Stuart J. Mundell

Agonist-Selective Mechanisms of μ-Opioid Receptor Desensitization in Human Embryonic Kidney 293 Cells

The ability of two opioid agonists, [d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) and morphine, to induce μ-opioid receptor (MOR) phosphorylation, desensitization, and internalization was examined in human embryonic kidney (HEK) 293 cells expressing rat MOR1 as well G protein-coupled inwardly rectifying potassium channel (GIRK) channel subunits. Both DAMGO and morphine activated GIRK currents, but the maximum response to DAMGO was greater than that of morphine, indicating that morphine is a partial agonist. The responses to DAMGO and morphine desensitized rapidly in the presence of either drug. Expression of a dominant negative mutant G protein-coupled receptor kinase 2 (GRK2), GRK2-K220R, markedly attenuated the DAMGO-induced desensitization of MOR1, but it had no effect on morphine-induced MOR1 desensitization. In contrast, inhibition of protein kinase C (PKC) either by the PKC inhibitory peptide PKC (19-31) or staurosporine reduced MOR1 desensitization by morphine but not that induced by DAMGO. Morphine and DAMGO enhanced MOR1 phosphorylation over basal. The PKC inhibitor bisindolylmaleimide 1 (GF109203X) inhibited MOR1 phosphorylation under basal conditions and in the presence of morphine, but it did not inhibit DAMGO-induced phosphorylation. DAMGO induced arrestin-2 translocation to the plasma membrane and considerable MOR1 internalization, whereas morphine did not induce arrestin-2 translocation and induced very little MOR1 internalization. Thus, DAMGO and morphine each induce desensitization of MOR1 signaling in HEK293 cells but by different molecular mechanisms; DAMGO-induced desensitization is GRK2-dependent, whereas morphine-induced desensitization is in part PKC-dependent. MORs desensitized by DAMGO activation are then readily internalized by an arrestin-dependent mechanism, whereas those desensitized by morphine are not. These data suggest that opioid agonists induce different conformations of the MOR that are susceptible to different desensitizing and internalization processes.

Distinct Clathrin‐Coated Pits Sort Different G Protein‐Coupled Receptor Cargo

Upon activation, many G protein‐coupled receptors (GPCRs) internalize by clathrin‐mediated endocytosis and are subsequently sorted to undergo recycling or lysosomal degradation. Here we observe that sorting can take place much earlier than previously thought, by entry of different GPCRs into distinct populations of clathrin‐coated pit (CCP). These distinct populations were revealed by analysis of two purinergic GPCRs, P2Y1 and P2Y12, which enter two populations of CCPs in a mutually exclusive manner. The mechanisms underlying early GPCR sorting involve differential kinase‐dependent processes because internalization of P2Y12 is mediated by GPCR kinases (GRKs) and arrestin, whereas P2Y1 internalization is GRK‐ and arrestin‐independent but requires protein kinase C. Importantly, the β2 adrenoceptor which also internalizes in a GRK‐dependent manner also traffics exclusively to P2Y12‐containing CCPs. Our data therefore reveal distinct populations of CCPs that sort GPCR cargo at the plasma membrane using different kinase‐dependent mechanisms.

Agonist-induced internalization of the metabotropic glutamate receptor 1a is arrestin- and dynamin-dependent

At present, little is known regarding the mechanism of metabotropic glutamate receptor (mGluR) trafficking. To facilitate this characterization we inserted a haemagglutinin (HA) epitope tag in the extracellular N‐terminal domain of the rat mGluR1a. In human embryonic kidney cells (HEK293), transiently transfected with HA‐mGluR1a, the epitope‐tagged receptor was primarily localized to the cell surface prior to agonist stimulation. Following stimulation with glutamate (10 µm; 30 min) the HA‐mGluR1a underwent internalization to endosomes. Further quantification of receptor internalization was provided by ELISA experiments which showed rapid agonist‐induced internalization of the HA‐mGluR1a. To determine whether agonist‐induced mGluR1a internalization is an arrestin‐ and dynamin‐dependent process, cells were cotransfected with HA‐mGluR1a and either of these dynamin‐K44A or arrestin‐2 (319–418). Expression of either dominant negative mutant constructs with receptor strongly inhibited glutamate‐induced (10 µm; 30 min) HA‐mGluR1a internalization. In addition, wild‐type arrestin‐2−green fluorescent protein (arrestin‐2−GFP) or arrestin‐3−GFP underwent agonist‐induced translocation from cytosol to membrane in HEK293 cells coexpressing HA‐mGluR1a. Taken together our observations demonstrate that agonist‐induced internalization of mGluR1a is an arrestin‐ and dynamin‐dependent process.

