Delineation of the intronless nature of the genes for the human and hamster beta 2-adrenergic receptor and their putative promoter regions

Pharmacology and molecular biology of adrenergic receptors

The recent cloning of multiple adrenergic receptors has moved our understanding of these receptors from a conceptual one (Alquist, 1948) to one based on well-defined unique cellular proteins. The biochemical and pharmacological properties of these receptors can now be studied in detail by expression of a single subtype in cells normally devoid of adrenergic receptors. By site-directed mutagenesis, the relationship between the structures of these receptors and their function is now being elucidated for each adrenergic receptor subtype. These functions include the binding of catecholamines and other ligands, G protein coupling and functional regulation.

Analysis of the promoter sequence and the transcription initiation site of the mouse 5-HT1C serotonin receptor gene

The serotonin 1c (5-HT1C) receptor is found in many brain regions, but is particularly enriched on the epithelial cells of the choroid plexus. A major challenge in neurobiology is to delineate the molecular processes that regulate the specific pattern of neuronal gene expression in the brain. As an initial step towards identifying cis-acting DNA sequences that control the expression of the 5-HT1C receptor, we have isolated the promoter sequence of its gene. Sequence analysis of a 1.8 kb fragment indicated that the 3' end of this fragment overlaps with the 5' untranslated region of the 5-HT1C receptor mRNA, and primer extension using mouse brain poly(A)+ RNA mapped the transcription initiation site within this fragment. There are a number of sequence elements upstream from the transcription initiation site that are homologous to regulatory elements found in other eucaryotic genes. To determine the promoter activity, a plasmid was constructed that contains this fragment as promoter region and the cDNA for the 5-HT1C receptor as the reporter. When injected into the nucleus of Xenopus oocytes, this construct resulted in functional expression of the reporter gene. Primer extension using the RNA extracted from the injected oocytes indicated a single transcription initiation site of the reporter mRNA. These results suggest that the 5-HT1C receptor was functionally expressed under the promoter activity of the 1.8 kb 5' sequence of its gene. This system will be useful for further analysis of the cis-acting elements in the promoter region of the 5-HT1C receptor gene and the trans-acting factors that regulate tissue-specific expression of the receptor.

Beta-adrenergic receptors in the hippocampal and retrohippocampal regions of rats and guinea pigs: autoradiographic and immunohistochemical studies

Species differences in the distribution of beta-adrenergic receptors in the hippocampal and retrohippocampal regions of rats and guinea pigs were examined using in vitro autoradiographic techniques. beta 1-receptors were found in the hippocampal area CA1 of both species, although guinea pigs had significantly lower receptor densities in comparison to rats. In guinea pigs, beta 2-adrenergic receptors were predominant in hippocampal area CA1. Hippocampal area CA3 had very low levels of beta 1- and beta 2-receptors in both species. The retrohippocampal area was also found to have a distinct topographic distribution of beta-receptors. In rats, the subiculum and parasubiculum (layers II-III) were heavily labeled for beta 1-receptors; in contrast, guinea pigs had few receptors in these regions. beta 2-receptors were particularly prominent in the parasubicular region in rats. The entorhinal cortex laminae was found to contain beta-receptors in both rats and guinea pigs. Immunohistochemical techniques were used to compare the pattern of catecholaminergic innervation with the receptor distribution within each hippocampal subregion. Despite the general lack of beta-receptors in area CA3, abundant catecholamine immunoreactive fibers were observed in CA3 of rat and guinea pig hippocampus. Significant species differences were found in the distribution of hippocampal beta-adrenergic receptor subtypes, and moreover, in both species the distribution of beta-adrenergic receptors did not coincide with the pattern of hippocampal adrenergic innervation.

Distribution of beta 2-adrenergic receptor mRNA expression along the hamster nephron segments

Distribution of beta 2-adrenergic receptor mRNA expression along the microdissected hamster nephron segments was examined by the reverse transcription-polymerase chain reaction (RT-PCR) technique. Conventional RT-PCR using a set of primers on separate exons could not be applied for the detection of beta 2-adrenergic receptor mRNA because of its intronless nature. We used the 'rapid amplification of cDNA ends' protocol [(1985) Proc. Natl. Acad. Sci. USA 85, 8998-9002] as a maneuver for RT-PCR of an intronless gene. Using this method, we successfully located hamster beta 2-adrenergic receptor mRNA only in glomeruli and early proximal convoluted tubule along the nephron segments tested.

