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Relationship of neural stem cells with their vascular niche: Implications in the malignant progression of gliomas
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Journal of Clinical Neuroscience 15 (2008) 1193–1197 www.elsevier.com/locate/jocn
Review
Relationship of neural stem cells with their vascular niche: Implications in the malignant progression of gliomas Kaveh Barami * Memorial Neuroscience Center, Memorial Hospital Jacksonville, 3625 University Boulevard South, Jacksonville, Florida 32216, USA Received 1 December 2007; accepted 8 January 2008
Abstract During embryogenesis and in regions of the adult brain undergoing post-natal neurogenesis, neural stem cells and endothelial precursors are found within a vascular niche, where the coordinated interactions between neurogenesis and vasculogenesis dictates development and responses to the environment. Moreover, recent evidence suggests that gliomas may arise from transformed neural stem cells and that angiogenesis is important in the malignant progression of these tumors. Taken together, these findings have led researchers to focus on the dynamic interaction between neural stem cells and their vascular niche so as to find new therapeutic strategies to halt the progression of gliomas. This review summarizes the cellular substrates responsible for the coordinated interactions between the nervous and vascular systems and how this relates to gliomagenesis. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Angiogenesis; Glioma; Neural stem cells; Neurogenesis; Vascular niche; Vasculogenesis
1. The developing and adult neurovascular unit There is compelling experimental evidence to suggest that there is a coordinated interaction between the nervous and vascular systems in the developing as well as the adult vertebrate brain.1 The interplay between these two parallel systems is responsible for creating a specialized microenvironment or ‘‘niche” in which neural stem cells reside, selfrenew, and differentiate. The neural tube gives rise to both nervous and vascular system components.2–4 Neurons and glia are derived from the neuroectodermal stem cells and cells destined for the vascular system are derived from the neural crest.2–4 Embryonic neuroectoderm vascularization is initiated by invasion of vascular sprouts from the perineural vascular plexus in the ventral neural tube. This process parallels the thickening of the proliferating neuroepithelium.5 Further evidence of the co-development of the two systems is demonstrated by endothelial cells and *
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neural stem cells being found in similar locations at various developmental stages in the neural germinal zones.6 The anatomic relationship between vasculogenesis and neurogenesis in the adult brain persists where brain blood vessels are intimately associated with the basal lamina of the subventricular zone, an area where new neurons are generated.7 Neurogenesis in the adult vertebrate brain is closely associated with active vascular recruitment. Palmer et al have demonstrated that in the adult rat hippocampus up to 37% of the cells proliferating in the subgranular zone are endothelial precursors and that neural precursors and angioblasts proliferate in clusters associated with the microvasculature of the subgranular zone,8 raising the possibility that circulating factors influence plasticity in the adult brain. Shen et al. have suggested a role for endothelial cells in the neurovascular unit. They found that endothelial cells secrete soluble factors, including Hes1 and Notch that act on the adult neural stem cells and stimulate neurogenesis.9 Louissaint et al. have shown that there is a coordinated interaction between neurogenesis and angiogenesis in the
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adult songbird brain. Testosterone not only modulates the integration and survival of newly generated neurons but also induces angiogenesis, upregulates vascular endothelial growth factor (VEGF) and its receptor and brain-derived neurotrophic factor (BDNF), all necessary for the recruitment of the new neurons in the neostriatum.10 In vitro studies have shown that the cross-talk between neural stem cells and endothelial cells seems to be mediated by VEGF and BDNF.11 The coordinated interaction between vasculogenesis and neurogenesis in the developing and adult brain may reflect a responsiveness of both systems to similar growth factors and receptors, including VEGF, neurotrophins, neuropilin, semaphorins, and ephrins/Eph molecules1 (Table 1). The maintenance of the neurovascular unit is dependant on soluble factors. Structural and ultrastructural studies on the subpendymal layer, the main site for adult neurogenesis, have revealed highly organized extravascular basal laminae, termed ‘‘fractones”, unique to this area.7 Fractones contact local blood vessels by their stems and also engulf astrocytes, ependymal cells, microglia and stem cells. This way, soluble factors such as cytokines and growth factors can exert an influence on the neurovascular unit. 2. Effect of hypoxia on angiogenesis and neurogenesis: Lessons learnt from ischemia Hypoxia increases cerebral blood flow, glucose consumption, and capillary density.12,13 Hypoxia-induced angiogenesis is mediated partly by the transcription factor hypoxia-inducible factor-1 (HIF-1), VEGF, angiopoietin1 or angiopoietin-2 (Ang1/Ang2), and cytochrome oxidase-2 (COX-2).1 This process is initiated with basement membrane breakdown by matrix metalloproteinases. Next, endothelial cells begin migrating and proliferating in response to angiogenic molecules such as Ang1 and VEGF. New capillaries are then formed and develop into functional vessels containing a lumen in which oxygen and blood
