Heterologous regulation of trafficking and signaling of G protein-coupled receptors: beta-arrestin-dependent interactions between neurokinin receptors

Biased agonism in peptide-GPCRs: A structural perspective

G protein-coupled receptors (GPCRs) are dynamic membrane receptors that transduce extracellular signals to the cell interior by forming a ligand-receptor-effector (ternary) complex that functions via allosterism. Peptides constitute an important class of ligands that interact with their cognate GPCRs (peptide-GPCRs) to form the ternary complex. "Biased agonism", a therapeutically relevant phenomenon exhibited by GPCRs owing to their allosteric nature, has also been observed in peptide-GPCRs, leading to the development of selective therapeutics with fewer side effects. In this review, we have focused on the structural basis of signalling bias at peptide-GPCRs of classes A and B, and reviewed the therapeutic relevance of bias at peptide-GPCRs, with the hope of contributing to the discovery of novel biased peptide drugs.

Noval insights and therapeutic strategies for tumor-induced kidney pathologies

Recent progress in elucidating the role of specific antidiuretic hormones in Drosophila models has provided valuable insights into the mechanisms underlying tumor-induced renal dysfunction. Xu et al. identified the mammalian neurokinin 3 receptor (TACR3), a homolog of the G protein-coupled receptor TkR99D in fruit flies, as a potential therapeutic target for alleviating renal tubular dysfunction in mice with malignant neoplasms. Here, we commented on these findings by emphasizing the structural and evolutionary significance of TACR3 and provided an in-depth analysis of cell type specific expression of TACR3 in response to renal injury and expressional dynamics during renal carcinoma progression. The implications of these findings for transforming the therapeutic approaches to renal complications associated with oncological disorders were highlighted. We also acknowledged the limitations of current experimental models in this study and emphasized the necessary clinical validation in the future. These insights could contribute to the advancement of diagnostic and therapeutic strategies for treating tumor-related renal pathologies.

Semi-synthetic nanobody-ligand conjugates exhibit tunable signaling properties and enhanced transcriptional outputs at neurokinin receptor-1

Antibodies have proven highly valuable for therapeutic development; however, they are typically poor candidates for applications that require activation of G protein-coupled receptors (GPCRs), the largest collection of targets for clinically approved drugs. Nanobodies (Nbs), the smallest antibody fragments retaining full antigen-binding capacity, have emerged as promising tools for pharmacologic applications, including GPCR modulation. Past work has shown that conjugation of Nbs with ligands can provide GPCR agonists that exhibit improved activity and selectivity compared to their parent ligands. The neurokinin-1 receptor (NK1R), a GPCR targeted for the treatment of pain, is activated by peptide agonists such as Substance P (SP) and neurokinin A (NKA), which induce signaling through multiple pathways (Gs , Gq and β-arrestin). In this study, we investigated whether conjugating NK1R ligands with Nbs that bind to a separate location on the receptor would provide chimeric compounds with distinctive signaling properties. We employed sortase A-mediated ligation to generate several conjugates consisting of Nbs linked to NK1R ligands. Many of these conjugates exhibited divergent and unexpected signaling properties and transcriptional outputs. For example, some Nb-NKA conjugates showed enhanced receptor binding capacity, high potency partial agonism, prolonged cAMP production, and an increase in transcriptional output associated with Gs signaling; whereas other conjugates were virtually inactive. Nanobody conjugation caused only minor alterations in ligand-induced upstream Gq signaling with unexpected enhancements in transcriptional (downstream) responses. Our findings underscore the potential of nanobody conjugation for providing compounds with advantageous properties such as biased agonism, prolonged duration of action, and enhanced transcriptional responses. These compounds hold promise not only for facilitating fundamental research on GPCR signal transduction mechanisms but also for the development of more potent and enduring therapeutics.

Neurokinin-2 receptor negatively modulates substance P responses by forming complex with Neurokinin-1 receptor

BACKGROUND

Tachykinins and their cognate receptors, neurokinin receptors (NKs) including NK1, NK2, and NK3 play vital roles in regulating various physiological processes including neurotransmission, nociception, inflammation, smooth muscle contractility, and stimulation of endocrine and exocrine gland secretion. Their abnormal expression has been reported to be associated with neurological disorders, inflammation, and cancer. Even though NKs are expressed in the same cells with their expression being inversely correlated in some conditions, there is no direct evidence to prove their interaction. Understanding the functional crosstalk between NKs in mediated downstream signaling and cellular responses may elucidate the roles of each receptor in pathophysiology.

RESULTS

In this study, we showed that NKs were co-expressed in some cells. However, different from NK3, which only forms homodimerization, we demonstrated a direct interaction between NK1 and NK2 at the protein level using co-immunoprecipitation and NanoBiT-based protein interaction analysis. Through heterodimerization, NK2 downregulated substance P-stimulated NK1 signals, such as intracellular Ca2+ mobilization and ERK phosphorylation, by enhancing β-arrestin recruitment, even at the ligand concentration that could not activate NK2 itself or in the presence of NK1 specific antagonist, aprepitant. In A549 cells with receptors deleted and reconstituted, NK2 exerted a negative effect on substance P/NK1-mediated cell migration.

CONCLUSION

Our study has provided the first direct evidence of an interaction between NK1 and NK2, which highlights the functional relevance of their heterodimerization in cellular responses. Our findings demonstrated that through dimerization, NK2 exerts negative effects on downstream signaling and cellular response mediated by NK1. Moreover, this study has significant implications for understanding the complexity of GPCR dimerization and its effect on downstream signaling and cellular responses. Given the important roles of tachykinins and NKs in pathophysiology, these insights may provide clues for developing NKs-targeting drugs.

Semi-synthetic nanobody-ligand conjugates exhibit tunable signaling properties and enhanced transcriptional outputs at neurokinin receptor-1

Antibodies have proven highly valuable for therapeutic development; however, they are typically poor candidates for applications that require activation of G protein-coupled receptors (GPCRs), the largest collection of targets for clinically approved drugs. Nanobodies (Nbs), the smallest antibody fragments retaining full antigen-binding capacity, have emerged as promising tools for pharmacologic applications, including GPCR modulation. Past work has shown that conjugation of Nbs with ligands can provide GPCR agonists that exhibit improved activity and selectivity compared to their parent ligands. The neurokinin-1 receptor (NK1R), a GPCR targeted for the treatment of pain, is activated by peptide agonists such as Substance P (SP) and neurokinin A (NKA), which induce signaling through multiple pathways (Gs, Gq and β-arrestin). In this study, we investigated whether conjugating NK1R ligands with Nbs that bind to a separate location on the receptor would provide chimeric compounds with distinctive signaling properties. We employed sortase A-mediated ligation to generate several conjugates consisting of Nbs linked to NK1R ligands. Many of these conjugates exhibited divergent and unexpected signaling properties and transcriptional outputs. For example, some Nb-NKA conjugates showed enhanced receptor binding capacity, high potency partial agonism, prolonged cAMP production, and an increase in transcriptional output associated with Gs signaling; whereas other conjugates were virtually inactive. Nanobody conjugation caused only minor alterations in ligand-induced upstream Gq signaling with unexpected enhancements in transcriptional (downstream) responses. Our findings underscore the potential of nanobody conjugation for providing compounds with advantageous properties such as biased agonism, prolonged duration of action, and enhanced transcriptional responses. These compounds hold promise not only for facilitating fundamental research on GPCR signal transduction mechanisms but also for the development of more potent and enduring therapeutics.

Allosteric Regulation of G-Protein-Coupled Receptors: From Diversity of Molecular Mechanisms to Multiple Allosteric Sites and Their Ligands

Allosteric regulation is critical for the functioning of G protein-coupled receptors (GPCRs) and their signaling pathways. Endogenous allosteric regulators of GPCRs are simple ions, various biomolecules, and protein components of GPCR signaling (G proteins and β-arrestins). The stability and functional activity of GPCR complexes is also due to multicenter allosteric interactions between protomers. The complexity of allosteric effects caused by numerous regulators differing in structure, availability, and mechanisms of action predetermines the multiplicity and different topology of allosteric sites in GPCRs. These sites can be localized in extracellular loops; inside the transmembrane tunnel and in its upper and lower vestibules; in cytoplasmic loops; and on the outer, membrane-contacting surface of the transmembrane domain. They are involved in the regulation of basal and orthosteric agonist-stimulated receptor activity, biased agonism, GPCR-complex formation, and endocytosis. They are targets for a large number of synthetic allosteric regulators and modulators, including those constructed using molecular docking. The review is devoted to the principles and mechanisms of GPCRs allosteric regulation, the multiplicity of allosteric sites and their topology, and the endogenous and synthetic allosteric regulators, including autoantibodies and pepducins. The allosteric regulation of chemokine receptors, proteinase-activated receptors, thyroid-stimulating and luteinizing hormone receptors, and beta-adrenergic receptors are described in more detail.

