A transmembrane accessory subunit that modulates kainate-type glutamate receptors

Abstract

Glutamate receptors play major roles in excitatory transmission in the vertebrate brain. Among ionotropic glutamate receptors (AMPA, kainate, NMDA), AMPA receptors mediate fast synaptic transmission and require TARP auxiliary subunits. NMDA receptors and kainate receptors play roles in synaptic transmission, but it remains uncertain whether these ionotropic glutamate receptors also have essential subunits. Using a proteomic screen, we have identified NETO2, a brain-specific protein of unknown function, as an interactor with kainate-type glutamate receptors. NETO2 modulates the channel properties of recombinant and native kainate receptors without affecting trafficking of the receptors and also modulates kainate-receptor-mediated mEPSCs. Furthermore, we found that kainate receptors regulate the surface expression of NETO2 and that NETO2 protein levels and surface expression are decreased in mice lacking the kainate receptor GluR6. The results show that NETO2 is a kainate receptor subunit with significant effects on glutamate signaling mechanisms in brain.

Figure 1. Native Kainate Receptor Complexes in Brain Contain NETO2

(A) Immunoprecipitation of GluR6/7 from rat cerebella membranes showed five specific bands (asterisks and arrows). Mass spectrometry demonstrated that these bands contained NETO2, AKAP8, and the kainate receptor subunits GluR6 and GluR7 (KARs). No protein was identified from two bands (asterisks: 130, 150 kDa). Keratin was identified from all five bands. (B) Coimmunoprecipitation from rat cerebella membranes showed that NETO2 interacts with kainate, but not NMDA and AMPA, receptors. (C) NETO2, GluR6/7, and KA2 are cofractionated and highly enriched in the postsynaptic density (PSD) fraction together with NMDA receptor subunit NR1 and PSD95, whereas synaptophysin (Syph) is not. (D) In rat cerebellum, NETO2 is sensitive to both PNGaseF and EndoH (similar to NR1). (E) Domain organization of kainate receptor with NETO2 and AMPA receptor with transmembrane AMPA receptor regulatory proteins (TARPs).

Figure 2. NETO2 Modulates Channel Properties, but Not Surface Trafficking, of Kainate Receptors

Oocytes were injected with cRNAs encoding GluR6 (0.5 ng), GluR1 (1 ng, 0.1 ng) or GluR1 (0.1 ng) and stargazin (STG; 0.5 ng) with or without NETO2 (0.5 ng), and responses to 10 mM glutamate were measured with TEVC. (A and B) NETO2 enhances maximal glutamate-evoked currents (I_Max) for GluR6, but not GluR1, channels (n = 8–10) (B). (C) Addition of NETO2 cRNA enhances currents in oocytes coinjected with HA-GluR6 cRNA (0.5 ng), but not surface expression of HA-GluR6 (0.5 ng). Data are shown as mean ± SEM. n = 10–11. ***p < 0.005.

Figure 3. NETO2 Modulates Kainate Receptor Kinetics and Peak Open Probability

(A and B) Responses to 10 mM glutamate in outside-out patches from tsA201 cells transfected with GluR6 alone or GluR6 and NETO2. Glutamate was applied for 200 ms (A) or 1 ms (B). The time constants (relative amplitude) are from monoexponential or biexponential fits to the decays of the currents. (C) The fits were used to calculate weighted time constants of desensitization and deactivation. NETO2 coexpression slowed both desensitization and deactivation significantly (n = 6–9). (D) Recovery from desensitization for GluR6 alone (n = 5) and GluR6 and NETO2 (n = 6). NETO2 markedly sped recovery at short intervals, but did not alter the slow component of recovery. (E) Examples of single-channel currents evoked by 10 mM glutamate in patches containing four GluR6 (left) or three GluR6+NETO2 channels (right). NETO2 increased the probability of channel opening and the duration of channel activity. (F) Burst-length distributions from the patches in (E). The similar fast component present in all such distributions is off-scale. NETO2 coexpression produced approximately a 2-fold increase in the time constant of the slow component. (G) Mean values for peak Popen obtained from patches containing GluR6 and GluR6+NETO2 channels (n = 6). (H) Mean percentage increases in the weighted time constants of deactivation and desensitization, as well as peak Popen, produced by stargazin coexpression with GluR1 (9) and NETO2 coexpression with GluR6. Data are given as mean ± SEM. *p < 0.05, **p < 0.01.