Cloning and sequence analysis of the human beta 1-adrenergic receptor 5'-flanking promoter region

We present 3.1 kb of the nucleotide sequence from the 5'-flanking region of the human beta 1-adrenergic receptor gene. The first 1.0 kb upstream from the translational start site is composed of 72% G + C residues. The sequence was analyzed for the presence of transcriptional regulatory elements and contains putative thyroid hormone, glucocorticoid hormone and cAMP response elements. These putative hormone response elements support physiological evidence that thyroid and glucocorticoid hormones regulate beta 1AR function by affecting receptor expression in tissues such as heart and adipose, where beta 1-adrenergic receptors are important regulators of heart rate and lipolysis, respectively.

Brain non-adenylated mRNAs

Most eukaryotic messenger RNA (mRNA) species contain a 3'-poly(A) tract. The histone mRNAs are a notable exception although a subclass of histone-encoding mRNAs is polyadenylated. A class of mRNAs lacking a poly(A) tail would be expected to be less stable than poly(A)+ mRNAs and might, like the histones, have a half-life that varied in response to changes in the intracellular milieu. Brain mRNA exhibits an unusually high degree of sequence complexity; studies published ten years ago suggested that a large component of this complexity might be present in a poly(A)- mRNA population that was expressed postnatally. The question of the existence of a complex class of poly(A)- brain mRNAs is particularly tantalizing in light of the heterogeneity of brain cells and the possibility that the stability of these poly(A)- mRNAs might vary with changes in synaptic function, changing hormonal stimulation or with other modulations of neuronal function. The mRNA complexity analyses, although intriguing, did not prove the existence of the complex class of poly(A)- brain mRNAs. The observed mRNA complexity could have resulted from a variety of artifacts, discussed in more detail below. Several attempts have been made to clone members of this class of mRNA. This search for specific poly(A)- brain mRNAs has met with only limited success. Changes in mRNA polyadenylation state do occur in brain in response to specific physiologic stimuli; however, both the role of polyadenylation and de-adenylation in specific neuronal activities and the existence and significance of poly(A)- mRNAs in brain remain unclear.

Bioamine receptors: evolutionary and functional variations of a structural leitmotiv

Bioamines act as neurohormonal messengers through their binding to receptors which belong to the largest membrane protein family known so far: the seven spanning membrane receptors. This class of receptors transmits the effect of agonist binding to intracellular effectors by interacting with an intermediary G-protein. The diversity of receptor subtypes inside the protein family, observed in many animal species, is the result of a long evolutionary process. The tendency to protein diversification depends upon gene duplications and upon the continuous accumulation of mutations. The maintenance of vital functions in organisms, however, strictly requires enough structural conservation to ensure the functionality of the corresponding proteins. Both forces cooperate to ensure the adaptation of organisms to a changing environment. We have reviewed here the main conformational and functional constraints exerted on the structure of the bioamine receptors. They are mainly the transmembrane conformation of the receptors, their ability to bind ligands, to interact with G-proteins and to desensitize. The molecular basis of the biochemical and pharmacological differences used to classify the members of the receptor family have also been examined. Interestingly, this classification is very close to that obtained by the molecular phylogeny methods, used to elucidate the evolutionary relationships between bioamine receptors. However, this latter classification allows to accurately distinguish between different receptor subtypes (paralogous genes) and species homologous (orthologous genes). In addition, the calculation of phylogenetical distances reveals two main periods of diversification: the first one occurred before the separation of arthropods from vertebrates, in the Precambrian, and corresponds to the appearance of the main subtypes of the bioamine receptors. The second one, which occurred about 400 million years ago, might accompany the cephalization of the CNS in vertebrates.

