Mutagenesis of cysteine residues in the human gonadotropin alpha subunit. Roles of individual disulfide bonds in secretion, assembly, and biologic activity

Rational Design of Hyper-glycosylated Human Chorionic Gonadotropin Analogs (A Bioinformatics Approach)

Background:

Protein pharmaceuticals routinely display a series of intrinsic physicochemical instabilities during their production and administration that can unfavorably affect their therapeutic effectiveness. Glycoengineering is one of the most desirable techniques to improve the attributes of therapeutic proteins. One aspect of glycoengineering is the rational manipulation of the peptide backbone to introduce new N-glycosylation consensus sequences (Asn-X-Ser/Thr, where X is any amino acid except proline).

Methods:

In this work, the amino acid sequence of human chorionic gonadotropin (hCG) was analyzed to identify suitable positions in order to create new N-glycosylation sites. This survey led to the detection of 46 potential N-glycosylation sites. The N-glycosylation probability of all the potential positions was measured with the NetNGlyc 1.0 server. After theoretical reviews and the removal of unsuitable positions, the five acceptable ones were selected for more analyses. Then, threedimensional (3D) structures of the selected analogs were generated and evaluated by SPDBV software. The molecular stability and flexibility profile of five designed analogs were examined using Molecular Dynamics (MD) simulations.

Results:

Finally, three analogs with one additional N-glycosylation site (V68T, V79N and R67N) were proposed as the qualified analogs that could be glycosylated at the new sites.

Conclusion:

According to the results of this study, further experimental investigations could be guided on the three analogs. Therefore, our computational strategy can be a valuable method due to the reduction in the number of the expensive, tiresome and time-consuming experimental studies of hCG analogs.

Minireview: Insights Into the Structural and Molecular Consequences of the TSH-β Mutation C105Vfs114X

Naturally occurring thyrotropin (TSH) mutations are rare, which is also the case for the homologous heterodimeric glycoprotein hormones (GPHs) follitropin (FSH), lutropin (LH), and choriogonadotropin (CG). Patients with TSH-inactivating mutations present with central congenital hypothyroidism. Here, we summarize insights into the most frequent loss-of-function β-subunit of TSH mutation C105Vfs114X, which is associated with isolated TSH deficiency. This review will address the following question. What is currently known on the molecular background of this TSH variant on a protein level? It has not yet been clarified how C105Vfs114X causes early symptoms in affected patients, which are comparably severe to those observed in newborns lacking any functional thyroid tissue (athyreosis). To better understand the mechanisms of this mutant, we have summarized published reports and complemented this information with a structural perspective on GPHs. By including the ancestral TSH receptor agonist thyrostimulin and pathogenic mutations reported for FSH, LH, and choriogonadotropin in the analysis, insightful structure function and evolutionary restrictions become apparent. However, comparisons of immunogenicity and bioactivity of different GPH variants is hindered by a lack of consensus for functional analysis and the diversity of used GPH assays. Accordingly, relevant gaps of knowledge concerning details of GPH mutation-related effects are identified and highlighted in this review. These issues are of general importance as several previous and recent studies point towards the high impact of GPH variants in differential signaling regulation at GPH receptors (GPHRs), both endogenously and under diseased conditions. Further improvement in this area is of decisive importance for the development of novel targeted therapies.

Bioengineering of coagulation factor VIII for efficient expression through elimination of a dispensable disulfide loop

BACKGROUND

Heterologous expression of factor VIII (FVIII) is about two to three orders of magnitude lower than similarly sized proteins. Bioengineering strategies aimed at different structural and biochemical attributes of FVIII have been successful in enhancing its expression levels.

OBJECTIVE

Disulfide bonds are vital to the proper folding, secretion and stability of most secretory proteins. In an effort to explore additional targeted bioengineering approaches, the role of disulfide bonds in FVIII secretion and function was probed in this study.

METHODS AND RESULTS

Single and paired cysteine mutants were generated by substituting with serine or glycine residues and analyzed by transient transfection into COS-1 and CHO cells. Seven of the eight disulfide bonds in FVIII were found to be indispensable for proper secretion and function. However, elimination of the disulfide bond formed by C1899 and C1903 within the conserved A3 domain improved the secretion of FVIII. The addition of the C1899G/C1903G mutations to a previously described FVIII variant, 226/N6, with high secretion efficiency increased its secretion by 2.2-fold. Finally, the addition of the A1-domain mutation, F309S, in conjunction with the disulfide mutation had an additive effect, resulting in a net improvement in secretion of between 35 and 45-fold higher than wild-type FVIII in CHO cells.

CONCLUSION

Such combined targeted bioengineering strategies may facilitate more efficient production of recombinant FVIII and contribute toward low-cost factor replacement therapy for hemophilia A.

Rapid Maturation of Glycoprotein Hormone Free α-Subunit (GPHα) and GPHαα Homodimers

The dynamics of glycoprotein hormone α-subunit (GPHα) maturation and GPHαα homodimer formation were studied in presence (JEG-3 choriocarcinoma cells) and absence (HeLa cells) of hCGβ. In both cases, the major initially occurring GPHα variant in [35S]Met/Cys-labeled cells carried two N-glycans (Mr app = 22 kDa). Moreover, a mono-N-glycosylated in vivo association-incompetent GPHα variant (Mr app = 18 kDa) was observed. In JEG-3 cells the early 22-kDa GPHα either associated with hCGβ, or showed self-association to yield GPHαα homodimers, or was later converted into heavily glycosylated large free GPHα (Mr app = 24 kDa). Micro-preparative isolation of intracellular GPHαα homodimers of JEG-3 cells and their conversion by reduction revealed that they consisted of 22-kDa GPHα monomers and not of large free GPHα. In HeLa cells, the large free GPHα variant was not observed, whereas GPHαα homodimers were present. Intracellularly, early GPHαα homodimers (35 kDa) and late variants (JEG-3: 44 kDa, HeLa: 39 kDa) were found. Both cell types secreted 45 kDa GPHαα homodimers. Large free GPHα and GPHαα homodimers were more rapidly sialylated than hCG αβ-heterodimers indicating a sequestration mechanism in the secretory pathway. In GPHαα homo- as well as hCG αβ-heterodimers the subunit interaction site, located on loop 2 of GPHα (amino acids 33–42), became immunologically inaccessible indicating similar spatial orientation of GPHα in both types of dimers. The studies demonstrate the formation, in vivo dynamics of GPHαα homodimers, and the pathways of the cellular metabolism of variants of GPHα, monoglycosylated GPHα and large free GPHα.

