Patterns of brain angiogenesis after vascular endothelial growth factor administration in vitro and in vivo

Encapsulated vascular endothelial growth factor—secreting cell grafts have neuroprotective and angiogenic effects on focal cerebral ischemia

Object

The authors evaluated the neuroprotective and angiogenic effects of a continuous and low-dose infusion of vascular endothelial growth factor (VEGF)-165 on cerebral ischemia in rats.

Methods

The authors introduced VEGF complementary (c)DNA into baby hamster kidney (BHK) cells and established a cell line that produces human VEGF165 (BHK-VEGF). The BHK-VEGF cells and BHK cells that had been transfected with an expression vector that did not contain human VEGF165 cDNA (BHK-control) were encapsulated. Both capsules were implanted into rat striata. Six days after capsule implantation, the right middle cerebral artery (MCA) was occluded. Some animals were killed 24 hours after occlusion to measure the volume of the resulting infarct and to perform immunohistochemical studies. Other animals were used for subsequent behavioral studies 1, 7, and 14 days after MCA occlusion.

The encapsulated BHK-VEGF cell grafts significantly reduced the volume of the infarct and the number of apoptotic cells in the penumbral area when compared with the effect of the BHK-control cell capsule. In addition, angiogenesis and gliogenesis significantly increased in the region around the capsule in animals that received BHK-VEGF cell capsules without an increase in focal cerebral blood flow; this did not occur in animals that received the BHK-control cell capsule. In behavioral studies rats that received the BHK-VEGF cell capsule displayed significant recovery while participating in the accelerating rotarod test after stroke.

Conclusions

Continuous intracerebral administration of low-dose VEGF165 through encapsulated grafts of VEGF-producing cells produces neuroprotective and angiogenic effects. These effects improve subsequent motor function.

Chronic stress in the adult dentate gyrus reduces cell proliferation near the vasculature and VEGF and Flk-1 protein expression

Recent evidence has shown that cell proliferation in the adult hippocampal dentate gyrus occurs in tight clusters located near the vasculature. Also, changes in neurogenesis often appear parallel to changes in angiogenesis. Moreover, both these processes share similar modulating factors, like vascular endothelial growth factor (VEGF) and its receptor Flk-1. In an earlier study we found that chronic stress decreased new cell proliferation in the adult dentate gyrus. We here questioned whether these effects of chronic stress are mediated through the vasculature and whether they involve an angiogenic-signaling pathway. We therefore measured the surface area covered by the vasculature, the proportion of vascular-associated newborn cells, and analysed VEGF and Flk-1 protein expression in the hippocampus of a control, chronically stressed and recovery group of rats. Our results show that 32% of the proliferating cells in the rat hippocampus is vascular associated. Chronic stress affected this population of newborn cells to a significantly larger extent than the non-associated cells. Interestingly, after 3 weeks of recovery, the decreased proliferation not associated with the vasculature was more effectively restored than vascular-associated proportion of proliferating cells. VEGF protein was expressed in high densities in GFAP-positive astrocytes located in the hilus, with VEGF-positive end feet extending into and often contacting the granule cells. After chronic stress, both VEGF and Flk-1 protein levels were significantly decreased in the granular cell layer, and again recovered after 3 weeks. This demonstrates that changes in angiogenic factors are implicated in the decreased adult proliferation found after chronic stress.

Astrocyte growth effects of vascular endothelial growth factor (VEGF) application to perinatal neocortical explants: receptor mediation and signal transduction pathways

The non-angiogenic role of vascular endothelial growth factor (VEGF), and its receptors flt-1 and flk-1, together with downstream signaling pathways were examined in fetal and postnatal rat cerebral cortical organotypic explants. VEGF application in both paradigms caused a significant increase in astroglial proliferation and a dose-dependent increase in GFAP and nestin immunoreactivity. The VEGF receptor flt-1 was observed on most, though not all astrocytes, while flk-1 receptor immunoexpression was absent. Treatment with antisense oligonucleotides (AS-ODNs) to flt-1 resulted in a dramatic decrease in GFAP and nestin immunoreactivity, which further confirmed the role of flt-1 in mediating VEGF's gliotrophic effects, while AS-ODNs to flk-1 had no effect. VEGF-induced gliotrophic effects were found to be mediated by the MAPK/ERK and PI-3 kinase signaling pathways, since the both the MEK1 inhibitor, PD98059 and the PI-3 kinase inhibitor, Wortmannin abolished VEGF-induced astrocytic GFAP(+) expression. Although high dose VEGF application resulted in strong upregulation of both GFAP and nestin immunoreactivity in astrocytes, overlap of the two proteins was not observed in all cells, suggesting that some of the nestin(+) cells might be neural progenitors. Exposure to VEGF resulted in upregulation of both VEGF and bFGF mRNA at the one-day time point, and bFGF protein by 3 days; VEGF activated astrocytes expressed bFGF to a much greater degree than those in untreated explants. The increased expression of bFGF induced by VEGF, may serve in the proliferation of multipotential neural stem/progenitor cells in vitro. VEGF, an established angiogenic factor, appears to play a significant role in the growth and differentiation of astrocytes in the CNS.

Molecular targets in radiation-induced blood-brain barrier disruption

Disruption of the blood-brain barrier (BBB) is a key feature of radiation injury to the central nervous system. Studies suggest that endothelial cell apoptosis, gene expression changes, and alteration of the microenvironment are important in initiation and progression of injury. Although substantial effort has been directed at understanding the impact of radiation on endothelial cells and oligodendrocytes, growing evidence suggests that other cell types, including astrocytes, are important in responses that include induced gene expression and microenvironmental changes. Endothelial apoptosis is important in early BBB disruption. Hypoxia and oxidative stress in the later period that precedes tissue damage might lead to astrocytic responses that impact cell survival and cell interactions. Cell death, gene expression changes, and a toxic microenvironment can be viewed as interacting elements in a model of radiation-induced disruption of the BBB. These processes implicate particular genes and proteins as targets in potential strategies for neuroprotection.

Angiogenesis in mucous retention cyst: a human in vivo-like model of endothelial cell differentiation in mucous substrate

BACKGROUND

Mucous retention cysts contain a mucous pool in the lumina, in which pure angiogenic processes are occasionally observed. By using this unique human material, our aim was to understand the in vivo angiogenic process.

METHODS

Fifteen surgical tissue samples of mucous retention cysts of the lip were examined for expression of vascular endothelial markers and extracellular matrix molecules by immunohistochemistry and in situ hybridization (ISH).

RESULTS

Endothelial cells forming new vascular channels showed immunopositivities for CD31, CD34, vascular endothelial growth factor (VEGF), and von Willebrand factor (vWF). These newly formed capillaries were surrounded by tenascin-positive matrices and further by a dense infiltration of CD68-positive cells with foamy to epitheloid appearances. Some of these cells were simultaneously positive for CD34, VEGF, and one of its receptors, Flk-1, and they showed definite mRNA as well as protein signals for tenascin. In addition, these cells often tended to be aligned, which suggested tubule formation.

CONCLUSION

The results suggest that monocyte/macrophage lineage cells are a major source for endothelial cells at least in mucous retention cysts and that tenascin produced by those cells plays an important role in differentiation of endothelial cells.

