Pharmacological characterization of mGlu1 receptors in cerebellar granule cells reveals biased agonism

Abstract

The majority of existing research on the function of metabotropic glutamate (mGlu) receptor 1 focuses on G protein-mediated outcomes. However, similar to other G protein-coupled receptors (GPCR), it is becoming apparent that mGlu1 receptor signaling is multi-dimensional and does not always involve G protein activation. Previously, in transfected CHO cells, we showed that mGlu1 receptors activate a G protein-independent, β-arrestin-dependent signal transduction mechanism and that some mGlu1 receptor ligands were incapable of stimulating this response. Here we set out to investigate the physiological relevance of these findings in a native system using primary cultures of cerebellar granule cells. We tested the ability of a panel of compounds to stimulate two mGlu1 receptor-mediated outcomes: (1) protection from decreased cell viability after withdrawal of trophic support and (2) G protein-mediated phosphoinositide (PI) hydrolysis. We report that the commonly used mGlu1 receptor ligands quisqualate, DHPG, and ACPD are completely biased towards PI hydrolysis and do not induce mGlu1 receptor-stimulated neuroprotection. On the other hand, endogenous compounds including glutamate, aspartate, cysteic acid, cysteine sulfinic acid, and homocysteic acid stimulate both responses. These results show that some commonly used mGlu1 receptor ligands are biased agonists, stimulating only a fraction of mGlu1 receptor-mediated responses in neurons. This emphasizes the importance of utilizing multiple agonists and assays when studying GPCR function.

Transduction of group I mGluR-mediated synaptic plasticity by β-arrestin2 signalling

“…Third, the convergence of G protein- and β-arrestin-mediated pathways on common downstream effectors, especially MAPK and Src kinases, likely obscured contributions from β-arrestin signalling in some experimental models. Fourth, the group I mGluR agonist DHPG appears to preferentially activate G protein signalling while minimally stimulating β-arrestin pathways downstream of mGlu1 receptors3738, which could also account for the divergence between DHPG- and synaptic stimulation-induced effects observed in our studies. It will be of interest to determine what roles β-arrestins have in other forms of G protein-independent signalling by mGluRs that have been described in hippocampal pyramidal cells235, hippocampal interneurons4 and cerebellar interneurons6.…”

“…However, it is also possible that DHPG activation of non-synaptic group I mGluRs led to induction of LTD via a divergence of signalling mechanisms compared with those regulated by PP-LFS. In contrast to DHPG, quisqualate, and ACPD, glutamate and aspartate elicit ‘balanced' G protein and arrestin responses, whereas succinate and glutarate were identified as arrestin-‘biased' agonists3738. We also attempted to test if the putative arrestin-biased agonist succinate altered mf-CA3 EPSCs, but 1 mM succinate application had no effect on mfEPSC amplitudes evoked at basal frequency or on the potentiation of EPSCs induced by PP-LFS (Supplementary Fig.…”

“…Biased signalling by mGlu1 receptors via G-protein or β-arrestin pathways has been proposed to be regulated by ligands via specific interactions within the ligand binding domain37. Whereas selective targeting of mGluR1-β-arrestin1 signalling could be used to regulate cell cycle and potentially treat some forms of cancer3865, targeting of specific mGluR1 signalling modes in the nervous system might have some utility in treating conditions such as cerebellar ataxia66 and chronic pain67. Drugs that interfere selectively with mGluR5-β-arrestin effectors, should those prove to exist, could have potential in therapeutic targeting of Fragile X syndrome68.…”

Biased allosteric agonism and modulation of metabotropic glutamate receptor 5: Implications for optimizing preclinical neuroscience drug discovery

“…Biased agonism offers the opportunity to design drugs that are tailored to activate the desired complement of receptor responses linked to therapeutic outcomes while avoiding those linked to adverse effects. Biased agonism is operative for orthosteric agonists at the related Group I mGlu 1 receptor between G-protein and β-arrestin pathways in both recombinant and natively expressing cells (Emery et al, 2010; Emery et al, 2012; Hathaway et al, 2015). Several studies have suggested that mGlu 5 biased agonism may be linked to distinct physiological responses (Gregory et al, 2013a; Gregory et al, 2012; Noetzel et al, 2013; Zhang et al, 2005).…”

Control of neuronal excitability by Group I metabotropic glutamate receptors

“…Interestingly, in this case, activation of both mGlu 1 and mGlu 5 receptors was required for full activation of the current; when applied separately, both LY367385 (mGlu 1 antagonist) and MPEP (mGlu5 antagonist) significantly reduced the Group I mGlu receptor current. DHPG has been reported to be a biased toward the canonical G-protein/PCL/IP 3 pathway compared to glutamate (Emery et al 2012;Hathaway et al 2015). This may explain some of the differences observed when receptors are stimulated with DHPG versus synaptic glutamate.…”