Time-dependent folding of immunological epitopes of the human chorionic gonadotropin beta-subunit

We have explored the possibility to use 14 different monoclonal antibodies in order to follow the formation of the respective epitopes during the biosynthesis of hCG subunits and their association in JEG-3 choriocarcinoma cells using pulse (30s to 5 min)-chase (0-180 min) experiments. We found central cystine knot epitope structures (epitope beta1) to be formed immediately and simultaneously with epitopes on the protruding hCG-beta loops 1 and 3. We found also differences in the time-dependent folding of beta2 and beta4 epitopes, which are highly overlapping structures on the loops 1+3. These differences were reinforced by decreasing the temperature during the pulse-chase experiments to 25 degrees C. Moreover, we describe for the first time an intracellular intact hCG beta-subunit form that showed the transient expression of the hCG-beta-core fragment epitope beta11 in the course of the maturation of this subunit which casts new light on the presence of hCG-beta-core fragment in Down's syndrome, tumors and pregnancy.

Mutational analysis of Norrin-Frizzled4 recognition

Norrin and Frizzled4 (Fz4) function as a ligand-receptor pair to control vascular development in the retina and inner ear. In mice and humans, mutations in either of the corresponding genes lead to defects in vascular development. The present work is aimed at defining the sequence determinants of binding specificity between Norrin and the Fz4 amino-terminal ligand-binding domain (the "cysteine-rich domain" (CRD)). The principal conclusions are as follows: 1) Norrin binds to the Fz4 CRD and does not detectably bind to the 14 other mammalian Frizzled and secreted Frizzled-related protein CRDs; 2) Norrin and Xenopus Wnt8 recognize largely overlapping regions of the Fz4 CRD; 3) surface determinants on the Fz4 and Fz8 CRDs that allow Norrin to distinguish between these two CRDs reside within several small regions on one face of the CRD; 4) Norrin function depends critically on three pairs of cysteines that form the highly conserved trio of disulfide bonds shared among all cystine knot proteins, but the remaining two putative disulfide bonds are less important; 5) Norrin-CRD binding depends on a largely contiguous group of amino acids in the extended beta-sheet domain of Norrin that are predicted to face away from the interface between the two monomers in the Norrin homodimer; 6) Norrin-CRD binding is strongly modulated by interactions involving charged amino acid side chains; and 7) Norrin-CRD binding is enhanced approximately 10-fold by the addition of heparin. These observations are discussed in the context of Frizzled signaling and the structure and function of other cystine knot proteins.

Single-chain, triple-domain gonadotropin analogs with disulfide bond mutations in the alpha-subunit elicit dual follitropin and lutropin activities in vivo

The human glycoprotein hormones chorionic gonadotropin (CG), TSH, LH, and FSH are heterodimers composed of a common alpha-subunit and a hormone-specific beta-subunit. The subunits assemble noncovalently early in the secretory pathway. LH and FSH are synthesized in the same cell (pituitary gonadotrophs), and several of the alpha-subunit sequences required for association with either beta-subunit are different. Nevertheless, no ternary complexes are observed for LH and FSH in vivo, i.e. both beta-subunits assembled with a single alpha-subunit. To address whether the alpha-subunit can interact with more than one beta-subunit simultaneously, we genetically linked the FSHbeta- and CGbeta-subunit genes to the common alpha-subunit, resulting in a single-chain protein that exhibited both activities in vitro. These studies also indicated that the bifunctional triple-domain variant (FSHbeta-CGbeta-alpha), is secreted as two distinct bioactive populations each corresponding to a single activity, and each bearing the heterodimer-like contacts. Although the data are consistent with the known secretion events of gonadotropins from the pituitary, we could not exclude the possibility whether transient intermediates are generated in vivo in which the alpha-subunit shuttles between the two beta-subunits during early stages of accumulation in the endoplasmic reticulum. Therefore, constructs were engineered that would direct the synthesis of single-chain proteins completely devoid of heterodimer-like interactions but elicit both LH and FSH actions. These triple-domain, single-chain chimeras contain the FSHbeta- and CGbeta-subunits and an alpha-subunit with cystine bond mutations (cys10-60 or cys32-84), which are known to prevent heterodimer formation. Here we show that, despite disrupting the intersubunit interactions between the alpha- and both CGbeta- and FSHbeta-subunits, these mutated analogs exhibit both activities in vivo comparable to nonmutated triple-domain single chain. Such responses occurred despite the absence of quaternary contacts due to the disrupted bonds in the alpha-subunit. Thus, gonadotropin heterodimer assembly is critical for intracellular events, e.g. hormone-specific posttranslational modifications, but when heterodimers are present in the circulation, the alpha/beta-contacts are not a prerequisite for receptor recognition.

Cloning and sequencing of feline thyrotropin (fTSH): heterodimeric and yoked constructs

The genes encoding the mature common glycoprotein alpha (CGA) and hormone-specific beta subunits of feline thyroid stimulating hormone (fTSH) were cloned and sequenced. The feline CGA gene was cloned from RNA extracted from the feline pituitary gland by the reverse transcription polymerase chain reaction (RT-PCR). The gene fragment that encodes mature TSHbeta was cloned from feline genomic DNA after direct polymerase chain reaction (PCR). In both cases, primers were based on the consensus sequences from TSH in other species. The resulting 510 bp PCR product for the CGA-subunit included the full coding sequence for the 96 amino acid mature subunit preceded by a 24 amino acid signal peptide. The 850 bp sequence of fTSHbeta genomic DNA consisted of two coding exons, an intron of 418 bp, and a 60 bp signal sequence. The octapeptide immunoaffinity tag FLAG was added to 3' end of the alpha gene to facilitate detection and purification. Both genes were cloned independently downstream from the EF1alpha promoter of the PEAK transfer vector to facilitate co-expression studies in PEAK cells (modified human embryonic kidney (HEK) cells). A single-chain analogue of fTSH termed yoked fTSH (yfTSH) was developed by fusing the nucleotides encoding the C-terminus of the beta-subunit fused to the N-terminus of the alpha-subunit with DNA encoding the C-terminal peptide (CTP) of human chorionic gonadotropin beta-subunit as a linker peptide. The resulting single-chain analogue encoded from N-terminus to C-terminus: beta-CTP-alpha-FLAG. The resulting DNA sequence was cloned, sequenced, ligated and recloned into expression vector PEAK. This report constitutes the first cloning and sequencing of the genes encoding the subunits of feline thyrotropin.

