Characterization of a follitropin-binding component prepared from immature bovine testes in the absence of detergent

The tale of follitropin receptor diversity: a recipe for fine tuning gonadal responses?

The original concept (dogma) of a single FSH receptor entity coupling to G(s) protein to activate adenylate cyclase and producing cAMP as second messenger appears inadequate to explain pleiotropic actions of the hormone. The identification and expression of alternatively spliced gonadotropin receptors, suggest that alternative splicing could serve as a mechanism for creating receptor diversity. Studies focused on sheep and mouse gonadal tissues show that the single large gene of approximately 250kb is a modular structure whose pre-mRNA undergoes alternative splicing creating several subtypes (at least four FSH-R1 to R4 identified to date). With segments of the N-terminus that are identical different topographies are generated by differing carboxyl termini. The same gene thus produces receptor types with different motifs that can display dominant positive, dominant negative, growth factor/cytokine type and potentially soluble binding protein features. Functional relevance is shown by modulation of receptor variants during hormonal stimulation. Presence of equivalent segments of the gene in the human and bovine suggests conservation and predicts similarity in structures and function. Thus, the complex cellular biology of follitropin receptors that may interact differently with polymorphic forms (glycosylation variants) of FSH represents an intricate scheme to regulate hormone signaling.

Novel splicing variants of the human thyrotropin receptor encode truncated polypeptides without a membrane-spanning domain

The thyrotropin receptor is of fundamental importance to normal thyroid function and is considered to be the predominant antigen affected by the autoantibodies of Graves' autoimmune hyperthyroidism. The identification of the epitopes on the receptor to which the autoantibodies bind or the mechanism by which the autoantibodies arise remain to be established. In this report we have analysed in detail thein vivo transcription of the human TSH receptor gene (hTSH-R), demonstrating the presence of numerous novel TSH receptor transcripts. Northern blot analysis of mRNA from human thyroid tissue using a radiolabelled cDNA probe specific for the extracellular domain of the hTSH-R revealed the presence of small polyadenylated mRNAs, in addition to the full-length hTSH-R mRNA. A PCR strategy devised to clone transcripts with 3' polyadenylation and 5' hTSH-R specific sequences was used to clone five different hTSH-R transcripts (hTSH-R. ST1 to ST5; 250bp-1.7 kb) from human thyroid tissue. Sequence analysis demonstrated that the small transcripts arose by alternative splicing of the hTSH-R mRNA. The transcripts were associated with polysomes and were demonstrated in human thyroid tissue from patients suffering from Graves' disease, sporadic goiter as well as in healthy lobes of thyroid tissue.In situ hybridization demonstrated that two of the alternative transcripts adopted a tissue distribution pattern identical to that of the full-length hTSH-R transcript. The two major truncated transcripts ST4 and ST5 contained unique sequences at the 3' end of the mRNAs and thus potentially represent the molecular origin of soluble TSH receptor variants which have been postulated on numerous occasions.

Thermodynamics of follitropin binding to solubilized calf testis receptor

Thermodynamic parameters of follitropin binding to solubilized testicular receptors were measured in order to assess the forces involved in the binding reaction. Reversibility of follitropin binding to solubilized receptor decreased only 20% over the temperature range 4-24 degrees C, whereas earlier studies indicated reversibility of binding to membrane-bound receptor decreased by more than 40% over the same range [Anderson, T. T., Curatolo, L. M., & Reichert, L. E., Jr. (1983) Mol. Cell. Endocrinol. 33, 37-52]. Thermodynamic analysis of follitropin binding to solubilized receptors showed that the hydrophobic effect was important in the binding reaction. The mean values, at 25 degrees C, for delta H and delta S were -31.8 kcal/mol and -66.0 cal mol-1 K-1, respectively, and delta Cp was -3.0 kcal mol-1 K-1. This is an unusually large heat capacity for protein-protein association reactions, indicating an enhanced role for the hydrophobic effect with the solubilized (compared to membrane-bound) receptor. Since glycerol was necessary to stabilize the solubilized receptor, we determined whether glycerol affected the thermodynamic parameters measured for the binding reaction. Control experiments, performed with membrane-bound receptor in the presence or absence of glycerol, indicated that delta Cp actually decreased upon addition of glycerol (-0.8 kcal mol-1 K-1 in the presence of glycerol compared to -2.3 kcal mol-1 K-1 in the absence of glycerol). Thus, the large negative delta Cp observed for the soluble receptor was a result of its removal from the membrane and was not due to the presence of glycerol.(ABSTRACT TRUNCATED AT 250 WORDS)

