Subcellular distribution of the alpha subunit(s) of Gi: visualization by immunofluorescent and immunogold labeling

Photolytic insertion of albumin on activated carbon modified with ozone

254nm photolyses of bovine serum albumin [BSA] in aqueous solutions, were carried out in the presence of activated carbons modified by reaction with ozone. The photolyses were monitored by fluorescence spectroscopy and UV spectrophotometry, and the products were characterized by elemental analysis, FTIR, TGA, total organic carbon analyses [TOC], and XPS. The ozonation reaction was carried out at room temperature with O₃ under dry and wet conditions. The carbon characterization showed that the reaction increased the amount of epoxide and carbonyl groups on the carbon matrix. The activated carbon modified with dry O₃ exhibited higher concentration of oxidized groups in its surface, smaller surface area and lower thermal stability. Characterization of the photolysis of ozonized carbons pointed to a small release of carbon organic groups during the reaction with elimination of epoxide groups and increase of carbonyl groups without change of thermal stability. Photolysis of BSA in aqueous solution occurred with fluorescence quenching due to changes of the local microenvironment and/or macromolecular conformational changes. Absorbance increase of the UV spectrum indicated a hyperchromic effect due to albumin structure modifications during photolysis. TGA analysis of the photolysed activated carbons in the presence of BSA suggested that ozonized carbon samples underwent insertion of BSA upon photolysis, in particular the sample ozonized under dry conditions. The changes observed for the FTIR and elemental analysis agreed with this conclusion, which was further supported by ¹³C SS-NMR, fluorescence emission and XPS.

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

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

Submembraneous microtubule cytoskeleton: regulation of microtubule assembly by heterotrimeric Gproteins

Heterotrimeric Gproteins participate in signal transduction by transferring signals from cell surface receptors to intracellular effector molecules. Gproteins also interact with microtubules and participate in microtubule-dependent centrosome/chromosome movement during cell division, as well as neuronal differentiation. In recent years, significant progress has been made in our understanding of the biochemical/functional interactions between Gprotein subunits (alpha and betagamma) and microtubules, and the molecular details emerging from these studies suggest that alpha and betagamma subunits of Gproteins interact with tubulin/microtubules to regulate the assembly/dynamics of microtubules, providing a novel mechanism for hormone- or neurotransmitter-induced rapid remodeling of cytoskeleton, regulation of the mitotic spindle for centrosome/chromosome movements in cell division, and neuronal differentiation in which structural plasticity mediated by microtubules is important for appropriate synaptic connections and signal transmission.

A specific role of AGS3 in the surface expression of plasma membrane proteins

Activator of G protein signaling 3 (AGS3), originally identified in a functional screen for mammalian proteins that activate heterotrimeric G protein signaling, is known to be involved in drug-seeking behavior and is up-regulated during cocaine withdrawal in animal models. These observations indicate a potential role for AGS3 in the formation or maintenance of neural plasticity. We have found that the overexpression of AGS3 alters the surface-to-total ratios of a subset of heterologously expressed plasma membrane receptors and channels. Further analysis of the endocytic trafficking of one such protein by a biotin-based internalization assay suggests that overexpression of AGS3 moderately affects the internalization or recycling of surface proteins. Moreover, AGS3 overexpression and siRNA-mediated knockdown of AGS3 both result in the dispersal of two endogenously expressed trans-Golgi network (TGN)-associated cargo proteins without influencing those in the cis- or medial-Golgi compartments. Finally, adding a TGN-localization signal to a CD4-derived reporter renders the trafficking of fusion protein sensitive to AGS3. Taken together, our data support a model wherein AGS3 modulates the protein trafficking along the TGN/plasma membrane/endosome loop.

Differential partitioning of Galphai1 with the cellular microtubules: a possible mechanism of development of Taxol resistance in human ovarian carcinoma cells

Background

Taxol binds to the cellular microtubules and suppresses their dynamic instability. Development of tumor cell resistance to taxol is typically associated with increased expression of the drug efflux pump P-glycoprotein and/or alterations in the microtubules. Recently, changes in the dynamic instability of the microtubules have also been associated with development of taxol resistance in a lung cancer cell line. We have established a 250-fold taxol-resistant human ovarian carcinoma subline (2008/13/4) that does not display the typical alterations associated with development of drug resistance.

Results

Utilizing the mRNA differential display technique, we observed increased expression of an alpha subunit of the guanine nucleotide-binding protein, Gαi1, in the taxol-resistant human ovarian carcinoma cell lines compared to the parental 2008 cells. Several isoforms of the α-subunit of the G protein have been identified and the Gαi (inhibitory) are so named because they inhibit the activity of adenylate cyclase leading to inactivation of the cAMP-dependent protein kinase A (PKA) pathway. In addition, Gαi1 is also known to bind to microtubules and activates their GTPase activity and thus induces depolymerization of the microtubules. In the present study we demonstrate that the intracellular level of cAMP and the PKA activity were higher in the taxol-resistant 2008/13/4 and the 2008/17/4 cells despite the increased expression of Gαi1 in these cells. Moreover, Gαi1 was found to be localized not on the cell membrane, but in intracellular compartments in both the taxol-sensitive and -resistant human ovarian carcinoma cells. Interestingly, increased association of the Gαi1 protein and the microtubules in the taxol-resistant cells compared to the parental 2008 cells was observed, both prior to and after treatment of these cells with taxol.

Conclusion

Based on the opposing effects of taxol and the Gαi1 protein on the microtubule dynamic instability (taxol suppresses microtubule dynamic instability whilst the Gαi1 protein inhibits the suppression) our results indicate the operation of a novel pathway that would enable the cells to escape the cytotoxic effects of taxol.

Heterotrimeric G-proteins associate with microtubules during differentiation in PC12 pheochromocytoma cells

Tubulin modifies G-protein signaling and heterotrimeric G-proteins regulate microtubule assembly. Here we report an interplay among G-protein-coupled receptor and receptor tyrosine kinase (such as nerve growth factor-NGF) signaling systems in PC12 pheochromocytoma cells that resulted in a translocation of Galpha(s), Galpha(i1), and Galpha(o) from cell bodies to cellular processes where they appear to localize with tubulin-containing structures. This relocation appeared to depend on the integrity of microtubules, as it was blocked and reversed by nocodazole. Latrunculin, which promotes actin filament depolymerization, had no effect. Both deconvolution microscopy and immunoprecipitation showed a significant increase of Galpha association with microtubules that was coincident with the extension of "neurites." There were distinctions among the Galpha subtypes, with Galpha(s) showing the most profound NGF-induced colocalization with tubulin. Translocation of Galpha was blocked by agents that inhibit the MAP kinases required for neuronal differentiation, suggesting that G-protein relocation is triggered by the intracellular signals for differentiation. Consistent with this, Galpha in Neuro-2A cells, which spontaneously differentiate, showed a similar translocation coincident with differentiation. Thus, diverse signals that promote neuronal differentiation and changes in cell morphology may use specific G-proteins to evoke cytoskeletal rearrangement.

