Comparative Effects of Domain-Specific Human Monoclonal Antibodies Against LGI1 on Neuronal Excitability

Abstract

Background and ObjectivesAutoantibodies to leucine-rich glioma inactivated protein 1 (LGI1) cause an autoimmune limbic encephalitis with frequent focal seizures and anterograde memory dysfunction. LGI1 is a neuronal secreted linker protein with 2 functional domains: the leucine-rich repeat (LRR) and epitempin (EPTP) regions. LGI1 autoantibodies are known to interfere with presynaptic function and neuronal excitability; however, their epitope-specific mechanisms are incompletely understood.MethodsWe used patient-derived monoclonal autoantibodies (mAbs), which target either LRR or EPTP domains of LGI1 to investigate long-term antibody-induced alteration of neuronal function. LRR- and EPTP-specific effects were evaluated by patch-clamp recordings in cultured hippocampal neurons and compared with biophysical neuron modeling. Kv1.1 channel clustering at the axon initial segment (AIS) was quantified by immunocytochemistry and structured illumination microscopy techniques.ResultsBoth EPTP and LRR domain-specific mAbs decreased the latency of first somatic action potential firing. However, only the LRR-specific mAbs increased the number of action potential firing together with enhanced initial instantaneous frequency and promoted spike-frequency adaptation, which were less pronounced after the EPTP mAb. This also led to an effective reduction in the slope of ramp-like depolarization in the subthreshold response, suggesting Kv1 channel dysfunction. A biophysical model of a hippocampal neuron corroborated experimental results and suggests that an isolated reduction of the conductance of Kv1-mediated K+currents largely accounts for the antibody-induced alterations in the initial firing phase and spike-frequency adaptation. Furthermore, Kv1.1 channel density was spatially redistributed from the distal toward the proximal site of AIS under LRR mAb treatment and, to a lesser extant, under EPTP mAb.DiscussionThese findings indicate an epitope-specific pathophysiology of LGI1 autoantibodies. The pronounced neuronal hyperexcitability and SFA together with dropped slope of ramp-like depolarization after LRR-targeted interference suggest disruption of LGI1-dependent clustering of K+channel complexes. Moreover, considering the effective triggering of action potentials at the distal AIS, the altered spatial distribution of Kv1.1 channel density may contribute to these effects through impairing neuronal control of action potential initiation and synaptic integration.

LGI1 autoantibodies enhance synaptic transmission by presynaptic Kv1 loss and increased action potential broadening

“…31 In contrast, some studies observed an increased neuronal excitability only with LRR but not with EPTP autoantibodies. 31,57,58 Our data indicate that the duration of the action potential is preferentially affected by EPTP autoantibodies, whereas LRR-targeting antibodies were previously shown to interfere with multimerization and cause internalization of the LGI1-ADAM22 complex. 13,30,31 A differential effect of autoantibodies targeting EPTP and LRR is conceivable because of the complex interplay of various types of potassium channels in controlling excitability and action potential repolarization.…”

“…Indeed, it was recently shown that LGI1 autoantibodies also alter the Kv1 cluster distribution at the axon initial segment. 57,58 The autoantibody-induced increase in neuronal excitability was mediated by antibodies specifically targeting the LRR-domain. 30,31,57,58 Consistently, structural analyses indicate that LGI1 can form protein complexes in a cis-configuration serving as an extracellular scaffold instead of a transsynaptic hub.…”

“…57,58 The autoantibody-induced increase in neuronal excitability was mediated by antibodies specifically targeting the LRR-domain. 30,31,57,58 Consistently, structural analyses indicate that LGI1 can form protein complexes in a cis-configuration serving as an extracellular scaffold instead of a transsynaptic hub. 15,59 It remains to be determined if the density of presynaptic Kv1 channel outside of the release site is controlled by LGI1 proteins in the cis-configuration.…”

Peripherally-derived LGI1-reactive monoclonal antibodies cause epileptic seizures in vivo

“… 10 In vitro primary neuronal cultures also incubated for 7 days with LRR- and EPTP-specific mAbs showed that LRR-directed mAbs could directly affect neuronal excitability, but this effect was less pronounced in EPTP-specific mAb exposed neurons. 23 In contrast to the action of LRR mAbs in vitro , the EPTP mAbs are reported to exert their effects by directly inhibiting the docking of LGI1 to the ADAM proteins. 14 In our study, EEG recordings have demonstrated increases in network and neuronal excitability in hippocampal CA3 in vivo by both LRR and EPTP domain directed LGI1 autoantibodies.…”

“… 14 In our study, EEG recordings have demonstrated increases in network and neuronal excitability in hippocampal CA3 in vivo by both LRR and EPTP domain directed LGI1 autoantibodies. 10 , 23 Studies have shown that LRR- and EPTP-specific antibodies co-occur in patients with LGI1-antibody encephalitis but the numbers of recorded rats with either mAb subclass do not allow for statistical comparison of epitope-specific clinical features ( Supplementary Fig. 1 ).…”