can flow through. Lastly, vascular pruning and regression occur, mediated by Ang2 in the absence of VEGF.1 Stroke leading to ischemia and hypoxia also induces neurogenesis in the adult mammalian brain.14 In rodents neurogenesis is enhanced bilaterally in the dentate gyrus and subventricular zone after global and focal experimental ischemia. Newly generated neurons migrate to the ischemic penumbra and penumbra of the ischemic cortex and differentiate into mature neurons of the host tissue, thus replacing lost neurons.14 Post-stroke neurogenesis is a dynamic process characterized by waves of newly generated neurons migrating to the ischemic areas and waves of cell death in the newly arrived neuroblast (young neuron) population.15 A conceptual model suggests that stroke and ischemia activate HIF-1 pathway, an early molecular signal after stroke in peri-infarct cortex. This induces endogenous production of the cytokine erythropoietin around the infarct. Neuroblasts respond to this ligand with their erythropoietin receptor. This interaction leads to neuroblast migration toward the peri-infarct cortex. Erythropoietin also stimulates angiogenesis near the infarct and promotes vascular production of growth factors that stimulate neurogenesis.15 Thus, the dynamic interplay between the vascular and neural systems seen in normal development is recapitulated in angiogenesis and neurogenesis ensuing an ischemic event. Fig. 1 shows the relevant anatomy, cell types, and their environment. 3. Angiogenesis and glioma formation The ‘‘angiogenic switch” refers to the transition of an avascular tumor to an angiogenic one and represents a distinct step in the malignant transformation of gliomas. Overexpression of angiogenic factors or hypoxia results in the production of certain growth factors and cytokines that have been shown to induce angiogenesis in gliomas.16 The VEGF family and their receptors seem to be the
Table 1 The coordinated effects of growth factors on vascular and nervous systems From: ‘‘The neurovascular unit and its growth factors: Coordinated response in the vascular and nervous systems” by: Ward and Lamanna. Neurological Research 2004; 26:870–83. Reproduced with permission Growth factor
Nervous system
Vascular system
Vascular endothelial growth factor (VEGF) Angiopoietin
Neurogenic and neuroprotective – stimulates neuron and axon growth Neurite outgrowth, neurite patterning
Semaphorin/neuropilin
Axon guidance, attraction and repulsion, repulsive growth cone guidance, neural patterning Neuronal survival and differentiation
Critical for angiogenesis, EC survival, proliferation, migration EC survival and migration, pericyte recruitment VEGF receptor, EC migration and proliferation EC survival , vessel stabilization
Neuronal survival and differentiation, neurite outgrowth Neuronal survival and differentiation Co-receptor for high affinity neurotrophin binding
Angiogenic Angiogenic Vascular tone, smooth muscle cell apoptosis
Guidance cues for growing axons, migration of neural crest cells
Angiogenesis, arterial versus venous identity
Brain derived neurotrophic factor (BDNF) Nerve growth factor (NGF) Neurotrophin-3 (NT-3) p75NGFR Low-affinity NGF receptor Ephs/ephrins EC, endothelial cell.
K. Barami / Journal of Clinical Neuroscience 15 (2008) 1193–1197
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Fig. 1. Neural stem cells and their vascular niche. Neural stem cells (NSCs) in the subventricular zone (SVZ) lie near the ependymal layer, which produces vascular endothelial growth factor (VEGF), at a vascular niche of numerous capillaries. A shortage of oxygen up-regulates VEGF levels, which stimulate both NSCs and capillaries to grow, and enhance neuronal differentiation, axon outgrowth, and neuron survival. The favorable effect of endothelial cells on neurogenesis is, at least partly, mediated by the release of brain-derived neurotrophic factor (BDNF). BV, blood vessel. From: ‘‘Blood vessels and nerves: common signals, pathways and diseases”, by: Carmeliet P. Nature Reviews Genetics 2003; 4: 710–20. Reproduced with permission.
central mediators of glioma angiogenesis. Other important angiogenic molecules are basic fibroblast growth factor (bFGF), platelet-derived growth factor (PDGF), hepatocyte growth factor/scatter factor (SF) and the angiopoietins.16 VEGF induces endothelial cell migration, proliferation, and tube formation. Moreover, it stimulates endothelial nitric oxide synthase and thus further activates the angiogenic cascade. VEGF also acts as a survival factor for endothelial cells and increases microvascular leakiness causing extravasation of plasma proteins.16 VEGF mediates the key differential steps in glioma angiogenesis and is responsible for many pathological hallmarks seen in malignant gliomas. The VEGF receptors (VEGFR) are tyrosine kinase receptors and mediate intracellular signaling via the mitogen-activated protein (MAP) kinase pathway. VEGFR-1 and VEGFR-2 expression is increased after ischemia and in malignant gliomas.16 VEGF production is mainly triggered by hypoxia yet it is also stimulated by other vascular cytokines such as FGF and PDGF. It may also be constitutively expressed due to genetic alterations.16 Glioma cell lines and human gliomas have been shown to express high levels of FGF mRNA. FGF has also been shown to stimulate endothelial cell migration, proliferation, sprouting, and tube formation.16 PDGF has a direct role in angiogenesis and its expression has also been correlated to glial malignancy.16 Angiopoietins regulate endothelial cell survival and blood vessel maturation. During glioma angiogenesis, angiopoietins and VEGF receptors are induced in tumor endothelial cells leading to increased blood vessel permeability, loss of blood brain barrier function, microvascular dilation, and sprout formation.16 Lastly, hepatocyte growth factor expression has been associated with increased malignancy and vascularity yet its exact contribution to glioma angiogenesis remains to be elucidated.16
4. Relationship of gliomas with neural stem cells and endothelial cells Recent compelling evidence suggests that gliomas may arise from transformed neural stem cells that reside in the walls of the lateral ventricular system.17 Astrocytes within this region, functioning as neural stem cells, self-renew and generate astrocytes, oligodendrocytes, and neurons throughout adult life. Neural stem cells behave similarly to glioma cells. Features common to both glioma cells and neural stem cells include expression of stem-cell markers, high motility, diversity of progeny, robust proliferation, and association with blood vessels and white matter tracts.17 Stem cells have also been isolated from gliomas.18 Gliomas frequently express the progenitor-cell marker nestin (an intermediate filament) and glioblastomas express higher nestin levels than less malignant ones.19 This finding suggests that gliomas share gene expression patterns with neural stem cells and may be derived from them. Despite data suggesting that neural stem cells may give rise to gliomas, the relationship of these cell types to gliomas remains controversial. As an example, endogenous neural precursor cells have also been shown to migrate from the SVZ towards the tumor in a murine experimental glioblastoma model.20 In this study, the stem cells exhibited a strong tropism for glioblastomas in vivo and in vitro. The precursor cells surrounded the tumors and their presence was associated with decreased tumor size and improved survival, suggesting an antitumorigenic response. Taken together, this study demonstrates the dual relation of SVZ stem cells to gliomas. Although neural stem cells have been shown to give rise to gliomas, beneficial effects have also been demonstrated by them. In contrast to normal endothelial cells, which appear small and plump, endothelial cells associated with high grade gliomas have a large, flat, and veil-like appearance. They are also resistant to cytotoxic drugs and undergo
