Conformational signatures in β-arrestin2 reveal natural biased agonism at a G-protein-coupled receptor

Engineering a NanoBiT biosensor for detecting angiotensin-converting enzyme-2 (hACE2) interaction with SARS-CoV-2 spike protein and screening the inhibitors to block hACE2 and spike interaction

Infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is facilitated by its trimeric surface spike protein, which binds to the human angiotensin-converting enzyme 2 (hACE2) receptor. This critical interaction facilitates viral entry and is a primary target for therapeutic intervention against COVID-19. However, it is difficult to fully optimize viral infection using existing protein-protein interaction methods. Herein, we introduce a nano-luciferase binary technology (NanoBiT)-based pseudoviral sensor designed to stimulate the dynamics of viral infection in both living cells and animals. Infection progression can be dynamically visualized via a rapid increase in luminescence within 3 h using an in vivo imaging system (IVIS). Inhibition of viral infection by baicalein and baicalin was evaluated using a NanoBiT-based pseudoviral sensor. These results indicate that the inhibitory efficacy of baicalein was strengthened by targeting the spike protein, whereas baicalin targeted the hACE2 protein. Additionally, under optimized conditions, baicalein and baicalin provided a synergistic combination to inhibit pseudoviral infection. Live bioluminescence imaging was used to evaluate the in vivo effects of baicalein and baicalin treatment on LgBiT-hACE2 mice infected with the BA.2-SmBiT spike pseudovirus. This innovative bioluminescent system functions as a sensitive and early-stage quantitative viral transduction in vitro and in vivo. This platform provides novel opportunities for studying the molecular biology of animal models.

A high-affinity, cis-on photoswitchable beta blocker to optically control β2-adrenergic receptors in vitro and in vivo

This study introduces (S)-Opto-prop-2, a second-generation photoswitchable ligand designed for precise modulation of β2-adrenoceptor (β2AR). Synthesised by incorporating an azobenzene moiety with propranolol, (S)-Opto-prop-2 exhibited a high PSScis (photostationary state for cis isomer) percentage (∼90 %) and a favourable half-life (>10 days), facilitating diverse bioassay measurements. In vitro, the cis-isomer displayed substantially higher β2AR binding affinity than the trans-isomer (1000-fold), making (S)-Opto-prop-2 one of the best photoswitchable GPCR (G protein-coupled receptor) ligands reported so far. Molecular docking of (S)-Opto-prop-2 in the X-ray structure of propranolol-bound β2AR followed by site-directed mutagenesis studies, identified D1133.32, N3127.39 and F2896.51 as crucial residues that contribute to ligand-receptor interactions at the molecular level. In vivo efficacy was assessed using a rabbit ocular hypertension model, revealing that the cis isomer mimicked propranolol's effects in reducing intraocular pressure, while the trans isomer was inactive. Dynamic optical modulation of β2AR by (S)-Opto-prop-2 was demonstrated in two different cAMP bioassays and using live-cell confocal imaging, indicating reversible and dynamic control of β2AR activity using the new photopharmacology tool. In conclusion, (S)-Opto-prop-2 emerges as a promising photoswitchable ligand for precise and reversible β2AR modulation with light. The new tool shows superior cis-on binding affinity, one of the largest reported differences in affinity (1000-fold) between its two configurations, in vivo efficacy, and dynamic modulation. This study contributes valuable insights into the evolving field of photopharmacology, offering a potential avenue for targeted therapy in β2AR-associated pathologies.

Primary cilia, A-kinase anchoring proteins and constitutive activity at the orphan G protein-coupled receptor GPR161: A tale about a tail

Primary cilia are non-motile antennae-like structures responsible for sensing environmental changes in most mammalian cells. Ciliary signalling is largely mediated by the Sonic Hedgehog (Shh) pathway, which acts as a master regulator of ciliary protein transit and is essential for normal embryonic development. One particularly important player in primary cilia is the orphan G protein-coupled receptor, GPR161. In this review, we introduce GPR161 in the context of Shh signalling and describe the unique features on its C-terminus such as PKA phosphorylation sites and an A-kinase anchoring protein motif, which may influence the function of the receptor, cAMP compartmentalisation and/or trafficking within primary cilia. We discuss the recent putative pairing of GPR161 and spexin-1, highlighting the additional steps needed before GPR161 could be considered 'deorphanised'. Finally, we speculate that the marked constitutive activity and unconventional regulation of GPR161 may indicate that the receptor may not require an endogenous ligand. LINKED ARTICLES: This article is part of a themed issue Therapeutic Targeting of G Protein-Coupled Receptors: hot topics from the Australasian Society of Clinical and Experimental Pharmacologists and Toxicologists 2021 Virtual Annual Scientific Meeting. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.14/issuetoc.

Multiplex Detection of Fluorescent Chemokine Binding to CXC Chemokine Receptors by NanoBRET

NanoLuc-mediated bioluminescence resonance energy transfer (NanoBRET) has gained popularity for its ability to homogenously measure ligand binding to G protein-coupled receptors (GPCRs), including the subfamily of chemokine receptors. These receptors, such as ACKR3, CXCR4, CXCR3, play a crucial role in the regulation of the immune system, are associated with inflammatory diseases and cancer, and are seen as promising drug targets. The aim of this study was to optimize NanoBRET-based ligand binding to NLuc-ACKR3 and NLuc-CXCR4 using different fluorescently labeled chemokine CXCL12 analogs and their use in a multiplex NanoBRET binding assay of two chemokine receptors at the same time. The four fluorescent CXCL12 analogs (CXCL12-AZD488, -AZD546, -AZD594, -AZD647) showed high-affinity saturable binding to both NLuc-ACKR3 and NLuc-CXCR4, with relatively low levels of non-specific binding. Additionally, the binding of all AZDye-labeled CXCL12s to Nluc receptors was inhibited by pharmacologically relevant unlabeled chemokines and small molecules. The NanoBRET binding assay for CXCL10-AZD488 binding to Nluc-CXCR3 was also successfully established and successfully employed for the simultaneous measurement of the binding of unlabeled small molecules to NLuc-CXCR3 and NLuc-CXCR4. In conclusion, multiplexing the NanoBRET-based competition binding assay is a promising tool for testing unlabeled (small) molecules against multiple GPCRs simultaneously.

Exploring the Potential Link between Acute Central Serous Chorioretinopathy and Trimethylamine N-Oxide, Phoenixin, Spexin, and Alarin Molecules

PURPOSE

Acute central serous chorioretinopathy (ACSCR) is a condition characterized by decreased visual acuity, macular thickening, and edema under the retinal layer. Although the underlying mechanisms of the disease are not fully understood, oxidative stress is considered to be a critical risk factor. The aim of this study was to shed light on the pathophysiology of ACSCR by investigating the levels of circulating trimethylamine N-oxide (TMAO), phoenixin (PNX), alarin (ALA), and spexin (SPX) molecules in ACSCR patients.

METHODS

The study included 30 ACSCR patients and 30 healthy individuals as controls. ACSCR was diagnosed using optical coherence tomography (OCT) imaging. Five mL blood samples were collected from all participants following overnight fasting. The levels of TMAO, PNX, ALA, and SPX in the blood samples were measured using the ELISA method.

