Recombinant tropomyosin from Penaeus aztecus (rPen a 1) for measurement of specific immunoglobulin E antibodies relevant in food allergy to crustaceans and other invertebrates

Immunoglobulin E (IgE)-mediated food allergy to crustaceans and mollusks is relatively common and affected individuals typically react to a range of different species. The only known major allergen of shrimp was first described over 20 years ago and later identified as the muscle protein tropomyosin. This protein may be useful as a defined and relevant diagnostic marker for allergic sensitization to invertebrate foods. In order to generate an assay reagent suitable for this purpose, tropomyosin from the shrimp Penaeus aztecus (Pen a 1) was produced as a recombinant protein in Escherichia coli and characterized with respect to IgE antibody binding properties in comparison to natural shrimp tropomyosin. Hexahistidine-tagged rPen a 1 accumulated as a predominantly soluble protein in the E. coli expression host and a two-step chromatographic procedure provided a high yield of pure and homogeneous protein. rPen a 1 displayed chromatographic and folding characteristics similar to those of purified natural shrimp tropomyosin. Serum preincubation with serial protein dilutions revealed similar capacity of recombinant and natural tropomyosin to compete with immobilized shrimp extract for IgE binding. rPen a 1 was further shown to extensively and specifically compete for IgE binding to extracts of other crustacean species, house dust mite and German cockroach.     

Cryo-electron microscopy of tropomyosin magnesium paracrystals

Magnesium paracrystals of tropomyosin have been examined in thin films of vitreous ice by electron microscopy. This material is radiation sensitive, with damage being located near the molecular ends. However, low-dose images show a repeating pattern of pairs of dark lines every 40 nm which indicates that the structure has been well preserved. These dark lines are separated by about 15 nm and probably represent extra protein density at the molecular ends. It has not been possible previously to see this feature by electron microscopy. We also demonstrate that the mercurial p-chloro-mercuribenzoate (PCMB) attached to the tropomyosin thiol at residue 190 can be detected in unstained paracrystals embedded in vitreous ice.     

Expression of tropomyosin alpha 4 chain is increased in esophageal squamous cell carcinoma as evidenced by proteomic profiling by two-dimensional electrophoresis and liquid chromatography-mass spectrometry/mass spectrometry

To identify proteins associated with esophageal carcinogenesis, we performed protein profiling of 16 esophageal squamous cell carcinomas (ESCCs) and paired noncancerous tissues by 2-DE and MS/MS. In cancerous tissues, three spots showed significant up-regulation in the amount of protein, while eight spots were significantly down-regulated. The identities of the spots were determined by PMF with LC-MS/MS and were confirmed by immunoblotting. The up-regulated proteins were tropomyosin alpha 4 chain, transgelin, and pyruvate kinase. The down-regulated proteins were serum albumin precursor, isoforms of annexin A1, tropomyosin beta chain, 14-3-3 protein sigma, and isoforms of serotransferrin precursor. In all 16 cases, up-regulation of the tropomyosin alpha 4 chain was confirmed by immunoblotting. Localization of the tropomyosin alpha 4 chain in ESCC cells and adjacent fibroblasts was confirmed by immunohistochemistry.     

Identification of cancer-associated proteins by proteomics and downregulation of β-tropomyosin expression in colorectal adenoma and cancer

Elucidating the molecular mechanism underlying the development of adenoma, the major precursor lesion of colorectal cancer (CRC), would provide a basis for early detection, prevention as well as treatment of CRC. Using the highly sensitive 2-D DIGE method coupled with MS, we identified 24 differentially expressed proteins in adenoma tissues compared with matched normal colonic mucosa and CRC tissues. Fifteen proteins were downregulated and three proteins were upregulated in adenoma tissues when compared with individual-matched normal colonic mucosa. Five proteins were downregulated, while one protein was upregulated in adenoma tissues when compared with matched CRC tissues. A protein, β-tropomyosin (TM-β), recently suggested to be a biomarker of esophageal squamous carcinoma, was downregulated in both adenoma and CRC tissues. Additionally, the reduction in the level of TM-β in adenoma and CRC tissues was further validated by Western blotting (p<0.05) and RT-PCR (p<0.001). Our findings suggest that downregulation of TM-β is involved in the early development of CRC and that differentially expressed proteins might serve as potential biomarkers for detection of CRC.     

Identification and characterisation of the major allergen of Chinese mitten crab (Eriocheir sinensis)

Tropomyosin (TM) from Chinese mitten crab (Eriocheir sinensis) was purified to homogeneity and TM genes were amplified from three species of crab (Chinese mitten crab, mud crab and swimming crab), respectively. Sequence analysis showed that all three cloned DNA fragments had open reading frames of 855 bp, predicted to encode proteins with 284 amino acid residues. Sequence alignment revealed that the three tropomyosins share high homology to tropomyosins from other crustaceans. Chinese mitten crab TM gene was further recombined with the vector of pGEX-4T-3 and expressed in Escherichia coli JM109. The expressed protein revealed a band of about 62 kDa on SDS–PAGE, suggesting the successful expression of glutathione S-transferase–tropomyosin (GST–TM) fusion protein. Immunoblotting analysis using sera from subjects with crustacean allergy confirmed that the expressed fusion protein reacted positively with these sera, indicating tropomyosin is a major allergen of Chinese mitten crab.     

