Allosteric modulation and G-protein selectivity of the Ca2+-sensing receptor

The calcium-sensing receptor (CaSR) is a family C G-protein-coupled receptor1 (GPCR) that has a central role in regulating systemic calcium homeostasis2,3. Here we use cryo-electron microscopy and functional assays to investigate the activation of human CaSR embedded in lipid nanodiscs and its coupling to functional Gi versus Gq proteins in the presence and absence of the calcimimetic drug cinacalcet. High-resolution structures show that both Gi and Gq drive additional conformational changes in the activated CaSR dimer to stabilize a more extensive asymmetric interface of the seven-transmembrane domain (7TM) that involves key protein-lipid interactions. Selective Gi and Gq coupling by the receptor is achieved through substantial rearrangements of intracellular loop 2 and the C terminus, which contribute differentially towards the binding of the two G-protein subtypes, resulting in distinct CaSR-G-protein interfaces. The structures also reveal that natural polyamines target multiple sites on CaSR to enhance receptor activation by zipping negatively charged regions between two protomers. Furthermore, we find that the amino acid L-tryptophan, a well-known ligand of CaSR extracellular domains, occupies the 7TM bundle of the G-protein-coupled protomer at the same location as cinacalcet and other allosteric modulators. Together, these results provide a framework for G-protein activation and selectivity by CaSR, as well as its allosteric modulation by endogenous and exogenous ligands.

Delineation of the GPR15 receptor-mediated Gα protein signalling profile in recombinant mammalian cells

The GPR15 receptor is a G protein‐coupled receptor (GPCR), which is activated by an endogenous peptide GPR15L(25–81) and a C‐terminal peptide fragment GPR15L(71–81). GPR15 signals through the G_i/o pathway to decrease intracellular cyclic adenosine 3′,5′‐monophosphate (cAMP). However, the activation profiles of the GPR15 receptor within G_i/o subtypes have not been examined. Moreover, whether the receptor can also couple to G_s, G_q/11 and G_12/13 is unclear. Here, GPR15L(25–81) and GPR15L(71–81) are used as pharmacological tool compounds to delineate the GPR15 receptor‐mediated Gα protein signalling using a G protein activation assay and second messenger assay conducted on living cells. The results show that the GPR15 receptor preferentially couples to G_i/o rather than other pathways in both assays. Within the G_i/o family, the GPR15 receptor activates all the subtypes (G_i1, G_i2, G_i3, G_oA, G_oB and G_z). The E_max and activation rates of G_i1, G_i2, G_i3, G_oA and G_oB are similar, whilst the E_max of G_z is smaller and the activation rate is significantly slower. The potencies of both peptides toward each G_i/o subtype have been determined. Furthermore, the GPR15 receptor signals through G_i/o to inhibit cAMP accumulation, which could be blocked by the application of the G_i/o inhibitor pertussis toxin.

G-protein activation by a metabotropic glutamate receptor

Family C G-protein-coupled receptors (GPCRs) operate as obligate dimers with extracellular domains that recognize small ligands, leading to G-protein activation on the transmembrane (TM) domains of these receptors by an unknown mechanism¹. Here we show structures of homodimers of the family C metabotropic glutamate receptor 2 (mGlu2) in distinct functional states and in complex with heterotrimeric G_i. Upon activation of the extracellular domain, the two transmembrane domains undergo extensive rearrangement in relative orientation to establish an asymmetric TM6-TM6 interface that promotes conformational changes in the cytoplasmic domain of one protomer. Nucleotide-bound G_i can be observed pre-coupled to inactive mGlu2, but its transition to the nucleotide-free form seems to depend on establishing the active-state TM6-TM6 interface. In contrast to family A and B GPCRs, G-protein coupling does not involve the cytoplasmic opening of TM6 but is facilitated through the coordination of intracellular loops 2 and 3, as well as a critical contribution from the C terminus of the receptor. The findings highlight the synergy of global and local conformational transitions to facilitate a new mode of G-protein activation.

