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2015-05-22 - Colloque/Présentation - communication orale - Anglais - 1 page(s)

Paci Paula , Ris Laurence , "Effects of GAD65-specific b78 antibody on synaptic plasticity in hippocampal organotypic slices" in 11th National Congress of the Belgian Society for Neuroscience , Mons, Belgique, 2015

  • Codes CREF : Sciences biomédicales (DI3200)
  • Unités de recherche UMONS : Neurosciences (M119)
  • Instituts UMONS : Institut des Sciences et Technologies de la Santé (Santé)

Abstract(s) :

(Anglais) It is well known that Arc, for Activity-Regulated Cytoskeleton-associated protein, is implicated in synaptic plasticity in the hippocampus. This protein is translated only in activated dendritic spines where it stabilizes F-actin polymers and induces structural modifications. However, the mechanisms underlying the regulation of its expression are still poorly understood. Moreover, very little is known about a possible role of GABAergic interneurons in this regulation. In this study, rat hippocampal organotypic slices were used to investigate Arc expression and synaptic plasticity in the presence of GAD65-specific monoclonal antibodies. This experimental model offers the advantages of an in vitro model together with the preservation of the structure of a complex neuronal network. Arc expression was induced in this model by forskolin and IBMX, two drugs increasing the level of cAMP, confirming the results obtained by other groups in vivo or in primary cell cultures. Two GAD65-specific human monoclonal antibodies (b96.11 and b78) arising from a patient with autoimmune polyendocrine syndrome type 2 were tested to determine their potential effects in hippocampal organotypic slices. GAD65 is the smaller isoform of glutamate decarboxylase which transforms glutamate to GABA at the level of synapses. It is the target of autoantibodies in various neurological disorders such as stiff-person syndrome, cerebellar ataxia or limbic encephalitis but also in type 1 diabetes. It has been demonstrated that b96.11 antibody recognizes an epitope located at amino acid residues 308-365 while b78 antibody recognizes an epitope located at the C-terminus (512-540). These antibodies induce, in vitro, a reduction in GAD65 enzymatic activity which is significantly stronger for b78 than for b96.11 (Raju et al. 2005). In agreement with this observation, an increase of extracellular glutamate concentration has been observed in the cerebellum after in vivo antibody administration with a significantly more pronounced effect of b78 as compared to b96.11 (Manto et al. 2011). Moreover, it has been recently discovered that the two antibodies affect the inhibitory synaptic transmission in cerebellar slices where b78 causes a significantly stronger long-term reduction contrary to b96.11. The same study has also proved that b78 only interferes with the association of GAD65 to GABAergic vesicles, disturbing GABA exocytosis in synapses (Manto et al. 2015). In hippocampal slices, after having demonstrated that GAD65-specific antibodies were able to enter living neurons without affecting cell viability, we observed that b78 antibody led to a significant microglial proliferation when added at 0.5µg/ml during 3 days. This effect was specific as it was not induced by b96.11 antibody. Moreover, b78 antibody, at the same concentration, prevented long-term potentiation (LTP) and inhibited the induction of Arc expression in culture. These results could be explained by an alteration of dendritic spines at the basal state or by the impairment of dendritic spines structural modifications normally induced by LTP and dependent on Arc expression. To answer this question, a confocal approach was realized on EGFP transfected neurons at the basal state. No disruption of dendritic spines density or morphology in slices incubated with b78 antibody was detected. However, it would be very interesting to reproduce this analysis after forskolin and IBMX application to determine if b78 could prevent the structural modifications of spines correlated to the increase of synaptic efficacy. Based upon these results, it would be also attractive to investigate the interaction between GABAergic interneurons and microglia in the context of synaptic plasticity. We sincerely thank the Professor C. Hampe of the University of Washington for having generously supplied b78 and b96.11 antibodies.