Crosstalk between Mu-Opioid receptors and neuroinflammation: Consequences for drug addiction and pain

Mu-Opioid Receptors (MORs) are well-known for participating in analgesia, sedation, drug addiction, and other physiological functions. Although MORs have been related to neuroinflammation their biological mechanism remains unclear. It is suggested that MORs work alongside Toll-Like Receptors to enhance the release of pro-inflammatory mediators and cytokines during pathological conditions. Some cytokines, including TNF-α, IL-1β and IL-6, have been postulated to regulate MORs levels by both avoiding MOR recycling and enhancing its production. In addition, Neurokinin-1 Receptor, also affected during neuroinflammation, could be regulating MOR trafficking. Therefore, inflammation in the central nervous system seems to be associated with altered/increased MORs expression, which might regulate harmful processes, such as drug addiction and pain. Here, we provide a critical evaluation on MORs' role during neuroinflammation and its implication for these conditions. Understanding MORs' functioning, their regulation and implications on drug addiction and pain may help elucidate their potential therapeutic use against these pathological conditions and associated disorders.

Dramatic Effect of Botulinum Toxin Type A on Hypertrophic Scar: A Promising Therapeutic Drug and Its Mechanism Through the SP-NK1R Pathway in Cutaneous Neurogenic Inflammation

Background

Hypertrophic scar formation may be related to cutaneous neurogenic inflammation (CNI) through the substance P-neurokinin 1 receptor (SP-NK1R) signaling pathway. As a widely used drug in aesthetic clinical work, botulinum toxin type A (BTX-A) has a therapeutic effect on scars, but the actual mechanism remains unclear. This study aimed to clarify the potential mechanism by which BTX-A inhibits CNI in hypertrophic scars both in vitro and in vivo.

Methods

Tissue samples were obtained from surgical excisions. Immunohistological analysis was used to locate SP in human hypertrophic scars and normal skin. RT-PCR and western blot analysis were used to evaluate the expression of collagens after SP/BTX-A treatment. A rabbit ear scar model was used to explore the in vivo effect of BTX-A on scar treatment.

Results

SP and NK-1R were overexpressed in hypertrophic scars compared to normal skin tissues. Collagen secretion of hypertrophic scar-derived fibroblasts increased with increasing doses of SP. However, BTX-A may downregulate collagen expression through SP-NK1R pathway with or without the presence of SP inducing agent capsaicin. Meanwhile, SP inhibited the expression of NK-1R, and this inhibition was blocked by pretreatment with BTX-A. In vivo, intralesional BTX-A injection can also reduce the volume of scars and inhibit collagen secretion. Capsaicin may cause more severe scar manifestations, while the therapeutic effect of BTX-A remains.

Conclusion

Our research confirms that CNI stimulates fibroblasts during scar formation, while BTX-A can reduce collagen secretion by inhibiting the SP-NK1R signaling pathway, thus identifying a novel therapeutic target for this benign solid skin tumor.

A Second Wave for the Neurokinin Tac2 Pathway in Brain Research

Despite promising advances in basic research of the neurokinin B/Tac2 pathway in both animals and humans, clinical applications are yet to be implemented. This is likely because of our limited understanding of the action of the pathway in the brain. While this system controls neuronal activity in multiple regions, the precise impact of Tac2-induced cellular responses on behavior remains unclear. Recently, elegant studies revealed a key contribution to stress-related behaviors and memory. Here, we discuss the crucial importance of bridging the gap between the Tac2 pathway's involvement in cell physiology and cognition to comprehend its role in health and disease. We propose that a better understanding of the Tac2 pathway in the brain could provide an essential perspective for basic investigations, which in turn will feed clinical research.

The orphan receptor GPR88 blunts the signaling of opioid receptors and multiple striatal GPCRs

GPR88 is an orphan G protein-coupled receptor (GPCR) considered as a promising therapeutic target for neuropsychiatric disorders; its pharmacology, however, remains scarcely understood. Based on our previous report of increased delta opioid receptor activity in Gpr88 null mice, we investigated the impact of GPR88 co-expression on the signaling of opioid receptors in vitro and revealed that GPR88 inhibits the activation of both their G protein- and β-arrestin-dependent signaling pathways. In Gpr88 knockout mice, morphine-induced locomotor sensitization, withdrawal and supra-spinal analgesia were facilitated, consistent with a tonic inhibitory action of GPR88 on µOR signaling. We then explored GPR88 interactions with more striatal versus non-neuronal GPCRs, and revealed that GPR88 can decrease the G protein-dependent signaling of most receptors in close proximity, but impedes β-arrestin recruitment by all receptors tested. Our study unravels an unsuspected buffering role of GPR88 expression on GPCR signaling, with intriguing consequences for opioid and striatal functions.

Mu and Delta Opioid Receptors Are Coexpressed and Functionally Interact in the Enteric Nervous System of the Mouse Colon

BACKGROUND & AIMS

Functional interactions between the mu opioid receptor (MOR) and delta opioid receptor (DOR) represent a potential target for novel analgesics and may drive the effects of the clinically approved drug eluxadoline for the treatment of diarrhea-predominant irritable bowel syndrome. Although the enteric nervous system (ENS) is a likely site of action, the coexpression and potential interaction between MOR and DOR in the ENS are largely undefined. In the present study, we have characterized the distribution of MOR in the mouse ENS and examined MOR-DOR interactions by using pharmacologic and cell biology techniques.

METHODS

MOR and DOR expression was defined by using MORmCherry and MORmCherry-DOR-eGFP knockin mice. MOR-DOR interactions were assessed by using DOR-eGFP internalization assays and by pharmacologic analysis of neurogenic contractions of the colon.

RESULTS

Although MOR was expressed by approximately half of all myenteric neurons, MOR-positive submucosal neurons were rarely observed. There was extensive overlap between MOR and DOR in both excitatory and inhibitory pathways involved in the coordination of intestinal motility. MOR and DOR can functionally interact, as shown through heterologous desensitization of MOR-dependent responses by DOR agonists. Functional evidence suggests that MOR and DOR may not exist as heteromers in the ENS. Pharmacologic studies show no evidence of cooperativity between MOR and DOR. DOR internalizes independently of MOR in myenteric neurons, and MOR-evoked contractions are unaffected by the sequestration of DOR.

CONCLUSIONS

Collectively, these findings demonstrate that although MOR and DOR are coexpressed in the ENS and functionally interact, they are unlikely to exist as heteromers under physiological conditions.

G-Protein-Coupled Receptors Are Dynamic Regulators of Digestion and Targets for Digestive Diseases

G-protein-coupled receptors (GPCRs) are the largest family of transmembrane signaling proteins. In the gastrointestinal tract, GPCRs expressed by epithelial cells sense contents of the lumen, and GPCRs expressed by epithelial cells, myocytes, neurons, and immune cells participate in communication among cells. GPCRs control digestion, mediate digestive diseases, and coordinate repair and growth. GPCRs are the target of more than one third of therapeutic drugs, including many drugs used to treat digestive diseases. Recent advances in structural, chemical, and cell biology research have shown that GPCRs are not static binary switches that operate from the plasma membrane to control a defined set of intracellular signals. Rather, GPCRs are dynamic signaling proteins that adopt distinct conformations and subcellular distributions when associated with different ligands and intracellular effectors. An understanding of the dynamic nature of GPCRs has provided insights into the mechanism of activation and signaling of GPCRs and has shown opportunities for drug discovery. We review the allosteric modulation, biased agonism, oligomerization, and compartmentalized signaling of GPCRs that control digestion and digestive diseases. We highlight the implications of these concepts for the development of selective and effective drugs to treat diseases of the gastrointestinal tract.

GPCRs and Signal Transducers: Interaction Stoichiometry

Until the late 1990s, class A G protein-coupled receptors (GPCRs) were believed to function as monomers. Indirect evidence that they might internalize or even signal as dimers has emerged, along with proof that class C GPCRs are obligatory dimers. Crystal structures of GPCRs and their much larger binding partners were consistent with the idea that two receptors might engage a single G protein, GRK, or arrestin. However, recent biophysical, biochemical, and structural evidence invariably suggests that a single GPCR binds G proteins, GRKs, and arrestins. Here we review existing evidence of the stoichiometry of GPCR interactions with signal transducers and discuss potential biological roles of class A GPCR oligomers, including proposed homo- and heterodimers.

Nitric oxide-heat shock protein axis in menopausal hot flushes: neglected metabolic issues of chronic inflammatory diseases associated with deranged heat shock response

BACKGROUND

Although some unequivocal underlying mechanisms of menopausal hot flushes have been demonstrated in animal models, the paucity of similar approaches in humans impedes further mechanistic outcomes. Human studies might show some as yet unexpected physiological mechanisms of metabolic adaptation that permeate the phase of decreased oestrogen levels in both symptomatic and asymptomatic women. This is particularly relevant because both the severity and time span of hot flushes are associated with increased risk of chronic inflammatory disease. On the other hand, oestrogen induces the expression of heat shock proteins of the 70 kDa family (HSP70), which are anti-inflammatory and cytoprotective protein chaperones, whose expression is modulated by different types of physiologically stressful situations, including heat stress and exercise. Therefore, lower HSP70 expression secondary to oestrogen deficiency increases cardiovascular risk and predisposes the patient to senescence-associated secretory phenotype (SASP) that culminates in chronic inflammatory diseases, such as obesities, type 2 diabetes, neuromuscular and neurodegenerative diseases.

