Figure 1.
Decreased brain glucose uptake and glucose transporter expressions at early stage of female aging.
A. Representative FDG-microPET images showed an age-related decline in brain glucose uptake in nonTg brain, which was maximal between 6 and 9 months of age. (Yellow indicates higher values and red indicates lower value). B. Quantitative analysis demonstrated an age-related decrease in brain glucose uptake in nonTg brain, which was significant between 6 and 9 months of age. C. GLUT155 Kda expression showed a significant increase after 3 months of age. This rise sustained across 6, 9 and 12 months of age and then decreased significantly at 15 months of age. D. In nonTg hippocampus, there was a trend towards increased expression of GLUT145 Kda from 3 to 12 months of age. However, it did not reach significance. E. GLUT3 expression in nonTg hippocampus demonstrated an age-related decline, which reached statistical significance between 6 and 9 months of age. F. There was no change in GLUT4 expression in nonTg hippocampus. G. Membrane GLUT4 expression increased significantly at 15 months of age. H. GLUT5 expression did not change in nonTg hippocampus. I. Representative FDG-microPET images showed an age-related decline in brain glucose uptake in 3xTgAD brain, which was maximal between 6 and 9 months of age. (Yellow indicates higher values and red indicates lower value). J. Quantitative analysis demonstrated an age-related decrease in brain glucose uptake in 3xTgAD mice, which was significant between 6 and 9 months of age. K. 3xTgAD hippocampus demonstrated an age-related decrease in the expression of GLUT155 Kda, which was significant after 6 months of age. L. In 3xTgAD hippocampus, GLUT145 Kda increased significantly during aging. M. The age-related decline in GLUT3 expression reached significance after 6 months of age. N. GLUT4 expression did not change in 3xTgAD hippocampus. O. Membrane GLUT4 expression did not change in 3xTgAD hippocampus. P. There was no change in GLUT5 expression in 3xTgAD hippocampus. * p<0.05, ** p<0.01, *** p<0.001, **** p<0.0001, bars represent mean value ± SEM.
Figure 2.
Decreased hexokinase activity at early stage in female aging.
A. In nonTg hippocampus, expression of hexokinase 2 decreased significantly at 15 months of age. B. Hexokinase activity in nonTg hippocampus decreased significantly after 6 months of age. C. There was no change in the expression of hexokinase 2 in 3xTgAD hippocampus. D. Hexokinase activity in 3xTgAD hippocampus decreased significantly after 6 months of age. * p<0.05, ** p<0.01, *** p<0.001, **** p<0.0001, bars represent mean value ± SEM.
Figure 3.
Increase in the phosphorylation of PDH at early stage of female aging.
A. Immunofluorescent labeling of phosphoPDH (Ser293) in the nonTg hippocampal CA3. B. NonTg hippocampus demonstrated an age-related increase in phosphoPDH/PDH ratio, which was significant after 6 months of age. C. Immunofluorescent labeling of phosphoPDH (Ser293) in the 3xTgAD hippocampal CA3. D. 3xTgAD hippocampus demonstrated an age-related increase in phosphoPDH/PDH ratio, which was significant after 6 months of age. * p<0.05, ** p<0.01, bars represent mean value ± SEM. Scale: 100 µm.
Figure 4.
Reproductive transition paralleled a significant decrease in LDH5 and LDH1 expressions in nonTg hippocampus.
A. NonTg hippocampus demonstrated a an age-related decrease in LDH5 expression from 6 to 15 months of age, which was significant between 6 and 9, 9 and 12 months of age. B. LDH1 expression decreased with age in nonTg hippocampus from 6 to 15 months of age, which was significant between 6 and 9, 9 and 15 months of age. C. LDH5/LDH1 ratio decreased significantly at 12 months of age. D. There was no change in 3xTgAD LDH5 expression. E. There was no change in 3xTgAD LDH1 expression. F. LDH5/LDH1 ratio decreased significantly at 15 months of age. * p<0.05, ** p<0.01, *** p<0.001, **** p<0.0001, bars represent mean value ± SEM.
Figure 5.
Plasma β-hydroxybutyrate level and its neuronal transporter increase with reproductive transition.
A. Plasma β-hydroxybutyrate level increase with age in nonTg mice (plasma samples were pooled from 5 different animals, n = 1–2). B. MCT1 expression showed a trend towards increasing from 3 to 9 months of age and decreased afterwards in nonTg hippocampus. C. The age-related decrease in MCT4 expression was significant between 3 and 6, 6 and 15 months of age. D. MCT2 increased significant from 3 to 12 months with age (linear regression: slope = 0.07, R2 = 0.3273, p<0.05). E. In 3xTgAD mice, plasma β-hydroxybutyrate level was highest at 3 months of age. The β-hydroxybutyrate level decreased after 3 months and increased again after 9 months of age (plasma samples were pooled from 5 different animals, n = 1–2). F. In 3xTgAD hippocampus, MCT1 expression showed an age-related decline, which was significant at 12 and 15 months of age. G. The age-related decrease in MCT4 expression was significant at 15 months of age. H. The age-related increase in MCT2 expression was significant between 6 and 9 months of age. * p < 0.05, ** p < 0.01, *** p < 0.001, **** p < 0.0001, bars represent mean value ± SEM.
Figure 6.
Schematic of brain bioenergetics and timeline of bioenergetic aging in female mammalian brain.
A. In neurons, glucose is transported through the blood-brain-barrier glucose transporter 1 (GLUT155 Kda), transported into neurons by glucose transporter 3 (GLUT3), phosphorylated by hexokinase and further metabolized into pyruvate. Pyruvate is converted to acetyl-CoA by pyruvate dehydrogenase to enter into TCA cycle to generate ATP. In astrocytes, glucose is transported by glucose transporter 1 (GLUT145 Kda) and phosphorylated by hexokinase to generate pyruvate, which can be converted to lactate by lactate dehydrogenase 5 (LDH5). In oligodentrocytes, glucose is transported into oligodendrocyte to generate ATP or serve as carbon skeleton in lipid/myelin synthesis. In microglia, glucose is transported by GLUT5 (mainly) and GLUT145 Kda for downstream metabolic pathways. The alternative fuels, lactate or ketone bodies, are transported through blood-brain-barrier monocarboxylate transporter 1 (MCT1). Lactate generated by astrocyte is transported by MCT1 or MCT4. Lactate and ketone bodies are transported by MCT2 into neuron, where lactate is converted to pyruvate by lactate dehydrogenase 1 (LDH1) and ketone bodies is converted to acetyl-CoA by 3-oxoacid-CoA transferase (SCOT) to generate ATP. In oligodendrocyte, lactate can be transported by MCT1 and is suggested to serve important role in energy production and lipid/myelin synthesis. In microglia, lactate is generated by LDH5 and transported by MCT1. B. In normal nonTg brain, the decline in brain glucose transport and glycolytic capacity occurred between 6 and 9 months of age, which temporally preceded mitochondrial dysfunction. The decline in brain bioenergetic was paralleled with the increase in peripheral ketone body concentration. The expression of BBB and glial ketone body transporter decreased after 9 months of age whereas the astrocytic ketone body transporter increased at 15 months of age. C. In 3xTgAD brain, the decline in brain glucose transport and glycolytic capacity also occurred between 6 and 9 months of age, which temporally preceded mitochondrial dysfunction. The activation of the ketogenic pathway occurred at both early age and early stage of female aging. The expression of BBB/glial and astrocytic ketone transporters was maximal at early age (3 months) but decreased at early stage of female aging (9 to 15 months).