Previous Article | Next Article ![]()
Clinical and Diagnostic Laboratory Immunology, September 2004, p. 874-878, Vol. 11, No. 5
1071-412X/04/$08.00+0 DOI: 10.1128/CDLI.11.5.874-878.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Department of Parasitology and Institute of Tropical Medicine,1 Department of Internal Medicine and Institute of Allergy, Brain Korea 21 Project for Medical Science, Yonsei University, College of Medicine, Seoul, Korea2
Received 29 April 2004/ Returned for modification 15 June 2004/ Accepted 28 June 2004
|
|
|---|
|
|
|---|
At least 18 different isoforms are known to be generated by alternative RNA splicing in mammalian cells. The synthesis of isoforms is developmentally regulated, and cells from different embryonic lineages express different isoforms (26). The alternate exon splicing patterns of Drosophila melanogaster were reported to involve 27 amino acids at the C terminus (3), which frequently contain IgE-binding regions (24). Specifically, eight different IgE-binding epitopes were identified in the American cockroach tropomyosin (Per a 7) by using a set of overlapping synthetic peptides (1).
The amino acid sequence diversity of individual allergens has been described in wild or cultured house dust mites (5, 29, 30, 32, 35) or storage mites (16). Small changes in the amino acid sequences of given allergens can influence their allergenicities (10). For example, certain natural isoforms of Bet v 1, the major birch pollen allergen, were found to have high T-cell reactivities and low or no IgE-binding activities (21). Analysis of these isoforms may lead us to a better understanding of the different allergenicities of many invertebrate tropomyosins and the development of immunotherapeutic strategies and products, such as hypoallergenic (low IgE-binding activity) products.
We have previously isolated the cDNA encoding German cockroach tropomyosin (15) and named it Bla g 7, according to the guidelines of the International Union of Immunological Societies Allergen Nomenclature Subcommittee (17). Recombinant tropomyosin expressed in Escherichia coli showed low levels of IgE-binding reactivity. Recombinant tropomyosin was also expressed as a nonfusion protein in Pichia pastoris, and its IgE reactivity was compared with that of its native counterpart. The structural differences of native and recombinant proteins did not seem to influence significantly the IgE reactivities of tropomyosins (14).
In order to better understand the different allergenicities of German cockroach tropomyosin, the cDNA sequence variations in German cockroach tropomyosin were investigated by reverse transcriptase PCR (RT-PCR). Fragmented recombinant proteins were also produced, and their IgE-binding reactivities were examined.
|
|
|---|
Two-dimensional gel electrophoresis and immunoblotting. Two-dimensional gel electrophoresis was performed with precast gels (Invitrogen, Carsbad, Calif.), according to the instructions of the manufacturer. Cockroach extract was prepared as described previously (15). Fifty micrograms of whole-body extracts mixed with an equal volume of the sample buffer was loaded into the first-dimension gel. After isoelectric focusing (pH 3 to 10), second-dimension gel electrophoresis was carried out in a 4 to 20% gradient polyacrylamide gel containing sodium dodecyl sulfate. The proteins were then electrophoretically transferred onto a nitrocellulose membrane (pore size, 0.45 µm; Osmonics, Westborough, Mass.). After the membrane was blocked overnight with 3% skim milk, it was incubated for 1 h with mouse anti-recombinant Bla g 7 serum. The blots were then incubated with goat anti-mouse IgG conjugated with alkaline phosphatase (Sigma, St. Louis, Mo.) for 1 h at room temperature and developed in a substrate solution of Nitro Blue Tetrazolium and 3-bromo-4-chloro-5-indolyl-phosphate (Promega, Madison, Wis.).
RT-PCR. cDNA encoding tropomyosin was amplified by using high-fidelity Pfu DNA polymerase (Stratagene, La Jolla, Calif.). A total of 150 mg of a German cockroach was pulverized in liquid nitrogen with a mortar and pestle, and total RNA was extracted by using the TRIzol reagent (Invitrogen), according to the manual prepared by the manufacturer. Reverse transcription was initiated with 6 µg of total RNA and an oligo(dT) (18 T residues) primer. Five microliters of single-stranded cDNA was used for each reaction. The primer sequences used were as follows: forward primer, 5'-ATGGATGCCATCAAGAAGAAG-3'; reverse primer, 5'-GTTTAGTTGCCAATAAGTTCGG-3'. The cDNA encoding tropomyosin from different cockroach species (Periplaneta fuliginosa), as well as Blattella germanica, was successfully cloned by RT-PCR with this specific primer set (13, 15). PCR was performed as follows: after an initial denaturation (5 min at 95°C), the samples were subjected to 35 cycles of amplification, each of which consisted of 1 min at 95°C, 1 min at 55°C, and 2 min at 72°C. The final extension was performed at 72°C for 8 min. The DNA fragment so obtained was cloned into a pPCR Script Amp SK (Stratagene) vector.