Expression of the FSH receptor in the testis

FSH has multiple and changing roles in the regulation of spermatogenesis. The first function of FSH is to increase the number of Sertoli cells by stimulation of their mitotic activity. During the prepubertal phase of development, FSH is important for the maturation of the Sertoli cells. Hormonal stimulation of tight junction formation and specific protein secretion are essential. In the adult rat, some of the functions carried out by FSH in prepubertal animals are assumed by testosterone. However, there is evidence that even in the adult rat, FSH is important for quantitatively normal spermatogenesis. The gene for the FSH receptor is large (greater than 85 kb) and complex (10 introns) and is structurally similar to the genes for the LH and TSH receptor. The promoter region of the FSHR gene has been identified and is active in the expression of transgenes in transfected Sertoli cells. We have shown that the FSH receptor mRNA is present in the testes of the adult rat and that the levels of this mRNA are changing during the cycle of the seminiferous epithelium. The presence of relatively high levels of FSHR mRNA in stages XIV-II of the cycle and the relatively low levels in stages VII-VIII suggest that the FSH receptor is carefully regulated in adult rats and presumably has an important function in spermatogenesis. The levels of FSHR mRNA in cultured Sertoli cells are immediately reduced in the presence of FSH or phorbol esters, but the levels soon return to normal.

Changes in cardiac beta 1- and beta 2-adrenoceptor densities after human cardiac transplantation: relation to transplant coronary vasculopathy and pretransplantation disease

In 100 patients 12 to 60 months after cardiac transplantation, the influence of transplant coronary vasculopathy and of the pretransplantation disease (end-stage heart failure caused by coronary artery disease or dilated cardiomyopathy) on the beta-adrenergic receptor (AR) numbers and beta 1/beta 2-AR ratio of right ventricular biopsies was determined. Patients with coronary vasculopathy (CVP) after cardiac transplantation had lower absolute numbers of beta 1-AR compared with patients without CVP. Since patients with CVP had increased left ventricular (LV) end-diastolic pressure and LV muscle mass, it is suggested that decreased beta 1-AR may be the result of an altered hemodynamic situation of the transplanted heart after development of CVP. Patients with dilated cardiomyopathy (DCM) before cardiac transplantation showed a decrease in total beta-AR and of the beta 1/beta 2-AR ratio as a result of an increase in beta 2-AR and a decrease in beta 1-AR numbers. The decreased beta 1/beta 2-AR ratio in patients with previous DCM may indicate that the beta-AR system of the transplanted heart might be influenced (at least in part) by pathophysiologic factors that are characteristic of the pretransplantation disease ultimately leading to cardiac transplantation and persisting after cardiac transplantation.

Food intake of lean and obese Zucker rats following ventricular infusions of adrenergic agonists

The effect on food intake of adrenergic agonists administered into the third cerebral ventricle was studied in Zucker fatty and lean rats. The alpha 2 agonist, clonidine, produced a larger dose-related increase in food intake in lean rats than in the fatty rats. Dose-response curves show similar sensitivity, but decreased responsiveness in the lean animal. The beta 2 adrenergic agonist, salbutamol, produced similar food effect in obese and lean rats reducing food intake in the lean rats at the highest dose (300 nmol), and in the fatty rats at the two highest doses. The effects were small in both groups. The beta 3 agonist, BRL 37344, ([4-(2-((2-hydroxy-2-(3-chlorophenyl)ethyl)amino)-propyl)-phenoxy acetate]) produced a larger dose-related decrease in food intake in the fatty rat than in the lean rats. Dose-response curves showed that sensitivity of beta-receptors was similar, but the lean animals were less responsive. The beta-adrenergic blocking drug propranolol blocked the anorectic effect of BRL 37344 in the fatty rat. These studies suggest that in the fatty rat, the alpha 2 receptor system is tonically more active and the beta 3 receptor system tonically less active, a relationship that would explain the hyperphagia and development of obesity in these animals.

Isolation of two distinct type I angiotensin II receptor genes

A rat genomic Southern blot, probed with a type I angiotensin II receptor probe, demonstrated that two highly homologous type I angiotensin II receptors were present. A rat genomic library was subsequently screened and four clones were isolated. From restriction mapping, differential hybridization, polymerase chain reaction amplification and sequence analyses we have determined that there are two unique type I angiotensin II receptor genes. The first of these genes corresponds to the published rat vascular complementary DNA sequence; the second, corresponds to a novel receptor not previously described.

