A Bird's-Eye View of Glycolytic Upregulation in Activated Brain: The Major Fate of Lactate Is Release From Activated Tissue, Not Shuttling to Nearby Neurons

Abstract

Glucose is the major, obligatory fuel for the brain, and nearly all glucose is oxidized in the awake, resting state. However, during activation, much of the glucose is not oxidized even though adequate oxygen is available, ATP demand is increased, and glycolysis generates less ATP than oxidation. The fate of the lactate produced by glycolysis is a highly debated topic, in part because its origin and fate in the living brain are difficult to measure. One idea has been that astrocytes generate lactate and shuttle it to neurons as a major fuel, but critical elements of the shuttle model are not validated, and there is no compelling evidence to support shuttling coupled with oxidation in vivo. Metabolic brain imaging reveals rapid loss of labeled metabolites of glucose from activated tissue that is mediated by lactate transporters and gap junctional trafficking among astrocytes. Lactate is highly labeled by [¹³C- and ¹⁴C]glucose, it is diffusible, and it is quickly released to blood and the perivascular-lymphatic drainage system. During intense sensory stimulation, astrocytic glycogen is consumed at half the rate of glucose by all brain cells; it is a major fuel. The oxygen-carbohydrate metabolic mismatch increases when glycogen is included in the calculation, revealing that glycogen is not oxidized. Although the energetics of brain activation is complex, metabolic modeling with comparison to a wide range of experimental data relating metabolism to neurotransmission strongly supports two concepts: (i) glycogenolysis in astrocytes spares blood-borne glucose for activated neurons, and (ii) the increase in cerebral blood flow in excess of oxygen consumption removes protons produced by glycolytic metabolism to maintain tissue pH, pO₂, and pCO₂ homeostasis. Several studies have identified processes and situations that involve neuronal aerobic glycolysis, and a better understanding of the roles of glycolysis in neuron-astrocyte interactions and functional metabolism in the normal and diseased brain is required.

Keywords: astrocyte; brain activation; glucose; glycogen; lactate; neuron.

FIGURE 1

Astrocyte‐neuron lactate shuttle (ANLS) model. The ANLS model states that transmitter glutamate uptake into astrocytes along with Na⁺ creates a demand for 2 ATP (one to extrude the Na⁺ by Na⁺,K⁺‐ATPase, one to synthesize glutamine from glutamate) that are generated by glycolytic metabolism of glucose; the two lactate are released and taken up and oxidized by neurons (Pellerin and Magistretti 1994). The model has the following explicit requirements (i) 1:1:2 stoichiometry for glutamate uptake: glucose metabolized glycolytically: lactate released. (ii) Oxidation of lactate as the major fuel for activated neurons in the living brain requires that oxygen consumption in neurons matches the glycolytic metabolism of glucose in astrocytes, i.e., ∆CMR_O2 = ∆CMR_glc. (iii) The carbon atoms derived from lactate oxidation via the tricarboxylic acid cycle are quickly and quantitatively incorporated into the large TCA cycle‐derived metabolite and amino acid pools (e.g., glutamate, aspartate, γ‐aminobutyric acid (GABA), and glutamine) (Boumezbeur et al. 2010). Thus, nearly all of the label corresponding to labeled glucose consumed by glycolysis in astrocytes is trapped in the neurons and accurately registers increases in metabolism during activation. Glc, glucose; Glc‐6‐P, glucose‐6‐phosphate; Gln, glutamine; Glu, glutamate; HK, hexokinase; Lac, lactate; Pyr, pyruvate; TCA, tricarboxylic acid.

FIGURE 2

Brain activation: Increased glucose utilization in neurons, glycogenolysis in astrocytes, and release of glucose‐ and glycogen‐derived lactate. Metabolic responses to brain activation include glutamate uptake and oxidation and stimulation of glycogenolysis in astrocytes, and increased glycolytic and oxidative metabolism of glucose in neurons (see text). Glycogenolysis blocks metabolism of blood‐borne glucose by astrocytes, so its pyruvate/lactate pool is essentially unlabeled by metabolism of blood‐borne glucose. When metabolic assays are carried out with labeled glucose during activation, the glucose is metabolized mainly in neurons, labeling their pyruvate/lactate pools and TCA cycle‐derived metabolites. Labeled lactate is released from neurons and unlabeled lactate is released from astrocytes. Compartmentation of glycogen in astrocytes separates glycolytic metabolism of glycogen from glycolytic metabolism of labeled glucose. Some of the glutamate taken up into astrocytes is subject to partial and/or complete oxidation to generate ATP that can support the energetics of the Na⁺,K⁺‐ATPase, glutamine synthetase, or other energy demands. If one glutamate molecule is partially oxidized and replaced, 15 ATP are generated, and if completely oxidized and resynthesized, 26.5–28.5 ATP are produced. These calculated ATP values are derived in the supplemental information section of Rothman et al. (2022). AcCoA, acetyl CoA; Cit, citrate; Glc, glucose; Glc‐6‐P, glucose‐6‐phosphate; Gln, glutamine; Glu, glutamate; HK, hexokinase; Lac, lactate; Mal, malate; OAA, oxaloacetate; Pyr, pyruvate; TCA, tricarboxylic acid; αKG, α‐ketoglutarate.

