Environmental Enrichment And Sodium Butyrate Reverse Synaptic Plasticity And Memory Dsficits In Presenilin1and Presenilin2Conditional Double Knockout Mice

Alzheimer’s disease (AD) is a progressive, irreversible neurodegenerative disease, and its cause is still not clear. The previous researches have reported that mutations of presenilin1(PS1) and/or presenilin2(PS2) change the APP splice sites of y-secretase, increase the production of Aβ42and result in AD symptoms. PS1/2conditional double knockout mice (DKO mice), which are used in the present study, also show many AD symptoms, such as brain atrophy, ventricles expand and neurofibrillary tangles. However, there is no Ap deposition in DKO mice. These suggest that there are other pathogenic mechanisms underlying AD symptoms. We combined electrophysiological, behavioral and molecular biological technologies to analyze the changes of Notch signaling, ErbB4signaling and histone acetylation in DKO mice. In addition, we investigated effects of environmental enrichment (EE) or sodium butyrate (SB) treatment on deficits in synaptic plasticity and memory function in DKO mice, and analyzed the mechanisms of their effects. The main results are as follows:1. Presenilin1and presenilin2double knockout impaired synaptic plasticity and memory function1) Effects of PS1and PS2double knockout on the basic synaptic transmission and long-term synaptic plasticity:Using electrophysiology technology, we recorded the field excited postsynaptic potential (fEPSP) at hippocampal CA3-CA1pathway. We found that input-output curve and paired-pulse facilitation were significantly impaired in6months-old DKO mice compared with Wt mice. Long-term potentiation (LTP) was also significantly impaired in DKO mice. These results indicated that PS1and PS2double knockout significantly impaired the basic synaptic transmission and LTP.2) Effects of PS1and PS2double knockout on hippocampus-dependent memory:Using the spatial reference memory, spatial working memory and aversive contextual fear memory tasks,we examined the hippocampus-dependent memory function in6months-old DKO mice. We found that spatial reference memory, spatial working memory and aversive contextual fear memory were significantly impaired in DKO mice compared with Wt mice. These results indicated that PS1and PS2double knockout significantly impaired memory function. 3) Mechanisms of the impairments of synaptic plasticity and memory in PS1and PS2double knockout mice:Using Western blotting technology, we examined the expression of some molecules related to synaptic plasticity and memory, a) The expression of NR2A and NR2B subunits of NMDA receptors were significantly reduced and the expression of GluRl and GluR2subunits of AMPA receptors also decreased significantly in hippocampus of DKO mice, b) The expression of CBP protein was significantly decreased in hippocampus of DKO mice, and CBP-dependent histone3(H3K27and H3K56) acetylation level, not CBP-independent histone3(H3K9) acetylation level, was also significantly declined. Furthermore, we did not observe changes in histone deacetylase (HDAC1and HDAC2) expression. These results indicated that lower level of histone acetylation is due to the reduction of CBP protein expression, rather than changing of histone deacetylase. c) The expression of brain-derived neurotrophic factor (BDNF), which is a CREB signaling dependent process, decreased significantly in DKO mice compared to Wt mice.d) The expression of Notch2intercellular domain was significantly increased in hippocampus of DKO mice, however, HES1, a downstream molecule of Notch, was expressed normally in DKO mice compared to Wt mice. These results indicated that our detected NICD band may be the immature precursor of NICD. No significant difference was found in ErbB4intercellular domain between DKO and Wt mice.2. EE reversed the deficits of synaptic plasticity and memory in presenilin1and presenilin2conditional double knockout miceEE has been shown to enhance learning and memory and synaptic plasticity in the brain regions, which are associated with learning and memory. In addition, EE could also increase histone acetylation, but the mechanisms are unclear. Thus, we study the effects of EE on synaptic plasticity and memory function in DKO mice and the mechanisms of these effects.1) EE significantly reversed the impairment of LTP at hippocampal CA3-CA1pathway in DKO mice.2) EE significantly reversed the impairments of spatial reference memory and spatial working memory as well as aversive contextual fear memory in DKO mice.3) Mechanisms by which EE reversed the deficits of synaptic plasticity and memory in DKO mice:a) EE significantly increased the expression of NR2A and NR2B subunits of NMDA receptors as well as GluR1and GluR2subunits of AMPA receptors at hippocampus in DKO mice, b) EE significantly increased CBP expression and CBP dependent histone acetylation (AcH3K27and AcH3K56) levels, but not CBP independent histone acetylation (AcH3K9) levels at hippocampus in DKO mice. In addition, we did not observe changes in histone deacetylase (HDAC1and HDAC2) expression. These results indicated that the increase of histone acetylation by EE may be due to the increase of CBP expression, rather than the changes of histone deacetylase. c) EE significantly reversed the reduction of CBP-dependent BDNF expression in DKO mice.3. Intraperitoneal injection of sodium butyrate reversed the deficits of synaptic plasticity and memory in presenilin1/2conditional double knockout miceSince SB is a broad-spectrum histone deacetylase inhibitor, we determined whether SB could reverse the reduction of histone acetylation in DKO mice and whether SB treatment could reverse the impairments of synaptic plasticity and memory in DKO mice. Our results suggested that:1) SB treatment significantly reversed the impairment of the basic synaptic transmission and LTP at hippocampal CA3-CA1pathway in DKO mice.2) SB treatment significantly reversed the impairments of hippocampus-dependent spatial reference memory, working memory and aversive contextual fear memory in DKO mice.3) Mechanisms by which SB reverse the impairments of synaptic plasticity and memory in DKO mice:a) SB treatment significantly increased the expression of NR2A and NR2B subunits of NMD A receptors as well as GluR1and GluR2subunits of AMPA receptors at hippocampus in DKO mice, b) SB treatment significantly increased the expression of AcH4K12in hippocampus of DKO mice. SB treatment also reversed the impairments of AcH3K27and AcH3K56expression. In particular, SB treatment significantly enhanced CBP expression, but did not change the expression of histone deacetylase (HDAC1and HDAC2) in DKO mice. These results indicated that the increases of cH3K27and AcH3K56expression were due to the increase of CBP expression, and it is also the first report showing that SB treatment can improve CBP expression.c) SB treatment significantly reversed the reduction of CBP-dependent BDNF expression at hippocampus in DKO mice.In conclusion, PS1/2double knockout can reduce the expression of CBP, and then result in reducing the expressions of histone acetylation and BDNF protein, which is CBP-dependent in DKO mice, and this might be the epigenetic mechanisms for the impairments of synaptic plasticity and memory function in DKO mice. In addition, the down-regulation of hippocampal NMDA receptors and AMPA receptors might be a direct reason for the impairments of synaptic plasticity and memory function in DKO mice. Environmental enrichment and sodium butyrate treatment can improve synaptic plasticity and memory function in DKO mice by reversing the changes of epigenetic mechanisms and the down-regulation of NMDA receptors and AMPA receptors.