G protein-coupled receptors (GPCRs) mediate physiological responses to various ligands, such as hormones, neurotransmitters and sensory stimuli. The signalling and trafficking properties of GPCRs are often highly malleable depending on the cellular context. Such fine-tuning of GPCR function can be attributed in many cases to receptor-interacting proteins that are differentially expressed in distinct cell types. In some cases these GPCR-interacting partners directly mediate receptor signalling, whereas in other cases they act mainly as scaffolds to modulate G protein-mediated signalling. Furthermore, GPCR-interacting proteins can have a big impact on the regulation of GPCR trafficking, localization and/or pharmacological properties.

GPR56 is an adhesion G protein-coupled receptor that plays a key role in cortical development. Mutations to GPR56 in humans cause malformations of the cerebral cortex, but little is known about the normal function of the receptor. We found that the large N terminus (NT) of GPR56 is cleaved from the rest of the receptor during processing but remains non-covalently associated with the seven-transmembrane region of the receptor, as indicated by coimmunoprecipitation of the two GPR56 fragments from both transfected cells and native tissue. We also found that truncation of the GPR56 NT results in constitutive activation of receptor signaling, as revealed by increased GPR56-stimulated signaling upon transfection of HEK-293 cells with truncated GPR56, greatly enhanced binding of ␤-arrestins by truncated GPR56 relative to the full-length receptor, extensive ubiquitination of truncated GPR56, and cytotoxicity induced by truncated GPR56 that could be rescued by cotransfection of cells with ␤-arrestin 2. Furthermore, we found that the GPR56 NT is capable of homophilic trans-trans interactions that enhance receptor signaling activity. On the basis of these findings, we suggest a model of receptor activation in which the large N terminus of GPR56 constrains receptor activity but N-terminal interactions (GPR56 NT with an extracellular ligand and/or GPR56 NT homophilic trans-trans associations) can remove this inhibitory influence of the N terminus to activate receptor signaling.During the development of the cerebral cortex, neuronal precursors proliferate in the ventricular and subventricular zones that line the cerebral cavity and then migrate outward to make connections with other neurons. Given the billions of cells involved and the requirements for temporal and spatial precision, it is perhaps not surprising that many different types of problems can arise during this process. Abnormalities in cortical development can lead to a range of distinct neurodevelopmental disorders, some of which are caused by mutations to a single gene. For example, bilateral frontoparietal polymicrogyria is a condition in which patients exhibit profound cognitive abnormalities and seizures because of disordered cortical connectivity in the frontoparietal area. Bilateral frontoparietal polymicrogyria is an autosomal recessive syndrome that results from mutations in the orphan receptor GPR56 (1). Thus, insights into the natural function of GPR56 might shed light on the specific pathology underlying bilateral frontoparietal polymicrogyria and also lead to new insights about the fundamental mechanisms controlling cortical development.GPR56 is a member of the adhesion family of G proteincoupled receptors (GPCRs) 2 , which are characterized by extremely large extracellular N termini (NT) exhibiting homology to adhesion proteins (2). There are approximately 30 adhesion GPCRs, all of which are still considered to be orphan receptors. Almost all members of the adhesion GPCR family possess an N-terminal region known as a "GPCR proteolytic site" or GPS dom...

SUMMARYBackground & Aims-Lysophosphatidic acid (LPA) is a potent inducer of colon cancer and LPA receptor type 2 (LPA2) is overexpressed in colon tumors. LPA2 interacts with membraneassociated guanylate kinase with inverted orientation-3 (MAGI-3) and the Na+/H+ exchanger regulatory factor 2 (NHERF-2), but the biological effects of these interactions are unknown. We investigated the roles of MAGI-3 and NHERF-2 in LPA2-mediated signaling in human colon cancer cells.