Regulator of G protein signaling Z1 (RGSZ1) interacts with Galpha i subunits and regulates Galpha i-mediated cell signaling

Regulator of G protein signaling (RGS) proteins constitute a family of over 20 proteins that negatively regulate heterotrimeric G protein-coupled receptor signaling pathways by enhancing endogenous GTPase activities of G protein alpha subunits. RGSZ1, one of the RGS proteins specifically localized to the brain, has been cloned previously and described as a selective GTPase accelerating protein for Galpha(z) subunit. Here, we employed several methods to provide new evidence that RGSZ1 interacts not only with Galpha(z,) but also with Galpha(i), as supported by in vitro binding assays and functional studies. Using glutathione S-transferase fusion protein pull-down assays, glutathione S-transferase-RGSZ1 protein was shown to bind (35)S-labeled Galpha(i1) protein in an AlF(4)(-)dependent manner. The interaction between RGSZ1 and Galpha(i) was confirmed further by co-immunoprecipitation studies and yeast two-hybrid experiments using a quantitative luciferase reporter gene. Extending these observations to functional studies, RGSZ1 accelerated endogenous GTPase activity of Galpha(i1) in single-turnover GTPase assays. Human RGSZ1 functionally regulated GPA1 (a yeast Galpha(i)-like protein)-mediated yeast pheromone response when expressed in a SST2 (yeast RGS protein) knockout strain. In PC12 cells, transfected RGSZ1 blocked mitogen-activated protein kinase activity induced by UK14304, an alpha(2)-adrenergic receptor agonist. Furthermore, RGSZ1 attenuated D2 dopamine receptor agonist-induced serum response element reporter gene activity in Chinese hamster ovary cells. In summary, these data suggest that RGSZ1 serves as a GTPase accelerating protein for Galpha(i) and regulates Galpha(i)-mediated signaling, thus expanding the potential role of RGSZ1 in G protein-mediated cellular activities.

Association and colocalization of G protein alpha subunits and Purkinje cell protein 2 (Pcp2) in mammalian cerebellum

Previously, we have demonstrated a novel interaction between Galpha(o) protein and Purkinje cell protein-2 (Pcp2, also known as L7) in vitro and in transfected cells (Luo and Denker [1999] J. Biol. Chem. 274:10685-10688). Pcp2 is uniquely expressed in cerebellar Purkinje cells and in retinal bipolar neurons, and it may function as a cell-type specific modulator for G protein-mediated cell signaling. This interaction has been further evaluated in the present studies. Coimmunoprecipitation experiments reveal that Pcp2 associates with Galpha(o) in vivo in mouse cerebellum and eye extract. Pcp2 also associate with Galpha(i2) in the cerebellum. No detectable associations of Pcp2 with Galpha(z) and Galpha(q) subunits are observed. The association of Galpha(o) and Pcp2 is detected at postnatal day 1 (P1), and the association remains stable from day 3 (P3) until adulthood. Further, immunofluorescent double labeling and confocal microscopy suggest that Pcp2 and Galpha(o) are colocalized in the distal processes of cerebellar Purkinje cells including axonal endings and dendritic spines. Taken together, these findings indicate colocalization and association of Galpha(o) and Pcp2 in cerebellum and suggest a functional role in regions of synaptic activity.

Calcium-induced alterations in mitochondrial morphology quantified in situ with optical scatter imaging

Optical scatter imaging (OSI), a technique we developed recently, was used to measure the ratio of wide-to-narrow angle scatter (OSIR) within endothelial cells subjected to calcium overload (1.6 mM) after permeabilization by ionomycin. Within a few minutes of calcium overload, the mitochondria, which started as elongated organelles, rounded up into spherically shaped particles. This change in morphology was accompanied by a statistically significant 14% increase in OSIR in the cells' cytoplasm. Mitochondrial rounding and OSIR increase were suppressed by cyclosporin A (25 microM), implying that the observed geometrical and scattering changes were directly attributable to the mitochondrial permeability transition. The angular scattering properties of a long mitochondrion rounding up were approximated by numerical simulations of light scatter from an ellipsoid rounding up into a sphere. The simulations predicted a relative increase in OSIR comparable to that measured experimentally for the case where the shape transition takes place with little or no volume increase. The simulations also suggested that mitochondrial refractive index changes could not account for the OSIR changes observed. Our data show that changes in OSIR correlate with mitochondrial morphology change in situ. OSI provides a new tool for subcellular imaging and complements other microscopy methods, such as fluorescence.

Oncocytic adenocarcinoma of the stomach: parietal cell carcinoma

We report 10 cases of an unusual type of gastric adenocarcinoma that occurred in elderly patients 58-81 years of age. Histologically, the tumors were well to moderately differentiated tubular adenocarcinomas with very eosinophilic, finely granular cytoplasm. Immunohistochemical stains for antimitochondrial antibody were strongly positive. Ultrastructurally, the tumor cells had numerous mitochondria in their cytoplasm and occasional intracytoplasmic lumina with associated long microvilli. These histologic and ultrastructural features are similar to those of parietal cells in normal gastric fundic mucosa, but immunohistochemical staining of the tumors using four different antiparietal cell antibodies (anti-H(+)-K(+)-adenosine triphosphatase antibodies) was negative in all cases. Therefore, we think that these tumors were not parietal cell carcinomas but could be termed oncocytic adenocarcinomas, or adenocarcinomas with oncocytic differentiation. Previously reported cases of parietal cell carcinoma have been said to have a favorable prognosis, but it will be necessary to study a larger number of cases to determine the prognosis of oncocytic adenocarcinoma.

Immunohistochemical detection of hepatocellular carcinoma in the setting of ongoing necrosis after radiofrequency ablation

After radiofrequency ablation (RFA), hepatocellular carcinoma undergoes complete necrosis and an ongoing necrosis that is irreversible and characterized histologically by disrupted cell outlines, homogenous cytoplasmic eosinophilia, and preserved nuclear staining, with the cells appearing quite distinct from viable cancer cells. Antibody to detect single-stranded DNA (ssDNA) specifically labeled nuclei in the setting of ongoing necrosis, but not viable tumor cells, whereas human mitochondrial antibody labeled the cytoplasm of viable cells but not cells of ongoing necrosis. The results demonstrate that RFA causes denaturation of both DNA and proteins and that the immunohistochemistry of ssDNA and mitochondrial protein is useful in detection of ongoing necrosis after RFA and provides pathological information on the validity of this procedure.

Age-related changes in the localization of presenilin-1 in cynomolgus monkey brain

Age-related changes in PS-1 localization were examined in the brains of 22 cynomolgus monkeys ranging in age from embryonic day 87 to 35 years. In embryonic monkey brains, anti-PS-1 antibody N12, which recognizes the PS-1 N-terminal fragment (Ntf) and holo protein, stained immature neuronal cells. In juvenile monkeys, N12 stained large pyramidal neurons, cerebral neocortical neurons, and cerebellar Purkinje's cells. Cytoplasmic staining of these cells was granular in appearance. In aged monkeys, N12 stained neurons in all layers of the neocortex. In contrast, regardless of the age of the animals examined, M5, an anti-PS-1 antibody that specifically recognizes only the PS-1 C-terminal fragment (Ctf), stained neurons in all layers of the neocortex and neurons in the cerebellum. M5 also stained neuropil and white matter, and in aged monkeys, M5 stained swollen neurites of mature senile plaques. Age-related changes in PS-1 expression were further examined using Western blot analysis of mitochondrial, myelin, microsomal, nuclear, synaptosomal, and cytosol fractions isolated from 10 monkey brains ranging in age from embryonic day 87 to 32 years. In all brains, Ntf and Ctf were expressed most abundantly in the microsome fraction. The amount of PS-1 in the nuclear fraction dramatically increased with age. We conclude that the transport of PS-1 diminished with age and that PS-1 fragments accumulated in endoplasmic reticulum (ER) associated with the nuclear membrane.

