Disabling the Gβγ-SNARE interaction disrupts GPCR-mediated presynaptic inhibition, leading to physiological and behavioral phenotypes

Biased Signaling in G Protein-Coupled Receptors: Understanding the Biological Relevance and Tools for Probing Functionally Selective Ligands

Biased signaling has transformed pharmacology by revealing that receptors, particularly G protein-coupled receptors (GPCRs), can activate specific intracellular pathways selectively rather than uniformly. This discovery enables the development of targeted therapeutics that minimize side effects by precisely modulating receptor activity. Functionally selective ligands, which preferentially activate distinct signaling branches, have become essential tools for exploring receptor mechanisms and uncovering the complexities of GPCR signaling. These ligands help clarify receptor function in various physiological and pathological contexts, offering profound implications for therapeutic innovation. GPCRs, which mediate a wide range of cellular responses through coupling to G proteins and arrestins, are key pharmacological targets, with nearly a third of FDA-approved drugs acting on them. Recent advancements in biosensor development, multiplex assay platforms, and deep mutational scanning methods are improving our ability to define GPCR signaling, allowing for a better understanding of biased signaling pathways.

Sex differences in metabolic regulation by Gi/o-coupled receptor modulation of exocytosis

Background

Presynaptic Gi/o coupled GPCRs can act as negative feedback regulators of neurotransmitter release via Gβγ effector modulation through two mechanisms: decreased calcium influx and direct inhibition of membrane fusion by soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor (SNARE). Previously, we discovered that truncation of the last three C-terminal amino acids of SNAP25 (SNAP25Δ3) prevents Gβγ-SNARE interaction, effectively removing the braking mechanism on neurotransmitter release. We have demonstrated enhanced metabolic protection in male SNAP25Δ3/Δ3 mice housed at room temperature (22°C), including increased adipose tissue beiging and glucose uptake and enhanced insulin sensitivity, rendering them resistant to diet-induced obesity (DIO). When male SNAP25Δ3/Δ3 mice were housed at thermoneutrality (30°C), all metabolic protection was abolished, suggesting sympathetic tone is important for the phenotypes.

Methods

We housed male and female mice at either standard room temperature (21°C) or at thermoneutrality (30°C) and fed them a high fat diet (HFD) for 8 weeks. Glucose tolerance tests were performed before and after the 8 weeks of HFD along with body composition analyses. Organs were then dissected for mass analysis as well as immunohistochemistry. Additionally, we ovariectomized female mice to investigate the role of sex hormones in our phenotypes. Finally, we housed mice in Sable Promethion chambers at various environmental temperatures to investigate the effect of environmental temperature on basal metabolic rates.

Results

We found SNAP25Δ3/Δ3 female mice exhibited the same metabolic protection at RT (22°C) and displayed enhanced metabolic protection from DIO compared to standard chow just as males did. However, female SNAP25Δ3/Δ3 mice display persistent metabolic protection even when housed at thermoneutrality. In this study, we investigate the mechanisms behind this sex dependent persistent phenotype. Thermoneutral set point did not differ between sexes nor genotype, suggesting that metabolic protection is not due to a difference in hypothalamic temperature regulation. Metabolic protection in SNAP25Δ3/Δ3 persisted in ovariectomized mice despite increased weight gain compared to mice receiving sham operations.

Conclusion

This study has identified that there is not a sex-dependent difference for thermoneutral set point in mice. Additionally, there is a sex hormone independent mechanism driving the persistent metabolic protection of female SNAP25Δ3/Δ3 mice housed in thermoneutrality.

Chemogenetic silencing reveals presynaptic Gi/o protein-mediated inhibition of developing hippocampal synchrony in vivo

Recent advances in understanding how neuronal activity shapes developing brain circuits increasingly rely on Gi/o-dependent inhibitory chemogenetic tools (Gi-DREADDs). However, their mechanisms of action and efficacy in neurons with immature Gi/o signaling are elusive. Here, we express the Gi-DREADD hM4Di in glutamatergic telencephalic neurons and analyze its impact on CA1 pyramidal neurons in neonatal mice. Using acousto-optic two-photon Ca2+ imaging, we report that activation of hM4Di leads to a complete arrest of spontaneous synchrony in CA1 in vitro. We demonstrate that hM4Di does not cause somatic hyperpolarization or shunting but rather mediates presynaptic silencing of glutamatergic neurotransmission. In vivo, inhibition through hM4Di potently suppresses early sharp waves (eSPWs) and discontinuous oscillatory network activity in CA1 of head-fixed mice before eye opening. Our findings provide insights into the role of Gi/o signaling in synchronized activity in the neonatal hippocampus and bear relevance for applying chemogenetic silencing at early developmental stages.