RESULTS

Visual acuity was found to be significantly reduced in ACSCR patients compared to the control group (<0.05), while macular thickness was increased (<0.05). Furthermore, TMAO, PNX, and ALA levels were significantly higher in ACSCR patients (<0.05), while SPX levels were significantly lower compared to the control group (<0.05). In ACSCR patients, there was a positive correlation between macular thickness and TMAO, PNX, and ALA; there was, however, a negative correlation with SPX. Additionally, visual acuity was negatively correlated with TMAO, PNX, and ALA, while SPX levels decreased as visual acuity decreased.

CONCLUSIONS

These results demonstrate a correlation between the TMAO, PNX, ALA, and SPX levels of ACSCR patients and their visual acuity and macular thickness. Given the role of these molecules in ACSCR's pathophysiology, they hold promise as potential diagnostic, therapeutic, and follow-up markers in the future.

Structural snapshots uncover a key phosphorylation motif in GPCRs driving β-arrestin activation

Agonist-induced GPCR phosphorylation is a key determinant for the binding and activation of β-arrestins (βarrs). However, it is not entirely clear how different GPCRs harboring divergent phosphorylation patterns impart converging active conformation on βarrs leading to broadly conserved functional responses such as desensitization, endocytosis, and signaling. Here, we present multiple cryo-EM structures of activated βarrs in complex with distinct phosphorylation patterns derived from the carboxyl terminus of different GPCRs. These structures help identify a P-X-P-P type phosphorylation motif in GPCRs that interacts with a spatially organized K-K-R-R-K-K sequence in the N-domain of βarrs. Sequence analysis of the human GPCRome reveals the presence of this phosphorylation pattern in a large number of receptors, and its contribution in βarr activation is demonstrated by targeted mutagenesis experiments combined with an intrabody-based conformational sensor. Taken together, our findings provide important structural insights into the ability of distinct GPCRs to activate βarrs through a significantly conserved mechanism.

Investigating the potential of GalR2 as a drug target for neuropathic pain

Neuropathic pain is a chronic and debilitating condition characterised by episodes of hyperalgesia and allodynia. It occurs following nerve damage from disease, inflammation or injury and currently impacts up to 17% of the UK population. Existing therapies lack efficacy and have deleterious side effects that can be severely limiting. Galanin receptor 2 (GalR2) is a G-protein coupled receptor (GPCR) implicated in the control and processing of painful stimuli. Within the nervous system it is expressed in key tissues involved in these actions such as dorsal root ganglia (DRG) and the dorsal horn of the spinal cord. Stimulation of GalR2 is widely reported to have a role in the attenuation of inflammatory and neuropathic pain. Several studies have indicated GalR2 as a possible drug target, highlighting the potential of specific GalR2 agonists to both provide efficacy and to address the side-effect profiles of current pain therapies in clinical use. A strong biological target for drug discovery will be well validated with regards to its role in the relevant disease pathology. Ideally there will be good translational models, sensitive probes, selective and appropriate molecular tools, translational biomarkers, a clearly defined patient population and strong opportunities for commercialisation. Before GalR2 can be considered as a drug target suitable for investment, key questions need to be asked regarding its expression profile, receptor signalling and ligand interactions. This article aims to critically review the available literature and determine the current strength of hypothesis of GalR2 as a target for the treatment of neuropathic pain.

The role of sensory and sympathetic nerves in craniofacial bone regeneration

Multiple factors regulate the regeneration of craniofacial bone defects. The nervous system is recognized as one of the critical regulators of bone mass, thereby suggesting a role for neuronal pathways in bone regeneration. However, in the context of craniofacial bone regeneration, little is known about the interplay between the nervous system and craniofacial bone. Sensory and sympathetic nerves interact with the bone through their neuropeptides, neurotransmitters, proteins, peptides, and amino acid derivates. The neuron-derived factors, such as semaphorin 3A (SEMA3A), substance P (SP), calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), and vasoactive intestinal peptide (VIP), possess a remarkable role in craniofacial regeneration. This review summarizes the roles of these factors and recently published factors such as secretoneurin (SN) and spexin (SPX) in the osteoblast and osteoclast differentiation, bone metabolism, growth, remodeling and discusses the novel application of nerve-based craniofacial bone regeneration. Moreover, the review will facilitate understanding the mechanism of action and provide potential treatment direction for the craniofacial bone defect.

Characterization of the chicken melanocortin 5 receptor and its potential role in regulating hepatic glucolipid metabolism

Melanocortin receptors (MC1R-MC5R) and their accessory proteins (MRAPs) are involved in a variety of physiological processes, including pigmentation, lipolysis, adrenal steroidogenesis, and immunology. However, the physiological roles of MC5R are rarely characterized in vertebrates, particularly in birds. In this work, we cloned the full-length cDNA of chicken MC5R and identified its core promoter region. Functional studies revealed that cMC5R was more sensitive to ACTH/α-MSH than β-MSH/γ-MSH, and was coupled to the cAMP/PKA signaling pathway. We demonstrated that MRAP2 decreased MC5R sensitivity to α-MSH, whereas MRAP1 did not have a similar effect, and that both MRAPs significantly reduced MC5R expression on the cell membrane surface. Transcriptome and qPCR data showed that both MRAP1 and MC5R were highly expressed in chicken liver. Additionally, we observed that ACTH might increase hepatic glucose production and decrease lipogenesis in primary hepatocytes, and dose-dependently downregulated the expression levels of ELOVL6 and THRSPA genes. These findings indicated that ACTH may act directly on hepatocytes to regulate glucolipid metabolism, which will help to understand the function of MC5R in avian.

β-arrestin1 and 2 exhibit distinct phosphorylation-dependent conformations when coupling to the same GPCR in living cells

β-arrestins mediate regulatory processes for over 800 different G protein-coupled receptors (GPCRs) by adopting specific conformations that result from the geometry of the GPCR–β-arrestin complex. However, whether β-arrestin1 and 2 respond differently for binding to the same GPCR is still unknown. Employing GRK knockout cells and β-arrestins lacking the finger-loop-region, we show that the two isoforms prefer to associate with the active parathyroid hormone 1 receptor (PTH1R) in different complex configurations (“hanging” and “core”). Furthermore, the utilisation of advanced NanoLuc/FlAsH-based biosensors reveals distinct conformational signatures of β-arrestin1 and 2 when bound to active PTH1R (P-R*). Moreover, we assess β-arrestin conformational changes that are induced specifically by proximal and distal C-terminal phosphorylation and in the absence of GPCR kinases (GRKs) (R*). Here, we show differences between conformational changes that are induced by P-R* or R* receptor states and further disclose the impact of site-specific GPCR phosphorylation on arrestin-coupling and function.

Here the authors present improved intramolecular sensors for β-arrestin2 and 1, which enable assessment of conformational changes of both isoforms in living cells. These reveal that the same GPCR induces differential conformational rearrangements that determine the functional diversity between the two β-arrestins.

