New insights into mechanisms of nuclear translocation of G-protein coupled receptors

Nuclear VANGL2 Inhibits Lactogenic Differentiation

Planar cell polarity (PCP) proteins coordinate tissue morphogenesis by governing cell patterning and polarity. Asymmetrically localized on the plasma membrane of cells, PCP proteins are also trafficked by endocytosis, suggesting they may have intracellular functions that are dependent or independent of their extracellular role, but whether these functions extend to transcriptional control remains unknown. Here, we show the nuclear localization of transmembrane, PCP protein, VANGL2, in undifferentiated, but not differentiated, HC11 cells, which serve as a model for mammary lactogenic differentiation. Loss of Vangl2 function results in upregulation of pathways related to STAT5 signaling. We identify DNA binding sites and a nuclear localization signal in VANGL2, and use CUT&RUN to demonstrate direct binding of VANGL2 to specific DNA binding motifs, including one in the Stat5a promoter. Knockdown (KD) of Vangl2 in HC11 cells and primary mammary organoids results in upregulation of Stat5a , Ccnd1 and Csn2 , larger acini and organoids, and precocious differentiation; phenotypes rescued by overexpression of Vangl2 , but not Vangl2 ΔNLS . Together, these results advance a paradigm whereby PCP proteins coordinate tissue morphogenesis by keeping transcriptional programs governing differentiation in check.

Localisation of oestrogen receptors in stem cells and in stem cell-derived neurons of the mouse

Oestrogen receptors (ER) transduce the effects of the endogenous ligand, 17β-estradiol in cells to regulate a number of important processes such as reproduction, neuroprotection, learning and memory and anxiety. The ERα or ERβ are classical intracellular nuclear hormone receptors while some of their variants or novel proteins such as the G-protein coupled receptor (GPCR), GPER1/GPR30 are reported to localise in intracellular as well as plasma membrane locations. Although the brain is an important target for oestrogen with oestrogen receptors expressed differentially in various nuclei, subcellular organisation and crosstalk between these receptors is under-explored. Using an adapted protocol that is rapid, we first generated neurons from mouse embryonic stem cells. Our immunocytochemistry approach shows that the full length ERα (ERα-66) and for the first time, that an ERα variant, ERα-36, as well as GPER1 is present in embryonic stem cells. In addition, these receptors typically decrease their nuclear localisation as neuronal maturation proceeds. Finally, although these ERs are present in many subcellular compartments such as the nucleus and plasma membrane, we show that they are specifically not colocalised with each other, suggesting that they initiate distinct signalling pathways.

G Protein-Coupled Receptor 37L1 Modulates Epigenetic Changes in Human Renal Proximal Tubule Cells

Renal luminal sodium transport is essential for physiological blood pressure control, and abnormalities in this process are strongly implicated in the pathogenesis of essential hypertension. Renal G protein-coupled receptors (GPCRs) are critical for the regulation of the reabsorption of essential nutrients, ions, and water from the glomerular filtrate. Recently, we showed that GPCR 37L1 (GPR37L1) is expressed on the apical membrane of renal proximal tubules (RPT) and regulates luminal sodium transport and blood pressure by modulating the function of the sodium proton exchanger 3 (NHE3). However, little is known about GPR37L1 intracellular signaling. Here, we show that GPR37L1 is localized to the nuclear membrane, in addition to the plasma membrane in human RPT cells. Furthermore, GPR37L1 signals via the PI3K/AKT/mTOR pathway to decrease the expression of DNA (cytosine-5)-methyltransferase 1 (DNMT1) and enhance NHE3 transcription. Overall, we demonstrate the direct role of a nuclear membrane GPCR in the regulation of renal sodium through epigenetic gene regulation.

