Mechanistic and structural studies reveal NRAP-1-dependent coincident activation of NMDARs.

N-methyl-D-aspartate (NMDA)-type ionotropic glutamate receptors have essential roles in neurotransmission and synaptic plasticity. Previously, we identified an evolutionarily conserved protein, NRAP-1, that is required for NMDA receptor (NMDAR) function in C. elegans. Here, we demonstrate that NRAP-1 was sufficient to gate NMDARs and greatly enhanced glutamate-mediated NMDAR gating, thus conferring coincident activation properties to the NMDAR. Intriguingly, vertebrate NMDARs-and chimeric NMDARs where the amino-terminal domain (ATD) of C. elegans NMDARs was replaced by the ATD from vertebrate receptors-were spontaneously active when ectopically expressed in C. elegans neurons. Thus, the ATD is a primary determinant of NRAP-1- and glutamate-mediated gating of NMDARs. We determined the crystal structure of NRAP-1 at 1.9-Å resolution, which revealed two distinct domains positioned around a central low-density lipoprotein receptor class A domain. The NRAP-1 structure, combined with chimeric and mutational analyses, suggests a model where the three NRAP-1 domains work cooperatively to modify the gating of NMDARs.

MAPK signaling and a mobile scaffold complex regulate AMPA receptor transport to modulate synaptic strength.

Synaptic plasticity depends on rapid experience-dependent changes in the number of neurotransmitter receptors. Previously, we demonstrated that motor-mediated transport of AMPA receptors (AMPARs) to and from synapses is a critical determinant of synaptic strength. Here, we describe two convergent signaling pathways that coordinate the loading of synaptic AMPARs onto scaffolds, and scaffolds onto motors, thus providing a mechanism for experience-dependent changes in synaptic strength. We find that an evolutionarily conserved JIP-protein scaffold complex and two classes of mitogen-activated protein kinase (MAPK) proteins mediate AMPAR transport by kinesin-1 motors. Genetic analysis combined with in vivo, real-time imaging in Caenorhabditis elegans revealed that CaMKII is required for loading AMPARs onto the scaffold, and MAPK signaling is required for loading the scaffold complex onto motors. Our data support a model where CaMKII signaling and a MAPK-signaling pathway cooperate to facilitate the rapid exchange of AMPARs required for early stages of synaptic plasticity.

NRAP-1 Is a Presynaptically Released NMDA Receptor Auxiliary Protein that Modifies Synaptic Strength.

NMDA receptors (NMDARs) are a subtype of postsynaptic ionotropic glutamate receptors that function as molecular coincidence detectors, have critical roles in models of learning, and are associated with a variety of neurological and psychiatric disorders. To date, no auxiliary proteins that modify NMDARs have been identified. Here, we report the identification of NRAP-1, an auxiliary protein in C. elegans that modulates NMDAR function. NMDAR-mediated currents were eliminated in nrap-1 mutants, as was NMDA-dependent behavior. We show that reconstitution of NMDA-gated current in Xenopus oocytes, or C. elegans muscle cells, depends on NRAP-1 and that recombinant NRAP-1 can convert silent NMDARs to functional channels. Our data indicate that NRAP-1, secreted from presynaptic neurons, localizes to glutamatergic synapses, where it associates with postsynaptic NMDARs to modify receptor gating. Thus, our studies reveal a novel mechanism for synaptic regulation via pre-synaptic control of NMDAR-mediated synaptic transmission.

Neuronal Activity and CaMKII Regulate Kinesin-Mediated Transport of Synaptic AMPARs.

