The role of the carbohydrate moiety in thyrotropin action

N-Glycosylation regulates endothelial lipase-mediated phospholipid hydrolysis in apoE- and apoA-I-containing high density lipoproteins

Endothelial lipase (EL) is a member of the triglyceride lipase gene family with high phospholipase and low triacylglycerol lipase activities and a distinct preference for hydrolyzing phospholipids in HDL. EL has five potential N-glycosylation sites, four of which are glycosylated. The aim of this study was to determine how glycosylation affects the phospholipase activity of EL in physiologically relevant substrates. Site-directed mutants of EL were generated by replacing asparagine (N) 62, 118, 375, and 473 with alanine (A). These glycan-deficient mutants were used to investigate the kinetics of phospholipid hydrolysis in fully characterized preparations of spherical reconstituted high density lipoprotein (rHDL) containing apolipoprotein E2 (apoE2) [(E2)rHDL], apoE3 [(E3)rHDL], apoE4 [(E4)rHDL], or apoA-I [(A-I)rHDL] as the sole apolipoprotein. Wild-type EL hydrolyzed the phospholipids in (A-I)rHDL, (E2)rHDL, (E3)rHDL, and (E4)rHDL to similar extents. The phospholipase activities of EL N118A, EL N375A, and EL N473A were significantly diminished relative to that of wild-type EL, with the greatest reduction being apparent for (E3)rHDL. The phospholipase activity of EL N62A was increased up to 6-fold relative to that of wild-type EL, with the greatest enhancement of activity being observed for (E2)rHDL. These data show that individual N-linked glycans have unique and important effects on the phospholipase activity and substrate specificity of EL.

Characterization of N-glycans of recombinant human thyrotropin using mass spectrometry

Thyroid-stimulating hormone is a vital component of the regulatory mechanism that maintains the structure and function of the thyroid gland and governs thyroid hormone release. In this paper we report the first detailed structural characterization of the N-linked oligosaccharides of recombinant human thyroid-stimulating hormone (rhTSH). Using a strategy combining mass spectrometric analysis and sequential exoglycosidase digestion, we have defined the structures of the N-glycans released from recombinant human thyrotropin by peptide N-glycosidase F. All glycans are complex-type glycans and are mainly of the bi- and triantennary type with variable degrees of fucosylation and sialylation. The major non-reducing epitope in the complex-type glycans is: NeuAcalpha2-3Galbeta1-4GlcNAc (sialylated LacNAc). The carbohydrate microheterogeneity at the three glycosylation sites was studied using reversed-phase high-performance liquid chromatography (RP-HPLC), concanavalin A affinity chromatography and mass spectrometric techniques, including both matrix-assisted laser desorption/ionization (MALDI) and electrospray. rhTSH was reduced, carboxymethylated and then digested with trypsin. The mixture of peptides and glycopeptides was subjected to RP-HPLC and the structures of the glycopeptides were determined by MALDI in conjunction with on-target exoglycosidase digestions. After PNGase F digestion, the peptide moiety of the glycopeptide was determined by the presence of the b- and y-series ions derived from its amino acid sequence in the quadrupole time-of-flight tandem mass (QTOF-MS/MS) spectrum. Glycosylation sites Asn-alpha52 and Asn-alpha78 contain mainly bi- and triantennary complex-type glycans. Only glycosylation site Asn-alpha52 bears fucosylated N-glycans. Minor tetraantennary complex structures were also observed on both glycosylation sites. Profiling of the carbohydrate moieties of Asn-beta23 indicates a large heterogeneity. Bi-, tri-, and tetraantennary N-glycans were present at this site. These data demonstrate site-specificity of glycosylation in the alpha subunit but not in the beta subunit of rhTSH with Asn-alpha52 bearing essentially di- and triantennary glycans with or without core fucosylation and bi- and triantennary glycans with no core fucosylation being attached to Asn-alpha78.