Stimulation by N6-Cyclopentyladenosine of A1 Adenosine Receptors, Coupled to Gαi2 Protein Subunit, Has a Capacitative Effect on Human Spermatozoa1

The effects of selective A(1) receptor agonist on human spermatozoa were examined to verify physiological responses and to investigate the signal transduction pathway. N6-Cyclopentyladenosine on uncapacitated spermatozoa did not induce spontaneous acrosome reaction after 5 h capacitation, whereas the number of capacitated spermatozoa, assessed by lysophosphatidylcholine-induced acrosome reaction with Pisum sativum agglutinin staining, was significantly increased. N6-Cyclopentyladenosine was also added to capacitated human spermatozoa to find out whether the agonist could induce the acrosome reaction. Results, although statistically significant, could not be considered biologically significant. A1-Mediated capacitation was followed by the increase of tyrosine phosphorylation of a protein subset ranging between M(r) = 200 000 and 30 000. Stimulation of A1 receptor with the selective agonist elicited an agonist-induced inositol phospholipid hydrolysis leading to a transient rise of inositol triphosphate (IP3). This increase was not induced by A(1) receptor antagonist and was blocked by phospholipase C inhibitor. Coimmunoprecipitation experiments showed that the A(1) receptor is coupled to Galphai2 subunit suggesting that the activation of phospholipase C is mediated by betagamma subunits. In conclusion, the A(1) adenosine receptor in human spermatozoa is coupled to Galphai2, signals via IP3, and affects the capacitative status of ejaculated spermatozoa.

Cloning and mitochondrial localization of full-length D-AKAP2, a protein kinase A anchoring protein

Differential compartmentalization of signaling molecules in cells and tissues is being recognized as an important mechanism for regulating the specificity of signal transduction pathways. A kinase anchoring proteins (AKAPs) direct the subcellular localization of protein kinase A (PKA) by binding to its regulatory (R) subunits. Dual specific AKAPs (D-AKAPs) interact with both RI and RII. A 372-residue fragment of mouse D-AKAP2 with a 40-residue C-terminal PKA binding region and a putative regulator of G protein signaling (RGS) domain was previously identified by means of a yeast two-hybrid screen. Here, we report the cloning of full-length human D-AKAP2 (662 residues) with an additional putative RGS domain, and the corresponding mouse protein less the first two exons (617 residues). Expression of D-AKAP2 was characterized by using mouse tissue extracts. Full-length D-AKAP2 from various tissues shows different molecular weights, possibly because of alternative splicing or posttranslational modifications. The cloned human gene product has a molecular weight similar to one of the prominent mouse proteins. In vivo association of D-AKAP2 with PKA in mouse brain was demonstrated by using cAMP agarose pull-down assay. Subcellular localization for endogenous mouse, rat, and human D-AKAP2 was determined by immunocytochemistry, immunohistochemistry, and tissue fractionation. D-AKAP2 from all three species is highly enriched in mitochondria. The mitochondrial localization and the presence of RGS domains in D-AKAP2 may have important implications for its function in PKA and G protein signal transduction.

Regulation of galpha i palmitoylation by activation of the 5-hydroxytryptamine-1A receptor

Nearly all alpha subunits of heterotrimeric GTP-binding regulatory proteins (G proteins) are palmitoylated at cysteine residues near the N terminus. A regulated cycle of palmitoylation could provide a mechanism for modulating G protein signaling by affecting protein interactions and localization of the subunit. In the present studies we utilized both [(3)H]palmitate incorporation and pulse-chase techniques to address the dynamics of alpha(i) palmitoylation in Chinese hamster ovary cells. Both techniques demonstrated a dose- and time-dependent change in [(3)H]palmitate labeling of alpha(i) upon activation of stably expressed 5-hydroxytryptamine-1A receptors by the agonist (+/-)-2-dipropylamino-8-hydroxy-1,2,3, 4-tetrahydronaphthalene hydrobromide (DPAT), with an EC(50) of approximately 10 nm. For the incorporation assay, DPAT elicited an approximate doubling in labeling at the earliest time point measured. For the pulse-chase assay, DPAT promoted a significant loss of radiolabel almost equally as fast. These data demonstrate that the exchange of palmitate on alpha(i) is increased upon stimulation of 5-hydroxytryptamine-1A receptors through the combined processes of depalmitoylation and palmitoylation. These results provide the basis for extending the concept of regulated exchange of palmitate beyond G(s) and provide a framework for exploring the specific functional attributes of the palmitoylated and depalmitoylated forms of subunit.

Heparin-binding protein targeted to mitochondrial compartments protects endothelial cells from apoptosis

Neutrophil-borne heparin-binding protein (HBP) is a multifunctional protein involved in the progression of inflammation. HBP is stored in neutrophil granules and released upon stimulation of the cells in proximity to endothelial cells. HBP affects endothelial cells in multiple ways; however, the molecular and cellular mechanisms underlying the interaction of HBP with these cells are unknown. Affinity isolation and enzymatic degradation demonstrated that HBP released from human neutrophils binds to endothelial cell-surface proteoglycans, such as syndecans and glypican. Flow cytometry indicated that a significant fraction of proteoglycan-bound HBP is taken up by the endothelial cells, and we used radiolabeled HBP to determine the internalization rate of surface-bound HBP. Confocal and electron microscopy revealed that internalized HBP is targeted to perinuclear compartments of endothelial cells, where it colocalizes with mitochondria. Western blotting of isolated mitochondria from HBP-treated endothelial cells showed that HBP is present in 2 forms - 28 and 22 kDa. Internalized HBP markedly reduced growth factor deprivation-induced caspase-3 activation and protected endothelial cells from apoptosis, suggesting that uptake and intracellular routing of exogenous HBP to mitochondria contributes to the sustained viability of endothelial cells in the context of locally activated neutrophils.

G protein alpha subunits activate tubulin GTPase and modulate microtubule polymerization dynamics

G proteins serve many functions involving the transfer of signals from cell surface receptors to intracellular effector molecules. Considerable evidence suggests that there is an interaction between G proteins and the cytoskeleton. In this report, G protein alpha subunits Gi1alpha, Gsalpha, and Goalpha are shown to activate the GTPase activity of tubulin, inhibit microtubule assembly, and accelerate microtubule dynamics. Gialpha inhibited polymerization of tubulin-GTP into microtubules by 80-90% in the absence of exogenous GTP. Addition of exogenous GTP, but not guanylylimidodiphosphate, which is resistant to hydrolysis, overcame the inhibition. Analysis of the dynamics of individual microtubules by video microscopy demonstrated that Gi1alpha increases the catastrophe frequency, the frequency of transition from growth to shortening. Thus, Galpha may play a role in modulating microtubule dynamic instability, providing a mechanism for the modification of the cytoskeleton by extracellular signals.

ADP-induced platelet aggregation and inhibition of adenylyl cyclase activity stimulated by prostaglandins: signal transduction mechanisms

ADP is the oldest and one of the most important agonists of platelet activation. ADP induces platelet shape change, exposure of fibrinogen binding sites, aggregation, and influx and intracellular mobilization of Ca2+. ADP-induced platelet aggregation is important for maintaining normal hemostasis, but aberrant platelet aggregation manifests itself pathophysiologically in myocardial ischemia, stroke, and atherosclerosis. Another important aspect of ADP-induced platelet activation is the ability of ADP to antagonize adenylyl cyclase activated by prostaglandins. ADP-induced inhibition of the stimulated adenylyl cyclase activity does not appear to play a role in ADP-induced platelet aggregation in vitro or in vivo. It is believed that a single ADP receptor mediates the above two ADP-induced platelet responses in platelets. The ADP receptor mediating ADP-induced platelet aggregation and inhibition of the stimulated adenylyl cyclase activity has not been purified. Therefore, the nature of molecular mechanisms underlying the two seemingly unrelated ADP-induced platelet responses remains either unclear or less well understood. The purpose of this commentary is to examine and make suggestions concerning the role of phospholipases and G-proteins in the molecular mechanisms of signal transduction underlying the two ADP-induced platelet responses. It is hoped that such discussion would stimulate thinking and invite future debates on this subject, and energize investigators in their efforts to advance our knowledge of the details of the molecular mechanisms of ADP-induced platelet activation.