Dissecting the roles of GRK2 and GRK3 in μ-opioid receptor internalization and β-arrestin2 recruitment using CRISPR/Cas9-edited HEK293 cells

Most G protein-coupled receptors (GPCRs) recruit β-arrestins and internalize upon agonist stimulation. For the μ-opioid receptor (μ-OR), this process has been linked to development of opioid tolerance. GPCR kinases (GRKs), particularly GRK2 and GRK3, have been shown to be important for μ-OR recruitment of β-arrestin and internalization. However, the contribution of GRK2 and GRK3 to β-arrestin recruitment and receptor internalization, remain to be determined in their complete absence. Using CRISPR/Cas9-mediated genome editing we established HEK293 cells with knockout of GRK2, GRK3 or both to dissect their individual contributions in β-arrestin2 recruitment and μ-OR internalization upon stimulation with four different agonists. We showed that GRK2/3 removal reduced agonist-induced μ-OR internalization and β-arrestin2 recruitment substantially and we found GRK2 to be more important for these processes than GRK3. Furthermore, we observed a sustained and GRK2/3 independent component of β-arrestin2 recruitment to the plasma membrane upon μ-OR activation. Rescue expression experiments restored GRK2/3 functions. Inhibition of GRK2/3 using the small molecule inhibitor CMPD101 showed a high similarity between the genetic and pharmacological approaches, cross-validating the specificity of both. However, off-target effects were observed at high CMPD101 concentrations. These GRK2/3 KO cell lines should prove useful for a wide range of studies on GPCR function.

Delineation of molecular determinants for FR900359 inhibition of Gq/11 unlocks inhibition of Gαs

Heterotrimeric G proteins are essential mediators of intracellular signaling of G protein-coupled receptors. The G_q/11 subfamily consists of G_q, G₁₁, G₁₄, and G₁₆ proteins, of which all but G₁₆ are inhibited by the structurally related natural products YM-254890 and FR900359. These inhibitors act by preventing the GDP/GTP exchange, which is necessary for activation of all G proteins. A homologous putative binding site for YM-254890/FR900359 can also be found in members of the other three G protein families, G_s, G_i/o, and G_12/13, but none of the published analogs of YM-254890/FR900359 have shown any inhibitory activity for any of these. To explain why the YM-254890/FR900359 scaffold only inhibits G_q/11/14, the present study delineated the molecular selectivity determinants by exchanging amino acid residues in the YM-254890/FR900359-binding site in G_q and G_s We found that the activity of a G_s mutant with a G_q-like binding site for YM-254890/FR900359 can be inhibited by FR900359, and a minimum of three mutations are necessary to introduce inhibition in G_s In all, this suggests that although the YM-254890/FR900359 scaffold has proven unsuccessful to derive G_s, G_i/o, and G_12/13 inhibitors, the mechanism of inhibition between families of G proteins is conserved, opening up the possibility of targeting by other, novel inhibitor scaffolds. In lack of a selective Gα_s inhibitor, FR900359-sensitive Gα_s mutants may prove useful in studies where delicate control over Gα_s signaling would be of the essence.

Structural Insights into the Process of GPCR-G Protein Complex Formation

The crystal structure of the β2-adrenergic receptor (β2AR) bound to the G protein adenylyl cyclase stimulatory G protein (Gs) captured the complex in a nucleotide-free state (β2AR-Gs^empty). Unfortunately, the β2AR-Gs^empty complex does not provide a clear explanation for G protein coupling specificity. Evidence from several sources suggests the existence of a transient complex between the β2AR and GDP-bound Gs protein (β2AR-Gs^GDP) that may represent an intermediate on the way to the formation of β2AR-Gs^empty and may contribute to coupling specificity. Here we present a structure of the β2AR in complex with the carboxyl terminal 14 amino acids from Gαs along with the structure of the GDP-bound Gs heterotrimer. These structures provide evidence for an alternate interaction between the β2AR and Gs that may represent an intermediate that contributes to Gs coupling specificity.

Structural insights into the activation of metabotropic glutamate receptors

Metabotropic glutamate receptors are family C G-protein-coupled receptors. They form obligate dimers and possess extracellular ligand-binding Venus flytrap domains, which are linked by cysteine-rich domains to their 7-transmembrane domains. Spectroscopic studies show that signalling is a dynamic process, in which large-scale conformational changes underlie the transmission of signals from the extracellular Venus flytraps to the G protein-coupling domains-the 7-transmembrane domains-in the membrane. Here, using a combination of X-ray crystallography, cryo-electron microscopy and signalling studies, we present a structural framework for the activation mechanism of metabotropic glutamate receptor subtype 5. Our results show that agonist binding at the Venus flytraps leads to a compaction of the intersubunit dimer interface, thereby bringing the cysteine-rich domains into close proximity. Interactions between the cysteine-rich domains and the second extracellular loops of the receptor enable the rigid-body repositioning of the 7-transmembrane domains, which come into contact with each other to initiate signalling.

