Retromer terminates the generation of cAMP by internalized PTH receptors

Biased signaling in drug discovery and precision medicine

Receptors are crucial for converting chemical and environmental signals into cellular responses, making them prime targets in drug discovery, with about 70% of drugs targeting these receptors. Biased signaling, or functional selectivity, has revolutionized drug development by enabling precise modulation of receptor signaling pathways. This concept is more firmly established in G protein-coupled receptor and has now been applied to other receptor types, including ion channels, receptor tyrosine kinases, and nuclear receptors. Advances in structural biology have further refined our understanding of biased signaling. This targeted approach enhances therapeutic efficacy and potentially reduces side effects. Numerous biased drugs have been developed and approved as therapeutics to treat various diseases, demonstrating their significant therapeutic potential. This review provides a comprehensive overview of biased signaling in drug discovery and disease treatment, highlighting recent advancements and exploring the therapeutic potential of these innovative modulators across various diseases.

Non-canonical signaling initiated by hormone-responsive G protein-coupled receptors from subcellular compartments

G protein-coupled receptors (GPCRs), the largest family of membrane receptors in the mammalian genomes, regulate almost all known physiological processes by transducing numerous extracellular stimuli including almost two-thirds of endogenous hormones and neurotransmitters. The traditional view held that GPCR signaling occurs exclusively at the cell surface, where the receptors bind with the ligands and undergo conformational changes to recruit and activate heterotrimeric G proteins. However, with the application of advanced biochemical and biophysical techniques, this conventional model is challenged by the elucidation of spatiotemporal GPCR activation with the evidence that receptors can signal from subcellular compartments to exhibit various molecular and cellular responses with physiological and pathophysiological relevance. Thus, this 'location bias' of GPCR signaling has become another layer of complexity of GPCR signal transduction. In this review, we generally introduce the development of the concept of compartmentalized GPCR signaling and comprehensively summarize the receptors reported to be localized on the membranes of different intracellular organelles. We review the physiological functions of these compartmentalized GPCRs with emphasis on some well-characterized prototypical hormone/neurotransmitter-binding receptors, including β2-adrenergic receptor, opioid receptors, parathyroid hormone type 1 receptor, thyroid-stimulating hormone receptor, cannabinoid receptor type 1, and metabotropic glutamate receptor 5, as examples. In addition, the therapeutic implications of compartmentalized GPCR signaling by introducing lipophilic or hydrophilic ligands for intracellular targeting, lipid conjugation anchor drugs, and strategy to modulate receptor internalization/resensitization, are highlighted and open new avenues in GPCR pharmacology and therapeutics.

Endomembrane GPCR signaling: 15 years on, the quest continues

G-protein-coupled receptors (GPCRs) are the largest family of cell receptors. They mediate the effects of a multitude of endogenous and exogenous cues, are deeply involved in human physiology and disease, and are major pharmacological targets. Whereas GPCRs were long thought to signal exclusively at the plasma membrane, research over the past 15 years has revealed that they also signal via classical G-protein-mediated pathways on membranes of intracellular organelles such as endosomes and the Golgi complex. This review provides an overview of recent advances and emerging concepts related to endomembrane GPCR signaling, as well as ongoing research aimed at a better understanding of its mechanisms, physiological relevance, and potential therapeutic applications.

Fast-diffusing receptor collisions with slow-diffusing peptide ligand assemble the ternary parathyroid hormone-GPCR-arrestin complex

The assembly of a peptide ligand, its receptor, and β-arrestin (βarr) into a ternary complex within the cell membrane is a crucial aspect of G protein-coupled receptor (GPCR) signaling. We explore this assembly by attaching fluorescent moieties to the parathyroid hormone (PTH) type 1 receptor (PTH1R), using PTH as a prototypical peptide hormone, along with βarr and clathrin, and recording dual-color single-molecule imaging at the plasma membrane of live cells. Here we show that PTH1R exhibits a near-Brownian diffusion, whereas unbound hormone displays limited mobility and slow lateral diffusion at the cell surface. The formation of the PTH-PTH1R-βarr complex occurs in three sequential steps: (1) receptor and ligand collisions, (2) phosphoinositide (PIP3)-dependent recruitment and conformational change of βarr molecules at the plasma membrane, and (3) collision of most βarr molecules with the ligand-bound receptor within clathrin clusters. Our results elucidate the non-random pathway by which PTH-PTH1R-βarr complex is formed and unveil the critical role of PIP3 in regulating GPCR signaling.

Exploring Bias in GPCR Signaling and its Implication in Drug Development: A One-Sided Affair

G protein-coupled receptors (GPCRs) play a pivotal role in regulating numerous physiological processes through their interactions with two key effectors: G proteins and β-arrestins (βarrs). This makes them crucial targets for therapeutic drug development. Interestingly, the evolving concept of biased signaling where ligands selectively activate either the G proteins or the βarrs has not only refined our understanding of segregation of physiological responses downstream of GPCRs but has also revolutionized drug discovery, offering the potential for treatments with enhanced efficacy and minimal side effects. This Review explores the mechanisms behind biased agonism, exploring it through various lenses, including ligand, receptor, cellular systems, location, and tissue-specific biases. It also offers structural insights into both orthosteric and allosteric ligand-binding pockets, structural rearrangements associated with the loops, and how ligand-engineering can contribute to biased signaling. Moreover, we also discuss the unique conformational signature in an intrinsically biased GPCR, which currently remains relatively less explored and adds a new dimension in biased signaling. Lastly, we address the translational challenges and practical considerations in characterizing bias, emphasizing its therapeutic potential and the latest advancements in drug development. By designing ligands that target specific signaling pathways, biased signaling presents a transformative approach to creating safer and more effective therapies. This Review focuses on our current understanding of GPCR-biased signaling, discussing potential mechanisms that lead to bias, the effect of bias on GPCR structures at a molecular level, recent advancements, and its profound potential to drive innovation in drug discovery.

PTH receptor signalling, osteocytes and bone disease induced by diabetes mellitus

Basic, translational and clinical research over the past few decades has provided new understanding on the mechanisms by which activation of the receptor of parathyroid hormone (parathyroid hormone 1 receptor (PTH1R)) regulates bone physiology and pathophysiology. A fundamental change in the field emerged upon the recognition that osteocytes, which are permanent residents of bone and the most abundant cells in bone, are targets of the actions of natural and synthetic ligands of PTH1R (parathyroid hormone and abaloparatide, respectively), and that these cells drive essential actions related to bone remodelling. Among the numerous genes regulated by PTH1R in osteocytes, SOST (which encodes sclerostin, the WNT signalling antagonist and inhibitor of bone formation) has a critical role in bone homeostasis and changes in its expression are associated with several bone pathologies. The bone fragility syndrome induced by diabetes mellitus is accompanied by increased osteocyte apoptosis and changes in the expression of osteocytic genes, including SOST. This Review will discuss advances in our knowledge of the role of osteocytes in PTH1R signalling and the new opportunities to restore bone health in diabetes mellitus by targeting the osteocytic PTH1R-sclerostin axis.