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less apoptosis than control cells.21 Ultrastructural studies have shown that capillary endothelial cell proliferation in malignant gliomas are composed of immature capillaries with narrow slit-like lumens, high nuclear to cytoplasmic ratio, and paucity of organelles. They resemble capillary buds seen in normal repair tissue.22 Endothelial cells associated with high-grade gliomas proliferate more slowly and undergo less apoptosis than their normal counterparts; however, they migrate faster than them.23 Glioblastoma associated endothelial cells constitutively produce high levels of VEGF and endothelin-1 yet are less responsive to them compared to normal endothelial cells.23 Glioma-associated endothelial cells express senescenceassociated beta-galactosidase, a signature of an end stage of differentiation known as ‘‘replicative senescence”. Characteristics of these cell types include increased levels of cell cycle inhibitors p21 and p27, increased resistance to cytotoxic drugs, increased growth factor production, and inability to proliferate.24 Calabrese et al. have shown that treating mice bearing orthotopic xenografts of glioma cells with the anti-angiogenic drug, bevacizumab (Avastin; Genentech, South San Francisco, CA, USA), resulted in a drastic reduction of vessel-associated tumor stem cells without impacting the actual tumor cells.25 This suggests that antiangiogenic drugs arrest brain tumor growth by disrupting a vascular niche microenvironment that is critical for the maintenance of tumor stem cells.26 5. Hypoxia and gliomas Necrosis and vascular proliferation are pathologic hallmarks that distinguish malignant infiltrative gliomas from the lower grade forms.27 In glioblastomas, hypercellular zones called pseudopalisades typically surround necrotic foci and secrete proangiogenic factors, further promoting tumor growth.28 At least a subset of cells that comprise pseudopalisades represent tumor cells migrating away from a vaso-occlusive event.28 The hypoxic microenvironment of high grade gliomas renders the tumors more resistant to cancer therapies and rescues endothelial cells from apoptosis induced by hypoxia.29 Activation of the HIF-1 pathway is a common feature of gliomas and most likely explains the intense vascular hyperplasia seen in high grade tumors.30 HIF-1 is one of the master regulators that orchestrates the cellular response to hypoxia and has a central role in promoting proangiogenic and invasive properties in gliomas.31 It is a heterodimeric transcription factor composed of alpha and beta subunits. The alpha subunit is stable in hypoxic conditions but is rapidly degraded in normoxia. Upon stabilization, HIF-1 translocates to the nucleus and induces transcription of its downstream target genes. Activation of HIF-1 results in the activation of VEGF and their receptors, PDGF-B, matrix metalloproteinases, plasminogen activator inhibitor, transforming growth factors, angiopoietin, endothelin-1, inducible nitric oxide synthase, adrenomedullin,
and erythropoietin, all factors capable of promoting angiogenesis and invasion.30 Silencing HIF-1 alpha inhibits cell migration and invasion in a hypoxic environment in malignant gliomas.31 Fujiwara et al. showed that introducing HIF-1 alpha-targeted small interfering RNA into glioma cell lines resulted in significant reduction of migration and invasiveness of the cells. This response was associated with downregulation of matrix metalloproteinases (MMP-2 and MMP-9), which are activated by hypoxia.31 This suggests that targeting the HIF-1 alpha molecule might be a novel therapeutic strategy for malignant gliomas by reducing cell motility through alteration of invasion-related molecules. Angiopoietins are crucial in angiogenesis in the developing nervous system and are also important in hypoxia-induced angiogenesis in gliomas. The angiopoietins Ang1 and Ang2 are ligands for the tyrosine kinase receptor expressed by endothelial cells (TIE2).32 Ang1 enhances vascular stability mainly by pericyte recruitment33 and Ang2 acts as an antagonist of Ang1 and leads to vessel destabilization and thus neovascularization in the presence of VEGF.34 Ang1 has been detected in the periphery of gliomas and is absent in perinecrotic areas where hypoxia down-regulates Ang1 expression and upregulates Ang2 expression. Ang2 is highly expressed in perinecrotic areas and the tumor periphery in endothelial cells as well as glioma cells.34 This pattern of expression suggests that Ang1 modulates angiogenesis in association with Ang2, and Ang2 is involved in vessel regression and neovascularization in the presence of VEGF.35 6. Conclusions The dynamic interplay between the vascular and nervous systems is responsible for creating a neurovascular unit that acts as a niche for maintaining neural stem cells. These interactions also seem to be operative in the malignant progression of gliomas. Elucidation of mechanisms underlying the balance of capillary density, cellular activity, and oxygen status might lead to the development of new therapeutic strategies against these deadly tumors. References 1. Ward NL, Lamanna JC. The neurovascular unit and its growth factors: coordinated response in the vascular and nervous systems. Neurol Res 2004;26:870–83. 2. Osterfield M, Kirschner MW, Flanagan JG. Graded positional information:interpretation for both fate and guidance. Cell 2003;113:425–8. 3. Panchision DM, McKay RD. The control of neural stem cells by morphogenic signals. Curr Opin Genet Dev 2002;12:478–87. 4. Temple S. The development of neural stem cells. Nature 2001;414:112–7. 5. Breier G, Albrecht U, Sterrer S, et al. Expression of vascular endothelial growth factor during embryonic angiogenesis and endothelial cell differentiation. Development 1992;114:521–32. 6. Zerlin M, Goldman JE. Interactions between glial progenitors and blood vessels during early postnatal corticogenesis: Blood vessel
K. Barami / Journal of Clinical Neuroscience 15 (2008) 1193–1197
7.