GPCRs in Intracellular Compartments: New Targets for Drug Discovery

The architecture of eukaryotic cells is defined by extensive membrane-delimited compartments, which entails separate metabolic processes that would otherwise interfere with each other, leading to functional differences between cells. G protein-coupled receptors (GPCRs) are the largest class of cell surface receptors, and their signal transduction is traditionally viewed as a chain of events initiated from the plasma membrane. Furthermore, their intracellular trafficking, internalization, and recycling were considered only to regulate receptor desensitization and cell surface expression. On the contrary, accumulating data strongly suggest that GPCRs also signal from intracellular compartments. GPCRs localize in the membranes of endosomes, nucleus, Golgi and endoplasmic reticulum apparatuses, mitochondria, and cell division compartments. Importantly, from these sites they have shown to orchestrate multiple signals that regulate different cell pathways. In this review, we summarize the current knowledge of this fascinating phenomenon, explaining how GPCRs reach the intracellular sites, are stimulated by the endogenous ligands, and their potential physiological/pathophysiological roles. Finally, we illustrate several mechanisms involved in the modulation of the compartmentalized GPCR signaling by drugs and endogenous ligands. Understanding how GPCR signaling compartmentalization is regulated will provide a unique opportunity to develop novel pharmaceutical approaches to target GPCRs and potentially lead the way towards new therapeutic approaches.

Positive allosteric regulation of PAC1-R up-regulates PAC1-R and its specific ligand PACAP

PAC1-R is a recognized preferential receptor for the neuropeptide of pituitary adenylate cyclase-activating polypeptide (PACAP), which mediates neuroprotective and nerve regenerative activities of PACAP. In this study, we found that in both PAC1R-CHO cells with high expression of PAC1R-eGFP and retinal ganglion cells (RGC-5) with the natural expression of PAC1-R, oligo-peptide PACAP(28-38) and the positively charged arginine-rich penetrating peptide TAT, as positive allosteric modulators of PAC1-R, significantly trigger the nuclear translocation of PAC1-R. The chromatin immunoprecipitation (ChIP)-PCR results show that the nuclear translocated PAC1-R binds with the promoter regions of PAC1-R and its specific ligand PACAP. The up-regulated promoter activities of PAC1-R and PACAP induced by PACAP(28-38) or TAT are positively correlative with the increase of the expression levels of PAC1-R and PACAP. Moreover, the nuclear translocation of PAC1-R induced by PACAP(28-38) or TAT is significantly inhibited by the mutation of PAC1-R on Cys25 and the palmitoylation inhibitor 2-bromopalmitate. Meanwhile, the increase in both PAC1-R and PACAP levels and the neuroprotective activities of PACAP(28-38) and TAT in MPP-induced cell model of Parkinson ' s disease are synchronously inhibited by 2-bromopalmitate, which are positively correlated with the nuclear translocation of PAC1-R induced by PACAP(28-38) or TAT. Bioinformatics analysis and motif enrichment analysis following ChIP-sequencing show that the transcription factors including SP1, Zic2, GATA1, REST and YY1 may be recruited by nuclear PAC1-R and involved in regulating the promoter activities of PAC1-R and PACAP. ChIP-sequencing and related bioinformatics analysis show that the downstream target genes regulated by the nuclear PAC1-R are mostly involved in the process of cellular stress and related to neuroprotection, neuronal genesis and development.

Human CD4+CD45RA+ T Cells Behavior after In Vitro Activation: Modulatory Role of Vasoactive Intestinal Peptide

Naїve CD4⁺ T cells, which suffer different polarizing signals during T cell receptor activation, are responsible for an adequate immune response. In this study, we aimed to evaluate the behavior of human CD4⁺CD45RA⁺ T cells after in vitro activation by anti-CD3/CD28 bead stimulation for 14 days. We also wanted to check the role of the VIP system during this process. The metabolic biomarker Glut1 was increased, pointing to an increase in glucose requirement whereas Hif-1α expression was higher in resting than in activated cells. Expression of Th1 markers increased at the beginning of activation, whereas Th17-associated biomarkers augmented after that, showing a pathogenic Th17 profile with a possible plasticity to Th17/1. Foxp3 mRNA expression augmented from day 4, but no parallel increases were observed in IL-10, IL-2, or TGFβ mRNA expression, meaning that these potential differentiated Treg could not be functional. Both VIP receptors were located on the plasma membrane, and expression of VPAC₂ receptor increased significantly with respect to the VPAC₁ receptor from day 4 of CD4⁺CD45RA⁺ T activation, pointing to a shift in VPAC receptors. VIP decreased IFNγ and IL-23R expression during the activation, suggesting a feasible modulation of Th17/1 plasticity and Th17 stabilization through both VPAC receptors. These novel results show that, without polarizing conditions, CD4⁺CD45RA⁺ T cells differentiate mainly to a pathogenic Th17 subset and an unpaired Treg subset after several days of activation. Moreover, they confirm the important immunomodulatory role of VIP, also on naїve Th cells, stressing the importance of this neuropeptide on lymphocyte responses in different pathological or non-pathological situations.

