Molecular cloning of a novel splice variant of the alpha subunit of the mammalian Go protein

Gαo1 and Gαo1/Gαo2 deletion differentially affect hippocampal mossy fiber tract anatomy and neuronal morphogenesis

The heterotrimeric G-protein αo subunit is ubiquitously expressed in the CNS as two splice variants Gαo1 and Gαo2, regulating various brain functions. Here, we investigated the effect of single Gαo1, Gαo2, and double Gαo1/2 knockout on the postnatal development of the murine mossy fiber tract, a central pathway of the hippocampal connectivity circuit. The size of the hippocampal synaptic termination fields covered by mossy fiber boutons together with various fiber length parameters of the tract was analyzed by immunohistochemical staining of the vesicular Zinc transporter 3 (ZnT3) or Synaptoporin at postnatal days 2, 4, 8, 12, 16, and in the adult. Ultimately, Gαo1 knockout resulted in a reduced developmental growth of synaptic mossy fiber terminal fields by 37% in the adult Stratum lucidum and by 30% in the total mossy fiber tract size. Other morphological parameters such as projection length of the infrapyramidal bundle of the tract were increased (+52% in Gαo1 -/- mice). In contrast, Gαo2 knockout had no effects on the mossy fiber tract. Moreover, by using primary heterozygous and homozygous Gαo1 knockout hippocampal cultures, we detected a strongly pronounced reduction in axon and dendrite length (-50% and -38%, respectively) as well as axon and dendrite arborization complexity (-75% and -72% branch nodes, respectively) in the homozygous knockout. Deletion of both splice variants Gαo1 and Gαo2 partially rescued the in vivo and completely reconstituted the in vitro effects, indicating an opposing functional relevance of the two Gαo splice variants for neuronal development and synaptic connectivity.

AAV9-Mediated Intrastriatal Delivery of GNAO1 Reduces Hyperlocomotion in Gnao1 Heterozygous R209H Mutant Mice

Mutations in the GNAO1 gene, which encodes the abundant brain G-protein Gα o, result in neurologic disorders characterized by developmental delay, epilepsy, and movement abnormalities. There are over 50 mutant alleles associated with GNAO1 disorders; the R209H mutation results in dystonia, choreoathetosis, and developmental delay without seizures. Mice heterozygous for the human mutant allele (Gnao1 +/R209H) exhibit hyperactivity in open field tests but no seizures. We developed self-complementary adeno-associated virus serotype 9 (scAAV9) vectors expressing two splice variants of human GNAO1 Gα o isoforms 1 (GoA, GNAO1.1) and 2 (GoB, GNAO1.2). Bilateral intrastriatal injections of either scAAV9-GNAO1.1 or scAAV9-GNAO1.2 significantly reversed mutation-associated hyperactivity in open field tests. GNAO1 overexpression did not increase seizure susceptibility, a potential side effect of GNAO1 vector treatment. This represents the first report of successful preclinical gene therapy for GNAO1 encephalopathy applied in vivo. Further studies are needed to uncover the molecular mechanism that results in behavior improvements after scAAV9-mediated Gα o expression and to refine the vector design. SIGNIFICANCE STATEMENT: GNAO1 mutations cause a spectrum of developmental, epilepsy, and movement disorders. Here we show that intrastriatal delivery of scAAV9-GNAO1 to express the wild-type Gα o protein reduces the hyperactivity of the Gnao1 +/R209H mouse model, which carries one of the most common movement disorder-associated mutations. This is the first report of a gene therapy for GNAO1 encephalopathy applied in vivo on a patient-allele model.

The α-subunit of the trimeric GTPase Go2 regulates axonal growth

The Goα splice variants Go1α and Go2α are subunits of the most abundant G-proteins in brain, Go1 and Go2. Only a few interacting partners binding to Go1α have been described so far and splice variant-specific differences are not known. Using a yeast two-hybrid screen with constitutively active Go2α as bait, we identified Rap1GTPase activating protein (Rap1GAP) and Girdin as interacting partners of Go2α, which was confirmed by co-immunoprecipitation. Comparison of subcellular fractions from brains of wild type and Go2α-/- mice revealed no differences in the overall expression level of Girdin or Rap1GAP. However, we found higher amounts of active Rap1-GTP in brains of Go2α deficient mutants, indicating that Go2α may increase Rap1GAP activity, thereby effecting the Rap1 activation/deactivation cycle. Rap1 has been shown to be involved in neurite outgrowth and given a Rap1GAP-Go2α interaction, we found that the loss of Go2α affected axonal outgrowth. Axons of cultured cortical and hippocampal neurons prepared from embryonic Go2α-/- mice grew longer and developed more branches than those from wild-type mice. Taken together, we provide evidence that Go2α regulates axonal outgrowth and branching.

Specific roles of G(i) protein family members revealed by dissecting SST5 coupling in human pituitary cells

Despite intensive investigation over the past 20 years, the specific role played by individual G(i) protein family members in mediating complex cellular effects is still largely unclear. Therefore, we investigated the role of specific G(i) proteins in mediating somatostatin (SS) effects in somatotroph cells. Because our previous data showed that SS receptor type 5 (SST5) carrying a spontaneous R240W mutation in the third intracellular loop had a similar ability to inhibit intracellular cAMP levels to the wild-type protein but failed to mediate inhibition of growth hormone (GH) release and cell proliferation, we used this model to check specific receptor-G-protein coupling by a bioluminescent resonance energy transfer analysis. In HEK293 cells, wild-type SST5 stimulated the activation of Gα(i1-3) and Gα(oA), B, whereas R240W SST5 maintained the ability to activate Gα(i1-3) and Gα(oB), but failed to activate the splicing variant Gα(oA). To investigate the role of the selective deficit in Gα(oA) coupling, we co-transfected human adenomatous somatotrophs with SST5 and a pertussis toxin (PTX)-resistant Gα(oA) (Gα(oA(PTX-r))) protein. In PTX-treated cells, Gα(oA(PTX-r)) rescued the ability of the selective SST5 analog BIM23206 to inhibit extracellular signal-related kinase 1/2 (ERK1/2) phosphorylation, GH secretion and intracellular cAMP levels. Moreover, we demonstrated that silencing of Gα(oA) completely abolished SST5-mediated inhibitory effects on GH secretion and ERK1/2 phosphorylation, but not on cAMP levels. In conclusion, by analysing the coupling specificity of human SST5 to individual Gα(i) and Gα(o) subunits, we identified a crucial role for Gα(oA) signalling in human pituitary cells.

Augmented glucose-induced insulin release in mice lacking G(o2), but not G(o1) or G(i) proteins

Insulin secretion by pancreatic β cells is a complex and highly regulated process. Disruption of this process can lead to diabetes mellitus. One of the various pathways involved in the regulation of insulin secretion is the activation of heterotrimeric G proteins. Bordetella pertussis toxin (PTX) promotes insulin secretion, suggesting the involvement of one or more of three G(i) and/or two G(o) proteins as suppressors of insulin secretion from β cells. However, neither the mechanism of this inhibitory modulation of insulin secretion nor the identity of the G(i/o) proteins involved has been elucidated. Here we show that one of the two splice variants of G(o), G(o2), is a key player in the control of glucose-induced insulin secretion by β cells. Mice lacking G(o2)α, but not those lacking α subunits of either G(o1) or any G(i) proteins, handle glucose loads more efficiently than wild-type (WT) mice, and do so by increased glucose-induced insulin secretion. We thus provide unique genetic evidence that the G(o2) protein is a transducer in an inhibitory pathway that prevents damaging oversecretion of insulin.

Gαo represses insulin secretion by reducing vesicular docking in pancreatic beta-cells

OBJECTIVE

Pertussis toxin uncoupling-based studies have shown that Gαi and Gαo can inhibit insulin secretion in pancreatic β-cells. Yet it is unclear whether Gαi and Gαo operate through identical mechanisms and how these G-protein-mediated signals inhibit insulin secretion in vivo. Our objective is to examine whether/how Gαo regulates islet development and insulin secretion in β-cells.

