Isolation and characterization of the gene encoding the alpha-subunit of the rat pituitary glycoprotein hormones

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.

CRIP1, a novel immune-related protein, activated by Enterococcus faecalis in porcine gastrointestinal epithelial cells

Cysteine-rich intestinal protein 1 (CRIP1) is an important transcriptional regulation factor during the tumor development. Although it was largely studied in the human or mouse, no report has provided functional evidence for it in the swine. To date, the real sequence of porcine CRIP1 (poCRIP1) was also still unknown. In this study, clear characteristics for the poCRIP1 were represented. A 552bp poCRIP1 cDNA was obtained from porcine brain tissue using real time reverse transcriptase PCR. The poCRIP1 showed 89% and 93% homologous with human and cattle, respectively. And it also contained one conserved domain, LIM-CRIP domain. Meanwhile, the genomic structure and promoter map was done and several conserved transcriptional regulatory sites were also predicted in this study. The expression pattern of poCRIP1 indicated that poCRIP1 is expressed in mucosal tissue. An infection experiment about the gut was designed to analyze whether or not poCRIP1 was functional in gut immunity, and an interesting result was that poCRIP1 was only activated by an opportunistic pathogen, Enterococcus faecalis FA2-2. It was the first report to identify the full-length sequence of poCRIP1 gene, represent a clear characteristic and immunologic role of CRIP1 in domestic animal until now.

Cloning of the genes for the pituitary glycoprotein hormone alpha and follicle-stimulating hormone beta subunits in the Japanese crested ibis, Nipponia nippon

We have isolated a part of the gene for the pituitary glycoprotein hormone common alpha subunit (PGHalpha) and the whole gene for the follicle-stimulating hormone beta subunit (FSHbeta) in the Japanese crested ibis (Nipponia nippon), a critically endangered bird species in East Asia. The nucleotide sequence of a part of the PGHalpha gene (5026 bp) contained three exons holding the whole coding and 3' untranslated regions, but lacked a 5' untranslated region. Its exon-intron structure was similar to that in mammals, but different from that in teleosts in the location of the second intron. For the FSHbeta gene, the nucleotide sequence of 7633 bp was assembled from two phage clones. The exon-intron structure of three exons and two introns was similar to that observed in mammals and teleosts. In the putative promoter region of the ibis FSHbeta gene, a progesterone responsive element (PRE)-like sequence and two AP-1 responsive element-like sequences reported in the ovine FSHbeta gene were not conserved in complete form. The increased number of ATTTA motifs in the putative 3' untranslated region in comparison with those in Japanese quail and chicken FSHbeta cDNA suggested that more rapid degradation of FSHbeta mRNA occurs in this species. Deduced amino acid sequences of the ibis PGHalpha and FSHbeta showed high similarities with those of the corresponding subunits of other avian species. This is the first report on the genomic sequences of the PGHalpha and FSHbeta in an avian species.

Sequence analysis of the human glycoprotein hormone alpha-subunit gene 5'-flanking DNA and identification of a potential regulatory element as an alu repetitive sequence

