A novel mechanism of neurokinin-1 receptor resensitization

Roles of proteolysis in regulation of GPCR function

The enzymatic activity of peptidases must be tightly regulated to prevent uncontrolled hydrolysis of peptide bonds, which could have devastating effects on biological systems. Peptidases are often generated as inactive propeptidases, secreted with endogenous inhibitors, or they are compartmentalized. Propeptidases become active after proteolytic removal of N-terminal activation peptides by other peptidases. Some peptidases only become active towards substrates only at certain pHs, thus confining activity to specific compartments or conditions. This review discusses the different roles proteolysis plays in regulating GPCRs. At the cell-surface, certain GPCRs are regulated by the hydrolytic inactivation of bioactive peptides by membrane-anchored peptidases, which prevent signalling. Conversely, cell-surface peptidases can also generate bioactive peptides, which directly activate GPCRs. Alternatively, cell-surface peptidases activated by GPCRs, can generate bioactive peptides to cause transactivation of receptor tyrosine kinases, thereby promoting signalling. Certain peptidases can signal directly to cells, by cleaving GPCR to initiate intracellular signalling cascades. Intracellular peptidases also regulate GPCRs; lysosomal peptidases destroy GPCRs in lysosomes to permanently terminate signalling and mediate down-regulation; endosomal peptidases cleave internalized peptide agonists to regulate GPCR recycling, resensitization and signalling; and soluble intracellular peptidases also participate in GPCR function by regulating the ubiquitination state of GPCRs, thereby altering GPCR signalling and fate. Although the use of peptidase inhibitors has already brought success in the treatment of diseases such as hypertension, the discovery of new regulatory mechanisms involving proteolysis that control GPCRs may provide additional targets to modulate dysregulated GPCR signalling in disease.

Substance P is a mechanoresponsive, autocrine regulator of human tenocyte proliferation

It has been hypothesised that substance P (SP) may be produced by primary fibroblastic tendon cells (tenocytes), and that this production, together with the widespread distribution of the neurokinin-1 receptor (NK-1 R) in tendon tissue, could play an important role in the development of tendinopathy, a condition of chronic tendon pain and thickening. The aim of this study was to examine the possibility of endogenous SP production and the expression of NK-1 R by human tenocytes. Because tendinopathy is related to overload, and because the predominant tissue pathology (tendinosis) underlying early tendinopathy is characterized by tenocyte hypercellularity, the production of SP in response to loading/strain and the effects of exogenously administered SP on tenocyte proliferation were also studied. A cell culture model of primary human tendon cells was used. The vast majority of tendon cells were immunopositive for the tenocyte/fibroblast markers tenomodulin and vimentin, and immunocytochemical counterstaining revealed that positive immunoreactions for SP and NK-1 R were seen in a majority of these cells. Gene expression analyses showed that mechanical loading (strain) of tendon cell cultures using the FlexCell© technique significantly increased the mRNA levels of SP, whereas the expression of NK-1 R mRNA decreased in loaded as compared to unloaded tendon cells. Reduced NK-1 R protein was also observed, using Western blot, after exogenously administered SP at a concentration of 10⁻⁷ M. SP exposure furthermore resulted in increased cell metabolism, increased cell viability, and increased cell proliferation, all of which were found to be specifically mediated via the NK-1 R; this in turn involving a common mitogenic cell signalling pathway, namely phosphorylation of ERK1/2. This study indicates that SP, produced by tenocytes in response to mechanical loading, may regulate proliferation through an autocrine loop involving the NK-1 R.

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.

Functional Selectivity of NK1Receptor Signaling: Peptide Agonists Can Preferentially Produce Receptor Activation or Desensitization

A cascade of events follows binding of an agonist ligand to the tachykinin NK(1) receptor. These events include activation of multiple signal transduction pathways as well as cellular modulation of receptor function by the process of desensitization. This study examines the differences in the abilities of naturally occurring peptide agonist ligands of the tachykinin NK(1) receptor to preferentially direct signaling through the receptor to produce signal activation versus receptor desensitization. The differential effects of tachykinin peptides with respect to ligand competition binding, receptor-G protein coupling, intracellular Ca(2+) elevations, and receptor desensitization have been measured. In relation to its potency in competition binding studies, substance P produces desensitization at lower concentrations, whereas higher concentrations are required to elicit a Ca(2+) response. In contrast to this, a related peptide, ranatachykinin C, is more effective at activating a Ca(2+) response relative to its ability to produce desensitization. This difference in functional selectivity is conserved for an amphibian and two mammalian ortholog tachykinin receptors. The present study demonstrates that peptide agonist ligands of NK(1) receptors can preferentially produce signal activation or desensitization.

