Vascular endothelial growth factor receptor 3 directly regulates murine neurogenesis

Neural stem cell survival factors

Neural stem and progenitor cells (NSCs and NPs) give rise to the central nervous system (CNS) during embryonic development. NSCs and NPs differentiate into three main cell-types of the CNS; astrocytes, oligodendrocytes, and neurons. NSCs are present in the adult CNS and are important in maintenance and repair. Adult NSCs hold great promise for endogenous or self-repair of the CNS. Intriguingly, NSCs have been implicated as the cells that give rise to brain tumors. Thus, the balance between survival, growth and differentiation is a critical aspect of NSC biology, during development, in the adult, and in disease processes. In this review, we survey what is known about survival factors that control both embryonic and adult NSCs. We discuss the neurosphere culture system as this is widely used to measure NSC activity and behavior in vitro and emphasize the importance of clonality. We define here NSC survival factors in their broadest sense to include any factor that influences survival and proliferation of NSCs and NPs. NSC survival factors identified to date include growth factors, morphogens, proteoglycans, cytokines, hormones, and neurotransmitters. Understanding NSC and NP interaction in response to these survival factors will provide insight to CNS development, disease and repair.

Induction of vascular endothelial growth factor receptor-3 expression in perivascular cells of the ischemic core following focal cerebral ischemia in rats

Vascular endothelial growth factor receptor (VEGFR)-3, a receptor for VEGF-C and VEGF-D, has recently been reported to be induced within vessel-like structures in the ischemic brain. The purpose of the present study was to characterize and define further the cellular phenotypes of vascular-associated cells that manifest induced VEGFR-3 expression in a rat model of ischemic stroke. Vessel-associated cells expressing VEGFR-3 were found to be perivascular astrocytes in the peri-infarct region, whereas in the ischemic core, where astrocytes had virtually disappeared, induction of VEGFR-3 mRNA and protein was still prominent in vascular structures 3-7 days after reperfusion. VEGFR-3 and nestin expression were colocated in almost all cells associated with the vasculature in the ischemic core, and most (~82%) of the VEGFR-3/nestin double-labeled cells were proliferative. A subpopulation of these VEGFR-3-expressing cells appeared to be included in two immunophenotypically distinct perivascular cells: NG2-positive pericytes and ED2- or OX6-perivascular macrophages. However, most of these cells did not show markers for vasculature-associated cell types such as endothelial cells, microglia/macrophages, and smooth muscle cells. Thus, our data indicated that vasculature-associated VEGFR-3-expressing cells in the ischemic core may represent a heterogeneous population of cells with functional diversity, rather than a uniform cell type.

Peripheral vascular endothelial growth factor level is associated with antidepressant treatment response: results of a preliminary study

BACKGROUND

Recent investigations have revealed multiple actions of vascular endothelial growth factor (VEGF) in the nervous system. The role of VEGF in the molecular background of mood disorders has also been proposed. In this study we were interested in investigating a possible association between VEGF levels and treatment response in patients with a current episode of major depression (MDE).

METHODS

34 patients with MDE were enrolled in our study. Depressive symptoms were monitored by the Montgomery-Åsberg Depression Rating Scale at baseline (V(1)) and after a 4-week treatment period (V(2)). Patients with less than a 50% improvement in MADRS total scores during this period were regarded as non-responders.

RESULTS

Plasma VEGF levels did not change during the treatment period in either the total sample or in the responder and non-responder subsamples. There was a strong trend for higher baseline VEGF levels in the non-responder group than in the responder group (p=0.055) and this difference-as a weak trend-was still detectable at the end of the treatment period (p=0.097). Regression analysis revealed that the baseline VEGF level was a significant predictor for the endpoint MADRS score (p=0.02).

LIMITATIONS

Sample size was relatively small; sample consists of both patients with MDD and bipolar disorder.

CONCLUSIONS

Our preliminary results raise the possibility that baseline levels of peripheral VEGF may predict treatment response in patients with mood disorders. Considering the limitations of our study, further investigations should resolve whether VEGF is a useful biomarker for treatment response in depression in clinical practice.

