Local control of AMPA receptor trafficking at the postsynaptic terminal by a small GTPase of the Rab family

The delivery of neurotransmitter receptors into the synaptic membrane is essential for synaptic function and plasticity. However, the molecular mechanisms of these specialized trafficking events and their integration with the intracellular membrane transport machinery are virtually unknown. Here, we have investigated the role of the Rab family of membrane sorting proteins in the late stages of receptor trafficking into the postsynaptic membrane. We have identified Rab8, a vesicular transport protein associated with trans-Golgi network membranes, as a critical component of the cellular machinery that delivers AMPA-type glutamatergic receptors (AMPARs) into synapses. Using electron microscopic techniques, we have found that Rab8 is localized in close proximity to the synaptic membrane, including the postsynaptic density. Electrophysiological studies indicated that Rab8 is necessary for the synaptic delivery of AMPARs during plasticity (long-term potentiation) and during constitutive receptor cycling. In addition, Rab8 is required for AMPAR delivery into the spine surface, but not for receptor transport from the dendritic shaft into the spine compartment or for delivery into the dendritic surface. Therefore, Rab8 specifically drives the local delivery of AMPARs into synapses. These results demonstrate a new role for the cellular secretory machinery in the control of synaptic function and plasticity directly at the postsynaptic membrane.     

Key amino acid residues within the third membrane domains of NR1 and NR2 subunits contribute to the regulation of the surface delivery of N-methyl-D-aspartate receptors

N-methyl-d-aspartate (NMDA) receptors are glutamate ionotropic receptors that play critical roles in synaptic transmission, plasticity, and excitotoxicity. The functional NMDA receptors, heterotetramers composed mainly of two NR1 and two NR2 subunits, likely pass endoplasmic reticulum quality control before they are released from the endoplasmic reticulum and trafficked to the cell surface. However, the mechanism underlying this process is not clear. Using truncated and mutated NMDA receptor subunits expressed in heterologous cells, we found that the M3 domains of both NR1 and NR2 subunits contain key amino acid residues that contribute to the regulation of the number of surface functional NMDA receptors. These key residues are critical neither for the interaction between the NR1 and NR2 subunits nor for the formation of the functional receptors, but rather they regulate the early trafficking of the receptors. We also found that the identified key amino acid residues within both NR1 and NR2 M3 domains contribute to the regulation of the surface expression of unassembled NR1 and NR2 subunits. Thus, our data identify the unique role of the membrane domains in the regulation of the number of surface NMDA receptors.     

Regulation of dopamine D1 receptor trafficking and desensitization by oligomerization with glutamate N-methyl-D-aspartate receptors

Activation of dopamine D1 receptors is critical for the generation of glutamate-induced long-term potentiation at corticostriatal synapses. In this study, we report that, in striatal neurons, D1 receptors are co-localized with N-methyl-d-aspartate (NMDA) receptors in the postsynaptic density and that they co-immunoprecipitate with NMDA receptor subunits from postsynaptic density preparations. Using modified bioluminescence resonance energy transfer, we demonstrate that D1 and NMDA receptor clustering reflects the existence of direct interactions. The tagged D1 receptor and NR1 subunit cotransfected in COS-7 cells generated a significant bioluminescence resonance energy transfer signal that was insensitive to agonist stimulation and that did not change in the presence of the NR2B subunit, suggesting that the D1 receptor constitutively and selectively interacts with the NR1 subunit of the NMDA channel. Oligomerization with the NR1 subunit substantially modified D1 receptor trafficking. In individually transfected HEK293 cells, NR1 was localized in the endoplasmic reticulum, whereas the D1 receptor was targeted to the plasma membrane. In cotransfected cells, both the D1 receptor and NR1 subunit were retained in cytoplasmic compartments. In the presence of the NR2B subunit, the NR1-D1 receptor complex was translocated to the plasma membrane. These data suggest that D1 and NMDA receptors are assembled within intracellular compartments as constitutive heteromeric complexes that are delivered to functional sites. Coexpression with NR1 and NR2B subunits also abolished agonist-induced D1 receptor cytoplasmic sequestration, indicating that oligomerization with the NMDA receptor could represent a novel regulatory mechanism modulating D1 receptor desensitization and cellular trafficking.     

