Abstract
Synapsin I is a phosphoprotein that coats the cytoplasmic side of synaptic vesicles and regulates their trafficking within nerve terminals. Autoantibodies against Syn I have been described in sera and cerebrospinal fluids of patients with numerous neurological diseases, including limbic encephalitis and clinically isolated syndrome; however, the effects and fate of autoantibodies in neurons are still unexplored. We found that in vitro exposure of primary hippocampal neurons to patient’s autoantibodies to SynI decreased the density of excitatory and inhibitory synapses and impaired both glutamatergic and GABAergic synaptic transmission. These effects were reproduced with a purified SynI antibody and completely absent in SynI knockout neurons. Autoantibodies to SynI are internalized by FcγII/III-mediated endocytosis, interact with endogenous SynI, and promote its sequestration and intracellular aggregation. Neurons exposed to human autoantibodies to SynI display a reduced density of SVs, mimicking the SynI loss-of-function phenotype. Our data indicate that autoantibodies to intracellular antigens such as SynI can reach and inactivate their targets and suggest that an antibody-mediated synaptic dysfunction may contribute to the evolution and progression of autoimmune-mediated neurological diseases positive for SynI autoantibodies.
Highlights
A large battery of autoantibodies directed to neuronal proteins have been discovered in sera and cerebrospinal fluid (CSF) of patients suffering from a variety of neurological diseases[1]
Autoantibodies directed against the amino-3-hydroxy-5methyl-4-isoxazolepropionic acid receptors (AMPARs), the N-methyl-D-aspartate receptor (NMDAR) and the γaminobutyric acid (GABA) type B receptor belong to the first group and are frequently detected in the serum and cerebrospinal fluid (CSF) of affected subjects[2]
We found that PitStop[2], to sodium azide, reduced the internalization of Synapsin I (SynI)-mAb (Fig. 5a, b), indicating that the antibody uptake mainly occurs via clathrin-dependent endocytosis
Summary
SynI autoantibodies from patient CSF impair excitatory and inhibitory synaptic transmission in hippocampal neurons To investigate potential pathogenic effects of SynI autoantibodies, we treated primary neurons with the CSF of a limbic encephalitis and clinical isolated syndrome patients ( referred to as LE-CSF and CIS-CSF, respectively) that were characterized by high titers of IgG and IgA autoantibodies against SynI13,14. Treatments with SynI-mAb induced a significant reduction in the frequency and amplitude of both mEPSCs and mIPSCs compared to control conditions (Fig. 2a, b), fully mimicking the results obtained with either LE-CSF or CIS-CSF (Fig. 1) These finding suggest that the impairment of excitatory and inhibitory synaptic. Addition of the nonspecific blocker of ATP-dependent endocytosis sodium azide[16] to neurons incubated with SynI-mAb induced a significant reduction of the antibody uptake (Fig. 5a, b) and abolished the effects of both SynI-mAb and LE-CSF treatments on mEPSC and mIPSC frequency and amplitude (Fig. 5c, d) To confirm this result using a distinct inhibitor of endocytosis, we pretreated neurons PitStop[2], an inhibitor of clathrin-mediated endocytosis[17]. This phenotype resembles the phenotype of SynI KO neurons[25,26,27] and indicates that SynI autoantibodies sequester endogenous SynI and alter its ability to form liquid phase and cluster SVs
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