Outcome of fetuses with short femur length detected at second-trimester anomaly scan: a national survey

OBJECTIVE

To assess the relationship between the finding of fetal femur diaphysis length (FL) below the 5(th) percentile at the second-trimester scan and pregnancy outcome, in a population in which more than 90% of women attend first-trimester screening.

METHODS

This was a retrospective study of all Danish singleton pregnancies with a 17-22-week anomaly scan between 1 January 2008 and 30 June 2011. Information on FL and gestational age (GA) at anomaly scan, on birth weight and GA at delivery and on chromosomal abnormalities was obtained from the Danish Fetal Medicine Database.

RESULTS

Short FL was identified in 2718 (1.8%) of 147,766 fetuses and was present in 11 (16.2%) of the 68 fetuses affected by trisomy 21 (positive likelihood ratio (LR+) 8.8 (95% CI, 5.1-15.2)). Trisomy 13/18 and unbalanced autosomal structural abnormalities were also associated with a short FL in three (12.0%, LR+ 6.5 (95% CI, 2.3-18.9)) and eight (32.0%, LR+ 17.4 (95% CI, 9.8-30.9)) of the cases, respectively. The risk of a fetus having trisomy 21, trisomy 18, trisomy 13 or an unbalanced autosomal structural abnormality was 1 : 123 (95% CI, 79-192), given a short FL. Pregnancies with a fetus with short FL were more often affected by early preterm delivery (before 34 weeks) (5.6%; odds ratio (OR) = 4.2 (95% CI, 3.5-4.9)) and small-for-gestational-age (SGA) infants (13.9%; OR = 4.3 (95% CI, 3.8-4.8)).

CONCLUSION

Short FL at the second-trimester anomaly scan is associated with a significantly higher relative risk of chromosomal abnormalities, and a substantially higher absolute risk for SGA and early preterm delivery.

Pharmacological characterization and modeling of the binding sites of novel 1,3-bis(pyridinylethynyl)benzenes as metabotropic glutamate receptor 5-selective negative allosteric modulators

Metabotropic glutamate receptor subtype 5 (mGluR5) is a potential drug target in neurological and psychiatric disorders, and subtype-selective allosteric modulators have attracted much attention as potential drug candidates. In this study, the binding sites of three novel 2-methyl-6-(phenylethynyl)pyridine (MPEP)-derived negative allosteric modulators, 2-, 3-, and 4-BisPEB, have been characterized. 2-, 3-, and 4-BisPEB are 1,3-bis(pyridinylethynyl)-benzenes and differ only by the position of the nitrogen atoms in the pyridine rings. Despite their high structural similarity, 2-BisPEB [1,3-bis(pyridin-2-ylethynyl)-benzene, nitrogen atoms in ortho positions], with an IC(50) value in the nanomolar range, is significantly more potent than the 3- and 4-pyridyl analogs. Mutational analysis, directed by a previously published mGluR5 homology model, was used to determine key residues for the ligand-receptor interactions that may explain the potency differences of 2-, 3-, and 4-BisPEB. Residues Ile651, Pro655, Tyr659, Asn747, Trp785, Phe788, Tyr792, Ser809, and Ala810 were found to have critical roles for the activity of one or more of the three BisPEBs and the reference compound MPEP. The mutagenesis data suggest that the higher potency of 2-BisPEB is due to hydrogen bonding to Ser809 because the S809A mutation made 2-BisPEB equipotent to 3- and 4-BisPEB (IC(50), 1-2.5 μM). The potency of MPEP was also greatly affected by S809A (52-fold), suggesting that a Ser809-mediated hydrogen bond is also a key interaction between MPEP and mGluR5. Potential binding modes of 2-, 3-, and 4-BisPEB obtained by molecular docking to the mGluR5 homology model provide a structural context for the reported major mutational effects.

