Roles of receptor phosphorylation and Rab proteins in G protein-coupled receptor function and trafficking

Lysophosphatidic Acid Receptor 3 (LPA3): Signaling and Phosphorylation Sites

LPA3 receptors were expressed in TREx HEK 293 cells, and their signaling and phosphorylation were studied. The agonist, lysophosphatidic acid (LPA), increased intracellular calcium and ERK phosphorylation through pertussis toxin-insensitive processes. Phorbol myristate acetate, but not LPA, desensitizes LPA3-mediated calcium signaling, the agonists, and the phorbol ester-induced LPA3 internalization. Pitstop 2 (clathrin heavy chain inhibitor) markedly reduced LPA-induced receptor internalization; in contrast, phorbol ester-induced internalization was only delayed. LPA induced rapid β-arrestin-LPA3 receptor association. The agonist and the phorbol ester-induced marked LPA3 receptor phosphorylation, and phosphorylation sites were detected using mass spectrometry. Phosphorylated residues were detected in the intracellular loop 3 (S221, T224, S225, and S229) and in the carboxyl terminus (S321, S325, S331, T333, S335, Y337, and S343). Interestingly, phosphorylation sites are within sequences predicted to constitute β-arrestin binding sites. These data provide insight into LPA3 receptor signaling and regulation.

Beneath the surface: endosomal GPCR signaling

G protein-coupled receptors (GPCRs) located at the cell surface bind extracellular ligands and convey intracellular signals via activation of heterotrimeric G proteins. Traditionally, G protein signaling was viewed to occur exclusively at this subcellular region followed by rapid desensitization facilitated by β-arrestin (βarr)-mediated G protein uncoupling and receptor internalization. However, emerging evidence over the past 15 years suggests that these βarr-mediated events do not necessarily terminate receptor signaling and that some GPCRs continue to activate G proteins after having been internalized into endosomes. Here, we review the recently elucidated mechanistic basis underlying endosomal GPCR signaling and discuss physiological implications and pharmacological targeting of this newly appreciated signaling mode.

LPA3 Receptor Phosphorylation Sites: Roles in Signaling and Internalization

Lysophosphatidic acid (LPA) type 3 (LPA3) receptor mutants were generated in which the sites detected phosphorylated were substituted by non-phosphorylatable amino acids. Substitutions were made in the intracellular loop 3 (IL3 mutant), the carboxyl terminus (Ctail), and both domains (IL3/Ctail). The wild-type (WT) receptor and the mutants were expressed in T-REx HEK293 cells, and the consequences of the substitutions were analyzed employing different functional parameters. Agonist- and LPA-mediated receptor phosphorylation was diminished in the IL3 and Ctail mutants and essentially abolished in the IL3/Ctail mutant, confirming that the main phosphorylation sites are present in both domains and their role in receptor phosphorylation eliminated by substitution and distributed in both domains. The WT and mutant receptors increased intracellular calcium and ERK 1/2 phosphorylation in response to LPA and PMA. The agonist, Ki16425, diminished baseline intracellular calcium, which suggests some receptor endogenous activity. Similarly, baseline ERK1/2 phosphorylation was diminished by Ki16425. An increase in baseline ERK phosphorylation was detected in the IL3/Ctail mutant. LPA and PMA-induced receptor interaction with β-arrestin 2 and LPA3 internalization were severely diminished in cells expressing the mutants. Mutant-expressing cells also exhibit increased baseline proliferation and response to different stimuli, which were inhibited by the antagonist Ki16425, suggesting a role of LPA receptors in this process. Migration in response to different attractants was markedly increased in the Ctail mutant, which the Ki16425 antagonist also attenuated. Our data experimentally show that receptor phosphorylation in the distinct domains is relevant for LPA3 receptor function.

