Cloning of cDNA encoding the pre-beta subunit of mouse thyrotropin

Cell signaling and transcriptional regulation of osteoblast lineage commitment, differentiation, bone formation, and homeostasis

The initiation of osteogenesis primarily occurs as mesenchymal stem cells undergo differentiation into osteoblasts. This differentiation process plays a crucial role in bone formation and homeostasis and is regulated by two intricate processes: cell signal transduction and transcriptional gene expression. Various essential cell signaling pathways, including Wnt, BMP, TGF-β, Hedgehog, PTH, FGF, Ephrin, Notch, Hippo, and Piezo1/2, play a critical role in facilitating osteoblast differentiation, bone formation, and bone homeostasis. Key transcriptional factors in this differentiation process include Runx2, Cbfβ, Runx1, Osterix, ATF4, SATB2, and TAZ/YAP. Furthermore, a diverse array of epigenetic factors also plays critical roles in osteoblast differentiation, bone formation, and homeostasis at the transcriptional level. This review provides an overview of the latest developments and current comprehension concerning the pathways of cell signaling, regulation of hormones, and transcriptional regulation of genes involved in the commitment and differentiation of osteoblast lineage, as well as in bone formation and maintenance of homeostasis. The paper also reviews epigenetic regulation of osteoblast differentiation via mechanisms, such as histone and DNA modifications. Additionally, we summarize the latest developments in osteoblast biology spurred by recent advancements in various modern technologies and bioinformatics. By synthesizing these insights into a comprehensive understanding of osteoblast differentiation, this review provides further clarification of the mechanisms underlying osteoblast lineage commitment, differentiation, and bone formation, and highlights potential new therapeutic applications for the treatment of bone diseases.

Discoveries Around the Hypothalamic-Pituitary-Thyroid Axis

Many members of the American Thyroid Association played prominent roles in discovering the various aspects of the hypothalamic-pituitary-thyroid axis. This axis is fundamental for maintaining the normal serum levels of circulating thyroid hormones (THs) and thus the euthyroid state. The pituitary glycoprotein hormone, thyrotropin (TSH), controls the activity of the thyroid gland. Thyrotropin-releasing hormone and the negative feedback mechanism of circulating TH regulate the synthesis and the secretion of TSH. The dynamic interplay of these two dominant mechanisms has essential effects on TSH release. Therefore, the finding of abnormal serum levels of TSH often indicates the presence of a disorder of thyroid gland function. A summary of key historical discoveries in the understanding of the hypothalamic-pituitary axis is presented.

Molecular cloning and sequence analysis of a cDNA encoding pituitary thyroid stimulating hormone beta-subunit of the Chinese soft-shell turtle Pelodiscus sinensis and regulation of its gene expression

A cDNA encoding thyroid stimulating hormone beta-subunit (TSHbeta) was cloned from pituitary of the Chinese soft-shell turtle, Pelodiscus sinensis, and its regulation of mRNA expression was investigated for the first time in reptile. The Chinese soft-shell turtle TSHbeta cDNA was cloned from pituitary RNA by reverse transcription and polymerase chain reaction (RT-PCR), and rapid amplification cDNA end (RACE) methods. The Chinese soft-shell turtle TSHbeta cDNA consists of 580-bp nucleotides, including 67-bp nucleotides of 5'-untranslated region (UTR), 402-bp of the open reading frame, and 97-bp of 3'-UTR followed by a 14 poly (A) trait. It encodes a precursor protein molecule of 133 amino acids with a putative signal peptide of 19 amino acids and a putative mature protein of 114 amino acids. The number and position of 12 cysteine residues, presumably forming six disulfide bonds, one putative asparagine-linked glycosylation site, and six proline residues that are found at positions for changing the backbone direction of the protein have been conserved in the turtle as in other vertebrate groups. The deduced amino acid sequence of the Chinese soft-shell turtle TSHbeta mature protein shares identities of 82-83% with birds, 71-72% with mammals, 49-57% with amphibians, and 44-61% with fish. The Chinese soft-shell turtle pituitaries were incubated in vitro with synthetic TRH (TSH-releasing hormone), thyroxine and triiodothyronine at doses of 10(-10) and 10(-8)M. TRH stimulated, while thyroid hormones suppressed, TSHbeta mRNA levels in dose-related manner. The sequences of cDNA and its deduced peptide of TSHbeta as well as the regulation of its mRNA level were reported for the first time in reptile.

Cloning of a partial cDNA for Japanese quail thyroid-stimulating hormone and effects of methimazole on the thyroid and reproductive axes

