Amphetamine modulates excitatory neurotransmission through endocytosis of the glutamate transporter EAAT3 in dopamine neurons

Abstract

Amphetamines modify the brain and alter behavior through mechanisms generally attributed to their ability to regulate extracellular dopamine concentrations. However, the actions of amphetamine are also linked to adaptations in glutamatergic signaling. We report here that when amphetamine enters dopamine neurons through the dopamine transporter, it stimulates endocytosis of an excitatory amino acid transporter, EAAT3, in dopamine neurons. Consistent with this decrease in surface EAAT3, amphetamine potentiates excitatory synaptic responses in dopamine neurons. We also show that the process of internalization is dynamin- and Rho-mediated and requires a unique sequence in the cytosolic C terminus of EAAT3. Introduction of a peptide based on this motif into dopamine neurons blocks the effects of amphetamine on EAAT3 internalization and its action on excitatory responses. These data indicate that the internalization of EAAT3 triggered by amphetamine increases glutamatergic signaling and thus contributes to the effects of amphetamine on neurotransmission.

Figure 1. AMPH stimulates internalization of the glutamate transporter EAAT3

A. Glutamate uptake in primary midbrain cultures after AMPH treatment EAAT2-mediated ³H-glutamate transport (DHK-sensitive, Na⁺-dependent uptake) was not significantly altered by AMPH pretreatment (112.2±15% of vehicle treated control). The EAAT3-mediated ³H-glutamate transport (DHK-insensitive, Na⁺-dependent uptake) in these cultures is mediated by EAAT3 and this component of the glutamate transport was significantly attenuated by AMPH treatment (38.4±16%). B. Characterization of DA neurons in culture Neurons in primary midbrain cultures that express tyrosine hydroxylase (TH, green) also express DAT and EAAT3 (red). All DAT(+) cells were also EAAT3(+). C.Glutamate uptake in HEK cells transiently transfected with EAATs1, 2 and 3 HEK cells transiently expressing DAT with either EAAT 1, 2 or 3 were treated with AMPH, 10μM, or vehicle for thirty minutes and then ³H-glutamate transport was measured. Only transport mediated by EAAT3 exhibited sensitivity to AMPH. (*p<0.05 by 2-way ANOVA). D. TIRF quantification of EAAT3 internalization EAAT2 or EAAT3 cells treated with vehicle or AMPH were monitored by TIRF. The vehicle treated traces indicate that both carriers are relatively stable over this time-course in these cultures. EAAT3 membrane localization decreased under AMPH conditions while the EAAT2 protein remained stable. (See also Figure S1.)

Figure 2. Endogenous EAAT3 in DA neurons in brain slices also internalize in response to AMPH

A. DA neurons in midbrain slices express EAAT3 Midbrain sections were immunolabeled for DAT (green) and EAAT3. Soma of DAT(+) neurons in the midbrain were also EAAT3 (top panel). Further out at the processes, these two transporters are still found in the same cells (bottom panel). B. Biotinylated EAAT3 from acute midbrain slices decreases in response to AMPH Acute midbrain slices from adult mice were exposed to AMPH, cocaine, or AMPH and cocaine together. All membrane proteins were biotinylated and analyzed by SDS-Page gel. Probing for EAAT3 indicates a loss of membrane accessible protein in response to AMPH that is prevented by cocaine. DAT protein demonstrates the same sensitivity. C. Quantification of changes in membrane expression of DAT and EAAT3 Densitometry of EAAT3 and DAT shows significant internalization of both carriers that is blocked by cocaine. (*p<0.05 and ***p<0.001 by 2-way ANOVA). D. Quantification of changes in membrane expression of several membrane proteins in response to AMPH Glutamate receptor subunits GluR1, GluR4 and NR1 remain at the cell membrane following exposure to AMPH. The glutamate transporter EAAT2 is also not mobilized by AMPH. EAAT3 and DAT data are the same samples shown in 2C (*p<0.05 by Paired t-tests for each protein; see also Figure S2).

Figure 3. EAAT3 sensitivity to AMPH depends on DAT co-expression

A. Glutamate transport in HEK293 cells expressing EAAT3 or EAAT3 and DAT HEK293 cells were transiently transfected with either EAAT3 or EAAT3 and DAT and exposed to increasing concentrations of AMPH. ³H-glutamate uptake was then measured. Glutamate transport in cells expressing only EAAT3 did not respond to AMPH pretreatment while those expressing both DAT and EAAT3 exhibited a decrease in transport capacity. B. Pharmacology of AMPH-mediated decrease in EAAT3 activity HEK293 cells expressing only EAAT3 were unaffected by AMPH but glutamate transport was greatly decreased in cells expressing both EAAT3 and DAT. Cocaine, 100 μM, co-application was sufficient to block AMPH-mediated effects on these cells but had no effect on its own. (***p<0.001 by 1-way ANOVA). C.Membrane labeling of eGFP-EAAT3-AP in primary neurons Primary neurons were transiently transfected with eGFP-EAAT3-AP constructs, treated with vehicle or AMPH for thirty minutes, and then labeled for surface localized transporter. Total protein (green, GFP) did not differ from vehicle treated control, however the proportion of carrier that could be labeled at the membrane (red, Streptavidin-Alexa Fluor 568) greatly decreased in response to AMPH. D. Quantification of membrane labeled GFP-EAAT3-AP in DAT(+) and DAT(-) neurons Membrane and total (green) expression in both DAT(+) and DAT(-) primary neurons in culture were quantized by photon counting. Data are expressed as a ratio of membrane fraction to total. (**p<0.01 by one-way ANOVA. Scale bars are 50μm.)