Adrenoceptor Desensitization: Current Understanding of Mechanisms

G-protein coupled receptors (GPCRs) transduce a wide range of extracellular signals. They are key players in the majority of biologic functions including vision, olfaction, chemotaxis, and immunity. However, as essential as most of them are to body function and homeostasis, overactivation of GPCRs has been implicated in many pathologic diseases such as cancer, asthma, and heart failure (HF). Therefore, an important feature of G protein signaling systems is the ability to control GPCR responsiveness, and one key process to control overstimulation involves initiating receptor desensitization. A number of steps are appreciated in the desensitization process, including cell surface receptor phosphorylation, internalization, and downregulation. Rapid or short-term desensitization occurs within minutes and involves receptor phosphorylation via the action of intracellular protein kinases, the binding of β-arrestins, and the consequent uncoupling of GPCRs from their cognate heterotrimeric G proteins. On the other hand, long-term desensitization occurs over hours to days and involves receptor downregulation or a decrease in cell surface receptor protein level. Of the proteins involved in this biologic phenomenon, β-arrestins play a particularly significant role in both short- and long-term desensitization mechanisms. In addition, β-arrestins are involved in the phenomenon of biased agonism, where the biased ligand preferentially activates one of several downstream signaling pathways, leading to altered cellular responses. In this context, this review discusses the different patterns of desensitization of the α 1-, α 2- and the β adrenoceptors and highlights the role of β-arrestins in regulating physiologic responsiveness through desensitization and biased agonism. SIGNIFICANCE STATEMENT: A sophisticated network of proteins orchestrates the molecular regulation of GPCR activity. Adrenoceptors are GPCRs that play vast roles in many physiological processes. Without tightly controlled desensitization of these receptors, homeostatic imbalance may ensue, thus precipitating various diseases. Here, we critically appraise the mechanisms implicated in adrenoceptor desensitization. A better understanding of these mechanisms helps identify new druggable targets within the GPCR desensitization machinery and opens exciting therapeutic fronts in the treatment of several pathologies.

Regulation of the pro-inflammatory G protein-coupled receptor GPR84

GPR84 is an understudied rhodopsin-like class A G protein-coupled receptor, which is arousing particular interest from a therapeutic perspective. Not least this reflects that gpr84 expression is significantly up-regulated following acute inflammatory stimuli and in inflammatory diseases, and that receptor activation plays a role in regulating pro-inflammatory responses and migration of cells of the innate immune system such as neutrophils, monocytes, macrophages and microglia. Although most physiological responses of GPR84 reflect receptor coupling to Gαi/o-proteins, several studies indicate that agonist-activated GPR84 can recruit arrestin adaptor proteins and this regulates receptor internalisation and desensitisation. To date, little is known on the patterns of either basal or ligand regulated GPR84 phosphorylation and how these might control these processes. Here, we consider what is known about the regulation of GPR84 signalling with a focus on how G protein receptor kinase-mediated phosphorylation regulates arrestin protein recruitment and receptor function. LINKED ARTICLES: This article is part of a themed issue GPR84 Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.10/issuetoc.

Ion channel trafficking implications in heart failure

Heart failure (HF) is recognized as an epidemic in the contemporary world, impacting around 1%-2% of the adult population and affecting around 6 million Americans. HF remains a major cause of mortality, morbidity, and poor quality of life. Several therapies are used to treat HF and improve the survival of patients; however, despite these substantial improvements in treating HF, the incidence of HF is increasing rapidly, posing a significant burden to human health. The total cost of care for HF is USD 69.8 billion in 2023, warranting a better understanding of the mechanisms involved in HF. Among the most serious manifestations associated with HF is arrhythmia due to the electrophysiological changes within the cardiomyocyte. Among these electrophysiological changes, disruptions in sodium and potassium currents' function and trafficking, as well as calcium handling, all of which impact arrhythmia in HF. The mechanisms responsible for the trafficking, anchoring, organization, and recycling of ion channels at the plasma membrane seem to be significant contributors to ion channels dysfunction in HF. Variants, microtubule alterations, or disturbances of anchoring proteins lead to ion channel trafficking defects and the alteration of the cardiomyocyte's electrophysiology. Understanding the mechanisms of ion channels trafficking could provide new therapeutic approaches for the treatment of HF. This review provides an overview of the recent advances in ion channel trafficking in HF.

Basal interaction of the orphan receptor GPR101 with arrestins leads to constitutive internalization

GPR101 is an orphan G protein-coupled receptor that promotes growth hormone secretion in the pituitary. The microduplication of the GPR101 gene has been linked with the X-linked acrogigantism, or X-LAG, syndrome. This disease is characterized by excessive growth hormone secretion and abnormal rapid growth beginning early in life. Mechanistically, GPR101 induces growth hormone secretion through constitutive activation of multiple heterotrimeric G proteins. However, the full scope of GPR101 signaling remains largely elusive. Herein, we investigated the association of GPR101 to multiple transducers and uncovered an important basal interaction with Arrestin 2 (β-arrestin 1) and Arrestin 3 (β-arrestin 2). By using a GPR101 mutant lacking the C-terminus and cell lines with an Arrestin 2/3 null background, we show that the arrestin association leads to constitutive clathrin- and dynamin-mediated GPR101 internalization. To further highlight GPR101 intracellular fate, we assessed the colocalization of GPR101 with Rab protein markers. Internalized GPR101 was mainly colocalized with the early endosome markers, Rab5 and EEA-1, and to a lesser degree with the late endosome marker Rab7. However, GPR101 was not colocalized with the recycling endosome marker Rab11. These findings show that the basal arrestin recruitment by GPR101 C-terminal tail drives the receptor constitutive clathrin-mediated internalization. Intracellularly, GPR101 concentrates in the endosomal compartment and is degraded through the lysosomal pathway. In conclusion, we uncovered a constitutive intracellular trafficking of GPR101 that potentially represents an important layer of regulation of its signaling and function.