The purposes of this study were to develop a probe for the detection of thyroid-stimulating hormone (TSH) beta subunit mRNA, to validate the usefulness of that probe in measuring TSH, and to use it to investigate the effects of thyroid suppression on TSH and the reproductive axis in Japanese quail. The objectives of experiment 1 were to isolate and characterize a partial cDNA for quail TSH and validate a riboprobe transcribed from this cDNA. This riboprobe was then used to assess changes in TSHbeta mRNA levels in Japanese quail. We isolated a cDNA of 168 bp with 94% identity to the corresponding sequence in chicken TSHbeta. The transcribed riboprobe was shown to be pituitary gland specific, and differences in TSHbeta mRNA levels were detectable with 2.5 microg of total RNA in Northern blot analysis. In experiment 2, our objective was to determine if thyroid inhibition would result in a detectable change in TSHbeta mRNA and alterations in the pituitary luteinizing hormone (LH) or indices of gonadal function. We used adult, reproductively active, male Japanese quail on a long-day photoperiod. Treatment with a goitrogen, methimazole (MMI), increased (P < 0.05) thyroid gland and liver weights and decreased (P < 0.05) serum thyroxine (T4) concentrations compared to control birds. We detected increased TSHbeta mRNA in the pituitaries of MMI-treated birds compared to controls. There was no effect of MMI treatment on the reproductive variables measured, including LHbeta mRNA levels, serum androgen and estradiol concentrations, gonad weight, or cloacal gland area. Therefore, it appears that thyroid axis inhibition and the consequent increase in TSHbeta mRNA did not have direct effects on reproductive axis function in male Japanese quail.

Thyroid-stimulating hormone and thyroid-stimulating hormone receptor structure-function relationships

This review focuses on recent advances in the structure-function relationships of thyroid-stimulating hormone (TSH) and its receptor. TSH is a member of the glycoprotein hormone family constituting a subset of the cystine-knot growth factor superfamily. TSH is produced by the pituitary thyrotrophs and released to the circulation in a pulsatile manner. It stimulates thyroid functions using specific membrane TSH receptor (TSHR) that belongs to the superfamily of G protein-coupled receptors (GPCRs). New insights into the structure-function relationships of TSH permitted better understanding of the role of specific protein and carbohydrate domains in the synthesis, bioactivity, and clearance of this hormone. Recent progress in studies on TSHR as well as studies on the other GPCRs provided new clues regarding the molecular mechanisms of receptor activation. Such advances are a result of extensive site-directed mutagenesis, peptide and antibody approaches, detailed sequence analyses, and molecular modeling as well as studies on naturally occurring gain- and loss-of-function mutations. This review integrates expanding information on TSH and TSHR structure-function relationships and summarizes current concepts on ligand-dependent and -independent TSHR activation. Special emphasis has been placed on TSH domains involved in receptor recognition, constitutive activity of TSHR, new insights into the evolution of TSH bioactivity, and the development of high-affinity TSH analogs. Such structural, physiological, pathophysiological, evolutionary, and therapeutic implications of TSH-TSHR structure-function studies are frequently discussed in relation to concomitant progress made in studies on gonadotropins and their receptors.

Cloning of bullfrog thyroid-stimulating hormone (TSH) beta subunit cDNA: expression of TSHbeta mRNA during metamorphosis

A thyroid-stimulating hormone beta subunit (TSHbeta) cDNA encoding both signal peptide and mature TSHbeta molecule was cloned from a cDNA library constructed from total RNA of the bullfrog (Rana catesbeiana) adenohypophysis. The bullfrog TSHbeta mRNA was estimated by Northern blot analysis to be approximately 1 kb. The deduced amino acid sequence showed 40-61% homologies with the sequences of TSH beta subunits of other vertebrates. Using the cDNA as a probe, we measured changes in mRNA expression in metamorphosing tadpoles of R. catesbeiana. The TSH beta subunit mRNA level increased progressively throughout prometamorphic stages, reaching its maximum at the end of prometamorphosis. The maximum level was maintained throughout early and mid climax, declining at late climax. These results, together with previously obtained data on plasma prolactin and pituitary prolactin mRNA levels, as well as thyroid hormone levels, are discussed in relation to metamorphic changes occurring in the bullfrog larvae.

Expression patterns of the hepatic leukemia factor gene in the nervous system of developing and adult mice

Hepatic leukemia factor (HLF) is a bZIP transcription factor related to the CES-2 protein, which controls apoptosis of the NSM serotoninergic neurons in Caenorhabditis elegans. Ectopic expression of HLF as an E2A-HLF fusion protein in t(17;19)-positive human pro-B cell acute lymphoblastic leukemias is believed to promote malignancy by interfering with apoptosis. While HLF has been linked to malignancies of the lymphoid system, it is not normally expressed in these cells. Rather, HLF transcripts are detected in the liver, kidney, lung and adult nervous system by Northern blotting. Despite the links to cell death, little is known of the distribution or function of HLF in the adult and developing mammalian nervous system. Therefore, we cloned mouse Hlf and studied its expression by in situ hybridization. During embryonic brain development, Hlf expression was restricted to the anterior pituitary and meninges. By early postnatal life, Hlf was highly expressed in somatosensory cortex, thalamic nuclei, and structures arising from ectodermal placodes. Subsequently, Hlf expression increased in the central nervous system and was found throughout the brain by adulthood. In the developing pituitary gland, Hlf was highly expressed in the rostral tip of the anterior lobe. This pattern is similar to that of Tef, an Hlf-related bZIP protein. However, while Tef is expressed in the anterior pituitary of the adult mouse, Hlf was detected in both the anterior and posterior pituitary. Hlf expression was not associated with cells undergoing programmed cell death in the nervous system. Hlf expression increased markedly with synaptogenesis and was coincident with barrel formation revealed by cytochrome oxidase staining. Together, these data suggest that Hlf plays a role in the function of differentiated neurons in the adult nervous system rather than programmed cell death.