The nucleotide sequence of the human glycoprotein hormone alpha-subunit (GPHalpha) gene 5'-flanking DNA was determined from -1637 to +49 relative to the cap site (+1). Comparison of the upstream sequence of the human gene with those of rhesus and mouse demonstrates regions with variable identity. When the 1.7 kb fragment was used to drive the expression of chloramphenicol acetyltransferase (CAT) in transiently transfected HeLa cells, it was found that CAT activity was elevated about 3-fold when the fragment was truncated from -1637 to -846, suggesting the presence of a negative regulatory element in the distal 5'-flanking DNA. This overlaps an Alu repetitive sequence (ARS) located between nucleotides -1330 and -1007. Gel mobility shift and DNase protection analyses identified a protein binding site centered around -1100 in the ARS second monomer. The GPHalpha upstream ARS was cloned in both orientations in positions upstream and downstream from the bacterial CAT gene under control of the herpes simplex virus thymidine kinase (tk) promoter. DNA-mediated transient transfection of these plasmids revealed a marked inhibition (79-82%) of CAT production by the ARS when it was cloned upstream from the tk promoter and in the same orientation as that found in the GPHalpha 5'-flanking DNA. Smaller decreases (29-57%) were produced by the ARS cloned upstream from the tk promoter in the reverse orientation. In marked contrast, the Alu repetitive element had little or no effect when cloned in either orientation downstream from the tk-CAT gene. Introduction of a second ARS downstream from the CAT reporter gene in vectors already containing an ARS upstream from the tk promoter significantly reduced the strong negative effect elicited by the upstream repetitive element. When compared to the Blur 8 Alu element, the GPHalpha upstream ARS differs markedly with respect to its effect on tk-CAT expression in transient assays and as a substrate for DNA binding proteins present in HeLa nuclear extracts. Together, the transient expression results demonstrate that ARS elements can influence expression of nearby class II promoters. The extent of this effect depends on element position and orientation, cell type, the particular ARS (e.g., GPHalpha or Blur 8), and whether copies were present both upstream and downstream from the transcription unit.

Cloning of cDNA encoding the common alpha subunit precursor molecule of pituitary glycoprotein hormones in the Australian lungfish, Neoceratodus forsteri

We have isolated cDNA clones encoding a putative glycoprotein hormone alpha subunit precursor molecule from a pituitary cDNA library of the Australian lungfish (Neoceratodus forsteri) by a two-step plaque hybridization technique initially using cDNA encoding the toad glycoprotein hormone alpha subunit precursor molecule as the hybridization probe. The inserts (799 bp) of two of the isolated cDNA clones contained sequences of 5' and 3' untranslated regions, including a poly(A) stretch, and the entire coding sequence of the alpha subunit precursor molecule. The deduced amino acid sequence showed that its signal peptide consists of 24 residues and its mature protein 97 residues. In the mature protein, there is an insert of one residue (Ala) just after the 9th residue. This insert is unique to the Australian lungfish among vertebrate species studied. The amino acid sequence of the mature protein shares the common, or the same-group, amino acid residues at 9 positions with tetrapod and not actinopterygian vertebrates, while only one residue is common to some teleosts and the lungfish to the exclusion of the tetrapods. The overall sequence of the mature protein of the Australian lungfish also shares more similarity with those of tetrapods (69 to 84%) than it does with teleosts (57 to 74%). These results on the relation of the alpha subunit precursor molecule between the lungfish and other vertebrates are consistent with the recent molecular phylogenetic studies suggesting a closer relationship between lungfish and tetrapods than between lungfish and teleosts. We also found that the primary structure of the lungfish alpha subunit is slightly but significantly more similar to that of homeothermic vertebrates than to that of amphibians. This may be due to specialization of the amphibian alpha subunit molecules.

Studies on the structure and function of the carp gonadotropin alpha subunit by site-directed mutagenesis

There are two genes encoding the alpha subunit of carp gonadotropin (alpha 1 and alpha 2 subunits). Our previous data have demonstrated that both alpha 1 and alpha 2 subunits expressed in insect cells are able to associate with the beta subunit, but only the alpha 1/beta heterodimer displays biological activity. In the mature protein, there are only four amino-acid residues different between the alpha 1 and alpha 2 subunits. In this study we used site-directed mutagenesis and expressed different mutant alpha subunits in insect cells to identify which residue might be important for biological activity of the alpha subunit. Our results suggested that the change of Arg-71 to Gln-71 affected the activity of the carp gonadotropin alpha subunit.