Finding the Perfect Fit: Conformational Biosensors to Determine the Efficacy of GPCR Ligands

G protein-coupled receptors (GPCRs) are highly druggable targets that adopt numerous conformations. A ligand's ability to stabilize specific conformation(s) of its cognate receptor determines its efficacy or ability to produce a biological response. Identifying ligands that produce different receptor conformations and potentially discrete pharmacological effects (e.g., biased agonists, partial agonists, antagonists, allosteric modulators) is a major goal in drug discovery and necessary to develop drugs with better effectiveness and fewer side effects. Fortunately, direct measurements of ligand efficacy, via receptor conformational changes are possible with the recent development of conformational biosensors. In this review, we discuss classical efficacy models, including the two-state model, the ternary-complex model, and multistate models. We describe how nanobody-, transducer-, and receptor-based conformational biosensors detect and/or stabilize specific GPCR conformations to identify ligands with different levels of efficacy. In particular, conformational biosensors provide the potential to identify and/or characterize therapeutically desirable but often difficult to measure conformations of receptors faster and better than current methods. For drug discovery/development, several recent proof-of-principle studies have optimized conformational biosensors for high-throughput screening (HTS) platforms. However, their widespread use is limited by the fact that few sensors are reliably capable of detecting low-frequency conformations and technically demanding assay conditions. Nonetheless, conformational biosensors do help identify desirable ligands such as allosteric modulators, biased ligands, or partial agonists in a single assay, representing a distinct advantage over classical methods.

A NanoBiT assay to monitor membrane proteins trafficking for drug discovery and drug development

Internalization of membrane proteins plays a key role in many physiological functions; however, highly sensitive and versatile technologies are lacking to study such processes in real-time living systems. Here we describe an assay based on bioluminescence able to quantify membrane receptor trafficking for a wide variety of internalization mechanisms such as GPCR internalization/recycling, antibody-mediated internalization, and SARS-CoV2 viral infection. This study represents an alternative drug discovery tool to accelerate the drug development for a wide range of physiological processes, such as cancer, neurological, cardiopulmonary, metabolic, and infectious diseases including COVID-19.

Membrane protein trafficking is monitored using split nanoluciferase. Receptor internalization leads to complementation on the early endosome and a bioluminescent response, and is applied to receptor internalization/recycling, antibody-mediated internalization and SARS-CoV2 entry.

Spexin ameliorates skeletal muscle insulin resistance through activation of GAL2 receptor

Skeletal muscle is a principal tissue involved in energy expenditure and glucose metabolism. Although the results of our and other studies show that spexin could decrease food intake and obesity, the specific metabolic effect of spexin on glucose metabolism of skeletal muscle is still unclear. The aim of this study is to investigate whether spexin might mitigate obesity-induced insulin resistance in skeletal muscles and to explore its underlying mechanisms. The high fat diet-fed mice were treated with 50 μg/kg/d spexin for 21 consecutive days, and the differentiated myotubes of L6 were treated with spexin (200, 400, 800 nM) in the absence or presence of M871 (800 nM) for 12 h respectively. Besides, the galanin type 2 (GAL2) receptor knockdown myotubes were treated with 800 nM spexin for 12 h in this study. The present findings showed that spexin reversed hyperglycemia and glucose intolerance as well as insulin intolerance and insulin resistance in the mice fed with high fat diet. Furthermore, spexin markedly augmented the peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α) expression and deacetylation, and further triggered glucose transporter 4 (GLUT4) expression and trafficking in myotubes through p38 mitogen-activated protein kinase (P38MAPK) and protein kinase B (AKT) activation. More importantly, the elevation of glucose consumption related genes by spexin were abolished by GAL2 receptor antagonist or silencing of GAL2 receptor in myotubes. In conclusion, our findings provide a novel insight that spexin can protect against insulin resistance and increase glucose consumption in skeletal muscles mainly through activation of GAL2/GLUT4 signal pathway. Spexin might therefore be a novel therapeutic target for hyperglycemia and insulin resistance in clinic.

Spexin Promotes the Proliferation and Differentiation of C2C12 Cells In Vitro—The Effect of Exercise on SPX and SPX Receptor Expression in Skeletal Muscle In Vivo

SPX (spexin) and its receptors GalR2 and GalR3 (galanin receptor subtype 2 and galanin receptor subtype 3) play an important role in the regulation of lipid and carbohydrate metabolism in human and animal fat tissue. However, little is still known about the role of this peptide in the metabolism of muscle. The aim of this study was to determine the impact of SPX on the metabolism, proliferation and differentiation of the skeletal muscle cell line C2C12. Moreover, we determined the effect of exercise on the SPX transduction pathway in mice skeletal muscle. We found that increased SPX, acting via GalR2 and GalR3 receptors, and ERK1/2 phosphorylation stimulated the proliferation of C2C12 cells (p < 0.01). We also noted that SPX stimulated the differentiation of C2C12 by increasing mRNA and protein levels of differentiation markers Myh, myogenin and MyoD (p < 0.01). SPX consequently promoted myoblast fusion into the myotubule (p < 0.01). Moreover, we found that, in the first stage (after 2 days) of myocyte differentiation, GalR2 and GalR3 were involved, whereas in the last stage (day six), the effect of SPX was mediated by the GalR3 isoform. We also noted that exercise stimulated SPX and GalR2 expression in mice skeletal muscle as well as an increase in SPX concentration in blood serum. These new insights may contribute to a better understanding of the role of SPX in the metabolism of skeletal muscle.

The Neuropeptide Spexin Promotes the Osteoblast Differentiation of MC3T3-E1 Cells via the MEK/ERK Pathway and Bone Regeneration in a Mouse Calvarial Defect Model

BACKGROUND

The neural regulation of bone regeneration has emerged recently. Spexin (SPX) is a novel neuropeptide and regulates multiple biological functions. However, the effects of SPX on osteogenic differentiation need to be further investigated. Therefore, the aim of this study is to investigate the effects of SPX on osteogenic differentiation, possible underlying mechanisms, and bone regeneration.

METHODS

In this study, MC3T3-E1 cells were treated with various concentrations of SPX. Cell proliferation, osteogenic differentiation marker expressions, alkaline phosphatase (ALP) activity, and mineralization were evaluated using the CCK-8 assay, reverse transcriptase-quantitative polymerase chain reaction (RT-qPCR), ALP staining, and alizarin red S staining, respectively. To determine the underlying molecular mechanism of SPX, the phosphorylation levels of signaling molecules were examined via western blot analysis. Moreover, in vivo bone regeneration by SPX (0.5 and 1 µg/µl) was evaluated in a calvarial defect model. New bone formation was analyzed using micro-computed tomography (micro-CT) and histology.

RESULTS

The results indicated that cell proliferation was not affected by SPX. However, SPX significantly increased ALP activity, mineralization, and the expression of genes for osteogenic differentiation markers, including runt-related transcription factor 2 (Runx2), Alp, collagen alpha-1(I) chain (Col1a1), osteocalcin (Oc), and bone sialoprotein (Bsp). In contrast, SPX downregulated the expression of ectonucleotide pyrophosphatase/phosphodiesterase 1 (Enpp1). Moreover, SPX upregulated phosphorylated mitogen-activated protein kinase kinase (MEK1/2) and extracellular signal-regulated kinase (ERK1/2). In vivo studies, micro-CT and histologic analysis revealed that SPX markedly increased a new bone formation.

CONCLUSION

Overall, these results demonstrated that SPX stimulated osteogenic differentiation in vitro and increased in vivo bone regeneration via the MEK/ERK pathway.