Excitatory glutamatergic synaptic transmission is critically dependent on maintaining an optimal number of postsynaptic AMPA receptors (AMPARs) at each synapse of a given neuron. Here, we show that presynaptic activity, postsynaptic potential, voltage-gated calcium channels (VGCCs) and UNC-43, the C. elegans homolog of CaMKII, control synaptic strength by regulating motor-driven AMPAR transport. Genetic mutations in unc-43, or spatially and temporally restricted inactivation of UNC-43/CaMKII, revealed its essential roles in the transport of AMPARs from the cell body and in the insertion and removal of synaptic AMPARs. We found that an essential target of UNC-43/CaMKII is kinesin light chain and that mouse CaMKII rescued unc-43 mutants, suggesting conservation of function. Transient expression of UNC-43/CaMKII in adults rescued the transport defects, while optogenetic stimulation of select synapses revealed CaMKII's role in activity-dependent plasticity. Our results demonstrate unanticipated, fundamentally important roles for UNC-43/CaMKII in the regulation of synaptic strength.

Kinesin-1 regulates synaptic strength by mediating the delivery, removal, and redistribution of AMPA receptors.

A primary determinant of the strength of neurotransmission is the number of AMPA-type glutamate receptors (AMPARs) at synapses. However, we still lack a mechanistic understanding of how the number of synaptic AMPARs is regulated. Here, we show that UNC-116, the C. elegans homolog of vertebrate kinesin-1 heavy chain (KIF5), modifies synaptic strength by mediating the rapid delivery, removal, and redistribution of synaptic AMPARs. Furthermore, by studying the real-time transport of C. elegans AMPAR subunits in vivo, we demonstrate that although homomeric GLR-1 AMPARs can diffuse to and accumulate at synapses in unc-116 mutants, glutamate-gated currents are diminished because heteromeric GLR-1/GLR-2 receptors do not reach synapses in the absence of UNC-116/KIF5-mediated transport. Our data support a model in which ongoing motor-driven delivery and removal of AMPARs controls not only the number but also the composition of synaptic AMPARs, and thus the strength of synaptic transmission.

Cornichons control ER export of AMPA receptors to regulate synaptic excitability.

The strength of synaptic communication at central synapses depends on the number of ionotropic glutamate receptors, particularly the class gated by the agonist AMPA (AMPARs). Cornichon proteins, evolutionarily conserved endoplasmic reticulum cargo adaptors, modify the properties of vertebrate AMPARs when coexpressed in heterologous cells. However, the contribution of cornichons to behavior and in vivo nervous system function has yet to be determined. Here, we take a genetic approach to these questions by studying CNI-1--the sole cornichon homolog in C. elegans. cni-1 mutants hyperreverse, a phenotype associated with increased glutamatergic synaptic transmission. Consistent with this behavior, we find larger glutamate-gated currents in cni-1 mutants with a corresponding increase in AMPAR number. Furthermore, we observe opposite phenotypes in transgenic worms that overexpress CNI-1 or vertebrate homologs. In reconstitution studies, we provide support for an evolutionarily conserved role for cornichons in regulating the export of vertebrate and invertebrate AMPARs.

The SOL-2/Neto auxiliary protein modulates the function of AMPA-subtype ionotropic glutamate receptors.

The neurotransmitter glutamate mediates excitatory synaptic transmission by gating ionotropic glutamate receptors (iGluRs). AMPA receptors (AMPARs), a subtype of iGluR, are strongly implicated in synaptic plasticity, learning, and memory. We previously discovered two classes of AMPAR auxiliary proteins in C. elegans that modify receptor kinetics and thus change synaptic transmission. Here, we have identified another auxiliary protein, SOL-2, a CUB-domain protein that associates with both the related auxiliary subunit SOL-1 and with the GLR-1 AMPAR. In sol-2 mutants, behaviors dependent on glutamatergic transmission are disrupted, GLR-1-mediated currents are diminished, and GLR-1 desensitization and pharmacology are modified. Remarkably, a secreted variant of SOL-1 delivered in trans can rescue sol-1 mutants, and this rescue depends on in cis expression of SOL-2. Finally, we demonstrate that SOL-1 and SOL-2 have an ongoing role in the adult nervous system to control AMPAR-mediated currents.

Large isoforms of UNC-89 (obscurin) are required for muscle cell architecture and optimal calcium release in Caenorhabditis elegans.