G protein beta subunit is closely associated with microtubules

Previously, we have identified the association of G protein beta subunit (Gbeta) with mitotic spindles in various mammalian cells. Since microtubules are the main component of mitotic spindles, here we have isolated bovine brain microtubules and purified Gbeta subunit to identify the close association of Gbeta subunit with purified brain microtubules and have shown the direct incorporation of Gbeta subunit into the microtubules both in vitro and in vivo. It was found that: (1) microtubular fraction isolated from bovine brain contained Gbeta subunit, (2) coimmunoprecipitation demonstrated that Gbeta subunit could be coprecipitated with tubulin, (3) addition of purified Gbeta subunit into cytosolic extract for microtubule assembly caused direct incorporation of Gbeta subunit into assembled microtubules and increased the association of microtubule-associated proteins with microtubules, and (4) incubation of exogenous Gbeta subunit with detergent-permeabilized cells resulted in direct incorporation of Gbeta subunit into microtubule fibers and depolymerized tubulin molecules. We conclude that G protein beta subunit is closely associated with microtubules and may play an important role in the regulation of microtubule formation in addition to its regulatory role in cellular signal transduction.

Heterotrimeric Gi protein is associated with the inositol 1,4,5-trisphosphate receptor complex and modulates calcium flux

In vascular smooth muscle, pertussis toxin (PT) inhibits thrombin-induced Ca2+ release by a mechanism independent of its effect on IP3 formation. Thus, the possibility of a direct role of G alpha i proteins in regulating IP3-sensitive Ca2+ release was investigated by examining whether G alpha i proteins are associated with the IP3 receptor complex. Purified microsomal membranes were prepared and separated by sucrose density gradient centrifugation. The relative density of [3H]-IP3 binding sites between the microsomal fractions was inversely related to the distribution of the plasma membrane marker. The relative distribution of G alpha i3 determined by immunoblotting was closely correlated with the density of [3H]-IP3 binding. Levels of G alpha i2 were more evenly distributed with highest levels present in plasma membrane-enriched fractions. IP3 receptor immunoprecipitated from triton-solubilized microsomal membranes contained G alpha i3 immunoreactivity. To determine whether G alpha i proteins influence IP3-induced Ca2+ release, the effect of PT on Ca2+ release from digitonin-permeabilized cell suspensions using Fluo-3 was examined. Exposure to PT (0.1 microgram/ml, 5 min) attenuated the initial rate of IP3 (1 microM)-induced Ca2+ release. Together, these findings are consistent with the hypothesis that a heterotrimeric G alpha i protein directly regulates IP3-dependent Ca2+ release.

G protein beta1gamma2 subunits promote microtubule assembly

alpha and betagamma subunits of G proteins are thought to transduce signals from cell surface receptors to intracellular effector molecules. Galpha and Gbetagamma have also been implicated in cell growth and differentiation, perhaps due to their association with cytoskeletal components. In this report Gbetagamma is shown to modulate the cytoskeleton by regulation of microtubule assembly. Specificity among betagamma species exists, as beta1gamma2 stimulates microtubule assembly, and beta1gamma1 is without any effect. Furthermore, a mutant beta1gamma2, beta1gamma2(C68S), which does not undergo prenylation and subsequent carboxyl-terminal processing on the gamma subunit, does not stimulate the formation of microtubules. beta immunoreactivity was detected exclusively in the microtubule fraction after assembly in the presence of beta1gamma2, suggesting a preferential association with microtubules rather than soluble tubulin. Crude microtubule fractions from ovine brain contain Gbetagamma, and electron microscopy reveals a specific association with microtubules. The decoration of microtubules by Gbetagamma appears to be strikingly similar to the periodic pattern observed for microtubule-associated proteins, suggesting a similar site of activation of microtubule assembly by both agents. It is suggested that reformation of the cytoskeleton represents an additional cellular process mediated by Gbetagamma.

Organization of G proteins and adenylyl cyclase at the plasma membrane

There is mounting evidence for the organization and compartmentation of signaling molecules at the plasma membrane. We find that hormone-sensitive adenylyl cyclase activity is enriched in a subset of regulatory G protein-containing fractions of the plasma membrane. These subfractions resemble, in low buoyant density, structures of the plasma membrane termed caveolae. Immunofluorescence experiments revealed a punctate pattern of G protein alpha and beta subunits, consistent with concentration of these proteins at distinct sites on the plasma membrane. Partial coincidence of localization of G protein alpha subunits with caveolin (a marker for caveolae) was observed by double immunofluorescence. Results of immunogold electron microscopy suggest that some G protein is associated with invaginated caveolae, but most of the protein resides in irregular structures of the plasma membrane that could not be identified morphologically. Because regulated adenylyl cyclase activity is present in low-density subfractions of plasma membrane from a cell type (S49 lymphoma) that does not express caveolin, this protein is not required for organization of the adenylyl cyclase system. The data suggest that hormone-sensitive adenylyl cyclase systems are localized in a specialized subdomain of the plasma membrane that may optimize the efficiency and fidelity of signal transduction.

Role of lipid polymorphism in G protein-membrane interactions: nonlamellar-prone phospholipids and peripheral protein binding to membranes

Heterotrimeric G proteins (peripheral proteins) conduct signals from membrane receptors (integral proteins) to regulatory proteins localized to various cellular compartments. They are in excess over any G protein-coupled receptor type on the cell membrane, which is necessary for signal amplification. These facts account for the large number of G protein molecules bound to membrane lipids. Thus, the protein-lipid interactions are crucial for their cellular localization, and consequently for signal transduction. In this work, the binding of G protein subunits to model membranes (liposomes), formed with defined membrane lipids, has been studied. It is shown that although G protein alpha-subunits were able to bind to lipid bilayers, the presence of nonlamellar-prone phospholipids (phosphatidylethanolamines) enhanced their binding to model membranes. This mechanism also appears to be used by other (structurally and functionally unrelated) peripheral proteins, such as protein kinase C and the insect protein apolipophorin III, indicating that it could constitute a general mode of protein-lipid interactions, relevant in the activity and translocation of some peripheral (amphitropic) proteins from soluble to particulate compartments. Other factors, such as the presence of cholesterol or the vesicle surface charge, also modulated the binding of the G protein subunits to lipid bilayers. Conversely, the binding of G protein-coupled receptor kinase 2 and the G protein beta-subunit to liposomes was not increased by hexagonally prone lipids. Their distinct interactions with membrane lipids may, in part, explain the different cellular localizations of all of these proteins during the signaling process.

Two-dimensional distribution of Gi2alpha in the plasma membrane: a critical evaluation by immunocytochemistry

Caveolae have been postulated as a center for signal transduction, because many signaling molecules are concentrated in caveolin-rich fractions. We took Gi2alpha as an example and examined whether it is constitutively concentrated in caveolae. First, the behavior of caveolin and Gi2alpha in density-equilibrium ultracentrifugation was reexamined. By collecting fractions efficiently, caveolin and Gi2alpha were found to distribute differently. Secondly, by novel immunocytochemical methods it was found that the labeling density of Gi2alpha was 2.29 times higher in caveolae than in the non-caveolar plasma membrane. The results indicate that the concentration of Gi2alpha in caveolae is lower than deduced from most biochemical studies.

