Heterogeneity in Synaptogenic Profile of Astrocytes from Different Brain Regions

Astrocytes, the most abundant glial cells in the central nervous system (CNS), comprise a heterogeneous population of cells. However, how this heterogeneity impacts their function within brain homeostasis and response to injury and disease is still largely unknown. Recently, astrocytes have been recognized as important regulators of synapse formation and maturation. Here, we analyzed the synaptogenic property of astrocytes from different regions of the CNS. The effect of conditioned medium derived from astrocytes (astrocyte-conditioned medium (ACM)) from cerebral cortex, hippocampus, midbrain and cerebellum, in synapse formation, was evaluated. Synapse formation was analyzed by quantification of pre- and postsynaptic proteins, synaptophysin, and postsynaptic density protein 95 (PSD-95). ACM from the four regions increased significantly the number of synaptophysin/PSD-95 puncta on neurons from the same and different brain regions. Differences on astrocytic synaptogenic potential between the regions were observed according to ACM protein concentration. Thus, cerebellar astrocytes have higher synaptogenic effect when ACM is less concentrated. Also, heterotypical co-culture assays revealed that neurons from cerebral cortex and midbrain equally respond to ACM, indicating that differences in synapse effect are unlike to be neuron-autonomous. The expression profile of the synaptogenic molecules secreted by astrocytes from distinct brain regions was analyzed by qPCR. Gene expression of glypicans 4 and 6, hevin, and secreted protein-acidic and rich in cysteine (SPARC) greatly varies between astrocytes from different brain regions. Furthermore, in vivo analysis of hevin protein confirmed that variance. These findings highlight the heterogeneity of astrocytes and suggest that their synaptogenic potential may be different in each brain region, mainly due to distinct gene expression profiles.