Genetic control of the spontaneous activation of CD4+ Th cells in systemic lupus erythematosus-prone (NZB x NZW) F1 mice

The F(1) hybrid of autoimmune hemolytic anemia-prone NZB and nonautoimmune NZW strains of mice has been studied as a murine model of systemic lupus erythematosus. Both NZB and F(1) hybrid mice show age-dependent spontaneous activation of peripheral CD4(+) T cells as reflected by the elevated frequencies of CD4(+) T cells positive for CD69 early activation marker. Both strains also show age-dependent abnormal decrease of the frequencies of CD62L(+) naive CD4(+) T cells and/or NTA260(+) memory CD4(+) T cells in the spleen. We studied the multigenic control of these abnormal features of peripheral CD4(+) T cells in (NZB x NZW) F(1) x NZW backcross mice by quantitative trait loci mapping and by association rule analysis. The abnormally elevated frequencies of CD69(+)CD4(+) T cells and decreased frequencies of CD62L(+) naive and/or NTA260(+) memory CD4(+) T cells were under the common genetic control, in which the interaction between MHC and a hitherto unknown locus, designated Sta-1 (spontaneous T-cell activation) on chromosome 12, plays a major role. The allelic effects of these loci likely predispose CD4(+) T cells to the loss of self-tolerance, and are responsible for the accelerated autoimmune phenotypes of (NZB x NZW) F(1) hybrid mice.

Synthesis and biological evaluation of boronic acid containing cis-stilbenes as apoptotic tubulin polymerization inhibitors

A series of boronic acid containing cis-stilbenes as potent inhibitors of tubulin polymerization was synthesized by the introduction of boronic acid as an acceptor-type functional group into the aromatic ring B of the combretastatin framework. High cell-growth inhibition was observed with boron compounds 13 c and 13 d, in which a hydroxy group on the aromatic ring B of combretastatin A-4 was replaced with boronic acid; IC50 values toward B-16 and 1-87 cell lines are 0.48-2.1 microM. Compounds 13 c and 13 d exhibited significant inhibitory activity toward tubulin polymerization (IC50=21-22 microM). The carboxylic acid derivative 17, which can be considered as a mimic of boronic acid 13 c, did not show significant inhibition of cell growth or tubulin polymerization. According to the FACScan analysis using Jurkat cells, apoptosis was induced after incubation for 8 h with 13 c at a concentration of >10(-8) M. Growth inhibitory experiments against a panel of 39 human cancer cell lines revealed 13 c to inhibit growth differently than combretastatin A-4; the correlation coefficient (r) between the two compounds was 0.553 in the COMPARE analysis.

Clathrin required for phosphorylation and internalization of beta2-adrenergic receptor by G protein-coupled receptor kinase 2 (GRK2)

Clathrin is a major component of clathrin-coated pits and serves as a binding scaffold for endocytic machinery through the binding of a specific sequence known as the clathrin-binding motif. This motif is also found in cellular signaling proteins other than endocytic components, including G protein-coupled receptor kinase 2 (GRK2), which phosphorylates G protein-coupled receptors and promotes uncoupling of receptor-G protein interaction. However, the functions of clathrin in the regulation of GRK2 are unknown. Here we demonstrated that overexpression of GRK2 mutated at the clathrin-binding motif with alanine (GRK2-5A) results in inhibition of phosphorylation and internalization of the beta2-adrenergic receptor (beta2AR). However, the interaction of beta2AR with GRK2-5A is the same as that of wild type GRK2 as determined by bioluminescence resonance energy transfer. Furthermore, GRK2-5A phosphorylates rhodopsin essentially to the same extent as wild type GRK2 in vitro. Depletion of the clathrin heavy chain using small interference RNA inhibits agonist-induced phosphorylation and internalization of beta2AR. Thus, clathrin works as a regulator of GRK2 in cells. These results indicate that clathrin is a novel player in cellular functions in addition to being a component of endocytosis.

