Receptor and effector interactions of Gs. Functional studies with antibodies to the alpha s carboxyl-terminal decapeptide

Gsα deficiency in the dorsomedial hypothalamus leads to obesity, hyperphagia, and reduced thermogenesis associated with impaired leptin signaling

OBJECTIVE

G_sα couples multiple receptors, including the melanocortin 4 receptor (MC4R), to intracellular cAMP generation. Germline inactivating G_sα mutations lead to obesity in humans and mice. Mice with brain-specific G_sα deficiency also develop obesity with reduced energy expenditure and locomotor activity, and impaired adaptive thermogenesis, but the underlying mechanisms remain unclear.

METHODS

We created mice (DMHGsKO) with G_sα deficiency limited to the dorsomedial hypothalamus (DMH) and examined the effects on energy balance and thermogenesis.

RESULTS

DMHGsKO mice developed severe, early-onset obesity associated with hyperphagia and reduced energy expenditure and locomotor activity, along with impaired brown adipose tissue thermogenesis. Studies in mice with loss of MC4R in the DMH suggest that defective DMH MC4R/G_sα signaling contributes to abnormal energy balance but not to abnormal locomotor activity or cold-induced thermogenesis. Instead, DMHGsKO mice had impaired leptin signaling along with increased expression of the leptin signaling inhibitor protein tyrosine phosphatase 1B in the DMH, which likely contributes to the observed hyperphagia and reductions in energy expenditure, locomotor activity, and cold-induced thermogenesis.

CONCLUSIONS

DMH G_sα signaling is critical for energy balance, thermogenesis, and leptin signaling. This study provides insight into how distinct signaling pathways can interact to regulate energy homeostasis and temperature regulation.

The stimulatory G protein Gsα is required in melanocortin 4 receptor-expressing cells for normal energy balance, thermogenesis, and glucose metabolism

Central melanocortin 4 receptors (MC4Rs) stimulate energy expenditure and inhibit food intake. MC4Rs activate the G protein G_sα, but whether G_sα mediates all MC4R actions has not been established. Individuals with Albright hereditary osteodystrophy (AHO), who have heterozygous G_sα-inactivating mutations, only develop obesity when the G_sα mutation is present on the maternal allele because of tissue-specific genomic imprinting. Furthermore, evidence in mice implicates G_sα imprinting within the central nervous system (CNS) in this disorder. In this study, we examined the effects of G_sα in MC4R-expressing cells on metabolic regulation. Mice with homozygous G_sα deficiency in MC4R-expressing cells (MC4RGsKO) developed significant obesity with increased food intake and decreased energy expenditure, along with impaired insulin sensitivity and cold-induced thermogenesis. Moreover, the ability of the MC4R agonist melanotan-II (MTII) to stimulate energy expenditure and to inhibit food intake was impaired in MC4RGsKO mice. MTII failed to stimulate the secretion of the anorexigenic hormone peptide YY (PYY) from enteroendocrine L cells, a physiological response mediated by MC4R-G_sα signaling, even though baseline PYY levels were elevated in these mice. In G_sα heterozygotes, mild obesity and reduced energy expenditure were present only in mice with a G_sα deletion on the maternal allele in MC4R-expressing cells, whereas food intake was unaffected. These results demonstrate that G_sα signaling in MC4R-expressing cells is required for controlling energy balance, thermogenesis, and peripheral glucose metabolism. They further indicate that G_sα imprinting in MC4R-expressing cells contributes to obesity in G_sα knockout mice and probably in individuals with Albright hereditary osteodystrophy as well.

Redistribution of Gαs in mouse salivary glands following β-adrenergic stimulation

OBJECTIVE

Signalling via β-adrenergic receptors activates heterotrimeric G-proteins, which dissociate into α and βγ subunits. In salivary glands, the α subunit of Gs stimulates adenylate cyclase, increasing cyclic AMP levels and promoting exocytosis. The goals of this study were to determine Gαs localization in salivary glands and whether it undergoes redistribution upon activation.

METHODS

Mouse parotid and submandibular (SMG) glands were fixed with paraformaldehyde and prepared for immunofluorescence labelling with anti-Gαs.

RESULTS

In unstimulated parotid and SMG acinar cells, Gαs was localized mainly to basolateral membranes. Some parotid acinar cells also exhibited cytoplasmic fluorescence. Isoproterenol (IPR) stimulation resulted in decreased membrane fluorescence and increased cytoplasmic fluorescence, which appeared relatively uniform by 30 min. Beginning about 2 h after IPR, cytoplasmic fluorescence decreased and membrane fluorescence increased, approaching unstimulated levels in SMG acini by 4 h. Some parotid acini exhibited cytoplasmic fluorescence up to 8 h after IPR. The IPR-induced redistribution of Gαs was prevented (SMG) or reduced (parotid) by prior injection of propranolol. Striated duct cells of unstimulated mice exhibited general cytoplasmic fluorescence, which was unchanged after IPR.

CONCLUSIONS

Gαs is localized to basolateral membranes of unstimulated salivary acinar cells. Activation of Gαs causes its release from the cell membrane and movement into the cytoplasm. Reassociation of Gαs with the membrane begins about 2 h after stimulation in the SMG, but complete reassociation takes several hours in the parotid gland. The presence of Gαs in striated duct cells suggests a role in signal transduction of secretion and/or electrolyte transport processes.

G(olf) in the basal ganglia

All neurotransmitter and hormone receptors that stimulate adenylyl cyclase are thought to do so via the alpha subunit of the guanine nucleotide binding (G) protein G(s). The basal ganglia contain a well-characterized dopamine-stimulated adenylyl cyclase and D1 dopamine receptors coupled to G(s) are thought to mediate this activity. We have found using immunohistochemistry, immunoblotting, and cholera toxin-dependent ADP ribosylation that the rat basal ganglia contain very high levels of a G(salpha)-like protein; however, it is distinct from the G(s) in other brain regions. Furthermore, in situ hybridization and Northern blot studies showed that the striatum contains remarkably low levels of G(salpha) mRNA. G(olf) is a G protein recently cloned from olfactory sensory neurons which can also stimulate adenylyl cyclase. We have now discovered high levels of G(olf) mRNA expression in the striatum, nucleus accumbens, and olfactory tubercle. Northern blot analyses indicate that in the striatum, G(olf) transcripts are approximately 10-fold more abundant than G(salpha) transcripts. Thus G(olf) is not an olfactory neuronspecific G protein. It is also the major stimulatory G protein in the basal ganglia, where it may couple D 1 dopamine receptors to adenylyl cyclase.

Characterization of ductal and lobular breast carcinomas using novel prolactin receptor isoform specific antibodies

BACKGROUND

Prolactin is a polypeptide hormone responsible for proliferation and differentiation of the mammary gland. More recently, prolactin's role in mammary carcinogenesis has been studied with greater interest. Studies from our laboratory and from others have demonstrated that three specific isoforms of the prolactin receptor (PRLR) are expressed in both normal and cancerous breast cells and tissues. Until now, reliable isoform specific antibodies have been lacking. We have prepared and characterized polyclonal antibodies against each of the human PRLR isoforms that can effectively be used to characterize human breast cancers.

METHODS

Rabbits were immunized with synthetic peptides of isoform unique regions and immune sera affinity purified prior to validation by Western blot and immunohistochemical analyses. Sections of ductal and lobular carcinomas were stained with each affinity purified isoform specific antibody to determine expression patterns in breast cancer subclasses.

RESULTS

We show that the rabbit antibodies have high titer and could specifically recognize each isoform of PRLR. Differences in PRLR isoform expression levels were observed and quantified using histosections from xenografts of established human breast cancer cells lines, and ductal and lobular carcinoma human biopsy specimens. In addition, these results were verified by real-time PCR with isoform specific primers. While nearly all tumors contained LF and SF1b, the majority (76%) of ductal carcinoma biopsies expressed SF1a while the majority of lobular carcinomas lacked SF1a staining (72%) and 27% had only low levels of expression.

CONCLUSIONS

Differences in the receptor isoform expression profiles may be critical to understanding the role of PRL in mammary tumorigenesis. Since these antibodies are specifically directed against each PRLR isoform, they are valuable tools for the evaluation of breast cancer PRLR content and have potential clinical importance in treatment of this disease by providing new reagents to study the protein expression of the human PRLR.

Pancreas-specific Gsalpha deficiency has divergent effects on pancreatic alpha- and beta-cell proliferation

The ubiquitously expressed G protein alpha-subunit G(s)alpha mediates the intracellular cAMP response to glucagon-like peptide 1 (GLP1) and other incretin hormones in pancreatic islet cells. We have shown previously that mice with beta-cell-specific G(s)alpha deficiency (betaGsKO) develop severe early-onset insulin-deficient diabetes with a severe defect in beta-cell proliferation. We have now generated mice with G(s)alpha deficiency throughout the whole pancreas by mating G(s)alpha-floxed mice with Pdx1-cre transgenic mice (PGsKO). PGsKO mice also developed severe insulin-deficient diabetes at a young age, confirming the important role of G(s)alpha signaling in beta-cell growth and function. Unlike in betaGsKO mice, islets in PGsKO mice had a relatively greater proportion of alpha-cells, which were spread throughout the interior of the islet. Similar findings were observed in mice with pancreatic islet cell-specific G(s)alpha deficiency using a neurogenin 3 promoter-cre recombinase transgenic mouse line. Studies in the alpha-cell line alphaTC1 confirmed that reduced cAMP signaling increased cell proliferation while increasing cAMP produced the opposite effect. Therefore, it appears that G(s)alpha/cAMP signaling has opposite effects on pancreatic alpha- and beta-cell proliferation, and that impaired GLP1 action in alpha- and beta-cells via G(s)alpha signaling may be an important contributor to the reciprocal effects on insulin and glucagon observed in type 2 diabetics. In addition, PGsKO mice show morphological changes in exocrine pancreas and evidence for malnutrition and dehydration, indicating an important role for G(s)alpha in the exocrine pancreas as well.

