A metabotropic glutamate receptor family gene in Dictyostelium discoideum

Attention-deficit/hyperactive disorder updates

Background

Attention-deficit/hyperactive disorder (ADHD) is a neurodevelopmental disorder that commonly occurs in children with a prevalence ranging from 3.4 to 7.2%. It profoundly affects academic achievement, well-being, and social interactions. As a result, this disorder is of high cost to both individuals and society. Despite the availability of knowledge regarding the mechanisms of ADHD, the pathogenesis is not clear, hence, the existence of many challenges especially in making correct early diagnosis and provision of accurate management.

Objectives

We aimed to review the pathogenic pathways of ADHD in children. The major focus was to provide an update on the reported etiologies in humans, animal models, modulators, therapies, mechanisms, epigenetic changes, and the interaction between genetic and environmental factors.

Methods

References for this review were identified through a systematic search in PubMed by using special keywords for all years until January 2022.

Results

Several genes have been reported to associate with ADHD: DRD1, DRD2, DRD4, DAT1, TPH2, HTR1A, HTR1B, SLC6A4, HTR2A, DBH, NET1, ADRA2A, ADRA2C, CHRNA4, CHRNA7, GAD1, GRM1, GRM5, GRM7, GRM8, TARBP1, ADGRL3, FGF1, MAOA, BDNF, SNAP25, STX1A, ATXN7, and SORCS2. Some of these genes have evidence both from human beings and animal models, while others have evidence in either humans or animal models only. Notably, most of these animal models are knockout and do not generate the genetic alteration of the patients. Besides, some of the gene polymorphisms reported differ according to the ethnic groups. The majority of the available animal models are related to the dopaminergic pathway. Epigenetic changes including SUMOylation, methylation, and acetylation have been reported in genes related to the dopaminergic pathway.

Conclusion

The dopaminergic pathway remains to be crucial in the pathogenesis of ADHD. It can be affected by environmental factors and other pathways. Nevertheless, it is still unclear how environmental factors relate to all neurotransmitter pathways; thus, more studies are needed. Although several genes have been related to ADHD, there are few animal model studies on the majority of the genes, and they do not generate the genetic alteration of the patients. More animal models and epigenetic studies are required.

Spatial transcriptomic and single-nucleus analysis reveals heterogeneity in a gigantic single-celled syncytium

In multicellular organisms, the specification, coordination, and compartmentalization of cell types enable the formation of complex body plans. However, some eukaryotic protists such as slime molds generate diverse and complex structures while remaining in a multinucleate syncytial state. It is unknown if different regions of these giant syncytial cells have distinct transcriptional responses to environmental encounters and if nuclei within the cell diversify into heterogeneous states. Here, we performed spatial transcriptome analysis of the slime mold Physarum polycephalum in the plasmodium state under different environmental conditions and used single-nucleus RNA-sequencing to dissect gene expression heterogeneity among nuclei. Our data identifies transcriptome regionality in the organism that associates with proliferation, syncytial substructures, and localized environmental conditions. Further, we find that nuclei are heterogenous in their transcriptional profile and may process local signals within the plasmodium to coordinate cell growth, metabolism, and reproduction. To understand how nuclei variation within the syncytium compares to heterogeneity in single-nucleus cells, we analyzed states in single Physarum amoebal cells. We observed amoebal cell states at different stages of mitosis and meiosis, and identified cytokinetic features that are specific to nuclei divisions within the syncytium. Notably, we do not find evidence for predefined transcriptomic states in the amoebae that are observed in the syncytium. Our data shows that a single-celled slime mold can control its gene expression in a region-specific manner while lacking cellular compartmentalization and suggests that nuclei are mobile processors facilitating local specialized functions. More broadly, slime molds offer the extraordinary opportunity to explore how organisms can evolve regulatory mechanisms to divide labor, specialize, balance competition with cooperation, and perform other foundational principles that govern the logic of life.