Separation of mu-opioid receptor desensitization and internalization: endogenous receptors in primary neuronal cultures

A close relationship between desensitization and internalization of mu-opioid receptors (MORs) has been proposed based on differential actions of series of agonists. The role that these two processes have in the development of tolerance and dependence to opioids has been a controversial subject that has been studied in a variety of model systems. Here, we examine desensitization and internalization of endogenous MORs simultaneously in primary cultures of locus ceruleus neurons using fluorescently tagged peptide agonists. With the use of two fluorescent opioid peptides, dermorphin-Bodipy Texas Red and dermorphin-Alexa594 (Derm-A594), desensitization was measured electrophysiologically and trafficking was followed by the accumulation of intracellular fluorescent puncta. Blocking endocytosis with concanavalin A eliminated the accumulation of fluorescent puncta but desensitization induced by Derm-A594 was unaffected. Likewise, after treatment with concanavalin A, there was no change in either desensitization or recovery from desensitization induced by [Met]5enkephalin. The results demonstrate that desensitization and the recovery from desensitization are not dependent on receptor internalization and suggest that the activity of endogenous MORs in primary neurons can be modulated at the level of the plasma membrane.

Neurokinin-1 receptor resensitization precedes receptor recycling

Following agonist binding, neurokinin-1 receptors undergo rapid desensitization followed by internalization and recycling. Desensitization requires receptor phosphorylation but does not require internalization, whereas resensitization is thought to require internalization and recycling. Our previous data, however, have suggested that, following activation and desensitization, the return of responsiveness to the neurokinin-1 agonist substance P (termed "resensitization") occurs hours before internalized receptors are recycled back to the plasma membrane. To further investigate this novel mechanism of neurokinin-1 receptor resensitization, we have studied the time courses of neurokinin-1 receptor responsiveness, recycling, and dephosphorylation by measuring cellular Ca(2+) responses, ligand-receptor binding, and receptor phosphorylation, respectively. Concentration-response curves and competition binding curves were obtained at various times following desensitization. The effects of the nonhydrolyzable GTP analog Gpp(NH)p on substance P binding were also studied to assess receptor-G protein coupling. After receptor activation and desensitization, Ca(2+) signaling in response to substance P occurred within 90 min, whereas the return of receptor binding required 240 min. Receptor dephosphorylation was greater than 90% complete 20 min after agonist washout. In addition, the return of substance P responsiveness coincided with a return in sensitivity of substance P binding to Gpp(NH)p, indicating a return in receptor-G protein coupling. These data show that the resensitization of responsiveness to substance P precedes receptor recycling. This may result from a conversion of nonfunctional neurokinin-1 receptors to functional receptors at the plasma membrane.

A library of 7TM receptor C-terminal tails. Interactions with the proposed post-endocytic sorting proteins ERM-binding phosphoprotein 50 (EBP50), N-ethylmaleimide-sensitive factor (NSF), sorting nexin 1 (SNX1), and G protein-coupled receptor-associated sorting protein (GASP)

Adaptor and scaffolding proteins determine the cellular targeting, the spatial, and thereby the functional association of G protein-coupled seven-transmembrane receptors with co-receptors, transducers, and downstream effectors and the adaptors determine post-signaling events such as receptor sequestration through interactions, mainly with the C-terminal intracellular tails of the receptors. A library of tails from 59 representative members of the super family of seven-transmembrane receptors was probed as glutathione S-transferase fusion proteins for interactions with four different adaptor proteins previously proposed to be involved in post-endocytotic sorting of receptors. Of the two proteins suggested to target receptors for recycling to the cell membrane, which is the route believed to be taken by a majority of receptors, ERM (ezrin-radixin-moesin)-binding phosphoprotein 50 (EBP50) bound only a single receptor tail, i.e. the beta(2)-adrenergic receptor, whereas N-ethylmaleimide-sensitive factor bound 11 of the tail-fusion proteins. Of the two proteins proposed to target receptors for lysosomal degradation, sorting nexin 1 (SNX1) bound 10 and the C-terminal domain of G protein-coupled receptor-associated sorting protein bound 23 of the 59 tail proteins. Surface plasmon resonance analysis of the binding kinetics of selected hits from the glutathione S-transferase pull-down experiments, i.e. the tails of the virally encoded receptor US28 and the delta-opioid receptor, confirmed the expected nanomolar affinities for interaction with SNX1. Truncations of the NK(1) receptor revealed that an extended binding epitope is responsible for the interaction with both SNX1 and G protein-coupled receptor-associated sorting protein as well as with N-ethylmaleimide-sensitive factor. It is concluded that the tail library provides useful information on the general importance of certain adaptor proteins, for example, in this case, ruling out EBP50 as being a broad spectrum-recycling adaptor.