FIGURE 3

Underestimation of metabolic activation in autoradiographic assays of glucose utilization with [1‐ or 6‐¹⁴C]glucose compared to [¹⁴C]deoxyglucose. (A) Rats were given unilateral visual stimulation with on–off flash to activate metabolic activity in the dorsal superior colliculus of conscious rats (white arrows). Autoradiographs illustrate images obtained from [¹⁴C]DG and [6‐¹⁴C]glucose at 8 Hz, and the graph shows the incremental increase in calculated glucose utilization above no stimulation as function of stimulus frequency. Reproduced from figures 1 and 3B of Collins et al. (1987) ©1987 with permission from John Wiley and Sons. (B) Seizures were induced in conscious rats by kainic acid prior to CMR_glc assays with [¹⁸F]fluorodeoxyglucose (FDG) and [6‐¹⁴C]glucose in the same rat. Glucose utilization increased in several brain structures (hippocampus, upper horizontal arrows; entorhinal cortex, large arrowheads; substantia nigra, small arrowheads and lower horizontal arrows). Registration of label accumulation by [¹⁸F]FDG was much higher than for [6‐¹⁴C]glucose; the table shows calculated CMR_glc and percent difference for the hippocampus. Reproduced from figure 4 and tabular data from table 1 of Ackermann and Lear (1989) ©1989 by permission from Macmillan Publishers Ltd. and Sage publishers of the Journal of Cerebral Blood Flow and Metabolism. (C) Unilateral topical application of KCl to the dorsal left hemisphere of conscious rats increases glucose utilization; metabolic activation is higher in the [¹⁴C]DG compared to [6‐¹⁴C]glucose autoradiographs. Color scales indicate metabolite rate for [¹⁴C]DG and ¹⁴C concentration for [¹⁴C]glucose. The scale from low to high CMR_glc or ¹⁴C concentration is blue, green, yellow, red. Quantitative values are shown in the table. Data are from Cruz et al. (1999); Adachi et al. (1995). (D) Unilateral (right) acoustic stimulation with an 8 kHz tone increases glucose utilization in groups of cells that preferentially respond to the tone. Tonotopic bands in the inferior colliculus are readily detected with [¹⁴C]DG compared to [1‐ or 6‐¹⁴C]glucose (horizontal arrows), and right–left differences are also greater with [¹⁴C]DG. Color scales indicate metabolic rate for [¹⁴C]DG and ¹⁴C concentration for [¹⁴C]glucose. The graph shows the percent increase in labeling in 5 min metabolic assays. IC‐mean denotes the mean percent increase for the entire inferior colliculus, and the peak value is for the major tonotopic band (arrow). Corresponding percent increases were also determined with [1‐ or 6‐¹⁴C]glucose. Because DG assays total glucose utilization at the hexokinase step and glucose registers mainly the oxidative pathways (see Figure 1), the missing label represents loss of diffusible labeled metabolites of glucose. Autoradiographs reproduced from figure 1 of Cruz et al. (2007) © 2007, with permission from John Wiley & Sons; calculated data in the bar graph are from the same study. The figure and legend are reprinted from figure 4 in Dienel (2012b) © 2012 The Author, with permission. The figures in each of the panels are reprinted with permission of the cited sources.

FIGURE 4

Schematics of the major metabolic pathways of the GSG model. (A) shows a schematic of the fluxes of neuronal and astrocytic glucose metabolism for Glu‐GABA/Gln cycle rates at or below the resting awake (RA) state, identified by V_NTcycle‐RA. Under the RA condition, the net rate of glycogenolysis (V_Gnet) is equal to 0. The pseudo‐malate–aspartate shuttle (PMAS) is a mechanism that couples glycolytic production of NADH in neuronal cytoplasm with NADH shuttling to and oxidation in mitochondria and the conversion of astrocyte‐derived glutamine into neurotransmitter glutamate (see Rothman et al. (2022), Supporting Information (SI), Section 6 and Figure SI‐3 for details); both the MAS and PMAS involve oxidation–reduction and transamination reactions. The dotted lines from Lac to the plasma membranes of astrocytes and neurons represent lactate release that corresponds to a small fraction (about 5%) of the glucose metabolized from the cells and from brain at or below the RA state. (B) shows the incremental fluxes of glucose and glycogen, as well as ATP synthesis, due to an increase in V_NTcycle above RA state (ΔV_NTcycle > 0). The flux values and predicted stoichiometries are derived in Rothman et al. (2022), SI Sections 2–5, respectively. The majority of pyruvate from glycogenolysis leaves the astrocyte as lactate, with a small fraction being oxidized as part of the glutamate and GABA oxidation and resynthesis pathways. Major pathways of ATP synthesis (ΔV_ATP) coupled to the rates of glucose and glycogen metabolism are indicated in green. Major pathways of ATP consumption are indicated in red, the largest being assigned to the neuronal and astrocytic Na⁺,K⁺‐ATPase (see SI Section 5). For the derivation of the coefficients for ATP synthesis rates, see Rothman et al. (2022), SI Section 2, Equations SI‐25 and SI‐26. For the derivation of the coefficients for astrocytic and neuronal ATP consumption by the Na⁺, K⁺‐ATPase, see Rothman et al. (2022), SI Section 5 and Equation SI‐64. The solid lines from Lac to the plasma membranes of astrocytes and neurons represent the release of larger amounts of lactate from the activated cells and from the brain. The figure and legend are reprinted from figure 1 in Rothman et al. (2022), with permission. CMR, cerebral metabolic rate; GABA, γ‐aminobutyric acid; Glc‐6‐P, glucose‐6‐phosphate; Gln, glutamine; Glu, glutamate; HK, hexokinase; Lac, lactate; MAS, malate–aspartate shuttle; Mito, mitochondria; Pyr, pyruvate; V denotes rates for different pathways identified by subscripts and is defined in the ‘Theory and Calculations’ within the main text of Rothman et al. (2022).