Reinsertion or degradation of AMPA receptors determined by activity-dependent endocytic sorting

Both acute and chronic changes in AMPA receptor (AMPAR) localization are critical for synaptic formation, maturation, and plasticity. Here I report that AMPARs are differentially sorted between recycling and degradative pathways following endocytosis. AMPAR sorting occurs in early endosomes and is regulated by synaptic activity and activation of AMPA and NMDA receptors. AMPAR intemalization triggered by NMDAR activation is Ca2+-dependent, requires protein phosphatases, and is followed by rapid membrane reinsertion. Furthermore, NMDAR-mediated AMPAR trafficking is regulated by PKA and accompanied by dephosphorylation and rephosphorylation of GluR1 subunits at a PKA site. In contrast, activation of AMPARs without NMDAR activation targets AMPARs to late endosomes and lysosomes, independent of Ca2+, protein phosphatases, or PKA. These results demonstrate that activity regulates AMPAR endocytic sorting, providing a potential mechanistic link between rapid and chronic changes in synaptic strength.     

Endocytic Adaptor Epidermal Growth Factor Receptor Substrate 15 (Eps15) Is Involved in the Trafficking of Ubiquitinated α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid Receptors

AMPA-type glutamate receptors (AMPARs) play a critical role in mediating fast excitatory synaptic transmission in the brain. Alterations in receptor expression, distribution, and trafficking have been shown to underlie synaptic plasticity and higher brain functions, including learning and memory, as well as brain dysfunctions such as drug addiction and psychological disorders. Therefore, it is essential to elucidate the molecular mechanisms that regulate AMPAR dynamics. We have shown previously that mammalian AMPARs are subject to posttranslational modification by ubiquitin, with AMPAR ubiquitination enhancing receptor internalization and reducing AMPAR cell surface expression. Here we report a crucial role for epidermal growth factor receptor substrate 15 (Eps15), an endocytic adaptor, in ubiquitination-dependent AMPAR internalization. We find that suppression or overexpression of Eps15 results in changes in AMPAR surface expression. Eps15 interacts with AMPARs, which requires Nedd4-mediated GluA1 ubiquitination and the ubiquitin-interacting motif of Eps15. Importantly, we find that Eps15 plays an important role in AMPAR internalization. Knockdown of Eps15 suppresses the internalization of GluA1 but not the mutant GluA1 that lacks ubiquitination sites, indicating a role of Eps15 for the internalization of ubiquitinated AMPARs. These results reveal a novel molecular mechanism employed specifically for the trafficking of the ubiquitin-modified AMPARs.     

Transmembrane and ubiquitin-like domain-containing protein 1 (Tmub1/HOPS) facilitates surface expression of GluR2-containing AMPA receptors

Some ubiquitin-like (UBL) domain-containing proteins are known to play roles in receptor trafficking. Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) undergo constitutive cycling between the intracellular compartment and the cell surface in the central nervous system. However, the function of UBL domain-containing proteins in the recycling of the AMPARs to the synaptic surface has not yet been reported.Here, we report that the Transmembrane and ubiquitin-like domain-containing 1 (Tmub1) protein, formerly known as the Hepatocyte Odd Protein Shuttling (HOPS) protein, which is abundantly expressed in the brain and which exists in a synaptosomal membrane fraction, facilitates the recycling of the AMPAR subunit GluR2 to the cell surface. Neurons transfected with Tmub1/HOPS-RNAi plasmids showed a significant reduction in the AMPAR current as compared to their control neurons. Consistently, the synaptic surface expression of GluR2, but not of GluR1, was significantly decreased in the neurons transfected with the Tmub1/HOPS-RNAi and increased in the neurons overexpressing EGFP-Tmub1/HOPS. The altered surface expression of GluR2 was speculated to be due to the altered surface-recycling of the internalized GluR2 in our recycling assay. Eventually, we found that GluR2 and glutamate receptor interacting protein (GRIP) were coimmunoprecipitated by the anti-Tmub1/HOPS antibody from the mouse brain. Taken together, these observations show that the Tmub1/HOPS plays a role in regulating basal synaptic transmission; it contributes to maintain the synaptic surface number of the GluR2-containing AMPARs by facilitating the recycling of GluR2 to the plasma membrane.     