Vascular endothelial growth factor counteracts NMDA-induced cell death of adult cholinergic neurons in rat basal nucleus of Meynert

Vascular endothelial growth factor (VEGF) plays an important role in angiogenesis in the brain and has recently been shown to possess neuroprotective activity. The aim of the present study was to observe if VEGF can counteract the excitotoxic N-methyl-D-aspartate (NMDA)-induced cell death of cholinergic neurons of the basal nucleus of Meynert in vivo in adult rats. Immunohistochemistry was used to detect vascular structures (RECA-1 or laminin staining) and cholinergic neurons (choline-acetyltransferase). To compare the in vivo VEGF effects also in vitro, VEGF was applied to axotomized cholinergic neurons in organotypic brain slices with or without NMDA. Our data provide evidence that VEGF counteracted cell death in vivo in adult rats but did not protect cholinergic neurons in developing brain slices. Nerve growth factor protected cholinergic neurons in vivo as well as in vitro. In conclusion, VEGF exhibits neuroprotective activity on adult cholinergic neurons of the basal nucleus of Meynert.

The differences between high and low-dose administration of VEGF to dopaminergic neurons of in vitro and in vivo Parkinson's disease model

Vascular endothelial growth factor (VEGF) has previously been shown to display neuroprotective effects on dopaminergic (DA) neurons. In this study, we investigated whether the effects of VEGF were dose-dependent or not. First, VEGF was shown to be neuroprotective on 6-hydroxydopamine (6-OHDA)-treated murine DA neurons in vitro, although the 1 ng/ml of VEGF displayed more neuroprotective effects than 100 ng/ml. Furthermore, using 2 sizes of capsules (small/large) with different secreting quantities, 6-OHDA-treated rats receiving the small capsule filled with VEGF-secreting cells (BHK-VEGF) into the striatum showed a significant decrease in amphetamine-induced rotational behavior in number and a significant preservation of TH-positive fibers compared to those receiving the large BHK-VEGF capsule as well as those receiving BHK-Control capsule. Rats receiving the large BHK-VEGF capsule showed much more glial proliferation, angiogenesis, and brain edema around the capsule than those with the small one. High-dose administration of VEGF might cause poor circulation related to brain edema, although low-dose administration of VEGF displays neuroprotective effects on DA neurons. Our results demonstrate the importance of administration dose of VEGF, suggesting that low-dose administration of VEGF might be desirable for Parkinson's disease.

Hypoxia and hypoxia-inducible factor-1 target genes in central nervous system radiation injury: a role for vascular endothelial growth factor

PURPOSE

Microvascular permeability changes and loss of blood-brain barrier integrity are important features of central nervous system (CNS) radiation injury. Expression of vascular endothelial growth factor (VEGF), an important determinant of microvascular permeability, was examined to assess its role in CNS radiation damage. Because hypoxia mediates VEGF up-regulation through hypoxia-inducible factor-1alpha (HIF1alpha) induction, we studied the relationships of hypoxia, HIF1alpha expression, and expression of VEGF in this damage pathway.

EXPERIMENTAL DESIGN

Expression of HIF1alpha, VEGF, and another hypoxia-responsive gene, glucose transporter-1, was assessed in the irradiated rat spinal cord using immunohistochemistry and in situ hybridization. Hypoxic areas were identified using the nitroimidazole 2-(2-nitro-1H-imidazole-L-yl)-N-(2,2,3,3,3,-pentafluoropropyl) acetamide. To determine the causal importance of VEGF expression in radiation myelopathy, we studied the response of transgenic mice with greater (VEGF-A(hi/+)), reduced (VEGF-A(lo/+)), and wild-type VEGF activity to thoracolumbar irradiation.

RESULTS

In rat spinal cord, the number of cells expressing HIF1alpha and VEGF increased rapidly from 16 to 20 weeks after radiation, before white matter necrosis and forelimb paralysis. A steep dose response was observed in expression of HIF1alpha and VEGF. HIF1alpha and VEGF expressing cells were identified as astrocytes. Hypoxia was present in regions where up-regulation of VEGF and glucose transporter-1 and increased permeability was observed. VEGF-A(lo/+) mice had a longer latency to development of hindlimb weakness and paralysis compared with wild-type or VEGF-A(hi/+) mice.

CONCLUSIONS

VEGF expression appears to play an important role in CNS radiation injury. This focuses attention on VEGF and other genes induced in response to hypoxia as targets for therapy to reduce or prevent CNS radiation damage.

Quantification of angiogenesis by functional computed tomography in a Matrigel model in rats

RATIONALE AND OBJECTIVES

The aim was to evaluate functional computed tomography (fCT) in the quantification of angiogenesis by comparing the tissue perfusion parameters measured by CT perfusion (CTP) software with histologic vascular parameters in a Matrigel model in rats. It was hypothesized that tissue perfusion parameters and histologic vascular parameters are related.

MATERIALS AND METHODS

In vivo angiogenesis assays were performed using Matrigel supplemented with escalating doses (0 ng [control group], 250 ng, and 1,000 ng) of recombinant rat vascular endothelial growth factor (VEGF164) subcutaneously injected into the backs of Sprague Dawley rats. On day 7, rats with Matrigel plug underwent fCT following a bolus injection of iodinated contrast medium. Using CTP software, fCT parameters were generated (blood flow [BF], blood volume [BV], mean transit time, and permeability-surface area product) and functional maps on the basis of a distributed parameter tracer kinetic model, the adiabatic approximation to the tissue homogeneity model. The animals were then sacrificed. Matrigel plug was sectioned into slices corresponding to the CT scan plane and stained with CD31 immunohistochemical stain. Histologic vascular parameters, including microvascular density (MVD), vessel number (VN), vascular area, and vascular perimeter, were measured. CTP and histologic parameters were correlated.

RESULTS

The Matrigel plugs with the 1,000-ng VEGF group exhibited a higher MVD than the 250-ng VEGF and control groups (P < .05). VN differed significantly between the control versus the 250-ng VEGF groups and 250-ng versus 1,000-ng VEGF groups (P < .05), with the highest VN in the 250-ng VEGF group. BF, mean transit time, and permeability-surface area product each differed significantly to VEGF levels. Changes in BF and BV did not correspond with increases in MVD or VN; however, in the 250-ng VEGF group, there was a strong positive correlation (r = 0.9) between BV and VN, vascular area, and vascular perimeter, which was not seen in the control or 1,000-ng VEGF group. All fCT parameters significantly correlated with each other (P < .05), with strong correlations between BF and mean transit time (r = -0.7) and between BF and permeability-surface area product (r = 0.7) and a weak correlation between BF and BV (r = 0.3).

CONCLUSION

These results validate the VEGF-induced endothelial cell in a rat Matrigel model. In addition, histologic vascular parameter MVD does not correlate with fCT parameters measured by CTP software.

Vascular endothelial growth factor is increased in cerebrospinal fluid after traumatic brain injury in infants and children

OBJECTIVE

Vascular endothelial growth factor (VEGF) is an important regulator of angiogenesis, the formation of which is triggered by hypoxia, cytokines, and growth factors and is also induced by activation of the adenosine 2B receptor. VEGF is neuroprotective in several models of experimental brain injury and is increased in brain after traumatic brain injury (TBI) in humans and experimental animals. Adenosine is a neuroprotective purine metabolite that increases in cerebrospinal fluid (CSF) after clinical TBI in children. We hypothesized that VEGF levels would 1). be increased in CSF after TBI in infants and children, and 2). be preceded by increases in CSF adenosine. To test this hypothesis, we designed a case-control study to compare the CSF of infants and children after severe TBI with that of uninjured children.

