Molecular characterization of a disease associated conformational epitope on GAD65 recognised by a human monoclonal antibody b96.11

Structure and dynamics of GAD65 in complex with an autoimmune polyendocrine syndrome type 2-associated autoantibody

The enzyme glutamate decarboxylase (GAD) produces the neurotransmitter GABA, using pyridoxal-5'-phosphate (PLP). GAD exists as two isoforms, GAD65 and GAD67. Only GAD65 acts as a major autoantigen, frequently implicated in type 1 diabetes and other autoimmune diseases. Here we characterize the structure and dynamics of GAD65 and its interaction with the autoimmune polyendocrine syndrome type 2-associated autoantibody b96.11. Using hydrogen-deuterium exchange mass spectrometry (HDX), X-ray crystallography, cryo-electron microscopy, and computational approaches, we examine the conformational dynamics of apo- and holoGAD65 and the GAD65-autoantibody complex. HDX reveals local dynamics accompanying autoinactivation, with the catalytic loop promoting collective motions at the CTD-PLP domain interface. In the GAD65-b96.11 complex, heavy chain CDRs dominate the interaction, with a long CDRH3 bridging the GAD65 dimer via electrostatic interactions with the 260PEVKEK265motif. This bridging links structural elements controlling GAD65's conformational flexibility to its autoantigenicity. Thus, intrinsic dynamics, rather than sequence differences within epitopes, appear to be responsible for the contrasting autoantigenicities of GAD65 and GAD67. Our findings elucidate the structural and dynamic factors that govern the varying autoantibody reactivities of GAD65 and GAD67, offering a revised rationale for the autoimmune response to GAD65.

Structure and substrate specificity determinants of the taurine biosynthetic enzyme cysteine sulphinic acid decarboxylase

Pyridoxal 5́-phosphate (PLP) is an important cofactor for amino acid decarboxylases with many biological functions, including the synthesis of signalling molecules, such as serotonin, dopamine, histamine, γ-aminobutyric acid, and taurine. Taurine is an abundant amino acid with multiple physiological functions, including osmoregulation, pH regulation, antioxidative protection, and neuromodulation. In mammalian tissues, taurine is mainly produced by decarboxylation of cysteine sulphinic acid to hypotaurine, catalysed by the PLP-dependent cysteine sulphinic acid decarboxylase (CSAD), followed by oxidation of the product to taurine. We determined the crystal structure of mouse CSAD and compared it to other PLP-dependent decarboxylases in order to identify determinants of substrate specificity and catalytic activity. Recognition of the substrate involves distinct side chains forming the substrate-binding cavity. In addition, the backbone conformation of a buried active-site loop appears to be a critical determinant for substrate side chain binding in PLP-dependent decarboxylases. Phe94 was predicted to affect substrate specificity, and its mutation to serine altered both the catalytic properties of CSAD and its stability. Using small-angle X-ray scattering, we further showed that CSAD presents open/close motions in solution. The structure of apo-CSAD indicates that the active site gets more ordered upon internal aldimine formation. Taken together, the results highlight details of substrate recognition in PLP-dependent decarboxylases and provide starting points for structure-based inhibitor design with the aim of affecting the biosynthesis of taurine and other abundant amino acid metabolites.

Comprehensive mapping of immune perturbations associated with severe COVID-19

Although critical illness has been associated with SARS-CoV-2-induced hyperinflammation, the immune correlates of severe COVID-19 remain unclear. Here, we comprehensively analyzed peripheral blood immune perturbations in 42 SARS-CoV-2 infected and recovered individuals. We identified extensive induction and activation of multiple immune lineages, including T cell activation, oligoclonal plasmablast expansion, and Fc and trafficking receptor modulation on innate lymphocytes and granulocytes, that distinguished severe COVID-19 cases from healthy donors or SARS-CoV-2-recovered or moderate severity patients. We found the neutrophil to lymphocyte ratio to be a prognostic biomarker of disease severity and organ failure. Our findings demonstrate broad innate and adaptive leukocyte perturbations that distinguish dysregulated host responses in severe SARS-CoV-2 infection and warrant therapeutic investigation.

