IL-34/TREM2 modulates microglia-mediated inflammation and provides neuroprotection in a mouse model of sporadic Alzheimer's disease

Background As a recently identified cytokine, interleukin-34 (IL-34) is predominantly produced by neurons and functions as a modulator for glial functions. Emerging evidence indicates that IL-34 exerted neuroprotective effects in Alzheimer's disease (AD), but the underlying mechanism remained elusive. Objective To uncover the mechanisms by which IL-34 provides neuroprotection in AD. Methods Using senescence-accelerated mouse prone substrain 8 (SAMP8) mice, a well-established model for sporadic AD, we investigated the dynamic changes in brain IL-34 concentrations during AD progression. Afterwards, SAMP8 mice received a 4-week continuous intracerebroventricular infusion of IL-34. Morris water maze test was employed to assess the spatial cognitive functions. Neuronal and synaptic markers, oxidative stress makers, pro-inflammatory cytokines and glial activation markers in the brains of SAMP8 mice were measured. Finally, amyloid-β (Aβ) 42 -stimulated primary microglia, lentivirus-mediated gene knockdown strategy and co-immunoprecipitation assay were utilized to uncover the possible mechanisms by which IL-34 exerted neuroprotection in AD. Results In SAMP8 mice, we revealed that brain IL-34 concentrations gradually decreased during AD progression. A 4-week continuous intracerebroventricular infusion of IL-34 rescued spatial cognitive impairments, ameliorated neuronal and synaptic damage, and suppressed oxidative stress and microglia-mediated inflammation in the brains of SAMP8 mice. Using Aβ 42 -stimulated primary microglia, we demonstrated for the first time that IL-34 suppressed microglial NLRP3 inflammasome activation and pro-inflammatory cytokines release by interacting with triggering receptor expressed on myeloid cells 2 (TREM2), a key regulator of microglial functions. Conclusions These findings uncover the mechanisms by which IL-34 provides neuroprotection in AD, indicating that IL-34/TREM2 signaling may represent a novel therapeutic strategy for this devastating disease.