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.

Redox-modulated SNX25 as a novel regulator of GPCR-G protein signaling from endosomes

GPCR-G protein signaling from endosomes plays a crucial role in various physiological and pathological processes. However, the mechanism by which endosomal G protein signaling is terminated remains largely unknown. In this study, we aimed to investigate the regulatory mechanisms involved in terminating the signaling of Gα subunits from endosomes. Through structural analysis and cell-based assays, we have discovered that SNX25, a protein that targets endosomes via its PXA or PXC domain, interacts with regulator of G protein signaling (RGS) proteins (including RGS2, RGS4, RGS8, and RGS17) in a redox-regulated manner. The interaction between SNX25 and these RGS proteins enhances their GTPase-accelerating activity towards Gαi/q and their ability to bind GDP-bound (inactive form) Gαi/q. As a result, SNX25 recruits these RGS proteins to endosomes, leading to the termination of endosomal Gαi/q signaling. Furthermore, we have found that the SNX25/RGS complex also exerts a negative regulatory effect on Gαi/q signaling from the plasma membrane. This is achieved by recruiting Gαi/q to endosomes and preventing its activation on the plasma membrane. Our findings shed light on the previously unknown role of redox-modulated SNX25 in inhibiting Gαi/q signaling, thereby uncovering a novel mechanism for terminating Gαi/q signaling from endosomes. Importantly, this study expands our understanding of the regulation of GPCR-Gαi/q signaling beyond the plasma membrane.

Generation of Comprehensive GPCR-Transducer-Deficient Cell Lines to Dissect the Complexity of GPCR Signaling

G-protein-coupled receptors (GPCRs) compose the largest family of transmembrane receptors and are targets of approximately one-third of Food and Drug Administration-approved drugs owing to their involvement in almost all physiologic processes. GPCR signaling occurs through the activation of heterotrimeric G-protein complexes and β-arrestins, both of which serve as transducers, resulting in distinct cellular responses. Despite seeming simple at first glance, accumulating evidence indicates that activation of either transducer is not a straightforward process as a stimulation of a single molecule has the potential to activate multiple signaling branches. The complexity of GPCR signaling arises from the aspects of G-protein-coupling selectivity, biased signaling, interpathway crosstalk, and variable molecular modifications generating these diverse signaling patterns. Numerous questions relative to these aspects of signaling remained unanswered until the recent development of CRISPR genome-editing technology. Such genome editing technology presents opportunities to chronically eliminate the expression of G-protein subunits, β-arrestins, G-protein-coupled receptor kinases (GRKs), and many other signaling nodes in the GPCR pathways at one's convenience. Here, we review the practicality of using CRISPR-derived knockout (KO) cells in the experimental contexts of unraveling the molecular details of GPCR signaling mechanisms. To mention a few, KO cells have revealed the contribution of β-arrestins in ERK activation, Gα protein selectivity, GRK-based regulation of GPCRs, and many more, hence validating its broad applicability in GPCR studies. SIGNIFICANCE STATEMENT: This review emphasizes the practical application of G-protein-coupled receptor (GPCR) transducer knockout (KO) cells in dissecting the intricate regulatory mechanisms of the GPCR signaling network. Currently available cell lines, along with accumulating KO cell lines in diverse cell types, offer valuable resources for systematically elucidating GPCR signaling regulation. Given the association of GPCR signaling with numerous diseases, uncovering the system-based signaling map is crucial for advancing the development of novel drugs targeting specific diseases.

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.

The Intricacies of Renal Phosphate Reabsorption-An Overview

To maintain an optimal body content of phosphorus throughout postnatal life, variable phosphate absorption from food must be finely matched with urinary excretion. This amazing feat is accomplished through synchronised phosphate transport by myriads of ciliated cells lining the renal proximal tubules. These respond in real time to changes in phosphate and composition of the renal filtrate and to hormonal instructions. How they do this has stimulated decades of research. New analytical techniques, coupled with incredible advances in computer technology, have opened new avenues for investigation at a sub-cellular level. There has been a surge of research into different aspects of the process. These have verified long-held beliefs and are also dramatically extending our vision of the intense, integrated, intracellular activity which mediates phosphate absorption. Already, some have indicated new approaches for pharmacological intervention to regulate phosphate in common conditions, including chronic renal failure and osteoporosis, as well as rare inherited biochemical disorders. It is a rapidly evolving field. The aim here is to provide an overview of our current knowledge, to show where it is leading, and where there are uncertainties. Hopefully, this will raise questions and stimulate new ideas for further research.

EGFR-dependent endocytosis of Wnt9a and Fzd9b promotes β-catenin signaling during hematopoietic stem cell development in zebrafish

Cell-to-cell communication through secreted Wnt ligands that bind to members of the Frizzled (Fzd) family of transmembrane receptors is critical for development and homeostasis. Wnt9a signals through Fzd9b, the co-receptor LRP5 or LRP6 (LRP5/6), and the epidermal growth factor receptor (EGFR) to promote early proliferation of zebrafish and human hematopoietic stem cells during development. Here, we developed fluorescently labeled, biologically active Wnt9a and Fzd9b fusion proteins to demonstrate that EGFR-dependent endocytosis of the ligand-receptor complex was required for signaling. In human cells, the Wnt9a-Fzd9b complex was rapidly endocytosed and trafficked through early and late endosomes, lysosomes, and the endoplasmic reticulum. Using small-molecule inhibitors and genetic and knockdown approaches, we found that Wnt9a-Fzd9b endocytosis required EGFR-mediated phosphorylation of the Fzd9b tail, caveolin, and the scaffolding protein EGFR protein substrate 15 (EPS15). LRP5/6 and the downstream signaling component AXIN were required for Wnt9a-Fzd9b signaling but not for endocytosis. Knockdown or loss of EPS15 impaired hematopoietic stem cell development in zebrafish. Other Wnt ligands do not require endocytosis for signaling activity, implying that specific modes of endocytosis and trafficking may represent a method by which Wnt-Fzd specificity is established.

cAMP signaling: a remarkably regional affair

Louis Pasteur once famously said 'in the fields of observation chance favors only the prepared mind'. Much of chance is being in the right place at the right time. This is particularly true in the crowded molecular environment of the cell where being in the right place is often more important than timing. Although Brownian motion argues that enzymes will eventually bump into substrates, this probability is greatly enhanced if both molecules reside in the same subcellular compartment. However, activation of cell signaling enzymes often requires the transmission of chemical signals from extracellular stimuli to intracellular sites of action. This review highlights new developments in our understanding of cAMP generation and the 3D utilization of this second messenger inside cells.

