Expression of recombinant glutamic acid decarboxylase in insect larvae and its application in an immunoassay for the diagnosis of autoimmune diabetes mellitus

Autoimmune Diabetes Mellitus (DM) is a chronic disease caused by the selective destruction of insulin producing beta cells in human pancreas. DM is characterized by the presence of autoantibodies that bind a variety of islet-cell antigens. The 65 kDa isoform of glutamate decarboxylase (GAD65) is a major autoantigen recognized by these autoantibodies. Autoantibodies to GAD65 (GADA) are considered predictive markers of the disease when tested in combination with other specific autoantibodies. In order to produce reliable immunochemical tests for large scale screening of autoimmune DM, large amounts of properly folded GAD65 are needed. Herein, we report the production of human GAD65 using the baculovirus expression system in two species of larvae, Rachiplusia nu and Spodoptera frugiperda. GAD65 was identified at the expected molecular weight, properly expressed with high yield and purity in both larvae species and presenting appropriate enzymatic activity. The immunochemical ability of recombinant GAD65 obtained from both larvae to compete with [³⁵S]GAD65 was assessed qualitatively by incubating GADA-positive patients’ sera in the presence of 1 μM of the recombinant enzyme. All sera tested became virtually negative after incubation with antigen excess. Besides, radiometric quantitative competition assays with GADA-positive patients’ sera were performed by adding recombinant GAD65 (0.62 nM–1.4 µM). All dose response curves showed immunochemical identity between proteins. In addition, a bridge-ELISA for the detection of GADA was developed using S. frugiperda-GAD65. This assay proved to have 77.3% sensitivity and 98.2% of specificity. GAD65 could be expressed in insect larvae, being S. frugiperda the best choice due to its high yield and purity. The development of a cost effective immunoassay for the detection of GADA was also afforded.

Prokaryotic expression and characterization of the heterodimeric construction of ZnT8 and its application for autoantibodies detection in diabetes mellitus

Background

In the present work we described the recombinant production and characterization of heterodimeric construction ZnT8-Arg-Trp325 fused to thioredoxin using a high-performance expression system such as Escherichia coli. In addition, we apply this novel recombinant antigen in a non-radiometric method, with high sensitivity, low operational complexity and lower costs.

Results

ZnT8 was expressed in E. coli as a fusion protein with thioredoxin (TrxZnT8). After 3 h for induction, recombinant protein was obtained from the intracellular soluble fraction and from inclusion bodies and purified by affinity chromatography. The expression and purification steps, analyzed by SDS-PAGE and western blot, revealed a band compatible with TrxZnT8 expected theoretical molecular weight (≈ 36.8 kDa). The immunochemical ability of TrxZnT8 to compete with [³⁵S]ZnT8 (synthesized with rabbit reticulocyte lysate system) was assessed qualitatively by incubating ZnT8A positive patient sera in the presence of 0.2–0.3 μM TrxZnT8. Results were expressed as standard deviation scores (SDs). All sera became virtually negative under antigen excess (19.26–1.29 for TrxZnT8). Also, radiometric quantitative competition assays with ZnT8A positive patient sera were performed by adding TrxZnT8 (37.0 pM–2.2 µM), using [³⁵S]ZnT8. All dose–response curves showed similar protein concentration that caused 50% inhibition (14.9–0.15 nM for TrxZnT8). On the other hand, preincubated bridge ELISA for ZnT8A detection was developed. This assay showed 51.7% of sensitivity and 97.1% of specificity.

Conclusions

It was possible to obtain with high-yield purified heterodimeric construction of ZnT8 in E. coli and it was applied in cost-effective immunoassay for ZnT8A detection.

Electronic supplementary material

The online version of this article (10.1186/s12934-017-0816-4) contains supplementary material, which is available to authorized users.

