Hexahydroquinoline Derivatives Are Selective Agonists for the Adhesion G Protein-Coupled Receptor ADGRG1/GPR56.

GPR56 is a widely expressed adhesion GPCR (AGPCR) that has pleotropic roles in brain development, platelet function, cancer, and more. Nearly all AGPCRs possess extracellular regions that bind protein ligands and conceal a cryptic tethered peptide agonist. AGPCR reception of mechanical or shear force is thought to release the tethered agonist permitting its binding to the AGPCR orthosteric site for consequent activation of G protein signaling. This multistep mechanism of AGPCR activation is difficult to target, emphasizing the need for tool compounds and potential therapeutics that modulate AGPCRs directly. We expanded our cell-based pilot screen for GPR56 small molecule activators to screen >200,000 compounds and identified two promising agonists: 2-(furan-2-yl)-1-[(4-phenylphenyl)carbonyl]pyrrolidine, or compound 4, and propan-2-yl-4-(2-bromophenyl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate, or compound 36. Both compounds activated GPR56 receptors enginered to have impaired tethered agonists and/or be cleavage deficient. Compound 4 activated a subset of group VIII AGPCRs while compound 36 had exclusive specificity for GPR56 among the GPCRs tested. Compound 36 SAR analysis identified an analog with the isopropyl R group replaced with a cyclopentyl ring and the electrophilic bromine replaced with a CF3 group. Analog 36.40 had 40% increased potency over compound 36 and was 20-fold more potent than synthetic peptidomimetics designed from the GPR56 tethered agonist. The new GPCR56 tool compounds discovered in this screen may be used to further advance understanding of GPR56 function and aid development of AGPCR-targeted therapeutics. SIGNIFICANCE STATEMENT: Adhesion G protein coupled receptors (AGPCRs) are a large, clinically relevant class of GPCRs with no available therapeutics, in part due to their unique mechanism of activation. GPR56 is a widely expressed model AGPCR involved in cancer metastasis, hemostasis, and neuron myelination. In the present study, we identified novel small molecule agonists for GPR56. These molecules are among the most potent identified thus far and may become useful leads in the development of a GPR56-targeted therapeutic.

Structures of Ric-8B in complex with Gα protein folding clients reveal isoform specificity mechanisms.

Mammalian Ric-8 proteins act as chaperones to regulate the cellular abundance of heterotrimeric G protein α subunits. The Ric-8A isoform chaperones Gαi/o, Gα12/13, and Gαq/11 subunits, while Ric-8B acts on Gαs/olf subunits. Here, we determined cryoelectron microscopy (cryo-EM) structures of Ric-8B in complex with Gαs and Gαolf, revealing isoform differences in the relative positioning and contacts between the C-terminal α5 helix of Gα within the concave pocket formed by Ric-8 α-helical repeat elements. Despite the overall architectural similarity with our earlier structures of Ric-8A complexed to Gαq and Gαi1, Ric-8B distinctly accommodates an extended loop found only in Gαs/olf proteins. The structures, along with results from Ric-8 protein thermal stability assays and cell-based Gαolf folding assays, support a requirement for the Gα C-terminal region for binding specificity, and highlight that multiple structural elements impart specificity for Ric-8/G protein binding.

GPR56/ADGRG1 is a platelet collagen-responsive GPCR and hemostatic sensor of shear force.

Circulating platelets roll along exposed collagen at vessel injury sites and respond with filipodia protrusion, shape change, and surface area expansion to facilitate platelet adhesion and plug formation. Various glycoproteins were considered to be both collagen responders and mediators of platelet adhesion, yet the signaling kinetics emanating from these receptors do not fully account for the rapid platelet cytoskeletal changes that occur in blood flow. We found the free N-terminal fragment of the adhesion G protein-coupled receptor (GPCR) GPR56 in human plasma and report that GPR56 is the platelet receptor that transduces signals from collagen and blood flow-induced shear force to activate G protein 13 signaling for platelet shape change. Gpr56-/- mice have prolonged bleeding, defective platelet plug formation, and delayed thrombotic occlusion. Human and mouse blood perfusion studies demonstrated GPR56 and shear-force dependence of platelet adhesion to immobilized collagen. Our work places GPR56 as an initial collagen responder and shear-force transducer that is essential for platelet shape change during hemostasis.

Mechanisms of adhesion G protein-coupled receptor activation.

