Beneath the surface: endosomal GPCR signaling

The ins and outs of connexins and pannexins beyond the cell surface

Classically implicated in the transport of small molecules between neighboring cells or into the extracellular milieu, emerging evidence implicates connexins and pannexins in other noncanonical biological functions. Propelled by recent technological advances and genetic approaches, this review aims to provide a comprehensive and thought-provoking perspective on unconventional functions of connexins and pannexins that could be shared with other cell surface proteins. Although multiple studies have linked dysfunctional channel activity with human disorders, the contribution of non-junctional roles of connexins and pannexins during disease pathophysiology is now beginning to blossom. We highlight the potential regulatory signals and players involved in unfamiliar connexin and pannexin activities that can pave the way to design novel therapeutic tools.

Signal termination of the chemokine receptor CCR9 is governed by an arrestin-independent phosphorylation mechanism

The C-C chemokine receptor type 9 (CCR9) coordinates immune cell migration from the thymus to the small intestine along gradients of the chemokine CCL25. Receptor dysregulation is associated with a variety of inflammatory bowel diseases such as Crohn's and ulcerative colitis, whereas aberrant CCR9 overexpression correlates with tumor metastasis. Despite being an attractive therapeutic target, attempts to clinically antagonize CCR9 have been unsuccessful. This highlights the need for a deeper understanding of its specific regulatory mechanisms and signaling pathways. CCR9 is a G protein-coupled receptor (GPCR) and activates Gi and Gq pathways. Unexpectedly, live-cell bioluminescence resonance energy transfer assays reveal only limited G protein activation, and signaling is rapidly terminated. Truncating the receptor C terminus significantly enhanced G protein coupling, highlighting a regulatory role of this domain. Signal suppression was not because of canonical arrestin-coordinated desensitization. Rather, removal of GPCR kinase phosphorylation led to sustained and robust G protein activation by CCR9. Using site-directed mutagenesis, we identified specific phosphorylation motifs that attenuate G protein coupling. Receptor internalization did not correlate with G protein activation capabilities. Instead, CCR9 phosphorylation disrupted the interaction of G protein heterotrimers with the receptor. This interference may lead to rapid loss of productive coupling and downstream signaling as phosphorylation would effectively render the receptor incapable of G protein coupling. An arrestin-independent, phosphorylation-driven deactivation mechanism could complement arrestin-dependent regulation of other GPCRs and have consequences for therapeutically targeting these receptors.

Opioid receptors reveal a discrete cellular mechanism of endosomal G protein activation

Many GPCRs initiate a second phase of G protein-mediated signaling from endosomes. This inherently requires the GPCR to increase cognate G protein activity on the endosome surface. Gs-coupled GPCRs are thought to achieve this by internalizing and mediating a second round of allosteric coupling to G proteins on the endosome membrane. Here, we provide evidence that the μ-opioid receptor (MOR), a Gi-coupled GPCR, is able to increase endosomal G protein activity in a different way. Leveraging conformational biosensors, we show that MOR activation triggers a transient increase of active-state Gi/o on the plasma membrane that is followed by a prolonged increase on endosomes. Contrary to the Gs-coupled GPCR paradigm, however, we show that the MOR-induced increase of active-state Gi/o on endosomes requires neither internalization of MOR nor the presence of activated MOR in the endosome membrane. We propose a distinct and additional cellular mechanism of endosomal signaling by Gi/o that is communicated through trafficking of the activated G protein rather than its activating GPCR.

Pain Signaling by GPCRs and RTKs

Chronic pain is common and debilitating, yet is inadequately treated by current therapies, which can have life-threatening side effects. Treatments targeting G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs), key pain mediators, often fail in clinical trials for unknown reasons. Here, we discuss the recent evidence that GPCRs and RTKs generate sustained signals from multiprotein signaling complexes or signalosomes in intracellular compartments to control chronic pain. We evaluate the evidence that selective antagonism of these intracellular signals provides more efficacious and long-lasting pain relief than antagonism of receptors at the surface of cells. We highlight how the identification of coreceptors and molecular scaffolds that underpin pain signaling by multiple receptors has identified new therapeutic targets for chronic pain, surmounting the redundancy of the pain signaling pathway.

Improving effects of melatonin on memory and synaptic potentiation in a mouse model of Alzheimer's-like disease: the involvement of glutamate homeostasis and mGluRs receptors

BACKGROUND

Alzheimer's disease (AD) is characterized by progressive cognitive decline and synaptic dysfunction, largely driven by amyloid plaques and neurofibrillary tangles (NFTs) composed of hyperphosphorylated tau. These pathological hallmarks disrupt glutamate signaling, which is essential for synaptic plasticity and memory consolidation. This study investigates the therapeutic potential of melatonin on memory and synaptic plasticity in an AD-like mouse model, with a focus on its regulatory effects on glutamate homeostasis and metabotropic glutamate receptors (mGluRs).

METHODS

The study began with an in-silico bioinformatics analysis of RNA-seq datasets from hippocampal tissues of AD patients to identify differentially expressed genes (DEGs) related to glutamate signaling and tau pathology. An AD-like model was induced via intra-hippocampal injection of cis-phospho tau in C57BL/6 mice. Memory function was assessed using behavioral tests. Synaptic plasticity was evaluated using in vitro field potential recording of hippocampal slices. Histological analyses included Nissl staining for neuronal density, Luxol Fast Blue for myelin integrity, and immunofluorescence for tau hyperphosphorylation. Molecular studies employed qPCR and Western blot to assess glutamate-related markers and tau phosphorylation. Melatonin (10 mg/kg) was administered intraperitoneally, starting either two weeks (early intervention) or four weeks (late intervention) post-induction.

