Expression of the extracellular domain of the thyrotropin receptor in the baculovirus system using a promoter active earlier than the polyhedrin promoter. Implications for the expression of functional highly glycosylated proteins

Temporal characterization of protein production levels from baculovirus vectors coding for GFP and RFP genes under non-conventional promoter control

The ease of use and versatility of the Baculovirus Expression Vector System (BEVS) has made it one of the most widely used systems for recombinant protein production However, co-expression systems currently in use mainly make use of the very strong very late p10 and polyhedron (polh) promoters to drive expression of foreign genes, which does not provide much scope for tailoring expression ratios within the cell. This work demonstrates the use of different Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) promoters to control the timing and expression of two easily traceable fluorescent proteins, the enhanced green fluorescent protein (eGFP), and a red fluorescent protein (DsRed2) in a BEVS co-expression system. Our results show that gene expression levels can easily be controlled using this strategy, and also that modulating the expression level of one protein can influence the level of expression of the other protein within the system, thus confirming the concept of genes "competing" for limited cellular resources. Plots of "expression ratios" of the two model genes over time were obtained, and may be used in future work to tightly control timing and levels of foreign gene expression in an insect cell co-expression system.

Utility of temporally distinct baculovirus promoters for constitutive and baculovirus-inducible transgene expression in transformed insect cells

Genetically transformed lepidopteran insect cell lines have biotechnological applications as constitutive recombinant protein production platforms and improved hosts for baculovirus-mediated recombinant protein production. Insect cell transformation is often accomplished with a DNA construct(s) encoding a foreign protein(s) under the transcriptional control of a baculovirus immediate early promoter, such as the ie1 promoter. However, the potential utility of increasingly stronger promoters from later baculovirus gene classes, such as delayed early (39K), late (p6.9), and very late (polh), has not been systematically assessed. Hence, we produced DNA constructs encoding secreted alkaline phosphatase (SEAP) under the transcriptional control of each of the four temporally distinct classes of baculovirus promoters, used them to transform insect cells, and compared the levels of SEAP RNA and protein production obtained before and after baculovirus infection. The ie1 construct was the only one that supported SEAP protein production by transformed insect cells prior to baculovirus infection, confirming that only immediate early promoters can be used to isolate transformed insect cells for constitutive recombinant protein production. However, baculovirus infection activated transgene expression by all four classes of baculovirus promoters. After infection, cells transformed with the very late (polh) and late (p6.9) promoter constructs produced the highest levels of SEAP RNA, but only low levels of SEAP protein. Conversely, cells transformed with the immediate early (ie1) and delayed early (39K) promoter constructs produced lower levels of RNA, but equal or higher levels of SEAP protein. Unexpectedly, the 39K promoter construct provided tightly regulated, baculovirus-inducible protein production at higher levels than the later promoter constructs. Thus, this study demonstrated the utility of the 39K promoter for insect cell engineering, particularly when one requires higher levels of effector protein production than obtained with ie1 and/or when constitutive transgene expression adversely impacts host cell fitness and/or genetic stability.

Co-expression vs. co-infection using baculovirus expression vectors in insect cell culture: Benefits and drawbacks

The baculovirus expression vector system (BEVS) is a versatile and powerful platform for protein expression in insect cells. With the ability to approach similar post-translational modifications as in mammalian cells, the BEVS offers a number of advantages including high levels of expression as well as an inherent safety during manufacture and of the final product. Many BEVS products include proteins and protein complexes that require expression from more than one gene. This review examines the expression strategies that have been used to this end and focuses on the distinguishing features between those that make use of single polycistronic baculovirus (co-expression) and those that use multiple monocistronic baculoviruses (co-infection). Three major areas in which researchers have been able to take advantage of co-expression/co-infection are addressed, including compound structure-function studies, insect cell functionality augmentation, and VLP production. The core of the review discusses the parameters of interest for co-infection and co-expression with time of infection (TOI) and multiplicity of infection (MOI) highlighted for the former and the choice of promoter for the latter. In addition, an overview of modeling approaches is presented, with a suggested trajectory for future exploration. The review concludes with an examination of the gaps that still remain in co-expression/co-infection knowledge and practice.

Central dogma in thyroid dysfunction: a review on structure modification of TSHR as a cornerstone for thyroid abnormalities

Thyroid stimulating hormone receptor (TSHR) is a vital thyrocyte membrane protein in the thyroid gland. Thyroid Stimulating Hormone (TSH) which is a pituitary hormone is the main stimulator of thyroid gland to produce thyroid hormones, it binds with high affinity to the TSHR through weak bonds including hydrophobic, ionic, hydrogen bonds and trigger the initial steps in thyroid gland stimulation to produce the related hormones. This study was carried out at department of biochemistry of Golestan university of medical sciences. All the related articles related to TSHR modification happened due to mutations and any other alterations which affect the level of TSH-TSHR complex were studied and the main points were extracted out of the pile of information and were organized as present review. TSH-TSHR is the initial and vital step of a long process of thyroid hormone production within the thyroid gland. Any alteration on the TSH-TSHR affinity which may happen due to the direct effect of TSHR modification eventually lead to the serious adverse effects of either hypothyroidism or hyperthyroidism if the TSH-TSHR level are suppressed or elevated, respectively. The prime cause of the thyroid disorders relay on the possible modification on the biochemical structure of TSHR with subsequent alteration on the level of TSH-TSHR complex. TSHR mutation accompanied by biochemical modification, unable it to bind properly to TSH. In some other conditions such mutation leave a TSHR with either of higher affinity towards to TSH or even TSHR which can be activated in the absence of TSH. The structural modification of TSHR and alteration in the level of TSH-TSHR in the thyroid gland eventually lead to thyroid disorders either of hypothyroidism or hyperthyroidism.

Factors affecting recombinant Western equine encephalitis virus glycoprotein production in the baculovirus system

In an effort to produce processed, soluble Western equine encephalitis virus (WEEV) glycoproteins for subunit therapeutic vaccine studies, we isolated twelve recombinant baculoviruses designed to express four different WEEV glycoprotein constructs under the transcriptional control of three temporally distinct baculovirus promoters. The WEEV glycoprotein constructs encoded full-length E1, the E1 ectodomain, an E26KE1 polyprotein precursor, and an artificial, secretable E2E1 chimera. The three different promoters induced gene expression during the immediate early (ie1), late (p6.9), and very late (polh) phases of baculovirus infection. Protein expression studies showed that the nature of the WEEV construct and the timing of expression both influenced the quantity and quality of recombinant glycoprotein produced. The full-length E1 product was insoluble, irrespective of the timing of expression. Each of the other three constructs yielded soluble products and, in these cases, the timing of expression was important, as higher protein processing efficiencies were generally obtained at earlier times of infection. However, immediate early expression did not yield detectable levels of every WEEV product, and expression during the late (p6.9) or very late (polh) phases of infection provided equal or higher amounts of processed, soluble product. Thus, while earlier foreign gene expression can provide higher recombinant glycoprotein processing efficiencies in the baculovirus system, in the case of the WEEV glycoproteins, earlier expression did not provide larger amounts of high quality, soluble recombinant glycoprotein product.

High-level expression and improved folding of proteins by using the vp39 late promoter enhanced with homologous DNA regions

Some recombinant proteins expressed by baculovirus expression vector systems (BEVS) aggregate because the BEVS can produce large amounts of protein late during infection, when post-translational modification and protein quality control mechanisms are inactive. For expression during earlier stages than that driven by the polyhedrin (polh) very late promoter, transfer vectors were generated in which this promoter was replaced with a green fluorescent protein (GFP) gene controlled by a vp39 late promoter modified to contain HR3, one of the homologous DNA regions (HRs) of Bombyx mori nuclear polyhedrosis virus (BmNPV). The rise times of the fluorescence of GFP expressed by using recombinant viruses carrying the modified vp39 promoter were earlier than those associated with either the polh promoter or the native vp39 promoter lacking HR3. In transient expression assays, the vp39 late promoter in transfer vectors behaved like a delayed-early promoter, and was enhanced by HR3, and required IE-1 protein and various viral gene products encoded on both sides of BmNPV polh. When the vp39 promoter with HR3 was used, the aggregation of several foreign proteins expressed by the BEVS was markedly decreased. This study provides a new option for the expression of sufficiently quality-controlled proteins by using the vp39 promoter and HR3 in BEVS early in baculovirus infection, when the infection has caused little damage in the host cells.

