NLRP3 inflammasome inhibition and M1-to-M2 microglial polarization shifting via scoparone-inhibited TLR4 axis in ovariectomy/D-galactose Alzheimer's disease rat model

Neuroinflammation mediated by microglia activation is a critical contributor to Alzheimer's disease (AD) pathogenesis. Dysregulated microglia polarization in terms of M1 overactivation with M2 inhibition is involved in AD pathological damage. Scoparone (SCO), a coumarin derivative, displays several beneficial pharmacological effects including anti-inflammatory and anti-apoptotic properties, however, its neurological effect in AD is still elusive. This study investigated the neuroprotective potential of SCO in AD animal model focusing on determining its effect on M1/M2 microglia polarization and exploring the plausible mechanism involved via investigating its modulatory role on TLR4/MyD88/NF-κB and NLRP3 inflammasome. Sixty female Wistar rats were randomly allocated into four groups. Two groups were sham-operated and treated or untreated with SCO, and the other two groups were subjected to bilateral ovariectomy (OVX) and received D-galactose (D-Gal; 150 mg/kg/day, i.p) alone or with SCO (12.5 mg/kg/day, i.p) for 6 weeks. SCO improved memory functions of OVX/D-Gal rats in the Morris water maze and novel object recognition tests. It also reduced the hippocampal burden of amyloid-β42 and p-Tau, additionally, the hippocampal histopathological architecture was prominently preserved. SCO inhibited the gene expression of TLR4, MyD88, TRAF-6, and TAK-1, additionally, p-JNK and NF-κBp65 levels were significantly curbed. This was associated with repression of NLRP3 inflammasome along with M1-to-M2 microglia polarization shifting as exemplified by mitigating pro-inflammatory M1 marker (CD86) and elevating M2 neuroprotective marker (CD163). Therefore, SCO could promote microglia transition towards M2 through switching off TLR4/MyD88/TRAF-6/TAK-1/NF-κB axis and inhibiting NLRP3 pathway, with consequent mitigation of neuroinflammation and neurodegeneration in OVX/D-Gal AD model.