8. 9.
10.
11.
12.
13.
14. 15. 16.
17. 18. 19.
20.
21.
contact represents an early stage of astrocyte differentiation. J Comp Neurol 1997;387:537–46. Mercier F, Kitasako JT, Hatton GI. Anatomy of the brain neurogenic zones revisited: fractones and the fibroblast/macrophage network. J Comp Neurol 2002;451:170–88. Palmer TD, Willhoite AR, Gage FH. Vascular niche for adult hippocampal neurogenesis. J Comp Neurol 2000;425:479–94. Shen Q, Goderie SK, Jin L, et al. Endothelial cells stimulate selfrenewal and expand neurogenesis of neural stem cells. Science 2004;304:1338–40. Louissaint Jr A, Rao S, Leventhal C, et al. Coordinated interaction of neurogenesis and angiogenesis in the adult songbird brain. Neuron 2002;34:945–60. Li Q, Ford MC, Lavik EB, et al. Modeling the neurovascular niche: VEGF- and BDNF-mediated cross-talk between neural stem cells and endothelial cells: an in vitro study. J Neurosci Res 2006;84:1656–68. Beck T, Krieglstein J. Cerebral circulation, metabolism, and bloodbrain barrier of rats in hypocapnic hypoxia. Am J Physiol 1987;252: H504–12. Lauro KL, LaManna JC. Adequacy of cerebral vascular remodeling following three weeks of hypobaric hypoxia. Examined by an integrated composite analytical model. Adv Exp Med Biol 1997;411: 369–76. Taupin P. Stroke-induced neurogenesis: physiopathology and mechanisms. Curr Neurovasc Res 2006;3:67–72. Carmichael ST. Cellular and molecular mechanisms of neural repair after stroke: making waves. Ann Neurol 2006;59:735–42. Tuettenberg J, Friedel C, Vajkoczy P. Angiogenesis in malignant glioma – a target for antitumor therapy? Crit Rev Oncol Hematol 2006;59:181–93. Sanai N, Alvarez-Buylla A, Berger MS. Neural stem cells and the origin of gliomas. N Engl J Med 2005;353:811–22. Singh SK, Hawkins C, Clarke ID, et al. Identification of human brain tumor initiating cells. Nature 2004;432:396–401. Dahlstrand J, Collins VP, Lendahl U. Expression of the class VI intermediate filament nestin in human central nervous system tumors. Cancer Res 1992;52:5334–41. Glass R, Synowitz M, Kronenberg G, et al. Glioblastoma-induced attraction of endogenous neural precursor cells is associated with improved survival. J Neurosci 2005;25:2637–46. Charalambous C, Chen TC, Hofman FM. Characteristics of tumorassociated endothelial cells derived from glioblastoma multiforme. Neurosurg Focus 2006;20:E22.
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22. Miyagami M, Katayama Y. Angiogenesis of glioma:evaluation of ultrastructural characteristics of microvessels and tubular bodies (Weibel-Palade) in endothelial cells and immunohistochemical findings with VEGF and p53 protein. Med Mol Morphol 2005;38:36–42. 23. Charalambous C, Hofman FM, Chen TC. Functional and phenotypic differences between glioblastoma multiforme-derived and normal human brain endothelial cells. J Neurosurg 2005;102:699–705. 24. Charalambous C, Virrey J, Kardosh A, et al. Glioma-associated endothelial cells show evidence of replicative senescence. Exp Cell Res 2007;313:1192–202. 25. Calabrese C, Poppleton H, Kocak M, et al. A perivascular niche for brain tumor stem cells. Cancer Cell 2007;11:69–82. 26. Gilbertson RJ, Rich JN. Making a tumour’s bed:glioblastoma stem cells and the vascular niche. Nat Rev Cancer 2007;7:733–6. 27. Daumas-Duport C, Scheithauer B, O’Fallon J, et al. Grading of astrocytomas. a simple and reproducible method. Cancer 1988;62: 2152–65. 28. Brat DJ, Castellano-Sanchez AA, Hunter SB, et al. Pseudopalisades in glioblastoma are hypoxic, express extracellular matrix proteases, and are formed by an actively migrating cell population. Cancer Res 2004;64:920–7. 29. Ezhilarasan R, Mohanam I, Govindarajan K, et al. Glioma cells suppress hypoxia-induced endothelial cell apoptosis and promote the angiogenic process. Int J Oncol 2007;30:701–17. 30. Kaur B, Khwaja FW, Severson EA, et al. Hypoxia and the hypoxiainducible-factor pathway in glioma growth and angiogenesis. Neuro Oncol 2005;7:134–53. 31. Fujiwara S, Nakagawa K, Harada H, et al. Silencing hypoxiainducible factor-1 alpha inhibits cell migration and invasion under hypoxic environment in malignant gliomas. Int J Oncol 2007;30: 793–802. 32. Kim I, Kim JH, Ryu YS, et al. Characterization and expression of a novel alternatively spliced human angiopoietin-2. J Biol Chem 2000;275:18550–6. 33. Jansen M, de Witt Hamer PC, Witmer AN, et al. Current perspectives on antiangiogenesis strategies in the treatment of malignant gliomas. Brain Res Brain Res Rev 2004;45:143–63. 34. Lobov IB, Brooks PC, Lang RA. Angiopoietin-2 displays VEGFdependent modulation of capillary structure and endothelial cell survival in vivo. Proc Natl Acad Sci USA 2002;99:11205–10. 35. Koga K, Todaka T, Morioka M, et al. Expression of angiopoietin-2 in human glioma cells and its role for angiogenesis. Cancer Res 2001;61:6248–54.