Purification by Strep-Tactin affinity chromatography of a delete envelope gp51 protein of Bovine Leukaemia virus expressed in Sf21 insect cells

Bovine leukaemia virus (BLV) causes disease in cattle and it is related to human T lymphotrofic viruses HTLV-1 and HTLV-2. The objective of this study was to express and purify deleted and stable forms of the gp51 envelope glycoprotein of BLV using a baculovirus system. Two forms of the gp51 were synthesised: one comprised the gp51 N-terminal 174 amino acids and a single 6xHis tag (Delta(175-268)gp51-His) and the second form contained the same amino acid sequence and a C-terminal Strep-tag II in addition to the 6xHis tag (Delta(175-268)gp51-STH). The two proteins were expressed and purified by immobilized metal-affinity chromatography (IMAC) or by Strep-Tactin column. The Strep-Tactin technology was more efficient than IMAC method and achieved a high pure recombinant deleted gp51. In addition the Delta(175-268)gp51-STH protein was further concentrated by IMAC. This purified antigen could be used for the isolation of immunoreactive molecules and to develop a competitive ELISA test.

Baculovirus-insect cell expression systems

In the early 1980s, the first-published reports of baculovirus-mediated foreign gene expression stimulated great interest in the use of baculovirus-insect cell systems for recombinant protein production. Initially, this system appeared to be the first that would be able to provide the high production levels associated with bacterial systems and the eukaryotic protein processing capabilities associated with mammalian systems. Experience and an increased understanding of basic insect cell biology have shown that these early expectations were not completely realistic. Nevertheless, baculovirus-insect cell expression systems have the capacity to produce many recombinant proteins at high levels and they also provide significant eukaryotic protein processing capabilities. Furthermore, important technological advances over the past 20 years have improved upon the original methods developed for the isolation of baculovirus expression vectors, which were inefficient, required at least some specialized expertise and, therefore, induced some frustration among those who used the original baculovirus-insect cell expression system. Today, virtually any investigator with basic molecular biology training can relatively quickly and efficiently isolate a recombinant baculovirus vector and use it to produce their favorite protein in an insect cell culture. This chapter will begin with background information on the basic baculovirus-insect cell expression system and will then focus on recent developments that have greatly facilitated the ability of an average investigator to take advantage of its attributes.

Protein expression in the baculovirus system

Insect cell-recombinant baculovirus co-cultures offer a protein production system that complements microbial systems by providing recombinant proteins in soluble form and with most post-translational modifications. Moreover, the large size of the viral genome enables cloning of large segments of DNA and consequent expression of complex protein aggregates. This unit describes methods associated with the large-scale production of recombinant proteins in the baculovirus expression system. A method for large-scale production of viral stocks is described and methods for titration of virus are provided (a plaque assay and an end-point assay). Once viral stocks have been prepared and titered, a protocol for testing the virus in small-scale cultures is provided to determine the kinetics of expression, which allows evaluation of various cell culture and infection conditions aimed at developing optimal levels of protein production (e.g., comparisons of different host cell lines, media, and environmental parameters). Support protocols provide instructions for preparing culture samples for protein analysis by SDS-PAGE and discuss analytical methods for monitoring nutrient levels in cell culture fluids. Once optimal process parameters are identified, protocols describe production of the target protein on a large scale in fermentors using either regular batch production in bioreactors or a fed-batch procedure of production in perfusion cultures. Techniques for harvesting cultures from bioreactors are also provided.

Protein expression in the baculovirus system

Insect cell-recombinant baculovirus co-cultures offer a protein production system that complements microbial systems by providing recombinant proteins in soluble form and with most post-translational modifications. Moreover, the large size of the viral genome enables cloning of large segments of DNA and consequent expression of complex protein aggregates. This unit describes methods associated with the large-scale production of recombinant proteins in the baculovirus expression system. A method for large-scale production of viral stocks is described and methods for titration of virus are provided (a plaque assay and an end-point assay). Once viral stocks have been prepared and titered, a protocol for testing the virus in small-scale cultures is provided to determine the kinetics of expression, which allows evaluation of various cell culture and infection conditions aimed at developing optimal levels of protein production (e.g., comparisons of different host cell lines, media, and environmental parameters). Support protocols provide instructions for preparing culture samples for protein analysis by SDS-PAGE and discuss analytical methods for monitoring nutrient levels in cell culture fluids. Once optimal process parameters are identified, protocols describe production of the target protein on a large scale in fermentors using either regular batch production in bioreactors or a fed-batch procedure of production in perfusion cultures. Techniques for harvesting cultures from bioreactors are also provided.

Protein N-glycosylation in the baculovirus-insect cell expression system and engineering of insect cells to produce "mammalianized" recombinant glycoproteins

Baculovirus expression vectors are frequently used to express glycoproteins, a subclass of proteins that includes many products with therapeutic value. The insect cells that serve as hosts for baculovirus vector infection are capable of transferring oligosaccharide side chains (glycans) to the same sites in recombinant proteins as those that are used for native protein N-glycosylation in mammalian cells. However, while mammalian cells produce compositionally more complex N-glycans containing terminal sialic acids, insect cells mostly produce simpler N-glycans with terminal mannose residues. This structural difference between insect and mammalian N-glycans compromises the in vivo bioactivity of glycoproteins and can potentially induce allergenic reactions in humans. These features obviously compromise the biomedical value of recombinant glycoproteins produced in the baculovirus expression vector system. Thus, much effort has been expended to characterize the potential and limits of N-glycosylation in insect cell systems. Discoveries from this research have led to the engineering of insect N-glycosylation pathways for assembly of mammalian-style glycans on baculovirus-expressed glycoproteins. This chapter summarizes our knowledge of insect N-glycosylation pathways and describes efforts to engineer baculovirus vectors and insect cell lines to overcome the limits of insect cell glycosylation. In addition, we consider other possible strategies for improving glycosylation in insect cells.

Secretion of human glucocerebrosidase from stable transformed insect cells using native signal sequences

The lysosomal hydrolase, glucocerebrosidase (GBA), catalyses the penultimate step in the breakdown of membrane glycosphingolipids. An inherited deficiency of this enzyme activity leads to the onset of Gaucher disease, the most common lysosomal storage disorder. Affected individuals range from adults with hepatosplenomegaly, haematological complications, and bone pain (type 1 disease) to children and neonates with severe neuronopathy leading to neurological degradation and premature death (type 2 and type 3 disease). Enzyme replacement therapy has become the standard of treatment for type I Gaucher disease but remains an expensive option, in part because of the cost of recombinant enzyme production using mammalian cell culture. Using a nonlytic integrative plasmid expression system, we have successfully produced active human GBA in stable transformed Sf9 (Spodoptera frugiperda) cells. Both the 39 and 19 amino acid native GBA signal sequences were capable of endoplasmic reticulum targeting, which led to secretion of the recombinant protein, although approximately 30% more enzyme was produced using the longer signal sequence. The secreted product was purified to apparent electrophoretic homogeneity using hydrophobic interaction chromatography and found to be produced in a fully glycosylated and a hypoglycosylated form, both of which cross-reacted with a human GBA-specific monoclonal antibody. The pH optimum (at pH 5.5) for activity of the recombinant enzyme was as expected for human GBA using the artificial substrate 4-methyl-umbelliferyl-β-D-glycopyranoside. With initial nonoptimized expression levels estimated at 10–15 mg/L using small-scale batch cultures, stable transformed insect cells could provide a viable alternative system for the heterologous production of human GBA when grown under optimized perfusion culture conditions.Key words: Gaucher disease, glucocerebrosidase, protein expression, enzyme purification, Sf9 cells.

