Differential Activity of Orthosteric Agonists and Allosteric Modulators at Metabotropic Glutamate Receptor 7.

Metabotropic glutamate receptor 7 (mGlu 7 ) is a G protein coupled receptor that has demonstrated promise as a therapeutic target across a number of neurological and psychiatric diseases. Compounds that modulate the activity of mGlu 7 , such as positive and negative allosteric modulators (PAMs and NAMs), may represent new therapeutic strategies to modulate receptor activity. The endogenous neurotransmitter associated with the mGlu receptor family, glutamate, exhibits low efficacy and potency in activating mGlu 7 and surrogate agonists, such as the compound L-AP4, are often used for receptor activation and compound profiling. To understand the implications of the use of such agonists in the development of PAMs, we performed a systematic evaluation of receptor activation using a system in which mutations can be made in either protomer of the mGlu 7 dimer; we employed mutations that prevent interaction with the orthosteric site as well as the G-protein coupling site of the receptor. We then measured increases in calcium levels downstream of a promiscuous G protein to assess the effects of mutations in one of the two protomers in the presence of two different agonists and three positive allosteric modulators. Our results reveal that distinct PAMs, for example VU0422288 and VU6006459, do exhibit different maximal levels of potentiation with L-AP4 versus glutamate, but there appear to be common stable receptor conformations that are shared among all of the compounds examined here. Significance Statement This manuscript describes the systematic evaluation of the mGlu 7 agonists glutamate and L-AP4 in the presence and absence of three distinct potentiators examining possible mechanistic differences. These findings demonstrate that mGlu 7 potentiators display subtle variances in response to glutamate versus L-AP4.