The delayed response of the SVZ vasculature to grow

The delayed response of the SVZ vasculature to grow toward and into the polyps, occurring after more than 7 days of EGF infusion, suggests an indirect mechanism, such as hypoxia, rather than a direct angiogenic effect of EGF infusion. In the cerebral cortex, hypoxia-induced angiogenesis is initiated 7 to 14 days after induction of hypoxia (Masamoto et al., 2013). The accumulation of microglia/macrophages and dying cells in the core of stage III polyps and the subsequent reduction in angiogenic stage IV polyps further strengthens this notion. Whether the microglia/macrophages are attracted by hypoxia-induced signaling or by cell death-related signals remains to be elucidated. However, in the angiogenic stage III polyps, microglia/macrophages adopt an amoeboid morphology, engulf nuclei of dying cells, and display ultrastructural features, suggesting an active role in scavenging dead or dying cells. Microglia and macrophages occasionally express EGFR, and EGF can stimulate microglia migration, acting as a motility factor (Nolte et al., 1997; Planas et al., 1998; Lamb et al., 2004; Qu et al., 2012). However, EGFR expression and chemotactic effects of EGF on microglia/macrophages in vivo appears to be limited to disease contexts like trauma, neuroinflammation, and atherosclerosis (Lamb et al., 2004; Qu et al., 2012). This could help explain the local, rather than general, microglia/macrophage recruitment after EGF/ TGFα infusion, specifically to hyperproliferative areas with extensive cell death in the SVZ demonstrated here and by others (de Chevigny et al., 2008). One possible mechanism behind the microglia/macrophage recruitment could be through production of cytokines like CCL2/MCP-1, TNFα, or IL-1β by microglial cells resident in the growing polyps in response to local hypoxia, like those previously described after hypoxia in the neonatal purchase Lomefloxacin HCl (Deng et al., 2008, 2009). Interestingly, amoeboid microglial cells produce MCP-1 under hypoxia in vitro and express the MCP-1 receptor CCR2 in vivo. When MCP-1 was injected intracerebrally amoeboid microglia cells migrated toward the injection site (Deng et al., 2009). Furthermore, the altered morphology of microglia/macrophages and close association to angiogenic vessels we observe, which occur in stage IV polyps, could represent a change from an angiogenesis-promoting to a vessel-stabilizing role of microglia/macrophages. In the retina, transplanted bone marrow-derived cells differentiate into microglia and promote vessel normalization in a HIF1α-dependent manner (Ritter et al., 2006). SVZ expansion after EGF treatment is to a large extent caused by extensive proliferation of dysplastic SOX2/OLIG2-expressing cells (Lindberg et al., 2012a). This dysplastic cell type is highly enriched in the polyps and could be involved in microglia/macrophage accumulation and polyp angiogenesis, via paracrine angiogenic signaling. For instance, neural stem cells are known to produce substantial amounts of vascular endothelial growth factor (VEGF) in vitro (Schänzer et al., 2004). Although VEGF does not appear to induce further proliferation of neural stem cells in vitro in the presence of growth factors, neural stem cell-derived VEGF could induce angiogenesis through endothelial cell proliferation (Fabel et al., 2003). The polyps are not only unique in their composition of cells. They lack an ependymal cell layer and the polyps are freely exposed to molecules present in the cerebrospinal fluid (Lindberg et al., 2012a). This could alter the availability of both chemoattractants and angiogenic growth factors in the polyps. Adult neural stem cells have been suggested as the cells of origin of certain brain tumors (Vescovi et al., 2006). Neural stem cells from animals deficient in p16 (Ink4a) and p19 (Arf) that are retrovirally induced to express constitutively active EGFR persistently form glioma-like tumors in vivo (Bachoo et al., 2002). Moreover, overexpression of wild-type Egfr in white matter glial progenitors keeps cells in an immature state, induces extensive migration, and leads to formation of tumor-like white matter hyperplasias (Ivkovic et al., 2008). A small population of stem cell-like cells has been suggested to exist in brain tumors that are treatment resistant and capable of generating secondary tumors (Singh et al., 2004). Much like the supportive niche of neural stem cells, brain tumors appear to modulate the microenvironment to provide conditions promoting stemness (Calabrese et al., 2007). A study investigating human glioblastoma multiforme tumor progression and brain location demonstrated increased risk of multifocal growth and tumor recurrence in tumors located close to the SVZ (Lim et al., 2007). The present study indicates that the SVZ neural stem cell niche can respond to the extensive hyperproliferation by inducing angiogenesis in hyperproliferative areas, possibly supported by microglia/macrophages. Microglia/macrophages in polyps could play a role similar to tumor-associated macrophages, known to contribute to angiogenic processes in tumors (Movahedi et al., 2010; Casazza et al., 2013).