Emerging modes of regulation of neuromodulatory G protein-coupled receptors

In the nervous system, G protein-coupled receptors (GPCRs) control neuronal excitability, synaptic transmission, synaptic plasticity, and, ultimately, behavior through spatiotemporally precise initiation of a variety of signaling pathways. However, despite their critical importance, there is incomplete understanding of how these receptors are regulated to tune their signaling to specific neurophysiological contexts. A deeper mechanistic picture of neuromodulatory GPCR function is needed to fully decipher their biological roles and effectively harness them for the treatment of neurological and psychiatric disorders. In this review, we highlight recent progress in identifying novel modes of regulation of neuromodulatory GPCRs, including G protein- and receptor-targeting mechanisms, receptor-receptor crosstalk, and unique features that emerge in the context of chemical synapses. These emerging principles of neuromodulatory GPCR tuning raise critical questions to be tackled at the molecular, cellular, synaptic, and neural circuit levels in the future.

GPCR-mediated regulation of beige adipocyte formation: Implications for obesity and metabolic health

Obesity and its associated complications pose a significant burden on health. The non-shivering thermogenesis (NST) and metabolic capacity properties of brown adipose tissue (BAT), which are distinct from those of white adipose tissue (WAT), in combating obesity and its related metabolic diseases has been well documented. However, beige adipose tissue, the third and relatively novel type of adipose tissue, which emerges in extensive presence of WAT and shares similar favorable metabolic properties with BAT, has garnered considerable attention in recent years. In this review, we focused on the role of G protein-coupled receptors (GPCRs), the largest receptor family and the most successful class of drug targets in humans, in the induction of beige adipocytes. More importantly, we highlight researchers' clinical treatment attempts to ameliorate obesity and other related metabolic diseases through the formation and activation of beige adipose tissue. In summary, this review provides valuable insights into the formation of beige adipose tissue and the involvement of GPCRs, based on the latest advancements in scientific research.

The Mechanism of α2 adrenoreceptor-dependent Modulation of Neurotransmitter Release at the Neuromuscular Junctions

α2-Adrenoreceptors (ARs) are main Gi-protein coupled autoreceptors in sympathetic nerve terminals and targets for dexmedetomidine (DEX), a widely used sedative. We hypothesize that α2-ARs are also potent regulators of neuromuscular transmission via G protein-gated inwardly rectifying potassium (GIRK) channels. Using extracellular microelectrode recording of postsynaptic potentials, we found DEX-induced inhibition of spontaneous and evoked neurotransmitter release as well as desynchronization of evoked exocytotic events in the mouse diaphragm neuromuscular junction. These effects were suppressed by SKF-86,466, a selective α2-AR antagonist. An activator of GIRK channels ML297 had the same effects on neurotransmitter release as DEX. By contrast, inhibition of GIRK channels with tertiapin-Q prevented the action of DEX on evoked neurotransmitter release, but not on spontaneous exocytosis. The synaptic vesicle exocytosis is strongly dependent on Ca2+ influx through voltage-gated Ca2+ channels (VGCCs), which can be negatively regulated via α2-AR - GIRK channel axis. Indeed, inhibition of P/Q-, L-, N- or R-type VGCCs prevented the inhibitory action of DEX on evoked neurotransmitter release; antagonists of P/Q- and N-type channels also suppressed the DEX-mediated desynchronization of evoked exocytotic events. Furthermore, inhibition of P/Q-, L- or N-type VGCCs precluded the frequency decrease of spontaneous exocytosis upon DEX application. Thus, α2-ARs acting via GIRK channels and VGCCs (mainly, P/Q- and N-types) exert inhibitory effect on the neuromuscular communication by attenuating and desynchronizing evoked exocytosis. In addition, α2-ARs can suppress spontaneous exocytosis through GIRK channel-independent, but VGCC-dependent pathway.

