I. Introduction IN classical physiology, the main regulation of the thyroid gland involves a positive control by pituitary TSH. The pituitary thyrotrophs that synthesize and secrete TSH are submitted to tonic stimulation by the hypothalamic TRH and to negative feedback by T3, which is generated in these cells from plasma T4 (1, 2). The predominant role of TSH in thyroid regulation is exemplified by physiological experiments and pathological situations. Hypophysectomy, or treatment with thyroid hormones that reduces plasma TSH levels without interfering with other pituitary functions, almost abolishes thyroid function and greatly reduces the weight and cell mass of the gland. Conversely, chronic inhibition of the synthesis of thyroid hormones, and thus of their secretion, which greatly enhances TSH plasma levels, enhances thyroid growth and iodine metabolism upstream from the inhibited step. In pathology, hyperthyrotropinemia caused by pituitary adenomas induces hyperthyroidism and goiter. Hypopituitarism ...

The thyrotropin receptor and the regulation of thyrocyte function and growth.

Cloned cDNA encoding the rat Sertoli cell receptor for FSH was isolated from a cognate library and functionally expressed in cultured mammalian cells. The FSH receptor (FSH-R), as predicted from the cDNA, is a single 75K polypeptide with a 348 residue extracellular domain which contains three N-linked glycosylation sites. This domain is connected to a structure containing seven putative transmembrane segments which displays sequence similarity to G protein-coupled receptors. Thus, the FSH-R is identical in its structural design to the LH/CG receptor (LH/CG-R). Furthermore, both receptors display 50% sequence similarity in their large extracellular domains and 80% identity across the seven transmembrane segments. Expression of the cloned cDNA in mammalian cells conferred FSH-dependent cAMP accumulation. The selectivity for FSH is attested by the fact that the related human glycoprotein hormones human CG and human TSH do not stimulate adenylyl cyclase in FSH-R expressing cells even when these hormones are p...

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The testicular receptor for follicle stimulating hormone: structure and functional expression of cloned cDNA

We report a woman with primary amenorrhoea and infertility associated with an isolated deficiency of pituitary follicle–stimulating hormone (FSH), but normal luteinizing hormone (LH) secretion. Ovulation was induced by administration of exogenous FSH and resulted in a successful pregnancy. Sequence analysis of the FSHβ–subunit gene indicated that she is homozygous for a two nucleotide frameshift deletion in the coding sequence. Her mother and son are heterozygous for this mutation. This deletion results in an alteration of amino acid codons 61–86 followed by a premature termination codon. The predicted truncated β–subunit peptide lacks regions which are important for association with the α subunit and for binding to and activation of the FSH receptor. Abnormalities of FSH structure or function might be an under recognised but treatable cause of infertility.

Primary amenorrhoea and infertility due to a mutation in the beta-subunit of follicle-stimulating hormone

Cells from the chorionic girdle of the equine trophoblast invade the maternal endometrium at day 36 of gestation and become established as secretory elements known as the endometrial cups. These structures, which persist for 40-60 days, produce a gonadotropin which can be found in circulation until about day 130 of gestation. This glycoprotein has been identified in the horse and the donkey, with the former having received much better characterization. It consists of 2 noncovalently linked peptide chains; an alpha-subunit of 96 amino acids, which is common to that found in other horse glycoprotein hormones. The beta-subunit of 149 amino acids is identical to horse LH beta. Horse CG is the most heavily glycosylated of the known pituitary and placental glycoprotein hormones. The alpha-subunit has two and the beta-subunit one N-linked glycosylation site, and the beta-chain has in excess of four O-linked glycosylation sites. The N-linked glycans have some oligosaccharides that are not found on other glycoprotein hormones. The sialic component of glycosylation confers an exceptionally long half-life on CG compared to other glycoprotein hormones. Horse CG has LH-like activity in horse receptor and in vitro bioassays. In spite of the amino acid homology, it has lower LH activity than does horse LH. Its most intriguing, and as yet unexplained, characteristic is its pronounced FSH and LH activity in species other than the horse. Horse CG binds to FSH receptors of virtually all mammalian species, other than the horse, in which it has been tested and will produce biological effects peculiar to FSH. It has similar and potent interaction with LH receptors. The structural basis of this duality is not known but may be related to the region 90-110 of the beta-chain. Horse CG is believed to be constitutively expressed by the trophoblastic cells until the endometrial cups degenerate. The role of CG in equine gestation is not completely understood. It is believed to act as an LH-like hormone to induce supplementary ovulation and/or luteinization of follicles in the mare. It has not been established whether CG or the accessory corpora lutea are necessary for successful horse pregnancy. They may serve as a redundant system to assure that there is sufficient secretion of the primary corpus luteum to maintain pregnancy until the placenta assumes its role as the principal steroidogenic organ of gestation.

Equine Chorionic Gonadotropin

Glycosylation at specific sites of erythropoietin is essential for biosynthesis, secretion, and biological function.

