Mechanisms of differential desensitization of metabotropic glutamate receptors

Robust GRK2/3/6-dependent desensitization of oxytocin receptor in neurons

Oxytocin plays critical roles in the brain as a neuromodulator, regulating social and other affective behavior. However, the regulatory mechanisms controlling oxytocin receptor (OXTR) signaling in neurons remain unexplored. In this study, we have identified robust and rapid-onset desensitization of OXTR response in multiple regions of the mouse brain. Both cell autonomous spiking response and presynaptic activation undergo similar agonist-induced desensitization. G-protein-coupled receptor kinases (GRK) GRK2, GRK3, and GRK6 are recruited to the activated OXTR in neurons, followed by recruitment of β-arrestin-1 and -2. Neuronal OXTR desensitization was impaired by suppression of GRK2/3/6 kinase activity but remained unaltered with double knockout of β-arrestin-1 and -2. Additionally, we observed robust agonist-induced internalization of neuronal OXTR and its Rab5-dependent recruitment to early endosomes, which was impaired by GRK2/3/6 inhibition. This work defines distinctive aspects of the mechanisms governing OXTR desensitization and internalization in neurons compared to prior studies in heterologous cells.

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.

ArreSTick motif controls β-arrestin-binding stability and extends phosphorylation-dependent β-arrestin interactions to non-receptor proteins

The binding and function of β-arrestins are regulated by specific phosphorylation motifs present in G protein-coupled receptors (GPCRs). However, the exact arrangement of phosphorylated amino acids responsible for establishing a stable interaction remains unclear. We employ a 1D sequence convolution model trained on GPCRs with established β-arrestin-binding properties. With this approach, amino acid motifs characteristic of GPCRs that form stable interactions with β-arrestins can be identified, a pattern that we name "arreSTick." Intriguingly, the arreSTick pattern is also present in numerous non-receptor proteins. Using proximity biotinylation assay and mass spectrometry analysis, we demonstrate that the arreSTick motif controls the interaction between many non-receptor proteins and β-arrestin2. The HIV-1 Tat-specific factor 1 (HTSF1 or HTATSF1), a nuclear transcription factor, contains the arreSTick pattern, and its subcellular localization is influenced by β-arrestin2. Our findings unveil a broader role for β-arrestins in phosphorylation-dependent interactions, extending beyond GPCRs to encompass non-receptor proteins as well.

Control of G protein-coupled receptor function via membrane-interacting intrinsically disordered C-terminal domains

G protein-coupled receptors (GPCRs) control intracellular signaling cascades via agonist-dependent coupling to intracellular transducers including heterotrimeric G proteins, GPCR kinases (GRKs), and arrestins. In addition to their critical interactions with the transmembrane core of active GPCRs, all three classes of transducers have also been reported to interact with receptor C-terminal domains (CTDs). An underexplored aspect of GPCR CTDs is their possible role as lipid sensors given their proximity to the membrane. CTD-membrane interactions have the potential to control the accessibility of key regulatory CTD residues to downstream effectors and transducers. Here we report that the CTDs of two closely related family C GPCRs, metabotropic glutamate receptor 2 (mGluR2) and mGluR3, bind to membranes and that this interaction can regulate receptor function. We first characterize CTD structure with NMR spectroscopy, revealing lipid composition-dependent modes of membrane binding. Using molecular dynamics simulations and structure-guided mutagenesis, we then identify key conserved residues and cancer-associated mutations that modulate CTD-membrane binding. Finally, we provide evidence that mGluR3 transducer coupling is controlled by CTD-membrane interactions in live cells, which may be subject to regulation by CTD phosphorylation and changes in membrane composition. This work reveals a novel mechanism of GPCR modulation, suggesting that CTD-membrane binding may be a general regulatory mode throughout the broad GPCR superfamily.