G(s)alpha deficiency in adipose tissue leads to a lean phenotype with divergent effects on cold tolerance and diet-induced thermogenesis

G(s)alpha, the G protein that mediates receptor-stimulated cAMP generation, has been implicated as a regulator of adipogenesis and adipose tissue function. Heterozygous G(s)alpha mutations lead to obesity in Albright hereditary osteodystrophy (AHO) patients and in mice. In this study, we generated mice with adipose-specific G(s)alpha deficiency. Heterozygotes had 50% loss of G(s)alpha expression in adipose tissue and no obvious phenotype, suggesting that adipose-specific G(s)alpha deficiency is not the cause of obesity in AHO. Homozygotes (FGsKO) had severely reduced adipose tissue, indicating that G(s)alpha is required for adipogenesis. Although FGsKO mice had impaired cold tolerance and reduced responsiveness of brown adipose tissue (BAT) to sympathetic signaling, diet-induced thermogenesis and fatty acid oxidation in skeletal muscle were increased. In normal mice, high-fat diet raised sympathetic nerve activity in muscle, but not in BAT. Our results show that cold- and diet-induced thermogenesis may occur in separate tissues, especially when BAT function is impaired.

Central nervous system imprinting of the G protein G(s)alpha and its role in metabolic regulation

In Albright hereditary osteodystrophy, a monogenic obesity disorder linked to heterozygous mutations of G(s)alpha, the G protein that mediates receptor-stimulated cAMP generation, obesity develops only when the mutation is on the maternal allele. Likewise, mice with maternal (but not paternal) germline G(s)alpha mutation develop obesity, insulin resistance, and diabetes. These parent-of-origin effects are due to G(s)alpha imprinting, with preferential expression from the maternal allele in some tissues. As G(s)alpha is ubiquitously expressed, the tissue involved in this metabolic imprinting effect is unknown. Using brain-specific G(s)alpha knockout mice, we show that G(s)alpha imprinting within the central nervous system underlies these effects and that G(s)alpha is imprinted in the paraventricular nucleus of the hypothalamus. Maternal G(s)alpha mutation impaired melanocortin stimulation of energy expenditure but did not affect melanocortin's effect on food intake, suggesting that melanocortins may regulate energy balance in the central nervous system through both G(s)alpha-dependent and -independent pathways.

G(s)alpha deficiency in skeletal muscle leads to reduced muscle mass, fiber-type switching, and glucose intolerance without insulin resistance or deficiency

The ubiquitously expressed G protein alpha-subunit G(s)alpha is required for receptor-stimulated intracellular cAMP responses and is an important regulator of energy and glucose metabolism. We have generated skeletal muscle-specific G(s)alpha-knockout (KO) mice (MGsKO) by mating G(s)alpha-floxed mice with muscle creatine kinase-cre transgenic mice. MGsKO mice had normal body weight and composition, and their serum glucose, insulin, free fatty acid, and triglyceride levels were similar to that of controls. However, MGsKO mice were glucose intolerant despite the fact that insulin sensitivity and glucose-stimulated insulin secretion were normal, suggesting an insulin-independent mechanism. Isolated muscles from MGsKO mice had increased basal glucose uptake and normal responses to a stimulator of AMP-activated protein kinase (AMPK), which indicates that AMPK and its downstream pathways are intact. Compared with control mice, MGsKO mice had reduced muscle mass with decreased cross-sectional area and force production. In addition, adult MGsKO mice showed an increased proportion of type I (slow-twitch, oxidative) fibers based on kinetic properties and myosin heavy chain isoforms, despite the fact that these muscles had reduced expression of peroxisome proliferator-activated receptor coactivator protein-1alpha (PGC-1alpha) and reduced mitochondrial content and oxidative capacity. Therefore G(s)alpha deficiency led to fast-to-slow fiber-type switching, which appeared to be dissociated from the expected change in oxidative capacity. MGsKO mice are a valuable model for future studies of the role of G(s)alpha signaling pathways in skeletal muscle adaptation and their effects on whole body metabolism.

Activation of the progesterone-signaling pathway by methyl-beta-cyclodextrin or steroid in Xenopus laevis oocytes involves release of 45-kDa Galphas

Treatment of Xenopus laevis oocytes with cholesterol-depleting methyl-beta-cyclodextrin (MebetaCD) stimulates phosphorylation of mitogen-activated protein kinase (MAPK) and oocyte maturation, as reported previously [Sadler, S.E., Jacobs, N.D., 2004. Stimulation of Xenopus laevis oocyte maturation by methyl-beta-cyclodextrin. Biol. Reprod. 70, 1685-1692.]. Here we report that treatment of oocytes with MebetaCD increased levels of immunodetectable 39-kDa mos protein. The protein synthesis inhibitor, cycloheximide, blocked the appearance of Mos, blocked MebetaCD-stimulated phosphorylation of MAPK, and inhibited MebetaCD-induced oocyte maturation. These observations suggest that MebetaCD activates the progesterone-signaling pathway. Chemical inhibition of steroid synthesis and mechanical removal of follicle cells were used to verify that MebetaCD acts at the level of the oocyte and does not require production of steroid by surrounding follicle cells. Cortical Galpha(s) is contained in low-density membrane; and treatment of oocytes with progesterone or MebetaCD reduced immunodetectable levels of Galpha(s) protein in cortices and increased internal levels of 45-kDa Galpha(s) in cortical-free extracts. Dose-dependent increases in internal Galpha(s) after treatment of oocytes with progesterone correlated with the steroid-induced maturation response, and the increase in internal Galpha(s) after hormone treatment was comparable to the decrease in cortical Galpha(s). These results are consistent with a model in which release of Galpha(s) from the plasma membrane is involved in activation of the progesterone-signaling pathway that leads to amphibian oocyte maturation.

Antibodies that identify only the active conformation of G(i) family G protein alpha subunits

Production of antisera able to recognize individual heterotrimeric G protein alpha subunits resulted in rapid expansion of information on their distribution and function. However, no antibodies that specifically recognize the active state have been available. Four-way primary screening of 763 hybridomas generated from mice immunized with guanosine 5'-O-(3-thio)triphosphate-loaded G alpha(i1) and isolated using an automated robotic colony picker identified three antibodies that interacted with the constitutively active, Q(204)L, mutant but neither the constitutively inactive, G(203)A, mutant nor wild-type G alpha(i1). This profile extended to other closely related G(i) family G proteins but not to the less closely related G alpha(s) and G alpha(q)/G alpha(11) families. Each antibody was, however, also able to identify wild-type, GDP-bound G(i) family G proteins in the presence of fluoroaluminate, which mimics the presence of the terminal phosphate of GTP and hence generates an active/transition state conformation. Stimulation of cells coexpressing a wild-type G alpha(i) subunit and the dopamine D2 receptor with the agonist ligand nor-apomorphine also allowed these conformationally selective antibodies to bind the G protein. Such reagents allow the specific identification of activated G proteins in a native environment and may allow the development of label-free screening assays for G protein-coupled receptor-mediated activation of G(i) family G proteins.

Meiotic arrest in human oocytes is maintained by a Gs signaling pathway

In mammalian oocytes, the maintenance of meiotic prophase I arrest prior to the surge of LH that stimulates meiotic maturation depends on a high level of cAMP within the oocyte. In mouse and rat, the cAMP is generated in the oocyte, and this requires the activity of a constitutively active, Gs-linked receptor, GPR3 or GPR12, respectively. To examine if human oocyte meiotic arrest depends on a similar pathway, we used RT-PCR and Western blotting to look at whether human oocytes express the same components for maintaining arrest as rodent oocytes. RNA encoding GPR3, but not GPR12, was expressed. RNA encoding adenylate cyclase type 3, which is the major adenylate cyclase required for maintaining meiotic arrest in the mouse oocyte, was also expressed, as was Galphas protein. To determine if this pathway is functional in the human oocyte, we examined the effect of injecting a function-blocking antibody against Galphas on meiotic resumption. This antibody stimulated meiotic resumption of human oocytes that were maintained at the prophase I stage using a phosphodiesterase inhibitor. These results demonstrate that human oocytes maintain meiotic arrest prior to the LH surge using a signaling pathway similar to that of rodent oocytes.

Beta cell-specific deficiency of the stimulatory G protein alpha-subunit Gsalpha leads to reduced beta cell mass and insulin-deficient diabetes

The G protein alpha-subunit G(s)alpha is required for hormone-stimulated cAMP generation. In pancreatic beta cells, G(s)alpha mediates the signaling of glucagon-like peptide 1 and other incretin hormones, which are implicated as important regulators of beta cell survival and insulin release. Studies have suggested that G(s)alpha/cAMP mediates these actions by stimulating insulin receptor substrate 2 (IRS2) expression. Mice with beta cell-specific G(s)alpha deficiency (betaGsKO) were generated by mating G(s)alpha-floxed mice to rat insulin II promoter-cre recombinase mice. betaGsKO mice had poor survival and postnatal growth with low serum insulin-like growth factor 1 levels. betaGsKO mice also developed severe hyperglycemia and glucose intolerance with severe hypoinsulinemia and reduced islet insulin content and glucose-stimulated insulin release. betaGsKO mice had markedly reduced average islet size and beta cell mass, which was partially explained by reduced beta cell size. In addition, betaGsKO mice had significantly reduced beta cell proliferation and increased beta cell apoptosis and markedly reduced expression of the cell cycle protein cyclin D2. The effects on beta cell mass and proliferation, but not apoptosis, were present from birth. Unexpectedly expression of Irs2 and the downstream gene Pdx1 were unaffected. These results show that G(s)alpha/cAMP pathways are critical regulators of beta cell function and proliferation that can work through IRS2-independent mechanisms.