Regulation of endogenous neural stem/progenitor cells for neural repair-factors that promote neurogenesis and gliogenesis in the normal and damaged brain

Neural stem/precursor cells in the adult brain reside in the subventricular zone (SVZ) of the lateral ventricles and the subgranular zone (SGZ) of the dentate gyrus in the hippocampus. These cells primarily generate neuroblasts that normally migrate to the olfactory bulb (OB) and the dentate granule cell layer respectively. Following brain damage, such as traumatic brain injury, ischemic stroke or in degenerative disease models, neural precursor cells from the SVZ in particular, can migrate from their normal route along the rostral migratory stream (RMS) to the site of neural damage. This neural precursor cell response to neural damage is mediated by release of endogenous factors, including cytokines and chemokines produced by the inflammatory response at the injury site, and by the production of growth and neurotrophic factors. Endogenous hippocampal neurogenesis is frequently also directly or indirectly affected by neural damage. Administration of a variety of factors that regulate different aspects of neural stem/precursor biology often leads to improved functional motor and/or behavioral outcomes. Such factors can target neural stem/precursor proliferation, survival, migration and differentiation into appropriate neuronal or glial lineages. Newborn cells also need to subsequently survive and functionally integrate into extant neural circuitry, which may be the major bottleneck to the current therapeutic potential of neural stem/precursor cells. This review will cover the effects of a range of intrinsic and extrinsic factors that regulate neural stem/precursor cell functions. In particular it focuses on factors that may be harnessed to enhance the endogenous neural stem/precursor cell response to neural damage, highlighting those that have already shown evidence of preclinical effectiveness and discussing others that warrant further preclinical investigation.

Nerve growth factor-mediated vascular endothelial growth factor expression of astrocyte in retinal vascular development

The angiogenic aspect of neurotrophins and their receptors rather than the neuroscientific aspect has been focused. However, their role in retinal vascular development is underdiscovered. The purpose of this study is to understand the role of neurotrophin receptors in retinal vascular development and the mechanisms of their action. To identify the expression of tropomyosin receptor kinase receptor (Trk) in developing retina, tissues of 4, 8, 12, 16 and 26 day-old mice were prepared for experiments. Immunohistochemistry and immunofluorescence double staining against glial fibrillary acidic protein and type IV collagen were performed. TrkA was expressed mainly along the vessel structure in inner part of retina, especially in retinal astrocyte. In cultured primary astrocyte, recombinant nerve growth factor (NGF) was used to activate TrkA. NGF induced the phosphorylation of TrkA, and it also enhanced the level of activated Akt and vascular endothelial growth factor (VEGF) mRNA. Inhibition of phosphoinositide 3-kinase (PI3K) reversed the NGF-induced activation of these two molecules. This study demonstrated that TrkA activation on NGF leads to VEGF elevation by PI3K-Akt pathway and therefore suggested that TrkA could be a stimulator of retinal vascular development.

Molecular parallels between neural and vascular development

The human central nervous system (CNS) features a network of ~400 miles of blood vessels that receives >20% of the body's cardiac output and uses most of its blood glucose. Many human diseases, including stroke, retinopathy, and cancer, are associated with the biology of CNS blood vessels. These vessels originate from extrinsic cell populations, including endothelial cells and pericytes that colonize the CNS and interact with glia and neurons to establish the blood-brain barrier and control cerebrovascular exchanges. Neurovascular interactions also play important roles in adult neurogenic niches, which harbor a unique population of neural stem cells that are intimately associated with blood vessels. We here review the cellular and molecular mechanisms required to establish the CNS vascular network, with a special focus on neurovascular interactions and the functions of vascular endothelial growth factors.

VEGF-dependent continuous angiogenesis in the median eminence of adult mice

Brain vasculature forms the blood-brain barrier (BBB) that restricts the movement of molecules between the brain and blood, but the capillary of the median eminence (ME) lacks the BBB for secretion of adenohypophysial hormone-releasing peptides. In the present study, we aimed to elucidate whether continuous angiogenesis occurs in the ME of adult mice. By using a mitotic marker, bromodeoxyuridine (BrdU), we demonstrated that new endothelial cells were born continuously in the ME of adults. Prominent expression of NG2, platelet-derived growth factor receptor B (PDGFRB), and delta-like ligand 4 was observed at pericytes of adults, although the expression of these angiogenesis-associated proteins has been shown to be at low or trace levels in adult mature capillary. In addition, vascular endothelial growth factor (VEGF), a key regulator of angiogenesis, was expressed highly in the nervous parenchyma of the ME. Expression of VEGF receptor 2 (VEGFR2) was observed at endothelial cells in the external zone and at somatodendrites in the internal zone. Finally, a VEGFR- and PDGFR-associated tyrosine kinase inhibitor, SU11248, significantly decreased the number of BrdU-positive proliferating endothelial cells and parenchyma cells. In conclusion, the present study demonstrates VEGF-dependent continuous angiogenesis in the ME of adult mouse brains under normal conditions, which provides new insight into our understanding of neurosecretion in the ME.