The protective effects of methylene blue on astrocytic swelling after cerebral ischemia-reperfusion injuries are mediated by Aquaporin-4 and metabotropic glutamate receptor 5 activation

Methylene blue (MB) was found to exert neuroprotective effect on different brain diseases, such as ischemic stroke. This study assessed the MB effects on ischemia induced brain edema and its role in the inhibition of aquaporin 4 (AQP4) and metabotropic glutamate receptor 5 (mGluR5) expression. Rats were exposed 1 h transient middle cerebral artery occlusion (tMCAO), and MB was injected intravenously following reperfusion (3 mg/kg). Magnetic resonance imaging (MRI) and 2,3,5-triphenyltetrazolium chloride (TTC) staining was performed 48 h after the onset of tMCAO to evaluate the brain infarction and edema. Brain tissues injuries as well as the glial fibrillary acidic protein (GFAP), AQP4 and mGluR5 expressions were detected. Oxygen and glucose deprivation/reoxygenation (OGD/R) was performed on primary astrocytes (ASTs) to induce cell swelling. MB was administered at the beginning of reoxygenation, and the perimeter of ASTs was measured by GFAP immunofluorescent staining. 3,5-dihydroxyphenylglycine (DHPG) and fenobam were given at 24 h before OGD to examine their effects on MB functions on AST swelling and AQP4 expression. MB remarkably decreased the volumes of T2WI and ADC lesions, as well as the cerebral swelling. Consistently, MB treatment significantly decreased GFAP, mGluR5 and AQP4 expression at 48 h after stroke. In the cultivated primary ASTs, OGD/R and DHPG significantly increased ASTs volume as well as AQP4 expression, which was reversed by MB and fenobam treatment. The obtained results highlight that MB decreases the post-ischemic brain swelling by regulating the activation of AQP4 and mGluR5, suggesting potential applications of MB on clinical ischemic stroke treatment.

Profiling the proximal proteome of the activated μ-opioid receptor

The μ-opioid receptor (μOR) represents an important target of therapeutic and abused drugs. So far, most understanding of μOR activity has focused on a subset of known signal transducers and regulatory molecules. Yet μOR signaling is coordinated by additional proteins in the interaction network of the activated receptor, which have largely remained invisible given the lack of technologies to interrogate these networks systematically. Here we describe a proteomics and computational approach to map the proximal proteome of the activated μOR and to extract subcellular location, trafficking and functional partners of G-protein-coupled receptor (GPCR) activity. We demonstrate that distinct opioid agonists exert differences in the μOR proximal proteome mediated by endocytosis and endosomal sorting. Moreover, we identify two new μOR network components, EYA4 and KCTD12, which are recruited on the basis of receptor-triggered G-protein activation and might form a previously unrecognized buffering system for G-protein activity broadly modulating cellular GPCR signaling.

Projection-Targeted Photopharmacology Reveals Distinct Anxiolytic Roles for Presynaptic mGluR2 in Prefrontal- and Insula-Amygdala Synapses

Dissecting how membrane receptors regulate neural circuit function is critical for deciphering basic principles of neuromodulation and mechanisms of therapeutic drug action. Classical pharmacological and genetic approaches are not well-equipped to untangle the roles of specific receptor populations, especially in long-range projections which coordinate communication between brain regions. Here we use viral tracing, electrophysiological, optogenetic, and photopharmacological approaches to determine how presynaptic metabotropic glutamate receptor 2 (mGluR2) activation in the basolateral amygdala (BLA) alters anxiety-related behavior. We find that mGluR2-expressing neurons from the ventromedial prefrontal cortex (vmPFC) and posterior insular cortex (pIC) preferentially target distinct cell types and subregions of the BLA to regulate different forms of avoidant behavior. Using projection-specific photopharmacological activation, we find that mGluR2-mediated presynaptic inhibition of vmPFC-BLA, but not pIC-BLA, connections can produce long-lasting decreases in spatial avoidance. In contrast, presynaptic inhibition of pIC-BLA connections decreased social avoidance, novelty-induced hypophagia, and increased exploratory behavior without impairing working memory, establishing this projection as a novel target for the treatment of anxiety disorders. Overall, this work reveals new aspects of BLA neuromodulation with therapeutic implications while establishing a powerful approach for optical mapping of drug action via photopharmacology.