GADL1 is a multifunctional decarboxylase with tissue-specific roles in β-alanine and carnosine production

Carnosine and related β-alanine-containing peptides are believed to be important antioxidants, pH buffers, and neuromodulators. However, their biosynthetic routes and therapeutic potential are still being debated. This study describes the first animal model lacking the enzyme glutamic acid decarboxylase-like 1 (GADL1). We show that Gadl1^-/- mice are deficient in β-alanine, carnosine, and anserine, particularly in the olfactory bulb, cerebral cortex, and skeletal muscle. Gadl1^-/- mice also exhibited decreased anxiety, increased levels of oxidative stress markers, alterations in energy and lipid metabolism, and age-related changes. Examination of the GADL1 active site indicated that the enzyme may have multiple physiological substrates, including aspartate and cysteine sulfinic acid. Human genetic studies show strong associations of the GADL1 locus with plasma levels of carnosine, subjective well-being, and muscle strength. Together, this shows the multifaceted and organ-specific roles of carnosine peptides and establishes Gadl1 knockout mice as a versatile model to explore carnosine biology and its therapeutic potential.

Optimized purification strategies for the elimination of non-specific products in the isolation of GAD65-specific monoclonal autoantibodies

Autoantibodies against antigens expressed by insulin-producing β cells are circulating in both healthy individuals and patients at risk of developing Type 1 diabetes. Recent studies suggest that another set of antibodies (anti-idiotypic antibodies) exists in this antibody/antigen interacting network to regulate auto-reactive responses. Anti-idiotypic antibodies may block the antigen-binding site of autoantibodies or inhibit autoantibody expression and secretion. The equilibrium between autoantibodies and anti-idiotypic antibodies plays a critical role in mediating or preventing autoimmunity. In order to investigate the molecular mechanisms underlying such a network in autoimmunity and potentially develop neutralizing reagents to prevent or treat Type 1 diabetes, we need to produce autoantibodies and autoantigens with high quality and purity. Herein, using GAD65/anti-GAD65 autoantibodies as a model system, we aimed to establish reliable approaches for the preparation of highly pure autoantibodies suitable for downstream investigation.

Open conformation of human DOPA decarboxylase reveals the mechanism of PLP addition to Group II decarboxylases

DOPA decarboxylase, the dimeric enzyme responsible for the synthesis of neurotransmitters dopamine and serotonin, is involved in severe neurological diseases such as Parkinson disease, schizophrenia, and depression. Binding of the pyridoxal-5'-phosphate (PLP) cofactor to the apoenzyme is thought to represent a central mechanism for the regulation of its activity. We solved the structure of the human apoenzyme and found it exists in an unexpected open conformation: compared to the pig kidney holoenzyme, the dimer subunits move 20 Å apart and the two active sites become solvent exposed. Moreover, by tuning the PLP concentration in the crystals, we obtained two more structures with different conformations of the active site. Analysis of three-dimensional data coupled to a kinetic study allows to identify the structural determinants of the open/close conformational change occurring upon PLP binding and thereby propose a model for the preferential degradation of the apoenzymes of Group II decarboxylases.

Immunoglobulin subclass profiles of anti-idiotypic antibodies to GAD65Ab differ between type 1 diabetes patients and healthy individuals

Previously we reported the presence of anti-idiotypic antibodies (anti-Id)-specific to autoantibodies against GAD65 (GAD65Ab) in healthy individuals while the activity of anti-Id directed to GAD65Ab in type 1 diabetes (T1D) patients was significantly lower. These anti-Id recognize the antigen-binding site of GAD65Ab, thus preventing their binding to GAD65. Here, we characterized the IgG subclass profile of these anti-Id (GAD65Ab specific) and of the associated GAD65Ab themselves. The IgG subclass response of anti-Id in healthy individuals (n = 16) was IgG3-dominated, while in T1D patients (n = 8) IgG1 was the major IgG subclass. The GAD65Ab bound by anti-Id in both healthy individuals (n = 38) and GAD65Ab-negative T1D patients (n = 35) showed a predominant rank order of IgG1 > IgG2 > IgG4 > IgG3. However, the frequency of GAD65Ab of the IgG4 subclass was significantly higher in T1D patients (P < 0.05). We conclude that the IgG subclass profile of anti-Id (GAD65Ab specific) in healthy individuals differs from that in T1D patients. These differences may provide insights into the development of these antibodies.

Characterisation of peptide microarrays for studying antibody-antigen binding using surface plasmon resonance imagery

Background

Non-specific binding to biosensor surfaces is a major obstacle to quantitative analysis of selective retention of analytes at immobilized target molecules. Although a range of chemical antifouling monolayers has been developed to address this problem, many macromolecular interactions still remain refractory to analysis due to the prevalent high degree of non-specific binding. We describe how we use the dynamic process of the formation of self assembling monolayers and optimise physical and chemical properties thus reducing considerably non-specific binding and allowing analysis of specific binding of analytes to immobilized target molecules.