FLT4/VEGFR3 and Milroy disease: novel mutations, a review of published variants and database update

Milroy disease (MD) is an autosomal dominantly inherited primary lymphedema. In 1998, the gene locus for MD was mapped to 5q35.3 and variants in the VEGFR3 (FLT4) gene, encoding vascular endothelial growth factor receptor 3 (VEGFR3), were identified as being responsible for the majority of MD cases. Several reports have since been published detailing pathogenic FLT4 mutations. To date, a total of 58 different variants in FLT4, 20 of which are unpublished, have been observed in 95 families with MD. A review of published mutations is presented in this update. Furthermore, the unpublished variants are presented including clinical data. Comparison of clinical features in patients and their families with the same mutations reveals incomplete penetrance and variable expression, making genotype-phenotype correlations difficult. Most mutations are missense, but a few deletions and one splicing variant have also been reported. Several animal models have confirmed the role of VEGFR3 in lymphangiogenesis and studies show mutant VEGFR3 receptors are not phosphorylated. Here, an MD patient with the same p.Ile1053Phe change as seen in the Chy mouse is presented for the first time. This finding confirms that this mouse lineage is an excellent model for MD. All the data reviewed here has been submitted to a database based on the Leiden Open (source) Variation Database (LOVD) and is accessible online at www.lovd.nl/flt4.

Upregulation of vascular endothelial growth factor receptor-3 in the spinal cord of Lewis rats with experimental autoimmune encephalomyelitis

We investigated the spatiotemporal expression of vascular endothelial growth factor receptor-3 (VEGFR-3) in the spinal cord of Lewis rats with experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis. VEGFR-3 mRNA and protein were constitutively expressed in gray matter neurons and in a few white matter astrocytes. Induction of VEGFR-3 occurred predominantly in perivascular infiltrated macrophages in the spinal cord white matter during the inductive phase of EAE. VEGFR-3 expression was also induced in activated microglial cells in the gray and white matter, mainly in the peak phase. In addition, reactive astrocytes in the white matter, but not in the gray matter, expressed VEGFR-3 as disease severity increased. These data suggest that VEGFR-3 is involved in the recruitment of monocytic macrophages and in glial reactions during EAE.

Semaphorin3A, Neuropilin-1, and PlexinA1 are required for lymphatic valve formation

RATIONALE

The lymphatic vasculature plays a major role in fluid homeostasis, absorption of dietary lipids, and immune surveillance. Fluid transport depends on the presence of intraluminal valves within lymphatic collectors. Defective formation of lymphatic valves leads to lymphedema, a progressive and debilitating condition for which curative treatments are currently unavailable. How lymphatic valve formation is regulated remains largely unknown.

OBJECTIVE

We investigated if the repulsive axon guidance molecule Semaphorin3A (Sema3A) plays a role in lymphatic valve formation.

METHODS AND RESULTS

We show that Sema3A mRNA is expressed in lymphatic vessels and that Sema3A protein binds to lymphatic valves expressing the Neuropilin-1 (Nrp1) and PlexinA1 receptors. Using mouse knockout models, we show that Sema3A is selectively required for lymphatic valve formation, via interaction with Nrp1 and PlexinA1. Sema3a(-/-) mice exhibit defects in lymphatic valve formation, which are not due to abnormal lymphatic patterning or sprouting, and mice carrying a mutation in the Sema3A binding site of Nrp1, or deficient for Plxna1, develop lymphatic valve defects similar to those seen in Sema3a(-/-) mice.

CONCLUSIONS

Our data demonstrate an essential direct function of Sema3A-Nrp1-PlexinA1 signaling in lymphatic valve formation.