Calcium, a ubiquitous intracellular signaling molecule, controls a diverse array of cellular processes. Consequently, cells have developed strategies to modulate the shape of calcium signals in space and time. The force generating machinery in muscle is regulated by the influx and efflux of calcium ions into the muscle cytoplasm. In order for efficient and effective muscle contraction to occur, calcium needs to be rapidly, accurately and reliably regulated. The mechanisms underlying this highly regulated process are not fully understood. Here, we show that the Caenorhabditis elegans homolog of the giant muscle protein obscurin, UNC-89, is required for normal muscle cell architecture. The large immunoglobulin domain-rich isoforms of UNC-89 are critical for sarcomere and sarcoplasmic reticulum organization. Furthermore, we have found evidence that this structural organization is crucial for excitation-contraction coupling in the body wall muscle, through the coordination of calcium signaling. Thus, our data implicates UNC-89 in maintaining muscle cell architecture and that this precise organization is essential for optimal calcium mobilization and efficient and effective muscle contraction.

Wnt signaling regulates acetylcholine receptor translocation and synaptic plasticity in the adult nervous system.

The adult nervous system is plastic, allowing us to learn, remember, and forget. Experience-dependent plasticity occurs at synapses--the specialized points of contact between neurons where signaling occurs. However, the mechanisms that regulate the strength of synaptic signaling are not well understood. Here, we define a Wnt-signaling pathway that modifies synaptic strength in the adult nervous system by regulating the translocation of one class of acetylcholine receptors (AChRs) to synapses. In Caenorhabditis elegans, we show that mutations in CWN-2 (Wnt ligand), LIN-17 (Frizzled), CAM-1 (Ror receptor tyrosine kinase), or the downstream effector DSH-1 (disheveled) result in similar subsynaptic accumulations of ACR-16/α7 AChRs, a consequent reduction in synaptic current, and predictable behavioral defects. Photoconversion experiments revealed defective translocation of ACR-16/α7 to synapses in Wnt-signaling mutants. Using optogenetic nerve stimulation, we demonstrate activity-dependent synaptic plasticity and its dependence on ACR-16/α7 translocation mediated by Wnt signaling via LIN-17/CAM-1 heteromeric receptors.

In a pickle: is cornichon just relish or part of the main dish?

The recent discovery that vertebrate homologs of Drosophila cornichon associate with AMPA receptors led to the unexpected notion that cornichons play a role in synaptic transmission. In this issue of Neuron, Kato et al. find that cornichons modulate the gating of TARP-associated AMPA receptors by preventing their resensitization to glutamate.

A novel Conus snail polypeptide causes excitotoxicity by blocking desensitization of AMPA receptors.

BACKGROUND: Ionotropic glutamate receptors (iGluRs) are glutamate-gated ion channels that mediate excitatory neurotransmission in the central nervous system. Based on both molecular and pharmacological criteria, iGluRs have been divided into two major classes, the non-NMDA class, which includes both AMPA and kainate subtypes of receptors, and the NMDA class. One evolutionarily conserved feature of iGluRs is their desensitization in the continued presence of glutamate. Thus, when in a desensitized state, iGluRs can be bound to glutamate, yet the channel remains closed. However, the relevance of desensitization to nervous system function has remained enigmatic. RESULTS: Here, we report the identification and characterization of a novel polypeptide (con-ikot-ikot) from the venom of a predatory marine snail Conus striatus that specifically disrupts the desensitization of AMPA receptors (AMPARs). The stoichiometry of con-ikot-ikot appears reminiscent of the proposed subunit organization of AMPARs, i.e., a dimer of dimers, suggesting that it acts as a molecular four-legged clamp that holds the AMPAR channel open. Application of con-ikot-ikot to hippocampal slices caused a large and rapid increase in resting AMPAR-mediated current leading to neuronal death. CONCLUSIONS: Our findings provide insight into the mechanisms that regulate receptor desensitization and demonstrate that in the arms race between prey and predators, evolution has selected for a toxin that blocks AMPAR desensitization, thus revealing the fundamental importance of desensitization for regulating neural function.

Neuron-specific regulation of associative learning and memory by MAGI-1 in C. elegans.