METHODS

Using an Institutional Review Board-approved protocol, we compared CSF concentrations of VEGF (by enzyme-linked immunosorbent assay) and adenosine (by high-performance liquid chromatography) in 73 samples from 14 infants and children with severe TBI (Glasgow Coma Scale score <or=8) with those in CSF from 5 noninjured control subjects. Patients received standard neurointensive care.

RESULTS

Mean VEGF levels were increased after TBI versus control (39.8 +/- 6.2 versus 14.9 +/- 1.5 ng/dl, mean +/- standard error of the mean, P = 0.0002) and peaked at 91.6 +/- 26.4 ng/dl, approximately 6 times control (P = 0.001). Peak VEGF occurred at 22.4 hours after injury. There was a trend toward increased adenosine concentration after TBI versus control (18.3 +/- 3.5 versus 11.5 +/- 2.3 nmol/L), but this did not reach statistical significance. A multivariate regression model showed an independent, significant association between the concentrations of VEGF and adenosine.

CONCLUSION

VEGF is increased in CSF after pediatric TBI, and this increase is associated with an increase in CSF adenosine. These results may imply that a component of the vascular regenerative response of the brain is initiated rapidly after TBI and continues for several days after injury. Further investigation is warranted to determine 1). whether this association is causative, 2). the role of adenosine in triggering the increase in CSF VEGF concentration, and 3). the exact role VEGF that plays after injury.

Synergistic effect of estrogen and VEGF on the proliferation of hemangioma vascular endothelial cells

PURPOSE

The aim of this study was to observe the influences of estradiol (E2), vascular endothelial growth factor (VEGF), and 4OH-tamoxifen (TAM) on the proliferation of hemangioma vascular endothelial cells (HVECs).

METHODS

Two strawberry hemangiomas with positive estrogen receptor staining from 2 infants (cases 1 & 2) were used for HVECs culture. The HVECs of passage 3 were cultured in estrogen-free improved minimum essential medium and divided into 5 groups based on different treatments: group 1, no treatment; group 2, treated with E2; group 3, treated with VEGF; group 4, treated with both E2 and VEGF; group 5, treated with E2, VEGF, and TAM. Cell count (CC) and DNA proliferation index (PI) were determined on culture days 0, 3, 6, and 9.

RESULTS

On day 9 in case 1, CC and PI were the following: in group 1, 1.15 +/- 0.18 x 10(5) mL and 19.96% +/- 1.45%, respectively, presenting no statistically significant changes; in group 2, 1.38 and 1.61 times those of group 1, respectively (P <.01); in group 3, 2.10 and 1.61 times those of group 2, respectively (P <.01); in group 4, 1.62 and 1.40 times as high as with group 3, respectively (P <.01); in group 5, down to the levels of group 1. The results in case 2 were similar to those in case 1.

CONCLUSIONS

In vitro, the promoting effect of VEGF on HVECs proliferation is stronger than that of estrogen. Estrogen and VEGF enhance this proliferation in a synergistic fashion, which can be inhibited by tamoxifen.

Neuroprotection of ischemic brain by vascular endothelial growth factor is critically dependent on proper dosage and may be compromised by angiogenesis

Vascular endothelial growth factor (VEGF) is currently considered a potential pharmacologic agent for stroke therapy because of its strong neuroprotective and angiogenic capacities. Nonetheless, it is unclear how neuroprotection and angiogenesis by exogenous VEGF are related and whether they are concurrent events. In this study, the authors evaluated by stereology the effect of VEGF on neuronal and vascular volume densities of normal and ischemic brain cortices of adult male Sprague-Dawley rats. Ischemia was induced by a 4-hour occlusion of the middle cerebral artery. Low, intermediate, and high doses of VEGF165 were infused through the internal carotid artery for 7 days by an indwelling osmotic pump. The low and intermediate doses, which did not induce angiogenesis, significantly promoted neuroprotection of ischemic brains and did not damage neurons of normal brains. In contrast, the high dose that induced angiogenesis showed no neuroprotection of ischemic brains and damaged neurons of normal brains. These findings suggest that in vivo neuroprotection of ischemic brains by exogenous VEGF does not necessarily occur simultaneously with angiogenesis. Instead, neuroprotection may be greatly compromised by doses of VEGF capable of inducing angiogenesis. Stroke intervention efforts attempting to induce neuroprotection and angiogenesis concurrently through VEGF monotherapy should be approached with caution.

Regulation of hypoxia-inducible factor 1alpha and induction of vascular endothelial growth factor in a rat neonatal stroke model

Stroke is a devastating condition occurring in at least 1 in 4000 live births in the neonatal period. Since hypoxia-inducible factor (HIF)-1alpha can modulate ischemic injury via induction of target genes that may protect cells against ischemia, and is induced after preconditioning by hypoxia in the neonatal rat brain hypoxia-ischemia model, we evaluated whether HIF-1alpha is induced after focal ischemia-reperfusion, a model for neonatal stroke. We developed an ischemia-reperfusion model in postnatal day 10 (P10) rats by transiently occluding the middle cerebral artery (MCA) for 1.5 h. The MCA territory was reperfused for 0, 4, 8, or 24 h and the expression of HIF-1alpha and its target gene, vascular endothelial growth factor (VEGF), were delineated. HIF-1alpha protein and VEGF protein peaked at 8 h, and declined subsequently at 24 h in injured cortex following 1.5 h of MCA occlusion. Double-immunolabeling indicated that both HIF-1alpha and VEGF are expressed together in neurons with a similar time course of expression. The presence of HIF-1alpha and VEGF after moderate ischemia-reperfusion injury suggests potential avenues to exploit for neuroprotection.

Vascular endothelial growth factor promotes neurite maturation in primary CNS neuronal cultures

Recent studies have demonstrated that vascular endothelial growth factor (VEGF) and its receptor VEGFR2 (flk-1) are expressed by neurons during development and following hypoxic-ischemic events. Moreover, fetal CNS tissue explants exposed to exogenous VEGF exhibit increased neuronal Map-2 expression, suggesting that VEGF could have an effect on neuronal maturation. To determine whether this effect is of a direct nature, we examined the expression of Map-2 in the presence of VEGF in primary CNS neuronal cultures. After 3 days in culture, a statistically significant dose-dependent increase in the length of Map-2(+) processes was observed, with the peak occurring at 10 ng/ml of VEGF. Immunohistochemical analysis of the cultures demonstrated the presence of VEGFR2 after VEGF treatment, as well as the expression of the VEGF receptor VEGFR1 (flt-1). Treatment of the cultures with antisense oligonucleotides against VEGFR2, but not against VEGFR1, abolished the effect of VEGF on the length of Map-2(+) processes. RT-PCR analyses of Map-2 and VEGFR1 indicated that mRNAs of these two genes are upregulated in the presence of VEGF. The addition of wortmannin, an inhibitor of PI3K/Akt signal-transduction pathway, to the media did not affect the VEGF-dependent increase in Map-2(+) length. In contrast PD98059, which inhibits the MAPK pathway, partially abolished this effect of VEGF. These experiments suggest that VEGF has a direct effect on neuronal growth and maturation under normoxic conditions during CNS development, which is mediated by the VEGFR2 receptor via the MAPK pathway.