β-Arrestin-independent endosomal cAMP signaling by a polypeptide hormone GPCR

Many G protein-coupled receptors (GPCRs) initiate a second phase of stimulatory heterotrimeric G protein (Gs)-coupled cAMP signaling after endocytosis. The prevailing current view is that the endosomal signal is inherently β-arrestin-dependent because β-arrestin is necessary for receptor internalization and, for some GPCRs, to prolong the endosomal signal. Here we revise this view by showing that the vasoactive intestinal peptide receptor 1 (VIPR1), a secretin-family polypeptide hormone receptor, does not require β-arrestin to internalize or to generate an endosomal signal. β-Arrestin instead resolves the plasma membrane and endosomal signaling phases into sequential cAMP peaks by desensitizing the plasma membrane phase without affecting the endosomal phase. This appears to occur through the formation of functionally distinct VIPR1-β-arrestin complexes at each location that differ in their phosphorylation dependence. We conclude that endosomal GPCR signaling can occur in the absence of β-arrestin and that β-arrestin sculpts the spatiotemporal profile of cellular GPCR-G protein signaling through location-specific remodeling of GPCR-β-arrestin complexes.

Functional diversification of cell signaling by GPCR localization

G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors and a critical class of regulators of mammalian physiology. Also known as seven transmembrane receptors (7TMs), GPCRs are ubiquitously expressed and versatile, detecting a diverse set of endogenous stimuli, including odorants, neurotransmitters, hormones, peptides, and lipids. Accordingly, GPCRs have emerged as the largest class of drug targets, accounting for upward of 30% of all prescription drugs. The view that ligand-induced GPCR responses originate exclusively from the cell surface has evolved to reflect accumulating evidence that receptors can elicit additional waves of signaling from intracellular compartments. These events in turn shape unique cellular and physiological outcomes. Here, we discuss our current understanding of the roles and regulation of compartmentalized GPCR signaling.

Endosomal signaling via cAMP in parathyroid hormone (PTH) type 1 receptor biology

Compartmentalization of GPCR signaling is an emerging topic that highlights the physiological relevance of spatial bias in signaling. The parathyroid hormone (PTH) type 1 receptor (PTH1R) was the first GPCR described to signal via heterotrimeric G-protein and cAMP from endosomes after β-arrestin mediated internalization, challenging the canonical GPCR signaling model which established that signaling is terminated by receptor internalization. More than a decade later, many other GPCRs have been shown to signal from endosomes via cAMP, and recent studies have proposed that location of cAMP generation impacts physiological outcomes of GPCR signaling. Here, we review the extensive literature regarding PTH1R endosomal signaling via cAMP, the mechanisms that regulate endosomal generation of cAMP, and the implications of spatial bias in PTH1R physiological functions.

Characterization of the real-time internalization of nine GPCRs reveals distinct dependence on arrestins and G proteins

G protein-coupled receptors (GPCRs) are seven transmembrane receptors that respond to external stimuli and undergo conformational changes to activate G proteins and modulate cellular processes leading to biological outcomes. To prevent overstimulation and prolonged exposure to stimuli, GPCRs are regulated by internalization. While the canonical GPCR internalization mechanism in mammalian cells is arrestin-dependent, clathrin-mediated endocytosis, more diverse GPCR internalization mechanisms have been described over the years. However, there is a lack of consistent methods used in the literature making it complicated to determine a receptor's internalization pathway. Here, we utilized a highly efficient time-resolved Förster resonance energy transfer (TR-FRET) internalization assay to determine the internalization profile of nine distinct GPCRs representing the GPCR classes A, B and C and with different G protein coupling profiles. This technique, coupled with clustered regularly interspaced palindromic repeats (CRISPR) engineered knockout cells allows us to effectively study the involvement of heterotrimeric G proteins and non-visual arrestins. We found that all the nine receptors internalized upon agonist stimulation in a concentration-dependent manner and six receptors showed basal internalization. Yet, there is no correlation between the receptor class and primary G protein coupling to the arrestin and G protein dependence for GPCR internalization. Overall, this study presents a platform for studying internalization that is applicable to most GPCRs and may even be extended to other membrane proteins. This method can be easily applicable to other endocytic machinery of interest and ultimately will lend itself towards the construction of comprehensive receptor internalization profiles.

From membrane to nucleus: A three-wave hypothesis of cAMP signaling

For many decades, our understanding of G protein-coupled receptor (GPCR) activity and cyclic AMP (cAMP) signaling was limited exclusively to the plasma membrane. However, a growing body of evidence has challenged this view by introducing the concept of endocytosis-dependent GPCR signaling. This emerging paradigm emphasizes not only the sustained production of cAMP but also its precise subcellular localization, thus transforming our understanding of the spatiotemporal organization of this process. Starting from this alternative point of view, our recent work sheds light on the role of an endocytosis-dependent calcium release from the endoplasmic reticulum in the control of nuclear cAMP levels. This is achieved through the activation of local soluble adenylyl cyclase, which in turn regulates the activation of local protein kinase A (PKA) and downstream transcriptional events. In this review, we explore the dynamic evolution of research on cyclic AMP signaling, including the findings that led us to formulate the novel three-wave hypothesis. We delve into how we abandoned the paradigm of cAMP generation limited to the plasma membrane and the changing perspectives on the rate-limiting step in nuclear PKA activation.

Location bias: A "Hidden Variable" in GPCR pharmacology

G protein-coupled receptors (GPCRs) are the largest family of transmembrane receptors and primarily signal through two main effector proteins: G proteins and β-arrestins. Many agonists of GPCRs promote "biased" responses, in which different cellular signaling pathways are activated with varying efficacies. The mechanisms underlying biased signaling have not been fully elucidated, with many potential "hidden variables" that regulate this behavior. One contributor is "location bias," which refers to the generation of unique signaling cascades from a given GPCR depending upon the cellular location at which the receptor is signaling. Here, we review evidence that GPCRs are expressed at and traffic to various subcellular locations and discuss how location bias can impact the pharmacologic properties and characterization of GPCR agonists. We also evaluate how differences in subcellular environments can modulate GPCR signaling, highlight the physiological significance of subcellular GPCR signaling, and discuss the therapeutic potential of exploiting GPCR location bias.