BACKGROUND: Identifying the molecular mechanisms and neural circuits that control learning and memory are major challenges in neuroscience. Mammalian MAGI/S-SCAM is a multi-PDZ domain synaptic scaffolding protein that interacts with a number of postsynaptic signaling proteins and is thereby thought to regulate synaptic plasticity [1], [2], [3]. PRINCIPAL FINDINGS: While investigating the behavioral defects of C. elegans nematodes carrying a mutation in the single MAGI ortholog magi-1, we have identified specific neurons that require MAGI-1 function for different aspects of associative learning and memory. Various sensory stimuli and a food deprivation signal are associated in RIA interneurons during learning, while additional expression of MAGI-1 in glutamatergic AVA, AVD and possibly AVE interneurons is required for efficient memory consolidation, i.e. the ability to retain the conditioned changes in behavior over time. During associative learning, MAGI-1 in RIA neurons controls in a cell non-autonomous fashion the dynamic remodeling of AVA, AVD and AVE synapses containing the ionotropic glutamate receptor (iGluR) GLR-1 [4]. During memory consolidation, however, MAGI-1 controls GLR-1 clustering in AVA and AVD interneurons cell-autonomously and depends on the ability to interact with the beta-catenin HMP-2. SIGNIFICANCE: Together, these results indicate that different aspects of associative learning and memory in C. elegans are likely carried out by distinct subsets of interneurons. The synaptic scaffolding protein MAGI-1 plays a critical role in these processes in part by regulating the clustering of iGluRs at synapses.

Evolutionary conserved role for TARPs in the gating of glutamate receptors and tuning of synaptic function.

Neurotransmission in the brain is critically dependent on excitatory synaptic signaling mediated by AMPA-class ionotropic glutamate receptors (AMPARs). AMPARs are known to be associated with Transmembrane AMPA receptor Regulatory Proteins (TARPs). In vertebrates, at least four TARPs appear to have redundant roles as obligate chaperones for AMPARs, thus greatly complicating analysis of TARP participation in synaptic function. We have overcome this limitation by identifying and mutating the essential set of TARPs in C. elegans (STG-1 and STG-2). In TARP mutants, AMPAR-mediated currents and worm behaviors are selectively disrupted despite apparently normal surface expression and clustering of the receptors. Reconstitution experiments indicate that both STG-1 and STG-2 can functionally substitute for vertebrate TARPs to modify receptor function. Thus, we show that TARPs are obligate auxiliary subunits for AMPARs with a primary, evolutionarily conserved functional role in the modification of current kinetics.

Action potentials contribute to neuronal signaling in C. elegans.

Small, high-impedance neurons with short processes, similar to those found in the soil nematode Caenorhabditis elegans, are predicted to transmit electrical signals by passive propagation. However, we have found that certain neurons in C. elegans fire regenerative action potentials. These neurons resembled Schmitt triggers, as their potential state appears to be bistable. Transitions between up and down states could be triggered by application of the neurotransmitter glutamate or brief current pulses.

Memory in Caenorhabditis elegans is mediated by NMDA-type ionotropic glutamate receptors.

Learning and memory are essential processes of both vertebrate and invertebrate nervous systems that allow animals to survive and reproduce. The neurotransmitter glutamate signals via ionotropic glutamate receptors (iGluRs) that have been linked to learning and memory formation; however, the signaling pathways that contribute to these behaviors are still not well understood. We therefore undertook a genetic and electrophysiological analysis of learning and memory in the nematode Caenorhabditis elegans. Here, we show that two genes, nmr-1 and nmr-2, are predicted to encode the subunits of an NMDA-type (NMDAR) iGluR that is necessary for memory retention in C. elegans. We cloned nmr-2, generated a deletion mutation in the gene, and showed that like nmr-1, nmr-2 is required for in vivo NMDA-gated currents. Using an associative-learning paradigm that pairs starvation with the attractant NaCl, we also showed that the memory of a learned avoidance response is dependent on NMR-1 and NMR-2 and that expression of NMDARs in a single pair of interneurons is sufficient for normal memory. Our results provide new insights into the molecular and cellular mechanisms underlying the memory of a learned event.

