Stromal cell-derived factor 1 is secreted by meningeal cells and acts as chemotactic factor on neuronal stem cells of the cerebellar external granular layer.

The cerebellar external granular layer (EGL) is an unusually long-lasting neural proliferative zone positioned immediately beneath the pial surface. Its position and stability critically depend on meningeal cells, as their selective destruction leads to its rapid dispersal, creating massive cortical ectopia. Similar ectopias have recently been described as a side effect of deficiency for stromal cell-derived factor 1 (SDF-1), a chemoattractant for haematopoietic precursor cell migration. Here we show that SDF-1 is present in meningeal cells in vivo and in vitro, where it is secreted in functionally relevant concentrations into the medium. Correspondingly, the SDF-1 receptor (termed CXCR4) can be demonstrated on stem cells of the external granular layer, but is absent on postmitotic cells commencing their final inward migration. We show that SDF-1 is concentrated by heparan sulphate proteoglycans highly expressed in the EGL in a laminar fashion, which thus might act to locally restrict SDF-1 action to the EGL in a kind of step gradient. In vitro, SDF-1 chemotactically attracts neuronal cells isolated from the external, but not from the internal granular layer, in a Boyden chamber assay in concentrations found in meningeal cell-conditioned medium. Selective removal of SDF-1 from conditioned media by immunoprecipitation abolishes their chemoattractive action, which can be reconstituted again by the addition of recombinant SDF-1. Meningeal cells are thus an important source for the expression of SDF-1 during brain development, which--comparable to its role in haematopoiesis--appears to be a key factor attracting precursor cells to their proliferative compartment.