5-Oxo-Eicosatetraenoic Acid-Induced Chemotaxis: Identification of a Responsible Receptor hGPCR48 and Negative Regulation by G Protein G12/13

While screening genes encoding G protein-coupled receptors (GPCRs) in the human genome, we and other groups have identified a GPCR named hGPCR48 as a high affinity receptor for 5-oxo-6,8,11,14-eicosatetraenoic acid (5-oxo-ETE), which is arachidonic acid metabolite and an endogenous chemoattractant for granulocytes. Using Chinese hamster ovary (CHO) cells stably expressing hGPCR48, we show here that activation of the receptor causes the chemotaxis of the cells toward 5-oxo-ETE. We also show that the chemotaxis of human granulocytes toward 5-oxo-ETE is inhibited by pretreatment with anti-hGPCR48 antibodies, indicating that hGPCR48 is an endogenous receptor responsible for chemotaxis of granulocytes toward 5-oxo-ETE. In addition, we show that the chemotaxis of CHO cells expressing hGPCR48 is suppressed by pretreatment with pertussis toxin, and enhanced by overexpression of the carboxy terminal peptides of Galpha (12/13) subunits or a regulator of the G protein signaling domain of p115RhoGEF, both of which are known to suppress G(12/13)-dependent signaling pathways. These results indicate that hGPCR48 couples with G(i/o) and G(12/13) proteins, which then initiate or attenuate the chemotaxis of the cells toward 5-oxo-ETE, respectively.

The structure of the third intracellular loop of the muscarinic acetylcholine receptor M2 subtype

We have examined whether the long third intracellular loop (i3) of the muscarinic acetylcholine receptor M2 subtype has a rigid structure. Circular dichroism (CD) and nuclear magnetic resonance spectra of M2i3 expressed in and purified from Escherichia coli indicated that M2i3 consists mostly of random coil. In addition, the differential CD spectrum between the M2 and M2deltai3 receptors, the latter of which lacks most of i3 except N- and C-terminal ends, gave no indication of secondary structure. These results suggest that the central part of i3 of the M2 receptor has a flexible structure.

klavier (klv), a novel hypernodulation mutant of Lotus japonicus affected in vascular tissue organization and floral induction

A novel hypernodulation mutant line was isolated from Lotus japonicus Miyakojima MG-20 by irradiation with a helium ion beam. This mutant, named klavier (klv), had roots that were densely covered with small nodules. The nodulation zone of klv was significantly wider than that of the wild type. Grafting experiments showed that klv is impaired in the long-distance shoot-to-root autoregulatory mechanism. Thus the shoot genotype was found to be responsible for the negative regulation of nodule development by KLV. Nodulation of klv showed a higher tolerance to nitrogen (KNO3) than the wild type, which is a common feature of hypernodulating mutants. In addition to an increased number of nodules, the klv mutant showed convex leaf veins on the adaxial leaf surface, markedly delayed flowering and dwarf phenotypes. Microscopic examination of the leaf veins revealed that they were discontinuous. Other phenotypes such as fasciated stems, increased number of flowers and bifurcated pistils were also frequently observed in the klv mutant. Among these phenotypes, hypernodulation, aberrant leaf vein formation and significantly delayed flowering were all linked in a monogenic and recessive manner, indicating that these phenotypes are caused by either a single mutation, or tightly linked mutations. KLV was mapped within 0.29 cM on the long arm of chromosome 1.

Low pH facilitates uptake of proteins by cells through a non-endocytic pathway

We previously noted that bovine apolipoprotein A-II (apoA-II) had a bactericidal effect causing morphological changes in the cytoplasm. To determine whether and how apoA-II and apoA-I, which have acidic isoelectric points (pIs), enter cells, we determined the rates of uptake of FITC-labeled proteins by fibroblast cells and found that they entered cells more easily at low pH than at neutral pH under conditions where endocytosis was inhibited. The enhanced uptake of proteins at low pH was also observed for other proteins examined regardless of the molecular weight (M(r)) or pI in a time-dependent manner, although the efficiency of uptake varied among the proteins. Furthermore, a pH gradient was shown to be the main driving force for the translocation. As cells were viable above pH 4 for 2 h at 4 degrees C and internalized beta-galactosidase was active under these conditions, we suggest that this procedure is applicable to the injection of proteins into cells without the use of an apparatus such as a microinjector.

