GPCRs represent the largest family of integral membrane proteins and were first identified as receptor proteins that couple via heterotrimeric G-proteins to regulate a vast variety of effector proteins to modulate cellular function. It is now recognized that GPCRs interact with a myriad of proteins that not only function to attenuate their signalling but also function to couple these receptors to heterotrimeric G-protein-independent signalling pathways. In addition, intracellular and transmembrane proteins associate with GPCRs and regulate their processing in the endoplasmic reticulum, trafficking to the cell surface, compartmentalization to plasma membrane microdomains, endocytosis and trafficking between intracellular membrane compartments. The present review will overview the functional consequence of b-arrestin, receptor activity-modifying proteins (RAMPS), regulators of G-protein signalling (RGS), GPCR-associated sorting proteins (GASPs), Homer, small GTPases, PSD95/Disc Large/Zona Occludens (PDZ), spinophilin, protein phosphatases, calmodulin, optineurin and Src homology 3 (SH3) containing protein interactions with GPCRs. LINKED ARTICLESThis article is part of a themed section on the Molecular Pharmacology of G Protein-Coupled Receptors (GPCRs). To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-6. To view the 2010 themed section on the same topic visit http://onlinelibrary.wiley.com/doi/10.1111/bph.2010.159.issue-5/issuetoc Abbreviations 5-HT2AR, serotonin receptor subtype 2; 5HT2CR, serotonin 2C receptor; a1DAR, a1D adrenergic receptor; Arf6, ADP-ribosylation factor 6; ARNO, ARF nucleotide-binding site opener; AT1R, angiotensin II type 1 receptor; b2AR, b2-adrenergic receptor; CAL, cystic fibrosis transmembrane conductance regulator-associated ligand; CASK, calcium/calmodulin-dependent serine protein kinase; CaSR, calcium sensing receptor; cGRP1/cGRP2, calcitonin gene-related peptides; CIPP, channel-interacting PDZ protein; CRF, corticotrophin-releasing factor; CRFR1, corticotropin releasing factor receptor 1; CRLR, calcitonin-like receptor; CXCR2, IL-8 receptor B; CXCR4, chemokine receptor type 4; EBP50, ERM-binding phosphoprotein 50; EEA1, effector early endosome antigen 1; EVH, ENA/VASP homology domain; GABAB, GABA type B receptor; GAPs, GTPase activating proteins; GASP, GPCR-associated sorting protein; GDP, guanosine diphosphate; GEFs, guanine nucleotide exchange factors; GIPC, GIPC PDZ domain containing family member 1; GRB2, growth factor receptor-bound protein 2; GRKs, GPCR kinases; MAGI, membrane-associated guanylate kinase inverted; MAGUK, membrane-associated guanylate kinase; mGluRs, metabotropic glutamate receptors; MINT1, Munc-18-interacting protein 1; MPP3, p55 subfamily member 3; MT1, melatonin type 1 receptor; MUPP1, multi-PDZ-domain protein; Nck, non-catalytic region of tyrosine kinase adaptor protein; NHERF, Na + /H + exchanger regulatory factor 1; NSF, N-ethylmaleimide-sensitive factor; P2Y1, P2Y1 purinergic receptors; PAK, p21-activated kinase...

Stress and anxiety disorders are risk factors for depression and these behaviours are modulated by corticotropin releasing factor (CRFR1) and serotonin (5-HT2R) receptors. However, the potential behavioral and cellular interaction between these two receptors is unclear. Here, we showed that pre-administration of CRF into the prefrontal cortex of mice sensitized 5-HT2R-mediated anxiety behaviours in response to 2,5-dimethoxy-4-iodoamphetamine. In both heterologous cell cultures and mouse cortical neurons, the activation of CRFR1 also sensitized 5-HT2 receptor-mediated inositol phosphate formation. CRFR1-mediated increases in 5-HT2R signaling were dependent upon receptor internalization and receptor recycling via rapid recycling endosomes resulting in increased cell surface 5-HT2R expression. The sensitization of 5-HT2R signaling by CRFR1 required intact PDZ domain binding motifs at the end of the C-terminal tails of both receptor types. These data reveal a novel mechanism by which CRF, a peptide known to be released by stress, sensitized anxiety-related behaviour via sensitization of 5-HT2R signaling.

