Minocycline alleviates abnormal microglial phagocytosis of synapses in a mouse model of depression

As antidepressants commonly used in the clinic have proved to be problematic, it is urgent to gain an updated understanding of the pathogenesis of depression and find potential therapeutic targets. Since both functional brain imaging studies and autopsy reports indicated that there is indeed a loss of synapses in depressed patients, it is necessary to explore the mechanism of this process. We firstly investigated the effect of chronic social defeat stress (CSDS), a mouse model of depression, on behaviors, synapses, microglia, and microglial phagocytosis of synapses in mice. Then, as it is unclear whether microglial phagocytosis leads to synaptic loss, or synaptic loss induces the microglial clearance in CSDS mice, we used minocycline, a microglial activation inhibitor, to inhibit the microglial phagocytosis of synapses and study its effect on synapses and behaviors in CSDS mice. Our results showed that the expression levels of PSD-95 in the hippocampal dentate gyrus (DG) of CSDS mice were significantly reduced, while the microglia were significantly activated and the Iba1+CD68+ cell (phagocytic microglia) density was increased. We co-labeled the synaptic protein PSD-95 with the microglia marker Iba1 and found that the microglia in the hippocampal DG of CSDS mice contained significantly more PSD-95 engulfed puncta, which revealed that microglia in CSDS mice abnormally phagocytized synapses. Moreover, our results indicated that minocycline treatment dampened microglial activation, decreased the phagocytic microglia density, reduced abnormal microglial phagocytosis of synapses, reversed synaptic loss, and alleviated behavioral impairment in CSDS mice. Under depressive pathological conditions, the activated microglia may abnormally engulf neuronal synapses causing synaptic loss and behavioral impairments. Thus, microglial phagocytosis may be a novel therapeutic target for the treatment of depression.