Presynaptic Targeting of α4β2 Nicotinic Acetylcholine Receptors Is Regulated by Neurexin-1β

Abstract

The mechanisms involved in the targeting of neuronal nicotinic acetylcholine receptors (AChRs), critical for their functional organization at neuronal synapses, are not well understood. We have identified a novel functional association between ␣4␤2 AChRs and the presynaptic cell adhesion molecule, neurexin-1␤. In non-neuronal tsA 201 cells, recombinant neurexin-1␤ and mature ␣4␤2 AChRs form complexes. ␣4␤2 AChRs and neurexin-1␤ also coimmunoprecipitate from rat brain lysates. When exogenous ␣4␤2 AChRs and neurexin-1␤ are coexpressed in hippocampal neurons, they are robustly targeted to hemi-synapses formed between these neurons and cocultured tsA 201 cells expressing neuroligin-1, a postsynaptic binding partner of neurexin-1␤. The extent of synaptic targeting is significantly reduced in similar experiments using a mutant neurexin-1␤ lacking the extracellular domain. Additionally, when ␣4␤2 AChRs, ␣7 AChRs, and neurexin-1␤ are coexpressed in the same neuron, only the ␣4␤2 AChR colocalizes with neurexin-1␤ at presynaptic terminals. Collectively, these data suggest that neurexin-1␤ targets ␣4␤2 AChRs to presynaptic terminals, which mature by trans-synaptic interactions between neurexins and neuroligins. Interestingly, human neurexin-1 gene dysfunctions have been implicated in nicotine dependence and in autism spectrum disorders. Our results provide novel insights as to possible mechanisms by which dysfunctional neurexins, through downstream effects on ␣4␤2 AChRs, may contribute to the etiology of these neurological disorders.The clustering of ion channels or receptors and precise targeting to pre-and postsynaptic specializations in neurons is critical to efficiently regulate synaptic transmission. Within the central nervous system, neuronal nicotinic acetylcholine receptors (AChRs) 5 regulate the release of neurotransmitters at presynaptic sites (1) and mediate fast synaptic transmission at postsynaptic sites of neurons (2). These receptors are part of a family of acetylcholine-gated ion channels that are assembled from various combinations of ␣2-␣10 and ␤2-␤4 subunits (3). AChRs participate in the regulation of locomotion, affect, reward, analgesia, anxiety, learning, and attention (4, 5).The ␣4␤2 subtype is the most abundant AChR receptor expressed in the brain. Multiple lines of evidence support a major role for ␣4␤2 AChRs in nicotine addiction. ␣4␤2 AChRs show high affinity for nicotine (6) and are located on the dopaminergic projections of ventral tegmental area neurons to the medium spiny neurons of the nucleus accumbens (7,8). Furthermore, ␤2 AChR subunit knock-out mice lose their sensitivity to nicotine in passive avoidance tasks (9) and show attenuated self-administration of nicotine (10). ␣4 AChR subunit knock-out mice also exhibit a loss of tonic control of striatal basal dopamine release (11). Finally, experiments with knock-in mice expressing ␣4␤2 AChRs hypersensitive to nicotine demonstrate that ␣4␤2 AChRs indeed mediate the essential features of nicotine addiction including reward, tolerance, and sen...

Endogenously Expressed Muscarinic Receptors in HEK293 Cells Augment Up-regulation of Stably Expressed α4β2 Nicotinic Receptors

“…It will also be important to determine if this muscarinic receptor contribution to nAChR up-regulation occurs in native tissues, including the brain, which could indicate the potential physiological importance of these findings. In any case, these findings, along with other recent studies (38,40,71), reveal the possibility of modulating ␣4␤2 nAChR expression in the brain by drugs and mechanisms other than nicotine.…”

Modulation of synaptic function through the α-neurexin–specific ligand neurexophilin-1

“…In addition, research has identified several molecules, mostly ion channels and receptors, which are functionally impaired when Nrxn expression is altered. Nrxn variants have been found to affect voltage-dependent calcium channels (9,10), GABA A R (7, 37), NMDAR (44), GABA B R (36,45), nicotinergic acetylcholine receptors (69), and AMPAR (47). These ionotropic and metabotropic receptors represent "target molecules" of Nrxn that may include some physical association, but it appears unlikely that the functional link to all these requires stable protein-protein interactions.…”

Processing of the Synaptic Cell Adhesion Molecule Neurexin-3β by Alzheimer Disease α- and γ-Secretases

“…Yet, NRXN3␤-ICD contains the PDZ II binding motif that is required for its binding to cytoplasmic partners, such as CASK and Mint, and functions as a scaffold protein for vesicle clustering (50,57). Moreover, the NRXN␤ intracellular domain has recently been reported to interact with and participate in the proper targeting of nicotinic receptors (58). Whether NRXN3␤-ICD also triggers signaling pathways and/or affects neurotransmitter vesicle clustering and release remains unknown and needs further investigation.…”