Studies of the regulation and function of the Gs alpha gene Gnas using gene targeting technology

The heterotrimeric G protein alpha-subunit G(s)alpha is ubiquitously expressed and mediates receptor-stimulated intracellular cAMP generation. Its gene Gnas is a complex imprinted gene which uses alternative promoters and first exons to generate other gene products, including the G(s)alpha isoform XL alpha s and the chromogranin-like protein NESP55, which are specifically expressed from the paternal and maternal alleles, respectively. G(s)alpha itself is imprinted in a tissue-specific manner, being biallelically expressed in most tissues but paternally silenced in a few tissues. Gene targeting of specific Gnas transcripts demonstrates that heterozygous mutation of G(s)alpha on the maternal (but not the paternal) allele leads to early lethality, perinatal subcutaneous edema, severe obesity, and multihormone resistance, while the paternal mutation leads to only mild obesity and insulin resistance. These parent-of-origin differences are the consequence of tissue-specific G(s)alpha imprinting. XL alpha s deficiency leads to a perinatal suckling defect and a lean phenotype with increased insulin sensitivity. The opposite metabolic effects of G(s)alpha and XL alpha s deficiency are associated with decreased and increased sympathetic nervous system activity, respectively. NESP55 deficiency has no metabolic consequences. Other gene targeting experiments have shown Gnas to have 2 independent imprinting domains controlled by 2 different imprinting control regions. Tissue-specific G(s)alpha knockout models have identified important roles for G(s)alpha signaling pathways in skeletal development, renal function, and glucose and lipid metabolism. Our present knowledge gleaned from various Gnas gene targeting models are discussed in relation to the pathogenesis of human disorders with mutation or abnormal imprinting of the human orthologue GNAS.

A G(s)-linked receptor maintains meiotic arrest in mouse oocytes, but luteinizing hormone does not cause meiotic resumption by terminating receptor-G(s) signaling

The maintenance of meiotic prophase arrest in fully grown vertebrate oocytes depends on the activity of a G(s) G-protein that activates adenylyl cyclase and elevates cAMP, and in the mouse oocyte, G(s) is activated by a constitutively active orphan receptor, GPR3. To determine whether the action of luteinizing hormone (LH) on the mouse ovarian follicle causes meiotic resumption by inhibiting GPR3-G(s) signaling, we examined the effect of LH on the localization of Galpha(s). G(s) activation in response to stimulation of an exogenously expressed beta(2)-adrenergic receptor causes Galpha(s) to move from the oocyte plasma membrane into the cytoplasm, whereas G(s) inactivation in response to inhibition of the beta(2)-adrenergic receptor causes Galpha(s) to move back to the plasma membrane. However, LH does not cause a change in Galpha(s) localization, indicating that LH does not act by terminating receptor-G(s) signaling.

Regulation of renin in mice with Cre recombinase-mediated deletion of G protein Gsα in juxtaglomerular cells

By crossing mice with expression of Cre recombinase under control of the endogenous renin promoter (Sequeira Lopez ML, Pentz ES, Nomasa T, Smithies O, Gomez RA. Dev Cell 6: 719–728, 2004) with mice in which exon 1 of the Gnas gene was flanked by loxP sites (Chen M, Gavrilova O, Liu J, Xie T, Deng C, Nguyen AT, Nackers LM, Lorenzo J, Shen L, Weinstein LS. Proc Natl Acad Sci USA), we generated animals with preferential and nearly complete excision of Gsα in juxtaglomerular granular (JG) cells. Compared with wild-type animals, mice with conditional Gsα deficiency had markedly reduced basal levels of renin expression and very low plasma renin concentrations. Furthermore, the acute release responses to furosemide, hydralazine, and isoproterenol were virtually abolished. Consistent with a state of primary renin depletion, Gsα-deficient mice had reduced arterial blood pressure, reduced levels of aldosterone, and a low glomerular filtration rate. Renin content and renin secretion of JG cells in primary culture were drastically reduced, and the stimulatory response to the addition of PGE2or isoproterenol was eliminated. Unexpectedly, Gsα recombination was also observed in the renal medulla, and this was associated with a vasopressin-resistant concentrating defect. Our study shows that Cre recombinase under control of the renin promoter can be used for the excision of floxed targets from JG cells. We conclude that Gsα-mediated signal transduction is essential and nonredundant in the control of renin synthesis and release.

Structural features of parathyroid hormone receptor coupled to Galpha(s)-protein

The molecular basis of the activation of G-proteins by the G-protein coupled receptor for parathyroid hormone (PTH) is unknown. Employing a combination of NMR methods and computer-based structural refinement, structural features involved in the activation of Galpha(s) by the PTH receptor (PTH1R) have been determined. Focusing on the C-terminus of the third intracellular loop (IC3), previously shown to be important for Galpha(s) activation by PTH1R, the structure of this region, PTH1R(402-408), while bound to Galpha(s), was determined by transferred nuclear Overhauser effect spectroscopy. The relative topological orientation of the IC3 while associated with Galpha(s) was determined by saturation transfer difference spectroscopy. These experimental data were incorporated into molecular dynamics simulations of the PTH1R and Galpha(s) to provide atomic insight into the receptor-protein interactions important for PTH signaling and a structural framework to analyze previous mutagenesis studies of Galpha(s). These data provide the first step toward development of a molecular mechanism for the signaling profile of PTH1R, an important regulator of calcium levels in the bloodstream.

Regulation of meiotic prophase arrest in mouse oocytes by GPR3, a constitutive activator of the Gs G protein

The arrest of meiotic prophase in mouse oocytes within antral follicles requires the G protein G(s) and an orphan member of the G protein-coupled receptor family, GPR3. To determine whether GPR3 activates G(s), the localization of Galpha(s) in follicle-enclosed oocytes from Gpr3(+/+) and Gpr3(-/-) mice was compared by using immunofluorescence and Galpha(s)GFP. GPR3 decreased the ratio of Galpha(s) in the oocyte plasma membrane versus the cytoplasm and also decreased the amount of Galpha(s) in the oocyte. Both of these properties indicate that GPR3 activates G(s). The follicle cells around the oocyte are also necessary to keep the oocyte in prophase, suggesting that they might activate GPR3. However, GPR3-dependent G(s) activity was similar in follicle-enclosed and follicle-free oocytes. Thus, the maintenance of prophase arrest depends on the constitutive activity of GPR3 in the oocyte, and the follicle cell signal acts by a means other than increasing GPR3 activity.

Effects of ageing on muscarinic receptor subtypes and function in rat urinary bladder

We compared the density and function of M2 and M3 muscarinic acetylcholine receptor subtypes in the urinary bladder of young adult (3 months) and old (23 months) male Wistar rats. Old rats had a reduced density of muscarinic receptors (96+/-10 vs. 156+/-21 fmol/mg protein), but competition experiments with the M3-selective darifenacin did not indicate alterations in the relative roles of M2 and M3 receptors, with the former being more abundant. The amount of immunodetectable alpha-subunits of various G-proteins potentially linked to muscarinic receptor function was unchanged. The potency of carbachol to contract bladder strips was also unaltered; its maximum effects as well as those of a single KCl concentration were unchanged if raw data or those corrected for strip length were analysed, but somewhat reduced when those corrected for strip weight were analysed. Antagonistic effects of atropine, the M2-selective Ro 320-6206 and the M3-selective darifenacin were unchanged. Agonistic effects of the M3-sparing agonist 4-(2-oxo-2,3-dihydro-benzoimidazol-1-yl)-[1,4']bipiperidinyl-1'-carboxylic acid ethyl ester were similarly poor in young and old rats. Additional experiments were concomitantly performed in submandibular glands from the same animals. While total muscarinic receptor density in submandibular glands was not significantly affected by age (56+/-5 vs. 61+/-4 fmol/mg protein), the relative contribution of M3 receptors significantly declined from 68+/-3% to 57+/-2% based upon darifenacin competition curves. We conclude that aged Wistar rats express fewer muscarinic receptors in their urinary bladder, but there is no change in the relative abundance of M2 and M3 receptors; this is accompanied by only minor if any alterations in receptor responsiveness. In contrast, submandibular gland expresses similar receptor numbers in young and old rats, but slightly fewer M3 receptors in old animals.

Identification of the control region for tissue-specific imprinting of the stimulatory G protein alpha-subunit

Gnas is a complex gene with multiple imprinted promoters. The upstream Nesp and Nespas/Gnasxl promoters are paternally and maternally methylated, respectively. The downstream promoter for the stimulatory G protein alpha-subunit (G(s)alpha) is unmethylated, although in some tissues (e.g., renal proximal tubules), G(s)alpha is poorly expressed from the paternal allele. Just upstream of the G(s)alpha promoter is a primary imprint mark (1A region) where maternal-specific methylation is established during oogenesis. Pseudohypoparathyroidism type 1B, a disorder of renal parathyroid hormone resistance, is associated with loss of 1A methylation. Analysis of embryos of Dnmt3L(-/-) mothers (which cannot methylate maternal imprint marks) showed that Nesp, Nespas/Gnasxl, and 1A imprinting depend on one or more maternal primary imprint marks. We generated mice with deletion of the 1A differentially methylated region. These mice had normal Nesp-Nespas/Gnasxl imprinting, indicating that the Gnas locus contains two independent imprinting domains (Nespas-Nespas/Gnasxl and 1A-G(s)alpha) controlled by distinct maternal primary imprint marks. Paternal, but not maternal, 1A deletion resulted in G(s)alpha overexpression in proximal tubules and evidence for increased parathyroid hormone sensitivity but had no effect on G(s)alpha expression in other tissues where G(s)alpha is normally not imprinted. The 1A region is a maternal imprint mark that contains one or more methylation-sensitive cis-acting elements that suppress G(s)alpha expression from the paternal allele in a tissue-specific manner.