Expression of alpha-subunits of G proteins in failing human heart: a reappraisal utilizing quantitative polymerase chain reaction

A decrease in the density of the beta 1-adrenergic receptor and an increase in the functional activity of the G inhibitory protein Gi accompany human heart failure; however, the molecular and biochemical mechanisms responsible for these changes are unclear. We previously reported that the steady-state levels of the mRNAs encoding both alpha Gi-3 and alpha Gs were significantly increased in failing human heart. However, these results are not consistent with recent studies demonstrating that immunodetectable levels of alpha G proteins are not different in failing human hearts when compared with non-failing controls. In addition, analysis of the 5' flanking regions of alpha Gi and alpha Gs suggests that these two genes are unlikely to be co-regulated as their regulatory domains are quite different. Therefore, we hypothesized that the disparity between the measurements of alpha G protein gene expression and assessment of the actual levels of alpha G proteins might be due to technical limitations of the Northern blot technique utilized in previous studies for assessment of the mRNA levels; (i) cytoskeletal beta-actin mRNA was used as a standard for normalization; and (ii) only relative levels of alpha G mRNAs were measured. The recent application of the polymerase chain reaction to quantification of mRNA levels in small quantities of human heart provided the tool with which to test this hypothesis. When expressed in molecules of mRNA per microgram of total RNA, there were no differences in the levels of alpha Gi and alpha Gs mRNAs in failing human heart when compared with non-failing controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of beta 2-adrenergic receptors in keratinocytes: glucocorticoids increase steady-state levels of receptor mRNA in foetal rat keratinizing epidermal cells (FRSK cells)

Glucocorticoids increase the beta-adrenergic adenylate cyclase response of epidermal keratinocytes. Using FRSK cells, a cultured cell line of foetal rat keratinocytes, the regulatory mechanism of the beta-adrenergic augmentation effect was investigated. Treatment with dexamethasone (1 x 10(-6) M increase by 1.5-fold the beta-adrenergic adenylate cyclase response of FRSK cells. The effect was observed at 6 h incubation and remained for at least 48 h. The prostaglandin E-adenylate cyclase response was also increased 1.5-fold by glucocorticoid treatment. Neither the adenosine-adenylate cyclase response nor cholera toxin- or forskolin-induced cyclic AMP accumulations were altered. Northern blot hybridization showed that levels of the beta 2-adrenergic receptor mRNA increased within 3 h, while actin-, Gs-alpha, Gi-2 alpha, Gi-3 alpha mRNA levels were unchanged. Testosterone, 17 beta-oestradiol, and progesterone had no effect on either the beta 2-adrenergic adenylate cyclase response or the expression of beta 2-adrenergic receptor mRNA. The increase in the numbers of the beta-adrenergic receptors was visualized by immunofluorescence with an antibody specific for the beta 2-adrenergic receptor. Our results indicate that glucocorticoids regulate the beta 2-adrenergic adenylate cyclase response of FRSK cells through the enhanced expression of the receptor.

Supersensitivity of the adenylyl cyclase system in acute myocardial ischemia: evaluation of three independent mechanisms

Malignant arrhythmias and the spreading of the infarcted zone in acute myocardial ischemia may be influenced by the sympathetic system. It has been known for quite some time that acute ischemia leads to an increased release of endogenous catecholamines. Adaptive mechanisms at the postsynaptic level such as receptor desensitization, which are operative under normoxic conditions, are abolished in acute myocardial ischemia. On the contrary, three newly characterized, distinct mechanisms lead to a transiently increased activity of the beta-adrenergic system in the early phase of acute ischemia: 1) Functionally coupled beta-adrenergic receptors are rapidly and persistently increased at the cell surface due to the impairment of beta-agonist-promoted uncoupling and internalization. 2) Despite the reversible increase of inhibitory, muscarinic M2 receptors, the inhibitory pathway of the adenylyl cyclase systems becomes ineffective since the coupling protein, Gi, is rapidly impaired. Both the Gi-linked GTPase-activity and the binding of [gamma-35S]GTP are reduced by 25-30% without any loss of the total protein. Stimulatory effects prevail at the G-protein level since in the early period of ischemia the stimulatory G-protein, Gs, remains intact. 3) The adenylyl cyclase is transiently sensitized by about 30%. This increased activity is closely associated with the partially purified enzyme and may be due to a rapidly reversible covalent modification. Prolonged ischemia, in contrast, results in a general decrease of the cyclase activity notwithstanding any changes at the receptor or G-protein level. The individual mechanisms may play distinct and/or complimentary roles in the early sensitization of the adenylyl cyclase system in acute myocardial ischemia.