RESEARCH DESIGN AND METHODS

Immunoassays were used to analyze the Gαo expression in mouse pancreatic cells. Gαo was specifically inactivated in pancreatic progenitor cells by pancreatic cell-specific gene deletion. Hormone expression and insulin secretion in response to different stimuli were assayed in vivo and in vitro. Electron microscope and total internal reflection fluorescence-based assays were used to evaluate how Gαo regulates insulin vesicle docking and secretion in response to glucose stimulation.

RESULTS

Islet cells differentiate properly in Gαo(-/-) mutant mice. Gαo inactivation significantly enhances insulin secretion both in vivo and in isolation. Gαo nullizygous β-cells contain an increased number of insulin granules docked on the cell plasma membrane, although the total number of vesicles per β-cell remains unchanged.

CONCLUSIONS

Gαo is not required for endocrine islet cell differentiation, but it regulates the number of insulin vesicles docked on the β-cell membrane.

Coordinated control of sensitivity by two splice variants of Gα(o) in retinal ON bipolar cells

The high sensitivity of scotopic vision depends on the efficient retinal processing of single photon responses generated by individual rod photoreceptors. At the first synapse in the mammalian retina, rod outputs are pooled by a rod “ON” bipolar cell, which uses a G-protein signaling cascade to enhance the fidelity of the single photon response under conditions where few rods absorb light. Here we show in mouse rod bipolar cells that both splice variants of the G_o α subunit, Gα_o1 and Gα_o2, mediate light responses under the control of mGluR6 receptors, and their coordinated action is critical for maximizing sensitivity. We found that the light response of rod bipolar cells was primarily mediated by Gα_o1, but the loss of Gα_o2 caused a reduction in the light sensitivity. This reduced sensitivity was not attributable to the reduction in the total number of G_o α subunits, or the altered balance of expression levels between the two splice variants. These results indicate that Gα_o1 and Gα_o2 both mediate a depolarizing light response in rod bipolar cells without occluding each other’s actions, suggesting they might act independently on a common effector. Thus, Gα_o2 plays a role in improving the sensitivity of rod bipolar cells through its action with Gα_o1. The coordinated action of two splice variants of a single Gα may represent a novel mechanism for the fine control of G-protein activity.

Differentiation of human T cells alters their repertoire of G protein alpha-subunits

Because T cell differentiation leads to an expanded repertoire of chemokine receptors, a subgroup of G protein-coupled receptors, we hypothesized that the repertoire of G proteins might be altered in parallel. We analyzed the abundance of mRNA and/or protein of six G protein α-subunits in human CD4(+) and CD8(+) T cell subsets from blood. Although most G protein α-subunits were similarly expressed in all subsets, the abundance of Gα(o), a protein not previously described in hematopoietic cells, was much higher in memory versus naive cells. Consistent with these data, activation of naive CD4(+) T cells in vitro significantly increased the abundance of Gα(o) in cells stimulated under nonpolarizing or T(H)17 (but not T(H)1 or T(H)2)-polarizing conditions. In functional studies, the use of a chimeric G protein α-subunit, Gα(qo5), demonstrated that chemokine receptors could couple to Gα(o)-containing G proteins. We also found that Gα(i1), another α-subunit not described previously in leukocytes, was expressed in naive T cells but virtually absent from memory subsets. Corresponding to their patterns of expression, siRNA-mediated knockdown of Gα(o) in memory (but not naive) and Gα(i1) in naive (but not memory) CD4(+) T cells inhibited chemokine-dependent migration. Moreover, although even in Gα(o)- and Gα(i1)-expressing cells mRNAs of these α-subunits were much less abundant than Gα(i2) or Gα(i3), knockdown of any of these subunits impaired chemokine receptor-mediated migration similarly. Together, our data reveal a change in the repertoire of Gα(i/o) subunits during T cell differentiation and suggest functional equivalence among Gα(i/o) subunits irrespective of their relative abundance.

Cannabinoid system in the budgerigar brain

Cannabinoid receptor density and cannabinoid receptor-mediated G protein stimulation were studied by autoradiographic techniques throughout the budgerigar (Melopsittacus undulatus) brain. The maximal CB(1) receptor density value (using [(3)H]CP55,940 as radioligand) was found in the molecular layer of the cerebellum (Mol), and high binding values were observed in the nucleus taeniae amygdalae (TnA), nucleus preopticus medialis, and nucleus pretectalis. The highest net-stimulated [(35)S]GTPgammaS binding values induced by the selective CB(1) receptor agonist WIN55,212-2 were observed in the nucleus paramedianus internus thalami, and high values of [(35)S]GTPgammaS binding were observed in the TnA, Mol, arcopallium dorsale and arcopallium intermedium. The distribution data suggest that in the budgerigar, as previously indicated in mammals, cannabinoid receptors may be related to the control of several brain functions in the motor system, memory, visual system, and reproductive behavior. The discrepancies between the cannabinoid receptor densities and the cannabinoid receptor-mediated stimulation found in several budgerigar brain nuclei support the hypothesis, previously described for mammals, of the existence of different G(i/o) protein populations able to associate with the cannabinoid receptors, depending on the brain structure, and could reflect the relative importance that cannabinoid transmission could exerts in each cerebral area.

Expression of novel splice variants of the G protein subunit, Go alpha, is tissue-specific and age-dependent in the rat

Heterotrimeric G proteins play an essential role in coupling numerous surface membrane receptors to intracellular signal transduction pathways. Relatively little is known about the splice variants of G proteins, including whether they undergo differential expression as a function of aging. We screened for splice variants of the alpha subunit of the dominant inhibitory G protein family member G(o) in a variety of tissues from rat and examined the expression of these splice variants during development. The splice variants were characterized using specific primers for G(o)alpha1 and G(o)alpha2 in conjunction with reverse transcription-polymerase chain reaction, and subsequently sequenced. G(o)alpha1 expression dominated over G(o)alpha2 in all neuronal tissues screened, including cerebral cortex, pituitary, spinal cord, colon myenteric plexus, dorsal root ganglion, and prenatal cortex. The sequence data of G(o)alpha1 supports the presence of three splice variants: G(o)alpha1a, G(o)alpha1b, G(o)alpha1c. The G(o)alpha1a variant was reported previously [J. Biol. Chem. 262 (1987) 14241], whereas G(o)alpha1b and G(o)alpha1c represent novel variants. The G(o)alpha1b splice variant demonstrates a 94 bp deletion using a cryptic donor site in exon 10. The G(o)alpha1c variant demonstrates a complete deletion of exon 10. A protein product with a molecular weight of approximately 34 kDa consistent with that expected for G(o)alpha1c was identified using Western blot analysis and two-dimensional gel electrophoresis. The expression of G(o)alpha1a decreased postnatally, supporting a potential physiological role during fetal development, whereas G(o)alpha1c expression increased postnatally. The age-dependent and tissue-specific expression of the G(o)alpha1 splice variants presage a broader functional role than has been observed historically with G(o).

The Identification and Localization of the Guanine Nucleotide Binding Protein G0 in the Auditory System

The identification of guanine nucleotide binding proteins (G proteins) in guinea-pig tissues was assessed by the adenosine diphosphate-ribosylation of the alpha subunit by Bordetella pertussis toxin using [alpha32P]nicotinamide adenine dinucleotide as the substrate followed by sodium dodecyl sulphate - polyacrylamide gel electrophoresis and autoradiography. Three tissues (inferior colliculus, neuroblastoma cells, and the organ of Corti) contained G0alpha (39 kD), as well as Gi2alpha (40 kD) and Gi1alpha and/or Gi3alpha (41 kD). The stria vascularis and the VIIIth nerve contained mainly Gi2alpha, Gi1alpha and/or Gi3alpha, but G0alpha was barely detectable. A purified preparation of outer hair cells from the organ of Corti contained all three pertussis toxin substrates including G0alpha, with the Gi2alpha (40 kD) subunit being the most prominent. The immunocytochemical localization of the G0alpha subunit was determined by light microscopy after incubating isolated outer hair cells, Hensen cells and the stria vascularis with affinity-purified anti-G0alpha antibodies. In hair cells a positive reaction was observed along the plasma membrane and around the perimeter of the cuticular plate (zona adherens). Positive reaction was also observed within the infracuticular network extending from the cuticular plate towards the nucleus in outer hair cells. Finally, the base of the outer hair cells also contained G0alpha. However, it is likely that the G0alpha that is present in this cell region is not within the hair cell itself, but rather in nerve terminals which remained attached during dissection.