The Association of Serum Circulating Neuropeptide Q and Chemerin Levels with Cardiometabolic Risk Factors among Patients with Metabolic Syndrome

The potential involvement of neuropeptide Q (NPQ) and chemerin (CHEM) in metabolic disorders is yet to be fully elucidated. The aim of this study was to evaluate serum concentrations of NPQ and CHEM and to establish their relationship with cardiometabolic risk factors among individuals with metabolic syndrome. A total of 66 patients with metabolic syndrome (MetS) and 83 healthy volunteers (non-MetS) underwent biochemical, blood pressure, and anthropometric measurements. The concentration of NPQ in the MetS group was significantly lower (0.47 (0.34 ; 0.54) vs. 0.52 (0.43 ; 0.60) ng/mL, p = 0.015) than in non-MetS, while there were no differences in CHEM level. In the entire study population, we observed several negative correlations between NPQ concentration and waist-hip ratio (WHR), visceral adipose tissue, diastolic blood pressure (DBP), triglycerides (TG) along with a positive correlation with high-density lipoprotein (HDL), total muscle mass, and CHEM. Moreover, a negative correlation was observed in the MetS group between NPQ and glycemia. CHEM showed no significant correlations with cardiometabolic risk factors in the study population. In a multiple regression model, the total muscle mass proved to be an independent factor determining NPQ concentration in the population (p < 0.00000001, R²_adj = 28.6%). NPQ seems to protect against metabolic disorders correlated with obesity. Thus, it is worth considering NPQ level as a candidate protective biomarker of metabolic syndrome complications.

Spexin: Its role, regulation, and therapeutic potential in the hypothalamus

Spexin is the most recently discovered member of the galanin/kisspeptin/spexin family of peptides. This 14-amino acid peptide is highly conserved and is implicated in homeostatic functions including, but not limited to, metabolism, energy homeostasis, and reproduction. Spexin is expressed by neurons in the hypothalamus, which coordinate energy homeostasis and reproduction. Critically, levels of spexin appear to be altered in disorders related to energy homeostasis and reproduction, such as obesity, diabetes, and polycystic ovarian syndrome. In this review, we discuss the evidence for the involvement of spexin in the hypothalamic control of energy homeostasis and reproduction. The anorexigenic properties of spexin have been attributed to its effects on the energy-regulating neuropeptide Y/agouti-related peptide neurons and proopiomelanocortin neurons. While the role of spexin in reproduction remains unclear, there is evidence that gonadotropin-releasing hormone expressing neurons may produce and respond to spexin. Furthermore, we discuss the disorders and concomitant treatments, which have been reported to alter spexin expression, as well as the underlying signaling mechanisms that may be involved. Finally, we discuss the biochemical basis of spexin, its interaction with its cognate receptors, and how this information can be adapted to develop therapeutics for disorders related to the alteration of energy homeostasis and reproduction.

Novel insights on the role of spexin as a biomarker of obesity and related cardiometabolic disease

Spexin (SPX) is a 14-amino acid neuropeptide, discovered recently using bioinformatic techniques. It is encoded by the Ch12:orf39 gene that is widely expressed in different body tissues/organs across species, and secreted into systemic circulation. Recent reports have highlighted a potentially important regulatory role of SPX in obesity and related comorbidities. SPX is also ubiquitously expressed in human tissues, including white adipose tissue. The circulating concentration of SPX is significantly lower in individuals with obesity compared to normal weight counterparts. SPX's role in obesity appears to be related to various factors, such as the regulation of energy expenditure, appetite, and eating behaviors, increasing locomotion, and inhibiting long-chain fatty acid uptake into adipocytes. Recent reports have also suggested SPX's relationship with novel biomarkers of cardiovascular disease (CVD) and glucose metabolism and evoked the potential role of SPX as a key biomarker/player in the early loss of cardiometabolic health and development of CVD and diabetes later in life. Data on age-related changes in SPX and SPX's response to various interventions are also emerging. The current review focuses on the role of SPX in obesity and related comorbidities across the life span, and its response to interventions in these conditions. It is expected that this article will provide new ideas for future research on SPX and its metabolic regulation, particularly related to cardiometabolic diseases.

Emerging roles of kisspeptin/galanin in age-related metabolic disease

Age is a major risk factor for developing metabolic diseases such as obesity and diabetes. There is an unprecedented rise in obesity and type 2 diabetes in recent decades. A convincing majority of brain-gut peptides are associated with a higher risk to develop metabolic disorders, and may contribute to the pathophysiology of age-related metabolic diseases. Accumulating basic studies revealed an intriguing role of kisspeptin and galanin involved in the amelioration of insulin resistance in different ways. In patients suffered from obesity and diabetes a significant, sex-related changes in the plasma kisspeptin and galanin levels occurred. Kisspeptin is anorexigenic to prevent obesity, its level is negatively correlative with obesity and insulin resistance. While galanin is appetitive to stimulate food intake and body weight, its level is positively correlative with obesity, HOMA-IR and glucose/triglyceride concentration. In turn, kisspeptin and galanin also distinctly increase glucose uptake and utilization as well as energy expenditure. This article reviews recent evidence dealing with the role of kisspeptin and galanin in the pathophysiology of age-related metabolic diseases. It should be therefore taken into account that the targeted modulation of those peptidergic signaling may be potentially helpful in the future treatment of age-related metabolic diseases.

Engineering with NanoLuc: a playground for the development of bioluminescent protein switches and sensors

The small engineered luciferase NanoLuc has rapidly become a powerful tool in the fields of biochemistry, chemical biology, and cell biology due to its exceptional brightness and stability. The continuously expanding NanoLuc toolbox has been employed in applications ranging from biosensors to molecular and cellular imaging, and currently includes split complementation variants, engineering techniques for spectral tuning, and bioluminescence resonance energy transfer-based concepts. In this review, we provide an overview of state-of-the-art NanoLuc-based sensors and switches with a focus on the underlying protein engineering approaches. We discuss the advantages and disadvantages of various strategies with respect to sensor sensitivity, modularity, and dynamic range of the sensor and provide a perspective on future strategies and applications.

Mouse Spexin: (III) Differential Regulation by Glucose and Insulin in Glandular Stomach and Functional Implication in Feeding Control

Spexin (SPX), a neuropeptide with diverse functions, is a novel satiety factor in fish models and its role in feeding control has been recently confirmed in mammals. In mouse, food intake was shown to trigger SPX expression in glandular stomach with parallel rise in serum SPX and these SPX signals could inhibit feeding via central actions within the hypothalamus. However, the mechanisms for SPX regulation by food intake are still unclear. To examine the role of insulin signal caused by glucose uptake in SPX regulation, the mice were IP injected with glucose and insulin, respectively. In this case, serum SPX was elevated by glucose but not altered by insulin. Meanwhile, SPX transcript expression in the glandular stomach was up-regulated by glucose but the opposite was true for insulin treatment. Using in situ hybridization, the differential effects on SPX gene expression were located in the gastric mucosa of glandular stomach. Co-injection experiments also revealed that glucose stimulation on serum SPX and SPX mRNA expressed in glandular stomach could be blocked by insulin. In gastric mucosal cells prepared from glandular stomach, the opposite effects on SPX transcript expression by glucose and insulin could still be noted with similar blockade of the stimulatory effects of glucose by insulin. In this cell model, SPX gene expression induced by glucose was mediated by glucose uptake via GLUT, ATP synthesis by glycolysis/respiratory chain, and subsequent modulation of KATP channel activity, but the voltage-sensitive Ca2+ channels were not involved. The corresponding inhibition by insulin, however, was mediated by PI3K/Akt, MEK1/2/ERK1/2, and P38MAPK cascades coupled to insulin receptor but not IGF-1 receptor. Apparently, glucose uptake in mice can induce SPX expression in the glandular stomach through ATP synthesis via glucose metabolism and subsequent modification of KATP channel activity, which may contribute to SPX release into circulation to act as the satiety signal after food intake. The insulin rise caused by glucose uptake, presumably originated from the pancreas, may serve as a negative feedback to inhibit the SPX response by activating MAPK and PI3K/Akt pathways in the stomach.

