BACKGROUND AND PURPOSEMorphine activates the m-opioid receptor without causing its rapid endocytosis. In contrast, full agonists such as [D-Ala 2 -MePhe 4 -Gly-ol]enkephalin (DAMGO) or etonitazene stimulate a rapid and profound internalization. However, the detailed molecular events underlying the differential regulation of receptor trafficking by distinct opioid agonists remain incompletely understood. EXPERIMENTAL APPROACHHere, we have generated phosphosite-specific antibodies for the carboxyl-terminal residues serine 363 (Ser363), threonine 370 (Thr370) and serine 375 (Ser375), which enabled us to selectively detect either the Ser363-, Thr370-or Ser375-phosphorylated form of the receptor. KEY RESULTSWe showed that agonist-induced phosphorylation occurs at Thr370 and Ser375, whereas Ser363 is constitutively phosphorylated in the absence of agonist. We further demonstated that DAMGO and etonitazene stimulated the phosphorylation of both Thr370 and Ser375. In contrast, morphine promoted the phosphorylation of Ser375, but failed to stimulate Thr370 phosphorylation. In the presence of DAMGO, Ser375 phosphorylation occurred at a faster rate than phosphorylation of Thr370, indicating that Ser375 is the primary site of agonist-dependent phosphorylation. Activation of PKC by phorbol 12-myristate 13-acetate increased receptor phosphorylation only on Thr370, but not on Ser375, indicating that Thr370 can also undergo heterologous PKC-mediated phosphorylation. We also showed that m receptor dephosphorylation can occur within minutes at or near the plasma membrane, and that agonist removal is a major prerequisite for Thr370 and Ser375 dephosphorylation. CONCLUSIONS AND IMPLICATIONSTogether, we showed for the first time that distinct agonists stimulate site-specific patterns of phosphorylation, which are intimately related to their ability to elicit m-opioid receptor sequestration. LINKED ARTICLE

BACKGROUND AND PURPOSEThe molecular basis of agonist-selective signalling at the m-opioid receptor is poorly understood. We have recently shown that full agonists such as [D-Ala ), and that is followed by a robust receptor internalization. In contrast, morphine promotes a selective phosphorylation of Ser 375 without causing rapid receptor internalization. EXPERIMENTAL APPROACHHere, we identify kinases and phosphatases that mediate agonist-dependent phosphorylation and dephosphorylation of the m-opioid receptor using a combination of phosphosite-specific antibodies and siRNA knock-down screening in HEK293 cells. KEY RESULTSWe found that DAMGO-driven phosphorylation of Thr 370 and Ser 375 was preferentially catalysed by G-protein-coupled receptor kinases (GRKs) 2 and 3, whereas morphine-driven Ser 375 phosphorylation was preferentially catalysed by GRK5. On the functional level, inhibition of GRK expression resulted in enhanced m-opioid receptor signalling and reduced receptor internalization. Analysis of GRK5-deficient mice revealed that GRK5 selectively contributes to morphine-induced Ser 375 phosphorylation in brain tissue. We also identified protein phosphatase 1g as a m-opioid receptor phosphatase that catalysed Thr 370 and Ser 375 dephosphorylation at or near the plasma membrane within minutes after agonist removal, which in turn facilitates receptor recycling. CONCLUSIONS AND IMPLICATIONSTogether, the morphine-activated m-opioid receptor is a good substrate for phosphorylation by GRK5 but a poor substrate for GRK2/3. GRK5 phosphorylates m-opioid receptors selectively on Ser 375 , which is not sufficient to drive significant receptor internalization.