OBJECTIVE AND RATIONALE

This review focuses on HSP70 and its accompanying heat shock response (HSR), which is an anti-inflammatory and antisenescent pathway whose intracellular triggering is also oestrogen-dependent via nitric oxide (NO) production. The main goal of the manuscript was to show that the vasomotor symptoms that accompany hot flushes may be a disguised clue for important neuroendocrine alterations linking oestrogen deficiency to the anti-inflammatory HSR.

SEARCH METHODS

Results from our own group and recent evidence on hypothalamic control of central temperature guided a search on PubMed and Google Scholar websites.

OUTCOMES

Oestrogen elicits rapid production of the vasodilatory gas NO, a powerful activator of HSP70 expression. Whence, part of the protective effects of oestrogen over cardiovascular and neuroendocrine systems is tied to its capacity of inducing the NO-elicited HSR. The hypothalamic areas involved in thermoregulation (infundibular nucleus in humans and arcuate nucleus in other mammals) and whose neurons are known to have their function altered after long-term oestrogen ablation, particularly kisspeptin-neurokinin B-dynorphin neurons, (KNDy) are the same that drive neuroprotective expression of HSP70 and, in many cases, this response is via NO even in the absence of oestrogen. From thence, it is not illogical that hot flushes might be related to an evolutionary adaptation to re-equip the NO-HSP70 axis during the downfall of circulating oestrogen.

WIDER IMPLICATIONS

Understanding of HSR could shed light on yet uncovered mechanisms of menopause-associated diseases as well as on possible manipulation of HSR in menopausal women through physiological, pharmacological, nutraceutical and prebiotic interventions. Moreover, decreased HSR indices (that can be clinically determined with ease) in perimenopause could be of prognostic value in predicting the moment and appropriateness of starting a HRT.

Developments in the field of allergy mechanisms in 2015 through the eyes of Clinical & Experimental Allergy

In the first of two papers we described the development in the field of allergy mechanisms as described by Clinical and Experimental Allergy in 2015. Experimental models of allergic disease, basic mechanisms, clinical mechanisms and allergens are all covered. A second paper will cover clinical aspects.

β-arrestin signalling and bias in hormone-responsive GPCRs

G protein-coupled receptors (GPCRs) play crucial roles in the ability of target organs to respond to hormonal cues. GPCRs' activation mechanisms have long been considered as a two-state process connecting the agonist-bound receptor to heterotrimeric G proteins. This view is now challenged as mounting evidence point to GPCRs being connected to large arrays of transduction mechanisms involving heterotrimeric G proteins as well as other players. Amongst the G protein-independent transduction mechanisms, those elicited by β-arrestins upon their recruitment to the active receptors are by far the best characterized and apply to most GPCRs. These concepts, in conjunction with remarkable advances made in the field of GPCR structural biology and biophysics, have supported the notion of ligand-selective signalling also known as pharmacological bias. Interestingly, recent reports have opened intriguing prospects to the way β-arrestins control GPCR-mediated signalling in space and time within the cells. In the present paper, we review the existing evidence linking endocrine-related GPCRs to β-arrestin recruitement, signalling, pathophysiological implications and selective activation by biased ligands and/or receptor modifications. Emerging concepts surrounding β-arrestin-mediated transduction are discussed in the light of the peculiarities of endocrine systems.

Development of a BRET2 Screening Assay Using β-Arrestin 2 Mutants

This study has focused on enhancing the signal generated from the interaction between a G-protein-coupled receptor (GPCR) and β-arrestin 2 (β-arr2), measured by the bioluminescence resonance energy transfer (BRET2) technology. Both class A (β2-adrenergic receptor [β2-AR]) and class B (neurokinin-type 1 receptor [NK1-R]) GPCRs, classified based on their internalization characteristics, have been analyzed. It was evaluated whether the BRET2 signal can be enhanced by using (1) β-arr2 phosphorylation-independent mutant (β-arr2 R169E) and (2) β-arr2 mutants deficient in their ability to interact with the components of the clathrin-coated vesicles (β-arr2 R393E, R395E and β-arr2 373 stop). For the class B receptor, there was no major difference in the agonist-promoted BRET2 signal when comparing results obtained with wild-type (wt) and mutant β-arr2. However, with the class A receptor, a more than 2-fold increase in the BRET2 signal was observed with β-arr2 mutants lacking the AP-2 or both AP-2 and clathrin binding sites. This set of data suggests that the inability of these β-arr2 mutants to interact with the components of the clathrin-coated vesicle probably prevents their rapid dissociation from the receptor, thus yielding an increased and more stable BRET2 signal. The β-arr2 R393E, R395E mutant also enhanced the signal window with other members of the GPCR family (neuropeptide Y type 2 receptor [NPY2-R] and TG1019 receptor) and was successfully applied in full-plate BRET2-based agonist and antagonist screening assays.

The pharmacology of neurokinin receptors in addiction: prospects for therapy

Addiction is a chronic disorder in which consumption of a substance or a habitual behavior becomes compulsive and often recurrent, despite adverse consequences. Substance p (SP) is an undecapeptide and was the first neuropeptide of the neurokinin family to be discovered. The subsequent decades of research after its discovery implicated SP and its neurokinin relatives as neurotransmitters involved in the modulation of the reward pathway. Here, we review the neurokinin literature, giving a brief historical perspective of neurokinin pharmacology, localization in various brain regions involved in addictive behaviors, and the functional aspects of neurokinin pharmacology in relation to reward in preclinical models of addiction that have shaped the rational drug design of neurokinin antagonists that could translate into human research. Finally, we will cover the clinical investigations using neurokinin antagonists and discuss their potential as a therapy for drug abuse.

G Protein-Coupled Receptor Multimers: A Question Still Open Despite the Use of Novel Approaches

Heteromerization of G protein-coupled receptors (GPCRs) can significantly change the functional properties of involved receptors. Various biochemical and biophysical methodologies have been developed in the last two decades to identify and functionally evaluate GPCR heteromers in heterologous cells, with recent approaches focusing on GPCR complex stoichiometry and stability. Yet validation of these observations in native tissues is still lagging behind for the majority of GPCR heteromers. Remarkably, recent studies, particularly some involving advanced fluorescence microscopy techniques, are contributing to our current knowledge of aspects that were not well known until now, such as GPCR complex stoichiometry and stability. In parallel, a growing effort is being applied to move the field forward into native systems. This short review will highlight recent developments to study the stoichiometry and stability of GPCR complexes and methodologies to detect native GPCR dimers.

Inflammation-induced abnormalities in the subcellular localization and trafficking of the neurokinin 1 receptor in the enteric nervous system

Activated G protein-coupled receptors traffic to endosomes and are sorted to recycling or degradative pathways. Endosomes are also a site of receptor signaling of sustained and pathophysiologically important processes, including inflammation. However, the mechanisms of endosomal sorting of receptors and the impact of disease on trafficking have not been fully defined. We examined the effects of inflammation on the subcellular distribution and trafficking of the substance P (SP) neurokinin 1 receptor (NK1R) in enteric neurons. We studied NK1R trafficking in enteric neurons of the mouse colon using immunofluorescence and confocal microscopy. The impact of inflammation was studied in IL10(-/-)-piroxicam and trinitrobenzenesulfonic acid colitis models. NK1R was localized to the plasma membrane of myenteric and submucosal neurons of the uninflamed colon. SP evoked NK1R endocytosis and recycling. Deletion of β-arrestin2, which associates with the activated NK1R, accelerated recycling. Inhibition of endothelin-converting enzyme-1 (ECE-1), which degrades endosomal SP, prevented recycling. Inflammation was associated with NK1R endocytosis in myenteric but not submucosal neurons. Whereas the NK1R in uninflamed neurons recycled within 60 min, NK1R recycling in inflamed neurons was delayed for >120 min, suggesting defective recycling machinery. Inflammation was associated with β-arrestin2 upregulation and ECE-1 downregulation, which may contribute to the defective NK1R recycling. We conclude that inflammation evokes redistribution of NK1R from the plasma membrane to endosomes of myenteric neurons through enhanced SP release and defective NK1R recycling. Defective recycling may be secondary to upregulation of β-arrestin2 and downregulation of ECE-1. Internalized NK1R may generate sustained proinflammatory signals that disrupt normal neuronal functions.