The possible contribution of anti-Gal to Graves' disease

Anti-Gal is a natural antibody specific for the alpha-galactosyl epitope. Previous studies suggested that Graves' disease (GD) patients had elevated anti-Gal titers compared to normal controls, but titers returned to normal after treatment. We developed an anti-Gal enzyme-linked immunosorbent assay (ELISA) using the property of anti-Gal to bind tightly to mouse laminin. We found no significant correlations between anti-Gal and thyroidstimulating immunoglobulin (TSI) or free thyroxine (T(4)) in untreated hyperthyroid GD patients (n = 15) without clinical ophthalmopathy or euthyroid, previously treated GD patients with ophthalmopathy. There was a significant regression between TSI and free T(4) in the hyperthyroid patients (p < 0.01). Addition of total anti- Gal antibody to the regression showed a trend toward improved correlation (p = 0.15 for improved correlation relative to TSI and free T(4) alone), suggesting it may stimulate GD thyroid tissue. However, in contrast to previous studies we found hyperthyroid patients (n = 20) had lower levels of anti-Gal immunoglobulin G (IgG) (18.4 +/- 4.0 vs. 41.8 +/- 8.9) than normals (n = 36 p < 0.05). Interestingly, hyperthyroid patients without clinical ophthalmopathy tended to have lower IgG anti-Gal levels than euthyroid patients with ophthalmopathy (p = 0.1). Hyperthyroidism significantly lowers anti-Gal, but the possible increase of anti-Gal in patients with ophthalmopathy suggests anti-Gal may play a role in ophthalmopathy, or may reflect the euthryoid status of these patients. This trend needs further study.

Current perspective on the pathogenesis of Graves' disease and ophthalmopathy

Graves' disease (GD) is a very common autoimmune disorder of the thyroid in which stimulatory antibodies bind to the thyrotropin receptor and activate glandular function, resulting in hyperthyroidism. In addition, some patients with GD develop localized manifestations including ophthalmopathy (GO) and dermopathy. Since the cloning of the receptor cDNA, significant progress has been made in understanding the structure-function relationship of the receptor, which has been discussed in a number of earlier reviews. In this paper, we have focused our discussion on studies related to the molecular mechanisms of the disease pathogenesis and the development of animal models for GD. It has become apparent that multiple factors contribute to the etiology of GD, including host genetic as well as environmental factors. Studies in experimental animals indicate that GD is a slowly progressing disease that involves activation and recruitment of thyrotropin receptor-specific T and B cells. This activation eventually results in the production of stimulatory antibodies that can cause hyperthyroidism. Similarly, significant new insights have been gained in our understanding of GO that occurs in a subset of patients with GD. As in GD, both environmental and genetic factors play important roles in the development of GO. Although a number of putative ocular autoantigens have been identified, their role in the pathogenesis of GO awaits confirmation. Extensive analyses of orbital tissues obtained from patients with GO have provided a clearer understanding of the roles of T and B cells, cytokines and chemokines, and various ocular tissues including ocular muscles and fibroblasts. Equally impressive is the progress made in understanding why connective tissues of the orbit and the skin in GO are singled out for activation and undergo extensive remodeling. Results to date indicate that fibroblasts can act as sentinel cells and initiate lymphocyte recruitment and tissue remodeling. Moreover, these fibroblasts can be readily activated by Ig in the sera of patients with GD, suggesting a central role for them in the pathogenesis. Collectively, recent studies have led to a better understanding of the pathogenesis of GD and GO and have opened up potential new avenues for developing novel treatments for GD and GO.

Calnexin co-expression and the use of weaker promoters increase the expression of correctly assembled Shaker potassium channel in insect cells

Voltage-gated potassium channels control the membrane potential of excitable cells. To understand their function, knowledge of their structure is essential. However, these channels are scarce in natural sources, and overexpression is necessary to generate material for structural studies. We have compared functional expression of the Drosophila Shaker H4 potassium channel in stable insect cell lines and in baculovirus-infected insect cells, using three different baculovirus promoters. Stable insect cell lines expressed correctly assembled channel, which was glycosylated and found predominantly at, or close to, the cell surface. In comparison, the majority of baculovirus-overexpressed Shaker was intracellular and incorrectly assembled. The proportion of functional Shaker increased, however, if the weaker basic protein promoter was used rather than the stronger p10 or polyhedrin promoters. In addition, co-expression of the molecular chaperone, calnexin, increased the quantity of correctly assembled channel protein, suggesting that calnexin can be used to increase the efficiency of channel expression in insect cells.

Rescue of intracellularly trapped lutropin receptor exodomain by endodomain and reconstitution of a functional membrane receptor: interaction between exo- and endodomains

The lutropin receptor consists of an extracellular N-terminal half and a membrane-associated C-terminal half. hCG initially binds the exodomain with a high affinity and the resulting complex is thought to interact with the endodomain through a secondary contact generating a hormonal signal. Therefore, the exodomain and endodomain are likely to associate directly or indirectly with each other, but lack of fruitful materials and technology has hampered knowledge about their physical relationship and contact sites. In this work, we engineered a double-recombinant (separate exodomain and endodomain) baculovirus system successfully expressing on the surface of insect cells high levels of split LH receptor, binding the hormone with high affinity and inducing cAMP synthesis. In contrast, the exodomain and endodomain expressed separately were mostly trapped in cells. Our data indicate that the exodomain and endodomain are disulfide linked in the split receptor. When the disulfide links were reduced, the split receptor still induced cAMP up to 60%, which raises the intriguing possibility of a residual induction activity of the endodomain in the absence of high-affinity ligand binding. Our results also underscore that the targeting and transport of the LH receptor to plasma membrane require both domains, whereas each domain is independently sufficient for folding. The expression level of functional lutropin receptors is the highest ever reported. Our system may also be useful for future studies requiring a high amount of soluble secreted exodomain.

Thyroid-stimulating hormone and thyroid-stimulating hormone receptor structure-function relationships

This review focuses on recent advances in the structure-function relationships of thyroid-stimulating hormone (TSH) and its receptor. TSH is a member of the glycoprotein hormone family constituting a subset of the cystine-knot growth factor superfamily. TSH is produced by the pituitary thyrotrophs and released to the circulation in a pulsatile manner. It stimulates thyroid functions using specific membrane TSH receptor (TSHR) that belongs to the superfamily of G protein-coupled receptors (GPCRs). New insights into the structure-function relationships of TSH permitted better understanding of the role of specific protein and carbohydrate domains in the synthesis, bioactivity, and clearance of this hormone. Recent progress in studies on TSHR as well as studies on the other GPCRs provided new clues regarding the molecular mechanisms of receptor activation. Such advances are a result of extensive site-directed mutagenesis, peptide and antibody approaches, detailed sequence analyses, and molecular modeling as well as studies on naturally occurring gain- and loss-of-function mutations. This review integrates expanding information on TSH and TSHR structure-function relationships and summarizes current concepts on ligand-dependent and -independent TSHR activation. Special emphasis has been placed on TSH domains involved in receptor recognition, constitutive activity of TSHR, new insights into the evolution of TSH bioactivity, and the development of high-affinity TSH analogs. Such structural, physiological, pathophysiological, evolutionary, and therapeutic implications of TSH-TSHR structure-function studies are frequently discussed in relation to concomitant progress made in studies on gonadotropins and their receptors.

Antithyrotropin receptor antibody: an update

Numerous studies have reported the characteristics and significance concerning antithyrotropin receptor antibodies (TSHR-Abs), which cause Graves' disease and in some cases primary hypothyroidism. However, many unsolved questions concerning those antibodies remain. Here, recent developments in the study of TSHR-Abs are reviewed based on three aspects: mechanisms of TSHR-Ab production, antibody binding epitopes, and clinical TSHR-Ab assays. Mechanisms of TSHR-Ab production are discussed from five points of view: aberrant expression of the major histocompatibility complex, dysregulation of T cells, molecular mimicry, bystander effect, and expansion of autoreactive B cells. Regarding epitopes, unique TSHR-Abs have been reported that may explain the complicated pathophysiology of patients with TSHR-Ab diseases. Finally, recent efforts to improve TSHR-Ab measurements are introduced. Such efforts will contribute to clinical examinations and treatments for thyroid diseases as well as experimental methods of thyroidology.