The luteinizing hormone receptor: influence of buffer composition on ligand binding and signaling of wild type and mutant receptors

There is evidence that ligand binding to and ligand-mediated signaling by the luteinizing hormone receptor (LHR) are influenced by buffer conditions, including ionic type and strength, an issue that becomes important in comparing functional parameters obtained on receptor mutants under different conditions. In order to study this phenomenon, we performed binding (kinetic and saturation) and signaling studies of human chorionic gonadotropin (hCG) with wild type (wt) LHR and several mutants expressed in COS-7 cells using two common buffer systems. One buffer was of low ionic strength and contained a low concentration of Na+, while the other had a near-physiological concentration of Na+. Emphasis was placed on mutations of two amino acid residues in the hinge region of the ectodomain (E332 and D333). It was found that the buffer of higher ionic strength, primarily from Na+, led to an increase of about 4-fold in the Kd of hCG binding to wt and mutant LHRs. The reduced binding affinities were attributable to a comparable reduction in the rate constants of association, with no significant differences in the calculated rate constants of dissociation in the two buffers. Analysis of the signaling properties of these mutants showed that, when corrected for the amount of hCG bound under the conditions of the signaling assay, the maximal ligand-mediated cAMP produced in cells maintained in the buffer of low ionic strength was comparable for wt LHR and the mutants, only the D333A mutant being somewhat elevated. In the buffer of higher ionic strength, however, the response by wt LHR was significantly greater than that of the mutants. These results show that E332 and D333 are important in hormone-mediated signaling, but only in the buffer of higher Na+ concentration. In addition to mutants of these two residues, the buffer of higher ionic strength also led to reduced binding to a number of mutants throughout the receptor. Since these mutants included additional replacements in the ectodomain and transmembrane helices 6 and 7, the general nature of the buffer effect on wt and mutant LHRs suggests that electrostatic effects are contributing to ligand binding and/or that the LHR ectodomain may exist in two conformations, one being more accessible to ligand at reduced ionic strength.

Inositol phosphate stimulation by LH requires the entire alpha Asn56 oligosaccharide

Lentil lectin-bound, fucose-enriched hTSH was reported to stimulate both cAMP and inositol phosphate (IP) intracellular signalling pathways, whereas fucose-depleted hTSH stimulated only the cAMP pathway. Gonadotropins activate the cAMP pathway and in several studies higher concentrations activate the IP pathway. Since only the 10% of alpha subunit Asn(56) oligosaccharides (Asn(52) in humans) are fucosylated, the higher glycoprotein hormone concentrations required for IP pathway activation might be related to the abundance of competent hormone isoforms. Lentil lectin-fractionated equine (e)LHalpha and eFSHalpha preparations were combined with a truncated, des(121-149)eLHbeta preparation. All four hybrid hormone preparations induced IP accumulation in porcine theca cells, suggesting that activation of the IP pathway was not dependent on fucosylation at alpha subunit Asn(56). However, the presence of Asn(56) carbohydrate was necessary for increased IP accumulation. Intact, rather than Asn(56)-deglycosylated eLH preparations provoked a biphasic steroidogenic response by rat testis Leydig cells, suggesting that Galpha(i) stimulation was also sensitive to loss of Asn(56) carbohydrate. While rat granulosa cells responded to human FSH preparations in a biphasic manner, a classical sigmoidal response was obtained to eFSH and Asn(56)-deglycosylated eFSH, suggesting that the equine preparations did not activate Galpha(i). Purified oLHalpha Asn(56) oligosaccharides inhibited FSH-stimulated steroidogenesis in rat granulosa cell cultures indicating a direct role for carbohydrate in FSH action. The same carbohydrate preparation inhibited hCG-stimulated fluorescence energy transfer suggesting oligosaccharide involvement in activated LH receptor self-association.

Conversion of thyrotropin heterodimer to a biologically active single-chain

TSH and the gonadotropins, FSH, LH, and CG are a family of heterodimeric glycoprotein hormones composed of a common alpha-subunit noncovalently linked to a hormone specific beta-subunit. Assembly of alpha- and beta-subunits is essential for hormone-specific posttranslational modifications, receptor binding, and bioactivity. Structure-function studies of TSH and gonadotropins using site-directed mutagenesis can often affect folding, assembly, and secretion of the hormone. To circumvent these difficulties, recently, the gonadotropin heterodimers were converted to single chains. Here we converted the hTSH heterodimer to a biologically active single chain by genetically fusing the amino terminal end of the common alpha-subunit to the carboxyl terminal end of hTSHbeta in the presence or absence of hCGbeta carboxyl terminal peptide (CTP), which was used as a linker. Wild-type hTSH and the single chains were expressed in Chinese hamster ovary (CHO) cells, and they were efficiently secreted. Although the secretion rate of the single chain was 3-fold higher than that of hTSH wild-type. Moreover, the secretion of the single chain in the presence of the CTP linker was dramatically increased. On the other hand, receptor binding and in vitro bioactivity of the single chains were similar to that of hTSH wild-type. These data indicate the potential of the single chain approach to further investigate structure-function relationships of TSH.