Metazoan evolution of glutamate receptors reveals unreported phylogenetic groups and divergent lineage-specific events

Glutamate receptors are divided in two unrelated families: ionotropic (iGluR), driving synaptic transmission, and metabotropic (mGluR), which modulate synaptic strength. The present classification of GluRs is based on vertebrate proteins and has remained unchanged for over two decades. Here we report an exhaustive phylogenetic study of GluRs in metazoans. Importantly, we demonstrate that GluRs have followed different evolutionary histories in separated animal lineages. Our analysis reveals that the present organization of iGluRs into six classes does not capture the full complexity of their evolution. Instead, we propose an organization into four subfamilies and ten classes, four of which have never been previously described. Furthermore, we report a sister class to mGluR classes I-III, class IV. We show that many unreported proteins are expressed in the nervous system, and that new Epsilon receptors form functional ligand-gated ion channels. We propose an updated classification of glutamate receptors that includes our findings.

The physiological regulation of macropinocytosis during Dictyostelium growth and development

Macropinocytosis is a conserved endocytic process used by Dictyostelium amoebae for feeding on liquid medium. To further Dictyostelium as a model for macropinocytosis, we developed a high-throughput flow cytometry assay to measure macropinocytosis, and used it to identify inhibitors and investigate the physiological regulation of macropinocytosis. Dictyostelium has two feeding states: phagocytic and macropinocytic. When cells are switched from phagocytic growth on bacteria to liquid media, the rate of macropinocytosis slowly increases, due to increased size and frequency of macropinosomes. Upregulation is triggered by a minimal medium containing three amino acids plus glucose and likely depends on macropinocytosis itself. The presence of bacteria suppresses macropinocytosis while their product, folate, partially suppresses upregulation of macropinocytosis. Starvation, which initiates development, does not of itself suppress macropinocytosis: this can continue in isolated cells, but is shut down by a conditioned-medium factor or activation of PKA signalling. Thus macropinocytosis is a facultative ability of Dictyostelium cells, regulated by environmental conditions that are identified here.This article has an associated First Person interview with the first author of the paper.

A polycystin-type transient receptor potential (Trp) channel that is activated by ATP

ATP and ADP are ancient extra-cellular signalling molecules that in Dictyostelium amoebae cause rapid, transient increases in cytosolic calcium due to an influx through the plasma membrane. This response is independent of hetero-trimeric G-proteins, the putative IP3 receptor IplA and all P2X channels. We show, unexpectedly, that it is abolished in mutants of the polycystin-type transient receptor potential channel, TrpP. Responses to the chemoattractants cyclic-AMP and folic acid are unaffected in TrpP mutants. We report that the DIF morphogens, cyclic-di-GMP, GABA, glutamate and adenosine all induce strong cytoplasmic calcium responses, likewise independently of TrpP. Thus, TrpP is dedicated to purinergic signalling. ATP treatment causes cell blebbing within seconds but this does not require TrpP, implicating a separate purinergic receptor. We could detect no effect of ATP on chemotaxis and TrpP mutants grow, chemotax and develop almost normally in standard conditions. No gating ligand is known for the human homologue of TrpP, polycystin-2, which causes polycystic kidney disease. Our results now show that TrpP mediates purinergic signalling in Dictyostelium and is directly or indirectly gated by ATP.

Summary: We show that a Trp channel related to the mammalian polycystin channel, rather than a P2X receptor, is responsible for the purinergic stimulation of cytosolic calcium levels in Dictyostelium cells.

The C-Terminal SynMuv/DdDUF926 Domain Regulates the Function of the N-Terminal Domain of DdNKAP

NKAP (NF-κB activating protein) is a highly conserved SR (serine/arginine-rich) protein involved in transcriptional control and splicing in mammals. We identified DdNKAP, the Dictyostelium discoideum ortholog of mammalian NKAP, as interacting partner of the nuclear envelope protein SUN-1. DdNKAP harbors a number of basic RDR/RDRS repeats in its N-terminal domain and the SynMuv/DUF926 domain at its C-terminus. We describe a novel and direct interaction between DdNKAP and Prp19 (Pre mRNA processing factor 19) which might be relevant for the observed DdNKAP ubiquitination. Genome wide analysis using cross-linking immunoprecipitation-high-throughput sequencing (CLIP-seq) revealed DdNKAP association with intergenic regions, exons, introns and non-coding RNAs. Ectopic expression of DdNKAP and its domains affects several developmental aspects like stream formation, aggregation, and chemotaxis. We conclude that DdNKAP is a multifunctional protein, which might influence Dictyostelium development through its interaction with RNA and RNA binding proteins. Mutants overexpressing full length DdNKAP and the N-terminal domain alone (DdN-NKAP) showed opposite phenotypes in development and opposite expression profiles of several genes and rRNAs. The observed interaction between DdN-NKAP and the DdDUF926 domain indicates that the DdDUF926 domain acts as negative regulator of the N-terminus.