Insights into Nuclear G-Protein-Coupled Receptors as Therapeutic Targets in Non-Communicable Diseases

G-protein-coupled receptors (GPCRs) comprise a large protein superfamily divided into six classes, rhodopsin-like (A), secretin receptor family (B), metabotropic glutamate (C), fungal mating pheromone receptors (D), cyclic AMP receptors (E) and frizzled (F). Until recently, GPCRs signaling was thought to emanate exclusively from the plasma membrane as a response to extracellular stimuli but several studies have challenged this view demonstrating that GPCRs can be present in intracellular localizations, including in the nuclei. A renewed interest in GPCR receptors' superfamily emerged and intensive research occurred over recent decades, particularly regarding class A GPCRs, but some class B and C have also been explored. Nuclear GPCRs proved to be functional and capable of triggering identical and/or distinct signaling pathways associated with their counterparts on the cell surface bringing new insights into the relevance of nuclear GPCRs and highlighting the nucleus as an autonomous signaling organelle (triggered by GPCRs). Nuclear GPCRs are involved in physiological (namely cell proliferation, transcription, angiogenesis and survival) and disease processes (cancer, cardiovascular diseases, etc.). In this review we summarize emerging evidence on nuclear GPCRs expression/function (with some nuclear GPCRs evidencing atypical/disruptive signaling pathways) in non-communicable disease, thus, bringing nuclear GPCRs as targets to the forefront of debate.

Location Bias as Emerging Paradigm in GPCR Biology and Drug Discovery

GPCRs are the largest receptor family that are involved in virtually all biological processes. Pharmacologically, they are highly druggable targets, as they cover more than 40% of all drugs in the market. Our knowledge of biased signaling provided insight into pharmacology vastly improving drug design to avoid unwanted effects and achieve higher efficacy and selectivity. However, yet another feature of GPCR biology is left largely unexplored, location bias. Recent developments in this field show promising avenues for evolution of new class of pharmaceuticals with greater potential for higher level of precision medicine. Further consideration and understanding of this phenomenon with deep biochemical and molecular insights would pave the road to success. In this review, we critically analyze this perspective and discuss new avenues of investigation.

Angiotensin Receptors Heterodimerization and Trafficking: How Much Do They Influence Their Biological Function?

G-protein–coupled receptors (GPCRs) are targets for around one third of currently approved and clinical prescribed drugs and represent the largest and most structurally diverse family of transmembrane signaling proteins, with almost 1000 members identified in the human genome. Upon agonist stimulation, GPCRs are internalized and trafficked inside the cell: they may be targeted to different organelles, recycled back to the plasma membrane or be degraded. Once inside the cell, the receptors may initiate other signaling pathways leading to different biological responses. GPCRs’ biological function may also be influenced by interaction with other receptors. Thus, the ultimate cellular response may depend not only on the activation of the receptor from the cell membrane, but also from receptor trafficking and/or the interaction with other receptors. This review is focused on angiotensin receptors and how their biological function is influenced by trafficking and interaction with others receptors.

Regulation of sister chromatid cohesion by nuclear PD-L1

Programmed death ligand-1 (PD-L1 or B7-H1) is well known for its role in immune checkpoint regulation, but its function inside the tumor cells has rarely been explored. Here we report that nuclear PD-L1 is important for cancer cell sister chromatid cohesion. We found that depletion of PD-L1 suppresses cancer cell proliferation, colony formation in vitro, and tumor growth in vivo in immune-deficient NSG mice independent of its role in immune checkpoint. Specifically, PD-L1 functions as a subunit of the cohesin complex, and its deficiency leads to formation of multinucleated cells and causes a defect in sister chromatid cohesion. Mechanistically, PD-L1 compensates for the loss of Sororin, whose expression is suppressed in cancer cells overexpressing PD-L1. PD-L1 competes with Wing Apart-Like (WAPL) for binding to PDS5B, and secures proper sister chromatid cohesion and segregation. Our findings suggest an important role for nuclear PD-L1 in cancer cells independent of its function in immune checkpoint.

