While many orthosteric ligands have been developed for the mGlu2 receptor, little is known about their target binding kinetics and how these relate to those of the endogenous agonist glutamate.... Show moreWhile many orthosteric ligands have been developed for the mGlu2 receptor, little is known about their target binding kinetics and how these relate to those of the endogenous agonist glutamate. Here, the kinetic rate constants, i.e. kon and koff, of glutamate were determined for the first time followed by those of the synthetic agonist LY354740 and antagonist LY341495. To increase the understanding of the binding mechanism and impact of allosteric modulation thereon, kinetic experiments were repeated in the presence of allosteric modulators. Functional assays were performed to further study the interplay between the orthosteric and allosteric binding sites, including an impedance-based morphology assay. We found that dissociation rate constants of orthosteric mGlu2 ligands were all within a small 6-fold range, whereas association rate constants were ranging over more than three orders of magnitude and correlated to both affinity and potency. The latter showed that target engagement of orthosteric mGlu2 ligands is kon-driven in vitro. Moreover, only the off-rates of the two agonists were decreased by a positive allosteric modulator (PAM), thereby increasing their affinity. Interestingly, a PAM increased the duration of a glutamate-induced cellular response. A negative allosteric modulator (NAM) increased both on- and off-rate of glutamate without changing its affinity, while it did not affect these parameters for LY354740, indicating probe-dependency. In conclusion, we found that affinity- or potency-based orthosteric ligand optimization primarily results in ligands with high kon values. Moreover, positive allosteric modulators alter the binding kinetics of orthosteric agonists mainly by decreasing koff, which we were able to correlate to a lengthened cellular response. Together, this study shows the importance of studying binding kinetics in early drug discovery, as this may provide important insights towards improved efficacy in vivo. Show less
Doornbos, M.L.J.; Linden, I. van der; Vereyken, L.; Tresadern, G.; IJzerman, A.P.; Lavreysen, H.; Heitman, L.H. 2018
Label-free cellular assays using a biosensor provide new opportunities for studying G protein-coupled receptor (GPCR) signaling. As opposed to conventional in vitro assays, integrated receptor... Show more Label-free cellular assays using a biosensor provide new opportunities for studying G protein-coupled receptor (GPCR) signaling. As opposed to conventional in vitro assays, integrated receptor-mediated cellular responses are determined in real-time rather than a single downstream signaling pathway. In this study, we examined the potential of a label-free whole cell impedance-based biosensor system (i.e. xCELLigence) to study the pharmacology of one GPCR in particular, the mGlu2 receptor. This receptor is a target for the treatment of several psychiatric diseases such as schizophrenia and depression. After optimization of assay conditions to prevent interference of endogenous glutamate in the culture medium, detailed pharmacological assessments were performed. Concentration-response curves showed a concentration-dependent increase in impedance for agonists and positive allosteric modulators, whereas receptor inhibition by an antagonist or negative allosteric modulator resulted in a concentration-dependent decrease in cellular impedance. Interestingly, constitutive receptor activity was observed that was decreased by LY341495, which therefore behaved as an inverse agonist here, a property that was heretofore unappreciated. This was confirmed by concentration-dependent modulation of LY341495 potency and efficacy by a allosteric modulators. In summary, the use of the xCELLigence system to study mGlu2 receptor pharmacology was validated. This is the first class C GPCR to be characterized extensively by such method, opening new avenues to study receptor pharmacology including inverse agonism and demonstrating its value for future drug discovery efforts of mGlu receptors as well as other GPCRs. Show less