The Potential Role of Vascular Endothelial Growth Factor in the Central Nervous System

During the past decade, vascular endothelial growth factor (VEGF) has been widely investigated, and reported to have pleiotropic functions in the central nervous system (CNS) and its supporting physiological environment. VEGF is involved in not only such well-known functions as angiogenesis, accentuation of vessel permeability, and glial proliferation, but also more recently acknowledged functions such as neuroprotection and even neurogenesis itself. Most recently, the neurogenesis function has attracted much attention, and a number of research groups have taken up the challenge of elucidating this activity. In keeping with this trend, our knowledge of VEGF receptors has increased, and certain suggestions concerning the mechanisms of neuroprotection have come to light in the course of the ongoing work, though at times what the researchers had to work with was only a tiny percent of the signal transduction of VEGF. Together with flt-1 (VEGF receptor 1) and flk-1 (VEGF receptor 2), neuropilin (NP) is frequently described as being involved in the neuroprotective effects of VEGF. In this review, both the direct and indirect neuroprotective effects of VEGF, including various signaling pathways as well as the neurogenesis induced by this factor, are discussed in the context of the newly emerging insights into the biological mechanisms of VEGF and closely related, interacting molecules.

Vascular endothelial growth factor (VEGF) in seizures: a double-edged sword

Vascular endothelial growth factor (VEGF) is a vascular growth factor which induces angiogenesis (the development of new blood vessels), vascular permeability, and inflammation. In brain, receptors for VEGF have been localized to vascular endothelium, neurons, and glia. VEGF is upregulated after hypoxic injury to the brain, which can occur during cerebral ischemia or high-altitude edema, and has been implicated in the blood-brain barrier breakdown associated with these conditions. Given its recently-described role as an inflammatory mediator, VEGF could also contribute to the inflammatory responses observed in cerebral ischemia. After seizures, blood-brain barrier breakdown and inflammation is also observed in brain, albeit on a lower scale than that observed after stroke. Recent evidence has suggested a role for inflammation in seizure disorders. We have described striking increases in VEGF protein in both neurons and glia after pilocarpine-induced status epilepticus in the brain. Increases in VEGF could contribute to the blood-brain barrier breakdown and inflammation observed after seizures. However, VEGF has also been shown to be neuroprotective across several experimental paradigms, and hence could potentially protect vulnerable cells from damage associated with seizures. Therefore, the role of VEGF after seizures could be either protective or destructive. Although only further research will determine the exact nature of VEGF's role after seizures, preliminary data indicate that VEGF plays a protective role after seizures.

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.

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.

Activation of receptor-mediated angiogenesis and signaling pathways after VEGF administration in fetal rat CNS explants

The angiogenic role of vascular endothelial growth factor (VEGF) receptors, flk-1 and flt-1, and their downstream signaling pathways, MAPK/ERK and PI-3 kinase, were examined in a fetal rat cortical explant model after exposure to exogenous VEGF. Treatment with VEGF resulted in substantial neovascularization characterized by increased vascular flk-1 receptor expression, whereas flt-1 receptor protein expression was absent. The specific role of flk-1 receptors in the angiogenic process was confirmed by the addition of antisense oligonucleotides (AS-ODNs) to flk-1, which blocked angiogenesis, whereas AS-ODNs to flt-1 had no effect. These results were further supported by the finding that specific chemical inhibition of flk-1 receptors caused disruption of the angiogenic response, whereas inhibition of the flt-1 receptors had no effect. Application of either MAPK/ERK or PI-3 kinase pathway inhibitors disrupted VEGF-induced angiogenesis, thereby indicating that both signaling pathways mediate this process. Thus VEGF binding to the endothelial flk-1 receptor activates the MAPK/ERK and PI-3 kinase pathways, resulting in neoangiogenic events. Of interest is the fact that although VEGF is regarded as a vascular permeability factor, its application to nascent cortical tissue caused an increase in a key physiologic protein of the blood-brain barrier function, glucose transporter-1, suggesting that the cytokine may have a role in blood-brain barrier development.

VEGF-induced neuroprotection, neurogenesis, and angiogenesis after focal cerebral ischemia

Vascular endothelial growth factor (VEGF) is an angiogenic protein with therapeutic potential in ischemic disorders, including stroke. VEGF confers neuroprotection and promotes neurogenesis and cerebral angiogenesis, but the manner in which these effects may interact in the ischemic brain is poorly understood. We produced focal cerebral ischemia by middle cerebral artery occlusion for 90 minutes in the adult rat brain and measured infarct size, neurological function, BrdU labeling of neuroproliferative zones, and vWF-immunoreactive vascular profiles, without and with intracerebroventricular administration of VEGF on days 1-3 of reperfusion. VEGF reduced infarct size, improved neurological performance, enhanced the delayed survival of newborn neurons in the dentate gyrus and subventricular zone, and stimulated angiogenesis in the striatal ischemic penumbra, but not the dentate gyrus. We conclude that in the ischemic brain VEGF exerts an acute neuroprotective effect, as well as longer latency effects on survival of new neurons and on angiogenesis, and that these effects appear to operate independently. VEGF may, therefore, improve histological and functional outcome from stroke through multiple mechanisms.

Angiogenic and neurotrophic effects of vascular endothelial growth factor (VEGF165): studies of grafted and cultured embryonic ventral mesencephalic cells

The present series of experiments investigated the effects of vascular endothelial growth factor (VEGF165) on adult rat striatal cerebrovasculature and embryonic dopamine (DA) neuron allografts in a rat model of Parkinson's disease (PD). We examined VEGF165's ability to (1) alter the vascular network of the adult rat striatum, (2) influence the vascular growth of solid embryonic day 14 (E14) ventral mesencephalic (VM) grafts when placed into a VEGF-pretreated host striatum, (3) alter the function and survival of E14 VM grafts when transplanted into an adult DA-deleted striatum, and (4) influence cell survival and neurite growth in cultures of E14 VM cells. We demonstrate here that a single bolus injection of VEGF165 into the adult rat striatum significantly increases the amount of vasculature in the vicinity of the injection site in a delayed and transient manner when compared to saline controls. Transplanting solid E14 VM grafts into the VEGF165-pretreated striatum resulted in a homogeneous distribution of small blood vessels throughout the graft, a pattern that closely resembles mature adult vasculature. In contrast, grafts in the control condition contained a patchy distribution of heavily dilated vessels. Behavioral measurements indicate that VEGF pretreatment of the intrastriatal graft site accelerates recovery of amphetamine-induced rotational asymmetry in unilateral 6-OHDA lesioned rats. Unexpectedly, however, VEGF pretreatments failed to increase survival of tyrosine hydroxylase-immunoreactive (THir) neurons in the grafts. In contrast to this finding in vivo, adding VEGF165 to glial-reduced E14 rat VM cultures produced a fourfold increase in THir cell survival and a doubling in the length of THir neurites. We conclude that with the proper method of delivery, VEGF165 may prove to be one of several strategies necessary to significantly improve the survival and function of fetal VM tissue grafts.

Angiogenesis factors in gliomas: a new key to tumour therapy?

Angiogenesis, the formation of new blood vessels, is required for the growth and expansion of tumours. Gliomas, the most common brain tumours, are particularly highly vascularized and, therefore, serve as a model to elucidate the process of tumour angiogenesis and to investigate new anti-angiogenic therapies. This review describes the role of angiogenic factors in glioma angiogenesis and new strategies to inhibit glioma growth by application of anti-angiogenic substances. We focus on vascular endothelial growth factor (VEGF), but also examine the role of angiopoietin and pleiotropic factors such as platelet-derived growth factor (PDGF), pleiotrophin and transforming growth factor-beta (TGF-beta). Strategies to inhibit glioma growth by reducing the action of angiogenic factors, by the application of anti-angiogenic substances such as angiostatin or endostatin, or inactivation of endothelial cells, are discussed. These new anti-angiogenic therapies appear to have a high potential not only for the treatment of gliomas, but also of other tumours.