Characterization of zinc transporter 8 (ZnT8) antibodies in autoimmune diabetic patients from Argentinian population using monomeric, homodimeric, and heterodimeric ZnT8 antigen variants

OBJECTIVE

In order to gain further knowledge of the structure of zinc transporter 8 (ZnT8) epitopes, we studied the role of the amino acid at position 325 in the antigen and its dimeric conformation for autoantibodies to ZnT8 (ZnT8A) recognition.

METHODS

For this purpose, several ZnT8 C-terminal domain variants were designed: monomer carrying Arg325 or Trp325, homo-dimers ZnT8-Arg-Arg325 and ZnT8-Trp-Trp325, and hetero-dimer ZnT8-Arg-Trp325. Two groups of Argentinian diabetic patients were subjected to analysis using [(35)S]-ZnT8 variants by radioligand binding assay (RBA): i) 100 new-onset, insulin-dependent, type 1 diabetic patients and ii) 282 slowly progressing to insulin requirement, non-obese adult-onset diabetic patients. In addition, 50 type 1 diabetic patients and 100 normal control sera provided by the American Diabetes Association (ADA) were evaluated in order to calculate the sensitivity and specificity of ZnT8A assays for each antigenic variant. Other routine β-cell autoantibodies were also tested by RBA.

RESULTS

Of the 100 Argentinian type 1 diabetic patients, 65 were ZnT8A+. Out of them, 8 patients recognized all recombinant forms of ZnT8 and most patients (56) reacted against the heterodimer. Additionally, out of 282 non-obese adult-onset diabetic patients 46 were ZnT8A+, whereas 29 patients recognized only dimers. Besides, exclusive reactivity against ZnT8A was found in 9.0% for type 1 diabetes mellitus and 10.3% for non-obese adult-onset diabetic patients.

CONCLUSIONS

Significantly higher signal values in RBA were obtained with the heterodimeric variant. An increased detection of humoral autoimmunity was found in both groups when ZnT8A was employed in combination with the other β-cell autoantibodies. The inclusion of homodimeric immunoreactive peptides revealed the existence of quaternary structure-defined epitopes probably resembling the actual state of the autoantigen in vivo. Finally, the differential profiles of ZnT8A exhibited by type 1 and non-obese adult-onset diabetic patients suggest the different nature of autoimmune processes underlying both pathologies.

Flow cytometric microsphere-based immunoassay as a novel non-radiometric method for the detection of glutamic acid decarboxylase autoantibodies in type 1 diabetes mellitus

The first measurable sign of arising autoimmunity in type 1 diabetes mellitus is the detection of autoantibodies against beta-cell antigens, such as glutamic acid decarboxylase (GAD65). GAD65 autoantibodies (GADA) are usually measured by the Radioligand Binding Assay (RBA). The aim of this work was to develop protocols of flow cytometric microsphere-based immunoassays (FloCMIA) which involved glutamic acid decarboxylase fused to thioredoxin (TrxGAD65) adsorbed on polystyrene microspheres. Detection of bound GADA was accomplished by the use of anti-human IgG-Alexa Fluor 488 (protocol A), anti-human IgG-biotin and streptavidin-dichlorotriazinyl aminofluorescein (DTAF) (protocol B) or TrxGAD65-biotin and streptavidin-DTAF (protocol C). Serum samples obtained from 46 patients assayed for routine autoantibodies at Servicios Tecnológicos de Alto Nivel (STAN-CONICET) were analyzed by RBA, ELISA and three alternative FloCMIA designs. Protocol C exhibited the highest specificity (97.8%) and sensitivity (97.4%) and a wide dynamic range (1.00-134.40 SDs). Samples obtained from 40 new-onset diabetic patients were also analyzed to further evaluate the performance of protocol C. The latter protocol showed a sensitivity of 58.6% and a prevalence of 47.5%. Two patients resulted positive only by FloCMIA protocol C and its SDs were higher than those of RBA and ELISA, showing a significantly wide dynamic range. In conclusion, FloCMIA proved to be highly sensitive and specific, requiring a low sample volume; it is environmentally adequate, innovative and represents a cost-effective alternative to traditional GADA determination by RBA and/or ELISA, making it applicable to most medium-complexity laboratories.