The thyrotropin beta-subunit gene is repressed by thyroid hormone in a novel thyrotrope cell line, mouse T alphaT1 cells

TSH is expressed in two populations of thyrotropes in the pituitary: one in the pars distalis and a second in the pars tuberalis. Pars distalis thyrotropes exhibit classical endocrine inhibition of TSH by thyroid hormone, whereas pars tuberalis thyrotropes do not. The majority of our understanding of TSH subunit gene regulation has come from studies conducted in dispersed pituitary, dispersed thyrotropic tumors, or the GH3 somatolactotrope cell line. However, the dispersed pituitary model is limited because of its inherent heterogeneity, thyrotropic tumors are difficult to grow and maintain, and the GH3 cells lack endogenous TSH expression. The recent derivation of a clonal thyrotrope cell line, T alphaT1, that expresses thyrotrope-specific markers, overcomes these limitations. However, because it was not possible to distinguish whether the tumor from which the T alphaT1 cells are derived originated in the pars distalis or the pars tuberalis, it was necessary to define their cellular origin and thereby establish their status as representative thyrotrope cells for future molecular studies. In this study, we demonstrate that the T alphaT1 cells express thyroid hormone receptors (beta1 and beta2) and their heterodimeric partner, retinoid X receptor-gamma. Treatment with T3 causes a dose- and time-dependent decrease in the expression of the TSH beta-subunit messenger RNA. In contrast to previous reports in rat pituitary cultures, T3 does not alter TSH beta-subunit messenger RNA stability in the T alphaT1 cells. Based on these data and the presence of thyrotrope-specific isoforms of the transcription factor Pit-1, we conclude that the T alphaT1 cells represent differentiated thyrotropes of the pars distalis and will be a useful model system for future analysis of the cis- and trans-acting factors necessary for thyrotrope-specific and thyroid hormone-regulated TSH gene expression.

Cloning and sequence analysis of a cDNA for the beta subunit of chicken thyroid-stimulating hormone

The beta subunit of thyroid-stimulating hormone (TSHbeta) has been isolated and sequenced in many species, including several mammals and the frog, but not in any avian species. Therefore, the objective of this study was to isolate and sequence a cDNA for chicken TSHbeta. Degenerate oligonucleotide primers were designed, based on conserved regions of TSHbeta from four other species, and used for reverse transcription and polymerase chain reaction amplification of a cDNA fragment from total cellular RNA of pituitary glands from 7-day-old chicks. The remaining sequence was completed by rapid amplification of cDNA ends. The predicted amino acid sequence was 70. 4% identical between bovine and chicken, 69.6% identical between chicken and rat, and 57.4% identical between chicken and frog. To test for tissue specificity of the cDNA, total cellular RNA samples from testicle, liver, pituitary, lung, and heart were analyzed by Northern blot. The 32P-labeled antisense riboprobe hybridized to an RNA species of approximately 600-700 bases in pituitary RNA alone, corresponding with the length of TSHbeta mRNA in other species. Gene expression in Day 1 posthatch chickens was then analyzed by ribonuclease protection assay. Anterior pituitary cells of Day 1 chickens were treated for 20 to 24 hr in serum-free medium alone or with medium containing either thyrotropin-releasing hormone (TRH) (10(-8) M) or triiodothyronine (T3) (10(-9) M). The RNA was then harvested from these cells and hybridized with a 32P-labeled antisense riboprobe. Treatment with TRH had no effect on TSHbeta mRNA levels, while T3 significantly decreased (P < 0.05; n = 6 trials) TSHbeta mRNA levels by 45%. Taken together these results indicate that the cDNA sequence derived represents chicken TSHbeta mRNA, and that TSHbeta gene expression is downregulated by thyroid hormones as it is in mammals.

Quantification of salmon alpha- and thyrotropin (TSH) beta-subunit messenger RNA by an RNase protection assay: regulation by thyroid hormones

In order to study salmon thyroid-stimulating hormone (TSH), we designed a highly specific, sensitive, and rapid RNase protection assay (RPA) for quantification of steady-state levels of salmon TSH beta-subunit mRNA expression. The cDNA encoding the beta-subunit of TSH was isolated from coho salmon pituitary total RNA by reverse-transcriptase PCR, partially sequenced, and used as template for synthesizing a radioactively labeled, sequence-specific, antisense probe, and sense standard for the RPA. This assay, along with a similar RPA previously designed for coho salmon total alpha-subunit mRNA, was used to examine the effects of feeding T3 (0, 10, 100 micrograms/g) and methimazole (a thyroid inhibitor) (2.5 mg/g) on TSH subunit gene expression after 2 and 4 weeks. The low dose of T3 (10 micrograms/g) caused no change in TSH beta mRNA after 2 and 4 weeks and a transient increase in alpha mRNA after 2 weeks, followed by no significant effect after 4 weeks. The high dose of T3 (100 micrograms/g) caused a decrease in TSH beta mRNA after 4 weeks and no change in total alpha mRNA after 2 and 4 weeks. In contrast, methimazole treatment caused significant increases in both TSH beta mRNA (250%) and alpha mRNA (50%) levels after 4 weeks. These findings confirm that, as in mammals, TSH alpha- and beta-subunit expression in teleosts may be differentially regulated by negative feedback from the thyroid hormones.

