependent effect of vascular endothelial growth factor, resulting in increased vessel neogenesis, branching and lumen size by three days in vitro. This effect was blocked by addition of an anti-vascular endothelial growth factor antibody. At higher doses of vascular endothelial growth factor, the effect was attenuated, though a statistically significant increase in both astrocyte, and neuronal density was observed using antisera to glial and neuronal markers. Tyrosine hydroxylase-immunoreactive (i.e. dopaminergic) neurons, particularly, exhibited increased survival in response to vascular endothelial growth factor application. Vascular endothelial growth factor had a mitogenic effect on endothelial cells and astrocytes, but not dopaminergic neurons, as demonstrated by the addition of [3H]thymidine to the cultures 2 h after the cultures were established. Similarly, results of a radioreceptor assay indicated that specific vascular endothelial growth factor binding sites were present on blood vessels and astrocytes, and were up-regulated by exposure to vascular endothelial growth factor. We conclude that, in explants of the ventral mesencephalon, exogenously applied vascular endothelial growth factor is mitogenic for endothelial cells and astrocytes, and promotes growth/survival of neurons in general and dopaminergic neurons in particular.