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. 2010 Oct 6;68(1):99-112.
doi: 10.1016/j.neuron.2010.09.002.

Metabolic control of vesicular glutamate transport and release

Narinobu Juge  1 , John A Gray, Hiroshi Omote, Takaaki Miyaji, Tsuyoshi Inoue, Chiaki Hara, Hisayuki Uneyama, Robert H Edwards, Roger A Nicoll, Yoshinori Moriyama
Affiliations

Metabolic control of vesicular glutamate transport and release

Narinobu Juge et al. Neuron. .

Abstract

Fasting has been used to control epilepsy since antiquity, but the mechanism of coupling between metabolic state and excitatory neurotransmission remains unknown. Previous work has shown that the vesicular glutamate transporters (VGLUTs) required for exocytotic release of glutamate undergo an unusual form of regulation by Cl(-). Using functional reconstitution of the purified VGLUTs into proteoliposomes, we now show that Cl(-) acts as an allosteric activator, and the ketone bodies that increase with fasting inhibit glutamate release by competing with Cl(-) at the site of allosteric regulation. Consistent with these observations, acetoacetate reduced quantal size at hippocampal synapses and suppresses glutamate release and seizures evoked with 4-aminopyridine in the brain. The results indicate an unsuspected link between metabolic state and excitatory neurotransmission through anion-dependent regulation of VGLUT activity.