Diverse roles for VEGF-A in the nervous system

Vascular endothelial growth factor A (VEGF-A) is best known for its essential roles in blood vessel growth. However, evidence has emerged that VEGF-A also promotes a wide range of neuronal functions, both in vitro and in vivo, including neurogenesis, neuronal migration, neuronal survival and axon guidance. Recent studies have employed mouse models to distinguish the direct effects of VEGF on neurons from its indirect, vessel-mediated effects. Ultimately, refining our knowledge of VEGF signalling pathways in neurons should help us to understand how the current use of therapeutics targeting the VEGF pathway in cancer and eye disease might be expanded to promote neuronal health and nerve repair.

Vascular endothelial growth factor regulates adult hippocampal cell proliferation through MEK/ERK- and PI3K/Akt-dependent signaling

Vascular endothelial growth factor (VEGF) is a hypoxia-induced angiogenic protein that exhibits a broad range of neurotrophic and neuroprotective effects in the central nervous system. Given that neurogenesis occurs in close proximity to blood vessels, increasing evidence has suggested that VEGF may constitute an important link between neurogenesis and angiogenesis. Although it is known that VEGF can directly stimulate the proliferation of neuronal progenitors, the underlying signaling pathways responsible in this process are not fully understood. Thus, in the present study, we set out to examine the requirement of two downstream targets of the VEGF/Flk-1 signaling network, the phosphatidylinositol 3-kinase (PI3K)/Akt and extracellular signal-regulated kinase (ERK) pathways, in producing the mitogenic effects of VEGF. Both in vivo and in vitro experiments showed that a single treatment of VEGF activated Erk1/2 and Akt signaling pathways in the adult rat hippocampus and in cultured hippocampal neuronal progenitor cells. This effect was blocked with the VEGF/Flk-1 inhibitor SU5416. Importantly, microinfusion of VEGF into the rat brain also induced pCREB expression in the dentate gyrus and increased the number of BrdU-labeled cells in the dentate subgranular zone. Double immunofluorescence labeling revealed that a large proportion of BrdU-labeled cells expressed activated forms of Flk-1, Erk1/2, and Akt. Interestingly, treatment with the SSRI fluoxetine, which is well known to stimulate neurogenesis and VEGF-signaling, also produced a similar expression pattern of Erk1/2 and Akt in proliferating cells. Finally, pharmacological experiments showed that administration of inhibitors of either MAPK/ERK (U0126) or PI3K (LY294002) blocked VEGF-stimulation of hippocampal cell proliferation in vivo and in vitro. Taken together, our findings demonstrate that the proliferative actions of VEGF require activation of both ERK and Akt signaling cascades and that these intracellular pathways are stimulated almost exclusively in actively proliferating neuronal progenitor cells of the adult hippocampus.

VEGF signaling inside vascular endothelial cells and beyond

Vascular endothelial growth factor-A (VEGF-A) has long been recognized as the key regulator of vascular development and function in health and disease. VEGF is a secreted polypeptide that binds to transmembrane tyrosine kinase VEGF receptors on the plasma membrane, inducing their dimerization, activation and assembly of a membrane-proximal signaling complex. Recent studies have revealed that many key events of VEGFR signaling occur inside the endothelial cell and are regulated by endosomal receptor trafficking. Plasma membrane VEGFR interacting molecules, including vascular guidance receptors Neuropilins and Ephrins also regulate VEGFR endocytosis and trafficking. VEGF signaling is increasingly recognized for its roles outside of the vascular system, notably during neural development, and blood vessels regulate epithelial branching morphogenesis. We review here recent advances in our understanding of VEGF signaling and its biological roles.

Betacellulin promotes cell proliferation in the neural stem cell niche and stimulates neurogenesis

Neural stem cells (NSCs) reside in specialized niches in the adult mammalian brain, including the subventricular zone and the dentate gyrus, which act to control NSC behavior. Among other cell types within these niches, NSCs are found in close proximity to blood vessels. We carried out an analysis of the interaction between endothelial cells and NSCs, and show that betacellulin (BTC), a member of the EGF family and one of several signaling molecules made by the former, induces NSC proliferation and prevents spontaneous differentiation in culture. When infused into the lateral ventricle, BTC induces expansion of NSCs and neuroblasts, and promotes neurogenesis in the olfactory bulb and dentate gyrus, whereas specific blocking antibodies reduce the number of stem/progenitor cells. BTC-null mice are less able to regenerate neuroblast numbers compared with WT littermates following depletion of proliferating cells using cytosine-β-d-arabinofuranoside. BTC acts via both the EGF receptor, located on NSCs, and ErbB4, located on neuroblasts, with the latter explaining why its effects are distinct from those of EGF itself. Our results suggest that BTC could be a good candidate to aid regenerative therapies.