Characterization of insulin antibodies by Surface Plasmon Resonance in two clinical cases: brittle diabetes and insulin autoimmune syndrome

In this study, the characterization of insulin (auto)antibodies has been described, mainly in terms of concentration (q), affinity (K_a) and Ig (sub)isotypes by Surface Plasmon Resonance (SPR) in two particular clinical cases of individuals with severe episodes of impaired glycemia. Subject 1 suffers from brittle diabetes associated with circulating insulin antibodies (IA) due to insulin treatment. Subject 2 has insulin autoantibodies (IAA) associated with hypoglycemia in spite of not being diabetic and not having ever received exogenous insulin therapy. After conventional screening for IA/IAA by radioligand binding assay (RBA), we further characterized IA/IAA in sera of both patients in terms of concentration (q), affinity (K_a) and Ig (sub)isotypes by means of SPR technology. In both cases, q values were higher and Ka values were lower than those obtained in type 1 diabetic patients, suggesting that IA/IAA:insulin immunocomplexes could be responsible for the uncontrolled glycemia. Moreover, subject 1 had a predominat IgG₁ response and subject 2 had an IgG₃ response. In conclusion, SPR technology is useful for the complete characterization of IA/IAA which can be used in special cases where the simple positive/negative determination is not enough to achieve a detailed description of the disease fisiopathology.

Detection and characterization of ZnT8 autoantibodies could help to screen latent autoimmune diabetes in adult-onset patients with type 2 phenotype

Autoantibodies to zinc transporter 8 (ZnT8A) constitute an additional marker of autoimmune diabetes, complementing those already used in diagnosis support. ZnT8A could also be found in latent autoimmune diabetes of adults (LADA). The aim of this study was to evaluate the prevalence of ZnT8A in adult-onset diabetic patients in Argentinian population. A total of 271 patients diagnosed for diabetes at mean age 53.4 ± 10.9, body mass index ≤ 30, without insulin treatment for the first year of disease, and initially classified as type 2 diabetic patients were tested for ZnT8A using cDNA plasmids encoding the C-terminal domains (aa 268-369) carrying 325Arg, 325Trp, and a dimeric cDNA construct carrying both 325Arg and 325Trp (ZnT8 Arg-Trp325). We also analyzed proinsulin autoantibodies (PAA), glutamic acid decarboxylase autoantibodies (GADA), and protein tyrosine phosphatase IA-2 autoantibodies (IA-2A). A subset of 101 patients was followed during 6 years in order to analyze insulin requirement. Out of the 271 patients, 22.1% presented at least one humoral marker, 2.6% were PAA+, 12.5% were GADA+, 3.3% were IA-2A+, and 10.7% were ZnT8A+. Among the latter, 7.0% were ZnT8A-Arg325, 51.7% were ZnT8A-Trp325, and 62.1% were ZnT8A-Arg-Trp325. Furthermore, the prevalence of autoantibodies in the group of patients treated with insulin (n = 18) was 55.6%. These results demonstrated that a significant proportion of autoimmune adult-onset diabetic patients presented ZnT8A as the only humoral marker. Between them, the higher prevalence was for ZnT8A-Trp325. We suggest that screening for LADA patients, best performed with a minimal set of marker determination, must include at least the screening of GADA and ZnT8A-Arg-Trp325.

Development of 2 alternative enzyme-linked immunosorbent assays for routine screening of glutamic acid decarboxylase autoantibodies

BACKGROUND

Antibodies to GAD65 (GADA) are considered highly predictive humoral markers of the type 1 diabetes mellitus and also of the insulin requirement in adult-onset patients presumptively classified as type 2 diabetics or LADA.