Gβγ-SNAP25 exocytotic brake removal enhances insulin action, promotes adipocyte browning, and protects against diet-induced obesity

Negative regulation of exocytosis from secretory cells is accomplished through inhibitory signals from Gi/o GPCRs by Gβγ subunit inhibition of 2 mechanisms: decreased calcium entry and direct interaction of Gβγ with soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor (SNARE) plasma membrane fusion machinery. Previously, we disabled the second mechanism with a SNAP25 truncation (SNAP25Δ3) that decreased Gβγ affinity for the SNARE complex, leaving exocytotic fusion and modulation of calcium entry intact and removing GPCR-Gβγ inhibition of SNARE-mediated exocytosis. Here, we report substantial metabolic benefit in mice carrying this mutation. Snap25Δ3/Δ3 mice exhibited enhanced insulin sensitivity and beiging of white fat. Metabolic protection was amplified in Snap25Δ3/Δ3 mice challenged with a high-fat diet. Glucose homeostasis, whole-body insulin action, and insulin-mediated glucose uptake into white adipose tissue were improved along with resistance to diet-induced obesity. Metabolic protection in Snap25Δ3/Δ3 mice occurred without compromising the physiological response to fasting or cold. All metabolic phenotypes were reversed at thermoneutrality, suggesting that basal autonomic activity was required. Direct electrode stimulation of sympathetic neuron exocytosis from Snap25Δ3/Δ3 inguinal adipose depots resulted in enhanced and prolonged norepinephrine release. Thus, the Gβγ-SNARE interaction represents a cellular mechanism that deserves further exploration as an additional avenue for combating metabolic disease.

Vesicle-associated membrane protein 2 is a cargo-selective v-SNARE for a subset of GPCRs

Vesicle fusion at the plasma membrane is critical for releasing hormones and neurotransmitters and for delivering the cognate G protein-coupled receptors (GPCRs) to the cell surface. The SNARE fusion machinery that releases neurotransmitters has been well characterized. In contrast, the fusion machinery that delivers GPCRs is still unknown. Here, using high-speed multichannel imaging to simultaneously visualize receptors and v-SNAREs in real time in individual fusion events, we identify VAMP2 as a selective v-SNARE for GPCR delivery. VAMP2 was preferentially enriched in vesicles that mediate the surface delivery of μ opioid receptor (MOR), but not other cargos, and was required selectively for MOR recycling. Interestingly, VAMP2 did not show preferential localization on MOR-containing endosomes, suggesting that v-SNAREs are copackaged with specific cargo into separate vesicles from the same endosomes. Together, our results identify VAMP2 as a cargo-selective v-SNARE and suggest that surface delivery of specific GPCRs is mediated by distinct fusion events driven by distinct SNARE complexes.

SNAP25 differentially contributes to Gi/o-coupled receptor function at glutamatergic synapses in the nucleus accumbens

The nucleus accumbens (NAc) guides reward-related motivated behavior implicated in pathological behavioral states, including addiction and depression. These behaviors depend on the precise neuromodulatory actions of Gi/o-coupled G-protein-coupled receptors (GPCRs) at glutamatergic synapses onto medium spiny projection neurons (MSNs). Previous work has shown that discrete classes of Gi/o-coupled GPCR mobilize Gβγ to inhibit vesicular neurotransmitter release via t-SNARE protein, SNAP25. However, it remains unknown which Gαi/o systems in the NAc utilize Gβγ-SNARE signaling to dampen glutamatergic transmission. Utilizing patch-clamp electrophysiology and pharmacology in a transgenic mouse line with a C-terminal three-residue deletion of SNAP25 (SNAP25Δ3) weaking the Gβγ-SNARE interaction, we surveyed a broad cohort of Gi/o-coupled GPCRs with robust inhibitory actions at glutamatergic synapses in the NAc. We find that basal presynaptic glutamate release probability is reduced in SNAP25Δ3 mice. While κ opioid, CB1, adenosine A1, group II metabotropic glutamate receptors, and histamine H3 receptors inhibit glutamatergic transmission onto MSNs independent of SNAP25, we report that SNAP25 contributes significantly to the actions of GABAB, 5-HT1B/D, and μ opioid receptors. These findings demonstrate that presynaptic Gi/o-coupled GPCRs recruit heterogenous effector mechanisms at glutamatergic synapses in the NAc, with a subset requiring SNA25-dependent Gβγ signaling.

Key differences in regulation of opioid receptors localized to presynaptic terminals compared to somas: Relevance for novel therapeutics