Neurokinin B induces c-fos transcription via protein kinase C and activation of serum response factor and Elk-1 in immortalized GnRH neurons

Mutations in neurokinin B (NKB) and its receptor, NK3R, were identified in human patients with hypogonadotropic hypogonadism, a disorder characterized by lack of puberty and infertility. Further studies have suggested that NKB acts at the level of the hypothalamus to control GnRH neuron activity, either directly or indirectly. We recently reported that treatment with senktide, a NK3R agonist, induced GnRH secretion and expression of c-fos mRNA in GT1-7 cells. Here, we map the responsive region in the murine c-fos promoter to between -400 and -200 bp, identify the signal transducer and activator of transcription (STAT) (-345) and serum response element (-310) sites as required for induction, a modulatory role for the Ets site (-318), and show that induction is protein kinase C dependent. Using gel shift and Gal4 assays, we further show that phosphorylation of Elk-1 leads to binding to DNA in complex with serum response factor at serum response element and Ets sites within the c-fos promoter. Thus, we determine molecular mechanisms involved in NKB regulation of c-fos induction, which may play a role in modulation of GnRH neuron activation.

Endothelin-converting enzyme 1 and β-arrestins exert spatiotemporal control of substance P-induced inflammatory signals

Although the intracellular trafficking of G protein-coupled receptors controls specific signaling events, it is unclear how the spatiotemporal control of signaling contributes to complex pathophysiological processes such as inflammation. By using bioluminescence resonance energy transfer and superresolution microscopy, we found that substance P (SP) induces the association of the neurokinin 1 receptor (NK1R) with two classes of proteins that regulate SP signaling from plasma and endosomal membranes: the scaffolding proteins β-arrestin (βARRs) 1 and 2 and the transmembrane metallopeptidases ECE-1c and ECE-1d. In HEK293 cells and non-transformed human colonocytes, we observed that G protein-coupled receptor kinase 2 and βARR1/2 terminate plasma membrane Ca(2+) signaling and initiate receptor trafficking to endosomes that is necessary for sustained activation of ERKs in the nucleus. βARRs deliver the SP-NK1R endosomes, where ECE-1 associates with the complex, degrades SP, and allows the NK1R, freed from βARRs, to recycle. Thus, both ECE-1 and βARRs mediate the resensitization of NK1R Ca(2+) signaling at the plasma membrane. Sustained exposure of colonocytes to SP activates NF-κB and stimulates IL-8 secretion. This proinflammatory signaling is unaffected by inhibition of the endosomal ERK pathway but is suppressed by ECE-1 inhibition or βARR2 knockdown. Inhibition of protein phosphatase 2A, which also contributes to sustained NK1R signaling at the plasma membrane, similarly attenuates IL-8 secretion. Thus, the primary function of βARRs and ECE-1 in SP-dependent inflammatory signaling is to promote resensitization, which allows the sustained NK1R signaling from the plasma membrane that drives inflammation.

Genome-wide investigation and expression analysis suggest diverse roles and genetic redundancy of Pht1 family genes in response to Pi deficiency in tomato

BACKGROUND

Phosphorus (P) deficiency is one of the major nutrient stresses limiting plant growth. The uptake of P by plants is well considered to be mediated by a number of high-affinity phosphate (Pi) transporters belonging to the Pht1 family. Although the Pht1 genes have been extensively identified in several plant species, there is a lack of systematic analysis of the Pht1 gene family in any solanaceous species thus far.

RESULTS

Here, we report the genome-wide analysis, phylogenetic evolution and expression patterns of the Pht1 genes in tomato (Solanum lycopersicum). A total of eight putative Pht1 genes (LePT1 to 8), distributed on three chromosomes (3, 6 and 9), were identified through extensive searches of the released tomato genome sequence database. Chromosomal organization and phylogenetic tree analysis suggested that the six Pht1 paralogues, LePT1/3, LePT2/6 and LePT4/5, which were assigned into three pairs with very close physical distance, were produced from recent tandem duplication events that occurred after Solanaceae splitting with other dicot families. Expression analysis of these Pht1 members revealed that except LePT8, of which the transcript was undetectable in all tissues, the other seven paralogues showed differential but partial-overlapping expression patterns. LePT1 and LePT7 were ubiquitously expressed in all tissues examined, and their transcripts were induced abundantly in response to Pi starvation; LePT2 and LePT6, the two paralogues harboring identical coding sequence, were predominantly expressed in Pi-deficient roots; LePT3, LePT4 and LePT5 were strongly activated in the roots colonized by arbuscular mycorrhizal fungi under low-P, but not high-P condition. Histochemical analysis revealed that a 1250-bp LePT3 promoter fragment and a 471-bp LePT5 promoter fragment containing the two elements, MYCS and P1BS, were sufficient to direct the GUS reporter expression in mycorrhizal roots and were limited to distinct cells harboring AM fungal structures. Additionally, the four paralogues, LePT1, LePT2, LePT6 and LePT7, were very significantly down-regulated in the mycorrhizal roots under low Pi supply condition.

CONCLUSIONS

The results obtained from this study provide new insights into the evolutionary expansion, functional divergence and genetic redundancy of the Pht1 genes in response to Pi deficiency and mycorrhizal symbiosis in tomato.

Tachykinins and their receptors: contributions to physiological control and the mechanisms of disease

The tachykinins, exemplified by substance P, are one of the most intensively studied neuropeptide families. They comprise a series of structurally related peptides that derive from alternate processing of three Tac genes and are expressed throughout the nervous and immune systems. Tachykinins interact with three neurokinin G protein-coupled receptors. The signaling, trafficking, and regulation of neurokinin receptors have also been topics of intense study. Tachykinins participate in important physiological processes in the nervous, immune, gastrointestinal, respiratory, urogenital, and dermal systems, including inflammation, nociception, smooth muscle contractility, epithelial secretion, and proliferation. They contribute to multiple diseases processes, including acute and chronic inflammation and pain, fibrosis, affective and addictive disorders, functional disorders of the intestine and urinary bladder, infection, and cancer. Neurokinin receptor antagonists are selective, potent, and show efficacy in models of disease. In clinical trials there is a singular success: neurokinin 1 receptor antagonists to treat nausea and vomiting. New information about the involvement of tachykinins in infection, fibrosis, and pruritus justifies further trials. A deeper understanding of disease mechanisms is required for the development of more predictive experimental models, and for the design and interpretation of clinical trials. Knowledge of neurokinin receptor structure, and the development of targeting strategies to disrupt disease-relevant subcellular signaling of neurokinin receptors, may refine the next generation of neurokinin receptor antagonists.

GPCR signaling along the endocytic pathway

Many G protein-coupled receptors (GPCRs) internalize after agonist-induced activation. While endocytosis has long been associated with homeostatic attenuation of cellular responsiveness, accumulating evidence from study of a wide range of eukaryotes reveals that the endocytic pathway also contributes to generating receptor-initiated signals themselves. Here we review recent progress in this area, discussing primarily but not exclusively GPCR signaling in mammalian cells.

Downscaling the analysis of complex transmembrane signaling cascades to closed attoliter volumes

Cellular signaling is classically investigated by measuring optical or electrical properties of single or populations of living cells. Here we show that ligand binding to cell surface receptors and subsequent activation of signaling cascades can be monitored in single, (sub-)micrometer sized native vesicles with single-molecule sensitivity. The vesicles are derived from live mammalian cells using chemicals or optical tweezers. They comprise parts of a cell's plasma membrane and cytosol and represent the smallest autonomous containers performing cellular signaling reactions thus functioning like minimized cells. Using fluorescence microscopies, we measured in individual vesicles the different steps of G-protein-coupled receptor mediated signaling like ligand binding to receptors, subsequent G-protein activation and finally arrestin translocation indicating receptor deactivation. Observing cellular signaling reactions in individual vesicles opens the door for downscaling bioanalysis of cellular functions to the attoliter range, multiplexing single cell analysis, and investigating receptor mediated signaling in multiarray format.

Agonist-biased trafficking of somatostatin receptor 2A in enteric neurons

Somatostatin (SST) 14 and SST 28 activate somatostatin 2A receptors (SSTR2A) on enteric neurons to control gut functions. SST analogs are treatments of neuroendocrine and bleeding disorders, cancer, and diarrhea, with gastrointestinal side effects of constipation, abdominal pain, and nausea. How endogenous agonists and drugs differentially regulate neuronal SSTR2A is unexplored. We evaluated SSTR2A trafficking in murine myenteric neurons and neuroendocrine AtT-20 cells by microscopy and determined whether agonist degradation by endosomal endothelin-converting enzyme 1 (ECE-1) controls SSTR2A trafficking and association with β-arrestins, key regulators of receptors. SST-14, SST-28, and peptide analogs (octreotide, lanreotide, and vapreotide) stimulated clathrin- and dynamin-mediated internalization of SSTR2A, which colocalized with ECE-1 in endosomes and the Golgi. After incubation with SST-14, SSTR2A recycled to the plasma membrane, which required active ECE-1 and an intact Golgi. SSTR2A activated by SST-28, octreotide, lanreotide, or vapreotide was retained within the Golgi and did not recycle. Although ECE-1 rapidly degraded SST-14, SST-28 was resistant to degradation, and ECE-1 did not degrade SST analogs. SST-14 and SST-28 induced transient interactions between SSTR2A and β-arrestins that were stabilized by an ECE-1 inhibitor. Octreotide induced sustained SSTR2A/β-arrestin interactions that were not regulated by ECE-1. Thus, when activated by SST-14, SSTR2A internalizes and recycles via the Golgi, which requires ECE-1 degradation of SST-14 and receptor dissociation from β-arrestins. After activation by ECE-1-resistant SST-28 and analogs, SSTR2A remains in endosomes because of sustained β-arrestin interactions. Therapeutic SST analogs are ECE-1-resistant and retain SSTR2A in endosomes, which may explain their long-lasting actions.