Expression of human thyrotropin in cell lines with different glycosylation patterns combined with mutagenesis of specific glycosylation sites. Characterization of a novel role for the oligosaccharides in the in vitro and in vivo bioactivity

We used a novel approach to study the role of the Asn-linked oligosaccharides for human thyrotropin (hTSH) activity. Mutagenesis of Asn (N) within individual glycosylation recognition sequences to Gln (Q) was combined with expression of wild type and mutant hTSH in cell lines with different glycosylation patterns. The in vitro activity of hTSH lacking the Asn alpha 52 oligosaccharide (alpha Q52/TSH beta) expressed in CHO-K1 cells (sialylated oligosaccharides) was increased 6-fold compared with wild type, whereas the activities of alpha Q78/TSH beta and alpha/TSH beta Q23 were increased 2-3-fold. Deletion of the Asn alpha 52 oligosaccharide also increased the thyrotropic activity of human chorionic gonadotropin, in contrast to previous findings at its native receptor. The in vitro activity of wild type hTSH expressed in CHO-LEC2 cells (sialic acid-deficient oligosaccharides), CHO-LEC1 cells (Man5GlcNAc2 intermediates), and 293 cells (sulfated oligosaccharides) was 5-8-fold higher than of wild type from CHO-K1 cells. In contrast to CHO-K1 cells, there was no difference in the activity between wild type and selectively deglycosylated mutants expressed in these cell lines. Thus, in hTSH, the oligosaccharide at Asn alpha 52 and, specifically, its terminal sialic acid residues attenuate in vitro activity, in contrast to the previously reported stimulatory role of this chain for human chorionic gonadotropin and human follitropin activity. The increased thyrotropic activity of alpha Q52/CG beta suggests that receptor-related mechanisms may be responsible for these differences among the glycoprotein hormones. Despite their increased in vitro activity, alpha Q52/TSH beta, and alpha Q78/TSH beta from CHO-K1 cells had a faster serum disappearance rate and decreased effect on T4 production in mice. These findings highlight the importance of individual oligosaccharides in maintaining circulatory half-life and hence in vivo activity of hTSH.

Inhibition of TSH bio-activity by synthetic beta TSH peptides

The in vitro bioactivity of the human beta TSH subunit was investigated utilizing eleven overlapping synthetic peptides representing the entire 112 residue sequence. The peptides were tested for both stimulatory and inhibitory activity in two sensitive bioassay systems: the first based on cAMP production in FRTL-5 rat thyroid cells, and the second based on stimulation of iodine trapping by the same continuous cell line. Peptides from three distinct regions of the beta-subunit showed concentration dependent inhibition of TSH bio-activity, including beta 1-15, beta 11-25, beta 31-45, beta 81-95, and beta 91-105 with IC50 values ranging from 150 to 304 microM. An additional peptide representing the entire sequence of the "intercysteine loop" region of beta TSH, beta 31-52, also inhibited TSH activity with somewhat higher potency than its fragment peptide beta 31-45 (IC50 of 87.5 +/- 14.7 microM for beta 31-52 versus 207 +/- 92.4 microM for beta 31-45). Three of these, beta 1-15, beta 31-45, and beta 31-52, also inhibited binding of TSH to the receptor in a radio-receptor assay, as previously reported (1), supporting their importance in receptor interaction. None of the synthetic peptides stimulated either cAMP production or iodine trapping. Two other overlapping peptides, beta 81-95 and beta 91-105, possessed bio-inhibitory activity but did not inhibit binding of labeled TSH. Computer analysis of this sequence predicted an extended turn structure for this region. This region has been referred to as the "determinant loop" as it is bounded by cysteine residues at positions 88 and 95 that many believe form a disulfide bond in the native subunit. The current data suggests the beta 88-95 region may play a role in receptor activation after initial binding of hormone to receptor.

Differential effect of glycosylation on the expression of antigenic and bioactive domains in human thyrotropin