Increased production of active human beta(2)-adrenergic/G(alphas) fusion receptor in Sf-9 cells using nutrient limiting conditions

Using the baculovirus/insect-cell expression vector system, we succeeded in obtaining a high yield of active human beta(2)-adrenergic receptor/G(alphas) fusion protein. This was achieved following high cell density production under nutrient-limiting conditions using a very low multiplicity of infection (MOI). This approach was found to significantly reduce inactive protein accumulation that occurred when production was done using conventional high MOI procedures. The maximum specific and volumetric yields of active receptor using this strategy increased by factors of two- and sixfold, respectively. Our results suggest that the increase in the ratio of active/total protein produced results from production under nutrient limitation. Since low multiplicity of infection offers many advantages for large-scale applications, we suggest that this simple production method should be considered when optimizing expression of G-protein-coupled receptors and other complex proteins.

Characterization of the secretable ectodomain of thyrotropin receptor produced by the recombinant baculovirus system

Thyrotropin receptor (TSHR) is a member of the glycoprotein hormone receptor family and an autoantigen of Graves' disease. Various attempts have been made to obtain a large amount of soluble ectodomain of TSHR in insect or mammalian cells, but most of them failed to secrete the overexpressed ectodomain. In the present study, we observed that about one-third of the ectodomain protein (sTSHR-gp), in which the signal peptide of TSHR was replaced by the baculovirus-encoded glycoprotein 67-signal peptide, was secreted into the culture medium and the remainder stayed within cells in the recombinant baculovirus system. Microsequencing the N-terminal of the purified protein confirmed that the baculovirus signal peptide was cleaved at the expected site. Carbohydrate studies using several glycosidases and lectins revealed that the secreted form of the ectodomain had biantennary carbohydrate, whereas the non-secreted form had high-mannose. Moreover, the secreted form of sTSHR-gp exhibited high-affinity ligand binding, whereas the non-secreted form did not show any significant ligand binding. Regarding the interactions of TSHR ectodomains with anti-TSHR antibodies, both the secreted and non-secreted forms of sTSHR-gp, almost completely neutralized the stimulatory and inhibitory anti-TSHR antibody activities. In conclusion, we succeeded in secreting the ectodomain of TSHR into culture medium, which was capable of binding to TSH and neutralizing anti-TSHR antibody activities.

Purification and characterization of a soluble bioactive amino-terminal extracellular domain of the human thyrotropin receptor

The amino-terminal ectodomain of the human TSH receptor has been expressed at the surface of CHO cells as a glycosylphosphatidylinositol-anchored molecule containing a 10-residue histidine tag close to its C terminus. The soluble ectodomain could be released from the cells by treatment with a glycosylphosphatidylinositol-phospholipase C and purified to apparent homogeneity by cobalt-Sepharose chromatography. Two nanomoles of material was obtained, which was suitable for analysis by mass spectrometry. This allowed the identification of four out of the six potential N-glycosylation sites as being effectively glycosylated. A proportion of the purified soluble ectodomain displayed specific binding of (125)I-labeled TSH, allowing for the first time performance of classical saturation binding experiments. Two classes of high-affinity binding sites were identified: site A, K(d) 0.014 nM; site B, K(d) 0.83 nM. The significance of site A, whose affinity is much higher than for the holoreceptor at the surface of intact cells, remains to be clarified. The purified ectodomain was capable of inhibiting efficiently the thyroid stimulating activity of immunoglobulins from patients with Graves' disease. It allowed computation of the amounts of these immunoglobulins in patient's serum, giving values up to 10 microg/mL. Contrary to all currently available assays, the soluble ectodomain of the TSH receptor purified in a functionally competent conformation allows direct studies of its interactions with TSH and autoantibodies and opens the way to structural studies.

Hormone-induced conformational change of the purified soluble hormone binding domain of follitropin receptor complexed with single chain follitropin

Human follicle-stimulating hormone receptor (hFSHR) belongs to family I of G protein-coupled receptors. FSHR extracellular domain (ECD) is predicted to have 8-9 alphabeta or leucine-rich repeat motif elements. The objective of this study was to identify elements of the FSHR ECD involved in ligand binding. Preincubation of recombinant hFSHR ECD with rabbit antisera raised against synthetic peptides of hFSHR ECD primary sequence abolished follitropin binding primarily in the region of amino acids 150-254. Accessibility of hFSHR ECD after hormone binding, captured by monoclonal antibodies against either ECD or FSH, was decreased for the region of amino acids 150-220 but additionally for amino acids 15-100. Thus, when hFSH bound first, accessibility of antibody binding was decreased to a much larger extent than if antibody was bound first. This suggestion of a conformational change upon binding was examined further. Circular dichroism spectra were recorded for purified single chain hFSH, hFSHR ECD, and hFSHR ECD-single chain hFSH complex. A spectral change indicated a small but consistent conformational change in the ECD.FSH complex after hormone binding. Taken together, these data demonstrate that FSH binding requires elements within the leucine-rich repeat motifs that form a central region of hFSHR ECD, and a conformational change occurs upon hormone binding.

Role of complex asparagine-linked oligosaccharides in the expression of a functional thyrotropin receptor

To evaluate the functional role of complex asparagine-linked oligosaccharides of the human thyrotropin receptor (TSHR), a Chinese hamster ovary cell line (JP09) and a K562 cell line (K562-TSHR) expressing this receptor were treated with deoxymannojirimycin (dMM), a mannosidase I inhibitor. dMM blocks the formation of complex-type structures and leads to the formation of high-mannose-type structures. Treatment of cells with dMM led to a decrease in the number of thyrotropin (TSH)-binding sites at the cell surface. Detection of the TSHR at the cell surface using a monoclonal antibody directed against the A subunit showed that this decrease was not due to a decrease in the number of TSHRs expressed at the cell surface. However the recognition of TSHR by a monoclonal antibody directed against the C peptide was greatly decreased. On immunoblotting, after deglycosylation using peptide N-glycanase F, the A subunit was visualized as a doublet (36 and 41 kDa). In control cells the species of higher molecular mass was more abundant whereas after dMM treatment the species of lower molecular mass became more abundant. This difference in molecular mass between the two peptides is compatible with the removal of the C peptide. In conclusion, the results show that inhibition of complex-type structure formation leads to (i) an incapacity for TSHR to bind TSH, without affecting its intracellular transport and (ii) an increase of TSHR susceptibility to proteases that remove the C peptide. We then hypothesized that removal of the C peptide could contribute to the formation of a non-functional TSHR.

Induction of Experimental Autoimmune Graves’ Disease in BALB/c Mice

We immunized BALB/c mice with M12 cells (H-2d) expressing either mouse (mM12 cells) or human thyrotropin receptor (TSHR) (hM12 cells). Immunized mice developed autoantibodies to native TSHR by day 90 and, by day 180, showed considerable stimulatory Ab activity as measured by their ability to enhance cAMP production (ranging from 6.52 to 20.83 pmol/ml in different treatment groups relative to 1.83 pmol/ml for controls) by TSHR-expressing Chinese hamster ovary cells. These mice developed severe hyperthyroidism with significant elevations in both tetraiodothyronine and triiodothyronine hormones. Tetraiodothyronine levels in different experimental groups ranged from a mean of 8.66–12.4 μg/dl, relative to 4.8 μg/dl in controls. Similarly, mean triiodothyronine values ranged from 156.18 to 195.13 ng/dl, relative to 34.99 ng/dl for controls. Next, we immunized BALB/c mice with a soluble extracellular domain of human TSHR (TBP), or TBP expressed on human embryonic kidney cells (293 cells) (293-TBP cells). These mice showed severe hyperthyroidism in a manner very similar to that described above for mice immunized with the mouse TSHR or human TSHR, and exhibited significant weight loss, with average weight for treatment groups ranging from 20.6 to 21.67 g, while controls weighed 24.2 g. Early after onset of the disease, histopathological examination of thyroids showed enlargement of colloids and thinning of epithelial cells without inflammation. However, later during disease, focal necrosis and lymphocytic infiltration were apparent. Our results showed that conformationally intact ectodomain of TSHR is sufficient for disease induction. Availability of a reproducible model in which 100% of the animals develop disease should facilitate studies aimed at understanding the molecular pathogenesis of Graves’ disease.