Subcellular localization of G-proteins in primary-cultured mouse preadipocytes and adipocytes

The subcellular localization of G5 alpha, Gi alpha 1&2, Gi alpha 3, and G beta was studied in primary-cultured undifferentiated and differentiated, lipid replete, adipose cells. The results show a distinct distribution for each of these G-proteins and differences between differentiated and undifferentiated cells. All the G-proteins examined had a cytoplasmic localization; only Gi alpha 1 and 2 showed a significant colocalization with the plasma membrane and this only in differentiated cells. Most studies using cells in culture have reported an intracellular localization for G-proteins, whereas in tissue sections the localization has been reported to be largely with the plasma membrane, with some intracellular localization. The results suggest that the cell-cell interactions or the specific geometry imposed by culture conditions favor the intracellular compared to peripheral localization of G-proteins. Alternately, the posttranslational modifications necessary for G-protein insertion in the plasma membrane may be deficient in cultured cells.

Multiple G-protein coupling of the dog thyrotropin receptor

We investigated, in dog thyroid membranes, the ability of the dog thyrotropin (TSH) receptor to interact with the endogenous G proteins expressed in this tissue. Activation of the receptor led to increased incorporation of the photoreactive GTP analog [alpha-(32)P]GTP azidoanilide into immunoprecipitated alpha subunits of three G protein families: G(s), G(q/11), G(i/o). This effect was not due to a general loss of receptor G protein specificity since carbamylcholine, in the same membrane preparations, only stimulated the binding of the GTP analog to the alpha subunits of G(q/11) proteins. To investigate the multiple coupling of the dog TSH receptor in intact cells, cyclic AMP accumulation, IP(3) formation and (45)Ca2+ efflux experiments were performed. When thyrocytes were pretreated with pertussis toxin (PTX), the TSH receptor-mediated accumulation of cAMP increased by approximately 45% with TSH at 1 mU/ml, suggesting that the TSH receptor coupled to both G(s) and G(i) in vivo. On the other hand, no increase in IP(3) accumulation nor Ca2+ efflux was observed in the presence of thyrotropin. These data in intact cells are thus in contradiction with those obtained in membranes, suggesting that receptor-mediated transmembrane signalling may implicate a specificity which itself may reflect a localization and organization of the different components (receptors, G proteins, ...) in the plasma membrane of intact cells. As in some cells, G(i) activates mitogenesis by hormone activated G-protein-coupled receptors, we tested its role in the stimulation by TSH of the proliferation of thyrocytes. This was not affected by PTX, suggesting that the mitogenic effect of TSH does not involve G(i)-proteins.

Comparison of the subcellular distribution of G-proteins in hepatocytes in situ and in primary cultures

The subcellular localization of the heterotrimeric G-proteins in hepatocytes in situ was compared to that in hepatocytes in primary culture. The ability of various ligands to activate adenylyl cyclase (AC) in membrane preparations was also investigated. In hepatocytes in situ the G proteins were mainly localized at the plasma membrane while in hepatocytes in culture they were predominantly cytoplasmic. The localization of the G-proteins in hepatocytes in situ correlates with their role in signal transduction. In homogenates prepared from the cultured cells, ligands which stimulate AC via Gs alpha were without effect, which was consistent with the localization of Gs alpha in the cytoplasmic and nuclear compartments. The "relocalization" of the G proteins to the cytoplasm when cells are cultured suggests that transmembrane signalling may be regulated by cell differentiation and cell-cell and cell-extracellular matrix interactions.

Mechanism of inhibition of calcium channels in rat nucleus tractus solitarius by neurotransmitters

1. High-threshold Ca2+ channel currents were measured every 15 s following a 200 ms voltage step from -80 mV to 0 mV in order to study the coupling mechanism between neurotransmitter receptors and Ca2+ channels in neurones acutely isolated from the nucleus tractus solitarius (NTS) of the rat. 2. Application of 30 microM baclofen (GABAB receptor agonist) caused 38.9 +/- 1.2% inhibition of the peak inward Ba2+ current (IBa2+) in most NTS cells tested (n = 85 of 88). Somatostatin, 300 nM, also reduced IBa2+ by 31.3 +/- 1.6% in 53 cells of 82 tested. 3. Activation of mu-opioid-, GABAB- or somatostatin-receptors inhibited both N- and P/Q-type Ca2+ channels. 4. The inhibition of Ca2+ currents by DAMGo (mu-opioid receptor agonist), baclofen and somatostatin was reduced by treatment with pertussis toxin and partially relieved by application of a 50 ms conditioning prepulse to +80 mV. This suggests that a pertussis toxin-sensitive G-protein was involved in the neurotransmitter-mediated action in the observed inhibition of Ca2+ currents. 5. Intracellular loading with an antiserum raised against the amino terminus of Go alpha (GC/2) markedly attenuated the somatostatin-induced inhibition, but did not block the DAMGO- and baclofen-induced inhibition. 6. These findings suggest at least two different pertussis toxin-sensitive G-protein-mediated pathways are involved in receptor-induced inhibition of Ca2+ currents in the NTS.

A novel regulatory mechanism for trimeric GTP-binding proteins in the membrane and secretory granule fractions of human and rodent beta cells

Recently we described roles for heterotrimeric and low-molecular-mass GTP-binding proteins in insulin release from normal rat islets. During these studies, we observed that a protein with an apparent molecular mass (37 kDa) similar to that of the beta subunit of trimeric GTP-binding proteins underwent phosphorylation in each of five classes of insulin-secreting cells. Incubation of the beta cell total membrane fraction or the isolated secretory granule fraction (but not the cytosolic fraction) with [gamma-32P]ATP or [gamma-32P]GTP resulted in the phosphorylation of this protein, which was selectively immunoprecipitated by an anti-serum directed against the common beta subunit of trimeric G-proteins. Disruption of the alpha beta gamma trimer (by pretreatment with either fluoroaluminate or guanosine 5'(-)[gamma-thio]triphosphate) prevented beta subunit phosphorylation. Based on differential sensitivities to pH, heat and the histidine-selective reagent diethyl pyrocarbonate (and reversal of the latter by hydroxylamine), the phosphorylated amino acid was presumptively identified as histidine. Incubation of pure beta subunit alone or in combination with the exogenous purified alpha subunit of transducin did not result in the phosphorylation of the beta subunit, but addition of the islet cell membrane fraction did support this event, suggesting that membrane localization (or a membrane-associated factor) is required for beta subunit phosphorylation. Incubation of phosphorylated beta subunit with G alpha.GDP accelerated the dephosphorylation of the beta subunit, accompanied by the formation of G alpha-GTP. Immunoblotting detected multiple alpha subunits (of Gi, G(o) and Gq) and at least one beta subunit in the secretory granule fraction of normal rat islets and insulinoma cells. These data describe a potential alternative mechanism for the activation of GTP-binding proteins in beta cells which contrasts with the classical receptor-agonist mechanism: G beta undergoes transient phosphorylation at a histidine residue by a GTP-specific protein kinase; this phosphate, in turn, may be transferred via a classical Ping-Pong mechanism to G alpha.GDP (inactive), yielding the active configuration G alpha.GTP in secretory granules (a strategic location to modulate exocytosis).

Disruption of cellular signaling pathways by daunomycin through destabilization of nonlamellar membrane structures

Albeit anthracyclines are widely used in the treatment of solid tumors and leukemias, their mechanism of action has not been elucidated. The present study gives relevant information about the role of nonlamellar membrane structures in signaling pathways, which could explain how anthracyclines can exert their cytocidal action without entering the cell [Tritton, T. R. & Yee, G. (1982) Science 217, 248-250]. The anthracycline daunomycin reduced the formation of the nonlamellar hexagonal (HII) phase (i.e., the hexagonal phase propensity), stabilizing the bilayer structure of the plasma membrane by a direct interaction with membrane phospholipids. As a consequence, various cellular events involved in signal transduction, such as membrane fusion and membrane association of peripheral proteins [e.g., guanine nucleotide-binding regulatory proteins (G proteins and protein kinase C-alpha beta)], where nonlamellar structures (negative intrinsic monolayer curvature strain) are required, were altered by the presence of daunomycin. Functionally, daunomycin also impaired the expression of the high-affinity state of a G protein-coupled receptor (ternary complex for the alpha 2-adrenergic receptor) due to G-protein dissociation from the plasma membrane. In vivo, daunomycin also decreased the levels of membrane-associated G proteins and protein kinase C-alpha beta in the heart. The occurrence of such nonlamellar structures favors the association of these peripheral proteins with the plasma membrane and prevents daunomycin-induced dissociation. These results reveal an important role of the lipid component of the cell membrane in signal transduction and its alteration by anthracyclines.

