Inactivation of Galpha(z) causes disassembly of the Golgi apparatus

Gpr176 is a Gz-linked orphan G-protein-coupled receptor that sets the pace of circadian behaviour

G-protein-coupled receptors (GPCRs) participate in a broad range of physiological functions. A priority for fundamental and clinical research, therefore, is to decipher the function of over 140 remaining orphan GPCRs. The suprachiasmatic nucleus (SCN), the brain's circadian pacemaker, governs daily rhythms in behaviour and physiology. Here we launch the SCN orphan GPCR project to (i) search for murine orphan GPCRs with enriched expression in the SCN, (ii) generate mutant animals deficient in candidate GPCRs, and (iii) analyse the impact on circadian rhythms. We thereby identify Gpr176 as an SCN-enriched orphan GPCR that sets the pace of circadian behaviour. Gpr176 is expressed in a circadian manner by SCN neurons, and molecular characterization reveals that it represses cAMP signalling in an agonist-independent manner. Gpr176 acts independently of, and in parallel to, the Vipr2 GPCR, not through the canonical Gi, but via the unique G-protein subclass Gz.

The suprachiasmatic nucleus (SCN) is the central regulator of circadian rhythms. Here the authors identify mouse Gpr176 as a pace modulator of this circadian clock and characterize its mode of action as coupling to Gz rather than Gi subunits.

Regulators of G-protein-signaling proteins: negative modulators of G-protein-coupled receptor signaling

Regulators of G-protein-signaling (RGS) proteins are a category of intracellular proteins that have an inhibitory effect on the intracellular signaling produced by G-protein-coupled receptors (GPCRs). RGS along with RGS-like proteins switch on through direct contact G-alpha subunits providing a variety of intracellular functions through intracellular signaling. RGS proteins have a common RGS domain that binds to G alpha. RGS proteins accelerate GTPase and thus enhance guanosine triphosphate hydrolysis through the alpha subunit of heterotrimeric G proteins. As a result, they inactivate the G protein and quickly turn off GPCR signaling thus terminating the resulting downstream signals. Activity and subcellular localization of RGS proteins can be changed through covalent molecular changes to the enzyme, differential gene splicing, and processing of the protein. Other roles of RGS proteins have shown them to not be solely committed to being inhibitors but behave more as modulators and integrators of signaling. RGS proteins modulate the duration and kinetics of slow calcium oscillations and rapid phototransduction and ion signaling events. In other cases, RGS proteins integrate G proteins with signaling pathways linked to such diverse cellular responses as cell growth and differentiation, cell motility, and intracellular trafficking. Human and animal studies have revealed that RGS proteins play a vital role in physiology and can be ideal targets for diseases such as those related to addiction where receptor signaling seems continuously switched on.

Gas1 is a receptor for sonic hedgehog to repel enteric axons

The myenteric plexus of the enteric nervous system controls the movement of smooth muscles in the gastrointestinal system. They extend their axons between two peripheral smooth muscle layers to form a tubular meshwork arborizing the gut wall. How a tubular axonal meshwork becomes established without invading centrally toward the gut epithelium has not been addressed. We provide evidence here that sonic hedgehog (Shh) secreted from the gut epithelium prevents central projections of enteric axons, thereby forcing their peripheral tubular distribution. Exclusion of enteric central projections by Shh requires its binding partner growth arrest specific gene 1 (Gas1) and its signaling component smoothened (Smo) in enteric neurons. Using enteric neurons differentiated from neurospheres in vitro, we show that enteric axon growth is not inhibited by Shh. Rather, when Shh is presented as a point source, enteric axons turn away from it in a Gas1-dependent manner. Of the Gαi proteins that can couple with Smo, G protein α Z (Gnaz) is found in enteric axons. Knockdown and dominant negative inhibition of Gnaz dampen the axon-repulsive response to Shh, and Gnaz mutant intestines contain centrally projected enteric axons. Together, our data uncover a previously unsuspected mechanism underlying development of centrifugal tubular organization and identify a previously unidentified effector of Shh in axon guidance.

