Supply and demand in cerebral energy metabolism: the role of nutrient transporters

Abstract

Glucose is the obligate energetic fuel for the mammalian brain, and most studies of cerebral energy metabolism assume that the majority of cerebral glucose utilization fuels neuronal activity via oxidative metabolism, both in the basal and activated state. Glucose transporter (GLUT) proteins deliver glucose from the circulation to the brain: GLUT1 in the microvascular endothelial cells of the blood-brain barrier (BBB) and glia; GLUT3 in neurons. Lactate, the glycolytic product of glucose metabolism, is transported into and out of neural cells by the monocarboxylate transporters (MCT): MCT1 in the BBB and astrocytes and MCT2 in neurons. The proposal of the astrocyte-neuron lactate shuttle hypothesis suggested that astrocytes play the primary role in cerebral glucose utilization and generate lactate for neuronal energetics, especially during activation. Since the identification of the GLUTs and MCTs in brain, much has been learned about their transport properties, that is capacity and affinity for substrate, which must be considered in any model of cerebral glucose uptake and utilization. Using concentrations and kinetic parameters of GLUT1 and -3 in BBB endothelial cells, astrocytes, and neurons, along with the corresponding kinetic properties of the MCTs, we have successfully modeled brain glucose and lactate levels as well as lactate transients in response to neuronal stimulation. Simulations based on these parameters suggest that glucose readily diffuses through the basal lamina and interstitium to neurons, which are primarily responsible for glucose uptake, metabolism, and the generation of the lactate transients observed on neuronal activation.

Figure 1

A schematic representation of the cellular localization of glucose transporter (GLUTs) and monocarboxylate transporters (MCTs) in mammalian brain (McKenna et al. 2005).

Figure 2. A representative electron micrograph illustrating the relative distribution of immunogold labeling of GLUT1 glucose transporters in endothelial cells and astrocytic endfeet

GLUT1 glucose transporters were detected by a combination of an antibody raised against purified human erythrocyte GLUT1 (Wheeler et al. 1982) and Alexa Fluronanogold Fab fragment goat anti rabbit Nanoprobe. Arrows, endothelial GLUT1; arrowheads, astrocytic GLUT1.

Figure 3. Electron micrograph depiction of barriers and pathways for solute delivery from serum to brain

A. A typical electron micrograph depicting endothelial cell interactions with surrounding astrocytes and neurons. Electron micrograph kindly provided by Dr. Robert Page (Hawkins et al. 2006). B. Electron micrograph illustrating interstitial diffusion of horseradish peroxidase. Horseradish peroxidase was injected directly into the lateral ventricles of the rat and diffused throughout the interstitium, between neurons and glia (arrowheads), up to, and including, the basal lamina (arrows). Electron micrograph was kindly provided by Dr. Milton Brightman (Brightman and Reese 1969). C. Diagrammatic representation of potential routes of glucose diffusion from blood to the neuron. Route 1 depicts glucose traversing the basal lamina whence it is transported into the astrocytic endfoot. Astrocytic glucose is either directly exported into the interstitium or is metabolized to lactate which is subsequently exported to the interstitium. Route 2 depicts the diffusion of glucose throughout the basal lamina, into the interstitium, and subsequently to the respective neurons and astrocytes.

Figure 4. Compartment model for glucose transport and metabolism in mammalian brain

Glucose (Glc) and lactate distribute among 6 compartments in the brain: serum; endothelial cells; basal lamina; interstitium; astrocytes; and neurons. Glc distributions are shown on the left and lactate distributions on the right. Flows between compartments (transport is bidirectional and metabolism is unidirectional) are indicated by the arrows (double or single-headed, respectively). Each flow is assigned a number and the transport process is indicated. For example, the astrocyte shows four flows for Glc: GLUT1-mediated uptake from basal lamina (flow 3); GLUT1-mediated uptake from interstitium (flow 5); metabolic conversion to lactate (flow 7); and to other metabolites (XX) such as glycogen (flow 8). Direct flow of Glc and lactate between the basal lamina and interstitium is diffusion-mediated and is described by flows 4 (Glc) and 14 (lactate), respectively. The number assigned to each flow is cross-referenced in Appendix Tables 1 and 2. Appendix Table 2 presents the mass transport equation describing each individual net flow and Appendix Table 3 describes each parameter contributing to each flow equation and the volume of each compartment.

Figure 5. Simulations of basal, steady-state brain total glucose and interstitial lactate versus serum glucose

A. Brain total [Glc] (black lines) or interstitial [lactate] (blue lines) (mM) are determined as a function of serum [Glc] in mM. The circles show measurements of steady-state glucose levels in rat (filled circles) and human (open circles) brain observed at varying serum [Glc] (Choi et al. 2001; Gruetter et al. 1998) and fixed serum [lactate] (1 mM). The solid black line indicates close agreement between predicted and experimental data. When paracellular diffusion is prevented (Pathway 1, Figure 3C, dotted lines), predicted total glucose and interstitial lactate levels fall significantly. B. Contributions to brain total [Glc] and interstitial [lactate] at 6 mM serum glucose and 1 mM serum lactate. The ordinate indicates net flux (mmol/sec) of glucose and lactate at 3 membrane sites: neuron; astrocytic endfoot; and the astrocyte/interstitium prior to stimulation. The neuron is the primary source of lactate production: 14% is exported to interstitium, the remainder is metabolized by the neuron. See Figure 8A for the origins of the lactate transients. The astrocyte imports 50% of consumed lactate from the interstitium.