Enzymatic deglycosylation of human thyroid-stimulating hormone (hTSH) was shown to result in a mixture of partially and fully deglycosylated forms of the hormone by gel electrophoresis, silver staining and immunoblotting. Radioiodination of the enzymatic digest, followed by gel filtration and concanavalin A-Sepharose chromatography allowed to separate two different forms of partially deglycosylated [125I]hTSH and a fully deglycosylated hormone. The final recovery was of approx. 60% for [125I]hTSH deglycosylated in its beta-subunit, of 30% for [125I]hTSH missing the oligosaccharide in beta and one in alpha but only of 10% for [125I]hTSH deglycosylated in both the alpha- and beta-subunits. Gel electrophoresis under non-denaturing conditions showed that each form migrated distinctly from free subunits and reverse-phase high performance liquid chromatography after reduction and carboxymethylation identified the presence of the two subunits. Mapping of [125I]hTSH derivatives with polyclonal, monoclonal and anti-peptide antibodies allowed to identify two novel glycosylation-independent epitopes preserved in deglycosylated hTSH while the main immunogenic determinant was lost. When assayed in a bioassay with FRTL-5 cells, the hormone deprived of its beta-linked carbohydrate chain was found to be as effective as the native hormone on cAMP production and cell growth. In contrast, the fully deglycosylated derivative proved to stimulate cAMP release but appeared to be definitely less potent on thyroid cell growth. Our findings thus demonstrate that glycosylation of the alpha-subunit but not that of the beta-subunit is essential to express the domains involved in hTSH immunoreactivity as well as those controlling the post-receptor biological activity of the hormone.

Post-translational modifications of recombinant human tissue factor

Recombinant human tissue factor (rTF) purified from transfected mammalian cells is a glycoprotein that contains N-linked, but not O-linked oligosaccharides. Two of the three potential N-linked sites in the extracellular portion are fully glycosylated, while one site is approximately 90% utilized. These sites have complex-type oligosaccharides attached. The potential N-linked site in the cytoplasmic domain near the C-terminus is not glycosylated. Characterization of the tryptic map of rTF confirmed most of the proposed amino acid sequence. In addition, the disulfide bonds (between Cys-49 and Cys-57 and between Cys-186 and Cys-209) were demonstrated by FAB-MS analysis of cysteine-containing fragments obtained from the tryptic map.

Role of N-linked glycans of envelope glycoproteins in infectivity of human immunodeficiency virus type 1

We have shown that enzymatic removal of N-linked glycans from human immunodeficiency virus type 1 (HIV-1) recombinant envelope glycoproteins gp160 and gp120 produced in BHK-21 cells did not significantly reduce their ability to bind to CD4, the cellular receptor for the virus. Because recombinant proteins may behave differently from proteins present on virions, we investigated whether such viral envelope glycoproteins either in a purified form or present on viral particles could be deglycosylated by treatment with an endoglycosidase F-N-glycanase mixture which cleaves all accessible glycan moieties. Endoglycosidase analysis of the carbohydrate composition of purified viral gp120 (vgp120) indicated a glycosylation pattern similar to that for recombinant gp120 (rgp120), and treatment with endoglycosidase F-N-glycanase resulted in comparable molecular weight (MW) reduction for both molecules. Similarly, after immunoblotting of the deglycosylated viral preparation, the characteristic 160- and 120-kilodalton (kDa) bands were replaced by 90- and 60-kDa bands, respectively. The apparent MW of gp41 shifted to 35 kDa. These results are consistent with complete deglycosylation. The immunoreactive conformation of envelope glycoproteins remained unaltered after deglycosylation: they were recognized to the same extent by specific human polyclonal or mouse monoclonal antibodies, and no proteolysis of viral proteins occurred during enzymatic treatment. Deglycosylation of vgp120 resulted in a less than 10-fold reduction of the ability to bind to CD4, presented either in a soluble form or at the cell membrane. In addition, deglycosylation significantly reduced, but did not abolish, HIV-1 binding to and infectivity of CD4+ cells as determined, respectively, by an indirect immunofluorescence assay and a quantitative dose-response infection assay. Taken together, these results indicate that removal of glycans present on mature envelope glycoproteins of HIV-1 diminishes but does not abolish either virus binding to CD4 or its capacity to infect CD4+ cells.

Changes in the sialylation and sulfation of secreted thyrotropin in congenital hypothyroidism

