Bikkavilli Rama Kamesh. / "August 2004." / Thesis (Ph.D.)--Chinese University of Hong Kong, 2004. / Includes bibliographical references (p. 212-223) / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.

The apelin system is an evolving transmitter system consisting of the G protein coupled apelin receptor and two endogenous peptide ligands, apelin and elabela. It is implicated as a potential therapeutic for a number of diseases; however, the endogenous peptides are limited by half-life and bioavailability. This study aims to identify and pharmacologically characterise apelin agonists in vitro and in vivo and to evaluate their therapeutic potential in pulmonary arterial hypertension as a model disease. CMF-019 was identified as the first G protein biased apelin agonist. To date, suitable small molecule apelin agonists as experimental tool compounds have been limited and CMF-019 represents an important advance. CMF-019 was active in vivo, producing an increase in cardiac contractility and vasodilatation, similar to apelin. These effects were achieved without receptor desensitisation, supporting the remarkable G protein bias observed in vitro. Furthermore, it was disease-modifying in vitro in an endothelial cell apoptosis assay but despite this, did not prevent pulmonary arterial hypertension in a monocrotaline rat model of the disease. An apelin mimetic peptide possessing an unnatural amino acid, MM202, conjugated chemically via a polyethylene glycol linker to an anti-serum domain antibody (AlbudAb) was also characterised. The product MM202-AlbudAb represents the first time an AlbudAb has been conjugated chemically to an unnatural peptide mimetic, providing protection from proteolysis and glomerular filtration. Importantly, it retained binding to albumin and demonstrated in vitro and in vivo activity at the apelin receptor. In conclusion, this thesis has identified and pharmacologically characterised two novel apelin agonists that possess significant advantages over the endogenous peptides. CMF-019 is suitable as an experimental tool compound and, as the first G protein biased small molecule, provides a starting point for more suitable therapeutics. In addition, MM202-AlbudAb proves that unnatural peptides can be conjugated to AlbudAb, supporting use of this technology in other small-peptide ligand transmitter systems.

Regulator of G-protein Signaling (RGS) proteins temporally regulate the G protein signaling cascades initiated by GPCR activation. Reports have established dysregulation of RGS expression in a variety of disease states including several cancers. Additionally, use of genetic ablation techniques has implicated RGS proteins in a variety of other disease states through the native action of the RGS i.e. not a consequence of dysregulation of RGS expression. Therefore identification and optimization of small molecule lead compounds that alter RGS protein function has emerged as a promising therapeutic strategy. In this thesis, we use high throughput screening to interrogate small molecule libraries targeting two RGS proteins, RGS6 and RGS17. RGS6 has been reported as an essential mediator of doxorubicin induced cardiotoxicity, alcohol induced cardio and hepatotoxicity, anxiety, depression, and alcohol dependence. RGS17 has largely been implicated in a variety of cancer pathogenesis, with reported over expression in prostate, lung, breast, and hepatocellular carcinomas. Chapter 2 of this work focuses on the screening efforts targeting RGS6. Three separate screening campaigns interrogating over 20K compounds led to the identification of 3 small molecules that inhibit the RGS6: Gαo protein protein interaction with appreciable selectivity over control assays. The development of a cell based protein interaction assay is discussed, and the compounds were investigated using this system. All compounds tested did not appreciably alter signal over control, meaning that the cellular activity of these compounds remains ambiguous. Chapter 3 details the screening and follow up efforts targeting RGS17. The primary screening and/or follow up of four separate screening campaigns interrogating over 110K compounds is discussed. In total, 10 identified leads and a panel of analogs were subjected to significant follow up evaluation. All compounds were found to be cysteine dependent. The second generation RGS17 inhibitors (UI series) were determined to be both cytostatic and cytotoxic against lung and prostate cancer cell lines in culture, although whether this is due to RGS17 dependent mechanisms or due to general promiscuity of the compounds remains to be determined. Lead compounds from a library provided by the NCI were found to have cellular activity and were subjected to an investigation of structure activity relationships via commercially available compounds. The active form of three of these compounds was found to be a degradation product, which is likely due to decomposition of furan or methyl furan moieties that these compounds shared. One compound demonstrated robust SAR which allowed for the generation of schemes detailing putative inhibitory mechanisms. Finally, the role of RGS17 in the transition from epithelial to mesenchymal phenotypes is investigated. RGS17 was found to cause a sub population of PC3 cells to shift to mesenchymal phenotype, indicating that RGS17 may indeed play a role in this transition. Chapter 4 focuses on efforts to investigate variable potencies of published RGS4 inhibitors against a panel of RGS proteins, with the goal of gleaning insight in to structural characteristics that influence the inhibitability of RGS proteins. Most compounds tested were found to be more potent inhibitors of RGS14 rather than RGS4 in biochemical assays. We developed the NanoBit protein complementation assay to assess the interaction of RGS proteins with either Gαi1 or Gαq in a cellular context, and used this system to investigate compound selectivity in a cellular context. The compounds tested showed selectivity for RGS2, RGS4, and RGS14 over the other RGS proteins tested. The structural differences between the RGS proteins is discussed. Chapter 5 focuses on the future directions the lab may take with respect to the projects outlined in the previous chapters. This includes the screening of more targeted libraries or even virtual screening for RGS6, the development of in vivo assessment tools for RGS17, and an expanded structural examination of RGS proteins including NMR and crystal structure analysis. Additionally, the development of the NanoBit system to interrogate RGS protein interactions that are not RGS: Gα interactions is discussed.