The association of heterotrimeric GTP-binding protein (Go) with microtubules

The heterotrimeric GTP-binding regulatory proteins (G proteins) play an important role in the regulation of membrane signal transduction. Recently, we identified the association of Go protein with mitotic spindles. Here we have investigated the relationship between Go protein and microtubules. We used temperature-dependent reversible assembly and taxol methods to purify microtubules from bovine brains. Goalpha and Gbeta proteins were identified in the microtubular fraction by both methods. The Goalpha subunit in the microtubular fraction could be ADP ribosylated by pertussis toxin. Co-immunoprecipitation data also revealed that Go protein can interact with microtubules. Exogenous Go protein could be incorporated into the assembled microtubular fraction, and 5 microg/ml (60 nM) of Go protein inhibited 40% of microtubule assembly. Western blot analysis of Goalpha-1 and Goalpha-2 in microtubular fractions showed that only Goalpha-1 is associated with microtubules. We conclude that the Goalpha-1betagamma proteins are associated with microtubules and may play some role in regulating the assembly and disassembly of microtubules.

Most central nervous system D2 dopamine receptors are coupled to their effectors by Go

We reported previously that Go-deficient mice develop severe neurological defects that include hyperalgesia, a generalized tremor, lack of coordination, and a turning syndrome somewhat reminiscent of unilateral lesions of the dopaminergic nigro-striatal pathway. By using frozen coronal sections of serially sectioned brains of normal and Go-deficient mice, we studied the ability of several G protein coupled receptors to promote binding of GTPgammaS to G proteins and the ability of GTP to promote a shift in the affinity of D2 dopamine receptor for its physiologic agonist dopamine. We found a generalized, but not abolished reduction in agonist-stimulated binding of GTPgammaS to frozen brain sections, with no significant left-right differences. Unexpectedly, the ability of GTP to regulate the binding affinity of dopamine to D2 receptors (as seen in in situ [(35)S]sulpiride displacement curves) that was robust in control mice, was absent in Go-deficient mice. The data suggest that most of the effects of the Gi/Go-coupled D2 receptors in the central nervous system are mediated by Go instead of Gi1, Gi2, or Gi3. In agreement with this, the effect of GTP on dopamine binding to D2 receptors in double Gi1 plus Gi2- and Gi1 plus Gi3-deficient mice was essentially unaffected.

Selective coupling of G protein beta gamma complexes to inhibition of Ca2+ channels

Several mechanisms couple heterotrimeric guanine nucleotide-binding proteins (G proteins) to cellular effectors. Although alpha subunits of G proteins (Galpha) were the first recognized mediators of receptor-effector coupling, Gbetagamma regulation of effectors is now well known. Five Gbeta and 12 Ggamma subunit genes have been identified, suggesting through their diversity that specific subunits couple selectively to effectors. The molecular determinants of Gbetagamma-effector coupling, however, are not well understood, and most studies of G protein-effector coupling do not support selectivity of Gbetagamma action. To explore this issue further, we have introduced recombinant Gbetagamma complexes into avian sensory neurons and measured the inhibition of Ca(2+) currents mediated by an endogenous phospholipase Cbeta- (PLCbeta) and protein kinase C-dependent pathway. Activities of Gbetagamma in the native cells were compared with enzyme assays performed in vitro. We report a surprising selective activation of the PLCbeta pathway by Gbetagamma complexes containing beta(1) subunits, whereas beta(2)-containing complexes produced no activation. In contrast, when assayed in vitro, PLCbeta and type II adenylyl cyclase did not discriminate among these same Gbetagamma complexes, suggesting the possibility that additional cellular determinants confer specificity in vivo.

High expression of the gamma5 isoform of G protein in neuroepithelial cells and its replacement of the gamma2 isoform during neuronal differentiation in the rat brain

High concentrations of G proteins, which include multiple isoforms of each subunit, alpha, beta, and gamma, are expressed in the adult brain. In this study, we concentrated attention on changes of these isoforms during embryonic development in the rat brain. Concentrations of gamma2 as well as GoAalpha, GoBalpha, and beta2 were low in early embryogenesis and then increased, whereas expression of gamma5, in contrast, was initially high followed by a drop, with only very low levels observed throughout postnatal development. Among the other isoforms, Gi1alpha, G(s)alpha-short, G12alpha, G13alpha, beta4, gamma3, gamma7, and gamma12 were present in the embryonic brain at low levels, but their levels markedly increased after birth. In contrast, the levels of Gi2alpha, G(s)alpha-long, Gq/11alpha, and beta1 were essentially constant throughout. Immunohistochemical staining of the brain vesicles in the embryos showed gamma5 to be specifically expressed in the proliferative region of the ventricular zone, whereas gamma2 was mainly present in differentiated neuronal cells of the marginal zone. Furthermore, differentiation of P19 mouse embryonal carcinoma cells to neuronal cells with retinoic acid induced the expression of gamma2 and a decrease of gamma5, the major isoform in the undifferentiated state. These results suggest that neuronal differentiation is responsible for the on/off switch of the expression of gamma2 and gamma5 subunits.

Cyclic AMP regulates G(omicronalpha) protein and mRNA levels by modulating the transcriptional rate of G(omicronalpha) gene

In rat astroglial cells, four G(omicronalpha) transcripts were found: G(omicron2alpha) mRNA (5.7 kb) and three G(omicron1alpha) mRNAs (4.0, 3.0 and 2.3 kb). However, G(omicron2alpha) but little G(omicronalpha1) proteins were present in membrane-enriched fractions. Culturing astroglial cells with forskolin (10 microM) or isoproterenol (10 microM) a beta-adrenergic agonist increased transiently in a time-dependent manner the levels of G(omicronalpha) proteins. The degradation rate of G(omicronalpha) proteins was slightly decreased by the cAMP treatment. In parallel, forskolin (10 microM) treatment increased transiently the amounts of both G(omicron1alpha) and G(omicron2alpha) mRNAs. The relative transcription rate of G(omicronalpha) gene was increased by 1.7-fold in forskolin-treated cells whereas the half-lives of G(omicron1alpha) and G(omicron2alpha) mRNAs were not significantly changed. These results suggest that cAMP regulates the transcription rate of G(omicronalpha) gene and this is compatible with the existence of a cAMP responsive element in the promoter of the G(omicronalpha) gene.

The relationship of G(o)alpha subunit deamidation to the tissue distribution, nucleotide binding properties, and betagamma dimer interactions of G(o)alpha subunit isoforms

The distribution and properties in brain of the alpha subunits of the major bovine brain Go isoforms, GoA, GoB and GoC, were characterized. The alpha(o)A and alpha(o)B isoforms arise from alternative splicing of RNAs from a single alpha(o) gene, whereas alpha(o)C is a deamidated form of alpha(o)A. All three Go isoforms purify from brain with different populations of betagamma dimers. This variable subunit composition of Go heterotrimers is likely a consequence of their functional differences. This study examined the biochemical properties of the alpha(o) isoforms to see if these properties explain the variable betagamma composition of their heterotrimers. The brain distribution of alpha(o)B differed substantially from that of alpha(o)A and alpha(o)C, as did its guanine nucleotide binding properties. The unique subunit composition of GoB can be explained by its expression in different brain regions. The alpha(o)A and alpha(o)C showed slight differences in guanine nucleotide binding properties but no preference for particular betagamma dimers when reassociated with a heterogeneous betagamma pool. The alpha(o)C protein occurred in a constant ratio to alpha(o)A throughout the brain, but was a much larger percent of total brain alpha(o) than previously thought, approximately 35%. These results suggest that alpha(o)A is a precursor of alpha(o)C and that the association of G(o)alpha subunits with different betagamma dimers reflects the function of an adaptive, G-protein signaling mechanism in brain.