Nse1 and Nse4, subunits of the Smc5-Smc6 complex, are involved in Dictyostelium development upon starvation

The Smc5-Smc6 complex contains a heterodimeric core of two SMC proteins and non-Smc elements (Nse1-6), and plays an important role in DNA repair. We investigated the functional roles of Nse4 and Nse1 in Dictyostelium discoideum. Nse4 and Nse3 expressed as Flag-tagged fusion proteins were highly enriched in nuclei, while Nse1 was localized in whole cells. Using yeast two-hybrid assays, only the interaction between Nse3 and Nse1 was detected among the combinations. However, all of the interactions among these three proteins were recognized by co-immunoprecipitation assay using cell lysates prepared from the cells expressing green fluorescent protein (GFP)- or Flag-tagged fusion proteins. GFP-tagged Nse1, which localized in whole cells, was translocated to nuclei when co-expressed with Flag-tagged Nse3 or Nse4. RNAi-mediated Nse1 and Nse4 knockdown cells (Nse1 KD and Nse4 KD cells) were generated and found to be more sensitive to UV-induced cell death than control cells. Upon starvation, Nse1 and Nse4 KD cells had increases in the number of smaller fruiting bodies that formed on non-nutrient agar plates or aggregates that formed under submerged culture. We found a reduction in the mRNA level of pdsA, in vegetative and 8 h-starved Nse4 KD cells, and pdsA knockdown cells displayed effects similar to Nse4 KD cells. Our results suggest that Nse4 and Nse1 are involved in not only the cellular DNA damage response but also cellular development in D. discoideum.

Neurotransmitter-mediated control of neurogenesis in the adult vertebrate brain

It was long thought that no new neurons are added to the adult brain. Similarly, neurotransmitter signaling was primarily associated with communication between differentiated neurons. Both of these ideas have been challenged, and a crosstalk between neurogenesis and neurotransmitter signaling is beginning to emerge. In this Review, we discuss neurotransmitter signaling as it functions at the intersection of stem cell research and regenerative medicine, exploring how it may regulate the formation of new functional neurons and outlining interactions with other signaling pathways. We consider evolutionary and cross-species comparative aspects, and integrate available results in the context of normal physiological versus pathological conditions. We also discuss the potential role of neurotransmitters in brain size regulation and implications for cell replacement therapies.

Identification of G protein-coupled receptor signaling pathway proteins in marine diatoms using comparative genomics

BACKGROUND

The G protein-coupled receptor (GPCR) signaling pathway plays an essential role in signal transmission and response to external stimuli in mammalian cells. Protein components of this pathway have been characterized in plants and simpler eukaryotes such as yeast, but their presence and role in unicellular photosynthetic eukaryotes have not been determined. We use a comparative genomics approach using whole genome sequences and gene expression libraries of four diatoms (Pseudo-nitzschia multiseries, Thalassiosira pseudonana, Phaeodactylum tricornutum and Fragilariopsis cylindrus) to search for evidence of GPCR signaling pathway proteins that share sequence conservation to known GPCR pathway proteins.

RESULTS

The majority of the core components of GPCR signaling were well conserved in all four diatoms, with protein sequence similarity to GPCRs, human G protein α- and β-subunits and downstream effectors. There was evidence for the Gγ-subunit and thus a full heterotrimeric G protein only in T. pseudonana. Phylogenetic analysis of putative diatom GPCRs indicated similarity but deep divergence to the class C GPCRs, with branches basal to the GABAB receptor subfamily. The extracellular and intracellular regions of these putative diatom GPCR sequences exhibited large variation in sequence length, and seven of these sequences contained the necessary ligand binding domain for class C GPCR activation. Transcriptional data indicated that a number of the putative GPCR sequences are expressed in diatoms under various stress conditions in culture, and that many of the GPCR-activated signaling proteins, including the G protein, are also expressed.