Arresting inflammation: contributions of plasma membrane and endosomal signalling to neuropeptide-driven inflammatory disease

GPCR (G-protein-coupled receptor) signalling at the plasma membrane is under tight control. In the case of neuropeptides such as SP (substance P), plasma membrane signalling is regulated by cell-surface endopeptidases (e.g. neprilysin) that degrade extracellular neuropeptides, and receptor interaction with β-arrestins, which uncouple receptors from heterotrimeric G-proteins and mediate receptor endocytosis. By recruiting GPCRs, kinases and phosphatases to endocytosed GPCRs, β-arrestins assemble signalosomes that can mediate a second wave of signalling by internalized receptors. Endosomal peptidases, such as ECE-1 (endothelin-converting enzyme-1), can degrade SP in acidified endosomes, which destabilizes signalosomes and allows receptors, freed from β-arrestins, to recycle and resensitize. By disassembling signalosomes, ECE-1 terminates β-arrestin-mediated endosomal signalling. These mechanisms have been studied in model cell systems, and the relative importance of plasma membrane and endosomal signalling to complex pathophysiological processes, such as inflammation, pain and proliferation, is unclear. However, deletion or inhibition of metalloendopeptidases that control neuropeptide signalling at the plasma membrane and in endosomes has marked effects on inflammation. Neprilysin deletion exacerbates inflammation because of diminished degradation of pro-inflammatory SP. Conversely, inhibition of ECE-1 attenuates inflammation by preventing receptor recycling/resensitization, which is required for sustained pro-inflammatory signals from the plasma membrane. β-Arrestin deletion also affects inflammation because of the involvement of β-arrestins in pro-inflammatory signalling and migration of inflammatory cells. Knowledge of GPCR signalling in specific subcellular locations provides insights into pathophysiological processes, and can provide new opportunities for therapy. Selective targeting of β-arrestin-mediated endosomal signalling or of mechanisms of receptor recycling/resensitization may offer more effective and selective treatments than global targeting of cell-surface signalling.

The bile acid receptor TGR5 does not interact with β-arrestins or traffic to endosomes but transmits sustained signals from plasma membrane rafts

TGR5 is a G protein-coupled receptor that mediates bile acid (BA) effects on energy balance, inflammation, digestion, and sensation. The mechanisms and spatiotemporal control of TGR5 signaling are poorly understood. We investigated TGR5 signaling and trafficking in transfected HEK293 cells and colonocytes (NCM460) that endogenously express TGR5. BAs (deoxycholic acid (DCA), taurolithocholic acid) and the selective agonists oleanolic acid and 3-(2-chlorophenyl)-N-(4-chlorophenyl)-N, 5-dimethylisoxazole-4-carboxamide stimulated cAMP formation but did not induce TGR5 endocytosis or recruitment of β-arrestins, as assessed by confocal microscopy. DCA, taurolithocholic acid, and oleanolic acid did not stimulate TGR5 association with β-arrestin 1/2 or G protein-coupled receptor kinase (GRK) 2/5/6, as determined by bioluminescence resonance energy transfer. 3-(2-chlorophenyl)-N-(4-chlorophenyl)-N, 5-dimethylisoxazole-4-carboxamide stimulated a low level of TGR5 interaction with β-arrestin 2 and GRK2. DCA induced cAMP formation at the plasma membrane and cytosol, as determined using exchange factor directly regulated by cAMP (Epac2)-based reporters, but cAMP signals did not desensitize. AG1478, an inhibitor of epidermal growth factor receptor tyrosine kinase, the metalloprotease inhibitor batimastat, and methyl-β-cyclodextrin and filipin, which block lipid raft formation, prevented DCA stimulation of ERK1/2. Bioluminescence resonance energy transfer analysis revealed TGR5 and EGFR interactions that were blocked by disruption of lipid rafts. DCA stimulated TGR5 redistribution to plasma membrane microdomains, as localized by immunogold electron microscopy. Thus, TGR5 does not interact with β-arrestins, desensitize, or traffic to endosomes. TGR5 signals from plasma membrane rafts that facilitate EGFR interaction and transactivation. An understanding of the spatiotemporal control of TGR5 signaling provides insights into the actions of BAs and therapeutic TGR5 agonists/antagonists.

Modulating neuromodulation by receptor membrane traffic in the endocytic pathway

Cellular responsiveness to many neuromodulators is controlled by endocytosis of the transmembrane receptors that transduce their effects. Endocytic membrane trafficking of particular neuromodulator receptors exhibits remarkable diversity and specificity, determined largely by molecular sorting operations that guide receptors at trafficking branchpoints after endocytosis. In this Review, we discuss recent progress in elucidating mechanisms mediating the molecular sorting of neuromodulator receptors in the endocytic pathway. There is emerging evidence that endocytic trafficking of neuromodulator receptors, in addition to influencing longer-term cellular responsiveness under conditions of prolonged or repeated activation, may also affect the acute response. Physiological and pathological consequences of defined receptor trafficking events are only now being elucidated, but it is already apparent that endocytosis of neuromodulator receptors has a significant impact on the actions of therapeutic drugs. The present data also suggest, conversely, that mechanisms of receptor endocytosis and molecular sorting may themselves represent promising targets for therapeutic manipulation.

G protein-coupled receptors: walking hand-in-hand, talking hand-in-hand?

Most cells express a panel of different G protein-coupled receptors (GPCRs) allowing them to respond to at least a corresponding variety of extracellular ligands. In order to come to an integrative well-balanced functional response these ligand-receptor pairs can often cross-regulate each other. Although most GPCRs are fully capable to induce intracellular signalling upon agonist binding on their own, many GPCRs, if not all, appear to exist and function in homomeric and/or heteromeric assemblies for at least some time. Such heteromeric organization offers unique allosteric control of receptor pharmacology and function between the protomers and might even unmask 'new' features. However, it is important to realize that some functional consequences that are proposed to originate from heteromeric receptor interactions may also be observed due to intracellular crosstalk between signalling pathways of non-associated GPCRs.

Protein phosphatase 2A mediates resensitization of the neurokinin 1 receptor

Activated G protein-coupled receptors (GPCRs) are phosphorylated and interact with β-arrestins, which mediate desensitization and endocytosis. Endothelin-converting enzyme-1 (ECE-1) degrades neuropeptides in endosomes and can promote recycling. Although endocytosis, dephosphorylation, and recycling are accepted mechanisms of receptor resensitization, a large proportion of desensitized receptors can remain at the cell surface. We investigated whether reactivation of noninternalized, desensitized (phosphorylated) receptors mediates resensitization of the substance P (SP) neurokinin 1 receptor (NK(1)R). Herein, we report a novel mechanism of resensitization by which protein phosphatase 2A (PP2A) is recruited to dephosphorylate noninternalized NK(1)R. A desensitizing concentration of SP reduced cell-surface SP binding sites by only 25%, and SP-induced Ca(2+) signals were fully resensitized before cell-surface binding sites started to recover, suggesting resensitization of cell-surface-retained NK(1)R. SP induced association of β-arrestin1 and PP2A with noninternalized NK(1)R. β-Arrestin1 small interfering RNA knockdown prevented SP-induced association of cell-surface NK(1)R with PP2A, indicating that β-arrestin1 mediates this interaction. ECE-1 inhibition, by trapping β-arrestin1 in endosomes, also impeded SP-induced association of cell-surface NK(1)R with PP2A. Resensitization of NK(1)R signaling required both PP2A and ECE-1 activity. Thus, after stimulation with SP, PP2A interacts with noninternalized NK(1)R and mediates resensitization. PP2A interaction with NK(1)R requires β-arrestin1. ECE-1 promotes this process by releasing β-arrestin1 from NK(1)R in endosomes. These findings represent a novel mechanism of PP2A- and ECE-1-dependent resensitization of GPCRs.