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 neuroprotection, neurogenesis, and angiogenesis after focal cerebral ischemia

Vascular endothelial growth factor (VEGF) is an angiogenic protein with therapeutic potential in ischemic disorders, including stroke. VEGF confers neuroprotection and promotes neurogenesis and cerebral angiogenesis, but the manner in which these effects may interact in the ischemic brain is poorly understood. We produced focal cerebral ischemia by middle cerebral artery occlusion for 90 minutes in the adult rat brain and measured infarct size, neurological function, BrdU labeling of neuroproliferative zones, and vWF-immunoreactive vascular profiles, without and with intracerebroventricular administration of VEGF on days 1-3 of reperfusion. VEGF reduced infarct size, improved neurological performance, enhanced the delayed survival of newborn neurons in the dentate gyrus and subventricular zone, and stimulated angiogenesis in the striatal ischemic penumbra, but not the dentate gyrus. We conclude that in the ischemic brain VEGF exerts an acute neuroprotective effect, as well as longer latency effects on survival of new neurons and on angiogenesis, and that these effects appear to operate independently. VEGF may, therefore, improve histological and functional outcome from stroke through multiple mechanisms.

Antiangiogenic therapy against experimental glioblastoma using genetically engineered cells producing interferon-alpha, angiostatin, or endostatin

Inhibition of angiogenesis has been considered among the most promising approaches to treat highly vascularized solid tumors such as glioblastoma. In this study, we designed and validated a new in vitro assay system based on the implantation of tumor cells into organotypic brain slice cultures. We evaluated the effects of local production of three endogenous inhibitors of angiogenesis, angiostatin, endostatin, and interferon (IFN)-alpha(1), using stably transfected rat (9L) and human (GL15) glioblastoma cells on tumor vascularization and growth. Despite similar effectiveness of the three proteins in a classic in vitro endothelial cell migration assay, IFN-alpha(1) demonstrated the most potent antiangiogenic effect in organotypic brain slice cultures. In vivo, after intracerebral implantation of such genetically modified glioblastoma cells, IFN-alpha(1) caused a dramatic decrease in tumor volume revealed by magnetic resonance imaging and by postmortem histology. The mechanisms of this antitumor effect were most likely caused by the major antiangiogenic action of the cytokine, because IFN-alpha(1) expression provoked a pronounced decrease in blood vessel density, which was accompanied by extensive necrosis in the body mass of the tumors. The median survival time of rats implanted intracerebrally with IFN-alpha-expressing 9L cells tripled, and was still significantly increased when these constituted only 1% of transplanted tumor cells. A similar effect was seen when 50% of the transplanted cells were replaced by IFN-alpha-expressing bone marrow stromal cells. These data point to the local delivery of IFN-alpha(1) using cell vectors as a potent tool for the inhibition of tumor-induced angiogenesis.

Inhibition of endogenous VEGF impedes revascularization and astroglial proliferation: roles for VEGF in brain repair

Vascular endothelial growth factor (VEGF) is upregulated following injury to the CNS. Our previous work has shown that exogenous application of VEGF promotes angiogenesis, blood-brain barrier permeability, and astroglial mitogenicity in the traumatized brain. To develop a model that could link endogenously secreted VEGF to brain tissue repair, a specific neutralizing antibody to VEGF was infused by osmotic minipump directly into the neocortex and striatum for up to 1 week. Tissues adjacent to the infusion/wound site were analyzed for specific vascular and astroglial protein markers and proliferation, necrosis/apoptosis (via TUNEL staining), VEGF, the VEGF receptors flt-1 and flk-1, and bFGF expression using immunohistochemistry and semi-quantitative RT-PCR. Neutralization of native VEGF caused significant decreases in angiogenic activity, astroglial proliferation, and nestin immunoexpression, while vascular and astroglial degeneration was substantially increased, resulting in much larger wound cavities when compared to controls. The hindrance of brain tissue repair occurred despite an increase in bFGF expression at the wound sites. VEGF appears to be an integral factor in CNS wound healing that is essential for vascular endothelial proliferation and survival and may also be necessary for astroglial proliferation and maintenance during the repair of brain injury.

Microvascular plasticity in aging

Understanding the bases of aging-related cognitive decline remains a central challenge in neurobiology. Quantitative studies reveal little change in the number of neurons or synapses in most of the brain but their ongoing replacement is reduced, resulting in a significant loss of neuronal plasticity with senescence. Aging also may alter neuronal function and plasticity in ways that are not evident from anatomical studies of neurons and their connections. Since the nervous system is dependent upon a consistent blood supply, any aging-related changes in the microvasculature could affect neuronal function. Several studies suggest that, as the nervous system ages, there is a rarefaction of the microvasculature in some regions of the brain, as well as changes in the structure of the remaining vessels. These changes contribute to a decline in cerebral blood flow (CBF) that reduces metabolic support for neural signaling, particularly when levels of neuronal activity are high. In addition to direct effects on the microvasculature, aging reduces microvascular plasticity and the ability of the vessels to respond appropriately to changes in metabolic demand. This loss of microvascular plasticity has significance beyond metabolic support for neuronal signaling, since neurogenesis in the adult brain is regulated coordinately with capillary growth.

Heterologous expression of human VEGF165 in rat brain: dose-dependent, heterogeneous effects on CBF in relation to vascular density and cross-sectional area

Vascular endothelial growth factor (VEGF) induces increased vessel permeability and formation of abnormal vessels. To investigate cerebral blood flow (CBF) during local overexpression of VEGF recombinant adenoviruses carrying the human VEGF165 complementary DNA (2.3 to 23. 108 pfu/mL) were injected stereotactically into the caudate nucleus of anesthetized rats. Saline and adenoviruses carrying the beta-galactosidase gene served as controls. Eleven days later (1) size and density of vessels were assessed in hematoxylin-eosin-stained sections, (2) vascular permeability was measured by intravenous Evans blue injections, and (3) local CBF (lCBF) was quantified using the iodo-[14C]antipyrine technique. Dose-dependent increases were found in (1) vessel density and size (only vessels >43 microm could be quantified morphologically), (2) Evans blue extravasation and brain edema formation, and (3) lCBF (up to eightfold). At medium doses, hyperemic areas and smaller areas of decreased lCBF were found. In low flow areas, vascular cross-sectional areas were increased 223-fold and vessel density up to 10-fold. In high flow areas, these parameters were increased 32-fold and up to 15-fold, respectively. Adenovirus mediated VEGF overexpression results in (1) increased vessel size and density, (2) areas of increased and of decreased flow, and (3) more and smaller vessels in high flow than in low flow areas. These results indicate a diverging flow pattern of newly formed vessels.

Temporal profile of angiogenesis and expression of related genes in the brain after ischemia

Angiogenesis is an intricately regulated phenomenon. Its mechanisms in the ischemic brain have not been clearly elucidated. The authors investigated expression of angiogenesis-related genes using a complementary DNA (cDNA) array method as well as Western blotting and immunohistochemistry, and compared these studies with a temporal profile of angiogenesis in mouse brains after ischemia. The number of vessels significantly increased 3 days after injury, and proliferating endothelial cells increased as early as 1 day. This means that angiogenesis occurs immediately after the injury. Ninety-six genes implicated in angiogenesis were investigated with a cDNA array study. It was found that 42, 29, and 13 genes were increased at 1 hour, 1 day, and 21 days, respectively. Most of the well-known angiogenic factors increased as early as 1 hour. Vessel-stabilizing factors such as thrombospondins also increased. At 1 day, however, thrombospondins decreased to lower levels than in the control, indicating a shift from vascular protection to angiogenesis. At 21 days, many genes were decreased, but some involved in tissue repair were newly increased. Western blotting and immunohistochemistry showed findings compatible with the cDNA array study. Many molecules act in an orchestrated fashion in the brain after ischemia and should be taken into account for therapeutic angiogenesis for stroke.

