Mitofusin-2 mediates cannabidiol-induced neuroprotection against cerebral ischemia in rats

Cannabidiol (CBD) reportedly exerts protective effects against many psychiatric disorders and neurodegenerative diseases, but the mechanisms are poorly understood. In this study, we explored the molecular mechanism of CBD against cerebral ischemia. HT-22 cells or primary cortical neurons were subjected to oxygen-glucose deprivation insult followed by reoxygenation (OGD/R). In both HT-22 cells and primary cortical neurons, CBD pretreatment (0.1, 0.3, 1 μM) dose-dependently attenuated OGD/R-induced cell death and mitochondrial dysfunction, ameliorated OGD/R-induced endoplasmic reticulum (ER) stress, and increased the mitofusin-2 (MFN2) protein level in HT-22 cells and primary cortical neurons. Knockdown of MFN2 abolished the protective effects of CBD. CBD pretreatment also suppressed OGD/R-induced binding of Parkin to MFN2 and subsequent ubiquitination of MFN2. Overexpression of Parkin blocked the effects of CBD in reducing MFN2 ubiquitination and reduced cell viability, whereas overexpressing MFN2 abolished Parkin's detrimental effects. In vivo experiments were conducted on male rats subjected to middle cerebral artery occlusion (MCAO) insult, and administration of CBD (2.5, 5 mg · kg-1, i.p.) dose-dependently reduced the infarct volume and ER stress in the brains. Moreover, the level of MFN2 within the ischemic penumbra of rats was increased by CBD treatment, while the binding of Parkin to MFN2 and the ubiquitination of MFN2 was decreased. Finally, short hairpin RNA against MFN2 reversed CBD's protective effects. Together, these results demonstrate that CBD protects brain neurons against cerebral ischemia by reducing MFN2 degradation via disrupting Parkin's binding to MFN2, indicating that MFN2 is a potential target for the treatment of cerebral ischemia.

Ferroptosis mediated by the interaction between Mfn2 and IREα promotes arsenic-induced nonalcoholic steatohepatitis

Exposure to arsenic is a risk factor for nonalcoholic steatohepatitis (NASH). Ferroptosis is a form of regulated cell death defined by the accumulation of lipid peroxidation. In the current study, we observed the occurrence of ferroptosis in arsenic-induced NASH by assessing ferroptosis related hallmarks. In vitro, we found that ferrostatin-1 effectively attenuated the executing of ferroptosis and NASH. Simultaneously, the expression of ACSL4 (acyl-CoA synthetase long-chain family member 4) was upregulated in rat's liver and L-02 cells exposed to arsenic. While, suppression of ACSL4 with rosiglitazone or ACSL4 siRNA remarkably alleviated arsenic-induced NASH and ferroptosis through diminishing 5-hydroxyeicosatetraenoic acid (5-HETE) content. Additionally, Mitofusin 2 (Mfn2), a physical tether between endoplasmic reticulum and mitochondria, has rarely been explored in the ferroptosis. Using Mfn2 siRNA or inositol-requiring enzyme 1 alpha (IRE1α) inhibitor, we found NASH and ferroptosis were obviously mitigated through reducing 5-HETE content. Importantly, Co-IP assay indicated that Mfn2 could interact with IRE1α and promoted the production of 5-HETE, ultimately led to ferroptosis and NASH. Collectively, our data showed that ferroptosis is involved in arsenic-induced NASH. These data provide insightful viewpoints into the mechanism of arsenic-induced NASH.

3-Nitropropionic acid induces autophagy by forming mitochondrial permeability transition pores rather than activating the mitochondrial fission pathway

BACKGROUND AND PURPOSE

Huntington's disease is a neurodegenerative process associated with mitochondrial alterations. Inhibitors of the electron-transport channel complex II, such as 3-nitropropionic acid (3NP), are used to study the molecular and cellular pathways involved in this disease. We studied the effect of 3NP on mitochondrial morphology and its involvement in macrophagy.

EXPERIMENTAL APPROACH

Pharmacological and biochemical methods were used to characterize the effects of 3NP on autophagy and mitochondrial morphology. SH-SY5Y cells were transfected with GFP-LC3, GFP-Drp1 or GFP-Bax to ascertain their role and intracellular localization after 3NP treatment using confocal microscopy.

KEY RESULTS

Untreated SH-SY5Y cells presented a long, tubular and filamentous net of mitochondria. After 3NP (5 mM) treatment, mitochondria became shorter and rounder. 3NP induced formation of mitochondrial permeability transition pores, both in cell cultures and in isolated liver mitochondria, and this process was inhibited by cyclosporin A. Participation of the mitochondrial fission pathway was excluded because 3NP did not induce translocation of the dynamin-related protein 1 (Drp1) to the mitochondria. The Drp1 inhibitor Mdivi-1 did not affect the observed changes in mitochondrial morphology. Finally, scavengers of reactive oxygen species failed to prevent mitochondrial alterations, while cyclosporin A, but not Mdivi-1, prevented the generation of ROS.

CONCLUSIONS AND IMPLICATIONS

There was a direct correlation between formation of mitochondrial permeability transition pores and autophagy induced by 3NP treatment. Activation of autophagy preceded the apoptotic process and was mediated, at least partly, by formation of reactive oxygen species and mitochondrial permeability transition pores.

Signal transduction through Ras-GTPase and Ca2+/ calmodulin-dependent protein kinase II contributes to development of diabetes-induced renal vascular dysfunction

This study examined the role of Ca2+/calmodulin-dependent protein kinase II (CaMKII) and Ras-GTPase in the development of abnormal reactivity to vasoactive agents in the renal artery of diabetic rats. The vasoconstrictor response induced by norepinephrine (NE), endothelin-1 (ET-1) or angiotensin II (Ang II) was significantly increased whereas vasodilator response to carbachol, histamine or sodium nitroprusside (SNP) was not altered in the renal artery segments of the streptozotocin (STZ)-diabetic rats. Chronic intraperitoneal administration of KN-93 (5 mg/kg/ alt diem), an inhibitor of CaMKII or FPTIII (1.5 mg/kg/ alt diem), an inhibitor of Ras-GTPase, produced significant normalization of the altered agonist-induced vasoconstrictor responses without affecting blood glucose levels. All the inhibitors were administered for four weeks starting from day one of diabetes induction. Inhibition of Ras-GTPase or CaMKII did not affect the agonist-induced vasoconstrictor and vasodilator responses in the non-diabetic control animals. These data suggest that inhibition of signal transduction involving CaMKII and Ras-GTPase can prevent development of diabetes-induced abnormal vascular reactivity in the renal artery.

Anti-cancer therapy: targeting the mevalonate pathway

The mevalonate pathway has become an important target for anti-cancer therapy. Manipulation of this pathway results in alteration of malignant cell growth and survival in cell culture and animal models, with promising potential for application in human cancers. Mevalonate is synthesized from 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA). Mevalonate is further metabolized to farnesyl pyrophosphate (FPP), which is the precursor for sterols. In addition, the farnesyl moiety from FPP is utilized for post-translational modification of proteins including small GTPases, such as Ras and Ras related proteins, which play a role in malignant transformation of cells. FPP is a precursor for geranylgeranyl pyrophosphate (GGPP), which is similarly involved in post-translational modification of proteins. There has been intense interest in manipulating the pathway through HMG-CoA reductase inhibition. More recently, the focus has been on manipulating the pathway by post-translational modification of key regulatory proteins through farnesyl prenyl transferase (FPTase) or geranylgeranyl prenyl transferase (GGPTase) inhibition. This review focuses on the mevalonate pathway and the application of rational drug therapies to manipulate this pathway. Included in the review are a summary of agents demonstrating success in preclinical investigations such as; farnesyl transferase inhibitors, geranylgeranyl transferase inhibitors, dual inhibitors, statins, bisphosphonates, histone deacetylase inhibitors and other compounds. While these agents have shown preclinical success, translation to success in clinical trials has been more difficult. These clinical trials are reviewed along with evaluation of some of the potential problems with these agents in their clinical application.