RAB31 drives extracellular vesicle fusion and cancer-associated fibroblast formation leading to oxaliplatin resistance in colorectal cancer

Epithelial-mesenchymal transition (EMT) is associated with tumorigenesis and drug resistance. The Rab superfamily of small G-proteins plays a role in regulating cell cytoskeleton and vesicle transport. However, it is not yet clear how the Rab family contributes to cancer progression by participating in EMT. By analysing various in silico datasets, we identified a statistically significant increase in RAB31 expression in the oxaliplatin-resistant group compared to that in the parental or other chemotherapy drug groups. Our findings highlight RAB31's powerful effect on colorectal cancer cell lines when compared with other family members. In a study that analysed multiple online meta-databases, RAB31 RNA levels were continually detected in colorectal tissue arrays. Additionally, RAB31 protein levels were correlated with various clinical parameters in clinical databases and were associated with negative prognoses for patients. RAB31 expression levels in all three probes were calculated using a computer algorithm and were found to be positively correlated with EMT scores. The expression of the epithelial-type marker CDH1 was suppressed in RAB31 overexpression models, whereas the expression of the mesenchymal-type markers SNAI1 and SNAI2 increased. Notably, RAB31-induced EMT and drug resistance are dependent on extracellular vesicle (EV) secretion. Interactome analysis confirmed that RAB31/AGR2 axis-mediated exocytosis was responsible for maintaining colorectal cell resistance to oxaliplatin. Our study concluded that RAB31 alters the sensitivity of oxaliplatin, a supplementary chemotherapy approach, and is an independent prognostic factor that can be used in the treatment of colorectal cancer.

Lysophosphatidic acid receptor LPA1 trafficking and interaction with Rab proteins, as evidenced by Förster resonance energy transfer

LPA₁ internalization to endosomes was studied employing Förster Resonance Energy Transfer (FRET) in cells coexpressing the mCherry-lysophosphatidic acid LPA₁ receptors and distinct eGFP-tagged Rab proteins. Lysophosphatidic acid (LPA)-induced internalization was rapid and decreased afterward: phorbol myristate acetate (PMA) action was slower and sustained. LPA stimulated LPA₁-Rab5 interaction rapidly but transiently, whereas PMA action was rapid but sustained. Expression of a Rab5 dominant-negative mutant blocked LPA₁-Rab5 interaction and receptor internalization. LPA-induced LPA₁-Rab9 interaction was only observed at 60 min, and LPA₁-Rab7 interaction after 5 min with LPA and after 60 min with PMA. LPA triggered immediate but transient rapid recycling (i.e., LPA₁-Rab4 interaction), whereas PMA action was slower but sustained. Agonist-induced slow recycling (LPA₁-Rab11 interaction) increased at 15 min and remained at this level, whereas PMA action showed early and late peaks. Our results indicate that LPA₁ receptor internalization varies with the stimuli.

Cell Trafficking and Function of G Protein-coupled Receptors

The G protein-coupled receptors (GPCRs) are plasma membrane proteins that function as sensors of changes in the internal and external milieux and play essential roles in health and disease. They are targets of hormones, neurotransmitters, local hormones (autacoids), and a large proportion of the drugs currently used as therapeutics and for "recreational" purposes. Understanding how these receptors signal and are regulated is fundamental for progress in areas such as physiology and pharmacology. This review will focus on what is currently known about their structure, the molecular events that trigger their signaling, and their trafficking to endosomal compartments. GPCR phosphorylation and its role in desensitization (signaling switching) are also discussed. It should be mentioned that the volume of information available is enormous given the large number and variety of GPCRs. However, knowledge is fragmentary even for the most studied receptors, such as the adrenergic receptors. Therefore, we attempt to present a panoramic view of the field, conscious of the risks and limitations (such as oversimplifications and incorrect generalizations). We hope this will provoke further research in the area. It is currently accepted that GPCR internalization plays a role signaling events. Therefore, the processes that allow them to internalize and recycle back to the plasma membrane are briefly reviewed. The functions of cytoskeletal elements (mainly actin filaments and microtubules), the molecular motors implicated in receptor trafficking (myosin, kinesin, and dynein), and the GTPases involved in GPCR internalization (dynamin) and endosomal sorting (Rab proteins), are discussed. The critical role phosphoinositide metabolism plays in regulating these events is also depicted.