Local hormone networks and intestinal T cell homeostasis

Neuroendocrine hormones of the hypothalamus-pituitary-thyroid axis can exert positive or negative immunoregulatory effects on intestinal lymphocytes. Small intestine epithelial cells were found to express receptors for thyrotropin-releasing hormone (TRH) and to be a primary source of intestine-derived thyroid-stimulating hormone (TSH). The gene for the TSH receptor (TSH-R) was expressed in intestinal T cells but not in epithelial cells, which suggested a hormone-mediated link between lymphoid and nonhematopoietic components of the intestine. Because mice with congenitally mutant TSH-R (hyt/hyt mice) have a selectively impaired intestinal T cell repertoire, TSH may be a key immunoregulatory mediator in the intestine.

Immortalization of pituitary cells at discrete stages of development by directed oncogenesis in transgenic mice

Targeted expression of oncogenes in transgenic mice can immortalize specific cell types to serve as valuable cultured model systems. Utilizing promoter regions from a set of genes expressed at specific stages of differentiation in a given cell lineage, we demonstrate that targeted oncogenesis can produce cell lines representing sequential stages of development, in essence allowing both spatial and temporal immortalization. Our strategy was based on our production of a committed but immature pituitary gonadotrope cell line by directing expression of the oncogene SV40 T antigen using a gonadotrope-specific region of the human glycoprotein hormone alpha-subunit gene in transgenic mice. These cells synthesize alpha-subunit and gonadotropin-releasing hormone (GnRH) receptor, yet are not fully differentiated in that they do not synthesize the beta-subunits of luteinizing hormone (LH) or follicle-stimulating hormone (FSH). This observation lead to the hypothesis that targeting oncogenesis with promoters that are activated earlier or later in development might immortalize cells that were more primitive or more differentiated, respectively. To test this hypothesis, we used an LHbeta promoter to immortalize a cell that represents a subsequent stage of gonadotrope differentiation (expression of alpha-subunit, GnRH receptor, and LH beta-subunit but not FSH beta-subunit). Conversely, targeting oncogenesis with a longer fragment of the human alpha-subunit gene (which is activated earlier in development) resulted in the immortalization of a progenitor cell that is more primitive, expressing only the alpha-subunit gene. Interestingly, this transgene also immortalized cells of the thyrotrope lineage that express both alpha- and beta-subunits of thyroid-stimulating hormone and the transcription factor GHF-1 (Pit-1). Thus, targeted tumorigenesis immortalizes mammalian cells at specific stages of differentiation and allows the production of a series of cultured cell lines representing sequential stages of differentiation in a given cell lineage.

Recessive resistance to thyroid hormone in mice lacking thyroid hormone receptor beta: evidence for tissue-specific modulation of receptor function

The diverse functions of thyroid hormone (T3) are presumed to be mediated by two genes encoding the related receptors, TRalpha and TRbeta. However, the in vivo functions of TRalpha and TRbeta are undefined. Here, we report that targeted inactivation of the mouse TRbeta gene results in goitre and elevated levels of thyroid hormone. Also, thyroid-stimulating hormone (TSH), which is released by pituitary thyrotropes and which is normally suppressed by increased levels of thyroid hormone, was present at elevated levels in homozygous mutant (Thrb-/-) mice. These findings suggest a unique role for TRbeta that cannot be substituted by TRalpha in the T3-dependent feedback regulation of TSH transcription. Thrb-/- mice provide a recessive model for the human syndrome of resistance to thyroid hormone (RTH) that exhibits a similar endocrine disorder but which is typically caused by dominant TRbeta mutants that are transcriptional inhibitors. It is unknown whether TRalpha, TRbeta or other receptors are targets for inhibition in dominant RTH; however, the analysis of Thrb-/- mice suggests that antagonism of TRbeta-mediated pathways underlies the disorder of the pituitary-thyroid axis. Interestingly, in the brain, the absence of TRbeta may not mimic the defects often associated with dominant RTH, since no overt behavioural or neuroanatomical abnormalities were detected in Thrb-/- mice. These data define in vivo functions for TRbeta and indicate that specificity in T3 signalling is conferred by distinct receptor genes.

cDNA cloning of the beta subunit of teleost thyrotropin

cDNA clones encoding the beta subunit of thyrotropin (thyroid-stimulating hormone; TSH) were isolated from a cDNA library made from the pituitaries of immature rainbow trout and sequenced. The precursor of rainbow trout TSH beta consists of 147 aa, which can be cleaved into a signal peptide (20 aa) and a mature protein (127 aa) containing one potential N-glycosylation site and 12 cysteine residues. The protein showed highest homology with human TSH beta (51%) and lesser homology with human follitropin (42%), human lutropin (32%), and salmon gonadotropin (31-33%) beta subunits. The identification of TSH in addition to two gonadotropins (gonadotropins I and II) in the teleost fish suggests that the divergence of three kinds of glycoprotein hormones from an ancestral molecule took place earlier than the time of divergence of teleosts from the main line of evolution leading to tetrapods. Northern blot analysis showed that the expression of the rainbow trout TSH beta gene is specific to the pituitary gland and is significantly higher in immature fish than in mature fish, suggesting that TSH plays some role in the biological processes of immature fish.