Identification and expression of Go1 and Go2 alpha-subunit transcripts in human myometrium in relation to pregnancy

The 39-kDa Goalpha protein, the alpha subunit of a major heterotrimeric G protein of brain and neuroendocrine cells, was found to be present in human myometrium. Using three different antisera, we showed its strong expression in myometrium from pregnant patients as compared to nonpregnant ones. This is in agreement with the high expression level of its two isoforms (alphao1 and alphao2), previously described in late pregnancy. To better ascertain the nature of these immunoreactive isoforms, we investigated transcripts of the Goalpha gene in myometrium from pregnant and nonpregnant patients by reverse transcription-polymerase chain reaction (RT-PCR). In this tissue, the amplified cDNA product of a region common to both Go1alpha and Go2alpha mRNA variants was recognized as the Goalpha nucleotide sequence. Transcripts of Go1alpha and Go2alpha were identified by sequencing. A partial cDNA Go2alpha sequence was described, which differed from the Goalpha gene by two nucleotides in exon 8B. Levels of Go1alpha and Go2alpha transcripts analyzed by semi-quantitative RT-PCR were significantly higher in myometrium from pregnant than from nonpregnant patients. It is suggested that Goalpha gene expression in this tissue may contribute to modifications seen in the signaling pathways observed at the end of pregnancy.

Posttranslational modification of Galphao1 generates Galphao3, an abundant G protein in brain

Galphao, the most abundant G protein in mammalian brain, occurs at least in two subforms, i.e., Galphao1 and Galphao2, derived by alternative splicing of the mRNA. A third Galphao1-related isoform, Galphao3, has been purified, representing about 30% of total Go in brain. Initial studies revealed distinct biochemical properties of Galphao3 as compared with other Galphao isoforms. In matrix-assisted laser desorption/ionization peptide mass mapping of gel-isolated Galphao1 and Galphao3, C-terminal peptides showed a difference of +1 Da for Galphao3. Nanoelectrospray tandem mass spectrometry sequencing revealed an Asp instead of an Asn at position 346 of Galphao3. Gel electrophoretic analysis of recombinant Galphao3 showed the same mobility as native Galphao3 but distinct to Galphao1. The conversion of 346Asn-->Asp changed the signaling properties, including the velocity of the basal guanine nucleotide-exchange reaction, which points to the involvement of the C terminus in basal guanosine 5'-[gamma-thio]triphosphate binding. No cDNA coding for Galphao3 was detected, suggesting an enzymatic deamidation of Galphao1 by a yet-unidentified activity. Therefore, Galpha heterogeneity is generated not only at the DNA or RNA levels, but also at the protein level. The relative amount of Galphao1 and Galphao3 differed from cell type to cell type, indicating an additional principle of G protein regulation.

D2S, D2L, D3, and D4 dopamine receptors couple to a voltage-dependent potassium current in N18TG2 x mesencephalon hybrid cell (MES-23.5) via distinct G proteins

We utilized the approach of stably expressing different dopamine (DA) receptors into identified cell lines in an attempt to better understand the coupling of these receptors to membrane ion channels via second messenger systems. Recently, we examined the N18TG2 x mesencephalon (MES-23.5) cell line that is phenotypically similar to mesencephalic dopamine-containing neurons. Whole-cell voltage-clamp methods were used to investigate a voltage-dependent K+ current present in these cells. Untransfected MES-23.5 cells displayed a voltage-dependent slow-onset, slowly inactivating outward current which was not altered by bath application of either the D2 DA receptor agonist quinpirole (QUIN; 10-100 microM) or the D1 DA receptor agonist SKF38393, indicating that these cells were devoid of DA receptors. The K+ current studied was activated upon depolarization from a holding potential of -60 mV to a level more positive than -20 mV and was observed to be sensitive to bath application of tetraethylammonium. When MES-23.5 cells were transfected to stably express the D2S, D2L, D3, and D4 receptors, the same current was observed. In cells expressing D2L, D2S, and D3 receptors, application of the DA receptor agonists QUIN (1-80 microM), 7-hydroxy-dipropylaminoteralin (7-OH-DPAT, 1-80 microM), and dopamine (DA, 1-80 microM), increased the peak outward current by 35-40%. In marked contrast, cells stably expressing the D4 receptor demonstrated a significant DA agonist-induced reduction of the peak K+ current by 40%. For all four receptor subtypes, the D2-like receptor antagonist sulpiride (SUL 5 microM), when coapplied with QUIN (10 microM), totally abolished the change in K+ current normally observed, while coapplication of the D1-like receptor antagonist SCH23390 was without effect. The modulation of K+ current by D2L, D3, and D4 receptor stimulation was prevented by pretreatment of the cells with pertussis toxin (PTX, 500 ng/ml for 4 h). In addition, the intracellular application of a polyclonal antibody which specifically recognizes Goalpha completely blocked the ability of D2L, D3, and D4 receptors to modulate outward K+ currents. In contrast, the intracellular application of an antibody directed against Goalpha was without effect, whereas intracellular application of an antibody recognizing Gsalpha abolished the ability of the D2S receptor to enhance K+ current. These findings demonstrate that different members of the D2 DA receptor family may couple in a given cell to a common effector in dramatically different ways.

Characterization of the major bovine brain Go alpha isoforms. Mapping the structural differences between the alpha subunit isoforms identifies a variable region of the protein involved in receptor interactions

Go is the major G protein in bovine brain, with at least three isoforms, GoA, GoB, and GoC. Whereas alphaoA and alphaoB arise from a single Goalpha gene as alternatively spliced mRNAs, alphaoA and alphaoC are thought to differ by covalent modification. To test the hypothesis that alphaoA and alphaoC have different N-terminal lipid modifications, proteolytic fragments of alphao isoforms were immunoprecipitated with an N terminus-specific antibody and analyzed by matrix-assisted laser desorption ionization mass spectrometry. The major masses observed in immunoprecipitates were the same for all three alphao isoforms and corresponded to the predicted mass of a myristoylated N-terminal fragment. Structural differences between alphaoA and alphaoC were also compared before and after limited tryptic proteolysis using SDS-polyacrylamide gel electrophoresis containing 6 M urea. Based upon the alphao subunit fragments produced under activating and nonactivating conditions, differences between alphaoA and alphaoC were localized to a C-terminal fragment of the protein. This region, involved in receptor and effector interactions, implies divergent signaling roles for these two alphao proteins. Finally, the structural difference between alphaoA and alphaoC is associated with a difference of at most 2 daltons based upon measurements by electrospay ionization mass spectrometry.

Association of GAP-43 with detergent-resistant membranes requires two palmitoylated cysteine residues

GAP-43 is an abundant protein in axonal growth cones of developing and regenerating neurons. We found that GAP-43 was enriched in detergent-resistant membranes (DRMs) isolated by Triton X-100 extraction from PC12 pheochromocytoma cells and could be detected in detergent-insoluble plasma membrane remnants after extraction of cells in situ. GAP-43 is palmitoylated at Cys-3 and Cys-4. Mutation of either Cys residue prevented association with DRMs. A hybrid protein containing the first 20 amino acid residues of GAP-43 fused to beta-galactosidase was targeted to DRMs even more efficiently than GAP-43 itself. We conclude that tandem palmitoylated Cys residues can target GAP-43 to DRMs, defining a new signal for DRM targeting. We propose that tandem or closely spaced saturated fatty acyl chains partition into domains or "rafts" in the liquid-ordered phase, or a phase with similar properties, in cell membranes. These rafts are isolated as DRMs after detergent extraction. The brain-specific heterotrimeric G protein Go, which may be regulated by GAP-43 in vitro, was also enriched in DRMs from PC12 cells. Targeting of GAP-43 to rafts may function to facilitate signaling through Go. In addition, raft association may aid in sorting of GAP-43 into axonally directed vesicles in the trans-Golgi network.

