[Absence of BAX differentially affects astrocyte density in the mouse cortex and hippocampus]

Astrocytes are a heterogenous group of macroglia present in all regions of the brain and play critical roles in many aspects of brain development, function and disease. Previous studies suggest that the B-cell lymphoma-2 associated X protein (BAX)-dependent apoptosis plays essential roles in regulating neuronal number and achieving optimal excitation/inhibition ratio. The aim of the present paper was to study whether BAX regulates astrocyte distribution in a region-specific manner. Immunofluorescence staining of SOX9 was used to analyze and compare astrocyte density in primary somatosensory cortex, motor cortex, retrosplenial cortex and hippocampus in heterozygous and homozygous BAX knockout mice at age of six weeks when cortical development has finished and glia development has reached a relatively steady state. The results showed that astrocyte density varied significantly among different cortical subdivisions and between cortex and hippocampus. In contrast to the significant increase in GABAergic interneurons, the overall and region-specific astrocyte density remained unchanged in the cortex when BAX was absent. Interestingly, a significant reduction of astrocyte density was observed in the hippocampus of BAX knockout mice. These data suggest that BAX differentially regulates neurons and astrocytes in cortex as well as astrocytes in different brain regions during development. This study provided important information about the regional heterogeneity of astrocyte distribution and the potential contribution of BAX gene during development.