Doreen Thor

Altered Immune Response in Mice Deficient for the G Protein-coupled Receptor GPR34

The X-chromosomal GPR34 gene encodes an orphan Gi protein-coupled receptor that is highly conserved among vertebrates. To evaluate the physiological relevance of GPR34, we generated a GPR34-deficient mouse line. GPR34-deficient mice were vital, reproduced normally, and showed no gross abnormalities in anatomical, histological, laboratory chemistry, or behavioral investigations under standard housing. Because GPR34 is highly expressed in mononuclear cells of the immune system, mice were specifically tested for altered functions of these cell types. Following immunization with methylated BSA, the number of granulocytes and macrophages in spleens was significantly lower in GPR34-deficient mice as in wild-type mice. GPR34-deficient mice showed significantly increased paw swelling in the delayed type hypersensitivity test and higher pathogen burden in extrapulmonary tissues after pulmonary infection with Cryptococcus neoformans compared with wild-type mice. The findings in delayed type hypersensitivity and infection tests were accompanied by significantly different basal and stimulated TNF-α, GM-CSF, and IFN-γ levels in GPR34-deficient animals. Our data point toward a functional role of GPR34 in the cellular response to immunological challenges.

Structural aspects of M3 muscarinic acetylcholine receptor dimer formation and activation

To explore the structural mechanisms underlying the assembly and activation of family A GPCR dimers, we used the rat M3 muscarinic acetylcholine receptor (M3R) as a model system. Studies with Cys‐substituted mutant M3Rs expressed in COS‐7 cells led to the identification of several mutant M3Rs that exclusively existed as cross‐linked dimers under oxidizing conditions. The cross‐linked residues were located at the bottom of transmembrane domain 5 (TM5) and within the N‐terminal portion of the third intracellular loop (i3 loop). Studies with urea‐stripped membranes demonstrated that M3R disulfide cross‐linking did not require the presence of heterotrimeric G proteins. Molecular modeling studies indicated that the cross‐linking data were in excellent agreement with the existence of a low‐energy M3R dimer characterized by a TM5‐TM5 interface. [35S]GTPγS binding/Gαq/11 immunoprecipitation assays revealed that an M3R dimer that was cross‐linked within the N‐terminal portion of the i3 loop (264C) was functionally severely impaired (∼50% reduction in receptor‐G‐protein coupling, as compared to control M3R). These data support the novel concept that agonist‐induced activation of M3R dimers requires a conformational change of the N‐terminal segment of the i3 loop. Given the high degree of structural homology among family A GPCRs, these findings should be of broad significance.—Hu, J., Thor, D., Zhou, Y., Liu, T., Wang, Y., McMillin, S. M., Mistry, R., Challiss, R. A. J., Costanzi, S., Wess, J. Structural aspects of M3 muscarinic acetylcholine receptor dimer formation and activation. FASEB J. 26, 604–616 (2012). www.fasebj.org

Involvement of the V2 Vasopressin Receptor in Adaptation to Limited Water Supply

Mammals adapted to a great variety of habitats with different accessibility to water. In addition to changes in kidney morphology, e.g. the length of the loops of Henle, several hormone systems are involved in adaptation to limited water supply, among them the renal-neurohypophysial vasopressin/vasopressin receptor system. Comparison of over 80 mammalian V2 vasopressin receptor (V2R) orthologs revealed high structural and functional conservation of this key component involved in renal water reabsorption. Although many mammalian species have unlimited access to water there is no evidence for complete loss of V2R function indicating an essential role of V2R activity for survival even of those species. In contrast, several marsupial V2R orthologs show a significant increase in basal receptor activity. An increased vasopressin-independent V2R activity can be interpreted as a shift in the set point of the renal-neurohypophysial hormone circuit to realize sufficient water reabsorption already at low hormone levels. As found in other desert mammals arid-adapted marsupials show high urine osmolalities. The gain of basal V2R function in several marsupials may contribute to the increased urine concentration abilities and, therefore, provide an advantage to maintain water and electrolyte homeostasis under limited water supply conditions.

