Embryonic microglia influence developing hypothalamic glial populations

Abstract

Background: Although historically microglia were thought to be immature in the fetal brain, evidence of purposeful interactions between these immune cells and nearby neural progenitors is becoming established. Here, we examined the influence of embryonic microglia on gliogenesis within the developing tuberal hypothalamus, a region later important for energy balance, reproduction, and thermoregulation. Methods: We used immunohistochemistry to quantify the location and numbers of glial cells in the embryonic brain (E13.5-E17.5), as well as a pharmacological approach (i.e., PLX5622) to knock down fetal microglia. We also conducted cytokine and chemokine analyses on embryonic brains in the presence or absence of microglia, and a neurosphere assay to test the effects of the altered cytokines on hypothalamic progenitor behaviors. Results: We identified a subpopulation of activated microglia that congregated adjacent to the third ventricle alongside embryonic Olig2+ neural progenitor cells (NPCs) that are destined to give rise to oligodendrocyte and astrocyte populations. In the absence of microglia, we observed an increase in Olig2+ glial progenitor cells that remained at the ventricle by E17.5 and a concomitant decrease of these Olig2+ cells in the mantle zone, indicative of a delay in migration of these precursor cells. A further examination of maturing oligodendrocytes in the hypothalamic grey and white matter area in the absence of microglia revealed migrating oligodendrocyte progenitor cells (OPCs) within the grey matter at E17.5, a time point when OPCs begin to slow their migration. Finally, quantification of cytokine and chemokine signaling in ex vivo E15.5 hypothalamic cultures +/− microglia revealed decreases in the protein levels of several cytokines in the absence of microglia. We assayed the influence of two downregulated cytokines (CCL2 and CXCL10) on neurosphere-forming capacity and lineage commitment of hypothalamic NPCs in culture and showed an increase in NPC proliferation as well as neuronal and oligodendrocyte differentiation. Conclusion: These data demonstrate that microglia influence gliogenesis in the developing tuberal hypothalamus.

Subventricular zone/white matter microglia reconstitute the empty adult microglial niche in a dynamic wave

“…Accumulating evidence indicates that WM and developing microglia, contribute to myelinogenesis/oligodendrocyte progenitor maintenance 64–66 ; thus, we next explored the effects of filling the brain with repopulating cells on WM and WM-associated cells. However, unlike during development in which these cells participate in oligo/myelinogenesis, in 28d recovery mice we observe no changes in Olig2, a marker for cells of oligodendrocyte lineage (Figure S5A, D), PDGFRα, a marker for oligodendrocyte progenitor cells (Figure S5B, E), or MBP, a marker for myelin (Figure S5C, F-G).…”

Novel Tools and Investigative Approaches for the Study of Oligodendrocyte Precursor Cells (NG2-Glia) in CNS Development and Disease

“…Similar studies, again using DT-mediated ablation of NG2-expressing cells, have provided the first indications of a role of OPCs in the maintenance of brain immune homeostasis and, while it has been known for some time that microglia can regulate the behavior and differentiation of OPCs (reviewed by Domingues et al, 2016 ), these studies have shown that OPCs can, in turn, modulate microglia (Zhang S.-Z. et al, 2019 ; Liu and Aguzzi, 2020 ; Marsters et al, 2020 ; Mecha et al, 2020 ). In the absence of NG2-positive OPCs, microglia become hypersensitive to lipopolysaccharide (LPS) injection, resulting in microglial activation and neuroinflammation (Zhang S.-Z.…”

Immovable Object Meets Unstoppable Force? Dialogue Between Resident and Peripheral Myeloid Cells in the Inflamed Brain

“…CD11c + microglia, which expand in the postnatal brain, express a neurosupportive gene signature and IGF-1 and are required for myelinogenesis during development ( 75 ). More recently, a new role in gliogenesis has been identified for microglia at the later embryonic stages of E15.5 and E17.5 ( 76 ). A subpopulation of amoeboid microglia lining the tuberal hypothalamic third ventricle have been found to influence glial precursors via chemokine signaling, namely, CCL2 and CXCL10, which are required for the migration and maturation of oligodendrocytes, but not astrocytes ( 76 ).…”

“…More recently, a new role in gliogenesis has been identified for microglia at the later embryonic stages of E15.5 and E17.5 ( 76 ). A subpopulation of amoeboid microglia lining the tuberal hypothalamic third ventricle have been found to influence glial precursors via chemokine signaling, namely, CCL2 and CXCL10, which are required for the migration and maturation of oligodendrocytes, but not astrocytes ( 76 ). An additional unique microglial subset ( i.e .…”

“…Microglial-expressed TAM receptor kinases, MER Proto-Oncogene Tyrosine Kinase (MerTK), and Axl have revealed an important role for neuronal progenitor cell clearance ( 90 ), which may be required for efficient neurogenesis, whereas CD11b, TREM2, TIM-4, and BAI1 appear to be required for the phagocytosis of apoptotic neurons ( 53 , 91 , 92 ). Microglia can recognize a number of “eat-me” signals, including phosphatidylserine, components of the complement system, thrombospondin and uridine 5′-diphosphate ( 93 ), which stimulate phagocytosis ( 68 , 76 , 93 ). Although microglia are the principle phagocytes in the CNS, other glia, including oligodendrocytes and astrocytes, are also thought to contribute to this function ( 94 , 95 ).…”