Der Fluoreszenz-Resonanz-Energie-Transfer ist ein Phänomen, welches erstmals 1948 von Theodor Förster beschrieben wurde. Mit der Entwicklung von Fluoreszenzproteinen konnten in Kombination mit Mikroskopietechniken Einblicke in zellbiologische Vorgänge gewonnen werden, die durch biochemische oder physiologische Experimente nicht möglich sind. Dabei spielt die hohe zeitliche und räumliche Auflösung eine wichtige Rolle. Auf dem Forschungsgebiet der GPCR, welche die größte Gruppe von Membranproteinen bei den Säugetieren darstellen, wurden insbesondere Erkenntnisse über Konformationsänderungen der Rezeptoren, die Kinetik der Rezeptoraktivierung und die Interaktion mit intrazellulären Signalproteinen gewonnen. Der µ-Opioidrezeptor gehört zur Familie der GPCR und stellt aufgrund seiner analgetischen Wirkungen eine wichtige pharmakologische Zielstruktur dar. Das Ziel dieser Arbeit war sowohl den Rezeptor als auch seine Signalwege mittels FRET-Mikroskopie zu untersuchen. Zunächst sollte ein intramolekularer FRET-Sensor des µ-Opioidrezeptors entwickelt werden, dazu wurden basierend auf den Kenntnissen über die Tertiärstruktur und dem Aufbau bereits bekannter GPCR-Sensoren verschiedene Rezeptorkonstrukte kloniert. Bei den Konstrukten wurden entweder zwei Fluoreszenzproteine oder ein Fluoreszenzprotein und ein Fluorophor-bindendes Tetracysteinmotiv kombiniert. Auch die Positionen der eingefügten Sequenzen wurden in den intrazellulären Domänen variiert, da der Rezeptor auf die Modifikationen mit beeinträchtigter Membranlokalisation reagierte. Durch die Optimierung wurden Rezeptoren konstruiert, die an der Zellmembran lokalisiert waren. Jedoch zeigte keines der Rezeptorkonstrukte Funktionalität im Hinblick auf die Rezeptoraktivierung. Im zweiten Teil wurden die pharmakologischen Effekte der Metabolite von Morphin am humanen µ-Opioidrezeptor systematisch analysiert. Dazu wurde die Fähigkeit der Metabolite, Gi-Proteine zu aktivieren und β-Arrestin2 zu rekrutieren, mittels FRET-basierter Messungen an lebenden Zellen untersucht. Außerdem wurde die Affinität der Metabolite zum humanen µ Opioidrezeptor anhand der Verdrängung eines radioaktiven Liganden analysiert. Meine Experimente identifizierten eine Gruppe mit stark agonistischen und eine mit schwach agonistischen Eigenschaften. Die starken Partialagonisten aktivieren den Rezeptor bereits bei nanomolaren Konzentrationen, während die schwachen Metabolite den Rezeptor erst bei Konzentrationen im mikromolaren Bereich aktivieren. Die Metabolite Normorphin, Morphin-6-Glucuronid und 6-Acetylmorphin zeigen geringere Potenz als Morphin bei der Gi-Aktivierung aber überraschenderweise höhere Potenz und Effizienz für die β-Arrestin-Rekrutierung. Dies deutet auf eine bevorzugte Aktivierung von β-Arrestin2 hin. Die aus diesen Studien gewonnenen Ergebnisse liefern Hinweise darauf, welche Metabolite bei der Signalverarbeitung am µ Opioidrezeptor in vivo beteiligt sind. / Fluorescence resonance energy transfer was first described by Theodor Förster in 1948. The discovery and development of intrinsic fluorescent proteins revolutionized cell and molecular biology. The FRET-technique allows the analysis of protein-protein interactions and intramolecular conformational changes. In this method, its high temporal and spatial resolution plays a crucial role. Especially in the research field of GPCR, which are the largest family of membrane proteins in mammals, insights into receptor conformational changes, kinetics of receptor activation and the interaction with intracellular proteins were obtained. The µ-opioid receptor belongs to the GPCR family and is involved in analgesia. Therefore, the receptor is an important pharmacological target. Its pharmacological properties were extensively analyzed in the current thesis by FRET. Engineering of an intramolecular MOR-biosensor was initially planned. Based on the knowledge about the tertiary receptor structure and earlier GPCR-sensors, different receptor constructs were cloned. For each receptor construct either two fluorescent proteins or one fluorescent protein and one fluorophore binding tetracysteine motif were combined. The insertion of the additional amino acid sequences prevented the membrane localization for some constructs. Hence, the insertion site of the amioacid sequences was varied in the intracellular loops. Ultimately, the optimization resulted in some membrane localized receptor constructs with the tetracysteine motif in the third intracellular loop. Nevertheless, none of the receptor constructs was functional in terms of measurable conformational change upon receptor activation. In the second part of this thesis, the pharmacological effects of morphine and its metabolites were studied. The analgesic effects of morphine are mainly mediated via the activation of the µ opioid receptor. This receptor activates inhibitory G-proteins and induces the recruitment of β-arrestin2. Therefore I analyzed activation of these two pathways induced by morphine metabolites using FRET-microscopy in living cells. Furthermore, radioligand binding studies were used to determine the affinity of each compound to the human µ-opioid receptor. This approach identified two groups of metabolites, which were classified into strong and weak ligands. Strong partial agonists showed efficacies in the nanomalar range. In contrast, weak metabolites activated µ opioid receptor pathways in the micromolar range. Normorphine, morphine-6-glucuronide and 6 acetylmorphine had lower potencies regarding Gi-protein activation but higher potencies and efficacies for β-arrestin2 recruitment than morphine. These findings indicate that these metabolites are biased towards β-arrestin2 pathways.