Expression of HKalpha2 protein is increased selectively in renal medulla by chronic hypokalemia

Our laboratory has demonstrated by Northern analysis that chronic hypokalemia increases HKalpha2 (i.e., alpha-subunit of the colonic H+-K+-ATPase) mRNA abundance in the rat. To determine whether the increase in mRNA correlated with an increase in HKalpha2 protein, an antibody was raised against a synthetic peptide derived from amino acids 686-698 of the HKalpha2 sequence. The anti-HKalpha2 antibody hybridized to rat distal colon membranes which migrated at approximately 100 kDa (expected mobility of HKalpha2). HKalpha2 protein was not detected in plasma membranes from rat whole kidney or stomach (100 microg) derived from control animals. The antibody was then used to investigate changes in expression of HKalpha2 in renal cortex, renal medulla, and distal colon in two pathophysiological conditions: 1) chronic hypokalemia (LK) and 2) chronic metabolic acidosis (CMA). In LK rats there was a marked, but selective, increase in the abundance of HKalpha2 protein in membranes prepared from renal medulla. Nevertheless, a corresponding increase in HKalpha2 protein abundance was not observed in membranes prepared from the distal colon of LK rats. HKalpha2 protein abundance in CMA was indistinguishable from controls. Moreover, chronic hypokalemia had no effect on expression of alpha1-Na+-K+-ATPase or HKalpha1 in kidney or distal colon under any experimental condition. Therefore, HKalpha2 protein is tissue- and site-specifically upregulated in response to chronic hypokalemia but not by CMA. Furthermore, this regulatory response is localized to the renal medulla.

Odorants selectively activate distinct G protein subtypes in olfactory cilia

Chemoelectrical signal transduction in olfactory neurons appears to involve intracellular reaction cascades mediated by heterotrimeric GTP-binding proteins. In this study attempts were made to identify the G protein subtype(s) in olfactory cilia that are activated by the primary (odorant) signal. Antibodies directed against the alpha subunits of distinct G protein subtypes interfered specifically with second messenger reponses elicited by defined subsets of odorants; odor-induced cAMP-formation was attenuated by Galphas antibodies, whereas Galphao antibodies blocked odor-induced inositol 1,4, 5-trisphosphate (IP3) formation. Activation-dependent photolabeling of Galpha subunits with [alpha-32P]GTP azidoanilide followed by immunoprecipitation using subtype-specific antibodies enabled identification of particular individual G protein subtypes that were activated upon stimulation of isolated olfactory cilia by chemically distinct odorants. For example odorants that elicited a cAMP response resulted in labeling of a Galphas-like protein, whereas odorants that elicited an IP3 response led to the labeling of a Galphao-like protein. Since odorant-induced IP3 formation was also blocked by Gbeta antibodies, activation of olfactory phospholipase C might be mediated by betagamma subunits of a Go-like G protein. These results indicate that different subsets of odorants selectively trigger distinct reaction cascades and provide evidence for dual transduction pathways in olfactory signaling.

The alpha-subunit of the colonic H+,K+-ATPase assembles with beta1-Na+,K+-ATPase in kidney and distal colon

Previous experiments from our laboratory (Codina, J., Kone, B. C., Delmas-Mata, J. T., and DuBose, T. D., Jr. (1996) J. Biol. Chem. 271, 29759-29763) demonstrated that the alpha-subunit of the colonic H+, K+-ATPase (HKalpha2) requires coexpression with a beta-subunit to support H+/K+ transport in a heterologous expression system (Xenopus laevis oocytes). In these studies, HKalpha2 formed stable and functional alpha.beta complexes when coexpressed with either the rat beta1-subunit of the Na+,K+-ATPase or the beta-subunit of the gastric H+,K+-ATPase, suggesting that different beta-subunits may interact with HKalpha2. The present studies tested this hypothesis by development and application of a specific antibody against HKalpha2 peptide. Subsequently, immunoprecipitation experiments were performed to determine if HKalpha2 co-precipitates with the same beta-subunit in organs known to express HKalpha2 protein. The data demonstrate that HKalpha2 assembles with beta1-Na+,K+-ATPase in the renal medulla and in distal colon.

Multiple neurological abnormalities in mice deficient in the G protein Go

The G protein Go is highly expressed in neurons and mediates effects of a group of rhodopsin-like receptors that includes the opioid, alpha2-adrenergic, M2 muscarinic, and somatostatin receptors. In vitro, Go is also activated by growth cone-associated protein of Mr 43,000 (GAP43) and the Alzheimer amyloid precursor protein, but it is not known whether this occurs in intact cells. To learn about the roles that Go may play in intact cells and whole body homeostasis, we disrupted the gene encoding the alpha subunits of Go in embryonic stem cells and derived Go-deficient mice. Mice with a disrupted alphao gene (alphao-/- mice) lived but had an average half-life of only about 7 weeks. No Goalpha was detectable in homogenates of alphao-/- mice by ADP-ribosylation with pertussis toxin. At the cellular level, inhibition of cardiac adenylyl cyclase by carbachol (50-55% at saturation) was unaffected, but inhibition of Ca2+ channel currents by opioid receptor agonist in dorsal root ganglion cells was decreased by 30%, and in 25% of the alphao-/- cells examined, the Ca2+ channel was activated at voltages that were 13.3 +/- 1.7 mV lower than in their counterparts. Loss of alphao was not accompanied by appearance of significant amounts of active free betagamma dimers (prepulse test). At the level of the living animal, Go-deficient mice are hyperalgesic (hot-plate test) and display a severe motor control impairment (falling from rotarods and 1-inch wide beams). In spite of this deficiency, alphao-/- mice are hyperactive and exhibit a turning behavior that has them running in circles for hours on end, both in cages and in open-field tests. Except for one, all alphao-/- mice turned only counterclockwise. These findings indicate that Go plays a major role in motor control, in motor behavior, and in pain perception and also predict involvement of Go in Ca2+ channel regulation by an unknown mechanism.

Pertussis toxin-sensitive GTP-binding proteins characterized in synaptosomal fractions of embryonic avian cerebral cortex

Pertussis toxin (PTX)-sensitive GTP-binding proteins (G proteins) are essential intermediaries subserving neuronal signal transduction pathways that regulate excitation-secretion coupling. Despite this established role, relatively little is known regarding the identity, subcellular distribution, and relative abundance of this class of G proteins in synaptic nerve endings. Here, sucrose density gradient centrifugation was combined with 1- and 2-dimensional gel electrophoresis to characterize PTX-sensitive G protein alpha subunits in synaptosomal fractions of embryonic (day 12) chick cerebral cortical homogenates. These findings demonstrate multiple isoforms of M(r) 40-41 kDa Gi alpha and G(o) alpha subunits that can be identified on the basis of PTX-catalyzed ADP-ribosylation and immunoblot analysis.

Na+/H+ exchange in vascular smooth muscle cells is controlled by GTP-binding proteins

This study examines the involvement of GTP-binding proteins (Gps) in the regulation of Na+/H+ exchange and Ca2+ influx, which are increased in vascular smooth muscle cells from spontaneously hypertensive rats. Gp activity was modulated by fluoride, GTPgammaS, GDPbetaS, and antisense oligodeoxynucleotides complementary to conserved regions of the alpha- and beta-subunits of Gps (alpha-comm and beta-comm, respectively). Beta-adrenergic-induced Gs-mediated cAMP production was used as a positive control to estimate the efficiency of these compounds. Na+/H+ exchange, measured as ethylisopropyl amiloride-sensitive 22Na influx, was activated by 5- to 6-fold by a 30-minute preincubation of cells with 10 mmol/L NaF with a K0.5 for NaF of approximately 13 mmol/L. In contrast, no activation of 45Ca influx was observed under preincubation of vascular smooth muscle cells with NaF in Ca2+-free medium, whereas at [Ca2+]o >0.5 mmol/L, simultaneous addition of 45Ca and 10 mmol/L NaF led to sharply increased isotope uptake. NaF-induced 45Ca influx did not reach saturation up to 3 mmol/L [Ca2+]o and 20 mmol/L NaF and was correlated with the formation of calcium-fluoride complexes measured by light scattering. GTPgammaS increased basal cAMP production and Na+/H+ exchange, whereas GDPbetaS decreased isoproterenol-induced cAMP production and Na+/H+ exchange. Alpha-comm reduced whereas beta-comm augmented isoproterenol-induced cAMP production by 70%. Both oligodeoxynucleotides decreased basal Na+/H+ exchange by 40% to 50%. NaF-induced Na+/H+ exchange was not sensitive to alpha-comm but was inhibited by 60% in beta-comm-loaded cells. Neither basal nor NaF-induced 45Ca uptake was affected by GTPgammaS, GDPbetaS, and the oligodeoxynucleotides. Our results show that 45Ca uptake is activated by NaF in vascular smooth muscle cells by nonspecific accumulation of calcium-fluoride complexes and is not related to modification of Gps. On the contrary, the Na+/H+ exchanger is controlled by Gps, and Gp beta-subunits are involved in [Ca2+]o-independent activation of this carrier by NaF.