Opioid receptors are G protein-coupled receptors (GPCRs) that regulate activity within peripheral, subcortical and cortical circuits involved in pain, reward, and aversion processing. Opioid receptors are expressed in both presynaptic terminals where they inhibit neurotransmitter release and postsynaptic locations where they act to hyperpolarize neurons and reduce activity. Agonist activation of postsynaptic receptors at the plasma membrane signal via ion channels or cytoplasmic second messengers. Agonist binding initiates regulatory processes that include phosphorylation by G protein receptor kinases (GRKs) and recruitment of beta-arrestins that desensitize and internalize the receptors. Opioid receptors also couple to effectors from endosomes activating intracellular enzymes and kinases. In contrast to postsynaptic opioid receptors, receptors localized to presynaptic terminals are resistant to desensitization such that there is no loss of signaling in the continuous presence of opioids over the same time scale. Thus, the balance of opioid signaling in circuits expressing pre- and postsynaptic opioid receptors is shifted toward inhibition of presynaptic neurotransmitter release during continuous opioid exposure. The functional implication of this shift is not often acknowledged in behavioral studies. This review covers what is currently understood about regulation of opioid/nociceptin receptors, with an emphasis on opioid receptor signaling in pain and reward circuits. Importantly, the review covers regulation of presynaptic receptors and the critical gaps in understanding this area, as well as the opportunities to further understand opioid signaling in brain circuits. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Distinct mechanisms of CB1 and GABAB receptor presynaptic modulation of striatal indirect pathway projections to mouse globus pallidus

Presynaptic modulation is a fundamental process regulating synaptic transmission. Striatal indirect pathway projections originate from A2A-expressing spiny projection neurons (iSPNs), targeting the globus pallidus external segment (GPe) and control the firing of the tonically active GPe neurons via GABA release. It is unclear if and how the presynaptic G-protein-coupled receptors (GPCRs), GABAB and CB1 receptors modulate iSPN-GPe projections. Here we used an optogenetic platform to study presynaptic Ca2+ and GABAergic transmission at iSPN projections, using a genetic strategy to express the calcium sensor GCaMP6f or the excitatory channelrhodopsin (hChR2) on iSPNs. We found that P/Q-type calcium channels are the primary voltage-gated Ca2+ channel (VGCC) subtype controlling presynaptic calcium and GABA release at iSPN-GPe projections. N-type and L-type VGCCs also contribute to GABA release at iSPN-GPe synapses. GABAB receptor activation resulted in a reversible inhibition of presynaptic Ca2+ transients (PreCaTs) and an inhibition of GABAergic transmission at iSPN-GPe synapses. CB1 receptor activation did not inhibit PreCaTs but inhibited GABAergic transmission at iSPN-GPe projections. CB1 effects on GABAergic transmission persisted in experiments where NaV and KV 1 were blocked, indicating a VGCC- and KV 1-independent presynaptic mechanism of action of CB1 receptors. Taken together, presynaptic modulation of iSPN-GPe projections by CB1 and GABAB receptors is mediated by distinct mechanisms. KEY POINTS: P/Q-type are the predominant voltage-gated Ca2+ channels controlling presynaptic Ca2+ and GABA release on the striatal indirect pathway projections. GABAB receptors modulate iSPN-GPe projections via a VGCC-dependent mechanism. CB1 receptors modulate iSPN-GPe projections via a VGCC-independent mechanism.

Presynaptic mechanisms underlying GABAB-receptor-mediated inhibition of spontaneous neurotransmitter release

Inhibition of neurotransmitter release by neurotransmitter substances constitutes a fundamental means of neuromodulation. In contrast to well-delineated mechanisms that underlie inhibition of evoked release via suppression of voltage-gated Ca²⁺ channels, processes that underlie neuromodulatory inhibition of spontaneous release remain unclear. Here, we interrogated inhibition of spontaneous glutamate and GABA release by presynaptic metabotropic GABA_B receptors. Our findings show that this inhibition relies on Gβγ subunit action at the membrane, and it is largely independent of presynaptic Ca²⁺ signaling for both forms of release. In the case of spontaneous glutamate release, inhibition requires Gβγ interaction with the C terminus of the key fusion machinery component SNAP25, and it is modulated by synaptotagmin-1. Inhibition of spontaneous GABA release, on the other hand, is independent of these pathways and likely requires alternative Gβγ targets at the presynaptic terminal.

Specificities of Gβγ subunits for the SNARE complex before and after stimulation of α2a-adrenergic receptors

Ligand binding to G protein–coupled receptors (GPCRs), such as the α2a-adrenergic receptor (α2aAR), results in the activation of heterotrimeric G proteins, which consist of functionally distinct Gα subunits and Gβγ dimers. α2aAR-dependent inhibition of synaptic transmission regulates functions such as spontaneous locomotor activity, anesthetic sparing, and working memory enhancement and requires the soluble NSF attachment protein receptor (SNARE) complex, a Gβγ effector. To understand how the Gβγ-SNARE complex underlies the α2aAR-dependent inhibition of synaptic transmission, we examined the specificity of Gβγ subunits for the SNARE complex in adrenergic neurons, in which auto-α2aARs respond to epinephrine released from these neurons, and nonadrenergic neurons, in which hetero-α2aARs respond to epinephrine released from other neurons. We performed a quantitative, targeted multiple reaction monitoring proteomic analysis of Gβ and Gγ subunits bound to the SNARE complex in synaptosomes from mouse brains. In the absence of stimulation of auto-α2aARs, Gβ1 and Gγ3 interacted with the SNARE complex. However, Gβ1, Gβ2, and Gγ3 were found in the complex when auto-α2aARs were activated by epinephrine. Further understanding of the specific usage of distinct Gβγ subunits in vivo may provide insights into the homeostatic regulation of synaptic transmission and the mechanisms of dysfunction that occur in neurological diseases.