A novel luminescence-based β-arrestin recruitment assay for unmodified receptors

G protein-coupled receptors (GPCRs) signal through activation of G proteins and subsequent modulation of downstream effectors. More recently, signaling mediated by β-arrestin has also been implicated in important physiological functions. This has led to great interest in the identification of biased ligands that favor either G protein or β-arrestin-signaling pathways. However, nearly all screening techniques for measuring β-arrestin recruitment have required C-terminal receptor modifications that can in principle alter protein interactions and thus signaling. Here, we have developed a novel luminescence-based assay to measure β-arrestin recruitment to the membrane or early endosomes by unmodified receptors. Our strategy uses NanoLuc, an engineered luciferase from Oplophorus gracilirostris (deep-sea shrimp) that is smaller and brighter than other well-established luciferases. Recently, several publications have explored functional NanoLuc split sites for use in complementation assays. We have identified a unique split site within NanoLuc and fused the corresponding N-terminal fragment to either a plasma membrane or early endosome tether and the C-terminal fragment to β-arrestins, which form the basis for the MeNArC and EeNArC assays, respectively. Upon receptor activation, β-arrestin is recruited to the membrane and subsequently internalized in an agonist concentration-dependent manner. This recruitment promotes complementation of the two NanoLuc fragments, thereby reconstituting functional NanoLuc, allowing for quantification of β-arrestin recruitment with a single luminescence signal. Our assay avoids potential artifacts related to C-terminal receptor modification and has promise as a new generic assay for measuring β-arrestin recruitment to diverse GPCR types in heterologous or native cells.

Dopamine regulates pancreatic glucagon and insulin secretion via adrenergic and dopaminergic receptors

Dopamine (DA) and norepinephrine (NE) are catecholamines primarily studied in the central nervous system that also act in the pancreas as peripheral regulators of metabolism. Pancreatic catecholamine signaling has also been increasingly implicated as a mechanism responsible for the metabolic disturbances produced by antipsychotic drugs (APDs). Critically, however, the mechanisms by which catecholamines modulate pancreatic hormone release are not completely understood. We show that human and mouse pancreatic α- and β-cells express the catecholamine biosynthetic and signaling machinery, and that α-cells synthesize DA de novo. This locally-produced pancreatic DA signals via both α- and β-cell adrenergic and dopaminergic receptors with different affinities to regulate glucagon and insulin release. Significantly, we show DA functions as a biased agonist at α_2A-adrenergic receptors, preferentially signaling via the canonical G protein-mediated pathway. Our findings highlight the interplay between DA and NE signaling as a novel form of regulation to modulate pancreatic hormone release. Lastly, pharmacological blockade of DA D₂-like receptors in human islets with APDs significantly raises insulin and glucagon release. This offers a new mechanism where APDs act directly on islet α- and β-cell targets to produce metabolic disturbances.

Functions of galanin, spexin and kisspeptin in metabolism, mood and behaviour

The bioactive peptides galanin, spexin and kisspeptin have a common ancestral origin and their pathophysiological roles are increasingly the subject of investigation. Evidence suggests that these bioactive peptides play a role in the regulation of metabolism, pancreatic β-cell function, energy homeostasis, mood and behaviour in several species, including zebrafish, rodents and humans. Galanin signalling suppresses insulin secretion in animal models (but not in humans), is potently obesogenic and plays putative roles governing certain evolutionary behaviours and mood modulation. Spexin decreases insulin secretion and has potent anorectic, analgesic, anxiolytic and antidepressive-like effects in animal models. Kisspeptin modulates glucose-stimulated insulin secretion, food intake and/or energy expenditure in animal models and humans. Furthermore, kisspeptin is implicated in the control of reproductive behaviour in animals, modulation of human sexual and emotional brain processing, and has antidepressive and fear-suppressing effects. In addition, galanin-like peptide is a further member of the galaninergic family that plays emerging key roles in metabolism and behaviour. Therapeutic interventions targeting galanin, spexin and/or kisspeptin signalling pathways could therefore contribute to the treatment of conditions ranging from obesity to mood disorders. However, many gaps and controversies exist, which must be addressed before the therapeutic potential of these bioactive peptides can be established.

Mouse Spexin: (II) Functional Role as a Satiety Factor inhibiting Food Intake by Regulatory Actions Within the Hypothalamus

Spexin (SPX) is a pleiotropic peptide with highly conserved protein sequence from fish to mammals and its biological actions are mediated by GalR2/GalR3 receptors expressed in target tissues. Recently, SPX has been confirmed to be a novel satiety factor in fish species but whether the peptide has a similar function in mammals is still unclear. Using the mouse as a model, the functional role of SPX in feeding control and the mechanisms involved were investigated. After food intake, serum SPX in mice could be up-regulated with elevations of transcript expression and tissue content of SPX in the glandular stomach but not in other tissues examined. As revealed by immunohistochemical staining, food intake also intensified SPX signals in the major cell types forming the gastric glands (including the foveolar cells, parietal cells, and chief cells) within the gastric mucosa of glandular stomach. Furthermore, IP injection of SPX was effective in reducing food intake with parallel attenuation in transcript expression of NPY, AgRP, NPY type 5 receptor (NPY5R), and ghrelin receptor (GHSR) in the hypothalamus, and these inhibitory effects could be blocked by GalR3 but not GalR2 antagonism. In agreement with the central actions of SPX, similar inhibition on feeding and hypothalamic expression of NPY, AgRP, NPY5R, and GHSR could also be noted with ICV injection of SPX. In the same study, in contrast to the drop in NPY5R and GHSR, SPX treatment could induce parallel rises of transcript expression of leptin receptor (LepR) and melanocortin 4 receptor (MC4R) in the hypothalamus. These findings, as a whole, suggest that the role of SPX as a satiety factor is well conserved in the mouse. Apparently, food intake can induce SPX production in glandular stomach and contribute to the postprandial rise of SPX in circulation. Through GalR3 activation, this SPX signal can act within the hypothalamus to trigger feedback inhibition on feeding by differential modulation of feeding regulators (NPY and AgRP) and their receptors (NPY5R, GHSR, LepR, and MC4R) involved in the feeding circuitry within the CNS.

NanoLuc-Based Methods to Measure β-Arrestin2 Recruitment to G Protein-Coupled Receptors

Cytosolic β-arrestins are key regulators of G protein-coupled receptors (GPCRs) by sterically uncoupling G protein activation, facilitating receptor internalization, and/or acting as G protein-independent signaling scaffolds. The current awareness that GPCR ligands may display bias toward G protein signaling or β-arrestin recruitment makes β-arrestin recruitment assays important additions to the drug discovery toolbox. This chapter describes two NanoLuc-based methods to monitor β-arrestin2 recruitment to the human histamine H₁ receptor by measuring bioluminescence resonance energy transfer and enzyme-fragment complementation in real-time on living cells with reasonable high throughput. In addition to the detection of agonism, both assay formats can be used to qualitatively evaluate the binding kinetics of antihistamines on the human histamine H₁ receptor.