Journal of Clinical Neuroscience 15 (2008) 1193–1197 www.elsevier.com/locate/jocn
Review
Relationship of neural stem cells with their vascular niche: Implications in the malignant progression of gliomas Kaveh Barami * Memorial Neuroscience Center, Memorial Hospital Jacksonville, 3625 University Boulevard South, Jacksonville, Florida 32216, USA Received 1 December 2007; accepted 8 January 2008
Abstract During embryogenesis and in regions of the adult brain undergoing post-natal neurogenesis, neural stem cells and endothelial precursors are found within a vascular niche, where the coordinated interactions between neurogenesis and vasculogenesis dictates development and responses to the environment. Moreover, recent evidence suggests that gliomas may arise from transformed neural stem cells and that angiogenesis is important in the malignant progression of these tumors. Taken together, these findings have led researchers to focus on the dynamic interaction between neural stem cells and their vascular niche so as to find new therapeutic strategies to halt the progression of gliomas. This review summarizes the cellular substrates responsible for the coordinated interactions between the nervous and vascular systems and how this relates to gliomagenesis. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Angiogenesis; Glioma; Neural stem cells; Neurogenesis; Vascular niche; Vasculogenesis
1. The developing and adult neurovascular unit There is compelling experimental evidence to suggest that there is a coordinated interaction between the nervous and vascular systems in the developing as well as the adult vertebrate brain.1 The interplay between these two parallel systems is responsible for creating a specialized microenvironment or ‘‘niche” in which neural stem cells reside, selfrenew, and differentiate. The neural tube gives rise to both nervous and vascular system components.2–4 Neurons and glia are derived from the neuroectodermal stem cells and cells destined for the vascular system are derived from the neural crest.2–4 Embryonic neuroectoderm vascularization is initiated by invasion of vascular sprouts from the perineural vascular plexus in the ventral neural tube. This process parallels the thickening of the proliferating neuroepithelium.5 Further evidence of the co-development of the two systems is demonstrated by endothelial cells and *
Tel.: +1 904 296 2522; fax: +1 904 296 8173. E-mail address: [email protected]
0967-5868/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jocn.2008.01.002
neural stem cells being found in similar locations at various developmental stages in the neural germinal zones.6 The anatomic relationship between vasculogenesis and neurogenesis in the adult brain persists where brain blood vessels are intimately associated with the basal lamina of the subventricular zone, an area where new neurons are generated.7 Neurogenesis in the adult vertebrate brain is closely associated with active vascular recruitment. Palmer et al have demonstrated that in the adult rat hippocampus up to 37% of the cells proliferating in the subgranular zone are endothelial precursors and that neural precursors and angioblasts proliferate in clusters associated with the microvasculature of the subgranular zone,8 raising the possibility that circulating factors influence plasticity in the adult brain. Shen et al. have suggested a role for endothelial cells in the neurovascular unit. They found that endothelial cells secrete soluble factors, including Hes1 and Notch that act on the adult neural stem cells and stimulate neurogenesis.9 Louissaint et al. have shown that there is a coordinated interaction between neurogenesis and angiogenesis in the
1194
K. Barami / Journal of Clinical Neuroscience 15 (2008) 1193–1197
adult songbird brain. Testosterone not only modulates the integration and survival of newly generated neurons but also induces angiogenesis, upregulates vascular endothelial growth factor (VEGF) and its receptor and brain-derived neurotrophic factor (BDNF), all necessary for the recruitment of the new neurons in the neostriatum.10 In vitro studies have shown that the cross-talk between neural stem cells and endothelial cells seems to be mediated by VEGF and BDNF.11 The coordinated interaction between vasculogenesis and neurogenesis in the developing and adult brain may reflect a responsiveness of both systems to similar growth factors and receptors, including VEGF, neurotrophins, neuropilin, semaphorins, and ephrins/Eph molecules1 (Table 1). The maintenance of the neurovascular unit is dependant on soluble factors. Structural and ultrastructural studies on the subpendymal layer, the main site for adult neurogenesis, have revealed highly organized extravascular basal laminae, termed ‘‘fractones”, unique to this area.7 Fractones contact local blood vessels by their stems and also engulf astrocytes, ependymal cells, microglia and stem cells. This way, soluble factors such as cytokines and growth factors can exert an influence on the neurovascular unit. 2. Effect of hypoxia on angiogenesis and neurogenesis: Lessons learnt from ischemia Hypoxia increases cerebral blood flow, glucose consumption, and capillary density.12,13 Hypoxia-induced angiogenesis is mediated partly by the transcription factor hypoxia-inducible factor-1 (HIF-1), VEGF, angiopoietin1 or angiopoietin-2 (Ang1/Ang2), and cytochrome oxidase-2 (COX-2).1 This process is initiated with basement membrane breakdown by matrix metalloproteinases. Next, endothelial cells begin migrating and proliferating in response to angiogenic molecules such as Ang1 and VEGF. New capillaries are then formed and develop into functional vessels containing a lumen in which oxygen and blood