β-Arrestin-dependent and -independent endosomal G protein activation by the vasopressin type 2 receptor

The vasopressin type 2 receptor (V2R) is an essential G protein-coupled receptor (GPCR) in renal regulation of water homeostasis. Upon stimulation, the V2R activates Gαs and Gαq/11, which is followed by robust recruitment of β-arrestins and receptor internalization into endosomes. Unlike canonical GPCR signaling, the β-arrestin association with the V2R does not terminate Gαs activation, and thus, Gαs-mediated signaling is sustained while the receptor is internalized. Here, we demonstrate that this V2R ability to co-interact with G protein/β-arrestin and promote endosomal G protein signaling is not restricted to Gαs, but also involves Gαq/11. Furthermore, our data imply that β-arrestins potentiate Gαs/Gαq/11 activation at endosomes rather than terminating their signaling. Surprisingly, we found that the V2R internalizes and promote endosomal G protein activation independent of β-arrestins to a minor degree. These new observations challenge the current model of endosomal GPCR signaling and suggest that this event can occur in both β-arrestin-dependent and -independent manners.

GNAS locus: bone related diseases and mouse models

GNASis a complex locus characterized by multiple transcripts and an imprinting effect. It orchestrates a variety of physiological processes via numerous signaling pathways. Human diseases associated with the GNAS gene encompass fibrous dysplasia (FD), Albright's Hereditary Osteodystrophy (AHO), parathyroid hormone(PTH) resistance, and Progressive Osseous Heteroplasia (POH), among others. To facilitate the study of the GNAS locus and its associated diseases, researchers have developed a range of mouse models. In this review, we will systematically explore the GNAS locus, its related signaling pathways, the bone diseases associated with it, and the mouse models pertinent to these bone diseases.

β-arrestin-dependent and -independent endosomal G protein activation by the vasopressin type 2 receptor

The vasopressin type 2 receptor (V2R) is an essential GPCR in renal regulation of water homeostasis. Upon stimulation, the V2R activates Gαs and Gαq/11, which is followed by robust recruitment of β-arrestins and receptor internalization into endosomes. Unlike canonical GPCR signaling, the β-arrestin association with the V2R does not terminate Gαs activation, and thus, Gαs-mediated signaling is sustained while the receptor is internalized. Here, we demonstrate that this V2R ability to co-interact with G protein/β-arrestin and promote endosomal G protein signaling is not restricted to Gαs, but also involves Gαq/11. Furthermore, our data implies that β-arrestins potentiate Gαs/Gαq/11 activation at endosomes rather than terminating their signaling. Surprisingly, we found that the V2R internalizes and promote endosomal G protein activation independent of β-arrestins to a minor degree. These new observations challenge the current model of endosomal GPCR signaling and suggest that this event can occur in both β-arrestin-dependent and -independent manners.

An insulin-regulated arrestin domain protein controls hepatic glucagon action

Glucagon signaling is essential for maintaining normoglycemia in mammals. The arrestin fold superfamily of proteins controls the trafficking, turnover, and signaling of transmembrane receptors as well as other intracellular signaling functions. Further investigation is needed to understand the in vivo functions of the arrestin domain-containing 4 (ARRDC4) protein family member and whether it is involved in mammalian glucose metabolism. Here, we show that mice with a global deletion of the ARRDC4 protein have impaired glucagon responses and gluconeogenesis at a systemic and molecular level. Mice lacking ARRDC4 exhibited lower glucose levels after fasting and could not suppress gluconeogenesis at the refed state. We also show that ARRDC4 coimmunoprecipitates with the glucagon receptor, and ARRDC4 expression is suppressed by insulin. These results define ARRDC4 as a critical regulator of glucagon signaling and glucose homeostasis and reveal a novel intersection of insulin and glucagon pathways in the liver.

Molecular Mechanisms of PTH/PTHrP Class B GPCR Signaling and Pharmacological Implications

The classical paradigm of G protein-coupled receptor (GPCR) signaling via G proteins is grounded in a view that downstream responses are relatively transient and confined to the cell surface, but this notion has been revised in recent years following the identification of several receptors that engage in sustained signaling responses from subcellular compartments following internalization of the ligand-receptor complex. This phenomenon was initially discovered for the parathyroid hormone (PTH) type 1 receptor (PTH1R), a vital GPCR for maintaining normal calcium and phosphate levels in the body with the paradoxical ability to build or break down bone in response to PTH binding. The diverse biological processes regulated by this receptor are thought to depend on its capacity to mediate diverse modes of cyclic adenosine monophosphate (cAMP) signaling. These include transient signaling at the plasma membrane and sustained signaling from internalized PTH1R within early endosomes mediated by PTH. Here we discuss recent structural, cell signaling, and in vivo studies that unveil potential pharmacological outputs of the spatial versus temporal dimension of PTH1R signaling via cAMP. Notably, the combination of molecular dynamics simulations and elastic network model-based methods revealed how precise modulation of PTH signaling responses is achieved through structure-encoded allosteric coupling within the receptor and between the peptide hormone binding site and the G protein coupling interface. The implications of recent findings are now being explored for addressing key questions on how location bias in receptor signaling contributes to pharmacological functions, and how to drug a difficult target such as the PTH1R toward discovering nonpeptidic small molecule candidates for the treatment of metabolic bone and mineral diseases.

Molecular insights into peptide agonist engagement with the PTH receptor

The parathyroid hormone (PTH) 1 receptor (PTH1R) is a G protein-coupled receptor (GPCR) that regulates skeletal development and calcium homeostasis. Here, we describe cryo-EM structures of the PTH1R in complex with fragments of the two hormones, PTH and PTH-related protein, the drug abaloparatide, as well as the engineered tool compounds, long-acting PTH (LA-PTH) and the truncated peptide, M-PTH(1-14). We found that the critical N terminus of each agonist engages the transmembrane bundle in a topologically similar fashion, reflecting similarities in measures of Gαs activation. The full-length peptides induce subtly different extracellular domain (ECD) orientations relative to the transmembrane domain. In the structure bound to M-PTH, the ECD is unresolved, demonstrating that the ECD is highly dynamic when unconstrained by a peptide. High resolutions enabled identification of water molecules near peptide and G protein binding sites. Our results illuminate the action of orthosteric agonists of the PTH1R.