Signaling Specificity and Kinetics of the Human Metabotropic Glutamate Receptors

Metabotropic glutamate receptors (mGluRs) are obligate dimer G protein coupled receptors that can all function as homodimers. Here, each mGluR homodimer was examined for its G protein coupling profile using a bioluminescence resonance energy transfer-based assay that detects the interaction between a split YFP-tagged Gβ 1γ2 and a Nanoluciferase tagged free Gβγ sensor, MAS-GRK3-ct- nanoluciferase with 14 specific Gα proteins heterologously expressed, representing each family. Canonically, the group II and III mGluRs (2 and 3 and 4, 6, 7, and 8, respectively) are thought to couple to Gi/o exclusively. In addition, the group I mGluRs (1 and 5) are known to couple to the Gq/11 family and generally thought to also couple to the pertussis toxin-sensitive Gi/o family some reports have suggested Gs coupling is possible as cAMP elevations have been noted. In this study, coupling was observed with all eight mGluRs through the Gi/o proteins and only mGluR1 and mGluR5 through Gq/11, and, perhaps surprisingly, not G14 None activated any Gs protein. Interestingly, coupling was seen with the group I and II but not the group III mGluRs to G16 Slow but significant coupling to Gz was also seen with the group II receptors. SIGNIFICANCE STATEMENT: Metabotropic glutamate receptor (mGluR)-G protein coupling has not been thoroughly examined, and some controversy remains about whether some mGluRs can activate Gαs family members. Here we examine the ability of each mGluR to activate representative members of every Gα protein family. While all mGluRs can activate Gαi/o proteins, only the group I mGluRs couple to Gαq/11, and no members of the family can activate Gαs family members, including the group I receptors alone or with positive allosteric modulators.

Distinct beta-arrestin coupling and intracellular trafficking of metabotropic glutamate receptor homo- and heterodimers

The metabotropic glutamate receptors (mGluRs) are family C, dimeric G protein-coupled receptors (GPCRs), which play critical roles in synaptic transmission. Despite an increasing appreciation of the molecular diversity of this family, how distinct mGluR subtypes are regulated remains poorly understood. We reveal that different group II/III mGluR subtypes show markedly different beta-arrestin (β-arr) coupling and endocytic trafficking. While mGluR2 is resistant to internalization and mGluR3 shows transient β-arr coupling, which enables endocytosis and recycling, mGluR8 and β-arr form stable complexes, which leads to efficient lysosomal targeting and degradation. Using chimeras and mutagenesis, we pinpoint carboxyl-terminal domain regions that control β-arr coupling and trafficking, including the identification of an mGluR8 splice variant with impaired internalization. We then use a battery of high-resolution fluorescence assays to find that heterodimerization further expands the diversity of mGluR regulation. Together, this work provides insight into the relationship between GPCR/β-arr complex formation and trafficking while revealing diversity and intricacy in the regulation of mGluRs.

Heterodimerization of Chemoreceptors TAS1R3 and mGlu2 in Human Blood Leukocytes

The expression of canonical chemosensory receptors of the tongue, such as the heteromeric sweet taste (TAS1R2/TAS1R3) and umami taste (TAS1R1/TAS1R3) receptors, has been demonstrated in many extra-oral cells and tissues. Gene expression studies have revealed transcripts for all TAS1 and metabotropic glutamate (mGlu) receptors in different types of immune cells, where they are involved, for example, in the chemotaxis of human neutrophils and the protection of T cells from activation-induced cell death. Like other class-C G protein-coupling receptors (GPCRs), TAS1Rs and mGlu receptors form heteromers within their families. Since mGlu receptors and TAS1R1/TAS1R3 share the same ligand, monosodium glutamate (MSG), we hypothesized their hitherto unknown heteromerization across receptor families in leukocytes. Here we show, by means of immunocytochemistry and co-IP/Western analysis, that across class-C GPCR families, mGlu2 and TAS1R3 co-localize and heterodimerize in blood leukocytes. Expressing the recombinant receptors in HEK-293 cells, we validated their heterodimerization by bioluminescence resonance energy transfer. We demonstrate MSG-induced, mGlu2/TAS1R3 heteromer-dependent gain-of-function and pertussis toxin-sensitive signaling in luminescence assays. Notably, we show that mGlu2/TAS1R3 is necessary and sufficient for MSG-induced facilitation of N-formyl-methionyl-leucyl-phenylalanine-stimulated IL-8 secretion in neutrophils, using receptor-specific antagonists. In summary, our results demonstrate mGlu2/TAS1R3 heterodimerization in leukocytes, suggesting cellular function-tailored chemoreceptor combinations to modulate cellular immune responses.