CONCLUSIONS

The presence of sequences in all four diatoms that code for the proteins required for a functional mammalian GPCR pathway highlights the highly conserved nature of this pathway and suggests a complex signaling machinery related to environmental perception and response in these unicellular organisms. The lack of evidence for some GPCR pathway proteins in one or more of the diatoms, such as the Gγ-subunit, may be due to differences in genome completeness and genome coverage for the four diatoms. The high divergence of putative diatom GPCR sequences to known class C GPCRs suggests these sequences may represent another, potentially ancestral, subfamily of class C GPCRs.

Systematic analysis of γ-aminobutyric acid (GABA) metabolism and function in the social amoeba Dictyostelium discoideum

While GABA has been suggested to regulate spore encapsulation in the social amoeba Dictyostelium discoideum, the metabolic profile and other potential functions of GABA during development remain unclear. In this study, we investigated the homeostasis of GABA metabolism by disrupting genes related to GABA metabolism and signaling. Extracellular levels of GABA are tightly regulated during early development, and GABA is generated by the glutamate decarboxylase, GadB, during growth and in early development. However, overexpression of the prespore-specific homologue, GadA, in the presence of GadB reduces production of extracellular GABA. Perturbation of extracellular GABA levels delays the process of aggregation. Cytosolic GABA is degraded by the GABA transaminase, GabT, in the mitochondria. Disruption of a putative vesicular GABA transporter (vGAT) homologue DdvGAT reduces secreted GABA. We identified the GABAB receptor-like family member GrlB as the major GABA receptor during early development, and either disruption or overexpression of GrlB delays aggregation. This delay is likely the result of an abolished pre-starvation response and late expression of several "early" developmental genes. Distinct genes are employed for GABA generation during sporulation. During sporulation, GadA alone is required for generating GABA and DdvGAT is likely responsible for GABA secretion. GrlE but not GrlB is the GABA receptor during late development.

Cross-Genome Clustering of Human and C. elegans G-Protein Coupled Receptors

G-protein coupled receptors (GPCRs) are one of the largest groups of membrane proteins and are popular drug targets. The work reported here attempts to perform cross-genome phylogeny on GPCRs from two widely different taxa, human versus C. elegans genomes and to address the issues on evolutionary plasticity, to identify functionally related genes, orthologous relationship, and ligand binding properties through effective bioinformatic approaches. Through RPS blast around 1106 nematode GPCRs were given chance to associate with previously established 8 types of human GPCR profiles at varying E-value thresholds and resulted 32 clusters were illustrating co-clustering and class-specific retainsionship. In the significant thresholds, 81% of the C. elegans GPCRs were associated with 32 clusters and 27 C. elegans GPCRs (2%) inferred for orthology. 177 hypothetical proteins were observed in cluster association and could be reliably associated with one of 32 clusters. Several nematode-specific GPCR clades were observed suggesting lineage-specific functional recruitment in response to environment.

The repertoire of G protein-coupled receptors in the human parasite Schistosoma mansoni and the model organism Schmidtea mediterranea

Background

G protein-coupled receptors (GPCRs) constitute one of the largest groupings of eukaryotic proteins, and represent a particularly lucrative set of pharmaceutical targets. They play an important role in eukaryotic signal transduction and physiology, mediating cellular responses to a diverse range of extracellular stimuli. The phylum Platyhelminthes is of considerable medical and biological importance, housing major pathogens as well as established model organisms. The recent availability of genomic data for the human blood fluke Schistosoma mansoni and the model planarian Schmidtea mediterranea paves the way for the first comprehensive effort to identify and analyze GPCRs in this important phylum.

Results

Application of a novel transmembrane-oriented approach to receptor mining led to the discovery of 117 S. mansoni GPCRs, representing all of the major families; 105 Rhodopsin, 2 Glutamate, 3 Adhesion, 2 Secretin and 5 Frizzled. Similarly, 418 Rhodopsin, 9 Glutamate, 21 Adhesion, 1 Secretin and 11 Frizzled S. mediterranea receptors were identified. Among these, we report the identification of novel receptor groupings, including a large and highly-diverged Platyhelminth-specific Rhodopsin subfamily, a planarian-specific Adhesion-like family, and atypical Glutamate-like receptors. Phylogenetic analysis was carried out following extensive gene curation. Support vector machines (SVMs) were trained and used for ligand-based classification of full-length Rhodopsin GPCRs, complementing phylogenetic and homology-based classification.

Conclusions

Genome-wide investigation of GPCRs in two platyhelminth genomes reveals an extensive and complex receptor signaling repertoire with many unique features. This work provides important sequence and functional leads for understanding basic flatworm receptor biology, and sheds light on a lucrative set of anthelmintic drug targets.