Methodology/Principal Findings

We illustrate this approach by the production of specific protein arrays for the analysis of interactions between the 65kDa isoform of human glutamate decarboxylase (GAD65) and a human monoclonal antibody. Our data illustrate that we have effectively eliminated non-specific interactions with the surface containing the immobilised GAD65 molecules. The findings have several implications. First, this approach obviates the dubious process of background subtraction and gives access to more accurate kinetic and equilibrium values that are no longer contaminated by multiphase non-specific binding. Second, an enhanced signal to noise ratio increases not only the sensitivity but also confidence in the use of SPR to generate kinetic constants that may then be inserted into van't Hoff type analyses to provide comparative ΔG, ΔS and ΔH values, making this an efficient, rapid and competitive alternative to ITC measurements used in drug and macromolecular-interaction mechanistic studies. Third, the accuracy of the measurements allows the application of more intricate interaction models than simple Langmuir monophasic binding.

Conclusions

The detection and measurement of antibody binding by the type 1 diabetes autoantigen GAD65 represents an example of an antibody-antigen interaction where good structural, mechanistic and immunological data are available. Using SPRi we were able to characterise the kinetics of the interaction in greater detail than ELISA/RIA methods. Furthermore, our data indicate that SPRi is well suited to a multiplexed immunoassay using GAD65 proteins, and may be applicable to other biomarkers.

High-titer GAD65 autoantibodies detected in adult diabetes patients using a high efficiency expression vector and cold GAD65 displacement

Adult type 2 diabetes patients with GAD65 autoantibodies (GADA) are known as latent autoimmune diabetes in adults (LADA). It has been suggested that GADA in LADA patients preferentially bind to the N-terminal end of GAD65. Using the N-terminal end extension of ³⁵S-GAD65 generated by the pEx9 plasmid, we tested the hypothesis that GADA in LADA patients preferentially react with ³⁵S-GAD65 from the pEx9 plasmid compared to the normal length pThGAD65 plasmid. Healthy control subjects (n = 250) were compared with type 1 (n = 23), type 2 (n = 290), and unspecified (n = 57) diabetes patients. In addition, radio-binding assays for GADA with ³⁵S-GAD65 generated from both the pEx9 and pThGAD65 plasmids were used in displacement assays with an excess of recombinant human GAD65 (2 μg/mL) to correct for non-specific binding. ³⁵S-GAD65 produced by either pEx9 or pThGAD65 did not differ in binding among the healthy controls and among the type 1 diabetes patients. Among the type 2 and unspecified patients, there were 4/290 and 3/57 patients, respectively, with binding to the pEx9 but not to the pThGAD65 generated ³⁵S-GAD65. In the displacement assay, we discovered 14 patients with very high-titer GADA among the type 1 (n = 3, 12,272-29,915 U/mL), type 2 (n = 7; 12,398-334,288 U/mL), and unspecified (n = 4; 20,773-4,053,580 U/mL) patients. All samples were fully displaced following appropriate dilution. We conclude that pThGAD65 is preferred for the coupled in vitro transcription translation of ³⁵S-GAD65 and that displacement with recombinant GAD65 may detect very high-titer GADA with possible clinical relevance.

Heritability of levels of autoantibodies to thyroid antigens using the method of plotting regression of offspring on midparent (ROMP)

Only a few methods can be applied in a simple manner to estimate the genetic control of autoimmunity in humans. Here we examined the heritability of autoantibodies to two thyroid antigens; thyroglobulin (Tg) and thyroperoxidase (TPO, formerly known as thyroid microsomal antigen), using methods of regression of offspring on mid-parental values (ROMP). With the data sets available, affected and unaffected siblings were compared by this rapid screening method using results determined by hemagglutination (HA). The presence of both types of autoantibodies showed positive heritability in patients with Graves' thyrotoxicosis (TT), but it was not observed in chronic lymphocytic or Hashimoto's thyroiditis (CLT) patients. Since these assays have been extensively used over the years by most diagnostic and research laboratories, they should provide some insight as to which quantifiable parameters may be usefully accumulated to help select groups of patients and their families for further genetic study. ROMP may also be useful to determine the sequential appearance of different types of antibody in predicting disease onset in other family members, and in distinguishing maternal and paternal effects on imprinting. The method may be extended to study epitope spreading and other measures of disease progression.

