TrkB promotes the neuronal secretion of soluble Siglec-2 (CD22) to mitigate microglial activation and alleviate depression-like behaviors in male mice.

Microglia-neuron contacts have been shown to regulate neural network activity through the formation and elimination of synapses. The pathogenesis of major depressive disorder is accompanied by a decline in brain-derived neurotrophic factor (BDNF) signaling, associated with increased microglia activity that disrupts cognitive function. The actions of both typical and rapid-acting antidepressant drugs, which have been shown to increase BDNF signaling through the tropomyosin receptor kinase B (TrkB) receptor, decrease microglia activation and the levels of pro-inflammatory cytokines. Examining the link between BDNF signaling and the microglial pro-inflammatory response, we demonstrate that TrkB signaling elicits the neuronal secretion of CD22 (Siglec-2), a sialic acid-binding immunoglobulin-type lectin, to inhibit microglial activation and alleviate depression-like symptoms. In a male chronic mild stress (CMS) mouse model of depression decreased expression of the postsynaptic scaffolding protein PSD-95 and Gαi1/3 were found to compromise TrkB signaling leading to reduced CD22 levels in hippocampal tissue. Restoration of TrkB-Gαi1/3-Akt signaling with dSyn3, a peptidomimetic compound targeting the PDZ3 domain of PSD-95, enhanced CD22 expression to inhibit microglial activation, promote dendritic spine formation and rapidly mitigate depression-like symptoms. Furthermore, hippocampal overexpression of CD22 in neurons was sufficient to reduce microglial activation and depressive-like behaviors in male CMS mice. S-ketamine, a rapid-acting antidepressant, increased CD22 expression to mitigate depression-like symptoms. While neuronal knockdown of CD22 in the hippocampus did not significantly impair the rapid (within 4 h) antidepressant effects typically observed with S-ketamine or dSyn3 administration, strikingly, knockdown of CD22 attenuated the long-acting (within 3 days) antidepressant effects of S-ketamine or dSyn3, as evidenced by sustained immobility in the TST (tail suspension test) and FST (forced swim test), and a lack of improvement in sucrose preference. In contrast, a single dose of fluoxetine failed to increase CD22 expression or inhibit microglia activity. These results suggest that rapidly-acting anti-depressant drugs enhance TrkB-induced neuronal expression and secretion of CD22 to promote the homeostatic state of microglia required for antidepressant actions. In male depression mice, dSyn3 facilitates BDNF-induced TrkB-PSD-95-Gαi1/3 complex formation to increase Akt-mTOR activation as well as synaptic and spine density in the hippocampus. TrkB signaling increases CD22 expression and secretion from neurons blocking microglial activation in the hippocampal region of male CMS mice.