Determination of L-AP4-bound human mGlu8 receptor amino terminal domain structure and the molecular basis for L-AP4’s group III mGlu receptor functional potency and selectivity

Novel Inhibitory Site Revealed by Mode of Action on the Metabotropic Glutamate 7 Receptor Venus Flytrap Domain

Metabotropic glutamate (mGlu) receptors play a key role in modulating most synapses in the brain. The mGlu7 receptors inhibit presynaptic neurotransmitter release and offer therapeutic possibilities for post-traumatic stress disorders or epilepsy. Screening campaigns provided mGlu7-specific allosteric modulators as the inhibitor XAP044 (Gee et al. J. Biol. Chem. 2014). In contrast to other mGlu receptor allosteric modulators, XAP044 does not bind in the transmembrane domain but to the extracellular domain of the mGlu7 receptor and not at the orthosteric site. Here, we identified the mode of action of XAP044, combining synthesis of derivatives, modeling and docking experiments, and mutagenesis. We propose a unique mode of action of these inhibitors, preventing the closure of the Venus flytrap agonist binding domain. While acting as a noncompetitive antagonist of L-AP4, XAP044 and derivatives act as apparent competitive antagonists of LSP4-2022. These data revealed more potent XAP044 analogues and new possibilities to target mGluRs.

Complex N-glycosylation of mGluR6 is required for trans-synaptic interaction with ELFN adhesion proteins

Synaptic transmission from retinal photoreceptors to downstream ON-type bipolar cells (BCs) depends on the postsynaptic metabotropic glutamate receptor mGluR6, located at the BC dendritic tips. Glutamate binding to mGluR6 initiates G-protein signaling that ultimately leads to BC depolarization in response to light. The mGluR6 receptor also engages in trans-synaptic interactions with presynaptic ELFN adhesion proteins. The roles of post-translational modifications in mGluR6 trafficking and function are unknown. Treatment with glycosidase enzymes PNGase F and Endo H demonstrated that both endogenous and heterologously expressed mGluR6 contain complex N-glycosylation acquired in the Golgi. Pull-down experiments with ELFN1 and ELFN2 extracellular domains revealed that these proteins interact exclusively with the complex glycosylated form of mGluR6. Mutation of the four predicted N-glycosylation sites, either singly or in combination, revealed that all four sites are glycosylated. Single mutations partially reduced, but did not abolish, surface expression in heterologous cells, while triple mutants had little or no surface expression, indicating that no single glycosylation site is necessary or sufficient for plasma membrane trafficking. Mutation at N445 severely impaired both ELFN1 and ELFN2 binding. All single mutants exhibited dendritic tip enrichment in rod BCs, as did the triple mutant with N445 as the sole N-glycosylation site, demonstrating that glycosylation at N445 is sufficient but not necessary for dendritic tip localization. The quadruple mutant was completely mislocalized. These results reveal a key role for complex N-glycosylation in regulating mGluR6 trafficking and ELFN binding, and by extension, function of the photoreceptor synapses.

Metabotropic glutamate receptor orthosteric ligands and their binding sites

Metabotropic glutamate receptors (mGluRs) have been discovered almost four decades ago. Since then, their pharmacology has been largely developed as well as their structural organization. Indeed mGluRs are attractive therapeutic targets for numerous psychiatric and neurological disorders because of their modulating role of synaptic transmission. The more recent drug discovery programs have mostly concentrated on allosteric modulators. However, orthosteric agonists and antagonists have remained unavoidable pharmacological tools as, although not expected, many of them can reach the brain, or can be modified to reach the brain. This review focuses on the most common orthosteric ligands as well as on the few allosteric modulators interacting with the glutamate binding domain. The 3D-structures of these ligands at their binding sites are reported. For most of them, X-Ray structures or docked homology models are available. Because of the high conservation of the binding site, subtype selective agonists were not easy to find. Yet, some were discovered when extending their chemical structures in order to reach selective sites of the receptors.