Luciferase Complementation Approaches to Measure GPCR Signaling Kinetics and Bias

An understanding of the kinetic contributions to G protein-coupled receptor pharmacology and signaling is increasingly important in compound profiling. Nonequilibrium conditions are commonly present in vivo, for example, as the drug competes with dynamic changes in hormone or neurotransmitter concentration for the receptor. Under such conditions individual binding kinetic properties of the ligands can influence duration of action, local ligand concentration, and functional properties such as the degree of insurmountable inhibition. Mapping the kinetic patterns of GPCR signaling events elicited by agonists, rather than a peak response at a single timepoint, is often key to predicting their functional impact. This is also a path to a better understanding of the origins of ligand bias, and whether such ligands demonstrate their effects through selection of distinct GPCR conformations, or via their kinetic properties. Recent developments in complementation approaches, based on a small bright shrimp luciferase Nanoluc, provide a new route to kinetic analysis of GPCR signaling in living cells that is amenable to the throughput required for compound profiling. In the NanoBiT luciferase complementation system, GPCRs and effector proteins are tagged with Nanoluc fragments optimized for their low interacting affinity and stability. The interactions brought about by GPCR recruitment of the effector are reproduced by a rapid and reversible increase in NanoBiT luminescence, in the presence of its substrate furimazine. Here we discuss the methods for optimizing and validating the GPCR NanoBiT assays, and protocols for their application to study endpoint and kinetic aspects of agonist and antagonist pharmacology. We also describe how timecourse families of agonist concentration response curves, derived from a single NanoBiT assay experiment, can be used to evaluate the kinetic components in operational model derived parameters of ligand bias.

Palmitate differentially regulates Spexin, and its receptors Galr2 and Galr3, in GnRH neurons through mechanisms involving PKC, MAPKs, and TLR4

The function of the gonadotropin-releasing hormone (GnRH) neuron is critical to maintain reproductive function and a significant decrease in GnRH can lead to disorders affecting fertility, including hypogonadotropic hypogonadism. Spexin (SPX) is a novel hypothalamic neuropeptide that exerts inhibitory effects on reproduction and feeding by acting through galanin receptor 2 (GALR2) and galanin receptor 3 (GALR3). Fatty acids can act as nutritional signals that regulate the hypothalamic-pituitary-gonadal (HPG) axis, and elevated levels of circulating saturated fatty acids associated with high fat diet (HFD)-feeding have been shown to induce neuroinflammation, endoplasmic reticulum stress and hormonal resistance in the hypothalamus, as well as alter neuropeptide expression. We previously demonstrated that palmitate, the most common saturated fatty acid in a HFD, elevates the expression of Spx, Galr2 and Galr3 mRNA in a model of appetite-regulating neuropeptide Y hypothalamic neurons. Here, we found that Spx, Galr2 and Galr3 mRNA were also significantly induced by palmitate in a model of reproductive GnRH neurons, mHypoA-GnRH/GFP. As a follow-up to our previous report, we examined the molecular pathways by which Spx and galanin receptor mRNA was regulated in this cell line. Furthermore, we performed inhibitor studies, which revealed that the effect of palmitate on Spx and Galr3 mRNA involved activation of the innate immune receptor TLR4, and we detected differential regulation of the three genes by the protein kinases PKC, JNK, ERK, and p38. However, the intracellular metabolism of palmitate to ceramide did not appear to be involved in the palmitate-mediated gene regulation. Overall, this suggests that SPX may play a role in reproduction at the level of the hypothalamus and the pathways by which Spx, Galr2 and Galr3 are altered by fatty acids could provide insight into the mechanisms underlying reproductive dysfunction in obesity.

CXCR7: a β-arrestin-biased receptor that potentiates cell migration and recruits β-arrestin2 exclusively through Gβγ subunits and GRK2

BACKGROUND

Some chemokine receptors referred to as atypical chemokine receptors (ACKRs) are thought to non-signaling decoys because of their inability to activate typical G-protein signaling pathways. CXCR7, also known as ACKR3, binds to only two chemokines, SDF-1α and I-TAC, and recruits β-arrestins. SDF-1α also binds to its own conventional receptor, CXCR4, involving in homeostatic modulation such as development and immune surveillance as well as pathological conditions such as inflammation, ischemia, and cancers. Recently, CXCR7 is suggested as a key therapeutic target together with CXCR4 in such conditions. However, the molecular mechanisms underlying cellular responses and functional relation with CXCR7 and CXCR4 have not been elucidated, despite massive studies. Therefore, we aimed to reveal the molecular networks of CXCR7 and CXCR4 and compare their effects on cell migration.

METHODS

Base on structural complementation assay using NanoBiT technology, we characterized the distinct mechanisms underlying β-arrestin2 recruitment by both CXCR4 and CXCR7. Crosslinking and immunoprecipitation were conducted to analyze complex formation of the receptors. Gene deletion using CRISPR and reconstitution of the receptors were applied to analysis of ligand-dependent ERK phosphorylation and cell migration. All experiments were performed in triplicate and repeated more than three times. Unpaired Student's t-tests or ANOVA using PRISM5 software were employed for statistical analyses.

RESULTS

Ligand binding to CXCR7 does not result in activation of typical signaling pathways via Gα subunits but activation of GRK2 via βγ subunits and receptor phosphorylation with subsequent β-arrestin2 recruitment. In contrast, CXCR4 induced Gα_i activation and recruited β-arrestin2 through C-terminal phosphorylation by both GRK2 and GRK5. SDF-1α-stimulated ERK phosphorylation was facilitated by CXCR4, but not CXCR7. Heterodimerization of CXCR4 and CXCR7 was not confirmed in this study, while homodimerization of them was verified by crosslinking experiment and NanoBiT assay. Regarding chemotaxis, SDF-1α-stimulated cell migration was mediated by both CXCR4 and CXCR7.

CONCLUSION

This study demonstrates that SDF-1α-stimulated CXCR7 mediates β-arrestin2 recruitment via different molecular networking from that of CXCR4. CXCR7 may be neither a simple scavenger nor auxiliary receptor but plays an essential role in cell migration through cooperation with CXCR4.

CRISPR-Mediated Protein Tagging with Nanoluciferase to Investigate Native Chemokine Receptor Function and Conformational Changes

G protein-coupled receptors are a major class of membrane receptors that mediate physiological and pathophysiological cellular signaling. Many aspects of receptor activation and signaling can be investigated using genetically encoded luminescent fusion proteins. However, the use of these biosensors in live cell systems requires the exogenous expression of the tagged protein of interest. To maintain the normal cellular context here we use CRISPR/Cas9-mediated homology-directed repair to insert luminescent tags into the endogenous genome. Using NanoLuc and bioluminescence resonance energy transfer we demonstrate fluorescent ligand binding at genome-edited chemokine receptors. We also demonstrate that split-NanoLuc complementation can be used to investigate conformational changes and internalization of CXCR4 and that recruitment of β-arrestin2 to CXCR4 can be monitored when both proteins are natively expressed. These results show that genetically encoded luminescent biosensors can be used to investigate numerous aspects of receptor function at native expression levels.