We have examined the oligosaccharide structure of secreted thyrotropin (TSH) in perinatal and mature rats with congenital primary hypothyroidism. Rat pituitaries from euthyroid control animals and those rendered hypothyroid by methimazole treatment were incubated with [3H]glucosamine in vitro. Secreted TSH was purified, and oligosaccharides were enzymatically released and characterized by anion-exchange HPLC. In perinatal hypothyroid animals compared with control animals, oligosaccharides from TSH alpha and beta subunits contained more species with three or more negative charges. Moreover, perinatal hypothyroid animals demonstrated a dramatic increase in the ratio of sialylated to sulfated species within oligosaccharides of the same negative charge (2.9- to 7.4-fold increase for TSH-alpha; 15.1- to 25.5-fold increase for TSH-beta). In mature hypothyroid 9-week-old animals compared with control animals, changes were less pronounced, suggesting that endocrine regulation of oligosaccharide structure is dependent upon the maturational state of the animal. These changes were specific for TSH because glycosylation of free alpha subunit (synthesized by the thyrotroph and gonadotroph) and of total glycoproteins was minimally altered by hypothyroidism. Together, these data provide direct evidence and characterization of specific changes in the structure of a secreted pituitary glycoprotein hormone occurring as a result of in vivo endocrine alterations during early development. Moreover, they provide a potential structural basis to explain the delayed clearance of both TSH and the gonadotropins with end-organ deficiency, which may have important implications for the in vivo biological activities of these hormones. Specifically, such posttranslational changes may be an important adaptive response to prevent the consequences of endocrine deficiency during early development.

The charge characterization of native and deglycosylated thyrotropin

Chromatofocusing was used to characterize the charge microheterogeneity of crude pituitary, highly purified native bovine (b) and deglycosylated (dg) thyrotropin (TSH) preparations. Greater than 90% of crude pituitary TSH and native bTSH-I-1 bound to concanavalin-A (conA) columns while dgbTSH was excluded from the column. Extracts of ovine (o) pituitaries contained at least nine species (isohormones) of immunoreactive TSH when chromatofocused on pH 7.5-4 gradients. Highly purified native bTSH-I-1 displayed a similar elution profile. In contrast, dgbTSH eluted as a single homogeneous species with an apparent pI greater than 7.5. When subjected to chromatofocusing on a pH 10.5-7 gradient, 68% of crude pituitary oTSH and 96% of native bTSH-I-1 was bound to the column but could be eluted with NaCl indicating acidic species, while at least three peaks of dgbTSH could be resolved having apparent pI's of 9.12, 9.03 and 8.98. These data suggest that although removal of the carbohydrate moieties markedly alters the isohormone pattern of TSH, chemical deglycosylation does not completely eliminate the charge microheterogeneity of bTSH.

The subcellular sites of sulfation of mouse thyrotropin and free alpha subunits: studies employing subcellular fractionation and inhibitors of the intracellular translocation of proteins

To determine the subcellular sites of sulfation of thyrotropin (TSH) and free alpha-subunits, mouse thyrotropic tumor minces were incubated simultaneously with [3H]Met and [35S]SO4 for 1 or 3h, homogenized, and fractionated by discontinuous sucrose gradient ultracentrifugation. Dual-labeled TSH or free alpha-subunits were immunoprecipitated, and analyzed by SDS-gel electrophoresis. Endoglycosidase F released all [35S], but little [3H], from the dual-labeled species, indicating that [35S]SO4 was incorporated into oligosaccharides of TSH and free alpha-subunits. Both [35S]TSH and [35S] free alpha-subunits were predominantly in Golgi fractions at 1 and 3 h, but small amounts were also detected in fractions enriched in rough endoplasmic reticulum (RER). Similar distributions of [35S]SO4-labeled species were noted in cell fractions prepared from mouse pituitaries. Pituitaries from hypothyroid mice were incubated with [3H]Met and [35S]SO4 for 2 h, then chased for 4 or 16 h in the absence or presence of 2 uM monensin (Mon) or 10 uM carboxyl cyanide m-chlorophenylhydrazone (CCCP). At 4h, release into the medium of [3H]TSH was inhibited 59% and 86% by Mon and CCCP, respectively; release of [35S]TSH was inhibited 28% and 46%. At 4h, release of [3H]free alpha-subunits was inhibited 58% and 81% by these drugs, respectively; release of [35S]free alpha-subunits was inhibited 6% and 50%. Thus, Mon and CCCP inhibited the release of each [3H] species more than the [35S] species, indicating that most sulfation occurred in Golgi.

Enzymatic deglycosylation of thyroid-stimulating hormone with peptide N-glycosidase F and endo-beta-N-acetylglucosaminidase F

We investigated the ability of two enzymes, peptide N-glycosidase F (PNGase F) and endo-beta-N-acetylglucosaminidase F (Endo F), to deglycosylate microgram quantities of bovine TSH and its subunits under nondenaturing conditions. One oligosaccharide chain could be selectively removed from the alpha subunit by PNGase F, and all the oligosaccharide chains from both subunits could be removed by Endo F. These methods of enzymatic deglycosylation should permit study of the functional role of each N-linked carbohydrate chain of various glycoprotein hormones.