Activation of Th lymphocytes alters pattern expression and cellular location of VIP receptors in healthy donors and early arthritis patients

Vasoactive Intestinal Peptide (VIP) is an important immunomodulator of CD4⁺ cells in normal and pathological conditions, which exerts its anti-inflammatory and immunomodulatory actions through VPAC receptors, VPAC₁ and VPAC₂. Only a decrease in the expression of VPAC₁ mRNA on Th cells upon activation has been reported. Thus, the deepening in the knowledge of the behavior of these receptors may contribute to the design of new therapies based on their activation and/or blockade. In this study, we describe the expression pattern, cellular location and functional role of VIP receptors during the activation of human Th cells in healthy conditions and in early arthritis (EA). The protein expression pattern of VPAC₁ did not change with the activation of Th lymphocytes, whereas VPAC₂ was up-regulated. In resting cells, VPAC₁ was located on the plasma membrane and nucleus, whereas it only appeared in the nucleus in activated cells. VPAC₂ was always found in plasma membrane location. VIP receptors signaled through a PKA-dependent pathway in both conditions, and also by a PKA-independent pathway in activated cells. Both receptors exhibit a potent immunomodulatory capacity by controlling the pathogenic profile and the activation markers of Th cells. These results highlight a novel translational view in inflammatory/autoimmune diseases.

Nucleoligands-repurposing G Protein-coupled Receptor Ligands to Modulate Nuclear-localized G Protein-coupled Receptors in the Cardiovascular System

There is significant evidence that internal pools of G protein-coupled receptors (GPCRs) exist and may be affected by both endogenous signaling molecules and hydrophobic pharmaceutical ligands, once assumed to only affect cell surface versions of these receptors. Here, we discuss evidence that the biology of nuclear GPCRs in particular is complex, rich, and highly interactive with GPCR signaling from the cell surface. Caging existing GPCR ligands may be an excellent means of further stratifying the phenotypic effects of known pharmacophores such as β-adrenergic, angiotensin II, and type B endothelin receptor ligands in the cardiovascular system. We describe some synthetic strategies we have used to design ligands to go from in cellulo to in vivo experiments. We also consider how surface and intracellular GPCR signaling might be integrated and ways to dissect this. If they could be selectively targeted, nuclear GPCRs and their associated nucleoligands would represent a completely novel area for exploration by Pharma.

Nuclear localization of PD-L1: artifact or reality?

BACKGROUND

The levels of expression and membrane localization of programmed cell death ligand 1 (PD-L1), an immune checkpoint type I transmembrane glycoprotein, are related to the clinical response of anti-PD-L1/PD-1 therapy. Although the biologically relevant localization of PD-L1 is on the plasma membrane of cancer cells, it has also been reported to be in the cytoplasm and sometimes in the nucleus. Furthermore, it has been claimed that chemotherapeutics can modify PD-L1 expression and/or its nuclear localization.

RESULTS

Data from our group suggest that the nuclear localization of PD-L1, and other plasma membrane proteins as well, could be an artifact resulting from inadequate experimental conditions during immunocytochemical studies. Mild detergent and rigorous fixation conditions should be used in order to preserve the membrane localization and to prevent an erroneous translocation of PD-L1 and other non-interconnected membrane proteins, such as CD24, into other cellular compartments including the nucleus, of untreated and chemotherapeutically treated breast cancer cells.

CONCLUSION

We propose that well-specified and rigorously followed protocols should be applied to immunocytochemical diagnostic techniques, especially to those related to individualized diagnosis and treatment.

An Overview of VPAC Receptors in Rheumatoid Arthritis: Biological Role and Clinical Significance