G protein-coupled receptors (GPCRs) are the largest family of signaling proteins in animals and represent the largest family of druggable targets in the human genome. Therefore, it is of no surprise that the molecular mechanisms of GPCR activation and signal transduction have attracted close attention for the past few decades. Several stabilizing interactions within the GPCR transmembrane (TM) domain helices regulate receptor activation. An example is a salt bridge between 2 highly conserved amino acids at the bottom of TM3 and TM6 that has been characterized for a large number of GPCRs. Through structural modeling and molecular dynamics (MD) simulations, we predicted several electrostatic interactions to be involved in metabotropic glutamate receptor 2 (mGlu2R) activation. To experimentally test these predictions, we employed a charge reversal mutagenesis approach to disrupt predicted receptor electrostatic intramolecular interactions as well as intermolecular interactions between the receptor and G proteins. Using two electrode voltage clamp in Xenopus laevis oocytes expressing mutant receptors and G-proteins, we revealed novel electrostatic interactions, mostly located around intracellular loops 2 and 3 of mGlu2R, that are critical for both receptor and G-protein activation. These studies contribute to elucidating the molecular determinants of mGluRs activation and conformational coupling to G-proteins, and can likely be extended to include other classes of GPCRs.

Medulloblastoma is the most common malignant brain tumor in children. Genetic profiling has identified four principle tumor subgroups; each subgroup is characterized by different initiating mutations, genetic and clinical profiles, and prognoses. The two most well-defined subgroups are caused by overactive signaling in the WNT and SHH mitogenic pathways; less is known about Groups 3 and 4 medulloblastomas. Identification of tumor subgroup using molecular classification is poised to become an important component of the medulloblastoma diagnosis and staging and will likely guide therapeutic options. G-protein coupled receptors (GPCR) possess characteristics that make them ideal targets for molecular imaging and therapeutics. While expression patterns of many proteins in human medulloblastoma subgroups have been discerned, the expression pattern of GPCRs in medulloblastoma has not been investigated. We have found that clusters of medulloblastoma tumors arise based solely on differential GPCR expression patterns. Further, two of these clusters correspond with high fidelity to the WNT and SHH subgroups. Distinct over-expressed GPCRs emerge; for example, LGR5 and GPR64 are significantly and uniquely over-expressed in the WNT subgroup of tumors, while PTGER4 is over-expressed in the SHH subgroup. Uniquely under-expressed GPCRs were also observed. Our results identify GPCRs with potential to act as imaging and therapeutic targets; elucidating tumorigenic mechanisms is a secondary benefit to identifying differential GPCR expression patterns in medulloblastoma tumors. Current imaging for diagnosis, staging, and measuring response to therapy for medulloblastoma patients relies heavily on MRI; single photon emission tomography (SPECT) using 111In-DTPA-Octreotide targeting the somatostatin type 2 receptor (SSTR2) is also available. Positron emission tomography (PET) affords a more sensitive and specific imaging modality than SPECT; however, the most common tracer 18FDG, is of limited usefulness for the delineation of brain tumors. Smoothened (SMO) is a GPCR that is overexpressed in a subset of medulloblastoma; we hypothesized that SMO overexpression could be exploited as a specific PET target in these tumors. Genentech generously provided the synthetically-derived small-molecule SMO ligand, GDC-0449, for use as the lead compound for development of a PET tracer. GDC-0449 has already been demonstrated to localize in brain tumors and has Cl- atoms incorporated in positions that are predicted to readily exchange with fluorine-18 to generate a fluorinated analog of the compound. We have successfully fluorinated GDC-0449, with very high radiochemical purity. Binding assays reveal affinities of the fluorinated analog of GDC-0449 for SMO to be comparable to precursor GDC-0449, and biodistribution experiments demonstrate accumulation of the fluorinated compound in tumors. The fluorinated analog of GDC-0449 holds promise as a novel PET imaging agent in medulloblastoma, providing highly specific and sensitive imaging for use in diagnosis, staging and measurement of response-to-treatment.

