Elucidating the molecular logic of a metabotropic glutamate receptor heterodimer.

Metabotropic glutamate (mGlu) receptor protomers can heterodimerize, leading to different pharmacology compared to their homodimeric counterparts. Here, we use complemented donor-acceptor resonance energy transfer (CODA-RET) technology that distinguishes signaling from defined mGlu heterodimers or homodimers, together with targeted mutagenesis of receptor protomers and computational docking, to elucidate the mechanism of activation and differential pharmacology in mGlu heteromers. We demonstrate that positive allosteric modulators (PAMs) that bind an upper allosteric pocket in the mGlu transmembrane domain are active at both mGlu homomers and mGlu heteromers, while those that bind a lower allosteric pocket within the same domain are efficacious in homomers but not heteromers. We further demonstrate that both protomers of mGlu heteromers are cis-activated by their orthosteric agonists, signaling independently with no trans-activation detected. Intriguingly, however, upper pocket mGlu PAMs enable trans-activation in mGlu heteromers from mGlu to the mGlu protomer and also enhance cis-activation of the mGlu protomer. While mGlu PAMs enhanced mGlu cis-activation in the heterodimer, we were unable to detect trans-activation in the opposite direction from mGlu to the mGlu protomer, suggesting an asymmetry of signaling. These insights into the molecular logic of this receptor heteromer are critical to building toward precision targeted therapies for multiple neuropsychiatric disorders.