Neural expression of a novel alternatively spliced and polyadenylated Gs alpha transcript

The Distinct Role of the Extra-Large G Protein ɑ-Subunit XLɑs

GNAS is one of the most complex gene loci in the human genome and encodes multiple gene products including Gsα, XLαs, NESP55, A/B, and AS transcripts. XLαs, the extra-large G protein ɑ-subunit, is paternally expressed. XLɑs and Gsɑ share the common 2-13 exons with different promoters and first exons. Therefore, XLɑs contains most of the functional domains of Gsα including receptor and effector binding sites. In vitro studies suggest a "Gsɑ"-like function of XLɑs regarding the stimulation of cAMP generation in response to receptor activation with different cellular actions. However, it is unclear whether XLαs has an important physiological function in humans. Pseudopseudohypoparathyroidism (PPHP) and progressive osseous heteroplasia (POH) are caused by paternally inherited mutations of GNAS. Maternal uniparental disomy of chromosome 20 [UPD(20)mat] lacks paternal chromosome 20. Therefore, the phenotypes of these diseases may be secondary to the abnormal functions of XLɑs, at least partly. From the phenotypes of human diseases like PPHP, POH, and UPD(20)mat, as well as some animal models with deficient XLɑs functions, it could be seen that XLɑs is involved in the growth and development of the mammalian fetus, plays a different role in glucose, lipid, and energy metabolism when compared with Gsɑ, and could prevent heterotopic ossification in humans and mice. More in vivo and in vitro studies, especially the development of conditional XLɑs knockout mice, are needed to clarify the physiopathologic roles and related signal pathways of XLɑs.

Gene Dosage Effects at the Imprinted Gnas Cluster

Genomic imprinting results in parent-of-origin-dependent monoallelic gene expression. Early work showed that distal mouse chromosome 2 is imprinted, as maternal and paternal duplications of the region (with corresponding paternal and maternal deficiencies) give rise to different anomalous phenotypes with early postnatal lethalities. Newborns with maternal duplication (MatDp(dist2)) are long, thin and hypoactive whereas those with paternal duplication (PatDp(dist2)) are chunky, oedematous, and hyperactive. Here we focus on PatDp(dist2). Loss of expression of the maternally expressed Gnas transcript at the Gnas cluster has been thought to account for the PatDp(dist2) phenotype. But PatDp(dist2) also have two expressed doses of the paternally expressed Gnasxl transcript. Through the use of targeted mutations, we have generated PatDp(dist2) mice predicted to have 1 or 2 expressed doses of Gnasxl, and 0, 1 or 2 expressed doses of Gnas. We confirm that oedema is due to lack of expression of imprinted Gnas alone. We show that it is the combination of a double dose of Gnasxl, with no dose of imprinted Gnas, that gives rise to the characteristic hyperactive, chunky, oedematous, lethal PatDp(dist2) phenotype, which is also hypoglycaemic. However PatDp(dist2) mice in which the dosage of the Gnasxl and Gnas is balanced (either 2∶2 or 1∶1) are neither dysmorphic nor hyperactive, have normal glucose levels, and are fully viable. But PatDp(dist2) with biallelic expression of both Gnasxl and Gnas show a marked postnatal growth retardation. Our results show that most of the PatDp(dist2) phenotype is due to overexpression of Gnasxl combined with loss of expression of Gnas, and suggest that Gnasxl and Gnas may act antagonistically in a number of tissues and to cause a wide range of phenotypic effects. It can be concluded that monoallelic expression of both Gnasxl and Gnas is a requirement for normal postnatal growth and development.

A mouse model for osseous heteroplasia

GNAS/Gnas encodes G_sα that is mainly biallelically expressed but shows imprinted expression in some tissues. In Albright Hereditary Osteodystrophy (AHO) heterozygous loss of function mutations of GNAS can result in ectopic ossification that tends to be superficial and attributable to haploinsufficiency of biallelically expressed G_sα. Oed-Sml is a point missense mutation in exon 6 of the orthologous mouse locus Gnas. We report here both the late onset ossification and occurrence of benign cutaneous fibroepithelial polyps in Oed-Sml. These phenotypes are seen on both maternal and paternal inheritance of the mutant allele and are therefore due to an effect on biallelically expressed G_sα. The ossification is confined to subcutaneous tissues and so resembles the ossification observed with AHO. Our mouse model is the first with both subcutaneous ossification and fibroepithelial polyps related to G_sα deficiency. It is also the first mouse model described with a clinically relevant phenotype associated with a point mutation in G_sα and may be useful in investigations of the mechanisms of heterotopic bone formation. Together with earlier results, our findings indicate that G_sα signalling pathways play a vital role in repressing ectopic bone formation.