Mechanoinduction of lymph vessel expansion

In the mammalian embryo, few mechanical signals have been identified to influence organ development and function. Here, we report that an increase in the volume of interstitial or extracellular fluid mechanically induces growth of an organ system, that is, the lymphatic vasculature. We first demonstrate that lymph vessel expansion in the developing mouse embryo correlates with a peak in interstitial fluid pressure and lymphatic endothelial cell (LEC) elongation. In 'loss-of-fluid' experiments, we then show that aspiration of interstitial fluid reduces the length of LECs, decreases tyrosine phosphorylation of vascular endothelial growth factor receptor-3 (VEGFR3), and inhibits LEC proliferation. Conversely, in 'gain-of-fluid' experiments, increasing the amount of interstitial fluid elongates the LECs, and increases both VEGFR3 phosphorylation and LEC proliferation. Finally, we provide genetic evidence that β1 integrins are required for the proliferative response of LECs to both fluid accumulation and cell stretching and, therefore, are necessary for lymphatic vessel expansion and fluid drainage. Thus, we propose a new and physiologically relevant mode of VEGFR3 activation, which is based on mechanotransduction and is essential for normal development and fluid homeostasis in a mammalian embryo.

Molecular control of endothelial cell behaviour during blood vessel morphogenesis

The vertebrate vasculature forms an extensive branched network of blood vessels that supplies tissues with nutrients and oxygen. During vascular development, coordinated control of endothelial cell behaviour at the levels of cell migration, proliferation, polarity, differentiation and cell-cell communication is critical for functional blood vessel morphogenesis. Recent data uncover elaborate transcriptional, post-transcriptional and post-translational mechanisms that fine-tune key signalling pathways (such as the vascular endothelial growth factor and Notch pathways) to control endothelial cell behaviour during blood vessel sprouting (angiogenesis). These emerging frameworks controlling angiogenesis provide unique insights into fundamental biological processes common to other systems, such as tissue branching morphogenesis, mechanotransduction and tubulogenesis.

Vascular damage in the central nervous system: a multifaceted role for vascular-derived TGF-β

The brain function depends on a continuous supply of blood. The blood-brain barrier (BBB), which is formed by vascular cells and glia, separates components of the circulating blood from neurons and maintains the precisely regulated brain milieu required for proper neuronal function. A compromised BBB alters the transport of molecules between the blood and brain and has been associated with or shown to precede neurodegenerative disease. Blood components immediately leak into the brain after mechanical damage or as a consequence of a compromised BBB in brain disease changing the extracellular environment at sites of vascular damage. It is intriguing how blood-derived components alter the cellular and molecular constituents of the neurovascular interface after BBB opening. We recently identified an unexpected role for the blood protein fibrinogen, which is deposited in the nervous system promptly after vascular damage, as an initial scar inducer by promoting the availability of active TGF-β. Fibrinogen-bound latent TGF-β interacts with astrocytes, leading to active TGF-β formation and activation of the TGF-β/Smad signaling pathway. Here, we discuss the pleiotropic effects of potentially vascular-derived TGF-β on cells at the neurovascular interface and we speculate how these biological effects might contribute to degeneration and regeneration processes. Summarizing the effects of the components derived from the brain vascular system on nervous system regeneration might support the development of new therapeutic approaches.

Neural stem cell-based therapy for ischemic stroke

Stem cell-based approaches for the treatment of stroke have been the subject of intensive research over the past decade. Based on accumulated experimental evidence, stem cell-based therapy is a very promising prospect for the development of a novel treatment to restore stroke-damaged brain and impaired neurological function. Studies performed on experimental animal models of stroke employed a variety of stem cell types from diverse sources and have demonstrated their ability to replace lost neurons and functionally integrate into the brain, modulate inflammation, and stimulate angiogenesis and neurogenesis from an endogenous stem cell pool, most likely through trophic actions. A few clinical trials in stroke patients using stem cell transplantation have been completed or are on-going but the results have not yet proven the effectiveness of the stem cell-based approaches. A joint effort of stroke researchers and clinicians is needed to further optimize treatment protocols using safe and reproducible stem cell sources tested in relevant animal models of stroke and showing substantial neurological recovery of stroke-impaired function.