Pivotal function for cytoplasmic protein FROUNT in CCR2-mediated monocyte chemotaxis

Ligation of the chemokine receptor CCR2 on monocytes and macrophages with its ligand CCL2 results in activation of the cascade consisting of phosphatidylinositol-3-OH kinase (PI(3)K), the small G protein Rac and lamellipodium protrusion. We show here that a unique clathrin heavy-chain repeat homology protein, FROUNT, directly bound activated CCR2 and formed clusters at the cell front during chemotaxis. Overexpression of FROUNT amplified the chemokine-elicited PI(3)K-Rac-lamellipodium protrusion cascade and subsequent chemotaxis. Blocking FROUNT function by using a truncated mutant or antisense strategy substantially diminished signaling via CCR2. In a mouse peritonitis model, suppression of endogenous FROUNT markedly prevented macrophage infiltration. Thus, FROUNT links activated CCR2 to the PI(3)K-Rac-lamellipodium protrusion cascade and could be a therapeutic target in chronic inflammatory immune diseases associated with macrophage infiltration.

Effects of partial agonists and Mg2+ ions on the interaction of M2 muscarinic acetylcholine receptor and G protein Galpha i1 subunit in the M2-Galpha i1 fusion protein

We have expressed a M(2)-Galpha(i1) fusion protein in insect cells, in which the G protein alpha(i1) subunit was fused with a mutant of the muscarinic receptor M(2) subtype without glycosylation sites and the central part of the third intracellular loop. The M(2)-Galpha(i1) fusion protein showed GTP-sensitive, high-affinity agonist binding. Displacement curves by GDP of [(35)S]GTPgammaS binding shifted to the right in the presence of muscarinic agonists. The extent of the shift was greater for full agonists (120-150 fold) than for partial agonists (25-35 fold), and virtually no shift was observed for antagonists. The affinity for GDP decreased with increasing MgCl(2) concentration in the presence of an agonist but was not affected by MgCl(2) in the presence of an antagonist. These results indicate that the apparent affinity for GDP of the M(2)-Galpha(i1) fusion protein bound to a ligand represents the efficacy of the given ligand, and that Mg(2+) is required for the agonist-bound M(2) to interact with Galpha(i1), reducing its affinity for GDP. We propose that the agonist-M(2)-Galpha(i1) complex represents the transition state for the GDP-GTP exchange reaction catalyzed by agonist-bound receptors, and that the complex has different affinities for GDP depending on the species of the ligand bound to M(2) receptors.

Stimulation of Increases in Intracellular Calcium and Prostaglandin E2 Generation in Chinese Hamster Ovary Cells Expressing Receptor-G 16 Fusion Proteins

We examined whether fusion proteins of G protein-coupled receptors with the alpha subunit of G(16) (Galpha(16)) could activate downstream signals. We expressed fusion proteins of G(i)-coupled receptors, i.e. CX(3)C chemokine receptor 1 (CX(3)CR1) and M(2) receptor, in Chinese hamster ovary cells. An agonist for CX(3)CR1 induced greater increases in intracellular Ca(2+) and prostaglandin E(2) generation in cells expressing CX(3)CR1-Galpha(16) fusion protein than in cells expressing CX(3)CR1 alone or both CX(3)CR1 and Galpha(16) separately. Similarly, agonist-induced prostaglandin E(2) generation was greater in cells expressing M(2)-Galpha(16) fusion protein than ones expressing M(2) alone or both M(2) and Galpha(16) separately. In cells expressing fusion proteins with Galpha(16) of G(q)-coupled receptors, i.e. urotensin II receptor and M(1) receptor, the relevant agonists induced similar increases in intracellular Ca(2+) and prostaglandin E(2) generation as in ones expressing the receptor alone. In cells expressing urotensin II receptor-Galpha(16) fusion protein, prostaglandin E(2) generation exhibited a lower EC(50) value than the intracellular Ca(2+) increase. These results indicate that agonist-stimulated receptor-Galpha(16) fusion proteins are coupled to downstream signaling pathways, and suggest that receptor-Galpha(16) fusion proteins may be useful for screening for ligands of orphan G protein-coupled receptors and G(i)-coupled receptors.