New mutations at the imprinted Gnas cluster show gene dosage effects of Gsα in postnatal growth and implicate XLαs in bone and fat metabolism but not in suckling

The imprinted Gnas cluster is involved in obesity, energy metabolism, feeding behavior, and viability. Relative contribution of paternally expressed proteins XLαs, XLN1, and ALEX or a double dose of maternally expressed Gsα to phenotype has not been established. In this study, we have generated two new mutants (Ex1A-T-CON and Ex1A-T) at the Gnas cluster. Paternal inheritance of Ex1A-T-CON leads to loss of imprinting of Gsα, resulting in preweaning growth retardation followed by catch-up growth. Paternal inheritance of Ex1A-T leads to loss of imprinting of Gsα and loss of expression of XLαs and XLN1. These mice have severe preweaning growth retardation and incomplete catch-up growth. They are fully viable probably because suckling is unimpaired, unlike mutants in which the expression of all the known paternally expressed Gnasxl proteins (XLαs, XLN1 and ALEX) is compromised. We suggest that loss of ALEX is most likely responsible for the suckling defects previously observed. In adults, paternal inheritance of Ex1A-T results in an increased metabolic rate and reductions in fat mass, leptin, and bone mineral density attributable to loss of XLαs. This is, to our knowledge, the first report describing a role for XLαs in bone metabolism. We propose that XLαs is involved in the regulation of bone and adipocyte metabolism.

Transgenic overexpression of the extra-large Gsα variant XLαs enhances Gsα-mediated responses in the mouse renal proximal tubule in vivo

XLαs, a variant of the stimulatory G protein α-subunit (Gsα), can mediate receptor-activated cAMP generation and, thus, mimic the actions of Gsα in transfected cells. However, it remains unknown whether XLαs can act in a similar manner in vivo. We have now generated mice with ectopic transgenic expression of rat XLαs in the renal proximal tubule (rptXLαs mice), where Gsα mediates most actions of PTH. Western blots and quantitative RT-PCR showed that, while Gsα and type-1 PTH receptor levels were unaltered, protein kinase A activity and 25-hydroxyvitamin D 1-α-hydroxylase (Cyp27b1) mRNA levels were significantly higher in renal proximal tubules of rptXLαs mice than wild-type littermates. Immunohistochemical analysis of kidney sections showed that the sodium-phosphate cotransporter type 2a was modestly reduced in brush border membranes of male rptXLαs mice compared to gender-matched controls. Serum calcium, phosphorus, and 1,25 dihydroxyvitamin D were within the normal range, but serum PTH was ∼30% lower in rptXLαs mice than in controls (152 ± 16 vs. 222 ± 41 pg/ml; P < 0.05). After crossing the rptXLαs mice to mice with ablation of maternal Gnas exon 1 (E1(m-/+)), male offspring carrying both the XLαs transgene and maternal Gnas exon 1 ablation (rptXLαs/E1(m-/+)) were significantly less hypocalcemic than gender-matched E1(m-/+) littermates. Both E1(m-/+) and rptXLαs/E1(m-/+) offspring had higher serum PTH than wild-type littermates, but the degree of secondary hyperparathyroidism tended to be lower in rptXLαs/E1(m-/+) mice. Hence, transgenic XLαs expression in the proximal tubule enhanced Gsα-mediated responses, indicating that XLαs can mimic Gsα in vivo.

Diseases associated with genomic imprinting

Genomic imprinting is the phenomenon where the expression of a locus differs between the maternally and paternally inherited alleles. Typically, this manifests as transcriptional silencing of one of the alleles, although many genes are imprinted in a tissue- or isoform-specific manner. Diseases associated with imprinted genes include various cancers, disorders of growth and metabolism, and disorders in neurodevelopment, cognition, and behavior, including certain major psychiatric disorders. In many cases, the disease phenotypes associated with dysfunction at particular imprinted loci can be understood in terms of the evolutionary processes responsible for the origin of imprinting. Imprinted gene expression represents the outcome of an intragenomic evolutionary conflict, where natural selection favors different expression strategies for maternally and paternally inherited alleles. This conflict is reasonably well understood in the context of the early growth effects of imprinted genes, where paternally inherited alleles are selected to place a greater demand on maternal resources than are maternally inherited alleles. Less well understood are the origins of imprinted gene expression in the brain, and their effects on cognition and behavior. This chapter reviews the genetic diseases that are associated with imprinted genes, framed in terms of the evolutionary pressures acting on gene expression at those loci. We begin by reviewing the phenomenon and evolutionary origins of genomic imprinting. We then discuss diseases that are associated with genetic or epigenetic defects at particular imprinted loci, many of which are associated with abnormalities in growth and/or feeding behaviors that can be understood in terms of the asymmetric pressures of natural selection on maternally and paternally inherited alleles. We next described the evidence for imprinted gene effects on adult cognition and behavior, and the possible role of imprinted genes in the etiology of certain major psychiatric disorders. Finally, we conclude with a discussion of how imprinting, and the evolutionary-genetic conflicts that underlie it, may enhance both the frequency and morbidity of certain types of diseases.