Brassinosteroids stimulate plant tropisms through modulation of polar auxin transport in Brassica and Arabidopsis

Brassinosteroids (BRs) are important plant growth regulators in multiple developmental processes. Previous studies have indicated that BR treatment enhanced auxin-related responses, but the underlying mechanisms remain unknown. Using (14)C-labeled indole-3-acetic acid and Arabidopsis thaliana plants harboring an auxin-responsive reporter construct, we show that the BR brassinolide (BL) stimulates polar auxin transport capacities and modifies the distribution of endogenous auxin. In plants treated with BL or defective in BR biosynthesis or signaling, the transcription of PIN genes, which facilitate functional auxin transport in plants, was differentially regulated. In addition, BL enhanced plant tropistic responses by promoting the accumulation of the PIN2 protein from the root tip to the elongation zone and stimulating the expression and dispersed localization of ROP2 during tropistic responses. Constitutive overexpression of ROP2 results in enhanced polar accumulation of PIN2 protein in the root elongation region and increased gravitropism, which is significantly affected by latrunculin B, an inhibitor of F-actin assembly. The ROP2 dominant negative mutants (35S-ROP2-DA/DN) show delayed tropistic responses, and this delay cannot be reversed by BL addition, strongly supporting the idea that ROP2 modulates the functional localization of PIN2 through regulation of the assembly/reassembly of F-actins, thereby mediating the BR effects on polar auxin transport and tropistic responses.

A retromerlike complex is a novel Rab7 effector that is involved in the transport of the virulence factor cysteine protease in the enteric protozoan parasite Entamoeba histolytica

Vesicular trafficking plays an important role in a virulence mechanism of the enteric protozoan parasite Entamoeba histolytica as secreted and lysosomal cysteine protease (CP) contributes to both cytolysis of tissues and degradation of internalized host cells. Despite the primary importance of intracellular sorting in pathogenesis, the molecular mechanism of CP trafficking remains largely unknown. In this report we demonstrate that transport of CP is regulated through a specific interaction of Rab7A small GTPase (EhRab7A) with the retromerlike complex. The amoebic retromerlike complex composed of Vps26, Vps29, and Vps35 was identified as EhRab7A-binding proteins. The amoebic retromerlike complex specifically bound to GTP-EhRab7A, but not GDP-EhRab7A through the direct binding via the carboxy terminus of EhVps26. In erythrophagocytosis the retromerlike complex was recruited to prephagosomal vacuoles, the unique preparatory vacuole of digestive enzymes, and later to phagosomes. This dynamism was indistinguishable from that of EhRab7A, and consistent with the premise that the retromerlike complex is involved in the retrograde transport of putative hydrolase receptor(s) from preparatory vacuoles and phagosomes to the Golgi apparatus. EhRab7A overexpression caused enlargement of lysosomes and decrease of the cellular CP activity. The reduced CP activity was restored by the coexpression of EhVps26, implying that the EhRab7A-mediated transport of CP to phagosomes is regulated by the retromerlike complex.

Preclinical evidence for the effect of bisphosphonates and cytotoxic drugs on tumor cell invasion

Bisphosphonates (BPs) are stable pyrophosphate analogs currently used in the treatment of patients with metastatic bone disease, known to affect bone resorption by reducing osteoclast activity. Use of these drugs in adjuvant therapy is currently under investigation following reports of an effect of BPs on tumor cell apoptosis in preclinical models. Recent evidence has suggested that BPs might also affect tumor cell invasion in vitro, and the component processes of adhesion, migration and degradation, through mechanisms including inhibition of prenylation of intracellular small GTPases such as Ras and Rho. The effects potentially may be enhanced through co-administration with chemotherapy agents, as both synergistic and additive effects have been described in vitro. This review discusses the preclinical evidence for the potential use of BPs and cytotoxic drugs for inhibiting tumor cell invasion, a key process in cancer progression.

The small GTPase Rac is involved in clustering of hippocampal neurons and fasciculation of their neurites

In hippocampal neurons cultured from brains of newborn rats, the glutamate receptor agonist N-methyl-D-aspartate induced the clustering of neuronal perikarya and the fasciculation of neurites. In addition, N-methyl-D-aspartate activated the small GTPase Rac1. Other stimuli of Rac activity, such as the Rho kinase inhibitors Y-27632, H-1152, and H89, as well as the cytotoxic necrotizing factor-1 from Escherichia coli, also caused neuronal clustering and neurite bundling. In neurons transiently transfected with dominant negative Rac1N17 neither N-methyl-D-aspartate nor Y-27632 induced clustering and fasciculation. In addition, the PI3-kinase inhibitors wortmannin and LY-294002 prevented these effects, as did a dominant negative form of p110PI3-Kgamma. Time-lapse microscopy showed that lethal toxin from Clostridium sordellii, which inhibits Rac, and wortmannin blocked the neuronal migration induced by Y-27632. In contrast, only lethal toxin reversed the clustering and fasciculation induced by pre-treatment with Y-27632. This effect of the toxin may be due to inactivation of Ras, since FTI-277, which prevents the farnesylation of Ras and thereby inactivates the GTPase, also dissolved the preformed clusters. We suggest that active Rac and a PI3-kinase synergistically induce neuronal migration, whereas a Ras isoform is responsible for the lasting attachment of neurons necessary for clustering and neurite fasciculation.

The GTP/GDP cycling of rho GTPase TCL is an essential regulator of the early endocytic pathway

Rho GTPases are key regulators of actin dynamics. We report that the Rho GTPase TCL, which is closely related to Cdc42 and TC10, localizes to the plasma membrane and the early/sorting endosomes in HeLa cells, suggesting a role in the early endocytic pathway. Receptor-dependent internalization of transferrin (Tf) is unaffected by suppression of endogenous TCL by small interfering RNA treatment. However, Tf accumulates in Rab5-positive uncoated endocytic vesicles and fails to reach the early endosome antigen-1-positive early endosomal compartments and the pericentriolar recycling endosomes. Moreover, Tf release upon TCL knockdown is significantly slower. Conversely, in the presence of dominant active TCL, internalized Tf accumulates in early endosome antigen-1-positive early/sorting endosomes and not in perinuclear recycling endosomes. Tf recycles directly from the early/sorting endosomes and it is normally released by the cells. The same phenotype is generated by replacing the C terminus of dominant active Cdc42 and TC10 with that of TCL, indicating that all three proteins share downstream effector proteins. Thus, TCL is essential for clathrin-dependent endocytosed receptors to enter the early/sorting endosomes. Furthermore, the active GTPase favors direct recycling from early/sorting endosomes without accumulating in the perinuclear recycling endosomes.

Dynamic changes in the osteoclast cytoskeleton in response to growth factors and cell attachment are controlled by beta3 integrin

The beta3 integrin cytoplasmic domain, and specifically S752, is critical for integrin localization and osteoclast (OC) function. Because growth factors such as macrophage colony-stimulating factor and hepatocyte growth factor affect integrin activation and function via inside-out signaling, a process requiring the beta integrin cytoplasmic tail, we examined the effect of these growth factors on OC precursors. To this end, we retrovirally expressed various beta3 integrins with cytoplasmic tail mutations in beta3-deficient OC precursors. We find that S752 in the beta3 cytoplasmic tail is required for growth factor-induced integrin activation, cytoskeletal reorganization, and membrane protrusion, thereby affecting OC adhesion, migration, and bone resorption. The small GTPases Rho and Rac mediate cytoskeletal reorganization, and activation of each is defective in OC precursors lacking a functional beta3 subunit. Activation of the upstream mediators c-Src and c-Cbl is also dependent on beta3. Interestingly, although the FAK-related kinase Pyk2 interacts with c-Src and c-Cbl, its activation is not disrupted in the absence of functional beta3. Instead, its activation is dependent upon intracellular calcium, and on the beta2 integrin. Thus, the beta3 cytoplasmic domain is responsible for activation of specific intracellular signals leading to cytoskeletal reorganization critical for OC function.