The evidence for and consequences of metabotropic glutamate receptor heterodimerization

Metabotropic glutamate receptors (mGluRs) are an essential component of the mammalian central nervous system. These receptors modulate neuronal excitability in response to extracellular glutamate through the activation of intracellular heterotrimeric G proteins. Like most other class C G protein-coupled receptors, mGluRs function as obligate dimer proteins, meaning they need to form dimer complexes before becoming functional receptors. All mGluRs possess the ability to homodimerize, but studies over the past ten years have demonstrated these receptors are also capable of forming heterodimers in specific patterns. These mGluR heterodimers appear to have their own unique biophysical behavior and pharmacology with both native and synthetic compounds with few rules having been identified that allow for prediction of the consequences of any particular mGluR pair forming heterodimers. Here, we review the relevant literature demonstrating the existence and consequences of mGluR heterodimerization. By collecting biophysical and pharmacological data of several mGluR heterodimers we demonstrate the lack of generalizable behavior of these complexes indicating that each individual dimeric pair needs to be investigated independently. Additionally, by combining sequence alignment and structural analysis, we propose that interactions between the β4-A Helix Loop and the D Helix in the extracellular domain of these receptors are the structural components that dictate heterodimerization compatibility. Finally, we discuss the potential implications of mGluR heterodimerization from the viewpoints of further developing our understanding of neuronal physiology and leveraging mGluRs as a therapeutic target for the treatment of pathophysiology.

Whole-genome sequencing identifies missense mutation in GRM6 as the likely cause of congenital stationary night blindness in a Tennessee Walking Horse

BACKGROUND

The only known genetic cause of congenital stationary night blindness (CSNB) in horses is a 1378 bp insertion in TRPM1. However, an affected Tennessee Walking Horse was found to have no copies of this variant.

OBJECTIVES

To identify the genetic cause for CSNB in an affected Tennessee Walking Horse.

STUDY DESIGN

Case report detailing a whole-genome sequencing (WGS) approach to identify a causal variant.

METHODS

A complete ophthalmic exam, including an electroretinogram (ERG), was performed on suspected CSNB-affected horse. WGS data were generated from the case and compared with data from seven other breeds (n = 29). One hundred candidate genes were evaluated for coding variants homozygous in the case and absent in all other horses. Protein modelling was used to assess the functional effects of the identified variant. A random cohort of 90 unrelated Tennessee Walking Horses and 273 horses from additional breeds were screened to estimate allele frequency of the GRM6 variant.

RESULTS

ERG results were consistent with CSNB. WGS analysis identified a missense mutation in metabotropic glutamate receptor 6 (GRM6) (c.533C>T p.Thr178Met). This single nucleotide polymorphism (SNP) is predicted to be deleterious and protein modelling supports impaired binding of the neurotransmitter glutamate. This variant was not detected in 273 horses from three additional breeds. The estimated allele frequency in Tennessee Walking Horses is 10%.

MAIN LIMITATIONS

Limited phenotype information for controls and no additional cases with which to replicate this finding.

CONCLUSIONS

We identified a likely causal recessive missense variant in GRM6. Based on protein modelling, this variant alters GRM6 binding, and thus signalling from the retinal rod cell to the ON-bipolar cell, impairing vision in low light conditions. Given the 10% population allele frequency, it is likely that additional affected horses exist in this breed and further work is needed to identify and examine these animals.

Molecular docking studies of N-substituted 4-methoxy-6-oxo-1-aryl-pyridazine-3-carboxamide derivatives as potential modulators of glutamate receptors

Introduction: The virtual target-oriented screening is a necessary stage of modern drug-design. In the present study, the affinity of pyridazine derivatives for the most promising antiparkinsonian biotargets – I–III groups of metabotropic and ionotropic NMDA-glutamate receptors – was evaluated.

Materials and methods: Docking of the studied ligands to the active sites of biotargets – mGluR5, mGluR3, mGluR8, NMDA GluN2B – was performed using AutoDockVina. Base of the preparation of ligands and proteins – AutoDockTools-1.5.6. A Discovery Studio Visualizer 2017/R2 was used to visualize the interpretation of the results.