The role of GNAS and other imprinted genes in the development of obesity

Genomic imprinting is an epigenetic phenomenon affecting a small number of genes, which leads to differential expression from the two parental alleles. Imprinted genes are known to regulate fetal growth and a 'kinship' or 'parental conflict' model predicts that paternally and maternally expressed imprinted genes promote and inhibit fetal growth, respectively. In this review we examine the role of imprinted genes in postnatal growth and metabolism, with an emphasis on the GNAS/Gnas locus. GNAS is a complex imprinted locus with multiple oppositely imprinted gene products, including the G-protein alpha-subunit G(s)alpha that is expressed primarily from the maternal allele in some tissues and the G(s)alpha isoform XLalphas that is expressed only from the paternal allele. Maternal, but not paternal, G(s)alpha mutations lead to obesity in Albright hereditary osteodystrophy. Mouse studies show that this phenomenon is due to G(s)alpha imprinting in the central nervous system leading to a specific defect in the ability of central melanocortins to stimulate sympathetic nervous system activity and energy expenditure. In contrast mutation of paternally expressed XLalphas leads to opposite metabolic effects in mice. Although these findings conform to the 'kinship' model, the effects of other imprinted genes on body weight regulation do not conform to this model.

Physiological Dysfunctions Associated with Mutations of the Imprinted Gnas Locus

The ubiquitous Gαs-subunit of the trimeric, stimulatory G-protein plays a central role in receptor-mediated signal transduction, coupling receptor activation with the production of cAMP. The Gαs-encoding locus Gnas is now known to consist of a complex arrangement of several protein-coding and noncoding transcripts. We provide an overview of its genomic organization, its regulation by genomic imprinting, and a summary of the physiological roles of the alternative protein variants Gαs and XLαs as determined from deficient mouse models.

Control of imprinting at the Gnas cluster

Genomic imprinting is a form of epigenetic regulation in mammals whereby a small subset of genes is silenced according to parental origin. Early work had indicated regions of the genome that were likely to contain imprinted genes. Distal mouse chromosome 2 is one such region and is associated with devastating but ostensibly opposite phenotypes when exclusively maternally or paternally derived. Misexpression of proteins encoded at the Gnas complex, which is located within the region, can largely account for the imprinting phenotypes. Gnas is a complex locus with extraordinary transcriptional and regulatory complexity. It gives rise to alternatively spliced isoforms that show maternal-, paternal- and biallelic expression as well as a noncoding antisense transcript. The objective of our work at Harwell is to unravel mechanisms controlling the expression of these transcripts. We have performed targeted deletion analysis to test candidate regulatory regions within the Gnas complex and, unlike other imprinted domains, two major control regions have been identified. One controls the imprinted expression of a single transcript and is subsidiary to and must interact with, a principal control region that affects the expression of all transcripts. This principal region contains the promoter for the antisense transcript, expression of which may have a major role in controlling imprinting at the Gnas cluster.

Genomic imprinting and dermatological disease

Imprinting is the process whereby genetic alleles responsible for a phenotype are derived from one parent only. It is an epigenetic phenomenon resulting from DNA methylation or modification of protruding histones. When imprinted genes are disrupted, syndromes with characteristic patterns of inheritance and multisystem phenotype occur. Those detailed in this article have some quite characteristic cutaneous features and patterns of inheritance. These diseases include Beckwith-Wiedmann, Silver-Russell, Prader-Willi, McCune-Albright and Angelman syndromes, Albright's hereditary osteodystrophy, and progressive osseous heteroplasia. In the case of Von Hippel-Lindau syndrome, hypomelanosis of Ito and dermatopathia pigmentosa reticularis, imprinting may play a part in the inheritance. With neurofibromatosis type 1, a nonimprinted condition, the expression of the phenotype could be affected by interaction with imprinted gene loci. Imprinted genes could also play a part in the polygenetic inheritance of more common diseases also, as atopic eczema and psoriasis may have predominantly maternal and paternal modes of transmission, respectively.