RESULTS

Key molecular targets in glutamate signaling pathways were identified using bioinformatics. AD-like mice displayed memory deficits and synaptic dysfunction. Melatonin improved cognitive function, especially with early intervention, as confirmed by behavioral tests. Histological studies revealed reduced neuronal loss, improved myelin integrity, and decreased tau hyperphosphorylation. Molecular findings showed restored mGluR expression and reduced GSK3 activity. Early intervention yielded superior outcomes, with partial restoration of synaptic plasticity observed in LTP recordings.

CONCLUSIONS

These findings underscore the neuroprotective properties of melatonin, mediated by its ability to modulate glutamate signaling and mGluR activity, offering new insights into its potential as a therapeutic agent for AD. Additionally, the results suggest that earlier administration of melatonin may significantly enhance its efficacy, highlighting the importance of timely intervention in neurodegenerative diseases.

Endosomal chemokine receptor signalosomes regulate central mechanisms underlying cell migration

Chemokine receptors are GPCRs that regulate the chemotactic migration of a wide variety of cells including immune and cancer cells. Most chemokine receptors contain features associated with the ability to stimulate G protein signaling during β-arrestin-mediated receptor internalization into endosomes. As endosomal signaling of certain non-GPCR receptors plays a major role in cell migration, we chose to investigate the potential role of endosomal chemokine receptor signaling on mechanisms governing this function. Applying a combination of pharmacological and cell biological approaches, we demonstrate that the model chemokine receptor CCR7 recruits G protein and β-arrestin simultaneously upon chemokine stimulation, which enables internalized receptors to activate G protein from endosomes. Furthermore, spatiotemporal-resolved APEX2 proteome profiling shows that endosomal CCR7 uniquely enriches specific Rho GTPase regulators as compared to plasma membrane CCR7, which is directly associated with enhanced activity of the Rho GTPase Rac1 and chemotaxis of immune T cells. As Rac1 drives the formation of membrane protrusions during chemotaxis, our findings suggest an important integrated function of endosomal chemokine receptor signaling in cell migration.

Understanding the impact of nuclear-localized GPCRs on cellular signalling

G protein-coupled receptors (GPCRs) have historically been associated with signalling events driven from the plasma membrane. More recently, signalling from endosomes has been recognized as a feature of internalizing receptors. However, there was little consideration given to the notion that GPCRs can be targeted to distinct subcellular locations that did not involve an initial trafficking to the cell surface. Here, we focus on the evidence for and the potential impact of GPCR signalling specifically initiated from the nuclear membrane. We also discuss the possibilities for selectively targeting this and other internal pools of receptors as novel venues for drug discovery.

Opioid receptors reveal a discrete cellular mechanism of endosomal G protein activation

Many GPCRs initiate a second phase of G protein-mediated signaling from endosomes, which inherently requires an increase in G protein activity on the endosome surface. Gs-coupled GPCRs are thought to achieve this by internalizing and allosterically activating cognate G proteins again on the endosome membrane. Here we demonstrate that the μ-opioid receptor (MOR), a Gi-coupled GPCR, increases endosomal G protein activity in a different way. Leveraging conformational biosensors, we resolve the subcellular activation dynamics of endogenously expressed MOR and Gi/o-subclass G proteins. We show that MOR activation triggers a transient increase of active-state Gi/o on the plasma membrane that is followed by a prolonged increase on endosomes. Contrary to the Gs-coupled GPCR paradigm, however, we show that the MOR-induced increase of active-state Gi/o on endosomes requires neither internalization of MOR nor activation of MOR in the endosome membrane. We propose a distinct and additional cellular mechanism for GPCR-triggered elevation of G protein activity on endosomes that is mediated by regulated trafficking of the activated G protein rather than its activating GPCR.

Engineered mini-G proteins block the internalization of cognate GPCRs and disrupt downstream intracellular signaling

Mini-G proteins are engineered, thermostable variants of Gα subunits designed to stabilize G protein-coupled receptors (GPCRs) in their active conformations. Because of their small size and ease of use, they are popular tools for assessing GPCR behaviors in cells, both as reporters of receptor coupling to Gα subtypes and for cellular assays to quantify compartmentalized signaling at various subcellular locations. Here, we report that overexpression of mini-G proteins with their cognate GPCRs disrupted GPCR endocytic trafficking and associated intracellular signaling. In cells expressing the Gαs-coupled GPCR glucagon-like peptide 1 receptor (GLP-1R), coexpression of mini-Gs, a mini-G protein derived from Gαs, blocked β-arrestin 2 recruitment and receptor internalization and disrupted endosomal GLP-1R signaling. These effects did not involve changes in receptor phosphorylation or lipid nanodomain segregation. Moreover, we found that mini-G proteins derived from Gαi and Gαq also inhibited the internalization of GPCRs that couple to them. Finally, we developed an alternative intracellular signaling assay for GLP-1R using a nanobody specific for active Gαs:GPCR complexes (Nb37) that did not affect GLP-1R internalization. Our results have important implications for designing methods to assess intracellular GPCR signaling.