Identification and characterization of a novel P2Y12 variant in a patient diagnosed with type 1 von Willebrand disease in the European MCMDM-1VWD study

We investigated whether defects in the P2Y(12) ADP receptor gene (P2RY12) contribute to the bleeding tendency in 92 index cases enrolled in the European MCMDM-1VWD study. A heterozygous mutation, predicting a lysine to glutamate (K174E) substitution in P2Y(12), was identified in one case with mild type 1 von Willebrand disease (VWD) and a VWF defect. Platelets from the index case and relatives carrying the K174E defect changed shape in response to ADP, but showed reduced and reversible aggregation in response to 10 muM ADP, unlike the maximal, sustained aggregation observed in controls. The reduced response was associated with an approximate 50% reduction in binding of [(3)H]2MeS-ADP to P2Y(12), whereas binding to the P2Y(1) receptor was normal. A hemagglutinin-tagged K174E P2Y(12) variant showed surface expression in CHO cells, markedly reduced binding to [(3)H]2MeS-ADP, and minimal ADP-mediated inhibition of forskolin-induced adenylyl cyclase activity. Our results provide further evidence for locus heterogeneity in type 1 VWD.

Distinct roles for protein kinase C isoforms in regulating platelet purinergic receptor function.

ADP is a critical regulator of platelet activation, mediating its actions through two G protein-coupled receptors (GPCRs), P2Y1 and P2Y12. We have shown previously that the receptors are functionally desensitized, in a homologous manner, by distinct kinase-dependent mechanisms in which P2Y1 is regulated by protein kinase C (PKC) and P2Y12 by G protein-coupled receptor kinases. In this study, we addressed whether different PKC isoforms play different roles in regulating the trafficking and activity of these two GPCRs. Expression of PKCα and PKCδ dominant-negative mutants in 1321N1 cells revealed that both isoforms regulated P2Y1 receptor signaling and trafficking, although only PKCδ was capable of regulating P2Y12, in experiments in which PKC was directly activated by the phorbol ester phorbol 12-myristate 13-acetate (PMA). These results were paralleled in human platelets, in which PMA reduced subsequent ADP-induced P2Y1 and P2Y12 receptor signaling. PKC isoform-selective inhibitors revealed that novel, but not conventional, isoforms of PKC regulate P2Y12 function, whereas both novel and classic isoforms regulate P2Y1 activity. It is also noteworthy that we studied receptor internalization in platelets by a radioligand binding approach showing that both receptors internalize rapidly in these cells. ADP-induced P2Y1 receptor internalization is attenuated by PKC inhibitors, whereas that of the P2Y12 receptor is unaffected. Both P2Y1 and P2Y12 receptors can also undergo PMA-stimulated internalization, and here again, novel but not classic PKCs regulate P2Y12, whereas both novel and classic isoforms regulate P2Y1 internalization. This study therefore is the first to reveal distinct roles for PKC isoforms in the regulation of platelet P2Y receptor function and trafficking.

Endomorphin-2: A Biased Agonist at the μ-Opioid Receptor

Previously we correlated the efficacy for G protein activation with that for arrestin recruitment for a number of agonists at the μ-opioid receptor (MOPr) stably expressed in HEK293 cells. We suggested that the endomorphins (endomorphin-1 and -2) might be biased toward arrestin recruitment. In the present study, we investigated this phenomenon in more detail for endomorphin-2, using endogenous MOPr in rat brain as well as MOPr stably expressed in HEK293 cells. For MOPr in neurons in brainstem locus ceruleus slices, the peptide agonists [d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) and endomorphin-2 activated inwardly rectifying K+ current in a concentration-dependent manner. Analysis of these responses with the operational model of pharmacological agonism confirmed that endomorphin-2 had a much lower operational efficacy for G protein-mediated responses than did DAMGO at native MOPr in mature neurons. However, endomorphin-2 induced faster desensitization of the K+ current than did DAMGO. In addition, in HEK293 cells stably expressing MOPr, the ability of endomorphin-2 to induce phosphorylation of Ser375 in the COOH terminus of the receptor, to induce association of arrestin with the receptor, and to induce cell surface loss of receptors was much more efficient than would be predicted from its efficacy for G protein-mediated signaling. Together, these results indicate that endomorphin-2 is an arrestin-biased agonist at MOPr and the reason for this is likely to be the ability of endomorphin-2 to induce greater phosphorylation of MOPr than would be expected from its ability to activate MOPr and to induce activation of G proteins.

A novel thromboxane A2 receptor D304N variant that abrogates ligand binding in a patient with a bleeding diathesis.