Multiple potential regulatory elements in the 5' flanking region of the beta 3-adrenergic receptor

Since the promoter regions of several genes which modulate intracellular cAMP contain cAMP responsive elements which participate in the regulation of transcription, we assessed whether similar elements were present in the 5' flanking region of the recently cloned human beta 3-adrenergic receptor. This receptor is coupled to cAMP formation, and may play a key role in adrenergic receptor-mediated lipolysis and thermogenesis. Therefore, from a human genomic library we cloned the beta 3-AR coding block and a 1,320 base pair region immediately 5' upstream from the initiation of translation. Based on the classic consensus sequence, we found four potential cAMP responsive elements in this 5' flanking region: CGAGGTCA, TCACATCA, TGAGGTCT and TGACTCCA.

The biology of beta-adrenergic receptors: analysis in human epidermoid carcinoma A431 cells

1. G-protein-linked transmembrane signaling has emerged as a major pathway for information transduction across the cell membrane. 2. In addition to photopigments that propagate the signal from light, cell-surface receptors for hormones, neurotransmitters, and autacoids propagate signals from ligand binding to membrane-bound effector units via G-proteins. 3. Biochemical and molecular features of one prominent member of these receptors, the beta-adrenergic receptor, will be highlighted in the present article. 4. The role of the human epidermoid carcinoma A431 cells as a model for the study of the structure and biology of beta-adrenergic receptors will be emphasized. 5. A model for receptor regulation, gleaned from recent advances in the biochemistry, cell and molecular biology of beta-adrenergic receptors, is discussed.

RDC8 codes for an adenosine A2 receptor with physiological constitutive activity

The cDNA of an unidentified recently cloned G protein-coupled receptor, RDC8, has been expressed in Y1 adrenal cells, in dog thyrocytes in primary culture and in Xenopus oocytes. In all these systems this resulted in the activation of adenylyl cyclase and of the cyclic AMP cascade in the absence of any added external signal. However, this physiologically constitutive activator was inhibited by adenosine deaminase and by inhibitors of the adenosine A2 receptor. Cos 7 cells transfected with RDC8 cDNA constructs acquired binding characteristics of an adenosine A2 receptor. Moreover, RDC8 mRNA and adenosine A2 receptors display a very similar distribution in the brain. RDC8 therefore codes for an A2 adenosine receptor. Whether the physiologically constitutive activation of this receptor is entirely explained by endogeneously produced adenosine is as yet unknown.

Structure and chromosome localization of the human eosinophil-derived neurotoxin and eosinophil cationic protein genes: evidence for intronless coding sequences in the ribonuclease gene superfamily

Human genomic DNAs for the eosinophil granule proteins, eosinophil-derived neurotoxin (EDN) and eosinophil cationic protein (ECP), were isolated from genomic libraries. Alignment of EDN (RNS2) and ECP (RNS3) gene sequences demonstrated remarkable nucleotide similarities in noncoding sequences, introns, and flanking regions, as well as in the previously known coding regions. Detailed examination of the 5'-noncoding regions yielded putative TATA and CAAT boxes, as well as similarities to promoter motifs from unrelated genes. A single intron of 230 bases was found in the 5' untranslated region and we suggest that a single intron in this region and an intronless coding region are features common to many members of the RNase gene superfamily. The RNS2 and RNS3 genes were localized to the q24-q31 region of human chromosome 14. It is likely that these two genes arose as a consequence of a gene duplication event that took place approximately 25-40 million years ago and that a subset of anthropoid primates possess both of these genes or closely related genes.

