Loss of Brain Aerobic Glycolysis in Normal Human Aging

Abstract

The normal aging human brain experiences global decreases in metabolism, but whether this affects the topography of brain metabolism is unknown. Here we describe PET-based measurements of brain glucose uptake, oxygen utilization, and blood flow in cognitively normal adults from 20 to 82 years of age. Age-related decreases in brain glucose uptake exceed that of oxygen use, resulting in loss of brain aerobic glycolysis (AG). Whereas the topographies of total brain glucose uptake, oxygen utilization, and blood flow remain largely stable with age, brain AG topography changes significantly. Brain regions with high AG in young adults show the greatest change, as do regions with prolonged developmental transcriptional features (i.e., neoteny). The normal aging human brain thus undergoes characteristic metabolic changes, largely driven by global loss and topographic changes in brain AG.

Keywords: aerobic glycolysis; brain aging; brain metabolism; neoteny.

Figure 1. Meta-analysis of human whole brain metabolism

We performed and fit loess curves to a meta-analysis of prior measurements of whole brain CMRGlc (“total glucose use”, blue) and CMRO₂ converted to glucose use equivalents based on the stoichiometric oxygen-to-glucose ratio of 6 (“oxidative glucose use”, orange) (see Figure S1 for details). This shows that CMRGlc decreases significantly with age, but CMRO₂ does not change or only minimally so, resulting in gradual decrease in apparent whole brain AG (yellow shaded region). Apparent whole brain AG approximates zero near the age of 60. Notably, similar changes in whole brain AG between young (mean 21 years old) and older (mean 71 years old) individuals were noted by Darab Dastur in 1985 based on multiple Kety-Schmidt measurements (Dastur, 1985), when restricting the older cohort to “physically and mentally active” healthy participants.

Figure 2. Brain AG topography changes with normal human aging

The normalized data for each metabolic parameter and each individual was Spearman rank correlated with an average data set comprising participants aged 20 – 23 years. For CBF (yellow), CMRGlc (pink), and CMRO₂ (orange), the Spearman rank correlation remained high throughout the lifespan (CMRGlc minimum Spearman’s rho = 0.92, CMRO₂ min rho = 0.89, CBF min rho = 0.91), suggesting that the topography for these aspects of brain metabolism remains relatively stable throughout the adult lifespan. The Spearman rank correlations for AG (blue) for each participant instead shows significant decreases with age (Pearson’s r = −0.64, p < 2×10⁻¹⁶), remaining only modestly similar to the young adults among the oldest participants. High inter-individual variability is evident for AG, particularly among the older participants. These changes in AG topography are in part due to whole brain changes in AG and in part due to topographical changes between CMRGlc and CMRO₂ (see Figure S4 for details).

Figure 3. Summary of regional quantitative metabolic change in the human brain with aging

Literature-based whole brain estimates for CMRO₂, CMRGlc, CBF, and AG were combined with local-to-global ratios to determine a quantitative value in every individual in the normative cohort, for each metabolic parameter in 42 gray matter regions and the corpus callosum as a representative white matter region. These were then fit with loess curves to demonstrate the trajectory for each metabolic parameter in each of these regions. The thin lines represent each of the regions with superimposed thick dotted lines representing representative regions, as shown in the bottom legend. The shaded gray regions show the standard error about the loess curves, though it should be noted that using literature-based whole brain estimates will necessarily obscure quantitative inter-individual variability. It is evident that with rare exception, all regions decrease in all aspects of metabolism. However, the loss of CMRO₂ is slight as compared to the more dramatic decrease in AG, which regionally varies in rate of change with aging. This whole-brain normalized regionally quantitative data is provided as a normative data set in the Supplemental Table.

Figure 4. The topography of AG flattens with aging

Quantitative values for AG were determined by combining literature-based whole brain values for CMRGlc and CMRO₂ with local-to-global values within each individual (see Methods). This demonstrates that in a young cohort of adults (21–35 yo) AG varies considerably throughout the brain as has been described previously (Vaishnavi et al. 2010). In an older cohort (60–76 yo) matched for sex / gender, the overall topography of AG is flattened and depressed. Slight regional variation of AG persists in a manner similar to that seen in young adults, accounting for the modestly positive Spearman rank correlations for older adults (Figure 2). As our measurement of AG does not include other carbohydrate use for oxidative phosphorylation—such as lactate—some values of AG extend below zero, in particular in the cerebellum in the older cohort.

Figure 5. Regional rate of decreased AG correlates with regional metabolic and transcriptional neoteny

(A) AG was calculated for each region using the combined literature-based whole brain and local-to-global values (see Methods). Change in AG per year was then calculated for each region and compared to the AG in that region in young adults aged 20 – 23 years. Regions with the highest AG in young adults showed the most rapid loss of AG per year (Pearson’s r = −0.87, p < 3×10⁻¹⁴), including such regions as the medial frontal cortex and the precuneus. (B) The BrainSpan lifespan human brain transcriptional data was used to calculate a regional neoteny index, which is a measure of the persistence of developmentally related gene expression (Goyal et al., 2014), for each of the 15 regions assessed by the BrainSpan data set as compared to the cerebellum. In addition to the cerebellum, the 15 regions include dorsolateral prefrontal cortex; ventrolateral, medial and orbital frontal cortex; primary motor, somatosensory, auditory and visual cortex; posterior inferior parietal cortex; posterior superior and inferior temporal cortex; hippocampus and amygdala; striatum; and thalamus (Kang et al., 2011). The loss of AG per year for these 15 regions inversely correlates with their regional neoteny index (Pearson’s r = −0.79, 95% CI −0.47 to −0.93, p < 0.0005). These results suggest that the largest aging-related change in AG occurs in the most metabolic and transcriptionally neotenous regions of the human brain, accounting for the flattening of AG seen in Figure 4.