Effect of additional N-glycosylation signal in the N-terminal region on intracellular function of the human gonadotropin alpha-subunit

hCG, LH, FSH, and TSH are a family of heterodimeric glycoprotein hormones that contain a common alpha-subunit, but differ in their hormone-specific beta-subunits. The alpha-subunit has two N-glycosylation sites at Asn52 and Asn78. To obtain more information on the relationship between the structure and function of the alpha-subunit, we introduced a novel N-glycosylation site in the N-terminal region by mutating Asp3 and Gln5 into Asn and Thr, respectively. Glycosylation mutants were expressed alone or with hCGbeta-subunit in Chinese hamster ovary cells. New N-linked oligosaccharides were efficiently added to the wild-type and mutant alpha-subunits lacking N-glycan at Asn52 (alpha deltaAsn1), Asn78 (alpha deltaAsn2), and both (alpha deltaAsn(1 + 2)). The new sugar chain did not affect secretion and assembly except that 1) it increased the intracellular degradation of alpha deltaAsn(1 + 2), and 2) it augmented the assembly of alpha deltaAsn1 with hCGbeta-subunit. Amino acid changes generated the attachment of O-glycosylation in free alpha-subunit but not in assembled form. These data indicate that the newly introduced N-glycosylation consensus sequence is functional, and that the N-terminal region of the alpha-subunit is flexible and can be modified without affecting the intracellular function. Furthermore, amino acid sequences in the N-terminus are involved in the O-glycosylation in free alpha-subunit.

Cloning and sequence analysis of FSH and LH in the giant panda (Ailuropoda melanoleuca)

The giant panda (Ailuropoda melanoleuca) is an endangered species and indigenous to China. It has been proposed that it has a highly specialized reproductive pattern with low fecundity, but little is known about its basic reproductive biology at the molecular level. In this report the genes encoding gonadotropin subunits alpha, follicle-stimulating hormone (FSH) beta and luteinizing hormone (LH) beta of the giant panda were amplified for the first time by RT-PCR from pituitary total RNA, and were cloned, sequenced and analyzed. The results revealed that the open reading region (ORF) of gonadotropin subunits alpha, FSH beta and LH beta are 363, 390 and 426 bp long, respectively. They displayed a reasonably high degree (74-94, 85-93, 75-91%, for alpha, FSH beta and LH beta subunits, respectively) of identity when deduced amino acids were compared with homologous sequences from partial available mammals including human, cattle, sheep, pig, rat, mouse. Three distinct differences were found at the site of 59 aa of the alpha subunit and 55 aa, 68 aa of FSH beta subunit. Our results provide an insight into understanding the mechanism of reproduction regulation and genetic characteristics of giant panda which will make an actual contribution to its conservation. In addition they lay a foundation for a further study towards producing recombinant panda FSH and LH which can be used in artificial breeding aimed to increase its captive reproductive efficiency.

The position of the alpha and beta subunits in a single chain variant of human chorionic gonadotropin affects the heterodimeric interaction of the subunits and receptor-binding epitopes

The glycoprotein hormone family represents a class of heterodimers, which include the placental hormone human chorionic gonadotropin (CG) and the anterior pituitary hormones follitropin, lutropin, and thyrotropin. They are composed of common alpha subunit and a hormone-specific beta subunit. Based on the CG crystal structure, it was suggested that the quaternary subunit interactions are crucial for biological activity. However, recent observations using single chain glycoprotein hormone analogs, where the beta and alpha subunits are linked (NH(2)-CGbeta-alpha; CGbetaalpha orientation), implied that the heterodimeric-like quaternary configuration is not a prerequisite for receptor binding/signal transduction. To study the heterodimeric alignment of the two subunit domains in a single chain and its role in the intracellular behavior and biological action of the hormone, a single chain CG variant was constructed in which the carboxyl terminus of alpha was fused to the CGbeta amino terminus (NH(2)-alpha-CGbeta; alphaCGbeta orientation). The secretion rate of alphaCGbeta from transfected Chinese hamster ovary cells was less than that seen for CGbetaalpha. The alphaCGbeta tether was not recognized by dimer-specific monoclonal antibodies and did not bind to lutropin/CG receptor. To define if one or both subunit domains were modified in alphaCGbeta, it was co-transfected with a monomeric alpha or CGbeta gene. In each case, alphaCGbeta/alpha and alphaCGbeta/CGbeta complexes were formed indicating that CG dimer-specific epitopes were established. The alphaCGbeta/alpha complex bound to receptor indicating that the beta domain in the alphaCGbeta tether was still functional. In contrast, no significant receptor binding of alphaCGbeta/CGbeta was observed indicating a major perturbation in the alpha domain. These results suggest that although dimeric-like determinants are present in both alphaCGbeta/alpha and alphaCGbeta/CGbeta complexes, the receptor binding determinants in the alpha domain of the tether are absent. These results show that generating heterodimeric determinants do not necessarily result in a bioactive molecule. Our data also indicate that the determinants for biological activity are distinct from those associated with intracellular behavior.

Partial restoration of lutropin activity by an intersubunit disulfide bond: implications for structure/function studies

Gonadal function is controlled by lutropins and follitropins, heterodimeric cystine knot proteins that have nearly identical alpha-subunits. These heterodimeric proteins are stabilized by a portion of the hormone-specific beta-subunit termed the "seatbelt" that is wrapped around alpha-subunit loop 2 (alpha 2). Here we show that replacing human chorionic gonadotropin (hCG) alpha 2 residue Lys51 with cysteine or alanine nearly abolished its lutropin activity, an observation that implies that alpha Lys51 has a key role in hormone activity. The activity of the heterodimer containing alpha K51C, but not that containing alpha K51A, was increased substantially when beta-subunit seatbelt residue beta Asp99 was converted to cysteine. As had been reported by others, heterodimers containing alpha K51C and beta D99C were crosslinked by a disulfide. The finding that an intersubunit disulfide restored some of the activity lost by replacing alpha Lys51 suggests that this residue is not crucial for receptor binding or signaling and also that hCG and related hormones may be particularly sensitive to mutations that alter interactions between their subunits. We propose the unique structures of hCG and related family members may permit some subunit movement in the heterodimer, making it difficult to deduce key residues involved in receptor contacts simply by correlating the activities of hormone analogs with their amino acid sequences.