Structural basis of allosteric modulation of metabotropic glutamate receptor activation and desensitization

The metabotropic glutamate receptors (mGluRs) are neuromodulatory family C G protein coupled receptors which assemble as dimers and allosterically couple extracellular ligand binding domains (LBDs) to transmembrane domains (TMDs) to drive intracellular signaling. Pharmacologically, mGluRs can be targeted either at the LBDs by glutamate and synthetic orthosteric compounds or at the TMDs by allosteric modulators. Despite the potential of allosteric TMD-targeting compounds as therapeutics, an understanding of the functional and structural basis of their effects on mGluRs is limited. Here we use a battery of approaches to dissect the distinct functional and structural effects of orthosteric versus allosteric ligands. We find using electrophysiological and live cell imaging assays that both agonists and positive allosteric modulators (PAMs) can drive activation and desensitization of mGluRs. The effects of PAMs are pleiotropic, including both the ability to boost the maximal response to orthosteric agonists and to serve independently as desensitization-biased agonists across mGluR subtypes. Conformational sensors reveal PAM-driven inter-subunit re-arrangements at both the LBD and TMD. Motivated by this, we determine cryo-electron microscopy structures of mGluR3 in the presence of either an agonist or antagonist alone or in combination with a PAM. These structures reveal PAM-driven re-shaping of intra- and inter-subunit conformations and provide evidence for a rolling TMD dimer interface activation pathway that controls G protein and beta-arrestin coupling.

Highlights

-Agonists and PAMs drive mGluR activation, desensitization, and endocytosis-PAMs are desensitization-biased and synergistic with agonists-Four combinatorial ligand conditions reveal an ensemble of full-length mGluR structures with novel interfaces-Activation and desensitization involve rolling TMD interfaces which are re-shaped by PAM.

Signaling specificity and kinetics of the human metabotropic glutamate receptors

Metabotropic glutamate receptors (mGluRs) are obligate dimer G protein coupled receptors that can all function as homodimers. Here, each mGluR homodimer was examined for its G protein coupling profile using a BRET based assay that detects the interaction between a split YFP-tagged Gβ1γ2 and a Nanoluc tagged free Gβγ sensor, MAS-GRK3-ct-NLuc with 14 specific Ga proteins heterologously expressed, representing each family. Canonically, the group II and III mGluRs (2&3, and 4, 6, 7&8, respectively) are thought to couple to Gi/o exclusively. In addition, the group I mGluRs (1&5) are known to couple to the Gq/11 family, and generally thought to also couple to the PTX-sensitive Gi/o family; some reports have suggested Gs coupling is possible as cAMP elevations have been noted. In this study, coupling was observed with all 8 mGluRs through the Gi/o proteins, and only mGluR1&5 through Gq/11, and perhaps surprisingly, not G14. None activated any Gs protein. Interestingly, coupling was seen with the group I and II, but not the group III mGluRs to G16. Slow but significant coupling to Gz was also seen with the group II receptors.