Chondrocyte-specific knockout of the G protein G(s)alpha leads to epiphyseal and growth plate abnormalities and ectopic chondrocyte formation

G(s)alpha is a ubiquitously expressed G protein alpha-subunit that couples receptors to adenylyl cyclase. Mice with chondrocyte-specific ablation of the G(s)alpha gene had severe epiphyseal and growth plate abnormalities and ectopic cartilage formation within the metaphyseal region of the tibia. These results show that G(s)alpha negatively regulates chondrocyte differentiation and is the critical signaling mediator of the PTH/PTH-rP receptor in growth plate chondrocytes.

INTRODUCTION

G(s)alpha is a ubiquitously expressed G protein alpha-subunit that mediates signaling through G protein-coupled receptors to activate the cAMP/protein kinase A signaling pathway. Although studies suggest an important role for G(s)alpha in regulating growth plate development, direct in vivo results examining this role are lacking.

MATERIALS AND METHODS

The G(s)alpha gene was ablated in murine cartilage by mating mice with loxP sites surrounding the G(s)alpha promoter and first exon with collagen 2a1 promoter-Cre recombinase transgenic mice. Skeletal tissues were studied by gross and microscopic pathology, and gene expression was determined by in situ hybridization.

RESULTS AND CONCLUSIONS

Mice with complete chondrocyte-specific G(s)alpha deficiency (homozygotes) died within minutes after birth and had severe epiphyseal and growth plate defects with shortening of the proliferative zone and accelerated hypertrophic differentiation of growth plate chondrocytes, a phenotype similar to that of PTH/PTH-related peptide (PTHrP) receptor knockout mice. Indian hedgehog and PTH/PTHrP receptor expression in prehypertrophic chondrocytes was unaffected in mutant mice. PTHrP expression in periarticular cartilage was increased in the mutant mice, probably because of the closer proximity of Ihh-secreting chondrocytes to the periarticular zone. In addition, these mice developed ectopic cartilage at the anterior side of the metaphyseal region in the tibia. Mice with partial G(s)alpha deficiency (heterozygotes) exhibited no phenotype. These results show that G(s)alpha negatively regulates chondrocyte differentiation and is the critical signaling mediator of the PTH/PTHrP receptor in epiphyseal and growth plate chondrocytes.

Deficiency of the G-protein alpha-subunit G(s)alpha in osteoblasts leads to differential effects on trabecular and cortical bone

The G-protein alpha-subunit G(s)alpha is required for the intracellular cAMP responses to hormones and other agonists. G(s)alpha is known to mediate the cAMP response to parathyroid hormone and other hormones and cytokines in bone and cartilage. To analyze the in vivo role of G(s)alpha signaling in osteoblasts, we developed mice with osteoblast/osteocyte-specific G(s)alpha deficiency (BGsKO) by mating G(s)alpha-floxed mice with collagen Ialpha1 promoter-Cre recombinase transgenic mice. Early skeletal development was normal in BGsKO mice, because formation of the initial cartilage template and bone collar was unaffected. The chondrocytic zones of the growth plates also appeared normal in BGsKO mice. BGsKO mice had a defect in the formation of the primary spongiosa with reduced immature osteoid (new bone formation) and overall length, which led to reduced trabecular bone volume. In contrast, cortical bone was thickened with narrowing of the bone marrow cavity. This was probably due to decreased cortical bone resorption, because osteoclasts were markedly reduced on the endosteal surface of cortical bone. In addition, the expression of alkaline phosphatase, an early osteoblastic differentiation marker, was normal, whereas the expression of the late osteoblast differentiation markers osteopontin and osteocalcin was reduced, suggesting that the number of mature osteoblasts in bone is reduced. Expression of the osteoclast-stimulating factor receptor activator of NF-kappaB ligand was also reduced. Overall, our findings have similarities to parathyroid hormone null mice and confirm that the differential effects of parathyroid hormone on trabecular and cortical bone are primarily mediated via G(s)alpha in osteoblasts. Osteoblast-specific G(s)alpha deficiency leads to reduced bone turnover.

Maintenance of meiotic prophase arrest in vertebrate oocytes by a G s protein-mediated pathway

Maintenance of meiotic prophase arrest in fully grown vertebrate oocytes depends on an elevated level of cAMP in the oocyte. To investigate how the cAMP level is regulated, we examined whether the activity of an oocyte G protein of the family that stimulates adenylyl cyclase, Gs, is required to maintain meiotic arrest. Microinjection of a dominant negative form of Gs into Xenopus and mouse oocytes, or microinjection of an antibody that inhibits the Gs G protein into zebrafish oocytes, caused meiosis to resume. Together with previous studies, these results support the conclusion that Gs-regulated generation of cAMP by the oocyte is a common mechanism for maintaining meiotic prophase arrest in vertebrate oocytes.

G(alpha)s levels regulate Xenopus laevis oocyte maturation

Progesterone, produced by follicular cells, induces Xenopus laevis oocyte maturation through a very early event that inhibits the activity of the adenylyl cyclase effector system. The participation of a G-protein has been implicated, based on the fact that the inhibitory effect of the steroid is GTP-dependent, and it has been proposed that progesterone acts interfering with G(alpha)s function at the plasma membrane. Here we investigate whether the change in oocyte G(alpha)s levels affects the maturation process induced by progesterone. Overexpression of X. laevis wild type (wt) G(alpha)s and the constitutive activated G(alpha)s(QL) mutant, both blocked progesterone-induced maturation, G(alpha)s(QL) being much more effective than the wt protein. On the other hand, depletion of G(alpha)s, by the use of antisense oligonucleotides, caused spontaneous maturation measured as MAPK activation, indicating clearly that the presence of G(alpha)s is necessary to keep oocytes arrested. Overexpression of three different G-protein coupled receptors (GPCR), the beta2-adrenergic receptor and the m4 and m5 muscarinic receptors, all caused inhibition of MAPK activation induced by progesterone. These receptors, upon their activation with the respective ligands, might be inducing the release of G(beta)gamma from their respective G(alpha), which together with endogenous G(alpha)s-GTP, activate adenylyl cyclase. Our results indicate that G(alpha)s plays an important role in the maturation process and support previous findings of G(beta)gamma participation, suggesting the presence of a mechanism where a constitutively activated G(alpha)s subunit, together with the G(beta)gamma heterodimer, both maintain high levels of intracellular cAMP levels, blocking the G2/M transition.

Meiotic arrest in the mouse follicle maintained by a Gs protein in the oocyte

The mammalian ovarian follicle consists of a multilayered complex of somatic cells that surround the oocyte. A signal from the follicle cells keeps the oocyte cell cycle arrested at prophase of meiosis I until luteinizing hormone from the pituitary acts on the follicle cells to release the arrest, causing meiosis to continue. Here we show that meiotic arrest can be released in mice by microinjecting the oocyte within the follicle with an antibody that inhibits the stimulatory heterotrimeric GTP-binding protein Gs. This indicates that Gs activity in the oocyte is required to maintain meiotic arrest within the ovarian follicle and suggests that the follicle may keep the cell cycle arrested by activating Gs.

Accumulation of cyclic AMP in Swiss 3T3 cells adhering to a cellulose biomaterial substratum through interaction with adenylyl cyclase

Controlling cell shape induced by cell-substrata interaction appears of prime importance to influence subsequent biological processes such as cell migration, proliferation, differentiation or apoptosis. Studies on Swiss 3T3 fibroblasts have recently provided evidence that cell spreading is mediated by integrins and RhoA. Our previous studies showed that on Cuprophan, a cellulose membrane (CU) to which vitronectin adhesive protein is poorly adsorbed, Swiss 3T3 cells are rounded and undergo cAMP-dependent aggregation. In contrast, on a polyacrylonitrile membrane (AN69) that favours the adsorption of vitronectin and fibronectin, cells spread out and contain low concentrations of cAM P. We have now examined the parts played by the three components in the cAMP pathway (receptors, G-proteins and adenylyl cyclase itself) in cAM P-dependent cell aggregation on CU. Experiments with intact cells showed no interaction between the CU and receptors, or between the CU and G-proteins. Assays on membrane preparations plus the Mn-ATP substrate, which uncouples G-proteins and adenylyl cyclase, demonstrated that activation of the cAMP pathway by CU depends primarily on the catalytic activity of the adenylyl cyclase. These investigations provide essential data for the development of biomaterials that favour cell functionality.