Convergent, functionally independent signaling by mu and delta opioid receptors in hippocampal parvalbumin interneurons

Functional interactions between G protein-coupled receptors are poised to enhance neuronal sensitivity to neuromodulators and therapeutic drugs. Mu and Delta opioid receptors (MORs and DORs) can interact when overexpressed in the same cells, but whether co-expression of endogenous MORs and DORs in neurons leads to functional interactions is unclear. Here, in mice, we show that both MORs and DORs inhibit parvalbumin-expressing basket cells (PV-BCs) in hippocampal CA1 through partially occlusive signaling pathways that terminate on somato-dendritic potassium channels and presynaptic calcium channels. Using photoactivatable opioid neuropeptides, we find that DORs dominate the response to enkephalin in terms of both ligand-sensitivity and kinetics, which may be due to relatively low expression levels of MOR. Opioid-activated potassium channels do not show heterologous desensitization, indicating that MORs and DORs signal independently. In a direct test for heteromeric functional interactions, the DOR antagonist TIPP-Psi does not alter the kinetics or potency of either the potassium channel or synaptic responses to photorelease of the MOR agonist DAMGO. Thus, aside from largely redundant and convergent signaling, MORs and DORs do not functionally interact in PV-BCs in a way that impacts somato-dendritic potassium currents or synaptic transmission. These findings imply that crosstalk between MORs and DORs, either in the form of physical interactions or synergistic intracellular signaling, is not a preordained outcome of co-expression in neurons.

Mice Expressing Regulators of G protein Signaling-insensitive Gαo Define Roles of μ Opioid Receptor Gαo and Gαi Subunit Coupling in Inhibition of Presynaptic GABA Release

Regulators of G protein signaling (RGS) proteins modulate signaling by G protein-coupled receptors. Using a knock-in transgenic mouse model with a mutation in Gαo that does not bind RGS proteins (RGS-insensitive), we determined the effect of RGS proteins on presynaptic μ opioid receptor (MOR)-mediated inhibition of GABA release in the ventrolateral periaqueductal gray (vlPAG). The MOR agonists [d-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) and met-enkephalin (ME) inhibited evoked inhibitory postsynaptic currents (eIPSCs) in the RGS-insensitive mice compared with wild-type (WT) littermates, respectively. Fentanyl inhibited eIPSCs similarly in both WT and RGS-insensitive mice. There were no differences in opioid agonist inhibition of spontaneous GABA release between the genotypes. To further probe the mechanism underlying these differences between opioid inhibition of evoked and spontaneous GABA release, specific myristoylated Gα peptide inhibitors for Gαo1 and Gαi1-3 that block receptor-G protein interactions were used to test the preference of agonists for MOR-Gα complexes. The Gαo1 inhibitor reduced DAMGO inhibition of eIPSCs, but Gαi1-3 inhibitors had no effect. Both Gαo1 and Gαi1-3 inhibitors separately reduced fentanyl inhibition of eIPSCs but had no effects on ME inhibition. Gαi1-3 inhibitors blocked the inhibitory effects of ME and fentanyl on miniature postsynaptic current (mIPSC) frequency, but both Gαo1 and Gαi1-3 inhibitors were needed to block the effects of DAMGO. Finally, baclofen-mediated inhibition of GABA release is unaffected in the RGS-insensitive mice and in the presence of Gαo1 and Gαi1-3 inhibitor peptides, suggesting that GABAB receptor coupling to G proteins in vlPAG presynaptic terminals is different than MOR coupling. SIGNIFICANCE STATEMENT: Presynaptic μ opioid receptors (MORs) in the ventrolateral periaqueductal gray are critical for opioid analgesia and are negatively regulated by RGS proteins. These data in RGS-insensitive mice provide evidence that MOR agonists differ in preference for Gαo versus Gαi and regulation by RGS proteins in presynaptic terminals, providing a mechanism for functional selectivity between agonists. The results further define important differences in MOR and GABAB receptor coupling to G proteins that could be exploited for new pain therapies.