Functional contributions of noncysteine residues within the cystine knots of human chorionic gonadotropin subunits

Human chorionic gonadotropin (hCG) is a heterodimeric member of a family of cystine knot-containing proteins that contain the consensus sequences Cys-X(1)-Gly-X(2)-Cys and Cys-X(3)-Cys. Previously, we characterized the contributions that cystine residues of the hCG subunit cystine knots make in folding, assembly, and bioactivity. Here, we determined the contributions that noncysteine residues make in hCG folding, secretion, and assembly. When the X(1), X(2), and X(3) residues of hCG-alpha and -beta were substituted by swapping their respective cystine knot motifs, the resulting chimeras appeared to fold correctly and were efficiently secreted. However, assembly of the chimeras with their wild type partner was almost completely abrogated. No single amino acid substitution completely accounted for the assembly inhibition, although the X(2) residue made the greatest individual contribution. Analysis by tryptic mapping, high performance liquid chromatography, and SDS-polyacrylamide gel electrophoresis revealed that substitution of the central Gly in the Cys-X(1)-Gly-X(2)-Cys sequence of either the alpha- or beta-subunit cystine knot resulted in non-native disulfide bond formation and subunit misfolding. This occurred even when the most conservative change possible (Gly --> Ala) was made. From these studies we conclude that all three "X" residues within the hCG cystine knots are collectively, but not individually, required for the formation of assembly-competent hCG subunits and that the invariant Gly residue is required for efficient cystine knot formation and subunit folding.

Intracellular folding pathway of the cystine knot-containing glycoprotein hormone alpha-subunit

Three of the five disulfide bonds in the glycoprotein hormone alpha-subunit (GPH-alpha) form a cystine knot motif that stabilizes a three-loop antiparallel structure. Previously, we described a mutant (alpha(k)) that contained only the three knot disulfide bonds and demonstrated that the cystine knot was necessary and sufficient for efficient GPH-alpha folding and secretion. In this study, we used alpha(k) as a model to study the intracellular GPH-alpha folding pathway. Cystine knot formation proceeded through a 1-disulfide intermediate that contained the 28-82 disulfide bond. Formation of disulfide bond 10-60, then disulfide bond 32-84, followed the formation of 28-82. Whether the two non-cystine knot bonds 7-31 and 59-87 could form independent of the knot was also tested. Disulfide bond 7-31 formed rapidly, whereas 59-87 did not form when all cysteine residues of the cystine knot were converted to alanine, suggesting that 7-31 forms early in the folding pathway and that 59-87 forms during or after cystine knot formation. Finally, loop 2 of GPH-alpha has been shown to be very flexible, suggesting that loop 2 does not actively drive GPH-alpha folding. To test this, we replaced residues 36-55 in the flexible loop 2 with an artificially flexible glycine chain. Consistent with our hypothesis, folding and secretion were unaffected when loop 2 was replaced with the glycine chain. Based on these findings, we describe a model for the intracellular folding pathway of GPH-alpha and discuss how these findings may provide insight into the folding mechanisms of other cystine knot-containing proteins.

Disulfide bond mutations in follicle-stimulating hormone result in uncoupling of biological activity from intracellular behavior

The crystal structure of human CG reveals that each subunit is a member of the superfamily of cystine-knot growth factors. Although the distribution of the cysteine residues in all the beta-subunits is conserved, the conformation of the human FSH dimer differs from that of the CG/LH dimers. This suggests that the function of the cystine bonded loops in the human FSHbeta-subunit may differ from that in the CGbeta-subunit. To address this issue, we deleted two disulfide bonds in the FSHbeta domain: cys 20-104 and cys 28-82, which correspond to the disulfide bonds 26-110 and 34-88, respectively, in the CGbeta-subunit. The cys 26-110 bond is associated with the "seat-belt" region and cys 34-88 is a bond in the cystine knot. Coexpression of the wild-type alpha-subunit with the FSHbeta cysteine mutants in CHO cells revealed no detectable heterodimer. The FSHbeta mutants were then incorporated into a single chain where the beta-subunit is genetically fused to the alpha-subunit. In such a model, the rate-limiting subunit assembly step is by-passed and mutations that otherwise block heterodimer formation can be evaluated in terms of biological activity. Compared with the nonmutated single chain, the single-chain 28-82 mutant is secreted more slowly and its recovery is substantially reduced, whereas secretion and recovery of the 20-104 mutant was not significantly affected. The receptor binding affinity of the cys 28-82 mutant did not differ from wild-type and binding of the cys 20-104 mutant was decreased only 2-fold. The signal transduction data parallel the binding affinities, although the maximal accumulation of cAMP is less for the cys 20-104 mutant than that seen for cys 28-82 and nonmutated single-chains variants. These data support the hypothesis that the determinants for intracellular behavior and bioactivity of the gonadotropins are not the same, and that the cystine knot is a critical determinant for the formation of a stable, assembly-competent subunit. In addition, the data imply that the "seat-belt" conformation does not play a prominent role in the bioactivity of FSH.