Endocrine manifestations of stimulatory G protein alpha-subunit mutations and the role of genomic imprinting

The heterotrimeric G protein G(s) couples hormone receptors (as well as other receptors) to the effector enzyme adenylyl cyclase and is therefore required for hormone-stimulated intracellular cAMP generation. Receptors activate G(s) by promoting exchange of GTP for GDP on the G(s) alpha-subunit (G(s)alpha) while an intrinsic GTPase activity of G(s)alpha that hydrolyzes bound GTP to GDP leads to deactivation. Mutations of specific G(s)alpha residues (Arg(201) or Gln(227)) that are critical for the GTPase reaction lead to constitutive activation of G(s)-coupled signaling pathways, and such somatic mutations are found in endocrine tumors, fibrous dysplasia of bone, and the McCune-Albright syndrome. Conversely, heterozygous loss-of-function mutations may lead to Albright hereditary osteodystrophy (AHO), a disease characterized by short stature, obesity, brachydactyly, sc ossifications, and mental deficits. Similar mutations are also associated with progressive osseous heteroplasia. Interestingly, paternal transmission of GNAS1 mutations leads to the AHO phenotype alone (pseudopseudohypoparathyroidism), while maternal transmission leads to AHO plus resistance to several hormones (e.g., PTH, TSH) that activate G(s) in their target tissues (pseudohypoparathyroidism type IA). Studies in G(s)alpha knockout mice demonstrate that G(s)alpha is imprinted in a tissue-specific manner, being expressed primarily from the maternal allele in some tissues (e.g., renal proximal tubule, the major site of renal PTH action), while being biallelically expressed in most other tissues. Disrupting mutations in the maternal allele lead to loss of G(s)alpha expression in proximal tubules and therefore loss of PTH action in the kidney, while mutations in the paternal allele have little effect on G(s)alpha expression or PTH action. G(s)alpha has recently been shown to be also imprinted in human pituitary glands. The G(s)alpha gene GNAS1 (as well as its murine ortholog Gnas) has at least four alternative promoters and first exons, leading to the production of alternative gene products including G(s)alpha, XLalphas (a novel G(s)alpha isoform that is expressed only from the paternal allele), and NESP55 (a chromogranin-like protein that is expressed only from the maternal allele). A fourth alternative promoter and first exon (exon 1A) located approximately 2.5 kb upstream of the G(s)alpha promoter is normally methylated on the maternal allele and transcriptionally active on the paternal allele. In patients with isolated renal resistance to PTH (pseudohypoparathyroidism type IB), the exon 1A promoter region has a paternal-specific imprinting pattern on both alleles (unmethylated, transcriptionally active), suggesting that this region is critical for the tissue-specific imprinting of G(s)alpha. The GNAS1 imprinting defect in pseudohypoparathyroidism type IB is predicted to decrease G(s)alpha expression in renal proximal tubules. Studies in G(s)alpha knockout mice also demonstrate that this gene is critical in the regulation of lipid and glucose metabolism.

Specific combinations of G-protein subunits discriminate hormonal signalling in rat pituitary (GH(3)) cells in culture

It was previously shown that hormone receptor coupling to voltage-dependent calcium channels in prolactin and growth hormone-producing GH(3) cells was heavily dependent on the specific heterotrimeric combinations of alpha, beta, and gamma subunits of the guanosine triphosphate (GTP)-binding protein family. Consequently, we assessed whether this was also the case for hormonal modulation of the adenylate cyclase (AC) and phospholipase C (PL-C) effector enzymes in GH(3) cells in culture. By employing polyclonal antibodies directed towards C-terminal decapeptides of various alpha subunits in membrane assays, as well as antisense oligonucleotides towards certain beta- and gamma-subunit genes in whole-cell incubations, it was possible to unravel a tentative profile of heterotrimers preferred by some of the seven-transmembrane-stretch receptors in their modulation of AC and PL-C activities. Vasoactive intestinal peptide (VIP) and thyroliberin (TRH) activate membrane-bound AC through alpha(s)beta(2)gamma(2), while somatostatin (SRIH) and dopamine (DA) inhibited the AC through alpha(i2)beta(1)gamma(3). TRH activated membrane-bound PL-C through alpha(q/11)beta(4)gamma(2), while DA inhibition of the PL-C was accomplished via alpha(o)beta(3)gamma(4). Hence, it seems that not only the specificity of alpha subunits determines the coupling between G protein-associated receptors in GH cells, the receptor binding to G proteins also requires certain combinations of beta and gamma subunits.

Selective resistance to parathyroid hormone caused by a novel uncoupling mutation in the carboxyl terminus of G alpha(s). A cause of pseudohypoparathyroidism type Ib

G(s) is a heterotrimeric (alpha, beta, and gamma chains) G protein that couples heptahelical plasma membrane receptors to stimulation of adenylyl cyclase. Inactivation of one GNAS1 gene allele encoding the alpha chain of G(s) (G alpha(s)) causes pseudohypoparathyroidism type Ia. Affected subjects have resistance to parathyroid hormone (PTH) and other hormones that activate adenylyl cyclase plus somatic features termed Albright hereditary osteodystrophy. By contrast, subjects with pseudohypoparathyroidism type Ib have hormone resistance that is limited to PTH and lack Albright hereditary osteodystrophy. The molecular basis for pseudohypoparathyroidism type Ib is unknown. We analyzed the GNAS1 gene for mutations using polymerase chain reaction to amplify genomic DNA from three brothers with pseudohypoparathyroidism type Ib. We identified a novel heterozygous 3-base pair deletion causing loss of isoleucine 382 in the three affected boys and their clinically unaffected mother and maternal grandfather. This mutation was absent in other family members and 15 additional unrelated subjects with pseudohypoparathyroidism type Ib. To characterize the signaling properties of the mutant G alpha(s), we used site-directed mutagenesis to introduce the isoleucine 382 deletion into a wild type G alpha(s) cDNA, transfected HEK293 cells with either wild type or mutant G alpha(s) cDNA, plus cDNAs encoding heptahelical receptors for PTH, thyrotropic hormone, or luteinizing hormone, and we measured cAMP production in response to hormone stimulation. The mutant G alpha(s) protein was unable to interact with the receptor for PTH but showed normal coupling to the other coexpressed heptahelical receptors. These results provide evidence of selective uncoupling of the mutant G alpha(s) from PTH receptors and explain PTH-specific hormone resistance in these three brothers with pseudohypoparathyroidism type Ib. The absence of PTH resistance in the mother and maternal grandfather who carry the same mutation is consistent with current models of paternal imprinting of the GNAS1 gene.

Characterization of the extra-large G protein alpha-subunit XLalphas. II. Signal transduction properties

In the preceding paper (Pasolli, H. A., Klemke, M., Kehlenbach, R. H. , Wang, Y., and Huttner, W. B. (2000) J. Biol. Chem. 275, 33622-33632), we report on the tissue distribution and subcellular localization of XLalphas (extra large alphas), a neuroendocrine-specific, plasma membrane-associated protein consisting of a novel 37-kDa XL domain followed by a 41-kDa alphas domain encoded by exons 2-13 of the Galphas gene. Here, we have studied the signal transduction properties of XLalphas. Like Galphas, XLalphas undergoes a conformational change upon binding of GTPgammaS (guanosine 5'-O-(thio)triphosphate), as revealed by its partial resistance to tryptic digestion, which generated the same fragments as in the case of Galphas. Two approaches were used to analyze XLalphas-betagamma interactions: (i) ADP-ribosylation by cholera toxin to detect even weak or transient XLalphas-betagamma interactions and (ii) sucrose density gradient centrifugation to reveal stable heterotrimer formation. The addition of betagamma subunits resulted in an increased ADP-ribosylation of XLalphas as well as an increased sedimentation rate of XLalphas in sucrose density gradients, indicating that XLalphas interacts with the betagamma dimer. Surprisingly, however, XLalphas, in contrast to Galphas, was not activated by the beta2-adrenergic receptor upon reconstitution of S49cyc(-) membranes. Similarly, using photoaffinity labeling of pituitary membranes with azidoanilide-GTP, XLalphas was not activated upon stimulation of pituitary adenylyl cyclase-activating polypeptide (PACAP) receptors or other Galphas-coupled receptors known to be present in these membranes, whereas Galphas was. Despite the apparent inability of XLalphas to undergo receptor-mediated activation, XLalphas-GTPgammaS markedly stimulated adenylyl cyclase in S49cyc(-) membranes. Moreover, transfection of PC12 cells with a GTPase-deficient mutant of XLalphas, XLalphas-Q548L, resulted in a massive increase in adenylyl cyclase activity. Our results suggest that in neuroendocrine cells, the two related G proteins, Galphas and XLalphas, exhibit distinct properties with regard to receptor-mediated activation but converge onto the same effector system, adenylyl cyclase.

Selective stabilization of the high affinity binding conformation of glucagon receptor by the long splice variant of Galpha(s)

To analyze functional differences in the interactions of the glucagon receptor (GR) with the two predominant splice variants of Galpha(s), GR was covalently linked to the short and the long forms Galpha(s)-S and Galpha(s)-L to produce the fusion proteins GR-Galpha(s)-S and GR-Galpha(s)-L. GR-Galpha(s)-S bound glucagon with an affinity similar to that of GR, while GR-Galpha(s)-L showed a 10-fold higher affinity for glucagon. In the presence of GTPgammaS, GR-Galpha(s)-L reverted to the low affinity glucagon binding conformation. Both GR-Galpha(s)-L and GR-Galpha(s)-S were constitutively active, causing elevated basal levels of cAMP even in the absence of glucagon. A mutant GR that failed to activate G(s) (G23D1R) was fused to Galpha(s)-L. G23D1R-Galpha(s)-L bound glucagon with high affinity, but failed to elevate cAMP levels, suggesting that the mechanisms of GR-mediated Galpha(s)-L activation and Galpha(s)-L-induced high affinity glucagon binding are independent. Both GR-Galpha(s)-S and GR-Galpha(s)-L bound the antagonist desHis(1)[Nle(9),Ala(11),Ala(16)]glucagon amide with affinities similar to GR. The antagonist displayed partial agonist activity with GR-Galpha(s)-L, but not with GR-Galpha(s)-S. Therefore, the partial agonist activity of the antagonist observed in intact cells appears to be due to GRs coupled to Galpha(s)-L. We conclude that Galpha(s)-S and Galpha(s)-L interact differently with GR and that specific coupling of GR to Galpha(s)-L may account for GTP-sensitive high affinity glucagon binding.