We investigated the cause of mild mucocutaneous bleeding in a 14-year-old male patient (P1). Platelet aggregation and ATP secretion induced by arachidonic acid and the thromboxane A(2) receptor (TxA(2)R) agonist U46619 were reduced in P1 compared with controls, whereas the responses to other platelet agonists were retained. P1 was heterozygous for a transversion within the TBXA2R gene predictive of a D304N substitution in the TxA(2)R. In Chinese hamster ovary-K1 cells expressing the variant D304N TxA(2)R, U46619 did not increase cytosolic free Ca(2+) concentration, indicating loss of receptor function. The TxA(2)R antagonist [(3)H]-SQ29548 showed an approximate 50% decrease in binding to platelets from P1 but absent binding to Chinese hamster ovary-K1 cells expressing variant D304N TxA(2)R. This is the second naturally occurring TxA(2)R variant to be associated with platelet dysfunction and the first in which loss of receptor function is associated with reduced ligand binding. D304 lies within a conserved NPXXY motif in transmembrane domain 7 of the TxA(2)R that is a key structural element in family A G protein-coupled receptors. Our demonstration that the D304N substitution causes clinically significant platelet dysfunction by reducing ligand binding establishes the importance of the NPXXY motif for TxA(2)R function in vivo.

Developing oligodendrocytes express functional GABAB receptors that stimulate cell proliferation and migration

GABAB receptors (GABABRs) are involved in early events during neuronal development. The presence of GABABRs in developing oligodendrocytes has not been established. Using immunofluorescent co‐localization, we have identified GABABR proteins in O4 marker‐positive oligodendrocyte precursor cells (OPCs) in 4‐day‐old mouse brain periventricular white matter. In culture, OPCs, differentiated oligodendrocytes (DOs) and type 2 astrocytes (ASTs) express both the GABAB1abcdf and GABAB2 subunits of the GABABR. Using semiquantitative PCR analysis with GABABR isoform‐selective primers we found that the expression level of GABAB1abd was substantially higher in OPCs or ASTs than in DOs. In contrast, the GABAB2 isoform showed a similar level of expression in OPCs and DOs, and a significantly higher level in ASTs. This indicates that the expression of GABAB1 and GABAB2 subunits are under independent control during oligodendroglial development. Activation of GABABRs using the selective agonist baclofen demonstrated that these receptors are functionally active and negatively coupled to adenylyl cyclase. Manipulation of GABABR activity had no effect on OPC migration in a conventional agarose drop assay, whereas baclofen significantly increased OPC migration in a more sensitive transwell microchamber‐based assay. Exposure of cultured OPCs to baclofen increased their proliferation, providing evidence for a functional role of GABABRs in oligodendrocyte development. The presence of GABABRs in developing oligodendrocytes provides a new mechanism for neuronal–glial interactions during development and may offer a novel target for promoting remyelination following white matter injury.

Regulation of P2Y1 Receptor Traffic by Sorting Nexin 1 is Retromer Independent

The activity and traffic of G‐protein coupled receptors (GPCRs) is tightly controlled. Recent work from our laboratory has shown that P2Y1 and P2Y12 responsiveness is rapidly and reversibly modulated in human platelets and that the underlying mechanism requires receptor trafficking as an essential part of this process. However, little is known about the molecular mechanisms underlying P2Y receptor traffic. Sorting nexin 1 (SNX1) has been shown to regulate the endosomal sorting of cell surface receptors either to lysosomes where they are downregulated or back to the cell surface. These functions may in part be due to interactions of SNX1 with the mammalian retromer complex. In this study, we investigated the role of SNX1 in P2Y receptor trafficking. We show that P2Y1 receptors recycle via a slow recycling pathway that is regulated by SNX1, whereas P2Y12 receptors return to the cell surface via a rapid route that is SNX1 independent. SNX1 inhibition caused a dramatic increase in the rate of P2Y1 receptor recycling, whereas inhibition of Vps26 and Vps35 known to be present in retromer had no effect, indicating that SNX1 regulation of P2Y1 receptor recycling is retromer independent. In addition, inhibition of SNX4, 6 and 17 proteins did not affect P2Y1 receptor recycling. SNX1 has also been implicated in GPCR degradation; however, we provide evidence that P2Y receptor degradation is SNX1 independent. These data describe a novel function of SNX1 in the regulation of P2Y1 receptor recycling and suggest that SNX1 plays multiple roles in endocytic trafficking of GPCRs.

An intact PDZ-motif is essential for correct P2Y12 purinoceptor traffic in human platelets

The platelet P2Y(12) purinoceptor (P2Y(12)R), which plays a crucial role in hemostasis, undergoes internalization and subsequent recycling to maintain receptor responsiveness, processes that are essential for normal platelet function. Here, we observe that P2Y(12)R function is compromised after deletion or mutation of the 4 amino acids at the extreme C-terminus of this receptor (ETPM), a putative postsynaptic density 95/disc large/zonula occludens-1 (PDZ)-binding motif. In cell line models, removal of this sequence or mutation of one of its core residues (P341A), attenuates receptor internalization and receptor recycling back to the membrane, thereby blocking receptor resensitization. The physiologic significance of these findings in the regulation of platelet function is shown by identification of a patient with a heterozygous mutation in the PDZ binding sequence of their P2Y(12)R (P341A) that is associated with reduced expression of the P2Y(12)R on the cell surface. Importantly, platelets from this subject showed significantly compromised P2Y(12)R recycling, emphasizing the importance of the extreme C-terminus of this receptor to ensure correct receptor traffic.