can flow through. Lastly, vascular pruning and regression occur, mediated by Ang2 in the absence of VEGF.1 Stroke leading to ischemia and hypoxia also induces neurogenesis in the adult mammalian brain.14 In rodents neurogenesis is enhanced bilaterally in the dentate gyrus and subventricular zone after global and focal experimental ischemia. Newly generated neurons migrate to the ischemic penumbra and penumbra of the ischemic cortex and differentiate into mature neurons of the host tissue, thus replacing lost neurons.14 Post-stroke neurogenesis is a dynamic process characterized by waves of newly generated neurons migrating to the ischemic areas and waves of cell death in the newly arrived neuroblast (young neuron) population.15 A conceptual model suggests that stroke and ischemia activate HIF-1 pathway, an early molecular signal after stroke in peri-infarct cortex. This induces endogenous production of the cytokine erythropoietin around the infarct. Neuroblasts respond to this ligand with their erythropoietin receptor. This interaction leads to neuroblast migration toward the peri-infarct cortex. Erythropoietin also stimulates angiogenesis near the infarct and promotes vascular production of growth factors that stimulate neurogenesis.15 Thus, the dynamic interplay between the vascular and neural systems seen in normal development is recapitulated in angiogenesis and neurogenesis ensuing an ischemic event. Fig. 1 shows the relevant anatomy, cell types, and their environment. 3. Angiogenesis and glioma formation The ‘‘angiogenic switch” refers to the transition of an avascular tumor to an angiogenic one and represents a distinct step in the malignant transformation of gliomas. Overexpression of angiogenic factors or hypoxia results in the production of certain growth factors and cytokines that have been shown to induce angiogenesis in gliomas.16 The VEGF family and their receptors seem to be the
Table 1 The coordinated effects of growth factors on vascular and nervous systems From: ‘‘The neurovascular unit and its growth factors: Coordinated response in the vascular and nervous systems” by: Ward and Lamanna. Neurological Research 2004; 26:870–83. Reproduced with permission Growth factor
Nervous system
Vascular system
Vascular endothelial growth factor (VEGF) Angiopoietin
Neurogenic and neuroprotective – stimulates neuron and axon growth Neurite outgrowth, neurite patterning
Semaphorin/neuropilin
Axon guidance, attraction and repulsion, repulsive growth cone guidance, neural patterning Neuronal survival and differentiation
Critical for angiogenesis, EC survival, proliferation, migration EC survival and migration, pericyte recruitment VEGF receptor, EC migration and proliferation EC survival , vessel stabilization
Neuronal survival and differentiation, neurite outgrowth Neuronal survival and differentiation Co-receptor for high affinity neurotrophin binding
Angiogenic Angiogenic Vascular tone, smooth muscle cell apoptosis
Guidance cues for growing axons, migration of neural crest cells
Angiogenesis, arterial versus venous identity
Brain derived neurotrophic factor (BDNF) Nerve growth factor (NGF) Neurotrophin-3 (NT-3) p75NGFR Low-affinity NGF receptor Ephs/ephrins EC, endothelial cell.
K. Barami / Journal of Clinical Neuroscience 15 (2008) 1193–1197
1195
Fig. 1. Neural stem cells and their vascular niche. Neural stem cells (NSCs) in the subventricular zone (SVZ) lie near the ependymal layer, which produces vascular endothelial growth factor (VEGF), at a vascular niche of numerous capillaries. A shortage of oxygen up-regulates VEGF levels, which stimulate both NSCs and capillaries to grow, and enhance neuronal differentiation, axon outgrowth, and neuron survival. The favorable effect of endothelial cells on neurogenesis is, at least partly, mediated by the release of brain-derived neurotrophic factor (BDNF). BV, blood vessel. From: ‘‘Blood vessels and nerves: common signals, pathways and diseases”, by: Carmeliet P. Nature Reviews Genetics 2003; 4: 710–20. Reproduced with permission.
central mediators of glioma angiogenesis. Other important angiogenic molecules are basic fibroblast growth factor (bFGF), platelet-derived growth factor (PDGF), hepatocyte growth factor/scatter factor (SF) and the angiopoietins.16 VEGF induces endothelial cell migration, proliferation, and tube formation. Moreover, it stimulates endothelial nitric oxide synthase and thus further activates the angiogenic cascade. VEGF also acts as a survival factor for endothelial cells and increases microvascular leakiness causing extravasation of plasma proteins.16 VEGF mediates the key differential steps in glioma angiogenesis and is responsible for many pathological hallmarks seen in malignant gliomas. The VEGF receptors (VEGFR) are tyrosine kinase receptors and mediate intracellular signaling via the mitogen-activated protein (MAP) kinase pathway. VEGFR-1 and VEGFR-2 expression is increased after ischemia and in malignant gliomas.16 VEGF production is mainly triggered by hypoxia yet it is also stimulated by other vascular cytokines such as FGF and PDGF. It may also be constitutively expressed due to genetic alterations.16 Glioma cell lines and human gliomas have been shown to express high levels of FGF mRNA. FGF has also been shown to stimulate endothelial cell migration, proliferation, sprouting, and tube formation.16 PDGF has a direct role in angiogenesis and its expression has also been correlated to glial malignancy.16 Angiopoietins regulate endothelial cell survival and blood vessel maturation. During glioma angiogenesis, angiopoietins and VEGF receptors are induced in tumor endothelial cells leading to increased blood vessel permeability, loss of blood brain barrier function, microvascular dilation, and sprout formation.16 Lastly, hepatocyte growth factor expression has been associated with increased malignancy and vascularity yet its exact contribution to glioma angiogenesis remains to be elucidated.16
4. Relationship of gliomas with neural stem cells and endothelial cells Recent compelling evidence suggests that gliomas may arise from transformed neural stem cells that reside in the walls of the lateral ventricular system.17 Astrocytes within this region, functioning as neural stem cells, self-renew and generate astrocytes, oligodendrocytes, and neurons throughout adult life. Neural stem cells behave similarly to glioma cells. Features common to both glioma cells and neural stem cells include expression of stem-cell markers, high motility, diversity of progeny, robust proliferation, and association with blood vessels and white matter tracts.17 Stem cells have also been isolated from gliomas.18 Gliomas frequently express the progenitor-cell marker nestin (an intermediate filament) and glioblastomas express higher nestin levels than less malignant ones.19 This finding suggests that gliomas share gene expression patterns with neural stem cells and may be derived from them. Despite data suggesting that neural stem cells may give rise to gliomas, the relationship of these cell types to gliomas remains controversial. As an example, endogenous neural precursor cells have also been shown to migrate from the SVZ towards the tumor in a murine experimental glioblastoma model.20 In this study, the stem cells exhibited a strong tropism for glioblastomas in vivo and in vitro. The precursor cells surrounded the tumors and their presence was associated with decreased tumor size and improved survival, suggesting an antitumorigenic response. Taken together, this study demonstrates the dual relation of SVZ stem cells to gliomas. Although neural stem cells have been shown to give rise to gliomas, beneficial effects have also been demonstrated by them. In contrast to normal endothelial cells, which appear small and plump, endothelial cells associated with high grade gliomas have a large, flat, and veil-like appearance. They are also resistant to cytotoxic drugs and undergo
