Interaction between astrocytes and adult subventricular zone precursors stimulates neurogenesis

Abstract

Neurogenesis continues in the mammalian subventricular zone (SVZ) throughout life. However, the signaling and cell-cell interactions required for adult SVZ neurogenesis are not known. In vivo, migratory neuroblasts (type A cells) and putative precursors (type C cells) are in intimate contact with astrocytes (type B cells). Type B cells also contact each other. We reconstituted SVZ cell-cell interactions in a culture system free of serum or exogenous growth factors. Culturing dissociated postnatal or adult SVZ cells on astrocyte monolayers-but not other substratessupported extensive neurogenesis similar to that observed in vivo. SVZ precursors proliferated rapidly on astrocytes to form colonies containing up to 100 type A neuroblasts. By fractionating the SVZ cell dissociates with differential adhesion to immobilized polylysine, we show that neuronal colonyforming precursors were concentrated in a fraction enriched for type B and C cells. Pure type A cells could migrate in chains but did not give rise to neuronal colonies. Because astrocyteconditioned medium alone was not sufficient to support SVZ neurogenesis, direct cell-cell contact between astrocytes and SVZ neuronal precursors may be necessary for the production of type A cells.Throughout life, the subventricular zone (SVZ) retains a neurogenic population of cells (reviewed in refs. 1-5). Cells born in the neonatal (6) and adult rodent (7) SVZ migrate along a restricted pathway called the rostral migratory stream to the olfactory bulb where they differentiate into interneurons. The SVZ of adult mice lies directly under the ependyma of the lateral wall of the lateral ventricle (8).Three SVZ cell types have been identified based on their ultrastructural and immunocytochemical characteristics (2, 7, 9): (i) type A cells are migratory neuroblasts; (ii) type B cells are astrocytes; and (iii) type C cells are putative precursors. The spatial relationship between these cells is depicted in Fig. 1 Inset. Both type A and type C cells are apposed to the type B astrocytes. Type B cells also have intercellular contacts with each other. Type A cells migrate in chains through glial tubes composed of type B cells, and type C cells are found as small clusters along these chains of migrating neuroblasts. Type A cells are immunopositive for neuronal markers including neuron-specific tubulin (Tuj1) and the polysialylated form of the neural cell-adhesion molecule (PSA-NCAM). Type B cells contain glial-fibrillary acidic protein (GFAP), a marker of astrocytes. Type C cells are ultrastructurally immature and do not stain for either neuronal or glial markers.The type B cells can be classified further: B1 cells contact the ependyma and B2 cells do not. Type A, B2, and C cells are mitotic. Based on their immature characteristics, mitotic activity, and juxtaposition to the chains of type A cells, the type C cells have been proposed to be the immediate precursors of the migratory type A neuroblasts (9).How SVZ neurogenesis is maintained in the adult brain is not well understoo...

Adult Neurogenesis and the Ischemic Forebrain

“…Astrocytes appear to influence all phases of adult neurogenesis, including neural precursor cell proliferation, migration, and differentiation. Astrocytes promote neuroblast proliferation and neuronal differentiation in cultured SVZ stem cells and hippocampal progenitors (Lim and Alvarez-Buylla, 1999;Song et al, 2002a). In addition, the RMS structural arrangement of neuroblast chain migration through astrocytic tubes suggests that astrocytes direct migration of neuroblasts from the SVZ to the olfactory bulb (Thomas et al, 1996;Peretto et al, 1997).…”

Multipotent Neural Stem Cells Reside into the Rostral Extension and Olfactory Bulb of Adult Rodents

“…This shows that the SVZ-derived neuroblasts migrating in the RE are not the source of the RE-derived stem cells and further suggests that these cells reside within the RE itself. To provide support for this conclusion, cells from the RE of untreated mice were separated into type A (migrating) and nontype A (resident) fractions (Lim and Alvarez-Buylla, 1999), and neurospheres were prepared from each fraction. Immunocytochemistry performed on cultures after dissociation showed that 77-83% of the cells in the type A fraction-derived cultures were IR to Tuj1 antibody (known to label type A precursors), and 1-2% of the cells were IR for GFAP.…”

“…Freshly dissociated RE cells were fractionated by differential adhesion to poly-D-lysine-treated (Sigma) plastic substrates as previously described (Lim and Alvarez-Buylla, 1999). Briefly, fraction 1 (highly enriched for type A cells) was collected by washing the nonadherent cells off the plate.…”

Neural stem cells: mechanisms and modeling

“…Furthermore, astrocytes can promote the differentiation of NSCs into neurons and the development of immature neurons (Lim and Alvarez-Buylla, 1999;Song et al, 2002aSong et al, , 2002bYuan et al, 2012), probably through the secretion of Wnt3, a neurogenic factor (Muroyama et al, 2004;Lie et al, 2005;Yoshinaga et al, 2010). Notably, this Wnt3-secreting function of astrocytes is only limited to the cells in the hippocampus but not from other areas.…”