The structural features of the heterodimeric glycoprotein hormones (LH, FSH, TSH, and hCG) are briefly reviewed. Removal of carbohydrate chains does not reduce binding of the hormones to membrane receptors, but markedly reduces biological responses. The glycopeptides from the hormone do not reduce binding of native hormone to receptors but do reduce biological responses. Newer data concerned with replication of different regions of the peptide chains of these molecules using synthetic peptides are reviewed and presented. These studies indicate that two regions on the common alpha subunit are involved with receptor binding of the LH, hCG, and TSH molecules. These regions are alpha 26 to 46 and alpha 75-92. Two synthetic disulfide loop peptides from the hCG beta subunit beta 38-57 and beta 93-100 also block binding of hCG to its receptor. In addition, the beta 38-57 peptide stimulates testosterone production by Leydig cells. These data indicate that glycoprotein hormone binding to plasma membrane receptors ...

https://faseb.onlinelibrary.wiley.com/doi/pdf/10.1096/fasebj.2.11.2456242

The glycoprotein hormones: recent studies of structure-function relationships.

Publisher Summary The gonadotropins, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) of pituitary origin, and chorionic gonadotropin (hCG, eCG) of placental origin, along with thyroid-stimulating hormone (TSH) constitute a family of glycoprotein hormones. The α-subunit is common to all of the hormones and shows considerable homology from one species to another. The β-subunits are unique for LH, FSH, and TSH. The s-subunits of hLH and hCG are highly homologous (85%) through the first 114 residues, but hCG β differs in that it has a C-terminus extension rich in serine and proline. This offers a chemical basis for their common biological activities. Baboon chorionic gonadotropin, and eLH and chorionic gonadotropin s-subunits also have C-terminal extensions. The glycoprotein hormone s-subunits also show regions of homology between themselves and among different species. A wide range of chemical and enzymatic modifications have been employed in efforts to define residues and sequences in the s-subunits that may be essential for receptor binding. In general, chemical modifications of reactive amino acids appear to be more tolerated in the s- than in the α-subunits.

Structure-function relationships of gonadotropins.

Most anti-nicotinic acetylcholine receptor (AChR) antibodies in myasthenia gravis are directed against an immunodominant epitope or epitopes [main immunogenic region (MIR)] on the AChR alpha-subunit. Thirty-two synthetic peptides, corresponding to the complete Torpedo alpha-subunit sequence and to a segment of human muscle alpha-subunit, were used to map the epitopes for 11 monoclonal antibodies (mAbs) directed against the Torpedo and/or the human MIR and for a panel of anti-AChR mAbs directed against epitopes on the alpha-subunit other than the MIR. A main constituent loop of the MIR was localized within residues alpha 67-76. Residues 70 and 75, which are different in the Torpedo and human alpha-subunits, seem to be crucial in determining the binding profile for several mAbs whose binding to the peptides correlated very well with their binding pattern to native Torpedo and human AChRs. This strongly supports the identification of the peptide loop alpha 67-76 as the actual location of the MIR on the intact AChR molecule. Residues 75 and 76 were necessary for binding of some mAbs and irrelevant for others, in agreement with earlier suggestions that the MIR comprises overlapping epitopes. Structural predictions for the sequence segment alpha 67-76 indicate that this segment has a relatively high segmental mobility and a very strong turning potential centered around residues 68-71. The most stable structure predicted for this segment, in both the Torpedo and human alpha-subunits, is a hairpin loop, whose apex is a type I beta-turn and whose arms are beta-strands. This loop is highly hydrophilic, and its apex is negatively charged. All these structural properties have been proposed as characteristic of antibody binding sites. We also localized the epitopes for mAbs against non-MIR regions. Among these, the epitope for a monoclonal antibody (mAb 13) that noncompetitively inhibits channel function was localized within residues alpha 331-351.

Main Immunogenic Region of Torpedo Electroplax and Human Muscle Acetylcholine Receptor: Localization and Microheterogeneity Revealed by the Use of Synthetic Peptides

Neurotransmitter receptors are key components of neuronal function. A major breakthrough in their study has been provided by cloning and sequencing of their genes and deduction of the amino acid sequence of their precursors.

Use of Synthetic Peptides and High Affinity Protein Ligands for Structural Studies of Central and Peripheral Nicotinic Receptors

Several regions on both the α- and β-subunits of human LH comprise the receptor-binding domain of the hormone One of these, a disulfide loop peptide containing residues 38–57 on the βsubunit, also stimulated steroidogenesis in rat Leydig cells Circular dichroism analysis and a Schiffer-Edmundson helical wheel projection of β-(38–57) revealed the possibility of an amphipathic αhelical structure through its N-terminal region Secondary structure prediction algorithms do not predict αhelix in β-(38–57), but, rather, suggest a sheet-coil-sheet topology Homology searches between this peptide and proteins with known structure revealed that the two best matches are with prealbumin-(10–30) and melittin-(1–26) Based on hydrophobic moment calculations, we suggest that β-(38–57) more closely resembles melittin, a known example of an amphipathic helix Molecular models were constructed that included an α-helix between Pro-39 and Pro-50 producing a hydrophilic face involving Thr-40, Arg-43, and Gln-46 Loop closur

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Robert P. Milius

Papers

The glycoprotein hormones: recent studies of structure-function relationships.

Structure-function relationships of gonadotropins.

Main Immunogenic Region of Torpedo Electroplax and Human Muscle Acetylcholine Receptor: Localization and Microheterogeneity Revealed by the Use of Synthetic Peptides

Use of Synthetic Peptides and High Affinity Protein Ligands for Structural Studies of Central and Peripheral Nicotinic Receptors

Molecular Modeling of Residues 38–57 of the β-Subunit of Human Lutropin