Processing of follitropin and its receptor by cultured pig Sertoli cells. Effect of monensin

Immature pig Sertoli cells, cultured in a chemically defined medium, are able to maintain many of their functional characteristics for at least two weeks. This model was used to investigate the binding, internalization and degradation of 125I-labelled human follitropin (hFSH) and the effects of pig FSH (pFSH) on its own receptors. The binding of 125I-labelled hFSH was dependent on time, temperature and concentration. At 4 degrees C, the apparent steady state was reached in 8-12 h and remained constant for at least 24 h, whereas at 33 degrees C the apparent equilibrium was reached in 4-6 h. Thereafter the total binding declined and by 24 h it was less than 50% of the maximum binding. At 33 degrees C the binding for the hormone to its surface receptor was followed by internalization of the hormone (half-life approximately equal to 1 h) and its degradation (half-life approximately equal to 3 h). The receptor-mediated internalization of hFSH was blocked by phenylarsine oxide. In the presence of the ionophore monensin (20 microM) the rates of binding and internalization were not modified but the degradation rate was much lower (half-life approximately equal to 18 h). Thus, in the presence of monensin, maximum binding increased twofold to threefold, and remained constant for 24 h. This increase was mainly due to an increase of the internalized hormone. When Sertoli cells were exposed to pFSH there was a loss of its own receptor, which was both dose-dependent (ED50 = 250 ng/ml) and time-dependent (t 1/2 = 14 h). Cycloheximide did not modify the FSH-induced down-regulation, whereas monensin enhanced the down-regulation process. These results show that FSH, like other ligands, is internalized and degraded by its target cells and indicate that the hormone-mediated down-regulation is related to the internalization process. However, the discrepancy between the rate of internalization and of hormone-induced down-regulation, suggests that some of the internalized receptors are recycled.

Demonstration of a beta-casomorphin immunoreactive material in the plasma of newborn calves after milk intake

Blood was collected from newborn calves before and after their first milk intake after birth; extracts of plasma were assayed by radioimmunoassay for the presence of beta-casomorphin-7 immunoreactive materials. No beta-casomorphin immunoreactivity was found in samples collected before milk ingestion; however, in samples collected after milk ingestion a beta-casomorphin-7 immunoreactive material was detected. Chromatographic characterization showed that this material was not identical with beta-casomorphin-7 but might rather represent a precursor thereof. The material proved resistant to enzymatic attack during a 30-min incubation period at 37 degrees C in the plasma of newborn calves, whereas beta-casomorphin-7 was degraded under these conditions. A physiological significance of beta-casomorphin-7 eventually cleaved from such a precursor material at any site in the newborn mammal is suggested.

Correlation of B coefficient of viscosity for monovalent salts with effects on binding of human follitropin to receptor