Inhibitory Role of Endophilin 3 in Receptor-mediated Endocytosis

Endophilin 1 (Endo1) participates in synaptic vesicle biogenesis through interactions of its Src homology 3 domain with the polyphosphoinositide phosphatase Synaptojanin and the GTPase Dynamin. Endo1 has also been reported to affect endocytosis by converting membrane curvature via its lysophosphatidic acid acyltransferase activity. Here we report that a closely related isoform of Endo1, Endo3, inhibits clathrin-mediated endocytosis. Mutational analyses showed that the variable region of Endo3 is important in regulating transferrin endocytosis. In the brain, Endo3 is co-localized with dopamine D2 receptor in olfactory nerve terminals and inhibits its clathrin-mediated endocytosis in COS-7 cells. Furthermore, overexpression of Endo3 in an olfactory epithelium-derived cell line suppressed dopamine D2 receptor-mediated endocytosis and therefore accelerated its dopamine-induced differentiation. These results indicate that Endo3 may act as a negative regulator of clathrin-mediated endocytosis in brain neurons.

The receptor-Galpha fusion protein as a tool for ligand screening: a model study using a nociceptin receptor-Galphai2 fusion protein

As a model system to screen endogenous ligands for G(i)-coupled receptors, we have prepared and characterized a fusion protein of nociceptin receptor and alpha subunit of G(i2). We detected nociceptin binding to the fusion protein by measuring stimulation of [(35)S]GTPgammaS binding with an EC(50) of 2.0 nM and a gain of approximately five times. The stimulation by nociceptin of [(35)S]GTPgammaS binding to the fusion protein was clearly observed in the presence of an appropriate concentration of GDP, because the affinity for GDP was decreased in the presence of agonist. Full and partial agonists differed in their effects on apparent the affinity of the fusion protein for GDP: the IC(50) values for GDP to displace 100 pM [(35)S]GTPgammaS were estimated to be 2 micro M, 0.4 micro M, and 0.05 micro M in the presence of full agonist (nociceptin), partial agonist (F/G-NC), and antagonist (NBZH), respectively. We also detected the activity to stimulate [(35)S]GTPgammaS binding to the fusion protein in the brain extract derived from 2-3 g wet weight tissue without false-positive results. The active component was identified as endogenous nociceptin itself. These results indicate that the fusion protein of GPCR and Galpha(i) is useful for screening of endogenous ligands.

Production of N-Lauroylated G Protein -Subunit in Sf9 Insect Cells: The Type of N-Acyl Group of G Influences G Protein-Mediated Signal Transduction

The alpha-subunit of rod photoreceptor G protein transducin (G(t1)alpha) is heterogeneously modified at the N-terminus by a mixture of acyl groups, laurate (C12:0), myristate (C14:0), and two unsaturated fatty acids (C14:1 and C14:2). Although the N-fatty acylation of G(t1)alpha plays important roles in protein-protein and protein-membrane interactions in light signaling, the biological significance of the heterogeneous acylation remains unclear due to the difficulty in isolating each G(t1)alpha isoform from the retinal rod cells. Here we found that G(t1)alpha/G(i1)alpha chimera (G(t/i)alpha) expressed in Sf9 cells is also heterogeneously modified by myristate (approximately 90%) and laurate (approximately 10%), raising the possibility that the N-acyl group of recombinant G(t/i)alpha may be manipulated by modifying culture media. In fact, addition of myristic acid to the medium decreased the relative content of lauroylated G(t/i)alpha to an undetectable level, whereas exogenously added lauric acid significantly increased the relative content of lauroylated G(t/i)alpha in a concentration-dependent manner. By culturing the G(t/i)alpha-virus infected Sf9 cells with fatty acids, we obtained four different preparations of N-acylated G(t/i)alpha, in which the relative abundance of lauroylated isoform was 0%, 20%, 33% and approximately 70%, respectively. Functional analysis of these proteins showed that an increase in the relative content of the lauroylated isoform remarkably slowed down the steady-state GTP hydrolysis rate of G(t/i)alpha; the steady-state GTPase activity of the lauroylated isoform was estimated to be one order of magnitude lower than that of the myristoylated isoform. These results suggest that the retinal G(t1)alpha is composed of isoforms having functionally heterogeneous signaling properties.