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less apoptosis than control cells.21 Ultrastructural studies have shown that capillary endothelial cell proliferation in malignant gliomas are composed of immature capillaries with narrow slit-like lumens, high nuclear to cytoplasmic ratio, and paucity of organelles. They resemble capillary buds seen in normal repair tissue.22 Endothelial cells associated with high-grade gliomas proliferate more slowly and undergo less apoptosis than their normal counterparts; however, they migrate faster than them.23 Glioblastoma associated endothelial cells constitutively produce high levels of VEGF and endothelin-1 yet are less responsive to them compared to normal endothelial cells.23 Glioma-associated endothelial cells express senescenceassociated beta-galactosidase, a signature of an end stage of differentiation known as ‘‘replicative senescence”. Characteristics of these cell types include increased levels of cell cycle inhibitors p21 and p27, increased resistance to cytotoxic drugs, increased growth factor production, and inability to proliferate.24 Calabrese et al. have shown that treating mice bearing orthotopic xenografts of glioma cells with the anti-angiogenic drug, bevacizumab (Avastin; Genentech, South San Francisco, CA, USA), resulted in a drastic reduction of vessel-associated tumor stem cells without impacting the actual tumor cells.25 This suggests that antiangiogenic drugs arrest brain tumor growth by disrupting a vascular niche microenvironment that is critical for the maintenance of tumor stem cells.26 5. Hypoxia and gliomas Necrosis and vascular proliferation are pathologic hallmarks that distinguish malignant infiltrative gliomas from the lower grade forms.27 In glioblastomas, hypercellular zones called pseudopalisades typically surround necrotic foci and secrete proangiogenic factors, further promoting tumor growth.28 At least a subset of cells that comprise pseudopalisades represent tumor cells migrating away from a vaso-occlusive event.28 The hypoxic microenvironment of high grade gliomas renders the tumors more resistant to cancer therapies and rescues endothelial cells from apoptosis induced by hypoxia.29 Activation of the HIF-1 pathway is a common feature of gliomas and most likely explains the intense vascular hyperplasia seen in high grade tumors.30 HIF-1 is one of the master regulators that orchestrates the cellular response to hypoxia and has a central role in promoting proangiogenic and invasive properties in gliomas.31 It is a heterodimeric transcription factor composed of alpha and beta subunits. The alpha subunit is stable in hypoxic conditions but is rapidly degraded in normoxia. Upon stabilization, HIF-1 translocates to the nucleus and induces transcription of its downstream target genes. Activation of HIF-1 results in the activation of VEGF and their receptors, PDGF-B, matrix metalloproteinases, plasminogen activator inhibitor, transforming growth factors, angiopoietin, endothelin-1, inducible nitric oxide synthase, adrenomedullin,
and erythropoietin, all factors capable of promoting angiogenesis and invasion.30 Silencing HIF-1 alpha inhibits cell migration and invasion in a hypoxic environment in malignant gliomas.31 Fujiwara et al. showed that introducing HIF-1 alpha-targeted small interfering RNA into glioma cell lines resulted in significant reduction of migration and invasiveness of the cells. This response was associated with downregulation of matrix metalloproteinases (MMP-2 and MMP-9), which are activated by hypoxia.31 This suggests that targeting the HIF-1 alpha molecule might be a novel therapeutic strategy for malignant gliomas by reducing cell motility through alteration of invasion-related molecules. Angiopoietins are crucial in angiogenesis in the developing nervous system and are also important in hypoxia-induced angiogenesis in gliomas. The angiopoietins Ang1 and Ang2 are ligands for the tyrosine kinase receptor expressed by endothelial cells (TIE2).32 Ang1 enhances vascular stability mainly by pericyte recruitment33 and Ang2 acts as an antagonist of Ang1 and leads to vessel destabilization and thus neovascularization in the presence of VEGF.34 Ang1 has been detected in the periphery of gliomas and is absent in perinecrotic areas where hypoxia down-regulates Ang1 expression and upregulates Ang2 expression. Ang2 is highly expressed in perinecrotic areas and the tumor periphery in endothelial cells as well as glioma cells.34 This pattern of expression suggests that Ang1 modulates angiogenesis in association with Ang2, and Ang2 is involved in vessel regression and neovascularization in the presence of VEGF.35 6. Conclusions The dynamic interplay between the vascular and nervous systems is responsible for creating a neurovascular unit that acts as a niche for maintaining neural stem cells. These interactions also seem to be operative in the malignant progression of gliomas. Elucidation of mechanisms underlying the balance of capillary density, cellular activity, and oxygen status might lead to the development of new therapeutic strategies against these deadly tumors. References 1. Ward NL, Lamanna JC. The neurovascular unit and its growth factors: coordinated response in the vascular and nervous systems. Neurol Res 2004;26:870–83. 2. Osterfield M, Kirschner MW, Flanagan JG. Graded positional information:interpretation for both fate and guidance. Cell 2003;113:425–8. 3. Panchision DM, McKay RD. The control of neural stem cells by morphogenic signals. Curr Opin Genet Dev 2002;12:478–87. 4. Temple S. The development of neural stem cells. Nature 2001;414:112–7. 5. Breier G, Albrecht U, Sterrer S, et al. Expression of vascular endothelial growth factor during embryonic angiogenesis and endothelial cell differentiation. Development 1992;114:521–32. 6. Zerlin M, Goldman JE. Interactions between glial progenitors and blood vessels during early postnatal corticogenesis: Blood vessel
K. Barami / Journal of Clinical Neuroscience 15 (2008) 1193–1197
7.
