Microglia maintain the normal structure and function of the hippocampal astrocyte network

Abstract

Microglial control of activity‐dependent plasticity and synaptic remodeling in neuronal networks has been the subject of intense research in the past several years. Although microglia–neuron interactions have been extensively studied, less is known about how microglia influence astrocyte‐dependent control over neuronal structure and function. Here, we explored a role for microglia in regulating the structure and function of the astrocyte syncytium in mouse hippocampus. After depleting microglia using a CSF1R antagonist (PLX5622, Plexxikon), we observed severe disruption of astrocyte syncytial isopotentiality and dye coupling. A decrease in astrocyte‐specific gap junction connexin (Cx) 30 and 43 expression, at least partially accounts for these microglia‐dependent changes in astrocytes. Because neuronal function requires intact astrocyte coupling, we also evaluated the effects of microglia depletion on synaptic transmission in the hippocampus. Without microglia, the strength of synaptic transmission was reduced at baseline and after long‐term potentiation (LTP). Conversely, priming microglia with systemic injections of lipopolysaccharide enhanced CA3‐CA1 synaptic transmission. This microglia‐induced scaling of synaptic transmission was associated with increased expression of post‐synaptic scaffold proteins (Homer1) in CA1. However, astrocyte network function was not affected by microglia priming, indicating that microglia‐dependent effects on astrocytes and neurons vary across functional states. Through manipulation of microglia in the brain, our results reveal the importance of microglia in homeostatic regulation of the astrocyte syncytium and scaling of synaptic transmission. These novel mechanisms uncover a new direction for future studies interrogating microglia function in various physiological and pathological contexts.

Genesis of a functional astrocyte syncytium in the developing mouse hippocampus

“…To identify the time hippocampal astrocytes fully connect into a syncytial network, we combined LY dye coupling and syncytial isopotentiality analyses to trace the functional maturation of the astrocyte syncytium in the mouse hippocampal CA1 region (Du et al, 2022; Kiyoshi & Zhou, 2019; Stephan et al, 2021).…”

“…The syncytial isopotentiality measurement has already been used to study the mechanism among microglia‐astrocyte‐neurons crosstalk (Du et al, 2022), and changes in the functional state of an astrocyte syncytium in diseased conditions (Aten, Du, et al, 2022; Wang, Wang, et al, 2020). Now a variation of this method has been established and validated to examine the efficacy of the K + buffering within a syncytium under both physiological and pathological conditions in the future.…”

“…To examine this question further in the developing brain, we loaded LY to the individual astrocytes after the syncytium was uncoupled by 100 μM MFA in bath aCSF for over 1 h (Schools et al, 2006;Zhou et al, 2006), which effectively inhibits gap junctional coupling by 99.3% (Ma et al, 2016). The uncoupling enhanced LY accumulation in the recorded cell, which, therefore, facilitated the visualization of the delicate processes for morphological analysis (Du et al, 2022;Xu et al, 2010Xu et al, , 2014.…”

“…Additionally, a strong electrical coupling constantly equalizes the membrane potentials of individual astrocytes into an isopotential network, termed syncytial isopotentiality (Huang et al, 2018;Kiyoshi et al, 2018;Ma et al, 2016). Recent evidence further shows the essential role of syncytial isopotentiality for the coordination of microglia-astrocyte-neuron coupling in normal brain function (Du et al, 2022).…”

What microglia depletion approaches tell us about the role of microglia on synaptic function and behavior

“…Notably, GFAP alterations are maintained upon microglia repopulation when treatment is discontinued (Basilico et al, 2021). Recently, it has been reported that CSF1R-i treatment weakens astrocyte gap junctional coupling (Du et al, 2022). Importantly, alterations in astrocytes have also been reported with genetic microglia depletion models (Bruttger et al, 2015;De Luca et al, 2020).…”