Mechanisms underlying large-leaf yellow tea mediated inhibition of cognitive impairment in the 5xFAD model of Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia and is characterized by amyloid-β (Aβ) peptides and hyperphosphorylated Tau proteins. Evidence indicates that AD and type 2 diabetes mellitus (T2DM) share pathophysiological characteristics, including impaired insulin sensitivity. Large-leaf yellow tea (LYT) has been widely recognized for its health benefits, and we previously found that LYT can improve peripheral insulin resistance. This study aimed to investigate the protective effects and underlying mechanisms of LYT in the 5xFAD mouse model of AD. HPLC and spectrophotometric methods determined the chemical composition of the LYT extract. 5xFAD mice were treated with LYT supplementation (2 and 4 mg/ml) in drinking water for six months. Barnes and Y mazes were used to evaluate cognitive function, and the open field test assessed anxiety-like behavior. Immunofluorescence, silver, and Nissl staining were used to evaluate the pathological effects of LYT extract. A FRET-based assay assessed β-site APP cleavage enzyme 1 (BACE1) activity, ELISA measured Aβ levels in the brain, and Western blot analyses explored protein expression levels. Our results revealed that LYT significantly attenuated memory impairment and anxiety levels and alleviated cerebral neural damage. A reduction of senile plaques was also observed in both the cortex and hippocampus. LYT significantly inhibited the activity of BACE1, which resulted in a lower Aβ protein level. In addition, LYT enhanced insulin receptor substrate 1 (IRS-1)-mediated phosphorylation of phosphoinositide 3-kinase (PI3K) and protein kinase B (AKT), further suppressed glycogen synthase kinase-3β (GSK3β), and ultimately inhibited hyperphosphorylation of the protein Tau. The inhibitory effect of the LYT extract on the phosphorylation of Tau and BACE1 activity was dose-dependent. LYT improves cognitive ability and reduces Aβ production by inhibiting BACE1 activity. Decreases of Tau protein hyperphosphorylation upon LYT treatment appear to be associated with the regulation of the IRS-1/PI3K/AKT/GSK3β axis. Thus, the findings of this study also provide new evidence that LYT regulates insulin signaling pathways within the central nervous system.