Limitations of the semisynthetic library approach for obtaining human monoclonal autoantibodies to the thyrotropin receptor of Graves' disease

Graves' disease (GD) is characterized by the presence of autoantibodies against the TSH-receptor (TSH-R) which are pathogenic and, upon binding to the receptor, trigger intracellular signal transduction. The autoantibodies are oligoclonal and as they are responsible for disease activity, their characterization would lead to a better understanding of the development of GD. Attempts to isolate anti-TSH-R antibodies from patients have proved to be difficult due to the exceedingly low serum levels due to rarity of these B cells, together with difficulties in obtaining purified TSH-R capable of interacting with patients autoantibodies. We employed phage antibody display technology and performed selection with a previously characterized semisynthetic antibody library on the purified extracellular ectodomain of the TSH-R. We report the isolation of six different anti-TSH-R monoclonal phage antibodies (moPhabs) from this library. All the moPhabs recognized TSH-R and its recombinant fragments by Western blotting, but failed to recognize the native TSH-R by flow cytometry. Consequently, the moPhabs did not lead to TSH-R activation. As these were the first moPhabs to TSH-R, they were analysed in terms of nucleotide and amino acid sequence and epitope specificity on the receptor. The moPhabs used immunoglobulin VH1 and VH3 germ line genes, all associated with Vlambda3 genes. Interestingly, the CDR3 regions of all moPhabs were remarkably similar, though not identical. In light of the common CDR3 usage, the epitopes recognized on TSH-R appeared to be restricted to amino acids residues 405-411 and 357-364. In summary, our results show that semisynthetic libraries may be limited in isolating human monoclonal antibodies that resemble pathogenic antithyrotropin receptor autoantibodies present in patients with GD. It is likely that until preparations of purified TSH-R that can be recognized by patients autoantibodies become available, similar to the recently described glycosylphosphatidylinositol (GPI) anchored TSH-R ectodomain, monoclonal antibodies from phage antibody display to TSH-R will be limited for isolating the rare, pathogenic antibodies of GD.

The luteinizing hormone receptor activates phospholipase C via preferential coupling to Gi2

Binding of lutropin/choriogonadotropin (LH/CG) to its cognate receptor results in the activation of adenylyl cyclase and phospholipase C. This divergent signaling of the LH receptor is based on the independent activation of distinct G protein subfamilies, i.e. , Gs, Gi, and potentially also Gq. To examine the selectivity of LH receptor coupling to phospholipase C beta-activating G proteins, we used an in vivo reconstitution system based on the coexpression of the LH receptor and different G proteins in baculovirus-infected insect cells. In this paper, we describe a refined expression strategy for the LH receptor in insect cells. The receptor protein was inserted into the cell membrane at an expression level of 0.8 pmol/mg of membrane protein. Sf9 cells expressing the LH receptor responded to hCG challenge with a concentration-dependent accumulation of intracellular cAMP (EC50 = 630 nM) but not of inositol phosphates, whereas stimulation of the histamine H1 receptor in Sf9 cells led to increased phospholipase C (PLC) activity. Immunoblotting experiments using G protein-specific antisera revealed the absence of quantitative amounts of alpha i in Sf9 cells, whereas alpha s and alpha q/11 were detected. We therefore attempted to restore the hCG-dependent PLC activation by infection of Sf9 cells with viruses encoding the LH receptor and different G protein alpha subunits. HCG stimulation of cells coexpressing the LH receptor and exogenous alpha i2 resulted in stimulation of PLC activity. In cells coinfected with an alpha i3-baculovirus, hCG challenge led to a minor activation of PLC, whereas no hCG-dependent PLC stimulation was observed in cells coexpressing alpha i1. Most notably, coinfection with baculoviruses encoding alpha q or alpha 11 did not reproduce the PLC activation by the LH receptor. Thus, the murine LH receptor activates adenylyl cyclase via Gs and PLC via selective coupling to Gi2.

Selective binding of thyrotropin receptor autoantibodies to recombinant extracellular domain of thyrotropin/lutropin-chorionic gonadotropin receptor chimeric proteins

The extracellular domain of the glycosylated human thyrotropin receptor (ET-gp) contains epitopes that can adsorb pathogenic antibodies from sera of patients with Graves' disease (GD). In an attempt to define the regions within the ETSHR with which autoantibodies interact, we expressed extracellular domains of eight thyrotropin receptor/chorionic gonadotropin receptor (TSHR/LH-CGR) chimeric proteins in insect cells. The levels of expression were high and chimeric proteins were glycosylated. Chimeric proteins designated as EMc2+4 and EMc2+3+4, in which amino acids (aa) 90-165 and 261-370, and aa 90-370, respectively, of TSHR were replaced with corresponding aa of LH-CGR, partially reversed the thyrotropin binding inhibitory immunoglobulin (TBII) activity of experimental anti-TSHR antisera (anti-ET-gp). The other six chimeras almost completely reversed the TBII activity of these anti-ET-GP antisera. Next, we tested the ability of these chimeric proteins to reverse the TBII activity of GD patients' sera. Similar to our earlier study, ET-gp protein reversed the TBII activity of all eight GD patients' sera tested. Chimera EMc2, in which aa 90-165 of TSHR has been replaced with corresponding aa of LH-CGR, and EMc2+4 partially reversed the TBII activity of only three of the eight GD patients' sera. However, the other six chimeric proteins failed to neutralize the TBII activity of any of GD patients' sera. These data showed the following: (1) There is considerable heterogeneity amongst autoantibodies in GD patients' sera, (2) The TBII activity of some, but not others, is dependent on aa 90-165 and 261-370, and (3) Most Graves' sera, with TBII activity, failed to react with chimeric proteins in which either N-terminal or C-terminal regions of the extra cellular domain of the TSHR were replaced with corresponding regions of LH-CGR. These results suggest that the TBII activity of GD patients' sera is dependent on conformational epitopes and replacement of certain regions of TSHR with homologous regions of LH-CGR results in sufficient alteration in the conformation of the protein leading to loss of reactivity.

Differences in the expression and localization of human melanotransferrin in lepidopteran and dipteran insect cell lines

The ability of several lepidopteran and dipteran insect cell lines to express human melanotransferrin (p97), a glycosyl phosphatidylinositol (GPI)-anchored, iron-binding sialoglycoprotein, was assessed. Spodoptera frugiperda-derived (Sf9) cell lines, transformed with the p97 gene under control of a baculovirus immediate-early promoter, were able to constitutively express the protein and correctly attach it to the outer cell membrane via a GPI anchor as demonstrated by PI-PLC treatment. In contrast, stable constitutive expression could not be demonstrated with cell lines derived from either Drosophila melanogaster (Kc1 or SL2) or Lymantria dispar (Ld652Y) despite the observation that p97 could be detected in transient expression assays. This may indicate that the long-term expression and accumulation of p97 is inhibitory to Drosophila cells, possibly due to improper localization of the protein and resultant competition for cellular iron. In stably transformed Sf9 cells, p97 was expressed on the cell at a maximal level of 0.18 microg/10(6) cells and was secreted at a maximal rate of 9.03 ng/10(6) cells/h. This level was comparable to the amount expressed with the baculovirus system (0.37 microg/10(6) cells and 31.2 ng/10(6) cells/h) and transformed CHO cells (0.88 microg/10(6) cells and 7.8 ng/10(6) cells/h). Deletion of the GPI cleavage/attachment site resulted in an eightfold increase in the secretion rate of p97, when compared to the intact construct suggesting that the rate-limiting step involves processing of the GPI anchor.

