Structural insight to mutation effects uncover a common allosteric site in class C GPCRs

Evolutionary history of calcium-sensing receptors unveils hyper/hypocalcemia-causing mutations

Despite advancements in understanding the structure and functions of the Calcium Sensing Receptor (CaSR), gaps persist in our knowledge of the specific functions of its residues. In this study, we used phylogeny-based techniques to identify functionally equivalent orthologs of CaSR, predict residue significance, and compute specificity-determining position (SDP) scores to understand its evolutionary basis. The analysis revealed exceptional conservation of the CaSR subfamily, emphasizing the critical role of residues with high SDP scores in receptor activation and pathogenicity. To further enhance the findings, gradient-boosting trees were applied to differentiate between gain- and loss-of-function mutations responsible for hypocalcemia and hypercalcemia. Lastly, we investigated the importance of these mutations in the context of receptor activation dynamics. In summary, through comprehensive exploration of the evolutionary history of the CaSR subfamily, coupled with innovative phylogenetic methodologies, we identified activating and inactivating residues, providing valuable insights into the regulation of calcium homeostasis and its connections to associated disorders.

The Lateral Metalation of Isoxazolo[3,4-d]pyridazinones towards Hit-to-Lead Development of Selective Positive Modulators of Metabotropic Glutamate Receptors

Isoxazolo[3,4-d] pyridazinones ([3,4-d]s) were previously shown to have selective positive modulation at the metabotropic glutamate receptor (mGluR) Subtypes 2 and 4, with no functional cross-reactivity at mGluR1a, mGluR5, or mGluR8. Additional analogs were prepared to access more of the allosteric pocket and achieve higher binding affinity, as suggested by homology modeling. Two different sets of analogs were generated. One uses the fully formed [3,4-d] with an N6-aryl with and without halogens. These underwent successful selective lateral metalation and electrophilic quenching (LM&EQ) at the C3 of the isoxazole. In a second set of analogs, a phenyl group was introduced at the C4 position of the [3,4-d] ring via a condensation of 4-phenylacetyl-3-ethoxcarbonyl-5-methyl isoxazole with the corresponding hydrazine to generate the 3,4-ds 2b and 2j to 2n.

EMX2-GPR156-Gαi reverses hair cell orientation in mechanosensory epithelia

Hair cells detect sound, head position or water movements when their mechanosensory hair bundle is deflected. Each hair bundle has an asymmetric architecture that restricts stimulus detection to a single axis. Coordinated hair cell orientations within sensory epithelia further tune stimulus detection at the organ level. Here, we identify GPR156, an orphan GPCR of unknown function, as a critical regulator of hair cell orientation. We demonstrate that the transcription factor EMX2 polarizes GPR156 distribution, enabling it to signal through Gαi and trigger a 180° reversal in hair cell orientation. GPR156-Gαi mediated reversal is essential to establish hair cells with mirror-image orientations in mouse otolith organs in the vestibular system and in zebrafish lateral line. Remarkably, GPR156-Gαi also instructs hair cell reversal in the auditory epithelium, despite a lack of mirror-image organization. Overall, our work demonstrates that conserved GPR156-Gαi signaling is integral to the framework that builds directional responses into mechanosensory epithelia.

Allosteric Molecular Switches in Metabotropic Glutamate Receptors

Metabotropic glutamate receptors (mGlu) are class C G protein-coupled receptors of eight subtypes that are omnipresently expressed in the central nervous system. mGlus have relevance in several psychiatric and neurological disorders, therefore they raise considerable interest as drug targets. Allosteric modulators of mGlus offer advantages over orthosteric ligands owing to their increased potential to achieve subtype selectivity, and this has prompted discovery programs that have produced a large number of reported allosteric mGlu ligands. However, the optimization of allosteric ligands into drug candidates has proved to be challenging owing to induced-fit effects, flat or steep structure-activity relationships and unexpected changes in theirpharmacology. Subtle structural changes identified as molecular switches might modulate the functional activity of allosteric ligands. Here we review these switches discovered in the metabotropic glutamate receptor family..