GHF-1-promoter-targeted immortalization of a somatotropic progenitor cell results in dwarfism in transgenic mice

During pituitary development, the homeo domain protein GHF-1 is required for generation of somatotropes and lactotropes and for growth hormone (GH) and prolactin (PRL) gene expression. GHF-1 mRNA is detectable several days before the emergence of GH- or PRL-expressing cells, suggesting the existence of a somatotropic progenitor cell in which GHF-1 transcription is first activated. We have immortalized this cell type by using the GHF-1 regulatory region to target SV40 T-antigen (Tag) tumorigenesis in transgenic mice. The GHF-Tag transgene caused developmental entrapment of somatotropic progenitor cells that express GHF-1 but not GH or PRL, resulting in dwarfism. Immortalized cell lines derived from a transgenic pituitary tumor maintain the characteristics of the somato/lactotropic progenitor in that they express GHF-1 mRNA and protein yet fail to activate GH or PRL transcription. Using these cells, we identified an enhancer that activates GHF-1 transcription at this early stage of development yet is inactive in cells representing later developmental stages of the somatotropic lineage or in other cell types. These experiments not only demonstrate the potential for immortalization of developmental progenitor cells using the regulatory regions from cell type-specific transcription factor genes but illustrate the power of such model systems in the study of developmental control.

Thyroid hormone regulation of thyrotropin gene expression

Thyroid hormones suppress the synthesis and release of thyrotropin from thyrotropes in the anterior pituitary gland, a feature that is critical in the classic negative-feedback loop of the pituitary-thyroid endocrine axis. The major effect of thyroid hormones in this system is exerted at the transcriptional level. The molecular mechanisms by which there is negative regulation of TSH subunit gene expression by thyroid hormone have been elucidated. The TSH subunit genes have isolated and characterized. Structure-function analyses using fusion genes and DNA transfection approaches have defined the putative negative TREs among the promoters of the rat, mouse, and human alpha and TSH beta genes. These sequences are either largely overlapping direct TRE half-sites, TRE half-sites as direct repeats gapped by two nucleotides, or single TRE half-sites. These arrangements are distinct from those seen in positive TREs. Recent knowledge regarding the molecular mechanisms of thyroid action in general forces consideration of multiple TR isoforms, TR heterodimer partners (TRAPs), and thyroid hormones in the ultimate mechanisms of negative action. Several models have been proposed, but none has yet been proved. In addition, the role of thyroid hormone in the regulation of gene expression at the posttranscriptional level is beginning to be addressed. Future work should continue to illuminate these important facets of gene regulation.

Isolation and characterization of two distinct gonadotropins, GTHI and GTHII, from bonito (Katsuwonus plelamis) pituitary glands

Two distinct glycoproteins homologous to chum salmon GTHI and GTHII were isolated from the pituitary glands of a marine fish, the bonito (Katsuwonus plelamis), and characterized by amino acid sequence analysis in order to obtain additional evidence for duality of teleost GTHs. Glycoproteins were extracted from the pituitary glands, and intact GTHI and GTHII, consisting of two distinct subunits, were purified by ion-exchange chromatography on DEAE-cellulose, rpHPLC on Asahipak C4P-50 in alkaline buffer, and gel filtration on Superdex 75. The association of the subunits was stable in GTHI (39 kD) and unstable in GTHII (30 kD) in acidic conditions. Immunoblotting revealed that antisera against beta subunits of chum salmon GTHs reacted with GTHII, but not with GTHI. In addition, none of the GTHs was stained with antiserum against human TSH beta. Sequence analysis demonstrated that bonito GTHI beta is homologous to salmon GTHI beta with 43% sequence identity, and bonito GTHII beta is homologous to salmon GTHII beta with 67% identity. Sequence identity between bonito GTHI beta and GTHII beta was only 28%. Thus, it is evident that the bonito pituitary gland produces two chemically distinct gonadotropins homologous to chum salmon GTHs.

Molecular structures of glycoprotein hormones and functions of their carbohydrate components

Images

Mutations of the human thyrotropin-beta subunit glycosylation site reduce thyrotropin synthesis independent of changes in glycosylation status

In recent studies, site-directed mutagenesis has been used to alter the tripeptide glycosylation recognition sequences of glycoprotein hormone subunits, thereby affecting their structure and function. However, it is not known whether these effects result from changes in glycosylation status, amino acid sequence, or both. We therefore studied the synthesis of wild-type and mutant recombinant human thyrotropins produced by transient transfection of a human cell line. Mutating the TSH-beta subunit glycosylation recognition sequence, Asn-Thr-Thr (codons 23-25), to either Gln-Thr-Thr or Asn-Thr-Tyr abolished subunit glycosylation, as demonstrated by the inability to incorporate 3H-carbohydrates. However, a third mutation (Asn-Thr-Ser) contained an intact glycosylation recognition sequence site, and was shown to retain glycosylation. The mutations that abolished TSH-beta subunit glycosylation resulted in greater than 90% decreases in TSH synthesis. However, the glycosylation recognition sequence mutant that retained beta subunit glycosylation exhibited a 70% decrease in TSH production. These decreases were not attributable to the intracellular accumulation of TSH or its free beta subunit. We also engineered two TSH-beta subunit mutations that did not alter the glycosylation recognition sequence. A glycine to arginine mutation adjacent to the glycosylation recognition sequence, in a region thought to be critical for heterodimer formation, abolished TSH production. In contrast, shortening the TSH-beta subunit carboxyterminus by six amino acids increased TSH synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Pituitary cell phenotypes involve cell-specific Pit-1 mRNA translation and synergistic interactions with other classes of transcription factors

