An ER-resident membrane protein complex regulates nicotinic acetylcholine receptor subunit composition at the synapse

Abstract

Nicotinic acetylcholine receptors (nAChRs) are homo-or heteropentameric ligand-gated ion channels mediating excitatory neurotransmission and muscle activation. Regulation of nAChR subunit assembly and transfer of correctly assembled pentamers to the cell surface is only partially understood. Here, we characterize an ER transmembrane (TM) protein complex that influences nAChR cell-surface expression and functional properties in Caenorhabditis elegans muscle. Loss of either type I TM protein, NRA-2 or NRA-4 (nicotinic receptor associated), affects two different types of muscle nAChRs and causes in vivo resistance to cholinergic agonists. Sensitivity to subtype-specific agonists of these nAChRs is altered differently, as demonstrated by whole-cell voltage-clamp of dissected adult muscle, when applying exogenous agonists or after photo-evoked, channelrhodopsin-2 (ChR2) mediated acetylcholine (ACh) release, as well as in singlechannel recordings in cultured embryonic muscle. These data suggest that nAChRs desensitize faster in nra-2 mutants. Cell-surface expression of different subunits of the 'levamisole-sensitive' nAChR (L-AChR) is differentially affected in the absence of NRA-2 or NRA-4, suggesting that they control nAChR subunit composition or allow only certain receptor assemblies to leave the ER.

Optogenetic analysis of GABABreceptor signaling inCaenorhabditis elegansmotor neurons

“…Thus no plastic changes were apparent, and ACh transmission remained sufficiently high to cause sustained full body contraction. As we and others previously showed, constant photostimulation of cholinergic MNs causes an initially large postsynaptic inward current (ϳ1,200 pA) that quickly declines to a steady-state current of roughly 80 pA (Almedom et al 2009;Liewald et al 2008;Liu et al 2009). This is in large part due to desensitization of postsynaptic nicotinic ACh receptors and somewhat to depression of cholinergic MNs and partial inactivation of ChR2.…”

NRA-2, a Nicalin Homolog, Regulates Neuronal Death by Controlling Surface Localization of Toxic Caenorhabditis elegans DEG/ENaC Channels

“…Later studies revealed a role for nra-2 in AChR subunit selection and/or surface expression in muscle (16). More specifically, this study showed that specific AChR subunits co-expressed in muscle were selected against being included into drug-sensitive AChR complexes when NRA-2 was active (16).…”

“…Receptor Neurons-NRA-2 encodes a transmembrane ER-resident protein implicated in regulation of acetylcholine receptor (AChR) subunit composition in C. elegans muscle, possibly by allowing only specific subunit combinations of that receptor to exit the ER (16). This role in channel assembly raises the question as to whether NRA-2 might similarly regulate distribution, assembly, or surface expression of the touch-sensing MEC channel.…”

“…Here we report that disruption of mammalian nicalin homolog nra-2, an ER protein implicated in C. elegans muscle acetylcholine receptor (AChR) channel maturation (16), enhances MEC-10(d)-induced necrosis via a mechanism that increases surface expression of the MEC-10(d) channel. Our data further underscore the role of excessive channel activity in necrosis induction, identify NRA-2/nicalin as a new player in DEG/ENaC channel biogenesis, and reveal an unexpected focused surface distribution of the MEC-10(d) channel in touch neurons.…”

Biosynthesis of ionotropic acetylcholine receptors requires the evolutionarily conserved ER membrane complex

“…Interestingly, expression of L-AChRs in Xenopus oocytes not only requires the five receptor subunits but the three auxiliary proteins RIC-3, UNC-50, and UNC-74 as well (36). Recently, the ER resident transmembrane proteins NRA-2/ nicalin (nicastrin-like protein) and NRA-4/nodal modulator (NOMO), were proposed to regulate L-AChR subunit composition and stoichiometry (40).…”