Interferon-induced antiviral Mx1 GTPase is associated with components of the SUMO-1 system and promyelocytic leukemia protein nuclear bodies

Mx proteins are interferon-induced large GTPases, some of which have antiviral activity against a variety of viruses. The murine Mx1 protein accumulates in the nucleus of interferon-treated cells and is active against members of the Orthomyxoviridae family, such as the influenza viruses and Thogoto virus. The mechanism by which Mx1 exerts its antiviral action is still unclear, but an involvement of undefined nuclear factors has been postulated. Using the yeast two-hybrid system, we identified cellular proteins that interact with Mx1 protein. The Mx1 interactors were mainly nuclear proteins. They included Sp100, Daxx, and Bloom's syndrome protein (BLM), all of which are known to localize to specific subnuclear domains called promyelocytic leukemia protein nuclear bodies (PML NBs). In addition, components of the SUMO-1 protein modification system were identified as Mx1-interacting proteins, namely the small ubiquitin-like modifier SUMO-1 and SAE2, which represents subunit 2 of the SUMO-1 activating enzyme. Analysis of the subcellular localization of Mx1 and some of these interacting proteins by confocal microscopy revealed a close spatial association of Mx1 with PML NBs. This suggests a role of PML NBs and SUMO-1 in the antiviral action of Mx1 and may allow us to discover novel functions of this large GTPase.

Similar apparent constitutive activity of human histamine H(2)-receptor fused to long and short splice variants of G(salpha)

Fusion proteins allow for the analysis of receptor/G protein coupling under defined conditions. The beta(2)-adrenoceptor (beta(2)AR) fused to the long splice variant of G(salpha) (G(salphaL)) exhibits a higher apparent constitutive activity than the beta(2)-adrenoceptor fused to the short splice variant of G(salpha) (G(salphaS)). Experimentally, this results in higher efficacy and potency of partial agonists and in higher efficacy of inverse agonists at the beta(2)AR fused to G(salphaL) relative to the beta(2)AR fused to G(salphaS), indicating that the agonist-free beta(2)AR and the beta(2)AR occupied by partial agonists promote GDP dissociation from G(salphaL) more efficiently than from G(salphaS). In fact, the GDP affinity of G(salphaS) fused to the beta(2)AR is higher than the GDP affinity of G(salphaL) fused to the beta(2)AR. We asked the question whether the histamine H(2)-receptor (H(2)R) exhibits similar coupling to G(salpha) splice variants as the beta(2)AR. To address this question, we studied H(2)R-G(salpha) fusion proteins expressed in Sf9 cells. In contrast to beta(2)AR-G(salpha) fusion proteins, the potencies and efficacies of partial agonists and the efficacies of inverse agonists were similar at the H(2)R fused to G(salphaL) and G(salphaS) as assessed by guanosine-5'-O-(3-thio)triphosphate binding and/or steady-state GTPase activity. However, the time course analysis of guanosine-5'-O-(3-thio)triphosphate binding indicated that G(salphaS) fused to the H(2)R possesses a higher GDP-affinity than G(salphaL) fused to the H(2)R. Our data show that the H(2)R fused to G(salphaL) and G(salphaS) possesses similar constitutive activity and is insensitive to differences in GDP affinity of G(salpha) splice variants. Thus, GDP affinity of G proteins does not generally determine constitutive activity of receptors.

Searching new targets for anticancer drug design: the families of Ras and Rho GTPases and their effectors

The Ras superfamily of low-molecular-weight GTPases are proteins that, in response to diverse stimuli, control key cellular processes such as cell growth and development, apoptosis, lipid metabolism, cytoarchitecture, membrane trafficking, and transcriptional regulation. More than 100 genes of this superfamily grouped in six subfamilies have been described so far, pointing to the complexities and specificities of their cellular functions. Dysregulation of members of at least two of these families (the Ras and the Rho families) is involved in the events that lead to the uncontrolled proliferation and invasiveness of human tumors. In recent years, the cloning and characterization of downstream effectors for Ras and Rho proteins have given crucial clues to the specific pathways that lead to aberrant cellular growth and ultimately to tumorigenesis. A direct link between the functions of some of these effectors with the appearance of transformed cells and their ability to proliferate and invade surrounding tissues has been made. Accordingly, drugs that specifically alter their functions display antineoplasic properties, and some of these drugs are already under clinical trials. In this review, we survey the progress made in understanding the underlying molecular connections between carcinogenesis and the specific cellular functions elicited by some of these effectors. We also discuss new drugs with antineoplastic or antimetastatic activity that are targeted to specific effectors for Ras or Rho proteins.

Functional coupling of GABA(B) receptors with G proteins that are sensitive to N-ethylmaleimide treatment, suramin, and benzalkonium chloride in rat cerebral cortical membranes

Although it is known that GABA(B) receptors are negatively coupled to adenylyl cyclase, the detailed selectivity of functional interaction between GABA(B) receptors and Gi subfamily members is still ambiguous. (+/-)-Baclofen-stimulated high-affinity GTPase activity, which was competitively antagonized by 2-hydroxy-saclofen, was attenuated by pretreatment of the membranes with N-ethylmaleimide (NEM) in a concentration- and incubation period-dependent manner. The NEM-pretreated (50 microM, 15 min at 4 degrees C) membranes restored the (+/-)-baclofen-sensitive high-affinity GTPase activity when reconstituted with pertussis toxin-sensitive bovine brain G proteins. Among recombinant rat Galpha subunits, G(i alpha(-2)) appeared most effective as compared with other subunits (G(i alpha(-2)) > G(i alpha(-3) > G(i alpha(-1) = G(o alpha). The GABA(B) receptor-mediated high-affinity GTPase activity was also completely eliminated by 100 microM suramin and by 100 microM benzalkonium chloride. These results indicate that GABA(B) receptors in rat cerebral cortex couple to NEM-sensitive G proteins, in particular Gi2, which are sensitive to suramin and benzalkonium chloride.

Stimulation of high-affinity GTPase activity through group II metabotropic glutamate receptors in rat hippocampal and striatal membranes

The stimulation of high-affinity GTPase activity through metabotropic glutamate receptors (mGluRs) was pharmacologically characterized with the use of a series of agonists for mGluRs in rat hippocampal and striatal membranes. The pharmacological profile of the response was almost identical to each other between both brain regions. Thus, the high-affinity GTPase activities were stimulated by several mGluR-related compounds with the following rank order of potency: (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) = (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I) > L-glutamate = 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate [(2R,4R)-APDC] > (S)-4-carboxy-3-hydroxyphenylglycine [(S)-4C3HPG] = 1S,3R-1-aminocyclopentane-1,3-dicarboxylate [(1S,3R)-ACPD] > (S)-3-carboxy-4-hydroxyphenylglycine [(S)-3C4HPG] = ibotenate. The negative logarithmically transformed EC50 (pEC50) values of these compounds in both brain regions were significantly correlated with those reported previously in the cerebral cortical membranes (N. Nishi et al., Br. J. Pharmacol., 130, 1664-1670, 2000). On the contrary, other reagents including a selective group I mGluRs agonist, (RS)-3,5-dihydroxyphenylglycine [(RS)-3,5-DHPG], and selective group III mGluRs agonists such as L(+)-2-amino-4-phosphonobutylate (L-AP4) and L-serine-O-phosphate (L-SOP) had little or no effects even at the highest concentration examined. Quisqualate was also a very weak agonist in both regions. These results indicate that mGluR-mediated high-affinity GTPase activity derives from the Gi proteins associated with adenylyl cyclase inhibition through group II mGluRs, in particular the mGluR2 subtype, in rat hippocampal and striatal membranes.

Distinct molecular mechanisms and divergent endocytotic pathways of AMPA receptor internalization

Internalization of postsynaptic AMPA receptors depresses excitatory transmission, but the underlying dynamics and mechanisms of this process are unclear. Using immunofluorescence and surface biotinylation, we characterized and quantified basal and regulated AMPA receptor endocytosis in cultured hippocampal neurons, in response to synaptic activity, AMPA and insulin. AMPA-induced AMPA receptor internalization is mediated in part by secondary activation of voltage-dependent calcium channels, and in part by ligand binding independent of receptor activation. Although both require dynamin, insulin- and AMPA-induced AMPA receptor internalization are differentially dependent on protein phosphatases and sequence determinants within the cytoplasmic tails of GluR1 and GluR2 subunits. AMPA receptors internalized in response to AMPA stimulation enter a recycling endosome system, whereas those internalized in response to insulin diverge into a distinct compartment. Thus, the molecular mechanisms and intracellular sorting of AMPA receptors are diverse, and depend on the internalizing stimulus.