Characterization of the ovine LH beta-subunit gene: the promoter directs gonadotrope-specific expression in transgenic mice

The alpha- and beta-subunits of the gonadotropin hormones are expressed in the gonadotrope cells of the anterior pituitary. There are no adequate in vitro systems for the analysis of beta-subunit gene expression. In this study, therefore, transgenic mice have been used to investigate the regulation of expression of the ovine luteinizing hormone beta-gene (oLH beta) in vivo. oLH beta was isolated, characterized, and 1.9 kb of the promoter fused to the bacterial reporter chloramphenicol acetyl-transferase (CAT). Three lines of transgenic mice were generated. CAT enzyme was detected in the pituitary of two lines, whereas the third line did not express. Measurement of endogenous luteinizing hormone and follicle stimulating hormone levels in both expressing lines revealed small differences when compared to controls, but these did not affect the fertility of the animals. Immunostaining of the anterior pituitary revealed that the oLH beta CAT transgene was expressed specifically in gonadotrope cells.

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.

Organization and nucleotide sequence of carp gonadotropin α subunit genes

We have used PCR to amplify and align the sequence of two genes encoding cGTH alpha. Both genes comprise four exons and three introns. The organization of cGTH alpha genes is very similar to that of mammalian GTH alpha genes. However, the cGTH alpha genes only span a region of 1.2 kb which is much smaller than those mammalian GTH alpha genes.

Cloning and sequence analysis of the common alpha-subunit complementary deoxyribonucleic acid of turkey pituitary glycoprotein hormones

Two cDNA clones of nearly full length that encode the turkey pituitary common alpha-subunit glycoprotein hormone have been isolated from a pituitary cDNA library and their nucleotide sequences have been determined. The longer alpha-subunit clone was 777 bp in length. It contained 88 bp of the 5'-untranslated region (UTR), an open reading frame of 360 bp (that encodes the turkey alpha-subunit 24 amino acid leader polypeptide fragment and 96 amino acid apoprotein), and a 312-bp 3'-UTR followed by a 17-bp poly A tract. When the nucleotide sequence of the turkey alpha-subunit was compared with the sequence of the chicken alpha-subunit clone, the coding region was greater than 98% homologous, but was only 69 to 76% homologous when compared with mammalian alpha-subunit sequences. Northern blot analysis showed an approximate 800 bp processed transcript that hybridized to the labeled turkey alpha-subunit cDNA. There was a greater than fourfold up-regulation of the steady-state levels of turkey alpha-subunit transcription when intact cultured pituitaries were treated with chicken gonadotropin-releasing hormone I.

Molecular cloning of the rhesus glycoprotein hormone alpha-subunit gene

A rhesus monkey genomic library was screened with a cDNA for the glycoprotein hormone alpha-subunit. Genomic clones hybridizing with exon-specific probes were selected and the DNA sequences were determined for 1.6 kb of 5'-flanking DNA, all four exons, the second and third introns, all exon-intron junctions, and 357 bp of 3'-flanking DNA. Comparison with the 236 bp of 5'-flanking sequence data available for the human alpha gene indicates an overall homology of 95%. Primer extension analysis of rhesus placental and pituitary mRNA demonstrated that transcription initiation is identical to that in the human placenta. The rhesus gene contains an element nearly identical (21/22 bases) to the placental tissue-specific element described for the human alpha gene. The rhesus gene has only one copy of the cAMP-response element (CRE), which is present as a direct repeat in the human gene. The rhesus CRE contains the consensus core sequence TGACG-TCA with the cytosine in the fourth position that is essential for placental expression of the human gene. The 5'-flanking region also has elements highly homologous to the consensus estrogen and progesterone/glucocorticoid response elements, as well as thyrotrope-specific and Pit-1-like binding sites described in rodent genes. The nucleotide sequence of four exons (predicted mRNA) have an aggregate homology of 92.7% with the human sequence. However, a 12-bp insertion to the second exon results in the addition of 4 amino acids to the amino-terminal end of the protein; these are homologous with the proteins of nonprimates but are lacking in the human alpha-subunit. The amino acid sequence of the deduced protein was slightly more homologous with the bovine than the human protein (91.6% vs. 89.6%). Thus, the rhesus glycoprotein alpha-subunit gene codes for a protein whose structure somewhat more closely resembles that of lower species, but the 5'-flanking DNA of the gene has gained the elements necessary for transcription in the placental syncytiotrophoblast which distinguishes the primate placenta from the other species examined.