G(o)2 and Gi3 proteins mediate the action of somatostatin on membrane Ca2+ and K+ currents in ovine pituitary somatotrophs

1. Growth hormone (GH) secretion from the anterior pituitary gland is mainly regulated by hypothalamic GH-releasing hormone (GHRH) and somatostatin (SRIF). Somatostatin reduces both spontaneous and GHRH-stimulated GH secretion. 2. Exocytosis of GH is mainly determined by the intracellular free Ca2+ concentration ([Ca2+]i), which is regulated by the influx of Ca2+ via membrane Ca2+ channels. Somatostatin reduces the influx of Ca2+ through two separate mechanisms, namely a direct action on Ca2+ channels and an indirect action on membrane potentials through the activation of K+ channels. 3. In the present experiments, somatotroph-enriched cells were obtained from the ovine pituitary gland by means of collagenase dissociation and Percoll-gradient centrifugation. Further identification was based on the effect of SRIF (10 nmol/L) on Ca2+ or K+ currents. 4. A significant reduction in Ca2+ currents and an increase in K+ currents was obtained in response to local application of SRIF (10 nmol/L), but vehicle application had no effect. The responses of Ca2+ and K+ currents to SRIF were reversible after removal of SRIF. 5. Dialysis of GTP-gamma-s (200 mumol/L) abolished the recovery phase of K+ current response to SRIF after its removal, whereas GDP-beta-s (200 mumol/L) totally blocked the response. Pretreatment of the cells with pertussis toxin (100 nmol/L) overnight abolished the Ca2+ current response to SRIF. 6. Intracellular dialysis of antibodies to alpha o, alpha i1-3, alpha i1-2 and alpha i3 subunits of the G-proteins into cells via whole-cell patch-clamp pipettes was confirmed by immunofluorescent staining of the antibodies. 7. Dialysis of anti-alpha i1-3 or anti-alpha i3 antibodies significantly attenuated the increase in the K+ current in response to 10 nmol/L SRIF, whereas neither anti-alpha o nor anti-alpha i1-2 antibodies diminished the effect of SRIF on the K+ current. 8. Dialysis of anti-alpha o antibodies significantly attenuated the reduction in the Ca2+ current that was obtained upon application of 10 nmol/L SRIF. Neither anti-alpha i1-2 nor anti-alpha i3 antibody dialysis diminished the effect of SRIF on the Ca2+ current. 9. Dialysis of the alpha o common antisense oligonucleotides (ASm) but not the alpha i3 AS significantly diminished the inhibitory effect of SRIF on the Ca2+ current. This effect of alpha o ASm dialysis occurred at 12 h incubation after dialysis, reaching a maximal level at 48 h and partially recovering at 72 h incubation. Antisense oligonucleotides specific for alpha o1 (alpha o1 AS) or alpha o2 (alpha o2 AS) were dialysed into somatotrophs and only alpha o2 AS significantly attenuated the inhibition of SRIF on the Ca2+ current. 10. It is concluded that the Gi3 protein mediates the effect of SRIF on the K+ current and that the G(o)2 protein mediates the effect of SRIF on the Ca2+ current in primary cultured ovine somatotrophs.

Role of subunit diversity in signaling by heterotrimeric G proteins

The heterotrimeric G proteins are extensively involved in the regulation of cells by extracellular signals. The receptors that control them are often the targets of drugs. There are many isoforms of each of the three subunits that make up these proteins. Thus far, genes for at least sixteen alpha subunits, five beta subunits, and eleven gamma subunits have been identified. In addition, some of these proteins have splice variants or are differentially modified. Based upon what is already known, there are well over a thousand possible G protein heterotrimer combinations. The role of subunit diversity in heterotrimer formation and its effect on signaling by G proteins are still not well understood. However, many current lines of research are leading toward an understanding of these roles. The functional significance of subunit heterogeneity is related to the mechanisms used by G proteins to transmit and integrate the many signals coming into cells through this system. Described here are the basic mechanisms by which G proteins integrate cellular responses, the possible role of subunit heterogeneity in these mechanisms, and the evidence for and against their physiological significance. Recent studies suggest the likely possibility that subunit heterogeneity plays an important role in signaling by G proteins. This role has the potential to extend substantially the flexibility of G proteins in mediating cellular responses to extracellular signals. However, the details of this are yet to be worked out, and they are the subject of many different avenues of research.

Dynorphin A-mediated reduction in multiple calcium currents involves a G(o) alpha-subtype G protein in rat primary afferent neurons

We examined the effect of antisera directed at specific G-protein subtype(s) on dynorphin A (Dyn A)-mediated reduction of calcium currents in rat dorsal root ganglia (DRG) neurons. Whole cell patch-clamp recordings were performed on acutely dissociated neurons. Dyn A (1 microM)-mediated decrease in calcium currents was inhibited > 90% by the preferential kappa-receptor antagonist norbinaltorphimine. Dyn A (300-1,000 nM)-mediated reduction in calcium currents was examined during intracellular administration of antisera directed against specific regions of G(o) alpha, G(i) 1 alpha/G(1) 2 alpha, and G(i) 3 alpha subunits. Intracellular dialysis with an antiserum specific for G(o) alpha for 20 min decreased calcium current inhibition by Dyn A (1 microM) in 13 of 15 neurons by an average of 75%. Dialysis with nonimmune serum did not affect Dyn A's action to reduce calcium currents. Intracellular dialysis with either anti-G(i) 1 alpha/G(i) 2 alpha or anti-G(i) 3 alpha antisera did not affect Dyn A-induced changes in calcium currents. In the presence of the N-type calcium channel antagonist omega-conotoxin GVIA, the P-type calcium channel antagonist omega-Aga IVA, and omega-Aga MVIIC applied subsequent to the other toxins, the effect of Dyn A to reduce calcium currents was inhibited by 52, 28, and 16%, respectively. The L channel antagonist nifedipine did not affect the ability to Dyn A to inhibit calcium currents. These results suggest that in rat DRG neurons coupling of kappa-opioid receptors to multiple transient, high-threshold calcium currents involves the G(o) alpha subclass of G proteins.

Molecular cloning of heterotrimeric G-protein alpha-subunits in chicken pineal gland

The avian pinealocytes have an intrinsic circadian clock function that controls rhythmic synthesis of melatonin, and an environmental light signal can reset the phase of the clock. In addition to the photoendocrine function, the melatonin synthesis of the pinealocytes is regulated by neural signals from sympathetic nerves. Thus the avian pinealocytes show diagnostic characters which seem to represent an evolutionary transition from photosensory cells of lower vertebrates to the neuroendocrinal cells of mammals. To understand the evolutionary background of the regulatory mechanism for the melatonin synthesis in this organ, we screened the chicken pineal cDNA library to find alpha-subunits of heterotrimeric G-proteins involved in the photic and neural regulations. In addition to the transducin-like alpha-subunit (Gt alpha) supposed to mediate the photic pathway, we isolated cDNA clones encoding Gi2 alpha, Gi3 alpha, and Go1 alpha and its splicing variant Go2 alpha. The deduced amino acid sequence of each G alpha had a potential site for pertussis toxin-catalyzed ADP-ribosylation. As it is known that adrenergic receptor-mediated inhibition of melatonin synthesis is blocked by pertussis toxin, the G-proteins identified in the present study are likely to contribute to this neuroendocrine function of the chicken pineal cells.