G–Protein Coupled Receptors are one of the most important targets in drug development, making up over 60% of drug targets. Recent studies have implicated a role of Regulator of G–Protein Signaling (RGS) proteins in the development and progression of pathologies, including some cancers. RGS17, the most–recently identified family member of the RZ family of RGS proteins, has been implicated in the growth, proliferation, metastasis and migration of prostate tumors as well as small–cell and non–small cell lung cancers. In neoplastic tumor tissues RGS17 is up–regulated 13 fold over patient–matched normal tissues in prostate cancer. Studies have shown that RGS17 RNAi knockdown inhibits colony formation and decreases tumorigenesis in nude mice. Based on these findings, this thesis explores the research undertaken to develop small molecule inhibitors of the RGS17: Gαo protein: protein interaction. In this thesis, we implemented AlphaScreen® technology to develop a high–throughput screening method for interrogating small molecule libraries for inhibitors of RGS17. Chapter 3 focuses on the initial results of the AlphaScreen® in 384–well format. The screen utilizes a measurement of the Gα: RGS17 protein: protein interaction (PPI) and with an excellent Z–score exceeding 0.73, a signal to noise ratio >70 and a screening time of 1,100 compounds per hour. Chapter 3 presents the development, validation and initial high–throughput screening for inhibitors of Gα: RGS17 interaction as well as preliminary characterization of the RL series of hits. In this pilot screen the NCI Diversity Set II was interrogated, yielding 35 initial hits of which 16 were confirmed after screening against controls. The 16 compounds exhibited IC50 <10 ΜM in dose–response experiments for inhibiting the Gα: RGS17 interaction. Four exhibited IC50 values <6 ΜM while inhibiting the Gα: RGS17 interaction >50% when compared to a biotinylated GST control (TrueHits). Compounds RL–1 and RL–2 were confirmed by flow cytometry protein interaction assay (FCPIA) while RL–3 and RL–4 were unable to disrupt this PPI in FCPIA. All four compounds were tested using the differential scanning fluorimetry (DSF) method, which is based on energetic coupling between ligand binding and protein unfolding and found compounds RL–1 to RL–4 all slightly increased protein stability upon ligand binding. Chapter 4 focuses on the miniaturization and optimization of AlphaScreen® to a 1536–well format and screening of the MicroSource SPECTRUM and NDL3000 small molecule libraries. This increased throughput 11–fold and decreased our working volumes from 45 ΜL to 10 ΜL, which reduced reagent cost. After optimization, we retained in an excellent Z–factor ≥0.70 with S/N>5.77 and increased the screening rate to more than 12,000 compounds per hour. In this format, the initial screening of the SPECTRUM and NDL3000 libraries was completed and filtered the initial hits by counter screening and PAINs filtering as well as developing four powerful orthogonal assays for the characterization of potential lead molecules. Chapter 6 focuses on the future directions, which include the screening the in–house 50,000 compound library in the University of Iowa HTS Core facility as well as the development of cell based assays to determine the activity of these leads in the cellular milieu. These screens are the first step to developing novel pharmacophores for further optimization of structure with the focus on RGS17 activity in enzymatic, whole cell, xenograft and whole animal models as well as providing new avenues for the development of anticancer therapies.