METHODS

We present 2 methods for GADA assessment. The first one (fluid phase, ELISA Protocol A) is carried out in a 2-step procedure in which serum GADA are first allowed to react with a fixed dose of GAD65-biotin in solution and the residual free antigen is later assayed by a conventional ELISA. In the second test (solid phase, ELISA Protocol B) GADA are measured in an ELISA that depends on the ability of divalent autoantibodies to form a bridge between immobilized TrxGAD65 and liquid-phase biotinylated TrxGAD65.

RESULTS

All normal control samples scored negative in both variants of ELISA and RBA, hence specificity was 100% for all methods; the relative sensitivity of ELISA Protocol A respect of the RBA was 94% and that of ELISA Protocol B was 76%.

CONCLUSIONS

Although ELISA Protocol A exhibited a better performance in terms of relative sensitivity than ELISA Protocol B, the simplicity of execution and the intended use of the assay must also be taken in consideration for the final choice.

Single-tube test for autoantibodies to glutamic acid decarboxylase and proinsulin as first-line screening for autoimmunity in adult-onset diabetic patients

The aim of this work was develop a new combined radioligand-binding assay (RBA-combi) for the rapid and simultaneous determination of two autoimmunity markers, GADA and PAA, known to be differentially distributed in young and in some adult diabetic patients. The methodology was applied to sera from 85 young type 1 and 98 adult-onset diabetic patients with different marker profiles and insulin requirements, and to 53 normal control sera. Among type 1 diabetes sera used as autoimmunity controls, 100% of those with at least one positive marker by single methods and 17.7% of those with double negative markers were positive by RBA-combi (RBA-combi+). Among sera from adult-onset diabetes, 100% of those PAA+ (GADA+ or GADA-), 92.3% of GADA+/PAA-, and 1.3% of GADA-/PAA- were RBA-combi+. In conclusion, the new RBA-combi allowed the simultaneous detection of GADA and PAA markers with acceptable performance. Moreover, 16 out of 18 (88.9%) of adult patients RBA-combi+ evolved to insulin requirement, suggesting that this test is a valuable tool for assessing autoimmune processes associated to future impairment of insulin secretion.

A radioligand-binding assay for detecting antibodies specific for proinsulin and insulin using 35S-proinsulin

A new radioligand-binding assay (RBA) is described for the detection of insulin/proinsulin-specific antibodies using 35S-labeled proinsulin produced by a cell-free reticulocyte extract. Direct use of the crude expression product in the RBA was not feasible because the protein failed to fold properly (or had incorrectly paired disulphide bridges) and purification was hindered by interfering by-products. A refolding protocol and a chromatographic procedure were devised that readily allowed production of purified and immunochemically competent 35S-labeled proinsulin. The new RBA was compared with the reference test, in which the tracer was standard 125I-insulin. The analysis included sera from 41 diabetic patients and 25 healthy controls. Twenty-six (63.4%) and 29 (70.7%) patients scored positive by RBA using 35S-PI and 125I-insulin, respectively. The methods showed a satisfactory correlation with r(2)=0.77 and a slope not significantly different from unity (m=1.16+/-0.10; 95% confidence interval). Since the nuclide used in the assay is 35S, the procedure is compatible with standard assays for GADA and IA-2A, and thus may permit combined assays for the major early markers of autoimmune diabetes.