Parathyroid Hormone-Related Protein/Parathyroid Hormone Receptor 1 Signaling in Cancer and Metastasis

PTHrP exerts its effects by binding to its receptor, PTH1R, a G protein-coupled receptor (GPCR), activating the downstream cAMP signaling pathway. As an autocrine, paracrine, or intracrine factor, PTHrP has been found to stimulate cancer cell proliferation, inhibit apoptosis, and promote tumor-induced osteolysis of bone. Despite these findings, attempts to develop PTHrP and PTH1R as drug targets have not produced successful results in the clinic. Nevertheless, the efficacy of blocking PTHrP and PTH1R has been shown in various types of cancer, suggesting its potential for therapeutic applications. In light of these conflicting data, we conducted a comprehensive review of the studies of PTHrP/PTH1R in cancer progression and metastasis and highlighted the strengths and limitations of targeting PTHrP or PTH1R in cancer therapy. This review also offers our perspectives for future research in this field.

Structural details of a Class B GPCR-arrestin complex revealed by genetically encoded crosslinkers in living cells

Understanding the molecular basis of arrestin-mediated regulation of GPCRs is critical for deciphering signaling mechanisms and designing functional selectivity. However, structural studies of GPCR-arrestin complexes are hampered by their highly dynamic nature. Here, we dissect the interaction of arrestin-2 (arr2) with the secretin-like parathyroid hormone 1 receptor PTH1R using genetically encoded crosslinking amino acids in live cells. We identify 136 intermolecular proximity points that guide the construction of energy-optimized molecular models for the PTH1R-arr2 complex. Our data reveal flexible receptor elements missing in existing structures, including intracellular loop 3 and the proximal C-tail, and suggest a functional role of a hitherto overlooked positively charged region at the arrestin N-edge. Unbiased MD simulations highlight the stability and dynamic nature of the complex. Our integrative approach yields structural insights into protein-protein complexes in a biologically relevant live-cell environment and provides information inaccessible to classical structural methods, while also revealing the dynamics of the system.

CRHR1 endocytosis: Spatiotemporal regulation of receptor signaling

Corticotropin releasing hormone (CRH) is crucial for basal and stress-initiated reactions in the hypothalamic-pituitary-adrenal axis (HPA) and extrahypothalamic brain circuits, where it acts as a neuromodulator to organize behavioral and humoral responses to stress. We review and describe cellular components and molecular mechanisms involved in CRH system signaling through G protein-coupled receptors (GPCRs) CRHR1 and CRHR2, under the current view of GPCR signaling from the plasma membrane but also from intracellular compartments, which establish the bases of signal resolution in space and time. Focus is placed on latest studies of CRHR1 signaling in physiologically significant contexts of the neurohormone function that disclosed new mechanistic features of cAMP production and ERK1/2 activation. We also introduce in a brief overview the pathophysiological function of the CRH system, underlining the need for a complete characterization of CRHRs signaling to design new and specific therapies for stress-related disorders.

Membrane phosphoinositides regulate GPCR-β-arrestin complex assembly and dynamics

Binding of arrestin to phosphorylated G protein-coupled receptors (GPCRs) is crucial for modulating signaling. Once internalized, some GPCRs remain complexed with β-arrestins, while others interact only transiently; this difference affects GPCR signaling and recycling. Cell-based and in vitro biophysical assays reveal the role of membrane phosphoinositides (PIPs) in β-arrestin recruitment and GPCR-β-arrestin complex dynamics. We find that GPCRs broadly stratify into two groups, one that requires PIP binding for β-arrestin recruitment and one that does not. Plasma membrane PIPs potentiate an active conformation of β-arrestin and stabilize GPCR-β-arrestin complexes by promoting a fully engaged state of the complex. As allosteric modulators of GPCR-β-arrestin complex dynamics, membrane PIPs allow for additional conformational diversity beyond that imposed by GPCR phosphorylation alone. For GPCRs that require membrane PIP binding for β-arrestin recruitment, this provides a mechanism for β-arrestin release upon translocation of the GPCR to endosomes, allowing for its rapid recycling.

Endosomal parathyroid hormone receptor signaling

The canonical model for G protein-coupled receptors (GPCRs) activation assumes that stimulation of heterotrimeric G protein signaling upon ligand binding occurs solely at the cell surface and that duration of the stimulation is transient to prevent overstimulation. In this model, GPCR signaling is turned-off by receptor phosphorylation via GPCR kinases (GRKs) and subsequent recruitment of β-arrestins, resulting in receptor internalization into endosomes. Internalized receptors can then recycle back to the cell surface or be trafficked to lysosomes for degradation. However, over the last decade, this model has been extended by discovering that some internalized GPCRs continue to signal via G proteins from endosomes. This is the case for the parathyroid hormone (PTH) type 1 receptor (PTHR), which engages on sustained cAMP signaling from endosomes upon PTH stimulation. Accumulative evidence shows that the location of signaling has an impact on the physiological effects of GPCR signaling. This mini-review discusses recent insights into the mechanisms of PTHR endosomal signaling and its physiological impact.

Biased Agonism and Polymorphic Variation at the GLP-1 Receptor: Implications for the Development of Personalised Therapeutics

Glucagon-like peptide-1 receptor (GLP-1R) is a well-studied incretin hormone receptor and target of several therapeutic drugs for type 2 diabetes (T2D), obesity and, more recently, cardiovascular disease. Some signalling pathways downstream of GLP-1R may be responsible for drug adverse effects such as nausea, while others mediate therapeutic outcomes of incretin-based T2D therapeutics. Understanding the interplay between different factors that alter signalling, trafficking, and receptor activity, including biased agonism, single nucleotide polymorphisms and structural modifications is key to develop the next-generation of personalised GLP-1R agonists. However, these interactions remain poorly described, especially for novel therapeutics such as dual and tri-agonists that target more than one incretin receptor. Comparison of GLP-1R structures in complex with G proteins and different peptide and non-peptide agonists has revealed novel insights into important agonist-residue interactions and networks crucial for receptor activation, recruitment of G proteins and engagement of specific signalling pathways. Here, we review the latest knowledge on GLP-1R structure and activation, providing structural evidence for biased agonism and delineating important networks associated with this phenomenon. We survey current biased agonists and multi-agonists at different stages of development, highlighting possible challenges in their translational potential. Lastly, we discuss findings related to non-synonymous genomic variants of GLP1R and the functional importance of specific residues involved in GLP-1R function. We propose that studies of GLP-1R polymorphisms, and specifically their effect on receptor dynamics and pharmacology in response to biased agonists, could have a significant impact in delineating precision medicine approaches and development of novel therapeutics.