Protein sequence and gene structure for mouse leukosialin (CD43), a T lymphocyte mucin without introns in the coding sequence

A partial cDNA clone for mouse leukosialin was isolated by use of a rat leukosialin cDNA probe. The mouse cDNA was then used to isolate genomic clones that corresponded to the two mouse genes detected in Southern blots. One gene encoded an open reading frame for the homologue of rat leukosialin and this gene was notable for the absence of introns within the coding sequence. A lack of introns has previously been observed for the human leukosialin gene (Shelley, C. S., Remold-O'Donnell, E., Rosen, F. S. and Whitehead, A. S., Biochem. J., submitted). The other mouse gene was an intronless pseudogene for a leukosialin-related sequence. The presence of only one functional gene that lacked coding-region introns established that molecular heterogeneity in mouse leukosialin could not arise from multiple genes or alternative splicing of exons. The sequence of mouse leukosialin suggested an extracellular segment with a high content of O-linked carbohydrate, as is the case in the rat and human. In addition the mouse molecule had one possible N-linked glycosylation site. The cytoplasmic domain of 124 amino acids was highly conserved between rodent and human leukosialins for the functional genes but not for the pseudogene. This suggests an important functional role for the cytoplasmic domain.

Human arylamine N-acetyltransferase genes: isolation, chromosomal localization, and functional expression

N-Acetylation by hepatic arylamine N-acetyltransferase (NAT, EC 2.3.1.5) is a major route in the metabolism and detoxification of numerous drugs and foreign chemicals. NAT is the target of a common genetic polymorphism of clinical relevance in human populations. We have used our recently isolated rabbit cDNA rnat to clone three human NAT genes from human leukocyte DNA. None of the three genomic coding sequences was interrupted by introns. Two genes, designated NAT1 and NAT2, each possessed open reading frames of 870 bp. Both genes have been assigned to human chromosome 8, pter-q11. Following transfection they were transiently expressed in monkey kidney COS-1 cells. NAT1 and NAT2 gave rise to functional NAT proteins, as judged by their NAT enzyme activity with the arylamine substrate sulfamethazine. Western blots with NAT-specific antisera detected proteins of apparent molecular weight of 33 and 31 kD in NAT1- and NAT2-transfected cultures, respectively. The product of NAT2 had an identical apparent molecular weight as that of NAT detected in human liver cytosol. The deduced amino acid sequence of NAT2 also contained 6 peptide sequences which had previously been determined from tryptic peptides of the polymorphic NAT purified from human liver. These data suggest that NAT2 encodes the polymorphic NAT protein. The third gene, NATP, had multiple deleterious mutations and did not encode a functional NAT protein; it most likely represents a pseudogene.

Adrenergic receptors. Models for regulation of signal transduction processes

Adrenergic receptors are prototypic models for the study of the relations between structure and function of G protein-coupled receptors. Each receptor is encoded by a distinct gene. These receptors are integral membrane proteins with several striking structural features. They consist of a single subunit containing seven stretches of 20-28 hydrophobic amino acids that represent potential membrane-spanning alpha-helixes. Many of these receptors share considerable amino acid sequence homology, particularly in the transmembrane domains. All of these macromolecules share other similarities that include one or more potential sites of extracellular N-linked glycosylation near the amino terminus and several potential sites of regulatory phosphorylation that are located intracellularly. By using a variety of techniques, it has been demonstrated that various regions of the receptor molecules are critical for different receptor functions. The seven transmembrane regions of the receptors appear to form a ligand-binding pocket. Cysteine residues in the extracellular domains may stabilize the ligand-binding pocket by participating in disulfide bonds. The cytoplasmic domains contain regions capable of interacting with G proteins and various kinases and are therefore important in such processes as signal transduction, receptor-G protein coupling, receptor sequestration, and down-regulation. Finally, regions of these macromolecules may undergo posttranslational modifications important in the regulation of receptor function. Our understanding of these complex relations is constantly evolving and much work remains to be done. Greater understanding of the basic mechanisms involved in G protein-coupled, receptor-mediated signal transduction may provide leads into the nature of certain pathophysiological states.