The endogenous neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) exerts various neuromodulatory functions in mammalian brain. Enhancement of synaptic activity, mediation of chronic inflammatory and neuropathic pain, and neuroprotection in cerebral ischemia reperfusion injury constitute some of the exemplary functions of PACAP. However, it remains unclear whether PACAP signaling can directly influence the function of critical voltage-gated ion channels, which could profoundly alter the excitability of neurons. Voltage-gated K+ (Kv) channels are critical regulators of neuronal excitability. The major Kv channel in the dendrites of mammalian neurons, Kv4.2, contributes most of the fast-activating and rapidly-inactivating K+ currents (IA), and is a key regulator of dendritic excitability, as well as modulation of synaptic inputs. In addition, the major somatic Kv channel Kv2.1 that contributes the bulk of slow-activating and non-inactivating K+ currents (IK), acts as an integrator of neuronal inputs and limits high frequency firing in neurons. As such, it provides homeostatic control of excitability under hyperexcitable and ischemic conditions. Both these Kv channels are known to undergo extensive post-translational modifications mainly by phosphorylation that alters their localization and biophysical properties. PACAP can activate its specific receptor PAC1 that could result in downstream activation of various kinases including protein kinase A (PKA), protein kinase C (PKC), extracellular signal-regulated kinase (ERK1/2). Therefore, I hypothesize that PACAP activation of PAC1 receptor can cause phosphorylation-dependent modulation of somatodendritic Kv4.2 and Kv2.1 channels, resulting in altered neuronal excitability. First, I identified the various PAC1 receptor isoforms expressed in rat and mouse brain and elucidated that their activation by PACAP caused downstream PKA- and PKC-dependent signaling pathways, ultimately converging on ERK1/2 activation. Further, PACAP caused reduction in IA that was mediated by phosphorylation-dependent internalization of the channel protein from the plasma membrane. These effects were mediated by direct phosphorylation of the channel by ERK1/2 at the cytoplasmic C-terminus of the channel. Although PACAP did not significantly alter the voltage-dependence of Kv4.2 channel activation/inactivation, I observed distinct ERK1/2- and PKA-dependent changes in the extent and kinetics of channel inactivation. Next, I observed that PACAP induced dephosphorylation of the Kv2.1 channel in CHN that was mediated by protein phosphatase 2A (PP2A), and was dependent on PKC activation but was independent of the effects of PACAP on Kv4.2 currents. Rapid but reversible dephosphorylation of Kv2.1 was also observed following induction of ischemia in neurons by oxygen-glucose deprivation (OGD). PACAP prolonged the dephosphorylation of Kv2.1 following in vitro ischemia-reperfusion and also reduced neuronal death. My results therefore suggest a novel PACAP/PAC1-PKC-PP2A-Kv2.1 signaling axis that provides neuroprotection during ischemia reperfusion injury. In summary, my results suggest that PACAP can induce direct phosphorylation-dependent modulation of the Kv4.2 and Kv2.1 channel localization and function in mammalian brain neurons. The effect of PACAP on these two critical somatodendritic ion channels occurs via distinct signaling - convergent PKA-PKC-ERK-mediated phosphorylation of Kv4.2 channel, and PKC-PP2A-mediated dephosphorylation of the Kv2.1 channel. Such distinct modulations of these ion channels are presumably responsible for the multifarious roles of PACAP in the central nervous system.

Regulator of G Protein Signaling 4 (RGS4) mediates motor defects in Parkinson's disease. Small molecule RGS4 inhibitors (e.g. CCG-50014) modify buried cysteine residues, but the structural and dynamic mechanisms underpinning specificity of inhibitors for RGS4 within the RGS family are poorly understood. We used NMR and other biophysical methods to examine ligand-induced structural changes and the dynamics of unliganded RGS4 and RGS8 that allow ligand binding. NMR and fluorescence spectroscopy data reveal details of the hidden, excited conformational state of RGS4 that exposes Cys148, one of the buried cysteines bound by inhibitors. We further show that specificity of RGS4 inhibitors is driven by differential accessibility of the target cysteine compared to its equivalent in RGS8. Cys148 is buried beneath the lid at the center the α4-α7 helix bundle, and this bundle is destabilized in RGS4 compared to RGS8. Notably, helix 6 is highly destabilized in RGS4 compared to RGS8 and is likely the key mediator of access to Cys148. Our findings provide key insight into the mechanism of allosteric RGS4 inhibition and show that dynamics drive inhibitory specificity among RGS proteins.