Imprinting the Gnas locus

Gnas is an enigmatic and rather complex imprinted gene locus. A single transcription unit encodes three, and possibly more, distinct proteins. These are determined by overlapping transcripts from alternative promoters with different patterns of imprinting. The canonical Gnas transcript codes for Gsalpha, a highly conserved signalling protein and an essential intermediate in growth, differentiation and homeostatic pathways. Monoallelic expression of Gnas is highly tissue-restricted. The alternative transcripts encode XLalphas, an unusual variant of Gsalpha, and the chromogranin-like protein Nesp55. These transcripts are expressed specifically from the paternal and maternal chromosomes, respectively. Their existence in the Gnas locus might imply functional connections amongst them or with Gsalpha. In this review, we consider how imprinting of Gnas was discovered, the phenotypic consequences of mutations in each of the gene products, both in the mouse and human, and provide some conjectures to explain why this elaborate imprinted locus has evolved in this manner in mammals.

Interactions between imprinting effects: summary and review

Mice with uniparental disomies (uniparental duplications) for defined regions of certain chromosomes, or certain disomies, show a range of developmental abnormalities most of which affect growth. These defects can be attributed to incorrect dosages of maternal or paternal copies of imprinted genes lying within the regions involved. Combinations of certain partial disomies result in interactions between the imprinting effects that seemingly independently affect foetal and/or placental growth in different ways or modify neonatal and postnatal development. The findings are generally in accord with the 'conflict hypothesis' for the evolution of genomic imprinting but do not demonstrate common growth axes within which imprinted genes may interact. Instead, it would seem that any gene that favours embryonic/foetal development, at consequent cost to the mother, will have been subject to evolutionary selection for only paternal allele expression. Reciprocally, any gene that reduces embryonic/foetal growth to limit disadvantage to the mother will have been selected for only maternal allele expression. It is concluded that survival of the placenta is core to the evolution of imprinting.

Interactions between imprinting effects in the mouse

Mice with uniparental partial or complete disomies for any one of 11 identified chromosomes show abnormal phenotypes. The abnormalities, or imprinting effects, can be attributable to an incorrect dosage of maternal or paternal copies of imprinted gene(s) located within the regions involved. Here we show that combinations of partial disomies may result in interactions between imprinting effects that seemingly independently affect fetal and/or placental growth in different ways or modify neonatal and postnatal imprinting effects. Candidate genes within the regions have been identified. The findings are generally in accord with the "conflict hypothesis" for the evolution of genomic imprinting but do not clearly demonstrate common growth axes within which imprinted genes may interact. Instead, it would seem that any gene that represses or limits embryonic/fetal growth to the advantage of the mother--by any developmental means--will have been subject to evolutionary selection for paternal allele repression. Likewise, any gene that favors embryonic/fetal development at consequent cost to the mother--by any developmental means--will have faced selection for maternal allele repression. The classical Igf2-Igf2r axis may therefore be unique. The findings involve reinterpretation of older imprinting data and consequently revision of the mouse imprinting map.

The imprinted signaling protein XLαs is required for postnatal adaptation to feeding

Genomic imprinting, by which maternal and paternal alleles of some genes have different levels of activity, has profound effects on growth and development of the mammalian fetus. The action of imprinted genes after birth, in particular while the infant is dependent on maternal provision of nutrients, is far less well understood. We disrupted a paternally expressed transcript at the Gnas locus, Gnasxl, which encodes the unusual Gs alpha isoform XL alpha s. Mice with mutations in Gnasxl have poor postnatal growth and survival and a spectrum of phenotypic effects that indicate that XL alpha s controls a number of key postnatal physiological adaptations, including suckling, blood glucose and energy homeostasis. Increased cAMP levels in brown adipose tissue of Gnasxl mutants and phenotypic comparison with Gnas mutants suggest that XL alpha s can antagonize Gs alpha-dependent signaling pathways. The opposing effects of maternally and paternally expressed products of the Gnas locus provide tangible molecular support for the parental-conflict hypothesis of imprinting.