GAD65 as a prototypic autoantigen

The repertoire of known autoantigens is limited to a very small proportion of all human proteins, and the reason why only some proteins become autoantigens is unclear, but is likely associated with structural features. The 65kDa isoform of the enzyme glutamic acid decarboxylase (GAD65) is a major autoantigen in type I diabetes, and in various neurological diseases, whereas the closely related isoform, GAD67, is rarely antigenic. Conformational epitopes of GAD65 have been mapped using human monoclonal antibodies to GAD65 and GAD mutated by GAD65/67 sequence exchanges or point mutations, but these studies have been limited by a lack of structural information. The recent publication of crystal structures for the two isoforms has shown that the N-, C- and middle domains that have been identified previously as likely epitope regions are closely associated within the GAD dimer. Two major epitope regions, ctc1 and ctc2, have been identified in the C-terminal domain of GAD65, that encompass N- and C-terminal residues, and middle and C-terminal residues respectively. These regions are highly flexible compared with the equivalent regions in GAD67, and T cell epitopes have been localized to the same surface region of GAD65. Comparative analysis of these two structurally similar isoforms, GAD65 and GAD67, only one of which is autoantigenic should provide new insights into the provocations to autoimmunity.

COOH-terminal clustering of autoantibody and T-cell determinants on the structure of GAD65 provide insights into the molecular basis of autoreactivity

OBJECTIVE

To gain structural insights into the autoantigenic properties of GAD65 in type 1 diabetes, we analyzed experimental epitope mapping data in the context of the recently determined crystal structures of GAD65 and GAD67, to allow "molecular positioning" of epitope sites for B- and T-cell reactivity.

RESEARCH DESIGN AND METHODS

Data were assembled from analysis of reported effects of mutagenesis of GAD65 on its reactivity with a panel of 11 human monoclonal antibodies (mAbs), supplemented by use of recombinant Fab to cross-inhibit reactivity with GAD65 by radioimmunoprecipitation of the same mAbs.

RESULTS

The COOH-terminal region on GAD65 was the major autoantigenic site. B-cell epitopes were distributed within two separate clusters around different faces of the COOH-terminal domain. Inclusion of epitope sites in the pyridoxal phosphate-and NH(2)-terminal domains was attributed to the juxtaposition of all three domains in the crystal structure. Epitope preferences of different mAbs to GAD65 aligned with different clinical expressions of type 1 diabetes. Epitopes for four of five known reactive T-cell sequences restricted by HLA DRB1*0401 were aligned to solvent-exposed regions of the GAD65 structure and colocalized within the two B-cell epitope clusters. The continuous COOH-terminal epitope region of GAD65 was structurally highly flexible and therefore differed markedly from the equivalent region of GAD67.

CONCLUSIONS

Structural features could explain the differing antigenicity, and perhaps immunogenicity, of GAD65 versus GAD67. The proximity of B- and T-cell epitopes within the GAD65 structure suggests that antigen-antibody complexes may influence antigen processing by accessory cells and thereby T-cell reactivity.

Modulation of diabetes in NOD mice by GAD65-specific monoclonal antibodies is epitope specific and accompanied by anti-idiotypic antibodies

Type 1 diabetes is caused by the autoimmune destruction of pancreatic beta cells. Here we show that administration of a human monoclonal antibody (b96.11) specific to the 65-kDa isoform of glutamate decarboxylase (GAD65) to prediabetic non-obese diabetic (NOD) mice significantly delays the onset of autoimmune diabetes. We found this effect to be epitope-specific, as only b96.11 showed this therapeutic property, while a GAD65-specific human monoclonal control antibody (b78) derived from the same patient, but specific to a different determinant of GAD65, had no significant effect on the progression of disease. Administration of b96.11 or b78 to NOD mice was accompanied by the generation of anti-idiotypic antibodies. Importantly, the induced anti-idiotypic antibodies were specific for the immunizing antibody and blocked the binding of GAD65 by the respective antibody. These findings suggest a potential role for the internal image of the GAD65 determinant recognized by b96.11 in the anti-idiotypic antibody, supporting an immunomodulatory role for GAD65-specific autoantibodies, as originally postulated by Jerne.

Construction of nanogold hollow balls with dendritic surface as immobilized affinity support for protein adsorption

This contribution introduced the construction of nanosize gold hollow balls (NGB) with dendritic surface as the immobilized affinity support for aflatoxin B(1) antibody (anti-AFB(1)) adsorption, as a model protein, in biorecognition interface. The interaction between nanogold hollow balls and anti-AFB(1) was characterized by cyclic voltammetry, electrochemical impedance spectroscopy, and quartz crystal microbalance (QCM) technique. The deposited mass of anti-AFB(1) on the nanogold hollow ball-modified electrodes was more than that on the nanogold particle-modified electrodes via QCM analysis. The cyclic voltammogram tends to be more irreversible with anti-AFB(1) concentration increased. The antigen-antibody interaction was examined by using the nanogold hollow ball-modified QCM probes. Experimental results show that the developed protein assay system is sensitive to the concentration of AFB(1) as low as 0.05 ng mL(-1). Thus, the nanogold hollow ball is a useful matrix, and can be used as the immobilized affinity support for the adsorption of other proteins.