The biogenesis of the Golgi ribbon: the roles of membrane input from the ER and of GM130

The Golgi complex in mammalian cells forms a continuous ribbon of interconnected stacks of flat cisternae. We show here that this distinctive architecture reflects and requires the continuous input of membranes from the endoplasmic reticulum (ER), in the form of pleiomorphic ER-to-Golgi carriers (EGCs). An important step in the biogenesis of the Golgi ribbon is the complete incorporation of the EGCs into the stacks. This requires the Golgi-matrix protein GM130, which continuously cycles between the cis-Golgi compartments and the EGCs. On acquiring GM130, the EGCs undergo homotypic tethering and fusion, maturing into larger and more homogeneous membrane units that appear primed for incorporation into the Golgi stacks. In the absence of GM130, this process is impaired and the EGCs remain as distinct entities. This induces the accumulation of tubulovesicular membranes, the shortening of the cisternae, and the breakdown of the Golgi ribbon. Under these conditions, however, secretory cargo can still be delivered to the Golgi complex, although this occurs less efficiently, and apparently through transient and/or limited continuities between the EGCs and the Golgi cisternae.

RGSZ1 interacts with protein kinase C interacting protein PKCI-1 and modulates mu opioid receptor signaling

Protein kinase C interacting protein (PKCI-1) was identified among the potential interactors from a yeast two hybrid screen of human brain library using N terminal of RGSZ1 as a bait. The cysteine string region, unique to the RZ subfamily, contributes to the observed interaction because PKCI-1 interacted with N-terminus of RGS17 and GAIP, but not with that of RGS2 or RGS7 where cysteine string motif is absent. The interaction between RGSZ1 and PKCI-1 was confirmed by coimmunoprecipitation and immunofluorescence. PKCI-1 and RGSZ1 could be detected by coimmunoprecipitation using 14-3-3 antibody in cells transfected with PKCI-1 or RGSZ1 respectively, but when transfected with PKCI-1 and RGSZ1 together, only RGSZ1 could be detected. Phosphorylation of Galphaz by protein kinase C (PKC) reduces the ability of the RGS to effectively function as GTPase accelerating protein for Galphaz, and interferes with ability of Galphaz to interact with betagamma complex. We investigated the roles of 14-3-3 and PKCI-1 in phosphorylation of Galphaz. Phosphorylation of Galphaz by PKC was inhibited by 14-3-3 and the presence of PKCI-1 did not provide any further inhibition. PKCI-1 interacts with mu opioid receptor and suppresses receptor desensitization and PKC related mu opioid receptor phosphorylation [W. Guang, H. Wang, T. Su, I.B. Weinstein, J.B. Wang, Mol. Pharmacol. 66 (2004) 1285.]. Previous studies have also shown that mu opioid receptor co-precipitates with RGSZ1 and influence mu receptor signaling by acting as effector antagonists [J. Garzon, M. Rodriguez-Munoz, P. Sanchez-Blazquez, Neuropharmacology 48 (2005) 853., J. Garzon, M. Rodriguez-Munoz, A. Lopez-Fando, P. Sanchez-Blazquez Neuropsychopharmacology 30 (2005) 1632.]. Inhibition of cAMP by mu opioid receptor was significantly reduced by RGSZ1 and this effect was enhanced in combination with PKCI-1. Our studies thus provide a link between the previous observations mentioned above and indicate that the major function of PKCI-1 is to modulate mu opioid receptor signaling pathway along with RGSZ1, rather than directly mediating the Galphaz RGSZ1 interaction.

The physiology of membrane transport and endomembrane-based signalling

Some of the important open questions concerning the physiology of the secretory pathway relate to its homeostasis. Secretion involves a number of separate compartments for which their transport activities should be precisely cross-coordinated to avoid gross imbalances in the trafficking system. Moreover, the membrane fluxes across these compartments should be able to adapt to environmental 'requests' and to respond to extracellular signals. How is this regulation effected? Here, we consider evidence that endomembrane-based signalling cascades that are similar in organization to those used at the plasma membrane coordinate membrane traffic. If this is the case, this would also represent a model for a more general inter-organelle signalling network for functionally interconnecting different intracellular activities, a necessity for the maintenance of cellular homeostasis and to express harmonic global cellular responses.

