Effects Of Simvastatin On Aβ_25-35-impaired Hippocampal Neurogenesis And Underlying Molecular Mechanisms

IntroductionStatins, inhibitors of 3-hydroxy-3-methyl-glytarylcoenzyme A(HMG-Co A) reductase, are widely used to prevent or treat cardiovascular diseases. In the general population, the use of statins is associated with a lower risk of Alzheimer’s disease(AD). Clinical epidemiology study has shown that the treatment with SV can decrease risk for AD and improve cognition in AD patients. However, the underlying mechanism still remains unclear.A large body of evidence has established that the mammalian brain produces continuously newborn neurons in hippocampal dentate gyrus(DG), termed “adult neurogenesis” or “neuronal regeneration”. The newborn neurons can connect to hippocampal CA3 field and entorhinal cortex to integrate into the neural network, which can instead of degeneration and death of neurons. This is reinforced by evidence that hippocampus-dependent memory is strongly correlated with increased levels of newborn neurons, while the inhibition of neurogenesis has adverse effects on cognitive behaviors. The main component of senile plaques is β-amyloid 25-35 peptide(Aβ25-35). Our previous studies have reported that Aβ25-35 injection(i.c.v.) can impair the survival and neurite growth of newborn neurons through down-regulating PI3K-Akt-m TOR signaling. The treatment with SV in Aβ25-35-mice through increasing phosphorylation of PI3K-Akt and extracellular ERK1/2 can reduce the death of neuronal cells in a α7n ACh R-dependent manner. Thus, it is proposed that the treatment with SV through enhancing α7n ACh R can correct the Aβ25-35-induced decrease of Akt phosphorylation, which protects the hippocampal neurogenesis.ObjectiveThe aims of the present study were(1) to investigate the effects of SV on the Aβ25-35-impaired neurogenesis and spatial cognitive function;(2) to explore the molecular mechanisms underlying SV-protected neurogenesis in Aβ25-35-mice.Materials and Methods1. Preparation of AD animal model: ICR male mice were intracerebroventriculy injected(i.c.v.) with the “aggregated” Aβ25-35(3 nmol/3 μl) to prepare the Aβ25-35-mice.2. SV administration: Oral administration(p.o.) of SV at dose of 20 mg/kg daily was given starting at 4 h after Aβ25-35-injection for consecutive 13 days.3. Histological examination: 5-bromodeoxyuridine(Brd U) was intraperitoneally injected thrice at 24 h before Aβ25-35-injection to label proliferating cells. Brd U immuno-staining was performed on day 14 and 28 after the last Brd U injection, respectively. Doublecortin(DCX) immuno-staining was used to quantify the dendritic growth of newborn neurons. Brd U and Neu N double immuno-fluorescence staining were used to evaluate the maturation of newborn neurons.4. Cleaved caspase-3 immuno-staining was used to examine the number of apoptotic cells.5. Western Blot was used to analysis the levels of Akt and ERK1/2 phosphorylation in hippocampal.6. Brain derived neurotrophic factor(BDNF) levels were determined by enzyme-linked immunosorbent assay(ELISA).7. Behavioral examination: Morris water maze task and Y-maze task were used to test the cognitive performance.Results1. The number of 14-day-old Brd U positive(Brd U+) cells, 28-day-old Brd U+ cells and 28-day-old Brd U+/Neu N+ cells in Aβ25-35-mice was decreased significantly relative to control mice, which could be rescued by the SV-treatment in a dose-dependent manner. The findings indicate that the SV-treatment can protect the survival of newborn neurons in Aβ25-35-mice.2. In comparison with control mice, not only the number of DCX positive(DCX) cells but also the number of DCX+ fibers per DCX+ cell was reduced in Aβ25-35-mice, which was corrected by the SV-treatment. The findings indicate that the SV-treatment can protect the neurite growth of newborn neurons in Aβ25-35-mice.3. A large number of caspase-3 positive(caspase-3+) cells was observed in hippocampal SGZ and inner 1/3 granular cell layer of Aβ25-35-mice, which was significantly attenuated by the SV-treatment. The findings indicate that the SV-treatment can attenuate the apoptosis of newborn neurons in Aβ25-35-mice.4. The administration of farnesol(FOH) that can convert to farnesyl pyrophosphate(FPP) in Aβ25-35-mice could attenuate the SV-protected survival or neurite growth of newborn neurons. The findings indicate that the SV-treatment in Aβ25-35-mice can protect neurogenesis through reducing FPP levels.5. The effects of SV-protected survival and neurite growth of newborn neurons in Aβ25-35-mice were blocked by the α7n ACh R antagonist MLA in a dose-dependent manner, but not the α2β4n ACh R antagonist DHβE.6. In comparison with controls, the level of phosphor-Akt in Aβ25-35-mice was reduced. The SV-treatment could rescue the decline of phosphor-Akt in Aβ25-35-mice, which was blocked by the application of MLA or FOH. The application of PI3 K inhibitor LY294002 in Aβ25-35-mice could block the SV-protected survival and neurite growth of newborn neurons. The findingsindicate that the SV-treatment can correct the Aβ25-35-induced decline of hippocampal phosphor-Akt through the reduction of FPP and the activation of α7n ACh R to protect neurogenesis.7. The level of phosphor-ERK2 was increased by the SV-treatment in Aβ25-35-mice, which was attenuated by MLA or FOH. However, the effects of SV-protected survival and neurite growth of newborn neurons in Aβ25-35-mice were not affected by MEK inhibitor U0126.8. The SV-treatment could correct the decline of hippocampal BDNF concentration in Aβ25-35-mice, which was blocked by MLA and FOH.9. The SV-treatment in Aβ25-35-mice could attenuate the prolongation of escape latency and improve the decline of alternation ratio, which were sensitive to the FOH-application. The findings indicate that the SV-protected neurogenesis contributes to the spatial cognitive improvement in Aβ25-35-mice.ConclusionThe results indicate that treatment with SV through the reduction of FPP and the activation of α7n ACh R can correct the Aβ25-35-induced decline of hippocampal PI3K-Akt and BDNF to protect neurogenesis, which is accompanied by the improvement of spatial cognitive deficits.