The expanding roles and mechanisms of G protein-mediated presynaptic inhibition

Abstract

Throughout the past five decades, tremendous advancements have been made in our understanding of G protein signaling and presynaptic inhibition, many of which were published in the Journal of Biological Chemistry under the tenure of Herb Tabor as Editor-in-Chief. Here, we identify these critical advances, including the formulation of the ternary complex model of G protein-coupled receptor signaling and the discovery of Gβγ as a critical signaling component of the heterotrimeric G protein, along with the nature of presynaptic inhibition and its physiological role. We provide an overview for the discovery and physiological relevance of the two known Gβγ-mediated mechanisms for presynaptic inhibition: first, the action of Gβγ on voltage-gated calcium channels to inhibit calcium influx to the presynaptic active zone and, second, the direct binding of Gβγ to the SNARE complex to displace synaptotagmin downstream of calcium entry, which has been demonstrated to be important in neurons and secretory cells. These two mechanisms act in tandem with each other in a synergistic manner to provide more complete spatiotemporal control over neurotransmitter release.

Keywords: G protein; G protein-coupled receptor (GPCR); SNARE proteins; neuroscience; pharmacology.

Figure 1.

Gβγ–SNARE and Gβγ–VGCC act synergistically to inhibit vesicle release. Serotonin (blue) from paracrine sources acts on inhibitory presynaptic 5-HT_1B receptors (blue) to liberate Gβγ to bind SNAREs, displacing the fusogenic calcium sensor synaptotagmin I and inhibiting the release of glutamate (green) This mechanism is distal to and synergistic with GABA_B heteroreceptors, which in the presence of axo-axonic GABA (red), inhibit calcium fluxes into the terminal (75). Class 2/3 mGluRs, acting as autoreceptors, may also signal via Gβγ–SNARE, as has been demonstrated to occur at the synapse between cone photoreceptors and horizontal cells of the inner nuclear layer of the retina (86).

Figure 2.

Synergy between presynaptic 5-HT_1B and GABA_B receptors at the CA1-subicular synapse. A, schematic of recording paradigm within mouse hippocampus for CA1-subicular synapses. At these terminals, 5-HT_1B receptors release Gβγ to bind SNAREs, and GABA_B receptors release Gβγ to inhibit Ca²⁺ channels. B, stimulation of the CA1-subicular pathway evoked whole-cell recorded EPSCs in subicular pyramidal neurons. During repetitive stimulation, CP93129 (400 nm; blue) substantially inhibited the first response, but in subsequent responses, amplitudes recovered almost 5-fold to their original height. The ratio of inhibition of the first versus the fifth response was 4.6 ± 0.8. C, baclofen (1 μm; green) uniformly inhibited EPSCs throughout the stimulus train. The addition of CP93129 in tandem with baclofen (pink) substantially inhibited responses throughout the stimulus train to a greater extent than baclofen alone (***, p = 0.0002). D, bar graph showing the ratio of the amplitudes of the first and fifth stimulation of the effects of CP93129 alone and after the addition of baclofen (1 μm) (112). Error bars represent mean ± S. E. Adapted from Ref. . This research was originally published in Science Signaling. Zurawski et al. Disabling Gβγ SNARE interaction in transgenic mice disrupts GPCR-mediated presynaptic inhibition leading to physiological and behavioral phenotypes. Science Signaling 2019. ©American Association for the Advancement of Science.

Figure 3.

Potential synergy between GPCR antagonists and Gβγ–SNARE inhibitors. Potential mechanism for a synergistic relationship between existing G_i/o-coupled GPCR antagonists (orange) used clinically to inhibit release presynaptically and hypothetical to-be-developed inhibitors of the Gβγ–SNARE interaction (yellow). The usage of two different inhibitors acting on different targets within the same pathway should more potently drive release than either drug used alone and may permit lower concentrations of antagonist to be utilized, minimizing potential side effects from specificity.