Structural and functional evolution of the P2Y12-like receptor group

Metabotropic pyrimidine and purine nucleotide receptors (P2Y receptors) belong to the superfamily of G protein-coupled receptors (GPCR). They are distinguishable from adenosine receptors (P1) as they bind adenine and/or uracil nucleotide triphosphates or diphosphates depending on the subtype. Over the past decade, P2Y receptors have been cloned from a variety of tissues and species, and as many as eight functional subtypes have been characterized. Most recently, several members of the P2Y12-like receptor group, which includes the clopidogrel-sensitive ADP receptor P2Y12, have been deorphanized. The P2Y12-like receptor group comprises several structurally related GPCR which, however, display heterogeneous agonist specificity including nucleotides, their derivatives, and lipids. Besides the established function of P2Y12 in platelet activation, expression in macrophages, neuronal and glial cells as well as recent results from functional studies implicate that several members of this group may have specific functions in neurotransmission, inflammation, chemotaxis, and response to tissue injury. This review focuses specifically on the structure-function relation and shortly summarizes some aspects of the physiological relevance of P2Y12-like receptor members.

Structural and functional evolution of the P2Y(12)-like receptor group.

Metabotropic pyrimidine and purine nucleotide receptors (P2Y receptors) belong to the superfamily of G protein-coupled receptors (GPCR). They are distinguishable from adenosine receptors (P1) as they bind adenine and/or uracil nucleotide triphosphates or diphosphates depending on the subtype. Over the past decade, P2Y receptors have been cloned from a variety of tissues and species, and as many as eight functional subtypes have been characterized. Most recently, several members of the P2Y12-like receptor group, which includes the clopidogrel-sensitive ADP receptor P2Y12, have been deorphanized. The P2Y12-like receptor group comprises several structurally related GPCR which, however, display heterogeneous agonist specificity including nucleotides, their derivatives, and lipids. Besides the established function of P2Y12 in platelet activation, expression in macrophages, neuronal and glial cells as well as recent results from functional studies implicate that several members of this group may have specific functions in neurotransmission, inflammation, chemotaxis, and response to tissue injury. This review focuses specifically on the structure-function relation and shortly summarizes some aspects of the physiological relevance of P2Y12-like receptor members.

Using ortholog sequence data to predict the functional relevance of mutations in G-protein-coupled receptors

Evaluating the functional relevance of naturally occurring gene variants usually requires experimental testing or is even impossible because of the lack of appropriate functional assays. Here we have analyzed whether comparative sequence data from orthologs are suitable to predict the functional relevance of mutations in a model protein, a G‐proteincoupled receptor for ADP (P2Y12). The functional effect of every possible substitution at each amino acid position within a portion of P2Y12 (1254 mutants) was individually determined. Sequence analysis of >70 P2Y12 vertebrate orthologs revealed that this amino acid variability ensuring proper receptor function in vivo highly correlates (>90%) with the in vitro experimental data. Therefore, ortholog sequence data are helpful to predict the functional relevance of individual positions and mutations for P2Y12. It is likely that similar conclusions may be extended for other GPCRs and conserved proteins as well.—Cöster, M., Wittkopf, D., Kreuchwig, A., Kleinau, G., Thor, D., Krause, G., Schöneberg, T. Using ortholog sequence data to predict the functional relevance of mutations in G‐protein‐coupled receptors. FASEB J. 26, 3273–3281 (2012). www.fasebj.org

Generation of an agonistic binding site for blockers of the M3 muscarinic acetylcholine receptor