Pancreatic beta-cell receptors and G proteins coupled to adenylyl cyclase

Glucagon and tGLP-1 receptors can be either coexpressed or selectively expressed in beta-cell models. Our results indicate that both these peptides can regulate insulin secretion from beta-cells through their own specific receptors. The finding of a selective expression of G proteins in insulin and glucagon cells indicates a clear difference in their transduction pathways. A key role of the G alpha s family in beta-cell function is further supported by its conserved cell distribution between different species. In conclusion, one could postulate that in the human beta-cells, tGLP-1 and glucagon receptors could mediate their action through different G protein alpha-subunits of the G alpha s family.

Secretion of protease nexin-1 by C6 glioma cells is under the control of a heterotrimeric G protein, Go1

Heterotrimeric Go proteins have recently been described as regulators of vesicular traffic. The Goalpha gene encodes, by alternative splicing, two Goalpha polypeptides, Go1alpha and Go2alpha. By immunofluorescence and electron microscopy, we detected Go1alpha on the membrane of small intracellular vesicles in C6 glioma cells. After stable transfection of these cells, overexpression of Go1alpha but not Go2alpha was followed by a rise in the secretion of a serine protease inhibitor, protease nexin-1 (PN-1). This secretion was enhanced as a function of the amount of expressed Go1alpha. Metabolic cell labeling indicated that this increase in PN-1 secretion was not the result of an enhancement in PN-1 biosynthesis or a decrease in its uptake, but revealed a potential role of Go1alpha in the regulation of vesicular PN-1 trafficking. Furthermore, activators of Go proteins, mastoparan and a peptide derived from the amino terminus of the growth cone-associated protein GAP43, increased PN-1 secretion in parental and Go1alpha-overexpressing cells. Brefeldin A, an inhibitor of vesicular traffic, inhibited both basal and mastoparan-stimulated PN-1 secretions. These results indicate, that in C6 glioma cells, PN-1 secretion could be regulated by both Go1alpha expression and activation.

D2L, D2S, and D3 dopamine receptors stably transfected into NG108-15 cells couple to a voltage-dependent potassium current via distinct G protein mechanisms

The D2-like dopamine (DA) receptor family has continued to expand and now includes the D2-short (D2S) and D2-long (D2L) receptor isoforms and the D3 and D4 receptors. The D2 receptor isoforms differ in length by 29 amino acids within the third cytoplasmic loop, a region of the receptor believed to be important for G protein coupling. This observation has led to the hypothesis that the two isoforms of the D2 receptor may utilize different signal transduction pathways when present in the same cell. The D2 and D3 receptors, although mostly different, show some common amino acid sequences within the third cytoplasmic loop. Thus, it is possible that the D2 and D3 receptors may employ similar signal transduction pathways. To test these hypotheses directly, NG108-15 neuroblastoma-glioma hybrid cells were stably transfected to express either the D2S, D2L, or D3 DA receptors. All transfected but not untransfected NG108-15 cells demonstrated a dose-dependent reduction in the peak whole-cell potassium (K+) current in response to receptor activation by DA or the DA receptor agonists quinpirole (QUIN) and apomorphine (APO). The modulation of K+ current by D2S receptor stimulation was prevented by pretreatment of the cells with cholera toxin (20 micrograms/ml for 18 h), whereas pertussis toxin pretreatment (500 ng/ml for 4 h) completely blocked the effects of D2L and D3 receptor activation. These observations suggest that the signal transduction mechanisms involved in coupling the two isoforms of the D2 receptor to the K+ current are different, whereas the D2L and D3 receptor coupling mechanisms may be similar. In direct support of this hypothesis, it was observed that the intracellular application of a polyclonal antibody that is specific for the GO alpha subunit completely blocked the ability of D2L and D3 receptors to modulate outward K+ currents. In contrast, the D2S-mediated modulation of K+ currents was blocked by intracellular application of an antibody recognizing GS alpha but not GO alpha. These findings demonstrate that D2S and D2L receptors are able to couple to a common effector in a cell via two G protein pathways.

Differences in G-protein activation by mu- and delta-opioid, and cannabinoid, receptors in rat striatum

Receptor activation of G-proteins can be measured by agonist-stimulated [35S]GTP gamma S binding in the presence of excess guanosine diphosphate (GDP). To determine whether opioid and cannabinoid receptor-mediated G-protein activation correlate with their receptor densities, this study compared opioid- and cannabinoid-stimulated [35S]guanylyl-5'-O-(gamma-thio)-triphosphate (GTP gamma S) binding with the corresponding Bmax values of receptor binding in rat striatum. Scatchard analysis revealed that the Bmax of cannabinoid receptor binding was approximately ten times higher than that of mu- or delta-opioid receptor binding. However, comparable levels of cannabinoid- and mu- and delta-opioid-stimulated [35S]GTP gamma S binding were observed in the caudate-putamen by [35S]GTP gamma S autoradiography in brain sections. Scatchard analysis of net agonist-stimulated [35S]GTP gamma S binding in membranes showed that the Bmax of cannabinoid-stimulated binding was only twice that of mu- or delta-opioid-stimulated binding. Thus, the calculated amplification factors for mu- and delta-opioid receptors are seven times that of cannabinoid receptors.

ADP-ribosylation of G alpha i and G alpha o in pituitary cells enhances their recognition by antibodies directed against their carboxyl termini

Using antibodies raised against synthetic peptides of heterotrimeric GTP binding proteins, we demonstrate the presence of G alpha s, G alpha i1,2, G alpha i3, G alpha o2, and G beta subunits in pituitary cells. Pretreatment of pituitary cells with cholera toxin diminished the immunoreactivity of G alpha s and this decrease was kinetically coupled to the rate of G alpha s ADP-ribosylation. ADP-ribosylation by islet activating protein (IAP or Bordetella pertussis toxin) of G alpha i and G alpha o enhanced their immunoreactivities to antibodies raised against synthetic decapeptides that correspond to the G alpha carboxyl termini. Such enhancement was not observed when antibodies directed against the NH2-termini were used. These findings are consistent with the fact that ADP-ribosylation by IAP occurs on the cysteine located in the carboxyl terminal part of G alpha i and G alpha o. These observations mean that the kinetics and extent of Gi and Go ADP-ribosylation by IAP in whole pituitary cells and membrane preparations can be followed. It could be that ADP-ribosylation causes conformational changes in G alpha i and G alpha o. Indeed, we observed that ADP-ribosylated G alpha i was more sensitive to trypsin proteolysis and that the ADP-ribosylation rates of G alpha i and G alpha o in whole cells were comparable to the rate of loss of coupling between inhibitory neurohormone receptors and adenylyl cyclase.

Ulcerative colitis and adenocarcinoma of the colon in G alpha i2-deficient mice

G proteins are involved in cellular signalling and regulate a variety of biological processes including differentiation and development. We have generated mice deficient for the G protein subunit alpha i2 (G alpha i2) by homologous recombination in embryonic stem cells. G alpha i2-deficient mice display growth retardation and develop a lethal diffuse colitis with clinical and histopathological features closely resembling ulcerative colitis in humans, including the development of adenocarcinoma of the colon. Prior to clinical symptoms, the mice show profound alterations in thymocyte maturation and function. The study of these animals should provide important insights into the pathogenesis of ulcerative colitis as well as carcinogenesis.

Bovine brain GO isoforms have distinct gamma subunit compositions

The gamma subunit composition of the major bovine brain Go and Gi proteins (GOA, GOB, GOC, Gi1, and Gi2) was characterized using antibodies against specific gamma isoforms. Each of the purified G protein heterotrimers contained a heterogeneous population of gamma subunits, and the profiles of the gamma subunits found with Gi1, Gi2, and GOA were similar. In contrast, each GO isoform had a distinct pattern of associated gamma subunits. These differences were surprising given that all three alpha O isoforms are thought to share a common amino-terminal sequence important for the binding of beta gamma dimers and that the alpha OA and alpha OC proteins may come from the same alpha O1 mRNA. The free alpha OA and alpha OC subunits had unique elution behaviors during MonoQ chromatography, compatible with differences in their post-translational processing. These results indicate that both the alpha and gamma subunit compositions of heterotrimers define the structure of an intact G protein. Furthermore, the exact subunit composition of G protein heterotrimers may depend upon regulated expression of different subunit isoforms or upon cellular processing of alpha subunits.