Various monovalent salts have been shown to inhibit the binding of radioiodinated human follitropin [( 125I]hFSH) to receptors present in calf testis. We examined the effects of salts on the affinity constant (Ka) and number of binding sites (R0) in the FSH-calf testis receptor system. A constant amount of [125I]hFSH and increasing amounts of unlabeled hFSH were allowed to bind to a constant amount of receptor in the absence or presence (0.10 M) of halides, nitrates or acetates of the alkali ions at 20.0 degrees C. There was an appreciable variation of the affinity constant depending on the salt being used, but there was no change in receptor number (R0). Within an alkali cation series (e.g. NaCH3CO2, NaF, NaCl, NaBr, NaNO3, NaI or a similar potassium series) the affinity decreased with decreasing B coefficient of viscosity. Within a halide series (e.g. LiCl, NaCl, KCl, RbCl or the similar bromide series) the minimum value of the affinity constant occurred with the Na+ salt. The special inhibitory potency of Na+ may suggest that it has a unique interaction with calf testis FSH receptor or with follitropin. The studies indicate a correlation between the B coefficient of viscosity and the ability of salts to inhibit binding of [125I]hFSH to receptor.

Cellular processing of follicle-stimulating hormone by Sertoli cells in serum-free culture

Specific binding of radioiodinated human follicle-stimulating hormone [( 125I]hFSH) to receptors on the cell surface of immature rat Sertoli cells in serum-free culture (4-7 days) was concentration-, time- and temperature-dependent with an apparent steady state of binding stained after 2 h at 4, 18 or 32 degrees C. Analysis of [125I]hFSH binding data indicated an apparent Ka of 3 X 10(9) M-1 with approximately 670 fmoles binding sites/mg protein. The radioiodinated hFSH (60-70 microCi/micrograms) bound to receptor and stimulated estradiol production, thus confirming its biological activity and Sertoli cell viability. Approximately 50% of the specifically bound radioactivity was lost from the cell surface after 0.25 h at 32 degrees C, 1 h at 18 degrees C or 4 h at 4 degrees C. Most (greater than 90%) of the receptor-bound hormone was removed from the cell surface and degraded to [125I]mono- and diiodotyrosine within 4 h at 32 degrees C. FSH degradation was inhibited at low temperatures (4 degrees C) and by the lysosomotropic agents chloroquine, NH4Cl or concanavalin A. Bacitracin and methylamine, inhibitors of internalization, were effective in blocking degradation but not loss of membrane-bound [125I]hFSH. Thus it appears that, as with small peptide and simple protein hormones, complex glycoprotein hormone-receptor complexes, such as FSH Sertoli cell receptors, are endocytosed by normal target cells and subsequently degraded, most probably within the lysosomes.

Follitropin binding to receptors in testis: studies on the reversibility and thermodynamics of the reaction

Studies were designed to assess the extent to which binding of follitropin (FSH) to its receptor is a reversible process. [125I]hFSH was allowed to interact with membrane receptors from calf testis for 2 h at various temperatures by which time significant specific binding of [125I]hFSH had taken place in all instances. Unlabeled FSH was then added (delayed addition) and the amount of [125I]hFSH remaining bound was monitored as a function of time. In order to assess reversibility, [125I]hFSH binding in the latter samples was compared to that occurring when unlabeled FSH had not been added or when unlabeled FSH was present from the start of the incubation. Binding was essentially fully reversible at 4 degrees C, but reversibility decreased with increasing temperature. Reversibility of FSH binding decreased markedly at temperatures greater than 26 degrees C and was considered irreversible at temperatures above 30 degrees C. At 4 degrees C essentially full reversibility (greater than 90%) was observed when the unlabeled hormone was added after 7 h of incubation, but decreased when added after 12 h. At warmer temperatures (22 or 30 degrees C) 'there was a progressive decrease in reversibility as the time of delay before addition of unlabeled hormone was lengthened. By determining the affinity constant (in separate experiments) at various temperatures, a thermodynamic analysis was possible. This analysis was restricted to the temperature range 4-26 degrees C in order to minimize complications arising from irreversible binding. The reaction was endothermal at low temperatures (T less than 12.5 degrees C) and exothermal at higher temperatures (T greater than 12.5 degrees C) and was associated with a decrease in heat capacity of 1800 cal [mol deg]-1 at 25 degrees C. The results are consistent with the concept that the hydrophobic effect plays an important role in FSH binding, but that the reaction is complex and may be composed of more than one step.