Development of the anterior pituitary gland involves proliferation and differentiation of ectodermal cells in Rathke's pouch to generate five distinct cell types that are defined by the trophic hormones they produce. A detailed ontogenetic analysis of specific gene expression has revealed novel aspects of organogenesis in this model system. The expression of transcripts encoding the alpha-subunit common to three pituitary glycoprotein hormones in the single layer of somatic ectoderm on embryonic day 11 established that primordial pituitary cell commitment occurs prior to formation of a definitive Rathke's pouch. Activation of Pit-1 gene expression occurs as an organ-specific event, with Pit-1 transcripts initially detected in anterior pituitary cells on embryonic day 15. Levels of Pit-1 protein closely parallel those of Pit-1 transcripts without a significant lag. Unexpectedly, Pit-1 transcripts remain highly expressed in all five cell types of the mature pituitary gland, but the Pit-1 protein is detected in only three cell types--lactotrophs, somatotrophs, and thyrotrophs and not in gonadotrophs or corticotrophs. The presence of Pit-1 protein in thyrotrophs suggests that combinatorial actions of specific activating and restricting factors act to confine prolactin and growth hormone gene expression to lactotrophs and somatotrophs, respectively. A linkage between the initial appearance of Pit-1 protein and the surprising coactivation of prolactin and growth hormone gene expression is consistent with the model that Pit-1 is responsible for the initial transcriptional activation of both genes. The estrogen receptor, which has been reported to be activated in a stereotypic fashion subsequent to the appearance of Pit-1, appears to be capable, in part, of mediating the progressive increase in prolactin gene expression characteristic of the mature lactotroph phenotype. This is a consequence of synergistic transcriptional effects with Pit-1, on the basis of binding of the estrogen receptor to a response element in the prolactin gene distal enhancer. These data imply that both transcriptional and post-transcriptional regulation of Pit-1 gene expression and combinatorial actions with other classes of transcription factors activated in distinct temporal patterns, are required for the mature physiological patterns of gene expression that define distinct cell types within the anterior pituitary gland.

Molecular cloning and sequence analysis of chum salmon gonadotropin cDNAs

cDNAs encoding alpha and beta subunits of salmon gonadotropins, sGTHI and sGTHII, have been isolated from the cDNA library prepared from salmon pituitary mRNA. sGTHI alpha, sGTHI beta, and sGTHII beta cDNAs encode polypeptides of 114, 137, and 142 amino acids, including signal peptides of 22, 24, and 23 amino acids, respectively. The deduced amino acid sequence for sGTHI alpha revealed rather high homology (66-69%) to mammalian alpha chains, whereas sGTHI beta and sGTHII beta show lower homology (30%) to each other and to mammalian beta subunits. The existence of two distinct beta-subunit cDNAs in the teleost suggests that divergence of the GTH gene took place earlier than divergence of teleosts from the main line of evolution leading to tetrapods.

The human thyrotropin beta-subunit gene differs in 5' structure from murine TSH-beta genes

The gene encoding the beta-subunit of human thyrotropin (hTSH-beta) was isolated, and its nucleotide sequence was determined. The gene is 4.3 kb in length, consists of three exons and two introns, and is present as a single copy as determined by Southern blot analysis of total genomic DNA. The protein coding portion of the gene, which includes exons 2 and 3, was isolated from a human genomic phage library, while exon 1, which encodes only 5' untranslated mRNA sequence, was isolated from a plasmid library of size-selected genomic DNA fragments. Here we describe the isolation of the 5' untranslated exon of the hTSH-beta subunit and 5'-flanking region. The structure of the hTSH-beta gene is very similar to the previously characterized TSH-beta genes from mouse and rat. The genes from all three species have two distinct promoter regions, but while both promoters are utilized by the murine TSH-beta genes, the human TSH-beta gene apparently utilizes only the proximal promoter for transcription initiation. A striking difference in hTSH-beta gene structure compared to the murine genes is that exon 1 of the human gene is 36 nucleotides. An analysis of the mouse, rat, and human exon 1 and 5'-flanking region shows a high percentage of sequence homology, with the exception of a 9-nucleotide insertion 13 bases 3' from the proximal TATA box found in the human gene but not found in the other two species. We propose that this insertion results in the additional length of human exon 1 compared to the mouse and rat genes. By isolating the promoter region of the hTSH-beta gene, we can begin to identify specific sequences involved in the regulation of hTSH gene expression.