A comprehensive transcript map of the mouse Gnas imprinted complex

The recent publication of the FANTOM mouse transcriptome has provided a unique opportunity to study the diversity of transcripts arising from a single gene locus. We have focused on the Gnas complex, as imprinting loci themselves provide unique insights into transcriptional regulation. Thirteen full-length cDNAs from the FANTOM2 set were mapped to the Gnas locus. These represented one previously described transcript and 12 putative new transcripts. Of these, eight were found to be differentially expressed from either the maternal or paternal allele. Two clones extended Nespas in the 3' direction, providing evidence of antisense transcription spanning a 30-kb genomic region from a single allele. The transcripts were summarized into six transcriptional units, Nespas, Nesp, Gnasxl, F7, exon 1A, and Gnas. The resolution of the Gnas transcript map by the FANTOM2 clones revealed a pattern of alternate splicing. In addition to the transcripts described previously as splicing onto exon 2 of Gnas, each new sense transcript had an alternate short 3'UTR independent of Gnas. Both spliced and unspliced variants of the new imprinted sense transcripts were found. Whereas the functional significance of these alternate transcripts is not known, the availability of the FANTOM clones has provided remarkable insights into the repertoire of transcripts in the Gnas complex locus.

Analysis of GNAS1 and overlapping transcripts identifies the parental origin of mutations in patients with sporadic Albright hereditary osteodystrophy and reveals a model system in which to observe the effects of splicing mutations on translated and untranslated messenger RNA

Albright hereditary osteodystrophy (AHO) is caused by heterozygous deactivating GNAS1 mutations. There is a parent-of-origin effect. Maternally derived mutations are usually associated with resistance to parathyroid hormone termed "pseudohypoparathyroidism type Ia." Paternally derived mutations are associated with AHO but usually normal hormone responsiveness, known as "pseudo-pseudohypoparathyroidism." These observations can be explained by tissue-specific GNAS1 imprinting. Regulation of the genomic region that encompasses GNAS1 is complex. At least three upstream exons that splice to exon 2 of GNAS1 and that are imprinted have been reported. NESP55 is exclusively maternally expressed, whereas exon 1A and XL alphas are exclusively paternally expressed. We set out to identify the parental origin of GNAS1 mutations in patients with AHO by searching for their mutation in the overlapping transcripts. This information would be of value in patients with sporadic disease, for predicting their endocrine phenotype and planning follow-up. In doing so, we identified mutations that resulted in nonsense-mediated decay of the mutant Gs alpha transcript but that were detectable in NESP55 messenger RNA (mRNA), probably because they lie within its 3' untranslated region. Analysis of the NESP55 transcripts revealed the creation of a novel splice site in one patient and an unusual intronic mutation that caused retention of the intron in a further patient, neither of which could be detected by analysis of the Gs alpha complementary DNA. This cluster of overlapping transcripts represents a useful model system in which to analyze the effects that mutant sequence has on mRNA-in particular, splicing-and the mechanisms of nonsense-mediated mRNA decay.

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.

Lysophospholipid receptors

Lysophospholipids (LPs), including lysophosphatidic acid and sphingosine 1-phosphate, produce many cellular effects. However, the prolonged absence of any cloned and identified LP receptor has left open the question of how these lipids actually bring about these effects. The cloning and functional identification of the first LP receptor, lp(A1)/vzg-1, has led rapidly to the identification and classification of multiple orphan receptors/expression sequence tags known by many names (e.g. edg, mrec1.3, gpcr26, H218, AGR16, nrg-1) as members of a common cognate G protein-coupled receptor family. We review features of the LP receptor family, including molecular characteristics, genomics, signaling properties, and gene expression. A major question for which only partial answers are available concerns the biological significance of receptor-mediated LP signaling. Recent studies that demonstrate the role of receptor-mediated LP signaling in the nervous system, cardiovascular system, and other organ systems indicate the importance of this signaling in development, function, and pathophysiology and portend an exciting time ahead for this growing field.

Characterization of the extra-large G protein alpha-subunit XLalphas. I. Tissue distribution and subcellular localization

Our group previously described a new type of G protein, the 78-kDa XLalphas (extra large alphas) (Kehlenbach, R. H., Matthey, J., and Huttner, W. B. (1994) Nature 372, 804-809 and (1995) Nature 375, 253). Upon subcellular fractionation, XLalphas labeled by ADP-ribosylation with cholera toxin was previously mainly detected in the bottom fractions of a velocity sucrose gradient that contained trans-Golgi network and was differentially distributed to Galphas, which also peaked in the top fractions containing plasma membrane. Here, we investigate, using a new antibody specific for the XL domain, the tissue distribution and subcellular localization of XLalphas and novel splice variants referred to as XLN1. Upon immunoblotting and immunofluorescence analysis of various adult rat tissues, XLalphas and XLN1 were found to be enriched in neuroendocrine tissues, with a particularly high level of expression in the pituitary. By both immunofluorescence and immunogold electron microscopy, endogenous as well as transfected XLalphas and XLN1 were found to be predominantly associated with the plasma membrane, with only little immunoreactivity on internal, perinuclear membranes. Upon subcellular fractionation, immunoreactive XLalphas behaved similarly to Galphas but was differentially distributed to ADP-ribosylated XLalphas. Moreover, the bottom fractions of the velocity sucrose gradient were found to contain not only trans-Golgi network membranes but also certain subdomains of the plasma membrane, which reconciles the present with the previous observations. To further investigate the molecular basis of the association of XLalphas with the plasma membrane, chimeric proteins consisting of the XL domain or portions thereof fused to green fluorescent protein were analyzed by fluorescence and subcellular fractionation. In both neuroendocrine and non-neuroendocrine cells, a fusion protein containing the entire XL domain, in contrast to one containing only the proline-rich and cysteine-rich regions, was exclusively localized at the plasma membrane. We conclude that the physiological role of XLalphas is at the plasma membrane, where it presumably is involved in signal transduction processes characteristic of neuroendocrine cells.