Invasion of the nervous system and neuronal spread of infection are critical, but poorly understood, steps in the pathogenesis of transmissible spongiform encephalopathies or prion diseases. To characterize pathways for the uptake and intraneuronal trafficking of infectious, protease-resistant prion protein (PrP-res), fluorescent-labeled PrP-res was used to infect a neuronally derived murine cell line (SN56) and adult hamster cortical neurons in primary culture. Concurrent with the establishment of persistent scrapie infection, SN56 cells internalized PrP-res aggregates into vesicles positive for markers for late endosomes and/or lysosomes but not synaptic, early endocytic, or raft-derived vesicles. Internalized PrP-res was then transported along neurites to points of contact with other cells. Similar trafficking was observed with dextran, Alzheimer's A␤1-42 fibrils and noninfectious recombinant PrP fibrils, suggesting that PrP-res is internalized by a relatively nonspecific pinocytosis or transcytosis mechanism. Hamster cortical neurons were also capable of internalizing and disseminating exogenous PrP-res. Similar trafficking of exogenous PrP-res by cortical neurons cultured from the brains of PrP knock-out mice showed that uptake and neuritic transport did not require the presence of endogenous cellular PrP. These experiments visualize and characterize the initial steps associated with prion infection and transport within neuronal cells.

Acetylcholine (ACh) is an important neuromodulator in the nervous system implicated in many forms of cognitive and motor processing. Recent studies have used bacterial artificial chromosome (BAC) transgenic mice expressing channelrhodopsin-2 (ChR2) protein under the control of the choline acetyltransferase (ChAT) promoter (ChAT-ChR2-EYFP) to dissect cholinergic circuit connectivity and function using optogenetic approaches. We report that a mouse line used for this purpose also carries several copies of the vesicular acetylcholine transporter gene (VAChT), which leads to overexpression of functional VAChT and consequently increased cholinergic tone. We demonstrate that these mice have marked improvement in motor endurance. However, they also present severe cognitive deficits, including attention deficits and dysfunction in working memory and spatial memory. These results suggest that increased VAChT expression may disrupt critical steps in information processing. Our studies demonstrate that ChAT-ChR2-EYFP mice show altered cholinergic tone that fundamentally differentiates them from wild-type mice.

The prion protein (PrP(C)) is highly expressed in the nervous system, and its abnormal conformer is associated with prion diseases. PrP(C) is anchored to cell membranes by glycosylphosphatidylinositol, and transmembrane proteins are likely required for PrP(C)-mediated intracellular signaling. Binding of laminin (Ln) to PrP(C) modulates neuronal plasticity and memory. We addressed signaling pathways triggered by PrP(C)-Ln interaction in order to identify transmembrane proteins involved in the transduction of PrP(C)-Ln signals. The Ln γ1-chain peptide, which contains the Ln binding site for PrP(C), induced neuritogenesis through activation of phospholipase C (PLC), Ca(2+) mobilization from intracellular stores, and protein kinase C and extracellular signal-regulated kinase (ERK1/2) activation in primary cultures of neurons from wild-type, but not PrP(C)-null mice. Phage display, coimmunoprecipitation, and colocalization experiments showed that group I metabotropic glutamate receptors (mGluR1/5) associate with PrP(C). Expression of either mGluR1 or mGluR5 in HEK293 cells reconstituted the signaling pathways mediated by PrP(C)-Ln γ1 peptide interaction. Specific inhibitors of these receptors impaired PrP(C)-Ln γ1 peptide-induced signaling and neuritogenesis. These data show that group I mGluRs are involved in the transduction of cellular signals triggered by PrP(C)-Ln, and they support the notion that PrP(C) participates in the assembly of multiprotein complexes with physiological functions on neurons.