Phosphorylation of glutamate receptor interacting protein 1 regulates surface expression of glutamate receptors

The number of synaptic alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type glutamate receptors (AMPARs) controls the strength of excitatory transmission. AMPARs cycle between internal endosomal compartments and the plasma membrane. Interactions between the AMPAR subunit GluR2, glutamate receptor interacting protein 1 (GRIP1), and the endosomal protein NEEP21 are essential for correct GluR2 recycling. Here we show that an about 85-kDa protein kinase phosphorylates GRIP1 on serine 917. This kinase is present in NEEP21 immunocomplexes and is activated in okadaic acid-treated neurons. Pulldown assays and atomic force microscopy indicate that phosphorylated GRIP shows reduced binding to NEEP21. AMPA or N-methyl-D-aspartate stimulation of hippocampal neurons induces delayed phosphorylation of the same serine 917. A wild type carboxy-terminal GRIP1 fragment expressed in hippocampal neurons interferes with GluR2 surface expression. On the contrary, a S917D mutant fragment does not interfere with GluR2 surface expression. Likewise, coexpression of GluR2 together with full-length wild type GRIP1 enhances GluR2 surface expression in fibroblasts, whereas full-length GRIP1-S917D had no effect. This indicates that this serine residue is implicated in AMPAR cycling. Our results identify an important regulatory mechanism in the trafficking of AMPAR subunits between internal compartments and the plasma membrane.     

AMPA and NMDA glutamate receptor trafficking: multiple roads for reaching and leaving the synapse

Glutamate receptor trafficking in and out of synapses is one of the core mechanisms for rapid changes in the number of functional receptors during synaptic plasticity. Recent data have shown that the fast gain and loss of receptors from synaptic sites are accounted for by endocytic/exocytic processes and by their lateral diffusion in the plane of the membrane. These events are interdependent and regulated by neuronal activity and interactions with scaffolding proteins. We review here the main cellular steps for AMPA and NMDA receptor synthesis, traffic within intracellular organelles, membrane exocytosis/endocytosis and surface trafficking. We focus on new findings that shed light on the regulation of receptor cycling events and surface trafficking and the way that this might reshape our thinking about the specific regulation of receptor accumulation at synapses.     

AMPA受体转运及其调控的分子机制

α-氨-3-羟基-5-甲基-4-异恶唑丙酸受体（α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors,AMPA receptors）介导中枢神经系统（CNS）绝大多数快兴奋性突触传递,在学习、记忆和认知等方面具有重要功能. 突触AMPA受体的数量、分布和亚基组成是调节突触传递强度的一个主要机制,与AMPA受体转运密切相关. 最新研究显示,异常的AMPA受体转运与阿尔茨海默病（Alzheimer’s disease,AD）、脆性X综合征(fragile X syndrome, FXS)等神经疾病有关. 本文主要针对AMPA受体转运及其调控的分子机制做一综述,以期为AD、FXS等神经疾病提供新的治疗靶点和途径.     

蛋白质棕榈酰化对离子型谷氨酸受体运输的调节作用

棕榈酰化是一种可逆的翻译后修饰,其对蛋白质的定位和功能具有重要的调节意义.离子型谷氨酸受体有N-甲基-D-天冬氨酸(NMDA)受体、α-氨基羟甲基恶唑丙酸(AMPA)受体和人海藻酸受体.近期研究发现,它们的棕榈酰化修饰对其膜表面分布和内化均具有重要的意义.其中NMDA受体在其C末端有2个不同的棕榈酰化位点.1个位于C末端近膜区(CysclusterⅠ),它的棕榈酰化可以增高酪氨酸的磷酸化水平,增加受体膜表面分布,影响神经元中NMDA受体的组构性内化;另1个位于C末端中部(CysclusterⅡ),它受到蛋白质酰基转移酶GODZ的调节,使得受体在高尔基体大量积聚,从而影响受体的膜表面分布.与NMDA受体相似,AMPA受体也存在2个棕榈酰化位点.1个位于在第2跨膜域,受蛋白质酰基转移酶GODZ的调节,能导致AMPA受体在高尔基体的积聚.另1个位点在受体C末端近膜区,它的棕榈酰化能降低AMPA受体和4.1N蛋白的相互作用,并调节受体的内化.这两种离子型谷氨酸受体在棕榈酰化机制上虽然存在差异,但均对受体的运输、膜表面分布和内化具有十分重要的作用.     