Structural biology of GABAB receptor

Metabotropic GABAB receptor is a G protein-coupled receptor (GPCR) that mediates slow and prolonged inhibitory neurotransmission in the brain. It functions as a constitutive heterodimer composed of the GABAB1 and GABAB2 subunits. Each subunit contains three domains; the extracellular Venus flytrap module, seven-helix transmembrane region and cytoplasmic tail. In recent years, the three-dimensional structures of GABAB receptor extracellular and intracellular domains have been elucidated. These structures reveal the molecular basis of ligand recognition, receptor heterodimerization and receptor activation. Here we provide a brief review of the GABAB receptor structures, with an emphasis on describing the different ligand-bound states of the receptor. We will also compare these with the known structures of related GPCRs to shed light on the molecular mechanisms of activation and regulation in the GABAB system, as well as GPCR dimers in general. This article is part of the "Special Issue Dedicated to Norman G. Bowery".

"Selective" Class C G Protein-Coupled Receptor Modulators Are Neutral or Biased mGlu5 Allosteric Ligands

Numerous positive and negative allosteric modulators (PAMs and NAMs) of class C G protein-coupled receptors (GPCRs) have been developed as valuable preclinical pharmacologic tools and therapeutic agents. Although many class C GPCR allosteric modulators have undergone subtype selectivity screening, most assay paradigms have failed to perform rigorous pharmacologic assessment. Using mGlu5 as a representative class C GPCR, we tested the hypothesis that allosteric modulator selectivity was based on cooperativity rather than affinity. Specifically, we aimed to identify ligands that bound to mGlu5 but exhibited neutral cooperativity with mGlu5 agonists. We additionally evaluated the potential for these ligands to exhibit biased pharmacology. Radioligand binding, intracellular calcium (iCa2+) mobilization, and inositol monophosphate (IP1) accumulation assays were undertaken in human embryonic kidney cells expressing low levels of rat mGlu5 (HEK293A-mGlu5-low) for diverse allosteric chemotypes. Numerous "non-mGlu5" class C GPCR allosteric modulators incompletely displaced allosteric mGlu5 radioligand [3H]methoxy-PEPy binding, consistent with a negative allosteric interaction. Affinity estimates for CPCCOEt (mGlu1 ligand), PHCCC (mGlu4 ligand), GS39783 (GABAB ligand), AZ12216052 (mGlu8 ligand), and CGP7930 (GABAB ligand) at mGlu5 were within 10-fold of their target receptor. Most class C GPCR allosteric modulators had neutral cooperativity with both orthosteric and allosteric mGlu5 agonists in functional assays; however, NPS2143 (calcium-sensing receptor (CaSR) NAM), cinacalcet (CaSR PAM), CGP7930, and AZ12216052 were partial mGlu5 agonists for IP1 accumulation, but not iCa2+ mobilization. By using mGlu5 as a model class C GPCR, we find that for many class C GPCR allosteric modulators, subtype selectivity is driven by cooperativity and misinterpreted owing to unappreciated bias.

Covalent Allosteric Probe for the Metabotropic Glutamate Receptor 2: Design, Synthesis, and Pharmacological Characterization

Covalent labeling of G protein-coupled receptors (GPCRs) by small molecules is a powerful approach to understand binding modes, mechanism of action, pharmacology, and even facilitate structure elucidation. We report the first covalent positive allosteric modulator (PAM) for a class C GPCR, the mGlu₂ receptor. Three putatively covalent mGlu₂ PAMs were designed and synthesized. Pharmacological characterization identified 2 to bind the receptor covalently. Computational modeling combined with receptor mutagenesis revealed T791^7.29×30 as the likely position of covalent interaction. We show how this covalent ligand can be used to characterize the PAM binding mode and that it is a valuable tool compound in studying receptor function and binding kinetics. Our findings advance the understanding of the mGlu₂ PAM interaction and suggest that 2 is a valuable probe for further structural and chemical biology approaches.

A Structural Framework for GPCR Chemogenomics: What's In a Residue Number?

The recent surge of crystal structures of G protein-coupled receptors (GPCRs), as well as comprehensive collections of sequence, structural, ligand bioactivity, and mutation data, has enabled the development of integrated chemogenomics workflows for this important target family. This chapter will focus on cross-family and cross-class studies of GPCRs that have pinpointed the need for, and the implementation of, a generic numbering scheme for referring to specific structural elements of GPCRs. Sequence- and structure-based numbering schemes for different receptor classes will be introduced and the remaining caveats will be discussed. The use of these numbering schemes has facilitated many chemogenomics studies such as consensus binding site definition, binding site comparison, ligand repurposing (e.g. for orphan receptors), sequence-based pharmacophore generation for homology modeling or virtual screening, and class-wide chemogenomics studies of GPCRs.