Evidence for a G protein-coupled gamma-hydroxybutyric acid receptor

gamma-Hydroxybutyric acid (GHB) is a naturally occurring metabolite of GABA that has been postulated to exert ubiquitous neuropharmacological effects through GABA(B) receptor (GABA(B)R)-mediated mechanisms. The alternative hypothesis that GHB acts via a GHB-specific, G protein-coupled presynaptic receptor that is different from the GABA(B)R was tested. The effect of GHB on regional and subcellular brain adenylyl cyclase in adult and developing rats was determined and compared with that of the GABA(B)R agonist (-)-baclofen. Also, using guanosine 5'-O:-(3-[(35)S]thiotriphosphate) ([(35)S]GTPgammaS) binding and low-K:(m) GTPase activity as markers the effects of GHB and (-)-baclofen on G protein activity in the brain were determined. Neither GHB nor baclofen had an effect on basal cyclic AMP (cAMP) levels. GHB significantly decreased forskolin-stimulated cAMP levels by 40-50% in cortex and hippocampus but not thalamus or cerebellum, whereas (-)-baclofen had an effect throughout the brain. The effect of GHB on adenylyl cyclase was observed in presynaptic and not postsynaptic subcellular tissue preparations, but the effect of baclofen was observed in both subcellular preparations. The GHB-induced alteration in forskolin-induced cAMP formation was blocked by a specific GHB antagonist but not a specific GABA(B)R antagonist. The (-)-baclofen-induced alteration in forskolin-induced cAMP formation was blocked by a specific GABA(B)R antagonist but not a specific GHB antagonist. The negative coupling of GHB to adenylyl cyclase appeared at postnatal day 21, a developmental time point that is concordant with the developmental appearance of [(3)H]GHB binding in cerebral cortex, but the effects of (-)-baclofen were present by postnatal day 14. GHB and baclofen both stimulated [(35)S]GTPgammaS binding and low-K:(m) GTPase activity by 40-50%. The GHB-induced effect was blocked by GHB antagonists but not by GABA(B)R antagonists and was seen only in cortex and hippocampus. The (-)-baclofen-induced effect was blocked by GABA(B)R antagonists but not by GHB antagonists and was observed throughout the brain. These data support the hypothesis that GHB induces a G protein-mediated decrease in adenylyl cyclase via a GHB-specific G protein-coupled presynaptic receptor that is different from the GABA(B)R.

Natriuretic peptides inhibit G protein activation. Mediation through cross-talk between cyclic GMP-dependent protein kinase and regulators of G protein-signaling proteins

Atrial natriuretic peptide (ANP) inhibits the proliferation of many cells, in part through interfering with signal transduction enacted by G protein-coupled growth factor receptors. Signaling interactions between ANP and the G protein-coupled growth factor receptor ligand, endothelin-3 (ET-3), regulate astrocyte proliferation at a very proximal but undefined point. Here, we find that ANP inhibits the ability of ET-3 to activate Galpha(q) and Galpha(i) in these cells. ANP stimulated the translocation of endogenous regulators of G protein-signaling (RGS) proteins 3 and 4 from the cytosol to the cell membrane, and enhanced their association with Galpha(q) and Galpha(i). ANP effects were significantly blocked by HS-142-1, an inhibitor of guanylate cyclase activation, or by ET-3. KT5823, an inhibitor of cyclic GMP-dependent protein kinase (PKG) reversed the RGS translocation induced by ANP; conversely, expression of an active catalytic subunit of PKG-I, or 8-bromo-cyclic GMP stimulated RGS translocation. ANP caused the phosphorylation of both RGS proteins in a PKG-dependent fashion, and the expressed PKG (in the absence of ANP) also stimulated RGS phosphorylation. A novel cross-talk between PKG and RGS proteins is stimulated by ANP and leads to the increased translocation and association of RGS proteins with Galpha. The rapid inactivation of G proteins provides a mechanism by which ANP inhibits downstream signaling to the cell proliferation program.

Formate protects stationary-phase Escherichia coli and Salmonella cells from killing by a cationic antimicrobial peptide

For a sustained infection, enteric bacterial pathogens must evade, resist or tolerate a variety of antimicrobial host defence peptides and proteins. We report here that specific organic acids protect stationary-phase Escherichia coli and Salmonella cells from killing by a potent antimicrobial peptide derived from the human bactericidal/permeability-increasing protein (BPI). BPI-derived peptide P2 rapidly halted oxygen consumption by stationary-phase cells preincubated with glucose, pyruvate or malate and caused a 109-fold drop in cell viability within 90 min of addition. In marked contrast, O2 consumption and viability were not significantly affected in stationary-phase cells preincubated with formate or succinate. Experiments with fdhH, fdoG, fdnG, selC and sdhO mutants indicate that protection by formate and succinate requires their oxidation by the Fdh-N formate dehydrogenase and succinate dehydrogenase respectively. Protection was also dependent on the BipA GTPase but did not require the RpoS sigma factor. We conclude that the primary lesion caused by this cationic peptide is not gross permeabilization of the bacterial cytoplasmic membrane but may involve specific disruption of the respiratory chain. Because P2 shares sequence similarity with a range of other antimicrobial peptides, its cytotoxic mechanism has broader significance. Additionally, protective quantities of formate are secreted by E. coli and Salmonella during growth suggesting that such compounds are important determinants of bacterial survival in the host.

Effects of the active aldehyde group generated by RNA N-glycosidase in the sarcin/ricin domain of rat 28S ribosomal RNA on peptide elongation

Effects of the active aldehyde group of ribose C1' at position 4324 of rat 28S rRNA, in the inactivated ribosome generated by RNA N-glycosidases (trichosanthin, A-chain of cinnamomin and ricin), on peptide elongation have been studied. The aldehyde group inhibits the activities of eEF1A-dependent aminoacyl-tRNA binding to the inactivated ribosome and eEF1A-dependent GTPase, but increases eEF2-dependent activity. At a high concentration of RNA N-glycosidase, the generated aldehyde group also inhibits aminoacyl-tRNA binding to the inactivated ribosome in the absence of elongation factor and translocation activity. When the aldehyde group is reduced into a hydroxyl group by sodium borohydride or blocked with an amino acid through nucleophilic addition, the activities of eEF1A-dependent aminoacyl-tRNA binding and eEF1A-dependent GTPase of the inactivated ribosome are partially restored, but the altered activities of eEF2-dependent GTPase, translocation and aminoacyl-tRNA binding in the absence of elongation factor are not normalized. Thus, reduction or blockage of the aldehyde group with sodium borohydride or amino acids might change the conformation of the S/R domain in rat 28S ribosomal RNA to meet the requirement for eEF1A-dependent reactions, but not eEF2-involved reactions.

Myr+-Gi2 alpha and Go alpha subunits restore the efficacy of opioids, clonidine and neurotensin giving rise to antinociception in G-protein knock-down mice

In mice whose Gi/o-protein function had been impaired by antisense 'knock-down' or pertussis toxin treatment, i.c.v. injection of myr+-Gi/o alpha subunits restored the effectiveness of beta-endorphin, morphine, DPDPE, clonidine and neurotensin to produce antinociception. Myr+-G alpha subunits of the class of G-proteins actually impaired were more effective than unlike but related myr+-G alpha subunits. Selectivity was noted in that only exogenous myr+-G alpha subunits affected (enhanced) the activity of agonists in G alpha-deficient signalling systems. This treatment had little effect on agonist potency when the impairment resided at the receptor level. The potential of the opioids, clonidine and R-PIA to increase G alpha-related in vitro hydrolysis of GTP was also re-established after injecting myr+-Gi2 alpha subunits into Gi2-knocked-down mice. Myr+-Gi2 alpha subunits pre-incubated with GTPgammaS or GDPbetaS before i.c.v. injection did not improve the activity of agonists in vivo (antinociception) or in vitro (regulation of low Km GTPase). After impairing the function of PKCbeta1 by antisense treatment or with the inhibitor H7, the effect of myr+-G alpha subunits on agonist potency was prevented. Electron microscope analysis showed the entry of gold-conjugated myr+-G alpha subunits into neural cells. These particles were found in the cytoplasm, associated with the plasma membranes of different neuronal processes and also in synaptic junctions. In cultured neurons and astrocytes myr+-Gi2 alpha-associated fluorescence was internalised in a dose-dependent manner and distributed in the plasma membrane and cytosol, as well as in nuclei of dividing astrocytes. Thus, G alpha subunits in CSF enter into neurons and functionally couple to the receptor-triggered signalling cascade. As G-proteins have been implicated in the pathophysiology of several neural disorders, this finding may be valuable in the therapy of such dysfunctions.