Characterization of the stimulatory action of insulin on insulin-like growth factor II binding to rat adipose cells. Differences in the mechanism of insulin action on insulin-like growth factor II receptors and glucose transporters

Insulin is known to increase the number of cell surface insulin-like growth factor II (IGF-II) receptors in isolated rat adipose cells through a subcellular redistribution mechanism similar to that for the glucose transporter. The effects of insulin on these two processes, therefore, have now been directly compared in the same cell preparations. 1) Insulin increases the steady state number of cell surface IGF-II receptors by 7-13-fold without affecting receptor affinity; however, insulin stimulates glucose transport activity by 25-40-fold. 2) The insulin concentration required for half-maximal stimulation of cell surface IGF-II receptor number is approximately 30% lower than that for the stimulation of glucose transport activity. 3) The half-time for the achievement of insulin's maximal effect at 37 degrees C is much shorter for IGF-II receptor number (approximately 0.8 min) than for glucose transport activity (approximately 2.6 min). 4) Reversal of insulin's action at 37 degrees C occurs more rapidly for cell surface IGF-II receptors (t1/2 congruent to 2.9 min) than for glucose transport activity (t1/2 congruent to 4.9 min). 5) When the relative subcellular distribution of IGF-II receptors is examined in basal cells, less than 10% of the receptors are localized to the plasma membrane fraction indicating that most of the receptors, like glucose transporters, are localized to an intracellular compartment. However, in response to insulin, the number of plasma membrane IGF-II receptors increases only approximately 1.4-fold while the number of glucose transporters increases approximately 4.5-fold. Thus, while the stimulatory actions of insulin on cell surface IGF-II receptors and glucose transport activity are qualitatively similar, marked quantitative differences suggest that the subcellular cycling of these two integral membrane proteins occurs by distinct processes.     

Modulation of the insulin growth factor II/mannose 6-phosphate receptor in microvascular endothelial cells by phorbol ester via protein kinase C

Phosphorylation of hormone receptors by protein kinase C (PKC) may be involved in the regulation of receptor recycling. We have studied the recycling and the phosphorylation state of the insulin growth factor (IGF) II/mannose 6-phosphate (Man-6-P) receptor in microvascular endothelial cells from rat adipose tissue. Scatchard analysis showed these cells have over 2 x 10(6) receptors/cell with an affinity constant of 1 x 10(9) M-1. In the presence of phorbol myristate acetate (PMA), an activator of PKC and analog of diacylglycerol, IGF-II receptor number increased in the plasma membrane by 60% without changes in the binding affinity. This increase in cell surface receptor number was confirmed by affinity cross-linking and 125I-surface labeling studies, occurred with a half-time of 20 min, and was reversible upon withdrawal of PMA. The redistribution of IGF-II/Man-6-P receptors was not due to an inhibition of internalization which was in fact stimulated by PMA. The effect of PMA on IGF-II receptor recycling correlated with its stimulation of PKC activity. Furthermore, after down-regulation of cellular PKC levels by preincubation with PMA, PMA was unable to activate residual PKC activity in the membranous pool or increase IGF-II receptor number at the cell surface. The phosphorylation state of the IGF-II/Man-6-P receptor was determined by 32P labeling of intact cells and immunoprecipitation with anti-receptor antibodies. In the basal state, the receptor was phosphorylated only on serine residues which was increased by 75% after treatment with PMA. In contrast, IGF-II decreased receptor phosphorylation and plasma membrane binding in a parallel and dose-dependent manner. Thus, PKC-stimulated serine phosphorylation of IGF-II/Man-6-P receptor may promote the translocation of the receptor to the cell surface, whereas IGF-II-stimulated dephosphorylation of the receptor may lead to a decrease in the number of cell surface receptors. These data suggest a role for PKC-mediated serine phosphorylation in the regulation of intracellular trafficking of receptors in endothelial cells.     

Endosomal accumulation of the activated epidermal growth factor receptor (EGFR) induces apoptosis

Endocytosis positively and negatively regulates cell surface receptor signaling by temporally and spatially controlling interactions with downstream effectors. This process controls receptor-effector communication. However, the relationship between receptor endocytic trafficking and cell physiology is unclear. In MDA-MB-468 cells, cell surface EGF receptors (EGFRs) promote cell growth, whereas intracellular EGFRs induce apoptosis, making these cells an excellent model for studying the endocytic regulation of EGFR signaling. In addition, MDA-MB-468 cells have limited EGFR degradation following stimulation. Here, we report that in MDA-MB-468 cells the phosphorylated EGFR accumulates on the limiting membrane of the endosome with its carboxyl terminus oriented to the cytoplasm. To determine whether perturbation of EGFR trafficking is sufficient to cause apoptosis, we used pharmacological and biochemical strategies to disrupt EGFR endocytic trafficking in HeLa cells, which do not undergo EGF-dependent apoptosis. Manipulation of HeLa cells so that active EGF·EGFRs accumulate on the limiting membrane of endosomes reveals that receptor phosphorylation is sustained and leads to apoptosis. When EGF·EGFR complexes accumulated in the intraluminal vesicles of the late endosome, phosphorylation of the receptor was not sustained, nor did the cells undergo apoptosis. These data demonstrate that EGFR-mediated apoptosis is initiated by the activated EGFR from the limiting membrane of the endosome.     