Large pleiomorphic traffic intermediates in the secretory pathway

There are two main classes of traffic intermediates that operate in intracellular trafficking pathways: small round vesicles, and large pleiomorphic carriers (LPCs). While both are essential, the LPCs appear to be responsible for moving the bulk of the secretory traffic between distant compartments. LPCs are much larger and more variable in shape than vesicles, and they have evident interconnected tubular and saccular/cisternal components. They appear to form by en bloc extrusion and cleavage of large membrane areas of the donor organelle. Although many proteins and lipids that are involved in LPC formation have been identified, the intrinsic complexity of these carriers and current technical limitations mean that a coherent picture of the process of of LPC formation is only just beginning to emerge.

p125 is localized in endoplasmic reticulum exit sites and involved in their organization

Transport vesicles coated with the COPII complex, which is assembled from Sar1p, Sec23p-Sec24p, and Sec13p-Sec31p, are involved in protein export from the endoplasmic reticulum (ER). We previously identified and characterized a novel Sec23p-interacting protein, p125, that is only expressed in mammals and exhibits sequence homology with phosphatidic acid-preferring phospholipase A(1) (PA-PLA(1)). In this study, we examined the localization and function of p125 in detail. By using immunofluorescence and electron microscopy, we found that p125 is principally localized in ER exit sites where COPII-coated vesicles are produced. Analyses of chimeric proteins comprising p125 and two other members of the mammalian PA-PLA(1) family (PA-PLA(1) and KIAA0725p) showed that, for localization to ER exit sites, the p125-specific N-terminal region is critical, and the putative lipase domain is interchangeable with KIAA0725p but not with PA-PLA(1). RNA interference-mediated depletion of p125 affected the organization of ER exit sites. The structure of the cis-Golgi compartment was also substantially disturbed, whereas the medial-Golgi was not. Protein export from the ER occurred without a significant delay in p125-depleted cells. Our study suggests that p125 is a mammalian-specific component of ER exit sites and participates in the organization of this compartment.

Galphaz inhibits serum response factor-dependent transcription by inhibiting Rho signaling

Galpha12/13 or Galphaq signals induce activation of Rho GTPase, leading to serum response factor (SRF)-mediated gene transcription and actin cytoskeletal organization; however, less is known regarding how Rho pathway signals are down-regulated. Here we report that Galphaz signals inhibit serum response factor (SRF)-dependent transcription. Galphaz expression inhibits Galpha12/13-, Galphaq-, and Rho guanine nucleotide exchange factor (GEF)-induced serum response element (SRE) reporter activation in human embryonic kidney 293T and PC-12 cells. Expression of Galphaz mutants with defective fatty acylation has no inhibitory effect. Expression of Galphaz, but not Galphai, attenuates serum-induced SRE reporter activation, suggesting that Galphaz can down-regulate endogenous signals leading to SRF. Whereas Galphaz also blocks SRE reporter induction by the activated mutant RhoAL63, it does not affect Galpha12- or Rho GEF-induced RhoA activation or RhoAL63-GTP binding in vivo. Moreover, Galphaz does not inhibit SRE reporter induction by an activated form of Rho kinase. Because Galphaz inhibits RhoAL63/A188-induced reporter activation, phosphorylation of RhoA on serine 188 does not seem to be involved; furthermore, RhoA subcellular localization was not affected. Use of pharmacologic inhibitors implies that Galphaz-induced reduction of SRE reporter activation occurs via a mechanism other than adenylate cyclase modulation. These findings suggest that Galphaz signals may attenuate Rho-induced stimulation of SRF-mediated transcription.

Finding fusion genes resulting from chromosome rearrangement by analyzing the expressed sequence databases

Chromosomal rearrangements resulting in gene fusions are frequently involved in carcinogenesis. Here, we describe a semiautomatic procedure for identifying fusion gene transcripts by using publicly available mRNA and EST databases. With this procedure, we have identified 96 transcript sequences that are derived from 60 known fusion genes. Also, 47 or more additional sequences appear to be derived from 20 or more previously unknown putative fusion genes. We have experimentally verified the presence of a previously unknown IRA1/RGS17 fusion in the breast cancer cell line MCF7. The fusion gene encodes the full-length RGS17 protein, a regulator of G protein-coupled signaling, under the control of the IRA1 gene promoter. This study demonstrates that databases of ESTs can be used to discover fusion genes resulting from structural rearrangement of chromosomes.

