Increasing evidence links mitochondrial dysfunction in multiple neurodegenerative disorders, such as Alzheimer’s disease (AD) [11
]. We previously demonstrated that mitochondrial bioenergetic deficits precede AD pathology in the female triple transgenic AD mouse model [7
], suggesting a potential causal role of mitochondrial bioenergetic deficiency in AD pathogenesis. Clinically, Alzheimer’s pathology is accompanied by a decrease in expression and activity of enzymes involved in mitochondrial bioenergetics, which would be expected to lead to compromised electron transport chain complex activity and reduced ATP synthesis [28
]. Further, in AD there is a generalized shift from glycolytic energy production towards use of an alternative fuel, ketone bodies. This is evidenced by a 45% reduction in cerebral glucose utilization in AD patients [31
], which is paralleled by decrease in the expression of glycolytic enzymes coupled to a decrease in the activity of the pyruvate dehydrogenase complex [28
]. In the current study, reproductive senescent nonTg female mice exhibited a brain metabolic profile comparable to the metabolic phenotype observed early in AD. In normal aging female nonTg mice, reproductive senescence paralleled a significant decline in mitochondrial bioenergetic function including decreased mitochondrial respiration, compromised PDH and COX activity, and activation of ketogenic pathway. In 3xTgAD mice, reproductive senescence paralleled the exacerbation of the existing impairment of mitochondrial bioenergetics that occurred early in the disease progression.
PDH is the key rate-limiting enzyme in mitochondria to convert pyruvate, the end product of glycolysis, into acetyl-CoA, which subsequently condenses with oxaloacetate to initiate the TCA cycle for energy production. Complex IV is the terminal enzyme of electron flow and reduces O2
O. Compromised activity of PDH and COX reduces the substrate input and driving force for oxidative phosphorylation, resulting in increased ATP demand via other pathways that leads to the activation of ketogenic pathway. Our findings of decreased activity of PDH and COX with reproductive senescence are consistent with the clinical observation of significantly decreased PDH and COX activity in postmortem Alzheimer’s brain tissue [28
In contrast to decreased PDH and COX activity, reproductive senescence paralleled a significant increase in HADHA and SCOT protein expression in nonTg mice and a moderate increase in 3xTgAD mice, indicative of activation of ketogenic pathway in brain which would be required to generate an alternative fuel source to compensate for the decline in glucose driven metabolic activity. These findings are also consistent with clinical observations that patients with incipient AD exhibit a utilization ratio of 2:1 glucose to alternative fuel whereas comparably aged controls exhibit a ratio of 29:1 while young controls exclusively use glucose as with a ratio of 100:0 ratio [33
Together, these data indicate a critical role for ovarian hormones in sustaining and enhancing glucose driven brain metabolism and mitochondrial function. Ovarian hormones, particularly estrogen, has been demonstrated to positively enhance glucose driven mitochondrial bioenergetics [34
]. Estrogen promotes the coupling of glycolysis to oxidative phosphoyrlation (OXPHOS) by increasing the activity of both glycolytic enzymes, including hexokinase, phosphofructokinase, and phosphoglycerate kinase [35
], and the expression and activity of proteins involved in OXPHOS, including pyruvate dehydrogenase, aconitase, and ATP synthase [19
]. Loss of ovarian hormones due to reproductive senescence could induce a systematic decrease in glucose driven ATP generation. The increase in ketogenic enzyme expression is reflective of compensatory mechanism to generate an alternative pathway to utilize ketone body for ATP generation. Noticeably, at 12 month nonTg mice expressed significantly higher level of SCOT relative to 3xTgAD mice but which were commensurate with SCOT levels at the early preAD pathology stage of AD. Collectively, findings from this study indicate that loss of ovarian hormones due to reproductive senescence accompanies a bioenergetic phenotype in the female brain suggestive of the prodromal AD metabolic phenotype. These findings provide a potential mechanism underlying increased risk for AD observed in postmenopausal women.
To assess whether all reproductively senescing mice bioenergetically aged the same, we analyzed the bioenergetic responses from individual nonTg and 3xTgAD mice. Results of this analysis, indicated segregation of metabolic profiles among individual mice through reproductive senescence transition. The variability of PDH remains largely the same throughout reproductive senescence (). Consistent with our previous findings that decline in PDH expression occurred early in 3xTgAD female mouse brain [7
], data from this study indicate that compromised PDH associated with loss of ovarian hormones in reproductive senescence likely induces a variety of adaptive responses, including activation of ketogenic pathway and fatty acid oxidation, to compensate for the loss of substrate availability. COX activity, in contrast, showed greater variability between individual mice at the onset of reproductive senescence in both nonTg and 3xTgAD mice at 9 month (). However, variability in COX activity collapsed at 12 month, when mice are reproductively senescent, and converged to cluster at a much reduced level of activity. These data indicated that despite the compensatory pathways that supply alternative substrates, the net catalytic reactivity and energy transducing capacity relies stringently on COX activity as COX serves as the terminal point of electron transport. Although the variance in COX activity at earlier stage likely reflects the differential adaptation of compensatory pathways, the convergent collapse in COX activity at 12 month likely indicates a point of no return as far as energy generation by mitochondria is concerned. However, the decline between 9 and 12 month opens a therapeutic, if not a preventative, window to alleviate the detrimental impact of loss of ovarian hormones on bioenergetics during reproductive senescence.
Collectively, findings from this study demonstrate that reproductive senescence is paralleled by a significant decline in bioenergetics and mitochondrial function in normal nonTg mice and the exacerbation of the impaired mitochondrial bioenergetics pre-existed in 3xTgAD mice. These data indicate that reproductive senescence might accelerate the decline in mitochondrial bioenergetics, linking reproductive senescence to the development of a hypometabolic brain phenotype clinically observed in prodromal AD brains [36
]. Further, multiple bioenergetic metabolic phenotypes evident at the onset of reproductive senescence indicates the activation of a combination of adaptive responses. Together with our previous findings that mitochondrial bioenergetic deficits precede AD pathology, the current study provides a plausible mechanism underlying the increased risk for AD in menopausal women and importantly indicates a critical window of opportunity to prevent development of bioenergetic phenotype of AD.