Metabotropic glutamate receptor 5 (mGlu5 )-positive allosteric modulators differentially induce or potentiate desensitization of mGlu5 signaling in recombinant cells and neurons

Allosteric modulators of metabotropic glutamate receptor 5 (mGlu₅ ) are a promising therapeutic strategy for a number of neurological disorders. Multiple mGlu₅ -positive allosteric modulator (PAM) chemotypes have been discovered that act as either pure PAMs or as PAM-agonists in recombinant and native cells. While these compounds have been tested in paradigms of receptor activation, their effects on receptor regulatory processes are largely unknown. In this study, acute desensitization of mGlu₅ mediated intracellular calcium mobilization by structurally diverse mGlu₅ orthosteric and allosteric ligands was assessed in human embryonic kidney 293 cells and primary murine neuronal cultures from both striatum and cortex. We aimed to determine the intrinsic efficacy and modulatory capacity of diverse mGlu₅ PAMs [(R)-5-((3-fluorophenyl)ethynyl)-N-(3-hydroxy-3-methylbutan-2-yl)picolinamide (VU0424465), N-cyclobutyl-6-((3-fluorophenyl)ethynyl)picolinamide (VU0360172), 1-(4-(2,4-difluorophenyl)piperazin-1-yl)-2-((4-fluorobenzyl)oxy)ethanone (DPFE), ((4-fluorophenyl) (2-(phenoxymethyl)-6,7-dihydrooxazolo[5,4-c]pyridin-5(4H)-yl)methanone) (VU0409551), 3-Cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB)] on receptor desensitization and whether cellular context influences receptor regulatory processes. Only VU0424465 and VU0409551 induced desensitization alone in human embryonic kidney 293-mGlu₅ cells, while all PAMs enhanced (S)-3,5-dihydroxyphenylglycine (DHPG)-induced desensitization. All mGlu₅ PAMs induced receptor desensitization alone and enhanced DHPG-induced desensitization in striatal neurons. VU0424465 and VU0360172 were the only PAMs that induced desensitization alone in cortical neurons. With the exception of (CDPPB), PAMs enhanced DHPG-induced desensitization in cortical neurons. Moreover, differential apparent affinities, efficacies, and cooperativities with DHPG were observed for VU0360172, VU0409551, and VU0424465 when comparing receptor activation and desensitization in a cell type-dependent manner. These data indicate that biased mGlu₅ allosteric modulator pharmacology extends to receptor regulatory processes in a tissue dependent manner, adding yet another layer of complexity to rational mGlu₅ drug discovery.

Kinetic and system bias as drivers of metabotropic glutamate receptor 5 allosteric modulator pharmacology

Allosteric modulators of the metabotropic glutamate receptor subtype 5 (mGlu₅) have been proposed as potential therapies for various CNS disorders. These ligands bind to sites distinct from the orthosteric (or endogenous) ligand, often with improved subtype selectivity and spatio-temporal control over receptor responses. We recently revealed that mGlu₅ allosteric agonists and positive allosteric modulators exhibit biased agonism and/or modulation. To establish whether negative allosteric modulators (NAMs) engender similar bias, we rigorously characterized the pharmacology of eight diverse mGlu₅ NAMs. Radioligand inhibition binding studies revealed novel modes of interaction with mGlu₅ for select NAMs, with biphasic or incomplete inhibition of the radiolabeled NAM, [³H]methoxy-PEPy. We assessed mGlu₅-mediated intracellular Ca²⁺ (iCa²⁺) mobilization and inositol phosphate (IP₁) accumulation in HEK293A cells stably expressing low levels of mGlu₅ (HEK293A-rat mGlu₅-low) and mouse embryonic cortical neurons. The apparent affinity of acetylenic NAMs, MPEP, MTEP and dipraglurant, was dependent on the signaling pathway measured, agonist used, and cell type (HEK293A-rat mGlu₅-low versus mouse cortical neurons). In contrast, the acetylenic partial NAM, M-5MPEP, and structurally distinct NAMs (VU0366248, VU0366058, fenobam), had similar affinity estimates irrespective of the assay or cellular background. Biased modulation was evident for VU0366248 in mouse cortical neurons where it was a NAM for DHPG-mediated iCa²⁺ mobilization, but neutral with DHPG in IP₁ accumulation assays. Overall, this study highlights the inherent complexity in mGlu₅ NAM pharmacology that we hypothesize may influence interpretation when translating into preclinical models and beyond in the design and development of novel therapeutics for neuropsychiatric and neurological disorders.

Structure-Based Design and Discovery of New M2 Receptor Agonists

Muscarinic receptor agonists are characterized by apparently strict restraints on their tertiary or quaternary amine and their distance to an ester or related center. On the basis of the active state crystal structure of the muscarinic M₂ receptor in complex with iperoxo, we explored potential agonists that lacked the highly conserved functionalities of previously known ligands. Using structure-guided pharmacophore design followed by docking, we found two agonists (compounds 3 and 17), out of 19 docked and synthesized compounds, that fit the receptor well and were predicted to form a hydrogen-bond conserved among known agonists. Structural optimization led to compound 28, which was 4-fold more potent than its parent 3. Fortified by the discovery of this new scaffold, we sought a broader range of chemotypes by docking 2.2 million fragments, which revealed another three micromolar agonists unrelated either to 28 or known muscarinics. Even pockets as tightly defined and as deeply studied as that of the muscarinic reveal opportunities for the structure-based design and the discovery of new chemotypes.

β2-adrenoceptor signaling regulates invadopodia formation to enhance tumor cell invasion

Introduction

For efficient metastatic dissemination, tumor cells form invadopodia to degrade and move through three-dimensional extracellular matrix. However, little is known about the conditions that favor invadopodia formation. Here, we investigated the effect of β-adrenoceptor signaling - which allows cells to respond to stress neurotransmitters - on the formation of invadopodia and examined the effect on tumor cell invasion.

Methods

To characterize the molecular and cellular mechanisms of β-adrenergic signaling on the invasive properties of breast cancer cells, we used functional cellular assays to quantify invadopodia formation and to evaluate cell invasion in two-dimensional and three-dimensional environments. The functional significance of β-adrenergic regulation of invadopodia was investigated in an orthotopic mouse model of spontaneous breast cancer metastasis.

Results

β-adrenoceptor activation increased the frequency of invadopodia-positive tumor cells and the number of invadopodia per cell. The effects were selectively mediated by the β₂-adrenoceptor subtype, which signaled through the canonical Src pathway to regulate invadopodia formation. Increased invadopodia occurred at the expense of focal adhesion formation, resulting in a switch to increased tumor cell invasion through three-dimensional extracellular matrix. β₂-adrenoceptor signaling increased invasion of tumor cells from explanted primary tumors through surrounding extracellular matrix, suggesting a possible mechanism for the observed increased spontaneous tumor cell dissemination in vivo. Selective antagonism of β₂-adrenoceptors blocked invadopodia formation, suggesting a pharmacological strategy to prevent tumor cell dissemination.

Conclusion

These findings provide insight into conditions that control tumor cell invasion by identifying signaling through β₂-adrenoceptors as a regulator of invadopodia formation. These findings suggest novel pharmacological strategies for intervention, by using β-blockers to target β₂-adrenoceptors to limit tumor cell dissemination and metastasis.