Adhesion G protein-coupled receptors (AGPCRs) are a thirty-three-member subfamily of Class B GPCRs that control a wide array of physiological processes and are implicated in disease. AGPCRs uniquely contain large, self-proteolyzing extracellular regions that range from hundreds to thousands of residues in length. AGPCR autoproteolysis occurs within the extracellular GPCR autoproteolysis-inducing (GAIN) domain that is proximal to the N terminus of the G protein-coupling seven-transmembrane-spanning bundle. GAIN domain-mediated self-cleavage is constitutive and produces two-fragment holoreceptors that remain bound at the cell surface. It has been of recent interest to understand how AGPCRs are activated in relation to their two-fragment topologies. Dissociation of the AGPCR fragments stimulates G protein signaling through the action of the tethered-peptide agonist stalk that is occluded within the GAIN domain in the holoreceptor form. AGPCRs can also signal independently of fragment dissociation, and a few receptors possess GAIN domains incapable of self-proteolysis. This has resulted in complex theories as to how these receptors are activated in vivo, complicating pharmacological advances. Currently, there is no existing structure of an activated AGPCR to support any of the theories. Further confounding AGPCR research is that many of the receptors remain orphans and lack identified activating ligands. In this review, we provide a detailed layout of the current theorized modes of AGPCR activation with discussion of potential parallels to mechanisms used by other GPCR classes. We provide a classification means for the ligands that have been identified and discuss how these ligands may activate AGPCRs in physiological contexts.

Computational and Experimental Studies of Inhibitor Design for Aldolase A.

Glycolytic enzyme fructose-bisphosphate aldolase A is an emerging therapeutic target in cancer. Recently, we have solved the crystal structure of murine aldolase in complex with naphthalene-2,6-diyl bisphosphate (ND1) that served as a template of the design of bisphosphate-based inhibitors. In this work, a series of ND1 analogues containing difluoromethylene (-CF2), methylene (-CH2), or aldehyde substitutions were designed. All designed compounds were studied using molecular dynamics (MD) simulations with the AMOEBA force field. Both energetics and structural analyses have been done to understand the calculated binding free energies. The average distances between ligand and protein atoms for ND1 were very similar to those for the ND1 crystal structure, which indicates that our MD simulation is sampling the correct conformation well. CF2 insertion lowers the binding free energy by 10-15 kcal/mol, while CF2 substitution slightly increases the binding free energy, which matches the experimental measurement. In addition, we found that NDB with two CF2 insertions, the strongest binder, is entropically driven, while others including NDA with one CF2 insertion are all enthalpically driven. This work provides insights into the mechanisms underlying protein-phosphate binding and enhances the capability of applying computational and theoretical frameworks to model, predict, and design diagnostic strategies targeting cancer.

Production of Phosphorylated Ric-8A proteins using protein kinase CK2.

Resistance to Inhibitors of Cholinesterase-8 (Ric-8) proteins are molecular chaperones that fold heterotrimeric G protein α subunits shortly after biosynthesis. Ric-8 proteins also act as test tube guanine nucleotide exchange factors (GEF) that promote Gα subunit GDP for GTP exchange. The GEF and chaperoning activities of Ric-8A are regulated by phosphorylation of five serine and threonine residues within protein kinase CK2 consensus sites. The traditional way that Ric-8A proteins have been purified is from Spodoptera frugiperda (Sf9) or Trichoplusia ni (Tni) insect cells. Endogenous insect cell kinases do phosphorylate the critical regulatory sites of recombinant Ric-8A reasonably well, but there is batch-to-batch variability among recombinant Ric-8A preparations. Additionally, insect cell-production of some Ric-8 proteins with phosphosite alanine substitution mutations is proscribed as there seems to be interdependency of multi-site phosphorylation for functional protein production. Here, we present a method to produce wild type and phosphosite mutant Ric-8A proteins that are fully occupied with bound phosphate at each of the regulatory positions. Ric-8A proteins were expressed and purified from E. coli. Purified Ric-8A was phosphorylated in vitro with protein kinase CK2 and then re-isolated to remove kinase. The phosphorylated Ric-8A proteins were ∼99% pure and the completeness of phosphorylation was verified by chromatography, phos-tag SDS-PAGE mobility shifts, immunoblotting using phospho-site specific antibodies, and mass spectrometry analysis. E. coli-produced Ric-8A that was phosphorylated using this method promoted a faster rate of Gα subunit guanine nucleotide exchange than Ric-8A that was variably phosphorylated during production in insect cells.