Organization and nucleotide sequence of the gene encoding the beta-subunit of murine thyrotropin

We constructed a murine genomic DNA library in lambda EMBL3 and have isolated and determined the nucleotide sequence of the murine thyrotropin beta-subunit (TSH beta) gene. The cloned gene was derived from a thyrotropic tumor and had no detectable rearrangements when compared to the murine TSH beta gene in total genomic DNA. The murine TSH beta gene is 5 kb in size and consists of five exons and four introns. The 5' untranslated region of the mRNA is encoded except for a single nucleotide by exons 1, 2, and 3. The protein-coding regions are encoded by exons 4 and 5 while the 3' untranslated region is entirely contained in exon 5. Primer extension analysis using an exon 1-specific primer was used to map the 5' end of the gene. Two transcriptional start sites are present in the murine TSH beta gene which appear to be positioned by two TATAAA sequences located 40 bp apart. In all, 99% of transcripts initiate at the downstream site. Transcription from both start sites is affected by thyroidal status in both murine pituitaries and in TtT97 thyrotropic tumors. Finally, sequences homologous with putative thyroid-responsive elements and cyclic AMP-responsive elements are present in the 5'-flanking region and may be important in regulating negative and positive effects on TSH beta gene expression.

Isolation and nucleotide sequence analysis of a cloned cDNA encoding the beta-subunit of bovine follicle-stimulating hormone

Two different cDNAs containing sequences coding for the beta-subunit of bovine follicle stimulating hormone (FSH-beta) have been isolated from a phage lambda gt11 bovine pituitary cDNA library. The complete nucleotide sequence of both clones was determined, and the combined sequence represents most of FSH-beta mRNA. The combined sequence contains 46 nucleotides of 5'-untranslated sequence followed by 387 nucleotides of coding sequence. The coding sequence predicts a 19-amino-acid amino-terminal precursor segment followed by the 110-amino-acid sequence of mature bovine FSH-beta. The cDNA sequence demonstrates the presence of a long 3'-untranslated region containing 1295 bases followed by a segment representing the poly(A) portion of the mRNA. Thus, the combined sequence of the cDNAs suggests a minimal size of 1.7 kb for FSH-beta mRNA. Analysis of FSH-beta sequences present in bovine pituitary mRNA demonstrated the presence of an mRNA with a size of about 2.0 kb. This apparent discrepancy is probably due to the presence of a several-hundred nucleotide tract of poly(A) at the 3' terminus of the mRNA. Comparison of the amino acid sequence predicted from the cDNA with the known amino acid sequence of the beta-subunit of FSH from several different species demonstrates that the protein has been highly conserved.

Cloning and DNA sequence analysis of the cDNA for the precursor of the beta chain of bovine follicle stimulating hormone

Follicle stimulating hormone (FSH) plays essential roles in the maintenance and development of oocytes and spermatozoa in normal reproductive physiology. FSH possesses two subunits, alpha and beta, the latter being responsible for FSH biological specificity. We have cloned and sequenced the cDNA encoding the FSH beta chain from a bovine anterior pituitary cDNA library. The mature molecule is 109 amino acids long and is preceded by a 20-amino acid putative signal peptide. RNA gel blot analysis of bovine pituitary RNA shows that the mRNA encoding beta chain of FSH is approximately 1.7 kilobases in length.

Assignment of the gene for the beta subunit of thyroid-stimulating hormone to the short arm of human chromosome 1

The chromosomal locations of the genes for the beta subunit of human thyroid-stimulating hormone (TSH) and the glycoprotein hormone alpha subunit have been determined by restriction enzyme analysis of DNA extracted from rodent-human somatic cell hybrids. Human chorionic gonadotropin (CG) alpha-subunit cDNA and a cloned 0.9-kilobase (kb) fragment of the human TSH beta-subunit gene were used as hybridization probes in the analysis of Southern blots of DNA extracted from rodent-human hybrid clones. Analysis of the segregation of 5- and 10-kb EcoRI fragments hybridizing to CG alpha-subunit cDNA confirmed the previous assignment of this gene to chromosome 6. Analysis of the patterns of segregation of a 2.3-kb EcoRI fragment containing human TSH beta-subunit sequences permitted the assignment of the TSH beta-subunit gene to human chromosome 1. The subregional assignment of TSH beta subunit to chromosome 1p22 was made possible by the additional analysis of a set of hybrids containing partially overlapping segments of this chromosome. Human TSH beta subunit is not syntenic with genes encoding the beta subunits of CG, luteinizing hormone, or follicle-stimulating hormone and is assigned to a conserved linkage group that also contains the structural genes for the beta subunit of nerve growth factor (NGFB) and the proto-oncogene N-ras (NRAS).

Mechanisms and regulation of TSH glycosylation

Thyroid-stimulating hormone provides an interesting model to study the glycosylation and carbohydrate processing of a heterodimeric glycoprotein with a clear physiological function. The carbohydrate moiety on TSH is required for subunit combination, protection from intracellular proteolysis and aggregation, and for attainment of full biological activity. Recent work, summarized herein, has studied mechanisms and kinetics of TSH carbohydrate maturation and has contrasted processing rates and composition of free and combined subunits. Neuroendocrine factors, such as thyrotropin-releasing hormone, appear to modulate the carbohydrate structure of secreted TSH, which results in a change in the relative bioactivity of the circulating hormone. The biochemical mechanisms by which these carbohydrate alterations occur and how they affect hormone-receptor interaction are currently under investigation.