PSD-95-like membrane associated guanylate kinases (PSD-MAGUKs) and synaptic plasticity

Activity-dependent modification of excitatory synaptic transmission is a fundamental mechanism for developmental plasticity of the neural circuits and experience-dependent plasticity. Synaptic glutamatergic receptors including AMPA receptors and NMDA receptors (AMPARs and NMDARs) are embedded in the postsynaptic density, a highly organized protein network. Overwhelming data have shown that PSD-95-like membrane associated guanylate kinases (PSD-MAGUKs), a major family of scaffold proteins at glutamatergic synapses, regulate basal synaptic AMPAR function and trafficking. It is now clear that PSD-MAGUKs have multifaceted functions in regulating both basal synaptic transmission and synaptic plasticity. Here we discuss recent advancements in understanding the roles of PSD-95 and other family members of PSD-MAGUKs in synaptic plasticity, both as an anchoring protein for synaptic AMPARs and as a signaling scaffold for mediating the interaction of the signaling complex and NMDARs.     

Mechanism of abnormal growth in astrocytes derived from a mouse model of GM2 gangliosidosis

P2X receptors mediate a variety of physiological actions, including smooth muscle contraction, neuro-endocrine secretion and synaptic transmission. Among P2X receptors, the P2X₃ subtype is expressed in sensory neurons of dorsal root- and trigeminal-ganglia, where it performs a well-recognized role in sensory and pain transmission. Recent evidence indicates that the strength of P2X₃-mediated responses is modulated in vivo by altering the number of receptors at the plasma membrane. In the present study, we investigate the trafficking properties of P2X₃ receptor in transfected HEK293 cells and in primary cultures of dorsal root ganglion neurons, finding that P2X₃ receptor undergoes rapid constitutive and cholesterol-dependent endocytosis. We also show that endocytosis is accompanied by preferential targeting of the receptor to late endosomes/lysosomes, with subsequent degradation. Furthermore, we observe that at steady state the receptor localizes predominantly in lamp1-positive intracellular structures, with a minor fraction present at the plasma membrane. Finally, the level of functional receptor expressed on the cell surface is rapidly up-regulated in response to agonist stimulation, which also augments receptor endocytosis. The findings presented in this work underscore a very dynamic trafficking behavior of P2X₃ receptor and disclose a possible mechanism for the rapid modulation of ATP-mediated responses potentially relevant during physiological and pathological conditions.     

A role for cGMP-dependent protein kinase II in AMPA receptor trafficking and synaptic plasticity

Regulated trafficking of AMPA receptors (AMPARs) is an important mechanism that underlies the activity-dependent modification of synaptic strength. Trafficking of AMPARs is regulated by specific interactions of their subunits with other proteins. Recently, we have reported that the AMPAR subunit GluR1 binds the cGMP-dependent kinase type II (cGKII) adjacent to the kinase catalytic site, and that this interaction is increased by cGMP. In this complex, cGKII phosphorylates GluR1 at serine 845 (S845), a site known to be phosphorylated also by PKA. S845 phosphorylation leads to an increase of GluR1 on the plasma membrane. In neurons, cGMP is produced by soluble guanylate cyclase (sGC), which is activated by nitric oxide (NO). Calcium flux through the NMDA receptor (NMDAR) activates neuronal nitric oxide synthase (nNOS), which produces NO. Using a combination of biochemical and electrophysiological experiments, we have shown that trafficking of GluR1 is under the regulation of NO, cGMP and cGKII. Moreover, our study indicates that the interaction of cGKII with GluR1, which is under the regulation of the NMDAR and NO, plays an important role in hippocampal plasticity.     