Expression of the glycoprotein hormone alpha-subunit gene in the placenta requires a functional cyclic AMP response element, whereas a different cis-acting element mediates pituitary-specific expression

The single-copy gene encoding the alpha subunit of glycoprotein hormones is expressed in the pituitaries of all mammals and in the placentas of only primates and horses. We have systematically analyzed the promoter-regulatory elements of the human and bovine alpha-subunit genes to elucidate the molecular mechanisms underlying their divergent patterns of tissue-specific expression. This analysis entailed the use of transient expression assays in a chorionic gonadotropin-secreting human choriocarcinoma cell line, protein-DNA binding assays, and expression of chimeric forms of human or bovine alpha subunit genes in transgenic mice. From the results, we conclude that placental expression of the human alpha-subunit gene requires a functional cyclic AMP response element (CRE) that is present as a tandem repeat in the promoter-regulatory region. In contrast, the promoter-regulatory region of the bovine alpha-subunit gene, as well as of the rat and mouse genes, was found to contain a single CRE homolog that differed from its human counterpart by a single nucleotide. This difference substantially reduced the binding affinity of the bovine CRE homolog for the nuclear protein that bound to the human alpha CRE and thereby rendered the bovine alpha-subunit promoter inactive in human choriocarcinoma cells. However, conversion of the bovine alpha CRE homolog to an authentic alpha CRE restored activity to the bovine alpha-subunit promoter in choriocarcinoma cells. Similarly, a human but not a bovine alpha transgene was expressed in placenta in transgenic mice. Thus, placenta-specific expression of the human alpha-subunit gene may be the consequence of the recent evolution of a functional CRE. Expression of the human alpha transgene in mouse placenta further suggests that evolution of placenta-specific trans-acting factors preceded the appearance of this element. Finally, in contrast to their divergent patterns of placental expression, both the human and bovine alpha-subunit transgenes were expressed in mouse pituitary, indicating differences in the composition of the enhancers required for pituitary- and placenta-specific expression.

Expression of the glycoprotein hormone alpha-subunit gene in the placenta requires a functional cyclic AMP response element, whereas a different cis-acting element mediates pituitary-specific expression

The single-copy gene encoding the alpha subunit of glycoprotein hormones is expressed in the pituitaries of all mammals and in the placentas of only primates and horses. We have systematically analyzed the promoter-regulatory elements of the human and bovine alpha-subunit genes to elucidate the molecular mechanisms underlying their divergent patterns of tissue-specific expression. This analysis entailed the use of transient expression assays in a chorionic gonadotropin-secreting human choriocarcinoma cell line, protein-DNA binding assays, and expression of chimeric forms of human or bovine alpha subunit genes in transgenic mice. From the results, we conclude that placental expression of the human alpha-subunit gene requires a functional cyclic AMP response element (CRE) that is present as a tandem repeat in the promoter-regulatory region. In contrast, the promoter-regulatory region of the bovine alpha-subunit gene, as well as of the rat and mouse genes, was found to contain a single CRE homolog that differed from its human counterpart by a single nucleotide. This difference substantially reduced the binding affinity of the bovine CRE homolog for the nuclear protein that bound to the human alpha CRE and thereby rendered the bovine alpha-subunit promoter inactive in human choriocarcinoma cells. However, conversion of the bovine alpha CRE homolog to an authentic alpha CRE restored activity to the bovine alpha-subunit promoter in choriocarcinoma cells. Similarly, a human but not a bovine alpha transgene was expressed in placenta in transgenic mice. Thus, placenta-specific expression of the human alpha-subunit gene may be the consequence of the recent evolution of a functional CRE. Expression of the human alpha transgene in mouse placenta further suggests that evolution of placenta-specific trans-acting factors preceded the appearance of this element. Finally, in contrast to their divergent patterns of placental expression, both the human and bovine alpha-subunit transgenes were expressed in mouse pituitary, indicating differences in the composition of the enhancers required for pituitary- and placenta-specific expression.