Molecular architecture and biorecognition processes of the cystine knot protein superfamily: part I. The glycoprotein hormones

In this review article, the reader is introduced to recent advances in our knowledge on a subset of the cystine knot superfamily of homo- and hetero-dimeric proteins, from the perspective of the endocrine glycoprotein hormone family of proteins: follitropin (FSH), Iutropin (LH), thyrotropin. (TSH) and chorionic gonadotropin (CG). Subsequent papers will address the structure-function behaviour of other members of this increasingly significant family of proteins, including various members of the transforming growth factor-beta (TGF-beta) family of proteins, the activins, inhibins, bone morphogenic growth factor, platelet derived growth factor-beta, nerve growth factor and more than 35 other proteins with similar topological features. In the present review article, specific emphasis has been placed on advances with the glycoprotein hormones (GPHs) that have facilitated greater insight into their physiological functions, molecular structures and most importantly the basis of the molecular recognition events that lead to the formation of hetero-dimeric structures as well as their specific and selective recognition by their corresponding receptors and antibodies. Thus, this review article focuses on the structural motifs involved in receptor recognition and the current techniques available to identify these regions, including the role of immunological methodology, peptide fragment design and synthesis and mutagenesis to delineate their structure-function relationships and molecular recognition behaviour.

Cystine knot mutations affect the folding of the glycoprotein hormone alpha-subunit. Differential secretion and assembly of partially folded intermediates

The common glycoprotein hormone alpha-subunit (GPH-alpha) contains five intramolecular disulfide bonds, three of which form a cystine knot motif (10-60, 28-82, and 32-84). By converting each pair of cysteine residues of a given disulfide bond to alanine, we have studied the role of individual disulfide bonds in GPH-alpha folding and have related folding ability to secretion and assembly with the human chorionic gonadotropin beta-subunit (hCG-beta). Mutation of non-cystine knot disulfide bond 7-31, bond 59-87, or both (leaving only the cystine knot) resulted in an efficiently secreted folding form that was indistinguishable from wild type. Conversely, the cystine knot mutants were inefficiently secreted (<25%). Furthermore, mutation of the cystine knot disulfide bonds resulted in multiple folding intermediates containing 1, 2, or 4 disulfide bonds. High performance liquid chromatographic separation of intracellular and secreted forms of the folding intermediates demonstrated that the most folded forms were preferentially secreted and combined with hCG-beta. From these studies we conclude that: (i) the cystine knot of GPH-alpha is necessary and sufficient for folding and (ii) there is a direct correlation between the extent of GPH-alpha folding, its ability to be secreted, and its ability to heterodimerize with hCG-beta.

cDNA cloning of canine common alpha gene and its co-expression with canine thyrotropin beta gene in baculovirus expression system

The common alpha gene of the canine glycoprotein hormones was cloned, sequenced and co-expressed with the canine thyrotropin beta (TSH beta) gene in the baculovirus expression system, and a bioactive recombinant canine TSH was purified. The canine common alpha gene was cloned from the total RNA extracted from the canine pituitary gland by the reverse transcription polymerase chain reaction (RT-PCR) using primers that were designed based on the consensus sequences from other species. The resulting 476 bp PCR product is consisted of the full coding sequence for the 96 amino acid mature alpha subunit, and a sequence encoding a 24 amino acid signal peptide. Homology analysis with other species revealed that the canine common alpha subunit potentially contains five disulfide bonds and two oligosaccharide chains N-linked to Asn residues located at positions 56 and 82. For expression in the baculovirus expression system, the common alpha gene was cloned downstream of the p10 promoter of the pAcUW51 transfer vector, and the previously cloned canine TSH beta gene was inserted under the polyhedrin promoter of the same vector. The recombinant virus containing both alpha and beta genes was generated and propagated before being used to transfect the Sf9 insect cells for expression. The medium from the Sf9 cultures, presumably containing canine TSH alpha and beta in native heterodimer confirmation, exhibited TSH bioactivity as indicated in the cAMP stimulation assay in FRTL-5 cells. The expressed recombinant protein was purified from the culture medium with an affinity column that was coupled with IgG purified from the polyclonal antibodies generated against the partially purified native canine TSH.

Definition and measurement of follicle stimulating hormone

FSH has a key role in the development and function of the reproductive system and is widely used both diagnostically and therapeutically in developmental and reproductive medicine. The accurate measurement of FSH levels, in patients for diagnosis and monitoring and in therapeutic preparations for clinical use, is essential for safe and successful treatment. Historically, FSH was defined on the basis of classical in vivo endocrine activity, and early therapeutic preparations were calibrated using in vivo bioassays. There was early recognition that reference preparations were required for calibration if the results from different laboratories were to be comparable. In response to the perceived need, the World Health Organization established the first standard for such preparations in 1959. Subsequent developments in biotechnology have led to recognition that there is no single molecule that can be uniquely defined as FSH, and that FSH can induce a range of biological activities. Several highly purified standards for FSH are now available, but discontinuity and heterogeneity of estimates of FSH activity in terms of these standards made using in vitro assays and binding assays have been noted. It is thus essential that any measurement of FSH include specification both of the standard with which the measured FSH is compared and the assay method used for that comparison.

The biological action of choriogonadotropin is not dependent on the complete native quaternary interactions between the subunits

Human CG (hCG) is a member of the glycoprotein hormone family characterized by a heterodimeric structure consisting of a common alpha-subunit noncovalently bound to a hormone-specific beta-subunit. The two subunits are highly intertwined and only the heterodimer is functional, implying that the quaternary structure is critical for biological activity. To assess the dependence of the bioactivity of hCG on the heterodimeric interactions, alpha- and beta-subunits bearing mutations that prevent assembly were covalently linked to form a single chain hCG. Receptor binding and signal transduction of these analogs were tested and their structural integrity analyzed using a panel of monoclonal antibodies (mAbs). These included dimer-specific mAbs, which react with at least four different epitope sites on the hormone, and some that react only with the free beta-subunit. We showed that there was significant loss of quaternary and tertiary structure in several regions of the molecule. This was most pronounced in single chains that had one of the disulfide bonds of the cystine knot disrupted in either the alpha- or beta-subunit. Despite these structural changes, the in vitro receptor binding and signal transduction of the single chain analogs were comparable to those of the nonmutated single chain, demonstrating that not all of the quaternary configuration of the hormone is necessary for biological activity.

