The Effect Of Apolipoprotein E4 On Hippocampal Synaptic And Neural Network In Young Mice Carried Human ApoE4 Gene

Objective: To investigate the alterations of hippocampal dendritic and spine morphology and neural network in young ApoE4 transgenic(TG)mice.Methods: 22 young(4-month-old)male ApoE4TG mice,weighting 30g±5g,and 22 young(4-month-old)male ApoE3TG mice,weighting 30g±5g were divided into two groups: experiment group(ApoE4TG mice)and control group(ApoE3TG mice).All the transgenic mice expressed human ApoE4 or ApoE3 gene.The whole mice were sacrificed under isoflurane anesthesia.Then brain tissue was quickly removed and bathed in cold(4?C)artificial cerebrospinal fluid(ACSF)which was continuously bubbled with 95%O2.Hippocampal coronal slices containing dorsal sections were cut using a vibratome.Then slices were transferred to a holding chamber for incubation for at least 1 hour at t room temperature.Thereafter,single slice was transferred to a liquid-air interface chamber to record field potentials,which were performed from the hippocampal CA1 region using recording electrode and stimulating electrode.Several parameters were collected,including Input-output relationship curve(I/O),paired-pulse facilitation(PPF),Long-term Potentiation(LTP),high frequency stimulations(HFSs)and carbachol-induced θ oscillation(CCh-induced θ oscillation).Besides,I/O,PPF and LTP were also compared between young(4-month-old)and aged(12-month-old)male ApoE4TG mice.Fluorescent dye,Biocytin was injected into recorded hippocampal CA1 pyramidal neuron after patch-clamp whole-cell recording.Then these labeled neurons were reconstructed for quantified analysis of hippocampal dendritic and spine morphology.Result:1 Hippocampal neuronal dendritic and spine density comparison between young ApoE3 and ApoE4TG mice ApoE4 neurons exhibited significantly reduced spine head volume(P<0.01)compared to ApoE3 neurons.Spine density in ApoE4 neurons is significantly lower overall(Fig.AE;9.2 ± 0.31 spines per 10μm dendrite in ApoE4 neurons compared with 13.7 ± 0.9 in ApoE3 neurons;P<0.05)(Fig.2).2 Field potential Comparison of the linear slopes of input-output relationship curves between young ApoE4 and ApoE3TG mice,the results revealed a significant difference between these two groups(ApoE4: 0.89 ± 0.08,ApoE3: 0.49 ± 0.04,P<0.0001)(Fig.3).The ratio of P2/P1 at all tested P1 and P2 intervals in young ApoE4TG mice showed a significant enhancement in the PPF when compared to young ApoE3TG mice(P<0.05)(Fig.4).Compared to young ApoE3TG mice,both of the LTP induction(LTP 0-10 min)and maintenance(after LTP 50-60 min)were impaired after theta-burst stimulation in young ApoE4TG mice(LTP induction: ApoE4 group: 1.38±0.02,ApoE3 group: 1.52±0.02,P<0.0001;LTP maintenance: ApoE4 group: 1.31 ± 0.01,ApoE3 group: 1.47 ± 0.01,P<0.0001)(Fig.5).The aged ApoE4TG mice exhibited an impaired synaptic transmission.The averaged linear slope of aged ApoE4TG mice between 4 and 10 V-induced response showed a significant difference when compared to young ApoE4TG mice(aged: 0.49±0.04,young: 0.89±0.07,P<0.001).The ratio of all P1 and P2interval was reduced in aged ApoE4TG mice compared to young ApoE4TG mice.For LTP,there was no significant difference of LTP induction(LTP 0-10min)between aged and young ApoE4TG mice(aged: 1.34±0.04,young ApoE4 1.38±0.02,P>0.05).But aged ApoE4 mice exhibited a reduced LTP maintenance(after LTP 50-60min)(aged: 1.20±0.006,young 1.31±0.007,P<0.0001)(Fig.6).For the CCh-induced θ oscillation,the initiation time of burst events was prolonged(P<0.05)and the burst event numbers was reduced(P<0.01),which demonstrated a significant impairment of CCh-induced theta oscillations in young ApoE4TG mice compared to young ApoE3TG mice(Fig.7). The results of HFSs in the hippocampus reflected the synaptic vesicle recycling.There was no significant difference of the ratio of last and first stimulation-induced f EPSPs and the ratio of maximal and first peak responses between in young ApoE4TG mice and young ApoE3TG mice in three stimulatory parameters(Fig.8).Conclusion: 1 The young ApoE4TG mice show impaired dendritic length and reduced spine density in the hippocampal CA1 area.2 The basic synaptic transmission and paired-pulse facilitation are enhanced,but long term potentiation and carbachol-induced theta oscillations are impaired in the hippocampal CA3-CA1 of young ApoE4TG mice.