The adenosine A2A receptor (A2AR) is a potential drug target for the treatment of Parkinson’s disease and other neurological disorders. In rodents, the therapeutic efficacy of A2AR modulation is improved by concomitant modulation of the metabotropic glutamate receptor 5 (mGluR5). To elucidate the anatomical substrate(s) through which these therapeutic benefits could be mediated, pre-embedding electron microscopy immunohistochemistry was used to conduct a detailed, quantitative ultrastructural analysis of A2AR localization in the primate basal ganglia and to assess the degree of A2AR/mGluR5 colocalization in the striatum. A2AR immunoreactivity was found at the highest levels in the striatum and external globus pallidus (GPe). However, the monkey, but not the rat, substantia nigra pars reticulata (SNr) also harbored a significant level of neuropil A2AR immunoreactivity. At the electron microscopic level, striatal A2AR labeling was most commonly localized in postsynaptic elements (58% ± 3% of labeled elements), whereas, in the GPe and SNr, the labeling was mainly presynaptic (71% ± 5%) or glial (27% ± 6%). In both striatal and pallidal structures, putative inhibitory and excitatory terminals displayed A2AR immunoreactivity. Striatal A2AR/mGluR5 colocalization was commonly found; 60–70% of A2AR-immunoreactive dendrites or spines in the monkey striatum coexpress mGluR5. These findings provide the first detailed account of the ultrastructural localization of A2AR in the primate basal ganglia and demonstrate that A2AR and mGluR5 are located to interact functionally in dendrites and spines of striatal neurons. Together, these data foster a deeper understanding of the substrates through which A2AR could regulate primate basal ganglia function and potentially mediate its therapeutic effects in parkinsonism.

The astrocytic glutamate transporter GLAST (also known as EAAT1) is a key regulator of extracellular glutamate levels in many regions of vertebrate brains. To identify novel interacting partners that might regulate the localization and function of GLAST in astrocytes, we screened the transporter's C-terminus (GLAST-CT) against a proteomic array of 96 different PDZ domains. The GLAST-CT robustly and specifically interacted with PDZ domains from two related scaffolding proteins, the Na + /H + exchanger regulatory factors 1 and 2 (NHERF-1 and NHERF-2). Studies on cultured rat cortical astrocytes revealed that these cells are highly enriched in NHERF-2 relative to NHERF-1. Endogenous GLAST and NHERF-2 from cultured astrocytes were found to robustly co-immunoprecipitate, and further co-immunoprecipitation studies on mutant versions of GLAST expressed in transfected cells revealed the GLAST/NHERF-2 interaction to be dependent on the last amino acid of the GLAST-CT. Knockdown of endogenous NHERF-2 in astrocytes via siRNA treatment resulted in a significant reduction in GLAST activity, which corresponded to significantly reduced total expression of GLAST protein and reduced halflife of GLAST, as assessed in pulse-chase metabolic labeling studies. These findings reveal that NHERF-2 can interact with GLAST in astrocytes to enhance GLAST stability and activity.Glutamate is the most abundant neurotransmitter in the mammalian central nervous system and the mediator of excitatory neurotransmission at the majority of synapses in the brain. The extracellular concentration of glutamate must be tightly regulated, however, as excessive glutamate signaling can lead to excitotoxic cellular death [4]. Five transporters have been identified as the principal regulators of extracellular glutamate levels and therefore have been named excitatory amino acid transporters EAAT-1 (rat homologue known as GLAST), EAAT-2 (rat homologue known as GLT-1), EAAT-3, EAAT-4, and EAAT-5. Genetic studies have shed light on the functions of the individual transporter subtypes, revealing a dominant role for GLAST and GLT-1 in controlling synaptic glutamate levels [16,21]. Interestingly, GLAST and GLT-1 are exclusively expressed in astrocytes [3,12], thereby speaking to the importance of astrocytes in clearing synaptic glutamate. Moreover, mutations in GLAST are linked with a variety of disease states, © 2010 Elsevier Ireland Ltd. All rights reserved.Address for correspondence: Randy A. Hall, Rollins Research Center, room 5113, 1510 Clifton Rd., Emory University School of Medicine, Atlanta, GA, USA,. rhall@pharm.emory.edu. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content...