Changes of beta-adrenergic signaling in compensated human cardiac hypertrophy depend on the underlying disease

In human heart failure, desensitization of the beta-adrenergic signal transduction has been reported to be one of the main pathophysiological alterations. However, data on the beta-adrenergic system in human compensated cardiac hypertrophy are very limited. Therefore, we studied the myocardial beta-adrenergic signaling in patients suffering from hypertrophic obstructive cardiomyopathy (HOCM, n = 9) or from aortic valve stenosis (AoSt, n = 8). beta-Adrenoceptor density determined by [(125)I]iodocyanopindolol binding was reduced in HOCM and AoSt compared with nonhypertrophied, nonfailing myocardium (NF) of seven organ donors. In HOCM the protein expression of stimulatory G protein alpha-subunit (G(s)alpha) measured by immunoblotting was unchanged, whereas the inhibitory G protein alpha-subunit (Galpha(i-2)) was increased. In contrast, in AoSt, Galpha(i-2) protein was unchanged, but G(s)alpha protein was increased. Adenylyl cyclase stimulation by isoproterenol was reduced in HOCM but not in AoSt. Plasma catecholamine levels were normal in all patients. In conclusion, both forms of hypertrophy are associated with beta-adrenoceptor downregulation but with different changes at the G protein level that occur before symptomatic heart failure due to progressive dilatation of the left ventricle develops and are not due to elevated plasma catecholamine levels.

Regulation of NHE3 activity by G protein subunits in renal brush-border membranes

NHE3 activity is regulated by phosphorylation/dephosphorylation processes and membrane recycling in intact cells. However, the Na(+)/H(+) exchanger (NHE) can also be regulated by G proteins independent of cytoplasmic second messengers, but the G protein subunits involved in this regulation are not known. Therefore, we studied G protein subunit regulation of NHE3 activity in renal brush-border membrane vesicles (BBMV) in a system devoid of cytoplasmic components and second messengers. Basal NHE3 activity was not regulated by G(s)alpha or G(i)alpha, because antibodies to these G proteins by themselves were without effect. The inhibitory effect of D(1)-like agonists on NHE3 activity was mediated, in part, by G(s)alpha, because it was partially reversed by anti-G(s)alpha antibodies. Moreover, the amount of G(s)alpha that coimmunoprecipitated with NHE3 was increased by fenoldopam in both brush-border membranes and renal proximal tubule cells. Furthermore, guanosine 5'-O-(3-thiotriphosphate) but not guanosine 5'-O-(2-thiodiphosphate), the inactive analog of GDP, increased the amount of G(s)alpha that coimmunoprecipitated with NHE3. The alpha(2)-adrenergic agonist, UK-14304 or pertussis toxin (PTX) alone had no effect on NHE3 activity, but UK-14304 and PTX treatment attenuated the D(1)-like receptor-mediated NHE3 inhibition. The ability of UK-14304 to attenuate the D(1)-like agonist effect was not due to G(i)alpha, because the attenuation was not blocked by anti-G(i)alpha antibodies or by PTX. Anti-Gbeta(common) antibodies, by themselves, slightly inhibited NHE3 activity but had little effect on D(1)-like receptor-mediated NHE3 inhibition. However, anti-Gbeta(common) antibodies reversed the effects of UK-14304 and PTX on D(1)-like agonist-mediated NHE3 inhibition. These studies provide concrete evidence of a direct regulatory role for G(s)alpha, independent of second messengers, in the D(1)-like-mediated inhibition of NHE3 activity in rat renal BBMV. In addition, beta/gamma dimers of heterotrimeric G proteins appear to have a stimulatory effect on NHE3 activity in BBMV.

G(s) and adenylyl cyclase in transverse tubules of heart: implications for cAMP-dependent signaling

The transverse tubules are highly specialized invaginations of the cardiac sarcolemmal membrane involved in excitation-contraction (EC) coupling. Several proteins directly involved in EC coupling have been shown to reside either in the transverse tubular membrane or in closely associated structures. With the use of immunofluorescence microscopy, we have found that G(S) and adenylyl cyclase, key elements in the beta-adrenergic signal transduction cascade, are essentially homogeneously distributed throughout the transverse tubular network of isolated rat ventricular myocytes. G(S), in particular, was much more abundant within the transverse tubular membrane than in the peripheral sarcolemma. Furthermore, both proteins are also present in the intercalated disk region. The location of these elements of the cAMP-signaling cascade within a few micrometers of every inotropic target suggests that control and action of this second messenger are quite local. Furthermore, a similar distribution is likely for negatively inotropic receptor systems that oppose G(S)-linked receptors at the level of adenylyl cyclase. Thus, in addition to their role in EC coupling, transverse tubules appear to be the primary site for signaling by inotropic agents.

Decreased renal Na-K-2Cl cotransporter abundance in mice with heterozygous disruption of the G(s)alpha gene

Transport processes along the nephron are regulated in part by hormone stimulation of adenylyl cyclases mediated by the heterotrimeric G protein G(s). To assess the role of this pathway in the regulation of Na-K-2Cl cotransporter abundance in the renal thick ascending limb (TAL), we studied mice with heterozygous disruption of the Gnas gene, which codes for the alpha-subunit of G(s). Outer medullary G(s)alpha protein abundance (as assessed by semiquantitative immunoblotting) and glucagon-stimulated cAMP production were significantly reduced in the heterozygous G(s)alpha knockout mice (GSKO) relative to their wild-type (WT) littermates. Furthermore, Na-K-2Cl cotransporter protein abundance in the outer medulla was significantly reduced (band density, 48% of WT). In addition, GSKO mice had a significantly reduced (72% of WT) urinary osmolality in response to a single injection of 1-deamino-[8-D-arginine]vasopressin (DDAVP), a vasopressin analog. In contrast, outer medullary protein expression of the type 3 Na/H exchanger (NHE-3) or Tamm-Horsfall protein did not differ between the GSKO mice and their WT littermates. However, abundance of type VI adenylyl cyclase was markedly decreased in the outer medullas of GSKO mice, suggesting a novel feed-forward regulatory mechanism. We conclude that expression of the Na-K-2Cl cotransporter of the TAL is dependent on G(s)alpha-mediated hormone stimulation, most likely due to long-term changes in cellular cAMP levels.

Alternate coupling of receptors to Gs and Gi in pancreatic and submandibular gland cells

Many Gs-coupled receptors can activate both cAMP and Ca2+ signaling pathways. Three mechanisms for dual activation have been proposed. One is receptor coupling to both Gs and G15 (a Gq class heterotrimeric G protein) to initiate independent signaling cascades that elevate intracellular levels of cAMP and Ca+2, respectively. The other two mechanisms involve cAMP-dependent protein kinase-mediated activation of phospholipase Cbeta either directly or by switching receptor coupling from Gs to Gi. These mechanisms were primarily inferred from studies with transfected cell lines. In native cells we found that two Gs-coupled receptors (the vasoactive intestinal peptide and beta-adrenergic receptors) in pancreatic acinar and submandibular gland duct cells, respectively, evoke a Ca2+ signal by a mechanism involving both Gs and Gi. This inference was based on the inhibitory action of antibodies specific for Galphas, Galphai, and phosphatidylinositol 4,5-bisphosphate, pertussis toxin, RGS4, a fragment of beta-adrenergic receptor kinase and inhibitors of cAMP-dependent protein kinase. By contrast, Ca2+ signaling evoked by Gs-coupled receptor agonists was not blocked by Gq class-specific antibodies and was unaffected in Galpha15 -/- knockout mice. We conclude that sequential activation of Gs and Gi, mediated by cAMP-dependent protein kinase, may represent a general mechanism in native cells for dual stimulation of signaling pathways by Gs-coupled receptors.

A mutation in the heterotrimeric stimulatory guanine nucleotide binding protein alpha-subunit with impaired receptor-mediated activation because of elevated GTPase activity

It has been reported that substitution of Arg258, a residue within the GTPase domain of the heterotrimeric guanine nucleotide binding protein (G protein) alpha-subunit (alphas), to alanine (alphas-R258A) results in decreased activation by receptor or aluminum fluoride (AlF4-) and increased basal GDP release. Arg258 interacts with Gln170 in the helical domain, and, presumably, loss of this interaction between the GTPase and helical domain leads to more rapid GDP release, resulting in decreased activation by AlF4- and increased thermolability. In this study, we mutate Gln170 to alanine (alphas-Q170A) and demonstrate that this mutant, like alphas-R258A, has decreased activation by AlF4-, increased thermolability (both reversed in the presence of excess guanine nucleotide), and an increased rate of GDP release. However, unlike alphas-R258A, alphas-Q170A does not have impaired receptor-mediated activation. Therefore, this interdomain interaction is critical to maintain normal guanine nucleotide binding (and hence normal activation by AlF4-) but is not important for receptor-mediated activation. In single turnover GTPase assays, the catalytic rate for GTP hydrolysis of alphas-R258A was 14-fold higher than normal whereas that of alphas-Q170A was unaffected. Examination of the alphas crystal structure suggests that Arg258, through interactions with Glu50, might constrain the position of Arg201, a residue critical for catalyzing the GTPase reaction. This is an example of a mutation in a heterotrimeric G protein that results in an increased intrinsic GTPase activity and provides another mechanism by which G protein mutations can impair signal transduction.