Identification of surrogate ligands for orphan G protein-coupled receptors

We prepared fusion proteins with an alpha subunit of G protein Gi (Gi1alpha) of 26 orphan G protein-coupled receptors (GPCRs) and with Gsalpha of 10 orphan GPCRs, most of which had been identified from the human genome previously [FEBS Lett 520 (2002) 97]. Ligands for these fusion proteins were screened from a library consisting of approximately 1000 authentic compounds by measuring their effect on [35S]GTPgammaS binding to membrane preparations of insect Sf9 cells expressing these fusion proteins. Eleven compounds were found to act as surrogate agonists for a GPCR-Gsalpha and four GPCR-Gialpha fusion proteins, a compound as an inverse agonist for two GPCR-Gsalpha fusion proteins, and a compound as an endogenous agonist for a GPCR-Gialpha fusion protein.

High-affinity choline transporter

The cholinergic neurons have long been a model for biochemical studies of neurotransmission. The components responsible for cholinergic neurotransmission, such as choline acetyltransferase, vesicular acetylcholine transporter, nicotinic and muscarinic acetylcholine receptors, and acetylcholine esterase, have long been defined as functional units and then identified as molecular entities. Another essential component in the cholinergic synapses is the one responsible for choline uptake from the synaptic cleft, which is thought to be the rate-limiting step in acetylcholine synthesis. A choline uptake system with a high affinity for choline has long been assumed to be present in cholinergic neurons. Very recently, the molecular entity for the high-affinity choline transporter was identified and is designated CHT1. CHT1 mediates Na(+)- and Cl(-)-dependent choline uptake with high sensitivity to hemicholinium-3. CHT1 has been characterized both at the molecular and functional levels and was confirmed to be specifically expressed in cholinergic neurons.

Detection of the high-affinity choline transporter in the MOLT-3 human leukemic T-cell line

We previously showed that lymphocytes possess the necessary components to constitute an independent, non-neuronal cholinergic system; these include acetylcholine (ACh) itself, choline acetyltransferase (the ACh-synthesizing enzyme), and both muscarinic and nicotinic ACh receptors (AChRs). In addition, we showed that stimulation of AChRs with their respective agonists elicits a variety of biochemical and functional effects, suggesting that lymphocytic cholinergic system is involved in the regulation of immune function. In nerve terminals, choline taken up via the high-affinity choline transporter (CHT1) is exclusively utilized for ACh synthesis. In the present study, therefore, we investigated the expression of CHT1 in T-lymphocytes. Reverse transcription-polymerase chain reaction analysis revealed that MOLT-3 cells, a human leukemic T-cell line used as a T-lymphocyte model, expressed CHT1 mRNA, but that the CEM and Jurkat T-cell lines did not. Consistent with that finding, specific binding of [3H]hemicholinium-3 (HC-3), an inhibitor of CHT1, and HC-3-sensitive [3H]choline uptake were also detected in MOLT-3 cells. These results suggest that CHT1 plays a role in mediating choline uptake in T-lymphocytes and provides further evidence for the presence of an independent lymphocytic cholinergic system.

Identification of sites of phosphorylation by G-protein-coupled receptor kinase 2 in beta-tubulin