A role for cGMP-dependent protein kinase II in AMPA receptor trafficking and synaptic plasticity

Regulated trafficking of AMPA receptors (AMPARs) is an important mechanism that underlies the activity-dependent modification of synaptic strength. Trafficking of AMPARs is regulated by specific interactions of their subunits with other proteins. Recently, we have reported that the AMPAR subunit GluR1 binds the cGMP-dependent kinase type II (cGKII) adjacent to the kinase catalytic site, and that this interaction is increased by cGMP. In this complex, cGKII phosphorylates GluR1 at serine 845 (S845), a site known to be phosphorylated also by PKA. S845 phosphorylation leads to an increase of GluR1 on the plasma membrane. In neurons, cGMP is produced by soluble guanylate cyclase (sGC), which is activated by nitric oxide (NO). Calcium flux through the NMDA receptor (NMDAR) activates neuronal nitric oxide synthase (nNOS), which produces NO. Using a combination of biochemical and electrophysiological experiments, we have shown that trafficking of GluR1 is under the regulation of NO, cGMP and cGKII. Moreover, our study indicates that the interaction of cGKII with GluR1, which is under the regulation of the NMDAR and NO, plays an important role in hippocampal plasticity.     

A Monte Carlo model reveals independent signaling at central glutamatergic synapses

We have developed a biophysically realistic model of receptor activation at an idealized central glutamatergic synapse that uses Monte Carlo techniques to simulate the stochastic nature of transmission following release of a single synaptic vesicle. For the a synapse with 80 AMPA and 20 NMDA receptors, a single quantum, with 3000 glutamate molecules, opened approximately 3 NMDARs and 20 AMPARs. The number of open receptors varied directly with the total number of receptors, and the fraction of open receptors did not depend on the ratio of co-localized AMPARs and NMDARs. Variability decreased with increases in either total receptor number or quantal size, and differences between the variability of AMPAR and NMDAR responses were due solely to unequal numbers of receptors at the synapse. Despite NMDARs having a much higher affinity for glutamate than AMPARs, quantal release resulted in similar occupancy levels in both receptor types. Receptor activation increased with number of transmitter molecules released or total receptor number, whereas occupancy levels were only dependent on quantal size. Tortuous diffusion spaces reduced the extent of spillover and the activation of extrasynaptic receptors. These results support the conclusion that signaling is spatially independent within and between central glutamatergic synapses.     

Differential roles of NR2A- and NR2B-containing NMDA receptors in Ras-ERK signaling and AMPA receptor trafficking

NMDA receptors (NMDARs) control bidirectional synaptic plasticity by regulating postsynaptic AMPA receptors (AMPARs). Here we show that NMDAR activation can have differential effects on AMPAR trafficking, depending on the subunit composition of NMDARs. In mature cultured neurons, NR2A-NMDARs promote, whereas NR2B-NMDARs inhibit, the surface expression of GluR1, primarily by regulating its surface insertion. In mature neurons, NR2B is coupled to inhibition rather than activation of the Ras-ERK pathway, which drives surface delivery of GluR1. Moreover, the synaptic Ras GTPase activating protein (GAP) SynGAP is selectively associated with NR2B-NMDARs in brain and is required for inhibition of NMDAR-dependent ERK activation. Preferential coupling of NR2B to SynGAP could explain the subtype-specific function of NR2B-NMDARs in inhibition of Ras-ERK, removal of synaptic AMPARs, and weakening of synaptic transmission.     

AMPA receptors commandeer an ancient cargo exporter for use as an auxiliary subunit for signaling

Fast excitatory neurotransmission in the mammalian central nervous system is mainly mediated by ionotropic glutamate receptors of the AMPA subtype (AMPARs). AMPARs are protein complexes of the pore-lining α-subunits GluA1-4 and auxiliary β-subunits modulating their trafficking and gating. By a proteomic approach, two homologues of the cargo exporter cornichon, CNIH-2 and CNIH-3, have recently been identified as constituents of native AMPARs in mammalian brain. In heterologous reconstitution experiments, CNIH-2 promotes surface expression of GluAs and modulates their biophysical properties. However, its relevance in native AMPAR physiology remains controversial. Here, we have studied the role of CNIH-2 in GluA processing both in heterologous cells and primary rat neurons. Our data demonstrate that CNIH-2 serves an evolutionarily conserved role as a cargo exporter from the endoplasmic reticulum (ER). CNIH-2 cycles continuously between ER and Golgi complex to pick up cargo protein in the ER and then to mediate its preferential export in a coat protein complex (COP) II dependent manner. Interaction with GluA subunits breaks with this ancestral role of CNIH-2 confined to the early secretory pathway. While still taking advantage of being exported preferentially from the ER, GluAs recruit CNIH-2 to the cell surface. Thus, mammalian AMPARs commandeer CNIH-2 for use as a bona fide auxiliary subunit that is able to modify receptor signaling.