Pasireotide (SOM230) is currently under clinical evaluation as a successor compound to octreotide for the treatment of acromegaly, Cushing's disease, and carcinoid tumors. Whereas octreotide acts primarily via the sst(2A) somatostatin receptor, pasireotide was designed to exhibit octreotide-like sst(2A) activity combined with enhanced binding to other somatostatin receptor subtypes. In the present study, we used phophosite-specific antibodies to examine agonist-induced phosphorylation of the rat sst(2A) receptor. We show that somatostatin and octreotide stimulate the complete phosphorylation of a cluster of four threonine residues within the cytoplasmic (353)TTETQRT(359) motif in a variety of cultured cell lines in vitro as well as in intact animals in vivo. This phosphorylation was mediated by G protein-coupled receptor kinases (GRK) 2 and 3 and followed by rapid cointernalization of the receptor and ss-arrestin into the same endocytic vesicles. In contrast, pasireotide failed to promote substantial phosphorylation and internalization of the rat sst(2A) receptor. In the presence of octreotide or SS-14, SOM230 showed partial agonist behavior, inhibiting phosphorylation, and internalization of sst(2A). Upon overexpression of GRK2 or GRK3, pasireotide stimulated selective phosphorylation of Thr356 and Thr359 but not of Thr353 or Thr354 within the (353)TTETQRT(359) motif. Pasireotide-mediated phosphorylation led to the formation of relatively unstable beta-arrestin-sst(2A) complexes that dissociated at or near the plasma membrane. Thus, octreotide and pasireotide are equally active in inducing classical G protein-dependent signaling via the sst(2A) somatostatin receptor. Yet, we find that they promote strikingly different patterns of sst(2A) receptor phosphorylation and, hence, stimulate functionally distinct pools of beta-arrestin.

Termination of signaling of activated G protein-coupled receptors (GPCRs) is essential for maintenance of cellular homeostasis. It is well established that ␤-arrestin redistributes to phosphorylated GPCRs and thereby facilitates desensitization of classical G protein-dependent signaling. ␤-Arrestin in turn serves as a scaffold to initiate a second wave of signaling. Here, we report a molecular mechanism that regulates the termination of unconventional ␤-arrestin-dependent GPCR signaling. We identify protein phosphatase 1␤ (PP1␤) as a phosphatase for the cluster of phosphorylated threonines ( 353 TTETQRT 359 ) within the sst 2A somatostatin receptor carboxyl terminus that mediates ␤-arrestin binding using siRNA knock-down screening. We show that PP1␤-mediated sst 2A dephosphorylation is initiated directly after receptor activation at or near the plasma membrane. As a functional consequence of diminished PP1␤ activity, we find that somatostatin-and substance P-induced but not epidermal growth factor-induced ERK activation was aberrantly enhanced and prolonged. Thus, we demonstrate a novel mechanism for fine tuning unconventional ␤-arrestin-dependent GPCR signaling in that recruitment of PP1␤ to activated GPCRs facilitates GPCR dephosphorylation and, hence, leads to disruption of the ␤-arrestin-GPCR complex. Desensitization of GPCR2 signaling causes a reduction of receptor response to repeated or long-lasting stimuli. It usually involves agonist-induced phosphorylation of cytoplasmic parts of the receptor by GRKs or second messenger-dependent protein kinases such as protein kinase A or protein kinase C (1). Agonist-induced phosphorylation allows binding of ␤-arrestin to the receptor (2). It is well established that ␤-arrestin promotes desensitization of G protein signaling and induces receptor internalization. More recent work has established that ␤-arrestin binding in turn stimulates a second wave of signaling (3,4).Although the regulation of agonist-induced phosphorylation has been studied in detail for many GPCRs, the molecular mechanisms and functional consequences of receptor dephosphorylation are far from understood. Earlier studies have identified retinal degeneration C as the phosphatase required for rhodopsin dephosphorylation in Drosophila melanogaster. The catalytic domain of retinal degeneration C exhibits high homology to PP1, PP2, and PP3 (5-7). Loss of retinal degeneration C causes disturbance of light-signal transduction and leads to light-dependent retinal degeneration (7). At the same time, a PP2-related phosphatase that dephosphorylates the ␤ 2 -adrenergic receptor was identified and named GPCR phosphatase (8, 9). It was proposed that GPCR phosphatase is tethered to vesicular membranes and that receptors have to internalize into an acidic endosomal compartment to become dephosphorylated (8, 9). However, later it was shown that inhibition of ␤ 2 -adrenergic receptor internalization with dominant negative dynamin or hypertonic sucrose did not affect the rate of receptor dephosphorylation. Similar, ...