Combined measurement of diabetes mellitus immunological markers: an assessment of its benefits in adult-onset patients

The convenience of combining the measurement of antibodies to glutamic acid decarboxylase (GADA), protein tyrosine phosphatase (IA-2A), and autoantibodies to insulin (IAA) in diabetic patients was assessed. We analysed 71 type 1 and 115 adult-onset diabetic patients. The latter were grouped into three categories according to the time of evolution to insulin dependence. The main findings were as follows: (i) in type 1 diabetes, the combined analysis of GADA and IA-2A showed a sensitivity of 87.4% and was not appreciably improved by adding IAA; (ii) out of 31 adults who required insulin immediately or within the first two years of diagnosis, 41.9, 29.0, and 6.5% were positive for at least one, two or all three, and all three markers, respectively; GADA was the most prevalent (35.5%) and IA-2A the least represented (16.1%); (iii) 34 adult patients with slow evolution to insulin dependence showed a completely different profile: 5.9% were GADA positive and 23.5% were IAA positive and no double or triple positivity was observed as all patients were IA-2A negative; and (iv) 50 type 2 patients who had not required insulin treatment showed a low incidence of GADA (4%) as the only marker present. We conclude that a combined double-antigen test for GADA and IA-2A is a useful strategy for prospective screening of type 1 diabetes. However, in adults, the profile of individual markers discloses the course to insulin dependence. Therefore, it seems advisable to measure the markers separately, to allow a better classification of these patients, and help define their treatment.

Immunologic and genetic markers in insulin-dependent diabetes mellitus (type 1) in an Argentine population

The objective was to evaluate the prevalence and association of several markers (islet cell antibodies: ICA, insulin autoantibodies: IAA, glutamic acid decarboxylase antibodies: GADA and ICA512 antibodies: ICA512A) along with HLA DQB1 genotype in type 1 diabetes mellitus of recent onset, including siblings and individuals without any history of this disease, in an Argentine population. A total of 79 children with type 1 diabetes mellitus of recent onset were studied, as well as 79 control children, and 68 healthy siblings of type 1 diabetic cases. IAA, ICA, GADA, ICA512A and HLA DQB1 alleles were determined. Sensitivity was 67.1% for ICA, 36.7% for IAA, 74.6% for GADA and 63.4% for ICA512A. None of the control subjects was positive for the immunological markers. Combined sensitivity of ICA-IAA-GADA was 89.8%, similar to the ICA512A-GADA (87.3%) or ICA512A-GADA-IAA combination (91.1%). GADA correlated positively with ICA, but no such correlation was found between IAA, ICA512A and ICA. IAA correlated negatively and GADA positively with age. IAA was associated to DQB1*0201, whereas ICA and ICA512A associated to DQB1*0302. Among siblings, 3/68 (4.4%) were positive for IAA and a single case (1.5%) was positive for GADA and one for ICA512A. Our findings show that the combination of multiple tests increases the sensitivity for prediction, with the ICA512A-GADA combination proving highly sensitive and equivalent to other proposed combinations, such as ICA-IAA-GADA.

Replacement of methionine-161 with threonine eliminates a major by-product of human glutamate decarboxylase 65-kDa variant expression in Escherichia coli

Most insulin-dependent diabetes mellitus patients gen-erate conformational autoantibodies to the islet-cell 65-kDa variant of human glutamate decarboxylase (GAD65), and several immunochemical tests for the early detection of type-1 diabetes rely on GAD65 antibody (GADA) assessment using properly folded recombinant GAD65 as the antigen. In addition, preventive therapies based on tolerization by GAD65 administration may be available in the near future. Therefore, there exists a strong interest in a facile and economically sound expression procedure for this antigen. Several attempts to produce, in native form, wild-type GAD65 in Escherichia coli have failed. However, this difficulty was recently surmounted in our laboratory by expressing GAD65 as a fusion protein with thioredoxin [Papouchado, Valdez, Ghiringhelli, Poskus and Ermácora (1997) Eur. J. Biochem. 246, 350-359]. In this work, a new GAD65 hybrid gene was prepared by joining engineered cDNA obtained from human and rat tissues. The new gene was modified additionally to finally code for human GAD65 with a single amino-acid substitution: Met-161-->Thr. This change impeded the co-expression of a 48-kDa by-product from an internal translation site. Also, a second 58-kDa by-product was identified as a GAD65 C-terminal proteolytic fragment that co-purifies with thioredoxin-M161T GAD65. The new GAD65 variant was expressed and easily purified, yielding an antigen that performed equally or better than wild-type GAD65 in the reference radiobinding assay for GADA. The procedure provides an inexpensive source of large amounts of fully active and immunochemically competent GAD65.