Asymmetric activation of dimeric GABAB and metabotropic glutamate receptors

G protein-coupled receptors (GPCRs) represent the largest family of membrane proteins and are important drug targets. GPCRs are allosteric machines that transduce an extracellular signal to the cell by activating heterotrimeric G proteins. Herein, we summarize the recent advancements in the molecular activation mechanism of the γ-aminobutyric acid type B (GABAB) and metabotropic glutamate (mGlu) receptors, the most important class C GPCRs that modulate synaptic transmission in the brain. Both are mandatory dimers, this quaternary structure being needed for their function The structures of these receptors in different conformations and in complexes with G proteins have revealed their asymmetric activation. This asymmetry is further highlighted by the recent discovery of mGlu heterodimers, where the eight mGlu subunits can form specific and functional heterodimers. Finally, the development of allosteric modulators has revealed new possibilities for regulating the function of these receptors by targeting the transmembrane dimer interface. This family of receptors never ceases to astonish and serve as models to better understand the diversity and asymmetric functioning of GPCRs.NEW & NOTEWORTHY γ-aminobutyric acid type B (GABAB) and metabotropic glutamate (mGlu) receptors form constitutive dimers, which are required for their function. They serve as models to better understand the diversity and activation of G protein-coupled receptors (GPCRs). The structures of these receptors in different conformations and in complexes with G proteins have revealed their asymmetric activation. This asymmetry is further highlighted by the recent discovery of specific and functional mGlu heterodimers. Allosteric modulators can be developed to target the transmembrane interface and modulate the asymmetry.

The role of glutamate receptors in the regulation of the tumor microenvironment

Glutamate, as one of the most important carbon sources in the TCA cycle, is central in metabolic processes that will subsequently influence tumor progression. Several factors can affect the expression of glutamate receptors, playing either a tumor-promoting or tumor-suppressor role in cancer. Thus, the activation of glutamate receptors by the ligand could play a role in tumor development as ample studies have demonstrated the expression of glutamate receptors in a broad range of tumor cells. Glutamate and its receptors are involved in the regulation of different immune cells' development and function, as suggested by the receptor expression in immune cells. The activation of glutamate receptors can enhance the effectiveness of the effector's T cells, or decrease the cytokine production in immunosuppressive myeloid-derived suppressor cells, increasing the antitumor immune response. These receptors are essential for the interaction between tumor and immune cells within the tumor microenvironment (TME) and the regulation of antitumor immune responses. Although the role of glutamate in the TCA cycle has been well studied, few studies have deeply investigated the role of glutamate receptors in the regulation of cancer and immune cells within the TME. Here, by a systematic review of the available data, we will critically assess the physiopathological relevance of glutamate receptors in the regulation of cancer and immune cells in the TME and provide some unifying hypotheses for futures research on the role of glutamate receptors in the immune modulation of the tumor.

Kinetic fingerprinting of metabotropic glutamate receptors

Dimeric metabotropic glutamate receptors (mGluRs) are abundantly expressed in neurons. In mammals, eight subunit isoforms, mGluR1-8, have been identified, forming the groups I, II, and III. We investigated receptor dimerization and kinetics of these mGluR isoforms in excised membrane patches by FRET and confocal patch-clamp fluorometry. We show that 5 out of 8 homodimeric receptors develop characteristic glutamate-induced on- and off-kinetics, as do 11 out of 28 heterodimers. Glutamate-responsive heterodimers were identified within each group, between groups I and II as well as between groups II and III, but not between groups I and III. The glutamate-responsive heterodimers showed heterogeneous activation and deactivation kinetics. Interestingly, mGluR7, not generating a kinetic response in homodimers, showed fast on-kinetics in mGluR2/7 and mGluR3/7 while off-kinetics retained the speed of mGluR2 or mGluR3 respectively. In conclusion, glutamate-induced conformational changes in heterodimers appear within each group and between groups if one group II subunit is present.

Glutamate receptor endocytosis and signaling in neurological conditions

The non-essential amino acid glutamate acts as a major excitatory neurotransmitter and plays a significant role in the central nervous system (CNS). It binds with two different types of receptors, ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs), responsible for the postsynaptic excitation of neurons. They are important for memory, neural development and communication, and learning. Endocytosis and subcellular trafficking of the receptor are essential for the regulation of receptor expression on the cell membrane and excitation of the cells. The endocytosis and trafficking of the receptor are dependent on its type, ligand, agonist, and antagonist present. This chapter discusses the types of glutamate receptors, their subtypes, and the regulation of their internalization and trafficking. The roles of glutamate receptors in neurological diseases are also briefly discussed.