G-protein-coupled receptor kinase 2 (GRK2) is known to specifically phosphorylate the agonist-bound forms of G-protein-coupled receptors (GPCRs). This strict specificity is due at least partly to activation of GRK2 by agonist-bound GPCRs, in which basic residues in intracellular regions adjacent to transmembrane segments are thought to be involved. Tubulin was found to be phosphorylated by GRK2, but it remains unknown if tubulin can also serve as both a substrate and an activator for GRK2. Purified tubulin, phosphorylated by GRK2, was subjected to biochemical analysis, and the phosphorylation sites in beta-tubulin were determined to be Thr409 and Ser420. In addition, the Ser444 in beta III-tubulin was also indicated to be phosphorylated by GRK2. The phosphorylation sites in tubulin for GRK2 reside in the C-terminal domain of beta-tubulin, which is on the outer surface of microtubules. Pretreatment of tubulin with protein phosphatase type-2A (PP2A) resulted in a twofold increase in the phosphorylation of tubulin by GRK2. These results suggest that tubulin is phosphorylated in situ probably by GRK2 and that the phosphorylation may affect the interaction of microtubules with microtubule-associated proteins. A GST fusion protein of a C-terminal region of beta I-tubulin (393-445 residues), containing 19 acidic residues but only one basic residue, was found to be a good substrate for GRK2, like full-length beta-tubulin. These results, together with the finding that GRK2 may phosphorylate synuclein and phosducin in their acidic domains, indicate that some proteins with very acidic regions but without basic activation domains could serve as substrates for GRK2.

Single nucleotide polymorphism of the human high affinity choline transporter alters transport rate

High affinity choline uptake plays a critical role in the regulation of acetylcholine synthesis in cholinergic neurons. Recently, we succeeded in molecular cloning of the high affinity choline transporter (CHT1), which is specifically expressed in cholinergic neurons. Here we demonstrate the presence of functionally relevant, nonsynonymous single nucleotide polymorphism in the human CHT1 gene by comprehensive sequence analysis of the exons and the intron/exon boundaries including the transcription start site. The deduced amino acid change for the polymorphism is isoleucine to valine at amino acid 89 (I89V) located within the third transmembrane domain of the protein. The allele frequency of I89V was 6% for Ashkenazi Jews. Functional assessment of the I89V transporter in mammalian cell lines revealed a 40-50% decrease in V(max) for choline uptake rate compared with the wild type, whereas there was no alteration in the apparent affinities for choline, sodium, chloride, and the specific inhibitor hemicholinum-3. There also was no change in the specific hemicholinum-3 binding activity. The decreased choline uptake was not associated with the surface expression level of the protein as assessed by biotinylation assay. These results suggest an impaired substrate translocation in the I89V transporter. The Caenorhabditis elegans ortholog of CHT1 has a valine residue at the corresponding position and a single replacement from valine to isoleucine caused a decrease in the choline uptake rate by 40%, suggesting that this hydrophobic residue is generally critical in the choline transport rate in CHT1. This polymorphism in the allelic CHT1 gene may represent a predisposing factor for cholinergic dysfunction.

Ascidian arrestin (Ci-arr), the origin of the visual and nonvisual arrestins of vertebrate

Arrestin is one of the key proteins for the termination of G protein signaling. Activated G protein-coupled receptors (GPCRs) are specifically phosphorylated by G protein-coupled receptor kinases (GRKs) and then bind to arrestins to preclude the receptor/G protein interaction, resulting in quenching of the following signal transduction. Vertebrates possess two types of arrestin; visual arrestin expressed exclusively in photoreceptor cells in retinae and pineal organs, and beta-arrestin, which is expressed ubiquitously. Unlike visual arrestin, beta-arrestin contains the clathrin-binding domain at the C-terminus, responsible for the agonist-induced internalization of GPCRs. Here, we isolated a novel arrestin gene (Ci-arr) from the primitive chordate, the ascidian Ciona intestinalis larvae. The deduced amino acid sequence suggests that Ci-Arr be closely related to vertebrate arrestins. Interestingly, this arrestin has the feature of both visual and beta-arrestin. Whereas the expression of Ci-arr was restricted to the photoreceptors in the larvae similarly to visual arrestin, the gene product, containing the clathrin-binding domain, promoted the GPCR internalization in HEK293tsA201 cells similarly to beta-arrestin. The phylogenetic tree shows that Ci-Arr is branched from a common root of visual and beta-arrestins. Southern analysis suggests that the Ciona genome contains only one gene for the arrestin family. These results suggest that the visual and beta-arrestin genes were generated by the duplication of the prototypical arrestin gene like Ci-arr in the early evolution of vertebrates.