Zahlreiche experimentelle Befunde lassen vermuten, dass G-Protein gekoppelte Rezeptoren (GPCR) nach ihrer Aktivierung einer ligandenselektiven Änderung der Rezeptorkonformation unterliegen. Ziel der vorliegenden Arbeit war es dieses Phänomen am Subtyp 2 der muskarinischen Acetylcholinrezeptoren (M2 AChR) zu untersuchen. Muskarinische Acetylcholinrezeptoren (mAChR) können in fünf Subtypen (M1-M5) unterschieden werden. Durch die Beteiligung der mAChR an zahlreichen physiologischen Prozessen stellen sie wichtige Zielstrukturen pharmakologischer Therapien dar. Da die orthosterische Ligandenbindestelle (= Bindestelle des endogenen Liganden) in allen fünf Subtypen hoch konserviert ist, wird ihr pharmakologischer Nutzen derzeit allerdings durch die unselektive Rezeptormodulation und dem damit verbundenen Auftreten unerwünschter Arzneimittelwirkungen stark limitiert. Ein Ansatz zur Erzielung subtypselektiver Effekte besteht in der Verwendung allosterischer Modulatoren. Da die allosterische Bindestelle der mAChR eine geringere Sequenzhomologie aufweist, können so gezielt einzelne Subtypen der mAChR reguliert werden. Der M2 AChR stellt hinsichtlich allosterischer Modulation ein gut charakterisiertes Modellsystem dar. Für ihn wurde bereits eine Vielzahl allosterischer Liganden entwickelt. Auch bitopische Liganden, die sowohl einen allosterischen als auch einen orthosterischen Anteil enthalten, wurden für den M2 AChR bereits beschrieben. Im ersten Teil der vorliegenden Arbeit wurden verschiedene FRET-Sensoren des M2 AChR generiert und charakterisiert. Als FRET-Paar wurden das cyan fluoreszierende Protein (CFP) und der niedermolekulare fluorescein-basierte Fluorophor FlAsH (fluorescein arsenical hairpin binder) gewählt. CFP wurde in den Sensoren am Ende des C-Terminus angefügt. Die zur Markierung mit FlAsH nötige Tetracysteinsequenz wurde in verschiedenen Bereichen der dritten intrazellulären Rezeptorschleife (IL) eingebracht. Die auf diese Weise erstellten Re-zeptorsensoren trugen das Tetracysteinmotiv in der N terminalen (M2i3-N) bzw. in der C terminalen Region (M2i3-C) von IL 3. Die Charakterisierung der Rezeptorsensoren bezüglich Ligandenbindung, Gi-Protein Aktivierung und β-Arrestin2 Translokation ergab keine signifikanten Unterschiede zwischen M2i3-N, M2i3 C und M2CFP oder Wildtyp M2 AChR. Zunächst wurden sowohl unterschiedliche orthosterische, als auch allosterische Liganden hinsichtlich ihrer mittleren effektiven Konzentration und ihrer maximalen Wirkstärke an den Rezeptorsensoren untersucht. Mit Hilfe von FRET-Messungen konnte ein superago-nistisches Verhalten des orthosterischen Testliganden Iperoxo gezeigt werden. Die Eigenschaften der allosterischen Substanzen wurden durch Messung der Rezeptordeakti-vierungskinetik und des maximalen Hemmeffekts auf einen vorstimulierten Rezeptor charakterisiert. Alle allosterischen Liganden deaktivierten den vorstimulierten M2 AChR mit einer schnelleren Kinetik als Atropin. Die EC50-Werte der unterschiedlichen Substanzen waren unabhängig von der Markierungsposition im verwendeten Rezeptorsensor vergleich-bar. Ausnahmen bildeten die allosterischen Liganden JK 289, JK 338, ½ W84 und EHW 477, die liganden- und sensorabhängig unterschiedliche mittlere effektive Konzentrationen aufwie-sen. Bei der Untersuchung der Konformationsänderung des M2 AChR konnte kein liganden-selektiver Unterschied zwischen den FRET-Signalen für 19 getestete orthosterische Liganden beobachtet werden. Dies deutet darauf hin, dass alle orthosterischen Testliganden eine dem Acetylcholin (ACh) vergleichbare Änderung der M2 AChR Konformation induzier-ten. Um zu untersuchen, ob für die orthosterischen Testliganden eine Korrelation zwischen ihrer maximalen Wirkstärke hinsichtlich Rezeptoraktivierung in FRET-Experimenten und der Aktivierung nachgeschalteter Signalwege besteht, wurde die orthosterisch-vermittelte Translokation von β-Arrestin2 mit Hilfe der Konfokalmikroskopie bestimmt. Bis auf 5-Methyl-furmethiodid translozierten alle orthosterischen Liganden β-Arrestin2 in einem Ausmaß, das mit der maximalen Rezeptoraktivierung vergleichbar war. Dagegen rief 5 Methylfurmethiodid verglichen mit dem endogenen Liganden ACh zwar eine ca. halbmaximale Rezeptorakti-vierung, aber nur eine äußerst geringe β-Arrestin2 Translokation hervor. Im zweiten Teil der Arbeit wurde der Einfluss verschiedener Allostere auf eine ligandenselektive Konformationsänderung des M2 AChR untersucht. Die allosterischen Liganden JK 337 und Seminaph beeinflussten den M2i3-C Sensor signifikant stärker, als das M2i3-N Konstrukt. Dagegen zeigte EHW 477 eine stärkere Beeinflussung der Rezeptorkon-formation des M2i3-N-, als des M2i3-C Sensors. Dies erlaubt die Vermutung, dass JK 337 und Seminaph eine stärkere Bewegung unterhalb von Transmembrandomäne (TM) 6, als unterhalb von TM 5 hervorriefen. Die Ergebnisse für EHW 477 legen nahe, dass TM 5 eine größere Bewegung eingeht, als TM 6. FRET-basierte Untersuchungen der Einflüsse der allosterischen Testliganden auf nachgeschaltete Signalwege ergaben, dass sowohl die Akti-vierung des Gi Proteins, als auch die β-Arrestin2 Translokation selektiv durch einzelne allosterische Liganden beeinflusst werden. Auch ein Zusammenhang zwischen Rezeptor-aktivierung