Early events in glutamate receptor trafficking

doi:10.1016/j.ceb.2004.01.003

Current Opinion in Cell Biology

Volume 16, Issue 2, April 2004, Pages 134-139

https://doi.org/10.1016/j.ceb.2004.01.003 Get rights and content

Abstract

Glutamate receptors are the primary mediators of excitatory synaptic transmission in the mammalian central nervous system. Activity-dependent changes in the number of postsynaptic glutamate receptors underlie aspects of synaptic plasticity and provide a mechanism for information storage in the brain. Recent work shows that receptor exit from the endoplasmic reticulum represents a critical regulatory step in glutamate receptor trafficking to the neuronal cell surface.

Introduction

Glutamate, the principal excitatory neurotransmitter in the mammalian central nervous system, activates two types of postsynaptic receptors: ionotropic receptors (iGluRs), which are glutamate-gated cation channels, and metabotropic receptors (mGluRs), which are linked through G-proteins to second messenger systems 1., 2., 3., 4.. Ionotropic GluRs can be subdivided into three families on the basis of pharmacology, electrophysiology and sequence homology: AMPA receptors, which are homo- or hetero-tetramers composed of the subunits GluR1–4; kainate receptors, homo- or hetero-tetramers of the subunits GluR5–7, KA1 and KA2; and NMDA receptors, hetero-tetramers that contain both NR1 and NR2 (NR2A–D) subunits, and in some cases NR3 subunits. Among the mGluR subunits, mGluR1 and mGluR5 are predominant at postsynaptic sites, and these receptors probably exist on the cell surface as homodimers. Metabotropic GluR subunits are seven-transmembrane proteins, whereas iGluR subunits contain three transmembrane domains and a pore-lining, re-entrant membrane loop (Figure 1). All GluR subunits contain an extracellular N-terminal domain and a cytoplasmic C terminus.

GluRs are highly concentrated at postsynaptic sites, where they interact with a host of structural and signaling proteins collectively referred to as the postsynaptic density (PSD). Tremendous progress has been made in dissecting the molecular anatomy of the PSD [5]. In addition, it has become clear that changing the numbers of postsynaptic glutamate receptors, especially AMPA receptors, is a key mechanism of activity-dependent modulation of synaptic strength, a phenomenon thought to underlie aspects of learning and memory 6., 7., 8.. Accordingly, GluR trafficking at the postsynaptic membrane has been intensively studied.

Classical studies in non-neuronal cells have shown that exit from the endoplasmic reticulum (ER) is often the most stringently controlled step in integral membrane protein transport to the cell surface [9]. Similarly, recent work reveals that GluR trafficking through this early compartment of the secretory pathway is tightly regulated and that mechanisms controlling ER exit have a major impact on synaptic GluR abundance. This review highlights these recent discoveries of rules governing ER exit of GluRs (Figure 2).

Section snippets

ER quality control of GluRs

Like other multimeric cell membrane proteins, GluRs are synthesized, folded and assembled in the ER. For large multi-subunit membrane proteins such as ion channels, folding of individual subunits often continues throughout formation of the multimeric protein complex [10]. N-glycosylation and disulphide bond formation, which often promote proper protein folding 11., 12., also occur in the ER. All GluR subunits possess N-glycosylation sites 2., 4., and iGluR [13] and mGluR [4] subunits contain

ER retention/retrieval signals in GluRs

ER retrieval signals, such as the KDEL and KKXX motifs, help anchor ER resident proteins 9., 22.. Current evidence suggests that these signals mediate cargo retrieval from the ER–Golgi intermediate compartment (ERGIC) or from the Golgi to the ER by direct or indirect interactions with coat protein complex I (COPI), a set of cytosolic proteins involved in retrograde vesicle transport through the secretory pathway.