Characterization of RO4583298 as a novel potent, dual antagonist with in vivo activity at tachykinin NK₁ and NK₃ receptors

BACKGROUND AND PURPOSE

Clinical results of osanetant and talnetant (selective-NK₃ antagonists) indicate that blocking the NK₃ receptor could be beneficial for the treatment of schizophrenia. The objective of this study was to characterize the in vitro and in vivo properties of a novel dual NK₁/NK₃ antagonist, RO4583298 (2-phenyl-N-(pyridin-3-yl)-N-methylisobutyramide derivative).

EXPERIMENTAL APPROACH

RO4583298 in vitro pharmacology was investigated using radioligand binding ([³H]-SP, [³H]-osanetant, [³H]-senktide), [³H]-inositol-phosphate accumulation Schild analysis (SP- or [MePhe⁷]-NKB-induced) and electrophysiological studies in guinea-pig substantia nigra pars compacta (SNpc). The in vivo activity of RO4583298 was assessed using reversal of GR73632-induced foot tapping in gerbils (GFT; NK₁) and senktide-induced tail whips in mice (MTW; NK₃).

KEY RESULTS

RO4583298 has a high-affinity for NK₁ (human and gerbil) and NK₃ (human, cynomolgus monkey, gerbil and guinea-pig) receptors and behaves as a pseudo-irreversible antagonist. Unusually it binds with high-affinity to mouse and rat NK₃, yet with a partial non-competitive mode of antagonism. In guinea-pig SNpc, RO4583298 inhibited the senktide-induced potentiation of spontaneous activity of dopaminergic neurones with an apparent non-competitive mechanism of action. RO4583298 (p.o.) robustly blocked the GFT response, and inhibited the MTW.

CONCLUSIONS AND IMPLICATIONS

RO4583298 is a high-affinity, non-competitive, long-acting in vivo NK₁/NK₃ antagonist; hence providing a useful in vitro and in vivo pharmacological tool to investigate the roles of NK₁ and NK₃ receptors in psychiatric disorders.

Activation of corticotropin-releasing factor receptor 2 mediates the colonic motor coping response to acute stress in rodents

BACKGROUND & AIMS

Corticotropin-releasing factor receptor-1 (CRF(1)) mediates the stress-induced colonic motor activity. Less is known about the role of CRF(2) in the colonic response to stress.

METHODS

We studied colonic contractile activity in rats and CRF(2)-/-, CRF-overexpressing, and wild-type mice using still manometry; we analyzed defecation induced by acute partial-restraint stress (PRS), and/or intraperitoneal injection of CRF ligands. In rats, we monitored activation of the colonic longitudinal muscle myenteric plexus (LMMP) neurons and localization of CRF(1) and CRF(2) using immunohistochemical and immunoblot analyses. We measured phosphorylation of extracellular signal-regulated kinase 1/2 by CRF ligands in primary cultures of LMMP neurons (PC-LMMPn) and cyclic adenosine monophosphate (cAMP) production in human embryonic kidney-293 cells transfected with CRF(1) and/or CRF(2).

RESULTS

In rats, a selective agonist of CRF(2) (urocortin 2) reduced CRF-induced defecation (>50%), colonic contractile activity, and Fos expression in the colonic LMMP. A selective antagonist of CRF(2) (astressin(2)-B) increased these responses. Urocortin 2 reduced PRS-induced colonic contractile activity in wild-type and CRF-overexpressing mice, whereas disruption of CRF(2) increased PRS-induced colonic contractile activity and CRF-induced defecation. CRF(2) colocalized with CRF(1) and neuronal nitric oxide synthase in the rat colon, LMMP, and PC-LMMPn. CRF-induced phosphorylation of extracellular signal-regulated kinase in PC-LMMPn; this was inhibited or increased by a selective antagonist of CRF(1) (NBI35965) or astressin(2)-B, respectively. The half maximal effective concentration, EC(50), for the CRF-induced cAMP response was 8.6 nmol/L in human embryonic kidney-293 cells that express only CRF(1); this response was suppressed 10-fold in cells that express CRF(1) and CRF(2).

CONCLUSIONS

In colon tissues of rodents, CRF(2) activation inhibits CRF(1) signaling in myenteric neurons and the stress-induced colonic motor responses. Disruption of CRF(2) function impairs colonic coping responses to stress.

Activation of corticotropin-releasing factor receptor 2 mediates the colonic motor coping response to acute stress in rodents

BACKGROUND & AIMS

Corticotropin-releasing factor receptor-1 (CRF(1)) mediates the stress-induced colonic motor activity. Less is known about the role of CRF(2) in the colonic response to stress.

METHODS

We studied colonic contractile activity in rats and CRF(2)-/-, CRF-overexpressing, and wild-type mice using still manometry; we analyzed defecation induced by acute partial-restraint stress (PRS), and/or intraperitoneal injection of CRF ligands. In rats, we monitored activation of the colonic longitudinal muscle myenteric plexus (LMMP) neurons and localization of CRF(1) and CRF(2) using immunohistochemical and immunoblot analyses. We measured phosphorylation of extracellular signal-regulated kinase 1/2 by CRF ligands in primary cultures of LMMP neurons (PC-LMMPn) and cyclic adenosine monophosphate (cAMP) production in human embryonic kidney-293 cells transfected with CRF(1) and/or CRF(2).

RESULTS

In rats, a selective agonist of CRF(2) (urocortin 2) reduced CRF-induced defecation (>50%), colonic contractile activity, and Fos expression in the colonic LMMP. A selective antagonist of CRF(2) (astressin(2)-B) increased these responses. Urocortin 2 reduced PRS-induced colonic contractile activity in wild-type and CRF-overexpressing mice, whereas disruption of CRF(2) increased PRS-induced colonic contractile activity and CRF-induced defecation. CRF(2) colocalized with CRF(1) and neuronal nitric oxide synthase in the rat colon, LMMP, and PC-LMMPn. CRF-induced phosphorylation of extracellular signal-regulated kinase in PC-LMMPn; this was inhibited or increased by a selective antagonist of CRF(1) (NBI35965) or astressin(2)-B, respectively. The half maximal effective concentration, EC(50), for the CRF-induced cAMP response was 8.6 nmol/L in human embryonic kidney-293 cells that express only CRF(1); this response was suppressed 10-fold in cells that express CRF(1) and CRF(2).

CONCLUSIONS

In colon tissues of rodents, CRF(2) activation inhibits CRF(1) signaling in myenteric neurons and the stress-induced colonic motor responses. Disruption of CRF(2) function impairs colonic coping responses to stress.

Substance P activates both contractile and inflammatory pathways in lymphatics through the neurokinin receptors NK1R and NK3R

OBJECTIVE

The aim of this study was to elucidate the molecular signaling mechanisms by which substance P (SP) modulates lymphatic muscle contraction and to determine whether SP stimulates both contractile as well as inflammatory pathways in the lymphatics.

METHODS

A rat mesenteric lymphatic muscle cell culture model (RMLMCs) and known specific pharmacological inhibitors were utilized to delineate SP-mediated signaling pathways in lymphatics.

RESULTS

We detected expression of neurokinin receptor 1 (NK1R) and neurokinin receptor 3 (NK3R) in RMLMCs. SP stimulation increased phosphorylation of myosin light chain 20 (MLC₂₀) as well as p38 mitogen associated protein kinase (p38-MAPK) and extracellular signal regulated kinase (ERK1/2) indicating activation of both a contractile and a pro-inflammatory MAPK pathway. Pharmacological inhibition of both NK1R and NK3R significantly affected the downstream SP signaling. We further examined whether there was any crosstalk between the two pathways upon SP stimulation. Inhibition of ERK1/2 decreased levels of p-MLC₂₀ after SP activation, in a PKC dependent manner, indicating a potential crosstalk between these two pathways.

CONCLUSIONS

These data provide the first evidence that SP-mediated crosstalk between pro-inflammatory and contractile signaling mechanisms exists in the lymphatic system and may be an important bridge between lymphatic function modulation and inflammation.

Inhibition of dynamin prevents CCL2-mediated endocytosis of CCR2 and activation of ERK1/2

The magnitude and duration of G protein-coupled receptor (GPCR) signals are regulated through desensitization mechanisms. In leukocytes, ligand binding to chemokine receptors leads to Ca2+ mobilization and ERK activation through pertussis toxin-sensitive G proteins, as well as to phosphorylation of the GPCR. After interaction with the endocytic machinery (clathrin, adaptin), the adaptor beta-arrestin recognizes the phosphorylated GPCR tail and quenches signaling to receptors. The molecular mechanisms that lead to receptor endocytosis are not universal amongst the GPCR, however, and the precise spatial and temporal events in the internalization of the CCR2 chemokine receptor remain unknown. Here we show that after ligand binding, CCR2 internalizes rapidly and reaches early endosomes, and later, lysosomes. Knockdown of clathrin by RNA interference impairs CCR2 internalization, as does treatment with the dynamin inhibitor, dynasore. Our results show that CCR2 internalization uses a combination of clathrin-dependent and -independent pathways, as observed for other chemokine receptors. Moreover, the use of dynasore allowed us to confirm the existence of a dynamin-sensitive element that regulates ERK1/2 activation. Our results indicate additional complexity in the link between receptor internalization and cell signaling.