Nucleotide sequence determination. A ThermoSequenase kit (Amersham Life Science, Cleveland, Ohio) was used for nucleotide sequence determination. Reaction mixtures were run on a Long ReadIR 4200 DNA sequencer (LI-COR, Inc., Lincoln, Nebr.). All reactions (both forward and reverse) were performed in duplicate.
Generation of fragments by PCR amplification. For epitope analysis, rBla g 7 was divided into five fragments containing 50 overlapping amino acids, i.e., fragments A (residues 1 to 100), B (residues 51 to 150), C (residues 101 to 200), D (residues 151 to 250), and E (residues 201 to 284) (Fig. 1). The oligonucleotides used in the PCR are listed in Table 1. Each cDNA fragment was amplified by PCR and ligated into the pGEM-T Easy vector (Promega). The cDNA of Bla g 7 cloned in pET-28b was used as the template, and restriction enzyme cleavage sites were incorporated into each oligonucleotide primer to create restriction sites for subcloning (BamHI for forward primers and XhoI for reverse primers). The PCR was carried out with an initial denaturation at 95°C for 5 min and then 35 cycles of amplification were done under following conditions: denaturation at 94°C for 30 s, annealing at 52°C for 30 s, and extension at 72°C for 1 min. The PCR product was cloned into the pGEM-T Easy vector (Promega) and, after restriction digestion, into the pET-28b vector. Recombinant proteins were expressed in E. coli BL21(DE3) and purified by Ni-NTA agarose (Qiagen) affinity column chromatography.
![]() View larger version (14K): [in a new window] |
FIG. 1. Schematic representation of tropomyosin fragments for epitope analysis of German cockroach tropomyosin.
|
|
View this table: [in a new window] |
TABLE 1. Sequences of oligonucleotides used for production of fragmented tropomyosin
|
IgE epitope analysis of subjects' sera. The reactivities of specific IgE antibodies to fragmented recombinant Bla g 7 were examined by ELISA with seven serum samples obtained from recombinant Bla g 7-positive patients. Purified recombinant proteins (0.2 µg/well) were coated (0.1 M sodium carbonate [pH 9.6]) onto a polystyrene microtiter plate (Corning, Corning, N.Y.) and incubated overnight at 4°C. After the plate was blocked with 3% skim milk for 1 h, the plate was washed with phosphate-buffered saline containing 0.05% Tween 20 and incubated for 1 h with human serum (1:4 dilution). After the plate was washed, IgE antibody was detected by using biotinylated goat anti-human IgE (epsilon chain specific; Vector, Burlingame, Calif.) diluted 1:1,000 with streptavidin-peroxidase (Sigma) diluted 1:1,000. The signal was developed by adding 3,3',5,5'-tetramethylbenzidine (Kirkegaard & Perry Laboratories, Gaithersburg, Md.), and the optical density at 450 nm was determined after the addition of 1% H2SO4 on an automatic microplate reader (Tecan, Salzburg, Austria). The mean absorbance level plus 2 standard deviations for the sera from eight healthy controls was used as the cutoff value.
|
|
|---|
![]() View larger version (45K): [in a new window] |
FIG. 2. Two-dimensional immunoblot analysis of German cockroach tropomyosin with mouse anti-rBla g 7 sera. The numbers on the left indicate the sizes of the molecular mass standards (in kilodaltons).
|
![]() View larger version (75K): [in a new window] |
FIG. 3. Amino acid sequence variations in German cockroach tropomyosin identified by RT-PCR. The frequency of each sequence is shown in parentheses. Shaded areas indicate the IgE-binding epitopes identified from P. aztecus tropomyosin.
|
IgE reactivities of recombinant peptides by ELISA. Recombinant fragments of Bla g 7 were expressed in E. coli and assayed for their IgE reactivities (Fig. 4). The IgE reactivities to intact Bla g 7 and recombinant proteins were determined by ELISA with Bla g 7-sensitized sera (Fig. 5) and are summarized in Table 2. The results obtained showed that IgE-binding regions were heterogeneously distributed among the different serum samples. The IgE antibodies from patients 5 and 6 were able to bind to all recombinant proteins tested, indicating that the serum of a single patient can recognize at least three different IgE-binding epitopes. IgE antibodies from patients 1, 4, 5, and 6 were able to recognize fragments A and B; and the IgE antibodies of patients 5, 6, and 7 were able to recognize fragment C. The IgE antibodies of patients 3, 4, 5, 6, and 7 recognized fragments D and E. The IgE antibody from patient 2 recognized fragment E only.
