Sex Differences in Presynaptic Density and Neurogenesis in Middle-Aged ApoE4 and ApoE Knockout Mice

Abstract

Atherosclerosis and apolipoprotein E ε4 (APOE4) genotype are risk factors for Alzheimer's disease (AD) and cardiovascular disease (CVD). Sex differences exist in prevalence and manifestation of both diseases. We investigated sex differences respective to aging, focusing on cognitive parameters in apoE4 and apoE knockout (ko) mouse models of AD and CVD. Presynaptic density and neurogenesis were investigated immunohistochemically in male and female apoE4, apoE ko, and wild-type mice. Middle-aged female apoE4 mice showed decreased presynaptic density in the inner molecular layer of the dentate gyrus of the hippocampus. Middle-aged female apoE ko mice showed a trend towards increased neurogenesis in the hippocampus compared with wild-type mice. No differences in these parameters could be observed in middle-aged male mice. Specific harmful interactions between apoE4 and estrogen could be responsible for decreased presynaptic density in female apoE4 mice. The trend of increased neurogenesis found in female apoE ko mice supports previous studies suggesting that temporarily increased amount of synaptic contacts and/or neurogenesis is a compensatory mechanism for synaptic failure. To our knowledge, no other studies investigating presynaptic density in aging female apoE4 or apoE ko mice are available. Sex-specific differences between APOE genotypes could account for some sex differences in AD and CVD.

Figure 1

Placement of contours for the analysis of the number of synaptophysin-immunoreactive presynaptic boutons in the hippocampus. The squares indicate the randomly chosen areas in the inner (IML) and outer molecular layer (OML) of the dentate gyrus, the stratum lucidum (SL) of the CA3, and the stratum radiatum (SR) of the CA1.

Figure 2

Representative doublecortin-immunostained neurons in the dentate gyrus of the hippocampus (coronal section).

Figure 3

Number of synaptophysin-immunoreactive presynaptic boutons (SIPBs) in the prelimbic area (PRL), the cingulate gyrus (GC), and the stratum radiatum (SR), stratum lucidum (SL) and the inner (IML) and outer molecular layer (OML) of the dentate gyrus in the hippocampus. In female mice, there was a significant effect of genotype on the number of SIPBs in the IML (n = 7/group, P < 0.01). ApoE4 female mice have less SIPBs than female wild-type mice (∗P < 0.01). In other brain regions, no significant differences were found (n = 7/group, P > 0.05). In male mice, there were no significant effects found in any of the brain regions (n = 6–8/group, P > 0.05). Error bars show mean ± SEM.

Figure 4

Neurogenesis in the dentate gyrus of the hippocampus. In female mice, a trend was observed in the number of doublecortin-positive newly formed neurons between genotypes (n = 6–7 mice/group, ^# P = 0.052). The data strongly indicate that neurogenesis is increased in female apoE knockout mice compared with female wild-type mice. In males, no differences in neurogenesis were found (n = 5–7 mice/group, P > 0.05). Error bars show mean ± SEM.