Dynamic changes in cerebral blood flow and angiogenesis after transient focal cerebral ischemia in rats. Evaluation with serial magnetic resonance imaging

BACKGROUND AND PURPOSE

Angiogenesis occurs after cerebral ischemia, but the relationship between angiogenesis and cerebral hemodynamic change is unknown. The aim of the present study was to investigate the relationship between ischemia-induced angiogenesis and hemodynamics in a well-defined 3-vessel occlusion model of the rat by using diffusion- (DWI), perfusion-, and T2-weighted MRI (T2WI).

METHODS

Rats were subjected to 60 minutes of transient middle cerebral artery occlusion or sham operation. DWI and T2WI were used to characterize the extent of the ischemic lesion from 4.5 hours to 14 days after reperfusion. A flow-sensitive alternating inversion recovery method and dynamic susceptibility contrast MRI were used to evaluate the temporal changes in relative cerebral blood flow (CBF) and cerebral blood volume (CBV), respectively. Rats were randomly selected and killed at each time point for investigation of vascular density and for hematoxylin-eosin staining.

RESULTS

Ischemic lesions developed in the ipsilateral cortex, as demonstrated by DWI and T2WI. CBF was significantly increased in the ipsilateral cortex, especially in the cortical outer layer from day 1 to day 14, and peaked on day 7 (P<0.05), while CBV was significantly increased on day 7 (P<0.01). The vascular density on the ipsilateral brain surface was gradually increased from day 1 to day 5, peaked on day 7, and then decreased on day 14. Histology study showed pannecrosis in the cortex from day 1 to day 5 and partial liquefaction of the necrotic tissues on days 7 and 14.

CONCLUSIONS

A delayed increase in both CBF and CBV is documented in the ipsilateral cortex after transient focal brain ischemia, and such an increase may be associated with angiogenesis.

Differential expression of protein tyrosine kinase genes during microglial activation

Protein tyrosine kinase (PTK) activity is abundant in microglia, but the PTKs that participate in their activation have not been identified. For these studies, we used three paradigms to characterize PTK expression during microglial activation: resting and activated microglia were bulk fractionated from the adult brain, cultured newborn microglia were treated with lipopolysaccharide (LPS) to model the transition from activated toward phagocytic microglia, and PTK expression was examined in activated microglia in situ after facial nerve axotomy. Two PCR-based strategies were used to show that 21 different PTK genes are expressed by rat brain microglia: 5 receptor PTKs, 10 nonreceptor PTKs, and 6 members of the src family. Seven of the 21 PTKs were examined in greater detail. Five PTK mRNAs (fgr, hck, fak, jak-2, and flk-1) increased expression across all three models of activation. We conclude that they represent key components in the cascades that participate in microglial activation. In contrast, expression of fes and fms correlated with stimuli that affect microglial proliferation. Four of the PTKs (hck, fgr, fes, and fms) are believed to be myeloid cell specific and were not expressed by cultured astrocytes. HCK and FAK protein were also not expressed in lysates of immature astrocytes and oligodendrocytes. Because of their putative specificity, these kinases represent potential targets for inhibitors of microglial activation. Because reactive microglia can exacerbate the severity of neurological diseases, the identification of specific kinases that participate in microglial activation represents an important advance toward the development of new therapeutics.

Cellular replacement therapy for Parkinson's disease--where we are today?

The concept of replacing lost dopamine neurons in Parkinson's disease using mesencephalic brain cells from fetal cadavers has been supported by over 20 years of research in animals and over a decade of clinical studies. The ambitious goal of these studies was no less than a molecular and cellular "cure" for Parkinson's disease, other neurodegenerative diseases, and spinal cord injury. Much research has been done in rodents, and a few studies have been done in nonhuman primate models. Early uncontrolled clinical reports were enthusiastic, but the outcome of the first randomized, double blind, controlled study challenged the idea that dopamine replacement cells can cure Parkinson's disease, although there were some significant positive findings. Were the earlier animal studies and clinical reports wrong? Should we give up on the goal? Some aspects of the trial design and implantation methods may have led to lack of effects and to some side effects such as dyskinesias. But a detailed review of clinical neural transplants published to date still suggests that neural transplantation variably reverses some aspects of Parkinson's disease, although differing methods make exact comparisons difficult. While the randomized clinical studies have been in progress, new methods have shown promise for increasing transplant survival and distribution, reconstructing the circuits to provide dopamine to the appropriate targets and with normal regulation. Selected promising new strategies are reviewed that block apoptosis induced by tissue dissection, promote vascularization of grafts, reduce oxidant stress, provide key growth factors, and counteract adverse effects of increased age. New sources of replacement cells and stem cells may provide additional advantages for the future. Full recovery from parkinsonism appears not only to be possible, but a reliable cell replacement treatment may finally be near.

VEGF analysis induced by endothelialized gas-plasma treated D,L-PLA scaffolds

PURPOSE

Vascular endothelial growth factor (VEGF) isoforms play different roles in the temporal sprouting of endothelial-lined vessels in a nude mouse peritoneal model as cells respond to nontreated control and gas-plasma-treated bioresorbable poly-D,L-lactide acid 3D scaffolds with human aortic endothelial cells (HAEC).

METHODS AND MATERIALS

Nude mice peritoneums were incubated with HAEC (CW = control; TW = gas-plasma treated) or polymer scaffolds (Cp = control; Tp = treated) for 12, 24 and 72 days. Cytoplasmic and nuclear protein fractions were isolated using NER, electrophoresized using NuPAGE-MES and analyzed by WesternBreeze Chemiluminescent.

RESULTS

Prominent VEGF bands included 28, 45 and 62 kDa; 52-kDa VEGF observed in cytoplasmic TW fractions contributed about 18.6% at 12 days, 20.0% at 24 days and 13.1% at 72 days of the total VEGF signal. Yet, it was only noted in CW at 72 days where it accounted for 6.9%. A unique 32-kDa band appeared in both Cp (24.6%) and Tp (18.3%). Significant differences between band densities occurred for cytoplasmic nuclear CW24- TW24 (P = .022), CW72-TW72 (P = .011) and, also, cytoplasmic Cp24-Tp24 (P = .038).

CONCLUSIONS

The temporal and spatial organization of the TW isoforms results in more angiogenesis.

Coordinated interaction of neurogenesis and angiogenesis in the adult songbird brain

Neurogenesis proceeds throughout life in the higher vocal center (HVC) of the adult songbird neostriatum. Testosterone induces neuronal addition and endothelial division in HVC. We asked if testosterone-induced angiogenesis might contribute importantly to HVC neuronal recruitment. Testosterone upregulated both VEGF and its endothelial receptor, VEGF-R2/Quek1/KDR, in HVC. This yielded a burst in local HVC angiogenesis. FACS-isolated HVC endothelial cells produced BDNF in a testosterone-dependent manner. In vivo, HVC BDNF rose by the third week after testosterone, lagging by over a week the rise in VEGF and VEGF-R2. In situ hybridization revealed that much of this induced BDNF mRNA was endothelial. In vivo, both angiogenesis and neuronal addition to HVC were substantially diminished by inhibition of VEGF-R2 tyrosine kinase. These findings suggest a causal interaction between testosterone-induced angiogenesis and neurogenesis in the adult forebrain.

