The Unfolded Protein Response Regulates Glutamate Receptor Export from the Endoplasmic Reticulum

Abstract

␣-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptors mediate the majority of excitatory signaling in the CNS, and the functional properties and subcellular fate of these receptors depend on receptor subunit composition. Subunit assembly is thought to occur in the endoplasmic reticulum (ER), although we are just beginning to understand the underlying mechanism. Here we examine the trafficking of Caenorhabditis elegans glutamate receptors through the ER. Our data indicate that neurons require signaling by the unfolded protein response (UPR) to move GLR-1, GLR-2, and GLR-5 subunits out of the ER and through the secretory pathway. In contrast, other neuronal transmembrane proteins do not require UPR signaling for ER exit. The requirement for the UPR pathway is cell type and age dependent: impairment for receptor trafficking increases as animals age and does not occur in all neurons. Expression of XBP-1, a component of the UPR pathway, is elevated in neurons during development. Our results suggest that UPR signaling is a critical step in neural function that is needed for glutamate receptor assembly and secretion. INTRODUCTIONIon channels conduct electrochemical signaling in the nervous system, and ␣-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type ionotropic glutamate receptors (AMPAR) in particular mediate the bulk of excitatory transmission in the CNS. AMPAR subunits (up to 4 in mammals, referred to as GluR1-R4) are multi-transmembrane-spanning proteins that can assemble into tetrameric channels of differing subunit composition (Hollmann and Heinemann, 1994;Dingledine et al., 1999). The specific subunit composition of a given AMPAR channel plays a critical role in determining the functional properties of that channel, including its channel opening probability, ion selectivity, and cytosolic binding partners (Hollmann and Heinemann, 1994;Dingledine et al., 1999;Sheng, 2001;Shi et al., 2001;Malinow, 2003). Subunit composition can also control the subcellular localization and regulated cell biological fate of a channel (Beattie et al., 2000;Lin et al., 2000). For example, heteromeric complexes of GluR1-R2 receptors have been shown to be added to hippocampal synapses in an activity-dependent manner, whereas GluR2-R3 complexes appear to cycle into synaptic membranes in a constitutive manner (Passafaro et al., 2001;Shi et al., 2001). To better understand AMPAR function in the nervous system, it is important to determine how individual subunits assemble into specific complexes of channels.Multisubunit channel assembly in general is tied to the movement of channels through the secretory pathway. Most proteins exit the ER without the need for specialized export signals (Wieland et al., 1987). However, many channels and receptors seem to defy this general rule by requiring channel assembly, sometimes specific export signals, and sometimes the interaction of scaffolding proteins and certain signal transduction events before exiting the ER (Ma and Jan, 2002). One explanation is ...

RAB-10 Regulates Glutamate Receptor Recycling in a Cholesterol-dependent Endocytosis Pathway

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“…To explore this possibility, we examined the localization of the synaptic vesicle protein synaptobrevin (SNB-1) (Nonet et al, 1998). SNB-1::GFP, when expressed from the glr-1 promoter, displays a punctate localization pattern, labeling presynaptic boutons along neurites of the command interneurons in wild-type animals (Rongo et al, 1998;Shim et al, 2004;Glodowski et al, 2005). We observed a similar punctate localization for SNB-1::GFP in rab-10 mutant animals (data not shown).…”

“…18, November 2007 4389 (White et al, 1986;Kaplan and Horvitz, 1993). Mutants with reduced GLR-1 activity have a lower frequency of spontaneous reversals and are nose-touch insensitive, whereas mutants with increased GLR-1 activity (e.g., higher levels of synaptic GLR-1 at the cell surface) have a higher frequency of spontaneous reversals (Hart et al, 1995;Maricq et al, 1995;Burbea et al, 2002;Mellem et al, 2002;Juo and Kaplan, 2004;Shim et al, 2004;Zheng et al, 2004;Umemura et al, 2005;Schaefer and Rongo, 2006). We examined GLR-1-mediated behaviors in wild-type animals and in animals with mutations in lin-10 and rab-10.…”

KEL-8 Is a Substrate Receptor for CUL3-dependent Ubiquitin Ligase That Regulates Synaptic Glutamate Receptor Turnover

“…To observe the localization of different synaptic proteins, we used several previously published integrated transgenes: nuIs25 [glr-1::gfp], odIs1 [snb-1::gfp], odIs16 [glr-1::rfp], odIs22 [lin-10::gfp], nuIs68 [unc-43::gfp], and nuIs89 [MUb, (Rongo et al, 1998;Rongo and Kaplan, 1999;Burbea et al, 2002;Shim et al, 2004). Other transgenic strains were isolated by microinjecting various plasmids (typically at 50 ng/ml) using rol-6dm (a gift from C. Mello, University of Massachusetts Medical School, Worcester, MA) as a marker.…”

Approaches to imaging unfolded secretory protein stress in living cells

“…Like the transcriptional reporters, fluorescence levels of SR-GFP can be measured by fluorescent microscopy and flow cytometry. Similar approaches have been used in other organisms including C. elegans [76], D. melanogaster [77], and mammalian cells [78, 79]. …”