8. 9.
10.
11.
12.
13.
14. 15. 16.
17. 18. 19.
20.
21.
contact represents an early stage of astrocyte differentiation. J Comp Neurol 1997;387:537–46. Mercier F, Kitasako JT, Hatton GI. Anatomy of the brain neurogenic zones revisited: fractones and the fibroblast/macrophage network. J Comp Neurol 2002;451:170–88. Palmer TD, Willhoite AR, Gage FH. Vascular niche for adult hippocampal neurogenesis. J Comp Neurol 2000;425:479–94. Shen Q, Goderie SK, Jin L, et al. Endothelial cells stimulate selfrenewal and expand neurogenesis of neural stem cells. Science 2004;304:1338–40. Louissaint Jr A, Rao S, Leventhal C, et al. Coordinated interaction of neurogenesis and angiogenesis in the adult songbird brain. Neuron 2002;34:945–60. Li Q, Ford MC, Lavik EB, et al. Modeling the neurovascular niche: VEGF- and BDNF-mediated cross-talk between neural stem cells and endothelial cells: an in vitro study. J Neurosci Res 2006;84:1656–68. Beck T, Krieglstein J. Cerebral circulation, metabolism, and bloodbrain barrier of rats in hypocapnic hypoxia. Am J Physiol 1987;252: H504–12. Lauro KL, LaManna JC. Adequacy of cerebral vascular remodeling following three weeks of hypobaric hypoxia. Examined by an integrated composite analytical model. Adv Exp Med Biol 1997;411: 369–76. Taupin P. Stroke-induced neurogenesis: physiopathology and mechanisms. Curr Neurovasc Res 2006;3:67–72. Carmichael ST. Cellular and molecular mechanisms of neural repair after stroke: making waves. Ann Neurol 2006;59:735–42. Tuettenberg J, Friedel C, Vajkoczy P. Angiogenesis in malignant glioma – a target for antitumor therapy? Crit Rev Oncol Hematol 2006;59:181–93. Sanai N, Alvarez-Buylla A, Berger MS. Neural stem cells and the origin of gliomas. N Engl J Med 2005;353:811–22. Singh SK, Hawkins C, Clarke ID, et al. Identification of human brain tumor initiating cells. Nature 2004;432:396–401. Dahlstrand J, Collins VP, Lendahl U. Expression of the class VI intermediate filament nestin in human central nervous system tumors. Cancer Res 1992;52:5334–41. Glass R, Synowitz M, Kronenberg G, et al. Glioblastoma-induced attraction of endogenous neural precursor cells is associated with improved survival. J Neurosci 2005;25:2637–46. Charalambous C, Chen TC, Hofman FM. Characteristics of tumorassociated endothelial cells derived from glioblastoma multiforme. Neurosurg Focus 2006;20:E22.
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22. Miyagami M, Katayama Y. Angiogenesis of glioma:evaluation of ultrastructural characteristics of microvessels and tubular bodies (Weibel-Palade) in endothelial cells and immunohistochemical findings with VEGF and p53 protein. Med Mol Morphol 2005;38:36–42. 23. Charalambous C, Hofman FM, Chen TC. Functional and phenotypic differences between glioblastoma multiforme-derived and normal human brain endothelial cells. J Neurosurg 2005;102:699–705. 24. Charalambous C, Virrey J, Kardosh A, et al. Glioma-associated endothelial cells show evidence of replicative senescence. Exp Cell Res 2007;313:1192–202. 25. Calabrese C, Poppleton H, Kocak M, et al. A perivascular niche for brain tumor stem cells. Cancer Cell 2007;11:69–82. 26. Gilbertson RJ, Rich JN. Making a tumour’s bed:glioblastoma stem cells and the vascular niche. Nat Rev Cancer 2007;7:733–6. 27. Daumas-Duport C, Scheithauer B, O’Fallon J, et al. Grading of astrocytomas. a simple and reproducible method. Cancer 1988;62: 2152–65. 28. Brat DJ, Castellano-Sanchez AA, Hunter SB, et al. Pseudopalisades in glioblastoma are hypoxic, express extracellular matrix proteases, and are formed by an actively migrating cell population. Cancer Res 2004;64:920–7. 29. Ezhilarasan R, Mohanam I, Govindarajan K, et al. Glioma cells suppress hypoxia-induced endothelial cell apoptosis and promote the angiogenic process. Int J Oncol 2007;30:701–17. 30. Kaur B, Khwaja FW, Severson EA, et al. Hypoxia and the hypoxiainducible-factor pathway in glioma growth and angiogenesis. Neuro Oncol 2005;7:134–53. 31. Fujiwara S, Nakagawa K, Harada H, et al. Silencing hypoxiainducible factor-1 alpha inhibits cell migration and invasion under hypoxic environment in malignant gliomas. Int J Oncol 2007;30: 793–802. 32. Kim I, Kim JH, Ryu YS, et al. Characterization and expression of a novel alternatively spliced human angiopoietin-2. J Biol Chem 2000;275:18550–6. 33. Jansen M, de Witt Hamer PC, Witmer AN, et al. Current perspectives on antiangiogenesis strategies in the treatment of malignant gliomas. Brain Res Brain Res Rev 2004;45:143–63. 34. Lobov IB, Brooks PC, Lang RA. Angiopoietin-2 displays VEGFdependent modulation of capillary structure and endothelial cell survival in vivo. Proc Natl Acad Sci USA 2002;99:11205–10. 35. Koga K, Todaka T, Morioka M, et al. Expression of angiopoietin-2 in human glioma cells and its role for angiogenesis. Cancer Res 2001;61:6248–54.