Pseudoginsenoside-F11 attenuates cognitive dysfunction and tau phosphorylation in sporadic Alzheimer's disease rat model

Abstract

We previously reported that pseudoginsenoside-F11 (PF11), an ocotillol-type saponin, significantly ameliorated Alzheimer's disease (AD)-associated cognitive defects in APP/PS1 and SAMP8 mice by inhibiting Aβ aggregation and tau hyperphosphorylation, suggesting a potential therapeutic effect of PF11 in the treatment of AD. In the present study we further evaluated the therapeutic effects of PF11 on relieving cognitive impairment in a rat model of sporadic AD (SAD). SAD was induced in rats by bilateral icv infusion of streptozotocin (STZ, 3 mg/kg). The rats were treated with PF11 (2, 4, 8 mg·kg^-1·d^-1, ig) or a positive control drug donepezil (5 mg·kg^-1·d^-1, ig) for 4 weeks. Their cognitive function was assessed in the nest building, Y-maze, and Morris water maze tests. We showed that STZ icv infusion significantly affected the cognitive function, tau phosphorylation, and insulin signaling pathway in the hippocampus. Furthermore, STZ icv infusion resulted in significant upregulation of the calpain I/cyclin-dependent protein kinase 5 (CDK5) signaling pathway in the hippocampus. Oral administration of PF11 dose-dependently ameliorated STZ-induced learning and memory defects. In addition, PF11 treatment markedly reduced the neuronal loss, protected the synapse structure, and modulated STZ-induced expression of tau phosphorylation by regulating the insulin signaling pathway and calpain I/CDK5 signaling pathway in the hippocampus. Donepezil treatment exerted similar beneficial effects in STZ-infused rats as the high dose of PF11 did. This study highlights the excellent therapeutic potential of PF11 in managing AD.

Keywords: Alzheimer’s disease; Tau hyperphosphorylation; calpain I/CDK5 signaling pathway; donepezil; insulin signaling pathway; pseudoginsenoside-F11.

Fig. 1. Experimental design.

SAD was induced in rats by bilateral icv infusion of STZ (3 mg/kg). The rats were treated with PF11 (2, 4, 8 mg·kg⁻¹· d⁻¹, ig) or a positive control drug donepezil (5 mg·kg⁻¹· d⁻¹, ig) for 4 weeks. Their cognitive function was assessed in the behavioral tests. All rats were sacrificed after the behavioral tests for Nissl staining, TEM, immunochemical analysis and Western blot analysis.

Fig. 2. PF11 ameliorated STZ-induced cognitive decline.

a Chemical structure of PF11 (C₄₂H₇₂O₁₄, molecular weight = 801.02). b Self-care activity was evaluated in the nest building test. c Working memory was assessed in the Y-maze test. d–h The spatial memory of the mice was tested in the Morris water maze test. d Distance (distance traveled to find the platform), e escape latency (time to find the platform), f swimming velocity, g number of platform crossings during the probe test, and h representative swimming paths in the probe test. The data are reported as the mean ± SEM (n = 8–10). ^*P < 0.05, ^**P < 0.01, and ^***P < 0.001 vs. the sham group; ^#P < 0.05, ^##P < 0.01, and ^###P < 0.001 vs. the STZ group.

Fig. 3. PF11 alleviated STZ-induced neuronal death and synaptic damage.

a Nissl staining (×40, ×400) was used to evaluate neuronal death in the hippocampal DG and CA1 regions. Sections of the hippocampal DG and CA1 regions (scale bar = 50 μm). b Quantitative analysis of the number of surviving neurons in the hippocampal DG and CA1 regions with Image-Pro Plus (Media Cybernetics, Inc., Rockville, MD, USA) (n = 9). c Transmission electron microscopy (TEM) (×900) was performed to observed synapse structure (scale bar = 500 nm). The red arrow indicates a synapse. d Quantitative analysis of the number of synapses, PSD thickness and PSD length (n = 3). The data are reported as the mean ± SEM. ^**P < 0.01, ^***P < 0.001 vs. sham rats; ^#P < 0.05, ^##P < 0.01 vs. the STZ group.

Fig. 4. PF11 inhibited STZ-induced tau hyperphosphorylation in the hippocampus.

a Representative figures of coronal sections from each group after immunofluorescence staining against p-tau (scale bar = 100 μm, the red arrow indicates positive staining). b Levels of p-tau in the hippocampus were measured by Western blotting. c The integral optical density of p-tau (Ser396), p-tau (Ser199/202), and Tau5 in the hippocampus analyzed with ImageJ (NIH Image, Bethesda, MD, USA). The data are reported as the mean ± SEM (n = 3). ^**P < 0.01 vs. the sham group; ^#P < 0.05, ^##P < 0.01, and ^###P < 0.001 vs. the STZ group.

Fig. 5. PF11 attenuated STZ-induced tau hyperphosphorylation by reversing dysregulation of the IRS-1/PI3K/AKT/GSK-3β and calpain I/CDK5 signaling pathways.

a–d Expression of proteins related to the IRS-1/PI3K/AKT/GSK-3β and calpain I/CDK5 signaling pathways. a, c Protein was extracted from the hippocampus for each group and evaluated by Western blot analysis. b, d The integral optical densities of proteins related to the signaling pathways in the hippocampus were analyzed with ImageJ. The data are reported as the mean ± SEM (n = 3).^*P < 0.05, ^**P < 0.01, ^***P < 0.001 vs. the sham group; ^#P < 0.05, ^##P < 0.01, and ^###P < 0.001 vs. the STZ group.