Calcium-sensing Receptor-mediated ERK1/2 Activation Requires Gαi2 Coupling and Dynamin-independent Receptor Internalization

The calcium-sensing receptor (CaR) recently has been shown to activate MAP kinase (ERK1/2) in various cell types as well as in heterologous expression systems. In this study we show that the CaR agonist NPS R-467 (1 microm), which does not activate the CaR by itself, robustly activates ERK1/2 in the presence of a low concentration of Ca(2+) (0.5 mm CaCl(2)) in human embryonic kidney (HEK) cells permanently expressing the human CaR (HEK-hCaR). Ca(2+) (4 mm) also activates ERK1/2 but with differing kinetics. CaR-dependent ERK1/2 activation begins to desensitize to 4 mm Ca(2+) after 10 min, whereas there is no desensitization to NPS R-467/CaCl(2) as late as 4 h. Moreover, recovery from desensitization occurs as rapidly as 30 min with 4 mm CaCl(2). Pretreatment of HEK-hCaR cells with concanavalin A (250 microg/ml) to block CaR internalization completely eliminated the NPS R-467/CaCl(2)-mediated ERK1/2 activation but did not block the 2-min time point of 4 mm Ca(2+)-mediated ERK1/2 activation. Neither dominant-negative dynamin (K44A) nor dominant-negative beta-arrestin inhibited ERK1/2 activation by either CaR agonist treatment, suggesting that CaR-elicited ERK1/2 signaling occurs via a dynamin-independent pathway. Pertussis toxin pretreatment partially attenuated the 4 mm Ca(2+)-ERK1/2 activation; this attenuated activity was completely restored by co-expression of the Galpha(i2) (C351I) but not Galpha(i1) (C351I) or Galpha(i3) (C351I) G proteins, PTX-insensitive G protein mutants. Taken together, these data suggest that both 4 mm Ca(2+) and NPS R-467/CaCl(2) activate ERK1/2 via distinguishable pathways in HEK-hCaR cells and may represent a nexus to differentially regulate differentiation versus proliferation via CaR activation.

Phosphorylation of a carboxyl-terminal serine within the kappa-opioid receptor produces desensitization and internalization

G-protein receptor kinase and beta-arrestin mediated desensitization of the rat kappa-opioid receptor (KOR) was previously shown using Xenopus oocyte expression to require serine 369 within the C terminus of KOR. To define the effects of phosphorylation of this residue in desensitization and internalization processes in mammalian expression systems, wild-type KOR-green fluorescent protein (KOR-GFP) and KOR(S369A)-GFP were stably expressed in AtT-20 and HEK293 cells. Using whole-cell patch clamp recording in transfected AtT-20 cells, agonist activation of either kappa receptor form produced equivalent activation of the intrinsic G-protein-gated inwardly rectifying potassium channel. Incubation for 60 min with the kappa agonist U50,488 (100 nm) desensitized the response in cells expressing wild-type KOR-GFP by 86% but had no effect on KOR(S369A)-GFP-expressing cells. Phosphorylation of serine 369 was detected using a phosphospecific antibody (KOR-P) able to distinguish the phosphorylated form of the receptor. The agonist-induced increase in KOR-P labeling was dose-dependent, blocked by co-treatment with the kappa antagonist norbinaltorphimine, and prevented by co-expression of the dominant negative form of the G-protein receptor kinase, GRK2(K220R). In contrast, agonist-induced increase in KOR-P labeling was not evident in KOR(S369A) expressing cells. Prolonged activation resulted in receptor internalization that was also blocked by KOR(S369A) substitution, but interestingly, KOR-P labeling was evident at lower agonist concentrations than required to induce internalization. Following the removal of agonist, receptor dephosphorylation detected by loss of KOR-P labeling was complete within 60 min, could be blocked by okadaic acid, and was not blocked by sucrose inhibition of receptor internalization. These results demonstrate that GRK-mediated phosphorylation of serine 369 mediates rat KOR desensitization and internalization.