Is regulation of microglial functions by T3 dependent on the micro‐environment?: Insights from the brainstem respiratory control network of newborn mice

We previously showed that thyroid hormones (THs) facilitate respiratory control development in newborn rodents. To understand the underlying mechanisms, we tested the hypotheses that TH, more precisely T3, 1) influences microglial functions in the brainstem of newborn mice and 2) this effect is region dependent. Measurements were performed on primary cultured mouse microglia isolated from the mixed glial cultures of postnatal day 3 C57BL/6J mice. Primary cultured neurons from cortex and brainstem were obtained from embryonic day 14–16 C57BL/6J mice. Microglia from cortex and brainstem were either cultured alone (DMEM) or co‐cultured with respective neurons (DMEM + neurobasal medium) under standard conditions (37°C, 5% CO 2 / 90% humidity). Microglial motility (μm traveled) was monitored using a time‐lapse video‐microscopy system over 3h with or without T3 (1μM). Phagocytosis was quantified and observed in brainstem microglial cells pre‐incubated with 54 μg/ml Dextran, Rhodamine B (10 000 MW, neutral) in DMEM for 1h followed by incubation in DMEM with or without T3 (1 μM) for 3h. The cells were then fixed by 4% PFA. In the presence of neurons, but not in single culture, exposure to T3 increased brainstem microglial motility by 350% relative to control over the 3 h period (13.0 ± 4.5 vs 45.2 ± 7.8 μm; P=0.004). Exposing cortex cells to T3 also increased microglial motility; however, the effect was less important (156%; 46.0 ± 4.9 vs 72.1 ± 8.8 μm; P=0.009). The region of origin could also influence microglial functions in co‐culture as brainstem microglia showed decreased motility compared to cortex microglia (with and without T3). By comparison with controls, brainstem microglial cells cultured alone responded to T3 exposure by a ~4‐fold increase in phagocytosis as indicated by the fluorescence intensity of Dextran Rhodamine B (average arbitrary unit/area; 5.4 ± 1.1 vs 1.3 ± 0.2 AU/μm 2 ; P=0.001). We conclude that 1) T3 facilitates multiple functional responses of microglia including motility and phagocytosis; both functions are important in regulation of synaptic function during development. 2) Microglial activation is specific to the surrounding environment, such as presence of thyroid hormones and brain regions. Microglia therefore appear as important effectors of TH action in the development of the respiratory network. Understanding the mechanisms by which these cells influence the generation and regulation of respiratory activity offers new approaches to alleviate respiratory disorders in newborn, especially those born pre‐term. Support or Funding Information Supported by NSERC, Fonds de Recherche en Santé du Québec and Research Support Center, Graduate School of Medical Sciences, Kyushu University. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .

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