ER retention/retrieval signals are also present in some proteins destined for

ER export signals in GluRs

Vesicles export cargo from the ER to the Golgi. Budding of export vesicles is driven by interactions of ER membranes with coat protein complex II (COPII), a polymer formed by ordered assembly of cytosolic subunits [36]. According to the bulk-flow model, a default pathway packages properly folded and assembled membrane proteins into COPII-coated vesicles at their prevailing concentration in the ER. On the other hand, some membrane proteins contain specific export signals that interact with COPII

Regulation of ER exit of GluRs by phosphorylation

The RRR retention motif in NR1-1 and NR1-3 is adjacent to two serine residues that are phosphorylated by protein kinase A and protein kinase C (PKC). Mutation of these serines to negatively charged residues to mimic phosphorylation suppresses ER retention 28., 29.. In addition, pharmacological activation of PKC increases NR1 surface expression 27., 28., 29.. The effect of phosphorylation on NR1 export from the ER adds an activity-dependent dimension to the control of this trafficking step.

Regulation of ER exit of GluRs by GluR-binding proteins

In the past eight years a large number of GluR-binding proteins have been identified. Most of these interacting proteins localize to the PSD [5]; however, some interacting proteins associate with GluRs in the proximal secretory pathway and affect ER export. As mentioned earlier, binding of PDZ proteins suppresses the NR1-3 ER retention signal 27., 28., 29.. Synapse-associated protein 97, a PDZ domain-containing binding partner of GluR1, preferentially binds immature GluR1 in the ER and cis

ER in dendrites and spines

Most GluRs are synthesized and processed in somatic ER and then trafficked to dendrites and spines. This is illustrated by the subcellular redistribution of NMDA receptor subunits in mice specifically lacking NR1 in hippocampal CA1 pyramidal neurons [25^••]. As discussed above, NR2 subunits cannot exit the ER without NR1. In CA1 pyramidal cells of CA1–NR1 knockout mice, NR2 subunits are nearly absent from dendrites and accumulate in somatic ER [25^••], implying that somatic ER represents the

Conclusions

The complexity of ER export of GluRs is only beginning to be understood. GluR exit from ER is regulated by diverse mechanisms including mRNA splicing and editing, as well as receptor phosphorylation, association with specific binding partners, and interaction with the QC machinery. By altering the efficiency of GluR surface expression, these events influence neuronal responsiveness to glutamate and vulnerability to excitotoxicity. In addition, the presence of ER in dendrites and spines allows

Update

Mu and colleagues recently reported that neuronal activity regulates ER export of NMDA receptors by controlling alternative splicing of the C-terminal tail of the NR1 subunit [50^••]. Chronically decreasing neuronal activity favors production of splice variants that efficiently exit the ER, whereas chronically increasing activity has the opposite effect. Consistent with previous studies 27., 28., 29., accelerated ER export of NR1 depends on a C-terminal –TVV sequence present in NR1-3 and NR1-4

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

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of special interest
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of outstanding interest

Acknowledgements

WV is supported by a postdoctoral fellowship of the Fund for Scientific Research-Flanders. DSB is supported by grants from the National Institutes of Health and is an established investigator of the American Heart Association.

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Early events in glutamate receptor trafficking

Abstract

Introduction

Section snippets

ER quality control of GluRs

ER retention/retrieval signals in GluRs

ER export signals in GluRs

Regulation of ER exit of GluRs by phosphorylation

Regulation of ER exit of GluRs by GluR-binding proteins

ER in dendrites and spines

Conclusions

Update

References and recommended reading

Acknowledgements

Neuron

Trends Neurosci.

Trends Cell. Biol.

J. Biol. Chem.

Cell

Cell

Curr. Opin. Neurobiol.

Annu. Rev. Pharmacol. Toxicol.

Proc. Natl. Acad. Sci. U S A

Neuropharmacology

Mol. Cell. Neurosci.

Neuron

Neuron

J. Biol. Chem.

J. Biol. Chem.

J. Neurosci.

Nat. Cell. Biol.

J. Biol. Chem.

J. Neurosci.

Proc. Natl. Acad. Sci. U S A

J. Neurosci.

Cloned glutamate receptors

Annu. Rev. Neurosci.

The glutamate receptor ion channels

Pharmacol. Rev.

The structure and function of glutamate receptor ion channels

Nat. Rev. Neurosci.

Structural, signalling and regulatory properties of the group I metabotropic glutamate receptors: prototypic family C G-protein-coupled receptors

Biochem. J.