Control of Gαq signaling dynamics and GPCR cross-talk by GRKs

Numerous processes contribute to the regulation of G protein-coupled receptors (GPCRs), but relatively little is known about rapid mechanisms that control signaling on the seconds time scale or regulate cross-talk between receptors. Here, we reveal that the ability of some GPCR kinases (GRKs) to bind Gα_q both drives acute signaling desensitization and regulates functional interactions between GPCRs. GRK2/3-mediated acute desensitization occurs within seconds, is rapidly reversible, and can occur upon local, subcellular activation. This rapid desensitization is kinase independent, insensitive to pharmacological inhibition, and generalizable across receptor families and effectors. We also find that the ability of GRK2 to bind G proteins also enables it to regulate the extent and timing of Gα_q-dependent signaling cross-talk between GPCRs. Last, we find that G protein/GRK2 interactions enable a novel form of GPCR trafficking cross-talk. Together, this work reveals potent forms of Gα_q-dependent GPCR regulation with wide-ranging pharmacological and physiological implications.

Use of CRISPR/Cas9-edited HEK293 cells reveals that both conventional and novel protein kinase C isozymes are involved in mGlu5a receptor internalization

The internalization of G protein-coupled receptors (GPCRs) can be regulated by protein kinase C (PKC). However, most tools available to study the contribution of PKC isozymes have considerable limitations, including a lack of selectivity. In this study, we generated and characterized human embryonic kidney 293A (HEK293A) cell lines devoid of conventional or novel PKC isozymes (ΔcPKC and ΔnPKC) and employ these to investigate the contribution of PKC isozymes in the internalization of the metabotropic glutamate receptor 5 (mGlu₅). Direct activation of PKC and mutation of rat mGlu_5a Ser⁹⁰¹, a PKC-dependent phosphorylation site in the receptor C-tail, both showed that PKC isozymes facilitate approximately 40% of the receptor internalization. Nonetheless, we determined that mGlu_5a internalization was not altered upon the loss of cPKCs or nPKCs. This indicates that isozymes from both classes are involved, compensate for the absence of the other class, and thus fulfill dispensable functions. Additionally, using the Gαq/11 inhibitor YM-254890, GPCR kinase 2 and 3 (GRK2 and GRK3) knock-out cells and a receptor containing a mutated putative adaptor protein complex 2 (AP-2) interaction motif, we demonstrate that internalization of rat mGlu_5a is mediated by Gαq/11 proteins (77% of the response), GRK2 (27%) and AP-2 (29%), but not GRK3. Our PKC knock-out cell lines expand the repertoire of knock-out HEK293A cell lines available to research GPCR pharmacology. Moreover, since pharmacological tools to study PKC isozymes generally lack specificity and/or potency, we present the PKC knock-out cell lines as more specific research tools to investigate PKC-mediated aspects of cell biology.

GPCR-mediated β-arrestin activation deconvoluted with single-molecule precision

β-arrestins bind G protein-coupled receptors to terminate G protein signaling and to facilitate other downstream signaling pathways. Using single-molecule fluorescence resonance energy transfer imaging, we show that β-arrestin is strongly autoinhibited in its basal state. Its engagement with a phosphopeptide mimicking phosphorylated receptor tail efficiently releases the β-arrestin tail from its N domain to assume distinct conformations. Unexpectedly, we find that β-arrestin binding to phosphorylated receptor, with a phosphorylation barcode identical to the isolated phosphopeptide, is highly inefficient and that agonist-promoted receptor activation is required for β-arrestin activation, consistent with the release of a sequestered receptor C tail. These findings, together with focused cellular investigations, reveal that agonism and receptor C-tail release are specific determinants of the rate and efficiency of β-arrestin activation by phosphorylated receptor. We infer that receptor phosphorylation patterns, in combination with receptor agonism, synergistically establish the strength and specificity with which diverse, downstream β-arrestin-mediated events are directed.

Targeting GPCRs and Their Signaling as a Therapeutic Option in Melanoma

Simple Summary

Sixteen G-protein-coupled receptors (GPCRs) have been involved in melanogenesis or melanomagenesis. Here, we review these GPCRs, their associated signaling, and therapies.