The neurokinin-3 (NK3) and the neurokinin-1 (NK1) receptors are differentially targeted to mesocortical and mesolimbic projection neurons and to neuronal nuclei in the rat ventral tegmental area

Tonic activation of neurokinin-3 (NK(3)) receptors in dopamine neurons of the ventral tegmental area (VTA) has been implicated in the pathophysiology of schizophrenia. This psychiatric disorder is associated with a dysfunctional activity in VTA projection neurons that can affect cognitive function at the level of the medial prefrontal cortex (mPFC) as well as motor and motivational states controlled in part by mesolimbic output to the nucleus accumbens (Acb). To determine the relevant sites for NK(3) receptor activation within this neuronal network, we used confocal and electron microscopy to examine NK(3) receptors (Cy5; immunogold) and retrograde labeling of fluorogold (FG, FITC; immunoperoxidase) in the VTA of rats receiving either Acb or mPFC injections of FG. Comparison was made with neurokinin-1 (NK(1)) receptors, which are also present, but less abundant then NK(3) receptors, in dopaminergic and GABAergic VTA neurons. There were no observable differences between NK(3) and NK(1) receptors in their primary locations in the cytoplasm and on the plasma membrane of VTA somata and dendrites with or without FG. Dendrites labeled with FG retrogradely transported from mPFC, however, contained more NK(3) or less NK(1) immunogold particles (plasmalemmal + cytoplasmic) then those retrogradely labeled following FG injection in the Acb. Moreover, only the NK(3) receptors were detected in neuronal nuclei in the VTA and in the nuclei of human HEK-293T NK(3)-transfected cells. The enrichment of NK(3) receptors in mesocortical projection neurons and nuclear distribution of these receptors may provide insight for understanding the selective antipsychotic effectiveness of NK(3) antagonists.

Neurokinin 1 receptors regulate morphine-induced endocytosis and desensitization of mu-opioid receptors in CNS neurons

mu-Opioid receptors (MORs) are G-protein-coupled receptors (GPCRs) that mediate the physiological effects of endogenous opioid neuropeptides and opiate drugs such as morphine. MORs are coexpressed with neurokinin 1 receptors (NK1Rs) in several regions of the CNS that control opioid dependence and reward. NK1R activation affects opioid reward specifically, however, and the cellular basis for this specificity is unknown. We found that ligand-induced activation of NK1Rs produces a cell-autonomous and nonreciprocal inhibition of MOR endocytosis induced by diverse opioids. Studies using epitope-tagged receptors expressed in cultured striatal neurons and a neuroblastoma cell model indicated that this heterologous regulation is mediated by NK1R-dependent sequestration of arrestins on endosome membranes. First, endocytic inhibition mediated by wild-type NK1Rs was overcome in cells overexpressing beta-arrestin2, a major arrestin isoform expressed in striatum. Second, NK1R activation promoted sequestration of beta-arrestin2 on endosomes, whereas MOR activation did not. Third, heterologous inhibition of MOR endocytosis was prevented by mutational disruption of beta-arrestin2 sequestration by NK1Rs. NK1R-mediated regulation of MOR trafficking was associated with reduced opioid-induced desensitization of adenylyl cyclase signaling in striatal neurons. Furthermore, heterologous regulation of MOR trafficking was observed in both amygdala and locus ceruleus neurons that naturally coexpress these receptors. These results identify a cell-autonomous mechanism that may underlie the highly specific effects of NK1R on opioid signaling and suggest, more generally, that receptor-specific trafficking of arrestins may represent a fundamental mechanism for coordinating distinct GPCR-mediated signals at the level of individual CNS neurons.

The tachykinin NK3 receptor agonist senktide induces locomotor activity in male Mongolian gerbils

The tachykinin family of receptors has been of strong interest recently due to the potential of the tachykinin NK(3) receptor antagonism in treatment of schizophrenia. However, critical differences in the tachykinin NK(3) receptor between rats, mice and humans make rats and mice less acceptable species for testing tachykinin NK(3) receptor antagonism. This has led to testing of tachykinin NK(3) receptor activity in gerbils and guinea pigs. As these species are much less common laboratory animals than rats and mice, there is a relative paucity of in vivo testing models for tachykinin NK(3) receptor activation. In the present study, locomotor activity induced by the tachykinin NK(3) receptor agonist senktide was characterized. Injection of senktide i.c.v. was found to dose-dependently induce hyperlocomotion from a dose of 0.06 nmol to the maximal dose tested, 0.6 nmol. Locomotion induced by 0.1 nmol of senktide could be blocked by injection of the tachykinin NK(3) receptor antagonists SB222200 (10 and 30 mg/kg i.p.) and talnetant (SB223412; 10 and 30 mg/kg i.p.), as well as by osanetant (SR142801; 10 and 30 mg/kg i.p.) when administered in a vehicle containing vitamin E and glycofurol. Senktide-induced activity was also reversed by the antipsychotics haloperidol (0.3 and 1 mg/kg p.o.) and risperidone (1 mg/kg p.o.), but not by the serotonin 5HT(2a/c) receptor antagonist MDL100907 (tested at 0.1, 0.3 and 1 mg/kg p.o.). Hyperlocomotion induced by 0.03 nmol of senktide was potentiated by antagonism of the tachykinin NK(1) receptor with aprepitant (1, 3 and 10 mg/kg, p.o.). Thus, hyperlocomotion induced by senktide in gerbils is a tachykinin NK(3) receptor-mediated behavior that is appropriate for use in testing tachykinin NK(3) receptor activity of novel compounds.

Differential expression of Nk1 and NK3 neurokinin receptors in neurons of the nucleus tractus solitarius and the dorsal vagal motor nucleus of the rat and mouse

Tachykinins (substance P, neurokinin A and neurokinin B) influence autonomic functions by modulating neuron activity in nucleus tractus solitarius (NTS) and dorsal motor nucleus of the vagus (DMV) through activation of neurokinin receptors NK1 and NK3. Our purpose was to identify and define by neurochemical markers, the subpopulations of NK1 and NK3 expressing neurons in NTS and DMV of rat and mouse. Because the distribution of the NK1 and NK3 expressing neurons overlaps, co-expression for both receptors was tested. By double labeling, we show that NK1 and NK3 were not co-expressed in NTS neurons. In the DMV, most of neurons (87%) were immunoreactive for only one of the receptors and 34% of NK1 neurons, 7% of NK3 neurons and 12% of NK1-NK3 neurons were cholinergic neurons. None of the neurons immunoreactive for NK1 or NK3 were positive for tyrosine hydroxylase, suggesting that catecholaminergic cells of the NTS (A2 and C2 groups) did not express neurokinin receptors. The presence of NK1 and NK3 was examined in GABAergic interneurons of the NTS and DMV by using GAD65-EGFP transgenic mouse. Immunoreactivity for NK1 or NK3 was found in a subpopulation of GAD65-EGFP cells. A majority (60%) of NK3 cells, but only 11% of the NK1 cells, were GAD65-EGFP cells. In conclusion, tachykinins, through differential expression of neurokinin receptors, may influence the central regulation of vital functions by acting on separate neuron subpopulations in NTS and DMV. Of particular interest, tachykinins may be involved in inhibitory mechanisms by acting directly on local GABAergic interneurons. Our results support a larger contribution of NK3 compared with NK1 in mediating inhibition in NTS and DMV.

Evidence for a role of caveolin-1 in neurokinin-1 receptor plasma-membrane localization, efficient signaling, and interaction with beta-arrestin 2

This study was focused on the relationship between the plasma-membrane localization of neurokinin-1 receptor (NK1-R) and its endocytic and signaling properties. First, we employed electron paramagnetic resonance (EPR) to study the domain structure of HEK-293 cells and NK1-R microlocalization. EPR spectra and the GHOST condensation routine demonstrated that NK1-R was distributed in a well-ordered domain of HEK-293 cells possibly representing lipid raft/caveolae microdomains, whereas the impairment of caveolae changed the NK1-R plasma-membrane distribution. Internalization and second messenger assays combined with bioluminescence resonance energy transfer were employed subsequently to evaluate the functional importance of the NK1-R microlocalization in lipid raft/caveolae microdomains. The internalization pattern was delineated through the use of dominant-negative mutants (DNM) of caveolin-1 S80E (Cav1 S80E), dynamin-1 K44A (Dyn K44A), and beta-arrestin (beta-arr 319-418) and by means of cell lines that expressed various endogenous levels of beta-arrestins. NK1-R displayed rapid internalization that was substantially reduced by DNMs of dynamin-1 and beta-arrestin and even more profoundly in cells lacking both beta-arrestin1 and beta-arrestin2. These internalization data were highly suggestive of the predominant use of the clathrin-mediated pathway by NK1-R, even though NK1-R tended to reside constitutively in lipid raft/caveolae microdomains. Evidence was also obtained that the proper clustering of the receptor in these microdomains was important for effective agonist-induced NK1-R signaling and for its interaction with beta-arrestin2.