Glycosylated ectodomain of the human thyrotropin receptor induces antibodies capable of reacting with multiple blocking antibody epitopes

Recently, we showed that the glycosylated ectodomain of the human thyrotropin receptor (hET-gp) reacts with autoantibodies from autoimmune thyroid disease (AITD) patients' sera. To better understand the effects of glycosylation of thyrotropin receptor (TSHR) in antibody induction, we immunized rabbits with hET-gp protein. The rabbits developed relatively high titers of antibodies with highly potent TSH binding inhibitory immunoglobulin (TBII) and thyroid stimulatory blocking antibody (TSBAb) activities. Both the hET-gp and a nonglycosylated ectodomain of the human TSHR (hETSHR) protein significantly reversed the TBII as well as TSBAb activity. Based on the ability of synthetic peptides to significantly reverse the functional activity of these rabbit antisera, we identified three discrete regions of the TSH R, represented by amino acids 202-221, 292-311 and 367-386, as TBII epitopes and four regions represented by amino acids 352-371, 367-386, 382-401 and 392-415 as TSBAb epitopes. These data demonstrate that rabbit antibodies that bind to amino acids 367-386 mediate their TSBAb activity by inhibiting the binding of TSH to TSHR; whereas, antibodies to regions 352-415, excluding aa 367-386, exert their TSBAb activity by affecting a step subsequent to TSH binding. Coincident with the elevation of TBII and TSBAb activity, serum total T4 levels declined and thus suggested that the antibodies exerted functional effects on thyroid in vivo. Together, these data demonstrate that glycosylated hET-gp protein is a more potent immunogen and it can induce a broader antibody response directed against multiple TBII and TSBAb epitopes.

Critical relationship between glycosylation of recombinant lutropin receptor ectodomain and its secretion from baculovirus-infected insect cells

The lutropin receptor ectodomain overexpressed under the control of the powerful polyhedrin promoter in baculovirus-infected Sf9 insect cells, is mainly found in an inactive, intracellularly-aggregated form. It is secreted in an active form under the control of the P10 promoter, a somewhat weaker and earlier promoter, at the price of a lower production. The apparent molecular masses of the two species encoded by the same cDNA are 48 kDa and 60-68 kDa, respectively. The relationship between the extent and type of glycosylation and the extracellular targeting for the recombinant lutropin receptor ectodomains was investigated precisely with endoglycosidases, lectins of various specificities, and a glycosylation inhibitor, and tested with monoclonal and polyclonal antibodies. The results indicate that the strong polyhedrin promoter probably overwhelms the processing capacity of the ER in Sf9 cells, so that only a high-mannose precursor is expressed in large amounts. Only a minute amount of protein is secreted, which has been processed by Sf9 exoglycosidases/glycosyltransferases and bears complex/hybrid oligosaccharides. The weaker P10 promoter allows secretion of a mature and active receptor ectodomain, bearing complex glycosylation. An important O-linked glycosylation is also added post-translationally on this species. In particular, beta-galactose and sialic acid residues were specifically detected in the secreted species, evidence of the induction of the corresponding glycosyltransferases or of their genes. These results suggest that Sf9 cells should eventually be engineered with chaperones and glycosyltransferases in order to improve the production of demanding glycoproteins such as the porcine lutropin ectodomain, so as to open the way to resolution of the three-dimensional structures of these receptors.

Protein glycosylation: implications for in vivo functions and therapeutic applications

The glycosylation machinery in eukaryotic cells is available to all proteins that enter the secretory pathway. There is a growing interest in diseases caused by defective glycosylation, and in therapeutic glycoproteins produced through recombinant DNA technology route. The choice of a bioprocess for commercial production of recombinant glycoprotein is determined by a variety of factors, such as intrinsic biological properties of the protein being expressed and the purpose for which it is intended, and also the economic target. This review summarizes recent development and understanding related to synthesis of glycans, their functions, diseases, and various expression systems and characterization of glycans. The second section covers processing of N- and O-glycans and the factors that regulate protein glycosylation. The third section deals with in vivo functions of protein glycosylation, which includes protein folding and stability, receptor functioning, cell adhesion and signal transduction. Malfunctioning of glycosylation machinery and the resultant diseases are the subject of the fourth section. The next section covers the various expression systems exploited for the glycoproteins: it includes yeasts, mammalian cells, insect cells, plants and an amoeboid organism. Biopharmaceutical properties of therapeutic proteins are discussed in the sixth section. In vitro protein glycosylation and the characterization of glycan structures are the subject matters for the last two sections, respectively.

Different bioactivities of human thyrotropin receptors with different signal peptides

For investigation of the mechanism and pathogenesis of Graves' disease, availability of a large amount of functional human thyrotropin receptor (TSHR) capable of recognition by Graves' autoantibodies is essential. Many attempts have been made to produce the extracellular domain of TSH receptor (TSHRE) in a baculovirus expression system. However, the receptor is expressed as an insoluble form and the refolded protein is often not recognized by the autoantibodies. In this study, we found that the TSHRE expressed with its own signal peptide (VL3-RE) in insect cells is retained inside of the cells and found in both soluble and insoluble fractions in equal proportion. The signal peptide is not removed. The receptor in the soluble fraction is not recognized by either TSH or Graves' autoantibodies. The TSHRE with an insect-specific mellitin signal peptide (Mel-RE) is also retained inside of the cell and found in both the soluble and insoluble fractions in equal proportion. However, the signal peptide is removed and the receptor is recognized by the Graves' autoantibodies but not by TSH. Also, the amount of Mel-RE expressed was 5-10-fold higher than VL3-RE. The two receptor preparations apparently have the same degree of glycosylation as evidenced by the same increased mass (approximately 15 kDa) due to glycosylation. However, the two receptors have different affinity for an anion-exchange resin and different pI. Deglycosylated receptors have the same pI. This suggests that the composition of sugars may be different. Taken together, the results suggest that the two receptors are modified and folded differently by different pathways due to the presence of different signal peptides. Use of an insect-specific signal peptide is recommended for expression of TSHR that is recognized by Graves' autoantibodies in a baculovirus system.

Production of bioactive amino-terminal domain of the thyrotropin receptor via insertion in the plasma membrane by a glycosylphosphatidylinositol anchor

A chimeric cDNA construct encoding the extracellular amino-terminal domain (ECD) of the thyrotropin receptor fused to the signal for addition of glycosylphosphatidylinositol from the Thy-1 gene directs efficient expression of the ECD at the plasma membrane of transfected CHO cells. A cell line (GT14) expressing over 106 receptors/cell was isolated, which allows direct detection, by flow cytometry, of autoantibodies from the majority of patients with Graves' disease or autoimmune idiopathic myxedema. Treatment of GT14 cells with a glycosylphosphatidylinositol-specific phospholipase C (PI-PLC) releases a soluble 80 kDa molecule which neutralizes the autoantibodies from Graves patients. Whereas it does not bind TSH when released from the cells by PI-PLC in free form, the soluble ECD displays clear TSH binding activity when it is released as a complex with a monoclonal antibody recognizing a conformational epitope of the ECD. Our results allow production of bioactive ECD of the thyrotropin receptor in high yield, with possible applications in structural analyses.

Effect of promoters and signal sequences on the production of secreted HIV-1 gp120 protein in the baculovirus system

We compared insect cell production levels of secreted HIV-1 gp120 glycoprotein encoded by five different baculovirus expression constructs. Combinations consisting of one of two baculovirus promoters (very late or hybrid late/very late) and one of three different signal sequences [human tissue plasminogen activator (tpa), human placental alkaline phosphatase (pap), or baculovirus envelope glycoprotein (gp67)] were constructed. Production of secreted gp120 from these constructs was analyzed in two enzyme-linked immunosorbent assay formats, one detecting the total amount of secreted gp120 protein and the other measuring the level of "active" gp120 (as defined by the ability to bind to CD4). We found that for all of the constructs, approximately 50 to 90% of the secreted gp120 protein was active. Furthermore, our results indicated that expression from either promoter yielded comparable production of secreted protein, despite the fact that transcription from the hybrid promoter begins at an earlier time. By contrast, the signal sequence had a much greater effect on the levels of secreted gp120: the tpa leader yielded the highest level of secreted protein, followed by the gp67 and pap sequences. This result suggests that transcription is not a limiting factor in the production of secreted gp120, but rather that downstream processing of the protein is more critical. Furthermore, these results confirm the notion that the "optimal" signal sequence is protein dependent and that an insect-derived signal sequence is not optimal in all cases.

