The main finding of this study showed that the cleavage of AIF into cell death-inducing tAIF increased in the course of aging and Aβ accumulation selectively in the cortex of the Tg mouse model of AD studied. Moreover, the age-dependent increase in tAIF was not found in other vulnerable (hippocampus) or relatively spared (cerebellum) brain areas further pointing to the specificity of the observed alterations in the cortex. These results are the first to indicate the contribution of the key mediator of caspase-independent programmed cell death, AIF, to the cell loss associated with age-related progression in a model of AD-like pathology.
Furthermore in our study, cortical oxidative stress in Tg mice was found to occur as early as the pre-plaque stage (i.e. 2 months), thus greatly preceding the onset of plaque burden (3–4 months)
[34, 40] and cell loss (6–7 months)
[3, 40]. These findings validate our methodological approaches, as they are in agreement with previously published data in other AD mouse models demonstrating the relationship between oxidative stress, Aβ and Aβ plaque accumulation
[52–56]. Interestingly, the first signs of mitochondrial dysfunction, including signs of oxidative stress such as increased ROS production, coincide with the intracellular accumulation of Aβ and continue to accumulate with further increases in Aβ and its deposition into plaques in Thy-1 APP mice
. In addition, our results point to a higher Aβ load in the cortex than in the hippocampus and cerebellum of TgCRND8 mice. Moreover, despite a general trend towards the up-regulated production of ROS in both hippocampus and cortex of the youngest (2-month-old) mice, this increase was only significant in the cortex. These results are in line with the recently published data in APP/PS1 mice where oxidative stress is specifically increased in the cortex before evidence of plaque burden
Besides, the increase in the level of ROS was accompanied by decreased ATP production only in the cortex of the youngest (i.e. 2 months) age group studied, further suggesting early mitochondrial dysfunction in this brain region. These results may not be surprising since creatine levels have been reported to rise with age, and this might function to buffer ATP production
. Remarkably, the study of the mitochondrial proteome has recently identified an early alteration in expression of more than 20 proteins, among others involved in oxidative stress and apoptosis, specifically in the cortex of another mouse model of Alzheimer’s disease
The signs of mitochondrial dysfunction observed in our study preceded the increased production of tAIF and its nuclear translocation, both of which were augmented in cortices of mid-aged (4- and 6-month-old) Tg mice. Taken together, these data may suggest a causal relationship between mitochondrial impairment/oxidative stress and AIF-mediated caspase-independent PCD in this transgenic AD mouse model. In addition, these findings confirm and extend previous reports on the causal relationship between early mitochondrial dysfunction and multiple AD-associated cell death pathways including caspase-dependent apoptosis and autophagic pathways
[52, 56, 58].
Despite the observed genotypic differences in ROS levels, the production of ATP was similar between transgenic and non-Tg mice during aging (excluding 2-month-old transgenics). Previous studies have described differences in distinct ROS scavenging capacity as well as metabolic and intracellular signalling between the cortex and hippocampus
[52, 55, 59, 60]. Therefore, the differential sensitivity of mitochondrial ATP production and increased ROS between these two regions reported herein may be associated with their respective level of expression of AIF. In agreement with this hypothesis, our previous study revealed significant differences in the level of AIF expression between brain regions of the rodent brain
. Furthermore, low level of AIF expression in Harlequin mice has been linked to mitochondrial respiratory chain dysfunction
 and AIF has been implicated in ROS production
. Therefore, it is plausible that the tissue level of AIF may be related to the degree of Aβ-induced oxidative stress and subsequent AIF-mediated cell death.
Age-related cell death in Tg mice was demonstrated in this study by increased levels of cortical tAIF and Bax in the mid-aged groups (4- and 6-month old). However, in the oldest group (9-month-old Tg mice) AIF-mediated cell death was not evident, as indicated by the absence of increased tAIF and Bax. This apparent paradoxical observation may be related to the limited, yet still significant, neuronal loss observed in our model, starting by 6–7 months of age
Cell death in the hippocampus has been documented at 6 months of age in TgCRND8 mice
. Interestingly in this study, the hippocampal level of Bax was not increased before 9 months and no significant age-dependent increase was seen in the level of tAIF, suggesting that cell death does not involve AIF and is Bax-independent, at least up to 9 months of age. In the hippocampus of the two mid-aged groups, the absence of deleterious effects (despite high levels of ROS) is suggested to be the result of compensated production of ATP, which may be related to the increased expression of the mitochondrial, 62 kDa, form of AIF. An increased expression of this form of AIF may provide more efficient respiratory chain function due to, at least in part, more efficient complex I and III function
Cyclin D1 expression, used here as an early marker of neuronal cell death, was similarly expressed between different age groups in all brain regions. Although cyclin D1 induction has been associated with Aβ-induced cell death, in vitro and in human post-morten AD brain tissues
[45, 48–50], the data are much less conclusive regarding the relationship of cyclin D1/Aβ in AD mouse models