GPCRs in the regulation of the functional activity of multipotent mesenchymal stromal cells

Adipose tissue is one of the tissues in the human body that is renewed during the whole life. Dysregulation of this process leads to conditions such as obesity, metabolic syndrome, and type 2 diabetes. The key role in maintaining the healthy state of adipose tissue is played by a specific group of postnatal stem cells called multipotent mesenchymal stromal cells (MSCs). They are both precursors for new adipocytes and key paracrine regulators of adipose tissue homeostasis. The activity of MSCs is tightly adjusted to the needs of the organism. To ensure such coordination, MSCs are put under strict regulation which is realized through a wide variety of signaling mechanisms. They control aspects of MSC activity such as proliferation, differentiation, and production of signal molecules via alteration of MSC sensitivity to hormonal stimuli. In this regard, MSCs use all the main mechanisms of hormonal sensitivity regulation observed in differentiated cells, but at the same time, several unique regulatory mechanisms have been found in MSCs. In the presented review, we will cover these unique mechanisms as well as specifics of common mechanisms of regulation of hormonal sensitivity in stem cells.

Biased GPCR signaling by the native parathyroid hormone-related protein 1-141 relative to its N-terminal fragment 1-36

The parathyroid hormone (PTH)–related protein (PTHrP) is indispensable for the development of mammary glands, placental calcium ion transport, tooth eruption, bone formation and bone remodeling, and causes hypercalcemia in patients with malignancy. Although mature forms of PTHrP in the body consist of splice variants of 139, 141, and 173 amino acids, our current understanding on how endogenous PTHrP transduces signals through its cognate G-protein coupled receptor (GPCR), the PTH type 1 receptor (PTHR), is largely derived from studies done with its N-terminal fragment, PTHrP_1-36. Here, we demonstrate using various fluorescence imaging approaches at the single cell level to measure kinetics of (i) receptor activation, (ii) receptor signaling via Gs and Gq, and (iii) receptor internalization and recycling that the native PTHrP_1-141 displays biased agonist signaling properties that are not mimicked by PTHrP_1-36. Although PTHrP_1–36 induces transient cAMP production, acute intracellular Ca²⁺ (iCa²⁺) release and β-arrestin recruitment mediated by ligand–PTHR interactions at the plasma membrane, PTHrP_1-141 triggers sustained cAMP signaling from the plasma membrane and fails to stimulate iCa²⁺ release and recruit β-arrestin. Furthermore, we show that the molecular basis for biased signaling differences between PTHrP_1-36 and properties of native PTHrP_1-141 are caused by the stabilization of a singular PTHR conformation and PTHrP_1-141 sensitivity to heparin, a sulfated glycosaminoglycan. Taken together, our results contribute to a better understanding of the biased signaling process of a native protein hormone acting in conjunction with a GPCR.

Structural pharmacology of PTH and PTHrP

Parathyroid hormone (PTH) and PTH-related peptide (PTHrP) regulate extracellular phosphate and calcium homeostasis as well as bone remodeling. PTH is a classic endocrine peptide hormone whose synthesis and negative feedback by multiple factors control release from the parathyroid glands. PTHrP is ubiquitously expressed (pre- and postnatally) and acts in an autocrine/paracrine manner. This review considers the structural pharmacology and actions of PTH and PTHrP, biological consequences of inherited mutations, engineered analogs that illuminate similarities and differences in physiologic actions, and targeted therapeutic opportunities.

[β-arrestins, their mechanisms of action and multiple roles in the biology of G protein-coupled receptors]

The stimulation of G protein-coupled receptors (GPCRs) induces biological responses to a wide range of extracellular cues. The heterotrimeric G proteins, which are recruited to the active conformation of GPCRs, lead to the generation of various diffusible second messengers. Only two other families of proteins exhibit the remarkable characteristic of recognizing and binding to the active conformation of most GPCRs: GPCR kinases (GRKs) and β-arrestins. These two families of proteins were initially identified as key players in the desensitization of G protein activation by GPCRs. Over the years, β-arrestins have been implicated in an increasing number of interactions with non-receptor proteins, expanding the range of cellular functions in which they are involved. It is now well established that β-arrestins, by scaffolding and recruiting protein complexes in an agonist-dependent manner, directly regulate the trafficking and signaling of GPCRs. Remarkable advances have been made in recent years which have made it possible i) to identify biased ligands capable, by stabilizing particular conformations of a growing number of GPCRs, of activating or blocking the action of β-arrestins independently of that of G proteins, some of these ligands holding great therapeutic interest; ii) to demonstrate β-arrestins' role in the compartmentalization of GPCR signaling within the cell, and iii) to understand the molecular details of their interaction with GPCRs and of their activation through structural and biophysical approaches.

Precise druggability of the PTH type 1 receptor

Class B G protein-coupled receptors (GPCRs) are notoriously difficult to target by small molecules because their large orthosteric peptide-binding pocket embedded deep within the transmembrane domain limits the identification and development of nonpeptide small molecule ligands. Using the parathyroid hormone type 1 receptor (PTHR) as a prototypic class B GPCR target, and a combination of molecular dynamics simulations and elastic network model-based methods, we demonstrate that PTHR druggability can be effectively addressed. Here we found a key mechanical site that modulates the collective dynamics of the receptor and used this ensemble of PTHR conformers to identify selective small molecules with strong negative allosteric and biased properties for PTHR signaling in cell and PTH actions in vivo. This study provides a computational pipeline to detect precise druggable sites and identify allosteric modulators of PTHR signaling that could be extended to GPCRs to expedite discoveries of small molecules as novel therapeutic candidates.

Endomembrane-Based Signaling by GPCRs and G-Proteins

G-protein-coupled receptors (GPCRs) and G-proteins have a range of roles in many physiological and pathological processes and are among the most studied signaling proteins. A plethora of extracellular stimuli can activate the GPCR and can elicit distinct intracellular responses through the activation of specific transduction pathways. For many years, biologists thought that GPCR signaling occurred entirely on the plasma membrane. However, in recent decades, many lines of evidence have proved that the GPCRs and G-proteins may reside on endomembranes and can start or propagate signaling pathways through the organelles that form the secretory route. How these alternative intracellular signaling pathways of the GPCR and G-proteins influence the physiological and pathological function of the endomembranes is still under investigation. Here, we review the general role and classification of GPCRs and G-proteins with a focus on their signaling pathways in the membrane transport apparatus.

