Dual signaling is differentially activated by different active states of the metabotropic glutamate receptor 1[alpha]

The metabotropic glutamate receptor 1[alpha] (mGluR1[alpha]) is crucial for some forms of synaptic plasticity, by inducing various cell responses via coupling to various types of G proteins. Upon glutamate binding, an actie conformation, closed-open/active, of the extracellular domain is stabilized, which induces dimeric rearrangement in the intracellular domains, resulting in the initiation of downstream signals. We have confirmed that mGluR1[alpha] functionally interacts with both Gq and Gs pathways; a combination of fluorescent indicators showed that glutamate increased intracellular [Ca.sup.2+] and cAMP concentration ([[[Ca.sup.2+]].sub.i] and [[cAMP].sub.i]). By contrast, [Gd.sup.3+], a different type of ligand whose recognition site on mGluR1[alpha] is distinct from the glutamate site, increased only [[[Ca.sup.2+]].sub.i] and the concentration-activation curve was bell-shaped. FRET analysis revealed that a low concentration of [Gd.sup.3+] induced dimeric rearrangement of the intracellular domains of mGluR1[alpha] as does glutamate, whereas a high concentration of [Gd.sup.3+] reversed the FRET efficiency, which was consistent with a bell-shaped relationship between concentration and Gq activation. These results suggest that [Gd.sup.3+] induces an active and a sort of 'inactivated' conformation in mGluR1[alpha]. The [Gd.sup.3+]-induced active state is considered to correspond to the closed-closed/active conformation, revealed by previous x-ray crystallographic studies. In conclusion, the glutamate-induced closed-open/active state coupled both to Gs and Gq proteins whereas the [Gd.sup.3+]-induced closed-closed/active conformation state preferred Gq to Gs, suggesting that mGluR1[alpha] serves not only as a simple on/off switch but also as a multiple signaling path regulator. FRET | G protein-coupling receptor