Figure 4. NETO2 Modulates Kainate-Receptor-Mediated mEPSCs

mEPSCs were recorded from stargazer cerebellar granule cells transfected with GluR6 (K696R) alone (69 events from 14 neurons) or GluR6 (K696R) and NETO2 (233 events from 17 neurons). (A) NETO2 coexpression significantly increased the frequency of kainate receptor mEPSCs. ***p = 0.01. (B and C) Representative mEPSCs (eight traces on left) and ensemble averages (right) from neurons transfected with GluR6 (K696R) alone (B) or with GluR6 (K696R) and NETO2 (C). Addition of the AMPA receptor antagonists GYKI 53655 (50 μM) and SYM 2206 (100 μM) did not alter mEPSC kinetics. (D) Ensemble mEPSCs from neurons transfected with GluR6 (K696R) with or without NETO2 in the presence of the AMPA receptor antagonists GYKI 53655 (50 μM) and SYM 2206 (100 μM). NETO2 slows the decay kinetics of mEPSCs. (E) Kinetic parameters (half-width, decay time, and charge transfer) calculated from mEPSCs from neurons transfected with GluR6 (K696R) alone or with GluR6 (K696R) and NETO2. Data are given as mean ± SEM. *p < 0.05.

Figure 5. NETO2 Modulates the Agonist Sensitivity of Kainate Receptors

(A) Oocytes injected with NETO2 (0.5 ng) and GluR6 (0.1 ng) cRNAs were pre-exposed to concanavalin A and currents evoked by glutamate (500 μM; I_Glu) or kainate (20 μM; I_KA) were measured with TEVC (n = 7–8). Coinjection of NETO2 increases the relative efficacy of kainate. (B) CHO cells were cotransfected with expression vectors for wild-type (wt) or mutant (mut) NETO2 and NETO2 shRNA vector. NETO2 shRNAs eliminated detectable protein for NETO2 wild-type (wt) without affecting actin expression. The knockdown was prevented by silent point mutations in the NETO2 cDNA (NETO2.mut). shRNA parent vector (Control.shRNA) had no effect. (C) Inward whole-cell currents evoked in cultured hippocampal neurons (−80 mV) by the successive application of 500 μM glutamate and 20 μM kainate. The neurons (DIV 10–11) were transfected at DIV 7. Agonists were applied by local superfusion with a large-bore glass pipette. Solution exchange times were about 1 s. Neurons were pre-exposed to concanavalin A to reduce kainate receptor desensitization and all solutions contained SYM 2206 (100 μM), AP-5 (50 μM), and tetrodotoxin (2 μM) to block AMPA receptors, NMDA receptors, and sodium channels, respectively. (D) Currents evoked by glutamate (500 μM; I_Glu) or kainate (20 μM; I_KA) in hippocampal cultured neurons transfected as indicated. NETO2.shRNA decreased the ratio of kainate- and glutamate-evoked currents (n = 8). Coexpression of NETO2.mut rescued this suppression (n = 6). Data are given as mean ± SEM. ***p < 0.005.

Figure 6. Kainate Receptors Modulate Cell Surface Expression of NETO2

(A) Oocytes were injected with HA-NETO2 (20 pg) and various amounts of GluR6 cRNA as indicated, and surface expression of NETO2 was quantified by chemiluminescence (n = 8). GluR6 specifically and dose-dependently increased the surface expression of NETO2, whereas GluR1 or GluR1 with stargazin (STG) did not. (B–E) Quantitative analysis of protein expression (B and C) and mRNA levels (D and E) in GluR6 heterozygous (+/−) and homozygous (−/−) littermate mice. (B and C) NETO2 expression was reduced in GluR6^−/− mice, whereas expression of the NMDA receptor subunit NR1 and TARP was unaffected (n = 4). Arrow and asterisk indicate NETO2 and nonspecific band, respectively. Data are given as mean ± SEM. ***p < 0.005. (D and E) mRNA level of NETO2 and NR1 was unaffected in GluR6^−/− mice (n = 3). (F and G) Cerebellar cultures from GluR6 littermates were biotinylated, solubilized, and precipitated with neutravidin-beads. Twice as much total protein was loaded for the sample from GluR6 homozygous mice to account for the reduced NETO2 expression in these cultures relative to cultures from heterozygous littermates. The surface expression of NETO2 is dramatically decreased in GluR6 homozygous cultures, whereas NMDA receptor subunit NR1 is unaltered (n = 3). Data are given as mean ± SEM. **p < 0.01.