Dissociation of RalA from synaptic membranes by Ca2+/calmodulin

Ras-related small GTP-binding proteins execute many cellular functions, such as cell growth, differentiation, cytoskeletal reorganization, membrane trafficking, and membrane fusion. RalA belongs to the superfamily of Ras-related small GTP-binding proteins. Synaptic vesicles (SV) contain small GTP-binding proteins, where RalA, Rab3A, and Rab5A are the major GTP-binding proteins. It has been postulated that a cycling of these proteins between membrane-bound and soluble states is required for regulating cellular functions. Calmodulin (CaM) was found to dissociate Rab3A from SV membranes by forming a 1:1 complex with Ca2+/CaM. RalA was also found to be a Ca2+/CaM-binding protein. Therefore, we examined if Ca2+/CaM can also cause the RalA to dissociate from SV membranes. In this study, we identified that Ca2+/CaM dissociates RalA as well as Rab3A from synaptic vesicles.

Potential drug targets: small GTPases that regulate leukocyte function

Leukocytes are not only important mediators of innate immunity, but they also induce and perpetuate inflammatory responses that are harmful to the host. Although inflammatory mediators activate leukocytes through a common heterotrimeric G protein (Gi) signalling intermediate, many downstream inflammatory functions are regulated by distinct small GTPases, which suggests that pharmacological modulation of small GTPase activity would be useful in developing specific anti-inflammatory therapies. The recent identification of multiple small GTPase effectors, the recognition of the role of GTPase regulatory proteins in directing downstream signalling from small GTPases, and detailed structural information on the GTPases themselves suggests new possibilities for the development of effective and selective anti-inflammatory drugs.

Effects of vasopressin-mastoparan chimeric peptides on insulin release and G-protein activity

Two chimeric peptides, consisting of the linear vasopressin receptor V1 antagonist PhAc-D-Tyr(Me)-Phe-Gln-Asn-Arg-Pro-Arg-Tyr, in the N-terminus and mastoparan in the C-terminus connected directly (M375) or via 6-aminohexanoic acid (M391), have been synthesised. At 10 microM concentration, these novel peptides increased insulin secretion from isolated rat pancreatic islet cells 18-26-fold at 3.3 mM glucose and 3.5-5-fold at 16.7 mM glucose. PTX pretreatment of the islets decreased the peptide-induced insulin release. M375 and M391 bind to V1a vasopressin receptors with affinities lower than the unmodified vasopressin antagonist, but with K(D) values of 3.76 nM and 9.02 nM, respectively, both chimeras are high affinity ligands. The GTPase activity and GTPgammaS binding in the presence of these peptides has been characterised in Rin m5F cells. Comparison of the influence of the peptides M375 and M391 on GTPase activity in native and pertussis toxin-treated cells suggests a selective activation of G alpha(i)/G alpha(o) subunits, combined with a suppression of other GTPases, primarily G alpha(s). However, the GTPgammaS binding data show that the peptides retain some of the activating property even in PTX-treated cell membranes. In conclusion, the conjugation of mastoparan with the V1a receptor antagonists produce peptides with properties different from the parent peptides that could be used to elucidate the role of different G proteins in insulin release.

The amyloid precursor protein interacts with Go heterotrimeric protein within a cell compartment specialized in signal transduction

The function of the beta-amyloid protein precursor (betaAPP), a transmembrane molecule involved in Alzheimer pathologies, is poorly understood. We recently reported the presence of a fraction of betaAPP in cholesterol and sphingoglycolipid-enriched microdomains (CSEM), a caveolae-like compartment specialized in signal transduction. To investigate whether betaAPP actually interferes with cell signaling, we reexamined the interaction between betaAPP and Go GTPase. In strong contrast with results obtained with reconstituted phospholipid vesicles (Okamoto et al., 1995), we find that incubating total neuronal membranes with 22C11, an antibody that recognizes an N-terminal betaAPP epitope, reduces high-affinity Go GTPase activity. This inhibition is specific of Galphao and is reproduced, in the absence of 22C11, by the addition of the betaAPP C-terminal domain but not by two distinct mutated betaAPP C-terminal domains that do not bind Galphao. This inhibition of Galphao GTPase activity by either 22C11 or wild-type betaAPP cytoplasmic domain suggests that intracellular interactions between betaAPP and Galphao could be regulated by extracellular signals. To verify whether this interaction is preserved in CSEM, we first used biochemical, immunocytochemical, and ultrastructural techniques to unambiguously confirm the colocalization of Galphao and betaAPP in CSEM. We show that inhibition of basal Galphao GTPase activity also occurs within CSEM and correlates with the coimmunoprecipitation of Galphao and betaAPP. The regulation of Galphao GTPase activity by betaAPP in a compartment specialized in signaling may have important consequences for our understanding of the physiopathological functions of betaAPP.

Peptitergent PD1 affects the GTPase activity of rat brain cortical membranes

Peptitergent PD1 shows complex effects on GTPase activity of rat brain cortical membranes: inhibition in the presence of lower concentrations of GTP and activation at a higher concentration, above 0.5 microM, of GTP. Its effect is dose dependent and is characterized by an EC50 of 1.8 +/- 0.2 microM and a Hill coefficient of 1.6 +/- 0.3, and it increases both Km and Vmax of the GTP hydrolysis. PD1 that was unable to solubilize G-proteins from the membranes probably acts on them by direct binding near the C-terminal alpha-helical region of the Galpha subunit, similarly to mastoparan.

Regulation of phospholipase C-delta by GTP-binding proteins-rhoA as an inhibitory modulator

The regulation of Phospholipase C (PLC)delta activity remains obscure. These studies show that PLCdelta1 activity is significantly enhanced by both guanosine thiotriphosphate (GTPgammaS) and Clostridium botulinum exoenzyme C3 (C3) but not by aluminium fluoride. C3 ADP ribosylated a 21-kDa protein in the PLCdelta1 preparation and Western blotting identified rhoA in these samples. RhoA acts as an inhibitory modulator of PLCdelta activity.

G-protein activation and expression in lung injury

Lung injury and repair are characterized by changes in the phenotypic and functional characteristics of parenchymal cells. It is not known to what extent these changes are regulated by alterations in total G-protein activity or expression of G-protein subfamilies. G-proteins (GTPases) are receptor-associated proteins that act as molecular switches for signal transduction pathways. They are involved in regulating ribosomal protein synthesis, transmembrane signaling of hormones and growth factors, cellular differentiation and proliferation and guiding organelle movement within cells. In this study, we validated, modified and optimized the experimental conditions needed to evaluate GTPase activity in the lung. We also measured changes in GTPase activity and the expression of G-protein subfamilies in lungs undergoing paraquat lung injury. Total GTPase activity as well as the expression of Gi, Gs, and Go subfamilies demonstrated a sharp increase on day 1 after injury, a time previously shown to coincide with rapid cellular proliferation and increased protein synthesis. In contrast to the other subfamilies, expression of the alpha subunit of the Gs subfamily was more intense and sustained during the repair phase (day 14). These findings implicate G-proteins in general, and the Gs subfamily, in particular, in regulating cellular function during lung repair.

