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

The metabotropic glutamate receptor 1alpha (mGluR1alpha) 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 active 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 mGluR1alpha functionally interacts with both Gq and Gs pathways; a combination of fluorescent indicators showed that glutamate increased intracellular Ca(2+) and cAMP concentration ([Ca(2+)](i) and [cAMP](i)). By contrast, Gd(3+), a different type of ligand whose recognition site on mGluR1alpha is distinct from the glutamate site, increased only [Ca(2+)](i) and the concentration-activation curve was bell-shaped. FRET analysis revealed that a low concentration of Gd(3+) induced dimeric rearrangement of the intracellular domains of mGluR1alpha as does glutamate, whereas a high concentration of Gd(3+) reversed the FRET efficiency, which was consistent with a bell-shaped relationship between concentration and Gq activation. These results suggest that Gd(3+) induces an active and a sort of "inactivated" conformation in mGluR1alpha. The Gd(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(3+)-induced closed-closed/active conformation state preferred Gq to Gs, suggesting that mGluR1alpha serves not only as a simple on/off switch but also as a multiple signaling path regulator.