Bimodal regulation of cyclic AMP by muscarinic receptors. Involvement of multiple G proteins and different forms of adenylyl cyclase

In membranes of rat olfactory bulb, muscarinic receptor agonists stimulate basal adenylyl cyclase activity . This response is inhibited by a number of muscarinic receptor antagonists with a rank order of potency suggesting the involvement of the M4 muscarinic receptor subtype. The stimulatory effect does not require Ca2+ and occurs independently of activation of phosphoinositide hydrolysis. Pretreatment with pertussis toxin completely prevents the muscarinic stimulation of adenylyl cyclase, indicating the participation of G proteins of the Gi/Go family. Immunological impairment of the G protein, Gs, also reduces the muscarinic response, whereas concomitant activation of Gs-coupled receptors by CRH or VIP results in a synergistic stimulation of adenylyl cyclase activity. Although these data suggest a role for Gs, a body of evidence indicates that the muscarinic receptors do not interact directly with this G protein. Moreover, the Ca2+/calmodulin (Ca2+/CaM)- and forskolin-stimulated enzyme activities are inhibited by muscarinic receptor activation in a pertussis toxin-sensitive manner and with a pharmacological profile similar to that observed for the stimulatory response. These data indicate that in rat olfactory bulb M4 muscarinic receptors exert a bimodal control on cyclic AMP formation through a sequence of events that may involve activation of Gi/Go proteins, synergistic interaction with Gs and differential modulation of Ca2+/CaM-independent and -dependent forms of adenylyl cyclase.

Mutagenesis of the conserved residue Glu259 of Gsalpha demonstrates the importance of interactions between switches 2 and 3 for activation

We previously reported that substitution of Arg258 within the switch 3 region of Gsalpha impaired activation and increased basal GDP release due to loss of an interaction between the helical and GTPase domains (Warner, D. R., Weng, G., Yu, S., Matalon, R., and Weinstein, L. S. (1998) J Biol. Chem. 273, 23976-23983). The adjacent residue (Glu259) is strictly conserved in G protein alpha-subunits and is predicted to be important in activation. To determine the importance of Glu259, this residue was mutated to Ala (Gsalpha-E259A), Gln (Gsalpha-E259Q), Asp (Gsalpha-E259D), or Val (Gsalpha-E259V), and the properties of in vitro translation products were examined. The Gsalpha-E259V was studied because this mutation was identified in a patient with Albright hereditary osteodystrophy. S49 cyc reconstitution assays demonstrated that Gsalpha-E259D stimulated adenylyl cyclase normally in the presence of GTPgammaS but was less efficient with isoproterenol or AlF4-. The other mutants had more severely impaired effector activation, particularly in response to AlF4-. In trypsin protection assays, GTPgammaS was a more effective activator than AlF4- for all mutants, with Gsalpha-E259D being the least severely impaired. For Gsalpha-E259D, the AlF4--induced activation defect was more pronounced at low Mg2+ concentrations. Gsalpha-E259D and Gsalpha-E259A purified from Escherichia coli had normal rates of GDP release (as assessed by the rate GTPgammaS binding). However, for both mutants, the ability of AlF4- to decrease the rate of GTPgammaS binding was impaired, suggesting that they bound AlF4- more poorly. GTPgammaS bound to purified Gsalpha-E259D irreversibly in the presence of 1 mM free Mg2+, but dissociated readily at micromolar concentrations. Sucrose density gradient analysis of in vitro translates demonstrated that all mutants except Gsalpha-E259V bind to beta gamma at 0 degreesC and were stable at higher temperatures. In the active conformation Glu259 interacts with conserved residues in the switch 2 region that are important in maintaining both the active state and AlF4- in the guanine nucleotide binding pocket. Although both Gsalpha Arg258 and Glu259 are critical for activation, the mechanisms by which these residues affect Gsalpha protein activation are distinct.

A decrease in the amount and function of inhibitory GTP-binding protein in the resistance small artery from spontaneously hypertensive rats

The inhibitory GTP-binding protein (Gi protein) plays an important role in regulation of vascular tone. Many studies have implicated the role of Gi protein in conduit vessels. However, the physiological role of Gi protein in the control of peripheral microvascular tone in hypertension has not been established yet. Therefore, we investigated the concentration of Gi protein in the peripheral resistance arteries and aorta in the spontaneously hypertensive rats (SHR), normotensive Wistar-Kyoto rats (WKY) and renovascular hypertensive rats (RHR), using immunohistochemical methods semiquantitatively. Changes in the function of Gi protein in relation to alpha2-adrenoceptor were also investigated by microcannulation techniques. We have shown that the amount of alpha2 subunits of Gi protein in the cremaster small artery was significantly lower in SHR aged 4 weeks and older than in age-matched WKY and that there were no significant differences between RHR and WKY. We also demonstrated that the function of Gi protein in relation to alpha2-adrenoceptor was already lower in SHR before the onset of hypertension. The quantitative and functional decline in Gi protein in the smooth muscle cells of peripheral small arteries were observed in SHR even before the onset of hypertension, whereas rats with secondary hypertension did not exhibit this finding.

Conformational changes at the carboxyl terminus of Galpha occur during G protein activation

To understand the dynamics of conformational changes during G protein activation, surface exposed cysteine residues on Galpha were fluorescently labeled. Limited trypsinolysis and mutational analysis of recombinant Galphat/Galphai1 determined that two cysteines are the major fluorescent labeling sites, Cys210, located in the switch II region, and Cys347 at the C terminus. Mutants with serines replacing Cys210 (Chi6a) and Cys347 (Chi6b) were single fluorescently labeled with lucifer yellow (LY), while a double mutant (Chi6ab) was no longer labeled. When Chi6b was labeled with LY on Cys210, AlF4- caused a 220% increase in LY fluorescence, indicating that the fluorescent group at Cys210 is a reporter of conformational change in the switch II region. Chi6a labeled at Cys347 also showed an AlF4--dependent increase in LY fluorescence (91%), indicating that Galpha activation leads to a conformational change at the COOH terminus. Preactivation of the protein with AlF4- before labeling led to a decreased incorporation of LY into Cys347 suggesting that Galpha activation buries Cys347. This COOH-terminal conformational change may provide the structural basis for communication between the GDP-binding site on Galpha and activated receptors, and may contribute to dissociation of activated Galpha subunit from activated receptor.

A novel mutation in the switch 3 region of Gsalpha in a patient with Albright hereditary osteodystrophy impairs GDP binding and receptor activation

Albright hereditary osteodystrophy (AHO), a disorder characterized by skeletal abnormalities and obesity, is associated with heterozygous inactivating mutations in the gene for Gsalpha. A novel Gsalpha mutation encoding the substitution of tryptophan for a nonconserved arginine within the switch 3 region (Gsalpha R258W) was identified in an AHO patient. Although reverse transcription-polymerase chain reaction studies demonstrated that mRNA expression from wild type and mutant alleles was similar, Gsalpha expression in erythrocyte membranes from the affected patient was reduced by 50%. A Gsalpha R258W cDNA, as well as one with arginine replaced by alanine (Gsalpha R258A), was generated, and the biochemical properties of in vitro transcription/translation products were examined. When reconstituted with cyc- membranes, both mutant proteins were able to stimulate adenylyl cyclase normally in the presence of guanosine- 5'-O-(3-thiotriphosphate) (GTPgammaS) but had decreased ability in the presence of isoproterenol or AlF4- (a mixture of 10 microM AlCl3 and 10 mM NaF). The ability of each mutant to bind and be activated by GTPgammaS or AlF4- was assessed by trypsin protection assays. Both mutants were protected normally by GTPgammaS but showed reduced protection in the presence of AlF4-. The addition of excess GDP (2 mM) was able to rescue the ability of AlF4- to protect the mutants, suggesting that they might have reduced affinity for GDP. A Gsalpha R258A mutant purified from Escherichia coli had decreased affinity for GDP and an apparent rate of GDP release that was 10-fold greater than that of wild type Gsalpha. Sucrose density gradient analysis demonstrated that both Gsalpha R258W and Gsalpha R258A were thermolabile at higher temperatures and that denaturation of both mutants was prevented by the presence of 0.1 mM GTPgammaS or 2 mM GDP. The crystal structure of Gsalpha demonstrates that Arg258 interacts with a conserved residue in the helical domain (Gln170). Arg258 substitutions would be predicted to open the cleft between the GTPase and helical domains, allowing for increased GDP release in the inactive state, resulting in enhanced thermolability and reduced AlF4--induced adenylyl cyclase stimulation and trypsin protection, since activation by AlF4- requires bound GDP.

Variable and tissue-specific hormone resistance in heterotrimeric Gs protein alpha-subunit (Gsalpha) knockout mice is due to tissue-specific imprinting of the gsalpha gene

Albright hereditary osteodystrophy (AHO), an autosomal dominant disorder characterized by short stature, obesity, and skeletal defects, is associated with heterozygous inactivating mutations of GNAS1, the gene encoding the heterotrimeric G protein alpha-subunit (Gsalpha) that couples multiple receptors to the stimulation of adenylyl cyclase. It has remained unclear why only some AHO patients present with multihormone resistance and why AHO patients demonstrate resistance to some hormones [e.g., parathyroid hormone (PTH)] but not to others (e.g., vasopressin), even though all activate adenylyl cyclase. We generated mice with a null allele of the mouse homolog Gnas. Homozygous Gs deficiency is embryonically lethal. Heterozygotes with maternal (m-/+) and paternal (+/p-) inheritance of the Gnas null allele have distinct phenotypes, suggesting that Gnas is an imprinted gene. PTH resistance is present in m-/+, but not +/p-, mice. Gsalpha expression in the renal cortex (the site of PTH action) is markedly reduced in m-/+ but not in +/p- mice, demonstrating that the Gnas paternal allele is imprinted in this tissue. Gnas is also imprinted in brown and white adipose tissue. The maximal physiological response to vasopressin (urinary concentrating ability) is normal in both m-/+ and +/p- mice and Gnas is not imprinted in the renal inner medulla (the site of vasopressin action). Tissue-specific imprinting of Gnas is likely the mechanism for variable and tissue-specific hormone resistance in these mice and a similar mechanism might explain the variable phenotype in AHO.