The Role of Genomic Imprinting of Galpha in the Pathogenesis of Albright Hereditary Osteodystrophy

Albright hereditary osteodystrophy (AHO) is caused by heterozygous inactivating mutations of the gene encoding the alpha-subunit of the G protein Gs. The Gsalpha gene is a complex gene that uses various alternative promoters and produces various protein products. Recently, it has been shown that this gene is imprinted in a tissue-specific manner. The role of tissue-specific imprinting of Gsalpha in the pathogenesis of AHO is discussed.

On the role of c-Jun in the induction of PAI-1 gene expression by phorbol ester, serum, and IL-1alpha in HepG2 cells

We have characterized the regulation of plasminogen activator inhibitor-1 (PAI-1) gene expression by phorbol 12-myristate 13-acetate (PMA), serum, and interleukin-1alpha (IL-1alpha) in the human hepatoma cell line HepG2. PMA, serum, and IL-1alpha induced a rapid and transient 28-fold (PMA), 9-fold (serum), and 23-fold (IL-1alpha) increase in PAI-1 mRNA, peaking after approximately 4 hours. These inductions of PAI-1 mRNA accumulation were reduced by pretreatment of the HepG2 cells with the protein tyrosine kinase inhibitor genistein. Conversely, stimulation of tyrosine phosphorylation by sodium orthovanadate, an inhibitor of protein tyrosine phosphatases, caused an increase in PAI-1 mRNA levels. The effects of PMA, serum, and IL-1alpha on PAI-1 mRNA expression have been compared with their ability to modulate the expression of a chloramphenicol acetyltransferase (CAT) reporter plasmid, which was under control of the -489 to +75 region of the PAI-1 promoter, and stably transfected into HepG2 cells. This region of the PAI-1 promoter was previously found to contain a tetradecanoyl phorbol acetate-response element (TRE; between -58 and -50) necessary for PMA responsiveness and with a high affinity for c-Jun homodimers. Whereas incubation of these transfected HepG2 cells with PMA and serum showed an induction profile of CAT mRNA similar to that of PAI-1 mRNA, hardly any induction of CAT mRNA was found with IL-1alpha. In line with these findings, IL-1alpha poorly induced c-Jun homodimer binding to the PAI-1 TRE in gel mobility-shift assays. Pretreatment of HepG2 cells with the protein kinase C inhibitor Ro 31-8220 or the mitogen-activated protein kinase kinase (MAPKK)1,2 activity blocker PD98059 selectively suppressed the induction of PAI-1 (and CAT) expression by PMA, but not that by IL-1alpha. In contrast, the protein tyrosine kinase inhibitor herbimycin A blocked PAI-1 mRNA induction by IL-1 alpha only. We propose 2 separate PAI-1 inductory pathways for PMA and IL-1alpha in HepG2, both involving protein tyrosine kinase activation; the serum-induced signaling pathway may (partially) overlap with the PMA-activated protein kinase C/mitogen-activated protein kinase kinase pathway, leading to c-Jun homodimer binding to the PAI-1 TRE.

The human GNAS1 gene is imprinted and encodes distinct paternally and biallelically expressed G proteins

The GNAS1 gene encodes the alpha subunit of the G protein Gs, which couples receptor binding by several hormones to activation of adenylate cyclase. Null mutations of GNAS1 cause pseudohypoparathyroidism (PHP) type Ia, in which hormone resistance occurs in association with a characteristic osteodystrophy. The observation that PHP Ia almost always is inherited maternally has led to the suggestion that GNAS1 may be an imprinted gene. Here, we show that, although Gsalpha expression (directed by the promoter upstream of exon 1) is biallelic, GNAS1 is indeed imprinted in a promoter-specific fashion. We used parthenogenetic lymphocyte DNA to screen by restriction landmark genomic scanning for loci showing differential methylation between paternal and maternal alleles. This screen identified a region that was found to be methylated exclusively on a maternal allele and was located approximately 35 kb upstream of GNAS1 exon 1. This region contains three novel exons that are spliced into alternative GNAS1 mRNA species, including one exon that encodes the human homologue of the large G protein XLalphas. Transcription of these novel mRNAs is exclusively from the paternal allele in all tissues examined. The differential imprinting of separate protein products of GNAS1 therefore may contribute to the anomalous inheritance of PHP Ia.

