Synaptic and Cognitive Improvements by Inhibition of 2-AG Metabolism Are through Upregulation of MicroRNA-188-3p in a Mouse Model of Alzheimer's Disease

Abnormal accumulation of β-amyloid (Aβ) is the major neuropathological hallmark of Alzheimer's disease (AD). However, the mechanisms underlying aberrant Aβ formation in AD remain unclear. We showed previously that inhibition of monoacylglycerol lipase (MAGL), the primary enzyme that metabolizes the endocannabinoid 2-arachidonoylglycerol (2-AG) in the brain, robustly reduces Aβ by inhibiting β-site amyloid precursor protein cleaving enzyme 1 (BACE1), a key enzyme responsible for Aβ formation. However, the molecular mechanisms responsible for suppression of BACE1 by inhibition of 2-AG metabolism are largely unknown. We demonstrate here that expression of the noncoding small RNA miR-188-3p that targets BACE1 was significantly downregulated both in the brains of AD humans and APP transgenic (TG) mice, a mouse model of AD. The downregulated miR-188-3p expression was restored by MAGL inhibition. Overexpression of miR-188-3p in the hippocampus reduced BACE1, Aβ, and neuroinflammation and prevented deteriorations in hippocampal basal synaptic transmission, long-term potentiation, spatial learning, and memory in TG mice. 2-AG-induced suppression of BACE1 was prevented by miR-188-3p loss of function. Moreover, miR-188-3p expression was upregulated by 2-AG or peroxisome proliferator-activated receptor- (PPAR) agonists and suppressed by PPAR antagonism or NF-B activation. Reducing Aβ and neuroinflammation by MAGL inhibition was occluded by PPAR antagonism. In addition, BACE1 suppression by 2-AG and PPAR activation was eliminated by knockdown of NF-B. Our study provides a novel molecular mechanism underlying improved synaptic and cognitive function in TG mice by 2-AG signaling, which upregulates miR-188-3p expression through PPAR and NF-B signaling pathway, resulting in suppressions of BACE1 expression and Aβ formation.