Method for forming two-dimensional paracrystals of biological filaments on lipid monolayers

A method is described for electron microscopic observation of two-dimensional paracrystals on unsupported lipid monolayers. The method uses a hydrophobic holey C-coated grid placed on a monolayer made positively charged by the inclusion of stearylamine (SA) and has been used to align scallop thin filaments and reconstituted actin/tropomyosin filaments to form paracrystals. The use of unsupported monolayers allows the paracrystals to be viewed in either negative stain or with cryoelectron microscopy. Those paracrystals in frozen hydrated specimens have better order than those with negative stain. It was found that varying the lipid composition between the less fluid distearolyphosphotidylcholine/SA and the more fluid egg yolk phosphotidylcholine/SA alters the size and order of the paracrystals, the more fluid system having smaller, more ordered paracrystalline domains. The advantage of the technique for studying actin/thin filaments is the ability to form large two-dimensional paracrystals under physiological conditions of [Mg²⁺] and pH.     

Impact of heat processing on the detection of the major shellfish allergen tropomyosin in crustaceans and molluscs using specific monoclonal antibodies

The major heat-stable shellfish allergen, tropomyosin, demonstrates immunological cross-reactivity, making specific differentiation of crustaceans and molluscs for food labelling very difficult. The aim of this study was to evaluate the application of allergen-specific monoclonal antibodies in differential detection of shellfish-derived tropomyosin in 11 crustacean and 7 mollusc species, and to study the impact of heating on its detection. Cross-reactive tropomyosin was detected in all crustacean species, with partial detection in molluscs: mussels, scallops and snails but none in oyster, octopus and squid. Furthermore, we have demonstrated that heating of shellfish has a profound effect on tropomyosin detection. This was evident by the enhanced recognition of multiple tropomyosin variants in the analysed shellfish species. Specific monoclonal antibodies, targetting the N-terminal region of tropomyosin, must therefore be developed to differentiate tropomyosins in crustaceans and molluscs. This can help in correct food labelling practices and thus protection of consumers.     

Detection of tropomyosin and determination of proteins in crustacean oils

Tropomyosin is known to be the main allergen in crustaceans and the objective of this study was to investigate if this protein could be detected in commercial crustacean oils from Antarctic krill (Euphausia superba) and the zooplankton Calanus finmarchicus. We also examined the possibility of determining the protein content in the oils by direct amino acid analysis. Western blotting showed that a commercial antibody against shrimp tropomyosin cross-reacted with a protein of similar size in Antarctic krill and C. finmarchicus. The protein tentatively identified as tropomyosin, was also detected in krill oil products, but not in oils from C. finmarchicus. The acetone–heptane method used for extracting proteins in the oils is however not optimal. Other extraction methods should therefore be considered when investigating the presence of allergenic proteins in oils. Direct amino acid analysis on oils should be further explored as a method for determining the total amount of proteins present.     

基于定向标记抗体的虾原肌球蛋白量子点免疫层析方法的建立及应用

目的 利用量子点定向标记的单克隆抗体,建立竞争荧光免疫层析方法快速检测食品中虾原肌球蛋白(tropomyosin, TM)。方法 使用杂交瘤细胞株免疫Balb/c小鼠,从小鼠腹水中制备得到单克隆抗体,将氨基化量子点通过具有抗体Fc端生物亲和性的重组蛋白共价固定于抗体Fc端。以量子点定向标记抗体作为检测抗体,以南美白对虾中的TM作为检测靶标,基于竞争法原理组装免疫层析试纸条。在对条件参数进行优化后,应用于市售食物样品中TM的检测。结果 所建立方法的灵敏度为0.96μg/mL (%P=50%),检出限为0.15μg/mL(%P=20%),检测结果与文献中报道的相当。试纸条批内差和批间差的变异系数分别为6.81%～8.86%和8.27%～14.67%,均小于15%,在可接受范围之内。同时试纸条显示出了良好的特异性与准确性,实际样品的检测结果与过敏原标签一致。结论 本研究所建立的荧光免疫层析试纸条检测方法适用于TM的快速、高效检测。     

Immunological characteristics of monoclonal antibodies against shellfish major allergen tropomyosin

Two types of murine monoclonal antibodies (MAbs) against American lobster (Homarus americanus) were generated and characterized. Three purified MAbs were characterized to be specific to the shellfish major allergen tropomyosin. MAbs 5G5E1 and 1A3A7 were reactive to tropomyosin from crustacean species only, whereas MAb 2A7H6 was reactive to both crustacean and mollusk tropomyosins. None of the antibodies reacted to vertebrate tropomyosins. Competitive ELISA indicated that the antigenic epitopes recognized by the two types of MAbs were different from each other. In addition, competitive immunoblot results showed that the binding of shellfish-allergic patient IgEs to lobster tropomyosin was inhibited by the MAb 2A7H6 only. This finding suggests that the antigenic epitope for the 2A7H6 antibody might be similar or close to the allergenic epitope shared by crustaceans and mollusks. Consequently, the MAbs recognizing the different common antigenic epiotopes obtained in the present study would not only facilitate the allergen characterization of shellfish, but may also be useful for the development of specific and sensitive immunoassays for allergen quantification or epitope mapping.