Antagonists of the receptor-G protein interface block Gi-coupled signal transduction

The carboxyl terminus of heterotrimeric G protein alpha subunits plays an important role in receptor interaction. We demonstrate that peptides corresponding to the last 11 residues of Galphai1/2 or Galphao1 impair agonist binding to A1 adenosine receptors, whereas Galphas or Galphat peptides have no effect. Previously, by using a combinatorial library we identified a series of Galphat peptide analogs that bind rhodopsin with high affinity (Martin, E. L., Rens-Domiano, S., Schatz, P. J., and Hamm, H. E. (1996) J. Biol. Chem. 271, 361-366). Native Galphai1/2 peptide as well as several analogs were tested for their ability to modulate agonist binding or antagonist-agonist competition using cells overexpressing human A1 adenosine receptors. Three peptide analogs decreased the Ki, suggesting that they disrupt the high affinity receptor-G protein interaction and stabilize an intermediate affinity state. To study the ability of the peptides to compete with endogenous Galphai proteins and block signal transduction in a native setting, we measured activation of G protein-coupled K+ channels through A1 adenosine or gamma-aminobutyric acid, type B, receptors in hippocampal CA1 pyramidal neurons. Native Galphai1/2, peptide, and certain analog peptides inhibited receptor-mediated K+ channel gating, dependent on which receptor was activated. This differential perturbation of receptor-G protein interaction suggests that receptors that act on the same G protein can be selectively disrupted.

Escape from vasopressin-induced antidiuresis: role of vasopressin resistance of the collecting duct

Previously, we demonstrated that escape from vasopressin-induced antidiuresis ("vasopressin escape") in rats is associated with a large, selective decrease in whole kidney expression of aquaporin-2, the vasopressin-regulated water channel. Here, we show that isolated perfused inner medullary collecting ducts (IMCDs) from vasopressin-escape rats desamino-[D-arginine]vasopressin (DDAVP)/water-loaded have dramatically reduced vasopressin-dependent osmotic water permeabilities [46% of control rats (DDAVP alone)], which coincides with a fall in inner medullary aquaporin-2 protein abundance as measured by immunoblotting in the opposite kidney. Furthermore, we demonstrate in IMCD suspensions that cAMP accumulation in response to DDAVP is substantially reduced in the vasopressin-escape rats both in the presence and absence of the phosphodiesterase inhibitor IBMX. By immunoblotting, we show that the abundance of two proteins important in cAMP generation: the stimulatory heterotrimeric G protein subunit Gs and adenylyl cyclase type VI, do not change. We conclude that vasopressin escape is associated with relative vasopressin resistance of the collecting duct cells manifested by decreased intracellular cAMP levels. The decreased cAMP levels can contribute to the demonstrated decrease in collecting duct water permeability in two ways: 1) by causing a decrease in aquaporin-2 expression and 2) by limiting the acute action of vasopressin to increase collecting duct water permeability.

Functional alteration of guanine nucleotide binding proteins (Gs and Gi) in psoriatic epidermis

Psoriatic involved epidermis has been characterized by a defective beta-adrenergic adenylate cyclase response. It is also characterized by increased cholera toxin- and forskolin-induced cyclic AMP accumulations. Due to the fact that receptor signals are transduced to adenylate cyclase through guanine nucleotides binding proteins (G-proteins), that affect cholera toxin- and forskolin-induced cyclic AMP accumulations, possible alterations of G-proteins of psoriatic involved and perilesional uninvolved epidermis by using toxin-catalyzed ADP-ribosylation and immunoblot analyses was investigated. Cholera toxin catalyzes ADP-ribosylation of stimulatory guanine nucleotides binding protein (Gs) in either trimeric (inactive) or monomeric (active) form, while islet activating protein (IAP) catalyzes ADP ribosylation of inhibitory guanine nucleotide binding protein (Gi) in only trimeric (inactive) form. Results indicate that although the psoriatic involved epidermis shows increased cholera toxin- and IAP-catalyzed ADP-ribosylations, the amounts of immunoreactive Gs alpha or Gi alpha are not significantly altered. The increase in IAP-catalyzed ADP-ribosylation indicates increased inactive Gi, explaining the increased forskolin-induced cyclic AMP accumulation. The increase in cholera toxin-catalyzed ADP-ribosylation of Gs explains the increased cholera toxin-induced cyclic AMP accumulation in the psoriatic involved epidermis.

Impaired aquaporin and urea transporter expression in rats with adriamycin-induced nephrotic syndrome

Nephrotic syndrome is associated with abnormal regulation of renal water excretion. To investigate the role of collecting duct water channels and solute transporters in this process, we have carried out semiquantitative immunoblotting of kidney tissues from rats with adriamycin-induced nephrotic syndrome. These experiments demonstrated that adriamycin-induced nephrotic syndrome is associated with marked decreases in expression of aquaporin-2, aquaporin-3, aquaporin-4, and the vasopressin-regulated urea transporter in renal inner medulla, indicative of a suppression of the capacity for water and urea absorption by the inner medullary collecting duct. In contrast, expression of the alpha(1)-subunit of the Na,K-ATPase in the inner medulla was unaltered. Light and electron microscopy of perfusion-fixed kidneys demonstrated that the collecting ducts are morphologically normal and unobstructed. Inner medullary expression of the descending limb water channel, aquaporin-1, was not significantly altered, pointing to a selective effect on the collecting duct. Aquaporin-2 and aquaporin-3 expression was also markedly diminished in the renal cortex, indicating that the effect is not limited to the inner medullary collecting duct. Differential centrifugation studies and immunocytochemistry in inner medullary thin sections demonstrated increased targeting of aquaporin-2 to the plasma membrane, consistent with the expected short-term action of vasopressin on aquaporin-2 trafficking. The extensive down-regulation of aquaporin and urea transporter expression may represent an appropriate renal response to the extracellular volume expansion observed in nephrotic syndrome, but may occur at the expense of decreased urinary concentrating and diluting capacity.

Evidence for the shuttle model for Gs alpha activation of adenylyl cyclase

Knowledge of the nature of the interaction between the stimulatory G protein (Gs) and the adenylyl cyclase catalytic unit (C) is essential for interpreting the effects of Gs mutations and expression levels on cellular response to a wide variety of hormones, drugs, and neurotransmitters. It has been proposed that beta-adrenergic receptor activation of adenylyl cyclase occurs either by a two-step "shuttle" mechanism where the receptor activates Gs independently of cyclase followed by Gs alpha activation of cyclase independent of the receptor; or the receptor activates a "precoupled" Gs-C complex in a single step. Simulations of the two models revealed that the two forms of activation are distinguishable by the effect of Gs levels on epinephrine-stimulated EC50 values for cyclase activation; specifically, the shuttle model predicts an increased potency of epinephrine stimulation as levels of Gs alpha increase. To address this problem, S49 cyc- cells were stably transfected with the gene for Gs alpha(long) regulated by the MMTV LTR promoter, which allowed for an induction of Gs alpha(long) expression levels over a 40-fold range by incubation of the cells for various times with 5 microM dexamethasone. Expression of Gs alpha was strongly correlated to the appearance of GTP shifts in the competitive binding of epinephrine with [125I]iodocyanopindolol to the beta-adrenergic receptors and epinephrine-stimulated adenylyl cyclase activity. Most importantly, high expression of Gs alpha resulted in lower EC50 values for epinephrine and prostaglandin E1 stimulation of adenylyl cyclase activity. The decrease in EC50 did not occur as a result of a change in beta2-adrenergic receptor, Gi alpha, G betagamma, or adenylyl cyclase levels. These novel findings demonstrate that a change in the level of a protein downstream of a plasma membrane receptor can influence hormone potency. We explain these results by using kinetic arguments to suggest that some fraction of hormone-activated adenylyl cyclase occurs via a shuttle mechanism, and not a purely precoupled mechanism.

The stoichiometry of G alpha(s) palmitoylation in its basal and activated states

Palmitoylation is the dynamic modification of proteins by the addition of palmitate to cysteine residues. The alpha subunits of heterotrimeric G proteins undergo palmitoylation on their amino terminus, and activation of alpha(s) accelerates its palmitate turnover. In previous studies, palmitoylation was assessed by incorporation or turnover of [3H]palmitate. These studies did not determine the fraction of alpha(s) that is palmitoylated because the specific activity of [3H]palmitoyl-CoA within cells is indeterminate. We developed an HPLC method to determine the fraction of alpha(s) that was palmitoylated in the basal and activated states. COS and S49 cells were radiolabeled with [35S]methionine, and alpha(s) was immunoprecipitated from the particulate fraction. The immunoprecipitated proteins were separated by reverse phase HPLC into two peaks that were determined to contain the modified and unmodified forms of alpha(s). Approximately 77% of the endogenous alpha(s) in COS cells and 70% in S49 lymphoma cells were palmitoylated. The fraction of alpha(s) that was modified did not change after treatment with isoproterenol, a beta-adrenergic receptor agonist that causes turnover of palmitate on alpha(s). These results suggest that receptor activation of alpha(s) caused a rapid turnover of palmitate to maintain most of alpha(s) in its palmitoylated form.