und der Regulation nachgeschalteter Signalwege war erkennbar. Allerdings waren auf Grund der Versuchsbedingungen keine quantitativen Aussagen möglich. Im Folgenden wurden die bitopischen Liganden Hybrid 1 und 2 (H 1, H 2) hinsichtlich ihres Effekts auf die Konformationsänderung des M2 AChR untersucht. Während eine Stimulation mit H 1 vergleichbare FRET-Signale an beiden Sensoren ergab, konnte mit H 2 weder am M2i3-N-, noch an M2i3-C Sensor eine FRET-Änderung detektiert werden. Um den mangeln-den Effekt der Hybridsubstanzen in FRET-mikroskopischen Untersuchungen aufzuklären, wurden verschiedene Ansätze gewählt: Mit kettenverlängerten Derivaten der Hybridsubstanzen konnte in FRET-Messungen eine Änderung des FRET-Signals detektiert werden. Die Entfernung des allosterischen Bausteins führte in FRET-Experimenten zu einer verglichen mit dem endogenen Liganden ACh etwa halbmaximalen Aktivierung beider Sensoren. Dagegen resultierte die Mutation der alloste-rischen Bindestelle in nachfolgenden FRET-Untersuchungen mit H 1 und H 2 in keiner Signaländerung des FRET-Ratio. Diese Beobachtungen führten zu der Annahme, dass die Linkerkette, die orthosterischen und allosterischen Baustein der Hybride miteinander verbindet, zu kurz war um eine gleichzeitige Bindung an die allosterische und orthosterische Bindestelle zu ermöglichen. Ein anderer Erklärungsansatz besteht darin, dass nach Bindung des Orthosters der Kanal zwischen orthosterischer und allosterischer Bindestelle durch die Konformationsänderung des Rezeptors verschlossen wird, weshalb keine dauerhafte, dualsterische Bindung der Hybridsubstanzen an den M2 AChR möglich ist. Im Rahmen der vorliegenden Arbeit ist es gelungen mittels FRET-Experimenten die Existenz einer ligandenselektiven Rezeptorkonformation des M2 AChR mit allosterischen Liganden nachzuweisen. Darüber hinaus konnte auch ein Bezug zum Auftreten einer funktionellen Selektivität mit allosterischen Liganden hergestellt werden. Die Untersuchung von 19 orthosterischen Liganden hinsichtlich ihres Einflusses auf die Rezeptorkonformation des M2 AChR ergab keinen Hinweis auf eine ligandenselektive Konformationsänderung. Die Betrachtung der orthosterisch-vermittelten Translokation von β-Arrestin2 zeigte, dass zwischen der Effizienz der orthosterischen Testliganden, den M2 AChR zu aktivieren und dem Ausmaß, in dem sie eine β Arrestin2 Translokation induzierten eine direkte Korrelation besteht. Lediglich 5-Methylfurmethiodid rief eine ungleich geringere β-Arrestin2 Translokation hervor, verglichen mit dem Ausmaß an Rezeptoraktivierung. Diese Beobachtung deutet auf die Existenz eines signaling-bias für diesen Liganden hin. Die Untersuchung der dualsterischen Liganden H 1 und 2 bezüglich ihrer Fähigkeit zur Rezeptoraktivierung ergab, dass erst durch eine Verlängerung der Linkerkette, durch die orthosterischer und alloste-rischer Baustein miteinander verbunden sind eine Konformationsänderung des M2 AChR hin zu einer aktiven Konformation erreicht werden kann. Es kann somit angenommen werden, dass in den ursprünglichen Hybridsubstanzen H 1 und H 2 eine zu kurze Linkerkette, durch die keine dualsterische Bindung der Hybride an die allosterische und orthosterische Bindestelle möglich ist, ursächlich für die mangelnde Rezeptoraktivierung des M2 AChR war. / A large body of experimental evidence suggests that upon receptor activation G-protein coupled receptors are subject to ligandspecific changes of receptor conformation. The aim of this study was to investigate this phenomenon using the muscarinic M2 acetylcholine receptor (M2 AChR). Muscarinic acetylcholine receptors (mAChR) can be subdivided into five different subtypes (M1-M5). Their involvement in various physiological processes makes them an important target of pharma-cological therapies. With the orthosteric binding site (= binding site of the endogenous ligand) being highly conserved across all five mAChR subtypes, the unselective receptor modulation can lead to severe side effects. Thus the clinical use of drugs modulating muscarinic receptors is currently limited. Allosteric modulation is one attempt to achieve subtype-selective receptor regulation. Since the allosteric binding site of mAChR is less well conserved, it is possible to selectively target one mAChR subtype. As far as allosteric modulation is concerned, the M2 AChR represents a well characterized model with a large number of allosteric modulators being available. For the M2 AChR bitopic ligands which contain an allosteric as well as an orthosteric binding block have been developed as well. In the first part of this study several FRET-sensors of the M2 AChR were designed and characterized. The cyan fluorescent protein (CFP) was fused to the C-terminus of both sensors while the FlAsH (fluorescein arsenical hairpin binder) binding site was inserted into the N-terminal (M2i3-N) or the C terminal (M2i3-C) region of the third interacellular loop (IL). The receptor sensors were characterized concerning ligand affinity, activation of the Gi protein and -arrestin2 translocation and did not display any significant differences compared to the wildtype M2 or the M2 CFP receptor. Various orthosteric as well as allosteric ligands were investigated