Results and discussion: A high degree of the affinity is predicted for group III of the metabotropic mGlu8 receptors – binding energy from -5.0 to -8.7 kcal/mol, compared to -6.1 kcal/mol of that of the reference drug (L-AP4), as well as for the ionotropic NMDA GluN2B receptors –binding energy from -8.7 to -11.6 kcal/mol, compared to -11.3 kcal/mol of that of ifenprodil.

Conclusion: The prospects of the searching for glutamate receptor modulators in a number of n-substituted 4-methoxy-6-oxo-1-aryl-pyridazine-3-carboxamide derivatives are proved. Some aspects of the structure-affinity relationship are discussed.

Insights into the Structural Aspects of the mGlu Receptor Orthosteric Binding Site

The amino terminal domain (ATD) of the metabotropic glutamate (mGlu) receptors contains the orthosteric glutamate recognition site, which is highly conserved across the eight mGlu receptor subtypes. In total, 29 X-ray crystal structures of the mGlu ATD proteins have been reported to date. These structures span across 3 subgroups and 6 subtypes, and include apo, agonist- and antagonist-bound structures. We will discuss the insights gained from the analysis of these structures with the focus on the interactions contributing to the observed group and subtype selectivity for select agonists. Furthermore, we will define the full expanded orthosteric ligand binding pocket (LBP) of the mGlu receptors, and discuss the macroscopic features of the mGlu ATD proteins.

Computational Drug Design Applied to the Study of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors are a family of eight GPCRs that are attractive drug discovery targets to modulate glutamate action and response. Here we review the application of computational methods to the study of this family of receptors. X-ray structures of the extracellular and 7-transmembrane domains have played an important role to enable structure-based modeling approaches, whilst we also discuss the successful application of ligand-based methods. We summarize the literature and highlight the areas where modeling and experiment have delivered important understanding for mGlu receptor drug discovery. Finally, we offer suggestions of future areas of opportunity for computational work.

On the origin of the 2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylate scaffold's unique group II selectivity for the mGlu receptors

Analogs based on the 2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylate scaffold showed high potency and selectivity as both group II mGlu receptors orthosteric agonists and antagonists. This scaffold was initially designed to mimic the fully extended glutamate backbone conformation that was hypothesized to be the active conformation for the group II mGlu receptors. With the availability of crystal structures of l-Glu-bound amino terminal domain proteins from multiple mGlu receptor subtypes spanning all three subgroups, a new steric hindrance hypothesis was proposed to account for the scaffold's unique group II selectivity that explores the subtle distance differences between the α-carbon of l-Glu and the center of the tyrosine phenyl ring from the bottom lobe (e.g. Y216 of mGlu₂).

Structural Basis for ( S)-3,4-Dicarboxyphenylglycine (DCPG) As a Potent and Subtype Selective Agonist of the mGlu8 Receptor

( S)-3,4-Dicarboxyphenylglycine (DCPG) was first reported in 2001 as a potent orthosteric agonist with high subtype selectivity for the mGlu₈ receptor, but the structural basis for its high selectivity is not well understood. We have solved a cocrystal structure of recombinant human mGlu₈ amino terminal domain (ATD) protein bound to ( S)-DCPG, which possesses the largest lobe opening angle observed to date among known agonist-bound mGlu ATD crystal structures. The binding conformation of ( S)-DCPG observed in the crystal structure is significantly different from that in the homology model built from an l-glutamate-bound rat mGlu₁ ATD crystal structure, which has a smaller lobe opening angle. This highlights the importance of considering various lobe opening angles when modeling mGlu ATD-ligand complex. New homology models of other mGlu receptors based on the ( S)-DCPG-bound mGlu₈ ATD crystal structure were explored to rationalize ( S)-DCPG's high mGlu₈ receptor subtype selectivity.