Different effects of Gsalpha splice variants on beta2-adrenoreceptor-mediated signaling. The beta2-adrenoreceptor coupled to the long splice variant of Gsalpha has properties of a constitutively active receptor

The beta2-adrenoreceptor (beta2AR) couples to the G-protein Gs to mediate adenylyl cyclase activation. The splice variants of Gsalpha differ by a 15-amino acid insert between the Ras-like domain and the alpha-helical domain. The long splice variant of Gsalpha (GsalphaL) binds GDP with lower affinity than the short splice variant (GsalphaS), but the impact of this difference on the interaction of Gsalpha with the beta2AR is not known. We studied the beta2AR/Gsalpha interaction using receptor/G-protein fusion proteins (beta2ARGsalphaS and beta2ARGsalphaL) expressed in Sf9 cells. Fusion of the beta2AR to Gsalpha promotes efficient coupling as shown by high-affinity agonist binding and GTPase and adenylyl cyclase activation and ensures fixed stoichiometry between receptor and G-protein. Importantly, fusion does not change the fundamental properties of the beta2AR or Gsalpha. The beta2AR in beta2ARGsalphaL showed hallmarks of constitutive activity (increased potency and intrinsic activity of partial agonists, increased efficacy of inverse agonists, and increased basal GTPase activity) compared with the beta2AR in beta2ARGsalphaS. The apparent constitutive activity of the beta2AR in beta2ARGsalphaL may be due to the lower GDP affinity of GsalphaL compared with GsalphaS, i.e. GsalphaL is more often nucleotide-free than GsalphaS and, therefore, more frequently available to stabilize the beta2AR in the active (R*) state. This study demonstrates that subtle structural differences between closely related G-protein alpha-subunits can have important consequences for the functional properties of a G-protein-coupled receptor.

Substance P-related inhibitors of mast cell exocytosis act on G-proteins or on the cell surface

[p-Glu5,D-Trp(7,9,10)]substance P-(5-11) inhibited mastoparan-stimulated GTPase activity in homogenized rat peritoneal mast cells and decreased histamine secretion induced by mastoparan from streptolysin O-permeabilized mast cells (IC50 of about 30 microM), but not from intact cells. In contrast, [D-Pro4,D-Trp(7,9,10)]substance P-(4-11) inhibited the secretion from intact cells (IC50 of about 10 microM) but had no effect on histamine secretion from permeabilized cells, suggesting that this peptide exerts its inhibitory effect on the plasma membrane, whereas [p-Glu5,D-Trp(7,9,10)]substance P-(5-11) interacts with G proteins. Pretreatment of mast cells with neuraminidase led to an inhibition of the secretory response to mastoparan and related triggers. This response was restored following cell permeabilization, demonstrating the role of the cell surface on the entry of mastoparan and related triggers and on their ability to reach G proteins sensitive to pertussis toxin and [p-Glu5,D-Trp(7,9,10)]substance P-(5-11).

Sigma ligands stimulate GTPase activity in mouse prefrontal membranes: evidence for the existence of metabotropic sigma receptor

We studied effects of various sigma ligands on GTPase activity in mouse prefrontal membranes. Some representative sigma agonists, such as (+)-pentazocine, SA4503 and (+)-3-PPP, stimulated the GTPase activity in a concentration-dependent manner in ranges of 10 nM to 10 microM. Maximal effect was almost 10% increase to the control without treatment of drugs. However, another representative agonist, (+)-SKF10,047 showed only a partial activity with maximal effect 5% at 1 microM. NE-100, a representative antagonist, showed no effect at concentrations not more than 100 nM, while it did stimulate GTPase activity at 1 and 10 microM. Furthermore, these stimulative effects of both (+)-pentazocine and SA4503 on GTPase activity were significantly antagonized by NE-100 at 100 nM, suggesting that NE-100 possesses agonist-antagonist property. These findings suggest the possibility that there exist metabotropic sigma receptors.

Activation of R-Ras GTPase by GTPase-activating proteins for Ras, Gap1(m), and p120GAP

The enzymatic properties of Gap1(m) were characterized using three Ras and R-Ras proteins as substrates and were compared with those of p120GAP. Gap1(m) stimulated the GTPase of Ras better than that of R-Ras, in contrast to p120GAP which promoted the GTPase of R-Ras better than that of Ras. The EC50 values of Gap1(m) for Ha-Ras and R-Ras were 0.48 +/- 0.02 and 1.13 +/- 0.12 nM, respectively, whereas the EC50 values of p120GAP for Ha-Ras and R-Ras were 23.1 +/- 1.9 and 3.86 +/- 0.38 nM, respectively. The affinities of Gap1(m) and p120GAP to the substrates determined by competitive inhibition by using Ha-Ras.GTPgammaS (guanosine 5'-O-(3-thiotriphosphate)) or R-Ras.GTPgammaS as a competitor agreed well with the substrate specificities of these GTPase-activating proteins. The Km values of Gap1(m) for Ha-Ras and R-Ras were 1.53 +/- 0.27 and 3.38 +/- 0.53 microM, respectively, which were lower than that of p120GAP for Ha-Ras (145 +/- 11 microM) by almost 2 orders of magnitude. The high affinity of Gap1(m) to the substrates and its membrane localization suggest that Gap1(m) may act as a regulator of the basal activity of Ha-Ras and R-Ras.

Morphine and muscle relaxants are receptor-independent G-protein activators and cromolyn is an inhibitor of stimulated G-protein activity

Morphine and muscle relaxants are classical mast cell activators and cromolyn is a mast cell inhibitor. However, the mechanisms underlying the effects of these drugs are obscure. We asked the question whether morphine and muscle relaxants may activate heterotrimeric guanine nucleotide-binding proteins (G-proteins), and whether cromolyn may prevent this activation. Morphine activated Gi-proteins in HL-60 membranes and purified transducin (TD) at concentrations above 1 mM, but the effects on morphine did not reach saturation up to 10 mM. d-Tubocurarine activated Gi-proteins and TD in a saturable manner, with EC50 values of 0.3 mM and 4.2 mM, respectively. Gallamine and succinylcholine were less effective activators of TD than d-tubocurarine, Morphine and d-tubocurarine were about similarly effective activators of Gi-proteins, whereas d-tubocurarine was a more effective activator of TD than morphine. Cromolyn at 10 microM and 100 microM had little effect on TD activity but reduced the stimulatory effect of morphine by 50% and 80%, respectively. Our data suggest the following: (1) Receptor-independent G-protein activation by morphine and muscle relaxants presumably accounts for their mast cell-activating properties. (2) Cromolyn may act by preventing G-protein activation. (3) The variability in responsiveness of mast cells towards morphine and muscle relaxants could be due to differential expression of G-proteins with different sensitivity to activation by these drugs.

[3-[4-(4,5-Diphenyl-2-oxazolyl)-5-oxazolyl]phenoxy]acetic acid (BMY 45778) is a potent non-prostanoid prostacyclin partial agonist: effects on platelet aggregation, adenylyl cyclase, cAMP levels, protein kinase, and iloprost binding

[3-[4-(4,5-diphenyl-2-oxazolyl)-5-oxazolyl]phenoxy]acetic acid (BMY 45778) inhibits human (IC50 = 35 nM), rabbit (136 nM) and rat (1.3 microM) platelet aggregation. This compound activates adenylyl cyclase (ED50 = 6-10 nM) and stimulates GTPase in human platelet membrane preparations. The potency (EC50) of BMY 45778 stimulating adenylyl cyclase is comparable to iloprost. However, maximal stimulation of GTPase by BMY 45778 is approximately half the iloprost-stimulated activity, and BMY 45778 limits the GTPase stimulation by iloprost suggesting that BMY 45778 is a partial agonist at the IP receptor. BMY 45778 completely prevents [3H]]Iloprost binding to platelet membranes (IC50 = 7 nM). In whole platelets, BMY 45778 causes elevation of platelet cAMP levels (cAMP content doubles at 13 nM) and activation of the cAMP-dependent protein kinase (cAMP-protein kinase ratio is twice basal at 2 nM). BMY 45778 treatment of whole platelets also desensitizes the adenylyl cyclase activation by iloprost. These results indicate that BMY 45778, which is structurally different from prostacyclin and most prostacyclin agonists, acts by stimulating prostacyclin (IP) receptors.