Toll-Like Receptor Signaling Alters the Expression of Regulator of G Protein Signaling Proteins in Dendritic Cells: Implications for G Protein-Coupled Receptor Signaling

Conserved structural motifs on pathogens trigger pattern recognition receptors present on APCs such as dendritic cells (DCs). An important class of such receptors is the Toll-like receptors (TLRs). TLR signaling triggers a cascade of events in DCs that includes modified chemokine and cytokine production, altered chemokine receptor expression, and changes in signaling through G protein-coupled receptors (GPCRs). One mechanism by which TLR signaling could modify GPCR signaling is by altering the expression of regulator of G protein signaling (RGS) proteins. In this study, we show that human monocyte-derived DCs constitutively express significant amounts of RGS2, RGS10, RGS14, RGS18, and RGS19, and much lower levels of RGS3 and RGS13. Engagement of TLR3 or TLR4 on monocyte-derived DCs induces RGS16 and RGS20, markedly increases RGS1 expression, and potently down-regulates RGS18 and RGS14 without modifying other RGS proteins. A similar pattern of Rgs protein expression occurred in immature bone marrow-derived mouse DCs stimulated to mature via TLR4 signaling. The changes in RGS18 and RGS1 expression are likely important for DC function, because both proteins inhibit G alpha(i)- and G alpha(q)-mediated signaling and can reduce CXC chemokine ligand (CXCL)12-, CC chemokine ligand (CCL)19-, or CCL21-induced cell migration. Providing additional evidence, bone marrow-derived DCs from Rgs1(-/-) mice have a heightened migratory response to both CXCL12 and CCL19 when compared with similar DCs prepared from wild-type mice. These results indicate that the level and functional status of RGS proteins in DCs significantly impact their response to GPCR ligands such as chemokines.

Fluoride causes reversible dispersal of Golgi cisternae and matrix in neuroendocrine cells

A role for heterotrimeric G proteins in the regulation of Golgi function and formation of secretory granules is generally accepted. We set out to study the effect of activation of heterotrimeric G proteins by aluminum fluoride on secretory granule formation in AtT-20 corticotropic tumor cells and in melanotrophs from the rat pituitary. In AtT-20 cells, treatment with aluminum fluoride or fluoride alone for 60 min induced complete dispersal of Golgi, ER-Golgi intermediate compartment and Golgi matrix markers, while betaCOP immunoreactiviy retained a juxtanuclear position and TGN38 was unaffected. Electron microscopy showed compression of Golgi cisternae followed by conversion of the Golgi stacks into clusters of tubular and vesicular elements. In the melanotroph of the rat pituitary a similar compression of Golgi cisternae was observed, followed by a progressive loss of cisternae from the stacks. As shown in other cells, brefeldin A induced redistribution of the Golgi matrix protein GM130 to punctate structures in the cytoplasm in AtT-20 cells, while mannosidase II immunoreactivity was completely dispersed. Fluoride induced a complete dispersal of mannosidase II and GM130 immunoreactivity. The effect of fluoride was fully reversible with reestablishment of normal mannosidase II and GM130 immunoreactivity within 2 h. After 1 h of recovery, showing varying stages of reassembly, the patterns of mannosidase II and GM130 immunoreactivity were identical in individual cells, indicating that Golgi matrix and cisternae reassemble with similar kinetics during recovery from fluoride treatment. Instead of a specific aluminum fluoride effect on secretory granule formation in the trans-Golgi network, we thus observe a unique form of Golgi dispersal induced by fluoride alone, possibly via its action as a phosphatase inhibitor.

Endoplasmic reticulum retention, degradation, and aggregation of olfactory G-protein coupled receptors

The mammalian olfactory G-protein coupled receptor family is comprised of hundreds of proteins that mediate odorant binding and initiate signal transduction cascades leading to the sensation of smell. However, efforts to functionally express olfactory receptors and identify specific odorant ligand-olfactory receptor interactions have been severely impeded by poor olfactory receptor surface expression in heterologous systems. Therefore, experiments were performed to elucidate the cellular mechanism(s) responsible for inefficient olfactory receptor cell surface expression. We determined that the mouse odorant receptors mI7 and mOREG are not selected for export from the ER and therefore are not detectable at the Golgi apparatus or plasma membrane. Specifically, olfactory receptors interact with the ER chaperone calnexin, are excluded from ER export sites, do not accumulate in ER-Golgi transport intermediates at 15 degrees C, and contain endoglycosidase H-sensitive oligosaccharides, consistent with olfactory receptor exclusion from post-ER compartments. A labile pool of ER-retained olfactory receptors are post-translationally modified by polyubiquitination and targeted for degradation by the proteasome. In addition, olfactory receptors are sequestered into ER aggregates that are degraded by autophagy. Collectively, these data demonstrate that poor surface expression of olfactory receptors in heterologous cells is attributable to a combination of ER retention due to inefficient folding and poor coupling to ER export machinery, aggregation, and degradation via both proteasomal and autophagic pathways.