Addition of an N-terminal dimerization domain promotes assembly of hCG analogs: implications for subunit combination and structure-function analysis

Human chorionic gonadotropin (hCG) is a heterodimeric placental glycoprotein hormone that acts through ovarian lutropin receptors (LHR) to maintain early pregnancy. Its ability to distinguish LHR and follitropin receptors (FSHR) is controlled by 20 beta-subunit 'seatbelt' residues that surround alpha-subunit loop 2. Positively charged amino acids between residues 93-100, a small loop within the seatbelt, have been postulated to make essential LH receptor contacts. Previous studies showed that analogs containing negatively charged amino acids in this small loop had 5-10% the activity of hCG and 1-10% the lutropin activities of hCG/hFSH chimeric analogs capable of binding LHR and FSHR. These effects might be due to the influence of these residues on receptor contacts or on hormone conformation. During efforts to distinguish these possibilities, we increased and decreased the number of residues in this loop, mutations we anticipated would distort its conformation. Consistent with this supposition, these changes inhibited dimer formation, precluding assessment of these mutations on hormone activity. Addition of Fos and Jun dimerization domains to the N-termini of hCGalpha- and hCG/hFSHbeta-subunit chimeras overcame the effects of the seatbelt mutations on subunit combination and enabled preparation of heterodimers containing six, seven, or nine residues in their seatbelt loops. These had 0.1-10% the lutropin and 3-60% the follitropin activities of bifunctional chimeras containing 8 residues derived from hCG in the seatbelt loop. The abilities of N-terminal dimerization domains to promote subunit combination may permit structure/function analysis of other residues that influence heterodimer formation.

Disruption of disulfide bonds is responsible for impaired secretion in human complement factor H deficiency

Factor H, a secretory glycoprotein composed of 20 short consensus repeat modules, is an inhibitor of the complement system. Previous studies of inherited factor H deficiency revealed single amino acid substitutions at conserved cysteine residues, on one allele arginine for cysteine 518 (C518R) and on the other tyrosine for cysteine 941 (C941Y) (Ault, B. H., Schmidt, B. Z., Fowler, N. L., Kashtan, C. E., Ahmed, A. E., Vogt, B. A., and Colten, H. R. (1997) J. Biol. Chem. 272, 25168-25175). To ascertain if the phenotype, impaired secretion of factor H, is due to the C518R substitution or the C941Y substitution and to ascertain the mechanism by which secretion is impaired, we studied COS-1 and HepG2 cells transfected with wild type and several mutant factor H molecules. The results showed markedly impaired secretion of both C518R and C941Y factor H as well as that of factor H molecules bearing alanine or arginine substitutions at the Cys518-Cys546 disulfide bond (C518A, C546A, C546R, C518A-C546A). In each case, mutant factor H was retained in the endoplasmic reticulum and degraded relatively slowly as compared with most other mutant secretory and membrane proteins that are retained in the endoplasmic reticulum. These data indicate that impaired secretion of the naturally occurring C518R and C941Y mutant factor H proteins is due to disruption of framework-specific disulfide bonds in factor H short consensus repeat modules.

Suppression by beta-mercaptoethanol of the intracellular hormonal dynamics of human chorionic gonadotropin-beta subunit (hCG-beta) in BeWo choriocarcinoma cells

The effects of beta-mercaptoethanol (ME) on the steady-state level of mRNA of the human chorionic gonadotropin-beta subunit (hCG-beta) and the intracellular hormonal dynamics of the product protein were examined in BeWo cells, a choriocarcinoma cell line, using Northern blot analysis and a radioimmunoassay (RIA) specific for hCG-beta. ME reduced both medium and intracellular contents of hCG-beta in a dose-dependent manner, with its minimum effective dose being 0.01 per cent. The highest dose used (0.1 per cent) caused more than 90 per cent inhibition with both parameters, without affecting the cell number and the cell viability as verified by trypan blue exclusion. Significant reductions in both the medium and intracellular contents began to occur 6 h after the onset of incubation with ME. The ME-induced suppressions were reversible. Northern blot analysis showed that ME had no effects on the steady-state level of hCG-beta mRNA. When medium and cell lysates collected from ME-free incubations were incubated with 0.03 per cent ME, there were significant reductions of immunoreactive hCG-beta with both the medium and cell lysates. The magnitude of reduction, however, was much greater with the latter (75 per cent) than with the former (25 per cent). In contrast, the hCG-beta immunoreactivity of the RIA reference preparation was unaffected by incubation with ME. These results suggested that the major target(s) of ME action were the intracellularly located hCG-beta molecule, presumably its intramolecular disulphide bonds. It must also be pointed out that the hCG-beta molecule synthesized and secreted by BeWo cells have some structural deviation from the reference standard molecule of normal trophoblastic origin to explain the differential susceptibility to ME.

Expression of biologically active beta subunit of bovine follicle-stimulating hormone in the methylotrophic yeast Pichia pastoris

Follicle-stimulating hormone (FSH), a pituitary gonadotropin, is a heterodimer composed of an alpha subunit, which is common to all the glycoprotein hormones, noncovalently associated with the hormone-specific beta subunit. The objective of the present study is to develop a recombinant DNA expression system for the beta subunit of FSH that can be applied to study structure-function relationships while producing large quantities of the hormone subunit for immuno-contraceptive, clinical, and veterinary purposes. We report here the expression of biologically active bovine FSH beta (bFSH beta) in the methylotrophic yeast Pichia pastoris. The Pichia-expressed FSH beta (pFSH beta) was secreted into the culture medium and was found to be immunologically very similar to pituitary-derived ovine FSH beta. Replacement of cognate signal peptide with the yeast alpha mating factor signal peptide increased the level of expression from 230 ng/ml (cognate signal peptide) to 4 micrograms/ml (alpha mating factor signal peptide) of the culture supernatant. pFSH beta His.tag (pFSH beta with six histidine residues at the C terminus) was purified to apparent homogeneity using one-step nickel affinity chromatography. The molecular weight of purified pFSH beta His.tag was approximately 22,000, which was slightly higher than that of the pituitary-derived ovine FSH beta. pFSH beta His.tag could assemble with the alpha subunit to yield a heterodimer capable of binding to the FSH receptors and also elicit biological response. These data show that pFSH beta His.tag is properly folded and biologically active.