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Figures

Figure 1
Figure 1. Cl− activates VGLUT2 externally
A. Simple assay system for VGLUT. Proteoliposomes containing purified VGLUT2 (10 μg protein) were analyzed in an 11% polyacrylamide gel in the presence of SDS and visualized by Coomassie brilliant blue staining. The positions of molecular markers are indicated. The principle of Val-evoked glutamate uptake is also shown. B. Time course of glutamate uptake by reconstituted VGLUT2 was measured as described in Materials and Methods in the presence (closed circle) or absence (open circle) of Val. Kinetics of Val-evoked glutamate uptake at 1 min is shown in the inset. See also Figure S1. C. Energetics of glutamate uptake. Glutamate uptake was measured under the indicated ionic conditions. Nig, nigericin. D. Concentration dependence of Cl− on glutamate uptake. A sample was taken after 1 min. Δψ, measured as oxonol-V fluorescence quenching, is shown. The inset shows the Hill plot of glutamate uptake as a function of [Cl−]. E. Effect of internal and external Cl− on glutamate uptake. Proteoliposomes were prepared at the indicated concentrations of Cl− and suspended in buffer containing the indicated Cl− concentration. Val-evoked glutamate uptake was measured at 1 min. F. Cl− was replaced with the indicated anion and Val-evoked uptake of glutamate was measured at 1 min. Error bars represent mean ± SEM; n = 3.
Figure 2
Figure 2. VGLUT2 does not transport Cl− during glutamate transport
A. Uptake of radiolabeled Cl−. In the presence of 5 mM glutamate, 36Cl− uptake by liposomes or VGLUT2-containing proteoliposomes was measured in the presence (+) or absence (−) of Val. In some experiments, nystatin was included to estimate [36Cl−] at equilibrium. 36Cl− uptake by VGAT at 1 min is shown as a positive control for active Cl− transport. Essentially the same results were obtained when 10 % (w/w) cholesterol was included in the proteoliposomes. Error bars represent mean ± SEM; n = 3. B. Comparison of glutamate or GABA-evoked Cl− uptake by VGLUT or VGAT, respectively, as revealed by SPQ fluorescence. SPQ-trapped VGLUT2-containing proteoliposomes were prepared and SPQ fluorescence intensity was monitored. Proteoliposomes containing either VGAT or the E213A mutant VGAT were also measured. Additions: KCl, 10 mM; GABA, 5 mM; glutamate, 5 mM; glycine, 5 mM; Val, 2 μM; nystatin, 250 μM. A principle of assay system was also illustrated. See also Figure S2.
Figure 3
Figure 3. Acetoacetate is a physiological modulator of VGLUT2 through competition with Cl−
A. Dose-dependent inhibition of VGLUT2 by various compounds as indicated in the presence of 10 mM Cl−. For results of other substances, see Figure S4. Glutamate uptake at 1 min is shown. See also Figure S3 and S4. B. Acetoacetate did not affect Δψ. Val-evoked formation of Δψ in the absence or presence of 10 mM acetoacetate. Error bars represent mean ± SEM; n = 3. C. Glutamate uptake at 1 min at various [Cl−] in the presence of the indicated concentrations of acetoacetate. Glutamate uptake in the absence of acetoacetate (Figure 1D) was also shown as a dashed line for comparison. D. Hill plot of glutamate uptake at the indicated concentrations of acetoacetate. Data were taken from Figure 3C. E. Eadie-Hofstee plot of glutamate uptake in the presence or absence of 0.2 mM acetoacetate. F. Inhibition caused by 5 mM acetoacetate was fully reversed by washing the proteoliposomes. Error bars represent mean ± SEM; n = 3. G. Protection of glutamate uptake activity by Cl− or acetoacetate against DIDS inactivation. Proteoliposomes were incubated in the absence or presence of 1 μM DIDS (DIDS treated VGLUT2) and in two cases, in the presence of 5 mM acetoacetate (acetoacetate and DIDS treated VGLUT2) for 1 min. These proteoliposomes were then incubated in buffer containing 10 or 100 mM Cl− in the presence or absence of DIDS. After 1 min, Val-evoked uptake of glutamate was measured. Error bars represent mean ± SEM; n = 3.
Figure 4
Figure 4. Cl− dependence and its modulation by acetoacetate is a specific property of SLC17 vesicular neurotransmitter transporters
A. Val-evoked uptake of glutamate (0.1 mM) by purified, reconstituted VGLUT1 was assayed. (left) Coomassie brilliant blue-stained SDS gel of reconstituted proteoliposomes (10 μg protein each) showing the purified transporters. (center) Time course in buffer containing 10 mM Cl− (closed circles). (right) Val-evoked glutamate uptake at 1 min in the presence or absence of the indicated concentration of Cl−. B–D. Purification, time course, and Cl− dependence of VGLUT3 (B), VEAT (C) and VMAT2 (D). Glutamate (0.1 mM) and aspartate (0.1 mM) were used as substrates for VGLUT3 and VEAT, respectively. In the case of VMAT2, ΔpH-dependent uptake of serotonin (10 μM) was measured by the pH jump method. See also Figure S5. E. The effect of acetoacetate in the presence of 10 mM Cl−. In “washout” experiments, the proteoliposomes were washed with buffer lacking acetoacetate after incubation in 5 mM acetoacetate. Full activity (100%) of VNUT and VGAT was 16.0 ± 3.2 and 6.9 ± 1.0 n mol/min/mg protein, respectively. Full activity of VGLUT1, VGLUT3, VEAT and VMAT2 was as indicated above. Error bars represent mean ± SEM; n = 3. See also Figure S6. F. 36Cl− uptake at 1 min during the uptake of transport substrates by the respective transporters. Error bars represent mean ± SEM; n = 3.
Figure 5
Figure 5. Acetoacetate modulates vesicular glutamate release from neurons but not from astrocytes
A. Cultured hippocampal neurons were treated with the indicated concentrations of acetoacetate 20 min before KCl stimulation. For the “washout” experiment, acetoacetate-treated cells were washed with medium and incubated a further 1 h. KCl-dependent glutamate release was assessed for 20 min. Error bars represent mean ± SEM; n = 6. B. Hippocampal astrocytes were also treated with 5 mM acetoacetate. Glutamate secretion upon the addition of KCl was measured as described above. Error bars represent mean ± SEM; n = 6; asterisks indicate statistical significance, ***P<0.001.
Figure 6
Figure 6. Acetoacetate reduces quantum glutamate release
Whole-cell voltage clamp recordings from CA1 pyramidal neurons in acute mouse hippocampal slices incubated in either 10 mM LiCl or 10 mM lithium acetoacetate for >2h. A. mEPSC amplitude is significantly reduced following incubation with acetoacetate. Cumulative distribution and average mEPSC amplitudes (n=15) representative averaged traces; scale bars, 2 pA, 10 msec. B. mEPSC frequency is significantly reduced following incubation with acetoacetate. Cumulative distribution of mEPSC inter-event intervals (n=15), average mEPSC frequency, and sample traces; scale bars, 10 pA, 2 sec. C. Paired pulse ratio (PPR) is unchanged (n=16). Average PPR and representative traces; scale bars, 20 pA, 15 msec. D. AMPA:NMDA EPSC ratio is significantly reduced (n=16). Average ratio and representative traces; scale bars, 40 pA, 20 msec. Error bars = mean ± SEM; asterisks indicate statistical significance, **p<0.01.
Figure 7
Figure 7. Acetoacetate inhibited 4AP-evoked seizures and glutamate secretion
A. Schematic diagram and typical protocol of 4AP-evoked seizures. At the times and duration indicated, 10 mM 4AP was added. Acetoacetate (10 mM) was added during the indicated period. B. 4AP-evoked seizures and the protective effect of 10 mM acetoacetate were observed for 1 h after each addition of 4AP and given a total seizure severity score (TSSS) based on behavioral analysis. Error bars represent mean ± SEM; n = 6. Asterisks indicate statistical significance (Student's t-test), *P<0.1, ** P<0.01, *** P<0.001. C. Typical HPLC chromatogram of glutamate and dopamine secreted at the indicated sampling times. * o-phospho-L-serine at 40 pmol as an internal marker. D. Dose dependence of acetoacetate on 4AP-evoked glutamate secretion.
Figure 8
Figure 8. Proposed mode of action of ketone bodies on VGLUT-mediated suppression of glutamatergic neurotransmission
A. The activity state of VGLUT is strongly dependent on the positive cooperativity of Cl− binding. Acetoacetate may replace Cl−, which may lead to inactivation of VGLUT, causing suppression of vesicular glutamate release. B. The concentration of circulating ketone bodies (acetoacetate) increases upon ketosis and can effectively suppress VGLUT activity leading to a shut down in excitatory neurotransmission. Once the state of ketosis is ended, for instance by eating carbohydrates, this suppression is readily released and excitatory neurotransmission recovers.
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