Measurement of tryptophan hydroxylase mRNA levels by competitive RT-PCR

Tryptophan hydroxylase (TPH) is the rate limiting enzyme in serotonin biosynthesis [D.G. Grahame-Smith, Tryptophan hydroxylation in brain, Biochem. Biophys. Res. Commun. 16 (1964) 586-592 [19]]. As such, the TPH gene is a likely target for modulation of serotonergic function, which has been associated with several psychiatric disorders [E.C. Azmitia, P.M. Whitaker-Azmitia, Awakening the sleeping giant: anatomy and plasticity of the brain serotonergic system, J. Clin. Psychiatry 52 (12, Suppl.) (1991) 4-16 [1]; R.P. Hart, R. Yang, L.A. Riley., T.L. Green, Post-transcriptional control of tryptophan hydroxylase gene expression in rat brain stem and pineal gland, Mol. Cell. Neurosci. 2 (1991) 71-77 [20]; M.J. Owens, C.B. Numeroff, Role of serotonin in the pathophysiology of depression: focus on the serotonin transporter, Clin. Chem. 40 (1994) 288-295 [24]]. Unfortunately, it has been technically difficult to measure TPH mRNA levels in central serotonergic neurons due to its low levels. For example, detection with ribonuclease protection assays requires pooling of 5-10 dissected brainstems [M.C. Darmon, B. Guibert, V. Leviel, M. Ehret, M. Maitre, J. Mallet, Sequence of two mRNAs encoding active rat tryptophan hydroxylase, J. Neurochem. 51 (1988) 312-316 [15]; B.L. Jacobs, E.C. Azmitia, Structure and function of the brain serotonin system, Physiol. Rev. 72 (1992) 165-229 [21]]. This protocol describes the use of competitive RT-PCR to measure TPH mRNA levels from rat brain. First described in 1988, competitive RT-PCR has become an accepted method of measuring RNA abundance [M. Clementi, S. Menzo, P. Bagnarelli, A. Manzin, A. Valenza, P.E. Varaldo, Quantitative PCR and RT-PCR in virology, PCR Methods Appl. 2 (1994) 191-196 [12]; N.C.P. Cross, Quantitative PCR techniques and applications, Br. J. Haematol. 89 (1995) 693-697 [14]; K.P. Foley, M.W. Leonard, J.D. Engel, Quantitation of RNA using the polymerase chain reaction, Trends Genet. 9 (1993) 380-385 [17]; P.D. Siebert, J.W. Larrick, Competitive PCR, Nature 359 (1992) 558 [27]]. Competitive RT-PCR uses co-amplification with a known quantity of an in vitro transcribed RNA which amplifies using the same primers and thus competes for reactants with the product of interest. As the two products amplify with the same efficiency, the relative abundance of the two amplification products remains constant, and thus can be used to determine initial tissue TPH mRNA levels [G. Gilliland, S. Perrin, K. Blanchard, H.F. Bunn, Analysis of cytokine mRNA and DNA: detection and quantitation by competitive polymerase chain reaction, Proc. Natl. Acad. Sci. U.S.A. 87 (1990) 2725-2729 [18]; A.M. Wang, M. V. Doyle, D.F. Mark, Quantitation of mRNA by the polymerase chain reaction, Proc. Natl. Acad. Sci. U.S.A. 86 (1989) 9717-9721 [31]]. We first demonstrate equivalent results between RNA slot blots and competitive RT-PCR using the CA77 thyroid C cell line [M.S. Clark, A. F. Russo, Tissue-specific glucocorticoid regulation of tryptophan hydroxylase mRNA levels, Mol. Brain Res. 48 (1997) 346-354 [9]]. We then describe the use of competitive RT-PCR to measure TPH mRNA levels in RNA isolated from rat brain poly-A+ RNA.