Exploring the molecular structures that confer ligand selectivity for galanin type II and III receptors

Galanin receptors (GALRs) belong to the superfamily of G-protein coupled receptors. The three GALR subtypes (GALR1, GALR2, and GALR3) are activated by their endogenous ligands: spexin (SPX) and galanin (GAL). The synthetic SPX-based GALR2-specific agonist, SG2A, plays a dual role in the regulation of appetite and depression-like behaviors. Little is known, however, about the molecular interaction between GALR2 and SG2A. Using site-directed mutagenesis and domain swapping between GALR2 and GALR3, we identified residues in GALR2 that promote interaction with SG2A and residues in GALR3 that inhibit interaction with SG2A. In particular, Phe¹⁰³, Phe¹⁰⁶, and His¹¹⁰ in the transmembrane helix 3 (TM3) domain; Val¹⁹³, Phe¹⁹⁴, and Ser¹⁹⁵ in the TM5 domain; and Leu²⁷³ in the extracellular loop 3 (ECL3) domain of GALR2 provide favorable interactions with the Asn⁵, Ala⁷, Phe¹¹, and Pro¹³ residues of SG2A. Our results explain how SG2A achieves selective interaction with GALR2 and inhibits interaction with GALR3. The results described here can be used broadly for in silico virtual screening of small molecules for the development of GALR subtype-specific agonists and/or antagonists.

β-Arrestin-2 BRET Biosensors Detect Different β-Arrestin-2 Conformations in Interaction with GPCRs

β-Arrestins are critical regulators of G protein-coupled receptors (GPCRs) that desensitize G protein signaling, promote receptor internalization, and initiate signaling on their own. Recent structural findings indicate that β-arrestins adopt different conformations upon interaction with agonist-activated GPCRs. Here, we established a β-arrestin-2 conformational bioluminescence resonance energy transfer (BRET) sensor composed of the bright Nanoluc BRET donor and the red-shifted CyOFP1 BRET acceptor. The sensor monitors early intramolecular conformational changes of β-arrestin-2 in complex with a wide panel of different class A and class B GPCRs upon agonist activation and with orphan GPCRs known to spontaneously recruit β-arrestin-2. The introduction of the R170E mutant in the β-arrestin-2 sensor allowed the detection of a partially active β-arrestin-2 conformation, which is not dependent on receptor phosphorylation, while the deletion of the β-arrestin-2 finger-loop region detected the "tail-conformation" corresponding to the interaction of β-arrestin with the carboxyl-terminal domain of GPCRs. The new sensors combine the advantages of the BRET technique in terms of sensitivity, robustness, and suitability for real-time measurements with a high responsiveness toward early conformational changes to help to elucidate the different conformational states of β-arrestins associated with GPCR activation in living cells.

Spexin and a Novel Cichlid-Specific Spexin Paralog Both Inhibit FSH and LH Through a Specific Galanin Receptor (Galr2b) in Tilapia

Spexin (SPX) is a 14 amino acid peptide hormone that has pleiotropic functions across vertebrates, one of which is involvement in the brain-pituitary-gonad axis of fish. SPX(1) has been identified in each class of vertebrates, and a second SPX (named SPX2) has been found in some non-mammalian species. We have cloned two spexin paralogs, designated as Spx1a and Spx1b, from Nile tilapia (Oreochromis niloticus) that have varying tissue distribution patterns. Spx1b is a novel peptide only identified in cichlid fish, and is more closely related to Spx1 than Spx2 homologs as supported by phylogenetic, synteny, and functional analyses. Kisspeptin, Spx, and galanin (Gal) peptides and their corresponding kiss receptors and Gal receptors (Galrs), respectively, are evolutionarily related. Cloning of six tilapia Galrs (Galr1a, Galr1b, Galr2a, Galr2b, Galr type 1, and Galr type 2) and subsequent in vitro second-messenger reporter assays for Gα_s, Gα_q, and Gα_i suggests that Gal and Spx activate Galr1a/Galr2a and Galr2b, respectively. A decrease in plasma follicle stimulating hormone and luteinizing hormone concentrations was observed with injections of Spx1a or Spx1b in vivo. Additionally, application of Spx1a and Spx1b to pituitary slices decreased the firing rate of LH cells, suggesting that the peptides can act directly at the level of the pituitary. These data collectively suggest an inhibitory mechanism of action against the secretion of gonadotropins for a traditional and a novel spexin paralog in cichlid species.

Galanin peptide family regulation of glucose metabolism

Recent preclinical and clinical studies have indicated that the galanin peptide family may regulate glucose metabolism and alleviate insulin resistance, which diminishes the probability of type 2 diabetes mellitus. The galanin was discovered in 1983 as a gut-derived peptide hormone. Subsequently, galanin peptide family was found to exert a series of metabolic effects, including the regulation of gut motility, body weight and glucose metabolism. The galanin peptide family in modulating glucose metabolism received recently increasing recognition because pharmacological activiation of galanin signaling might be of therapeutic value to improve insuin resistance and type 2 diabetes mellitus. To date, however, few papers have summarized the role of the galanin peptide family in modulating glucose metabolism and insulin resistance. In this review we summarize the metabolic effect of galanin peptide family and highlight its glucoregulatory action and discuss the pharmacological value of galanin pathway activiation for the treatment of glucose intolerance and type 2 diabetes mellitus.

Genetic code expansion and photocross-linking identify different β-arrestin binding modes to the angiotensin II type 1 receptor

The angiotensin II (AngII) type 1 receptor (AT1R) is a member of the G protein-coupled receptor (GPCR) family and binds β-arrestins (β-arrs), which regulate AT1R signaling and trafficking. These processes can be biased by different ligands or mutations in the AGTR1 gene. As for many GPCRs, the exact details for AT1R-β-arr interactions driven by AngII or β-arr-biased ligands remain largely unknown. Here, we used the amber-suppression technology to site-specifically introduce the unnatural amino acid (UAA) p-azido-l-phenylalanine (azF) into the intracellular loops (ICLs) and the C-tail of AT1R. Our goal was to generate competent photoreactive receptors that can be cross-linked to β-arrs in cells. We performed UV-mediated photolysis of 25 different azF-labeled AT1Rs to cross-link β-arr1 to AngII-bound receptors, enabling us to map important contact sites in the C-tail and in the ICL2 and ICL3 of the receptor. The extent of AT1R-β-arr1 cross-linking among azF-labeled receptors differed, revealing variability in β-arr's contact mode with the different AT1R domains. Moreover, the signature of ligated AT1R-β-arr complexes from a subset of azF-labeled receptors also differed between AngII and β-arr-biased ligand stimulation of receptors and between azF-labeled AT1R bearing and that lacking a bias signaling mutation. These observations further implied distinct interaction modalities of the AT1R-β-arr1 complex in biased signaling conditions. Our findings demonstrate that this photocross-linking approach is useful for understanding GPCR-β-arr complexes in different activation states and could be extended to study other protein-protein interactions in cells.