The heterotrimeric G-protein Gi is localized to the insulin secretory granules of beta-cells and is involved in insulin exocytosis

Mastoparan, a tetradecapeptide found in wasp venom that stimulates G-proteins, increases insulin secretion from beta-cells. In this study, we have examined the role of heterotrimeric G-proteins in mastoparan-induced insulin secretion from the insulin-secreting beta-cell line beta-TC3. Mastoparan stimulated insulin secretion in a dose-dependent manner from digitonin-permeabilized beta-TC3 cells. Active mastoparan analogues mastoparan 7, mastoparan 8, and mastoparan X also stimulated secretion. Mastoparan 17, an inactive analogue of mastoparan, did not increase insulin secretion from permeabilized beta-TC3 cells. Mastoparan-induced insulin secretion from permeabilized beta-TC3 cells was inhibited by pretreatment of the cells with pertussis toxin, suggesting that mastoparan-induced insulin secretion is mediated through a pertussis toxin-sensitive G-protein present distally in exocytosis. Enriched insulin secretory granules (ISG) were prepared by sucrose/nycodenz ultracentrifugation. Western immunoblotting performed on beta-TC3 homogenate and ISG demonstrated that G alpha i was dramatically enriched in ISG. Levels of G alpha o and G alpha q were comparable in homogenate and ISG. Mastoparan stimulated ISG GTPase activity in a pertussis toxin-sensitive manner. Mastoparan 7 and mastoparan 8 also stimulated GTPase activity in the ISG, while the inactive analogue mastoparan 17 had no effect. Selective localization of G alpha i to ISG was confirmed with electron microscopic immunocytochemistry in beta-TC3 cells and beta-cells from rat pancreas. In contrast to G alpha o and G alpha q, G alpha was clearly localized to the ISG. Together, these data suggest that mastoparan may act through the heterotrimeric G-protein G alpha i located in the ISG of beta-cells to stimulate insulin secretion.

Muscarinic regulation of Alzheimer's disease amyloid precursor protein secretion and amyloid beta-protein production in human neuronal NT2N cells

The Alzheimer amyloid precursor protein (APP) undergoes complex processing resulting in the production of a 4-kDa amyloid peptide (A beta) which has been implicated in the pathogenesis of Alzheimer's disease. Recent studies have shown that cells can secrete carboxyl terminus truncated APP derivatives (APP-S) in response to physiological stimulus. We have used human central nervous system neurons (NT2N) derived from a teratocarcinoma cell line (NT2) to study the signal transduction pathways involved in APP-S secretion and A beta production. Muscarinic receptors (m2 and m3) as well as the heterotrimeric GTP-binding protein Gq and the beta 1 isoform of phospholipase C were present in NT2N neurons. Stimulation of the muscarinic receptor with carbachol resulted in phospholipase C activation as shown by a transient increase in the second messengers 1,2-diacyl-sn-glycerol and inositol 1,4,5-trisphosphate. Carbachol also caused an increase in intracellular Ca2+ levels measured in single NT2N neurons. Under these conditions, carbachol caused a time-dependent 2-fold increase in APP-S secretion into the medium. In contrast, prolonged treatment with carbachol caused a decrease in A beta production into the medium. These results suggest that APP-S secretion and A beta production in NT2N neurons are regulated by the muscarinic/phospholipase C signal transduction pathway. Furthermore, activation of this pathway results in dissociation of APP-S secretion and A beta production.

Lateral mobility of tetramethylrhodamine (TMR) labelled G protein alpha and beta gamma subunits in NG 108-15 cells

Multi-step signal transducing events, such as those mediated by G proteins, have been difficult to study in intact cells. We prepared fluorescently labelled G protein subunits, tetramethylrhodamine-alpha o (TMR-alpha o) and TMR-beta gamma, in order to study their subcellular distribution and lateral mobility. Heterotrimeric G proteins labelled in the alpha (TMR-alpha o/beta gamma) or beta (TMR-beta gamma/alpha o) subunit were reconstituted into lipid vesicles and fused to NG-108-15 cells using polyethylene glycol (PEG). Vesicles fused completely to the cells as determined by dequenching of a fluorescent lipid probe, octadecyl rhodamine B. The orientation of G protein beta gamma subunits after fusion followed the expected random distribution; the quenching of surface fluorescence with anti-fluorescein antibodies showed that about 50% of the label was accessible extracellularly. G proteins incorporated by the fusion method were able to couple to endogenous alpha 2 adrenergic receptors based on the restoration of high affinity agonist binding to pertussis toxin-treated cells. The subcellular localization of TMR-alpha o and TMR-beta gamma determined by differential centrifugation and confocal microscopy indicated that TMR-alpha o was present in the plasma membrane and in intracellular membranes, whereas TMR-beta gamma was mainly localized in the plasma membrane. The lateral mobility of TMR-alpha o and TMR-beta gamma measured using fluorescence recovery after photobleaching (FRAP) demonstrated low mobile fractions of 0.34 +/- 0.03 and 0.16 +/- 0.03, respectively. The translational diffusion coefficients of the mobile components were similar, 4.0 x 10(-9) and 2.0 x 10(-9) cm2/s, for alpha and beta gamma respectively. Neither activation of Gi-linked receptors nor cytoskeletal disruption with nocodozole or cytochalasin D changed the mobile fraction or diffusion coefficient of the alpha or beta gamma subunits. The FRAP data combined with the localization of fluorescent subunits by confocal microscopy suggest that the beta gamma subunits are highly constrained to localized regions of the plasma membrane while the alpha subunit may diffuse in intracellular regions to transmit signals from receptors to effector proteins.

Anti-tumor antibody BR96 blocks cell migration and binds to a lysosomal membrane glycoprotein on cell surface microspikes and ruffled membranes

BR 96 is an internalizing antibody that binds to Lewis Y (Le(y)), a carbohydrate determinant expressed at high levels on many human carcinomas (Hellström, I., H. J. Garrigues, U. Garrigues, and K. E. Hellström. 1990. Cancer Res. 50:2183-2190). Breast carcinoma cell lines grown to confluence bind less BR96 than subconfluent cultures (Garrigues, J., U. Garrigues, I. Hellström, and K. E. Hellström. 1993. Am. J. Path. 142:607-622). However, when the confluent cells are induced to migrate by scratch wounding, they again bind BR96 suggesting that antigens bearing the Le(y) determinant may promote cell migration. In the present study, BR96 was found to be highly enriched on microspikes and ruffled membranes, cell surface structures involved in cell migration. In addition, BR96 was a potent inhibitor of cell migration in vitro. When stationary BR96 treated cells were exposed to fresh culture media, membrane ruffles and microspikes developed at the cell margin and migration resumed. Immunogold microscopy showed that BR96 antigens were enriched on these membrane protrusions. BR96 cell surface immunoprecipitation analysis of 3H-glucosamine labeled breast carcinoma cells identified antigens with approximate molecular weights of 135 kd (upper antigen) and 85 kd (lower antigen). A short amino terminal sequence (8 residues) of the upper antigen matched that of human lysosomal membrane glycoprotein 1 (LAMP-1). In addition, the upper antigen was detected on immunoblots probed with anti-LAMP-1, and within the intracellular compartment BR96 was found predominantly in endosomes and lysosomes. A soluble LAMP-1/immunoglobulin fusion protein (LAMP-1/Ig) was transiently expressed in both BR96 binding and nonbinding cell lines. Immunoblot analysis of LAMP-1/Ig's from the various cell lines showed that (a) acquisition of the BR96 epitope is probably controlled at the level of polylactosamine modification (e.g., fucosylation) rather than LAMP-1 gene expression; (b) alternate forms of LAMP-1/Ig comigrate with the lower BR96 antigen raising the possibility that it may be a degradation product of the upper antigen; and (c) LAMP-1/Ig expressed in 3396 breast carcinoma cells has approximately 30-fold more BR96 epitopes than LAMP-1/Ig from non-tumorigenic mammary epithelial cells. Together these data indicate that a major BR96 antigen, LAMP-1, is present on unique cell surface domains involved in cell locomotion as well as membranes of the endocytic compartment. Altered glycosylation of LAMP-1 expressed in transformed cells may contribute to their ability to disseminate.