Community Guidelines for GPCR Ligand Bias: IUPHAR Review XX

G protein-coupled receptors modulate a plethora of physiological processes and mediate the effects of one-third of FDA-approved drugs. Depending on which ligand activates a receptor, it can engage different intracellular transducers. This 'biased signaling' paradigm requires that we now characterize physiological signaling not just by receptors but by ligand-receptor pairs. Ligands eliciting biased signaling may constitute better drugs with higher efficacy and fewer adverse effects. However, ligand bias is very complex, making reproducibility and description challenging. Here, we provide guidelines and terminology for any scientists to design and report ligand bias experiments. The guidelines will aid consistency and clarity, as the basic receptor research and drug discovery communities continue to advance our understanding and exploitation of ligand bias. Scientific insight, biosensors, and analytical methods are still evolving and should benefit from and contribute to the implementation of the guidelines, together improving translation from in vitro to disease-relevant in vivo models.

Activity-based, bioorthogonal imaging of phospholipase D reveals spatiotemporal dynamics of GPCR-Gq signaling

Canonically, G-protein-coupled receptor (GPCR) signaling is transient and confined to the plasma membrane (PM). Deviating from this paradigm, the parathyroid hormone receptor (PTHR1) stimulates sustained G_s signaling at endosomes. In addition to G_s, PTHR1 activates G_q signaling; yet, in contrast to the PTHR1-G_s pathway, the spatiotemporal dynamics of the G_q branch of PTHR1 signaling and its relationship to G_s signaling remain largely ill defined. Recognizing that a downstream consequence of G_q signaling is the activation of phospholipase D (PLD) enzymes, we leverage activity-based, bioorthogonal imaging tools for PLD signaling to visualize and quantify the G_q branch of PTHR1 signaling. We establish that PTHR1-G_q signaling is short lived, exclusively at the PM, and antagonized by PTHR1 endocytosis. Our data support a model wherein G_q and G_s compete for ligand-bound receptors at the PM and more broadly highlight the utility of bioorthogonal tools for imaging PLDs as probes to visualize GPCR-G_q signaling.

Parkinson's in the bone

Patients with Parkinson’s disease (PD) exhibit systemic deficits, including arthritis and osteoporosis-like symptoms. However, the questions, how the deficits in periphery organs or tissues occur in PD patients, and what are the relationship (s) of the periphery tissue deficits with the brain pathology (e.g., dopamine neuron loss), are at the beginning stage to be investigated. Notice that both PD and osteoporosis are the products of a complex interaction of genetic and environmental risk factors. Genetic mutations in numerous genes have been identified in patients either with recessive or autosomal dominant PD. Most of these PD risk genes are ubiquitously expressed; and many of them are involved in regulation of bone metabolism. Here, we review the functions of the PD risk genes in regulating bone remodeling and homeostasis. The knowledge gaps in our understanding of the bone-to-brain axis in PD development are also outlined.

Spatial bias in cAMP generation determines biological responses to PTH type 1 receptor activation

[Figure: see text].

Physiological and Pharmacological Roles of PTH and PTHrP in Bone Using Their Shared Receptor, PTH1R

Parathyroid hormone (PTH) and the paracrine factor, PTH-related protein (PTHrP), have preserved in evolution sufficient identities in their amino-terminal domains to share equivalent actions upon a common G protein-coupled receptor, PTH1R, that predominantly uses the cyclic adenosine monophosphate-protein kinase A signaling pathway. Such a relationship between a hormone and local factor poses questions about how their common receptor mediates pharmacological and physiological actions of the two. Mouse genetic studies show that PTHrP is essential for endochondral bone lengthening in the fetus and is essential for bone remodeling. In contrast, the main postnatal function of PTH is hormonal control of calcium homeostasis, with no evidence that PTHrP contributes. Pharmacologically, amino-terminal PTH and PTHrP peptides (teriparatide and abaloparatide) promote bone formation when administered by intermittent (daily) injection. This anabolic effect is remodeling-based with a lesser contribution from modeling. The apparent lesser potency of PTHrP than PTH peptides as skeletal anabolic agents could be explained by lesser bioavailability to PTH1R. By contrast, prolongation of PTH1R stimulation by excessive dosing or infusion, converts the response to a predominantly resorptive one by stimulating osteoclast formation. Physiologically, locally generated PTHrP is better equipped than the circulating hormone to regulate bone remodeling, which occurs asynchronously at widely distributed sites throughout the skeleton where it is needed to replace old or damaged bone. While it remains possible that PTH, circulating within a narrow concentration range, could contribute in some way to remodeling and modeling, its main physiological role is in regulating calcium homeostasis.

AM833 Is a Novel Agonist of Calcitonin Family G Protein-Coupled Receptors: Pharmacological Comparison with Six Selective and Nonselective Agonists

Obesity and associated comorbidities are a major health burden, and novel therapeutics to help treat obesity are urgently needed. There is increasing evidence that targeting the amylin receptors (AMYRs), heterodimers of the calcitonin G protein-coupled receptor (CTR) and receptor activity-modifying proteins, improves weight control and has the potential to act additively with other treatments such as glucagon-like peptide-1 receptor agonists. Recent data indicate that AMYR agonists, which can also independently activate the CTR, may have improved efficacy for treating obesity, even though selective activation of CTRs is not efficacious. AM833 (cagrilintide) is a novel lipidated amylin analog that is undergoing clinical trials as a nonselective AMYR and CTR agonist. In the current study, we have investigated the pharmacology of AM833 across 25 endpoints and compared this peptide with AMYR selective and nonselective lipidated analogs (AM1213 and AM1784), and the clinically used peptide agonists pramlintide (AMYR selective) and salmon CT (nonselective). We also profiled human CT and rat amylin as prototypical selective agonists of CTR and AMYRs, respectively. Our results demonstrate that AM833 has a unique pharmacological profile across diverse measures of receptor binding, activation, and regulation. SIGNIFICANCE STATEMENT: AM833 is a novel nonselective agonist of calcitonin family receptors that has demonstrated efficacy for the treatment of obesity in phase 2 clinical trials. This study demonstrates that AM833 has a unique pharmacological profile across diverse measures of receptor binding, activation, and regulation when compared with other selective and nonselective calcitonin receptor and amylin receptor agonists. The present data provide mechanistic insight into the actions of AM833.