Genome-wide copy number variation study associates metabotropic glutamate receptor gene networks with attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD) is a common, heritable neuropsychiatric disorder of unknown etiology. We performed a whole-genome copy number variation (CNV) study on 1,013 cases with ADHD and 4,105 healthy children of European ancestry using 550,000 SNPs. We evaluated statistically significant findings in multiple independent cohorts, with a total of 2,493 cases with ADHD and 9,222 controls of European ancestry, using matched platforms. CNVs affecting metabotropic glutamate receptor genes were enriched across all cohorts (P = 2.1 × 10(-9)). We saw GRM5 (encoding glutamate receptor, metabotropic 5) deletions in ten cases and one control (P = 1.36 × 10(-6)). We saw GRM7 deletions in six cases, and we saw GRM8 deletions in eight cases and no controls. GRM1 was duplicated in eight cases. We experimentally validated the observed variants using quantitative RT-PCR. A gene network analysis showed that genes interacting with the genes in the GRM family are enriched for CNVs in ∼10% of the cases (P = 4.38 × 10(-10)) after correction for occurrence in the controls. We identified rare recurrent CNVs affecting glutamatergic neurotransmission genes that were overrepresented in multiple ADHD cohorts.

Exciting times beyond the brain: metabotropic glutamate receptors in peripheral and non-neural tissues

Metabotropic glutamate (mGlu) receptors are G-protein-coupled receptors expressed primarily on neurons and glial cells, where they are located in the proximity of the synaptic cleft. In the central nervous system (CNS), mGlu receptors modulate the effects of l-glutamate neurotransmission in addition to that of a variety of other neurotransmitters. However, mGlu receptors also have a widespread distribution outside the CNS that has been somewhat neglected to date. Based on this expression, diverse roles of mGlu receptors have been suggested in a variety of processes in health and disease including controlling hormone production in the adrenal gland and pancreas, regulating mineralization in the developing cartilage, modulating lymphocyte cytokine production, directing the state of differentiation in embryonic stem cells, and modulating gastrointestinal secretory function. Understanding the role of mGlu receptors in the periphery will also provide a better insight into potential side effects of drugs currently being developed for neurological and psychiatric conditions. This review summarizes the new potential roles of mGlu receptors and raises the possibility of novel pharmacological targets for various disorders.

Neurotransmitter receptor homologues of Dictyostelium discoideum

The social amoeba Dictyostelium discoideum is a genetically amenable eukaryotic cell which displays many animal cell traits and has been used to study cellular signalling for over 30 years. Recently studies have highlighted the roles that molecules associated with synaptic transmission in animals, such as glutamate, GABA and ATP play in cellular differentiation and homeostasis in this simple organism. This short review summarises the evidence for the existence of both ionotropic and metabotropic families of neurotransmitter receptors in Dictyostelium.

The effects of extracellular calcium on motility, pseudopod and uropod formation, chemotaxis, and the cortical localization of myosin II in Dictyostelium discoideum

Extracellular Ca(++), a ubiquitous cation in the soluble environment of cells both free living and within the human body, regulates most aspects of amoeboid cell motility, including shape, uropod formation, pseudopod formation, velocity, and turning in Dictyostelium discoideum. Hence it affects the efficiency of both basic motile behavior and chemotaxis. Extracellular Ca(++) is optimal at 10 mM. A gradient of the chemoattractant cAMP generated in the absence of added Ca(++) only affects turning, but in combination with extracellular Ca(++), enhances the effects of extracellular Ca(++). Potassium, at 40 mM, can partially substitute for Ca(++). Mg(++), Mn(++), Zn(++), and Na(+) cannot. Extracellular Ca(++), or K(+), also induce the cortical localization of myosin II in a polar fashion. The effects of Ca(++), K(+) or a cAMP gradient do not appear to be similarly mediated by an increase in the general pool of free cytosolic Ca(++). These results suggest a model, in which each agent functioning through different signaling systems, converge to affect the cortical localization of myosin II, which in turn effects the behavioral changes leading to efficient cell motility and chemotaxis. Cell Motil. Cytoskeleton 2009. (c) 2009 Wiley-Liss, Inc.