Distinct specificity in the recognition of phosphoinositides by the pleckstrin homology domains of dynamin and Bruton's tyrosine kinase

Pleckstrin homology (PH) domains may act as membrane localization modules through specific interactions with phosphoinositide phospholipids. These interactions could represent responses to second messengers, with scope for regulation by soluble inositol polyphosphates. A biosensor-based assay was used here to probe interactions between PH domains and unilamellar liposomes containing different phospholipids and to demonstrate specificity for distinct phosphoinositides. The dynamin PH domain specifically interacted with liposomes containing phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] and, more weakly, with liposomes containing phosphatidylinositol-4-phosphate [PI(4)P]. This correlates with phosphoinositide activation of the dynamin GTPase. The functional GTPase of a dynamin mutant lacking the PH domain, however, cannot be activated by PI(4,5)P2. The phosphoinositide-PH domain interaction can be abolished selectively by point mutations in the putative binding pocket predicted by molecular modelling and NMR spectroscopy. In contrast, the Bruton's tyrosine kinase (Btk)PH domain specifically bound liposomes containing phosphatidylinositol-3,4,5-trisphosphate [PI(3,4,5)P3]: an interaction requiring Arg28, a residue found to be mutated in some X-linked agammaglobulinaemia patients. A rational explanation for these different specificities is proposed through modelling of candidate binding pockets and is supported by NMR spectroscopy.

The SpoIIAA protein of Bacillus subtilis has GTP-binding properties

SpoIIAA is the first protein of the spoIIA operon. Here we show that SpoIIAA can bind and hydrolyze GTP. The protein also accepts ATP, but with lower affinity. GDP competes poorly for binding of GTP. The GTPase activity of SpoIIAA is within the range found for other GTP-binding proteins.

Subcellular localization of the small GTPase Rab5a in resting and stimulated human neutrophils

The evidence that small GTPases of the Rab family are regulators of vesicle traffic which can influence various cell functions prompted us to investigate the potential role of one of these proteins, Rab5a, in human neutrophils. In this paper we show that a large amount of Rab5a is present in the cytosol of peripheral blood mature neutrophils. The remaining protein was found to be membrane and azurophilic granule associated. Upon neutrophil challenge with PMA for 10 min the amount of membrane-associated Rab5a was upregulated while the cytosolic content of the protein concomitantly decreased. These findings support the hypothesis that Rab5a could be involved in the mechanism of neutrophil activation by modulating the rate of endocytosis and/or vesicle fusion.

Noradrenaline stimulation of high-affinity GTPase activity in membranes from rat aorta and caudal artery

The ability of noradrenaline (NA) to stimulate increases in high-affinity GTPase activity in sarcolemma-enriched rat aorta and caudal artery membranes was examined in the present study. In aortic membranes, NA significantly (P < 0.05; N = 5) increased the Vmax from a basal value of 103 +/- 29 to 156 +/- 38 pmol Pi/min/mg protein, but did not affect the Km which was 0.32 +/- 0.08 microM in the absence and 0.58 +/- 0.16 microM in the presence of NA. However, in caudal artery membranes, NA significantly (P < 0.05; N = 6) increased both the Vmax and the Km from basal values of 69 +/- 12 pmol Pi/min/mg protein and 0.24 +/- 0.05 microM, respectively, to 205 +/- 54 pmol Pi/min/mg protein and 1.01 +/- 0.25 microM, respectively. Removing the endothelium from both artery preparations did not alter significantly basal GTPase activity or the magnitude of the increase stimulated by NA. Prazosin significantly inhibited NA-stimulated increases in GTPase activity in membranes from endothelium-denuded caudal artery and aorta, and in endothelium-intact caudal artery membranes. However, yohimbine significantly inhibited NA-stimulated increases in GTPase activity only in preparations from endothelium-intact caudal arteries. Therefore, in endothelium-intact caudal artery membranes, NA stimulated increases in GTPase activity that were apparently mediated by both alpha 1-adrenoceptors and alpha 2-adrenoceptors, while in endothelium-denuded aortic and caudal artery membranes this increase was mediated solely by alpha 1-adrenoceptors. Western blotting of these arteries confirmed the presence of both Gi alpha 2,3 and Gq/11 alpha, which are candidates for mediating the alpha 1-adrenoceptor-stimulated increases in GTPase activity.

GapIII, a new brain-enriched member of the GTPase-activating protein family

Ras GTPase-activating proteins (GAPs) are negative regulators of ras, which controls proliferation and differentiation in many cells. Ras GAPs have been found in a variety of species from yeast to mammals. We describe here a newly identified mammalian GAP, GapIII, which was obtained by differential screening of a rat oligodendrocyte cDNA library. GapIII putatively encodes a 834 amino acid protein with a predicted molecular weight of 96 kDa, which contains a consensus GAP-related domain (GRD). The protein encoded by this cDNA has high homology with Gap1m, which was recently identified as a putative mammalian homolog of Drosophila Gap1. These proteins contain three structural domains, an N-terminal calcium-dependent phospholipid binding domain, GRD, and a C-terminal PH/Btk domain. Because of the sequence homology and the structural similarities of this protein with Gap1m, we hypothesize that GapIII and Gap1m may be members of a mammalian GAP gene family, separate from p120GAP, neurofibromin (NF1), and IQGAP. To confirm the GapIII protein activity, constructs containing different GapIII-GRD domains were transformed into iral mutant yeast to determine their relative ability to replace IRA1 functionally. Constructs that contained essentially the full-length protein (all three domains), the GRD alone, or the GRD plus PH/Btk domain suppressed heat shock sensitivity of ira1, whereas constructs that contained the GRD with part of the PH/Btk domain had only a weak ability to suppress heat shock sensitivity. These results suggest that the GapIII GRD itself is sufficient to down-regulate ras proteins in yeast. Expression of GapIII mRNA (4.2 kb) was examined by Northern analysis and in situ hybridization. This mRNA was expressed at highest levels in the brain, where its expression increased with development. Lower levels of the mRNA were expressed in the spleen and lung. Among neural cells, GapIII mRNA was expressed in neurons and oligodendrocytes, but not in astrocytes. Interestingly, the expression pattern in brain is reminiscent of type 1 NF1 expression reported by Gutmann et al. (Cell Growth Differ in press, 1995). We propose that in addition to p120GAP and neurofibromin, the GapIII/Gap1m family may be important for modulating ras activity in neurons and oligodendrocytes during normal brain development and in particular in the adult brain.

Differential regulation of G protein alpha-subunit GTPase activity by peptides derived from the third cytoplasmic loop of the alpha 2-adrenergic receptor

The effect of peptides homologous to segments of a G protein-coupled receptor on the GTPase activity of recombinant Go alpha (rGo alpha) and Gs alpha (rGs alpha) has been tested. These peptides contain overlapping sequences spanning from amino acid 212 of the putative fifth transmembrane domain to amino acid 229 of the third cytoplasmic loop of the alpha 2 adrenergic receptor. Interestingly, two peptides (comprising residues 212-227 and 214-227) strongly inhibit the basal GTPase activity of both rGo alpha and rGs alpha. Instead, a C-terminally extended peptide (residues 216-229) stimulates rGo alpha but slightly inhibits rGs alpha. Circular dichroism spectroscopy of the peptides reveals that an a helical structure is more easily inducible in the inhibitory ones. These findings constitute an example of peptides representing cytoplasmic receptor sequences that differentially modulate the GTPase activity of recombinant G protein alpha-subunits.

Gq/11 communicates with thromboxane A2 receptors in human astrocytoma cells, rabbit astrocytes and human platelets

Western blot analysis was performed to clarify the presence of trimeric G protein subfamily in membranes derived from human astrocytoma cells (1321N1), cultured rabbit astrocytes and human platelets, using G protein antisera GS alpha, Gi alpha, Gq/11 alpha, and G beta were found to exist in membranes derived from human astrocytoma cells and rabbit astrocytes as well as human platelets. However, only small amount of G(o) alpha was detected in any membranes. Gq/11 alpha was expressed much more in human platelets than in human astrocytoma cells or rabbit astrocytes. 9,11-Epithio-11,12-methanothromboxane A2 (STA2), a stable analogue of thromboxane A2 (TXA2), activated GTPase in membranes derived from human astrocytoma cells, rabbit astrocytes and human platelets with different potencies. STA2-induced GTPase activation in human platelet membranes was partly inhibited by treatment with QL antibody at 0 degrees C for 90 min. Furthermore, STA2-induced GTPase activation in membranes derived from human astrocytoma cells and rabbit astrocytes were potently inhibited by treatment with QL antibody. The results obtained indicate that TXA2 receptors in human astrocytoma cells and rabbit astrocytes communicate with Gq/11 as well as in human platelets.