GPCRs (G-protein-coupled receptors) exist in a spontaneous equilibrium between active and inactive conformations that are stabilized by agonists and inverse agonists respectively. Because ligand binding of agonists and inverse agonists often occurs in a competitive manner, one can assume an overlap between both binding sites. Only a few studies report mutations in GPCRs that convert receptor blockers into agonists by unknown mechanisms. Taking advantage of a genetically modified yeast strain, we screened libraries of mutant M(3)Rs {M(3) mAChRs [muscarinic ACh (acetylcholine) receptors)]} and identified 13 mutants which could be activated by atropine (EC50 0.3-10 microM), an inverse agonist on wild-type M(3)R. Many of the mutations sensitizing M(3)R to atropine activation were located at the junction of intracellular loop 3 and helix 6, a region known to be involved in G-protein coupling. In addition to atropine, the pharmacological switch was found for other M(3)R blockers such as scopolamine, pirenzepine and oxybutynine. However, atropine functions as an agonist on the mutant M(3)R only when expressed in yeast, but not in mammalian COS-7 cells, although high-affinity ligand binding was comparable in both expression systems. Interestingly, we found that atropine still blocks carbachol-induced activation of the M(3)R mutants in the yeast expression system by binding at the high-affinity-binding site (Ki approximately 10 nM). Our results indicate that blocker-to-agonist converting mutations enable atropine to function as both agonist and antagonist by interaction with two functionally distinct binding sites.

Luciferase activity under direct ligand-dependent control of a muscarinic acetylcholine receptor

BackgroundControlling enzyme activity by ligand binding to a regulatory domain of choice may have many applications e.g. as biosensors and as tools in regulating cellular functions. However, until now only a small number of ligand-binding domains have been successfully linked to enzyme activity. G protein-coupled receptors (GPCR) are capable of recognizing an extraordinary structural variety of extracellular signals including inorganic and organic molecules. Ligand binding to GPCR results in conformational changes involving the transmembrane helices. Here, we assessed whether ligand-induced conformational changes within the GPCR helix bundle can be utilized to control the activity of an integrated enzyme.ResultsAs a proof of principle, we inserted the luciferase amino acid sequence into the third intracellular loop of the M3 muscarinic acetylcholine receptor. This fusion protein retained both receptor and enzyme function. Receptor blockers slightly but significantly reduced enzyme activity. By successive deletion mutagenesis the enzyme activity was optimally coupled to ligand-induced conformational helix movements.ConclusionOur results demonstrate that in engineered GPCR-enzyme chimeras, intracellular enzyme activity can be directly controlled by a GPCR serving as the extracellular ligand-binding domain.

Deciphering and modulating G protein signalling in C. elegans using the DREADD technology

G-protein signalling is an evolutionary conserved concept highlighting its fundamental impact on developmental and functional processes. Studies on the effects of G protein signals on tissues as well as an entire organism are often conducted in Caenorhabditis elegans. To understand and control dynamics and kinetics of the processes involved, pharmacological modulation of specific G protein pathways would be advantageous, but is difficult due to a lack in accessibility and regulation. To provide this option, we designed G protein-coupled receptor-based designer receptors (DREADDs) for C. elegans. Initially described in mammalian systems, these modified muscarinic acetylcholine receptors are activated by the inert drug clozapine-N-oxide, but not by their endogenous agonists. We report a novel C. elegans-specific DREADD, functionally expressed and specifically activating Gq-protein signalling in vitro and in vivo which we used for modulating mating behaviour. Therefore, this novel designer receptor demonstrates the possibility to pharmacologically control physiological functions in C. elegans.

Structural and functional evolution of the P2Y12

Metabotropic pyrimidine and purine nucleotide receptors (P2Y receptors) belong to the superfamily of G protein-coupled receptors (GPCR). They are distinguishable from adenosine receptors (P1) as they bind adenine and/or uracil nucleotide triphosphates or diphosphates depending on the subtype. Over the past decade, P2Y receptors have been cloned from a variety of tissues and species, and as many as eight functional subtypes have been characterized. Most recently, several members of the P2Y12-like receptor group, which includes the clopidogrel-sensitive ADP receptor P2Y12, have been deorphanized. The P2Y12-like receptor group comprises several structurally related GPCR which, however, display heterogeneous agonist specificity including nucleotides, their derivatives, and lipids. Besides the established function of P2Y12 in platelet activation, expression in macrophages, neuronal and glial cells as well as recent results from functional studies implicate that several members of this group may have specific functions in neurotransmission, inflammation, chemotaxis, and response to tissue injury. This review focuses specifically on the structure-function relation and shortly summarizes some aspects of the physiological relevance of P2Y12-like receptor members.