Mature astrocytes transform into transitional radial glia within adult mouse neocortex that supports directed migration of transplanted immature neurons

Neuronal migration is an essential step in normal mammalian neocortical dev elopment, and the expression of defined cellular and molecular signals with in the developing cortical microenvironment is likely crucial to this proce ss. Therapy via transplanted or manipulated endogenous precursors for disea ses which involve neuronal loss may depend critically on whether newly inco rporated cells can actively migrate to repopulate areas of neuronal loss wi thin the adult brain. Previous studies demonstrated that embryonic neurons and multipotent precursors transplanted into the neocortex of adult mice un dergoing targeted apoptosis of pyramidal neurons migrate long distances int o neuron-deficient regions, undergo directed differentiation, accept affere nt synaptic input, and make appropriate long-distance projections. The expe riments presented here: (1) use time-lapse digital confocal imaging of neur onal migration in living slice cultures to assess cellular mechanisms utili zed by immature neurons during such long distance migration, and (2) identi fy changes within the host cortical astroglial population that may contribu te to this migration. Prelabeled embryonic day 17 mouse neocortical neurons were transplanted into adult mouse primary somatosensory cortex undergoing targeted apoptotic degeneration of callosal projection neurons. Four to 7 days following transplantation, living slice cultures containing the region of transplanted cells were prepared and observed. Sequential time-lapse im ages were recorded using a video-based digital confocal microscope. Transpl anted cells displayed bipolar morphologies characteristic of migrating neur oblasts and moved in a saltatory manner with mean rates of up to 14 mu m/h. To investigate whether a permissive glial phenotype may provide a potentia l substrate for this directed form of neuronal migration, slice cultures we re immunostained with the RC2 monoclonal antibody, which identifies radial glia that act as a substrate for neuronal migration during corticogenesis. RC2 does not label mature stellate astrocytes, which express glial fibrilla ry acidic protein (GFAP). RC2 expression was observed in glial cells closel y apposed to migrating donor neurons within the slice cultures. The timing and specificity of RC2 expression was examined immunocytochemically at vari ous times following transplantation. RC2 immunostaining within regions of n euronal degeneration was transient, with peak staining between 3 and 7 days following transplantation. Strongly RC2-immunoreactive cells that did not express GFAP were found within these regions, but not in distant cortical r egions or within control brains. RC2-positive cells were identified in reci pient transgenic mice which express beta-galactosidase under a glial specif ic promoter, Coexpression of RC2 and beta-galactosidase identified these ce lls as host astroglia. These results demonstrate that adult cortical astroc ytes retain the capacity to reexpress an earlier developmental phenotype th at may partially underlie the observed active migration of transplanted neu rons and neural precursors. Further understanding of these processes could allow directed migration of transplanted or endogenous precursors toward th erapeutic cellular repopulation and complex circuit reconstruction in neoco rtex and other CNS regions. (C) 1999 Academic Press.