Modulation of Neural Microcircuits That Control Complex Dynamics in Olfactory Networks

Neuromodulation influences neuronal processing, conferring neuronal circuits the flexibility to integrate sensory inputs with behavioral states and the ability to adapt to a continuously changing environment. In this original research report, we broadly discuss the basis of neuromodulation that is known to regulate intrinsic firing activity, synaptic communication, and voltage-dependent channels in the olfactory bulb. Because the olfactory system is positioned to integrate sensory inputs with information regarding the internal chemical and behavioral state of an animal, how olfactory information is modulated provides flexibility in coding and behavioral output. Herein we discuss how neuronal microcircuits control complex dynamics of the olfactory networks by homing in on a special class of local interneurons as an example. While receptors for neuromodulation and metabolic peptides are widely expressed in the olfactory circuitry, centrifugal serotonergic and cholinergic inputs modulate glomerular activity and are involved in odor investigation and odor-dependent learning. Little is known about how metabolic peptides and neuromodulators control specific neuronal subpopulations. There is a microcircuit between mitral cells and interneurons that is comprised of deep-short-axon cells in the granule cell layer. These local interneurons express pre-pro-glucagon (PPG) and regulate mitral cell activity, but it is unknown what initiates this type of regulation. Our study investigates the means by which PPG neurons could be recruited by classical neuromodulators and hormonal peptides. We found that two gut hormones, leptin and cholecystokinin, differentially modulate PPG neurons. Cholecystokinin reduces or increases spike frequency, suggesting a heterogeneous signaling pathway in different PPG neurons, while leptin does not affect PPG neuronal firing. Acetylcholine modulates PPG neurons by increasing the spike frequency and eliciting bursts of action potentials, while serotonin does not affect PPG neuron excitability. The mechanisms behind this diverse modulation are not known, however, these results clearly indicate a complex interplay of metabolic signaling molecules and neuromodulators that may fine-tune neuronal microcircuits.

A photoswitchable GPCR-based opsin for presynaptic inhibition

Optical manipulations of genetically defined cell types have generated significant insights into the dynamics of neural circuits. While optogenetic activation has been relatively straightforward, rapid and reversible synaptic inhibition has proven more elusive. Here, we leveraged the natural ability of inhibitory presynaptic GPCRs to suppress synaptic transmission and characterize parapinopsin (PPO) as a GPCR-based opsin for terminal inhibition. PPO is a photoswitchable opsin that couples to Gi/o signaling cascades and is rapidly activated by pulsed blue light, switched off with amber light, and effective for repeated, prolonged, and reversible inhibition. PPO rapidly and reversibly inhibits glutamate, GABA, and dopamine release at presynaptic terminals. Furthermore, PPO alters reward behaviors in a time-locked and reversible manner in vivo. These results demonstrate that PPO fills a significant gap in the neuroscience toolkit for rapid and reversible synaptic inhibition and has broad utility for spatiotemporal control of inhibitory GPCR signaling cascades.

Biased ligands at opioid receptors: Current status and future directions

The opioid crisis represents a major worldwide public health crisis that has accelerated the search for safer and more effective opioids. Over the past few years, the identification of biased opioid ligands capable of eliciting selective functional responses has provided an alternative avenue to develop novel therapeutics without the side effects of current opioid medications. However, whether biased agonism or other pharmacological properties, such as partial agonism (or low efficacy), account for the therapeutic benefits remains questionable. Here, we provide a summary of the current status of biased opioid ligands that target the μ- and κ-opioid receptors and highlight advances in preclinical and clinical trials of some of these ligands. We also discuss an example of structure-based biased ligand discovery at the μ-opioid receptor, an approach that could revolutionize drug discovery at opioid and other receptors. Last, we briefly discuss caveats and future directions for this important area of research.

Sexual Dimorphism in Stress-induced Hyperthermia in SNAP25Δ3 mice, a mouse model with disabled Gβγ regulation of the exocytotic fusion apparatus