Comparative immunocytochemical demonstration of G proteins in rat heart tissue

Localization of G proteins in the rat heart tissue was investigated using primary affinity-purified antibodies against synthetic peptides with amino acid sequences corresponding to alpha-subunits (alpha i common and alpha i 1, 2) of G proteins. Detection of immunoreactivity was performed with the peroxidase-anti-peroxidase complex (PAP), avidin-biotin complex (ABC) and fluorescein-labelled secondary antibodies for light microscopy and the protein A-gold technique for electron microscopy. In ventricles and atria, immunostaining for G proteins was detected in the sarcolemma and perinuclear space of cardiomyocytes. In endotheliocytes and fibroblasts, immunoreactivity was present also in the endoplasmic reticulum. All four immunocytochemical methods permit to demonstrate the same localization of G proteins in heart tissue. The ABC method and fluorescein labelled secondary antibodies technique showed the same sensitivity which is higher than that of the PAP method. Nomarski contrast microscopy enhanced the visualization of the final reaction product formed by the peroxidase reaction developed with diaminobenzidine in the ABC method. The results are discussed in terms of the role of G proteins in signal transduction via plasma membrane and membranes of the intracysternal space of the cell.

Demonstration by two-color flow cytometry that tyrosine kinase activity is required for down-modulation of the oncogenic neu receptor

Expression of rat oncogenic neu receptor, p185T-neu (a growth factor receptor with constitutive tyrosine kinase activity), causes cells to become transformed. Treatment with anti-neu receptor monoclonal antibodies reverts the transformed phenotype by down-modulation of p185T-neu. Monoclonal antibody treatment of cells expressing normal neu receptor, p185C-neu (which lacks constitutive tyrosine kinase activity), does not result in down-modulation of p185C-neu. To understand further the role the biochemical activity of p185T-neu plays in transformation and endocytosis, we created a series of mutations in p185T-neu. We found that fibroblasts expressing the tyrosine kinase-defective mutants cannot form foci in culture, colonies in soft agar, or tumors in immunocompromised mice. To follow the antibody-induced endocytosis of neu receptors expressed in these transfectants, we developed a novel two-color flow cytometric assay and confirmed receptor localization by electron microscopy. Cells were treated with mAb7.16.4 over time. After 4 hr of antibody treatment, less than 50% of full-length p185T-neu and of mutant T691 remained on the cell surface, whereas internal expression of the neu receptors within these cells initially increased and then decreased to the original internal receptor level. In contrast, the level of kinase-deficient mutated neu receptors remaining on the cell surface initially decreased by 35%, but, after 4 hr of antibody treatment, the cell surface expression level returned to approximately the original level. Concurrently, fluctuations in expression levels were seen internally over time as well. These cell lines were also treated with gold-conjugated mAb7.16.4. Using electron microscopy, we consistently found the gold particles within multivesicular bodies of cell lines expressing full-length or mutated neu receptor. These data strongly suggest that the fate of the neu receptor, once internalized, is directed by its tyrosine kinase activity. When the kinase activity of the neu receptor is disrupted, the receptor is internalized but recycled to the cell surface, whereas neu receptors which have constitutive kinase activity are internalized and presumably degraded when engaged with anti-neu receptor mAb. Understanding the regulation of receptor endocytosis, degradation, and recycling will contribute to the development of novel therapeutic protocols to combat human malignancies, particularly those associated with the overexpression of the human homologue of the neu receptor, c-erbB2.

Cell-specific coupling of the cloned human 5-HT1F receptor to multiple signal transduction pathways

We recently described the cloning of a fifth member of the 5-hydroxytryptamine (5-HT)1 (serotonin1) receptor class that inhibits adenylyl cyclase, namely the human 5-HT1F receptor (Adham et al. 1993a). In the present study we have examined in greater detail the functional coupling of the 5-HT1F receptor in two different cell lines, NIH-3T3 and LM(tk-) fibroblasts (receptor densities of 1.7 and 4.4 pmol/mg protein, respectively). The maximal inhibitory response elicited by 5-HT was significantly greater in NIH-3T3 as compared to LM(tk-) cells, whereas the EC50 values were comparable. To investigate the relationship between receptor occupancy and inhibition of cAMP accumulation mediated by 5-HT1F receptors in NIH-3T3 cells (and hence the degree of receptor reserve), we used the irreversible receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). The half-maximal response required only about 10% receptor occupancy, consistent with a receptor reserve of 90% (88 +/- 2.1%, n = 4) for 5-HT-induced inhibition of FSCA. Despite the presence of such a high degree of receptor reserve, a range of intrinsic activities was displayed by structurally diverse classes of compounds. For example, sumatriptan and lysergol were as efficacious as 5-HT itself and thus acted as full agonists, whereas metergoline and 1-NP behaved as partial agonists and as shown previously (Adham et al. 1993a), methiothepin was a silent antagonist (Kb = 438 nM). We have also investigated activation of additional signal transduction pathways by the 5-HT1F receptor and found that the responses differ in the two cell lines with respect to stimulation of phospholipase C. For example, in NIH-3T3 cells no elevation of inositol phosphates (IP) of [Ca2+]i was observed even at very high agonist concentrations (100 microM). In contrast, in LM(tk-) cells concentrations of 5-HT as low as 10 nM induced stimulation of IP and a rapid increase of [Ca2+]i. The 5-HT1F receptor failed to alter arachidonic acid release in either cell line. The maximal increase in IP accumulation in LM(tk-) cells was modest, averaging about 100% above basal. The increases of IP and [Ca2+]i required 5-HT concentrations less than one order of magnitude greater than those inhibiting FSCA (EC50 = 17, 55 and 8 nM, respectively), and both responses were blocked by 100 microM methiothepin. All three responses (cAMP, IP, and [Ca2+]i) were sensitive to pertussis toxin pre-treatment, suggesting the involvement of Gi/Go protein(s) in these signal transduction pathways.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification and localization of G-proteins in the clonal adipocyte cell lines HGFu and Ob17

HGFu and Ob17 are cell lines derived from adipose tissue of lean (+/?) and ob/ob mice, respectively. Neither adenylyl cyclase activity nor G protein abundance and subcellular distribution have been assessed previously in these cells. Cyclase activity was low and resistant to catecholamine stimulation in both cell lines. However, the enzyme could be stimulated to high levels by forskolin and Mn2+. Gs alpha (largely the long isoform), Gi alpha 2, and G beta were the major G protein subunits identified. The levels of G protein mRNA expression were similar in both cell lines and, unlike actin expression, did not change as a result of differentiation. Immunoblotting and ADP-ribosylation of the G peptides corroborated these results. Assessment of the subcellular localization of the subunits by indirect epifluorescence and scanning confocal microscopy showed that each of the subunits had a characteristic subcellular pattern. Gs alpha showed vesicular cytoplasmic and nuclear staining; Gi alpha 2 colocalized with actin stress fibers and disruption of these structures altered the distribution of Gi alpha 2; beta subunits showed some colocalization with the stress fibers as well as a cytoplasmic vesicular and nuclear pattern. As a result of differentiation, there was reorganization of the actin, together with the Gi alpha 2 and beta fibrous patterns. Both cell lines showed similar modifications. The induction of differentiation in these cells is therefore not associated with changes in adenylyl cyclase activity nor of the abundance of G-protein subunits, although reorganization of some of these subunits does accompany actin reorganization.

