Molecular Neurobiology Synaptic Plasticity
Regulation of Neuronal Functions by Auxiliary Subunits of G-protein Coupled Receptors
We are interested in the mechanisms that control neuronal excitability, and to exploit these mechanisms for the treatment of neurological and psychiatric diseases. We are giving emphasis to the control of neuronal excitability by G-protein coupled receptors (GPCRs), in particular GABAB receptors, mGlu5 receptors, dopamine receptors and Trace Amine-Associated Receptor 1 (TAAR1).
GABAB receptors are the GPCRs for the inhibitory neurotransmitter γ-aminobutyric acid (GABA). Their activity influences many neural systems and behavioral states (Gassmann and Bettler, 2012). GABAB receptors have been implicated in a variety of neurological and psychiatric conditions, including epilepsy, anxiety, depression, schizophrenia, obsessive compulsive disorder, addiction and pain. Despite the involvement of GABAB receptors in mental health disorders, the clinical use of GABAB receptor agonists is currently limited to the treatment of narcolepsy, neuropathic pain, spasticity and dystonia. One reason for this is that the main therapeutic effect of baclofen – the prototypical GABAB receptor agonist in clinical use – has unwanted side effects for mental health indications.
A large body of work suggests that native GABAB receptors vary in their kinetic and pharmacological properties. The origin of this variation is unclear. To some extent, it may be explained by the existence of associated proteins that alter receptor properties. In collaboration with B. Fakler (University Freiburg iBr) we affinity-purified native GABAB receptor complexes and identified their constituents using tandem mass-spectrometry. We found that GABAB receptors not only comprise principal GABAB1a, GABAB1b and GABAB2 subunits (Fig. 1A,C) but also auxiliary KCTD8, 12, 12b and 16 sub- units (Fig. 1B,C). The KCTDs seem to be the missing components that confer fast activation kinetics, variation in the desensitization kinetics and distinct agonist potencies to native GABAB receptor responses. In the presence of KCTD12, GABAB receptor activation elicits a strongly desensitizing response (Fig. 1C). By contrast, in the presence of KCTD16, the activated receptors induce largely non-desensitizing responses (Fig. 1C). We found that distinct KCTD protein domains promote and inhibit receptor-mediated desensitization (Seddik et al, 2012). These differential effects, together with the distinct spatial and temporal KCTD distribution patterns (Metz et al., 2011), support the view that KCTDs contribute to the variation in native GABAB receptor responses (Ivankova et al., 2013). The discovery that KCTDs confer subtype-specificity on GABAB receptors presents opportunities for drug discovery. Indeed, drugs that target individual receptor subtypes would allow more-specific therapeutic interference with GABAB receptor signaling. The advantages of such drugs could include a reduction in side effects as well as entirely new therapeutic applications. To support drug discovery we are analyzing the mechanism of action of the KCTD proteins. Moreover, we are using a combination of knock-down and overexpression strategies to analyze whether GABAB receptor-associated proteins other than the KCTDs influence receptor distribution, neuronal processes and higher brain functions.
In collaboration with L. Lindemann (Roche, Basel) we have identified novel mGlu5 receptor-associated proteins. We are currently characterizing the newly identified proteins for their effects on mGlu5 receptor functions in vitro and in vivo.
In collaboration with B. Fakler (University Freiburg iBr.) and C. Lüscher we have been awarded a Sinergia grant from the Swiss National Science Foundation to identify dopamine receptor-associated proteins. We are analyzing several receptor-associated proteins for their effects on dopamine receptor functions in vitro and in vivo.
Trace Amine Receptor 1
In collaboration with M. Höhner (Roche, Basel) we have found a cross- talk between TAAR1 and dopamine receptors (Revel et al., 2011). We are currently studying the underlying mechanism.
Constitutive Notch2 Signaling in Hepatic Tumors and Neural Stem Cells
Hepatocellular carcinoma (HCC) and cholangiocarcinoma (CCC) are the most common liver tumors and a leading cause for cancer-related death in men. Notch2 regulates cellular differentiation in the liver. Notch signaling is implicated in various cancers, but it is unclear whether Notch2 contributes to HCC and CCC formation. We generated mice that ectopically express activated Notch2 in the liver. In collaboration with M. Heim we found that liver-specific expression of Notch2 is sufficient to induce HCC formation and biliary hyperplasia. Using the diethylnitrosamine (DEN) HCC carcinogenesis model, we further showed that Notch2 signaling accelerates DEN-induced HCC formation (Dill et al., 2013). DEN-induced HCCs with constitutive Notch2 signaling exhibit a marked increase in size, and proliferation when compared with HCCs from DEN-induced control mice. Additionally, DEN treated mice constitutively expressing Notch2 eventually develop CCC. Our data establish an oncogenic role for constitutive Notch2 signaling in liver cancer development. In collaboration with A. Merlo (Neurosurgery) and B. Hemmings (FMI Basel) we found that constitutive Notch2 signaling in neural stem cells promotes tumorigenic features and astroglial lin- eage entry in mice (Tchorz et al., 2012).