Molecular cloning of the thyrotropin receptor

The pituitary hormone thyrotropin, or thyroid-stimulating hormone (TSH), is the main physiological agent that regulates the thyroid gland. The thyrotropin receptor (TSHR) was cloned by selective amplification with the polymerase chain reaction of DNA segments presenting sequence similarity with genes for G protein-coupled receptors. Out of 11 new putative receptor clones obtained from genomic DNA, one had sequence characteristics different from all the others. Although this clone did not hybridize to thyroid transcripts, screening of a dog thyroid complementary DNA (cDNA) library at moderate stringency identified a cDNA encoding a 4.9-kilobase thyroid-specific transcript. The polypeptide encoded by this thyroid-specific transcript consisted of a 398-amino acid residue amino-terminal segment, constituting a putative extracellular domain, connected to a 346-residue carboxyl-terminal domain that contained seven putative transmembrane segments. Expression of the cDNA conferred TSH responsiveness to Xenopus oocytes and Y1 cells and a TSH binding phenotype to COS cells. The TSHR and the receptor for luteinizing hormone-choriogonadotropin constitute a subfamily of G protein-coupled receptors with distinct sequence characteristics.

A molecular modelling study of the interaction of noradrenaline with the beta 2-adrenergic receptor

A model of the beta 2-adrenergic receptor binding site is built from the primary structure of the receptor, experimental evidence for key binding residues and analogy with a homologous protein of partially determined structure. It is suggested that residues Trp-109, Thr-110 and Asp-113 are involved in ligand binding. Noradrenaline is successfully docked into this model, and the results of an INDO molecular orbital calculation on the complex indicate that a charge transfer interaction between Trp-109 and noradrenaline is possible.

Cytoplasmic loop of beta-adrenergic receptors: synaptic and intracellular localization and relation to catecholaminergic neurons in the nuclei of the solitary tracts

Pharmacological studies suggest that beta-adrenergic receptors (beta AR) in the medial nuclei of the solitary tracts (m-NTS) facilitate presynaptic release of catecholamines and also function at postsynaptic sites. We have localized the antigenic sites for a monoclonal antibody against a peptide corresponding to amino acids 226-239 of beta AR in the m-NTS of rat brain. By light microscopy, immunoperoxidase labeling for this antibody was detected in somata and proximal processes of many small cells that were distributed throughout the rostrocaudal extent of the m-NTS. Electron microscopy confirmed the cytoplasmic localization of beta AR in perikarya and proximal dendrites of neurons. Immunoreactivity occurred as discrete patches associated with cytoplasmic surfaces of plasma membrane and with irregularly-shaped saccules with clear lumen in the immediate vicinity. Select regions of nuclear envelopes, mitochondrial membranes, and rough endoplasmic reticulum were also immunoreactive along their cytoplasmic surfaces. In contrast, the Golgi apparatus was labeled, but infrequently. Immunoreactivity was also detected at numerous post- and occasional presynaptic membrane specializations of select axodendritic junctions. Dual labeling for the beta AR-antibody by the immunoperoxidase method and for a rabbit antiserum against the catecholamine-synthesizing enzyme, tyrosine hydroxylase (TH), by the immunoautoradiographic method within the same sections, further established the precise cellular relations between beta AR and catecholaminergic neurons. Immunoreactivity for beta AR was detected in numerous perikarya and proximal dendrites that did not show detectable levels of TH. However, a few cells were dually labeled for both antigens, as seen by both light and electron microscopy. The TH-labeled terminals formed synapses at junctions both with and without beta AR-like immunoreactivity. These results from the single and dual labeling studies: (1) confirm biochemical predictions that amino acids 226-239 of beta AR protein reside intracellularly; (2) provide the first ultrastructural evidence for beta AR localization within both pre- and postsynaptic membrane specializations of a subset of catecholaminergic synapses; and (3) suggest select intracellular sites that may be involved with synthesis and/or internalization and degradation of the receptor protein.