Glutamate decarboxylase solubilized from the rat cerebral cortex by two different concentrations of Triton X-100: effects of glutamate analogues and analysis by SDS-PAGE/western blotting using GAD6 and K2 antibodies

Characterisation of an autoreactive conformational epitope on GAD65 recognised by the human monoclonal antibody b78 using a combination of phage display, in vitro mutagenesis and molecular modelling

Autoantibodies to the diabetes autoantigen, the 65kDa isoform of glutamic acid decarboxylase (GAD65), react with conformational epitopes defined according to linear sequences but not according to structural information, or contact sites with the antibody paratope. To ascertain such information for an exemplary human monoclonal antibody (mAb) to GAD65, b78, we combined antibody screening of phage-displayed peptide libraries, alanine mutagenesis of selected motifs, homology modelling of the PLP and C-terminal regions of GAD65, and molecular dynamics to examine for structural effects of mutagenesis. By phage display, mAb b78 selected phagotopes containing acidic residues (D, E), hydrophobic residues (Y, F or W) and LRS that localised to a possible surface-exposed conformational epitope on the combined homology model. Alanine mutants of GAD65 based on deduced contact residues were examined for binding with b78 and control sera. Mutation of (524)SRL(526), (572)DF(573) and (498)KPQ(500) reduced reactivity of b78 with mutant GAD65 > 50%. Molecular dynamics indicated that mutation of (498)KPQ(500) caused structural changes that could account for effects of this mutation. Thus phage display in combination with molecular modelling identified contact residues within a highly conformational epitope for mAb b78 in the C-terminus of GAD65. These techniques should have broad applicability to definition of epitope structure.

Effects of L-glutamate transport inhibition by a conformationally restricted glutamate analogue (2S,1'S,2'R)-2-(carboxycyclopropyl)glycine (L-CCG III) on metabolism in brain tissue in vitro analysed by NMR spectroscopy

(2S,1'S,2'R)-2-(Carboxycyclopropyl)glycine (L-CCG III) was a substrate of Na(+)-dependent glutamate transporters (GluT) in Xenopus laevis oocytes (IC50 to approximately 13 and to approximately 2 microM for, respec tively, EAAT 1 and EAAT 2) and caused an apparent inhibition of [3H]L-glutamate uptake in "mini-slices" of guinea pig cerebral cortex (IC50 to approximately 12 microM). In slices (350 microM) of guinea pig cerebral cortex, 5 microM L-CCG III increased both the flux of label through pyruvate carboxylase and the fractional enrichment of glutamate, GABA, glutamine and lactate, but had no effect on total metabolite pool sizes. At 50 microM L-CCG III decreased incorporation of 13C from [3-13C]-pyruvate into glutamate C4, glutamine C4, lactate C3 and alanine C3. The total metabolite pool sizes were also decreased with no change in the fractional enrichment. Furthermore, L-CCG III was accumulated in the tissue, probably via GluT. At lower concentration, L-CCG III would compete with L-glutamate for GluT and the changes probably reflect a compensation for the "missing" L-glutamate. At 50 microM, intracellular L-CCG III could reach > 10 mM and metabolism might be affected directly.