Morroniside Modulates Microglia Polarization via the CX3CL1/CX3CR1/PU.1 Axis in <i>ApoE4</i> Transgenic Mice

Microglia monitor disease stimulation, neuronal apoptosis, and neural repair, and their overactivation-induced inflammation plays a key role in the pathogenesis of Alzheimer's disease (AD). Morroniside (Mor), an iridoid glycoside compound in Cornus officinalis, is one of the effective active components. The effects of Mor on antioxidant stress, antiapoptosis, and nerve repair function have been widely studied, but the mechanism of Mor in AD treatment remains unclear. To study the neuroprotective effects of Mor and elucidate the molecular mechanisms underlying its improvement of AD symptoms, we used ApoE4 transgenic mice and ApoE4-transfected BV2 cells as models of AD, focusing on microglia phenotype, function, and neuroinflammation. The 10-month-old mice were randomly divided into the ApoE3 control group (ApoE3 + Veh), the ApoE4 model group (ApoE4 + Veh), and the ApoE4 + Mor 10, 20, and 40 mg/kg groups as in vivo models. The in vitro BV2-ApoE model was constructed via lentiviral transfection. The effects of Mor on cognitive function of AD models were assessed through behavioral tests, western blot, immunofluorescence staining, and ELISA to measure changes of related pathological and inflammatory factors. Mor improved the cognitive function of ApoE4 transgenic mice by reducing Aβ plaques in the brain, improving the structural lesions of hippocampal neurons, and increasing synaptic plasticity in the brain of AD mice. In addition, Mor promoted the transformation of microglia from the M1 to the M2 phenotype, inhibited the activation of the CX3CR1/PU.1 signaling axis, and alleviated the dysfunction of microglia both in vitro and in vivo. CX3CR1 siRNA and PU.1 siRNA were used further to verify the regulatory effect of Mor on microglia phenotype. Our findings indicate that Mor can inhibit neuroinflammation, reduce Aβ accumulation, and improve synaptic damage in ApoE4 mice via the CX3CL1/CX3CR1/PU.1 pathway regulating the phenotype and function of microglia. This study provides a new therapeutic candidate for the prevention and treatment of AD.