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.

Intraventricular infusion of vascular endothelial growth factor promotes cerebral angiogenesis with minimal brain edema

OBJECTIVE

Therapeutic cerebral angiogenesis, i.e., using angiogenic factors to enhance collateral vessel formation within the central nervous system, is a potential method for cerebral revascularization. Vascular endothelial growth factor (VEGF) is a potent endothelial cell mitogen that also increases capillary permeability, particularly in ischemic tissue. The purpose of this study was to assess the angiogenic and capillary permeability effects of chronic intraventricular infusion of exogenous VEGF in nonischemic brain tissue, because many patients with impaired cerebrovascular reserve do not exhibit chronic cerebral ischemia.

METHODS

Recombinant human VEGF(165) was infused into the right lateral ventricle of rats at a rate of 1 microl/h for 7 days, at concentrations of 1 to 25 microg/ml, with osmotic minipumps. Control animals received vehicle only. Vessels were identified in laminin immunohistochemical analyses. Capillary permeability and brain edema were assessed with Evans blue extravasation, [(3)H]inulin permeability, and brain water content measurements.

RESULTS

Vessel density was dose-dependently increased by VEGF(165) infusions, with significant increases occurring in animals treated with 5 or 25 microg/ml, compared with control animals (P h 0.01). Significant enlargement of the lateral ventricles was observed for the highest-dose group but not for animals treated with other doses. Capillary permeability was assessed in animals treated with a dose of 5 microg/ml. An increase in capillary permeability in the diencephalon was identified with Evans blue extravasation and [(3)H]inulin permeability assessments; however, the brain water content was not significantly increased.

CONCLUSION

Chronic intraventricular infusions of VEGF(165) increased vascular density in a dose-dependent manner. There seems to be a therapeutic window, because infusion of VEGF(165) at a concentration of 5 microg/ml resulted in a significant increase in vessel density with minimal associated brain edema and no ventriculomegaly.

Morphological characteristics of the microvasculature in healing myocardial infarcts

Myocardial infarction (MI) is associated with an angiogenic response, critical for healing and cardiac repair. Using a canine model of myocardial ischemia and reperfusion, we examined the structural characteristics of the evolving microvasculature in healing MI. After 7 days of reperfusion, the infarcted territory was rich in capillaries and contained enlarged, pericyte-poor "mother vessels" and endothelial bridges. During scar maturation arteriolar density in the infarct increased, and a higher percentage of microvessels acquired a pericyte coat (60.4 +/- 6.94% after 28 days of reperfusion vs 30.17 +/- 3.65% after 7 days of reperfusion; p<0.05). The microvascular endothelium in the early stages of healing showed intense CD31/PECAM-1 and CD146/Mel-CAM immunoreactivity but weak staining with the Griffonia simplicifolia lectin I (GS-I). In contrast, after 28 days of reperfusion, most infarct microvessels demonstrated significant lectin binding. Our findings suggest that the infarct microvasculature undergoes a transition from an early phase of intense angiogenic activity to a maturation stage associated with pericyte recruitment and formation of a muscular coat. In addition, in the endothelium of infarct microvessels CD31 and CD146 expression appears to precede that of the specific sugar groups that bind the GS-I lectin. Understanding of the mechanisms underlying the formation and remodeling of the microvasculature after MI may be important in designing therapeutic interventions to optimize cardiac repair.

The role of oxidative stress in the pathophysiology of cerebrovascular lesions in Alzheimer's disease

Alzheimer's disease (AD) and stroke are two leading causes of age-associated dementia. A rapidly growing body of evidence indicates that increased oxidative stress from reactive oxygen radicals is associated with the aging process and age-related degenerative disorders such as atherosclerosis, ischemia/reperfusion, arthritis, stroke, and neurodegenerative diseases. New evidence has also indicated that vascular lesions are a key factor in the development of AD. This idea is based on a positive correlation between AD and cardiovascular and cerebrovascular diseases such as arterio- and atherosclerosis and ischemia/reperfusion injury. In this review we consider recent evidence supporting the existence of an intimate relationship between oxidative stress and vascular lesions in the pathobiology of AD. We also consider the opportunities for therapeutic interventions based on the molecular pathways involved with these causal relationships.

Collateral growth and angiogenesis around cortical stroke

BACKGROUND AND PURPOSE

We tested the hypothesis that there are significant long-term local vascular changes after ministroke that could form a basis for functional recovery.

METHODS

A 6- to 8-mm cranial window was opened over the barrel cortex, which was identified by an intrinsic optical signal during mechanical stimulation of the whiskers in anesthetized female Wistar rats. Branches of the middle cerebral artery (MCA) to this region were ligated. Fluorescein isothiocyanate (FITC) transits were recorded by videomicroscopy in each rat just before, immediately after, and 30 days after ligation. Changes in surface vessels and parenchymal perfusion were measured. In similarly prepared rats, angiogenesis was identified by 5-bromo-2-deoxyuridine labeling and immunohistochemistry for the integrin family member alpha(v)beta(3).

RESULTS

The intrinsic optical signal disappeared immediately after MCA ligations. FITC injection just after ligation demonstrated 3 concentric regions: 1 region of unchanged perfusion, surrounding 1 region of reduced perfusion (the ischemic border) surrounding a central core with little observable perfusion. At 30 days, the following had taken place: (1) diameters and lengths of surface collaterals in the ischemic border had grown significantly, but no new surface vessels were detected, (2) FITC entered occluded MCA segments, (3) arteriocapillary latencies in the ischemic border were shortened compared with latencies just after ligation, and (4) small infarcts were virtually identical to the poorly perfused core. Angiogenesis was confined to the ischemic border.

CONCLUSIONS

Arteriolar collateral growth and new capillaries support restored perfusion in the ischemic border after ministroke and could support long-term functional recovery.

Vascular growth factors in cerebral ischemia

During the past decade, there has been a surge of interest in growth factors (GFs) that act selectively on vascular endothelium and perivascular cells. Studies employing mutant mice or the administration of recombinant proteins have suggested that these factors not only mediate the proliferation of endothelial cells, but also regulate vascular differentiation, regression, and permeability. During and after cerebral ischemia, brain vasculature becomes leaky and unstable, and the normally impermeable blood-brain barrier breaks down. Several days after the ischemic insult, endothelial cells begin to proliferate, and angiogenesis occurs. Expression studies have shown that key vascular GFs are regulated, during these processes, in a complex and coordinated manner. The distinct pattern of regulation exhibited by each vascular GF suggests a unique role for each factor during the initial vascular destabilization and subsequent angiogenesis that occurs after cerebral ischemia. Data from studies in other biological systems support these suggested roles. Thus, manipulation of vascular GFs may prove to be an effective means of stabilizing or enriching brain vasculature after ischemia, and ameliorating the detrimental effects of blood-brain barrier breakdown and vessel regression after stroke.