CRE-binding proteins interact cooperatively to enhance placental-specific expression of the glycoprotein hormone alpha-subunit gene

The 18-bp direct repeat occurring between positions -146 and -111 in the 5'-flanking region of the human alpha-subunit gene serves two functions: it mediates the transcriptional effect of cAMP and it acts in conjunction with an adjacent cis-acting element (URE) to confer properties of placental-specific expression to the alpha-subunit promoter. Functional activity of the URE and CRE requires binding of a trans-acting factor; each element binds a different factor. Analysis of saturation isotherms provides good evidence that cooperativity is involved in binding of CREB to the 18-bp direct repeat. This cooperativity could account for the synergistic effect of two CRE on both basal and cAMP-stimulated transcription. It remains to be determined whether heterotropic cooperativity is involved in binding of trans-acting factors to the URE and CRE. A major difference between the 5'-flanking region of the human alpha-subunit gene and comparable regions from bovine, rat, and mouse alpha-subunit genes is that the latter contain a single CRE homolog which appears incapable of binding the trans-acting factor that binds to the human alpha CRE. Lack of a functional CRE provides at least one explanation for inactivity of the bovine alpha-subunit promoter in choriocarcinoma cells and probably in bovine placenta as well. Yet, the same bovine promoter-regulatory region that lacks a functional CRE is capable of conferring pituitary-specific expression to the CAT gene in transgenic mice (data not shown). This suggests that the CRE is not required for pituitary-specific expression of the bovine alpha-subunit gene. Instead, another cis-acting element(s) must confer this property to the alpha-subunit promoter. While it is tempting to suggest that bovine, rat, and mouse alpha-subunit genes are not regulated by cAMP because of their inactive CRE homolog, it is also quite possible that other CRE are located further upstream. Accordingly, it will be of interest to obtain additional 5'-flanking sequence and determine whether functional homologs of the human alpha CRE are present in the bovine, rat, and mouse alpha-subunit genes, or whether another class of cis-acting elements provide cAMP-responsiveness.

Isolation and characterization of the gene encoding the beta-subunit of rat follicle-stimulating hormone

Follicle-stimulating hormone (FSH) is a pituitary glycoprotein hormone that is comprised of two dissimilar subunits, alpha and beta, encoded by separate genes. We have isolated the gene encoding the beta-subunit of rat FSH by screening a rat genomic DNA-library by filter hybridization with bovine FSH-beta cDNA. Southern blot analysis of rat genomic DNA suggests that there is a single copy of the FSH-beta gene per haploid genome in the rat. The nucleotide sequence of the rat FSH-beta gene was determined and the amino acid sequence of the subunit was deduced. The gene is composed of three exons and two introns. The predicted amino acid sequence reveals that there is a 20-amino-acid signal peptide followed by a mature protein of 110 amino acid residues. Exons I, II, and III are 36, 187, and 1221 bp, respectively. Intron 1 (640 bp) interrupts the 5'-untranslated (UT) region (61 bp) and intron 2 (approximately 1 kb) interrupts the coding region between amino acid residues +34 and +35. Comparison of the amino acid sequence to those of the human and bovine FSH-beta subunits reveals 80% similarity to both species. The "CAGY" sequence, or the Cys-Ala-Gly-Tyr quartet of amino acids encoded by exon II is present in every other glycoprotein beta-subunit sequenced thus far, is altered in rat FSH-beta, with the Ala residue replaced by Glu. Primer extension analysis demonstrated that there is a single transcriptional start site. The nucleotide sequence of the 5'-flanking region (1 kb) was determined and compared to the nucleotide sequences of corresponding regions in the bovine and human genes. This analysis revealed that there are three regions in the 5'-flanking region of the rat FSH-beta gene that display greater than 80% sequence similarity to regions in the bovine and human genes. The second of these regions also shares similarity to segments within the 5'-flanking regions of the rat alpha- and LH beta-subunit genes. The characterization of the rat FSH-beta gene will enable further study of the regulation of rat FSH using in vitro systems.