Membrane depolarization controls mGlu5 receptor activity, as a
NAM.
The scope of this study did not include the identification of the
voltage sensor within the mGlu5 receptor. However, previous studies on
other GPCRs have identified molecular determinants of Vmsensitivity, such as the binding sites of orthosteric or allosteric
ligands, G protein binding domains, or Na+ ions within
the 7TM . These studies mainly concern class A GPCRs, where the
molecular sites mentioned above are located in the 7TM. Unfortunately,
very little is known about the voltage-sensing domains of class C GPCRs,
to which the mGlu5 receptor belongs. Nevertheless, it is reasonable to
assume that the voltage sensor is also located in the 7TM, directly
exposed to the magnetic field. In the atypical structure of class C
GPCRs, the extracellular domains of the receptor (and the orthosteric
binding site) are approximately 10 nm distant from the membrane , out of
the membrane electric field, which excludes their involvement as a
voltage sensor. In contrast, allosteric modulators bind to the 7TM of
the receptor. Interestingly, many studies show that Vmaffects the efficiency or agonist potency of class A GPCRs in a manner
similar to allosteric modulators . Specifically, tyrosine residues
located in the allosteric site are involved in Vmdetection by GPCRs . Of note, the allosteric binding site of the mGlu5
receptor also contains a tyrosine residue (Tyr6593.44) that enables NAM
inhibitory effects .
Functional data collected in this study suggests that membrane
depolarization causes a NAM effect on the mGlu5 receptor. First, NAMs
cause a decrease in agonist efficacy with virtually no changes on
agonist potency , similar to the effect of depolarization on the
dose-response curve (Figure 1E ). Second, mGlu5
receptor-mediated Ca2+ oscillations are known to
undergo frequency reduction by NAMs, such as MPEP, supported by a
decrease in the proportion of oscillating cells . Interestingly, a
similar process is induced by membrane depolarization, namely, a
reduction in frequency when depolarization is applied before or after
the initiation of oscillations, correlated with a decrease in the
proportion of oscillating cells (Figure 2 ). Finally, the
competitive antagonist LY341495 binds to the Venus Flytrap (VFT)
domains, and its binding is favored by membrane depolarization, acting
like a NAM that stabilizes the inactive conformation of the receptor.
Similarly, mGlu4 and mGlu5 allosteric modulators binding to the 7-TM of
the mGlu receptor can drive changes in VFT domain conformation , meaning
that 7-TM modulation by Vm could, as a NAM, be
transduced on VFT domain conformation. Hence, membrane depolarization
would reduce the activity of receptors expressed at the membrane, like a
NAM. These results support to search for the voltage sensor of mGlu5 in
the NAM binding site.
In conclusion, this study has identified mGlu5 receptor as a member of
the small yet expanding group of voltage-sensitive GPCRs. The high
sensitivity of mGlu5 receptor to changes in membrane potential at
synapses suggests that its effects extend beyond canonical signaling and
may even impact its cognate NMDA receptor. Optimal functioning of mGlu5
receptor at hyperpolarized potentials could have far-reaching
consequences, such as restricting its activation to specific locations
by integrating glutamate spill-over and electrical propagation at
synapses. Additionally, mGlu5 receptor may exert cell-type-dependent
actions, conditioned by the resting potential of cells. Finally, given
the crucial role of this receptor in neuronal physiology, it is likely
that its voltage sensitivity plays a critical role in pathological
conditions associated with variations in intrinsic excitability.