Nerve growth factor changes G protein levels and localization in PC12 cells

Growth cones at the growing tips of developing neurites contain the machinery to transmit information from receptors to a variety of intracellular enzymes and ion channels. In order to understand how signals are transmitted across the membrane, we asked whether the multiplicity of signalling pathways in the growth cone is reflected by the diversity of G proteins found in this organelle. Our immunohistochemical analysis indicated that growth cones of differentiated PC12 cells contain at least 4 alpha G protein subunits, 3 that are pertussis toxin substrates (alpha o, alpha i-1, alpha i-2) and 1 that is not (alpha q). In addition to localization in the neurites and growth cones, alpha o, alpha i-1, alpha i-2, and alpha q were detected in intracellular perinuclear structures. We also analyzed the temporal change in G proteins in PC12 cells differentiated by treatment with nerve growth factor (NGF). Time course experiments have shown that alpha o and beta proteins coordinately increase after 2 days of treatment with NGF, reach a maximum at 4 days, and remain elevated. In contrast to alpha o, alpha i-2 reached a peak at 4 days, then declined to almost the basal level by day 7 of treatment with NGF. These data indicated that the levels of alpha o, alpha i-2, and beta are differentially regulated during NGF-induced neuronal differentiation in PC12 cells. The alpha o protein was highly concentrated at the tips of the growth cones before the cellular level of alpha o had increased appreciably, suggesting that the alpha subunits are translocated during the first stage of neurite development. In addition, not every neural process has the same high level of alpha o, suggesting that G proteins may help define the specialized functions of particular neurites within a single cell.

Agonist receptors and G proteins as mediators of platelet activation

Recent studies have helped to define the earliest events of signal transduction in platelets, particularly those involved in the generation of second messengers. The best-understood of these events are those which involve guanine nucleotide binding regulatory proteins. G proteins are heterotrimers comprised of alpha, beta and gamma subunits, each of which can exist in multiple forms. Some, but not all, of the known variants of G alpha are substrates for ADP-ribosylation by pertussis toxin, a modification which disrupts the flow of information from receptor to effector. The G proteins that have been identified in platelets to date are Gs, Gi1, Gi2, Gi3, Gz and Gq. Gs and one or more of the Gi family members regulate cAMP formation by adenylylcyclase. Gi may also be responsible for the pertussis toxin-sensitive activation of phospholipase C which occurs when platelets are activated by thrombin. Gq is thought to be responsible for the pertussis toxin-resistant activation of phospholipase C by TxA2. Gz does not have an established role, but has the unique property of being phosphorylated by protein kinase C during platelet activation. Recent efforts to clone the receptors that interact with G proteins in platelets have been successful for epinephrine, thrombin, TxA2 and platelet activating factor. Each of these resembles other G protein-coupled receptors, being comprised of a single polypeptide with 7 transmembrane domains. In the case of thrombin, receptor activation is thought to involve a unique mechanism in which thrombin cleaves its receptor, creating a new N-terminus that can serve as a tethered ligand. Peptides corresponding to the tethered ligand can mimic the effects of thrombin, while antibodies to the same domain inhibit platelet activation. Shortly after activation, thrombin receptors become resistant to re-activation by thrombin. This desensitization, which appears to be due to a combination of proteolysis, phosphorylation and internalization, provides a potential mechanism for limiting the duration of thrombin-initiated signals in platelets.

High level expression of transfected G protein alpha i3 subunit is required for plasma membrane targeting and adenylyl cyclase inhibition in NIH 3T3 fibroblasts

The alpha subunits of pertussis toxin-sensitive G proteins Gi1, Gi2 and Gi3 have been shown to inhibit adenylyl cyclase in transfected cells. However, Gi3 has recently been associated with protein transport and localized to the Golgi apparatus in a number of cell lines, rather than to the plasma membrane. We studied NIH 3T3 clones stably expressing different levels of a constitutively activated mutant of the alpha subunit of Gi3 (alpha i3-Q204L). Transfected alpha i3 subunits were localized to the Golgi apparatus in all NIH 3T3 clones. In clones expressing alpha i3-Q204L at high levels, alpha i3 subunits were also localized to the plasma membrane. Those clones which demonstrated expression of alpha i3 at the plasma membrane showed a 40% to 60% inhibition of forskolin-induced cAMP accumulation. Transfected NIH 3T3 clones in which plasma membrane alpha i3 was undetectable, did not show inhibition of forskolin-induced cAMP accumulation. These data suggest that, unless high expression is achieved in transfected cells, alpha i3 is targeted predominantly to the Golgi, not to the plasma membrane, and does not control adenylyl cyclase activity in NIH 3T3 fibroblasts.

Developmental expression of G protein alpha subunits in mouse spermatogenic cells: evidence that G alpha i is associated with the developing acrosome

Guanine nucleotide-binding proteins (G proteins) are important signal transducing molecules found in all cells. G proteins are associated with the plasma membrane/outer acrosomal membrane region of acrosome-intact sperm and at least one G protein is involved in the zona pellucida-induced acrosome reaction. With the goal of elucidating the functions of these proteins during spermatogenesis, we investigated the types of G proteins present in spermatogenic cells and when they first become associated with the developing acrosome. Using bacterial toxin-catalyzed [32P]ADP-ribosylation in conjunction with immunoprecipitation and immunofluorescence utilizing antibodies directed against specific regions of various G protein isotypes, the alpha subunits of Gi1, Gi2, Gi3, and G(o) were detected in mouse spermatocytes and spermatids. An antiserum recognizing a conserved sequence of G alpha i subtypes localized to the proacrosomal granules of spermatocytes and the developing acrosome of spermatids. Levels of G alpha o diminished as spermatocytes developed into spermatids such that G alpha o was not detected in cauda epididymal sperm. Immunoreactivity using G alpha o-specific antisera did not display a distinct regionalization within any of the spermatogenic cell types. G alpha s was not detected in the developing spermatogenic cells or sperm. The association of G alpha i with the developing acrosome suggests a role for G proteins may have a role in acrosome biogenesis as well as being part of a complex required later for signal transduction leading to acrosomal exocytosis.

Two distinct members of the ADP-ribosylation factor family of GTP-binding proteins regulate cell-free intra-Golgi transport

We have used an intra-Golgi transport assay to identify GTP-binding proteins involved in regulation of protein traffic. Two soluble proteins of 20 kd were purified by their ability to mediate GTP gamma S-dependent inhibition of transport. These GTP-dependent Golgi binding factors, or GGBFs, exhibit a 3-fold difference in activity and are differentiated by their hydrophobicity, isoelectric points, and apparent size. Removal of 80% of GGBFs from cytosol abolishes GTP gamma S sensitivity but does not affect inhibition by aluminum fluoride. We demonstrate that GGBFs are members of the ADP-ribosylation factor (ARF) family. Recombinant ARF1 exhibits GGBF activity and myristoylation is required. The distinct biochemical properties of GGBFs indicate that members of the ARF family may have related but distinct functions in intracellular transport.