Expression and recovery of functional G-protein-coupled receptors using baculovirus expression systems

Baculovirus expression systems have been used for more than ten years as the tool of choice to over-express G-protein-coupled receptors. Although this expression system has also been used to study the signaling mechanisms of the receptors at the cellular level, it was found to be a most useful method to produce large quantities of receptors for biochemical and biophysical studies. Methods that allow easy and selective recovery of properly folded and mature receptors in viral particles open new perspectives for such applications.

Expression and characterization of human glutamate-cysteine ligase

Glutamate-cysteine ligase (GLCL) catalyzes the rate-limiting step in glutathione biosynthesis. GLCL comprises regulatory (GLCLR) and catalytic (GLCLC) subunits. To understand better the structure-function relationship of GLCL subunits and holoenzyme, human GLCLR and GLCLC genes were inserted into the baculovirus genome. Recombinant hGLCLR andhGLCLC were produced in cells infected with recombinant baculoviruses, and homogeneous hGLCL subunits and holoenzyme were purified from cell lysates with a Ni-NTA resin. Purified recombinant hGLCL holoenzyme was catalytically more active than hGLCLC with L-glutamate, L-alpha-aminobutyrate, and ATP as substrates. The selectivity of purified hGLCL holoenzyme for L-glutamate, L-alpha-aminobutyrate, or L-cysteine was significantly higher than for hGLCLC. Glutathione was a noncompetitive inhibitor for both hGLCL holoenzyme and hGLCLC. hGLCLC was more sensitive to inhibition by glutathione than hGLCL holoenzyme. Deletion of the first 25 amino acid residues at the amino terminus of GLCLC dramatically decreased GLCL activity, indicating that the amino terminus of GLCLC is required for full catalytic activity. Expressed and purified hGLCL provides a useful tool to investigate glutathione biosynthesis in vitro.

Production of the thyrotrophin receptor extracellular domain as a glycosylphosphatidylinositol-anchored membrane protein and its interaction with thyrotrophin and autoantibodies

The thyrotrophin (TSH) receptor (TSHR) is synthesized as a single polypeptide with a predicted large extracellular domain (ECD), a seven-transmembrane pass region and a C-terminal intracellular tail. It is a common target for production of autoantibodies. To investigate whether the ECD is solely responsible for ligand interaction, we directed the expression of this domain in isolation on the cell surface by means of a glycosylphosphatidylinositol (GPI) anchor sequence. Immunoblotting detected TSHR material of Mr 70,000 expressed at high levels. In immunoprecipitation studies, the GPI-anchored ECD was recognized by experimental and pathological antibodies. The molecule was detected on the cell surface by flow cytofluorimetry at up to 10-fold higher amounts than the highest expressing full-length receptor clone. Radioligand binding studies confirmed this and showed that the recombinant molecule bound TSH with high affinity similar to full-length receptor; however, studies with human autoimmune sera indicated differences in the degree of inhibition when compared with full-length receptor. The existence of the GPI anchor was confirmed by cleavage with a GPI-specific phospholipase C and biosynthetic labeling with [3H]ethanolamine. TSHR material was also present inside the cell in both soluble and membrane-bound forms. Thus, the recombinant GPI-anchored ECD is the smallest known fragment of the TSHR that retains high-affinity TSH binding and is expressed at high levels on the cell surface as well as internally; this approach may well be useful for other membrane proteins.

Genetic Immunization Against the Human Thyrotropin Receptor Causes Thyroiditis and Allows Production of Monoclonal Antibodies Recognizing the Native Receptor

The generation of Abs recognizing the native structure of the human thyrotropin receptor (hTSHR) has been difficult because there is currently no method allowing the purification of correctly folded Ag in the amounts required by classical immunization protocols. The majority of Abs made against the hTSHR react preferentially with denatured molecules. We report that a humoral response against the native hTSHR, compatible with mAb production, is elicited in mice by immunization with a DNA construct encoding the receptor. BALB/c mice were inoculated in the anterior tibialis muscle with 100 μg of plasmid DNA harboring the hTSHR cDNA. Eleven weeks after the first injection, 10 mice of 14 showed by FACS analysis a strong IgG response against the hTSHR expressed at the surface of Chinese hamster ovary cells. A clear TSH-binding inhibiting Ig and thyrotropin-blocking Ab activity (competition with TSH binding and TSH activity, respectively) was demonstrated in the majority of sera tested. One serum exhibited a clear stimulating activity. Despite the maintenance of normal circulating free T4 levels in all mice, these bioactivities persisted until 18 wk, in which mice were sacrificed, their thyroids were examined histologically, and spleens from two animals were used for mAb production. All mice displayed a severe lymphocytic infiltration of their thyroids, composed mostly of activated B cells. Three mAbs were produced against conformational epitopes of the hTSHR. We conclude that genetic immunization is an efficient method of generating Abs recognizing the native structure of the hTSHR and a new way of inducing thyroiditis in mice murine.

A series of broad host range shuttle vectors for constitutive and inducible expression of heterologous proteins in insect cell lines

A series of shuttle vectors have been constructed that allow expression of heterologous proteins in either dipteran or lepidopteran insect cell lines. Constitutive expression in a broad range of host cells is mediated by the Orgyia pseudotsugata multicapsid nucleopolyhedrosis virus (OpMNPV) immediate-early 2 (ie2) promoter. Alternatively, if inducible expression is required, for example to express cytotoxic proteins, a vector has been constructed that uses the Drosophila metallothionein (Mtn) promoter for metal-inducible protein expression in dipteran cell lines. A chimeric synthetic bacterial-OpMNPV ie promoter-Zeocin resistance gene cassette has been included to facilitate cloning in E. coli as well as the generation of stably transformed insect cell lines. The utility of the system is demonstrated by the constitutive and inducible expression of the highly processed glycosylphosphatidylinositol-anchored glycoprotein, human melanotransferrin, in transformed insect cell lines.

C-Mannosylation of human RNase 2 is an intracellular process performed by a variety of cultured cells

C2-alpha-Mannosyltryptophan was discovered in RNase 2 from human urine, representing a novel way of attaching carbohydrate to a protein. Here, we have addressed two questions related to the biosynthesis of this modification: (i) is C-mannosylation part of the normal intracellular biosynthetic route, and (ii) how general is it, i.e. which organisms perform this kind of glycosylation? To answer the first question, RNase 2, which is identical to the eosinophil-derived neurotoxin, was isolated from intracellular stores of cultured human HL-60 cells. The enzyme was C-mannosylated at Trp-7, showing that the modification occurs intracellularly, before secretion of the protein. The second question was investigated by immunological and chemical analysis of RNase 2 purified from the supernatant of transiently transformed cells from different organisms. This revealed that C-mannosylation occurs in cells from man, green monkey, pig, mouse, and hamster. The observation that pig kidney cells contain the machinery for C-mannosylation of Trp-7 of human RNase 2 but that the homologous RNase from porcine kidney is not a substrate, since it does not contain a tryptophan at position 7, strongly suggests that C-mannosylated proteins other than RNase 2 exist. Recombinant RNase 2 isolated from insect cells, plant protoplasts, and Escherichia coli was not C-mannosylated. These results not only form the basis for further studies on the biochemical aspects of C-mannosylation but also have implications for the choice of cells for production of recombinant glycoproteins.

Understanding the thyrotropin receptor function-structure relationship

The thyrotropin (TSH) receptor (TSHR) is a key protein in the control of thyroid function and a major thyroid autoantigen. Recently, molecular cloning of the receptor has been carried out and we now review the impact of this work on our understanding of the physiology and pathophysiology of the TSHR. Analysis of recombinant TSHR proteins expressed in prokaryotic and eukaryotic systems has indicated that post-translational processing is important for the formation of active receptors. Studies of TSHR glycosylation have shown that a 'mature' form of the receptor containing mainly complex-type sugar residues is principally involved in TSH and TSHR autoantibody (TRAb) binding. In addition, the processing of the TSHR peptide chain into two subunits observed with native TSHR has been confirmed using recombinant TSHR. However, despite considerable efforts in many laboratories, the binding site(s) for TSH and TRAb on the TSHR have not been well characterized as yet and lessons learned from the discovery of naturally occurring amino acid mutations of the TSHR confirm the complexity of the hormone and autoantibody binding sites. Future progress in producing large amounts of pure TSHR as well as monoclonal TRAbs, followed by crystallographic analysis of TSHR-TSH complexes and TSHR-TRAb complexes, should be helpful in providing a better insight into the relationship between TSHR structure and function.

