Transcriptome analysis reveals the contribution of oligodendrocyte and radial glia-derived cues for maintenance of microglia identity

Microglia are increasingly being recognized as druggable targets in neurodegenerative disorders, and good in vitro models are crucial to address cell biological questions. Major challenges are to recapitulate the complex microglial morphology and their in vivo transcriptome. We have therefore exposed primary microglia from adult rhesus macaques to a variety of different culture conditions including exposure to soluble factors as M-CSF, IL-34, and TGF-beta as well as serum replacement approaches, and compared their morphologies and transcriptomes to those of mature, homeostatic in vivo microglia. This enabled us to develop a new, partially serum-free, monoculture protocol, that yields high numbers of ramified cells. We also demonstrate that exposure of adult microglia to M-CSF or IL-34 induces similar transcriptomes, and that exposure to TGF-beta has much less pronounced effects than it does on rodent microglia. However, regardless of culture conditions, the transcriptomes of in vitro and in vivo microglia remained substantially different. Analysis of differentially expressed genes inspired us to perform 3D-spherical coculture experiments of microglia with oligodendrocytes and radial glia. In such spheres, microglia signature genes were strongly induced, even in the absence of neurons and astrocytes. These data reveal a novel role for oligodendrocyte and radial glia-derived cues in the maintenance of microglial identity, providing new anchor points to study microglia in health and disease.