Cystine knot of the gonadotropin alpha subunit is critical for intracellular behavior but not for in vitro biological activity

The common alpha subunit of glycoprotein hormones contains five disulfide bonds. Based on the published crystal structure, the assignments are 7-31, 59-87, 10-60, 28-82, and 32-84; the last three comprise the cystine knot, a structure also seen in a variety of growth factors. Previously, we demonstrated that the efficiency of secretion and the ability to form heterodimers by alpha subunits bearing single cysteine residue mutants in the cystine knot were significantly reduced. These results suggested that the cystine knot is critical for the intracellular integrity of the subunit. To assess if the presence of the free thiol affected the secretion kinetics, we constructed paired cysteine mutants of each disulfide bond of the alpha subunit. The secretion rate for these monomers was comparable with wild type except for the alpha-10-60 mutant, which was 40% lower. The recovery of the alpha7-31 and alpha59-87 mutants was greater than 95%, whereas for the cystine knot mutants, it was 20-40%. Co-expression of the wild-type chorionic gonadotropin beta subunit with double cysteine mutants did not enhance the recovery of alpha mutants in the media. Moreover, compared with wild-type, the efficiency of heterodimer formation of the alpha10-60 or alpha32-84 mutants was less than 5%. Because subunit assembly is required for biological activity, studies on the role of these disulfide bonds in signal transduction were not possible. To bypass the assembly step, we exploited the single chain model, where the alpha and beta subunits are genetically fused. The recovery of secreted tethered gonadotropins bearing mutations in the cystine knot was increased significantly. Although dimer-specific monoclonal antibodies discriminated the conformation of single chain alpha10-60 and alpha32-84 mutants from the native heterodimer, these mutants were nevertheless biologically active. Thus, individual bonds of cystine knot are important for secretion and heterodimer formation but not for in vitro bioactivity. Moreover, the data suggest that the native heterodimer configuration is not a prerequisite for receptor binding or signal transduction.

Mutational analysis of extracellular cysteine residues of rat secretin receptor shows that disulfide bridges are essential for receptor function

We attempted to express point-mutant secretin receptors where each of the 10 extracellular Cys residues was replaced by a Ser residue, in Chinese hamster ovary (CHO) cells. Six of the point-mutant receptors (C24-->S, C44-->S, C53-->S, C67-->S, C85-->S and C101-->S) could not be detected by binding or functional studies: the mutations resulted in functional inactivation of the receptor. In contrast, the four other point-mutant receptors (C11-->S, C186-->S, C193-->S and C263-->S) were able to bind poorly 125I-secretin, and to activate adenylate cyclase with high secretin EC50 values. These results suggest that cysteine residues 24, 44, 53, 67, 85 and 101 are necessary for receptor function, and that the two putative disulfide bridges formed by cysteine residues 11, 186, 193 and 263 are functionally relevant, but not essential for receptor expression. Secretin activated the adenylate cyclase through the quadruple mutant (C11,186,193,263-->S), the four triple mutants, and through double mutants C186,193-->S and C186,263-->S with a very high (microM) EC50 value, suggesting that, in the wild-type receptor, disulfide bridges are formed between C11-C186, and between C193-C263. Prior treatment with dithiothreitol resulted in a marked EC50 increase of the wild-type receptor and of those receptors with at least the two cysteine residues in positions 11 and 186, suggesting that the C11-C186 (but not the C193-C263) disulfide bridge was accessible to this reducing agent. Several results nevertheless indicated that, in mutant receptors, alternative disulfide bridges can be formed between cysteine 186 and cysteine 193 or 263, suggesting that these three residues are in close spatial proximity in the wild-type receptor.

Evaluation of subunit truncation and the nature of the spacer for single chain human gonadotropins

Three single chain gonadotropins were designed based on the three-dimensional-structure of human choriogonadotropin and structure/activity relationships of the glycoprotein hormones. In each single chain, the C-terminal end of the human choriogonadotropin beta subunit is connected via Ser-Gly repeats to the N-terminal end of the alpha subunit. In addition, two of the single chains have truncated subunits. The three mutants were expressed in CHO cells. In vitro binding of two of the three mutants to the human lutropin/choriogonadotropin receptor was found to be comparable to wild-type lutropin. In contrast, both the receptor binding and the in vitro bioactivity of the mutant with truncated alpha and beta subunits in which the beta:26-110 disulphide bond cannot be formed, are lowered relative to wild-type lutropin. The fact that this mutant still displays biological activity shows that the seat-belt arrangement proposed by Isaacs and coworkers [Lapthorn, A. J., Harris, D. C., Littlejohn, A., Lustbader, J. W., Canfield, R. E., Machin, K. J.. Morgan, F. J. & Isaacs, N. W. (1994) Nature 369, 455-461] is important but not essential for receptor binding and biological activity in the context of single chain gonadotropins. Single chains in which Ser-Gly spacers are combined with truncated subunits, provide an attractive approach towards the design and generation of novel, biologically active gonadotropins.

Redox modulation of synaptic responses and plasticity in rat CA1 hippocampal neurons

Effects of redox reagents on excitatory and inhibitory synaptic responses as well as on the bidrectional plasticity of alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor-mediated synaptic responses were studied in CA1 pyramidal neurons in rat hippocampal slices. The oxidizing agent 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB, 200 microM) did not affect AMPA, GABAA or GABAB receptor-mediated synaptic responses or the activation of presynaptic metabotropic receptors. However, DTNB irreversibly decreased (by approximately 50%) currents evoked by focal application of NMDA. DTNB also decreased the NMDA component of the EPSC. The reversal potential of NMDA currents and the Mg2+ block were not modified. In the presence of physiological concentrations of Mg2+ (1.3 mM), DTNB did not affect the NMDA receptor-dependent induction of long-term potentiation (LTP) or long-term depression (LTD) expressed by AMPA receptors. In contrast, DTNB fully prevented LTP and LTD induced and expressed by NMDA receptors. Plasticity of NMDA receptor-mediated synaptic responses could be reinstated by the reducing agent tris-(2-carboxyethyl) phosphine (TCEP, 200 microM). These results suggest that persistent, bidirectional changes in synaptic currents mediated by NMDA receptors cannot be evoked when these receptors are in an oxidized state, whereas NMDA-dependent LTP and LTD are still expressed by AMPA receptors. Our observations raise the possibility of developing therapeutic agents that would prevent persistent excitotoxic enhancement of NMDA receptor-mediated events without blocking longterm modifications of AMPA receptor-mediated synaptic responses, thought to underlie memory processes.