Using the novel HiBiT tag to label cell surface relaxin receptors for BRET proximity analysis

Relaxin family peptide 1 (RXFP1) is the receptor for relaxin a peptide hormone with important therapeutic potential. Like many G protein-coupled receptors (GPCRs), RXFP1 has been reported to form homodimers. Given the complex activation mechanism of RXFP1 by relaxin, we wondered whether homodimerization may be explicitly required for receptor activation, and therefore sought to determine if there is any relaxin-dependent change in RXFP1 proximity at the cell surface. Bioluminescence resonance energy transfer (BRET) between recombinantly tagged receptors is often used in GPCR proximity studies. RXFP1 targets poorly to the cell surface when overexpressed in cell lines, with the majority of the receptor proteins sequestered within the cell. Thus, any relaxin-induced changes in RXFP1 proximity at the cell surface may be obscured by BRET signal originating from intracellular compartments. We therefore, utilized the newly developed split luciferase system called HiBiT to specifically label the extracellular terminus of cell surface RXFP1 receptors in combination with mCitrine-tagged receptors, using the GABAB heterodimer as a positive control. This demonstrated that the BRET signal detected from RXFP1-RXFP1 proximity at the cell surface does not appear to be due to stable physical interactions. The fact that there is also no relaxin-mediated change in RXFP1-RXFP1 proximity at the cell surface further supports these conclusions. This work provides a basis by which cell surface GPCR proximity and expression levels can be specifically studied using a facile and homogeneous labeling technique such as HiBiT.

Emerging Roles of NPQ/Spexin in Physiology and Pathology

Spexin (SPX), also called neuropeptide Q (NPQ), is a novel endogenous neuropeptide. Spexin gene and protein are widely expressed in central nervous system and peripheral tissues in humans, rodents, goldfish, etc. A few of physiological and pathological roles of spexin are gradually emerged recently. This article summarized the roles of spexin in feeding behavior, gastrointestinal motility, obesity, diabetes, energy metabolism, endocrine, mental diseases, and cardiovascular function. Given the broad roles of spexin, this neuropeptide has attracted much interest from investigators and will be as a promising future target for novel therapeutic research and drug design.

Spexin-Based Galanin Receptor Type 2 Agonist for Comorbid Mood Disorders and Abnormal Body Weight

Despite the established comorbidity between mood disorders and abnormal eating behaviors, the underlying molecular mechanism and therapeutics remain to be resolved. Here, we show that a spexin-based galanin receptor type 2 agonist (SG2A) simultaneously normalized mood behaviors and body weight in corticosterone pellet-implanted (CORTI) mice, which are underweight and exhibit signs of anhedonia, increased anxiety, and depression. Administration of SG2A into the lateral ventricle produced antidepressive and anxiolytic effects in CORTI mice. Additionally, SG2A led to a recovery of body weight in CORTI mice while it induced significant weight loss in normal mice. In Pavlovian fear-conditioned mice, SG2A decreased contextual and auditory fear memory consolidation but accelerated the extinction of acquired fear memory without altering innate fear and recognition memory. The main action sites of SG2A in the brain may include serotonergic neurons in the dorsal raphe nucleus for mood control, and proopiomelanocortin/corticotropin-releasing hormone neurons in the hypothalamus for appetite and body weight control. Furthermore, intranasal administration of SG2A exerted the same anxiolytic and antidepressant-like effects and decreased food intake and body weight in a dose-dependent manner. Altogether, these results indicate that SG2A holds promise as a clinical treatment for patients with comorbid mood disorders and abnormal appetite/body weight.

NanoBRET: The Bright Future of Proximity-Based Assays

Bioluminescence resonance energy transfer (BRET) is a biophysical technique used to monitor proximity within live cells. BRET exploits the naturally occurring phenomenon of dipole-dipole energy transfer from a donor enzyme (luciferase) to an acceptor fluorophore following enzyme-mediated oxidation of a substrate. This results in production of a quantifiable signal that denotes proximity between proteins and/or molecules tagged with complementary luciferase and fluorophore partners. BRET assays have been used to observe an array of biological functions including ligand binding, intracellular signaling, receptor-receptor proximity, and receptor trafficking, however, BRET assays can theoretically be used to monitor the proximity of any protein or molecule for which appropriate fusion constructs and/or fluorophore conjugates can be produced. Over the years, new luciferases and approaches have been developed that have increased the potential applications for BRET assays. In particular, the development of the small, bright and stable Nanoluciferase (NanoLuc; Nluc) and its use in NanoBRET has vastly broadened the potential applications of BRET assays. These advances have exciting potential to produce new experimental methods to monitor protein-protein interactions (PPIs), protein-ligand interactions, and/or molecular proximity. In addition to NanoBRET, Nluc has also been exploited to produce NanoBiT technology, which further broadens the scope of BRET to monitor biological function when NanoBiT is combined with an acceptor. BRET has proved to be a powerful tool for monitoring proximity and interaction, and these recent advances further strengthen its utility for a range of applications.

Distribution and neuronal circuit of spexin 1/2 neurons in the zebrafish CNS

Spexin (SPX) is a highly conserved neuropeptide that is widely expressed in mammalian brain and peripheral tissue. In teleost, SPX1 is mainly expressed in the brain and ovary, and is involved in reproduction and food intake. A second form of SPX, SPX2, was recently identified in chick, Xenopus, and zebrafish. The expression pattern and roles of SPX2 are unknown. SPX (spx1) is highly expressed in the vertebrate brain, but its distribution, circuits, and interactions with its putative receptor are unknown. Here, we observed expression of spx1 in the midbrain and hindbrain, and spx2 in the hypothalamic preoptic area using in situ RNA hybridization in zebrafish. Analysis of transgenic reporter zebrafish revealed that hindbrain SPX1 neurons are PAX2⁺ inhibitory interneurons and project to the spinal cord, where they interact with galanin receptor 2b (GALR2b) neurons, suggesting that hindbrain SPX1 neurons are reticulospinal neurons. spx1 mRNA and SPX1 reporter expression were observed in dorsal habenula (dHb). SPX1 neurons in the dHb project to the interpeduncular nucleus (IPN), where GALR2a and GALR2b expression was also observed, suggesting that habenula SPX1 neurons may interact with GALR2a/2b in the IPN.

Evolution of Structural and Functional Diversity of Spexin in Mammalian and Non-mammalian Vertebrate Species

Spexin (SPX) is a novel neuropeptide, which was first identified in the human genome using bioinformatics. Since then, orthologs of human SPX have been identified in mammalian and non-mammalian vertebrates. The mature sequence of SPX, NWTPQAMLYLKGAQ, is evolutionally conserved across vertebrate species, with some variations in teleost species where Ala at position 13 is substituted by Thr. In mammals, the gene structure of SPX comprises six exons and five introns, however, variation exists within non-mammalian species, goldfish and zebrafish having five exons while grouper has six exons. Phylogenetic and synteny analysis, reveal that SPX is grouped together with two neuropeptides, kisspeptin (KISS) and galanin (GAL) as a family of peptides with a common evolutionary ancestor. A paralog of SPX, termed SPX2 has been identified in non-mammalians but not in the mammalian genome. Ligand-receptor interaction study also shows that SPX acts as a ligand for GAL receptor 2 (2a and 2b in non-mammalian vertebrates) and 3. SPX acts as a neuromodulator with multiple central and peripheral physiological roles in the regulation of insulin release, fat metabolism, feeding behavior, and reproduction. Collectively, this review provides a comprehensive overview of the evolutionary diversity as well as molecular and physiological roles of SPX in mammalian and non-mammalian vertebrate species.