Identification of a Subpopulation of Astrocyte Progenitor Cells in the Neonatal Subventricular Zone: Evidence that Migration is Regulated by Glutamate Signaling

In mice engineered to express enhanced green fluorescent protein (eGFP) under the control of the entire glutamate transporter 1 (GLT1) gene, eGFP is found in all 'adult' cortical astrocytes. However, when 8.3 kilobases of the human GLT1/EAAT2 promoter is used to control expression of tdTomato (tdT), tdT is only found in a subpopulation of these eGFP-expressing astrocytes. The eGFP mice have been used to define mechanisms of transcriptional regulation using astrocytes cultured from cortex of 1-3 day old mice. Using the same cultures, we were never able to induce tdT⁺ expression. We hypothesized that these cells might not have migrated into the cortex by this age. In this study, we characterized the ontogeny of tdT⁺ cells, performed single-cell RNA sequencing (scRNA-seq), and tracked their migration in organotypic slice cultures. At postnatal day (PND) 1, tdT⁺ cells were observed in the subventricular zone and striatum but not in the cortex, and they did not express eGFP. At PND7, tdT⁺ cells begin to appear in the cortex with their numbers increasing with age. At PND1, scRNA-seq demonstrates that the tdT⁺ cells are molecularly heterogeneous, with a subpopulation expressing astrocytic markers, subsequently validated with immunofluorescence. In organotypic slices, tdT⁺ cells migrate into the cortex, and after 7 days they express GLT1, NF1A, and eGFP. An ionotropic glutamate receptor (iGluR) antagonist reduced by 50% the distance tdT⁺ cells migrate from the subventricular zone into the cortex. The pan-glutamate transport inhibitor, TFB-TBOA, increased, by sixfold, the number of tdT⁺ cells in the cortex. In conclusion, although tdT is expressed by non-glial cells at PND1, it is also expressed by glial progenitors that migrate into the cortex postnatally. Using this fluorescent labeling, we provide novel evidence that glutamate signaling contributes to the control of glial precursor migration.