Molecular cloning of the human thyrotropin-beta subunit gene

Genomic DNA fragments that carried a gene for human thyrotropin-beta (hTSH beta) subunit were isolated. Nucleotide sequence analysis of the gene showed that the hTSH beta subunit precursor consists of 138 amino acid residues. There is an N-terminal sequence of 20 amino acids as a signal peptide, followed by 112 amino acids, whose sequence is in agreement with that known for the secretory form of hTSH beta subunit. This is followed by an additional stretch of 6 hydrophobic amino acids, which may be eliminated post-translationally. The coding region is separated by an intron of about 460 bp. Genomic Southern blot hybridization analysis suggested that the hTSH beta gene is a unique single copy gene.

Thyroid hormone regulation of thyrotropin alpha- and beta-subunit gene transcription

We studied the effect of thyroid hormone on the transcription of the genes for the alpha- and beta-subunits of thyrotropin (TSH) in thyrotropic tumors (IAK 109D and 109F) carried in hypothyroid mice. Gene transcription was measured in isolated nuclei by allowing completion of RNA chains initiated in vivo in the presence of [alpha-32P]UTP and by hybridization of labeled RNA transcripts to filter-bound plasmids containing alpha or TSH-beta cDNA sequences. Treatment of animals carrying tumor IAK 109D with 3,5,3'-triiodo-L-thyronine (T3) (5 micrograms/100 g body weight) for 2 hr reduced TSH-beta gene transcription to less than 10% of control levels, whereas alpha RNA synthesis was reduced to 59% of control. The inhibition of TSH-beta gene activity was maintained after 6 hr of T3 treatment, whereas alpha gene transcription rose slightly to 77% of control. The tumor content of alpha and TSH-beta mRNA, determined by dot blot hybridization with 32P-labeled plasmid probes containing alpha or TSH-beta cDNAs, was unchanged after 2 hr of T3 treatment, and each was reduced by approximately 25% at 6 hr. These untreated tumors contained approximately equal amounts of alpha and TSH-beta mRNA. However, the basal rate of TSH-beta gene transcription was threefold greater than that of alpha gene transcription. Treatment of animals bearing tumor IAK 109F with the same dose of T3 for 30 min did not significantly affect alpha or TSH-beta gene transcription, but at 2 hr alpha and TSH-beta RNA synthesis had decreased to 50% and 10% of control values, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

[Application of the technics of molecular genetics in neurochemistry]

Research in molecular neurobiology has recently entered a new phase of rapid development as a result of the application of the techniques of molecular genetics. This is illustrated by recent work on the electric ray (Torpedo marmorata and T. californica), whose electric organ is a rich source of cholinergic synapses. Other examples from recent literature of the application of the recombinant DNA technique to the mammalian central nervous system are given and possible future developments are discussed.

Thyroid hormone decreases thyrotropin subunit mRNA levels in rat anterior pituitary

Recombinant DNA clones containing sequences for the subunits of rat thyrotropin have been isolated, characterized, and used to examine the hormonal regulation of subunit mRNA levels. A library of rat cDNA sequences cloned in plasmid pBR322 was constructed and screened for thyrotropin sequences using cloned bovine cDNAs which had been previously identified. The identity of rat thyrotropin subunit cDNAs was confirmed by nucleotide sequence analysis. The cloned cDNAs were then used as hybridization probes to examine the effects of triiodothyronine on the levels of the alpha- and beta-subunit mRNAs of thyrotropin. The results demonstrate that treatment with thyroid hormone decreases the levels of both alpha- and beta-subunit mRNA in rat anterior pituitary.

Compilation and analysis of sequences upstream from the translational start site in eukaryotic mRNAs

5-Noncoding sequences have been tabulated for 211 messenger RNAs from higher eukaryotic cells. The 5'-proximal AUG triplet serves as the initiator codon in 95% of the mRNAs examined. The most conspicuous conserved feature is the presence of a purine (most often A) three nucleotides upstream from the AUG initiator codon; only 6 of the mRNAs in the survey have a pyrimidine in that position. There is a predominance of C in positions -1, -2, -4 and -5, just upstream from the initiator codon. The sequence CCAGCCAUG (G) thus emerges as a consensus sequence for eukaryotic initiation sites. The extent to which the ribosome binding site in a given mRNA matches the -1 to -5 consensus sequence varies: more than half of the mRNAs in the tabulation have 3 or 4 nucleotides in common with the CCACC consensus, but only ten mRNAs conform perfectly.

Assignment of the genes for the alpha and beta subunits of thyrotropin to different mouse chromosomes

A series of mouse-hamster somatic cell hybrids, containing reduced numbers of mouse chromosomes and a complete set of hamster chromosomes, was used to determine the chromosomal locations of the genes for the alpha and beta subunits of mouse thyrotropin. Cloned cDNA probes for each subunit, in conjunction with Southern blot analysis of DNA treated with the restriction enzyme BamHI, allowed for assignment of the alpha-subunit gene to mouse chromosome 4 and of the beta-subunit gene to chromosome 3. Mouse alpha-subunit gene sequences always segregated with chromosome 4 (concordant in 14 hybrids) and the enzyme markers phosphoglucomutase 2 and 6-phosphogluconate dehydrogenase. Mouse beta-subunit gene sequences always segregated with chromosome 3 (concordant in 15 hybrids). Thus, the genes for at least one of the glycoprotein hormones, thyrotropin, are on different chromosomes.