Mechanistic effects of Lycium barbarum polysaccharides against rat hippocampal injuries induced by chronic intermittent hypoxia

Abstract

Background: Chronic intermittent hypoxia (CIH), highlighting clinical manifestations of obstructive sleeping apnea (OSA), induces oxidative stress, inflammation, endoplasmic reticulum (ER) stress and apoptosis in hippocampus in which regenerative mechanism is subsequently activated for repairing damages. Lycium barbarum polysaccharides (LBP), bioactive fraction of traditional Chinese herbal medicine Goji, are demonstrated to possess anti-oxidative and anti-inflammatory properties, and promote hippocampal neurogenesis inhibited by corticosterone. Objectives: Our study aims to elucidate the mechanistic effects of LBP in CIH rats. We hypothesize that LBP attenuate oxidative stress, inflammation, endoplasmic reticulum stress, apoptosis, and facilitate hippocampal regeneration in rat exposed to CIH. Methods: Adult Sprague-Dawley rats (180-200 g) were exposed to air as normoxic control (Nx) or to intermittent hypoxic treatment (IH) with normobaric oxygen content alternating between 5 to 21% 8 hr/day for 1 week. Rats were orally fed LBP solution (1 mg/kg body weight) or vehicle 2 hours prior to hypoxic treatment. Lipid peroxidation extent was measured by MDA assay. Western blot was employed to examine the expressions level of antioxidant enzymes (SOD-1, SOD-2, GPx-1); inflammatory mediators (IL-1β, TNFα, COX-2); redox sensitive transcriptional factor Nuclear factor kappa B (NFκB) p65 and p50; negative regulator of NFκB (IκBα); ER stress markers (GRP78/Bip, PERK and CHOP); caspase-dependent extrinsic (FADD, caspase 8, Bid) and intrinsic apoptotic (Bax, Bcl2, cytochrome C) cell death (cleaved caspase 3) cascades; endogenous cell cycles markers (PCNA); phosphorylation of survival molecule Akt (p-Akt(Ser 473)). In situ cell death staining (TUNEL) was utilized to reveal the apoptotic situation of hippocampal subfields (DG, CA1 and CA3). PCNA and BrdU DAB immunostaining were performed to demonstrate cellular proliferation in subgranular zone (SGZ) of dentate gyrus in hippocampus. Results: LBP administration significantly decreased and restored, respectively, the elevated MDA level and depleted antioxidant enzymes in the hypoxic treated group. Besides, degradation of IκBα, activation and translocation of NFκB p65 and p50 were observed in hypoxic treated groups but were significantly inhibited by LBP pre-treatment. Additionally, LBP pre-treatment markedly attenuated ER stress sensors activated in hypoxic treated groups. The number of TUNEL positive labelled cells was found significantly increased when compared with that of controls but dramatically reduced by LBP pre-treatment. Importantly, LBP antagonized CIH-induced hippocampal cell death through mitigation of caspase mediated intrinsic and extrinsic cascades. On the other hand, the numbers of PCNA- and BrdUpositive labelled cells were elevated in hypoxic groups and were further augmented by LBP administration. Consistently, protein levels of PCNA and phosphorylated Akt were increased in the hypoxic group, which were further enhanced by LBP pre-treatment. Conclusion: The anti-oxidant, anti-apoptotic and pro-regenerative properties of LBP could explain its protective effects against neurocognitive deficit induced by severe OSA conditions.