Conserved SOL-1 proteins regulate ionotropic glutamate receptor desensitization.

The neurotransmitter glutamate mediates excitatory synaptic transmission by activating ionotropic glutamate receptors (iGluRs). In Caenorhabditis elegans, the GLR-1 receptor subunit is required for glutamate-gated current in a subset of interneurons that control avoidance behaviors. Current mediated by GLR-1-containing iGluRs depends on SOL-1, a transmembrane CUB-domain protein that immunoprecipitates with GLR-1. We have found that reconstitution of glutamate-gated current in heterologous cells depends on three proteins, STG-1 (a C. elegans stargazin-like protein), SOL-1, and GLR-1. Here, we use genetic and pharmacological perturbations along with rapid perfusion electrophysiological techniques to demonstrate that SOL-1 functions to slow the rate and limit the extent of receptor desensitization as well as to enhance the recovery from desensitization. We have also identified a SOL-1 homologue from Drosophila and show that Dro SOL1 has a conserved function in promoting C. elegans glutamate-gated currents. SOL-1 homologues may play critical roles in regulating glutamatergic neurotransmission in more complex nervous systems.

Reconstitution of invertebrate glutamate receptor function depends on stargazin-like proteins.

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (AMPARs) are a major subtype of ionotropic glutamate receptors (iGluRs) that mediate rapid excitatory synaptic transmission in the vertebrate brain. Putative AMPARs are also expressed in the nervous system of invertebrates. In Caenorhabditis elegans, the GLR-1 receptor subunit is expressed in neural circuits that mediate avoidance behaviors and is required for glutamate-gated current in the AVA and AVD interneurons. Glutamate-gated currents can be recorded from heterologous cells that express vertebrate AMPARs; however, when C. elegans GLR-1 is expressed in heterologous cells, little or no glutamate-gated current is detected. This finding suggests that other receptor subunits or auxiliary proteins are required for function. Here, we identify Ce STG-1, a C. elegans stargazin-like protein, and show that expression of Ce STG-1 together with GLR-1 and the CUB-domain protein SOL-1 reconstitutes glutamate-gated currents in Xenopus oocytes. Ce STG-1 and homologues cloned from Drosophila (Dro STG1) and Apis mellifera (Apis STG1) have evolutionarily conserved functions and can partially substitute for one another to reconstitute glutamate-gated currents from rat, Drosophila, and C. elegans. Furthermore, we show that Ce STG-1 and Apis STG1 are primarily required for function independent of possible roles in promoting the surface expression of invertebrate AMPARs.

SOL-1 is an auxiliary subunit that modulates the gating of GLR-1 glutamate receptors in Caenorhabditis elegans.

Most rapid excitatory synaptic signaling in the brain is mediated by postsynaptic ionotropic glutamate receptors (iGluRs) that are gated open by the neurotransmitter glutamate. In Caenorhabditis elegans, sol-1 encodes a CUB-domain transmembrane protein that is required for currents that are mediated by the GLR-1 iGluR. Mutations in sol-1 do not affect GLR-1 expression, localization, membrane insertion, or stabilization at synapses, suggesting that SOL-1 is required for iGluR function. Here, we provide evidence that SOL-1 is an auxiliary subunit that modulates the gating of GLR-1 receptors. We show that mutant variants of GLR-1 with altered gating partially restore glutamate-gated current and GLR-1-dependent behaviors in sol-1 mutants. Domain analysis of SOL-1 indicates that extracellular CUB domain 3 is required for function and that a secreted variant partially restores glutamate-gated currents and behavior. Also, we show that endogenous glutamatergic synaptic currents are absent in sol-1 mutants. Our data suggest that GLR-1 iGluRs are not simply stand-alone molecules and require the SOL-1 auxiliary protein to promote the open state of the receptor. Our analysis presents the possibility that glutamatergic signaling in other organisms may be similarly modified by SOL-1-like transmembrane proteins.