Endothelin-converting enzyme 1 degrades neuropeptides in endosomes to control receptor recycling

Neuropeptide signaling requires the presence of G protein-coupled receptors (GPCRs) at the cell surface. Activated GPCRs interact with beta-arrestins, which mediate receptor desensitization, endocytosis, and mitogenic signaling, and the peptide-receptor-arrestin complex is sequestered into endosomes. Although dissociation of beta-arrestins is required for receptor recycling and resensitization, the critical event that initiates this process is unknown. Here we report that the agonist availability in the endosomes, controlled by the membrane metalloendopeptidase endothelin-converting enzyme 1 (ECE-1), determines stability of the peptide-receptor-arrestin complex and regulates receptor recycling and resensitization. Substance P (SP) binding to the tachykinin neurokinin 1 receptor (NK1R) induced membrane translocation of beta-arrestins followed by trafficking of the SP-NK1R-beta-arrestin complex to early endosomes containing ECE-1a-d. ECE-1 degraded SP in acidified endosomes, disrupting the complex; beta-arrestins returned to the cytosol, and the NK1R, freed from beta-arrestins, recycled and resensitized. An ECE-1 inhibitor, by preventing NK1R recycling in endothelial cells, inhibited resensitization of SP-induced inflammation. This mechanism is a general one because ECE-1 similarly regulated NK3R resensitization. Thus, peptide availability in endosomes, here regulated by ECE-1, determines the stability of the peptide-receptor-arrestin complex. This mechanism regulates receptor recycling, which is necessary for sustained signaling, and it may also control beta-arrestin-dependent mitogenic signaling of endocytosed receptors. We propose that other endosomal enzymes and transporters may similarly control the availability of transmitters in endosomes to regulate trafficking and signaling of GPCRs. Antagonism of these endosomal processes represents a strategy for inhibiting sustained signaling of receptors, and defects may explain the tachyphylaxis of drugs that are receptor agonists.

Predominant surface distribution of neurokinin-3 receptors in non-dopaminergic dendrites in the rat substantia nigra and ventral tegmental area

Neurokinin-3 (NK(3)) receptors are prevalent within the substantia nigra (SN) and ventral tegmental area (VTA), where their activation can affect motor and motivational behaviors as well as cardiovascular function and stress responses. These actions are mediated, in part, by dopaminergic neurons in each region. To determine the relevant sites for activation of these receptors, we examined the electron microscopic localization of NK(3) receptors and tyrosine hydroxylase (TH), the catecholamine synthesizing enzyme in dopaminergic neurons in the SN and VTA of rat brain. In each region, immunogold-silver labeling for NK(3) receptors was detected in many somatodendritic profiles, some of which contained TH-immunoreactivity. NK(3)-immunogold particles were largely associated with endomembranes resembling smooth endoplasmic reticulum, and only occasionally located on the plasma membrane in TH-labeled dendrites. In comparison with these dendrites, non-TH immunoreactive dendrites contained significantly more total (VTA) and more plasmalemmal (VTA and SN) NK(3)-immunogold particles. In each region, NK(3) gold particles also were seen in axonal as well as glial profiles, some of which contacted TH-immunoreactive dendrites. The NK(3)-labeled axon terminals formed either symmetric or asymmetric, excitatory-type synapses, the latter of which were significantly more prevalent in the VTA, compared with SN. These results provide the first ultrastructural evidence indicating that NK(3) receptors are available in cytoplasmic reserve in dopaminergic neurons, but more immediately accessible at the plasmalemmal surface of non-dopaminergic dendrites in both the SN and VTA. The activation of these receptors, together with the NK(3) receptors in either the presynaptic axon terminals or glia may contribute to the diverse physiological effects of tachykinins in each region, and most prominently involving excitatory inputs to the VTA.

Arrestin binding to calmodulin: a direct interaction between two ubiquitous signaling proteins

Arrestins serve as multi-functional regulators of G-protein coupled receptors, interacting with hundreds of different receptor subtypes and a variety of other signaling proteins. Here we identify calmodulin as a novel arrestin interaction partner using three independent methods in vitro and in cells. Arrestin preferentially binds calcium-loaded calmodulin with a Kd value of approximately 7 microM, which is within range of endogenous calmodulin concentrations. The calmodulin binding site is localized on the concave side of the C-domain and a loop in the center of the arrestin molecule, significantly overlapping with receptor and microtubule-binding sites. Using purified proteins, we found that arrestins sequester calmodulin, preventing its binding to microtubules. Nanomolar affinity of arrestins for their cognate receptors makes calmodulin an ineffective competitor for arrestin binding at relatively high receptor concentrations. The arrestin-calmodulin interaction likely regulates the localization of both proteins and their availability for other interaction partners.

Expression of NK-1 and NK-3 tachykinin receptors in pancreatic acinar cells after acute experimental pancreatitis in rats

Activation of neurokinin (NK)-1 receptors but not of NK-3 stimulates amylase release from isolated pancreatic acini of the rat. Immunofluorescence studies show that NK-1 receptors are more strongly expressed than NK-3 receptors on pancreatic acinar cells under basal conditions. No studies have examined the expression of the two NK receptor populations in pancreatic acini during pancreatitis in rats. We therefore investigated the relationships between expression of these two tachykinin receptors and experimental acute pancreatitis induced by stimulating pancreatic amylase with caerulein (CK) in rats. Hyperstimulation of the pancreas by CK caused an increase in plasma amylase and pancreatic water content and resulted in morphological evidence of cytoplasmic vacuolization. Immunofluorescence analysis revealed a similar percentage of NK-1 receptor antibody immunoreactive acinar cells in rats with pancreatitis and in normal rat tissue but a larger percentage of NK-3 receptor immunoreactive cells in acute pancreatitis than in normal pancreas. Western blot analysis of NK-1 and NK-3 receptor protein levels after CK-induced pancreatitis showed no change in NK-1 receptors but a stronger increase in NK-3 receptor expression in pancreatic acini compared with normal rats thus confirming the immunofluorescence data. These new findings support previous evidence that substance P-mediated functions within the pancreas go beyond sensory signal transduction contributing to neurogenic inflammation, and they suggest that substance P plays a role in regulating pancreatic exocrine secretion via acinar NK-1 receptors. The significant increase in NK-3 receptors during pancreatic stimulation suggests that NK-3 receptors also intervene in the pathogenesis of mild acute pancreatitis in rats.

Ubiquitin-dependent down-regulation of the neurokinin-1 receptor

Transient stimulation with substance P (SP) induces endocytosis and recycling of the neurokinin-1 receptor (NK(1)R). The effects of sustained stimulation by high concentrations of SP on NK(1)R trafficking and Ca(2+) signaling, as may occur during chronic inflammation and pain, are unknown. Chronic exposure to SP (100 nm, 3 h) completely desensitized Ca(2+) signaling by wild-type NK(1)R (NK(1)Rwt). Resensitization occurred after 16 h, and cycloheximide prevented resensitization, implicating new receptor synthesis. Lysine ubiquitination of G-protein-coupled receptors is a signal for their trafficking and degradation. Lysine-deficient mutant receptors (NK(1)RDelta5K/R, C-terminal tail lysines; and NK(1)RDelta10K/R, all intracellular lysines) were expressed at the plasma membrane and were functional because they responded to SP by endocytosis and by mobilization of Ca(2+) ions. SP desensitized NK(1)Rwt, NK(1)RDelta5K/R, and NK(1)RDelta10K/R. However, NK(1)RDelta5K/R and NK(1)RDelta10K/R resensitized 4-8-fold faster than NK(1)Rwt by cycloheximide-independent mechanisms. NK(1)RDelta325 (a naturally occurring truncated variant) showed incomplete desensitization, followed by a marked sensitization of signaling. Upon labeling receptors in living cells using antibodies to extracellular epitopes, we observed that SP induced endocytosis of NK(1)Rwt, NK(1)RDelta5K/R, and NK(1)RDelta10K/R. After 4 h in SP-free medium, NK(1)RDelta5K/R and NK(1)RDelta10K/R recycled to the plasma membrane, whereas NK(1)Rwt remained internalized. SP induced ubiquitination of NK(1)Rwt and NK(1)RDelta5K/R as determined by immunoprecipitation under nondenaturing and denaturing conditions and detected with antibodies for mono- and polyubiquitin. NK(1)RDelta10K/R was not ubiquitinated. Whereas SP induced degradation of NK(1)Rwt, NK(1)RDelta5K/R and NK(1)RDelta10K/R showed approximately 50% diminished degradation. Thus, chronic stimulation with SP induces ubiquitination of the NK(1)R, which mediates its degradation and down-regulation.