Induction of vascular endothelial growth factor and hypoxia-inducible factor-1alpha by global ischemia in rat brain

Vascular endothelial growth factor is an angiogenic and neurotrophic peptide whose expression is transcriptionally induced in hypoxic tissues through the action of hypoxia-inducible factor-1alpha. To determine if this signaling pathway is activated in the ischemic brain, and might therefore participate in adaptive processes such as angiogenesis and neuroprotection, we examined the expression of vascular endothelial growth factor and hypoxia-inducible factor-1alpha in cerebral cortex and hippocampus following transient global cerebral ischemia in the rat. Northern analysis showed ischemia-inducible expression of multiple vascular endothelial growth factor messenger ribonucleic acid splice variants between 4 and 24h. Western analysis and immunocytochemistry demonstrated the concerted induction of vascular endothelial growth factor and hypoxia-inducible factor-1alpha in the same, apparently neuronal, cells in vulnerable regions of cortex and hippocampus after 15min of ischemia, which persisted for as long as 4 to 72h of reperfusion. These findings demonstrate that hypoxia-sensitive vascular endothelial growth factor signaling can be induced in neurons in global cerebral ischemia in vivo, and are consistent with the hypothesis that ischemic insults trigger hypoxia-sensing and adaptive downstream molecular responses in central neurons.

Induction of vascular endothelial growth factor receptors and phosphatidylinositol 3'-kinase/Akt signaling by global cerebral ischemia in the rat

Vascular endothelial growth factor is an angiogenic peptide that binds to tyrosine kinase receptors on target cells to activate signal transduction pathways involving phosphatidylinositol 3'-kinase and the serine-threonine protein kinase, Akt. To determine whether this signaling pathway is activated in cerebral ischemia, we examined the expression of vascular endothelial growth factor receptors 1 and 2, and phosphatidylinositol 3'-kinase-activated phospho-Akt, in the cerebral cortex and hippocampus following transient global cerebral ischemia in the rat. Western blot analysis and immunocytochemistry demonstrated induction of vascular endothelial growth factor receptor 1 and 2 expression, and of anti-phosphatidylinositol 3'-kinase-immunoprecipitated phospho-Akt, in vulnerable regions of the cortex and hippocampus after 15 min of global ischemia and 4-72 h of reperfusion. These findings demonstrate that vascular endothelial growth factor receptors and receptor-coupled signal transduction pathways are induced in ischemic brain in vivo, and could therefore participate in endogenous neuroprotective responses to ischemia.

Induction of VEGF and VEGF receptors in the spinal cord after mechanical spinal injury and prostaglandin administration

Vascular endothelial growth factor (VEGF) is an angiogenetic factor that promotes endothelial cell proliferation during development and after injury to various types of tissue, including the central nervous system (CNS). Using immunohistochemical and in situ hybridization methods we have here demonstrated that VEGF and its receptors Flk-1, Flt-1 and Neuropilin-1 mRNAs and proteins are induced after incisions in the rat spinal cord. The inducible enzyme for prostaglandin synthesis cyclooxygenase-2 (COX-2) is known to be upregulated after spinal injury, cerebral ischemia and to stimulate angiogenesis. To test the hypothesis that prostaglandins may be involved in the VEGF response after lesion we investigated whether intraspinal microinjections of prostaglandin F2alpha (PGF2alpha) alters VEGF expression in the spinal cord. Such treatment was followed by a strong upregulation of VEGF mRNA and protein in the injection area. Finally, by use of an in vitro model with cell cultures of meningeal fibroblast and astrocyte origin, resembling the lesion area cellular content after spinal cord injury but devoid of inflammatory cells, we showed that VEGF is expressed in this in vitro model cell system after treatment with PGF2alpha and prostaglandin E2 (PGE2). These data suggest that cells within a lesion area in the spinal cord are capable of expressing VEGF and its receptors in response to mechanical injury and that prostaglandins may induce VEGF expression in such cells, even in the absence of inflammatory cells.

VEGF enhances angiogenesis and promotes blood-brain barrier leakage in the ischemic brain

VEGF is a secreted mitogen associated with angiogenesis and is also a potent vascular permeability factor. The biological role of VEGF in the ischemic brain remains unknown. This study was undertaken to investigate whether VEGF enhances cerebral microvascular perfusion and increases blood-brain barrier (BBB) leakage in the ischemic brain. Using magnetic resonance imaging (MRI), three-dimensional laser-scanning confocal microscope, and functional neurological tests, we measured the effects of administrating recombinant human VEGF(165) (rhVEGF(165)) on angiogenesis, functional neurological outcome, and BBB leakage in a rat model of focal cerebral embolic ischemia. Late (48 hours) administration of rhVEGF(165) to the ischemic rats enhanced angiogenesis in the ischemic penumbra and significantly improved neurological recovery. However, early postischemic (1 hour) administration of rhVEGF(165) to ischemic rats significantly increased BBB leakage, hemorrhagic transformation, and ischemic lesions. Administration of rhVEGF(165) to ischemic rats did not change BBB leakage and cerebral plasma perfusion in the contralateral hemisphere. Our results indicate that VEGF can markedly enhance angiogenesis in the ischemic brain and reduce neurological deficits during stroke recovery and that inhibition of VEGF at the acute stage of stroke may reduce the BBB permeability and the risk of hemorrhagic transformation after focal cerebral ischemia.

Hypoxia-induced vascular endothelial growth factor expression precedes neovascularization after cerebral ischemia

We investigated the hypothesis that hypoxia induces angiogenesis and thereby may counteract the detrimental neurological effects associated with stroke. Forty-eight to seventy-two hours after permanent middle cerebral artery occlusion we found a strong increase in the number of newly formed vessels at the border of the infarction. Using the hypoxia marker nitroimidazole EF5, we detected hypoxic cells in the ischemic border of the neocortex. Expression of vascular endothelial growth factor (VEGF), which is the main regulator of angiogenesis and is inducible by hypoxia, was strongly up-regulated in the ischemic border, at times between 6 and 24 hours after occlusion. In addition, both VEGF receptors (VEGFRs) were up-regulated at the border after 48 hours and later in the ischemic core. Finally, the two transcription factors, hypoxia-inducible factor-1 (HIF-1) and HIF-2, known to be involved in the regulation of VEGF and VEGFR gene expression, were increased in the ischemic border after 72 hours, suggesting a regulatory function for these factors. These results strongly suggest that the VEGF/VEGFR system, induced by hypoxia, leads to the growth of new vessels after cerebral ischemia. Exogenous support of this natural protective mechanism might lead to enhanced survival after stroke.

Improved vascularization of planar membrane diffusion devices following continuous infusion of vascular endothelial growth factor

Improving blood vessel formation around an immunobarrier device should improve the survival of the encapsulated tissue. In the present study we investigated the formation of new blood vessels around a planar membrane diffusion device (the Baxter Theracyte System) undergoing a continuous infusion of vascular endothelial growth factor through the membranes and into the surrounding tissue. Each device (20 microl) had both an inner immunoisolation membrane and an outer vascularizing membrane. Human recombinant vascular endothelial growth factor-165 was infused at 100 ng/day (low dose: n = 6) and 500 ng/day (high dose: n = 7) for 10 days into devices implanted s.c. in Sprague-Dawley rats; noninfused devices transplanted for an identical period were used as controls (n = 5). Two days following the termination of VEGF infusion, devices were loaded with 20 microl of Lispro insulin (1 U/kg) and the kinetics of insulin release from the lumen of the device was assessed. Devices were then explanted and the number of blood vessels (capillary and noncapillary) was quantified using morphometry. High-dose vascular endothelial growth factor infusion resulted in two- to threefold more blood vessels around the device than that obtained with the noninfused devices and devices infused with low-dose vascular endothelial growth factor. This increase in the number of blood vessels was accompanied by a modest increase in insulin diffusion from the device in the high-dose vascular endothelial growth factor infusion group. We conclude that vascular endothelial growth factor can be used to improve blood vessel formation adjacent to planar membrane diffusion devices.