Highly-sensitive and specific enzyme-linked immunosorbent assays for GAD65 autoantibodies using a thioredoxin-GAD65 fusion antigen

Autoantibodies against glutamic acid decarboxylase (GAD65) are present in the sera of most patients with recently diagnosed insulin-dependent diabetes mellitus (IDDM). These antibodies appear years before the clinical symptoms, and they are considered to be early markers of the disease. To detect GAD65 autoantibodies (GADA), we developed new enzyme-linked immunosorbent assays (ELISA) with a fusion protein thioredoxin-GAD65 (Trx-GAD65) produced in E. coli as the antigen. These assays were compared with the reference radiobinding assay (RBA). Since most GADA are directed against native epitopes, and adsorption of GAD65 to plastic may cause disruption of its native conformation, the new assays rely on the following immobilization procedures: (a) capture ELISA (c-ELISA) with Trx-GAD65 (protocol A) or biotin-Trx-GAD (protocol B) indirectly immobilized by a non-adsorptive process; (b) ELISA with antigen-antibody preincubation in solution (p-ELISA) in which GADA were reacted first with Trx-GAD65 (protocol C) or biotin-Trx-GAD (protocol D) and the free antigen was determined by conventional ELISA. The results obtained with 42 newly diagnosed IDDM patients and 30 normal individuals were as follows: RBA had 79% sensitivity (percentage of IDDM patients detected) and 97% specificity (100% minus the percentage of false positives). c-ELISA showed low sensitivity (36 and 50%, respectively for protocols A and B), and high specificity (100 and 97%, respectively). p-ELISA were highly-sensitive (74 and 79%, respectively) and specific (97 and 93% for protocols C and D, respectively). Thus, protocols C and D had a performance similar to the reference method. The results reported here provide the basis for simple, highly-sensitive, specific, and widely-applicable tests for GADA that eliminate many of the drawbacks of the radioactive methods.

Expression of properly folded human glutamate decarboxylase 65 as a fusion protein in Escherichia coli

Autoantibodies to the islet-cell 65-kDa variant of glutamate decarboxylase (GAD65) are found in most insulin-dependent diabetes mellitus (IDDM) patients many years before the appearance of clinical symptoms of the disease. As IDDM-preventive therapies may be available in the future, an international effort is taking place to develop widely applicable anti-GAD immunochemical tests. These tests would help to detect individuals at risk before the full installation of the disease and to enroll them in prevention programs. Autoantibodies to GAD65 are mostly directed to conformational epitopes, and the enzyme is a complex molecule with a prosthetic group and 15 cysteine residues. Thus, the conformational integrity of GAD65 is essential for an appropriate anti-GAD assay. Isolation of large amounts of GAD65 from pancreas or other tissues is impractical, and no successful production of properly folded GAD65 has been reported in bacteria. Native recombinant GAD65 for immunochemical tests is usually obtained from eukaryotic expression systems. Since the large-scale production of a recombinant protein in an eukaryotic system is expensive and technically difficult, we investigated the expression of GAD65 in Escherichia coli as an alternative. A number of DNA constructs intended to export the enzyme to the periplasmic space or to improve its cytoplasmic solubility were designed and tested. Our results provide a solution to the two main problems associated with the expression of GAD65 in E. coli: misfolding, leading to the formation of inclusion bodies; and the presence of alternative initiation sites for translation that causes the preferential production of truncated variants of GAD65. We describe here the production of properly folded, fully active, and immunochemically competent GAD65 as an N-terminal fusion protein with thioredoxin. An account of the reactivity of the produced protein with sera of six IDDM patients is also presented.