The alpha-subunit mRNAs for Gs and Go2 are differentially regulated by protein kinase A and protein kinase C in rat Sertoli cells

In the present study, we have examined regulatory effects of protein kinase A and protein kinase C activation by 8-CPTcAMP and TPA, respectively, on mRNAs for various G protein alpha-subunits and corresponding immunoreactive proteins in rat Sertoli cells. Gs alpha and Go alpha mRNA levels were transiently increased 1.5-fold and 4-fold, respectively, by 8-CPTcAMP in cultured Sertoli cells. This up-regulation of mRNAs for Gs alpha and Go alpha was also observed when Sertoli cells were incubated in the presence of FSH. When protein synthesis was inhibited by cycloheximide, the cAMP-mediated stimulation of Gs alpha mRNA was abolished, whereas Go alpha mRNA was superinduced to a 50- to 100-fold higher level than basal. Activation of protein kinase C with TPA had a strong, synergistic effect on cAMP-mediated stimulation of Gs alpha mRNA, whereas the cAMP-mediated stimulation of Go alpha mRNA was completely blocked. Surprisingly, changes in mRNA levels were not accompanied by any alterations in the levels of immunoreactive Gs alpha and Go alpha proteins.

Up-regulation in late pregnancy of both Go1 alpha and Go2 alpha isoforms in human myometrium

The nature of the 39 kDa pertussis toxin substrate previously detected in human pregnant myometrium was investigated. Comparison of membranes from non-pregnant and from 39-40 week pregnant myometrium revealed a higher level at 39 kDa of pertussis toxin ADP-ribosylation and Go alpha immunoreactivity in late pregnancy. Furthermore, quantification of both Go alpha isoforms with specific anti-alpha o1- and alpha o2-antibodies revealed an increase in their expression in late pregnancy. At the same time, only limited changes in the levels of Gi1,2, Gi3 and Gq were observed. In rat myometrial membranes, neither pertussis toxin substrate nor Go alpha immunoreactivity could be detected at the 39 kDa level. These results demonstrate that, in the human myometrium in late pregnancy, there is an increase in the expression of both isoforms of the Go alpha subunits, suggesting a role for these proteins throughout gestation and/or near term for parturition.

Dopamine neuron membrane physiology: characterization of the transient outward current (IA) and demonstration of a common signal transduction pathway for IA and IK

Dopamine neurons derived from the mesencephalon of embryonic rats were maintained in primary culture, identified and studied with whole-cell patch recording techniques. These neurons demonstrated a rapidly activating and inactivating voltage-dependent outward current which required the presence of K+ ions. This current was termed IA because of its transient nature. It was elicited by step depolarizations from holding potentials more negative than -50 mV and exhibited steady-state inactivation at a membrane potential more positive than -40 mV and half-maximal inactivation observed at -65 mV. This current rapidly achieved peak activation in less than 8 msec and decayed with a time constant (tau) of 58 +/- 5 msec. This current was observed in the presence of tetraethylammonium but was readily blocked by 4-aminopyridine (2-4 mM). This current was also observed to be modulated by stimulation of D2 dopamine receptors (DA autoreceptors) located on the dopamine neurons. Thus, both DA and the D2 receptor agonist quinpirole enhanced the peak IA observed, while the partial D1 receptor agonist SKF 38393 was without effect. The enhancement of IA was confirmed to be due to the activation of D2 receptors as the effects of either DA or quinpirole were blocked by the D2 receptor antagonists eticlopride and sulpiride, but not by the D1 receptor antagonist SCH 23390. Since we have previously demonstrated that the IK present in these cells is also enhanced by D2 receptor stimulation, we investigated the signal transduction pathways involved in coupling DA autoreceptors to both IA and IK. The response of both these potassium currents to DA autoreceptor stimulation was completely abolished by the preincubation of cultures with pertussis toxin, indicating the possible involvement of the G proteins Gi and G(o). In an attempt to further characterize which G protein may be involved, additional experiments were performed. The ability of DA autoreceptor stimulation to augment both currents was also blocked completely when G protein activation was prevented by the intracellular application of GDP beta S (100 microM). In contrast, irreversible activation of G proteins by intracellular application of the nonhydrolyzable GTP analog GTP gamma S (100 microM) mimicked the effects of DA autoreceptor stimulation on both IA and IK. In addition, the intracellular application of a polyclonal antibody that was selective for the alpha-subunit of G(o) completely abolished the DA autoreceptor modulation of both currents while preimmune serum was without effect.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential requirements of sodium for coupling of cannabinoid receptors to adenylyl cyclase in rat brain membranes

Sodium is generally required for optimal inhibition of adenylyl cyclase by Gi/o-coupled receptors. Cannabinoids bind to specific receptors that act like other members of the Gi/o-coupled receptor superfamily to inhibit adenylyl cyclase. However, assay of cannabinoid inhibition of adenylyl cyclase in rat cerebellar membranes revealed that concentrations of NaCl ranging from 0 to 150 mM had no effect on agonist inhibition. This lack of effect of sodium was not unique to cannabinoid receptors, because the same results were observed using baclofen as an agonist for GABAB receptors in cerebellar membranes. The lack of sodium dependence was region-specific, because assay of cannabinoid and opioid inhibition of adenylyl cyclase in striatum revealed an expected sodium dependence, with 50 mM NaCl providing maximal inhibition levels by both sets of agonists. This difference in sodium requirements between these two regions was maintained at the G protein level, because agonist-stimulated low Km GTPase activity was maximal at 50 mM NaCl in striatal membranes, but was maximal in the absence of NaCl in cerebellar membranes. Assay of [3H]WIN 55212-2 binding in cerebellar membranes revealed that the binding of this labeled agonist was sensitive to sodium and guanine nucleotides like other Gi/o-coupled receptors, because both NaCl and the nonhydrolyzable GTP analogue Gpp(NH)p significantly inhibited binding. These results suggest that differences in receptor-G protein coupling exist for cannabinoid receptors between these two brain regions.

Retinoic acid-induced differentiation of human neuroblastoma SH-SY5Y cells is associated with changes in the abundance of G proteins

Western blot analysis, using subtype-specific anti-G protein antibodies, revealed the presence of the following G protein subunits in human neuroblastoma SH-SY5Y cells: Gs alpha, Gi alpha 1, Gi alpha 2, Go alpha, Gz alpha, and G beta. Differentiation of the cells by all-trans-retinoic acid (RA) treatment (10 mumol/L; 6 days) caused substantial alterations in the abundance of distinct G protein subunits. Concomitant with an enhanced expression of mu-opioid binding sites, the levels of the inhibitory G proteins Gi alpha 1 and Gi alpha 2 were found to be significantly increased. This coordinate up-regulation is accompanied by functional changes in mu-opioid receptor-stimulated low-Km GTPase, mu-receptor-mediated adenylate cyclase inhibition, and receptor-independent guanosine 5'-(beta gamma-imido)triphosphate [Gpp(NH)p; 10 nmol/L]-mediated attenuation of adenylate cyclase activity. In contrast, increased levels of inhibitory G proteins had no effect on muscarinic cholinergic receptor-mediated adenylate cyclase inhibition. With respect to stimulatory receptor systems, a reciprocal regulation was observed for prostaglandin E1 (PGE1) receptors and Gs alpha, the G protein subunit activating adenylate cyclase. RA treatment of SH-SY5Y cells increases both the number of PGE1 binding sites and PGE1-stimulated adenylate cyclase activity, but significantly reduced amounts of Gs alpha were found. This down-regulation is paralleled by a decrease in the stimulatory activity of Gs alpha as assessed in S49 cyc- reconstitution assays.(ABSTRACT TRUNCATED AT 250 WORDS)