Crystal structure of the µ-opioid receptor bound to a morphinan antagonist

Opium is one of the world's oldest drugs, and its derivatives morphine and codeine are among the most used clinical drugs to relieve severe pain. These prototypical opioids produce analgesia as well as many undesirable side effects (sedation, apnoea and dependence) by binding to and activating the G-protein-coupled µ-opioid receptor (µ-OR) in the central nervous system. Here we describe the 2.8 Å crystal structure of the mouse µ-OR in complex with an irreversible morphinan antagonist. Compared to the buried binding pocket observed in most G-protein-coupled receptors published so far, the morphinan ligand binds deeply within a large solvent-exposed pocket. Of particular interest, the µ-OR crystallizes as a two-fold symmetrical dimer through a four-helix bundle motif formed by transmembrane segments 5 and 6. These high-resolution insights into opioid receptor structure will enable the application of structure-based approaches to develop better drugs for the management of pain and addiction.

1,2,3-triazolyl amino acids as AMPA receptor ligands

The central nervous system glutamate receptors are an important target for drug discovery. Herein we report initial investigations into the synthesis and glutamate receptor activity of 1,2,3-triazolyl amino acids. Two compounds were found to be selective AMPA receptor ligands, which warrant further investigation.

Granulocyte-macrophage colony-stimulating factor delays neutrophil constitutive apoptosis through phosphoinositide 3-kinase and extracellular signal-regulated kinase pathways

Activated neutrophils play an important role in the pathogenesis of sepsis, glomerulonephritis, acute renal failure, and other inflammatory processes. The resolution of neutrophil-induced inflammation relies, in large part, on removal of apoptotic neutrophils. Neutrophils are constitutively committed to apoptosis, but inflammatory mediators, such as GM-CSF, slow neutrophil apoptosis by incompletely understood mechanisms. We addressed the hypothesis that GM-CSF delays neutrophil apoptosis by activation of extracellular signal-regulated kinase (ERK) and phosphoinositide 3-kinase (PI 3-kinase) pathways. GM-CSF (20 ng/ml) significantly inhibited neutrophil apoptosis (GM-CSF, 32 vs 65% of cells p < 0. 0001). GM-CSF activated the PI 3-kinase/Akt pathway as determined by phosphorylation of Akt and BAD. GM-CSF-dependent Akt and BAD phosphorylation was blocked by the PI 3-kinase inhibitor LY294002. A role for the PI 3-kinase/Akt pathway in GM-CSF-stimulated delay of apoptosis was indicated by the ability of LY294002 to attenuate apoptosis delay. GM-CSF-dependent inhibition of apoptosis was significantly attenuated by PD98059, an ERK pathway inhibitor. LY294002 and PD98059 did not produce additive inhibition of apoptosis delay. To determine whether PI 3-kinase and ERK are used by other ligands that delay neutrophil apoptosis, we examined the role of these pathways in IL-8-induced apoptosis delay. LY294002 blocked IL-8-dependent Akt phosphorylation. PD98059 and LY294002 significantly attenuated IL-8 delay of apoptosis. These results indicate IL-8 and GM-CSF act, in part, to delay neutrophil apoptosis by stimulating PI 3-kinase and ERK-dependent pathways.

Regulation of expression of type II sodium-phosphate cotransporters by protein kinases A and C

The purpose of the present study was to determine the effect of protein kinase A and protein kinase C activation on the membrane expression of NaPi-4, the type II sodium-phosphate cotransporter in OK cells. NaPi-4 expression was measured using polyclonal antisera produced in rabbits against a peptide identical to the carboxy-terminal 12-amino acid sequence of NaPi-4. The antisera identified an apically localized protein by confocal imaging of intact OK cells and a broad band of 110-140 kDa by immunoblot analysis of OK cell membranes. Treatment of OK cells with parathyroid hormone (PTH) decreased the intensity of the 110- to 140-kDa band, which was detectable by 2 h, maximal by 4 h at 62%, and sustained for 24 h. 8-Bromo-cAMP (8-BrcAMP) inhibited NaPi-4 expression for up to 24 h by over 90%. However, phorbol 12-myristate 13-acetate inhibited NaPi-4 expression by less than 10%. PTH-(3-34), a fragment which stimulates only protein kinase C, inhibited phosphate transport but also had no effect on NaPi-4 expression. We conclude that protein kinase A but not protein kinase C inhibits sodium-phosphate uptake in OK cells by downregulation of NaPi-4 expression.