Poly(ADP-Ribose) Polymerase Inhibitors for Arsenic Trioxide-Resistant Acute Promyelocytic Leukemia: Synergistic In Vitro Antitumor Effects with Hypomethylating Agents or High-Dose Vitamin C

Arsenic trioxide (ATO) is an anticancer agent used for the treatment ofacute promyelocytic leukemia (APL). However, 5%-10% of patients fail to respond or experience disease relapse. Based on poly(ADP-ribose) polymerase (PARP) 1 involvement in the processing of DNA demethylation, here we have tested the in vitro susceptibility of ATO-resistant clones (derived from the human APL cell line NB4) to PARP inhibitors (PARPi) in combination with hypomethylating agents (azacitidine and decitabine) or high-dose vitamin C (ascorbate), which induces 5-hydroxymethylcytosine (5hmC)-mediated DNA demethylation. ATO-sensitive and -resistant APL cell clones were generated and initially analyzed for their susceptibility to five clinically used PARPi (olaparib, niraparib, rucaparib, veliparib, and talazoparib). The obtained PARPi IC50 values were far below (olaparib and niraparib), within the range (talazoparib), or above (rucaparib and veliparib) the C max reported in patients, likely as a result of differences in the mechanisms of their cytotoxic activity. ATO-resistant APL cells were also susceptible to clinically relevant concentrations of azacitidine and decitabine and to high-dose ascorbate. Interestingly, the combination of these agents with olaparib, niraparib, or talazoparib resulted in synergistic antitumor activity. In combination with ascorbate, PARPi increased the ascorbate-mediated induction of 5hmC, which likely resulted in stalled DNA repair and cytotoxicity. Talazoparib was the most effective PARPi in synergizing with ascorbate, in accordance with its marked ability to trap PARP1 at damaged DNA. These findings suggest that ATO and PARPi have nonoverlapping resistance mechanisms and support further investigation on PARPi combination with hypomethylating agents or high-dose ascorbate for relapsed/ATO-refractory APL, especially in frail patients. SIGNIFICANCE STATEMENT: This study found that poly(ADP-ribose) inhibitors (PARPi) show activity as single agents against human acute promyelocytic leukemia cells resistant to arsenic trioxide at clinically relevant concentrations. Furthermore, PARPi enhance the in vitro efficacy of azacitidine, decitabine, and high-dose vitamin C, all agents that alter DNA methylation. In combination with vitamin C, PARPi increase the levels of 5-hydroxymethylcytosine, likely as a result of altered processing of the oxidized intermediates associated with DNA demethylation.

The Retromer Complex: From Genesis to Revelations

The retromer complex has a well-established role in endosomal protein sorting, being necessary for maintaining the dynamic localisation of hundreds of membrane proteins that traverse the endocytic system. Retromer function and dysfunction is linked with neurodegenerative diseases, including Alzheimer's and Parkinson's disease, and many pathogens, both viral and bacterial, exploit or interfere in retromer function for their own ends. In this review, the history of retromer is distilled into a concentrated form that spans the identification of retromer to recent discoveries that have shed new light on how retromer functions in endosomal protein sorting and why retromer is increasingly being viewed as a potential therapeutic target in neurodegenerative disease.

Subcellular Organization of the cAMP Signaling Pathway

The field of cAMP signaling is witnessing exciting developments with the recognition that cAMP is compartmentalized and that spatial regulation of cAMP is critical for faithful signal coding. This realization has changed our understanding of cAMP signaling from a model in which cAMP connects a receptor at the plasma membrane to an intracellular effector in a linear pathway to a model in which cAMP signals propagate within a complex network of alternative branches and the specific functional outcome strictly depends on local regulation of cAMP levels and on selective activation of a limited number of branches within the network. In this review, we cover some of the early studies and summarize more recent evidence supporting the model of compartmentalized cAMP signaling, and we discuss how this knowledge is starting to provide original mechanistic insight into cell physiology and a novel framework for the identification of disease mechanisms that potentially opens new avenues for therapeutic interventions. SIGNIFICANCE STATEMENT: cAMP mediates the intracellular response to multiple hormones and neurotransmitters. Signal fidelity and accurate coordination of a plethora of different cellular functions is achieved via organization of multiprotein signalosomes and cAMP compartmentalization in subcellular nanodomains. Defining the organization and regulation of subcellular cAMP nanocompartments is necessary if we want to understand the complex functional ramifications of pharmacological treatments that target G protein-coupled receptors and for generating a blueprint that can be used to develop precision medicine interventions.

Biased signaling in naturally occurring mutations of G protein-coupled receptors associated with diverse human diseases

G protein-coupled receptors (GPCRs) play critical roles in transmitting a variety of extracellular signals into the cells and regulate diverse physiological functions. Naturally occurring mutations that result in dysfunctions of GPCRs have been known as the causes of numerous diseases. Significant progresses have been made in elucidating the pathophysiology of diseases caused by mutations. The multiple intracellular signaling pathways, such as G protein-dependent and β-arrestin-dependent signaling, in conjunction with recent advances on biased agonism, have broadened the view on the molecular mechanism of disease pathogenesis. This review aims to briefly discuss biased agonism of GPCRs (biased ligands and biased receptors), summarize the naturally occurring GPCR mutations that cause biased signaling, and propose the potential pathophysiological relevance of biased mutant GPCRs associated with various endocrine diseases.

Retromer retrieves the Wilson disease protein ATP7B from endolysosomes in a copper-dependent manner

The Wilson disease protein, ATP7B maintains copper (herein referring to the Cu+ ion) homeostasis in the liver. ATP7B traffics from trans-Golgi network to endolysosomes to export excess copper. Regulation of ATP7B trafficking to and from endolysosomes is not well understood. We investigated the fate of ATP7B after copper export. At high copper levels, ATP7B traffics primarily to acidic, active hydrolase (cathepsin-B)-positive endolysosomes and, upon subsequent copper chelation, returns to the trans-Golgi network (TGN). At high copper, ATP7B colocalizes with endolysosomal markers and with a core member of retromer complex, VPS35. Knocking down VPS35 did not abrogate the copper export function of ATP7B or its copper-responsive anterograde trafficking to vesicles; rather upon subsequent copper chelation, ATP7B failed to relocalize to the TGN, which was rescued by overexpressing wild-type VPS35. Overexpressing mutants of the retromer complex-associated proteins Rab7A and COMMD1 yielded a similar non-recycling phenotype of ATP7B. At high copper, VPS35 and ATP7B are juxtaposed on the same endolysosome and form a large complex that is stabilized by in vivo photoamino acid labeling and UV-crosslinking. We demonstrate that retromer regulates endolysosome to TGN trafficking of copper transporter ATP7B in a manner that is dependent upon intracellular copper.