Tissue-specific glucocorticoid regulation of tryptophan hydroxylase mRNA levels

A potential long-term target of glucocorticoid modulation of serotonin (5-HT) production is tryptophan hydroxylase (TPH) gene expression. However, studies on TPH gene expression have been hampered by the extremely low levels of TPH mRNA in the brain, and there have been contradictory reports on the effects of glucocorticoids on 5-HT levels. To overcome these obstacles, we have developed a sensitive competitive RT-PCR assay to directly measure TPH mRNA levels from the rat brain. We observed a tissue-specific modulation of TPH mRNA levels in the melatonin producing pineal gland and the serotonin producing raphe nuclei of the brain. Following chronic treatment of adrenalectomized rats with the synthetic glucocorticoid dexamethasone for 1 week, there was a 16-fold increase in TPH mRNA in the pineal gland that was contrasted by a decrease in TPH mRNA to 16% of the control levels in the brain. To address the mechanism of dexamethasone repression of TPH mRNA levels, we then tested a serotonergic neuronal-like cell line derived from rat thyroid C cells. Dexamethasone caused a rapid decrease in TPH mRNA levels to approximately 20% of control values in CA77 C cells. This was measured by both competitive RT-PCR and a standard hybridization assay, which confirmed the validity of the RT-PCR assay. Furthermore, the reduction of TPH mRNA levels was associated with a decrease in 5-HT levels in the CA77 C cells. Hence, glucocorticoids may alter serotonin and melatonin biosynthetic capacity by cell-specific modulation of the TPH gene.

Molecular analysis of the multiple Golf alpha subunit mRNAs in the rat brain

The alpha subunit of GTP-binding protein Golf (G alpha olf), identified in the olfactory epithelium, in which it is supposed to mediate odorant-generated adenylyl cyclase activations, is much more expressed in the striatum than G alpha s, the classical stimulatory G protein alpha subunit (Hervé et al., J Neurosci., 13 (1993) 2237-2248). Four species of G alpha olf messenger RNA (mRNA) were observed by Northern blot in the rat striatum. Analysis of striatal G alpha olf cDNA clones demonstrated that despite extensive variations in their 5' and 3' untranslated regions, these four G alpha olf mRNAs encode the same G alpha olf polypeptide. One of the four mRNA species, which was selectively observed in the striatum, is generated by a novel promoter whose activity was barely detectable in the olfactory epithelium. Surprisingly, this promoter generates an unexpectedly high proportion of transcripts in which the first intron is unspliced. The retention of intron was found to abolish the translation of G alpha olf mRNA in the reticulocyte lysate system, suggesting that it could be involved in the down regulation of G alpha olf expression in the striatum. Furthermore, a new polyadenylation site with a non canonical sequence, AATACA, was found to be responsible of the two shortest forms of G alpha olf mRNA. In conclusion, we have shown that the G alpha olf proteins present in the striatum and the olfactory epithelium are identical and that multiple variations in the untranslated parts of the mRNAs could affect G alpha olf expression.

Mice lacking Snrpn expression show normal regulation of neuronal alternative splicing events

The SmN protein is closely related to the constitutively expressed RNA splicing protein SmB but is expressed only in brain and heart tissue. Mice which lack expression of SmN die shortly after birth suggesting a critical role for this protein possibly in the regulation of neuronal-specific alternative splicing events. We show here however that the neuronal-specific alternative splicing of the RNAs encoding several different classes of protein proceeds normally in mice lacking SmN expression. The potential role of SmN and the reasons for the lethal effect observed in non-expressing mice are discussed.

Developmental changes in Gs and G(olf) proteins and adenylyl cyclases in mouse brain membranes

Guanine nucleotide-binding (G) proteins, Gs and G(olf) mediate the increase in cAMP formation through the activation of adenylyl cyclases. The developmental profiles of Gs, G(olf) and adenylyl were determined in mouse striatum and whole brain using immunobloting with specific antisera. Gs and the 115 kDa and 150 kDa adenylyl cyclases were present at the earliest age tested, embryonic day (E) 14.5 G(olf) and the 160 kDa adenylyl cyclase emerged in parallel, postnatally; during this period the increase in the relative abundance of the 150 kDa was observed. Gpp[NH]p activated Gs/G(olf) in a dose dependent manner, with a smaller response observed in embryos compared to adults. Mn2+ and forskolin activated the adenylyl cyclases and this activation increased during development. At E 14.5, maximal activation with Mn2+ and forskolin elicited a similar increase in cAMP levels, but from postnatal day 1, a nearly two fold higher response was obtained with forskolin compared to Mn2+; at the same time the 160 kDa adenylyl cyclase was detected. These data suggest that the appearance of certain forms of stimulatory G proteins was developmentally correlated with the expression of specific adenylyl cyclases.