Positive and negative early life experiences differentially modulate long term survival and amyloid protein levels in a mouse model of Alzheimer's disease

Abstract

Stress has been implicated as a risk factor for the severity and progression of sporadic Alzheimer's disease (AD). Early life experiences determine stress responsivity in later life, and modulate age-dependent cognitive decline. Therefore, we examined whether early life experiences influence AD outcome in a bigenic mouse model which progressively develops combined tau and amyloid pathology (biAT mice).Mice were subjected to either early life stress (ELS) or to 'positive' early handling (EH) postnatally (from day 2 to 9). In biAT mice, ELS significantly compromised long term survival, in contrast to EH which increased life expectancy. In 4 month old mice, ELS-reared biAT mice displayed increased hippocampal Aβ levels, while these levels were reduced in EH-reared biAT mice. No effects of ELS or EH were observed on the brain levels of APP, protein tau, or PSD-95. Dendritic morphology was moderately affected after ELS and EH in the amygdala and medial prefrontal cortex, while object recognition memory and open field performance were not affected. We conclude that despite the strong transgenic background, early life experiences significantly modulate the life expectancy of biAT mice. Parallel changes in hippocampal Aβ levels were evident, without affecting cognition of young adult biAT mice.

Keywords: Alzheimer; Gerotarget; biAT; early handling; early life stress; glucocorticoids.

Figure 1. Early life experiences acutely affect body weight of biAT mice

Body weight gain measured from PND 2 and 9 in A. ELS litters (Ctrl: n = 7; ELS: n = 7; t(11) = 2.96; p < 0.05), and B. EH animals (Ctrl: n = 5; EH: n = 4; t(4) = −2.93; p < 0.05) each compared to control reared biAT mice. Data are expressed as mean ± SEM.

Figure 2. ELS exacerbates early death while EH prolongs survival of young biAT mice

A. ELS significantly decreased survival of biAT mice (Ctrl: n = 32; ELS: n = 28; χ²(1) = 4.35; p < 0.05). B. EH prolonged survival compared to control mice (Ctrl: n = 15; EH: n = 23; χ²(1) = 4.15; p < 0.05). C. Tau.P301L littermates were not significantly affected by ELS (Ctrl: n = 31; ELS: n = 25; χ²(1) = 1.17, ns) or D. by EH (Ctrl: n = 17; EH: n = 6; χ²(1) = 0.62, ns).

Figure 3. Early life experience alters hippocampal Aβ levels at PND 120 in biAT mice

A. Western blot of hippocampal protein extracts demonstrated elevated Aβ levels after ELS B., whereas EH led to a reduction in Aβ levels. C.. No effects of ELS or EH were observed on levels of APP. B. protein tau (AT8) C. or PSD-95 (H,I). ELS vs Ctrl, Ctrl: n= 7, ELS: n = 5; EH vs Ctrl, Ctrl: n = 3, EH: n = 5. Data are expressed as mean ± SEM.

Figure 4. Explorative behaviour and cognition was not affected by early life experiences

Time spent in the centre of the open field was not significantly different between ELS and Ctrl biAT (t(12) = 0.41, ns). A. or EH and Ctrl biAT mice (t(35) = −0.70, ns. B.. Total locomotor activity was also resistant to either ELS (Ctrl vs ELS: t(12) = 0.86, ns) C. or EH (Ctrl vs EH: t(35) = −0.38, ns). D.. Although all groups explored the novel object significantly above chance level (Ctrl vs ELS experiment: Ctrl: t(5) = 17.99, p < 0.000; ELS: t(5) = 3.16, p < 0.05; Ctrl vs EH experiment: Ctrl: t(15) = 2.96, p < 0.05; EH: t(19) = 4.60, p < 0.000) cognitive performance was comparable for all groups (Ctrl vs ELS: t(5.58) = 1.16, ns; Ctrl vs EH: t(34) = 0.43, ns). E.,F.. ELS vs Ctrl: Ctrl: n = 7, ELS: n = 7; EH vs Ctrl: Ctrl: n = 15, EH: n = 21. Data are expressed as mean ± SEM.

Figure 5. Dendritic morphology of mPFC subregions is differentially affected by ELS and EH

Sholl plots of the distribution of apical and basal dendritic length at increasing distances from the centre of the cell body. In the basal dendrites of the infralimbic cortex, a trend towards a reduction in dendritic distribution was observed after both ELS and EH (ELS vs Ctrl: F(1,7) = 5.11, p = 0.058; EH vs Ctrl: F(1,17) = 4.31, p < 0.05). A.,B.. In the infralimbic cortex no effects of ELS or EH were reported on apical dendrites C.,D.. As a consequence of ELS, basal dendritic distribution of the prelimbic area was increased (F(1,11) = 4.92, p < 0.05), with significant post hoc differences at 60 μm from the centre of the cell body E.. No effects were observed in basal dendritic distribution in the prelimbic area after EH F.. The apical branches of the prelimbic cortex were not different between ELS and EH animals and their respective controls G.,H.. In the cingulate cortex, dendritic length at increasing distance from the centre of the cell body is comparable between ELS and Ctrl I.,K. and between EH and Ctrl J.,L. for both the apical and basal tree. 3-5 neurons from one animal were averaged (3-12 animals/group). Data are expressed as mean ± SEM.

Figure 6. Early life stress increased neuronal complexity in the basolateral amygdala

A. Representative image of the basolateral nucleus of the amygdala from Golgi-Cox-stained coronal brain section. B. ELS significantly increased the segmental complexity (F(1,7) = 6.69, p < 0.05) between 70 and 120 μm, and between 150 and 160 μm from the soma. C. Neuronal complexity was not affected by EH. Data of 3-5 neurons from one animal were averaged (3-12 animals/group) and expressed as mean ± SEM.

Figure 7. Neuronal morphology of CA1 and CA3 pyramidal neurons is unaffected by early life experiences

Representative images from pyramidal neurons in CA1 A. and CA3 F. areas from Golgi-Cox staining of coronal sections. Sholl plots indicated that the distribution in CA1 of basal B.,C. and apical D.,E. dendritic length at increasing distances from the centre of the cell body is comparable between groups. In the CA3, the segmental distribution of basal branches of the neuron is not affected by ELS G. and EH H.. The segmental distribution of the apical branches is also comparable between ELS and Ctrl I. and EH and Ctrl J. animals. Data from 3-5 neurons from one animal were averaged (n = 5-12 animals/group) and expressed as mean ± SEM.