![]() View larger version (57K): [in a new window] |
FIG. 4. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of full-length and fragmented recombinant Bla g 7. Lanes: M, molecular mass standard; F, full-length fragment; A, fragment of 1 to 100 amino acid; B, fragment of 51 to 150 amino acids; C, fragment of 101 to 200 amino acids; D, fragment of 151 to 250 amino acids; E, fragment of 201 to 284 amino acids; S, bovine serum albumin.
|
![]() View larger version (29K): [in a new window] |
FIG. 5. Profiles of IgE antibody binding to recombinant Bla g 7 and relevant recombinant proteins obtained by ELISA. Dotted line, cutoff value (mean absorbance plus 2 standard deviations for the sera from eight healthy controls); 1 to 7, serum samples from seven allergic patients, respectively; N, Bla g 7-negative serum sample; B, buffer control.
|
|
View this table: [in a new window] |
TABLE 2. IgE-binding reactivities of peptide fragments of German cockroach tropomyosin
|
|
|
|---|
IgE is thought to be a key molecule in the mediation of many allergic diseases (22). It was reported that the IgE-binding capacity of the German cockroach extract was totally abolished by Atlantic shrimp extract, which was found to have strong IgE-binding components between 30 and 43 kDa (presumably tropomyosin) by IgE blot inhibition (6). However, in the previous study (6), recombinant German cockroach tropomyosin was able to inhibit only 32.4% of IgE binding to cockroach extract (15).
The first approach required in the study of the relationship between structure and allergenicity is epitope identification. At present the SPOTs system (Genosys, The Woodland, Tex.) and the Novatope system (Novagen, Madison, Wis.) are extensively used to identify IgE-binding epitopes, and the results obtained with the two systems have been extensively compared (25). Moreover, fragmented peptides are reported to have higher IgE-binding capacities than whole molecules in the case of paramyosin, Der f 11 (33). These were not real peptide fragments presented by professional phagocytes of the immune system; however, these peptide fragments could have been made by the proteolytic enzymes derived from mites or the cockroaches themselves.
In the present study, we tried to determine whether the low allergenicity of German cockroach tropomyosin is affected or not by amino acid sequence variations of its isoforms. For convenience, the German cockroach tropomyosin amino acid sequences deduced from RT-PCR analysis were compared with those of P. aztecus tropomyosin (Fig. 3). Only two variant German cockroach tropomyosins resulting from amino acid substitutions in the IgE epitope regions were different from P. aztecus tropomyosin, which is one of the well-studied tropomyosin molecules (1), and 11 different amino acid sequence variations were identified (Fig. 3). The IgE-binding reactivities of intact or fragmented Bla g 7 were analyzed to investigate IgE epitopes in the Korean patient population (Fig. 5). All sera tested showed different patterns of IgE reactivity. Analyses of IgE epitopes from different patient groups or tropomyosin from different organisms showed that the epitopes exhibited different IgE-binding regions (1, 12, 23, 27), which implies the presence of various epitope regions, which are influenced by genetic backgrounds and environmental factors. The structural basis for bending tropomyosin around actin filaments is attributed to the structural regularity of the molecule (31). The tropomyosin coiled coil consists of two
-helices, which are characterized by the occurrence of tandem (heptad) repeats (28). The structural regularity of tropomyosin may be a possible explanation for the existence of multiple IgE-binding epitopes. Specific immunotherapy is an efficient treatment for subjects with IgE-mediated allergic reactions. Studies of IgE epitopes have led to a better understanding of the mechanisms underlying successful immunotherapy and the proposed use of hypoallergenic forms of allergens for immunotherapy (9).
In conclusion, the low allergenicity of previously reported German cockroach tropomyosin does not seem to be due to amino acid sequence variations. The IgE-binding epitope regions were found to be distributed over the whole molecule. It is not advisable to use a fragment for diagnostic or therapeutic purposes in case of tropomyosin. Invertebrate tropomyosin could provide a molecular model for investigation of the genetic and environmental factors affecting sensitization and the onset of allergic disorders.
This work was supported by Korea Science and Engineering Foundation grant R01-2002-000-00243-0.
|
|
|---|
-tropomyosin ablation in mouse. Circ. Res. 82:116-123.
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Copyright © 2009 by the American Society for Microbiology. For an alternate route to Journals.ASM.org, visit: http://intl-journals.asm.org | More Info»