Figure 6. Lactate transients during neuronal stimulation

Data obtained from Hu and Wilson (Hu and Wilson, 1997) of extracellular measurements of lactate made during and following a short period of neuronal stimulation are shown in red. The blue curve depicts simulated brain interstitial [lactate] (mM) obtained upon applying the model presented in Figure 4 with parameters summarized in Appendix Table 3. Neuronal and astrocytic lactate oxidation were simulated to undergo a 1.5-fold increase at t = 1000 sec (duration = 25 sec) followed by a 3.2-fold increase in glycolysis at t = 1028 sec (duration = 45 sec). Serum [Glc] and [lactate] are 6 mM and 1 mM, respectively. Elimination of paracellular diffusion (blue dotted lines), causes predicted interstitial lactate levels to fall significantly and eliminates the lactate transient.

Figure 7. Simulations of brain interstitial lactate and total glucose versus serum glucose when astrocytic glucose transport capacity is increased to match neuronal glucose transport

The experiments of Figure 5 and 6 were simulated allowing for paracellular flux with two changes: 1) astrocytic glucose transport capacity was increased 12-fold to match neuronal glucose transport; 2) neuronal and astrocytic metabolic parameters were adjusted in order to reproduce the variation of brain glucose with serum glucose and lactate transients during glycolytic stimulation A) Total brain glucose and interstitial lactate (mM) as a function of serum [Glc]. Simulations report total brain [Glc] (black lines) or interstitial [lactate] (blue lines). Data points are as described in Figure 5. The inset shows the predicted lactate transient during a 14.4-fold stimulation of astrocytic (but not neuronal) glycolysis for 45 sec preceded by a 25 sec 1.5-fold increase in neuronal and astrocytic lactate oxidation (ordinate and abscissa as in Figure 6). The red line indicates the experimental data of Hu and Wilson (see Figure 6); the blue line represents simulated interstitial lactate levels when serum [Glc] and [lactate] are 6 mM and 1 mM, respectively. B) Adjustments in transport and metabolic flows prior to stimulation to simulate the data in panel 7A. The ordinate indicates change in net flow of glucose and lactate at 3 membrane sites: neuron; astrocytic endfoot; astrocyte/interstitium; serum glucose 6 mM and lactate 1mM. The dashed lines indicate no change (unity). Prior to stimulation the neuron continues to export lactate and the astrocyte remains a lactate importer. Astrocytic endfoot glucose import is increased 6-fold. Astrocyte lactate import accounts for 38% of astrocytic lactate consumption. However, upon, stimulation the lactate transient is created by the astrocyte (See Figure 8B) and the lactate is taken up by the neuron thus recapitulating the postulates of the ANLS model.

Figure 8. Contributions to interstitial lactate before and after neuronal stimulation

A) Net lactate flow in and out of the interstitium during the glycolytic step increase of Figure 6. Three lactate flows are represented: 1) net flow from astrocytes to interstitium (green line); 2) net flow from the basal lamina to interstitium (red line); 3) net flow from neurons to interstitium (blue line). Positive values indicate the direction of net flow is into the interstitium. Negative values indicate the direction of net flow is out of the interstitium. The neuron releases lactate both at rest and during stimulation while the astrocyte imports lactate both at rest and during stimulation. B) Net lactate flow into the interstitium when astrocytic glucose transport rates are increased 12-fold (see Fig 7). The neuron exports lactate before stimulation but imports lactate during stimulation of astrocytic lactate production. The astrocyte imports lactate at rest but releases lactate during stimulation thus fulfilling the predictions of ALNS.

Figure 9. Simulations of total brain glucose versus serum glucose: adjustments required when diffusion is eliminated

The experiments illustrated in Figures 5 and 6 were simulated with two changes: 1) diffusion is eliminated (path 2 of Figure 3C; flows 4 and 14 of Figure 4); 2) astrocytic glucose transport, and neuronal and astrocytic metabolic parameters were adjusted in order to reproduce experimental behavior. A) Total brain [Glc] (black lines) or interstitial [lactate] (blue lines) as a function of serum [Glc]. The data points are as described in Figure 5. The inset shows the predicted lactate transient during an 8.5-fold stimulation of astrocytic (but not neuronal) glycolysis for 45 sec preceded by a 25 sec 1.5-fold increase in neuronal lactate oxidation (ordinate and abscissa as in Figure 6). The blue line represents simulated interstitial lactate levels. The red line indicates the experimental data of Hu and Wilson (see Figure 6). B) Contributions to steady-state, basal brain total [Glc] and interstitial [lactate] levels at 6 mM serum glucose and 1 mM serum lactate with adjustments in transport and metabolic flows prior to stimulation to simulate the data in panel 9A. The ordinate indicates net change in flow of glucose and lactate at 3membrane sites: neuron; astrocytic endfoot; astrocyte/interstitium. The dashed lines indicate no change (unity). The simulation required that astrocytic glucose transport capacity be increased 100-fold, astrocytic glycolytic capacity be increased by 75%, and astrocytic oxidative capacity be reduced by 66%. The neuron now imports lactate while the astrocyte exports lactate. Neuronal glycolytic and oxidative capacities were increased by 31 and 71% respectively. The astrocyte now exports glucose into the interstitium where it is imported and metabolized by the neuron to lactate.