Behavioral assays in the mouse can show marked differences between male and female animals of a given genotype. These differences identified in such preclinical studies may have important clinical implications. We recently made a mouse model with impaired presynaptic inhibition through Gβγ-SNARE signaling. Here, we examine the role of sexual dimorphism in the severity of the phenotypes of this model, the SNAP25Δ3 mouse. In males, we already reported that SNAP25Δ3 homozygotes demonstrated phenotypes in motor coordination, nociception, spatial memory and stress processing. We now report that while minimal sexually dimorphic effects were observed for the nociceptive, motor or memory phenotypes, large differences were observed in the stress-induced hyperthermia paradigm, with male SNAP25Δ3 homozygotes exhibiting an increase in body temperature subsequent to handling relative to wild-type littermates, while no such genotype-dependent effect was observed in females. This suggests sexually dimorphic mechanisms of Gβγ-SNARE signaling for stress processing or thermoregulation within the mouse. Second, we examined the effects of heterozygosity with respect to the SNAP25Δ3 mutation. Heterozygote SNAP25Δ3 animals were tested alongside homozygote and wild-type littermates in all of the aforementioned paradigms and displayed phenotypes similar to wild-type animals or an intermediate state. From this, we conclude that the SNAP25Δ3 mutation does not behave in an autosomal dominant manner, but rather displays incomplete dominance for many phenotypes.

The regulatory G protein signaling complex, Gβ5-R7, promotes glucose- and extracellular signal-stimulated insulin secretion

G protein-coupled receptors (GPCRs) are important modulators of glucose-stimulated insulin secretion, essential for maintaining energy homeostasis. Here we investigated the role of Gβ5-R7, a protein complex consisting of the atypical G protein β subunit Gβ5 and a regulator of G protein signaling of the R7 family. Using the mouse insulinoma MIN6 cell line and pancreatic islets, we investigated the effects of G protein subunit β 5 (Gnb5) knockout on insulin secretion. Consistent with previous work, Gnb5 knockout diminished insulin secretion evoked by the muscarinic cholinergic agonist Oxo-M. We found that the Gnb5 knockout also attenuated the activity of other GPCR agonists, including ADP, arginine vasopressin, glucagon-like peptide 1, and forskolin, and, surprisingly, the response to high glucose. Experiments with MIN6 cells cultured at different densities provided evidence that Gnb5 knockout eliminated the stimulatory effect of cell adhesion on Oxo-M-stimulated glucose-stimulated insulin secretion; this effect likely involved the adhesion GPCR GPR56. Gnb5 knockout did not influence cortical actin depolymerization but affected protein kinase C activity and the 14-3-3ϵ substrate. Importantly, Gnb5-/- islets or MIN6 cells had normal total insulin content and released normal insulin amounts in response to K+-evoked membrane depolarization. These results indicate that Gβ5-R7 plays a role in the insulin secretory pathway downstream of signaling via all GPCRs and glucose. We propose that the Gβ5-R7 complex regulates a phosphorylation event participating in the vesicular trafficking pathway downstream of G protein signaling and actin depolymerization but upstream of insulin granule release.

Disabling Gβγ-SNAP-25 interaction in gene-targeted mice results in enhancement of long-term potentiation at Schaffer collateral-CA1 synapses in the hippocampus

Three SNARE proteins, SNAP-25, syntaxin 1A, and VAMP2 or synaptobrevin 2, constitute the minimal functional machinery needed for the regulated secretion of neurotransmitters. Dynamic changes in the regulated release of neurotransmitters are associated with the induction of long-term plasticity at central synapses. In-vitro studies have validated the C-terminus of SNAP-25 as a target for inhibitory Gi/o-coupled G-protein coupled receptors at a number of synapses. The physiological consequences of the interaction between Gi/o proteins and SNAP-25 in the context of activity-dependent long-term synaptic plasticity are not well understood. Here, we report direct ex-vivo evidence of the involvement of the C-terminus of SNAP-25 in inducing long-term potentiation of synaptic strength at Schaffer collateral-CA1 synapses using a gene-targeted mouse model with truncated C-terminus (carboxyl terminus) of SNAP-25. It has been shown previously that truncation of the three extreme C-terminal residues in SNAP-25[INCREMENT]3 homozygote mice reduces its interaction with the inhibitory Gβγ subunits two-fold. In in-vitro hippocampal slices, we show that these SNAP-25[INCREMENT]3 mice express significantly larger magnitude of long-term potentiation at hippocampal Schaffer collateral-CA1 synapses.

