We also highlight how neuronal co-culture can further modulate microglial gene expression and how interactions with the neuronal glycocalyx increase Siglec expression. Interestingly, iMGLs not cultured with neurons differentially expressed early microglia genes including FFAR2 (Erny et al., 2015; Matcovitch-Natan et al., 2016) suggesting that other factors are needed to further educate microglia as tissue-resident macrophages of the brain. In accordance with this notion, iMGLs cultured in 3D brain cultures actively migrate, tile and encompass the volume of the BORGs, extending processes reminiscent of early microglia development. We also show that iMGLs transplanted in mice, engraft, survive, and display characteristic ramified processes that have increased branch order complexity, closely resembling quiescent microglia (Andreasson et al., 2016). We also note that transplanted, highly ramified iMGLs were morphological heterogeneous within the brain (Figure 7I–L). This morphological diversity is indicative of microglia responding to distinct cortical layers/brain-regions and potentially reflect microglia subtypes found within the brain(Grabert et al., 2016). Our results also demonstrate that iMGLs can respond appropriately to the neurotoxic build-up of Aβ, thus providing a model system to investigate the role of microglia in various proteinopathies.