The axis comprised by the Vasoactive Intestinal Peptide (VIP) and its G protein-coupled receptors (GPCRs), VPAC1, and VPAC2, belong to the B1 family and signal through Gs or Gq proteins. VPAC receptors seem to preferentially interact with Gs in inflammatory cells, rather than Gq, thereby stimulating adenylate cyclase activity. cAMP is able to trigger various downstream pathways, mainly the canonical PKA pathway and the non-canonical cAMP-activated guanine nucleotide exchange factor (EPAC) pathway. Classically, the presence of VPACs has been confined to the plasma membrane; however, VPAC1 location has been described in the nuclear membrane in several cell types such as activated Th cells, where they are also functional. VPAC receptor signaling modulates a number of biological processes by tipping the balance of inflammatory mediators in macrophages and other innate immune cells, modifying the expression of TLRs, and inhibiting MMPs and the expression of adhesion molecules. Receptor signaling also downregulates coagulation factors and acute-phase proteins, promotes Th2 over Th1, stimulates Treg abundance, and finally inhibits a pathogenic Th17 profile. Thus, the VIP axis signaling regulates both the innate and adaptive immune responses in several inflammatory/autoimmune diseases. Rheumatoid arthritis (RA) is a complex autoimmune disease that develops on a substrate of genetically susceptible individuals and under the influence of environmental factors, as well as epigenetic mechanisms. It is a heterogeneous disease with different pathogenic mechanisms and variable clinical forms between patients with the same diagnosis. The knowledge of VIP signaling generated in both animal models and human ex vivo studies can potentially be translated to clinical reality. Most recently, the beneficial effects of nanoparticles of VIP self-associated with sterically stabilized micelles have been reported in a murine model of RA. Another novel research area is beginning to define the receptors as biomarkers in RA, with their expression levels shown to be associated with the activity of the disease and patients-reported impairment. Therefore, VPAC expression together VIP genetic variants could allow patients to be stratified at the beginning of the disease with the purpose of guiding personalized treatment decisions.

Intracellular GPCRs Play Key Roles in Synaptic Plasticity

The trillions of synaptic connections within the human brain are shaped by experience and neuronal activity, both of which underlie synaptic plasticity and ultimately learning and memory. G protein-coupled receptors (GPCRs) play key roles in synaptic plasticity by strengthening or weakening synapses and/or shaping dendritic spines. While most studies of synaptic plasticity have focused on cell surface receptors and their downstream signaling partners, emerging data point to a critical new role for the very same receptors to signal from inside the cell. Intracellular receptors have been localized to the nucleus, endoplasmic reticulum, lysosome, and mitochondria. From these intracellular positions, such receptors may couple to different signaling systems, display unique desensitization patterns, and/or show distinct patterns of subcellular distribution. Intracellular GPCRs can be activated at the cell surface, endocytosed, and transported to an intracellular site or simply activated in situ by de novo ligand synthesis, diffusion of permeable ligands, or active transport of non-permeable ligands. Current findings reinforce the notion that intracellular GPCRs play a dynamic role in synaptic plasticity and learning and memory. As new intracellular GPCR roles are defined, the need to selectively tailor agonists and/or antagonists to both intracellular and cell surface receptors may lead to the development of more effective therapeutic tools.

Endocytosis of G Protein-Coupled Receptors and Their Ligands: Is There a Role in Metal Trafficking?

The prevalence of metal dysregulation in many neurodegenerative and neurocognitive disorders has compelled many studying such diseases to investigate the mechanisms underlying metal regulation in the central nervous system. Metal homoeostasis is often complex, with sophisticated, multilayered pathways in operation. G protein-coupled receptors are omnipresent on cell membranes and have intriguing mechanisms of endocytosis and trafficking that may be useful in metal homoeostasis. Indeed, many receptors and/or their cognate ligands are able to bind metals, and in many cases metals are considered to have neuromodulatory roles as a result of receptor binding. In this mini-review, we outline the structural and functional aspects of G protein-coupled receptors with a focus on the mechanisms leading to endocytosis and cellular trafficking. We further highlight how this may help in the trafficking of metal ions, notably copper.

GPCR signalling from within the cell

Traditionally, signal transduction from GPCRs is thought to emanate from the cell surface where receptor interactions with external stimuli can be transformed into a broad range of cellular responses. However, emergent data show that numerous GPCRs are also associated with various intracellular membranes where they may couple to different signalling systems, display unique desensitization patterns and/or exhibit distinct patterns of subcellular distribution. Although many GPCRs can be activated at the cell surface and subsequently endocytosed and transported to a unique intracellular site, other intracellular GPCRs can be activated in situ either via de novo ligand synthesis, diffusion of permeable ligands or active transport of nonpermeable ligands. Current findings reinforce the notion that intracellular GPCRs play a dynamic role in various biological functions including learning and memory, contractility and angiogenesis. As new intracellular GPCR roles are defined, the need to selectively tailor agonists and/or antagonists to both intracellular and cell surface receptors may lead to the development of more effective therapeutic tools.

LINKED ARTICLES

This article is part of a themed section on Molecular Pharmacology of GPCRs. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.21/issuetoc.