Abstract

G-protein-coupled receptors (GPCRs) serve prominent roles in melanocyte lineage physiology, with an impact at all stages of development, as well as on mature melanocyte functions. GPCR ligands are present in the skin and regulate melanocyte homeostasis, including pigmentation. The role of GPCRs in the regulation of pigmentation and, consequently, protection against external aggression, such as ultraviolet radiation, has long been established. However, evidence of new functions of GPCRs directly in melanomagenesis has been highlighted in recent years. GPCRs are coupled, through their intracellular domains, to heterotrimeric G-proteins, which induce cellular signaling through various pathways. Such signaling modulates numerous essential cellular processes that occur during melanomagenesis, including proliferation and migration. GPCR-associated signaling in melanoma can be activated by the binding of paracrine factors to their receptors or directly by activating mutations. In this review, we present melanoma-associated alterations of GPCRs and their downstream signaling and discuss the various preclinical models used to evaluate new therapeutic approaches against GPCR activity in melanoma. Recent striking advances in our understanding of the structure, function, and regulation of GPCRs will undoubtedly broaden melanoma treatment options in the future.

The evidence for and consequences of metabotropic glutamate receptor heterodimerization

Metabotropic glutamate receptors (mGluRs) are an essential component of the mammalian central nervous system. These receptors modulate neuronal excitability in response to extracellular glutamate through the activation of intracellular heterotrimeric G proteins. Like most other class C G protein-coupled receptors, mGluRs function as obligate dimer proteins, meaning they need to form dimer complexes before becoming functional receptors. All mGluRs possess the ability to homodimerize, but studies over the past ten years have demonstrated these receptors are also capable of forming heterodimers in specific patterns. These mGluR heterodimers appear to have their own unique biophysical behavior and pharmacology with both native and synthetic compounds with few rules having been identified that allow for prediction of the consequences of any particular mGluR pair forming heterodimers. Here, we review the relevant literature demonstrating the existence and consequences of mGluR heterodimerization. By collecting biophysical and pharmacological data of several mGluR heterodimers we demonstrate the lack of generalizable behavior of these complexes indicating that each individual dimeric pair needs to be investigated independently. Additionally, by combining sequence alignment and structural analysis, we propose that interactions between the β4-A Helix Loop and the D Helix in the extracellular domain of these receptors are the structural components that dictate heterodimerization compatibility. Finally, we discuss the potential implications of mGluR heterodimerization from the viewpoints of further developing our understanding of neuronal physiology and leveraging mGluRs as a therapeutic target for the treatment of pathophysiology.

mGluR5 Negative Modulators for Fragile X: Treatment Resistance and Persistence

Fragile X syndrome (FXS) is caused by silencing of the human FMR1 gene and is the leading monogenic cause of intellectual disability and autism. Abundant preclinical data indicated that negative allosteric modulators (NAMs) of metabotropic glutamate receptor 5 (mGluR5) might be efficacious in treating FXS in humans. Initial attempts to translate these findings in clinical trials have failed, but these failures provide the opportunity for new discoveries that will improve future trials. The emergence of acquired treatment resistance ("tolerance") after chronic administration of mGluR5 NAMs is a potential factor in the lack of success. Here we confirm that FXS model mice display acquired treatment resistance after chronic treatment with the mGluR5 NAM CTEP in three assays commonly examined in the mouse model of FXS: (1) audiogenic seizure susceptibility, (2) sensory cortex hyperexcitability, and (3) hippocampal protein synthesis. Cross-tolerance experiments suggest that the mechanism of treatment resistance likely occurs at signaling nodes downstream of glycogen synthase kinase 3α (GSK3α), but upstream of protein synthesis. The rapid emergence of tolerance to CTEP begs the question of how previous studies showed an improvement in inhibitory avoidance (IA) cognitive performance after chronic treatment. We show here that this observation was likely explained by timely inhibition of mGluR5 during a critical period, as brief CTEP treatment in juvenile mice is sufficient to provide a persistent improvement of IA behavior measured many weeks later. These data will be important to consider when designing future fragile X clinical trials using compounds that target the mGluR5-to-protein synthesis signaling cascade.