Despite extensive research on neuroimaging correlates of human brain aging, there is little mechanistic insight into how they are linked to loss of brain function. Previous studies on the role of cerebral blood flow (CBF) in supporting brain function have focused on delivery of nutrients, namely oxygen and glucose. However, CBF is required also to clear the byproducts of energy metabolism, namely CO2 and protons. With the goal of determining whether age-associated reduction in regional CBF may lead to abnormal brain partial pressure of carbon dioxide (pCO2) and pH levels that are sufficient to alter brain activity and cognitive function, we applied a recently introduced homeostatic modeling of nutrients and waste products to human neuroimaging PET data acquired in young and older adults (Goyal et al. in Cell Metab 26(2):353-360, 2017). Our results demonstrate that age-associated reductions in CBF, in the presence of virtually unaltered oxygen consumption rates, show concurrent regional age-associated increases in pCO2 and associated pH acid-shifts of possible functional relevance. We conclude that the implications of altered vascular health in older adults needs to be revisited in light of its central role in removing waste products from energy metabolism at resting state and, in future studies, during external stimulations.

• MeSH
• Adult MeSH
• Energy Metabolism * MeSH
• Hydrogen-Ion Concentration MeSH
• Middle Aged MeSH
• Humans MeSH
• Young Adult MeSH
• Brain * metabolism   diagnostic imaging   physiology   MeSH
• Cerebrovascular Circulation physiology   MeSH
• Carbon Dioxide metabolism   MeSH
• Positron-Emission Tomography MeSH
• Aged MeSH
• Oxygen Consumption MeSH
• Aging * metabolism   physiology   MeSH
• Check Tag
• Adult MeSH
• Middle Aged MeSH
• Humans MeSH
• Young Adult MeSH
• Male MeSH
• Aged MeSH
• Female MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Carbon Dioxide MeSH

N-Methyl-D-aspartate receptors (NMDARs) are ionotropic glutamate receptors essential for synaptic plasticity and memory. Receptor activation involves glycine- and glutamate-stabilized closure of the GluN1 and GluN2 subunit ligand binding domains that is allosterically regulated by the amino-terminal domain (ATD). Using single molecule fluorescence resonance energy transfer (smFRET) to monitor subunit rearrangements in real-time, we observe a stable ATD inter-dimer distance in the Apo state and test the effects of agonists and antagonists. We find that GluN1 and GluN2 have distinct gating functions. Glutamate binding to GluN2 subunits elicits two identical, sequential steps of ATD dimer separation. Glycine binding to GluN1 has no detectable effect, but unlocks the receptor for activation so that glycine and glutamate together drive an altered activation trajectory that is consistent with ATD dimer separation and rotation. We find that protons exert allosteric inhibition by suppressing the glutamate-driven ATD separation steps, and that greater ATD separation translates into greater rotation and higher open probability.

• MeSH
• Allosteric Regulation MeSH
• Glycine chemistry   metabolism   MeSH
• HEK293 Cells MeSH
• Kinetics MeSH
• Microscopy, Confocal MeSH
• Protein Conformation * MeSH
• Glutamic Acid chemistry   metabolism   MeSH
• Humans MeSH
• Models, Molecular MeSH
• Protein Multimerization * MeSH
• Receptors, N-Methyl-D-Aspartate chemistry   genetics   metabolism   MeSH
• Fluorescence Resonance Energy Transfer methods   MeSH
• Protein Binding MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Research Support, N.I.H., Extramural MeSH
• Names of Substances
• Glycine MeSH
• Glutamic Acid MeSH
• N-methyl D-aspartate receptor subtype 2A MeSH Browser
• NR2B NMDA receptor MeSH Browser
• Receptors, N-Methyl-D-Aspartate MeSH