Functional serotonin-2B receptors are expressed by a teratocarcinoma-derived cell line during serotoninergic differentiation

Among immortalized teratocarcinoma-derived cells, the clone 1C11 is a committed precursor of the neuronal lineage. On day 2 of its serotoninergic differentiation, this clone expresses only one subtype of serotonin [5-hydroxytryptamine (5-HT)] receptor, which is functionally coupled to phosphatidylinositol hydrolysis. The identity of these receptors was established by comparing their properties with those of 5-HT2B receptors expressed by LMTK- fibroblasts stably transfected with the recently cloned murine cDNA NP75 (LM5 cells). In both cell types, the analysis of (+/-)-1-(2,5-dimethoxy-4-[125I]iodophenyl)- 2-aminopropane HCl ([125I]DOI) binding revealed the presence of a single class of sites, the affinity of which was 1 order of magnitude lower than that reported for 5-HT2A receptors. In 1C11 cells differentiated for 2 days, as well as in LM5 cells, DOI binding was decreased by nonhydrolyzable analogs of GTP, indicating that the 5-HT2B receptor is functionally coupled to a G protein. The DOI-induced increase of phosphoinositide hydrolysis, which was correlated with both GTPase activity and binding data, is mediated by a Gq protein. This work demonstrates that the 5-HT2B receptor is functionally expressed before complete serotoninergic differentiation of 1C11 cells. The inducible 1C11 clone thus provides an in vitro model to investigate the possible role of the 5-HT2B receptor in the expression of the serotoninergic phenotype.

Influence of chronic opioid treatment on low Km GTPase activity in rat brain: evidence for the involvement of G-proteins in opioid tolerance

To investigate G protein function during the initial state of opioid tolerance, low Km GTPase activity was measured following chronic treatment with morphine (mu agonist) and butorphanol (mu/delta/kappa mixed agonist) in rats. Chronic opioid administration (20 mg/kg, IP) was performed once a day for 7 consecutive days. Under these conditions, antinociceptive tolerance to morphine but not butorphanol was developed. Chronic morphine treatment enhanced basal low Km GTPase activity in the pons/medulla, but not in the cortex and midbrain. On the other hand, chronic butorphanol treatment had no effect on basal low Km GTPase activity. These results suggest that chronic in vivo treatment of rats with mu agonists leads to an increase in the hydrolysis of GTP to GDP, by a basal low Km GTPase activity of G-proteins in the pons/medulla and that an enhancement of GTPase activity in this specific area may contribute to the development of antinociceptive tolerance to mu agonists.

Signal transduction by Ras-like GTPases: a potential target for anticancer drugs

Members of the ras family of GTPases are involved in a wide variety of cellular processes including cell proliferation, differentiation, apoptosis, and transformation. The ras oncogene is one of the most frequently mutated genes in human cancer. In addition, other oncogene and tumor suppressor gene products are components of the signal transduction pathways in which Ras or other Ras-like GTPases play key regulatory functions. Current progress in the elucidation of these signal transduction pathways will be reviewed and the potential use of these insights for the development of novel therapeutic agents for the treatment of cancer will be discussed.

Area specific alterations in muscarinic stimulated low Km GTPase activity in aging and Alzheimer's disease: implications for altered signal transduction

Carbachol-stimulated low Km GTPase activity (an index of muscarinic receptor-G protein coupling) was examined in hippocampus, basal ganglia, orbital frontal gyrus and superior frontal gyrus obtained from mature, aged and Alzheimer's Disease (AD) groups. Results indicated that carbachol-stimulated low Km GTPase activities in basal ganglia were as follows: mature controls > aged > AD, and there was a trend toward a similar pattern of decline in the hippocampus. No differences were seen in the two cortical areas examined; however, carbachol-stimulated low Km GTPase activity was small in the mature controls. Importantly, there were significant negative correlations between disease duration and carbachol-stimulated low Km GTPase activity in all areas examined except the orbital frontal gyrus. The longer the duration of the disease the lower the carbachol-stimulated low Km GTPase activity. Results suggest that age and disease-related changes in mAChR-G protein interactions in the basal ganglia may contribute to reduced signal transduction (ST). In addition, since decreased carbachol-stimulated low Km GTPase activity has also been observed in the aged rat; thus, investigations of the factors involved in decrements in signal transduction in the aged rat may be useful for understanding these alterations in aged humans or victims of AD.

Stimulation of phosphoinositide hydrolysis by glyceraldehyde-3-phosphate in electroporated HIT-T15 cells

HIT-T15 cells labeled with myo-[3H] inositol were permeabilized by electroporation and subsequently stimulated with various glycolytic intermediates in the presence of 20 mM LiCl in a buffer mimicking cytosolic ionic composition. Of the various glycolytic intermediates tested, only D-glyceraldehyde-3-phosphate (G3-P) stimulated the formation of labeled inositol phosphates. The half-maximal response to G3-P occurred at a concentration of 0.75 mM. Formation of inositol phosphates in electroporated cells was also observed in response to GTP. G3-P further potentiated the formation of inositol phosphates in response to GTP, however, the interaction between G3-P and GTP was additive rather than synergistic, indicating that G3-P stimulates phosphoinositide hydrolysis in a manner different than the receptor mediated GTP-dependent activation of phospholipase C. The potentiation of the GTP response by G3-P did not appear to involve inhibition of the GTPase activity of a phosphoinositide-specific G protein, since G3-P also potentiated the formation of inositol phosphates in response to GTP-gamma-S or NaF in a nearly additive manner.

BW373U86: a nonpeptidic delta-opioid agonist with novel receptor-G protein-mediated actions in rat brain membranes and neuroblastoma cells

BW373U86 is a potent and highly selective nonpeptidic agonist for delta-opioid receptors. To determine its ability to couple with G protein-linked second messenger systems, this study examined the effects of BW373U86 on the inhibition of adenylyl cyclase and the stimulation of low-Km GTPase activity. In rat striatal membranes, BW373U86 inhibited basal adenylyl cyclase activity in a GTP-dependent manner, with maximal inhibition levels similar to those of the prototypic delta agonist [D-Ser2,Thr6]Leu-enkephalin (DSLET). However, BW373U86 was approximately 100 times more potent than DSLET in inhibiting adenylyl cyclase. Analysis of the inhibitory activity across 10 brain regions revealed that both low and high concentrations of BW373U86 inhibited adenylyl cyclase activity in a manner similar to that of DSLET. Inhibition of adenylyl cyclase by BW373U86 was delta receptor selective, because the delta receptor-selective antagonist naltrindole was significantly more potent than naloxone and the mu receptor-selective antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 was ineffective in blocking BW373U86 inhibition. BW373U86 also inhibited adenylyl cyclase activity in membranes prepared from NG108-15 cells, with an IC50 value 5 times lower than that of DSLET. This increased potency was not observed in concentration-effect curves for agonist-stimulated low-Km GTPase in NG108-15 membranes. BW373U86 is a competitive inhibitor of [3H]diprenorphine at delta receptors of NG108-15 cell membranes. However, unlike DSLET, BW373U86 displacement of [3H]diprenorphine binding to NG108-15 cell membranes was not affected by sodium and guanine nucleotides. This lack of GTP effect on binding apparently produced slow dissociation rates for this agonist, because naltrindole was less potent in blocking BW373U86 inhibition of adenylyl cyclase when membranes were preincubated with this agonist. These results demonstrate the novel finding that the binding of a full agonist to a G protein-coupled receptor is not regulated by GTP, and they also show how the lack of regulation in receptor binding affects agonist potency.