Purinergic-mediated Ca2+ influx in Dictyostelium discoideum

The presence of five P2X-like genes (p2xA–E) in Dictyostelium suggests that nucleotides other than cAMP may act as extracellular signalling molecules in this model eukaryote. However, p2xA was found to have an exclusively intracellular localisation making it unclear whether Dictyostelium utilise P2 receptors in a manner analogous to vertebrates. Using an apoaequorin expressing strain we show here that Dictyostelium do possess cell surface P2 receptors that facilitate Ca²⁺ influx in response to extracellular ATP and ADP (EC₅₀ = 7.5 μM and 6.1 μM, respectively). Indicative of P2X receptor activation, responses were rapid reaching peak within 2.91 ± 0.04 s, required extracellular Ca²⁺, were inhibited by Gd³⁺, modified by extracellular pH and were not affected by deletion of either the single Gβ or iplA genes. Responses also remained unaffected by disruption of p2xA or p2xE showing that these genes are not involved. Cu²⁺ and Zn²⁺ inhibited purine-evoked Ca²⁺ influx with IC₅₀ values of 0.9 and 6.3 μM, respectively. 300 μM Zn²⁺ completely abolished the initial large rapid rise in intracellular Ca²⁺ revealing the presence of an additional smaller, slower P2Y-like response. The existence of P2 receptors in Dictyostelium makes this organism a valuable model to explore fundamental aspects of purinergic signalling.

GrlJ, a Dictyostelium GABAB-like receptor with roles in post-aggregation development

BACKGROUND

The G-protein-coupled receptor (GPCR) family represents the largest and most important group of targets for chemotherapeutics. They are extremely versatile receptors that transduce signals as diverse as biogenic amines, purins, odorants, ions and pheromones from the extracellular compartment to the interior via biochemical processes involving GTP-binding proteins. Until recently, the cyclic AMP receptors (cARs) were the only known G protein coupled receptors in Dictyostelium discoideum. The completed genome sequence revealed the presence of several families of GPCRs in Dictyostelium, among them members of the family 3 of GPCRs, the GABAB/glutamate like receptor family, which in higher eukaryotes is involved in neuronal signaling.

RESULTS

D. discoideum has seventeen Family 3 members of GPCRs, denoted GrlA through GrlR. Their transcripts are detected throughout development with increased levels during early and late development. We have examined here GrlJ. GFP-tagged GrlJ localises to the plasma-membrane and to internal membranes. Inactivation of the grlJ gene leads to precocious development, and the mutant completes development ~6 hours earlier. Alterations were also noted at the slug stage and in spore formation. grlJ- slugs were longer and broke apart several times on their way to culmination forming smaller but proportionate fruiting bodies. Spores from grlJ- fruiting bodies were malformed and less viable, although the spore differentiation factors were synthesized and sensed normally. Expression of a GFP-tagged full length GrlJ rescued the phenotype.

CONCLUSION

Our data suggest that GrlJ acts at several stages of Dictyostelium development and that it is a negative regulator in Dictyostelium development.

Ancestral reconstruction of the ligand-binding pocket of Family C G protein-coupled receptors

The metabotropic glutamate receptors (mGluRs) within the Family C subclass of G protein-coupled receptors are crucial modulators of synaptic transmission. However, their closest relatives include a diverse group of sensory receptors whose biological functions are not associated with neurotransmission, raising the question of the evolutionary origin of amino acid-binding Family C receptors. A common feature of most, if not all, functional Family C receptors is the presence of an amino acid-binding site localized within the large extracellular Venus flytrap domain. Here, we used maximum likelihood methods to infer the ancestral state of key residues in the amino acid-binding pocket of a primordial Family C receptor. These residues were reconstructed in the background of the fish 5.24 chemosensory receptor, a broad-spectrum amino acid-activated receptor. Unlike the WT 5.24 receptor, which was not activated by mGluR agonists and displayed low sensitivity toward l-glutamate, the reconstructed ancestral receptor possessed a pharmacological profile characterized by high affinity for both l-glutamate and selective Group I mGluR agonists. This pharmacological phenotype could be largely recapitulated by mutating only two residues in the 5.24 receptor-binding pocket. Our results suggest that this primordial Family C receptor may have arisen early in metazoan evolution and that it already was preadapted as a glutamate receptor for its later use at excitatory synapses in glutamate-mediated neurotransmission.