regarding their affinity and efficacy at both sensors. Using FRET-measurements iperoxo was proven to behave as a superagonist. The characteristics of the allosteric ligands were investigated by measuring the receptor deactivation kinetics and their maximum inhibitory effect on a pre-stimulated receptor. All allosteric test substances displayed faster deactivation kinetics compared to the antagonist atropine and similar EC50 values at both receptor sensors. When investigating the change of receptor conformation of the M2 AChR upon ligand binding there were no ligand selective differences in the FRET-signal detected for either of the 19 orthosteric ligands at both M2 sensors. This data suggest that all orthosteric ligands induced a change in receptor conformation comparable to acetylcholine (ACh). In order to correlate the efficacy of various orthosteric ligands to activate the M2 AChR in FRET-experiments with their effect on downstream signaling pathways, the translocation of  arrestin2 upon receptor activation with orthosteric ligands was investigated using confocal microscopy. Except for 5 methylfurmethiodide all orthosteric ligands induced -arrestin2 translocation to an extent which was comparable to the maximal receptor activation observed with each other ligand, respectively. In contrast 5-methylfurmethiodide evoked a half maximal receptor activation compared to the endogenous ligand ACh while only a minimal translocation of -arrestin2 was observed. The second aim of this study was to investigate the effects of allosteric ligands on the change of receptor conformation of the M2 AChR. The allosteric ligands JK 337 and seminaph more strongly influenced the M2i3-C than the M2i3-N, whilst EHW 477 behaved just the opposite way. This data suggest that the orthosteric ligands induce a conformation of the M2 AChR comparable to ACh. JK 337 and seminaph seem to evoke a greater movement underneath TM 6 compared to TM 5 whereas EHW 477 probably induces a larger movement beneath TM 5. The allosteric ligands were tested via FRET-measurements concerning their ability to activate the Gi protein and to translocate  arrestin2. The activation of the Gi protein as well as the -arrestin2 translocation were selectively influenced by all allosteric ligands. However, due to the experimental setup, a quantification of the effects was not possible. Furthermore the bitopic ligands hybrid 1 and 2 (H 1, H 2) were tested regarding their effect on the receptor conformation of the M2 AChR. While stimulation with H 1 induced FRET signals that were comparable for both receptor sensors, it wasn’t possible to detect any change in the FRET ratio neither of the M2i3-N nor of the M2i3-C with H 2. The lack of effect of H 1 and H 2 in the FRET-experiments was explored using two different approaches: Derivatives of H 1 and H 2, in which the carbon linker between the allosteric and the orthosteric building block had been elongated, were able to induce changes in the FRET ratio. Upon the removal of the allosteric building block a half-maximal activation of both receptor sensors could be detected. However, the mutation of the allosteric binding site did not result in any change of the FRET-signals upon stimulation of the receptor mutants with H 1 or H 2. These data suggest that the carbon linker, which connects the allosteric and the orthosteric building block, is not long enough to enable a simultaneous binding to the allosteric and the orthosteric binding site. Another explanation would be that upon binding of an orthoster the channel between the orthosteric and the allosteric binding site of the M2 AChR is closed because of the change in receptor conformation, hence a stable, dual-steric binding of the hybrid substances to the M2 AChR would not be possible. In the course of this study it was possible to prove the existence of a ligand selective receptor conformation of the M2 AChR with allosteric ligands using FRET-experiments. In addition a connection was found to the occurrence of a functional selctivity with allosteric ligands. The investigation of 19 orthosteric ligands regarding their influence on the receptor conformation of the M2 AChR did not reveal any evidence of the existence of a ligand selective change of the receptor conformation. Regarding the translocation of β arrestin2 induced by orthosteric ligands there was a direct correlation between the efficency of the orthosteric ligands to activate the receptor and the extend of β-arrestin2 translocation observed. With the only exception being 5-methylfurmethiodide which induced far less β arrestin2 translocation compared to the magnitude of the conformational change of the receptor. This data suggest the existence of a signaling bias for this ligand. The analysis of the dualsteric ligands H 1 and H 2 concerning their ability to activate the M2 AChR revealed that an activation of the M2 AChR could just be observed upon elongation of the linker which connects the orthosteric with the allosteric building block. This suggests that the short linker chain of the original hybrid substances inhibited a dualsteric binding to the orthosteric and the allosteric binding site and thus caused the difficency of H 1 and H 2 to activate the M2 AChR.