Figure 4. Mechanism of AMPH-Mediated EAAT3 internalization

A. AMPH stimulated movement of CypHer5-labled EAAT3 protein into intracellular vesicles HEK293 were transiently transfected with DAT and eGFP-EAAT3-AP. The uniquely biotinylated eGFP-EAAT3-AP was labeled with streptavidin conjugated to the pH-sensitive dye, CypHer5. Over 10 minutes under vehicle conditions, cells did not exhibit robust CypHer5 labeling. However, 10 minutes of AMPH, 10 μM, lead to an increase in CypHer5 fluorescence. B. Quantification of CypHer-5 labeling at 10 minutes following vehicle or AMPH treatment (*p<0.05 by Paired t-test.) C. AMPH-Induced EAAT3 internalization is dynamin-mediated ³H-glutamate uptake in HEK293 cells transiently transfected with DAT and EAAT3 and pretreated with AMPH were unaffected by pretreatment with brefeldin A. However the dynamin inhibitor Dynasore prevented AMPH-mediated internalization of EAAT3. D. EAAT3 internalization by AMPH is RhoA mediated Co-transfection of EAAT3 and DAT with wildtype RhoA did not alter EAAT3 sensitivity to AMPH. However, coexpression of the dominant negative T17N attenuated EAAT3 AMPH-mediated endocytosis. The constitutively active mutant, V14, also exhibited a diminished effect of AMPH pretreatment on ³H-glutamate uptake, however the baseline transporter capacity of these cells was significantly diminished initially under vehicle control conditions. E. Coexpression of the exotoxin, C3, also blocked AMPH sensitivity in these cells. (*p<0.05, ***p<0.001 and **** p<0.0001 by one-way ANOVA; see also Figure S3).

Figure 5. Introduction of the EAAT3 C-terminal motif disrupts the effect of AMPH on EAAT3

A. Schematic of TAT-peptide constructs B. Responses to AMPH in HEK293 cells exposed to TAT-EAAT3 HEK293 cells transfected with DAT and EAAT3 were assessed for EAAT3 activity by ³H-glutamate uptake. Cells treated with TAT-scrambled were still sensitive to AMPH, but TAT-EAAT3 abolished the AMPH-mediated decrease in carrier activity. C and D. EAAT3 internalization is blocked by TAT-EAAT3 in HEK293 cells C. HEK293 cells were transiently transfected with DAT and eGFP-EAAT3-AP, treated with TAT-EAAT3 (top panel) or TAT-scrambled (bottom panel), and then AMPH. Total eGFP-EAAT3-AP expression was not altered (green), but the surface localized transporters labeled with streptavidin-Alexa568 were decreased only in cells exposed to the TAT-scrambled peptide. TAT-EAAT3 treated cells still had robust surface localized eGFP-EAAT3-AP. D. Quantification of vehicle treated TAT-EAAT3 or TAT-scrambled eGFP-EAAT3-AP membrane localization demonstrates no baseline effect on the transporter. The response to AMPH, however, is blocked in cells treated with TAT-EAAT3 only. Inset In parallel assays, DAT membrane expression is decreased in response to AMPH with both the scrambled as well as the EAAT3 TAT peptides. E and F. Biotinylation of endogenous EAAT3 in an acute brain slice Western blot of biotinylated EAAT3 in acute brain slices treated with TAT-E3 (top panel) or TAT-scrambled (bottom panel) demonstrate EAAT3 trafficking induced by AMPH in the TAT-scrambled treated slices only. TAT-EAAT3 abolished the AMPH-mediated internalization of EAAT3. F. Quantification of biotinylated EAAT3 in acute brain slices. Inset DAT membrane expression is decreased in response to AMPH under both conditions. (Scale bar represents 25μm for merged images, 50μm for single color images. * p<0.05 by one-way ANOVA.)

Figure 6. AMPH potentiates glutamatergic synaptic transmission

A. Averaged AMPA-mediated evoked excitatory postsynaptic currents (eEPSCs) are increased following AMPH (10μM) superfusion. B. Averaged NMDA-mediated eEPSCs are also potentiated by AMPH. C. Bar graphs with compiled data for AMPA and NMDA-mediated eEPSCs in control and in the presence of AMPH (10μM). D. The effect of AMPH on AMPA-mediated eIPSCs was abolished with the EAAT3 peptide in the intracellular solution compared to a scrambled peptide. E. Bar graph showing the AMPH-induced potentiation normalized to baseline AMPA-mediated eIPSCs (*p<0.05 by student’s t-test, see also Figure S4).

Figure 7. AMPH modulation of EAAT3 is involved in the potentiation of NMDA EPSCs by AMPH

A. Amplitudes of NMDA-mediated EPSCs (every 30s) were measured over time after breaking into whole-cell mode (t=0 min) with pipette solutions containing vehicle (DMSO) control (n=8). AMPH was superfused during the time denoted by bar. B. Time course after breaking into whole-cell mode (t=0 min) with pipette solutions containing TBOA (200 μM; n = 10). C. Bar graph showing the changes in eEPSC amplitude with either vehicle (DMSO) or TBOA added to the intracellular recording solution (Two way Repeated Measures ANOVA, F(2,16) = 8.548, p<0.05). AMPH increased the amplitude of NMDA eEPSCs in cells recorded with vehicle but had no effect in cells recorded with intracellular TBOA. In those cells, TBOA alone significantly increased eEPSCs (*p<0.05).