Thyrotropin-receptor-mediated diseases: a paradigm for receptor autoimmunity

Autoantibodies to the thyrotropin receptor (TSHR) can act as thyrotropin agonists or antagonists, or can cause thyroid hypertrophy. Neither the autoantibody-binding sites on the TSHR nor the intracellular mechanisms by which the autoantibodies mediate their diverse functional effects are completely understood. This article reviews how cloning of the TSHR has contributed to our understanding of its structure and function, and has allowed induction of experimental autoimmunity to the TSHR.

Engineering the human thyrotropin receptor ectodomain from a non-secreted form to a secreted, highly immunoreactive glycoprotein that neutralizes autoantibodies in Graves' patients' sera

Previous attempts to generate autoantibody-reactive, secreted thyrotropin receptor (TSHR) ectodomain in mammalian cells have failed because of retention within the cell of material with immature carbohydrate. We have overcome this difficulty by performing progressive carboxyl-terminal truncations of the human TSHR ectodomain (418 amino acid residues including signal peptide). Three ectodomain variants (TSHR-261, TSHR-289, and TSHR-309) were truncated at residues 261, 289, and 309, respectively. Unlike the full ectodomain, ectodomain variants were secreted with an efficiency inversely proportional to their size. Secreted ectodomain variants contained approximately 20 kDa of complex carbohydrate. TSHR-261 was chosen for further study because it was secreted very efficiently and neutralized autoantibodies in Graves' patients' sera. This ectodomain variant was partially purified using sequential lectin and nickel-chelate chromatography, permitting the first direct visualization and quantitation of the mammalian TSHR. Most important, very small (nanogram) quantities of this material neutralized 70-100% of TSHR autoantibody activity in all 18 Graves' sera studied. In summary, carboxyl-terminal truncation of the human TSHR ectodomain generates a secreted protein with complex carbohydrate that neutralizes autoantibodies in Graves' patients' sera. Antigenically active TSHR will be valuable for future studies on the diagnosis, pathogenesis, and immunotherapy of Graves' disease.

Folding-dependent binding of thyrotropin (TSH) and TSH receptor autoantibodies to the murine TSH receptor ectodomain

The mouse TSH receptor ectodomain (mTSHR-ecd) was amplified from murine thyroid complementary DNA and ligated into the pAcGP67B insect cell vector, and the nucleotide sequence was confirmed. Employing a baculovirus-insect cell system, the mTSHR-ecd (amino acids 22-415) was expressed as a fusion protein with the gp67 insect cell signal sequence at the NH2-terminus and a C-terminal six-histidine tag. Protein expression was assessed by Western blot using a murine monoclonal antibody (recognizing amino acids 22-35) and a rabbit antipeptide antibody (recognizing amino acids 397-415). These antibodies detected two principal species of mTSHR-ecd, one glycosylated (66 kDa) and one nonglycosylated (52 kDa), in cell lysates of infected insect cells. More than 10% of these species were present in a water-soluble (cytosolic) fraction. This fraction was then used to purify, under native conditions, 100-microg amounts of mTSHR-ecd using nickel-nitrilo-triacetic (Ni-NTA) resin chromatography. The purified cytosolic mTSHR-ecd migrated as a homogeneous 66-kDa band visible on Coomassie blue-stained gels and was confirmed by Western blotting. We also purified the mTSHR-ecd from total cell lysates under denaturing conditions, followed by "in vitro" refolding on the Ni-NTA column. Under these conditions, milligram amounts of soluble mTSHR-ecd were obtained. This material consisted primarily of the 66-kDa glycosylated form, but in addition contained four or five lower molecular mass, partially glycosylated intermediates and the 52-kDa nonglycosylated form. Deglycosylation with either endoglycosidase F or H, reduced all mTSHR-ecd glycosylated species to a 52-kDa nonglycosylated form. Both the cytosolic and refolded mTSHR-ecd preparations inhibited the binding of [125I]TSH to the full-length human TSHR expressed in Chinese hamster ovary cells in a dose-dependent manner, with similar affinities. The affinity of such interactions was 3 orders of magnitude less than observed with native porcine TSHR and was further reduced by unfolding the mTSHR-ecd preparations. The cytosolic and refolded mTSHR-ecd were also recognized by hTSHR autoantibodies in the serum of patients with hyperthyroid Graves' disease. Such autoantibody binding to mTSHR-ecd was also markedly reduced by unfolding the antigen. These results demonstrated the successful production of large quantities of well characterized, biologically active, mTSHR-ecd antigen. In addition, the data showed that although the ectodomain of the mTSHR bound TSH, intact holoreceptor may be required for high affinity ligand binding. Whether the transmembrane region is required for direct ligand binding, as seen for other G protein-linked receptors, or whether it is needed to stabilize the ligand binding to the ectodomain and maintain a correctly folded state, remains unclear.

Baculovirus vectors for expression in insect cells

Recombinant baculoviruses now represent a mature technology in which vector development, particularly for the control of expression level, has reached a plateau. However, other aspects of expression, such as the production of multiple proteins, improved product purification or maximizing protein processing, remain areas for novel vector and host cell development. This year has seen these topics come to the fore in descriptions of new expression systems.

Photoaffinity labeling of the human brain cholecystokinin receptor overexpressed in insect cells. Solubilization, deglycosylation and purification

The human cholecystokinin B (CCKB) receptor was expressed in Sf9 cells by infection with recombinant baculovirus. For immunodetection a c-myc epitope tag (EQKLISEEDL) was fused at the amino-terminus of the CCKB receptor. In a second construct an additional hexa-histidine tag was introduced at the C-terminus of the CCKB receptor to enable employment of metal affinity chromatography. The two receptor constructs were expressed at densities of 6.0 +/- 1.1 pmol/mg protein and 7.2 +/- 1.1 pmol/mg protein, respectively which are 100-200-fold higher compared with the receptor amounts found in natural sources. Saturation of the binding sites with [3H]propionyl-CCK8 revealed Kd values of 4.5 +/- 0.5 nM and 7.8 +/- 0.6 nM for the CCKB receptor without or with histidine tag. In SDS/PAGE and subsequent immunodetection the histidine-tagged CCKB receptor migrated as a 55-kDa band, whereas the CCKB receptor without C-terminal modification revealed apparent molecular masses of 45 kDa and 49 kDa. The differences in the mass values observed for the two constructs suggest that the histidine tag could protect the CCKB receptor against proteolytical degradation from its C-terminus. Furthermore two new photoreactive derivatives of cholecystokinin octapeptide residues 26-33 (CCK8) with high labeling efficiency and specificity for the cholecystokinin receptor subtype B were developed: [3H]BzBz-des-Met28-[p-NH2Bz29]-CCK8 and [3H]BzBz-biotinyl-des-Met28-[p-NH2Bz29]-CCK8. Both contain the p-benzoyl-benzoyl (BzBz) residue at the N-terminus for photoactivation and a p-aminobenzoyl (p-NH2BZ) residue instead of Met28-Gly29 in cholecystokinin. Enzymatic deglycosylation of the CCKB receptor with N-glycosidase F after photoaffinity labeling demonstrated that the CCKB receptor with three potential glycosylation sites was slightly glycosylated, amounting to a molecular mass of about 4 kDa. Using the biotinylated cholecystokinin derivative the photoaffinity-labeled CCKB receptor could be purified 1260-fold by a two-step procedure including affinity chromatography on a streptavidin/avidin agarose matrix. For purification of the native receptor, an improved solubilization protocol for the CCKB receptor using dodecyl beta-D-maltopyranoside was developed. The solubilized CCKB receptors with C-terminal histidine tag retained their ligand binding characteristics after chromatography on a nickel affinity matrix.