Diverse effects of anti-GluN1 antibodies in hippocampal excitatory synapses

Abstract

Antibodies are highly specialised immunological proteins produced by our immune system upon exposure to foreign, pathogenic protein material, which interact with specific epitope sites on their respective immunogen. The penetrative ability of antibodies to cross the blood-brain barrier and into the central nervous system has opened new avenues for immunotherapy development in various neurological disorders. We have previously reported neuroprotective and cognitive-enhancing effects of antibodies against the obligatory GluN1 subunit of the NMDA subtype glutamate receptor (NMDAR) in animal immunisation studies. This present study aimed to elucidate the underlying cellular mechanisms of these therapeutic effects and identify possible epitope-dependent responses by examining changes in protein expression as well as excitatory synaptic transmission in the glutamatergic synapse of cultured hippocampal neurons. Anti-GluN1 antibodies which bind to the glycine-binding site of GluN1 (recNR1 IgG) increased the total number of excitatory synapses and NMDARcontaining synapses but not AMPA receptor (AMPAR)-containing synapses in a concentration-dependent manner. This change in receptor expression was paralleled by an increase in NMDAR-mediated but not AMPAR-mediated synaptic response. On the other hand, anti-GluN1 antibodies which bind to the N-terminal domain of GluN1 (NR1.NTD IgG) did not alter the number of excitatory synapses or NMDAR expression, but did increase synaptic AMPAR expression. However, a coordinated increase in AMPAR-mediated synaptic response was not observed following NR1.NTD IgG treatment. Differences in treatment response between recNR1 IgG and NR1.NTD IgG signify possible epitopedependent effects. Although the precise pharmacological actions of anti-GluN1 antibodies remain unclear, the increased proportion of NMDAR-containing synapses in hippocampal neurons treated with recNR1 IgG suggests an increased presence of morphologically silent synapses, which could be implicated in plasticity changes relating to the improvements in learning and memory previously observed in our animal studies.