Developmental Time Course of SNAP-25 Isoforms Regulate Hippocampal Long-Term Synaptic Plasticity and Hippocampus-Dependent Learning

SNAP-25 is essential to activity-dependent vesicle fusion and neurotransmitter release in the nervous system. During early development and adulthood, SNAP-25 appears to have differential influences on short- and long-term synaptic plasticity. The involvement of SNAP-25 in these processes may be different at hippocampal and neocortical synapses because of the presence of two different splice variants, which are developmentally regulated. We show here that the isoform SNAP-25a, which is expressed first developmentally in rodent brain, contributes to developmental regulation of the expression of both long-term depression (LTD) and long-term potentiation (LTP) at Schaffer collateral-CA1 synapses in the hippocampus. In one month old mice lacking the developmentally later expressed isoform SNAP-25b, Schaffer collateral-CA1 synapses showed faster release kinetics, decreased LTP and enhanced LTD. By four months of age, SNAP-25b-deficient mice appeared to have compensated for the lack of the adult SNAP-25b isoform, now exhibiting larger LTP and no differences in LTD compared to wild type mice. Interestingly, learning a hippocampus-dependent task reversed the reductions in LTP, but not LTD, seen at one month of age. In four month old adult mice, learning prevented the compensatory up-regulation of LTD that we observed prior to training. These findings support the hypothesis that SNAP-25b promotes stronger LTP and weakens LTD at Schaffer collateral-CA1 synapses in young mice, and suggest that compensatory mechanisms can reverse alterations in synaptic plasticity associated with a lack of SNAP-25b, once mice reach adulthood.

CD13 regulation of membrane recycling: implications for cancer dissemination

Membrane recycling is critical to numerous cell functions and its dysregulation contributes to cancer and metastasis. We established that activation of the transmembrane molecule aminopeptidase N (ANPEP, also known as CD13) tethers the IQ motif containing, guanosine triphosphate hydrolase activating protein 1 (IQGAP1) scaffolding protein at the plasma membrane, thus stimulating the recycling regulator ADP-ribosylation factor 6 (ARF6) to ensure proper recycling of β1-integrin and other membrane components impacting cell attachment.

Heterosynaptic GABAB Receptor Function within Feedforward Microcircuits Gates Glutamatergic Transmission in the Nucleus Accumbens Core

Complex circuit interactions within the nucleus accumbens (NAc) facilitate goal-directed behavior. Medium spiny neurons (MSNs) mediate NAc output by projecting to functionally divergent brain regions, a property conferred, in part, by the differential projection patterns of D1- and D2 dopamine receptor-expressing MSNs. Glutamatergic afferents to the NAc direct MSN output by recruiting feedforward inhibitory microcircuits comprised of parvalbumin (PV)-expressing interneurons (INs). Furthermore, the GABA_B heteroreceptor (GABA_BR), a G_i/o-coupled G-protein-coupled receptor, is expressed at glutamatergic synapses throughout the mesolimbic network, yet its physiological context and synaptic mechanism within the NAc remains unknown. Here, we explored GABA_BR function at glutamatergic synapses within PV-IN-embedded microcircuits in the NAc core of male mice. We found that GABA_BR is expressed presynaptically and recruits a noncanonical signaling mechanism to reduce glutamatergic synaptic efficacy at D1(+) and D1(-) (putative D2) MSN subtypes. Furthermore, PV-INs, a robust source of neuronal GABA in the NAc, heterosynaptically target GABA_BR to selectively modulate glutamatergic transmission onto D1(+) MSNs. These findings elucidate a new mechanism of feedforward inhibition and refine mechanisms by which GABA_B heteroreceptors modulate mesolimbic circuit function.SIGNIFICANCE STATEMENT Glutamatergic transmission in the nucleus accumbens (NAc) critically contributes to goal-directed behaviors. However, intrinsic microcircuit mechanisms governing the integration of these synapses remain largely unknown. Here, we show that parvalbumin-expressing interneurons within feedforward microcircuits heterosynaptically target GABA_B heteroreceptors (GABA_BR) on glutamate terminals. Activation of presynaptically-expressed GABA_BR decreases glutamatergic synaptic strength by engaging a non-canonical signaling pathway that interferes with vesicular exocytotic release machinery. These findings offer mechanistic insight into the role of GABA_B heteroreceptors within reward circuitry, elucidate a novel arm to feedforward inhibitory networks, and inform the growing use of GABA_BR-selective pharmacotherapy for various motivational disorders, including addiction, major depressive disorder, and autism (Cousins et al., 2002; Kahn et al., 2009; Jacobson et al., 2018; Stoppel et al., 2018; Pisansky et al., 2019).

Sp3-cificity of TNF-α expression promotes the Smac mimetic-mediated killing of cancer cells

A genome-wide small-interfering RNA-based screen identified the transcription factor Specificity Protein 3 (SP3) as a critical factor for Second mitochondrial-derived activator of caspase (Smac) mimetic-mediated killing of cancer cells. In concert with Nuclear Factor kappa B (NF-κB,) SP3 is required for the expression of the cytokine Tumor Necrosis Factor alpha (TNF-α) under basal and Smac mimetic-stimulated conditions.

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

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.