Disease-specific monoclonal antibodies targeting glutamate decarboxylase impair GABAergic neurotransmission and affect motor learning and behavioral functions

Abstract

Autoantibodies to the smaller isoform of glutamate decarboxylase (GAD) can be found in patients with type 1 diabetes and a number of neurological disorders, including stiff-person syndrome, cerebellar ataxia and limbic encephalitis. The detection of disease-specific autoantibody epitopes led to the hypothesis that distinct GAD autoantibodies may elicit specific neurological phenotypes. We explored the in vitro/in vivo effects of well-characterized monoclonal GAD antibodies. We found that GAD autoantibodies present in patients with stiff person syndrome (n = 7) and cerebellar ataxia (n = 15) recognized an epitope distinct from that recognized by GAD autoantibodies present in patients with type 1 diabetes mellitus (n = 10) or limbic encephalitis (n = 4). We demonstrated that the administration of a monoclonal GAD antibody representing this epitope specificity; (1) disrupted in vitro the association of GAD with γ-Aminobutyric acid containing synaptic vesicles; (2) depressed the inhibitory synaptic transmission in cerebellar slices with a gradual time course and a lasting suppressive effect; (3) significantly decreased conditioned eyelid responses evoked in mice, with no modification of learning curves in the classical eyeblink-conditioning task; (4) markedly impaired the facilitatory effect exerted by the premotor cortex over the motor cortex in a paired-pulse stimulation paradigm; and (5) induced decreased exploratory behavior and impaired locomotor function in rats. These findings support the specific targeting of GAD by its autoantibodies in the pathogenesis of stiff-person syndrome and cerebellar ataxia. Therapies of these disorders based on selective removal of such GAD antibodies could be envisioned.

The Role of Pathogenic Autoantibodies in Autoimmunity

“…Moreover, in support of this, mAb b78, but not mAb b96.11 has been shown to disrupt the association of GAD with GABA-containing synaptic vesicles from rat brain in vitro [230]. Also, the human mAb b78 and to a lesser extent b96.11, have been shown to cause neurological changes when infused into the brains of animals [230]. Thus, there may be a major epitope region specifically associated with anti-GAD positive neurological diseases where autoantibodies could interfere with the GABAergic system in the central nervous system not only by direct inhibition of GABA synthesis, but also by interfering with membrane binding of GAD65 with the GABA transporter.…”

“…The b78 epitope is in the C-terminal region that moves during catalysis, and co-locates with the N-terminal residues (from amino acid 84) in the crystal structure, close to the membrane binding domain. Moreover, in support of this, mAb b78, but not mAb b96.11 has been shown to disrupt the association of GAD with GABA-containing synaptic vesicles from rat brain in vitro [230]. Also, the human mAb b78 and to a lesser extent b96.11, have been shown to cause neurological changes when infused into the brains of animals [230].…”

“…GAD65 conformational Catalytic region [223] Catalytic region [222] GAD65 N-terminal linear None ascertained [210,224] Amino acids 4-22 [210,224,225] GAD65 C-terminal linear Rare Amino acids 475-585 [222] GAD67 conformational GAD67 or GAD65 [216] GAD67 specific [210,224] Cross-inhibition (see text) b96.11 > b78 [226] b78 > b96.11 [210] Anti-GAD65 transfer to animals (see text) Diabetes: no Not recorded Neurological: yes [227][228][229][230][231] Other autoimmune diseases Thyrogastric cluster [232] Diabetes 30%-60% Thyrogastric cluster [207,208] IvIg Plasmapheresis Rituximab ……..…”

GABA and Glutamate: Their Transmitter Role in the CNS and Pancreatic Islets

“…Furthermore, SPS-like symptoms were reproduced in experimental rats and mice injected intrathecally or intraventricularly [84] or intracerebellarly [80] with IgGs obtained from the CSF of CA and SPS patients. Specifically, IgGs obtained from the CSF of patients with anti-GAD65Ab-associated CA depressed GABA release in cerebellar brain slices [77,78,82,85], and their intracerebellar injection interfered with cerebellar control of the motor cortex and resulted in ataxic gait in rats [79]. These actions were reproduced by human anti-GAD65 monoclonal Ab b78, which binds to an epitope similar to that recognized in SPS patients positive for anti-GAD65Ab [80][81][82].…”

“…However, recent studies have shed new light on this issue. First, the pathogenic actions of anti-GAD65Ab have been clarified both in in vitro and in vivo preparations [77][78][79][80][81][82], for example, the Cerebrospinal fluid (CSF) of patients with SPS inhibited GABA synthesis [83]. Furthermore, SPS-like symptoms were reproduced in experimental rats and mice injected intrathecally or intraventricularly [84] or intracerebellarly [80] with IgGs obtained from the CSF of CA and SPS patients.…”