hCG-receptor binding and transmembrane signaling

The technique of site-directed mutagenesis has proven to be quite powerful in elucidating contact sites involved in the interaction of the heterodimeric glycoprotein hormones and their respective seven transmembrane (TM) G protein-coupled receptors. Our laboratory has focused on identification of the minimum core sequences of the alpha and beta subunits required for bioactivity, the minimum length of a conjoined (yoked) single-chain hCG, the amino acid residues on hCG and the LH/CG-receptor (LH/CG-R) responsible for high-affinity binding, and the regions of the receptor that are involved in TM signaling. A number of amino acid residues have been mapped on the alpha and beta subunits of hCG that appear important in receptor binding. When projected onto the crystal structure of HF-treated hCG, these residues, by and large, cluster on one side of the molecule and cover a sizeable surface area, indicating that the hormone-receptor binding interface is rather extensive. Based on mutagenesis studies of several conserved ionizable amino acid residues in the extracellular domain (ECD) of LH/CG-R and a model that we, in collaboration with Drs Lapthorn and Isaacs, have developed for this region based on the crystal structure of porcine ribonuclease inhibitor, a charged region that appears to play an important role in hormone-receptor recognition has been identified. We have also delineated several regions of LH/CG-R that do not appear to participate in hCG binding but are involved in hCG-mediated signaling. These regions are located in the ECD and extracellular loop III just prior to entry into the membrane via TM helices I and VII, respectively, and in TM helices VI and VII. Similarly, a homologous region in the ECD of the FSH receptor, located with ten residues of TM helix I, is important in signaling but not hormone binding. These results suggest that ligand binding and ligand-mediated receptor activation are quasi-distinct, albeit sequential phenomena. Collectively, our mutagenesis and modeling studies, coupled with results from other laboratories, argue for a ligand-induced conformational change of the receptor that may involve a relative reorientation of the TM helices.

Expression of biologically active fusion genes encoding the common alpha subunit and either the CG beta or FSH beta subunits: role of a linker sequence

The gonadotropin/thyrotropin hormone family is characterized by a heterodimeric structure composed of a common alpha subunit non-covalently linked to a hormone-specific beta subunit. The conformation of the heterodimer is essential for controlling secretion, hormone-specific post-translational modifications and signal transduction. Structure-function studies of FSH and the other glycoprotein hormones are often hampered by mutagenesis induced defects in subunit combination. Thus, the ability to overcome the limitation of subunit assembly would expand the range of structure activity relationships that can be performed on these hormones. Here we converted the FSH heterodimer to a single chain by genetically fusing the carboxyl end of the FSH beta subunit to the amino end of the alpha subunit in the presence or absence of a natural linker sequence. In the absence of the CTP linker, the secretion rate was decreased over three fold. (The CTP sequence is the last 28 amino acids of the CG beta sequence and contains four serine-linked oligosaccharides). Unexpectedly however receptor binding/signal transduction was unaffected by absence of the linker. Molecular modelling of the tethers lacking the linker sequence show that the alignment of the alpha/beta domains in the single chain differ substantially from that seen in the heterodimer. These data show that the single chain FSH was secreted efficiently and is biologically active and that the conformation determinants required for secretion and biologic activity are not the same.

Expression of biologically active fusion genes encoding the common alpha subunit and the follicle-stimulating hormone beta subunit. Role of a linker sequence

The gonadotropin/thyrotropin hormone family is characterized by a heterodimeric structure composed of a common alpha subunit noncovalently linked to a hormone-specific beta subunit. The conformation of the heterodimer is essential for controlling secretion, hormone-specific post-translational modifications, and signal transduction. Structure-function studies of follicle-stimulating hormone (FSH) and the other glycoprotein hormones are often hampered by mutagenesis-induced defects in subunit combination. Thus, the ability to overcome the limitation of subunit assembly would expand the range of structure-activity relationships that can be performed on these hormones. Here we converted the FSH heterodimer to a single chain by genetically fusing the carboxyl end of the FSH beta subunit to the amino end of the alpha subunit in the presence or absence of a linker sequence. In the absence of the CTP linker, the secretion rate was decreased over 3-fold. Unexpectedly, however, receptor binding/signal transduction was unaffected by the absence of the linker. These data show that the single-chain FSH was secreted efficiently and is biologically active and that the conformation determinants required for secretion and biologic activity are not the same.

Converting heterodimeric gonadotropins to genetically linked single chains: new approaches to structure activity relationships and analogue design

One of the distinguishing features of the gonadotropin and thyrotropin hormone family is their heterodimeric structure; the subunits combine early in the secretory pathway and only the dimers are capable of binding to receptors. Therefore, assembly is rate limiting in the production of functional heterodimers, a problem encountered when removing the carbohydrates from one or both subunits as discussed in this review. If the heterodimers can be expressed as single chains, this might avoid mutagenesis-induced defects in secretion and combination of individual subunits for structure-function studies and analogue design. Here we discuss the feasibility of this approach for such problems.

Paired natural cysteine mutation mapping: aid to constraining models of protein tertiary structure

This paper discusses the benefit of mapping paired cysteine mutation patterns as a guide to identifying the positions of protein disulfide bonds. This information can facilitate the computer modeling of protein tertiary structure. First, a simple, paired natural-cysteine-mutation map is presented that identifies the positions of putative disulfide bonds in protein families. The method is based on the observation that if, during the process of evolution, a disulfide-bonded cysteine residue is not conserved, then it is likely that its counterpart will also be mutated. For each target protein, protein databases were searched for the primary amino acid sequences of all known members of distinct protein families. Primary sequence alignment was carried out using PileUp algorithms in the GCG package. To search for correlated mutations, we listed only the positions where cysteine residues were highly conserved and emphasized the mutated residues. In proteins of known three-dimensional structure, a striking pattern of paired cysteine mutations correlated with the positions of known disulfide bridges. For proteins of unknown architecture, the mutation maps showed several positions where disulfide bridging might occur.

Cystine knots

Recent reports of the crystal structure of the glycoprotein hormone human chorionic gonadotrophin show that cystine knots are proving to be versatile structural motifs that enable the construction of a variety of proteins with different functional properties. Because of their shape, there appears to be an intrinsic requirement for the cystine-knot growth factors to form dimers. This extra level of organization increases the variety of structures built around this simple structural motif.